For the preparation of amines in organic synthesis

For the preparation of amines in organic synthesis, reduction of organic nitro compounds is one of the most important reaction. There is a large number of important applications that these amines possess in the preparation of many chemicals such as dyes, antioxidants, medicines and agrochemicals. There are number of methods to reduce the aromatic nitro compounds, such as sodium borohydride (NaBH4) in the presence of metal nanoparticles. We cannot use this catalyst directly under normal conditions instead, we have to use transition metal halides to reduce aromatic nitro compounds completely. Rate of reduction can be increased by using bimetallic system as Pd-Ni or Pd-Ag. In the field of catalyst research, heterogeneous catalysis got a great importance as with increase in the importance of green chemistry in daily life. By applying a number of heterogeneous catalysts we can select the best combination for specific reaction.
As the development in the field of nanotechnology introduce such nanomaterials which have properties depending on size and shape. The nanomaterials having the properties of variation in band gap with size are known as quantum dots. These quantum dots become good visible-light photocatalysts by doping some heteroatoms (like B N S and F). {Sajjad, 2016 #1}. Nitrobenzene is converted to aniline by using liquid phase hydrogenation in the existence of a Ni-based catalysts or noble metal- based catalysts. The properties of high activity and low price make the Ni-based catalysts important whereas high temperature and high pressure is required. The catalytic activity of Ni-based catalysts is affected by crystallite size and degree of dispersion of Ni. To get the best catalytic activity it is valuable to enhance the argument of dispersion of Ni on the surface of a catalyst. if carbon is used as support then silica, increase in the reduction of nitrobenzene is observed. {Yingmin, 2017 #2}.
In the photoreduction of nitrobenzene, the noble metals with small size and tractable morphology are extensively used. But selectivity and catalytic activity can be reduced by lowering the size of metal particles, which results in a rise in surface energy. The reducing potential of a catalyst can be declined by using the long chain molecule on the surface of noble metal nanoparticle because it can reduce its ability to gain or loss the electron. So a supporting substrate is used to improve this problem. It is observed that the catalytic activity can be affected by the nature of substrate selected. It is also be noted that layer of noble metal remains if we use the sp2 hybrid carbon as the supporting substrate.
Graphene substrate:
Grapheme is considered as an ideal supporting substrate used for Pd, Pt and Au nanoparticles. It is a new carbon allotrope and have unique properties as large surface area, high ability to carry a charge, ultrathin thickness and large thermal conductivity. {Bocheng, 2016 #3}. It is also valuable due to its property of better resistance to photobleaching, less toxic, cheap and easily available. To boost the electrical and optical properties of graphene quantum dots (GQDs), doping of some hetero atoms such as B, N, F and S is applied. If we use N as doping material, then due to similarity in size with carbon, it is very fruitful for its electronic modification. It is also observed that N-GQDs is also a better supporting substrate for the reduction of nitro compounds in catalytic reduction using Pd as catalyst. Cellulose is used as green support for the recycling of N-GQDs. There is also another unique property exists in N-GQDS is photoluminescence behavior.{Sajjad, 2016 #1}.At room temperature, for the reduction of nitrobenzene, reduced graphene oxide (rGO) is used. It is observed that nanoparticles attached with GQDs are important to increase the catalytic activity and stability of a catalyst.{Jiali, 2017 #4}
CdS Quantum dots:
The semiconductors having the direct band gap are used in photocatalysis in visible range as heterogeneous catalysis. CdS is a good example of photocatalysts. It reduce the nitrobenzene to aniline by six photoinduced electron-proton couple transfer in a sequenced array. With no change in catalytic activity at pH 3.6 – 4.3, it gives the internal quantum yield of 37.1%.{Stephen, 2016 #5}.
PbBiO2X NPs:
It is another method to reduce nitroarenes with blue light. When illuminated with blue light by high power LED or sunlight, these give the effective heterogeneous photoreduction of nitrobenzene. Because of transfer of six equivalents of hydrogen, reduction of nitrobenzene derivative was selected to measure the catalytic activity and efficiency of photocatalysts.{Stefan, 2011 #8}
TiO2 NPs:
Nanosized particles of TiO2 are the important photocatalyst with major properties of high surface area and low recombination rate of photogenerated electron-hole pairs. With noble metals it become a type of interesting nanocomposite due to the property of metal to increase the charge separation and transportation.{Bocheng, 2016 #3}
In water medium the Ni-based TiO2 is used to reduce nitrobenzene and production of aniline and other important chemicals used in the manufacturing of dyes agrochemicals and pharmaceutics.

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