Bimetallic Zeolite Beta Beads with Hierarchical Porosity as Brønsted-Lewis Solid Acid Catalysts for the Synthesis of Methyl Lactate

Abstract

Bimetallic zeolite Beta in bead format and containing Al sites with Brønsted acid behavior and Sn, Zr or Hf sites with Lewis acid character, were prepared using a two-step synthetic route. First, zeolite Beta in the format of macroscopic beads (400 to 840 μm) with hierarchical porosity (micropores accessed through meso- and macropores in the range of 30 to 150 nm) were synthesized by hydrothermal crystallization in the presence of anion-exchange resin beads as hard template and further converted into their H-form. Next, the zeolite beads were partially dealuminated using different concentrations of HNO3 (i.e., 1.8 or 7.2 M), followed by grafting with one of the above-mentioned metals (Sn, Zr or Hf) to introduce Lewis acid sites. These bimetallic zeolites were tested as heterogeneous catalysts in the conversion of dihydroxyacetone (DHA) to methyl lactate (ML). The Sn-containing zeolite Beta beads treated by 1.8 M HNO3 and grafted with 27 mmol of SnCl4 (Sn-deAl-1.8-Beta-B) demonstrated the best catalytic activity among the prepared bimetallic zeolite beads, with 99% selectivity and 90% yield of ML after 6 h at 90 °C. This catalyst was also tested in combination with Au-Pd nanoparticles supported on functionalized carbon nanotubes (CNTs) as multifunctional catalytic system for the conversion of glycerol to ML, achieving 29% conversion of glycerol and 67% selectivity towards ML after 4.5 h at 140 °C under 30 bar air. The catalytic results were rationalized by means of a thorough characterization of the zeolitic beads with a combination of techniques (XRD, N2-physisorption, SEM, XRF, TEM, UV-vis spectroscopy and pyridine-FT-IR).