Chemistry Reference
In-Depth Information
MTH reaction is believed to proceed through an indirect mechanism,
wherein hydrocarbon species act as reaction centers for product for-
mation.
47,77-79
The hydrocarbons that act as reaction centers may be
alkenes,
80,81
aromatic species,
82-87
or both alkenes and aromatics sim-
ultaneously.
82
In the following section, the efforts leading to the current
mechanistic understanding of the MTH reaction are presented in a
roughly chronological order. Isotopic labeling experiments have been
instrumental in several breakthrough contributions, and the first section
is therefore dedicated to a brief introduction to such studies.
2.2.1 Isotopic labeling studies. Two types of isotopic labeling studies
are commonly used. The first is co-feed studies, in which
13
CH
3
OH is
co-fed with unlabeled hydrocarbons and the products of reaction are
studied with respect to
13
C content and distribution. Products which are
formed by simple methylation reactions will contain the same number of
13
C as the number of carbons introduced in the product by sequential
methylation, whereas products which are formed by a more complex
mechanism, or by several parallel mechanisms, will contain a statistical
distribution of
13
C atoms.
The other type of isotopic labeling studies is steady-state isotope
transient experiments, in which the feed is switched from unlabeled to
labeled reactant after a predetermined time on stream, and the
13
C
contents of reactant and products are followed as a function of time after
switching the feed (Fig. 10). Such experiments are commonly used in
catalysis research to distinguish between reaction intermediates and
spectator molecules. In general, an active intermediate will incorporate
the labelled component more rapidly than, or equally fast as, the product
molecules. On the other hand, a spectator molecule will incorporate the
labelled component more slowly than the products.
2.2.2 Alkene based mechanism. To the best of our knowledge,
Dessau and coworkers from Mobil were the first to use
13
CH
3
OH/
12
C-
alkene and -arene co-feed studies to elucidate mechanistic details about
the MTH reaction. Based on such co-feed studies over H-ZSM-5 (MFI)
zeolite, they proposed that the reaction proceeds by an alkene methyl-
ation/cracking mechanism, illustrated in Scheme 1.
80,81
Fig. 10 Illustration of steady-state isotopic transient experiments with switching from
12
C
methanol to
13
C methanol after a predefined time on stream, X.
