Abstract
Alumina-lanthana (La at 1, 3, or 5 wt%) supports were prepared by sol-gel from Al alkoxide sol where La(NO3)3 was added. Annealed (550 °C) xerogels were characterized by N2 physisorption, thermal analysis (TG-DTA), X-ray diffraction (XRD), scanning electron microscopy- energy dispersive spectroscopy (SEM-EDS), CO2-adsorption studied in IR region, Raman and ultraviolet-vis (UV-vis) spectroscopies. The texture of amorphous binary matrices of high La dispersion was adequate to applications in catalysts for middle distillates hydrodesulfurization (HDS). Generally, the amount and strength of surface basic sites increased with La content in solids. Mo (at 2.8 at. nm−2) and Co (at Co/(Co+Mo) = 0.3) were deposited over carriers by one-pot simultaneous impregnation in the presence of PO43− (P2O5/(NiO+MoO3) = 0.2 mass ratio). Calcined (400 °C) Co-Mo-P impregnated precursors had decreased basicity as to that of corresponding carriers, suggesting strong La-deposited species interaction. As La content in carriers increased Mo=O Raman stretching vibrations shifted to lower wave-numbers (949 to 935 cm−1) suggesting octahedral molybdates coordination change to tetrahedral. Although La at the lowest concentration (1 wt%) enhanced dibenzothiophene, HDS (~38% higher as to the Al2O3-supported formulation) desulfurization was significantly diminished at augmented content. Presence of hardly sulfidable tetrahedral Mo originated during impregnation at basic conditions in pores of La-modified carriers seemed to dictate observed behavior. Rare earth content in formulations enhanced selectivity to biphenyl.
Highlights
Improved catalytic formulations of enhanced properties in hydrodesulfurization (HDS) reactions that allow for compliance with ever stricter environmental regulations on S content in internal combustion engine fuels remains a challenging research field
From their studies on molybdenum supported on La-modified alumina, Massoth et al [13] found that strong interaction of rare earth oxidic domains with Al2 O3 resulted in LaO monolayer dispersion
The chemical composition of the supports and the various impregnated materials prepared were determined from energy dispersive spectroscopy (EDS)
Summary
Improved catalytic formulations of enhanced properties in hydrodesulfurization (HDS) reactions that allow for compliance with ever stricter environmental regulations on S content in internal combustion engine fuels remains a challenging research field. Diminished activity in thiophene HDS of sulfided CoMo/alumina catalysts modified by La has been attributed to enhanced proportions of isolated MoO4 2− refractory and less-sulfidable species adsorbed on lanthana domains [14] From their studies on molybdenum supported on La-modified alumina, Massoth et al [13] found that strong interaction of rare earth oxidic domains with Al2 O3 resulted in LaO monolayer dispersion. A diminished proportion of disintegration reactions was observed, decreased cracking being originated by the neutralization of surface acidic sites over alumina, due to the basic agent deposition Considering those contradictory results, the influence of basic La as a carrier additive on HDS catalyst properties deserves further investigation. Some advances from this investigation have been recently reported [16]
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