Duen-Wu Hua

Characterization of Porous Solids.

Progress in the synthesis and engineering of advanced porous materials demands better pore structure characterization. The analysis of pore structure is complicated by (1) the wide range in pore sizes observed, from molecular ( 1 mm) dimensions, (2) complex pore shapes and connectivities, (3) chemical and physical heterogeneities, and (4) pore structure changes that can occur during characterization. The required pore structure information varies with application. Bulk density and the pore-size distribution are needed for thermal insulation. In this case, the dimension of interest is the so-called hydraulic radius since, for small pores, the gas-phase conductivity is proportional to the mean hydraulic radius to the mean free path. A few large but isolated pores will significantly affect conductivity but will go undetected in typical gas-absorption methods. In contrast, for separations, bottlenecks control performance. For transport, such as migration through geologic formations, both the pore-size distribution and pore connectivity are important. For adsorption, surface area and pore size are the relevant factors. Finally, the conventional concepts of pore structure lose meaning as the pore size approaches molecular dimensions, typical of adsorbents and gas-separation membranes.

Pore structure evolution in silica gel during aging/drying. III. Effects of surface tension*

A two-step acid/base-catalyzed silica gel has been aged in alcohol and water baths followed by various aprotic solvents with a wide range of surface tensions. Low temperature (CO 2 ) and high temperature (ethanol) aerogels were also prepared. The physical and chemical structures of gels dried from aprotic solvents were studied by a series of techniques ( 29 Si MAS-NMR, nitrogen adsorption, SAXS, elemental analysis, TGA). The aprotic solvents isolated the effects of pore fluid surface tension during drying since they do not participate in condensation and other reactions. For aprotic solvents, a linear decrease in xerogel surface area was observed with increasing surface tension. Pore volume and pore size distribution followed a similar trend. Depending upon whether the gel had been washed in ethanol or water prior to the aprotic solvent, the final pore volume was changed significantly for a given surface tension. This indicates that both surface area and pore volume may be independently controlled.

Thermal and temporal aging of TMOS-based aerogel precursors in water

Silica aerogels with densities <0.2 g/cm 3 have low mechanical strength and stiffness and improved mechanical properties are of interest to increase their commercial viability. Silica solubility in water is dependent on particle diameter, temperature and pH and, hence, these parameters affect aging. The manner in which the stiffness of a tetramethoxysilane-based gel can be increased by aging in water has been investigated. Aging at 40, 70 and 100°C as a function of time has been studied by measuring wet gel shear modulus and modulus of rupture. Surface area, density and small-angle X-ray scattering were measured on aerogels produced by aging, washing with acetone and liquid CO 2 , and supercritically drying. The observed increase in shear modulus, G, to 1.6 MPa from 0.7 MPa is caused by neck growth between primary particles, dissolution of smaller primary particles followed by deposition into regions of negative curvature and continued condensation reactions. The highest G modulus was obtained after 3 h of aging at 100°C compared with 132 h at 40° C. For longer times, the stiffness actually decreased, presumably a result of continued coarsening.

Formation of microporous silica gels from a modified silicon alkoxide. I. Base-catalyzed gels

Base-catalyzed silica gels have been prepared from mixtures of TEOS and a modified silicon alkoxide, methyltriethoxysilane. The ratio of modified ester to TEOS was varied in order to study the effect on gel structure. Low level additions of the modified ester resulted in an increase in the surface area of dried gels, but higher additions caused a decrease in the accessible area. This decrease in surface area corresponds to a significant change in physical morphology as probed with small angle X-ray scattering. 29 Si MAS-NMR was used to determine the Q and T distributions of the gels and to estimate surface area and skeletal density based on the terminal group size. As the methyltriethoxysilane content was increased, the fraction of T 3 and T 2 species increased and the difference between the nitrogen surface area (accessible) and ‘Q-derived’ (total) surface area increased. Adsorption of carbon dioxide, methane, and water at 298 K was also measured.

Structural analysis of silica aerogels

Abstract Silica aerogels have numerous properties which suggest many applications such as ultrahigh-efficiency thermal insulation, and performance in these applications relates directly to the aerogel pore size distribution. The micro and meso pore size range can be investigated by normal small-angle X-ray scattering and possibly nitrogen adsorption. The measurement of larger pores (> 500 A) is accomplished by using combined small-angle scattering (both light and X-ray) techniques. These techniques were employed to study the change in wet gel pore structure during partial drying of alkoxide-prepared aerogel precursor gels as well as for aerogels produced from those wet gels of different solids content prepared by supercritical CO2 drying. Aerogels made with density varying from 0.17 to ⋍ 0.6 g/cm3 as well as the wet gel precursors showed a loss of the largest pores first during partial drying. As the degree of drying shrinkage increased, both the large pores surrounding individual aggregates and mesopores within the aggregates decreased in size. These results agree with earlier scattering studies on colloidal silica aerogels which had been isostatically compacted to different densities.

Adsorption Studies Of Pure And Modified Imogolite As A Potential Pore Size Standard.

Abstract Imogolite is a microporous tubular aluminosilicate having the unique property of forming tubes of a single length and fixed inner diameter. With proper processing, the tubes can self assemble into aligned, densely-packed arrays exhibiting a high degree of microporosity orientated in a single dimension. These tubes have a nominal inner diameter of 0.8 nm but the diameter may be increased by replacing a portion of the silicon with germanium, or chemically modified via silylation reactions with the active silanol groups on the internal tube surface. These materials are attractive as pore size standards since they form continuous tubes (not throats and cavities typical of a zeolite), the pores are uniform in size, and the pore size may be varied or modified in a controlled fashion. We discuss the characterization of imogolite tube bundles by a range of adsorption experiments including nitrogen and carbon dioxide and by 129xe NMR. Gas phase silylation experiments are also explored in relation to changes in surface area, pore size, size distribution, total pore volume, and adsorption kinetics.

Advanced NMR-based techniques for pore structure analysis of coal. Final project report

During the 3 year term of the project, new methods have been developed for characterizing the pore structure of porous materials such as coals, carbons, and amorphous silica gels. In general, these techniques revolve around; (1) combining multiple techniques such as small-angle x-ray scattering (SAXS) and adsorption of contrast-matched adsorbates or {sup 129}Xe NMR and thermoporometry (the change in freezing point with pore size), (2) combining adsorption isotherms over several pressure ranges to obtain a more complete description of pore filling, or (3) applying NMR ({sup 129}Xe, {sup 14}N{sub 2}, {sup 15}N{sub 2}) techniques with well-defined porous solids with pores in the large micropore size range (>1 nm).