Bronsted centers

The coordinates of crystallographically different atomic types of Si-0(H)-Al Bronsted centers within five H-form aluminosilicate frameworks have been optimized using a full periodic ab initio Hartree-Fock scheme at the STO-3G level. Single-point calculations have been carried out to obtain the H, Al, and respective QCCs and e.f.g. anisotropies. The latter have been discussed and compared to the experimental values measured for different zeolites. 

It has been established from these studies that the different catalytic properties of transition metal oxides (chromium, cobalt) on zirconium dioxide are attributed to their different acidic properties determined by TPDA and IR-spectroscopy. The most active catalyst is characterized by strong acidic Bronsted centers. The cobalt oxide deposited by precipitation on the zirconium-containing pentasils has a considerable oxidative activity in the reaction N0+02 N02, and for SCR-activity the definite surface acidity is necessary for methane activation. Among the binary systems, 10% CoO/(65% H-Zeolite - 35% Z1O2)... 

Frequency of the fundamental stretching vibration of the OH group in a Bronsted center, =Si-OH-Al=, of H-ZSM-5 zeolite. 

For surface acids a distinction is made between protic (Bronsted centers) and non-protic (Lewis centers). Bronsted centers can release surface protons, while Lewis centers represent surface acceptor sites for electron pairs and thus bind nucleophiles. 

Infrared spectroscopy is a powerful method that allows the direct determination of the Bronsted centers. When pyridine (py) is adsorbed on the catalyst simultaneous determination of both types of center is possible, since it is bound to Bronsted centers in the form of a pyridinium ion through a hydrogen bond (Eq. 5-63), whereas on Lewis acid centers, adsorption occurs by a coordinative acid-base interaction (Eq. 5-64). 

According to Equation 5-66, the Al center can form its fourth bond with a free electron pair of a hydroxide anion. At the same time, the proton can react with a free electron pair of a neighboring O atom, and the formation of a partial bond results in a Bronsted acid center. The Si" " center, which is more electopositive than Al, weakens the 0-H bond and increases the acidity. Experimentally it was found that maxumun acidity occurs at ca. 30 % AI2O3. This model also allows the chemisorption of ammonia on Bronsted centers to be explained (Eq. 5-67). 

In order to unravel adsorption mechanisms, a detailed knowledge of the composition and reactivity of the adsorption centers on the initial adsorbent is imperative [35-37]. It is well known [37-39] that fractured surfaces can be covered by cations and anions with unoccupied orbitals that act as Lewis acid and Lewis base centers, respectively. After adsorption of water molecules, with the evolution of hydroxyl and hydrogen species, the Lewis centers are transformed into Bronsted centers, which will influence surface properties. In many cases, characterizing the adsorption sites is complicated, and controversy stiU exists in the interpretation of IR spectra of functional groups, even for extensively studied oxides such as silica and alumina [6, 40, 41]. 

Two points arise when studying the interaction of mildly basic molecifles with the H-Al-MTS surface with the purpose of identifying the acidic Bronsted centers through the formation of H-bonded adducts. A first point is to check whether the missing acidic OH species absorb at the very same frequency of silanols, and are, thus, masked by the latter, being nonetheless characterized by a more marked acidity, as proposed by both Busca and... 

No definite assignment of these two bands was made. Lavalley and coworkers, however, report a correlation between the frequency of CO interacting with a Bronsted center and the shift suffered by the related O - H stretch, which helps in proposing an interpretation for the hidden species [70]. From this correlation, it results that the corresponding CO frequency of 2173 cm would imply a shift of ca. 200cm . This means that the 0-H stretch of the acidic hydroxyls before interaction with CO would fall, roughly speaking, around 3680 and 3620 cm respectively. 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

The nature (Bronsted or Lewis centers), the number, and the strength of the acidic sites of the Pd/Al203 and Pd/Zr02 solids have been checked using infixed spectroscopy of adsorbed pyridine and thermoprogrammed desorption of ammonia. 

The infrared spectra were recorded after equilibrating the reduced and evacuated solids with an excess of pyridine vapor and further evacuation at various temperatures. After evacuation at 423 K there is no more physically adsorbed pyridine. There is no characteristic band of pyridine adsorbed on Bronsted acid sites (no appearance of the 19b vibration at 1540-45 cm" ) [11,12]. The OH groups observed on the solids are thus non acidic. The existence of Lewis acid centers (coordinatively unsatured Al " or Zr ) is proven by the presence of the 19b vibration at 1440-50 cm" and of the 8a vibration at 1610-1620 cm". The absorbances of the 1440-50 cm" band show that the acidity difference between the Pd/Al203 and PdyZr02 solids is not significant. 

As corroborated by deuterium labeling studies, the catalytic mechanism likely involves oxidative dimerization of acetylene to form a rhodacyclopen-tadiene [113] followed by carbonyl insertion [114,115]. Protonolytic cleavage of the resulting oxarhodacycloheptadiene by the Bronsted acid co-catalyst gives rise to a vinyl rhodium carboxylate, which upon hydrogenolysis through a six-centered transition structure and subsequent C - H reductive elimina-... 

Figure 2 Model for the Bronsted sites in the supercage of a dealuminated HY, depending on the nature and location of the extraframework aluminic phase (the drawing of the extraframework phase is only schematic). In the center is the unperturbed Bronsted site.

Recrystallization procedure applied to the amorphous aluminosilicates of different chemical composition resulted in the formation of the dispersed zeolitic domains of the FAU and BEA structure in porous matrices. The structural transformation into the composite material was proved with TEM, XRD and 27Al and 29Si MAS NMR spectroscopies. The IR data revealed that strong Bronsted acid centers were main active sites generated in the composite materials, irrespectively of the Al content. 

In summary, there now exists a body of data for the reactions of carbocations where the values of kjkp span a range of > 106-fold (Table 1). This requires that variations in the substituents at a cationic center result in a >8 kcal mol-1 differential stabilization of the transition states for nucleophile addition and proton transfer which have not yet been fully rationalized. We discuss in this review the explanations for the large changes in the rate constant ratio for partitioning of carbocations between reaction with Bronsted and Lewis bases that sometimes result from apparently small changes in carbocation structure. 

The LLB catalyst system needs a rather long reaction time and the presence of excess ketone to get a reasonable yield. Yamada and Shibasaki63 found that another complex, BaBM (91), was a far superior catalyst. Complex 91 also contains a Lewis acidic center to activate and control the orientation of the aldehyde, but it has stronger Bronsted basic properties than LLB. The preparation of BaBM is shown in Scheme 3-35. 

The development of catalytic asymmetric reactions is one of the major areas of research in the field of organic chemistry. So far, a number of chiral catalysts have been reported, and some of them have exhibited a much higher catalytic efficiency than enzymes, which are natural catalysts.111 Most of the synthetic asymmetric catalysts, however, show limited activity in terms of either enantioselectivity or chemical yields. The major difference between synthetic asymmetric catalysts and enzymes is that the former activate only one side of the substrate in an intermolecular reaction, whereas the latter can not only activate both sides of the substrate but can also control the orientation of the substrate. If this kind of synergistic cooperation can be realized in synthetic asymmetric catalysis, the concept will open up a new field in asymmetric synthesis, and a wide range of applications may well ensure. In this review we would like to discuss two types of asymmetric two-center catalysis promoted by complexes showing Lewis acidity and Bronsted basicity and/or Lewis acidity and Lewis basicity.121... 

Most of the catalytic interest in the AlP04-based molecular sieves have centered on the SAPOs which have weak to moderate Bronsted acidity, and two have been commerciahzed SAPO-11 in lube oil dewaxing by Ghevron and SAPO-34 in methanol-to-olefins conversion by UOP/Norsk Hydro. Spurred on by the success of TS-1 in oxidation catalysis, there is renewed interest in Ti, Co, V, Mn and Cr substituted AlP04-based materials, for a review of recent developments in the AlP04-based molecular sieves see [35]. 

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