Effect of surface silicon modification of H-beta zeolites for alkylation of benzene with 1-dodecene

Article abstract of DOI:10.1039/d0ra00393j

H-beta zeolites of 100-200 nm (named BEA-L) and 20-30 nm (named BEA-S) were treated by chemical liquid deposition (CLD) of tetraethyl orthosilicate (TEOS) to improve the selectivity of 2-phenyl linear alkylbenzene (2-LAB) from benzene alkylation with 1-dodecene. The results indicate that H-beta zeolite with a smaller crystal size has a longer lifetime due to shorter channels and less diffusion limitation. The deposited SiO₂ layers passivated the external surface acid sites of the zeolite and made the pores narrower. BEA-L lost more external Br?nsted acid sites than BEA-S with the same added amount of TEOS, which was due to the severe aggregation of BEA-S grains. This increased passivation gave BEA-L increased 2-LAB selectivity. And when the added amount of SiO₂ was 7.20 wt% of the parent zeolite, the selectivity of 2-LAB over BEA-L significantly increased from 41.9% to 54.7% while that of BEA-S only increased by 2%.

Full text of DOI:10.1039/d0ra00393j

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Effect of surface silicon modification of H-beta
zeolites for alkylation of benzene with 1-dodecene
Cite this: RSC Adv., 2020, 10, 10006
*
and Minghan Han
Ruimin Li,
Shiyong Xing, Shuai Zhang
H-beta zeolites of 100–200 nm (named BEA-L) and 20–30 nm (named BEA-S) were treated by chemical
liquid deposition (CLD) of tetraethyl orthosilicate (TEOS) to improve the selectivity of 2-phenyl linear
alkylbenzene (2-LAB) from benzene alkylation with 1-dodecene. The results indicate that H-beta zeolite
with a smaller crystal size has a longer lifetime due to shorter channels and less diffusion limitation. The
deposited SiO2 layers passivated the external surface acid sites of the zeolite and made the pores
narrower. BEA-L lost more external Brønsted acid sites than BEA-S with the same added amount of
TEOS, which was due to the severe aggregation of BEA-S grains. This increased passivation gave BEA-L
increased 2-LAB selectivity. And when the added amount of SiO2 was 7.20 wt% of the parent zeolite, the
selectivity of 2-LAB over BEA-L significantly increased from 41.9% to 54.7% while that of BEA-S only
increased by 2%.
Received 14th January 2020
Accepted 3rd March 2020
DOI: 10.1039/d0ra00393j
rsc.li/rsc-advances
pollution; (3) special handling is required not only for the
transportation and storage of these catalysts, but also for the
1. Introduction
separation of homogeneous catalyst, reactants, and products,
leading to a high cost.3 Thus, it was a remarkable achievement
when U.S. UOP Co. and Spain CEPSA Co. developed noncorro-
sive uorided silica-alumina catalysts in 1992. The noncorrosive
uorided silica-alumina catalysts were much better than the
homogeneous catalysts, but the selectivity of 2-LAB was only
about 30%.1,6 In recent years, more environmentally friendly
catalysts, including ionic liquids,7–9 clays,10–12 zirconia,10,11 het-
eropolyacids13,14 and zeolites,1,5,6,12,15–19 have been extensively
studied. Among these catalysts, zeolites have attracted the most
attention for the target of attaining a high selectivity of 2-LAB
owing to their shape-selectivity catalysis.20 The best zeolites for
the alkylation of benzene with long chain olens contain 12-
membered ring (12 MR) channels, and are Y (pore size of 7.4
Linear alkylbenzene sulfonate (LAS) is a widely used anionic
surfactant.1 Extensive use of detergents affects water quality and
causes damage to environment, thus it is necessary to minimize
these. Linear alkylbenzenes (LABs) with long chains (C10–C14
)
produced industrially by the alkylation of benzene with a-
olens are the key intermediates for the synthesis of LAS.2,3 Five
isomers of LABs (2-, 3-, 4-, 5-, 6-LAB) can be obtained by the
alkylation of benzene with 1-dodecene, via a carbenium ion
reaction mechanism in the presence of acidic catalysts, as
shown in Fig. 1. 1-LAB is unstable and hard to form on account
of the Markovnikov rule. The molecular size of these ve
isomers is less than 0.529 nm, and 2-LAB is the smallest one.2
He et al. reported that the surface activity and biodegradability
of LAS are greatly inuenced by the position of the phenyl group
in the LAB intermediate used.2 Among all LAB isomers, the 2-
phenyl isomer (2-LAB) is the most biodegradable and environ-
mentally favorable.4 Therefore, much research has been
devoted to nding a suitable catalyst to attain a high yield of 2-
LAB.5
AlCl3, as the early alkylation catalyst used in industrial
development, was gradually replaced by a HF catalyst in 1960s.1
Despite a lot of weakness itself, HF has been the preferred
catalyst worldwide for LAB synthesis until now. The weakness
includes that (1) the selectivity of 2-LAB in the HF process is less
than 20%; (2) the high corrosivity of these homogeneous cata-
lysts causes the corrosion of equipment and environmental
18
5,19
ꢀ
ꢀ
ꢀ
4,19,21
ꢀ
ꢀ
A), beta (pore size of 6.7 A ꢀ 7.3 A and 5.6 A ꢀ 5.6 A) and
ꢀ
ꢀ
MOR (pore size of 6.5 A ꢀ 7.0 A).
However, the rapid deactivation of the zeolite is the primary
barrier to industrial application.22 Small crystal size zeolites
have been researched to solve the deactivation problem of
zeolites. Zeolites with small grains have large specic surface
area, short channels and low diffusion limitation, which effec-
tively increase anti-coking capability.23–26 Therefore, small grain
zeolite has broad application prospects in the alkylation of
benzene with long chain olens.
In addition, with the stricter environmental protection
requirements, increasing the yield of 2-LAB to make detergents
more easily degraded is necessary. The acid sites of zeolites are
distributed in the channels and external surface. The diffusion
limitation of the channels makes it easier for 2-LAB to diffuse
out than the other isomers, which facilitates the selectivity to 2-
Department of Chemical Engineering, Beijing Key Laboratory of Green Reaction
Engineering and Technology, Tsinghua University, Beijing 100084, P. R. China.
E-mail: hanmh@tsinghua.edu.cn
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Fig. 1 Alkylation reaction of benzene with 1-dodecene.
LAB. However, acid sites on the external surface have no diffu- 99.5%, Aladdin Chemical Reagent Co. Ltd, China), n-hexane
sion restriction so that they need to be passivated. Chemical (C6H14, 97%, Shanghai Titan technology Co. Ltd, China) and
silicon deposition is a common method for passivating the acid tetraethoxysilane (TEOS, Tianjin Heowns technology Co. Ltd,
sites on the external surface of zeolites.27–33 Shang et al. inves- China).
tigated HMCM-22 zeolite silylated with silicon tetrachloride
(SiCl4), tetraethoxysilane (TEOS) and 8-hydroxyquinoline
(C9H7NO) to passivate the external surface acidity, which
sharply increased the selectivity of isobutene.34 Li et al. reported
2.2. Preparation of catalysts
2.2.1. Synthesis of zeolite beta. b Zeolite was synthesized by
hydrothermal treatment with following chemical molar ratio:
that silanization modication of HZSM-5 zeolites by CLD not
only passivated the acidity of the external surface, but also gave
narrower pore opening and promoted shape-selectivity.35
Therefore, the passivation of the external surface acid sites of
the zeolite is also likely to improve the selectivity of 2-LAB.
H-beta zeolite is potentially the best catalyst for the alkyl-
ation of long chain olens. b Zeolite has a three-dimensional
SiO2 : Al2O3 : TEAOH : Na2O : H2O
¼
1 : 0.03 : 0.2 : 0.07 : 30.
The materials used were silica sol, sodium aluminate, TEAOH
and sodium hydroxide, which were uniformly mixed in order.
The mixture was aged for 2 h under vigorous stirring at room
temperature to obtain a homogenous gel and then placed in
a 100 ml Teon-lined stainless steel autoclave. Two kinds of
b zeolites with crystal size 100–200 nm and 20–30 nm were
obtained by crystallizing at 145 ꢁC for 100 h and 170 ꢁC for 65 h,
respectively. Aer crystallization, the solid product was sepa-
rated using centrifuge, washed several times with deionized
water, dried overnight at 80 ꢁC and calcined in air at 550 ꢁC for
4 h. The H-beta zeolite was obtained aer three times ion-
exchange with 1.0 mol Lꢂ1 NH4Cl aqueous solution at 80 ꢁC
for 2 h, drying overnight at 120 ꢁC and calcining in air at 550 ꢁC
for 4 h. Smaller crystal size H-beta zeolite was named BEA-S.
Another one was named BEA-L.
ꢀ
ꢀ
twelve-membered ring (12-MR) ([100] and [010]: 6.7 A ꢀ 7.3 A,
21
ꢀ
ꢀ
[001]: 5.6 A ꢀ 5.6 A), which is smaller than the size of TEOS
ꢀ
(10.3 A). TEOS cannot enter the internal channels of the zeolite,
and can be used to modify the external surface. In this work,
different crystal size H-beta zeolites modied by the CLD
treatment with TEOS were used as the catalyst for the alkylation
of benzene with 1-dodecene. This study focused on the effect of
crystal size on the deactivation of catalysts and surface silicon
modication on the selectivity of 2-LAB, to devise efficient
catalysts for the benzene alkylation with long chain olens in
further studies.
2.2.2. Modication of zeolite beta. 4 g zeolite BEA-L was
suspended in 200 ml of n-hexane. Thorough mixing was con-
ducted with an ultrasonic treatment for 2 h to ll the pores with
n-hexane. Then, 0.2 g TEOS was dropwise added to the mixture.
Aer that, the new mixture was stirred at ambient temperature
2. Experimental
2.1. Chemicals and reagents
The reagents used were silica sol (40% SiO2, Qingdao Haiyang for 8 h. A rotary evaporator was used to evaporate n-hexane at
Chemical Co. Ltd, China), sodium aluminate (NaAlO2, AR, 80 ꢁC. Then, the obtained product was dried at 80 ꢁC overnight
Macklin Chemical Co. Ltd, China), tetraethylammonium and calcined in air at 550 ꢁC for 4 h. The resulting sample was
hydroxide (TEAOH, 35% in water, Hangzhou FineCHEM, Co. named 1.44% SiO2/g BEA-L, (1.44 is the mass percentage of
Ltd, China), sodium hydroxide (NaOH, AR, Sinopharm Chem- deposited SiO2 per gram zeolite). The catalysts 4.32% SiO2/g
ical Reagent Co. Ltd, China), ammonium chloride (NH4Cl, BEA-L and 7.20% SiO2/g BEA-L were prepared by the same
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method except the added amount of TEOS were 0.6 g and 1.0 g, proles were recordeꢁd from 40 ꢁC to 600 ꢁC in owing He with
respectively.
a heating rate of 10 C minꢂ1. The amount of total acid from
The above method was used to modify zeolite BEA-S to NH3-TPD was calculated aer calibrating the corresponding
obtain the catalysts 1.44% SiO2/g BEA-S, 4.32% SiO2/g BEA-S relation between acid quantity and peak area.
and 7.20% SiO2/g BEA-S.
2.4. Catalyst reaction
2.3. Characterization of catalysts
The alkylation reaction of benzene with 1-dodecene was inves-
tigated with a xed bed microreactor. The catalyst (2 g, 10–20
The crystal structure of the catalysts was determined by a Bruker
D8 Advance powder X-ray diffractometer with Cu Ka radiation.
XRD patterns were obtained in the 2q range of 5–50ꢁ recorded at
mesh) was loaded in a reaction tube (i.d. 10 mm, length 50 cm)
ꢁ
and pretreated at 300 C for 3 h under N2 ow. Aer that, the
2ꢁ minꢂ1. The Si/Al atomic ratio of the catalysts was measured
reactor was cooled down to 136 ꢁC. Benzene and 1-dodecene, as
by XRF analysis on ARL PERFORM X instrument from Thermo
reactants, were mixed in a molar ratio of 15 : 1 and the mixture
Fisher. TEM images of the catalysts were recorded by a JEOL
was pumped into the reaction tube by a micro ow pump at
JSM-7401F instrument. The scanning transmission electron
a WHSV of 4 hꢂ1. The pressure was maintained at 4 MPa. The
microscopy (STEM) images were collected using a FEI Titan
products collected by a centrifuge tube every hour or two were
Cubed Themis G2 300 spherical aberration corrected micro-
scope. The specic surface area and pore size distribution of the
catalysts were measured with a Quantachrome Autosorb iQ and
AsiQwin instrument using N2 adsorption isotherms at 77 K.
analyzed by gas chromatography (GC) to get the conversion of 1-
dodecene and selectivity of products (2-LAB, 3-LAB, 4-LAB, 5-
LAB and 6-LAB). The conversion of 1-dodecene and selectivity of
LAB isomers were dened as follows:
The Brønsted and Lewis acid site analysis was carried out by
1-Dodecene conversion ð%Þ ¼
IR spectroscopy with pyridine/2,4,6-trimethylpyridine adsorp-
tion between 1350 and 1750 cmꢂ1 using a Nicolet 6700 model
total amount of products ðall LAB isomersÞ
Fourier transform infrared spectrometer. The amount of
ꢀ 100%
total amount of products þ unreacted 1-dodecene
Brønsted and Lewis acid sites was calculated by the following
equation: C (pyridine on B or L acid sites) ¼ IA(B,L)$pR2/
(IMEC(B,L) ꢀ W); IA(B,L) ¼ integrated absorbance of Brønsted
Target LAB isomer selectivity ð%Þ ¼
acid sites at 1540 cmꢂ1 or Lewis acid sites at 1455 cmꢂ1; R ¼
radius of catalyst disk (cm); IMEC(B,L) ¼ integrated molar
the amount of target LAB isomer
extinction coefficient of Brønsted acid sites at 1540 cmꢂ1
(1.67 cm mmolꢂ1) or Lewis acid sites at 1455 cmꢂ1 (2.22 cm
mmolꢂ1); W ¼ weight of disk (mg).36 2,4,6-Trimethylpyridine
adsorbed on the external Brønsted acid sites gave a band at
1638 cmꢂ1. NH3-TPD (temperature programmed desorption)
experiments were performed on an Auto Chem II 2920 to
characterize the acid amount of the catalysts. 0.2 g sample was
ꢀ 100%
the amount of all LAB isomers
3. Results and discussion
3.1. Basic physicochemical properties characterization
pretreated in a continuous helium ow at 400 ꢁC for 30 min and Fig. 2 shows the XRD patterns of the BEA-L, BEA-S and theirs
then cooled to 40 ꢁC. Aerwards, the sample was saturated with modied H-beta zeolites. Fig. 3 shows the TEM images of the
NH3 for 45 min. The gas was switched to helium for 30 min to BEA-L, BEA-S and theirs modied H-beta zeolites. The Si/Al
remove excess NH3 and make the baseline stable. The NH3-TPD atomic ratio of the catalysts analyzed by XRF is given in Table
Fig. 2 XRD patterns of BEA-L, BEA-S and theirs modified H-beta zeolites.
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Fig. 3 TEM images of BEA-L, BEA-S and theirs modified H-beta zeolites. (a) BEA-L, (b) 7.20% SiO2/g BEA-L, (c) BEA-S, (d) 7.20% SiO2/g BEA-S.
Table 1 Si/Al molar ratio of BEA-L, BEA-S and theirs modified H-beta
atomic ratio increased signicantly with increased SiO2
zeolites after silanization
loading, clearly conrming that SiO2 was effectively deposited
on parent zeolites. Moreover, it is worth mentioning that the
Samples
X% SiO2/g BEA-L
X% SiO2/g BEA-S
surface silicon leads to the increase in surface Si/Al atomic ratio
without changing in internal Si/Al atomic ratio. TEM images of
BEA-L, 7.20% SiO2/g BEA-L, BEA-S and 7.20% SiO2/g BEA-S are
presented in Fig. 3. A regular crystal structure and clear pores
can be seen in the TEM images (Fig. 3(a) and (c)) of the parent
zeolites. The crystal size of the H-beta zeolite named BEA-L was
in the range of 100–200 nm while that of BEA-S was in the range
of 20–30 nm which showed severe aggregation of the crystals. In
the case of Fig. 3(b) and (d) of the modied zeolites, the crystal
structure of H-beta zeolite was keep intact although the pore
structure became blurred aer modication, which was
presumably because the external surface of the modied cata-
lysts was covered with a layer of amorphous SiO2.
SiO2 loading
Si/Al
0
1.44
4.32
16.6
7.20
16.9
0
1.44
4.32
14.7
7.20
15.1
15.7 16.2
13.8 14.2
1. As shown in Fig. 2, all modied zeolites exhibited the char-
acteristic diffraction peaks of H-beta zeolite without the char-
acteristic peaks of SiO2, indicating that the CLD treatment with
TEOS did not change the crystal structure of the H-beta zeolite.
This also shows that the SiO2 deposited during modication
was highly dispersed and present in an amorphous phase. The
Si/Al atomic ratio of BEA-L and BEA-S zeolites were 15.7 and
13.8, respectively. For the modied H-beta zeolites, the Si/Al
Fig. 4 Nitrogen adsorption isotherms of BEA-L, BEA-S and theirs modified H-beta zeolites. (a) BEA-L and modified BEA-L zeolites; (b) BEA-S and
modified BEA-S zeolites.
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Table 2 Textural properties of BEA-L, BEA-S and theirs modified H-beta catalystsa
Samples
SBET (m2 gꢂ1
)
Vtol (cm3 gꢂ1
)
Vmicro (cm3 gꢂ1
)
Vmeso (cm3 gꢂ1
)
Vmacro (cm3 gꢂ1
)
BEA-L
636
565
547
545
685
595
620
612
0.354
0.333
0.265
0.251
0.565
0.509
0.506
0.480
0.192
0.165
0.160
0.164
0.179
0.149
0.161
0.165
0.043
0.045
0.039
0.032
0.188
0.159
0.160
0.183
0.119
0.122
0.066
0.055
0.198
0.201
0.185
0.132
1.44% SiO2/g BEA-L
4.32% SiO2/g BEA-L
7.20% SiO2/g BEA-L
BEA-S
1.44% SiO2/g BEA-S
4.32% SiO2/g BEA-S
7.20% SiO2/g BEA-S
a
SBET: surface area by the BET method; Vtol, Vmicro, Vmeso, Vmacro: volume of total pore, micropore, mesopore, macropore by the DFT method.
Fig. 5 SiO2 layer on the external surface of the catalysts.
The adsorption isotherms of parent and modied H-beta block micropores because of the modied hydroxyl groups
zeolites are exhibited in Fig. 4. The N2 adsorption isotherms located on the external surface. The volume of the macropores
of BEA-L is typical I while that of BEA-S is typical IV, indicating was essentially unchanged because the thin SiO2 layer on the
that BEA-L has a mainly microporous structure and BEA-S has external surface of zeolites obtained from the modication did
more mesoporous structure. Table 2 shows the textural not change the intergranular pore size. As can be seen in
parameters of parent and the modied catalysts. The surface Fig. 5,37 with the further increase of silylating reagent, a thicker
areas of BEA-L and BEA-S were almost the same. As can be seen SiO2 layer was formed, resulting in the decrease of intergranular
from Fig. 3, compared with BEA-L, BEA-S exhibit more pore size, which contributed to the increase in micropores. The
agglomeration due to the small size of the crystals, which STEM images of modied H-beta zeolite are shown in Fig. 6,
represents more intergranular pores. Therefore, BEA-S contains presuming that the silica layer was presented in an amorphous
more mesopores and macropores. When the added amount of phase and deposited on the external surface and intergranular
modifying SiO2 was 1.44 wt% of the parent zeolites, the pores, which is consistent with the above results.
micropore volume decreased, implying that silanization can
Fig. 6 STEM images of modified H-beta zeolite. (a) SiO2 layer on the external surface; (b) SiO2 layer on the intergranular pore.
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Fig. 7 NH3-TPD profiles of BEA-L and modified BEA-L zeolites (a), BEA-S and modified BEA-S zeolites (b).
total acid amount and Py-IR is used to distinguish Brønsted and
Lewis acid sites. The amounts of acid determined by these two
methods provide a reference to analyze the trend of acid aer
modication. The amounts of total acid sites, Brønsted and
Lewis acid sites are listed in Table 3.
3.2. Acidity characterization
Fig. 7 and 8 show the ammonia temperature programmed
desorption (NH3-TPD) proles and the FT-IR spectra of pyridine
at different desorption temperatures of the BEA-L, BEA-S and
theirs modifed zeolites. NH3-TPD is a method to analyze the
Fig. 8 FT-IR spectra of pyridine adsorbed on the BEA-L, BEA-S and theirs modified zeolites under different desorption temperature (200 ꢁC and
350 ꢁC). A-200 ꢁC, A-350 ꢁC: BEA-L and modified BEA-L zeolites; B-200 ꢁC, B-350 ꢁC: BEA-S and modified BEA-S zeolites.
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Table 3 Acidic properties of BEA-L, BEA-S and theirs modified H-beta zeolitesa
Brønsted acid sites (mmol gꢂ1
)
Lewis acid sites (mmol gꢂ1
)
Samples
Total acid (mmol gꢂ1
)
200 ꢁC
350 ꢁ
C
200 ꢁ
C
350 ꢁC
BEA-L
197
159
73
204
234
1.44% SiO2/g BEA-L
4.32% SiO2/g BEA-L
7.20% SiO2/g BEA-L
BEA-S
170(ꢂ14.4%)
167(ꢂ16.6%)
164(ꢂ16.9%)
197
75(ꢂ52.8%)
75(ꢂ52.8%)
61(ꢂ61.0%)
263
72(ꢂ1.4%)
14(ꢂ80.8%)
24(ꢂ67.1%)
211
255(+25.0%)
304(+49.3%)
212(+3.7%)
117
291(+24.3%)
318(+35.9%)
265(+13.2%)
112
1.44% SiO2/g BEA-S
4.32% SiO2/g BEA-S
7.20% SiO2/g BEA-S
182(ꢂ7.6%)
167(ꢂ14.8%)
160(ꢂ18.7%)
254(ꢂ3.4%)
250(ꢂ4.9%)
237(ꢂ9.9%)
183(ꢂ13.3%)
179(ꢂ15.2%)
172(ꢂ18.5%)
131(+12.0%)
157(+34.2%)
119(+1.7%)
109(ꢂ2.7%)
125(+11.6%)
95(ꢂ15.2%)
a
( ): Change in the amount of acid sites.
Fig. 9 Mechanism of SiO2 deposition.
As shown in the NH3-TPD curves (Fig. 7), two desorption increased, more reactions between TEOS with bridging hydroxyl
ꢁ
ꢁ
peaks at 200 C (weak acid sites) and 400 C (strong acid sites) groups were involved, which led to a reduction of Brønsted acid
were observed on all samples. BEA-S exhibited stronger signal sites. In addition, more Si–O bonds were formed on the surface
than BEA-L at the strong acid sites. The strong acids sites of of the zeolites, resulting in an increase in Lewis acid sites. When
BEA-L and BEA-S were both reduced aer modication. NH3 can the silanization reagent was too much (7.20% SiO2/g BEA-L or
be adsorbed on the internal and external acid sites of zeolites on BEA-S), blockage caused by reaction around the pore mouth led
account of its small size (0.26 nm). Therefore, the adsorption to the inability of pyridine molecules to enter the pore, resulting
peaks areas correspond to the total amount of acid content. As in the apparent decrease of Brønsted and Lewis acid sites. The
can be seen in Table 3, the acidity of BEA-L and BEA-S were results from the FT-IR spectra Brønsted and Lewis acid sites
signicantly altered aer surface silicon modication. The total showed the trend of decreasing Brønsted acid sites, and
acid amount of the parent zeolites decreased with increasing increasing-decreasing Lewis acid sites. In addition, the results
ꢁ
SiO2 loading used in the modication. 16.9% acid sites of BEA-L from desorption temperature 200 and 350 C of Py-IR usually
and 18.7% acid sites of BEA-S were lost when the added amount represent total acid and strong acid. As the desorption
of modied SiO2 was 7.20 wt% of the parent zeolites. This can temperature increases, the BrØnsted and Lewis acid sites of
be attributed to the passivation of external surface acid sites and BEA-S obviously decrease. And the conversion of BrØnsted acid
the blockage of pores aer modication.
sites to strong Lewis acid sites might be the reason for the
The FT-IR spectra of zeolites BEA-L and BEA-S with different increase in Lewis acid amount and decrease in BrØnsted acid
loading of SiO2 were investigated in the range of 1650– amount of BEA-L as the desorption temperature increases from
1400 cmꢂ1 and desorption temperature of pyridine of 200 ꢁC 200 C to 350 ꢁC.38,39
ꢁ
and 350 ꢁC. The adsorption bands at 1540 cmꢂ1 and 1455 cmꢂ1
The FT-IR spectra with 2,4,6-trimethylpyridine adsorption
correspond to Brønsted and Lewis acid sites of H-beta zeolites, was used to measure the external surface acid sites of the BEA-L,
respectively. The quantity of Brønsted and Lewis acid sites of BEA-S and theirs modied zeolites since it has a larger size than
the BEA-L, BEA-S and theirs modied zeolites are presented in the H-beta zeolite channels. As shown in Fig. 10, 2,4,6-trime-
Table 3. Brønsted acid sites of zeolites BEA-L and BEA-S thylpyridine adsorbed on external Brønsted acid sites gave the
decreased with increased SiO2 loading. Lewis acid sites of the band at 1638 cmꢂ1 while the infrared adsorption peak on Lewis
modied zeolites increased rst, then decreased. In the case of acid sites was not exhibited due to steric constraints.21,40–44 Aer
silylation modication, TEOS was hydrolyzed which resulted in surface silicon modication, the quantity of external Brønsted
triethoxy reacting with bridging hydroxyl groups on the acid sites was signicantly reduced, indicating that the silylat-
zeolites.27 Aer calcination, new Al–O–Si bonds were formed, ing reagent TEOS has neutralized hydroxyl groups located on
leading to the reduction of skeleton aluminum. The mechanism the external surface of the zeolites. As can be seen in Fig. 3, BEA-
of SiO2 deposit is displayed in Fig. 9.37 As the SiO2 loading S showed more crystal agglomeration compared with BEA-L. As
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Fig. 10 FT-IR spectra of 2,4,6-trimethylpyridine adsorbed on the BEA-L and modified BEA-L zeolites (a), BEA-S and modified BEA-S zeolites (b)
at desorption temperature of 200 ꢁC.
shown in Table 2, when the added amount of modied SiO2 was
4.32 wt% and 7.20 wt% of the parent zeolites, micropores and
mesopores of BEA-S was increased due to the decrease of
intergranular pore size. It can be speculated that the silylating
reagent TEOS was hard to access the interior of the agglomer-
ated particles of BEA-S. The acid sites located on the external
surface of the agglomerated particles were easier to passivate.
Almost all external surface acid sites of BEA-L could be passiv-
ated by TEOS due to the large size of the crystals. Therefore,
more external Brønsted acid sites of BEA-L were lost than that of
BEA-S at the same added amount of TEOS, as shown in Fig. 11.
Paul J. Kunkeler et al. also proposed that the restriction of
surface silicon modication due to extremely rough surface and
the destruction of silica layers during calcination were also the
reasons for less efficient external acidity passivation of small
size crystal zeolite.45
3.3. Alkylation of benzene with 1-dodecene over H-beta
catalysts
Fig. 11 Relative external acid sites of the BEA-L, BEA-S and theirs
modified H-beta zeolites with different loading of SiO2.
The alkylation reaction of benzene with 1-dodecene was studied
over the BEA-L, BEA-S and theirs modied H-beta catalysts.
Fig. 12 Influence of time on stream (TOS) on the catalytic conversion of 1-dodecene and selectivity of 2-LAB over BEA-L, BEA-S and theirs
modified H-beta zeolites.
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Fig. 13 1-Dodecene conversion (left) and selectivity of products (2-LAB, 3-LAB, 4 + 5 + 6-LAB) (right) over BEA-L, BEA-S and theirs modified H-
beta zeolites.
Fig. 12 presents the inuence of time on stream (TOS) on the
Five dodecylbenzene isomers (except unstable 1-LAB) can be
conversion of 1-dodecene and selectivity of products. The produced during the alkylation of benzene with 1-dodecene.
conversion of 1-dodecene and selectivity of products before the The molecular size of 2-LAB is the smallest while 6-LAB is the
activity of the catalysts declined are displayed in Fig. 13. 1- largest. Diffusion limitation in the zeolite channels makes it
Dodecene conversion over zeolites BEA-L and BEA-S were more difficult for larger products to diffuse out. Among the
$98%. As can be seen in Fig. 12, the lifetime of BEA-S could isomers obtained from the alkylation of benzene with 1-dode-
attain 65 h while that of BEA-L was 14 h, which can be attributed cene, the selectivity of 2-LAB was the highest. The selectivity of
to the small crystal size and abundant Brønsted acid sites of 2-LAB over zeolites BEA-L and BEA-S are similar (41.9% and
BEA-S. Compared to BEA-L, the shorter channels of BEA-S make 40.0%, respectively) which was attributed to the same channel
it easier for reactants and products to diffuse into and out of structure of H-beta zeolite. On BEA-L, as the SiO2 loading
channels. Moreover, BEA-S was active for a longer time than increased, the selectivity of products 2-LAB and 3-LAB
BEA-L before it started deactivation due to more Brønsted acid increased, and that of products (4 + 5 + 6)-LAB markedly
sites.
decreased. The selectivity to 2-LAB increased from 41.9% (BEA-
With the increase of SiO2 loading, the conversion of 1- L) to 54.7% (7.20% SiO2/g BEA-L), while that of (4 + 5 + 6)-LAB
dodecene over modied zeolites BEA-L and BEA-S gradually decreased from 34.2% (BEA-L zeolites) to 19.2% (7.20% SiO2/g
dropped off and eventually reached 92.7% and 96.9% when the BEA-L). However, aer modication, the product selectivity of
added amount of modifying SiO2 was 7.20 wt% of the BEA-L and BEA-S was only slightly changed. When the added amount of
BEA-S, respectively. Based on the acidity characterization modied SiO2 was 7.20 wt% of BEA-S, the selectivity of 2-LAB
results, the decrease of 1-dodecene conversion was attributed to was only increased by 2% and that of (4 + 5 + 6)-LAB decreased
the passivation of external surface acid sites, which led to the by 3.9%.
reduction of active sites. In addition, more Brønsted acid sites
The active sites of zeolites are located on the internal and
of BEA-L were lost than that of BEA-S, resulting in the faster external surfaces. External surface acid sites have no shape
decrease of conversion of BEA-L aer modication. This also selective catalysis. The passivation of the external surface area
indicated that the alkylation of benzene with 1-dodecene mainly by CLD treatment with TEOS led to a decrease in nonselective
occurred on Brønsted acid sites.
acid sites, resulting in the increase of 2-LAB selectivity. The
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characterization studies showed that more external Brønsted added amount of modifying SiO2 was 7.20 wt% of the parent
acid sites of BEA-L were lost than that of BEA-S at same added zeolites, the selectivity of 2-LAB over BEA-L signicantly
amount of TEOS. However, the main effect was probably that increased from 41.9% to 54.7% while that of BEA-S only
the modication procedure has narrowed the pore mouth and increased by 2%. This is because the silylating reagent TEOS is
restricted the diffusion of larger products, giving rise to the difficult to access the interior of the agglomerated particles of
increase of 2-LAB selectivity. It is hard for the silylating reagent BEA-S. According to our study, the silanization modication for
TEOS to access the interior of agglomerated particles of BEA-S. external surfaces is not a suitable method for improving the 2-
Orices of the crystals inside agglomerated particles have nearly LAB selectivity of small-grain b zeolites. It is a feasible method
no change. As a result, the selectivity of 2-LAB from modied of to add silanization reagent during synthesis procedure of
BEA-S increased only slightly. It was easier to make the orices b zeolite to passivate the acidity of the external surface of crys-
of BEA-L narrower, leading to the signicant increase in selec- tallites, which will be our research topic in the future.
tivity of 2-LAB.
The lifetimes of the zeolites decreased sharply with
increased SiO2 loading, as shown in Fig. 12. The lifetime of BEA-
Conflicts of interest
L decreased from 14 h (BEA-L) to 5 h (7.20% SiO2/g BEA-L) while
that of BEA-S decreased from 65 h (BEA-S) to 20 h (7.20% SiO2/g
There are no conicts to declare.
BEA-S). One reason is that passivation of external surface acidity
led to fewer active sites for the alkylation. Another reason is the Acknowledgements
narrowing of orice and pores, such that reactants cannot easily
We are grateful for the discussion of the experimental results
enter the pore to react on the active sites, and products are
with our group members.
difficult to diffuse out and stay in the channels.
For benzene alkylation with long chain olens, the rapid
deactivation of H-beta zeolites is an obstacle to the industrial
application. The main reason for inactivation is that the shape
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