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A Mesoporous Aluminosilicate Nanoparticle-Supported Nickel-Boron Composite for the Catalytic Reduction of Nitroarenes

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A Mesoporous Aluminosilicate Nanoparticle-Supported Nickel-Boron Composite for the Catalytic Reduction of Nitroarenes Lawrence Berkeley National Laboratory Recent Work Title A Mesoporous Aluminosilicate Nanoparticle-Supported Nickel-Boron Composite for the Catalytic Reduction of Nitroarenes Permalink https://escholarship.org/uc/item/02m7d916 Journal ACS Applied Nano Materials, 2(3) ISSN 2574-0970 Authors Hauser, JL Amberchan, G Tso, M et al. Publication Date 2019-03-22 DOI 10.1021/acsanm.8b02351 Peer reviewed eScholarship.org Powered by the California Digital Library University of California Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX www.acsanm.org A Mesoporous Aluminosilicate Nanoparticle-Supported Nickel− Boron Composite for the Catalytic Reduction of Nitroarenes † † † † ‡ Jesse L. Hauser, Gabriella Amberchan, Monique Tso, Ryan Manley, Karen Bustillo, § † † † Jason Cooper, Josh H. Golden, Bakthan Singaram,*, and Scott R. J. Oliver*, † Department of Chemistry and Biochemistry, University of California, Santa Cruz, Santa Cruz, California 95064, United States ‡ § National Center for Electron Microscopy, Molecular Foundry, and Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States *S Supporting Information ABSTRACT: An amorphous nickel and boron composite (NBC) was synthesized from nickel chloride hexahydrate · (NiCl2 6H2O) and sodium borohydride (NaBH4) in absolute ethanol, both in bulk and supported on mesoporous alumi- nosilicate nanoparticles (MASN). Comparatively, NBC-MASN demonstrated better catalytic activity for the selective reduction of the nitro group on a variety of polysubstituted nitroarenes, · using hydrazine hydrate (N2H4 H2O) as the reducing agent at 25 °C. Reuse and regeneration of NBC-MASN for the reduction of p-nitrotoluene to p-toluidine were studied with NaBH4 acting as a regeneration agent. Good catalytic activity was sustained through nine reuse cycles when equimolar NaBH4 was present in situ with · − N2H4 H2O (99% 67% isolated aniline yield). The structure and composition of NBC and NBC-MASN were examined by electron microscopy, energy dispersive X-ray spectroscopy (EDS), powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and inductively coupled plasma optical emission spectroscopy (ICP-OES). The results for NBC-MASN show that a thin (<10 nm) amorphous coating forms over the MASN surface, consisting of a mixture of metallic and oxidized nickel (9 wt % Ni), and various species of boron (atomic ratio of Ni:B = 2). For unsupported NBC, metallic nickel nanocrystals (1−3 nm) were ° discovered imbedded within an amorphous matrix of a similar composition. Upon calcination at 550 CinaN2 atmosphere, partial conversion of unsupported NBC to crystalline Ni3B was observed, whereas only crystalline metallic Ni was observed for NBC-MASN. To explain these differences, further evidence is given to suggest the presence of residual boron hydrides encapsulated in the bulk unsupported NBC, suggesting Ni3B was an artifact of processing rather than an initial product. KEYWORDS: nitroarene, catalysis, nickel, boron, hydrazine, borohydride, mesoporous silica 0 ■ INTRODUCTION such as NiCl2 and CoBr2, to their corresponding metallic (M ) phase, while undergoing hydrolysis to form borate species and See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. The reduction of nitroarenes to aromatic amines is of general 10 interest for industrial as well as biological applications.1 liberate hydrogen gas (eq 2). Downloaded via LAWRENCE BERKELEY NATL LABORATORY on February 25, 2019 at 18:04:16 (UTC). Traditionally, the synthesis of aromatic amines from their −+++2 →+++ −+ corresponding nitro compounds is performed at high temper- BH4 2M 2H222 O 2M BO 4H 2H atures and pressures, in the presence of hydrogen gas and a (2) metallic catalyst such as Pt, Pd, Fe, Sn, or the well-known 2−4 The resulting black precipitates could be best described as Raney nickel (eq 1). nickel- or cobalt-based borate composites, both of which perform excellently in the catalytic hydrogenation of a variety Arylnitro reduction by Raney Ni to arylamine products:5 of organic substrates containing reducible functional groups. − 11−13 Furthermore, the hydrolytic degradation of BH4 and nickel-based composites is particularly successful at selectively reducing nitroarene compounds to their corresponding aniline 14,15 Alternatively, amorphous heterogeneous catalysts formed derivatives. However, the reusability of such catalysts is from reacting transition metal salts with sodium borohydride (NaBH ) in protic solvents have a long history of success in a Received: December 27, 2018 4 − wide variety of reduction reaction schemes.6 9 In aqueous Accepted: February 8, 2019 media, sodium borohydride will generally reduce metal salts, Published: February 8, 2019 © XXXX American Chemical Society A DOI: 10.1021/acsanm.8b02351 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX ACS Applied Nano Materials Article Scheme 1. Schematic Illustration of (a) the Synthesis and (b) Use of NBC-MASN Catalyst for the Reduction of Nitroarenes with Hydrazine Hydrate typically limited due to particle sintering, agglomeration, and therefore increases catalytic activity through electron enrich- surface oxidation. It has recently been shown that supporting ment of the metal surface, while also sacrificially protecting the − these composites on inert scaffolds (such as titania, silica gel, metal from oxidation.31 38 However, a review of the previously organic polymers, or mesoporous silica nanoparticles) can published XPS data reveals a certain amount of ambiguity in enhance activity and reusability, compared to their bulk assigning both the B 1s and Ni or Co 2p binding energy shifts powders, by increasing catalyst surface area and stabilizing to that of the corresponding borides as well as debate about − against particle agglomeration.16 18 These catalysis schemes the direction of electron donation between the metal and have also benefited from the use of safer and more stable boron, in both the amorphous and crystalline compounds.39 chemical sources of hydrogen, such as hydrazine hydrate With careful use of electron microscopy and elemental · 14,16,19,20 (N2H4 H2O). analysis, some authors have shown that bulk NBC and CBC The motivation for our study was to investigate the safe and powders are nanocomposites comprising single nanometer ffi · economically e cient reduction of nitroarenes, using N2H4 sized crystalline metal particles imbedded in an amorphous H O and a reusable amorphous nickel boron composite matrix containing boron oxides (transition or alkali metal 2 − (NBC) catalyst supported on mesoporous aluminosilicate borates, (poly)borates, etc.).21 25,40 Although many of these nanoparticles (MASN) (Scheme 1). In the course of this work, authors invoke the presence of borides, they presume that the fundamental questions arose concerning the difference in main function of the amorphous matrix is to physically prevent physical character between supported and unsupported sintering and rapid oxidation of the catalytically active metal versions of NBC. These questions are discussed in light of particles during synthesis, thereby preserving the large catalytic considerable uncertainty in the literature as to the actual surface area claimed to be responsible for high activity. − composition of such amorphous composites.21 26 While the exact nature of the metal−boron interaction in In protic solvents at ambient pressure and temperature, most these amorphous materials remains uncertain, it is known that salts of the first row transition metals to the left of copper crystalline borides of transition metals are routinely formed generate precipitates containing various boron species when under much more energetically demanding conditions than reacted with excess NaBH4, whereas copper and noble metal those of protic solvent synthesis at near-ambient conditions. salts with more favorable reduction potentials tend to give pure Traditional nickel borides are synthesized in solid state metallic phases.27 In the case of nickel (and similarly for reactions such as laser ablation or melt quenching of elemental cobalt), these precipitates have historically been termed Ni and B41 or in nonprotic solvent systems at elevated ∼ ° 27 amorphous nickel borides (Ni3B, Ni2B, etc.), a designation temperatures ( 90 C). Given that borohydrides are known 38 initially based on a combination of elemental analysis and to decompose readily in protic solvents to produce H2(g) and powder X-ray diffraction studies of the crystalline structures various borates (B(OH) −,BO−,BO , polyborates, etc.), and − 4 2 2 3 which evolved upon heating in an inert atmosphere.28 30 given that this process is autocatalyzed in situ by the very same These amorphous precipitates are now known to be more precipitates formed during the reaction of borohydrides with structurally complex, possessing a variety of possible certain transition metal salts, particularly Ni2+ and Co2+,12 it compositions and morphologies, depending on the choice of seems self-evident that protic solvent synthesis routes cannot synthetic parameters (solvent system, pH, temperature, mixing produce true transition metal borides. It is much more rate, and the presence of oxygen in solution), the ratio of plausible that such reactions primarily produce oxides of borohydride to nickel reacted, and the choice of nickel salt boron, which may then interact strongly with metallic precursor.27,31 precipitates to form amorphous transition metal and
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