Synergetic effect of H-ZSM-5/Silicalite-1@Pt/Al2O3 core–shell catalyst to enhance the selective hydrogenation of p-xylene
Graphical abstract
A H-ZSM-5/Silicalite-1@Pt/Al2O3 core–shell catalyst was successfully synthesized by coating one inner Silicalite-1 layer and one outer H-ZSM-5 layer onto the surface of Pt/Al2O3 catalyst. The bi-layered core–shell catalyst, with the synergetic effect of equilibrium shift and shape-selectivity, showed enhanced efficiency for the selective hydrogenation of p-xylene in comparison with monolayer core–shell catalysts.
Introduction
P-xylene (PX) is an important raw material for its high value downstream products in petrochemical industry [1], [2], and primarily produced from naphtha cracking and catalytic reforming steam, in admixture with two other xylene isomers, m-xylene (MX) and o-xylene (OX), due to thermodynamic equilibrium [3]. However, separation of PX from xylene isomers by fractional distillation seems to be infeasible because of their close boiling points (138.3 °C for PX, 139.1 °C for MX and 144.4 °C for OX). Currently, commercial processes for separating xylene isomers to obtain PX, such as crystallization, extractive distillation and selective adsorption, are highly energy intensive. Therefore, developing a novel approach for the separation and recovery of PX from xylene mixtures has moved into the spotlight in recent years.
MFI-type zeolite has a three-dimensional pore system with straight channels in the b-direction (5.4×5.6 Å) and sinusoidal channels in the a-direction (5.1×5.5 Å) [4], [5]. PX with a kinetic diameter of 5.8 Å can enter and diffuse through these zeolite channels more readily than the bulkier MX and OX isomers, which have kinetic diameter of 6.8 Å [6], [7]. Due to molecular sieving through zeolitic pores, defect-free MFI zeolite membranes have higher permselectivity for PX than for MX and OX [8], [9]. Owing to this unique property, MFI zeolite has been one of the most promising candidates for developing a new approach of xylenes separation.
Recent studies on xylenes separation by MFI zeolite membranes have been explored by several groups [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. Although these approaches were successful in demonstrating high selectivity for xylene separation, they are of limited practical potential because of prohibitive low fluxes [20]. Bakker et al. [21] indicated that a maximum permeation flux could be described by the equilibrium adsorption amount for xylenes through the MFI membrane. Thus, a combination of separation with catalysis is required to attain compatible separation fluxes and reaction rates. The selective conversion of reactant can break the equilibrium limitation on the membrane, which will enhance the diffusion efficiency of PX. Besides, diffusion is an activated process and the diffusivity increases with the reaction temperature, thus a catalytic reaction under relative high temperature will further enhance the diffusion efficiency through the membrane.
To solve these problems, we propose a novel approach to selectively separate PX from xylene isomers. This process consists of selective hydrogenation of PX from xylene isomers to 1,4-dimethylcyclohexane (1,4-DMC) as an intermediate hydrogenation product, distillation 1,4-DMC from xylene mixtures and 1,4-DMC further dehydrogenation to produce high purity PX, shown in Scheme 1. As the boiling point of 1,4-DMC is 119.5 °C, which is much lower than those of xylenes, it is can be easily separated from xylene isomers by distillation. Among this process, the key and challenging step is the selective hydrogenation of PX from xylene mixtures. That is, PX can be selectively separated to the hydrogenation reaction zone. To achieve this tough goal, we delicately designed a core–shell catalyst coupling separation and selective hydrogenation properties. A Pt/Al2O3 hydrogenation catalyst was prepared as the core catalyst. Al-substituted MFI zeolite (ZSM-5) membrane could be used as a bi-functional membrane to simultaneously separate PX and isomerize xylenes. It would be very interesting that, if the selective separation and subsequent hydrogenation of PX to 1,4-DMC could break the thermodynamic equilibrium of xylenes outside the catalyst, the equilibrium between xylene mixtures will shift continually to generate more PX.
However, the quality of ZSM-5 membrane is difficult to control due to the introduction of aluminum in the MFI zeolite framework. Noack et al. [22] also suggested that aluminum present in synthesis solution could affect the growth of defect-free MFI membranes because the surface charge of the zeolite framework becomes more negative when the aluminum content is high. In our previous studies, a Silicalite-1@Pt/Al2O3 core–shell catalyst was successfully synthesized by coating Silicalite-1 layer onto the surface of Pt/Al2O3 pellet. The obtained core–shell catalyst showed higher efficiency for the hydrogenation of PX than for that of MX or OX, showing that Silicalite-1 membrane could effectively act as a molecular sieving layer and impose sterical restriction on the transport of MX and OX molecules [23]. Thus, we propose a bi-layered membranes coated H-ZSM-5/Silicalite-1@Pt/Al2O3 core–shell catalyst with an outer H-ZSM-5 shell and an inner Silicalite-1 shell to enhance the selective hydrogenation of PX.
Several different types of membrane reactors have been studied for separation [24], [25] and catalysis reactions [26], [27]. Lin and co-workers [28] synthesized ZSM-5/Silicalite bilayer membrane on a porous α-alumina support coated with a yttria stabilized zirconia intermediate barrier layer to improve the membrane stability. During the membrane synthesis, a thin and high quality ZSM-5 top layer was synthesized on a thick Silicalite bottom layer to form a ZSM-5/Silicalite bilayer membrane. The CCD modified bilayer membrane exhibited a H2/CO separation factor of 1.85×10−7 to 1.28×10−7 mol m−2 s Pa at 450 °C. The high H2/CO permselectivity achieved was attributed to the controlled CCD modification in the thin and high quality ZSM-5 top layer. The bilayer MFI membrane also showed H2 permeance of about 1.2×10−7 mol m−2 s Pa−1 and H2 to CO2, CO and H2O vapor selectivity respectively of about 23, 28 and 180 [29]. Tsubaki et al. [30], [31] reported dual membrane capsule catalysts with H-ZSM-5 and Silicalite-1 membranes and different cores (such as Fe/SiO2 and Pd/Silica), and successfully employed them for Fischer–Tropsch synthesis and dimethyl ether synthesis. In their works, the Silicalite-1 zeolite membrane was first synthesized to avoid the corrosion damage of the core catalyst and favored the in-situ growth of H-ZSM-5 zeolite membrane. The H-ZSM-5 shell was then directly synthesized without Na+ ion in the precursor solution and acted as a functional membrane for catalytic reactions. As a result, the excellent separation and catalysis properties of the membrane capsule catalyst were ascribed to the well-designed bi-layered structure, which was very effective and available for the controlled synthesis of target products with high selectivity.
To our best knowledge, the bi-layered H-ZSM-5/Silicalite-1 membrane capsule catalyst has not been studied so far for the selective hydrogenation of xylenes. In this study, The H-ZSM-5 membrane could act as a bifunctional membrane displaying PX selective separation and isomerization catalytic activity. While the Silicalite-1 membrane acts as an inert separation layer to enhance the PX separation performance. The physiochemical structures and catalytic activities of this H-ZSM-5/Silicalite-1@Pt/Al2O3 core–shell catalyst, especially the isomerization and selective hydrogenation performance, are shown and discussed here.
Section snippets
Core catalyst preparation
α-Al2O3 pellets (Sasol, 2–2.3 mm, 92.2 m2 g−1) were used as support for the preparation of a Pt/Al2O3 core catalyst (named PA) by impregnation with an aqueous solution of chloroplatinic acid hexahydrate (H2PtCl4·6H2O). The impregnated core catalyst was dried at 80 °C for 12 h, and then calcined in air at 500 °C for 4 h. The platinum loading on the core catalyst was 1.0 wt%.
Synthesis of MFI membrane coated core–shell catalyst
In this paper, four different core–shell catalysts were synthesized for comparison. (1) The single Silicalite-1 layer coated Pt/Al2O
Core–shell catalyst characterization
X-ray diffraction (XRD) patterns of the naked PA catalyst, S@PA catalyst, Z@PA catalyst, SZ@PA catalyst and pure MFI zeolite are presented in Fig. 1. For the naked PA catalyst, only the peaks of crystalline α-Al2O3 phase were observed. For the S@PA, Z@PA and SZ@PA catalysts, X-ray diffraction peaks belonging to MFI zeolite appeared at 2θ=7.9, 8.9, and 23.5° by comparing with the pure MFI zeolite, indicating that MFI membranes had successfully crystallized on the PA core catalyst for each
Conclusions
A bi-functional core–shell ZS@PA catalyst was successfully synthesized by coating one inner Silicalite-1 layer and one outer H-ZSM-5 layer onto the surface of Pt/Al2O3 catalyst using a hydrothermal synthesis method. In the hydrogenation of xylene isomers, the ZS@PA catalyst showed much higher hydrogenation conversion and selectivity of PX than those over the monolayer S@PA and Z@PA core–shell catalysts. The PX hydrogenation conversion and 1,4-DMC selectivity on the ZS@PA catalyst of as high as
Acknowledgments
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).
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