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A novel silicalite-1 zeolite shell encapsulated iron-based catalyst for controlling synthesis of light alkenes from syngas

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A novel silicalite-1 zeolite shell encapsulated iron-based catalyst for controlling synthesis of light alkenes from syngas
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  Short Communication A novel silicalite-1 zeolite shell encapsulated iron-based catalyst for controllingsynthesis of light alkenes from syngas Nan Jiang a , Guohui Yang a,c , Xiongfu Zhang a, ⁎ , Lei Wang b, ⁎⁎ , Chunyan Shi b , Noritatsu Tsubaki c a State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China b State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China c Department of Applied Chemistry, School of Engineering, University of Toyama, Toyama 930 – 8555, Japan a b s t r a c ta r t i c l e i n f o  Article history: Received 24 October 2010Received in revised form 18 February 2011Accepted 22 February 2011Available online 2 March 2011 Keywords: Zeolite capsule catalystSilicalite-1 shellFTSIron-based catalystLight alkenes A well-designed zeolite capsule catalyst with a Core (Fe/SiO 2 )-Shell (Silicalite-1) structure was successfullyprepared by zeolite seeding and then zeolite shell growing via secondary hydrothermal method. Thecharacterization on this zeolite capsule catalyst indicated that it had a compact, defect-free zeolite shellenwrapping core catalyst tightly. The application of this zeolite capsule catalyst was the direct synthesis of light alkenes from syngas via Fischer – Tropsch synthesis (FTS) reaction. This zeolite capsule catalyst exhibitedexcellent abilitiescompared with thetraditional FTScatalyst, bothonthecontrolled synthesisofthedesirablelight alkenes and the suppressing formation of the undesired long-chain hydrocarbons.© 2011 Elsevier B.V. All rights reserved. 1. Introduction Light alkenes (ethylene, propylene, etc.) are very importantorganic chemical materials and produced mainly by steam crackingof naphtha from petroleum, here the naphtha is derived frompetroleum. With the soaring price of crude oil and dwindlingresources, it is necessary to develop a non-petroleum way for lightalkenes production [1,2]. Fischer – Tropsch synthesis (FTS) reaction isone of the promising processes for hydrocarbons synthesis [3], and the syngas (CO+H 2 ) used for FTS reaction is widely derived frombiomass, natural gas, or coal [4]. Hydrocarbons produced by FTSreaction are sulfur-free, nitrogen-free, and aromatics-free, whichmakes the FTS products inexpensive and environment-friendly. Butthe light alkenes selectivity of FTS reaction is very low as the mainproducts are normal paraf  󿬁 n [5].In order to improve the conventional FTS products composition,that is, increase the light alkenes selectivity, some metals (potassium,manganese, zinc and so on) are usually adopted as the promoters of conventional FTS catalysts to improve their activity and/or alkenesselectivity [6 – 8]. In addition, some metals directly supported onzeoliteascatalysts,alsoexhibitgoodselectivityinlightalkenesviaFTSreaction, but they usually show bad activity [9,10]. Unfortunately,former researchers only focused on the catalyst's composition ratherthanitsstructure.Asweknow,aheterogeneouscatalystwillshowthegood performance only when it has a well-designed structure [11].Recently, the core-shell concept for the combination of two differenttypesofmaterialshasarousedagreatinterestbecauseofitspromisingapplications in catalysis, electronics, sensors and semiconductors[12 – 16]. In this report, different from traditional catalysts, a novelcatalyst with a well-designed Core (Fe/SiO 2 )-Shell (Silicalite-1membrane) structure is successfully prepared via secondary growthmethod.Thezeoliteshellenwrappingcorecatalysttightly,asshowninFig. 1, provides a tailor-made con 󿬁 ned reaction environment, andresultsinthespatiallycon 󿬁 nedeffectandshapeselectivityfunctioninFTSreaction,whichhelpslightalkenesgenerationandsuppressingtheformation of undesired long-chain hydrocarbons. 2. Experimental  2.1. Preparation of Conventional FTS Catalyst Fe/SiO  2 Silica pellets (CARIACT Q-10, Fuji Silysia Chemical Ltd., pellet size:1.81 – 2.36 mm, surface area: 282 m 2 g − 1 ) were used as supports for20wt.%Fe/SiO 2 catalystpreparationbyincipientwetnessimpregnationfrom a ferric nitrate solution (i.e., (Fe(NO 3 ) 3 ·9H 2 O, 98.5%, Kermel).Catalystprecursorswere 󿬁 rstlydriedat378 Kfor2 h,andthencalcinedinairat823 Kfor6 h.The 󿬁 nalsampleswereiron-based20wt.%Fe/SiO 2 catalysts. Catalysis Communications 12 (2011) 951 – 954 ⁎  Corresponding author. Tel./fax: +86 411 84986155. ⁎⁎  Corresponding author. Tel./fax: +86 10 82627080. E-mail addresses:  xfzhang@dlut.edu.cn (X. Zhang), lwang@home.ipe.ac.cn(L. Wang).1566-7367/$  –  see front matter © 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.catcom.2011.02.021 Contents lists available at ScienceDirect Catalysis Communications  journal homepage: www.elsevier.com/locate/catcom   2.2. Tailor-made Zeolite Capsule Catalyst Fe/SiO  2 -S  Zeolite capsule catalyst was prepared by secondary growthmethod. Firstly, Silicalite-1 seeds with a size of about 180 nm wereprepared from a solution with a molar composition of 1.0 tetraethylorthosilicate (TEOS, 28%, Kermel):0.25 tetrapropylammonium hy-droxide (TPAOH, 25%, Aldrich):27 H 2 O, then hydrothermal synthesiswas performed at 448 K for 24 h. The obtained Silicalite-1 seeds wererecovered as 0.3 wt.% suspension in ethanol after repeated washingand centrifugation steps. Secondly, 20 wt.% Fe/SiO 2  catalyst pelletswere seeded using an organic linker, 3-aminopropyltrimethoxysilane(APTS, 98.0%, Jianghan Fine Chemical Co., LTD) according to theprocedure reported in our previous work [17]. The seeded Fe/SiO 2 core catalyst as support for zeolite shell growth on its surface wasmixed with a synthesis solution of 1.0 TEOS:0.25 TPAOH:350 H 2 O in astainless steel autoclave and  󿬁 xed in an oven at 448 K for 24 h. Afterhydrothermal synthesis, the sample named Fe/SiO 2 -S was washed,dried and calcined in air at 923 K for 4 h to remove the TPA + mol-ecules from the zeolite shell.  2.3. Characterization of Catalysts by XRD and SEM  SiO 2  support, Fe/SiO 2  catalyst, Fe/SiO 2 -S zeolite capsule catalystand pure Silicalite-1 zeolite powder were characterized by X-raydiffraction (XRD), respectively. The XRD patterns were collected onPANalytical X'Pert PRO MPD using Cu Ka  radiation operated at 40 kV and 50 mA. Scanning electron microscopy (SEM) analysis on nakedFe/SiO 2 , seeded Fe/SiO 2  and Fe/SiO 2 -S zeolite capsule catalyst,respectively, was performed using JEOL JSM-6700 F scanning electronmicroscope.  2.4. Catalysts Performance Test  FTS reactions were conducted in a continuous  󿬂 ow type  󿬁 xed-bedreactor at 380 °C under 1.0 MPa. The catalysts (naked Fe/SiO 2  orzeolite capsule catalyst Fe/SiO 2 -S) were loaded in the center of thestainless steel reactor (i.d. 14 mm) and then reduced in situ at 623 Kfor 4 h in pure H 2  󿬂 ow, followed by cooling down to roomtemperature before exposure to syngas. When the desired reactiontemperature was reached, pressurized syngas (CO/H 2 =1/2 in molar,1.0 MPa, W Fe/SiO2 /F CO+H2 =28 g h mol − 1 ) was introduced. The pro-ducts from the reactor were analyzed using an online gas chromato-graph (Agilent GC-7890, FID and TCD). Fig. 1.  Schematic drawing of Fe/SiO 2 -S zeolite capsule catalyst in light alkenes directsynthesis via FTS reaction. Fig. 2.  SEM images of (a) the Fe/SiO 2  core catalyst; (b) the seeded Fe/SiO 2  pellet; (c) the surface and (d) cross-section SEM images of zeolite capsule catalyst Fe/SiO 2 -S.952  N. Jiang et al. / Catalysis Communications 12 (2011) 951 – 954  3. Results and Discussion  3.1. Preparation and Characterization of Zeolite Capsule Catalyst  In order to get a defect-free Silicalite-1 zeolite shell on the nakedFe/SiO 2  core catalyst, the secondary synthesis method involvingseeding the naked Fe/SiO 2  and zeolite shell growing was adoptedhere. The seeding step was similar to our previous reported method[17]. After this seeding process, the surface of Fe/SiO 2  was uniformlycovered by one layer of Silicalite-1 seeds, as shown in Fig. 2b. Zeoliteseeds and core catalyst surface linked up tightly by using a covalentbridge of APTS,and the zeolite seedsattached on the corecatalyst hadequal opportunities to absorb nutrition from synthesis solution for itsgrowingtoformanintegratedzeoliteshell[17,18].SurfaceSEMimageof zeolite capsule catalyst Fe/SiO 2 -S in Fig. 2c showed that the corecatalyst was covered well by zeolite shell. Cracks or pinhole could notbe found on this zeolite shell even if it was calcined under harshconditions. The cross-section SEM image of Fe/SiO 2 -S in Fig. 2d alsoindicated that the zeolite shell was in a good condition: core catalystwas perfectly enwrapped by zeolite shell and no damage emerged onthe core catalyst after hydrothermal synthesis.In Fig. 3, the XRD pattern of this zeolite capsule catalyst con 󿬁 rmedthat the zeolite shell was a typical Silicalite-1 structure. SiO 2 , Fe/SiO 2 and pure Silicalite-1 zeolite powder were also characterized asreferences, as shown in Fig. 3. The zeolite peaks of the Fe/SiO 2 -Sbelongedto Silicalite-1 couldbe easilydistinguishedand identi 󿬁 edbycomparing with that of Fe/SiO 2  catalyst and pure Silicalite-1 zeolite,respectively. All these results indicated that this secondary growthmethod was successful and also had great potential application forconstructing other types of zeolite capsule catalysts.  3.2. Light Alkenes Direct Synthesis on Zeolite Capsule Catalyst  Light alkenes direct synthesis from syngas via FTS reaction wasperformed on both the zeolite capsule catalyst Fe/SiO 2 -S and thenaked Fe/SiO 2  catalyst. Table 1 shows the reaction results of thecatalysts activity and products selectivity. The time on stream resultsof CO conversion of these two catalysts are presented in Fig. 4. Thezeolite capsule catalyst Fe/SiO 2 -S gives a lower CO conversion of 21.18%, while the naked Fe/SiO 2  catalyst presents a higher CO con-version of 40.97%. The lower catalyst activity for the capsule catalystshould be attributed to hydrothermal synthesis process. Hydrother-mal synthesis might do some damage to the core catalyst or formsome zeolite crystals covering part of active sites of the core catalyst.In addition, as mentioned by former researchers [19], the decreasedactivity induced by the diffusion-limited reactant CO should also beconsidered. Clearly, the structure of this zeolite capsule catalyst iscompletely different from the conventional Co/SiO 2  FTS catalyst.Therefore, the transport restriction of CO becomes more severeespecially within the zeolite shell region compared with the nakedFe/SiO 2  catalyst, which leads to the higher ratio of H 2 /CO in the corecatalyst region than the srcinal syngas. As a result, a lower COconversion may be obtained. However, the deactivated catalyst ac-tivity canbeeasily overcomeonly byincreasing the metal'scontentof core catalyst or extending the residence time of syngas with zeolitecapsule catalyst.In Table 1, it is clear that the products selectivity of the zeolitecapsule catalyst Fe/SiO 2 -S is different from the naked Fe/SiO 2  catalyst.The capsule catalyst Fe/SiO 2 -S exhibits better ability in the directsynthesisoflightalkenes.Theratioofalkenestoalkanes(n ≤ 4)fortheFe/SiO 2 -S is 0.46 higher than that of the naked Fe/SiO 2  catalyst (0.37).Furthermore, the formation of long-chain hydrocarbons with carbonnumber more than  󿬁 ve has been suppressed obviously, bene 󿬁 ttingfrom the space con 󿬁 nement function. The C 5+ selectivity for the nakedFe/SiO 2  is as high as 44.94%, however, it is only 35.20% for the capsulecatalystFe/SiO 2 -S. Fig.5clearlyshowsthat thezeolitecapsulecatalystFe/SiO 2 -S presents a better ability to the direct synthesis of lightalkenes, and also spends less time into steady state in FTS reactioncomparedwiththenakedFe/SiO 2 catalyst.Inaddition,theauthorshadalso done the long-time continuous FTS reaction using this zeolite 0 20 40 60 Silicalite-1Fe/SiO2-SFe/SiO2    I  n   t  e  n  s   i   t  y 2 Theta SiO2 Fig. 3.  XRD patterns of SiO 2 , Fe/SiO 2 , capsule catalyst Fe/SiO 2 -S, and pure Silicalite-1zeolite. ( ● Fe 2 O 3 ,  ❚ Silicalite-1 zeolite).  Table 1 Reaction performance of conventional FTS catalyst and zeolite capsule catalyst. a Catalyst CO Conversion(mol%)Selectivity (C-mol%) C n= /C n (n ≤ 4) b CH 4  C 2 H 4  C 2 H 6  C 3 H 6  C 3 H 8  C 4 H 8  C 4 H 10  C 5+ Fe/SiO 2  40.97 22.64 5.19 6.52 12.01 3.40 3.02 2.28 44.94 0.37Fe/SiO 2 -S c 21.18 20.59 11.76 9.34 14.87 2.06 3.31 2.87 35.20 0.46 a Reaction conditions: T=653 K, P=1.0 MPa, H 2 /CO=1/2, W Fe/SiO2 /F Syngas =28 g mol h − 1 , 6 h. b The C n= /C n  (n ≤ 4) means the ratio of alkenes to hydrocarbons in the range of C 1 – C 4 . c The  “ S ”  in the name of zeolite capsule catalyst stands for the Silicalite-1 zeolite shell. Fig. 4.  The time on stream results of CO conversion of the bare Fe/SiO 2  catalyst andzeolite capsule catalyst Fe/SiO 2 -S.953 N. Jiang et al. / Catalysis Communications 12 (2011) 951 – 954  capsule catalyst, and the results showed that the CO conversion,methane and CO 2  selectivity were all stable without obviousdeactivation after entering the steady state.All these results should be attributed to the special core-shellstructure of zeolite capsule catalyst. The Silicalite-1 zeolite shell of zeolite capsule catalyst Fe/SiO 2 -S plays multifunction in FTS reaction:adjusting the CO concentration in core catalyst; sieving FTS productswith different size and shape; and prohibiting the re-adsorption of light alkenes. In FTS reaction on this zeolite capsule catalyst, theSilicalite-1 zeolite shell can reduce the diffusion rate of CO obviouslywithin its region. Especially in the core catalyst, the H 2 /CO ratio maybe higher than that of feed gas, which will lead to the decreasing of heavy hydrocarbons selectivity. Furthermore, in the core section, theconcentration of light alkenes/alkanes produced by FTS reaction canbe reduced effectively due to their non-blocked diffusion throughSilicalite-1 zeolite shell. As a result, the formation probability of lighthydrocarbons can be promoted obviously, depressing the generationof heavy hydrocarbons at the same time. The diffusion speed of hydrocarbons depends on their molecular size, shape and composi-tions. It is also affected clearly by the type of zeolite shell. Lightalkenes with smaller molecular diffuse by a faster rate passingthroughzeoliteshell,whichbreaksupthereactionequilibriumincorecatalyst to promote the formation of light hydrocarbons. Here, thespace con 󿬁 nement and separation function of Silicalite-1 zeolite shellcan easily control FTS reaction equilibrium, improving the lightalkenes selectivity and suppressing the formation of heavy hydro-carbons simultaneously. 4. Conclusion Anovelzeolitecapsulecatalystwithaspeci 󿬁 cCore(Fe/SiO 2 )-Shell(Silicalite-1 zeolite membrane) structure was successfully preparedby secondary growth method. Seeding the millimeter-sized corecatalyst by a self-assembly method with an organic linker effectivelyconduce to the formation of an integrated Silicalite-1 zeolite shell.Light alkenes direct synthesis via FTS reaction was used to test thecatalytic performance of this zeolite capsule catalyst. In FTS reaction,this capsule catalyst gave excellent ability for higher light alkenesdirect synthesis than that of the naked Fe/SiO 2  catalyst. Bene 󿬁 tingfromthespecialcon 󿬁 nedstructureandshapeselectivityfunction,thiszeolite capsule catalyst can realize the controlled direct synthesis of light alkenes with lower selectivity on heavy hydrocarbons. Thepreparation and application of zeolite capsule catalyst in this reportalso have widely potential applications, both in zeolite membranesynthesisandmultifunctionalcatalystdesignforconsecutivereaction.  Acknowledgments The authors gratefully acknowledge the  󿬁 nancial support of Innovation Project of Institute of Process Engineering, ChineseAcademy of Sciences (082702), Knowledge Innovation Program of the Chinese Academy of Science, China National 863 Program (GrantNo. 2007AA05Z137), Program for Liaoning Excellent Talents inUniversity (Grant no. LR201008) and Universities Science andResearch Project of Liaoning Province Education Department (Grantno. 2009S019). Reference [1] L. Xu, Q. Wang, D. Liang, X. Wang, L. Lin, W. Cui, Y. Xu, Appl. Catal. A 173 (1998)19 – 25.[2] D. Song, J. Li, J. Mol. Catal. A: Chem. 247 (2006) 206 – 212.[3] H.G. Olive, S. Olive, The Chemistry of the Catalyzed Hydrogenation of CarbonMonoxide, 1984, p. 144.[4] C. Zhang, Y. Yang, B. Teng, T. Li, H. Zheng, H. 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