On the textural and morphological properties of crystalline and amorphous α-tin phosphate

Abstract

The synthesis as well as the textural and morphological properties of microporous amorphous and crystalline tin(IV) phosphate, Sn(HPO₄)₂·H₂O (α-SnP), prepared by the sol–gel technique are presented. Tin tetra-tert-amyloxide, Sn(OAm^t)_4, is first synthesized and a solution of this reactant in tert-amyl alcohol reacts with ortho-phosphoric acid to obtain in due course both the amorphous and the crystalline α-SnP forms. The N₂ sorption isotherm of the crystalline powder attains a hysteresis cycle proper of lamellar void structures over a long interval of relative pressures. The differential t-plot of this isotherm reveals the presence of two kinds of supermicropores: (i) a group of narrow slit-like pores with interlamellar void maxima arising at 0.712 nm, 0.774 nm, and 0.828 nm (these values suggest that the primary micropore filling of the interlayer space occurs with at most two layers of N₂ indicating likewise the non-rigid character of the plates that are delimiting each slit-like pore), (ii) a second microporous contribution that involves the cooperative micropore filling of wider slit-like pores having widths in the interval 1.01–1.57 nm. The narrow supermicropores are formed by the interlamellar spacing between a pair of elementary α-Sn-P sheets, while the wider micropores are delimited by solid plates that may be built by the parallel stacking of several elementary sheets of α-Sn-P. BET and t surface areas of the α-SnP specimen both correspond to 20.2 m² g⁻¹, therefore confirming adsorption on flat surfaces and the absence of voids other than supermicropores. Application of Bragg’s equation to the X-ray diffraction pattern of the α-SnP sample renders an elementary interlayer distance of 0.80 nm that compares well with the theoretical 0.78 nm value. NMR spectra depict sharp crystalline peaks at −11.9 and −12.1 ppm for the amorphous and crystalline forms, respectively. SAXS measurements render crystallite sizes between 4 and 32 nm for the amorphous and crystalline forms. Crystalline α-SnP also depicts a wide-ranging fractal structure; this fact is confirmed by the consistent values of the surface fractal dimension obtained from application of the Neimark–Kiselev sorption approach over a long range of relative pressures.