Hierarchical AlPO₄-5 and SAPO-5 microporous molecular sieves with mesoporous connectivity for water sorption applications

Herein, we report the synthesis of hierarchical AlPO₄-5 and SAPO-5 molecular sieves that exhibit varying degrees of mesoporous interconnectivity by controlling the amount of triethylamine (TEA) (used as the structure directing agent) and the Si-to-Al ratio (Si/Al); the gel mixture had a composition of (1 − y) Al₂O₃:y SiO₂:1.3 P₂O₅:x TEA:200 H₂O. The molecular sieves were characterized by X-Ray powder Diffraction (XRD) and Scanning Electron Microscopy/Energy Dispersive X-Ray Spectroscopy (SEM/EDX), and their sorption properties were studied by N₂ adsorption-desorption at 77 K and water adsorption at 316 K. In the synthesis gel with x = 1.6 ((1 − y) Al₂O₃:y SiO₂:1.3 P₂O₅:1.6 TEA:200 H₂O), SAPO-5 and AlPO₄-5 exhibit high Brunauer-Emmett-Teller specific surface areas of ~380 m² g⁻¹ and 325 m² g⁻¹, respectively. SAPO-5, having a composition of Si_0.46Al_0.6P_0.804, exhibits significant water uptake at low relative pressures (3.4 mmol/g at P/P₀ = 0.03 vs. 0.2 mmol/g for pure AlPO₄-5) attributed to the contribution of hydrophilic Brønsted acid sites, arising from the charge imbalance created by the isomorphic substitution of Si into the aluminophosphate structure predominantly via the SM2 mechanism. Furthermore, SAPO-5 exhibited a hierarchical pore structure having distinct pore size distributions centred on 0.7 nm and 15.8 nm, attributed to the AFI framework and the induced mesoporosity, respectively. At higher relative pressures, SAPO-5 exhibited the highest total water adsorption capacity of 14.1 mmol/g at a relative pressure of 0.38, compared to 11 mmol/g for AlPO₄-5. The higher overall capacity of SAPO-5 is attributed to capillary condensation into the relatively large mesopore volume.