Enhanced lactide yield in catalytic conversion of L-lactic acid from passivation of external acidic sites on aluminosilicate Beta zeolites by coating siliceous Beta zeolite

The development of efficient Beta zeolite for conversion of L-lactic acid (L-LA) to L,L-lactide (L-LT) is in urgent need but still remains challenging. Herein, we reported an enhancement of the L-LT yield by passivation of external acidic sites on aluminosilicate Beta zeolites by coating pure silica Beta (Al-Beta@Si-Beta). The passivation of external acidic sites was strongly evidenced by in situ 2,4,6-tritertbutylpyridine (TTBP) adsorption FT-IR spectra, where the external acidic sites of Al-Beta@Si-Beta were effectively covered. In the tests for conversion of L-LA to L-LT, the Al-Beta@Si-Beta exhibited much higher L-LT yield than commercial microporous Beta zeolite, which is reasonably attributed to that the passivation of external acidic sites of the Beta zeolite significantly decreases molecular weight of the reaction intermediates, favoring for reduction of coke formation. Furthermore, this strategy is extended to passivate mesoporous Beta zeolite (Al-Beta-meso@Si-Beta). As expected, the Al-Beta-meso@Si-Beta displayed higher L-LT yield than the Al-Beta@Si-Beta because the presence of mesoporosity in the zeolite catalyst is favorable for mass transfer in the conversion of L-LA to L-LT. The strategy for coating pure siliceous zeolite might offer a new opportunity to develop highly efficient zeolite catalysts for the production of L-LT in the future.