Enhancing multi-phase catalytic process efficiency with supercritical reaction media.

摘要

Heterogeneously catalyzed multi-phase processes are usually limited by severe gas-liquid mass-transfer resistances, poor heat-transfer rates, and catalyst deactivation. The objective of this study is to exploit the pressure-tunable properties (diffusivity, density, heat-capacity, etc.) of supercritical fluids (SCFs) to enhance reaction rates, reduce “hot spot” temperatures, and mitigate coking.; The exothermic hydrogenation of cyclohexene was investigated over Pd/C at near-critical (nc) conditions (70°C, 138 bar, 90% CO₂, 1:1 H₂/cyclohexene) in a fixed-bed reactor. Mitigation of the organic peroxides in cyclohexene to 2 ppm resulted in a stable activity (187 mmol/h/gcat) ∼45 times greater than that reported in trickle-bed reactors. Deactivation by catalyst poisons such as palladium-formate, and CO (from reverse water-gas shift reaction) was insignificant at these conditions. Liquid-like heat capacity of the nc fluid aided in mitigating the “hot spot” temperature and its parametric sensitivity to the feed temperature and pressure.; Pressure-tunable solubility and diffusivity of SCFs was exploited to improve the selectivity toward methylcyclohexene during the series hydrogenation of toluene over Ru/Al₂O₃ (60°C, 138 bar, 90% CO ₂). Enhanced methylcyclohexene selectivity (10–25%), relative to the gas or liquid phase, in the absence of modifiers, was observed. Space velocity was found to be the most sensitive variable affecting methylcyclohexene selectivity.; High-pressure in situ FTIR studies to probe catalyst deactivation during sc hydrogenation revealed the susceptibility of palladium to CO-poisoning upon exposure to CO₂ and H₂ at reaction conditions. Temporal evolution of bridged-CO (1950 cm−1) suggests that catalyst poisoning might be insignificant in short residence-time continuous reactors compared to long residence-time batch reactors. Ruthenium and nickel were found to be in active for bridged-CO formation.; The steady activity obtained during the geometric isomerization of 1-hexene over Pt/γ-Al₂O₃ at sc conditions was exploited to investigate the intrinsic kinetics and effective diffusivities (D_eff). The activation energy was found to be 184 kJ/mol, while the D_eff was shown to increase by two orders of magnitude (6.3 × 10−6 cm2/s to 1.4 × 10 −4 cm2/s) on moderate changes in pressure at nc conditions (235°C–310°C, 35–75 bar).; The results of this dissertation have contributed to a better understanding of the strengths and limitations of sc reaction media in enhancing the efficiency of a multi-phase catalytic process.