Catalysis, which provides sustainable, economical, and efficient ways to convert raw materials into valuable chemicals and fuels, is essential to the development of modern society. With the aim to avoid the use of volatile organic solvents, toxic reagents, hazardous, and/or harsh reaction conditions as well as challenging and time-consuming wasteful separations, greener and environmentally benign catalytic protocols have recently become more popular. In this regard, the development of nanoscience, undreamt of a century ago, has made the greening of chemistry possible.
The seemingly magical properties of nanoparticles have been unknowingly utilized for centuries. Early uses of nanostructured materials include the third-century Lycurgus Cup,1 made up of dichroic glass of gold and silver nanoparticles, which makes the cup look opaque green when lit from the outside and glowing red when lit from the inside. Similar shiny and sparkling ceramic glazes, which were used in the Islamic world between the ninth and seventeenth centuries,2 also contained various metallic nanoparticles. The qualities of “Damascus” saber blades, used from 300 BCE to 1700 CE, including their excellent strength, toughness, resistance to shattering, and the ability to remain extremely sharp, came from their construction out of carbon nanotubes and cementite nanowires.3
We use huge varieties of nanomaterials in our daily lives,4 and they are now being utilized in the field of catalysis. As is often said, “nature makes and chemistry reshapes.” Nanocatalysis is becoming an important part of nanoscience.5 Moreover, nanocatalysis bridges homogeneous catalysis, in which catalytic reactions usually proceed at a single metal site from reactants to products, and heterogeneous catalysis, in which microscopic powders are used. Even in the century-old Haber–Bosch process for ammonia synthesis,6 iron nanoparticles were already present.7 It also in the beginning of the twentieth century that Ostwald’s simple demonstration indicated the considerable increase of surface upon dividing cubes and consequences for surface-dependent catalysis.
Nanocatalysts are extremely structure sensitive and their catalytic efficiency and selectivity dramatically depend on the size, shape, and composition of the nanoparticle as well as the support material; as demonstrated by the largely unexpected discovery that gold nanoparticles smaller than 5 nm were very active catalysts even at sub-ambient temperatures.8,9 We now know the benefit of the increased surface-to-volume ratio of nanocatalysts as well as accessibility of specific sites (e.g., steps, edges, and corners) of catalytic nanoparticles.10, 11 The approaches to nanocatalysts are multiple and extremely varied from supported to unsupported nanocrystals of metals, metal oxides, and others. Nanocatalytic reaction mechanisms are complex and not yet well understood,12 however, which makes this science exciting.
Modern directions that take into account the greenness of the nanocatalysts include magnetic nanocatalyst recovery,14 the use of ionic liquids, the mix of metal nanoparticles15 in core–shell bimetallic nanoparticles and alloys, the derivatization of electrodes with nanocatalysts for improved redox catalysis for energy-relevant applications, the encapsulation of nanoparticles in solid zeolite-type cavities, such as molecular organic frameworks, and morphologically controlled synthesis of metals and metal oxide nanoparticles with varied shapes.16
This special issue is devoted to development of sustainable and green catalytic protocols using nanochemistry, what we call “Green Chemistry by Nanocatalysis”. The issue includes two Reviews, two Minireviews, one Communication and seven Full Papers, all of which were invited contributions by experts in the field. The first article is a Review by Ranu et al. on use of copper-based nanocatalysts for carbon–carbon and carbon–heteroatom bond formation, both of which are used extensively in the chemical, material, and industrial communities. They have provided an exclusive account of the developments in this field and also their green perspective.
In the next article, Garcia and co-workers provide a critical Review on the use of metal nanoparticles for the Fenton reaction. The generation of hydroxyl radicals from hydrogen peroxide, known as the Fenton reaction, is one of the best ways to tackle organic pollutants, which are notorious sources of environmental pollution. They discuss a range of metal particles, both supported and unsupported, as Fenton catalysts and highlight their efficiency, stability and mechanism of action for degradation of organic pollutants.
The use of a catalyst is a central principle of green chemistry,17 but “nano” makes catalysis “greener”.18 In their Minireview, Kalidindi and Jagirdar explain how catalysis and nanoscience are linked and describe the recent trend in the use of nanomaterials for the development of various green catalytic protocols. They also discuss the use of nanocatalysts for clean energy applications, such as hydrogen generation and storage and fuel cell applications.
A more detailed discussion of the design and use of nanocatalysts for clean hydrogen production, especially from green feedstock, bioethanol, is provided by Bion, Duprez, and Epron in their Minireview. In addition to the basics of hydrogen production from ethanol, these authors review ways to design better nanocatalysts, both the active phase and the catalyst support and how their designs affect hydrogen production.
In their Communication, Polshettiwar and his coworkers discuss how their recently discovered fibrous nano-silica (KCC-1) can be an excellent support to design nano-palladium (Pd) catalyst. They found that KCC-1/Pd nanocatalysts are very active in Suzuki coupling reactions, even for chloroaromatics, with excellent stability, which they claim is due to the fibrous nature of the support that restricts particle growth or aggregation.
Chiral ammonium-coated rhodium nanoparticles were synthesized by Nowicki, Roucoux, and co-workers, as reported in their Full Paper. They prepared a family of optically active molecules based on N-methylephedrine, N-methylprolinol derivatives and then used them as a stabilizer as well as chiral inducers, to prepare rhodium nanoparticles. These particles showed good catalytic activity for hydrogenation reaction in green solvent, water.
Selective oxidation can be achieved without using expensive metal-based catalysts. Pham-Huu and colleagues report in their Full Paper on the use of nitrogen-doped carbon nanotubes for oxidation of noxious H2S gas. They also shaped the catalysts on the macroscopic scale using silicon carbide foam, which not only enhances the accessibly of the nanotubes to the reactants but also eases the handling of the nanocatalysts. This is clearly a green, sustainable and industry-ready catalytic system.
Ionic liquids are used to prepare stable gold and palladium nanoparticles, as described in the Full Paper by Banerjee, Theron, and Scott. The lack of use of any organic solvent or external stabilizer during the synthesis makes this protocol truly green. Their nanoparticle suspensions were found to be stable over several months and also showed good activity as well as recyclability in hydrogenation reactions.
Ionic liquids are also used in several fields, including for the synthesis of nanomaterials. Swadzba-Kwasny, Seddon, and co-workers report, in their Full Paper, an electrochemical synthesis of indium nanoparticles using haloindate ionic liquid. In their synthetic methods, the ionic liquid alone acts as the metal source, template, and stabilizer while the electron acts as the reducing agent. Interestingly, the size distribution and morphology of metal nanoparticles is dictated by the nature of the ionic-liquid cation.
Oxidation reactions are important processes for synthesis of chemical intermediates in the manufacture of high-tonnage commodities and high-value fine chemicals. Hutchings et al. report on the use of gold nanoparticles supported on cerium oxide for oxidation of benzyl alcohol. They used naturally abundant L-asparagine to produce cerium oxide foam and solvent-free conditions during catalysis, making this protocol green and sustainable. They also observed that the open structure of the support (oxide foam) enhances the mobility of the surface oxygen, increasing the overall activity of the nanocatalysts.
A method to stop aggregation and sintering is also reported by Wang, Biradar, and Asefa in their Full Paper. They designed highly stable supported Pd nanoparticles by creating hollow and porous zirconia shells around them. The resulting catalyst system demonstrated excellent activity for hydrogenation of olefins and nitro groups. These porous shells not only provide easy access of the Pd to the reactants but also stopped the Pd nanoparticles from growing, making the catalysts system active and stable.
Along a similar line, Gorte and co-workers developed a new synthetic protocol for a core–shell catalyst using a self-assembly technique. In their Full Paper, these authors report their study on the synthesis of various nanocomposites with metals (Pt, Pd) as core and oxides (ceria, zirconia, titania) as shell. Great enhancement in the stability of metal nanocatalysts due to the metal-oxide shells was observed.
As can be seen form the papers in this special issue, scientists around the globe are working very hard to discover a variety of nanocatalysts to develop green and sustainable protocols. There are large numbers of excellent publications coming out daily in this field, and it is now a “hot topic”. However, key challenges for the future involve not only improved efficiency and selectivity but also the recovery and re-use of the catalysts, the discovery of catalytic processes with abundant and cheap metals, the preferred use of non-toxic “bio”-metals, and, of course, the use of “green” solvents such as water. Fortunately, modern spectroscopy, microscopy and computational tools and techniques are being improved every day and becoming more commonly available in laboratories, which leads to optimistic promises for future developments and breakthroughs in this challenging field.