Spatial confinement effects of zeolite-based micro-capsule catalyst on tuned Fischer-Tropsch synthesis product distribution

Accepted Manuscript Spatial confinement effects of zeolite-based micro-capsule catalyst on tuned Fischer-Tropsch synthesis product distribution

Wenze Li, Yingluo He, Hangjie Li, Dongming Shen, Chuang Xing, Ruiqin Yang PII: DOI: Reference:

S1566-7367(17)30201-7 doi: 10.1016/j.catcom.2017.05.008 CATCOM 5042

To appear in:

Catalysis Communications

Received date: Revised date: Accepted date:

21 January 2017 19 April 2017 7 May 2017

Please cite this article as: Wenze Li, Yingluo He, Hangjie Li, Dongming Shen, Chuang Xing, Ruiqin Yang , Spatial confinement effects of zeolite-based micro-capsule catalyst on tuned Fischer-Tropsch synthesis product distribution. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Catcom(2017), doi: 10.1016/j.catcom.2017.05.008

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ACCEPTED MANUSCRIPT Spatial confinement effects of zeolite-based micro-capsule catalyst on tuned Fischer-Tropsch synthesis product distribution Wenze Li a, Yingluo He c , Hangjie Li b, Dongming Shen b, Chuang Xing b,*, Ruiqin Yang a

b,

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School of Applied Chemistry, Shenyang University of Chemical Technology, Shenyang 110142, China b

School of Biological and Chemical Engineering, Zhejiang University of Science and Technology, Hangzhou 310023, China

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Department of Applied Chemistry, Graduate School of Engineering, University of Toyama, Gofuku 3190, Toyama 9308555, Japan

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*E-mail address: [email protected] (C. Xing)

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**E-mail address: [email protected] (R. Yang)

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ABSTRACT Selectivity control of target product(s) is an enormous challenging in cascade catalytic or tandem system. We demonstrated a facile and fast route to directly construct zeolite-based micro-capsule catalyst with mesoporous

nano-silica shell. The as-prepared out-

Co/HY@SiO2 catalyst has achieved an excellent performance of CH4 and C 5-11 selectivity with 16.4 and 65.0 % respectively, as well as low CO2 selectivity of 2.1 %. As expected,

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the mesoporous shell of micro-capsule catalyst could offer the spatial confinement effects, especial the stay zone of intermedium products for long chain hydrocarbons formation,

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which were hydrocracked by acidic HY core, consequently tuning the products selectivity.

Keywords: Mesoporous shell, Micro-capsule catalyst, Encapsulation, Fischer-Tropsch

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synthesis, Confinement effects

ACCEPTED MANUSCRIPT 1. Introduction Cascade catalytic or tandem system is an energy-efficient, economic and time-saving strategy, realizing by one-pot process for the joint action of two or more reactions [1]. CO hydrogenation is a conventional strategy for clean liquid fuels synthesis. At present, the transformation of syngas (CO+H2) over Fischer-Tropsch (FT) synthesis process into hydrocarbons, such as olefin, liquefied petroleum gas, gasoline, diesel and wax, has attracted great attention because it provides an economical and environmentally friendly

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alternative route instead of the dwindling depletion [2]. However, tunable selectivity of FT products is still challenging since it often contains C1-50 hydrocarbons, oxy-compounds and

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water. Capsule catalysts with efficient and sustainable characterization, could be proposed as reasonable candidate catalyst on oriented synthesis of target products [3].

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As well known, the middle hydrocarbons (C5-11) can be directly used as gasoline additive due to high octane value [4]. Up to now, according to the positional relation

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between metal particles and zeolite, there are two types of the direct C5-11 hydrocarbons synthesis catalysts from syngas over capsule catalysts: accumulational zeolite shell

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encaged metal@support core [5] and single crystal zeolite encapsulated metal nanoparticles [6-8]. A series of millimeter-level capsule catalysts with accumulational zeolite as shell and Co/Al2 O3 as core respectively, for improving tremendously the middle

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hydrocarbons selectivity as well as suppressing the formation of heavy hydrocarbons have already been studied by former work [9,10]. The completed encapsulation cobalt clusters

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inside the channels of mesoporous HZSM-5 zeolite catalyst was synthesized using mesoporous carbon as a hard template and evaluated C5-11 selectivity [11]. However, the prepared process of catalyst was multi-steps and complicated, especially a dissatisfied FT

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activity. The relative low catalytic activity depended upon the strong metal-support interaction effect during high calcination temperature to remove zeolite template. Single crystal zeolite completed e ncapsulation of metallic catalyst, as a novel micrometer-level reactor has a more interesting research to orient synthesis reaction [12]. More recent, Xiao group designed a generalized strategy for furfural hydrogenation by achieving the completed encapsulation of Pd nanoparticles inside of silicalite-1 zeolite [13]. A series of silica shell covered zeolite catalysts have been developed by Zhao et al., through a facile sol-gel strategy to construct the mesoporous silica core-shell structure [1417]. Uniform mesoporous silica shells coated zeolite was synthesized by using cetyltrimethylammonium bromide (CTAB) as template and tetraethyl-orthosilicate (TEOS) as silica precursor. In this work, the Co-based zeolite-based micro-capsule catalyst with mesoporous

ACCEPTED MANUSCRIPT silica shell as micrometer-level reactor has been successfully constructed and applied in middle hydrocarbons synthesis from syngas. Owning the poor diffusion of zeolite micropores, the direct connection of microporous zeolite and mesoporous silica shell could be realized to solve the defect, offering rise to a highly opened pore structure. Spatial confinement effects of zeolite-based micro-capsule catalyst play a vital role in selective conversion of syngas to middle hydrocarbons.

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2. Experimental 2.1 Catalyst preparation

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Co/HY catalyst was prepared by incipient wetness (IW) impregnation of the HY support (SiO2/Al2 O3=5.2, Catalyst Plant of Nankai University) with the aqueous solution of

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cobalt nitrate. The impregnated solid was subsequently dried at 120 oC and calcined at 400 oC for 2 h.

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HY or Co/HY zeolite was added a mixture containing CTAB (0.7 g), ethanol (40 g) and ammonia (10 g, 25 wt%). Subsequently, TEOS (1.2 g) was added dropwise and stirred for

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4 h at 25 oC. The composite sample was collected by centrifugation and washed with water and ethanol, then dried and calcined at 550 oC for 2 h, denoted as HY@SiO2 and inCo/HY@SiO2, respectively. By using above IW method, out-Co/HY@SiO2 catalyst was

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also prepared by impregnation on HY@SiO2 support. The final Co content was 10 wt% for all the catalysts.

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2.2 Characterization

The N2 sorption experiments were measured by an automatic gas adsorption system (Autosorb-1, Quantachrome). Transmission electron microscopy (TEM) images were

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observed on JEOL JEM-2100 UHR instrument with an acceleration voltage of 200 kV. 2.3. Catalyst test

FT synthesis reaction was performed at a continuous following fixed-bed reactor with the inner diameter of 6 mm. Prior to reaction, the as-prepared catalyst was in-situ reduced at 400 oC for 4 h in pure H2 flow (40 mL/min). The reaction conditions were 260°C, 1.0 MPa, H2/CO=2/1, W/F=4 gh/mol and 10wt%Co loading. The gaseous and light liquid products were detected by two on-line gas chromatographs (GC-TCD, GC-FID) with TDX01 and aluminium oxide columns, respectively. The heavy hydrocarbons were captured by an ice trap between reactor and back pressure regulator, which were analyzed using an offline gas chromatograph (Shimadzu GC-2014, FID).

ACCEPTED MANUSCRIPT 3. Results and discussion 3.1. Structure properties of the catalysts N2 adsorption-desorption isotherms are shown in Fig. 1. All the samples exhibit steep uptakes in a low-pressure region (p/p0<0.1), representing a characteristic microporous framework as well as the non-destructive zeolite. The HY and Co/HY display an abrupt capillary condensation at the high relative pressures (P/P 0>0.90), which is attribution to the accumulation pores of zeolite grains. Whereas, the H1 hysteresis loops of typical IV

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isotherm with a capillary condensation at P/P 0 between 0.4 and 0.8 appear clearly on micro-capsule catalyst (in- and out-Co/HY@SiO2), which is a typical adsorption

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phenomenon for mesoporous materials. The BET surface areas and pore volumes of asprepared catalysts are shown in Table 1. The Co/HY shows a specific surface area of 603

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m2/g contributed from micropores surface area. The total pore volume is calculated to be 0.46 cm3/g, mainly deriving from the present macropores between the aggregated zeolite

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grains, in agreement with its N2 sorption isotherm. After the construction of silica shell, the decreased BET surface area is observed. Moreover, the BET surface area and pore

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volume of in-Co/HY@SiO2 are larger than out-Co/HY@SiO2 one, because the mesoporous silica channels of HY@SiO2 are blocked after the impregnation of Co particles. These phenomena indicate the combinative sorption behavior of micropores and

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mesopores for out-Co/HY@SiO2 and in-Co/HY@SiO2 capsule catalysts.

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Table 1 Summary of the textural properties of different samples. S(m2/g)a V(cm3/g) Sample Total Microporeb Totalc Microporeb HY 620 539 0.49 0.27 Co-HY 603 504 0.46 0.25 in-Co-HY@SiO2 459 380 0.32 0.20 out-Co-HY@SiO2 409 354 0.27 0.17 a BET surface area. b Microporous surface area and pore volume evaluated by the t-plot method. c Total pore volume estimated from the adsorbed amount at P/P0=0.995. The TEM images of in-Co-HY@SiO2 and out-Co-HY@SiO2 catalysts are presented in Fig. 2. After the construction of mesoporous silica shell on HY substrate by using CTAB as a template, two catalysts show very uniform core-shell structure, and its shell with an almost average thickness about 10 nm. The in-Co-HY@SiO2 was obtained from silica shell coated Co/HY catalyst, meaning that the Co partilces were completely encapsulated inside silica shell (Fig. 2a). For out-Co-HY@SiO2 , as displayed in Fig. 2b, Co particles are

ACCEPTED MANUSCRIPT well-dispersity on silica shell. The distinct capsule structure will tune controllably product distribution in FT synthesis.

3.2. FT synthesis performances Capsule catalyst with core-shell structure, owning multi-functional effects, could be proposed as reasonable candidate catalyst on oriented synthesis of target products. Cobased HY@SiO2 micro-capsule catalyst with nano-shell, which not only affacts catalytic

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activity but also to tune product distribution. Table 2 lists reation pefromances of CO hydrogenation over catalysts with differently positional relation between metal particles

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and zeolite. Co/HY exhibits the lowest CO conversion with 20.5 % among all the catalysts. The CH4 selectivity is high to 25.3 % due to the strong acid of HY zeolite. The CO

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conversion with 63.8 % obvious enhances after coating with silica shell on SiO2@HY (inCo/HY@SiO2). The selectivity of the shorter-chain hydrocarbons, including CH4 and C 2-4

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hydrocarbons is the highest over the completedly encapsulated Co particles inside silica shell than other catalysts. Fig. 3 schematically summarizes the comparison of catalytic

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activity and product distribution. As illustrated in-Co/HY@SiO2, the enhanced CO conversion could be attributed to fact that the syngas easily enriches inside the intracavity of silica shell and contacts more cobalt clusters with more stay time [18]. But, the

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intermedium products of FT on zeolite substrate are also in favor of hydrogenation and hydrocracking, further resulting in the formation of the abundant shorter-chain

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hydrocarbons (C1-5) [19]. Moreover, a CO2 selectivity of 27.7 % is also obtained over inCo/HY@SiO2 catalyst due to the confinement of silica shell. It is known that CO2 is mainly derived form water-gas shift (WGS) reaction during FTS, where water remain aggravates

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WGS reaction under shell coverage. A volcano-like ditribution of FT products is obtained when Co particles were impregnated on silica shell (out-Co/HY@SiO2). The C 5-11 selectivity reaches up to 65.0 %, whereas CH4 selective decrease to 16.4 %. As illustrated in Fig. 3, syngas first diffuses and contacts the active sites, subsequently the formed intermedium products further diffuses and stays through the mesopores of silica shell, where the intracavity of mesoporous silica is critical to product heavy hydrocarbons as stay zone. As well known, FT synthesis is a representative monomer polymerization process, which the stay time of monomer on catalyst channels promotes heavy hydrocarbons formation. Ultimately, the C12+ hydrocarbons are hydrocracked over the acidic HY core to generate C 5-11 hydrocarbons, realizing the tuned product distribution by the micro-capsule catalyst.

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Table 2 Catalytic performance of the different catalysts. Sel. / % Catalysts CO Conv. / % CO2 CH4 C2-4 C 5-11 C12+ Co/HY 20.5 1.8 25.3 16.3 58.2 0.2 in-Co/HY@SiO2 63.8 27.7 34.6 44.0 21.2 0.1 out-Co/HY@SiO2 39.7 2.1 16.4 18.4 65.0 0.2 Reduction conditions: 0.1 MPa, pure H2, 60 mL/min, 400 oC, 4h; Reaction conditions: 10wt% Co loading, T=260 oC; P=1.0 MPa; H2/CO=2/1, catalyst weight=0.3 g, W/F=4 gh/mol.

4. Conclusions

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A FT micro-capsule catalyst is a coupling of a mesoporous silica shell and HY core

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that provides (1) intermedium products to polymerization zone and (2) long chain hydrocarbons to hydrocracking zone. The as-prepared out-Co/SiO2@HY catalyst has achieved an excellent performance of CH4 and C 5-11 selectivity with 16.4 and 65.0 %

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respectively, as well as low CO2 selectivity of 2.1 %. The mesoporous shell of microcapsule catalyst core could offer a high diffusion rate, especial the stay zone of

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intermedium products for long chain hydrocarbons formation, where were hydrocracked by HY core with acidic sites, consequently tuning the products selectivity. The spatial confinement effects of the silica shell acted as an important role for the improvement of C5selectivity. The newly developed hierarchical shell for constructed micro-capsule catalyst

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will play a vital application in selective conversion of syngas, which open the way for other

Acknowledgements

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combinations of hydrogenation catalysts and zeolite-based catalysts.

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The authors acknowledge the financial s upport from National Natural Science Foundation of China (21528302) and Zhejiang Province Natural Science Foundation (LQ15B060004). Science research project of Liaoning Provincial Department of Education (LZ2016007) and Natural Science Foundation of Liaoni ng Province (201602587) are also gratefully acknowledged.

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ACCEPTED MANUSCRIPT HY Co/HY in-Co/HY@SiO2

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out-Co/HY@SiO2

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Volume adsorption / cm3g-1

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P/P0

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Fig. 1. N2 sorption isotherms of the catalysts.

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Fig. 2. TEM images of samples: (a) in-Co/HY@SiO2, (b) out-Co/HY@SiO2.

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Fig. 3. Schematic diagram showing product distribution from syngas over as-prepared

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catalysts.

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Graphical abstract

ACCEPTED MANUSCRIPT Highlights Micro-capsule catalyst with ∼10 nm silica shell was designed successfully.

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The mesoporous silica shell coated completely single crystal zeolite.

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Facilely realizing the direct middle hydrocarbons synthesis via FT synthsis reaction.

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The tuned product distribution was offered by spatial confinement effects.

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