Applied Catalysis A, General (v.289, #2)
Affinity order among noble metals and CeO2 by Saburo Hosokawa; Makoto Taniguchi; Kazunori Utani; Hiroyoshi Kanai; Seiichiro Imamura (115-120).
The affinity order among four kinds of major noble metals (Ru, Rh, Pd, and Pt) and CeO2 was investigated by XRD and XAFS techniques. The states of noble metals supported on CeO2 and the sintering behavior of CeO2 on calcination differed depending upon the kinds of noble metals. Rh and Pd retained their Rh―O―Ce and Pd―O―Ce bonds even after calcination at 800 °C, and Pt kept its Pt―O―Ce bond at 500 °C, while Ru on CeO2 formed bulk RuO2 on calcination at temperatures as low as 500 °C. Rh effectively prevented sintering of CeO2. The order of the affinity of the noble metals toward CeO2 was assumed to be Rh > Pd > Pt > Ru.
Keywords: Noble metal–ceria interaction; Rh/CeO2; Pt/CeO2; Pd/CeO2; Ru/CeO2; XAFS; XRD;
Hydrogen production from methane by steam reforming in a periodically operated two-layer catalytic reactor by Vladimir Galvita; Kai Sundmacher (121-127).
Almost CO-free hydrogen gas, which could be directly used as a feed for a PEM fuel cell, can be produced by a novel steam reforming process of methane in a fixed bed reactor which contains two different catalysts layers which go through a periodic reduction/re-oxidation cycle. During the reduction phase, water-free methane is converted at the first layer (Pt–Ce0.5Zr0.5O2) into a mixture of СО and Н2 by partial oxidation. This mixture is fed directly to a second layer of Fe2O3–Ce0.5Zr0.5O2 where the products of the methane partial oxidation reduce the iron-containing catalyst whereby giving CO2 and H2O. During the re-oxidation phase of the cycle, the two-layer reactor is fed with pure steam which is converted into a CO-free hydrogen gas whereby leading to the re-oxidation of the catalytic materials in both layers so that that they can be reused in the next redox cycle. The present study reports the preparation and characterization of suitable catalytic materials, related activity tests and kinetic studies which demonstrate the feasibility of the proposed process concept.
Keywords: Methane; Hydrogen; Steam reforming; Periodic process; Redox process; Iron; Ce doped by Zr;
Zeolite-based organic–inorganic hybrid catalysts for phosgene-free and solvent-free synthesis of cyclic carbonates and carbamates at mild conditions utilizing CO2 by R. Srivastava; D. Srinivas; P. Ratnasamy (128-134).
As-synthesized zeolite-beta exhibits high catalytic activity for the synthesis of cyclic carbonates and alkyl and aryl carbamates by a phosgene-free route, utilizing the greenhouse effect gas CO2. The reaction occurs with high yields of the desired products at mild conditions and without using any solvent or cocatalyst. Cyclic carbonates are synthesized by cycloaddition reaction of CO2 with oxiranes (epichlorohydrin, propene oxide, styrene oxide and n-butene oxide) at 393 K and 6.9 bar. Alkyl and aryl carbamates are synthesized by the reaction of the corresponding amines, CO2 and n-butyl bromide at 353 K and 3.4 bar. The as-synthesized zeolite-beta containing the encapsulated quaternary ammonium ions is not only reusable in several recycling experiments, but also shows superior activity to that of the corresponding homogeneous, quaternary ammonium halide salt generally used in the commercial synthetic practice. The microporous silica (inorganic) acting in concert with the encapsulated organic component constitutes an efficient, recyclable catalyst for this reaction.
Keywords: Carbon dioxide utilization; Cyclic carbonate; Polycarbonate; Alkyl and aryl carbamates; Zeolite-beta; Phosgene-free synthetic route;
Fischer–Tropsch synthesis: Water effects on Co supported on narrow and wide-pore silica by Ajay K. Dalai; Tapan K. Das; Karuna V. Chaudhari; Gary Jacobs; Burtron H. Davis (135-142).
The effect of water on the performance of narrow and wide-pore silica-supported cobalt catalysts was investigated during Fischer–Tropsch synthesis in a continuously stirred tank reactor (CSTR). In these studies the added water replaced an equivalent amount of inert gas so that all other reaction conditions remained the same before, during and after water addition. A low cobalt loading of 12.4 wt.% on wide-pore silica exhibited a beneficial effect on CO conversion with the addition of water up to 25 vol.% of the total feed. In contrast, the addition of up to 20 vol.% water to a 20 wt.% Co on narrow- or wide-pore silica did not significantly alter the CO conversion. It appears that the CO conversion mainly increases when cobalt clusters are small enough to fit inside the silica pores.
Keywords: Fischer–Tropsch synthesis; Wide-pore silica; Cobalt catalyst; Water, effect on FTS; Silica, pore size effect;
Fries rearrangement of phenyl acetate over sulfonic modified mesostructured SBA-15 materials by Rafael van Grieken; Juan A. Melero; Gabriel Morales (143-152).
Arenesulfonic modified mesostructured SBA-15 is shown as an active catalyst in the liquid-phase Fries rearrangement of phenyl acetate. The arenesulfonic acid-centers located within the structure of SBA-15 show high catalytic performance as compared to other homogeneous and heterogeneous acid catalysts. This high activity is accompanied with a remarkable stability without leaching of sulphur species during the reaction. Reaction conditions have been investigated in order to optimize production of hydroxyacetophenones. Strong adsorption of reaction products and/or coke deposition on the sulfonic acid sites rapidly deactivates the catalyst. Interestingly, when dichloromethane is used as solvent the deactivation process is slowed down drastically. Finally, this contribution supports a new application of these sulfonated mesostructured materials in the production of fine chemicals.
Keywords: Fries rearrangement; Phenyl acetate; Sulfonic acid; SBA-15 and hydroxyacetophenones;
Methanol steam reforming over Pd/ZnO and Pd/CeO2 catalysts by Easwar S. Ranganathan; Shyamal K. Bej; Levi T. Thompson (153-162).
The goal of work described in this paper was to better understand the methanol steam reforming (MSR) activity and selectivity patterns of ZnO and CeO2 supported Pd catalysts. This reaction is being used to produce H2-rich gas for a number of applications including hydrogen fuel cells. The Pd/ZnO catalysts had lower MSR rates but were more selective for the production of CO2 than the Pd/CeO2 catalysts. The CH3OH conversion rates were proportional to the H2 chemisorption uptake suggesting that the rate determining step was catalyzed by Pd. The corresponding turnover frequencies averaged 0.8 ± 0.3 s−1 and 0.4 ± 0.2 s−1 at 230 °C for the Pd/ZnO and Pd/CeO2 catalysts, respectively. The selectivities are explained based on the reaction pathways, and characteristics of the support. The key surface intermediate appeared to be a formate. The ZnO supported catalysts had a higher density of acidic sites and favored pathways where the intermediate was converted to CO2 while the CeO2 supported catalysts had a higher density of basic sites and favored the production of CO.
Keywords: Methanol steam reforming; Pd/ZnO; Pd/CeO2; HCHO; HCOOH; HCOOCH3; Acid–Base; NH3 TPD; CO2 TPD; In situ DRIFTS;
Hydrodesulfurization of sulfur-containing polyaromatic compounds in light gas oil using noble metal catalysts by Atsushi Ishihara; Franck Dumeignil; Jeayoung Lee; Kouhei Mitsuhashi; Eika W. Qian; Toshiaki Kabe (163-173).
We systematically monitored the hydrodesulfurization (HDS) activity of dibenzothiophene (DBT) and groups of substituted DBTs present in a SR-LGO over various noble metal catalysts (Ru, Rh, Ru-Rh, Pt, Pd and Pt-Pd) supported on alumina. The catalytic performances were compared to those obtained over a conventional CoMo catalyst. The Pd-based catalysts exhibited excellent HDS performances, especially for desulfurizing the refractory compounds. In particular, the 4,6-dimethyldibenzothiophene (4,6-DMDBT) HDS activity over the Pd or the Pd-Pt catalyst was equivalent to that over the CoMo catalyst. This was attributed to the exceptional hydrogenation (HYD) properties of the Pd-based catalysts, which enable desulfurization of the refractory compounds by considerably minimizing the effect of the steric hindrance due to their substituents. The synergetic effect observed on the bimetallic Pt-Pd system at low temperature for all the sulfur compounds was largely attenuated in the high temperature range, probably due to shifting of the HYD/dehydrogenation equilibrium to dehydrogenation. Despite a much lower metal loading (0.25 wt.%), the performances of the Rh catalyst were superior to those of the optimized 16 wt.% Ru catalyst. A synergetic effect was observed on the Ru-Rh catalyst, on which the DBT HDS activity was significantly enhanced compared to the activity of each corresponding monometallic catalyst.Further, we recently showed that the use of SiO2 or SiO2-Al2O3 as a support allows higher optimal Rh contents, making the Rh-based HDS catalysts even more promising. In brief, we obtained encouraging results, which showed for instance that the excellent properties of the noble metal catalysts in model HDS reactions are still observed during the HDS treatment of real feeds.
Keywords: Platinum; Palladium; Ruthenium; Rhodium; CoMo; Hydrodesulfurization; SR-LGO; DBT; 4-MDBT; 4,6-DMDBT; Substituted dibenzothiophenes;
Highly selective methoxycarbonylation of aliphatic diamines with methyl phenyl carbonate to the corresponding methyl N-alkyl dicarbamates by Tsutomu Yoshida; Masaaki Sasaki; Fumiaki Hirata; Yukio Kawamani; Koji Inazu; Akio Ishikawa; Kazuhito Murai; Tsuneo Echizen; Toshihide Baba (174-178).
The reactivity parameters of aliphatic diamines such as 1,6-hexanediamine, 1,3-bis(aminomethyl)benzene and 1,3-bis(aminomethyl)cyclohexane, with methyl phenyl carbonate, were investigated under mild conditions. Methoxycarbonylation of aliphatic diamines with methyl phenyl carbonate proceeds selectively in the absence of a catalyst, quantitatively converting aliphatic diamines to the corresponding methyl N-alkyl dicarbamates at 323–363 K.
Keywords: Methoxycarbonylation; Methyl phenyl carbonate; 1,6-Hexanediamine; 1,3-Bis(aminomethyl)benzene; 1,3-Bis(aminomethyl)cyclohexane; 5-Amino-1,3,3-trimethylcyclohexanemethylamine;
A novel BN supported bi-metal catalyst for selective hydrogenation of crotonaldehyde by Jeffrey C.S. Wu; Wei-Chih Chen (179-185).
A series of boron nitride (BN) supported Pt-Sn catalysts was prepared with a co-incipient wetness method employing hexachloroplatinic acid and tin(II) chloride. Pt loading was fixed at 1.1 wt%; Sn loading varied from 0.25 to 0.75 wt% on BN support. Selective hydrogenation of gas-phase crotonaldehyde was conducted in a steady-state flow reactor with temperatures ranging from 40 to 100 °C. The selectivity of crotyl alcohol reached over 80%. An optimum yield of crotyl alcohol reached 38% at 60% conversion of crotonaldehyde at 80 °C using Pt-Sn(0.75)/BN catalyst, while Pt-Sn(0.75)/γ-Al2O3 yielded less crotyl alcohol and a lower rate of crotonaldehyde conversion. The maximum yield rate of crotyl alcohol was 2.4 mmol/(g-cat. h) at 80 °C. Negligible deactivation was found during reaction for 4–6 h. The crystalline phases of PtSn and SnPt3 alloys were observed from the XRD spectra of Pt-Sn/BN catalysts with various Sn loadings. The selectivity of crotyl alcohol increased with Sn loadings but the activity values of the catalysts went through a maximum. The H2 reduction at 300 °C gave an optimum Pt–Sn alloy particle size so that the selectivity of crotyl alcohol increased without losing catalyst activity. The C＝O bond of crotonaldehyde was preferentially hydrogenated and the hydrogenation of C＝C bond was suppressed, resulting in the increase of crotyl alcohol selectivity.
Keywords: Pt-Sn/BN catalyst; Non-oxide support; Selective hydrogenation; Crotonaldehyde;
Enhancement of photocatalytic activity of mesoporous TiO2 by using carbon nanotubes by Ying Yu; Jimmy C. Yu; Jia-Guo Yu; Yuk-Chun Kwok; Yan-Ke Che; Jin-Cai Zhao; Lu Ding; Wei-Kun Ge; Po-Keung Wong (186-196).
Titanium dioxide/carbon nanotubes (TiO2/CNTs) composites were prepared with the aid of ultrasonic irradiation. Products of different TiO2:CNTs molar ratio were characterized by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) adsorption analysis, thermogravimetric and differential thermal analysis (TGA–DSC), photoluminescence (PL) and UV–vis spectroscopy measurements (UV–vis). The photocatalytic activity was evaluated by the degradation of acetone and by the detection of the hydroxyl radical (·OH) signals using electron paramagnetic resonance (EPR). It is found that the crystalline TiO2 is composed of both anatase and brookite phases. The agglomerated morphology and the particle size of TiO2 in the composites change in the presence of CNTs. The CNTs in the composites are virtually all covered by TiO2. Other than an increase of the surface area, the addition of CNTs does not affect the mesoporous nature of the TiO2. Meanwhile, more hydroxyl groups are available on the surface of the composite than in the case of the pure TiO2. The higher the content of CNTs, there is more effective in the suppression of the recombination of photo-generated e−/h+ pairs. However, excessive CNTs also shield the TiO2 from absorbing UV light. The optimal amount of TiO2 and CNTs is in the range of 1:0.1 and 1:0.2 (feedstock molar ratio). These samples have much more highly photocatalytic activity than P25 and TiO2/activated carbon (AC) composite. The mechanism for the enhanced photocatalytic activity of TiO2 by CNTs is proposed.
Keywords: Carbon nanotubes; Photocatalytic activity; Organic pollutant; TiO2; Sonochemistry; Enhancement; Hydroxyl radical;
Selective oxidation of 5-hydroxymethyl-2-furaldehyde to furan-2,5-dicarboxaldehyde by catalytic systems based on vanadyl phosphate by Carlo Carlini; Pasquale Patrono; Anna Maria Raspolli Galletti; Glauco Sbrana; Vitezslav Zima (197-204).
The selective oxidation of 5-hydroxymethyl-2-furaldehyde (HMF) to furan-2,5-dicarboxaldehyde (FDA) has been studied under different conditions, in terms of reaction temperature, oxidant agent (air, oxygen and their pressure), type of solvent (water, organic), in the presence of heterogeneous catalysts based on VOPO4·2H2O (VOP). The attempt of obtaining in a one-pot reaction FDA from fructose passing through HMF as an intermediate product failed both in water and in a mixed water/methyl isobutyl ketone (MIBK) medium. The best performances in the direct oxidation of HMF to FDA have been obtained when VOP and N,N-dimethylformamide (DMF) were used as a heterogeneous catalyst and a reaction medium, respectively, under mild conditions (100 °C and room pressure of O2). Productivity up to 8.13 mmol of FDA/(g catalyst h) was achieved with 95% selectivity at low HMF conversion. The modification of the heterogeneous catalysts, obtained by partial substitution of VO3+ with different metal cations (Fe3+, Cr3+, Ga3+, Mg2+, Cu2+ and Pd2+), did not cause any improvement on the performances in this reaction.
Keywords: 5-Hydroxymethyl-2-furaldehyde oxidation; Furan-2,5-dicarboxaldehyde; Heterogeneous catalysis; Vanadyl phosphate catalysts; Metals modified vanadyl phosphate catalysts;
Influence of the Si/Al ratio in the hydroisomerization of n-octane over platinum and palladium beta zeolite-based catalysts with or without binder by Antonio de Lucas; María Jesús Ramos; Fernando Dorado; Paula Sánchez; José Luís Valverde (205-213).
The influence of the Si/Al ratio on catalytic performance of palladium and platinum beta zeolite-based catalysts with or without binder in the hydroisomerization of n-octane was studied.In order to characterize the samples, temperature-programmed desorption of ammonia, surface area measurements, atomic absorption spectroscopy, inductively coupled plasma emission spectrophotometry and chemisorption measurements were used.The catalytic activity of beta zeolite catalysts decreased with increasing Si/Al ratios in samples with or without binder. As expected, a high Si/Al ratio means a low acid site density. However, the acid strength of the medium-strong acid sites increased with increasing values of this ratio.The isomer selectivity in the palladium catalysts increased with the Si/Al ratio (from 54.3 to 67.8 mol% in samples without binder). The medium-strong acid sites strength of the beta catalysts increased with increasing Si/Al ratios allowing relatively longer residence time of the intermediate carbenium ions on the acid sites. The opposite effect was observed in the platinum catalysts (isomer selectivity shifts from 96.6 towards 78.2 mol% in samples without binder). The presence of hydrogenolysis products in samples with low aluminium content decreased the octane isomer formation. The same effect was observed in the agglomerated catalysts but not as pronounced as in the non-agglomerated ones.
Keywords: Beta zeolite; Hydroisomerization; Binder; Si/Al;
Partial oxidation of methane to syngas over Ni/MgO and Ni/La2O3 catalysts by J. Requies; M.A. Cabrero; V.L. Barrio; M.B. Güemez; J.F. Cambra; P.L. Arias; F.J. Pérez-Alonso; M. Ojeda; M.A. Peña; J.L.G. Fierro (214-223).
Lanthana- and magnesia-supported nickel catalysts with Ni-loading ranging from 10 to 30 wt.% and calcined at temperatures 1073–1273 K are prepared and tested in the catalytic partial oxidation of methane. These catalysts are characterized by means of X-ray diffraction (XRD), N2 adsorption–desorption measurements, temperature-programmed reduction (TPR), thermogravimetric analysis (TG) and X-ray photoelectron spectroscopy (XPS) techniques. The main objective of this work is the development of more efficient catalysts to produce syngas (H2 + CO) from methane. For the catalysts developed the influence of the nickel content and the support on the activity are studied in a series of nickel catalysts.
Keywords: Methane; Catalytic partial oxidation; Nickel catalysts; Magnesia; Lanthana; Catalyst characterization; Syngas; Hydrogen;
Photocatalysed reaction of few selected organic systems in presence of titanium dioxide by M. Muneer; D. Bahnemann; M. Qamar; M.A. Tariq; M. Faisal (224-230).
The photocatalysed reaction of five selected organic systems, namely 2,2′-dinitro biphenyl (1), N,N′-dimethyl-4-nitroso aniline (2), 4-dimethyl amino benzaldehyde (3), phthalaldehyde (4) and tetramethyl benzoquinone (5) has been investigated in acetonitrile/water mixture in the presence of titanium dioxide and oxygen. An attempt has been made to identify the products formed during the photooxidation process through GC/MS analysis technique. The photolysis of 2,2′-dinitro biphenyl (1) leads to the formation of 2-hydroxy-2′-nitro biphenyl (6), 2,2′-dihydroxy biphenyl (7) and 2-biphenyl amine (11); whereas, N,N′-dimethyl-4-nitroso aniline (2) gave rise to N,N′-dimethyl-4-nitro aniline (14) and N,N′-dimethyl-1,4-benzenediamine (15). However, the photolysis of 4-dimethyl amino benzaldehyde (3) yielded only a demethylated product (18). The illumination of phthalaldehyde (4), on the other hand, led to the formation of two products such as 1(3H)-isobenzofuranone (24) and 2-formyl-benzoic acid (21). Two intermediate products, tetramethyl hydroquinone (35) and 2,5-furandione-3,4-dimethyl (34) have been identified in case of tetramethyl benzoquinone (5). The products have been identified by comparing the molecular ion and mass fragmentation peaks of the products with those reported in the GC/MS library. A probable mechanism for the formation of the products has been proposed.
Keywords: Photocatalysis; Titanium dioxide; Phthalaldehyde; 2,2′-Dinitro biphenyl; 4-Dimethyl amino benzaldehyde;
Fe:Co/TiO2 bimetallic catalysts for the Fischer–Tropsch reaction by D.J. Duvenhage; N.J. Coville (231-239).
A series of catalysts, Fe:Co/TiO2, were made with different total metal loadings (Fe:Co wt.% loading; 1:1–5:5), with different Fe:Co ratios (at 10% metal loading) and using different synthetic procedures (impregnation versus physical mixing). The catalysts were characterised by BET, CO-chemisorption and Mössbauer spectroscopy. Their Fischer–Tropsch (FT) activity/selectivity properties were also measured. The co-impregnated catalysts revealed that the 5:5 Fe:Co catalyst showed higher activity than catalysts prepared with either 2.5:7:5 or 7.5:2.5 Fe:Co ratios. Physically mixed samples of Fe/TiO2 and Co/TiO2 showed different selectivities and also gave better FT activity than equivalent co-impregnated Fe:Co samples. The reducibility behaviour and Mössbauer spectra of the Fe:Co samples suggested that the catalyst activity and selectivity changes related to the Co phase, Fe–Co alloy formation and the ability of the Co to enhance the reducibility of the Fe. A relationship between reduction level and activity was observed. The bimetallic systems also showed good stability and only modest changes in selectivity with time on line.
Keywords: Fischer–Tropsch; Bimetallic; Iron; Cobalt; Mössbauer;
Kinetics of the partial oxidation of methanol over a Fe-Mo catalyst by S.A.R.K. Deshmukh; M. van Sint Annaland; J.A.M. Kuipers (240-255).
The intrinsic steady-state kinetics of the partial oxidation of methanol to formaldehyde over a commercial Fe-Mo catalyst has been studied experimentally in a differentially operated reactor at temperatures of 230–260 °C, over a wide range of methanol and oxygen concentrations. The principal products found were formaldehyde, water, dimethyl ether (DME) and dimethoxymethane (DMM). The kinetics of the formaldehyde formation from methanol could be well described with Langmuir–Hinshelwood kinetics, assuming two different metal oxide sites, one containing adsorbed oxygenates and the other one containing lattice oxygen. The presence of water vapor lowered the formaldehyde formation rate significantly, especially at lower water partial pressures. These results could be well explained in terms of competitive adsorption of water with methanol on the free active catalyst sites.For the most important side reactions, i.e. dimethyl ether formation as well as dimethoxymethane formation the forward reaction rates were determined from the selectivity data. The conversion rate of dimethyl ether to formaldehyde was also measured with separate experiments in the differential reactor. Carbon monoxide was not formed during the differential kinetic measurements of formaldehyde formation from methanol. Therefore, the rate of formaldehyde oxidation to CO was studied separately in a dual bed catalyst, where formaldehyde was formed in the first integral reactor at low temperatures and subsequently converted to CO in a differential reactor. The rate of CO formation was first order in formaldehyde and the oxygen dependency was the same as that for the formaldehyde formation from methanol.Rate expressions for all reactions were formulated based on the above assumptions and a multivariate Levenberg–Marquardt method was used to fit all the model constants to all the experiments for all reaction rates simultaneously, while additionally accounting for axial concentration profiles in the reactor. The observed influences of composition and temperature on the reaction rates could be well described.
Keywords: Catalytic oxidation; Methanol; Formaldehyde; Carbon monoxide; Kinetics; Differential reactor;
Catalytic investigation of Pd particles supported on MCM-41 for the selective hydrogenations of terminal and internal alkynes by Attila Papp; Árpád Molnár; Ágnes Mastalir (256-266).
Pd-MCM-41 samples with different Pd contents were synthetized by using PdCl2, tetraethyl orthosilicate (TEOS) and the cationic surfactant cetyltrimethylammonium bromide (CTAB). The template-free materials were characterized by inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray diffraction (XRD), N2 sorption, H2 chemisorption and TEM measurements. The representative samples Pd-MCM(1.39) and Pd-MCM(5.85) had Pd contents 1.39 and 5.85%, respectively. Although the formation of the Pd particles was found to decrease the crystalline character of the host material, the structure of the MCM-41 framework was retained for both samples. The evidence that the Pd crystallite sizes exceeded 5 nm suggested that the Pd particles were mainly situated on the surface of MCM-41, although some of the Pd content of Pd-MCM(5.85) may also be embedded in the mesopores. Accordingly, the Pd-MCMs can essentially be regarded as MCM-41-supported Pd materials. Both samples proved to be active and selective catalysts for the liquid-phase semihydrogenations of phenylacetylene, 3-butyn-1-ol, 4-octyne and 1-phenyl-1-butyne. The initial activity of Pd-MCM(1.39) surpassed that of Pd-MCM(5.85) for each reaction, indicating that the catalytic activity was dependent on the Pd crystallite size. In contrast, the selectivity of alkene formation was irrespective of the particle diameter for most reactants. For the semihydrogenation of 4-octyne, Pd-MCM(1.39) proved to be an extremely efficient catalyst. The pronounced (Z)-alkene stereoselectivities obtained for the hydrogenations of internal alkynes over both Pd-MCMs may be attributed to the participation of high-coordination terrace atoms as active sites. The catalytic activities of the Pd-MCMs were considerably higher than those of silica-supported Pd catalysts.
Keywords: Palladium; MCM-41; X-ray diffraction; N2 sorption; Transmission electron microscopy; Alkyne hydrogenation; (Z)-alkene; Stereoselectivity; Silica-supported palladium catalyst;
Simplified kinetic models of methanol oxidation on silver by A. Andreasen; H. Lynggaard; C. Stegelmann; P. Stoltze (267-273).
Recently the authors developed a microkinetic model of methanol oxidation on silver [A. Andreasen, H. Lynggaard, C. Stegelmann, P. Stoltze, Surf. Sci. 544 (2003) 5–23]. The model successfully explains both surface science experiments and kinetic experiments at industrial conditions applying physically realistic parameters. Unfortunately the rate expression based on this microkinetic model is complex and impractical to apply for reactor engineering purposes. In this paper the rate expression is simplified by a number of approximations to make it suitable for practical applications without loosing significant accuracy. This is achieved by converting from a statistical thermodynamic description to a classical thermodynamic description resulting in a reduction of model parameters from more than 100 to a total of only 26. Further, other assumptions are applied including the most abundant reaction intermediate (MARI) approximation and the irreversible step (IS) approximation. This leads to further reduction in model complexity and in number of parameters. Computations verify that the performance of the model is preserved despite the reduction of the vast amount of parameters.
Keywords: Methanol oxidation; Formaldehyde synthesis; Microkinetic modeling; Catalysis; Silver; Quasi-equilibrium approximation; MARI approximation; IS approximation;