Microwave-assisted thermochemical and primary

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Apr 22, 2014 -
Biomass Conv. Bioref. DOI 10.1007/s13399-014-0124-8

REVIEW ARTICLE

Microwave-assisted thermochemical and primary hydrolytic conversions of lignocellulosic resources: a review Aurore Richel & Nicolas Jacquet

Received: 21 February 2014 / Revised: 22 April 2014 / Accepted: 24 April 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract Faced with the inevitable depletion of fossil resources, agricultural productions have rapidly emerged as promising renewable alternatives. Particularly, the conversion of lignocellulosic materials has nowadays opened new vistas for the production of energy, biofuels, and chemicals. In this literature review, microwave technology is described as an original heating source either for the thermochemical conversions (at temperatures up to 400 °C) of lignocellulose into biofuels or the pretreatment (below 400 °C) and further hydrolysis of lignocellulose into bioethanol and other valuable chemicals. Advantages of microwave approaches include a commonly observed acceleration in reaction rate and improved selectivities and yields. Keywords Microwaves . Biomass . Lignocellulose . Green chemistry . Pyrolysis . Pretreatment . Hydrolysis

1 Introduction As an answer to the impending problems related to fossil fuels, agricultural raw materials have gained interest as possible alternatives to replace a wide diversity of petroleum-based products [1]. Competitive in price with petroleum, lignocellulosic feedstock, with a yearly supply of approximately 200 billion metric tons worldwide, appear indeed as plentiful and renewable resources [2]. However, despite this huge potential, nearly all bio-based fuels and chemicals produced at the present are obtained from edible resources such as starch and vegetable oils (first generation biorefinery). The challenge A. Richel (*) : N. Jacquet Unit of Biological and Industrial Chemistry, University of Liege— Gembloux Agro-Bio Tech, Passage des Déportés, 2, 5030 Gembloux, Belgium e-mail: [email protected]

imposed by notoriously heterogeneous and recalcitrant inedible lignocellulosic feedstock is the close association that exists between the structural components of lignocellulose, namely polymeric carbohydrates (cellulose and hemicelluloses) and the polyphenolic lignin component [3]. Thus, the conversion of lignocellulose into fuels, energy, or high-added value materials relies generally on two distinct steps, namely the thermochemical conversions at high temperatures (up to 400 °C) or the hydrolytic conversions of lignocellulosic biomass (moderate temperatures), to liberate the main structural components for further transformations [4, 5] (Scheme 1). The need of environmentally benign, innovative, and energy-efficient processes has progressively encouraged the development of the microwave (MW) technology as a nonconventional heating source [6]. This microwave heating has been reported for the processing of lignocellulose (mainly, pyrolysis or pretreatment, and subsequent, hydrolysis into valuable carbohydrates monomers). In this account, we describe the scientific literature covering this topic using mostly domestic or lab-scale microwave reactors.

2 Fundamentals of microwave technology Since the pioneering works of Gedye and Giguere in 1986, MW heating has been used for a plethora of organic synthetic conversions wherein reactions are accelerated due to the selective and efficient interaction of the MW electromagnetic wave, via dipolar polarisation and/or ionic conduction, with the polar constituents of the reaction medium [7, 8]. The ability of a given substance to convert electromagnetic energy into heat, at both selected frequency and temperature, is determined by the loss tangent factor (tanδ) [9]. This factor is expressed as the ratio between the dielectric loss (ε”) and the dielectric constant (ε’). Practically, substances with high tanδ values are well-suited candidates for microwave

Biomass Conv. Bioref. Scheme 1 Overview of the production of fuels and chemicals from lignocellulosic biomass

Thermochemical conversions

Lignocellulose

(up to 400°C)

Pyrolysis

Bio-oils

Gasification

Syngas

Hydrolytic transformations

Fuels Chemicals Materials

(moderate temperatures)

PRIMARY CONVERSIONS

Pretreatment and Hydrolysis Fermentation

Bioalcohols

Mono/di/polysaccharides Lignin, Cellulose

applications. In this sense, ethylene glycol and ethanol are convenient microwave solvents, by opposition with hexane (Table 1). Even if constituted of very polar oxygenated functions, cellulose has a very small tanδ. Indeed, for isolated dry cellulose fibers, this value reaches 5×10−3 at 20 °C and 50 Hz. Interestingly, it is the presence of lignin and residual water that improves considerably the ability of “fresh” lignocellulosic materials to absorb microwave incident energy. For instance, a 30 wt.% of residual water allows the tanδ of pinewood to shift to 0.35 (220 °C and 3 GHz), whilst this tanδ value does not exceed 0.05 for a 5 wt.% of residual water [10]. Therefore, for lignocellulosic transformations, two distincts modus operandi could be investigated. Either lignocellulosic feedstock is submitted under solventless conditions to microwave without any Table 1 Loss factor value (tanδ) of several materials estimated at 20 °C Material

Frequency

tanδ

Solvents Ethylene glycol Ethanol DMSO Methanol Glycerol Water Hexane Cellulose paper fibers Wood (% water)

2.45 GHza 2.45 GHz 2.45 GHz 2.45 GHz 2.45 GHz 2.45 GHz 2.45 GHz 50 Hz

1.350 0.941 0.825 0.659 0.651 0.123 0.020 0.005

3 GHz 1 MHz 1 MHz 10 MHz 10 MHz

0.014 0.058 0.004 0.035 0.140

Balsa (0 %) Beech (16 %) Birch (10 %) Walnut (0 %) Walnut (17 %) a

2.45 GHz is the operating frequency of commercial microwave ovens. However, for lignocellulosic components, the value of tanδ at 2.45 GHz is not referred in the literature. The values given in the Table 1 are thus given between 1 MHz to 3 GHz, depending on the specificity of measurement apparatus. Usually, tanδ increases proportionally with frequency

Abiological processes

Platform chemicals

Materials

previous drying (residual contained water allowing an efficient MW heating), or lignocellulosic materials (with low tanδ), are suspended into highly microwave-effective solvents to ensure fast and performant MW heating.

3 Thermochemical conversions Thermochemical conversion occurs at temperatures up to 400 °C and designates both gasification and pyrolysis technology. These processes are quite versatile and run on a wide variety of input biomass materials to generate a wide variety of output biofuels [11]. Biomass gasification produces hydrogen or syngas (H2 + CO) through catalytic ( 1 kWatt

Rice straw / sugar cane bagasse Beechwood

Glycerol/H2O H2O2–(NH4)2MoO4

No temperature control, 10 min 140 °C, 30 min

240 Watt Not specified

Barley straw

0.5 % H2SO4 or H2O only

200 °C, 5 min or 210 °C, 10 min

Not specified

Rice straw

1 % NaOH

No temperature control, 15 min–2 h

300–700 Watt

economically viable yields when a catalyst is employed. Due to their high efficiency and their ability to operate under mild conditions, enzymes have been selected as promising catalysts. However, the presence of residual lignin can be inhibitory to such enzyme hydrolysis processes. Other deficiencies include prohibitive costs of enzymes as well as the long reaction times required to perform convenient hydrolysis [33]. Some research has also been focused on the use of dilute or concentrated (Brönsted) acid catalysts. These chemical promoters induce a “de-crystallisation” of cellulose, which allows hydrolysis reaction with a high rate constant at moderate temperatures [58, 59]. Nevertheless, these homogeneous acids provide work-up disadvantages as the need for neutralisation, thus generating wastes, and corrosion of reactor materials. Solid acid catalysts, including H-form zeolites or sulfated and sulfonated-activated species, were thus investigated as environmentally benign alternatives [60–62]. Kabza and co-workers were the first to investigate in 1996 the effect of microwave irradiation on the hydrolysis of “model” cellulose [63]. To offer a comprehensive view of the process, the authors selected cellobiose hydrolysis to model β-glycosidic bond stability and scission under the influence of

Scheme 4 Most common cellulose conversion pathways: a cleavage into water-soluble sugars, platforms, and fuels; b non-invasive conversion into biopolymers. In route a, asterisk refers to chemical-catalysed processes

Cellulose

Yields (%)

Ref.

82 % glucose 63 % xylose 78–88 % glucose 87–96 % xylose Not specified 54 % glucose 5 % xylose 85 % glucose 46 % xylose 65 % glucose

[51] [52] [47] [52] [53] [54]

electromagnetically moderated enzymatic reactions (Scheme 5). Using cellulase from Penicillinum funiculosum as the promoter, at a temperature of 35 °C, microwave and non-microwave treatments were otherwise equipotent with a glucose production culminating at 600 mg/dL after a runtime of 6 min. For sake of comparison, Orozko et al. explored the acidpromoted hydrolysis of pure cellulose into water-soluble monosaccharides under microwave conditions. Their results highlighted a quite rapid decomposition of cellulose into glucose using 2 to 7.5 vol.% of phosphoric acid at a fixed temperature of 160 °C. Evolution of glucose yield as a function of time followed a bell curve with a maximum yield of approximately 90 % (w/w) glucose after about 5 min of microwave exposure. Further increase in reaction time was accompanied by charring or browning of produced glucose [64]. Kinetic analysis assumed a pseudo-homogeneous consecutive first order reaction and allowed an accurate appreciation of the benefits of MW by comparison with conventional non-microwave hydrolysis processes. When switching to a truly heterogeneous catalytic system, consisting of biomass char sulfonic acid particles (BC–SO3H),

Biomass Conv. Bioref. OH

OH

O HO HO

OH

OH HO O

OH O

Cellulase 35°C, aq.

2

O HO HO

OH OH

HO (α, β)

Scheme 5 Enzymatic microwave-assisted hydrolysis of cellobiose. Scheme inspired from Kabza et al.

Wu et al. offered recently a convenient sustainable insight into catalytic hydrothermal cellulose saccharification. After 60 min of microwave dielectric heating, at a constant power of 350 W, cellulose was converted nearly quantitatively into D-glucose. This BC–SO3H promoter displayed a much higher turnover number (TON, 1.33–1.73) for this microwaveassisted process compared to a diluted aq. H2SO4 system (TON, 0.02). The strong affinity of these new carboncontaining acid catalysts to β-1,4-glycosidic linkages seemed to be the key explanation of this remarkable acceleration effect [65]. Due to its hydrogen-bonded supramolecular structures, cellulose is insoluble in water. Therefore, the driving force of the aforementioned hydrolysis processing methodology is the progressive solubilisation of sugars into water. In order to accelerate the hydrolysis rate, solubilisation of cellulose was considered. Swatloski et al. disclosed in 2002 on the dissolution of cellulose in ionic liquids (ILs) at temperatures up to 70 °C [66]. In particular, ILs incorporating anions which are strong hydrogen bond acceptors were the more suited for such purpose [67]. In this context, Zhao et al. reported on the efficient depolymerisation of cellulose in ionic liquid 1-butyl-3-methylimidazolium chloride ([C4mim]Cl) catalysed by mineral acids (i.e. H2SO4/ cellulose mass ratio of 0.92 to 0.11). A noticeable acceleration of reaction rates was encountered at 100 °C after a few hours, under atmosphere and without pretreatment [68]. Noteworthy also is the approach devised by Amarasekara in 2009, with the employment of Brönsted acid ionic liquids acting as both the solvent and the catalyst. The d i s s ol u t i o n of c e l l u l o s e w a s e n s u r ed i n 1 - ( 1 propylsulfonic)-3-methylimidazolium chloride or 1-(1butylsulfonic)-3-methylimidazolium chloride (up to 20 g cellulose/100 g ILs). Hydrolysis of commercial cellulose samples (DP~450) operated, after addition of 2.0 equiv. of water per glucose unit of cellulose, at temperature up to 70 °C, and afforded glucose (14 % yield) alongside with other reducing sugars (62 %) after 30 min [69]. Zhang et al. reported on the tandem application of ILs and microwave as the heating source for saccharification of Avicel-type crystalline cellulose (DP~220) [70]. H-form zeolites with a low Si/Al ratio and a high surface area displayed a high hydrolytic activity in ionic liquid [C4mim]Cl, providing glucose in around 37 % yield (with a 47.5 % total reducing sugars yield) within 8 min at a microwave constant power of

240 Watts. Compared with classical oil-bath heating, microwave exhibited a significant reduction in reaction times and increased yields. For instance, after 8 min in an oil bath at 180 °C, glucose reached only a maximum yield of about 4.5 % (12.7 % total sugars yield). Attempt to consider enzymatic-promoted cellulose depolymerisation in ILs under microwave conditions was unconvincing. The main reason was related to denaturation of enzymes (cellulases from T. reesei), in ionic tris-(2hydroxyethyl)methylammonium methylsulfate liquid, mostly due to superheating by microwaves, producing a detrimental local temperature at the enzymes [71]. Beside these examples dedicated to the decomposition of commercial cellulose crystalline samples, few efforts have been directed toward the treatment of “more complex” and more “realistic” cellulosic systems. De Castro et al. developed recently a high-pressure batch microwave reactor for total hydrolysis of sugar cane bagasse, without pretreatment step. Using phosphoric or sulphuric acids as chemical catalyst, carbohydrates filled up to 68 % of the bagasse and 64 % of the bagasse was hydrolysed after 10 min in the microwave cavity at 12 atm [http://kongress.achema. de/]. When using a continuous flow microwave apparatus (with a 4.9 kWatt and a fixed temperature of 240 °C) with a feed capacity of 8–20 L/h of raw materials in suspension in water (1 kg dry material / 15 L water), Tsubaki et al. evidenced interest of microwaves for efficient separation of biomass components [72]. Their results include practical applications such microwaves pretreatment followed by enzymatic hydrolysis of substrates such as sugar cane bagasse, rice hulls, and woody residues. Even if microwave technology was demonstrated to provide better results than conventional methods, it is still necessary to assess whether these MW-assisted conversions are economically feasible, and hence, to evaluate the energy balance [73]. Continuous flow-type microwave units are expected to more effective in terms of energy/cost ratio. Depending on the pathway investigated (thermo-chemical or biochemical), the use of additives is demonstrated to reduce energy demand. Further works are, however, required in this “microwave biorefinery” field and the design of appropriate efficient, high-volume and continuous microwave reactors is, thus, needed.

5 Conclusion In conclusion, microwaves are depicted as a convenient heating for the conversion of lignocellulose into fuels, raw materials, and fermentable sugars. Nowadays, two distinct options are technically assumed. The first one involves the thermochemical transformation, at temperature up to 300 °C,

Biomass Conv. Bioref.

of raw materials into high calorific value chars, biogas, and bio-oils. The biomass is preferentially used without drying or dispersed in water, with or without additives, to ensure a convenient microwave heating. The second option relies on a two-step approach, namely the pretreatment of lignocellulose to liberate polysaccharides and their subsequent hydrolysis into fermentable sugars. Usually, microwaves present advantages in terms of yields, decrease in reaction times and/or processing temperatures.

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