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samples had similar Si/Al ratio, Brønsted acid strength (they were all
Si-Al-O zeolites) and crystal size, but varying topology.
Since it is only possible to prepare samples with similar, but not
identical, material parameters, a summary of previous studies about the
influence of Si/Al ratio, acid strength and crystal size is given in Section
3.2. The experimental set-up for the catalytic tests is described in Section
3.3, and test data are described and discussed in Section 3.4.
3.2 Influence of material parameters other than topology
In the literature, the effect of Si/Al ratio and crystal size has mainly been
studied for H-ZSM-5 (MFI). In general, the literature studies conclude
that a higher Si/Al ratio (which corresponds to a lower density of acid
sites) leads to higher selectivity towards C
2
-C
4
alkenes and lower selec-
tivities toward aromatic products, in accordance with a predominantly
sequential reaction scheme. In such a scheme, alkenes are formed first
and sequentially converted into aromatic and alkane products, as
originally suggested by Chang and Silvestri.
55
Chang et al. studied the
MTH reaction over MFI with two Si/Al ratios, i.e., 70 and 3340,
while intermediate Brønsted acid site densities were obtained by partial
H
þ
Na
þ
cationic exchange. At 370 1C and full conversion, the C
2
-C
4
alkene selectivity was 30% and 46% for Si/Al
=
70 and 3340, respectively.
Lower Si/Al ratios led to higher ethene selectivities and lower propene
selectivities.
156
Prinz and Riekert tested MFI catalysts with varying Si/Al
ratios (34-660) or crystal sizes (0.5-8 mm) for the MTH reaction at 290-
360 1C in a batch reactor, and reported that increasing Si/Al ratios led to
increasing light alkene selectivities and propene-to-ethene ratios. A
similar effect was reported for decreasing crystal sizes (0.5 versus
2
2
6 micron crystals); however, it is worth noting that for con-
versions in excess of 80% the selectivity differences due to crystal size
vanished.
157
Recently, Liu et al. tested MFI samples with Si/Al ratios
ranging from 12-360 at 460 1C, and observed a gradual increase in
propene selectivity from 16% to 52%, respectively, at full methanol
conversion. Concurrently, the propene to ethene ratio increased from
2.0 to 6.5.
158
We recently studied the MTH reaction over three MFI
catalysts with different Si/Al ratios (12, 50 and 140, respectively) under
the conditions reported in Figs. 12-16, and observed that the C
3
selectivity decreased, while the HTI (hydrogen transfer index, see
Section 3.4.2) and aromatics selectivity increased, with a decrease in
Si/Al ratio (i.e. an increase in acid site density), throughout the
conversion range tested (50-100% conversion). This behaviour is in line
with the cited literature reports.
156-158
Forethenethepicturewasmore
complex: The samples with the highest density of acid sites gave an
intermediate selectivity to ethene, compared to the two samples of
medium (highest ethene selectivity) and low (lowest ethene selectivity)
acid site density. These observations suggest that correlating ethene
selectivity with material parametersisnotastraightforwardtaskand
should be performed with care.
Very recently, Westgård Erichsen et al.
51
elucidated the effect of
acid strength in MTH catalysts by comparing two isostructural materials,
