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incorporated into silmethylenes, higher hydrocarbons, and coke. It should be pointed
out that this scheme does not represent the actual mechanisms involved these
reactions. Indeed, understanding the actual mechanisms of these side reactions is
critical for controlling the desired ones or suppress the undesired ones to optimize
process performance. Two key questions are: (a) what are the rate-determining-steps
(RDS) for these side-reactions? And (b) what are the active-sites that catalyze these
side-reactions during the direct process?
Scheme 1.
Summary of side-reactions and by-products related to unproductive methyl
groups. HC stands for hydrocarbons.
C-H bond cleavage as a rate-determining-step
Formation of all of the by-products summarized in Scheme 1 requires cleaving C-H
bonds. As C-H bonds in a free methyl radical are very strong (~105 kcal/mol),
16
one
would expect that C-H bond cleavage is a major component of the rate-determining-
step (RDS). One way to test this hypothesis is replacing the proton with a deuterium.
If C-H bond cleavage is a major component of the RDS, the reaction rate should be
changed (reduced in most cases). This phenomenon is called kinetic isotope effect
(KIE).
17
Scheme 2 explains the origin of KIE in C-H bond homolytic cleavage.
Changing from a proton to a deuterium doubles its mass and reduces the stretching
vibration frequency from 2900 cm
-1
for a C-H bond to 2100 cm
-1
for a C-D bond. As
vibration frequency reduces, the zero-point-energy (ZPE) also reduces from 4.15
kcal/mol for C-H stretch to 3.00 kcal/mol for C-D stretch, and causes the same
difference for homolytic cleavage activation energies (
ǻ
G
‡
). Based on the Eyring
equation, that will reduce the reaction rate by a factor of about 7.
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150