Levulinic acid hydrogenolysis on Al₂O₃-based Ni-Cu bimetallic catalysts

Abstract

Inexpensive γ-alumina-based nickel-copper bimetallic catalysts were studied for the hydrogenolysis of levulinic acid, a key platform molecule for biomass conversion to biofuels and other valued chemicals, into γ-valerolactone as a first step towards the production of 2-methyltetrahydrofurane. The activities of both monometallic and bimetallic catalysts were tested. Their textural and chemical characteristics were determined by nitrogen physisorption, elemental analysis, temperature- programmed ammonia desorption, and temperature-programmed reduction. The monometallic nickel catalyst showed high activity but the highest by-product production and significant amounts of carbon deposited on the catalyst surface. The copper monometallic catalyst showed the lowest activity but the lowest carbon deposition. The incorporation of the two metals generated a bimetallic catalyst that displayed a similar activity to that of the Ni monometallic catalyst and significantly low by-product and carbon contents, indicating the occurrence of important synergetic effects. The influence of the preparation method was also examined by studying impregnated- and sol-gel-derived bimetallic catalysts. A strong dependency on the preparation procedure and calcination temperature was observed. The highest activity per metal atom was achieved using the sol-gel-derived catalyst that was calcined at 450 °C. High reaction rates were achieved; the total levulinic acid conversion was obtained in less than 2 h of reaction time, yielding up to 96% γ-valerolactone, at operating temperature and pressure of 250 °C and 6.5 MPa hydrogen, respectively.

Introduction

Owing to diminishing oil reserves, increasing energy demands from emerging economies, and increasing global warming concerns, researchers have focused their attention on biomass-derived chemicals to replace non-renewable energy resources [1, 2, 3, 4, 5]. The suitability of biomass-derived products, such as liquid fuels, is being established by currently used first-generation biofuels (bioethanol and biodiesel) that are easily produced from edible biomass [5, 6]. However, this leads to expensive feedstocks and competition with the food market. In contrast, second-generation biofuels are derived from non-edible, inexpensive, and abundant lignocellulosic biomass. However, their production is more challenging than that of first-generation biofuels [6].

Levulinic acid (LA) is a key chemical towards the development of these new biofuels because it can be produced in good yields at both laboratory scale [7, 8] and semi-industrial scale by acid hydrolysis of lignocellulosic materials and urban residues [6, 9, 10]. In 2004, the US Department of Energy classified LA as a “top ten” bio-based product obtained from biorefinery carbohydrates in accordance with characteristics such as potential as a platform chemical and building block, petrochemical replacement capability, and potential production scale-up [10].

LA can be converted into suitable and sustainable fuels such as γ-valerolactone (GVL) and 2-methyltetrahydrofuran (MTHF), as shown in Scheme 1. GVL possesses many of the properties of an ideal liquid both for energy and chemical product manufacture, as reported [11], and shows similar behavior to ethanol when added to gasoline. MTHF is a well- known fuel additive that can be added to gasoline in proportions of up to 70 vol% [12] in the absence of engine modification or used as part of new fuel formulations for flexible fuel engines, as reported by Paul [13] and the US Department of Energy [14]. However, direct production of MTHF from LA is rarely reported. To our knowledge only two published reports addressed this issue: Elliot and Frye [15] achieved an 89.8% MTHF yield using operating conditions of 242 °C and 10 MPa H₂, and a Pd(5%)–Re(5%)/C catalyst. Upare and co-workers [16] reported a maximum MTHF yield of 89% using operating conditions of 265 °C and 2.5 MPa H₂ and a Cu(72%)-Ni(8%)/ SiO₂ catalyst. In both cases, 1,4-dioxane was used as a solvent.

In contrast, GVL production is widely reported and carried out under milder conditions. Carbon-based ruthenium catalysts are used by many authors under reaction conditions that vary from 25 to 215 °C and from 1.2 to 5.5 MPa hydrogen [17, 18, 19, 20] in solvents such as 1,4-dioxane or methanol. Temperatures of 70–150 °C and pressures of 0.5–3.5 MPa H₂ have been successfully used when water is employed as a solvent [21, 22, 23]. Other noble metals on different supports have also been reported to effectively activate the reaction process such as Pt over titania, silica, and zirconia [24, 25, 26], Au over zirconia [27], and Ir supported on carbon nanotubes [28]. However, reports using non-noble metals for LA hydrogenation are rare. GVL production in liquid phase [29, 30] using copper supported on Cr₂O₃, Al₂O₃, or ZrO₂ has been reported but requires harsher conditions, i.e., temperatures of 200–265 °C, pressures of 3.0–7.0 MPa H₂, and long reaction times of 5–10 h.

Non-noble metal catalysts are generally preferred owing to their higher availability and lower price that are more appropriate for large-scale operations such as biofuel production [29, 30]. Hence, this work investigates the use of sustainable non-noble metal-based catalysts for the production of LA derivatives.