Noncatalyzed Liquefaction of Celluloses in Hydrothermal Conditions

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Non-catalyzed liquefaction of celluloses in hydrothermal conditions: influence of reactant physico-chemical characteristics and modeling studies Pascal Fongarland, Nadine Essayem, and Franck Rataboul Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03846 • Publication Date (Web): 15 Dec 2016 Downloaded from http://pubs.acs.org on December 21, 2016

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Industrial & Engineering Chemistry Research

Non-catalyzed liquefaction of celluloses in hydrothermal conditions: influence of reactant physico-chemical characteristics and modeling studies

Pascal Fongarland,a,b Nadine Essayema and Franck Rataboula,*

a

Prof. Pascal Fongarland, Dr. Nadine Essayem, Dr. Franck Rataboul, Université Lyon 1,

CNRS, UMR 5256, IRCELYON, Institut de recherches sur la catalyse et l’environnement de Lyon, 2 avenue Albert Einstein, 69626 Villeurbanne, France b

Present address: Université Lyon 1, CNRS, CPE Lyon, UMR 5285, Laboratoire de Génie des

Procédés Catalytiques, 43 boulevard du 11 novembre 1918, 69616 Villeurbanne, France

ABSTRACT This paper reports a fundamental study on the hydrothermal liquefaction of cellulose into light products. First it aimed at comparing the non-catalyzed intrinsic reactivity of model celluloses such as Avicel and Sigmacell with various physico-chemical characteristics (degree of polymerization, crystallinity, particle size, morphology). This information is of importance to dissociate the chemo-physical phenomenon from the catalytic one in the case of catalytic liquefaction to better evaluate the added-value of a catalyst. Under various conditions (3-70 g.L1

, 175-200 °C, 0-120 h) despite a higher degree of polymerization Sigmacell reacted faster

compared to Avicel and formed a higher amount of light products. The reactivity was preferentially influenced by the initial morphology of cellulose. The liquefaction in the absence of catalyst was never complete and led to insoluble hydrochar formation (20-30 wt%). Then a model was built based of three reaction pathways with kinetics and thermodynamic

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investigation. Although liquefaction of Sigmacell seemed to be more complex to predict, good correlation with the experimental data was obtained for Avicel both in terms of liquefaction and products formation.

1. INTRODUCTION Cellulose is recognised as a potential renewable resource for the production of chemicals. For such transformation cellulose is liquefied, implying the use of a solvent, water in the large majority of cases. Although the non-catalytic liquefaction in supercritical water has been studied,1,2 the process most often involves subcritical conditions in association with a catalyst to promote a selective transformation. Liquefaction is usually performed with Brønsted acids along with other kinds of catalysts depending on the desired product.3 During the last decade research has focused on the use of solid catalysts in order to develop more sustainable ways of cellulose transformation. Many catalytic studies are being reported and are regularly reviewed.4-9 Literature reveals that very different conditions are used: cellulosic charge, catalyst, reaction temperature and pressure…, making result comparison difficult. As recently stated by Cinlar et al. for monosaccharide hydrolysis,10 there is a lack of fundamental studies in this area: in the majority of the articles dealing with these catalytic transformations little information is reported on the intrinsic behaviour of cellulose in the reported conditions. Fundamental studies include the simple treatment of cellulose in water, in the absence of a catalyst.11,12 Such information would be of importance for example to dissociate the chemo-physical phenomenon from the catalytic one. This would be of benefit to better understand and evaluate the added-value of the catalyst. Few papers reported such kind of investigations under these conditions. Yu et al. showed significant influence of the crystalline and amorphous portions during hydrolysis of


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microcrystalline cellulose in a semi-continuous reactor.13,14 A general study from Möller et al. on the reactivity of saccharides and polysaccharides in subcritical water with the influence of temperature and pressure has been reported but no detailed information other than conversions was given.15 Jasiukaityte-Grojzdek et al. reported the influence of polymerization degree, crystallinity and crystallites size on cellulose liquefaction in ethylene glycol in the presence of ptoluene sulfonic acid. For example, it was shown that reactivity of amorphous cellulose regions was faster than for crystalline ones.16 More recently Wan et al. correlated the crystallinity index of cellulose in waste paper with 5-HMF yield in subcritical water.17 Besides, modeling and determination of cellulose possible liquefaction pathways is of great interest for a deeper understanding of the overall transformation process. For the treatment of cellulose in water, studies mainly concern modeling in the presence of catalyst18-34 while only few describe the case without catalyst.35 Sasaki et al. reported the reactivity of cellulose in sub and supercritical water. Cellulose conversion rate was investigated and the first order rate constant was evaluated.36 Based on a sort of comparison between soluble oligomers and cellulose, the authors suggested that cellulose hydrolysis takes place at the surface of the particles.2,37 Sidiras et al. modeled the influence of pH decrease due to acid formation during cellulose and hemicellulose autohydrolysis in batch reactors giving also the optimized temperature for glucose et xylose formation.38 Mohd Shafie et al. modeled the non-catalyzed reactivity of cellobiose (cellulose monomer) in hydrothermal conditions showing the influence of cellobiose initial concentration on the kinetics of decomposition.39 Recently, molecular dynamics simulations were performed to study solvents accessibility on cellulose chains40 and the solubility of cellulose in supercritical water compared with ambient conditions.41


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We wish here to report our contribution in this area. We compared the reactivity in water and in the absence of catalyst of various type of celluloses, in the conditions used for catalyzed transformation on cellulose into chemicals. We particularly tried to identify what can be the factors that intrinsically influence the liquefaction of the celluloses. We also modeled our experimental data in order to propose possible liquefaction pathways in these specific conditions.

2. MATERIALS AND METHODS 2.1. Materials. The three different celluloses, Cellulose microcrystalline, powder, 20 µm (Aldrich); Avicel PH-101 (Fluka); Sigmacell Cellulose type 101 (Sigma) were obtained from the corresponding provider and used as received. 2.2. Hydrothermal Treatments. Cellulose was suspended in deionized water (60 mL) in a 100 mL Parr autoclave equipped with a mechanical agitation. The system was purged and pressurized with argon (2 MPa). The mixture was heated up to the desired temperature (4 °C.min-1) and the reaction was considered as started when reaching this temperature. The reaction was stopped by cooling the autoclave in an ice bath. The solid residue was separated from the liquid by filtration over Teflon filter (Millipore®, 0.45 µm) and dried at room temperature for one day. The initial mass of cellulose (mi) and the mass of the solid residue (mr) (whatever its nature) were used to determine the liquefaction: liquefaction (%) = 100*(mi-mr)/mi. 2.3. Solid Sample Analysis. 2.3.1. Average Degree of Polymerization (DP). Average degree of polymerization was estimated using the NF G06-037 norm. This norm is based on the international norm ISO/DIS5351/1 and is used for the conventional determination of average polymerisation degree (DP) of natural or regenerated cellulose using viscometry. This determination is realised from the


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measure of the viscosity index of cellulose in diluted solutions of bis(ethylenediamine)copper hydroxide (EDC). The principle is based on comparing the flowing time in the capillary viscometer of single CED solution (t0) with that of cellulose-CED solution (t, at known concentration, c). From this measure one can deduce the relative viscosity increase (ηa): ηa = (tt0)/t0. The average polymerisation degree can be directly determined using calibration curves present in the NF G06-037 norm: ηa = f(DP.c). In the present study, the relative viscosity increase was measured using a capillary viscometer (K = 0.005539 mm2.s-2) after dissolution of 0.125 g of cellulose in 50 mL of 0.5 M aqueous solution of EDC (2 hours, 25 °C). 2.3.2. Crystallinity Index (CI). Experiments were performed on a Bruker D5005 X-ray diffractometer and sample crystallinity (CI) was determined adapting published procedures based on the amorphous subtraction method (see Figure S1 in Supporting Information).42,43 2.3.3. Solid State CP-MAS

13

C NMR Spectroscopy. The solid residues recovered after each

experiment were analysed on a Avance DSX400 Bruker spectrometer (MAS 10 kHz, T2 4 s, 6144 scans). 2.3.4. Scanning Electron Microscopy. Analysis were performed on a ESEM-FEG FEI XL30 apparatus at the Consortium Lyon Saint-Etienne de Microscopie (CLYM). 2.4. Liquid Sample Analysis. HPLC analysis was performed on a Shimadzu system (Refractive Index Detector, water eluent, 0.5 mL.min-1, COREGEL 107C column, 70 °C). The global yield of detected products was estimated by applying to the overall signal area the average of the response coefficient of the main identified products, glucose, 5-HMF and levulinic acid, quantified by external calibration. Values of pH were determined using a Waterproof pHTestr20 pH meter at 25 °C. The viscosity of the liquid phase obtained after liquefaction treatments was compared to that of pure deionized water using a capillary viscometer (K = 0.005539 mm2.s-2).


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3. RESULTS AND DISCUSSION 3.1. Physico-Chemical Properties of Celluloses. It is difficult to appreciate accurately the characteristics of native celluloses because they are inevitably modified during their extraction from the original source. Nevertheless, one can globally say that cellulose is a polymer of glucoside units forming primary fibrils.44-46 These are assembled together through an extensive network of H-bonds giving larger fibrils. A long-range organisation exists creating ordered (crystalline) and disordered (amorphous) regions within the material. These features make cellulose very resistant to chemical attacks and prevent its solubilisation in usual solvents under mild conditions. Various celluloses of different physico-chemical characteristics are commercially available. Microcrystalline celluloses are obtained generally by treating wood pulp with mineral acids leading to partial depolymerization and increased the crystallinity.47,48 Moreover, treatments homogenise the particles size giving celluloses with varied granulometry. Although these celluloses are certainly far from those that will be used in future biorefineries, some are recognised to be “pure”. For that they are studied as “models” of native cellulose by the research groups working on the catalytic transformation of cellulose willing a well-defined reactant. For the present work, we choose to study three different commercial celluloses. They are labelled by the suppliers as: Microcrystalline powder 20 µm; Sigmacell Cellulose type 101; Avicel PH101. These celluloses have been chosen due to different characteristics that could play a role


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on their reactivity under hydrothermal treatments (Table 1 and Figures S2 and S3 in Supporting Information).

Table 1. Considered Main Physico-Chemical Features of the Studied Celluloses average DPa

CI (%)a

average particle size (µm)b

microcrystalline powder

155

80

20

Sigmacell type 101

560

70

fibers

Avicel PH101

155

80

50

cellulose

a

b

This work. Supplier data.

SEM analysis of the cellulose samples (Figure S3) indicates that the particles are not roundshaped and globally confirms the particle sizes given in Table 1. The fiber morphology announced for Sigmacell is less obvious from this analysis but the sample presents smaller particles with higher level of aggregation. The values for average polymerization degree (DP) and crystallinity index (CI) fall in the range of those available from literature.25,42,49-51 Microcrystalline powder cellulose and Avicel differ only from the particle sizes. The characteristics of Sigmacell are quite different with notably a much higher DP consistent with the fiber morphology and a lower CI. The respective difference of these three celluloses may let us discriminate some characteristics for the liquefaction process studied here.

3.2. Reactivity in Hydrothermal Conditions. Initial experiments were performed with 2 grams of cellulose at 190 °C. This temperature corresponds to that usually employed in the catalytic transformation studies (see Introduction for references). Figure 1 shows that at 190 °C similar liquefaction was obtained for the three celluloses after a prolonged reaction time. In all cases liquefaction tends to reach a plateau. We noticed that fast pH decrease was observed after the


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first three hours of treatment, before a slow increase (Figure S4 in Supporting Information). Therefore the hydrothermal treatment rapidly formed carboxylic acids. Their presence first helps the cellulose liquefaction as previously noted for cellobiose.39 Carboxylic acids formation would be then in competition with their consumption due to other reactions (dehydration, condensations…) explaining the pH increase. This would also explain the observed liquefaction plateau by the concentration decrease of these acids overtime. An additional possible explanation is that water auto-protolysis at 190 °C cannot be sufficient for full liquefaction which would need additional active acid species. 100

80 Liquefaction (%)

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60 Sigmacell 40 Avicel microcrystalline 20

0 0

20

40

60

80

100

120

Time (h)

Figure 1. Liquefaction as a function of time of the three different celluloses (2 g, 60 mL water, 190 °C).

Samples of microcrystalline cellulose and Avicel, having identical DP and CI, present the same reactivity profile and a liquefaction of 80% was obtained after 120 h of treatment. Therefore the initial particle size had here no influence. Sigmacell with a much higher DP reacted faster during the first part of the treatment and a plateau was reached after about 24 hours to give finally the


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same liquefaction. Therefore the characteristics of the celluloses seem to have an influence (if any) on the kinetics rather than on the final liquefaction. At first glance, this result could be surprising if one admits that the reactivity of a given cellulose decreases with higher initial DP. Here we show that it is not necessarily the case and intrinsic reactivity may depend on other factors, for example, the crystallinity index, which one is lower for Sigmacell sample. In their study of acid-catalyzed celluloses liquefaction on ethylene glycol, Jasiukaityte-Grojzdek et al. showed differences of behaviour between cotton-based cellulose and microcrystalline ones. Cotton cellulose liquefaction was partly dependent on DP and CI. Moreover lowest DP celluloses reacted faster due to the highest number of terminal moieties.16 In our case it is difficult to conclude at this stage on the impact of one parameter among others. The residues obtained after each run were analysed by

13

C solid-state CP-MAS NMR

spectroscopy and powder XRD (Figures 2 and 3). In both cases, NMR spectroscopy shows that significant changes occurred only after 15 hours. This indicates that during this first part of the treatment, the recovered solid still corresponds to cellulose as shown by the presence of its fingerprint.42,53 Interesting is to correlate the NMR spectra and X-ray diffractograms. In the case of Sigmacell, after 24 hours XRD indicates that this residual solid possesses a higher CI than initially. This would show that for this sample the amorphous component reacted preferentially during the first part of the treatment explaining the initial higher reactivity of Sigmacell (see Figure 1) while DP had no influence.13 Differently the CI of Avicel sample remained constant up to 24 h indicating a similar reactivity of both amorphous and crystalline phases as already noticed by Tolonen at al. (although in different conditions).54 Note that crystalline-to-amorphous transformation can be excluded here due to the mild conditions we used.55,56) The origin of this different behaviour is not obvious. Indeed the structure of cellulose is not simply limited to


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amorphous and crystalline regions and other intermediate parts are present.57 Moreover, it is established that the large range of chain lengths within these areas plays also a role.13 We rather propose that for Sigmacell the fiber structure preserves the crystalline regions while for Avicel sample amorphous and crystalline areas are equally accessible to hydrolysis. Nevertheless no definitive conclusions can be drawn on structural changes and we can propose at this stage that the initial morphology (fibers vs. particles) explained the different liquefaction behaviours. NMR cellulose patterns displayed decreasing intensity (signal/noise ratio) to completely disappear after 120 h of treatment. The absence of significant peaks indicates that this remaining solid does not correspond to the so-called humins58 but clearly to full decomposition of cellulose into hydrothermal carbon (hydrochar).59 It seems difficult to determine if this recovered material formed directly from the solid reactant, or if it corresponds to a solid precipitated from secondary reactions of soluble species during the course of the treatment (see Scheme 1 in Part 3.3). Nevertheless all applied treatments were unable to solubilize all the carbon content from cellulose and always a 20 wt% fraction remained as solid.


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cellulose fingerprint

(b)

(a) CI 80 % Avicel

CI 79,5 % 15 h

CI 81 % 24 h

48 h 120 h 200

150

100 δ (ppm)

50

0

0

10

20

30

40

50

60

70

80

2θ

Figure 2. Analysis of solid residue obtained after the hydrothermal treatment of Avicel (190 °C) for different times (a) 13C solid state NMR. (b) X-ray diffraction (CI: crystallinity index).


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cellulose fingerprint

(b)

(a) CI 70 % Sigmacell

CI 85 % 15 h

CI 98 % 24 h 48 h 120 h 200

150

100 δ (ppm)

50

0

0

10

20

30

40

50

60

70

80

2θ

Figure 3. Analysis of solid residue obtained after the hydrothermal treatment of Sigmacell (190 °C) for different times (a) 13C solid state NMR. (b) X-ray diffraction (CI: crystallinity index).

The influence of temperature was studied for two celluloses having a significant different reactivity at 190 °C. Sigmacell and Avicel were treated at 175 °C and 200 °C (2 grams). Results presented in Figure 4 show that the difference is also present at these temperatures. Interestingly temperature had a more significant influence on initial rates rather than on final extent of liquefaction after 120 hours which kept in the 70-90% range, and much higher temperatures should be needed for non-catalyzed full cellulose liquefaction.12


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100

100 (a) Avicel

(b) Sigmacell 80

Liquefaction (%)

Liquefaction (%)

80

60

40

Model

Exp.

60

40

200 °C

190 °C

20

Model

Exp.

200 °C

190 °C

20

175 °C

175 °C

0

0 0

20

40

60

80

100

120

0

20

40

Time (h)

60

80

100

120

Time (h)

Figure 4. Experimental and modeled liquefaction as a function of time at various temperatures (2 grams).

The influence of the initial amount of reactant was studied and the example with Sigmacell is presented here. Figure 5a shows that the same profiles were obtained for 0.2, 1.25, 2 and 4 grams, and surprisingly, even over a concentration range with a factor 20 close cellulose liquefaction is obtained after each treatment time and reached around 80%. Figure 5b shows that the absolute amount of liquefied cellulose increased with the charged amount and also that the initial rate of liquefaction increased with the initial mass of cellulose, but in all cases liquefaction reached a maximum of 80%. Cellulose hydrothermal liquefaction in the absence of catalyst can be described as a first order process as already proposed in previous studies (see Introduction for references). 100

4

(a)

(b)

3.5 80

Liquefaction (g)

3

Liquefaction (%)

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60 4g

40

2g

2 1.5 1

1.25 g

20

2.5

0.2 g

0.5

0

0 0

10

20

30

40

50

0

10

20

30

40

50

Time (h)

Time (h)


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Figure 5. Liquefaction as a function of time of various initial amounts of Sigmacell at 190 °C: (a) Relative amount of the initial mass (%). (b) Liquefied amount of cellulose (g).

3.3. Cellulose Liquefaction Model Cellulose dissolution in an isothermal batch reactor was modeled based on the experimental data presented above in order to represent cellulose liquefaction, and formation of soluble/non-soluble oligomers and global detectable products quantified by HPLC. Based on our experimental results and inspired from literature, we supposed cellulose to be converted through a three reaction pathway as presented in Scheme 1.34,36

kR solid residue cellulose

kO

hydrochar

kP

kD

liquid phase hydrosoluble oligomers

HPLC detectable products « monomers »

Scheme 1. Reaction pathway proposed for cellulose liquefaction.

As expected, HPLC analysis indicated that this non-catalytic transformation was clearly not selective and a complex mixture of light products was obtained (corresponding to P route in Scheme 1). Among others, we observed the presence of common products from acid-catalysed treatment of cellulose in water: glucose, 5-HMF, and organic acids like levulinic and lactic acids. Due to the impossibility to identify all compounds, we combined the total response signal area


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due to detected light products, obtained for each treatment (see Materials and Methods part for details). Figure 6 indicates different behavior for the two celluloses.

25

25 (a) Avicel

(b) Sigmacell

20

20

15

15

10

Yield (%)

Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Model

Exp.

10

190 °C

5

Model

Exp.

200 °C

200 °C

5

190 °C

175 °C

175 °C 0

0 0

20

40

60

80

100

120

0

20

Time (h)

40

60

80

100

120

Time (h)

Figure 6. Experimental and modeled evolution of estimated global yield of HPLC detectable products as a function of time at various temperatures (2 grams).

The maximum amount of HPLC detected products was much faster from Sigmacell than from Avicel. This fits with the highest liquefaction rate (Figure 1): Sigmacell was liquefied more rapidly as was the transformation into light products. The maximum global mass yield was aroud 25 wt% and 20 wt% for Sigmacell and Avicel, respectively. For both celluloses, the rate of formation depended on the temperature in the first part of the reaction. For 190 and 200 °C, the concentration of such compounds was maximum after the first hours and decreased for prolonged reaction times. This indicates that these species formed higher molecular mass compounds that became not detectable. Note that this phenomenon is less marked with Avicel, indicating here also a different behavior. At 175 °C, the formation of initial products tends to reach a plateau. This temperature seems therefore not sufficient and/or would need more time for re-condensation reactions. As for liquefaction extent (Figure 1), the total amount of detected products falls in the same range after 120 hours for both celluloses and each temperature. Globally, the selectivity of these products decreased with liquefaction increase. This confirms


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that condensation reactions leading to heavier molecules occurred after long reaction times. These secondary reactions can be reasonably attributed to hydrothermal carbon formation (corresponding to D route in Scheme 1) consistent with our observation (see part 3.2). Moreover, after the first minutes of treatment, we observed no relative viscosity increase between pure water, and water-containing liquefied cellulose (ηa = 0) and did not change with time. This confirms that as soon as part of the polymer was transferred into the liquid phase, significant depolymerization of cellulose chains occurred leading to low molecular weight species along with short length oligomers having no impact on the viscosity of the aqueous solution. In order to better model experimental data, we have also assumed and introduced this oligomer family obtained from cellulose depolymerization, soluble in water, not HPLC detectable, but also being difficult to be further hydrolysed into light monomers34 (corresponding to O route in Scheme 1). Finally, according to characterization performed on solid residues after hydrothermal liquefaction presented above (Part 3.2), we have assumed that cellulose could rapidly be depolymerized and then re-polymerized until obtaining a residue completely different from initial celluloses (corresponding to R route in Scheme 1). This allows explaining why a solid fraction will always be present although cellulose was apparently completely converted after a treatment in our conditions. To model the cellulose depolymerization in aqueous phase, mass balance was written for each family assuming a pseudo-first order for each step. We also supposed that the dissolution mechanism was controlled as a chemical reaction according to the high value of activation energy (see Table 2) calculated from data of Figure 4. The mass balance is then:

= − +

+ ∗ = − ∗

(1)


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=

∗

(2)

= ∗ − ∗

= ∗ + ∗

(3) (4)

Assuming no gaseous compounds were produced during liquefaction, overall mass balance at each time is then: = + !" # + + !" #

(5)

And mass of solids will be equal to: #! = + !" #

(6)

This ordinary system of equations can be integrated analytically allowing expressing each family concentration as a function of time. However, it was possible to analyze quantitatively only the remaining solid and the products detected by HPLC labeled "monomers" family. Assuming that the remaining solid consists in un-reacted cellulose and non-soluble/non-hydrolysable materials, we have modeled this remaining solid fraction using the simplified kinetic model presented previously in order to estimate properly the kinetic constant. The advantage of this kinetic analysis is to give a quantitative estimation of the remaining cellulose that is not directly accessible except qualitatively using NMR and XRD techniques. Temporal evolution of mass of each fraction is then given: = ∗ 1 − % &'( = ∗ )'

'*

+ &'(

(7)

, % &'( − % &'+

'

!" # = ∗ '- ∗ 1 − % &'( (

(8) (9)


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Non-hydrosoluble oligomers produced from direct cellulose conversion and monomers recombination were deduced from mass balance as expressed in equation (5).

All parameters have been adjusted using simple non-linear regression method minimizing without constraints the following objective function: 23 5

1 . = ∑6 − ! !780!

4

(10)

Adjustments have been performed using mass of solid fraction and "monomers" fraction for both celluloses and three temperatures using an Arrhenius term for the kinetic constants. It results that experimental data are correctly represented by this formal model as shown in Figure 4 for the liquefaction and Figure 6 for the yield of HPLC detectable products. Note that for Sigmacell, the model predicts a greater decrease of the global yields than observed experimentally and we observed a deviation for liquefaction modeling after prolonged reaction times. This indicates that for Sigmacell the model fits less with the experimental data. Additionally, this model lets determine the evolution of the cellulose fraction in the residual solid (Figure 7).

(a) Avicel

Model

(b) Sigmacell

200 °C 190 °C 175 °C

60

Model 200 °C

80

Cellulose fraction (%)

80

Cellulose fraction (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40

20

190 °C 175 °C

60

40

20

0

0 0

20

40

60

80

100

120

0

20

Time (h)

40

60

80

100

120

Time (h)

Figure 7. Modeled cellulose fraction in the solid residue as a function of time at various temperatures (2 grams).


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For both celluloses this fraction decreased faster at 200 °C. In the case of Avicel, this model fits well with NMR data (Figure 2) from which we can say qualitatively that for example after 48 hours this fraction became low. This fits correlation is less true for Sigmacell since the model predicts that cellulose conversion (that can be different to cellulose liquefaction) was faster and no cellulose would be present in the residue after 15 hours. This tendency was not confirmed by NMR monitoring which showed that a significant amount of cellulose was still present in the solid residue after this reaction time (Figure 3). The liquefaction of Sigmacell seems to be more complex to be modeled due probably to important heterogeneity in the biomass matrix. According to kinetic constants values reported in Table 2, the three parallel pathways seem to present similar “reactivity” for a given cellulose, the highest value of ki being for cellulose depolymerization into soluble oligomers non-hydrolysable into HPLC detectable products. Avicel is more refractory to hydrothermal liquefaction process as observed with systematic lower values for kinetic constants (factor 5 to 10) in comparison to Sigmacell. Then, a lower maximum amount of HPLC products is obtained from Avicel and also in term of cellulose degradation into hydrochar. Note that the kinetic constant for overall cellulose liquefaction (kA) for Sigmacell has a value close to that calculated by Sasaki et al. extrapolated to the operating conditions of this study (temperature between 175 and 200 °C).36

Table 2: Kinetic Constants Values at 200 °C and Activation Energy for Sigmacell and Avicel kinetic constant (h-1) Sigmacell Avicel

activation energy (kJ.mol-1) Sigmacell Avicel

kR

0.140

0.031

16.4

57.3

kP

0.185

0.024

75.5

96.2

kO

0.362

0.086

73.9

100.6


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a

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kD

0.020

0.002

19.8

44.2

kAa

0.687

0.140

57.8

88.0

Global cellulose conversion

In term of activation energy, values are higher for Avicel compared to Sigmacell suggesting for the latter that depolymerization process could involve acidic catalysis (H3O+ present in water), while Avicel depolymerization would be preferentially controlled by thermal decomposition with an excepted higher value of activation energy. Values are of most of the time between 80 and 100 kJ.mol-1 similar to those reported in the literature. Mok et al. obtained an activation energy between 100 and 140 kJ.mol-1 for a two pathways mechanism leading to the formation of soluble oligomers that are not hydrolysable.34 Olanrewaju has obtained a value of 99 kJ.mol-1 for microcrystalline cellulose hydrolysis in water using a shrinking-core model.60 Some other studies have founded higher value but generally experiments were conducted at higher temperature leading to a significant change of H3O+ concentration reaching near-critical state. For example, Cantero et al. obtained an activation energy of 154 kJ.mol-1 for a temperature between 280 and 370 °C.61 This higher value could be a consequence of the increase of pure thermal activation process (pyrolysis) competing with “acid hydrolysis” as suggested before.

4. CONCLUSION In this study we compared the behavior of model celluloses with various physico-chemical characteristics during their non-catalyzed hydrothermal liquefaction in order to determine which properties can influence the intrinsic reactivity. We used liquefaction conditions generally employed when a catalyst is present for a transformation of cellulose into chemicals. We showed


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that among some differencing parameters, polymerization degree, particle size, crystallinity index and initial morphology, the latter may play the most significant role. Under various conditions Sigmacell reacted faster compared to Avicel and formed a higher amount of light products. The liquefaction in the absence of catalyst was never complete with always a 20-30 wt% formation of insoluble hydrochar. We built a liquefaction model based on a three reaction pathways leading to light products, hydrosoluble oligomers and hydrochar. Although liquefaction of Sigmacell seemed to be more complex to be modeled, this fitted well with the experimental data in terms of liquefaction and various products formation. This study giving kinetic and thermodynamic information is a fundamental approach towards the water liquefaction of cellulose into light products. These kinetic data obtained in the absence of catalyst will serve as a basis for comparison with that obtained in the presence of a catalyst to clearly determine the added-value of a catalyst on the overall transformation.

SUPPORTING INFORMATION Schematic representation of the method used for CI determination., Initial powder X-ray diffraction patterns of microcrystalline, Avicel and Sigmacell celluloses, Scanning electron microscopy of microcrystalline, Avicel and Sigmacell celluloses, and Evolution of pH value over time for Avicel and Sigmacell celluloses.

AUTHOR INFORMATION Corresponding Author [email protected]


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ACKNOWLEGMENTS We thank the CNRS and University Lyon 1 for funding.

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Table of Contents/Abstract graphic Study of un-catalyzed hydrothermal cellulose liquefaction kR cellulose kO

solid residue hydrochar

kP

kD

liquid phase hydrosoluble oligomers

monomers


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Noncatalyzed Liquefaction of Celluloses in Hydrothermal Conditions

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