Bound Zeolite Forms

Exposure of the zeolite-bound 23 to dioxygen at low temperatures indicated (as evidenced by a very weak axial electron spin resonance, ESR, signal) that a weakly bound oxygen adduct formed that was similar to the solution-phase adducts observed in noncoordinating sol-... 

With XPS it is possible to obtain good analytical information on the amount of metal adsorbed and, in favourable cases, to identify the chemical form of that metal. Oxidation states are readily determined and it can be shown, for example, that adsorption of Co(II) on manganese oxides results in oxidation to Co(III) (38,39), whereas adsorption of Co(II) on zirconia and alumina leads to the formation of cobalt(II) hydroxide (40). With Y-type zeolites hexaaquacobalt(II) is adsorbed as Co(II), and cobalt(III) hexaammlne is adsorbed as Co(III). The XPS spectrum of Co(II) adsorbed on chlorite was consistent with the presence of the hexaaquacobalt(II) ion for pH 3-7 and indicated that no cobalt(II) hydroxide was present (41). With kaollnlte and llllte, Co is adsorbed as Co(II) over the pH range 3-10 (39,42), it being bound as the aqua ion below pH 6 and as the hydroxide above pH 8. Measurements involving Pb have... 

Later reports (58) have questioned whether the earlier report (55) was correct in concluding that the planar cobalt(II) complex of salen was formed in zeolite Y. The characteristics of the supposedly zeolite-entrapped [Con(salen)] are apparently not as similar to the same species in solution as previously reported. For example, planar [Con(salen)] and its adducts with axially disposed bases are generally ESR-detect-able low-spin complexes (59), and cyclic voltammetry of the entrapped complex revealed a Co3+/Co2+ redox transition that is absent in solution (60). These data, and more recent work (58), indicate that, in the zeolite Y environment, [Con(salen)] is probably not a planar system. Further, the role of pyridine in the observed reactivity with dioxygen is unclear, since, once the pyridine ligand is bound to the cobalt center, it is doubtful that the complex could actually even fit in the zeolite Y cage. The lack of planarity may account for the differences in properties between [Con(salen)] entrapped in zeolite Y and its properties in solution. 

Fig. 4. 45-MHz 2H spectra of CD3I on zeolites CsX, HY, and HZSM-5. On CsX the adsorbate forms a framework-bound CD3 group. Methyl iodide is much less reactive on acidic zeolites. It tumbles isotropically in HY and shows restricted motion in HZSM-5. (Courtesy of Larry W. Beck.)...

A carbenoid-type mechanism with free or surface-bound species formed by a elimination from methanol promoted by the strong electrostatic field of zeolites was proposed first.433,456,457 Hydrocarbons then can be formed by the polymerization of methyl carbene, or by the insertion of a surface carbene (8) into a C-O bond453-455,458,459 (Scheme 3.2, route a). If surface methoxyl or methyloxonium species are also present, they may participate in methylation of carbene454,455,460,461 depicted here as a surface ylide (9) (Scheme 3.2, route b). A concerted mechanism with simultaneous a elimination and sp3 insertion into methanol or dimethyl ether was also suggested 433,454,457... 

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 ... 

The nature of the surface acidity is dependent on the temperature of activation of the NH4-faujasite. With a series of samples of NH4—Y zeolite calcined at temperatures in the range of 200° to 800°C, Ward 148) observed that pyridine-exposed samples calcined below 450°C displayed a strong infrared band at 1545 cm-1, corresponding to pyridine bound at Brpnsted (protonic) sites. As the temperature of calcination was increased, the intensity of the 1545-cm 1 band decreased and a band appeared at 1450 cm-1, resulting from pyridine adsorbed at Lewis (dehydroxylated) sites. The Brtfnsted acidity increased with calcination temperature up to about 325°C. It then remained constant to 500°C, after which it declined to about 1/10 of its maximum value (Fig. 19). The Lewis acidity was virtually nil until a calcination temperature of 450°C was reached, after which it increased slowly and then rapidly at calcination temperatures above 550°C. This behavior was considered to be a result of the combination of two adjacent hydroxyl groups followed by loss of water to form tricoordinate aluminum atoms (structure I) as suggested by Uytterhoeven et al. 146). Support for the proposed dehydroxylation mechanism was provided by Ward s observations of the relationship of Brpnsted site concentration with respect to Lewis site concentration over a range of calcination tem-... 

Lefrancois and Malbois (227) determined the types of acidity present on H-mordenite and various cationic forms by obtaining infrared spectra of pyridine adsorbed on the zeolite. H-mordenite activated at 400° contained both Br0nsted and Lewis acid sites. Upon addition of water, the band due to Lewis-bound pyridine disappeared and the Br0nsted site concentration increased. Removal of the added water by evacuation restored some of the Lewis acid sites. 

The structure proposed by Bradley (1940) has three forms of water a zeolitic water, bound water (at the edges of the octahedral sheet), and structural hydroxyls. Using thermogravimetric curves, Caillere and Henin (1961a) attempted to measure the amount of these three types of water for several attapulgites (Table LV). The amount of bound water and hydroxyls differs considerably from that calculated on the basis of the ideal structure (column 5) and suggests there are more structural hydroxyls than proposed for the ideal structure. 

The sheets formed by the apices of the tetrahedra are completed by hydroxyls and magnesium ions in octahedral coordination link the sheets. The one structural arrangement has nine octahedral sites and the other only eight. Both structures have channels on both sides and top and bottom of each ribbon which contains water molecules (zeolitic water). Additional water is bound to the edge of the ribbons and hydroxyls occur in the structure proper. 

A recent patent (to UOP) (228) claims that active hydroformylation catalysts can be prepared by reacting an aluminated zeolite (prepared from a hydrosol) with HCo(CO)4 vapor. The HCo(CO)4 complex apparently reacts with surface hydroxyl groups releasing hydrogen and yielding a surface-bound cobalt carbonyl complex. The catalyst so formed is claimed to hydroformylate higher olefins to aldehydes and alcohols at 120°C and 240 atm pressure. 

The relatively high activities of these catalysts can in most cases be attributed to the high dispersions of the active species. These are normally incorporated as cations via an ion-exchange process and thus remain bound onto the extensive inner surface of the zeolites by electrostatic forces. The selectivities observed, for example, in oligomerization reactions where, in general, dimers are formed in preference to higher oligomers, may be a direct consequence of the spatial limitations imposed on transition-state complexes within the small zeolite cavities. 

The tetrahedral unit that forms the basis of the network of the zeolite makes many different structures possible. a-Quartz, the low temperature form of SiC>2, is one of the dense polymorphs. Many low-density structures can be formed, which are microporous and contain interconnected channels, bound by oxygen atoms that connect the cation-containing lattice tetrahedra. Approximately 60 different structures exist two examples of are given in Fig. 4.61a and 4.61b. The oxygen atoms are located in the middle of the lines connecting tetrahedrally... 

The data in Table III for the photochemical isomerization of 1-pentene show that photochemical activation is also a viable means of sample activation. During these reactions, CO gas is given off and it is believed from solution studies that an Fe(C0)4L complex is initially formed. The Fe(C0) L complex, where L is a bound pentene, can then undergo isomerization to the cis and trans isomers of 2-pentene. The data in Table III show that the incorporation of a zeolite not only changes the product distribution from a 2.0 ratio of the trans to the cis, as observed in solution studies, but that the photolysis time is relatively short. It should be recognized here that high energy ultraviolet radiation is used, but the photon flux is relatively low. The kinetics of this reaction are surely different from that of the solution reactions and it is not inconceivable that there are steric constraints administered by the zeolite... 

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