Jianhua Yang et al. / Chinese Journal of Catalysis 35 (2014) 49–57
catalysts [17]. The Brönsted acid sites act as catalytic centers
The final mixture was poured into a stainless‐steel auto‐
clave and then subjected to a crystallization reaction for 144 h
at 438 K under agitation. After the crystallization, the samples
were washed with deionized water and dried at 373 K over‐
night. The organic templates and carbon particles were elimi‐
nated by calcination at 823 K for 10 h at a heating and cooling
rate of 1.8 K/min. A white MCM‐22‐FC material was obtained.
For comparison, a conventional flaky MCM‐22 was prepared by
the same synthesis procedure from a gel but without the addi‐
tion of fluoride ions and carbon particles, and the MCM‐22‐F
and MCM‐22‐C zeolites were obtained from a gel with the addi‐
tion of fluoride ions and carbon particles, respectively.
In our previous work, MCM‐22 zeolite with high crystallinity
[11,25] was synthesized at the lower temperature of 423 K
with 144 h for the crystallization reaction using the structure
directing agent HMI from Aldrich. However, use of HMI from
Shanghai Longsheng Chemical Corporation led to the formation
of MCM‐22 with much lower crystallinity and poor MDA cata‐
lytic performance (not shown here) under the same crystalliza‐
tion reaction conditions. The different purity and synthesis
routes of the two HMIs are responsible for their different in‐
fluence on the MCM‐22 product.
The different H‐formed MCM‐22 zeolite samples are denot‐
ed HMCM‐22‐FC, HMCM‐22, HMCM‐22‐F, and HMCM‐22‐C and
were obtained by successive exchange with NH4NO3 aqueous
solution (1 mol/L) and calcination. Different Mo‐based cata‐
lysts (6 wt% Mo) denoted Mo/HMCM‐22‐FC, Mo/HMCM‐22,
Mo/HMCM‐22‐F, and Mo/HMCM‐22‐C were then prepared by
impregnation with an aqueous solution of (NH4)6Mo7O24·4H2O.
After impregnation, the catalysts were dried at 373 K for 4 h
and calcined in air at 823 K for 6 h.
for aromatic products and also as sites for aromatic‐type car‐
bon deposition [18]. Successive chemical steps occur along the
diffusion path within the zeolite crystallites, and condensation
reactions are favorable, resulting in a fast coking rate [19].
Modifying the acidic properties and shape of the character‐
istic pore structure in the MCM‐22 zeolite is effective in inhib‐
iting carbonaceous deposits. The acidic properties of zeolite
catalysts can be modified by the fluoride synthesis route. Fluo‐
ride ions have previously been proposed to act as mineralizing
agents or as a promoter for the formation of Si–O–Si bonds
[20–22]. By interacting with the zeolite skeleton, fluoride ions
with high electronegativity change the electron density, the
acid strength, and the acid quantity. Zeolites with a hierarchical
pore structure, and that possess both micropores and meso‐
pores are a new generation of zeolite catalyst. The introduction
of mesopores to the microporous zeolite provides fast diffu‐
sion, overcoming the mass transport limitation, and resulting in
improved catalytic activity [23,24].
In this work, we explored a feasible method to combine a
hierarchical structure and acidic modification in the MCM‐22
zeolite catalyst to improve its catalytic performance in the MDA
reaction. Hierarchical MCM‐22 zeolite aggregate (MCM‐22‐FC)
were synthesized from gel containing carbon particles and
fluoride ions as a template and an acidic property modifier,
respectively. The effect of carbon particles and fluoride ions on
the morphology and properties of MCM‐22 zeolite was exam‐
ined. The MCM‐22 zeolite was modified using ammonium mo‐
lybdate ((NH4)6Mo7O24·4H2O) to form Mo/MCM‐22 catalysts.
Among the three investigated catalysts, the hierarchical
Mo/MCM‐22‐FC catalyst showed improved aromatic selectivity
and benzene yield and a long catalyst life in the MDA reaction.
The structure‐property relationship of the Mo‐based MCM‐22
catalysts in the MDA reaction was investigated by N2 adsorp‐
tion, NH3 temperature‐programmed desorption (NH3‐TPD),
pyridine adsorption infrared (Py‐IR) spectroscopy, and ther‐
mogravimetric (TG) analysis.
2.2. Catalyst characterization
X‐ray powder diffraction (XRD) measurements were ob‐
tained on a Rigaku‐Dmax 2400 X‐ray diffractometer using Cu
Kα radiation with a 2θ angle range from 5° to 50°.
The morphology of the as‐synthesized samples was ob‐
2. Experimental
served by scanning electron microscopy (SEM) using a
QUANTA‐450 at an acceleration voltage of 20 kV. Transmission
electron microscopy (TEM) measurement was carried out on a
Tecnai F30 instrument, operating at 300 kV.
2.1. Catalyst preparation
Hierarchical MCM‐22‐FC zeolites with a Si/Al ratio of 15
were prepared by one‐pot hydrothermal synthesis from a gel
containing carbon particles and fluoride ions. Typically, a mix‐
ture containing 0.08 g sodium hydroxide (96% NaOH, Si‐
nopharm Chemical Reagent Co, Ltd.), 0.53 g sodium metaalu‐
minate (NaAlO2, Tianjin Bodi Chemical Co, Ltd.), 0.81 g potas‐
sium fluoride (KF·2H2O, Tianjin Kemiou Chemical Reagent Co,
Ltd.), 3.15 g carbon R400R (purchased from Cabot Corpora‐
tion), 3.47 g HMI (> 98%, Shanghai Longsheng Chemical Co,
Ltd.), and 31.52 g deionized water was agitated at room tem‐
perature for 6 h. A total of 10.52 g Ludox (AS‐40, 40% SiO2,
Aldrich) was added to the mixture dropwise and vigorously
stirred for 24 h to obtain a gel with a composition of 1.0 SiO2:
0.033 Al2O3:0.06 Na2O:0.5 HMI:30 H2O:0.12 KF. The mass frac‐
tion of carbon R400R in the precursor solution was 10%.
Nitrogen adsorption analyses were carried out with a Mi‐
cromeritics ASAP‐2020M adsorption analyzer at 77 K. Before
the adsorption measurements, the sample was degassed at 473
K for 6 h. The surface area and pore size distribution were ob‐
tained by multipoint BET analysis and the BJH method, respec‐
tively.
The elements contained in the samples were detected by
energy dispersive X‐ray (EDX) measurements with a QUANTA‐
450 instrument. The relative content of the elements was
measured by the intensity of emitted characteristic X‐rays of
the sample elements.
The acidic properties of the samples were investigated by
NH3‐TPD and measured on a Quantachrome CHEMBET‐3000
chemisorption analyzer. Py‐IR spectra were collected using a
Bruker TENSOR27 Fourier transform infrared spectrometer in