Study On DME Synthesis Over Zeolite SUZ-4

Over the past few decades much research has been focused on synthesis, characterization, and catalysis on zeolites. Only few of these materials, however, are used commercially as ionexchange builders, adsorbents, or catalysts. There is consequently a significant incentive (i) to utilize known zeolites in novel applications and (ii) to produce new materials with potentially useful properties. In this context, synthesis of SUZ-4 zeolite and its application in catalysis have been studied in this dissertation. Zeolite SUZ-4 is a new synthetic aluminosilicate patented by British Petroleum Co. (BPC) in 1992. Zeolite SUZ-4 shows high activity, selectivity and stability in methanol dehydration to dimethyl ether (DME), however, it was deactivated easily in syngas to DME. Therefore, technologies of XRD, SEM, MAS NMR, FT-IR, TG-DTA,XPS, NH3-TPD,EDS and N2 adsorption are employed for catalysts characterizations, and, based on the results of catalysts characterizations, mechanisms of DME formation and the reasons for high DME selectivity in methanol dehydration over HK-SUZ-4 zeolites are proposed. The reason for HK-SUZ-4 deactivation in syngas to DME is also reported here.Generally, ion-exchange is employed in modifying acidity of zeolites, and many methods were proposed in previous papers. It is proposed that some potassium ions in SUZ-4 zeolite are locked in a small cage, as a result, their exchange would be difficult. Consequently, we tried to improve ion-exchange degree of SUZ-4 by performing ion exchange using multicycle of ion-exchange/calcination. Through three exchange/calcination cycles under general hydrothermal circumstance, the resulting proton exchanged zeolite with ion-exchange degree of only 76% was obtained. HK-SUZ-4 zeolite with different ion-exchange degree possesses variou H+ contents, and, consequently, their acidities are different. As a result, they present distinct catalytic properties in methanol dehydration to DME.It is well known that DME has many advantageous properties as a propersubstitute for diesel or refrigerant in future. Presently, DME is made by catalytic dehydration of methanol over a solid-acid catalyst. As an alternating process, the direct synthesis of DME from syngas was proposed over bifunctional catalysts, and, moreover, this process is more favorable in view of thermodynamics and economy. In this study, methanol dehydration to DME over HK-SUZ-4 and syngas to DME over HC/HK-SUZ-4 catalyst have been investigated. DME formation in methanol dehydration on the surface of HK-SUZ-4 takes place via mechanism of acidic catalysis. Interestingly, HK-SUZ-4 zeolites give high DME selectivity, although they possess strong acid sites on the surface. It can be ascribed to unique channel system and acid sites location of HK-SUZ-4 zeolite. Furthermore, the very low concentration of AlOCH3 species on surface of HK-SUZ-4 might lead to the less formation of hydrocarbons over HK-SUZ-4, accordingly a high DME selectivity was achieved.The optimum ratio of HK-SUZ-4 to HC (commercial methanol synthesis catalyst) must be changed with increasing reaction temperature. In the case of 553 K, the optimum amount of HK-SUZ-4 in bifunctional catalysts is 7.5 wt.%. Bifunctional catalysts HC/HK-SUZ-4 also show a high DME selectivity in syngas to DME; however its stabilities are very poor. It might be attributed to deactivation of HK-SUZ-4, which might be poisoned by hydrocarbons formed on the surface of HC catalysts. Fortunately, HK-SUZ-4 can be regenerated by a simple method.The evaluation of H-ZSM-5 and modified ones for methanol dehydration to DME has also been investigated. Zeolites H-ZSM-5 modified by Na and MgO exhibit high DME selectivity, but its activity are low compared with HK-SUZ-4.