Bu çalışmada ZSM-5 destekli molibden ve renyum katalizörler üzerinde ve oksijensiz ortamda metanın aromatizasyonu reaksiyonu araştırılmıştır. Katalizörlerin deaktivasyondan en az etkilendiği koşullarda (düşük metan kısmi basıncı, yüksek akış hızı) elde edilen aktivite, gerçek aktivitedir ve katalizörler bu koşullar altında incelendiklerinde, seçici yüzey dealüminasyonu ile katalizörün aktivitesinin azaldığı gözlemlenmiştir. Bunun nedeni dış yüzey dealüminasyonu sonucunda katalizörün aktif merkez sayısının azalmasıdır. Yüzeydeki aktif merkezlerin eliminasyonu, gözenek ağızlarının tıkanmasına neden olan yüksek aromatik bileşiklerin oluşumunu kısıtladığından, yüzey dealüminasyonu sonucunda katalizörün kararlılığı artmıştır. $CaC_2$ ile ön işlem sonucu elde edilen düşük indüksiyon ve $t_{maks}$ süreleri, $CaC_2$ ile işlemin katalizörü reaksiyon öncesi karbürize ettiğinin göstergesidir. $CaC_2$ ile hazırlanan örnek ile elde edilen yüksek aktivitenin, $(MoO_2)^{2+}/(Mo_2O_5)^{2+}$ ile $CaC_2$’ün seçimli reaksiyonu sonucunda zeolitin iskelet yapısında daha iyi dağılmış $Mo_2C$’ün oluşumundan kaynaklandığı düşünülmektedir. Hangi yöntem ile karbürize edildiğine bağlı olmaksızın tüm örnekler, benzer şekilde deaktive olmuştur. ZSM-5’in dış yüzey asit merkezi başına düşen maksimum reaksiyon hızları zeolitin dış yüzey alanı ile doğrusal olarak artmıştır. Bu durum, ZSM-5 morfolojisine bağlı olarak değişen kütle taşınım sınırlamalarının katalitik performans üzerinde önemli bir etkisi bulunduğunu göstermiştir. Asit merkez içeren hidrojen formundaki Re/Mo-ZSM-5 katalizörlerine oranla kalsiyum formundaki Re/Mo-ZSM-5 katalizörleri ile çok daha düşük aktivite değerleri elde edilmiştir. Kalsiyum formundaki ZSM-5 örneğinde $C_2$ bileşenlerinin aromatizasyonunu gerçekleştirebilecek asit merkezler bulunmamaktadır. Metanın aromatizayonu reaksiyonunda yüksek aktivitede bir katalizör hazırlanabilmesi için ZSM-5’in yeterli miktarda asit merkezlere de sahip olması gerektiği belirlenmiştir.

Conversion of methane into useful chemicals has been and still is a subject of worldwide renewed efforts. Several routes have been explored over the previous two decades. In addition to methane conversion into syngas, which is of real industrial significance, more ambitious routes such as direct conversion into oxygenates, oxidative coupling of methane and conversion of methane into benzene and hydrogen were explored. The latest developments were concerned with the conversion of methane into benzene and hydrogen in the presence of various transition metal oxide catalyst precursors. Mainly molybdenum and rhenium based active components deposited over HZSM-5 were investigated. In the case of molybdenum-based catalysts, the activation procedure was scrutinized, and it was generally agreed that molybdenum was converted, in the presence of methane, under the reaction conditions, into molybdenum carbide. The reaction proceeded via the formation of $C_2H_2$ (or $C_2H_4$) over the molybdenum carbide species and further cyclization and aromatization would occur over the acid sites. The present study was conducted to understand the different aspects of the aromatization reaction of methane under non-oxidative conditions. In an effort to understand the specific effect of external acid sites on the catalytic performance, the activities of catalysts with molybdenum supported on HZSM-5 and surface dealuminated HZSM-5 were compared under two different reaction conditions. Molybdenum carbide associated with a surface dealuminated HZSM-5 appeared to be more active and more stable than when associated to the parent HZSM-5, contrary to the expectation, in the conversion of a 1:1 mixture of methane and argon at atmospheric presssure and low flow rate. By contrast, under a low methane partial pressure and a high flow rate, the expected order of activity was found; the carbide associated with the parent zeolite showed a significantly higher activity. The discrepancy shown by the results of the two sets of experiments was interpreted in terms of a more rapid deactivation of the most active catalyst under a high methane load, resulting in a lower apparent activity. The higher stability of the carbide associated with the surface dealuminated zeolite resulted from the elimination of the surface sites which produce bulky aromatics with low vapor pressure leading to a rapid clogging of the pore mouths of the zeolite, in line with the general background on zeolite catalysis. Treatment with $CaC_2$, a possible carburization agent, in addition to carburization with a $CH_4/H_2$ mixture as was routinely carried out in the previous studies, enhanced the activities of the catalyst. The higher dispersion of $Mo_2C$ species in the zeolite framework by preferred reaction of $CaC_2$ with $(MoO_2)^{2+}$ or $(Mo_2O_5)^{2+}$ species was proposed as a plausible explanation for the higher activities. Induction period seen in the case of the sample prepared without precarburization decreased with $CaC_2$ treatment This result shows that carburization of $MoO_3$/HZSM-5 catalyst took place to a considerable extent prior to the reaction with CaC2 treatment. All samples, precarburized with $CaC_2$ or $CH_4/H_2$ mixture or carburized in the course of the reaction, deactivated in a similar manner after the variable induction period. In order to study the influence of the catalyst support on the growth and location of the molybdenum carbide and coke precursors in this reaction, the activities of molybdenum catalysts supported on three HZSM-5 samples, which differ in their morphology and aluminum content, were compared and it was seen that the rates of conversion of methane into hydrocarbons appeared not to vary simply with the number of acid sites as usually expected. Turnover frequencies of the surface acid sites, calculated using the Si/Al ratios determined from XPS measurements, were observed to vary linearly with the external surface area, indicating that transport phenomena were determining the overall rate. In an effort to see the possible effect of the acidity of the inorganic carrier, calcium and hydrogen forms of MFI were used as support for rhenium and molybdenum catalysts. HZSM-5-supported molybdenum and rhenium catalysts were observed to be more active for the aromatization of methane, compared to those supported on CaZSM-5. As evident from the formation of aromatics, the reaction also proceeded, but to a lower extent on CaMFI-supported molybdenum and rhenium catalysts, which were shown to have only residual acidity. Although highly dispersed rhenium is expected to form over CaZSM-5, lower conversion values obtained with CaZSM-5-supported rhenium catalyst underline the importance of acid sites on the formation of aromatics, in line with the commonly accepted bifunctional mechanism.