Difficulties Associated with Regard to the Analysis of Micropore Adsorption Data
Methods for Micropore Analysis
Physical adsorption in materials consisting of micropores, as for instance in zeolites etc. occurs at relative pressures substantially lower than in the case of adsorption phenomena in mesopores, but spans several orders of magnitude in relative pressure. In particular, the characterization of zeolites with nitrogen at 77.4 K is difficult, because the filling of pores of dimension 0.5 - 1 nm occurs at relative pressures of 10-7 to 10-5, where the rate of diffusion and adsorption equilibration is very slow. Hence, it is advantageous to analyze zeolites consisting of such narrow micropores by using argon as adsorptive at liquid argon temperature (87.3 K). Argon fills micropores of dimensions 0.5 – 1nm at much higher relative pressures (i.e., at relative pressures 10-5 to 10-3) compared to nitrogen, which leads to accelerated diffusion and equilibration processes and thus also to a reduction in analysis time.
Figure 1(a) shows the different pore filling ranges for argon adsorption at 87.3 K and nitrogen adsorption at 77.4 K based on adsorption data obtained on a faujasite-type zeolite. Argon adsorption at liquid argon temperature allows to obtain accurate and high-resolution adsorption isotherm data for the most important zeolites. This is demonstrated in Figure 1(b), which shows argon adsorption isotherms on some selected zeolite samples of different texture (i.e., different pore geometries and pore sizes).
Figure 1(a). Nitrogen and argon sorption isotherms at 77.4 K and 87.3 K on a faujasite-type zeolite.
Figure 1(b). Argon adsorption isotherms at 87.3 K in various zeolites.
In addition, difficulties are associated with regard to the analysis of micropore adsorption data. Classical, macroscopic theories like for instance the Dubinin-Radushkevich approach and semiempirical treatments such those of Horvath and Kawazoe (HK) and Saito and Foley do not give a realistic description of micropore filling. This leads to an underestimation of pore sizes for micropores and even for materials with narrow mesopores . Further, in order to determine the pore volume classical methods usually assume that the adsorbed fluid has a bulk-like liquid density. However this assumption is more than questionable for the adsorbate confined to the narrow micropores in zeolites. In order to achieve a more realistic description, microscopic theories which describe the sorption and phase behavior of fluids in narrow pores on a molecular level, are necessary. Thus, treatments such as the Density Functional Theory (DFT) or methods of molecular simulation (Monte Carlo Simulation (MC), Molecular Dynamics (MD)) provide a much more accurate approach for pore size analysis.
To overcome the above mentioned problems associated with zeolite characterization, Quantachrome introduced in 2001 a new method for micropore analysis based on Nonlocal Density Functional Theory (NLDFT). This NLDFT method is designed for pore size characterization of zeolitic materials with cylindrical pores using high-resolution low-pressure argon adsorption isotherms at 87.3 K. The materials covered include zeolites (e.g., silicalite, ZSM-5), catalysts, separation membranes, sensors and other zeolite-based systems.
Recently (i.e, in 2004) Quantachrome introduced another method, which is applicable to zeolitic materials with spherical pores, i.e. cage-like strucures (e.g., Faujasite, 13X). It is important to note that in case of zeolites consisting of cage-like structures, the position of the pore filling step is controlled by the adsorption potential exerted by the cage. Hence, from an analysis of argon or nitrogen adsorption data obtained on zeolites with cage-like structures one can calculate the cage diameter, but cannot determine the width of the aperture (which is however often erroneously reported in the literature). Many zeolites contain also secondary mesoporosity, i.e., also with regard to this aspect it is advantageous to apply NLDFT based methods because they allow to obtain a accurate pore size analysis over the complete micro/mesopore size range. This is demonstrated in Figures 2 and 3. Figure 2 shows high-resolution argon adsorption isotherms on zeolitic catalyst ZSM-5, mesoporous MCM-41 material, and the combined isotherm on a mixed micro-mesoporous material, containing 50% of ZSM-5 and 50% of MCM-41. Fig. 3 reveals the NLDFT pore size distributions of the combined” material (by applying a zeolite/silica hybrid NLDFT kernel based on a cylindrical pore model), which shows two distinct groups of pores: the micropores of ZSM-5 and the mesopores of MCM-41. The reported mode pore diameter of ZSM-5 zeolite obtained from structural considerations is 0.51-0.55 nm, which agrees very well with the pore size distribution obtained from Ar adsorption by the NLDFT method. The MCM-41 material is non-microporous and the pore size obtained by independent methods is 3.2 nm, in excellent agreement with the NLDFT method. Further, as shown in figure 4, the NLDFT-fit to the experimental adsorption isotherm is excellent.
Figure 2. High-resolution argon adsorption isotherms at 87.3 K on MCM-41 mesoporous molecular sieve, ZSM-5 zeolite and a mixed mixed micro-mesoporous material, containing 50% of ZSM-5 and 50% MCM- 41.
Figure 3. NLDFT pore size distribution obtained from argon adsorption at 87.3 K on a mixed micro-mesoporous material, containing 50% of ZSM-5 and 50% MCM- 41.
Figure 4. Comparison of NLDFT fit and experimental argon adsorption isotherm on a mixed micro-mesoporous material, containing 50% of ZSM-5 and 50% MCM- 41.
A complete list of references is available by referring to the source document.
Source: State-Of-The-Art Zeolite Characterization: Argon Adsorption At 87.3 K and Nonlocal Density Functional Theory (NLDFT)
For more information on this source, please visit Quantachrome Instruments.