Co-aromatization Of N-hexane And Methanol

Aromatics are important basic materials and solvents for chemical industry.At least 90%of aromatics is traditionally from petroleum cracking.China is rich in coal-based methanol,and the added value of light hydrocarbons from coal processing low.The co-aromatization of methanol with light hydrocarbons for the production of aromatics has attracted attention.On the one hand,the heat released from methanol aromatization can directly supply the endothermic aromatization of light hydrocarbons.On the other hand,light hydrocarbons can be realized better.In this thesis,the catalytic performance of Zn,Cr and P modified HZSM-5 molecular sieve with on the coupling co-aromatization of n-hexane with methanol in a fixed bed reactor was studied,aiming at improvement in the catalytic stability.The relationships between the surface acidity of the catalyst and the activity,and influences of reaction conditions on the product distribution were also revealed.1.For the aromatization of n-hexane over Zn-modified HZSM-5,the yield of aromatic hydrocarbons increased from 31%to 38.2%with the increase of Zn content,but the actcivity stability was poor.Further Cr modification to prepare[Zn,Cr]/HZSM-5 by incipient wetness impregnation method,which improved the dehydroaromatization and inhibited the hydrogen transfer reaction due to remarkedly decreased Bronsted acid site(BAS)and significantly increased Lewis acid site(LAS),thus enhanced the active stability and the conversion of n-hexane reached over 97%.Especially,over the Zn-5Cr/ZSM-5 catalyst,the aromatics yield reached 49%and the yield of benzene,toluene and xylene(BTX)reached 8.1%,27%and 12.9%,respectively,which all were higher than Zn/HZSM-5 and verifying the coactivation mechanism of n-hexane cracking and alkane dehydrogenation catalyzed by both of BAS and LAS.2.The contact time,reaction temperature,feed ratio and feed position of methanol influenced the product distribution obviously in the co-aromatization of n-hexane and methanol.The Studies had shown that when the reaction temperature of 450℃,the LHSV of 2 h-1,the feed ratio of n-hexane(m):methanol(m)=3:7,methanol feeding from the top was better for the co-aromatization and the yield of aromatics over Zn-5Cr/HZSM-5 reached 40.1%.When feeding with methanol in the middle position,the methylation was enhanced and the selectivity of xylene was improved from 40.4%to 59.6%although the n-hexane conversion and aromatics yield decreased.Compared with aromatization of n-hexane,the feed of methanol in co-aromatization didn’t change the aromatics yield but significantly increased the selectivity of xylene from 26.3%to 52.3%.It is verified that methanol occurred both aromatization and alkylation.2.Over Zn-5Cr/HZSM-5 catalyst,raising the reaction temperature improved the aromatic yield but decreased the stability.While[Zn,Cr,P]/HZSM-5 catalyst further improved the aromatics yield,especially for benzene and toluene to 39.5%and 36.8%,respectively.The modification with P species prevented the removal of the framework A1 and carbon deposition to improve the catalyst stability,and simultaneously modulated the product distributions resulting in the two extremes of C1-C3 gaseous hydrocarbons and liquid aromatic hydrocarbons(the yield reached 87.5%).