Catalytic performance of a phosphorus-modified H-IM-5@meso-SiO2 composite in the alkylation of toluene with methanol

Article abstract of DOI:10.1039/c9nj02440a

Herein, a core-shell H-IM-5@meso-SiO₂ hybrid material was synthesized by reversing the charge of the external surface of IM-5 crystals to a positive charge by PDDA that could induce an oriented self-assembly formation of the meso-SiO₂ shell. The as-synthesized H-IM-5@meso-SiO₂ composite was thereafter modified with phosphorus species to further enhance the selectivity of the target product. The sample morphology and structure were analyzed by electron microscopy technology, XRD, XPS and N₂ adsorption-desorption. The effects of the meso-SiO₂ shell and phosphorus species on sample acidity were investigated by NH₃-TPD, FTIR spectroscopy and solid-state NMR measurements. The catalytic properties and characterization results revealed that the meso-SiO₂ shell could passivate the external surface acids of the H-IM-5 crystals. The phosphorus modification could simultaneously adjust the surface acidity as well as the size of the pores and pore-openings. The TPD analysis of xylene showed that the sample surface acidity and geometric factors played important roles in the diffusion process of the diffusing molecules.

Full text of DOI:10.1039/c9nj02440a

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DOI: 10.1039/C9NJ02440A
Catalytic Performance of Phosphorus Modified H-IM-5@meso-SiO2
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Composite in Alkylation of Toluene with Methanol
Dezhi Yi, Xuan Meng, Naiwang Liu, and Li Shi 
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The State Key Laboratory of Chemical Engineering, East China University of Science and
Technology, Shanghai 200237, China.
ABSTRACT
A core-shell H-IM-5@meso-SiO2 hybrid material was synthesized via a method of
reversing the external surface of IM-5 crystals to positively charged by PDDA, which
could induce an oriented self-assembly formation of meso-SiO2 shell. The
as-synthesized H-IM-5@meso-SiO2 composite was thereafter modified with
phosphorus species to further enhance the selectivity of target product. The sample
morphology and structure were analyzed by electron microscopy technology, XRD,
XPS and N2 adsorption-desorption. The effects of meso-SiO2 shell and phosphorus
species on sample acidity were investigated by NH3-TPD, FT-IR and solid-state NMR
measurement. The catalytic properties and characterization results revealed that the
meso-SiO2 shell could passivate the external surface acids of H-IM-5 crystals. The
phosphorus modification could adjust the surface acidity as well as the size of pores
and pore-opening simultaneously. The TPD of xylene analysis showed that the sample
surface acidity and geometric factors play important roles in the diffusion process of
the diffusing molecules.

To whom correspondence should be addressed. E-mail: yyshi@ecust.edu.cn Tel.:
021-64252274.
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Keywords: H-IM-5; meso-SiO2; phosphorus; toluene; alkylation; diffusion activation
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1. Introduction
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P-xylene is the main raw material for the production of terephthalic acid (PTA) and
dimethyl terephthalate (DMT), which are used in the synthesis of polyesters such as
polyethylene terephthalate (PET) and polybutylene terephthalate (PBT).1 The
selectivity synthesis of p-xylene by methylation of toluene using zeolite catalysts has
been developed.2,3 As an important commercial zeolite for industrial catalysis, ZSM-5
has been proved to be very appropriate for the selective production of aromatics with
para-alkyl groups since its pore dimension approximates the size of these aromatic
moleculars.4 However, the external surface acid sites of the catalyst exhibit different
catalytic behaviors and reduce the selectivity of target product. In the methylation
reaction of toluene, the xylene isomerization which is favorable to produce m-xylene
was roughly correlated with the external surface of the catalyst particles.5,6 Hence,
surface passivation and pore size control have been proposed in order to improve the
selectivity, such as silicon modification by methods of chemical vapor/liquid
deposition (CVD/CLD)7-12 and impregnation with P, Mg or B et al.13-17 Although these
modification techniques could increase para-selectivity, they are also accompanied by
pore blockage, which reduce the catalytic activity significantly.18 In recent years,
core-shell composites with excellent special functions have broadened the application
areas of these materials from electrode19 and biomaterial20 to catalysis21 and
separation.22 A number of core-shell zeolites have been synthesized, among which
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MFI@MFI composites with excellent para-selectivity have been synthesizedDtOhI:r1o0u.1g03h9/C9NJ02440A
heterogeneous nucleation methods.23-25 In addition, core-shell composites with an
active zeolite core and an inactive mesoporous silica shell were also synthesized for
selective catalytic reactions.26-28
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As a new zeolite reported by Benazzi et al.,29,30 IM-5 zeolite consists of an unusual
2D 10-MR channel system with large internal 3D cavities,31 which is different from
the MFI zeolites. IM-5 zeolite is considered as a promising catalyst alternative to
ZSM-5 zeolite due to its strong acidity, excellent hydrothermal stability and catalytic
cracking activity.32-34 As described by He et al.,34 the excellent physico-chemical
properties of IM-5 zeolite may be related to its numerous high acidic Al, H-sites.
Previous studies have investigated the catalytic properties of IM-5 zeolite by reactions
of aromatization,35 selective reduction36,37 and alkylation.38,39
In the present study, a novel H-IM-5@meso-SiO2 composite with hierarchical pores
was prepared to passivate the external acid sites of the core H-IM-5 zeolite. Thereafter,
the phosphorus species were incorporated into the composite to adjust the surface
acidity as well as the size of pores and pore-opening simultaneously, and enhance the
selectivity of target product eventually. The texture and acid properties were analyzed
by N2 adsorption-desorption, NH3-TPD and Py/Coll-IR. The Al and P species were
tested by solid-state NMR measurement. The catalytic performance of modified
samples were studied by alkylation of toluene with methanol under different reaction
conditions. Moreover, the diffusion activation energies of p/m-xylene on the samples
were determined by the TPD technique.
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2. Experimental
2.1. Reagents
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ZSM-5 zeolite (SiO2/Al2O3 = 50) was obtained from Nankai University Catalyst
Co., Ltd. Tetraethylorthosilicate (TEOS, 28 wt% SiO2, Greagent), sodium hydroxide
(NaOH, 98 wt%, Greagent), aluminum nitrate nonahydrate (Al(NO3)3ꞏ9H2O, 99 wt%,
Greagent), cetyltrimethylammonium bromide (CTAB, 99 wt%, Adamas),
poly(diallyldimethylammonium chloride) (PDDA, 20 wt% in water, Aldrich),
ammonium chloride (NH4Cl, 99.5 wt%, Greagent), phosphorus acid (H3PO4, 85 wt%
in water, Greagent), ammonium aqueous solution (NH3ꞏH2O, 28 wt%, Greagent),
2,4,6-collidine (C8H11N, 99 wt%, Adamas), pyridine (C5H5N, 99 wt%, Adamas),
toluene (C7H8, 99.5 wt%, Greagent), methanol (CH3OH, 99.8 wt%, Greagent) and
ethanol (C2H5OH, 99.7%, Greagent) were purchased from Shanghai Titan Scientific
Co., Ltd. The structure-directing agent was supplied by Guangzhou Dayou Fine
Chemical Plant. The silica sol (40 wt% in water) was supplied by Qingdao Haiyang
Chemical Co., Ltd.
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2.2. Synthesis of H-IM-5@meso-SiO2 Core-shell Composite
Firstly, the raw Na-IM-5 zeolite was synthesized according the procedure reported
in the literature40 with a precursor mixture of 30SiO2:11Na2O:0.6Al2O3:3
(1,5-MPPBr2):1200 H2O. The synthesized Na-IM-5 was calcined at 550 ℃ for 5 h to
eliminate the organic template. And then the calcined sample was ion-exchanged with
1.0 mol/L NH4Cl solution at 80 ℃ three times and dried at 110 ℃ overnight. The
white powder named NH4-IM-5 was obtained. The H-IM-5 zeolite was obtain by the
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calcination of NH4-IM-5 at 500 ℃ for 5 h. Secondly, the external suDrfOaI:c1e0.10o3f9/C9NJ02440A
NH4-IM-5 crystals was functionalized to positively charged by an aqueous solution of
polycation agent (PDDA). The modification procedure was as follows: NH4-IM-5
mixed with a 2 g/L aqueous solution of PDDA with a mass ratio of NH4-IM-5: PDDA
aqueous solution = 1: 50. After 1 h stirring, the functionalized NH4-IM-5 was
obtained after centrifugal washing three times and drying at 80 ℃ overnight. Thirdly,
1.2 g of as-prepared functionalized NH4-IM-5 was added to the mixture solution (240
mL ethanol, 300 mL deionized water, 4.0 g ammonia aqueous solution (28 wt%) and
1.2 g CTAB). The suspension was dispersed by ultrasonic treatment for 1.0 h. Then,
TEOS (1.2-3.6 g) was added into the suspension dropwise under vigorous stirring at
room temperature. After stirring for 5 h, the sample was filtered and washed with
ethanol and deionized water, dried at 110 ℃ overnight and finally calcined at 550 ℃
for 6 h to eliminate the organic template. Five H-IM-5@meso-SiO2 samples (IM-S)
with different amounts of TEOS (1.2, 1.8, 2.4, 3.0 and 3.6 g) were synthesized and
named IM-1.2S, IM-1.8S, IM-2.4S, IM-3.0S and IM-3.6S respectively. For
comparison, the functionalized NH4-IM-5 (1.2 g) was modified with amorphous silica
by chemical liquid deposition using TEOS (2.4 g) by referring to the method reported
by Zheng et al.41 and named as IM-2.4S-CLD.
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2.3. Synthesis of the Phosphorus Modified H-IM-5@meso-SiO2 Samples
Five phosphorus modified IM-2.4S samples (IM-2.4S-P) with different phosphorus
content (0.5, 1.0, 1.5, 2.0 and 2.5 wt%) were synthesized by impregnating with certain
amounts of phosphoric acid on the IM-2.4S sample and named as IM-2.4S-0.5P,
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IM-2.4S-1.0P, IM-2.4S-1.5P, IM-2.4S-2.0P and IM-2.4S-2.5P respDeOcIt:i1v0e.10ly39./C9NJ02440A
Meanwhile, a comparison sample with 2.0 wt% phosphorus content was prepared by
the same impregnation method on the H-IM-5 zeolite without coating of meso-SiO2
shell. The comparison sample was named as IM-2.0P. All the impregnated samples
were dried at 110 ℃ overnight, and calcined at 500 ℃ for 5 h.
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2.4. Materials Characterization
The X-ray diffraction (XRD) patterns were collected using a Rigaku Ultima IV
diffractometer with Cu Kα radiation (40 kV, 44 mA). The chemical composition of
samples were obtained on an Agilent 725 ICP-OES. N2 adsorption-desorption
isotherms were collected using a Micromeritics ASAP-2020 system. The BET specific
surface area (SBET), micropore specific surface area (Smicro), mesopore specific surface
area (Smeso), micropore volume (Vmicro) and mesopore volume (Vmeso) were derived
from the corresponding N2 adsorption-desorption isotherms. The pore size
distributions were derived from adsorption branches of isotherms using the NL-DFT
method. Scanning electron micrographs (SEM) were obtained on Hitachi S-4800
operating at a field emission electron acceleration voltage of 5-15.0 kV. Transmission
electron microscopy (TEM) images were obtained on a JEOL-2010F electron
microscope operated at 200 kV. The X-ray photoelectron spectroscopy (XPS) was
obtained by using a Perkin-Elmer PHI 5000C ESCA system equipped with Mg anode.
The binding energy was calibrated by C 1s at 284.6 eV. The solid-state MAS NMR
spectra was performed on a Bruker AVANCE III 500 spectrometer.
NH3-TPD was performed on Auto Chem 2910 (Micromeritics, USA) and the
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desorbed NH3 were detected using a thermal conductivity detector (TCD). The acid
type and accessibility of samples were tested by pyridine/2,4,6-collidine-FTIR
(Py/Coll-IR) using a Nicolet IS-10 equipped with an in situ reaction cell. The number
of Lewis and Brønsted acid sites were quantified according to the Lambert-Beer
formula: ξ = A•S/n, in which ξ is extinction coefficient, A is the corresponding
absorbance, S is surface area of the wafer, and n is amount of the adsorbate substance.
For Py-IR, ξBrønsted (1541) = 1.02 cm/μmol and ξLewis (1443-1453) = 0.89 cm/μmol
were used to quantify Brønsted and Lewis acid sites, respectively.42-44 The total and
strong acids were measured at 200 and 450 ℃, and their difference was the weak
acids. For Coll-IR, ξ(1632-1648) = 10.1 cm/μmol was applied to quantify external
surface acids of the samples.42,44
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Temperature programmed desorption (TPD) of p/m-xylene was performed on Auto
Chem 2910 (Micromeritics, USA). The diffusion of m-xylene into ZSM-5 pores is
limited at lower temperatures. Therefore, the adsorption of p/m-xylene was carried out
at 80 ℃ to ensure that p/m-xylene could diffuse into the pores. The sample was
pretreated at 450 ℃ with helium flow for 60 min, and then the temperature cooled to
80 ℃ for the adsorption of p-xylene or m-xylene. Then the temperature cooled to
40 ℃. Finally, the TPD curves were collected from 40 to 500 ℃ with varying heating
rates (8-20 ℃/min).
2.5. Catalytic Measurements
The characteristic of modified samples were studied by alkylation of toluene with
methanol in a quartz tube reactor (length: 500 mm; internal diameter: 9 mm). A
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certain amount of samples (20-40 mesh) were packed into the constant temperature
section of the reactor and the rest spaces were filled with quartz sands (20-40 mesh).
The samples were preheated at the reaction temperature for 1 h in N2 flow to ensure
that the water and volatile organic compounds adsorbed in the pores were completely
removed. The reactants, mixture of toluene with methanol, were then pumped into the
reactor at certain reaction temperatures and WHSV with N2 flow (40 mL/min).
Products were directly delivered into a gas chromatograph equipped with a flame
ionization detector and innowax column.
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3. Results and Discussion
3.1. Texture and Catalytic Properties of H-IM-5 and H-ZSM-5
Fig. 1A exhibited the XRD patterns of H-IM-5 and H-ZSM-5 zeolites, which peak
positions matched the characteristics of IMF and MFI topologies as described in the
literatures, respectively.40,45 As exhibited in Fig. 1B, the N2 adsorption-desorption
isotherms of H-IM-5 and H-ZSM-5 zeolites can be attributed the mixture of type I and
IV isotherms,44 in which the micropores were filled at P/P0 < 0.1 and the addition N2
uptake at P/P0 of 0.45-0.95 confirmed the presence of irregularly shaped intergranular
mesopores. The Smicro and Vmicro of H-IM-5, shown in Table S1, were close proximity
to that of the H-ZSM-5. The NH3-TPD curves (Fig. 1C) of these two zeolites
exhibited two same NH3 desorption peaks at 202 and 406 ℃, and the peak areas were
basically the same, indicating similar acid sites amount and acid strength on these two
zeolites. The catalytic performance of toluene alkylation with methanol on these two
zeolites were obtained as shown in Fig. 1D. Compared with H-ZSM-5, H-IM-5
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possessed higher toluene conversion and xylene selectivity, which may be related to
its unusual channel system with large internal cavities and numerous high acidic Al,
H-sites.34 The selectivity of p-xylene on these two zeolites were basically the same,
which were closely to thermodynamic equilibrium value. In the subsequent study,
H-IM-5 was modified with coating of mesoporous silica and impregnation of H3PO4
to improve its para-selectivity.
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Fig. 1 (A) XRD patterns, (B) N2 adsorption-desorption isotherms isotherms, (C) NH3-TPD
profiles, (D) Catalytic performance of H-IM-5 and H-ZSM-5 zeolites. SX-Selectivity of xylene;
SPX-Selectivity of p-xylene.
3.2. Texture and Acid Properties of H-IM-5@meso-SiO2 Samples
Fig. 2 exhibited the XRD results of H-IM-5@meso-SiO2 samples with different
TEOS weight varying from 1.2 g to 3.6 g. The wide-angle XRD patterns exhibited
apparent diffraction peaks contributed by the core IM-5 zeolite, which were
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completely consistent with the topological structure of IM-5. However, tDhOeI: 1p0.e10a3k9/C9NJ02440A
intensities decreased with the increasing of TEOS weight, i.e. the increase of
meso-SiO2/IM-5 mass ratio, which may be caused by the coating of meso-SiO2. Since
the framework of meso-SiO2 shell is amorphous, there was no obvious diffraction
peak of meso-SiO2 in the wide-angle XRD patterns of H-IM-5@meso-SiO2 samples.
As shown in Fig. 2, the low-angle XRD of H-IM-5@meso-SiO2 samples presented
(100) peak at about 2.4° without the appearance of (110) and (200) peaks, suggesting
the existence of short-range ordered mesoporous structure.46,47 The intensity of this
(100) peak increased with the increasing of TEOS weight, indicating the proportion of
meso-SiO2 shell to the core H-IM-5 became higher. As shown in the electron
microscopy images (Fig. 3), the parent H-IM-5 was in the morphology of rod-like
crystals. After the coating of meso-SiO2, the crystal size increased obviously and the
crystal particles were prone to agglomeration. The TEM images of IM-2.4S sample,
shown in Fig. 3 (T2 and T3), confirmed that the core-shell structure with H-IM-5
crystals for the core and meso-SiO2 for the shell. XPS spectra of Al 2p was carried out
and displayed in Fig. S1. It can be clearly observed that the characteristic binding
energy at 74.5 eV of Al 2p was easily recognized in the spectra of H-IM-5 zeolite.48,49
However, the Al 2p signal on IM-2.4S sample was barely detectable by XPS
measurement, which suggested that the core H-IM-5 crystals were completely coated
with the meso-SiO2 shell.
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Fig. 2 XRD patterns of H-IM-5 and H-IM-5@meso-SiO2 samples: (a) H-IM-5, (b) IM-1.2S, (c)
IM-1.8S, (d) IM-2.4S, (e) IM-3.0S, (f) IM-3.6S.
Fig. 3. SEM images: (A) H-IM-5 (×20.0K), (B) IM-1.2S (×30.0K), (C) IM-1.8S (×30.0K), (D) IM-2.4S
(×30.0K), (E) IM-3.0S (×30.0K), (F) IM-3.6S (×30.0K). TEM images: (T1) H-IM-5, (T2, T3) IM-2.4S.
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A
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IM-2.4S
IM-3.0S
IM-3.6S
30
25
20
2.0
1.5
1.0
0.5
0.0
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
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30
31
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43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
15
H-IM-5
IM-1.2S
IM-1.8S
IM-2.4S
IM-3.0S
IM-3.6S
10
5
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
0
0
0.0
0.2
0.4
0.6
0.8
1.0
0
2
4
6
8
10
Pore width (nm)
Relative pressure (P/P0)
Fig. 4 (A) N2 adsorption-desorption isotherms of the samples, (B) The corresponding pore size
distributions based on the adsorption isotherms. The pore size distributions in the inset were offset
along the y-axis for visual clarity.
Table 1 Structural properties of H-IM-5 and H-IM-5@meso-SiO2 samples
b
c
c
c
c
SiO2/Al2O3
ratio a
SBET
Smicro
Smeso
Vtotal
Vmicro
Vmeso
Samples
(m2 g-1)
382.477
499.009
547.098
598.365
638.119
707.043
(m2 g-1)
286.107
194.133
184.067
174.993
166.772
159.288
(m2 g-1) (cm3 g-1)
(cm3 g-1) (cm3 g-1)
H-IM-5
IM-1.2S
IM-1.8S
IM-2.4S
IM-3.0S
IM-3.6S
47
96.370
304.876
363.031
423.372
471.347
547.755
0.355
0.404
0.423
0.445
0.477
0.485
0.142
0.106
0.097
0.089
0.077
0.071
0.213
0.298
0.326
0.356
0.400
0.414
58
65
73
79
86
a Calculated by ICP-OES; b Calculated by the BET method; c Calculated using the t-plot method.
For the sample without PDDA pretreatment, the coating of meso-SiO2 onto the
H-IM-5 crystals also happened. Nonetheless, partial silica self-assembled in sphere
form outside the bulk phase of the desirable H-IM-5@meso-SiO2 core-shell
composite (Fig. S2). Therefore, the external surface of H-IM-5 crystals functionalized
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to positively charged was more conducive to attract negatively charged silica species
in basic media. And then H-IM-5 crystals were coated with the meso-SiO2 shell by
layer-by-layer electrostatic adsorption.
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11
12
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16
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23
24
25
26
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29
30
31
32
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40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
N2 adsorption-desorption was carried out to determine the texture properties of the
samples. As exhibited in Fig. 4A, the isotherms for all the samples can be attributed to
the mixture of type I and IV isotherms. The isotherms for H-IM-5@meso-SiO2
samples showed a steep increase at P/P0 = 0.2-0.4 confirmed the presence of
meso-SiO2 corresponding to pore size distributions at 2.0 ~ 3.5 nm (Fig. 4B inset).
Same as the H-IM-5 isotherm, H-IM-5@meso-SiO2 samples also exhibited obvious
H4 hysteresis loops at P/P0 of 0.45-0.95, which indicates the presence of irregularly
shaped intergranular mesopores caused by H-IM-5 crystals. In contrast to the parent
H-IM-5 zeolite, the pore size distributions of H-IM-5@meso-SiO2 exhibited two
peaks, in which the peak at ~ 0.87 nm was ascribed to the micropores of core H-IM-5
and the other at around 2.5 nm was related to the meso-SiO2. As shown in Table 1, the
SBET and total pore volume (Vtotal) of H-IM-5 zeolite were 382.477 m2 g-1 and 0.355
cm3 g-1, respectively, which were mainly derived from micropores. In the
H-IM-5@meso-SiO2 samples, the contribution of micropores to SBET and Vtotal
decreased with the increase of TEOS weight, while the contribution of mesopores at
the range of 2.0~3.5 nm increased obviously and became the main contributor. The
results of N2 adsorption-desorption suggested that the core H-IM-5 was
well-connected with the meso-SiO2 shell and micropores of the core were accessible
to the outside. For further confirmation of the accessibility of core H-IM-5, NH3-TPD
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and FT-IR measurements were carried out.
6
7
8
9
As exhibited in Fig. 5A, the NH3-TPD curves presented two NH3 desorption peaks
which were related to weak and strong acids. For H-IM-5, the peaks at 203 and
409 ℃ were attributed to weak and strong acids, respectively.50 Compared with
H-IM-5, the peak intensity of H-IM-5@meso-SiO2 samples decreased and the peak
value shifted to lower temperature slightly, indicating a decrease of acid amount and
decline of acid strength. It should be attributed to the coating of meso-SiO2 shell
10
11
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14
15
16
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18
19
20
21
22
23
24
25
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28
29
30
31
32
33
34
35
36
37
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41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
which are extremely low in acidity.
1632
A
B
1648
f
a
b
d
b
c
d
e
f
a
100
200
300
400
500
1740
1710
1680
1650
1620
1590
1560
1530
Temperature (C)
200 C
Wavenumbers (cm-1)
450 C
C
D
1540
1540
1451
1451
f
f
e
d
e
d
c
c
b
a
b
a
1400
1450
1500
1550
1600 1400
1450
1500
1550
1600
Wavenumbers (cm-1)
Wavenumbers (cm-1)
Fig. 5 A) NH3-TPD, B) Coll-IR at 200 ℃, C) Py-IR at 200 ℃ and D) Py-IR at 450 ℃. The
spectra of samples: a) H-IM-5, b) IM-1.2S, c) IM-1.8S, d) IM-2.4S, e) IM-3.0S and f) IM-3.6S.
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Table 2 The Brønsted/Lewis amount determined by Py-IR (mmol Py g-1-cat) and Coll-IR (mmol
5
6
7
Coll g-1-cat) for the samples.
8
9
Acidity a
WB SL
Adsorbed pyridine External
per core H-IM-5 c
surface
(mmol-Pyꞏg-1-IM-5) acidity b (mmol-collꞏg-1-IM-5)
Adsorbed collidine
per core H-IM-5 c
Sample
T a
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
SB
WL
H-IM-5
IM-1.2S
IM-1.8S
IM-2.4S
IM-3.0S
IM-3.6S
a
1.235
0.889
0.796
0.725
0.662
0.610
0.349 0.584 0.190 0.112
0.243 0.41 0.142 0.094
0.213 0.373 0.122 0.088
0.201 0.326 0.112 0.086
0.189 0.287 0.104 0.082
0.178 0.260 0.091 0.081
b
1.235
1.138
1.130
1.131
1.125
1.122
0.231
0.112
-
0.231
0.143
-
0.088
-
0.137
-
0.069
0.127
Determined by Py-IR. Determined by Coll-IR. c The number of adsorbed pyridine or collidine
per core H-IM-5 was calculated by dividing the amount of total acid or external acid with the
proportion of core H-IM-5 in the samples, respectively. T-Total acid; SL and SB-Strong Lewis and
Brønsted acid; WL and WB-Weak Lewis and Brønsted acid.
The acidic type and accessibility of the samples were determined by Py-IR and
Coll-IR, in which Py-IR results reflected the sample acidity without differentiating the
internal or external surface acids, while Coll-IR can only access to the external
surface acidity of H-IM-5 crystals.44,51-53 The Py-IR spectra at 200 and 450 ℃ were
obtained as shown in Fig. 5C and D. The peak at 1451 cm-1 can be related to the
Lewis acid sites. Another characteristic peak at around 1540 cm-1 was related to the
vibration of pyridine molecules bonded to Brønsted acid sites.54 The relative
intensities of these two peaks decreased obviously as the temperature rose from 200 to
450 ℃, indicating desorption of weakly adsorbed pyridines. Fig. 5B presents Coll-IR
spectra of the samples at 200 ℃. As demonstrated by N. S. Nesterenko et al.,42 peaks
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at 1632 and 1648 cm-1 were related to adsorption of 2,4,6-collidine at Lewis and
Brønsted acid, which were subsequently used to quantify the external surface acidity.
As indicated by Coll-IR spectra in Fig. 5B, the coating of meso-SiO2 shell resulted in
a dramatically decrease of peak intensities at 1632 and 1648 cm-1, which can be due to
the increase of meso-SiO2/IM-5 mass ratio and passivation of the external surface acid
sites by silica shell. The detailed Py/Coll-IR results were analyzed with the previously
established methods and compiled in Table 2. The total acid amount calculated by
pyridine exhibited the same tendency as the results of NH3-TPD. In contrast to
H-IM-5, the total acids of IM-2.4S decreased by 41%. It was assumed that the
meso-SiO2 shell had no acid center and only the core H-IM-5 could adsorb the
pyridine molecules. In that way, the number of adsorbed pyridine per core H-IM-5 in
the IM-2.4S sample was 1.131 mmol-Pyꞏg-1-IM-5, which was about 8% less than that
of parent H-IM-5 zeolite. The external surface acids on the IM-2.4S sample calculated
by Coll-IR was 0.088 mmol Collꞏg-1-cat and the adsorbed collidine per core H-IM-5
was 0.137 mmol-Collꞏg-1-IM-5. The relative decrement of external surface acids was
0.094 mmol-Collꞏg-1-IM-5 which accounted for 7.6% of the total acids. Therefore, it
can be deduced that the decrement of the number of adsorbed pyridine per core
H-IM-5 was mainly due to the decrease of external surface acids caused by the
coating of meso-SiO2. The results of N2 adsorption-desorption and acid analysis
confirmed that the meso-SiO2 shell did not result in blockage of pores of core crystals
and the micropores of the core H-IM-5 were accessible to the outside.
5
6
7
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14
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22
23
24
25
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28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
3.3. Texture Properties of Phosphorus Modified H-IM-5@meso-SiO2 Samples
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N2 adsorption-desorption was carried out to determine the texture properties of the
phosphorus modified samples. The isotherms for phosphorus modified IM-2.4S-P
samples (Fig. S3) were similar to that of IM-2.4S which can be attributed to the
mixture of type I and IV isotherms. As illustrated in Table 3, the SBET and Vtotal of the
IM-2.4S-P samples, as well as the Smicro and Vmicro, decreased with the increasing of
phosphorus impregnation amount. It indicated that the phosphate ions entered into
pores of the sample and resulted in the narrowing of the channel. The XRD patterns
(Fig. S4) exhibited the typical IMF topology without distinct peaks for phosphorus
species. Nevertheless, compared with the IM-2.4S sample, the crystallinity of
phosphorus modified samples decreased with the increasing of phosphorus
impregnation amount, which was mainly attributed to the dealumination caused by
H3PO4.
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 3 Structural properties of IM-2.4S and phosphorus modified samples
b
c
c
c
c
P content a
(wt%)
SBET
(m2 g-1)
Smicro
Smeso
Vtotal
Vmicro
Vmeso
Samples
(m2 g-1) (m2 g-1) (cm3 g-1) (cm3 g-1) (cm3 g-1)
IM-2.4S
-
598.365 174.993 423.372
577.267 159.742 417.525
559.013 149.972 409.041
539.072 137.351 401.721
510.365 117.427 392.938
479.253 100.712 378.541
0.445
0.426
0.405
0.389
0.371
0.350
0.089
0.079
0.072
0.065
0.055
0.047
0.356
0.347
0.333
0.324
0.316
0.303
IM-2.4S-0.5P
IM-2.4S-1.0P
IM-2.4S-1.5P
IM-2.4S-2.0P
IM-2.4S-2.5P
0.43
0.95
1.41
2.07
2.42
a Determined by ICP-OES; b Calculated by BET method; c Calculated by t-plot method.
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27Al
31P
55
-14
-6
-32
-14
0
-40
36
8
9
e
e
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11
12
13
14
15
16
17
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20
21
22
23
24
25
26
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30
31
32
33
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41
42
43
44
45
46
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48
49
50
51
52
53
54
55
56
57
58
59
60
d
c
d
c
b
a
b
150
100
50
0
-50
-100
-150
60
40
20
0
-20 -40 -60 -80 -100
ppm
ppm
Fig. 6 27Al and 31P MAS NMR spectra of the samples: a) IM-2.4S, b) IM-2.4S-1.0P, c)
IM-2.4S-1.5P, d) IM-2.4S-2.0P, e) IM-2.4S-2.5P.
27
As exhibited in Fig. 6, the Al spectrum of IM-2.4S showed an intense peak at 54
ppm corresponds to the tetra-coordinated framework aluminum (FAL) and the weaker
peak at 0 ppm typically related to the octahedral extra-framework aluminum
(EFAL).55 Similar spectra for the IM-2.4S-P samples were observed, while the
relatively weak peak of octahedral Al shifted from 0 to -14 ppm, indicating the
octahedral Al attached to phosphorous and the higher signal at -14 ppm of IM-2.4S-P
samples indicated more octahedral Al due to the dealumination from zeolite caused by
phosphoric acid.56 Meanwhile, a broad band appeared at ~ 36 ppm in the spectrum of
IM-2.4S-2.5P sample can be attributed to either twisted tetra-coordinated FAL and/or
classical penta-coordinated aluminium.55,57 As shown in the 31P MAS NMR spectrum
(Fig. 6), the -6 ppm signal was assigned to phosphorus in H4P2O7, pyrophosphates or
short-chain polyphosphates not connected to Al,55 whereas the -14 ppm signal was
related to the short-chain polyphosphates connected to Al.55,56 The peaks at -32 and
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-40 ppm were related to highly condensed polyphosphates or polyphosphoric species
connected to Al.55,58 It is clear that the phosphoric acid on the samples decomposed
into H4P2O7 and its derivatives after the calcination treatment. As shown in Fig. 6,
when the phosphorus content was less than 2.0 wt%, there was a predominance of the
peaks in the range of -6 ~ -14 ppm, corresponding to more phosphorus existed as
pyrophosphoric acid or short-chain polyphosphates. The proportion of peak area in
the range of -32 ~ -40 ppm increased with the increase of phosphorus content,
indicating more highly condensed polyphosphates bonded to Al atoms.
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The acidity of the phosphorus modified IM-2.4S-P samples were measured using
Py-IR and NH3-TPD. Fig. S5 displayed the Py-IR spectra of phosphorus modified
IM-2.4S-P samples at 200 and 450 ℃, and the detailed acid amounts were listed in
Table 4. As shown in Table 4, both the total and strong acids decreased with the
increase of phosphorus content, while the weak acids exhibited a variation tendency
to increase first and then decrease. These two different variation tendencies had been
confirmed in the NH3-TPD profiles (Fig. S6) that the peak intensity of strong acid
decreased with increasing of phosphorus content, while the weak acid peak intensity
increased first and then decreased. Furthermore, the peak value shifted to lower
temperature slightly after the modification of phosphorus. Combined with the analysis
of 31P NMR, the variation of acidity due to the phosphorus impregnation can be
explained by the model proposed by Lercher et al.59 that a bond was formed between
the P and O of the Si-O-Al group and the P-OH had a lower acid strength than the
Si-OH-Al group. When the P content was lower than 1.0 wt%, the quantity of P-OH
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could compensate for the loss of Si-OH-Al. And as a consequence, the amount of
weak acids increased, while the strong acids and acid strength decreased. However,
when the P content was higher than 1.0 wt%, the P species not only interacted with Al
but also with the P species via the oxygen bridge, indicating more condensed
polyphosphates which resulted in the decrease of P-OH amount. Eventually, the acid
sites decreased as the condensed polyphosphates increased.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 4 The Brønsted/Lewis amount determined by Py-IR for the samples after modification of
phosphorus.
Samples
T
TB
TL
SB
WB
SL
WL
IM-2.4S
0.729 0.530 0.199
0.723 0.557 0.166
0.719 0.576 0.143
0.637 0.517 0.120
0.462 0.387 0.075
0.364 0.298 0.066
0.201
0.135
0.091
0.054
0.015
0.009
0.329
0.422
0.485
0.463
0.372
0.289
0.113
0.075
0.051
0.036
0.012
0.007
0.086
0.091
0.092
0.084
0.063
0.059
IM-2.4S-0.5P
IM-2.4S-1.0P
IM-2.4S-1.5P
IM-2.4S-2.0P
IM-2.4S-2.5P
T-Total acid; TB and TL-Total Brønsted and Lewis acid; SB and SL-Strong Brønsted and Lewis acid;
WB and WL-Weak Brønsted and Lewis acid.
3.4. Catalytic Performance of H-IM-5@meso-SiO2 and Phosphorus Modified
H-IM-5@meso-SiO2 Samples in Alkylation of Toluene with Methanol
3.4.1. Catalytic performance of H-IM-5@meso-SiO2 samples
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48
60
54
48
42
36
30
24
18
12
6
A
B
45
42
39
a
b
c
d
e
f
g
36
10
11
12
13
14
15
16
17
18
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23
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30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
24
23
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
Time on stream (h)
Time on stream (h)
92
88
84
80
76
72
68
64
60
xylene
p-xylene
D
40
35
30
25
20
15
10
5
C
0
0
1
2
3
4
5
6
7
8
9
a
b
c
d
e
f
g
Time on stream (h)
Fig. 7 Catalytic properties of H-IM-5 and H-IM-5@meso-SiO2 samples: a) H-IM-5, b) IM-1.2S,
c) IM-1.8S, d) IM-2.4S, e) IM-3.0S, f) IM-3.6S, g) IM-2.4S-CLD. Reaction temperature = 440 ℃,
T/M = 2.0 and WHSV = 2.0 h-1.
The texture and acidic properties of catalyst have significant effects on the
para-selectivity in the alkylation of toluene with methanol. As shown in Fig. 7A, the
parent H-IM-5 exhibited the highest catalytic activity of toluene conversion, but the
worst selectivity of xylene and p-xylene. For the H-IM-5@meso-SiO2 samples, the
average toluene conversion decreased from 55.2% to 44.4% with the increase of
TEOS weight from 1.2 g to 3.6 g, which was mainly due to the decrease of core
H-IM-5 proportion in the composite. Meanwhile, due to the passivation of meso-SiO2
shell to the external surface acid sites, the p-xylene selectivity improved above the
thermodynamic equilibrium (24%) with the increase of meso-SiO2/IM-5 mass ratio.
For the IM-2.4S sample, the average toluene conversion, selectivity of p-xylene and
xylene were 51.6%, 40.3% and 74.3%, respectively. As demonstrated in Fig. 7D, the
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yield of p-xylene was significantly improved for samples with TEOS weight less than
2.4g, and the yield of xylene was basically unchanged. In contrast, the IM-2.4S-CLD
sample exhibited higher selectivity of xylene and p-xylene, but its toluene conversion
decreased to 9.5% after 8 hours of catalytic reaction, indicating that the amorphous
silica obtained by chemical liquid deposition blocked the entrance of pores to a large
extent as exhibited in Fig. S7. Although the coating of meso-SiO2 shell can improve
the yield of p-xylene, the selectivity of p-xylene still needs to be further improved.
The IM-2.4S sample showed the best yield of p-xylene among all the
H-IM-5@meso-SiO2 samples. Therefore, IM-2.4S sample was selected and further
modified with phosphorus.
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3.4.2. Effects of phosphorous content on the activity and selectivity of the samples
A
B
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Time on stream (h)
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C
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D
xylene
p-xylene
0
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a
b
c
d
e
f
g
Time on stream (h)
Fig. 8 Catalytic properties of IM-2.4S and phosphorus modified IM-2.4S-P samples: a) IM-2.4S,
b) IM-2.4S-0.5P, c) IM-2.4S-1.0P, d) IM-2.4S-1.5P, e) IM-2.4S-2.0P, f) IM-2.4S-2.5P, g) IM-2.0P.
Reaction temperature = 440 ℃, T/M = 2.0 and WHSV = 2.0 h-1.
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Fig. 8 exhibited the effects of phosphorous content on catalytic properties of the
phosphorus modified IM-2.4S-P samples. It can be seen from Fig. 8 that the p-xylene
selectivity of the phosphorus modified samples improved significantly with loss of
catalytic activity. The p-xylene selectivity of IM-2.4S-2.0P sample was above 60%
with a xylene selectivity of more than 88%, and its p-xylene yield reached the
maximum. However, the yield of xylene decreased with increasing of phosphorus
content, mainly due to the loss of acid sites which resulted in the weaken of catalytic
activity. Further increasing phosphorus content to 2.5 wt% resulted in an increase of
p-xylene selectivity to ~ 70%, while the toluene conversion decreased to 14.6% after
8 hours of catalytic reaction. As shown in Fig. 8, the IM-2.0P sample obtained by
phosphorus modification without coating of meso-SiO2 shell exhibited the worst
selectivity and yield of p-xylene with a relatively high yield of xylene.
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Combined with the analysis of N2 adsorption-desorption and acidity, it can be
deduced that the meso-SiO2 shell could passivate the external surface acids of sample
crystals, which reduced the isomerization of p-xylene and improved the selectivity of
p-xylene. The phosphorus modification method was mainly used to reduce the strong
acids, which could inhibit the occurrence of side reactions. In addition, the
phosphorus species could enter the pores of sample and narrow the pore-opening size,
which ultimately improved the selectivity of target product. However, the selectivity
and yield of p-xylene cannot be improved effectively only by coating of meso-SiO2
shell or phosphorus modification alone. Therefore, the combination modification of
meso-SiO2 shell and phosphorus could adjust the surface acidity as well as the size of
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pores and pore-opening simultaneously, and enhance the selectivity of targetDpOrI:o10d.1u0c39t/C9NJ02440A
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eventually.
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3.4.3. Effects of methanol/toluene (T/M) feed ratios on the activity and selectivity of
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the sample
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SX
80
70
SPX
65
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T/M = 0.5
T/M = 1.0
T/M = 2.0
T/M = 3.0
0
1
2
3
4
5
6
7
8
9
Time on stream (h)
Fig. 9 Effects of T/M feed molar ratios on the activity and selectivity of the IM-2.4S-2.0P
sample. Reaction temperature = 440 ℃ and WHSV = 2.0 h-1. SX-Selectivity of xylene;
SPX-Selectivity of p-xylene.
Fig. 9 showed the catalytic performance of IM-2.4S-2.0P sample at different T/M
ratios. The toluene conversion decreased with the increase of T/M molar ratio, while
the selectivity of xylene and p-xylene were improved. In the absence of side reactions,
the maximum toluene conversion was theoretically 100% at a T/M = 1.0. But the
actual toluene conversion was around 36.1%, indicating that only 36.1% of methanol
could involve in the alkylation of toluene. When the T/M molar ratio was 2.0,
approximately 64.1% methanol involved in the alkylation reaction, indicating that
methanol could be more effective for the alkylation reaction at a higher T/M molar
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ratio. In addition, the higher methanol feed concentration could accelerate
deactivation of the sample caused by dehydration and/or dehydrogenation of methanol.
Fig. S8 illustrated the TG curves of IM-2.4S-2.0P sample after 24 h alkylation
reaction at different T/M feed molar ratios. And the weight loss ratio increased with
the decline of T/M molar ratio, which confirmed a faster coke formation rate at lower
T/M feed molar ratios.
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3.4.4. Effects of reaction temperature on the activity and selectivity of the sample
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SX
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SPX
360 C
400 C
440 C
480 C
0
1
2
3
4
5
6
7
8
9
Time on stream (h)
Fig. 10 Effects of reaction temperature on the activity and selectivity of the IM-2.4S-2.0P
sample. T/M = 2.0 and WHSV = 2.0 h-1. SX-Selectivity of xylene; SPX-Selectivity of p-xylene.
Fig.10 exhibited the effects of reaction temperatures carried out in the range of
360-480 ℃ on the IM-2.4S-2.0P sample. The results revealed that the catalytic
activity increased due to the improvement of diffusion rate and reaction rate of
toluene at higher reaction temperature, while p-xylene exhibited higher selectivity due
to the restrain of isomerization at the lower reaction temperature. But the sample
could deactivate faster at higher temperatures. Hence, the optimal reaction
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temperature for the IM-2.4S-2.0P sample could be selected at 440 ℃.
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3.4.5. Effects of WHSV on the activity and selectivity of the sample
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Fig. 11 showed the effects of WHSV ranging from 1.0 to 4.0 h-1 at 440 ℃ on the
IM-2.4S-2.0P sample and the T/M molar ratio was 2.0. As shown in Fig. 11, the
average toluene conversion decreased from 34.9% to 25.1% when the WHSV
increased from 1.0 to 4.0 h-1, which was mainly due to the shorten of contact time
between reactants and acid sites. Meanwhile, the shorten of contact time between
products and acid sites inhibited the isomerization of products, which was conducive
to improve the selectivity of p-xylene.
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0
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Time on stream (h)
Fig. 11 Effects of WHSV on the activity and selectivity of the IM-2.4S-2.0P sample. Reaction
temperature = 440 ℃ and T/M = 2.0. SX-Selectivity of xylene; SPX-Selectivity of p-xylene.
3.5. Diffusion of p/m-xylene in the samples
The TPD methods have been acknowledged a useful technique for surface analysis,
which can be applied to the study of desorption and diffusion processes.60,61 The
experiments were carried out on the TPD device at different heating rates using the
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diffusion equation reported by D. Fraenkel et al.,60 in which the activation energy for
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RTm2
r2E
E






0
diffusion (E) can be formulated, ln
 ln


1.44 2D0
RTm
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where D0 is the intrinsic diffusion coefficient, E is the activation energy for diffusion,
β is the heating rate, R is a gas constant, r0 is radius of zeolite particle, Tm is the peak
desorption temperature. The calculation of D0 involves the radius of the crystal which



2 
RTm
is difficult to achieve accurately. Therefore, a plot of ln
versus 1/Tm could



obtain the line with slop E/R, which can be used to calculate the E.
Table 5 Tm of p/m-xylene at different heating rates and E on the H-IM-5, H-ZSM-5, IM-2.4S
and IM-2.4S-2.0P samples.
Tm (K) of p-xylene at
different β (K/min)
Tm (K) of m-xylene at
different β (K/min)
E of
E of
Samples
p-xylene
(kJ/mol)
m-xylene
(kJ/mol)
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H-IM-5
H-ZSM-5
IM-2.4S
419.0 427.9 435.3 442.2
400.8 412.3 420.9 430.3
419.5 428.6 435.7 442.5
427.5 436.3 444.2 450.1 53.9 ± 2.6 57.1 ± 2.1
407.6 418.9 427.7 435.5 38.1 ± 2.2 41.6 ± 2.0
428.9 437.4 445.1 451.0 54.7 ± 2.2 59.0 ± 2.4
426.7 435.8 442.8 449.3 50.0 ± 2.5 57.7 ± 2.0
IM-2.4S-2.0P 416.7 426.6 433.8 441.5
The TPD curves of p/m-xylene on the H-IM-5, H-ZSM-5, IM-2.4S and
IM-2.4S-2.0P samples at the different heating rates were shown in Fig. S9. As shown
in the Fig. S9, the TPD patterns for p-xylene exhibited only one obvious peak.
Whereas, the TPD patterns for m-xylene showed two distinct peaks, one of which
appeared at lower temperature was attributed to the m-xylene adsorbed on the external
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surface of sample crystals and the other peak arose at high temperature was due to the
m-xylene adsorbed in the zeolite channels.62 So the diffusion parameters of m-xylene
in the zeolite channels were calculated by the second desorption peak temperatures. In
addition, the adsorption capacity of xylene on the IM-2.4S-2.0P samples decreased
obviously, especially the m-xylene, indicating that the diffusion of m-xylene into the
micropores was hindered by the phosphorus modification. The values of E obtained
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


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RTm
from the linear ln
versus 1/Tm plots according to the equation were given in



Table 5. The E of p-xylene on H-ZSM-5 was 38.1 ± 2.2 kJ/mol, which was higher
than the literature data (30.0 kJ/mol).63,64 The differences can be explained from the
interaction between π-electrons of benzene ring and protonic acidity on H-ZSM-5
zeolite.65 Moreover, the asymmetrical distribution of methyl groups enables m-xylene
higher polarity and alkaline, which can be used to explain the higher E of m-xylene on
the samples than that of p-xylene.62 Compared with H-ZSM-5 zeolite, xylene on
H-IM-5 zeolite showed higher diffusion activation energy, which may be caused by its
unusual channel system. As shown in Table 5, there was a slight increase in the E of
p/m-xylene after the coating of meso-SiO2, which was mainly due to the diffusion
resistance of the silica shell. Compared with IM-2.4S sample, the E of p/m-xylene
after phosphorus modification decreased due to the neutralization of acid sites on
H-IM-5, which was consistent with the reported literature.66 In addition, the
phosphorus modification had little effect on the E of m-xylene. It may be attributed to
the reason that phosphorus deposition narrowed the pore-opening size and increased
the diffusion resistance of m-xylene in the pores of sample, which could partially
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compensate for the loss of E caused by acid neutralization. Thus, the interactions
between diffusing molecule and sample surface acidity as well as the geometric
factors play important roles in the diffusion process.
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4. Conclusions
A core-shell H-IM-5@meso-SiO2 composite with hierarchical pores was
successfully obtained via a method of reversing the external surface of IM-5 crystals
to positively charged by PDDA, which could induce an oriented self-assembly
formation of meso-SiO2 shell. The as-synthesized H-IM-5@meso-SiO2 composite
was thereafter modified with phosphorus species to adjust the acidity on the internal
surfaces as well as the size of pores and pore-opening. Combined with the
physicochemical structure and catalytic performance evaluation, the meso-SiO2 shell
passivated the external surface acidity of zeolite crystals without blockage of pores
and the phosphorus modification method reduced the strong acids which could inhibit
the side reactions. Therefore, the combination modification of meso-SiO2 shell and
phosphorus could adjust the surface acidity as well as the size of pores and
pore-opening simultaneously, and enhance the selectivity of target product eventually.
The TPD of xylene analysis showed that the interactions between diffusing molecule
and sample surface acidity as well as the geometric factors play important roles in the
diffusion process.
Conflicts of Interest
There are no conflicts to declare.
Acknowledgement
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