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Amine/Hydrido Bifunctional Nanoporous Silica with Small Metal Nanoparticles Made Onsite: Efficient Dehydrogenation Catalyst Yang Zhu,†,‡ Takahiro Nakanishi,†,‡ Kazuyoshi Kanamori,‡ Kazuki Nakanishi,*,‡ Shun Ichii,§ Kohji Iwaida,§ Yu Masui,§ Toshiyuki Kamei,§ Toyoshi Shimada,*,§ Akihito Kumamoto,⊥ Yumi H. Ikuhara,∥ Mina Jeon,# George Hasegawa,¶ Masamoto Tafu,□ Chang Won Yoon,*,#,○ and Tewodros Asefa*,△,▲ ‡
Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto, Japan Department of Chemical Engineering, Nara National College of Technology, 22-Yata-cho, Yamatokoriyama, Nara, Japan ⊥ Institute of Engineering Innovation, School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo, Japan ∥ Nanostructures Research Laboratory, Japan Fine Ceramics Center, 2-4-1, Mutsuno, Atsuta-ku, Nagoya, Japan # Fuel Cell Research Center, Korea Institute of Science and Technology, Hwarangno 14-gil 5, Seongbuk-gu, Seoul 02792, Republic of Korea ¶ The Institute of Scientific and Industrial Research, Osaka University, Mihogaoka, Ibaraki 567-0047, Japan □ Toyama National College of Technology, 13, Hongo-cho, Toyama, 939-8630, Japan ○ KHU-KIST Department of Converging Science and Technology, Kyunghee University, Seoul, Republic of Korea △ Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, 610 Taylor Road, Piscataway, New Jersey 08854, United States ▲ Department of Chemical and Biochemical Engineering, Rutgers, The State University of New Jersey, 98 Brett Road, Piscataway, New Jersey 08854, United States §
S Supporting Information *
ABSTRACT: Multifunctional catalysts are of great interest in catalysis because their multiple types of catalytic or functional groups can cooperatively promote catalytic transformations better than their constituents do individually. Herein we report a new synthetic route involving the surface functionalization of nanoporous silica with a rationally designed and synthesized dihydrosilane (3-aminopropylmethylsilane) that leads to the introduction of catalytically active grafted organoamine as well as single metal atoms and ultrasmall Pd or Ag-doped Pd nanoparticles via on-site reduction of metal ions. The resulting nanomaterials serve as highly effective bifunctional dehydrogenative catalysts for generation of H2 from formic acid.
KEYWORDS: amino/hydrido bifunctional, nanoporous silica, onsite synthesis, metal nanoparticles, dehydrogenation catalyst, formic acid dehydrogenation, bifunctional catalyst, supported nanoparticles
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complex processes, including physiologically relevant catalytic transformations by using their different functional groups together. Scientists try to mimic many of such tricks applied by natural systems in order to produce various synthetic analogues, or efficient catalysts. One notable synthetic method to do so involves the cografting of multiple types of functional groups or molecular catalytic moieties onto high surface area support materials (e.g., nano/mesoporous silica) to generate bi- or
he design and synthesis of new and efficient catalytically active materials can directly or indirectly contribute to the solutions of our limited renewable energy resources. In this endeavor, multifunctionality, a concept involving the assembly of different active functional groups within one material, is of immense significance. This is because cooperative activities by two or more types of catalytically active groups can improve the pathways by which various reactants transform to products (e.g., with high yield, desired selectivity), which are sometimes hard to attain by a single type of catalytic group.1 This approach is applied by many natural systems evolved over millions of years. For example, enzymes effectively perform various © XXXX American Chemical Society
Received: October 12, 2016 Accepted: December 19, 2016 Published: December 19, 2016 A
DOI: 10.1021/acsami.6b12972 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces multifunctional, cooperative heterogeneous catalytic systems.2 Besides their possible high catalytic activity and selectivity, the resulting catalysts are more advantageous than their molecular counterparts because they are much easier to separate or recover from reaction mixtures, and to recycle and reuse.3 However, so far, there have been only few reports on the synthesis of materials with geometrically controlled, well aligned bifunctional groups.4 Moreover, only limited choices of combinations of functional groups/catalytic sites are available. For instance, while the close proximity of incompatible yet cooperative acidic and basic groups can mimic the structures of natural amino acids, such as L-proline, which are highly effective homogeneous catalysts for reactions such as aldol and nitroaldol condensations,5 putting such incompatible groups together on solid materials by conventional chemical surface modification methods is unsurprisingly tricky. To incorporate such types of incompatible but cooperative groups into various nanostructured materials (e.g., silica), researchers have had to use or often consider various delicate, intricate, or relatively lengthy synthetic routes, such as solvent-mediated grafting,6,7 protection−deprotection chemistry,8−10 and multistep reactions.11 Meanwhile, although the reduction power of molecular hydrosilanes in homogeneous media is known for decades, and also utilized in various chemical syntheses,12 until recently their solid counterparts, i.e., surface-immobilized hydrido groups, have not been capitalized as much in materials synthesis. Such surface-immobilized hydrosilanes have, for instance, the ability to reduce metal ions to metal nanoparticles (NPs).13 Our recent work also showed that hydrido groups incorporated in hydrogen silsesquioxane monolithic materials could reduce metallic ions and give surface-immobilized monometallic or multimetallic (bi-, tri-, and even tetra-metallic) alloy NPs.14,15 Moreover, we recently demonstrated a surface modification synthetic strategy for silica materials using a reaction between the surface silanol groups of the silica materials and different monohydrosilanes, catalyzed by tris(pentafluorophenyl)borane (B(C6F5)3).16 Herein we show, for the first time, the synthesis of a novel, rationally designed dihydrosilane (3-aminopropylmethylsilane or APMS) and its grafting on silica surfaces producing two different groups coplaced with geometrically controlled interspacing (Scheme 1). This is demonstrated using SBA-15 mesoporous silica, generating a bifunctional nanoporous material, dubbed NH2-H-SBA-15. It is worth noting here that the two groups (hydrido and amino groups) are incompatible,
especially in the presence of water (physisorbed or otherwise), and are thus normally difficult to coplace on a single surface, let alone control their interspacing. Furthermore, thanks to its residual surface-grafted hydrido groups, the resulting amine/ hydrido bifunctional nanoporous silica can reduce metallic ions onsite and form silica-supported single metal atoms as well as small metal (Ag, Au, Pd, Ag-doped Pd) NPs. This way, bifunctional mesoporous silicas containing amine and small metallic species that can cooperatively, and thus efficiently, catalyze the dehydrogenation of formic acid (FA) at ambient temperature without the presence of external base are obtained. The successful functionalization of the surfaces of SBA-15 with the dihydrosilane precursor (APMS) is confirmed by FTIR spectroscopy and CHN elemental analysis. Figure 1a shows the FT-IR spectra of SBA-15 and NH2-H-SBA-15 (or the APMS-modified SBA-15) samples. Peaks corresponding to C− H vibration at ca. 3000 cm−1 and Si-CH3 vibration at ca. 1,430 cm−1 are observed in the spectrum of the latter material, suggesting the presence of methyl and propyl groups in it. A peak at ca. 2166 cm−1, which can be attributed to Si−H groups, is also observed in the same spectrum indicating the presence of residual Si−H groups on the surface of the material. These Si−H groups are present because the two Si−H bonds in the dihydrosilane precursor are closer to each other than the typical distance between most, two neighboring silanol groups on SBA-15; so, after the reaction between one of the silane’s hydrido groups and a surface silanol takes place, further reaction between the second hydride and a neighboring silanol is unlikely to occur. Consequently, a large number of residual hydrido groups remain intact after the surface modification reaction. The density of 3-aminopropyl groups in the SBA-15 is found to be 1.89 mmol g−1, as determined by CHN elemental analysis. This amount is higher than the density of grafted groups typically achievable with common monohydrosilane precursors under similar reaction conditions, which is typically 1.15 mmol g−1.16 This indicates the higher reactivity of the dihydrosilane precursor, or one of the advantages of the surface modification method reported herein. N2 adsorption−desorption isotherms (Figure 1b) show a drastic decrease in the microporosity and total pore volume, and a decrease in the surface area (from 671 to 320 m2 g−1), in SBA-15 material after its surface modification with APMS. These results suggest that the dihydrosilane molecules are grafted inside the mesopores of the material, most likely in the micropores of the material, especially given the fact that SBA-15 possesses a substantial amount of microporosity and its mesostructure has not been compromised, as seen by electron microscopy (vide infra). The residual Si−H groups in NH2-H-SBA-15 are then used as reducing agents for metallic ions to produce noble metal NPs on-site on the walls of the material. The size of the particles is controlled by the limited number of Si−H groups, the spatial hindrance rendered to the SBA-15 by its cylindrical mesopores, and the coordination effect exerted by the 3-aminopropyl groups on the pore walls of the material.17,18 Figure S1 shows the TEM and HRTEM images of Ag-NH2-SBA-15 and AuNH2-SBA-15 (the SBA-15 materials containing metallic NPs and amino groups), which are synthesized by reducing the corresponding metal ions with the hydrido groups. The formation of a large number of Ag and Au NPs inside the cylindrical mesopores of the material is evident in Figure S1a, c. The NPs inside the mesopores are found to be
