Titania-Silica Catalysts for Lactide Production from Renewable Alkyl Lactates: Structure–Activity Relations

Different Ti-Si catalysts, viz. TiO 2 supported on amorphous SiO 2 or Si-MCM-41, TiO 2 -SiO 2 xerogels, and Ti zeolites (TS-1 and Ti-beta), were compared in terms of activity and selectivity for the direct conversion of methyl lactate to lactide in the gas phase. Except for Ti-beta, all catalysts exhibit a high lactide selectivity of 88–92% at conversions below 50%. From DR UV–vis spectroscopy, it is evidenced that the catalytic activity of tetrahedral TiO 4 sites is higher than those of polymerized TiO 5 or the octahedral TiO 6 counterparts, irrespective of the catalyst structure, an analysis supported by ToF-SIMS measurements. A kinetic analysis shows that the catalytic activity is proportional to the number of vacant sites on the catalyst surface. Thus, the activity increase observed for tetrahedral TiO 4 sites may be attributed to an increased number of vacant sites (e.g., two for TiO 4, zero for TiO 6 ). Lactide productivity thus highly benefits from an increased dispersion of Ti sites on the catalyst surface and could be increased by a factor of 2.5 (up to 10 g LD g cat –1 h –1 ) when TiO 2 is dispersed on a Si-MCM-41 support, with higher surface areas in comparison to amorphous SiO 2 gels.