The Synergistic Effect Of Acid Sites In HZSM-5 Zeolite For Alkane Cracking

Using acidic zeolites for catalytic cracking is one of the most commonly used methods in petrochemical industry.Zeolites have been extensively employed as solid acid catalysts in petroleum refining and petrochemicals production,to catalyze a variety of reactions such as fluid catalytic cracking,hydrocracking,paraffin isomerization,aromatic alkylation,xylene isomerization,and toluene disproportionation.The structure of zeolites is composed of silicon and aluminum oxide tetrahedral and they exhibit both Bronsted and Lewis acidic properties.The acidity and pore size of zeolites have a significant influence on the conversion and product distributions of cracking reaction.Considering pore size,acidity and stability of HZSM-5,it is very suitable to be the catalyst for alkane cracking.The location of Br(?)nsted acid sites within zeolite channels strongly influences reactivity because of the extent to which spatial constraints determine the stability of reactants and of cationic transition states.Monomolecular alkane cracking and dehydrogenation turnovers occurred with strong preference on acid sites contained within smaller 8-MR pockets in H-MOR,while rates on sites located within 12-MR channels were much lower and often undetectable.The effect of density of Bronsted acid sites and distribution in the zeolite channel on the alkane transformation and activation has not been clarified up to date.In the present work,the cracking experiment of propane,n-butane and n-pentane has been carried out on HZSM-5 zeolites with varied Si/Al ratios(16-140)and the conversions are strictly controlled to be less than 5%.The cracking rate of the monomolecular alkane increase with the density of framework Al,i.e.Bronsted acid site exponentially.1H DQ MAS NMR has proved the spatial proximities between Bronsted acid sites in the channel when the density of Br(?)nsted acid site is high enough.The results of 13C MAS NMR results reveal that adjacent Bronsted acid sites have a polarization on the adsorbed alkane molecules,which re-disperses the electron cloud and results in peak splitting.And this polarization changes the transition state of the activated alkane and leads to the enhancement of conversion and cracking rate of alkanes and the increasing of cracking products with Haag-Dessau mechanism.Alkane-TPD experiments indicate more stronger adsorption of the alkane on the zeolite which offers the synergic effect between Br(?)nsted acid sites.Careful steam treatment(753K,1h)has been carried out on HZSM-5 zeolites with varied Si/Al ratios(16-140)to produce adjacent pair sites of Lewis acid sites and Br(?)nsted acid sites in different concentrations from dealumination,which are tested for the methylcyclohexane transformation reactions at 703 K under strictly controlled as differential conditions.The enhanced conversion and cracking rate of the methylcyclohexane molecules are observed on the zeolites with pair sites of Lewis acid sites and Bronsted acid sites.The synergy between them is proved to contribute to the enhanced reaction rate and has been positively correlated with the results of 1H DQ MAS NMR experiments.Above results have offered new concept for the mechanism of the alkane cracking and a strategy to further innovate the new effective catalysts.