Reducibility and CO hydrogenation over Pt and Pt-Co bimetallic catalysts encaged in NaY-zeolite

Temperature programmed techniques (TPR, TPD) and X-ray diffraction (XRD) have been used to study ion migration and location as well as reducibility of platinum and cobalt ions encapsulated in Pt/NaY, Co/NaY and Pt-Co/NaY zeolites prepared by ion exchange. The temperature required to reduce Co²⁺ in NaY was significantly lowered by the presence of Pt and dependent upon the relative locations of Pt and Co ions in zeolite cages. The exact location was controlled by the calcination condition and the metal contents. For bimetallic catalyst with low Pt content (0.5 wt% Pt and 0.9 wt% Co), the TPR results indicated that reduction of Co²⁺ ions in the vicinity of Pt shifted toward lower temperature, while that of Co²⁺ staying alone was not affected. With high Pt loading (4.5 wt% Pt, 0.7 and 2.6 wt% Co), however, most of the Co²⁺ ions were reduced by means of Pt at temperature below 723 K after calcination at 573 K. The temperature for Pt reduction in bimetallic catalysts was somewhat higher than Pt/NaY and increased with Co atomic fraction, indicating that mixed oxide, PtCo _x O _y , might be formed during calcination. After reduction in hydrogen at 723 K, highly dispersed metal particles were formed. These fine particles were most probably confined inside zeolite cages as indicated by the absence of XRD peak for all samples after calcination and reduction. Surface composition of the bimetallic particles may be different for catalysts with similar Pt content but different Co loading. Accordingly, H/Pt ratios of 1.0 and 0.72 for catalysts with low and high Co content, respectively, were shown by hydrogen chemisorption. It was further supported by the increase in TPD peak intensity with Co loading in the high temperature range, which was related to the reoxidation of Co in bimetallic particles by surface hydroxyl groups. Preliminary results on CO hydrogenation demonstrated that activity and methanol selectivity were higher on Pt-Co bimetallic catalysts than either over monometallic Pt or Co catalyst, which was consistent with the Pt enhanced Co reduction and formation of Pt-Co bimetallic particles.     

几种环己醇脱氢催化剂性能的比较

对几种国内外工业用环己醇脱氢催化剂的性能及使用情况进行了比较。结果显示,国产脱氢催化剂选择性、转化率和寿命等主要性能已完全达到甚至优于国外同类催化剂。     

环己醇脱氢催化剂及催化机理研究进展

对环己醇脱氢制环己酮的研究现状以及可能发生的副反应作了简要的介绍,认为催化剂性能的优劣是影响环己醇脱氢的主要因素。从主要活性组分、助剂和载体几个部分详细评述了环己醇脱氢制环己酮催化剂的研究进展,并对铜催化剂催化环己醇脱氢制环己酮的反应机理进行了介绍,解释了亚铜离子比金属铜具有更佳活性和选择性的原因。最后对环己醇脱氢制环己酮催化剂的发展方向进行了展望,提出了载体是环己醇脱氢催化剂需要突破的重点。     

Oxidative dehydrogenation of cyclohexane over alumina-supported vanadium oxide nanoliths

Featuring highly ordered one-dimensional nanopores, anodic aluminum oxide (AAO) makes an ideal substrate for fabrication of catalysts by atomic layer deposition (ALD). Vanadium oxide (VOx) catalysts supported on AAOs and prepared by ALD and incipient wetness impregnation are characterized by X-ray fluorescence and ultraviolet–visible (UV–Vis) spectroscopy. At low loadings (<4 V/nm²) the supported VOx are mostly isolated monomers; polyvanadate domains are gradually formed as the surface vanadium content increases. The catalytic performance at a series of loadings (<3–32 V/nm²), and hence different forms of VOx, for the oxidative dehydrogenation (ODH) of cyclohexane are investigated. Compared to the catalysts prepared by incipient wetness impregnation, the ALD VOx catalysts show specific activities that are between 2 and 7.5 times higher. This reflects a better dispersion of the catalytic species on the surface as synthesized by ALD. In the cyclohexane ODH reaction with the supported ALD VOx, the kinetic orders and activation energies are comparable to kinetics data reported previously for the supported VOx. The results indicate that the ALD technique can be applied as an alternative approach to synthesize the supported VOx catalysts and achieves very good dispersion even at loadings above one monolayer (8 V/nm²). In the ODH reaction, polyvanadate sites are shown to be more active, overall, than monovanadate sites. However, numerical modeling of the reaction pathways indicates that the olefin formation rate is ～3 times faster on monomeric VOx sites than on polymeric VOx. By comparing the ODH of cyclohexane and the oxidations of cyclohexene and benzene, we find that both the sequential path and the direct path (the direct conversion from cyclohexane to benzene) are important in the oxidation process of cyclohexane.     

Hydrodenitrogenation of pyridine over alumina-supported iridium catalysts

The catalytic properties of alumina-supported Ir catalysts (≈1 wt% Ir) were studied in the hydrodenitrogenation (HDN) of pyridine at 320°C and 20 bar of pressure in the absence as well as presence of parallel hydrodesulfurization (HDS) of thiophene. The effects of Ir precursor (Ir(AcAc)₃, Ir₄(CO)₁₂, H₂IrCl₆, (NH₄)₂IrCl₆), metal dispersion and sulfur addition were investigated. Ir₄(CO)₁₂ gave the most active catalyst which was ascribed to a lower amount of contaminants originated from the starting Ir compounds rather than to a better Ir dispersion. The decrease of Ir dispersion by sintering in air led to much higher decrease of the rate of C–N bond hydrogenolysis than that of pyridine hydrogenation. The Ir dispersion determined partly the HDN selectivity; a better dispersed Ir phase gave a lower amount of intermediate piperidine. Presulfidation of the reduced catalyst led to 20% decline of the rates of both consecutive HDN steps. An additional and much larger activity decline was caused by the simultaneous execution of HDS. The competitive adsorption of thiophene (or H₂S) was selectively affecting C–N bond hydrogenolysis more than pyridine hydrogenation. The alumina-supported Ir catalysts possessed much higher HDN activity and HDN/HDS selectivity than a conventional NiMo system.     

Selective propane dehydrogenation to propylene on novel bimetallic catalysts

Platinum catalysts (0.1–0.2% of Pt) supported on corundum and promoted by indium and tin were prepared, characterized by XPS, XRD, TEM and chemisorption and tested in propane dehydrogenation at 500–580°C with steam and hydrogen as diluents. The characteristics of samples based on corundum were compared with catalysts of similar composition supported on θ-alumina. The catalytic performance in steam was superior by far to that in hydrogen. Coking of corundum-supported catalysts decreased by a factor of 30–60 so their stability was much higher than with θ-supported catalyst. For the first time high dispersion and high activity of corundum-based dehydrogenation catalysts are reported.     

Thiophene hydrodesulfurization over modified alumina-supported molybdenum sulfide catalysts

Methanethiol has been synthesized by one‐step catalytic reaction from H₂S‐content syngas on K₂MoS₄/SiO₂ catalyst with selectivity over 95% under the optimum reaction conditions of 563 K, 2.0–3.0 MPa and 5–6% H₂S content in the feed syngas. The results of XRD and XPS showed that Mo–S–K phase on the surface of the catalyst K₂MoS₄/SiO₂ was responsible for the high activity and selectivity to methanethiol, and which may be restrained by the existence of (S–S)^2- species. This revised version was published online in July 2006 with corrections to the Cover Date.     

Oxidative dehydrogenation of ethane to ethylene over alumina-supported vanadium oxide catalysts: Relationship between molecular structures and chemical reactivity

The influence of vanadium oxide loading in the supported VO_x/Al₂O₃ catalyst system upon the dehydrated surface vanadia molecular structure, surface acidic properties, reduction characteristics and the catalytic oxidative dehydrogenation (ODH) of ethane to ethylene was investigated. Characterization of the supported VO_x/Al₂O₃ catalysts by XPS surface analysis and Raman spectroscopy revealed that vanadia was highly dispersed on the Al₂O₃ support as a two-dimensional surface VO_x overlayer with monolayer surface coverage corresponding to 9 V/nm². Furthermore, Raman revealed that the extent of polymerization of surface VO_x species increases with surface vanadia coverage in the sub-monolayer region. Pyridine chemisorption-IR studies revealed that the number of surface Brønsted acid sites increases with increasing surface VO_x coverage and parallels the extent of polymerization in the sub-monolayer region. The reducibility of the surface VO_x species was monitored by both H₂-TPR and in situ Raman spectroscopy and also revealed that the reducibility of the surface VO_x species increases with surface VO_x coverage and parallels the extent of polymerization in the sub-monolayer region. The fraction of monomeric and polymeric surface VO_x species has been quantitatively calculated by a novel UV–Vis DRS method. The overall ethane ODH TOF value, however, is constant with surface vanadia coverage in the sub-monolayer region. The constant ethane TOF reveals that both isolated and polymeric surface VO_x species possess essentially the same TOF value for ethane activation. The reducibility and Brønsted acidity of the surface VO_x species, however, do affect the ethylene selectivity. The highest selectivity to ethylene was obtained at a surface vanadia density of 2.2 V/nm², which corresponds to a little more than 0.25 monolayer coverage. Below 2.2 V/nm², exposed Al support cations are responsible for converting ethylene to CO. Above 2.2 V/nm², the enhanced reducibility and surface Brønsted acidity appear to decrease the ethylene selectivity, which may also be due to higher conversion levels. Above monolayer coverage, crystalline V₂O₅ nanoparticles are also present and do not contribute to ethane activation, but are responsible for unselective conversion of ethylene to CO. The crystalline V₂O₅ nanoparticles also react with the Al₂O₃ support at elevated temperatures via a solid-state reaction to form crystalline AlVO₄, which suppresses ethylene combustion of the crystalline V₂O₅ nanoparticles. The molecular structure–chemical characteristics of the surface VO_x species demonstrate that neither the terminal VO nor bridging VOV bonds influence the chemical properties of the supported VO_x/Al₂O₃ catalysts, and that the bridging VOAl bond represents the catalytic active site for ethane activation.     

Hydrodesulfurization of dibenzothiophene over alumina-supported nickel molybdenum phosphide catalysts

The activity of nickel molybdenum phosphide catalysts was studied for the hydrodesulfurization of dibenzothiophene at 573 K and total pressure of 2.0 MPa. The Al₂O₃-supported NiMo phosphide catalysts were prepared by successive and simultaneous methods. The effect of the reduction temperature on the catalyst activity was also studied. The simultaneous preparation was determined to be the best method for the preparation of the active supported catalyst for dibenzothiophene HDS. The 623 K-reduced catalyst had the highest HDS rate of the catalysts. Nickel migrated from the inside to the surface during the reaction and promoted the HDS activity. The active species in the dibenzothiophene HDS and the oxidation states of Mo, Ni and P in the catalyst before and after reaction and of S after the reaction were studied on the basis of an XPS analysis.     

钌钯双金属催化剂催化L-氨基丙酸加氢特性

通过添加金属Pd对Ru/C催化剂进行改性,提高了L-氨基丙酸加氢制备L-氨基丙醇的收率和光学选择性。采用ICP-MS、XRD、TEM和XPS对所制备的催化剂进行了系统的表征,结果表明：RuPd物种在载体活性炭上分散均匀粒径较小,Pd原子的加入可显著改善Ru的电子特性,调变不同的RuPd原子比,可影响RuPd物种的存在状态,进而影响L-氨基丙醇的收率和光学选择性。在氨基丙酸浓度为0.9 mol·L^-1,磷酸浓度0.69 mol·L^-1,催化剂用量为反应物的20%,反应温度95℃,压力4.0 MPa的反应条件下,RuPd原子比为3∶1时催化剂表现出最好的催化性能,L-氨基丙醇的收率达到99.7%,产品的光学选择性同样达到了99.7%,同时催化剂能循环使用20次,催化性能基本保持不变。     

Role of CO/CO2 co-feeding in the dehydrogenation of cyclohexanol to cyclohexanone over Cu–ZnO based catalysts

The present work highlights the role of CO/CO₂ co-feeding in the dehydrogenation of cyclohexanol to cyclohexanone over Cu–ZnO–Cr₂O₃ and Cu–ZnO–Cr₂O₃–La₂O₃ catalysts in the temperature range of 448–523 K at atmospheric pressure under vapor phase conditions. Both the catalysts are prepared by coprecipitation technique and are characterized by BET surface area, XRD, TPR and N₂O pulse chemisorption under dynamic conditions. The co-feeding of CO/CO₂ along with cyclohexanol results in enhanced conversion of cyclohexanol and additional formation of methanol. The hydrogen generated in the dehydrogenation of cyclohexanol to cyclohexanone promotes the methanol formation from CO/CO₂. This is the first report where in methanol formation is observed at atmospheric pressure from CO/CO₂ co-feeding in cyclohexanol dehydrogenation.     

n-decane dehydrogenation on bimetallic PtSn and PtGe catalysts prepared by dip-coating

The catalytic performance of Pt, PtSn and PtGe supported on γ-Al₂O₃ (γ-A) deposited by dipcoating of spheres of α-Al₂O₃ (α-A) was studied in the n-decane dehydrogenation. The effect of Sn and Ge addition to Pt on the activity and selectivity was analyzed. The catalytic characterization was carried out using cyclohexane dehydrogenation (CHD), cyclopentane hydrogenolysis (CPH), temperature-programmed reduction (TPR), hydrogen chemisorption, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA) and scanning electronic microscopy (SEM). Pt(0.5)Sn/γ-A/α-A catalyst had the best catalytic performance and showed a low electronic interaction between the metals, with a surface segregation of Sn and the presence of oxidized Sn stabilized on the support. PtGe catalysts presented strong interactions with probable alloy formation. The catalytic performance of these catalysts is comparable to that reported in the patents.     

Propane dehydrogenation to propylene over Pt-based catalysts

Propane catalytic dehydrogenation to propylene was investigated over novel platinum-based catalysts, and the BET, H₂-chemisorption, H₂-TPR, H₂-TPD, C₃H₆-TPD and O₂-pulse techniques were used for catalyst characterization in this paper. The results showed that the addition of Zn and Ce promoters into Pt-based catalyst could remarkably improve the performances of dehydrogenation. Highly propylene selectivity, about 99%, was obtained in the dehydrogenation reaction. This improvement in catalytic performances could be attributed to the strong surface interactions, which modified the surface properties of the active platinum metal and the support, and further improved the catalytic performance of Pt-based catalyst.     

The effect of lanthanum in dehydrogenation of propane on Pt-Sn bimetallic catalysts

Four catalysts, H2PtCl6-5SnCl2/γ-Al2O3 (cat-1), { H2PtCl6-5SnCl2 + xLaCl3 } / γ-Al2O3 (cat-2), (Me4N)[Pt(SnCl3)5]/ γ-Al2O3 (cat-3), and {(Me4N)[Pt(SnCl3)5] + xLaCl3} / γ-Al2O3 (cat-4) were prepared and their catalytic activities and stabilities in dehydrogenation of propane were investigated. It has been shown that the stabilities of the Pt-Sn bimetallic catalysts were improved by introducing lanthanum into the catalysts either by the conventional impregnation method with Pt- and Sn-containing salts or by using precursors derived from Pt-Sn bimetallic clusters. Results from XPS analysis indicated that addition of lanthanum could inhibit the reduction of tin in the catalysts, especially in cat-2. The distribution of pore capacity in a used cat-2 was mainly in the large and medium size range, but in a used cat-1 the distribution was mainly in the small to medium size range. Thermal gravimetric analysis (TGA) showed that weight loss in the used cat-1 was much more than that in the used cat-2. The results suggested that introducing lanthanum into the Pt-Sn bimetallic catalysts could effectively prevent them from coking. This revised version was published online in July 2006 with corrections to the Cover Date.     

Oxidative dehydrogenation of isobutane over magnesium molybdate catalysts

A series of Mo-Mg-O catalysts with different crystalline phases (pure and mixtures) have been studied in the oxidative dehydrogenation of isobutane. X-ray diffraction, UV–Vis spectroscopy and temperature programmed reduction were applied to characterise the samples. In addition, the kinetics of the lattice oxygen exchange with labelled C¹⁸O₂ was measured for all samples. It has been found that the MgMoO₄ phase contains the surface active sites suitable for the adsorption of isobutane and for its dehydrogenation to isobutene with the minimum of cracking reaction. Moreover, for this reaction the migration of lattice oxygen ions is not a key parameter in the control of the catalytic properties of these materials.     

Oxidative dehydrogenation of propane over magnesium molybdate catalysts

Catalytic activities of magnesium molybdates were investigated for the oxidative dehydrogenation of propane with and without molecular oxygen under atmospheric pressure. Catalytic properties drastically changed with the catalyst composition, and it turned out that Mg_0.95MoO_x catalysts having slight excess molybdenum showed the highest activity in the oxidative dehydrogenation of propane, which gave 61% selectivity to propene at 22% conversion of propane at 515°C. The catalytic activities strongly depended on the acidic properties of the catalysts. It was also revealed that the lattice oxide ions of the catalysts participated as an active oxygen in the oxidative dehydrogenation of propane.     

Thiophene hydrodesulfurization over bimetallic and promoted nitride catalysts

Alumina-supported bimetallic (Co₃Mo₃N/Al₂O₃” and promoted (Co–Mo₂N/Al₂O₃) nitride catalysts have been prepared and characterized. Thiophene hydrodesulfurization (HDS) measurements show that the Co₃Mo₃N/Al₂O₃ and Co–Mo₂N/Al₂O₃ catalysts are significantly more active than a Mo₂N/Al₂O₃) catalyst with the same Mo loading. Furthermore, the Co–Mo₂N/Al₂O₃ catalyst has a substantially higher HDS activity than a sulfided Co–MoO₃/Al₂O₃ catalyst with an identical metal loading. This revised version was published online in July 2006 with corrections to the Cover Date.