Abstract
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-V 2O 5 system. When heated near 760°C in the presence of steam, sepiolite decomposes into enstatite (MgSiO 3) and silica. If the hydrothermal treatment is repeated in the presence of vanadium, a disordered microcrystalline phase of α-Mg 2V 2O 7 forms. Steam treatment at higher temperatures results in the formation of β-Mg 2V 2O 7. 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-Mg 2V 2O 7. An MgV 2O 6-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.
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