| The isomerization of alkanes is one of the most effective and green way to increase the octane number of gasoline and reduce the pour point and freezing point of biomass fuel.It has got rapid development in recent years in order to satisfy the requirements of high quality clean gasoline.Among numberous isomerization catalysts,bifunctional catalysts with noble metal loading on acid molecular sieve has attracted widespread attention because of the high selectivity,low reaction temperature and low pollution.However,the reaction mechanism of such catalysts has not been widely accepted.Based on the density functional theory（DFT）,the Pt based catalysts(Pt（111）and Pt55)has been chosen as metallic center of bifunctional catalyst to study the adsorption and dehydrogenation reaction of C2-C6 paraffin.Besides,we choose HZSM-5 as the acid center of bifunctional catalyst to study the skeleton isomerization mechanism of hexene.The further study on the reaction mechanisms of the two active centers of the bifunctional catalyst can provide good theoretical support for the upgrading and design of alkane isomerization catalyst in the future.C2-C6 alkanes physisorbed on both Pt（111）and Pt55 without chemical bond.The van der Waals interaction had a great influence on the adsorption energy.Considering the van der Waals force,there is no special adsorption site on Pt（111）surface while the energy differences is less than 0.08 e V,and the optimal adsorption configuration on Pt55 is edge.The alkane adsorption energies increase linearly with chain length,and the adsorption on the Pt55 cluster is more stable than that on Pt（111）due to the lower coordination number of cluster atoms.Alkenes can be adsorbed on Pt catalyst stably by di-σandπmodes.The most stable adsorption structure of C2-C6 olefin on Pt（111）surface is di-σmode while that on Pt55 isπmode and is more stable.Under the same adsorbent and adsorption mode,the adsorption energy of olefin does not change much with the increase of chain length,and there is no linear relationship as the case of alkane adsorption.Therefore,the adsorption of olefin on Pt is independent of the chain length.The Rate determining step（RDS）of paraffin dehydrogenation on Pt（111）is the first dehydrogenation step,and the RDS energy barrier increases with the chain length.However,the barrier differences between the first step and the second step are very small（<0.10 e V）on Pt55.Therefore,the RDS can not be well determined,and the reaction barriers fluctuate with the chain length.The dehydrogenation barrier on the Pt55 cluster was significantly lower than that on the Pt（111）surface,showing higher reactivity.The BEP（Br（?）nsted-Evans-Polanyi）linear relationship between the final state and the transition state of alkane dehydrogenation was analyzed based on the large calculated data.The linear relationship is not affected by the catalyst and chain length,which can provide support for the subsequent dehydrogenation of alkanes on different Pt based catalysts.There are three adsorption mode of hexene in HZSM-5,π-complex,alkoxy group and carbocation.We have studied the adsorption of both n-hexene and i-hexene.It is found that tert-carbonium can stably exist in ZSM-5 with an adsorption energy of 0.82 e V.We studied the isomerization process of hexene following the PCP reaction mechanism with alkoxy and carbonium mode i-hexene as final structures.The RDS of hexene’s skeleton isomerization in ZSM-5 is the formation of protonated cyclopropane intermediate MECP+,with a barrier of 1.08e V,which is consistent with the experimental results and other theoretical studies.The barriers for the formation of alkoxy group and carbocation product from the same intermediate MECP are 0.16 e V and 0.40 e V,respectively.The alkoxy mechanism has a lower reaction barrier,but the two are both much lower than the RDS.Therefore,when the experimental conditions satisfy the RDS,both mechanisms are able to carry out,generation two hexene isomers with different branched-chain locations. |
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