Minimizing Energy Demand in the Conversion of Levulinic Acid to γ‑Valerolactone via Photothermal Catalysis Using Raney Ni

Abstract

The valorization of lignocellulosic wastes emerges as a prime strategy to mitigate the global carbon footprint. Among the multiple biomass derivatives, γ-valerolactone is particularly attractive as precursor of high-value chemicals, biofuel, green solvent or perfumery. γ-Valerolactone can be synthesized through a hydrogenation reaction from levulinic acid, obtained from cellulose. However, the high energy requirements of this synthetic pathway have hindered its industrial viability. To drastically reduce the reaction energy requirements, here a novel synthetic strategy, based on solvothermal-photothermal processes using cost-effective Raney-Ni as photothermal catalyst, is proposed. First, the use of hydrogen gas is avoided by selecting isopropanol as a safer and greener H-source. Second, a photothermocatalytic process is used to minimize the reaction temperature and time with respect to conventional reactions. This approach exploits the broadband optical absorption of the Raney®-Ni, due to its highly damped plasmonic behavior, to achieve fast and efficient catalyst heating inside the reactor. The photothermal reaction required less than 2 h and just 132 °C to reach over 95% conversion, thereby drastically reducing the reaction time and energy consumption compared to conventional reactions. Importantly, these conditions granted high catalyst reusability. This solvothermal-photothermal approach could offer a sustainable alternative for the industrial production of γ-valerolactone.

Keywords: Raney Ni; biomass valorization; lignocellulosic biomass; photothermo‐catalysis; γ‐valerolactone.

Figure 1

Evolution of the yield of γ‐valerolactone over time at different temperatures under hydrothermal conditions, using a) formic acid and b) isopropanol as the H‐source. Reaction conditions: catalyst mass = 120 mg; volume = 4 mL; [LA]₀ = 0.5 M. Each condition was performed in triplicate.

Figure 2

Evaluation of the yield of γ‐valerolactone obtained under solvothermal conditions in isopropanol in an autoclave, in the dark, at different reaction times, depending on: a) the oven temperature, b) the amount of catalyst and c) the initial concentration of levulinic acid (the total production in mmol is indicated with circles and associated with the right axis). Unless otherwise stated, the reference reaction conditions imply: catalyst = 120 mg; [LA]₀ = 0.5 M; T = 160 °C. Each condition was performed in triplicate.

Figure 3

Photothermal catalytic experiments. a) Evolution of the temperature and yield of γ‐valerolactone as a function of reaction time using 120 mg of catalyst. b) Yield of the produced γ‐valerolactone (bars) and maximum obtained temperature [dots; light symbols – 60 min, and dark symbols – longer reaction times (100 or 120 min)] depending on the amount of catalyst for two different reaction times. Note that the longer reactions were conducted for 120 min for 60–120 mg of catalyst, while they were carried out for 100 min in the cases of 150 and 180 mg because of the faster completion of the reaction for higher concentrations. The reactions were conducted under laser‐induced heating and solvothermal conditions in isopropanol, [LA]₀ = 0.5 M. The laser power was maintained constant at 18 W until the reactor reached a pressure 10 bars, and then the power was reduced to keep a stable temperature and avoid overpressure (blue region in (a)). Each result was performed in triplicate.

Figure 4

Evaluation of the yield (bars) and production per hour (dots) of γ‐valerolactone at different reaction times under photothermal conditions (T_max = 132 °C; laser initial power = 18 W): a) [LA]₀ = 0.5 M; V_sol = 4 mL; b) increasing [LA]₀ to 1.5 M (V_sol = 4 mL), and c) increasing V_sol to 10 mL ([LA]₀ = 0.5 M). Each condition was performed in triplicate.

Figure 5

Reusability and stability analyses performed under laser‐induced heating and solvothermal conditions in isopropanol: a) conversion of levulinic acid and yield of γ‐valerolactone through 5 consecutive cycles. FE‐SEM images comparing the morphology of b,c) the pristine catalyst and d,e) the reused one (scale bar 5 µm). Conditions: catalyst = 120 mg; [LA]₀ = 0.5 M; T_max = 132 °C; laser initial power = 18 W.