Ewelina Kuna

Ronan Behling

Sabine Valange

Gregory Chatel
email: gregory.chatel@univ-smb.fr

Juan Carlos Colmenares
email: jcarloscolmenares@ichf.edu.pl

Carol Sze

Ki Lin

Sonocatalysis: a potential sustainable pathway for the valorization of lignocellulosic biomass and derivatives

Keywords: Sonochemistry, Sonocatalysis, Biomass upgrading, Lignocellulosic waste valorization, Lignocellulose depolymerization

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The worldwide accessibility of biomass undoubtedly plays a fundamental role in sustainable energy production. In favorable circumstances, around 25% of global energy requirements can be supplied by biomass [START_REF] Perlack | Biomass as feedstock for a bioenergy and bioproducts industry: The technical feasibility of a billion-ton annual supply[END_REF]. The enormous potential of plant materials resides in the presence of lignocellulosic residues coming from forest residues, energy crops, municipal and industrial wastes [2]. This type of renewable resource is now increasingly used for the production of value-added chemicals and transport fuels [START_REF] Briens | Biomass valorization for fuel and chemicals production. A review[END_REF]. Nevertheless, plant dry matter possesses a sturdy structure, which complicates the effective conversion process of lignocellulosic biomass into platform chemicals. Lignocellulose is a recalcitrant biopolymer composed of the semi-crystalline polysaccharide cellulose, the polysaccharide hemicellulose, and the three-dimensional amorphous phenylpropanoid lignin polymer [START_REF] Chatel | Sonochemistry: what potential for conversion of lignocellulosic biomass into platform chemicals?[END_REF][START_REF] Mora-Pale | Room temperature ionic liquids as emerging solvents for the pretreatment of lignocellulosic biomass[END_REF]. The conversion of lignocellulosic feedstock leads to the production of a variety of added value platform chemicals, including phenolic compounds (pcoumaryl alcohol, coniferyl alcohol, and sinapyl alcohol), aliphatic acids (formic acid, acetic acid, levulinic acid), and furan aldehydes (hydroxymethyl furfural) [START_REF] Jo ¨nsson | Bioconversion of lignocellulose: inhibitors and detoxification[END_REF]. It is worth mentioning that the accessibility to desired products is dependent on biomass pretreatment methods, which have a significant influence on the production of their derived feedstocks for further valorization strategies. Notwithstanding, the selection of the most favorable pretreatment process depends predominantly on the target molecule and many other factors such as economical and environmental aspects [START_REF] Harmsen | Literature review of physical and chemical pretreatment processes for lignocellulosic Biomass[END_REF]. The depolymerization process of lignocellulosic biomass includes physical, chemical, and biological treatments, as well as various combinations thereof (Fig. 1) [START_REF] Harmsen | Literature review of physical and chemical pretreatment processes for lignocellulosic Biomass[END_REF][START_REF] Zakzeski | The catalytic valorization of lignin for the production of renewable chemicals[END_REF][START_REF] Silveira | Current pretreatment technologies for the development of cellulosic ethanol and biorefineries[END_REF]. The physical pretreatment of biomass as a first step for further upgrading is achieved by mechanical comminution of lignocellulosic materials through a combination of chipping, grinding, or milling. In the particular case of cellulosic biomass, such physical process leads to a size reduction due to a decrease of both the degree of polymerization and the crystallinity, resulting in the increase of the mass transfer and the improvement of the hydrolyzation reaction. However, the energy consumption required for physical treatment is higher than the theoretical energy content available in lignocellulose, which makes it prohibitively expensive for large-scale uses [START_REF] Kumar | Methods for pretreatment of lignocellulosic biomass for efficient hydrolysis and biofuel production[END_REF]. An alternative for deconstructing lignocellulosic biomass is chemical pretreatment. This method is based on catalytic processes such as acid/alkaline hydrolysis, oxidative delignification, cracking, reduction reaction, among others [START_REF] Brodeur | Chemical and physicochemical pretreatment of lignocellulosic biomass: a review[END_REF]. Acid pretreatment allows converting hemicellulose into monomeric sugars (e.g. glucose, xylose) and soluble oligomers, whereas alkaline hydrolysis renders lignin recoverable. Delignification of lignocellulosic biomass can also be performed by treating in the presence of oxidizing agents (e.g. hydrogen peroxide, ozone, and oxygen) [START_REF] Bussemaker | Effect of ultrasound on lignocellulosic biomass as a pretreatment for biorefinery and biofuel applications[END_REF]. In most of these cases, catalysis increases the efficiency of the process and is responsible for the major effects achieved by pretreatment [START_REF] Nitsos | The role of catalytic pretreatment in biomass valorization toward fuels and chemicals[END_REF]. The application of catalysts provides a more effective approach in biological processing, where the yield of hydrolysis is relatively low along with long pretreatment times. Enzymatic hydrolysis can also be improved by combining ultrasonic pretreatment with the organosolv process. Ultrasound has the potential to enhance the separation and hydrolysis of lignocellulosic materials [START_REF] Bussemaker | Effect of ultrasound on lignocellulosic biomass as a pretreatment for biorefinery and biofuel applications[END_REF]. By contrast, the combined use of organic solvents (including ionic liquids) or their mixtures with water was shown to enhance dissolution of biomass and increase depolymerization rates [START_REF] Akhtar | Recent advances in pretreatment technologies for efficient hydrolysis of lignocellulosic biomass[END_REF].

The immensity of available pretreatment methods creates opportunity to choose those which enable downstream processing of lignocellulosic biomass. Recovered carbohydrate polymers such as cellulose and hemicellulose can further be transformed into fermentable sugars and then into fuels or feedstock chemicals. Valuable products can also be obtained from lignin through development and integration of current and new technologies such as sonocatalysis, heterogeneous photocatalysis, or microwave-assisted conversion [START_REF] Gallezot | Conversion of biomass to selected chemical products[END_REF][START_REF] Li | A sustainable approach for lignin valorization by heterogeneous photocatalysis[END_REF]. According to a classification proposed in previous papers [START_REF] Zakzeski | The catalytic valorization of lignin for the production of renewable chemicals[END_REF][START_REF] Li | Catalytic Transformation of Lignin for the Production of Chemicals and Fuels[END_REF] a simplified summary of conversion strategies is given in Fig. 2. Thermal technologies can be used to produce solid feedstocks (e.g. biochar), liquids (e.g. oils and viscous tars), and gaseous products. However, methods such as pyrolysis and gasification require large energy inputs due to the high processing temperatures. Chemical conversion techniques (e.g. catalyzed depolymerization, hydrotreating, oxidation, liquid-phase reforming) constitute a more energy efficient and environmentally sustainable way to valorize biomass [START_REF] Li | Catalytic Transformation of Lignin for the Production of Chemicals and Fuels[END_REF].

In the present review, we focus on the recent literature advances on sonocatalytic valorization of lignocellulosic biomass and their platform molecules. It is evident that catalysis is regarded as a key route enabling technology for pretreatment and conversion of lignocellulosic biomass [START_REF] Zakzeski | The catalytic valorization of lignin for the production of renewable chemicals[END_REF]. Over the past few years, research on the development and optimization of highly active and selective catalytic systems has been an ongoing activity to overcome drawbacks associated with harsh chemical conditions, low yield production, and high processing cost [START_REF] Subhedar | Ultrasound assisted intensification of biodiesel production using enzymatic interesterification[END_REF]. Additionally, Fig. 2 Biorefinery strategies for lignocellulosic biomass valorization to fuels and chemicals. (Adapted and modified from Refs. [START_REF] Zakzeski | The catalytic valorization of lignin for the production of renewable chemicals[END_REF][START_REF] Li | Catalytic Transformation of Lignin for the Production of Chemicals and Fuels[END_REF]) sonochemical-assisted reactions offer opportunities to develop environmentally friendly and cost-effective processes for biomass upgrading [START_REF] Chatel | Sonochemistry: what potential for conversion of lignocellulosic biomass into platform chemicals?[END_REF][START_REF] Amaniampong | Selective and Catalyst-free Oxidation of D-Glucose to D-Glucuronic acid induced by High-Frequency Ultrasound[END_REF].

Generalities on Sonochemistry

Sonochemical effects arise from cavitation, which is defined as the phenomenon of formation, growth, and implosive collapse of bubbles under the influence of an ultrasonic field in liquids [START_REF] Gogate | Theory of cavitation and design aspects of cavitational reactors[END_REF][START_REF] Xu | Sonochemical synthesis of nanomaterials[END_REF]. Cavitation can be categorized into various forms (acoustic, hydrodynamic, optic, and particle cavitation) depending on the method of generation and associated ultrasonic/experimental parameters (frequency, acoustic power, shape of the reactor, solvents, temperature, pressure, etc.). Acoustic and hydrodynamic cavitation may generate physical and chemical changes in solution in contrast to optic and particle cavitations. Numerous ''hot spots'' can be created by acoustic and hydrodynamic cavitation due to the accumulation of a huge amount of energy which in turn results in immense pressures and temperatures [START_REF] Agarwal | Principle and applications of microbubble and nanobubble technology for water treatment[END_REF][START_REF] Suslick | Hot spot conditions during multi-bubble cavitation[END_REF]. The pressure fluctuation induced by changing the geometry of the flow system produces hydrodynamic cavitation, while the pressure fluctuation in the passageway of sound waves induced acoustic cavitation [START_REF] Colmenares | Sonication-induced pathways in the synthesis of light-active catalysts for photocatalytic oxidation of organic contaminants[END_REF].

Acoustic cavitation takes place within collapsing bubbles (gas-phase chemistry), on the outer side of the bubbles (solution-phase chemistry) and at the liquid-solid interface (physical modification) [START_REF] Xu | Sonochemical synthesis of nanomaterials[END_REF]. The chemical and physical effects of ultrasound eventuate from the cavitation phenomenon and not from direct interaction between chemical species and ultrasonic waves [START_REF] Xu | Sonochemical synthesis of nanomaterials[END_REF][START_REF] Suslick | Sonoluminescence and sonochemistry[END_REF]. The chemical effect of ultrasound is the consequence of the implosive collapse of microbubbles, producing free-radicals, whereas the physical effects are the result of shock waves Fig. 3 Acoustic cavitation mechanism and microjet generated during symmetric and asymmetric cavitation, respectively (Fig. 3) [START_REF] Hagenson | Comparison of the effects of ultrasound and mechanical agitation on a reacting solid-liquid system[END_REF].

The rate of motion of ultrasound is significantly greater than the molecular scale [START_REF] Suslick | Hot spot conditions during multi-bubble cavitation[END_REF]. Generally, when the sound velocities in a liquid are around 1000-1500 m s -1 the power ultrasound (in opposition to diagnostic ultrasound, particularly used in medical imaging) can oscillate from approximately 10 to 10 -4 cm over the frequency range of 20-2000 kHz [START_REF] Xu | Sonochemical synthesis of nanomaterials[END_REF][START_REF] Agarwal | Principle and applications of microbubble and nanobubble technology for water treatment[END_REF]. The compression and expansion waves put the liquid under dynamic tensile stress. As a result, the local pressure decreases adequately below the saturated vapor pressure and initiate cavitation [START_REF] Xu | Sonochemical synthesis of nanomaterials[END_REF][START_REF] Hickel | DNS and LES of two-phase flows with cavitation[END_REF]. Microbubbles present in a liquid absorb energy from ultrasound waves and undergo a rapid overgrowth leading to violent collapse. The final stage of implosion is almost adiabatic and provides extreme conditions [START_REF] Xu | Sonochemical synthesis of nanomaterials[END_REF][START_REF] Lorimer | Sonochemistry. Part 1-The physical aspects[END_REF].

Homogeneous and Heterogeneous Sonochemical Systems

Sonochemical reactions can be classified into three categories, namely homogeneous sonochemistry of liquids, heterogeneous sonochemistry of liquid-liquid or liquid-solid systems, and sonocatalysis (which involves the aforementioned systems) (Fig. 4) [START_REF] Price | Sonochemistry and sonoluminescence[END_REF][START_REF] Mason | Ultrasound in synthetic organic chemistry[END_REF]. Homogeneous systems include radical reactions, which are accelerated by sonication and that follow via radical or radical-ion intermediates [START_REF] Cella | Ultrasound in heterocycles chemistry[END_REF]. In this case, the chemical bonds are broken under the high temperature and pressure generated during cavitation [START_REF] Colmenares | Sonication-induced pathways in the synthesis of light-active catalysts for photocatalytic oxidation of organic contaminants[END_REF]. The short-lived chemical species are turned back to the bulk liquid and react with other species [START_REF] Cella | Ultrasound in heterocycles chemistry[END_REF]. Sometimes homogeneous sonochemistry followed by secondary reactions taking place in the liquid, especially in the case of compounds of low volatility, which can interact with Fig. 4 Classification of sonochemistry reactions radical species produced from solvent sonolysis [START_REF] Hussein | Sonochemistry: synthesis of bioactive heterocycles[END_REF]. In homogeneous systems, where the surroundings are uniform, the cavity remains spherical. Cavity collapse in heterogeneous system may proceed via two fundamentally different mechanisms such as microjet impact and shock wave damage. Deformation in the cavity is caused by asymmetric motions of the molecules in liquid during cavity collapse.

The expanded bubble's potential energy is converted into kinetic energy of a highspeed liquid jet that passes through the bubble's interior and pierces the opposite bubble wall. The available energy is predominantly handed over to the accelerating jet rather than the bubble wall itself [START_REF] Suslick | Encyclopedia of physical science and technology[END_REF][START_REF] Suslick | Sonochemistry. Van Nostrand's encyclopedia of chemistry[END_REF]. High energy concentration can cause an intense damage to the boundary surface. The stress fracture on the surface can be invoked by shockwaves generated through cavity collapse in the liquid. The impingement of microjets and shockwaves form the localized erosion, which is in charge of ultrasonic cleaning and another sonochemical effects such as particle size reduction or improved mass transfer on heterogeneous reactions [START_REF] Colmenares | Sonication-induced pathways in the synthesis of light-active catalysts for photocatalytic oxidation of organic contaminants[END_REF]. In heterogeneous systems, the use of ultrasound accelerates chemical reactions, drawing on mechanical effects of cavitation. The dynamics of the cavity collapse changes dramatically when cavitation takes place in a liquid nearby a solid surface [START_REF] Cella | Ultrasound in heterocycles chemistry[END_REF]. The imposition of the heterogeneous and homogeneous sonochemistry includes a radical and an ionic reaction mechanism. Indeed, depending of the ultrasonic frequency (see Sect. 2.2), the sonication enhances radical formation and mechanical effects (e.g. mass transfer) [START_REF] Mason | Ultrasound in synthetic organic chemistry[END_REF].

More Important Parameters in Sonochemistry

The selection of ultrasonic parameters (such as frequency, acoustic power, temperature, pressure, solvent, design of reactors, etc.) is a crucial issue in order to optimize the system and thus influence efficiency of a chemical process. There are different ways to optimize these parameters depending on the target experimental outcome; they can be chosen by taking into account the data concerning studies on bubble cavitation characteristics [START_REF] Luo | Ultrasound-enhanced conversion of biomass to biofuels[END_REF]. The problem connected with cavitation distribution and quantification might be addressed by using different experimental or theoretical mapping techniques [START_REF] Gogate | Sonochemical reactors: important design and scale up considerations with a special emphasis on heterogeneous systems[END_REF]. The basic aspects of each method, their applicability, pros and cons have been highlighted by Sutkar and Gogate [START_REF] Gogate | Sonochemical reactors: important design and scale up considerations with a special emphasis on heterogeneous systems[END_REF][START_REF] Sutkar | Design aspects of sonochemical reactors: techniques for understanding cavitational activity distribution and effect of operating parameters[END_REF]. Their analysis allows one to determine the behaviour of cavitational activity regarding reactor geometry over a range of operating parameters [START_REF] Colmenares | Sonication-induced pathways in the synthesis of light-active catalysts for photocatalytic oxidation of organic contaminants[END_REF]. In order to estimate mutual relation, bubble dynamics analysis has been also employed by other researchers [START_REF] Gogate | Sonochemical reactors: important design and scale up considerations with a special emphasis on heterogeneous systems[END_REF][START_REF] Flannigan | Plasma formation and temperature measurement during singlebubble cavitation[END_REF]. The study includes quantifying the correlation between the pressure and temperature linked with cavity implosion as a function of intensity, frequency, and initial radius of the nuclei [START_REF] Colmenares | Sonication-induced pathways in the synthesis of light-active catalysts for photocatalytic oxidation of organic contaminants[END_REF].

The cavity dynamics is defined by two elements: the maximum magnitude accomplished by the cavity before implosion (that determines the pressure and temperature generating during collapse) and the lifetime of the cavity (that defines the distance travelled by the cavity from the place where it is formed) [START_REF] Gogate | Sonochemical reactors: important design and scale up considerations with a special emphasis on heterogeneous systems[END_REF][START_REF] Gogate | Design and scale-up of sonochemical reactors for food processing and other applications[END_REF]. Both elements are of paramount importance for the design of sonochemical reactors and should be optimized by appropriately adjusting the various geometric and operating parameters [START_REF] Gogate | Sonochemical reactors: important design and scale up considerations with a special emphasis on heterogeneous systems[END_REF]. The reactor design has significant effects on the cavitational activity, in terms of the reactor and horn tip diameters (including the ratio of both diameters) and the position of the horn tip immersed in the liquid [START_REF] Colmenares | Sonication-induced pathways in the synthesis of light-active catalysts for photocatalytic oxidation of organic contaminants[END_REF][START_REF] Gogate | Cavitation Generation and usage without ultrasound: hydrodynamic cavitation[END_REF].

Another factor that has an influence on the cavitation is the amount of energy, which is supplied to the bulk solution. The power dissipation rate varies on the extent of temperature growth, which causes direct changes in the gas solubility and vapour pressure, generating active cavitation sites. Energy efficiency is expressed by the amount of energy dissipated into the liquid and is generally calculated by a calorimetric method (monitoring the temperature as a function of time allows to estimate the acoustic power) [START_REF] Colmenares | Sonication-induced pathways in the synthesis of light-active catalysts for photocatalytic oxidation of organic contaminants[END_REF][START_REF] Gogate | Sonochemical reactors: important design and scale up considerations with a special emphasis on heterogeneous systems[END_REF].

The effect on the bubble cavitation is directly connected to the frequency of the ultrasound. We can distinguish frequencies at low range (20-80 kHz) and high range ([150 kHz) [START_REF] Kardos | Sonochemistry of carbohydrate compounds[END_REF]. High frequency does not promote the occurrence of active cavitation, because of insufficient duration of the ultrasonic cycle, which is required for the growth, radial motion, and collapse of bubbles. The short-lived bubble can boost the concentration of free radicals and may have a higher probability to get out the cavitation site to the bulk mixture. Compared to lower frequency ultrasound, high ultrasonic frequencies produce less violent cavitation and lead to chemical effects. Low frequencies are responsible for physical effects, where rapid cavitation leading to enormous temperatures and pressures at the cavitation site [START_REF] Thompson | Sonochemistry: science and engineering[END_REF]. It is noteworthy that physical properties of the liquid phase have also many effects on ultrasonic cavitation. Relative low volatility, viscosity, and high surface tension of liquid solvents are preferred for favoring efficient cavitation [START_REF] Gogate | Sonochemical reactors: important design and scale up considerations with a special emphasis on heterogeneous systems[END_REF]. Active cavitation occurs also in heterogeneous sites in liquids such as impurities, gas microbubbles, non-volatile additives, etc. [START_REF] Luo | Ultrasound-enhanced conversion of biomass to biofuels[END_REF].

The physical and chemical effects produced by ultrasonic cavitation in a liquid phase provide extreme local conditions such as immense local heating (*5000 °C), pressures (*1000 atm), and heating/cooling rates (10 10 °C s -1 ) [START_REF] Suslick | Encyclopedia of physical science and technology[END_REF]. Microjet streams and shock waves created by cavitation promote better energy and mass transfer, which has an impact on accelerating chemical reaction, increasing conversion, improving the yield, and enhancing the selectivity in both homogeneous and heterogeneous systems [START_REF] Saito | Ultrasound field and bubles[END_REF]. For this reason, sonochemistry has found wide applications in chemical synthesis used for the preparation of nanostructured materials (e.g. hybrid lignocellulosic materials) [START_REF] Dong | Environmentally-friendly sonochemistry synthesis of hybrids from lignocelluloses and silver[END_REF] and modification of inorganic materials (e.g. clay minerals) [START_REF] Chatel | How efficiently combine sonochemistry and clay science?[END_REF]. Additionally, the benefits of the use of ultrasound in organic synthesis are also highlighted by interesting review articles [START_REF] Baig | Alternative energy input: mechanochemical, microwave and ultrasound-assisted organic synthesis[END_REF][START_REF] Cravotto | Power ultrasound in organic synthesis: moving cavitational chemistry from academia to innovative and large-scale applications[END_REF][START_REF] Luche | Synthetic organic sonochemistry, 1st edn[END_REF].

Some key examples where synthetically useful sonochemical organic transformations carried out in homogenous and heterogeneous conditions include: hydrolysis [START_REF] Piiskop | Sonification effects on ester hydrolysis in alcohol-water mixtures[END_REF], cycloaddition (e.g. Diels-Alder reaction) [START_REF] Javed | Influence of ultrasound on the Diels-Alder cyclization reaction: synthesis of some hydroquinone derivatives and lonapalene, an antipsoriatic agent[END_REF], coupling (e.g. Suzuki reaction, Mitsunobu reaction) [START_REF] Baig | Alternative energy input: mechanochemical, microwave and ultrasound-assisted organic synthesis[END_REF], isomerization (e.g. glucose to fructose) [START_REF] Wang | Combination use of ultrasound irradiation and ionic liquid in enzymatic isomerization of glucose to fructose[END_REF], alkylation [START_REF] Hofmann | Ultrasound promoted C-alkylation of benzyl cyanide -Effect of reactor and ultrasound parameters[END_REF], esterification [START_REF] Gro ¨nroos | Ultrasound accelerated esterification of bile acids[END_REF], and polymerization reactions [START_REF] Paulusse | Ultrasound in polymer chemistry: revival of an established technique[END_REF].

3 Ultrasound-Assisted Catalysis for Lignocellulosic Biomass Valorization

Principles of Sonocatalysis

The combination of sonochemistry with catalysis can be used to accomplish a number of chemical reactions with convenient workup conditions (e.g. shorter reaction times) in contrast to more conventional methods [START_REF] Gawande | Solvent-free and catalystsfree chemistry: a benign pathway to sustainability[END_REF]. Heterogeneous reactions follow via ionic intermediates provoked by mechanical effects, whereas radical reaction enhanced mainly by sonication. In the case when radical and ionic mechanisms lead to other products, ultrasound might promote the radical reaction, which can also provide new synthetic pathways [START_REF] Mason | Ultrasound in synthetic organic chemistry[END_REF]. The fundamental rule of sonocatalysis is diffusion and sorption of the main components on a solid surface followed by a series of heterogeneous chemical reactions on active sites [START_REF] Ince | Single and hybrid applications of ultrasound for decolorization and degradation of textile dye residuals in water[END_REF]. In a heterogeneous reaction system, the improvement of chemical reaction is mainly caused by physical effects. The physical phenomena improve mass transfer from turbulent mixing and acoustic streaming, generate cavitation erosion at liquid-solid interfaces, and are responsible for deformation of solid surfaces (Fig. 5) [START_REF] Suslick | Sonoluminescence and sonochemistry[END_REF].

The effect of ultrasonic irradiation on a heterogeneous catalyst may cause physical and chemical modifications (e.g. changes in crystallization, dispersion, and surface properties, as well as changes on catalytic reactivity during reaction (Fig. 5) [START_REF] Suslick | Sonocatalysis[END_REF]. The chemical rate increases due to enhancement of external transport phenomena and the increase in temperature at the catalyst surface. Acoustic Fig. 5 Effect of ultrasound irradiation on a heterogeneous catalyst surface cavitation can induce the breaking of the catalytic particle and gives more accessibility to the internal surface for the reagents. In the gas-liquid-solid system (e.g. hydrogenation reactions) sonication increases the interphase surface and favors the removal of outer oxide or other passivating layers from the catalyst surface [START_REF] Luche | Synthetic organic sonochemistry, 1st edn[END_REF].

Homogenous and Heterogeneous Sonocatalysis

The application of ultrasound in homogeneous and heterogeneous reaction systems in the presence of catalysts is viewed as a convenient technique for lignocellulosic valorization. Catalysis assisted by ultrasound includes a variety of reactions such as hydrolysis, hydrogenation, oxidation [START_REF] Bonrath | Ultrasound supported catalysis[END_REF]. Sonication improves hydrolysis of lignocellulosic materials into sugars and their subsequent fermentation into bioethanol. The main reason for enhanced conversion is the substantial improvement of mass transfer in reacting systems, as well as the activation of chemical and biological catalysts [START_REF] Gogate | Sonochemical reactors: important design and scale up considerations with a special emphasis on heterogeneous systems[END_REF]. Yunus et al. reported that the acid hydrolysis of palm oil to xylose was increased from 22 to 52% under ultrasound pretreatment (20 kHz, 2 kW), in comparison to silent conditions (Table 1; Entry 1) [START_REF] Yunus | Effect of ultrasonic pre-treatment on low temperature acid hydrolysis of oil palm empty fruit bunch[END_REF]. An improvement of the reaction rate under ultrasound (25 kHz, 600 W) was noticed also by Choi and Kim during the acid-catalyzed hydrolysis of starch (Table 1; Entry 2) [START_REF] Choi | Effect of ultrasound on sulfuric acid-catalysed hydrolysis of starch[END_REF]. Ultrasonic energy can have a direct influence on hydrolysis and fermentation reactions of cellulosic materials, while the application of ultrasound with enzymes accelerates saccharification and the fermentation rate [START_REF] Luo | Ultrasound-enhanced conversion of biomass to biofuels[END_REF]. Cavitation effects enhance the transport of enzyme macromolecules to the surface of the substrate, whereas the substrate surface is opening up to the action of enzymes due to the mechanical effect of cavitation [START_REF] Chatel | Sonochemistry: what potential for conversion of lignocellulosic biomass into platform chemicals?[END_REF][START_REF] Yachmenev | Intensification of enzymatic reactions in heterogeneous systems by low intensity, uniform sonication: new oad to ''Green Chemistry[END_REF]. Additionally, the sono-assisted enzymatic conversion of cellulose performed in solvents such as ionic liquids (ILs) yielded high performances by promoting high conversion, yield, and selectivity [START_REF] Luo | Ultrasound-enhanced conversion of biomass to biofuels[END_REF]. The combination of ultrasound with ILs has indeed recently attracted much interest for lignocellulosic biomass valorization [START_REF] Luo | Ultrasound-enhanced conversion of biomass to biofuels[END_REF]. For example, enzymatic hydrolysis of lignocellulose assisted by ultrasound (45 kHz, 100 W) in imidazolium-based ionic liquid media improves cellulosic conversion from 75 to 95% [START_REF] Yang | Enhancement of enzymatic in situ saccharification of cellulose in aqueous-ionic liquid media by ultrasonic intensification[END_REF] and saccharification ratio to 92% from 55% [START_REF] Ninomiya | Combined use of completely bio-derived cholinium ionic liquids and ultrasound irradiation for the pretreatment of lignocellulosic material to enhance enzymatic saccharification[END_REF] in 60 min (24 kHz, 35 W). The benefits of ultrasound-assisted enzymatic processes are the enhanced solvation and the increased reactivity of biomass reactants, coupled to reactions taking place at lower temperature within shorter time and with less requirement for acid or base catalysts [START_REF] Luo | Ultrasound-enhanced conversion of biomass to biofuels[END_REF].

The nature of solvents fulfils a crucial role in the lignocellulose depolymerization process. In some cases, ionic liquids were used only selectively to dissolve lignin rather than hemicellulose or cellulose [START_REF] Chatel | Review: oxidation of lignin using ionic liquids-an innovative strategy to produce renewable chemicals[END_REF]. For example, the catalytic hydroprocessing of lignin under ultrasound conditions resulted in a higher efficiency when conducted in an ionic liquid (1-butyl-3-methylimidazolium acetate) in comparison to organic solvents and water. Considerable enhancement in conversion up to 90% was noticed with nano-Ni and NiO nanosheets catalysts [START_REF] Finch | Catalytic hydroprocessing of lignin under thermal and ultrasound conditions[END_REF]. The conversion rate and the mass distribution of products depends on the procedure used for the pretreatment (acidic or alkali) of lignin. This means that in most cases, acidic extraction leads to a larger extent of depolymerization reaction (Table 1; Entry 3) [66]. Additionally, work by Napoly and co-workers shows that it is possible to obtain vanillin-based monomers with yield equal to 0.51 wt% in the presence of a tungsten-based catalyst and an oxidizing agent such as H 2 O 2 [START_REF] Napoly | H2O2-mediated kraft lignin oxidation with readily available metal salts: what about the effect of ultrasound?[END_REF]. Na 2 WO 4 acted as the most promising catalyst, which promoted an effective system, where ultrasound generated sufficient oxidation conditions (20 kHz, 11 W) and involved strengthened oxidative coupling of phenoxy radicals [START_REF] Napoly | H2O2-mediated kraft lignin oxidation with readily available metal salts: what about the effect of ultrasound?[END_REF].

A relevant study about the degradation of cellulose was recently published by Taghizadeh (Table 1; Entry 4) [START_REF] Taghizadeh | Sonocatalytic degradation of 2-Hydroxyethyl Cellulose in the presence of some nanoparticles[END_REF]. The degradation behaviour of 2-hydroxyethyl cellulose was conducted in the presence of a variety of heterogeneous catalysts (such as titanium oxide, montmorillonite clay, zinc oxide, and iron oxide) under ultrasound irradiation (24 kHz). Sonolytic degradation (without catalyst) increases with increasing of ultrasonic power (in the range of 30-90 W); however, it is remarkably lower in comparison to the efficiency of the sonocatalytic degradation. The results obtained revealed that the combined use of catalysts and US irradiation improved the degree of cellulose depolymerization. Among all the catalysts tested, the most efficient was Fe 3 O 4 , which provided a better ability for radical generation through electron transfer between the metal ion and the water molecules during the sonication process. The possibility of combining ultrasound irradiation with heterogeneous photocatalysis was also studied by the same research group (Table 1; Entry 5) [START_REF] Taghizadeh | Sonolytic, sonocatalytic and sonophotocatalytic degradation of chitosan in the presence of TiO2 nanoparticles[END_REF]. The complete degradation of chitosan (with cellulose-based structure) was achieved during 1 h in the presence of titanium oxide at 24 kHz. In this case, sonophotocatalysis enhanced the production of reactive radicals as well as increasing the active sites of the catalyst surface.

Behling et al. recently investigated the low frequency (20 kHz) ultrasoundassisted aqueous-phase oxidation of vanillyl alcohol to vanillin using a heterogeneous Co 3 O 4 catalyst with hydrogen peroxide as the primary oxidizing agent under mild reaction conditions (low temperature and atmospheric pressure). The outcome of this work was that the ultrasound-assisted catalytic reaction is faster (4x), more selective (2.3x), and more efficient (2.7x) than the corresponding reaction carried out under silent conditions [START_REF] Behling | Sonochemical oxidation of vanillyl alcohol to vanillin in the presence of a cobalt oxide catalyst under mild conditions[END_REF]. Additionally, a large decrease of the overall energy consumption was observed under ultrasound (36 vs. 288 kJ). Moreover, from an environmental point of view, green metrics indicators such as the E factor and the process mass intensity (PMI) calculated for both activation systems clearly showed the benefit of the ultrasound-mediated reaction [START_REF] Behling | Sonochemical oxidation of vanillyl alcohol to vanillin in the presence of a cobalt oxide catalyst under mild conditions[END_REF]. The ultrasound-microwave assisted process is also an interesting approach for lignocellulosic biomass valorization. The simultaneous microwave (100 W) and ultrasound irradiations was shown to improve the hydrolysis reaction rates of glucose [START_REF] Li | Study on preparation of levulinic acid by hydrolysis of glucose using ultrasound-microwave assisted hydrochloric acid[END_REF] and corn starch [START_REF] Zhou | Study on technology of ultrasound-microwave assisted generates levulinic acid by hydrolysis of corn starch[END_REF] (Table 1; Entry 6) in 60 min. In both cases, the reaction yield to levulinic acid was high (49 and 23% from glucose and corn starch, respectively) in comparison with those reported in the open literature.

Kardos and Luche [START_REF] Kardos | Sonochemistry of carbohydrate compounds[END_REF] have investigated interesting approaches to obtain high value-added chemicals through the conversion of biomass feedstocks such as polymeric carbohydrates to lower weight molecules. In the case of polysaccharides, the partial or total depolymerization has to be taken into account. Hydrolytic procedures, already mentioned before, have been widely examined to accomplish this aim. Nevertheless, particular attention should be paid to oxidation reactions. For example, glucose being selectively oxidized into glucuronic acid in the presence of iron sulfate under ultrasound irradiation (100 kHz), whereas hexoses are oxidized to the corresponding uronic acids. It is interesting to note that this type of reaction cannot be performed without oxygen or acoustic activation [START_REF] Kardos | Sonochemistry of carbohydrate compounds[END_REF].

The sonocatalytic oxidation of primary benzyl alcohols into the corresponding aldehydes was reported by Naik et al. [START_REF] Naik | An efficient sonochemical oxidation of benzyl alcohols into benzaldehydes by FeCl3/HNO3 in acetone[END_REF]. They noted that the mixture of HNO 3 / FeCl 3 provides high yields (80-94%) of aldehydes within 10-25 min (Table 1; Entry 7). Reactions carried out under silent conditions showed fourfold lower rates and yields than those performed under sonication. The application of ultrasound (35 kHz, 120 W) gave excellent yields with short reaction times and allowed to avoid over-oxidized products.

The work of Qin et al. [START_REF] Qin | Preparation of ultrasonic-assisted high carboxylate content cellulose nanocrystals by tempo oxidation[END_REF] showed that the TEMPO/NaBr/NaOCl oxidation system assisted by ultrasound (40 kHz, 300 W) can be used to prepare cellulose nanocrystals with high carboxylate content from cotton linter pulp (Table 1; Entry 8). This is consistent with the study carried out by Brochette-Lemoine et al. [START_REF] Brochette-Lemoine | Ultrasound in carbohydrate chemistry: sonophysical glucose oligomerisation and sonocatalysed sucrose oxidation[END_REF]. The results of these investigations indicated that the rate of the oxidation of methyl a-D-glucopyranoside or sucrose was increased in the presence of ultrasound. Moreover, the reaction can then occur without the commonly used sodium bromide owing to the ability of ultrasound to generate the active oxidizing species during the catalytic cycle. Additionally, sonication accelerated the oxidation reaction, especially when the frequency of ultrasound was increased from 20 up to 500 kHz [START_REF] Lemoine | Sucrose tricarboxylate by sonocatalysed TEMPO-mediated oxidation[END_REF]. In order to scale up the oxidation of cellulose, Paquin et al. [START_REF] Paquin | The use of Weissler method for scale-up a Kraft pulp oxidation by TEMPO-mediated system from a batch mode to a continuous flow-through sonoreactor[END_REF] proposed the use of a continuous flow-through system instead of the classical standard batch mode. The flow-through reactor increased the reaction rate (*36%) in comparison to the batch reactor while decreasing the overall energy consumption (*87%) [START_REF] Chatel | Sonochemistry: what potential for conversion of lignocellulosic biomass into platform chemicals?[END_REF].

Another reaction worth interest for the industry is the production of gluconic acid from D-glucose. Rinsant et al. [START_REF] Rinsant | Efficient and selective oxidation of D-Glucose into Gluconic acid under low-frequency ultrasonic irradiation[END_REF] have described a way to selectively oxidize glucose via a sono-Fenton process with hydrogen peroxide in the presence of iron (II) sulfate as catalyst. In contrast to preconceived ideas, they proved that sonochemistry does not constitute an ''intensive energy consuming'' technology. Furthermore, the energy consumption could be minimized when ultrasound-based processes are optimized. Energy consumption (under ultrasound) was lower than that attained under conventional reactions. Remarkably high conversion (*100%) and selectivity (*95%) values were obtained only after 15 min (20 kHz, 0.25 W mL -1 ). This example on D-glucose oxidation relies on the efficient combination of an eco-friendly oxidant (hydrogen peroxide) and ultrasound, which constitutes a promising strategy for the valorization of biomass. In the same strategy of sugar oxidation by sonocatalysis, Bujak et al. [START_REF] Bujak | Highly efficient room-temperature oxidation of cyclohexene and D-glucose over nanogold Au/SiO2 in water[END_REF] observed that silica-supported gold catalysts are extremely active and selective for D-glucose oxidation to gluconic acid at ambient temperature and under ultrasound conditions (35 kHz) (Table 1; Entry 9). The application of ultrasound is of crucial importance to provide not only high conversion of glucose into gluconic acid with 100% yield, but also high reproducibility.

The effect of sonication [START_REF] Hagenson | Comparison of the effects of ultrasound and mechanical agitation on a reacting solid-liquid system[END_REF][START_REF] Rinsant | Efficient and selective oxidation of D-Glucose into Gluconic acid under low-frequency ultrasonic irradiation[END_REF] and 130 W cm -2 ) on D-fructose hydrogenation in the presence of heterogeneous catalysts was examined by Toukoniitty et al. [START_REF] Toukoniitty | Effect of ultrasound on catalytic hydrogenation of D-Fructose to D-Mannitol[END_REF]. The reaction rate and selectivity were investigated at various conditions of temperature (70-110 °C), pressure (10, 30, and 50 bar), and ultrasonic power (0-50 W) (Table 1; Entry 10). The application of sonication during the hydrogenation reaction considerably accelerated the reaction rate in the presence of the Cu/SiO 2 catalyst. High temperature and pressure had a moderate effect on the catalyst activity whereas the variation of nominal ultrasonic power input effectively improved the reaction rates.

Challenges and Future Perspectives

In most cases, combining catalysis with sonication has interesting effects on reactions course. In the present mini-review, we have shown that the use of sonocatalysis allows avoiding harsh chemical conditions, along with reducing reaction times and improving heat and mass transfer, thereby increasing chemical rate constants, yields, and selectivities. Hence, the ultrasound-assisted catalysis can be successfully applied for the pretreatment and chemical conversion of lignocellulosic biomass and its derivatives in a variety of processes such as hydrolysis, oxidation, and hydrogenation reactions. The recent studies on the use of ultrasound to assist catalytic reactions have clearly shown great advantages and technological potential of this concept for the chemical industry, especially when thinking about processes under flow. More significant scientific breakthroughs for biomass valorization are expected to occur in this innovative field in the near future. Last, but not least, and continuing in the same lines, the important role of ultrasound on photocatalysis (ultrasound and photocatalysis together) for the valorization of lignocellulosic biomass and its derivatives might be also a promising research avenue worth broad interest in the huge spectrum of possibilities offered by lignocellulose-based processes, for instance, sonophotocatalytic proof concepts for: lignocellulosic biomass depolymerization [81], biohydrogen [START_REF] Penconi | Hydrogen Production from Water by Photolysis, Sonolysis and Sonophotolysis with Solid Solutions of Rare Earth, Gallium and Indium Oxides as Heterogeneous Catalysts[END_REF], and biomethane production [START_REF] Jafari | Enhanced biohydrogen and subsequent biomethane production from sugarcane bagasse using nano-titanium dioxide pretreatment[END_REF].

Fig. 1

 1 Fig. 1 Methods for pretreatment of lignocellulosic biomass. (Adapted and modified from Ref. [9])

Table 1

 1 Homogeneous and heterogeneous sonocatalytic biomass valorization

	References		[60]			[61]							[66]							[68]
	Experimental Sonocatalytic behaviour	details	Reaction time 15, °C The highest yield of xylose (58%) was 45, 60 min Solvent H 2 SO 4 concentration (2%) Temperature 25 obtained under ultrasound irradiation (90% amplitude) during 45 min, whereas without ultrasonic pretreatment yield was equal to 22%	Reaction time The reaction yield in the presence of	100 min. Solvent ultrasound is higher than in the	dilute sulfuric reaction without sonication	acid (1-5 wt%) (increased *75 % at 90 °C)	Temperature	90-100 °C	Reaction time: 6 h/ Catalysts exhibited a slight activity	24 h. Solvent: whereas considerable growth in	ethyl acetate/ conversion (up to 90% under US	methanol/ionic irradiation) was noticed on nano-Ni	liquid (0) and NiO (111) nanosheets samples	[BMIM]OAc	Temperature room	temperature/	180 °C	Reaction time °C The sonocatalytic activity (rate 100 min Solvent water Temperature 25 constants) of nanoparticles catalysts 2 , ZnO, Anatase-increases in the following order: Fe TiO 3 O 4 , Rutile-TiO 2 , and MMT
	Sonochemical	parameters	2 kW	(20 kHz)		600 W	(25 kHz)						(35 kHz)							100 W	(24 kHz)
	Substrate		Oil palm empty fruit	bunch		Starch	Maltose	Maltotriose			Miscanthus (lignin,	glucose, xylose,	arabinose,	galactose,	mannose,	extractives, and	ash)	2-hydroxyethyl	cellulose
	Catalyst		2% sulphuric acid	(aqueous	solution)	1-5% sulphuric	acid (aqueous	solution)				Fe 3 O 4 (NiAlO) x	Fe 3 O 4 (NiMgAlO)	x			Rutile-TiO 2	Anatase-TiO 2	Montmorillonite	Clay (MMT),	ZnO and Fe 3 O 4
	Entry Process		1 Hydrolysis			2 Hydrolysis							3 Hydroprocessing							4 Degradation

Top Curr Chem (Z) (2017) 375:41

Acknowledgements Prof. Dr. Colmenares would like to thank the National Science Centre (Poland) for the support within the project Sonata Bis Nr. 2015/18/E/ST5/00306 and the support from COST Action FP1306 for networking and possibilities for meetings and future students exchange. M.Sc. Kuna would like to acknowledge the COST Association (Action FP1306) for the Trainee Grant (COST-STSM-ECOST-STSM-FP1306-010216-068720). Ronan Behling acknowledges the CAPES Foundation (Brazilian Ministry of Education) for awarding his PhD scholarship (13342-13-4) through the Science without Borders program.

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