2,3-Butanediol dehydration catalyzed by silica-supported sodium phosphates

Catalytic gas phase dehydration of 2,3 butanediol (BDO) was studied in the fixed bed reactor over silica-supported sodium phosphates for a wide range of Na/P ratios (Na/P = 0-3) and variable content of phosphates. Major products were 1,3-butadiene (BD) and methyl ethyl ketone (MEK) for long contact times. 3-Buten-2-ol (3B2OL) and MEK are major products for the short contact times. Achieved yields of elimination products (BD + 3B2OL) were over 60% per pass at Na/P = 1.8-1.9 which is the optimum combination of acidic and basic components for the long term production of BD or 3B2OL from BDO. Minor products detected at short contact times (2,3-epoxybutane, 2,3-butanedione and acetoin) imply that hydrogen transfer and dehydrative epoxidation are minor pathways. Mass transport limitations are more apparent for BDO -> 3B2OL -> BD route (elimination) than for BDO MEK route (rearrangement). This leads to the lower selectivity of elimination products at long contact time and/or insufficient flow rate of the carrier gas. 2-butanol dehydration using the same catalysts shows considerable selectivity to 1-butene (1-BT) providing evidence for Eta elimination mechanism that requires concerted action of acid-base pairs. NH3 and CO2 TPD studies confirmed existence of weak acid and basic sites for the sodium phosphates supported on silica. High affinity adsorption of pyridine (Py) and benzoic acid (BA) shows maximum of acid/base ratio at Na/P similar to 1.5-2. According to XRD and P-31 MAS NMR data the catalysts providing the best performance are composed of pyrophosphates and tripolyphosphates finely dispersed on the surface of silica. XPS shows considerable migration of surface sodium cations into the bulk when Na/P ratio exceeds 1.4. (C) 2015 Elsevier B.V. All rights reserved.