n-Butene skeletal isomerization over HFER zeolites: Influence of Si/Al ratio and of carbonaceous deposits

The effect of time-on-stream TOS (from 1 min to 21.5 h) on the rate and selectivity of n-butene transformation at 350 °C was determined over a series of HFER zeolites with different Si/Al ratios (6, 9.5, 10, 19 and 32). The porosity and acidity of the fresh and aged samples were characterized by nitrogen adsorption and by NH3 adsorption at 50 °C followed by FTIR, respectively. The main differences between the samples were the following: (i) increase with Si/Al in the activity of the protonic sites (turnover frequency) of fresh samples and in their selectivity for isobutene production, (ii) decrease in the rate of isobutene production with increasing TOS for high Si/Al ferrierites as could be expected from coke formation and complex change for low Si/Al ferrierites with in particular an unexpected increase. Furthermore with all the samples, the turnover frequency for isobutene production increases with TOS whereas the one for propene + pentenes decreases or remains constant. All these observations can be explained by considering the zeolite pore system as a series of non-interconnected nanoreactors (the 10-MR channels) in which the molecules diffuse without any possibility of desorption before their exit while undergoing successive bimolecular reactions. In those with few protonic sites, n-butene isomerizes selectively through an autocatalytic process. In those with many sites a thermodynamic equilibrium mixture of propene, butenes and pentenes is formed.