On the role of boron in the direct dehydration of 1,3-butanediol into butadiene over Al,B-MFI catalysts

Direct dehydration of biomass-derived 1,3-butanediol (BDO) offers a potentially simple and bio-based route to produce 1,3-butadiene (BD). This is an acid-catalyzed dehydration reaction, which is usually carried out over either zeolites or metal oxides. While most of the studies are focused on initial activity, the relationship between nature of the active sites, stability and reaction mechanism remains poorly understood. In this study we show presence of the boron atoms in the zeolite framework along with aluminum enhances stability of the reaction. Intermediates conversion experiments identified 3-buten-1-ol (3B1OL) as a key problematic intermediate leading to C-C cracking products. IR spectroscopy and density functional theory (DFT) calculations suggest that trigonal framework boron generates weakly Brønsted acidic silanol groups and that B sites provide additional covalent stabilization of the primary carbocation formed from 3B1OL, favoring dehydration over cracking. Al sites, in contrast, appear more efficient at converting BDO but provide less stabilization in 3B1OL conversion. We therefore propose that the complementary roles of these two sites facilitate the simultaneous improvement in BD selectivity and slower deactivation of MFI zeolites containing both Al and B atoms in the framework.