| Propylene is an important building block, mainly used for the manufacture of polypropylene, acrylonitrile, acrylic acid, acrylic aldehyde, etc. Traditional ways of producing it-steam cracking and catalytic cracking, however, can no longer meet the ever increasing world demands in recent years. Propane dehydrogenation (PDH) as an alternative has become the third propylene producing way. However, the endothermic and thermodynamic limited nature of this reaction make industrial processes energy intensive. Coupling dehydrogenation process with selective hydrogen combustion (SHC) reaction would help provide heat in situ and at the same time break equilibrium limitations by removal of hydrogen from product mixtures, thus drawing a lot of attentions nowadays.In the present study, with the aim of developing this coupled process, we started from exploring the basic principles for the rational design of selective hydrogen combustion catalysts. Combing experiments and theoretical calculation, we emphasized on the study of the factors determining the activity and selectivity of supported Pt catalysts for selective hydrogen combustion. Then Pt clusters were introduced into the cages of LTA zeolites and the relationship between zeolites’pore size, adsorption properties and catalytic performances, the effects of surface modification of guest zeolites on the catalysts’activity and selectivity were studied. After that, the effects of steam generated from hydrogen combustion on the catalysts’activity and structure of Pt and Pt-Sn catalysts were elucidated. The main results were summarized as follows:(1) Low selectivity and stability of supported Pt catalysts for SHC. Alloying Pt with Sn would help weaken adsorption of C3 species on Pt surfaces, thus improving O2 selectivity to H2O. It is found that after reacting 25 h, the selectivities decrease from 96.3% and 94.5% to 84.2%and 75.0% for Pt and Pt-Sn catalysts, respectively. The oxidation of Sn was found responsible for the even lower selectivity for Pt-Sn catalyst. The formation of more highly graphited coke caused by oxygen introduction would be the reason for the selectivity drop with time on stream over both catalysts.(2) Pore size-dependent selectivity of encapsulated Pt/LTA catalysts for SHC. Propylene deep oxidation and coking reactions would be restricted when Pt clusters are encapsulated in zeolites with pore size small enough to exclude propylene. As expected, O2 selectivity to H2O increase with decreasing pore size of LTA zeolites in the sequence of Pt/CaA (77.0%)< Pt/NaA (81.6%)< Pt/KA (94.2%). Coking reactions can be significantly suppressed over Pt/KA and Pt/NaA catalysts with smaller pore size. In addition, most Pt clusters still remained to be around1.1 nm even after 120 h long-run test.(3) Effects of zeolites’ adsorption properties on catalysts activity. The activities of Pt/LTA catalysts depend on propylene partial pressure due to strong interactions between propylene and zeolites. As a result, oxygen conversions for all catalysts are below 51.0% in presence of 30 kPa of propylene. DFT calculation indicated that propylene adsorption on 8-member rings of NaA with specific configurations tend to blocked the access of reactants and that the insertion of one more Na into the 8-member ring help weaken propylene adsorption by lowering the acidity of charge balancing Na+ and intervening the formation of H-bonds with framework oxygen ions. When modifying the surfaces of LTA zeolites with alkali or alkaline earth metals, it was found that the propylene adsorption amount were lowered and the interactions between propylene and zeolites were weakened. At the same time, the pore size of these zeolites were decreased as well. Accordingly, for Pt-3K/KA and Pt-3Na/NaA catalysts, the oxygen conversion soared up to 92.0%, and the selectivities of them increased to 98.5% and 95.6%, respectively.(4) Effects of steam on the propane dehydrogenation mechanism. DFT investigation of propane dehydrogenation on Pt(111) surface revealed that co-adsorbing H2O,-OH and -O with propane and (n,i)-propyl would shift dehydrogenation energy barriers to higher values at the extent of 0.01-0.06 eV. The involving of surface -OH or -O in dehydrogenating transition states through Langmuir-Hinshelwood and Eley-Rideal mechanism would make propane activation becomes the rate limiting step and the activation energies higher than that on clean Pt(111) surface, thus frustrating propylene formation. Steam would help remove cokes deposited on uncoordinated Pt sites, which leads to higher propane converting rates and lower apparent activation energies.(5) Effects of steam on the activity and structure of Pt-Sn catalysts. Promoting effects of steam were observed on Pt-Sn catalysts. It is found that steam would promote the spreading of SnOx species to supports to expose more active sites, and lead to Sn oxidation and segregation from Pt-Sn alloys to transform inactive alloy phases (PtSn) to active ones (Pt3Sn or Pt), and lower the coking rates. Which contributes to higher propane conversion and lower apparent activation energies. However, in kinetic point of view, steam would retard propylene formation as discussed in (4). That is why there exist an optimum amount of steam for Pt-Sn catalysts, which increases with increasing Sn loadings. |
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