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Authors: Viswanadham, Nagabhatla
Issue Date: 1996
Abstract: Zeolites which are crystalline aluminosilicates having three dimensional porous framework of silica and alumina have been attracting the attention of researchers working in the area of catalysis worldwide, by virtue of their properties like shape-selectivity, acidity, thermal stability and ionexchange capacity. More over, non-corrosive, environment-friendly and economically viable nature of the processes based on zeolite materials, make them suitable for industrial exploitation. With the advent of new synthesis and characterization techniques, these zeolite materials are emerging as catalysts in many more frontier areas like petroleum, petrochemicals, fine chemicals and in synthetic organic processes and, replacing the traditional amorphous and liquid acid catalysts. Among these materials, ZSM-5 occupies a unique place and ZSM-5 based catalysts used in many process such as xylene isomerization, methanol to gasoline and diesel, methanol to olefins and conversion of lower alkanes to aroamtics. Conversion of lower paraffins into BTX (Benzene, Toluene and Xylenes) aromatics is an important process for the valorization of the low value commercial feed stocks. The process attracts significance, because the aromatic rich product can be used as raw material for the production of various petrochemicals, or it can also be used as gasoline blending feed stock. ZSM-5 zeolite of pentasil family was considered to be a promising catalyst for the purpose, when it is used in combination with metal dehydrogenation function (e.g. Zinc and Gallium). The aromatization activity of the catalyst depends on the various factors like 1) Nature, number and strength of acid sites 2) Amount, nature and the nature of interaction of dehydrogenating component, and its distribution in zeolite channels, 3) Nature of reactants and other reaction conditions. Active research has been going on to understand the influence of above mentioned factors on the reaction pathways of the reactant molecules. II In this thesis a detailed account of influence of synthesis variables of the ZSM-5 zeolite, post synthesis modifications of the ZSM-5 zeolite by hydrothermal and metal loading on various physicochemical properties and catalytic activity for aromatization of light alkanes have been discussed. Among the synthesis variables, the Si/Al ratio and synthesis without using template were studied in detail. In the post synthesis modification hydrothermal treatment and effect of metals were studied. In both the cases aromatization of n-heptane was chosen as model reaction. The crux of the whole endeavour has been to evaluate the number and strength of acid sites of the above said catalyts and relate them to catalytic activity and selectivity. In the case of hydrothermal treatment the role of extra framework alumina in generation of acid sites of enhanced strength and their relation to catalytic activity for aromatization reaction were discussed at length. In the case of Si/Al variation a linear correlation between acid sites > 90 KJ/mole and catalytic activity was realized, emphasising the importance of strong acid sites in such reactions. The aromatization reaction can be optimized with the help of metals. In order to understand the role of metal in this transformations, metals such as zinc and gallium were incorporated in the catalyst by various methods and consequent gains in aromatization were assessed. The catalysts based on some of the above mentioned formulations were studied in pilot plant reactor. The basic research reported in the thesis led to the development of a process for aromatization of light naphtha and natural gas liquids. The contents of the thesis are classified into eight chapters. Chapter 1 reviews historical development of zeolite materials with special emphasis on their synthesis characterization and applications to various catalytic processes. Chapter 2 gives the detailed description of the experimental methods adopted for synthesis, characterization and activity evaluation of ZSM-5 zeolites. Ill In Chapter 3 synthesis without aid of organic template and its effect on aromatization activity of n-heptane was discussed. Generally organic templates like tetrapropyl ammonium bromide (TPA Br) act as structure-directing agents for the zeolite synthesis. Since, these organic reagents are expensive, corrosive and to be removed before the zeolites are used for actual applications, synthesizing zeolites without using any template is advantageous. These studies indicated that the formation of small, uniform size zeolite crystals with good crystallinity, in presence of a small amount of (~ 2 wt %) seed materials. The activity and physico-chemical properties of these zeolites are comparable to those where the template was used in ZSM-5 synthesis. The effect of variation of Si/Al ratios on acidity and catalytic activity was discussed in Chapter 4. Since, aluminum is the source of acidity and activity of the zeolite catalyst, making balance between silicon and aluminum ions is the key step in interplay with acidity and activity of zeolites, to obtain the desired products. The results indicated that 2.6 Al/U.C. is the optimum requirement of framework aluminum at a typical process condition for the maximization of aromatic products. The effect of hydrothermal treatment on acidity and aromatization activity was discussed in Chapter 5. Attempts made to estimate the nature and role of enhanced acid sites created by mild steaming indicatedthat these sites are formed by the synergistic interaction between bridging hydroxylsand acid insoluble extra framework aluminum, and play a positive role in aromatization of paraffins. These catalysts were also active for the conversion of low-paraffin-containing commercial feed stocks like NGL and light naphtha. In addition to the acidity, aromatization activity also depends on the nature and amount of dehydrogenating function and these effects are discussed in Chapter 6. Various metal incorporation methods have been adopted for the synthesis of metal loaded ZSM-5 catalysts. Results indicated that IV the initial activity of the all the zinc containing catalysts was independent of the method of metal incorporation, where as the rate of deactivation of these catalysts differ in the order Zn(imp) > Zn(ion) > Zn-Al-Silicate. In case of Ga-Al-Silicate, the formation of extra-framework gallium was observed to give positive influence on aromatization activity. Catalyst stability and regenerability are the two important factors, that govern the economic viability and commercialization of the any given process. In order to gain insight in this direction catalyst deactivation studies (Chapter 7) on the above mentioned catalysts were carried out and the results indicated that, strong acid sites deactivate faster during the coke formation, where as binder materials like a-alumina have a positive influence on the catalyst stability. As C3, C4, are the major by-products in the aromatization reaction, keeping in view of LPG demand in mind, two processes NTGG-1 and NTGG-2 (Naphtha to gas and gasoline) were developed. The pilot plant studies on promising catalysts are discussed in Chapter 8 and economic evaluation suggested the viability of the process. In conclusion, the thesis deals with interesting observations from academic point-of-view, as well as industrial application of these materials for aromatization process.
Other Identifiers: Ph.D
Appears in Collections:DOCTORAL THESES (chemistry)

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