Thermal Decomposition of Citric Acid

Depending on heating rate, citric acid monohydrate loses hydration water in the 70-100 °C temperature range and melts from 135 to 152 °C. Decomposition of citric acid starts above 175 °C. Early description of the decomposition process is given in 1877 by Fittig and Landolt [56] who during rapid distillation of anhydrous citric acid obtained as main products itaconic, citraconic and mesaconic acids and anhydrides. This observation was supported in 1880 by Anschutz [57] who detected itaconic and citraconic anhydrides. These compounds were formed between 200 and 215 °C and identified after distillation under reduced pressure. Shriner et al. [58] performed syntheses of itaconic anhydride and itaconic acid from citric acid 

Because citric acid is considered as relatively cheap and abundant material, it was catalytically dehydrated to aconitic acid in the 120-150 °C temperature range by Umbdenstock and Bruin [61]. Aconitic acid can be readily decaiboxylated to a mixture of isomeric itaconic acids (itaconic, citraconic and mesaconic acids). These acids and their esters are nsed to produce alkyl resins and plasticizers. The mechanism of thermal rearrangement of citraconic acid to itaconic acid in aqueous solution was in a great detail investigated by Sakai [62]. In some cases, the applied catalyst caused excessive pyrolysis of citric acid and in the dehydration and decarboxylation reactions acetone dicarboxylic acid (P-ketoglutaric acid) was initially formed and from it acetone. The catalytic pyrolysis of citric acid monohydrate heated up to 140 °C to obtain itaconic and citraconic acids was reported by Askew and Tawn [63], 

Thermal analysis studies of decomposition process started with the Wendlandt and Hoiberg [67] investigatiom Differential thermal analysis (DTA) showed three peaks at 170, 185 and 210 °C, all of them indicating endothermic reactions. First peak was attributed to the fusion of citric acid and other peaks to deeomposition 

They observed that citric acid decomposes slowly above 148 °C and the decomposition rate significantly increases only after the melting point (153 0.1 °C), especially after 165 °C having a maximum at 188 °C, and decreases above 212 C. Above 480 °C, under oxidizing atmosphere, the decomposition process is exothermic. The thermal pyrolysis of citric acid depends on heating rate and particle size in heated samples. The interpretation of DSC experiments (the rate of weight loss of citric acid samples) is also consistent with competitive reactions when the final product of thermal decomposition is acetone. 

Thermoanalytical characteristics of citric acid were also studied by Trask-Morrell and Kottes Andrews [74] in the 60-600 °C temperature range. They found that anhydrous citric acid melted at 152-154 °C and then was decomposed at 228-242 °C. The weight loss of about 96 % of sample size was observed (in an apparent single peak), and at 575 °C, the residue of sample was very small. Thermal analysis of binary systems included in the Krebs tricarboxylic acids cycle was performed by UsoTtseva et al. [75-78]. They investigated systems of fumaric acid, malic acid, succinic acid, cis-aconitic acid and a-ketoglutaric acid with citric acid. They 

---

Fig. 1. Thermal decomposition of citric acid (1) to aconitric acid (2), citraconic acid (3), itaconic acid (4), and oxidation to acetonedicarboxylic acid (5).

Barbooti MM, Al-Sammerrai DA (1986) Thermal decomposition of citric acid. Thermochim Acta 98 119-123... 

Popov A, Micev L (1962) Paper chromatographic detection of some acids formed by thermal decomposition of citric acid. Dokl Bulg AkadNauk 15 37-40... 

It should be emphasized that citric acid is not the only possible acid employed in Pechini-type syntheses. Other polybasic carboxylic hydroxy acids (malic, tartaric, hydroxyglutaric, etc.) and polybasic carboxylic acids (e.g., succinic) have been probed in Refs. [4, 13-16], As far as amino acids are concerned, glycine seems to remain the only representative [13, 14]. However, the choice of each particular organic acid has never been justified, and no comparative studies are performed in order to find possible dependencies of the process (ability to form a sol, a gel, or a resin, easiness of thermal decomposition of precursors, etc.) on the steric factors, specifically, on the number of hydroxy and carboxylic groups in the molecule of an acid, as well as on the length of its carbon skeleton. 

A study of the relaxational transitions and related heat capacity anomalies for galactose and fructose has been described which employs calorimetric methods. The kinetics of solution oxidation of L-ascorbic acid have been studied using an isothermal microcalorimeter. Differential scanning calorimetry (DSC) has been used to measure solid state co-crystallization of sugar alcohols (xylitol, o-sorbitol and D-mannitol), and the thermal behaviour of anticoagulant heparins. Thermal measurements indicate a role for the structural transition from hydrated P-CD to dehydrated P-CD. Calorimetry was used to establish thermodynamic parameters for (1 1) complexation equilibrium of citric acid and P-CD in water. Several thermal techniques were used to study the decomposition of p-CD inclusion complexes of ferrocene and derivatives. DSC and derivative thermogravimetric measurements have been reported for crystalline cytidine and deoxycytidine. Heats of formation have been determined for a-D-glucose esters and compared with semiempirical quantum mechanical calculations. ... 

Tsay J, Fang T (1999) Effects of molar ratio of citric acid to cations and of pH value on the formation and thermal decomposition behavior of barium titanium citrate. J Amer Ceramic Soc 82 1409-1415... 

Figure 5.9 DTA-EGD curves for Cu-Cr-O spinel in the basic statel201. The sample was prepared by the complexing method using citric acid. Sample mass, 2.18 mg reference material empty crucible. Measuring conditions as in Figure 5.7, Three exotherms at 157,260 and 305 C thermal decomposition and oxidation. Owing to the decomposition reaction of nitrate and citric acid, CuO and Cr,0- are formed...

Nanocrystalline yttria (Y2O3) powders with most suitable characteristics for the fabrication of yttria crucibles were synthesized by the sol-gel method [85]. The combustion synthesis method was used starting with high-purity (99.9%) yttrium nitrate hexahydrate and citric acid. Different fuel to oxidant (citric acid/nitrate) ratios (0.25, 0.5, 0.75, 1.0, and 1.1) were tested. These mixtures were heated at 373 K until a viscous gel was formed, and its decomposition into powder formation was realized by thermal treatment at 473 K for 3 h. In all mixtures, yttria powders were obtained by thermal treatment at 1073 K. Scanning electron microscopy (SEM) showed that these powders were porous, whereas high-resolution transmission electron microscopy (HRTEM) revealed that they consist of randomly oriented cuboidal nanocrystallites with an average crystallite size of about 17-30 nm. In order to obtain dense ceramics, the powders were compacted as pellets at 120 MPa and were sintered at 1673 K. Pellets with a sintered density as high as 98-99% of the theoretical density could be obtained from powders prepared with fuel to oxidant ratios (/ ) of 0.75 and 1.0. 

J. Y. Luo et al synthesized the mesoporous La-Co-Ce-O composit oxide by citric acid complexation-organic template decomposition method. The catalysts prepared by this method had a high specific surface area about 95-156.6 m. g with very uniform pore diameter (3.7-3.95 nm) (changing with the different component and calcination temperature). The catalysts were used in catalytic CO oxidation and propane oxidation, and the excellent catalytic properties and thermal stability can be observed. In the case of CO oxidation, the reaction temperatures for 50% (Tso) and 90% CO (Tgo) conversion are 140 °C and 169 °C, respectively, over mesoporous La-Co-Ce-0 eomposit oxide catalysts, which are 39 and 30 lower than those over the catalyst prepared by co-preeipitation method (LCC (0.5)-CP-500 catalyst) in the case of propane oxidation, the 301 °C and 341 °C reaction temperature are required for the Tso and Tgo, which are 29 and 64 lower than those over the LCC (0.5)-CP-500 catalyst. In addition, the ordered mesoporous strueture of catalyst was conductive to the dispersion and transmission of the reactants, and therefore the inner surface of catalyst was fully utilized, which benefited the increase in catalytic activity and efficiency by the easy accessibility, and as well as the decrease in resistance of mass-transfer. As a result, excellent catalytic properties for CO oxidation and propane oxidation can be obtained over mesoporous lanthanum-containing composite oxide catalysts [87],... 

---