Gallium-modified zeolites for the selective conversion of bio-based dihydroxyacetone into C1–C4 alkyl lactates

• Alkyl lactates production is studied over gallium-modified FAU zeolites.
• Alkaline-assisted galliation outperforms dry impregnation and ion exchange of Ga3+.
• The method is successfully applied to MOR, MFI, FER, and BEA zeolites.
• Galliated FAU is the best tin-free catalyst for C2–C4 alkyl lactates production.
• Retained performance upon 5-fold increase in substrate feed concentration.

Recently, we have demonstrated that post-synthetic alkaline-assisted galliation of FAU-type zeolites generates outstanding Lewis-acid tin-free catalysts for the production of ethyl lactate from dihydroxyacetone under mild conditions. Herein, we show that this preparation method is superior with respect to conventionally practiced strategies for metal incorporation in zeolites, such as dry impregnation and ion exchange with gallium salts. By means of IR spectroscopy of adsorbed pyridine and 71Ga MAS NMR, it is evidenced that, although gallium migration to the zeolite framework likely occurs also during the traditional preparation, the highly selective tetra-coordinated gallium centers identified for the galliated zeolite in alkaline medium are less abundant in the impregnated and exchanged samples. Furthermore, alkaline-assisted galliation was successfully applied to other zeolite frameworks including MFI, MOR, FER, and BEA, thus evidencing the versatility of the approach. The GaUSY catalyst also proved to be the best tin-free system for the production of n-propyl and n-butyl lactates. In addition, its remarkable performance in ethanol remained unaltered even upon a 5-fold increase of the DHA feed concentration, i.e. from the typically applied 3 wt.% to 15 wt.%, thus strengthening its potential for an industrial application.

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