Novel support for catalysts

#1. Field of the Invention

#This invention relates to catalysts for the oxidation of sulfur dioxide. More particularly it relates to supports for such catalysts.

#Sulfuric acid is produced on a commercial scale by the vanadium-catalyzed oxidation of sulfur dioxide to sulfur trioxide in the gaseous phase followed by absorption of the sulfur trioxide in an aqueous solution.

#The FIGURE is a diagrammatic drawing of the apparatus for measuring the activity of the catalyst.

#A supported vanadium composition remains after 60 years the standard catalyst for oxidizing sulfur dioxide to make sulfuric acid or for environmental improvement. The mineral support or carrier plays several roles. Firstly, the support provides a surface on which expensive vanadium moieties, often "promoted" by other metal salts, can be deposited in a thin layer to minimize cost. Secondly, a support of high surface area maximizes the activity of the catalyst. A support of high porosity and pore volume maximizes the rate of diffusion of both gaseous reactants and product. Thermal stability of the support allows the vanadium to be in a molten or plastic state, in which its activity is enhanced, while the shaped article maintains its microporous nature without collapse. Chemical stability allows the fragile microporous structure of the support to withstand the hot reactants and product formed by oxidation of sulfur dioxide, that is sulfur trioxide.

#The durability of a catalyst can be measured by its resistance to abrasion after heating at about 816.degree. C. for 24 hours in air and cooling to ambient. It is the figure of merit for defining physical thermal stability; resistance to dusting in a hot, gaseous stream; resistance to self-abrasion in a bed of shaped pellets; and spalling, cracking, or crumbling in use.

#The preferred chemical composition for high durability and high catalytic activity has been found to be high silica content coupled with moderate alumina content. High silica content is used herein to denote a support containing at least 85 percent silica by weight. The siliceous carriers historically used, listed above in the Description of the Prior Art, have ranged from about 75 to almost 100 percent silica. The presence of alumina enhances carrier durability, but too much lowers its chemical resistance and catalytic activity. In the past carriers with up to 12 weight percent alumina have been used. It has now been discovered that about 3 to about 5 percent is preferred with from about 3.3 to about 3.7 percent highly preferred for the instant invention. Iron content should be below about 2 weight percent to maintain high activity of the catalyst.