Chinese Journal of Catalysis

Table of Contents for VOL.37 No.1

2016, 37 (1): 0-0.

Abstract ( 191 )

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Preface to Special Column on New Porous Catalytic Materials

Ying Wan, Serge Kaliaguine, Dongyuan Zhao

2016, 37 (1): 1-2. DOI: 10.1016/S1872-2067(15)61027-0

Abstract ( 284 ) [Full Text(HTML)] ()

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Ethylbenzene to styrene over ZrO₂-based mixed metal oxide catalysts with CO₂ as soft oxidant

Nanzhe Jiang, Abhishek Burri, Sang-Eon Park

2016, 37 (1): 3-15. DOI: 10.1016/S1872-2067(15)60901-9

Abstract ( 543 ) [Full Text(HTML)] ()

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ZrO₂-based mixed metal oxide catalysts for the industrially important dehydrogenation process of ethylbenzene to styrene monomer have been explored by our group for the past 20 years. These efforts were subjected to the activation of CO₂ over mixed metal oxide catalysts and resulted in several promising benefits to the dehydrogenation processes, such as stabilized conversion and selectivity, suppressed coke formation and commercially-acceptable longevity. In this review, we summarize the most recent developments on ZrO₂-based mixed metal oxide catalysts, including the further optimization of sol-gel process in the synthesis of catalysts, rationalizing acid-base properties by doping, co-operative properties between redox and acid-base active sites and additional promoters towards the effective improvement of the longevity of catalysts.

Advances in development and industrial applications of ethylbenzene processes

Weimin Yang, Zhendong Wang, Hongmin Sun, Bin Zhang

2016, 37 (1): 16-26. DOI: 10.1016/S1872-2067(15)60965-2

Abstract ( 902 ) [Full Text(HTML)] ()

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The benzene alkylation process for the production of ethylbenzene has undergone significant improvements during recent decades. Various environmentally benign zeolite-catalyzed ethylbenzene processes, including ZSM-5-zeolite-based vapor-phase ethylbenzene processes and Y-, β-, and MCM-22-zeolite-based liquid-phase processes, have been developed and commercialized. Pure ethylene, ethanol, and dilute ethylene have been used as ethylation agents. Here, the development and industrial application of alkylation catalysts and benzene ethylation techniques are summarized, and some other promising innovations are discussed. Recent advances in benzene alkylation over hierarchical zeolites with improved access to active sites and molecular transport are also covered. Zeolites with short diffusion lengths are promising candidates as better alkylation catalysts. The key point is how to obtain such materials easily and economically. The structure-activity relationships of commercial zeolites in these processes are discussed. Liquid-phase processes catalyzed by β and MCM-22 are more profitable than vapor-phase processes catalyzed by ZSM-5.

Synthesis of three-dimensional ordered mesoporous MnO₂ and its catalytic performance in formaldehyde oxidation

Bingyang Bai, Qi Qiao, Junhua Li, Jiming Hao

2016, 37 (1): 27-31. DOI: 10.1016/S1872-2067(15)61026-9

Abstract ( 611 ) [Full Text(HTML)] ()

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Three-dimensional (3D) ordered mesoporous MnO₂ was prepared using KIT-6 mesoporous molecular sieves as a hard template. The material was used for catalytic oxidation of HCHO. The material has high surface areas and the mesoporous characteristics of the template, with cubic symmetry (ia3d). It consists of a β-MnO₂ crystalline phase corresponding to pyrolusite, with a rutile structure. Transmission electron microscopy and X-ray photoelectron spectroscopy showed that the 3D-MnO₂ catalyst has a large number of exposed Mn⁴⁺ ions on the (110) crystal plane surfaces, with a lattice spacing of 0.311 nm; this enhances oxidation of HCHO. Complete conversion of HCHO to CO₂ and H₂O was achieved at 130 ℃ on 3D-MnO₂; the same conversions on α-MnO₂ and β-MnO₂ nanorods were obtained at 140 and 180 ℃, respectively, under the same conditions. The specific mesoporous structure, high specific surface area, and large number of surface Mn⁴⁺ ions are responsible for the catalytic activity of 3D-MnO₂ in HCHO oxidation.

Pore structure effects on the kinetics of methanol oxidation over nanocast mesoporous perovskites

Mahesh M. Nair, Freddy Kleitz, Serge Kaliaguine

2016, 37 (1): 32-42. DOI: 10.1016/S1872-2067(15)60909-3

Abstract ( 319 ) [Full Text(HTML)] ()

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Mesoporous LaMnO₃ perovskite catalysts with high surface area were synthesized by using the recently developed hard templating method designated as “nanocasting”. Ordered mesoporous silica designated as SBA-15 was used as the hard template. It was found that the surface area of the nanocast perovskites can be tuned (80-190 m²/g) by varying the aging temperature of the SBA-15 template. Nanocast LaMnO₃ catalysts showed high conversion efficiencies for the total oxidation of methanol under steady state conditions, the one with the highest value of surface area being the best catalysts, as expected. Kinetic studies were performed for all of the synthesized catalysts. Rate constants were found to vary in accordance with the specific surface area of the nanocast catalyst which depends on the aging temperature of the parent template. From the rate constants obtained from experimental conversions at various space velocities (19500 to 78200 h^-¹), values of activation energy and pre-exponential factor for the three nanocast LaMnO₃ catalysts were determined by the linear regression of the Arrhenius plot. It is observed that the activation energy for all the catalysts remain constant irrespective of the variation in specific surface area. Further, a linear relationship was found to exist between the pre-exponential factor and specific surface areas of the catalysts indicating that the rates per unit surface area remains the same for all the catalysts.

Ordered mesoporous carbon supported bifunctional PtM (M = Ru, Fe, Mo) electrocatalysts for a fuel cell anode

Chin-Te Hung, Zih-Hao Liou, Pitchaimani Veerakumar, Pei-Hao Wu, Tuan-Chi Liu, Shang-Bin Liu

2016, 37 (1): 43-53. DOI: 10.1016/S1872-2067(15)60878-6

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The deposition onto an ordered mesoporous carbon (OMC) support of well dispersed PtM (M = Ru, Fe, Mo) alloy nanoparticles (NPs) were synthesized by a direct replication method using SBA-15 as the hard template, furfuryl alcohol and trimethylbeneze as the primary carbon sources, and metal acetylacetonate as the alloying metal precursor and secondary carbon source. The physicochemical properties of the PtM-OMC catalysts were characterized by N₂ adsorption-desorption, X-ray diffraction, transmission electron microscopy, X-ray absorption near edge structure, and extended X-ray absorption fine structure. The alloy PtM NPs have an average size of 2-3 nm and were well dispersed in the pore channels of the OMC support. The second metal (M) in the PtM NPs was mostly in the reduced state, and formed a typical core (Pt)-shell (M) structure. Cyclic voltammetry measurements showed that these PtM-OMC electrodes had excellent electrocatalytic activities and tolerance to CO poisoning during the methanol oxidation reaction, which surpassed those of typical activated carbon-supported PtRu catalysts. In particular, the PtFe-OMC catalyst, which exhibited the best performance, can be a practical anodic electrocatalyst in direct methanol fuel cells due to its superior stability, excellent CO tolerance, and low production cost.

A Pd-metalated porous organic polymer as a highly efficient heterogeneous catalyst for C-C couplings

Zhifeng Dai, Fang Chen, Qi Sun, Yanyan Ji, Liang Wang, Xiangju Meng, Feng-Shou Xiao

2016, 37 (1): 54-60. DOI: 10.1016/S1872-2067(15)60952-4

Abstract ( 582 ) [Full Text(HTML)] ()

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An efficient catalyst system based on a Pd-metalated porous organic polymer bearing phenanthroline ligands was designed and synthesized. This catalyst was applied to various C-C bond-forming reactions, including the Suzuki, Heck and Sonogashira couplings, and afforded the corresponding products while exhibiting excellent activities and selectivities. More importantly, this catalyst can be readily recycled. These features show that such catalysts have significant potential applications in the future.

Size-control growth of thermally stable Au nanoparticles encapsulated within ordered mesoporous carbon framework

Shuai Wang, Jie Wang, Xiaojuan Zhu, Jianqiang Wang, Osamu Terasaki, Ying Wan

2016, 37 (1): 61-72. DOI: 10.1016/S1872-2067(15)60917-2

Abstract ( 419 ) [Full Text(HTML)] ()

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Simultaneously controlling the size of Au nanoparticles and immobilizing their location to specific active sites while hindering migration and sintering at elevated temperatures is a current challenge within materials chemistry. Typical methods require the use of protecting agents to control the properties of Au nanoparticles and therefore it is difficult to decouple the influence of the protecting agent and the support material. By functionalizing the internal surface area of mesoporous carbon supports with thiol groups and implementing a simple acid extraction step, we are able to design the resulting materials with precise control over the Au nanoparticle size without the need for the presence of any protecting group, whilst simultaneously confining the nanoparticles to within the internal porous network. Monodispersed Au nanoparticles in the absence of protecting agents were encapsulated into ordered mesoporous carbon at various loading levels via a coordination-assisted self-assembly approach. The X-ray diffractograms and transmission electron microscopy micrographs show that the particles have controlled and well-defined diameters between 3 and 18 nm at concentrations between 1.1 and 9.0 wt%. The Au nanoparticles are intercalated into the pore matrix to different degrees depending on the synthesis conditions and are stable after high temperature treatment at 600 ℃. N₂ adsorption- desorption isotherms show that the Au functionalized mesoporous carbon catalysts possess high surface areas (1269-1743 m²/g), large pore volumes (0.78-1.38 cm³/g) and interpenetrated, uniform bimodal mesopores with the primary larger mesopore lying in the range of 3.4-5.7 nm and the smaller secondary mesopore having a diameter close to 2 nm. X-ray absorption near extended spectroscopy analysis reveals changes to the electronic properties of the Au nanoparticles as a function of reduced particle size. The predominant factors that significantly determine the end Au nanoparticle size is both the thiol group concentration and subjecting the as-made materials to an additional concentrated sulfuric acid extraction step.

Rh₂O₃/mesoporous MO_x-Al₂O₃ (M = Mn, Fe, Co, Ni, Cu, Ba) catalysts: Synthesis, characterization, and catalytic applications

Huan Liu, Yi Lin, Zhen Ma

2016, 37 (1): 73-82. DOI: 10.1016/S1872-2067(15)60951-2

Abstract ( 412 ) [Full Text(HTML)] ()

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Recently, a one-pot self-assembly method was proposed for the synthesis of mesoporous Al₂O₃ and MO_x-Al₂O₃ composite materials. However, few attempts have been made to use mesoporous MO_x-Al₂O₃ composites to support metal oxides for catalysis. In the present work, mesoporous MO_x-Al₂O₃ (M = Mn, Fe, Co, Ni, Cu, Ba) materials were prepared by a one-pot self-assembly method using Pluronic P123 as a structure-directing agent. The obtained mesoporous materials were loaded with Rh₂O₃ nanoparticles via impregnation with Rh(NO₃)₃ followed by calcination in air at 500 ℃. The resulting catalysts were characterized by X-ray diffraction, N₂ adsorption-desorption measurements, transmission electron microscopy, inductively coupled plasma optical emission spectrometry, X-ray photoelectron spectroscopy, and their catalytic activity and stability for CO oxidation and N₂O decomposition were tested. The Rh₂O₃ nanoparticles were found to be on the order of 1 nm in size and were highly dispersed on the high surface area mesoporous MO_x-Al₂O₃ supports. A number of the Rh₂O₃/mesoporous MO_x-Al₂O₃ catalysts exhibited higher catalytic activity than the Rh₂O₃/mesoporous Al₂O₃ prepared for comparison.

Synthesis of Pd nanoparticles supported on CeO₂ nanotubes for CO oxidation at low temperatures

Jingmi Wu, Liang Zeng, Dangguo Cheng, Fengqiu Chen, Xiaoli Zhan, Jinlong Gong

2016, 37 (1): 83-90. DOI: 10.1016/S1872-2067(15)60913-5

Abstract ( 515 ) [Full Text(HTML)] ()

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Developing efficient supported Pd catalysts and understanding their catalytic mechanism in CO oxidation are challenging research topics in recent years. This paper describes the synthesis of Pd nanoparticles supported on CeO₂ nanotubes via an alcohol reduction method. The effect of the support morphology on the catalytic reaction was explored. Subsequently, the performance of the prepared catalysts was investigated toward CO oxidation reaction and characterized by Nitrogen sorption, X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and CO-temperature-programmed desorption techniques. The results indicated that the catalyst of Pd on CeO₂ nanotubes exhibits excellent activity in CO oxidation at low temperatures, due to its large surface area, the high dispersion of Pd species, the mesoporous and tubular structure of the CeO₂-nanotube support, the abundant Ce³⁺, formation of Pd-O-Ce bonding, and enhanced metal-support interaction on the catalyst surface.

Ni@Pd core-shell nanoparticles supported on a metal-organic framework as highly efficient catalysts for nitroarenes reduction

Siping Jian, Yingwei Li

2016, 37 (1): 91-97. DOI: 10.1016/S1872-2067(15)60940-8

Abstract ( 561 ) [Full Text(HTML)] ()

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Ni@Pd core-shell nanoparticles with a mean particle size of 8-9 nm were prepared by solvothermal reduction of bivalent nickel and palladium in oleylamine and trioctylphosphine. Subsequently, the first-ever deposition of Ni@Pd core-shell nanoparticles having different compositions on a metal-organic framework (MIL-101) was accomplished by wet impregnation in n-hexane. The Ni@Pd/MIL-101 materials were characterized by powder X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy and also investigated as catalysts for the hydrogenation of nitrobenzene under mild reaction conditions. At 30 ℃ and 0.1 MPa of H₂ pressure, the Ni@Pd/MIL-101 gives a TOF as high as 375 h^-1 for the hydrogenation of nitrobenzene and is applicable to a wide range of substituted nitroarenes. The exceptional performance of this catalyst is believed to result from the significant Ni-Pd interaction in the core-shell structure, together with promotion of the conversions of aromatics by uncoordinated Lewis acidic Cr sites on the MIL-101 support.

Recent advances of remote selective C-H activation: Ligand and template design

Guoqiang Yang, Nicholas Butt, Wanbin Zhang

2016, 37 (1): 98-101. DOI: 10.1016/S1872-2067(15)61008-7

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Progress in research on catalysts for catalytic oxidation of formaldehyde

Bingyang Bai, Qi Qiao, Junhua Li, Jiming Hao

2016, 37 (1): 102-122. DOI: 10.1016/S1872-2067(15)61007-5

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Formaldehyde (HCHO) is carcinogenic and teratogenic, and is therefore a serious danger to human health. It also adversely affects air quality. Catalytic oxidation is an efficient technique for removing HCHO. The development of highly efficient and stable catalysts that can completely convert HCHO at low temperatures, even room temperature, is important. Supported Pt and Pd catalysts can completely convert HCHO at room temperature, but their industrial applications are limited because they are expensive. The catalytic activities in HCHO oxidation of transition-metal oxide catalysts such as manganese and cobalt oxides with unusual morphologies are better than those of traditional MnO₂, Co₃O₄, or other metal oxides. This is attributed to their specific structures, high specific surface areas, and other factors such as active phase, reducibility, and amount of surface active oxygens. Such catalysts with various morphologies have great potential and can also be used as catalyst supports. The loading of relatively cheap Ag or Au on transition-metal oxides with special morphologies potentially improves the catalytic activity in HCHO removal at room temperature. The preparation and development of new nanocatalysts with various morphologies and structures is important for HCHO removal. In this paper, research progress on precious-metal and transition-metal oxide catalyst systems for HCHO oxidation is reviewed; topics such as oxidation properties, structure-activity relationships, and factors influencing the catalytic activity and reaction mechanism are discussed. Future prospects and directions for the development of such catalysts are also covered.

Flower-like 3D CuO microsphere acting as photocatalytic water oxidation catalyst

Xiaoqiang Du, Jingwei Huang, Yingying Feng, Yong Ding

2016, 37 (1): 123-134. DOI: 10.1016/S1872-2067(15)61012-9

Abstract ( 555 ) [Full Text(HTML)] ()

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Flower-like 3D CuO microspheres were synthesized and used to photo-catalyze water oxidation under visible light. The structure of the CuO microspheres was characterized by scanning electron microscopy, transmission electron microscopy, infrared, powder X-ray diffraction, electron dispersive spectroscopy, Raman and X-ray photoelectron spectroscopy (XPS). This is the first time that a copper oxide was demonstrated as a photocatalytic water oxidation catalyst under near neutral conditions. The catalytic activity of CuO microspheres in borate buffer shows the best performance with O₂ yield of 11.5%. No change in the surface properties of CuO before and after the photocatalytic reaction was seen by XPS, which showed good catalyst stability. A photocatalytic water oxidation reaction mechanism catalyzed by the CuO microspheres was proposed.

Oxidation of 4-chloro-3-methylphenol using zeolite Y-encapsulated iron(III)-, nickel(II)-, and copper(II)-N,N'-disalicylidene- 1,2-phenylenediamine complexes

Solomon Legese Hailu, Balachandran Unni Nair, Mesfin Redi-Abshiro, Isabel Diaz, Rathinam Aravindhan, Merid Tessema

2016, 37 (1): 135-145. DOI: 10.1016/S1872-2067(15)61010-5

Abstract ( 309 ) [Full Text(HTML)] ()

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The degradation of 4-chloro-3-methylphenol (PCMC) using zeolite-encapsulated iron(III), nickel(II), and copper(II) complexes of N,N'-disalicylidene-1,2-phenylenediamine as catalysts, in a heterogeneous Fenton-like advanced oxidation process, was studied. The physicochemical properties of the catalysts were determined using powder X-ray diffraction, thermogravimetric analysis, Brunauer-Emmett-Teller surface area analysis, Fourier-transform infrared spectroscopy, elemental analysis, and scanning electron microscopy. The effects of four factors, namely initial H₂O₂ concentration, catalyst dosage, temperature, and pH, on the degradation of a model organic pollutant were determined. The results show that at low acidic pH, almost complete removal of PCMC was achieved with the iron(III), nickel(II), and copper(II) catalysts after 120 min under the optimum reaction conditions: catalyst dosage 0.1 g, H₂O₂ concentration 75 mmol/L, initial PCMC concentration 0.35 mmol/L, and 50 ℃. The reusability of the prepared catalysts in PCMC degradation was also studied and a possible catalyst deactivation mechanism is proposed. The possible intermediate products, degradation pathway, and kinetics of PCMC oxidation were also studied.

Ultrasound assisted multicomponent reactions: A green method for the synthesis of N-substituted 1,8-dioxo-decahydroacridines using β-cyclodextrin as a supramolecular reusable catalyst in water

Asha V. Chate, Umesh B. Rathod, Jagdish S. Kshirsagar, Pradip A. Gaikwad, Kishor D. Mane, Pravin S. Mahajan, Mukesh D. Nikam, Charansingh H. Gill

2016, 37 (1): 146-152. DOI: 10.1016/S1872-2067(15)61005-1

Abstract ( 410 ) [Full Text(HTML)] ()

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We demonstrate a superficial method for the synthesis of N-substituted 1,8-dioxo-decahydroacridines using β-cyclodextrin as a supramolecular, biodegradable, and reusable catalyst in aqueous medium. The reaction product is in excellent yield with moderate to excellent selectivity. The mechanistic transformation presumably proceeds via a one-pot, multicomponent cyclization of dimedone in the presence of aromatic aldehydes and aromatic amines/INH, undergoing a tandem Michael addition reaction. The proposed approach in this study provides a highly efficient and environmentally benign route to N-substituted 1,8-dioxo-decahydroacridines.

Ruthenium nanoparticles supported on mesoporous MCM-41 as an efficient and reusable catalyst for selective oxidation of arenes under ultrasound irradiation

Alireza Khorshidi

2016, 37 (1): 153-158. DOI: 10.1016/S1872-2067(15)61001-4

Abstract ( 400 ) [Full Text(HTML)] ()

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Mesoporous MCM-41 was used as a support for the uniform dispersion of ruthenium nanoparticles having an average particle size of ~5 nm. The obtained nanocomposite, MCM-41-Ru, was characterized using inductively coupled plasma, transmission electron microscopy, energy dispersive X-ray analysis, X-ray diffraction, and BET surface area measurements. The material was employed as an efficient and recyclable catalyst in the ultrasound-assisted oxidation of arenes. It was observed that ultrasound irradiation in combination with KBrO₃ as the oxidant, in the presence of MCM-41-Ru nanoparticles, accelerates the oxidation reaction to afford the desired products in good yields. The recovered catalyst retained activity for successive runs, with a continuous change in the nature of its active sites.

Preparation of a novel supported electrode comprising a nickel (II) hydroxide-modified carbon paste electrode (Ni(OH)₂-X/CPE) for the electrocatalytic oxidation of formaldehyde

Safura Kavian, Seyed Naser Azizi, Shahram Ghasemi

2016, 37 (1): 159-168. DOI: 10.1016/S1872-2067(15)60990-1

Abstract ( 383 ) [Full Text(HTML)] ()

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We prepared a novel nickel (II) hydroxide-modified carbon paste electrode (Ni(OH)2-X/CPE) for the electrocatalytic oxidation of formaldehyde. The electrode was prepared by a simple method without the use of linking chemicals. The prepared Ni(OH)₂-X/CPE material was characterized by scanning electron microscopy and energy dispersive X-ray spectrometry. The electrochemical performance of the proposed electrode was investigated using cyclic voltammetry, electrochemical impedance spectroscopy, and chronoamperometry. The results indicate that Ni(OH)₂-X/CPE exhibits good electrocatalytic activity with regards to formaldehyde oxidation owing to its nanoporous structure and the large surface area of zeolite X. The values of the electron transfer coefficient and the catalytic rate constant were 0.7 and 6.1 × 10⁴ cm³/(mol·s), respectively. Therefore, the proposed electrode, which showed remarkable electroactivity with regards to formaldehyde oxidation with long-term stability and good reproducibility, could be useful in fuel cells.

Comparison of growth mechanisms of undoped and nitrogen-doped carbon nanofibers on nickel-containing catalysts

Vladimir V. Chesnokov, Olga Yu. Podyacheva, Alexander N. Shmakov, Lidiya S. Kibis, Andrei I. Boronin, Zinfer R. Ismagilov

2016, 37 (1): 169-176. DOI: 10.1016/S1872-2067(15)60982-2

Abstract ( 296 ) [Full Text(HTML)] ()

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The growth mechanisms of carbon nanofibers on Ni catalysts and nitrogen-doped carbon nanofibers on Ni and Ni-Cu catalysts were studied. The growth of both types of nanofibers was found to occur by a mechanism that included the formation of surface non-stoichiometric nickel carbide followed by the dissolution and diffusion of carbon, or carbon and nitrogen into the bulk of the catalyst particles.

Effects of alkaline additives on the formation of lactic acid in sorbitol hydrogenolysis over Ni/C catalyst

Junjie Zhang, Fang Lu, Weiqiang Yu, Rui Lu, Jie Xu

2016, 37 (1): 177-183. DOI: 10.1016/S1872-2067(15)60976-7

Abstract ( 342 ) [Full Text(HTML)] ()

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Lactic acid is produced as a major byproduct during sorbitol hydrogenolysis under alkaline conditions. We investigated the effects of two different alkaline additives, Ca(OH)₂ and La(OH)₃, on lactic acid formation during sorbitol hydrogenolysis over Ni/C catalyst. In the case of Ca(OH)₂, the selectivity of lactic acid was 8.9%. In contrast, the inclusion of La(OH)₃ resulted in a sorbitol conversion of 99% with only trace quantities of lactic acid being detected. In addition, the total selectivity towards the C2 and C4 products increased from 20.0% to 24.5% going from Ca(OH)₂ to La(OH)₃. These results therefore indicated that La(OH)₃ could be used as an efficient alkaline additive to enhance the conversion of sorbitol. Pyruvic aldehyde, which is formed as an intermediate during sorbitol hydrogenolysis, can be converted to both 1,2-propylene glycol and lactic acid by hydrogenation and rearrangement reactions, respectively. Notably, these two reactions are competitive. When Ca(OH)₂ was used as an additive for sorbitol hydrogenolysis, both the hydrogenation and rearrangement reactions occurred. In contrast, the use of La(OH)₃ favored the hydrogenation reaction, with only trace quantities of lactic acid being formed.

Solvent-free selective oxidation of cyclohexane with molecular oxygen over manganese oxides: Effect of the calcination temperature

Mingzhou Wu, Wangcheng Zhan, Yun Guo, Yunsong Wang, Yanglong Guo, Xueqing Gong, Li Wang, Guanzhong Lu

2016, 37 (1): 184-192. DOI: 10.1016/S1872-2067(15)60983-4

Abstract ( 457 ) [Full Text(HTML)] ()

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The effects of calcination temperature on the physicochemical properties of manganese oxide catalysts prepared by a precipitation method were assessed by X-ray diffraction, N₂ adsorption-desorption, X-ray photoelectron spectroscopy, H₂ temperature-programmed reduction, O₂ temperature-programmed desorption, and thermogravimetry-differential analysis. The catalytic performance of each of these materials during the selective oxidation of cyclohexane with oxygen in a solvent-free system was subsequently examined. It was found that the MnO_x-500 catalyst, calcined at 500 ℃, consisted of a Mn₂O₃ phase in addition to Mn₅O₈ and Mn₃O₄ phases and possessed a low surface area. Unlike MnO_x-500, the MnO_x-400 catalyst prepared at 400 ℃ was composed solely of Mn₃O₄ and Mn₅O₈ and had a higher surface area. The pronounced catalytic activity of this latter material for the oxidation of cyclohexene was determined to result from numerous factors, including a higher concentration of surface adsorbed oxygen, greater quantities of the surface Mn⁴⁺ ions that promote oxygen mobility and the extent of O₂ adsorption and reducibility on the catalyst. The effects of various reaction conditions on the activity of the MnO_x-400 during the oxidation of cyclohexane were also evaluated, such as the reaction temperature, reaction time, and initial oxygen pressure. Following a 4 h reaction at an initial O₂ pressure of 0.5 MPa and 140 ℃, an 8.0% cyclohexane conversion and 5.0% yield of cyclohexanol and cyclohexanone were achieved over the MnO_x-400 catalyst. In contrast, employing MnO_x-500 resulted in a 6.1% conversion of cyclohexane and 75% selectivity for cyclohexanol and cyclohexanone. After being recycled through 10 replicate uses, the catalytic activity of the MnO_x-400 catalyst was unchanged, demonstrating its good stability.

Theoretical studies of CO oxidation with lattice oxygen on Co₃O₄ surfaces

Yang-Gang Wang, Xiao-Feng Yang, Jun Li

2016, 37 (1): 193-198. DOI: 10.1016/S1872-2067(15)60969-X

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Low-temperature CO oxidation has attracted extensive interest in heterogeneous catalysis because of the potential applications in fuel cells, air cleaning, and automotive emission reduction. In the present study, theoretical investigations have been performed using density functional theory to elucidate the crystal plane effect and structure sensitivity of Co₃O₄ nano-catalysts toward catalyzing CO oxidation. It is shown that the surface Co-O ion pairs are the active site for CO oxidation on the Co₃O₄ surface. Because of stronger CO adsorption and easier removal of lattice oxygen ions, the Co₃O₄(011) surface is shown to be more reactive for CO oxidation than the Co₃O₄(001) surface, which is consistent with previous experimental results. By comparing the reaction pathways at different sites on each surface, we have further elucidated the nature of the crystal plane effect on Co₃O₄ surfaces and attributed the reactivity to the surface reducibility. Our results suggest that CO oxidation catalyzed by Co₃O₄ nanocrystals has a strong crystal plane effect and structure sensitivity. Lowering the vacancy formation energy of the oxide surface is key for high CO oxidation reactivity.

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