Calcium/strontium chloride impregnated zeolite A and X granules as optimized ammonia sorbents

Calcium chloride (CaCl2) impregnated zeolite A and strontium chloride (SrCl2) impregnated zeolite A and X composite granules were evaluated as ammonia sorbents for automotive selective catalytic reduction systems. The SrCl2-impregnated zeolite A granules showed a 14% increase in ammonia uptake capacity (8.39 mmol g−1) compared to zeolite A granules (7.38 mmol g−1). Furthermore, composite granules showed 243% faster kinetics of ammonia sorption (0.24 mmol g−1 min−1) compared to SrCl2 (0.07 mmol g−1 min−1) in the first 20 min. The composite CaCl2/SrCl2 impregnated zeolite A granules combined the advantages of the zeolites and CaCl2/SrCl2, where the rapid physisorption from zeolites can reduce the ammonia loading and release time, and chemisorption from the CaCl2/SrCl2 offers abundant ammonia capacity. Moreover, by optimizing the content of SrCl2 loading, the composite granules maintained the granular form with a crushing load of 17 N per granule after ammonia sorption–desorption cycles. Such structurally stable composite sorbents offer an opportunity for fast ammonia loading/release in automotive selective catalytic reduction systems.


Electronic Supplementary Information (ESI) S0: Safety protocol on ammonia operation and experiments
The equipment and operation involved ammonia followed lab safety protocol. The connection of the ammonia cylinder to the simultaneous IsoSORP ® sorption analyzer (TA Instruments, United States) was set up with ammonia corrosion-resistant O-rings and stainless-steel pipelines. The vacuum pump for the IsoSORP ® sorption analyzer was also ammonia-resistant (VA MD 1C, Vacuubrand, Germany). All the ammonia exhaust from the experiments was conducted directly to the ventilation system by pipelines and the extraction arm. The measurement setup and operation were checked and monitored by an ammonia gas detector (GAXT-A-DL, Honeywell, UK) with a sensitivity of 1 ppm.

S1: Ion-exchange process
To avoid the unexpected salt crystal produced in the impregnation process, the NaX and CaA zeolite granules were first ion-exchanged with Sr 2+ . Based on the previously reported ionexchange process, 1,2 the ion-exchange process was carried out by loading zeolite granules into Electronic Supplementary Material (ESI) for RSC Advances. This journal is © The Royal Society of Chemistry 2022 SrCl 2 solution with stirring. The magnet was put in the center bottom of the beak with a low stirring rate at 50 rpm to avoid breaking the granules. The concentration of the SrCl 2 solution and the bath time were investigated. The concentration of the SrCl 2 solution was tested at 0.14 g mL -1 , 0.27 g mL -1 , 0.40 g mL -1 , and 0.54 g mL -1 . As shown in Figure S1, at 0.54 g mL -1 , cracks were observed in both zeolite granules. By reducing the SrCl 2 concentration, the SrCl 2 solution was optimized at 0.40 g mL -1 for zeolite X and 0.27 g mL -1 for zeolite A.

Figure S1 Ion-exchanged zeolite A and X granules with SrCl 2 solution at different concentration
The Sr atomic percentage in the ion-exchanged granules was measured with scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDS, JSM-IT300LV, JEOL GmbH, Germany) with 5 granules for each type of zeolite to achieve statistic reliability. 30 min was regarded as one batch time for ion exchange. The repeat times of ion exchange (replace with new SrCl 2 solution) were tested at 1, 2, 3, 6, and 12 cycles. From the Sr atomic percentage shown in Figure S2, we noticed that the Sr increased by 50% when comparing 1 cycle (8 at.%) to 3 cycles (12 at.%), while maintaining relatively stable at 12 cycles (14 at.%). Therefore, the ion-exchange time was repeated 3 times with 30 min each time.

S2: Impregnation process
The AEMHs loading in the final composite is calculated based on the mass fraction according to Equation (S1), where , the mass fraction of AEMHs; , the mass of the AEMHs; , the mass of zeolite.
The SrCl 2 loading can be adjusted by dripping different amounts of SrCl 2 solution into the granules.
For the zeolite granule X, the 0.54 g ml -1 SrCl 2 solution was used, and 0.27 g ml -1 SrCl 2 solution was used for zeolite A as verified in section S1. As shown in Figure S3, by dripping 1.5 mL 0.54 g ml -1 SrCl 2 solution into 1 g ion-exchanged zeolite X, we obtained 0.81 g SrCl 2 loading (45 wt%) in Sr_X. For Sr_A, 1 mL 0.27 g ml -1 SrCl 2 solution into 1 g ion-exchanged zeolite A. Figure S3 The impregnated zeolite X granules with different SrCl 2 loading

S4: Estimation of the SrCl 2 loading after ammonia sorption and sieving
The ammonia uptake capacity of the impregnated granules has been contributed from two parts, the SrCl 2, and the zeolite. If we assume the ammonia uptake capacity of the impregnated granules ( ) is composed proportional to the SrCl 2 ( ) and ammonia uptake capacity of the zeolite 2 ( ) and according to their corresponding mass fraction ( and , where 2 ) in equation (S2), we can calculate the actual SrCl 2 loading ( ) based on the 2 + = 1 2 measured , 46.97 mmol g -1 , , 9.44 mmol g -1 and 7.15 mmol g -1 for ion-exchanged X 2 and A, respectively, yielding 4 wt% and 3 wt% SrCl2 loading in Sr_X and Sr_A after removing the falling and loose salts on the granule surface.

S5: Isobaric curves of ammonia desorption by temperature swing adsorption (TSA) method
To mimic the current practical ammonia desorption process, the ammonia desorption performance of the materials was characterized by the temperature swing adsorption (TSA) method. 3 The saturated ammonia sorbents were maintained at a 3 bar ammonia atmosphere, and the temperature of the reaction chamber was increased to 120 °C by the electrical heater to achieve the fast ramp speed in the machine (~3 °C min -1 ). As shown in Figure S5(a), in the first 10 min, zeolite X released 7.1% ammonia (0.69 mmol g -1 ), which is around 4 times higher than 0.3% in SrCl 2 (0.14 mmol g -1 ). The Sr_X after 2-cycle was observed to have faster kinetics in the ammonia sorption kinetics before the temperature reaches the Sr(NH 3 ) 8 Cl 2 decomposition temperature of 60 °C as shown in the blue regime in Figure S5(b). After the temperature surpasses 60 °C, each Sr(NH 3 ) 8 Cl 2 releases 7 ammonia molecules (87.5%), giving abundant ammonia dosing. Figure S5 The isobaric ammonia sorption percentage curves in the ammonia desorption at 3 bar, with temperature increasing from room temperature (~20 °C) to 120 °C. (a) ammonia sorption percentage of SrCl 2 and zeolite X in the first 10 min before the temperature reaches 60 °C, (b) ammonia sorption percentage of SrCl 2 , zeolite X, and Sr_X after 2-cycle test, the blue region below 60 °C, the orange region above 60°C.

S6: Structural stability of Sr_X after 10 cycles of ammonia sorption and desorption
After removing the detached SrCl 2 from the first ammonia test, some cracks were found in the zeolite granule. By repeating 10 more cycles after that, no obvious extra cracks were found, as shown in Figure S6, suggesting good cyclic structural stability.