Study On Molecular Sieves Catalyst For Methanol-to-Olefins Reaction

The methanol-to-olefins (MTO) technology becomes interesting on account of the shortness of petroleum resource. The research and design of catalysts is one of the crucial steps.In this work, SAPO were synthesized hydrothermally in systems of single template, bi-templates and tri-templates, respectively, and the characterization and modifications were studied. The catalytic performance and coking behavior in MTO reaction was tested in a fixed bed reactor for an excellent catalyst.Using single template, SAPO-34 were synthesized with different templates, template amounts, pH, crystallization time and metal ions. In comparison, TEAOH was the most proper template for the preparation of SAPO-34 with larger BET surface areas, homogeneous cubic crystals in narrow size distribution and proper ratio of strong to weak acid sites. Synthesized in the range of TEAOH:Al2O3=2.02～1.35, the product was pure SAPO-34, while reducing TEAOH amount could lead SAPO-5 co-generated. As to metal ions modification, the acid strength was enhanced by Mn2+ and Ni2+ incorporation, simultaneously with less acid amount, while by Cr3+ incorporation, the crystallinity was increased and the ratio of strong to weak acid sites was adjusted. CrAPSO-34 was tested to have good hydrothermal stability, excellent catalytic performance (methanol conversion:100%, ethylene and propylene selectivity:85.02%, C2=7C3==1.39, lifetime:275min) and 17.0wt%coke capacity.Using bi-templates, different combinations DEA/TPABr, DEA/TEA, DEA/Mor and TEAOH/DEA were applied. DEA/Mor and TEAOH/DEA were the templates for SAPO-34, but DEA/TPABr and DEA/TEA were inclined to direct the formation of SAPO-5. In comparison with DEA/Mor, the SAPO-34 synthesized with TEAOH/DEA exhibited better catalytic performance.Using TEAOH/DEA as bi-templates, the influences of combinations of TEAOH/DEA on SAPO-34 synthesis and their catalytic performance were studied. XRD indicated the good crystallinity directed by single or bi-templates for CHA structure. The morphology of crystals was shown to be cubic shape of typical SAPO-34, but distinct in crystal size, increasing with the amount of DEA in the gel. The catalyst synthesized with single DEA had large crystal size of 3.1-9.1μm and had strong acidity with small acid amount, which was caused by the existence of siliceous islands of large size. However, bi-templates were demonstrated to introduce more amounts of silicon into framework which dispersed finely in siliceous islands of smaller size, and kept good acidity. In the MTO reaction, the catalyst prepared in the ratio of n(TEAOH):n(DEA)=0.5:0.5 showed the longest lifetime.The preparation of SAPO-34 using bi-templates in the ratio of n(TEAOH):n(DEA)=0.5:0.5 was optimized in terms of template amount, SiO2/Al2O3 and P2O5/Al2O3 molar ratios, and the crystallization temperature. SAPO-34 and SAPO-5 were competing phases at low template concentration. Varing the SiO2/Al2O3 ratio at 0.1～1.0 did not change the purity of SAPO-34, but affected its crystallinity and acidity. The temperature program crystallization was favorable for the synthesis of transition phase in the temperature of 90～150℃, which influenced the crystallinity of product greatly. The chosen silicon content of 0.60SiO2:Al2O3 was not favorable for the Cr3+ incorporation in the framework. An optimal synthesis procedure consisting of heating a gel of composition Al2O3:P2O5:0.6SiO2:1.0TEAOH:1.0DEA :53H2O at 100℃for 24h firstly and 200℃for 72h later, gave rise to an acidic catalyst active in the MTO reactions with 100% methanol conversion,85.2% ethylene and propylene selectivity, C2=/C3= molar ratio 1.85 and a longest lifetime 540min.Using tri-templates, the influence of molar ratio of TEAOH/DEA/TEA was investigated in SAPO synthesis. Pure SAPO-34 could only be synthesized at low TEA template concentration (<40%), which had small crystal size (0.8～1.4μm). But higher TEA concentration resulted in SAPO-5 and small amount of SAPO-34 co-generating, and the morphology of crystals changed from slice of small size to aggregations of slice in lager size with TEA content increased. The acidity was different with TEAOH/DEA/TEA molar ratio. TEAOH:DEA:TEA=0.67:0.67:0.67 was proper to give a catalyst with plenty acidity and proper acid strength, which exhibited excellently in MTO reaction with 100% methanol conversion,89.39% ethylene and propylene selectivity, C2=/C3= molar ratio 2.27, a long lifetime 560min, and 24.2wt% coke capacity. ZSM-5 and SAPO-44 was also tested in MTO reaction for comparison with SAPO-34. Low methanol conversion and complex product with much C5～C8 hydrocarbons was obtained on ZSM-5, while incomplete methanol conversion and quick deactivation was shown on SAPO-44.Based on the SAPO-34 catalyst synthesized with tri-templates TEAOH/DEA/TEA, the intrinsic kinetics of MTO reaction were obtained in a TGA integral fixed-bed reactor at 375～475℃and space velocity of 8-32h-1. The kinetics equations of generate rate were built corresponding to the 5 lumps (methane, ethene, propene, butene and propane). The ordinary differential function was resolved by Rung-Kutta method, while the model parameters were estimated by Levenberg-Marquardt optimization method. The kinetic model was proved to be satisfied with statistical tests.