Characterization of Aluminophosphate-Based Tubular Mesoporous Molecular Sieves

Abstract

A series of aluminophosphate-based tubular mesoporous molecular sieves without (UHM-1) and with substituted silicon (UHM-3) have been synthesized at room temperature in the presence of a cationic surfactant cetyltrimethylammonium chloride (CTACl) and an organic base tetramethylammonium hydroxide (TMAOH). Characterization by powder X-ray diffraction (XRD), electron probe microanalysis (EPMA), transmission electron microscopy (TEM), N2 adsorption, and 27Al, 31P, and 29Si magic-angle-spinning (MAS) NMR spectroscopies has been carried out to understand both the pore structures and the local atomic arrangements of these solid products. The synthesis of UHM-1 and UHM-3 is successful only at room temperature within the following molar gel composition: Al2O3:(0.6−3.4) P2O5:(0.0−1.0) SiO2:(0.24−0.50) CTACl:(8.5−47.0) TMAOH:(200−642) H2O. The presence of TMAOH is essential to maintain the gel pH at 8.5 or above. The pH greatly influences the nature of the final “crystalline” products, particularly the coordination of aluminum and phosphorus. XRD, EPMA, TEM, and N2 adsorption measurements show that these solid products have a unique composition and disordered tubular mesopores with an average pore diameter of about 35 Å in the calcined form. The overall P/Al ratio of UHM-1 is less than unity, especially in gels of higher pH, indicating incomplete condensation of phosphorus and a nonideal aluminophosphate framework. This may account for their poorer thermal stability relative to microporous aluminophosphate molecular sieves and aluminosilicate-based MCM-41 materials. 27Al and 31P MAS NMR reveal that aluminum in UHM-1 with a P/Al ratio of 0.60 synthesized at a gel pH of 8.5 is present in both tetrahedral and octahedral environments while phosphorus is present in a tetrahedral environment and also as hydroxylated species. As the P/Al ratio of UHM-1 decreases together with increasing gel pH, the relative amounts of tetrahedral aluminum and tetrahedral phosphorus decrease. This change suggests some Al−O−P linkages in the tetrahedral regions of the UHM-1 samples, and an increasing octahedral aluminum content at lower P/Al ratio indicates that octahedral aluminum alone can construct a tubular mesoporous structure. After incorporation of silicon into UHM-1, 29Si, 27Al, and 31P MAS NMR of UHM-3 reveal that silicon is atomically dispersed and that the relative amounts of tetrahedral aluminum and tetrahedral phosphorus increase monotonically with increasing silicon incorporation. Virtually all the aluminum is tetrahedral, and all the phosphorus is tetrahedral after silicon incorporation reaches a Si/Al ratio of unity. This indicates more ordered atomic arrangements of tetrahedral silicon, aluminum, and phosphorus via oxygen bridges in UHM-3 than in UHM-1. The incorporation of silicon appears to make the aluminophosphate framework more flexible and facilitates the formation of an aluminophosphate-based mesoporous structure.