Abstract
The present study focuses on synthesis of SAPO-34 zeolite membrane on the surface of CuO–ZnO–Al2O3 (CZA) catalyst particles to form CZA@SAPO-34 core@shell structured catalyst. In contrast to the traditional support of porous alumina, CZA catalyst particles have a relatively brittle surface, which leads to a big challenge to coat SAPO-34 zeolite membrane on their surface. Moreover, the hydrothermal synthesis of SAPO-34 zeolite membrane is carried out under weakly alkaline condition at 200 °C for hours, which causes part of the surface of CZA to be fragmented. To overcome these shortcomings, the intermediate layer of alumina is introduced to the surface of the CZA particles and acts as a barrier to the high-temperature hydrothermal and alkaline condition. It also takes as a transition to enhance SAPO-34 zeolite seeds adherence to the surface of CZA particles. With the help of an alumina layer, a continuous and dense zeolite membrane has been obtained on the surface of CZA particles. The prepared core@shell structured catalyst has better selectivity in CO hydrogenation for producing light hydrocarbons because of the synergetic effects between the membrane and core catalyst.
Introduction
Concept of core@shell structure has been widely explored as approaches to improved performance of materials in a variety of fields, such as TiO2/C and Si/C core@shell nanocomposites, which can improve cycling behavior of lithium ion batteries [1–3]. Design and fabrication of core@shell structured catalyst by coating a continuous zeolite membrane shell on the surface of individual particles has received much attention due to its potential applications in catalysis [4, 5]. Yang et al. [6] have successfully synthesized HZSM-5 zeolite membrane on the surface of CuO–ZnO–Al2O3 (CZA) particles, named as CZA@HZSM-5, and employed it in the direct synthesis of dimethyl ether (DME) from syngas (CO and H2). The high selectivity of product was obtained due to the acidic property of zeolite membrane exhibited in dehydration of formed methanol from the core. The compactness of zeolite membrane plays a great role in this process.
The in situ hydrothermal synthesis and the secondary growth method are often used to prepare zeolite membrane on porous Al2O3, SiO2 and ceramic. There are some factors which are crucial to form a continuous membrane. Firstly, the components of synthetic solution may have some side effects on the core catalyst. For example, reagent containing NaOH, KOH, Cl–, or Br– cannot be used for HZSM-5 zeolite membrane synthesis since they may cause some damage on the core CZA catalyst during the hydrothermal synthesis process [6]. Secondly, the physical and chemical properties of the supports, here it means the core catalyst, also affect the synthesis of zeolite membrane [7, 8]. It is difficult to synthesize zeolite membrane onto activated carbon (AC) particles because of its hydrophobic and rough surfaces [9].
Previously, our group has developed an intermediate layer modified secondary growth method in synthesis of ZSM-5 membrane on the surface of AC [10–12]. AC particles were firstly coated by an alumina layer before the hydrothermal synthesis. The introduction of intermediate layer has significantly improved the rough surface and increased the functional groups on AC surface. In addition, the layer provides a smooth surface for seeds dispersion and a binder for seeds fixation. The intermediate layer method is an efficient way for zeolite membrane supporting on various kinds of supports, especially the inert ones.
SAPO-34 is typical microporous zeolite which has been used successfully in methanol to olefin (MTO) process. The synthesis of SAPO-34 membrane on stainless-steel or porous alumina tubes can be used in gas separation and catalysis [13–15]. The encapsulation of CZA by continuous and uniform SAPO-34 zeolite membrane (CZA@SAPO-34) is expected to realize the direct transformation of syngas to light hydrocarbons. However, the hydrothermal synthesis of SAPO-34 zeolite membrane is often carried out under weakly alkaline condition at 200 °C for hours. Such wild synthetic conditions of zeolite membrane may influence the catalytic activity of core catalyst. Thus, in this work, we plan to use the intermediate layer as a protector of CZA in the preparation of CZA@SAPO-34. The function of the layer in the synthesis will be analyzed and discussed, the activity of CZA@SAPO-34 in the conversion of syngas to light hydrocarbons as well.
Experimental section
CZA was prepared by the co-precipitation method and the molar ratios of Cu/Zn/Al were 45/45/10. An aqueous solution of sodium carbonate (0.3 M) and aluminum nitrate (0.3 M) was added dropwise into a beaker at 70 °C and the pH value was kept at 7.0. The solution of sodium carbonate and copper and zinc nitrate (0.3 M) was added dropwise into the prepared precursor at 82 °C and the pH value of 8.0. The precipitate was aged for 5 h. After filtration and washing, the precipitated gel was then dried at 120 °C for 4 h and calcined at 450 °C for 6 h. The obtained CZA was then pressed, crushed and sieved to 20–40 meshes.
The original material of the intermediate layer is boehmite sol (0.36 M) which is prepared by the Yoldas process, using aluminum isopropoxide as the precursor. The irregular CZA particles were coated by boehmite sol using the spray-coating method and the coated boehmite sol was converted into alumina after calcination at 450 °C. Prior to coating, the pre-weighted CZA particles were dried at 100 °C in a vacuum oven overnight. The boehmite sol was diluted in ethanol and sprayed on CZA particles uniformly. After two times of spraying, the CZA particles were dried at 60 °C in an oven for 30 min. Then the CZA particles were redispersed and sprayed for two times. This procedure was repeated several times. The thickness of the alumina layer on CZA particles can be controlled by the times of spraying. The CZA with alumina intermediate layer was marked as CZA/Al.
SAPO-34 seeds were synthesized through a typical hydrothermal process as stated elsewhere [13]. The seeded process is similar to the preparation of intermediate layer. SAPO-34 seeds and boehmite sol dispersed in ethanol and followed by ultrasonic treatment for 30 min. Then the mixture was sprayed on CZA/Al particles. The preparation process is shown in Fig. 1. The prepared particles with core-shell structure were denoted as CZA@SAPO-34. The mass content of coated SAPO-34 membrane can be estimated by the mass difference of the particles before and after the hydrothermal synthesis. According to this, the CZA@SAPO-34 particles comprise of 10 wt. % SAPO-34 zeolite membrane.
Schematic flow chart for the preparation of CZA@SAPO-34 catalyst.
X-ray diffraction (XRD) patterns were determined on an X-ray diffractometer (Shimadzu, XRD-6000) using Cu Kα radiation with a working voltage of 40 kV and current of 40 mA. The scanning electron microscopy (SEM) of the view of CZA particles was obtained from Carl Zeiss Ultra55. The morphology of other samples was examined by Hitachi TM-1000. A SIRION-100 SEM and GENESIS4000 energy dispersive spectroscopy (EDS) were used for SEM imaging and EDS analyzing on the cross-section of CZA@SAPO-34 catalyst.
The catalytic tests were performed using a fixed-bed stainless steel reactor embedded with 3 mL catalyst at 3.0 MPa. The reaction temperature was 400 °C. The total flow rate of the syngas (H2/CO = 2) was 150 mL min–1, corresponding to a GSHV of 3000 h–1. The catalyst was reduced at 240 °C by 10 % hydrogen in nitrogen flow (60 mL min–1) for 10 h before reaction. An online GC 9790 gas chromatograph with a TCD detector and a Porapak-Q column was used to determine the products on line. All analysis lines and valves were heated to prevent possible condensation of the products before entering the gas chromatograph.
Results and discussion
Characterization of CZA@SAPO-34 catalyst
CZA is a conventional catalyst usually used for methanol synthesis [16]. In this study, it was selected as the core catalyst. CZA particles are observed uniformly with a pellet size of 0.85–1.70 μm (Fig. 2a). The detailed morphology of CZA shows that CZA particles are composed of nanoparticles (Fig. 2b). The corresponding XRD patterns of CZA confirm that CZA is composed of CuO and ZnO (Fig. 2c), the diffraction peaks of Al2O3 can not be detected due to its high dispersion [17]. The synthesized SAPO-34 zeolite seeds are cubic crystals and uniform in size and shape, with a mean diameter of 1–2 μm (Fig. 3a). There are three strong characteristic diffraction peaks locating at 2θ = 9.5, 16.0 and 20.8° and double peaks at 26.0° and 30.5° for SAPO-34 zeolite seeds, which has a good agreement with standard XRD of SAPO-34 zeolite (PDF card 47-0429).
SEM images (a) and detailed SEM images (b) and XRD patterns (c) of CZA particles.
SEM images (a) and XRD patterns (b) of SAPO-34 zeolite seeds.
Secondary growth of SAPO-34 zeolite membrane on seeded CZA particles was employed to fabricate CZA@SAPO-34 catalyst. If the seeded CZA particles were put into the synthetic solution and took 24 h hydrothermal treatment in order to synthesize SAPO-34 zeolite membrane onto it directly. There are only some scattered SAPO-34 crystallites spreading on the surface of CZA particles, and no continuous zeolite membrane shell, as shown in Fig. 4a. Besides, copper particles attached to the surface of CZA particles are observed, as indicated in Fig. 4b. That is, a small amount of copper is extracted from the core catalyst during hydrothermal synthetic process. It may be due to the interaction between the physically rough surface of CZA particles and the chemical components of synthetic solution of zeolite membrane. The related mechanism would be discussed later.
SEM images (a) and detailed SEM images (b) of CZA particles after direct synthesis of SAPO-34 membrane.
In order to enhance zeolite membrane adherence, modifications like air-oxidation and chemical treatment were employed to improve the surface of supports [9, 18]. Khan et al. [5] prepared ZnO@silicalite-1 core@shell structured particles through modifying the ZnO surface by a layer by layer self-assembly of polyelectrolyte. At the present study, the intermediate layer is introduced to the surface of CZA particles which acts as a transition between the core of CZA catalyst and the shell of zeolite membrane (Fig. 5). The intermediate layer is a γ-alumina layer. CZA particles are covered completely by a homogeneous alumina layer (Fig. 6a). The thickness of the intermediate alumina layer is about 2 μm (Fig. 6b). Mesoporous structure of the alumina layer makes gas or liquid permeate easily [19].
Schematic illustration of the preparation of CZA@SAPO-34 catalyst by intermediate layer method.
SEM images (a) and cross-sectional images (b) of CZA particles with intermediate layer.
The key factor to secondary growth method is depositing and fixing a uniform and continuous layer of seeds on the surface of support [20]. The uniformly dispersed SAPO-34 zeolite seeds can accelerate the interaction between seeds and synthetic solution and promote the formation of SAPO-34 zeolite membrane. However, the challenge is that the millimeter-size CZA particles have uneven and irregular surfaces which are not favorable to the fixation and deposition of seeds. It is difficult to coat a uniform and continuous SAPO-34 zeolite seeds layer onto the surface of CZA particles directly (Fig. 7a). Most of seeds are located on the low-lying area of the surface and most of the surface of CZA particles is bare with no seeds (Fig. 7b). However, CZA particles with intermediate layer can be coated by a uniform and continuous layer of SAPO-34 seeds (Fig. 7c and 7d) due to the surface of CZA with intermediate layer is smoother than that of CZA particles.
SEM images of seeded CZA particles (a) and seeded CZA particles with intermediate layer (c), detailed SEM images of seeded CZA particles (b) and seeded CZA particles with intermediate layer (d).
With the introduction of intermediate layer, a well-intergrowth zeolite membrane shell encapsulated CZA particles is obtained (Fig. 8a). The shell is compact and integrated which indicates that the CZA@SAPO-34 catalyst has been prepared successfully. And there is no crack on it (Fig. 8b). The cross-section morphology and elemental composition of CZA@SAPO-34 core@shell catalyst are given in Fig. 8c and 8d. The related EDS line analysis along the black line in the SEM image exhibits the change of element signals from the zeolite shell to the core part. The Al and P Kα signals increase obviously in the zeolite shell and the Si Kα signals increase slightly for the small amount of Si in SAPO-34 zeolite shell. The extent of Al, P and Si Kα signals also represents that the thickness of the zeolite shell is about 7 μm. The Al Kα signals increase sharply at the junction between zeolite shell and core catalyst, indicating the intermediate layer of γ-Al2O3 with the thickness of about 2 μm. The Cu and Zn Kα signals are close to zero in the zeolite shell and keep a stable level in the core along the direction from the shell to the core, which is consistent with the cross sectional SEM image of CZA@SAPO-34.
SEM images (a), detailed SEM images (b), cross sectional SEM images (c) and EDS line analysis (d) of CZA@SAPO-34 catalyst.
Mechanism of the intermediate layer
Traditionally, SAPO-34 zeolite membrane is synthesized on surface of disk or tubular supports made of stainless steel or porous alumina, which possess smooth surface and a large amount of functional groups [21, 22]. However, there is a big challenge in synthesizing zeolite membrane on irregular particles or the one with fragile surface, such as CZA particles, which are easy to be destroyed during the synthetic process of zeolite shell. The intermediate layer acts as protector of CZA particles against wild synthetic conditions of zeolite membrane on the one hand. On the other hand, it also acts as promoting factor of synthesis of zeolite membrane.
At the beginning of the synthetic process, the solution contains components of H+, Al3+, SiO2, PO43-, TEAOH ((CH3 CH2)4 NOH) and H2 O, and the value of pH is about 6.0, as shown in Fig. 9a. When the temperature rises above 110 °C, a small amount of TEAOH begins to decompose into TEA ((CH3 CH2)3 N) (reaction 1), and furthermore into NH3 (reaction 2) [23]. Correspondingly, the value of pH changes into 11.0. If CZA particles are put into the synthetic solution directly, the reductive NH3 formed may reduce parts of CZA particles into copper metal (reaction 3), as shown in Fig. 9b. On the contrary, as shown in Fig. 9c, if CZA particles with intermediate layer are put into the synthetic solution, quite large amounts of Al–O bonds on the surface prefer to combine with the tetrahedral unit T–O4 of SAPO-34 zeolite forming the initial form of SAPO-34 zeolite membrane shell [24–28], and T-O4 is composed of P–O4, A–O4 and Si–O4. Simultaneously, the formed Al–O–T–O4 may also prevent core CZA reduced by NH3, as shown in Fig. 9c.
Composition changes of synthesis sol with crystallization temperatures (a), the influence of synthesis sol on CZA particles (b) and CZA particles with intermediate layer (c).
The detailed mechanism of chemical bonds of the intermediate layer is shown in Fig. 10. The interaction between the intermediate layer of γ-Al2O3 and the core CZA is mainly Van der Waals force (VDW) for the original material of γ-Al2O3 are boehmite sol [29]. The structural collapse of boehmite sol occurs after calcination, and γ-Al2O3 characteristics appear [19]. The chemical bonds play an important role in the interaction between intermediate layer and SAPO-34 zeolite shell. The surface of intermediate layer contains large amounts of Al–O bonds [26, 28], and SAPO-34 zeolite shell is composed of tetrahedral units, such as Al–O4, P–O4 and Si–O4. The oxygen atoms of Al–O bonds prefer to involve in the formation of tetrahedral units of SAPO-34 zeolite shell, which is similar to the overgrowth of zeolite X onto crystals of zeolite A [30].
Structure models of interaction among the core, intermediate layer, and the shell. VDW denotes Van der Waals force.
Catalytic performance of CZA@SAPO-34 core@shell catalyst
CZA catalyst is typical catalyst for methanol synthesis from syngas. And SAPO-34 zeolite can catalyze methanol into light hydrocarbons. Therefore, CZA@SAPO-34 catalyst may be used in CO hydrogenation to produce hydrocarbons directly. Firstly, syngas passes through the zeolite shell to enter the core catalyst, where the syngas is converted into methanol. Then the methanol formed in the core catalyst is converted into hydrocarbons through contacting with active sites of SAPO-34 zeolite shell during diffusion process. According to the results in Table 1, CZA@SAPO-34 catalyst enjoys higher selectivity of CH4 and C2-C4 and lower selectivity of CH3 OH than the physical mixing catalyst. The better performance of CZA@SAPO-34 is attributed to that the core@shell structure ensure each methanol molecular that diffused from the core catalyst possesses enough opportunities to contact active sites of SAPO-34 zeolite shell. The byproduct CO2 comes from water gas shift reaction. The lower conversion of CO of core@shell structured catalyst may due to slight deactivation of CZA during preparation [7]. In addition, the alumina layer and SAPO-34 zeolite membrane may affect the diffusion of CO, H2 and products which may also lead to the lower CO conversion.
Comparison of the catalytic performance of CZA@SAPO-34 core@shell structured catalyst and physical mixing catalyst.
| Catalyst | Conversion of CO/% | Selectivity of production/% | |||||
|---|---|---|---|---|---|---|---|
| CO2 | CH4 | C2 | C3 | C4 | CH3 OH | ||
| CZA@SAPO-34 | 20.4 | 49.5 | 23.6 | 10.7 | 10.9 | 5.1 | 0.2 |
| CZA + 10 wt. % SAPO-34* | 35.5 | 42.2 | 18.8 | 11.3 | 8.5 | 3.5 | 15.7 |
*CZA + 10 wt. % SAPO-34 represents the physical mixture of CZA catalyst and SAPO-34 zeolite.
Conclusion
After the intermediate layer introduced to the CZA particles, a well-intergrown and continuous zeolite membrane encapsulated CZA particles, it means CZA@SAPO-34 core@shell structured catalyst, are obtained, and the thickness of SAPO-34 zeolite membrane is about 7 μm. The intermediate layer is γ-Al2O3 and its thickness is about 2 μm. CZA particles with intermediate layer enjoy uniformly dispersed SAPO-34 zeolite seeds which facilitate the interaction between seeds and synthetic solution and promote the formation of SAPO-34 zeolite membrane. The Al–O bonds on the surface of the intermediate layer prefer to combine with the tetrahedral units of T-O4 of SAPO-34 zeolite and act as protector of CZA particles against wild synthetic conditions of zeolite membrane. Simultaneously, Al–O bonds of the intermediate layer involve in the formation of tetrahedral units of SAPO-34 zeolite membrane which may promote the adherence of the zeolite membrane. The prepared catalysts show good performance in CO hydrogenation to produce light hydrocarbons due to the core@shell structure.
Article note: Paper based on a presentation at the 9th International Symposium on Novel Materials and their Synthesis (NMS-IX) and the 23rd International Symposium on Fine Chemistry and Functional Polymers (FCFP-XXIII), Shanghai, China, 17–22 October 2013.
Acknowledgments
Financial support from National Natural Science Foundation of China (21376209) and Zhejiang Provincial Natural Science Foundation (LZ13B060004) are gratefully acknowledged.
References
[1] L. J. Fu, H. Liu, H. P. Zhang, C. Li, T. Zhang, Y. P. Wu, R. Holze, H. Q. Wu. Electrochem. Commun.8, 1 (2006).Search in Google Scholar
[2] T. Zhang, L. J. Fu, J. Gao, L. C. Yang, Y. P. Wu, H. Q. Wu. Pure Appl. Chem.78, 1889 (2006).Search in Google Scholar
[3] Q. T. Qu, Y. S. Zhu, X. W. Gao, Y. P. Wu. Adv. Energy Mater.2, 950 (2012).Search in Google Scholar
[4] E. A. Khan, A. Rajendran, Z. Lai. Ind. Eng. Chem. Res.49, 12423 (2010).Search in Google Scholar
[5] E. A. Khan, E. Hu, Z. Lai. Micropor. Mesopor. Mat.118, 210 (2009).Search in Google Scholar
[6] G. Yang, N. Tsubaki, J. Shamoto, Y. Yoneyama, Y. Zhang. J. Am. Chem. Soc.132, 8129 (2010).Search in Google Scholar
[7] R. Brent, S. M. Stevens, O. Terasaki, M. W. Anderson. Cryst. Growth Des.10, 5182 (2010).Search in Google Scholar
[8] R. Brent, P. Cubillas, S. M. Stevens, K. E. Jelfs, A. Umemura, J. T. Gebbie, B. Slater, O. Terasaki, M. A. Holden, M. W. Anderson. J. Am. Chem. Soc.132, 13858 (2010).Search in Google Scholar
[9] J. García-Martínez, D. Cazorla-Amorós, A. Linares-Solano, Y. S. Lin. Micropor. Mesopor. Mat.42, 255 (2001).Search in Google Scholar
[10] F. Q. Chen, C. C. Cai, D. G. Cheng, X. L. Zhan. Ind. Eng. Chem. Res.52, 3653 (2013).Search in Google Scholar
[11] W. Y. Jin, D. G. Cheng, F. Q. Chen, X. L. Zhan. Acta Phys.-Chim. Sin.29, 139 (2013).Search in Google Scholar
[12] F. Q. Chen, W. Y. Jin, D. G. Cheng, X. L. Zhan, Y. S. Lin. J. Chem. Technol. Biotechnol.88, 2133 (2013).Search in Google Scholar
[13] M. Hong, S. Li, J. L. Falconer, R. D. Noble. J. Membrane Sci.307, 277 (2008).Search in Google Scholar
[14] D. Chen, K. Moljord, T. Fuglerud, A. Holmen. Micropor. Mesopor. Mat.29, 191 (1999).Search in Google Scholar
[15] J. C. Poshusta, V. A. Tuan, J. L. Falconer, R. D. Noble. Ind. Eng. Chem. Res.37, 3924 (1998).Search in Google Scholar
[16] M. Behrens, F. Studt, I. Kasatkin, S. Kühl, M. Hävecker, F. Abild-Pedersen, S. Zander, F. Girgsdies, P. Kurr, B. Kniep, M. Tovar, R. W. Fischer, J. K. Nørskov, R. Schlögl. Science336, 893 (2012).10.1126/science.1219831Search in Google Scholar
[17] S-H. Kang, J. W. Bae, H-S. Kim, G. M. Dhar, K-W. Jun. Energy Fuels. 24, 804 (2010).Search in Google Scholar
[18] C. Moreno-Castilla, M. A. Ferro-Garcia, J. P. Joly, I. Bautista-Toledo, F. Carrasco-Marin, J. Rivera-Utrilla. Langmuir11, 4386 (1995).10.1021/la00011a035Search in Google Scholar
[19] X. Krokidis, P. Raybaud, A. Gobichon, B. Rebours, P. Euzen, H. Toulhoat. J. Phys. Chem. B105, 5121 (2001).10.1021/jp0038310Search in Google Scholar
[20] Y. Bouizi, L. Rouleau, V. P. Valtchev. Chem. Mater.18, 4959 (2006).Search in Google Scholar
[21] S. Li, J. L. Falconer, R. D. Noble. Adv. Mater.18, 2601 (2006).Search in Google Scholar
[22] J. C. Poshusta, V. A. Tuan, E. A. Pape, R. D. Noble, J. L. Falconer. AIChE J.46, 779 (2000).Search in Google Scholar
[23] E. Bourgeat-Lami, F. Di Renzo, F. Fajula, P. H. Mutin, T. Des Courieres. J. Phys. Chem.96, 3807 (1992).Search in Google Scholar
[24] D. V. Vu, M. Miyamoto, N. Nishiyama, S. Ichikawa, Y. Egashira, K. Ueyama. Micropor. Mesopor. Mat.115, 106 (2008).Search in Google Scholar
[25] D. Van Vu, M. Miyamoto, N. Nishiyama, Y. Egashira, K. Ueyama. J. Catal.243, 389 (2006).Search in Google Scholar
[26] M. Digne, P. Sautet, P. Raybaud, P. Euzen, H. Toulhoat. J. Catal.226, 54 (2004).Search in Google Scholar
[27] T. Wakihara, S. Yamakita, K. Iezumi, T. Okubo. J. Am. Chem. Soc.125, 12388 (2003).Search in Google Scholar
[28] M. Digne, P. Sautet, P. Raybaud, P. Euzen, H. Toulhoat. J. Catal.211, 1 (2002).Search in Google Scholar
[29] P. Raybaud, M. Digne, R. Iftimie, W. Wellens, P. Euzen, H. Toulhoat. J. Catal.201, 236 (2001).Search in Google Scholar
[30] E. de Vos Burchart, J. C. Jansen, H. van Bekkum. Zeolites9, 432 (1989).10.1016/0144-2449(89)90099-7Search in Google Scholar
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