Zeolites electron deficiency

Y.Z. Zhang, T.T. Wong, and W.M.H. Sachtler, The effect of Ca + andMg + ions on the formation of electron-deficient palladium-proton adducts in zeolite, J. Catal. 128, 13-22 (1991). 

The oxonium ylide mechanism requires a bifunctional acid-base catalyst. The validity of the oxonium ylide mechanism on zeolites was questioned459,461,464 because zeolites do not necessarily possess sufficiently strong basic sites to abstract a proton from the trimethyloxonium ion to form an ylide. It should, however, be pointed out, as emphasized by Olah,447,465 that over solid acid-base catalysts (including zeolites) the initial coordination of an electron-deficient (i.e., Lewis acidic) site of the catalysts allows formation of a catalyst-coordinated dimethyl ether complex. It then can act as an oxonium ion forming the catalyst-coordinated oxonium ylide complex (10) with the participation of surface bound CH30 ions ... 

Concentrations of proton and non-proton sites in zeolites were changed by thermal treatment of Na, NH zeolites at different temperatures (.100°, 250°, 350°, 450°, 550°, 650°, and 750°C). Molecules of N,N-dimethylaniline interact at 20°C with both the proton-donating and electron-deficient zeolite sites. Effects of these interactions are evident in the spectra of the adsorbed species. 

Cince the catalytic activity of synthetic zeolites was first revealed (1, 2), catalytic properties of zeolites have received increasing attention. The role of zeolites as catalysts, together with their catalytic polyfunctionality, results from specific properties of the individual catalytic reaction and of the individual zeolite. These circumstances as well as the different experimental conditions under which they have been studied make it difficult to generalize on the experimental data from zeolite catalysis. As new data have accumulated, new theories about the nature of the catalytic activity of zeolites have evolved (8-9). The most common theories correlate zeolite catalytic activity with their proton-donating and electron-deficient functions. As proton-donating sites or Bronsted acid sites one considers hydroxyl groups of decationized zeolites these are formed by direct substitution of part of the cations for protons on decomposition of NH4+ cations or as a result of hydrolysis after substitution of alkali cations for rare earth cations. As electron-deficient sites or Lewis acid sites one considers usually three-coordinated aluminum atoms, formed as a result of dehydroxylation of H-zeolites by calcination (8,10-13). 

The influence of both heat treatment of decationized zeolites and the nature of cations on the proton-donating and electron-deficient zeolite properties has been studied (13-16). However, these works do not allow one to follow clearly the mutually dependent changes in proton-donating and... 

The object of this work was to study the influence of pretreated, decationized NH4-zeolites on adsorbed A,iV-dimethylaniline molecules. Such influence is caused by, proton-donating and electron-deficient active sites in decationized zeolites. Interaction of an aromatic amine molecule (M) with the proton-donating site leads to the formation of the MH+ molecule ion interaction with the electron-deficient site results in the M+ cation radical. Stabilization of these states by adsorption leads to the... 

Scheme I illustrates the appearance of proton-donating sites in the temperature region for stable OH groups when the NH4+ ions are still partially in the zeolites at this point electron-deficient sites are not formed (low temperature region). Schemes II and III describe the successive breakdown of proton-donating sites when the temperature is raised, according to ...

Schemes I—III do not differ significantly from those reported in the literature (8,12). First, the electron-deficient centers in the zeolites must arise at the expense of proton-donating sites. Secondly, the nonproton centers formed in decationized zeolites are essentially different from each other. Both facts are confirmed by the results of our investigations on the electronic spectra of decationized zeolites.

For diborane, also reactive towards surface hydroxyl groups, and because of its electron deficient character reactive towards the oxygen bridges -Si-O-Si- in the zeolite structure, the primary and secondary reactions on H-zeolites can be summarized as ... 

From this discussion one may conclude that, in spite of the present difficulties in interpreting XPS and UPS spectra, small Pd clusters, unsupported or supported (no matter what support is used), would contain fewer d electrons relative to the bulk metal. Next we shall furnish electron spin resonance and infrared evidence for the existence of electron-deficient Pd species in alumina- and zeolite-supported Pd catalysts. 

The conclusion that palladium particles in zeolites may carry a partial positive charge follows from the IR study of CO adsorption. This adsorbate can be considered to be a probe of the electronic state of palladium. Namely, the shift toward higher frequencies of the CO linear band (for Pd°-CO it appears at <2100 cm ) reflects a decrease in the back donation of electrons from Pd to CO. Along with such an interpretation, Figueras et al. (138) detected the presence of electron-deficient Pd species in Pd/ HY but not in Pd/Si02. More recently, Lokhov and Davydov (139) confirmed the presence of positively charged Pd species apart from Pd° in reduced (at 300°C) Pd/Y samples and ascribed a 2120- to 2140-cm"1 band to Pd+-CO complexes (Fig. 7). Similarly, Romannikov et al. (140) report that adsorption of CO on Pd/Y samples reduced at 300°C produces IR bands at >2100 cm 1 ascribed to Pd+-CO and Pdzeolite protons, because the IR band of the zeolite O-H group decreases when CO is released and increases when CO is added to the cluster (141, 142). 

Although by 1972 Dalla Betta and Boudart (168) and later Foger and Anderson (169) showed that electron-deficient platinum (in zeolites Y LaY, CaY, and NaY) is much more active than Pt/Si02 (169) and Pt/Al203... 

Nitrogen dioxide, N02, is a fairly small molecule with an unpaired electron and may be expected to be a selective molecule for electron-deficient or Lewis acid sites. Nevertheless, only very little spectroscopic information on the nature of surface species formed on adsorption of N02 is available. Naccache and Ben Taarit (242) have shown by infrared spectroscopy and ESR that N02 forms Cr+N02+ and Ni+N02+ complexes on chromium and nickel zeolites. Thus, N02 behaves as an electron donor and reducing agent in these zeolites. Boehm (243) has studied the adsorption of N02 on anatase and on tj-A1203, which were pretreated at very low temperatures of only 100°-150°C. At 1380 cm-1, a band which is to be attributed to a free nitrate ion, was observed. Boehm (243) has explained the formation of the nitrate ion by the disproportionation by basic OH ions ... 

Figueras et al. (105) found some direct evidence for electron-deficient palladium clusters on various cation-exchanged forms of zeolite Y from CO adsorption experiments. In particular, a correlation was observed between the turnover number for benzene hydrogenation and the CO stretching frequency. The shift toward higher frequency with increasing support acidity was considered as evidence for increased electron acceptor properties of the support. Further studies will, however, be required to provide a more detailed understanding of this phenomenon. 

There does seem to be some quite good evidence for the existence of small electron-deficient metal clusters in zeolites (101-105), which may be related to their increased resistance to sulfur poisoning. Further studies are, however, necessary in order to provide a more detailed understanding in this area. 

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. 

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. 

Different charge-compensating cations in zeolite L have been tested for their promotional effect in n-hexane aromatization. Apparently, high basicity of the alkaline and alkaline earth promoter favors n-hexane aromatization. Basicity and selectivity both increase from Li and Cs 331) and from Mg to Ba (22,25). Bezouhanova et al. studied the FTIR bands of linearly adsorbed CO in the range of 2060-2075 cm . One band at 2075 cm", which is also found on unsupported Pt, is attributed to extrazeolite Pt particles, a second band shifts from 2060 cm" for Li to lower wavenumbers with K and Rb 331). Another criterion, used by Larsen and Haller, is the measured rate of competitive hydrogenation of benzene and toluene, which has been found to correlate with the zeolite basicity (25). As described in a previous section, this method had previously been used by Tri el al. to probe for the electron deficiency of Pt particles in acidic zeolites 332). The rate data are analyzed in terms of a Langmuir-Hinshelwood model and the ratio of the adsorption coefficients of toluene and benzene, A, /b, is determined. It was found to decrease from 8.6 for Pt/Si02, and 5.4 for Pt/MgL, to 4.4 for Pt/BaL. As direct electron transfer from the cations to neutral Pt particles is unlikely, an interaction of Pt with the zeolite framework or with... 

This process is often described not in terms of Fermi-level quantities but in terms of total charge if the metal particle is electron deficient, it has fewer electrons available for the Blyholder mechanism. Initially, the expression referred to the experimental fact that catalysts of platinum or palladium in zeolites, when compared to the same metals on oxides, often seemed to behave like their neighbors in the periodic system, iridium and rhodium. 

---