Metal redox molecular sieves

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]. 

The as-synthesized and calcined CrAPO-5 and CrS-1 were characterized by XRD which showed that the samples were pure and had an API and MFI structure respectively. ICP analysis showed that both catalysts contained about 1 % chromium. The results observed in the decomposition of cyclohexenyl hydroperoxide over several redox active moleular sieves are presented in Table 1. CrAPO-5 and CrS-1 displayed rougly equal activity and selectivity in the decomposition of cyclohexenyl hydroperoxide. Blank reactions carried out with Silicalite-1 (S-1) and silicon incorporated Aluminophosphate-5 (SAPO-5) show low conversions confirming that the chromium was responsible for the catalysis. Other transition- metal subsituted molecular sieves showed low conversions. 

The finding of an active solid redox system resulted in a flourish of activity in the development and application of diverse redox molecular sieves containing titanium (IV) and other metal ions [378-380]. Like the earlier ion-exchanged zeolites, many of the resulting catalysts, however, also suffered from loss by leaching, even when the redox element was substituted in the framework [102]. Ti-substituted zeolites remain special because of then stability. 

Framework substitution of Al, Si or P by a redox metal ion leads, in general to more stable redox molecular sieves (Figure 4). So-called isomorphous substitution, in udiich the metal ion is coordinated tetrahedrally by four oxygen atoms should be possible when the cuion/ oxygn between 0.22S and 0.414 [25]. It should be noted, however, tiiat the oxidation state of the metal and, hence, structure and charge of tire frameworic, may change substantially when an as-synthesized material is calcined. For example, Cr-substituted sieves generally contain Ci in the as-synthesized material but on calcination this is transformed to Cr. Since the latter contains two extra-framework Cr == O bonds it can only be anchored to a surface defect site rather than isomorphously substituted. By the same token, as-synthesized... 

Figure 4. Types of redox molecular sieves and oxidation states of the metal ions...

An important lesson leamt fiom this work is that with some metals, e.g. chromium, minute amounts of leadied metal ions, in our case 0.3% of tiie available chromium, can account for the observed catalysis. This means that the catalyst could, for example, be recycled ten or even a hunderd times while still obsoving the same activity. Hence, we conclude that, in the absence of unambiguous evidence to the contrary, many literature claims for heterogeneous catalysis by redox molecular sieves are, to say the least, questionable. Indeed, solubilization by reaction wih ROjH appears to be widespread phenomenon. For example, van Hooff and coworkers [63] observed this with VAPO catalysts and Schuchardt and coworkers similarly observed lead g with V, Cr, Mn, Fe and Co-substituted MCM-41 [84]. 

The discovery, in the mid-eighties, of the remarkable activity of TS-1 as a catalyst for selective oxidations with aqueous H2O2 fostered the expectation that this is merely the progenitor of a whole family of redox molecular sieve catalysts with unique activities. However, the initial euphoria has slowly been tempered by the realization that framework substitution/attachment of redox metal ions in molecular sieves does not, in many cases, lead to a stable heterogeneous catalyst. Nevertheless, we expect that the considerable research effort in this area, and the related zeolite-encapsulated complexes, will lead to the development of synthetically usefril systems. In this context the development of chiral ship-in-a-bottle type catalysts for intrazeolitic asymmetric oxidation is an important goal. Such an achievement would certainly justify the appellation mineral enzyme . 

Another option that sometimes enables immobilization of isolated metal ions stable to leaching, and avoidance of the formation of oligomers, is the synthesis of zeolites or zeotypes containing isolated metal ions in framework positions. In these the oxidation properties of the metal atoms are associated with the main characteristics of zeolites which involve shape-selective effects and unique adsorption properties which can be tuned in terms of their hydrophobicity-hydrophi-licity, enabling selection of the proportions of reactants with different polarities that will be adsorbed in the pores. Researchers at ENI succeeded in introducing Ti into silicalite producing the TS-1 redox molecular sieve oxidation catalyst [64]. TS-1 has an MFI structure formed by a bidimensional system of channels with 0.53 nm X 0.56 nm and 0.51 nm X 0.51 nm pore dimensions. The incorporation of Ti into the framework has been demonstrated by use of several techniques-XRD, UV-visible spectrophotometry, EXAFS-XANES a good review has been published by Vayssilov [65]. 

A final comment is necessary on the stability of redox molecular sieves, because in this chapter emphasis was placed on studies in which the heterogeneity of catalysis was most plausible (but in many cases not rigorously proven). Yet, catalysts for which metal leaching was once detected should be not automatically excluded from further consideration, because the extent of metal solubilization depends, inter alia, on the kind of reaction performed [44]. 

The quest for selective catalysts for the dream reactions discussed at the beginning of this article continues unabated. There is still a great need for systems that create gas-phase conditions in the liquid phase. One approach is maybe to isolate redox metal ions, by isomorphous substitution, in the lattice of molecular sieves [67]. Such redox molecular sieves may be viewed as inorganic en mes containing an active site in which there is no room for solvent molecules in addition to the substrate, i.e. gas phase conditions in the liquid phase. 

One approach to creating heterogeneous oxidation catalysts with novel activities and selectivities is to incorporate redox metals, by isomorphous substitution, into the lattice framework of zeolites and related molecular sieves. Site-isolation of redox metals in inorganic lattices prevents the dimerization or oligomerization of active oxometal species which is characteristic of many homogeneous oxometal complexes and leads to their deactivation in solution. We coined the term redox molecular sieves to describe such catalysts . The first and most well-known example is titanium silicalite (TS-1) which has been shown to catalyze a variety of systhetically useful oxidations with H202. ... 

As part of an ongoing programme on redox molecular sieves we are investigating the use of metal substituted alumino-phosphates (MeAPOs) in liquid phase oxidations. We have found that CrAPO-5 is an active and selective catalyst for the liquid phase oxidation of secondary alcohols with TBHP or O2. 

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. 

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 reductive/oxidative properties of transitional metal elements in these zeolite catalysts were also examined by TPR and TPO, and it is shown that metallic species in certain cation locations may migrate under calcination, reduction, and reaction conditions [7], The different treatment, e g, coking or even the oxidative regeneration, will produce metallic species of varied oxidation states with different distributions in the molecular sieve structures as exemplified by the above XPS data. The redox properties of these metallic cations exhibit the influence of hydrogen and/or coke molecules, and it is further postulated that the electron transfer with oxygen species are considered responsible for their catalyzed performance in the TPO regeneration processes, as shown in Figure 2. 

Unfortunately, the use of TS1 (as well as TS2 discovered in 1990 by the group of Ratnasamy (27)) in catalytic oxidations is restricted to the relatively small substrates able to enter the pores of these zeolites (apertures 0.55 nm). Therefore, many research groups attempted to incorporate titanium in large pore molecular sieves BEA zeolites, mesoporous molecular sieves MCM41 and MCM48. Other transition metal zeolites were also synthesized and tested in oxidation one of the main problems of these systems is the release of redox cations in liquid phase (24). Progress remains to be made to develop molecular sieves catalyzing the oxidation... 

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