Biomass-Waste-Derived Silica for Solvent-Lean Cu-SSZ-13 Porous Catalysts in NH₃-SCR NO_x Remediation

The development of biomass-waste-derived functional porous materials can support resource-conscious strategies for environmental remediation. In this study, Cu-SSZ-13 porous catalysts were prepared from rice husk ash as a biomass-derived silica source and metakaolin as the aluminosilicate precursor using solvent-lean vapor-assisted crystallization under OSDA-free or reduced-OSDA conditions. The S1-S8 catalyst matrix was prepared at nominal Si/Al ratios of 12 and 25, with crystallization conducted in the absence or presence of Cu, followed by a common post-synthetic Cu ion-exchange procedure. XRD showed predominant CHA formation across the matrix, while ²⁷Al and ²⁹Si MAS NMR confirmed predominantly tetrahedral Al environments and systematic differences in the silicate-framework response. Final Cu contents ranged from 1.50 to 2.61 wt%, with Cu/Al ratios of 0.22-0.27, and samples crystallized in the presence of Cu showed a consistent tendency toward slightly higher final Cu contents than their paired Cu-free-crystallization counterparts. The fresh catalysts exhibited substantial NH₃-SCR activity, with apparent T₅₀ values of 201.3-278.4 °C and maximum NO_x conversions of 89.9-97.0%; S4 showed the earliest light-off. Accelerated hydrothermal aging shifted T₅₀ by 32.4-65.3 °C while retaining 91.1-94.9% of the fresh maximum NO_x-conversion response. Estimated N₂ selectivity remained high throughout most of the principal activity region, whereas relative N₂O formation remained limited and increased mainly at elevated temperature. Overall, the results demonstrate that a rice-husk-ash/metakaolin precursor platform can be converted into functional CHA-type Cu-SSZ-13 porous catalysts for NO_x remediation using OSDA-free or reduced-OSDA, solvent-lean synthesis while retaining substantial catalytic functionality after accelerated hydrothermal exposure.