Rare earth cation-modified X zeolites for isobutane alkylation: The influence of ionic radius

A series of rare-earth cation-modified X zeolites (RE3+: La3+, Ce3+, Pr3+, Nd3+, Sm3+) were synthesized and evaluated for butene/isobutane alkylation. Based on the hypothesis that small RE3+ ions could more effectively polarize the tertiary C-H bond of isobutane, the alkylation performances of REX zeolite are anticipated to improve as the RE3+ radius decreases. However, our study reveals an opposite result. With the exception of Ce3+, which underwent oxidation during calcination, small RE3+ ions showed greater efficacy in facilitating framework dealumination, resulting in lower relative crystallinities, micropore structures, and acid concentrations. Consequently, the stability of REX zeolite essentially declined as the RE3+ radius decreased. Furthermore, it was observed that small RE3+ ions exhibited a preference for occupying sites within small cages. As a result, the concentration of accessible RE3+ ions reduced with decreasing RE3+ radius. In conjunction with the escalating electrostatic field of small RE3+ ions, the hydride transfer activity of REX zeolites displayed a non-linear relationship with reducing RE3+ radius, following a parabolic trend. PrX showed the highest activity in the hydride transfer reaction, as evidenced by its remarkable selectivity towards n-butane and trimethylpentanes.