The Modification Of SAPO-34 And Its Catalytic Performances For Methanol To Olefins Reaction

Since SAPO-34 has special pore structure, proper numbers of acid sites and acid strength, conducting high selectivity for olefins and good stability in methanol to olefins(MTO) reaction, it has been recognized as the most excellent active component in MTO catalyst. However, owing to its small pore diameter, coke could be easily formed, which will cause the deactivation of the catalyst and decrease olefins selectivity and catalyst lifetime. The modification of zeolite is the most commonly used and effective way to tailor zeolite’s physicochemical properties, which can improve its performance greatly. So it is necessary to study the modification of SAPO-34.The paper used SAPO-34 as catalyst in MTO reaction, aiming at improving selectivity for ethylene and propylene, as well as prolonging lifetime of the catalyst. Different modification methods, such as hydrothermal treatment, acid leaching, phosphorous modification and metal ions(La, Ni, and Mg) were studied. XRD, XRF, SEM, N2 adsorp-desorption and NH3-TPD were employed to investigate the physicochemical properties of SAPO-34 before and after modification. The performances of the above SAPO-34 zeolites in MTO reaction were studied on a fixed bed. Furthermore, the modification method to SAPO-34 and their performances were correlated.In hydrothermal-modification method, SAPO-34 was treated at 800 ℃ with 100 % steam with different time. After steaming treatment, the numbers and strength of the acidity became lower and more mesopores were formed. MTO test showed that with the increase of steaming time, the lifetime of SAPO-34 prolonged to three times at most and the selectivity for the olefins(C2=+C3=) increased by 14.4 %. Acid-leaching was to treat SAPO-34 by different concentration of citric acid. After leaching, Si species were decreased and the pore volume decreased, however the external surface increased. In MTO reaction, the lifetime of SAPO-34 prolonged by 33 % and the selectivity for olefins(C2=+C3=) increased by 7.9 %.Phosphorous-modification was to treat molecule sieves with ion-exchange and impregnation method. The evaluation results indicated that the selectivity for olefins and lifetime both decreased by impregnating with P, while the selectivity for olefins(C2=+C3=) increased by 12.9 % with 0.9 % P by ion-exchange method. The two methods, ion exchange and impregnation were also used in La, Ni, and Mg modification to SAPO-34. As for La, the selectivity for olefins(C2=+C3=) could increase by 5.1 % and lifetime remained the same by ion-exchange, while the selectivity for olefins(C2=+C3=) increased by 13.8 % and lifetime prolonged 2 h by impregnation; As for Ni, by ion-exchange method the olefins(C2=+C3=) selectivity increased by 12.5 % and lifetime remained the same, while the olefins selectivity(C2=+C3=) promoted by 12.8 % and lifetime decreased by 3 h by impregnating P; As for Mg, the selectivity for olefins(C2=+C3=) increased by 6.6 % and lifetime remained the same by ion-exchange, while the selectivity increased by 12.0% and lifetime decreased by 1 h by impregnation method.In summary, hydrothermal and acid modification could promote the selectivity for olefins and prolong lifetime by changing acid sites and tailoring pore structures of SAPO-34, which are expected effective ways to modify SAPO-34 in MTO reaction. However, when modifing SAPO-34 with phosphorous and metal ions(La, Ni and Mg), proper methods, ion-exchange or impregnation, should be chosen cautiously.