Highly efficient synthesis of γ-valerolactone by catalytic conversion of biomass-derived levulinate esters over support-free mesoporous Ni

Highlights

• A simple and green method was used to prepare support-free mesoporous Ni catalysts.

• Ni catalysts were efficient for the transfer hydrogenation of levulinates to γ-valerolactone.

• An excellent γ-valerolactone yield of 99.1% was obtained over Ni-1.0 catalyst.

• Metallic Ni and co-existing surface NiO contribute to the high catalytic activity.

• Ni-1.0 catalyst preserved above 95% of conversion activity after ten consecutive cycles.

Abstract

A series of mesoporous nickel catalysts with different molar ratios of citric acid and nickel nitrate were synthesized via a simple two-step method by solid-state grinding and in-situ reduction; these catalysts were further employed for the catalytic transfer hydrogenation of levulinate esters to γ-valerolactone (GVL) with 2-propanol as the hydrogen donor. The effects of C₆H₈O₇/Ni molar ratios, catalyst dosage, reaction temperature and time on the catalytic performance were investigated. An excellent GVL selectivity of 99.1% and an ethyl levulinate (EL) conversion of 100% could be achieved at 180 °C in 6 h over Ni-1.0 catalyst with a 1.0:1 M ratio of C₆H₈O₇/Ni. The characterization results showed that the Ni-1.0 catalyst was composed of mesoporous metallic Ni bulk and a surface Ni–NiO composite, and the acid sites of NiO played a synergistic catalytic role in producing GVL from EL. The superior mesoporous structure and large surface area of Ni-1.0 increased the atomic utilization rate of nickel metallic active centers. Moreover, Ni-1.0 catalyst could be easily separated and reused ten times, with an EL conversion above 95% remaining in each recycle. This work provids a highly efficient and environmentally friendly catalytic procedure for the conversion of biomass into GVL, a fuel additive compound.

Introduction

The depletion of traditional fossil resources and environmental degradation have spurred researchers worldwide to find sustainable alternatives for petroleum-based products [1,2]. In recent years, biomass-derived γ-valerolactone (GVL) is widely used as an additive for fuels, such as gasoline, diesel and bio-oil [[3], [4], [5]], as a green solvent in biomass conversion [6], and as an intermediate for the production of fine chemicals [7]. In particular, GVL can be employed as a precursor to produce gasoline and diesel fuels, such as C8–C18 alkanes [8,9] and 2-methylte-trahydrofuran [10]. Due to its outstanding physicochemical properties (high stability, low toxicity and low volatility) and wide application in biofuel products, it is considered to be one of the most promising renewable alternative fuel compounds.

Extensive research has been conducted to investigate the synthesis of GVL from biomass [[11], [12], [13]]. GVL can be synthesized from raw materials such as cellulose or hemicellulose as well as their degradation products through a multistep catalysis (shown in Scheme 1). A direct catalytic hydrogenation-cyclization of levulinic acid (LA) or its esters, which is produced from carbohydrates, is a critical step in the production of GVL from biomass [14,15]. Unlike LA, levulinate esters possess relatively lower boiling points, as well as easy recovery and acid-free characteristics. Moreover, relatively higher yields of alkyl levulinate can be achieved from acid-catalyzed alcoholysis of carbohydrates [16]. In this regard, the catalytic conversion of levulinate esters to GVL seems to be more attractive [17]. Currently, noble metals, such as Ru [18], Pd [19], Pt [20], and non-noble metals, such as Ni [21,22], Co [23], Cu [24], have been reported as catalysts for the hydrogenation of LA or esters to GVL. Noble metals exhibited excellent catalytic activity, especially Ru-based catalysts [25]. However, the high cost and scarcity of noble metals restrict their practical application in large-scale production. Therefore, the research focus has shifted to develop highly efficient non-noble metal catalysts.

Low-cost Ni-based catalysts showed high activity in this reaction and had attracted widespread attention. Mohan et al. found that 100% LA conversion and 92.2% GVL selectivity were achieved in 1 h using 30 wt% Ni/H-ZSM-5 at 250 °C and 1 bar H₂ [21]. Hengst et al. reported 92% LA conversion with 100% GVL selectivity after 4 h over 15 wt% Ni/Al₂O₃ at 200 °C and 50 bar H₂ [26]. In addition, Kumar et al. examined Ni (20 wt%) supported on SiO₂, γ-Al₂O₃ and ZrO₂ catalysts for the hydrogenation of aqueous LA or esters to GVL at 270 °C and ambient pressure [27]. Sakakibara et al. reported Ni/ZrO₂ catalyst for the catalytic transfer hydrogenation (CTH) of methyl levulinate to GVL using 2-propanol as a hydrogen donor, and a GVL yield of 94% was achieved in 20 h at 100 °C [15]. Although these Ni-based catalysts can efficiently produce GVL from LA or its esters, the reaction may often require stringent reaction conditions, such as a high reaction temperature, long reaction time and/or high hydrogen pressure. It is worth noting that all the Ni catalysts mentioned above are supported catalysts. Support-free metal catalysts are rarely used in the synthesis of GVL. This is because bulk metal easily crystallizes, which results in a nonporous surface with a small surface area [28]; thus, the utilization rate of the metal is reduced.

Recently, Singh et al. reported that an unsupported mesoporous Ni/NiO catalyst synthesized using an organosilane as a mesopore template showed high catalytic performance in the hydrogenation of glucose to sorbitol [29]. Generally, the drawback of the chemical reduction method for catalyst preparation is that the method consumes considerable hydrogen and energy, and the use of solvents cannot be avoided. Yao Fu’s group used Raney Ni as a catalyst for the CTH of EL to GVL at room temperature, and a GVL yield of up to 99% was obtained [22]. However, Raney Ni is easily deactivated and must be kept in a solvent. Moreover, the abovementioned shortcomings of bulk metals remain as problems, and the cumbersome and high-energy-consuming preparation processes of Raney-type nickel catalysts are expected to be improved [30]. Therefore, it is highly desirable to develop a simple, low-cost and environmentally friendly strategy for the preparation of support-free metal catalysts. Venkatesha & Ramesh have synthesized copper-based catalysts by an in situ reduction method, and the catalysts showed good activities in the hydrogenation of furfural to furfuryl alcohol [31]. This method is simple and pollution-free,with no solvent use, no waste liquid discharge, and low energy consumption; additionally, a metal self-reduction process is realized during calcination to shorten the preparation time. It will be economically viable for sustainable development and suitable for large-scale production of support-free metal catalysts.

Inspired by the above findings, in this work, a simple solid-state grinding and in-situ reduction method was first used for support-free mesoporous Ni catalysts preparation without the use of templates or solvents. The prepared Ni catalysts were employed for the CTH of levulinate esters to GVL using alcohols as hydrogen donors. The catalysts (Ni-1.0) showed very high catalytic activity (100% EL conversion with 99.1% GVL selectivity) and stability in the above reaction, which made the subsequent separation procedure easy and low energy cost. The effect of the molar ratios of citric acid and nickel nitrate (C₆H₈O₇/Ni) on the catalytic activities of the Ni catalysts was investigated, and a detailed characterization of the catalysts was conducted to show the relationship between the catalyst structure and catalytic activity. Additionally, a variety of experimental parameters as well as recycling experiments were systematically studied. This work provides a low-cost, highly efficient and environmentally friendly catalytic procedure for the large-scale production of GVL, a fuel additive compound. Moreover, support-free metal catalysts with large specific surface areas and high activities have potential application prospects in other hydrogenation reactions for the synthesis of valuable fuel compounds.