Amination with Zeolites
Zeolites have attracted considerable attention as catalysts as a result of their high activity and unusual selectivity for acid-catalyzed reactions. However, these properties have been applied almost exclusively to hydrocarbon processing. In this chapter, we summarize initial results in the exploration of a new and potentially broad area of acid catalysis with zeolites, i.e., synthesis of amines. Zeolite-catalyzed amination of ethylene, propylene, and isobutylene to the corresponding primary amines is described. Despite the high reaction temperatures employed, selectivity to total amine products, and specifically to the primary amines, is high. Olefin amination occurs via Markownikoff addition. Reaction is believed to involve a carbocationic intermediate which is formed by the interaction of the olefin with a surface proton or ammonium ion. Catalyst activity is directly proportional to the total number of strongly acidic sites as measured by ammonia chemisorption. The highest activities were obtained with small- to medium-pore acidic zeolites, such as H-clinoptilolite, H-erionite, and H-offretite. The necessity of 242strongly acidic sites for catalytic amination is demonstrated by the negligible activity of alkali-exchanged zeolites and amorphous silica alumina. Thermodynamic constraints on the reaction are discussed. In the presence of a wide range of acidic zeolites, methanol is aminated to form mixtures of mono-, di-, and trimethylamine. All zeolites examined show higher rates of methanol conversion than amorphous silica-alumina as a result of their higher acidity. High selectivities to mono- and dimethylamine are obtained with the small-pore zeolites H-chabazite and H-erionite. With the latter catalysts, shape selectivity induced by the zeolite structure completely suppresses formation of trimethylamine under typical operating conditions.