Chromia-pillared montmorillonites

Catalytic Properties of Chromia-Pillared Montmorillonite and Preliminary Study Results... 

The catalytic and structural properties of two chromia-pillared montmorillonites were compared in an effort to establish structure-reactivity relationships in these materials. The basal spacings of pillared products, prepared by reaction of Na+-montmorillonite with base-... 

In an effort to more fully elucidate the structure and reactivity of metal oxide pillared clays, we have been investigating the structure-reactivity properties of chromia-pillared derivatives (17). In the following sections, we provide an example of the structure-catalytic reactivity properties of chromia-pillared montmorillonites. Also, we report our initial efforts to structurally characterize the intercalated chromia aggregates by Extended X-ray Absorption Fine Structure (EXAFS) Spectroscopy. Unlike previously reported metal oxide pillared clays, chromia-pillared clay exhibits strong K-edge absorption and fine structure suitable for determination of metal-oxygen bond distances in the pillars. 

Figure 4. Conversion of cyclohexane to benzene over chromia pillared montmorillonites at 550°C (A)...

Figure 5. Uptake of cyclohexane at 25°C and p/pQ = 0.95 by chromia pillared montmorillonites (A) Cr3 53-...

The relatively small size of the pillars and the relatively low long-range ordering of intercalated clays exclude many surface and crystallographic techniques as suitable means of characterization. The present study is the first attempt to obtain interatomic distances in these systems from EXAFS data. Our results clearly show the utility of this method for local structural analysis of these clay materials. The interatomic distances obtained for chromia-pillared montmorillonite suggest similarities between the structure of the pillars and the structure of Cr203 (22). This result is in agreement... 

Chromia pillared montmorillonite with 1.0 -1.1 nm gallery height and a high specific surface area has been prepared successfully by using an elevated temperature (368 K) for the formation of hydroxy-chromium polycations [2]. This catalyst possesses significant activity in the hydrocracking of n-decane. Previously, we synthesized... 

Similarly, chromia-pillared montmorillonite catalyst reportedly [39] catalyzes the selective oxidation of alcohols with TBHP and titanium-montmorillonite in conjunction with tartaric acid esters functions as a heterogeneous catalyst for asymmetric epoxidations with TBHP [40]. 

Oxidative cleavage of allyl phenyl ethers in dichloromethane containing some chromia-pillared montmorillonite, stirred initially for 30 mins., and then treated with 2 moles of an iso-octane solution of 2.8M tert-butyl hydroperoxide followed by reaction under nitrogen during 12 hours, afforded phenol in 80-90% yield in the case of ailyoxybenzene in dichloromethane solution (ref.97). The oxidative aspect of the method, described as general for N-allyl and 0-allyl ethers, appears in the conversion of 2-allyloxyoctane to octan-2-one Dealkoxylations are less widely used but in recent years a number of new approaches have been adopted. 

The similarities in catalytic reactivity between Cr3 53-montmorillonite and chromia supported on alumina suggest that the structure of the intercalated chromia particles may resemble the structure of the bulk oxide. In order to obtain structural information for the chromia aggregates in pillared clays, we have initiated structural studies of these materials. Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy is being recognized as an effective tool for determining the local structure of a variety of materials. The basic principles and utility of this technique have been discussed elsewhere (18.). ... 

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