Synthesis of cyclophosphazene bridged mesoporous organosilicas for CO2 capture and Cr (VI) removal

Abstract

Cyclophosphazene bridged mesoporous organosilicas (CPMOs) were synthesized by condensation of (3-aminopropyl)triethoxysilane (APTES) and phosphonitrilic chloride trimer (PNC) using cetyltrimethylammonium bromide (CTABr) as structure directing and tetraethyl orthosilicate (TEOS) as co-condensing agents under basic condition. Five samples were prepared by varying the molar ratios of APTES:TEOS (1:0, 1:2, 1:3, 1:4 and 1:8). The specific surface area of the CPMOs varies from 34 to 1034 m2 g−1 and pore volume between 0.05 and 0.99 cm3 g−1, depending upon the APTES:TEOS ratios. The CPMO-8 show maximum CO2 uptake of 2.19 mmol g−1 at 273 K. Adsorption potential of the CPMOs for Cr (VI) removal in the aqueous solution was investigated by varying contact time, pH, initial metal ion concentration and temperature. The equilibrium data were analyzed using the Langmuir and Freundlich isotherm by linear regression analysis. In addition, the kinetic analysis revealed that the overall adsorption process was successfully fitted with the pseudo-second-order kinetic model. The maximum Cr (VI) adsorption capacity of 99.24 mg g−1 was observed for the CPMO-4. The thermodynamics studies indicate a spontaneous exothermic process with negative ΔG, and ΔH of −1.808 kJ mol−1. The ΔS was calculated to be 10.718 J mol−1 K−1.