Acid-Base Properties of Zeolites

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... 

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. 

The Lewis acid-base properties of zeolites received a growing interest recently. A series of works " suggested that the infrared and XPS spectra of adsorbed pyrrole can characterize both the Lewis basicity and the Lewis acidity of alkali-exchanged zeolites. The zeolite base strength decreases with an increase in Si/AI ratio of the framework, and increases when the electropositivity of the counter alkali cation is raised. The Lewis acid strength of counter cations increases with Si/AI ratio and decreases with electropositivity. In other words, the alkali-exchanged zeolites have both Lewis acid and Lewis base centres and provide an easy way to tune up the relative strengths of these two sites. 

Acid-base properties of zeolites are probed by studying their interactions with basic/acidic molecules by appropriate techniques (IR, NMR, calorimetry, TPD). For the external surface region, analogous methods based on XPS detection were developed following the pioneering work of Defosse and Canesson [65]. The probe molecules used are pyridine [55,56,66-72], ammonia [21,43,44,73], and pyrrole and chloroform for basic sites [70,74,75]. Kaliaguine [59] has published a review of the results. 

Fig. 11. Study of acidity and basicity with adsorbed probe molecules. Left column N Is lines of pyridine chemisorbed on Al-ZSM-22 (A),Fe-ZSM-22 (B),Al-ZSM-5 (C),Fe-ZSM-5 (D),B-ZSM-5 ( ) right column N Is lines of pyrrole chemisorbed on alkali-exchanged X. Taken from Borade RB, Huang M, Adnot A, Sayari A, Kaliaguine S (1993) Acid-base properties of zeolites an XPS approach using pyridine and pyrrole probe molecule. In New frontiers in catalysis, Proc 10th Int Congr Catal, Budapest, Hungary 1992, p 1625, with kind permission from Elsevier Science NL, Sara Burgerhartstraat 25,1055 KV Amsterdam, The Netiierlands...

The acid-base properties of zeolites or oxides are often studied by measuring the selectivities to the different products in the decomposition of alcohols and particularly isopropanol. The rate of propene formation can very often be correlated to the number of acidic sites determined by ammonia adsorption. A relationship has been found between the strength of the acid sites of bulk oxides, as determined by ammonia adsorption microcalorimetry [95], and the activation energy of dehydration, while the activation energy of dehydrogenation was independent of the strength of the sites [149]. 

Evaluation and tailoring of acid-base properties of zeolites... 

EVALUATION AND TAILORING OF ACID-BASE PROPERTIES OF ZEOLITES. PART 1... 

The acid-base properties of zeolites are further eomplicated by the confinement of host molecules in narrow mieropores. Several eomponents of the interaction energy between (basic) host molecules and the walls or acidic sites of zeolite cages may be strongly enhaneed in micropores with molecular dimensions. 

The acid/base properties of zeolites can be systematically changed by acting on various factors nature of the exchange cation, cation loading, decationa-tion, dealumination, which have been discussed in the previous section, and introduction of various elements (Ge, B, P, Ga,...) into the crystal framework, the effects of which we will now describe in more detail... 

The acid/base properties of zeolites are often studied by controlling the selectivity to the different products in the decomposition of alcohols and particularly isopropanol. The rate of propene formation can very often be correlated to the number of acidic sites determined from ammonia adsorption. 

Abstract Test reactions are one of the key methods to characterize the complex acid-base properties of zeolites. Although such reactions wiU not probe solely the acid or the base properties, observations in test reactions give a unique fingerprint of the catalytic properties of a material. They are best when the application of the zeolite requires similar conversion steps. The present review compiles currently used test reactions together with examples of their applications and also discusses the associated limitations. 

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. 

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... 

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. 

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. 

The reaction of phenol alkylation with methanol over oxides and zeolites to produce anisol and cresols presents some of the features of the reaction under investigation. Studies of phenol alkylation revealed that the reaction was sensitive to the acid-base properties of the catalyst [5-13]. The catalytic activity increased with acidity, but the selectivity towards O- or C-methylated products did not follow a simple correlation with observed acid-base properties. According to [7,12,16] the catalysts with basic sites favour C-methylation. Other authors [6,11] recently reported that an increase in catalyst acidity promote C-methylation. Therefore, a variance in the results concerning acidity and catalytic properties exists in the literature. 

Methanol is a key compound in Cl chemistry because it allows the conversion of raw materials, from which it is produced, into more valuable organic chemicals. However, the main application of heterogeneous catalysts for the activation of CH3OH is related to their transformation into hydrocarbons. For these technologies, the catalytic reactions are based on the acid-base properties of surfaces, and the catalytic materials consist of zeolites (such... 

The changes of the acid - base properties of metal substituted aluminophosphate based molecular sieves (MeAPO) as function of the chemical composition and the crystal structure are proposed to be complicated and to be substantially different compared to zeolites (1,2). 

The examples given below are mainly on zeolites and oxides, but the acid-base properties of all kinds of catalyst surfaces can be examined using adsorption calorimetry, including, for example, activated carbons [72], which are often used as supports of active phases due to their very high surface-area, oxynitrides [73] or hydrotalcites [74-76], which act as base catalysts [73], or heteropolyacids, which behave as strong acid catalysts [77],... 

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