Microwave-assisted hydrothermal treatments for biomass valorisation : a critical review

The sustainable conversion of biomass into biofuels, chemicals and biomaterials has gained increasing attention to ensure the well-being of present and future generations. Among the different technologies available to date for the valorisation of biomass, microwave-assisted hydrothermal conversion has recently appeared as a state-of-the-art technology, capable of furnishing a wide range of reaction products for the energy, pharmaceutical and chemistry sectors. This emerging technology combines the inherent benefits of microwave heating and the sustainable features of biomass hydrothermal valorisation. Herein, for the first time, this critical review summarises and analyses all the work conducted to date on the use of microwave-assisted hydrothermal processes (including microwave-assisted carbonisation, liquefaction and treatment/hydrolysis) for the conversion of biomass into hydrochar, bio-crude (bio-oil) and valuable chemicals. In particular, this work has put together vital information addressing the influences of the reaction conditions (temperature, time, amount and type of catalyst, biomass loading and type, and microwave power) on the yields and key properties of the reaction products. The relationships between these processing parameters and the chemical transformations involved in the processes (hydrolysis, dehydration, decarboxylation, condensation and re-polymerisation) have been described in detail, and reliable comparisons have also been established between microwave-assisted and conventional hydrothermal technologies when data were available. As a result, this critical review collects essential information on the use of this cutting-edge, recently appeared microwave-assisted hydrothermal technology, and paves the way for its expansion and future development and commercialisation

Microwave-assisted conversion of lignocellulosic polysaccharides into platform chemicals

Lignocellulosic biomass consists for 70% of polysaccharides (cellulose microfibrils and hemicellulose). Therefore, it is a valuable source for platform chemical production from polysaccharides. In this research, the possibility of microwave-assisted acid hydrolysis to convert lignocellulosic polysaccharides into platform chemicals (i.e. HMF and furfural) was investigated. A biphasic reaction system is applied to avoid humin formation and thereby increasing the yields of the platform chemicals produced. The biphasic reaction system consists of an acidified water phase (where the acid functions as the catalyst) and an organic solvent phase. In a first stage, the process parameters (temperature, acid concentration and reaction time) for the conversion of cellulose to HMF were optimized using response surface methodology (i.e., Box Behnken design). Then, under these optimized parameters, the simultaneous conversion of C5 and C6 polysaccharides to furfural and HMF was investigated. From the results it is clear that no humins are formed during the microwave treatment and that it is possible to convert C5 and C6 sugars into furfural and HMF respectively. An maximal HMF yield is found to be 32,98 w%(± 1,11w%). Furfural yields were within the same range.status: publishe

Production of levulinic acid and furfural by microwave-assisted hydrolysis from model compounds: effect of temperature, acid concentration and reaction time

In this study a system for the production of levulinic acid and furfural from lignocellulosic polysaccharides was investigated using cellulose and xylan as model compounds. The goal was to determine the optimum process conditions for a dilute acid hydrolysis of cellulose and xylan to levulinic acid and furfural, respectively. The yields of levulinic acid and furfural were investigated by optimization of the main process parameters, i.e., HCl concentration (cHCl), reaction temperature (T) and reaction time (t), via Response Surface Methodology. The reaction rate was increased by the use of microwave irradiation as an efficient heating method, allowing significant energy and time savings. The following optimal conditions for the conversion of cellulose to levulinic acid were obtained: T of 200 °C, t of 3.32 min and cHCl of 1.37 M. Similarly, the optimal conditions for the conversion of xylan to furfural were: T of 195°C, t of 1 min and cHCl of 0.36 M. The value of all three process variables under scrutiny were higher for the production of levulinic acid. It can hence be concluded that (i) the optimal conversion of cellulose and xylan takes place at different process conditions, hampering the simultaneous conversion of both components present in lignocellulosic biomass, (ii) a further optimization of the process parameters is needed. However, the overall results of this study provide useful information to developing more cost-effective and efficient systems for the production of platform chemicals from lignocellulosic biomass.status: publishe

Simultaneous conversion of C5 and C6 lignocellulosic polysaccharides to platform chemicals through microwave-assisted hydrolysis

Lignocellulosic biomass consists for ca. 70% of polysaccharides (cellulose microfibrils and hemicellulose) and ca. 30% lignin. Therefore, it is a valuable source for platform chemical production from polysaccharides. Even though biomass is relatively cheap and readily available, it is still a major challenge to break down the energy rich polysaccharides into platform chemicals that can be used in further processes. In this research, the possibility of microwave-assisted acid hydrolysis to convert lignocellulosic polysaccharides into platform chemicals (i.e. HMF and furfural) was investigated. A biphasic reaction system is applied to prevent the rapid rehydration of hydroxymethylfurfural (HMF) to levulinic acid and avoid polymerization to disperse polymers, the so-called humins. The biphasic reaction system consists of an acidified water phase (where the acid functions as the catalyst) and an organic solvent phase. The system has been heated by microwave radiation, which allows a quicker heating process and is said to have direct positive effects on the production of HMF. These advantages of a microwave heated biphasic reaction system contribute to the increase of the yields of the platform chemicals. In a first stage, the process parameters (temperature, acid concentration and reaction time) for the conversion of cellulose to HMF were optimized using response surface methodology (i.e., Box Behnken design). Then, under these optimized parameters, the simultaneous conversion of C5 and C6 polysaccharides (using xylan and cellulose as model components) to furfural and HMF was investigated. From the results it is clear that no humins are formed during the microwave treatment and that it is possible to convert C5 and C6 sugars into furfural and HMF respectively. A maximal HMF yield is found to be 32,98 w%(± 1,11w%). Furfural yields were within the same range. Previously named results were obtained using xylan and cellulose as model compounds for lignocellulosic polysaccharides. However, the lignocellulosic matrix itself (including lignin) can have a significant influence on the conversion pathways to the desired platform chemicals. Therefore the effect of the lignocellulosic matrix has also been investigated. In this research bamboo was applied as lignocellulosic biomass. In advance the cellulose and hemicellulose content of bamboo was determined using the Van Soest analysis. Also in this case C5 and C6 polysaccharides were simultaneous converted to furfural and HMF, while lignin remained after reaction.status: publishe

Microwave effects in the dilute acid hydrolysis of cellulose to 5-hydroxymethylfurfural

In this study, the effect of microwaves on the production of 5-hydroxymethylfurfural (HMF) in a biphasic system was evaluated via a kinetic analysis. The reaction system consisted of an acidified aqueous phase and methyl isobutyl ketone (MIBK) as an organic phase, in which HMF is extracted directly upon formation during the reaction. Two identically shaped reactors were used to assess the influence of microwaves on the production of HMF. A borosilicate glass reactor was used to heat the reaction mixture via microwaves directly, whereas the silicon carbide (SiC) wall of the second reactor absorbed all microwaves and hence the reactor content was heated via convective heat transfer. An identical temperature profile was imposed on both reactors. Cellulose, glucose and fructose were chosen as feedstocks for the conversion to HMF. It was observed that microwaves have a significant effect on the reactions. The hydrolysis of cellulose to glucose was a 2.3 folds faster in the presence of microwaves at the process conditions (0.046 M HCl, 177 °C). The isomerization of glucose to fructose showed a similar increase (factor 2.5). The required energy input for the reaction was systematically higher for the SiC reactor.status: publishe

Optimization of hydrothermal conversion of bamboo (Phyllostachys aureosulcata) to levulinic acid via response surface methodology

In this study, the dilute acid hydrolysis of lignocellulosic bamboo (Phyllostachys aureosulcata) particles to levulinic acid in a hydrothermal synthesis reactor is reported. The aim of the study was to optimize the reaction conditions for maximum levulinic acid production in terms of reaction time (t), reaction temperature (T) and HCl concentration (cHCl) via Response Surface Methodology (RSM). A maximum levulinic acid yield of 9.46 w% was predicted at the following reaction conditions: t of 3 h, T of 160 °C and cHCl of 0.37 M. A maximal experimental yield of levulinic acid of 10.13 w% was observed, which in respect to the cellulose fraction of the bamboo particles corresponds to 34.60 w% or 48.05 mol%. Furfural, which is formed by the hemicellulose fraction of bamboo, has not been observed within the boundaries of the RSM model, since it is already degraded under the given reaction conditions. The conversion of levulinic acid and furfural occurred more or less simultaneously, however, furfural was more vulnerable to degradation reactions at the given process conditions. Therefore, if both fractions (cellulose + hemicellulose) are required to be valorized, further optimization is required. However, the global results of this study provide insight in the potential of lignocellulosic bamboo as an alternative platform to fossil sources.status: publishe

A microwave-assisted process for the in-situ production of 5-hydroxymethylfurfural and furfural from lignocellulosic polysaccharides in a biphasic reaction system

© 2018 Elsevier Ltd In this study, a biphasic reaction system was developed for the microwave-assisted production of 5-hydroxymethylfurfural (HMF) and furfural. The proposed biphasic reaction system is a possible solution in the transition of current industrially state-of-the-art technologies which use C6 carbohydrates to an industrial process where lignocellulosic biomass can be processed, thus being a true sustainable process. In this respect, cellulose and xylan were chosen as model compounds since they are considered as the major building blocks of lignocellulosic polysaccharides. The reaction system consisted of an acidified aqueous phase and methyl isobutyl ketone (MIBK) as the organic phase. Upon the formation of HMF and furfural in the water phase, an instantaneous extraction into MIBK took place. The proposed biphasic reaction system offers many advantages over a monophasic reaction system, including (i) upon formation, furfural and HMF are continuously extracted into the MIBK layer, leading to the suppression of unwanted rehydration reactions, (ii) no unwanted solid particles are formed, and (iii) HMF and furfural can more easily be recovered from the reaction mixture. The goal of this study was to develop the microwave-assisted process in such a way that both cellulose and xylan were simultaneously converted. To this end, optimum process parameters in terms of HCl concentration (cHCl), temperature (T) and reaction time (t) were determined. A maximum HMF yield of 33.65 ± 0.46 w% (or 43.27 mol%) and a maximal furfural yield of 33.30 ± 1.30 w% (or 45.79 mol%) were achieved. To validate the proposed biphasic system, bamboo (Phyllostachys aureosulcata) was applied as a lignocellulosic feedstock, which resulted in a HMF yield of 42.44 mol% and a furfural yield of 48.90 mol%, based upon the cellulose and hemicellulose fraction of the bamboo particles. Since similar yields are observed compared to the model compounds, it is concluded that the lignocellulosic structure does not influence the reaction pathway. This means that the proposed biphasic system can be generalized for a wide variety of lignocellulosic feedstocks.status: publishe