5506 J. Phys. Chem. A, Vol. 101, No. 30, 1997
Herzler et al.
subsequent to the heating period on the cold walls of the shock
tube or sampling system to produce the ethanol. However, the
thermodynamic constraints of Ho and Melius mean that in no
case can ethanol be produced in a process (surface or gas) that
involves the formation of the silicate since this is the source of
the high endothermicity. A possible alternative would involve
the reaction of the silanols with water leading to the formation
of the alcohol and diol. This implies water as an impurity in
the system. In this regard, TEOS itself is readily hydrolyzed
in solution to produce silanol and ethanol, yet in control
experiments with unshocked mixtures no such reaction was
observed. This would require either that significant water is
not intrinsically present in our system, or that such reactions
do not readily occur on the surfaces. Of course water may also
be produced in the latter stages of TEOS decomposition and
could conceivably have an effect. However, in the studies with
t-butanol as the standard, an additional source of water was
present but no effect on ethanol yields was observed. This sets
a lower limit to how much water is needed to cause any effects.
radical and Figure 8 contains a possible mechanism for its
decomposition. In the case of the hydrocarbon analog, C-O
â-bond fission would clearly be the prevalent channel. How-
ever, in view of the strength of the Si-O bond, a more likely
pathway for the silicon compound is isomerization to form (CH3-
CH2O)2Si(OCH3)OCH2CH2 and (CH3CH2O)2(OCH3)SiOCHCH3
radicals via 1-6 or 1-5 hydrogen transfers. For hydrocarbon
systems the 1-5 H transfer would be the preferred channel and
would lead to C-O â-bond fission and acetaldehyde formation.
However, in the silicon compounds the strength of the C-O
and Si-O bonds leads to the conclusion that decomposition
through the ejection of a hydrogen atom would be favored.
Rate Constants. Molecular Elimination. The data given
above on TMEOS suggest that near 1250 K the ethylene
elimination reaction makes at least an 85% contribution to the
overall decomposition of TEOS. This is equivalent to a rate
constant for C2H4 elimination from TEOS of 1600 s-1. In our
work11 on diethoxymethane decomposition, the rate expression
for ethylene elimination leads to a rate constant of 1200 s-1 at
1250 K. Since diethoxymethane contains only two ethoxy
groups, this suggests that silicon for carbon substitution
decreases the rate constant by a factor of 1.5. Assuming that
this is largely due to a change in the activation energy, we derive
the rate expression
Our failure to find any indication of bimolecular processes
or surface effects leads to the conclusion that ethanol is directly
formed in a gas phase unimolecular process. This belief rests
essentially on the reproducibility of the results despite variations
in conditions and, indeed, in the molecules studied. Past
experience has shown that for similar situations where subse-
quent surface effects may be important, as in the decomposition
of ethyl phosphates,14 the yields of ethanol showed enormous
variations and were in effect irreproducible. It surface reactions
are unimportant, the process responsible for ethanol production
must be molecular in nature, since we are unaware of any radical
process that could lead to its formation under our conditions.
k[TEOS f C2H4 + (C2H5O)3SiOH] )
2.0 × 1015 exp(-34 900 K/T) s-1
This rate expression is somewhat different from that derived
by Chu et al. at temperatures near 800 K, k(TEOS) ) 4.9 ×
1013 exp(-31 500 K/T) s-1. Extrapolating their results to our
temperature range leads to a rate constant that is a factor of 2
smaller than ours. On the other hand, extrapolation of our
expression to their temperatures leads to a rate constant that is
a factor of 2 smaller than their value. Part of this deviation
may arise from the curvature in the Arrhenius plot over 7 orders
of magnitude in the rate constant. Indeed, a T1.1 curvature in
the Arrhenius plot will accommodate all the results. This then
leads to a rate expression of
An interesting aspect of the data in Figure 2 is that the
ethylene yields increase rapidly at temperatures above 1250 K,
while those for ethanol level off. Our experiments with ethanol
demonstrate that it is stable under our conditions, so this
behavior is directly related to the decomposition mechanism. It
is further significant that the yields of ethanol from DMDEOS
and TEOS are very similar. This suggests that only one ethoxy
group in TEOS is available for conversion to ethanol. When
this is coupled with the increase in ethylene yields in TEOS
decomposition at temperatures in excess of 1250 K and the
absence of such effects in DMDEOS decomposition, the
implication is that one ethanol is expelled from TEOS in a
relatively early process, but thereafter the reactions favor
ethylene formation. One possible explanation is that the ethanol
arises from decomposition of the silanol, but that products of
this reaction do not easily produce further ethanol.
k[TEOS f C2H4 + (C2H5O)3SiOH] )
1.04 × 1010T1.1 exp(-30 950 K/T)
Bond Fission. If we assume that the remaining component
is purely bond breaking, then the rate constant at 1250 K is
400 s-1. With a typical A-factor for breaking four C-C bonds
of 4 × 1017 s-1 we find that an activation energy of 360 kJ/mol
is required. This is in reasonable accord with what can be
expected from Figure 1. However, this rate constant is based
on the subtraction of two large numbers, and errors as large as
a factor of 2 are possible.
The sum of the elimination and bond-breaking processes
unfortunately does not reproduce the temperature dependence
that we have determined from the total disappearance of TEOS.
The deviations are about 15% at each end of the temperature
range. This should have minimal consequences on our assigned
rate constant for elimination. It may have more serious
consequences regarding the bond-breaking process. It is
conceivable that there are other but unknown processes con-
tributing to TEOS decomposition. Two possibilities are that
there are contributions from radical chain induced decomposition
at the highest temperatures and that our estimated rate expression
for bond breaking may be too small.
The formation of ethanol from silanol is accompanied by the
formation of a silicate. The analogous carbon compound is
diethyl carbonate (DEC). DEC has a very facile decomposition
channel,11 the retro-ene process, leading to the formation of
ethylene and C2H5OCO2H, which decomposes immediately to
ethanol and CO2. The silicate may also undergo such a process:
Note that the products are the same as for the four-center
elimination of ethylene, but that the reaction may be very much
faster. Except, by analogy with the carbon compounds, there
is unfortunately no basis for assigning rate constants for such a
process. A similar reaction path is also available for decom-
position of C2H5OSiO2H.
Results from Simulations. Models with Gas Phase Forma-
tion of Ethanol. The mechanism in Figure 8 involves numerous
reactions. The experiments described here yield direct results
Fission of the C-C bond was shown to be one of the initial
processes. In TEOS, this leads to the (CH3CH2O)3SiOCH2