Actually, there is a growing interest in the biotechnological production of lactic acid by fermentation aiming to substitute fossil fuel routes. The development of an efficient method for its separation and purification from fermentation broth is very important to assure the economic viability of production. Due to its high reactivity and tendency to decompose at high temperatures, the study of lactic acid thermal behavior is essential for its separation processes and potential application. In the present study, differential scanning calorimetry (DSC) analyses showed endothermic peaks related to the process of evaporation. Data of thermogravimetry (TG/DTG) were correlated to Arrhenius and Kissinger equations to provide the evaporation kinetic parameters and used to determine the vaporization enthalpy. Activation energies were 51.08 and 48.37 kJ·mol−1 and frequency values were 859.97 and 968.81 s−1 obtained by Arrhenius and Kissinger equations, respectively. Thermogravimetry, coupled with mass spectroscopy (TG-MS), provided useful information about decomposition products when lactic acid was heated at 573 K for approximately 30 min.
Lactic acid or 2-hydroxypropionic acid, an organic acid manufactured by fermentation from renewable sources, such as corn, potato, and other agriculture products, is an alternative to conventional petroleum raw materials routes. It has a wide variety of applications, in cosmetics (moisturizers, skin-lightening agents, skin-rejuvenating agents, pH regulators, antiacne agents, humectants, and antitartar agents), in pharmaceutical products (dialysis solution, mineral preparations, pills, prostheses, surgical sutures, and controlled drug delivery systems), in chemistry (descaling agents, pH regulators, neutralizers, chiral intermediates, green solvents, cleaning agents, slow acid release agents, and metal complexing agents), and in food (acidulants, preservatives, flavours, pH regulators, microbial quality improving agents, and mineral fortification) [
The thermal sensitivity and high reactivity of lactic acid can be explained due the presence of two adjacent functional groups, acid and alcohol, in a small molecule with only three carbon atoms, as well as their tendency to decompose at high temperatures [
The development of an efficient method for separation and purification of carboxylic acids from fermentation broth is very important to allow its biotechnological production in commercial scale, since these steps are responsible for 30–40% of the total production costs [
DSC is performed by difference in energy supplied to the sample and the reference pan in function of temperature. By varying the energy of the sample as a function of temperature, physical and chemical phenomena can be observed. In general, phase transitions, dehydration, reduction, and decomposition reactions produce endothermic effects, whereas crystallization, oxidations, and some decomposition reactions produce exothermic effects [
TG is a technique that records variation of mass while the sample is heated. The mass loss can be related to many phenomena such as dehydration, sublimation, evaporation, and decomposition. TG has been used to estimate the kinetic parameters, such as activation energies, reaction orders, and the Arrhenius preexponential factor, of degradation [
TG-MS is a powerful hyphenated technique combining the direct measurement of weight loss as a function of temperature with the use of sensitive spectroscopic detectors. Such detectors permit qualitative and quantitative determination of the evolved volatile products to provide kinetic information about the specific reaction mechanisms [
So, in this work, the objective was to study the lactic acid thermal behavior using DSC, TG/DTG, and TG-MS. The obtained results provided a better understanding of the lactic acid thermal behavior, allowing the development of adequate separation processes.
DL-lactic acid (molecular weight 90.08 kg·mol−1, CAS number 50-21-5) ~90% standard supplied by Sigma-Aldrich (St Louis, Missouri, EUA) was used in the thermal analysis by DSC, TG/DTG, and TG-MS without prior purification. Water and residual substances are impurities reported by the supplier.
DSC experiment was carried out using Shimadzu equipment (Japan), model DSC-50. A dynamic scan was performed at heating rate of 10 K·min−1 over a temperature range from 294 K to 773 K. The sample was analyzed under a nitrogen dynamic atmosphere at flow rate of 50 mL min−1. The experiments were carried out with a sample size of ~7 mg. The samples were weighed into open aluminum pans and hermetically sealed.
TG experiments were carried out using Shimadzu thermal analyser (Japan), model TGA-50. Constant heating rates of 5, 10, 15, 20, 25, and 30 K·min−1 were applied. Data were collected in the temperature range from 296 K to 773 K under a nitrogen dynamic atmosphere (50 mL·min−1). The masses of samples were nearly 15 mg. The equipment recorded TG and DTG (derivative thermogravimetric analysis) data simultaneously. Data obtained by TG were used to determine evaporation kinetic parameters and evaporation enthalpy. Data obtained by DTG were useful to show the evaporation stages and the effect of heating rate.
TG-MS analysis was carried out using Setaram SetSys Evolution 16/18 coupled with a mass spectrometer (MS) Thermostar Pfeiffer Vacuum GSD 301T. MS was responsible for monitoring the masses of compounds that have evolved from the sample during heating. Initially, the sample was heated from 298 K to 773 K at heating rate of 10 K·min−1 under nitrogen atmosphere (16 mL·min−1) in order to observe the temperature range in which the evolution of mass occurred. Afterwards, the analysis was performed with a mass spectrometer coupled, heating the sample from 293 K to 573 K at heating rate of 10 K·min−1 under nitrogen atmosphere (16 mL·min−1). In addition, an isothermal at 573 K was built during 2 h. In this case, a scan of all fragments evolved from the sample during the heating time was used to obtain the thermal and mass spectrum. Finally, TG-MS was performed monitoring fragments of higher intensity. Data were collected and used to obtain the lactic acid thermal behavior and degradation products as a function of temperature and time.
Thermogravimetry is the most common technique used for kinetic analysis. Sample mass variation by temperature data obtained by TG/DTG was used to determine evaporation kinetic parameters. The frequency factor (
Calculations are based on the following kinetic equation [
The amount of vaporized material (
Considering (
This approach assumes Arrhenius behavior and zero-order reaction kinetics. Taking the natural logarithm in (
For zero-order reactions (
Thus, plotting
Kissinger developed a model-free nonisothermal method where there is no need to calculate
The activation energy is determined from the TG data at different heating rates by liner regression of the
Thermogravimetry is a rapid and convenient technique used for determination of vaporization enthalpy. The theoretical basic of the TG procedure is the Langmuir equation [
Rearranging the Langmuir equation gives
A plot of
The DSC curve of lactic acid is presented in Figure
DSC lactic acid profile.
The TG and DTG thermal profiles are shown in Figures
TG (a) and DTG (b) curves of lactic acid as a function of temperature and heating rates (5, 10, 15, 20, 25, and 30 K⋅min−1).
The weight loss profile in Figure
The effect of heating rate is shown in Figures
DTG profiles showed three evaporation stages. The evaporation stages (heating rate of 10 K·min−1) occurred at ~347 K, ~459 K and ~612 K and mass loss percentage of 12.119%, 85.520%, and 1.483%, respectively. First was the evaporation of water and methanol impurities (~347 K) followed by lactic acid (~459 K) and probably lactide (~612 K).
The Arrhenius plot for the calculation of activation energy is shown in Figure
Evaporation kinetic parameters of lactic acid by Arrhenius and Kissinger models.
Model | Heating rate/K⋅min−1 |
|
|
|
---|---|---|---|---|
Arrhenius | 10 | 51.08 | 859.97 | 0.9962 |
Kissinger | 48.37 | 968.81 | 0.9712 |
Arrhenius plot (a) and Kissinger plot (b) for the calculation of the activation energy.
Kissinger plot of
The values of activation energy and frequency factor obtained from the Kissinger method are consistent with the values obtained by Arrhenius method (deviation less than 10%). Kinetic parameters of the lactic acid evaporation process have not been reported in literature review, as described in this work.
The plot of
The plot of
TG-MS analysis enabled the identification of vaporized molecules and thermal decomposition products from lactic acid at high temperatures. TG-MS profiles are shown in Figure
TG-MS profiles of lactic acid.
Lactic acid can be conveniently converted into various value-added products, such as acrylic acid. For acrylic acid production, direct dehydration of lactic acid in high temperatures is an alternative. For better understanding the lactic acid thermal behavior at high temperatures, TG-MS isotherm of lactic acid at 573 K was performed as shown in Figure
TG-MS lactic acid isothermal at 573 K during the time. (a):
Figure
According to the literature, the formation of self-reaction products such as lactide, which are subsequently more readily decomposed into fragments (e.g., carbon monoxide, acetaldehyde, and water), is the main reaction that competes with the dehydration process [
Additionally, at the end of the analysis, the sample was found graphitized indicating that a quantity of the sample was reduced, also confirming the analysis of the mass profiles. It can be concluded that the decomposition process of lactic acid is related to temperature and the time of exposure to heat. In addition, only after 30 min of heating at 573 K lactic acid was decomposed into water, hydroxyl, methanol, acetaldehyde or/and carbon dioxide, formate, acrylic acid, and propionic acid as can be seen by increasing the current ionic at 30 min.
Therefore, it is important work with separation processes, which minimize problems of thermal decomposition to avoid the formation of byproducts. Vacuum operations provide a substantial decrease in the boiling points of substances due to operating pressure reduction. This characteristic allows the separation of compounds that would be destroyed if the mixture is processed at normal pressures [
In this work, thermal characterization of lactic acid using thermoanalytical techniques was presented. DSC analyses showed endothermic peaks related to the process of evaporation. Effect of heating rate on TG and DTG curves was also presented and the data were correlated to Arrhenius and Kissinger equations to provide the evaporation kinetic parameters. Activation energies were 51.08 and 48.37 kJ·mol−1 and frequency values were 859.97 and 968.81 s−1 obtained by Arrhenius and Kissinger equations, respectively. Enthalpy of vaporization calculated by TG was 56.9 kJ·mol−1. TG-MS curve profiles showed many decomposition products when lactic acid was heated at 573 K in approximately 30 min. It can be concluded that decomposition process of lactic acid is related to temperature and the time of exposure to heat. Therefore, separation and purification processes which work with vacuum and short residence time are favored.
The authors declare that they have no conflicts of interest.
The authors are acknowledging the financial support from São Paulo Research Foundation (FAPESP) (Project nos. 2012/17501-0 and 2015/12783-5).