| Hydroisomerization of n-alkane is one of the most economical and effective methods of increasing gasoline octane number. It has developed rapidly since the1990s in order to meet the requirement of environmental protection. We choose an experimental metal/zeolite bifunctional catalyst in our research, the rate-determining step is the skeletal isomerization reaction of2-pentene in acidic zeolites. In this essay, first-principle calculations based on density functional theory are used to investigate the reaction mechanism for2-pentene isomerization on β-zeolite, MOR zeolite and ZSM-5zeolite, using periodic boundary conditions. The main conclusions of the work are summarized as follows:1. The results of PBE functional and BEEF functional (van der Waals Correction) calculation show that the smaller the channel of zeolites, the more significant changes of adsorption energies will be caused by van der Waals force.2. The calculations of β-zeolite show that the product of2-pentene chemisorption tends to be pent-2-oxide rather than pent-3-oxide due to the stability of pent-2-oxide. In the five different isomerization mechanisms, the near-in-plane DMCP (path a) mechanism and the out-of-plane DMCP mechanism are more likely to take place, due to formation of more stable secondary carbenium ions and stable DMCP intermediates, which are the two major mechanisms.3. With the comparision of the activation energies of2-pentene isomerization on three different zeolites, we can conclude that the12-membered and larger rings have little steric hindrance effect for2-pentene isomerization. While the10-membered ring and smaller rings have apparent steric hindrance effect, increasing the reaction activation energies.4. Due to different pore structures and acid sites, the energies required to complete each mechanism on different zeolites are different. The most favorable pathway might change from case to case. Even for the same mechanism, the rate-determining step may vary on different zeolites. |
PDF Full Text Request