Role of the nature of copper sites in the activity of copper-based catalysts for no conversion

The activities of the copper-based catalysts, Cu²⁺ /SiO²,Cu²⁺ /Vycor and Cu²⁺/ZSM-5, and V²O⁵/TiO² for NO conversion to N² in the presence or absence of NH³ and/or O² have been investigated. The Cu²⁺ /ZSM-5 catalyst exhibited the highest activity, even higher than that of V²O⁵/TiO². Photoluminescence studies of the dehydrated copper-based catalysts have suggested that the copper ions anchored onto ZSM-5 locate as isolated copper species near Brönsted sites in the zeolite channels while the copper ions anchored onto Vycor and SiO² locate mainly as copper dimer forms. These results suggest the role of copper ions which are stabilized with near-lying oxygen vacancies created by dehydroxylation of the zeolite, in NO conversion. As a result, it may be concluded that the isolated copper ions near Brönsted sites play a significant role in NO conversion but dimeric or polynuclear copper species are less effective for the reaction.     

Untersuchungen an oxidischen Katalysatoren. XXV. Katalytische Aktivität und Aktivitäts-Zeit-Verhalten modifizierter Mordenite beim Spalten von n-Octan

Studies on Oxide Catalysts. XXV. Catalytic Activity and Aging Properties of Modified Mordenites in the Cracking of n-Octane MeH-mordenites (Me = Li, K, Mg, Ca, Ba) were prepared by ion exchange starting with H-mordenite (SiO₂/Al₂O₃ mole ratio = 14). To characterize these samples the cracking of n-octane was used as catalytic test reaction. Surface OH groups and the adsorption of NH₃ on these samples were investigated by i. r. spectroscopy. Unaffected by the kind of the exchanged cation the Brönsted acidity of the H-mordenite decreases monotonously with increasing content of the incorporated cation. The catalytic activity and (to a much higher degree) the rate of deactivation by coking during the reaction decrease as the Brönsted acidity decreases. The strong dependence of the Brönsted acidity on the deactivation rate points to a multi-site mechanism of the coking process.     

Surface Structure and Catalytic Performance of CuCl/SiO2-Al2O3 Catalysts for Methanol Oxidative Carbonylation

Aluminum was doped into amorphous silica gel to modify its surface structure. The obtained SiO₂-Al₂O₃ support was used to prepare the CuCl/SiO₂-Al₂O₃ catalyst by solid-state ion exchange, and the catalyst activity for liquid-phase oxidative carbonylation of methanol to dimethyl carbonate was investigated. The results showed that the prepared SiO₂-Al₂O₃ support kept the amorphous structure of the silica gel. The BET specific surface area of the silica gel was decreased to 200 m²/g, and the surface acid sites (including Brønsted acid sites) were increased. In the CuCl/SiO₂-Al₂O₃ catalyst, CuCl was not only dispersed on surface but also was ion exchanged with surface Brønsted acid sites of the SiO₂-Al₂O₃ support to form Cu⁺ species, which resulted in a decrease in BET specific surface area to 148 m²/g. These two kinds of Cu⁺ species on the catalyst surface were both active centers for the oxidative carbonylation of methanol to dimethyl carbonate. When the catalyst was prepared with Si/Al molar ratio of 5 and was calcined at 500 °C, the selectivity and space-time yield of dimethyl carbonate reached 74% and 1.27 g/(g·h), respectively.     

Activity of palladium sulfide catalysts in the reaction of gas-phase hydrogenation of 2-methylthiophene

In the interaction of hydrogen with 2-methylthiophene in the gas phase over palladium sulfide catalysts at 180–260?C and 0.1–0.8 MPa, the saturation of the thiophene ring resulting in 2-methylthiolane and the hydrogenolysis of 2-methylthiophene occurs. When the conversion is lower than 60%, these reactions occur independently; at higher conversions, methylthiolane also undergoes hydrogenolysis. The specific catalytic activity of PdS supported on γ-Al₂O₃, TiO₂, and carbon and without support is much lower in the hydrogenation of 2-methylthiophene than the activity of PdS supported on SiO₂, aluminosilicate, and zeolite HNaY having strong Brönsted acid surface sites.     

Dehydrogenation of Isobutane Over V2O5/γ-Al2O3 Catalyst

The effect of the type of the support and the amount of V₂O₅ loading on the activity of V₂O₅/γ-Al₂O₃ catalyst for the dehydrogenation of isobutane have been investigated. Based on the experimental results of TPR, XRD and ESR spectroscopy, it is suggested that there are strong interactions between vanadia and carrier and that the V⁴⁺ species on the surface is the active site of V₂O₅/γ-Al₂O₃ for this reaction. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Significance of Lewis Acid Sites for the Selective Catalytic Reduction of Nitric Oxide on Vanadium‐Based Catalysts

The long debated reaction mechanisms of the selective catalytic reduction (SCR) of nitric oxide with ammonia (NH₃) on vanadium‐based catalysts rely on the involvement of Brønsted or Lewis acid sites. This issue has been clearly elucidated using a combination of transient perturbations of the catalyst environment with operando time‐resolved spectroscopy to obtain unique molecular level insights. Nitric oxide reacts predominantly with NH₃ coordinated to Lewis sites on vanadia on tungsta–titania (V₂O₅‐WO₃‐TiO₂), while Brønsted sites are not involved in the catalytic cycle. The Lewis site is a mono‐oxo vanadyl group that reduces only in the presence of both nitric oxide and NH₃. We were also able to verify the formation of the nitrosamide (NH₂NO) intermediate, which forms in tandem with vanadium reduction, and thus the entire mechanism of SCR. Our experimental approach, demonstrated in the specific case of SCR, promises to progress the understanding of chemical reactions of technological relevance.     

Effects of binder, coking and regeneration on acid properties of H-mordenite during TDP reaction

The effects of binder, coking and regeneration on the acid properties of H-mordenite zeolite during toluene disproportionation reaction (TDP) have been investigated by solid-state ³¹P-MAS-NMR of various adsorbed phosphorous probe molecules in conjunction with elemental analysis by ICP-MS technique. A series of fresh, spent and regenerated mordenite-based commercial catalysts were examined and the results were also compared with binder-free H-mordenite zeolite and unformulated γ-alumina binder. It is found that parent H-mordenite zeolite possessed only Brønsted acidity, which is responsible for the observed catalytic activity. In contrast, the γ-Al₂O₃ binder exhibited only Lewis acidity and plays a minor role during the catalytic reaction. While the amount of strong Brønsted acid sites decreased rapidly during initial coking, it reached a plateau at a total coke content of ca. 7 wt%, corresponding to ca. 80% decrease in total acidity. That the catalyst remained active even under deep coke deposition (>7 wt%) condition indicated catalytic activity may be invoked by subsequent coking taking place on the external surface rather than intracrystalline channels of the zeolite catalyst. Furthermore, upon catalyst regeneration treatment, ca. 75% of the total acidity could be effectively recovered.     

Methanol to Gasoline over ZSM-5 Zeolite Treated with Alkaline Mixture with Different Ratios of TBA<Superscript>+</Superscript>/OH<Superscript>–</Superscript>

The influence of ZSM-5 (SiO₂/Al₂O₃ = 50) treatment with a tetrabutylamine hydroxide (TBAOH)/NaOH mixture having different mole ratios on its physicochemical properties and catalytic performance in the reaction of methanol to gasoline (MTG) was investigated. It was found that, with increasing ratio TBA⁺/OH^–, the crystallinities, micropore surface areas, micropore volumes, the amounts of strong acid sites and Brönsted acid sites gradually increased, and the mesopore volumes decreased. The treatment with pure TBAOH (TBA⁺/OH^– = 1.0) ensured the formation of narrow and uniform intracrystalline mesoporosity and the large amounts of strong or Brönsted acid sites on the zeolite, which contribute to the highest liquid hydrocarbon yield in the reaction of MTG.     

Dispersion state and catalytic properties of vanadia species on the surface of V<Subscript>2</Subscript>O<Subscript>5</Subscript>/TiO<Subscript>2</Subscript> catalysts

The dispersion state and catalytic properties of anatase-supported vanadia species are studied by means of X-ray diffraction (XRD), laser Raman spectroscopy (LRS), H₂ temperature-programmed reduction (TPR) and the selective oxidation of o-xylene to phthalic anhydride. The almost identical values of the experimental dispersion capacity of V₂O₅ on anatase and the surface vacant sites available on the preferentially exposed (001) plane of anatase suggest that the highly dispersed vanadium cations are bonded to the vacant sites on the surface of anatase as derived by the incorporation model. When the loading amount of V₂O₅ is far below its dispersion capacity, the dispersed vanadia species might mainly consist of isolated VO_x species bridging to the surface through V-O-Ti bonds. With the increase of V₂O₅ loading the isolated vanadia species interact with their nearest neighbors (either isolated or polymerized vanadia) through bridging V-O-V at the expenses of V-O-Ti bonds, resulting in the increase of the ratio of polymerized to isolated vanadia species and the decrease of the reactivity of the associated surface oxygen anions and, consequently, although the activity increases with loading to reach a maximum value, the turn over number (TON) of the V₂O₅/TiO₂ catalyst decreases linearly. When the loading amount of V₂O₅ is higher than its dispersion capacity, the turn over number decreases more rapidly with the increase of V₂O₅ loading due to the formation of V₂O₅ crystallites in which the oxygen anions associated with V-O-V bonds are less reactive and only partially exposed on the surface.     

Beeinflussung der katalytischen Eigenschaften von Al2O3?SiO2-Katalysatoren durch Alkalivergiftung

Effect of Alkali Contamination on the Catalytic Properties of Al₂O₃? Si₂ Catalytic properties of amorphous Al₂O₃? SiO₂ catalysts containing different amounts of Al₂O₃ in dehydration of isopropanol and cracking of cumene were examined after a defined contamination of the acid centers by sodium ethylate from alcoholic solution. In both reactions, the catalytic activity is decreased by treatment with sodium ethylate, the cracking of cumene being suppressed at a lower alkali concentration than the dehydration of isopropanol. In dehydration of isopropanol, the dependence of the catalytic activity on the alkali content is influenced strongly by the Al₂O₃ content of the catalysts. In the cracking of cumene, strongly acid Brönsted centers are active, whereas the dehydration of isopropanol proceeds by joint action of acid Lewis or Brönsted centers, respectively, with basic centers at the surface of the catalyst (hydroxide groups or oxygen anions).     

Bifunctional catalysis of Mo/HZSM-5 in the dehydroaromatization of methane with CO/CO2 to benzene and naphthalene

The catalytic dehydrocondensation of methane to aromatics such as benzene and naphthalene was studied on the Mo carbide catalysts supported on micro- and mesoporous materials such as HZSM-5 (0.6 nm) and FSM-16 (2.7 nm). The Mo catalysts supported on H-ZSM-5 having appropriate micropores (0.6 nm size) and Si/Al ratios (20-70) exhibit higher yields (90-150 nmol/g-cat/s) and selectivities (higher than 74% on the carbon basis) in methane conversion to aromatic products such as benzene and naphthalene at 973 K and 1 atm, although they are drastically deactivated because of substantial coke formation. It was demonstrated that the CO/CO₂ addition to methane effectively improves the catalyst performance by keeping a higher methane conversion and selectivities of benzene formation in the prolonged time-on-stream. The oxygen derived from CO and CO₂ dissociation suppresses polycondensation of aromatic products and coke formation in the course of methane conversion. XAFS and TG/DTA/mass-spectrometric studies reveal that the zeolite-supported Mo oxide is endothermally converted under the action of methane around 955 K to nanosized particles of molybdenum carbide (Mo₂C) (Mo-C, coordination number = 1,R- 2.09 å; Mo-Mo, coordination number = 2.3–3.5;R = 2.98 å). The SEM pictures showed that the nanostructured Mo carbide particles are highly dispersed on and inside the HZSM-5 crystals. On the other hand, it was demonstrated by IR measurements of pyridine adsorption that the Mo/HZSM-5 catalysts having the optimum SiO₂/Al₂O₃ ratios around 40 show the maximum Brönsted acidity among the catalysts with the SiO₂/Al₂O₃ ratios of 20–1900. There is a close correlation between the activity of benzene formation in the methane aromatization and the Brönsted acidity of HZSM-5 due to the bifunctional catalysis.     

Mechanism by which Tungsten Oxide Promotes the Activity of Supported V2O5/TiO2 Catalysts for NOX Abatement: Structural Effects Revealed by 51V MAS NMR Spectroscopy

The selective catalytic reduction (SCR) of NO_x with NH₃ to N₂ with supported V₂O₅(‐WO₃)/TiO₂ catalysts is an industrial technology used to mitigate toxic emissions. Long‐standing uncertainties in the molecular structures of surface vanadia are clarified, whereby progressive addition of vanadia to TiO₂ forms oligomeric vanadia structures and reveals a proportional relationship of SCR reaction rate to [surface VO_x concentration]², implying a 2‐site mechanism. Unreactive surface tungsta (WO₃) also promote the formation of oligomeric vanadia (V₂O₅) sites, showing that promoter incorporation enhances the SCR reaction by a structural effect generating adjacent surface sites and not from electronic effects as previously proposed. The findings outline a method to assess structural effects of promoter incorporation on catalysts and reveal both the dual‐site requirement for the SCR reaction and the important structural promotional effect that tungsten oxide offers for the SCR reaction by V₂O₅/TiO₂ catalysts.     

A FTIR assessment of surface acidity and dispersion of surface species in titania and alumina-supported molybdena

Adsorption of pyridine on the MoO₃/TiO₂ and MoO₃/Al₂O₃ systems has been studied by FTIR spectroscopy, in order to identify surface acid sites existing in samples with different molybdena loadings. The results show that both Lewis and Brönsted surface acid sites exist, whatever the molybdena loading. The percentage of Brönsted sites is larger for loadings below the theoretical monolayer, and should correspond to bidimensional molybdenum oxides species, while for loadings above the monolayer these sites are associated with bulk MoO₃.     

Nickel Poisoning of a Cracking Catalyst Unravelled by Single-Particle X-ray Fluorescence-Diffraction-Absorption Tomography

Ni contamination from crude oil in the fluid catalytic cracking (FCC) process is one of the primary sources of catalyst deactivation, thereby promoting dehydrogenation–hydrogenation and speeding up coke growth. Herein, single-particle X-ray fluorescence, diffraction and absorption (μXRF-μXRD-μXAS) tomography is used in combination with confocal fluorescence microscopy (CFM) after thiophene staining to spatially resolve Ni interaction with catalyst components and study zeolite degradation, including the processes of dealumination and Brønsted acid sites distribution changes. The comparison between a Ni-lean particle, exposed to hydrotreated feedstock, and a Ni-rich one, exposed to non-hydrotreated feedstock, reveals a preferential interaction of Ni, found in co-localization with Fe, with the γ-Al₂O₃ matrix, leading to the formation of spinel-type hotspots. Although both particles show similar surface zeolite degradation, the Ni-rich particle displays higher dealumination and a clear Brønsted acidity drop.     

Configuration of MFI-type zeolite interacting with the probe molecule P(CH<Subscript>3</Subscript>)<Subscript>3</Subscript> – a theoretical study

With P(CH₃)₃ as the probe molecule adsorbed on titanium silicalite (TS-1) zeolite, the special and important role of T12 site in MFI-type zeolite was clearly elucidated. There are altogether three active sites present in TS-1 zeolite with Ti at the T12 site. Owing to the preferential adsorption of probe molecules on the first Brönsted acidic site, the Ti12 center will probably fail to show Lewis acidity. The ionic [HP(CH₃)₃]⁺ species can be stabilized by the first or second Brönsted acidic site, with the former energetically favored. The latter was formed through the transfer of the ionic [HP(CH₃)₃]⁺ species from the first to the second Brönsted acidic site.     

TPD study of NH3 adsorption/desorption on the surface of V/Ti, V/Al based catalysts for selective catalytic reduction of NOx by ammonia 1. TPD test of γ-Al2O3, TiO2 (anatase) and alumina-supported vanadia catalysts

The effect of temperature on the adsorption/desorption of ammonia from the air mixture on the surface of γ-Al₂O₃, TiO₂ (anatase) and alumina-supported vanadia catalyst samples has been investigated using temperature-programmed desorption (TPD). When the vanadia loading was increased, the fraction of the acid sites providing the NH₃ adsorption in the high-temperature state decreased. At the same time, the fraction of the medium temperature state significantly increased.     

Infrared study of surface species arising from ammonia adsorption on oxide surfaces

Infrared spectra of ammonia adsorbed on CoO, NiO, SiO₂, CaO, MgO, ZrO₂, ZnO, TiO₂, BeO and Al₂O₃, have been studied in the NH stretching and bending vibration regions at various stages of sample dehydroxylation. Several types of adsorption were found: hydrogen bonding to surface oxygen atoms or hydroxyl groups, coordination to Lewis acid sites and coordination plus hydrogen bonding; on some oxides ammonia molecules dissociate to produce surface NH₂ and OH groups. Frequencies characteristic of the distinct adsorbed species were determined. Except for Al₂O₃, no evidence was found for Brönsted acid sites on the surface of the above oxides.     

V2O5/TiO2 Catalyst Xerogels: Method of Preparation and Characterization

This work describes a modified sol-gel method for the preparation of V₂O₅/TiO₂ catalysts. The samples have been characterized by N₂ adsorption at 77 K, X-ray Diffractometry (XRD), Scanning Electronic Microscopy (SEM/EDX) and Fourier Transform Infrared Spectroscopy (FT-IR). The surface area increases with the vanadia loading from 24 m² g^–1 for pure TiO₂ to 87 m² g^–1 for 9 wt% of V₂O₅. The rutile form is predominant for pure TiO₂ but becomes enriched with anatase phase when vanadia loading is increased. No crystalline V₂O₅ phase was observed in the diffractograms of the catalysts. Analysis by SEM showed heterogeneous granulation of particles with high vanadium dispersion. Two species of surface vanadium were observed by FT-IR spectroscopy: a monomeric vanadyl and polymeric vanadates. The vanadyl/vanadate ratio remains practically constant. Ethanol oxidation was used as a catalytic test in a temperature range from 350 to 560 K. The catalytic activity starts around 380 K. For the sample with 9 wt% of vanadia, the conversion of ethanol into acetaldehyde as the main product was approximately 90% at 473 K.