THE INTERACTION OF VANADIA WITH SEPIOLITE - STRUCTURAL STUDIES BY V-51 SOLID-STATE NMR AND RAMAN-SPECTROSCOPY

The interaction of vanadium with sepiolite, a layered magnesium silicate used to stabilize cracking catalysts against metal contaminants in crude oils, is investigated. To this end, field-dependent 51IV wideline, magic-angle-spinning (MAS), and nutation NMR studies, together with X-ray diffraction (XRD) and laser Raman spectroscopy (LRS) results, are reported on sepiolite samples impregnated with a solution of vanadyl napthenate in benzene. These results are discussed in connection with benchmark NMR data of crystallographically well-defined model compounds in the M9O-V2O5 system. When heated near 760°C in the presence of steam, sepiolite decomposes into enstatite (MgSiO3) and silica. If the hydrothermal treatment is repeated in the presence of vanadium, a disordered microcrystalline phase of α-Mg2V2O7 forms. Steam treatment at higher temperatures results in the formation of β-Mg2V2O7. In addition, all samples examined contain an amorphous surface phase. Based on 51V nutation NMR spectroscopy, this amorphous constituent is inferred to have a structural environment similar to β-Mg2V2O7. An MgV2O6-like, distorted octahedral environment is present only in samples with very high surface coverages and can be suppressed by choosing multiple-step over single-step impregnation. © 1992.