Zeolites surface Bronsted acid sites

Based on the above results, we conclude that the diazomethane modification of zeolites is an effective method to change selectively the amount and strength of the surface Bronsted acidic sites. Therefore the method could be used to study the role of Bronsted and Lewis acidic sites preferably for low temperature 300 0 catalytic reactions. 

Other useful classes of basic probe molecules used to examine silica, alumina, and silica-alumina surfaces (as well as zeolite systems) include small organic phosphines and phosphine oxides, which rely on the highly convenient P nuclide (/ = 1/2, 100% natural abundance). As Lunsford and coworkers demonstrated for zeolites [89], the P NMR signal of trimethylphosphine is a useful probe for Bronsted acid sites on surfaces. The basis for this approach is the formation of R3P -H B( ) sites at surface Bronsted acid sites, H-B(. ... 

Surface-inactive HZSM-5 whose surface Bronsted acid sites have been inactivated with a sterically-hindered basic organophosphorus compound, the organophosphorus compound cross section is larger than zeolite pore (e.g., treated with 10% Ph4PBr)... 

Lewis acid sites may be formed following dehydroxylation of zeolite surface in H-form. At sufficiently high temperatures two Bronsted acid sites can drive off a water molecule and leave behind a coordinatively unsaturated Al site, as illustrated in Figure 13.16 [32]. Here not only the resulting tri-coordinated Al but also the tri-coordinated positively charged Si can act as a Lewis acid. Furthermore dehydroxylation may be followed by framework dealumination, leading to cationic extra-framework species like AlO AlfOHij that can act as Lewis acids [33-37]. 

Besides standard characterization all the samples were further explored by applying FTIR spectroscopy. The scope was to qnantify the acidity of the samples and to discriminate Bronsted from Lewis acid sites. The results of the acidities of the metalated samples are presented in correlation to the corresponding specific snrface areas (Figures 9.1 and 9.2). That is the zeolite snrface area for the Bronsted acid sites and the total surface area for the Lewis acid sites, obviously because the Bronsted acid sites exist only on the zeolitic component of the catalyst, while the Lewis acid sites are present on both matrix and zeolite. 

Krossner, M., Sauer, J., 1996, Interaction of Water With Bronsted Acidic Sites of Zeolite Catalysts. Ah Initio Study of 1 1 and 2 1 Surface Complexes , J. Phys. Chem., 100, 6199. 

The catalytic activity of amorphous silica-alumina ([Si—Al]) in reactions via carbonium ions is due to the existence of Bronsted acid sites on their surface. Consequently, amorphous [Si-Al] acid catalysts provide acid sites and transport to the active sites easily. As a result, amorphous [Si-Al] acid catalysts have been widely operated as cracking catalysts. Acid zeolites have been successfully applied as cracking catalysts. However, in some industrial applications of acid catalysts, for example, in the cracking of hydrocarbons of high molecular weight, zeolites are not useful, since... 

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. 

Before treatment, acid-washed LZ-105 shows two distinct bands in the OH region I. R. spectrum one at 3745 cm and the other at 3615 cm-l. The former is attributable to OH groups on terminal zeolite surfaces or amorphous impurity material. The 3615 cm- band is believed to be related to the Bronsted acid sites in LZ-105. After fluorination and 600°C calcination, both the 3615 and 3745 cm l bands disappear. 

Ward (24) has suggested that with ZnX the active centers are essentially Bronsted acid sites associated with residual hydroxyl groups. -Partial hydration could promote movement of cations and result in the formation of additional such sites. The activity of the ZnX-IV zeolite can be explained satisfactorily in this manner, but with ZnX-I other factors must be involved. It is possible that with the higher exchanged zeolite, the increase in surface cations provides stronger adsorption sites for the butenes. If this were the case, then a slow rate of desorption of the products (relative to the surface reaction) could allow the 2-butene product ratio to approach that of thermodynamic equilibrium (13). 

It is seen from the data in Table 3 that the (Si/Al)s ratio is higher than the bulk value (2.33). This suggests an aluminium depleted surface region. The low values observed for the ratio (N/AI)s reflect the fact that only part of the BrOnsted acid sites are accessible to pyridine. The pyridine molecule kinetic diameter of 5.9 A does not allow it to enter the sodalite cages with 2.2 A openings. Thus only the acid sites protruding in the supercages can chemisorb pyridine. The number of these molecules is estimated to be 24 per unit cell [41] and since the Y zeolite with a Si/Al ratio of 2.33 has 57 A1 atoms per unit cell, the maximum N/Al ratio is 0.42. This value is reasonably close to the 0.38 value obtained after calcination at 300°C. 

The most important conclusion concerns the application of cyclohexanol reactions to study the heterogeneity of acid sites in zeolite-like materials or on surfaces of other solids. Dehydration is the only reaction in the presence of weak Bronsted acid sites (such as Si—OH—B) and cyclohexene and water are the only products. The appearance of cyclopen-tenes, cyclopentanes and cyclohexane in the products indicates that strong BrOnsted acid sites, such as Si—OH— A1 are present. Cyclohexanol conversion may therefore be a convenient test reaction to study both weak and strong BrCnsted acid sites in one simple catalytic test. 

Gas-phase synthesis of 2MN can be carried out efficiently over H-ZSM-5 and H-ZSM-11 type zeolites. The results are consistent with the Rideal type mechanism for alkylation of naphthalene with methanol. The first step in the alkylation reaction of naphthalene is the chemisorption of methanol on the Bronsted acid sites. Methoxy groups are formed on the surface and according to TPD analysis, naphthalene reacts with them impacting, directly from the gas phase. The reaction seems to occur on the external surface of the crystallites of the medium pore zeolites. Using large pores zeolites, the reaction also takes place also in the channel space, and the selectivity of B derivatives is suppressed. 

The temperature conditions under which Bronsted acid sites are converted into Lewis acids have been determined in the Princeton laboratories. It was established that the ammonium zeolite completely decomposes at about 340° and leaves protonic acid (Bronsted sites) on the surface. When such a material is heated to over 480°, the protons are removed as water, two Bronsted acid sites producing one Lewis acid site and a Bronsted base site. It was thought desirable to determine which of these acidic sites were responsible for cracking of cumene and to extend the investigations to the study of the cracking of a more demanding molecule, in the sense of Boudart, i.e., 2,3-dimethyl-butane. 

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 results from surface acidity measurements by NH3 TPD in the catalysts show that the total number of acid sites was proportional to the accessible zeolite area (Tables 1 and 2). The same trend was found also for the Bronsted acid sites quantified by FT-IR of Py. 

In their hydrated forms zeolites are used for ion exchange purposes, for example, water softening by replacement of Ca2+ with Na+ or another ion (see Topic J4Y When dehydrated they have important catalytic applications, promoted by the Bronsted acid sites, and by the large area of internal surface . They are used for the cracking of petroleum and for the isomerization of hydrocarbons, where limited pore size exerts a shape selectivity , which allows one desirable product to be formed in high yield (see Topic J5). 

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