J. Am. Chem. SOC. 1990, 112, 8349-8364 8349 A 95Mo Solid-state NMR Study of Hydrodesulfurization Catalysts. 1. Formation of Fresh HDS Catalyst Precursors by Adsorption of Polyoxomolybdates onto ?-Alumina John C. Edwards, Richard D. Adams,* and Paul D. Ellis* Contribution from the Department of Chemistry, University of South Carolina. Columbia, South Carolina 29208. Received November 6, 1989 Abstract: The solid-state9sMo NMR of oxomolybdates and polyoxomolybdates adsorbed to y-alumina was utilized to characterize the molybdenum species present at circa monolayer coverages of molybdenum. This represents the first attempt at using solid-state NMR to look directly at the molybdenum species on the surface of “fresh” hydrodesulfurization catalyst precursors. The selectively excited central &I/, transition line shapes were obtained, by the solid-echo technique, for several polyoxomolybdates, which are considered model compounds of the surface molybdena species, as well as for the uncalcined and calcined catalysts at various loadings. The spectra were obtained for static and magic angle spinning samples. Static sample spectra of the catalysts reveal inhomogenously broadened lines owing to a range of surface species being present. Magic angle spinning spectra show the presence of four possible species on the surface of the uncalcined catalysts and two possible species present on the surface of calcined catalysts. Spikelet echo spectra identified static adsorbed species [speculated to be predominantly adsorbed tetrahedral/octahedral molybdena, and perhaps AI,(MoO,)~], and a dynamic species (speculated to be disordered, surface- interactive tetrahedral and octahedral molybdates), in the uncalcined catalyst samples. Upon calcination the line widths of the static and MAS spectra are increased, indicating a polymerization of the surface species to form a Moo3-like phase. The spikelet echo spectra reveal the loss of the dynamic species upon calcination, indicating that it is adsorbed to the surface or polymerized with the molybdenum surface species. The AI2(MoO4), phase is stable to calcination and also appears in the calcined catalysts. These qualitative speculations are in close agreement with the results of other researchers using other characterization techniques. An interactive graphical curve-fitting program was used to calculate, from the various line shapes, the values of the quadrupolar coupling constant, the asymmetry parameter of the electric field gradient, the three principal values of the chemical shielding tensor, and the three Euler angles that relate the noncoincident quadrupole and chemical shielding principal axis systems. Assignments of the quadrupole tensors to the different sites present in the model species were made based on the relationship between the Q, value and molecular distortion of the octahedral sites. These results have shown the feasibility of using solid-state 95Mo NMR to investigate heterogeneous and homogeneous catalysts containing active molybdenum species. introduction Catalysts prepared by the impregnation of y-A1203 with a solution of ammonium heptamolybdate have been the subject of many investigations due to the importance of Mo0,-y-A1203 as an oxidation catalyst, as well as being involved in olefin metathesis and hydrogenation catalysts. We will be studying, in this case, molybdena/alumina catalysts in the context of them being the precursors to hydrodesulfurization (HDS)’-* and hydro- denitrogenation ( HDN)9 catalysts. HDS catalysts are used in- dustrially to remove sulfur from petroleum feedstocks and coal by its conversion to H2S and hydrocarbon products. The com- mercial catalysts consist of either molybdenum or tungsten, promoted by either nickel or cobalt, supported on y-alumina, and are usually prepared by incipient wetness impregnation with an ammonium heptamolybdate solution, (i.e., by filling only the pore volume of the alumina). The “fresh” catalyst is reduced and sulfided to form the active catalyst, which is the subject of ongoing research in our laboratory, the results of which will be dealt with in future publications. We present here the results of our en- deavors to characterize the oxomolybdenum surface species present on the y-alumina surface of the fresh molybdena catalyst. To date, a variety of spectroscopic techniques have been used in attempts to characterize the interaction between the molybdenum species and the alumina support. XPS,1*12 y-ray di~persion,’~ (I) Schuman, S. C.; Shalit, H. Caral. Reo. 1970, 4, 245. (2) Weisser, 0.; Landa, S. In Sulphide Catalysts. Their Properties and (3) Amberg. C. H. J. Less-Common Met. 1974, 36, 339. (4) Grange, P.; Delmon, B. J. Less-Common Met. 1974, 36, 353. (5) de Beer, V. H. J.; Schuit, G. C. A. In Preparation of Catalysts; Del- mon. B., Jacobs, P. A., Poncelet, G., Eds.; Elsevier: Amsterdam, 1976; p 343. (6) Ohtsuka, T. Catal. Reo.-Sei. Eng. 1977, 16, 291. (7) Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. In Chemistry of Catalytic Processes; McGraw-Hill: New York, 1979; p 390. (8) Grange, P. Catal. Reo. 1980, 21, 135. (9) Ho, T. C. Catal. Reo.-Sei. Eng. 1988, 30, 117. Applications: Pergamon: Oxford, 1973. Raman’”Je” and lR21*22 spectroscopy, NO/C02 chemi~orption,~~ and UV-vis refle~tance~~*~~J* have provided the best information on the molecular nature of molybdena on alumina, but there is still disagreement as to the mechanism by which the hepta- molybdate anions present in the impregnation solution are ad- sorbed onto the alumina surface. It has been suggested by Wang and Ha11I4 that the heptamolybdate species, present in solutions with low initial pH, are adsorbed intact during impregnation to give a supported heptameric species. This adsorption occurs due to an electrostatic attraction between the positively charged alumina surface (at a pH lower than the isoelectric point) and the polyoxomolybdate anion. It is thought that these supported clusters are stable even after calcination. Another possibility, put forward by Knozinger and Jeziorowski,I6 is that exchange of [ Mo04I2- with the surface hydroxyl groups increases the pH at the surface, which in turn causes the dissociation of the heptameric species to the monomer, [Mo0,I2-. Thus, islands of tetrahedral molybdate are formed on the alumina surface. Upon calcination (10) Zing, D. S.; Makovsky, L. E.; Tischer, R. E.; Brown, F. R.; Hercules, (11) Clausen, B. S.; Lengeler, B.; Topsm, H. Polyhedron 1987,5, 199. (12) Hayden, T. F.; Dumesic, J. A. J. 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