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ARTICLE IN PRESS
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N. Zilková et al. / Catalysis Today xxx (2015) xxx–xxx
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amorphous silica, thus, the concentration of acid sites is diluted in
comparison with other zeolites. As it will be seen vida infra, despite
the same conversions of MFI and IPC-1PI the selectivities of both
catalysts were substantially different. The lowest conversions were
achieved over IPC-6 and IPC-4 (PCR). Channel structure of IPC-6 is
formed by two non-intersecting channel systems (12-10-R and 10-
8-R). In IPC-6 we assume that the active sites are mostly located in
the smaller 10-8-R channel system, where they are not available
for aromatic molecules as proved to be the case on IPC-4 zeolite
(with only 10-8-R). Therefore, the catalysis proceeded only on the
external surface similarly to IPC-4. The 10-R channels in IPC-4 have
Down flow reactor, 250°C, toluene/iso-prOH 9.6
10
8
100
80
A
B
6
4
2
60
0
600
D
C
70
60
50
40
30
˚
˚
distinctive elliptical shape (5.8 A × 3.8 A) compared to more spher-
400
200
0
˚
˚
˚
˚
ical 10-R channels of MFI (5.1 A × 5.5 A and 5.3 A × 5.6 A). The 8-R
channels are too small to contribute to the alkylation reaction of
aromatics. This means that particularly IPC-4 (PCR) can be con-
sidered in this reaction as zeolite with one-dimensional channel
system.
0
60
120
180
0
60
120
180
Selectivities to cymenes, the primary reaction products for o-
and p-cymenes and isomerization product for m-cymene, depend
nel system (Fig. 6B). In the case of MFI, selectivity to cymenes is
rather constant within 180 min T-O-S at only around 60% as a result
of bimolecular transalkylation between cymenes and toluene to n-
propyl toluenes. This reaction is strongly enhanced by intersecting
10-ring channels [35,36]. In the case of IPC-1PI, IPC-4 and IPC-6,
selectivity to cymenes is higher than 90% after 120 min of T-O-S.
Since IPC-1PI does not possess any microporosity, the active cen-
tres are located only in the open layer surface. We assume that
Al-IPC-1PI acts similar to solid acids where only cymenes are almost
exclusively formed. On the other hand, acid centres in IPC-4 and
IPC-6 channels are not accessible for aromatic molecules (discus-
sion vide supra), accordingly the reaction proceeds only on the
external surface (similarly to IPC-1PI). Lower selectivities for UTL,
IPC-7, and IPC-2 are mostly due to secondary alkylation of primary
formed cymenes or at a lesser extent toluene disproportionation.
The isopropyl (cymene)/n-propyl toluene ratio (Fig. 6C) is again
different among individual catalysts. While the isopropyl/n-propyl
toluene ratio is close to 2 for MFI zeolite, this value is between 70
and 100 for other zeolites after 120 min of T-O-S. In contrast, IPC-1PI
without any micropores reaches the ratio about 200. The very low
activity of IPC-4 and IPC-6 zeolites is responsible for almost neg-
ligible rate of this bimolecular reaction providing values around
500–600. It evidences that the steric constraints of 10-ring inter-
secting channels are a primary requisite for the performance of this
bimolecular reaction.
Selectivity to p-cymene reflects primarily diffusion constraints
in the channel system and first alkylation step directly providing
90%. Zeolites BEA and UTL form thermodynamic concentration of
p-cymene close to 30% indicating that the rate of isomerization
controls the final selectivity.
Fig. 7 provides the same relationship stressing the effect of acid
sites concentrations for IPC-2 zeolite (OKO). Three IPC-2 samples
with Si/Al molar ratios 48, 81 and 109 were synthesized. Fig. 7A
shows that toluene conversions at 250 ◦C are lower than original
UTL and BEA but higher than MFI. Conversions values over IPC-2
catalysts reflect the concentrations of acid sites, IPC-2 (48) > IPC-2
(81) > IPC-2 (109). As for the selectivities to mixture of cymenes,
para-selectivity and isopropyl/n-propyl toluene ratio, the relation-
ship is just opposite. With decreasing concentration of acid sites,
para-selectivity increases. With decreasing aluminium content the
density of acid sites decreases. Hence, the rate of subsequent reac-
tions like bimolecular transalkylation of primarily formed cymenes
to n-propyl toluenes decreased [36]. It can be also speculated that
T-O-S (min)
T-O-S (min)
Fig. 6. Time-on-stream dependence of toluene conversion (A), selectivity to
cymenes (B), iso-/n-propyltoluene ratio (C) and para-cymene selectivity (D) in
toluene alkylation with isopropyl alcohol at 250 ◦C. MFI (
).
),
IPC-1PI ( ), IPC-7 ( ), IPC-2 with Si/Al 48 ( ), IPC-6 (
), IPC-4 (
ing effect of diffusion constraints (Figs. 6 and 7). The highest toluene
conversion was achieved for zeolite BEA followed by UTL. Three
important factors influence these results, (i) zeolite BEA possesses
a higher concentration of acid sites (cf. Table 2), (ii) zeolite BEA
exhibits much smaller crystals in contrast to sheet-like crystals of
UTL, (iii) the three-dimensional system of zeolite BEA is more easily
accessible than 14-12-R system of UTL (Fig. 6A). This is in a good
agreement with results of Corma tested in the catalytic cracking
of n-decane, triisopropylbenzene, and vacuum gas oil [34]. Toluene
conversion over IPC-7 decreases substantially with time-on-stream
(T-O-S), which can be explained in terms of a high concentration
of Lewis acid sites (Table 2) promoting deactivation of this zeo-
lite. IPC-2 (OKO, 12-10-R channels) showed a higher conversion
than IPC-1PI and MFI. Although the structure of pillared IPC-1PI
and MFI are so different, both catalysts exhibited similar conver-
sions. It should be noted that around 30% of IPC-1PI is formed by
Down flow reactor, 250°C, toluene/iso-prOH 9.6
10
100
A
B
D
8
6
4
2
80
60
0
600
C
70
60
50
40
30
400
200
0
0
60
120
180
0
60
120
180
T-O-S (min)
T-O-S (min)
Fig. 7. Time-on-stream dependence of toluene conversion (A), selectivity to
cymenes (B), iso-/n-propyltoluene ratio (C) and para-cymene selectivity (D) in
toluene alkylation with isopropyl alcohol at 250 ◦C. MFI (
IPC-2 with Si/Al = 48 ( ), IPC-2 with Si/Al = 81 ( ), IPC-2 with = Si/Al 109 (
), BEA (
), UTL (
),
).
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Please cite this article in press as: N. Zilková, et al., The effect of UTL layer connectivity in isoreticular zeolites on the catalytic performance