Kinetic and Mechanistic Aspects of the Acetylation of Wood with Acetic Anhydride

Transcription

1 C. A.S.Hill et al.: Kinetics of the Acetylation of Wood 623 Holzforschung 52(1998) Kinetic and Mechanistic Aspects of the Acetylation of Wood with Acetic Anhydride By C.A.S.Hill, D. Jones, G. Strickland and N.S.Cetin School of Agricultural and Forest Sciences, University of Wales Bangor, Gwynedd UK Keywords Chemical modification Acetic anhydride Acetylation of pine Kinetics of acetylation Activation energy Corsican pine Finns nigra Summary The kinetics of the reaction of acetic anhydride (AA) and acetic anhydride in pyridine with Corsican pine (Finns nigra) sapwood have been investigated, and activation energies determined. For the pyridine catalysed system the chemical activation energy has been determined using the method of initial rates at 41.6 (±8.0) kj/mol, and for the non-catalysed reaction as 50.9 (±3.0) kj/mol. The reaction kinetics obey diffusion rate laws and the corresponding activation energies for diffusion have been evaluated as 20,5 (±0.9) kj/mol (pyridine catalysed) and 34.2 (± 1.0) kj/mol for the non-catalysed system. Introduction Acetylation has long been considered as a method of protecting wood from moisture and fungal decay, and has been the subject of numerous publications (Rowell 1983; Kumar 1994). Recently, there have been moves towards commercialisation of this technology (Sheen 1992). There has, however, been relatively little research directed towards the reaction kinetics of the process. It has been shown that the rate of reaction with the wood cell wall polymers decreases in the order Lignin > Hemicellulose > Cellulose, within both the wood (Rowell 1982), and the isolated cell wall polymers (Rowell et al. 1994). Furthermore SEM- EDXA studies of the chloroacetic anhydride modification have shown that the highest levels of reaction (at low weight percent gain (wpg)) are observed in the S 2 layer of the ceil wall but at high wpg the greatest substitution occurs in the middle lamella, indicating that the reaction kinetics of this reagent with wood is diffusion limited (Rowell et al. 1994). A recent study (Ramsden and Blake 1997) on the acetylation (using 1 : 1 acetic anhydride (AA) in xylene) of four wood species (Larch, Southern yellow pine, Deal and Sitka spruce) found a variation in the activation energy of the process from 42 to 107kJ/mol. There is, at present, a poor understanding of the kinetics of acetylation reactions, the research reported in this paper represents part of an extensive study into this phenomenon. The purpose of this study was three-fold: i To compare the nature of the kinetic processes of pyridine catalysed acetylation with a study of propionylation previously reported (Hill and Jones 1996) ii To determine and compare the activation energies for pyridine catalysed acetylation, and modification using neat anhydride iii To examine the effect that pyridine has upon the kinetic processes Materials and Methods Corsican pine (Finns nigra) sapwood was selected for this study. Sample sizes of 20mm χ 20mm χ 5mm (Radial χ tangential χ longitudinal) were cut from kiln dried wood, extracted using a mixture of toluene : methanol: acetone (4 : 1 : 1, by volume) for six hours, and dried in an oven overnight (105 C) before use. A weight loss of 3 % was recorded due to such treatment. For modification with the pyridine/aa system the following method was adopted (Hill and Jones 1996). Pre-weighed samples (five replicates) were vacuum impregnated with water free pyridine using a rotary vacuum pump, then added to a round bottom flask containing 100ml of pyridine set in an oil bath regulated to the desired temperature, i.e. 50, 60, 70, 80, 90, 100, or 110 C (± 0.5 C). An hour was allowed for the samples to achieve the required temperature, then one equivalent of anhydride (based on the OH content of the wood, estimated at 14.9mmoles/gm, calculated according to previously published methods) (Hill and Jones 1996) dissolved in pyridine (pre-heated to the required temperature) to make a volume of 25ml was added to the flask to initiate the reaction, making a total volume of reagent plus solvent of 125ml. At the termination of the reaction, the pyridine solution was decanted off and the blocks added to acetone at room temperature to quench the reaction. The blocks were left to stand in the acetone for one hour, then transferred to a Soxhlet apparatus, extracted as previously and dried in an oven at 105 C overnight. Such a treatment was found to be adequate to ensure that all traces of solvent, reactant and by-product were removed. A further set of experiments was performed in order to determine the effect of exposing modified and unmodified samples to hot pyridine solvent in order to determine whether extraction occurred due to such treatment. For this purpose experiments were performed under identical conditions to those detailed above, but with no anhydride reagent present. Acetylated samples modified to weight percent gains (wpg's) of 6, 10, and 14% were also heated in pyridine for a total of five hours at 120 C, in order to determine whether any weight loss of modified wood occurred due to extraction of acetylated components in the wood matrix. For modification with the neat anhydride system, the blocks were vacuum impregnated with AA then added to the flask containing AA at the desired temperature. Reaction was terminated Copyright 1998 Walter de Gruyter Berlin New York

2 624 C. A. S.Hill et al.: Kinetics of the Acetylation of Wood by decanting off the hot reagent, and adding the blocks to acetone at room temperature. After one hour, the samples were transferred to a Soxhlet apparatus extracted as described previously and dried in an oven at 105 C overnight. Results and Discussion Extraction studies Although the wood used for this study was extractive-free, there remains the possibility that there are components within the wood that are removed by hot pyridine which are not extracted by the solvent system in the Soxhlet. If this occurs, the kinetic profiles as determined by weight gain will not represent the true nature of the process since simultaneous weight loss will also be occurring. In order to determine the magnitude of this effect a comprehensive series of experiments was performed. A typical extraction plot is shown in Figure 1. Unmodified wood exhibited complex behaviour when subjected to pyridine extraction experiments. In general, following extraction for several hours a weight loss of 2 % was recorded. However, the data exhibited considerable scatter, particularly during the first hour and it was not possible to determine an average profile for the solvent extraction at any of the temperatures studied. When acetic anhydride modified wood was subjected to pyridine extraction at 120 C for six hours the following percentage weight losses were recorded: 0.69 (6%), 0.74 (11 %), 0.83 (13 %), weight percent gain given in brackets. This indicates that either acetylation of the wood stabilises the components to hot pyridine extraction, or that some of the extractable components are removed during the acetylation in neat anhydride. Due to the high degree of scatter of the experimental data, it was not possible to determine extraction profiles, the kinetic data is thus not corrected for any weight loss. However, the weight loss was less than 0.5 % during the first hour of experiment, and the temperature of the extraction solution did not affect the rate of extraction within the temperature range studied. It is therefore unlikely that extraction had any serious affect upon the kinetic data or activation energies. \ Acetylation kinetics The reaction of any reagent with a complex substrate such as wood will be inherently complicated. Chemically, the I"c (ι ' ο > -2.5 ν I ' 1 «1 «1 ι Time (sees) Fig. 1. Plot showing the extraction data for heating of wood samples in pyridine at 70 C. hydroxyl groups can be distinguished as being phenolic, benzylic, or alcoholic on the lignin regions, and alcoholic in the carbohydrate. The alcoholic hydroxyl's may be either primary or secondary, the phenolic hydroxyl's are attached to an aromatic ring which has various substituents attached. Thus, each of these groups will exhibit a different reactivity towards AA. For example, it has been demonstrated that the primary hydroxyls of cellulose in cotton linters are more reactive to acetylation (Malm et al 1953). The initial step in the mechanism for the reaction of AA with an hydroxyl group involves the nucleophilic attack on the acyl carbon centre of the AA molecule by a lone pair of the alcoholic (or phenolic) hydroxyl group (Fig. 2 a), subsequent loss of acetic acid generates the ester. The rate of the reaction will therefore depend upon the nucleophilicity of the relevant OH group. In addition, it is known that the solvent polarity affects the reaction mechanism and hence the rate and activation energy. This is due to the possibility of the AA molecule forming an iori pair (Fig. 2b). The equilibrium of this step is affected by the solvent, but the forward reaction only becomes strongly favoured in highly acidic environments. The positive charge on the CH 3 CO + cation strongly favours nucleophilic attack at the acyl carbon. The use of pyridine promotes the formation of a pyridinium salt intermediate, the positive charge on the pyridinium nitrogen adjacent to the acyl carbon centre endowing the latter with enhanced electrophilic character (Fig. 2c), the mechanism for this reaction is then analogous to (b). Two comprehensive reviews have been written on the mechanism of the nucleophilic substitution at acyl carbon (Bender 1960; Satchell 1963). Steric hindrance will also affect the rate of reaction, the stereochemical environment surrounding an OH group will be dependent upon the immediate environment of the molecule and due to neighbouring groups, this is of particular importance when the reaction is performed in a solid substrate. In addition, the reaction of acetic anhydride with wood generates acetic acid (HOAc) as a by-product. At low concentrations up to ca. 10% HOAc in the anhydride, it has been observed that the reaction is accelerated, but at higher concentrations a retardation occurs (Rowell et al. 1990). Thus reaction rate within the wood bulk is expected to be affected by the rate of reaction of AA units (and hence production of HOAc), and the rate at which HOAc diffuses out of the wood matrix into the surrounding solvent. The wood ultrastructure is also a factor, since the reagent molecules have to diffuse through the wood matrix to reach the reactive sites. Thus apart from in the initial stages, the reaction will be expected to be dominated by diffusion processes. The hydroxyl groups of the cell wall polymers form extensive hydrogen bonding networks within the matrix, and reaction of a reagent with an OH group requires the breaking of an Η-bond. If the breaking of hydrogen bonds is a slow process compared with the reaction of AA with the OH group, then this will be the determining factor in the kinetics and any evaluation of (say) the activation energy will measure this process rather than the anhydride/wood-polymer reaction. In a previous discussion

3 C. A.S. Hill ft nl.: Kinetics of the Acetylation of Wood 625 J A HJ, XOH (a) Rate β d OH /clt B ~ k' [OH) (2) The assumption that anhydride reagent is present in large excess is valid during the early stages of the reaction when it is assumed that surface sites arc only available for reaction. This represents only a small proportion of the total OH content. Equation (2) is a pseudo first-order rate expression, with k' being the related rale constant. Such an expression can be rearranged and integrated to yield the following expression: Ο O Si.-?---«* J. Ρ ι A HjC x *0 (c) Fig. 2. Reaction mechanisms referred to in the text. (a) Nuclcopliilic attack of acyl carbon by oxygen lone pair of OH group, (b) Formation of ion pair from acetic anhydride, (c) Pyridinc catalysed acctylation, (Hill and Jones 1996), the reaction of a reagent with wood was considered to be dominated by two processes, namely surface and bulk effects. The former was associated with reaction at or near the surface of the substrate and was assigned to the domination of the reaction kinetics by first-order process (with respect to the wood OH groups). As the reaction proceeds the bulk reactions assume increasing importance, and the kinetics will therefore be expected to display diffusion dominated processes. Whether diffusion kinetics arc indeed observed will depend critically upon the relative rales of diffusion and reaction, if the former is the slower process, then the reaction will be diffusion limited, but if the latter is slower then an alternative kinetic process will be observed. If it is assumed that during the initial stages of the process that the reaction sites are located at or near the wood surface, then the rate of reaction will be dependent upon the concentration of the AA molecules (.anhyd ) and the wood cell wall polymer hydroxyl groups ([OH]), and the rate constant (k). A rate expression for this reaction can be written (Atkins 1987): Rate = d OH]/dt = -k [OHlJanhyd] (1) This is a conventional second-order rate equation. If it is further assumed lhat during the initial stages of the reaction the anhydride reagent is present in a large excess, then the rate becomes dependent upon the concentration of the hydroxyl groups only: ln([oh],/ OH 0 )»- k't (3) Where fohl, is the concentration of OH groups remaining unreacted at time t, and [OH 0 the concentration of OH groups at lime /.cro. Thus, it is possible to determine the pseudo first-order rate constant by plotting the natural logarithm of OH],/ OII 0 against time, which yields a straight line of slope -k' if first-order kinetics is obeyed. It was reported that a linear relationship is found when propionic anhydride in pyridine is reacted with Corsican pine (Hill and Jones 1996), from which rate constants were obtained. However, when the data obtained from cither acetylation system investigated here is plotted in this way, no such linear relationship is obtained. Prom (his it is apparent that reactions at the surface during acetylation do not contribute to the kinetic profile to any appreciable extent. The activation energy for the reaction can be determined by measuring the rale constant at different temperatures, and evaluating using the Arrhenius expression: k a A exp (-En/RT) (4) Where Ea is the activation energy of the process, R the gas constant, T the absolute temperature, and A the collision factor. Thus, by plotting the natural logarithm of the rate constant against reciprocal absolute temperature, a straight line of slope -Ea/R is obtained if the Arrhenius expression is obeyed. Although the rate constants cannot be determined for the acetylation process, it is nonetheless still possible to evaluate an activation energy for surface reactions by using the method of initial rates first developed for determining the Ea of swelling of wood by various solvents (West 1988). This method relics upon determining the gradient of the rate curve at zero time to give the initial rate (Ro). The initial rate can then be substituted for k (or k') in the Arrhenius expression. This then determines the activation energy for the reaction before diffusion begins to influence the reaction profile. A comparison of the two methods for propionylation gave lia = 23.8kJ/mol using rate constants, and Ea = 31.2kJ/mol using initial rates. The difference in these values was attributed to a change in the hydroxyl concentration with temperature due to swelling of the substrate, since: Where, [OHJ 0 is the concentration of hydroxyl groups at time 1 = 0. Thus, the initial rate is proportional to the rate Hol/lbrscluing / Vol. 52 / 1998 / No. 6

4 626 C. A.S.Hill et αϊ: Kinetics of the Acetylation of Wood constant at all temperatures providing [OH] 0 is invariant with temperature, which cannot be assumed when a swelling solvent such as pyridine is used. The invariance of the hydroxyl group concentration with temperature cannot be assumed, since it is known that wood exhibits temperature dependent swelling with solvents such as pyridine (West 1988). The time to half-swell with this solvent has been estimated, and varies from 13 minutes at 40 C to 0.8 minutes at 100 C, thus allowing the samples one hour to equilibrate should be sufficient to remove this variable from the experiment, however the results suggest that there is a temperature dependent phenomenon affecting the hydroxyl availability, at least when using a reaction system with pyridine as a swelling agent. Pyridine catalysed acetylation A typical rate curve for the reaction of AA at 100 C is shown in Figure 3. An exponential fitting function was used for the curve fitting, and the fit was limited to the first hour of the experiment, since it was found that data points obtained thereafter distorted the fit. From such a fit, it is possible to determine the initial rate of the reaction, this has been done at a variety of temperatures to give the values reported in Table 1, where the results are compared with propionylation under identical conditions. The initial rates for acetylation are of the order ΙΟ 2 χ faster than for propionylation, at 100 C reflecting the affect of the larger size or molecular weight of the latter reagent. An Arrhenius plot using the initial rate data is shown in Figure 4, the data points exhibit a reasonable linear fit (R 2 = 0.842), yielding a value of Ea of 41.6 (+/-8.0) kj/mol. This value is some 10-20kJ/mol lower than solvent derived literature values (Table 2). Two factors may be responsible for this. Since the system under study here is a catalysed process, a lower activation energy for this process may be expected. In addition, the value has been obtained from the initial rate data, and it is possible that the rate constant derived Ea would be lower by some 8kJ/mol, as was noted with pyridine catalysed propionylation, which would then yield an Ea of ca. 34kJ/mol, ^8Ο Time (sees) Fig. 3. Reaction profile of pyridine catalysed acetylation, plotted as number ofmmoles of unsubstituted OH groups per gram <[OH] t ). Table 1. A comparison of the initial rates of reaction of propionic and acetic anhydride with Corsican pine (pyridine catalysed) Temperature (degreescelsius) Ro (acetic) χ 10'V ' ' Ro (propionic) x 10'V Table 2. Literature values for the activation energy for acetylation of ethanol with acetic anhydride Solvent system CC1 4 Ethanol Hexane CC1 4 CHC1 3 CC1 4 Ethanol Heptane Activation energy (kj/mol) 57.7 a 78.7 b 51.9 b 66.1 b 70.6 C 58.1 C 72.3 C 47.2 C a: Janelli and Beretta 1959; b: Moelwyn-Hughes and Rolfe 1932; c: Ono substantially lower than that observed in non-catalysed homogeneous systems. Studies of heterogeneous acetylation have also yielded values of Ea of the same order of magnitude. A study of perchloric acid catalysed aeetylation of cellulose derived from wood pulp found that the kinetics of the process followed pseudo first order rate laws, and an Ea = 60.6kJ/mol was measured (Frith 1963). In an investigation of the acetylation (perchloric acid catalysed) of jute and cotton it was found that the rate of reaction was diffusion controlled and activation energies of 54.3kJ/mol and 71.1kJ/mol respectively, were obtained (Sen and Ramaswamy 1957). The scatter of the data points in the Arrhenius plot, emphasises the necessity of performing the experiment at a sufficient number of temperatures to minimise error from this source. The value of Ea for acetylation compared with propionylation under identical conditions is approximately 10-15kJ/mol higher. This indicates that a different mechanism operates in the two reactions. The value for propionylation is close to that for the energy of the hydrogen bond in the wood matrix (Morrison and Dzieciuch 1959), which has been estimated at 25kJ/mol, suggesting that hydrogen bond breaking is the rate determining step in this reaction. With acetylation, a higher value for Ea is obtained and as such is an argument against such a process being the rate determining step. It is not yet known whether this difference is due to the larger size of the reacting species in propionylation. It may be postulated that the propionyl group requires a larger free-volume in the matrix in which to react, requiring

5 C. A.S.Hill et al.: Kinetics of the Acetylation of Wood η Fig. 4. Arrhenius plot obtained by using initial rate data. changes in the cell wall polymer configuration adjacent to a reacting site, but this remains speculation until work on other anhydrides is complete. West discussed the importance of Η-bond breaking with reference to the reaction of isocyanates with wood, and considered the importance of creating a void space within the substrate in order to allow the reagent molecule to align itself with the reactive OH group (West 1988). It should be noted that that the rate of reaction is not related to the activation energy since propionylation is some 10 2 times slower than acetylation at 100 C and ca. 20 times slower at 50 C, yet has the lower activation energy. Only the temperature dependence of the reaction rate is affected by the activation energy. Diffusion As the reaction proceeds, the diffusion of the reagent into the wood matrix assumes greater importance, and in the situation where the chemical reaction is rapid, compared with diffusion rate, the reaction will be under diffusion cpntrol. The process of heterogeneous kinetics limited by diffusion has previously been described (Pannitier and Souchay 1967). In this treatment, a derivation for the rate of reaction is obtained using Pick's law to describe the rate of diffusion. By considering the case of reaction on a plane front, the following expression is obtained: dimensional change was temperature dependant (West 1988). Since the samples were allowed to equilibrate for one hour before reagent was added, ample time was allowed for the blocks to achieve maximum swelling before reagent was added. Other treatments based upon Pick's law also give a square root time dependence for mass increase with diffusion dominated processes (e.g. Comstock 1963). Thus a plot of the mass gain against square root time will give a linear relationship of gradient a. Figure 5 is a diffusion plot for pyridine catalysed acetylation at temperatures of 100 C and 70 C. Both sets of data exhibit a linear relationship which projects through the origin in this plot, indicating that the reaction kinetics are dominated by diffusion processes for the time period studied here. This is an important observation since it suggests that the degree of acetylation within the wood ultrastructure will depend upon the local wood density, since the rate of diffusion is inversely proportional to the density (Dinwoodie 1981). This observation is in accordance with the results obtained measuring the distribution of chlorine using SEM-EDXA in choloroacetic anhydride modified wood, where it was found that the S 2 layer was modified more readily than the middle lamella at low wpg values. Thus the reaction rate and hence the degree of acetylation will be sensitive to the ultrastucture rather than the chemical composition of the wood. Diffusion can be considered to be an activated process, and expression (7) above may be rewritten as: 1/2 m = A.exp(- Ea/RT).t Which can be rearranged thus: In (m/t 172 ) = ln(a) = ln(a) - Ea/RT Therefore the activation energy of the diffusion process may be obtained by plotting the natural logarithm of (a) against reciprocal absolute temperature, where if this relationship is obeyed, a straight line will be obtained with an intercept of ln(a) and gradient -Ea/R. Such a plot is shown in Figure 6, for the pyridine catalysed acetylation of Corsican pine. From this data an activation energy for the diffusion process of 20.5 (± 0.9) kj/mol (R 2 = 0.939) is obtained. This (8) (9) 2 = -2.D.p.S 2.c.t (6) Where D is the diffusion coefficient, r the density of the reagent, S the surface area through which the reagent is diffusing, c the concentration gradient, and t the time. Assuming D, S, r and c, to be constant, the expression may be rewritten more simply as: m = a.t 1/2 (7) Where a is a constant related to the rate of diffusion. The surface area S through which the reagent diffuses may not be a constant since the wood may swell to different degrees at different temperatures. However, in a study of pyridine swelling of wood it was noted that the ultimate degree of swelling was constant, although the time for maximum ε 1.5 l 0.0 -φ 20 Sqr. Root Time (s 1 ' 2 ) Fig. 5. Diffusion plot for pyridine catalysed acetylation at 100 C (squares) and 70 C (circles).

6 628 C. A.S.Hill et al.: Kinetics of the Acetylation of Wood B Fig. 6. Arrhenius plot derived from the diffusion plots for pyridine catalysed acetylation. value is larger than that obtained from work on acid catalysed acetylation of cellulose, where a value of ll.7kj/mol was determined for the diffusion of acetic acid into cellulose (Hiller 1954). Conclusions The method of initial rates has been found to yield values for the activation energy which are reasonably close to literature reported activation energies for acetylation of ethanol in solution. This value is attributed to the reaction of acetic anhydride with surface hydroxyl sites. The reaction kinetics of acetylation of Corsican pine are diffusion dominated in catalysed (pyridine) and non-catalysed (neat anhydride) conditions. Activation energies for the diffusion process have been estimated for this process, since the rates of diffusion obey the Arrhenius relationship. The use of pyridine as a solvent lowers the activation energy of both acetylation at the surface, and diffusion within the bulk. The increase in reaction rate is due to a synergistic effect between catalysis and substrate swelling, since the use of a non-catalytic but swelling solvent (acetone) does not appreciably increase the rate of reaction when compared with a non participatory solvent (toluene). The activation energy of pyridine catalysed acetylation is higher than that of propionylation by ca kJ/mol, indicating that the kinetics are different for the two processes. Non-catalysed^acetylation Attempts to acetylate Corsican pine using toluene or acetone under identical conditions to the pyridine experiment did not produce rate curves that could be analysed due to the low rate of reaction observed. The data for the acetylation of Corsican pine with neat anhydride reagent was analysed in the same way as that for the pyridine catalysed acetylation to yield the Arrhenius type plots from the initial rate (Fig. 7), and diffusion data (Fig. 8). The corresponding values for the activation energies were Ea = 50.9 (± 3.0) kj/mol (R 2 = 0.929) from initial rates data, and 34.2 (±1.0) kj/mol (R 2 = 0.982) from diffusion data. Both values are larger when compared with pyridine catalysed acetylation. The initial rate data presumably reflects the catalytic effect that the pyridine has upon the reaction. The diffusion data illustrates the dramatic effect that the pyridine has as a swelling agent leading to a reduction in the activation energy for diffusion by one third. A value of 59.8kJ/mol for the activation energy of acetylation of the surface groups of cotton fibres with acetic acid under non-catalytic conditions has been reported (Hiller 1954). The Ea attributed to reaction of surface sites in this study using the method of initial rates is of a similar order. The magnitude of Ea for diffusion through the wood is approximately 20kJ/mol larger than the literature value derived for cellulose, this difference may be due to differences in the substrate. It should be noted that the value for diffusion obtained in this study is close to the value for the hydrogen bond energy. Since the non-catalysed acetylation does not take place on a pre-swollen substrate, the matrix will swell as the reaction proceeds, requiring disruption of the hydrogen bonding network. The activation energy for diffusion in this experiment may be a reflection of this process. O -8 DC Fig. 7. Arrhenius plot from intial rate data for non-catalysed acetylation reaction ^ f Θ Fig. 8. Arrhenius plot from diffusion data for non-catalysed acetylation reaction.

7 O.A.S.Hill et al: Kinetics of the Acetylation of Wood 629 Acknowledgements This work was carried out as part of a research contract funded by the Building Research Establishment, Garston, Watford, whose financial support we acknowledge. The support of the Nuffield foundation for the award of a grant for newly appointed lecturers (to CH) is also acknowledged. The University of Kahramanmaras Sutcu Imamn Turkey is thanked for the award of a PhD grant to NSC. References Atkins, P.W Physical chemistry', third edition. Oxford University Press, Oxford, pp Bender, M.L Mechanisms of catalysis of nucleophilic reactions of carboxylic acid derivatives. Chem. Revs., 60, Comstock, G.L Moisture diffusion coefficients in wood as calculated from adsorption, desorption, and steady state data. For. Prod. J., 13 (3), Dinwoodie, J.M Timber its nature and behaviour. Van Nostrand Reinhold, New York. pp Frith, W.C Kinetics of acid catalysed acetylation of cellulose. Tappi, 46 (12), Hill. C. A.S.and D. Jones A chemical kinetics study of the propionic anhydride modification of corsican pine (1). Determination of activation energies. J. Wood Chem. TechnoL, 16 (3), Hi Her, L. A Reaction of cellulose with acetic acid. J. Polym. Sei., 74, Janelli, U. and L. Beretta Kinetics of slow reactions. Influence of the substituent on the rate constant. Acetylation of methyl alcohol. Rend. Acad. Sei. Fis. Mat., 25, Kumar, S Chemical modification of wood. Wood and Fiber Sei., 26 (2), Malm, C.J., L.J. Tanghe, B.C. Laird and G.D. Smith Relative rates of acetylation of the hydroxyl groups in cellulose acetate. J. Amer. Chem. Soc., 75, Moelwyn-Hughes, E.A. and A.C. Rolfe The kinetics of esterification of acetic anhydride in ethyl-alcoholic solution. J. Chem. Soc., 135, Morrison, J.L. and M.A. Dzieciuch The thermodynamic properties of the system cellulose-water vapor. Can. J. Chem,, 37(9X Ono, Y Liquid phase reactions. Kinetics of esterification of ethyl alcohol with acetic anhydride. Reports Himeji Inst. Techno!., 2, Pannitier, G. and P. Souchay Chemical kinetics. Elsevier, Amsterdam, pp Ramsden, M.J. and F.S.R. Blake A kinetic study of the acetylation of cellulose hemicellulose and lignin components in wood. Wood Sei. and Technol., 31 (2), Rowell, R.M Distribution of acetyl groups in southern pine reacted with acetic anhydride. Wood Sei., 75 (2), Rowell, R.M Chemical modification of wood. For. Prod. Abstr., 5(12), Rowell, R.M., R. Simonsen and A.M. Tillman Acetyl balance for the acetylation of wood particles by a simplified procedure. Holzforschung, 44 (4), Rowell, R.M., R. Simonsen, S.Hess, D.V. Plackett, D. Cronshaw and. Dunningham Acetyl distribution in acetylated whole wood and reactivity of isolated cell wall components to acetic anhydride. Wood and Fiber Sei., 26 (1), Satchell, D.P.N An outline of acylation. Quart. Rev. (London), 77, Sen, M. K. and M. Ramaswamy Kinetics of fibrous acetylation of cotton and jute. J. Textile Inst.. 48 (3). T75-T80. Sheen, A.D The preparation of acetylated wood on a commercial scale. FRI Bulletin, 776, Chemical modification of lignocellulosics, pp West, H Kinetics and mechanism of wood-isocyanate reactions. PhD Thesis, University of Wales Bangor. Received July 14th 1997 Dr. Callum Hill Dr. Dennis Jones Dr. Gary Strickland Nihat S. Cetin School of Agricultural and Forest Sciences University of Wales Bangor, LL57 2UW United Kingdom Holzlbrschung / Vol. 52 / 1998 / No. 6