Metal Altlığın Isıtılması Yöntemiyle Zeolit Kaplamaların Hazırlanması

Abstract

Zeolitler, mikrogözeneklilikleri, düzenli kristal yapıları ve özgün kimyasal ve fiziksel özellikleri sayesinde adsorpsiyon, kataliz ve iyon değiştirme proseslerinde yaygın olarak kullanılmaktadır. Zeolitler, kafes yapılarında alüminyum, silis ve oksijen, gözeneklerinde ise katyon ve su içermektedir. Zeolitlerin önemi, özellikle son 50 yılda artmıştır ve yapılan çalışmalar sayesinde, gün geçtikçe artmaya devam etmektedir. Doğal zeolitlerin safsızlıklar içermeleri ve istenen gözenek çapı ve/veya geometrisine sahip olan doğal zeolit çeşitlerinin sınırlı olması sebebiyle yapay zeolit sentezine yönelinmiştir. Zeolitler, jel veya berrak çözeltilerden çeşitli yöntemlerle sentezlenebilmektedir. Kullanılan reaksiyon karışımı bileşimi, sentez sıcaklığı ve sentez süresi, sentezlenen zeolit türünü etkilemektedir. Bununla birlikte, zeolitlerin sentez mekanizması oldukça karmaşıktır ve hala tam olarak anlaşılamamıştır. A, X ve Y tipi zeolitler, yüksek hidrofilik özelliklere sahiptir ve endüstriyel açıdan önemleri gün geçtikçe artmaktadır. Bu zeolitlerin başta adsorpsiyon uygulamaları olmak üzere bazı özel uygulama alanlarında kullanılmalarına yönelik olarak metal altlıklar üzerinde kalın kaplamalarının hazırlanması ticari açıdan önemlidir. Ancak, metastabil yapılarından dolayı, söz konusu zeolitlerin kalın kaplamalarının tek aşamada hazırlanması kolay değildir ve bu önemli bir araştırma konusudur. Bölümümüz laboratuvarlarında, bu amaca yönelik olarak, yalnızca metal altlığın kondüksiyon ya da indüksiyon yoluyla ısıtıldığı, sentez çözeltisinin ise daha düşük bir sıcaklıkta tutulduğu bir kaplama yöntemi geliştirilmiştir. Bu çalışmada; metal altlıkların indüksiyon ile ısıtılması yoluyla geniş altlık yüzeyleri üzerinde kalın Zeolit A ve Zeolit X kaplamalarının hazırlanması hedeflenmiştir. Bu amaçla, bir reaktör ve kreostat arasında reaksiyon çözeltisinin sirküle edildiği bir deney sistemi kurulmuştur. Bu sistemde, istenen zeolit türünün elde edilmesi için gerekli sıcaklıklar üç parametre değiştirilerek, su deneyleri yardımıyla elde edilmiştir. Bu parametreler; indüksiyon cihazının gücü, pompanın debisi ve kreostat sıcaklığıdır. İstenen zeolit fazlarının sentezlenebileceği sıcaklıkların elde edilebildiği koşullar belirlendikten sonra, kaplama deneyleri gerçekleştirilmiş ve sentez süresinin, metal altlığın elek boyutunun, tekrarlı sentezlerin, ara besleme ile sentez çözeltisi eklemenin ve sentez çözeltisinin yaşlandırılmasının kaplama özellikleri (faz saflığı, kristalinite, ağırlık/kalınlık) üzerindeki etkileri incelenmiştir. Sentezlenen kaplamalara, Termogravimetrik Analiz (TGA) uygulanarak, adsorpladıkları nem oranları hesaplanmıştır. Gerekli görüldüğünde, X-Işını Kırınımı (XRD) analizi uygulanarak kaplamanın saflık ve kristalinitesi belirlenmiştir. Sentez süresinin artırılması ve sentez çözeltisine yaşlandırma uygulanması kaplama kalınlığı ve kristalinitesinde artış sağlamıştır. Tekrarlı sentez ve ara besleme yapılması da kaplama miktarında artış sağlamıştır. Yaşlandırma uygulanmadan yapılan sentez ile kıyaslandığında, çözeltinin sirkülasyonsuz olarak yaşlandırılması az da olsa kaplama ağırlığını artırırken, sistemde sirküle edilerek uygulanan yaşlandırma az da olsa kristaliniteyi artırmıştır. Altlığın elek açıklığı ve tek/çift katlı olması kaplama ağırlığını etkilemiştir.

Zeolites are microporous crystalline aluminosilicates, composed of AlO4 and SiO4 tetrahedra linked by common oxygen atoms. The zeolite framework is formed of cages and channels which can be defined as open cavities and these cavities are filled by water molecules and extraframework cations. Zeolites contain aluminum, silica and oxygen in the lattice structure; their pores which are uniform in size contain cations and water. The crystal structure of zeolites is three dimensional. The primary building units join together in various forms leading to secondary building units which in turn form the zeolite framework. Zeolites have specific properties owing to their large internal volume, large internal surface area, regular lattice structure and various geometries or chemical composition of framework. Zeolites can be used in many applications such as adsorption, ion exchange and catalysis processes owing to their unique physical and chemical properties. Owing to their high hydrophilicity, the importance of A, X and Y-type zeolites in the industry is increasing day by day. Preparation of zeolite coatings may also be important for commercial purposes, for example for use in adsorption applications. A type zeolite has low Si/Al ratio (approximately 1) and it is one of the most widely used type of zeolite as adsorbent and ion-exchanger. X and Y-type zeolites are named as faujasites. Faujasites have great importance for various applications and they are synthesized by using a variety of natural or synthetic aluminosilicate based materials. Faujasites are widely used in chemical industry especially in the petrochemical industry as catalysts and adsorbents. Zeolite X has low Si/Al ratio (between 1 and 1.5). Zeolite X has higher aluminium content than Zeolite Y therefore it contains more exchangeable cations in its pores. X-type zeolite crystals have wider pore openings than A type zeolite and it has the highest theoretical adsorption capacity and mass transfer rate in common zeolite adsorbents. Preparation of zeolite coatings to expand the applications of zeolite is an important research field in recent years. Although significiant amount of natural zeolite sources exist, these materials contain impurities and their physical and chemical properties vary according to their source and it is hard to obtain zeolites with desired pore diameter. Synthetic zeolites are more pure and homogeneous with respect to natural zeolites and this leads to choice of using synthetic zeolites in many commercial applications. Additionally, the presence of numerous synthetic zeolites with different pore size, physical and chemical properties and thermal stability facilitates to find convenient zeolite structure for the intended application. Zeolites are synthesized under hydrothermal conditions by using convenient reagents. The silica source is provided by sodium or other alkali silicate solution. Aluminum is provided by sodium aluminate, aluminum sulfate solution or hydrous aluminum oxide. Conventionally, synthesis is performed by crystallization of zeolites from alkali hydroxide and hydrous aluminasilicate gel mixtures in closed hydrothermal systems. Factors that affect the zeolite type formed are the composition of reaction mixture, type of the reagent and applied pretreatment, synthesis temperature, reaction time, pH of reaction mixture and type of operating system such as batch, continuous or semi-continuous. In conventional zeolite synthesis, gel composition is used as a reaction mixture. Instead of the gel composition, using clear solution provides significant advantages particularly in terms of the preparation of the coating. In order to synthesize zeolite crystals with narrow grain size distributions, controlled and homogeneous nucleation in the crystallization system is required. However, it is hard to provide all this when synthesis mixture is gel. In contradiction to synthesis from the gel mixture, nucleation doesn’t start in the clear reaction mixture, it occurs directly on support. Crystallization mechanisms of zeolites are still not fully understood. However, zeolite crystals obtained from clear solutions are almost uniform and nucleation may occur in a short time. Nuclei usually form with a rapid heterogeneous nucleation step in the liquid phase from impurities in the silica source. It is believed that zeolite crystallization from clear solution does not involve amorphous aluminasilicate formation. Competition between nucleation and crystal growth plays important role on properties of zeolite coatings such as homogeneity. Zeolite coatings are formed by adhesion to a surface of the zeolite by physical or chemical bonding. The preparation of zeolites in the form of coatings ensures more efficient heat and mass transfer and provides shortening of the diffusion path of reactants and easier access to the catalytic centers for various applications. Zeolite coatings may be used as membranes in separation processes, as catalysts in chemical reactions, as adsorbents in heat pumps or selective adsorbents in sensor technologies. Zeolite coatings may be prepared with a variety of single or multistep synthesis methods. It is hard to prepare thick zeolite coatings by conventional single step synthesis methods because of metastability, which means that unstable phases tend to transform into more stable phases. Multistep methods can be used for preparation of thick zeolite coatings but these methods are not useful and economical. Increasing the thickness of zeolite coatings are quite important for some special applications such as heating/cooling applications especially heat pumps. Synthesis methods for direct heating of supports were developed for the preparation of thick zeolite coatings in one-step synthesis particularly on metal surfaces. In these methods, temperature of metal support and solution can be controlled separately. In direct heating methods, the metal surface to be coated is directly heated by conduction or induction while synthesis solution is kept at lower temperature. With this temperature difference between metal support and solution, the reaction (nucleation and crystal growth) is provided to take place on the support surface preferably. Due to inhibition of crystallization in synthesis solution, by these methods, changes in solution composition and phase transformation resulting from the metastability can be delayed. Thus, they allow the preparation of thicker coatings on the support compared with conventional methods. Also with this method, as various parameters such as temperatures of support and reaction mixture, synthesis time, etc. are changed, coating properties can be varied. In this study, metal substrates were heated by using an induction device and it was aimed to prepare thick zeolite coatings. For this purpose, an experimental system, in which the reaction solution was circulated between a reactor and cryostat, was constructed. In this system, the required temperature for obtaining the desired type of zeolite was obtained by varying three parameters. These parameters were the power of the induction device, temperature of cryostat and the flow rate of the reaction mixture. After the determination of temperatures allowing the formation ofthe desired zeolite phases, coating experiments were performed and effects of the synthesis time, mesh size of metal support, performing repeated synthesis, intermediate feed and aging of the synthesis solution on coating properties (phase purity, crystallinity, weight/thickness) were examined. In the experimental system, preparation of Zeolite X and Zeolite A coatings on large support surfaces was intended. The materials used for synthesis were sodium hydroxide, sodium alluminate, sodium silicate and deionized water. For coatings of Zeolite X, two different synthesis compositions were used. These were 70 Na2O : Al2O3 : 15 SiO2 : 2100 H2O and 56 Na2O : Al2O3 : 12 SiO2 : 1680 H2O. For coatings of Zeolite A, 50 Na2O : Al2O3 : 5 SiO2 : 1000 H2O synthesis composition was used. Thermogravimetric Analysis (TGA) was applied to the synthesized coatings and the desorption capacities of the coatings were determined. When necessary, samples were characterized by X-Ray Diffraction (XRD) analysis and the formed phases were determined. Weight and crystallinity of coatings were increased by increasing time. Results of experiments that examined the effect of time were analyzed and it was determined that five hours are sufficient for preparation of sufficiently thick Zeolite X and Zeolite A coatings. Repeated synthesis promoted nucleation and crystal growth, thus the weight of coating was increased. When repeated synthesis was applied, coatings thicker than those obtained in the previous synthesis step were obtained in the next step. By feeding solution at an intermediate step during synthesis, the coating weight was increased. It is thought that better results might be obtained by investigating for the most suitable interval when the intermediate feed is to be supplied. Applying aging to synthesis solution resulted inan increase in the coating weight or crystallinity. It is believed that the effect of the aging period of solution should be investigated in more detail. It was observed that the mesh size of the support and having single or double layer of mesh clothing affected the coating weight although the temperatures of the solution at the entrance of the reactor and at the exit of the reactor were approximately the same in each case. This situation can originate from the geometric properties of the substrate as well as the different degrees of heating in the magnetic field for metal supports with different properties. Mesh openings and using single/double layer mesh can make a difference in heat transfer properties of supports and the amount of surface accessible to crystallized zeolite particles. According to the results obtained in this study, 60/100 mesh combination seems to be preferable because it requires relatively low induction power and it has more favorable geometric properties.