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Gözenekli destekler üzerinde zeolit kaplamaların hazırlanması ve karakterizasyonu

The Preparation and characterization of zeolite a coatings on prous support

  1. Tez No: 46504
  2. Yazar: FİLİZ ÖMEROĞLU
  3. Danışmanlar: PROF.DR. AYŞE ŞENATALAR ERDEM
  4. Tez Türü: Yüksek Lisans
  5. Konular: Kimya Mühendisliği, Chemical Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1995
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 76

Özet

DZET Moleküler boyutlarda mikrogözenekli malzemeler olan zeolitlerin şekil/boyut seçici adsorban, katalizör ve iyon değiştirici olarak alternatifsiz oldukları bilinmektedir. Son yıllarda gözenekli veya gözeneksiz destekler üzerinde, membranların, basınç düşüşü az reaktörlerin ve kimyasal gaz sensörlerinin geliştirilmesine yönelik olarak kullanılabilecek, bir kristal tabakasından oluşan zeolit kaplamaların hazırlanabileceği görülmüştür. Bu çalışmanın amacı, uygun destekler üzerinde zeolit A kaplamalarının hazırlanması ve karakterizasyonudur. Bu amaçla, farklı koşullarda, biri jel diğeri berrak çözelti karakterinde iki farklı bileşim kullanılarak, sentez deneyleri yapılmıştır. Sentez deneyleri 3D mi kapasiteli PET şişelerde sodyum silikat, sodyum aluminat, sodyum hidroksit, alüminyum oksit, silika reaktanları ile paslanmaz çelik ve alumina altlıklar kullanılarak gerçekleştirilmiştir. Destek yüzeyinin sentez çözeltisi içindeki konumunun, sürenin ve ultrasonik banyo ortamının etkileri incelenmiştir. Hazırlanan kaplamalar taramalı elektron mikroskopisi, termogravimetrik analiz ve geçirgenlik deneyleriyle karakterize edilmiştir. Bu amaçla bir geçirgenlik sistemi tasarlanmıştır. Yapılan sentez deneylerinin sonucunda, berrak çözelti sentez bileşimi kullanılarak, sentez karışımı içinde düşey tutulan paslanmaz çelik teller üzerinde sürekli zeolit A kaplamaları hazırlanabilmiştir. Geleneksel jel içeren sentez karışımları ve destek yüzeyinin yatay tutulması, bu tip kaplamaların hazırlanması için uygun bulunmamıştır. Ultrasonik titreşimlerden, kristal çekirdeklenme ve büyüme hızlarının arttırılmasında ve film kalınlığının denetlenmesinde yararlanılabileceği görülmüştür. TEA yardımıyla, metal üzerinde büyütülen filmin %100 kristalin olduğu belirlenmiştir. Hazırlanan kaplamalar basınç düşüşü az adsorban yatakların ya da reaktörlerin geliştirilmesine yönelik çalışmalarda kullanılabilir. Ancak geçirgenlik deneyleri, membran olarak kullanılabilmeleri için, tellerin birbirine temas ettiği noktalardaki boşlukların kapatılmasına yönelik olarak filmin kalınlaştırılması gerektiğini göstermiştir. -v-

Özet (Çeviri)

SUMMARY THE PREPARATION AND CHARACTERIZATION OF ZEOLITE A COATINGS ON POROUS SUPPORTS. Zeolites are hydrated, microporous crystalline aluminosilicates, commonly used as adsorbents, catalysts and-ion exchangers, which may be represented by the following empirical formula. M2/n0, A12Q3* xSi02* y H2° In this oxide formula, x is generally equal to or greater than 2..Since A1S, tetrahedra are known to be joined only to SiO, tetrahedra, n is the cation valance. Zeolites are constructed from TÛV tetrahedra (T=tetrahedral atom, e.g., Si, Al). The framework contains channels and interconnected voids which are occupied by cation and water molecules. Because of the difference in atomic charge“ between Al and Si, extra-lattice cations (M ) must be present to preserve electrical neutrality, a single positive change needed for each (A10”) unit. These cations are not part of the framework and may be easily exchanged. There are natural zeolites, synthetic analogues of natural zeolites and synthetic zeolites with no natural counterparts. The most important properties of zeolites that form the basis to their induslrial applications are their ability to exchange ions, adsorb gases and liquids. and catalyze reactions. Zeolites are useful as adsorbents primarily, because they (i) are molecular sieves, (ii) contain large void fractions (zeolites A and X have almost a 50 % void fraction), and (iii) most are hydrophilic. Zeolite adsorbents are used in both purification and bulk saparation operations:. -VI-The most important reason of zeolites being used as catalysts and catalyst supports is that their catalytic properties can be tailored by dealumination, decationization/cation exchange, hydration/dehydration and other modifications. Zeolites are also used as ion-exchangers. The cation-exchange behavior of zeolites depends on the characteristics of the zeolite-cation pair and the temperature. Preparation of zeolitic coatings on porous/non-porous supports for separation and catalysis applications, is a novel and important research topic of the recent years. Zeolitic coatings are used to develop zeolitic membranes, low-pressure drop reactors and chemical gas sensors. Most of the limited number of studies carried out on the preparation of zeolite membranes has concentrated on the growth of high silica or pure silica zeolite membranes of ZSM-5/silicalite type due to their hydrophobic character and ease of the synthesis conditions. Similar studies related to other zeolites that bear commercial importance in separation and catalytic applications are seen to lack in the literature. If the zeolitic coating is continuous and is prepared on a porous support, it can be used for the development of zeolitic membranes. Otherwise, it can be used for the development of low-pressure drop reactors or adsorption beds. Zeolites have had limited applications as membrane materials for continuous separation processes- Only in recent years have studies demonstrated the potential of zeolites as filling materials for improving the performance of polymeric membranes. Zeolite membranes provide in principle a means of continuous separation based on differences in the shapes and sizes of molecules. If they contain catalytic centers they can also be envisaged as membrane reactors. Membrane separation technology is a rapidly expanding field. So far, a number of organic and inorganic materials with pore sizes ranging from ID to 1DDD A have been used in a variety of membrane separation processes, such as microfiltration, ultrafiltration, dialysis, -vii-electrodialysis, reverse osmosis and gas permeation. Ze olites with less than 10 A pores are also desirable membrane materials, due to their crystallinity, resistance to high temperature and chemical inertness. For many applications, zeolite crystals are compounded with an amorphous matrix due to obtain particles with sizes on the order of magnitude of a milimeter, to prevent an intolerably high pressure drop across the reactor and fast plugging of the catalyst bed uith dust. Unfortunately, however, this counteracts the advantage of the high zeolite activity because of mass transfer limitations in the particles. Zeolite coatings can be used in the development of low-pressure beds for such adsorption and catalysis applications, especially when dust-proof systems are necessary and diffusion limitations are significant. Legein et al. - in their study on the application of zeolite coatings as structured catalysts for low-pressure drop reactors-observed that both the selectivity and activity of the Z5M-5 crystals grown on metal wire gauze proved to be superior to conventional vanadia /titania catalysts. Zeolite coatings can also be used in the development of chemical gas sensors. The two main methods for the preparation of supported zeolites are: 1) Covering an existing support with a zeolite layer by contacting it with a zeolite slurry (Dip-coating technique), and 2) Growth of a layer of zeolite crystals on an existing support by in-situ zeolite synthesis. Total coverage of an existing support with a closed layer of zeolite crystals will hardly ever be obtained by dip-coating. Main problems for the second method are the formation of a dense layer of crystals, their mutual orientation and prevention of the occurence of non- covered support areas. After the desired period of time, the substrate coated with crystalline material, is removed from contact -Vlll-with the chemical mixture, washed with distilled water and allowed to dry. The time of contacting of the surface with the reaction mixture (for ZSM-5 coating) may be from about G.5 hrs. to about 1DDD hrs., preferably from about lhr. to about 10D hrs.; at a temperature of from about 50 C, to about 250DC., preferably from about 110°C. to about 200°C.; and at a pressure from about 1 atm. to about 100 atm., preferably from about 1 atm. to about 15 atm. Zeolitic coatings can be produced on non-porous substrate forming surfaces such as polymer, silver and nickel surfaces or porous substrates such as alumina membranes and stainless steel sieves. The layer of crystalline material may be removed from the non-porous surface by various means. The layer may be separated from polymeric surfaces, for example, by mechanical means such as scraping or peeling. Removal from metal surfaces may be accomplished with the use of solvents such as acetone or by dissolving the metal with an acid such as aqueous hydrochloric or nitric acids. The purpose of this study is the growth of zeolite A coatings by hydrothermal crystallization on porous supports and their characterization. Synthesis runs were carried out at the temperatures of 65, B5 and 52 C and at different times. The reactants used for zeolite A synthesis were sodium aluminate, sodium silicate, sodium hydroxide, aluminum oxide, silica (fumed) and deionized water. Two different types of zeolite A synthesis mixtures compositions were used. One was a conventional zeolite A synthesis composition which forms gel on mixing and the other was a“clear”solution compositions. 2 Na2D. A1203. 2Si02. 143 H20 (gel) 50Na2D. A12G3. 5SiD2. 1000 H“0 (clear solution) Synthesis experiments were carried out in 30 ml. PET autoclaves in both static and ultrasonic conditions. Teflon holders were used for adjusting the position of the porous alumina and stainless steel supports. The synthesized zeolite coatings were characterized by scanning electron microscopy (SEM), thermogravimetric -IX-analysis (TGA) and permeation experiments, Continuous zeolite A coatings could be prepared in this study from ”clear" solution synthesis mixtures on stainless steel wire gauze supports hold vertically in the reaction media. The conventional zeolite A gel compositions and horizontal positioning of the supports were observed to be unsuitable for the preparation of zeolite coatings. Nucleation and growth rates of the crystals in the coating could be controlled by the use of ultrasonic vibrations. The zealitic coatings prepared after appropriate periods of reaction time were observed to be highly crystalline, as determined from the weight loss taking place in thermogravimetric analyses. The zeolite coatings prepared in this study can be used in low-pressure drop configurations for dust-proof adsorption and catalysis applications, especially for those in which micropore diffusion rates may be limiting. From the permeation data, further thickening of the prepared coatings was concluded to be necessary for membrane applications to fill the remaining voids between the wires in the gauze. -x-

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