Structural Transformations of Hydrolysates Obtained from Ti-, Zr-, and Ti, Zr-Solutions Used for Clay Pillaring: Towards Understanding of the Mixed Pillars Nature.

Krzysztof Bahranowski,Agnieszka Klimek,Alicja Michalik,Katarzyna Górniak,Ewa Serwicka-Bahranowska,Adam Gaweł

doi:10.3390/ma12010044

Abstract

Structural characteristics of hydrolysates formed from the aqueous Ti-, Zr-, and Ti, Zr-pillaring solutions prepared from inorganic precursors (TiCl4 and ZrOCl2), was investigated and compared with that of precipitates obtained from the same solutions after a slight alkalization of pH to the values reported for the conditions of clay pillaring. The materials were recovered by lyophilization and subsequently subjected to calcination at 500, 800 and 1000 °C. Of special interest was the effect of pH on the possible formation of mixed Ti, Zr-oxide species. Powder X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscope/energy dispersive spectrometer (SEM/EDS) analysis showed that even a relatively moderate alteration of pH in Ti-, Zr-, or Ti, Zr-precursor solutions caused substantial changes in the outcome of hydrolytic transformations, manifested by different phase and/or chemical composition of the resulting hydrolysates. Analysis of thermal evolution of hydrolysates showed that alkalization facilitated the transformation of anatase into rutile in materials obtained from Ti-pillaring solution, but retarded tetragonal to monoclinic zirconia conversion in samples derived from Zr-pillaring agent. The most striking effect was observed for the mixed Ti, Zr-pillaring solution, where an increase of pH enabled the formation of zirconium titanate as the only crystalline phase, rather than a multiphase mixture of anatase, monoclinic zirconia and zirconium titanate obtained from the more acidic precursor. The finding supports the model of mixed Ti-O-Zr network in props generated in Ti, Zr-pillared montmorillonites.

Highlights

Pillaring of clays is a process in which common interlayer hydrated cations compensating the layer charge are exchanged with large hydrated inorganic polymeric oxy-hydroxy cationic species which prop open the silicate layers [1]
The study suggested that pillars in [Ti, Zr]-pillared interlayered clays (PILCs) consisted of nano-clusters of a quasi-amorphous Ti-Zr mixed oxide, rather than separate titania and zirconia pillars, of short-range order similar to that of zirconium titanate phase
The X-ray diffraction (XRD) patterns of precipitates obtained by lyophilization of the as received Ti-pillaring solution are presented in Figure 1a, while Figure 1b shows XRD diagrams of lyophilized precipitates obtained from solutions whose pH has been adjusted to 1.2, the value observed upon preparation of Ti-pillared montmorillonite

Summary

Introduction

Pillaring of clays is a process in which common interlayer hydrated cations compensating the layer charge are exchanged with large hydrated inorganic polymeric oxy-hydroxy cationic species which prop open the silicate layers [1]. The resulting solids, referred to as pillared interlayered clays (PILCs), are characterized by large surface area, high pore volume and pore size tunable from micropore to low mesopore range. Appropriate modification of the chemical nature of pillars enables control of the materials acid–base and redox properties. Materials 2019, 12, 44 yielded material of unique structural, textural and surface acid–base properties, differing from those of reference Ti-PILC and Zr-PILC structures [2]. The study suggested that pillars in [Ti, Zr]-PILC consisted of nano-clusters of a quasi-amorphous Ti-Zr mixed oxide, rather than separate titania and zirconia pillars, of short-range order similar to that of zirconium titanate phase. Das, et al [4] reached a similar conclusion as to the binary nature of

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Conclusion