Synthesis of zeolite ZSM-57 and its catalytic evaluation for the 1-butene skeletal isomerization and n-octane cracking

The synthesis of the medium-pore zeolite ZSM-57 in the presence of sodium a nd N, N,N,N', N', N'-hexaethylpentanediammonium cations is described. Under the synthesis conditions studied here, crystallization of pure ZSM-57 was possible only from synthesis mixtures with a very narrow range of SiO2/Al2O 3 and NaOH/SiO2 ratios. The catalytic properties of H-ZSM-57 are evaluated for the skeletal isomerization of l-butene to isobutene and the cracking of n-octane and compared to those obtained from H-ZSM-5 and H-ZSM-35 with the same pore architecture but different pore sizes. A high initial 1-butene c onversion is observed for H-ZSM-57, while its selectivity to isobutene is m uch lower than that of H-ZSM-35. This trend remains almost unchanged with t ime on stream. Thus, the nonselective behavior of H-ZSM-57 for isobutene fo rmation may be due not only to the presence of very strong acid sites in th is zeolite as evidenced by ammonia TPD experiments, but also to its channel intersections that are large enough to allow undesired side reactions such as 1-butene dimerization followed by cracking to light olefins. By contras t, H-ZSM-57 shows considerably higher n-octane cracking activity than ZSM-5 known as one of the most active catalysts for this reaction. However, it i s found that the poisoning of strong acid sites in H-ZSM-57 by coke formati on occurs within the pores, leading to a continuous decrease in n-octane co nversion. Such an aging feature of ZSM-57 suggests that medium-pore zeolite s with intersecting channels of two different pore sizes may be less stable as acid catalysts than those with uniform pore size, due to the geometrica l constraints imposed by the dual pore system, even in the case that their 10-ring pore sizes are similar to each other. (C) 2000 Academic Press.