Jacques C. Vedrine

Methanol conversion on acidic ZSM-5, offretite, and mordenite zeolites: A comparative study of the formation and stability of coke deposits

The deposition of carbonaceous residues, leading to aging and modifications in the acidic properties of three zeolite samples differing by the size and shape of their interconnecting channel networks (ZSM-5, offretite, and mordenite), has been investigated during the conversion reaction of methanol to hydrocarbons. Catalytic tests, thermogravimetry, and microcalorimetry are used as complementary techniques. For zeolite ZSM-5, it is observed that carbonaceous residues are primarily formed on the outer surface of the crystallites, resulting only in a slight modification of its molecular shape-selective properties and producing a high resistance to aging. For offretite and mordenite, by contrast, the channels are large enough to accommodate carbonaceous residues, which leads to a drastic loss of catalytic activity and a very fast aging. The intimate nature of the channel network is shown to play a determining role in the coke formation and in the ease of removal of carbonaceous deposit. The role played by the shape-selective properties of the zeolites on their behavior towards coking and aging and also the effect of coking on the availability of Bronsted acidic sites have been particularly emphasized.

Reaction pathways for the conversion of methanol and olefins on H-ZSM-5 zeolite

The conversions of methanol, dimethyl ether, ethylene, propene, 1-butene, and 3,3-dimethyl-1-butene by reaction on various H-ZSM-5 catalysts at 370–380 °C demonstrate the importance of a carbenium ion mechanism in the formation of various aliphatic (C1C6) and aromatic (C6C10) hydrocarbons. C2C4 olefin reactions occur in a way very similar to the classical conjunct polymerization of olefins. The initial step in the formation of aromatics is a “concerted” cycloaddition of an olefin and a carbenium ion which is favored by the unique structural properties of the zeolite. From correlations between the SiAl ratio and yields in aromatics and C4 aliphatics, it is proposed that strong acid sites are responsible for the dehydrocyclization of C6+ olefins into aromatics. Some evidence is also presented for alkylation reactions. Some of the unique properties of H-ZSM-5 are apparently due to the combination of strongly acidic and molecular sieving properties.

Catalytic and physical properties of phosphorus-modified ZSM-5 zeolite

H-ZSM-5 zeolite modified by phosphorus (1.1 and 2.0 wt%) was studied by means of infrared spectroscopy and microcalorimetry for acidity characterization, by ESCA for determining elementary composition in the outer layers (depth ∼- 50 A) with respect to the bulk (φ ∼- several μm), and lastly by catalytic reaction analysis of methanol conversion. It was found that phosphorus neutralizes acidic sites primarily at the entrance of the channels of the zeolite particles in the same manner as do carbonaceous residues after some time of reaction. However, the strongest acid sites remain unmodified, which leads to the suggestion that the aluminium distribution and subsequently the acid site strength distribution along the zeolite channels is heterogeneous. Phosphorus-modified zeolites gave a higher yield of light olefins (C2=C4=) and subsequently a smaller yield of saturated aliphatics and aromatics than the parent zeolite in the methanol conversion reaction. The phosphorus-modified zeolite gave similar yields of the meta + para isomers of xylene and of ethyltoluene and a smaller yield of the ortho isomers. Lastly the yield of heavier aromatics A9+ was greatly decreased by the addition of phosphorus. The changes in selectivity were tentatively assigned to slight modifications in channel size and to increased tortuosity due to phosphorus compounds bonding to the zeolite framework, rather than to changes in acid strength.

Correlations between X-ray photoelectron spectroscopy data and catalytic properties in selective oxidation on SbSnO catalysts

Abstract A series of SbSnO catalysts has been studied by X-ray photoelectron spectroscopy (XPS) as a function of Sb content (from 1.7 to 39.7 atom%) and activation temperature. At low activation temperature (500 °C) surface and bulk compositions are comparable, while at higher activation temperatures strong surface enrichment in Sb occurs. Detailed analysis of the XPS line shapes shows that two phases may exist at the surface of the catalyst. The first one, which is observed at low Sb content and low activation temperature (500 °C), is assigned to Sb(V) in solid solution in the SnO 2 lattice. The second one, which is observed for Sb contents larger than 5% or for high-temperature activations, is assigned to an Sb 2 O 4 phase. Catalytic properties for selective oxidation of propylene were studied in the 350 to 400 °C range. The selectivity for acrolein increases when surface Sb content is increased either by enhancement of the Sb concentration or by higher activation temperature. It is then postulated that the catalytic phase, which gives selective oxidation, consists of an Sb 2 O 4 phase lying at the surface of a solid solution of Sb (V) in SnO 2 .

Acidic and catalytic properties of CsxH3−xPW12O40 heteropolyacid compounds

The heteropolyacid of Keggin structure (H3PW12O40) and several of its cesium salts have been synthesized and characterized for their acidic properties. Chemical analysis, TGA, NH3 adsorption-desorption and31P MAS-NMR techniques permit the characterization of the Brønsted acidity. The catalytic properties were studied forn-butane isomerization and for methanol conversion to dimethyl ether at 200 and 180°C, respectively. Our conclusion is that, in term of conversion, the more acidic catalysts are in the range Cs2/2.7 if considering a wide range of acid strengths evidenced by methanol conversion. The range is Cs2/Cs2.1 if one considers only very strong acidity as evidenced byn-butane isomerization.

Molecular description of active sites in oxidation reactions: Acid-base and redox properties, and role of water

Abstract Oxidation reactions in heterogeneous catalysis usually involve a Mars and van Krevelen mechanism which includes activation of the substrate on a metallic cation, insertion of oxygen from lattice oxygen ions, a redox mechanism on the catalyst surface, and the transfer of several electrons. It follows that such a reaction necessitates both acid-base and redox properties of a catalyst the acid site being of Lewis type (cations) and the basic sites being the surface O 2- or OH - species which could exhibit electrophilic or nucleophilic properties. The active site should be able to fulfil the following requirements: H abstraction from the substrate, oxygen insertion, and electron transfer. It has been shown to correspond to an ensemble of atoms of limited size in an inorganic molecular complex. It could correspond to local structural defects including steps, kinks, coordinatively unsaturated cations or to clusters of atoms on the surface. Some examples are described namely: 1. (i) n-butane oxidation to maleic anhydride on (VO) 2 P 2 O 7 catalyst where four dimers of vanadyl cations on the (100) face were suggested to form the active site; 2. (ii) isobutyric acid oxidative dehydrogenation to methacrylic acid on iron hydroxy phosphates where trimers of iron oxide octahedra were shown to constitute the most efficient and selective catalytic site while water was observed to be absolutely necessary to facilitate the reaction which corresponds to hydroxylated surface sites ensuring the redox mechanism; 3. (iii) propane oxidative dehydrogenation to propene on VMgO samples which was shown to depend both on VO x arrangements with respect to MgO and on the basicity of the material induced by MgO while vanadium cations induced acidic features.

Properties of boron-substituted ZSM-5 and ZSM-11 zeolites

Using NMR and IR spectroscopy boron-substituted pentasil zeolites are found to contain four-coordinated B in the hydrated state and three-coordinated B in the dehydrated state. In addition a new tetrahedral B site assigned to (HO)2B(H2O)(OSi) was characterized by its NMR quadrupole coupling constant and asymmetry parameter. Ammonia adsorption microcalorimetry gave heats of adsorption of ~65 kJ/mol for HBZSM-11 and showed that B-substituted pentasils have only very weak acidity. Calcination at 800 °C increased the heat of NH3 adsorption to ~170 kJ/mol by creation of strong Lewis acid sites. The lack of strong Bronsted acid sites in HBZSM-11 was confirmed by poor catalytic activity in methanol conversion and in toluene alkylation With methanol.

Basicity and basic catalytic properties of zeolites

In the present review article one describes basicity for zeolites from theoretical and experimental points of view. Zeolites appear to be rather soft bases, their basicity increasing within the series Li < Na < K < Rb < Cs. The physical methods used for characterizing basic sites either themselves (anionic) species: 02−, A104−, OH−) or through adsorption of acidic probes, as CO2, SO2, organic acids, pyrrole, etc., are described. A review of basic type reactions catalyzed by zeolites is also given. The influence of Lewis acidity of exchangeable cations and of basicity of framework oxide ions on adsorption processes is discussed.

The effect of boron on ZSM-5 zeolite shape selectivity and activity: II. Coincorporation of aluminium and boron in the zeolite lattice

ZSM-5 zeolite samples have been synthesized in the presence of NaAlO2 and NaBO2 to yield a crystalline phase of almost unchanged Al + B site population, but with increased BAl ratio. The presence of boron in the zeolitic lattice is evident by MAS NMR. Transformation of boron (upon NH3 sorption) within trigonal and tetrahedral symmetries, which is revealed by IR spectroscopy as persistence and disappearance of the trigonal BO stretching mode at 1385 cm−1, together with the appearance of the zSiOB skeletal vibration at 920 cm−1 for the zeolite as synthesized, confirms the incorporation of boron by synthesis with no evidence of occluded amorphous boron oxide phase. IR spectroscopy reveals a distinct absorption at 3700 cm−1 assigned to Bronsted sites associated with lattice boron. NH3 sorption, studied by IR spectroscopy and microcalorimetry, reveals weak acidity for boron-associated sites; however, they do certainly sorb ammonia. Kinetic data of 3-Me-pentane sorption suggest a role for lattice boron in modifying the interior part of the zeolite, which is also indicated by catalysis of toluene disproportionation as improved selectivity toward p-xylene formation. Zeolite modification by the present method, although exhibiting a minor effect on ZSM-5 shape selectivity, has a major role in drastically decreasing the zeolite acidity and hence activity.

Rhodium exchange in zeolites

Abstract Rhodium exchange was attempted by using solutions of RhCl 3 , [Rh(NH 3 ) 5 Cl]Cl 2 , and Rh(ClO 4 ) 3 in a series of zeolites with decreasing channel size: Y, mordenite, ZSM-34, ZSM-11, erionite, A, and ZK-5. Facile exchange was found in the large-pore zeolites Y and mordenite. Rh is found in the channels and on the surface of the medium-pore zeolites ZSM-34 and ZSM-11, suggesting that both exchange and surface hydrolysis occur. Surface hydrolysis to form islands of what is probably a Rh-oxy-hydroxy hydrate complex appears to be the primary type of deposition on the small-pore zeolites erionite, A, and ZK-5. Exchange in Y with the use of RhCl 3 (aq) solutions at 90 °C, occurs via the Rh(H 2 O) 6 3+ complex, does not result in an appreciable Cl − concentration in the zeolite and, contrary to previous work, does not result in Rh on the external surface of the zeolite. RhY prepared from RhCl 3 (aq) solutions can be used to prepare highly dispersed Rh in Y without the preliminary preoxidative treatment necessary for Rh(NH 3 ) 5 ClY. Reaction of CO with Rh-exchanged forms of Y, mordenite, ZSM-34, and ZSM-11 produces several species: Rh(I)(CO) 2 (O z ) 2 in mordenite and Y; Rh(I)(CO) 2 (O z )(H 2 O) in all four zeolites; and Rh(I)(CO) 3 (O z ) in mordenite, ZSM-34, and ZSM-11.