Template-regulated Si Distribution In SAPO-35 Zeolites And Its Impact On Performance

Zeolites are important heterogeneous catalysts in chemical industry,which have always occupied a crucial position in the fields of petrochemical and energy because of the regular and uniform pore system,high specific surface area,unique catalytic shape selectivity,outstanding framework stability,adjustable active sites and suitable acidity.In the 1980s,while traditional aluminosilicate zeolites are booming,another important family,aluminophosphate zeolites,has also been developed in succession.Aluminophosphate zeolites have rich and various composition of framework elements and topologies.In particular,the introduction of heteroatoms brings them excellent catalytic and adsorption/separation performance,etc.Among these,silicoaluminophosphate(SAPO)zeolites,as a relatively important member of aluminophosphate zeolites family,have arouse widely attention and in-depth study.The introduction of silicon atoms endows the electroneutral aluminophosphate zeolites frameworks with acidity.The differences on contents and positions of the introduced silicon atoms will affect the strength and distribution of active sites,further affecting the catalytic and gas adsorption and separation performance of SAPO zeolites.However,silicon atoms tend to agglomerate to form silicon islands and the isolated silicon atoms are difficult to determine with conventional characterization techniques.Hence,regulating the specific distribution of silicon atoms in SAPO zeolites on the atomic scale is formidably challenging.This thesis focuses on an 8-membered ring small pore zeolite,SAPO-35(LEV topology),as the research object.For the first time,a new organic template N-methylpiperidine(NMP)is selected in the regulation of the silicon contents and distribution of SAPO-35 zeolites.At the same time,the influence on methanol-to-olefin(MTO)catalytic performance and gas adsorption and separation performance of the corresponding regulation is also investigated.The results suggest that the catalytic performance of SAPO-35 zeolite is closely related to the specific distribution of silicon atoms,which is determined by the host-guest interaction between the template molecule and SAPO framework.And the framework silicon content is the key factor of gas adsorption and separation performance and MTO catalytic performance.This work provides guidance for the design,synthesis and performance optimization of SAPO zeolites.The main results are as follows:1.We synthesized Si-island-free SAPO-35 zeolites with isolated Si atoms by using NMP as a new template under hydrothermal condition.Compared to conventional SAPO-35 zeolites synthesized by hexamethyleneimine(HMI),SAPO-35?NMP exhibited excellent thermal stability and substantially improved methanol-to-olefin catalytic lifetime.SAPO-35 zeolites synthesized by NMP and HMI showed different performance although they possess the same silicon content and synthesis procedure,also similar crystal size and structural properties.With the aid of high-throughput computations on 44,697 structure models combined with various state-of-the-art characterization techniques such as ²⁹Si MAS NMR and synchrotron radiation XRD crystal structure analysis,we reveal that the host-guest interactions between template molecules and SAPO frameworks determine the specific distribution of silicon atoms,which are responsible for the improvement of the chemical properties of zeolites.Our work provides an insight into the template-based regulation of isolated Si atoms in SAPO zeolites,which opens a new avenue to the design and synthesis of promising zeolite catalysts with optimal silicon distribution.2.By using NMP as template,a series of SAPO-35 zeolites,SAPO-35?x(x:Si/(Si+P+Al)in molar ratio)with different Si contents were synthesized under hydrothermal conditions.Among these,SAPO-35?0.14 with the strongest acidity and moderate Si content showed the highest CO₂ uptake,which appeared to be one of the best CO₂ adsorbents among the reported zeolites.Calculation results showed that the enhancement of acidity led to the increase of adsorption heat,which strengthened the interaction between the inorganic framework and CO₂,thereby enhancing the CO₂uptakes of SAPO-35?0.14.Moreover,increased Br?nsted acidity can greatly improve the adsorption selectivities of CO₂ over N₂.The IAST(ideal adsorbed solution theory)was used for calculated the selectivity of SAPO-35?x for the separation of CO₂/N₂.SAPO-35?0.14 showed the highest CO₂/N₂ selectivities.Transient binary breakthrough experiments on SAPO-35?0.14 further proved the efficient separation performance and stable circulation,exhibiting a potential for industrial process.The overall results of this study prove that the framework Si contents of SAPO-35 zeolites is key to governing their CO₂ adsorption performance.This work demonstrates that modulating silicon content in SAPO zeolites via a proper choice of template,further acidity,is of great significance for the rational synthesis of zeolites with superior CO₂ adsorption and separation abilities.3.A series of nanoscale SAPO-35 zeolites with various silicon contents were synthesized by seed-assisted microwave irradiation using NMP as template.The nanoscale crystals exhibited the crystal size as small as about 100-fold smaller than SAPO-35 samples synthesized under conventional hydrothermal conditions.The range of silicon contents that NMP regulated was much wider than that of traditional template.In particular,the minimum silicon content of nano-sized SAPO-35 zeolites could be reached 5%,which exhibited significantly prolonged catalytic lifetime in MTO reactions because of the decrease of the crystal size and the acidity.Compared with other nano-sized samples with higher silicon contents,the sample with minimum silicon content showed the best MTO catalytic lifetime,which was longer than that of reported in previous literatures,and even exceeded that of conventional SAPO-34zeolites.So far,conventional SAPO-35 has not been considered as a good catalytic material for MTO reaction because of their limited catalytic lifetime.However,by optimizing the synthesis method and regulating silicon contents,nano-sized SAPO-35has become another promising candidate for MTO reactions.The strategy of combining the seed-assisted microwave irradiation and template-regulated silicon contents will provide guiding principles for the synthesis and perfomance enhancement of SAPO zeolites.