Effects Of Pore Structure And Silylation Modification On The Benzene Adsorption Performance Of Diatomite And The Related Mechanisms

The air pollution problems caused by volatile organic compounds(VOCs) have attracted increasing attention worldwide. Among the available technologies for VOCs control, adsorption is the most applicable technology because of the flexibility of the system, low energy, and inexpensive operation costs. The performance of adsorbents is the most critical factor in adsorption technology application. Activated carbon has been recognized as the most versatile adsorbent due to its low cost and excellent adsorption capacity. However, several drawbacks, such as pore clogging, low thermal stablility, and hygroscopicity are associated with its use in adsorption process. Therefore, it is important to develop new, cheap, durable, and high-efficient adsorbents to adsorb and separate VOCs from polluted air streams.Diatomite is a natural biogenetic mineral, which consists essentially of amorphous hydrated silica(Si O2·n H2O) derived from the frustules of diatoms. Diatoms are unicellular algae, microscopic plants that existed widely in ocean or lake. Diatomite has many unique physical and chemical characteristics, such as a highly developed mesoporosity and macroporosity, high adsorbability, strong acid resistance, low density, and excellent thermal resistance. Consequently, diatomite has been used in a variety of applications, including uses as adsorbents, filters, and supports. However, Diatomite exhibited ordinary adsorption capacity for organic molecules. This is due to the surface silanols of diatom shells do not possess a strong adsorption affinity for organics; moreover, the low specific surface area(SBET) of diatomite is disadvantageous to adsorption.In order to solve the above problems, the pore structure was modified to increase the SBET of diatomite, and surface silylation was performed to improve the adsorption affinity between diatomite and organic molecules.1. Pore structure modification. Two routes were used to modify the pore structure of diatomite:(i) Diatomite was first etched by Na OH solution to enlarge the pores on diatom frustules, followed by hydrothermal growth of MFI-type zeolite at the surface of the etched diatom frustles previously seeded with nanocrystalline silicatlite-1 to prepare diatomite/MFI-type zeolite composites.(ii) A diatomite/MFI-type zeolite composite was first prepared through a vapor-phase transport(VPT) method, in which the diatomaceous silica was partially tra nsformed into zeolite. Then, the composite was treated by desilication in an alkaline medium to create mesopores in the zeolite crystals coated on the surface of the diatomite. Various characterization techniques and benzene adsorption tests were performed to study the influences of pore structure modification on the pore structure, SBET, and benzene adsorption performance of diatomite and the related mechanisms.2. Silylation modification. Phenyltriethoxysilane(PTES) with phenyl functional groups was used to modify the diatomite surface to improve its hydrophobicity and its affinity to organic molecules. Various characterization techniques and benzene adsorption tests were performed to study the influences of silylation modification on the structure, surface property, and benzene adsorption performance of diatomite as well as the related mechanisms.The main conclusions of this work are listed as follows:(1) A ne w method of modifying the pore structure of diatomite was proposed, in which diatomite was first etched by Na OH solution to enlarge the pores on diatom frustules, followed by hydrothermal growth of MFI-type zeolite at the surface of the etched diatom frustles previously seeded with nanocrystalline silicatlite-1. Moreover, the influences of pore structure modification using this ne w method on the pore structure and benzene adsorption performance of diatomite as well as the related mechanisms were studied.1) Na OH etching enlarged significantly the pores on diatom frustules, with the center pore zise increasing from 200~500 nm to 400~1000 nm.2) Diatomite/MFI-type zeolite composites were prepared through enlarging the pores on diatom frustules by Na OH etching, followed by hydrothermal growth of MFI-type zeolite at the surface of the etched diatom frustles previously seeded with nanocrystalline silicatlite-1. The prepared diatomite/MFI-type zeolite composites possessed hierarchically porous structure, sourcing from the macroposity of diatomite, mesoporosity from the stacking of zeolite particles, and microporosity of MFI-type zeolite. The SBET and micropore volume(Vmicro) of the diatomite/MFI-type zeolite composite is as high as 325.4 m2/g and 0.125 cm3/g, respectively, which are significantly higher than those of diatomite(SBET, 16.8m2/g and Vmicro, 0.006 cm3/g).3) The presence of micropores in the diatomite/MFI-type zeolite composites was an important parameter for the dynamic adsorption of benzene. The largest dynamic adsorption capacity for benzene of the diatomite/MFI-type zeolite composites is 0.649 mmol/g, which is significantly higher than that of diatomite(0.087 mmol/g).4) A samller particle size of zeolite coated at the surface of diatom frustules led to a higher adsorption capacity per unit mass of zeolite(qs), due to its having the larger external surface. Moreover, the macroporous diatom frustules increased the qs of the diatomite/MFI-type zeolite composites. This result is suggested to be due to the zeolite particles coated on the surface of the macroporous diatom frustules obtaining better dispersion than pure zeolite.(2) The other ne w method of modifying the pore structure of diatomite was proposed, in which a diatomite/MFI-type zeolite composite was prepared through transforming diatomaceous silica into MFI-type zeolite, followe d by desilication treatment to create mesopores in the zeolite crystals coated on the surface of the diatomite. In addition, the influences of pore structure modification using this ne w method on the pore structure and benzene adsorption performance of diatomite as well as the related mechanis ms were studied.1) A diatomite/MFI-type zeolite composite was prepared through a VPT method, in which the diatomaceous silica was partially transformed into zeolite. The composite possessed the original disk morphology, the macroporous structure of diatomite, and the microporous structure of zeolite. The SBET and Vmicro of the diatomite/MFI-type zeolite composite is 295.7 m2/g and 0.095 cm3/g, respectively, which are significantly higher than those of diatomite(SBET, 16.8m2/g and Vmicro, 0.006 cm3/g).2) Treatment of the diatomite/MFI-type zeolite composite by an optimization desilication condition(0.2 M Na OH solution at 60℃ for 1 h) resulted in a mesopore size distribution centered at approximately 7 nm, without significantly altering the macroporosity and microporosity of the resulting composite. The total pore volume(Vtot al) of the desilication-treated diatomite/MFI-type zeolite composite can reach 0.255 cm3/g, which is significantly higher than that of diatomite(0.164 cm3/g).3) The diatomite/MFI-type zeolite composite prepared by an optimization desilication condition exhibited excellent static adsorption performance for benzene, with a higher benzene adsorption capacity and a faster adsorption kinetics. This result is due to the increase in porosity and the formation of terminal silanol groups on the newly developed pore surface with a size of 7 nm after the desilication treatment.(3) The effect of PTES-modification on the performance of benzene static adsorption.1) After silylation, a functional group(-C6H5, phenyl) was successfully introduced onto the surface of diatomite. PTES- modified diatomite(PTES-Dt) exhibited hydrophobic properties with a water contact angle(WCA) as high as 120±1o, whereas the original diatomite(Dt) was superhydrophilic with a WCA of 0o due to the different hydroxyl species on its surface.2) The benzene adsorption data on both Dt and PTES-Dt fit well with the Langmuir isotherm equation. The Langmuir adsorption capacity of benzene on the PTES-Dt is 28.1 mg/g, 4.5- fold greater than that on Dt. Moreover, the adsorption kinetics results show that equilibrium was achieved faster for PTES-Dt than for Dt, over the relative pressure range of 0.118–0.157. The excellent benzene adsorption performance of PTES-Dt is attributed to strong π-system interactions between the phenyl groups and the benzene molecules.In addition, a series of diatomite-based materials with excellent benzene adsorption performance were prepared, based on the pore structure and silylation modification. These materials exhibited excellent benzene adsorption performance, with high benzene adsorption capacity and fast adsorption kinetics. The results of this work provided the theoretical and experimental data for using diatomite in VOCs adsorption and recycling. Moreover, this work is valuable for the efficient use of diatomite mineral resource and for the development of VOCs adsorption theory and method.