Understanding the effect of modifying elements in supported vanadia bilayered catalysts for methanol oxidation to formaldehyde

The field of heterogeneous catalysis has long been interested in understanding the role of site structure on reactivity and selectivity for the rational design of catalysts. Vanadia is of particular interest because of its potential to be highly active and selective for a variety of reactions, such as oxidative dehydrogenation of alkanes to alkenes, or the oxidation of n-butane to maleic anhydride./nWhen supported, vanadia can exist in several environments depending on its surface coverage. At the lowest loadings, below 2 V/nm2, the vanadium exists predominantly in well dispersed, tetrahedral structures with 3 V-O-support bonds and 1 V=O bond. At higher loadings, above 2 V/nm2, V-O-V bonds form on the surface, and at loadings above 7 V/nm2, the vanadia begins to form 3 dimensional domains of V2O5. Methanol oxidation rates over catalysts with varying vanadia loadings have shown no significant effect of the V surface density on the formaldehyde formation rate. However, significant differences in the formaldehyde production rates are observed for different supports. Changing the support from silica to titania or zirconia, will result in increases in the production of formaldehyde from methanol by over an order of magnitude for similar vanadia surface coverages. These differences in rate are observed even though the reaction mechanism is believed to be the same regardless of the support. The mechanism is thought to proceed as follows. First methanol dissociatively adsorbs across a V-O-support bond, producing V-OCH3 and M-OH (M = Si, Ti, Zr, Ce) in a quasi-equilibrated step. Next a surface oxygen abstracts hydrogen from the methoxy group in the rate determining step, and formaldehyde desorbs. The final steps are fast and involve the production of H2O from neighboring hydroxyls and the reoxidation of the catalyst by gas-phase oxygen./nWhen vanadia is supported on bulk TiO2, ZrO2, or CeO2, the support surface area is relatively small (~200 m2/g at its highest), and the bulk support causes side reactions which make it difficult to understand the role of the vanadia. Furthermore, by using a bulk support, only the vanadia surface coverage can be varied, such that the effect of different V structures can be elucidated, but not that of V-O-M bonds. Therefore, high surface area silica with a variable coverage of two-dimensional TiO2, ZrO2, and CeO2 layers are used to support isolated vanadate structures to vary the quantity of vanadia bound to the modifying layer. These bilayered catalysts can be used to determine the effect of the V-O-support bonds on the formaldehyde production rate./nThree mesoporous silica supports, MCM-48 (1550 m2/g), MCM-41 (1353 m2/g), and SBA-15 (700 m2/g) were used as the high surface area silica. Ti was grafted to the MCM-48 surface using Ti(OiPr)4 and a maximum surface coverage of 2.8 Ti/nm2 was obtained after 3 graftings. The grafting of zirconium was performed using Zr-2-methyl-2-butoxide on MCM-41, and a maximum loading of 2.1 Zr/nm2 was achieved after 3 graftings. The final modifying element, cerium, was grafted onto SBA-15 using Ce(OtBu)4 for a maximum surface coverage of 0.9 Ce nm-2. After treating the MOx/SiO2 (M = Ti, Zr, Ce) samples in air to remove any organic ligands, OV(OiPr)3 was grafted onto the MOx/SiO2 (M = Ti, Zr, Ce) support to achieve the desired V surface coverage of approx. 0.7 V/nm2./nThe resulting catalysts contain amorphous two-dimensional layers of TiO2, ZrO2, or CeO2 with V existing in a pseudo-tetrahedral structure on the surface. As the surface density of the modifying element layer increases, the quantity of vanadia bound to TiO2, ZrO2, or CeO2 increases. For the VOx/ZrO2/SiO2 catalysts, the fraction of vanadia bound to the zirconia layer was able to be quantified and determined to be 35% of all V for a Zr surface density of 2.8 Zr nm-2./nEven for small quantities of modifying elements (0.2 M nm-2), the apparent rate constant for formaldehyde production on VOx/MO2/SiO2 (M = Ti, Zr, Ce) is an order of magnitude higher than for VOx/SiO2 catalysts at the same V surface density. Regardless of the modifying element used, the increase in apparent rate constant is comparable for all catalysts. As the modifying element surface density is increased, the apparent rate constant also increases, which is a result of an increasing fraction of V bound to the MOx layer. Each of these bilayered catalysts can be described using a two-site model of VOx/SiO2 and VOx/MO2 (M = Ti, Zr, Ce) with the latter being responsible for the increased apparent rate constant./nThis higher activity for the VOx/MO2 site is due to a lower apparent activation energy. For VOx/SiO2, the apparent activation energy is 23 kcal mol-1, but is approximately 17 kcal mol-1 for VOx/MO2 sites. The apparent activation energy can be expressed as the sum of the heat of methanol adsorption and the activation energy for H-abstraction. My results indicate that the lower apparent activation energy observed for the bilayered catalysts is a result of a decrease in the activation energy for H-abstraction. This lower energy pathway occurs because the MOx layer can abstract H from surface methoxy groups. For VOx/SiO2, however, the vanadyl oxygen abstracts H in a higher energy step.

Advisor: Bell, Alexis T