Study On Zeolite-based Catalysts For Isobutane/butene Alkylation

Isobutane/butene alkylation plays a vital role in the petrochemical industry.It produces alkylate oil,which is an ideal high-octane gasoline blending stock because of its high octane number,low sensitivity,and low volatility.The alkylate oil is also known for its clean-burning characteristics because of the absence of sulfur,olefins,and aromatics.The alkylation reaction is commercially catalyzed by sulfuric acid and hydrofluoric acid,which pollute the environment and corrode the equipment.Developing solid acid catalysts is the primary solution to solve these problems.For this purpose,this work focused on the FAU zeolites and systematically studied the relationship between physicochemical properties and the alkylation performance.Specifically,the main work of this article is as follows:（1）La³⁺exchanged FAU zeolites with varying framework Si/Al ratios were prepared and tested in the alkylation reaction.The catalyst lifetime was found positively related to the strong Br（?）nsted acid concentration.Combined with the extrapolations that further maxima may be obtained beyond the scope of this study,the catalyst lifetime was found to change in a volcano-shaped pattern with respect to the framework Si/Al ratio.This implies that there were at least two extreme Si/Al ratios in terms of catalyst stability:one is ca.1.2 while the other is no less than 6.7.Considering the product selectivity,the optimal Si/Al ratio was 1.2.The influence of process parameters on catalyst stability was established on this basis.At the best conditions(activation temperature:200°C;reaction temperature:60°C;olefin space:0.05 h^-1;stirring speed:1500 rpm),the lifetime of La FAU-1.2 was as long as 74.5 h.（2）X zeolites were modified with different rare-earth cations(RE³⁺:La³⁺,Pr³⁺,Nd³⁺,Sm³⁺),and the relationship of RE³⁺radius on the physicochemical properties and hydride transfer activity of REX zeolite has been studied.The progressively higher dealumination of REX zeolites with reducing RE³⁺radius has been confirmed.Such a phenomenon was explained by the stronger electrostatic field induced by smaller RE³⁺,which facilitates the hydrolysis of framework aluminum.The increasing dealumination degree made the acidities of REX zeolites decline with the reducing RE³⁺radius.Consequently,X zeolite modified with small RE³⁺showed poorer stability in isobutane alkylation.Further,small RE³⁺cations tend to migrate into the sodalite cages,making the hydride transfer activity of REX zeolite non-monotonously changed with the RE³⁺size.Pr X showed a higher hydride transfer activity than other samples,reflected in the high n-butane selectivity and the highest TMP/DMH ratio in the alkylation product.（3）Four Ce X zeolites with varying Ce⁴⁺content have been successfully prepared by changing the calcination atmosphere between nitrogen and air.The correlation between Ce⁴⁺content and physicochemical properties has been established.A great reduction of relative crystallinity and micropore structures was observed when the zeolite was calcined in air.This phenomenon was ascribed to the in situ generated water during Ce³⁺oxidation,facilitating framework dealumination.Further,the oxidation of Ce³⁺also consumes acidic protons,and as a result,the Br（?）nsted acid concentration was significantly reduced with the increasing Ce⁴⁺content.This made the samples that suffered air calcination show much poorer stability in the alkylation reaction:Ce X that calcined in the protection of nitrogen exhibited a lifetime of 12 hours,while that suffered two-step air calcination deactivated at the very beginning of the alkylation reaction.（4）By preparing La X zeolites with varying crystal sizes,the influence of intraparticle diffusion has been studied.It showed that reducing the crystal size can efficiently improve the catalyst stability when the zeolite crystal is larger than 3.2μm.However,further reducing the crystal size to the nanoscale will significantly increase the structural defects,which weaken the stability of the zeolite framework.As a result,the sample with nanosized crystals suffered higher dealumination towards ion exchange,and consequently,it exhibited a shorter lifetime and yielded a poorer product in the alkylation reaction.Given the tradeoff effect between diffusion resistance and catalyst acidity,there is speculated to be an optimal particle size for the X zeolite when used in the alkylation reaction.