Oleik asit ile gliserin arasındaki esterleşme reaksiyonunda sulfatlanmış demir oksit kullanımı
Sulfated iron oxide catalysts for the esterification of oleic acid with glyerol
- Tez No: 46506
- Danışmanlar: PROF.DR. A. TUNCER ERCİYES
- Tez Türü: Yüksek Lisans
- Konular: Kimya Mühendisliği, Chemical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1995
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 66
Özet
Bu çalışmada oleik asit ile gliserin arasındaki esterleşme reaksiyon kinetiği 180-240 C sıcaklık aralığında süperasit karakterindeki sülfatlanmış demir oksit katalizör kullanılarak incelenmiştir. Laboratuvarda üretilen bir seri katalizör sülfat içerikleri, asit merkez miktarları, kristalinite, morfoloji ve yüzey alanı açısından karakterize edilmiştir. Reaksiyonlar ekivalent miktarda reaktan kullanılarak gerçekleştirilmiş ve her reaksiyon için reaksiyon mertebesi belirlenmiştir. Ayrıca katalizör miktarı ve SO, ~ içeriğinin reaksiyon kinetiği üzerindeki etkisi de araştırılmıştır. Çeşitli koşullarda katalizörün katalitik davranışı katalizör yüzeyindeki asit merkezlerinin karakterleri de gözonüne alınarak saptanmıştır. Katalizörsüz reaksiyonlarda üç olduğu bilinen reaksiyon mertebesi katalizör kullanarak gerçekleştirilen deneylerin tümünde iki olarak bulunmuştur. 1B0 ve 200 C'daki katalizörlü reaksiyonlarda oluşan ürünün sülfat anyonunun oluşturduğu kuvvetli asit merkezler tarafından desorplanmadığı ve reaksiyonun yalnızca zayıf asit merkezler üzerinde gerçekleştiği; 220 ve 240 C'da ise oluşan ürünün kuvvetli asit merkezlerce desorplandığı ve bu merkezlerin etkin oldukları belirlenmiştir. Bu nedenle bu reaksiyonda sülfatlanmış demir oksit katalizörü kullanarak yüksek dönüşüm elde edilmesi için 220 C ve daha yüksek sıcaklıklarda çalışılması gerektiği sonucuna varılmıştır. -v-
Özet (Çeviri)
Esterification reaction between fatty acids and palyhydric alcohols is an industrially important tool for obtaining the required polyhidric esters. Free acids obtained by splitting the trigliserides are fractioned with respect to molecular weight or degree of unsaturation and then reesterified with glycerol. Thus, a synthetic oil with improved properties may be produced by this way. Other widely used fatty acid esters are the partial glycerides and can be produced by direct esterification as well. Considerable amount of work has been done on the esterification of fatty acids with glycerol. In the most of these studies either with a catalyst or without a catalyst, the conditions necessary to obtain a specified partial glyceride in a high yield and to reach a lower acid value in a reasonable time period, were determined. The reaction was also investigated by several researchers with respect to its kinetics. In the present study the kinetics of esterification of oleic acid with glycerol is investigated by using the sulfated iron oxide as catalyst. Sulfated metal oxides have been studied in coal liquefaction process and acid catalyzed-reactions. Due to their unusual properties, the most important one of these being their so-called“superacidity”, sulfate promoted iron oxide seemed to be worth investigating in the present study, as an esterification catalyst in the reaction between glycerol and oleic acid. For this purpose, sulfated iron oxide catalysts were prepared, characterized and used in the above mentioned reaction. The reaction was carried out at different temperatures in 1B0-24D C range with equivalent amounts of reactants and the kinetics for each case was determined. Additionally, the effects of the catalyst amount and SD, ~ content on the reaction kinetics were investigated. The obtained results were evaluated in view of the catalytic behavior of sulfated iron oxide at different conditions by considering the nature of the acidic centers present on the catalyst surfaces. A series of sulfated iron oxide catalysts of different sulfate contents were prepared according to the -VI-homo-geneous precipifcsüi'on method. Appropriate amounts of ferric alum and urea were dissolved in distilled water with stirring at room temperature. The solution was heated to 95 C and kept at this temperature for 2 hours. The ammonium hydroxide formed during the heating period caused iron oxyhydroxide, FeDDH, to precipitate. The mixture was cooled to room temperature and the resulting precipitate was separated and washed with distilled water until no sulfate ion was observed in the washings. The precipitate was dried in an oven at 110 C for 24 hours for converting the oxyhydroxides into the stable oxides. The sulfated iron oxides thus obtained were stored dry and heated again to 450 C for 1 hour prior to their use as catalyst in the reaction. Detailed characterization was carried out on a sulfated iron oxide catalyst with a sulfate content of 5.10 % by weight. Other catalyst samples were only characterized for their sulfate content by the thermogravimetric method. Conductometric titration and NH“-TPD methods were used for the characterization of the acid-catalytic centers. 0.1 ml portions of 0.1 NaOH solution was used in the former method to titrate the catalyst suspension prepared by dispersing the catalyst in double distilled water. Acid site concentration was calculated from the base consumption corresponding to the turning point of the titration curve. A Shimadzu TGA-50 thermogravimetric analyzer was used for NbU-TPD analyses. Initially, the sample was activated by heating to 450 C with heating rate of 10 C/ min under a nitrogen flow of 45 cc/min. Activation was carried out at this temperature for 1 hour. The sample was then cooled and NH, adsorption was carried out at 30 C, Far the desorptian of physically bonded and weakly chemisorbed ammonia the sample heated to 170 C and was flushed at this temperature for 1 hour with a nitrogen flow of 45 ml/min. The temperature was then raised to 400 C at a rate of 10 C/min and kept at this value for 1 hour for the complete removal of strongly chemisorbed ammonia. The concentration of strong acid sites was calculated from the weight loss that took place in the temperature range of 170 to 400°C. The sulfate content was determined by both TGA (Shimadzu TGA-50) and gravimetric methods. In the former method, the sample was heated up to B00 C with a heating rate of 20 C/min under a nitrogen flow of 50 ml/min. The sulfate content was determined from the weight loss recorded within the heating period. -vii-For the gravimetric determination, sulfated iron oxide was dissolved in HC1 solution and the sulfate was precipitated as BaSO, with BaCl”solution. Sulfate percentage was calculated from the weight of BaSO, precipi tate. The presence and nature of the sulfate groups were also confirmed by IR spectroscopy using a Mattson 1 000. FTIR equipment in the wavenumber range of the 400-4000cm.. For this purpose the catalyst sample was homogeneously mixed with KBr, which was previously dried at 110 C, at a ratio of 1/30D, and the mixture, was pressed into wafer form under a pressure of 10 kg/cm. Specific BET surface area of the catalyst was determined by nitrogen adsorption at -196 C. A constant volume adsorption system was used for _Uiis purpose. The sample was heated under a vacuum of 10~ mbar to 400 C at a rate of 2 C/min and was activated at this temperature for 1 hour prior to the adsorption experiment. Scanning electron micrographs were taken using a JEO L-0SM-T330 scanning electron microscope. Samples were coated with carbon in a JED L-3EF-4X vacuum evaporator. Finally, a Rigaku X-ray dif f actometer was used to obtain to XRD patterns in the 5-75° 29 range with CuKoc radiation, at 50 kV and 20 mA. A scan rate of lD/min was used. Esterif ication reaction was carried out in a fourneeked flask equipped with a stirrer, a thermometer and an air condenser. In the reaction, glycerol and oleic acid were used in equivalent proportions. Oleic acid was placed into the reaction flask and heated under agitation to the reaction temperature. Glycerol was heated separately to the reaction temperature and then added to the reaction flask. For catalyzed reactions a given amount of sulfated iron oxide catalyst was initially added to the oleic acid. The stirring rate was adjusted to 200 rqm, and nitrogen was passed over the surface of the reaction mixture at a rate of 200 ml/min to provide an inert at mosphere and to remove water. Samples were withdrawn at predetermined time intervals and cooled immediately by immersion into cold water. Acid values of the samples were determined. Oleic acid concentration was expressed in term of weight percentage as determined from the acid value and equivalent weight of the acid. A correction for loss of water was applied to each sample. -viii-The sulfate contents of the four catalyst samples prepared in this study were determined thermogravi- metrically to be 2.44, 3.47, 5.10 and 7.62% by weight. The weight loss due to the sulfate groups took place above 5DDDC with the temperature program employed. The sample with % 7.62 sulfate was also analyzed gravimetrically and from the weight of the BaSO, precipitate, its sulfate concent was determined to be 7.20%. The values obtained by both methods are seen to be sufficiently close. The amounts of strong and weak acid sites were determined to be 0.4B4 mmole/g cat. and 0.668 mmole/9 cat., respectively, from conductometric titration. The sample was calculated to contain D.531 mmole/g cat. of strong acid sites, assuming that a sulfate group is associated with each strong site. The presense of sites of different strenght was also evident in the TPD of ammonia. The concentration of strong acid sites determined by this method, from the amount of NH, desorbed in the temperature range of 170-400QC, was 0.647 mmole/g cat. The fact that this value is slightly higher may be related to the tailing of the lower temperature peak, due to the continuing desorption of NH- adsorbed on weaker sites, under the temperature program applied. Accurate quantification of the weak acid sites was not possible by NH,-TPD, since it proved to be difficult to separate the weakly chemisorbed NH, from the amount physically adsorbed on the catalyst surfaces. The IR spectrum of the catalyst contained adsorption bands at 1219, 1137, 10B0, 103B, 1000 and 920 cm“ wavenumbers, indicating a bidentate chelating complex structure for the sulfate groups with the metal ions. The sample was concluded to be highly amorphous from the XRD analysis. Fram the SEM micrographs aggregates in the size range of 1-6 urn are seen to exist, which consist of much smaller particles, with sizes below about 100 nanometers. The”BET surface area of the sample was determined to be 9B m /g cat. from the N" adsorption isotherm. All the catalyzed reactions between oleic acid and glycerol were found to follow second order kinetics at all temperatures under the conditions used in this study, whereas it was known from previous studies the uncatalyzed reaction follows a third order kinetics under the same conditions. -IX-The reaction rates were observed to increase with the amount of catalyst used and the sulfate content of the catalyst at 220 and 240DC, whereas at 1B0 and 200 C the rates were independent of both parameters. The reaction rate constants obtained are given in the following table together with the information about each experiment. TABLE 1: Results of Catalyzed and Uncatalyzed Reactions. -1 -1 (a) Unit of rate constant is (wt%) ( min ) for second order and (wt%)~ (min)~ for third order. (b) These results were taken from previous studies for comparison. the o, The results were interpreted to indicate the use of strong acid sites, the number of which depend on sulfate content, only at the temperatures 220 and 240LJC. The reaction seems to be catalyzed by only the weak acid sites at 1B0 and 200 DC experiments. Adsorption is thought to be too strong at lower temperatures, limiting the desorption of the products from the strong sites of the catalyst under these conditions. -x-
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