Su Buharı İle Modifiye Edilen H-zsm-5 Katalizörlerle Mtbe Sentezi

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Tarih
1998
Yazarlar
Sarıoğlan, Alper
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Yayınevi
Fen Bilimleri Enstitüsü
Institute of Science and Technology
Özet
Benzine katkı maddesi olarak eklenen oksijenli oktan yükselticilerden biri olan MTBE'nin (metil tersiyer bütil eter) kullanımı son zamanlarda büyük bir artış göstermiştir. Bu artışın en önemli sebeplerinden biri ABD "de yürürlüğe giren temiz hava yönetmeliği ile kurşunlu oktan yükselticilerin kullanımına getirilen büyük kısıtlamadır. MTBE, ticari olarak asidik iyon değiştirici bir reçine olan Amberlyst-15'in katalizör olarak kullanıldığı izobütilen ile metanol arasındaki reaksiyon ile üretilmektedir. Fakat termodinamik sınırlamalar, reçine katalizörün yüksek sıcaklıklarda kararsız olması ve 90°'nin üzerindeki sıcaklıklarda yan ürün oluşumu nedeniyle çok düşük olan seçicilik, MTBE sentezinde alternatif bir katalizör geliştirmeyi zorunlu kılmıştır. Bu çalışmada, HZSM-5 tipi katalizörlerin MTBE sentez aktivitelerinin su buharı ile işlem yoluyla artınlabilme olanağı araştırılmıştır. Bu amaçla, farklı SİO2/AI2O3 oranlanna(30,50,80) sahip bir seri HZSM-5 katalizörü biri daha yüksek sıcaklıkta (300°C,535°C ) olmak üzere iki farklı koşulda su buharı ile modifiye edilerek bu örnekler ile MTBE sentezi deneyleri gerçeMeştirilmiştir. Elde edilen sonuçlar modifiye edilmemiş HZSM-5 örnekleri ve Amberlyst-15 ile gerçekleştirilen MTBE sentezi deney sonuçlan ile karşılaştınlmıştır. 300°C'de modifiye edilmiş HZSM-5 örnekleri ile MTBE sentezinde elde edilen düşük dönüşme değerleri, 300°C'de uygulanan su buharı modifikasyonunun HZSM-5 zeolitlerinin aktivitesini attırmadığını göstermektedir. 535°C'de modifiye edilen HZSM-5 örnekleri arasında yalnızca SİO2/AI2O3 oranı 50 olan HZSM-5 ile elde edilen dönüşme değerleri modifiye edilmemiş örnek ile elde edilen dönüşmelerden yüksek çıkmıştır. Daha düşük SİO2/AI2O3 oranında (30) modifikasyon işleminin dönüşmeyi artırmamış olması, su buharı ile işlem sonucu oluşan yapı dışı alüminyum atomlarının sayısının daha yüksek olması sonucunda bu atomların zeolitin gözenek sistemini tıkamış olmasına bağlanmıştır. SİO2/AI2O3 oranı daha yüksek olan (80) örnek için ise, zaten az olan asit merkez sayışırım, modifikasyon sonucu daha da azalmış olduğu ve kuvvetli asit merkezleri oluşturan Al çiftlerinden yeterince oluşamadığı düşünülmektedir. MTBE sentez aktivitesi modifikasyonla artınlabilmiş SİO2/AI2O3 oranı 50 olan HZSM-5 örneğinde, asit merkez sayısının azalmasına karşın, asit merkez kuvvetlerindeki artış, katalizörün aktivitesini artırabilmiştir.Modifikasyon sonucu gücü artan aktif merkezlerin sayısının zeolitin alüminyum içeriğinin kuvvetli bir fonksiyonu olduğu ve modifikasyon koşullarının, orjinal örneğin SİO2/AI2O3 oranına göre optimize edilebileceği anlaşılmaktadır.
The use of oxygenates as octane enhancing additives has gained great importance in the recent years due to environmental problems caused by the conventionally employed octane enhancing additives which are mainly responsible for the presence of lead in the commercial gasoline. The Clean Air Act Amendment, put into effect in the U.S.A., is the main reason for the limitations brought about to the use of the conventional additives since these lead to the production of gasoline that contains lead. On the other hand, the oxygenates may supply gasoline free from lead and have an excellent antiknocking performance. They also play a very important role in reducing the toxic gas emissions such as carbonmonoxide. Ethers such as methyl tertiary butyl ether (MTBE), ethyl tertiary butyl ether (ETBE), and tert-amyl methyl ether (TAME) and alcohols such as methanol and ethanol are the main types of oxygenates used. Among the various types of oxygenates, ethers are the best choice for improving the quality of gasoline since they can be much more perfectly mixed with gasoline with respect to alcohols. MTBE and gasoline have densities very close to each other so they can make a perfect mixture. This fact ensures MTBE to be the most commonly used oxygenate among ethers. As a result, the production of MTBE has substantially increased during the last few years. MTBE is commercialy produced by the liquid phase chemical reaction taking place between methanol and isobutylene in the presence of the Amberlyst-15 catalyst, a commercial acidic ion exchange resin, at temperatures of 40-80°C and a pressure of 1.5 Mpa. The reaction which is reversible and highly exothermic may be expressed as: CH3 CH3 I I H3C-C=CH2 + CH3OH o CH3- C -O-CH3 I CH3 The heat of reaction is -37 kJ/mol. The maximum amount of conversion is determined by the thermodynamic equilibrium limitations. Since the reaction is exothermic, at high temperatures although reaction rate increases the equilibrium conversion decreases. The following side reactions may also take place at high temperatures: - The reaction of isobutylene with water to give tertiary-butyl alcohol (TBA). S,H2 CH3 CH3-C-CH3 + H20 < ? CH3-C-CH3 OH - Isobutylene dimerization resulting in diisobutylene (DIB). CH2 CH2 CH3-C-CH3 + CH3-C-CH3 < ? CH2 11 " C H 3- C- C H3 The reaction of methanol with another methanol to give dimethyl ether and water. CH3-OH + CH3-OH « » CH3-O-CH3 + H20 The unstability of the Amberlyst-15 resin at temperatures above 90°C, the low selectivity due to side reactions occurring at high temperatures and the thermodynamic equilibrium limitations have made the development of an alternative catalyst necessary. Among the catalysts tested for the synthesis of MTBE, the zeolites HZSM-5 and HZSM-1 1 are the most promising ones. There are many advantages of using ZSM-5 for the MTBE synthesis. First of all, İt has a high thermal stability and is not corrosive. Moreover, ZSM-5 has a high catalytic activity as well as a high MTBE selectivity. It is also possible to obtain high MTBE yields at high temperatures. Besides the many advantages of HZSM-5, low MTBE conversion obtained in comparison with Amberlyst-15 presents the main difficulty concerning its commercial use in the MTBE syntesis. To solve this problem, modification methods may be tested in order to enhance MTBE syntesis activity. Since the activity of zeolites is originating from their acidic nature and the acidity of zeolites is related to the number and strength of acid sites, various modification methods are employed to provide change in the number and strength of acid sites. In this way, changes in the activity of the zeolites may be provided. The modifications applied to the zeolites generally involve the dealumination of the framework of the zeolites. The dealumination process increases the SİO2/AI2O3 ratios of the zeolites because the aluminum atoms in the framework turn into extraframework aluminum located in the channels and cavities of the zeolites. The catalytic properties Ml of the high silica zeolites obtained by dealumination are expected to differ from those of the samples with the same SİO2/AI2O3 ratios which are synthesized directly. The reasons for this fact are the presence of nonframework aluminum as well as the crystal defects. Steam treatment is a well known modification method leading to dealumination. It is a hydrothermal treatment which causes high temperature-hydrogenolysis of Si-O-Al bonds resulting in limited damage to the structure thus causing the formation of mesapores. The pores formed are partially filled by silicon atoms from the amorphous SİO2 originating from the damaged crystal regions. Steam treatment is controlled by four factors, namely the temperature of the catalyst bed, the partial pressure of the water vapor in the inert gas that is fed onto the catalyst bed, the flow of the carrier gas and the duration of the steam treatment process. The studies on the effects of steaming conditions applied have shown that a high partial pressure of the water vapor, a high temperature of the catalyst bed and a long period of steaming time all cause severe steaming leading to a loss in the activity. On the other hand, milder steaming conditions favor the activity enhancement of the zeolites. The activity enhancement occurring after the steaming process is thought to arise from the interaction between framework and nonframework aluminum. This activity enhancement is directly related to a decrease in the number of acid sites since it is known that the strong interactions between the acid sites of the unmodified zeolites result in a loss of their strength. On the other hand, a decrease in the number of the acid sites occuring after the steam treatment process leads to a decrease in the strength of the interaction between them and results in the formation of stronger acid sites than the ones found in the structure of the unmodified zeolites. The purpose of this study is to investigate the possibility of enhancing the activity of the HZSM-5 catalyst to be used in MTBE synthesis. For this aim, a series of HZSM-5 catalysts with different Si02/Al203 ratios have been treated with steam and the test reactions for MTBE synthesis have been carried out. The results obtained for the steam-treated HZSM-5 samples have been compared with those obtained with the ones that have not been treated with steam but have the same SİO2/AI2O3 ratios as well as Amberlyst-15. The process of steam treatment was carried out under two different conditions. Before the treatment, the catalyst was activated under N2 stream for 4 hours. The catalyst bed was held either at 300°C or at 535°C. The carrier gas was fed into the saturator which was full of distilled water and thus was saturated with water vapor. Then the carrier gas was fed onto the catalyst bed under predetermined conditions. The vapor pressure of the carrier gas was controlled by the temperature of the water bath in which the stainless steel saturator was placed. The temperature of the catalyst bed was measured by means of a Ni-CrNi thermocouple while the temperature of the water bath was controlled by a contact thermometer. Mil MTBE synthesis was carried out in a stainless steel reaction system. The gases were fed from pressurized tanks while methanol was fed by a dosage pump. The reactants were first sent to a preheating region full of glass beads and then to the reactor. In the preheating region, methanol was vaporised and a perfect mixing of the reactants took place. Afterwards, the mixture was sent to the gas cromotography instrument which was equipped with a FID and a Porapak Q column, via the by-pass line in order to control the composition of the mixture. After verifying the mixture which was a prerequisite for determining the reaction conditions, the reactants were fed to the reactor. The inner and outer temperatures of the reactor were measured by Ni-CrNİ thermocouples, one placed inside the catalyst bed and the other placed on the outer surface of the reactor close to the catalyst. MTBE synthesis test reactions were carried at the temperatures of 60, 70, 80 and 90°C, using a methanol to isobutylene ratio of 1 and a GHSV of 17h"\ For the unsteamed HZSM-5 samples, the MTBE yield and the reaction rates increased with the SİO2/AI2O3 ratio until a maximum value corresponding to a SİO2/AI2O3 ratio of 80 was attained. It has been determined that a SİO2/AI2O3 ratio of 80 is an optimum value for MTBE synthesis. The same behavior has been observed for the HZSM-5 catalysts steamed at 535°C and 300°C, respectively but the maximum amount of conversion of MTBE for the former one has been obtained at a SİO2/AI2O3 ratio of 50 while for the latter one it has been obtained at 80. Since the same optimum SİO2/AI2O3 value has been obtained for the unsteamed and steamed samples at 300°C, it is apparent that steaming at 300°C is a mild treatment and no improvement was obtained in the MTBE conversions. On the other hand, steaming at 535°C provides a different optimum SİO2/AI2O3 ratio. When a HZSM-5 sample having a SİO2/AI2O3 ratio of 50 was employed, a higher amount of MTBE conversion was attained at reaction temperatures of 80°C and 90°C. It is thought that steaming at 535°C has a more severe effect and thus provides a more effective dealumination in the zeolite structure. As a result, formation of acid sites stronger than the ones pertaining to both of the HZSM-5 samples steamed at 300°C and unsteamed was observed. The fact that steaming at 535°C of the HZSM-5 sample with a Si02/Al203 ratio of 30 did not provide a catalyst with higher activity may be related to the higher number of acid sites in the original sample which may have resulted in higher amount of extraframework aluminum species that might have blocked the diffusion pathways in the pore network. On the other hand, steaming of the HZSM-5 sample with a SİO2/AI2O3 ratio of 80 did not result in an activity increase either. This may be related to the already smaller number of acid sites in the original catalyst, that must have further decreased as the result of dealumination provided by steam treatment. Very few acid sites must have remained and as a result the aluminum pairs required to enhance the activity probably could not form in the treated sample. As mentioned before, the number of the acid sites and the distance between them are indications of the level of the activity of the zeolite. Accordingly, as the number of acid sites increases, the distance between them shortens. A short distance results in an excess amount of interaction between the acid sites and thus in the loss of strength of the acid sites. For this reason, the enhancement of the number of the acid sites is directly related with the acid site concentration of the original zeolites. XIV The low amount of MTBE conversions obtained for the HZSM-5 catalysts having Si02/Al203 ratios of 150 and 280 indicate that the increased strength of the acid sites in these samples was not sufficient to compansate for the decrease in their number. The fact that activity increase was achieved for the sample with a SİO2/AI2O3 ratio of 50 and not for those with 30 or 80 indicates that the modification conditions could be varied to optimize the MTBE activity, with respect to the Si02/Al203 ratio of the original HZSM-5 samples.
Açıklama
Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 1998
Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 1998
Anahtar kelimeler
Katalizörler, Metil tersiyer bütil eter, Su buharı, Zeolitler, Catalysts, Methyl tertiary butyl ether, Water vapor, Zeolites
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