Abstract
Background: Chemical industry has increased the investment into and innovation capacity to supply chemicals from safe and sustainable sources, which will be essential to offering new solutions and supporting the green transition of the global economy and society. In this sense, the use of green solvents and reusable heterogeneous catalysts has emerged as a promising sustainable process strategy for engineering, chemistry and the environment. In this work, different homogeneous (copper bromide, CuBr and copper(II) acetate, Cu (CH3COO)2·H2O) and heterogeneous (Cu Wire, Cu Plate, Cu/β-SiC, pre-treated Cu Wire and pre-treated Cu Plate) copper catalysts were tested for the copper(I)-catalyzed alkyne–azide cycloaddition (CuAAC) reaction. In addition, the influence of different reaction media was analyzed, comparing the use of an organic solvent such as toluene and a green solvent such as supercritical CO2 (scCO2). Methods: Characterization of the catalysts includes by X-ray diffraction (XRD), Scan Electron Microscopy (SEM), Atomic absorption spectrophotometry (AA) and Temperature Programmed Reduction (TPR). Parameters such as catalyst loading, reaction time, reusability and leaching of the catalysts were studied to obtain more information on the CuAAC reaction in scCO2. Results: The pre-treated copper plate achieved a 57% increase in reaction yield compared to the non pre-treated copper plate. However, the recovery and reuse of the pre-treated copper plate showed a severe deterioration and a considerable change in its surface. Cu Wire (without pre-treatment) achieved yields of up to 94.2% after reusing it for five cycles. Conclusions: These results suggest the possibility to exploit the combination of heterogeneous catalysts and scCO2 and justify further research to highlight green solvents and simultaneously address the challenges of reaction, purification and recycling.
Highlights
Cycloaddition using azides and alkynes is an important method for the synthesis of 1,2,3 triazoles, which was firstly reported by Huisgen et al in 1960 [1]
All reactions were monitored by Gel permeation chromatography (GPC), from which the rate of loss of starting reagents was determined and yields of the reaction for each particular CuAAC reaction were calculated based on the normalized weight distribution form GPC traces (GPCs are included in the Supplementary Material, Figures S4–S7)
The CuAAC reaction has been widely used in the synthesis of polymers
Summary
Cycloaddition using azides and alkynes is an important method for the synthesis of 1,2,3 triazoles, which was firstly reported by Huisgen et al in 1960 [1]. Optimal conditions of pressure and temperature of scCO2 for the CuAAC reaction were fixed after the previous optimization study [35] Both homogeneous and heterogeneous catalysts were considered for this work. As the reaction was not carried out within 48 h, the amount of catalyst was not further increased due to the difficulty in purification This fact indicates that scCO2 at 35 ◦C and 13 MPa leads to the decomposition of Cu(CH3COO)2·H2O to different copper species with different oxidation states, such as Cu, Cu2O and CuO, including the catalytically active species Cu(I) in CuAAC that arises from Cu2O. 2 and the formation of Cu(I) from metal copper is due to the unfilled valences of the surface atom Both heterogeneous catalysts used in the CuAAC reaction, in scCO2 and in the absence of a ligand showed good catalytic behavior. It can be concluded that the three heterogeneous catalysts considered and scCO2 as the reaction media offer a very promising result in the CuAAC model reaction
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