| RESEARCH |
A different approach to improving solid acid catalysis and mitigating the problem of catalyst deactivation is the use of supercritical carbon dioxide as the reaction medium. Carbon dioxide has moderate critical properties (71.8 bar, 31.1 °C), and the supercritical CO2 region provides a continuum of properties There is a window near the critical point where there are unique combinations of fluid properties For example, the density within the window is liquidlike and suitable for solvating heavy hydrocarbons. The supercritical fluid also retains the diffusivity properties of a gas that allows the transport of heavy hydrocarbons or their precursors out of the catalyst pores before they consolidate and foul the catalyst. "By tuning the pressure, one can determine the optimum combination of gaslike and liquidlike properties that is essential for in situ decoking," he added. Supercritical CO2 properties can be further enhanced by adding auxiliaries, such as cosolvents, he noted.group studied alkylation reactions using zeolite catalysts in a lab-scale supercritical CO2 system. Although supercritical CO2 was found to be good at dissolving the heavy coke material on the outer catalyst surface, difficulties remained in removing the material from inside the small pores (5 to 10 Å) of the catalyst. next tried Nafion on silica--which has a larger mean pore size in the range of 50 to 70 Å--as the catalyst. Nafion is a DuPont perfluoropolymer that has sulfonic acid groups along its backbone; The acidity of Nafion is similar to sulfuric acid, and there have been some reports of using Nafion as an alkylation catalyst. These attempts were not successful, he noted, because the catalyst tends to rapidly deactivate in the liquid phase. AS A TEST of the supercritical CO2 system reacted isobutane with 1-butene in a 5:1 ratio in a slurry reactor using silica-supported Nafion particles suspended in supercritical CO2. Typically a 10- to 20-fold excess of isobutane is needed in conventional alkylation, Subramaniam explained, but in this system part of the isobutane is effectively replaced with CO2. This saves on costs, has a lower flammability hazard, and lowers environmental concerns, he said. Operating at 95 °C, reaction conditions were optimized by tuning the pressure of the reactor to 80 bar. Under those conditions, C8 alkylate selectivity was maximized at 30% with a butene conversion rate of 80%. On-line gas chromatographic analysis of the effluent was used to look at butene conversion and product composition. The researchers were able to maintain steady C8 alkylate production for up to two days before the catalyst began to show signs of deactivation--compared with a few hours operating at subcritical conditions. "In terms of product quality, these are not the most ideal results," but certainly in terms of changing from a microporous to a mesoporous catalyst, we see dramatic improvement in conversion and selectivity. Clearly, milder supercritical pressures provide the optimum combination of liquidlike densities and gaslike transport properties to effectively remove C8 products and coke precursors from the catalyst pores." The initial results led to the hypothesis that catalyst pores could periodically be cleaned by dissolving the heavier hydrocarbons at high pressure before catalyst reactivity is significantly diminished. In this way, the catalyst could be directly rejuvenated in the reaction vessel. The Kansas researchers tested this idea by running daylong cycles of alkylation, then stopping the feed of reactants and increasing the CO2 pressure. After four days of cycling, the catalyst activity was retained. By alternating the reaction/regeneration cycle between a pair of reactors, an uninterrupted production process could be established These results imply that CO2-based reaction mixtures and solid acids tailored for optimum pore structure and acidity offer an excellent opportunity for developing environmentally benign alternatives to conventional alkylation," he concluded. The reaction system should be amenable to other solid-acid-catalyzed reactions and isomerization reactions, he said |
| UPDATE | 10.01 |
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