Synthesis Of MFI Zeolites With Macropores Via The Dry-gel Conversion Of Mesoporous Silica Spheres

As a class of important inorganic catalytic materials,microporous zeolites have been widely applied in petrochemical industries,coal chemical industries and fine chemistry.However,their intrinsic and sole microporous system would undoubtedly incur the molecular transportation and diffusion restrictions in catalytic reactions involving bulky molecules,and therefore hinder the effective utilization of zeolites.Hierarchical zeolites combine the superior diffusion properties of macropores and/or mesopores,and exhibit bright prospect in improving the catalytic performance of zeolites as well as expanding their applications.Nowadays,most of the studies of hierarchical zeolites focus on meso/microporous zeolites,while few researches on zeolites with macropores have been reported.In this dissertation,efforts are devoted to the dry-gel conversion(DGC)synthesis of hierarchical MFI zeolites with macropores by using the self-made mesoporous silica spheres both as precursors and macroporous templates.Moreover,the influence of different synthetic conditions on formation of macropores were further studied in detail,the catalytic performance of MFI zeolites with macropores were evaluated with some probe reactions like Beckmann rearrangement.Main contents and findings are summarized as follows:Firstly,Silicalite-1 zeolite with macropores(HM-Silicalite-1(500))was synthesized through DGC method by using amorphous mesoporous silica spheres(MSS)impregnated with tetrapropylammonium hydroxide(TPAOH)as precursors.The synthesized sample possessed uniform size(3.5 ?m),and macropores ranging from 150 nm to 350 nm distributed uniformly from the external surface to the interior of particles could be observed.In the vapor-phase Beckmann rearrangement of cyclohexanone oxime(CHO),HM-Silicalite-1(500)exhibited high initial CHO conversion(96%)and a relatively high catalytic activity(40%CHO conversion)could still be maintained when the reaction proceeded to 57 hours,presenting much higher catalytic activity and stability in comparison with conventional microporous Silicalite-1 catalyst.The crystallization process of HM-Silicalite-1(500)was explored systematically and as revealed,MSS dissolved outside-in under the effect of TPAOH and was simultaneously crystallized into Silicalite-1 crystals near the external surface.It was the cooperation between MSS dissolution and Silicalite-1 crystallization processes that facilitate the in situ generation of macropores in Silicalite-1 during the DGC synthesis process.Furthermore,the amounts of TPAOH,crystallization temperature and water contents were all found to affect the generation of macropores.Based on our recognition of the formation of macropores,Silicalite-1 zeolite with more abundant macropores(HM-Silicalite-1(150))were synthesized through reducing the size of MSS precursors,and the macropore size was reduced to 50?100 nm.The catalytic activity and stability of HM-Silicalite-1(150)were further improved in the Beckmann rearrangement reaction,a 61%CHO conversion could still be maintained when the reaction proceeded to 57 hours.Secondly,with an attempt to introduce urea into the aforementioned DGC system,Silicalite-1 zeolite sheets(HMS-Silicalite-1-1.0)with intracrystalline macropores embedded and short b-axis dimension were successfully synthesized.The size of the synthesized sample was about 4 ?m × 2 ?m and the b-axis dimension was about 190 nm.Besides,macropores ranging from 150 nm to 400 nm were observed distributed inside the Silicalite-1 sheets.Due to the combination of rich intracrystalline macropores and short b-axis diffusion path,HMS-Silicalite-1-1.0 exhibited further improved catalytic stability than HM-Silicalite-1(150)during the vapor-phase Beckmann rearrangement of CHO,the CHO conversion could still be maintained at about 69%when the reaction proceeded to 57 hours.The crystallization process of HMS-Silicalite-1-1.0 as well as the influence of synthetic conditions on that were explored systematically.As revealed,when the MSS precursors were transformed into Silicalite-1 with macropores through the "dissolution-crystallization" process,the introduced urea would inhibit the growth of crystals along b-axis.Moreover,the dimension of HMS-Silicalite-1 crystals along b-axis could be tuned through varying the introduced amounts of urea.Thirdly,to expand the synthesis and application area of MFI zeolite with macropores,synthesis of ZSM-5 zeolites with macropores(HM-ZSM-5)were tried and were also successfully synthesized through a similar DGC method,utilizing aluminosilicate spheres(MASS)(MSS impregnated with NaAlO2)as precursor.The synthesized ZSM-5 zeolites with macropores were spherical particles with size of 2.5-4 ?m and the macropore size ranged from 300 nm to 450 nm.The hydrogen form of HM-ZSM-5 exhibited better catalytic activity and stability during the 1,3,5-triisopropylbenzene cracking reaction.The crystallization process as well as the influence of synthetic conditions on that were explored systematically.Different from the transformation process from MSS to HM-Silicalite-1,MASS were preferentially dissolved in an outwards behavior due to the difficult dissolution of Al-rich shells.At the same time,ZSM-5 crystallization proceeded at the surface of shell,and consequently the Al-rich shells could reveal a "space-occupying" effect and finally help the in situ construction of macropores.The Si/Al ratio of HM-ZSM-5 zeolites could be tuned in a range of 20?120 by varying the Si/Al ratio of MASS precursors.