FIELD OF THE INVENTION

The present invention relates to a process for liquefying a cellulosic material. The process provides products which may be converted into biofuel components for use in fuel formulations. More specifically, the invention relates to a process comprising hydrolysing lignocellulosic material in the presence of an acid catalyst to give useful monomeric and oligomeric products which may be converted to biofuel components.

BACKGROUND OF THE INVENTION

Lignocellulosic materials which may be converted into valuable intermediates, which intermediates may be further processed into fuel components, are of

considerable interest as feedstocks for the production of sustainable biofuels.

Biofuels are combustible fuels, typically derived from biological sources, which result in a reduction of greenhouse gas emissions. Biofuels used for blending with conventional gasoline fuel components are alcohols, in particular ethanol. Biofuels such as fatty acid methyl esters derived from rapeseed and palm oil can be blended with conventional diesel components for use in diesel engines. However, these biofuels are derived from edible feedstock and so compete with food production.

Non-edible renewable feedstocks such as

lignocellulosic biomass are therefore becoming

increasingly important, both economically and

environmentally, and there has been much interest in developing improved methods for producing useful

compounds from such materials. Specific fuel components have been derived from lignocellulose derivatives using multistep processes, for instance levulinate (Bozels et al . , Resources, Conservation and Recycling 2000, vol. 28, page 227), valerate (WO 2006/067171) and pentenoate (WO 2005/058793) esters from levulinic acid or methyl-furan (Roman-Leshkov et al . , Nature 2007, vol.447, page 982) and ethyl furfuryl ether (WO 2009/077606) from furfural. It would, however, be advantageous to be able to convert solid lignocelluloses into a liquefied 'biocrude' product which could then be fed to an oil refinery for upgrading to final fuel components.

WO 2005/058856 describes a process for liquefaction of cellulosic material. In the process solid cellulosic material is heated in the presence of an acid catalyst and a solvent. The solvent comprises a compound having a gamma lactone group of a specific general molecular formula. Examples of such compounds that are mentioned include gamma-valerolactone . It is further indicated that also levulinic acid, furfural or compounds without a gamma lactone group that are obtainable from levulinic acid or furfural may be used as solvent in the process.

The process is suitable for its purpose, but

unfortunately the large amounts of expensive solvent that are needed make the process economically less attractive.

US 5608105 describes a process for producing levulinic acid from carbohydrate-containing materials. It describes as an example reacting a slurry of paper sludge containing 3.5% by weight of the aqueous portion sulfuric acid with steam in a series of two reactors. The liquid product outflow contains levulinic acid at a

concentration of 0.68%. A disadvantage of the described process is that the process produces large amounts of low-value insoluble humins and, therefore, offers poor utilization of the lignocellulosic feedstock. In addition the process comprises an extensive process using two reactors .

There remains a continuing need for the development of improved processes for liquefying lignocellulosic material to produce useful compounds for subsequent conversion into biofuels. It would for example be an advancement in the art to provide a process for

liquefying lignocellulosic material having an increased degree of liquefaction and a reduction in the amount of unwanted insoluble humins .

SUMMARY OF THE INVENTION

The above has been achieved with the process according to the invention.

Accordingly, the present invention provides

a process for liquefying a cellulosic material

comprising hydrolysing the cellulosic material in the presence of an acid catalyst in a solvent mixture to produce a liquefied product, wherein the solvent mixture contains water and in the range of 5 to 95 wt% of a co- solvent and wherein the co-solvent is present in an amount of less than or equal to 90% by weight based on the weight of water and co-solvent, which co-solvent comprises one or more polar solvents, and wherein the solvent mixture is at least partly recycled. It has now been found that liquefying a cellulosic starting

material using a solvent mixture containing water and one or more polar co-solvents is advantageous as it allows for increased liquefaction, even at high cellulose loading; leaves less solid humins; and delivers more valuable components in the desired molecular weight range. In addition it offers processing efficiencies as the solvent mixture is at least partly recycled. Further at least part of the solvent mixture, for example any make-up solvent mixture, can be generated in-situ during liquefaction (that is, preferably part of the solvent mixture may be generated in-situ during the hydrolysis of the cellulosic material).

With the process according to the invention, valuable monomeric and oligomeric products may

advantageously be prepared starting from materials which are readily available from biomass. These products may subsequently be converted into hydrocarbons or oxygen- lean biofuels by refinery technologies such as

hydrodeoxygenation or thermal-, catalytic- or hydro- cracking processes.

The invention therefore also provides a process for preparing a biofuel from the liquefied cellulosic

material produced.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure la shows a schematic diagram of one process according to the invention.

Figure lb shows a schematic diagram of another process according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

By liquefying is herein understood the conversion of a solid material, such as cellulosic material, into one or more liquefied products. Liquefying is sometimes also referred to as liquefaction.

By a liquefied product is herein understood a product that is liquid at ambient temperature (20°C) and pressure (1 bar absolute) and/or a product that can be converted into a liquid by melting (for example by applying heat) or dissolving in a solvent. Preferably the liquefied product is liquid at ambient temperature (20°C) and pressure (1 bar absolute) . Liquefaction of a cellulosic material can comprise cleavage of covalent linkages in that cellulosic

material. For example liquefaction of lignocellulosic material can comprise cleavage of covalent linkages in the cellulose, hemicellulose and lignin present and/or cleavage of covalent linkages between lignin,

hemicelluloses and/or cellulose.

As used herein, cellulosic material refers to material containing cellulose. Preferably the cellulosic material is a lignocellulosic material comprising lignin, cellulose and optionally hemicellulose.

Any suitable cellulose-containing material may be used in the process according to the present invention. Advantageously, cellulosic material for use according to the invention may be obtained from a variety of plants and plant materials including agricultural wastes, forestry wastes and sugar processing residues. Examples of suitable cellulose-containing materials include agricultural wastes such as corn stover, soybean stover, corn cobs, rice straw, rice hulls, oat hulls, corn fiber, cereal straws such as wheat, barley, rye and oat straw; grasses; forestry products such as wood and wood-related materials such as sawdust; waste paper; sugar processing residues such as bagasse and beet pulp; or mixtures thereof.

Before being used in the process of the invention, the cellulosic material is preferably comminuted into small pieces in order to facilitate liquefaction.

Conveniently, the lignocellulosic material can be

comminuted into pieces of average length of 0.5 to 30 mm.

The solvent mixture contains water and a co-solvent comprising one or more polar solvents. Preferably the co- solvent comprises one to three, more preferably one or two polar solvents.

The polar solvent, or mixture of two or more polar solvents, in the solvent mixture can be any polar solvent that is stable under the liquefaction reaction conditions used and for the duration of the reaction time.

Advantageously, the polar solvent or mixture of polar solvents, may be water-miscible at the reaction temperature employed.

A measure of the polarity of a solvent is its log P value, where P is defined as the partition coefficient of a compound in a two phase octanol-water system. The log P value can be determined experimentally or calculated according to standard procedures as discussed in Handbook of Chemistry and Physics, 83 ^rd Edition, pages 16-43 to

16-47, CRC Press (2002) .

In one embodiment the co-solvent, comprising one or more polar solvents, is a solvent having a polarity of log P less than +1.

In another embodiment, the co-solvent, comprising one or more polar solvents, is a solvent having a

polarity of log P less than +0.5.

In a further embodiment, the co-solvent, comprising one or more polar solvents, is a solvent having a

polarity of log P less than 0.

Preferably one or more of the polar solvents is derived from cellulosic, and preferably lignocellulosic, material. More preferably one or more of the polar solvents is a solvent obtainable by acid hydrolysis of cellulosic material, such as acetic acid, formic acid and levulinic acid. Polar solvents which are obtainable from such acid hydrolysis products by hydrogenation may also suitably be used. An example of such a hydrogenation product solvent is gamma-valerolactone which is

obtainable from levulinic acid by hydrogenation .

Preferably the co-solvent for use according to the invention may comprise two or more such solvents.

The one or more polar solvents may comprise one or more carboxylic acids. By a carboxylic acid is herein understood an organic compound comprising at least one carboxyl (-CO-OH) group. In a preferred embodiment the co-solvent comprises at least one or more carboxylic acids. More preferably the co-solvent comprises equal to or more than 5 wt% carboxylic acids, more preferably equal to or more than 10 wt% carboxylic acids, most preferably equal to or more than 20wt% of carboxylic acids, based on the total weight of co-solvent (i.e.

excluding water) . There is no upper limit for the

carboxylic acid concentration, but for practical purposes the co-solvent may comprise equal to or less than 90wt%, more preferably equal to or less than 80wt% of carboxylic acids, based on the total weight of co-solvent (i.e.

excluding water) . Preferably the co-solvent comprises at least acetic acid, formic acid, levulinic acid and/or pentanoic acid. Especially acetic acid may be useful for simultaneous use as a polar solvent as well as use as an acid catalyst.

In one embodiment, the co-solvent comprises one or more polar solvents selected from acetic acid, formic acid, levulinic acid and gamma-valerolactone and is substantially free of other compounds. In a preferred embodiment, the co-solvent consists essentially of acetic acid, levulinic acid, gamma-valerolactone, or mixtures thereof. In a most preferred embodiment the co-solvent consists essentially of acetic acid, levulinic acid or mixtures thereof. Especially acetic acid may be useful for simultaneous use as a polar solvent as well as use as an acid catalyst.

In one embodiment, the co-solvent comprises a polar solvent that may be obtained from the cellulosic, preferably lignocellulosic, material used in the

liquefaction process of the invention itself. Hence preferably one or more of the polar solvents is a polar solvent generated at least partly in-situ during the process of the invention, that is, generated at least partly in-situ during the hydrolysis of the cellulosic material. Preferably the polar solvent generated in-situ is acetic acid, formic acid, levulinic acid.

Any solvent obtainable from the cellulosic

material liquefied according to the process of the invention may advantageously be recycled and used as a make-up solvent in the liquefaction process, affording significant economic and processing advantages.

In a preferred embodiment the recycle comprises a weight amount of solvent mixture of 2 to 100 times the weight of the cellulosic material, more preferably of 5 to 20 times the weight of the cellulosic material.

Most preferably the co-solvent comprises one or more polar solvent (s), which one or more polar solvents are selected from the group consisting of acetic acid, formic acid, levulinic acid and gamma-valerolactone and which one or more polar solvents is/are generated in-situ by hydrolysis of the cellulosic material, and wherein such one or more polar solvent (s) is/are at least partly recycled .

In another preferred embodiment the co-solvent comprises one or more polar solvent (s) that are at least partly obtained and/or derived from a source other than the cellulosic material used as a feedstock in the liquefaction process of the invention itself, for example a petroleum source. These one or more polar solvent (s) may for example be mixed with the cellulosic material before starting the liquefaction process or may be added to the reaction mixture during the liquefaction process.

Preferably the co-solvent comprising one or more polar solvents is present in an amount of less than or equal to 90% by weight, more preferably less than or equal to 80% by weight, based on the total weight of water and co-solvent. Further the co-solvent comprising one or more polar solvents is preferably present in an amount of more than or equal to 10% by weight, more preferably 20% by weight, based on the total weight of water and co-solvent. Preferably, the co-solvent

comprising one or more polar solvents is present in an amount of from 20% to 60% by weight based on the total weight of the water and co-solvent.

Preferably water is present in an amount of less than or equal to 90% by weight, more preferably less than or equal to 80% by weight, based on the total weight of water and co-solvent. Further water is preferably present in an amount of more than or equal to 10% by weight, more preferably 20% by weight, based on the total weight of water and co-solvent. Preferably, water is present in an amount of from 40% to 80% by weight based on the total weight of the water and co-solvent.

Preferably the solvent mixture contains the co- solvent and water in a weight ratio of co-solvent to water of less than or equal to 9:1, more preferably less than or equal to 8:2. Further the solvent mixture

preferably contains the co-solvent and water in a weight ratio of co-solvent to water of more than or equal to 1:9 more preferably more than or equal to 2:8. The cellulosic material and the solvent mixture containing water and co-solvent, are preferably mixed in a solvent mixture-to-cellulosic material ratio of 2:1 to 20:1 by weight, more preferably in a solvent mixture-to- cellulosic material ratio of 3:1 to 15:1 by weight and most preferably in a solvent mixture-to-cellulosic material ratio of 4:1 to 10:1 by weight.

The acid catalyst for use in the process according to the invention may be any acid catalyst known in the art to be suitable for liquefying of cellulosic material.

For example, the acid catalyst may be a Bronsted acid or a Lewis acid. Preferably the acid catalyst remains liquid and stable under the process conditions of the invention and is sufficiently strong to mediate depolymerisation and dehydration of the cellulosic material.

Preferably the acid catalyst is a Bronsted acid and more preferably the acid catalyst is a mineral or organic acid, preferably a mineral or organic acid having a pKa value below 3.75, most preferably a mineral or organic acid having a pKa value below 3.

Examples of suitable mineral acids which may be employed in the process of the invention include

sulphuric acid, nitric acid, hydrochloric acid, para- toluene sulphonic acid and phosphoric acid, or mixtures thereof. In one particular embodiment, the acid catalyst used in the process of the invention is sulphuric acid.

Examples of suitable organic acids which may be used in the process of the invention include formic acid and trichloracetic acid.

Preferably, the acid catalyst is present in an amount of less than or equal to 10% by weight of acid based on the weight of solvent mixture and acid. It will of course be appreciated that for any given acid the amount of acid required will depend on the strength of the acid. In one embodiment, the acid catalyst is present in an amount of from 1% to 10% by weight, preferably from 2% to 5% by weight, based on the weight of solvent mixture and acid.

The liquefaction process according to the invention is preferably carried out at a temperature of from 100°C to 300°C. More preferably, the process is carried out at a temperature of from 150°C to 250°C, most preferably from 180°c to 220°C.

Preferably the liquefaction process is performed under autogeneous pressure.

Preferably, more than 60% by weight of the

cellulosic material is liquefied. The present inventors have found that by means of the present invention, even

80% or more by weight of the lignocellulosic material may advantageously be liquefied in less than three hours.

The solid material remaining after liquefication is sometimes also referred to as humin and/or char. The liquefied products prepared by the process of the

invention may conveniently be stabilized by

hydrogenation . Hydrogenation of the liquefied products can also lead to the formation of solvent compounds such as gamma-valerolactone (obtainable by hydrogenation of levulinic acid) which may be used as a make-up solvent in the liquefaction process.

Hydrogenation of the liquefied products may suitably be carried out under hydrogenation conditions as known in the art. Hydrogenation may for example be performed subsequent to liquefaction in an additional step .

In one embodiment, hydrogenation of the liquefied products may be performed in the presence of a homogeneous or a heterogenous metal catalyst and a catalyst support which is resistant to acidic medium.

In a preferred embodiment the hydrogenation of the liquefied products can be performed simultaneously with the liquefaction.

Any suitable heterogenous metal catalyst known in the art to be suitable for hydrogenation may be used, for example a platinum group metal such as ruthenium, rhodium, palladium, iridium, platinum and gold, or mixtures thereof. Suitable catalyst supports include carbon and oxides that are stable under acidic conditions such as titanium dioxide, zirconium dioxide and silicon dioxide and mixtures thereof; optionally the support may comprise mesoporous powder, granules or extrudates but in a preferred embodiment, the support is suitably a

megaporous structure such as a foam, honeycomb, mesh or cloth .

Preferably, an additional hydrogenation step may be carried out at a temperature in the range of from 30 °C to 300°C, more preferably 30 °C to 200°C, still more preferably at a temperature in the range of 50°C to

150°C, and at a pressure of from 1 to 100 bar, preferably 1 to 20 bar.

In another embodiment, hydrogenation of the

liquefaction products may be performed in the presence of a homogeneous hydrogenation catalyst under homogeneous hydrogenation catalysis conditions. A suitable

homogeneous catalyst which may be used is ruthenium trichloride with triphenyl phosphine.

The liquefied product (herein also referred to as liquefied products), or the corresponding hydrogenated compounds, may comprise monomeric and/or oligomeric products. These monomeric and/or oligomeric may be converted to biofuel components. Examples of the

monomeric products include for example furfural,

hydroxymethylfurfural , acetic acid, formic acid,

levulinic acid and/or gamma valero lactone. Examples of the oligomeric products include oligomers of these monomers and/or oligomers of other compounds present. In one embodiment, the liquefied products of the process according to the invention, or the corresponding

hydrogenated compounds, may be separated from the reaction effluents (including for example water, co- solvent and acid) for subsequent conversion to biofuels. The recovered solvent mixture and/or acid may

advantageously be recycled for use in the liquefaction process with any excess water produced during the liquefaction process firstly being removed by

distillation or pervaporation .

As used herein, a biofuel is a component or mixture of components that is derived from biomass and can be used as a fuel or fuel component.

Preferably, separation of the liquefied products, or of the corresponding hydrogenated products derived therefrom, can be effected by liquid/liquid separation techniques, optionally in the presence of an extractive solvent. In an alternative embodiment, separation of the liquefied products or of the corresponding hydrogenated products may be effected using a separation membrane followed by removal of any residual solvents by

distillation .

If desired the monomeric products and oligomeric products may be conveniently separated from eachother using one or more membranes. For example, monomeric products and/or optionally water can be separated from any C9-C20 oligomeric products and C20+ oligomeric products by a ceramic membrane (for example a T1O2

membrane) or a polymeric membrane (for example a Koch MPF 34 (flatsheet) or a Koch MPS-34 (spiral wound) membrane) . The C9-C20 oligomeric products and the C20+ oligomeric products can conveniently be separated from eachother with for example a polymer grafted Zr0 ₂ membrane. The use of membranes for these separations can advantageously improve the energy efficiency of the process. By a Cx product is herein understood a product comprising x carbon atoms.

In one embodiment, monomeric liquefied products and/or oligomeric liquefied products prepared by the process according to the invention may suitably be recovered and converted into gasoline components such as ethyl valerate, methyl furan or ethyl furfural ether or esterified and/or etherified oligomers by means of hydrogenation, esterification and/or etherification reactions .

In another embodiment, the liquefied products of the process of the invention and the hydrogenated

products derivable therefrom may be converted into biofuels .

Suitably, the liquefied products or the

hydrogenated products derivable therefrom may be

converted to biofuels using techniques such as

hydrodeoxygenation or thermal-, catalytic- or hydro- cracking processes.

In one embodiment, the optionally stabilized liquefied products are at least partially

hydrodeoxygenated, rendering them hydrocarbon soluble, prior to being blended with a refinery stream such as crude oil, (vacuum) gasoil or (heavy) cycle oil and being subjected to further hydrodeoxygenation or a thermal- catalytic- or hydro- cracking processes.

Suitably, the hydrodeoxygenation may be performed under conditions in the presence of a supported

heterogenous metal or metal sulfide catalyst. The metal catalyst suitably comprises a metal of any one of groups 8 to 11 of the Periodic Table of Elements such as iron, cobalt, nickel, ruthenium, rhodium, palladium, iridium or platinum. Metal sulfide catalysts suitably comprise sulfided molybdenum optionally promoted with cobalt or nickel .

Following the initial hydrodeoxygenation step, the at least partially deoxygenated liquefaction products can be recovered from the solvents, for example by

liquid/liquid separation techniques, prior to being subjected to upgrading to hydrocarbons by means of further hydrodeoxygenation or by thermal-, catalytic- or hydro- cracking processes.

DETAILED DESCRIPTION OF THE DRAWINGS

Figures 1A and IB show process schemes for two embodiments of the process according to the invention.

In the embodiment shown in Figure 1A, solid biomass 1 and a stream of acid and solvent mixture (water and co- solvent) 3 are supplied to hydrolysis reactor A. In reactor A the liquefaction process is carried out and the liquefied products are then supplied to hydrogenation reactor B. Hydrogen 2 is supplied to reactor B and the liquefied products are stabilized by hydrogenation . The hydrogenated products are then transferred to separation unit C and separated into unwanted solid residue 4 and hydrogenated liquefied products 5. A stream comprising water, co-solvent and/or acid 3 is withdrawn from the separation reactor C and recycled to the hydrolysis reactor A. Excess water, co-solvent and/or acid 6 is purged. The purge stream 6 can optionally be treated to recover most of the co-solvent and/or acid for recycling to the hydrolysis reactor A.

In the alternative embodiment shown in Figure IB, the liquefied products formed in hydrolysis reactor A are supplied to separation unit C prior to being transferred to hydrogenation reactor B.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", mean "including but not limited to", and do not exclude other moieties, additives, components, integers or steps.

EXAMPLES

The invention will now be further illustrated by means of the following non-limiting examples and

comparative examples.

Experiments to investigate acid-catalyzed birch wood hydrolysis using birch wood concentrations of 12% by weight and 20% by weight and various co-solvents and reaction times were performed according to the following general conditions :-

Birch wood (particle size <4mm, semi-dried at

105 °C) was loaded into an autoclave together with water and co-solvent, the mixture was stirred (1400 rpm) and the reactor content was heated in 40 minutes to reaction temperature and pressure (200°C, 13.6 bar). Hydrolysis was commenced by injecting a small aliquot of a 27% by weight solution of H ₂ S0 ₄ in water to the heterogenous reaction mixture and water was subsequently injected into the system to flush the catalyst feed line, giving a final H ₂ S0 ₄ concentration of 3% by weight. Samples were taken from the reactor at regular intervals of time and the hydrolysis was terminated after the desired reaction time by forced cooling of the reactor content to room temperature. Insoluble humins were separated by

filtration over a P3 filter, followed by washing with acetone and drying under vacuum (200mbar) at 50 °C

overnight .

Samples were analysed for organic acids, 5- hydroxymethylfurfural and furfural using ion exclusion chromatography (ICE) . A conductivity detector was used for the detection and quantification of the organic acids and HMF and furfural were detected with an UV detector (wavelength; 320nm) . The analytical column used was a Dionex AS1 (9 x 250mm) column, with a 1 mL/min mobile phase flow of 1 mM heptafluorobutyric acid in

demineralised water. For suppression of the conductivity signal a Dionex AMMS-ICE II suppressor, with a 2 mL/min flow of 5 mM tertiary-butyl-ammonium hydroxide in

demineralised water was used. Calculations were based on external calibration. Prior to the organic acid, furfural and HMF determination, water was added to the liquor. When precipitated, residual humins were removed by filtration .

The oligomeric compounds were analyzed by means of Size Exclusion Chromatography (SEC), using PL-gel polymer as immobile phase, THF as mobile phase and using

Ultraviolet (UV, 254 nm wave length) and refractive index (RI) detectors.

The weight amount of oligomeric compounds was determined by RI using a series of samples loaded with known weight amounts of lignin as calibration.

The molecular weight distribution of the oligomeric compounds was determined between 150 and 5000 Dalton, using a series of polystyrene oligomer samples of known molecular weight as calibration.

Specific conditions used and the results obtained are given in Table 1.

Abbreviations :

BW birch wood

FUR furfural

HMF hydroxymethylfurfural

AA acetic acid

FA formic acid

LA levulinic acid

GVL gamma-valerolactone

tol toluene

W water

C12 dodecane

The weight% figures for the yields of monomers, oligomers and solid char (insoluble lignins ) residues presented in Table 1 are based on the weight of birch wood feedstock.

Table 1

Table 1

Table 1

Data that were not measured are represented by '-'

Experiments 1, 7, 9 and 10 are comparative experiments in which the acid catalysed hydrolysis of birch wood

feedstock is performed either in the absence of co-solvent (experiments 1,7) or in the presence of a water-insoluble (log P > 1) co-solvent (experiments 9,10)

From the results presented in Table 1 it can be seen that the presence of a water-miscible polar co-solvent according to the present invention leads to an increased degree of liquefaction and a reduction in the amount of unwanted humins, even at high birch wood loadings, compared to the levels obtainable in the absence of co-solvent or where a water-insoluble or apolar co-solvent is used.

Where the hydrolysis is performed in the absence of co- solvent at a birch wood concentration of 20w%, 63w% of the feedstock is liquefied after 105 minutes (experiment 1);

under the same conditions at a birch wood concentration of 12w%, 66w% liquefaction is achieved after 145 minutes

(experiment 7) . Comparable or lower results are obtained with the water-insoluble co-solvent toluene (log P = 2.73) or dodecane (log P = 6.1) (experiment 9, 10) . By contrast, when the hydrolysis is conducted in the presence of a polar co- solvent (experiments 2-6 and 8), higher liquefaction w% levels are obtained after comparable or even shorter reaction times for both 12% and 20% birch wood loadings.

Comparing the results obtained in experiments 2-6 with those for experiment 1 and the results obtained in

experiments 9 and 10 with those of experiment 8, it can be seen that conducting the hydrolysis reaction in the presence of a polar co-solvent according to the present invention also leads to a reduction in the w% of unwanted humins which are formed, for a given birch wood loading, at comparable or even longer reaction times.

Performing the hydrolysis in the presence of a co- solvent according to the invention also leads to improvements in the yields obtained of desirable monomeric products furfural, hydroxymethylfufural and levulinic acid and to the increased production of oligomeric furanic components.

Comparing the total w% of monomeric and oligomeric products from experiment 2,3 where the polar co-solvent acetic acid is used with that achieved for the same w% birch wood when no co-solvent is present (experiment 1), for example, it can be seen that significantly higher levels are obtained even after much shorter reaction times. A similar increase in monomeric and oligomeric products (27%) is observed with gVL used as co-solvent (experiment 5) . The oligomeric products consists of components with molecular weight between 250 and 2500 Da (as measured by SEC) , when referred polystyrene standard. These components can be highly unsaturated, possibly even polyaromatic, as suggested by the similar chromatograms obtained using the UV and refractive index detectors.