Synthesis of zeolites by twin screw extrusion and CALF-20-based porous liquids for biogas upgrading

The study also presents the first solvent-free synthesis of organic-functionalized SOD phases and identifies key parameters for achieving high crystallinity in closed steel vessels. Although the reaction between NaSiO3·5H2O and Al(OAc)2OH proceeds rapidly within 0.5-4 h under closed vessel conditions, attempts to apply twin screw extrusion (TSE) were unsuccessful, which is primarily due to the rapid dehydration of NaSiO3·5H2O.

Based on these findings, optimal conditions for zeolite synthesis via TSE should include open-system compatibility without water loss, rapid reaction within 30 minutes, low erosivity and corrosivity toward steel, and controlled dehydration to prevent hard or sticky by-products. In Chapter 3, a continuous, scalable, and low-solvent method for the synthesis of sodalite (SOD) and Linde type-A (LTA) zeolite using TSE is reported.

The synthesis method has been developed with economical starting materials including metakaolin (MTK, Al2Si2O7) and NaOH. In terms of the reaction characteristics, a less corrosive, low hardness procedure has been verified as suitable for TSE. Using TSE, sodalite is obtained quantitatively and continuously and the obtained product only requires a simple water washing step to remove the excess amount of NaOH.

A discussion on porous liquids and their potential for CO2/CH4 separation is presented in Chapter 4. Chapter 5 presents a novel Type III porous liquid (T3PL) composed of the metalorganic framework (MOF) CALF-20 dispersed in Genosorb® 1843, developed for biogas upgrading applications. The resulting T3PLs exhibit low viscosity (4.88–8.16 mPa·S), making it suitable for chemical engineering applications.

Although the suspension exhibited low stability to sedimentation, with complete settling occurring within 1–2 hours, CALF-20 could be readily redispersed by simple manual stirring. Single gas adsorption measurements show II that the CO2 and CH4 uptakes of CALF-20 12.5wt% T3PL are high (0.99 mmol·g-1 for CO2 and 0.31 mmol·g-1 for CH4 compared with Genosorb® 1843 (0.61 mmol·g-1 for CO2 and 0.10 mmol·g-1 for CH4) at 5 bar and 25 °C when using single gases.

The mixed gas CO2/CH4 (35/65, 50/50, 60/40) competitive adsorption demonstrates high selectivity (from 5–351, depending on conditions). Additionally, benefiting from the water-resistant properties of CALF-20 and the hydrophobic nature of Genosorb® 1843, the CALF-20 PLs are water tolerant. Water tolerance was investigated using a continuous flow gas uptake system to simulate the biogas upgrading process using a 35/65 mixture of CO2 and CH4.

Since biogas contains moisture unless it is pre-dried, 1wt% water was added to the CALF-20 12.5wt% PL as a representative case, equivalent to 19–96 hours of water accumulation when online, depending on biogas humidity. Under these conditions, the T3PL showed enhanced CO2 capture (98.50% vs. 95.37%) although with a slightly higher CH4 loss (6.25% vs. 4.46%) compared to the dry CALF-20 12.5wt% PL. The outlet stream contains 98.72% CH4 and 1.27% CO2, demonstrating effective CO2 capture and good water tolerance.

These results suggest that CALF-20-based PLs are promising for application in biogas upgrading.<br><br><i>Thesis is embargoed until 31st December 2028</i>