Download Organic Reactions in Organised Media

THESIS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
Organic Reactions in Organised Media
TOMASZ WITUŁA
Department of Chemical and Biological Engineering
CHALMERS UNIVERSITY OF TECHNOLOGY
Göteborg, Sweden 2007
Organic reactions in organised media
TOMASZ WITUŁA
ISBN 978-91-7385-040-7
© TOMASZ WITUŁA, 2007. Doktorsavhandlingar vid Chalmers tekniska högskola Ny serie nr 2721 ISSN 0346-718X
Department of Chemical and Biological Engineering Chalmers University of Technology
SE-412 96 Göteborg Sweden Telephone + 46 (0)31-772 1000 Cover picture
A lipophilic reactant, 4-tert-butylbenzyl bromide, meets a hydrophilic reactant, an iodide ion, at the oil-water interface of a microemulsion. Chalmers Reproservice
Göteborg, Sweden 2007
ii
ABSTRACT
A common problem in synthetic organic chemistry is reactant incompatibility between
lipophilic organic compounds and inorganic salts. The thesis reports an investigation of some
organic reactions, involving incompatible substrate and nucleophile, performed in liquid
crystalline phases and also in slurries of mesoporous materials with different symmetry. In
such media, the reaction occurs at the hydrophilic/lipophilic interface, and, because the
interface is large, the reaction is fast. A considerable advantage with the use of slurries of
mesoporous materials, as reaction media is that the workup procedure is extremely facile.
After the reaction is completed, the solid is simply removed by filtering or centrifugation.
The mesoporous materials, as well as the different self-assemblies, such as surfactant liquid
crystals and microemulsions, can be seen as minireactors for organic reactions.
Different mesoporous materials were also used as host for a lipase and the enzyme-loaded
particles were employed as catalyst for esterification of caprylic acid using a mixture of
glycerol and water as reaction medium. Mesoporous materials loaded with cross-linked lipase
can be reused several times with only marginal loss of activity.
Keywords: Liquid crystal, mesoporous material, silica, alumina, titania, microemulsion,
organic synthesis, nucleophilic substitution, lipase, heterogenized, entrapment.
iii
List of Papers
I.
Use of a Mesoporous Material for Organic Synthesis Tomasz Witula and Krister Holmberg
Langmuir 2005, 21, 3782-3785 II.
Liquid crystalline phases and other microheterogeneous systems as media for
organic synthesis.
Tomasz Witula and Krister Holmberg
J. Dispersion Sci. Technol., 2007, 28, 73-79
III.
Use of a different types of mesoporous materials as tools for organic synthesis Tomasz Witula and Krister Holmberg
J. Coll. Int. Sci., 2007, 310, 536-545 IV.
Mesoporous materials as host for an entrapped enzyme
Pedro Reis, Tomasz Witula and Krister Holmberg
Microporous Mesoporous Mater., 2007, in press.
iv
Table of Content
1 Introduction…………………………………………………………………………………..1 2 Background…………………………………………………………………………………..4 2.1 Surfactants and surfactant self-assemblies…………………………………………….4 2.2 Mesoporous materials………………………………………………………………...10 2.3 Hydrophobation of mesoporous materials……………………………………………16 2.4 Mesoporous carbon…………………………………………………………………...17 3 Methods……………………………………………………………………………………..18 3.1 Nuclear Magnetic Resonance, 1H-NMR……………………………………………...18 3.2 BET - Nitrogen Physisorption………………………………………………………...19 3.3 Transmission Electron Microscopy (TEM)…………………………………………..21
3.4 Small Angle X-ray Scattering (SAXS)……………………………………………….24 4 Reactions in organised self-assemblies – some examples………………………………….26
4.1 Microemulsions and micellar solutions………………………………………………26 4.1.1 Detoxification of mustard gas…………………………………………………..28
4.1.2 SN2 reaction: Reaction of p-bromophenacylmethyl picryl ether with bromide...28
4.1.3 Maillard reactions………………………………………………………………29 4.2 Lyotropic liquid crystalline phases…………………………………………………...29 4.3 Mesoporous materials………………………………………………………………...30 5 Results and Discussion……………………………………………………………………...32 5.1 Mesoporous materials and self-assemblies used for a nucleophilic substitution…..…32 5.2 Mesoporous materials as host for an entrapped enzyme……………………………...50 6 Conclusions…………………………………………………………………………………57 Acknowledgements…………………………………………………………………………...59 References…………………………………………………………………………………….60 1 Introduction
One of the basic requirements for every reaction is attaining proper contact between reactants.
Insufficient contact between reactants will prevent the reaction from running efficiently. This
is almost always the case when immiscible hydrophilic and hydrophobic reagents are
involved in the process. Due to the relatively small interfacial area of the two-phase systems
that are formed, the rate of such reactions is low unless special measures are taken.
Typical reactions facing such a problem are nucleophilic substitution reactions between a
lipophilic organic compound and an inorganic ion, hydrolysis of organic compounds by
caustic, many electrophilic substitution reactions, and epoxidation of olefins. In all these
reactions one of the reactants prefers a polar phase – which is normally water – and the other
reactant prefers a nonpolar phase.
There are several methods to overcome the incompatibility between reactants. One of them is
to use an aprotic polar solvent like dimethylsulfoxide, dimethylformamide, acetonitrile, or
hexamethylphosphorotriamide, which is favourable for SN2 reactions, or a protic polar
solvent, which is favourable for SN1 reactions. This is a straightforward approach, however,
finding a suitable solvent that can dissolve both reactants is not always possible. Moreover,
most aprotic polar solvents have high boiling temperatures (above 150°C) and significant
toxicity and are therefore inadequate for scaled-up processes.
A simple approach to the problem of reactant incompatibility is to use a stirred two-phase
system. The contact area between the phases becomes relatively small, even with extensive
stirring, however, and the reaction rate obtained in stirred two-phase systems without
auxiliary agents added is usually low.
The two-phase system approach becomes much more favourable if a phase transfer catalysts,
most commonly a quaternary ammonium salt or a crown ether, is added. Such reagents
facilitate the migration of the polar reactant from the aqueous to the organic phase in two­
phase systems. Inorganic ions transported as an ion pair into a nonprotic organic solvent, such
as a chlorinated hydrocarbon, become very reactive due to lack of solvation. Such “naked
anions” are strongly nucleophilic.
1
Surfactant self-assembly systems like microemulsions or different liquid crystalline phases
are single-phase systems, still they contain both polar and non-polar domains in which
reactants of different solubility profiles can dissolve. Such systems are able to dissolve both
organic and inorganic components, which will meet at the interface where the reaction can
take place. The large internal interface of these systems significantly increases the probability
of contact between incompatible reagents as compared to the situation in macroscopic two­
phase systems. The improved contact leads to increased reactivity. It has been shown that the
total interfacial area governs the rate a nucleophilic reaction1. However, there is a negative
aspect of using surfactant self-assemblies: the surfactant needs to be separated from the
product after completed reaction, and preferably reused, and this type of work-up is not
always easy. A similar problem also exists with phase transfer catalysts.
Slurries of mesoporous materials, typically oxides like silica, alumina, or titania, in a nonpolar
medium
constitute
an
alternative
to
the
surfactant
self-assembly
systems
as
microheterogeneous media. From a work-up point of view they have a distinct advantage
since no surfactant needs to be removed from the reaction mixture. However, for the
preparation also these materials use surfactants as structure-directing agents. The self­
assembled surfactant acts as a template for formation of the mesoporous material. Removal of
the surfactant by leaching or by burning gives a porous material with the pore domain
corresponding to the space previously occupied by the surfactant network. The pores are
hydrophilic and can house a polar reactant dissolved in water. When fine particles of such
mesoporous materials, loaded with a polar reactant, are suspended in a nonpolar medium
containing the nonpolar reactant, the reaction may occur at the oil-water interface, i.e. the
pore openings. The approach offers possibilities for reuse of the material since the particles
can easily be removed from the reaction mixture by filtration or by centrifugation.
Another potential use of mesoporous materials in preparative organic chemistry is for
homogeneous catalysis. The catalyst, which may be an enzyme or a synthetic homogeneous
catalyst, is entrapped in the pores. The catalyst-loaded particles are suspended in a medium
that contains the reactants. Again, the reaction takes place at the pore openings.
The main objective of this work was to study the use of different mesoporous materials and
surfactant self-assemblies as tools in organic synthesis. Materials with different character and
structure such as surfactant liquid crystals with hexagonal or cubic structure and mesoporous
2
silica, alumina, and titania with either hexagonal or cubic geometry were investigated for the
purpose. Attempts were also made to use hydrophobic mesoporous materials, either
mesoporous graphite prepared from mesoporous silica as template or mesoporous silica that
had been hydrophobized by a post treatment procedure. The organic reactions chosen for the
investigations were well-known simple reactions since the aim was to explore the new
synthesis principle rather than developing a specific synthesis.
Paper I deals with the use of a hexagonal mesoporous silica and a hexagonal liquid
crystalline phase as media for the nucleophilic substitution reaction between 4-tert­
butylbenzyl bromide and iodide, comparing the results with those obtained in a regular two­
phase system and in a microemulsion2.
Paper II extends the previous work to some new materials with cubic structures and is more
focused on the liquid crystalline phases.
Paper III describes and compares the efficiency of three mesoporous oxide materials, silica,
alumina and titania. This paper also deals with various types of hydrophobic mesoporous
materials. It also includes a study on a material with extremely small particle size. Different
types of organic reactions are tested. The paper also demonstrates the potential of reuse of the
mesoporous material.
Paper IV shows the application of three mesoporous materials as host for an enzyme, a
lipase. Such enzyme-loaded particles were applied in the esterification reaction between
caprylic acid and glycerol. Both hydrophobic and hydrophilic mesoporous materials were
used. It was demonstrated that the mesoporous materials loaded with in-situ cross-linked
enzyme could be reused several times without significant loss of activity.
3
2 Background 2.1 Surfactants and surfactant self-assemblies The word “surfactant” is an abbreviation of surface active agent. Surfactants are almost
always organic compounds. They are amphiphilic compounds, which means that they have
both a hydrophilic (head group) and a hydrophobic (tail) region in the molecule. This dual
nature of the molecule ensures at least some solubility in both organic solvents and water.
One of the basic surfactants characteristics is their tendency to absorb at surfaces and
interfaces.
Figure 2.1.1 A surfactant.
Surfactants decrease the interfacial tension between water and oil lowering the free energy at
the interface. The reduction in the free energy of the interface is one driving force for the
adsorption.
The general classification of surfactants is due to the charge of the polar head groups.
Basically there are two main categories of surfactants: ionic and nonionic. Ionic surfactants
include anionic, cationic and zwitterionic species. (Zwitterionics, which have a net zero
charge, may alternatively be classified as nonionic surfactants.) Anionic surfactants like
sodium dodecyl sulfate (SDS), alkylbenzene sulfonates, alkyl phosphates, dialkyl
sulfosuccinates, fatty acid salts, etc. have a negatively charged head group.
Low raw material cost and simple manufacturing processes make anionic surfactants the most
widely used surfactants. Sodium dodecylbenzene sulfonate is the largest of all synthetic
surfactants.
4
Cationic surfactants like cetyltrimethylammonium bromide (CTAB), cetylpyridinium chloride
(CPC), benzalkonium chloride (BAC), etc. have a positively charged head group. Cationic
surfactants adsorb strongly to surfaces because the majority of surfaces carry a net negative
charge.
Zwitterionic surfactants have both appositive and a negative charge. Phospholipids, the most
abundant of the amphiphiles in the body, are examples of zwitterionics. Other examples are
betaine and imidazoline surfactants.
Non-ionic surfactants have either a polyether or a polyhydroxyl unit as the polar head group.
Most commonly this group is a polyoxyethylene chain. The polyoxyethylene moiety is made
by polymerization of ethylene oxide. Nonionic surfactants were used as structure directing
agents for the mesoporous materials used in this work and in the majority of cases triblock
copolymers
of
the
type
poly(ethylene
oxide)-block-poly(propylene
oxide)-block­
poly(ethylene oxide) with the general composition (EO)n(PO)m(EO)n, were used. These
substances, which have a molecular weight of 4-12 000, can be regarded as either an
unusually high molecular weight surfactant or an unusually low molecular weight surface
active polymer. They are often referred to as Pluronics but Pluronic is the BASF trade name
of these surfactants. Figure 2.1.2 shows structures of some common nonionic surfactants.
O
OH
5
A f atty alcohol ethoxylate, penta(ethylene glycol)monododecyl ether, C12E5
O
OH
O
COO
O
O
A f atty acid ethoxylate, dodecanoic acid monoester of pentaethylene glycol
O
O
O
H
H OH
n
H O
HO
HO
H
H
m
An EO-PO block copolymer
H
OH
O
An alkyl glucoside, dodecyl glycoside
Figure 2.1.2 Structures of some common nonionic surfactants.
5
OH
n
The block copolymers used in this work are Pluronic 105, 123 and 127. Their compositions
and molecular weights are shown in Table 2.1.1.
Pluronic
n
m
MW (Da)
P105
37
58
6 500
P123
20
70
5 800
P127
100
65
12 600
Table 2.1.1 EO-PO block copolymers of the general formula (EO)n(PO)m(EO)n used in this
work.
A fundamental property of surfactants is that in solution they create aggregates, called
micelles. The aggregation process, called micellization, starts at the so-called critical micellar
concentration (CMC). The value of the CMC is the highest possible concentration of non­
aggregated surfactant molecules, unimers, in a solution. Surfactant micelles may be spherical,
rodlike, disclike or (in rare cases) branched. Figure 2.1.3 illustrates a spherical micelle.
Figure 2.1.3 A schematic cross-section of a spherical micelle.
Micellization is caused by the hydrophobic effect3,4, which means that the hydrophobic part of
the surfactant tries to avoid contact with water. Minimization of Gibbs free energy is the
driving force for formation of structural order5. At higher surfactant concentration the
micelles start to sense each other and form structures with long range order, so-called liquid
crystals. The liquid crystals are of different types such as hexagonal, micellar cubic, lamellar,
bicontinuous cubic, and reversed hexagonal.
6
The aggregate structure is basically determined by the surfactant geometry. This can be
expressed by a dimensionless number called critical packing parameter (CPP), which relates
the head group area (a0) to the extended length (l) and the volume (v) of the hydrophobic part
of a surfactant molecule. The CPP concept is illustrated in Figure 2.1.4 and different types of
self-assembled structures are shown in Figure 2.1.5.
Figure 2.1.4 The critical packing parameter, CPP, of a surfactant.
7
Figure 2.1.5 Critical packing parameter and the corresponding self-assembly structures.
(Redrawn from Ref. 6.)
The surfactant liquid crystals, which are thermodynamically stable systems, are often referred
to as lyotropic liquid crystalline phases. The word “lyotropic” means that the ordering effect
is induced through changes in the concentration of the surfactant in water. The structure of the
surfactant aggregate is dependant on the concentration of the surfactant in the solution. At
very low concentrations, below the CMC, there is no ordering at all and the molecules are
freely dispersed in the solvent. At the CMC, which is usually at very low concentration,
aggregation starts and micelles are created.
8
Most micelles are spherical at low surfactant concentration. An increase in the concentration
causes the surfactants to form more elongated structures, which on a further raise in the
concentration gives a liquid crystalline phase that can have hexagonal or micellar cubic
geometry. At higher surfactant concentration a lamellar system, with planes of double-layers
of surfactant molecules separated by thin layers of water, may be created. At very high
concentrations, reversed (water-in-oil) systems can form. The phase diagram for the block
copolymer Pluronic 105, which displays a broad range of liquid crystalline phases, is shown
in Figure 2.1.6.7
Figure 2.1.6 The phase diagram for the block copolymer Pluronic 105. (Redrawn from Ref.7)
The liquid crystalline phases can be said to have long range order but short range disorder,
which is contrary to normal crystals, which have both long and short range order. In spite of
the diversity of structures, in all liquid crystalline phases the surfactants are arranged such that
the ends of the hydrophobic tails of two surfactant films come together.
Due to the presence of both a hydrophilic and a hydrophobic domain, liquid crystalline phases
can solubilize polar, water soluble substances as well as nonpolar, oil soluble compounds. The
9
interface between the polar and nonpolar compartments is very large, which improves the
contact between such incompatible reactants compared to the situation in standard two phase
system. This property can be utilized to increase significantly the rate of an organic reaction
between a polar and a nonpolar compound, as is shown in Papers I and II.
2.2 Mesoporous materials
According to the IUPAC classification, porous materials can be divided into three main
groups depending on the pore diameter (d): micro- (d < 2 nm), meso- (2 nm < d < 50 nm) and
macro- (d > 50 nm)8. The first publications on inorganic mesoporous oxides appeared in
1992-19939-11. Such materials with meso-sized pores have highly regular periodic structure
and often uniform pore size distribution. The synthesis of mesoporous materials is based on
the use of surfactants as structure-directing agents12,13. The introduction of self-assemblies
like micellar aggregates (rather than unimeric, non-self-assembled surfactant molecules) as
directing agents gave new possibilities in terms of formulation of new ordered materials14.
Because of their ordered structure, high porosity, narrow pore size distribution, and large
surface area such materials are of potential interest for separation, catalysis, absorption, and
several other applications15. We16-19 and others (see Chapter 4.3) have explored them as a tool
for organic synthesis. As an auxiliary agent in an organic reaction such materials possess one
distinct advantage over surfactant-based systems, such as microemulsions and liquid
crystalline phases, namely they are surfactant-free. This makes isolation of the product and
also reuse of the auxiliary agent extremely easy. This aspect is important, not least for large
scale synthesis where separation by chromatographic methods is difficult.
There are basically two ways of preparing the mesoporous materials. One may either use
surfactants at a concentration where they form micelles or one may use a concentration high
enough to be in the region where a liquid crystalline phase is formed in a binary surfactant­
water system. In the former case the micelles, which are elongated, need to align in structures
corresponding to the organization of the liquid crystals and this ordering is caused by the
presence of the inorganic prepolymers in solution. In the latter case, the surfactant liquid
crystals can be used directly as template. The two processes are illustrated in Figure 2.2.1.
10
Figure 2.2.1 Suggested templating routes for generation of mesoporous materials.
This means that the syntheses described in the literature cover a very wide range of surfactant
concentrations, from only slightly above the CMC to more than 10 weight %. The low
surfactant concentration route, in which the ordered structure is generated through an
interplay between the surfactant micelles and the growing inorganic prepolymer (the upper
route of Figure 2.2.1), is by far the most widely used synthesis method. The mechanism by
which the well-ordered composite material forms in solution has been extensively studied,
particularly for silicate, and is still under debate20,21.
Huo et al.22 presented four different synthesis routes (S+I–), (S–I+), (S+X–I+) and (S–M+I–),
where S = surfactant (+ cationic, – anionic), I = soluble inorganic (silica), X = halogen ion,
and M = metal cation. The models are based on electrostatic interactions and to ensure proper
conditions the reaction must occur either at very low pH, where the silica surface is positively
charged, or at pH = 7–10, where the surface is negatively charged.
Tanev and Pinnavaia23 have proposed another route (S0I0, see Figure 2.2.2) based on
hydrogen bonding between a neutral amphiphile (like a nonionic surfactant) S, and a neutral
11
precursor I (such as silica source). It is known from other studies that there is strong
interaction between surfactants containing polyoxyethylene segments and silica in solution24.
Figure 2.2.2 Schematic representation of silicate-surfactant-solvent interactions. Circles
represent the solvent, dashed lines represent H-bonding interactions, M-O represents the
precursor, e.g., silica.
The S0I0 templating route gives mesostructures with improved stability due to thicker pore
walls. Thicker framework walls are the result of weaker repulsive interactions between neutral
metal oxide precursors at the surfactant-solution interface than between charged precursors.
The most significant advantage of S0I0 templating route is that it allows the synthesis of
mesoporous structures that are not accessible by other, electrostatic templating routes25.
Attard et al.26 introduced the procedure of using a preformed liquid crystalline phase as a
template for the formation of mesoporous silica (so called direct-templating method)20. This
direct templating method does not require a specific surfactant-silicate interaction, as does the
method of synthesis in a micellar solution. An illustration of this route is the formation of
mesoporous alumina with bicontinuous cubic symmetry, synthesized by direct casting from
12
the corresponding liquid crystalline phase made up of the nonionic surfactant monoolein and
water27. Monoolein does not contain a polyoxyethylene chain, which means that it is unlikely
to interact with silicate polyanions. Highly ordered mesoporous materials have been obtained by the use of a lyotropic liquid crystalline mesophase as a direct template28 (Figure 2.2.3). Figure 2.2.3 The direct templating method for synthesis of mesoporous silica. (Redrawn from
Ref. 20.) Mesoporous materials can also be formed by allowing ethanol to slowly evaporate from a water/ethanol solution of a silica source and a surfactant. The method is called evaporation­
induced self-assembly29. As the surfactant concentration increases and as the silicate
prepolymer grows in size during the evaporation, an ordering of the aggregates in solution occurs and eventually a well-ordered composite material is formed. The specific parameters of the resulting mesoporous material like pore diameter and specific surface area are dependant on the length of the hydrophobic tail of the surfactant and on the type and amount of auxiliary organic compound, which are capable of swelling micelles11. The auxiliary organic compound 1,3,5-trimethylbenzene has been used to expand the pore size of MCM-41, which is a sort of mesostructured silica, up to 100 Å30. The pore diameter of a mesoporous material prepared by the help of self-assembled
surfactants having one alkyl chain and one polar head group corresponds approximately to
twice the extended length of the alkyl chain. Thus, the pore size is governed by the choice of surfactant, which means that the material can be design with high degree of precision. For EO-PO block copolymers as structure directing agent it is the polyoxypropylene block that determines the pore size. Mesoporous oxide materials like silica, alumina, titania, etc. are generally stable and durable and have very good mechanical properties. It is also possible to influence the basic surface 13
properties such as acidity through surface modifications, as will be discussed below. The
well- defined porous structure combined with the diversity with regard to both material type
and surface character makes mesoporous materials of interest for a wide range of potential
applications. The literature on preparation and use of such materials has grown rapidly since
the early 1990’s, see Figure 2.2.4. This thesis describes a novel application of mesoporous
materials: their use as a tool in organic synthesis.
1000
Number of articles
900
800
700
600
500
400
300
200
100
0
1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004
Year
Figure 2.2.4 The development of the research field of mesoporous silica presented as number
of articles published per year in journals covered by SciFinder. (Received from Andreas
Berggren).
There are some differences in the wall structures and other properties of mesoporous silica
materials templated by nonionic block copolymers and cationic surfactants31. Block
copolymers give more stable materials and larger pore diameters32, the size of which can be
influenced by the temperature30. A very wide variety of ordered structures and large pore sizes
can be synthesized by the use of block copolymers30,33,34 and, as mentioned above, the
properties can be controlled by the choice of the block copolymer. Since block copolymers
have been the main templating agent in this work, the materials characteristics obtained with
theses surfactants are discussed in some detail below.
14
An investigation was made on the influence of the EO chain length when block copolymers
with approximately the same length of the PO block was used34. The study showed that the
length of the EO block determines which meso-structure is created. It was found that
copolymers with very short EO blocks, such as Pluronic L101 with blocks of 4 EO units, gave
a material with lamellar structure. Pluronics with EO blocks of 17-37 units gave two­
dimensional hexagonal mesostructures. Block copolymers with very long EO chains, such as
Pluronic 108 with 132 EO units, formed body-centred cubic materials. A similar study was
made with a series of block copolymers with the same EO chain length but with different
sizes of the PO block32. An increase in the length of the PO segment was found to facilitate
the formation of ordered structures. Flodström et al32. observed more distinct Bragg peaks for
Pluronics with longer PO-blocks; however, there is no simple explanation for this
observation.
The amphiphilic properties of block copolymers are highly temperature-dependant. The more
hydrophilic polymers (long EO or short PO-chains), require a higher synthesis temperature
than the less hydrophilic ones.
Different types of metal oxides have been made as mesoporous materials. Also combined
mesoporous materials, such as silica-alumina, have been synthesized35. Many of the different
materials prepared have been evaluated and applied as catalyst carriers in heterogeneous
catalysis.
Most mesoporous materials have been made in the form of powders, but stable films, for
instance of silica36,37, have also been prepared. These are of interest as membranes.
15
2.3 Hydrophobation of mesoporous materials
Due to the presence of hydroxyl groups on the surface, mesoporous materials are hydrophilic
in nature. There are possible routes for surface modifications, which change the surface
characteristics from hydrophilic to hydrophobic. Silylation is one of the most common ways
of such conversion. Typically, trimethylsilane is used as hydrophobizing agent. Such
silanization is a well-known procedure, and the conversion of hydrophilic hydroxyl groups
into hydrophobic trimethylsilyl groups usually reaches a degree of about 85 %38. The silica
surface contains free silanol (SiOH) groups and siloxane bridges (Si-O-Si), which are formed
by condensation of two adjacent silanol groups and hydrogen-bonded SiOH groups. Only the
silanol groups can react with the silylating agent. Thus, in order to increase the silylation
efficiency, the siloxane bridges and hydrogen-bonded groups are first hydrolyzed back to
pairs of geminal silanols38. This can be made by the thermal heating or by the use of a strong
acid or base.
The silylation reaction can be monitored by FTIR. New absorption bands corresponding to
trimethylsilyl groups are easily identifiable at 2965 and 2907 cm-1. The modification also
leads to a decrease in the pore diameter of 8-10 Å (Paper 3), which roughly corresponds to
two monolayer of trimethylsilyl groups (one on each side of the pore). Figure 2.3.1 shows the
reactions involved in surface hydrophobation using trimethylsilyl chloride as hydrophobizing
agent.
Si
O
O
H
O
Si
O
Si
temperature,
strong acid or base
Si
O
O
Si
H
O
O
Cl-Si(CH3) 3
O
O
Si
O
H
Figure 2.3.1 Surface activation and hydrophobation of silica.
Si(CH 3) 3
O
O
Si
H
Si(CH 3) 3
O
Si
O
16
Si
H
O
Si(CH 3) 3
O
Si
H
O
O
O
O
O
H
Si
H
Si
H
O
O
O
O
Si
Si
Si
Si(CH 3) 3
O
Si(CH 3) 3
2.4 Mesoporous carbon
Mesoporous carbon can be synthesized by a double templating technique39, in which the pores
of ordered mesoporous silica are impregnated with a polymerizable organic species, such as
furfuryl alcohol40, serving as a source of carbon, and a polymerization catalyst. Upon heating,
the monomer polymerizes creating a network inside of the template. Pyrolysis in the presence
of an inert gas converts the organic polymer into carbon. Since the polymer contains oxygen,
carbon dioxide will also be released, creating cavities in the material. In order to fill these, the
material is again impregnated with the polymerizable compound and the polymerization
followed by pyrolysis is repeated. This procedure may be repeated several times. The
resulting carbon/oxide composite material is treated with hydrofluoric acid in order to remove
the silica matrix. A schematic illustration of the process is shown in Figure 2.4.1. This
technique gives mesoporous carbon with a high degree of order.
Figure 2.4.1 The double templating technique to prepare mesoporous carbon from
mesoporous silica.
17
3 Methods
3.1 Nuclear Magnetic Resonance, 1H-NMR
Nuclear magnetic resonance (NMR) spectroscopy is a powerful technique based on the
electromagnetic radiation absorption by molecules with nuclei with nonzero spins, such as 1H,
13
C, 15N, etc. The technique has a wide variety of practical applications but in this work it was
only used for identification of organic species. The different magnetic moments for every type
of nucleus make them easily distinguishable from each other. The chemical shift represents
the relation between the nuclear magnetic energy level and the electronic environment in a
molecule and is defined as the quotient of the difference in precession frequency of the spin
magnetic moment between two nuclei and the Larmor frequency. The precession concerns the
changes in the direction of the rotating object axis.
The Larmor frequency (sometimes referred to as operating frequency, ω0) of the precession is
given by the relation:
ω0 = γ · B0
where γ is the gyromagnetic ratio (ratio of magnetic dipole moment and its angular
momentum) and B0 is the strength of the magnetic field.
The chemical shift is the most valuable information from the structure analysis point of view.
Depending on the magnetic environment nuclei precess at different frequencies and this is the
basis for their identification. A schematic illustration of structure analysis in terms of
chemical shift is shown in Figure 3.1.1. All organic reactions presented in this work, except
those of Paper IV, have been analysed by means of 1H-NMR.
There are several factors influencing the chemical shift. One of them is the electron density of
surrounding groups. Electronegative or electropositive groups will greatly affect the electron
density around the nuclei and since different electron densities give differences in the local
magnetic field this results in chemical shift changes.
NMR is a non-destructive technique, and no calibration is needed as in the case of many other
analysis methods.
18
CH OH
O
C
H
N
R
CH NH 2
H
O
C
R
R
H
C
R
R
H
H
10
9
8
7
6
5
4
3
2
1
0
ppm σ
Figure 3.1.1 1H-NMR chemical shift range of hydrogen atoms in different environments.
3.2 BET - Nitrogen Physisorption
BET (after Brunauer, Emmett and Teller) nitrogen physisorption41 at 77 K is a technique
which allows determination of specific surface parameters like surface area, pore size
distribution and porosity of porous materials. The underlying processes are adsorption and
desorption of an inert gas at the surface of a porous material. The BET theory extends the
Langmuir theory to include the adsorption of two or more molecular layers. Often molecules
form multilayers at the surface and then the Langmuir isotherm is not valid. Brunauer,
Emmett and Teller proposed the following scheme:
A(g) + S ⇄ AS
A(g) + AS ⇄ A2S
A(g) + A2S ⇄ A3S …
where A is an adsorbate and S is a surface.
Adsorption is usually expressed in the form of isotherms, which relate amount of adsorbate on
the adsorbent with its pressure (gas) or concentration (liquid). Langmuir’s hypotheses are not
always true, especially the statement that only monolayers are formed at maximum
19
adsorption. Figure 3.2.1 shows the differences in Langmuir’s and BET’s assumptions of
adsorption layers42.
Figure 3.2.1 Illustration of Langmuir’s (top) and BET’s (bottom) theories concerning
adsorption of an adsorbate, A, on a solid surface, S.
The BET theory leads to the BET equation.
p
1
C −1 p
=
+
⋅
V ( p0 − p) Vm C Vm C p0
20
where:
- V is the amount of adsorbed gas
- Vm is the amount of adsorbed gas in a monolayer (monolayer capacity)
- p is the partial pressure of the adsorbate
- p0 is the saturation vapour pressure of the adsorbate
- C is a parameter indicating the interaction between the adsorbent and the adsorbate
Having Vm, we easily get the specific surface area of the porous material:
S=
Vm
1
⋅ N A ⋅ am ⋅
22400
m
where
- S is the specific surface area (m2/g)
- Vm/22400 is the number of moles of nitrogen adsorbed in a monolayer
- NA is the Avogadro number (6.023 ⋅10 23 )
- am is the molecular area of adsorbate included in a monolayer (for N2 equal to 16.2 Å2)
- m is the mass of the sample (g).
An adsorption isotherm is plotted as a quotient 1/[V(p0/p − 1)] in the function of p/p0. The
function is linear in the range 0.05 < p/p0 < 0.35. Values of Vm and C are calculated from the
plot. The isotherm is called a BET plot and the BET area is obtained by multiplying the area
per molecule, am (for N2 equal to 16.2 Å2) times the number of molecules.
3.3 Transmission Electron Microscopy (TEM)
Transmission electron microscopy (TEM) is a technique commonly used to characterize
different materials from a crystallographic or structural point of view. In TEM, contrary to
light microscopy, a beam of electrons is applied. Due to the low wavelength of accelerated
electrons (picometers), transmitted through a specimen, a large resolution, up to a million
times better than in the light microscope, can be obtained. The electron source (usually heated
tungsten wire) is placed at the top of the microscope column. The electron beam passes
through the electromagnetic lenses, focusing electrons into a very narrow beam. Such a
focused beam passes through a thin slice of a specimen (100-200 nm). Unscattered electrons
21
are projected on a fluorescent screen generating an image, which can be viewed or saved.
Moreover, the diffraction pattern can be obtained.
The sample preparation in the case of solid porous materials consists of thorough material
crushing, dispersion of the powder in a small volume of a volatile solvent, like ethanol, and
placing one or two drops of the dispersion on a copper grid.
Figure 3.3.1 A simplified view of a TEM microscope.
The objective lens is the most important part of the TEM microscope because it produces the
first intermediate image (diffraction pattern and image), and its quality is decisive for the
quality of the final image. The objective lens forms a diffraction pattern which can be useful
22
in crystallographic structure determination. Some typical TEM images obtained for different
porous oxides are shown in Figure 3.3.2.
Figure 3.3.2 TEM images of different porous oxides: (a) hydrophilic cubic silica, (b) and (c)
hydrophilic hexagonal silica, (d) hydrophobic hexagonal silica.
The acceleration voltage of the instrument determines wavelength and resolution of the
microscope. The acceleration voltage suitable for most applications is in the range of 120 –
200 kV.
23
3.4 Small Angle X-ray Scattering (SAXS)
Small Angle X-Ray Scattering (SAXS) is an analytical technique, in which information about
characteristic distances of ordered materials, pore sizes and structural regularities can be
obtained. “Small angle” means that X-ray scattering occurs only at low Bragg angles, in the
range of 0-10°.
The rays of the incident beam are always parallel to the top beam, which hits the external top
layer with z-atoms (see Figure 3.4.1). Every consecutive beam has to travel some extra
distance. In the case of the second beam, the distance is AB+BC. According to the theory, the
distance is always an integral multiple (n) of the wavelength (λ).
Figure 3.4.1 Graphical derivation of Bragg’s law43.
24
This leads to the following equation:
n λ = AB + BC and AB = BC
=>
n λ = 2 AB (1)
According to trigonometry:
AB = d ⋅ sin θ
(2)
Combining equations (2) and (1), we get Bragg’s equation in the final form:
n λ = 2 d ⋅ sin θ
n is always an integer, λ is the wavelength of the X-rays
d is the distance between the planes in the atomic lattice θ is the angle between the incident ray and the scattering planes The distance d obtained by SAXS measurement can be used to calculate the unit cell
parameter (the repeat distance), which includes the pore size together with walls thickness44. In the case of a hexagonal material the following dependency exists:
a = d(100) ⋅ 2 /
3
a is the unit cell parameter, which is a basic building block of a structure, repeated infinitely
in three dimensions
For porous materials ordered domains usually exist together with nonordered ones. This is the
reason for the noise in the X-ray patterns. Hence, it is not always possible to find the proper
peaks in partially ordered materials.
25
4 Reactions in organised self-assemblies – some examples
Due to their unique properties (large surface/interfacial area and ability to dissolve
compounds with different solubility profiles) organised self-assemblies have been frequently
employed in organic synthesis. This chapter will present a brief overview of such systems and
reaction types.
4.1 Microemulsions and micellar solutions
Mackay was one of the first to use microemulsions as media for chemical reactions. In the
mid-70s he explored oil-in-water (L2) microemulsions as reaction media for alkaline
hydrolysis of p-nitrophenyl diphenyl phosphate and other nucleophilic substitution
reactions45-48. In the years to come there was a broad activity in the field and a large number
of reaction types were explored49-60. Schomäcker described the use of microemulsions as
media for a large number of reactions including nucleophilic substitutions, alkylations,
Knoevenagel condensations, ester hydrolyses, oxidations and reductions55,61. By preparing
microemulsions based on water, hydrocarbon and a nonionic surfactant, and by adjusting the
temperature during the course of the reaction most reactions were completed in less than 2
hours. The work-up procedure took advantage of the effect of temperature on the solution
behaviour of the surfactant. If the reaction product was lipophilic, then the microemulsion was
cooled to induce phase separation, the lower water phase was removed, and the product
isolated by evaporation of the organic solvent. If, on the other hand, the reaction product was
more soluble in water than in hydrocarbon, heating was applied. The upper phase containing
the surfactant was separated from the aqueous phase and the product was isolated from the
latter phase by evaporation of water.
Microemulsions have also been applied to induce regiospecifity62,63,
for bioorganic
reactions64-66, for photochemical reactions67,68, for electrochemical reactions69, in technical
processes70, for ultrafine particles synthesis71, for electron-transfer reactions72, for inorganic
reactions73, for surfactants synthesis74 etc. Matson et al. patented a way of conducting
chemical reactions in reversed micellar or microemulsion systems75. Reactivity of organic
species in reversed micelles has been also investigated by Ryzhkina et al76. Bhowmik et al.
have studied photophysical properties of a dye in a self-organised reversed micellar system77.
26
Such reversed systems have also been utilized to entrap enzymes. An example of a reaction
using an enzyme entrapped in such water pools is oxidation of bisphenol A catalyzed by
laccase hosted in reversed micelles78. Self-assembled surfactant systems have been
successfully used for epoxidation of an unsaturated enone - trans-chalcone by hydrogen
peroxide79,80. A colour-forming reaction in a reversed microemulsion system based on AOT
(sodium bis-(2-ethylhexyl)sulfosuccinate) as surfactant has been studied by Liu et al81.
Garcia-Rio et al. studied the influence of cyclodextrins on the hydrolysis kinetics of selected
reactions in water and in micellar systems82.
The effect of micelle formation on reaction rates is primarily a consequence of reactant
compartmentalization66. The pseudophase kinetic model assuming that micelles act as a phase
(pseudophase) apart from water is commonly used. Further, it is assumed that effects on
reaction rates are due primarily to the distribution of reactants between the micellar and
aqueous pseudophases83. Thus, the rate of a reaction is the sum of adjusted rates in the
aqueous and micellar pseudophases. The use of micellar catalysis suffers from one drawback.
Its preparative value is of limited importance since the reactant concentration is normally too
low. A micellar solution has a limited solubilization capacity.
In their ability to overcome reactant incompatibility microemulsions can be seen as an
alternative to two-phase systems with added phase transfer catalyst. It has also been
demonstrated that the two approaches can be combined1,84.
Oehme and coworkers have recently published a review on organic reactions in
microemulsions with more than 200 references85 and there are several other good overviews
on the use and potential of microemulsions within organic synthesis. Other reviews, such as
one by Martinek et al86 (325 references) and one by Yatsimirskii87 (235 references), deal with
the use of microemulsions for bioorganic reactions. Some representative examples of organic
reactions performed in microemulsions are shown below.
27
4.1.1 Detoxification of mustard gas.
OCl S
S
Cl
Cl
O
Figure 4.1.1.1 Oxidation of half-mustard with hypochlorite.
Menger and Elrington have used microemulsions for the detoxification of the water insoluble
compound called mustard88 (a highly potent chemical warfare agent). In order to reduce
toxicity, half-mustard, see Figure 4.1.1.1, was used in the laboratory work. Oxidation with
aqueous sodium hypochlorite, which took months in a regular two phase systems and could be
speeded up to minutes by addition of a phase transfer catalyst, was completed in less than 15
seconds in a properly formulated microemulsion.
4.1.2 SN2 reaction: Reaction of p-bromophenacylmethyl picryl ether with bromide
O2N
Br
COCH 2 O
NO2
+
Br-
O2N
O2N
Br
COCH 2Br
+
O
NO2
O2N
Figure 4.1.2.1 Nucleophilic substitution of p-bromophenacylmethyl picryl ether with bromide
ions.
Bunton and Buzzaccarini studied the SN2 reaction between p-bromophenacylmethyl picryl
ether and bromide ions89. The reaction was followed in a microemulsion based on the cationic
28
surfactant cetyltrimethylammoniumbromide and it was concluded that the reaction ran
smoothly and was of second order. Bunton has also worked out a method of determining the
ion concentration in the micellar region based on the micellar electrostatic surface potential90.
The counterion of cationic surfactants is also of importance for the rate of reaction in
microemulsions and micellar systems. Large, polarizable counterions, such as bromide and
iodide, bind more strongly to the interface where they may expel anionic reactants, thus
giving rise to lower reaction rates than when the same surfactant but with a smaller
counterion, such as acetate or chloride, is used91-93.
4.1.3 Maillard reactions
The Maillard reaction is a reaction between a reducing sugar and an amino acid. The resulting
molecules are of importance in the flavouring industry. Depending on the type of amino acid
used, different flavours are formed. The reaction between furfural and cysteine, leading to 2­
furfurylthiol, as well as the reaction between ribose and cysteine, has been studied in self­
assembled surfactant systems94,95. It was found that the rate of the former reaction was about
five times faster in a water-in-oil microemulsion and about six times faster in a liquid
crystalline phase with cubic geometry than in water.
O
O
O
HOOC
+
H
SH
SH
NH2
Figure 4.1.3.1 Synthesis of 2-furfurylthiol from furfural and cysteine.
4.2 Lyotropic liquid crystalline phases
Lyotropic liquid crystalline phases (LLCP) have also been used as media for chemical
synthesis17,94,96, however, not as often as microemulsions. Liquid crystalline phases are related
to the structures that build up cell membranes. They have been used in the simulation of
29
natural biochemical processes97 and they have also been employed as media for both organic
and inorganic synthesis. Surfactant liquid crystals have also been used in biocatalysis98-101 and
for synthesis of fine polymer lattices102. An extensive review by Batyuk et al. with 395
references, titled: “Kinetics of chemical reactions in liquid crystals” deals with
physicochemical characteristics of both lyotropic and thermotropic liquid crystalline phases
and reactions in these103.
4.3 Mesoporous materials
Since the first synthesis of a mesoporous material in the late 80’s by Mobil Oil Company,
research in the field has continuously grown (see Figure 2.2.4). Due to their unique properties,
mesoporous materials have been applied in catalysis, sorption, as separation media etc. A
novel application is as host for entrapped enzyme.
In bioorganic synthesis, the catalyst, i.e., the enzyme, can be used in either free or
immobilized state. This parallels chemical reactions in general for which the catalyst can be
either specific atoms at the surface (heterogeneous catalysis) or catalytically active molecules
in solution (homogeneous catalysis), the latter usually being metal-organic substances.
Homogeneous catalysts have some attractive properties, such as high selectivity and good
accessibility to catalytically active sites104. However, heterogeneous catalysts have many
advantages over the homogeneous ones, such as easy catalyst regeneration and good stability
to harsh conditions.
A specific problem with enzymes when used free in solution, i.e., as homogeneous catalysts,
is their susceptibility to protease catalyzed breakdown105. Most of the problems associated
with free enzyme can be circumvented by immobilization of the enzyme. The degradation rate
will usually be much decreased and the work-up becomes equivalent to that when
heterogeneous catalysts are used. The carrier-bound enzyme can simply be removed from the
reaction mixture by filtration or centrifugation and then reused.
Immobilization of enzymes to a support material is usually carried out by either of three
routes: covalent binding, adsorption, or encapsulation/entrapment106. Immobilization by
covalent bonds and also adsorption by electrostatic interactions may severely affect the active
30
site, which can lead to loss of activity107. These procedures may also create diffusional
restrictions that may impair the activity108. Immobilization by adsorption to a hydrophobic
surface will usually not directly affect the active site but may have other consequences. Such
an interaction is based on van der Waals forces between the enzyme and the support and the
nature of the surface is crucial for successful immobilization, as well as for long term
retention of enzymatic activity109. Too weak interaction between the enzyme and the surface
may lead to leakage of the enzyme; too strong interaction may cause the enzyme to gradually
change its conformation such that the activity decreases, a phenomenon known as
denaturation.
Encapsulation or entrapment relates to techniques of trapping the catalyst in the pore space of
a carrier material110. The technique can be used with both organic and inorganic support
materials and a variety of encapsulation procedures have been developed. An encapsulated
enzyme is normally resistant to peptidase catalyzed biodegradation and can usually be
recovered and reused many times.
There are not many publications dealing with use of mesoporous materials in organic
synthesis. Choi et al. have investigated organic bases immobilized on mesoporous materials
and their catalytic activity in the Knoevenagel condensation between ethylcyanoacetate and
benzaldehyde. An imidazole catalyst immobilized on a material with three-dimensional
channels showed good activity and could be reused111.
Zhou et al. have used sulfonic acid-functionalized mesoporous oxide material (MCM-SO3H)
for the pinacol rearrangement reaction (converting an 1,2-diol to a ketone) under mild
condition112. Mesoporous materials with an extremely high content of sulfonic acid groups
were used by Feng at al.113 for catalytic esterification of cyclohexanol with acetic acid. The
results regarding activity were comparable with those obtained with concentrated sulfuric
acid.
Goettmann at al.114 have studied the highly regioselective hydroformylation of terminal
alkynes and the Pauson-Khand reaction catalyzed by mesoporous zirconium oxide powder.
Mesoporous materials have proven to have a potential in hydrodesulfurisation and metathesis
reactions115. Several chemical conversions over MCM-41-type materials have been described
by Van Bekkum et al116.
31
5 Results and Discussion
5.1 Mesoporous materials and self-assemblies used for a nucleophilic
substitution.
Paper I deals with the use of a hexagonal mesoporous silica and a hexagonal liquid
crystalline phase as media for the nucleophilic substitution reaction between 4-tert­
butylbenzyl bromide and iodide (see Figure 5.1.1). The reaction between 4-tert-butylbenzyl
bromide and KI, which is very sluggish in a simple two-phase system, is fast in the system
based on a slurry of a mesoporous material with hexagonal geometry. It was found to be fast
also in a microemulsion and very fast when the hexagonal liquid crystalline phase used as
template for the mesoporous material was used as reaction medium. Even if the slurry of
mesoporous material did not give higher reaction rate than the systems based on surfactant
self-assembly, the former reaction medium is more attractive from a practical point of view.
The extremely facile work-up after completed reaction, filtering followed by evaporation of
the solvent, makes it a very useful tool in preparative organic synthesis.
Figure 5.1.1 A nucleophilic substitution reaction between 4-tert-butylbenzyl bromide and
potassium iodide.
Mesoporous materials are today being explored for a variety of applications, out of which
catalysis is the most prominent. In Paper I it has been demonstrated that such materials also
have a potential for use as medium for reactions between incompatible reactants.
The mesoporous material, as well as the corresponding hexagonal liquid crystalline phase,
was used as medium for the reaction. Two reference systems were used: a microemulsion
based on a nonionic surfactant and a surfactant-free oil-water two-phase system. The reaction
profiles are shown in Figure 5.1.2. The curves for the microemulsion and the two-phase
reactions are taken from previous work at Chalmers2.
32
% starting material
100
90
80
70
60
50
40
30
20
10
0
microemulsion
mesoporous
hexagonal LC
0
10
20
30
40
50
60
70
80
Time [min]
2-phase
% starting materials
microemulsion
100
90
80
70
60
50
40
30
20
10
0
mesoporous
hexagonal LC
0
10
20
30
40
50
60
70
80
Time [min]
Figure 5.1.2 Reaction profiles for reaction between 4-tert-butylbenzyl bromide and potassium
iodide using a 1:1 (top) or a 1:10 (bottom) molar ratio of the reactants. The reactions were
performed in a hexagonal liquid crystalline phase (LC), a microemulsion (microemulsion), a
mesoporous material with hexagonal symmetry (mesoporous), and, for the reaction where a
1:10 molar ratio of reactants is used, a two-phase system.
Surfactant self-assembly systems, such as liquid crystalline phases and microemulsions, are
frequently used in industrial and household products. Mesoporous materials are exploited as
catalysts or catalyst carriers.
33
While the use of liquid crystalline phases or other organized surfactant systems may give a
large rate enhancement, it suffers from one major drawback: the problem associated with
separation of the product from the surfactant. Although there are ways to facilitate the work­
up procedure, elimination of the surfactant is always tedious and often expensive. An
analogous problem exists for reactions performed using phase transfer catalysis; removing the
catalyst from the product is not always simple. Use of mesoporous materials, which are made
through self-assemblies of surfactants but which are in the end surfactant-free, is one way to
circumvent the problem. Such materials can be said to be the replica of the surfactant liquid
crystals and the dimensions are roughly the same. After completed reaction the solid is just
filtered off or centrifuged off and then reused.
Paper II extends the previous work to some new materials with cubic structures and is more
focused on the liquid crystalline phases. All the microheterogeneous systems studied were
shown to have a potential for use in organic synthesis. For the process studied, reaction
between 4-tert-butylbenzyl bromide and potassium iodide, the following reactivity order was
obtained: liquid crystalline phase > slurry of mesoporous silica > microemulsion > two-phase
system. There were only small differences between systems with hexagonal and with cubic
geometry for both the liquid crystalline phases and the mesoporous materials. The reaction
studied is a typical nucleophilic substitution involving a lipophilic organic compound and an
inorganic salt. It is chosen because the reacting species, the nucleophile and the substrate,
have radically different solubility characteristics. The nucleophile, potassium iodide (KI), is
water soluble and completely insoluble in the organic phase, while the substrate, 4-tert­
butylbenzyl bromide (4-TBBB), is soluble in hydrocarbon and virtually insoluble in water.
Hence the reaction will only take place at the interface between the polar and the nonpolar
domains.
The reaction has previously been investigated in some detail using different types of
microemulsion as reaction media, and it was shown that the reaction proceeds well in such
systems2,117. The reaction has also been performed in a range of organic solvents with
different polarity (and without surfactant), and by comparing the reaction rates obtained in the
different solvents it was established that the mechanism is that of a second-order nucleophilic
substitution reaction2.
34
In all cases the reaction was monitored by 1H NMR, following the rise of the -CH2I signal and
the decay of the -CH2Br signal, as illustrated in Figure 5.1.3. There was good correspondence
between the increase of the -CH2I signal and the decrease of the -CH2Br signal, which
indicates that side reactions are not important. The most likely side reaction is probably
hydrolysis of 4-TBBA into the corresponding benzyl alcohol. If this reaction would occur in
parallel to the formation of the benzyl iodide, the monitoring of the latter reaction would be
distorted because there would then not be full equivalence in terms of NMR signal intensity
between disappearance of-CH2Br and appearance of -CH2I. The peak of -CH2OH methylene
protons appears somewhat more downfield than that of -CH2Br, at 4.58 ppm. No peak
appeared at that frequency in either the system based on mesoporous material or the liquid
crystal-based system.
Figure 5.1.3 The reaction between 4-tert-butylbenzyl bromide and potassium iodide was
studied by 1H NMR, monitoring the decrease of the -CH2Br signal and the increase of the CH2I signal.
35
As shown in Figure 5.1.2, the reaction runs well in the slurry of mesoporous material. The
surfactant-based reaction media, and in particular microemulsions, are dynamic systems, and
their usefulness as media for organic reactions may be due both to the large oil-water interface
and to the dynamics of the interface, with surfactants going in and out and surfactant
monolayers disintegrating and reforming. No such dynamics are present in the medium based
on the solid silica material. The porous material is filled with the aqueous solution of KI, and
the material is mechanically dispersed in a nonpolar medium, which is either the lipophilic
reactant as such or a solution of the lipophilic reactant in hydrocarbon. Potassium iodide has
negligible solubility in the nonpolar medium and 4-tert-butylbenzyl bromide is virtually
insoluble in water, which means that the reactants must meet and react at the pore openings.
The product obtained, 4-tert-butylbenzyl iodide, is lipophilic and will partition into the
nonpolar medium. An attractive feature of the reaction in the slurry of mesoporous material is
the ease of the workup procedure. After completed reaction, the solid particles are filtered off
(and washed with hydrocarbon to remove adsorbed product and/or starting material). The
residue remaining after evaporation of the solvent is the reaction mixture with no
contamination of auxiliary substances, such as surfactants or phase transfer agents. As will be
demonstrated in another paper, discussed below, the mesoporous solid can be reused several
times without much loss of efficiency. A facile workup procedure may seem trivial but should
not be ignored. Surfactant self-assembly systems, such as microemulsions and liquid
crystalline phases, contain high concentrations of amphiphiles that may give rise to foaming
or persistent emulsions during the workup. Likewise, phase transfer agents are sometimes
difficult to quantitatively remove from the product. If separation of surfactants, quaternary
ammonium compounds, crown ethers, or other reaction aids involves time-consuming
operations, the organic chemist will appreciate this alternative approach to overcome reactant
incompatibility.
The very high reactivity in the liquid crystalline phases (Paper I and Paper II) is believed to
be associated with the very high interfacial area of these systems. The papers show that
surfactant liquid crystals can be used as minireactors for organic reactions.
The short range disorder of a liquid crystal facilitates swelling of the tail region. Thus, the
system can solubilize nonpolar compounds by incorporation in the hydrophobic domains. At
the same time polar, water-soluble substances can be accommodated in the aqueous domains.
Thus a liquid crystalline system can solubilize and compartmentalize both hydrophilic and
36
hydrophobic substances and the interface between the polar and the nonpolar domains is very
large. These properties make liquid crystalline phases interesting candidates as media for
reactions between reactants with widely different solubility characteristics. Such reactions are
frequently carried out in oil-water two-phase systems and since such an interface is not very
large, the reaction is usually sluggish. Liquid crystalline phases are highly ordered reaction
media and this can also be taken advantage of for regiochemical control of a reaction.
In Paper II, a systematic comparison of the reaction rates obtained in the following
microheterogeneous media has been made: a hexagonal and a cubic liquid crystalline phase,
slurries of hexagonal and cubic mesoporous silica, an oil-in-water microemulsion, and a two­
phase system. The reaction profiles are shown in Figure 5.1.4 for a 1:10 (top) and a 1:1
% starting material
(bottom) ratio between 4-TBBB and KI.
2-phase
microemulsion
hex. mesoporous
cubic mesoporous
hexagonal LC
cubic LC
100
90
80
70
60
50
40
30
20
10
0
0
10
20
30
40
t/min
37
50
60
70
80
% starting material
100
90
80
70
60
50
40
30
20
10
0
hex mesoporous
microemulsion
2-phase system
cubic mesoporous
cubic LC
hexagonal LC
0
10
20
30
40
50
60
70
80
t/min
Figure 5.1.4 Reaction profiles for reaction between 4-tert-butylbenzyl bromide and potassium
iodide using a 1:10 (top) or a 1:1 (bottom) molar ratio of the reactants. The reactions were
performed in a hexagonal or cubic liquid crystalline phases, in slurries of finely divided
mesoporous silica with hexagonal or cubic geometry, in a microemulsion, and in a decane­
water two-phase system.
The reaction is very fast in the liquid crystalline phases, intermediate in the slurries of
mesoporous silica, somewhat slower in the microemulsion and extremely sluggish in the
hydrocarbon-water two-phase systems. One can also note that there are only small, and not
entirely consistent, differences in reactivity between the cubic and the hexagonal geometries
for both the liquid crystalline phases and the mesoporous silica systems.
It is interesting to compare the rates obtained in the two types of surfactant systems, the liquid
crystalline phases and the microemulsion. First of all one can note that the rate is high in both
these systems compared to the rate in the two-phase system, which is a hydrocarbon-water
mixture exposed to vigorous stirring. It is reasonable to assume that the increase in rate
compared to the surfactant-free system is due to a much larger interfacial area in the
surfactant systems. For all the systems the reaction will occur at the interface between the
polar and the nonpolar domains. For the surfactant systems, the liquid crystalline phases as
well as the microemulsion, one may assume that the reactants will meet in the palisade layer
consisting of highly hydrated polyoxyethylene chains. 4-TBBA will enter the reaction zone
from the hydrocarbon tail region and KI will approach from the aqueous domain. It has
38
previously been demonstrated that the reaction is considerably faster in a microemulsion
based on an alcohol ethoxylate than in one based on a sugar surfactant117. Both these
surfactants are nonionic so the effect cannot be related to charges at the interface. It was
postulated that the difference in reactivity is due to a difference in the degree of hydration of
the headgroup layer of the surfactants. Sugar is a more polar headgroup than a
polyoxyethylene chain and the water activity in the interfacial zone of a microemulsion based
on a sugar surfactant is likely to be higher than in the interfacial zone of a microemulsion
based on an ethoxylated surfactant. (It is known that micelles of sugar-based surfactants have
a higher dielectrical constant than micelles of alcohol ethoxylates118.) An SN2 reaction of the
type studied here, i.e., a reaction involving a neutral substrate and an anionic nucleophile, runs
slower the more polar the reaction medium. Most likely, the high reactivity obtained in this
work in both the liquid crystalline phases and in the microemulsion is related to the
nucleophile being poorly solvated when present in the polyoxyethylene chain layer of such
systems. Poorly solvated anions are highly reactive nucleophiles.
There are at least two differences between liquid crystalline systems and microemulsions that
may play a role. Firstly, the liquid crystalline phases possess a much larger interface between
the polar and the nonpolar domains. Since all the surfactant can be assumed to reside at the
interface, the ratio in interfacial area between the systems is roughly the same as the ratio in
surfactant concentration. The liquid crystalline phases contain approximately three times as
much surfactant as the microemulsion. This difference should work in favour of the liquid
crystalline systems.
Secondly, a microemulsion is a more dynamic system than a liquid crystal. The interface of a
microemulsion disintegrates and reforms constantly. Liquid crystals are much less dynamic in
character, i.e., the residence time of a surfactant at the interface is longer. The highly dynamic
character of the microemulsion could be a factor in favour of the reaction. One may anticipate
that the probability of the reactants to meet in the interfacial zone (in this case the
polyoxyethylene chain layer) would be enhanced by such a dynamic situation.
The two effects discussed above, interfacial area and interface dynamics, will then work in
different directions when it comes to relative reactivity in the two surfactant systems. Since
the reaction is considerably faster in the liquid crystalline phases than in the microemulsion,
39
one may conclude that the large interface between the polar and the nonpolar domains of the
liquid crystals is the dominating factor.
Paper III describes and compares the efficiency of three mesoporous oxide materials, silica,
alumina and titania. The mesoporous materials were made by a surfactant-templated
procedure using a block copolymer as amphiphile. Block copolymers with different EO to PO
ratios were used for making the two different types of geometries. Figure 5.1.5 shows that the
pore size distribution is narrow, indicating proper control of the synthesis.
Figure 5.1.5 Pore size distribution of the mesoporous materials: Hexagonal alumina (top) and
cubic silica (bottom).
40
Paper III also deals with various types of hydrophobic mesoporous materials. It also includes
a study on a material with extremely small particle size. Different types of organic reactions
are tested (see Figure 5.1.6). The paper demonstrates the potential of reuse of the mesoporous
material.
Figure 5.1.6 Reaction I: A nucleophilic substitution reaction between 4-tert-butylbenzyl
bromide and potassium iodide. Reaction II: A nucleophilic substitution reaction between
benzyl mercaptane and chloroacetate. Reaction III: Epoxidation of an α,β-unsaturated enone,
trans-chalcone, by hydrogen peroxide.
Reactions I and II of Scheme 1 are typical nucleophilic substitution reactions and Reaction III
is an epoxidation reaction. The latter reaction has recently been found to run well in a
microemulsion based on a nonionic surfactant80. The pair of reactants in all three reactions is
chosen such that their solubility characteristics are completely different. The intention is that
the reaction will only take place at the interface between the polar and the apolar domains. In
the case of Reaction I, the nucleophile, the iodide ion, is the anion of a salt that is highly water
soluble and completely insoluble in the organic phase. The electrophilic reactant, 4-tert­
butylbenzyl bromide (4-TBBB), is soluble in hydrocarbon and insoluble in water. In the case
of Reaction II, the electrophile is the chloroacetate ion (ClCH2COO-), which as its sodium salt
is soluble in water but insoluble in an apolar phase, while the reverse is true for the
41
nucleophilic species, benzyl mercaptane. Thus, the solubility characteristics are reversed for
the reactants in these two nucleophilic substitution reactions. In Reaction III the oxidizing
agent, hydrogen peroxide, is soluble in water while the α,β-unsaturated enone, trans­
chalcone, is only oil soluble.
All tree mesoporous structures, silica, alumina and titania, were made with an EO-PO block
copolymer as template. We have also used extremely small particles of hexagonal
mesoporous silica synthesized by a special procedure from a metasilicate solution and using a
cationic surfactant as structure directing agent119. The purpose of this was to assess the role of
the particle size on the reaction rate.
The reaction profiles for the hexagonal mesoporous silica, alumina, and titania using either a
1:1 or a 10:1 ratio of KI to 4-TBBB are shown in Figure 5.1.7 and Figure 5.1.8, respectively.
The corresponding reaction profiles for the cubic mesoporous materials are depicted in Figure
5.1.9. Figure 5.1.8 also contains the curve for the slurry of the hexagonal silica obtained with
a cationic surfactant as templating agent that is known to give particles of very small size. The
profiles for reaction in the liquid crystalline phase with the corresponding geometry, in the
microemulsion, and in the two-phase system are also included in Figure 5.1.9. Whereas the
curves for the liquid crystals and for the microemulsion are rather smooth, many of the curves
for the mesoporous materials are irregular, which is the result of difficulties in collecting
representative samples from the reaction mixture, which is a very concentrated suspension of
the particles in an apolar medium. Nevertheless, the curves should give relevant qualitative
information about reaction rates. The purpose was to obtain relative values of reaction speeds
rather than absolute values of rate constants.
42
Figure 5.1.7 Reaction profiles for reaction between 4-tert-butylbenzyl bromide and potassium
iodide using a 1:1 molar ratio of the reactants. The reactions were performed in slurries of
hexagonal mesoporous silica, alumina and titania at room temperature unless otherwise stated.
Figure 5.1.8 Reaction profiles for reaction between 4-tert-butylbenzyl bromide and potassium
iodide using a 1:10 molar ratio of the reactants. The reactions were performed in slurries of
hexagonal mesoporous silica, alumina and titania, as well as in a slurry of finely divided,
small particle-sized hexagonal silica at room temperature.
43
Figure 5.1.9 Reaction profiles for reaction between 4-tert-butylbenzyl bromide and potassium
iodide using a 1:1 molar ratio of the reactants. The reactions were performed in slurries of
mesoporous silica, alumina and titania with cubic geometry, as well as in a cubic liquid
crystalline phase, in a microemulsion and in a decane-water two-phase system.
The type of oxide material used has a pronounced effect on the reactivity and it is clear that
the rate is not, as one may have expected, correlated to the specific surface area of the
material. In general silica and alumina work well, with alumina being somewhat better than
silica in most instances, while the reaction is slow when titania is used. It can also be seen that
even if the reaction rate is high when a slurry of silica or alumina is used, it is even higher in a
surfactant liquid crystal. The rate is somewhat lower in the microemulsion and, as expected,
the reaction is extremely sluggish in the hydrocarbon-water two-phase systems.
There are no major differences in reactivity between the systems with hexagonal and with
cubic geometry. This applies to reactions in both the mesoporous materials and in the liquid
crystals. This indicates that the higher connectivity of the bicontinuous cubic phase over the
hexagonal phase – three-dimensional vs. one-dimensional pore system – is not important.
This is reasonable assuming that the reaction occurs only at the pore openings, i.e., at the
interface between the water incorporated into the porous material (or in the polar domains of
the liquid crystal) and the surrounding apolar domain and that diffusion of the polar reactant is
fast enough so that transport inside the porous system is not rate limiting.
Figure 5.1.7 shows the rate acceleration for alumina-based system when increasing the
temperature from 20 to 40 °C. Even if the plots cannot be used to calculate rate constants, the
44
initial rate difference seems to correlate quite well with the Arrhenius rate expression, which
leads to approximately four times higher rate constant on increasing the temperature by 20
degrees.
It has been shown that there is no clear correlation between the rate of the organic reaction
and the specific surface area of the material (compare Table 5.1.1 and Figure 5.1.9). The
efficiency decreases in the order mesoporous alumina > mesoporous silica > mesoporous
titania; this order does not correlate with the BET surface area of the materials, which is silica
> alumina > titania.
Material
BET surface Pore size Pore volume
area [m2/g]
[Å]
[cm3/g]
Silica, hexagonal and hydrophilic
809
46
0,93
Silica, cubic and hydrophilic
819
70
0,91
Alumina, hexagonal and hydrophilic
567
37
0,51
Alumina, cubic and hydrophilic
432
39
0,43
Titania, hexagonal and hydrophilic
115
34
Titania, cubic and hydrophilic
121
36
Silica, hexagonal and hydrophobic
613
0,83
Silica, cubic and hydrophobic
613
58
Alumina, hexagonal and hydrophobic
216
28
0,21
Alumina, cubic and hydrophobic
206
29
0,19
Carbon
1380
1,60
Table 5.1.1 Surface area, pore size, and pore volume of the mesoporous materials.
In order to assess the effect of the particle size, mesoporous silica was also made by a
procedure that is known to give particles of smaller size than is usually obtained119, around 50
nm instead of the 100-200 nm that is typically obtained with an organic silica source and an
EO-PO block copolymer as structure directing agent. Metasilicate was used as silica source
instead of the more common alkoxides such as tetraethyl- or tetramethylorthosilicate and a
cationic surfactant was employed as templating molecule. The rationale behind this
experiment was that differences in the size of the oxide particles could be a possible reason
for the difference in reactivity of the different slurries of mesoporous material.
The finely divided silica was used as a carrier for the water-soluble reactant, i.e. KI, and the
reaction was performed with the particles dispersed in the apolar phase as with the other silica
materials. From the results presented in Figure 5.1.8 it seems that the particle size is a very
important parameter. The higher reactivity with the smaller sized silica is most likely due to
45
this material exposing more pore openings to the surrounding apolar medium, thus increasing
the interfacial area at which the reaction occurs.
Paper III also shows that the mesoporous material after completed reaction could be reused.
To this end reactions were performed a total of three times with the same hexagonal alumina,
removed by filtration and washed after each run. The results are shown in Figure 5.1.10.
Figure 5.1.11 shows results from another reuse approach with the mesoporous alumina
particles packed in a column. This procedure, schematically illustrated also in Figure 5.1.11,
would probably be the most appropriate for larger scale synthesis. As can be seen from the
figure, the rate and the yield of the column experiment are very good in the first run but only
moderate in the second and third runs. It is likely that there is a considerable potential to
improve the efficiency in the reuse of the material and that the column approach holds
promise for this.
46
Figure 5.1.10 Reaction profiles for reuse of hexagonal mesoporous materials, using a 1:10
molar ratio, for reaction between 4-tert-butylbenzyl bromide and potassium iodide in silica
(top) and in alumina (bottom).
Figure 5.1.11 Reaction profiles for reuse of hexagonal mesoporous alumina for reaction
between 4-tert-butylbenzyl bromide and potassium iodide, using a 1:10 molar ratio performed
in a column showed schematically to the right.
In the reactions discussed above the polar reactant was incorporated inside the pores of the
hydrophilic mesoporous material and the apolar reactant was present in the continuous phase,
in which the particulate solid was dispersed. An alternative procedure is to use a hydrophobic
mesoporous material as a carrier for the apolar reactant and to disperse this material in an
47
aqueous medium, in which the polar reactant is dissolved. This approach was tested for the
reaction between 4-TBBB and KI, i.e., Reaction I (see Figure 5.1.6). Both mesoporous silica
and mesoporous alumina were made hydrophobic by treatment with chlorotrimethylsilane and
used for the purpose.
Silylation of hexagonal silica and alumina was made by known methods38. Figure 5.1.12
shows FTIR spectra of silylated and unsilylated hexagonal silica and of silylated hexagonal
alumina. An absorption band at 2965(6) cm-1 and a small peak at 2907(8) cm -1 are indicative
of trimethylsilyl groups.
Figure 5.1.12 FTIR spectra of (a) silylated hexagonal silica, (b) unsilylated hexagonal silica,
and (c) silylated cubic silica. An absorption band at 2965–2966 cm−1 and a small peak at
2907–2908 cm−1 are indicative of trimethylsilyl groups.
In addition, mesoporous graphite was synthesized and used. The hydrophobic material was
impregnated with 4-TBBB and suspended in an aqueous solution of KI. However, in all cases
only traces of the expected product, 4-tert-butylbenzyl iodide, were obtained after a 2 h
reaction.
A possible reason for the poor reactivity in the “reverse” system is that of unfavourable
reaction equilibrium. The product formed, 4-tert-butylbenzyl iodide, is hydrophobic and will
48
remain inside the pores where it will be close to the reaction zone. In the “normal” system,
i.e., with dispersed hydrophilic particles, the product formed will partition into the continuous
phase, thus leaving the reaction zone. Such partitioning is likely to favour the reaction.
In order to test the influence of partitioning of the product, Reaction II (see Figure 5.1.6) was
performed in the reverse system. The hydrophobized mesoporous silica or alumina was
impregnated with benzyl mercaptane and the material was suspended in an aqueous solution
of sodium chloroacetate. The expected product, sodium benzylthioacetate, is likely to
partition into the continuous polar medium. Thus, the partitioning behaviour will be
analogous to that of Reaction I performed in the normal system, i.e., with hydrophilic
particles dispersed in an apolar medium. However, also this reaction failed. Almost no
product was obtained after 2 h reaction in spite of the fact that different external stimuli
(ultrasound or microwaves) were applied in order to accelerate the reaction.
Reactions I and II are nucleophilic substitution reactions. A third attempt to perform a
reaction using the reverse mode was made, Reaction III (see Figure 5.1.6). This is an
oxidation reaction involving a lipophilic substrate, trans-chalcone, and a polar oxidant,
hydrogen peroxide. The anticipated product is a lipophilic epoxide. The lipophilic reactant
was introduced into the pores of hydrophobized mesoporous silica or alumina and the
particles were suspended in an aqueous solution of the oxidizing agent. Also this reaction was
unsuccessful, however.
Thus, use of a slurry of a hydrophobic mesoporous material in water as medium for an
organic reaction comprising incompatible reactants has not been successful in spite of the fact
that a suspension of hydrophilic particles in an apolar phase is a good reaction medium. One
may speculate that the difference is due to a difference in the state of the two suspensions. In
the normal mode, that works well, hydrophilic particles filled with an aqueous solution are
dispersed in an apolar medium. In the reverse mode, which does not work well, hydrophobic
particles filled with an apolar solution are suspended in an aqueous phase. It is likely that the
particles in the former case, the nonaqueous dispersion, are much better dispersed than in the
latter case, the aqueous dispersion. Dispersing hydrophobic particles in water without the use
of a dispersing agent will invariably lead to severe agglomeration. Since the particles used in
this work are small, 100-200 nm, and since we have shown for the case of hydrophilic
particles that the size is important for the reaction rate, it seems reasonable to attribute the
49
poor reactivity obtained with hydrophobic mesoporous particles to agglomeration of the
primary particles into much larger entities, leading to a much diminished interfacial area
between the apolar medium inside the pores and the surrounding aqueous phase. Use of a
dispersion agent to prevent agglomeration seemed pointless since the whole idea of a
surfactant-free reaction medium would then have been put aside.
5.2 Mesoporous materials as host for an entrapped enzyme
In Paper IV three different mesoporous materials, silica, alumina and titania with different
geometries are used as host for a lipase and the enzyme-loaded particles were employed as
catalyst for esterification of caprylic acid using a mixture of glycerol and water as reaction
medium. The results are compared with results from the use of nonporous particles at which
surface the enzyme is merely adsorbed.
The paper shows that the lipase is not irreversibly entrapped in the pores of the mesoporous
materials. When the particles are removed by filtration after completed reaction and
subsequently washed with an aqueous buffer, the enzyme, which is not firmly attached to the
surface, is leached out. The lipase can be immobilized in the pores, however, by cross-linking
in-situ inside the pores using glutaraldehyde as cross-linking agent. Mesoporous materials
loaded with cross-linked lipase can be reused several times with only marginal loss of
activity. Paper IV describes heterogenized lipase used as catalyst for the chemical synthesis.
Immobilization can solve most of the problems associated with free enzyme, like in some
cases fast degradation rate, and it also facilitates the work-up procedure. As discussed below,
the immobilized enzyme can be removed from the reaction mixture by filtration or
centrifugation and then reused. It is shown that particles containing such cross-linked enzyme
can be recirculated with good retention of the catalytic activity.
Effect of water activity (aw) and of type of mesoporous material on the biocatalysis has been
studied. Figure 5.2.1 shows the effect of water activity on the degree of transformation of
caprylic acid into monocaprylin when a mixture of water and glycerol was used as reaction
medium. The lipase was entrapped into hexagonal silica and the reaction was run for 36 h.
50
Figure 5.2.2 shows the product composition after the corresponding reaction but with
monocaprylin instead of caprylic acid used as substrate. As can be seen, a similar pattern is
obtained with respect to product composition. Except for the case with the highest water
content a mixture of monoglyceride and fatty acid is formed and the ratio between the two
depends on the water activity of the reaction mixture. The fact that roughly the same product
mixture is obtained regardless of whether the reaction involves esterification (Figure 5.2.1) or
ester hydrolysis (Figure 5.2.2) indicates that equilibrium has been reached. Since appreciable
amounts of both monocaprylin and caprylic acid were obtained at an aw of 0.36,
corresponding to a water/glycerol ratio of 15:85 (by volume), this ratio was used in the
investigation of the effect of the type of mesoporous material used for entrapment of the
lipase.
Figure 5.2.1 Effect of the water activity, aw, on the relative amounts of caprylic acid and
monocaprylin formed when caprylic acid was used as substrate and different water/glycerol
mixtures used as reaction medium. The lipase was entrapped in mesoporous hexagonal silica.
51
Figure 5.2.2 Effect of the water activity, aw, on the relative amounts of caprylic acid and
monocaprylin formed when monocaprylin was used as substrate and different water/glycerol
mixtures used as reaction medium. The lipase was entrapped in mesoporous hexagonal silica.
The degree of conversion of caprylic acid into monocaprylin by esterification with glycerol
using lipase entrapped in different types of mesoporous materials as catalyst is shown in the
Figure 5.2.3.
52
Figure 5.2.3 Effect of the type of mesoporous material on the relative amounts of caprylic
acid and monocaprylin formed when caprylic acid was used as substrate and a water/glycerol
ratio of 15:85, corresponding to an aw of 0.36, was used as reaction medium. Reactions with
nonporous silicas were performed as references.
Similar to the experience with Reaction I of Paper III the reactivity seems not to be
correlated to pore size or specific surface area. However, reactions run with the same amount
of lipase adsorbed on non-porous silica particles did not give much conversion. Evidently,
there is an advantage with the pore structure and different mesoporous oxide materials
exhibited significant differences.
The mesoporous oxides differ widely with respect to surface character. Silica is an oxide with
a weak Brönsted activity (pKa of Brönsted’s sites of 7) and with a point of zero charge of 3.
Alumina is also a material with a weak Brönsted activity (pKa of Brönsted sites of 8.5) and its
point of zero charge is 8. Titania has strong Lewis acidity and basicity and also strong
Brönsted activity (pKa of 0.5–2.0). It has a point of zero charge of 5. Thus, under the
conditions used for the experiments (pH 7) silica will carry a strong negative charge, alumina
will have a small positive charge, and titania will carry a small negative charge. Since the
enzyme has a negative net charge under these conditions (the pKa of the lipase is 3.5), it is
reasonable to assume that it will give an attractive interaction with alumina and a repulsive
interaction with the other two oxides.
53
In the paper it was therefore postulated that the difference in degree of esterification that is
seen between alumina on the one hand and silica and titania on the other hand is related to the
type of interaction that the lipase has with the pore walls. When the walls consist of
negatively charged silica or titania, the also negatively charged enzyme will keep away from
the walls while for the slightly positively charged alumina it will at least partly be adsorbed at
the wall. From a dimensional point of view it is possible to discuss in these terms. The pore
diameters are 3.4–7.0 nm and the lipase is a small protein with a diameter of 3–4 nm109.
We believe that the difference in degree of esterification seen between silica, alumina, and
titania is due to a difference in the microenvironment around the enzyme. Variations in
enzymatic activity and specificity have previously been attributed to differences in the
microenvironment around the enzyme120,121. The water activity close to the walls will be
lower than in the middle of the pore. This is caused by the strong hydration of the oxide
surface.
The hydrophobized hexagonal silica has also been used in the study. The lipase can be
expected to adsorb strongly at the hydrophobic surface, which will not be hydrated. Figure
5.2.4 shows that a hydrophobized surface gives a high degree of esterification, which is in
accordance with the concept of low water activity around the enzyme favouring esterification.
54
Figure 5.2.4 Effect of the water activity, aw, on the relative amounts of caprylic acid and
monocaprylin formed when monocaprylin was used as substrate and different water/glycerol
mixtures used as reaction medium. The lipase was entrapped in hydrophobized mesoporous
hexagonal silica.
It is remarkable that at the lowest water activity, 0.15, corresponding to a water to glycerol
ratio of 5-95, caprylic acid was almost quantitatively transformed into tricaprylin. When the
same experiment was run with the enzyme loaded in normal, hydrophilic silica only traces of
the triglyceride was found.
In Paper IV the reusability of the entrapped lipase for hydrolysis of 4-nitrophenylpalmitate is
explored. Ester hydrolysis will generate 4-nitrophenol, which gives the reaction medium a
strong yellow colour, easy to monitor spectroscopically. The procedure was used for lipase
entrapped in hexagonal silica but almost no activity could be found in the second run. The
lipase had evidently leached out during the filtration and washing of the particles. To
overcome this problem, a gentle cross-linking was performed with glutaraldehyde, following
a known procedure122.
Sequential runs using 4-nitrophenol as substrate showed that the cross-linked lipase was very
well retained in the pores. As can be seen from Figure 5.2.5, the activity stayed at almost the
55
same level during five consecutive runs. The cross-linking is evidently a way to prevent the
enzyme from escaping the pores without impairing its catalytic activity.
Figure 5.2.5 Relative amounts of caprylic acid and monocaprylin formed during consecutive
runs using caprylic acid as substrate and a water/glycerol ratio of 15:85, corresponding to an
aw of 0.36, as reaction medium. The enzyme was embedded in either hexagonal silica or
hexagonal alumina and subsequently cross-linked. The enzyme-loaded particles were
separated by filtration and washed with aqueous buffer between each run.
56
6 Conclusions
This work demonstrates that the reaction between 4-tert-butylbenzyl bromide (4-TBBB) and
potassium iodide (KI), which is sluggish in a simple two-phase system, is fast in the system
based on a slurry of mesoporous oxides with different geometries. It was found to be fast also
in a microemulsion and very fast when the hexagonal liquid crystalline phase that
corresponded to the mesoporous material was used as reaction medium. Even if the slurry of
mesoporous material did not give a higher reaction rate than the systems based on surfactant
self-assembly, the former reaction medium is more attractive from a practical point of view.
The extremely facile workup after completed reaction, filtering followed by evaporation of
the solvent, makes it a useful tool in preparative organic synthesis. The size of the particles of
the mesoporous materials is important; the smaller the size, the faster the reaction, but there is
not much difference in efficiency between materials with hexagonal and with bicontinuous
cubic geometry.
The efficiency decreases in the order mesoporous alumina > mesoporous silica > mesoporous
titania; this order does not correlate with the BET surface area of the materials, which is silica
> alumina > titania.
The very high reactivity obtained in the liquid crystalline phases is believed to be due to the
very large interfacial area between the polar and the nonpolar domains of such systems. For
the process studied, reaction between 4-TBBB and KI, the following reactivity order was
obtained: liquid crystalline phase > slurry of mesoporous silica > microemulsion > two-phase
system.
Thus, this work shows that slurry of a hydrophilic mesoporous oxide in an apolar phase is a
useful medium for reaction between a polar reactant, situated inside the pores of the
particulate solid, and a nonpolar reactant that is present in the apolar phase. The reverse
system, using particles of a hydrophobized mesoporous material into which a lipophilic
reactant had been introduced suspended in an aqueous solution of the polar reactant, did not
work in spite of several attempts. The reason for this is not clear but may be due aggregation
of the hydrophobic particles.
57
We also showed that mesoporous oxide materials can be used as host for a lipase. The three
investigated materials, silica, alumina and titania, all worked well but alumina gave
considerably higher conversion of caprylic acid into monocaprylin than the other two oxides.
This effect is attributed to the positively charged alumina, but not the negatively charged
silica and titania, adsorbing the lipase, which carries a net negative charge. The enzyme will
experience a lower water activity when adsorbed at the highly hydrated mineral surface than
when present in the centre of the water-filled pores. A high local concentration of the fatty
acid salt, i.e., caprylate, around the alumina pore walls may also contribute to the high
conversion. A hydrophobized mesoporous material, which can be expected to have very low
water activity along its surface, also gave a high degree of esterification. The lipase-loaded
particles can only be used once because the enzyme leaks out during the filtration and
washing procedure. However, by in situ cross-linking the lipase inside the pores the enzyme
becomes immobilized. The particles containing cross-linked lipase were suitable for repeated
use with filtering and washing of the particles between each run. Such a procedure holds
considerable promise as a simple and versatile way of ‘‘heterogenizing’’ the lipase.
58
Acknowledgements
First of all, I would like to thank Professor Krister Holmberg for all your help, support and
giving me the opportunity to perform PhD studies at TYK. You are really a good supervisor.
One can learn a lot from you, not only in the area of surface chemistry.
Docent Lars Löwendahl is acknowledged for advice with nitrogen sorption measurements.
Dr Kjell Wikander is acknowledged for providing me with a mesoporous carbon for my
research.
Dr Martin Andersson is acknowledged for help with TEM.
Pedro Reis is acknowledged for a nice collaboration and accompanying me in the trip to
Copenhagen.
All people at TYK are gratefully acknowledged for a nice working environment and
enjoyable time spent in Sweden.
Ann Jakobsson and Britt Bergström are acknowledged for help with administrative problems.
My wife Kasia, my son Hubert and the rest of my family is acknowledged for all love and
support.
Thank you all!
59
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