Catalysts play a significant role in reduction processes and a low-cost nanocomposite catalyst containing copper oxide on graphene oxide support was made by a hydrothermal self-assembly process which was used for reduction of nitroaromatics. The reduction was carried in aqueous NaBH4 solution.
Nitroaromatics (NA) are reduced to produced amino-aromatics (AA). Several NAs are toxic in nature and cause environmental pollution and hence they are converted into AAs which serve as intermediates for accessing pharmaceuticals, pesticides, cosmetics, agrochemicals and dyes.
Reduction is carried via catalysts and nanoparticles have shown to provide better efficiency of catalytic systems. For a greener catalytic process, it is desirable to develop catalysts that can work in aqueous media and avoid the use of volatile organic solvents. Sodium borohydride (NaBH4) is one such water-soluble reductant and hence this was used by the author for the reduction process whereby the electrons from BH4- transfer to NA when the species is adsorbed on the surface of the catalyst.
Several metal nanoparticles produce nanocomposite catalyst for reduction. Heterogenous catalyst usually contains precious metal nano-catalyst like Pt, Pd, Rh etc. But their use is restricted due their high cost.
Catalysts can be monometallic where metals having diameters in the nanometer range shows better catalytic performance due to high surface area. This was also verified in Kaur et al. where he demonstrated reduction of NAs by using silver nano-catalysts. They are usually noble and transition metal nanoparticles and are directly employed without the need of any support. Several other nano-catalysts are bimetallic as they exhibit better activity due to synergetic effect which is indicated by the volcano shaped relationship due to the electronic interactions of the atomic orbitals of different metals.
The metallic source having higher activity is capable of transferring its extra electron from its outer orbital to the metallic atom of lower activity. This facilitates electron transfer from the adsorbed BH4- to the NA, thereby enhancing the reduction process. While there can be solid supported nanoparticles which provide vast surface area and it also inhibits the aggregation of active catalysts as compared to the unsupported nano-catalysts. Graphene is one such support which is extensively used as it easily facilitates the transfer of electrons from the reductants to the reaction media enhancing the reduction efficiency; this can be attributed to its conductivity. Hence this develops a synergetic effect between graphene and the nano-catalysts to result into a highly active hybrid nanocomposite catalyst. Due to the above-mentioned advantages of graphene it was used by the author to make a hybrid nanocomposite catalyst using copper oxide as it is cost effective, environment-friendly as well as provides good yield.
The main reagents and catalysts were purchased and made by following various techniques. Graphene oxide was synthesized from graphite using hummer’s process and water was deionized by nano pure system. CuO-GO nanocomposite catalyst was synthesized by a facile hydrothermal self-assembly process wherein CuCl2 was dissolved in deionized water and mixed with the previously made GO solution and then transferred to a Teflon-lined container which was filled with deionized water and heated up to 1500C for 12h. Then the product was gradually cooled and washed with deionized water and filtered to remove the loosely bound cooper oxide nanoparticles on the GO support. The reduction of NA to AA was carried at room temperature using the prepared nanocomposite catalyst with aqueous NaBH4. In the process the NA and NaBH4 was added to a dispersed solution (in deionized water) of catalyst and was stirred at room temperature for 30 mins and this marked the completion of the reaction after which the nano-catalyst was separated by a centrifuge and the AA product was measured.
Several physical methods were used for obtaining the results. Xray Diffraction was used to study the nanostructure of nano-catalyst wherein the peaks were well indexed with that of GO and CuO. The composition of the catalyst was analyzed by Raman technology and Fourier Transform-Infrared Radiation Spectroscopy whereby the peaks at 3450cm-1 indicates presence of water in GO and also the peaks for C=O (1725cm-1) and C=C( 1600cm-1) stretching disappeared in CuO-GO which are otherwise characteristic band of GO indicating the reduction of GO during the process.
XRD diffraction pattern, b Raman spectrum, c TGA analysis, and d FT-IR spectrum of the CuO–GO nanocomposite catalyst (Zhang et al.) High resolution transmission electron microscope and Fast Fourier transform images also showed the immobilization of homogenously confined and crystalline copper oxide nano-particles on GO support.
TEM and STEM images of-a, d GO. b, e CuO–GO nanocomposite catalyst. c HRTEM image of the CuO–GO nanocomposite catalyst. f EDX spectrum of the CuO–GO nanocomposite catalyst (Zhang et al.) The existence of Cu, O elements on GO support was proved by Energy Dispersive X-ray spectroscopy. Thermal gravimetric analysis was used to check the thermal stability of the catalyst while Gas chromatography-mass spectrometry was used to measure the yield of AA ie. it measured the conversion ratio of NA to AA. X-ray photoelectron spectroscopy was performed using an Al kα source to confirm the surface composition and the presence of Cu, O and C elements in the nanocomposite catalyst.
XPS analysis a survey scan, b C 1s, c Cu 2p, and d O 1s for the CuO–GO nanocomposite catalyst (Zhang et.al) The author first carried out the reduction on 4-nitrobenzene which gave a high yield of 4-aminobenzene. The reduction reaction was quite feasible with the prepared nanocomposite catalyst since selective reduction of a nitro moiety in the presence of another reducible functional group is difficult but the prepared nanocomposite catalyst could easily drive the reaction forward.
Reduction of Nitroaromatics using CuO-GO nanocomposite Catalyst in aqueous Media (Zhang et al.) However, the catalytic activity of CuO-GO nanocomposite catalyst was less than other heterogenous catalyst having noble metals like Pt, Pd etc. As reported in Chen et al. graphene oxide supported CdS hybrid photosensitive catalyst demonstrated a high reductive activity due to the high transfer of electrons for the reduction of NA to AA. Similarly, Li and his research group reported 96% reduction rate using mesoporous carbon nitride supported Au nanoparticles and Bhowmik et al. reported that the reduction of 4-nitrophenol to 4-aminophenol by carbon nitride supported ultrasmall Au nanoparticles was very fast and showed excellent catalytic activity and good stability. However, considering the reusability of the CuO-GO catalyst because of which it could be reused for six consecutive times and low price of copper, a competent, competitive and efficient catalytic activity was observed.
Catalysts with high specific surface area and reusability serve promising catalytic efficiency. CuO-GO nanocomposite catalyst formed by a cost-effective and facile hydrothermal self-assembly process proved to have excellent reduction performance giving high yields which is attributed to the synergetic effect. Moreover, it can be recycled up to six times. This makes it a promising heterogenous catalyst for future reduction of NAs in large scale where these properties are necessitated. However, the synthesis of sustainable nano-catalysts needs further research and innovation to increase its catalytic activity, yield and durability maintaining its cost effectiveness. More research on morphology dependent nano-catalysis can be done since reaction performance gets altered for catalyst particles with an anisotropic shape by selectively exposing specific crystal facets.