Hydrolysis of acetic anhydride is a fast and very exothermic reaction that has been used widely as a model reaction for studies of safety (Shukla and Pushpavanam, 1994; Westerterp et al., 2014, Fritzler et al., 2014; Gaviria et al., 2016), nonlinear dynamics (Halder et al., 2007; Gómez García et al., 2016), limit cycle (Haldar and Rao, 1991), multiplicity of steady states (Jayakumar et al., 2010, 2011a,b,2014), etc. The reaction has also kinetic and mechanistic similarities with other important industrial reactions of acetic anhydride, such as the acetylation of hydroxyl groups to form acetates, and with other different industrial processes, such as hydrolysis of esters and esterification of different alcohols using anhydrides.
The kinetics of the hydrolysis of acetic anhydride have been extensively studied in the literature using different techniques for measuring the extent of the reaction and the reaction rate, such as titration (Orton and Jones, 1912; Eldridge and Piret, 1950; Cleland and Wilhelm, 1956; Bunton et al., 1962; Butler and Bruice, 1964), colorimetry (Oakenfull, 1971), conductimetry (Rivett and Sidgwick, 1910; Wilsdon and Sidgwick, 1913; Asprey et al., 1996; Kralj, 2007); changes of pH (Wiseman, 2012; Wiseman et al., 2020), different spectroscopic techniques, such as Raman (Bell et al., 1998), IR (Plyler and Barr, 1953; Haji and Erkey, 2005), UV-vis (Bunton and Fendler, 1966; Davis and Hogg, 1983), 1H NMR (Susanne et al., 2012), and different calorimetric techniques, such as ice-calorimetry/dilatometry (Kilpatrick Jr, 1928; Gold, 1948; Smith, 1955); adiabatic calorimetry (Janssen et al., 1957; Dyne et al., 1967; Shatynski and Hanesian, 1993) and isoperiblic calorimetry (Gold and Hilton, 1955; King and Glasser, 1965; Glasser and Williams, 1971; Bisio and Kabel, 1985; Regenass, 1985; Zogg et al., 2003; Ampelli et al., 2005; Hirota et al., 2010; Asiedu et al., 2013; Westerterp et al., 2014; Garcia-Hernandez et al., 2019). Also, the combination of calorimetric and spectroscopic techniques has also been used (Ampelli et al., 2003; Zogg et al., 2004; Puxty et al., 2005; Torraga et al., 2019).
It is well accepted that the reaction can be regarded as first-order in acetic anhydride and also first-order with respect to water. However, several studies used very low concentrations of acetic anhydride so that, under these particular conditions of almost constant water concentration, the overall kinetics can be well represented as first order with respect to anhydride. The use of higher concentrations of acetic anhydride may result in incomplete miscibility with water and formation of a two-phase system. Some studies avoided these effects by adding acetic acid to the reaction mixture, which favors the miscibility but may introduce other effects, such as the catalytic effect of the acid generated by the dissociation of acetic acid. Some studies using higher concentrations of acetic anhydride tried to account for this partial miscibility effect by including a dissolution kinetics in the data treatment (Assirelli et al., 2011). A more rigorous approach would be to include a NRTL thermodynamic model for the liquid-liquid phase equilibrium, as done by Ramaswamy et al. (2010) in the study of the hydrolysis of isobutyric anhydride. On the other hand, Hirota et al. (2010), using a calorimetric technique, reported that the incomplete miscibility can be visually observed in the beginning of the reaction and the system becomes completely miscible after some extent of the reaction, but the effect of excluding the data collected during the two-phase period from the kinetic analysis did not significantly affect the resuts of the kinetic parameters (Torraga et al., 2019; Giudici, 2016).
Despite the large number of kinetic studies, the kinetic parameters obtained in different works show significantly different values. The values of the kinetic parameters seem to depend on the concentration of reactants (Westerterp et al., 2014; Hirota et al., 2010, Torraga et al., 2019), type and concentration of acid catalysts and/or inhibitor salts (Bunton et al., 1962; Butler and Bruice, 1964; Bunton and Fendler, 1966; Kilpatrick, 1928; Gold and Hilton, 1955) and types and concentrations of cosolvents (Wiseman et al., 2017, 2020; Cooper et al., 2017; Koskikallio et al., 1959; Koskikallio, 1959,1963). The effects of catalysts (e.g., strong acids) and salts on the kinetics of the hydrolysis of acetic anhydride reported in the literature are not completely self-consistent and differ widely for different strong acids and for different salts. Different solvents, or different mixtures of water and organic solvent (dioxane, methanol, acetone, acetonitrile, etc.) have been used to elucidate the effect of the solvent type. In addition, the effect of the concentration of the reactants seems to be another controversial effect. The variations in the degree of nonideality of the solution with the ionic strength, and also changes of the reaction mechanism, are some of the causes mentioned in the literature for the variability of the kinetic values. As a result, the kinetic parameters found in different works in the literature are limited to use under conditions close to those employed in the experiments. At the moment, there is no general kinetic model of sufficient generality to be used safely for different concentrations of reactants, accounting for the effects of catalyst and salts. Therefore, additional studies on the kinetics and mechanism of the reaction are still necessary.
The present work is a contribution in this direction and aims at determining the effects of strong acid (HCl) and salts (NaCl and KCl) on the kinetics of the hydrolysis of acetic anhydride. These monovalent acid and salts were chosen to start the study of these effects, and their concentration ranges were based on previous studies of the literature. The kinetics was measured using a very simple isoperibolic calorimetric tenhique.
Distilled water and reagent-grade acetic anhydride (minimum purity 97%), hydrochloric acid, sodium chloride and potassium chloride were used as received, without further purification.
The simple reaction calorimetric technique described in our previous works (Hirota et al., 2010; Giudici, 2016; Torraga et al., 2019) was employed for the assessment of the kinetic data. The reactor, shown in Fig. 1, was a simple vessel made of expanded polystyrene, equipped with magnetic stirrer, and a
Effect of strong acid catalyst (HCl)
Experiments A1 to A5 were performed with a total mass of ca. 200 g of reaction mixture and an initial concentration of acetic anhydride of ca. 2.5 mol/L, but with different concentrations of HCl in the range from 0 to 0.154 mol/L, as presented in Table 1. The sets of adjusted parameters in these runs are presented in Table 2.
Fig. 2 shows the temperature measurements and the values of conversion calculated from the calorimetric technique (Equation 7), along with the curves calculated by
This work studied the effects of strong acid (hydrochloric acid) and salts (sodium chloride and potassium chloride) on the kinetics of the hydrolysis of acetic anhydride, using a very simple isoperibolic calorimetric technique described in our previous works (Hirota et al., 2010, Giudici, 2016, Torraga et al., 2019).
The presence of NaCl and KCl in the range of 0 to 3 mol/L was found to promote a negative effect on the hydrolysis kinetics, with no detectable difference between the two salts. The
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Declaration of Competing Interest
The authors report no declarations of interest.
The financial support of FAPESP - Fundação de Amparo à Pesquisa do Estado de São Paulo (grant numbers 2015/50684-9 and 2013/50218-2), CNPq - Conselho Nacional de Desenvolvimento Científico e Tecnológico (grant number 309444/2016-0), and CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (Finance Code 001) are gratefully acknowledged. The authors also thank Prof. Frank Quina for revising the manuscript.
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