Adsorption properties of porous silicophosphate

Abstract

Isotherms of low-temperature physical adsorption-desorption of nitrogen for silicophosphate samples obtained at pH of the sol-gel transition varied from 5 to 9 in the presence of a template (cetylpyridinium chloride) were measured. The isotherms obtained were reconstructed in the form of t-graphs. The parameters of the microand mesopore structure of the samples were determined.Silicophosphates in the form of nanopowders, thinlayer membranes, and mesoporous materials are used in microelectronics, hydrogen power engineering, and electrophoretic separation of peptides and proteins [1]. An obvious problem concerning the new interpretation of the basic approaches to the formation of mesoporous silicophosphates, as well as silica gel, is directly related to the application and refi nement of variants of the solgel method.The goal of the study was the determination of the surface areas of mesopores and micropores in silicophosphate samples based on traditional and comparative analysis of low-temperature nitrogen sorption isotherms. It was suggested that the comparative analysis will make it possible to determine the true changes in the texture of silicophosphates obtained at different pH values of the sol-gel transition and in the presence of a template. EXPERIMENTALThe scheme of the silicophosphate synthesis included the preparation of solutions, the hydrolysis and condensation, aging, and drying. We used an aqueous sodium silicate solution containing 2 wt % of the salt (molar ratio SiO 2 :Na 2 O ≈ 3.0). Silica gel was precipitated with the phosphate anion under standard conditions at pH values of the sol-gel transition 5-9. As precipitants were used 2N sulfuric acid and ammonium hydrogen phosphate (NH 4 ) 2 HPO 4 (molar ratio Si : P = 1). Template was 1.0 wt % cetylpyridinium chloride (CPC). The resulting precipitate was aged for 0.5 h, separated onto a vacuum fi lter, thoroughly washed with distilled water, and dried in a desiccator at 323 K to constant weight.The adsorption properties of samples were estimated from the low-temperature (77 K) nitrogen physical adsorption-desorption isotherms measured by the volume method (ASAP 2020 МР surface area and porosity analyzer). The specifi c surface area was determined by the BET (A BET ) and Langmuir (A L ) methods and the micropore surface area (A micro ) and the external surface area (A ext ), by the comparative t-graph method. The desorption cumulative surface area (А BJH ) of pores with the diameter from 1.7 to 300 nm, the cumulative pore volume V BJH in the same range of the diameters, the average pore diameter D BJH , and the differential mesopore volume distribution over diameter were calculated by the Barrett, Joyner, and Halenda (BJH) method. Prior to the analysis, samples were evacuated at 523K under the residual pressure 133.3 × 10 -3 Pa for 1 h.The t-graph method consists in comparison of the isotherm being analyzed with the standard isotherm measured on a well-characterized reference [2]. We compared in the study the obtained nitrogen adsorption isotherms with...

Effect of aging and alkali activator on the porous structure of a geopolymer

“…The closeness of the hysteresis loop at P/P 0 > 0.40 indicates the absence of micropores and the formation of a well defined mesoporous texture (Radlinski et al, 2004;Sharma et al, 2002). According to IUPAC classification, the nitrogen adsorption-desorption isotherms show a typical type IV sorption behaviour with mixed type hysteresis loops (H1 + H2) (Rouquerol et al, 2003;Kuznetsova et al, 2012). The features of a hysteresis loop of type 1 on a loop of type 2 are usually attributed to a difference in mechanism between the condensation and evaporation processes occurring in interconnected pores with narrow necks and wide voids, often called 'ink-bottle pores' (Brunauer et al, 1938;Leofanti et al, 1998;Mami et al, 2008).…”