Cold Sintering of Zeolite 13X: Water versus Sodium Hydroxide Pathways to 99% Densification and Porosity Retention

This investigation demonstrates the cold sintering process (CSP) enabling remarkable densification of microporous zeolite 13X while preserving intrinsic structural integrity. Using sequential one-factor-at-a-time optimization, both H₂O- and NaOH-assisted CSP pathways are comprehensively evaluated through XRD, TGA, SEM, HR-TEM, BET analysis, and statistical validation. Water-assisted CSP (10 wt.%, 350 MPa, 220 °C, 90 min, 5 °C min⁻¹) achieves 82.1 ± 1% relative density via controlled dissolution–reprecipitation mechanisms. Heckel analysis reveals yield pressures (P_y) = 10.93–12.20 MPa (R² > 0.97), confirming a nonlinear deformation behavior. Hierarchical porosity is successfully maintained (BET surface area = 559.11 m² g⁻¹; micropore volume = 0.28 cm³ g⁻¹). NaOH-assisted CSP (5 M NaOH, 25 wt.%, 350 MPa, 200 °C, 60 min, 5 °C min⁻¹) achieves exceptional densification of 99.1 ± 0.5% (geometric) and 98.7 ± 0.5% (Archimedes) relative density. Enhanced nonlinear densification is evidenced by reduced P_y values (1.77–2.17 MPa, R² > 0.98). Despite near-theoretical densification, intrinsic porosity remains intact (BET = 422.01 m² g⁻¹; micropore volume = 0.21 cm³ g⁻¹). Statistical ANOVA analysis confirms excellent reproducibility (F < 1 × 10⁻⁵, p = 0.99) and significant pressure dependency. Both pathways preserve crystallographic integrity and thermal stability to 800 °C, achieving ≈80–90% energy reduction compared to conventional sintering, demonstrating CSP's potential for sustainable ceramic processing.