6728
Langmuir 1999, 15, 6728-6737
An Adsorption-Based Method for the Characterization of Pore Networks Containing Both Mesopores and Macropores K. L. Murray,† N. A. Seaton,*,‡ and M. A. Day§ Department of Chemical Engineering, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, United Kingdom, School of Chemical Engineering, University of Edinburgh, King’s Buildings, Mayfield Road, Edinburgh EH9 3JL, United Kingdom, and ICI Technology, Research and Technology Centre, PO Box 90, Wilton, Middlesborough, Cleveland TS90 8JE, United Kingdom Received February 16, 1999. In Final Form: May 13, 1999 Nitrogen sorption is a commonly used experimental technique for the characterization of solids that are primarily mesoporous. The standard nitrogen adsorption experiment can also detect pores at the lower end of the macropore range, up to perhaps 3000 or 4000 Å. However, some solids of industrial importance contain macropores beyond the size range probed by the standard nitrogen adsorption experiment. In this work, a modified experimental method is used to obtain a more complete characterization of the solid. The experiment involves the bulk condensation of nitrogen in the sample tube after completing the adsorption isotherm and before measurement of the desorption isotherm. Modifications to existing pore size distribution and connectivity analyses are proposed to incorporate data from the experimental desorption isotherms. These modified analyses enable an improved estimate for the pore size distribution and a lower bound on the mean coordination number of the pore network to be obtained for these samples.
1. Introduction Porous solids are widely used in industrial processes (e.g., as adsorbents and catalyst supports), in which they must perform efficiently and reproducibly. To ensure such behavior and to enable rational design of new processes and assist in troubleshooting existing ones, these solids need to be well characterized. The transport and reaction properties of a porous solid are determined by its morphology [i.e., the pore size distribution (PSD) and the connectivity of its pore network]. Conventional nitrogen adsorption and desorption isotherms, measured at 77 K, provide the input data for analysis methods to determine both the PSD and the connectivity of solids that are largely mesoporous [defined by the International Union of Pure and Applied Chemistry (IUPAC) as having pore widths ranging from 20 to 500 Å]. These solids have sorption hysteresis loops of IUPAC types H1 and H2 (illustrated in Figure 1, a and b). Well-established PSD analysis methods for mesoporous solids1-3 are based on the use of the Kelvin equation to analyze capillary condensation during the nitrogen adsorption process. The Kelvin equation determines the pressure at which nitrogen condenses in a pore of a given size; the presence of an adsorbed film on the pore wall before condensation is also taken into account. Because the Kelvin equation is a purely thermodynamic result and does not take explicit account of structure at the molecular level, it becomes inaccurate for small mesopores (below about 50 Å4,5) and fails to describe adsorption in micropores * To whom correspondence should be addressed.Tel. (+44) 131 650 4867. Fax (+44) 131 650 6551. E-mail
[email protected]. † University of Cambridge. ‡ University of Edinburgh. § ICI Technology. (1) Barrett, E. P.; Joyner, L. G.; Halenda, P. H. J. Am. Chem. Soc. 1951, 73, 373. (2) Cranston, R. W.; Inkley, F. A. Adv. Catal. 1957, 9, 143. (3) Brunauer, S.; Mikhail, R. Sh.; Bodor, E. E. J. Colloid Interface Sci. 1967, 24, 451.
(defined as having pore widths
