Carbon supported Ru-Ni bimetallic catalysts for the enhanced one-pot conversion of cellulose to sorbitol

Highlights

• Ru-based catalysts showed to be highly efficient for the conversion of cellulose.

• A synergic effect between Ru and Ni was observed.

• STEM-XEDS reveals a contact at nano-level between Ru and Ni particles.

• Mix-milling of cellulose and Ru-Ni catalysts greatly increased the activity.

Abstract

Ru and Ni mono- and bimetallic catalysts were prepared by impregnation of two different supports (activated carbon and carbon nanotubes) and were characterized by TPR, TEM and N₂ adsorption at −196 °C. The prepared Ru mono- and bimetallic catalysts were highly efficient for the hydrolytic hydrogenation of cellulose to sorbitol. Regardless of the support used, when nickel was incorporated into the supported ruthenium catalyst a promoting effect was observed, with an increase in both conversion and selectivity to sorbitol, which was explained by the interaction between both metals. Yields of sorbitol around 50–60% were achieved after 5 h of reaction when using Ru-Ni/AC and Ru-Ni/CNT as catalysts. Moreover, a yield of sorbitol over 70% could be reached in just 1 h of reaction if the Ru-Ni catalysts were ball-milled together with cellulose.

Introduction

Currently, the use of renewable feedstocks for chemicals, fuels and energy has achieved an increasing attention due to the depletion of fossil fuels and the problems arising from global warning [1], [2], [3]. Lignocellulosic biomass is considered a promising alternative source to fossil fuels for the sustainable production of fuels and chemicals [4], [5]. Cellulose is the most abundant form of biomass and its hydrolytic hydrogenation into sugar alcohols (such as sorbitol) is gaining increasing interest as an attractive alternative in its valorisation [6], [7]. Sorbitol has been identified as one of the twelve most important building blocks derived from biomass resources [8] as can be used in food industry, pharmaceuticals, cosmetics, textiles and industrial applications, or as reactant in processes that yield other high added-value products [9]. The conversion of cellulose into sugar alcohols consists of two consecutive steps: hydrolysis of cellulose to glucose via cellooligosaccharides [10], [11], [12] and hydrogenation of glucose to sugar alcohols over metals [13], [14]. Recently, it was shown that cellulose can be directly converted into sugar alcohols by the use of heterogeneous catalysts [15], [16], [17], instead of the traditional use of liquid acids required to perform the hydrolysis of cellulose to glucose [18].

In 2006, Fukuoka and Dhepe reported the first hydrolytic hydrogenation of cellulose using only solid catalysts and obtained a yield of sorbitol of 25% using Pt/γ-Al₂O₃ [19]. Being known as heat- and water-tolerant supports, carbons such as activated carbon (AC) and carbon nanotubes (CNT) have been extensively studied as catalyst supports in the conversion of cellulose [20]. Luo et al. reported 30% yield of sorbitol using Ru/AC [21] and Deng et al. attained 69% using Ru/CNT and H₃PO₄ pre-treated cellulose [22]. Geboers et al. obtained a yield of sugar alcohols of 85% from ball-milled cellulose by the mixture of Ru/AC and heteropoly acid H₄SiW₁₂O₄₀ [23]. Though this is one of the best sugar alcohols yield ever attained, it involved the use of acids, thus being a non-environmentally friendly approach. Kobayashi et al. presented an innovative method for an efficient hydrolysis of cellulose to glucose by ball-milling the cellulose together with the catalyst [10], denoted as mix-milling, and then the method was also applied for the hydrolytic hydrogenation of cellulose by Komanoya et al. using a ruthenium catalyst [24]. Later, we also reported a yield of sorbitol of 69% by mix-milling Ru/AC with cellulose as pre-treatment, without the use of any acids [25]. More recently, Ni catalysts have been also studied [20], [26]. Microcrystalline cellulose was converted to sorbitol over Ni₂P/AC [27] and Ni₁₂P₅/AC [28] with yields of 48 and 62%, respectively, but these catalysts showed to be not durable in hot water due to phosphorous leaching and Ni sintering. In contrast, Ni supported on carbon nanofibers (CNF) was stable for 24 h at 210 °C and gave sorbitol (64%) with 93% conversion of cellulose [29]. Bimetallic catalysts containing Ni have also shown to be effective for the hydrolytic hydrogenation of cellulose to sorbitol and a 58% yield of sugar alcohols was attained using Ir-Ni supported on mesoporous carbon (MC) [30].

An optimization of the catalyst can be done by the combination of active sites (metals), supports and promoters. Bimetallic catalysts are showing up as a promising option since the interaction between metals can modify the properties of the catalyst, which can result in an improvement of the catalytic activity, modification of the selectivity to the desired product and increase in the catalyst stability [31], [32], [33], [34]. Accordingly, new bimetallic catalysts need to be optimized ensuring high reaction rates and high selectivities to the desired products. Herein, we report an efficient one-pot conversion of cellulose to sorbitol using Ru-Ni catalysts supported on activated carbon and carbon nanotubes.