Solid acid catalysis in liquid, gas-expanded liquid and near-critical reaction media: Investigations of isobutane/butene alkylation and aromatic acylation reactions

The replacement of stoichiometric reagents based on Lewis acids like AlCl3 and toxic mineral acids like H2SO 4 and HF with safer alternatives based on solid acids as catalysts is one of the most important goals of green chemistry. While such a replacement has been commercially successful in processes like alkylation of aromatics, it has remained a Holy Grail in the case of isobutane/olefin (I/O) alkylation and aromatic acylation reactions, primarily due to the rapid deactivation of the catalyst by fouling. The objective of this study is to investigate the deactivation profiles of these reactions and explore the use of near-critical CO2 and 'CO2-expanded liquids' (CXLs) to effectively transport the coke precursors out of the pores, thereby preventing fouling.;Steady isobutane alkylation activity on mesoporous supported heteropolyacids (HPAs) and NafionRTM catalysts was demonstrated for several days time on stream, using dense CO2 (95°C, 80 bar, olefin space velocity of 0.05 h-1, feed I/O ratio = 5, 70 mol%CO 2). Using lumped kinetics and a slurry reactor model, effective rate constants were estimated from the steady conversion and selectivity data, thereby providing a framework for comparing the performance of various solid acids. On all the catalysts tested, effective rate constants for the desired reaction are two to three orders of magnitude lower than that of dimerization, the dominant side reaction, underlining the need to design more active solid acid catalysts. Model predictions reveal that a fixed bed reactor with olefin distributed along the bed is the optimal reactor configuration and that a high I/O ratio (2000+) inside the reactor is required to achieve the desired alkylate selectivity. Further studies on microporous zeolitic catalysts and mesoporous HPAs with dense CO2 and ethane reveal that the reaction media effects are appreciable for stabilizing dimerization activity, and to a limited extent for the desired alkylation activity, which is controlled by intermolecular hydride transfer step.;Acylation of anisole with acetic anhydride was demonstrated for the first time in a continuous mode slurry operation, on mesoporous supported HPAs and supported NafionRTM catalysts. These catalysts deactivate completely in less than 24 h during liquid phase operation at 70°C, with an anisole-rich feed molar ratio of 5:1 and a space velocity of 1.6 g acetic anhydride·gcat.-1 h-1 . It is found that the Keggin ions leach out into the solution from 70%HPA/SiO2 and to a lesser extent from 50%Cs2.5-HPA/SiO 2, as confirmed by elemental analysis. To guide experimental conditions for CXLs, high-pressure phase behavior studies with CO2 at 50-70°C indicate that the reaction mixtures can be isothermally expanded two-fold at pressures less than 100 bars. The use of CXLs on leach resistant supported NafionRTM catalysts is found to be detrimental despite the use of polar nitromethane as cosolvent. Post reaction analyses reveal that the deactivation may be due to the primary product and/or multiply acetylated products retained in the micropores of NafionRTM catalyst aggregates. The high turn over number (∼400) achieved before deactivation and their ability to regain complete activity by treating with HNO3 indicate that NafionRTM catalysts are promising alternatives to conventional acids. This work represents a first demonstration of a continuously operated CXL process.