Indicators, surface acidity

More information on the nature of active sites was obtained using some model catalysts obtained by incipient wetness impregnation of a commercial silica (Si-1803 with surface area = 300 nP g ). A preliminary test performed using the support (Table 39.6) showed a very low selectivity to MDB, with the preferential formation 2-EMP, indicating that acid sites alone are not able to promote the cyclization of the intermediate. 

Surface photovoltage spectroscopy (SPS) in Fig. 6.5 was used to determine the surface acidity of JML-1 by measuring transition of electrons between the interface and the surface. The JML-I40 calcined at 550°C exhibited two peaks at 596 nm and 677 nm, whereas the sample without calcination had only one peak at 330 nm. The peak at 330 nm is assigned to the band-band electron transition and those at 596 nm and 677 nm are attributed to the surface-related transitions. The observation of these surface-related transitions indicates the presence of positive charges on the surface of the calcined sample, suggesting that the acidity of JML-1 catalyst is resulted from a large amount of SZ acidic sites on the silica surface. 

It is important to understand the catalyst characteristics in detail, which in turn helps to understand the catalyst better and correlate the structure and composition of the catalysts with its performance, so that further improvement of the catalyst is possible. Acidity is an important property which influences the overall activity of the alkylation catalysts and the same was studied for Cui.xZnxFc204 by IR and TPD methods. The changes in acidity with respect to catalyst composition and temperature were studied through pyridine adsorption followed by IR measurements. In situ FTIR spectra of pyridine adsorbed on Cui xZnxFe204 between 100 and 400°C (Figme 23) indicated Lewis acidity is the predominant active centers available on the surface [14]. 

As shown in Table 1, the amount of irreversibly adsorbed pyridine dropped to one third its original value after modification, which is caused by the methylation of surface Bronsted acidic sites through equation (1). The results were confirmed by the presence of surface methoxyl groups and the absence of Bpy peaks of adsorbed pyridine in i.r. spectra. The TPD of ammonia in Figure 3 indicates that the modification influenced mainly the number of surface acidic sites. The results in Table 1 aslo show that the drop in acidity paralleled that... 

The preparation of precious metal supported catalysts by the HTAD process is illustrated by the synthesis of a wide range of silver on alumina materials, and Pt-, Pt-Ir, Ir-alumina catalysts. It is interesting to note that the aerosol synthesis of alumina without any metal loading results in a material showing only broad reflections by XRD. When the alumina sample was calcined to 900°C, only reflections for a-alumina were evident. The low temperature required for calcination to the alpha-phase along with TEM results suggest that this material was formed as nano-phase, a-alumina. Furthermore, the use of this material for hexane conversions at 450°C indicated that it has an exceptionally low surface acidity as evidenced by the lack of any detectable cracking or isomerization. 

Pernicone et al. [253,254] bring forward some evidence that surface acidity also plays a role with iron molybdate catalysts. Hammett indicators adsorbed over the molybdate assume the acid colour. Pyridine poisons the oxidation of methanol to formaldehyde. A correlation is reported between acidity and activity [253]. The authors agree with Ai that the acid sites are connected with Mo6+ ions. 

It can be inferred from table 1 that the activity of the catalyst with the coupling agent is the highest. The selectivity for formaldehyde has decreased however and a larger amount of demethylether is formed. This is an indication for a higher surface acidity, probably caused by the V-OH groups, surrounded by an hydrophobic environment. 

After evacuation at 25°C (spectrum C), the physisorbed fraction of acetonitrile-d3 and also the H-bonding effects with the silica surface -OH have disappeared, while the Bronsted acidity is still present (2309 cm 1). Subsequent evacuation at 60°C does not change the intensity of the Bransted acidity (spectrum D). Even at 120°C and at 150°C Bransted acid sites are still detected (spectra E, F). Therefore, it can be concluded that the Al-PCH is characterized by an important Bransted surface acidity. This type of acidity is expected since the initial Na+ ions on saponite have been replaced by surfactant cations and then by protons upon destruction of the surfactant through calcination. Besides, by the grafting of Al-species onto the support, Si-(OH)-Al bonds have been created, giving rise to the band at 2309 cm 1 indicative of Bransted acidity [10]. 

Values indicate number of surface acidic or basic sites, nm. ... 

Mason et al. [13] used indicator dyes to determine the effect of pretreatment on the surface acidity of Ti-6,4. They found that surfaces treated with a basic etchant left a basic surface with an isoelectric point between 7.3 and 9.2. A phosphate-fluoride etch left the surface acidic with an isoelectric point between 5.4 and 7.3. 

Nonaqueous methods for the determination of surface acidity represent a considerable improvement over aqueous methods because the solvents used (e.g., benzene, iso-octane) do not react with catalyst surfaces as previously described in the case of water. Of the available types of nonaqueous methods (1-3), the simplest is that employing adsorbed indicators. It can be used to determine acid strengths and amounts of surface acids as described in the following section. 

The indicator method is by far the easiest and quickest way of screening surface acidities of solid catalysts, but it has at least two drawbacks. First of all, the number of suitable indicators is limited because of the visual requirement that the color of the acid form mask that of the basic form. Second, the acid color of many of the Hammett indicators can be produced by processes other than simple proton addition. The first of these drawbacks can be overcome by using absorption spectroscopy to measure... 

Determination of acid strengths by means of fluorescent indicators (21) and still another class of indicators, the arylcarbinols (24), has also been reported. Comparison of these indicators for the titration of surface acidity will be discussed in the following section. 

After the acid strength of a catalyst surface has been bracketed by means of colors of adsorbed indicators, the next logical step in the determination of surface acidity is the measurement of the number of acidic groups. This is generally done by titrating a suspension of the catalyst with a solution of a suitable amine in an inert solvent the previously described indicators are used to determine endpoints. 

Hirschler (24) suggested the use of arylcarbinol indicators (see Table IV) for the visual measurement of surface acidity. He pointed out that a variety of physical measurements (27-29) show that arylcarbinols (ROH)... 

Fig. 2. Surface acidity of silica-alumina visual and spectrophotometric titrations (21, 24, 26). , A, Hammett indicators O, Fluorescent indicators , arylcarbinol indicators.

Lewis acid sites can coordinate with a given indicator molecule to produce an adsorption band identical in position with that produced through proton addition. Even if the indicators used are responsive only to Brpn-sted acids, most basic reagents used to titrate surface acidity (e.g., n-butylamine, pyridine) are strongly adsorbed on surface sites other than Br0nsted acid sites. In this connection, a recent study indicates that adsorption equilibrium is not fully established during titration of silica-alumina with n-butylamine because of the irreversible attachment of amine molecules by adsorption sites at which they first arrive (31). 

Rates of model reactions are more commonly used to determine relative rather than absolute surface acidities and a variety of acid-catalyzed reactions have been used for this purpose (1-3). Xylene isomerization is a particularly well-substantiated model reaction, thanks to work by Ward and Hansford (43). They demonstrated that the conversion of o-xylene to p- and /n-xylenes over a series of synthetic silica-alumina catalysts increases as the alumina content is increased from 1 to 7%. The number of strong Brdnsted acids in each member of the catalyst series was measured by means of infrared spectroscopy. Since conversion of o-xylene was found to be a straight-line function of the number of Br0nsted acids (see Fig. 9), rate of xylene isomerization appears to be a valid index of the amount of surface acidity for this catalyst series. This correlation also indicates that the acid strengths of these silica-alumina preparations are roughly equivalent. 

If the objective is to fingerprint surface acidity (so that one acidic solid can be distinguished more clearly from another), the nonaqueous titration method (26) would be appropriate. The original method could be made more selective for titration of Br0nsted acids by choosing recently recommended indicators (21) and using a sterically hindered amine (54) as the basic reagent. However, even an improved indicator method may have its drawbacks a sterically hindered amine is not perfectly selective,... 

Fluoride addition promotes the cracking (108-110) and isomerization (108, 111) activity of alumina, presumably, because of the formation of Brdnsted acid sites. In a comprehensive study of fluorided aluminas, Antipina et al. (110) demonstrated that there is a close parallelism between generation of cumene cracking activity at 430°C and surface acidity when fluoride content is increased from 0 to 7% wt (see Fig. 15). Acidity was measured by n-butylamine titration endpoints were determined by means of an arylcarbinol indicator that detected Br0nsted acids stronger than those corresponding to a pKa of — 13.3. In a separate article (112), Anti-... 

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