DFT Study On P-ZSM-5Zeolite Catalyst For Dehydration Of Bio-ethanol To Ethylene

Ethylene is an important basic organic chemical material, and its production is asymbol of the petrochemical development of a country. With the shortage of theworld’s energy and the growth of the oil prices, researchers are taking more attentionon the bio-ethanol to ethylene process. In recent years, the research has beenconcentrated on the catalyst of the ethanol dehydration. More research about themodification of ZSM-5zeolite has been done, and the P-ZSM-5zeolite catalyst hasshowed excellent catalytic activity. In this paper, the Density Functional Theory (DFT)was carried out to investigate the structural, electronic and acidity of P-ZSM-5zeolite.The resistance to coking of the P modified ZSM-5zeolite was also investigated.It was founded that when P-ZSM-5extra-framework modified, the hydrogenatoms in phosphoric acid molecule and the framework oxygen atoms in zeoliteshowed strong hydrogen bonds interactions, which was the most stable structure forextra-framework modified zeolite. When P-ZSM-5framework modified, P atoms andthe zeolite bridging oxygen atom were combined with single bond, rather thanreplacing the bridge hydroxyl with H2PO4-groups, which was because the skeletonstructure of the zeolite prevented the insertion of H2PO4-groups. The DFT was alsoused to calculate the binding energy of the extra-framework modified zeolite and thetotal energy of the framework modified zeolite. It was founded that the T12model wasthe most stable model for both extra-framework and framework modified zeolite. Inthis paper, the Mulliken charge and the deformation density were both analyzed. Itwas founded that the charge was transferred from the O atoms to the P, Al and Hatoms. This could enhance the interaction between phosphate and the zeolite, therebyenhancing the stability of the phosphate modified ZSM-5zeolite.The Proton Affinity (PA) and the interaction between NH3and the zeolite werealso investigated in this paper. It was founded that the acidity of ZSM-5wasdecreased after the introduction of P, especially the framework modification.The14T cluster model was selected after the comparison of the structural, ProtonAffinity and the ethanol adsorption energy of the8T,14T and18T cluster models.Ethylene dimerization reaction is one of the main reasons which lead to the coking ofthe ZSM-5zeolite. The results of the TS search showed that the activation energy ofthe ethylene dimerization reaction was increased after phosphate modified. This could inhibit the ethylene dimerization reaction occurs, possibly enhancing the performanceof resistance to carbon deposition, increasing the life of the zeolite catalyst.After the investigation of the structural, electronic, and acidity changes ofP-ZSM-5zeolite, the relationship between the microstructure and the macroscopicproperties was analyzed. Also the inhibition mechanism of the ethylene dimerizationwas investigated. This could provide a theory support for the catalysts design of theBio-ethanol to ethylene process.