Studies of cations in zeolites: adsorption of carbon monoxide; formation of Ni ions and Na 3+ 4 centres

Abstract

Zeolite cations lying on the intracrystalline pore surface of the Linde X and Y molecular sieves are linked to only three oxide ions and consequently are not well shielded electrically. They therefore create very large electrostatic fields, extending into the main zeolitic cavities, causing carboniogenic catalytic activity. This activity follows in magnitude the changes in the field, and is independent of the presenceof OH groups ; nochange in activity is observedeven after 99 %of OH protons present after ordinary activation in vacuum at 500°C are removed. Bivalent cations exposed on the surface of the main intracrystalline cavities adsorb carbon monoxide with infra-red frequencies specific to cation ; the adsorption follows Langmuir isotherms, suggesting that a single carbon monoxide molecule can be independently attached to every surface cation. Certain transitionmetal zeolites adsorb carbon monoxide much more tenaciously than alkaline earth cations, indicating a highly significant difference in their ability to form co-ordination bonds.Univalent nickel ions can be prepared both on the intracrystalline surface and at fully coordinated positions by heating NiIIY with alkali metal vapour. The surface NiI ions are chemically very reactive but thermally unstable ; those at fully co-ordinated positions are thermally stable even at 400"C, and are inert to H2, NH3, and CO although they react with oxygen to form 0;.The alkali-metal X and Y zeolites react with alkali metal vapour to form coloured products of non-stoichiometric compositions containing the paramagnetic centres Na 2 + and Naa +. The Na 2 + centres are stable up to 500°C and they react reversibly with gases ; with oxygen they form 0 , radicals. U.S.A. STRUCTURAL FRAMEWORKDespite differences in origin, composition, and chemical and physical properties, the mineral faujasite and the synthetic zeolites of interest in this paper-Linde Molecular Sieve Type X and Linde Molecular Sieve Type Y-are related in crystal structure to the extent that the basic framework is the same for all three. Their structures have been described in their general aspects 1-4 but are all too little known in their detail. Because the basic framework of these materials is primarily involved in the variations in the properties that are here our principal concern, namely, the net electric fields, valences, and electron affinities of the zeolitic (exchangeable) cations, its relevant aspects will now be reviewed. We must emphasize, however, that the only anhydrous material that has been studied by X-ray diffraction is single-crystal faujasite that had been repeatedly exchanged with calcium ion and then dehydrated. We of course cannot be sure of the extent to which we are justified in applying the resulting information to the synthetic materials. Any speculation on fine details of the structures, for example on the ordering of A1 relative to Si, immediately leads one to recognize many probable differences among faujasite, X and Y. As a probable result of the chemical as well as structural dif...

Catalytically important metal ion intermediates on zeolites and silica surfaces

“…Of particular interest is that the intermediate paramagnetic charge state nickel(I) can be stabilized in zeolite lattices and also on silica surfaces [22][23][24][25][26][27]. Photoreduction of nickel(II) by ultraviolet light under molecular hydrogen at low temperature has been used to generate the intermediate nickel(I) but this method is rather inefficient and is not used very much.…”

Insulating state of Na clusters and their metallic transition in low-silica X zeolite

“…1 There have been several studies on Na clusters generated in zeolites with the FAU-type structure. Rabo et al [7] exposed Na-type Y (Na-Y) to the Na vapor and observed ESR signal originated from Na 4 3 + clusters generated in the b cages, where one guest 3 s-electron was shared by 4 Na + ions. Harrison et al [8] observed the narrowing of the ESR signal in highly Na-loaded Na-Y and suggested the motional narrowing mechanism.…”

State of alkali azides in zeolites studied by thermal analysis

“…By the second method alkali metal clusters are produced in the zeolite cavities [8]. The method of metal introduction used in the practice are as follows: treatment in alkali metal vapour generated by evaporation of the alkali metal or decomposition of alkali metal azide furthermore, deposition of alkali metal or its complex from solution of ammonia or naphtalene containing tetrahydrofurane.…”