Brpnsted acidity characterization

The microcalorimetry of NH3 adsorption coupled with infrared spectroscopy was used to study the effect of the synthesis medium (OH or F ) on the nature and amount of acid sites present in Al,Si-MFl zeolites [103]. Both techniques revealed that H-MFl (F ) with Si/Al < 30 contained extra-framework aluminum species. Such species were responsible for the presence of Lewis acid sites and poisoning of the Brpnsted acidity. In contrast, MFl (F ) characterized by Si/Al > 30 presented the same behavior as H-MFl (OH ). 

On the other hand, Lewis acids are catalytically inactive for skeletal transformations unless proton donors are available at the same locality.1 Lewis acids consist of incompletely coordinated surface ions aluminum ion is the most frequently cited example. Since the relevance of acidity measurements for the prediction of catalytic activity is what we are trying to emphasize, we have concentrated on the determination of Brpnsted acidity in this critique. The problem of finding the most relevant method for acidity measurement has therefore been treated as an evolutionary process in which successive methods have been used more and more successfully for the characterization of a relatively small number of strong Br0nsted acids that are frequently accompanied by a multitude of other surface acids. 

Monocationic acyl ions are readily prepared as persistent species in solutions of low nucleophile strength.68 These acyl ions have been thoroughly characterized by IR and NMR spectroscopy, and several acyl ion salts have been characterized by X-ray crystallography. The monocationic acyl ions are often prepared in situ from carboxylic acids, esters, or anhydrides, by the action of a strong Brpnsted acid, or the ions can be prepared from ionization of an appropriate acid halide with a strong Lewis acid. Both methods have been used to prepare acyl-centered dications, some of which can be considered distonic superelectrophiles. As described previously, dicarboxylic acids cleave to the bis-acyl ions in superacid (FSChH-SbFs) provided that the acyl cations are separated by at least three methylene units (eq 54).55 The first bis-acyl dications were reported by Olah and Comisarow, being prepared by the reactions of dicarboxylic acid fluorides with superacidic SbFs (eq 72).69... 

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. 

For ferromagnetic cobalt particles in zeolite X, spin-echo ferromagnetic resonance has been used to obtain unique structural information (S6). In addition, study of the catalytic signature of metal/zeolite catalysts has been used by the groups of Jacobs (87), Lunsford (88), and Sachtler (47,73,89). Brpnsted acid protons are identified by their O—H vibration (90,91) in FTIR or indirectly, by using guest molecules such as pyridine or trimethylphosphine (92,93). An ingenious method to characterize acid sites in zeolites was introduced by Kazansky et al., who showed by diffuse reflection IR spectroscopy that physisorbed H2 clearly discerns different types of acid sites (94). Also, the weak adsorption of CO on Brpnsted and Lewis acid sites has been used for their identification by FTIR (95). The characterization of the acid sites was achieved also by proton NMR (96). 

Variable temperature MAS NMR was used to characterize the structure and dynamics of hydrogen bonded adsorption complexes between various adsorbates and the Brpnsted acid site in H ZSM-5 the Brpnsted proton chemical shift of the active site was found to be extremely sensitive to the amount of type of adsorbate (acetylene, ethylene, CO and benzene) introduced (105). Zscherpel and coworkers performed maS NMR spectroscopic measurements in order to investigate the interaction between Lewis acid sites in H ZSM-5 and adsorbed CO. A new measure for the "overall" Lewis acidity of zeolites was derived from the C MAS NMR spectroscopic data. In addition, the chemical shift of CO adsorbed... 

Other adsorbents have been used in an effort to measure the acid strength of the sites or eliminate diffusion limitations. Kubelkova et al. used low temperature adsorption of CO on H-ZSM-5, H-Y, NaH-Y, and various AlPO sieves to measure the shift in the acidic OH stretching frequency upon CO adsorption. The authors argue that this shift is related to the proton affinity of the zeolites and thus to the Brpnsted acid strength. Tvaruzkova et al. used d3-acetonitrile to characterize both the Brpnsted and Lewis acidity of a number of zeolites. Using the band intensities and the frequency of the C-N band they obtained relative concentrations and strengths of the various acid sites. 

The addition processes are characterized as Type II allylation reactions and featnre open transition states. Denmark and Keck have described mechanistic studies, which have provided basic information shaping current views regarding the Type II allylations of organostannanes. Denmark s models are briefly summarized by the cyclization of deuterium-labeled 4 (Scheme 5.2.2). Intra-molecular allylation proceeds with Lewis and Brpnsted acids to yield 6 with high selectivity. The alcohol 6 is rationalized throngh the synclinal transition state 5 via anti-Su substitution. To a lesser degree, various Lewis acids also produce the... 

In the field of snrface science and interfaces, it is well known that acid-base interactions play an important role for a large nnmber of phenomena, snch as adhesion on polymers, polishing, etc. The oxide surfaces in an aqneons solntion become charged due to amphoteric dissociation of surface M-OFl groups. The Brpnsted acidity or basicity of an oxide surface can be characterized by the point-of-zero charge (pzc), which corresponds to the pFI value required to achieve zero net surface charge. 

Table 4.2 lists a number of Brpnsted acids and their acid dissociation constants. Strong acids are characterized by values that are greater than that for hydronium ion (HsO, /fa = 55). Essentially every molecule of a strong acid transfers a proton to water in dilute aqueous solution. Weak acids have values less than that of H30 they are incompletely ionized in dilute aqueous solution. 

An efficient single-step catalytic process was recently developed for the conversion of glucan-type polysaccharides, especially starch, to sorbitol [15]. This process is characterized by the simultaneous hydrolysis of the polysaccharide and hydrogenation of the liberated monosaccharide. The catalyst used is Ru-loaded H-USY zeolite (3 % wlw Ru) in which the zeolitic material fulfils the role of metal carrier (Ru) and solid-acid catalyst. The zeolite provides the Brpnsted acidity required for the hydrolysis reaction either because of its outer surface or by introducing some homogeneous acidity, and the Ru catalyzes the hydrogenation of D-glucose to sorbitol (Scheme 2). 

Two main methods have been employed to characterize the acidity of solids temperature-programmed desorption of ammonia and infrared spectroscopy of adsorbed bases. Temperature-programmed desorption (TPD) over acid solids previously exposed to ammonia allows one to quantify the acid content and provides information on the strength of the acid sites. However, the distinction between Lewis and Brpnsted acidities is not straightforward and is often hampered by physisorbed ammonia. The validity of this method has recently been questioned (56). Infrared spectroscopy of adsorbed pyridine and ammonia have been most... 

Rhodium-Catalyzed Reactions via Zwitterionic Intermediates Diazo compounds are also known to undergo insertion into C—H bonds by action of a rhodium-based catalyst, giving rise to a zwitterionic species characterized by a similar reactivity to that of onium ylide species [170]. Recently, Hu et al. [171] have described that zwitterionic intermediates 150, obtained by carbene insertion into a C—H bond in indoles, can be trapped by imine 151 activated by a chiral Brpnsted acid. After optimization of conditions, three-component reactions carried out at -10 °C in toluene afforded the desired products 152 in high yields, >20 1 diastereoselectivities for the flnft-isomer, and 84-99% ee (Scheme 3.66). 

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