This invention relates to a process for isomerizing alkylaromatic hydrocarbons. More particularly, this invention relates to a process for isomerizing alkylaromatic hydrocarbons with a catalytic composite comprising a combination of a platinum or a palladium component, a rhodium component, a tin component and a halogen component with a porous carrier material. The present invention utilizes a dual-function catalytic composite having both a hydrogenation-dehydrogenation function and a cracking function which affords substantial improvements in hydrocarbon isomerization processes that have traditionally used dual-function catalysts.
Processes for the isomerization of hydrocarbons have acquired significant importance within the petrochemical and petroleum refining industry. For example, the demand for para-xylene has created a demand for processes to isomerize other xylene isomers and ethylbenzene to produce para-xylene.
Catalytic composites exhibiting a dual hydrogenation-dehydrogenation and cracking function are widely used in the petroleum and petrochemical industry to isomerize hydrocarbons. Such catalysts generally have a heavy metal component, e.g., metals or metallic compounds of Group V through VIII of the Periodic Table, to impart a hydrogenation-dehydrogenation function, with an acid-acting inorganic oxide to impart a cracking function. In catalysis of isomerization reactions, it is important that the catalytic composite not only catalyze the specific desired isomerization reaction by having its dual hydrogenation-dehydrogenation function correctly balanced against its cracking function, but also that the catalyst perform its desired functions well over prolonged periods of time.
The performance of a given catalyst in a hydrocarbon isomerization process is typically measured by the activity, selectivity, and stability of the catalyst. Activity refers to the ability of a catalyst to isomerize the hydrocarbon reactants into the corresponding isomers at a specified set of reaction conditions; selectivity refers to the percent of reactants isomerized to form the desired isomerized product and/or products; stability refers to the rate of change of the selectivity and activity of the catalyst.
The principal cause of instability (i.e., loss of selectivity and activity in an originally selective, active catalyst) is the formation of coke on the catalytic surface of the catalyst during the reaction. This coke is characterizable as a high molecular weight, hydrogen-deficient, carbonaceous material, typically having an atomic carbon to hydrogen ratio of about 1 or more. Thus, a problem in the hydrocarbon isomerization art is the development of more active and selective composites not sensitive to the carbonaceous materials and/or having the ability to suppress the rate of formation of these carbonaceous materials on the catalyst. A primary aim of the art is to develop a hydrocarbon isomerization process utilizing a dual-function catalyst having superior activity, selectivity and stability. In particular, it is desired to provide a process wherein hydrocarbons are isomerized without excess cracking or other decomposition reactions which lower the overall yield of the process and make it more difficult to operate.