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The synthesis of DDR-type zeolite membranes faces the problem of cracks that occur on the zeolite membrane due to differences in the thermal expansion coefficient between zeolite and the porous substrate during the detemplating process. In this study, Al-containing ZSM-58 zeolite membranes with DDR topology were prepared by rapid thermal processing (RTP), with the aim of developing a reproducible method for preparing DDR zeolite membrane without cracks. Moreover, we verified the influence of RTP before performing conventional thermal calcination (CTC) on ZSM-58 membranes with various silica-to-aluminum (Si/Al) molar ratios. Using the developed method, an Al-containing ZSM-58 membrane without cracks was obtained, along with complete template removal by RTP, and it had higher CO2/CH4 selectivity. An all-silica ZSM-58 membrane without cracks was obtained by only using the ozone detemplating method. ZSM-58 crystals and membranes with various Si/Al molar ratios were analyzed by using Fourier-transform infrared (FTIR) spectroscopy to confirm the effects of RTP treatment. Al-containing ZSM-58 zeolites had higher silanol concentrations than all-silica zeolites, confirming many silanol condensations by RTP. The condensation of silanol forms results in the formation of siloxane bonds and stronger resistance to thermal stress, therefore, RTP caused crack suppression in Al-containing ZSM-58 membranes. The results demonstrate that Al-containing ZSM-58 zeolite membranes with high CO2 permeance and CO2/CH4 selectivity and minimal cracking can be produced by using RTP.

First, we attempted to synthesize a DD3R zeolite membrane by using precursor solutions containing small amounts of a structure-directing agent and potassium fluoride (KF), with the aim of obtaining a membrane with high permeance toward CO2. The most dilute precursor solution (1-adamantylamine:KF:SiO2:H2O = 0.026:0.026:1:52) produced a thin and dense zeolite membrane that showed a high CO2 permeance of 12 × 10−8 mol m−2 s−1 Pa−1 and a high PCO2/PCH4 of 270 at a pressure drop of 1.0 MPa. We then studied the synthesis of DD3R membranes with various alkali metal halides (KCl, KBr, NaF, NaCl, or NaBr) as the mineralizing agent (MA). Regardless of the alkali metal halide selected, the addition of a small amount of MA permitted the preparation of DD3R membranes with a high separation selectivity of 190–380 at a pressure drop of 1.0 MPa. Addition of excessive amounts of MA caused gelling of the solution and a decrease in the separation selectivity. The presence of a few alkali metal cations promotes the growth of zeolite crystals on the support and permits the synthesis of dense DD3R zeolite membranes.

Utilising CO2 from biogas as a carbon source can introduce energy production systems incorporating microalgae cultivation into wastewater treatment plants (WWTPs). In this study, the effects of utilising membrane-separated CO2 (MSC; CO2 content: 983 ± 11 dm3 m−3)—obtained from biogas using a newly developed separation system—on indigenous microalgae cultivation with treated effluents was investigated. Assuming model cultivation systems, energy balance analysis was conducted to evaluate the feasibility of MSC utilisation and its superiority over flue gas CO2 (FC) from power plants (PPs). Scenarios 1 and 2 used generated biogas of 11.6 and 116 dm3 s−1, and produced MSC (carbon based) were 0.802 and 8.02 g s−1, respectively. Experiments comparing MSC (suspended solids (SS): 166 ± 23 mg dm−3; higher heating value (HHV): 21.1 ± 0.46 kJ g−1) with commercial CO2 (SS: 176 ± 16 mg dm−3; HHV: 21.2 ± 0.90 kJ g−1) revealed no negative effects on microalgae activity, biomass production, or energy content. Energy revenue of the MSC utilisation system was 27.0 MJ per 1 kg injected MSC, which is greater than the energy costs (18.2 and 17.1 MJ kg−1 in scenarios 1 and 2, respectively). MSC utilisation is energetically feasible, and energetically superior to FC utilisation when the PP-to-WWTP distance is over 0.6 and 1.9 km in scenarios 1 and 2, respectively.

We prepared all-silica ZSM-58 zeolite membranes on α-alumina tubular supports and optimized their synthesis conditions. High-purity ZSM-58 zeolite membranes were prepared using a concentrated precursor solution (methyltropinium iodide/SiO2 and NaOH/SiO2 molar ratios > 0.05) with a H2O/SiO2 molar ratio of 52 at 413 K for 24 h. A ZSM-58/DOH mixed layer or a pure DOH layer was generated with ZSM-58 when the synthesis temperature exceeded 413 K and the synthesis time was longer than 24 h. The membranes prepared with a H2O/SiO2 molar ratio higher than 52 were not covered ZSM-58 zeolite and/or generated DOH zeolite. Further, we investigated the separation performance of the ZSM-58 zeolite membrane for CO2/CH4 and CO2/N2 gas mixtures as a function of feed pressure. At a pressure drop of 0.2 MPa, the membrane exhibited a CO2 permeance and CO2/CH4 mixture selectivity of 17 × 10−8 mol s−1 m−2 Pa−1 and 290, respectively, for a CO2/CH4 system, and a CO2 permeance and CO2/N2 selectivity of 29 × 10−8 mol m−2 s−1 Pa−1 and 44, respectively, for a CO2/N2 system. The mixture selectivity was lower than the single gas selectivity in CO2/CH4 separation due to competitive adsorption since CH4 mainly passes through non-zeolite pores. In contrast, the mixture selectivity was higher than the single gas selectivity in CO2/N2 separation because N2 permeation was inhibited by CO2 adsorption in the zeolite pores since N2 mainly passes through the zeolite pores. Thus, high-purity ZSM-58 zeolite membranes with high CO2 separation performances were obtained by the optimized synthesis conditions.