| To adjust the porous and acidic properties of MCM-22zeolite, the precursors of MCM-22(MCM-22(P)) were delaminated, and pillared with different oxides, respectively, leading to a series of MCM-22derived materials. After loading10wt.%cobalt via the incipient impregnation method by using Co(NO3)2as a precursor, the catalysts were comparatively evaluated for the Fischer-Tropsch (FT) synthesis under the conditions of H2/CO=2, W/F=5.02g.h.mol-1,1.0MPa and508K. The samples were characterized by XRD, NH3-TPD, H2-TPR, and N2adsorption/desorption at low temperatures, and the acidity, porous properties, and reduction behavior of the catalysts/zeolites were correlated with the activity and product selectivity of FT synthesis. The work and main results are summarized as follows.1. After NH4+-exchanging and calcination of the MCM-22(P), H-MCM-22zeolites with a Si/Al ratio of20,30,40, and50were obtained. The10wt.%Co/H-MCM-22catalysts were comparatively studied for FT synthesis under the same reaction conditions. Results indicate that the CO conversion over all the catalysts was low (3-23%). However, as revealed from the product selectivity, cracking of the long-chain FT hydrocarbons was occurred, the extent of which is clearly dependent on the Si/Al ratio of H-MCM-22. The reaction results are well agreeable with the layered microporous structure of the zeolite and the reduction behavior of the catalysts.2. By using n-cetyltrimethylammonium brimide (CTAB) and tetrapropylammoniu-m hydroxide (TPAOH) as swelling agents, the delamination behavior of the MCM-22(P) was investigated under ultrasonic conditions. Under the same delamination conditions, the MCM-22(P) having a Si/Al ratio of25was significantly delaminated, leading to the formation of ITQ-2. On the contrary, for those having a Si/Al ratio of20,30,40, and50, low degree of delamination was observed, the extent of which is obviously dependent on the Si/Al ratio of the MCM-22(P). These results confirm that the crystalline property and Si/Al ratio are key factors for determining the delamination of MCM-22(P). In comparion with H-MCM-22, increased BET surface area and pore volume but decreased acidity were observed for the delaminated products. By comparing the FT results over cobalt loaded on the delaminated materials with those over Co/H-MCM-22, we found that (1) CO conversion was increased, and the more significant the delamination, the higher the CO conversion was;(2) selectivity of C4-C12hydrocarbons was clearly increased and selectivity of C21+hydrocarbons was obviously decreased. The FT activity was well explained based on the reduction behavior of the catalysts, and acidic and porous properties of the delaminated materials were determined to be responsible for the observed product selectivity.3. After swelling in the aqueous solution of CTAB and TPAOH, the pillaring behavior of MCM-22(P)(Si/Al=25) was comparatively investigated by using Si(OCH2CH3)4, Al(NO3)3, and ZrOCl2as the precursor of SiO2, A12O3, and ZrO2, respectively. Under the same swelling and pillaring conditions, SiO2-MCM-36was successfully prepared, but poorly structured Al2O3-MCM-36and ZrO2-MCM-36were formed. As indicated from NH3-TPD results, the amount of the stronger acidic sites over all the MCM-36zeolites was significantly decreaded in comparison with that over the H-MCM-22. Moreover, the total amount of acidic sites was decreased in the order of Al2O3-MCM-36> ZrO2-MCM-36> H-MCM-22> SiO2-MCM-36. The catalytic results indicate that the CO conversion was changed in the order of Co/SiO2-MCM-36≈Co/ZrO2-MCM-36>>Co/Al2O3-MCM-36≈Co/H-MCM-22. When the FT activity and selectivity of liquid fuels are taken into account, Co/ZrO2-MCM-36was the best among the catalysts investigated. The promotional effect of the oxide pillars, the acidity of the zeolite, the porous property and reduction behavior of the catalysts are responsible for the observed FT results. |
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