Kondüktametrik ve potansiyometrik titrasyon yöntemlerinin MFİ tipi zeolitlerin asit merkezlerinin karakterizasyonuna uygulanması
Başlık çevirisi mevcut değil.
- Tez No: 46383
- Danışmanlar: PROF.DR. AYŞE ERDEM ŞENATALAR
- 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: 60
Özet
ÖZET Son kırk yılda endüstride yaygın olarak kullanılan zeolit katalizörler, şekil-boyut seçicilikleri ve kuvvetli asit merkezleri ile katalitik uygulamalarda yerini almıştır. Bu tür katalizörlerin aktifliğini yüzeylerinde bulunan asit merkezlerin miktarı ve kuvveti belirlemekte, bu yüzden de asit merkezlerin sayısının belirlenmesi katalizör araştırmalarında oldukça önemli bir yer tutmaktadır. Zeolitlerin asit merkezlerinin belirlenmesinde birçok yöntem kullanılmaktadır. NMR ve İR spektroskopileri ile NH3-TPD yöntemleri bunlardan bazılarıdır. Ancak bu yöntemlerin zorluğu ve kantitatif analizlerde hassas sonuç alınamaması yeni yöntemlerin araştırılmasını zorunlu kılmıştır. Bu çalışmanın amacı kondüktometrik ve potansiyometrik titrasyon yöntemlerinin MFI tip zeolit katalizörlerin asit merkezlerinin belirlenmesinde uyguanabilirliğinin incelenmesi ve uygun çalışma aralığının belirlenmesidir. Bu amaçla Si/Al oranları, dolayısıyla da asit merkez sayıları farklı olan iki zeolit katalizör için iki ayrı seyreltme oranı ve üç ayrı bekleme süresinde 0.1 M NaOH kullanılarak kondüktometrik ve potansiyometrik titrasyon deneyleri yapılmıştır, örnekler kalsine edilmiş ve titrasyondan önceki karıştırma süresi 1 saate çıkarılarak aynı deneyler tekrar edilmiş ve asit merkez miktarları hesaplanmıştır. Kondüktometrik titrasyonda Si02/Al203 oranı 80 olan örnek için bazı deney koşullarında ortaya çıkan elektroforetik ve iyon relaksasyonu etkilerinin örnek kalsine edildiğinde ve/veya seyreltildiğinde kaybolduğu görülmüş, uygun koşullarda deney yapıldığında bekleme süresi etkisinin de ortadan kaybolduğu gözlenmiştir. Potansiyometrik titrasyonda da piklerde görülen dalgalanmalar, örneklerin kalsine edilmesi ile düzelmiş ve kalsine edilmiş örneklerin 5 dakikalık ölçümlerinden en yüksek asit merkez miktarı hesaplanmıştır. Deneyler kalsine edilmiş örneklerle yapıldığında potansiyometrik titrasyonun tüm koşullarında ve kondüktometrik titrasyonun 200 mi 5 dakikalık deneylerinde iki ayrı dönüm noktası olduğu görülmüş, bu durumda kondüktometrik ve potansiyometrik titrasyondan hesaplanan asit merkez miktarlarının birbirine yakın olduğu gözlenmiş ve 200 mi ve 5 dakikalık bekleme süresinin en uygun deney koşulları olduğu kanısına varılmıştır.
Özet (Çeviri)
THE APPLICATION OF CONÖUCTOMETRIC AND POTENTIOMETRIC TITRATION METHODS TO THE DETERMINATION OF THE ACID SITES OF MFI TYPE ZEOLITES SUMMARY Zeolites have found a wide range of industrial application areas as ion exchangers, adsorbents and catalysts. Although they were known since 1756, zeolites have been used in a large quantities for hydrocarbon conversion, size/shape selective catalysis, gas separation and purification as well as for ion exchange only after the first synthetic preparation of these aluminosilicate materials by Barrer and Milton in 1940's. Zeolites are alkali and alkaline earth metal aluminosilicates with the general formula; M2/nO. AI2O3. ySi02.wH20 where y is equal to or greater than two, M is an alkali or alkaline earth cation such as Na, K, Mg, Ca or Ba, n is the cation valence and w is the number of the water molecules contained in the voids of zeolites. Their structures are based on three dimensional network of AI04 and SİO4 tetrahedra which are linked to each other via doubly bridging oxygen atoms. VIBecause of the difference in charge between Al and Si, the negative charge of the AI02 units must be charge balanced by cation present in the structure to give electrical neutrality to the structure. In addition there are water molecules which are not covalently bonded to the lattice but are associated with the cations. The number of total exchangeable cation sites is determined by the silicon to aluminum ratio in the structure. Beside natural zeolites there are more than 200 synthetic zeolites that are synthesized and structurally identified some of them are counterparts to the natural zeolites whereas others have no natural analog. In the preparation of zeolites early investigators attempted to simulate the natural processes. The first synthetic zeolites were prepared by Union Carbide scientists based on the use of highly reactive aluminosilicate gels. Synthetic zeolites of ZSM-5, A, X, Y, L, Omega, Zeolon and natural zeolites such as mordenite, chabazite, erionite are the zeolites which are commercially utilized primarily as adsorbents, catalysts and ion exchangers in industry, agriculture and environmental protection. Zeolites are used very widely, particularly in the petroleum industry as catalysts. They are unique in their action as heterogeneous catalysts because they combine a size and shape selectivity controlled by the size of the channels together with a very strong catalytic activity. Probably the most widely known of the catalytic reactions of zeolites are those of zeolite ZSM-5 which is used in isomerization of n-paraffins and n-naphtenes, in polimerization of compounds containing an olefinic or acetylenic C to C linkage, such as isobutylene, in reforming and alkylation, in disproportination of aromatics, such as toluene to provide a mixture of benzene, xylene and methylbenzenes and finally in isomerization of polyalkyl substituted aromatics such as orthoxylene. The hydrogen form of ZSM-5 zeolites contains both Broensted and Lewis type acid sites in proportions that depend on the preparation, activation and modification conditions. VIIBroensted acidity is associated with the labile protons in the neigborhood of tetrahedrally coordinated lattice aluminum sites while Lewis acid centers are proposed to be associated with trigonal ly coordinated framework Al atoms formed by dehydroxylation of the Broensted hydroxyls or with extralattice AIO+ species the amount of which are determined by the synthesis and modification procedures. As the acidity of zeolite catalyst surfaces is related to their activity or selectivity, measurements of the acidity of zeolite catalysts have been of interest in catalyst research. Several techniques can be used for the characterization of the zeolites for the amount and nature of their acidity, such as NMR.IR and NH3- TPD methods. The microcaiorimetric and TPD studies of NH3 adsorption and desorption enabled estimation of the total concentration and strength of acid sites but not their types. On the other hand NMR spectroscopy is determined by the local magnetic environments around the nuclei and is extremely sensitive to short range ordering and structural effects but is much less affected by long range periodicities. From high resolution NMR spectroscopy in solution several critical pieces of information for the determination of molecular structures are obtained. A more direct study of the Broensted acidic OH groups in zeolites is possible by IR spectroscopy. The reaction with pyridine is used for this purpose. Adsorption of pyridine on ZSM-5 zeolites results in the appearance of IR bands characteristic of PyH+ or PyL complexes. However reliable quantitative information is difficult to obtain with this technique. While these techniques can, when used in combination, provide some VIIIdegree of information on the strength, number and distribution of the acid site present, the qualitative nature of many of the techniques means that it is often difficullt to obtain accurate quantitative data from them. Within this context, it is of interest to note that conductometric titration has been widely used by soil scientists for the determination of hydrogen ion concentrations in clays. However this simple technique has not been applied extensively to the measurement of Broensted acidity in zeolites. The purpose of this study is the application of conductometric and potentiometric methods to the determination of the acid sites of HZSM-5 type zeolites. For this purpose two catalysts with different Si/AI ratios were used. 0,2 ml portions of freshly prepared 0,1 N NaOH were added to a suspension of the zeolite (0,7 g.) in distilled water. For Valfor 8020 catalysts (Si/AI=40), 100 and 200 ml and Valfor 3020 catalysts (Si/AI=15) 200 and 300 ml of distilled water was used and with 2,5 and 10 min intervals, the conductance of the mixture was measured before each addition. Hydroxide addition was continiued until the readings indicated that the end point had been clearly reached. The potentiometric titration experiments were carried out under the same conditions. Samples were calcined at 500 °C for 5 h., after which the conductometric and potentiometric titration experiments were repeated. The effect of dilution and the intervals between the addition of portions of NaOH were studied for the original and calcined samples of both catalysts. In general an optimum dilution ratio and the time interval existed to obtain dependable values of acid sites from the turning points of the titration curves. IXElectrophoretic and ion relaxation effects were pronounced for concentrated solutions and diffusion limitations were thought to be present for the uncalcined samples especially for those with higher acid site concentration.
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