Solid acidity probe molecules

For basicity measurements, the number of acidic probes able to cover a wide range of strength is rather small [166]. The most common acidic probe molecules used are CO2 (p/fa = 6.37) and SO2 (p/fa = 1.89). Carboxylic acids such as acetic acid can also be used but dimmers can be formed, particularly at high coverage. Pyrrole may also be used, particularly at low adsorption temperature, but has sometimes shown some amphoteric character [103]. Hexafluoroisopropanol has also been used to characterize the surface basicity of some solids [145]. 

The snrface acid-base properties of supported oxides can be conveniently investigated by studying the adsorption of suitably chosen basic-acidic probe molecules on the solid. As shown, acidic and basic sites are often present simultaneously on solid surfaces. The knowledge of the detailed amphoteric character of supported metal oxides is of extreme interest due to the possibility of using them as catalysts in different reactions in which acidity governs the reaction mechanism. 

A wide variety of NMR methods are being applied to understand solid acids including zeolites and metal halides. Proton NMR is useful for characterizing Brpnsted sites in zeolites. Many nuclei are suitable for the study of probe molecules adsorbed directly or formed in situ as either intermediates or products. Adsorbates on metal halide powders display a rich carbenium ion chemistry. The interpretation of NMR experiments on solid acids has been greatly improved by Ae integration of theoretical chemistry and experiment. 

It is often said that the property of acidity is manifest only in the presence of a base, and NMR studies of probe molecules became common following studies of amines by Ellis [4] and Maciel [5, 6] and phosphines by Lunsford [7] in the early to mid 80s. More recently, the maturation of variable temperature MAS NMR has permitted the study of reactive probe molecules which are revealing not only in themselves but also in the intermediates and products that they form on the solid acid. We carried out detailed studies of aldol reactions in zeolites beginning with the early 1993 report of the synthesis of crotonaldehyde from acetaldehyde in HZSM-5 [8] and continuing through investigations of acetone, cyclopentanone [9] and propanal [10], The formation of mesityl oxide 1, from dimerization and dehydration of... 

The probe molecules of greatest historical interest in catalysis are the Hammett indicators [13]. The difficulty of making reliable visual or spectrophotometric observations of the state of protonation of these species on solids is well known. We have recently carried out the first NMR studies of Hanunett indicators on solid acids [ 14]. This was also the occasion of the first detailed collaboration between the authors of this article, and theoretical methods proved to strongly compliment the NMR experiments. The Hanunett story is told after first reviewing the application of theoretical chemistry to such problems. Central to the application of any physical method in chemistry is the process of modeling the relationship between the observables and molecular structure. However often one does this, it is rarely an exact process. One can rationalize almost any trend in isotropic chemical shift as a function of some variation in molecular structure - after the fact, but the quantitative prediction of such trends in advance defies intuition in most nontrivial cases. Even though the NMR spectrum is a function... 

Table 4.1. Conceptional criteria for the selection of probe molecule to characterize solid acids (from ref. [31])...

Wakabayashi, F. and Domen, K. (1997) A new method for characterizing solid surface acidity - an infrared spectroscopic method using probe molecules such as N2 and rare gases. Catalysis Surveys from Japan 1 181. 

The apparent acidities of zeolite catalysts are characterized by Av0h values induced by adsorption of hexane (Av0h.C6) under the same conditions than those applied during separate catalytic experiments. The Avoh,c6 values for the different zeolite samples shown in fig. 2 were determined as Figure 1. DRIFT spectra measured in the above for the nitrogen probe molecule. vOH region before (solid lines) and after (dashed lines) contacting the samples with N2 at 298 K and 9 bar equilibrium pressure. 

Besides the prediction of calcination temperatures during catalyst preparation, thermal analysis is also used to determine the composition of catalysts based on weight changes and thermal behavior during thermal decomposition and reduction, to characterize the aging and deactivation mechanisms of catalysts, and to investigate the acid-base properties of solid catalysts using probe molecules. However, these techniques lack chemical specificity, and require corroboration by other characterization methods. 

Besides the 29Si and 27 A1 NMR studies of zeolites mentioned above, other nuclei such as H, 13C, nO, 23Na, 31P, and 51V have been used to study physical chemistry properties such as solid acidity and defect sites in specific catalysts [123,124], 129Xe NMR has also been applied for the characterization of pore sizes, pore shapes, and cation distributions in zeolites [125,126], Finally, less common but also possible is the study of adsorbates with NMR. For instance, the interactions between solid acid surfaces and probe molecules such as pyridine, ammonia, and P(CH3)3 have been investigated by 13C, 15N, and 31P NMR [124], In situ 13C MAS NMR has also been adopted to follow the chemistry of reactants, intermediates, and products on solid catalysts [127,128],... 

Basic molecules such as pyridine and NH3 have been the popular choice as the basic probe molecules since they are stable and one can differentiate and quantify the Bronsted and Lewis sites. Their main drawback is that they are very strong bases and hence adsorb nonspecifically even on the weakest acid sites. Therefore, weaker bases such as CO, NO, and acetonitrile have been used as probe molecules for solid acid catalysts. Adsorption of CO at low temperatures (77 K) is commonly used because CO is a weak base, has a small molecular size, a very intense vc=0 band that is quite sensitive to perturbations, is unreactive at low temperature, and interacts specifically with hydroxyl groups and metal cationic Lewis acid sites.26... 

Prins reaction, heteropolyacid catalysis, 41 156 Probe molecules, 42 119 acidic dissociation constant, 38 210 NMR solid acidity studies, 42 139-140 acylium ions, 42 139, 160 aldehydes, 42 162-163 alkyl carbenium ions, 42 154-157 allyl cation, 42 143-144 ammonia, 42 172-174 arenium ions, 42 150-154 carbonium ions, 42 157-160 chalcogenenonium ions, 42 161-162 cyclopentenyl cations, 42 140-143 indanyl cations, 42 144-147 ketones, 42 162,163-165 nitrogen-containing compounds, 42 165-170... 

The choice of probe molecule for acidity determination is largely dictated by the sensitivity of the nuclei, and the high sensitivity of P nuclei has been exploited to quantify acidity in zeolites. The use of phosphorus containing bases as probe molecules to measure solid acidity circumvents insensitivity problems associated with (1.1% abundance) and N (0.4% abundance) in bases such as ammonia. 

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... 

The pretreatment temperature is an important factor that influences the acidic/ basic properties of solids. For Brpnsted sites, the differential heat is the difference between the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation of the probe molecule. For Lewis sites, the differential heat of adsorption represents the energy associated with the transfer of electron density toward an electron-deficient, coordinatively unsaturated site, and probably an energy term related to the relaxation of the strained surface [147,182]. Increasing the pretreatment temperature modifies the surface acidity of the solids. The influence of the pretreatment temperature, between 300 and 800°C, on the surface acidity of a transition alumina has been studied by ammonia adsorption microcalorimetry [62]. The number and strength of the strong sites, which should be mainly Lewis sites, have been found to increase when the temperature increases. This behavior can be explained by the fact that the Lewis sites are not completely free and that their electron pair attracting capacity can be partially modified by different OH group environments. The different pretreatment temperatures used affected the whole spectrum of adsorption heats... 

Since many NMR studies of solid acids entail the observation of a chemical shift change in a probe molecule, reactant, intermediate, or product upon complexation with an acid site, there is an opportunity to fundamentally impact the application of NMR to solid acids by making better use of this information. We therefore review the salient parts of the chemical shift in some detail. Since some of the more visible controversies in NMR studies of solid acids regard carbenium ions and related electrophilic species, this treatment will use such ions as examples wherever possible. 

V. NMR Studies of Solid Acidity Using Probe Molecules... 

As examples of probe molecules directly introduced onto solid acids, we consider ketones and aldehydes, amines and other nitrogen-containing compounds, phosphines, and molecules that form multiple hydrogen bonds. 

Ellis and co-workers published a 13C MAS study of ethylamine on solid acids in 1981 (157). Maciel and Haw (158, 159) published NMR studies of pyridine as a probe molecule on solid acids in 1983. We have recently begun to reexamine the 15N spectrum of pyridine on zeolites and other solid acids (160). At low temperatures pyridine is remarkably sensitive to the kinds of acid sites present. Figure 28 shows 15N spectra of pyridine adsorbed on HY samples before and after dealumination. Dealumination in this case seems to make four kinds of Lewis sites distinguishable by NMR of adsorbed pyridine, suggesting pyridine as a good candidate for... 

Here we report the synthesis and catalytic application of a new porous clay heterostructure material derived from synthetic saponite as the layered host. Saponite is a tetrahedrally charged smectite clay wherein the aluminum substitutes for silicon in the tetrahedral sheet of the 2 1 layer lattice structure. In alumina - pillared form saponite is an effective solid acid catalyst [8-10], but its catalytic utility is limited in part by a pore structure in the micropore domain. The PCH form of saponite should be much more accessible for large molecule catalysis. Accordingly, Friedel-Crafts alkylation of bulky 2, 4-di-tert-butylphenol (DBP) (molecular size (A) 9.5x6.1x4.4) with cinnamyl alcohol to produce 6,8-di-tert-butyl-2, 3-dihydro[4H] benzopyran (molecular size (A) 13.5x7.9x 4.9) was used as a probe reaction for SAP-PCH. This large substrate reaction also was selected in part because only mesoporous molecular sieves are known to provide the accessible acid sites for catalysis [11]. Conventional zeolites and pillared clays are poor catalysts for this reaction because the reagents cannot readily access the small micropores. 

Physicochemical methods, i.e. adsorption of probe molecules followed by varied analytical techniques (gravimetry, chromatography, calorimetry, spectroscopic techniques, etc.) are currently used for estimating more precisely the concentration of the potential active sites.[34 36] However, very few methods are well adapted for this purpose most of the methods employed for the characterization of the acidity of solid catalysts lead to values of the total concentrations of the acid sites (Brpnsted + Lewis) and to relative data on their strength, whereas few of them discriminate between Lewis and Brpnsted acid sites. It is however the case for base adsorption (often pyridine) followed by IR spectroscopy, from which the concentrations of Brpnsted and Lewis sites can be estimated from the absorbance of IR bands specific for adsorbed molecules on Brpnsted or Lewis sites. 

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