Abstract:Publication costs assisted by the Centre National de la Recherche ScientlfiqueThe catalytic activity for isooctane cracking, the surface acidity (using colored indicators and ir spectra of chemisorbed pyridine), and the concentrations in various OH groups (by means of ir and ESR spectroscopies) have been measured for Mg-Y zeolites having various H+ and Mg2+ contents and the same Na+ content. The influence of Mg2+ cations on these properties are discussed in terms of cation location and polarizing field. On the… Show more
“…Nevertheless other zeolite properties are characteristics of substituent effects such as the known increase of acid strength upon introduction of polyvalent cations. 30,31 The explanations proposed32,33 related to the field of the cation are quite relevant of substituents effects in solution.…”
Hypothesis on Zeolites Physicochemical Properties tribution to the actual mechanism of hydrogenation.Strictly, these conclusions apply only to the 11-pulse experiments at 0 and -31 °C on Pt/Si02 catalysts. The agreement between turnover frequencies in pulse and flow experiments suggests that these conclusions can be extended to flow experiments protracted for many minutes.The fact that the apparent activation energy is independent of temperature13'18 suggests that the conclusions can be extended to temperatures higher than those of the present experiments. However, the present paper provides no evidence as to whether reactions at much higher temperatures involve or do not involve transfer of hydrogen via carbonaceous residues.
“…Nevertheless other zeolite properties are characteristics of substituent effects such as the known increase of acid strength upon introduction of polyvalent cations. 30,31 The explanations proposed32,33 related to the field of the cation are quite relevant of substituents effects in solution.…”
Hypothesis on Zeolites Physicochemical Properties tribution to the actual mechanism of hydrogenation.Strictly, these conclusions apply only to the 11-pulse experiments at 0 and -31 °C on Pt/Si02 catalysts. The agreement between turnover frequencies in pulse and flow experiments suggests that these conclusions can be extended to flow experiments protracted for many minutes.The fact that the apparent activation energy is independent of temperature13'18 suggests that the conclusions can be extended to temperatures higher than those of the present experiments. However, the present paper provides no evidence as to whether reactions at much higher temperatures involve or do not involve transfer of hydrogen via carbonaceous residues.
“…Likewise, zeolites have shown highly distinctive behaviors of either electron acceptors (acids) or donors (bases), depending on the nature of the cation, the Si/Al ratio, and other modifications. Accordingly, a large number of experimental − and theoretical 49-55 studies have been directed to derive the factors that govern the donor−acceptor properties of zeolites and to develop the systematic methods to effectively monitor the properties.…”
Section: Introductionmentioning
confidence: 99%
“…As has been pointed out, however, special care should be taken in using the conventional methods. For example, the aforementioned probe molecules have been shown to be highly prone to various secondary reactions over the reactive zeolite surfaces. ,,− In particular, pyrrole has been well-known to be highly vulnerable to polymerization in the presence of acidic sites. ,, Intrinsically, XPS analysis also has its own limits since it is mainly designed for the analysis of the external surface. − In this regard, it is desirable to make a continuous effort to develop new experimental methods that are more viable in terms of simplicity and generality.…”
The visible absorption band of iodine adsorbed on zeolite
blue-shifted with increasing the electropositivity
of the countercation and the aluminum content in the framework.
This phenomenon was attributed to the increase
in the donor strength of the zeolite framework based on the analogous
spectral shift of iodine in solution. In support
of this, a negative linear correlation was observed between the
measured visible bands of iodine adsorbed on various
zeolites and their calculated Sanderson's intermediate
electronegativities. However, the simultaneous change in
the
electrostatic field strength within the zeolite pores, as a result of
the change in the number and the size of the cation,
has to be taken into account in order to interpret the overall spectral
shifts more precisely. The iodine band sensitively
blue-shifted with decreasing the moisture content in the framework but
red-shifted with the loss of NH3 from
NH4
+-exchanged zeolites. The visible band of iodine also progressively
red-shifted with increasing the adsorbed amount,
presumably due to the nature of iodine to deplete electron density from
the framework. The framework structure
also affected the spectral shift of the visible iodine band. The
overall results established that iodine can be used as
a novel molecular probe for the quantitative evaluation of the zeolite
donor strength.
“…According to Mirodatos et al (1976) and Ward (1968), magnesium addition to NaH-Y zeolites increased Brønsted and Lewis catalyst acidity. This fact may be attributed to the formation of H + and Mg(OH) + species resulting from cation hydrolysis followed by water molecules dissociation at high temperatures.…”
Sulfur removal aiming at attending environmental legislation standards has been the focus of many studies. The use of Fluid Catalytic Cracking, FCC, in the removal of gasoline-fraction sulfur contaminants is regarded as advantageous when compared to desulfurization processes downstream of the FCC process. This paper evaluates the effect of Mg addition to the Beta zeolite over n-hexane and thiophene cracking. The catalysts were characterized by X-ray diffraction (XRD), X-ray with dispersive energy fluorescence analysis (EDXRF), Fourier transform infrared spectroscopy (FTIR), ammonia temperature programmed desorption (NH 3-TPD), and Brunauer-Emmett-Teller method (BET). The magnesium incorporation into the zeolite increased the number of Brønsted active sites and total acidity in the catalyst, favoring an increase in selectivity for catalytic cracking reactions and a decrease of isomerization reactions in the n-hexane conversion step. In thiophene conversion, the greater selectivity for hydrogen transfer promoted the formation of H 2 S, and adsorptive capacity is a key factor in alkylated product formation due to the presence of Lewis active sites, which are more predominant in magnesium incorporated catalysts.
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