Framework atom substitution

Framework Atom Substitution and Inter/Intracluster Geometiy... 

Abstract We briefly underline the relevance of TS-1 catalyst for industrial applications in mild oxidation reactions using hydrogen peroxide as oxidant and review the experimental works employed over last two decades for imderstanding the structme of the Ti centers in the bare TS-1 material. After an animated and controversial debate that has lasted in the literature until 1994, several works (reviewed here in depth) have definitively assessed that Ti atoms occupy framework positions substituting a Si atom and forming tetrahedral... 

This can be related to the fact that the Si atoms substituting Al in the framework during the SiCl treatment originate outside the zeolite (i.e. from SiCl,), while in the steam/ acid treatment the corresponding silicon atoms originate in other parts of the zeolite crystals. This can also explain the absence of "secondary" pores in the material prepared with SiCl, as shown by sorption isotherms for different hydrocarbons (27). 

In substitutional disorder, two or more types of atoms randomly occupy one set of lattice nodes. Substitutional disorder of different atoms, in size and/or charge, would cause displacements not only in their lattice node, but also in the neighbor sites. The most striking effect of substitutional disorder is normally a thermal motion of the framework atoms which is apparently anomalously high. 

The aluminophosphate molecular sieve, AIPO4-5, itself has limited potential as catalyst, since its stnjcture is neutral and has neither catbn exchange capacities nor acidity [1-3]. There are two possibilities for utilizing the molecular sieves one is rrwdification of the framework by substitution of metal atoms such as silicon [3-6] and/or transition metals [5-11], and the other is introducing active site by impregnation. 

Supported non-framework elements, as well as substituted or doped framework atoms, have been important for zeolite catalyst regeneration. By incorporating metal atoms into a microporous crystalline framework, a local transition state selectivity can be built into the active site of a catalytic process that is not readily attainable in homogeneous catalysis. The use of zeolites for carrying out catalysis with supported transition metal atoms as active sites is just beginning. The local environment of transition metal elements as a function of reaction parameters is being defined by in situ Mossbauer spectroscopy, electron spin echo measurements, EXAFS, and other novel spectroscopic techniques. This research is described in the second part of this text. 

Figure 4. Deformation of an ideal framework after a T-atom substitution and protonation.

The common means of introducing redox catalytic activity in zeolites is by the substitution of framework atoms such as Si, A1 or P with redox-active metal cations. This has been accomplished by two different methods (1) hydrothermal synthesis and (2) post-synthesis modification. Irrespective of the method of preparation, with the notable exception of titanium silicalites, these redox metals in the framework are susceptible to leaching due to the solvolysis of M-O bonds [77]. Even the Ti silicalites suffer from leaching under basic conditions [76a]. 

Post-synthesis modification involves isomorphous substitution of framework atoms with the desired redox metals either in aqueous media with soluble metal salts or in the gas phase with volatile chlorides. Incorporation of Ti into the framework of faujasite, zeolite-) and ZSM-5 has been accomplished by treating the zeolite with ammonium titanyl oxalate, TiCU or Ti(0/-Pr)4. Substitution of V for framework atoms has been reported with VOCI3 vapor. A more generalized method involving the reoccupation of the silanol nests created by the deboronation of bor-osilicates (ZSM-5 and zeolite- ) shows considerable promise for the incorporation of redox metals into the framework [79]. 

This series also allowed an investigation of the influence of the position of the carbon atom substituting for nitrogen in the basic bis(terpyridine) framework. The highest efficiency for intervalence electron transfer was found when the carbon atom was located on the metal-metal C2 axis of the molecule. 

The unit cell dimensions of quality ZSM-5 materials can be meastired by powder X-ray diffraction almost routinely to an accuracy of O.OOSA or 0.05° (some 0.02%). The character of the changes in the unit cell dimensions that accompany framework T-atom substitution are known to depend on the framework topology and might be presumed to depend on the A1 T-site substitution pattern. Simulation was therefore applied in an attempt to relate changes in the unit cell dimensions, macroscopic observables, to particular patterns of local T-site substitution by aluminum in the MFI-framework [50]. 

It is clear that in the case of MFI, the zeolite pore entrances should not be considered as rigid apertures. Instead, zeolite framework topologies can show flexibility. While the O-Si-0 angle in the tetrahedral unit is rigid (109 + 1 °), the Si-O-Si angle between the units can vary between 145 and 180°. Based on isomorphous substitution of Si by other T-atoms in the framework [18], framework defects [19], cation positions, changes in the water content [16], external forces on the crystalline material [20] and upon adsorption of guest molecules [21] phase transitions can occur that have a dramatic influence in particular cases on the framework atom positions. 

Si is the only element exhibiting SM III substitution. Various possibilities to generate silicoaluminophosphate (SAPO) frameworks by substitution of P and Al atoms with Si are shown in Figure 2.11.[19] The substitution is conveniently explained by using a two-dimensional grid representation of T-atom configurations. 

Nearly all syntheses of zeolites and microporous aluminophosphates have limitations to gel composition and other parameters. For example, some zeolites with special compositions such as high-silica Y zeolite and low-silica ZSM-5 cannot be directly synthesized. A secondary framework modification is necessary for their preparation. For instance, dealuminization, isomorphous substitution of extraneous silicon for aluminum, and removal of the sodium process in Y zeolite are necessary to prepare ultra-stable zeolite Y (USY) isomorphous replacement of framework atoms of boron with aluminum in a presynthesized silicon-boron structure is often used to prepare some specific aluminosilicate zeolites that cannot be directly synthesized, such as Al-SSZ-24 (AFI) and Al-CIT-1. Secondary synthesis (post-treatment) will be discussed in detail in Chapter 6. 

The sp -bonded zeolite type structures are obtained from natural silica frameworks by substituting the silicon atoms with carbon and removal of oxygen. We demonstrate that considering n-type doping such structures [46-48] behave very much like diamond. A mid-gap state occurs after a substantial relaxation of the impurity which is localized on a nearby carbon atom. 

---