An Extension of the Corrugated Pore Structure Model to Simulate Composite Nitrogen Adsorption Isotherms of Porous Materials Exhibiting Two Discrete Random Pore Structure Regions

摘要

MCM-41 materials often exhibit pore structures giving rise to composite gas sorption isotherms forming three steeply rising sections i.e. one very close to the volume axis, a second at low relative pressures ca. P/P_0=0.4 (i.e. attributed to primary mesopores) and a third one at the high pressure range ca. P/P_0=0.85-0.99 (attributed to secondary mesopores). Composite gas sorption isotherms for conventional porous materials e.g. porous aluminas and zeolites have been also reported. Occasionally, composite isotherms show either one or two hysteresis loops that indicate the presence of two independent random pore structure regions of hierarchical nature. To investigate the pore structure characteristics of each region separately, a procedure is proposed in the present work that is based in the newly reported CPSM Theory. The latter, so far, was applied successfully in simulations of nitrogen sorption data for MCM-41 materials that did not exhibit hysteresis.. Composite gas isotherms bearing one or two hysteresis loops can be interpreted by means of CPSM Model as follows. Nitrogen sorption is conceived occurring simultaneously on the surface of both primary and secondary mesopores, whereas capillary condensation and evaporation phenomena take place in series (i.e. independently in each domain). In terms of the CPSM model, the appearance of a hysteresis loop at P/P_0=0.35-0.40 is of thermodynamic meta-stability origin and may be enhanced by pore structure networking effects, when N_(sp)>2 (i.e. N_(sp)nominal length of primary corrugated mesopores) and another at P/P_0=0.85-0.99 that is primarily due to pore structure networking (i.e. when the nominal pore length of secondary corrugated mesopores is ca. N_(ss).>2) and to a lesser extend to interface meta-stability effects. To validate the latter observations and demonstrate a new way of applying the CPSM model, a theoretical case of a composite isotherm showing two hysteresis loops resembling those of some MCM-41 materials, was worked out. A critical discussion of the results is provided and useful conclusions are drawn, especially with regard to the prediction of tortuosity factors and effective diffusion coefficients separately for each pore structure domain.