Synthesis, Catalytic Property Evaluation And Computer Simulation Study Of Hierarchical Zeolites

Hierarchical zeolites are class of novel porous materials, which combine the dual merits of mesoporosity and microporosity crystalline structure of zeolites. Therefore,hierarchical zeolites can be widely used in the field of adsorption, separation and catalysis as a type of advanced material. Herein, a series of hierarchical zeolites with ZSM-5, Beta and Y frameworks have been prepared on the basis of previous scientist ’s research works, and the structure of these hierarchical zeolites have been investigated by complementary combination of experiment and computer simulation technology.In this paper, a series of specially designed quaternary ammonium surfactants,which composed of hydrophobic long-chain alkane groups and hydrophilic quaternary ammonium groups have been synthesized successfully. The hydrophilic quaternary ammonium groups can interact with the synthesis precursor to induce the crystallization of zeolites, while the hydrophobic long-chain alkane groups can aggregate to produce the mesoporosity of the zeolites. Hierarchical ZSM-5, Beta and Y zeolites have been synthesized successfully by using obtained surfactants and organosilane surfactants, respectively. Subsequently, these resultant hierarchical zeolite samples were characterized by a complementary combination of X-ray diffraction, Fourier transform infrared spectroscopy, N2 adsorption and desorption,Scanning electron microscopy, Transmission electron microscopy, Thermogravimetric analysis, Solid-state magic-angle spinning nuclear magnetic resonance spectra, and Pyridine adsorption Fourier transform infrared spectroscopy. Experimental data showed that the synthesized hierarchical zeolites exhibit crystalline zeolite-like framework, and the mesopore penetrated the whole framework forming three dimensional networks. The morphology, arrangement of mesopores, acidity and textural properties of hierarchical zeolites can be tuned by manipulating the molecular structure of quaternary ammonium surfactants and organosilane surfactants. Aldolcondensations and esterification reaction demonstrated that the synthesized hierarchical zeolites possessed sufficient acidity to catalyze the reaction, whilst the additional mesoporous characteristic not only allowed large molecules to entering the interior of the zeolites so as to access the active sites, but also improved the mass transport ability of guest molecules. Therefore, the hierarchical zeolites exhibited higher activity and longer life time compared with commercial zeolites.This paper innovatively constructed and optimized the structure model of hierarchical ZSM-5 zeolite on the atomic scale by Material Studio software, using molecular force method. The model was further characterized by comparing the low/wide angle XRD, BET surface area and pore size distribution obtained from experiment with that from simulation. The adsorption behavior of VOC molecules such as benzene and toluene was investigated by Grand canonical Monte Carlo method. The data showed that the adsorption behavior of toluene and benzene exhibit some discrepancies at low pressure, Toluene molecule is an asymmetrical molecule with a relatively weak polar moment, thus its adsorption occurred in the ZSM-5micropores and on the surface of the mesopore channel at low pressures, attributing to the adsorption potential of the micropores and the coulombic interaction between toluene and the adsorbent. However, benzene adsorption occurred mainly in the micropores at low pressures. The simulation result revealed the adsorption behavior of benzene and toluene in the hierarchical ZEM-5 zeolite.This paper innovatively investigated the porogenic mechanism of templates using dissipative particle dynamics(DPD) method. Firstly, the coarse-grained models of surfactant, TEOS and water were constructed, and then the self-assemble process of the surfactant/TEOS/water system that is composed of synthesis precursor of zeolites was simulated. The DPD simulation showed that the self-assemble system formed a spherical micelle, in which the hydrophobic alkyl group formed the micellar core while the hydrophilic quaternary ammonium groups and the TEOS formed the micellar shell. The spherical micelle could form the order structure with rodmorphology under a suitable shear rate, which induced the formation of the final structure of hierarchical zeolites.In this paper, a novel bifunctional acid and base catalyst was successfully prepared through combination of nanosheet zeolites and organic functional molecules.Firstly, the templates were successfully removed by a combination of UV radiation and acid extraction, which can protect the silanol groups on the surface of the nanosheet zeolites from annihilation. Then, BTEB(bis(triethoxysilyl)benzene)molecules were intercalated into the layer spaces of nanosheet zeolites, which the BTEB molecules can form the covalent bound with the silanol groups located on the surface of the zeolite framework. Lastly, an amination process was employed to incorporate amino(-NH2) group into the sample, resulting in the formation of bifunctional acid and base catalyst. Acetal hydrolysis-Knoevenagel condensation cascade reaction demonstrated that the synthesized bifunctional hybrid materials exhibited extraordinary catalytic performance and higher product yield compared with commercial microporous zeolites since microporous zeolites contain only acid sites that catalyze the first step reaction in the cascade reaction, obtaining the medium products without the final products.The developed approaches for preparation of hierarchical zeolite and bifunctional acid-base catalyst in this paper can be extended to other hierarchical porous and hybrid materials. The obtained hierarchical zeolites and bifunctional acid-base catalyst with excellent property may find applications as advanced materials in various fields such as chemical industrial, medication, aerospace and so on.Computer simulation as an advanced technique can be used together with the experimental methods to eliminate the deficiency of sole experiment tool,contributing to the investigation of microstructural information of zeolites and porogenic mechanism of templates.