Revealing the effect of templates on atomic scale ordering and the hydrophilic properties of aluminophosphates

Abstract

Organic templates provide structure for the frameworks of porous materials via a panel of weak and strong electrostatic interactions, i.e., van der Waals forces, hydrogen bonds, and Coulomb interactions. The interactions between the inorganic framework and the organic templates are not obvious, especially when there is no geometrical specificity between them. Then, one molecule may lead to different framework topologies, and similarly, a framework topology may be obtained with more than one template. In this work, we used two different templates, i.e., triethylamine (TEA⁰) and tetraethylammonium (TEA⁺), to synthesize aluminophosphate-five (AFI) zeotypes (AlPO-5 and SAPO-5) under microwave conditions. The syntheses were carried out using both molecules under identical experimental conditions, i.e., with the same reagents, molar composition, crystallization time, and temperature. The structural and hydrophilic properties of the obtained materials were found to be highly dependent on the template used. A large set of characterization techniques was explored to understand short- and long-range order features. X-ray diffraction (XRD), scanning electron microscopy (SEM), inductively coupled plasma mass spectrometry (ICP-MS), energy dispersive X-ray (EDX) microanalysis, thermogravimetric analysis (TGA), nitrogen sorption, Fourier transform infrared (FT-IR) spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy were used. Through these investigations, we elucidated the impact of templates on the synthesis process and the resulting properties. Our findings reveal that the templates served as space-filling agents, occupying the channels within the framework. Furthermore, the charge of the template exerted a significant influence on various aspects, including morphology, silicon content, and atomic-scale ordering. These modifications contributed to the enhancement of hydrophilicity in the AlPO-5 and SAPO-5 materials. This discovery provides a promising method for tailoring the properties of aluminophosphate-based water adsorbents in a straightforward manner.