Modification Of Co-SAPO-34 Molecular Sieve Based On MTO Reaction And Carbon Accumulation Study

Methanol reforming to low carbon olefins(MTO)is an effective way to synthesize low carbon olefins from non-petroleum routes,which mainly depends on the selectivity and lifetime of the catalyst,and SAPO-34 molecular sieve is widely used in MTO catalytic reactions because of its special pore structure and moderate acidic sites,which perform well in the reactions.This thesis is mainly designed based on the MTO catalytic reaction,which can be further modified to improve the catalytic ability based on the synthesis of molecular sieves with dual template agents.In addition,the structural changes of molecular sieves before and after the reaction were analyzed to investigate the role of pore structure and acidity in the deactivation process of molecular sieve carbon accumulation.The effect of different template ratios on the physicochemical properties of SAPO-34 molecular sieve and the catalytic performance of methanol to olefin(MTO)was investigated by firstly synthesizing SAPO-34 molecular sieve crystals containing stacked structures and using tetraethylammonium hydroxide(TEAOH)and MORpholine(MOR)as mixed template agents.It was found that when the dosage of MOR was higher,the grains were smooth and regular cubic,with stronger acidity and MOR mesoporous content.With the increase of TEAOH dosage,the grain surface gradually changed into a multigrain stacking structure,the acidity slightly decreased,and the large-size mesopores slowly disappeared and were replaced by small-size mesopores with a pore size of 3-4 nm.However,due to the small amount of template agent,a small amount of Al PO4-5 heterocrystals appeared in the molecular sieve.When the molar ratio of each material is 1.0Al2O3:1.0P2O5:0.18 Si O2:43H2O:0.5MOR:0.125 TEAOH.The prepared molecular sieve has the most complex stacking structure with a high specific surface area of 723 m2/g,and at the same time the acidity is at a MOR moderate level.The performance of this sample in the MTO-catalyzed reaction was 100% methanol conversion at the beginning of the reaction,which could be maintained for a long time,while the product selectivity was up to 96.3% and the reaction lifetime was up to 360 min.Secondly,the prepared molecular sieve was subjected to two different modification methods to tune its structural properties,namely in situ synthesis and equal volume impregnation method.The modified molecular sieves were tested and analyzed by XRD,SEM and other characterization methods.It was found that the introduction of metallic Co by in situ synthesis method could inhibit the generation of Al PO4-5 heterocrystals and the acid strength increased,and the specific surface area decreased due to the increase of small size mesopore content in molecular sieve with the same pore volume.In contrast,for the sample modified by isovolume impregnation,the heterocrysts were still present,and the acidity increased significantly due to the introduction of metallic Co,while the pore volume remained unchanged,the mesopore content increased,and the specific surface area decreased.Among the two modified samples,the in situ modified molecular sieve could maintain a high level of methanol conversion in the MTO reaction for a long time,with the highest low carbon olefin selectivity of 96.8% and a better reaction lifetime of 410 min than the rest of the samples,while the impregnated modified sample performed poorly in terms of both product selectivity and catalytic lifetime,and was deactivated at 310 min.Finally,three groups of inactivated molecular sieves(blank,in situ modified and isovolume impregnated modified)were analyzed for carbon accumulation.The samples were characterized by XPS and GC-MS,respectively,and the results showed that the pore volume and acidic distribution of all three samples decreased before and after the reaction.Compared with the rest of the samples,the in situ modified samples showed a significant decrease in the weak acid sites after the reaction,while the strong and weak acid concentrations decreased simultaneously,which kept the acid distribution in the molecular sieve in a MOR balanced state.The in situ modified samples performed well in the reaction mainly due to the increased content of small-sized mesopores,which increased the capacity of carbon accumulation and made it MOR difficult for carbon accumulation to block the pore channels and deactivate.In summary,the acidic distribution and special pore structure of the in situ modified samples led to a deeper catalytic reaction,a reduction of reaction intermediates and a decrease of soluble carbon accumulation molecular weight,which reduced the carbon accumulation blockage rate of the pore channels and achieved the goal of enhancing the catalytic performance.