Montmorillonite pillared

The original epoxidation with titanium-tartrate is homogeneous, but it can be carried out heterogeneously without diminishing enantioselectivity by using titanium-pillared montmorillonite catalyst (Ti-PILC) prepared from titanium isopropoxide, (+)-DAT, and Na+-montmorillonite.38 Due to the limited spacing of Ti-PILC, the epoxidation becomes slower as the allylic alcohol gets bulkier. 

Microwave irradiation has also been applied to the preparation of Fe203/S04-superacid [20, 21] and high-surface aluminum pillared montmorillonites [20],... 

In 1990, Choudary [139] reported that titanium-pillared montmorillonites modified with tartrates are very selective solid catalysts for the Sharpless epoxidation, as well as for the oxidation of aromatic sulfides [140], Unfortunately, this research has not been reproduced by other authors. Therefore, a more classical strategy to modify different metal oxides with histidine was used by Moriguchi et al. [141], The catalyst showed a modest e.s. for the solvolysis of activated amino acid esters. Starting from these discoveries, Morihara et al. [142] created in 1993 the so-called molecular footprints on the surface of an Al-doped silica gel using an amino acid derivative as chiral template molecule. After removal of the template, the catalyst showed low but significant e.s. for the hydrolysis of a structurally related anhydride. On the same fines, Cativiela and coworkers [143] treated silica or alumina with diethylaluminum chloride and menthol. The resulting modified material catalyzed Diels-Alder reaction between cyclopentadiene and methacrolein with modest e.s. (30% e.e.). As mentioned in the Introduction, all these catalysts are not yet practically important but rather they demonstrate that amorphous metal oxides can be modified successfully. 

Influence of Preparation Conditions on the Catalytic Properties of Al-Pillared Montmorillonites... 

Owing to the possibility of tuning (1) their acidic and basic properties, (2) their surface hydrophilicity, and (3) their adsorption and shape-selectivity properties, catalytic activity of zeolites was investigated in the production of HMF from carbohydrates. Whatever the hexose used as starting material, acidic pillared montmorillonites and faujasite were poorly selective towards HMF, yielding levu-linic and formic acids as the main products [81-83]. 

Zirconium Pillared Montmorillonite Reduced Charge of the Clay... 

The relatively high Na content of the Zr modified pillared montmorillonites has to be noted (EIII-... 

The same evolution of the basal spacing (d 001) for the pillared montmorillonite in which the Li has been introduced after the Zr is illustrated in fig. 5. It has to be mentioed that, after saturation of the solids by ethylene glycol, the interlayer distance of the samples calcined at 400°C is always slightly higher than before saturation. 

The diffusion of Li+ in the octahedral cavities of the Na+montmorillonite allows to control the density of the pillars of the Zr pillared montmorillonite. The solids, stable up to 300°C, have larger surface area basal distancy than the pure Zir montmorillonite. The distance between the pillars increases while the interaction strength between the pillars and the clay layer decreases. 

SCHEME 119. Selective oxidation of alcohols using a chromium-pillared montmorillonite catalyst and TBHP... 

The sol-pillared clay was prepared following the procedures of Yamanaka et al (1). The mixing ratio of Afsi (mol)/A/T,(moI)/CEC equivalent of clay was 30/3/1. SiC -TiCh sol pillared montmorillonite, subjected to a calcination for 4h at 773 K, was labeled as sol-PILM. Portions of the wet cake prepared with bentonite were subjected to SCD or treatment with surfactants as described below. 

In situ monitoring of the activation of pillared montmorillonite with HP xenon. 

Fig. 4 shows a series of 129Xe NMR spectra taken from flowing HP gas in contact with a sample of pillared montmorillonite in the spinner of a MAS NMR probe. In the air-dry sample there is a weak line beside the gas line ( 0ppm) that can be attributed to xenon in... 

Titanium-pillared montmorillonite may be used as a heterogeneous catalyst for the Sharpless asymmetric epoxidation of allylic alcohols (Scheme 20) (46). The enantiomeric purities of the epoxy products are comparable with those achieved using homogeneous Ti isopropoxide with molecular sieves as water scavengers (Chapter 4). Since basal spacing of the recovered catalyst after the reaction is unaltered, the catalyst can be recycled. 

Al3+-exchanged synthetic hectorite is a good catalyst for these conversions, and the 13C NMR spectrum obtained in the interlamellar, proton-catalyzed addition of water to 2-methylpropene is indistinguishable (Fig. 79) from that of f-butanol. Doubtless studies of this kind, where natural-abundance, 3C NMR signals are used to probe the chemical identity and motional freedom of reactant and product species situated in the interlamellar spaces of clays or pillared clays (see below), will become increasingly popular. Using l3C NMR linewidths and spin-lattice relaxation studies, Matsumoto et al. (466) have succeeded in discriminating between the internal and external surfaces of 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... 

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