Confinement Effects in Lewis Acid-Catalyzed Sugar Conversion: Steering Toward Functional Polyester Building Blocks

• Published in: ACS catalysis Vol. 5; no. 10; pp. 5803 - 5811
• Main Authors: De Clercq, Rik, Dusselier, Michiel, Christiaens, Charles, Dijkmans, Jan, Iacobescu, Remus Ion, Pontikes, Yiannis, Sels, Bert F
• Format: Journal Article
• Language: English
• Published: American Chemical Society 02-10-2015
• Subjects: solid Lewis acid; Sn-β; biomass conversion; polylactic acid (PLA); α-hydroxy-γ-butyrolactone; α-hydroxy acids; methyl vinyl glycolate; functional PLA
• ISSN: 2155-5435; 2155-5435
• DOI: 10.1021/acscatal.5b01344

We report the use of solid Lewis acid catalysts for the conversion of tetrose sugars to four-carbon α-hydroxy acid esters (C4-AHA), which are useful as functional polyester building blocks. Sn-β was by far the most active and selective catalyst, yielding up to 80% methyl vinyl glycolate (MVG), methyl-4-methoxy-2-hydroxybutanoate (MMHB), and α-hydroxy-γ-butyrolactone (HBL) combined at 95% conversion. A very high turnover frequency (TOF) of 330 molC4‑AHA molSn h–1 was attained using Sn-β, a more than 6-fold increase compared with homogeneous SnCl4·5H2O. It is shown that, using different Sn-based catalysts with various pore sizes, the product distribution is strongly dependent on the size of the catalyst pores. Catalysts containing mainly mesopores, such as Sn-MCM-41 or Sn-SBA-15, prefer the production of the more bulky MMHB, whereas microporous catalysts such as Sn-β or Sn-MFI favor the production of MVG. This effect can be further enhanced by increasing the reaction temperature. At 363 K, only 20% MVG is attained using Sn-β, but at 433 K, this increases to 50%. Using a kinetic analysis, it was found that, in microporous catalysts, steric hindrance near the Sn active site in the catalyst pores plays a dominant role in favoring the reaction pathway toward MVG. Moreover, the selectivity toward both products is kinetically controlled.