Interaction of water with (silico)aluminophosphate zeotypes: a comparative investigation using dispersion-corrected DFT.

Michael Fischer

doi:10.1039/c6cp02289h

Abstract

Porous aluminophosphates (AlPOs) and silicoaluminophosphates (SAPOs) with zeolite-like structures have received considerable attention as potential adsorbents for heat transformation applications using water adsorption/desorption cycles. Since a detailed experimental characterisation of the water adsorption properties has only been performed for some of these materials, such as AlPO-18 (AEI topology) and SAPO-34 (CHA topology), more systematic insights regarding the influence of the pore topology and (for SAPOs) the arrangement of the framework protons on the affinity towards water are lacking. To study the relationships between structure and properties in more detail, the interaction of water with six structurally different AlPOs (with AEI, AFX, CHA, ERI, GIS, RHO topologies) and their SAPO analogues was investigated using dispersion-corrected density-functional theory (DFT-D) calculations. Different possible locations of silicon atoms and charge-balancing protons were considered for the SAPO systems. The calculations for SAPOs at low water loadings (one H2O molecule per framework proton) revealed that the interaction energies exhibit a considerable variation, ranging from -75 to -100 kJ mol(-1) (per water molecule). The differences in interaction energy were rationalised with the different structural environment of the framework protons at which the water molecules are adsorbed. At high water uptakes (near saturation), interaction energies in the range of -65 kJ mol(-1) were obtained for all AlPOs, and there was no evidence for a marked influence of pore size and/or topology on the interaction strength. The interaction of water with SAPOs was found to be approximately 5 kJ mol(-1) stronger than for AlPOs due to an increased contribution of electrostatic interactions. An analysis of the structural changes upon water adsorption revealed striking differences between the distinct topologies, with the materials with GIS and RHO topologies being distorted much more drastically than the systems based on double six-ring (d6r) units. Moreover, the direct coordination of water molecules to framework aluminium atoms occurs more frequently in these materials, an observation that points towards a reduced structural stability upon hydration.

Highlights

The use of water as working fluid in such applications is attractive, as water is readily available and environmentally benign
Since the characteristics of an ideal adsorbent depend strongly on the requirements of the process, there is no ‘‘onesize-fits-all’’ adsorbent for heat transformation applications, but different materials are best suited for different processes.[2]
The dispersion-corrected density-functional theory (DFT) calculations employed in this study have delivered detailed insights into the energetic and structural aspects of water adsorption in AlPOs and SAPOs with different topologies, which can be summarised as follows: (1) Due to the absence of strongly preferred adsorption sites in AlPOs, the interaction with small amounts of water was investigated only for SAPOs

Summary

Introduction

The use of water as working fluid in such applications is attractive, as water is readily available and environmentally benign. The remainder of this article is organised as follows: after a description of the model systems and the computational methodology, the results of the DFT-D optimisations of guestfree AlPO and SAPO systems are presented These are followed by a detailed discussion of the DFT-D results for water-containing systems: AlPOs typically exhibit a steep rise of the water adsorption isotherm at a certain relative pressure (corresponding to pore filling). This pressure, only very little water is adsorbed due to the absence of preferred adsorption sites. Besides reporting the DFT-D interaction energies (averaged over 5 snapshots for high water loadings), the structural changes upon water adsorption are assessed

Preparation of model systems

Details of DFT-D calculations

Optimisation of guest-free systems

Preliminary GCMC simulations

Conclusions