Selective oxidation of hydrocarbons employing tellurium containing heterogeneous catalysts

Abstract

Premier catalysts for the selective oxidation of hydrocarbons are multifunctional in nature and are based on complex mixed metal oxides containing at least one of the key elements Mo, Sb and/or V. Te is often added as a promoter to enhance selectivities of desired products. In the 4 + oxidation state its role is that of hydrogen abstraction, in the 6+ oxidation state that of selective oxygen or nitrogen insertion. It also suppresses by-product formation of unwanted cracked intermediates as well as CO 4x . The major drawback of using tellurium containing catalysts is tellurium's volatility when in its lower oxidation state (Te 4+) generated under the redox conditions of the catalytic operation. Unless immediately reoxidized to the 6 + oxidation state, it volatilizes and is lost from the catalyst with a concomitant loss of its beneficial properties originally imparted to the base catalyst. Many ingenious solutions have been found to minimize the losses from catalysts in commercial operation. A most elegant approach is intercatalyst redox stabilization of Te 6+ through strategic placing of powerful reoxidation couples (Cu, Fe, Ce, V, U) in the immediate proximetry of the tellurium in the lattice of a given catalytically active phase. This elegant but difficult approach alleviates the volatility problem only partially. A more pragmatic, practical approach in commercial scale operation is the addition of volatile tellurium compounds on a continuous or intermittent basis directly to the reactor. An optimum concentration of tellurium is thus maintained on the surface of the catalyst resulting in constant and optimized process operation. Similarly, other key covalent catalytic elements (e.g., Sb, Mo) also lost through volatilization from the catalysts can be replenished. Since (amm)oxidation catalyst surfaces are very dynamic in nature under operating conditions, the addition of controlled amounts of key catalytic elements and/or promoters by means of volatile compounds directly to the reactor not only effectively maintains catalyst life and product quality but also suggests the opportunity to create new surface states so chosen as to potentially alter the reaction channels at will.