Abstract
High-quality ZSM-5 zeolite membranes were successfully synthesized on the outer surface of tubular α-alumina substrates by manipulating the microstructure of the zeolite layer using a new different-sized seeding (DSSM) in combination with variable-temperature/time (VTT) methodd. In this method the microcracks/defects in the primary seed layer are filled by smaller seeds and effectively eliminated using a proper temperature/time profile, resulting in a significant increase in selectivity without scarifying permeance. The effects of number and combination of seed layers, seed size, and synthesis conditions on the microstructure, N2/SF6 gas permeation, and H2/CO2 separation performance of membranes are evaluated in detail. The synthesized ZSM-5 membranes show the microstructure of thin, fully inter-grown, and densified zeolite layer that is responsible for high selectivity without any negative effect on the permeance. The best synthesized defect-free membranes exhibited a very low H2/CO2 selectivity of 0.1 with CO2 permeance of 3×10-5mol m-2 s-1 Pa-1.
Keywords: ZSM-5, Zeolite membrane; Different-sized seeding; Variable temperature/time; CO2 selective
Introduction
Accumulation of carbon dioxide in the atmosphere significantly threatens human life due to its greenhouse effect [1] indicating the importance of the development of efficient separation processes for carbon dioxide removal [2]. MFI-type zeolite membranes due to their high thermal, mechanical, and chemical stabilities, proper hydrophobic properties, and high CO2 adsorption capacity are promising to control CO2 emission [3]. Among MFI-type zeolite membranes, its Al-containing analogue ZSM-5, if supported on porous ceramic tubes are attract for large scale separation of CO2 from gaseous mixtures involved in natural gas, biogas, and flu gas industries [4] due to its unique pore structure, which is similar to many industrially important molecules [5].
It is well-known that the performance of a tubular supported zeolite membrane in a gas separation process depends on the quality of the polycrystalline zeolite layer in terms of the number of defects/non-zeolitic pores and the thickness of the zeolite membrane. The former determines the membrane selectivity, while the latter responsible for permeability [6, 7]. Therefore, recent works have been focused on the elimination of the defects/non-zeolitic pores and reducing the membrane thickness [8, 9]. The defects generally form as a result of support defects, expansion mismatch problems, high-temperature heating/cooling cycles, especially during the template removal process, and a non-uniform seed layer [10].
Towards eliminate inter-crystalline gaps in the ZSM-5 zeolite framework, Zhang et al. [11] found that seed size has important effects on the formation of seed layers that determines the quality of regrown zeolite membranes. They showed that the zeolite layer synthesized from small seeds (100 nm) are more uniform and denser. An increase in seed size (600 nm – 1.5 μm) makes the zeolite layer coarser and the intergrowth of the membranes becomes poorer. Their results showed also that the crystal intergrowth of the membrane improves by increasing synthesis temperature reducing the voids and changing the orientation of crystal growth. Xia et al. [12] systematically manipulated the microstructure of the MFI-type zeolite membranes by tuning the seed size and seed morphology. They showed that small seeds (< 200 nm) tend to form c-oriented zeolite layer, whereas large seeds (> 500 nm) tend to grow in different directions and form more defective layer due to the low space limitation suppression under the same synthesis condition. As a result, small seeds tend to form a continuous, less defective, and dense zeolite layer, while larger seeds usually form a defective porous layer [11]. Shan et al. [13] found that the seeding procedure, as well as the quality/density of the seed layer, greatly influence on the MFI-type zeolite membrane formation and its separation performance. They showed that a continuous and dense zeolite seed layer is necessary to obtain a ZSM-5 membrane with good quality. However, their small seed size (70 nm) along with high seed concentrations (> 10 wt. %) led to a thick zeolite layer and caused the membrane to lose its permeability.
Looking from another perspective, Li et al. [14] tried to improve the performance of ZSM-5 zeolite membranes using a two-stage varying-temperature method. According to their method, membrane synthesis begins at 130˚C for 4 h, synthesis temperature then increases rapidly to 170˚C, and synthesis continues for another 4 h. They demonstrated that compared with synthesis at the constant temperature, this method is very effective for the preparation of high-quality membranes via controlling the rates of nuclei formation and crystal growth by changing the synthesis temperatures. The best ZSM-5 zeolite membrane obtained by Li et al. showed hydrogen permeance of 2.4×10-6 mol m-2s-1 Pa-1 along with an H2/n-butane selectivity of 129. The varying-temperature method was applied also by Kong et al. [15] to prepare thin and continuous MFI-type zeolite membranes on disc-shaped α-alumina supports. They showed that the inter-crystalline regions can effectively be eliminated using a proper synthesis temperature profile that leads to higher selectivities. However, due to application on unseeded supports, their selectivity (⁓22) for the same separation conditions was almost 6 times lower than that of Li et al. (⁓130). Considering all the above-mentioned facts, to further improve the selectivity of tubular ZSM-5 zeolite membranes, we need to minimize inter-crystalline gaps of the membrane layer via modifying the seeding method and combine it with an appropriate synthesis temperature/time profile. In this study, we report a new secondary-growth synthesis method consisting of multi-layer seeding in combination with the variable temperature/time profiles to prepare high-quality ZSM-5 zeolite membranes on alumina tubular supports. The schematic procedure of our proposed hybrid different-sized seeding / varying-temperature method is shown in Fig. 1.
In this method, three different sized seed particles are coated, fixed, and calcined to the support surface, in order of size. The hydrothermal synthesis process then is carried out under different varying temperature/time profiles. The as-synthesized zeolite membrane would contain a well inter-grown, densified, and uniform top layer which leads to higher selectivity, and a less inter-grown porous bottom layer that leads to higher permeance. This novel hybrid method enhances the selectivity of the tubular ZSM-5 zeolite membrane without sacrificing its permeance via the elimination of the inter-crystalline gaps/defects and decreasing the membrane thickness.