Promoting effect of Mo on the selective hydrogenation of cinnamaldehyde on Rh/SiO2 catalysts

MoO₃ was added to Rh/SiO₂ catalysts in order to improve both activity and selectivity towards cinnamyl alcohol during the hydrogenation of cinnamaldehyde in the liquid phase. In the present work, four catalysts were studied. Two Rh–Mo/SiO₂ catalysts and Rh/SiO₂, Mo/SiO₂ references were prepared by conventional impregnation techniques and characterised by hydrogen chemisorption, X-ray diffraction, transmission electron microscopy, temperature-programmed reduction and X-ray photoelectron spectroscopy. The catalytic behaviour in cinnamaldehyde hydrogenation, after in situ reduction treatments in flowing hydrogen at 773 K shows that Mo oxide clearly promotes the hydrogenation of the carbonyl bond. This is attributed to (i) the presence of both MoO_3−x and Rh^δ+ species which contribute to a polarization of the C=O bond, (ii) the presence of flat surfaces of molybdenum oxides strongly interacting with Rh particles, that provide the appropriate morphology to enhance the selectivity to the unsaturated alcohol, and (iii) the poisoning effect of MoO_x on rhodium species by partial coverage of the metallic particles that produce a decrease in the hydrogenation ability and therefore reduce the formation of the saturated alcohol. This revised version was published online in July 2006 with corrections to the Cover Date.     

The promoter function of molybdenum in Rh/Mo/SiO2 catalysts for CO hydrogenation

A series of Rh/Mo/SiO₂ catalysts with fixed Rh and different Mo contents were studied by FT-IR, chemisorption and CO hydrogenation. The FT-IR results at room temperature under CO atmosphere indicate that the addition of Mo to Rh/SiO₂ suppresses the linear and bridged CO species and promotes the twin CO species, which is consistent with the chemisorption results. It is suggested that the Mo promoter works via stabilization of Rh^1– ions and the coverage of Rh sites. The molybdenum promotes the formation of oxygenates and shifts the selectivity from hydrocarbons to oxygenates.     

Hydrogenation of Carbon Dioxide on Rh, Au and Au–Rh Bimetallic Clusters Supported on Titanate Nanotubes, Nanowires and TiO2

Supported gold, rhodium and bimetallic rhodium-core?Cgold-shell catalysts were prepared. The supports were TiO₂ as well as titanate nanotube and nanowire formed in the hydrothermal conversion of titania. The catalytic properties were tested in the CO₂ hydrogenation at 493?K. The amount and the reactivity of the surface carbonaceous deposit were determined by temperature-programmed reduction. The surfaces of the materials were characterized by X-ray photoelectron and low-energy ion scattering spectroscopy (LEIS). The surface forms during the catalytic reaction were identified by DRIFT spectroscopy. On the XP spectra of bimetallic catalysts the existence of highly dispersed gold particles could be observed besides the metallic form on all supports. Small Rh particles could also be identified on the titanate supports. LEIS spectra demonstrated that Rh-core?CAu-shell particles formed, since no scattering from Rh was detected. The main product of CO₂ hydrogenation was CH₄ on all catalysts. IR spectra revealed the existence of CO and formate species on the surface. In addition, a new band was observed around 1,770?cm^?1 which was assigned as tilted CO. It is bonded to Rh and interacts with a nearby the oxygen vacancy of the support. Agglomeration of highly dispersed Rh was observed on bimetallic samples induced by reaction or reactant.     

On the nature of catalyst promotion by manganese in CO hydrogenation to oxygenates over RhMn/NaY

CO hydrogenation over Mn promoted Rh/NaY catalysts was studied at 10 bar and 250°C. Significant selectivity to oxygenates, mainly ethanol and ethyl acetate, was obtained after neutralizing the protons that are formed during reduction of Rh ions. Layered bed experiments show that protons act as sites catalyzing secondary reactions. Protons also convert Mn(OH)₂ to Mn²⁺ ions; the catalysts with highest selectivity to oxygenates contain MnO particles and Rh clusters. The results suggest chemical interaction of adsorbates on Rh_n clusters with those on MnO.     

Selective hydrogenation of amides using Rh/Mo catalysts

Rh/Mo catalysts formed in situ from Rh₆(CO)₁₆ and Mo(CO)₆ are effective for the liquid phase hydrogenation of CyCONH₂ to CyCH₂NH₂ in up to 87% selectivity, without the requirement for ammonia to inhibit secondary amine formation. Use of in situ HP-FTIR spectroscopy has shown that decomposition of metal carbonyl precursors occurs during an extended induction period, with the generation of recyclable, heterogeneous, bimetallic catalysts. Variations in Mo:Rh content have revealed significant synergistic effects on catalysis, with optimum performance at values of ca. 0.6, and substantially reduced selectivities at ?1. Good amide conversions are noted within the reaction condition regimes 50–100 bar H₂ and 130–160 °C. Ex situ characterization of the catalysts, using XRD, XPS and EDX-STEM, has provided evidence for intimately mixed (ca. 2–4 nm) particles that contain metallic Rh and reduced Mo oxides, together with MoO₃. Silica-supported Rh/Mo analogues, although active, perform poorly at <150 °C and deactivate during recycle.     

Competitive Hydrogenation of Alkyl‐Substituted Arenes by Transition‐Metal Nanoparticles: Correlation with the Alkyl‐Steric Effect

The influence of substituents on rate constants of the hydrogenation of monoalkylbenzenes by transition metal nanoparticles or by classical heterogeneous catalysts can be rationalized in terms of the Taft rule. A series of the initial reaction rate constants obtained from various competitive toluene/benzene and toluene/monoalkylbenzene hydrogenation experiments catalyzed by transition‐metal nanoparticles prepared in the presence of imidazolium ionic liquids or surfactants [Ir(0), Rh(0) and Ru(0)] or by classical heterogeneous catalysts (PtO₂, Rh/C, Rh/Al₂O₃, Ru/C, Ru/Al₂O₃ and Pd/C) have been correlated with the Taft equation . Satisfactory correlation coefficients (r) (between 0.96 and 0.99) and positive slopes (ρ) between 0.38 and 0.83 have been obtained. The results clearly show that the reaction constants for the alkyl‐substituents can be expressed by steric factors and are independent of any other non‐steric factors. It is suggested that bulky alkylbenzene substituents, for both transition metal nanoparticles and classical heterogeneous hydrogenation reactions, lower the overall hydrogenation rate, implying a more disturbed transition state compared to the initial state of the hydrogenation (in terms of the Horiuti–Polanyi mechanism). This competitive method is suitable for the estimation of the constant selectivity for couples of alkylbenzenes in which the difference in hydrogenation rates are very high and experimentally difficult to measure and also useful for the design of more selective “nano” and classical catalysts for hydrogenation reactions.     

CO2 hydrogenation reactivity and structure of Rh/SiO2 catalysts prepared from acetate,chloride and nitrate precursors

Silica-supported Rh catalysts (Rh/SiO₂) were prepared from acetate, chloride and nitrate precursors by an impregnation method and were applied to CO₂ hydrogenation reaction. CO₂ conversion over the catalyst prepared from chloride precursor was lower than that over acetate or nitrate one, because of fewer active sites on catalysts, as estimated by H₂ chemisorption. The main product was CO over the catalysts prepared from acetate and nitrate, but it was CH₄ over the catalyst prepared from chloride precursor. Characterization of catalysts by TEM, FT-IR and XPS was carried out in order to elucidate the effect of metal precursor on the CO₂ hydrogenation reactivity. The results of XPS showed that the O atomic ratio to Rh on surface hydroxyl groups increased in the order: chloride<nitrate<acetate precursor. The ratio of hydroxyl groups to Rh particles on SiO₂ surface was expected to have a significant influence on the reactivity.     

Hydrogenation and Ring Opening of Aromatic and Naphthenic Hydrocarbons Over Noble Metal (Ir,Pt, Rh)/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Catalysts

Abstract  

The hydrogenation and ring opening of model hydrocarbons and of naphtha was studied over commercial noble metal (Ir, Pt, Rh)/Al₂O₃ catalysts. The experiments were performed in a fixed bed reactor at temperatures between 220 and 350 °C and pressures of 1.1 and 5.0 MPa, respectively. The product distribution was determined and the cetane number was calculated. The Pt catalyst is very active for hydrogenation of aromatics but does not catalyse the ring opening of naphthenes. The Ir and Rh catalysts are active for both hydrogenation of aromatics and ring opening of naphthenes. Experiments with toluene, m-xylene, propyl-benzene, and methylcyclohexane indicate that ring opening follows a selective mechanism, where the cleavage of bisecondary carbon bonds is favoured. This results in predominant formation of branched paraffins. The product distribution as well as cracking of long-chain hydrocarbons, which increase at temperatures above 260 °C, lead to an insignificant boost in the cetane number, as confirmed by experiments using real naphtha as feedstock.     

Correlation between metal oxidation state and catalytic activity: hydrogenation of crotonaldehyde over Rh catalysts

The effects of the rhodium (oxidation) state on the activity and selectivity for the crotonaldehyde hydrogenation reaction over Rh/Al₂O₃ and Rh/SiO₂ catalysts were examined using the techniques of temperature-programmed reduction, hydrogen chemisorption and X-ray absorption near-edge structure (XANES). In the alumina-supported system, the active phase-support interaction is shown to affect the chemical behavior of rhodium under the influence of a reductive atmosphere by stabilizing Rh³⁺ species. This behavior is not observed (as expected) for Rh/SiO₂ catalysts. The structural and electronic bases of the active phase-support interaction and the effect of the latter phenomenon on the hydrogenation of crotonaldehyde are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hydrodenitrogenation of quinoline over Ni–Mo/Al2O3 catalyst modified with fluorine and phosphorus

In this paper, the effects of fluorine and phosphorus on the physical and chemical properties of Ni–Mo/Al₂O₃ catalysts and the hydrodenitrogenation (HDN) activity of quinoline were investigated. The acidity, pore structure, and dispersion of Mo of the catalysts were analyzed with TG-DTA, BET, and XRD techniques. The activities of hydrodenitrogenation and hydrogenation of the catalysts were investigated using hydrogenation of quinoline at high pressure in a micro-reactor. Experimental results verified that phosphorus can promote the formation of moderate and strong acidic sites, the dispersion of Mo, and the formation of the active phases; therefore, the hydrogenation activity of aromatic rings and the hydrogenolysis activity of C–N bonds increase. The hydrogenation and hydrogenolysis accelerate each other, which results in the increase of HDN activity. It is concluded that phosphorus is a promoter for HDN activity of the Ni–Mo/Al₂O₃ catalysts. Fluorine can promote the formation of weak and moderate acidic sites and the dispersion of Mo, but inhibit the formation of the active phases. Therefore, the hydrogenation activity of aromatic rings and the hydrogenolysis activity of C–N bonds decrease, which results in the decrease of HDN activity. It is concluded that fluorine is not a promoter for HDN activity of the Ni–Mo/Al₂O₃ catalysts. The possible promoting mechanism of fluorine and phosphorus for the Ni–Mo/Al₂O₃ catalyst is put forward and discussed.     

Hydrogenation of CO2, acetone,and CO on a Rh foil promoted by titania overlayers

The effects of submonolayer deposits of titania on the hydrogenation of CO₂, acetone, and CO on a Rh foil have been investigated. Titania has been found to promote all three of the hydrogenation reactions, with each reaction exhibiting a maximum rate at a titania coverage of 0.5 ML. The maximum rate for CO₂ hydrogenation is 15 times that of the bare Rh surface. Acetone hydrogenation shows a 6-fold rate enhancement, while CO displays a 3-fold increase. Changes in the selectivities for each reaction are also observed upon titania promotion. The effects of titania on these reactions are attributed to an interaction between C-O bonds and Ti³⁺ ions located at the perimeter of titania islands.     

Polymer‐based catalysts for toluene hydrogenation

Crosslinked poly(4‐vinylpyridine‐co‐styrene) was synthesized by radical polymerization. Catalysts having 1 wt % Pd were obtained by impregnation of a copolymer, poly(4‐vinylpyridine‐co‐styrene) with a Pd colloidal dispersion. We modified metal particle sizes by changing the aging period of the colloidal dispersion, with the average size in the range of 2.5–4.3 nm. The most probable structure of the metal cluster attached to the polymers is described. X‐ray diffraction, transmission electron microscopy (TEM), and H₂? O₂ titrations were used as characterization techniques. The H₂ consumption during titration was extremely low, and the calculated metal dispersion was between 15 and 25 times lower than those estimated from TEM. This suggests that the Pd crystals were almost completely covered by the polymer. The vapor‐phase hydrogenation of toluene on resins supported Pd catalysts were studied. The catalysts in the hydrogenation of toluene exhibited low activity, and the obtention of significant selectivities to partial hydrogenation products (close to 60 mol %) was remarkable. The results are explained in terms of a significant decrease in the hydrogenation capacity due to the coverage of metal particles by the resin. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 381–385, 2002     

Conversion of syngas to C2+ oxygenates over Rh-based/SiO2 catalyst: The promoting effect of Fe

The effect of Fe promoter on the catalytic properties of Rh–Mn–Li/SiO₂ catalyst for CO hydrogenation was investigated. The catalysts were comprehensively characterized by means of X-ray diffraction (XRD), N₂ adsorption–desorption, temperature programmed reduction (TPR), temperature programmed desorption (TPD), temperature programmed surface reaction (TPSR), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Activity testing results showed that low loading of Fe (≤0.1 wt%) improved the reactivity and yield of C₂⁺ oxygenates; however, the opposite effect appeared at the high values of Fe (>0.1 wt%). Characterization results suggested that the addition of Fe strengthened the Rh–Mn interaction and increased the desorption/transformation rate of adsorbed CO, which could be responsible for the increase of CO conversion. But on the other hand, the existence of Fe might deposit over the Rh surface, and decreased the number of active sites, resulting in the decrease of CO conversion when the Fe amount was excessive. The selectivity to C₂⁺ oxygenates varied inversely with the reducibility of Rh oxide species. Moreover, it is proposed that the transformation of dicarbonyl Rh⁺(CO)₂ into H–Rh–CO is favorable for the formation of C₂⁺ oxygenates, and the hydrogenation ability of Fe can increase the hydrogenation of acetaldehyde to ethanol.     

DRIFT Studies of Adsorbed CO Over Sulfided K–Rh–Mo/Al2O3 Catalysts: Detection of Rh–Mo–S Phase

Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy was used to study the nature of active species in K–Rh–Co–MoS₂/Al₂O₃ catalyst by means of probing with CO molecule. The effects of K addition to Rh and interaction between Mo and Rh were studied with varying K and Mo loadings over 1 wt% Rh/Al₂O₃ catalyst. In sulfided Rh–Mo/Al₂O₃, the formation of Rh–Mo–S phase was evidenced first time by a band at 2,095 cm^?1. The introduction of Co to K–Rh–MoS₂/Al₂O₃ catalyst showed the existence of both Rh and Co promoted MoS₂ sites, but the CO absorption frequencies in DRIFT spectra are significantly at lower side compared to Co free Rh–Mo catalyst. The stabilities of CO band from Rh and Co promoted and unpromoted MoS₂ sites are studied at different temperatures. When activated carbon used as support, bands for both promoted and unpromoted MoS₂ sites were appeared, but the intensity of these bands were decreased largely compared to alumina based catalyst, resulted from the coverage of added K not only on the support surface but also on the active metal components due to the neutral nature of activated carbon.     

Olefin hydroformylation and selective hydrogenation of acetaldehyde on Mo-promoted Rh/SiO2 catalysts derived from metal salt and heteronuclear cluster precursors

Molybdenum promoted Rh/SiO₂ catalysts have been prepared by using the heteronuclear cluster (C₅H₅)₃RhMo₂(CO)₅ as well as metal salt precursors. The promoting effect of molybdenum has been studied for the hydroformylation of ethene and propene and the hydrogenation of acetaldehyde. It has been found that molybdenum, especially on the cluster-derived catalyst, increases both the hydrogenation and the hydroformylation rate of the olefins. No specific influence on the CO insertion reaction could be obtained. As an explanation, the promotion of the initial step to form intermediate surface alkyl groups has been proposed as the rate determining step for ethene hydroformylation. The promotion of the alcohol formation by bimetallic centers having Rh and Mo in close vicinity has been supported by the results of the hydrogenation of acetaldehyde.     

The kinetics of CO2 hydrogenation on a Rh foil promoted by titania overlayers

Submonolayer deposits of titania on a Rh foil have been found to increase the rate of CO₂ hydrogenation. The primary product, methane, exhibits a maximum rate at a TiO _x coverage of 0.5 ML which is a factor of 15 higher than that over the clean Rh surface. The rate of ethane formation displays a maximum which is 70 times that over the unpromoted Rh foil; however, the selectivity for methane remains in excess of 99%. The apparent activation energy for methane formation and the dependence of the rate on H₂ and CO₂ partial pressure have been determined both for the bare Rh surface and the titania-promoted surface. These rate parameters show very small variations as titania is added to the Rh catalyst. The methanation of CO₂ is proposed to start with the dissociation of CO₂ into CO_(a) and O_(a), and then proceed through steps which are identical to those for the hydrogenation of CO. The increase in the rate of CO₂ hydrogenation in the presence of titania is attributed to an interaction between the adsorbed CO, released by CO₂ dissociation, and Ti³⁺ ions located at the edge of TiO _x islands covering the surface. Differences in the effects of titania promotion on the methanation of CO₂ and CO are discussed in terms of the mechanisms that have been proposed for these two reactions.     

Modification of the catalytic properties of supported noble metal catalysts by carrier doping

The influence of altervalent cation doping of TiO₂ carriers on the chemisorptive and catalytic properties of supported Pt and Rh crystallites has been investigated. It was observed that doping of the carrier with higher valence cations leads to suppression of the H₂ and CO chemisorption capacity of Pt catalysts, while their activity in hydrogenation and oxidation reactions is significantly reduced. The opposite effects were observed in the case of Rh catalysts supported on higher valence doped TiO₂. These catalysts were found to possess higher activity in CO and CO₂ hydrogenation, in aromatics hydrogenation and in CO and C₂H₄ oxidation. Their stability characteristics were also found to be superior to those of the undoped Rh/TiO₂ catalyst. These effects are believed to originate from an electronic type interaction at the metal-support interface, induced by doping, which results in electron transfer from the support to the metal crystallites.     

Homogeneous Rh-Sn alkoxide coatings on silica surfaces: A novel route for preparation of bimetallic Rh-Sn catalysts

The reaction of [{(COD)Rh}₂Sn(OEt)₆], where COD = 1,5-cyclooctadiene and Et = ethyl, with silanol groups on silica surfaces is shown to lead to near-monolayer coverage of the silica by the Rh-Sn organometallic compound. Heating the supported compound at 498 K yields a catalyst that is active for benzene hydrogenation at room temperature. When the catalyst is reduced in H₂ at 823 K, the benzene hydrogenation activity increases with a simultaneous drop in the activity for n-butane hydrogenolysis. High temperature reduction leads to formation of Rh-Sn alloy particles with an average particle diameter of 2.5 nm. These particles are stable towards oxidation-reduction cycles involving oxidation at 773 K in 15% O₂. When normalized to the benzene hydrogenation activity, the n-butane hydrogenolysis activity of the bimetallic catalyst is suppressed by over 3 orders of magnitude when compared to a monometallic Rh catalyst.     

Ni catalysts supported on activated carbon from petcoke and their activity for toluene hydrogenation

Petroleum coke (petcoke) is an abundant resource that can potentially be converted to catalyst support materials through activation to increase the surface area and reduce the sulphur content. In this work, potassium hydroxide (KOH) catalysed activation was employed with petcoke to produce activated carbons, which were characterised with nitrogen physisorption, X‐ray diffraction, scanning electron microscopy and temperature‐programmed reduction. With activation temperatures between 500 and 800°C, the surface area increased from 4 m²/g to between 200 and 2400 m²/g while the sulphur content was reduced from 6.6 wt% to between 1 and 0.2 wt%. Nickel catalysts (nominally 5 wt%) were prepared on the activated carbon supports using wet impregnation. The activities of these catalysts were measured for toluene hydrogenation in a plug‐flow reactor with a toluene liquid hourly space velocity of 2.4/h, a pressure of 1.38 MPa, and a H₂/toluene mole ratio of 90. The catalytic activity varied between zero for nickel supported on petcoke to 98% conversion, with essentially 100% to methylcyclohexane for nickel supported on carbon activated at 750°C. Thus, activated carbon from petcoke was a suitable support for Ni‐based catalysts when used for toluene hydrogenation as a model reaction. © 2011 Canadian Society for Chemical Engineering     

Hydrogenation of carbon dioxide and carbon monoxide over supported rhodium catalysts under 10 bar pressure

The hydrogenation of carbon dioxide and carbon monoxide was carried out in a 10-atm flow reactor over supported rhodium catalysts. Rh/ZrO₂ and Rh/Nb₂O₅ showed the highest activity for carbon dioxide and hydrogenation, the main product being methane. Methanol was formed selectively from carbon dioxide hydrogen over a Rh/TiO₂ catalyst. The products of carbon monoxide hydrogenation over supported Rh catalysts contained higher hydrocarbons and ethanol. The rate of C-C bond formation was higher in carbon monoxide hydrogenation than in carbon dioxide hydrogenation. The effect of the support on hydrogenation of carbon dioxide and carbon monoxide is discussed.