Acid-base properties zeolites

Alumina is an amphoteric catalyst, which can difficult to characterize via chemical and physic methods. The transformation of cyclopentanol/cyclohexanone mixture allows us to estimate at the same time the acid-base properties of aluminas. From this transformation, it was shown that aluminas can be classified into two families only basic aluminas, such as theta, which were more basic than MgO, and acido-basic aluminas, eta, gamma and delta, which possess an acidic character less pronounced than dealuminated HMOR zeolite... 

In the present work ir, esr and microcalorimetry techniques were used to characterize the acid-base properties of acid ZSM-5 and ZSM-11 samples. Complementary studies by TEM, EDX-STEM and XPS were also carried out to determine the size and shape of zeolite particles and the A1 distribution within a particle. Catalytic properties for methanol conversion were also determined. 

It is well known that the elements in framework of zeolite molecular sieves greatly influence the properties and behaviors of these materials [1-3], The introduction of heteroatoms into the framework has become one of most active fields in study of zeolites. The investigations were mostly focused on the methods to introduce heteroatoms into the framework (for examples, hydrothermal synthesis and post-synthesis), the mechanisms for incorporations, the effect of heteroatoms on the acid-base properties and the catalytic features of modified samples [1-10]. Relatively less attention was paid to the effect of treatment process on the porous properties of samples although the incorporation of heteroatoms, especially by the so-called post-synthesis, frequently changes the distribution of pore size. Recently, we incorporated Al, Ga and B atoms into zeolites (3 by the post-synthesis in an alkaline medium named alumination, galliation and boronation, respectively. It was found that different trivalent elements inserted into the [3 framework at quite different level. The heteroatoms with unsuitable atom size and poor stability in framework were less introduced, leading to that a considerable amount of framework silicon were dissolved under the action of base and the mesopores in zeolite crystal were developed. As a typical case, the boronation of zeolites (3 and the accompanied formation of mesopores are reported in the present paper. 

Relatively few studies have focused on influence of the acid/base properties of the support on the chemisorption of reactants on supported metal clusters. A NMR study by Tong et al.23 showed that the stretching frequency of CO chemisorbed on zeolite supported Pt particles correlates with the surface local density of states (LDOS) of the Pt. The LDOS also showed a correlation with the faujasite framework acidity, but an explanation of this correlation is lacking. Several infrared studies on similar supported Pt catalysts show that the mode of CO... 

Zeolites containing 3d transition-metal ions were considered in Beran et al. (109-112). The peculiarities of the donor-acceptor interactions of these cations located within six-membered rings with a zeolite lattice were discussed in terms of atomic charges, bond orders, and orbital energies. The redox properties of the cations, the acid-base properties of zeolites, and the dependence of these characteristics on the Si/Al ratio were discussed as well. The authors noted that the forms containing univalent copper and nickel ions should possess the highest electron-donor ability and consequently the... 

The most important feature of zeolites (and zeotypes), in the context of catalysis, is not their range of acid-base properties, since that is also available with amorphous alumino-silicates. It is the presence of a regular structure containing... 

The acid-base properties of the decationated HY zeolites have been extensively studied with adsorption microcalorimetry. Tables II and III present a summary of calorimetric studies of the adsorption of ammonia and other probe molecules on HY zeolites with different Si/AI ratios, preparation methods, pretreatments, adsorption temperatures, and sodium contents. The large variety of conditions used in these studies complicates the comparison of the materials. For example, the initial differential heat of ammonia adsorption at... 

The acid-base properties of decationated ZSM-5 zeolite have been studied in some detail using adsorption microcalorimetry, as shown in Table VIII (169-173). As the calcination temperature for HZSM-5 zeolites was increased from room temperature to 1073 K, a maximum in acidity was observed while the initial differential heat of ammonia adsorption increased continuously. Vedrine et al. (92) also found a maximum in the intensity of the IR hydroxyl bands (169) of HZSM-5 at 673 K. The IR absorption band of pyridine adsorbed on Brpnsted sites followed the same trend as that found for the hydroxyl stretching bands, confirming that above 673 K the Bronsted acidity decreased as the dehydration temperature increased. 

To clarify the interrelation between the acid and base properties of a solid and its catalytic efficiency, the ring opening of epoxides was investigated by the use of zeolites with different acid-base properties (25). Table VI summarizes the results for ring openings of unsymmetrical epoxides with aniline. 

The most important aspect concerning the catalytic application of zeolites is not this range of potential acid-base properties since that is also available with the amorphous aluminosilicates. Instead it is the presence in these crystalline materials of molecular sized cavities and pores that make the zeolites effective as shape selective catalysts for a wide range of reactions. >53-59... 

The chemical selectivity obtained in the alkylation of aromatic molecules over zeolitic catalysts critically depends upon their acid base properties. While xylenes are the primary products in the methylation of toluene over acidic zeolites like HZSM5 [e.g. 1,2,3], ethylbenzene and styrene are formed over basic zeolites such as Rb-X and Cs-X [e g. 4,5,6]. Previous reports suggested the surface chemistry of chemisorbed methanol to be the most decisive parameter to determine the selectivity [7,8]. Recent experiments on toluene methylation to xylenes indicated, however, that various bimolecular precursors to the transition state in the rate determining step exist and may be important for the catalytic properties of zeolites [9,10],... 

Apart from zeolites and ion-exchange resins (Sections 4 and 5), the main classes of catalyst which are active for dehydration are salts (usually sulphates, phosphates, and to a lesser extent, carbonates) and oxides (single and mixed). A large proportion of research has been devoted to a correlation of acid-base properties with activity for dehydration and dehydrogenation of alcohols, the two reactions being closely associated. 

It is known [1,2] that zeolite-containing promoted catalysts are used widely in petroleum processing. They meet the strict standards of industry and ecology. The activities of these catalysts are usually related to their acid-base properties and the presence of structural OH-... 

ACID-BASE PROPERTIES OF SOME ZEOLITES AND THEIR ACTIVITY IN THE DECOMPOSITION OF N-HEXANE... 

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. 

In this paper, n-hexane was employed as a model hydrocarbon, and the decomposition of n-hexane over zeolites was investigated. Focus was centered on the relation between the product distributions of the reaction and the acid-base properties of zeolites. 

In HNaY (7.6%>), sodium ions replaced the minor portion of the protons, and the trans-convex type profile for HNaY (0%) was modified to the rake-type profile. In HNaY (33%) and HNaY (55 /o), on the other hand, sodium ions replaced the major portion of the protons, and the profiles became cis-convex in type. The type of profile produced is dependent on the degree of proton-sodium ion displacement. The acid-base property of zeolites can accordingly be characterized by these profiles. 

Iwamoto et al. (54) studied the activity of a series of metalion exchanged zeolites for the water-gas shift reaction. The lower water-gas shift activity of the acidic cations was explained in terms of hard-soft acid/base properties. In this model, carbon monoxide, which is a soft base, interacts more strongly with soft acid sites. The adsorption of CO is generally considered to be the rate controlling step in the water-gas shift reaction. Cations of lower acidity are generally softer acids and as such adsorb CO more readily. This would lead to higher surface concentrations of CO, thereby increasing the water-gas shift acitivity of the sample. 

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