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Product Selectivity Controlled by Nanoporous Environments in Zeolite Crystals Enveloping Rhodium Nanoparticle Catalysts for CO2 Hydrogenation Chengtao Wang, Erjia Guan, Liang Wang, Xuefeng Chu, Zhiyi Wu, Jian Zhang, Zhiyuan Yang, Yiwen Jiang, Ling Zhang, Xiangju Meng, Bruce C. Gates, and Feng-Shou Xiao J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019
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Journal of the American Chemical Society
Product Selectivity Controlled by Nanoporous Environments in Zeolite Crystals Enveloping Rhodium Nanoparticle Catalysts for CO2 Hydrogenation Chengtao Wang,#,b Erjia Guan,#,c Liang Wang,*,a Xuefeng Chu,e Zhiyi Wu,f Jian Zhang,f Zhiyuan Yang,b Yiwen Jiang,b Ling Zhang,b Xiangju Meng,b Bruce C. Gates,*,d and Feng-Shou Xiao*,a,b,f a
Key Lab of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China. b Key Laboratory of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University, Hangzhou 310028, China. c Department of Materials Science and Engineering, University of California, Davis, CA 95616, United States. d Department of Chemical Engineering, University of California, Davis, CA 95616, United States. e Key Laboratory of Architectural Cold Climate Energy Management, Ministry of Education, Jilin Jianzhu University, Changchun 130118, China. f Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China. KEYWORDS. CO2 hydrogenation; Selectivity; Zeolite; Core-shell structure; Rh nanoparticle ABSTRACT: Supported rhodium nanoparticles (NPs) are well known for catalyzing methanation in CO 2 hydrogenation. Now we have demonstrated that the selectivity in this process can be optimized for CO production by choice of molecular sieve crystals as supports. The NPs are enveloped within the crystals with controlled nanopore environments that allow tuning of the catalytic selectivity to minimize methanation and favor the reverse water-gas shift reaction. Pure silica MFI (S-1)-fixed rhodium NPs exhibited maximized CO selectivity at high CO2 conversions, whereas aluminosilicate MFI zeolite-supported rhodium NPs displayed high methane selectivity under the equivalent conditions. Strong correlations were observed between the nanoporous environment and catalytic selectivity, indicating that S-1 minimizes hydrogen spillover and favors fast desorption of CO to limit deep hydrogenation. Catalysts in this class appear to offer appealing opportunities for tailoring selective supported catalysts for a variety of reactions.
■ INTRODUCTION Supported metal nanoparticle (NP) catalysts continue to draw the attention of catalysis scientists because of their widespread applications in energy conversion and environmental protection.1-3 Nanoparticle sizes and morphologies are crucial to the catalyst performance in numerous processes, in part because they affect the structures of the NP surfaces and thereby their reactivities.4-10 Improved catalysts have emerged from efforts to (i) synthesize ultrasmall metal NPs (nanoclusters) and even isolated single atoms,4-7 (ii) prepare nanocrystallites with selected and unique facets,8-10 and (iii) select the compositions and structures of the supports.11-15 Thus, the focus has been on tuning the NPs and the metal oxide/hydroxide interfaces they reside on. 11,12 Significantly, recent results show that in reactions with liquid-phase reactants, the local environment of the metal NPs strongly influences the catalyst performance, illustrated by a significant enhancement of catalytic properties resulting from adjusting the hydrophobicity of the support.16,17 There is still much to be learned about how the local environment of a metal NP affects catalyst performance, in particular for challenging high-temperature reactions of gas-phase reactants such as in CO2 hydrogenation. There are, we posit,
opportunities for new approaches to tuning the local environments of metal NPs, and we address one of them here. Attention has been focused on increasing selectivity for CO formation relative to methane formation in CO2 hydrogenation,1822 because CO is a widely used feedstock for manufacturing chemicals and fuels in, for example, the Fischer-Tropsch process,2329 and some success has been achieved, exemplified by improvements in CO selectivity resulting from strong metalsupport interactions between the rhodium, iridium, and ruthenium NPs and oxide supports.7,11,12,20 However, it remains a challenge to synthesize NP catalysts that give high CO2 conversions, high CO selectivities, and low methane selectivities. Because the performance of catalysts for CO2 hydrogenation to give CH4 + CO is sensitive to catalyst composition and structure, this reaction is valuable for investigations of reaction mechanism,11-13,27-29 and we have relied on it in the work described here, working from the hypothesis that it provides a good test of our strategy for tuning catalyst selectivity and in prospect offering a basis for progress toward catalyst design. Herein, we report how to optimize the selectivity in CO2 hydrogenation by controlling the local environment around
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rhodium NP catalysts, and our approach is conceptually different from conventional strategies relying on the optimization of metal NP structures and metal–support interactions. Rhodium was chosen because rhodium NPs are active for CO2 hydrogenation,7,11,30,31 and zeolites were chosen as supports because they can be made to envelope the NPs rather than just providing platforms for them (in materials designated as Rh@zeolites).32 The data presented here show that the catalytic selectivity is strongly related to the environment of the NPs. Rhodium NPs enveloped within pure silica analogues of zeolites (Rh@S-1) have been found to exhibit maximized CO selectivities at high CO2 conversions, outperforming conventional metal oxide-supported rhodium NP catalysts. The S-1 support can minimize the hydrogen spillover from the rhodium and accelerate CO desorption that facilitates hydrogenation to CO. More important for potential applications, S1 zeolite-enveloped rhodium NPs are sinter-resistant, even under harsh reaction conditions. ■ RESULTS AND DISCUSSION
b
Scheme 1. Simplified models showing (a) Rh@zeolite and (b) Rh/zeolite.
Synthesis and characterization. In the work reported here, rhodium NPs were fixed within zeolite crystals prepared by a metal-containing zeolite seed-directing crystallization method,32 whereby the NPs were deposited onto zeolite crystals, giving seed crystals around which newly formed zeolite crystals grow to envelope the NP-containing zeolite crystals (Rh@zeolites). We chose MFI zeolites, because their nanoporous environments can be adjusted by changing the zeolite composition (Si:Al ratio). The
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molecular sieves used in this work included the pure silica S-1, hydroxyl modified pure silica (S-1-OH), the hydrogen form of ZSM-5 (HZSM-5, Si:Al ratio = 30, atomic), and the latter partially ion exchanged with potassium (K/Si/Al ratio = 0.3/30/1, atomic). For comparison, the rhodium NPs were also supported on the external surfaces of S-1 crystals, as synthesized by a controlled deposition method (giving Rh/S-1). Scheme 1 represents these catalysts in a simplified way. The catalysts Rh@S-1, Rh@HZSM-5, Rh@S-1-OH, Rh@KZSM-5, and Rh/S-1 were characterized by a set of complementary physical methods to provide compositions and structural information. Inductively coupled plasma (ICP) analyses show that the rhodium contents were similar for the various samples (0.4-0.52 wt% Rh, Table S1 in the Supporting Information, SI). X-ray diffraction (XRD) patterns demonstrate the synthesis of crystalline MFI structures, and the observation that no peaks characteristic of rhodium particles were evident is consistent with the low rhodium loadings and high rhodium dispersions in the matrices of the molecular sieves (Figure S1 in the SI). N2 sorption isotherms characterizing the samples (Figure S2) exhibit typical Langmuir behavior, consistent with uniform nanopore sizes and high internal surface areas (Table S1).33-39 Transmission electron microscopy (TEM) images of the samples represent views of tomogram-sectioned samples to provide evidence of the rhodium NPs within the molecular sieve matrices. The images of Rh@S-1, Rh@HZSM-5, Rh@S-1-OH, and Rh@KZSM-5 (Figures S3-S6, respectively) clearly show the NPs distributed in the matrices, confirming that they are fixed within crystalline regions.32 In contrast, Rh/S-1 is characterized by rhodium NPs mostly localized on the edges of the molecular sieve, showing that the NPs are on the external surfaces of the crystals (Figure S7). The size distributions of the rhodium NPs in the various samples, determined from the images, are summarized in Table S1, and most of these NPs have comparable average diameters, with distributions in the range of 4.2-5.0 nm (Figure S8). The Rh@zeolite samples are characterized by lower rhodium dispersions than the supported rhodium nanoparticle catalysts (e.g.,
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600
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Figure 1. Catalytic performance of (a) Rh@HZSM-5, (b) Rh@S-1, (c) Rh@S-1-OH, and (d) Rh@KZSM-5 in CO2 hydrogenation. Reaction conditions: 0.5 g of catalyst; 1 MPa feed gas pressure, with molar composition of CO2/H2/Ar = 1/3/1; feed flow rate 30 mL/min. (e) Dependence of CO selectivity on CO2 conversion for various catalysts. (f) Dependence of CO and methane formation rates on reaction temperatures for reaction catalyzed by Rh@HZSM-5 and by Rh@S-1. Error bounds: CO2 conversion, ±2%; CO selectivity, ±1%; methane selectivity, ±1%.
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Rh/S-1, Table S2), a result that also suggests the fixation of the rhodium NPs within the zeolite crystals—with the zeolite framework partially blocking the surface rhodium sites. The scanning electron microscopy (SEM) images of Rh@S-1, Rh@HZSM-5, Rh/S-1, Rh@S-1-OH, and Rh@KZSM-5 (Figure S9) show that the crystal diameters were mainly distributed in the range of 200-400 nm. Catalytic CO2 hydrogenation. The performance of each of the catalysts for CO2 hydrogenation was determined with a oncethrough flow reactor fed at steady state with a H2 + CO2 mixture having a molar ratio of 3 at a pressure of 1 MPa (Figure 1). Each catalyst was active for CO2 hydrogenation, giving CO2 conversions that increased as expected with increasing temperature at a given space velocity. The catalysts are distinguished by their selectivities. For example, methane was the predominant product with Rh@HZSM5 catalysts (Figure 1a), but markedly different selectivities were observed with S-1 as the support, with CO as the predominant product (Figure 1b). More important, the Rh@S-1 catalyst is characterized by excellent CO selectivity at high conversions of CO2 (Table S3). For example, Rh@S-1 operating at 500 °C at a CO2 conversion of 51.6% is characterized by a CO selectivity of 79.8%, whereas Rh@HZSM-5 at a CO2 conversion of 68.2% is characterized by a methane selectivity of 98.2%. The Rh@S-1 catalyst was improved by introducing defects (silanol groups, Si-OH) into the porous structure of the S-1 to give Rh@S-1-OH (Figure S10),40 as shown by the selectivity data (Figure 1c, Table S3). For example, CO was formed as the predominant product (selectivities in the range of 55.5–69.3%) at low CO2 conversions (
