Thermal and Catalytic Conversion of Inedible Vegetable Oils to Higher Value Products or Aromatics

Abstract

Jatropha oil is an inedible vegetable oil obtained from the Jatropha Curcas plant. It comprises triglyceride molecules consisting of glycerol and fatty acids, including oleic acid, which makes up 46 wt.% of jatropha oil. When subjected to pyrolysis, Jatropha oil components can yield high-value products such as biofuels and aromatic compounds such as benzene, toluene, or xylenes (BTX). BTX are used to manufacture a range of everyday products (i.e., paints, solvents, etc.), and its current production relies on processing finite fossil fuel-based feedstocks. Therefore, a renewable and sustainable feedstock to produce these chemicals is required in the near future. This work considers the non-catalytic and catalytic pyrolysis of the inedible feedstock jatropha oil to high-value products or aromatics, particularly BTX. Oleic acid was also used as a feedstock and a fatty acid model compound to better understand the jatropha oil decomposition pathways. Two reaction systems were used for the pyrolysis tests, including a small-scale batch Pyroprobe-GCMS system and a continuous fluidised bed system (90 g h¯¹). The non-catalytic pyrolysis of oleic acid and jatropha oil was undertaken in the Pyroprobe system at 400 °C, 500 °C and 600 °C. The influence of temperature on the yield and distribution of products was studied, focusing on the proportion of aromatics (including BTX). At all the temperatures and for both feedstocks, high proportions of acid compounds were observed in the products, attributed to unconverted fatty acids from the feedstocks. The lowest proportions of acids were obtained at 600 °C and accounted for 82.0% and 39.3% of the total peak area for oleic acid and jatropha oil. Aliphatics such as alkenes and alkanes were the second-largest group of compounds produced and were identified at all three temperatures. Aliphatics were identified as the main decomposition product in the absence of a catalyst and have been reported as aromatics precursors in the triglycerides decomposition pathway. Therefore, their identification in the product distribution is relevant when looking at aromatics production. A commercial ZSM-5 catalyst, alongside a range of Ni/ZSM-5 catalysts (1 wt.%, 2 wt.%, 5 wt.% and 10 wt. %), were used for the catalytic tests in the Pyroprobe. Oleic acid and jatropha oil were used as feedstocks at 400 °C, 500 °C and 600 °C. At 500 °C, it was observed that the aromatics yield was increased from 70.6% using ZSM-5 catalyst up to the maximum proportion of aromatics achieved at 76.1% when a small amount of nickel was added to the catalyst (1wt.% Ni/ZSM-5). This was attributed to a slight increase in pore diameter (1.505 nm for ZSM-5 and 1.528 nm for 1Ni/ZSM-5) as no significant changes were observed on the catalyst surface area (383.0 m²/g for ZSM-5 and 370.0 m²/g for 1Ni/ZSM-5). Other major groups of products identified in the catalytic decomposition of jatropha oil included aliphatics and polyaromatics. These aliphatics were highlighted earlier as precursors to aromatics, whilst the evolution of polyaromatics has likely come from further reactions between some of the aromatic compounds present. Finally, the non-catalytic fast pyrolysis of jatropha oil was carried out in the fluidised bed system at 450 °C. The reactor type selection was novel as no prior literature reported a continuous reactor used to process this feedstock. The conversion of the jatropha oil yielded on average 88.21 wt.% ±0.77 liquid products, with a small proportion of gases (3.12 wt.% ±0.78), and near to negligible solids (0.72 wt.% ±0.27). This system showed a preference for the decomposition of jatropha oil to ester compounds which made up on average 70.3% of the liquid products with just 4.2% acids. It has been reported that esters can be converted to aromatics in the presence of a catalyst, which has been suggested as part of future work. For the jatropha oil non-catalytic tests, the fluidised bed system showed a preference to yield esters, whilst the Pyroprobe showed a preference towards aliphatic compounds and small amounts of aromatics. The differences in the decomposition pathways were attributed to the different reactor type configurations. Nevertheless, the main functional groups identified (esters and aliphatics) can yield aromatics, including BTX via catalyst addition. For the Pyroprobe system, it was concluded that aromatics including BTX can be obtained from the catalytic pyrolysis of jatropha oil at 500 °C. The 1Ni/ZSM-5 catalyst is preferable to maximise the yield and distribution of these compounds. The two pyrolysis systems showed different decomposition pathways and product distribution; however, it could be feasible for aromatics to be produced in the fluidised bed reactor using ZSM-5 catalysts.