Effect of various metal-based halloysite nanotubes for the catalytic degradation of chitosan to low molecular weight chitosan

Abstract

This article describes the application of metal-based supported halloysite nanotube (HNT-M2+, M2+ = Ni2+ or Cu2+) catalysts as an alternative route for the degradation of chitosan (CS) into low molecular weight chitosan (LMWC). Results from scanning electron microscopy coupled with energy dispersive X-ray spectrometry (SEM/EDX) analysis reveals the surface morphology of the HNT-M2+ catalysts with certain amount of agglomeration, which may suggest the incorporation of the metal species into the HNT support. X-ray photoelectron spectroscopy (XPS) further clarifies that Ni(OH)2 or Cu(OH)2 and CuO exist as active species in the catalysts. X-ray diffraction (XRD) spectra reveal the presence of a low intensity Cu(OH)2 peak in the HNT-Cu2+ catalyst. The absence of other Cu2+ species in the HNT-Cu2+ catalysts is due to the peak overlap with HNT. The diffractogram of HNT-Ni2+ also shows only HNT peaks. The pH at zero point charge (pHzpc) of the HNT-M2+ catalysts at 3.8, which is slightly lower than the reaction mixture pH (~3.9), enhances the degradation of CS due to electrostatic attraction between the catalyst surface and CS. The crystallinity index (CrI), viscosity, [η] and viscosity average molecular weight, Mv of LMWC were lower than those of CS due to the chain scission of the polymer backbone and loss of crystallinity, thus accounting for their high water solubility. The results revealed that the HNT-Cu2+ catalyst exhibited a better catalytic performance than HNT-Ni2+ for the degradation of CS. This is probably due to the different active species available on HNT-Cu2+.