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mesopores.[12] On the other hand, a non-aqueous potentiomet-
ric titration measurement with tert-butylamine (the solvent di-
ameter, 0.68 nm, was larger than the micropore size of zeolite
ZSM-5, ꢀ0.54 nm) in Figure S6 showed a similar number of ac-
cessible acid sites on the external surface for the C-Z5, SM-Z5,
and Meso-Z5.[8] These experimental facts demonstrated that
1) the Meso-Z5 sample had abundant mesopores to reduce dif-
fusion limitations for increasing the conversion, 2) the special
intracrystal mesoporous structure (the mesopores were con-
nected to the gas phase by shape-selective MFI-type micro-
pores) was beneficial in maintaining the shape selectivity to p-
xylene, and 3) the few external surface acid sites benefiting
from this special structure would lead to few undesired secon-
dary reactions of the p-xylene product.
bly to the decreased size of mesopores or the small reduction
in their quantity.
In summary, a facile and fast synthesis of KF-aided OSDA-
free, seed-induced hydrothermal process is proposed for the
environmentally friendly and low-cost production of high-qual-
ity single-crystal ZSM-5 zeolite with enriched intracrystal meso-
pores (Meso-Z5). This Meso-Z5 displays catalytic performances
with a twofold p-xylene yield compared to C-Z5 in o-xylene
isomerization, owing to the greatly improved activity and well-
preserved selectivity. The quantity and size of mesopores and
the crystallinity can be controlled easily by changing the addi-
tion of KF and the duration of aging time. Notably, this
method can be easily extended to synthesize mesoporous
ZSM-5 zeolite with high Si/Al molar ratio (Si/Al>100, denoted
as Meso-EA-Z5) by simply adding a small amount of cheap eth-
ylamine (ethylamine/Si molar ratio=0.1) to the system. The
preliminary study indicates that the special intracrystal meso-
pore structure plays an important role in butene cracking reac-
tions to greatly enhance propene selectivity and well-preserve
butene conversion (see Table S1 and Figure S15).
Further investigation reveals that the addition of KF and the
duration of aging time are two crucial factors for the modula-
tion of the introduction of mesopores and the crystallinity of
samples. The XRD patterns for the samples with KF/Si=0–0.90
(Figure S7) all display characteristic patterns of the typical MFI
structure. The N2 isotherms and pore size distributions of the
samples are presented in Figures S8 and S9. No hysteresis loop
in N2 sorption isotherm is observed at KF/Si=0, indicating that
the special intracrystal mesopore structure cannot be obtained
in the absence of KF. The hysteresis loop appears with the ad-
dition of KF. Interestingly, the samples synthesized with differ-
ent KF content have different hysteresis loop sizes; the
amount and size of the mesopores in Meso-Z5 increase with
respect to KF/Si molar ratio in the order of 0.60>0.30>0.15,
as according to the TEM images in Figure S10. This sequence is
parallel to the size of the hysteresis loop. However, at KF/Si=
0.9, both surface area and hysteresis loop size decrease, imply-
ing partial damage of the framework owing to the structure-
breaking effect of K+.[13] To prepare Meso-Z5 with well-pre-
served microporous structure, the effect of prolonging the
aging time from 1.5 to 12 h is also investigated. The XRD pat-
terns in Figure S11 indicate that all products are of typical MFI
structure and longer aging time results in higher crystallinity.
According to the N2 isotherms and pore size distributions in
Figures S12 and S13, the suitable aging time is 6 h if the larger
amount of mesopores and the higher crystallinity are kept
synchronous.
Experimental Section
Syntheses: The seeds with the crystal size of 200 nm were presyn-
thesized by using a clear solution method.[14] Typically, for the syn-
thesis of intracrystal mesoporous single-crystal-like ZSM-5 zeolite
(Meso-Z5), a starting aluminosilicate mixture with a molar ratio of
SiO2/Al2O3/Na2O/KF/H2O 1:(0–0.025):0.15:(0–0.9):50 was prepared
by using Al2(SO4)3·18H2O as the alumina source and 40% colloidal
silica as the silica source. The preprepared silicalite-1 seed solution
was then added under stirring. The addition quantity of seed typi-
cally equaled 7.0 wt% of total SiO2 weight in the starting gel.
Then, the mixture was stirred at ambient temperature for 1.5–12 h
and treated hydrothermally at 1808C for 0.5–5 h. The obtained
zeolites were separated by filtration, then washed with deionized
water, dried at 1208C for 6 h, and finally calcined under static air at
5508C for 5 h. The intracrystal mesoporous single-crystal-like ZSM-
5 zeolite with high Si/Al molar ratio (Meso-EA-Z5) was prepared
under the same conditions as Meso-Z5, except that a small
amount of ethylamine (ethylamine/Si molar ratio=0.1) was added.
Reference samples: Two commercial H-ZSM-5 samples with low
Si/Al molar ratio of 36 (denoted as C-Z5) and high Si/Al molar ratio
of 145 (denoted as C-HS-Z5) (both from Nankai Catalyst Company)
were employed as references to investigate the advantages of
Meso-Z5 and Meso-EA-Z5. The SEM images in Figure S2 clearly
show the size and crystal morphology of the C-Z5 (4 mm, rectangu-
lar) and C-HS-Z5 samples (3 mm, coffin-shaped). The sub-microme-
ter sized HZSM-5 sample (denoted as SM-Z5) was synthesized by
a seed-induced method according to our previous work.[2b] This
sample did not contain mesopores but had a similar size, shape,
and number of acid sites (Si/Al=32) to Meso-Z5 (see Figure S4, in-
cluding SEM, XRD, N2 sorption isotherm, and catalytic performance
results).
The catalytic study has also been extended to four Meso-Z5
samples with different mesopore contents and sizes (Fig-
ure S10) to investigate their relationship to catalytic properties
in o-xylene isomerization. Considering the higher Si/Al ratio
(ꢀ44) of these Meso-Z5 samples, the reaction temperature was
increased to 723 K and the WHSV decreased to 2.5 hÀ1 to in-
vestigate the catalytic differences. As shown in Figure S14, the
p-xylene yield curve displayed a volcano curve in which
a better catalytic result was observed for the samples of KF/
Si=0.3–0.6 with higher conversion (ꢀ66%) and well-main-
tained p-xylene selectivity (ꢀ33%). On increasing the KF/Si
ratio to 0.9, the amount of mesopores decreased a lot, which
led to lower conversion (ꢀ49%) and similar selectivity
(ꢀ34%). At KF/Si=0.15, as more nano-particle stacked struc-
tures result in a large external surface, the selectivity (ꢀ30%)
decreased. The slightly lower conversion (ꢀ58%) owed proba-
Acknowledgements
This work is supported by the 973 programme (2013CB934101,
2009CB623506, and 2009CB930400) and STCMS (09DZ2271500
and 11JC1400400).
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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