Chemistry Reference
In-Depth Information
above, MTH product selectivity in 8-ring CHA has previously been
reported to be determined mainly by product shape selectivity,
106
hence
its selectivity pattern will not be further discussed here.
An internal comparison of the three 12-ring structures (BEA, AFI,
MOR) show that their C
4
HTIs and aromatics selectivities follow their
pore size, in the order: AFIWBEAWMOR. Furthermore, their C
4
/C
3
ratios
differ significantly and decrease in the order: BEAWAFIWMOR. The C
4
/
C
3
ratio differences are probably related to the aromatics selectivities,
since iso-C
4
(the main C
4
isomer) is more readily hydrogenated
than propene. The relative C
3
and C
5
þ
aliphatics selectivities of these
materials are similar and generally mirror each other, and their C
3
/C
2
ratios are also similar. It is interesting to note that the aromatics se-
lectivity is higher in 1D AFI than in 3D BEA. This observation suggests
that there is sucient space for bimolecular hydride transfer reactions
(See Section 2.3) in both of these structures, so that the extra space
related to channel intersections is no longer needed. In 1D MOR, with a
slightly smaller pore size (see Fig. 13), however, intermolecular hydride
transfer reactions seem to be slightly more restricted.
Proceeding to a comparison between 12-ring structures and 3D 10-ring
structures, several differences are observed. First, the aromatics selec-
tivities and hence, the C
4
HTIs, are significantly lower for the 3D 10-ring
structures than for the 12-ring structures. This observation suggests a
further hindrance of intermolecular hydride transfer reactions in 3D 10-
ring structures. Typically, among the 3D 10-ring materials, the highest
aromatics selectivity and C
4
HTI are observed for TUN, which has the
largest intersection volume among them. As a second observation, the C
3
selectivities, the C
2
selectivities (as indicated by the similar C
3
/C
2
ratios of
the two material groups) and the C
5
þ
aliphatics selectivities are signifi-
cantly higher for the 3D 10-ring materials than for the 12-ring materials,
whereas the C
4
selectivities are slightly lower. The observed differences in
C
4
selectivity may stem from the higher fraction of saturated C
4
products
from the 12-ring materials, since those products are inert toward further
reaction (see Sch. 4). The higher C
2
and C
3
selectivities generally observed
in the 3D 10-ring materials compared to the 12-ring materials might re-
late to the dominance of light methylbenzenes in 3D 10-ring materials
compared to the large fraction of the heaviest methylbenzene, hexa-
methylbenzene, in 12-ring materials. As referred to in Section 2.2,
previous mechanistic studies have suggested that the lower methyl-
benzenes favor formation of ethene and propene, whereas the highest
methylbenzenes favor formation of propene and butene in the arene
cycle (Sch. 4). Furthermore, the alkene cycle has been suggested to favor
C
3
þ
alkene formation (See Section 2.2). The data reported in Figs. 14-16
are in general agreement with those studies. It is furthermore interesting
to observe that the two 3D 10-ring structures with the largest intersection
volumes, TUN and IMF, yielded higher C
3
/C
2
ratios and hence lower C
2
selectivities than the other 3D 10-ring materials. Thorough inspection of
the aromatics fraction obtained over these materials showed that both
materials yielded hexamethylbenzene, which showed a contact time
behavior typical of a reaction intermediate
49
Hence, the aromatic cycle
