Metal-proton adducts

Br0nsted acidity of zeolite protons is essential for catalytic reactions such as isomerization and cracking and has been studied extensively 15,264). Several characterization methods for acid sites in zeolites have been developed this subject has been covered in recent reviews (265,266). Pyridine and other basic molecules are often used in IR work as probe molecules for Brpnsted and Lewis acid sites (267). Trimethylphosphine has also been used as a probe for the determination of zeolite acidity by IR or NMR (96,268). 

The term electron deficiency was introduced by Dalla Betta and Bou-dart to account for the anomalously high hydrogenation activity of small Pt particles in zeolite Y (50). The electron deficiency was ascribed to an electron transfer from small Pt particles to the zeolite. X-Ray absorption has been applied to measure the Pt Lm white line area as an indication of the electron deficiency because the white line is related to the number of unoccupied electronic states in the 5d and the 6j bands (273). For reduced Pt/NaHY it appeared that the white line area, and hence the electron deficiency of Pt particles, are closely related to the proton concentration of the zeolites. For example, the relative white line areas for Pt/H4gY, Pt/ H19Y, and Pt foil are 1.6, 1.2, and 1, respectively. White line areas at the Liii X-ray absorption threshold to determine the if-band occupancy of supported metal catalysts were first reported by Lytle 274). The use of the white line area as an indication for electron deficiency has been questioned by Lewis, who argues that a decrease of the metal particle size will also lead to an increase of the white line area (275). 

The electronic properties of Pt particles on different supports and in zeolites of different proton concentrations were also probed with the competitive hydrogenation of toluene and benzene (276). It was found that the ratio of the adsorption coefficients of toluene and benzene hr/6b, which can be obtained from a kinetic analysis of the hydrogenation rate data, can be used as a convenient empirical index for the electronic environment of Pt particles. For Pt in zeolite Y, the ratio was found to increase with increasing acidity, as does the electron deficiency. This trend was rationalized by considering that toluene is a stronger electron donor than benzene. 

Further evidence for a correlation between electron deficiency of reduced Pt or Pd and proton concentration was found in FTIR studies of adsorbed 

Tri et al. reported that the stretching frequency of CO adsorbed on reduced Pt/NaHY was shifted from 2090 to 2068 cm by neutralization with NaOH (277). A similar shift was found for Pd/NaHY and Pd/CaHY (152,189). The IR band for the linear mode of adsorbed CO on Pd/NaHY is at 2120 cm , which is considerably higher than that for CO adsorbed on Pd/SiOi. Neutralization with NH3 leads, again, to a red shift. 

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Metal phthalocyanines, 25 300-302 Metal porphyrins, 25 300-302 Metal-proton adducts, 39 175-180 catalytic propensities, 39 190-191 Metals see also specific elements... 

T. J. McCarthy, G.-D. Lei, and W. M. H. Sachtler, Methylcyclopentane conversion catalysis over zeolite Y encaged rhodium a test for the metal-proton adduct model, J. Catal. 159, 90-98... 

Although the effect of the support on the catalytic properties of the supported metal particles has been well established, the nature of this metal-support interaction has been the subject of much debate. Explanations have involved the formation of metal-proton adducts on Bronsted acidic supports5,12, electron transfer between support and particle8,13,14, the polarization of the metal particle by nearby cations15 and a rehybridization and polarization within the particle by the Madelung potential of the support16,17. 

Bivalent cations also affect the proton distribution if small cages have been filled, e.g., with Mg ions, the protons that are created during the reduction of a transition metal ion will predominantly stay in the supercages, where they interact with the metal clusters, as will be described in more detail below. Because such adducts can act as very active sites, a catalyst promoter effect of Ca and Mg on reduced Pd has therefore been attributed, in part, to the enhanced concentration of metal-proton adducts in accessible supercages (765). 

The dependence of the electron deficiency of Pd or Pt particles on the proton concentration of zeolites suggests a direct bond between metal particles and some protons (70). A model is the metal-proton adduct, e.g., [Pd Hm] , where m is the number of protons in the adduct. This leaves still two possibilities for the actual structure of this complex. One might assume that the protons in the adduct are either totally detached from the zeolite wall or that they act as bridges of the type Pd —O2, where Oj stands... 

In these samples the electron deficiency appears clearly related to the proton concentration of the supporting zeolite. The results indicate a direct interaction of Pd with protons, in accordance with the metal-proton adduct model. 

The catalytic propensities of the metal-proton adduct, [Pd — should be defined by the value of two parameters n and z. In the above examples n was kept constant and z was changed by varying the concentration of protons in the particular cage system accommodating the Pd particles. The model can, however, also be applied to rationalize data, where n has... 

Results on the characterization of coke on catalysts used in MCP conversion can also be rationalized in terms of the metal-proton adduct model. Temperature-programmed oxidation of such catalysts displays two distinct peaks of CO2 formation, as is shown in Fig. 26. One peak is characteristic of coke combustion catalyzed by metal oxide, and the second peak, at... 

Recent results are presented illustrating principal mechanistic differences between alkane isomerization in liquid acids and over solid acids, including bifunctional catalysts. Isotopic labeling shows that butane isomerization over solid acids proceeds preferentially as a bimolecular process, i.e. via a Cg intermediate, which subsequently decomposes, preferentially into two iso-Cn structures. Bronsted acid sites in zeolites form chemical bonds with metal clusters. The resulting metal-proton adducts function as "collapsed bifunctional sites". 

Key Words Acid Catalysts, Bifunctional Catalysts, Sulfated Zirconia, Metal-Proton Adducts, Carbenium Ions, Protonated Cyclopropane, Butane Isomerization, Collapsed Bifimctional Sites, Electron Deficiency. 

Catalysts which contain reduced transition metal clusters besides acid sites are able to catalyze reactions that are not observed on catalysts exposing one type of site only. The reaction network is inadequately described by models which assume only additivity of catalytic functions and shuttling of intermediates between sites. There is strong evidence that metal clusters and Bronsted sites form metal-proton adducts. These act as "collapsed bifunctional sites" all alkane isomerization steps can take place on such sites during one single residence of the adsorbed molecule. At low temperature, adsorption in a mode reminiscent of a carbenium ion can suppress pure metal catalysis. 

The metal protonated adducts (100) could not be detected directly because they are very acidic (p < ) This means they cannot be generated at the required pH because the thiolate ions are protonated under these conditions and the corresponding thiols are not reactive enough to add to the carbene complex. However there is kinetic evidence for 100. When 99 that has been generated at high pH is reacted with HCl, the pseudo-first-order rate constant for the conversion of 99 back to 98 shows a non-linear dependence on an+, as shown in Fig. 5 for some representative examples. This dependence is consistent with H -catalyzed... 

Guanidine, H2N(G=NH)NH2, is the amidine of carbamic acid, H2N(CO)OH. Guanidine forms three types of complexes with metals cationic (in which the guanidinium cation is formed by taking up a proton), adducts with neutral molecules or coordination products with ionic salts, and substitution products. A brief account of each type is presented below. 

Many metal hydrides protonate to give H2 complexes, but kinetic protonation can take place on M-H to give an M-(H2) complex, even when protonation is thermodynamically favored at the metal. Protonation of CpFeH(dppe) gave the dihydrogen complex at —80°, followed by rearrangement to the dihydride at 25°. Kinetic protonation by A-H at M-H is consistent with the presence of a dihydrogen bonded A-H...H-M precursor adduct as intermediate. Proton transfer in the adduct gives the H2 complex, and conversion to the trans dihydride is slower, because motion of the heavy atoms is needed. 

Its protonation state and site of protonation or deprotonatlon. Their Intensities, on the other hand, are largely a function of the metal binding site, but also depend somewhat on the nature of the ligand (16). Bands of this type have not been reported for any other metal-nucleoside adducts and contribute to making the present system one of the most convenient for study. 

Although so far only rarely investigated, cholesteryl esters possess characteristics similar to DAGs or TAGs, namely that they are exclusively detectable as alkali metal adducts [34, 69], and never as proton adducts. Even spectra of purified cholesteryl esters always contain a significant peak of free cholesterol (m/z 369.3), and this peak is normally more marked than that of the intact cholesteryl ester [70]. Although not yet carefully investigated, this unpHes a mechanism similar to TAG that is, a Umited stability of the adducts of cholesteryl esters. 

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