Microporous Volumes from Nitrogen Adsorption at 77 K: When to Use a Different Standard Isotherm?

João Pires,Moisés Pinto,Rafaela Fernandes,Mary Batista

doi:10.3390/catal11121544

Abstract

This work reviews the application of various standard isotherms to evaluate the micropore volume in a range of microporous materials. The selected materials have quite different surface chemistry, and are relevant due to their properties for adsorption and catalysis: zeolites, activated carbons, clay-based materials and MOFs. Some cases were analysed before and after being used as supports in the heterogenization of homogeneous catalysts. The discussion is centred, but not limited, to the three standard isotherms that are mostly employed in the literature (t-curve, non-porous carbon and non-porous hydroxylated silica) for the assessment of the micropore volume. For a given material the values of the micropore volumes from the different standard isotherms were compared, particularly against the values from the largely used t-curve. The cases where major discrepancies were found could normally be ascribed to samples that have a broad micropore size distribution.

Highlights

Nitrogen adsorption at 77 K is the standard technique for the characterization of microporosity in nanoporous materials
The effect of different standard isotherms in the determination of the microporous volumes from the nitrogen adsorption isotherm at 77 K, using standard isotherms in non-porous reference materials was reviewed for samples representative of various types of microporous materials, such as zeolites, clay-based materials, activated carbons and MOFs
Considering the three most employed standard isotherms in the literature (t-curve [24], carbon [25] and hydroxylated silica [1]), the microporous volumes using the standard isotherm for the hydroxylated silica were the highest for 58% of the materials studied in this work

Summary

Introduction

Nitrogen adsorption at 77 K is the standard technique for the characterization of microporosity in nanoporous materials. If for Cu-BTC and UiO-66 this relative invariance of the micropore volumes with the type of standard isotherm could be anticipated, in light of the above results that showed that this is relatively common for high rectangular isotherms, the case of MIL-101 would be less predictable For this material, the nitrogen isotherm is the least rectangular of those presented by the MOF materials (Figure 2d) but the structural regularity, that is, the uniformity of the pore dimensions, appears to be key. The network-accessible geometric volume obtained for MIL-101 is higher than that reported, pointing out that in the case of this MOF, where the pore openings are within the micropore dimensions but the internal dimensions of the cages are already in the range of the mesopores, the results obtained by t-plots and as-plots can provide values below the expected ones. (a) Difference between the value using the universal t-curve and using the non-porous carbon as standard. (b) Difference between the value using the universal t-curve and using the non-porous hydroxylated silica as standard

Materials and Methods

Findings

Conclusions