Bimetallic Catalysts Supported on Mesoporous Carbon for Hydrogenation of Succinic Acid to Butyrolactone and 1,4-Butanediol

Abstract

1,4-Butanediol (BDO) is a versatile chemical that can be used in a wide range of industrial applications. BDO has been used as an organic solvent and a fine chemical for production of adhesives, fibers, and polyurethanes. Recently, BDO has received much attention as an important raw material for thermoplastic polymers such as polybutylene succinate (PBS) and polybutylene terephthalate (PBT). As the consumption of these polymers is growing faster in electronics and automobile industries, in particular, the global demand for BDO is expected to increase rapidly. BDO has been produced through several conventional routes

hydrogenation of maleic anhydride, isomerization of propylene oxide, and acetoxylation of butadiene. These processes rely on petrochemical feedstocks derived from fossil fuel. Due to the limited amount of fossil fuel, however, current research trend for BDO production moves toward the utilization of renewable energy sources such as biomass. In this respect, conversion of succinic acid to BDO by catalytic hydrogenation has attracted recent attraction as a promising process, because succinic acid can be obtained from bio-refinery process. Hydrogenation of succinic acid to BDO occurs via two-step hydrogenation reactions as shown in Fig. 1. Succinic acid is first transformed into -butyrolactone (GBL) by hydrogenation, and then BDO or tetrahydrofuran (THF) is formed through consecutive hydrogenation of GBL. For the catalytic conversion of succinic acid, various noble metal catalysts such as Pd, Pt, Rh, Ru, and Re have been investigated. Among these catalysts, rhenium has been considered as the most efficient monometallic catalyst for the selective formation of BDO. However, several studies have shown that rhenium alone was not sufficient to obtain high yield for BDO. In an attempt to improve BDO production by hydrogenation of succinic acid, Re-based bimetallic catalysts, including Re-Pt/C, Re-Pd/C, Re-Pd/TiO2, and Re-Ru/C, have been investigated. Nonetheless, the researches have rarely elucidated the effect of interaction between rhenium and other metal on the selective formation of BDO from succinic acid. This is because combination of rhenium and noble metal causes difficulty in structural and chemical analyses. For example, rhenium can be miscible with noble metals such as Pt, Pd, and Ru to form a solid-solution due to their similar atomic sizes and surface energies, which complicates characterization. Moreover, since rhenium does not cause dissociative hydrogen chemisorption at low temperature, either modified hydrogen chemisorption or CO chemisorption method is essential for determining metal dispersion of Re-based catalyst. In addition, only a few studies have focused on the modification of carbon support for hydrogenation of succinic acid to BDO. It has been reported that addition of non-metal elements such as sulfur, boron, and phosphorous into carbon affects physicochemical properties of carbon support [24-30]. In particular, boron can be easily substituted for carbon atom during carbon growth process, which significantly changes structural and electronic properties of carbon lattice. Thus, it is expected that boron-containing carbon effectively interacts with active metal species to enhance reducibility and hydrogen adsorption behavior of supported metal catalyst. To utilize transition metals for production of BDO, we have studied for a bifunctional metal catalyst supported on mesoporous carbon, which contained rhenium and copper. Interestingly, this catalyst showed a considerable catalytic activity in the hydrogenation of succinic acid to GBL and BDO via dimethyl succinate (DMS) in the presence of methanol. Although the catalyst contains transition metal, yield for GBL and BDO over the catalyst was comparable to that over noble metal-based catalysts, due to its bifunctional catalysis. Thus, a systematic investigation on the catalyst based on a combination of transition metal and noble metal for hydrogenation of succinic acid to GBL and BDO would be worthwhile. In this work, Re-based metal catalysts were supported on mesoporous carbon, and they were applied to the liquid-phase hydrogenation of succinic acid to BDO. First of all, A series of Re-Ru bimetallic catalysts supported on mesoporous carbon (denoted as (0.6-x)Re-xRu/MC) were prepared by a single-step surfactant-templating method and a subsequent incipient wetness impregnation method with a variation of ruthenium loading (x, mol%), and they were applied to the liquid-phase hydrogenation of succinic acid to 1,4-butanediol (BDO). The effect of metal content on the catalytic activities and physicochemical properties of (0.6-x)Re-xRu/MC catalysts was investigated. It was found that a Re-Ru miscible phase was formed in the catalysts during the reduction process, and it was responsible for strong interaction between rhenium and ruthenium. It was also revealed that reducibility, metal dispersion, and oxidation state of (0.6-x)Re-xRu/MC catalysts were affected by Re:Ru molar ratio. In particular, the oxidation state was closely related to the hydrogen adsorption behavior of the catalysts. The amount of weak hydrogen-binding sites increased with increasing the ratios of metallic rhenium (Re0) and ruthenium (Ru0) with respect to total metallic species in the reduced (0.6-x)Re-xRu/MC catalysts. Catalytic performance in the hydrogenation of succinic acid to BDO over (0.6-x)Re-xRu/MC showed a volcano-shaped trend with respect to Re:Ru molar ratio. This result was well correlated with the amount of weak hydrogen-binding sites of the catalysts. Among the catalysts tested, 0.3Re-0.3Ru/MC with the largest amount of weak hydrogen-binding sites showed the best catalytic performance in the BDO production by hydrogenation of succinic acid. A series of Re-Ru bimetallic catalysts supported on mesoporous boron-modified carbon (denoted as Re-Ru/xBMC, x = B/C molar ratio) were prepared by a single-step surfactant-templating method and a subsequent incipient wetness impregnation method, and they were used for liquid-phase hydrogenation of succinic acid to 1,4-butandiol (BDO). The effect of boron addition on the catalytic activities and physicochemical properties of Re-Ru/xBMC catalysts was investigated. It was found that the addition of boron into carbon support affected surface area, metal dispersion, and reducibility of rhenium and ruthenium species in the Re-Ru/xBMC catalysts. It was also observed that boron species in carbon framework existed in several different phases such as substituted boron, partial oxidized boron, and boron oxide. In particular, the amount of substituted boron species was closely related to the hydrogen adsorption behavior of Re-Ru/xBMC catalysts. The amount of weak hydrogen-binding sites increased with increasing the amount of substituted boron species of the catalysts. Yield for BDO in the hydrogenation of succinic acid showed a volcano-shaped trend with respect to B/C molar ratio. This result was in good agreement with the amount of weak hydrogen-binding sites of the catalysts. It was revealed that TOFBDO increased with increasing the amount of weak hydrogen-binding sites of Re-Ru/xBMC catalysts. Among the catalysts, Re-Ru/0.04BMC with the largest amount of weak hydrogen-binding sites served as an efficient catalyst in the selective formation of BDO by hydrogenation of succinic acid. A mesoporous rhenium-copper-carbon composite catalyst (Re-Cu-MC) was prepared by a facile single-step surfactant-templating method. For comparison, a series of mesoporous carbon-supported catalysts (Re/Cu-MC, Cu/Re-MC, and Re-Cu/MC) were also prepared. The catalysts were applied to the liquid-phase hydrogenation of succinic acid to -butyrolactone (GBL) and 1,4-butanediol (BDO). The effect of preparation method on the physicochemical properties and catalytic activities of the catalysts was investigated. It was found that the catalysts based on metal-carbon composite (Re-Cu-MC, Re/Cu-MC, and Cu/Re-MC) were favorable for enhancing textural properties and metal-support interaction of the catalysts. Surface atomic ratios of metal species (Re/C and Cu/C) on the catalyst surface increased with increasing metal-support interaction. Yield for GBL and BDO increased with decreasing average metal particle size of the catalysts. It was revealed that metal particle size of the catalysts served as a key factor determining the catalytic activity and stability in the reaction. Among the catalysts tested, Re-Cu-MC catalyst with the smallest average metal particle size showed the best catalytic performance in the hydrogenation of succinic acid to GBL and BDO. In summary, various Re-based metal catalysts supported on mesoporous carbon were prepared, and they were applied to the liquid-phase hydrogenation of succinic acid to BDO. The catalysts were characterized by nitrogen adsorption-desorption, TPR, XRD, CO chemisorption, TEM, STEM-EDX mapping, Raman, XPS, and H2-TPD analyses.