Hydrogenation of Nitrobenzene with Supported Transition Metal Catalysts in Supercritical Carbon Dioxide

Transition metal catalysts such as Pd, Pt, Ru, and Rh supported on carbon, silica and alumina have been examined for the hydrogenation of nitrobenzene (NB) in supercritical carbon dioxide (scCO₂) and in ethanol. The order of hydrogenation activity is Pt>Pd>Ru, Rh in scCO₂ and in ethanol. The effectiveness of the support is C>Al₂O₃, SiO₂ for either Pt or Pd in scCO₂. For all the catalysts, higher selectivity to aniline has been obtained in scCO₂ compared with ethanol. Hydrogenation of nitrobenzene catalyzed with Pd/C and Pt/C catalysts was successfully conducted in scCO₂ with a 100% yield to aniline at a lower reaction temperature of 35 °C. The product aniline (organic phase) can be easily separated from the side‐product water (aqueous phase), solvent (scCO₂), and catalyst (solid) by a simple phase separation process. The hydrogenation of NB is a structure‐sensitive reaction in ethanol as well as in scCO₂ except for a few Pt/C catalysts in which the degree of metal dispersion is small (<0.08).     

Remarkable particle size effect in Rh-catalyzed enantioselective hydrogenations

A series of 0.5–4.3 wt% Rh/Al₂O₃ catalysts were prepared by flame synthesis. STEM indicated relatively narrow particle size distributions for all catalysts and the mean particle size increased almost linearly with the Rh content in the range 0.96–1.65 nm. A DRIFTS study of CO adsorption on as prepared Rh/Al₂O₃ and after heat treatment in hydrogen at 400 °C revealed that there was no Rh oxide present at the catalyst surface after the high temperature reduction, which procedure is commonly used prior to enantioselective hydrogenation. In the hydrogenation of ethyl pyruvate and ethyl 3-methyl-2-oxobutyrate the cinchona-modified 4.3 wt% Rh/Al₂O₃ gave considerably higher ee than those achieved with the best known Rh catalyst. A decrease of the metal loading and thus the mean Rh particle size, led to a loss of ee to (R)-lactate by a factor of up to seven at 1 bar and up to two at 10–100 bar. Our interpretation is that the performance of Rh/Al₂O₃ is strongly distorted at atmospheric pressure by catalyst deactivation due to the Al₂O₃-catalyzed aldol condensation of the substrate. During the fast reactions at 100 bar the contribution of strongly adsorbed impurities is small and the variation of ee is mainly due to an intrinsic particle size effect. The structure sensitivity observed under optimal conditions, at high surface hydrogen concentration, is mainly due to steric effects: a small, ca. 1 nm Rh particle cannot accommodate the enantiodifferentiating diastereomeric substrate–modifier complex and the hydrogenation on its surface leads to racemic product. A practical conclusion is that there is no advantage of using small nanoparticles and low metal loading in the enantioselective hydrogenation of α-ketoesters.     

Continuous Hydrogenation of Toluene over Sr‐ and PEG800‐Modified Ni/γ‐Al2O3 Catalysts

A series of γ‐Al₂O₃‐supported nickel‐based catalysts were evaluated in continuous hydrogenation of toluene. Sr‐ and poly(ethylene glycol) 800 (PEG800)‐modified Ni/γ‐Al₂O₃ catalysts provided the best activity with high conversion of toluene and selectivity for methylcyclohexane which was ascribed to the addition of Sr and PEG800 during the preparation process, resulting in smaller and highly dispersed Ni species on the surface and in the pores of γ‐Al₂O₃. Furthermore, the formation of SrCO₃ and NiAl₂O₄ is believed to be advantageous for the dispersion and stabilization of the active Ni species, accounting for its good stability.     

Enantioselective Hydrogenation of N‐Acetyldehydroamino Acids over Supported Palladium Catalysts

The enantioselective hydrogenation of two N‐acetyldehydroamino acids over Cinchona alkaloid‐modified, supported palladium catalysts has been studied. Moderate enantioselectivities, up to 36 %, were obtained in the hydrogenation of 2‐acetamidocinnamic acid over cinchonidine‐modified Pd/TiO₂ under low hydrogen pressure. Increase in the pressure or use of benzylamine as additive led to a gradual decrease in the enantiomeric excess and eventually inversion of the sense of the enantioselectivity. On the contrary, the optical purity of the product resulting from the hydrogenation of 2‐acetamidoacrylic acid was significantly increased by addition of benzylamine to the reaction mixture. Enantiomeric excess values up to 58 % and 60 % were obtained over Pd/Al₂O₃ modified by cinchonidine and cinchonine, respectively. These optical purities are the best obtained in the hydrogenation of dehydroamino acid derivatives over chirally modified heterogeneous metal catalysts.     

Millisecond catalytic reforming of monoaromatics over noble metals

The millisecond autothermal reforming of benzene, toluene, ethylbenzene, cumene, and styrene were independently studied over five noble metal‐based catalysts: Pt, Rh, Rh/γ‐Al₂O₃, Rh–Ce, and Rh–Ce/γ‐Al₂O₃, as a function of carbon‐to‐oxygen feed ratio. The Rh–Ce/γ‐Al₂O₃ catalyst exhibited the highest feedstock conversion as well as selectivities to both synthesis gas and hydrocarbon products (lowest selectivities to H₂O and CO₂). Experimental results demonstrate a high stability of aromatic rings within the reactor system. Benzene and toluene seem to react primarily heterogeneously, producing only syngas and combustion products. Ethylbenzene and cumene behaved similarly, with higher conversions than benzene and toluene, and high product selectivity to styrene, likely due to homogeneous reactions involving their alkyl groups. Styrene exhibited low conversions over Rh–Ce/γ‐Al₂O₃, emphasizing the stability of styrene in the reactor system. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Rate measurement by pulse technique for individual steps involved in CO hydrogenation and its application to catalyst design

By using pulse reaction technique, the rates were measured for the individual steps involved in CO hydrogenation over transition metal catalysts with and without promoters. On all the transition metals other than Rh, the rate constant for the dissociation of C-O bond (k ₁) was much smaller than that for the hydrogenation of surface carbon species (k ₂). The oxides of V, Nb, Mo, and W added to Ru/Al₂O₃ increasedk ₁ and decreasedk ₃. The Rh catalyst was unique in the sense that there was not much difference betweenk ₁ andk ₂. The characteristic feature observed was found to be useful in designing a catalyst for the production of liquid fuel or C₂-oxygenates from syngas.     

Study on liquid‐phase hydrogenation of paranitrotoluene over Ru‐based catalysts

This paper presented a study on the role of yttrium addition to Ru‐based catalysts for liquid phase paranitrotoluene hydrogenation reaction. An impregnation‐precipitation method was used for preparation of a series of yttrium doped Ru/NaY catalysts with yttrium content in the range of 0.0026–0.0052 g/g. Properties of the obtained samples were characterized and analyzed by X‐ray diffraction (XRD), H₂‐TPR, Transmission electron microscopy (TEM), ICP atomic emission spectroscopy, and Nitrogen adsorption‐desorption. The results revealed that catalytic activity of NaY supported Ru catalysts increased with the yttrium content at first, then decreased with the further increase of yttrium content. When yttrium content was 0.0033 g/g, a Ru‐Y/NaY² catalyst showed the most excellent performance of paranitrotoluene hydrogenation reaction (paranitrotoluene conversion and the selectivity toward P‐methyl‐cyclohexylamine reached 99.9 % and 82.5 %, respectively). In addition, to compare with the performance of Ru‐Y/NaY catalysts, the active carbon supported Ru catalysts were prepared using the same method in view of its higher surface area and adsorption capacity. Finally, the effect of solvent on the reaction over Ru‐Y/NaY² catalyst has been investigated, it was found that the best performance of paranitrotoluene hydrogenation reaction took place in protic solvents (isopropanol and ethanol). This was mainly ascribed to their polarity and hydrogen‐bond accepting capability.

     

Catalytic degradation of aqueous Fischer–Tropsch effluents to fuel gas over oxide‐supported Ru catalysts and hydrothermal stability of catalysts

BACKGROUND: The catalytic degradation of aqueous Fischer–Tropsch (FT) effluents to fuel gas over Ru/AC has been investigated. In order to understand the catalytic performance and stability of oxide‐supported Ru catalysts, several oxide supports (titania, zirconia, γ‐alumina and silica) were selected for study, with a focus on the hydrothermal stability of catalysts. RESULTS: The catalytic efficiency for transforming the oxygenates in aqueous FT effluents to C₁–C₆ alkanes decreased in the order: Ru/ZrO₂～ Ru/TiO₂ > Ru/SiO₂ > Ru/Al₂O₃. The conversion of alcohols was greatly suppressed over Ru/γ‐Al₂O₃. The former two catalysts (Ru/ZrO₂ and Ru/TiO₂) exhibited enhanced efficiency and long‐term stability (400 h) relative to Ru/SiO₂ and Ru/Al₂O₃. N₂‐physisorption, XRD and SEM showed that titania and zirconia exhibited high structural stability in an aqueous environment. However, the structures of γ‐alumina and silica were unstable due to significant drop in surface area and adverse changes in surface morphology. Especially for the case of the Ru/γ‐Al₂O₃ catalyst, the γ‐alumina was transformed into boehmite structure after reaction, and metal leaching and carbon deposition were extensive. CONCLUSION: Ru/ZrO₂ or Ru/TiO₂ may be a promising alternative for degrading aqueous FT effluents due to their long‐term stability. Copyright © 2012 Society of Chemical Industry     

Nitrogen Tolerant Hydrodesulfurization Catalysts Based on Rh and Ru Promoted Mo/Al2O3

Rh and Ru promoted Mo/Al₂O₃ catalysts were tested in HDS of thiophene in the presence of different amounts of pyridine and compared with CoMo/Al₂O₃. The Rh and Ru promoted catalysts were more nitrogen tolerant and in the presence of pyridine showed higher HDS activities than CoMo/Al₂O₃. This was explained by higher C–N bond hydrogenolysis activity and high nitrogen tolerance of the free Rh and Ru sulfides in the promoted catalysts.     

Selective hydrogenation of (E)-2-hexenal to (E)-2-hexen-1-ol over Co-based bimetallic catalysts

Hydrogenation of (E)-2-hexenal was carried out in a liquid phase using Co-based bimetallic catalysts (M–Co/Al₂O₃, M=Pd, Pt, Ru, Rh, Sn, Fe, or Cu). Pd–Co/Al₂O₃ showed the highest activity among the catalysts tested and catalyzed the hydrogenation of CC bond predominantly to produce hexanal and 1-hexanol. Pt–Co/Al₂O₃ was more active than monometallic Co/Al₂O₃ for the hydrogenation of CO bond. The excellent result, 92% selectivity to (E)-2-hexen-1-ol formation at 90% conversion, was obtained by the hydrogenation over Pt–Co/Al₂O₃ bimetallic catalyst. No improved activities were observed for the other bimetallic catalysts.     

Carbon‐Carbon Double Bond versus Carbonyl Group Hydrogenation: Controlling the Intramolecular Selectivity with Polyaniline‐Supported Platinum Catalysts

The use of polyaniline (PANI) as catalyst support for heterogeneous catalysts and their application in chemical catalysis is hitherto rather poorly known. We report the successful synthesis of highly dispersed PANI‐supported platinum catalysts (particle sizes between 1.7 and 3.7 nm as revealed by transmission electron microscopy, TEM) choosing two different approaches, namely (i) deposition‐precipitation of H₂PtCl₆ onto polyaniline, suspended in basic medium (DP method) and, (ii) immobilization of a preformed nanoscale platinum colloid on polyaniline (sol‐method). The PANI‐supported platinum catalysts were applied in the selective hydrogenation of the α,β‐unsaturated aldehyde citral. In order to benchmark their catalytic performance, citral hydrogenation was also carried out by using platinum supported on the classical support materials silica (SiO₂), alumina (Al₂O₃), active carbon and graphite. The relations of the structural characteristics and surface state of the catalysts with respect to their hydrogenation properties have been probed by EXAFS and XPS. It is found that the DP method yields chemically prepared PtO₂ on polyaniline and, thus, produces a highly dispersed and immobilized Adams catalyst (in the β‐PtO₂ form) which is able to efficiently hydrogenate the conjugated CC bond of citral (selectivity to citronellal=87%), whereas reduction of the CO group occurs with polyaniline‐supported platinum (selectivity to geraniol/nerol=78%) prepared via the sol‐method. The complete reversal of the selectivity between the preferred hydrogenation of the conjugated CC or CO group is not only particularly useful for the selective hydrogenation of α,β‐unsaturated aldehydes but also unveils the great potential of conducting polymer‐supported precious metals in the field of hitherto barely investigated chemical catalysis.     

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.     

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     

Catalytic Hydroprocessing of p-Cresol: Metal, Solvent and Mass-Transfer Effects

A systematic study of the comparative performances of supported Pt, Pd, Ru and conventional CoMo/Al₂O₃, NiMo/Al₂O₃, NiW/Al₂O₃ catalysts as well as the effects of solvent, H₂ pressure and temperature on the hydroprocessing activity of a representative model bio-oil compound (e.g., p-cresol) is presented. With water as solvent, Pt/C catalyst shows the highest activity and selectivity towards hydrocarbons (toluene and methylcyclohexane), followed by Pt/Al₂O₃, Pd and Ru catalysts. Calculations indicate that the reactions in aqueous phase are hindered by mass-transfer limitations at the investigated conditions. In contrast, with supercritical n-heptane as solvent at identical pressure and temperature, the reactant and H₂ are completely miscible and calculations indicate that mass-transfer limitations are eliminated. All the noble metal catalysts (Pt, Pd and Ru) show nearly total conversion but low selectivity to toluene in supercritical n-heptane. Further, conventional CoMo/Al₂O₃, NiMo/Al₂O₃ and NiW/Al₂O₃ catalysts do not show any hydrodeoxygenation activity in water, but in supercritical n-heptane, CoMo/Al₂O₃ shows the highest activity among the tested conventional catalysts with 97?% selectivity to toluene. Systematic parametric investigations with Pt/C and Pt/Al₂O₃ catalysts indicate that with water as the solvent, the reaction occurs in a liquid phase with low H₂ availability (i.e., low H₂ surface coverage) and toluene formation is favored. In supercritical n-heptane with high H₂ availability (i.e., high H₂ surface coverage), the ring hydrogenation pathway is favored leading to the high selectivity to 4-methylcyclohexanol. In addition to differences in H₂ surface coverage, the starkly different selectivities between the two solvents may also be due to the influence of solvent polarity on p-cresol adsorption characteristics.     

Immobilization of Optically Active Rhodium‐Diphosphine Complexes on Porous Silica via Hydrogen Bonding

The optically pure Rh(I)‐diphosphine complexes [((R)‐(R)‐BDPBzPSO₃)Rh(nbd)] ( 1 ), [((+)‐DIOP)Rh(nbd)]OTf ( 2 ), and [((S)‐BINAP)Rh(nbd)]OTf ( 3 ) have been tethered to silica via hydrogen‐bonding interaction of isolated silanols with a sulphonate group in either the chiral ligand or the counteranion. A preliminary study of the potential of the resulting supported hydrogen‐bonded (SHB) catalysts in the heterogeneous enantioselective hydrogenation of prochiral olefins is reported.     

Platinum group metals as catalysts in enantioselective 1-phenylpropane-1,2-dione hydrogenation

Different γ-Al₂O₃ supported Ir, Pd, Ru, Rh and Pt catalysts were tested in enantioselective 1-phenylpropane-1,2-dione hydrogenation using cinchona alkaloid modifiers. Activity and enantioselectivity over Ir and Ru catalysts were low. Pd catalyst was active in the hydrogenation of 1-phenylpropane-1,2-dione, however, the enantioselectivity over this catalyst was almost negligible. Over Pd hydrogenation proceeded mainly via hydrogenation of the C₁O₁ carbonyl group, which is attached to the phenyl ring. Hydrogenation over Pd did not proceed in the second hydrogenation step via an enol form as found for ethyl pyruvate hydrogenation over Pd. The structure-selectivity relationship and solvent effects are similar over Pt and Rh in the first hydrogenation step. However, in the second hydrogenation step of hydroxyketones to diols large mechanistical differences between Pt and Rh were observed. Although the activity over Rh catalysts was lower than over Pt after optimization the best result obtained with Rh/γ-Al₂O₃ (5754 Lancaster) was 60% ee in toluene at maximum yield of 28%, which makes Rh a promising metal for enantioselective hydrogenation.     

A Novel Ruthenium‐Phosphorus Amorphous Alloy Catalyst for Maltose Hydrogenation to Maltitol

A ruthenium‐phosphorus (Ru‐P) amorphous alloy catalyst was prepared by chemical reduction of ruthenium(III) ions [Ru³⁺] with hypophosphite [H₂PO₂⁻] in aqueous solution and was applied to the liquid‐phase hydrogenation of maltose. In comparison with other reference catalysts, Ru‐P showed significant activity as evident in the order: Ru‐P> Ru‐B≫ Ni‐P> Co‐P≫ Raney Ni. Furthermore, this catalyst was also found to be more durable during this hydrogenation process. Special emphasis was laid on a comparative study of Ru‐P and Ru‐B catalysts to get an insight into the excellent catalytic performances of Ru‐P.     

Comparative study of aromatization selectivity during N‐heptane reforming on sintered Pt/Al2O3 and Pt‐Re/Al2O3 catalysts

BACKGROUND: The metal dispersed over a support can be present as small crystallites with sizes less than 5 nm. The smaller crystallites favour aromatization while larger crystallites favour cracking/hydrogenolysis. Sintering results in the agglomerization of smaller metal crystallites. Correlation of size with aromatization selectivity was investigated. RESULTS: The primary products of n‐heptane reforming on fresh Pt were methane, toluene, and benzene, while on fresh Pt‐Re, the only product was methane. Both catalysts exhibited enhanced aromatization selectivity at different oxygen sintering temperatures. The reaction products ranged from only toluene at 500 °C sintering temperature to methane at a sintering temperature of 650 °C with no reaction at 800 °C for the Pt/Al₂O₃ catalyst. On Pt‐Re/Al₂O₃ catalyst, methane was the sole product at a sintering temperature of 500 °C while only toluene was produced at a sintering temperature of 800 °C. CONCLUSION: This is the first time that sintering has been used to facilitate aromatization of supported Pt and Pt‐Re catalysts. A superior selectivity behaviour associated with bi‐metallic Pt catalysts is established. It was found that no reaction occurred on Pt catalyst after sintering at 800 °C whereas sintering Pt‐Re at 800 °C promoted aromatization solely to toluene. Copyright © 2008 Society of Chemical Industry     

Catalytic hydrogenation of linoleic acid over platinum-group metals supported on alumina

Catalytic activity and selectivity for hydrogenation of linoleic acid (cis-9,cis-12 18:2) were studied on Pt, Pd, Ru, and Ir supported on Al₂O₃. Stearic acid (18:0) and 10 different octadecenoic isomers (18:1) in the products could be separated completely by using a new capillary column coated by isocyanopropyl trisilphenylene siloxane for gas-liquid chromatography. The monoenoic acid isomers and dienoic acid isomers in the products on the various catalysts showed different distributions. The catalysts exhibited nearly equal selectivity for stearic acid formation. The 12-position double bond in linoleic acid has higher reactivity than the 9-position double bond in catalytic hydrogenation on platinum-group metal catalysts. In addition to hydrogenation products of linoleic acid, geometrical and positional dienoic acid isomers (trans-9,trans-12; trans-8,cis-12; cis-9,trans-13; trans-9,cis-13; cis-9,trans-12 18:2), due to isomerization of linoleic acid during hydrogenation, were contained in the reaction products. Ru/Al₂O₃ exhibited the highest activity for isomerization of linoleic acid with the noble metal catalysts. Conjugated octadecadienoic acid isomers have been observed in products of the reaction on Pt/Al₂O₃, Ru/Al₂O₃, and Ir/Al₂O₃. Catalytic activities of noble metals for positional and geometric isomerization of linoleic acid during hydrogenation decreased in the sequence of Ru ≥ Pt > Ir » Pd.     

Catalytic roles of metals and supports on hydrodeoxygenation of lignin monomer guaiacol

Hydrodeoxygenation of the lignin monomer guaiacol was performed on the bifunctional catalysts of noble-metals supported on acidic matrices. The catalytic roles of metal nanoparticles and acidic supports were elucidated using the acid-site-measurement-dependent catalysis results, which demonstrated that metals were responsible for the hydrogenation of aromatic rings while metal-deposited acidic supports were indispensable to the deoxygenation of oxygenates. Among various combinations of metals and supports, Rh/SiO₂-Al₂O₃ and Ru/SiO₂-Al₂O₃ exhibited the highest cyclohexane yields.