Study On The Highly Stable Catalyst Of N-butene Skeletal Isomerization

The use of carbon four(C4)sources to produce higher value-added compounds has attracted widespread attention,and the isobutylene in C4 is an important intermediate for the production of fine chemicals.Therefore,the skeletal isomerization of n-butene is one of the most important steps to utilize C4 resources.The catalyst is the core of this technology,while the existing catalysts can hardly meet the requirements of industrial applications.Based on this,this thesis takes high concentration on n-butene as the treatment target,focusing on the development of a new type of high-efficiency and stable n-butene skeletal isomerization catalyst,and the in-depth study of the relationship between catalyst structure and performance.First,we used metal doping to change the acidity of Aluminophosphate-eleven(AEL)molecular sieves,and used templated coordination to achieve high-level metal substituted AEL defect adjustment.The performance of the obtained catalysts showed that increasing the Co content cannot effectively improve the catalytic activity of the nbutene skeletal isomerization of CoAPO-11 at high space velocity without the addition of imidazole.The results of UV-vis,ESR,TG-DSC and TPR proved that with the increase of Co content,the amount of CoxOy aggregates increased,while the addition of imidazole can effectively improve the dispersion of Co,changing from uneven CoxOy aggregates to isolated Co sites near the edge of the CoAPO-11 crystal.Co entered the APO-11 framework well under the guidance of imidazole,and formed unsaturated coordination once imidazole was removed(calcined).As Co2+and Al3+have different valence states,the Co2+in the framework brings in more Lewis acid sites,which has been confirmed by the pyridine infrared test.The change of zeolites’ pore structures significantly affects its catalytic performance.In the process of templated coordination strategy research,we further synthesized CoAPO-11 zeolites highly substituted with Co at the framework position by the method of slicing.We have developed a new synthesis method for AEL molecular sieve with thin layer thickness along the ten-membered ring direction.The obtained catalysts showed high activity and stability in catalyzing the skeletal isomerization reaction of n-butene under mild conditions(350 0C).The NH3-TPD characterization further confirmed the existence of a large number of weak acid sites in CoAPO-11 zeolites which effectively catalyzed the skeletal isomerization of n-butene.Using quasi-in-situ XPS,UV-Raman and ESR characterization techniques,we confirmed that through the improved synthesis strategy,Co can effectively enter the framework of the CoAPO-11 molecular sieve.Co was uniformly dispersed in the zeolite’s framework at stable state.Although the previous basic research has provided important information about the spin-crystal field interaction in CoAPO-11,as far as we know,there has been no research to reveal the local electronic and geometric structure changes around Co(the position of Co in the zeolite).Conventional characterization might not meet the requirements of searching for specific locations of acidic sites and their detailed information.Therefore,we carried out the X-ray absorption fine structure(XAFS)characterization of the CoAPO-11 zeolites based on the synchrotron radiation facility of the Photon Factory in Japan.Through the extended X-ray absorption fine structure(EXAFS)fitting results,we found that imidazole can indeed guide Co into the zeolite’s framework to form Co-O-P bonds thereby promoting the generation of zeolite’s acid sites.Similarly,small-sized zeolites synthesized through meso-construction also have a relatively rich framework of Co sites.Methods such as impregnation method and increasing Co content cannot utilize the empty coordination field of CoO and activate the π electrons of the n-butene double bond,so its activity in the n-butene skeletal isomerization was low.The templated coordination and meso-construction strategy enabled the good interaction between Co and the APO-11 framework due to its special nucleation guiding effect,which can effectively accept the π electrons transferred from n-butene.Therefore,its catalytic activity was much higher than that of CoAPO-11 synthesized by traditional methods.In addition,within a certain range,the coordination degree of Co-O and Co-O-P also had a great influence on the activity.