Hidrojen ayrıştırılması için sülfüre dayanıklı MOF kaplamalı PD bazlı yoğun metal membran geliştirilmesi
Development of PD-based dense metallic membrane with sulfur resistant MOF coated to separate hydrogen
Yazar: SEVGİ KALKAN
Danışman: DR. ÖĞR. ÜYESİ GAMZE GÜMÜŞLÜ GÜR
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Kimya Mühendisliği Ana Bilim Dalı / Kimya Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2022
Konu: Kimya Mühendisliği = Chemical Engineering
Dizin: Hidrojen üretimi = Hydrogen production ; Membranlar = Membranes ; Metal-organik çerçeveler = Metal-organic frameworks

Özet:
Hidrojen; günümüzde ulaşım sektörü, elektrik üretimi, termal enerji gibi alanlarda kullanıma sahip önemli bir kimyasaldır. Aynı zamanda amonyak, metanol gibi çok kullanılan kimyasallara hammadde olan H2, doğada saf halde değil, gaz karışımları halinde bulunmaktadır. Hidrojen, günümüzde en fazla doğalgazın reformasyonu ve fosil yakıtların gazlaştırılması olmak üzere iki şekilde elde edilmektedir. Bu üretim yöntemleri sonucunda oluşan sentez gazı başta H2, ve CO2 içerirken, daha az miktarda CO ve CH4 gibi gazları da içermektedir. Bunların yanında sentez gazı üretim hammaddesi kaynaklı olarak az miktarda kükürt bileşikleri içerebilmektedir. Hidrojenin etkin şekilde kullanılabilmesi için bu gaz karışımlarından ayrıştırılması gerekmektedir.
Basınç salınımlı adsorpsiyon, kriyojenik distilasyon gibi ticari hidrojen ayrıştırma yöntemleri yüksek kurulum ve işletim maliyetleri gerektirmektedir. Bu noktada, daha ekonomik şekilde ve daha az enerji tüketimi ile yüksek saflıkta hidrojen üretimi imkânı sağlayabilecek yoğun metalik membranlar öne çıkmaktadır. Hidrojen ayrıştırıcı yoğun metalik membranlar son zamanlarda üzerinde en çok durulan araştırma konularındandır. Yoğun metalik membran kullanımı sayesinde gaz karışımının basınç ve sıcaklığında değişime gidilmeden ayırma yapılması mümkün olabilmektedir. Bu konuda en başarılı metal, sonsuz H2 seçicigeçirgenliği ve yüksek H2 aktivitesinden dolayı Paladyum metalidir. Önemli bir geçiş metali olan Pd, hacminden yüzlerce kat fazla hacimde hidrojen absorblama kapasitesine sahiptir. Fakat Pd metali kükürde karşı çok hassastır ve ppm seviyelerindeki kükürt varlığında dahi, membran yapısı kolayca bozulmaktadır. Hidrojenin ayrılması istenilen gaz karışımlarında başta H2S olmak üzere farklı kükürt bileşikleri bulunabildiğinden, membranın ayrıştırma performansını etkilemeden membranın kükürt dayanıklılığının arttırılabileceği bir çözüm yoluna ihtiyaç vardır.
Bu çalışmada sentez gazı karışımından yüksek saflıkta hidrojen ayrıştırılmasını sağlayabilecek, aynı zamanda kükürt direncine sahip koruma katmanlı yoğun metalik membran geliştirilmesi hedeflenmektedir. Bu amaçla, destek malzemesi olarak Al2O3 tabletler kullanılarak paladyum yoğun metalik membranlar (PYMM) hazırlanmış, kükürt dayanıklılığı için metal organik kafes yapılı (MOF) bir malzeme kaplama uygulanması tercih edilmiştir.
MOF'lar metal düğümlerin organik ligandlarla birbirine bağlanmasıyla oluşmuş kristal yapıda ve gözenekli katı malzemelerdir. Olağanüstü geniş yüzey alanları ve metal düğümleri sayesinde kükürt bileşiklerini tutmada oldukça verimlilerdir. Ayrıca gözenek çapları metal membranlardaki kafes boşluklarından çok daha büyük olduğu için difüzyona karşı dirençleri metal membranlarınkinden daha düşüktür. Literatürde bazı MOF'ların yapılarında herhangi bir kusur olmaksızın yüksek H2S tutma kapasitesine sahip olduğu görülmektedir. Bu özellikteki MOF'lar arasında bulunan UiO-66, sıcaklık ve basınç artışına daha dayanıklı olduğundan ve sentezlenmesi pek çok diğer MOF çeşidine kıyasla kolay olduğundan bu çalışmada kükürde karşı koruyucu tabaka olarak UiO-66 kullanılması tercih edilmiştir.
Membran hazırlama için destek malzemesi olarak nispeten ucuz olan Al2O3 tercih edilmiştir. Toz α-Al2O3 (Alfa Aesar, aktif, nötr, Brockmann Grade II) tablet şekline getirilecek şekilde basınçlandırılmış, 1000°C'de sinterlenmiş ve kullanıma hazır hale getirilmiştir. Hazırlanan tabletlerin üzerine membran görevi görecek olan Pd film, elektriksiz kaplama (ELP) yöntemi ile kaplanmıştır. Bu yöntem Pd köklendirme ve Pd kaplama olmak üzere iki aşamada gerçekleştirilmektedir. Paladyum asetat içerikli çözelti ile Pd köklendirme, Paladyum klorür içerikli çözelti ile de Pd kaplama yapılmıştır. Çözeltide indirgen ajan olarak hidrazin tercih edilmiştir. Pd köklendirme ve kaplama aşamalarında, sonraki kaplama adımı için en etkili olacak yöntemler belirlenmiş ve kullanılmıştır. Son aşama olarak, Pd membrana kükürt dayanıklılığı sağlayacak UiO-66 kaplama uygulamasına geçilmiştir. Öncelikle, UiO-66 kristallerinin Pd kaplama üzerinde büyüyebileceği ortamı sağlamak amacıyla yüzey şartlandırma çalışmaları yapılmıştır. Hem Pd asetat hem de tereftalik asit (BDC) ile iki ayrı yüzey şartlandırmasından sonra, Pd kaplı numuneler zirkonyum klorür, tereftalik asit, dimetilformamid ve asetik asit içeren UiO-66 çözeltisinde 120°C'de 12 saat tutulmuştur. DMF kalıntılarından kurtulmak için aseton ile yıkandıktan sonra UiO-66 kaplı Pd yoğun metalik membran kullanıma hazır hale gelmiştir.
Bu çalışmada, H2 geçirgen Pd membrana kükürt dayanıklılığı sağlayan UiO-66 kaplamalı membran geliştirilmiştir. UiO-66 tabakanın Pd yüzeyi kükürde karşı koruma seviyesini test etmek için SO2 gaz testi uygulanmıştır. Geliştirilen membran otoklav reaktör içerisinde 2.5 bar basınç ve 150℃ sıcaklık altında SO2 gazına doğrudan maruz bırakılmıştır. Membran yüzeyin SO2 uygulaması öncesi ve sonrası XRD ve FTIR analizleri gerçekleştirilmiş, PdxSy bileşiklerinin oluşumunun engellendiği görülmüştür. Bunların yanı sıra membran üretiminin her aşamasında yüzey değişimleri FTIR sonuçlarıyla desteklenmiş, membran SEM görüntüleri paylaşılmıştır.

Abstract:
Nowadays, although the focus is on clean and renewable energy sources, fossil fuels are still used in large quantities all around the world. Especially in countries with large coal reserves, energy production will continue to be obtained by burning coal until clean energy processes are adapted to the industry. In energy production, with the current incline over the green energy system, gasification of coal is preferred over directly burning it. The gasification process transforms the coal into syngas (mainly CO and H2) that can be used in the energy or chemical industries, a gas mixture rich in H2 is obtained in the presence of water-gas shift reaction. To utilize the produced H2 in downstream processes, it must be obtained as pure as possible or prepared in such a way as to achieve certain ratios. Although it is not viable to separate all of the H2 in the stream using dense metallic membranes, they can be utilized in reducing the H2:CO ratio to the required levels by separating a certain amount of H2 from the syngas. In addition, the syngas produced via coal gasification comprises sulfurous compounds which are destructive/poisonous for the materials used in H2 utilization.
Commercial methods to separate hydrogen such as pressure swing adsorption (PSA), and cryogenic distillation require large capital and energy expenditures due to extensive changes in pressure and temperature. Separation with a membrane offers an alternative method that allows operation at the temperature and pressure of the inlet flow resulting in moderate expenses. Membranes are used in many different areas in the industry: separation of methane from other components of the biogas, enrichment of air with oxygen for medical or metallurgical purposes, removal of water vapor from natural gas and other gases, removal of CO2 from natural gas, removing H2S from natural gas. Apart from these, the ones related to hydrogen are as follows: separation of hydrogen from gases such as nitrogen and methane, recovery of hydrogen from the product stream in ammonia plants, and recovery of hydrogen in oil refinery processes. While a variety of polymeric, ceramic, and hybrid membranes are available, metallic membranes are also common. For these reasons, dense metallic membranes, which can separate the desired gases in high purity under pressure and temperature conditions of the gas mixture, without requiring an additional energy input or high budgets, have started to attract attention. Especially Pd (Palladium) membranes have come to the fore with their high hydrogen permeability and high activity for hydrogen. Pd is an important transition metal with a hydrogen absorption capacity of 900 times its volume. However, the sulfur in the form of different compounds existing in the gas mixtures poisons the Pd membrane. In this study, MOF (metal-organic framework) coating is applied over the Pd membrane to overcome this problem.
MOFs are a type of lattice-structured materials. Lattice-structured materials are crystalline and porous solid materials constructed from organic "struts" whose chemical structures are interconnected by "joints" (organic or metal oxide-based). MOFs are a new class of porous materials with a hybrid organic/inorganic structure that allows combining the properties of both organic and inorganic porous materials. Therefore, as they are called hybrid organic-inorganic materials, they can also be called porous coordination polymers (PCP). MOFs were discovered by Prof. Dr. Omar M. Yaghi (University of California, Berkeley - School of Chemistry) since then a lot of work has been done owing to their multi-porous structures, large surface areas, and thermal and mechanical stability. Considering the diversity of the periodic table and organic chemistry, it can be seen that there is a wide range of possibilities for the synthesis of metal-organic frameworks. The fact that different chemicals can be used for metal and organic parts is important in terms of showing that the framework structure can be changed and arranged as desired. Therefore, it is possible to create a large number of MOFs with very different properties. MOFs have the ability to hold and store various molecules with their pores. For this reason, they are used in many application areas such as CO2 capture, storage of other gases such as hydrogen and methane, drug transport systems, separation technologies, heterogeneous catalysis systems, and electronic devices. MOFs are highly efficient in retaining sulfide compounds, thanks to their extraordinarily large surface areas and metal knots. In addition, since their pore diameters are much larger than the lattice spaces in metal membranes, the resistance to diffusion is much lower than the use of metal membranes. In the literature, it can be seen that some of the MOFs show high H2S holding capacity without any defect in their structures. One of the studied MOFs with H2S holding capacity, UiO-66 is more resistant to temperature and pressure increase, and more importantly, it is much easier to synthesize. Therefore it was preferred to use UiO-66 as the Pd protective layer in the study.
UiO-66 is a zirconium-based MOF variant and has the formula of Zr6O4(OH)4(BDC)6. This MOF is formed by the combination of benzene-1,4-dicarboxylic acid (or terephthalic acid, shortly BDC) ligand with zirconium oxide by forming complex bridges. Since UiO-66 was first produced at the University of Oslo, it got its name from the initials of this place. UiO-66, which is most stable in the face-centered cubic (fcc) structure, has 12 BDC ligands attached to a zirconium knot. UiO-66 has high thermodynamic stability which is thought to originate from strong Zr-O bonds and has high mechanical stability as well as high acid, water, and liquid medium stability. Thanks to all these features and its ability to be easily synthesized by the solvothermal method, it has become a subject that has been studied recently. UiO-66 is especially used in gas separation processes, heterogeneous catalysts, and the removal of harmful chemicals such as NO2 and sulfur compounds.
In this study, the development of a sulfur-resistant double-layer Palladium-based dense metallic membrane for the separation of hydrogen from the sulfide-containing syngas is discussed. Pd is not resistant to sulfur, so the presence of sulfur causes defects in its structure and inhibits the operation of the membrane. To solve this problem, a double-layered structure is used. The first layer is Pd-based metal, while the second layer consists of a thin MOF structure. The aim is to keep the sulfur impurity in the syngas within the MOF structure and prevent it from reaching the membrane surface and thus destructing the structure. With this study, Pd membrane structure with MOF protection layer was introduced in the literature.
Al2O3 was preferred as the membrane support material in the study. Powder α-Al2O3 (Alfa Aesar, activated, neutral, Brockmann Grade II) was pressurized to form a tablet shape, sintered at 1000℃, and made ready for use. Pd film, which will act as a membrane, was coated on the prepared tablets by the electroless plating (ELP) method. This method takes place in two stages: Pd seeding and Pd coating, thus providing homogeneous coating. While a solution containing palladium acetate is used in Pd seeding, a solution with palladium chloride is used in the Pd coating step. In this solution, the use of hydrazine is preferred as the reducing agent. In the Pd seeding and Pd coating stages, the most efficient methods were selected with various experimental studies to make the last coating as desirable. The second layer, sulfur-resistant MOF, was prepared after the hydrogen-selective Pd dense membrane layer developed. First, experimental studies have been carried out for surface conditioning where UiO-66 crystals can grow on the surface. Pd acetate solution, hydrogen peroxide, isopropanol, and BDC were used for different test sets as conditioning solutions. Pd-coated membranes, which were kept in these solutions for a short time, were then placed in the UiO-66 solution and the surface examination was done by naked eyes. Because UiO-66 crystals are white, a white powder appearance should be seen on the Pd-coated surface. While hydrogen peroxide and isopropanol could not provide the desired appearance, acetate, and BDC solutions did in different experimental sets. The final method for the second coating is: after surface conditioning with both Pd acetate and terephthalic acid (BDC), the Pd-coated samples were kept in the UiO-66 solution which includes zirconium chloride, terephthalic acid, dimethylformamide, and acetic acid at 120°C for 12 hours. After washing with acetone to get rid of DMF residues, the UiO-66 coated Pd dense membrane became ready to use.
Non-homogeneous surface images were seen in the first samples obtained after UiO-66 enlargement studies, and homogeneity was tried to be achieved by repeating the experiments. As a result, it has been seen that clean, meticulous, and careful work is required to obtain a homogeneous UiO-66 coating on the membrane surface. Since UiO-66 crystals will grow by adhering to the conditioned surface, it has been noticed that no crystal growth is observed in contact with the membrane surface. For this reason, it was worked to minimize contact with the membrane surface, the UiO-66 solution was protected from possible contamination and the solution bottle was kept on a flat surface where there would be no difference in elevation. Intense and homogeneous surfaces could be obtained in the UiO-66 coating since the required appropriate physical conditions were also provided. The results were also supported by SEM images, and the entire Pd-coated membrane surface was homogeneously and densely coated with UiO-66 particles.
The study aims to form a UiO-66 layer on the Pd surface, thereby reducing the interaction of Pd with sulfur. With the methods developed within the scope of the thesis studies, the top of the Pd layer was coated with UiO-66 at the desired density and homogeneity. In order to measure the sulfur resistance of the obtained double-layer membrane structure, Pd and Pd-UiO-66 membranes surfaces were exposed to SO2 directly at 2.5 bar and 150℃ in an autoclave reactor to test the function of the UiO-66 layer formed on the surface. After this contact, surface deformations occurred on the Pd membrane, while no changes occurred on the Pd-UiO-66 membrane surface. The results are supported by SEM images, XRD, and FTIR analysis results. Moreover, the FTIR results of all steps of the membrane preparation and SEM images of the Pd layered and Pd-UiO-66 membranes are provided.