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Scheme 2 The hydrocarbon pool mechanism as proposed by Dahl and Kolboe for CHA
(SAPO-34). Adapted from Refs. 77-79.
2.2.3 The ''hydrocarbon pool''. In the mid-1990's, Dahl and Kolboe
proposed the "hydrocarbon pool mechanism" for the MTH reaction.
77,79
They carried out isotopic labeling experiments by co-feeding alkene
precursors (ethanol, propanol) and
13
C-methanol over a SAPO-34 (CHA)
catalyst. Analysis of the euent showed that most of the products were
formed exclusively from methanol under the applied reaction con-
ditions.
77-79
Hence, a parallel indirect mechanism, the ''hydrocarbon
pool'', was proposed. While their proposal shared many similarities with
previous works, this schematic concept had a greater immediate influ-
ence than the works of the previous decade.
92
The original hydrocarbon
pool model, as shown in Scheme 2, assumed that methanol was con-
tinuously added to a pool of adsorbed hydrocarbons, which successively
eliminated light alkenes.
The initial hydrocarbon pool was given an overall stoichiometry
(CH
2
)
n
, and the chemical structure was not specified.
77-79
Thus, the
concept of the hydrocarbon pool could cover both the alkene inter-
mediates proposed by Dessau,
80,81
the aromatic intermediates proposed
by Mole et al.,
90,91
and other types of intermediates. However, studies of
the hydrocarbon pool in the following decade focused mainly on
aromatic or cyclic intermediates. The group of Haw et al.
93-96
used MAS-
NMR spectroscopy to identify a number of benzenium and cyclopenta-
dienyl cations present inside the catalyst under working conditions,
while Mikkelsen et al.
83
found support for the hydrocarbon pool in large-
pore zeolites from co-reactions of aromatics and methanol. The groups of
Haw and Kolboe simultaneously concluded that polymethylbenzenes
were the main hydrocarbon pool species in H-SAPO-34 (CHA).
84,85,97
Additional evidence for the hydrocarbon pool mechanism in H-ZSM-5
(MFI), H-SAPO-34 (CHA) and H-SAPO-18 (AEI) was also provided by
Hunger et al.
98-100
Later studies of the MTH reaction with zeolite H-Beta
(BEA) cemented the importance of polymethylbenzene intermediates in
this catalyst.
101,102
2.2.4 The dual cycle concept. After the long period focusing on aro-
matic intermediates in the MTH reaction, steady-state isotope transient
studies (as described in Section 2.2.1) over the medium-pore H-ZSM-5
(MFI) catalyst revealed that not all alkenes were formed from aromatics.
While ethene and the lower polymethylbenzenes (toluene to tetramethyl
benzene) displayed similar contents of
13
C, the higher alkenes (C
3
รพ
)
displayed a higher reactivity for the incoming
13
C methanol than the
