EMT zeolitinin sentezi ve farklı ıyon formlarının su tutma kapasıtesı üzerine etkisi
Synthesis of EMT zeolite and the effect of fifferent ion forms on water holding capacity
Yazar: SEDAT DELEN
Danışman: DOÇ. DR. ÇİĞDEM ORAL
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Kimya Mühendisliği Ana Bilim Dalı / Kimya Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2023
Konu: Kimya Mühendisliği = Chemical Engineering

Özet:
Zeolitler yapılarında alkali veya toprak alkali elementler bulunduran kristal yapıya sahip alüminosilikatlarıdır. Zeolit kelimesi ilk kez İsveçli araştırmacı Axel F. Cronstedt tarafından kullanılmıştır. Toz veya pelet şeklinde elde edilen zeolitler başlıca olarak iyon değişimi, kataliz, adsorpsiyon ve ayırma gibi önemli uygulama alanlarında kullanılmaktadır.
Enerjiye olan ihtiyacın artması ve enerji kaynaklarının daha verimli kullanımının hedeflenmesiyle çevre duyarlılığı yüksek alternatif kaynaklara yönelim artmaktadır. Adsorpsiyon ısı pompaları, mekanik ısı pompalarıyla kıyaslandığında çevre dostu olup endüstriyel atık ısının tekrar kullanımını sağlamaktadır. Toz veya pelet halinde adsorban kullanıldığında adsorpsiyonlu ısı pompalarında verim oldukça düşmektedir. Zeolit gibi bir adsorbanın metal yüzeyler üzerinde kaplama şeklinde kullanılmasıyla, ısı ve kütle iletimindeki gelişmelerden dolayı çevrim süreleri çok kısalmaktadır. Adsorpsiyon ısı pompalarında, X, Y, A ve SAPO34 gibi su tutma kapasiteleri görece yüksek zeolitlerin kullanımı uygun gözükmektedir. Ayrıca düşük sıcaklıklarda rejenere olabilen zeolitlerin kullanımı daha avantajlıdır. Zeolitlerin su tutma kapasitesi bu malzemeleri değişik iyon formlarına getirerek de değiştirilebilir.
Bu çalışmada, öncelikle literatürde molar bileşimi bilinen bir reaksiyon karışımından organik bir yönlendirici kullanılarak EMT tipi toz zeolit Na formunda sentezlenmiş ve ardından farklı metal tuzlarının sulu çözeltileri kullanılarak değişik iyon formlarına getirilmiştir. EMT zeolitinin orijinal Na ve diğer iyonik formlardaki su desorpsiyon kapasiteleri termogravimetri (TG) ile belirlenmiştir. İkinci aşamada İTÜ Kimya Mühendisliği Bölümü Adsorpsiyon ve Kataliz Laboratuvarında daha önce yapılmış ön çalışmalardan yararlanılarak EMT zeolit kaplamalarının hazırlanması için değişik reaksiyon karışımı bileşimleri ile denemelere başlanmış ve sonrasında bazı sentez koşulları değiştirilerek incelemelere devam edilmiştir. Seçilen uygun kristalin ve kalınlıktaki kaplamalar da iyon değişimine tabii tutulmuştur. Elde edilen kaplamalar TG, N2 adsorpsiyonu, X-ışını kırınımı (XRD) ve alan emisyon tabancası taramalı elektron mikroskobu (FEGSEM) ile karakterize edilmiştir.
Çalışmalar sonucunda berrak reaksiyon karışımlarından nispeten yüksek kalınlığa sahip kristalin EMT zeoliti içeren kaplamalar elde edilmiştir. TG analizinden elde edilen sonuçlara göre toz ve kaplama formundaki EMT zeolitlerinin iyon değişimi yapıldıktan sonra kapasitelerinde önemli artışlar olduğu gözlenmiştir. İyon değişimi sırasında kaplamaların mekanik stabilitelerinin yüksek olduğu görülmüştür

Abstract:
Zeolites are crystalline aluminosilicates containing alkali or alkaline earth elements in their structure. The word zeolite was first used by Swedish researcher Axel F. Cronstedt. Zeolites obtained in powder or pellet form are mainly used in important application areas such as ion exchange, catalysis, adsorption and separation.
Zeolites are classified according to their crystal structure, chemical composition, effective pore diameter and formation. The Si/Al ratio is an important property of zeolites. The Si/Al ratio is inversely proportional to the cation concentration, but directly proportional to the thermal stability. As this ratio increases, the surface selectivity changes from hydrophilic to hydrophobic. Based on the number of atoms that define the pore openings, zeolites are classified as small, medium, large and extra-large, with rings of 8, 10, 12, and larger than 12, respectively.
During the eighteenth and nineteenth centuries, some researchers reported the discovery of new minerals classified as zeolites, as well as the description of some of their basic properties, and zeolites began to be applied in various sectors of industry. For instance, in 1857 A. Damour observed that when different natural zeolites desorbed water, there was no visible change in the transparency and morphology of their crystals. In 1858, German chemist H. Eichhorn reported reversible ion exchanges in natrolite and effortzite zeolites. In 1862, Henry Etienne Sainte-Claire Deville performed the first synthesis of levinite zeolite by heating an aqueous solution of potassium silicate and sodium aluminate at 170°C in a glass tube.
After the 1950s, the zeolitic "industrial era" began. Research on the properties of these materials increased, and the potential uses of zeolites in industrial processes became more apparent given the large number of patents on synthesis and applications that emerged during this time.For example, several types of zeolites with higher silicon content have been introduced, such as zeolite Y (FAU) by Union Carbide Corporation and highly porous mordenite marketed as Zeolon by Norton Company. It was accomplished in 1972 by Mobil Oil Corporation by synthesizing the zeolite ZSM-5 with various extraordinary properties. Compared to natural zeolites, synthetic zeolites have gained much more importance and application in the modern world due to their higher selectivity and purity, and modifying methods have been proposed to expand the application areas of natural zeolites.
Zeolites can be used in many fields such as gas and wastewater separation, construction, catalyst, medical uses, food, agriculture and pollutant removal. The pollutant types in wastewater are suitably used to remove ammonium, phosphorus, heavy metals, inorganic anions and organics from wastewater or gas processes. The use of modified zeolites is considered.
Since the 1960s, zeolites have been applied as catalysts in a variety of industrial chemical reactions (cracking, isomerization and alkylation reactions) for the production of chemicals, dyestuffs and fragrances. Due to the desirable properties of zeolites such as unique pore size, large surface areas, crystallinity and thermal stability, most reactions can be catalyzed.
There are commercial products based on zeolite that are said to cause "instant" bleeding when applied to wounds (in accidents or surgeries). Zeolites are also used in dialysis machines to absorb ammonia from the blood and prevent its accumulation in the body.
Zeolites are important materials with wide applications in agricultural engineering. It has been found that the incorporation of zeolite in soil increases crop yield and increases nutrient utilization efficiency.
Research on zeolite synthesis has a history of more than 100 years since the discovery of zeolites. During this process, not only did the types and quantities of zeolites increase, but also the synthesis methods were continuously improved. To give an example of known zeolite synthesis methods, Hydrothermal, Ionothermal, Solvothermal, Alkaline Fusion, Sol-Gel and Microwave methods can be mentioned.
With the increase in the need for energy and the more efficient use of energy resources, the tendency towards alternative sources with high environmental awareness is increasing. Compared to mechanical heat pumps, adsorption heat pumps are environmentally friendly and enable the reuse of industrial waste heat. When adsorbent is used in powder or pellet form, the efficiency of adsorption heat pumps decreases considerably. With the use of an adsorbent such as zeolite as a coating on metal surfaces, the cycle times are shortened due to the improvements in heat and mass conduction. It seems appropriate to use zeolites with relatively high water holding capacities such as X, Y, A and SAPO34 in adsorption heat pumps. In addition, the use of zeolites that can regenerate at low temperatures is more advantageous. The water holding capacity of zeolites can also be changed by bringing these materials into different ion forms.
EMT zeolite is a hexagonal polymorph of the cubic Faujasite (FAU) zeolite and has two types of lattices: a hyperlattice and a hypocache. Since pure EMT zeolite exhibits better catalytic performance than FAU zeolite, there is increasing interest in this large-pored mineral with high potential. Cubic FAU and hexagonal EMT structures contain the same structural layers of sodalite lattices; therefore, EMT often co-crystallises with the FAU phase.
The synthesis of EMT zeolite requires the use of an expensive and toxic compound known as an organic diverter, and its synthesis takes place over a long period of time. Therefore, many efforts have been made to reduce the consumption of crown ether by using cheaper organic modifiers and additional inorganic and organic additives. Although it has been reported recently that EMT zeolite can be obtained without the use of organic promoters, 18-crown-6 ether is still required for the synthesis of high crystallinity EMT zeolite.
In this study, first of all, EMT type powdered zeolite Na form was synthesized from a reaction mixture whose molar composition is known in the literature by using an organic router, and then it was brought into different ion forms using aqueous solutions of different metal salts. The water desorption capacities of the original Na and other ionic forms of the EMT zeolite were determined by thermogravimetry (TG). In the second stage, experiments were started with different reaction mixture compositions for the preparation of EMT zeolite coatings by utilizing the previous studies in ITU Chemical Engineering Department Adsorption and Catalysis Laboratory, and then some synthesis conditions were changed and investigations were continued. Selected suitable crystalline and thick coatings were also subjected to ion exchange. The resulting coatings were characterized by TG, N2 adsorption, X-ray diffraction (XRD) and field emission gun scanning electron microscopy (FEGSEM).
As a result of the studies, coatings containing crystalline EMT zeolite with relatively high thickness were obtained from the clear reaction mixtures. According to the results obtained from the TG analysis, it was observed that there was a significant increase in the capacity of EMT zeolites in powder and coating form after ion exchange. It has been observed that the mechanical stability of the coatings during ion exchange is high.