Catalytic upgrading of bio-oil via the hydrodeoxygenation of short chain carboxylic acids

Petroleum is non-renewable and contributes to environmental pollution, thus bio-oil can be substituted as a potential alternative. However, bio-oil in its crude form cannot be used directly as fuel since it contains a high proportion of oxygenated, acidic and reactive compounds such as carboxylic acids. These are known to cause corrosion of vessels and pipework, instability and phase separation. The oxygen content of bio-oil can be reduced through hydrodeoxygenation of oxygenated compounds. In this study, the hydrogenation of short chain (C2-C4) carboxylic acids typical of model compounds present in bio-oil was investigated using commercial Pt supported on Al2O3, SiO2, carbon and graphite, and prepared Pt and Pt-Re on TiO2 catalysts. This study reports the preparation of 4% Pt/TiO2 and 4% Pt-4%Re/TiO2 catalysts for alcohol production, which were screened against their commercial counterparts, the reaction space explored in the following ranges temperature 80-200 °C, pressure 10-40 bar, time 1-4 h, catalyst 0.1-0.4 g and stirring speed 400-1000 min-1 using 4%Pt/TiO2, and kinetic modelling of acetic acid hydrogenation. The catalysts were characterized using Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), Scanning electron microscopy (SEM), H2-Temperature Programmed Reduction (H2-TPR) and NH3-Temperature Programmed Desorption (NH3-TPD) techniques. BET analysis showed features of Type IV isotherm with Type 3 hysteresis for 4% Pt/TiO2, 4% Pt-4%Re/TiO2, 4% Pt/SiO2 and 5% Pt/graphite; in contrast, 5% Pt/C and 5% Pt/Al2O3 exhibited Type 2 hysteresis. Further, both commercial and synthesized catalysts are mesoporous. The XRD peaks present in 4% Pt/TiO2 and 4% Pt-4%Re/TiO2 were found to be rutile and anatase phases of TiO2 only. The incorporation of Pt and Re into TiO2 occupied more rutile than anatase phases. Catalyst morphology of spent 4% Pt/TiO2 and 4% Pt-4%Re/TiO2 showed the formation of compact mass and agglomerates after three reuse cycles. NH3-TPD analysis showed that 4%Pt/TiO2 had the highest acidity (0.48 mmol g-1) which favoured esterification reaction. Catalyst screening showed that 4% Pt/TiO2 and 4% Pt-4%Re/TiO2 outperformed the commercial catalysts, and favoured the production of ethyl acetate and ethanol respectively. Consequently, the production of alcohol over 4% Pt-4%Re/TiO2 increased with increasing Re loading from 1 to 4%. The achieved optimum conditions were 200 °C, 40 bar, 4 h, 0.4 g and 1000 min-1 for acetic acid conversion, and 160 c, 40 bar, 4 h, 0.4 g and 1000 min-1 for ethanol production. The hydrogenation of C2-C4 acids over 4% Pt/TiO2 and 4% Pt-4%Re/TiO2 as single acid feed showed that an increase in the molecular weight of the carboxylic acid from acetic to butanoic acid enhanced the selectivity towards the respective alcohol which can be summarized: butanol > propanol > ethanol. Conversely, ester selectivity is as follows: ethyl acetate > propyl propionate > butyl butyrate. The selectivity to alcohol decreased as the reaction temperature increases from 145 to 200 °C. Higher alcohol selectivities were achieved over 4% Pt-4%Re/TiO2 in all cases. The investigation of acids in a mixed acid feed system over 4% Pt/TiO2 showed that higher alcohol selectivity was attained compared to the single feed system. On the other hand, the conversion of propanoic and butanoic acids in the mixture containing acetic acid dropped from 94 to 77% and 88.2 to 70% respectively. The presence of acetic acid in the mixed feed inhibited the other acids due to competitive effect but favoured higher alcohol selectivity. Reaction kinetics of acetic acid hydrogenation was investigated using catalyst particle sizes < 65 μm and a stirring speed of 1000 min-1 at which negligible internal and external mass transfer resistances were experienced. The reaction order with respect to acetic acid and hydrogen were found to be 0.78 and 0.35 respectively, which indicated fractional order kinetics. Hence, the experimental data was fitted with a Langmuir-Hinshelwood model for dissociative H2 adsorption. The activation energy and pre-exponential factor were found to be 80.6 kJ mol-1 and 4.6 ×10-7 kmol.kgcat-1.min-1 respectively. Finally, 4% Pt/TiO2 catalyst favours the production of esters and alcohols from the hydrogenation of short chain carboxylic acids (C2-C4) in single and multiple feed systems. However, the addition of Re to form bimetallic catalyst (4% Pt-4%Re/TiO2) further increased the selectivity towards alcohols while maintaining a high acid conversion. Furthermore, the reaction kinetics of acetic acid using 4% Pt/TiO2 catalyst was adequately described using a model that assumes a dissociative adsorption of hydrogen.