REV. CHIM. (Bucharest) 61 Nr.8 2010http://www.revistadechimie.ro784

Esterification of Glycerol with Technical Olein inHeterogeneous Catalysis

DORIN BOMBOS1*, MIHAELA BOMBOS2, ION BOLOCAN1, GABRIEL VASILIEVICI2, EMIL ZAHARIA31 Petroleum-Gas University of Ploiesti, 39 Bucharest Blv., Ploiesti, Romania2 National Research and Development Institute for Chemistry Petrochemistry ICECHIM-Bucharest, 202 Splaiul Independetei,060021, Bucharest, Romania3 Atica Chemicals, 202 Cazanesti Str ., Rm. Valcea , Romania

The decrease of organic acidity of technical olein resulted from vegetable oil refining process was achievedby esterification with glycerol in presence of catalysts zinc oxide, ferrous oxide and stannous oxide, inpowder form. The study was made in batch regime, in a reactor provided with mechanical stirring andautomatically adjustment of temperature. The distribution of acid centers strength of tested inorganic catalystswas determined by thermodesorption of diethylamine. The assessment of tested catalysts performanceswas achieved by determination of the acidity and saponification indexes of the reaction product.

Keywords: glycerol, fatty acid, catalyst, esterification

Technical olein is the acid oil (a mixture of fatty acidsand neutral oil) obtained by neutralization of soapstockresulting in refining of edible fats.

Glycerol is produced during biodiesel production bytransesterification of fats with methanol. The increasingproduction of biodiesel has resulted in an increase ofglycerol production, which makes glycerol attractive forthe synthesis of other chemical products. For example, byreaction of glycerol with isobutene on acidic catalysts wereobtained ethers usefull as additives for motor fuels [1].Another direction of application of the glycerol isesterification with various fatty acids. Esterification is anequilibrium process favourized by the removal from thesystem of a reaction product [2]. The acceleration of theesterification process is achieved in presence of somecatalysts. Specialty literature accentuates the use of acidtype catalysts as oxides of some transitional metals ormetals in groups III A and IV A.

Mesoporous molecular sieves based on aluminium andzirconium (AlZrMMS-H) are used for the esterificiation ofglycerol with lauric acid, in presence of CO2 (supercriticalmedium). It was experimentally demonstrated that thecatalytic activity of these materials in supercritical mediumis higher than that presented in absence of solvents and inatmosphere of N2 [3]. It is obtained also a conversion ofabout 93%, resulting equal quantities of acid lauricmonogliceride and acid lauric digliceride.

Experimental studies demonstrated that mesoporousmaterials containing acid alkyl-sulphonic groups (HSO3-R-MCM-41) are efficiently catalysts for the esterificationprocess [4-6]. These materials, obtained by sulphonationreaction are active and selective in the esterificationprocess of glycerol with lauric and oleic acid.Themesoporous material with catalytic activity HSO3-etil-MCM-41 (V1H) lead to the highest conversion in either case,both for the esterification process of glycerol with lauricacid at 1000C and with oleic acid at 1200C, because of thehigher number of acid centers. The mesoporous materialHSO3-ethyl/methyl-MCM-41 (V3H) presented a lowercatalytic activity than V1H, and HSO3-methyl-MCM-41(V4H) presented the lowest catalytic activity.

The production of glycerol carboxylate derivatives is ofa really interest owing to the possibility of using the glycerolexcess resulted in biodiesel fabrication [7,8].

The esterification of glycerol with acetic acid was madein presence of ion exchange resins catalysts (Amberlyst35, Amberlyst 15) and zeolites type catalysts (HZSM-5,HUSY) [7,8].

By the esterification of some epoxidated vegetal oils(soya bean oil) with different polyols (sorbitol, pentaeritrole)on a Lewis type catalyst (tetraoctyl titanate, tetrabuthyltitanate, triphenyl phosphate and tetra-isopropyl titanate)are obtained oleochemical products recommended for thepreparation of rigid polyurethanic foams. Thecharacterization of polyols esters with fatty acids wasachieved by liquid chromatography (HPLC), bydetermination of the saponification index and of the acidityindex [9].

In this work, we describe the use of zinc oxide, ferrousoxide and stannous oxide, as catalysts for the esterificationof glycerol with technical olein, in order to obtain acomponent with surface characteristics for fuel.

Experimental partThe experiments were performed in steady regime, in a

round bottom flask provided with mechanical stirring andhorizontal condenser for removing water resulted fromprocess. Reaction temperature was 170 0C, and wasautomatically adjusted and the reaction time was 6 h.Glycerol used in these experiments was of analytical gradeand the main characteristics of technical olein arepresented in table 1.

The catalysts used for experiments were zinc oxide,ferrous oxide and stannous oxide, in powdery form. Zincoxide catalyst was tested both in commercial form andafter calcination at 550oC.

The distribution of acid centers strength of testedinorganic catalysts was determined by thermodesorptionof diethyl amine in the temperature domain 20-700oC.Thermal analyses (ATG, DSC) were made on an apparatusDuPont Instruments Thermal Analyst 2000/2100 coupledwith modules 912 Differential Scanning Calorimeter and951 Thermogravimetric Analyzer.

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REV. CHIM. (Bucharest) 61 Nr. 8 2010 http://www.revistadechimie.ro 785

Performances evaluation of tested catalysts wasachieved by the determination of the acidity and thesaponification indexes of the reaction products. Theexperiments were achieved at values of the molar ratioglycerol/free fatty acids from olein of 1-1.3 /1 and at catalystconcentrations between 0.2% and 0.8% to the reagentmixture.

Results and discussionsThermogravimetric and thermodifferential analysis of

not calcinated zinc oxide demonstrated a weight loss ofabout 15% at a temperature of 2800C, which means thatzinc oxide change its composition after calcination, theloss being probably generated by the anhydrization of zincoxi- hydroxide contained in the product (fig. 1).

Ferrous oxide catalyst treated with diethyl-aminepresents a variation curve of weight with the complextemperature; weight increases are probably due to theadsorption of oxygen from the atmosphere. This behaviourmakes difficult the evaluation of acid centers strength (fig.3).

Table 1THE CHARACTERISTICS OF TECHNICAL OLEIN

The thermogravimetric curve at the desorption of diethyl-amine on zinc oxide calcinated catalyst evidenced thepresence of weak acid centers with a large distributionand medium strength centers with a narrow distribution ofacid centers, having a maximum in the temperaturedomain 170-2500C (fig. 2).

Fig. 1. TGA-TGP curves of zinc oxide catalyst (not calcinated)

Fig. 2. Thermodesorption of diethyl amine on zinc oxide catalystcalcinated at 550oC for 5 h

Fig. 3. Thermogravimetric and thermodifferential curves of Fe2O3treated with diethyl-amine

Stannous oxide catalyst presents a large distribution ofacid centers strength, with a maximum at a temperatureof about 3000C, specific to strong acid centers. The weightincrease of the stannous oxide sample for temperatureslower than 76 0C and higher than 300 0C is due probably tothe adsorption of oxygen from air (fig. 4).

Fig. 4. Thermogravimetric curve of SnO treated with diethyl-amine

The influence of zinc oxide catalyst calcination uponperformances of technical olein esterification is presentedin figures 5 and 6.

Calcination of zinc oxide catalyst at the temperature of5500C determined the improvement of its performances;thus, the acidity index of technical olein decreased andthe saponification index increased after the calcination ofzinc oxide catalyst.

The influence of concentration of not calcinated zincoxide catalyst on the performances of technical oleinesterification process is presented in figures 7 and 8.

Fig. 5. The influence of zinc oxide catalyst calcinationupon the saponification index (equimolecular ratio glycerol/ fatty

acid; catalyst concentration 0.4%)

REV. CHIM. (Bucharest) 61 Nr.8 2010http://www.revistadechimie.ro786

Fig. 6. The influence of zinc oxide catalyst calcinationupon the acidity index (equimolecular ratio glycerol/ fatty acid;

catalyst concentration 0.4%)

Fig. 7. Variation of the acidity index with ZnO catalystcontent

Fig. 8. Variation of the saponification index with ZnOcatalyst content

It is observed that both the acidity index and thesaponification index present a variation with an ascendentslope at values of catalyst concentration less than 0.4%,after their variation follows a lower slope (figs. 7 and 8).Variation curves of the acidity and saponification indexhave a complementary shape, the decrease of acidity beingproportional to the increase of esters content.

The increase of glycerol excess, compared to free fattyacids from technical olein, at values up to 30% to theequimolecular ratio, determines a parabolic variation ofthe saponification index, with a maximum at an excess of10% glycerol, whereas the variation of acidity is notimportant for the glycerol excess which is used. (figs. 9and 10).

In case of ferrous oxide catalyst, both the acidity indexand the saponification index present an almost constantvariation with the slope on the studied domain of catalystconcentrations (figs. 11 and 12). Variation curves of theacidity and saponification index have a complementaryshape, the decrease of acidity being almost proportionalto the increase of esters content.

The acidity of technical olein presents a parabolicvariation with the excess of glycerol, with a maximum atan excess of 20% glycerol. Curve of the saponificationindex is similar, but the maximum is located at a lower

Fig. 11. Variation of the acidity index with Fe2O3 catalystcontent

Fig. 9. Variation of the acidity index with the molar ratioglycerol/fatty acids for ZnO catalyst

Fig. 10. Variation of the saponification index with the molarratio glycerol/fatty acids for ZnO catalyst

Fig. 12. Variation of the saponification index with Fe2O3catalyst content

Fig. 13. Variation of the acidity index with the molar ratioglycerol/fatty acids for Fe2O3 catalyst

REV. CHIM. (Bucharest) 61 Nr. 8 2010 http://www.revistadechimie.ro 787

excess of glycerol (10%), as shown in the figures 13 and14.

In case of stannous oxide catalyst the acidity index oftechnical olein presents an almost constant negativevariation with the slope on the studied domain of catalystconcentrations (fig. 15); the saponification index presentsa non significant variation on the studied domain (fig. 16).

Fig. 16. Variation of the saponification index with SnOcatalyst content

Fig. 14. Variation of the saponification index with themolar ratio glycerol/fatty acids for Fe2O3 catalyst

Fig. 15. Variation of the acidity index with SnO catalystcontent

Fig. 18. Variation of the saponification index with the molar ratioglycerol/fatty acids for SnO catalyst

Fig. 17. Variation of the acidity index with the molar ratio glycerol/fatty acids for SnO catalyst

The acidity of olein after esterification increases withthe excess of glycerol (fig. 17); considering that the reactionoccured in presence of air, it was probably that the catalystfavourized glycerol oxidation to carboxylic acids. Esterscontent presented a variation after a parabolic curve witha maximum, which presents the flattening tendency athigher values of glycerol excess compared to technicalolein (fig. 18).

ConclusionsThe diminution of organic acidity of technical olein

resulted from the process of vegetal oils refining can beachieved by direct esterification with glycerol on metaloxide catalysts with acid characteristics.

Zinc oxide catalyst favourized the obtaining of lowervalues of the acidity index and higher values of thesaponification index comparing to ferrous oxide andstannous oxide catalysts.

The increase of catalyst concentration influencesfavourably the process of technical olein esterification withglycerol. The use of a glycerol excess to the stoichiometricratio influences in different way the content of free acidsand esters depending on the nature of the catalysts tested.By correlation of the acidity index with the saponificationindex it was evidenced the tendency of some testedcatalyst to favourize oxidation reactions of glycerol withoxygen from air which is present in the esterification vessel.

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Manuscript received: 1.02.2010