Kobalt kompleksleri üzerinden kimyasal yolla tersinir moleküler oksijen depolanması
Reversible chemical storage of molecular oxygen via cobalt complexes
Yazar: AHMET İNCE
Danışman: PROF. DR. BÜNYAMİN KARAGÖZ
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Kimya Ana Bilim Dalı / Kimya Bilim Dalı
Derece: Doktora, 2023
Konu: Kimya = Chemistry ; Polimer Bilim ve Teknolojisi = Polymer Science and Technology
Dizin: Aşı kopolimerleri = Graft copolymers ; Enerji depolama = Energy storage ; Fotopolimerleşme = Photopolymerization ; Kimyasal depolama = Chemical deposition ; Kopolimerler = Copolymers ; Kopolimerleşme = Copolymerization

Özet:
Salisilaldimin'in kobalt kompleksi (CoSalene) ile tersinir oksijen bağlanabileceğinin keşfi, daha verimli oksijen depolama sistemleri için araştırmaların yapılmasını tetiklemiştir. CoSalene, oksijenle 2/1 katılma bileşiği oluşturarak uzun yıllar uçak ve denizaltılarda temiz hava sağlamak için kullanılmıştır. Araştırmalar genellikle CoSalene esaslı sistemler üzerinde yoğunlaşsa da diğer pek çok ligandın kobalt kompleksleri yüksek kapasiteli oksijen bağlayıcı cihazlarında kullanılmaktadır. Bununla birlikte havadan oksijen ve azot elde edilme prosesleri için, seçici geçirgen membranların üretiminde bu komplekslerin yararlı olduğu gösterilmiştir. Diğer taraftan ligandın hidrolize uğraması ve 50 ºC üstündeki sıcaklıklarda oksijen bağlamış kobalt komplekslerin kararlılıklarının düşük olması mevcut sistemlerin dezavantajlarıdır. 
Bu tez çalışmasında, yukarıda verilen bilgilerden yola çıkarak, kobalt kompleksleri üzerinden hava oksijenini bağlamak için farklı polimerik ligandlar geliştirilmiş ve oksijen bağlama özellikleri incelenmiştir. Dört farklı başlık altında yapılan çalışmalar, biri küçük molekül esaslı ligand olmak üzere, farklı polimerik ligandlar ve bunların kompleksler kullanılarak gerçekleştirilmiştir. 
İlk çalışmada, trietilen tetraminin glisidol ile reaksiyonu sonucu suda çözünebilir 1,1,4,7,10,10-hekzakis bis (2,3 dihidroksipropil)-1,4,7,10 tetraazadekan (HEPTETA) ligandı sentezlenmiş ve kobalt kompleksi elde edilmiştir. Bu kompleksin oksijen bağlama özellikleri gaz-volumetrik, titrasyon, oksijen sensörü, otoklav ve çözünür oksijen ölçme yöntemleri kullanılarak incelenmiştir. 
İkinci çalışmada metillenmiş poli(vinil amin) ile diallildimetil amonyumklorür (DADMAC) kopolimeri, N-vinil formamid (NVF) polimerizasyonu veya DADMAC ile kopolimerizasyonu yoluyla hazırlanmıştır. Polimerlerin asit hidrolizi ve ardından metilasyonu, ilgili poliaminlerin elde edilmesini sağlamıştır. Kobaltın karşı iyonları hidroksil iyonları ile değiştirildiğinde, poliaminlerin Co(II) kompleksleri, moleküler oksijen bağlanmasında oldukça hızlı olduğu gösterilmiştir. Oksijenasyon testleri, oksijen sensörü, FT-IR ve gaz volumetrik yöntemlerle yapılmış ve [amin]/[Co] oranları 4/1 veya 2/1 olan komplekslerin, 1:1 moleküler oksijen katılma ürünleri verdiği ortaya çıkmıştır. Hızlı oksijenasyon (yaklaşık 2-4 dakika), yüksek depolama kapasitesi ve 70-120 °C arasında düzgün oksijen salımı, oksijenasyon-deoksijenasyonun tipik özellikleridir.
Üçüncü çalışmada, N-vinil formamid'in (NVF) %1 ve %0,5 mol oranlarında çapraz bağlayıcı (tetra allil piperrazinyum dibromide) içeren yalın jelleri sentezlendi. Ayrıca monomerlere göre molce %1 çapraz bağlayıcı varlığında değişik DADMAC oranlarında (DADMAC %20, %10, %5% molce) NVF-DADMAC jelleri sentezlenmiştir. Bunların hidrolizleri ile elde edilen primer amin fonksiyonlu hidrojeller Eschweiler-Clarke yöntemiyle metillenerek ardından kobalt kompleksleri elde edilmiştir. Bunların tersinir oksijen bağlama özellikleri incelenmiştir. Deneysel sonuçlar lineer polimerlerin komplekslerinin oda sıcaklığında kısa sürede oksijen bağlayabildiğini, molce %3 çapraz bağlı jellerden elde edilen kobalt komplekslerinin ise daha yavaş oksijen bağladığını ve 55-60 dk'da ancak doygunluğa ulaşılabildiğini göstermiştir. Oksijen salınım şartları farklı sıcaklıklarda su ve etilen glikol çözücülerde volumetrik gaz ölçüm, basınç değişimi ve oksijen sensör teknikleri ile incelenmiştir. Deneysel sonuçlar her iki çözücüde oksijenin 70 oC civarında salınmaya başladığı, 120 oC civarında tamamen salındığını ve başlangıçtaki Co(II) kompleksinin rengine geri dönüldüğünü göstermiştir. Çapraz bağlı DADMAC jel komplekslerinde mol başına 0,37-0,41 mol tersinir oksijen bağlandığını yani bağlanmanın çoğunlukla mononükleer peroksi veya süperokso katılma bileşiği verecek şekilde gerçekleştiğini göstermiştir.
Son çalışmada ise, "Photo-iniferter" tekniği, PS-DVB mikrokürelerin yüzeyine bağlanmış ditiokarbamat gruplarından N-vinil formamid (NVF) veya DADMAC ile karıştırılmış NVF'nin yüzey başlatıcılı polimerizasyonu (SIP) için başarılı bir şekilde kullanılmıştır. Yoğun saçakların hidrolizi ve sonrasındaki metilasyon, mikroküreler üzerinde metillenmiş poli(vinil amin) saçaklarının kobalt ile şelat yapabilecek üniteler oluşturmuştur. Kobalt karşı iyonları hidroksil iyonları ile değiştirildiğinde, şelat saçaklarında Co(II) komplekslerinin oldukça hızlı oluştuğu gösterilmiştir. Oksijenleme testleri, oksijen sensörü, FT-IR, TGA, XPS teknikleri ve gaz volumetrik ölçüm yöntemleriyle yapılmış ve [amin]/[Co] oranı 4/1 veya 2/1 olan komplekslerin birbirine tek moleküllü oksijen katılma ürünlerini oluşturduğu ortaya çıkmıştır. İlk defa, [amin]/[Co] oranı 2/1 olan komplekslerin de özel aksiyal ligand kullanmadan moleküler oksijen bağlayabileceği (yaklaşık 2-4 dakikada) gösterilmiştir. Reçineli komplekslerin oksijen bağlamış halinin 70 oC'ye kadar stabil olduğu ve oda sıcaklığında iki haftadan daha uzun süre kuru halde saklanabileceği gözlemlenmiştir. Yüksek oksijen depolama kapasitelerine (80 mLg-1 kadar) sahip olan bu yeni polimer komplekslerinin, taze oksijen sağlamak için yeni tip katı matris olarak kullanışlı olabileceği gösterilmiştir.
Yapılan deneyler, oksijenlenmenin 1:1 katılma bileşiği oluşacak şekilde yürüdüğünü ve hava akımında 1-2 dakikada tamamlandığını göstermiştir. Bu komplekslerin yüksek kapasiteleri, oda sıcaklığında iki haftadan fazla bozunmadan kalabildikleri ve tekrar tekrar kullanılabilir oldukları düşünüldüğünde, mevcut kimyasal oksijen depolama sistemlerinden daha üstün ve pratikte kullanım amacı için ümit verici olduğu ortaya çıkmıştır.

Abstract:
The discovery of the reversible oxygen binding of the salicylaldimine-cobalt complex has indeed led to extensive research in designing more efficient oxygen storage-release systems. While most research has focused on salicylaldimine based systems, cobalt complexes of other chelating ligands have also been found useful for the fabrication of high capacity oxygen storage devices and perm-selective membranes for efficient harvesting of oxygen and nitrogen from air. However, the current systems have some drawbacks, such as ligand hydrolysis and low stability of the oxygenated complexes above 50 oC. To overcome these issues, researchers have been investigating the use of different ligands and metal ions to improve the stability and performance of these systems. For example, iron and manganese complexes have been studied as potential alternatives to cobalt complexes. Additionally, the use of more stable ligands, such as porphyrins and phthalocyanines, has been explored. Efforts have also been made to optimize the synthesis and preparation of these complexes to improve their stability and performance. For example, the use of encapsulation techniques, such as the incorporation of these complexes into zeolites or metal-organic frameworks (MOFs), has been shown to improve their stability and selectivity. Overall, the research on oxygen storage-release systems is a rapidly evolving field with significant potential for practical applications in various industries, such as aviation, healthcare, and energy production.
Water soluble cobalt complex of triethylenetetramine ligand with six bulky 2, 3-dihydroxy propyl groups is presented as versatile water-based reversible molecular oxygen binding system. The experiments conducted using gas-volumetric measurements, in combination with magnetic susceptibility, UV-Vis, DSC, and oxygen sensor techniques, revealed that, oxygenation occurs mostly via one-to-one oxygen adduct formation and completed within 1-2 min under air flow. The oxygen was determined to be driven off by heating of the loaded complex at 95-130 oC in water or in diethyleneglycol. It was found that, on contrary to common beliefs, no special axial ligand is essential to attain stable oxygen adducts when counter anion of cobalt is replaced with OH- ion during the complexation. In these conditions, oxygen binding capacity of the system did not change after 10 times of cycling, implying reversibility of the oxygenation and stability of the non-volatile ligand involved. The complex seemed to be promising also for harvesting of nitrogen as well as oxygen from air, due to its fast oxygenation. Non-volatile ligand, HEPTETA obtained by action of glycidol on triethylene tetramine yields water soluble cobalt complex undergoing rapid oxygenation by air flow, without using a special axial ligand when counter anions of cobalt is exchanged with hydroxyl ions in water or in ethylene glycol. No special axial ligand is essential to attain high oxygen uptakes within short times in these conditions.  The outer-sphere complex of the ligand with four electron donating amine nitrogen forms highly stable 1:1 oxygen adduct as inferred from gas volumetric measurements and oxygen sensor techniques. Bulky groups of HEPTETA ligand provide steric hindrance for the formation of dinuclear oxygen adducts, as expected. Heating of the oxygenated complex (superoxo complex) in 95-130 oC results in complete evolution of the molecular oxygen and pink color of the initial complex is restored. Having reasonable stability in the oxygenated form and showing no deterioration in more than ten times of recycling, the complex presented seems to be promising for designing large scale oxygen storage-release systems.
Methylated poly(vinyl amine) and its DADMAC copolymer were synthesized by polymerizing N-vinyl formamide (NVF) or copolymerizing it with DADMAC. Subsequently, the resulting polymers were subjected to acid hydrolysis followed by methylation to produce the targetted polyamines. When the counter anions of cobalt were exchanged with hydroxyl ions, the Co(II) complexes of these polyamines exhibited reasonably fast binding with molecular oxygen. Oxygenation tests, conducted using an oxygen sensor, FT-IR, and gas-volumetric methods, revealed that complexes with either a 4/1 or 2/1 [Amine] / [Cobalt] ratio formed 1:1 molecular oxygen adducts. The oxygenation-deoxygenation process displayed fast oxygenation (approximately 2-4 minutes), high storage capacity, and smooth oxygen release within the temperature range of 70-120 ℃. These characteristics make the system suitable for air oxygen harvesting and chemical storage. In summary, the structural characteristics of methylated polyvinyl amines and the oxygenation behavior of their cobalt complexes can be summarized as followsThe Co(II) complexes of the polymers are water-soluble and rapidly undergo oxygenation by aeration in water or ethylene glycol solvent when the cobalt counter ions are exchanged with hydroxyl ions.
Under these conditions, oxygenation is completed within minutes at room temperature, forming 1:1 oxygen-cobalt adducts (superoxo structure) as confirmed by gas volumetric measurements.
Deoxygenation occurs in the temperature range of 70-120 ℃.
The presence of DADMAC copolymer segments does not provide additional advantages in oxygenation.
The use of 4 and 2 amines per cobalt in complex formation does not alter the capacity, as a 1:1 oxygen adduct is formed in both cases.
The cobalt complex described appears to be superior to previous oxygen binding systems and is a promising candidate for the chemical storage of molecular oxygen.
Crosslinked NVF gels containing tetraallyl piperazine dibromide as a crosslinking agent were synthesized at mol ratios of 1% and 0.5%. Additionally, NVF-DADMAC gels were synthesized at different DADMAC ratios (20%, 10%, and 5% mol) relative to the monomers in the presence of a 1% mol crosslinking agent. Subsequently, hydrolysis of these gels was carried out, and the resulting amine-functionalized hydrogels were methylated using the Eschweiler-Clarke method to obtain cobalt complexes. The reversible oxygen binding properties of these complexes were investigated. Experimental results demonstrated that complexes formed from linear polymers exhibited rapid oxygen binding at room temperature, whereas complexes derived from gels with a 3% mol crosslinking agent exhibited slower oxygen binding, reaching saturation after 55-60 minutes. Oxygen release conditions were investigated using volumetric gas measurement, pressure change, and oxygen sensor techniques in water and ethylene glycol solvents at different temperatures. The results indicated that oxygen release occurred at approximately 70 ℃ in both solvents and was completed at around 120 ℃, with the Co(II) complex returning to its original color. In the complexes derived from crosslinked DADMAC gels, approximately 0.37-0.41 mol of reversible oxygen was bound per mol, suggesting that binding primarily occurred through mononuclear peroxo or superoxo complexes.Crosslinked NVF (N-vinylformamide) gels containing tetraallyl piperazine dibromide as a crosslinking agent were synthesized at 1% and 0.5% mol ratios. Furthermore, NVF-DADMAC (dimethyldiallylammonium chloride) gels were synthesized at different DADMAC ratios (20%, 10%, 5% mol) with respect to the monomers in the presence of 1% mol crosslinking agent. Hydrolysis of these gels was performed. The resulting amine-functionalized hydrogels were methylated using the Eschweiler-Clarke method to obtain cobalt complexes, and their reversible oxygen binding properties were investigated. The experimental results showed that complexes of linear polymers can bind oxygen rapidly at room temperature, while complexes obtained from gels with 3% mol crosslinking agent bind oxygen more slowly and reach saturation after 55-60 minutes. Oxygen release conditions were investigated using volumetric gas measurement, pressure change, and oxygen sensor techniques in water and ethylene glycol solvents at different temperatures. The experimental results showed that oxygen is released at around 70 °C in both solvents, and is completely released at around 120 °C, with the Co(II) complex returning to its original color. In crosslinked DADMAC gel complexes, 0.37-0.41 mol of reversible oxygen was bound per mol, indicating that binding mostly occurs through mononuclear peroxo or superoxo complexes.
Photo-iniferter technique was successfully employed for surface initiated polymerization (SIP) of N-vinyl formamide (NVF) or its mixture with DADMAC from ditiocarbamate groups on PS-DVB microspheres. Hydrolysis of the dense brushes and subsequent methylation yielded metal-chelating shell layer of methylated poly(vinyl amine) brushes on the microspheres. Co(II) complexes of the chelating brushes were demonstrated to be reasonably fast when counter anions of cobalt are exchanged with hydroxyl ions. The oxygenation tests carried out with oxygen sensor, FT-IR, TGA, XPS techniques and gas-volumetric methods, revealed that, complexes either with 4/1 or with 2/1 [amine]/[Co] ratios form one-to-one molecular oxygen adducts. We have demonstrated for the first time that, complexes with 2/1 [amine]/[Co] ratios can also bind molecular oxygen rapidly(c.a.2-4 min), without using special axial ligand. Oxygen adducts of the resinous complexes were determined to be stable up to 70 oC and storable in dry state for over two weeks at room temperature. It was observed that, oxygenation characteristics of the resin complexes are similar to those of the cobalt complexes with methylated linear poly(vinyl amine)'s.  Having high oxygen storage capacities (up to 80 mLg-1), these novel polymer complexes seemed to be useful as solid matrix for supplying fresh oxygen. Photoiniferter technique can be employed successfully for generation of thick P(NVF-co-DADMAC) brushes on morpholine dithiocarbamate functional PS-DVB microspheres. Formamido groups on the graft chains can be converted to amino groups in near quantitative yields by acid hydrolysis.  Followed methylation of the resulting amino groups yields tertiary amine functional copolymer brushes, cobalt complexes of which in 2/1 and 4/1 [amine]/[cobalt] ratios can form 1:1 oxygen adducts rapidly, upon aeration in diethylene glycol when counter anions of the cobalt are replaced by hydroxyl ions. The resin complexes consisting of two tertiary amino groups per cobalt atom ratio are especially useful in chemical storage of molecular oxygen, due their high oxygen binding capacities per unit mass (80 L/kg). Similar1:1 oxygen adducts formation are observed in cobalt complexes of methylated linear poly(vinyl amine) and in its copolymers with DADMAC. However their synthesis and purification needs tedious workup. Having appreciable stability, easy handling and manipulation facilities, complexes of PAGR series of resin materials are superior to the soluble poly(vinyl amine)'s and seem to be promising as high capacity solid matrix for reversible binding of molecular oxygen.