(711b) Tuning Oxygen Basicity and Metal Reducibility In Bismuth Vanadate-Molybdate Catalysts

The industrial use of bismuth molybdate-based catalysts dates back to 1955, when Standard Oil of Ohio obtained a patent for conversion of propylene to acrolein and acrylonitrile over bismuth molybdates doped with phosphate – a technology that is still in use today.  Yet even with the large body of research that has been done on bismuth molybdates in the subsequent decades, there is still not a full understanding of the structure of the active site(s), nor of the details of the chemical steps that link reactants to products on these catalysts [1].  Such understanding is even more lacking in the case of bismuth molybdate doped with vanadate, which shows promise for direct conversion of propane to acrolein [2] and which is also more selective for conversion of propylene to acrolein than undoped bismuth molybdate [3].  In order to better understand experimentally observed trends in selectivity vs. composition in these catalysts, our group has undertaken detailed physical and chemical characterization of a series of materials with composition Bi_1-x/3V_1-xMo_xO₄, where the ratio x = Mo/(Mo+V) ranges from 0 to 1.  Powder x-ray diffraction has been used to establish that a single-phase solid solution exists at all values of the ratio x.  Catalytic studies have shown that optimal selectivity of propylene to acrolein occurs near x=0.2: the apparent activation energy for acrolein formation is lower over bismuth vanadate (14.9 kcal/mol) than over bismuth molybdate (19.5 kcal/mol), but greater vanadium content also facilitates further oxidation to CO₂.  UV-Vis and x-ray absorption near edge (XANES) spectroscopies have shown that while only molybdenum centers are appreciably reduced under reaction conditions, molybdenum becomes more reducible with increasing vanadium content.  Raman spectroscopy, x-ray photoelectron spectroscopy (XPS), and probe molecule adsorption studies have shown that the catalyst also becomes more Lewis basic with increasing vanadium content.  Over bismuth molybdate, an initial hydrogen abstraction step is known to be rate limiting [4].  Since this step requires transfer of a proton to an oxygen and reduction of a molybdenum center, greater Lewis basicity and greater metal reducibility both facilitate the rate-limiting step, in agreement with the observed reduction in apparent activation energy in more basic, more reducible vanadium-doped materials.  Developing a fuller understanding of the interplay between oxygen Lewis basicity, metal reducibility, and catalyst composition in mixed metal oxide systems like the one studied here will enable rational formulation of the more selective partial oxidation catalysts that greener, more efficient industry requires.

[1]  Jung J.C.; Lee H.; Song I.K. Catalysis Surveys from Asia 2009, 13, 78 – 93.

[2]  Yang H.P.; Fan Y.N.; Feng L.Y.; Qiu J.H.; Lin M.; Xu B.L.; Chen Y. Acta Chimica Sinica 2002, 60 (6), 1006 – 1010.

[3]  Ueda W.; Asakawa K.; Chen C.L.; Moro-Oka Y.; Ikawa T. Journa of Catalysis 1986, 101, 360 – 368.

[4]  Grasselli R.K.; Burrington J.D. Industrial & Engineering and Chemistry Product Research and Development 1984, 23, 393 – 404.