Basic site characterization

In the present work low temperature adsoi ption of fluoroform and CO, were used to characterize surface basicity of silica, both pure and exposed to bases. It was found that adsorption of deuterated ammonia results in appearance of a new CH stretching vibration band of adsorbed CHF, with the position typical of strong basic sites, absent on the surface of pure silica. Low-frequency shift of mode of adsorbed CO, supports the conclusion about such basicity induced by the presence of H-bonded bases. 

Surampalli, Ong, Seagren, and Nuno compiled and edited a book by the American Society of Civil Engineers (ASCE) called Natural Attenuation of Hazardous Wastes.97 In addition to a discussion of the regulatory framework, this book covers major pollutants and basic scientific principles on physical, chemical, and biological processes involved in natural attenuation. It also contains an extensive review of literature, examples of applications of natural attenuation, and site characterization and monitoring requirements and procedures. 

Kustov, L.M. (1997) New trends in IR-spectroscopic characterization of acid and basic sites in zeolites and oxide catalysts, Top. Catal., 4, 131. 

It is known [4] that methylacetylene can be adsorbed dissociatively or not. The dissociation of methylacetylene is characterized by the appearance of a typical v(OH) vibration. Therefore, the absence of any zeolitic v(OH) band for the different LSX samples indicates that methylacetylene adsorbs mainly without dissociation The presence of non-dissociated adsorbed methylacetylene is also evidenced by the detection of specific v(C=C) and v(=CH) vibrations band. As expected, their wavenumbers decrease with increasing basicity. Moreover, the complex shape of the v(=CH) band reveals different environments of basic sites. 

This study permits to discuss the efficiency of new probe molecules for the characterization of basic sites in zeolites. For MBOH and for methylacetylene, the observed frequencies shift account for the variations in the basic strength of the zeolitic framework oxygen atoms. Interestingly, methylacetylene also informs on the environment of the basic sites and H2S dissociation brings information on the amount of strong basic sites. These results show the high potential and the complementarity of these protic probes to describe the strength, concentration and environment of basic sites. 

The aim of this study is to develop model reaction for the characterization of the acidity and basicity of various transition aluminas, the experimental conditions being close to that for catalysis use. Among various model reactions, the transformation of cyclopentanol and cyclohexanone mixture was chosen for this work. Indeed, this reaction was well known for estimating simultaneously the acid-base properties of oxide catalysts [1], Two reactions take place the hydrogen transfer (HT) on basic sites and the alcohol dehydration (DEH) on acid sites. The global reaction scheme is shown in Figure 1. 

Acidic properties of zeolitic materials have widely been investigated in the past fifteen years ( 1), particularly for Y-type zeolites. The presence of strong acid sites, as it is usually the case for acid zeolites, results in the presence of only weak basic sites, if any. Therefore, up to now, majority of the studies has dealt with the characterization of acid+rather than basic properties. The acid sites (Bronsted s H, Lewis Al) and basic sites (0, OH ) may be characterized directly by using physical... 

Another thermal analysis method available for catalyst characterization is microcalorimetiy, which is based on the measurement of the heat generated or consumed when a gas adsorbs and reacts on the surface of a solid [66-68], This information can be used, for instance, to determine the relative stability among different phases of a solid [69], Microcalorimetiy is also applicable in the measurement of the strengths and distribution of acidic or basic sites as well as for the characterization of metal-based catalysts [66-68], For instance, Figure 1.10 presents microcalorimetry data for ammonia adsorption on H-ZSM-5 and H-mordenite zeolites [70], clearly illustrating the differences in both acid strength (indicated by the different initial adsorption heats) and total number of acidic sites (measured by the total ammonia uptake) between the two catalysts. 

This review is a summary of the work done and potential opportunities for inexpensive and easily accessible base catalysts, such as alkaline earth metal oxides and hydroxides, as well as alkali metals and oxides supported on alkaline earth metal oxides. Preparation methods of these materials, as well as characterization of basic sites are reported. An extensive review of their catalytic applications for a variety of organic transformations including isomerization, carbon-carbon and carbon-oxygen bond formation, and hydrogen transfer reactions is presented. 

Adsorption of a specific probe molecule on a catalyst induces changes in the vibrational spectra of surface groups and the adsorbed molecules used to characterize the nature and strength of the basic sites. The analysis of IR spectra of surface species formed by adsorption of probe molecules (e.g., CO, CO2, SO2, pyrrole, chloroform, acetonitrile, alcohols, thiols, boric acid trimethyl ether, acetylenes, ammonia, and pyridine) was reviewed critically by Lavalley (50), who concluded that there is no universally suitable probe molecule for the characterization of basic sites. This limitation results because most of the probe molecules interact with surface sites to form strongly bound complexes, which can cause irreversible changes of the surface. In this section, we review work with some of the probe molecules that are commonly used for characterizing alkaline earth metal oxides. 

Ammonia and pyridine are frequently used as probe molecules for the characterization of acidic surfaces, but they also adsorb on strongly basic sites. Tsyganenko et al. (54) proposed various species resulting from NH3 adsorption on basic solids (Scheme 1). The formation of species I corresponds to hydrogen bonding to a basic surface oxygen, and species II, formed by dissociation to give NH2 and hydroxyl species, involves an acid-base site. Such adsorption requires... 

Isomerization of jS-isophorone to a-isophorone has been represented as a model reaction for the characterization of solid bases 106,107). The reaction involves the loss of a hydrogen atom from the position a to the carbonyl group, giving an allylic carbanion stabilized by conjugation, which can isomerize to a species corresponding to the carbanion of a-isophorone (Scheme 9). In this reaction, zero-order kinetics has been observed at 308 K for many bases, and consequently the initial rate of the reaction is equal to the rate constant. The rate of isomerization has been used to measure the total number of active sites on a series of solid bases. Figueras et al. (106,107) showed that the number of basic sites determined by CO2 adsorption on various calcined double-layered hydroxides was proportional to the rate constants for S-isophorone isomerization (Fig. 3), confirming that the reaction can be used as a useful tool for the determination of acid-base characteristics of oxide catalysts. 

On solids, the amount and strength of acid or basic sites are quite independent parameters, so both of them must be analyzed independently for a complete characterization. Additionally, several different families of acid sites may occur in the same solid surface, so their distribution must be characterized. The key to the effective utilization of microcalorimetry in heterogeneous catalysis is the judicious choice of gas-phase molecules for study. 

Bulk boron oxide was found to be much more acidic than basic [168], When SO2 adsorption microcalorimetry was used, no basic sites were observed, but some phy-sisorption occurred. Ammonia and pyridine adsorption microcalorimetry were used to characterize the acidity of B2O3. Boron oxide displays an initial heat for NH3 adsorption of 80 kJ/mol and can adsorb irreversibly a large amount of ammonia. The number of active sites determined by pyridine adsorption and the corresponding integral heats were found to be much lower than those determined by using ammonia. 

At the Idaho National Engineering and Environmental Laboratory (INEEL), the basic system installation cost was about 3.47/ft, for a total cost of about 34,700. This figure does not include site characterization and monitoring costs (D192921, p. 10). 

The role of the catalyst is very important in the latter step, which requires hard reaction conditions (M. Aresta et al. unpublished results). In particular, by using metal oxides characterized by a specific ratio between the acid and basic sites [242] as catalyst, conversion of the carbamate into the carbonate is very much improved, together with selectivity (the conversion into carbonate increases as the acid/basic site ratio decreases). 

Some very important surface properties of solids can be properly characterized only by certain wet chemical techniques, some of which are currently under rapid improvement. Studies of adsorption from solution allow determination of the surface density of adsorbing sites, and the characterization of the surface forces involved (the energy of dispersion forces, the strength of acidic or basic sites and the surface density of coul-ombic charge). Adsorption studies can now be extended with some newer spectroscopic tools (Fourier-transform infra-red spectroscopy, laser Raman spectroscopy, and solid NMR spectroscopy), as well as convenient modern versions of older techniques (Doppler electrophoresis, flow microcalorimetry, and automated ellipsometry). 

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