Confinement Effects in Lewis Acid-Catalyzed Sugar Conversion: Steering Toward Functional Polyester Building Blocks
We report the use of solid Lewis acid catalysts for the conversion of tetrose sugars to four-carbon α-hydroxy acid esters (C_4-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 mol_(C4-AHA) mol_(Sn) h^(–1) was attained using Sn-β, a more than 6-fold increase compared with homogeneous SnCl_4·5H_2O. 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.
© 2015 American Chemical Society. Received: June 26, 2015; Revised: August 20, 2015; Published: August 24, 2015. R.D.C. thanks IWT (Agency for Innovation by Science and Technology, project number 131404) for financial support. M.D. acknowledges FWO Vlaanderen for postdoctoral funding and the Belgian American Educational Foundation (BAEF) for his honorary fellowship. M.D. and B.F.S thank the Research Counsil of KU Leuven, IDO KP/14/003. The EPMA-WDS work has been feasible due to the support of the Hercules Foundation (project ZW09-09). We thank professor Johan Martens and Stef Kerkhofs for measurement of small-angle X-ray diffraction patterns.
Supplemental Material - cs5b01344_si_001.pdf