J . Phys. Chem. 1987, 91, 1526-1530
1526
Bransted Acid Sites in Transition Metal Oxide Catalysts: Modeling of Structure, Acid Strengths, and Support Effects J. Bernholc,*+J. A. Horsley, L. L. Murrell, L. G. Sherman, and S. Soled Corporate Research Science Laboratories, Exxon Research and Engineering Company, Clinton Township, Annandale, New Jersey 08801 (Received: October 14, 1986)
The trends and origin of Brernsted acidity of dispersed, y-alumina-supported transition metal oxides are investigated via a new theoretical method. The method is based on first-principles quantum mechanical calculations for the acid site and includes a realistic model of the acid-base interaction. Both free and supported transition metal (TM) oxide clusters of Ti, Nb, and W are considered. For tetrahedrally coordinated unsupported TM oxide clusters, it is found that the Br~lnstedacidity depends strongly on the number of terminal oxygen atoms (terminal 0 effect). For supported TM oxides, the calculations show that the charge delocalization extends into the support, leading to a strong additional increase in the Brernsted acidity. This novel “inductiueeffect” of the support together with the terminal 0 effect is responsible for the high Brernsted acidity of the catalysts. The strong support effect suggests that support modifications may significantly alter the catalytic properties of supported transition metal oxides. The calculated trends are in good agreement with the experimental measurements of relative Br~lnsted acidity of the catalysts.
I. Introduction A new class of solid acid catalysts has been developed at Exxon Research and Engineering Co. over the past few years.’*2 These catalysts consist of a transition metal oxide (TMO) supported on a high surface area solid such as A1203 or SO2. One of the supported T M O catalysts, W 0 3 supported on A1203,has been a candidate for residuum upgrading processes because of its high hydrocarbon cracking activity and its stability under high-temperature steam treatment^.^ Bulk tungsten oxide is known to be an acid catalyst for hydrocracking of n - h e ~ t a n e . ~In addition, tungsten oxide and reduced tungsten oxides have recently been studied as acid catalysts for olefin isomerization5 and for alcohol dehydrations6 A recent paper also reported the strong chemisorption of n-butylamine on reduced tungsten oxide.’ The acidic properties of W03-based mixed oxide catalysts have also been reported.* Tungsten oxide, reduced tungsten oxide, and mixed oxides containing W 0 3 have been recognized as possessing acidic character. In contrast to these studies with bulk tungsten oxide catalysts, much less work has been done employing supported tungsten oxide as an acid ~ a t a l y s t . ~ Two types of acidity occur on solid acid surfaces. The first type is Lewis acidity, which occurs at a coordinatively unsaturated metal atom which can accept an electron pair. The second one is Brernsted acidity, which occurs at sites that can donate a proton to a base. It is the Brernsted acid sites that are important in catalytic cracking.I0 A study of the gas oil cracking activity for a series of different transition metal oxide (TMO) catalysts’ has revealed interesting periodic trends. We have investigated one of the observed trends by carrying out first-principles calculations of the acid strengths of several TMO catalysts using cluster models of the Brernsted sites. 11. Periodic Trends in T M O Acid Strengths
The catalytic cracking activity of a series of T M O solid acids has been investigated by Murrell et for light gas oil cracking in fixed bed reactors. The relative activities obtained are shown in Table I. In the present work, we have focused on the periodic trends across the diagonal, Le., for Ti02/A1203, Nb2O5/AI2O3,and WO3/AI2O3. The relative catalytic cracking activity and the Brernsted acid strength for these three supported catalysts is Ti02
