Stabilization of fresh and aged simulated pyrolysis oil through mild hydrotreatment using noble metal catalysts

Journal article, 2024

The nature and reactivity of the oxygenates, containing different functional chemical groups, and especially carbonyl compounds, render pyrolysis oil unstable. Alterations in physical and chemical properties of pyrolysis oil during storage and the catalytic stabilization of this oil is therefore critical and is the objective of the current work. In this study, Pd/Al2O3, Pt/Al2O3, Rh/Al2O3, Re/Al2O3 and sulphided NiMo/Al2O3 catalysts were employed in the hydrotreatment (180 °C, 60 bar H2, 4 h) of simulated pyrolysis oil to examine their effect on stabilization and potential polymerization routes. Of all the catalysts used, Pd/Al2O3 with well-dispersed metal particles, and a high char-suppressing potential was the most effective catalyst. It had the highest bio-liquid yield and the highest selectivity to low molecular weight stabilized oxygenates and deoxygenated products. In addition, the acidity in the light fraction was low and a very low solid product formation was found that consisted mainly of soluble polymers composed predominantly of aliphatic compounds and sugars, whereas insoluble polymers were not fully developed char. The solid yield increased in the following order: Pd (3.3 wt%) < Rh (13.3 wt%) < NiMo (13.6 wt%) < Pt (21.5 wt%) < Re (25.8 wt%) < Blank (27.4 wt%). This trend was also accompanied by an enhanced yield of heavy oligomers in the corresponding liquid phase abundant in phenolic compounds compared to carboxylic acids and aliphatic compounds based on GPC and P-NMR analyses. The Pd loading necessary to obtain a high-quality product was also assessed, and the lower carbon loss when using catalysts with smaller contents of metal was revealed. Based on the results a detailed reaction network was proposed regarding the reactions during stabilization of sugars, aldehydes, ketones, furans, acids and phenols present in pyrolysis oil. To delve deeper into the simulated pyrolysis oil properties, it was subjected to accelerated aging. Interestingly as much as 79 % of the feed was converted during aging. According to GC/MS analysis only large oligomers were formed that could not be detected. When removing the most reactive components from the feed, i.e. the sugar and furan, the conversion was lowered to 53 %. Catalytic stabilization was conducted on the aged oil and compared with stabilization followed by aging. The results showed that the solid formation increased from 5.1 to 9.1 % when the pyrolysis oil was first aged, followed by stabilization. A suggested reason for this is the large amount of oligomers that were formed during the aging. Thus, aging before stabilization is very negative for an industrial process.