SIMULTANEOUS POLYMERIZATION AND OLIGOMERIZATION OF ACETYLENE ON ALUMINA AND FLUORIDATED ALUMINA CATALYSTS

The products of the reaction of acetylene over solid acid catalysts are gases, liquids, and solids at reaction conditions. Since the products of the reaction range from low to high molecular weight polymers the process is a combination of oligomerization and polymerization. This paper reports on the product selectivities measured in a fixed-bed continuous flow reactor using a series of fluoridated alumina catalysts (0 to 4.3 wt% F) under various reaction conditions (300 to 400°C, 1 to 3 g s ml-1, 5 to 15 mol% C2H2 in N2). A comparison of the liquid product distributions in the transformation of acetylene without catalysts, over fluoridated alumina catalysts (this work) and over the ZSM-5 zeolite, revealed that the noncatalytic and catalytic product distributions did not correspond to equilibrium conversion. A mechanism is proposed for the catalytic conversion of acetylene based on our experimental observations. The mechanism involves cationic chain growth polymerization leading to both linear and cyclic products. The initiation, propagation, transfer, and termination steps are discussed. The mechanism implies two requirements for promoting the cyclization reaction. First, a sufficient number of acidic sites must be present on the catalyst surface to initiate a polymeric chain. Second, an appropriate distribution of acid sites on the catalyst surface must exist to promote the transfer reaction. A fine balance is desired between the number of sites on the catalyst surface responsible for initiation and those responsible for isomerization of the active center on the growing chain. Therefore, although alumina and fluoridated alumina are both good initiators for acetylene oligomerization reactions, the yields of aromatic products, e.g., benzene, toluene, and xylene, are lower with the fluoridated alumina catalyst since the possibility for rearrangements of the growing chain on the catalyst surface is lower. © 1991.