Modification Of ZSM-5 Zeolite Catalysts And Their Catalytic Cracking Properties Of 1-butene

Propylene is one of the most important fundamental chemical intermediates, which mainly produced by steam cracking process. However, with the demand for propylene derivatives increasing steeply, the traditional methods for the production of propylene cannot satisfy the demand. Steam cracking of petroleum has low propylene yield and harsh operation conditions, and other techniques require high investment and economic viability is low. Recently, some new techniques on low olefins production have been proposed. The catalytic cracking ofhydrocarbon resources appeared to be one of the most promising methods for low olefins. C4 resources are mainly produced as the byproducts of steam cracking and fluid catalytic cracking process (FCC). With the technique of C4 catalytic cracking, not only waste can be reused, but also more propylene can be obtained. As a result, it becomes a hot research topic recently.Due to unique pore structure, good thermal stability, hydrothermal stability and adjustable acidity, ZSM-5 zeolites were widely used in catalysis materials and adsorption materials. Although C4 cracking process using ZSM-5 zeolites as catalysts has been applied industrially, there are still some serious problems to be resolved: dealumination at high temperature, easy to coke and so on. In this paper, Ag modified ZSM-5 zeolite catalysts for butene catalytic cracking were mainly investigated, and post treatment (alkaline treatment and hydrothermal treatment) was also used to study the influences of pore structures and acidities on catalytic performance of butene cracking. The physicochemical properties were characterized by N2 desorption, infrared spectroscopy with pyridine adsorption (PY-IR), NH3 temperature-programmed desorption (NH3-TPD), scanning electron microscope (SEM), X-ray diffraction (XRD), inductively coupled plasma mass spectrometry (ICP-MS)and transmission electron microscopy (TEM), and so on. This thesis is mainly focused on the following aspects:(1)The preparation of Ag modified HZSM-5 zeolite catalysts and their catalytic performance inbutene cracking. It is found that 550 ℃ and high space velocity of 20 h-1 are preferred reaction conditions for propylene selectivity in butene cracking. The influence of silica/alumina ratios (SAR) was studied to explore the relation of acidities with the performance of butene cracking. The results show that HZSM-5 zeolite of high silica/alumina ratio (SiO2/Al2O3=280) with low acidity and strength is less favor for hydrogen transfer reactions andbetter for high propylene selectivity. Ag modified HZSM-5(SAR=280) zeolites were prepared by ion-exchange method, and the influence of Ag loadings and steam treatment was investigated. The propylene selectivity over HZSM-5 is 33%, and the yield is 26.6%.The activity ofHZSM-5 catalyst significantly decreasesafter steam treatment, and the activity cannot be recovered after being regenerated under the mild condition.With proper Ag loadings and steam treatment, catalysts with high propylene selectivity in butene cracking and hydrothermal stability are obtained, for example,AgHZSM-5with a Ag loading of 0.43 wt%, after steam-treated for 24 h at 600 ℃,haspropylene selectivity of 42.2%, and yield of 30.4%, which also has good regeneration stability.(2)The preparation of Ag modified NaZSM-5 zeolites and their catalytic performance in butene cracking.In order to control the acidity of Ag-modified zeolite catalysts better and simplify the preparation process, the preparation of AgNaZSM-5 zeolite catalysts was proposed. To obtain catalysts with high propylene selectivity and hydrothermal stability, the influences of Ag modification methods(ion-exchange method, impregnation method and co-impregnation method) and Ag loadings on the zeolite properties and catalytic performance in butene cracking were investigated.AgNaZSM-5 catalyst prepared by ion-exchangemethod,with the Ag loading of 0.67 wt%, has propylene selectivity of 44.1% and yield of 31.6%.The Ag content kept unchanged after the zeolite was steam-treated for 24 h under 800 ℃,indicating that Ag species on the cation sites are stable. AgNaZSM-5 catalysts were prepared by traditional impregnation and rotating-impregnation of Ag salts in NaZSM-5 zeolites. However, AgNaZSM-5 catalysts prepared by these two methods hadlow Ag distribution andbad catalytic performance with high deactivation rate and low activity in butene cracking. AgNaZSM-5 zeolite catalysts prepared by co-impregnation of AgNO3 and NaNO3 in HZSM-5 had better Ag dispersion and similar catalytic performances with the catalysts prepared byion-exchange method. Due to the easier preparation process, the co-impregnation method is more perspective in industrialization.(3)Post treatmentof HZSM-5 zeolites and their catalytic performance in butene cracking. Mesopore HZSM-5 (SAR=80) zeolites with wide pore diameter distributions were prepared by alkaline treatment method. Their acidities increased and as a result, the conversion of butane cracking increased and selectivity of propylene decreased. HZSM-5 zeolite catalysts after steam treatment had lower acidities and showed higher propylene selectivity. The deactivation rate of HZSM-5 and the alkaline treated onewere almost the same. Alkaline treatment and steam treatment are combined in order to obtain catalysts with proper acidities and adequate mesopore volumes. The HZMS-5 zeolite treated by alkaline treatment sequenced by steam treatment is better than that treated by steam treatment followed by alkaline treatment.The former had adequate mesopores and acidity. In the long term test of butene cracking, the steam-treated HZSM-5 zeolite catalysts and alkaline+steam treated ones had a fast deactivation in the initial stage of the reactions. Combined with the BET data of used catalysts, it can be inferred that the fast deactivation is related to the smaller mesopores where coke reactions occur easily.