Cr-silicalite

The presence of methylenic bands shifted at higher frequency in the very early stages of the polymerization reaction has also been reported by Nishimura and Thomas [114]. A few years later, Spoto et al. [30,77] reported an ethylene polymerization study on a Cr/silicalite, the aluminum-free ZSM-5 molecular sieve. This system is characterized by localized nests of hydroxyls [26,27,115], which can act as grafting centers for chromium ions, thus showing a definite propensity for the formation of mononuclear chromium species. In these samples two types of chromium are present those located in the internal nests and those located on the external surface. Besides the doublet at 2920-2850 cm two additional broad bands at 2931 and 2860 cm are observed. Even in this favorable case no evidence of CH3 groups was obtained [30,77]. The first doublet is assigned to the CH2 stretching mode of the chains formed on the external surface of the zeolite. The bands at 2931 and... 

Figure 4.2. Effect of doping on catalyst morphology nsing SEM (a) Ge-doped silicahte and (b) Cr-silicalite.

Generally, the issue of whether a truly solid Cr catalyst has been created for the aforementioned reactions is unresolved. This point is illustrated most clearly by all the work that has been devoted, in vain, to Cr molecular sieves (55-57). Particularly the silicates Cr-silicalite-1 and Cr-sihcahte-2 and the aluminophosphate Cr-AlPO-5 have been investigated. These materials have been employed, among others, for alcohol oxidation with t-BuOOH, for allylic (aut)oxidation of olefins, for the autoxidation of ethylbenzene and cyclohexane, and even for the catalytic decomposition of cyclohexyl hydroperoxide to give mainly cyclohexanone ... 

If the polymerization reaction occurs in the presence of steric constraints, the activation energy associated with the monomer insertion grows with the progress of the reaction, in the same way as discussed for the oligomerization reactions catalyzed by Bronsted acid sites (Section LB and Fig. 2a), because the available space is progressively reduced. Under such conditions, the formation of polymeric species is limited, and small oligomeric species can become observable. An example of this situation is ethene polymerization on Cr/silicalite, in which the transition metal center is grafted to the internal surface of a cavity (Section VI.C.l). 

Fig. 23. Time-resolved FTIR spectra of ethene polymerization reaction on a l.Owt.% Cr/Silicalite sample oxidized at 923 K and CO-reduced at 623 K (ethene pressure = 2 kPa). The CH2 stretching region is reported. The last spectrum has been obtained after 10 min. (Reproduced with permission from Spoto et al. 234))...

Successful application of DRS together with complementary methods like ESR, XRD, XAS, IR, SEM, NMR etc. for characterization of the synthesized micro-porous materials with respect to incorporation into the framework has been further reported for MFI-type vanadium silicalites [74, 75], for VAPO-5, MgAPO-5 and MgVAPO-5 [76], for MFI-type ferrisilicalite [77] and for insertion of chromium in a number of molecular sieves [78]. In the case of the latter, essentially nonframework species could be identified and even in Cr-silicalite and CrAPO-5 Cr + showed no tendency to change from octahedral to tetrahedral coordination. 

Sugimoto et al. prepared Cr-silicalite-1 using morphohne (tetrahydro-1,4-oxazine) as template agent [195]. Incorporation of Cr was poorly supported by IR, XRD and EPR measurements in the H+-exchanged form, Cr(V) species were identified together with highly dispersed octahedrally coordinated Cr(III) species. TPD experiments showed a low Bronsted acidity, responsible for the high conversion of methanol to C2-C4 olefins [195]. 

Chromium-containing silicalite-2, CrS-2, has been synthesized and shown to catalyze similar reactions using TBHP as an oxidant [109]. Notably, TBHP has been reported to be an ineffective oxidant in TS-1 catalyzed oxidations [26]. However, it has been claimed that aTS-1-TBHP combination exhibits activity in the oxidative cleavage of the C=C double bond of silyl enol ethers to produce dicarboxylic acids [110]. 

Vibrational spectroscopies give rise to interesting information on the microscopic structure of soUd-solution mixed oxides. For example, the state of vanadium in soUd solution in Ti02 anatase catalysts [59], the partial ordering of cations in comndum-type Fe-Cr oxides [60], the real presence of Ti" in the silicalite framework of TSl catalysts [58] and the solubility of AT ions in the NiO rock-salt structure [61] have been objects of IR spectroscopic studies. 

Various redox metals, including Ti, V, Cr, Mn, Fe, Co, Cu, Zn, As, Zr and Sn, have been incorporated into microporous materials such as silicalites through hydrothermal synthesis by the addition of the respective cations to the synthesis gel. The disadvantages of this method include the time-consuming optimization of synthesis procedure for each metal-zeolite combination and the necessity of A1 for crystallization of certain structures. The presence of A1 leads to Bronsted acidity... 

The synthesis of cyclohexanone, which is an intermediate in the manufacture of nylon 6 and nylon 6,6 is an important industrial process [1], One of the major current routes for the synthesis of cyclohexanone is the liquid-phase autoxidation of cyclohexane at 125-160 °C and 10 bar followed by the selective decomposition of the intermediate cyclohexyl hydroperoxide, using a soluble cobalt catalyst, to a mixture of cyclohexanol and cyclohexanone [2]. These severe conditions are necessary due to the low reactivity of cyclohexane towards autoxidation. Due to the high reactivity of the products in the autoxidation step conversions must be kept low (<10%) [3,4]. Heterogeneous catalysts potentially offer several advantages over their homogeneous counterparts, for example, ease of recovery and recycling and enhanced stability. Recently we found that chromium substituted aluminophosphate-5 and chromium substituted silicalite-1 (CrS-1) are active, selective and recyclable catalysts for the decomposition of cyclohexyl hydroperoxide to cyclohexanone [5j. 

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. 

It is clear from Table 21.1 that the redox active centers (Mn , Cr [15]) in the AlPO microporons framework (pore diameter 7.3 A) is more effective than the Ti" active center in the microporons silicalite (TS-1 see US 4410501) framework (pore diameter 5.5 A), and that the yield of the desired product with the first of these catalysts are superior to those of the latter. The choice of solvent and substrate oxidant mole ratio has a significant inflnence on the activity and selectivity in these reactions. 

In an attempt to synthetize novel zeolite materials with different catalytic or thermal properties, zeolitic structures related to ZSM-5 zeolite have been obtained. The purpose was either to get better catalytic performances in shape selectivity and resistance to aging or to overcome Mobil patents. In such a way Al free ZSM-5 (designated silicalite) (67), B-ZSM-5 (Al replaced by B in boralite ( 60) or borosilicates ( )) Fe-ZSM-5 (Al replaced by Fe) ( ) zeolites have been prepared. Also, Cr, V, Ge, Zr, Ti,. .. were reported to have been used to replace Al or Si in zeolites. Some interesting improvements in catalytic properties have then been claimed. The key question is whether or not the substituted element is the active site or if residual Al is doing all the work. ... 

Following the success of TS-1 a variety of Ti-substituted molecular sieves were prepared by hydrothermal synthesis (Table 2) [28-32], Furthermore, various redox metals, e.g. V, Cr, Mn, Fe, Co, Cu, Zr, and Sn, have been reportedly incorporated into silicalites, zeolites,... 

In the liquid phase reaction of phenol with H2O2 in presence of titanium silicalite (TS-1), catechol and hydroquinone were obtained. This is the first reaction commercialized using the redox molecular sieve. A number of oxdation, hydroxylation and similar type of reactions were reported over Ti, V-, Cr, etc. silicalites, Ti, V, Cr, silicoaluminophosphates, TiY, Tip, etc. molecular sieves (Fig 8. ). Typical liquid phase reactions which we have canied out are given in Fig. 7. 

Similarly, the CrAPO-5- and chromium silicalite-1 (CrS-l)-catalyzed oxidation of aromatic side-chains with TBHP or O2 as the primary oxidant [27-31] almost certainly arises as a result of soluble chromium(VI) leached from the catalyst. The same probably applies to benzylic oxidations with TBHP catalyzed by chromium-pillared montmorillonite [32]. More recently, a chromium Schiff s base complex tethered to the mesoporous silica, MCM-41, was claimed [33] to be an active and stable catalyst for the autoxidation of alkylaromatic side-chains. It would seem unlikely, however, that Schiff s base ligands can survive autoxidation conditions. Indeed, on the basis of our experience with chromium-substituted molecular sieves we consider it unlikely that a heterogeneous chromium catalyst can be developed that is both active and stable to leaching under normal oxidizing conditions with O2 or RO2H in the liquid phase. Similarly, vanadium-substituted molecular sieves are also unstable towards leaching under oxidizing conditions in the liquid phase [6,34]. 

A series of Cr-modified silicalites-1 was prepared and characterized. Acidic and dehydrogenating properties were observed. Electron-acceptor Cr(III) species were detected by IR adsorption experiments. 

Syntheses of crystalline, XRD-single phase, Cr,Si-MFI materials have been achieved from low pH, Na free, reaction mixtures. Cr(III) framework substitution seems unlikely. No effects of Cr content on unit cell symmetry or on IR spectra compared with the silicalite are evident. Accessible Cr (III) cationic species have been detected by IR adsorption experiments. Cr... 

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