Hydrodeoxygenation of Pyrolysis Oil in Supercritical Ethanol with Formic Acid as an In Situ Hydrogen Source over NiMoW Catalysts Supported on Different Materials

Abstract

Hydrodeoxygenation (HDO) is one of the most promising approaches to upgrading pyrolysis oils, but this process normally operates over expensive noble metal catalysts (e.g., Ru/C, Pt/Al2O3) under high-pressure hydrogen gas, which raises processing costs and safety concerns. In this study, a wood-derived pyrolysis oil was upgraded in supercritical ethanol using formic acid as an in situ hydrogen source at 300 °C and 350 °C, over a series of nickel–molybdenum-tungsten (NiMoW) catalysts supported on different materials, including Al2O3, activated carbon, sawdust carbon, and multiwalled nanotubes (MWNTs). The upgrading was also conducted under hydrogen gas (an ex situ hydrogen source) for comparison. The upgrading process was evaluated by oil yield, degree of deoxygenation (DOD), and oil qualities. The NiMoW/MWNT catalyst showed the best HDO performance among all the catalysts tested at 350 °C, with 74.8% and 70.9% of oxygen in the raw pyrolysis oil removed under in situ and ex situ hydrogen source conditions, respectively, which is likely owing to the large pore size and volume of the MWNT support material, while the in situ hydrogen source outperformed the ex situ hydrogen source in terms of upgraded oil yields and qualities, regardless of the catalysts employed.

Catalytic Upgrading of Rice Straw Bio-Oil via Esterification in Supercritical Ethanol over Bimetallic Catalyst Supported on Rice Straw Biochar

“…Bio-oil typically consists of a complex mixture of organic compounds such as acids, alcohols, aldehydes, esters, ketones, sugars, guaiacols, syringols, furans, lignin-derived phenols, levoglucosans, and other compounds [9]. The presence of a high water content and the high percentage of oxygenated compounds (35)(36)(37)(38)(39)(40) wt.%) are the main reasons for bio-oil's negative properties, such as low heating value (13-20 MJ/kg) and immiscibility with fossil fuels [10]. Additionally, bio-oil has a strong tendency to polymerize during storage and transit and contains substantial quantities of carboxylic acids, such as formic, propanoic, and acetic acids, which results in low pH values (2)(3) and makes bio-oil more corrosive [11].…”

“…In a few instances, trimetallic catalysts, such as MgNiMo supported on activated charcoal, were used successfully for bio-oil esterification under supercritical ethanol conditions [14]. It is worth noting that most of these biocharbased catalysts were prepared using commercially purchased activated carbon, some of which had been utilized for the purpose of upgrading bio-oil derived from fast pyrolysis in supercritical ethanol [14,40]. It is therefore not surprising that recently biochar has become a more popular catalyst support, similar to zeolite, alumina, oxides etc.…”

Upgrading Fast Pyrolysis Oil with Vacuum Gas Oil by Catalytic Hydrodeoxygenation in Supercritical Ethanol

“…Supercritical fluid treatment is another promising approach for improving bio-oils into high-quality liquid products as a result of its liquid-like density, gas-like diffusivity and viscosity, and excellent mass and heating transfer efficiency . Supercritical ethanol ( T c of 241 °C and P c of 63 bar) has been reported as an effective hydrogen donor solvent for bio-oil upgrading. − Baloch et al investigated bio-oil upgrading under sub/supercritical ethanol conditions over Pt/AC and Pd/AC catalysts. Supercritical ethanol showed better upgrading efficiency than the subcritical ethanol condition, where the higher heating value (HHV) of bio-oil increased from 29.55 to 36.4 MJ/kg and the oxygen content decreased from 28.7 to 15.5 wt %.…”