Supercritical deoxygenation of a model bio-oil oxygenate

Abstract

A novel process for the fixed-bed deoxygenation of a model bio-oil compound in supercritical media at mild pressures (tens of bars) is presented. As an example, nonanal deoxygenation over Pt/Al₂O₃ catalyst in supercritical n-hexane (T_c = 234.45°C, P_c = 30.2 bar) with excess hydrogen (molar H₂/nonanol in feed = 57) at 300 °C provides steady conversions of approximately 60% over a pressure range of 7-80 bar with approximately 40% selectivity to C₈ and C₉ hydrocarbons, at liquid hourly space velocities of 80 g nonanal·(g Pt)^-1·h^-1. Further, the conversions display only a mild temperature dependence. This suggests that for the conditions under study, the reaction is limited by mass transfer of nonanal (limiting reactant) from the bulk fluid phase to the catalyst particle. Among the alkanes, the selectivity to the decarbonylation product (n-octane) is higher at all pressures relative to the hydrogention product (n-nonane). However, the selectivities toward nonane and nonanol increase relative to the decarbonylation product (n-octane) at higher reactor pressures, attributed to the enhanced hydrogen partial pressures at supercritical conditions. Effective rate constants, estimated from a simple isothermal packed-bed reactor model that assumes first-order dependence on nonanal concentration, decrease exponentially as the pressure is tuned from subcritical pressures (gaslike properties) to supercritical pressures (liquidlike properties). This decrease is characteristic of the pressure dependence of the external mass transfer coefficient. Mass transfer coefficients, predicted using published correlations, are of the same order of magnitude as the effective rate constants, further affirming that the measured reaction rates are controlled by external mass transfer.