C catalyst: Reaction pathway and kinetic model discrimination

Bioresource Technology 102 (2011) 3504–3511

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Disproportionation of rosin on an industrial Pd/C catalyst: Reaction pathway and kinetic model discrimination Juan Carlos Souto, Pedro Yustos, Miguel Ladero ⇑, Felix Garcia-Ochoa Dpt. Ingeniería Química, Fac. CC. Químicas, Universidad Complutense, 28040 Madrid, Spain

a r t i c l e

i n f o

Article history: Received 18 August 2010 Received in revised form 1 November 2010 Accepted 3 November 2010 Available online 12 November 2010 Keywords: Rosin acids Disproportionation Dehydrogenation Kinetics Pd/C catalyst

a b s t r a c t In this work, a phenomenological study of the isomerisation and disproportionation of rosin acids using an industrial 5% Pd on charcoal catalyst from 200 to 240 °C is carried out. Medium composition is determined by elemental microanalysis, GC–MS and GC-FID. Dehydrogenated and hydrogenated acid species molar amounts in the final product show that dehydrogenation is the main reaction. Moreover, both hydrogen and non-hydrogen concentration considering kinetic models are fitted to experimental data using a multivariable non-linear technique. Statistical discrimination among the proposed kinetic models lead to the conclusion hydrogen considering models fit much better to experimental results. The final kinetic model involves first-order isomerisation reactions of neoabietic and palustric acids to abietic acid, first-order dehydrogenation and hydrogenation of this latter acid, and hydrogenation of pimaric acids. Hydrogenation reactions are partial first-order regarding the acid and hydrogen. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Rosin is obtained from living trees (gum rosin), from pine stumps (wood rosin), and from the alkaline extraction of wood during the Kraft pulping process (tall-oil rosin). Rosin is mainly a mixture of C20 monocarboxylic diterpenic resin acids of the abietic-type acids (mainly abietic, palustric and neoabietic acids) and of the pymaric acid type (pimaric and isopimaric acids, usually). It also has a certain quantity of fatty acids (in tall oil) and other neutral components (5–15%). The main difference between rosin acids is a conjugated double bond system present in abietic type resin acids, but not in the pimaric family. Carboxylic and olefinic functionalities of resin acids are advantageously modified by hydrogenation, disproportionation, formation of adducts with dicarboxylic acids, dimerisation, polymerisation, esterification and saponification (Soltes and Zinkel, 1989). Some applications require a combination of reactions to obtain the desired product quality. For example, esterification with polyols, disproportionation and isomerisation reactions occur together, leading to esters of rosin acids stable towards oxidation and, thus, of a very clear colour – one of the desired properties of rosin and rosin products. Disproportionated rosin has good oxidation resistance, low brittleness, high thermal stability and light color, and maintains a high softening point, even higher than the original rosin due to the elimination of turpentine oils during disproportionation at

⇑ Corresponding author. Tel.: +34 913934164; fax: +34 913944179. E-mail address: [email protected] (M. Ladero). 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.11.022

200 to 240 °C. It is applied in several industries: adhesives (hotmelt and pressure-sensitive adhesives), solder flux, printing inks, paper neutral-size – after saponification – and more (Soltes and Zinkel, 1989). It has some important applications in the polymeric industry where abietic acid presence is not adequate, because it acts as an inhibitor (amounts as low as 0.5% in abietic acid are looked up) and very high percentages in dehydroabietic acid are desired (65% or more): as an emulsifier in the production of styrene–butadiene rubber and ABS resin and chloroprene rubber (Mayer et al., 1995, 1996). New fields where applications are being developed include the synthesis of non-ionic surfactants (Hu et al., 2006), applications in the coating industry in antifouling paints and in the synthesis of new polymers (Duan et al., 2009). Dehydroabietic acid, as other abietates, is a diterpene acid important in the defense system of conifers, against potential herbivores and pathogens (González et al., 2010). Dehydroabietic acid (DHA) and its derivatives show a broad spectrum of biological action: antiulcer, antimicrobial, anxiolytic, antiviral, antitumor, and cytotoxic activities (González et al., 2010; Tanaka et al., 2008; Fonseca et al., 2004). Recently, DHA structure has been proved to be a new scaffold for BK (large-conductance calcium-activated K+) channel openers, causing a dramatic increase of the channelopening activity. In fact, as it has shown one of the greatest activities ever reported, it seems that this structure will be the nucleus for the design of new BK channel modulators, of interest in the treatment of acute stroke, epilepsy, asthma, hypertension, gastric hypermotility, and psychoses (Ohwada et al., 2003). Moreover, DHA seems to be an anti-inflammatory agent with high PPARa/c dual activation potential and prevent the production of several

J.C. Souto et al. / Bioresource Technology 102 (2011) 3504–3511

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Nomenclature CAB CD CDi CH2 CN CP CPi DHA Ea EMA GCFID GC–MS H-RMN

abietic acid concentration (mol L1) dehydroabietic acid concentration (mol L1) dihydroabietic acid concentration (mol L1) hydrogen concentration (mol L1) neoabietic concentration (mol L1) palustric acid concentration (mol L1) pimaric acid concentration (mol L1) dehydroabietic acid activation energy (J mol1) elemental microanalysis gas chromatography with flame ionization detector gas chromatography–mass spectrometry hydrogen-nuclear magnetic resonance spectroscopy

proinflamatory mediators by activated macrophages and differentiated adipocytes. Inasmuch as obesity is associated to low-grade inflammation and the production of the mentioned proinflamatory mediators, DHA seems to be a compound with interest in the prevention of obesity, and the resistance to insulin associated to obesity (Kang et al., 2008). This anti-inflammatory activity can be related to its capacity to reduce cholesterol and arteriosclerosis. Disproportionation and isomerisation of rosin acids happen together as rosin is heated (Soltes and Zinkel, 1989). Disproportionation has been considered to be an exchange of hydrogen between acid molecules, though percentages of DHA higher than 50% are commonly reached. This reaction has traditionally been catalysed by sulphur alone or with some sulphur organic compounds (as sulphides, and disulphides), iron salts, iodide and iodide alkaline metals salts, and lithium salts (Soltes and Zinkel, 1989). In the last decades, due to the toxicity of the gases formed when using sulphur as catalyst, palladium supported on activated carbon or alumina has been used, though inactivation by the iron present in rosin leads to the necessity of reactivating the catalyst (Matsuo and Tsuchida, 1981). Nickel and selenium have been tested as catalysts, to replace Pd catalysts, being the search for a non-noble metal catalyst one of the most interesting research lines in this field (Soltes and Zinkel, 1989). Thus, at the present time, Pd catalysts show much interest due to the good properties that the disproportionated rosin feature (regarding colour, softening-point, low metal concentration, and resistance to oxidation). Several methods of analysis to determine the composition of rosin after disproportionation have been used: UV–vis spectrophotometry, GC-FID, GC–MS, HPLC, H NMR and MALDI-TOF (Brites et al., 1993; Gigante et al., 1995; Rigol et al., 2003; Mitani et al., 2007; Kumooka, 2008). UV–vis spectrophotometry, using 254 nm as wavelength, is a simple method able to give an average conversion of abietic acid to dehidroabietic acid, and has been the method of use during a long period of time. Modern chromatographic techniques have been applied in the rosin industry since the 80s–90s, allowing, from that date, to determine the exact composition of the disproportinated product. However, little information can be obtained in the literature regarding the kinetics of the reactions involved, with Pd/C or any other catalyst (Song et al., 1985; Vital and Lobo, 1992; Wang et al., 2009). Song et al. (1985) used gas chromatography to analyse reaction samples from a three-necked round-bottom flask in the disproportionation of gum rosin at 543 K, being dehydroabietic acid the main compound found in the final samples, with a low quantity of dihydroabietic acids (four of them were identified) and no trace of tetrahydroabietic acids. Wang et al. (2009) have proposed a kinetic model for the disproportionation of rosin on a home-made Pd 5%/C, based on a complex

HPLC high performance liquid chromatography ki kinetic constant preexponential term of the kinetic constant k0 MALDI-TOF matrix-assisted laser desorption/ionization RAb disappearance rate of abietic acid (mol L1 min1) RD appearance rate of dehydroabietic acid (mol L1 min1) appearance rate of dihydroabietic acid (mol L1 min1) RDi RN disappearance rate of neoabietic acid (mol L1 min1) RPi disappearance rate of pimaric acid (mol L1 min1) T temperature (°C, K) tR residence time (min) Xab abietic acid conversion

scheme of disproportionation and isomerisation reactions where several abietic- and pymaric-type rosin acids are considered. The reaction runs were performed on a batch reactor (usual in the industry) featuring a double-tier paddle agitator and reaction conditions were chosen so that kinetic control by chemical reactions was assured and oxidation was avoided (by using nitrogen). The kinetic model, able to fit well all data from 483 to 533 K, is of an empiric nature, as it is not capable of explaining the high quantity of dehydrogenated species compared to hydrogenated species in the final product and in samples taken during the kinetic runs. Being disproportionation the way to get a rosin of interest in classic and new applications, and a way of obtaining dehydroabietic acid of a technical grade, the knowledge of disproportionation kinetics will provide information on the composition of the reacting mixture under different conditions and time values. An empirical model can be of use in the design of reactors for a given catalyst, but a deeper knowledge of the phenomena underlying disproportionation will give more accurate information on the activity of each catalyst, being a sounder basis for the comparison of catalysts. Moreover, the use of commercial catalysts will be of interest to industry. Thus, in this work, the disproportionation of rosin with Pd (at 5% concentration) on activated carbon from Engelhardt, from 473 to 513 K, is studied, leading to the proposal of a kinetic model based on dehydrogenation, hydrogenation and isomerisation reaction pathway. Runs were performed at the mentioned temperatures and samples were withdrawn and analyzed by GC-FID and GC–MS. The proposed model and the empirical model by Wang et al. (2009) are used to fit data obtained with the Engelhardt catalyst, using a multiparametric and multivariable non-linear technique with a coupled integration of the ODEs of the kinetic models. 2. Experimental 2.1. Materials Rosin and glicerol were of technical and pharmaceutical grade, respectively, and were kindly supplied by LURESA. N,N-Dimethylformamide dimethyl acetal 92% for gas chromatography was purchased from Acros Organics. Pure oleic acid, from Fluka, was used as internal standard. The catalyst (Escat 111) was 5%Pd on active carbon and was a kind gift from Engelhard. 2.2. Catalyst characterisation The specific surface area (BET), total pore volume and average pore diameter were measured in duplicate by nitrogen adsorption

 C D  C H2 RD ¼ k3  C Ab  RP ¼ k2  C P RN ¼ k1  C N

0

0

RH2 ¼ k3  C D  C H2  k4  C Ab  C H2  k3  C D  C H2  k5  C Pi  C H2  k6  C H2

 C D  C H2 RD ¼ k3  C Ab þ k2  C P  RP ¼ k2  C P RN ¼ k1  C N  C D  C H2 RD ¼ k3  C Ab þ k2  C N  RP ¼ k1  C P RN ¼ k2  C N RDi ¼ k4  C Ab  C H2 RPi ¼ k5  C Pi  C H2

Palustric acid Neoabietic acid Dihydroabietic acids Pimaric acid Hydrogen

0

0 k3 0 k3 0 k3

Dehydroabietic acid

Model 5 Model 3

0

Abietic acid

Second-order kinetic models

RP ¼ k2  C P RN ¼ k1  C N RDi ¼ k4  C Ab RPi ¼ k5  C Pi Palustric acid Neoabietic acid Dihydroabietic acids Pimaric acid

Model 4

0

0

RAb ¼ k1  C N þ k3  C D  C H2  k3  C Ab  k4  C Ab  C H2

RAb ¼ k1  C P þ k3  C D  C H2  k3  C Ab  k4  C Ab  C H2

RD ¼ k3  C Ab  k3  C D

RD ¼ k3  C Ab Dehydroabietic acid

Model 2

0

RAb ¼ k1  C N þ k2  C P þ k3  C D  k3  C Ab  k4  C Ab

RAb ¼ k1  C N þ k2  C P  k3  C Ab  k4  C Ab

Two kinetic models were the base of the proposed models to be fitted to data from the reaction system: a first-order kinetic model (Wang et al., 2009) and a second-order kinetic model that considers dehydrogenation of abietic-type rosin acids followed by hydro-

Abietic acid

2.4. Kinetic modelling and simulation

First-order kinetic models

Kinetic runs were performed between 200 and 240 °C using 0.1% catalyst referred to the rosin mass. Batch runs were started by charging the reactor with 100 g of rosin into a 250 mL roundbottom flask with three necks with upper agitation by marine helix and a distillation head attached designed to condense the trementine. When the reactor was charged a flow of N2 was passed into the reactor to avoid oxidation. When reaching 100 °C agitation was set at 500 r.p.m. At the temperature of reaction, a zero time sample was withdrawn and catalyst was added, taking more samples during 6 h. Aliquots of the samples have also been crushed to powder to analyze their percentual mass content in C and H by elemental microanalysis (EMA) using each time a 1 mg aliquot and a Perkin Elmer CHN 2400 elemental microanalyser. For a qualitative and quantitative analysis of the rosin acids involved in the isomerisation and disproportionation reaction network, a GC–MS analytical procedure was also employed. A HP 6890 GC–MS chromatograph equipped with a mass detector MSD type 5973 was used to analyze the reaction compounds. The sample constituents were separated with a HP-INNOWAX (crosslinked PEG) 30 m  0.32 mm ØI  0.25 lm column. Helium (purity P99.999%) with a flow-rate of 16 mL min1, and a constant pressure of 24 kPa was used as the carrier gas. The oven temperature was T0 = 150 °C, 1 min, rate 5 °C/min, T1 = 190 °C, rate 10 °C/min, T2 = 240 °C, 25 min, and post run, 249 °C, 5 min. The injection port temperature was 210 °C, and the detector temperature was 250 °C. The injection volume was 1 ll (splitless mode). Data acquisition was performed in scan mode. Mass spectra were recorded from m/z 50–550 amu. To determine the concentration of the main rosin acids, GCFID analyses of withdrawn samples were performed. Samples were methylated with N,N-dimethylformamiddimethylacetal and analyzed on a HP model 5890 series II gas cromatograph with a flame ionization detector using a dimethylpolysiloxane capillary column (15 m  0.32 mm  0.5 mm, Zebron 1, Phenomenex). The operating conditions of the gas chromatography were as follows: (i) initial oven temperature 453 K for 0 min, program at 1 K min1 to 468 K, then at 20 K min1 to 558 K and hold for 5.5 min, with total run time of 25 min; (ii) N2 was used as carried gas with GC-FID system; (iii) the injection temperature was 523 K and the detectors temperature was 573 K (Rigol et al., 2003; HSE, 2006).

Model 1

2.3. Esterification runs and sample analysis

Compound

at 77 K in a Coulter SA 3100 Surface Area and Pore Size Analyzer (with outgassing of the samples at 573 K during 3 h). The mean particle size distribution of the catalyst was measured at several agitation speeds (to avoid aggregation and breakage of the catalyst) with a Focused Beam Reflectance Measurement with a LasentecÒ FBRMÒ system model M400L-316K. The morphology of the catalyst and the EDS was observed in a JEOL JM-6400 scanning electronic microscope (SEM), using an acceleration voltage of 20 kV. This was also the voltage for the quantitative Electron Dispersion Scanning X-ray microanalysis (EDS) controlled by the ISIS software. Powder X-ray diffraction (XRD) was performed at k = 1.54 nm in a multipurpose PANanalytical diffractometer model X’Pert MPD with a Cu–K anode, at 40 mV and 200 mA, while scanning the sample over a Bragg angle (2h) from10° to 90°.

RAb ¼ k1  C N þ k3  C D  C H2 þ k2  C P  k3  C Ab  k4  C Ab  C H2

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Table 1 First order kinetic models (empirical models) and second-order kinetic models (phenomenological models) for the disproportionation of rosin with a Pd 5%/C catalyst used to fit to experimental data in this work.

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genation of abietic- and pimaric-type rosin acids with the liberated hydrogen from the previous reactions. In the first-order kinetic model, it was assumed that the reactions just involve isomerization and exchange of hydrogen among molecules of rosin resin acids, a double bond rearrangement of rosin resin acids molecules (Wang et al., 2009). In the second-order kinetic model, the concentration of hydrogen able to perform hydrogenation of pimaric and abietic acids was considered. The proposed kinetic models included model 1 (the one by Wang et al.), the same with equilibrium between abietic and dehydroabietic acid (model 2) and three kinetic models based on second-order kinetic laws and the existence of free hydrogen molecules (models 3–5). Model 3 involves that palustric acid isomerised to abietic acid, while neoabietic acid is directly dehydrogenated. Model 4 considers exactly the opposite: neoabietic acid isomerise to abietic acid, while palustric acid is converted to dehydroabietic acid. Finally, model 5 involves the isomerisation of both acids to abietic acid prior to the dehydrogenation of the latter. Equations of all kinetic models are included in Table 1. Kinetic models were fitted using the Marquardt–Levenberg algorithm together with a Runge–Kutta method for the numerical integration of the kinetic equations. The selection of the most appropriated model was based on the usual physical criteria (positive value of the kinetic parameters and values for the activation energies within adequate ranges) and statistical criteria (Student’s t value for each kinetic parameter, Fischer’s F and SQR values for each kinetic model).

3. Results and discussion 3.1. Catalyst characterisation Basic characteristics of the Escat 111 catalyst were obtained in the Coulter SA 3100 Analyser. BET surface area was 582 m2/g, while total pore volume was 0.50 ml/g. BET surface area related to micropores (less than 5 nm diameter) was 200 m2/g, with an inner volume due to micropores of 0.088 ml/g. The pore size distribution shows a high percentage of volume related to the micropore region, while macropores in the 20–80 nm pore diameter region are present (data corroborated by SEM at 500–15,000 enlargement). Mesopores are also of importance, with a 35% of the total volume linked to pores in the 2–20 nm pore diameter region. Thus, pore distribution is almost trimodal, mainly bimodal, but highly dispersed. XRD shows little crystallinity but this is linked to the presence of Pd crystals (as SEM semiquantitative elemental analysis confirms). It can be seen that the sample exhibited four peaks at 2h of 34.04°, 43.53°, 55.05° and 80.01°, ascribed, respectively, to (1 1 1), (2 0 0), (2 1 0) and (3 1 1) reflections of Pd metal with a face centered cubic (fcc) structure. The average size of the Pd crystallite particles was estimated from the full width at half maximum of the diffraction peaks to be 2.92 nm through Scherrer equation. The dispersion values (D), the ratio of number of surface atoms to the total number of atoms is calculated from the equation D = 1.13/d, where d is the crystallite size (2.9 nm). Thus, %D = 1.13/2.92  100 = 38.6% (Pattabiraman, 1997). Particle size is small, as the FBRM measurements showed, with an average diameter of 1.6 ± 1.4 nm (high dispersion) and a totally asymmetric distribution, with a maximum at 0.48 nm. This is confirmed by SEM images, with a high number of small particles in the micro-meter range and some big particles with hydraulic diameters up to 100 lm. EDS measurements give an idea of the distribution of Pd on carbon particles, with a percentage of Pd between 2.9% and 4.3% depending on the particle and zone of particle studied.

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3.2. Reaction route and kinetic modelling To fit kinetic models to molar quantities of the involved rosin acids a non-linear regression technique based on the Marquardt–Levenberg algorithm has been applied. Several kinetic models have been considered, based on the isomerisation pathways in the literature as well as assumptions and facts in references regarding disproportionation. These kinetic models are shown on Table 1, based on an empirical first order model (models 1 and 2) and on second-order kinetics, of a more phenomenological nature (models 3–5). Identified compounds by GC–MS comprised abietic-type acids (abietic, palustric, neoabietic, dihydroabietic and dehydroabietic acids) as well as several pimaric and dihydropimaric acids (several peaks for each one, as the inner double bond can be in several positions inside the cyclic structure). Chromatograph profile was identical in GC-FID and GC–MS, so peaks were assigned in a straightforward way, though during the construction of the corresponding calibrates in GC-FID with several rosin acid standards, peak assignations were confirmed. Disproportionation of rosin involves dehydrogenation of abietic-type acids, as well as hydrogenation and isomerisation reactions, so that the mixture of acids evolves to a final composition that is more stable from a thermodynamical viewpoint. It is proposed that abietic acid is dehydrogenated to dehydroabietic acid (which is the actual sought after product in industry), so that two conjugated double bonds turn into an aromatic ring. At the same time, palustric and neoabietic acid convert to abietic acid or one of them can be dehydrogenated. Pymaric type acids (several of which are identified in the GC–MS chromatograms) are hydrogenated to dihydropymaric acids. The double bond external to the cyclic structure typical of such acids is prone to hydrogenation. The hydrogen molecules from the dehydrogenation step can react with pymaric and abietic-type acids, or can escape from the liquid phase where all reactions happen. Although this latter fact is more than possible, according to results from all authors (Song et al., 1985; Wang et al., 2009), it is also thought that hydrogen atoms are rearranged among the different acids involved so that dehydrogenation and hydrogenation are synchronized and no hydrogen leaves the liquid phase and, even, a very recent kinetic study seems to be based on such assumption (Wang et al., 2009). The equilibrium between abietic and dehydroabietic acids shows that the former is really stable at the high temperatures where disproportionation and other reactions involving rosin take place. In fact, at 200 °C, more than 80% of the rosin acids not yet dehydrogenated is abietic acid (in the absence of a disproportionation catalyst), so it is the most stable acid among the abietates. Moreover, a worse fitting and an even worse value of the F parameter are obtained when using a non-equilibrium considering kinetic model to fit the experimental data herein presented as well as the data featured in the paper by Wang et al. (2009), as shown in Table 2. If the empirical model involving only the redistribution of hydrogen atoms among the rosin acid molecules is modified in such a way that there is a chemical equilibrium between abietic and dehydroabietic acid, a model that fits considerably better to experimental data is obtained. (F changes from 1090 to 2649 although two kinetic parameters have to be added to those in model 1, and SQR is reduced by a factor of 2.5, as given in Table 2). All these results seem to show that not only abietic acid and pimaric acid react with free hydrogen molecules, but also that dehydroabietic acid itself is prone to be hydrogenated. Fitting of the proposed kinetic models to experimental data has been performed by first calculating parameters at given temperatures. Thus, values of the parameters at each temperature have been calculated for each model and, afterwards, by linearization of the Arrhenius equation, values for the neperian logarithm of

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Table 2 Fit of the empirical and phenomenological models to experimental data of disproportionation of rosin obtained at several temperatures (from 200 to 240 °C) with an industrial Pd 5%/C catalyst from Engelhardt. Kinetic parameters

k1 k2 k3 0

k3 k4 k5 k6 Model statistical parameters

Ln k0 Ea/R Ln k0 Ea/R Ln k0 Ea/R Ln k0 Ea/R Ln k0 Ea/R Ln k0 Ea/R Ln k0 Ea/R F SQR

Model 1

Model 2

Model 3

Model 4

Model 5

Valor ± STD error

Valor ± STD error

Valor ± STD error

Valor ± STD error

Valor ± STD error

21.8 ± 12.21 12,221 ± 5768 18.0 ± 5.13 10,422 ± 2481 18.7 ± 1.15 11,116 ± 558 – – 8.2 ± 2.27 6673 ± 1109 22.3 ± 10.62 13,434 ± 5244 – – 1090 6.62  102

21.9 ± 8.08 12,225 ± 3879 18.5 ± 3.35 10,635 ± 1619 18.4 ± 0.70 10,876 ± 340 5.6 ± 2.60 5901 ± 1301 14.0 ± 1.23 9407 ± 602 22.3 ± 6.59 13,434 ± 3253 – – 2649 2.53  102

19.4 ± 2.86 11,005 ± 1379 13.2 ± 5.21 8066 ± 2509 21.1 ± 0.85 12,276 ± 418 15.7 ± 8.01 8587 ± 3945 17.9 ± 4.89 9472 ± 2392 13.5 ± 8.63 8606 ± 4245 23.9 ± 7.63 12,327 ± 3751 3604 1.63  102

13.9 ± 2.43 8450 ± 1175 22.1 ± 8.03 12,166 ± 3847 22.2 ± 1.03 12,845 ± 504 23.2 ± 7.00 12,399 ± 3408 25.1 ± 4.10 13,195 ± 1994 18.8 ± 6.61 11,066 ± 3235 32.1 ± 6.77 16,534 ± 3327 3292 1.77  102

18.8 ± 2.76 10,797 ± 1333 23.0 ± 6.80 12,761 ± 3265 19.2 ± 0.76 11,221 ± 371 18.8 ± 6.83 10,361 ± 3342 20.0 ± 3.49 10,711 ± 1707 14.5 ± 6.36 9041 ± 3129 28.1 ± 6.33 14,637 ± 3118 3724 1.59  102

Note: k1–k6 in L mol1 min1. Ea/R (K).

the preexponential parameters and for the activation energies have been obtained. Finally, fitting of the kinetic models to all experimental data (at the several temperature values used) has been performed to calculate the optimal values of the kinetic parameters. As shown in Table 2, the empirical model based on first-order kinetic equations is able to reasonably fit experimental data (F value higher than the critical one for the given values of data and parameter numbers), but more complex models fit much better experimental data, reducing the SQR value by a factor of 4 and increasing by the same factor the F value when compared to model 1. Thus, both from a statistical and from a physical point of view, disproportionation of rosin proceeds through the dehydrogenation of rosin acids of the abietate kind, and the hydrogenation of all acids involved, affecting mainly to pimaric acids, as the hydrogenation of the exocyclic double bonds is favoured both by thermodynamics and kinetics (Song et al., 1985; Wang et al., 2009). As hydrogen molecules diffused fast, a great percentage of this gas is lost from the reacting mass. While palustric acid can be dehydrogenated directly, there is no evidence that it cannot be converted to abietic acid previously (model 5, SQR = 0.0159, F = 3724). In fact, while model 4 has a SQR value slightly worse than models 3 and 5 (SQR = 0.0177, F = 3292), model 3 is statistically equal to model 5 (SQR = 0.0163; F = 3604). Considering the work on isomerisation of levopimaric acid by Vital and Lobo (1992), this acid reacted directly to neoabietic, abietic and palustric acids, being these two latter acids related by a chemical balance. Vital and Lobo (1992) suggest that neoabietic acid is not related directly to abietic acid, but palustric acid converts to this latter acid, supporting model 3. In this work and another from some years later (Portugal et al., 1996), results suggest that levopimaric acid is stable only at relatively low temperatures (those of rosin distillation, 150–180 °C), and isomerised directly to abietic, neoabietic and palustric acid. In fact, levopimaric acid is the most abundant rosin acid in fresh oleoresin, while abietic acid is the prevalent in heat- or acid-treated oleoresin (Enoki, 1976). On the other hand, abietic acid isomerises to both palustric and neoabietic acids, with a higher percentage of the former, remaining the percentage of abietic acid higher than 80% at temperatures of 200 °C or higher (Takeda et al., 1969). Palustric acid isomerisation leads to abietic acid, the prevalent, and neoabietic acid (Joye and Lawrence, 1961), though it is suggested that neoabietic acid comes from levopimaric acid, being a product of isomerisation of palustric acid by a rearrangement of double bonds in one of the cycles of the diterpenic structure (Vital and Lobo,

1992). Thus, model 4 can be considered not valid based on literature and on the statistical analysis in Table 2. Models 3 and 5 are both statistically valid, and there are reasons in the literature for both of the chemical routes to be realistic. Model 5 involves the rearrangement of two double bonds in neoabietic acid to render abietic acid. This isomerisation is supported by some references in literature (Takeda et al., 1969; Perelson et al., 1990). This work and the one by Wang et al. (2009) show that abietic acid is the main acid of gum rosin and converts directly to dehydroabietic acid. The statistical study in this work shows that an equilibrium exists between abietic and dehydroabietic acid. Some authors suggest that levopimaric acid and palustric acid can be considered as the intermediates between abietic acid and dehydroabietic acid (Enoki, 1976; Portugal et al., 1996), and this seems to be the case when temperature is lower than 200 °C. The common industrial practice is, however, to work in the range 200–270 °C, depending on the catalyst used. Here, abietic acid and dehydroabietic acid seem to be the most stable acids. In fact, when using Pd on charcoal as catalyst during hydrogenation of abietic acid, the catalyst acts as a hydrogenation catalyst at temperatures lower than 150 °C and as a dehydrogenation catalyst at higher temperatures (Yu et al., 2005). Thus, considering the information in the literature and the results herein presented, model 5 seems to be more reasonable than model 3 (neoabietic and palustric acids react to abietic acid prior to the dehydrogenation/hydrogenation steps), and dehydrogenation is the most important reaction due to the high operational temperatures. In Fig. 1, experimental data are shown as points, while the fitting of model 5 to them is drawn in lines. Moreover, according to kinetic model 5 and as shown in Fig. 2, hydrogen molar amount grows initially until a maximum is reached and, afterwards, gradually decreases due to diffusion to the atmosphere, and this process is faster the higher temperature is. All references in literature regarding disproportionation of rosin, as well as the results from this work, lead to the proposal of simple potential kinetic models, though many reactions are involved in the final reaction scheme. Considering that reactions take place on or near the catalyst surface, hyperbolic kinetic equations could be expected (for example, regarding the adsorption and splitting of hydrogen molecules or the adsorption of rosin acid molecules). The macroscopic kinetic model shows, however, that these surface phenomena are too fast to be considered in the kinetic equations. As sulphur is the homogeneous catalyst with an activity similar to that of Pd/C, and its activity is related to its

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molar quantity (mol)

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Time (min) Fig. 1. The phenomenological kinetic model fitting to experimental data obtained at (a) 473 K; (b) 483 K; (c) 493 K; (d) 503 K and (e) 513 K. (j) Abietic acid, (N) dehydroabietic acid, (}) dihydroabietic acid, (s) palustric acid, (H) neoabietic acid, (h) pimaric acid, (–) kinetic model.

ability to withdraw hydrogen atoms and transfer them to other molecules, it can be envisaged a similar behaviour for the Pd atoms at the charcoal surface. There are a number of reasons why a macroscopic kinetic model has simply first-order kinetic equations even if heterogeneous catalysis is involved. However, all of them show that only one phenomenon is controlling the overall rate of the process and, what is more, no accumulation of reactants and

products is taking place near the surface, so the system is acting as a pseudo-homogeneous reacting system (Kanno et al., 1987; Zhang et al., 2010). When comparing kinetic parameters and model to those obtained by Wang et al. (2009), it can be observed that values for activation energies lay in the range 80–100 kJ mol1, regardless of the results considered (ours or those of Wang and coworkers). This

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0.06

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Time (min) Fig. 2. Simulated hydrogen molar amount temporal evolution in the reaction liquid using kinetic model 5.

could be expected as results from Wang and coworkers and our results are obtained in a similar range of particle size (less than 60 mm in diameter) and agitation speed (more than 400 rpm), where no hindrance due to mass transfer phenomena are expected and chemical reactions are the controlling step. As said previously, despite the empirical model (model 1) not being the best one, it is able to be fit to all empirical data reasonably well: residual sum of squares is low, F value is high and error ranges for the parameters are narrow. To compare results of both works, it is necessary to take into account the fact that rosin and catalyst concentration in Wang’s work are 55% the ones used in this work, as here solvent-less conditions are employed, while Wang and coworkers dilute rosin by using a mineral oil (oil no. 200) in a volume ratio 45:55 to rosin. Moreover, catalysts are similar but not identical (crystallinity and dispersion of palladium are different). In this situation, kinetic constants of isomerisation reactions (k1 and k2), the kinetic constant value for the dehydrogenation reaction (k3), and the kinetic constant for pimaric acids hydrogenation (k5) are similar in both works, while the hydrogenation reaction constant for dehydroabietic acid (k4) is eight times lower in this work. Thus, conditions in this paper are favourable to isomerisation and dehydrogenation of abietic acids and less favourable for hydrogenation reactions (mostly in the case of abietic acids). In Wang’s paper, the amount of hydrogenated acids is, more or less, between 60% and 75% that of dehydroabietic acid. Here, hydrogenated acids amount to less than 40% of the dehydrogenated acid, so, on the whole, both catalysts used are favourable to dehydrogenation. 3.3. Hydrogen fate during disproportionation Two approaches have been considered to prove the role of hydrogen in the disproportionation of rosin: elemental microanalysis (EMA) and gas chromatography, using both a quadrupole mass detector and a flame ionization detector (FID). The hydrogen to carbon mass ratio percentage can be calculated from data obtained by EMA and it changes from 1.25 to 1.17 from pure abietic to pure dehydroabietic acid. As the runs at all the temperatures proceed, this ratio changed from 1.22 to 1.19–1.18 in all cases, showing that certain dehydrogenation of the solid material happens in every run. However, as the material is not pure and some turpentine distilled off while disproportionation proceeds, this technique allows only for a qualitative assessment of the dehydrogenation.

GC–MS not only is employed for the identification of the compounds in the sample, depending on the value of a quality factor, but it can also be used to quantify the amount of each compound. Here again, dehydrogenation seems to happen, as the mass of the dehydrogenated species is considerably higher than the sum of the several dehydrogenated species that have been identified, even if both pymaric and abietate structures are taken into account. For an accurate identification of the several acids involved in the reactions taking place, GC-FID has been used, using standards of the most important rosin acid identified by CC–MS and oleic acid as an internal standard. Through this analytical strategy, quantification of the rosin acids concentration in the samples is performed. By adding amounts of dehydrogenated species on one hand, and performing the same calculation for the dehydrogenated ones on the other, it can be seen that the former is almost three times larger than the latter, and this is so independently of the reaction temperature. Though not so clearly, the same trend is observed in the paper of Wang and coworkers (2009). This fact is also due to the statistically identical values of activation energies in dehydrogenation and hydrogenation reactions (Wang et al., 2009). Thus, dehydrogenation is the prevailing reaction in the reaction network taking place during rosin disproportionation, and this reaction leads to the most stable rosin acid to oxidative degradation: dehydroabietic acid. Palladium on charcoal catalysts seems to behave as sulphur, highly increasing disproportionation rates at relatively low temperatures (comparing to other rosin processes, as esterification, which proceeds at 260–290 °C). This could be so as Pd atoms act as a hydrogen adsorbent, reservoir and a hydrogen-transfer vehicle, while the main reactions take place in the liquid phase, which is a mixture of rosin acids (it is a solvent-less reacting system). In fact, the formation of palladium hydrides and the well-known disruptive chemisorption of hydrogen on the palladium surface could be part of the explanation why there is a higher amount of dehydrogenated than hydrogenated rosin acids (Markus et al., 2007; Tomaszewska et al., 2008; Weng et al., 2010). On the other hand, hydrogen escapes from the reacting liquid aided by the nitrogen that flows onto and over the liquid surface, through an irreversible process that follows first-order kinetics and that highly depends on temperature, as the high activation energy of constant k6 of model 5 shows. In a similar way, but involving a chemical reaction, sulphur withdraws hydrogen atoms from rosin acids yielding hydrogen sulphide, that also desorbs partly from the liquid (Zhao et al., 2008). 4. Conclusions In this paper, the role of hydrogen in rosin disproportionation is clarified with the aid of recent literature and using several analytical techniques. Dehydrogenation of abietic acid extent is almost three times higher than that of hydrogenation reactions pooled all together, so, in the presence of Pd/C as a catalyst, dehydrogenation is the prevailing phenomenon. Based on this fact, a phenomenological kinetic model is selected, after discrimination among several kinetic models and reaction schemes was performed by non-linear regression techniques. This model considers isomerisation reactions of abietates, as well as dehydrogenation and hydrogenation of abietic acid and hydrogenation of pimaric acids. Acknowledgements Financial support from the Spanish Ministry of Education and from L.U.R.E.S.A. (through project PETRI 95-0821.OP) is gratefully acknowledged. Moreover, the authors want to extent this

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recognition to the Spanish Ministry of Environment (project MMA0392063-11.2) for further financial support. The authors also want to express their deepest gratitude to Engelhardt Inc. (now, branch of BASF) for the kind gift of a 5%Pd/C catalyst, as well as to L.U.R.E.S.A. for the generous gift of industrial rosin. References Brites, M.J., Guerreiro, A., Gigante, B., Marcelo-Curto, M.J., 1993. Quantitative determination of dehydroabietic acid methyl ester in disproportionated rosin. Journal of Chromatography 641 (1), 199–202. Duan, W., Shen, C., Fang, H., Li, G.H., 2009. Synthesis of dehydroabietic acidmodified chitosan and its drug release behavior. Carbohydrate Research 344 (1), 9–13. Enoki, A., 1976. Isomerization and autoxidation of resin acids. Wood Research: Bulletin of the Wood Research Institute Kyoto University 59 (60), 49–57. Fonseca, T., Gigante, B., Marques, M.M., Gilchrist, T.L., De Clercq, E., 2004. Synthesis and antiviral evaluation of benzimidazoles, quinoxalines and indoles from dehydroabietic acid. Bioorganic & Medicinal Chemistry 12 (1), 103–112. Gigante, B., Santos, L., Marcelo-Curto, M.J., Ascenso, J., 1995. 13C and 1H NMR assignments for a series of dehydroabietic acid derivatives. Magnetic Resonance in Chemistry 33 (4), 318–321. González, M.A., Pérez-Guaita, D., Correa-Royero, J., Zapata, B., Agudelo, L., MesaArango, A., Betancur-Galvis, L., 2010. Synthesis and biological evaluation of dehydroabietic acid derivatives. European Journal of Medicinal Chemistry 45 (2), 811–816. HSE, 2006. MDHS 83/3 resin acids in rosin (colophony) solder flux fume. Available from: . Hu, L.-h., Zhou, Y.-h., Song, Z.-q., 2006. Synthesis and properties of rosin-based polyglucoside. Linchan Huaxue Yu Gongye 26 (1), 11–14. Joye, N.M., Lawrence, R.V., 1961. The thermal isomerization of palustric acid. The Journal of Organic Chemistry 26 (4), 1024–1026. Kang, M.-S., Hirai, S., Goto, T., Kuroyanagi, K., Lee, J.-Y., Uemura, T., Ezaki, Y., Takahashi, N., Kawada, T., 2008. Dehydroabietic acid, a phytochemical, acts as ligand for PPARs in macrophages and adipocytes to regulate inflammation. Biochemical and Biophysical Research Communications 369 (2), 333–338. Kanno, T., Kimura, T., Onose, T., Hayashi, M., Kobayashi, M., 1987. Evaluation of the first order approximation in heterogeneous catalysis. Memoirs of the Kitami Institute of Technology 19 (1), 71–88. Kumooka, Y., 2008. Analysis of rosin and modified rosin esters in adhesives by matrix-assisted laser desorption/ionization time-of-flight. Forensic Science International 176 (2), 111–120. Markus, H., Plomp, A.J., Sandberg, T., Nieminen, V., Bitter, J.H., Murzin, D.Y., 2007. Dehydrogenation of hydroxymatairesinol to oxomatairesinol over carbon nanofibre-supported palladium catalysts. Journal of Molecular Catalysis A: Chemical 274 (1–2), 42–49. Matsuo, K., Tsuchida, S. 1981. Stabilized Rosin Ester and Pressure-Sensitive Adhesive and Hot-Melt Composition Based Thereon – US Patent Application: 06/169,619. Patent Number: 4302371. Mayer, M.J.J., Meuldijk, J., Thoenes, D., 1995. Influence of disproportionated rosin acid soap on the emulsion polymerization kinetics of styrene. Journal of Applied Polymer Science 56 (2), 119–126.

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