PILCs, acidity

Acidity of the PILC. Acidic properties were studied by infrared spectrometry adsorbing pyridine as probe molecule on self supported wafers prepared by pressing the PILC into thin films (15... 

The hydroamination of alkenes has been performed in the presence of heterogeneous acidic catalysts such as zeolites, amorphous aluminosilicates, phosphates, mesoporous oxides, pillared interlayered clays (PILCs), amorphous oxides, acid-treated sheet silicates or NafioN-H resins. They can be used either under batch conditions or in continuous operation at high temperature (above 200°C) under high pressure (above 100 bar). 

Oxidation intermediates and reaction pathways of wet hydrogen peroxide oxidation of p-coumaric acid over (Al-Fe)PILC catalyst... 

In the present study, we have focused our attention on the catalytic wet peroxide oxidation of p-coumaric acid over (Al-Fe)PILC. This phenolic molecule was chosen as a representative of the biologically recalcitrant polyphenolic compounds present in olive oil processing and wine distillery wastewaters. 

Fig 1 shows the rate of p-coumaric acid solution (500ppm) and TOC removals during the phenolic compound oxidation over (Al-Fe)PILC catalyst (0.5g/l), as well as the uncatalysed reaction in slurry at 70°C. 

It can be seen that p-coumaric acid was slowly oxidised without catalyst, only 20% of substrate was converted and 5% of TOC was removed in 4hours of reaction. The use of (Al-Fe)PILC catalyst remarkably increased the rates of both p-coumaric acid and TOC removals in the reaction mixture under mild reaction conditions (temperature of 70°C, atmospheric pressure, [H2O2]=2.10"2M). The fact that the fast removal of p-coumaric acid was accompanied by a rather slower TOC reduction implies that intermediates which are stable and resistant to further total oxidation had been formed. 

Figure 2. Concentration profile of p-Coumaric acid (A) p-hydroxybenzoic acid ( ) and p-hydroxybenzaldehyde ( ). ([p-coumaric] = 500 ppm, pHi= 3.5, T=343 K, [H202] = 2xl0 2 M, 0.5g/l (Al-Fe)PILC).

Such regioselectivities are unique and suggest that redox pillared clays may have broad scope and utility as selective, heterogeneous catalysts for liquid phase oxidations. Indeed, V-PILC also catalyzes the oxidation of benzyl alcohol (to a mixture of benzoic acid and benzylbenzoate) whilst a-methyl benzylalcohol is left completely untouched.71 Similarly, p-substituted benzyl alcohols are oxidized whilst o-substituted benzyl alcohols are inert.71... 

Finally, a titanium(IV) pillared clay (Ti-PILC) catalyst has been prepared.71 In the presence of tartaric acid esters as chiral ligands Ti-PILC is an effective, heterogeneous catalyst for the asymmetric epoxidation of allylic alcohols. Enantioselectivities were comparable to those observed in the homogeneous system - and reactions could be carried out at concentrations up to 2M with a simple work-up via filtration of the catalyst. 

Abbreviations AD, asymmetric dihydroxylation BPY, 2,2 -bipyridine DMTACN, 1,4-dimethyl-l,4,7-triazacyclonane EBHP, ethylbenzene hydroperoxide ee, enantiomeric excess HAP, hydroxyapatite LDH, layered double hydroxide or hydrotalcite-type structure mCPBA, meta-chloroperbenzoic acid MTO, methyltrioxorhenium NMO, A-methylmorpholine-A-oxide OMS, octahedral molecular sieve Pc, phthalocyanine phen, 1,10-phenantroline PILC, pillared clay PBI, polybenzimidazole PI, polyimide Por, porphyrin PPNO, 4-phenylpyridine-A-oxide PS, polystyrene PVP, polyvinylpyridine SLPC, supported liquid-phase catalysis f-BuOOH, tertiary butylhydroperoxide TEMPO, 2,2,6,6-tetramethyl-l-piperdinyloxy TEOS, tetraethoxysilane TS-1, titanium silicalite 1 XPS, X-ray photoelectron spectroscopy. 

Besides the Bronsted acidic sites created by the above explained methodology, in PILCs that are ion exchanged with multivalent cations and are partially dehydrated, Lewis acidic sites are produced [132], The main interest in developing acid PILCs was due to their potential applications as cracking catalysts. Certainly the prospect of making PILCs in which the big gaseous oil molecules can diffuse and meet the active acidic sites was a motivation for the development of these catalysts [122],... 

And after heating in an air flow, the acidic form of the PILCs is obtained as follows ... 

After this, the sample is heated in an air flow to obtain the acid form of the PILCS ... 

In addition to the Bronsted acid sites produced by the above-explained methodology, in PILCS ion exchanged with multivalent cations and partially dehydrated, Lewis acid sites are produced. 

Pillared clays (PILC) have been reported to show catalytic properties comparable to zeolites in several reactions, and therefore to have comparable acidities [107]. 

The distributions of acid strengths of various pillared clays obtained by calorimetry of NH3 adsorption showed that Al-PILC displays high acidity, with a small number of sites as strong as those measured on zeolites. Acidity should be directly related to the environment of the A1 ions in the pillars, which could be modified by the preparation. The adsorption of NH3 on pillared beidellite showed the presence of many strong acid sites (160 kJ mol" ) [108]. 

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... 

Selected spectra of pyridine on various samples at 473 K are depicted in Figure 2. It can be seen that both Brpnsted and Lewis acid sites were present on each sample. The ratio of these two types of add sites, measured by the ratio of absorbances of the respective bands, was different, however. Similar Brpnsted and Lewis acidity was found for the ion-exchanged samples (e.g., Ag/AL= 0.97 for Fc2Na-Mont). For the pillared samples, Lewis aridity predominated (Ag/AL= 0.27 and 0.12 for Al -PILC and Fe2Al j -PILC, respectively). Negligible Brpnsted acidity was found for the parent Na-Mont. 

When vanadia was added to sulfated Ti-PILC, the amount of NH3 desorbed decreased significantly. R. T. Yang [26] reported that the vanadia create more Bronsted acid sites with the increase of vanadia from 2 to 6%. Thus, the decrease of the acidity observed in our samples may be explained by the assumption that some of the ammonia desorbed from the surface was oxidized by lattice oxygen of the catalysts. It is known that lattice oxygen of V2O5 can oxidize ammonia to N2 and nitrogen oxides at high temperatures (27). 

Titanium-pillared montmorillonites (Ti-PILC) modified with tartrates were described as heterogeneous Sharpless epoxidation catalysts [33] as well as for the oxidation of aromatic sulfides [34]. Metal oxides modified with histamine showed modest efficiencies for the kinetic resolution of activated amino acid esters (kj /k5 2) [35]. Silica or alumina treated with diethylaluminium chloride and menthol catalyzed the Diels-Alder reaction between cylopentadiene and methacrolein with modest enantioselectivities of up to 31% ee [36]. ZeoHte HY, modified with chiral sulfoxides had remarkable selectivities for the kinetic resolution of 2-butanol (k /kj =39) but unfortunately the catalyst is not very stable... 

PILC have also been investigated in the acylation of mesitylene with acetyl chloride [62], showing in comparison with the starting clays (i) higher activities related to the presence of additional Lewis acid sites in the pillars... 

Pillared layered clays (PILC) containing different hydrated cations in the interlayer space are also selective catalysts. Pinacol and 2,3-diphenyl-2,3-butanediol have been studied in the presence of Al-PILC, Zr-PILC, and Cr-PILC [38], Zr-PILC, with the lowest acidity, gave rise to the largest amount of diene from pinacol (32 % and 22 %, depending on the method of preparation). The rearrangement was selective in the presence of a mixed-metal pillared layered clay (Fe Aln-PILC, sealed tube, 398 K, 1 h, catalyst/diol = 5) [40],... 

Because acidified titanium oxide is the catalyst usually employed commercially for the transformation of 1 into 2 [8] there has been much investigation of this catalytic system [9]. A 1995 paper by Stefanis et al. [10] reported an investigation of the reaction of 1 in several alumina-pillared clays (PILCs montmorillonite- and beidellite-based, and their and Ca" -exchanged congeners) under Lewis acid conditions (solid is activated by heat to remove all water). The results were compared with those obtained by use of medium-pore zeolites USY, NH4+-ZSM-5, and H-mordenite. Conversion to 2 > 50% was always observed. The aim of the work was to clarify differences between site availability and acidity for the two types of solid. 

The thermal stabilities of their corresponding pillar intmlayered montmorillonite clay minerals (PILCs) were characterized through the use of powder X-ray diffraction, differential thermal analyses and surface area measurements. These relative stabilities were also found to increase in the order Gaia-PILC < Alia-PILC < GaAl -PlLC. The relative Brdnsted and Lewis acidities of the montmorillonite itself, and of the clay mineral pillared with the different polyoxocations have been examined through infrared pyridine sorption studies. 

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