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Acid site distribution
cule, TPP, was used for 3DOm-i BEA. The acid site concentra-
tion on the external surface and the ratio of external acid sites
to the total acid site measured by CLD-TPD or TPP-TPD are
listed in Table 3. The fraction of external Brønsted acid sites ac-
cessible to the bulky base molecules in 3DOm-i MFI and
3DOm-i BEA is 14.2 and 10.1%, respectively, which is approxi-
mately twice as high as that of MCM-22 (6.4%). The higher dis-
tribution of the Brønsted acid sites on the external surface of
the 3DOm-i zeolites is mainly because of the hierarchical struc-
tures of the zeolite catalysts. The fraction of external Brønsted
acid sites accessible to CLD on ITQ-2 (55.1%) is much larger
than that on MCM-22 because of the delaminated structure of
ITQ-2. The values are also consistent with the theoretical
values calculated from the crystal morphologies with the as-
sumption of a random distribution of Brønsted acid sites. The
results support that the hierarchical structures created by the
hard-templating method and delamination method can signifi-
cantly improve the acid site distributions on the external sur-
face of the zeolite catalysts.
The distribution of Brønsted acid sites in the hierarchical zeo-
lites was investigated by using temperature-programmed de-
sorption (TPD). Isopropylamine (IPA), 2,4,6-collidine (CLD), and
triphenylphosphine (TPP) with different molecular sizes served
as probe molecules to access the total acid sites (IPA-TPD) and
acid sites on the external surface of the zeolite catalysts (CLD-
TPD or TPP-TPD).
IPA-TPD was used to determine the total number of Brønst-
ed acid sites in each catalyst, which is supported by the fact
that adsorption occurs with a 1:1 stoichiometry of IPA over the
Brønsted acid sites in zeolite catalysts.[15] The Si/Al ratio and Al
concentration of the zeolite catalysts determined by elemental
analysis (ICP-OES) are given in Table 3. The number of accessi-
ble Brønsted acid sites obtained from the IPA-TPD experiments
is similar to the amount of Al in 3DOm-i ZSM-5, which indi-
cates that most of the Al atoms are incorporated within the
framework of these zeolite catalysts to lead to Brønsted acidity.
This result is consistent with that of FTIR spectroscopy using
pyridine as the probe molecule (Figure S5). 3DOm-i BEA and
MCM-22 show lower Brønsted acid concentrations than the
amount of Al in the zeolites, which indicates the presence of
extra-framework Al. Interestingly, the total Brønsted acid sites
in 3DOm-i LTA measured by using IPA-TPD is significantly
lower than the amount of Al in this sample. This is because IPA
cannot diffuse into the 8 MR in the LTA structure and access
the acid sites located on the internal surface at the adsorption
temperature of 393 K. The observation is consistent with the
previous study of Pereira and Gorte.[15b] The acid site concen-
tration for 3DOm-i LTA reported in Table 3 is, thus, only related
to the accessible acid sites on the external surface of the
catalyst.
Catalytic activity
3DOm-i zeolites with imprinted mesoporous structures and
a large number of accessible Brønsted acid sites exhibit re-
markable benefits for the catalytic reactions in which diffusion
constraints and/or adsorption of reactant molecules onto the
active acid sites are the main concerns. In this study, the
liquid-phase catalytic conversion of benzyl alcohol in mesity-
lene was used to evaluate the catalytic performance of the hi-
erarchical zeolite catalysts. Previous studies showed that two
parallel reactions, the self-etherification of benzyl alcohol and
the alkylation of mesitylene with benzyl alcohol, can be cata-
lyzed by the Brønsted acid sites of zeolite catalysts.[8,18] The
products of the two reactions are dibenzyl ether (DE) and
To evaluate the acid sites on the external surface of the zeo-
lite samples, a bulky amine molecule, CLD, was used for
3DOm-i MFI, MCM-22, and ITQ-2 in the TPD measurements.[16]
As CLD, which has a kinetic diameter around 0.74 nm, can dif-
fuse easily into BEA micropores,[17] an even larger base mole-
1,3,5-trimethyl-2-benzylbenzene
(TMBB),
respectively
(Scheme 1). It has been revealed that the alkylation of mesity-
lene with benzyl alcohol is catalyzed exclusively by the acid
sites on the external surface of MFI zeolites because of the
steric resistance of the bulky reactant and product,
mesitylene and TMBB. The poisoning of the external
surface of the MFI zeolite catalysts with bulky amines
such as 2,6-di-tert-butylpyridine (DTBP) can eliminate
Table 3. Acid site distribution in the microporous and mesoporous catalysts.
Catalyst
Si/Al
Acid concentration [mmolgÀ1
]
fB,ext
(theoretical)
[%][e]
Total[a]
Total[b]
External
surface[c]
(actual)
[%][d]
the production of TMBB completely. However, the
production of DE from the self-etherification of
benzyl alcohol can be catalyzed by the acid sites lo-
cated on both the external and the internal surface
of MFI zeolite catalysts. As a result of the relatively
large molecular size of benzyl alcohol, diffusion con-
straints have been observed for the reaction in MFI
zeolites, and a diffusion–reaction model has been de-
(theoretical)
3DOm-i MFI
3DOm-i LTA
3DOm-i BEA
MCM-22
151
2.2
34
45
0.110
5.208
0.476
0.362
0.340
0.327
0.407
0.113
0.140
0.370
0.346
0.338
0.325
0.081
0.016
0.140
0.037
0.022
0.015
0.179
N.A.
14.2
100
10.0
6.4
2.4
55.1
N.A.
9.6
6.5
9.4
9.8
7.3
300 nm MFI
ITQ-2
48
50
100
N.A.
Al-MCM-41
204
veloped to interpret the reaction results.[18a,19]
A
[a] Number of total Brønsted acid sites calculated based on Si/Al ratio. [b] Number of
total Brønsted acid sites determined by IPA-TPD. [c] Number of external Brønsted acid
sites determined from CLD-TPD for 3DOm-i MFI, 3DOm-i LTA, MCM-22, and ITQ-2 and
TPP-TPD for 3DOm-i BEA. [d] Fraction of external Brønsted acid sites calculated by
(number of Brønsted acid sites by CLD-TPD or TPP-TPD/number of Brønsted acid sites
by IPA-TPD). For 3DOm-i LTA, Brønsted acid sites detected by IPA are assigned to the
acid sites on the external surface. [e] Calculated based on crystal shape and size and
assuming a random distribution of Brønsted acid sites.
study of the liquid-phase conversion of benzyl alco-
hol in mesitylene over the hierarchical zeolite cata-
lysts provides critical insights into the catalytic activi-
ty and selectivity of the hierarchical zeolite catalysts.
In this study, the conversion of benzyl alcohol in
mesitylene over various zeolite catalysts was per-
&
ChemCatChem 2016, 8, 1 – 10
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