Microporous silica analyses

A novel approach is reported for the accurate evaluation of pore size distributions for mesoporous and microporous silicas from nitrogen adsorption data. The model used is a hybrid combination of statistical mechanical calculations and experimental observations for macroporous silicas and for MCM-41 ordered mesoporous silicas, which are regarded as the best model mesoporous solids currently available. Thus, an accurate reference isotherm has been developed from extensive experimental observations and surface heterogeneity analysis by density functional theory the critical pore filling pressures have been determined as a function of the pore size from adsorption isotherms on MCM-41 materials well characterized by independent X-ray techniques and finally, the important variation of the pore fluid density with pressure and pore size has been accounted for by density functional theory calculations. The pore size distribution for an unknown sample is extracted from its experimental nitrogen isotherm by inversion of the integral equation of adsorption using the hybrid models as the kernel matrix. The approach reported in the current study opens new opportunities in characterization of mesoporous and microporous-mesoporous materials. 

Fig. XVII-30. Adsorption of Na on a silica gel at 77.3 K, expressed as a u-/ plot, illustrating a method for micropore analysis. (From Ref. 230.)...

Nitrogen adsorption was performed at -196 °C in a Micromeritics ASAP 2010 volumetric instrument. The samples were outgassed at 80 °C prior to the adsorption measurement until a 3.10 3 Torr static vacuum was reached. The surface area was calculated by the Brunauer-Emmett-Teller (BET) method. Micropore volume and external surface area were evaluated by the alpha-S method using a standard isotherm measured on Aerosil 200 fumed silica [8]. Powder X-ray diffraction (XRD) patterns of samples dried at 80 °C were collected at room temperature on a Broker AXS D-8 diffractometer with Cu Ka radiation. Thermogravimetric analysis was carried out in air flow with heating rate 10 °C min"1 up to 900 °C in a Netzsch TG 209 C thermal balance. SEM micrographs were recorded on a Hitachi S4500 microscope. 

Similarly, there is a great potential in the use of water vapour for the analysis of the porous texture, because it has considerable potential due to both the easy experimental conditions (at room temperature the whole range of relative pressures can be covered) and the characteristics of the molecule itself (polar molecule and small kinetic diameter-0.28 nm). This vapour is widely used in the characterisation of inorganic porous solids, such as zeolites, silicas, and clays. However, its interaction with carbon materials (microporous carbons coals, activated carbon fibres, carbon molecular sieves and porous carbons activated carbons), is more complex than the interaction of non-polar molecules [8]. 

As shown in Fig. 3, nitrogen adsorption isotherms of CMK-1 feature well-pronounced capillary condensation steps similar to those of ordered mesoporous silicas and indicative of high degree of mesopore size uniformity. The isotherms reveal that the CMK-1 carbon has high nitrogen BET specific surface area (1500-1800 m g ), and large total pore volume (0.9-1.2 cm g ) [14]. The adsorption capacity is comparable or larger than that of MCM-48 template. The pore-size analysis (calibrated BJH analysis) shows that typical CMK-1 has uniform mesopores about 3 nm in size, which is accompanied by a certain amount of micropores when sucrose is used as the carbon source. 

The new method allows one to evaluate not only pore size distributions, but also specific surface areas, primary mesopore volumes and micropore volumes. Moreover, it is applicable in the micropore range and appears to be essentially free from artefacts produced by many other methods of micropore analysis. Thus, a new approach provides a versatile and convenient tool for characterization of MCM-41, silica-based porous materials and other mesoporous and/or microporous oxides. 

The surface of ACF of w = 1.45 nm was modified with molecular adsorption-decomposition method using SiCU. SiCU was adsorbed on the ACF and then hydrolyzed by introduction of H2O vapour at 298 K. Afterwards, residual SiCU and produced HCl vapours were removed, and then the treated ACF was heated at 573 K. The amount of the produced hydrated silica was determined by the measurement of the weight change. The micropore structure of the silica-coated ACF was examined by N2 adsorption the t-plot analysis of the N2 adsorption isotherm showed that the micropore width decreases with the silica coating by 0.2 nm the silica coating decreased the micropore volume and surface area from 1.49 ml/g and 2280 m /g to 0.68 ml/g and 1100 m /g, respectively. No spherical silica particles were observed on the external surface of the silica-coated ACF by scanning electron microscopy with a resolution of 10 nm. Therefore, hydrated silica should be deposited entirely on the micropore walls of the ACF. 

The data in this table and of similar data sets provided by other authors (see Part II) have been interpreted with several degrees of sophistication in the simple model of electrostatic chemical shift When the data base is, however, extended to wide ranges of silica-to-alumina ratios, to other cations and to other microporous materials, the picture is less clear which is either due to inconsistent data acquisition (no single data source available) and/or to the breakdown of the simple shift analysis. 

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