Hydrothermal synthesize of modified Fe, Ag and K-SAPO-34 nanostructured catalysts used in methanol conversion to light olefins

Abstract

A series of modified SAPO-34 nanostructured catalysts have been prepared using hydrothermal treatment and ion exchange methods. The samples were characterized by XRD, FESEM, PSD, EDX, BET, FT-IR and NH3-TPD techniques. The X-ray diffraction patterns indicated a higher crystallinity for the catalyst synthesized using K+ ion exchange method. According to the XRD analysis, FeAPSO-34 catalyst has higher crystallinity and larger crystallite size than SAPO-34. The FESEM results indicated that the morphology of the nanostructured catalysts is similar to the chabazite phase. Incorporation of Fe in SAPO-34 framework leads to decrease acid strength as confirmed by NH3-TPD. By Ag+ ion exchange, it was also observed that acid strength of weaker and stronger acid sites increased. Catalytic performance of the parent and modified SAPO-34 zeolites for methanol to light olefins were evaluated on a fixed-bed reactor at different temperatures, atmospheric pressure, and methanol gaseous hourly space velocity (GHSV) of 4200 cm3/g.h. The resulting catalytic tests showed a remarkable improvement in activity and stability of FeAPSO-34 catalyst at 350 °C. The olefins selectivity of the SAPO-34 catalyst at 450 °C was also increased using ion exchange of catalyst in AgNO3 and KNO3 solutions. At 450 °C, the selectivity for ethylene on Ag-SAPO-34 increased from 56.34% to 68.56%, and compared with the SAPO-34, K-SAPO-34 lifetime was also prolonged from not more than 3–10 hs for selectivity up to 80% of light olefins due to ability of coke thermal cracking to propylene. It was found that, with K+ ion exchange of SAPO-34 catalyst, surface area increases and weak acid concentration, in contrary to strength of weak acid sites, decreases and the synthesized sample has exhibited the best olefins selectivity.