Johan C. Groen

Pore size determination in modified micro- and mesoporous materials. Pitfalls and limitations in gas adsorption data analysis

Physical gas adsorption is extensively used in the characterization of micro- and mesoporous materials and is often considered as a straightforward-to-interpret technique. However, physical phenomena like the tensile strength effect, adsorbate phase transitions, and monolayer formation in combined micro- and mesoporous materials frequently lead to extra contributions in the adsorption isotherm. Models for pore size determination mostly do not account for this, and assignment to real pores leads to improper analysis of adsorption data. In this review, common pitfalls and limitations in the analysis of pore size distributions derived from adsorption isotherms of micro- and mesoporous materials are identified and discussed based on new results and examples reported in the recent literature.

Hierarchical zeolites: Enhanced utilisation of microporous crystals in catalysis by advances in materials design

The introduction of synthetic zeolites has led to a paradigm shift in catalysis, separations, and adsorption processes, due to their unique properties such as crystallinity, high-surface area, acidity, ion-exchange capacity, and shape-selective character. However, the sole presence of micropores in these materials often imposes intracrystalline diffusion limitations, rendering low utilisation of the zeolite active volume in catalysed reactions. This critical review examines recent advances in the rapidly evolving area of zeolites with improved accessibility and molecular transport. Strategies to enhance catalyst effectiveness essentially comprise the synthesis of zeolites with wide pores and/or with short diffusion length. Available approaches are reviewed according to the principle, versatility, effectiveness, and degree of reality for practical implementation, establishing a firm link between the properties of the resulting materials and the catalytic function. We particularly dwell on the exciting field of hierarchical zeolites, which couple in a single material the catalytic power of micropores and the facilitated access and improved transport consequence of a complementary mesopore network. The carbon templating and desilication routes as examples of bottom-up and top-down methods, respectively, are reviewed in more detail to illustrate the benefits of hierarchical zeolites. Despite encircling the zeolite field, this review stimulates intuition into the design of related porous solids (116 references).

Desilication: on the controlled generation of mesoporosity in MFI zeolites

Recent studies have shown that desilication by treatment in alkaline medium is, with respect to other methods, a very suitable and reproducible methodology to obtain mesoporous ZSM-5 zeolites with preserved structural integrity. This feature article analyzes mechanistic and kinetic aspects associated with this post-synthesis treatment. Framework aluminium controls the process of framework silicon extraction and makes desilication selective towards intracrystalline mesopore formation. An optimal molar Si/Al ratio in the range of 25–50 has been identified. At higher Si/Al ratios non-selective and excessive extraction of framework silicon occurs, while minor extraction and limited mesopore formation occurs at lower ratios. The presence of non-framework aluminium species, for example obtained by steam treatment, inhibits mesopore formation by alkaline treatment due to reinsertion of these species into the zeolite framework. Additional kinetic optimization of the physicochemical properties of the hierarchical porous zeolite structures is achieved by variation of time and temperature of the alkaline treatment. A successive combination of post-treatments, in which desilication is followed by dealumination, enables a decoupled modification of the mesoporous and acidic properties, being interesting in catalyst design and optimization. Preliminary work on other zeolite framework types has shown a promising outlook of the alkaline treatment. Development of mesoporous zeolites via desilication should induce a more efficient usage of the zeolite crystal due to an improved accessibility and a facilitated transport to and from the active sites.

Liquid intrusion and alternative methods for the characterization of macroporous materials (IUPAC Technical Report)

This document deals with the characterization of porous materials having pore widths in the macropore range of 50 nm to 500 μm. In recent years, the development of advanced adsorbents and catalysts (e.g., monoliths having hierarchical pore networks) has brought about a renewed interest in macropore structures. Mercury intrusion–extrusion porosimetry is a well-established method, which is at present the most widely used for determining the macropore size distribution. However, because of the reservations raised by the use of mercury, it is now evident that the principles involved in the application of mercury porosimetry require reappraisal and that alternative methods are worth being listed and evaluated. The reliability of mercury porosimetry is discussed in the first part of the report along with the conditions required for its safe use. Other procedures for macropore size analysis, which are critically examined, include the intrusion of other non-wetting liquids and certain wetting liquids, capillary condensation, liquid permeation, imaging, and image analysis. The statistical reconstruction of porous materials and the use of macroporous reference materials (RMs) are also examined. Finally, the future of macropore analysis is discussed.

Facile synthesis of ZSM-5 composites with hierarchical porosity

Hierarchically structured composites (TUD-M) with ZSM-5 nanocrystals embedded in a well-connected mesoporous matrix were synthesized by employing only one organic templating/scaffolding molecule (TPAOH). Micro- and mesopores form separately under different synthesis conditions. Both the size of the zeolite crystals and the mesopore size in the amorphous matrix can be tuned. A lower Si/Al ratio results in a slower growth of zeolite crystals and improves the hydrothermal stability of this new type of composite. Solid state NMR reveals that the aluminium species are all tetrahedrally coordinated, and that silicon species condense further during crystal growth. A carbon replica of TUD-M proves that the mesopores are interconnected, and also hints at the similarities between the meso-structures of TUD-M and TUD-1. The scaffolding mechanism at the basis of the mesostructure is not limited to TPAOH. Other zeolite/meso-structure composites could also be synthesized based on the same concept.

Natural gas purification with a DDR zeolite membrane; permeation modelling with maxwell-stefan equations

Abstract The permeation of CO 2 , N 2 and CH 4 and their mixtures through a DDR membrane has been investigated over a wide range of temperatures and pressures. The synthesized DDR membrane exhibits a very high selectivity for CO 2 and a moderate selectivity for N 2 over CH 4 with good permeances. At a total pressure of 101 kPa and temperature of 225 K, the selectivity for CO 2 was found to be over 3000 and 40 for N 2 in a 50/50 feed mixture with CH 4 , both decrease with temperature. The N 2 /CH 4 selectivity remains constant with pressure, while that for CO 2 /CH 4 decreases. An engineering model, based on the generalized Maxwell-Stefan equations, has been used to interpret the transport phenomena in the membrane. The diffusivity of these permanent gases is strongly dependent on the loading in the membrane, severely complicating modelling work. A model developed by Reed and Ehrlich [ 1 ] could describe this phenomenon well for both the unary as the binary permeances. The feasibility of DDR membranes as applied to CO 2 and N 2 removal from natural gas is anticipated.

In situ monitoring of desilication of MFI-type zeolites in alkaline medium

In situ pH and Attenuated Total Reflection (ATR) infrared techniques have been successfully applied in order to gain insights into the dissolution process connected to mesopore formation occurring upon alkaline treatment of ZSM-5 zeolites. Online pH measurements reveal a similar consumption of OH(-) ions in the initial stage of the reaction independent of the Si/Al ratio of the zeolite. In view of the greatly different mesoporosity development, the extraction of polymeric silica entities is anticipated, its structure depending on the framework Si/Al ratio. In agreement, ATR-IR experiments have confirmed dissolution of polymeric silicon-containing species that in the course of the alkaline treatment disintegrate into smaller entities. A direct relation between the type of porosity developed and the process of silicon extraction as measured in the liquid phase cannot be drawn.

Single-template synthesis of zeolite ZSM-5 composites with tunable mesoporosity

Hierarchically structured composites (TUD-C) with ZSM-5 crystals embedded in a well-connected mesoporous matrix were synthesized by using only one organic templating/scaffolding molecule (TPAOH).

Role of intrinsic zeolite properties on mesopore formation by desilication of MFI structures

In this contribution the alkaline treatment of both commerical and synthesized MFI zeolites has been studied for creation of hierarchical porosity. The influence of Si/Al ratio, crystal size, and counter-cation on the mesoporosity development was investigated. The Si/Al ratio is of crucial importance in the mesoporosity development, as framework aluminium proves to be the pore directing agent. An optimal Si/Al range of 25–50 is found for Si extraction leading to well-controlled mesopore formation. At lower Si/Al ratios the extraction of framework Si is inhibited due to the protective role of Al, while at higher ratios excessive Si extraction leads to formation of large pores. Variations in the zeolite crystal size reveal a decreased mesoporosity development in crystals>3 μm, where both crystal size and an Al gradient can be held responsible. The type of counter-cation (Na, NH 4 , and H) that, balances the negative charge of the Al tetrahedra does not significantly affect the mesoporosity development.

The impact of mesoporosity on microporosity assessment by CO2 adsorption, revisited

The use of both CO 2 and N 2 physisorption measurements provide complementary information on the determination of micropore characteristics. The presence however of a substantial amount of external surface area, as e.g. present in hierarchically structured porous materials, can lead to an overestimation of the Dubinin-Radushkevich micropore volume as derived from CO 2 physisorption. A simplified method of correction to the already known isotherm subtraction approach is proposed here. This proposed correction is proportional to the amount of external surface area determined by N 2 adsorption and is based on correction of the DR-results. Advantages of this method are the ease of determining the correction factor and the simplicity of correction the obtained micropore volume under normal conditions using only sub-atmospheric CO 2 and N 2 adsorption measurements. The results of our method show a good similarity to the isotherm subtraction approach in the obtained micropore volumes. Novel adsorption models such as Density Functional Theory can only properly take into account the contribution of the external surface area if this has been actually measured during the analysis, which requires high-pressure CO 2 adsorption.