| With the adjustment of national energy structure and the successful implementation of the "West-East natural gas transmission" project, cheap natural gas has posed a huge impact on the liquefied petroleum gas (LPG), which has put more effect on refining enterprises. But with the development of LPG aromatization technology at home and abroad, the applications of aromatic hydrocarbons made of LPG have drawn greatly concern of petrochemical industry. The aromatic hydrocarbons can not only recombine high octane number gasoline, but also can be separated into benzene, toluene and xylene (BTX), which have been the most important chemical feed stocks with huge demand. Therefore, LPG aromatization technology has very important practical significance in the improvement of the added value of LPG and easing the tension of aromatics supply.In this paper, we mainly studied the transformation of C3and C4alkanes in LPG into predominant BTX aromatics on Zn/(HZSM-5+y-Al2O3) in a fixed bed microreactor and focused on the enhancement of the liquid yield and the stability of catalysts. Firstly, a series of HZSM-5+y-A12O3catalysts with different mass ratio of HZSM-5to pseudo-boehmite binder were prepared and their catalytic activity in LPG aromatization was investigated to sieve the optimal content of the binder. Then based on the above optimal catalysts, Zn/(HZSM-5+y-Al2O3) with different zinc content were prepared using impregnation method and their LPG aromatization activity were inspected to find the optimal content of Zn.Finally,the HZSM-5was calcined at700℃,800℃,900℃and1000℃followed by the addition of binder and the impregnation of Zn to optical the effect of heat treatment on catalytic activity. Catalysts were characterized by XRD, SEM, BET, FTIR, NH3-TPD and Py-FTIR to relate the micro-structure and acidity with aromatization of LPG. The results were shown as follows:1) Higher aromatics yield were obtained on HZSM-5+A(30%) due to its appropriate amount of Bronsted acid and total acid. The amount of total acid and Bronsted acid decreased dramatically, so did the ratio of B/L, at the same time, the amount and the strength of strong acid also had a little decrease.2)1.57%Zn/(HZSM-5+A(30%)) had the optimal aromatization activity among the catalysts with different zinc content. The addition of Zn led to the decrease of Bronsted acid sites and the increase of Lewis acid sites. The increase of Lewis acid sites and the decrease of B/L ratio were due to the formation of Zn-L acid sites which was the result of the combination of ZnO and Bronsted acid sites. With the increae of Zn content under the circumstance of lower Zn loading, the cracking of alkanes were weakened and dehydrogenation of alkanes and dehydrogenation aromatization of naphthenes were promoted by Zn-L acid accompanied with the decrease of hydrogen transfer aromatization, which led to higher aromatics yield. But the amount of Bronsted acid began to experience decrease sharply with the continuing increase of Zn content, which depressed the intermediate steps such as oligomerization and cyclization of alkenes on Bronsted acid, thereby made the aromatics yield decline.Synergy effect of Bronsted acid and Zn-L acid was the best when Zn m%was about1.57%, which led to expected yield of aromatics.3)1.57%Zn/(HZSM-5-700+A(30%)) exhibited better aromatization activity of C3and C4when compared with other heat treatment condition. After mild heat treatment of molecule sieve in700℃, Bronsted acid sites were reduced, accompany with the dealumination of frameworks. While after impregnated with Zn, the ratio of B/L of the catalyst were optimized further and the synergy effects of Bronsted and Lewis acid were strengthened, to some extent, which eventually improved the yield of aromatics. Heat treatment of elevated temperature of molecule sieves could cause serious dealumination of frameworks followed by the decrese of Bronsted acid sites and B/L ratio, which affected the improvement of liquid yield. |
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