Phenolic model compounds

Hill, C.A.S. and Papadopoulos, A.N. (2002). The pyridine-catalysed acylation of pine sapwood and phenolic model compounds with carboxylic acid anhydrides. Determination of activation energies and entropy of activation. Holzforschung, 56(2), 150-156. 

Figure 5. Possible mechanisms for Ca oxidation (A) and alkyl-aryl cleavage (B) reactions of a phenolic / - model compound 4 by laccase.

Oxidation Procedure (Standard Conditions). Monomeric Phenolic Model Compounds. 10 mmoles were dissolved in 50 ml. 0.2N NaOH (analytical grade, free of CO2), corresponding to 1 mole equivalent alkali per mole phenol. 

Such processes are always accompanied by a DP loss, either by electrophilic attack of ozone, by an ozone-catalyzed cleavage of the glycosidic bond or by attack of secondary radical species [15]. Residual lignin also plays a crucial role in ozone bleaching. Model studies showed that lignin with free phenolic hydroxyl groups accelerated carbohydrate oxidation, probably by activation of oxygen via phenoxyl radicals, whereas etherified phenolic model compounds had a protective effect [16,17]. 

Babcock, G. T., El-Deeb, M. K., Sandusky, P. O., Whittaker, M. M., and Whittaker, J. W., 1992, Electron paramagnetic resonance and electron nuclear double resonance spectroscopies of the radical site in galactose oxidase and of thioether-substituted phenol model compounds, J. Am. Chem. Soc. 114 372793734. 

The role of hydroxyl radicals in these ring and inter unit cleavage react ion s has been confirmed by adding peroxide stabilizers. When sodium silicate was present there was no reaction with either simple phenol model compounds or guaiacylglycerol-p-guaiacyl ether (Figure 12.1, Ila) [28,30,31]. 

Fast pyrolysis of biomass produces a phenol-rich oil suitable for incorporation into phenol-formaldehyde (PF) resins. To evaluate the reactivity and network formation characteristics of the compounds typically found in these oils, a series of phenolic model compounds was reacted with formaldehyde under conditions typically used to prepare PF resins. This study indicates that the substituted phenolics commonly found in pyrolysis oils are more reactive than phenol. It also showed that the network formation process for tiiese complex phenolic mixtures follows the predictions of the simple statistical approaches developed by Flory (23) and Stockmayer (24). These results show that the substituted phenolics commonly found in pyrolysis oils will be chemically bonded into the polymer network and that under the proper reaction conditions a highly stable network will be formed. 

Studies on the reactivity of phenolic model compounds are useful in understanding the reactivity of biomass-derived phenolic compounds with formaldehyde under various reaction conditions. Studies over a 40-year period have shown that the reaction of phenol with formaldehyde followed second-order kinetics (10,11). A series of kinetic studies of the reactions of 3-methylphenol,... 

To better understand how the complex phenolics produced by biomass pyrolysis can be chemically incorporated into the PF resin network, we used a series of phenolic model compounds to study the reaction rate and the gelation processes under conditions typically used to synthesize PF resins. The model compounds (Figure 1) include 2-methoxy-4-methylphenol, 4-methylcatechol, 2-methylphenol (o-cresol), 4-methylphenol (p-cresol), phenol, 4,4 -isopropylidenediphenol (Bisphenol A), and trisphenol. Figure 1 also shows the number of reactive sites (f) on each model compound. 

Figure 2. A comparison of the reactivity of phenol and substituted phenolic model compounds.

Figure 4. Changes in solution viscosity as a function of reaction time for simple model compound systems (a) the effect of change the ratio of phenol to formaldehyde reactive sites (r) for constant (f) and (p) (f=3, p=l), (b) the effect of changing the functionality of the phenolic model compound (f) for constant (r) and (p) (r=l, p=l), and (c) the effects of adding a monofunctional phenolic to the reactive system (p) for constant (f) and (r) (f=l, r=l).

The data in Figure 2 clearly show that the phenolic model compounds studied here are more reactive than monomeric phenol under standard phenol formaldehyde reaction conditions. This is not surprising, since a wealth of literature shows the phenols substituted with electron donating groups are more reactive than unsubstituted phenol (12-14). However, it is particularly important for these complex phenolic-rich pyrolysis oils. These results suggest that the pyrolysis oil phenolic will react rapidly under standard cooking conditions and become covalently bound into the polymer networic. This means that the pyrolysis oils contribute to the strength of the network and do not simply act as fillers or extenders. 

This study demonstrated that under the reaction conditions typically used to prepare commercial PF resins, the reaction of phenol (or phenolic model compounds) and formaldehyde follow second-order kinetics. The reaction rate of each compound increases when the reaction temperature and amount of base increases. The reactivity of 2-methoxy-4-methylphenol, 2-methylphenol, or 4-methylcatechol was higher than that of phenol at temperatures and sodium hydroxide concentration used in this study. This indicates that the phenolic compounds commonly found in pyrolysis oils will be highly reactive under traditional PF resin synthesis conditions, and should be chemically incorporated into the PF network. 

Figure 5. Changes in the solution viscosity as a function of reaction time fw complex mixtures of phenolic model compoimd systems (a) a series of phenolic model compounds with 20 mole percent of monofimctional phenolic and (b) a series of phenolic model compounds with 40 mole percent of monofimctional phenolic.

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