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might produce mainly propene and butene in these two materials. At the
same time, their aromatics content was lower than in the 12-ring ma-
terials, which probably means the alkene cycle, with its preference for
C
3
þ
alkene formation, was more abundant in these materials than in the
12-ring materials, together contributing to their particularly low C
2
selectivity.
Proceeding last to a comparison between the 3D 10-ring materials and
1D 10-ring TON, they gave similar C
3
selectivities. The C
4
selectivity of
TON was slightly higher, and the C
2
selectivity much lower, than for the
3D 10-ring materials. TON furthermore gave significantly lower C
4
HTI
and aromatics selectivity and a much higher C
5
þ
aliphatics selectivity
than the 3D 10-ring materials. This last observation shows that the ab-
sence of intersection volumes and cavities in the TON structure heavily
restricts its ability to form aromatic molecules by intermolecular hydride
transfer reactions. The high C
5
þ
aliphatics selectivity (which is higher
than the combined C
5
þ
aliphatics and aromatics selectivities of any
other material reported here) further suggests that alkene cracking re-
actions are sterically restricted in this material. The low C
2
selectivity is
further in line with mechanistic studies, which indicated that the alkene
cycle dominates in the MTH reaction over TON (See Section 2.2), and with
the low selectivity towards ethene in alkene cracking reactions reported
in literature.
168
4 Summary and outlook
Fundamental insight into the MTH reaction has now reached a level
where there is general agreement between the currently accepted dual
cycle mechanism, which results from decades of mechanistic studies and
is reviewed in Section 2 of this contribution and the main trends in shape
selectivity observed in the quasi-single parameter study of zeolite struc-
tures, reported in Section 3 of this contribution.
Shortly summarised, the product selectivities observed over the 8-ring
CHA structure are restricted by product shape selectivity, while the
product selectivities observed over 10- and 12-ring zeolites are further
restricted by transition-state or intermediate shape selectivity. In
particular, it is observed that zeolites with large pores and cavities favor
products formed from the arene cycle, with either light or heavy
methylbenzenes as main hydrocarbon pool species. Not unexpectedly,
these results further suggest that the more space-demanding reaction is
the intermolecular hydride transfer reaction, which determines the
relative fraction of aromatic versus alkene products, and, hence, influ-
ences the relative occurrence of the alkene versus the arene cycle. Within
the alkene cycle, cracking reactions appear as the more space-demanding
reactions, while methylation reactions are less space-demanding.
Moving from large to medium pore zeolites, the product spectrum may
thereby be altered from an aromatics- and alkanes-dominated product
mixture, via a balanced mixture of aliphatic and aromatic products in the
C
2
-C
10
range, and finally to a C
5
þ
-dominated alkenes product mixture,
controlled by transition state or intermediate shape selectivity.
