Nanoporous reactive membranes: The influence of pore structure of membrane selectivity.

摘要

Nanoporous membranes are thin films of molecular sieving materials having a narrow mode in pore size and remarkable gas separation capabilities. In this work, several catalytic nanoporous carbon membranes have been synthesized and investigated for high selectivity reaction and separation applications. Porous defects—larger, non-selective transport pathways within the film—have been shown to play a critical role in rationalizing membrane performance under both reactive and non-reactive conditions.; For example, highly disperse Pt within NPC films have been synthesized using a variation on a patented methodology and studied using the selective hydrogenation of mono-olefins (propylene, 1-butene, isobutylene). An analysis of the data using a solution of the reaction and diffusion problem within the film reveals ratios of intrinsic reaction rate constants of 62.8 for propylene/isobutylene and 5.3 and for propylene/1-butene. These ratios are much higher than intrinsic values measured over non-selective carbon catalysts. This suggests that reaction occurs in parallel with membrane permeation and is diffusion disguised in the classical sense. Similarly, results from the application of acid catalytic NPC membranes to the decomposition of methyl tert-butyl ether (MTBE) indicate that significant retention of the MTBE and subsequent conversion enhancement is highly sensitive to the membrane selectivity for small molecular separations. For example, membranes with O₂/N₂ permeability ratios of 0.94 to 6.5 span a wide range of reactor performance: from no retention in the former to complete retention with the latter.; Both of these results suggest a highly anisotropic pore distribution where a small fraction of larger pores influence membrane transport characteristics dramatically. To confirm this hypothesis, a transient analysis of membrane permeation, i.e. the time lag method, was performed to measure adsorption and diffusion in-situ for both catalytic and non-catalytic membranes. Heats of adsorption were found to be significantly lower than those for granular carbon adsorbents. Similarly, diffusion activation energies were closer to values reported for larger pore microporous glass (1.5 nm). The results generally point to the presence of two parallel pathways for transport—one through nanopores and a second through larger defect pathways that are few in absolute number but exert a disproportionate effect on membrane selectivity. It is also shown that their effect can be amplified dramatically by molecular templating providing a new route ultra- and nanofiltration carbon membranes. A parallel resistance model has been derived and applied to 19 different nanoporous carbon membranes published separately in the literature. Despite the wide variation in synthesis conditions reported, the model is able to describe the complete variation in separation performance of several membrane systems for the first time in the literature.