Single-phase catalysis for reductive etherification of diesel bioblendstocks

Abstract

Reductive etherification is a promising catalytic chemistry for coupling biomass derived alcohols and ketones to produce branched ethers that can be used as high cetane, low sooting blendstocks for diesel fuel applications. Previous catalyst materials examined for reductive etherification have typically been limited to binary physical mixtures of metal hydrogenation and acidic acetalization catalysts with limited thermal stability and industrial applicability. To address this, we developed a single-phase catalyst comprising Pd supported on acidic metal oxides with high catalytic activity, product selectivity, and regeneration stability. Batch reactor screening identified niobium phosphate (NbOPO₄) as the most active acidic metal oxide catalyst support, which was downselected to synthesize single-phase catalysts by Pd loading. Several branched ethers with favourable fuel properties were synthesized to demonstrate broad catalyst applicability. The fresh Pd/NbOPO₄ catalyst displayed a surface area of 130 m² g⁻¹, high acidity of 324 μmol g⁻¹ and Pd dispersion of 7.8%. The use of acidic metal oxide support allowed for elevated reaction temperatures with a mass selectivity to 4-butoxyheptane of 81% at 190 °C and an apparent activation energy of 40 kJ mol⁻¹. Continuous flow reactor testing demonstrated steady catalyst deactivation due to coke formation of 10 wt% after 117 h of time-on-stream. Four simulated catalyst regeneration cycles led to small changes in surface area and total acidity; however, a decrease in Pd site density from 18 to 8 μmol g⁻¹, in combination with an apparent Pd nanoparticle size effect, caused an increase in the production rate of 4-butoxyheptane from 138 to 190 μmol g_cat⁻¹ min⁻¹ with the regenerated catalyst. Lastly, technoeconomic analysis showed that higher H₂ equivalents and lower weight hourly space velocity values can reduce ether catalytic production costs.