| SAPO-34 zeolites were synthesized by the hydrothermal method and their structures were characterized by means of X-ray powder diffraction (XRD). The effects of Silica source, raw material ratio and synthesis conditions on the crystallinity were investigated. The results showed that: Taking morpholine (MOR), phosphate, pseudo-boehmite as template, phosphorus source and aluminum source, respectively, the crystallinity of SAPO-34 zeolite was better when synthesized at the following conditions: Took TEOS as silicon source, the raw material mole ratio was Al2O3:P2O5: SiO2:MOR:H2O=1.0:1.0:0.6:3.0:100, the aging time was 2 h, the crystallization temperature was 200℃and the crystallization time was 48 h.SAPO-34 zeolite was modified by means of adding alcohol additives such as methanol and metal ions such as zinc nitrate, nickel nitrate and (zinc nitrate + nitrate nickel) in its synthesis, respectively. The catalytic performance of these zeolites was experimentally evaluated taking methanol to olefins (MTO) as the model reaction in a fixed-bed reactor. The effeets of Zn on the physical chemistry characteristics of SAPO-34 zeolite were investigated based on the XRD, XPS, BET, NH3-TPD and FT-IR. The results showed that:(1) Adding methanol, ethanol, butanol, ethylene glycol and glycerol, respectively, all could synthesize SAPO-34 zeolites, but only could increase little of the total olefins (C2=+C3=) selectivity. (2) The MeAPSO-34 (Me=Ni, Zn, Fe, or Ni-Zn) zeolites prepared by adding metal ions when synthesized SAPO-34 had a higher selectivity of ethylene and total selectivity of ethylene and propylene than SAPO-34 zeolite, the total selectivity of ethylene and propylene was higher on Ni-ZnAPSO-34 than on other MeAPSO-34 (Me=Ni, Zn, Fe, Cu) and it was noteworthy that adding metal ions would lead to the decrease of the propylene selectivity. The NiAPSO-34 could make an increase of ethylene selectivity by 7.7% (from 40.7% over SAPO-34 upon to 48.4% over NiAPSO-34) and total selectivity of ethylene and propylene by 2.5%; the Ni-ZnAPSO-34 made an increase of ethylene selectivity by 6.5% and total selectivity of ethylene and propylene by 3.2%. (3) When the mass content of zinc nitrate was 0.41%, the total olefins (C2=+C3=) selectivity increased by 4.7% and was the best. (4) Zn incorporated into the framework of SAPO-34 zeolite, in addition, medium strength acid and appropriate pore size could increase the (C2=+C3=) selectivity.Then the effects of temperature, water/methanol weight ratio and methanol WHSV on MTO process were investigated and the optimal condition was determined as follows:the temperature was 500℃, the water/ methanol weight ratio was 5:1 and the methanol WHSV was 3.9 h-1. |
