Zeolit RHO katkılı matrimid® 5218 membranlarla gaz ayırımı
Gas separation with matrimid® 5218/zeolite RHO membranes
Yazar: BETÜL GAMZE ERTEKİN SURAL
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:
Doğalgaz, diğer fosil yakıtlar ile karşılaştırıldığında yüksek yanma verimine sahip olmasından dolayı temiz enerji kaynaklarından biri olarak kabul edilmektedir. Ancak doğalgaz, ham olarak elde edilirken içerisinde bulunan karbondioksit, azot, hidrojen sülfür gibi safsızlıklar yanma verimini düşürmektedir. Özellikle karbondioksit gazı, boru hattlarında korozyona sebep olmaktadır. Bu safsızlıkların giderilmesi için kullanılan konvansiyonel sistemler yüksek enerjiye ihtiyaç duyar ve doğaya zarar verebilecek solventler ile yürütülürler. Sıkça kullanılan ticari metotların aksine membran teknolojisi az enerji gerektirmesi, esnek olması ve kolay ölçeklendirilebilmesi sebebiyle son yıllarda araştırmacıların ilgisini çekmektedir. 
Literatüre bakıldığında çok çeşitli inorganik membran-polimer kombinasyonun, pek çok araştırmanın konusu olduğu görülmektedir. Özellikle karışık matrisli membranlar aracılığıyla, zeolit gibi yüksek seçiciliğe ve ısısal dayanıklılığa sahip inorganik maddeler, bir polimer fazına katılarak istenilen modüllerde membran üretilebilmekte ve yüksek seçici geçirgenliğine sahip olabilmektedirler.
Bu çalışmada içerdiği Si/Al oranından dolayı nispeten hidrofobik özellik gösteren RHO zeolit kristalleri, literatürde belirtilen organik yapı yönlendiricisi kullanılmadan, 8 günlük sentez süresinin aksine 2 günlük sentez süresi içinde,  zeolit/polimer kompozit membran üretmek amacıyla, membranhazırlama yöntemini iyi yönde etkileyebilmesi için mümkün olan en küçük partikül boyutunda sentezlenmiştir. Polimerik fazolarak, yüksek camsı geçiş sıcaklığına sahip ve CO2/CH4 gaz çifti için ayırma performansı yüksek olanMatrimid® 5218 polimeri seçilmiştir. Ara yüzey faz uyuşmazlığının ve yapışma sorununun  azaltılması amacıyla, membran sentezi sırasında yüzey modifikasyon tekniklerinden biri olan iyonik sıvı kullanılmıştır Çözücü olarak ise, çözücünün özellikle katkılı membranlar  içerisinde hızlı difüzyonu sonucu ortaya çıkacak stresi engelleyebilmek amacıyla kaynama noktası diğer çözücülerden nispeten yüksek (153°C) olan dimetilformamid'in kullanılması tercih edilmiştir. Deneylerde üretilen membranların ayırma performansı performansı incelenmiştir.
Elde edilen RHO zeolit yapısının belirlenmesi ve fazlarının tespitinin doğruluğu için XRD, lazer ışık saçınımı ölçüm yöntemi ile partikül boyutu analizi için Malvern Mastersizer, içerdiği fonksiyonel grupların ve molekül bağlarının analizi için FTIR-ATR analizi, ısıl kararlılığının ve içerdiği safsızlıkların tespiti için TGA analizi, zeolit partiküllerinin morfolojik yapıları ise FEGSEM ile karakterize edilmiştir ve sonuçlar literatürdeki değerler ile karşılaştırılmıştır. 
Zeolitlerin iyon değiştirme kapasitesine sahip olduğu ve Si/Al oranına bağlı olarak adsorpsiyon özelliklerinin geliştirilebilir olduğu bilinmektedir. Zeolit RHO'nun katyon değişiminin adsorpsiyon kapasitesine etkisini de görebilmek amacıyla, Li+, Na+, K+ ve Sr+2 katyonlarınca zengin iyon değişmiş formları çözelti içinde karıştırma yöntemiyle elde edilmiştir.
Katyon değişimi yapılan RHO formlarındaki iyon değişiminin gerçekleştiğini görebilmek amacıyla SEM-EDX analizi yapılmıştır. Ayrıca RHO ve katyon değişimli RHO kristal yapılı parçacıkların izoterm ve BET yüzey alanları adsorpsiyon ve desorpsiyon verilerinden elde edilmiş, RHO'ya ait BET yüzey alanı ve gözenek hacminin literatür ile uyumlu olduğu görülmüştür. 
Daha sonra elde edilen RHO zeolit, Matrimid® 5218 poliimid matrisine ağırlıkça %0, 10, 15 ve 20 oranlarında yükleme yapılarak karışık matrisli membranlar elde edilmiştir. Elde edilen membranlar karakteristik piklerindeki değişimlerin tespiti amacıyla FTIR-ATR analizi yapılmıştır. Matrimid® 5218/RHO karışık matrisli membranlarda hem Matrimid poliimidine hem de RHO zeolite ait karakteristik piklerin varlığı gözlemlenmiş ve ağırlıkça RHO dolgusunun artmasıyla doğru orantılı olarak, ona ait karakteristik pikin şiddetinde artış gözlemlenmiştir.
Matrimid® 5218/RHO karışık matrisli membranların ve saf matrimid membranın yüzey ve kesit morfolojileri SEM analizi ile incelenmiştir. SEM görüntülerinde zeolit/polimer arayüzey uyuşmazlığını engelleyebilmek amacıyla ilave edilen iyonik sıvı varlığının plastikleşme etkisi görülmekte olup, ağırlıkça %10 ve %20'lik Matrimid® 5218/RHO karışık matrisli membranlarda iyonik sıvı ile polimer/zeolit yapışmasının başarıyla sağlandığı görülmüştür.
Elde edilen membranların ısıl kararlılığı ve gözenekleri arasında kalan çözücü miktarının tespiti için TGA analizi gerçekleştirilmiştir. Matrimid® 5218/RHO karışık matrisli membranlarda yaklaşık 400℃'ye kadar ciddi bir kütle kaybının görülmemesi, elde edilen membranların iyi bir ısıl kararlılığa sahip olduğunu göstermektedir.
Membranların viskoelastik özelliklerinin ve camsı geçiş sıcaklığının tespiti için DMA analizi gerçekleştirilmiş olup, ağırlıkça RHO zeolit yüzdesinin artışı ile camsı geçiş sıcaklığının düştüğü gözlenmiştir.
RHO/Matrimid® 5218 membranlar CO2/CH4 gazlarının ayrımı için test edilmiş ve sonuçlar listelenmiştir.

Abstract:
It is aimed to remove impurities such as CO2 contained in natural gas by using mixed matrix membranes. CO2 is a corrosive gas present in large quantities in the natural gas stream. In order not to damage the piping systems and to increase the combustion efficiency of the natural gas, it is desired that the carbon dioxide component in the natural gas be less than 2%. Membrane processes as an alternative to existing methods have recently been the focus of attention. However, for this, it is necessary to determine the zeolite-polymer pair suitable for use in the relevant separation process.
Matrimid® 5218 polyimide, which has a high glass transition temperature and good permeability-selectivity values for the CO2/CH4 gas couple, was chosen for the organic phase of the mixed matrix membrane, and RHO zeolite, which has hydrophobic properties due to the Si/Al ratio it contains, was chosen as the inorganic phase, the dispersed phase. Mixed matrix membranes were synthesized at laboratory scale with 0%, 10%, 15% and 20% by weight RHO zeolite content. In order to eliminate the problem of zeolite/polymer interfacial adhesion, which is one of the biggest disadvantages of mixed matrix membranes, the zeolite surface modification method with an ionic liquid additive containing [absorption] [TF2N] (1-ethyl-3-methylimidazolium bis (trifluoromethanesulfonyl)imide) was used. In addition, it is expected in the literature that ionic liquids will have a positive effect on selectivity due to their CO2-philic structure. As a solvent, Dimethylformamide, which is a polymer organic solvent with a boiling point of 153°C, is preferred to prevent the stress that will arise as a result of the rapid diffusion of the solvent in the matrimide film.
RHO zeolite, which was synthesized to act as a molecular sieve, was obtained in a 2-day synthesis time using the ultrasonic method, unlike the traditional hydrothermal method that requires 8 days synthesis time. Depending on the mixing speed of the traditional method, it can cause uncontrolled crystal morphology to be obtained. It is thought that the use of ultrasonic allows more controlled nucleation by supporting the interaction between particles. Again, unlike the traditional method, RHO zeolite crystals were successfully obtained without using an organic support cage such as 18-crown-6 ether during the synthesis.
When the thermogravimetric analysis of the synthesized RHO particles was examined, it was proved that they were resistant to high temperatures. As a result of temperature-related mass changes in nitrogen environment, 14.31% mass loss up to 200°C is where the moisture trapped in the structure of RHO is removed, and 2.06% mass loss at 200-500°C is estimated to be due to the dissociation of the hydroxyl groups in the RHO zeolite particles.
The accuracy of RHO particle structures has been proven by XRD analysis results. The peaks of the RHO zeolite obtained by XRD analysis were compared with the studies in the literature and it was seen that the structure was obtained correctly. The synthesized RHO particles are approximately 8.3°, 16.6°, 18.6°, 25.1°, 26.4°, 30.2°, 32.5° and 35.7° consistent with the XRD model in the literature with characteristic diffraction peaks at respective 2θ angles. The highest intensity peak value of the obtained RHO particles occurred at 2θ: 25.1°, and again according to the literature, the high intensity sharp diffraction peaks represent a highly crystalline region in the synthesized sample. As a result, it was observed that high crystalline zeolite RHO was obtained with a synthesis time of 2 days without using an organic support cage.
It was aimed to obtain particles with the smallest possible particle size by optimizing the synthesis conditions of RHO selected for the inorganic phase of the mixed matrix membrane, and the average particle size (d50 value) of the particles was found to be 1.26 µm in the Malvern Mastersizer analysis.
FTIR-ATR analysis was performed to determine the organic bonds of the synthesized RHO crystal structures. From the peaks showing the correctness of the structure by FTIR-ATR analysis of RHO particles, the absorption peaks at 567 cm-1 and 580.03 cm-1 show the characteristic structure of the double eight-ring outer linkage in the RHO structure. The sharp and strong absorption peak at 1000.06 cm-1 is one of the characteristic peaks of the RHO zeolite, which corresponds to the asymmetric stretching vibration of Si–O and Al–O tetrahedra. The intense absorption peaks in the range of 3404.48 cm-1 and 1637.24 cm-1 indicate the presence of O–H stretching of water molecules adsorbed in the zeolite RHO sample and O–H bending of the lattice water.
In SEM images of RHO particles, the formed truncated octahedral structure was observed in accordance with the literature. In addition, it was observed that RHO particles were obtained homogeneously and without agglomeration.
It is known that zeolites have ion exchange capacity and their adsorption properties can be improved depending on the Si/Al ratio. It can adapt to many cation sizes and different adsorbate shapes such as Na+, Li+, Cd2+, Sr2+, Rb+ or Ba2+. In order to see the effect of cation exchange on the adsorption capacity of zeolite RHO, ion exchanged forms rich in Li+, Na+, K+ and Sr+2 cations were obtained by mixing in solution.
SEM-EDX analysis was performed in order to see the ion exchange in the cation exchanged RHO forms. Ion exchange degrees were calculated as 0.6%, 100%, 56.4% and 100% for Na-RHO, Li-RHO, K-RHO and Sr-RHO, respectively.
The isotherm and BET surface areas of RHO and cation exchanged RHO crystalline particles were obtained from adsorption and desorption data, and it was seen that the BET surface area and pore volume of RHO were compatible with the literature. It was also observed that all zeolites show the Type-I isotherm observed in microporous solids. The N2 adsorption capacity of RHO zeolites is directly related to their surface area. Accordingly, as a result of the analysis, we can list the adsorption capacities of RHO and cation exchanged forms as Na-RHO>RHO>Sr-RHO>Li-RHO>K-RHO.
In the preparation of zeolite doped membranes with different weight percentages, surface modification technique with ionic liquid was used in order to remove the non-selective empty pores that may occur at the Matrimid® 5218/RHO interfaces and to better wrap the Matrimid® 5218 polyimide on the RHO surface. While mixing the RHO zeolite in DMF solvent, after 6 hours, 2% ionic liquid was added into it and left to stir for 1 night. Then, Matrimid® 5218 was added gradually, corresponding to 15% of the total amount of the solution, and after a 1-day mixing-dissolving process, it was poured into the petri dish and mixed matrix membrane films were obtained by removing the solvent with the help of an oven.
Pure Matrimid® 5218, ionic liquid added Matrimid® 5218 and Matrimid® 5218/RHO mixed matrix membranes showed a harmonious and homogeneous distribution in SEM images. It was also observed that the addition of ionic liquid improved the zeolite/polymer interface and provided adhesion. Obtaining Matrimid® 5218 polyimide, which is known for its adhesion problems in the literature, compatible with an inorganic structure is a successful result for this second phase of the thesis. At the same time, it was determined that Matrimid® 5218/RHO 10% mixed matrix membrane thickness is 66.65 nm and 20% mixed matrix membrane thickness is 79.6 nm, and these thicknesses are quite suitable for gas permeability.
Characteristic bond structures of Matrimid® 5218 and ionic liquid and RHO structures in Matrimid® 5218/RHO mixed matrix membranes were investigated by performing FTIR-ATR analysis. The presence of characteristic peaks of RHO zeolite was observed in the membranes. With the increase in the amount of RHO incorporated into the Matrimid® 5218 polymer, an increase in the intensity of the peak of the asymmetric stretching vibration of Si–O and Al–O tetrahedra located at 1000.06 cm-1 was observed, thus proving the presence of the RHO structure in the polymer.
When the TGA curves given for the doped membranes were examined, it was observed that all Matrimid® 5218 membranes containing all RHO showed similar behavior. It was observed that the mass loss of the doped membranes was higher compared to the pure Matrimid® 5218. This was thought to be due to the degradation of organic binders (such as ionic liquid) in the RHO framework and organic structures in the membrane matrix. This idea was supported by the increase in mass loss rates as the amount of RHO particles increased in the TGA results. However, we can say that there is no serious decrease up to about 400°C, that the obtained mixed martis membranes show good thermal stability and are suitable for membrane operating conditions.
Tg, storage and loss modulus values of RHO doped Matrimid® 5218 mixed matrix membranes were investigated by DMA analysis. It was observed that the Tg value decreased with increasing RHO amount compared to the pure polymer. At first it was thought that the factor causing this was an inappropriate interfacial interaction between the polymer and the zeolite, but it was proved that this was not the real reason due to the SEM images showing the opposite. Secondly, it was evaluated that the plasticizing effect caused by the ionic liquid could cause a decrease in the glass transition temperature, and this latter possibility was proven by the determination of the amount of solvent remaining in the membrane in the TGA results.
Gas permeability and selectivity values of RHO/Matrimid® 5218 membranes have been tested and the results are listed.