Unmodified Catalyst Research Articles

Markedly different selectivity in the rhodium catalyzed hydroformylation of vinyl olefins containing a chiral alkoxy or alkyl group: good agreement between theory and experiment

A theoretical investigation on the regio- and stereoselective outcomes of the hydroformylation reaction with an unmodified rhodium catalyst (H–Rh(CO) 3) was carried out at the B3LYP/SBK(d) level on related chiral olefins, namely (1-vinyloxy-ethyl)-benzene ( 1) and (1-methyl-but-3-enyl)-benzene ( 2). A thorough and computationally expensive examination of the various possible H–Rh(CO) 3–olefin complex intermediates was performed, in order to determine, interpret, and eventually predict, the regio- and diastereoselectivity of the aforementioned reactions, whose products are a mixture of the linear aldehyde and of two diastereomers of the branched aldehyde. Regio- and diastereoselectivity of the reaction have been experimentally determined via hydroformylation runs at 20° and 100 °C for 1 and at 20 °C for 2. The theoretical results obtained are in good agreement with the experimental ones, which put forward a high chiral discrimination for chiral vinyl ether substrates in contrast to the lack of regio- and diastereoselectivity observed in the hydroformylation of 2. For the hydroformylation of 1, a regioselectivity ratio ( B: L) of 72:28 and a diastereoselectivity ratio ( b: b ′) of 97:3 have been computed which compare well to the corresponding experimental results (85:15 and 88:12). Therefore, theoretical calculations can give reliable estimates of regio- and diastereoselectivity provided a careful and accurate surface scan is performed for the alkyl-rhodium intermediates and the reaction is carried out at room temperature and, hence, in the absence of branched to linear alkyl isomerization.

Reforming of CH4with CO2and O2to produce syngas over CaO modified Ni/SiO2catalysts in a fluidized bed reactor

Methane reforming by carbon dioxide and oxygen was investigated over 5 wt.%Ni/CaO-SiO2 catalyst in a fluidized-bed reactor. The catalyst exhibited high activity and good stability at stoichiometric methane, carbon oxide and oxygen feed. Effects of calcium loading, space velocities, reaction temperatures and the feed gas compositions for this combined reaction were investigated. These results indicated that Ni/CaO-SiO2 is more effective and stable. The catalyst performance, stability, structure, dispersion of nickel and carbon deposition of the modified and unmodified catalysts were investigated by a series of characterization techniques. Results showed that catalysts modified with CaO improved their stability better than the pure nickel-based catalysts.

Effect of modifiers on the reactivity of Cr2O3/Al2O3 and Cr2O3/TiO2 catalysts for the oxidative dehydrogenation of propane

Chromium oxide supported on alumina and titania supports was modified with oxides of sodium, vanadium and molybdenum. The modified and unmodified chromium oxide catalysts were characterized by several techniques. The presence of surface chromium oxide and surface molybdenum and vanadium oxide species was detected in the unmodified and molybdenum and vanadium oxide modified supported chromium oxide catalysts. The reducibility (Tmax and H/Cr ratio) of the surface chromium species was not affected for the vanadium and molybdenum oxide modified catalysts; however, the reducibility changed noticeably for sodium modified supported chromium oxide catalysts. Studies of the reactivity of the ODH of propane revealed the effect of modifiers on the reactivity properties of the surface chromium oxide species. The activity and propene selectivity decreased for sodium modified supported chromium oxide catalysts. However, the activity increased for vanadium oxide modified catalysts and was similar for molybdenum oxide modified catalysts irrespective of the support. The propene selectivity was higher for molybdenum oxide modified chromium oxide catalysts. However, the propene selectivity for vanadium oxide modified catalysts depends on the support since it appears that the inherent selectivity of the surface vanadium oxide species is reflected.

Epoxidation on titania–silica aerogel catalysts studied by attenuated total reflection Fourier transform infrared and modulation spectroscopy

The epoxidation of cyclohexene over titania–silica aerogel catalysts using t-butylhydroperoxide (TBHP) was investigated by in situ attenuated total reflection (ATR) infrared spectroscopy. In order to distinguish between relevant and spectator species and to increase sensitivity, ATR was combined with modulation spectroscopy. In the latter technique the catalyst system is periodically perturbed by a forced concentration change. The interaction of cyclohexene with the aerogels is found to be weak. In contrast, TBHP adsorbs strongly on the catalysts on two different sites. Modulation experiments reveal that TBHP adsorbed on Si-OH groups is a spectator, whereas the one adsorbed on the Ti-sites is involved in the catalytic cycle. The latter species is stronger adsorbed and the associated signals increase with the Ti content of the catalyst. Adsorption of the TBHP on the Ti sites results in a strong shift of the C-O stretching vibration and changes in the Ti-O-Si modes of the catalyst. The study furthermore reveals vastly different pore diffusion for cyclohexene and TBHP due to their different interaction with the polar catalyst surface. In the modulation experiments the reaction product appears retarded with respect to the admittance of the reactants, which indicates that pore diffusion and kinetics of adsorption and desorption are important factors for the catalysis. Methylation of the aerogel has a beneficial effect on the catalysis, which can be traced to the different pore size distribution and polarity with respect to the unmodified catalyst. When the flow-through ATR cell is slowly heated, a change in the framework vibrations of the catalyst occurs simultaneously with the onset of reaction, indicating reaction induced structural changes.

Methane oxidation over vanadium-modified Pd/Al 2O 3 catalysts

Three different vanadium-modified Pd/Al 2O 3 catalysts were prepared and tested as catalysts for the deep oxidation of methane. Vanadium was added to the palladium catalyst by incipient wetness of palladium catalyst in order to modify its properties and improve its thermal stability and thioresistance. The behaviour of vanadium-modified catalysts depends on the concentration of this compound, being 0.5 wt.% the optimum amount. However, when strong catalyst poisons are present in the gas (SO 2), these modified catalysts do not show a better performance than unmodified catalyst. Bimetallic catalysts were tested with and without further reduction, being observed that reduced bimetallic catalysts perform worse than the non-reduced ones.

An Investigation of Promoter Effects in the Reduction of NO by H2 under Lean-Burn Conditions

The reduction of NO by H2 has been investigated under lean conditions at temperatures representative of automotive “cold-start” conditions (<200°C) using MoO3- and Na2O-modified Pt/Al2O3 and Pt/SiO2 catalysts. It has been found that small additions of sodium significantly increase the NO conversion while larger loadings of sodium have a severe poisoning effect. However, in the presence of excess O2 no enhancement in nitrogen selectivity at low temperatures has been observed for all loadings of Na. Indeed, an adverse effect has been found at higher temperatures. Addition of molybdenum as a promoter results in increases in NO conversion and nitrogen selectivity for all loadings tested. The optimal formulation was determined to be 1% Pt/10% MoO3/0.27% Na2O/Al2O3. Steady-state isotopic-transient kinetic (SSITK) experiments were performed on this and a “model” Pt/MoO3/Na2O/SiO2 catalyst using labelled nitric oxide in order to estimate the surface concentrations of species leading to N2, N2O, and retained NO. The data reveal significantly greater surface concentrations of N2 precursors over the modified catalysts for both the Pt/Al2O3 and Pt/SiO2 systems and this has been used to rationalise the increased selectivity to N2. An additional significant effect of the molybdenum promoter has been proposed because when the concentrations of N2 intermediates and the amount of available platinum in the modified catalysts are calculated, the “storage” of N2 precursors on the MoO3 seems to occur. This effect has been further explored using the modified SiO2 catalyst in non-steady-state transient experiments where the reductant supply (i.e., the H2) is cut off. It has been found that the decay in the production of N2 is very significantly delayed in comparison with the unmodified catalysts, and it is proposed that this is consistent with the trapping on the Mo of a reduced intermediate that can form N2 even in the absence of the normal H2 reductant. The mechanistic consequences of these novel results are discussed.

Effects of cobalt additive on amorphous vanadium phosphate catalysts prepared using precipitation with supercritical CO2as an antisolvent

The effect of addition of cobalt to an amorphous vanadium phosphate for the selective oxidation of n-butane to maleic anhydride is described and discussed. Cobalt is a well known promoter for crystalline vanadium phosphate catalysts and is most effective at a concentration of 1 atom % relative to vanadium. In contrast, for amorphous vanadium phosphate materials, prepared by precipitation using supercritical CO2 as an antisolvent, cobalt appears to act as a catalyst poison, decreasing both the catalyst activity and selectivity for maleic anhydride. Detailed analysis by transmission electron microscopy, 31P spin echo mapping NMR spectroscopy and X-ray absorption spectroscopy is described, which highlight differences with the unmodified catalyst. It is concluded that the addition of cobalt affects the morphology of the material and the oxidation state of vanadium, and that these changes deleteriously affect the catalytic performance.

Fischer—Tropsch synthesis over cobalt catalysts supported on zirconia-modified alumina

The effect of adding zirconia to the alumina support on supported cobalt Fischer-Tropsch catalysts has been studied. At 5 bar and H 2 :CO ratio 9:1 zirconia addition to the support leads to a significant increase in both activity and selectivity to higher hydrocarbons as compared to the unmodified catalysts. Reducibility and cobalt dispersion on the other hand are not improved by the presence of zirconia compared to the unmodified catalysts. SSITKA measurements have been performed in order to determine the intrinsic activity per active site. At constant temperature, zirconia-modified and unmodified catalysts showed basically the same intrinsic activity. Similar results were obtained with a noble metal (Pt) promoted catalyst. The promoting effect appears to be mainly due to coverage effects rather than a change in the intrinsic activity of the active sites. The turnover frequencies were found to be independent of pressure but strongly temperature dependent. However, the increase in turnover frequency did not account for the entire increase in reaction rate with temperature. This indicates that also the coverage of reactive intermediates increases with increasing temperature.

Hydrogenation of cinnamaldehyde over sol–gel Pd/SiO 2 catalysts: kinetic aspects and modification of catalytic properties by Sn, Ir and Cu additives

When 1% v/v solution of cinnamaldehyde/toluene was hydrogenated at 25 °C and 47 psi hydrogen pressure over sol–gel Pd/SiO 2 catalysts, a modification effect was observed. Sn and Ir were found to noticeably enhance the selectivity of Pd catalysts with respect to the hydrogenation of the CO bond relative to the favored hydrogenation of the CC bond. The produced cinnamyl alcohol, however, converts readily to phenylpropanol. On the other hand, Cu has caused a decrease in activity without any significant improvement on selectivity under the above conditions. The reaction kinetics were studied for the unmodified Pd catalysts, and the orders were determined to be first with respect to hydrogen, and zero with respect to cinnamaldehyde. The apparent activation energy was calculated and found to have a value of 30.1 kJ/mol. Also, the effects of metal loading and catalyst mass were studied.

ChemInform Abstract: Hydroformylation with Unmodified Rhodium Catalysts: Reaction Mechanism and Origin of Regioselectivity

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Olefin Insertion into the Rhodium−Hydrogen Bond as the Step Determining the Regioselectivity of Rhodium-Catalyzed Hydroformylation of Vinyl Substrates: Comparison between Theoretical and Experimental Results

A comparison between experimental and theoretical data on regioselectivity concerning the hydroformylation of several vinyl substrates (propene, 2-methylpropene, 1-hexene, 3,3-dimethylbutene, fluoroethene, 3,3,3-trifluoropropene, vinylmethylether, allylmethylether, styrene) with unmodified rhodium catalysts is reported. Various H−Rh(CO)3-olefin complexes are examined at the B3P86/3-21G or /6-31G* level (LANL2DZ for Rh) and compared to the adducts with modified catalysts, such as HRhPH3(CO)2. The computed geometries are in satisfactory agreement with the X-ray ones. The activation energies for the alkyl rhodium intermediate formation, computed at either level along the pathways to branched or linear aldehydes, allow one to predict the regioselectivity ratios, since they are in very good agreement with the experimental ones evaluated for the isomeric aldehydes.

Effect of Mg and Zr Modification on the Activity of VOx/Al2O3 Catalysts in the Dehydrogenation of Butanes

The initial activity of alumina-supported vanadium oxide catalysts with and without Mg and Zr was studied in the dehydrogenation of i-butane and n-butane at 580°C under atmospheric pressure. The catalyst activity was evaluated, after reduction with H2 and CO, as a function of the amount of the promotor. The dehydrogenation activity was highest after CO reduction, and the selectivity toward i-butene increased and the formation of coke decreased with the addition of Mg. According to X-ray photoelectron spectroscopy, electron spin resonance, temperature programmed reduction, and Fourier transform infrared measurements, during CO reduction, V3+ species and two kinds of V4+ species with different coordinations (A and B) were formed on the catalyst surface. During H2 reduction V3+ and some V4+ (B) species were formed. Apparently both V4+ and V3+ species were active in the dehydrogenation, and tentatively, we conclude that the activity of V4+ (A) was higher than that of V3+. For the modified and unmodified catalysts, the oxidation states after reduction, both with H2 and with CO, were similar and were not the main reason for the differences in activities. It is likely that the differences in activities induced by the promotors were due to the acidities of the catalysts, which had an influence on the side reactions of cracking, skeletal isomerization, and coke formation. The oxidative dehydrogenation during the first minutes on butane stream with calcined catalysts was also affected by the acidity of the catalyst.

Effect of addition of CaO on Ni/Al 2O 3 catalysts over CO 2 reforming of methane

The Ni/γ–Al 2O 3 catalyst was modified by CaO in order to improve the thermal stability and the carbon deposition resistance during the CO 2 reforming of methane to syngas reaction. The catalytic performance, thermal stability, structure, dispersivity of nickel and carbon deposition of the modified and unmodified catalysts were investigated by a series of characterization techniques (X-ray diffraction (XRD), BET, H 2 chemisorption, temperature programmed reduction (TPR) and flow-reaction). Results showed that catalysts modified with CaO improve their stability better than the nickel-based catalysts.

Oxidative dehydrogenation of ethylene glycol into glyoxal over phosphorus-doped ferric molybdate catalyst

Test data for the catalytic oxidation of ethylene glycol (EG) into glyoxal obtained with a laboratory continuous flow tubular reactor are reported. A ferric molybdate catalyst modified with triammonium phosphate was used. Reaction parameters (temperature, contact time) affect the product distribution. The yield of glyoxal for the Fe-Mo-P catalyst was 43.3% at 88.9% conversion, whereas the corresponding values for the Fe-Mo (unmodified catalyst) were 20.5 and 97.2%. The addition of phosphorus formed a new Fe 2M 0P 12 phase which restrains the reaction pathway towards carbon oxides.

Fischer–Tropsch synthesis over cobalt catalysts supported on zirconia-modified alumina

The effect of adding zirconia to the alumina support on supported cobalt Fischer–Tropsch catalysts has been studied. At 5 bar and H 2:CO ratio 9:1 zirconia addition to the support leads to a significant increase in both activity and selectivity to higher hydrocarbons as compared to the unmodified catalysts. Reducibility and cobalt dispersion on the other hand are not improved by the presence of zirconia compared to the unmodified catalysts. SSITKA measurements have been performed in order to determine the intrinsic activity per active site. At constant temperature, zirconia-modified and unmodified catalysts showed basically the same intrinsic activity. Similar results were obtained with a noble metal (Pt) promoted catalyst. The promoting effect appears to be mainly due to coverage effects rather than a change in the intrinsic activity of the active sites. The turnover frequencies were found to be independent of pressure but strongly temperature dependent. However, the increase in turnover frequency did not account for the entire increase in reaction rate with temperature. This indicates that also the coverage of reactive intermediates increases with increasing temperature.

Influence of CuO x additives on CO oxidation activity and related surface and bulk behaviours of Mn 2O 3, Cr 2O 3 and WO 3 catalysts

CuO x -modified and unmodified Mn 2O 3, Cr 2O 3 and WO 3 catalysts were prepared by impregnation with Cu(NO 3) 2 solution and/or calcination at 700°C of appropriate precursor compounds. Bulk phase composition and thermochemical stability were characterized by X-ray diffractometry, infrared spectroscopy and thermogravimetry. The surface area, chemical composition, oxidation state and chemisorption capacity were determined by X-ray photoelectron spectroscopy and sorptometry of N 2, O 2 and CO gas molecules. The catalytic CO oxidation activity was tested, using a gas circulating system equipped with a fixed-bed microreactor and a gas chromatograph. The CuO x additives were found to promote markedly the otherwise insignificant CO oxidation activity of WO 3 at 250–400°C. This was related to concomitant improvement primarily in the lattice oxygen reactivity, and in the O 2 chemisorption capacity. The already high CO oxidation activities of Mn 2O 3 and Cr 2O 3 catalysts at 150–250°C were found to be, respectively, slightly and hardly improved by CuO x additives, despite a marked improvement in oxygen chemisorption capacity. This was attributed to establishment on the unmodified catalysts of a favourable electron-mobile environment. In oxygen-rich reaction atmosphere, both the modified and unmodified catalysts were thermochemically stable. However, in lean oxygen atmosphere, Cr 2O 3 and CuO x -modified WO 3 were relatively the most stable test catalysts.

The gas phase hydrogenation of 2-butanone over supported nickel catalysts: introduction of enantioselectivity

The gas phase hydrogenation of 2-butanone to 2-butanol at 343 K promoted using a Y zeolite-supported nickel catalyst (2.2% w/w Ni) prepared by ion exchange and a range of Ni/SiO 2 catalysts (1.5–20.3% w/w Ni) prepared by precipitation/deposition and impregnation has been studied. The freshly activated catalysts generated racemic products and enantioselectivity was introduced to a negligible degree by in situ and to an appreciable degree by ex situ treatment with a methanolic solution of l-tartaric acid. The effectiveness of the latter modification was strongly dependent on modifier concentration and an optimum enantiomeric excess (ee) of 31% was achieved at an intermediate tartaric acid concentration (8×10 −3 mol dm −3) where the latter also served to raise the fractional conversion of the 2-butanone feedstock when compared with the unmodified catalyst under the same reaction conditions. Treatment with l-valine or l-glutamic acid did not result in the introduction of any appreciable level of product optical activity. Conversion of 2-butanone over Ni/SiO 2 was subject to a short term loss of catalytic activity which was however readily restored by heating in flowing hydrogen at 673 K. Deactivation of the Ni/Na-Y catalyst was more severe and was not reversed by such a heat treatment; loss of activity in this case is attributed to an irreversible pore blocking. Non-selective hydrogenation is approximated by pseudo-first-order kinetics, the possibility of thermodynamic limitations is considered and the effects of varying the hydrogen partial pressure and gas space velocity are addressed. The reaction exhibits a high degree of structure sensitivity and the relationship between specific rate constant and nickel particle diameter is presented wherein an optimum particle size of ca. 3 nm is identified.

Pt/CeO2 catalysts in selective hydrogenation of crotonaldehyde: high performance of chlorine‐free catalysts

The hydrogenation of crotonaldehyde was conducted in gaseous phase, at atmospheric pressure, on Pt/CeO2 catalysts prepared from metal precursors containing or not chlorine. The activities and selectivities were studied, at 253 K, as a function of the reduction temperature of the catalyst (473–993 K). The Pt/CeO2 catalyst, prepared from tetraammineplatinum nitrate, led to 5–20% crotyl alcohol selectivity when the catalyst was reduced at low temperature (473–673 K), while increasing the reduction temperature up to 973 K, the crotyl alcohol selectivity reached more than 80%. Repeating a series of experiments after a re‐calcination treatment at 673 K, the selectivity decreased to only 40% after 473 K reduction to reach again more than 80% after 673 K reduction temperature. A phase transformation of Pt to CePt5 was observed by XRD analysis after 973 K reduction treatment. Differently on Pt/CeO2 catalysts containing chlorine, prepared from either chloroplatinic acid or tetraammineplatinum chloride, the crotyl alcohol selectivity never exceeded 30% and did not form alloy up to 973 K reduction temperature. The main results are interpreted considering that the activity of CePt5 for C=C hydrogenation is low compared to unmodified platinum catalyst and the activation of the carbonyl bond is induced by the presence of oxygen vacancies sites located at the interface between ceria and the metallic particles. The results are in good accordance with the information known at the present time on the metal–support interactions in Pt deposited on CeO2.

Organically Modified Titania–Silica Aerogels for the Epoxidation of Olefins and Allylic Alcohols

Mesoporous titania–silica mixed oxides with varying amounts of covalently bound methyl groups were prepared from methyltrimethoxysilane and tetramethoxysilane using a sol-gel process and ensuing low temperature supercritical extraction with CO2. The aerogels containing 20 wt% TiO2were characterized by thermal analysis, N2and NH3adsorption, infrared spectroscopy, NMR, X-ray photoelectron spectroscopy, and transmission electron microscopy. The amorphous mesoporous structure of the aerogels did not depend markedly on the amount of modifier. The titania dispersion within the silica matrix was slightly lowered for methyl-modified materials.29Si CP-MAS NMR measurements indicated a more cross-linked network for the unmodified catalyst. Methyl groups located at the surface replaced part of the silanol groups present in the unmodified aerogel, resulting in a decreased ability to adsorb ammonia. The modified aerogels showed high thermal stability: a significant rate of Si–C bond breaking was detected only above 400°C. The materials were tested in the epoxidation of various olefins and allylic alcohols. The olefin epoxidation activity was generally lowered by the methyl modification, whereas in the epoxidation of allylic alcohol a maximum in activity was observed with increasing methyl content. In the epoxidation of cyclohexenol, 98% selectivity at 90% conversion was achieved in 10 min. Dependent on their basicity, inorganic salts and bases as additives exhibited a strong influence on the reaction over modified aerogels. This behavior is suggested to be due to: (i) different titanium complexes which form upon interaction with the reactant and basic additive and (ii) the resulting change of the stability/reactivity of the peroxo–titanium complex.

Rhodium-Catalyzed Hydroformylation in Supercritical Carbon Dioxide

Supercritical carbon dioxide (scCO2) is an environmentally benign reaction medium for highly efficient rhodium-catalyzed hydroformylation reactions. Olefinic substrates can be hydroformylated in scCO2 at 40−65 °C to give the corresponding aldehydes in practically quantitative yields. The reaction course of the hydroformylation of 1-octene in scCO2 was analyzed in detail by online-GC monitoring. The influence of reaction parameters such as temperature, synthesis gas pressure, and [P]/Rh ratio on reaction rates and selectivities is grossly similar to the effects observed in conventional solvents. Maximum turnover frequencies of 1375, 500, and 115 h-1 were determined as lower limits for the catalytic activities under the present conditions for the unmodfied, phosphine-modified, and phosphite-modified systems, respectively. With unmodified catalysts, the hydroformylation rates are considerably higher in scCO2 than in organic solvents or liquid CO2 under otherwise identical conditions. Modified catalytic syste...