Advantages and Limitations of Microwave Reactors: From Chemical

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Advantages and Limitations of Microwave Reactors, from Chemical Synthesis to the Catalytic Valorisation of Bio-based Chemicals Peter Priecel, and Jose Antonio Lopez-Sanchez ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03286 • Publication Date (Web): 12 Dec 2018 Downloaded from http://pubs.acs.org on December 13, 2018

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ACS Sustainable Chemistry & Engineering

Advantages and Limitations of Microwave Reactors, from Chemical Synthesis to the Catalytic Valorisation of Bio-based Chemicals Peter Priecel†,*, Jose Antonio Lopez-Sanchez†,* † Stephenson Institute for Renewable Energy, University of Liverpool, Department of Chemistry, Crown Street, L69 7ZD, Liverpool, United Kingdom. * corresponding author n.2: Peter Priecel, [email protected], tel. +44(0)1517958161 * corresponding author n.1: Jose Antonio Lopez-Sanchez, [email protected], tel. +44(0)1517943535

ABSTRACT This critical review examines recent scientific and patent literature in the application of microwave reactors for catalytic transformation of biomass and biomass-derived molecules with a particular emphasis on heterogeneous catalysis. Several recent reports highlight dramatic reductions in reaction time and even superior selectivity when microwaves are used. However, there are still many controversies and unexplained effects in this area that deserve attention. We critically review the available sources attempting to establish trends and elucidate the actual status of this area of research. Additionally, where possible, we discuss the potential for scale-up and commercial utilisation of microwaves and impediments that currently hold back their implementation. This critical review also aims at highlighting the opportunity of combining catalysis with microwave technology for biomass conversion but also to stimulate the reader to generate future understanding on the influence of the microwaves in catalytic processes in general.

KEYWORDS: microwave chemistry; scale-up; industry; microwave effect; catalysis; renewables; biomass;

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Introduction The use of microwave (MW) technology in chemical synthesis is well-established, and over 250 scientific papers are published each year as supported by the results of the Web of Science search on title microwave and topic chemistry being used in organic synthesis and materials processing. Microwaves have been demonstrated to heat chemical reactions effectively. They can be advantageous for the conversion of biomass and by-products, 1-3 but also waste 4 processing and reactions, especially using heterogeneous catalysts 5-8, both of which can be selectively heated by microwaves 9-10. Pyrolysis 11-14, torrefaction or carbonisation 15-18, bioactive compound extraction 19-20, hydrolysis 1, 21-28 or biodiesel production 29 are just a few examples of how many renewable sources could be valorised with the help of microwaves. Biomass and many of the compounds derived from it are susceptible to microwaves, but catalysts and especially those based on carbon can further enhance the absorption of MW 6-7, 30-32. Here, we critically analyse the biomass-related literature but also examine additional literature dedicated to microwaves and catalysis aiming to provide a perspective on the microwave effects and their controversies, advantages/disadvantages of this technology and pitfalls that follow microwave-assisted chemistry. Scale up and industrial utilisation of the microwave-assisted catalytic conversion will be presented whenever possible.

The interaction of microwaves with matter and solid catalysts Thanks to Albert Wallace Hull, inventor of the magnetron 33, microwave technology was developed first by the military in radar applications and later as microwave oven and wireless radio-communication 34. The first applications in chemistry followed in late 1950s/early 1960s mostly on decomposition or recombination of gases 35-38 or organics 39-41 and their polymerisation 42-43. Among the first were reports on gas phase and were said to be catalytic in nature - catalysed by one of the gases present in the mixture 38. The first heterogeneous catalytic study in microwaves was possibly the one by Greaves and Linnett 35 who investigated recombination of oxygen on different solid surfaces. Regarding microwaveassisted heterogeneous catalysis as we know it today, early reports were published by Kirkbride 44 as a patent for catalytic cracking of petroleum streams and by Wan 45 on hydrogenation of alkenes, water-gas shift reaction and hydrocracking/desulfurisation of crude oil. 2 ACS Paragon Plus Environment

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Microwave heating and various “microwave” effects rely on dipolar polarisation and ionic conduction which are connected to various properties of the compounds 46, for example, their permittivity and thus to their dielectric constant and dielectric/magnetic loss tangent, mathematical details of which can be found for example in the following publications 47-48. As microwave irradiation is electromagnetic and so both electric and magnetic parts need to be considered. Furthermore, the penetration depth of the material by microwave is critical, especially when scaling up a process and this will be discussed in more detail in the section 0. Also, it should be noted that the dielectric constant of a substance/material changes with temperature and this dependence is not known and tabulated for many compounds, which can complicate matters. Even more so, its determination is not so straightforward although there are efforts worldwide dedicated to this venture. Many books and reviews were published which detail the basics behind the interaction of microwaves and matter, some of which the reader is referred to48-51.

Microwave effect for organic and catalytic processes It is essential to be able to objectively assess the relevance and effect of reaction conditions to make the correct correlations, especially in the case of microwave reactors. There are many discussions in the literature and researchers are trying to prove/disprove the origin of the reaction rate enhancements observed in microwave-assisted processes vs. conventionally heated reactors. Some excellent books and reviews address thermal and non-thermal microwave effects 52-54.

Thermal vs non-thermal microwave effect When using microwave reactors, there are two main effects, which are illustrated well in the Figure 1.



Figure 1 Depiction of A) thermal microwave effect where heat dissipates from the friction of molecules induced by microwave irradiation, B) alignment of the dipoles of the molecules and charges of the electric field which could be interpreted as non-thermal microwave effect, C) effect of polar dimethylsulfoxide inducing dipoledipole interaction which could be seen as similar to what happens in B). Reproduced with permission from Perreux, L.; Loupy, A.; Petit, A., Nonthermal Effects of Microwaves in Organic Synthesis. In Microwaves in Organic Synthesis, de la Hoz, A.; Loupy, A., Eds. from Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Copyright 2013 55.

The first is the thermal effect which relates to the dielectric heating and results from the fact that molecular dipoles try to adjust to the changing electric field of the microwave radiation (Figure 1A) 49. This creates friction and collisions resulting in the generation of heat, which is spread from the molecules themselves by molecular friction and dielectric loss and therefore creates a more homogeneous thermal zone. This is followed by the secondary thermal phenomena, such as conduction, convection or radiation. However, in the cases of solid absorbers such as heterogeneous catalysts (e.g. carbon-based, containing metal nanoparticles), this heat can build up in these solids, causing overheating or hotspots. These might be challenging to measure or transfer into the bulk of the reaction mixture. A specific non-thermal microwave effect is related to the dipole-dipole-like interaction between the charges of the electric field and molecules with dipole moment 49. The energy that is created following the stabilisation is electrostatic and can be imagined to be similar to the alignment of the charges with passing electric current (Figure 1B) or the interaction of two dipoles brought about by a polar solvent (Figure 1C). Most investigations in the literature 56-66 could be described either as influenced by the thermal effect or by a combination of thermal and non-thermal effects, if existent. However, there is still much controversy in the literature, which culminated into several heated discussions. 67-71Most importantly, temperature measurement, homogeneity of the reaction components and reactor design are critical experimental variables for a true comparison of 4 ACS Paragon Plus Environment

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microwave-assisted reaction with conventional heating. These variables seem to be among the top reasons for the discrepancies in the literature and will be addressed subsequently.

Reality and controversy of the microwave effects The thermal effect induced by microwave irradiation is caused by inverse heat transfer, i.e. heating coming from the inside of the irradiated medium (e.g. solvent) towards the outside. This includes overheating of the polar solvent above its boiling point, hotspots such as those observed in solids (γ-alumina or MoS2) 8, 66 or selective overheating of the solvent 59, 72 or catalyst 65-66. Most of these phenomena can be noticed during any microwave-assisted reaction whether it is homogeneous or heterogeneous and liquid or gas phase. Discerning the contributions of different effects is not an easy task. Thermal and non-thermal microwave effects have been regularly reviewed 47, 50, 52-54, 63, 71, 7376.

The majority of the reviews or studies accepted the presence of thermal effects, i.e. direct

heating of the reaction components, whether it was solvent, substrate or catalyst, also leading to the formation of hot-spots in case of solid materials. On the other hand, most concluded there is little to no hard evidence on specific or non-thermal microwave effects, and such effects observed in the literature are at least in part explained due to irregularities in the temperature measurement, although contradictory findings appear as well 64. It was also suggested that non-thermal effects can be linked to the enhanced diffusion of the substrate 77, which was seemingly not yet challenged.

Temperature measurement challenges A precise and robust temperature measurement system is crucial not only for a fair comparison of the reactions, but more so between different reactor systems and laboratories. A recent tutorial review by Oliver Kappe provides a comprehensive and detailed discussion on the temperature control situation within microwave-assisted chemistry 78. The typical Pt100 thermocouple cannot be used in microwave systems due to its possible coupling with microwaves which results in its heating, thus showing misleading temperature measurements. This can be partly solved using a shielded thermocouple and positioning it in the cavity in a specific orientation to minimise the interaction. The next solution, which is widespread in microwave reactor technology is the infrared temperature sensor (IR). This is very useful in many cases as the sensor is not in direct contact with the reaction mixture. However, many 5 ACS Paragon Plus Environment


researchers voiced concerns of using only IR sensor to control temperature as it can easily report much lower temperature than that corresponding to reality 76, 79-80. For example, it was shown that the difference between IR and fibre optic-sensed (FO) temperature can be as high as 20-40 °C, depending on the set temperature Figure 2.

Figure 2 The difference between the measurement of temperature in ETHOS 1600 microwave reactor at reflux conditions as measured by fibre optic (FO), typical metal (MS) and infrared sensor (IR) when controlled by FO. Reproduced with permission from Nüchter, M.; Ondruschka, B.; Bonrath, W.; Gum, A., Microwave assisted synthesis - a critical technology overview. Green Chem. 2004, 6 (3), 128-141. from Royal Society of Chemistry, Copyright 2004.80

The reason is that the IR sensor reads the temperature of the outside of the vessel and it was shown the microwaves heat from the inside out, which can be further complicated by the composition of the reaction mixture. So far, the use of a fibre optic sensor appears the best option. The detector can be inserted directly in the reaction solution, protected either by glass/quartz or ceramic thermowell, which is microwave-transparent along with the material of the FO itself. Nevertheless, one has to be careful about the homogeneity of the reaction mixture and to make sure that the appropriate stirring is in place. Even better, more than one FO could be used to monitor the temperature at different positions in the reactor while comparing these to external IR sensor/camera measurements.


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Several heated discussions appeared in the literature in the early 2000s and continued until recently, e.g. 2002 highlight on non-thermal microwave effects by Kuhnert 67 vs. response by Strauss 68 and about a decade later, in 2013, an essay on “myth or reality” of microwave effects in organic synthesis by Kappe, Pieber and Dallinger 71 vs. correspondence by Dudley, Stiegman and Rosana 69 followed by a reply from Kappe 70. Our mission here is not to take a side and foster further animosity, but rather to show how important it is to be word-specific in the publications concerning microwave effects and how hard it is to compare and reproduce the results performed across laboratories and even by the same instrument. We want to emphasise the importance of setting rigorous rules and procedures in such studies and that the details in the experimental are often responsible for divergent results. This is to say that a specific terminology should be established and agreed upon, as well as means of, for example, carrying out reaction temperature measurements. Here, we give several examples where FO was used to measure the temperature of a heterogeneous catalyst bed for a gas phase reaction and how the temperature can be underestimated even in these cases. First, the publications of Mingos and co-workers focused on the both endothermic reactions such as decomposition of hydrogen sulfide 8, 66 or exothermic hydrodesulfurisation of thiophene 8, further exemplified by dry reforming of methane 81, catalytic reduction of sulfur dioxide by methane 82 or methane coupling 83, catalysed by MoS2/γ-Al2O3 or Pt/γ-Al2O3. Generally, it was shown that the microwaveassisted reaction occurred at temperatures >200 °C lower than under conventional heating conditions. This was explained by the selective heating of the catalyst causing hot-spots which were not possible to detect with fibre optic sensor. This was supported by the phase change of the alumina support (γ to α modification) and particle size/shape change of MoS2, both of which occurred well below the official temperature threshold. Bond et al. 84 and Chen et al. 85 further reported similar formation of hot-spots. Recently, Xu and co-workers 65 claimed a “microwave catalytic effect” to be responsible for the activity enhancement in oxidative decomposition of NO over mixed oxide catalysts based on Ba and Mn. Even though the authors didn’t observe formation of hot-spots, they compared the results from microwave-assisted reaction at 300 °C to the activity at 500 °C under conventional heating as it was previously reported in the literature that the hot-spots can get up to 100-200 K higher than the measured temperature. Still, the difference in the conversion of NO over BaMnO3 was significant (93.7% in MW vs. 28.1% heated conventionally). One thing to be considered in this example is the use of “modified thermocouple probe inserted to the catalyst bed” 7 ACS Paragon Plus Environment


which suggests shielded thermocouple rather than FO sensor. Comparing this for example to the oxidative methane coupling 83 in which no MW effect was observed with oxygen while gas-phase activation/hot-spots/arcing was seen in the absence of oxygen, this study definitely decreases the room for doubt surrounding the microwave effect. These examples illustrate the need to accurately and properly measure the temperature and to show that even FO sensor might not be suitable for all applications, especially in the heterogeneous catalytic system where both solid catalyst or polar substrate (liquid or gaseous) might be activated by microwaves separately. Within biorefinery, this could be used for example for activation of recycled CO2, such as in the dry reforming reaction 86. Furthermore, other ways of temperature sensing are necessary on the microscopic level to definitely separate the claims for different microwave effects. Another way to solve the issues with temperature measurement, reactor comparison or type of heating was suggested by Obermayer and co-workers 87-88. They suggested that pyrex glass reactor used in microwave-assisted experiments can be replaced by silicon carbide, which would stop microwaves from penetrating the vessel and interacting with the reaction mixture. That way, the microwave reactor and temperature measurement (e.g. FO) would stay the same and microwaves could be still used to heat the SiC, which would in turn heat the reaction solution. The reason behind this is a very strong absorption of dielectric heating by SiC. However, the same group showed later by combination of electromagnetic simulations and experiments that despite the ability of SiC to effectively block the microwave radiation, part of the electric field is still present inside the silicon carbide vessel, which in turn does not completely block the microwave effects 89. Recently, Horikoshi and co-workers 90 compared hydrolysis of cellulose with sulfonated activated carbon catalyst both in general microwave setup in Pyrex vessel and in SiC vessel to block the microwaves interacting directly with the reaction mixture. Comparing the glass and SiC reactors the authors found that at 150, 180 and 200 °C the yield of total reducing sugar was almost 1.7, 1.2 and 1 times higher, respectively, reflecting the efficiency of the acid-catalysed hydrolysis of cellulose and the heating regime change. This means that as the temperature increased, the rate of hydrolysis and penetration depth increased and the dielectric loss factor of water decreased, allowing selective heating of the carbon catalyst particles (and possibly the cellulose substrate). However, the yield of glucose alone was at all three temperatures cases similar within the experimental error. Similarly, Sweygers et al. 24 studied hydrolysis of cellulose for the production of HMF in a biphasic system catalysed by HCl and compared their reaction starting from cellulose, 8 ACS Paragon Plus Environment

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glucose and fructose also by reaction in SiC vessel. They found 2.3 times reaction rate increase for the hydrolysis of cellulose under microwave irradiation as compared to SiC reactor at 177 °C as well as 2.5 times rate increase for glucose to fructose isomerisation. One further point should be added for the case of heterogeneous catalysis, mainly gas-phase reactions. As the main issue is the temperature sensing inside the catalytic bed and hot-spots in the MW transparent reactor, SiC technology would probably not be as useful. However, it might be beneficial to largely block the majority of the dielectric heating if the issue with temperature sensing can be solved.

Microwave interaction with heterogeneous catalysts and biomass or biorefinery byproducts Microwave-assisted heterogeneous catalysis was reviewed by multiple groups in gas-phase 9192

and most recently and thoroughly in 2009 by Durka, Gerven and Sankiewicz 93 and in the

liquid-phase 5, 7, 30-32, 94-95, most recently in 2018 by the group of Lopez-Sanchez 95 focusing on catalytic conversion involving renewables, such as hydrolysis, (trans)esterification, oxidation, hydrogenation or dehydration and in 2014 briefly by Horikoshi and Serpone 6. Highlighting from the 2018 review 95, higher yields and/or shorter times were reported in cellulose, glucose, fructose and sucrose dehydration to 5-hydroxymethylfurfural 96-99 although comparison of dehydration of xylose to furfural in water/NaCl/HCl solution in SiC vessel (“conventional heating”) vs microwave by Xiouras and co-workers 100 showed similar reaction kinetics between the two reported heating methods although MW-based run required 30% less power to perform the reaction in comparison with conventional heating. In another study on hydrolysis of starch a 94.5% glucose yield was obtained in 60 minutes at 100 °C under MW which was five times higher than under conventional heating 101. As an example of use of pressurised hydrogen for the conversion of furfural to furfuryl alcohol, Romano et al. 102 reached >99% yield of the desired product at 125 °C and less than three hours as compared to 100 °C lower when using microwaves. However, it was also reported that this is possible mainly due to the absorbed moisture (water) 115, 123-124. Additionally, catalysts can be utilised to improve the product selectivity 125 and similarly the produced char/coal/carbon can serve as a microwave absorber and be reused. In cases of algal biomass, it was suggested that the use of better microwave susceptors such as mentioned carbon is advisable 117. Considering that wastes such as tyres or plastics could be processed as by-products in biorefineries, Appleton and co-workers 4 discussed options for greener and more energy-efficient processing at the same time as barriers such as replacing the established technologies. Although it might be expected that MW-assisted pyrolysis for example would produce higher yields of bio-oil than under conventional heating, it was concluded this can’t be confirmed as there were cases where the bio-oil yield was lower and



generally the whole process depended on many parameters, such as reactor geometry, heating power, water and ash content, feedstock composition, etc. 117.

Characterisation of microwave-assisted processes One of the important parameters that is hugely neglected in MW-based studies is the quantity of microwaves absorbed by the reaction system. This can be measured and quantified as microwaves reflected. It is all too easy to say a high power magnetron was used, especially when the temperature of the system is not followed or a system with non-polar solvent is used, which is mostly transparent to microwave irradiation. Excluding general microwave ovens transformed into crude reactors, most of the commercial microwave chemical reactors don’t include the option to measure the reflected waves. One of the exceptions is SAIREM a producer of industrial scale systems also providing laboratory/pilot scale reactors. In this case the measurement of the real power used and also reflected is included as a standard feature and it is suggested that this would become a standard practice for the microwave systems manufacturers. Characterisation of other aspects of the MW processes is to follow the extent of the reaction (e.g. conversion of the substrate) or catalyst (e.g. in-situ spectroscopy). Recently, Helen Kitchen and co-workers 126 reviewed such in-situ techniques that could be used in conjunction with MW as a heat source. The most important characteristic of such techniques is they need to be compatible with MW systems in terms of interaction. Secondly, depending on the construction of both MW reactor and characterisation system, one or both of them might need to be modified for effective use. The majority of in-situ studies for MW-assisted processes in the literature are performed to follow synthesis of the materials 126. For example, X-ray diffraction (XRD) was used to study the synthesis of zeolites 127-128 or zeotypes 129 using laboratory powder XRD but also synchrotron-based XRD during the formation of transition metal alloys 130-131. Although these are just examples of synthesis of potential catalysts, similar principle could be used to follow structural changes in catalysts, especially during gas-phase reactions. Similarly to XRD, neutron scattering was applied for example in the hydrolysis of iron to iron oxide and oxyhydroxide 132. On the other hand, spectroscopies such as UV-vis 133, FTIR 134 or Raman 135-138 were shown to be suitable for monitoring the progress of chemical reactions. These are also relatively simple to implement as for example in-situ Raman probe can be easily inserted almost inside 12 ACS Paragon Plus Environment

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the microwave cavity in a monomode instrument and allowed monitoring of, for example, the base-catalysed condensation of salicylaldehyde with ethylacetoacetate to give 3acetylcoumarin 135 or esterification of acetic acid and butanol 136 (Figure 4).

Figure 4 Photograph of CEM Discover SP microwave reactor with in-situ Raman spectroscopic probe and control unit (top) 135 and in-situ Raman spectra of the reaction mixture showing quantitative conversion of acetic acid and butanol to butyl acetate in 18 s (bottom) 136. Reproduced with permission from Leadbeater, N. E.; Schmink, J. R., Use of Raman spectroscopy as a tool for in situ monitoring of microwave-promoted reactions. Nature Protocols 2007, 3, 1. from Nature Publishing Group, Copyright 2007 and Leadbeater, N. E.; Smith, R. J.; Barnard, T. M., Using in-situ Raman monitoring as a tool for rapid optimisation and scale-up of microwavepromoted organic synthesis: esterification as an example. Org. Biomol. Chem. 2007, 5 (5), 822-825. from Royal Society of Chemistry, Copyright 2007.



In-situ Raman spectroscopy also helped to witness non-equilibrium local heating occurring at dimethylsulfoxide molecules in the close proximity of the Co particles caused by MW heating when the absorption of the incident power is faster than the heat loss from the domains involved 137. The challenge in characterising biomass lies in the fact that for example in the most utilised process, i.e. pyrolysis, one starts with solid material which is quickly transformed into a mixture of gas, liquid and solid, although the liquid is usually in the form of hot vapours and needs to be cooled to condense. Nevertheless, even simple characterisation such as temperature measurement becomes challenging in this case and the only option might be an infrared sensor. As soon as the reaction of soluble biomass by-products takes place in the liquid phase, the situation changes and especially spectroscopies such as those mentioned in the previous paragraph apply.

Reactor design for the use of microwave technology Reactor design plays an important role in its modelling and construction. Parameters such as type (batch/flow) mode (mono/multi), size, materials used, microwave frequency and homogeneity, penetration depth (shape) should be considered and in many cases these are interconnected. Mode influences both size and shape of the cavity, penetration depth and homogeneity of the field and also frequency 139. Monomode means the distribution of the MW is very well defined in the cavity or specific spot. Typically, smaller cavities are used and reactor is placed directly inside the waveguide where there is the highest concentration of MW. A disadvantage is that standing waves can be produced. Scalability in monomode can be an issue due to the size of homogeneous space and penetration depth although for example use of 915 MHz allows use of bigger waveguides thanks to the larger penetration depth, e.g. for continuous flow reactors (Figure 5) and can also decrease the price of the system as compared to 2450 MHz.


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Figure 5 Example of the continuous flow reactor with Auger-type stirring using 75 kW 915 MHz microwave generator, usable e.g. for extraction. Reproduced from Industrial microwave assisted processing in chemistry website of SAIREM 140, © Copyright SAIREM.

In multimode systems the distribution of MW is not defined as the cavity is a contained space that follows the waveguide in which MW reflect on the metallic surface in a random fashion. This is easier to apply in a larger cavity and the field is said to be more homogeneous although this depends on the waveguide and antenna. Kitchen microwaves are an example of the multimode appliance. Moving antenna can enhance the homogeneous distribution of the microwave field and stirring can improve the temperature uniformity of the sample. Both mode types of reactors can be used in both batch or flow reactor type. The choice of the reactor type will depend on the application as for example extraction of chemicals from plants or pyrolysis might benefit from batch while especially gas-phase reactions are typically carried out in the flow reactors. Regarding these reactor specifications, several reviews and articles were published that discuss in detail the different parts of the reactor and their applicability in various industries 80, 139, 141-144. For example, Nüchter and co-authors compared specifically CEM Discover unit vs. Milestone ETHOS as representatives of monomode and multimode equipment, respectively 80. As the operating frequency of magnetrons is not completely stable and can vary by as much as 50 MHz for 2450 MHz device, applications which would heavily depend on the frequency stability should be monitored and reconsidered. Alternatively, newly modified solid-state radio-frequency transmitters could replace classic magnetrons and decrease the size of the microwave source, significantly increase the stability of the microwave frequency, energy efficiency and the lifetime of the source 145. For the current magnetron-based systems, comparing 915 and 2450 MHz systems shows several other significant differences 146. While single 2450 MHz magnetron can provide up to 30 kW power, 915 MHz can give up to 100



kW. Furthermore, total system efficiency of 915 MHz vs. 2450 MHz systems is 55-70 vs. 82 %, respectively. Typical materials used for the construction of microwave reactors are metals (e.g. steel), which reflects the microwave irradiation and is therefore a suitable material for Faraday cage and waveguides. Elements of the insides of the microwave reactors are typically made of RFtransparent glass, quartz, ceramics and polymers, such as PTFE or PEEK. Concerning penetration depth, it was mentioned previously that 915 MHz system can be beneficial when compared to 2450 MHz if the chemistry or the transformation in question allows it and for example the penetration depth (depth at which the power density decreases to 37 % of the surface value) of 915 MHz is ca. three times that of 2450 MHz. For liquidphase applications, such as an example of hydrolysis of cellulose, it was shown that penetration depth also changes with temperature 90. While this depth of 20 mm was reported at 30 °C, it increased to 55 mm at 90 °C due to the decrease in dielectric loss of water. To provide the reader with an idea of how the penetration depth of various materials at 2450 MHz changes, Table 1 is presented.

Table 1 Penetration depths of various materials at 25 °C (unless specified otherwise) at 2450 MHz. Values taken from 147.

Material

Penetration depth [mm]

water (at 45 °C)

14

water (at 90 °C)

57

ice (at -12 °C)

11000

paper, cardboard

200-600

wood

80-3500

teflon

92000

quartz glass

160000

Batch vs. flow reactor configuration Typical domestic microwave is an example of multimode non-stirred batch system. The majority of laboratory-scale systems are of either mono- or multimode batch construction with implemented stirring and offering an option to perform the chemistry in an open vessel or pressurised reactor 148-151 and in some cases also with the pressure of reactive gas 151-153, 16 ACS Paragon Plus Environment

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which can be especially suited for gas-liquid (and solid) systems, e.g. heterogeneous catalyst for liquid phase reaction with excess of reactive gas. This type of reactor can accommodate a multitude of reactions as well as those in the catalytic valorisation in biorefinery. A limitation of the microwave-assisted batch system becomes apparent when scaling up, which will be discussed in the next section. However, as the high-throughput chemistry is becoming more common in research laboratories, batch configuration is the preferred microwave system for parallel and combinatorial approach 154-155. On the other hand, flow systems offer continuity of operation and higher throughput. The downside within microwave chemistry that one needs to make sure the temperature measurement is done at different positions, especially in the fixed-bed configuration utilising heterogeneous catalysts and that the microwave field is homogeneous to create an isothermal zone. Within the laboratory scale, both small systems employing plug-flow 151 as well as continually stirred tank reactor (CSTR) 156 or other types are available, e.g. catalyst-coating of the tubing internal wall. Commercial flow units ready to process higher throughputs are attainable 157-158 and multiple designs for flow systems were presented in the open literature 159-166.

Most of these systems are built to withstand a certain pressure of reactive gas and

therefore be used for oxidation/hydrogenation-type reactions. Although it is not typical, larger systems for processing of solid materials such as pellets, possibly usable for solid biomass, were reported employing an Archimedes-type screw to move the material in the tubular reactor 167.

Scale-up of microwave-assisted reactions towards bio-based processes When considering the type of reactor to be used in microwave chemistry, scale needs to be considered as it will determine the configuration that can be used 142. The important law here to remember is the definition of isobaric heat capacity dQ=m.Cp.dT which means there is no difference between conventional vs MW heating as the heat exchanged depends only on mass of the sample, it’s Cp and the temperature difference. However, efficiency of both heating types is different. Where during the conventional heating a significant part is lost and heating is from the outside to inside, MW irradiation creates friction between the molecules which means the heat is generated from the inside out and so the efficiency of the MW heating is better than conventional. To this end, Bermúdez et al. 168 studied several different processes, such as the torrefaction of wheat straw, heating of water, synthesis of xerogels, grinding of 17 ACS Paragon Plus Environment


metallurgical coke and high temperature heating of charcoal to identify the difference between the energy consumption of conventionally and microwave heated process. Moreover, a range of scale was investigated, from 5 to >200 g. The authors found that regardless of the process the energy consumption depends mainly on the amount of the sample. In the range of 5-100 g the decrease in specific energy consumption is 90-95 % which is pronounced further up to 200 g sample level. For scale of >200 g the specific consumption is constant. Furthermore, the difference between microwave and conventional process can be improved by optimisation of the MW reactor (i.e. frequency, reactor geometry, field density and homogeneity). The lower power consumption of microwaveassisted processes, even at bigger scale has been confirmed by other authors, mainly in the case of biodiesel production 169-170, which are reasonably easily being implemented in the continuous mode. Choedkiatsakul and co-workers 161 claimed total energy consumption of 116.7 Wh/L of produced biodiesel heated by microwave as measured by plug-in meter including premixing tank, peristaltic pump, MW system and cooling system. This means ca. 48% energy savings when compared to 222 Wh/L consumption during similar conventionally heated process 171-172. Furthermore, the authors compared 8 other intensification reactor types from literature (including plug flow, membrane or ultrasound) along with reaction conditions, yields and notes on energy consumption and productivity. Even though specific numbers are missing and would require further analysis, it was suggested that all of these reactors suffer from disadvantages such as low productivity, high costs or long reaction times. Further comparison of conventional vs. microwave vs. ultrasonic-based heating for biodiesel production was attempted by Gude and Martinez-Guerra who still regarded microwave process as most favourable in terms of energy consumption, simplicity, separation times, processing times or product quality and quantity with ultrasound being somewhere in between the other two 173. On the other hand, concerns over magnetron energy efficiency leading to similar or worse power efficiencies than conventionally heated processed were voiced 174. It was also shown that simple process optimisation can increase energy savings up to 93 % 175 in microwaveassisted dilute sulphuric acid pretreatment for bioethaol production in comparison with similar earlier MW-based reports 176-178. Energy savings are claimed also by multiple companies manufacturing microwave systems 179-180. Generally, penetration depth is one of the main factors said to determine the size to which microwave-assisted process can be scaled. As it was mentioned before, penetration depth 18 ACS Paragon Plus Environment

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depends on the temperature of the solvent used, but also on the microwave frequency, both of which need to be taken into account. Also, scaling up can be achieved in other ways as opposed to only increasing the size of the reactor and options include changing the shape of the reactor (i.e. batch to CSTR or flow) or parallel reactors 107, 181. Another way to overcome the challenge of penetration depth can be decreasing the frequency of microwaves or using antenna/part of waveguide directly inserted into the reactor to limit the blind spots 151 (Figure 6).

Figure 6 Photograph (left) and cross-section scheme (right) of 20 L SAIREM LABOTRON™ X reactor which can be used in both batch and continuous mode. Reproduced from SAIREM LABOTRON™ X brochure 182, © Copyright SAIREM.

Sourcing from the literature, Bowman and co-workers 183 studied scaling up from monomode CEM Discover reactor to multimode CEM MARS or Milestone Microsynth with Q-20 rotor (now Flexiwave), all controlled by FO temperature sensor and both in open-vessel or sealed vessel modes exclusively for homogeneous organic reactions plenty of examples. This includes synthesis of fluorescein (solid, 397 g, 85% yield) from two solids (330 g of resorcinol and 222.1 g of phthalic anhydride) in Milestone SPMR reactor (tilted rotating vessel for solid state synthesis and scalability up to 4 L vessel) as compared to slightly smaller scale giving 80% yield of fluorescein (250 g) in general monomode open vessel microwave; ethoxycarbonylation of iodobenzene over palladium(II) acetate in ethanol in 19 ACS Paragon Plus Environment


pressurised vessel with gas in CEM Discover with gas addition kit (ca. 5 mL solution) scaled up in Anton Paar Synthos 3000 with thick-glass quartz vessels able to hold 80 bar pressure (92 mL solution divided into 8 vessels). In this case, 11.2 mL of iodobenzene and 16.8 mL of 1,8-diazabicyclo[5.4.0]undec-7-ene were dissolved in 92 mL of ethanol containing 22.4 mg of palladium(II) acetate gave 81% of ethyl benzoate in 30 minutes at 125 °C. Another example was a continuous flow system (Milestone Flowsynth) with dedicated flow reactor and back-pressure regulator for esterification of 2.5 L of glacial acetic acid and 2 L of butanol at 150 °C and 200 mL/min. (1 min. residence time) giving 78% butyl acetate as compared to 89 % yield of acetate from 347 mL of glacial acetic acid and 553 mL of butanol in multimode open vessel system; synthesis of 85 g of 2-amino-4-phenylthiazole (97 % yield) in continuous multimode flow system from 77.25 g of phenacyl chloride and 57.1 g of thiourea in 2 L of ethanol; Beckmann rearrangement of benzophenone and hydroxylamine hydrochloride with sulfuric acid in acetic acid and water giving N-phenylbenzamide in 50% yield (270 g in flow and multimode) vs. 74% (730 mg) in monomode and sealed vessel vs. 90% (178 g) in multimode and open vessel or Suzuki coupling of phenylboronic acid and 4-bromotoluene giving 83% yield of 4-methylbiphenyl (34.8 g) using NaOH or 63% using KOH on ca. doubled scale (both multimode continuous flow and 40 mL/min. (5 min. residence time) with KOH at 140 °C and NaOH at 150 °C). These syntheses show that selection of the appropriate system is as important as any other reaction parameter and can hugely influence the effectiveness of the process and thus needs to be considered case by case when scaling up. De la Hoz et al. 181 reviewed reproducibility of the MW-assisted reactions also during the scale up (included works of Kappe, Bowman or Ondruschka) and concluded it is possible up to ca. 12 L volumes but for the reproducibility the full reaction specifications need to be provided. Authors also summarised options for scale up including kilogram-scale continuous flow system (Milestone Flowsynth) for which now alternatives exist both available commercially 157-158 and constructed or published in the literature 160-162, 184. They mentioned that further development for slurries (reactants or products) needs to be done although it was proven possible and that the energy efficiency as an important parameter needs to be considered. To help with process optimisation during scale up, in-situ Raman spectroscopy was demonstrated to be beneficial for esterification of acetic acid and butanol 136. To date, the best showcase of the possibility to scale up microwave-heated chemical processing is the effort of Japanese Microwave Chemical Company Ltd. who showcased the scale up of chemical production from 5-500 mL level through the pilot scale (50-200 L) to 20 ACS Paragon Plus Environment

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commercial process plant (500-2000 L) leading to the world’s first large-scale showcased microwave chemical plant with 10t/day production 185-186, which will be discussed in the following section in further detail.

Industrial applications of microwaves As mentioned above, there is scope to scale up microwave-assisted processes and learning from history, there are plenty of applications of radio-frequency radiation, primarily radar, radiocommunication 187 and food processing 188. From process perspective, the most common uses are heating, drying, cooking, baking, sterilisation, pasteurisation or tempering, of which some are being used in other industries such as ceramics (sintering), nanotechnology, photovoltaics or paper, wood, textile, medicinal or pharmaceutical processing 189-191. Separate is the application of microwave plasma 192-193, which is out of the scope of this perspective. Uses that are closer to chemistry include abovementioned synthesis of solid materials (perovskites, hydrotalcites, nitrides, carbides, chalcogenides, ceramics) 126, polymers 194, extraction of fragrances, flavours or bioactive compounds 19, distillation or treatment of soil, sludge or wastewater 180, 195. Within chemistry and catalysis and renewables where available, industrial chemical plant by Microwave Chemical Co. Ltd. 196 and examples from patent literature on gas abatement, biodiesel production, cracking or pyrolysis are included, which will be discussed in the following section 197-214. Following sections will detail both companies and industries utilising/offering microwave technology as well as patent literature and prospect for expansion and challenges.

Commercial avenues offering microwave technology and how this could be useful for future biorefineries There are a number of companies worldwide manufacturing standard and tailor-made microwave equipment, such as generators, waveguides, reactors/machinery, e.g. Advanced Microwave Technologies Ltd. (UK) 215, AMTek Microwaves (USA) 216, Ferrite Microwave Technologies LLC (USA) 217, Microwave Chemical Co. Ltd. (Japan) 185, MUEGGE GMBH (Germany) 191, PÜSCHNER GMBH + CO KG (Germany) 189, SAIREM SAS (France) 190,



Thermex Thermatron LP (USA) 218. Of course this list is not exhaustive but gives reader an idea of the leaders in this field for industrial applications. Most of these companies claim energy savings during the microwave-assisted process and decrease in the process footprint and reaction time. Looking into the area of renewables, Advanced Microwave Technologies applied their knowledge to continuously pretreat biomass which can be then better anaerobically digested to produce biogas 215. Recently, MUEGGE engineers published a paper on “emerging applications of MW technology in chemistry, polymers and waste” 145. They briefly review the current situation and suggest several possible future applications of microwaves, for example pyrolysis and production and processing of polymers such as polylactic acids, polyurethane and melamine foams or PTFE/PEEK recycling. Furthermore, an important part of the future development of RF technology is miniaturisation and making it safer. This can be achieved by utilising solidstate chip technology. Already a power of ca. 400 W is attainable with 1-2 chips which could be extended to several kilowatts using multi-chip arrays and this would improve in the future. As our main interest and focus here is chemistry/renewables/catalysis, two companies or applications should be highlighted, i.e. Bionic Laboratories BLG GmbH (Germany) 219 and Microwave Chemical Co. Ltd. (Japan) 185. The first is focused on the catalytic production of fuel-type hydrocarbon mixtures in a continuous pyrolysis reactor and the company claims to be able to process a wide variety of feedstocks, such as woody biomass, tyres or municipal waste thanks to the unique reactor design with ability to modify power, modulation or pulse period on the go 220 (Figure 7). Secondary product is a porous char with high calorific value, which can be utilised as fuel, catalyst, adsorbent or fertiliser/soil enhancer.


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Figure 7 Scheme (top) and photograph (bottom) of the Bionic µfuel mf60 plant for pyrolytic processing of variety of feedstocks such as biomass and wastes. Reproduced from Bionic µfuel technology website 220 and as provided by Bionic, © Copyright Bionic Laboratories BLG GmbH.

Furthermore, the reaction atmosphere can be adapted to the process and desirable products. Also, it is claimed that difficult samples as tar sands or shale oil can be transformed. The latter revolutionised the use of microwaves in chemical industry as Microwave Chemical was the first to build and operate continuous-flow manufacture of fatty acid butyl esters with 3200t/year production capacity that was supposed to be for biodiesel 221-222 (Figure 8). However, as Japan’s biofuel market is unprepared for this, the collaboration with Toyo Ink 23 ACS Paragon Plus Environment


gave fatty acid esters a different use, such as that in ink 223. Already, Microwave Chemical is developing a polymer manufacturing process together with a chemical giant BASF 186 and second plant with 1000t/year capacity to produce sucrose esters which are to be used as emulsifiers in food is to be built in Yokkaichi, some 130 km from Osaka, jointly with company Taiyo Kagaku, manufacturer of functional ingredients 224. Furthermore, Microwave Chemical said to use this plant also to produce silver nanowires for smartphone touch panels.

Figure 8 Photographs of commercial continuous microwave reactor (top left), plant (top right, provided by Microwave Chemical Co. Ltd.) and process scheme (bottom) of the Microwave Chemical Co. Ltd. for catalysed production of esters from waste oils and alcohol. Reproduced from Microwave Chemical Co. Ltd. production plant website 222, © Copyright Microwave Chemical Co. Ltd.


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Worth mention is also a joint multinational project led by Norway that focused on the development and utilisation of mobile pyrolysis plants (MICROFUEL) to turn forest floor residue and wood waste into bio-oil and charcoal 225. Although continuous operation was not realised at the end of the project, 400-500 kg/h feedstock throughput was estimated on the basis of showcased batch operation. Furthermore, 2 t/h scale was approximated in the case of full scale continuous operation using 400 kW power supply. One can be hopeful that ventures such as those presented above will create a spark in chemical industry and lead to further utilisation of microwave technology, especially in chemical industry. Unfortunately, many other applications which might be already being utilised in industry in areas such as extraction, pyrolysis or biomass pretreatment are protected by confidentiality and therefore not available to mention here. However, next we try to provide a short look into the patent literature which can afford a peek into the future.

Patent literature towards commercial microwave-assisted catalysis and conversion of renewables Information on many applications, of which some are of a special interest due to being applied in chemical and potentially catalytic or renewables industry might not be publicly available as they could be protected by confidentiality between the provider of the microwave and reactor technology and their recipient. However, we mentioned one application by Microwave Chemical Co. Ltd. in Japan which is publicly available. Furthermore, there are multiple patents on the use of microwaves in catalytic processes and renewables 44, 198-203, 205210, 212-213, 226-230,

and although they don’t mean the technology is actually used, it definitely

shows there was an interest in using microwaves in catalysis and by-product valorisation. The applications range from refinery processes claimed mostly by petrochemical companies 44, 200-201, 226 195

to gas phase reactions 199, 210-212, 214, cracking of plastics 202, wastewater treatment

to processes for renewables 197, 203-206, 208-209, 213, 228-234.

Plethora of reactions such as reforming, (hydro)cracking, hydrodealkylation or polymerization were reported to be sped up by microwave irradiation and further should be more economic. It was suggested by Kirkbride 44 that utilisation of MW allows one to operate at less severe reaction conditions and also using smaller vessels/catalyst amount while also 25 ACS Paragon Plus Environment


extending the catalyst lifetime. In this case, one might be slightly less restricted by the scale up question although we can only hypothesize on the size of the vessels that Kirkbride mentioned. In the gas phase reactions, microwaves are focused on heating of the solid catalyst or it’s combination with MW absorber, such as SiC. Even then, radio-frequency-based heating was shown to be advantageous, mostly in decomposition of NOx 199, 211, but also for methane coupling 210, 212 or dry reforming 214. Within renewable chemistry, the most utilised reaction is the production of biodiesel via transesterification 197, 203-205, 208-209 which was shown to be conveniently performed in continuous mode 203-204. In addition to this, Microwave Chemical Co. Ltd. demonstrated scale up to 2t/day in flow, said to be for a general chemical production but this could possibly be for biodiesel production. Further patents include liquefaction of biomass228-234, whether it’s a pyrolysis to prepare oil specifically rich in aromatics228-229, 233 or furfural 213, a reactive dissolution using ionic liquids, which are themselves a good microwave absorbents 202 or even torrefaction to carbonise the feedstock 227 (Figure 9). If an ever-increasing amount of man-made plastics were considered a renewable resource, similar pyrolysis concept was demonstrated for their conversion to produce fuel-like oil 202.


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Figure 9 Scheme of a unit for the torrefaction of pelleted biomass (top) 227 and computerised model of how a system that could possibly be used for such a torrefaction could look like (2.45 GHz, 6 kW Continuous Flow Reactor with Archimedes screw mechanism, SAIREM) 167. Reproduced from US20110219679A1 and SAIREM, © Copyright University of York (2011) and SAIREM.

Although not definitive, both companies and patent literature show that there is an interest in microwave technology and it’s use in chemical industry. It seems it is only a question of time when radio-frequency heating permeates the manufacture of chemicals and fuels on the industrial scale as it did in food processing. As principles of green chemistry and energy efficiency become more and more important, this might give industry yet another push toward commercialising MW technology. One challenge which however remains is the initial capital investment in the plants using microwaves as in many industries the conventional techniques still seem to be preferred 4.

Conclusions/Outlook Many instances of microwaves being used for treatment of biomass, reactions of intermediates and by-products and even waste such as plastics, sludges or water containing organics and associated transformations were shown to benefit from microwave heating. Although mostly on the laboratory scale, attempts to scale up patent literature and even a few examples of commercial chemical processes indicates that there is an interest in using microwaves on an industrial scale. This would be particularly suitable for future biorefineries where microwave technology could be incorporated relatively easily since these facilities would be a new structures. The fact that microwave systems can not easily be incorporated 27 ACS Paragon Plus Environment


into existing engineering is one of their seemingly big disadvantage, as the replacement of current similar conventional systems would require significant investment. Also, in many cases of the use of microwaves in chemical transformations, the real origin of the enhancement/effect is either uncertain or not transmitted well to the research community, which does not help in implementing MW technology more widely. Regarding many of the discussed parameters, issues and challenges, it is important to adopt the following precautions to avoid ambiguities during the microwave-assisted reactions. We advise to always compare MW with a conventionally heated reaction to show the MW promotion and it would be best to compare the process in the same vessel or with use of SiC if possible to eliminate the difference in the reactor construction (volume, geometry, stirring, heating rate, etc.) although even SiC was shown not to be completely blocking MW (electric part of the contribution). Regarding the temperature measurement, it is necessary to be critical with IR temperature sensing vs fibre optic and consider the possibility of local catalyst surface temperature sensing which could further help in the discussion and apparent unreliability of the IR sensor with respect to bulk liquid/reaction temperature. This is also connected to single vs. multiple points of temperature sensing even with fibre optic sensors as it is typical to measure the temperature at several reactor points in industrial flow reactors and especially as one would be expecting inhomogeneities in the microwave field within the reactor. What is the outlook for MW in the chemical industry? We know that it is in widespread use in homes and food processing (drying). Can MW succeed even if it had only a thermal MW effect simply because of the ease of getting a fast and direct heating of the catalyst, substrate or solvent and so promoting shorter reaction times? This is hard to address as a whole and will be industry-specific. We believe that it is in part addressed by the interest demonstarted by numerous companies and patents and applications published on the manufacturers’ websites. What about MW efficiency? Energy efficiency was reported to be multiple times better (>30%) for MW heated processes as compared to conventionally heated processes which could help to introduce MW processes in the industry. This offers a clear advantage in terms of reducing energy costs and in terms of decarbonisation of processes, which adds to the fact that the energy vector (electricity) could be produced with renewable sources. Still, magnetrons are not very efficient in converting electrical to microwave power. Solid-statebased microwave generators can improve this efficiency and decrease in microwave frequency (e.g. from 2450 to 915 MHz) can cut down both manufacturing and operating costs 28 ACS Paragon Plus Environment

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and might even be beneficial from the chemistry point of view. This will, however, be process dependent and a further barrier to the introduction of microwave-based processes would be the costs involved in the replacement of the existent and working technologies. Can scale up issues be answered by flow reactors and/or parallel reactor systems which would still make it possible for MW to penetrate the whole volume of the reactor or make the processes continuous? Several flow reactor designs and commercial lab scale/semi-pilot units are available and commercial scale process in Japan was showcased. Also, lowering the MW frequency can help to increase the penetration depth to design bigger even but still monomode cavities for example for flow reactor as for example shown by SAIREM. Regarding the many biomass sources/feedstock and heterogeneous catalysts (e.g. carbonbased), many of them are excellent absorbents of MW which adds to the advantage of using MW-based technology and further biomass and non-biomass wastes could be co-processed in the future biorefineries. We must conclude that even though catalytic reactions of biomass and bioderivatives are showing great promise and typically very high reaction rates and selectivities are achieved, we must remain critical and always question ourselves and explore the rationale behind such observations.

Acknowledgements Firstly, we want to thank Prof. Jean Paul Lange for his questions and critical thinking and openness during the 2017 CATBIOR conference in Lyon. His questioning during Prof. J.A. Lopez-Sanchez key-note presentation and follow-up discussions inspired us to research deeper into microwave effects and to write this critical review. The authors thank the Engineering and Physical Sciences Research Council (EPSRC) (grant EP/K014773/1) and Department of Business Skills and Innovation (Regional Growth Fund, MicroBioRefinery). We also thank Dr. Etienne Savary (SAIREM) for fruitful discussions on the various aspects of microwave technology and commercial use.

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TOC:

Synopsis: This review provides the challenges and advances in the microwave technology applied to catalytic valorisation of renewables and wastes to fuels and chemicals.



Peter Priecel obtained his Master degree in physical chemistry with focus on heterogeneous catalysis in 2009 and Ph.D. in 2013 at the University of Pardubice, Czech Republic under supervision of prof. Libor Čapek and Dr. David Kubička (Unipetrol Research and Education Centre (UniCRE)). After 4 years of postdoctoral research, he became and currently is Research Coordinator at the Department of Chemistry, University of Liverpool in the group of prof. Jose A. Lopez-Sanchez. His research interests are catalytic biomass conversion and development of metal nanoparticles and porous catalytic materials together along with the use of microwave technology for chemical synthesis.


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Prof Jose Antonio Lopez-Sanchez obtained his Master degree in Chemistry at U.R.V (Tarragona, Spain) and his PhD in 2003 with Prof. G.J. Hutchings in Cardiff University. After research positions in the Salerno, Glasgow and Cardiff he became Lecturer at the University of Liverpool (2011) where he promoted to Senior Lecturer (2013) and Professor (2016). He has 20 years expertise in applied catalysis and his research is currently focused in the development of Catalysis for Biomass transformations, Energy, Photocatalysis and Nanostructured Materials for Catalysis. He has also recently developed biomass-derived polymers and works in the development of catalytic routes for the valorisation of waste plastic. He is Director of an open-access MicroBioRefinary facility within the Chemistry Department dedicated to the use of high-throughput automation, the use of microwaves and photocatalysis for biomass conversion and other related catalytic applications.


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