Acidity in zeolites

In this way, the conjunct polymers serve as a reservoir of hydride ions. Under some conditions, the polymers are a source of hydride ions, but they accept these ions under other conditions. Substantial amounts of the saturated products are supposedly formed via this route with sulfuric acid. In zeolites, species similar to conjunct polymers also form. The heavy hydrocarbon molecules, which deactivate the catalyst by pore blocking or by site blocking, are generally termed soft coke or low-temperature coke , because of the absence of aromatic species. 

The choice of probe molecule for acidity determination is largely dictated by the sensitivity of the nuclei, and the high sensitivity of P nuclei has been exploited to quantify acidity in zeolites. The use of phosphorus containing bases as probe molecules to measure solid acidity circumvents insensitivity problems associated with (1.1% abundance) and N (0.4% abundance) in bases such as ammonia. 

Lunsford, ).H. (1997) Characterizabon of acidity in zeolites and related oxides using trimethylphosphine as a probe. Top. Catal., 4, 91-98. 

It is generally accepted that Lewis acidity in zeolites is due mainly to extraframework aluminum (16,17,18). Consequently, Lewis acid sites measured by pyridine adsorption must correlate with extra-framework aluminum. In Table I, the amount of pyridine coordinated to Lewis sites decreases for samples with the lowest Si/Al ratio, showing that, after thermal treatment, the amount of extraframework aluminum decreases with Si/Al ratio of the Beta zeolite. 

In addition to the Bronsted acidity in zeolites, in these materials the Lewis acidity is present as well. According to Lewis, an acid is an electron pair acceptor, a definition which is broader than that given by Bronsted, since a proton is a particular case of an electron pair acceptor. Then, the definition of Lewis covers practically all acid-base processes, whereas the definition of Bronsted represents only a particular type of process [128], The Lewis acidity is related to the existence of an extra-framework A1 (EFAL) species formed during the zeolite dealumination process [128],... 

Acidity in zeolite ZSM-5 was investigated via thermogravimetry and potentiometric titration measurements on NH4+- and H30+-exchanged samples subjected to various thermal treatments. Stoichiometric decomposition of the moiety... 

Acidity in zeolites, particularly Bronsted or proton acidity, was shown many years ago by Rabo, Pickert and co-workers (1 ) to be more complex than was originally proposed. Later it was shown that the thermal treatment of NH4Y zeolite produces a zeolite containing hydroxoaluminum cations as the stable end product, rather than solely protons, in order to balance framework negative charges following the decomposition of the ammonium ions, as established by Breck and Skeels in 1976 (2. ... 

Photoelectron spectroscopy (XPS) and other surface analysis XPS of zeolites Acidity in zeolites Basicity in zeolites... 

Netke SA, Sawant SB, Joshi JB, and Pangarkar VG. Sorption and permeation of acetic acid in zeolite filled membranes. J. Mem. Sci. 1995 107(l-2) 23-33. 

The purpose of this article is to review recent work concerning the characterization of the acid sites in zeolites, concentrating on the use of various techniques which can give detailed information concerning both the intensive and the extensive factors of acidity in zeolites. Due to the large number of papers being published in the area of zeolite catalysis, this review will certainly not be all-inclusive and the available literature will have greatly expanded by the time of publication. 

Infrared Spectroscopy. Infrared (IR) spectroscopy is probably the one instrumental technique that has been applied most often to the study of catalysts in general and to acidity in particular. IR spectroscopy can be used to detect Brpnsted acidity in zeolites directly by measurement of the OH stretching bands and to ascertain structural information based on the abundance of various OH bands and in the metal framework region. 

NMR spectroscopy has shown much promise as a technique for characterizing acid sites. In theory, it should enable the direct characterization of both the intensive and extensive factors of acidity in zeolites. The current limitations of the techniques have to do with the instrumentation not allowing for sufficient resolution for complete analysis of the proton sites. New NMR instrumentation is currently being developed that should allow for more complex experiments with higher accuracy. 

The protons thus formed as above are present as surface hydroxyl groups and is a cause of Broensted acidity in zeolites. This accounts for the high acidity and catalytic activity of HY. CuY showed a better activity than NaY. This as probably due to Cu" ions which constitute Lewis acid sites generating Broensted acid sites during activation treatment of CuY carried out prior to the dehydration reaction. The precursor for the above protons and hence the new Broensted sites could be the molecular water that gets evolved during the activation treatment as seen from TG/DTA profile. 

Quantum-Chemical Basis of Br0nsted Acidity in Zeolites. 

An additional interaction is observed if altuninum containing extra-framework cations or Lewis acid sites are present. Due to the nuclear spin of Al (f= 2)> each nitrogen hyperfine line is split into six lines, some of which may overlap. An example of such interaction is shown in Fig. 20 for NO-treated H-ZSM-5. Witzel et al. have used this property to determine the density and acidity of Lewis sites in H-ZSM-5,H-Mordenite and H-Y zeohtes [94-95]. The results obtained are in good agreement with those of more conventional methods and, consequently, ESR spectroscopy with NO as probe is a valuable tool for studying Lewis acidity in zeolites. It is important to stress that ESR spectroscopy of adsorbed NO is only... 

The same holds for protons, the origin of the outstanding Brdnsted acidity in zeolites [69],... 

Kassab, E., Seiti, K. and Allavena, M. (1991) Theoretical Determination of Relative Acidity in Zeolite (Faujasite). ]. Phys. Chem., 95, 9425-9431. 

TOWARDS A GENERAL SCALE FOR INTRINSIC ACIDITY IN ZEOLITES... 

Using qualitalive and quantitative models, including both short and bng range effects, progress has been made in understanding acidity in zeolites, but limitations that still exist in measuring that acidity by use of probe molecules. 

Lewis Acidity in Zeolites Cations Compared with Oxy-Cations... 

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