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 alpha-hydroxy acid esters (C-4-AHA), which are useful as functional polyester building blocks. Sn-beta was by far the most active and selective catalyst, yielding up to 80% methyl vinyl glycolate (MVG), methyl-4-methoxy-2-hydroxybutanoate (MMHB), and alpha-hydroxy-gamma-butyrolactone (HBL) combined at 95% conversion. A very high turnover frequency (TOF) of 330 mol(C4 center dot AHA) mol(Sn) h(-1) was attained using Sn-beta, a more than 6-fold increase compared with homogeneous SnCl4 center dot 5H(2)O. 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-beta or Sn-MF1 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-beta, 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.