Yves Simon
email: yves.simon@univ-lorraine.fr

Paul-Marie Marquaire

A unified mechanism for oxidative coupling and partial oxidation of methane

Keywords: Methane, kinetic mechanism, partial oxidation, oxidative coupling, La2O3

The oxidative coupling (OCM) and the partial catalytic oxidation (POM) of methane as well as the homogeneous oxidation of methane (HOM) differ only in the ratio CH4 / O2 used and of course in the use of a catalyst. It can therefore be considered that their overall reaction mechanism should be the same with elementary reactions of varying importance depending on the type of oxidation reaction studied. We have therefore tried to represent the results obtained during the experimental study of OCM and POM over La2O3 and that of the oxidation of methane in gas phase from a single mechanism. At high temperature, OCM and POM are catalytic reactions but their reaction mechanisms are very complex because the surface reactions are coupled to reactions in gas phase by the intermediary of radicals, so both homogeneous and heterogeneous mechanisms occur at the same time. In this work, the development and the validation of a hetero-homogeneous mechanism is proposed for the three reactions. This mechanism is based on elementary steps at the catalyst surface and elementary steps in gas phase for a large range of temperature (973K -1173K) and residence time (0.7s -5.5s).

INTRODUCTION

Fluctuating prices of fossil fuel and discovery of new natural gas deposits are increasing interest in the upgrading of methane to value-added products. To date, the world reserve of natural gas is estimated about 198.8 trillion cubic meters [START_REF]BP statistical review of world energy[END_REF] with steadily increasing in the future. Furthermore, over the past decade, increased production of shale gas has stimulated renewed interest to converting methane into valuable chemicals or liquid fuels.

The oxidative coupling of methane (OCM), first proposed by Keller and Bhasin [START_REF] Keller | Synthesis of Ethylene via Oxidative Coupling of Methane I. Determination of Active Catalysts[END_REF] in 1982, is a direct method of converting methane into higher hydrocarbons especially ethylene.

Ethylene is a critical intermediate in the petrochemical industry witch is currently produced by steam cracking of hydrocarbon, mainly ethane. It is a highly endothermic reaction that occurs at high-temperature. Unlike steam cracking, the oxidative coupling of methane is exothermic and allows the direct conversion of methane into ethylene at a lower energy cost compared to current indirect industrial processes. During the 1990s, many studies were devoted to the OCM reaction. These studies showed that the yield of the reaction was not really compatible with an industrial application. However, in the last few years, the interest in the OCM reaction has increased because catalysts with higher activity and selectivity have been found. Indeed, with oxides as La2O3 acceptable reaction performances have been reached [START_REF] Choudhary | Oxidative coupling of methane over alkaline earth oxides deposited on commercial support precoated with rare earth oxides[END_REF].

Another way of methane valorisation is the catalytic partial oxidation (POM). The catalytic partial oxidation of methane is an alternative to steam reforming [START_REF] Rostrup-Nielsen | Activity of steam reforming catalysts: role and assessment[END_REF][START_REF] Rostrup-Nielsen | Hydrogen and Synthesis Gas by Steam and CO2 Reforming[END_REF] for synthesis gas (CO + H2) production. One advantage of POM is the production of syngas [START_REF] Corbo | Hydrogen production by catalytic partial oxidation of methane and propane on Ni and Pt catalysts[END_REF][START_REF] Requies | Effect of redox additives over Ni/Al2O3 catalysts on syngas production via methane catalytic partial oxidation[END_REF][START_REF] Subramanian | Catalytic partial oxidation of higher hydrocarbons: reactivities and selectivities of mixtures[END_REF] with a H2/CO ratio ideal for a subsequent Fischer-Tropsch synthesis. In contrast to steam reforming, the partial oxidation is an exothermic reaction and therefore requires a smaller amount of heat energy. Furthermore, POM has a good dynamic response time and could therefore be used as an on-board hydrogen generator for fuel cells.

OCM and POM are catalytic reactions at high temperatures. Their reaction mechanisms are very complex because the surface reactions are coupled to reactions in gas phase by the intermediary of radicals [START_REF] Bistolfi | Kinetic Model for methane oxidative coupling reactors[END_REF][START_REF] Driscoll | Gas-Phase Radical Formation during the Reaction of Methane, Ethane, Ethylene and Propylene over selected Oxide Catalysts[END_REF][START_REF] Driscoll | Formation of Gas Phase Methyl Radicals over MgO[END_REF][START_REF] Sinev | Kinetic modeling of heterogeneous-homogeneous radical processes of the partial oxidation of low paraffins[END_REF][START_REF] Baerns | Catalytic partial oxidation of methane to synthesis gas -catalysis and reaction engineering[END_REF][START_REF] Deutschmann | Modeling the partial oxidation of methane in a shortcontact-time reactor[END_REF][START_REF] Goralski | Modeling homogeneous and heterogeneous chemistry in the production of syngas from methane[END_REF][16][START_REF] Thybaut | Catalyst design based on microkinetic models: oxidative coupling of methane[END_REF], so both homogeneous and heterogeneous mechanisms occur at the same time. Indeed, the general oxidation mechanism includes two sub-mechanisms, one that is related to gas-phase and another that contains the catalytic reactions. Obviously, the homogeneous oxidation of methane (HOM) requires only the gasphase mechanism. The modelling of the reaction by a detailed mechanism (composed of elementary steps describing the reaction as it occurs at the molecular level) has a lot of advantages. The mechanism and the kinetic analysis results allow a better understanding of the reaction. Indeed, based on this analysis, the reaction pathways, the importance of heterogeneous and homogeneous reactions and the limitative steps can be identified. Since 1982, the literature concerning the study of hetero-homogeneous reactions has been abundant [START_REF] Hickman | Production of syngas by direct catalytic oxidation of methane[END_REF][START_REF] Vlachos | Homogeneous-heterogeneous oxidation reactions over platinum and inert surfaces[END_REF]. However, there are few detailed mechanisms and much less kinetic data of surface reactions that were published.

The OCM and POM reactions differ only in the ratio CH4 / O2 used, so their overall reaction mechanism should be the same. We have therefore tried to represent the results obtained during the experimental study of OCM and POM over La2O3 and that of the oxidation of methane in gas phase [START_REF] Fleys | Detailed kinetic study of the partial oxidation of methane over La2O3 catalyst. Part 2: mechanism[END_REF][START_REF] Fleys | Detailed kinetic study of the partial oxidation of methane over La2O3 catalyst. Part 1: Experimental results[END_REF][START_REF] Fleys | Investigation of the reaction of partial oxidation of methane over Ni/La2O3 catalyst[END_REF][START_REF] Simon | Kinetic modeling of the oxidative coupling of methane[END_REF] from a single mechanism. The development and the validation of a hetero-homogeneous mechanism is proposed for the three reactions, based on elementary steps at the catalyst surface and on a homogeneous mechanism in gas phase for a large range of temperature and residence time. The experimental study was carried out in a perfectly stirred reactor in presence of lanthanum oxide. The simulations were done by using the Chemkin® software packages.

EXPERIMENTAL SECTION

The experimental setup was already described in previous articles [START_REF] Fleys | Detailed kinetic study of the partial oxidation of methane over La2O3 catalyst. Part 1: Experimental results[END_REF][START_REF] Simon | Kinetic modeling of the oxidative coupling of methane[END_REF]. The reactor developed for the investigation of hetero-homogeneous reactions [START_REF] Barbé | Competition between the gas and surface reactions for the oxidative coupling of methane. 1. "Non-isothermal" results in catalytic jet-stirred reactor[END_REF][START_REF] Barbé | A new "catalytic jet-stirred reactor" Application to the study of the oxidative coupling of methane[END_REF] is a continuous stirred tank reactor (CSTR) presented in Fig. 1. It consists of two parts: an important well-stirred gas-phase volume (110 cm 3 ) in contact with catalysts pellets laid on a removable cylindrical support. Four Thermocoax resistance wires that are in contact with the wall are used for heating the reactor. Before entering the reactor, the reactant mixture is preheated. The temperature of the first preheating part is 100°C lower than the reactor temperature while the temperature of the second preheating part equals the one of the reactor.

The temperature of the gas phase is measured thanks to a thermocouple located inside a quartz finger at the middle of the free volume.

The catalyst used was Lanthanum oxide. The powder of La2O3 was pelleted at a pressure of 20 kN into a cylindrical shape thanks to an electromechanical press Instron 5569.

Mass flow controlers

Ar or He The pellets are 12 mm wide (diameter) and 1 mm thick. In order to facilitate the desorption of water and also to decompose the carbonates present on the surface of the pellets, the La2O3 powder was heated for 8 hours at a temperature of 900 ° C. The BET surface areas are 1,8 m 2 .g -1 for catalyst pellets used in OCM and 5 m 2 .g -1 for pellets used in POM.

The outlet gases H2, CO, CH4, C2H4, C2H6 and CO2 are analysed in line or off line by gas phase chromatography [21 and 23]. Several standard bottles, containing a gas mixture at different concentrations, externally calibrate the chromatograph. All data were collected at a steady state regime and each test was repeated at least three times to verify the stability and repeatability of the measurements.

The experimental conditions used for this study are:

-Temperatures between 973K and 1173K -Outlet pressure fixed at 1 atm.

-Composition of the gas inlet with CH4/O2 = 5 for OCM and CH4/O2 = 2 for POM and HOM.

The reactants are highly diluted in argon or helium (94 % in OCM and 84% in POM or HOM) to better control the reaction temperature and to avoid hot spots.

KINETIC MODELING

Homogeneous mechanism

The mechanism used for simulation includes a heterogeneous part and a homogeneous one.

The homogeneous part is described by a set of over 450 elementary reactions. Due to its size, the mechanism cannot be fully presented here; we give only the most significant reactions in The homogeneous mechanism takes into account all elementary reactions between molecules and radicals including less than three carbon atoms. This gas-phase mechanism is well known and has been confirmed by a vast amount of experimental data for different hydrocarbon reactions for the homogeneous oxidation of methane [20; 23; 26].

This mechanism was generated by EXGAS software [START_REF] Warth | Computer based generation of reaction mechanisms for gas-phase oxidation[END_REF]. In the present work, this mechanism is used in combination with a heterogeneous mechanism for the simulation of the catalytic oxidation of methane reactions.

Heterogeneous mechanism 3.2.1. Description of the mechanism

The development of the heterogeneous mechanism takes into account the results from mechanistic studies reported in the literature [START_REF] Mccarty | Methane Conversion by Oxidative Processes[END_REF][START_REF] Martin | Surface chemistry in the oxidative coupling of methane[END_REF][30][START_REF] Couwenberg | Kinetics of a gas-phase chain reaction catalysed by a solid: The oxidative coupling of methane over Li/MgO-based catalysts[END_REF][START_REF] Wolf | Kinetic simulations of surface processes of the oxidative coupling of methane over a basic oxide catalyst[END_REF][START_REF] Coulter | The role of carbon dioxide in the oxidative dimerization of methane over Li/MgO[END_REF][START_REF] Shi | Origin of kinetic isotope effects during the oxidative coupling of methane over a lithium (1+)/magnesia catalyst[END_REF][START_REF] Myu | Kinetics of oxidative coupling of methane: Bridging the gap between comprehension an description[END_REF][START_REF] Karakaya | A detailled reaction mechanism for oxidative coupling of methane over Mn/Na2WO4/SiO2 catalyst for non-isothermal conditions[END_REF][37]. Despite the many studies carried out on this subject, the exact nature of the active sites on La 2 O 3 remains unclear. Yang et al.

suggested that O 2-ions could be the active site for methane activation [START_REF] Yang | Oxygen species on the surface of La2O3/CaO and its role in the oxidative coupling of methane[END_REF]. On the other hand for Hutchings et al. [START_REF] Hutchings | Partial oxidation of methane over oxide catalysts. Comments on the reaction mechanism[END_REF], the formation of CH 3 . radicals is due to the O -ions while O 2 2-lead to the formation of CH 2 : radicals. Lacombe et al. [START_REF] Lacombe | Oxidative coupling of methane over lanthana catalysts. I. Identification and role of specific active sites[END_REF] have identified various active sites on the surface of lanthanum oxide: a basic site associated with an anionic vacancy which would be responsible for the dissociation of gaseous oxygen into atomic species and an unsaturated site on which methyl radical would react to be further oxidized into CO 2 . In summary, there are at least two types of active sites on the La2O3 surface [START_REF] Huang | Adsorption and decomposition of NO on lanthanum oxide[END_REF].

In our mechanism, the two types of active sites considered are: a reduced site (s) and an oxygenated site O(s). Experimental studies of oxygen chemisorption [START_REF] Gambo | Recent advances and future prospect in catalysts for oxidative coupling of methane to ethylene : A review[END_REF] have shown that this reaction occurs with dissociation of a diatomic O 2 molecule to form two active atomic oxygen centres according to the following reaction:

O2(s) + s 2 O(s) 2 O(s) O2(s) + s
In these elementary reactions s represents a surface site and O(s) the atom O adsorbed on the surface of the La 2 O 3 crystal.

The heterogeneous mechanism was written systematically by considering the possible reactions between these two sites and the major reaction products and free radicals. These reactions were written according to the Eley-Rideal formalism involving the reactions between a gas phase molecule and an active site. For example, methane can react according to:

CH4 + O(s) CH3. + OH(s) CH4 + s CH3. + H(s)
The hydroxylated sites and hydrogenated sites can react according to the Langmuir-Hinshelwood formalism leading to the formation of hydrogen and water via surface steps:

H(s) +

10

To carry out the simulations, it is necessary to calculate the kinetics parameters of this heterogeneous mechanism. There are few models involving a detailed mechanism composed of elementary steps and with estimation of the kinetics parameters. We can quote for example Deutschmann [START_REF] Zerkle | Understanding homogeneous and heterogeneous contributions to the platinum-catalyzed partial oxidation of ethane in a shortcontact-time reactor[END_REF], Sinev [44] and Gent University [START_REF] Sun | Microkinetics of methane oxidative coupling[END_REF][START_REF] Thybaut | Catalyst design based on microkinetic models: oxidative coupling of methane[END_REF]. In our study, pre-exponential factor of elementary steps are calculated by methods derivate from Benson's techniques [START_REF] Benson | Thermochemical kinetics : methods for the estimation of thermochemical data and rate parameters[END_REF] whereas activation energies are chosen in first approximation by analogy with reactions in gas phase. Simulations were performed using the Chemkin® and Surface Chemkin® software packages in a CSTR reactor [START_REF] Glarborg | PSR: a Fortran program for modeling well-stirred reactors[END_REF][START_REF] Coltrin | Surface Chemkin (v 4.0): a Fortran package for 16 analyzing heterogeneous chemical kinetics at a solid surface -gas phase interface[END_REF]. The simulations were performed by simultaneously compiling the homogeneous and the heterogeneous sub-mechanisms so that the possible coupling could be taken into account. It should be noted that these methods give only an approximation of the initial numerical value of parameters, usually unknown, which must be adjusted during the optimization from experimental results.

Pre-exponential factor

The pre-exponential factor is calculated by using partition functions of gas molecule and activated complex. For the following reaction:

A + B(s) → AB ≠ → Products
where A is a gas molecule, B(s) a surface species and AB ≠ the activated complex, the preexponential factor is calculated by the following equation: 21

In Eq. ( 1), kB is the Boltzmann constant (1.38 10 -23 J.K -1 ), N the Avogadro constant (6.022 10 23 mol -1 ), T the temperature, h the Planck constant (6.6262 10 -34 J.s.) and qA, qB(s), q ≠ are the partition functions of A, B(s) and the activated complex. The difference between the degrees of freedom of B(s) and AB ≠ is mainly due to the vibrations of the molecule A in the configuration of the activated complex. Indeed, B(s) and AB ≠ are adsorbed species and have no rotational or translational degrees of freedom. So, to simplify the calculation, we suppose than the difference between partition function of B(s) and AB ≠ is only due to the vibrational component of the activated complex q ≠ v. Then the following expression is obtained:

The ). For easier use, these formulas can be rewritten:

1 2
In the first expression the unit of qt is m -3 and M is the molar mass (g.mol -1 ). In the second expression, the unit of D is amu 3 .Å 6 and that of I is amu.Å 2 .

Activation energy

The activation energy is the most difficult parameter to calculate, especially for surface reactions, because it depends on the nature of the adsorption site. Since these values are generally intended to be optimized by simulation of the experimental results, two simple methods of estimation depending on the type of reaction involved are proposed. For the adsorption/desorption reactions, the activation energy is calculated from the experimental value of adsorption enthalpy obtained from the literature. The activation energy of the other elementary surface reactions can be assumed in first estimate to be the same as the equivalent gas-phase reaction.

For example, the CO2 adsorption -desorption can be represented by the following reactions:

(3) CO2 + s → CO2(s)

(-3) CO2(s) → CO2 + s
The activation energy of the CO2 adsorption is low; so the activation energy of the step (3) is supposed to be equal to 0. The adsorption enthalpy of CO2 on the surface can be found in the literature; the adsorption enthalpy of CO2 on La2O3/CaO catalyst [START_REF] Toops | The effect of CO2 and H2O on the kinetics of NO reduction by CH4 over La2O3/γ-Al2O3 catalyst[END_REF] is about:

-∆Hads, CO2 = 44 ± 7 kcal.mol -1 = E-3.

For the other types of surface reactions, the activation energy can be chosen, by analogy with gas phase reactions. The Eley-Rideal reaction of an ethane molecule with the surface of the catalyst: C2H6 + s C2H5. + H(s) may be represented by the following homogeneous reaction: C2H6 + alkyl.

C2H5. + alkane

We can use, for example, the reaction: C2H6 + CH3.

C2H5. + CH4

The activation energy of this gas phase reaction [START_REF] Baulch | Evaluated kinetic data for combustion modelling[END_REF] is: 5800 cal.mol -1 . At a first approximation, this value for the activation energy of the surface reaction can be used. The activation energy depends on the catalyst surface and these activation energy values can only be considered as a starting point for the simulation. Finally, a hetero-homogeneous mechanism composed of many elementary steps and their kinetic constants (A and E) was obtained.

Sensitivity analysis

For all the reactions studied, a flow consumption analysis or a sensitivity analysis can be realized to better understand the reaction and to find optimal conditions. To determine the most sensitive reactions involved in the mechanism, a sensitivity analysis was performed for each of the three reactions. The analysis was performed for major products. Figures 18 and19 show sensitivity analysis for surface reactions for OCM and POM. The first order sensitivity coefficient for species n and reaction i was defined according to:

S i,n = k i dk i dx n x n ( 5
)
where ki is the kinetic constant of reaction i and xn is the molar fraction of species n. The higher the Si,n coefficient is, the more sensitive against species n the reaction i is. Moreover, a positive sensitivity coefficient Si,n means that an increase of the kinetic constant ki leads to an increase of the concentration of species n. Some reactions have sensitivity coefficients lower than |0.01| and can be considered negligible. Table 3 shows the final surface mechanism in which only the most important elementary reactions are present. This simplified mechanism consists of 17 elementary reactions and gives the same result as the complete mechanism with an error of less than 1%.

4-RESULTS AND DISCUSSION

For the three reactions of OCM, POM and HOM, the experimental curves of reactant consumption and product formation as a function of residence time were simulated. The kinetic parameters of the homogeneous reactions provide from literature [START_REF] Tsang | Chemical kinetic data base for combustion chemistry. Part I. methane and related compounds[END_REF][START_REF] Ranzi | A wide range modelling study of methane oxidation[END_REF][START_REF] Cavanagh | Computer modeling of cool flames and ignition of acetaldehyde[END_REF][START_REF] Schulz | Reaktionsmodell zur bruttokinetik der pyrolyse von methan im stosswellenrohr bei temperaturen von 1800 K bis 2500 K[END_REF][START_REF] Knyazev | Kinetics of the C2H3 + H2 = H + C2H4 and CH3 + H2 = H + CH4 reactions[END_REF][START_REF] Zhang | Rate constants for abstraction of hydrogen from benzene, toluene, and cyclopentane by methyl and ethyl radicals over the temperature range 650-770 K[END_REF][START_REF] Benson | Oxygen Initiated Combustion: Thermochemistry and Kinetics of Unsaturated Hydrocarbons[END_REF][START_REF] Mahmud | A High-Temperature Photochemistry Kinetics Study of the Reactions of O Atoms With Acetylene From 290 to 1510 K[END_REF][START_REF] Glarborg | Kinetic modeling and sensitivity analysis of nitrogen oxide formation in well-stirred reactors[END_REF] or are calculated by the Kingas [START_REF] Warth | Computed aided design of gas-phase oxidation mechanisms: Application to the modelling of normal-butane oxidation[END_REF] software (Table 1). The kinetic parameters of heterogeneous reactions are estimated using the methodology described here (Table 2) and modified to fit the model to the experimental results (Table 3). This unified hetero-homogeneous mechanism is used for the simulation of the three reactions.

Homogeneous oxidation of methane

Homogeneous oxidation of methane doesn't have a real industrial interest because of the stability of the methane molecule. However, the mechanism of this reaction is essential to represent the reactions of OCM and POM. Therefore, we have studied this reaction experimentally over a large temperature range (1083K to 1148 K) in order to determine its reaction mechanism. Figure 1 to 6 show the molar fraction of CH4, C2H4, C2H6, CO, CO2 and H2 versus residence time. The ratio CH4/O2 was fixed to 2. The simulation very well reproduces the conversion of CH4 and the production of the most important products.

Partial oxidation of methane

Among the proposed methods for the production of hydrogen, the partial oxidation of methane has many advantages: the reaction is exothermic and it may be carried out in autothermal conditions. This reaction produces syngas: CH4 + ½ O2 = CO + 2 H2

The reaction was studied at various temperatures between 973K and 1123K over La2O3 catalyst. Like for HOM, the ratio CH4/O2 was 2. The concentration of sites over the surface L was unknown. So we used an average value of L = 910 -11 mol.cm -2 as the starting point. L was thus considered as an adjustable parameter and the value used after adjustment was finally: L = 2.810 -11 mol.cm -2 . Figure 7 to 12 show the variation of the molar fraction of CH4, C2H4, C2H6, CO, CO2 and H2 according to the residence time at two temperatures. A good agreement between the experimental and theoretical results can be observed.

Oxidative coupling of methane

The reaction of oxidative coupling of methane has been investigated in a large number of laboratories because its development should have led to a direct process to obtain more valuable hydrocarbons such as ethylene for the chemical industry:

2 CH4 + 1/2O2 = C2H4 + 2H2O
This reaction was studied between 1023K and 1173K over La2O3 catalyst with a CH4/O2 ratio of 5. Like for POM, the concentration of sites over the surface was L = 910 -11 mol.cm -2 as the starting point and the value used after adjustment was L = 510 -11 mol.cm -2 . Figure 13 to 17

shows the molar fraction of CH4, C2H4, C2H6, CO and CO2 according to the residence time.

Although the kinetic parameters were only slightly modified the agreement between the theoretical curves and the experimental points was correct. Therefore the heterohomogeneous mechanism was validated in the experimental conditions.

Comparison of the three reactions

A simplification of the general mechanism can be represented according to figure 20.

The mechanism of oxidation of methane comprises two distinct reaction pathways: an oxidative route and a non-oxidative route.

The initiation of the oxidation of methane forms CH3. radicals (reaction 1). In the case of HOM, the activation occurs by reaction between CH4 and a gas phase radical while in OCM and POM the initiation takes place mainly on the catalyst by reactions involving surface species.

CH4 Then, the CH3. radicals can react according to the pyrolytic route (reaction 2) to form hydrocarbons or the oxidative route (reaction 3) to form oxygenated compounds. The pyrolytic route and therefore the formation of hydrocarbons is favored when the concentration of oxygen is low, which is the case for OCM. When the conversion is high, an oxidative step appears (reaction 4). This step decreases the concentration of C2H4 and may explain why the yield of hydrocarbons in OCM does not exceed 22% to 27% for all catalysts tested, like reported by numerous authors.

In POM and HOM, the oxygen concentration favors the oxidative pathway. However, lanthanum oxide is an excellent generator of free radical. Thus, in POM, the catalyst more easily generates radicals by reactions between CH4 and surface species and therefore

CONCLUSION

The study of catalytic oxidations of methane has met with renewed interest in the past 10 years due to the discovery of new natural gas fields and the rising price of oil. These catalytic reactions are carried out at high temperature and thus comport a homogeneous and heterogeneous part, which make their mechanism more complex. This reaction mechanism is essential for the development of industrial processes. We have therefore developed a reaction mechanism that allows representing, at the same time, three methane oxidation reactions: homogeneous oxidation, partial oxidation and oxidative coupling. The homogeneous mechanism may be found in the literature or determined from an automatic generator of mechanism. The heterogeneous mechanism was written following the Eley-Rideal and Langmuir-Hinshelwood formalisms. The pre-exponential factor was determined by methods derived from Benson's technics whereas the activation energy was chosen, in a first time, by analogy with gas phase reactions. Finally, the mechanism used for the simulation give good results although the values of the rate constants were only slightly modified. Thanks to the sensitivity analysis, the heterogeneous mechanism has been simplified and contains only 17 elementary reactions. Ired: reduced moment of inertia (amu. A 2 ) ω: frequency of vibration (cm -1 ) D: product of the moments of inertia (amu 3 .Å 6 or kg 3 . m 6 ) ∆Hads,i: heat of adsorption for species i (cal.mol -1 ) xn: molar fraction for species n Si,n: sensibility coefficient of reactions i for species n

Fig. 1 .

 1 Fig.1. Schematic representation of the catalytic jet-stirred reactor

  partition function of a molecule or a radical is the product of a translational partition function qt, an external rotational partition function qr, a vibrational partition function qv and an electronic partition function qe. These partition functions can be calculated by the Eq. (3). In these equations, σext is the external symmetry number, m (kg) the mass of the atom, s the spin and ωi (cm -1 ) the frequency of the tabulated vibration. The selection of the formula for calculating the rotational partition function depends on the molecule considered. A linear molecule only possesses two rotational degrees of freedom. The two moments of inertia about the two axes of rotation are equal to I (kg.m 2 ). In this case, the rotational partition function is qr 2D . However, there are 3 rotational degrees of freedom in a non-linear molecule and the calculation of the partition function qr 3D use the product of the moments of inertia: D (kg 3 .m 6

Figure 18 .Figure 19 .

 1819 Figure 18. Sensitivity analysis of surface reactions for POM at 1123 K Figure 19. Sensitivity analysis of surface reactions for OCM at 1173 K

Fig. 1 .

 1 Fig.1. Conversion of CH4 versus residence time. Comparison between experiment (symbol) and simulation (line).

Fig. 2 .Fig. 3 .

 23 Fig.2. Molar fraction of C2H6 versus residence time. Comparison between experiment (symbol) and simulation (line).

Fig. 4 .

 4 Fig.4. Molar fraction of CO2 versus residence time. Comparison between experiment (symbol) and simulation (line).

Fig. 5 .

 5 Fig.5. Molar fraction of CO versus residence time. Comparison between experiment (symbol) and simulation (line).

Fig. 6 .

 6 Fig.6. Molar fraction of H2 versus residence time. Comparison between experiment (symbol) and simulation (line).

Fig. 7 .

 7 Fig.7. Conversion of CH4 versus residence time. Comparison between experiment (symbol) and simulation (line).

Fig. 8 .

 8 Fig.8. Molar fraction of C2H6 versus residence time. Comparison between experiment (symbol) and simulation (line).

Fig. 9 .

 9 Fig.9. Molar fraction of C2H4 versus residence time. Comparison between experiment (symbol) and simulation (line).

Fig. 10 .

 10 Fig.10. Molar fraction of CO2 versus residence time. Comparison between experiment (symbol) and simulation (line).

Fig. 11 .

 11 Fig.11. Molar fraction of CO versus residence time. Comparison between experiment (symbol) and simulation (line).

Fig. 12 .

 12 Fig.12. Molar fraction of H2 versus residence time. Comparison between experiment (symbol) and simulation (line).

Fig. 13 .

 13 Fig.13. Conversion of CH4 versus residence time. Comparison between experiment (symbol) and simulation (line).

Fig. 14 .

 14 Fig.14. Molar fraction of C2H6 versus residence time. Comparison between experiment (symbol) and simulation (line).

2 Fig. 14 .

 214 Fig.14. Molar fraction of C2H4 versus residence time. Comparison between experiment (symbol) and simulation (line).

Fig. 16 .

 16 Fig.16. Molar fraction of CO2 versus residence time. Comparison between experiment (symbol) and simulation (line).

Fig. 20 .

 20 Fig.20. Schematic representation of the mechanism

  the reaction. In addition, a new source of H2 formed by heterogeneous reactions (reaction 5) appears on lanthanum oxide, which increases the yield of H2 in POM.

NOMENCLATUREki:

  Rate constant for elementary step i A: Arrhenius preexponential factor E: Activation energy (cal.mol -1 ) T: temperature (K) kB: Bolzmann constant: 1.381 10 -23 J.K -1 h: Planck constant : 6.6262 10 -34 J.s. N: Avogadro number: 6,022 10 23 qi: total partition function for species i (m -3 ) qiv: vibrational partition function for species i qir: rotational partition function for species i qit: translational partition function for species i (m -3 ) qie: electronic partition function for species i ≠: in relation with the activated complex m: molecule mass (kg) M: molar mass (g. mol -1 ) s: spin quantic number σext: external symmetry number σint: internal symmetry number I: moment of inertia (amu. A 2 or kg. m 2 )

Table 1 .

 1

	Reactions O2 + H = OH + O O2 + H (+M) = HO2 (+M) O2 + CHO = CO + HO2 O2 + CH3 = CH3O + O O2 + HCHO = CHO + HO2 O2 + CH3 = HCHO + OH O2 + CH4 = CH3 + HO2 O2 + C2H5 = C2H4 + HO2 O2 + C2H3 = HCHO + CHO 10 O2 + C2H3 = C2H2 + HO2 1 2 3 4 5 6 7 8 9 11 O2 + C2H2 = 2 CHO 12 O + H2 = OH + H 13 O + CH3 = HCHO + H 14 O + CH4 = CH3 + OH 15 O + C2H6 = C2H5 + OH 16 O + C2H4 = CH3 + CHO 17 O + C2H4 = HCHO + CH2 18 O + C2H4 = CH2CO + H2 19 O + C2H4 = CH2CHO + H 20 O + C2H4 = OH + C2H3 21 O + HCHO = CHO + OH 22 OH + H2 = H + H2O 23 OH + CH3 (+M) = CH3OH (+M) 24 OH + CH4 = CH3 + H2O 25 OH + CO = CO2 + H 26 OH + HCHO = CHO + H2O 27 OH + C2H4 = C2H3 + H2O 28 OH + C2H4 = CH3 + HCHO 29 OH + C2H6 = C2H5 + H2O 30 2 HO2 = H2O2 + O2 31 HO2 + H = 2 OH 32 HO2+ CH3 = CH3O + OH 33 HO2 + CH4 = CH3 + H2O2 34 HO2 + CO = CO2 + OH 35 HO2 + CHO = OH + H + CO2 36 HO2 + HCHO = CHO + H2O2 37 HO2 + C2H6 = C2H5 + H2O2 38 2 CH3 (+M) = C2H6 (+M) 39 2 CH3 = C2H5 + H 40 2CH3 = C2H4 + H2 41 CH3 + H (+M) = CH4 (+M) 42 CH3 + HCHO = CHO + CH4 43 CH4 + H = CH3 + H2 44 C2H2 + H (+M) = C2H3 (+M) 45 C2H3 (+M) = C2H2 + H 46 C2H4 (+ M) = C2H2 + H2 (+ M) 47 C2H4 + H = C2H3 + H2 48 C2H4 + CH3 = CH4 + C2H3 49 C2H4 + CH3 = C3H7	A (mol, cm 3 , s) 1.5 10 14 4.52 10 13 2.6 10 11 1.6 10 13 2.0 10 13 3.0 10 30 4.0 10 13 8.4 10 11 4.5 10 16 1.34 10 6 7.0 10 8 5.1 10 4 8.4 10 13 7.2 10 8 1.0 10 9 8.1 10 6 4.0 10 5 6.6 10 5 4.7 10 6 1.5 10 7 4.1 10 11 1.0 10 8 6.0 10 13 1.6 10 7 6.3 10 6 3.4 10 9 2.0 10 13 2.0 10 12 7.2 10 6 4.2 10 14 1.7 10 14 1.8 10 13 9.0 10 12 1.5 10 15 3.0 10 13 3.0 10 12 1.3 10 13 3.6 10 13 3.0 10 13 2.1 10 14 1.7 10 14 7.7 10 -8 1.3 10 4 8.4 10 12 2.0 10 14 1.0 10 17 5.1 10 7 6.3 10 11 2.1 10 10	n 0.0 0.0 0.0 0.0 0.0 -4.69 0.0 0.0 -1.39 1.61 1.8 2.67 0.0 1.56 1.5 1.88 1.88 1.88 1.88 1.91 0.57 1.6 0.0 1.83 1.5 1.18 0.0 0.0 2.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 6.1 3.0 0.0 0.0 0.0 1.93 0.0 0.0	Ea (cal. mol -1 ) 14810 0.0 410 31300 38800 36600 56700 3900 1000 -400 30600 6200 0 8400 5800 200 200 200 200 3700 2700 3300 0 2700 -500 -400 5900 900 900 11980 900 0 24600 23600 0 13000 20400 0.0 13500 19300 0.0 1970 8000 2610 39800 71600 12900 1600 7350

Table 1 .

 1 Adjusted kinetic parameters of most important homogeneous reactions in oxidation of methane.

k=A(T) n exp(-E/RT)

Table 2 .

 2 Kinetic parameters of surface reactions in oxidation of methane

	H(s) H2 + 2s
	OH(s) + H(s) H2O + 2s
	2OH(s)	O(s) + H2O + s

5

(s symbolizes surface site and (s) adsorbed species).

Table 3 .

 3 Simplified

	1
	Réactions		Afitted (mol, cm 2 , s)	Efitted (cal.mol -1 )
	O2 + s O2(s)	6.0 10 7	5000
	O2(s)	O2 + s	2,3 10 12	38000
	O2(s) + s 2O(s)	2.3 10 23	25000
	2O(s) O2(s) + s	3.3 10 23	33000
	CH 4 + O(s) CH 3 • + OH(s) C 2 H 6 + O(s) C 2 H 5 • + OH(s)	3.0 10 8.7 10	8 9	8840 3000
	CO + O(s) CO2 + s	8.3 10 8	0
	C2H4 + O(s) C2H3• + OH(s)	1.1 10 10	3000
	2OH(s) O(s) + H2O + s	3.0 10 23	2400
	CH 4 + s CH 3 • + H(s) C 2 H 6 + s C 2 H 5 • + H(s)	8.5 10 9.8 10	7 7	9850 5000
	C2H4 + s C2H3• + H(s)	1.9 10 7	5000
	H• + s H(s)	9.6 10 12	0
	OH(s) + H(s) H2O + 2s	1.0 10 24	0
	H(s) + H(s) H2 + 2s	4.0 10 23	0
	H2 + 2s 2H(s)	6.1 10 16	0
	H2 + O(s) OH(s) + H•	2.0 10 8	0
	2

surface mechanism of methane oxidation [s symbolizes surface site and (s) adsorbed 3 species].


Resa Martha

Mahdi Mubarok

Irmanida Batubara

Istie S Rahayu

Loedy Setiono

Wayan Darmawan
email: wayandar@indo.net.id

Obounou Firmin

Béatrice Akong

Christine George

Philippe Gérardin
email: philippe.gerardin@univ-lorraine.fr

Gérardin

Akong

Béatrice George

Christine Gérardin

Effect of furfurylation treatment on technological properties of short rotation teak wood

Keywords: dimensional stability, decay durability, FTIR and TGA analysis, furfuryl alcohol, lignin content, short rotation teak

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

methods have been proposed at laboratory scale, only two of them have led to the development of industrial application, namely acetylation and furfurylation [8]. Wood modification with furfuryl alcohol (FA) is a non-toxic alternative method to replace the toxic conventional preservation treatments [9]. FA is derived from renewable resource (primarily corn cobs) and is improved by the polymerisation process. Research on furfurylated wood has been carried out for several decades. The furfurylation process has been developed since 1990s with the use of a catalyst that can increase the entry of FA into the wood. Sejati et al. [10] treated beech wood by furfurylation process using five different catalysts: maleic anhydride, maleic acid, citric acid, itaconic acid, and tartaric acid. It was noted in the study that tartaric acid can be considered as a new promising catalyst to perform furfurylated wood and good alternative to other catalysts since able to react with wood components allowing also fixation of FA polymer.

Furfurylation treatment leads to an improvement of several wood properties like dimension stability, modulus of elasticity (MOE), modulus of rupture (MOR), hardness, decay resistance [10,[START_REF] Esteves | Properties of furfurilated wood (Pinus pinaster)[END_REF][START_REF] Dong | Flammability and physical-mechanical properties assessment of wood treated with furfuryl alcoholand nano-SiO2[END_REF]. Some studies have explained characteristics of furfurylated wood. Esteves et al. [START_REF] Esteves | Properties of furfurilated wood (Pinus pinaster)[END_REF] treated wood with 70% furfuryl alcohol mixture and obtained anti-swelling efficiency (ASE) as high as 45% at a weight percent gain (WPG) of approximately 38%. Li et al. [START_REF] Li | Wood modification with furfuryl alcohol catalysed by a new composite acidic catalyst[END_REF] reported a slight improvement of MOE and MOR. The furfurylation treatment has a much more significant effect on compressive strength than both MOE and MOR. Furfurylation treatment can improve durability against fungal decay [START_REF] Esteves | Properties of furfurilated wood (Pinus pinaster)[END_REF][START_REF] Li | Wood modification with furfuryl alcohol catalysed by a new composite acidic catalyst[END_REF][START_REF] Lande | Development of modified wood products based on furan chemistry[END_REF], termites [START_REF] Hadi | Termite resistance of furfuryl alcohol and imidacloprid treated short rotation tropical wood species as function of field test[END_REF], marine borers [START_REF] Westin | Field test of resistance of modified wood to marine borers[END_REF], and weathering [START_REF] Mantanis | Evaluation of Weathering of Furfurylated Wood Decks after a 3-year Outdoor Exposure in Greece[END_REF]. Recent studies regarding ecotoxicology and leachates from furfurylated wood have shown that furfurylated wood generally has a low ecotoxicity [9,[START_REF] Lande | Development of modified wood products based on furan chemistry[END_REF].

The present study was designed to investigate the effect of furfurylation treatment of short rotation teak to develop further industrial valorization on physical properties (WPG, density, and leachability), dimension stability (volumetric swelling, ASE, water uptake (WU), and bulking effect (BE)), mechanical properties (MOE and MOR), color change, decay resistance, and chemical compositions (holocellulose, hemicellulose, -cellulose, and lignin). Location of furfuryl alcohol in wood and in its different components was evaluated for the first time by Fourier Transform Infrared spectroscopy (FT-IR), Carbon 13 Nuclear Magnetic Resonance ( 13 C NMR), and Thermogravimetric Analysis (TGA).

Materials and Methods

Sample Preparation

The sample trees used were short rotation teak of 15 years with 25 cm diameter obtained from plantation forest managed by the state-owned enterprise (Perhutani) at Blora, Central Java (7°08'25.2"S/111°36 '19.5"E) and short rotation teak of 8 years with diameter 25 cm obtained from SEAMEO BIOTROP at Bogor, West Java (6°38'07.7"S/106°49'32.8"E). Differences in growing conditions (environment, genetics, and silviculture) between Central Java and West Java resulted in variations in the teak growth. Blora, Central Java has an average rainfall below 1496-2506 mm/year and dry conditions for 4-6 months with an average temperature of 27 °C. Bogor, West Java, has a high annual rainfall (average 3500-4000 mm/year), and dry conditions for 2-3 months with an average temperature of 26 °C. From the felled tree, the log sections in length of 1 m were taken from each tree at the bottom part of the trees stem. The sample logs were sawn into 3 cm thickness of boards by bandsaw. Board samples were air dried up to a moisture content of 12-15% and then sawn into smaller samples for studying various wood characteristics. Wood blocks for furfurylation treatment had a size of 200 x 20 x 50 mm 3 (longitudinal x tangential x radial). Furfurylation was performed only on sapwood samples considering that heartwood presented sufficient natural durability and poor impregnability. For this purpose, smaller wood samples were taken from preceding wood blocks presenting only from sapwood.

Furfurylation Treatment

A mixture of FA with 97% in purity and tartaric acid as catalyst were used as the impregnation solution. Five percent (w/w) tartaric acid (based on the FA solution) was added to 45% furfuryl alcohol in aqueous solution. All samples before treatment were dried at the temperature 103 ± 2 °C, the weight (m0) and volume (V0) were recorded. The impregnation process was initiated with vacuum conditions 8-10 kPa for 5 min in an autoclave and followed by introduction of the aqueous FA solution. Afterward, the pressure was brought to 100 kPa (ambient pressure), then the wood samples were exposed to a pressure of 1200 kPa for 10 min. The impregnated samples were kept at room temperature for 48 h to evaporate the excess of water avoiding the appearance of cracks after polymerization. Samples were wrapped in aluminium foil to avoid furfuryl alcohol evaporation during curing and placed again in the reactor for polymerization. The reactor was closed and placed under nitrogen. The reactor temperature was slowly increased by 0.5 °C min -1 from ambient to 40 °C and maintained at this temperature for 10 h. After this period, the reactor temperature was increased by 0.5 °C min -1 from 40 to 120 °C and the temperature was maintained for 16 h. Heating was then stopped and furfurylated wood samples were allowed to cool down to room temperature under inert atmosphere. All treated woods were measured for their weight (m1) and volume (V1).

WPG, density, and BE

WPG was calculated using the following equation (Eq. 1): WPG (%) = ((m1-m0) / m0) x 100 (1) where WPG is the percentage of weight gain due to the furfurylation treatment, m0 is the initial mass of wood at 103 °C before treatment, m1 is the mass of wood after treatment.

Density was calculated from the following equation (Eq. 2): Density (gr cm -3 ) = m0 / V0 or m1 / V1

(2) where m0 is the initial mass of wood at 103 °C before treatment, m1 is the mass of wood after treatment, V0 is the initial volume of wood at 103 °C before treatment, V1 is the volume of wood after treatment. BE during wood treated was calculated from the following equation (Eq. 3): BE (%) = ((V1-V0) / V0) x 100 (3) where BE is the percentage of bulk value of untreated or treated wood during treatment, V0 is the initial volume of wood at 103 °C before treatment, V1 is the volume of wood after treatment.

Leaching test

The leaching test of untreated and furfurylated woods were carried out according to a procedure adapted from NF X 41-568 [18]. Sample section of 30 × 15 × 5 mm 3 (L, R, T) was prepared in six replications for both untreated and furfurylated woods. The samples were submerged in test tubes of distilled water with a quantity of distilled water representing 5 times the volume of the introduced wood. The leaching test for the samples were carried out in six leaching periods (1, 2, 4, 8, 16, 48 h) under continuous shaking (175 linear shakes min -1 ) at 20 °C. Between the periods of 4 and 8 h, used distilled water was removed and the wood samples were kept without water for 16 h in tube test. After leaching periods were completed, samples were dried at 103 °C for 48 h and reweighed (m2).

Weight loss due to leaching (WLL) was calculated from the following equation (Eq. 4): WLL (%) = ((m1-m2) / m1) x 100 (4) where WLL is the percentage of weight loss value of untreated or furfurylated wood due to leaching, m1 is the mass of wood after treatment, m2 is the mass at 103 °C of untreated and treated wood after leaching process.

Chemicals compound analysis

Before the analysis, untreated and furfurylated wood samples were ground in Wiley mill to 40-60 mesh sawdust. The sawdust was extracted in Soxhlet apparatus for 24 h with different solvent of increasing polarity: dichloromethane, acetone, toluene/ethanol (2/1, v/v), and water. Two replicates of the extract free sawdust were measured for each wood in chemical composition. After determining chemical composition, holocellulose, cellulose, and lignin samples were then analysed with ATR-FTIR Perkin Elmer Spectrum 2000 instrument to indicate any reaction with the FA.

Holocellulose

The holocellulose was measured according to method developed by Rowell [START_REF] Rowell | Handbook of wood chemistry and wood composites[END_REF]. An 80 mL of hot distilled water was added to 2.5 g of wood sawdust, followed by 0.5 mL acetic acid, and 1 g of sodium chlorite in a 250 ml Erlenmeyer flask. The mixture was heated in a water bath at 70 °C. After 1 h, 0.5 mL of acetic acid and 1 g of sodium chlorite were added. Addition of 0.5 mL acetic acid and 1 g of sodium chlorite was repeated and it took 7 hours of chloriting. The sample could be left without further addition of acetic acid and sodium chlorite in the water bath for overnight. At the end of 24 hours of reaction, the sample was cooled and filtered on filter paper using a Buchner funnel until the yellow color and the odor of chlorine dioxide disappeared. The holocellulose was washed with acetone, vacuum oven dries at 105 °C for 24 hours, placed in a desiccator for an hour and weighed.

-cellulose

The -cellulose was obtained by Rowel method [START_REF] Rowell | Handbook of wood chemistry and wood composites[END_REF]. A 2 g of oven dried holocellulose was placed into a 250 ml Erlenmeyer flask. Ten ml of 17.5% NaOH solution was added to the 250 ml Erlenmeyer flask and placed in water bath at 20°C. Further, 5 ml of 17.5% NaOH solution was added every 5 minutes during 15 minutes. The mixtures were conditioned at 20°C for 30 min. The mixture was then diluted with 33 ml distilled water and was kept for 1 hour at 20°C before filtering. All of the holocellulose residue were transferred to the crucible, and washed with 100 ml of 8.3% NaOH solution, 100 ml of distilled water, 15 ml of 10% acetic acid solution and finally 250 ml distilled water. The residue of -cellulose (white residue) was dried in an oven at 103 °C for 24 h, and weighed.

Hemicellulose

The hemicellulose content was calculated using this equation (Eq. 5): Hemicellulose (%) = Holocellulose (%) --cellulose0 (%) (5)

Lignin content

Klason lignin content was measured in this research work [START_REF] Inari | Evidence of char formation during wood heat treatment by mild pyrolysis[END_REF]. A 0.3 g of sawdust (extractive-free) and 3 ml of 72% v/v sulfuric acid was added to the tubes.

The tubes were placed into the water bath at 30 °C for 1 h. After 1 hour of reaction, samples were diluted by 84 ml with distilled water. The diluted samples were placed into the autoclaved for 1 h at 120°C and cooled at the room temperature. Finally, the lignin was obtained by Büchner filtration. Before rinsing off the solution with water, the soluble lignin was measured. The filtrate was diluted 20 times by distilled water, then its absorbance was measured at 205 nm by UV spectrometry. UV white was 3% sulfuric acid. Once the absorbance of the filtrate was achieved, the Klason lignin extraction could be continued with water rinses. The samples were finally dried overnight in an oven at 103 °C and weighed.

Fourier Transform Infrared (FT-IR) Spectroscopy

FT-IR spectrums were recorded on an ATR-FTIR Perkin Elmer Spectrum 2000 instrument. The untreated and furfurylated samples were in the range of 600 -4000 cm -1 at a resolution of 4 cm -1 with an ATR cell.

Cross-Polarisation/Magic Angle Spinning (CP/MAS) 13 C NMR analysis

Solid state CP/MAS 13 C NMR spectra were recorded on a Bruker MSL300 spectrometer at a frequency of 75.47 MHz. Acquisition time was 0.026 s with number of transients of about 1200. All the spectra were run with a relaxation delay of 5 s, CP time of 1 ms and spectral width of 20,000 Hz. Spinning rates were 5 KHz. Interferograms were processed with Top Spin 3.6.2 using an apodization value of 21 Hz. Chemical shifts are expressed in parts per million (ppm).

Volumetric swelling, ASE, and WU measurement

The volumetric swelling, ASE, and WU measurement was measured by the method developed by Pfriem [START_REF] Pfriem | Furfuryl alcohol impregnation for improved plasticization and fxation during the densifcation of wood[END_REF]. The test samples with a dimension of 10 × 20 × 20 mm 3 (L, R, T) were prepared in five replications from untreated and furfurylated wood. The untreated and furfurylated wood were dried at 103 °C and their weights (Wd) and volumes were recorded (Vd). Samples were directly immersed in distilled water and placed under vacuum condition (10 kPa) for 1 hour. After 24 h releasing the vacuum, the sample was released from the distillate water and their wet weight (Ww) and volume (Vw) were measured. Four cycles (1, 2, 3, and 14 days) of dryingsoaking system were conducted and their weigh and volume (Wd, Vd, Ww, and Vw) for each cycle were recorded. The volumetric swelling (Sv) and ASE were calculated. The volumetric swelling (Sv) was determined using formula by equation (Eq. 6): Sv (%) = (Vw -Vd) / Vd (6) where Sv is the percentage of volumetric swelling of wood sample, Vw is the wet volumetric sample of the wood sample, and Vd is the dry volumetric of the oven-dried sample at 103 °C.

ASE was calculated by equation (Eq. 7): ASE (%) = [(Svu-Svt)/Svu] x 100 (7) where ASE is the percentage of anti-swelling efficiency of treated wood, Svu is volumetric swelling of untreated wood, and Svt is the volumetric swelling of the treated wood.

WU was determined using the following equation (Eq. 8): WU (%) = [(Ww-Wd)/Wd] x 100 (8) where WU is percentage of water uptake, Wd is the weight of the air-dried sample at 103 °C before immersion and Ww is the weight after immersion in water.

MOE and MOR

MOE and MOR of wood specimens were measured following the standard procedure of EN 310 [START_REF]EN 310[END_REF] with the sample size of 200 x 20 x 5 mm 3 (L, R, T). Wood samples of both untreated and furfurylated were conditioned at the room with a temperature of at 22 ± 2 °C and relative humidity of 65 ± 5% RH, until constant mass. MOE and MOR were determined using at one-point bending by device INSTRON 4467 universal testing machine (Buckinghamshire, UK).

Color Changes

Color measurements were conducted on the CIE L*, a*, b* system with X-Rite spectrophotometer. The test samples with a dimension of 70 × 50 × 20 mm 3 (L, R, T) were prepared in four replications from untreated and furfurylated wood. The color measurements were carried out on untreated and furfurylated wood samples. The L*, a*, and b* parameters for each specimen were measured at six contiguous locations on the wood surface. Color changes (∆ ) were calculated based on CIELab in Eq. 9:

∆ = (∆ ) + (∆ ) + (∆ ) (9
) where L* is the lightness, a* and b* describes the chromatic coordinates on the green-red color, and blue yellow axes, respectively. ΔL*, Δa*, Δb* are the differences between untreated and furfurylated wood and ∆ is color changes.

Decay resistance

The decay resistance after leaching test was evaluated by following a method from EN 113 [23] standard. The wood samples consisted of untreated teak wood, furfurylated teak sapwood, and beech as a control wood. Dimension of samples were 30 x 10 x 5 mm 3 (L, R, T) with 8 replications for each treatment and their weights (W0) were measured. White rot fungi od Coriolus versicolor (CV) and brown rot fungus of Pycnoporus sanguineus (PS) and Coniophora puteana (CP) were inoculated on potato agar culture medium in petri dish and cultivated in an incubator at 22 ± 2°C temperature, 70 ± 5 % of relative humidity for 14 days. After mycelium permeated the culture disc, wood samples were put in each petri dish and then placed in incubator for 12 weeks in a climatic chamber (at 22 ± 2 °C and 70 ± 5% RH). Once the fungal exposure was completed, samples were cleaned carefully from the fungus. All samples were then oven dried at 103 °C for 48 h and weighed (W1). Durability class was determined according to EN 350-1 [START_REF]EN 350-1, Durability of wood and wood-based products -Natural durability of solid wood[END_REF] standard usually used to classify wood species natural durability. The decay resistance was measured by weight loss (WL) according this equation (Eq. 10): WL (%) = ((W0 -W1) / W0) x 100 (10) where WL is percentage of weight loss, and W0 and W1 are dry weight of the samples before and after infection, respectively.

Thermogravimetric Analysis (TGA)

Thermal stability of untreated and furfurylated wood was carried out by TGA using a TGA/DSC1-TMA/SDTA 84Xe instrument from Mettler Toledo equipped with STARe v.11 fr System program. Sample powders (± 10 mg) were placed in alumina crucible. Thermogram were recorded using a heating rate of 10 °C /min from 25 to 700 °C under a nitrogen purge (20 ml.min -1 ).

Results and discussions 3.1 WPG, density, and leachability

The WPG, density, and leachability of untreated and furfurylated teak sapwoods are presented in Table 1. The average of WPG for furfurylated teak woods varied between 19.48-20.94%, similar result was also reported by Hadi et al. [START_REF] Hadi | Termite resistance of furfuryl alcohol and imidacloprid treated short rotation tropical wood species as function of field test[END_REF]. These authors noted that accacia mangium wood, which possess similar density value (0.60 g cm -3 ) with teak wood, pesented WPG value around 20.1% after furfurylation treatment. Density value was correlated with the WPG, in which the increase in density of 10% caused the increase in the WPG of 20%. The polymerization of FA in the cell wall is important to impart new properties on the treated wood. Polymerization FA with tartaric acid affected WPG value after treatment [10]. Furfurylation treatment improves density because the cell walls are filled with polymerized FA [START_REF] Dong | Assessment of the performance offurfurylated wood andacetylated wood: comparison among four short rotation wood species[END_REF]. Impregnation of wood with a polymer causes penetration of polymer and strengthen cell walls which occurs through the polymer filling the void space within the cell wall [7]. The result in Table 1 shows that the difference in leachability between untreated and furfurylated wood was observed to be 0.20 % for Plus Perhutani teak and 0.30% for Biotrop teak, which indicate a good fixation of poly-FA in the teak woods. Furfurylated Plus Perhutani teak presented relatively similar resistance to leaching compared to furfurylated Biotrop teak.

Chemical components

The effect of furfurylation treatment on chemical composition such as holocellulose, -cellulose, hemicellulose, and lignin are presented in Table 2. The result in Table 2 shows that the same tendency occurred on -celluloses, hemicellulose, and lignin contents between Plus Perhutani and Biotrop teak wood. Holocellulose content decreased from 65.22% to 58.99% for Plus Perhutani teak and from 64.28% to 56.51% for Biotrop teak. -cellulose content also decreased after furfurylation treatment from 41.91% to 32.47% for Plus Perhutani teak and from 43.68% to 31.33% for Biotrop teak. The decrease of holocellulose and -cellulose contents could be due to the introduction of an additional fraction of FA in the wood. The increase in lignin contents could be due to the interactions of FA polymer with lignin. The cross-linking reactions between the FA polymer and lignin are possible, although it has not been proven [7]. In this study, FTIR analysis was performed to prove the changes in native wood components due to furfurylation treatment.

FT-IR analysis

Figure 1 shows the FT-IR spectra of the untreated and furfurylated short rotation teak sapwoods over the 600 to 4000 cm -1 wavenumber range. The FT-IR absorbance at around 3400 cm -1 for the furfurylated teak wood, which assigned as the stretching vibration of the hydroxyl group in polysaccharides, showed a relative decrease in intensity compared to untreated teak wood. Furfurylated wood had a lower hydroxyl group intensity than untreated wood. The decrease in the intensity of the hydroxyl groups caused the wood more hydrophobic. The absorption band in the spectra between untreated and furfurylated wood were not significantly different especially in fingerprint area. There was no absorption band which indicates the presence of furan groups. This phenomenon indicates that the occurrence of a clear chemical interaction between FA and the wood should be proved. Native wood components from untreated and furfurylated wood were analyzed to determine potential reactions of FA with wood components after the furfurylation treatments. Holocellulose and α-cellulose fractions presented no significant differences in spectra between the untreated and furfurylated samples. Another analysis was also performed to determine the effect of furfurylation treatment on native klason lignin.

Figure 2 shows the FT-IR spectra of all the Klason lignin fractions for the untreated and the furfurylated short rotation teak sapwoods over the 600 to 2000 cm - 1 wavenumber range. The absorption bands of the furfurylated woods were different after furfurylation treatment. Two small bands were observed at 860 and 792 cm -1 . These two small bands are assigned to aromatic C-H out-of-plane deformation and 2,5-disubstituted furan ring, respectively [START_REF] Dong | Flammability and physical-mechanical properties assessment of wood treated with furfuryl alcoholand nano-SiO2[END_REF]. This result indicates that the polymerization of FA was completed and chemical interaction occurred between FA and lignin. Electrophilic aromatic substitutions involved in polymerization of FA were also very likely involved in reactions of FA or of its derivatives with lignin. These observations confirmed higher percentage of Klason lignin in furfurylated teak woods.

13 C (CP-MAS) NMR analysis

The CP/MAS 13 C NMR spectra of the untreated and furfurylated short rotation teak sapwoods is presented in Figures 3. Both untreated wood samples showed classical signals identifiable to main wood polymer components. Cellulose's carbons appeared between 60 and 105 ppm. The signals comprised between 72 and 90 ppm were assigned to C2, C3, C4, C5 and C6 carbons, while the signal at 105 ppm was assigned to hemiacetalic C1 carbon. Two signals at 89 and 84 ppm were endorsed to the C4 carbon of crystalline and amorphous cellulose, respectively. Similar behaviour was observed for the signals at 65 and 62 ppm characteristic of the C6 carbon of crystalline and amorphous cellulose. Signals of hemicelluloses were less obvious due to their overlapping with signals of cellulose. The shoulder at 102 ppm on the C1 signal of cellulose was assigned to hemiacetal carbon of hemicelluloses, while carbons of acetyl groups were detected at 20 and 170 ppm. Methoxyl groups of syringyl and guaiacyl units were detected at 56 ppm, while aromatic carbons appeared between 110 and 155 ppm: aromatic carbons linked to hydrogen appeared around 110 ppm, those linked to carbon around 135 ppm and those linked to oxygen around 150 ppm. Aliphatic carbons of phenylpropane units of lignin were masked by signals of cellulose and hemicelluloses. No difference was detected between untreated Plus Perhutani and Biotrop woods. Furfurylated samples presented in both case, signals characteristic of FA polymer present in the wood structure. The signal at 28 ppm allocated to methylene bridges between aromatic structures including furan rings as well as benzenic rings. The increase of the signal at 155 ppm was characteristic the two alpha carbons of furan ring close to the oxygen atom indicating clearly presence of FA polymer. The two beta carbons of furan ring appeared at 110 and 120 ppm as a shoulder the C1 signal of cellulose. To confirm observations made by FT-IR analysis on the regioselectivity of the reaction of FA with the different wood cell wall polymers, lignin fractions of untreated and furfurylated Plus Perhutani and Biotrop woods were also analyzed by CP/MAS 13 C NMR (Figure 4). Spectra of furfurylated samples clearly showed characteristic signal of FA polymer in addition to classical lignin signals. The signal at 28 ppm was assigned to methylene bridges due to reaction of FA with itself or lignin. Increase of signals at 110 and 155 ppm were characteristic carbons of furan ring. The CP/MAS 13 C NMR spectra confirmed the presence of FA polymer either mixed with lignin polymer or copolymerized with lignin.

Volumetric swelling, ASE, WU, and BE

The volumetric swelling, ASE, WU, and BE of the untreated and furfurylated teak sapwoods are shown in Table 1. The volumetric swelling of furfurylated wood was smaller than that of untreated wood. The decreasing in volumetric swelling value was followed by the increasing in ASE value after furfurylation treatment. ASE of furfurylated Plus Perhutani and Biotrop teak wood reached 64.17% and 60.01%, respectively. Sejati et al. [10] reported similar ASE (approximately 66%) on furfurylated beech wood containing 50% of FA and 5% of tartaric acid. Furfurylation treatment resulted in a decrease in WU values around 59% after furfurylation treatment. The previous result also reported the water uptake decrease after furfurylation treatment (around 50 %), which is due to the polymers filling the void spaces and blocking the water absorption [START_REF] Dong | Flammability and physical-mechanical properties assessment of wood treated with furfuryl alcoholand nano-SiO2[END_REF]. It was reported in another study that higher ASE generates lower WU [START_REF] Dong | Wood/polymer Nanocomposites prepared by impregnation with furfuril alcohol andnano-SiO2[END_REF][START_REF] Dirna | Improvement of short rotation wood species characteristics by MEG and nano SiO2 impregnation[END_REF]. In addition, WU and ASE are improved due to the decrease in crystallinity. This may be attributed to the penetration of poly-FA into the amorphous region of the wood cell wall [START_REF] Dong | Wood/polymer Nanocomposites prepared by impregnation with furfuril alcohol andnano-SiO2[END_REF]. Furfurylation treatment can stabilize wood dimensions due to the reduction of void volume in wood [START_REF] Hill | Wood modification: An update[END_REF]. The bulking agent will come in into the empty space in the cell wall to replace water, then the wood dimension becomes more stable [START_REF] Dirna | Improvement of short rotation wood species characteristics by MEG and nano SiO2 impregnation[END_REF][START_REF] Baysal | Dimensional stabilization of wood treated with furfuryl alcohol catalysed by borates[END_REF]. BE values increased in the average of 11% after furfurylation treatment. The increase of the BE values suggested that adding the FA polymer by the impregnation process can improve dimensional stability. This result was also confirmed by FTIR analysis, in which furfurylated wood was difficult to absorb water due to decreasing in hydroxyl groups lead to the increase in dimensional stability.

MOE and MOR

MOE and MOR of the untreated and furfurylated short rotation teak sapwood are presented in Figure 5. The mean MOE values for untreated Plus Perhutani and untreated Biotrop were 13703.13 kg cm -2 and 13941.98 kg cm -2 , respectively. The MOE value of furfurylated Plus Perhutani was almost similar as the untreated, meanwhile MOE of furfurylated Biotrop decreased by 14%. This result was in line with a report by Pfriem et al. [START_REF] Pfriem | Furfuryl alcohol impregnation for improved plasticization and fxation during the densifcation of wood[END_REF], who found reduction in MOE in furfurylated beech wood catalyzed by maleic anhydride. Another study also reported that furfurylation treatment did not significantly change the MOE values [START_REF] Esteves | Properties of furfurilated wood (Pinus pinaster)[END_REF][START_REF] Li | Wood modification with furfuryl alcohol catalysed by a new composite acidic catalyst[END_REF][START_REF] Epmeier | Differently modified wood: comparison of some selected properties[END_REF]. The MOR value decreased with furfurylation treatment by 20% from 128.12 kg cm -2 to 102.58 kg cm -2 for Plus Perhutani teak wood and by 37% from 153.90 kg cm -2 to 97.06 kg cm -2 for Biotrop teak wood. This result could be due to the decrease in the cellulose content.

Color changes

Table 3 presents the variation of ΔL*, Δa*, Δb*, and ΔE* of untreated and furfurylated short rotation teak sapwood. The color of untreated wood tended to change after furfurylation treatment. The change of L* value indicated the change in brightness in which the color of the furfurylated teak wood was darker. The furfurylated woods presented smaller L* value for both short rotation teak woods when compared to the untreated wood. The decrease in L* value of furfurylated pine wood was also reported by Acosta et al. [START_REF] Acosta | Dimensional stability and colour responses of pinus Elliottii wood subjected to furfurylation treatments[END_REF]. Furfurylated short rotation teak woods resulted in the increase in a* value, while the b* value was not changed after furfurylation treatment. Dong et al. [START_REF] Dong | Assessment of the performance offurfurylated wood andacetylated wood: comparison among four short rotation wood species[END_REF] reported that the a* value of furfurylated wood increases compared with untreated and b* value varies among species. Generally, a* value has corelation to the phenolic compound on the wood [START_REF] Mori | Caracterização da cor da madeira de clones de híbridos de Eucalyptus spp[END_REF]. Colors between untreated and furfurylated wood were prominently different. Mean value of total color difference (∆ ) changed dramatically after furfurylation treatment. The darker color is generated by the conjugated structure which causes the natural color of wood is covered by the color of poly-FA [START_REF] Choura | Acid-catalyzed polycondensation of furfuryl alcohol: Mechanisms of chromophore formation and cross-linking[END_REF].

Decay resistance

The weight losses of untreated and furfurylated short rotation teak sapwood against the white rot fungi and brown rot fungi are presented in Figure 6. Durability of the furfurylated wood against fungus increased after furfurylation treatment. Weight losses of Plus Perhutani teak wood due to PS, CV, and CP decreased after furfurylation treatment from 15.53% to 1.02%; from 29.63% to 2.92%; and from 1.87% to 1.06%, respectively. Weight losses of Biotrop teak wood due to PS, CV, and CP decreased after furfurylation treatment from 12.67% to 1.06%; from 19.76% to 1.23%; and from 1.63% to 1.16%, respectively. Beech wood controls presented the highest weight losses confirming the fungus tested in this work destroyed hardly the wood. Other studies also reported a decrease in weight loss of pine wood due to CP [START_REF] Esteves | Properties of furfurilated wood (Pinus pinaster)[END_REF][START_REF] Devetter | Combined evaluation of durability and ecotoxicity: A case study on furfurylated wood[END_REF] and CV [START_REF] Li | Wood modification with furfuryl alcohol catalysed by a new composite acidic catalyst[END_REF]. According to EN 350-1, furfurylated short rotation teak wood can be categorized in durability class 1 (very durable). Furfurylation treatment results in physical blocking of the entrance of decay and the reduction of equilibrium moisture content due to the penetration of resin into the wood cell wall and crosslinkage between FA and wood cell wall components [START_REF] Pilgard | Forest and Landscape Denmark University of Copenhagen, organizer[END_REF]. It contributes to inhibiting fungus growth in furfurylated wood to get the moisture required for decay [START_REF] Ringman | In vitrooxidative and enzymatic degradation of modified wood[END_REF].

TGA analysis

Thermal stability of untreated and furfurylated short rotation teak wood were investigated by the thermogravimetric analysis. Figure 7 shows the TGA and differential thermogravimetric (DTG) curves of the untreated and furfurylated short rotation teak sapwood. The details of major decomposition temperatures and the corresponding percentage of weight losses are summarized in Table 4. The first degradation region from 50 to 100 °C is related to the evaporation of moisture and adsorbed water from the wood. The weight losses of the furfurylated wood were lower than untreated wood at this temperature range (Figure 7). The second degradation occurs from 220 to 300 °C the weight loss decreases abruptly, while from 300 °C to above the weight loss decreases gradually. The final degradation occurs around 450 °C and corresponds to degradation of α-cellulose and lignin, meanwhile the carbonisation of lignin and furfuryl alcohol polymer were extended up to 650 °C [START_REF] Manimaran | Characterization of new cellulosic fiber: Dracaena reflexa as a reinforcement for polymer composite structures[END_REF][START_REF] Alshammari | A New Study on Characterization and Properties of Natural Fibers Obtained from Olive Tree (Olea europaea L.) Residues[END_REF]. DTG curve is attributed to lignin decomposition at a slower rate than hemicellulose and cellulose [START_REF] Sarvaramini | Pyrolysis kinetics of pre-torrefied woody biomass based on torrefaction severity-experiments and model verification[END_REF]. Compared with the untreated, the furfurylated wood showed a higher Tonset and Tmax (Table 4). For furfurylated woods, the Tonset of degradation (temperature at 5% weight loss) were 221 °C for Plus Perhutani teak wood and 220 °C for Biotrop teak wood, which are higher than untreated wood. Meanwhile, Tmax degradation temperatures were found at 356 °C in furfurylated Plus Perhutani and at 359 °C in furfurylated Biotrop. The weight losses for all the samples were about 70-80% at the end of the measurement (700 °C). Tonset and Tmax increased after furfurylation treatment. Furfurylated samples presented higher thermal stability due to higher stability of remaining carbonized lignin and furfuryl alcohol polymer. This result confirms with Acosta et al. [START_REF] Acosta | Juvenile and mature woods from pine subjected to in situ polymerization with furfuryl alcohol[END_REF] who reported that furfurylated juvenile pine wood with higher lignin is more thermally stable than furfurylated mature pine wood.

Conclusion

Furfurylation allows to improve properties of short rotation teak sapwood. Dimensional stability of the furfurylated wood is increased, though MOE and MOR are decreased. Furfurylation treatment causes the darker color on the short rotation teak sapwood. Durability against decaying fungi of the furfurylated short rotation teak wood is classified as very durable. The furfurylation treatment causes changes in the -cellulose, hemicellulose, and lignin contents of the short rotation teak sapwood, the latter one increasing, while the two first ones decrease. FT-IR and NMR analysis indicate the presence of chemical bonds between FA or FA-polymer and lignin. TGA and DTG analyses indicated that the furfurylated teak wood presents better thermal stability, compared to the untreated teak wood. The furfurylation treatment can be eco-friendly and valuable method for improving the quality of short rotation teaks for high quality wood product utilization.

Figure 1 .Figure 2 .Figure 3 .Figure 4 .Figure 5 .Figure 6 .

 123456 Figure 1. FT-IR spectra of untreated Plus Perhutani teak wood (a); and furfurylated Plus Perhutani teak wood (b); untreated Biotrop teak wood (c); and furfurylated Biotrop teak wood (d)

Funding This work was supported by Directorate for Research and Community Service of the Ministry of RISTEK DIKTI: [Grant Number 2686/IT3.L1/PN/2020].