A Study On Synthesis And Catalytic Performance Of Zeolites With Novel Oriented Morphology

Nanosized zeolite materials,featured by reduced length of channels and increased specific surface area,possess shortened diffusion path and improved accessibility of active sites by reactants,which improves the catalytic performance.Notably,the nanosized zeolites with oriented morphology of nanosheets or nanorods(especially with orientation along the channel direction)have more application value than nanoparticles,due to that they possess the advantage of nanoscale materials and meanwhile still have a relatively long-range crystalline order and complete pore systems.In addition,nanosized oriented zeolites usually exhibit the obvious hierarchical porosity due to stacking morphology,which also improves the mass transfer and diffusion in catalyst reaction space,and exhibits satisfactory catalytic performance especially for bulky molecular reactants.Summarily,reasonably designed oriented zeolites have considerable application potential,however,as for preparation,orientated zeolite materials are often difficult to synthesize,high cost,and single orientation.Therefore,with the purpose to design and synthesize new zeolite materials with special orientation,and explore their application advantages in catalytic reactions,we did the following research works.In the first part,previously,the nanoscaled oriented morphology of MFI zeolite has only been reported along the b-axis.referring to the monomer-conjugationcollaboration(MCC)synthesis mechanism,we succeeded in synthesizing highly aoriented ZSM-5 nanosheets(named NZ-6),using layered silicate H-kanemite as silica source.Meanwhile,1-(2-hydroxyethyl)-3-methylimidazolium hydroxide(HMMOH),a small molecular ionic liquid commercially available,was used as organic structuredirecting agent(OSDA),to reduce the synthesis cost.The length of crystal along a-axis is around 12 nm.This result provided another successful case of synthesizing nanosheets zeolite through the MCC mechanism.According to DFT calculation,the single imidazole ring conjugate can effectively inhibit the growth of ZSM-5 crystal along the a-axis.Up to now,a-oriented MFI nanosheets are seldom reported,NZ-6material making the effective study of high a-orientation influence on the catalytic selectivity impossible.With further prolonging crystallization time,we obtained a series of samples with different size along a-axis and similar size in other directions applied them to methanol-to-propylene(MTP)reaction after adjusting the catalysts acidity by dealumination.The thinnest nanosheets showed the excellent selectivity of propylene(54%)and lifetime.Therefore,we propose that a-oriented ZSM-5 nanosheets may be a potential MTP catalyst.In the second part,basing on the research in the first part,a-oriented ZSM-5nanosheets are of great significance in both experimental research and industrial application.Empirically,the introduced abundant mesoporous structures could further improve the reaction effect.However,the MCC mechanism has greatly reduced the cost of OSDA,it is still limited by the use of layered silicates as silicon source,which makes the synthesis method still has space to simplify.However,we found that the target materials were not obtained when imidazoles were used in synthesizing with common amorphous silicon sources.Fortunately,we found a commercial and available pyrroliane as OSDA to be used together with silica sol to obtain a-oriented ZSM-5nanosheets(denoted BMP-Z5).The length of crystal along a-axis is around 8 nm.Compared with the materials in the first part(NZ-6),the size of the primary particles is similar,but the “seaurchin”-like stacking method gives BMP-Z5 more abundant mesoporous volume and specific surface area.After the adjustment of aluminum content,the catalyst was probed in MTP reaction and showed superb lifetime,selectivity of propylene(56%)and P/E(>12)compared with other morphology ZSM-5(bulk,nanoparticle,b-oriented plate-like)and NZ materials.Basing on the computational simulation of diffusion velocities of different molecules in the two kinds of channels and experimental results,we supposed that well designed a-oriented ZSM-5 nanosheets are likely to be more advantageous than other morphologies in MTP reaction,and it is expected to have applications in other areas.The third part,same as ZSM-5,Beta is also zeolite material with great application ability.At present,the morphology of Beta is relatively simple.During the optimization of the synthesis composition of BMP-Z5 in the second part,we found the x-ray diffract peaks of a sample were similar with Beta zeolite but much broad.The morphology observed by SEM was not common diamond-like bulk or nano-particles.By adjusting the crystallize conditions,we obtained Beta nanorods zeolite with around 50 nm thickness named ECNU-41,nanorods are micrometers in length.In addition,the orderly knitted stacking modle of ECNU-41 was rarely reported in zeolite materials just like burr puzzle,and nanorods intersected each other with around 90° rotational angle.The stacking type not only afford stable macroscopic structure,but also form quite rich intergranular mesopores.In addition,when we explored a single nanorod,we found that it may possess twin crystal structure,which lead the corners were observed at every rod end.Compared with the commercial nano-particle Beta catalyst,ECNU-41 has the advantage of nanometer effect,and its overall morphology preserved micron size,which provided the complete pore system and better hydrothermal stability.Aluminosilicate and titanosilicate ECNU-41 exhibited remarkable and competitive catalytic performances in a variety of reactions,including alkylation,Beckmann rearrangement,MTP and epoxidation reactions.In the fourth part,we summarized the above three parts of research,and it is easy to found that suitable differences in zeolite morphology could alter the diffusion property and the catalytic performance.Thus,we also tried to use other strategies to obtain the zeolite materials with special oriented morphology.(1)A layered silicate with large interlayered space and nanosheet morphology(～200 nm)was synthesized by using hexamethonium hydroxide as template.According to the previous works in our group,the layered silicate could be silylated with suitable organosilane directly without swelling treatment,and finally,two new zeolite materials with stable 3D framework and 12 × 8 MR channels were obtained.These two zeolite materials preserve the nanosheet morphologies are same as precursor.At present,the exact structure analysis of the two zeolites is still in progress.(2)We also tried other larger molecular organics(with carbon chain or solo benzyl group)as templates to synthesize ultra-thin MFI-type zeolite nanosheets.Although the desired materials were not achieved at length,a variety of MFI zeolite samples with orientation or special morphologies were obtained,and their possible application and potential research value were supposed.