Chemistry Reference
In-Depth Information
H-SAPO-5 (AFI) at low temperatures, and concluded that a paring-type
reaction dominated alkene formation from aromatic intermediates over
both materials. The systematic incorporation of one carbon from the
aromatic ring, as observed in these studies, provides strong evidence that
a ring expansion or contraction step is involved in de-alkylation of
polymethylbenzenes at low temperature. Whether side-chain methylation
becomes more important at higher temperatures is an open question:
isotopic labeling experiments performed under typical MTH conditions
are very dicult to analyse, due to the possibility of independent
reactions leading to ring/methyl carbon exchange without de-
alkylation
101,102
combined with parallel alkene formation via the alkene
cycle.
The two dealkylation mechanisms have also been investigated
theoretically, but studies directly comparing the two mechanisms in the
same catalyst are still missing. McCann et al.
114
and Lesthaeghe et al.
115
investigated the paring reaction and side-chain methylation respectively
over H-ZSM-5 (MFI), but considered different end-products. While
McCann's paring cycle showed no major bottlenecks to produce alkenes,
Lesthaeghe found that an ethyl chain could grow from o-xylene, but with
barriers higher than 200 kJ/mol for ethene elimination. Later work by
Kolboe
116-118
revealed that elimination of an alkyl chain can occur with
much lower barriers through a p-complex between the benzene ring and
the alkyl fragment. This observation led de Wispelaere
119
to suggest a
complete low-barrier side-chain methylation mechanism, where all
barriers are below 100 kJ/mol.
A pertinent question regarding the side-chain methylation mechanism
is whether appreciable amounts of HeptaMB
þ
are ever deprotonated and
available for side-chain methylation under reaction conditions. The work
by Bjørgen et al.
101
suggests that this is not the case at low temperature,
while the deprotonation steps suggested by de Wispelaere et al.,
119
(supporting information) display very high reverse (protonation) rates.
Regarding the paring reaction, the observation of similar isotopic
labeling patterns (one ring carbon incorporated) for ethene, propene and
isobutene challenges the classical paring pathway, since this reaction is
normally not associated with ethene. The lack of an obvious pathway to
ethene from HeptaMB
þ
through a paring reaction leads to two possi-
bilities: Either ethene is formed from lower polymethylbenzenes, or chain
growth must proceed via another mechanism than hitherto proposed, that
can also account for the systematic carbon scrambling. One possibility is
expansion to a tropylium-type cation and subsequent contraction before
de-alkylation. This type of mechanism has been subjected to studies
of gas phase kinetics by Arstad et al.
120
(theory) and Sekiguchi et al.
121
(experiment). High barriers were found in both studies.
2.3 Recent research trends
2.3.1 Experimental studies. Due to the complexity of the MTH
reaction, several recent research efforts have focused on the kinetics of
elementary reaction steps. For a detailed description of the state of the
art, we refer to a recent review by Ilias and Bhan, who focused on six
