Study of n-butanol conversion to butenes : effect of Si/Al ratio on activity, selectivity and kinetics

As bio-butanol is gaining more and more interest as a commercially available bioresource, the dehydration of this alcohol towards butenes and higher carbons gains more of interest. In general HZSM-5 has shown to be the most promising catalyst for this conversion. The role of the zeolite’s Si/Al ratio in the butanol dehydration reaction is still not fully understood. Experimental data obtained for a series of HZSM-5 with decreasing Si/Al ratio revealed an increase in activity of the catalyst per active site without affecting the selectivity profile. To understand the underlying effects, a microkinetic model was constructed for H-ZSM-5 with a Si/Al ratio of 25, based on literature DFT calculations, and the model was further modified by fitting the key parameters to the measured data at the four different temperatures studied in this work. This resulted in an adequate model for the dehydration of butanol across the evaluated temperature range of 503K to 533K. Investigation of the occurring mechanisms indicated a inhibiting effect due to the strong adsorption of di-n-butylether. This ‘poisoning’ of the catalyst surface resulted in a peculiar S-like curve for the conversion site time relation, which was also experimentally observed. This newly fitted base model was used to obtain more insight in the effect of the Si/Al ratio by implementing an additional H parameter, which is related to the adsorption strength of n-butanol in the base model. H varies between -4.8 to +11.3 kJ/mol and provides a good fit for Si/Al ratios ranging from 15 to 140. The higher dehydration rates observed with decreasing Si/Al can be traced back to an increase in adsorption strength resulting in an overall increase in surface coverage. The constant selectivity-conversion profile can be explained by a similar dependency of all elementary steps on the adsorption strength. The model developed in this study enables to simulate and understand the experimentally observed effects of temperature and Si/Al ratio on the n-butanol dehydration