Geniş Metal Destekler Üzerinde Zeolit Kaplamaların Hazırlanması Ve Karakterizasyonu

Abstract

Zeolitler, moleküler boyutta kanallar ve/veya kafesler içeren sulu, kristal yapılı alüminosilikat mineralleridir. Adsorban, iyon değiştirici ve katalizör olarak çok çeşitli uygulamalarda kullanılmaktadırlar. Zeolit kaplamaların hazırlanması zeolitlerin kullanıldığı uygulamaları arttırmıştır. Zeolit kaplamalar, membran olarak ayırma işlemlerinde, katalizör olarak mikroreaktörlerde ve adsorban olarak soğutma/ısıtma uygulamalarında ve adsorpsiyonlu ısı pompalarında kullanılmaktadırlar. Farklı altlık türleri üzerinde hazırlanan zeolit kaplamalar, ısı ve kütle iletimini olumlu yönde etkilemektedir. Bu amaçla bölümümüz laboratuvarlarında geniş metal yüzeyler üzerinde kalın zeolit kaplamaların hazırlanması çalışılmaktadır. Kaplanması istenen altlığın doğrudan ısıtılıp, sentez karışımının soğutulması ile kalın zeolit kaplamalar hazırlanabilmektedir. Bu yöntemde, zeolit büyümesinin altlık üzerinde olması sağlanmaktadır. Bu çalışmada metal altlığın indüksiyon ile ısıtılması yöntemiyle zeolit kaplamalar hazırlanmıştır. Bu amaçla, indüksiyon cihazı, reaktör, kreostat ve pompadan oluşan bir deney sistemi kurulmuştur. Kurulan deney sisteminde, öncellikle sistem parametrelerinden indüksiyon gücü, debi, kreostat sıcaklığı ve metal elek türünün reaktör giriş ve çıkış sıcaklıklarına etkileri incelenmiştir. Daha sonra kaplama deneyleri gerçekleştirilmiş ve debi, indüksiyon gücü, sentez karışımı bileşimi ve metal elek türünün kaplama ağırlığı, dayanıklılık ve kristalinite üzerindeki etkileri incelenmiştir. Kaplamalardan alınan zeolit örneklerine termogravimetrik analiz (TGA) uygulanmış ve kristalinite tahmini yapılmıştır. Gerekli görüldüğünde X-ışını kırınımı (XRD) uygulanmış ve bu yolla elde edilen zeolit fazının saflığı ve kristalinitesi kontrol edilmiştir. Kullanılan sentez karışımı bileşiminin H2O ve Na2O içeriği ile SiO2/Al2O3 oranının azaltılması yoluyla kaplama ağırlığının (ve dolayısıyla da kalınlığının) ciddi miktarda artırılması sağlanmıştır. Ancak, karışım bileşimi jel bileşimlerine fazla yaklaştığında kaplama dayanıklılığının azaldığı görülmüştür. Debinin etkisinin incelenmesi için, denenenler arasından seçilen bileşimler kullanılarak yapılan deneylerde, debi artışının, hem kaplama ağırlığını, hem de kaplama dayanıklılığını artırdığı belirlenmiştir. Ancak, debinin artışıyla kaplamanın su tutma kapasitinde az da olsa bir miktar düşüş görülmüştür. Bu düşüş kaplamada, Zeolit X’in yanısıra az miktarda Zeolit A ve Zeolit P gibi fazların da bulunmasıyla ilgili olabilir. İndüksiyon gücünün artması ise kaplama ağırlığını olumlu, kaplamanın kristalinitesini olumsuz etkilemiştir. Çift katlı metal elek altlıklarda, dış tabakadaki tel kalınlığı inceldikçe, gerekli sentez sıcaklıklarına ulaşabilmek için indüksiyon gücünün artırılması gerekmiştir. Dış tabakadaki tel kalınlığındaki artışa paralel olarak, aynı reaktör giriş ve çıkış sıcaklıklarında elde edilen kaplama ağırlığı da bir miktar artmıştır.

Zeolites are microporous hydrated aluminosilicate crystals with unique properties which enable them to be used in a variety of industrial applications. About forty different types of zeolite minerals are found to occur in nature and more than 150 types of zeolites have been prepared synthetically. Synthetic zeolites have been found to be more suitable for many industrial applications when compared to natural zeolites, since the latter generally contain impurities varying in their amounts and types with respect to the location in the reserve. Alkali and/or alkaline earth metal cations reside in the micropores of the zeolite crystal structures, which can be exchanged with other cations in aqueous solutions of a number of salts, giving zeolites the property of selective ion-exchange. The ion-exchange property of zeolites was recognized very early in their development, and basically this was the only property that found industrial applications until the 1950s. The major industrial use of zeolites during these years was as ion-exchangers in water-softening processes. Today, zeolites are employed for a variety of ion-exchange applications, such as the removal of radioactive ions, heavy metal ions and NH4+ ion from different wastewaters. The microporous crystal structures of zeolites contain high void volumes and large internal surface areas, making them suitable for adsorptive applications. The pore diameters are on the same order of magnitude with commercially important molecules, giving zeolites their shape and size selective properties. This is the reason why zeolites are also called as zeolite molecular sieves. The size and shape selective properties of zeolites have been made use of in many adsorptive separation and purification applications. O2/N2 separation, linear/branched alkane separations, separation of xylene isomers, and removal of water, impurities and pollutants from various gas mixtures are some examples to the processes in which zeolites are used as adsorbents. The surfaces of zeolites may also contain strong catalytic sites depending on the Si/Al ratio of the zeolite, which allow them to be used as shape/size selective catalysts for various reactions in the industry, especially the petrochemical industry. Cracking, alkylation and isomerization reactions are some examples of the different groups of reactions, for which zeolites are used as catalysts. Most of the gasoline is produced today in the fluidized catalytic cracking (FCC) units utilizing zeolite cracking catalysts. In addition to the conventional applications of these materials, which employ pellets or granules of zeolites synthesized in powder form, new applications of zeolites prepared in coating configurations have been emerging recently. Preparation of zeolite coatings on various substrates to be used in novel membrane separations, catalytic microreactors, adsorption heat pumps and other adsorptive heating-cooling applications as well as in sensing applications have become new research areas of widespread interest. Improved heat and mass transfer is one of the main advantages of utilizing zeolite coatings in these applications. While some of these, such as the membrane separation and sensing applications require the preparation of thin zeolite coatings, others, such as the adsorptive heating-cooling applications require the preparation of relatively thick zeolite coatings. Preparation of relatively thick zeolite coatings have not been easy, especially for the hydrophilic aluminous zeolites, which are especially required for these applications. A method has recently been developed for the preparation of relatively thick zeolite coatings on metal substrates, where the substrate is heated selectively by conduction or induction, while the synthesis mixture is kept at a lower temperature. Since only the crystallization activity on the substrate is promoted in this method, while that in the synthesis solution is suppressed, the rate of change of the composition of the synthesis solution with time is decreased and the metastable phase transformations of the zeolites to more stable closed phases are postponed, allowing the preparation of thicker zeolite coatings. Zeolite coatings were prepared in this study on the surfaces of relatively large metal substrates, which were heated by induction. An experimental setup, consisting of an induction device, a reactor, a pump and a cryostat was assembled for this purpose. The pump was used to circulate the synthesis solution between the reactor and the cryostat, in order to keep its temperature at a value lower than that of the substrate. The temperatures of the synthesis solution entering and exiting the reactor were measured by resistance thermometers. Cylinders made of one or two layers of stainless steel (316L) wire mesh sheets of different mesh sizes were used as substrates. Initially the effects of the system parameters, namely the power of the induction device, the flow rate of the solution, and the temperature of the cryostat, on the inlet and exit temperatures were investigated using water instead of the synthesis solution. Later, with the information obtained from these experiments, coatings of Zeolite A and Zeolite X were prepared at temperatures required to crystallize these zeolites. Effects of the synthesis mixture composition, induction power, flow rate of the synthesis solution and the substrate type on the mass (thickness), crystallinity and mechanical stability of the coatings prepared were investigated. Coatings were weighed after being brought to constant weight under controlled humidity and samples taken from the coatings were analyzed by thermogravimetric analysis (TGA) and X-ray diffraction (XRD) techniques. The amount deposited on the substrate and thus the thickness of the coating prepared was observed to increase parallel to the decrease in the H2O and Na2O contents and the SiO2/Al2O3 ratio of the synthesis mixture, while mechanical stability was observed to decrease with the decrease in the SiO2/Al2O3 ratio, for the Zeolite X coatings. Both the mass and the stability of the coating prepared increased with increasing flow rate for similar inlet and exit temperatures, while the adsorption capacity of the coating was observed to decrease with increasing flow rate due to the formation of small amounts of Zeolite A and Zeolite P. Increase in the induction power also increased the mass and thus the thickness of the coating prepared, while it decreased its crystallinity. When a higher mesh size was used on the outer side of the two-layered substrates, a higher induction power was found to be necessary to heat the substrate to the required synthesis temperature. As the wire thickness on the outer layer increased, the mass of the coating obtained at similar inlet and exit temperatures also increased.