Vanadium pillared montmorillonite

In contrast to the lack of selectivity observed in the TS-1 catalyzed oxidation of 3-penten-2-ol (1) (Eqn. 21.5), the oxidation of 1 with tert-butyl hydroperoxide (TBHP) over Cr-PILC gave the unsaturated ketone, 3, in 82% yield (Eqn. 21.13)42 while the oxidation of 1 over a vanadium pillared montmorillonite (V-PILC) gave the epoxy alcohol, 2, in 94% yield.43 V-PILC, however, does promote the oxidation of primary benzyl alcohols to the acids with tert-butyl hydroperoxide. This reaction exhibits shape selectivity in that para-substituted benzyl alcohols are oxidized while the ortho- and meta- substituted species are essentially inert (Eqn. 21.14).44... 

There is considerable current interest in the design of new catalysts by interchelating clay minerals of the smectite type with redox metal ions, leading to the formation of oxidation catalysts with interesting (shape-selective) properties [38]. For example, vanadium-pillared montmorillonite (V-PILC) proved to be an... 

Hydrocarbon oxidation. A metal catalyst is usually required. By adding bis(/-riphenylsilyl) chromate," diphenylmethane is oxidized to benzophenone, and vanadium-pillared montmorillonite " catalyzes the conversion of arylacetic esters to arylglyoxylic esters by f-BuOOH. The presence of calcined ZnCrOj-hydrotalcite enables the selective generation of benzylic hydroperoxides from aralkanes."... 

Polymer-supported tetrabromooxomolybdate(V) was claimed to be a heterogeneous catalyst for alcohol oxidations with TBHP . However, it seems likely that molybdenum is leached from the surface and the observed catalysis may be, at least partially, homogeneous in nature. The same applies to Cr(III) and Ce(IV) catalysts supported on a perfluorinated sulfonic acid resin (Nafion K) which catalyze the oxidation of alcohols with TBHP . Similarly, vanadium-pillared montmorillonite clay (V-PILC) ° and a zeolite-encapsulated vanadium picolinate complex were shown to catalyze... 

V-PILC = vanadium-pillared montmorillonite catalyst rhoudary. B.M. Valli, V.L.K. Prasad, A.D. J. Chem. Soc., Ghent. Commun., 1990, 721. 

A successful example for this class of reaction was reported by Choudary et al. [53a]. With the combination of a dialkyl tartrate and titanium-pillared montmorillonite (Ti-PILC), excellent ee-values in the range of90-98% were achieved (Scheme 2.19). In contrast to the homogeneous conditions, this heterogeneous system was operational without the use of molecular sieves however, no recychng experiment was reported. Distinct from Ti-PILC, the use of vanadium-piUared montmorillonite catalyst for the AE of ( )-hex-2-enol, however, led to only 20% enantiomeric excess [53b]. 

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]. 

In 1992, a montmorillonite catalyst was used by Choudary et al in another catalytic system for selective oxidation in a liquid phase reaction [119]. A vanadium pillared clay (V-PILC), which was prepared by refluxing VOCI3 in benzene with H-montmorillonite, was used as the catalyst. The solid was filtered, and the resulting clay was found to have 14% of its weight of intercalated vanadium. x-Ray analysis showed an increase in dimensions of montmorillonite... 

Preparation, physicochemical characterization and catalytic properties of vanadium-doped alumina- and titania-pillared montmorillonites... 

Procedures leading to preparation of vanadium-doped alumina- and/or titania-pillared montmorillonites are described and physicochemical characterization (chemical analysis, XRD, BET, ESR) of the products is provided. Results show that introduction of vanadium into the pillared montmorillonites leads to a rigid association of the dopant with pillars, irrespective of the method of preparation. The mode of vanadyl attachment in alumina-pillared samples does not depend on the mode of preparation, while in titania-pillared montmorillonite it does. Certain degree of delocalization of the unpaired electron into ligands and increased in-plane 7t-covalent bonding is observed for vanadyl ions present in the co-pillared (V-Ti)-PILC samples which also show particularly high activity in catalytic ammoxidation of m-xylene to nitrile product, as monitored by IR. A hypothesis is advanced that this effect is due to the unique character of vanadyl species present in these catalysts. 

This paper describes preparation, physicochemical characterization and catalytic properties of a series of vanadium-doped alumina- and titania-pillared montmorillonites obtained by various methods The aim of this work was to investigate the influence of the preparation procedure and pretreatment on location of vanadium dopant within the PILC structure and to correlate the physicochemical characteristics of tiie samples with their catalytic activity in ammoxidation of m-xylene. 

ESR data presented in detail elsewhere [7] show that all vanadium-doped alumina-and/or titania-pillared montmorillonite samples contain immobilized vanadyl ions bound to the pillars. 

Fig. la shows typical ESR spectra of alumina-pillared samples obtained by different methods. Differences in the ESR parameters of vanadyl species introduced by exchange with uncalcined or calcined pillars are very small indicating that similar vanadium species are formed irrespective of the preparative procedure. Also in the case of titania-pillared montmorillonite the ESR parameters of vanadyl ions deposited onto uncalcined (V(Ti)-PILC) and calcined (V-(Ti-PILC)) pillars are similar In the co-pillared (V-Ti)-PILC sample, however, a vanadyl spectrum with different parameters is observed (e g. smaller value of Aj, Fig. lb) pointing to a different character of vanadyl-pillar bonding in this case Analysis of the ESR parameters [7] shows that in the co-pillared samples the unpaired electron is partially delocalized into ligands and the vanadyl species posses increased inplane 7t-covalent bonding. 

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