Download Isolating High Value Aromatics from Lignin Stockpiles

Isolating High Value Aromatics from Lignin
Stockpiles
Keyton Feller
Dr. Brian Barry, Advisor
Abstract
Selective, catalytic cleavage of aryl-heteroatom bonds found in lignin would allow for the
isolation of various valuable aromatics from a renewable resource. These catalysts
could also be used to remove heteroatoms found in heavy crude resources, increasing
liquid crude yields. The cleavage of aryl-heteroatom bonds requires a catalyst that has
an electron rich metal center that is coordinatively unsaturated and sterically protected.
This project uses guanidinate-like ligands to meet these requirements, while increasing
tunability of electron donation and resonance stabilization. A 2-coordinate, imidazolidin2-iminato Co(II) complex was successfully synthesized and tested for catalytic reactivity.
This test revealed the interaction of the exocyclic nitrogen lone pairs with test substrates
and encouraged the redesign of the ligand. The current design involves the bridging of
the previous ligand to create a neutral bidentate ligand and use the lone pairs for
coordination to the metal center, disallowing the unwanted C-N bond formation seen in
the previous design.
Introduction
The cleavage of aryl-heteroatom bonds has become an important research interest as
is could benefit the petroleum industry and make use of valuable lignin stockpiles.
Asphaltene has a high concentration aryl-heteroatoms throughout the structure and is a
byproduct from crude oil refinement. This material increases the viscosity of the crude
oil and precipitates causing low yields and clogging of pipes. Cleaving these arylheteroatom bonds and subsequent removal of the heteroatom will increase liquid
(transportable) crude yields. Lignin is a polymer found in woody plants bound to
cellulose and hemicellulose. It is produced as a biomass waste from the paper milling
industry and a majority is inefficiently burned as fuel.2 However, lignin contains many
aromatics and has the potential to produce high value chemicals through arylheteroatom bond cleavage. These aryl-heteroatom bonds are very stable and require
large amount of energy to cleave so catalysts have been implemented. The most
abundant and successful catalyst systems have been heterogeneous systems.
However, heterogeneous catalysts tend to be top-down methods that require harsh
reaction conditions and result in low yields and a plethora of different products.3 This
project focuses on creating a bottom up approach by synthesizing a homogenous
catalyst that may be employed under mild conditions.
Initial Catalyst Design
From the few successful homogeneous catalyst systems for cleaving aryl-heteroatom
bonds it has been determined that the catalysts requires an electron rich metal center 4
that is coordinately unsaturated5, and sterically protected6. The Cyclopentadienyl (Cp)
ligand class is ubiquitous in molecular inorganic chemistry due to its capability of acting
a 6 electron donor to metal centers.7 Our research offers an alternative to Cp that is
likely a better electron donor while also offering an ease in synthetic modifications. To
fulfill the requirements of an effective catalyst this project uses a guanidinate-like ligand
class that mimics the ring slip of the Cp ligand class, allowing for tunable electron
donation to the metal center. The positive charge stabilization in the imidazolium ring
allows for charge buildup at the exocyclic 2 position (Figure 1). Steric protection is
required to stabilize a coordinately unsaturated catalyst as well as to drive the reductive
elimination step. Currently this project is using diisopropylphenyl (Dipp) groups to
sterically protect the metal center.
Results of Initial Design
The first summer of research culminated in the successful synthesis of a 2-coordinate
Co(II) complex. This highly reactive, novel complex was crystallographically
characterized. We also synthesized several new imine ligands. In a test for catalytic
reactivity, the catalyst below was expected to undergo an oxidative addition with methyl
iodide at the metal center, however the lone pair on the exocyclic nitrogen acted as a
Lewis base and resulted in an unwanted reaction, cleaving the metal-nitrogen bond and
ultimately eliminating the catalyst’s identity (Figure 2). This encouraged a new design
which eliminates the lone pair from reacting.
Current Catalyst Design
In order to prevent interaction of the exocyclic nitrogen’s lone pair, the ligand has been
redesigned as a neutral bidentate ligand and utilizing the lone pairs to donate to the
metal center, leaving them unable to react with substrates. Since these neutral bisguanidinate analogs of the previously used monoanionic imidazolin-2-iminto ligands are
being employed, a new oxidation state pairing must be envisioned for the catalytic
cycle. (Figure 3)
These neutral ligands allow us to target a classic Pd0/PdII oxidation state cycle,
which we feel this is a superior design to the previously targeted Co II/CoIV pairing, as
CoIV is a rare unstable oxidation state. The proposed Pd0 catalyst can be seen directly
below, and highlights its potential to be an excellent electron donor and its ability to
moderate the degree of electron donation to accommodate the metal’s fluctuating
oxidation state. (Figure 4) Also seen is the proposed catalytic cycle of the new design
catalyst. (Figure 5)
Figure 5. Proposed catalytic cycle for the removal of sulfur from Dibenzothiophene.
Results of Current Design
This past semester an ethane bridged SIPr ligand was successfully synthesized and
crystallographically characterized. This achievement allows the coordination of multiple
different metals as well as rethinking and optimizing the synthesis of the ligands.
Figure 6. Crystal structure of ethane bridged SIPr ligand
Methods
Two of the ligands, EtN2 SIPr and IPr, have already been successfully synthesized,
purified and spectroscopically characterized in our labs. These two were synthesized
following a procedure for an analogous bisguanidine ligand, in which a 2iminoimidazoline is reacted with ethylene di(p-toluenesulfonate), and subsequently
reduced by K+ tBuO- (Figure 7d).8 A second route will mimic a reported route by
reacting a 2-bromoimidazolium bromide with ethylenediamine and subsequently
deprotonating with KOH (Figure 7c);9 this route will be tested for efficiency relative to
the aforementioned method. Both routes require the preparation of carbene precursors
followed by either imination to form the 2-iminoimadazoline or by bromination to form
the 2- bromoimidazolium bromide. Synthetic routes to all three precursor carbenes are
well established and have been successfully reproduced in our labs.10 From the
carbenes, a procedure developed by Tamm et. al. was repeated to form the previously
reported 2-iminoimidazoles of IPr and SIPr .11 This procedure was adopted in use with
the BIm carbene to successfully produce the previously unreported 2-imino-1,3diisopropylbenzimidazoline; this new compound was also characterized
crystallographically and can be seen in Figure 6. The reaction of all three carbenes with
Br2 at -78 °C results in the precipitation of an air stable 2- bromoimidazolium bromide
salt which have been reported as intermediates, but never isolated and characterized.
We have synthesized, purified and produced spectroscopically consistent results for all
three salts. The zero-valent metal catalyst complexes will be targeted through simple
ligand exchange. The weakly bound, easily displaced cyclooctadiene (COD) and
dibenzylideneacetone (dba) ligands found in Ni(COD)2 and Pd(dba)2 should be easily
replaced by the much stronger, chelating ligands found in the proposed bisguanidine
compounds. Once prepared and characterized, the catalysts will be exposed to the
previously mentioned test substrates.
Figure 6. Synthetic approaches to the bisguanidine M0 catalyst complexes
Conclusion
While the original catalyst design was proven to be catalytically unreactive, the
transition towards the neutral bridged ligands shows great potential to cleave arylheteroatom bonds. This new design should cause the electron rich metal center to be
the site of insertion of the heteroatom moiety rather than the exocyclic basic nitrogens.
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