Zeolites programmed desorption

Temperature Programmed Desorption (TPD). Chemisorbed molecules are bonded to the surface by forces dependent on the nature of the sites. For instance, ammonia will be strongly adsorbed on acid sites, whereas it is only weakly adsorbed on basic sites. Consequently, the adsorbate complex formed with the basic sites will decompose at lower temperatures than that formed with the acid sites. The following example regarding the NH.i-zeolite H-ZSM-5 system will illustrate this. 

Lok, B.M., Marcus, K.K., and AngeU, C.L (1986) Characterization of zeolite addity. 11. Measurement of zeolite acidity by ammonia temperature programmed desorption and FTIR spectroscopy techniques. Zeolites, 6, 185-194. 

Katada, N., Igi, H., Kim, J.H., and Niwa, M. (1997) Determination of the acidic properties of zeolite by theorecti-cal analysis of temperature programmed desorption of ammonia based on adsorption equilibrium. J. Phys. Chem. B, 101, 5969-5977. 

Torre-Abreu, C Ribeiro, MF Henriques, C Delahay, G. Characterisation of CuMFI catalysts by temperature programmed desorption of NO and temperature programmed reduction. Effect of the zeolite Si/Al ratio and copper loading, Appl. Catal, B Environmental, 1997, Volume 12, Issues 2-3, 249-262. 

With simple probe molecules, such as H2, information about the number of surface metal atoms is readily obtained by using adsorption measurements. However, even with such simple probe molecules further information about the heterogeneity of a surface may be obtained by performing temperature-programmed desorption measurements. With probe molecules which are chemically more specific (e.g., NH3 and organic amines, H2S and organic sulfides) it may be possible to obtain information about the number and nature of specific types of surface sites, for example, the number and strength of Lewis or Bronsted acid sites on oxides, zeolites or sulfides. 

The most likely reason for the high activity of zeolite BEA is the relatively high BET surface area of the catalyst (750 m2/g). Furthermore there are hints by temperature-programmed desorption (TPD) of ammonia that a large amount of acid sites are present. We assume that the alkoxylation of limonene takes place inside the pore structure of the beta zeolite. The high selectivity of zeolite BEA might originate from suitable acid sites in pores of its defined size and shape. 

The most likely reasons for the high activity of beta zeolite are the relatively high BET surface area of the catalyst (750 m2/g) and the large amount of acid sites measured by temperature-programmed desorption (TPD) of ammonia(58). 

In this paper, we will discuss the use of temperature programmed desorption as a complementary technique for obtaining lattice concentrations for high-silica materials. While TPD of ammonia is commonly used as a measure of the number of acid sites in zeolites, this paper will discuss the use of the reactive probe molecules, 2-propanamine and 2-propanol. It has previously been demonstrated that well-defined adsorption complexes, with a stoichiometry of one molecule/Al atom could be observed in high-silica zeolites for a number of adsorbates. These complexes were found to be independent of Si/Al2 ratio for a series of H-ZSM-5 zeolites and were unaffected by... 

Acid Strength Distribution After Stabilization Treatment. The acid strength distributions of the zeolites were determined by temperature-programmed desorption of ammonia equipped with a thermal conductivity detector. The detailed procedure has been given in the early paper (6). 

Exchanged zeolites were characterized by N2 adsorption at 77K, X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), immersion calorimetry and NHs-temperature programmed desorption (NH3-TPD). X-ray diffraction patterns (XRD) were obtained with a JSO Debye-Flex 2002 system, from Seifert, fitted with a Cu cathode and a Ni filter, using CuXa radiation (A,=1.5419) and 2°min of scanning rate. X-ray photoelectron spectroscopy (XPS) spectra were acquired with a VG-Microtech Multilab 3000 spectrometer equipped with a hemispherical electron analyzer and Mg Ka (1253.6 eV) 300W X-ray source. 

The adsorption property was measured by a static method at 30 °C with a conventional volumetric apparatus as well as by the temperature programmed desorption (TPD) method. The details of the pretreatment and adsorption procedures were shown in Results and Discussion section. Metal-loaded zeolite samples were characterized by XRD, diffuse reflectance UV-Vis spectroscopy (DRS) and electron spin resonance (ESR). 

Temperature-programmed desorption of ammonia from molybdenum-loaded Y-zeolites... 

Dealuminated M-Y zeolites (Si/Al = 4.22 M NH4, Li, Na, K, Cs) were prepared using the dealumination method developed by Skeels and Breck and the conventional ion exchange technique. These materials were characterised by infrared spectroscopy (IR) with and without pyridine adsorption, temperature-programmed desorption (t.p.d.) of ammonia. X-ray difiracto-metry (XRD) and differential thermoanalysis (DTA). They were used for encapsulation of Mo(CO)5. Subsequent decarbonylation and ammonia decomposition was monitored by mass spectrometry (MS) as a function of temperature. The oxidation numbers of entrapped molybdenum as well as the ability for ammonia decomposition were correlated to the overall acidity of the materials. It was found that the oxidation number decreased with the overall acidity (density and/or strength of Bronsted and Lewis acidity). Reduced acidity facilitated ammonia decomposition. 

The aim of this study was to measure the acidity of several dealuminated zeolitic supports and their Mo-loaded equivalents via temperature-programmed desorption (t.p.d.) of ammonia and correlate it with their ability to decompose ammonia. 

The samples, prepared as self-supporting wafers, were activated in vacuum for 1 h at 873 K (MOR and ZSM5) or for 4 h at 773 K (Y zeolites). The adsorption of thiophene at 1 mbar partial pressure and room temperature were investigated by microgravimetry and IR spectroscopy. After evacuation of the zeolites at room temperature, the temperature programmed desorption (TPD) of thiophene was carried out and followed by IR spectroscopy and mass spectrometry. 

The temperature-programmed desorption of low-molecular weight alcohols was studied over tungstated zirconia and acidic zeolites. Tungstated zirconia (WZ) acids have less tendency to deactivate due to carbon deposition and to form oligomeric products. Proton exchange is also less important in WZ. ... 

The hydrothermal deactivation of Y zeolite containing 0, 4, 7 and 12 wt.% of REO and its effects on catalytic activity, stability and selectivity were investigated. The Y zeolites were hydrothermally deactivated at 788°C in three consecutive cycles of two hours each. The fresh and deactivated zeolites were characterized by measuring Unit Cell Size (UCS) and surface area. The acidic properties were measured by the Temperature Programmed Desorption (TPD) of ammonia and IR-pyridine desorption. In order to correlate structural, textural and acid properties with catalytic behavior, the zeolites were evaluated in the conversion of cyclohexane. The Hydrogen Transfer Index (HTI) measured as a ratio of paraffins to olefins is a parameter of the selectivity. It was found that the REO was incorporated into zeolite structure up to high concentrations modifying to some extent XRD deflection, the acidic properties and the HTI ratio. After deactivation, the acidity and HTI were diminished and the Lewis/Bronsted acid ratio was modified. HTI decreased as REO concentration increased. 

The powder X-ray diffraction patterns were measured in a D-500 SIEMENS diffractometer with a graphite seeondary beam monochromator and CuKoj contribution was eliminated by the DIFFRAC/AT software to obtain a monochromatic CuKa,. The Unit Cell Size (UCS) was measured following the ASTM D-3942-90 procedure. The Surface areas were measured by nitrogen adsorption at 75 K on a Micromeritics Accusorb 2100 E equipment using the ASTM method D-3663-78. Temperature Programmed Desorption (TPD) of ammonia and pyridine adsorption by Infrared Spectroscopy (IR) were used to characterize the acidity of the zeolites. For IR-Pyridine the spectra were recorded each 100°C and the characteristic bands of Lewis and Bronsted acid sites (1444 cm" and 1540 cm, respectively) were integrated in order to obtain the total acid sites. 

The concentration of acid sites was determined by temperature programmed desorption (t.p.d.) of NH3. The degree of ion exchange was calculated from the difference in concentration of the strong Brensted acid sites present before and after ion exchange. The structure of the zeolites after synthesis or postsynthetic modification was verified by XRD. 

Since zeolites are typical acid-base catalysts, their acid-base properties are of great importance in investigating the catalytic decomposition of hydrocarbons. Three methods — titration, temperature-programmed desorption, and characterization by test reaction — are employed to measure acid-base properties. In this study, n-hexane was used as a model hydrocarbon and its decomposition over HY, HCeY, HSmY, and HCuY zeolites was investigated. Depending on the metal exchanged, n-hexane conversion and product distribution were observed to vary in the higher ccmversion r ion. The relation between product distribution and the acid-base properties of the zeolites are discussed. 

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