Synthesis Of The All-Silica DD3R And Their Performance In Adsorption And Separation

As the obligations of environmental protection, energy conservation, and emission reduction, adsorption-based separations by zeolite molecular sieves have received extensive attention due to their environmental friendliness and low energy consumption. With enrichment and development of zeolite synthesis and experimental techniques as well as with long-term improvement of molecular simulation, research on adsorption and separation becomes more prosperous.In the last decades, the all-silica DD3R, possessing8-ring pore apertures that match the critical dimensions of adsorbing molecules, displays a good prospect for application in many adsorption-based separation processes. However, the applications of the all-silica DD3R are largely limited to their low productivity or product quality in the synthesis, which is costly and cumbersome. This MSc thesis research has been focused on several aspects with respect to the associated scientific problems such as DD3R synthesis and characterization in terms of adsorption and diffusion, and the main obtained results are summarized as follows:From a fundamental point of view, the utilization of the synthesized pure DD3R crystals with uniform shape and size enables one to characterize the adsorption and diffusion properties of DD3R more accurately. From an application point of view, the development of a facile synthesis strategy makes the industrial application of DD3R as adsorbent to its adsorption-based separations possible. In this study, pure DD3R crystals with uniform shape and size were successfully synthesized in fluoride medium. By adding a small amount of seeds, of which DD3R has not been completely crystallized yet, the synthesis time can be reduced to one day. The results show that both type and size of the added seeds significantly affect the purity and uniformity of the outcome product. As a result, it is beneficial to obtain pure DD3R crystals by adding the DD3R seeds with tiny size possessing a high surface energy. Additionally, pure DD3R can be synthesized, when commercial fumed SiO2(silica) particles are used as "seeds". The effects of several synthesis parameters, including amount of seeds, mineralizing agent, and dosage of water, were systematically investigated. The gel composition plays a significant role in tuning the size and morphology of resulting DD3R crystals. The developed protocol for the synthesis of pure DD3R crystals is facile and the size and morphology of the resulting crystals are controllable.Apart from the traditional interest from the standpoint of separation, the understanding of adsorption, from a theoretical point of view, is of utmost importance. The single component adsorption isotherms of CH4, CO2, C2H4, C2H6, and C3H6on DD3R crystals were systematically investigated by means of a volumetric method at pressures up to120kPa and temperatures ranging from273K to348K. The dual-site Langmuir (DSL) model describes the measured adsorption isotherm data very well over the whole range of the investigated conditions. The extracted thermodynamic properties such as enthalpy and entropy associated with adsorption are discussed in detail. The sizes of the adsorbate molecules as well as the interactions between adsorbates play an important role in the adsorption on DD3R, of which the cavity volume is relatively limited. The approach using the DSL model provides an excellent interpretation of the adsorption of the adsorbing molecules on all-silica DD3R in terms of pore filling processes. The consistency between experimental results and model predictions emphasizes the strength of the proposed approach.Considering the different mechanisms of the conducting adsorption-based separation processes, both thermogravimetric and zero-length column (ZLC) techniques were used to characterize the diffusion properties of DD3R to explore its potential application. In addition, based on the results from the breakthrough-column experiments, both C3H6/C3H8and CO2/CH4mixtures can be well separated by DD3R. The results from the ZLC measurements, for the first time, show that C2H4diffuses much faster than C2H6in DD3R crystals, implying the feasible separation of C2H4/C2H6mixtures by DD3R adsorbent or membrane.