Parametric Study of Pt/C-Catalysed Hydrothermal Decarboxylation of Butyric Acid as a Potential Route for Biopropane Production

Abstract

Sustainable fuel-range hydrocarbons can be produced via the catalytic decarboxylation of biomass-derived carboxylic acids without the need for hydrogen addition. In this present study, 5 wt% platinum on carbon (Pt/C) has been found to be an effective catalyst for hydrothermally decarboxylating butyric acid in order to produce mainly propane and carbon dioxide. However, optimisation of the reaction conditions is required to minimise secondary reactions and increase hydrocarbon selectivity towards propane. To do this, reactions using the catalyst with varying parameters such as reaction temperatures, residence times, feedstock loading and bulk catalyst loading were carried out in a batch reactor. The highest yield of propane obtained was 47 wt% (close to the theoretical decarboxylation yield of 50 wt% on butyric acid basis), corresponding to a 96% hydrocarbon selectivity towards propane. The results showed that the optimum parameters to produce the highest yield of propane, from the range investigated, were 0.5 g butyric acid (0.57 M aqueous solution), 1.0 g Pt/C (50 mg Pt content) at 300 °C for 1 h. The reusability of the catalyst was also investigated, which showed little or no loss of catalytic activity after four cycles. This work has shown that Pt/C is a suitable and potentially hydrothermally stable heterogeneous catalyst for making biopropane, a major component of bioLPG, from aqueous butyric acid solutions, which can be sourced from bio-derived feedstocks via acetone-butanol-ethanol (ABE) fermentation.

Optimisation of Propane Production from Hydrothermal Decarboxylation of Butyric Acid Using Pt/C Catalyst: Influence of Gaseous Reaction Atmospheres

“…The experimental procedure has been reported previously [17]. For each individual experiment, a 10 wt% of butyric acid in water was used, with the deionised water (9 g) used to quantitatively transfer the butyric acid (1 g) into the reactor.…”

“…Decarboxylation of butyric acid has been identified as a potential route to produce commercial quantities of propane at high yields compared with existing routes [11,17]. Making butyric acid from biomass offers a green and simple route to make high yields of biopropane according to the following Reaction Equation (R1):…”

“…Indeed, researchers from NREL have recently published a paper on a potential commercial route to make bio-butyric acid that can deliver the product at about 55% of the cost of fossil the fossil-derived analogue [19]. In a recent publication, Razaq et al [17] showed that the decarboxylation of butyric acid could be achieved under hydrothermal conditions in the presence of 5 wt% Pt/C catalyst. The work showed that the highest yield of propane was obtained around 300 • C after 1 h reaction time under nitrogen atmosphere.…”

“…However, analysis of the reaction products showed the presence of other gases apart from propane and CO 2 . These other gases included hydrogen, methane, and ethane, and their compositions changed with reaction conditions [17]. The presence of these gases indicated that other competing reactions occurred alongside the main decarboxylation reaction (Reaction Equation (R1)).…”

“…Identifying such reactions and the conditions that favour them is important in determining the optimum reaction conditions for decarboxylation in order to enhance the selectivity towards propane. In addition, while experiments have been reported at temperatures above 300 • C [11,17], it is important to carry out tests at lower temperatures, which may have significant influence on production costs. Additionally, operating at lower temperatures and pressures may provide good compensation for the use of expensive Pt catalyst for this reaction.…”