Carbon oxides hydrogenation on RhNa-Y: Addition of 1-butene

At 4.0 MPa, CO₂ hydrogenation over a RhNA-Y catalyst yields only saturated and essentially linear hydrocarbons (up to C₇). In contrast, CO hydrogenation gives a more complex mixture including olefins ( and ), paraffins (linear and branched) and oxygenates. Addition of 1-butene provides a plausible interpretation of these differences.     

A method to determine the relative value of the barriers of carbon chain growth and termination in Fischer-Tropsch synthesis: elementary reaction kinetics for chain growth

A method is established, by which the difference of the reaction activation barriers of carbon chain growth and termination in Fischer-Tropsch (FT) synthesis can be determined from experiments. A FT synthesis is carried out on Fe/Zn catalyst. We apply the method to analyze the experimental result and obtain the difference of reaction activation barriers of carbon chain growth and termination of -olefins on the catalyst.     

Stepwise Conversion of Methane over Supported Metal-Boron Amorphous Alloy Catalysts

The stepwise conversion of CH₄ to higher hydrocarbons over HMCM-22 zeolite supported metal-boron amorphous alloy catalysts has been investigated, including: (1) the influence of metals (Co, Ni, Pt, Rh and Ru) of the catalysts on the reaction; (2) the promotional effect of V on the catalytic behavior of the catalysts; (3) the influence of hydrogenation pressure and CH₄ decomposition temperature; and (4) the nature of carbon species. It is found that the yield of C⁺ ₂ hydrocarbons is strongly dependent on the metals. Good yields of C⁺ ₂ hydrocarbon are reached only on the supported NiB and CoB catalysts. The probability of C--C chain growth is increased by V promotion without seriously affecting the activities of CH₄ decomposition and hydrogenation. The ease of carbon removal via hydrogenation is strongly affected by the CH₄ decomposition temperature. Increasing hydrogenation temperature has a negative effect on the yield of C⁺ ₂ hydrocarbons. XPS measurements show that a carbide(-like) carbon species is active and selective for hydrogenation to produce higher hydrocarbons. Its activity/selectivity is greatly reduced at high CH₄ decomposition temperatures, mainly due to transition of the reactive carbidic to unreactive graphitic form. Graphite/filamental carbons were found to be formed at high CH₄ decomposition temperature.     

Isotopic Studies on the Mechanism of Partial Oxidation of CH4 to Syngas over a Ni/Al2O3 Catalyst

Silica-supported PdZn catalysts have been studied in CO and CO₂ hydrogenation and in ethylene hydroformylation. The dilution of surface Pd by Zn lowers the hydrogenating capability of the catalysts and favours the production of higher hydrocarbons in CO hydrogenation. The catalyst with a molar ratio Pd:Zn = 3 showed an enhanced ability to insert CO into an M–alkyl bond; this catalyst produced higher oxygenates in the CO hydrogenation and was the most active in all reactions studied.     

Ethylene dimerization over a model Sn/Pt(111) catalyst

The dimerization reaction of ethylene was studied over Pt(111) and (3×3)R30°-Sn/ Pt(111) model catalysts at moderate pressures (20–100 Torr). The catalyst surfaces were prepared and characterized in a UHV surface analysis system and moderate pressure catalytic reactions were conducted with an attached batch reactor. The overall catalytic activity of the (3×3)R30°-Sn/Pt(111) surface alloy for C₄ products was slightly higher than that at Pt(111). In addition to the dimerization reaction, hydrogenolysis of ethylene to propane and methane was also observed, with the (3×3)R30°-Sn/Pt(111) surface alloy less active than Pt(111). Among the C₄ products, butenes andn-butane were the major components. Carbon buildup was observed to be significant above 500 K with the (3×3)R30°-Sn/Pt(111) surface alloy much more resistant than Pt(111). The dimerization of ethylene was not eliminated by the presence of surface carbonaceous deposits and even at significant surface coverages of carbon the model catalysts exhibited significant activities. The results are discussed in terms of the surface chemistry of ethylene and the previously reported catalytic reactions of acetylene trimerization andn-butane hydrogenolysis at these surfaces.     

Monte Carlo simulations combined with UHV-atmospheric pressure reaction studies on CO hydrogenation on cobalt

We show that useful information on catalytic reactions can be obtained using Monte Carlo simulations combined with experimental data from model catalysts. The experimental rate dependencies of CO hydrogenation on the partial pressures were used to guide the selection of different parameter values used in the simulations. The results give the following picture of the reaction conditions on the surface: hydrogen and carbon monoxide occupy different adsorption sites, the diffusion of hydrogen and the growth of hydrocarbon chains are fast processes, and the rate-limiting elementary reaction step is the termination of the hydrocarbon chains (-hydrogenation). The formation of longer chain hydrocarbons falls onto the line defined by the Anderson-Flory-Schulz distribution but the value of the chain growth parameter , obtained in the simulations, is higher than the experimental value.     

Selective synthesis of α-olefins on Fe-Zn Fischer-Tropsch catalysts

Fe/Zn oxides promoted with K and Cu selectively produce -olefins at typical Fischer-Tropsch synthesis conditions (2/1 H₂/CO, 1 MPa, and 270°C). The simultaneous presence of K and Cu introduces a synergistic activity enhancement while maintaining the high olefin selectivity obtained by alkali promotion. Structural and morphological differences in Fe-Zn oxides prepared from ammonium glycolate complexes or precipitated from nitrate solutions have only a small influence on catalytic properties. Catalyst behavior is strongly influenced by synergistic promoter effects (Cu, K) and by the controlled in situ conversion of iron oxide precursors to carbides.     

In situ observation of methoxide hydrogenation on the Ni(111) surface

The surface chemistry of methoxide (CH₃O-) on the Ni(111) surface has been studied in the presence of hydrogen pressures up to 2 Torr. During heating in vacuum methoxide decomposes to H₂ and CO, which desorb at 380 and 445 K, respectively. The CH₃O-decomposition process is rate limited by CH bond breaking and exhibits a strong deuterium kinetic isotope effect in CD₃O-. In the presence of ambient hydrogen pressures of 0.02–2.0 Torr both CH₃O- and CD₃O-are hydrogenated directly to methanol at 310 K. Methoxide is hydrogenated by adsorbed hydrogen, which nearly saturates the surface at these pressures and temperatures.     

Activity and selectivity control in iron catalyzed Fischer-Tropsch synthesis

We present results of a catalyst structure-function study that supports a CO hydrogenation model with -olefins formed as the principal primary products and n-paraffins formed during secondary hydrogenation reactions. The interplay of catalyst composition and reaction environment controls the extent of secondary reactions. Catalysts that contain mainly oxidic phases or carbides with large concentrations of excess matrix carbon favor secondary reactions. The relative concentrations of oxide and carbide phases depends on the ease of reduction of the catalyst, which can be changed by cation substitutions. For example, cobalt substitution into Fe₃O₄ lowers the reduction temperature by 20 ° C. Excess matrix carbon has been intentionally introduced (by treatment in high temperature H₂/CO) into model iron carbide catalysts produced by laser synthesis. Increased paraffin selectivity as matrix carbon is introduced suggests the influence of the diffusion constraints on product selectivity. Alkali promotion will affect secondary hydrogenation pathways. We illustrate how catalysts with low levels of alkali become increasingly more selective to paraffins at high conversions (and high effective H₂/CO ratios).Reaction environment also controls catalyst composition and selectivity. Mossbauer spectroscopy on spent catalysts suggests that oxide/carbide phase formation in iron catalysts are sensitive to reactor configuration (extent of backmixing). In integral fixed bed reactors, catalysts partition into carbide phases in the front of the bed but show increasing amounts of oxide near the exit, whereas the catalysts in the stirred tank reactor remain all carbides. Product selectivities reflect the phase differences.Other examples illustrating secondary hydrogenation phenomena will be presented.     

Synergistic Effect in the Dehydrogenation of Cyclohexene on C/W(111) Surfaces Modified by Submonolayer Coverage Pt

The dehydrogenation and decomposition of cyclohexene on the Pt-modified C/W(111) surfaces have been studied by temperature-programmed desorption (TPD), Auger electron spectroscopy (AES) and high-resolution electron energy loss spectroscopy (HREELS). The objective of the current study is to investigate how the surface reactivity of tungsten carbide is modified by the presence of submonolayer Pt. Similar to that observed on Pt(111), Pt(100) and C/W(111) surfaces, the characteristic reaction pathway on Pt/C/W(111) is the selective dehydrogenation of cyclohexene to benzene. At a Pt coverage of 0.52 monolayer, the selectivity to the gas-phase benzene product is 86±7%, which is slightly higher than that on Pt(111) (75%) and on C/W(111) (67±7%). More importantly, the desorption of benzene on Pt/C/W(111) is a reaction-limited process that occurs at 290 K, which is much lower than the benzene desorption temperature of 400 K from Pt(111).     

Characteristics of Fe/AlPO4-5 catalyst prepared with organic solution

The Fe/AlPO₄-5 catalysts are prepared by impregnation with aqueous and organic solution (acetic acid, alcohol and acetone) of iron(III) nitrite respectively. The characterization of catalyst by means of XPS, Mössbauer spectroscopy, TPR and CO hydrogenation is reported. The catalyst prepared with the aqueous solution has no activity for CO hydrogenation because the Fe(III) in the catalyst cannot be reduced to -Fe. However, the catalysts prepared with organic solution possess obvious hydrogenation activity, in which -Fe is present in the initial reduced catalyst besides Fe³⁺ and Fe²⁺. The results may be explained by the interaction degrees between the metal and the support induced by the different impregnation solvents.     

Carbon monoxide induced desorption of alkanes and alkenes up to C8 after chemisorption of methane on platinum

CH₄ homologation can proceed under reductive conditions by successive exposures of various supported metals to CH₄ and H₂. When a Pt/SiO₂ catalyst is submitted to a CO dose after an exposure to CH₄ at a moderate temperature, several hydrocarbons are released with a large proportion of olefins. It may therefore be concluded that C-C bonding processes take place during the mere exposure of platinum to CH₄ at moderate temperatures.     

Explanations of the Formation of Branched Hydrocarbons During Fischer–Tropsch Synthesis by Alkylidene Mechanism

After completely hydrogenation of Fischer–Tropsch synthesis (FTS) products on Pt/C catalyst, the branched C₈, C₁₀, C₁₁, and C₁₂ alkanes were identified using authentic samples to be methyl branched alkanes in both Co and Fe catalyzed reactions. No detectable ethyl branched and dimethyl branched alkanes were observed. The total amount of branched hydrocarbons in Co catalyzed FTS is about 5 % while the amount of branched hydrocarbons in Fe catalyzed FTS is about 25 %. The branched hydrocarbon distribution of Co catalyzed FTS does not obey the Anderson–Schulz–Flory (ASF) kinetics and the branched hydrocarbon distribution of Fe catalyzed FTS shows larger α value than that of straight chain alkanes. These results were explained by the formation of 2-alkylidene through readsorbed 1-alkenes, which can grow to form methyl branched hydrocarbons. Since the rate of propagation of 2-alkylidene is much slower than that of ethylidene and 1-alkylidenes, a “kink” will result in C₂ in the FT product distribution plot.     

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.     

Racemic poly(α-alkyl-α-phenyl-βpropiolactone) substituent dependence

Summary The -phenyl--alkyl--propiolactones (alkyl groups =-CH₂CH₃; -CH₂CH₂CH₃; -CH₂CH₂CH₂CH₃) were synthesized and polymerized by anionic ring-opening polymerization by tetra-ethylammonium benzoate (non-chiralic initiator). It was found that the molecular weight of polylactones increased as the size of the alkyl substituent increased. Poly(-propyl--phenyl--propiolactone) showed the highest melting temperature. Lactones polymerized using tetraethylammonium dibenzoiltartrate (chiralic initiator) gave polyesters with higher melting temperature than those obtained with a non-chiralic initiator under the same conditions.     

Kinetics and Selectivity of the Fischer–Tropsch Synthesis: A Literature Review

A critical review of the kinetics and selectivity of the Fischer–Tropsch synthesis (FTS) is given. The focus is on reaction mechanisms and kinetics of the water–gas shift and Fischer–Tropsch (FT) reactions. New developments in the product selectivity as well as the overall kinetics are reviewed. It is concluded that the development of rate equations for the FTS should be based on realistic mechanistic schemes. Qualitatively, there is agreement that the product distribution is affected by the occurrence of secondary reactions (hydrogenation, isomerization, reinsertion, and hydrogenolysis). At high CO and H₂O pressures, the most important secondary reaction is readsorption of olefins, resulting in initiation of chain growth processes. Secondary hydrogenation of α-olefins may occur and depends on the catalytic system and the process conditions. The rates of the secondary reactions increase exponentially with chain length. Much controversy exists about whether these chain-length dependencies stem from differences in physisorption, solubility, or diffusivity. Preferential physisorption of longer hydrocarbons and increase of the solubility with chain length influences the product distribution and results in a decreasing olefin-to-paraffin ratio with increasing chain length. Process development and reactor design should be based on reliable kinetic expressions and detailed selectivity models.     

Influence of alkali metal (Li and Cs) addition to Mo2N catalyst for CO hydrogenation to hydrocarbons and oxygenates

The aim of this work is to understand the catalytic behaviour of Li and Cs promoted Mo₂N for CO hydrogenation to hydrocarbons and oxygenates at the reaction conditions 275–325 °C, 7 MPa, and 30 000 h^?1 GHSV. Molybdenum nitrides were synthesized via temperature programmed treatment of ammonium heptamolybdate (AHM) and alkali metal (AM) precursors under continuous gaseous ammonia flow. Unpromoted Mo₂N and AM‐Mo₂N catalysts were characterized using BET‐pore size, X‐ray diffraction, TPD‐mass of CO, HR‐TEM, and XPS techniques. Nominal loadings of 1, 5, and 10 wt% of Li and Cs were selected for these studies. At a 10 % CO conversion level, the total oxygenate selectivity of 28, 11, and 6.5 % was observed on 5Cs‐Mo₂N, 5Li‐Mo₂N, and unpromoted Mo₂N, respectively. The decreased oxygenate selectivity for unpromoted Mo₂N was mainly associated with CO dissociative hydrogenation on Mo^δ+ sites. On the other hand, improved molecular CO insertion into ?C_xH_y intermediate accelerates the total oxygenate formation on the Cs‐Mo‐N catalyst. However, during nitridation, crystal structure changes were observed in Li‐Mo‐N and the obtained oxygenates selectivity was attributed to the Li₂MoO₄ phases. At lower AM loadings, the active sites corresponding to oxygenates formation were inadequate, and at higher AM loadings, surface metallic molybdenum decreased the total oxygenate selectivity.

     

Author Index

The enantioselective hydrogenation of ethyl benzoylformate to (R)-ethyl mandelate over dihydrocinchonidine (DHCD)-modified Pt/Al₂O₃ catalyst in acetic acid was studied as a function of modifier concentration, hydrogen pressure and reaction temperature. The maximum enantioselectivity obtained under optimized conditions (DHCD concentration 1 mmol dm^-3, 25 bar H₂, 0 °C, AcOH/toluene 1:1) was 98% ee. The difference between the rates of racemic and enantioselective hydrogenation was less significant than in the case of ethyl pyruvate. This indicates that the high reaction rate in the enantioselective heterogeneous hydrogenation of -ketoesters is not a necessary condition of the chiral induction.     

Syntheses of diblock copolymers: poly(2-methylpropene)-b-poly(α-amino acid)

Summary Diblock copolymer poly (2-methylpropene)-b-poly (-amino acid) was obtained by polymerization of the corresponding N-caboxy anhydride initiated by a poly (2-methylpropene) bearing a terminal amine in dioxane/CH₂Cl₂ mixture. Copolymers were analyzed by FT IR, ¹H and ¹³C NMR. of the -amino acid segment determined by ¹H NMR fits well with those obtained by a theoretical calculation.     

Marked Role of Carbon Monoxide in the Formation of Benzene During CH4-CO Reaction Over Rh/SiO2 Catalysts

Temperature-programmed desorption (He-TPD) and temperature-programmed reaction with hydrogen (H₂-TPR), carbon monoxide (CO-TPR) or methane (CH₄-TPR) were carried out to elucidate the benzene formation mechanism as well as the role of CO during CH₄-CO reaction over SiO₂-supported Rh catalysts. The steady-state surface for the CH₄-CO reaction was different from that of the CH₄ decomposition reaction. The existence of benzene-like adsorbed species as building blocks was demonstrated on the CH₄-CO reaction surface, while no such higher hydrocarbon adsorbed species was detected in the case of the CH₄ decomposition surface. On the contrary, in CO-TPR experiments various unsaturated hydrocarbons were released from the steady-state CH₄ decomposition surface, which was not the case from the CH₄-CO reaction surface. It is concluded that adsorbed CO may play an important role to enhance the C-C bond formation of carbonaceous species, which correlates deeply with the novel phenomenon of selective benzene formation in the CH₄-CO reaction.