Preparation of silicalite-1@Pt/alumina core-shell catalyst for shape-selective hydrogenation of xylene isomers

Article abstract of DOI:10.1016/j.catcom.2015.02.004

A silicalite-1@Pt/alumina core-shell catalyst that combined molecular sieving and hydrogenation was synthesized by coating silicalite-1 onto the surface of Pt/alumina pellet. While a Pt/alumina catalyst had no selectivity in the hydrogenation of xylene isomers, the silicalite-1@Pt/alumina core-shell catalyst showed much higher efficiency for the hydrogenation of p-xylene than for that of m- and o-xylene. The shape-selective hydrogenation catalyst has great potential for application in xylene separation.

Full text of DOI:10.1016/j.catcom.2015.02.004

Catalysis Communications 64 (2015) 110–113
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Preparation of silicalite-1@Pt/alumina core–shell catalyst for
shape-selective hydrogenation of xylene isomers
a
a
a
a,b,
Yilan Wu a, Yongming Chai a,b, , Jiangchuan Li , Hailing Guo , Ling Wen , Chenguang Liu
⁎
⁎
a
State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266580, PR China
Key Laboratory of Catalysis, China National Petroleum Corp. (CNPC), China University of Petroleum (East China), Qingdao 266580, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A silicalite-1@Pt/alumina core–shell catalyst that combined molecular sieving and hydrogenation was synthe-
sized by coating silicalite-1 onto the surface of Pt/alumina pellet. While a Pt/alumina catalyst had no selectivity
in the hydrogenation of xylene isomers, the silicalite-1@Pt/alumina core–shell catalyst showed much higher ef-
ficiency for the hydrogenation of p-xylene than for that of m- and o-xylene. The shape-selective hydrogenation
catalyst has great potential for application in xylene separation.
Received 22 December 2014
Received in revised form 30 January 2015
Accepted 3 February 2015
Available online 7 February 2015
© 2015 Published by Elsevier B.V.
Keywords:
Core–shell
Silicalite-1 membrane
Platinum
Xylene
Hydrogenation
1. Introduction
xylene vapor mixtures had a maximum value of about 60 between
room temperature and 473 K. Xomeritakis et al. [13,14] studied an
Zeolites are crystalline microporous aluminosilicates with periodic
arrangements of cages and channels that have been found with exten-
sive industrial applications as catalysts, adsorbents and ion exchangers
[1]. Recent success in the preparation of zeolite membranes with good
thermal, chemical, and structural stabilities has provided new opportu-
nities for applications in gas, vapor and liquid separations [2–4]. One
highly desirable process is the separation of p-xylene (kinetic
diameter ~ 5.8 Å) from its bulkier m- and o-xylene isomers (kinetic
diameter ~ 6.8 Å), which is of significant importance in petrochemical
industry for the high value downstream products from p-xylene such
as terephthalic acid, polyester resin and terylene [5]. The diameter of
the pore opening of MFI zeolite (5.5 × 5.1 Å sized elliptical channels run-
ning along the a axis in a sinusoidal manner and 5.6 × 5.3 Å sized ellip-
tical channels running straight along the b axis [6,7]) is approximately
of the size of p-xylene, while the bulkier m- and o-xylene isomer mole-
cules can face significant sterical hindrance within the confines of the
zeolite pores. Therefore, due to molecular sieving through zeolitic
pores, defect-free MFI zeolite membranes have high permselectivity
for p-xylene over o-xylene and m-xylene [8–11].
MFI-membrane synthesized on the surface of α-alumina disks to
evaluate the separation of xylenes in the 295–548 K temperature
range. The prepared membranes exhibited a p-xylene permeance of
2 × 10−8 molm−1 s−1 Pa−1 at 373–398 K and p/o separation factors
between 60 and 300. Lai et al. [15,16] reported the preparation of b-
Several groups have studied the separation of xylene isomers
through MFI zeolite membranes [12–20]. Keizer et al. [12] reported
that the separation factor of 0.62 kPa p-xylene and 0.52 kPa o-
⁎
Corresponding authors at: State Key Laboratory of Heavy Oil Processing, China
University of Petroleum (East China), Qingdao 266580, PR China.
E-mail addresses: ymchai@upc.edu.cn (Y. Chai), cgliu@upc.edu.cn (C. Liu).
Fig. 1. XRD patterns of PA-S, PA and pure silicalite-1.
http://dx.doi.org/10.1016/j.catcom.2015.02.004
1566-7367/© 2015 Published by Elsevier B.V.

Y. Wu et al. / Catalysis Communications 64 (2015) 110–113
111
Fig. 2. (a, b) Top-view SEM images of the PA catalyst, (c, d) top-view SEM images and (e, f) cross-section SEM images of the PA-S core–shell catalyst under low (left) and high (right)
magnifications.
Fig. 3. Surface EDS analysis of (a) the bare PA catalyst and (b) the PA-S core–shell catalyst.

112
Y. Wu et al. / Catalysis Communications 64 (2015) 110–113
Table 1
Table 3
Hydrogenation product yields over the PA and PA-S catalysts in the hydrogenation of sin-
Hydrogenation product yields over the PA-S catalyst in the hydrogenation of equimolar bi-
gle component xylene isomer.a
nary p/m xylene.a
Hydrogenation product yields (wt.%) b
Selectivity
p/o
Hydrogenation product yields (wt.%)
Selectivity
Catalysts
1,2-DC
1,3-DC
1,4-DC
p/m
1,4-DC
39.3
1,3-DC
2.9
p/m
13.6
PA
PA-S
80.7
0.8
83.9
2.4
81.8
75.6
1.0
94.5
1.0
31.5
a
Reaction conditions: 473 K, 1.0 MPa, Wcat = 2 g, feed = 4 ml h−1, WHSV = 1.0 h−1
,
FH = 2400 ml h−1. Feed composition: 50 wt.% p-xylene and 50 wt.% m-xylene.
Reaction conditions: 473 K, 1.0 MPa, Wcat = 2 g, feed = 4 ml h−1, WHSV = 1.0 h−1
,
a
2
FH = 2400 ml h−1. Feed composition: single component xylene isomer.
2
b
1,4-Dimethylcyclohexane (1,4-DC), 1,3-dimethylcyclohexane (1,3-DC) and 1,2-
dimethylcyclohexane (1,2-DC) are hydrogenation products of p-xylene, m-xylene
and o-xylene, respectively.
3. Results and discussion
3.1. Catalyst characterization
X-ray diffraction (XRD) patterns of the naked PA catalyst, PA-S core–
shell catalyst and pure MFI zeolite are presented in Fig. 1. For the PA cat-
alyst, only the peaks of crystalline α-Al2O3 were observed. For the
silicalite-1 coated PA-S catalyst, X-ray diffraction peaks belonging to
silicalite-1 appeared at 2θ = 7.9°, 8.9° and 23.5° by comparing with
pure silicalite-1 zeolite, indicating that silicalite-1 membrane had suc-
cessfully crystallized on the PA core catalyst.
The top-view SEM images in Fig. 2(a) and (c) show the whole sur-
face morphologies of the PA catalyst and the PA-S core–shell catalysts.
Both the PA and PA-S catalysts have spherical structures with the diam-
eters around 2 mm. From the magnified surface images of the PA and
PA-S catalyst in Fig. 2(b) and (d), it can be clearly observed that the sur-
face of the PA-S catalyst had been covered by one layer of silicalite-1.
Fig. 2(e) and (f) shows the cross-section SEM images of the PA-S catalyst
under different magnifications. A well-intergrown and uniform zeolite
shell coated on the surface of the PA core catalyst could be distin-
guished. The thickness of this silicalite-1 layer is about 4 μm. The ele-
mental distributions of the PA pellet surface before and after the MFI
zeolite coating are presented in Fig. 3(a) and (b). On the bare PA catalyst
surface, a Pt peak can be detected, but there is no Pt signal visible on the
surface of the PA-S core–shell catalyst after the zeolite coating step. This
indicates that the PA core catalyst is completely enwrapped by this inte-
grated silicalite-1 membrane.
oriented silicalite membranes synthesized on the surface of α-
alumina disks. These membranes showed p/o-xylene separation fac-
tors (0.5 kPa/0.45 kPa) as high as 483 at 493 K with a p-xylene
permeance of 2 × 10−7 molm−1 s−1 Pa−1
.
Although these approaches were successful in demonstrating
high selectivity for xylene separation, they are of limited practical
potential because of prohibitive low fluxes [15]. Bakker et al. [17] in-
dicated that the maximum permeation flux could be described by the
equilibrium adsorption amount for xylenes through the MFI mem-
brane. Thus, a combination of separation with catalysis is required
to attain compatible separation fluxes and reaction rates. The selec-
tive conversion of reactant can break the equilibrium limitation on
the membrane. Besides, diffusion is an activated process and the dif-
fusivity increases with the reaction temperature, thus a catalytic re-
action under high temperature will further enhance the diffusion
efficiency through the membrane.
In this work, we report a catalyst with a core–shell structure that
couples molecular sieving and hydrogenation. A Pt/Al2O3 hydrogena-
tion catalyst was prepared as a core catalyst. One layer of silicalite-1
shell was successfully developed on the core catalyst. The silicalite-1@
Pt/Al2O3 particles were then used for the hydrogenation of xylene iso-
mers. The product selectivity for the silicalite-1@Pt/Al2O3 catalyst was
compared with that for the naked Pt/Al2O3 core catalyst. This concept
of core–shell catalyst has great potential to develop into a novel ap-
proach for xylene separation.
3.2. Catalyst evaluation results
The PA-S catalyst was used for the selective hydrogenation of xylene
isomers to investigate the effect of the silicalite-1 coated core–shell struc-
ture. For comparison, the naked PA catalyst was also studied under the
same reaction conditions. The hydrogenation experiments were carried
out with single component xylene isomer and a mixture of equimolar bi-
nary p/o-xylene and binary p/m-xylene, respectively, as the feeds. Hydro-
genation product yields of single component xylene isomer over the two
catalysts are presented in Table 1. When silicalite-1 crystals without Pt
were used for the reaction, no catalytic activity was observed, so the
silicalite-1 coating on the catalyst acted not as a catalyst but as a mem-
brane. For each xylene isomer, there was only one corresponding hydro-
genation product. That is, p-xylene was hydrogenated to produce 1,4-
dimethylcyclohexane (1,4-DC), m-xylene was hydrogenated to produce
1,3-dimethylcyclohexane (1,3-DC) and o-xylene was hydrogenated to
produce 1,2-dimethylcyclohexane (1,2-DC). For the naked PA core cata-
lyst, the yield of 1,4-DC, 1,3-DC and 1,2-DC all exceeded 80 wt.%, demon-
strating that the bare PA core catalyst has no selectivity in the
hydrogenation of the three xylene isomers. In contrast, for the PA-S
core–shell catalyst, the hydrogenation conversion of p-xylene was only
slightly lower (75.6 wt.%) than that over the PA catalyst (81.8 wt.%),
while the yield of 1,3-DC and 1,2-DC was much lower than over the
bare PA catalyst. This indicates that the hydrogenation of m-xylene and
o-xylene was strongly suppressed on the PA-S catalyst. The ideal p/o hy-
drogenation selectivity on the PA-S catalyst (the ratio of the conversion
of p-xylene to that of o-xylene) was as high as 94.5, which was much
higher than that of the PA catalyst (about 1.0). The high ideal p/m
2. Experimental
Alumina (Al2O3) pellets (2.0–2.3 mm, Sasol Co.) were used as
support for the preparation of a 1.0 wt.% Pt/Al2O3 core catalyst by im-
pregnation with an aqueous solution of chloroplatinic acid hexahy-
drate (H2PtCl4·6H2O). The Pt/Al2O3 core catalyst, named PA, was
pre-modified with a 3.0 wt.% TPAOH solution, followed by the ad-
sorption of silicalite-1 nanoparticles. One layer of silicalite-1 shell
was synthesized on the thus-treated PA core catalyst using second-
ary hydrothermal synthesis, with a reaction solution composition
of 0.4TPAOH/1.0TEOS/160H2O, at 443 K for 3 days. The obtained
silicalite-1 membrane coated Pt/Al2O3 catalyst was named PA-S.
More detailed descriptions of the synthesis, characterization and
catalytic test were presented in the Supplementary material.
Table 2
Hydrogenation product yields over the PA-S catalyst in the hydrogenation of equimolar bi-
nary p/o xylene.a
Hydrogenation product yields (wt.%)
Selectivity
1,4-DC
34.7
1,2-DC
2.04
p/o
17.0
a
Reaction conditions: 473 K, 1.0 MPa, Wcat = 2 g, feed = 4 ml h−1, WHSV = 1.0 h−1
,
FH = 2400 ml h−1. Feed composition: 50 wt.% p-xylene and 50 wt.% o-xylene.
2

Y. Wu et al. / Catalysis Communications 64 (2015) 110–113
113
Scheme 1. Selective hydrogenation model of xylene isomers over the PA-S core–shell catalyst.
selectivity of 31.5 was also obtained on the PA-S catalyst. The selective hy-
Acknowledgments
drogenation performance of the PA-S catalyst using binary p/o xylene and
binary p/m xylene feeds is shown in Tables 2 and 3, respectively. The real
p/o and p/m hydrogenation selectivities of 17.0 and 13.6, respectively,
were obtained, indicating that the PA-S catalyst also realized high para-
selectivities for binary p/o and p/m xylene mixtures.
This work was financially supported by the National Natural Science
Fund of China (Grant No. U1162203) and the Fundamental Research
Funds for the Central Universities (14CX06049A and 14CX02058A).
The excellent para-selectivity is a consequence of diffusion resis-
tance, that is, the silicalite-1 membrane imposes sterical restriction on
the transport of m-xylene and o-xylene molecules relative to p-xylene
molecules. As illustrated in Scheme 1, when xylene isomers contact
the silicalite-1 shell, p-xylene is able to diffuse through the pores,
reach the Pt active site in the core, then undergo a hydrogenation, and
finally the produced 1,4-DC diffuses out. The bulkier isomers, m-
xylene and o-xylene, may be too large to diffuse into the pores easily
or at greatly reduced rates, suppressing their further hydrogenation to
generate 1,3-DC and 1,2-DC. As a result, the hydrogenation conversions
of m-xylene and o-xylene were decreased sharply over the PA-S catalyst
compared to those over the PA catalyst. The excellent catalysis and sep-
aration performance of the zeolite-coated PA-S catalyst confirmed that
this silicalite-1 membrane realized high-flux and high-selectivity of p-
xylene. Besides, the high permeability and reaction efficiency contribut-
ed to the combination of catalytic reaction and separation. The diffusion
limitation on the silicalite-1 membrane was broken by the selective
conversion of p-xylene to its hydrogenation product.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2015.02.004.
References
[1] M.E. Davis, Nature 417 (2002) 813–821.
[2] T.C. Bowen, R.D. Noble, J.L. Falconer, J. Membr. Sci. 245 (2004) 1–33.
[3] C.M. Braunbarth, L.C. Boudreau, M. Tsapasis, J. Membr. Sci. 174 (2000) 31–42.
[4] J. Coronas, R.D. Noble, J.L. Falconer, AICHE J. 43 (1997) 1797–1812.
[5] K. Li, M. Hsu, I. Wang, Catal. Commun. 9 (2008) 2257–2260.
[6] H.H. Funke, A.M. Argo, J.L. Falconer, R.D. Noble, Ind. Eng. Chem. Res. 36 (1997)
137–143.
[7] C. Bärlocher, W.M. Meier, D.H. Olson, Atlas of Zeolite Framework Types, 5th ed.
Structure Commission of the International Zeolite Association by Elsevier,
Amsterdam, 2001.
[8] C. Zhang, Z. Hong, J. Chen, X. Gu, W. Jin, N. Xu, J. Membr. Sci. 389 (2012) 451–458.
[9] L.B. Young, S.A. Butter, W.W. Kaeding, J. Catal. 76 (1982) 418–432.
[10] H. Karsh, A. Culfaz, H. Yucel, Zeolites 12 (1992) 728–732.
[11] D.M. Ruthven, M. Eic, E. Richard, Zeolites 11 (1991) 647–653.
[12] K. Keizer, A.J. Burggraaf, Z.A.E.P. Vroon, H. Verweij, J. Membr. Sci. 147 (1998)
159–172.
4. Conclusion
[13] G. Xomeritakis, Z. Lai, M. Tsapasis, Ind. Eng. Chem. Res. 40 (2001) 544–552.
[14] G. Xomeritakis, S. Nair, M. Tsapasis, Microporous Mesoporous Mater. 38 (2000)
61–73.
[15] Z. Lai, G. Bonilla, I. Diaz, J.G. Nery, K. Sujaoti, M.A. Amat, E. Kokkoli, O. Terasaki, R.W.
Thompson, M. Tsapasis, D.G. Vlachos, Science 300 (2003) 456–460.
[16] Z. Lai, M. Tsapasis, Ind. Eng. Chem. Res. 43 (2004) 3000–3007.
[17] W.J.W. Bakker, L.J.P. van den Broeke, F. Kapteijn, J.A. Moulijn, AICHE J. 43 (1997)
2203–2214.
A core–shell catalyst was shown to couple molecular sieving and hy-
drogenation. One layer of silicalite-1 membrane with a thickness of
4 μm was successfully coated onto the surface of Pt/Al2O3 catalyst by a
hydrothermal synthesis method. In the hydrogenation of xylene iso-
mers, the silicalite-1@Pt/Al2O3 catalyst showed much higher efficiency
for the hydrogenation of p-xylene than for that of m- and o-xylene.
The ideal p/o and p/m hydrogenation selectivities of 94.5 and 34.5, re-
spectively, as well as the real p/o and p/m hydrogenation selectivities
of 17.0 and 13.6, respectively, were obtained at 473 K due to the selec-
tive permeation of p-xylene into the Pt/Al2O3 particle through the
silicalite-1 layer.
[18] G. Xomeritakis, M. Tsapasis, Chem. Mater. 11 (1999) 875–878.
[19] X. Gu, J. Dong, T.M. Nenoff, D.E. Ozokwelu, J. Membr. Sci. 280 (2006) 624–633.
[20] A.M. Tarditi, G.I. Horowitz, E.A. Lombardo, J. Membr. Sci. 281 (2003) 692–699.