FTIR AND P-31-NMR SPECTROSCOPIC ANALYSES OF SURFACE SPECIES IN PHOSPHATE-CATALYZED LACTIC-ACID CONVERSION

The surface species present on silica/alumina-supported sodium phospha tes, active catalysts for the conversion of lactic acid to acrylic aci d and 2,3-pentanedione, are examined by pre- and postreaction MAS P-31 -NMR and FTIR spectroscopies, Species present following lactic acid co nversion are identified by transmission FTIR of phosphates supported o n silicon disks (as a model catalyst system) and verified by P-31-NMR and diffuse reflectance IR spectroscopy of actual catalysts used in re action. Monosodium phosphate (NaH2PO4) condenses to a mixture of sodiu m polyphosphate (NaPO3)(n) and sodium trimetaphosphate (Na3P3O9), whic h exhibit little catalytic activity for converting lactic acid to desi red products. Disodium phosphate (Na2HPO4) condenses to tetrasodium py rophosphate (Na4P2O7), and proton transfer from lactic acid to pyropho sphate results in the formation of sodium lactate. Trisodium phosphate (Na3PO4) accepts a proton from lactic acid to form sodium lactate and disodium phosphate, which condenses to pyrophosphate. The presence of pyrophosphate and sodium lactate on supported disodium and trisodium phosphates explains their similar catalytic properties; the larger qua ntity of sodium lactate present on trisodium phosphate leads to higher conversions at lower temperatures. (C) 1996 Academic Press, Inc.