Mechanism and kinetics of 1-dodecanol etherification over tungstated zirconia

- Tungstated zirconia exhibits high selectivity for dodecanol etherification vs dehydration.
- Brønsted-acid sites on the surface of WOx are active for alcohol etherification and dehydration.
- Etherification involves coupling of alcohols adsorbed on adjacent Brønsted- and Lewis-acid sites.
- Rates of etherification and dehydration scale with concentration of Brønsted-acid sites on WOx.
- Bimolecular etherification is enhanced by Lewis-acid sites adjacent to Brønsted-acid sites.

Growing interest in finding renewable alternatives to conventional fossil fuels and petroleum-derived specialty chemicals has motivated the investigation of biomass-derived alcohols to make ethers as diesel additives or lubricants. To optimize the direct etherification of long chain alcohols in the liquid phase, it is necessary to develop an understanding of the kinetics and mechanism of etherification and dehydration reactions. In this study, tungstated zirconia was identified as a selective solid-acid catalyst for the liquid-phase etherification of 1-dodecanol. Investigations of the mechanism and kinetics of this reaction suggest that cooperation between Brønsted- and Lewis-acid sites on tungstated zirconia enhances the selectivity to ether by increasing the surface concentration of adsorbed alcohol, thereby promoting bi-molecular ether formation relative to unimolecular alcohol dehydration. The suggested rate limiting step for etherification is the formation of a CO bond between two adsorbed alcohol molecules, and the suggested rate-limiting step for dehydration is the cleavage of the CH bond of the β-carbon atom in an adsorbed alcohol. Measurements of the kinetic isotope effects for etherification and dehydration support the proposed mechanism. A microkinetic model based on the proposed mechanism for dodecanol etherification and dehydration over tungstated zirconia accurately describes the observed effects of alcohol concentration and product inhibition.

94