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).
Highlights
A series of bimetallic and hierarchical zeolite Beta beads were synthesized using a tailored hydrothermal crystallization method followed by a post-synthetic treatment involving dealumination and metal-grafting steps
The heterogeneous catalytic system consisting of AuPd-NP/carbon nanotubes (CNTs)-F and Sn-deAl-1.8Beta-B was active in the conversion of glycerol to methyl lactate, though both product yield and selectivity were significantly lower compared to the state-of-the-art catalytic system for this reaction, in which nanosized Sn-MCM-41 particles in powder format are used instead of Sn-deAl-1.8-Beta-B (Table 4)
Zeolite Beta beads were synthesized by hydrothermal crystallization in the presence of anion-exchange resin beads as hard template
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. After removing the polymeric hard template by calcination, macroscopic beads were obtained, which contain microporous zeolite Beta crystals that are accessible through the meso- and macropores generated by removing the polymer (see Scheme 2) Such a hierarchical porous structure is expected to grant good accessibility to the zeolitic active sites [18,19,20], while the macroscopic bead format provides a binder-free shaping that facilitates the separation and recovery of the catalyst [21,22]. To the best of our knowledge, this is the first report of the synthesis of bimetallic, hierarchical and binder-free zeolite Beta beads possessing both Brønsted and Lewis acidity and of their application as heterogeneous catalysts for converting glycerol or its derivative dihydroxyacetone into methyl lactate
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
