Xiaochun Xu

Preparation and characterization of novel CO2 "molecular basket" adsorbents based on polymer-modified mesoporous molecular sieve MCM-41

Abstract Novel CO2 “molecular basket” adsorbents were prepared by synthesizing and modifying the mesoporous molecular sieve of MCM-41 type with polyethylenimine (PEI). The MCM-41-PEI adsorbents were characterized by X-ray powder diffraction (XRD), N2 adsorption/desorption, thermal gravimetric analysis (TGA) as well as the CO2 adsorption/desorption performance. This paper reports on the effects of preparation conditions (PEI loadings, preparation methods, PEI loading procedures, types of solvents, solvent/MCM-41 ratios, addition of additive, and Si/Al ratios of MCM-41) on the CO2 adsorption/desorption performance of MCM-41-PEI. With the increase in PEI loading, the surface area, pore size and pore volume of the PEI-loaded MCM-41 adsorbent decreased. When the PEI loading was higher than 30 wt.%, the mesoporous pores began to be filled with PEI and the mesoporous molecular sieve MCM-41 showed a synergetic effect on the adsorption of CO2 by PEI. At PEI loading of 50 wt.% in MCM-41-PEI, the highest CO2 adsorption capacity of 246 mg/g-PEI was obtained, which is 30 times higher than that of the MCM-41 and is about 2.3 times that of the pure PEI. Impregnation was found to be a better method for the preparation of MCM-41-PEI adsorbents than mechanical mixing method. The adsorbent prepared by a one-step impregnation method had a higher CO2 adsorption capacity than that of prepared by a two-step impregnation method. The higher the Si/Al ratio of MCM-41 or the solvent/MCM-41 ratio, the higher the CO2 adsorption capacity. Using polyethylene glycol as additive into the MCM-41-PEI adsorbent increased not only the CO2 adsorption capacity, but also the rates of CO2 adsorption/desorption. A simple model was proposed to account for the synergetic effect of MCM-41 on the adsorption of CO2 by PEI.

Synthesis of a high-permeance NaA zeolite membrane by microwave heating

Zeolite membranes with high permeance and separation factors are highly desirable for practical applications. Although, in the past, very good separation factors have been obtained, it has proved difficult to achieve a high permeance. Ken a comparative study of microwave versus conventional heating in the hydrothermal synthesis of NaA zeolite membranes is made. It is demonstrated that membranes prepared by microwave heating have not only a higher permeance but also a considerably shorter synthesis time. These observations are rationalized by examining the mechanism of membrane formation.

Synthesis of NaA zeolite membranes from clear solution

NaA zeolite membranes were successfully synthesized on a porous α-Al2O3 support from clear solution. The synthesis parameters, such as surface seeding, synthesis time, synthesis stages, etc. were investigated. Surface seeding can not only accelerate the formation of NaA zeolite on the support surface, but can also inhibit the transformation of NaA zeolite into other types of zeolites. A continuous NaA zeolite membrane formed on the seeded support after 2 h of synthesis. Gas permeation results showed that a synthesis time of 3 h produced the best NaA zeolite membrane. When the synthesis time was longer than 4 h, the NaA zeolite on the support surface began to transform into other types of zeolites, and the quality of the NaA zeolite membrane decreased. The quality of the NaA zeolite membrane can be improved by employing the multi-stage synthesis method. The NaA zeolite membrane with a synthesis time of 2 h after a two-stage synthesis showed the best gas permeation performance. The permeances of H2, O2, N2, and n-C4H10 decreased as the molecular kinetic diameter of the gases increased, which showed the molecular sieving effect of the NaA zeolite membrane. The permselectivities of H2/n-C4H10 and O2/N2 were 19.1 and 1.8, respectively. These values are higher than the Knudsen diffusion ratios of 5.39 and 0.94. However, the permeation of n-C4H10 also indicated that the NaA zeolite membrane had certain defects with diameters larger than the pore size of NaA zeolite. A synthesis model was proposed to clarify the effect of surface seeding.

Synthesis of NaA zeolite membrane by microwave heating

The synthesis of NaA zeolite membrane on a porous -Al2O3 support by microwave heating (MH) was investigated. The formation of a NaA zeolite membrane was drastically promoted by MH. The synthesis time was reduced from 3 h for conventional heating (CH) to 15 min for MH. Surface seeding cannot only promote the formation of NaA zeolite on the support, but also inhibit the transformation of NaA zeolite into other types of zeolites. The thickness of the NaA zeolite membrane synthesized by MH was about 4 m, thinner than that of NaA zeolite membrane synthesized by CH. The permeance of NaA zeolite membrane synthesized by MH was four times higher than that of the NaA zeolite membrane synthesized by CH, while their permselectivities were comparable. Multi-stage synthesis resulted in the transformation of NaA zeolite into other types of zeolites, and the perfection of the as-synthesized membrane decreased. The formation mechanism of NaA zeolite membrane on the porous -Al2O3 support by MH was proposed. The promotion effect on the formation of NaA zeolite membrane by MH can be divided into two parts: the ‘‘thermal effect’’ and the ‘‘microwave effect’’. The formation of a homogeneous and thin NaA zeolite membrane resulted from both the ‘‘thermal effect’’ and the ‘‘microwave effect’’, while the fast formation of NaA zeolite membrane was mainly caused by the ‘‘microwave effect’’.

Synthesis and gas permeation properties of an NaA zeolite membrane

A high quality NaA zeolite membrane, which shows a H2/n-C4H10 permselectivity of 106, has been synthesized on a seeded α-Al2O3 support by a multistage synthesis method.

Synthesis and perfection evaluation of NaA zeolite membrane

The synthesis of NaA zeolite membrane on a porous -Al2O3 support from clear solution and the evaluation of the perfection of the as-synthesized membrane by gas permeation were investigated. When an unseeded support was used, the NaA zeolite began to transform into other types of zeolites before a continuous NaA zeolite membrane formed. When the support was coated with nucleation seeds, not only the formation of NaA zeolite on the support surface was accelerated, but also the transformation of NaA zeolite into other types of zeolites was inhibited. A continuous NaA zeolite membrane can be formed. Perfection evaluation indicated that the NaA zeolite membrane with the synthesis time o f3hs howed the best perfection after a one-stage synthesis. The perfection of NaA zeolite membrane can be improved by employing the multi-stage synthesis method. The NaA zeolite membrane with a synthesis time of 2 h after a two-stage synthesis showed the best gas permeation performance. The permselectivity of H2/n-C4H10 and O2/N2 were 19.1 and 1.8, respectively, higher than those of the corresponding Knudsen diffusion selectivity of 5.39 and 0.94, which showed the molecular sieving effect of NaA zeolite. However, the permeation of n-C4H10 also indicated that the NaA zeolite membrane had certain defects, the diameter of which were larger than the NaA zeolite channels.

Synthesis of NaA zeolite membrane with high performance

In recent years, zeolite membranes have been hotly investigated due to their potential to be combined in reaction/separation devices such as membrane reactors and chemical sensors, and to achieve gas separation under steady-state operation [1–4]. Molecules with different sizes and shapes can be discriminated or separated by zeolites through their channels. A-type zeolite, due to the pore size (0.3–0.5 nm) of its channel system, can be selective for small molecules, such as H2, O2, N2, CO2 and H2O from other molecules or from each other. Aoki et al. [5] formed zeolite A-type membranes on a porous support tube, and obtained good results for gas separation, of which the permeance of H2 permeance was about 10−7 mol · s−1 · m−2 · Pa−1. Recently, we [6–8] reported the preparation of zeolite NaA membranes by multistage synthesis, the results of pure gas separations were very good, while the permeance of H2 was also about 10−7 mol · s−1 · m−2 · Pa−1. As mentioned above, zeolite NaA membranes usually have a good separation factor, but the permeance is too low for practical applications [9, 10]. The relationship of the permeance and the permselectivity of the membrane is a trade-off, which means to get a high permeance, the permselectivity has to be sacrificed to a certain extent, and reverse also affirms we have to sacrifice the permeance to improve permselectivity. Thus one of the challenges in the field of zeolite NaA membrane is to prepare zeolite membranes with high permeance, while maintaining high separation selectivity. The zeolite membrane is composed of three parts: the zeolite crystal layer, the intermediate layer and the substrate [11]. The intermediate layer is composed of the substrate and the zeolite crystals in the pore of the substrate. The permeance of the membrane is mainly controlled by the thickness of the zeolite layer and the intermediate layer. In order to increase the permeance of the membrane, it’s necessary to reduce the thickness of these two layers. As reported, an active mesoporous layer can assist the formation of zeolite membrane [12], thus possibly forming a continuous but thin zeolite membrane in a shorter time of reaction. And during the formation of the zeolite membrane, the reaction bulk may penetrate into the substrate to form a thick intermediate layer. In this paper, combining these two factors together by adding a more active mesoporous layer on the macroporous substrate before synthesis, we successfully synthesized zeolite A-type membrane with high permeance and gas selectivity (Fig. 1). The initially porous α-Al2O3 substrate used was made by casting technology, with 30 mm in diameter, 3 mm in thickness, 0.3–0.5 μm in pore radius and ca. 50% porosity. One face of the substrate was dipped into a solution of 1.0 M of AlOOH with 1 wt% of PEG400 and 2 wt% of PVA-72000. The dipping time was about 9–12 s. Then the substrate was dried at room temperature for 24 h, heated to 700◦C at a rate of 0.3◦C/min, held at 700◦C for 3 h and cooled down at 0.5◦C/min. The dipping-calcining process was repeated to the same face of the substrate for 3 times to form a continuous thin mesoporous top layer. Then the modified side was coated with NaA zeolite crystals as nucleation seeds. The synthesis mixture was prepared by mixing sodium aluminate, water glass, sodium hydroxide, and water under vigorous stirring for 24 h to form a homogenous gel with molar composition of 3Na2O : 2SiO2 : Al2O3 : 200H2O. After the precursor was poured into a stainless steel autoclave, the substrate was put vertically in the synthesis mixture supported by a Teflon holder, with the unseeded face protected from the reaction bulk. The autoclave was transferred into the oven preheated at 90◦C and reacted for 12 h. After the reaction, the membrane was cooled down, washed in water, and dried at 150◦C for 3 h. Finally, the formation of the zeolite membrane was confirmed by X-ray diffraction (XRD) using a Ragaku D max/b powder diffractometer with Cu Kα (λ = 0.154 nm) radiation and operating at 40 kV and 100 mA. The surface morphologies of the membrane were observed with scanning electron microscopy (SEM) on a JEM-1200E scanning electron microscope. The integrity of the membrane was evaluated by gas permeation tests. The membrane was sealed in a permeation module with the zeolite membrane on the high-pressure side. The gas permeances of the membrane were measured by a soap-film flowmeter under

Fast formation of NaA zeolite membrane in the microwave field

NaA zeolite membrane was successfully synthesized on the porous α-Al2O3 support by microwave heating. The synthesis of NaA zeolite membrane in the microwave field only needs 15 min and the synthesis time is 10 times shorter than that by conventional heating. SEM characterization indicates that the zeolite crystals in the NaA zeolite membrane synthesized by microwave heating are uniform in size; the membrane thickness is about 4 μm and is thinner than that of the NaA zeolite membrane synthesized by conventional heating. Gas permeation studies indicate that the permeances of the NaA zeolite membrane synthesized by microwave heating are 3–4 times higher than those of the NaA zeolite membrane synthesized by conventional heating, while their permselectivities are comparable.

Separation of butane isomer by tubular silicalite-1 zeolite membrane

The permeation properties ofn-butane,i-butane andn-butane/i-butane mixture (n-butane 24.3% (molar ratio),i-butane 75.7%) through a tubular silicalite-1 zeolite membrane were studied at 298 and 473 K respectively. The permselectivities ofn-butane andi-butane under pressure difference of 0.06 MPa at 298 and 473 K were 16.3 and 7.4 respectively. The separation factors ofn-butane/i-butane mixture were between 2.0 and 2.5 at 298 and 473 K. At 298 K, the permeances ofn-butane in the mixture were lower than those of single component while the permeances ofi-butane in the mixture were almost the same as those of single component. At 473 K, the permeances ofn-butane andi-butane in the mxiture were decreased compared with those of the single component, and the permeance ofn-butane decreased more rapidly than that ofi-butane.

THE DEVELOPMENT OF COAL-BASED TECHNOLOGIES FOR DEPARTMENT OF DEFENSE FACILITIES

The third phase of a three-phase project investigating the development of coal-based technologies for US Department of Defense (DOD) facilities was completed. The objectives of the project were to: decrease DODs dependence on foreign oil and increase its use of coal; promote public and private sector deployment of technologies for utilizing coal-based fuels in oil-designed combustion equipment; and provide a continuing environment for research and development of coal-based fuel technologies for small-scale applications at a time when market conditions in the US are not favorable for the introduction of coal-fired equipment in the commercial and industrial capacity ranges. The Phase III activities were focused on evaluating deeply-cleaned coals as fuels for industrial boilers and investigating emissions control strategies for providing ultra-low emissions when firing coal-based fuels. This was addressed by performing coal beneficiation and preparation studies, and bench- to demonstration-scale emissions reduction studies. In addition, economic studies were conducted focused on determining cost and market penetration, selection of incentives, and regional economic impacts of coal-based technologies.