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Microwave irradiation technique: Green potent energy source for sustainable applications

Synthesis of organic and inorganic compounds is a foundational concept in the preparation of bioactive material for drug development as well as provision of materials for other applications in everyday life of man. Conventional synthetic approach by heating under reflux has been in use since antiquity. This usually take a longer time to complete, it is not void of release of toxic chemical and could lead to decomposition of envisaged compounds some time. In recent time, microwave assisted reaction have been adopted in synthesis of numerous compounds which are essential templates in many industrial processes and various application. This present review deals with the recent advances of microwave irradiation techniques as sustainable and ecofriendly means of preparing compounds of interest to man. Microwave assisted technique approach is noted to be remarkably useful towards the synthesis of bioactive materials for drug design, polymeric materials for composite matrix, donor molecules for corrosion inhibition, material for battery application, nanomaterials as perovskites for supercapacitance electrodes, high wavelength compounds as solar cell sensitizers, dye materials in textile industries among others. Thus, microwave irradiation is an unavoidable technique for the accelerated eco-friendly synthesis of organic compounds which must be well explored for the benefit of man and his environment.


Introduction
The introduction of microwave synthesis is widely-recognized as a major revolution in synthetic chemistry due to the challenges posed by the conventional heating method [1]. Nowadays, microwave irradiation is being utilized in form of energy origin for most reactions and has been reported severally to be efficient, very rapid, environmentally friendly and leading to the production of greater and purer yields of the desired end product. It is not only an improvement to the conventional heating method, but it also provides new synthetic possibilities in chemical transformations that could not be accessed previously, due to its ability to transfer energy directly to the reactants and even to some activated complexes of reactions before relaxation and conversion to the product [2].
Heat energy required for chemical reactions to take place is transferred by conduction in the conventional method of heating. Absorption of heat is done by the walls of the container, before being transferred to solvent mixture and then the reactants. This conductive method is slow and inefficient; depending on the conductivity of the vessel material and uniform heating is achieved slowly [3].
Microwave heating on the other hand is independent of the conductivity of the vessel as thermal energy quantization allows the release of such energy to reacting substances directly and instantaneously.
Zirconium carbide, an ultra-high temperature ceramic [13], having multiple industrial applications due to its combination of metallic and ceramic properties [14] have been synthesized through carbothermal reduction route of Zirconium Oxide (ZrO2) in the presence of carbon (C) at a temperature of 1300 o C for 3 hours using microwave irradiation as heat source, a major improvement to the previous synthetic routes used which required a temperature of 1750 o C for 8 hours [15]. Similarly, the kinetics for the solution synthesis of chromium trioxide nanoparticles which has high near-infrared (NIR) reflectivity (>40%) from Cr(acac)3 as well as HO-CH2CH2-OH and can serve as reflection for roof-top cooling has been significantly improved using microwave irradiation. The reaction was completed in 40-60 mins, with a high yield of about 95% [16].
Fe3O4 doped zirconia nanoparticles have been synthesized simply and effectively following the microwave assisted sol-gel route. The pure stable tetragonally shaped doped nanoparticles (ZrO2) were obtained using microwave irradiation with power at 500 W. advantageous to other techniques, resulting in the formation of nanopowders of high purity in very short time and relatively low calcination temperature [29].

Synthesis of Polymers
Detoxification of water from contaminants such as heavy metals is of paramount importance. Arsenic imprinted polymer which is able to efficiently remove arsenic (As 3+ ) from water has been synthesized with the aid of 1-vinyl imidazole as pre-concentrated ligand [30], [31]. Reduction of polymerization's period time by 10-folds was brought about in microwave technique, resulting in an overall reaction time reduction [30].
Microwave preparation of polymeric hybrid nanocomposites (PHNCs) of graphene as well as the oxygenated derivatives has been established to be highly efficient than traditional methods of heating [32]- [34]. Specific advantages of this microwave approach in the synthesis of polymeric hybrid nanocomposites include quick monomers' melting, rapid improvement of hybridization of graphene and polymeric materials together as well as exfoliative process leading to the effortless product distribution in the polymer matrices [35]. González-Morones and co-workers [35] were able to elucidate the various steps accountable for in situ polymerization of Nylon-6 using graphite oxide, leading to the formation of the hybrid. The reaction was rapid and occurred in less than 60 seconds.
Q. Cao and co-workers [36] developed fluorescent polymeric nanoparticles (FPN) which has medical applications [37]- [39] using microwave irradiation under 5 minutes, the resultant FPN showing low toxicity was synthesized in the absence of solvent and catalyst under mild and environmentally friendly microwave assisted conditions.
The microwave-assisted synthesis of Diels-Alder poly(phenylene); a potential proton-conductor has been compared with the traditional heating approach [40]. The microwave assisted synthesis was 24 times faster than the conventional heating approach, also leading to a 7% increase in the tensile strength, but a 38.6% reduction in the elongation at break [40]. This has highlighted the efficiency of microwave-assistance in the synthesis of Diel-Alder poly(phenylene)s [40].
The polymerization of Methyl acrylate using azobisisobutryonitrile as the initiator was done at various microwave powers of 200, 300 and 500 W, with a constant temperature of 52 o C [41]. The reaction rate enhancement was compared to the traditional heating method at the same temperature; the reaction rate enhancement by the microwave assisted synthesis was 138%, 220%, 275% respectively for the various microwave intensities, this different reaction rate is expected due to the increased heating rate at higher microwave power [41].

Synthesis of corrosion inhibitors
Corrosion has plagued the industrial sector and the world at large. The development for development of corrosion inhibitors is gaining immense attention. Microwave irradiation is been considered as a faster and more efficient route for the synthesis of corrosion inhibitors.
Hydrazones synthesized from thiophene have shown anticorrosion properties against mild steel at different concentrations of concentrated acid, with the hydrazone showing strong affinity for the metal surface and showing mixed inhibition properties, majorly cathodic corrosion mitigation [42]. The microwave-assisted synthesis was compared with traditional synthetic approaches. The microwave route formed greater yield of the product at a much faster rate [42]. Corrosion inhibition in sodium chloride solution was exhibited by lanthanum transition coating produced upon Mg-Li alloy in lanthanum nitrate medium using microwave irradiation. Conversion coating was compared to that which was investigated in ambient condition; it was discovered with help of X-ray diffractions and Scanning electron microscopy that the microwave radiation makes the coating more compact, complexed and dispersed, eventually protecting the Mg-Li alloy [43]. Microwave sintering has been reported to form better corrosion resistant composites when compared with the traditional radiative heating, with much reduced sintering time [44]. The microwave sintering produced better densification and microstructural characteristics. Austenitic stainless steel (316L) and yttrium aluminum garnet (YAG)-reinforced 316L composited was sintering using both microwave and the traditional approach, with the microwave sintered product having higher corrosion resistance at 1400 o C [45]. Similarly, Fe-Al2O3 nano-composites where sintered with the aid of microwave irradiation at 600 W for 15 mins and the corrosion study carried out using the Tafel Polarization approach over 3.5% sodium chloride. The sintering resulted in uniform distribution and improvement of wear property in direct relation to the amount of alumina nanoparticles sintered [46]. Microwave irradiation have been applied for uniform sintering on metal matrix composites which have shown better corrosion inhibition and shorter reaction time [47], [48] Microwave-assisted deposition has been used to coat magnesium (Mg) alloy with strontium doped hydroxyapatite so as to increase the corrosion opposition as well as the biological activity [49]. The deposition was done in 10 mins and the result gotten from the scanning electron microscopy (SEM) showed uniform and homogenous deposition of a double layered structure. Judging from the weight loss of the coated sample, the microwave-assisted coating could provide effective long-term corrosion inhibition for the magnesium alloy [49]. Microwave-assisted combusted method has also been utilized in the synthesis of zinc polyphosphate nano powders and the nanoparticles coated on mild steel showed very improved inhibition efficiency of 94% in chloride solution [50].

Synthesis of materials for battery application
Tin oxide (SnO2) having flower-likely in it morphological feature and a rutile phase has been fabricated utilizing microwave-hydrothermal technique in the absence of surfactant. The conductivity of the tin oxide samples evaluated at room temperature has a conductivity of 1.11 x 10 -7 S cm -1 , having specific discharge capacity of 457 mAh g -1 after 50 cycles, that is noted to be better compared to graphite used in commercially available lithium ion battery [51]- [53], also having a current density of 100 mA g -1 . SnO2 flowers are potentially more acceptable anodic substrate Li-ion battery, having longer shelf-life than graphite used commercially for lithium ion [54]. Similarly, novel SnO2/ordered mesoporous carbon nanosphere (OMCS) for anode material has been synthesized using the microwave-assisted hydrothermal technique. The nanostructure showed outstanding reversible capacity of 1770 mAh g -1 at 35 mA g -1 initially, but maintaining energy capacity of 400 mAh g -1 at an ultra-high density of 3500 mAg -1 after 400 cycles. OMCS nanospheres supply enhanced mechanical and conductive stable quality SnO2 nanomaterials to limit expansion volume following delithiation-lithiation for 75 cycles [55].
Cathodic materials are crucial in evaluating the efficiency and safety of lithium ion batteries, with lithium cobalt oxide taking the lead as the dominant material used in cathode fabrication, although due to challenges such as low discharge capacity and safety problems associated with lithium cobalt oxide, the need for better cathodic materials are required [56]- [58]. Microwave irradiation is used as energy source for the rapid preparation process for encapsulated Li3V2(PO4)3, via carbon produced from starch as absorbing reagent. The synthesized Li3V2(PO4)3/C material possess outstanding specific capacities, possessing 131.1 mAh g -1 discharge capacity during the cycling rate of 0.5 C charge/discharge, retaining about 100% of the capacity over 250 cycles, making it an efficient material for fabrication of cathodes in lithium ion batteries (LIB) [59].
Microwave-assisted solid-state technique has been used for the facile synthesis of chevrel phase Cu2Mo6S8 in about 7 minutes as compared to the days required in the conventional route. The synthesized compound was tested for cathode material for Mg-battery, showing a reversible capacity exceeding 80 mAh/g, greater than the conventional solid-state methods [60].
Microwave-assisted solvothermal technique has been used for the simple one-pot preparation of SnC2O4/graphene nanocomposite in 30 mins. which is a promising anodic substance for Li-ion batteries. SnC2O4/graphene composite unveiled the discharge/charge capacity of 627/659 mAh g -1 upon completion of 100 cycles after density of 0.2 Ag -1 . Higher densities of the composite at 1.0 and 2.0 Ag -1 , the SnC2O4/graphene composite show good discharge/charge capacities of 553/552 and 418/414 mAh g -1 , respectively after 200 cycles in ambient temperature [61].

Synthesis for Fuel applications
Wang and co-workers [62] compared pyrolysis of soapstock in available and absence of microwave absorbent towards achieving hydrocarbon fuel. They reported the promising production of commercial fuel from pyrolysis of soapstock. The utilization of this absorbent led to a drastic rise in the alkane and aromatic hydrocarbon content. The microwave absorbent utilized for the synthesis is SiC, with optimal conditions found to be at a temperature of 550 o C, using 6 g/min as the feeding rate. These conditions led to synthesis of 64.74 wt.% of the hydrocarbon fuel, having a higher calorific value than biodiesel, however, the calorific value was lower for 0 # diesel; other parameters such as density and dynamic viscosity were comparable with 0 # diesel, having a better freezing point and cold filter plugging better than 0 # diesel.
Zhang and co-workers [63] reported novel pathway towards synthesis of jet fuel ranged cycloalkanes using lignocellulosic biomass as the precursor. The pathway warranted the adequate utilization of ZSM-5 in microwave-enhanced pyrolysis as well as homemade Raney 9® Ni catalyst giving optimal results, which lead reduction of hydrogen wastage. A 24.68% carbon aromatic yield was obtained which was converted to cycloalkanes for jet fuel, with approximately 94 wt% recovery of organics. Similarly, JP-5 navy fuel has synthesized from low-density polyethylene, using catalytic microwave degraded at a temperature of 375 o C and catalyst feed ration of 0.1, yielding a liquid carbon content which was 66.18%, with optimal conditions at 250 o C for 2 h through trnsformation of organics to jet fuel range cycloalkanes and aliphatic olefins. This serves as a potential route for the conversion of non-biodegradable plastic wastes [64].

Synthesis for Drug Design
Microwave irradiation has proven to be very vital drug design and syntheses of various antimicrobial motifs. Facile 2-quinoxalinone-3-hydrazone derivatives was carried out via microwave irradiative technique of hydrazide (1) to yield compounds (2) and (3) having significant antibacterial and antifungal activity [65]. Scheme 2: Synthesis of potent antimicrobial compounds from 2-quinoxaline-3-hydrazine In the same vein, compound (4), showing higher antibacterial activity than the gentamycin standard was synthesized using microwave-assisted Pfitzinger pathway from 2-propylquinoline-4-carbohydrazide [66]. Huang and coworkers [67] reported the rapid synthesis of 2-vinylquinolines (5), showing very potent antimalarial activities against Plasmodium falciparum. The reaction was done using microwave irradiation as energy source, leading to yields as high as 92% in 30 mins [67]. Scheme 4: Microwave-assisted synthesis of 2-vinylquinolines

Conclusion
The versatility of the microwave assisted technique as a fast and highly efficient means of synthesis of heterocyclic compounds and other valuable compounds is successfully highlighted. Microwave irradiation is remarkably good and proven to be a producer of the excellent pathways for the sustainable synthesis of highly functionalized materials for technological advances. Toxicity of the reacting solvents and discharge of poisonous chemicals associated with conventional synthesis has been eliminated in the microwave approach which makes it to be environmentally friendly method of synthesis. Thus, microwave irradiation technique is a laudable energy source for speedy as well as cost-effective preparation of materials for many industrial and domestic applications.