Tailoring Design Of Simple Organic Molecular Bifunctional Template And Preparation Of Hierarchical Molecular Sieve

Conventional microporous molecular sieves often exhibit large mass transfer resistance due to their own inherent microporous channels,eapecially when large molecules are involved in the reactions.Recent studies have shown that the preparation of hierarchical molecular sieves is one of the most effective ways to overcome diffusion restrictions induced by micro-pores in traditional molecular sieve.Soft template strategies has been widely used to synthesize hierarchical molecular sieves,because it can control the physicochemical properties and topological structure of the molecular sieves by rationally designing the molecular structure of organic template.However,limited species,time-consuming and complicated synthesizing procedure of this type of template are the main obstacles limit their practical applications.Therefore,a"molecular tailoring"strategy was used to tailor the commonly used bifunctional multi-quaternary surfactant molecules step by step for the simple organic molecule with long-chain alkyl-free hydrophobic group.Using the tailored simple organic molecule,H-*BEA,H-MTW and H-*MRE series of hierarchial molecular sieves with different topological structures were prepared.The result demonstrate that simple organic molecules containing quaternary ammonium groups can be used as a general way to act as a dual-functional template for hierarchical molecular sieves.The paper also innovatively established the process and guiding mechanism of the tailored simple organic molecule for the generation of hierarchical molecular sieves through nano-slice characterization and“acid-base theory”,determining the crucial factors affecting the orientation synthesis.The specific results are as follows:Firstly,we designed and synthesized a multi-quaternary ammonium surfactant C₁₈-N₂-P-N₂-P-N₂-C₁₈ to prepare hierarchical Beta molecular sieve（H-Beta）under hydrothermal conditions.Characterization results show that H-Beta has a polycrystalline structure and exhibites a narrow distributed mesopore with the size concentrated at about 8.2 nm.The mesopore pore volume and external surface area are 0.92 cm³ g^-1 and 452 m² g^-1,respectively.After being treated under 873 K 100%steam for two hours,the samples still maintained the basic crystal morphology,indicating its high hydrothermal stability.H-Beta shows almost the same catalysis activites（about 32%）with the traditional Beta catalyst for benzaldehyde in CBG（aldol-aldehyde condensation of formaldehyde and glycerol）when small molecules are involved.However,the conversion efficiency of benzyl alcohol was significantly higher than that of the traditional Beta catalyst（45.2%VS 24.6%）in ABB（alkylation of benzene with benzyl alcohol）reaction when bulky molecules are involved.Secondly,the polyquaternary ammonium molecule C₁₈-N₂-P-N₂-C₁₈ containing two hydrophobic chains was synthesized as a parent template,the single-hydrophobic chain molecule C₁₈-N₂-P-N₂ and non-hydrophobic chain molecule N₂-P-N₂ were subsequently tailored.Using these three different molecules as templates,three different hierarchical molecular sieves with*BEA,MTW and*MRE topologies were generated under the same conditions,respectively.²⁷Al MAS NMR spectrums and TG/DSC analysis show all the samples exhibit high thermal stability and the Al atoms are existing in the molecular sieve framework in the form of covalent tetrahedral aluminum.The alkylation of benzene with propylene reaction shows that H-*BEA,H-MTW and H-*MRE give higher catalytic activity and longer catalytic life than their corresponding microporous molecular sieves,respectively.These results indicate the success of using simple organic molrcule with no hydrophobic alkyl tail group to synthesis of hierarchically structured mesoporous molecular sieve,and which also suggest a new pathway to the synthesis of hierarchical materials via simple organic molecule.Thirdly,the tailored simple organic molecule N₂-P-N₂ can also be used as a template to generate highly crystalline hierarchical H-Beta and Nano-MTW molecular sieves in the different gel systems.Studies exhibit that the amount of template N₂-P-N₂ has a significant effect on the phase structure and physicochemical parameters of H-Beta-x.When x>1.6,the pore size,surface area,and total pore volume of H-Beta increase with the increasing x.In addition,the morphology of H-Beta samples gradually changed from quasi-spherical amorphous particles to inter-granular mesoporous Beta molecular sieves that are freely assembled by 20 nm nanocrystallites,with the crystallization time prolonged.A range of optimum synthesis parameters for synthesizing Nano-MTW were determined by evaluating the effects of the initial SiO₂/Al₂O₃ ratio and alkalinity of the synthetic gel on the crystalline phase and the morphology of final products.Properly control of SiO₂/Al₂O₃ and Na₂O/SiO₂molar ratios around 40 and 0.124,respectively,pure Nano-MTW molecular sieves can be prepared.Combination of XRD and SEM characterization techniques,Nano-MTW molecular sieves can be completely crystallized in 36 h,which is quicker than most of other methods previously reported.The alkylation of benzene and propylene shows H-Beta and Nano-MTW templated by the tailored simple organic molecule N₂-P-N₂ exhibit similar catalytic activities with H-*BEA and H-MTW,which are directed by C₁₈-N₂-P-N₂-C₁₈ and C₁₈-N₂-P-N₂,respectively.Fourthly,the growth process and directing mechanism of H-Beta templated by N₂-P-N₂was further systematically studied.SEM,TEM and selected area electron diffraction analyses indicate that the growth process of H-Beta can be divided into four stages:（i）the formation of the aluminosilicates after heating,（ii）the aggregation of aluminosilicates together with quaternary ammonium cations,yielding the formation of the amorphous quasi-nanoparticles with size of 400-800 nm by the Oswald ripening mechanism,（iii）the formation of H-Beta nuclei with size of 20-30 nm by solid-state reorganization of such quaternary ammonium cations in N₂-P-N₂ and aluminosilicate composites,and（iv）the crystallization process gradually expands from outer surface to the inner part until it is completely crystallized.In addition,C₆H₁₃-N⁺（CH₃）₂-CH₂-（p-C₆H₄）-CH₂-N⁺（CH₃）₂-C₆H₁₃(Cl^-1)₂（NPN）tailored from N₂-P-N₂ and N,N,N,N,N,-tetramethyl-1,6-hexamethylenediamine,the basic unit of N₂-P-N₂molecule were also used as templates under the same conditions as N₂-P-N_2.The results imply that the degree of freedom of the directing agent and the stability,balance of hydrophobicity are the key factors for the synthesis of hierarchical molecular sieves when the simple organic molecule was used as a dual-porogenic template.