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REVIEW - Microwave Technology: Niche Market in Enzymatic Reaction Systems.

Byline: Nik Nurfatmah Pz Nik Pauzi, Ahmad Syafiq Ahmad Hazmi, Haliza Abdul Aziz and Lim Wen Huei

Summary: Microwave irradiation is widely used as a source of heating in various applications such as food processing as well as many organic synthesis systems. The technique offers a simple, clean, fast, efficient, and economic process, providing momentum for many researches to switch from using traditional heating to the mmicrowave assisted irradiation method. In recent years, the microwave assisted organic reaction technique has emerged as a new tool and has enabled chemists to achieve cleaner and more efficient chemical reactions with higher yields compared to conventional heating methods. However, not many reviews have described the potential application of chemical synthesis that uses enzymes as catalyst, especially in relation to the microwave processing aspects and their dielectric properties. This paper unveils the synergism effect achieved using the microwave irradiation technique on enzymatic synthesis under various optimal conditions.

Keywords: Microwave; Irradiation; Organic synthesis; Enzymatic, Dielectric properties.

Introduction

Enzyme catalyzed reactions have gained considerable attention nowadays since heterogeneous catalysts promise the ease of catalyst removal from products and offer more reusability [1, 2]. Thus, enzymatic reaction is an alternative for heterogeneous catalytic reaction instead of using chemical catalyst [3-6]. In recent years, microwave (MW) irradiation has been accepted as an effective heating medium compared to conventional heating in organic synthesis reactions due to its advantages including a high reaction rate, higher selectivity, higher purity and a cleaner process [7-9]. Coupling between MW irradiation and enzymatic synthesis seems to be very interesting, but the utmost importance is the careful control of reaction parameters since MW is always associated with local hotspots formation that denatures the enzyme catalysis. Previous studies have demonstrated that a good synergism effect could be obtained between MW irradiation and enzymes [10-12].

Currently, MW technology has gained more attention due to its broad range of applications in communication, remote sensing, navigation, food processing, and electron paramagnetic resonance spectroscopy industries. Additionally, researchers have found that MW heating can be a suitable substitution to conventional heating due to its many advantages [13-17]. Conventional heating of a material is a big challenge due to the material's inter-surface thermal resistivity, and MW heating offers several advantages to overcome this obstacle. Unlike conventional heating which only heats the material at the surface, MW energy penetrates deep into the material and supplies energy. As a result, heat can be generated throughout the whole volume of the material. Therefore, rapid and uniform distribution of heat in the material can be achieved through microwave heating [18]. However, there are several rules to be observed.

There is a specific minimum volume needed for a reaction under MW irradiation in order to avoid the transparency of MW irradiation. However, small amount of material can also sometimes be heated using MW irradiation when MW penetration takes place in depth. Based on recent publications, MW-assisted organic synthesis is always associated with a shorter reaction time, increased product yield, reduced product impurity and has successfully been proven to be a solvent-free reaction which cannot be accomplished under classical heating [2, 19]. A detailed comparison of MW heating advantages over conventional heating is shown in Table-1 [20]. The use of MW-assisted synthesis methods is rapidly expanding in various industrial fields such as medical/drug discovery [21-24], polymer synthesis [25-29] and nanotechnology [30]. Fig. 1 shows the energy transfer comparison between MW irradiation and conventional heating [20].

Table-1: Advantages of MW heating over conventional heating [20].

###MW###Conventional

###Interaction of MW with matter characterized by two physical quantities; Induced by###Heat transmitted through conduction only. Thus,

1###dipolar rotation or ionic conductance. Thus, rapid and uniform volumetric heating###slower heating rate and only heat sample at surface

###(Internal heating) as MW penetrates at the speed of light###due to surface thermal resistivity

###More energy efficient by reducing energy consumption up to 50 %, reduce warm up

###Less energy efficient because heat loss to environment

2###and cool down time, diminish extra equipment (e.g. heated jackets, boiling pans and

###(e.g oil bath, heating mental, steam bath etc.)

###heating and cooling vessels)

###Specific heating of the subject material could be applied when using MW transmitter###Container and all the compound in the mixture heated

3

###as a container. Thus, more heat exchange could be generated.###uniformly

###Temperature limitation based on boiling point of the

4###Superheating possible

###subject material

Since 1998 to date, more than 4655 journal articles have been published in "Science Direct" on various topics e.g. polymer science, medicinal chemistry, coordination chemistry, fuel and energy, alloys and compounds, molecular catalysis, dyes and pigments, applied science, material chemistry and physics, ultrasonics sonochemistry, etc. on the use of MW as an energy source. The number of publications associated with keyword "MW, MW irradiation, MW assist and MW-assisted synthesis" plotted against the publication year extracted from Science Direct is shown in Fig. 2. Yet, these topics also led to many controversial issues, although during these several years many publications have come out with a completely revised edition on the advantages of MW irradiation.

Thus, this topic can be considered as one that prevails until today. Therefore, the purpose of this review is to highlight the literature reviews in the area of MW processing aspects, the importance of dielectric properties of materials for organic synthesis and the potential applications of MW technology in enzymatic reaction systems since current discussion on this topic is rather limited. The review paper explores works done by worldwide researchers and scientists on aligning organic reaction and MW technology. The application of a catalyst, however, requires a unique set of knowledge and experience as MW tends to develop various hot spots that might deactivate the catalyst (enzyme). Furthermore, the paper provides adequate background to the fundamental of MW irradiation. Without understanding this physics that correlates the dielectric concepts to molecular interaction at a microscopic level, it is almost impossible to exploit this technology.

Overall, the topic would be a good starting point for researchers interested in MW-assisted organic reaction with catalyst (enzyme).

Fundamentals of MW Materials Interaction

It is known that not all materials show interaction with MW irradiation. The ability of a material to interact with MW irradiation depends on many factors, primarily its dielectric properties. Thus, energy transfer can be calculated based on those properties. Surprisingly, MW has been found to be a new medium to induce a chemical reaction which was not feasible in conventional heating [31]. As a general rule, materials with higher polarity typically perform better in MW irradiation. Hence, it is imperative to evaluate the dielectric properties of a material desired to achieve efficient and selective reaction under MW irradiation [32].

The fundamental MW-material interaction is the heat generated by friction of molecules colliding with each other as a result of insufficient time to respond to the rapidly changing oscillating field of MW irradiation [33]. Interaction between electromagnetic waves with materials can be divided into three, namely transmission, reflection and absorption as shown in Fig. 3 (a), 3 (b) and 3 (c). By definition, materials that can be categorized as MW transmitters are materials which are translucent to MW such as several glasses, ceramics and Teflon [34]. In contrast, MW reflector is materials that can bend back MW irradiation from a material's surface with only a small amount or no energy penetrating into the system. Typically, bulk metals and alloys can be classified as MW reflectors. Materials that can absorb energy from MW efficiently and rapidly convert it into heat fall into the MW absorber category.

Usually, MW absorber materials can be characterized by its high dielectric properties as it will induce the heating rate of the medium [35]. As such, this is the kind of material needed for MW synthesis. Transmission, reflection and absorption may occur at the same time in varying ratios based on its dielectric properties [36]. Therefore, it is a very common practice to combine all three different categories of materials into MW to offer improved heat exchanges.

Theory on Dielectric Properties

Dielectric property is defined as the behavior of materials when subjected to a MW electromagnetic field in dielectric heating applications [37]. The dielectric heating induced by the dielectric field is the result of two mechanisms: dipolar rotation and ionic conductivity. Fig. 4 (a) shows the direction of dipoles when no electric field is applied. Fig. 4 (b) shows the direction of dipole movement when a too low frequency is applied. Different materials behave in different manners when subjected to different frequencies. Too low or too high frequencies could be determined by observing the graph pattern of frequency vs dielectric constant (Iu') measured using a dielectric measurement system. Data in Fig. 4 (c) shows the direction of dipole movement when too high of a frequency is applied.

Fig. 4 (d) shows the direction of dipole movement when a moderate frequency is applied. When dipoles are exposed to moderate electromagnetic field frequency, the polar molecules will try to orientate themselves by following delay according to the field polarity as shown in Fig. 4 (d). Once the applied field changes, the dipolar molecules try to orderly rearrange themselves to the new direction. During the rearrangement, friction between the molecules produces heat. In contrast to ionic conductivity, heat is generated by ion collision due to the change of charged ions with the polarity of the force lines of the electromagnetic field [38].

Interaction between the electromagnetic waves with dielectric substance is determined by the dielectric properties of the material, Iu = Iu' - jIu" where j is the square root of -1. Iu' is identified as the dielectric constant and is often expressed as relative permittivity (Iur) and counts as the ability of a material to store electromagnetic energy, while the imaginary part Iu" is dielectric loss which is described as the measure of energy absorbed from the applied field [39]. As known, Iu' is a frequency dependent variable and declines with increasing frequency [40]. Therefore, it is important to note that moderate frequency is needed to generate heat; no heat will be generated as the frequency is too low or too high. In addition, composition and temperature have an impact on these parameters [41].

The capability of a specific substance to transform electromagnetic energy into heat at a given frequency and temperature is described as loss factor, tan I'. This loss factor can be calculated using equation tan I' = Iu'/Iu". For effective absorption and rapid heating, a reaction medium with a high value of tan I' is required [42].

Almost all materials are polarizable in different manners and Iu' depends on frequency which has a somewhat complex relationship. The polarization of materials includes electronic, molecular and orientational. Electronic polarization is contributed by very light and highly responsive electron clouds. Molecular polarization occurs when atomic bonds are stretched under an electromagnetic field. Under an applied electromagnetic field, whole molecules will experience a dipole moment or orientational changes. The energy of a dipole is strongly dependent upon temperature. As shown in Fig. 5, both water and isopropyl alcohol show their highest Iu' at 200 MHz. The contribution from electronic polarization represented by the square of its refractive index [43] are 1.77 and 1.89 respectively. However, Iu' at 200 MHz for water and alcohol are 78 and 21, respectively.

The difference is contributed by molecular and orientational polarization. Higher values of Iu' of water suggest that water is able to store more energy than isopropyl alcohol. According to the Debye equation, relaxation of dielectric polarization occurs at a natural resonant frequency and when internal frequency has collapsed. The decay of the internal field describes the maximum energy transfer and is represented by Iu" peak. An example of frequency dependence of Iu' and Iu" of water and isopropyl alcohol is shown in Fig. 5.

Factors Influencing Dielectric Properties

The dielectric properties of a material are greatly influenced by the physical properties of the material such as molecular structure, density, moisture content, physical and chemical composition. A study by Ishida et al. [44] investigated the effects of various factors on the dielectric properties of cellulose fibers. On the other hand, a review by Perry [45] stated that dielectric properties of a material are greatly influenced by impurities, forming techniques and firing procedures, and only slightly affected by mechanical properties. A study by Hui [46] stated that based on pharmaceutical products, the frequency, electromagnetic field strength of MWs, moisture content, chemical composition, state, density, size, geometry and thermal properties do affect dielectric properties. For chemical composition, it is interesting to note that polar molecules perform better in MW. Dipole moments arise from two atoms with different electronegativities bond together asymmetrically.

Polar molecules can be acknowledged with the presence of these atoms N-H, O-H, C-O, C-N, C-Cl, etc. In contrast, atoms of similar electronegativities that bond together through an equal sharing of electrons are known as non-polar molecules. Examples of non-polar molecules are molecules that consist of C-H, C-C chains.

Another extensive study by Rao et al. [34] on the synthesis of inorganic solids using MWs highlighted that dielectric properties could be affected by impurities, aliovalent substitution and the chemical nature of the material. It is noted that the transition metal group has higher Iu, therefore can cope better with MW irradiation. Surface defects, surface charge and degree of polarization of materials in powder form lead to increases in dielectric parameters. The dielectric properties of agricultural materials have been of interest for many years, with several factors that affect these properties e.g. MW frequency, water content, temperature, and density of the materials had been extensively discussed by Nelson and Trabelsi [47], Jha et al. [48] and Venkatesh and Raghavan [49]. Past investigation have involved numerous agricultural and food products e.g. grain, peanuts, fruit, eggs, fresh chicken meat, whey protein gel, and a macaroni and cheese preparation.

Application of Dielectric Properties on Various Fields

Among all, the dielectric properties of food are widely investigated due to the broad range of MW applications on food e.g. pumpable foods, agri-foods, fluid foods, fruits like potato puree, grape juice, watermelons and apple, green coconut water (GCW), coffee, meat, salmon fillet, egg, chicken, fish, mashed potatoes, soybean protein, rice, garlic, bean, chickpea flour, honey, beef burger, milk, etc. [36, 48-77]. The dielectric properties of food are useful in determining the processing conditions, drying process, moisture control and the utmost importance of maintaining the quality of food. Franco et al. [77] offered useful information on the effect of temperature and frequency towards the dielectric properties of GCW. This data will be useful in determining the appropriateness of GCW for MW processing. Nevertheless, dielectric properties also play an important role in controlling the quality of frying oil.

It is well-known that the frying process involves changes in thermal temperature which probably affects the flavor and nutritional value of food. Thus, monitoring the quality of the frying oil is very important to maintain the food's quality and nutrition.

On the other hand, interest in applications of MW power for non-destructive techniques and moisture determination of agricultural products were carried out since between the late 1960s and early 1970s. Some applications of MW in agriculture are for the drying of corn, grain, seed, garlic slices, etc. [78-80]. Thus, the uses of dielectric properties for agricultural products are essential in determining the effectiveness of material when exposed to MW irradiation and the factors affecting their dielectric properties such as temperature, frequency and moisture content have been extensively discussed [61, 81-86]. Guo et al. studied the dielectric properties (Iu, Iu") of compressed chickpea flour samples within the frequency range of 10 to 1800 MHz at various moisture contents over a temperature range of 20 to 90 AdegC [61]. They revealed that dielectric constant and dielectric loss decrease with increases in frequency and increase with increases in temperature and moisture content.

The same findings was observed by the same group for temperature and moisture dependent dielectric properties of chickpea, green pea, lentil and soybean [85]. As expected, increases in moisture content contribute to increases in the ionic conduction of substances, correlating to the increases in the dielectric properties [61]. Other findings by Sacilik et al. were in agreement with the results reported by Guo et al. with regards to the relation between moisture content and the dielectric properties of safflower seed over a frequency range of 50 kHz to 10 MHz [81]. The same pattern was observed by Nelson et al. for the frequency and moisture dependence of hard red winter wheat [83] and corn at MW frequency [86]. Manickavasagan et al. studied the non-uniformity surface temperature of three different grains (barley, wheat and canola) after MW treatment in an industrial MW dryer.

Five different power levels (100, 200, 300, 400, 500 W) were used at 2.45 MHz, whereas the surface temperatures were measured using infrared thermal camera. The effects of five moisture contents (8, 12, 15, 18 and 21 % wet basis) of the grains to the non-uniformity of surface temperature were investigated [82]. They claimed that increases in the moisture level of barley and wheat contribute to decreases in the final temperature of grains measured after MW treatment.

In order to understand the interaction between vegetable oils and electromagnetic energy, the dielectric properties of various vegetable oils have been explored by a number of researchers [87-94]. Iu' of vegetable oils depend on temperature; increases in temperature decreases the Iu' [87]. However, a study by Sankarappa et al. revealed a reversed pattern of relation between temperature and Iu'. Moreover, the value of Iu' reported in this study is very high compared to those reported in previous literature, which can be explained by the different geographical origins of the oil sources and the higher water content within the molecules [89]. Other findings by Gjorgjevich et al. showed that higher monounsaturated fats in vegetable oils is accredited to more stable dielectric properties, thus are suitable candidates for MW irradiation due to the low deterioration observed during MW exposure [88]. Iu' was compared between two vegetables oils (corn oil and cotton seed oil) and mineral oil.

They found that vegetables oils have higher Iu than mineral oil at about 1+-0.2. This might be due to the polar nature of vegetables oil and the presence of triglycerides inside the structure. In contrast, mineral oil only consists of non-polar alkene molecules inside its structure, thus leading to a low dielectric constant since the dielectric properties of a material depends on the polarity of the molecule [90]. The summary of factors affecting dielectric properties of vegetable oils is shown in Table-2.

Table-2: Summary of factors affecting dielectric properties of vegetable oils

Parameter###Factors affecting parameter

Iu'###Temperature [88, 90], moisture content [90], polarity

###[91], monounsaturated fats in vegetable oils [89]

To date, many researchers have focused on the diversification of palm oil applications instead of those commonly used such as frying cooking oil and biodiesel. Among all, research on palm-based transformer oils as substitution for mineral oil seems to be an emerging field, and is beneficial since it is more abundant in nature and is greener to the environment [95-97]. Dielectric properties are one of the criteria observed in developing the next generation of insulating and cooling oil for high-voltage equipment such as transformers. Transformer oil has properties such as high dielectromagnetic field strength, low dielectric losses and reliable service lifespan.

Previously, many researchers have studied the dielectric properties of fibers and textile materials [98-100]. All published papers describe numerous measurement techniques to measure dielectric properties, thus providing a comprehensive understanding on the importance of these parameters. The dielectric properties of textile materials are of utmost importance in determining the process and quality of a textile. Dielectric properties provide information of a material's moisture content, unevenness, drying and static generation of the fibers and textiles [100]. There are several studies on the dielectric properties of fibers and textile industries such as to control the static generation of the textile [101], measure the moisture content of fabric [102], investigate moisture diffusion through textiles [103], etc.

Literature on dielectric properties in the pharmaceutical field have been one of the fields of interest for many years since the introduction of MW technology and has been exploited in many areas like MW synthesis in drug discovery/processing [23, 104], high and fast extraction tools for medical plant research [105, 106], drying of pharmaceutical granules and powders [107], and pharmaceutical actives [108]. Currently, the dielectric properties of polymer nanocomposites have utmost significance owing to the development of conductive polymer nanocomposites [55, 109-123]. Sadasivuni et al. [124] studied the effect of dielectric properties by incorporating graphene oxide (GO) and modified graphene oxide (mGO) fillers into a polyurethane (PU) matrix [81]. They found that Iu was greatly influenced by the incorporation of certain amounts of GO and mGO into the PU matrix. This is endorsed by the formation of a continuous conductive pathway in the PU matrix, thus increasing the motion of free charge carriers inside.

Correlation of Dielectric Properties to MW Irradiation for Enzymatic Organic Reactions

Reactions under MW irradiation are greatly influenced by two controlled variables; MW power and the frequency of the applied field. The conventional method for reaction rate enhancement is by controlling the MW power without considering the fundamental MW-material interaction. Adequate knowledge on this subject matter is useful to ensure a rapid reaction rate without sacrificing the yield and selectivity of the end products. The microwave dielectric heating phenomenon is a result of physical interaction between an electromagnetic field with the matter irradiated and characterized via intrinsic dielectric properties. This often needs empirical measurements that cover from the simple to the most complex materials such as reaction solutions.

This is how dielectric properties play its role. It is not a big issue to discuss for the dielectric properties of pure solvents and single components as the information are widely available in the literature. However, the issue that should be addressed is the measurement of reaction mixtures' dielectric properties, e.g. MW-assisted enzymatic organic reaction. The concept of MW heating is that when a material is exposed to MW irradiation, one of the molecules inside rotates excitedly and collides with other molecules inside the material. Upon the collision, the rotational energy of the molecule is generated and converted into kinetic energy, thus heat is generated. Polar solvents, e.g. dimethyl-formamide (DMF), dimethyl sulfoxide (DMSO), acetonitrile, methanol, and glyceryl usually performs better under MW irradiation because they can absorb MW irradiation better than non-polar solvents e.g. toluene and hexane.

A simple correlation could be used to quantify relative MW absorptivities to Iu', Iu" and tan I'. Iu' defines the polarizability of a molecule in the MW field, while Iu" describes the ability of a molecule to convert the incident of electromagnetic irradiation into molecular rotation and generated heat while tan I' refers to the value of Iu"/ Iu'. The easiest way to understand the correlation of dielectric properties and the behavior of a substance under MW irradiation is the closer the value of tan I' to "0", the more transparent the material is to MW irradiation, while values of tan I' approaching 1 represent that the material is a good MW absorber. For example, the value of tan I' of ethanol is 0.941, showing that ethanol is a good MW absorber while the tan I' value for hexane is 0.020, describing that hexane is transparent to MW irradiation [125].

Thus, organic synthesis consists of material that is transparent to MW irradiataion e.g. hexane which should be coupled with other MW absorbers to create hot spots to achieve efficiency.

In addition, the rate of reaction of a process being exposed to MW irradiation could be affected by several aspects such as specific heat capacity, heat of vaporization of the substance, and the penetration depth of MW irradiation into a substance. Iu" and tan I' provide information on irradiation wavelength as a function of temperature, specific heat changes as a function of temperature, and heat of vaporization changes as a function of pressure [125]. For example, a polar solvent e.g DMF and a non-polar solvent e.g toluene are exposed to MW irradiation to achieve 90AdegC at same MW frequency and power level. The polar solvent takes less time than the non-polar solvent, but higher specific heat capacities were calculated. This is the concept of how MW irradiation enhances the rate of reaction and improves selectivity. However, the polarity of a substance can be altered based on the temperature of reaction.

For a single substance being exposed to MW irradiation, the correlation between material and MW irradiation is easily understood compared to using a mixture of reactions under MW irradiation. Dall'Oglio et al. have studied the dielectric properties of various acids phosphorus acid (H3PO4), chlorosulfuric acid (CISO3H), methane sulfonic acid (CH3SO3H) and sulfuric acid (H2SO4) employed as catalysts in MW-assisted homogeneous transesterification reactions for the production of methylic and ethylic biodiesel. Although H3PO4 showed a higher value of tan I', it gave a lower conversion of maize oil to ethyl and methyl esters after being exposed to single monomode reactor MW irradiation. The relation of mixtures exposed under MW irradiation is somehow complex to understand.

It was explained by Dall' Oglio and his co-workers as the low activity of carbonyl protonation in a H3PO4 using ethanol and methanol which can be explained by the higher pKa value relative to the other catalysts used in the reaction. In certain circumstances under MW irradiation where non-polar solvent or substrate are needed to move the reaction forward, the addition of ionic liquids [126], WeflonTM9 could be practical as aid.

For the case of MW-assisted enzymatic reactions for organic synthesis, a study conducted by Young et al. had exposed different types of enzymes, namely pyrococcus furiosus [beta]-glucosidase (Pfu CelB), prunus dulcis (Pdu CelB), thermotoga maritima (Tm GaIA) and Sulfolobus solfataricus P1 (SsoP1 CE) to 300 W MW power [127]. It was surprising to note that different types of enzymes behave differently when being exposed to MW irradiation. Pfu Celb had showed increased enzyme activity (mol min-1 ug-1) to 4 folds at 300 W MW power compared to when no MW irradiation was applied. However, enzyme activity decreases as MW power decrease. In this case, temperature was increased from -20AdegC to 40AdegC [127]. This result showed that a good synergism effect was achieved between MW irradiation and enzymes under certain conditions that need to be explored on more.

MW technology is always associated with high reaction temperatures, shorter time, and increased selectivity and yield. In general, the allowable temperature limit for enzymatic reaction under MW irradiation should be not more than 60AdegC without sacrificing the enzyme activity and moving the reaction forward. Moreover, a stable and effective enzyme is needed to perform the reaction under MW. In the worst case, some modifications to the enzyme are needed to improve the stability and efficiency of the enzyme. The utmost important knowledge desirable for enzymatic catalysis is on the activity, selectivity and stability of the ezymes [11]. For MW enzymatic reactions, two parameters should be taken into consideration; i) hydration state, and ii) polarity of the reaction medium [11].

Since MW is often associated with the rapid heating of aqueous solutions under high-power MW irradiation, a careful consideration should be taken to maintain the required minimum amount of water needed by the enzyme to avoid inactivation and be a fully functioning enzyme as catalyst. Roy and Gupta studied enzyme catalyzed esterification and transesterication in six different solvent polarities at three tested temperatures under MW irradiation. They have conducted both enzymatic-esterification and enzymatic-transesterification reactions at 25AdegC, 40AdegC and 60AdegC under MW irradiation with six different log P solvents, namely n-octane (4.5), toluene (2.5), t-amyl alcohol (0.89), tetrahydrofuran (0.49), acetonitrile (20.33) and dioxane (21.1).

In general, the rate of reaction (mmol/h) for a-chymotrypsin-catalyzed-transesterification reaction increases from 2 to 4.9 times higher than conventional heating, while for subtilisin-catalyzed-esterification reaction, the reaction rate increases from 2.1 to 3.8 folds under MW irradiation. Results were also obtained on enzymes pH-tuned with sodium chloride (KCl). In this study, it can be observed that the best log for P ranges from 0.89 to 2.5 which are suitable to enhance the initial rates of the subtilisin-catalyzed reaction. On the other side, the finding revealed that regulated pH of enzymes and salt activation efficiently enhance initial reaction rates [128] in n-octane up to 4.3 times, and 7.6 times in t-amyl alcohol using unregulated enzyme with irradiation [129]. The pH-regulated enzymes were desirable because the protonation state of the enzymes in organic solvents is dependent on the pH of the aqueous buffer from which the enzymes are lyophilized.

Another finding by Porcelli et al. claimed that the denaturation of enzymes under MW irradiation is not dependent on the enzyme concentration [130], but is due to the non-thermal effects of MW [131].

MW In Organic Synthesis

The first MW application in organic synthesis was published in 1986 entitled " The use of MW ovens for rapid organic synthesis" [132, 133]. In the beginning, research on this topic was slower compared to inorganic synthesis because the applications of MW in inorganic synthesis had started earlier. However, in mid 1990s, the knowledge on MW dipolar heating with materials had increased, thus improving the number of publications [16]. Researchers' momentum on this topic has accelerated because a faster reaction time could be obtained by MW irradiation than conventional heating. Moreover, higher yields with better purity could be attained as MW offers a solvent-free synthesis technique, thus providing an easy purification step that fulfills the requirements for a green chemistry process. Besides that, improved commercial MW equipment and user-friendly software have also contributed to the widespread application of MW in organic synthesis.

Previously, MW was widely used in the chemical analysis of materials. However, in these recent years it is expected that the application may be diverted to chemical synthesis, a topic which is gaining more attention among researchers. This is due to MW's potential for industrial production as research has shifted from only using milligrams to kilograms. For the time being, there are four main suppliers in the market for MW synthesis reactors; Stoltz, CEM corporation, Biotage AB and Milestone. A wide variety of MW-assisted organic syntheses (MAOS) have been reported recently, from N-acylation, alkyl/aryl coupling, condensation, (de) protection, heterocycles, oxidation, reduction, radical reactions, organocatalysis, alkylation, nucleophilic substitution, cycloaddition, (trans) esterification, organometallic reactions, rearrangement, metathesis to olefination [134]. The effects of thermal and non-thermal MW irradiation on organic synthesis are discussed by De La Hoz et al. [135].

Perreux and Loupy extensively discussed on the non-thermal effects of MW in organic synthesis [136]. Lew et al. demonstrated a method to increase reaction rate through MW-assisted organic synthesis for combinational chemistry [137]. A critical assessment of the greenness and energy efficiency of MW-assisted organic synthesis was extensively described by Moseley and Kappe [138].

The esterification process always refers to the long reaction time associated with low yield. Efforts have been made to overcome this problem. Solvent free esterification of carboxylic acids with alcohols under MW irradiation in the presence of various Lewis acid catalysts was successfully reported by Shekarriz et al. [139] with shorter reaction time and high yield. The highest yield of 93% could be obtained by reacting phenyl acetic acid (1 mmol) with nonan-1-ol (1 mmol) in the presence of Zinc trifluoromethylsulfonate Zn(OTf)2 within 5 mins compared to only 10% of yield in the absence of the catalyst within 30 mins exposure to MW irradiation. A 68% reduction in yield could be observed when replacing Zn(OTf)2 with Trifluoroacetic acid CF3COOH/silica gel catalyst for 10 mins under MW irradiation.

Comparative attempts of quinoline derivatives synthesis using both MW-assisted and conventional methods were done by Corach et al. [87]. It was found that the MW method catalyzed by K-10 afforded interesting results, where a yield of 55-95% could be obtained within 6 mins than the 5-6 h needed to achieve 42% yield under the conventional method. Paul et al.[140] successfully conducted zinc mediated Friedel-Crafts acylation of aromatic compounds with acyl halides under solvent-free conditions via MW irradiation. It is interesting to note that yields of more than 80% could be achieved despite recycling the catalyst used four times. Thus, limitations associated with the reusability of acid catalyst (e.g. Anhydrous AlCl3) could be overcome by using Zinc (Zn) as a powder catalyst. Recovery could be achieved by a simple washing with diethyl ether and hydrochloric acid (HCl) after each use.

Zn powder is a suitable substituent to commonly used catalysts as it offers better synergism effects with MW irradiation. Moreover, higher yields of up to 95% could be attained within 30 seconds under MW irradiation than 8 mins by using conventional methods to achieve 79% yield at the same temperature of 60AdegC.

In the area of pharmaceutical and drug discovery, Vahabi and Hatamjafari [141] had successfully developed a new method based on the Pechmann condensation for the synthesis of substituted coumarins under solvent-free MW irradiation conditions and catalyzed by Iron III fluoride (FeF3) as an effective eco-friendly catalyst. It is noted that FeF3 could be re-used up to four times after simple washing with ethyl acetate and vacuum-drying with more than 80% yield. The highest yield of 95% could be obtained with the addition of 0.05 g FeF3 catalyst within 7 mins [141] compared to the 180 mins needed to reach 90% of conversion using 10% Pentafluorophenylammoniumtriflate (PFPAT) catalyst in the presence of toluene as solvent at 110 AdegC under MW irradiation [142].

Application of MW Synthesis in Oleochemicals Production

Nowadays, efforts are being devoted to synthesizing chemicals under green chemistry principles and alternative methods. Thus, the number of publications covering the synthesis and applications of "green solvents" is increasingly growing as a replacement method for hazardous common organic solvents [143]. Ionic liquid, water, polyfluorinated two phase systems, supercritical carbon dioxide and supercritical water could be suitable candidates for solvents [144]. On the other hand, the use of MW techniques or sonochemistry for organic synthesis seems to be a good replacement as shown by the growing number of research papers on the applications of MW organic synthesis with high yields, without solvents, low waste and very low energy requirements being published in recent years [145].

A study conducted by El Sherbiny et al. [146] revealed that the application of MW irradiation in lab scale biodiesel production could cut down reaction duration from 150 mins to 2 mins. This result can be accomplished as a synergism effect between MW irradiation and both polar and ionic components constituting a mixture of jatropha oil, methanol and potassium hydroxide. A 97.4% yield could be achieved at a methanol/jatropha oil molar ratio of 7.5: 1 (% w/w) and 1.5 % (w/w) potassium hydroxide as catalysts at 65 AdegC within 2 mins instead of 60 mins using conventional heating.

Several studies on the transesterification of glycerol (GLY) with dimethyl carbonate (DMC)/ ethylene carbonate (EC) for the production of glycerol carbonate (GC) are shown in Fig. 6 and Table-3 [147-151]. Hoong et al. [US 8816133 B2] have patented a process for faster preparation of polyglycerol from crude glycerol using MW irradiation as its heating element. The reaction was conducted in a 900 W MW oven in the presence of soap as catalyst and stirred using a magnetic stirrer. The complete reaction of crude glycerol to polyglycerol could be achieved within 20 to 30 mins under MW irradiation at temperature range of 250AdegC-270AdegC compared to 5 h to 72 h through conventional heating [152]. In accordance to this invention, Hoong et al. have patented that 90% conversion of crude glycerol to polyglycerol could be achieved after 3 h when subjected to MW irradiation at 270AdegC [152].

MW and its Potential in Enzymatic Synthesis

Currently, there is growing interest in enzymatic reaction as a replacement to commonly used chemical catalysts due to the ease of separation of the end product, reduced by-product formation, recyclable, the non-toxicity of the catalyst and milder reaction conditions [150]. Enzyme is a biocatalyst mainly derived from protein. Like other catalysts, enzymes increase the rate of a reaction by lowering the activation energy of a reaction. However, it is known that using enzyme as catalysts will typically slower the reaction rate than chemical catalysts. Thus, in order to overcome the obstacle, the use of MW will speed up the reaction rate while promising greener reaction conditions i.e. less energy consumption due to short reaction time and easy separation process. Coupling MW irradiation with enzyme seems to be a very interesting topic to discuss on since MW irradiation typically increases the reaction rate up to 100% under optimal conditions.

Table-3: Summary of the synthesis of GC from transesterification of GLY and DMC/EC using MW and conventional methods [147-151].

###Mode of###Stirrer###Catalyst###Temperature###DMC:GLY

###Title###Raw materials###Yield (%)

###heating###speed###loading###(AdegC)###/EC: GLY

###Crude glycerol

###MW-assisted

###(Gly) 70 % purity

###transesterification of###93.4 % could be

###and###1 wt % of Calcium oxide

###industrial grade crude###MW###NA###65###2:1 M###obtained within

###Dimethyl###(CaO)

glycerol for the production of###5 mins

###Carbonate

###glycerol carbonate [147]

###(DMC)

###Refined glycerol

###(99.8 % purity) and

###Synthesis of glyceryl###62 % Gly-C

###Ethylene

###carbonates via MW###0.5 wt. % MgO based on###yield with 96 %

###Carbonate###MW###NA###80###3:1

###irradiation###glycerol wt.###purity within 10

###(EC) and Dimethyl

###[148]###mins

###Carbonate

###(DMC)

###Extra pure glycerol

###90 % could be

Kinetic of the production of###(99.88 %) and

###Conventional###2500 ppm of###achieved within

###glycerol carbonate by###Dimethyl###1500

###batch###Potassium methoxide###70###3:1 M###75 mins and 96

transesterification of glycerol Carbonate (DMC)###rpm

###reaction###(CH3OK)###% within 240

with dimethyl and ethylene###purity 99 %

###mins

carbonate using potassium

methoxide, a highly active###90 % was

###Ethylene###Conventional###150 ppm of

###catalyst [149]###800###reached after

###Carbonate###batch###Potassium methoxide###60###3:1 M

###rpm###10 mins and 95

###(EC) purity 99 %###reaction###(CH3OK)

###% after 45 mins

Synthesis of glycidol from###97 %

###glycerol and dimethyl###Glycerol and###0.217 mmol of###conversion

carbonate using ionic liquid###Dimethyl###Conventional###NA###tetramethylammonium###80###3:1 M###could be

###as catalyst [150]###Carbonate (DMC)###hydroxide [TMA][OH]###achieved within

###90 mins.

Enzymatic production of###75 g /L of Novozym 435 in

###Glycerol and###Conventional###96.25 %

glycerol carbonate from by-###180###the presence of organic

###Dimethyl###(Water bath###60###2:1 M###conversion

###product of biodiesel###rpm###solvent acetonitrile and

###Carbonate (DMC)###shaker)###after 48 h

manufacturing process [151]###(10 % v/v of surfactant)

It is interesting to note that an extensive study was conducted by Basso et al. [151] regarding the effects of MW irradiation on enzymatic peptide bond synthesis. The variation of temperatures from 50, 60, 70, 80, 90 to 100 AdegC affects the percentage of Fmoc-Phe-Phe-PEGA1900 product conversion. The highest conversion could be obtained at 80AdegC within 60 mins and surprisingly the lowest conversion with the same duration could be obtained at 100AdegC. This might be due to the denaturation of enzymes at high temperatures. In addition, the initial rate of enzymatic reactions performed under MW irradiation is significantly higher than without MW irradiation, and the most efficient enzymatic activity could be obtained at 80AdegC. However, another study by Yadav and Lathi [153] on the intensification of enzymatic synthesis of propylene glycol monolaurate from 1,2-propanediol and lauric acid under MW irradiation claimed that Candida Antartica Lipase B (CALB) shows the highest enzymatic activity at 60AdegC.

Yu et al. [18] reported that MW irradiation could increase the rate of resolution of (R,S) - 2-octanol enzymatic transesterification up to 100% within the first two h of reaction compared to conventional heating. A comparison was made for reactions carried out in 50 ml of n-heptane, 50 mmol of (R,S)-2-octanol, and 100 mmol of vinyl acetate catalyzed by 60 mg of Novozym 435 with water activity of 0.56 at 40 AdegC and 200 rpm under MW irradiation and conventional heating. The conversion of 50.5% was obtained in 2 h under MW irradiation. In contrast, the same conversion rate can only be observed after 10 h under optimum conditions using conventional heating. However, the use of MW irradiation alone for the same process without the addition of enzyme catalyst showed no conversion after 12 h. Thus, it is proven that enzymatic reaction under MW irradiation is the best coupling condition. They also reported that the highest enzyme activity (275umol/min.mg) could be achieved at 80 AdegC.

As studied by Major et al. [154], the enzymatic esterification of lactic acid in ionic liquid conditions could shorten the reaction time from 24 h to 7 h under MW irradiation. The reaction was conducted at 40AdegC and 10 W in the presence of toluene as solvent. The reusability of the enzyme CALB in this study was 3 times with 80 % yield, proving that MW irradiation does not denature enzymes at 40 AdegC. Pilissao [155] and his co-workers had studied the effect of using free A. niger lipase and immobilizing A. niger lipase on the resolution of (RS)-sec-Butylamine. They found that higher degrees of conversion i.e. 21 % of (R)-amide with eep> 99% and enantioselectivity value (E value) > 200 could be achieved in 1 min under MW irradiation. In contrast, the production of (R)-amide was slower when using immobilized A. niger lipase where only 8-25% was attained after 3 or 5 mins of exposure to MW irradiation.

Yu et al. [156] presented an example of MW-assisted fatty acid methyl ester production from soybean oil catalyzed using Novozym 435. Under optimum conditions (aw of 0.53, tert-amyl alcohol/oil volume ratio of 1: 1, methanol/oil molar ratio of 6: 1, 3% Novozym 435 and 40 AdegC), the reaction rate was enhanced to about 100% compared to conventional heating. A study conducted by Yadav and Pawar [10] on the transesterification reaction of ethyl-3-phenylpropanoate with n-butanol in the presence of Novozym 435 as catalyst found that 67% of ethyl-3-phenylpropanoate could be obtained after 180 mins under MW irradiation. This shows a major difference than when using Rhizomucor miehei (Lipozyme RMIM) and Thermomyces lanuginosus (Lipozyme TMIM) which attribute to less than 20% conversion. Addition of hexane as solvent gave the highest conversion rate, followed by toluene, 1,4-dioxane and n-butanol.

The optimized conditions for catalyst loading are 0.0032 g cm-3, absolute reaction temperature of 70 AdegC and optimized mole ratio of n-butanol 1: 0.2. Work by Kidwai et al. [157] tabulated in Table-4 showed that the reaction time of the ethanolamine amidation process can be reduced to 50 mins using MW-assisted solid media and 5 mins using MW-assisted solution compared to 180 mins using conventional heating. In addition, a higher yield of 97% could be obtained with MW-assisted solid media instead of 87% using conventional heating under the same conditions as shown in Table-4. Furthermore, enzyme catalyst showed higher reusability in MW-assisted reaction than conventional as defined in Table-5.

Table-4: Amidation yield of ethanolamine with conventional vs. MW heating [157]

Type of heating###Conventional

###MW-assisted###MW-assisted

###solid media###solution

Molar ratio

(Lauric###1###1###1

Acid/Ethanolamine)

Catalyst CALB (mg)###20###20###20

Temperature (AdegC)###60###60###60

Time (mins)###180###50###5

Yield (%)###87###92###97

Table-5: Reusability data of CALB after several runs on yield of amidation [157]

###MW-assisted###MW-assisted

Run###Conventional

###solid media###solution

1###87###92###97

2###78###88###96

3###65###85###95

4###-###80###93

MW and its Potential in Enzymatic Synthesis Reactions using Different Solvent Systems

To date, MW-assisted enzymatic reactions are still a subject of controversy due to the lack of understanding on how it works and most probably the issue of enzyme denaturation. Thus, several efforts have been made based on previous researchers' findings to provide extensive and comprehensive knowledge on the use of MW irradiation without sacrificing enzyme activity. To understand that, different solvents interact in a different ways with MW irradiation based on their polarity. Dielectric properties such as Iu and tan I' are critical parameters for reaction under MW irradiation. High values of these two properties are preferable as they indicate mixtures' capability to absorb and convert MW energy into heat efficiently. However, the values can be dictated by subjecting to different temperatures and frequencies because it is well known that dielectric properties are frequency and temperature dependent.

Polar solvents such as alcohols, DMF, water, ketone and acid work well under MW and are able to reach high temperatures in a short time. Non-polar solvents e.g. toluene, chloroform and hexane are transparent to MW. Thus, two situations are possible:

1). Using a non-polar solvent, but polar reagents or at least one polar reagent: the reaction mixture is heated by MW.

2). Using a non-polar reaction mixture (both solvent and reagents). In this case, Weflon has to be added in order to absorb MW energy and heat the mixture.

Lin et al. [158] studied the accelerated enzymatic digestion of several proteins in various solvent systems under MW irradiation. They found that acetonitrile avoids the enzymes' inactivity at 10 mins of MW irradiation using a single-beam MW applicator, while methanol deactivates it. It is to note that the enzymatic digestion of proteins in the presence of acetonitrile as solvent under conventional heating contributes to the denaturation of enzyme and reduces enzyme activity. Lukasiewicz et al. [159] revealed that a MW-assisted enzymatic hydrolysis of starch could increase its initial rate of reaction up to 2.5 times compared to conventional heating.

Nevertheless, enzyme activity is strongly dependent on the MW power level and viscosity of the reaction mixture. For MW-assisted enzymatic reactions, the use of polar solvent is not preferable in order to avoid hot spots in the system, denaturation of enzymes and reduced reaction rates.

An investigation described by Da Ros et al. [160] using immobilized lipases in MW-assisted enzymatic synthesis of beef tallow for the production of biodiesel had successfully manipulated water content during the reaction, thus avoiding soap formation which is unnecessary in the reaction. However, the disadvantage of using immobilized lipases over chemical catalysts is its long reaction time of about 48 h. This drawback can be overcome by using MW irradiation and the reaction rate is expected to increase threefold in the initial 2 h. The reaction was completed at 45AdegC with a molar ratio of beef-tallow to ethanol of 1: 9 under 150 rpm agitation speed. It is assumed that in this study, ethanol can absorb MW irradiation very well due to its high Iu (24.6) and induce the active sites of the enzyme, thus accelerating the reaction conditions. Conversely, a higher ratio of ethanol to beef tallow can reduce the transesterification yield.

However, a study conducted by Leadbeater et al. [161] showed that no substantial effect could be observed for the lipase catalyzed transesterification reaction of methyl acetoacetate in toluene under MW irradiation compared to conventional heating.

Ribeiro et al. [162] presented an example of successful transesterification of fluorinated aromatic compounds catalyzed by CALB under MW irradiation in toluene as solvent and vinyl acetate as acylation agent. The result showed that under MW irradiation, a higher yield and selectivity could be achieved within shorter reaction conditions than conventional heating. The reaction temperatures used were 45, 65 and 80 AdegC. High selectivity (up to 99 % enantiomeric excess) was observed for kinetic resolution of organofluoro compounds catalyzed by immobilized lipase from C antartica under conventional and MW irradiation. Yadaw and Pawar [163] successfully studied lipase catalyzed kinetic resolution of (+-) - 1-(1-naphthyl) ethanol under MW irradiation.

Three different types of catalyst were used: (a) Novozym 435: Lipase B from Candida antarctica, supported on a macroporous acrylic resin with a water content of 1-2% (w/w) and enzyme activity 10,000 PLU/g, (b) lipozyme RMIM: Lipase from Rhizomucormiehei, supported on a macroporous anion exchange resin with a water content of 2-3% (w/w) and enzyme activity 6 BAU/g, (c) lipozyme TLIM: Lipase from Thermomyceslanuginosus, supported on porous silica granulates with water content 1-2% and enzyme activity 175 IU/g. The results showed that the use of Novozym 435 in the presence of n-heptane as solvent at 60 AdegC under MW irradiation contributed to an increase in percentage yield of about 47.74% of conversion, followed by RMIM~ 10 % and TLIM ~ 5%. In addition, an investigation was made on four different types of solvents.

Among the employed solvents, n-heptane gave the highest conversion and only a slight difference was seen while using n-hexane and cyclohexane, but a very significant difference could be observed while using toluene over a reaction time of three h. It is remarkable to note that the decrease in conversion was only 2.94% after three times reusing the lipase catalyst.

Kerep and Ritter [164] presented an example on the consequence of using various types of solvents on lipase catalyzed ring opening polymerization of Iu-caprolactone under MW irradiation. The results showed that toluene and benzene gave negative effects on the reaction rate, while diethyl ether could speed up reactions. This might be due to the different boiling points of solvent and the Iu of the solvents to that reaction mixture. Sterchi and Stocker [165] revealed that only a small amount of solvent is needed for any enzymatic reaction to avoid the denaturation of enzyme. Three types of organic solvents were used, such as methanol, acetonitrile, chloroform to study the effect of protein digestion under MW irradiation. The results found that increasing the amount of methanol gives a reverse effect to protein digestion, while acetonitrile gave better results.

However, works by Matos et al. [166] and Risso et al. [167] on MW-assisted lipase catalyzed of poly-o-caprolactone synthesis effectively demonstrated that MW irradiation could encourage enzymatic reaction in the presence of no solvents.

Reusability of Immobilized Enzymes Under MW Irradiation

As a rapid reaction rate can be accomplished by MW irradiation, the reusability data of enzymes remains a focal concern. Yu et al. [156] revealed that five time recycling of Novozym 435 (without significant loss on the enzyme activity) could be used for the transesterification reaction of soybean oil with methanol under MW irradiation to produce fatty acid methy ester (FAME). In their study, ionic liquid (IL) was used as a reaction medium and good synergism effect was achieved by the enzyme and IL in terms of enhancing enzyme reactivity. A study conducted by Da Ro' s [168] on the ethanolysis of vegetables oil by enzymatic catalysis accelerated by MW irradiation claimed that the activity of the enzyme remained at 75% after seven runs.

Five cycles of reusability data could be achieved by Novozym 435 under optimum conditions (aw of 0.53, tert-amyl alcohol/oil volume ratio of 1: 1, methanol/oil molar ratio of 6: 1, 3% Novozym 435 and 40 AdegC) for MW-assisted fatty acid methyl ester production from soybean oil. This supports the fact that Novozym 435 is highly stable and does not denature upon MW irradiation [154]. A study by Yadav and Pawar [10] discovered that immobilized Novozym 435 can be reused three times without affecting the percentage conversion of ethyl-3-phenylpropanoate. Thus, it is proven that enzyme stability does not decrease under MW irradiation. Moreover, a good synergism effect could be attained between enzyme and MW irradiation in this study.

MW Enhanced Chemistry

Liu et al. [169] reported a study on the effect of combining a small amount of Ionic Liquid (IL) with MW irradiation, resulting in tremendous MW-absorbing ability due to the increase in dielectric properties of the reaction mixture. Other findings reported by Abe et al. [170] and Yoshiyama et al. [171] revealed that activity, stability and enantioselectivity of the enzymes can be enhanced by the addition of small amounts of IL. However, the major drawback of using IL is the decreased enzyme performance due to the higher viscosity of the IL. Thus, a study has been conducted by Liu et al. [169] in which higher enzyme performance can be achieved by combining IL co-lyophilized lipase and MW irradiation for [alpha]-lipoic acid esterification. In addition, the effects of solvents along the Log P between -1.1 to 4.5 was also studied by Liu et al. [169] and the best result was obtained when using cyclohexane with a Log P value of 3.2.

Higher Log P values are not suitable since the desired amount of water from the enzyme was stripped off by the polar solvent, automatically reducing enzyme activity. The reusability of the IL co-lyophilized enzyme was reduced about 4.8% after 6 cycles of reaction conducted at 25 AdegC and 480 Watt under MW irradiation.

Another comparison was made between acetic acid, tan I' (0.174) with ethanol, tan I' (0.941) which had designated that ethanol can achieve high temperatures more rapidly than acetic acid, thus it is thought that ethanol interacts better with MW irradiation. In order to overcome the problem regarding poor Iu and tan I' of the solvent, a small amount of additives with high value of Iu and tan I' such as ionic salts could be added to ensure appropriate heating throughout the mixture. This is due to the nature of ion oscillation which enhances heat generation in MW irradiation rather than the motion of dipoles [2]. Another effective way as claimed by Hoz et al. [172] to enhance reaction conditions which is not favorable in MW or does not even occur under conventional heating is to overheat the polar substances and include some reflector materials to create hotspots. These similar effects have been supported by two other reviewed papers from Raner et al. [173] and Nilsson et al. [174].

An extensive study conducted by Hoz et al. [172] indicated that the selectivity of the organic synthesis product can be achieved by choosing the appropriate mode of heating, e.g. conventional or MW heating, proper selection of reaction conditions, solvent, temperature, time or using kinetic vs. thermodynamic control, protection and activation, and catalyst.

Trends, Challenges and Future Directions of MW Technology in Organic Synthesis

Most examples of MW-assisted chemistry published to date have been performed on a small scale. The transformation from small scale to industrial scale may still be a matter of debate since the control can be more at a larger scale. The measurement and control of temperatures, pressures, and flowrates can be more challenging as it involves pipelines, pumps and valves from one reactor to another. Absorption of irradiation by the surfaces of pipelines might be occurred and cause overheating as the power has to travel along the pipelines to the reaction mixture. Selection of materials to be used for scaled-up MW reactors is also critical in order to avoid overheating of wall surfaces since MW heating is always associated with high temperatures and pressure. Careful consideration is desirable for the selection of material used inside the reactors in order to certify that it is highly resistant to chemicals and solvents.

For the outer surface, transparent MW irradiation vessels are required to hinder the possibility of heat transfer. The vessels employed as the outer surface should have high strengths at high temperatures to resist the larger forces from internal pressure since the reaction scale has increased. Maintaining the reaction temperature is no less important during the equipment design. The major drawback of using current industrial scale MW reactors is the non-uniformity of surface temperature at the exit [82]. This proves that industrial scale MW reactors lose the benefit of volumetric heating enjoyed in lab scale-MW heating. It is suggested to design continuous flow reactors for industrial scale instead of batch processes to sustain the benefits of lab scale-MW heating because MW irradiation is always associated with penetration depth, which is defined as the distance that the MW irradiation can penetrate into the substance.

There are two things expected in the developments of MW technology in the next few years. The first one is that there will be more efforts on larger scale reactors, moving from grams to kilograms with better engineering approaches. The most challenging part of the scale-up is to retain the advantages that lab scale MW can offer. Second is the exploration of MW technology into new reactions since the relation between dielectric properties of mixture and MW irradiation is not easy to understand. Thus, more investigation should be devoted into this topic.

Conclusion

MW heating is a promising technology that has the potential to replace conventional heating in the future. This is due to its drastic rate of enhancement and reduction of solvent usage in the chemical reaction systems. However, knowledge in dielectric properties and MW energy of materials used are vital to achieve the desirable irradiation for heating. Under appropriate enzymatic reaction conditions, good synergism effects could be achieved when MW is introduced into reaction systems.

Acknowledgement

The authors would like to thank the Director-General of MPOB for permission to publish this article.

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Author:Pauzi, Nik Nurfatmah Pz Nik; Hazmi, Ahmad Syafiq Ahmad; Aziz, Haliza Abdul; Huei, Lim Wen
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