The Synthesis Of *BEA- And FAU- Type Heteroatoms-containing Zeolites And Their Application

Heteroatom-containing zeolites possess not only the textural properties of con-ventional aluminosilicate zeolites,but also redox and acid-base catalytic performances,which contribute to the promising application prospect on an industrial scale as solid heterogeneous catalysts.In order to increase the isolated metal contents,shorten syn-thetic periods,simplify the operation procedures,reduce the product cost,improve mass transport,and promote the industrialization process of heteroatoms-containing zeolites,in this thesis,two kinds of zeolites with*BEA and FAU topology are used as the research objects and we innovatively designed and efficiently synthesized all-silica Beta and corresponding heteroatoms-containing zeolites adsorption and cat-alytic materials.Additionally,the synthetic mechanism and adsorption as well as cat-alytic performances were studied systematially and deeply.Specially,it was divided into six parts as follows:In the first part,all-silica Beta zeolite,synthesized by conventional hydroxide route,usually possesses small crystal size of a few hundred nanometers but poor hy-drophobicity,whereas the fluoride-mediated one exhibits to be highly hydrophobic but microsized.To obtain nanosized all-silica Beta zeolite with excellent hydrophobi-city,an innovative and efficient hydrothermal route via interzeolite transformation for synthesizing all-silica Beta zeolite under fluoride-free conditions is proposed,which provide a new insight into the hydrothermal synthesis for all-silica Beta.With the as-sistance of Beta seeds and tetraethylammonium hydroxide as the structure-directing agent,siliceous Beta zeolite was well crystallized at a high solid yield via dissolu-tion-recrystallization of all-silica ITQ-1 crystals at an extremely low water content?H₂O/SiO₂ molar ratio of 1?.The obtained all-silica Beta crystals were composed of30–70 nm nanoparticles and highly hydrophobic,which was derived from relatively small amounts of internal defect sites.Thus,this Beta zeolite was superior to other pure silica Beta zeolites in the adsorption of large-sized volatile organic compounds?VOCs?.In the second part,an innovative hydrothermal synthesis strategy for preparing Sn-Beta zeolite in a short crystallization time via interzeolite transformation is re-ported.Most importantly,this synthesis route breaks through the Sn content limitation of the conventional approach and this prepared Sn-Beta zeolite possesses the highest isolated Sn contents?3.03 wt%or Si/Sn molar ratio of 63?in comparison to other hy-drothermally synthesized ones.The introduction of Beta seeds and the structural sim-ilarity between the parent zeolite?MWW?and the targeted zeolite?*BEA?were found to be indispensable factors in the crystallization of Sn-Beta zeolite.The obtained Sn-Beta zeolites possessed isolated tetrahedral Sn⁴⁺in the framework and high hy-drophobicity.Additionally,lowering the molar ratio of H₂O₂ to cyclohexanone and moderately reducing reaction time were proved to be effective methods to improve the selectivity of?-caprolactone in the Baeyer–Villiger oxidation of cyclohexanone with H₂O₂ using Sn-Beta as Lewis catalyst.Thus,the prepared Sn-Beta zeolite demonstrated promising catalytic properties,especially for the desired products selec-tivity,in the Baeyer–Villiger oxidation of ketones as well as the isomeriza-tion–esterification reaction of dihydroxyacetone in ethanol,which were mainly at-tributed to its high Sn contents in the framework and excellent diffusion properties resulting from the relatively small crystal size.Extending this strategy,well-crystallized Ti-Beta-DT zeolite was successfully synthesized in a extremely short time?2 h?using Ti-MWW zeolite as silica and titanium sources.The obtained Ti-Beta zeolite possessed nanosized particles and relatively high active sites stability.Com-pared with Ti-Beta-F prepared by the conventional fluoride method,Ti-Beta-OH syn-thesized via seeding route,and Ti-Beta-PS obtained by post-synthesized strategy,Ti-Beta-DT zeolite catalyst exhibited more excellent catalytic performances in oxida-tion reactions using bulky oganics?i.e.,cyclohexene and benzothiophene?as sub-strates.In the third part,an innovative and convenient directly hydrothermal synthesis strategy for heteroatom-containing zeolite,in this case Sn-Beta,via structural recon-struction is developed.The Sn-Beta-Re zeolite with incredibly high framework Sn contents?Si/Sn=33 or 6.06 wt%?,nanosized crystals?30-50 nm?,and outstanding hydrophobicity was crystallized completely within 1 h at 413 K,the time of which was shortened by two order of magnitude in comparison to conventional methods.Note that the isolated Sn content in this Sn-Beta-Re zeolite was beyond than that?3.03 wt%or Si/Sn molar ratio of 63?in Sn-Beta-Trans zelites synthesized by inter-zeolite transformation in the second part,which was the highest in comparison to oth-er hydrothermally synthesized ones.Furthermore,compared with post-synthesized Sn-Beta and state-of-the-art Sn-Beta-F materials,Sn-Beta-Re zeolites exhibited un-paralleled catalytic performances in terms of turnover frequency and space-time-yield in Baeyer-Villiger oxidation of 2-adamantanone,Meerwein-Ponndorf-Verley reac-tion of cyclohexanone,and isomerization-esterification reaction of dihydroxyacetone in ethanol.Additionally,the main raw materials required for this synthesis,including organic structure directing agent?tetraethylammonium hydroxide?and silica source?siliceous Beta from Beta aluminosilicate?,are simple and commercially available.This hydrothermal synthesis route bears the potential for highly productive heteroa-toms-containing zeolite synthesis on an industrial scale.In the fourth part,no matter directly hydrothermal synthesis method or post-synthesis approach for Sn-Beta zeolite,organic structure directing agents?OSDAs?are inevitably required.In this part,hierarchical Sn-Beta was successfully prepared via post-synthesis method under OSDA-and fluoride-free conditions,which consisted of the synthesis of aluminosilicate Beta,framework stabilization via steam-ing,deep dealumination,and SnCl₄ vapor treatment.Prior to deep dealumination,the OSDA-free aluminosilicate Beta was steamed at a mild temperature for a short time?823 K,30 min?,which allowed for the effectively stabilizing the Beta framework structure.Besides,the appearance of Al oxide particles generated by longer time steaming likely made the subsequent dealumination by acid leaching difficult as they were stable and hardly removed.Then Sn ions were tetrahedrally incorporated into the Beta framework at 773 K under vapor condition via the reaction between the Sn ions and silanol defects derived from the steaming and subsequent acid treatment.This Sn-Beta zeolite exhibited higher yields?89.1%?than conventional Sn-Beta-F?74.3%?in the conversion of dihydroxyacetone to ethyl lactate.In the fifth part,hierarchical Sn-Y zeolite with more open pore system was suc-cessfully prepared by vapor-solid isomorphous substitution to address the diffusion problem when Sn-Beta zeolite catalyzed reactions including bulky substrates.Sn ions were tetrahedrally incorporated into the zeolite framework by the reaction between the SnCl₄ molecules and silanol nests.The acid treatment time determined not only the quantity but also the quality of the inserted Sn species,the optimal time was 1 h.Thus prepared Sn-Y zeolite showed excellent catalytic properties in the Bae-yer-Villiger oxidation of 2-adamantanone using hydrogen peroxide or tert-butyl hy-droperoxide as oxidant,which was mainly attributed to the open pore system contrib-uted by 3-dimensional 12-membered ring channels and dealumination-derived in-tracrystal mesoporosity.However,a certain amount of Al ions?Si/Al ratio of ca.43?still existed in the Sn-Y zeolite,which would retard its catalytic activities.In the sixth part,Sn-Y zeolite,with hierarchical pore systems and extremely low Al content?Si/Al=150?,was readily obtained via a convenient postsynthetic route under mild aqueous condition to further lower Al ions content in the Sn-Y zeolite and correspondingly,enhance its Baeyer-Villiger oxidation catalytic performances.The acidic medium achieved by adding HCl in the Sn incorporation process affected not only the amount of incorporated Sn and residual Al but also the coordination state of inserted Sn ions.This Sn-Y exhibited more excellent catalytic properties in the Bae-yer-Villiger oxidation especially using tert-butyl hydroperoxide as the oxidant.