Zeolite thermal characterization

This book covers a wide range of lectures given at the Short Summer Course celebrated at La Laguna University about the new trends in the modern characterization methods chemical, microscopies, thermal. X-ray, resonance and nuclear methos for ceramics, glasses and related materials. This book also looks also at the zeolite thermal characterization methods as well as the new approaches in new microscopies for the investigation of surfaces in materials. 

The spectra of the typical acid catalyst indicators (diphenylamine, triphenylmethane, triphenylcarbinol) adsorbed on Na-zeolites are characterized by absorption bands of adsorbed molecules in the molecular state (7, 8, 9). The observed spectral behavior is in accordance with the general concepts of adsorption on zeolites (6). The spectrum of triphenylcarbinol adsorbed on Na-zeolite after thermal treatment in vacuum at 300°C (Figure 2, curve 1) is characterized by absorption bands at 260,... 

The thermal transformation of Type 4A zeolite begins at 550°C, as indicated by the decrease of water vapor adsorption capacity. This capacity is nonexistent after a thermal treatment at 670°C. We have demonstrated that, after grinding the samples which have been heated previously at this temperature, half of the zeolitic phase, characterized by its water retention capacity, remained. The residual zeolite is thermally unstable. It has the same x-ray diffraction pattern as the initial zeolite, but should not have the same chemical composition. We have shown that the solid-solid transformation is accompanied by a closed macroporosity which disappears gradually with sintering. There is every reason to believe that the solid-solid transformation begins at the periphery of the particles and progresses towards the center. 

This chapter discusses the synthesis, characterization and applications of a very unique mesoporous material, TUD-1. This amorphous material possesses three-dimensional intercoimecting pores with narrow pore size distribution and excellent thermal and hydrothermal stabilities. The basic material is Si-TUD-1 however, many versions of TUD-1 using different metal variants have been prepared, characterized, and evaluated for a wide variety of hydrocarbon processing applications. Also, zeolitic material can be incorporated into the mesoporous TUD-1 to take the advantage of its mesopores to facilitate the reaction of large molecules, and enhance the mass transfer of reactants, intermediates and products. Examples of preparation and application of many different TUD-1 are described in this chapter. 

Another thermal analysis method available for catalyst characterization is microcalorimetiy, which is based on the measurement of the heat generated or consumed when a gas adsorbs and reacts on the surface of a solid [66-68], This information can be used, for instance, to determine the relative stability among different phases of a solid [69], Microcalorimetiy is also applicable in the measurement of the strengths and distribution of acidic or basic sites as well as for the characterization of metal-based catalysts [66-68], For instance, Figure 1.10 presents microcalorimetry data for ammonia adsorption on H-ZSM-5 and H-mordenite zeolites [70], clearly illustrating the differences in both acid strength (indicated by the different initial adsorption heats) and total number of acidic sites (measured by the total ammonia uptake) between the two catalysts. 

Even if the metal cation exchanged zeolite is not dehydrated, the introduction of the ligand or its precursors frequently requires thermal treatment, which can redistribute the cations within the zeolite. Not all metal ions will become com-plexed and their presence can complicate the characterization of the materials as well as the interpretations of catalytic reactions. In principle, the uncomplexed metal ions should be removed by back-extraction. Unfortunately, however, this is a requirement more frequently honored in the breach, rather than the observance. 

CVD of Mo(CO)is into zeoHtes has been used as first step in the preparation of samples containing well-defined oxicarbide, oxinitride and sulfide used as catalysts in different reactions [25, 35-37]. Several cycles of adsorption of Mo(CO),5 in a NaY zeolite followed by thermal treatment afforded an oxicarbide dimer species in the zeolite cages that was characterized by EXAES [36]. 

The spectral behavior of CO bonded to metal atoms (metal carbonyls) has been used to characterize the surface of solids (61). For instance, it is known that metal carbonyl interacts with surface site of metal oxides and zeolites to form a Lewis-type adduct where a CO ligand of the metal carbonyl interacts (via the oxygen atom) with surface OH groups or with co-ordinatively unsaturated metal ions (surface Lewis acid sites) (62,63). On the other hand, thermal treatment of the metal carbonyl support adducts lead to loss of CO with formation of subcarbonyls, which are anchored to the support (64,65). Papile et al. (66) reported the characterization... 

TTigh silica zeolites attract great attention since they are characterized by relatively high thermal stability and considerable acid resistance. Physicochemical properties of high silica zeolites, despite a number of investigations, have not been sufficiently studied. The same is true for L- and clinoptilolite zeolite. The data on synthesis, structure, adsorption properties, decationization, dealuminization, adsorption heats, and other properties of the above-mentioned zeolites have been given (1-15). Results of studies of physicochemical properties of L zeolites and of natural and modified clinoptilolite are given here. 

NaY zeolite at 60.1 ppm. The image was obtained for a 3mm slice with full chemical shift imaging (note that for thermally polarised Xe this type of imaging experiment would be far more demanding in terms of experimental time even than chemical shift resolved imaging, as practiced for the Aerogel samples[30]), and was obtained in 30 min. Thus, the improvement in imaging with HP xenon over thermally polarized xenon is impressive, and indicates that there are real prospects for applications in the characterization of materials. 

The thermal and catalytic conversion of different hydrocarbon fractions, often with hydrotreating and other reaction steps, is characterized by a broad variety of feeds and products (Table 1, entry 4). New processes starting from natural gas are currently under development these are mainly based on the conversion of methane into synthesis gas, further into methanol, and finally into higher hydrocarbons. These processes are mainly employed in the petrochemical industry and will not be described in detail here. Several new processes are under development and the formation of BTX aromatics from C3/C4 hydrocarbons employing modified zeolite catalysts is a promising example [10],... 

Subsequently, it is possible to consider that the adsorbate-adsorbent interaction field inside these structures is characterized by the presence of sites of minimum potential energy for the interaction of adsorbed molecules with the zeolite framework and charge-compensating cations. A simple model of the zeolite-adsorbate system is that of the periodic array of interconnected adsorption sites, where molecular migration at adsorbed molecules through the array is assumed to proceed by thermally activated jumps from one site to an adjacent site, and can be envisaged as a sort of lattice-gas. 

In petrochemical and oil refining operations, the zeolite is primarily responsible for the catalyst s activity, selectivity and stability (catalytic, thermal and hydrothermal). The fluid catalytic cracking process (FCC) is the most widely used of the oil refining process and is characterized by the use of a finely divided catalyst, which is moved through the processing unit. The catalyst particles are of such a size (about 70 pm) that when aerated with air or hydrocarbon vapor, the catalyst behaves like a liquid and can be moved easily through pipes. 

The next stage of characterization focuses upon the different phases present within the catalyst particle and their nature. Bulk, component structural information is determined principally by x-ray powder diffraction (XRD). In FCC catalysts, for example, XRD is used to determine the unit cell size of the zeolite component within the catalyst particle. The zeolite unit cell size is a function of the number of aluminum atoms in the framework and has been related to the coke selectivity and octane performance of the catalyst in commercial operations. Scanning electron microscopy (SEM) can provide information about the distribution of crystalline and chemical phases greater than lOOnm within the catalyst particle. Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) can be used to obtain information on crystal transformations, decomposition, or chemical reactions within the particles. Cotterman, et al describe how the generation of this information can be used to understand an FCC catalyst system. 

The calcosilicate zeolite-like crystal material CAS-1 was hydrothermally synthesized and the thermal stability of the samples were investigated. The effects of composition of raw materials, reaction temperature and alkali metals on the synthesis of CAS-I were addressed. Cation exchange reactions and their influences on the thermal stability of CAS-I framework structure were also studied. The samples were characterized by XRD, TEM, SEM, DT-TGA, AAS and chemical analysis. The results showed that CAS-1 could be obtained from a wide range of composition of raw materials and reaction conditions. The cations have great influence on the thermal stability of the CAS-I framework structure. 

The success of the fluidized bed in eliminating interference from thermal or photochemical decomposition products is evident in the spectra characterizing benzene adsorbed in Fl-USY zeolite shown in Fig. 6. The strong peak at 990 cm in liquid benzene (6a) is assigned to a symmetric ring-breathing mode. This band is... 

Numerous reviews dealing with the adsorption capacities and acid-base proper-hes of zeolites have been published [4, 8, 10-14, 17] so we will not give a detailed description of these systems. Only some case studies will be given in order to assess the possibilihes of thermal techniques for characterizing such materials. In general the total number of acid sites is greater in zeolites than in amorphous silica-aluminas for a similar Si Al ratio. 

---