B. Chiche

Synthesis and characterisation of ibuprofen-anchored MCM-41 silica and silica gel

A non-steroidal anti-inflammatory drug (ibuprofen) has been anchored inside the mesoporous channels of MCM-41-type silica and on a silica gel surface. The relevant anchoring procedure through an ester function has been investigated. It uses the epoxide ring opening of 3-glycidoxypropylsilane grafted on the silica surface by the carboxylic-group-containing ibuprofen. The control of the surface modification and of the anchoring efficiency was achieved by comparison of spectroscopic data with those obtained using homogeneous counterparts. The use of nanostructured silica allowed an accurate verification of the different surface modifications and also a higher drug loading.

Butene oligomerization over mesoporous MTS-type aluminosilicates

Abstract The oligomerization of butene at 423 K and 1.5–2 MPa has been investigated over a series of zeolite, amorphous silica–alumina (ASA) and ordered mesoporous aluminium-containing micelle templated silica (MTS) catalysts. While olefin oligomerization into strongly adsorbed residue and fast deactivation prevailed on microporous catalysts and ASA, mesoporous aluminosilicates with uniform pore openings near 3 nm in size exhibited high selectivity and good stability with time for the production of branched dimers. The characterization of the surface properties of the solids, the nature of the adsorbed residue on the spent catalysts, and the identification of reaction intermediates by in situ infrared spectroscopy of co-adsorbed acetonitrile and butene, suggest that the unique catalytic behaviour of MTS-type catalysts is related to the moderate strength and the high dispersion of the acid sites in the mesoporous structure.

The effect of structure on reactivity in zeolite catalyzed acylation of aromatic compounds: A ϱ - σ+ relationship

Abstract A ϱ - σ+ relationship for zeolite catalyzed acylation of aromatic compounds by car☐ylic acids was studied. The large negative value of ϱ indicates that the mechanism is an electrophilic substitution in which the transition state resembles the Wheland intermediate. Obedience to the Brown selectivity relationship is also reported.

Mesoporous MCM-41 aluminosilicates as model silica-alumina catalysts: spectroscopic characterization of the acidity

The synthesis procedure brings about a low threshold value for the surface concentration of Al species and renders MCM-41 a model system for the study of silica-alumina phases. On severely outgassed samples, two types of Lewis sites are present, differing in their protrusion from the surface, as well as Bronsted sites engaged in lateral H-bonding, evidenced by the interaction with NH 3 and CO. The conversion of Lewis sites into Bronsted ones by water adsorption shows that the Bronsted species are Si(OH)Al, typical of zeolites, and Si-Al(OH)-Si, found in de aluminated zeolites. The flexibility of the amorphous framework accounts for the differences with zeolitic aluminosilicates.

CO adsorption on superacid sites on dealuminated mazzite

Abstract Spectra of CO adsorption on dealuminated mazzite activated at temperatures in excess of 400°C have revealed two Bronsted acid sites with OH peak positions at 3626 and 3606 cm −1 and corresponding CO peaks at 2180 and 2165 cm −1 . These Bronsted sites are located in the large and small channels of mazzite. Strong Lewis sites have been detected at low CO coverages at 2229 and 2188 cm −1 . For the first time, evidence is presented for the direct interaction of these Bronsted and Lewis sites. When Lewis sites are bare they interact with Bronsted sites and shift the OH peaks to 3590 and 3567 cm −1 . These are superacid OH sites located in the two types of channels in mazzite. When the Lewis sites are filled with CO their interaction with the Bronsted sites is attenuated and the OH peaks revert to their values at 3626 and 3606 cm −1 . CO adsorption on these Bronsted sites shifts the OH peaks to 3247 ( Δν =379 cm −1 ) and 3330 cm −1 ( Δν =276 cm −1 ) respectively. The high frequency OH site at 3626 cm −1 is the strongest acid site detected on mazzite. At high CO pressures a weaker Al–OH acid site detected at 3660 cm −1 is shifted to 3450 cm −1 ( Δν =210 cm −1 ) at the same time that a CO peak appears at 2160 cm −1 . The conversion rate for the isomerization of n -butane on mazzite is much higher than that on H-ZSM5 and consistent with very strong acid sites.

Influence of the aluminium content on the acidity and catalytic activity of MTW-type zeolites

Zeolite with MTW topology have been synthesized from gels containing different amounts of aluminium. The lowest value of Si/Al reached in the crystals, near 20, confirms the limits to the incorporation of aluminium. H-MTW-type zeolites contain highly acidic and readily accessible structural Bronsted sites and very strong Lewis sites associated with extra-framework species. In the cracking of n-hexane the materials show little deactivation with time-on-stream and an activity level directly proportional to the framework aluminium content.

Selective benzene isopropylation over Fe-containing zeolite beta

Ferrisilicate analogs of zeolite beta have been investigated as catalysts for benzene isopropylation. Activity, selectivity, reaction kinetics and catalyst deactivation have been examined. Results have been compared with those obtained over Al-beta catalysts. Mixed Fe-Al-beta catalysts showed remarkably high activity and selectivity to cumene and diisopropylbenzene.

Spectroscopic investigation of the state of aluminium in MCM-41 aluminosilicates

The synthesis procedure brings about a low threshold value for the surface concentration of Al species and renders MCM-41 a model system for the study of silicaalumina phases. On severely outgassed samples, two types of Lewis sites are present, differing in their protrusion from the surface, as well as Bronsted sites engaged in lateral H-bonding, evidenced by the interaction with NH3 and CO. The conversion of Lewis sites into Bronsted ones by water adsorption shows that the Bronsted species are Si(OH)Al, typical of zeolites, and Si-Al(OH)-Si, found in dealuminated zeolites. The flexibility of the amorphous framework accounts for the differences with zeolitic aluminosilicates.

Organic Reactions Catalyzed by Zeolites: Transfer of Concept from Homogeneous to Heterogeneous Catalysis

Abstract Four examples of acid-catalyzed organic reactions are presented which emphasize the similarities that exist between zeolites and liquid acids. Using Hammett correlations, it is shown that similar mechanisms operate. Thus a positively charged entity is formed in the transition state for acetal hydrolysis and aromatic acylation, with a similar charge transfer in the homogeneous and heterogeneous media. The efficiency and strength of the acid centers increases up to a constant value when the aluminium content of the zeolite decreases, as is observed by dilution of proton in solution. As a consequence, the activity of mordenites for olefin hydration and for dichlorobenzene isomerization follows volcano shaped curves as a function of the Al/(Al+Si)ratio.

How the stability of the microporous amorphous precursors of zeolites is affected by the Si/Al ratio

Abstract The microporosity of the precursor gels of zeolite synthesis is stabilized by CO 2 treatment by chemical condensation of the surface silanols. This effect is only observable in a given field of Si/Al ratio, corresponding to the appropriate density of silanols.