POROUS CHROMIA-PILLARED ALPHA-TIN PHOSPHATE MATERIALS

The reactions of Cr(OAc)3 (OAc- = CH3OCO-) solutions with colloidal suspensions of tetramethylammonium α-tin phosphate (α-Sn[Me4N]0.9 - 1.1H1.1-0.9(PO4)2 · nH2O = "Me4NSnP") have been investigated over a wide range of [Cr(OAc)3]:[Me4NSnP] ratios, Cr(OAc)3 concentrations, and heating conditions. On aging + reflux, Me4NSnP takes up oligomeric Cr3+ species to give crystalline, highly expanded materials having interlayer distances between 24 and 33 Å. Conversely, Cr(OAc)3 solutions treated in the same way, but separately, and then added to colloidal Me4NSnP gave composites with little expansion (interlayer distance, d002 = 15 Å), collapsing to nonporous oxide-phosphates (d002 = 10 Å) on calcination. Separately polymerized CrCl3 NaOH or [Cr3O(OAc)6(OH2)3]+ (standard precursors in clay-pillaring) also gave poorly defined delaminated materials. Chemical, TGA/DTA, and visible/near UV spectroscopic evidence shows the intercalates are polyhydroxyacetato-Cr3+ species, and some may be formulated as [Cr3(OH)6(OAc)]2+, [Cr4(OH)7(OAc)3+, and [Cr5(OH)7(OAc)3]5+. They are formed in situ on the α-tin phosphate surfaces; possible orientations within the layers (taking into account the presence of zeolite-type water) is discussed. Calcination under N2 at 400°C gives chromium oxide-pillared materials with d002 in the ranges 12.5-14.0 Å (from precursors prepared at higher Cr3+ concentrations) or 15-20 Å (from precursors at lower Cr3+ concentrations). The surface areas (BET, N2, 77K): 264-386 m2 g-1, compare well with analogues in clay chemistry (150-400 m2 g-1). Pore-sized distributions (cylindrical pores model) are narrow, >70% of pores having widths <40 Å. Calcination in air leads to segregation of Cr2O3 which, after washing gives materials having wider poresize distributions and lower surface areas. The ion-exchange properties of selected chromia-pillared materials with K+, Co2+, and Pr3+ have been investigated. The results are compared with models of porosity. © 1991.