Ion exchange of binderless zeolite A and X granules leads to high CO2/CH4 selectivity and CO2 uptake capacity.

Biogas is a potential renewable energy resource that can reduce the current energy dependency on fossil fuels. The major limitation of utilizing biogas fully in the various applications is the presence of a significant volume fraction of carbon dioxide in biogas. Here, we used adsorption-driven CO2 separation using the most prominent adsorbents, NaX (faujasite) and CaA (Linde Type A) zeolites. The NaX and CaA zeolites were structured into hierarchically porous granules using a low-cost freeze granulation technique to achieve better mass transfer kinetics. The freeze granulation processing parameters and the rheological properties of suspensions were optimized to obtain homogenous granules of NaX and CaA zeolites 2–3 mm in diameter with macroporosity of 77.9% and 68.6%, respectively. The NaX and CaA granules kept their individual morphologies, crystallinities with a CO2 uptake of 5.8 mmol/g and 4 mmol/g, respectively. The CO2 separation performance and the kinetic behavior were estimated by breakthrough experiments, where the NaX zeolite showed a 16% higher CO2 uptake rate than CaA granules with a high mass transfer coefficient, 1.3 m/s, compared to commercial granules, suggesting that freeze-granulated zeolites could be used to improve adsorption kinetics and reduce cycle time for biogas upgrading in the adsorption swing technology.

Hierarchical zeolite composite nanofibers are designed using an electrospinning technique with post‐carbonization processing to form mechanically strong pellets for biogas upgrading. A ZSM‐5 nanopowder (zeolite) and a polyvinylpyrrolidone (PVP) polymer are electrospun to form ZSM/PVP composite nanofibers, which are transformed into a ZSM and carbon composite nanofiber (ZSM/C) by a two‐step heat treatment. The ZSM/C nanofibers show a 30.4% increase in Brunauer–Emmett–Teller (BET) surface area compared with the non‐structured ZSM‐5 nanopowder. Using ideal adsorbed solution theory, CO2‐over‐CH4 selectivity of 20 and CO2 uptake of 2.15 mmolg−1 at 293 K at 1 bar for ZSM/C nanofibers are obtained. For the efficient use of adsorbents in pressure swing adsorption operation, the nanofibers are structured into ZSM/C pellets that offer a maximum tensile strength of 6.46 MPa to withstand pressure swings. In the breakthrough tests, the CO2 uptake of the pellets reach 3.18 mmolg−1 at 293 K at 4 bar after 5 breakthrough adsorption–desorption cycles, with a much higher mass transfer coefficient of 1.24 ms−1 and CO2 uptake rate of 2.4 mg of CO2 g−1s−1, as compared with other structured zeolite adsorbents.

Ammonia is one of the most common reductants for the automotive selective catalytic reduction (SCR) system owing to its high NO2 reduction (deNOx) efficiency. However, ammonia carriers for the SCR system have sluggishly evolved to achieve rapid ammonia dosing. In this study, the MOFs [M2(adc)2(dabco)] (M = Co, Ni, Cu, Zn) were synthesized and characterized as ammonia carriers. Among the four obtained MOFs, Ni2(adc)2(dabco) possessed the highest surface area, 772 m2/g, highest ammonia uptake capacity, 12.1 mmol/g, and stable cyclic adsorption-desorption performance. All the obtained MOFs demonstrated physisorption of ammonia and rapid kinetics of ammonia adsorption and desorption. Compared with halide ammonia carrier MgCl2, the obtained MOFs showed four times faster adsorption kinetics to reach 90% of the ammonia uptake capacity. For the ammonia desorption, the Ni2(adc)2(dabco) provided 6 mmol/g ammonia dosing when temperature reached 125 °C in the first 10 min, which was six times of the ammonia dosing from Mg(NH3)6Cl2. The results offer a solution to shorten the buffering time for ammonia dosing in the SCR system.