Active coke: Carbonaceous materials as catalysts for alkane dehydrogenation

The catalytic dehydrogenation (DH) and oxidative dehydrogenation (ODH) of light alkanes are of significant industrial importance. In this work both carbonaceous material deposited on VO_x/Al₂O₃ catalysts during reaction and unsupported carbon nanofibres (CNFs) are shown to be active for the dehydrogenation of butane in the absence of gas-phase oxygen. Their activity in these reactions is shown to be dependent upon their structure, with different reaction temperatures yielding structurally different coke deposits. Terahertz time-domain spectroscopy (THz-TDS), among other techniques, has been applied to the characterisation of these deposits – the first time this technique has been employed in coke studies. TEM and other techniques show that coke encapsulates the catalyst, preventing access to VO_x sites, without a loss of activity. Studies on CNFs confirm that carbonaceous materials act as catalysts in this reaction. Carbon-based catalysts represent an important new class of potential catalysts for DH and ODH reactions.     

Catalytic Oxidative Dehydrogenation of n‐Butane on Gallium Nitride‐Containing Titanosilicate Catalyst

GaN‐containing titanosilicate catalysts were used for the first time for the oxidative dehydrogenation (ODH) of n‐butane at a relatively low reaction temperature (460 °C). Commercially available GaN powder with a wurtzite crystal structure showed superior reactivity and stability for the ODH of n‐butane. The catalytic property of GaN catalyst for ODH strongly depends on the GaN particle size. The effects of the GaN weight percentage and GaN particle size on the catalytic performance are investigated in a fixed bed reactor. Based on the physicochemical properties of the catalyst characterized via TEM, DLS, N₂ adsorption‐desorption, XRF, O₂‐TPD, XRD, XPS, and in‐situ FTIR, the textural and structural properties of catalyst were obtained. The catalytic results reveal that the presence of GaN increases the activity of the catalysts, indicating that GaN can be used as a new active phase for the ODH of n‐butane. XRD, XPS, O₂‐TPD, DLS, TEM, and in‐situ FTIR results show that activated O species exist on the surface of the GaN catalyst and enhance the catalytic performance with a decreasing GaN particle size, suggesting that smaller GaN particles possess a remarkable capability to activate O species in O₂ and C‐H bonds in light alkanes.     

Ni–Nb–O mixed oxides as highly active and selective catalysts for ethene production via ethane oxidative dehydrogenation. Part II: Mechanistic aspects and kinetic modeling

In this work, transient and SSITKA experiments with isotopic ¹⁸O₂ were conducted to study the nature of oxygen species participating in the reaction of ethane oxidative dehydrogenation to ethylene and obtain insight in the mechanistic aspects of the ODH reaction over Ni-based catalysts. The study was performed on NiO, a typical total oxidation catalyst, and a bulk Ni–Nb–O mixed-oxide catalyst (Ni_0.85Nb_0.15) developed previously [E. Heracleous, A.A. Lemonidou, J. Catal., in press], a very efficient ethane ODH material (46% ethene yield at 400 °C). The results revealed that over both materials, the reaction proceeds via a Mars–van Krevelen-type mechanism, with participation of lattice oxygen anions. However, the ¹⁸O₂ exchange measurements showed a different distribution of isotopic oxygen species on the two materials. The prevalent formation of cross-labelled oxygen species on NiO indicates that dissociation of oxygen is the fast step of the exchange process, leading to large concentration of intermediate electrophilic oxygen species on the surface, active for the total oxidation of ethane. Larger amounts of doubly exchanged species were observed on the Ni–Nb–O catalyst, indicating that doping with Nb makes diffusion the fast step of the process and suppresses formation of the oxidizing species. Kinetic modeling of ethane ODH over the Ni_0.85Nb_0.15 catalyst by combined genetic algorithm and nonlinear regression techniques confirmed the above, since the superior model is based on a redox parallel-consecutive reaction network with the participation of two types of active sites: type I, responsible for the ethane ODH and ethene overoxidation reaction, and type II, active for the direct oxidation of ethane to CO₂. The kinetic model was able to successfully predict the catalytic performance of the Ni_0.85Nb_0.15 catalyst in considerably different experimental conditions than the kinetic experiments (high temperature and conversion levels).     

CO2氛围中低碳烷烃制烯烃催化剂的研究进展

CO₂作为温和的氧化剂有效抑制了低碳烷烃催化转化过程中深度氧化的发生,然而其广泛应用还依赖于高效、高稳定性催化剂的研究与开发。本文就近年来国内外有关利用CO₂作为氧化剂在低碳烷烃催化转化反应方面的研究成果进行了综述,主要涉及甲烷氧化偶联制乙烯和乙烷催化剂;乙烷氧化脱氢制乙烯催化剂、丙烷氧化脱氢制丙烯催化剂和丁烷氧化脱氢制丁烯催化剂,分析讨论了CO₂作为氧化剂的作用和作用机制,并提出了研究展望。     

MCM-41 supported CuO/Bi2O3 nanoparticles as potential catalyst for 1,4-butynediol synthesis

CuO/Bi₂O₃ (CuO/Bi₂O₃/MCM-41) nanoparticles supported on MCM-41 were synthesized by a facile impregnation method. The products were characterized by nitrogen adsorption/desorption, X-ray diffraction (XRD), H₂ temperature programmed reduction (H₂-TPR) and scanning electron microscopy (SEM). XRD patterns indicated the presence of crystalline CuO and Bi₂O₃ phase for CuO/Bi₂O₃/MCM-41 catalyst. TPR results revealed CuO nanoparticles were dispersed well on MCM-41. SEM results showed that the nanoparticles were located on the MCM-41. The activity of the catalysts towards ethynylation of formaldehyde for 1,4-butynediol synthesis was evaluated at atmospheric pressure. Compared with unsupported CuO/Bi₂O₃ and commercial Cu/Bi-based catalyst, CuO/Bi₂O₃/MCM-41 catalyst showed maximum conversion (51%) and selectivity (94%) towards 1,4-butynediol. The results show that CuO/Bi₂O₃ catalysts supported on MCM-41 have potential for 1,4-butynediol synthesis in industrial application.     

Ti-containing MCM-41 catalysts for liquid phase oxidation of cyclohexene with aqueous H2O2 and tert-butyl hydroperoxide

Framework Ti-substituted and Ti-grafted MCM-41 mesoporous material has been prepared by direct hydrothermal synthesis and a post-synthesis grafting method. The materials have been tested as catalysts for cyclohexene oxidation with aqueous H₂O₂ and tert-butyl hydroperoxide (TBHP). With aqueous H₂O₂ in methanol, the major products were cyclohexene diol and its methyl ethers. No cyclohexene oxide was produced. Titanium leaching was a serious problem, and the catalyst lost its activity irreversibly after only one cycle of reaction. With TBHP, the selectivity for cyclohexene oxide was near 100%, titanium leaching was negligible, and the catalyst could be repeatedly used after regeneration without suffering significant activity loss. However, the reaction rate was lower than when H₂O₂ was used. Framework substituted material and catalysts prepared by Ti-grafting onto a MCM-41 support behaved similar, but the Ti-grafted MCM-41 is somewhat more active. The turnover frequency (TOF) per mole of Ti decreases with an increase of Ti content in the catalyst. This is caused by a reduced Ti dispersion within the silica matrix. This revised version was published online in July 2006 with corrections to the Cover Date.     

An efficient microwave-assisted Suzuki reaction using Pd/MCM-41 and Pd/SBA-15 as catalysts in solvent-free condition

Palladium catalysts, Pd/MCM-41 and Pd/SBA-15 were prepared by impregnation of an aqueous solution of [Pd(NH₃)₄]Cl₂ on MCM-41 and SBA-15. Palladium contents of Pd/MCM-41 and Pd/SBA-15 are 8.4% and 8.7%, respectively. It has been shown that these catalysts are very suitable to microwave-assisted Suzuki reactions under solvent-free condition. It is also found that the base additives for this reaction are K₂CO₃, Cs₂CO₃ or CsF. Thus, phenylboronic acid and phenyl iodide with Pd/MCM-41 produce biphenyl by microwave irradiation for 10 min in 97.4% yield. Phenyl bromide, instead of phenyl iodide, also proceeds the reaction with phenylboronic acid using Pd/MCM-41 or Pd/SBA-15 yielding biphenyl by microwave irradiation for 10 min in excellent yield. Whereas the reaction of phenyl chloride with phenylboronic acid gives poor yield in same condition. Various aryl iodides and aryl bromides are tested. In this paper our recent results of microwave-assisted Suzuki reaction using Pd/MCM-41 and Pd/SBA-15 under solvent-free condition are described.     

Photodecomposition of polyphenols in <Emphasis Type="Italic">E. camaldulensis</Emphasis> leaves in the presence of hybrid catalyst of titania and MCM-41 synthesized from rice husk silica

Photocatalytic degradation (PCD) of polyphenols (gallic acid) from E. camaldulensis leaves on TiO₂/MCM-41 was investigated in order to get rid of substances harmful to aquatic life. The TiO₂/MCM-41 catalysts with titania loading of 2–40% were synthesized by hydrothermal method using rice husk silica and tetraethyl orthotitanate as silica and titania sources, respectively. The obtained catalysts were characterized by XRD, TEM, Zeta potential analyzer, N₂ adsorption-desorption and diffuse reflectance UV spectroscopy. Hexagonal array of MCM-41 was confirmed, but its crystallinity decreased dramatically with titania loading. Zeta potential of TiO₂/MCM-41s surface varied from 2.11 to 6.00 with the increase of TiO₂ from 0 to 100 wt%. Band gap energy of TiO₂ shifted from 394.1 to 425.1 nm after adding 60%MCM-41 (40%TiO₂/MCM-41), facilitating the ease of OH establishment. Gallic acid - a weak acid solution (pK_a=4.0) around 27 ppm was favorable to dissolve in water. PCD of gallic acid was carried out on irradiating of 400W of mercury lamp. The results showed gallic acid solution about 10 wt% properly adsorbed on 10%TiO₂/MCM-41 and effectively degraded at pH solution of 9.0. PCD completed at 60 minutes of irradiation time through catalyst concentration of 0.17 g/L and obeyed pseudo-first order. Intermediate products were formic, oxalic, pyruvic, malanic and maleic acids that finally mineralized to CO₂ and H₂O as downstream products.     

The synthesis and investigation of ruthenium phosphide catalysts

Highly active Ru₂P/MCM-41 and RuP/MCM-41 catalysts were synthesized by thermal decomposition of ruthenium chloride and hypophosphite precursors. The effect of experimental conditions on the final products is discussed. The results of investigation show that the reaction mechanism of ruthenium phosphide is different from nickel phosphide. Catalytic properties were tested for the hydrodesulfurization of dibenzothiophene and hydrodenitrogenation of quinoline. The results showed that both Ru₂P/MCM-41 and RuP/MCM-41 catalysts exhibited higher activities than Ru/MCM-41.     

Preparation of sulfated alumina supported on mesoporous MCM-41 silica and its application in esterification

New types of mesoporous SA/MCM-41 solid acid catalysts were prepared by loading sulfated alumina (SA) on MCM-41. The prepared catalysts were characterized by XRD, IR, N₂ physisorption, elemental analysis, FT-IR of adsorbed pyridine and NH₃-TPD. The esterification of acetic acid with n-butanol and citric acid with n-butanol were used as model reactions to test the catalytic activities and reusability of the SA/MCM-41 solid acid catalysts. Compared with SA catalyst, SA/MCM-41 catalysts exhibited higher catalytic performances, which were attributed to their high BET surface area and large pore volume. Moreover, 20SA/MCM-41 solid acid catalyst showed excellent reusability in both esterifications.     

Wet oxidation of wastewater containing hydrocarbons by novel supported Pd catalysts

Pd catalysts supported on TiO₂, ZrO₂, ZSM-5, MCM-41 and activated carbon were used in catalytic wet oxidation of hydrocarbons such as phenol, m-cresol and m-xylene. It was found that the Pd/TiO₂ catalyst was highly effective in the wet oxidation of hydrocarbon. The activities of catalysts with various hydrocarbon species, catalyst support, oxidation state of catalyst performed in a 3-phase slurry reactor show that reaction on Pd surface is more favorable than that in aqueous phase and that the active site is oxidized Pd in catalytic wet air oxidation of hydrocarbons. Based on the experimental results, a plausible reaction mechanism of wet oxidation of hydrocarbons catalyzed over Pd/TiO₂ catalyst was proposed. This catalyst is superior to other oxide catalysts because it suppressed the formation of hardly-degradable organic intermediates and polymer.     

Absorption of carbon dioxide into glycidyl methacrylate solution containing the triethylamine immobilized ionic liquid on MCM-41 catalyst

An ionic liquid (TEA-MS41), triethylamine-immobilized on chloropropyl-functionalized MCM-41, was synthesized by a grafting technique through a co-condensation method and used as a catalyst in the reaction of carbon dioxide with glycidyl methacrylate (GMA). CO₂ was absorbed into the heterogeneous system of the GMA solution and dispersed with solid particles of the catalyst in a batch stirred tank with a plane gas-liquid interface at 101.3 kPa. The absorption of CO₂ was analyzed by using mass transfer accompanied by chemical reactions based on film theory. The proposed model fits the measured data of the enhancement factor to obtain the reaction rate constants. Solvents such as N,N-dimethylacetamide, N-methyl-2-pyrrolidinone, and dimethyl sulfoxide influenced the reaction rate constants.     

Ni-Mo双金属催化剂的甲烷化性能与耐硫稳定性

采用沉淀-浸渍法合成一系列Ni-Mo基双金属催化剂,在固定床反应器中对其催化活性和耐硫性能进行评价,并辅以不同的表征手段阐释其作用机理。实验结果表明,以MCM-41分子筛为载体的催化剂活性和稳定性优于以NaY、γ-Al₂O₃和拟薄水铝石（PB）为载体的催化剂;反应前后催化剂的表征结果则说明Ni-Mo分子间适宜的相互作用是决定催化剂性能的关键。MCM-41载体催化剂中适宜的Ni-Mo分子间相互作用使得此催化剂中活性组分与载体间的相互作用适中,还原后的活性金属Ni能够均匀分散在载体表面,从而提高了催化剂的耐硫稳定性和抗积炭能力。不同Ni-Mo比同样会影响Ni-Mo分子间的相互作用,通过考察确定20Ni-10Mo/MCM-41的活性和稳定性最优。     

Alkynes Hydrogenation over Pd-Supported Catalysts

The liquid-phase stereoselective hydrogenation of phenyl alkyl acetylenics at 298 K and atmospheric pressure on Pd-supported catalysts has been studied. The catalysts were prepared by impregnation of Pd(acac)₂ precursor (1 wt% of Pd) on different siliceous substrates such as amorphous SiO₂, mesoporous MCM-41 and silylated MCM-41. The poisoning effect of lead incorporation on the supported palladium was also studied. All the catalysts displayed high selectivity to cis-alkene isomer, with Pd/MCM-41 being the most active catalyst. Deliberately adding lead to the base, palladium catalysts underwent changes in the selectivity to cis-alkene isomer and a significant drop in the activity. All the solids were characterized by nitrogen adsorption–desorption isotherms at 77 K, TGA, TPR, H₂ and CO chemisorption, XRD, XPS, and TEM.     

基于MCM-41的镍基甲烷化催化剂活性与稳定性

采用浸渍法分别以MCM-41,Al₂O₃和SiO₂ 为载体制备了不同镍负载量的甲烷化催化剂,并在连续流动固定床反应装置上对其甲烷化催化活性进行了评价。研究结果表明,与Ni/Al₂O₃和Ni/SiO₂相比,相同镍负载量的Ni/MCM-41催化剂具有更好的催化活性。同时研究了Ni含量对于Ni/MCM-41催化剂催化活性的影响,发现随着Ni含量的增加,CO转化率和CH₄收率逐渐升高,并且在Ni含量大于10%（质量分数）以后趋于稳定。在n（H₂）：n（CO）=3：1、反应压力1.5 MPa、反应温度350℃及质量空速12000 ml·h^-1·g^-1的反应条件下,10%Ni/MCM-41催化剂CH₄选择性达到94.9%,CO转化率接近100%。在100 h催化活性稳定性试验中,10%Ni/MCM-41催化活性无明显下降,表现出良好的催化活性稳定性。采用X射线衍射（XRD）、氮气物理吸附（BET）、热重分析（TG）及氢气程序升温还原（H₂-TPR）等技术手段对催化剂进行了表征,结果表明Ni颗粒大小是影响Ni/MCM-41催化剂催化活性的主要因素。     

Heterogeneously Catalysed Aldol Reactions in Supercritical Carbon Dioxide as Innovative and Non-Flammable Reaction Medium

Aldol reactions of several aldehydes have been investigated over acidic and basic catalysts in supercritical carbon dioxide at 180 bar and 100 °C. Both acidic (Amberlyst-15, tungstosilicic acid (TSA) on SiO₂ and MCM-41) and basic (hydrotalcite) materials showed interesting performance in this preliminary study under the entitled reaction conditions. Small and linear aldehydes, such as propanal, butanal, pentanal and hexanal, react more efficiently than the branched 3-methylbutanal, which is converted much slower. Whereas Amberlyst-15 showed the highest conversion based on the catalyst mass, tungstosilicic acid-based catalysts were significantly better if the rates were related to the number of acidic sites (>1000 h^?1). The rate depends both on the dispersion and the kind of support. Strikingly, tungstosilicic acid (TSA) on MCM-41 was also an effective catalysts for the selective C=C double bond hydrogenation of 2-butenal and is therefore a potential catalyst for the ??one-pot?? synthesis of 2-ethyl-2-hexenal and 2-ethylhexanal via combined hydrogenation and aldol reaction from 2-butenal. A number of characterisation techniques, such as temperature-programmed desorption of ammonia (NH₃-TPD), transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS), etc. were used to get an insight into the catalyst structure, which support a high dispersion and strong acidity of the tungsten based species on silica and MCM-41.     

Catalytic reduction of NOx over transition-metal-containing MCM-41

The catalytic properties of Pt, Rh and Co supported on mesoporous molecular sieves with MCM-41-type structure consisting of SiO₂ and Al₂O₃ were studied for the reduction of NO with propene. Pt supported on siliceous MCM-41 was the most active catalyst, however, significant quantities of undesirable N₂O were formed during the reaction. Pt supported on mesoporous Al₂O₃ and Rh supported on both mesoporous oxides showed a lower activity, but an improved selectivity towards N₂ formation. Co supported on MCM-41-type materials had only a low level of activity for the reduction of NO with propene. For Pt supported on MCM-41-type materials only a minor decrease in the activity was observed when water vapor was added into the reactant gas mixture, while on Rh- and Co-containing catalysts the activity strongly decreased. This revised version was published online in July 2006 with corrections to the Cover Date.     

Well-Dispersed Gallium-Promoted Sulfated Zirconia on Mesoporous MCM-41 Silica

Gallium-promoted sulfated zirconia (SZ) was confined inside pure-silica MCM-41 (abbreviated as SZGa/MCM-41), where the latter served as a host material. It was prepared by direct dispersion of metal sulfate in the as-synthesized MCM-41 materials, followed by thermal decomposition. The SZGa/MCM-41 catalysts were characterized by XRD, N₂ adsorption, HRTEM, DRIFT, NH₃-TPD, and TPR. The experimental results showed that the ordered porous host structure was still maintained in the catalyst. SZ was in meta-stable tetragonal phase and highly dispersed on the interior surface of MCM-41 even at a high loading of 50 wt%. Additionally, a small fraction of SZ nanoparticles on the external surface of MCM-41 was obtained. The catalytic activity of SZGa/MCM-41 was examined in n-butane isomerization. In comparison to SZ/MCM-41 without promoter, the catalytic activities of the Ga-promoted catalysts were greatly improved. The reason proposed for the higher activity of the Ga-promoted catalysts was that Ga enhances the oxidizing ability of the catalysts.     

Anchoring of Copper(II) Schiff Base Complex into Aminopropyl-Functionalised MCM-41: A Novel,Efficient and Reusable Catalyst for Selective Oxidation of Alcohols

A copper(II) complex containing tetradentate N₂O₂ Schiff base ligand immobilized into aminopropyl-functionalised MCM-41 (mobile crystalline material number 41), was prepared and characterized by Fourier-transform infrared, X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, N₂ adsorption–desorption and inductively coupled plasma analysis techniques. The novel heterogeneous catalyst, MCM-41-pr-NH₂-CuL, can be successfully applied for efficient and selective oxidation of different primary and secondary alcohols to the corresponding carbonyl compounds using hydrogen peroxide as an oxidant in acetonitrile at 60 °C. The effect of reaction parameters such as solvent, amount of catalyst, temperature and kind of oxidant on the oxidation of benzyl alcohol was also studied. The prepared catalyst could be recovered and reused four times without important loss of its catalytic performance. The heterogeneous MCM-41-pr-NH₂-CuL catalyst was found to be catalytically more active in the oxidation of alcohols compared to the similar type of copper(II) Schiff base complex in homogeneous media under the same reaction conditions.     

Mesoporous molecular sieve MCM-41 supported Co–Mo catalyst for hydrodesulfurization of dibenzothiophene in distillate fuels

A mesoporous aluminosilicate molecular sieve with MCM-41 type structure was synthesized using aluminum isopropoxide as the Al source. Supported Co–Mo/MCM-41 catalysts were prepared by co-impregnation of Co(NO₃)₂·6H₂O and (NH₄)₆Mo₇O₂₄ followed by calcination and sulfidation. For comparison, conventional Al₂O₃-supported sulfided Co–Mo catalysts were also prepared using the same procedure. These two types of catalysts were examined at two different metal loading levels in hydrodesulfurization of a model fuel containing 3.5 wt% sulfur as dibenzothiophene in n-tridecane. At 350–375°C under higher H₂ pressure (6.9 MPa), sulfided Co–Mo/MCM-41 catalysts show higher hydrogenation and hydrocracking activities at both normal and high metal loading levels, whereas Co–Mo/Al₂O₃ catalysts show higher selectivity to desulfurization. Co–Mo/MCM-41 catalyst at high metal loading level is substantially more active than the Co–Mo/Al₂O₃ catalysts.