Framework-substituted Molecular Sieves

As mentioned above the discovery of the remarkable activity of titanium silicalite-1 (TS-1) as a catalyst for a variety of synthetically useful oxidations, including epoxidation, with aqueous hydrogen peroxide constituted a major breakthrough in oxidation catalysis [14-20]. The success of TS-1 stimulated the search for related materials [21]. Titanium silicalite-2, with the MEL structure, was discovered by Rat-nasamy and coworkers in 1990 [41] and had properties similar to those of TS-1. 

Titanium was also incorporated into mesoporous silicas such as MCM-41 [50] and MCM-48 [51]. The resulting materials catalyze epoxidations with both H2O2 and TBHP. They are not, however, stable towards leaching under reaction conditions, particularly with H2O2 [52]. Different strategies have been employed [53] to enhance the hydrophobicity and, hence, the stability and overall performance of these materials as catalysts for epoxidations with H2O2, albeit with only moderate success. 

In an alternative approach, Maschmeyer et al. [26] grafted titanium(IV) species on to the internal snrface of MCM-41, by reaction with (Cp)2TiCl2 and subsequent calcination. Because all the Ti(IV) sites are at the surface one would expect this material to be more active than Ti-MCM-41 prepared by hydrothermal synthesis, and this proved to be the case [26]. Here again, this material was effective only in epoxidations with TBHP, not with H2O2, and its stability towards leaching was not rigorously established. 

Other transition metal-substituted molecular sieves have been synthesized and tested as epoxidation catalysts [21]. The stability of many of these materials towards leaching is, however, seriously in doubt [31]. Catalysis by vanadium-substituted molecular sieves has, in all cases studied, been shown to be homogeneous in nature, i.e. because of leached vanadium [31,55]. A priori, one would expect zirconium- [56] and tin- [57] substituted molecular sieves to be more stable but more rigorous proof is needed. 

Hydrotalcites are synthetic basic clays, so-called layered double hydroxides (LDHs) of magnesium and aluminum consisting of Brucite-like layers with an overall positive charge with anions (usually OH or in the interlamellar 

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Alternatively, framework substitution can be achieved by post-synthesis modification of molecular sieves, e.g. via direct substitution of A1 in zeolites by treatment with TiCl4 in the vapour phase [34] or by dealumination followed by reoccupation of the vacant silanol nests. Boron-containing molecular sieves are more amenable to post-synthesis modification than the isomorphous zeolites since boron is readily extracted from the framework under mild conditions [35]. Synthesis of framework-substituted molecular sieves via post-synthesis modification has the advantage that it is applicable to commercially available molecular sieves which have already been optimized for use as catalysts. 

CATALYTIC OXIDATIONS - FRAMEWORK-SUBSTITUTED MOLECULAR SIEVES... 

There are various Ti-substituted molecular sieves, e.g., TS-1 which is a ZSM-5 type molecular sieve.53 They are characterized by having isomorphous substitution of some SP by TP ions in the zeolite framework. These materials find wide use in synthesis and as oxidation catalysts. 

The difficulty of incorporating metal ions into the molecular sieve lattice results from the fact that actually two requirements have to be fulfilled, i.e., (i) the metal cation must have approximately the size of the atom it replaces (Si, A1 or P) and (ii) it must be able to coordinate in a tetrahedral position in the firamework. Fiuthermore, to function as a successful redox catalyst, a change in the valency and/or the coordination of the oxidant must be realized via reversible change of the coordination of the metal cation. Only a limited number of cations have been reported to be incorporated in the fiamework of zeolite and metal-aluminophosphate molecular sieves. These cations include Co, V, Mn, Cr. Ti [158,159] and a short compilation of the structures available (isomorphously substituted molecular sieves) is compiled in Table 1. Generally, it seems that aluminophosphate lattices are more easily adaptable for isomorphous substitution, but that the resulting materials have a lower stability than the corresponding zeolite frameworks [160]. 

The demonstration by Enichem workers [1] that titanium silicalite (TS-1) catalyzes a variety of synthetically useful oxidations with 30% aqueous hydrogen was a major breakthrough in the field of zeolite catalysis [2], The success of TS-1 prompted a flourish of activity in the synthesis of other titanium-substituted molecular sieves, such as titanium silicalite-2 (TS-2) [3], Ti-ZSM-48 [4] Ti-Al-mordenite [5], Ti-Al-beta [6]and Ti-MCM-41 [7]. Moreover, this interest has also been extended to the synthesis of redox molecular sieves involving framework substitution by other metals, e.g. chromium, cobalt, vanadium, etc. [8]. 

Infrared spectroscopy has proven to be a valuable tool in characterization of titanosiUcates. In the case of TS-1 and other Ti-substituted molecular sieves the infamous band at -960 cm was held to be evidence of titanium incorporation into the framework [20,21]. However, the same band was found with titanium free silicalite [22] and not observed when sihcalite was modified from aqueous fhiorotitanate solutions [23]. Most would agree now that this band is an Si-O stretch derived from defect sites in the sihcate yet there may be some overlap and contribution from Si-O-Ti stretches. It seems in some cases... 

It has been already emphasized that substitution of heteroelements into the framework of molecular sieves creates acidic sites. Incorporation of transition elements such as Ti, V, Mn, Fe, or Co, which have redox properties, provides molecular sieves with redox active sites that are involved in oxidation reactions (323-332). As mentioned in the beginning of the article, the transition metal-substituted molecular sieves, the so-called redox molecular sieves, exhibit several advantages compared with other types of heterogeneous redox catalysts (1) redox sites are isolated in a well-defined internal structure therefore, oligomerization of the active oxometal species is prevented (this is a major reason for the deactivation of homogeneous catalysts) (2) the site isolation (the so-called microenvironment) of redox centers prevents the leaching of the metal ions, which frequently happens in liquid-phase oxidations catalyzed by conventional transition metal-supported catalysts (3) well-defined cavities and channels of molecular dimensions endow the catalysts with unique performances such as the shape selectivity (and traffic control) toward reactants, intermediates, and/or products. 

Incorporation of titanium into high-silica frameworks has been the object of many studies, especially after the discovery of titanium-sihcalite-1 (also known as TS-1) and of its exceptional catalytic properties in oxidation reactions involving hydrogen peroxide. Unique among the transition metal-substituted molecular sieves, TS-1 has become an industrial catalyst... 

Propene is mainly formed over V-substituted molecular sieves, in particular, AlPO-5 and siUcalites. The mechanism of its formation is not very clear. However, there are several experimentally established aspects of successful ODH catalyst V has to be incorporated inside the framework as tetrahedral V +04 species [87,89]. StiU, there is no evidence of the actual existence of P-O-V and Al-O-V bonds [89] and seems that if there is any real substitution, it occurs at low extent. Furthermore, low vanadium content catalysts (IV/u.c. or 2 wt% V) show better performance, which is explained by extra-framework V2O5 species (less active) in the case of high loadings. This is experimentally demonstrated by preparing a VAPO-5 catalyst, where vanadium is added during the hydrothermal preparation of the molecular sieve and another, where V is impregnated onto AlPO-5 (entries 2 and 3 in Table 13.6) [85]. Both show similar conversions, but... 

These structures are unique for several reasons. First, they represent three new multidimensional 12-MR systems, which are rare even among zeolites. Second, the amount of framework substitution by metals such as Mn2+ and Mg2+ was unknown prior to this series. Also, the ease of forming both gallium and aluminum phosphates appear to be comparable. Finally, it would appear the charge-matching approach has proven to be a successful strategy for the synthesis of new molecular sieves. It is not clear whether these materials are thermally or hydrothermally stable but they do represent novel pore structures that should impart some unusual properties. 

Flanigen, E.M. and Grose, R.W. (1971) Phosphorus substitution in zeolite frameworks. Adv. Chem. Ser., 101 (Molecular Sieve Zeolites-I), 76-101. 

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. 

Takahashi et al. [25] reported that the dispersed tetravalent vanadium (l 7/2) showed a hyperfine structure but broad band could be observed in the agglomerated vanadium. Miyamoto et al. [8] and Jhung et al. [7] reported that EPR spectra of VAPO -S showed hyperfine structure. Miyamoto et al. [8] suggested that the hyperfine structure indicated atomically dispersion of vanadium in VAPO -S molecular sieve, in other words, vanadium was substituted in the framework of AIPO -S. 

Catalysis. - Aluminophosphate molecular sieves (A1PO) form a family of synthetic zeotypes, containing many three dimensional framework structures. Metal substituted aluminophosphates (MAPO) have important applications as catalysts and HFEPR has been used to determine the catalytically active sites. Two very detailed papers on various MAPO have been reported recently22,23 using both echo-detected HFEPR at 95 GHz and 3H and 31P ENDOR. 

It is appropriate and very instructive to briefly discuss a relatively new and very successful approach, namely, the development of catalysts with designed and atomically engineered active centers. Thomas and coworkers used micro- and meso-porous solids and carried out delicate structural and compositional variations to prepare specific catalysts capable of promoting regioselective, shape-selective, and enantioselective conversions.183-185 This strategy resulted in the development of framework-substituted CoALPO-18 and MnALPO-18 molecular sieves for the selective aerobic oxidation of linear alkanes to the corresponding monocarboxylic acids,186 and that of hexane to adipic acid.187 Framework-substituted MALPO-36... 

Synthesis of transition metal containing molecular sieves (microporous as well as mesoporous) is one of the fastest developing areas in molecular sieve science, as evidenced by recent published reviews [1,2] Several transition metals have been substituted into crystalline silica or aluminophosphate frameworks to yield the corresponding metallosilicate or metalloaluminophosphate molecular sieves, However, the location of the metal species and their state always remain uncertain, despite the employment of numerous different characterization methods comprising IR, NMR and ESR spectroscopy. 

Recently, there has been a growing interest into niobium- and tantalum-containing molecular sieves. The introduction of niobium into mesoporous molecular sieves has been studied by Ziolek et al [3,4], while Antonelli and Ying reported the synthesis of mesoporous niobium oxide [5], The synthesis and characterization of niobium- and tantalum-containing silicalite-1 (NbS-1 and TaS-1) was published recently [6,7,8] and some evidence has been presented for isomorphous substitution [6,8] of Nb and Ta into the silicalite-1 framework. The synthesis of NbS-2 (MEL) [9] and a new molecular sieve named NbAM-11 have been reported as well [10],... 

MCM-41 and silicalite-1 can be synthesized in the presence of niobium- and tantalum-containing compounds. The results indicated that Nb(V) and Ta(V) are well dispersed in the framework of silicalite-1 and in the amorphous walls of MCM-41 y-irradiation of activated niobium and tantalum molecular sieves show two radiation induced hole centers (V centers) located on Si-O-Si and M-O-Si (M = Nb, Ta) units. True isomorphous substitution as suggested in the literature for Ti(IV), however, is unlikely to be present Nevertheless, interesting chemical and catalytic properties can be expected from these systems and are subject to further studies... 

The development of mesoporous materials with more or less ordered and different connected pore systems has opened new access to large pore high surface area zeotype molecular sieves. These silicate materials could be attractive catalysts and catalyst supports provided that they are stable and can be modified with catalytic active sites [1]. The incorporation of aluminum into framework sites of the walls is necessary for the establishment of Bronsted acidity [2] which is an essential precondition for a variety of catalytic hydrocarbon reactions [3], Furthermore, ion exchange positions allow anchoring of cationic transition metal complexes and catalyst precursors which are attractive redox catalytic systems for fine chemicals [4]. The subject of this paper is the examination of the influence of calcination procedures, of soft hydrothermal treatment and of the Al content on the stability of the framework aluminum in substituted MCM-41. The impact on the Bronsted acidity is studied. 

The isomorphous substitution of T atoms by other elements produces novel hybrid atom molecular sieves with interesting properties. In the early 1980s, the synthesis of a zeolite material where titanium was included in the MFI framework of silicalite, that is, in the aluminum-free form of ZSM-5, was reported. The name given to the obtained material was titanium silicalite (TS-1) [27], This material was synthesized in a tetrapropylammonium hydroxide (TPAOH) system substantially free of metal cations. A material containing low levels (up to about 2.5 atom %) of titanium substituted into the tetrahedral positions of the MFI framework of silicalite was obtained [28], TS-1 has been shown to be a very good oxidation catalyst, mainly in combination with a peroxide, and is currently in commercial use. It is used in epoxidations and related reactions. TS-1, additionally an active and selective catalyst, is the first genuine Ti-containing microporous crystalline material. 

Breck has reviewed the early literature where Ga3+, P5+, and Ge4 were potentially incorporated into a few zeolite structures via a primary synthesis route (2). Evidence has also been presented to show that the small amounts of Fe3+, typically present in both natural and synthetic zeolites, are located in framework tetrahedral positions (3). A more recent review of "isomorphic substitution" in zeolites, via primary synthesis methods, speculates on the potential Impact of such substitutions on catalysis (4). The vast majority of work has been related to the high silica zeolites, particularly of the ZSM-5 type. Another approach to substitution of metal atoms into the open frameworks of zeolite structures has been to replace the typical silica alumina gel with gels containing other metal atoms. This concept has resulted in numerous unique molecular sieve compositions containing aluminum and phosphorus 5 silicon, aluminum and phosphorus (6) and with... 

The desire to synthesize molecular sieve compositions containing other than the typical silicon and aluminum atoms is evidenced by the large number of efforts, primarily in the primary synthesis area. The Secondary Synthesis process has now been extended to include substitution of both Fe3t and T1 + into the frameworks of a number of zeolites. This paper will describe substitution of iron or titanium ions into the frameworks of zeolites Y, L, W, mordenite, ZSM-5 and LZ-202. Zeolites Y, L, W and mordenite were obtained from Union Carbide Corporation. Zeolon, a synthetic mordenite, was obtained from The Norton Company. ZSM-5 was synthesized according to the procedures described by Argauer et al., (8). LZ-202 is an omega type zeolite, synthesized without the... 

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