Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Hydrogenation of CO2 to Formate using a Simple, Recyclable, and Efficient Heterogeneous Catalyst Gunniya Hariyanandam Gunasekar,†,‡ Kwang-Deog Jung,‡ and Sungho Yoon*,† †
Department of Applied Chemistry, Kookmin University, 861-1 Jeongneung-dong, Seongbuk-gu, Seoul 136-702, Republic of Korea Clean Energy Research Centre, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Republic of Korea
‡
Inorg. Chem. Downloaded from pubs.acs.org by WASHINGTON UNIV on 03/02/19. For personal use only.
S Supporting Information *
ABSTRACT: Today, one of the most imperative targets to realize the conversions of CO2 in industry is the development of practically viable catalytic systems that demonstrate excellent activity, selectivity, and durability. Herein, a simple heterogeneous Ru(III) catalyst is prepared by immobilizing commercially available RuCl3·xH2O onto a bipyridinefunctionalized covalent triazine framework, [bpy-CTF-RuCl3], for the first time. This novel catalyst efficiently hydrogenates CO2 into formate with an unprecedented turnover frequency (38800 h−1) and selectivity. In addition, the catalyst excellently maintains its efficiency over successive runs and produces a maximum final formate concentration of ∼2.1 M in just 2.5 h with a conversion of 12% in regard to CO2 feed. The apparent advantages of air stability, ease of handling, simplicity, the use of a readily available metal precursor, and the outstanding catalytic performance make [bpy-CTF-RuCl3] one of the possible candidates for realizing the large-scale production of formic acid/formate by CO2 hydrogenation.
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INTRODUCTION The development of green and sustainable chemical processes for the production of fuels and value-added products has become an inevitable target to control the global environmental menaces.1,2 In this regard, CO2 is one of the targeted C1 building blocks for transitioning chemical industries into sustainable feedstocks because it is a renewable, relatively nontoxic, abundant, and cheap carbon resource.3−5 Thus, a plethora of CO2 transformations have been identified in the past few decades.6−13 One of the prominent CO2 conversions that has garnered significant attention is the hydrogenation of CO2 into formate/formic acid because it has the capacity to store H2 in the liquid state and provides significant applications as a basic chemical in various industries.14,15 Production of formic acid through CO2 hydrogenation method can eminently reduce CO2 emission about three tons per ton of formic acid in comparison to the traditional CO-based production.6,7 In addition, it has significant economic benefits, as it is a one-step direct synthesis instead of a two-step indirect synthesis in the traditional CO-based production. Hence, the synthesis of formic acid through CO2 hydrogenation overcomes the inefficient and indirect methodology with a greener and simple direct pathway. Hydrogenation of CO2 to formate using homogeneous catalysts has seen considerable progress in the past few decades.16−20 Despite the impressive turnover numbers (TONs) and turnover frequencies (TOFs) achieved by several homogeneous catalysts19,20 (Table S1a), the obstacles in the separation of the formic acid and catalysts from the reaction © XXXX American Chemical Society
media has profoundly shifted research attention toward heterogeneous catalysts,21−24 which are in general wellknown for their obvious advantages of simple catalyst separation and continuous operations. Accordingly, numerous heterogeneous catalysts have been investigated for this transformation.24−42 In particular, Fachinetti et al. strongly emphasized the importance of heterogeneous catalysts for the successful separation of formic acid and catalysts from the reaction media using an Au nanoparticle supported TiO2 catalyst.25 However, the main drawback of their work was the poor catalytic efficiency; a TOF of 12 h−1 and a TON of 855 were observed in 3 days. In light of this work, other metal nanoparticle based catalysts such as Ru, Pd, and Au nanoparticles supported on alumina, reduced graphitic oxide, TiO2, and activated carbon were prepared; however, the TOFs and TONs were not significantly enhanced.27−31 Moreover, the preparation of these nanoparticle-based catalysts requires either toxic and hazardous metal hydrides or H2 at very high temperatures (>300 °C) to reduce the metal’s oxidation state, which are not environmentally benign processes. Alternatively, inspired by the high catalytic efficiency of homogeneous catalysts, the immobilization of homogeneous systems on solid supports has been reported.34−42 Several molecular complexes of Ru and Ir containing phosphine ligands have been immobilized on various conventional lowReceived: November 30, 2018
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DOI: 10.1021/acs.inorgchem.8b03336 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Scheme 1. Representative Synthesis and Structure of [bpy-CTF-RuCl3] (1)
catalysts with minimum or zero metal leaching during catalysis. Therefore, practically viable CTF-based catalysts can be developed if a pertinent molecular entity is designed and tailored. Indeed, efforts toward developing such catalysts for several conversions are steadily increasing. In this regard, a bipyridyl-functionalized covalent triazine framework (bpyCTF), reported by Hug et al.,46,47 is gaining particular interest, as the well-defined N^N sites available in the pore walls of bpyCTF can allow immobilization of various transition-metal complexes and construction of practically durable catalysts for numerous organic transformations.38,41,42,51,52 As one such effort, in this study, we explore a RuCl3 unit bound to bpyCTF via N^N chelation, prepared for the first time, as a practically viable CTF-based catalyst for the hydrogenation of CO2 to formic acid/formate. The scheme for the synthesis of 1 is outlined in Scheme 1. Briefly, a solution of RuCl3·xH2O in methanol was added to a methanolic suspension of bpy-CTF under an N2 atmosphere and stirred at 40 °C for 12 h to obtain 1 as a black solid. The solid is insoluble in water and common organic solvents and highly stable to moisture and air. Inductively coupled plasma optical emission spectrometry (ICP-OES) analysis showed that the Ru content in 1 was 2.1 wt %, which corresponds to ∼4.5% of the bpy units being coordinated with the Ru cation. The even dispersion of the C, N, Ru, and Cl atoms in the energy dispersive spectroscopic (EDS) mapping of 1 indicates uniform metalation throughout the solid block (Figure 1a,b and Figure S1). The 1:3 ratio of Ru to Cl indicates the expected formation of 1 (Table S2). The high-resolution transmission electron microscopy (HR-TEM) image shows the existence of a porous network in 1 (Figure 1c). N 2 adsorption−desorption measurement reveals that 1 has both microporous and mesoporous characters like those of bpyCTF (Figure 1d and Figure S2). Nevertheless, its BET surface area and total pore volume were reduced to 477 m2 g−1 and 0.17 cm3 g−1 from its bpy-CTF values of 684 m2 g−1 and 0.40 cm3 g−1, respectively (Table S3); this decrement is attributed to the partial occupancy of {RuCl3} species on the pore walls of bpy-CTF. Nevertheless, this high surface to volume ratio of 1 would provide an efficient diffusion of reaction species during the catalysis. Powder X-ray diffraction analysis showed no corresponding peaks for the Ru species, which might be due to the low content of Ru in 1 (Figure S3).53 To understand the coordination environment of the Ru ions, X-ray photoelectron spectroscopic measurement was performed (Figure 1e,f and Figure S4 and Table S4). The peak
surface-area supports, such as silica, polystyrene, and polyethylenimine (PEI).35−37 Even though the efficiencies of the reported heterogenized catalysts were better than those of the supported metal catalysts, they were still low in comparison to the homogeneous catalysts. Moreover, the recyclability of the catalysts was not attractive; for example, significant leaching of the metal species was observed in the PEIsupported Ir-phosphine-catalyzed hydrogenation.37 Furthermore, these catalysts require the use of phosphine ligands that are difficult to handle or modify and are expensive. Finally, the final formate concentrations ([HCO2−]) obtained by most of the reported heterogeneous catalysts remained low (Table S1b).24 Therefore, an industrially viable heterogeneous catalyst that produces significant amounts of formate with high efficiency and excellent durability is essential to realize the commercial production of formate via CO2 hydrogenation. In addition, the low market value of the fuels, such as formic acid, that are usually set by the petrochemical industry would impose the production of formic acid at low and affordable cost. Hence, the development of a simple, robust, easy to handle, and cheap catalyst composed of a relatively harmless metal and ligand precursors is the ultimate goal for creating an industrially viable catalyst for CO2 hydrogenation to formate. In this study, we report that the simple RuCl3·xH2O immobilized on a bipyridyl-functionalized covalent triazine framework [bpyCTF-RuCl3] (1) (Scheme 1), prepared for the first time, efficiently catalyzes the hydrogenation of CO2 to formate with an exceptional TOF of 38800 h−1 and produces a high [HCO2−] of 2.05 M in just 2.5 h with excellent recyclability. In comparison to the other reported catalysts, including CTFbased Ir and Ru heterogenized catalysts,34,38,41,42 the simplicity, air stability, good recyclability, and outstanding efficiency of 1 make it a highly attractive and practical catalyst for the hydrogenation of CO2 to formate.
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RESULTS AND DISCUSSION The interest in using CTFs as a catalytic support material is steadily increasing owing to their high stability in basic and acidic solutions under a broad range of temperatures and pressures.43−52 In addition, the large surface area to volume ratio of CTFs could significantly enhance the diffusion of reaction species in comparison to conventional support materials. Most interestingly, the formation of strongly bound coordination complexes on the well-defined coordination sites available in the functionalized CTFs can offer durable B
DOI: 10.1021/acs.inorgchem.8b03336 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Table 1. Hydrogenation of CO2 with Various Amine Basesa
base
T (°C)
P (MPa)b
[HCO2−] (M)
1-butylimidazole pentaethylenehexamine triethanolamine triethylamine (Et3N) tripropylamine tributylamine trihexylamine pyridine 4-dimethylaminopyridine
100 100 100 100 100 100 100 100 100
4 4 4 4 4 4 4 4 4
0.07(3) 0.35(12) 0.17(6) 0.50(11) 0.23(5) 0.03(4) 0.01(3)
