Nitrogen sorption porosimetry

Nitrogen sorption measurements were performed on a Quantachrome Autosorb 6B (Quantachrome Corporation, Boynton Beach, FL, USA). All samples were degassed at 423 K before measurement for at least 12 hours at 1 O 5 Pa. Mercury-porosimetrie has been measured on a Porosimeter 2000 (Carlo Erba Instruments) Scanning electron micrographs were recorded using a Zeiss DSM 962 (Zeiss, Oberkochen, Germany). The samples were deposited on a sample holder with an adhesive carbon foil and sputtered with gold. 

Rieckmann and Keil (1997) introduced a model of a 3D network of interconnected cylindrical pores with predefined distribution of pore radii and connectivity and with a volume fraction of pores equal to the porosity. The pore size distribution can be estimated from experimental characteristics obtained, e.g., from nitrogen sorption or mercury porosimetry measurements. Local heterogeneities, e.g., spatial variation in the mean pore size, or the non-uniform distribution of catalytic active centers may be taken into account in pore-network models. In each individual pore of a cylindrical or general shape, the spatially ID reaction-transport model is formulated, and the continuity equations are formulated at the nodes (i.e., connections of cylindrical capillaries) of the pore space. The transport in each individual pore is governed by the Max-well-Stefan multicomponent diffusion and convection model. Any common type of reaction kinetics taking place at the pore wall can be implemented. 

Nitrogen sorption measurements were performed by use of a Sorptomatic 1900 Turbo apparatus by Carlo Erba Instruments. All samples were degassed at 393 K before measurement for at least 24 hours at 10 mbar. The mercury porosimetry measurements were carried out on a Porosimeter 2000 apparatus by Carlo Erba Instruments. A contact angle of 141.3° for Hg was used. The samples were degassed at 393 K before measurement for 24 h. SEM of the porous glass membranes was carried out on a Phillips ESEM XL 30 PEG microscope. 

The texture properties of the ultrathin porous glass membranes prepared in our laboratory were initially characterized by the equilibrium based methods nitrogen gas adsorption and mercury porosimetry. The nitrogen sorption isotherms of two membranes are shown in Fig. 1. The fully reversible isotherm of the membrane in Fig. 1 (A) can be classified as a type I isotherm according to the lUPAC nomenclature which is characteristic for microporous materials. The membrane in Fig. 1 (B) shows a typical type IV isotherm shape with hysteresis of type FIl (lUPAC classification). This indicates the presence of fairly uniform mesopores. The texture characteristics of selected porous glass membranes are summarized in Tab. 1. The variable texture demanded the application of various characterization techniques and methods of evaluation. 

DVB were valid in this system as well. These concern the dependence of surface area and pore volume on the amount of diluent and cross-linker. The surface area increases with the amount of EDMA and goes through a maximum with increasing amount of diluent. Using cyclohexanol-dodecanol as a solvent-non-solvent pair, the factors of importance for the structure and morphology of the polymers were studied by experimental design [34]. In these experiments the concentration of the diluent mixture was varied between 20 and 77% (volume/total volume), the concentration of EDMA between 25 and 100% (volume/monomer volume), the concentration of initiator (AIBN) between 0.2 and 4% (w/w), the concentration of non-solvent (dodecanol), between 0 and 15% (v/v) and the polymerisation temperature between 70° and 90°C. The surface area (determined by nitrogen sorption), pore volume (determined by mercury porosimetry) (see Section 2.11.6.) and the mechanical properties were chosen as responses. 

As was observed for the C500 silica, reanalysis of the pore structure by nitrogen sorption (Figure 8, curve c) following intrusion and removal of mercury indicates that the porosimetry experiment results in a permanent loss in pore volume and a shift to smaller pore sizes. In C200, however, this loss in pore volume no longer approximates that associated with the broad-diffuse area of the intrusion trace. Instead, the loss represents only... 

Figure 46.6 depicts the internal cumulative pore volume as a function of pore diameter for Sorbsil C500. The total pore volumes as measured by the two techniques agree to within 0.05 cm /g the pore volume measured by mercury porosimetry is slightly higher, possibly because this is an extremely wide-pored silica, with some pores too wide to be measured by nitrogen sorption. 

Whereas from nitrogen sorption data a size distribution can only be extracted for mesopores (with pore diameter 2 nm < dp < 50 nm), standard mercury porosimetry is used to obtain complete pore size distributions in the pore diameter range from 7.5 nm to 150 [im. During the characterization experiment, the sample is first surrounded and then progressively intruded by mercury, as the pressure is increased. Experimental results are commonly plotted as invaded pore volume versus applied pressure (see Fig. 5.9a). The Washburn equation describes at which (capillary) pressure a cylindrical pore of diameter dp is invaded... 

Besides the determination of the porosity, pore size distribution and surface area, the (surface or mass) fractal dimension of dry gels may be of interest. To this purpose, small-angle X-ray scattering can be used nitrogen sorption and mercury porosimetry also offer possibilities to extract this structural information (see, for example, Blacher et al. (2000)). 

Lubda, D., Lindner, W Quaglia, M., du Fresne von Hohenesche, C., Unger, K K., 2005. Comprehensive pore stmcture characterization of silica monohths with controlled mesopore size and macropore size by nitrogen sorption, mercury porosimetry, transmission electron microscopy and inverse size exclusion chromatography. J. Chromatography A 1083 14-22. 

The model equations for the catalyst pellet also contain the tortuosities td, tk or a combination of both t and es. Their determination requires specific equipment. Well instrumented catalyst characterization equipment, including computerized data treatment, is commercially available, in particular for mercury porosimetry and nitrogen-sorption and -desorption. 

The nitrogen sorption (Fig. 3.5.1.2.A-lb) shows a steep increase of the amount adsorbed at a relative pressure of 0.9999. At that pressure the macropores are filled by nitrogen through capillary condensation. From the Washburn equation a total volume of adsorbed N2 of 0.6269 cm /g cat is calculated, in excellent agreement with that derived from Hg-porosimetry. The volume of N2 adsorbed until that sharp rise, 0.5453 cm /g cat, is the meso pore volume. Consequently, the macropores occupy 0.08165 cm /g cat, which is 13% of the total. 

H. Giesche, K.K. Unger, U. Muller, and U. Esser Hysteresis in nitrogen sorption and mercury porosimetry on mesoporous model adsorbents made of aggregated monodisperse silica spheres, CoUoids Surf., 37 (1989) 93-113... 

S. Bukowiecki, B. Straube, and K.K. Unger Pore structure analysis of close-packed silica spheres by means of nitrogen sorption and mercury porosimetry, in Principles and Applications of Pore Structural Characterization (Haynes J.M. Rossi-Doria P., eds) Arrowsmith,... 

Abdel-Jawad, Y., and W. Hansen (1989). Pore Structure of Hydrated Cement Determined by Various Porosimetry and Nitrogen Sorption Techniques . In Symposium Proceedings Vol. 137 Pore Structure and Permeability of Cementitious Material, edited by Robert, L. R., and J. P. Skalny, 105-118. Warrendale, Pennsylvania Materials Research Society. 

Hansen W. and J. Almudaiheem (1986). Pore structure of hydrated Portland cement measured by nitrogen sorption and mercury intrusion porosimetry . In MRS Proceedings Vol. 85, p. 105, Cambridge University Press. 

Figure 4 Comparison of PSDs obtainedfor MCM-41 type adsorbents from nitrogen and argon porosimetry using DFT (left). The pore wall thickness of five different MCM adsorbents, found by combining results from XRD and sorption DFT measurements, is consistent in four of the five samples (right) [20].

Gas sorption (nitrogen at 77 K), mercury intrusion (mercury porosimetry) Specific surface area (BET), pore size distribution, average pore diameter, specific pore volume, particle porosity Retention of solutes, mass loadability, column regeneration, column performance, mass loadability, pore and surface accessibility for solutes of given molecular weight, mechanical stability, column pressure drop, pore connectivity... 

Froment discusses pore network influences in deactivation. He concludes that, "Evidently, the parameters associated with the pore and network structure should be determined from independent physical measurements adsorption, mercury porosimetry, electron microscopy..." Unfortunately, no experimentally based studies have been published that have employed one or a combination of techniques to determine the pore network structure and its changes during deactivation. The few experimental determinations of pore structure are limited to determination of pore dimensions usually from the intrusion data in Hg porosimetry or the desorption data from nitrogen physical sorption (often incorrectly referred to as BET analyses). 

Microporosity (< 20 A) is best examined using nitrogen ad/desoiptioa While diffinent physics are at work in characterizing sorption data, the effects of network stroctuie yielding hysteresis in the adsorption and desorption profiles is similar to that found from mercury porosimetry (ref. 12) for mesopores. 

---