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"Guisnet, M"
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Bifunctional Hydrogenating/Acid Catalysis: Quantification of the Intimacy Criterion
n
-Alkane hydroisomerisation over Pt-tridimensional large pore Hzeolite catalysts was shown to be determined by only two parameters, easy to be quantified: the hydrogenating/acid balance and the number of acid steps undergone by olefinic intermediates during their diffusion between Pt sites, this last one being related to the Weisz’s intimacy criterion.
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Journal Article
Acetylation of 2-methoxynaphthalene with acetic anhydride over a series of dealuminated HBEA zeolites
The liquid phase acylation of 2-methoxynaphthalene with acetic anhydride was carried out in a batch reactor at 120°C over a series of HBEA zeolites resulting from dealumination of a commercial sample by treatment with chlorhydric acid or with ammonium hexafluorosilicate. Lewis acid sites which correspond to extraframework aluminium species and to transient-state species (aluminium atoms which are leaving the framework) were preferentially removed. With all the HBEA samples, acetylation of 2-methoxynaphthalene leads preferentially to 1-acetyl-2-methoxynaphtalene, this compound undergoing isomerization into 2-acetyl-6-methoxynaphthalene and 1-acetyl-7-methoxynaphthalene as well as deacylation. A maximum in acetylation and isomerization activities is found for a Si/Al ratio between 20 and 35. The low activity of the commercial sample can be related to limitations in the desorption of acetylated products from the zeolite micropores caused by the large amount of extraframework aluminium species present on this sample. Lewis acid sites were shown to be completely inactive for acetylation and isomerization reactions.
Journal Article
Alkylation of toluene with 1‐dodecene over HFAU zeolite. Deactivation and regeneration
The alkylation of toluene with 1‐dodecene was carried out over a HFAU zeolite (total and framework Si/Al ratio = 25) under the following conditions: fixed‐bed reactor, 90°C, molar toluene/dodecene ratio of 3, WHSV =10 h−. Monododecyltoluenes are selectively formed, bidodecyltoluenes appearing only in low amounts at a complete conversion of dodecene. Tridodecyltoluenes are also formed inside the supercages but cannot desorb from the zeolite. These compounds, mainly located in the outer part of the crystallites are responsible for catalyst deactivation. However, tridodecyltoluenes can be completely removed by treatment under toluene flow, which allows a complete regeneration of the catalyst. This removal occurs by transalkylation between tridodecyltoluenes and toluene molecules with a final formation of monododecyltoluenes. At least, the first transalkylation steps occur between toluene in the liquid phase and tridodecyltoluenes in the zeolite pores (pore mouth catalysis).
Journal Article
Shape Selectivity of HFER Zeolites as Responsible for the Positive Effect of Their Si/Al Ratio on the Selectivity to n-Butene Isomerization into Isobutene
The selectivity and turnover frequency for isobutene production was shown to increase with the Si/Al ratio of fresh HFER. This can be explained by considering the zeolite pore system as a series of non-interconnected nanoreactors (the 10-MR channels) in which the molecules diffuse and react bimolecularly without any possibility of desorption before the exit.
Journal Article
Acetone Transformation over PtCu/HAlZSM5 Catalysts. Effect of Copper Content
A series of bifunctional bimetallic PtCu/H[Al]ZSM5-type catalysts was prepared and acetone transformation was carried out over them at 160 °C, 1 atm, acetone/hydrogen molar ratio=3, and WHSV (weight of reactant injected per weight of catalyst per hour) ranging between 9.4 and 38 h−1. According to the results, as copper atomic fraction (X(Cu)) increases in the bimetallic catalyst, propane formation rate decreases and methyl isobutyl ketone (MIBK) formation rate increases until X(Cu) equals 0.40. These results suggest that active metallic centers for the olefin double bond hydrogenation in the α-β unsaturated ketone, which leads to the MIBK formation, do not appear to be those hydrogenating the acetone carbonyl double bond to form propane. When X(Cu)>0.40, MIBK formation rate considerably decreases, meaning that practically all platinum metallic centers have been passivated for those copper atomic fractions over the PtCu/H[Al]ZSM5-type catalyst. Therefore, catalysts begin to act only by means of the acid function, and as a result a logical increase in the mesityl oxide (MO) concentration occurs, a substance that is formed by means of an acid catalysis.
Journal Article
Molecular sieve catalysts as substitutes for metal chlorides in the chemical industry: Some selected examples
The current main concern of global sustainable development pushes toward the substitution of corrosive and highly waste-producing chlorine-containing conventional catalysts (such as the strongly acidic AlCl
–HCl mixture, still widely used in the production of chemicals) by environmentally friendly systems. The outstanding benefits in terms of environmental sustainability as well as of economic advantage related to the use of acid and micro- and/or mesoporous molecular sieves are here depicted, analyzing some selected examples in the field of oil refining, petrochemicals, and fine chemicals production: namely,
-C
–C
alkane isomerization, isobutane/butene (I/B) alkylation, benzene ethylation, aromatic acylation, and Fries rearrangement. In the large-scale processes of refining and petrochemistry, the shift toward this kind of green and efficient solid catalysts is an established reality. In contrast, in fine chemicals synthesis, most of the novelties are still at a lab-scale level, and additional hurdles, related to the large differences between the national legislations about environmental protection, strongly limit this substitution.
Journal Article
Selective isomerization of n-butenes into isobutene over aged H-ferrierite catalyst: nature of the active species
The nature of the active species responsible for butene isomerization over aged HFER samples is reexamined in the light of the change in the product yields at very short time-on-stream and of the reversible and irreversible increases in weight of the zeolite during the reaction. At very short time-on-stream, the selectivity of butene isomerization is that expected from a dimerization-cracking process, in particular simultaneous formation of isobutene, propene and pentenes. A rapid decrease of all the yields is observed with time-on-stream; however, for isobutene but not for the other products, the initial decrease is followed (after 10 minutes-on-stream) by an increase. The decrease in the yield can be related to the formation of carbonaceous compounds (``coke'') which block the access to the pores, while the increase in isobutene yield can be explained by the development of a new isomerization mode which is very selective to isobutene. This new mode could be catalyzed by carbonaceous compounds and/or by reaction products which are shown to be retained inside the pores during the reaction. It is proposed that at short time-on-stream the increase in isobutene yield is due to an autocatalytic reaction, n-butene isomerization occurring on t-butyl carbenium ions formed by adsorption of isobutene molecules (which are slowly desorbed from the pores) on the protonic sites of the zeolite. At long time-on-stream, the active species would be benzylic carbocations formed from carbonaceous compounds trapped in the pores near the outer surface of the crystallites.
Journal Article
Influence of carbonaceous deposits on the acidity of a zeolite with two non-interconnected pore systems: MCM-22
H-MCM-22 zeolite samples used in n-heptane cracking at 450 °C for time-on-stream between 2 min and 100 h, hence with different coke contents (from 0.45 to 6.9 wt%), were characterized by FTIR spectroscopy before and after pyridine adsorption. As is observed with the zeolites with only one pore system, coke molecules interact preferentially with the most acidic hydroxyl groups, which is confirmed by the large decrease in the number of the protonic sites able to retain pyridine adsorbed at high temperature. A decrease in the number and strength of the Lewis acid sites able to retain pyridine adsorbed is also observed above 0.5 wt% coke, which can be related to blockage by coke of the access of pyridine molecules to the supercage pore system.
Journal Article