Chemisorption of CO2 by chitosan oligosaccharide/DMSO: organic carbamato–carbonato bond formation

Abstract

A newly formed bond of organic carbamato–carbonato emerged upon bubbling CO₂ in a low molecular weight chitosan hydrochloride oligosaccharide CS·HCl/DMSO binary mixture. The aforementioned bond was detected and confirmed using attenuated total reflectance-Fourier transform Infrared (ATR-FTIR) spectroscopy, with two prominent peaks at 1551 cm⁻¹ and 1709 cm⁻¹ corresponding to ionic organic alkylcarbonate (RCO₃⁻) and carbamate (RNH–CO₂⁻ NH₃⁺–R), respectively. ¹H–, ¹³C–, and ¹H–¹⁵N heteronuclear single quantum coherence (HSQC) NMR experiments were also employed. According to ¹³C NMR, two newly emerged peaks at 157.4 ppm and 161.5 ppm attributed for the carbonyl carbon within the sequestered species RCO₃⁻ and RNH–CO₂⁻ NH₃⁺–R, respectively. Upon CO₂ bubbling, cross peaks obtained from ¹H–¹⁵N HSQC at 84.7 and 6.8 ppm correlated to the ammonium counterpart chemical shift bound to the proton resonances. Volumetric uptake of CO₂ was measured using an ATR-FTIR autoclave equipped with a silicon probe. The equilibrium sorption capacity was 0.6 and 0.2 bars through the formation of RCO₃⁻ and RNH–CO₂⁻ NH₃⁺–R, respectively. Moreover, physisorption by the dried DMSO contributed to additional 0.4 bars. Density functional theory (DFT) calculations supported the occurrence of the suggested dual mechanisms and confirmed the formation of carbonate at C-6 of the glucosamine co-monomer. Moreover, CS·HCl/DMSO showed a slight impact on cell proliferation after 48 hours; this was a clear evidence of its non-toxicity. The biodegradation test revealed that a degradation of about 80% of CS·HCl/DMSO was achieved after 33 days; these results indicated that this method is suitable for green industry. CS·HCl/DMSO showed modest activities against Staphylococcus aureus and Escherichia coli. In addition, CS·HCl/DMSO demonstrated a significant antifungal activity against Aspergillus flavus in comparison with Fluconazole.