Ethene and butene oligomerization over isostructural H-SAPO-5 and H-SSZ-24: Kinetics and mechanism

Abstract

Brønsted-acidic zeolite and zeotype materials are potential catalysts for the conversion of ethene to higher alkenes. In this study, two materials with AFI structure but different acid strength, H-SAPO-5 and H-SSZ-24, were subject to studies of ethene, cis-2-butene and ethene-butene mixture conversion under conditions where C 3 -C 5 alkene formation is thermodynamically favoured over higher hydrocarbons (673–823 K, 1 atm). Ethene and cis-2-butene partial pressures were varied in the range 9–60 and 0.9–8.1 kPa, respectively, and contact times were varied in the range 3.78–756 and 0.573–76.4 s.µmol H+/cm 3 over H-SAPO-5 and H-SSZ-24, respectively. Less than 1% conversion of ethene and less than 10% conversion of butene was obtained in the range of conditions used for elucidation of rate parameters. The ethene conversion rates were more than an order of magnitude higher over the more acidic H-SSZ-24 than over H-SAPO-5 (6.5 vs. 0.3 mmol/mol H + .s at 748 K, P ethene = 33 kPa), with corresponding lower reaction order in ethene (1.5 vs. 2.0 at 673 K) and lower apparent activation energy (52 vs. 80 kJ/mol at 698–823 K). Propene selectivity was substantially higher over H-SSZ-24 than over H-SAPO-5 (68% vs. 36% at 0.5% ethene conversion). A similar difference in apparent reaction rates was observed for cis-2-butene conversion over the two catalysts, and for co-feeds of ethene and cis-2-butene. However, the cis-2-butene conversion to C 3 -C 5 alkenes was found to be severely influenced by thermodynamic limitations, impeding a detailed kinetic analysis, and leading predominantly to isobutene formation at the highest temperatures. Reactor with three zones; representing the three measurement regimes. On the side; three typical conversion - selectivity graphs for H-SAPO-5 and H-SSZ-24, representing C3-C5 alkene selectivity in the three regimes.