Download catalytic activity of modified silicates: i. dehydration of ethanol

Clay Minerals (1987) 22, 423-433
C A T A L Y T I C A C T I V I T Y OF M O D I F I E D S I L I C A T E S :
I. D E H Y D R A T I O N OF E T H A N O L C A T A L Y S E D BY
ACIDIC SEPIOLITE
A. C O R M A AND J. P E R E Z - P A R I E N T E
Instituto de Cat6lisis y Petroleoquimica, CSIC, Serrano, 119, 28006-Madrid, Spain
(Received 13 August 1987)
A BST R A C T : The kinetics of the dehydration of ethanol on acidic AP §
sepiolite
has been studied. The product distribution indicates that the formation of diethylether and
ethylene takes place by a series of consecutive and parallel reactions. A mechanism for the
formation of both products has been derived by studying the influence of different operational
variables on the reaction rates.
Several solid acids have been reported to be successful in catalysing the dehydration of
ethanol into ethylene and ethyl ether at relatively low temperatures. For instance, )'-A1203
(Balaceanu & Jungers, 1951; Kn6zinger & K6hne, 1966), silica-alumina (Figueras et al.,
1969), zeolites (J acobs, 1977; Yue & Olaofe, 1984) and montmorillonites (Adams et al., 1981 ),
which all exhibit surface acidity, are active catalysts for alcohol dehydration.
For natural sepiolite, the catalytic activity for the above reaction is very low (FernfindezHernfindez & Fernfindez-Alvarez, 1983), as a consequence of its low surface acidity
(H0 > +4.0). This is not surprising if one takes into account the structure and composition of
sepiolite. The mineral is a fibrous magnesium silicate, the structure of which was first
reported by Brauner& Preisinger (1956). This mineral is constructed of talc-like ribbons
arranged in such a way that the tetrahedral sheet is continuous but inverts apical directions in
adjacent ribbons, generating channels along the c axis of the fibre. The dimensions of the
channels are approximately 10-8 x 4-0 A (estimated from the oxygen radius of 1.35/~ and a
water molecule size of 2.6 A) in a cross-section perpendicular to the length of the fibre (c axis).
The average fibre length in our sample is ~ 1 #m, and the reagent molecules have access only
to a small portion of the interior of the channels (Fern~ndez-Hernfindez & FernandezAlvarez, 1983; Serna & VanScoyoc, 1978). The tetrahedral sheet is formed of silicon only
with very little, if any, substitution by AP § and therefore with no acidity. The octahedral
sheet is composed mainly of Mg 2+ with a small amount of A13+ substitution. The framework
of sepiolite is therefore charge-balanced and consequently exhibits almost no ion exchange
capacity. This situation prevents an increase in surface acidity by classical exchange of extralattice cations. The only possible Br6nsted acidity present in natural sepiolite would be
generated by the two molecules of water bonded to each one of the Mg 2§ ions located at the
edges of the octahedral sheet which form the 'walls' of the channels running along the fibre.
However, these water molecules are not acidic enough, so that even NH3 cannot be
protonated (Serna & VanScoyoc, 1978). It has been shown recently (Corma et al., 1984) that,
as in montmorillonite (Morita et al., 1972; De Boer et al., 1967), substitution of octahedral
Mg 2+ in sepiolite by trivalent cations, with a high charge density, can generate medium and
9 1987 The Mineralogical Society
424
A. Corma and J. Perez-Pariente
TABLE1. Chemical composition of
natural sepiolite from Vallecas
(weight %).
SiO2
A1203
Fe203
CaO
MgO
Na20
60.31
1"88
0.48
0.27
25.48
0.12
K20
TiO2
Weight loss on
calcination at
1000~
0.26
0.22
10-88
strong acidity on the mineral surface. Thus, it was clearly of interest to study the catalytic
behaviour of acidic sepiolites in reactions involving carbonium ions.
In the present work, it is shown that when A13+ is introduced in the octahedral sheet,
sepiolite develops medium and strong acidity and becomes quite an active catalyst for the
dehydration of ethanol, with a reaction pathway and kinetics characteristic of a strong acid
catalyst.
EXPERIMENTAL
The sepiolite was a very pure sample from Vallecas (Spain). Its chemical composition is given
in Table 1. 1.8 mEq/g magnesium ions of the natural sample were exchanged by A13+ ions
following the procedure described previously (Corma et al., 1984), the final A1 content of the
exchanged sample being 2.60 wt%.
The catalytic experiments were performed in a tubular glass flow reactor, operated at
atmospheric pressure, using Nz as a carrier gas. Accumulated liquid and gas samples were
taken every 4 min for 60 min. The activity decreased slowly with time-on-stream, and the
value at time-on-stream zero, was obtained by fitting the experimental results to the decay
function :
(~), =
(x~)0 e -~'
(1)
where (xi)0 is the yield of product i at time-on-stream zero, (~)~ is the average yield of product i
at time-on-stream t, and k is a decay constant.
The reaction products were analysed by gas chromatography (GC), using a 2.5 m long
Porapak Q (100-120 mesh) column at 110~ for separation of the products. The acidity was
measured by infrared spectroscopy (IR) of the pyridine adsorbed on the sample. For that
purpose, wafers of 1 0 m g c m -z were heated for 2 h at 200~ under 10-4Torr in a
conventional greaseless pyrex IR cell. After pre-treatment and pyridine adsorption at room
temperature, the cell was evacuated for 1 h under 10-4 Torr at several temperatures. The
spectra were recorded at room temperature with a Perkin-Elmer 580B Spectrometer
equipped with a Data Station.
Ethanol dehydration catalysed by acidic sepiolite
425
RESULTS AND DISCUSSION
Reaction pathway
When reacting ethanol on solid acid catalysts, the products generally found are ethylene,
water and diethylether. Although there is no general agreement in the literature on the
reaction pathway, three schemes have been proposed, these depending to some extent on the
catalyst used:
(A) The olefin (O) is formed by a consecutive intramolecular dehydration reaction via the
intermediate ether (E) obtained by interdehydration of two molecules of alcohol (A):
2 A ~1- E + W L 2 0 + 2W
2
(B) The formation of both ether and olefin takes place by parallel reactions:
2A
~tE+W
2
"~ 2 0 + 2 W
3
(C) The ether and the olefin are formed by consecutive and parallel reactions:
J,4
A+O+ W
3"-~ 20 + 2W ~-~
In these schemes, W represents the molecules of water produced in the dehydration
processes.
Scheme A has been selected in some cases (Morita et al., 1972) on the basis of experimental
evidence that the concentration of ether in the reaction mixture goes through a maximum as a
function of contact time. Despite that, a consecutive mechanism is consistent with such an
observation, and so these results could also be explained by a B-type mechanism, provided
the formation of the ether is a reversible reaction (Yue & Olaofe, 1984).
The most general reaction pathway (scheme C) has been proposed by Kn6zinger & K6hne
(1966), and accounts for most of the results presented in the literature, especially those
obtained at reaction temperatures > 300~ (Balaceanu & Jungers, 1951; Kn6zinger &
K6hne, 1966). At those temperatures, reactions 5 and 6 are rather limited and may therefore
be neglected. Scheme C can be reduced to scheme B when the concentration of ether among
the reaction products is very small. Therefore, reactions 4 and 5 can be neglected, which is
common when using a differential reactor.
For the experiments being reported, the products observed when reacting ethanol on AP +exchanged sepiolite (Sep-A1) are diethylether, water, ethylene and very small amounts of
acetaldehyde at reaction temperatures > 400~ The first three products are typical of proton
catalysed dehydration, and indicate that Sep-A1 sample must have Br6nsted sites of at least
medium acid strength, as supported by the results of pyridine adsorption presented in Fig. 1.
Pyridine was adsorbed on Sep-A1 at room temperature and, after outgassing at 150~
under 10-4 Torr, the 1545 cm -1 band characteristic of pyridinium ions produced by the
protonation of a pyridine molecule developed. This band remained measurable even after
outgassing at 350~ indicating the presence of strong and medium Br6nsted acid sites
responsible for the behaviour of Sep-A1 during the dehydration of ethanol. The results are in
426
A. Corma and J. Perez-Pariente
u
o-
(I
n
i
1800
1700
i
i
1600 1500
cm-1
(__
i
1400
1300
FIG. 1. IR spectra of (a) AI3+ exchanged and (b) natural sepiolite after pyridine adsorption.
Samples were pretreated at 200°C and evacuated at 150°C after pyridine adsorption.
contrast to those of Dandy & Nadiye-Tabbiruka (1982) who, using natural sepiolite as a
catalyst, observed the formation of butadiene as a major product and small amounts of
ethylene on reacting ethanol at 300°C. The activity of natural sepiolite was about 200 times
lower than that of the A13+-exchanged sample reported here. In any case, the formation of
butadiene is more a consequence of the dehydrogenating activity of the cations than o£ an
acid catalysed reaction. Indeed, Br6nsted acidity by pyridine adsorption on natural sepiolite
has not been observed (Fig. 1). On the other hand, the role of the cations in the formation of
butadiene on sepiolite has been shown by Kitayama et al. (1984), who obtained mainly
butadiene as the reaction product of ethanol on a Mn2+-containing sepiolite.
To study the reaction pathway for the dehydration of ethanol on Sep-Al, the total
conversion and yields of the different reaction products were determined by changing the
contact time in the range 0.096-9-6 g h mo1-1. The yields of ethylene and diethylether vs.
total conversion obtained at 300 and 400°C are plotted in Figs 2a and b, respectively.
Ethanol dehydration catalysed by acidic sepiolite
427
0t~
~4
'~ / / o
~ 0 1
0
10
20
I
I
I
30
40
50
Xt(%)
/
50
o~
O
o/
40
30
o
X
~ 0
0
10
I_17 I
20
30
I
40
-171
-0--
50
Xt(%)
FIG. 2. Yields ((xi)0) of ethylene (O), diethylether (O) and acetaldehyde (Cl) as a function of the
total conversion (X,) of ethanol reacted on Sep-Al at (a) 300~ and (b) 400~ (Partial pressure of
ethanol = 0-14 atm.)
From the data given in Fig. 2, it is clear that ethyl ether and ethylene are both primary
products. Furthermore, the shape of the curves shows that ethylene is a primary plus
secondary stable product, while ether is a primary and unstable product. Taking this
observation into account, it appears that the dehydration of ethanol on Al3+-exchanged
sepiolite follows scheme C presented above:
x ethyl ether + water
ethanol ~
4~
3~" ethylene + water
It should be noted from Fig. 2 that the secondary character of ethylene and, consequently,
the instability of ether is only observed for conversions > 20%. This implies that, at low
428
A. Corma and J. Perez-Pariente
'~V
0
/o/
0.2
O.Z, Off
Pet (arm)
0.8
1.0
FIG. 3. Initial rates of formation of ethylene (O), diethylether (O) and acetaldehyde ([~) as a
function of ethanol partial pressure at 400~
conversion, the rate of reaction 4 is close to zero and scheme C can be reduced to the more
simple scheme B, in which the formation of ether and ethylene could be treated as
independent, simple parallel reactions.
Influence of the partial pressure of reagents
Influence of the ethanol partial pressure. The initial rates of formation of ethyl ether and
ethylene were obtained using differential conditions, i.e. at total conversion < 5%. The
reaction temperature was 400~ and the ethanol partial pressure was in the range of 0.070-46 atm. The results presented in Fig. 3 show that the rate of formation of ethylene and ethyl
ether increases when the partial pressure of the alcohol is increased. These results are in line
with those found for the dehydration of isopropanol on zeolites Y at reaction temperatures
> 150~ (Jacobs, 1977), and zeolite 4A at reaction temperatures > 280~ (Yue & Olaofe,
1984); they differ from the zero order with respect to ethanol found at T < 307~ on ~-A1203,
q-Al203 (Kn6zinger & Ress, 1967; De Boer et al., 1967) and on silica-alumina at T < 180~
(Figueras et al., 1969).
Influence of the water partial pressure. The influence of the partial pressure of water has been
studied at 400~ in the range 0-0-49 atm., while keeping constant the partial pressure of
ethanol (0.14 atm) by means of N2. The results presented in Fig. 4 do not show any effect of
the partial pressure of water, in the range investigated, on the rate of formation of ethylene,
and only a small inhibiting effect on the formation of diethylether.
Ethanol dehydration catalysed by acidic sepiolite
429
0.8
-0-O~
I.C
..L
o
E
0
0.6
0.4
0.2
0-
0
0.2
0.4
0.6
0.8
1.0
Pw(atm)
Fie. 4+ Initial rates of formation of ethylene (O) and diethylether (O) as a function of water
partial pressure at 400~ (Pet = 0+14atm.)
On other acid catalysts (Kitayama et al., 1984; Kn6zinger & Ress, 1967) it has been shown
that water strongly inhibits the rate of dehydration of ethanol.
In the case of the Sep-A1 catalyst, the absence of the inhibiting effect of water on the
formation of ethylene can be explained on the basis of the relative values of the adsorption
constants for ethanol (KA) and water (Kw). Indeed, if Kw ~ KA, the effect of water on the rate
of formation of ethylene should be practically negligible. It is obvious that this condition
could be achieved at the relatively high temperatures of reaction used if the heat of
adsorption of ethanol is smaller than the heat of adsorption of water on the Sep-A1 catalyst. In
this respect, the results reported in the literature for the adsorption of alcohols and water on
sepiolite clearly point in that direction, as it has been observed that the affinity of the Mg 2+
ions located at the edges of the channels for water is higher than for alcohols (Fern~ndezHern~mdez & Fern~mdez-Alvarez, 1983; Serna & VanScoyoc, 1978). If this is so for Mg 2§
ions, the difference for adsorption of water and ethanol would be even higher for A13§ ions,
which, as mentioned above, are the cause of the acidity.
For diethylether, the small inhibiting effect observed could be caused merely by an
increase in the rate of reaction 2 rather than by an inhibiting effect on reaction 1 (scheme C).
In any case, the small effect of the partial pressure of water on the rate of formation of
diethylether is consistent with the hypothesis made previously, that on Sep-Al, both
reactions--the formation of ethanol and diethylether--could be treated kinetically at low
conversions as parallel, irreversible reactions.
Kinetic models and reaction mechanism +for the dehydration of ethanol on Sep-A1
Formation of ethylene. It is well established in the literature that the dehydration of alcohols
on acid catalysts takes place on Br6nsted acid sites, the Lewis acid sites being inactive
(Jacobs, 1977; Morita et al., 1972; Venuto & Landis, 1968; Stone & Agudo, 1969; Levchuk &
430
A. Corma and J. Perez-Pariente
Ione, 1972). The production of ethylene from ethanol, as it has been postulated, occurs via the
formation of an oxonium and a carbonium ion as consecutive reaction intermediates:
+
(1)
CH 3 -
CH2OH
+
H - OS
--~ C H 3 -
CH 2 -
O -
H
I
H
OI
S
+
(2)
CH 3 -
CH 2 -
O -
+
H
--~ C H 3 -
CH 2 + H20
I
H
:
O-
-()
I
S
I
S
+
(3)
CH3 --
C H 2 --> C H 2 =
CH2 + H - OS
-O
I
S
where H - OS represents a surface Brfnsted site. In the above mechanisms, it is generally
assumed that the rate-limiting step is the formation of the carbocation (step 2) (Jacobs, 1977;
Yue & Olaofe, 1984; Gentry & Rudham, 1974). If this is correct, on a solid acid catalyst the
rate of the reaction should be given by:
r = k2[CH 3 -
CH2OH2OS]
(2)
where k2 is the kinetic rate constant for the formation of the carbocation.
Taking into account step 1, the microreversibility principle, and the Langmuir isotherm,
equation 2 is converted to:
k20K1 [ C H 3 -
CH2OH
]
r = 1 - K~ [CH 3 - CH2OH ]
For a differential reactor, k~~ = k2[HOSl, and /C~ is the
adsorption
(3)
constant of ethanol.
It should be noted that the concentration of water has not been introduced in equation
(3), since the reaction is performed in differential conditions, and furthermore, no effect of
the partial pressure of water on the rate of formation of ethylene was observed.
The values of the initial rate of the reaction at different partial pressures of ethanol (Fig. 3)
obtained at 400~ have been introduced in equation (3) and linearized by a least-square
fitting. The optimum values of the parameters found were: K20= 2-15 mol h -1 g - 1 cat;
Ethanol dehydration catalysed by acidic sepiolite
431
K1 = 2.98. The statistical tests applied (correlation coefficient (c.c.)=0.995, ~0 =0.147,
F = 138) clearly show that the fitting is statistically significant with a 95% level of confidence.
The results, therefore, are consistent with a mechanism in which the rate-limiting step is the
formation of a carbocation from an oxonium ion formed by protonation of ethanol.
Formation of ethyl ether. In the case of the intermolecular dehydration of alcohols on solid
acids leading to the formation of ethers, several kinetic equations corresponding to different
chemical mechanisms have been proposed:
Kinetic models based on Langmuir-Hinshelwood (LH) mechanisms.
(a) Model LH in which the rate-limiting step is the rate of the surface reaction of two
molecules of ethanol chemisorbed on two adjacent sites:
k/~A [CH3 - CH2OH] 2
r - (1 - KA[CH 3 - CH2OH]) 2
(4)
(b) Model L H D , in which the rate-limiting step is the surface reaction but with a
dissociative adsorption of ethanol. This model is based on the existence of acid-base pairs on
the surface of the catalyst (Figueras et al., 1969; Kn6zinger & Ress, 1967):
k(KA [CH3 - CH2OH]) 89
r-
(5)
1 ~- (KA[CH 3 - CH2OH]) ~
Kinetic models based on Rideal-Eley (RE) mechanisms (Yue & Olaofe, 1984; Venuto &
Landis, 1968).
(a) Model RE I, in which the rate-limiting step is the reaction between one molecule of
ethanol chemisorbed and another one in the gas phase:
kKA [CH 3 - CHEOH] 2
= _
r
1 - KA[CH 3 - CHEOH] 2
(6)
(b) Model RE II, in which the rate-limiting step is the reaction between one molecule of
ethanol chemisorbed on two sites and one molecule of ethanol in the gase phase:
kKA [CH3 - CH2OH] 2
r = (1 - KA[CH 3 - CH2OH]) 2
(7)
In the kinetic expressions (4) to (7), k is the kinetic rate constant which includes the
concentration of active sites, and KA is the adsorption constant for ethanol. The above kinetic
expressions have been developed taking into account that in this work the experiments have
been carried out in a differential reactor.
The values of the initial rate taken from Fig. 3 have been fitted to equations (4) to (7) and
linearized. The statistical parameters obtained are given in Table 2. These values clearly
indicate that the best fit is obtained with model RE I, namely the rate-controlling step is the
reaction between one molecule of ethanol adsorbed on one Br6nsted acid site and one
molecule of ethanol in the gas phase. The values of the parameters are k = 1.70 mol h -1 g-1
cat and KA = 45-0 atm. The worst fit is obtained with model L H D based on a dissociative
adsorption. Model LH also does not look probable, especially if one takes into account that it
should involve the direct interaction between two positively charged molecules (Gentry &
Rudham, 1974).
From the kinetic parameters obtained at 300 ~ and 400~ the apparent activation energies
for the two processes calculated according to the Arrhenius equation yield 28 and
432
A. Corma a n d J. Perez-Pariente
TABLE2. Statistical parameters obtained
in the fitting of models (4~(7).
Model
c.c.
~*
Ff
LH
LHD
RE I
RE II
0"991
0.982
0-997
0.991
0"195
0-267
0.130
0.195
104
54
146
104
* ~, of Exner (1966).
t F of Fisher (1955).
9 Kcal mol-1 for the formation of ethylene and diethylether, respectively. Despite their low
accuracy, these values correspond to those found for most solid acid catalysts (Balaceanu &
Jungers, 1951 ; Figueras et al., 1969; Yue & Olaofe, 1984).
CONCLUSIONS
Sepiolite in which some of the octahedral Mg 2+ ions have been substituted by A13§ develops
medium and strong Br6nsted acid sites, and is quite active for the dehydration of ethanol.
The product distribution and selectivities are similar to those obtained for silica-alumina and
zeolites. The formation of ethylene and diethylether on Al-sepiolite takes place by a series of
parallel and consecutive reactions, and the water formed during dehydration does not inhibit
the rate of formation of either product. Finally, the formation of ethylene fits a kinetic model
in which the rate-limiting step is the surface reaction of one molecule adsorbed on one
Br6nsted acid site, while the formation of diethylether is best explained by a model in which
the chemical reaction between one molecule of ethanol adsorbed on one Br6nsted acid site
and one molecule of ethanol in the gas phase is rate-limiting.
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