A study of the ageing and deactivation phenomena occurring during operation of an iron molybdate catalyst in formaldehyde production

Catalyst from a Perstorp Formox formaldehyde plant, operating with high inlet concentration of methanol (10.2 vol.%), was discharged from single tubes of a multi-tube reactor after half the expected lifetime of the catalyst and again after termination of the load. Each tube was filled with two different layers of catalyst. From the inlet of the reactor the first layer was a catalyst mixed with inert rings, which was followed by a second layer of pure catalyst extending from the middle to the outlet of the reactor. Catalyst fractions from the two layers were characterized with various techniques including BET, Fourier transform Raman (FT-Raman) spectroscopy, X-ray diffraction (XRD), elemental analysis with atomic absorption spectroscopy (AAS) and activity measurements. It was found that the surface area of the catalyst in the mixed layer increases during operation while a small decrease is noticeable for the catalyst below in the pure layer. Elemental analysis, XRD and FT-Raman show that during operation of the catalyst there is migration of Mo species from the upper part of the reactor towards the outlet. Activity measurements reveal severe deactivation of the catalyst in the mixed layer. It is concluded that the deactivation primarily is due to formation of volatile species formed by the MoO₃ surface reacting methanol, causing a decrease of the MoO₃/Fe₂(MoO₄)₃ mole ratio in the catalyst. Concerning the catalyst in the pure layer, the condensation of needle-like crystals of MoO₃ mainly occurs on the external surface of the catalyst ring.     

甲醇氧化制甲醛铁钼催化剂表面结构与活性

利用共沉淀法在不同搅拌速度下制备了相同Mo/Fe原子比的甲醇氧化制甲醛催化剂,采用SEM、XRD和拉曼光谱等对催化剂进行表征,在固定床微反上评价催化剂活性和选择性。结果表明,搅拌速度增大,催化剂比表面积增大,催化活性增强,甲醛收率由600 r·min^-1时的73.8%增加到10000 r·min^-1时的95.7%（280℃）。此外,催化剂由片状的MoO₃和颗粒状的Fe₂（MoO₄）₃两部分组成,游离的片状MoO₃无明显催化活性,只有与Fe₂（MoO₄）₃结合时才具有催化活性。     

Mechanism of deactivation of iron-molybdate catalysts prepared by coprecipitation and sol-gel techniques in methanol to formaldehyde oxidation

Stoichiometric iron-molybdate and an industrial like catalyst, with the usual Mo excess, were prepared by the normal coprecipitation technique. Additionally, a Mo rich catalyst was prepared by sol-gel like technique. The catalytic stability was tested in the presence and in the absence of water in the reactor feed. Only the stoichiometric catalyst deactivates appreciably in the tested conditions. Water in the reactor feed accelerates this process and exhibits a marked reaction inhibition effect. Water seems to hamper the catalyst reoxidation during reaction. The results of the applied characterisation techniques evidenced that the deactivation mechanism involves an increase of surface reduction (higher Fe²⁺/Fe³⁺ atomic ratio) and surface Mo loss by MoO₃ sublimation and formation of Mo-methanol and Mo-water volatile compounds. Mo species migrate from the bulk to the catalyst surface to compensate the Mo loss favoured by water. Mo excess is in fact required to have a stable, active and selective iron-molybdate catalyst for the methanol to formaldehyde oxidation.     

Selective oxidation of methane to formaldehyde on supported molybdate catalysts

A series of silica-supported molybdena catalysts with variable molybdenum content has been prepared and tested in the selective oxidation of methane to formaldehyde. The nature of the supported oxide phases has been studied by Fourier transform IR of chemisorbed NO molecule, X-ray photoelectron spectroscopy, UV-Visible reflectance and X-ray diffraction techniques. The results show that molybdenum oxide is highly dispersed on the silica at low molybdenum concentrations where two-dimensional polymolybdates are developed. Three-dimensional MoO₃ crystals grow in the region of high molybdenum concentrations. The analysis of the combined data suggests that there is a close relationship between methane conversion and formaldehyde selectivity and the presence of highly dispersed polymolybdate structures on the silica surface.     

甲醇氧化制甲醛用铁钼催化剂的研究

甲醇氧化制甲醛铁钼催化剂是由钼和铁的氧化物组成,其活性组分是钼酸铁和三氧化钼,添加不同的助剂可以提高该催化剂的性能。分别在催化剂制备工艺、催化剂活性评价和催化剂表征等方面对铁钼催化剂进行了探索。     

EQ-101型甲醇氧化制甲醛催化剂的性能评价

EQ-101型甲醛催化剂是一种铁钼系甲醇氧化制甲醛催化剂。考察反应温度、空速和进口甲醇含量等对EQ-101型甲醛催化剂性能的影响;通过500 h稳定性实验,考察EQ-101型甲醛催化剂的活性和稳定性。结果表明,在常压、反应温度300℃、空速12 000 h~(-1)和进口甲醇体积分数6.5%条件下,甲醇转化率99.5%,甲醛收率93.8%;500 h稳定性实验过程中,甲醛收率约稳定在93%,无破碎或粉化现象,催化剂活性和稳定性良好。     

甲醇氧化制甲醛铁钼催化剂研究

采用共沉淀法制备了不同Mo与Fe原子比的甲醇氧化制甲醛催化剂,在常压固定床反应器上对催化剂进行活性评价,采用BET、XRD和TEM对制备的催化剂进行表征。结果表明, Mo与Fe原子比为2.2~2.8时,催化剂具有较好的活性,(400~450) ℃焙烧的催化剂具有良好的比表面积、适宜的孔容和孔径,形成了较为稳定的MoO₃和Fe2(MoO₄)₃晶相,使催化剂具有更高的活性和选择性。对甲醇氧化制甲醛反应进行研究,结果表明,在反应温度(265~315) ℃,空速(8 500~13 000) h^-1时,甲醇转化率>98%,甲醛收率>93%, 500 h长周期考核,催化剂表现出良好的活性和稳定性。     

Determination of Optimum Conditions and the Kinetics of Methanol Oxidation

In this study, the catalytic oxidation of methanol to formaldehyde was investigated in a laboratory‐scale fixed‐bed catalytic reactor, under a large number of different conditions. Iron‐molybdate catalysts supported by silica or alumina with a molybdenium/iron (Mo/Fe) ratio of 1.5, 3 and 5 were studied for the gas phase reaction. In order to obtain the optimum conditions, six different temperatures in the range of 250–375 °C and three different space times of 50.63, 33.75 and 20.25 g/(mol/h) were investigated. After determining the optimum conditions for this reaction, experiments aimed at understanding the reaction kinetics, were carried out. These experiments were performed on the catalyst favoring the formation of formaldehyde, which has a (Mo/Fe) ratio of 5 on a silica support. Seven reaction models derived by the mechanisms cited in the literature were tested to elucidate the kinetics of the reaction and the surface reaction controlling model was found to be the most suitable reaction mechanism.     

硅源对纳米级ZSM-5分子筛结构及其对甲醇转化制丙烯与丁烯反应催化性能的影响

采用低温水热晶化法,以四丙基氢氧化铵为模板剂,分别使用98%粗孔固体硅胶和30%硅溶胶为硅源,制备不同硅铝物质的量比的纳米级ZSM-5分子筛,研究硅源对其物化结构及甲醇转化制丙烯与丁烯催化性能的影响。结果表明,硅源种类影响ZSM-5分子筛的结构及铝分布,进而影响其酸性和催化性能。固体硅胶为硅源,有利于形成弱酸性位点;硅溶胶为硅源,有利于形成强酸性位点。在相同硅铝物质的量比时,以固体硅胶为硅源的ZSM-5分子筛的总酸量小于以硅溶胶为硅源的样品。无论使用何种硅源,对ZSM-5分子筛的晶型结构影响不大,且ZSM-5分子筛颗粒形貌均呈现为由小晶粒堆积成(500~1 000)nm的类球形颗粒。以硅溶胶为硅源制备的样品颗粒尺寸大于以固体硅胶为硅源制备的样品。硅铝物质的量比为400时,两种硅源合成分子筛的丙烯与丁烯的选择性相近,但以硅溶胶为硅源的ZSM-5分子筛的寿命更长。     

Hydrothermal synthesis and crystal structure of the first layered iron molybdate KH3Fe2Mo2O10

The layered iron molybdate compound, KH₃Fe^II₂Mo^VI₂O₁₀, has been hydrothermally synthesized and structurally characterized for the first time. The compound crystallizes in monoclinic space group C2/m with a=9.6448(7), b=6.5946(4), , β=115.950(1)°, , Z=2, and R1=0.0313, wR2=0.0817. The structure consists of FeO₆ octahedra and MoO₄ tetrahedra. Each FeO₆ octahedron shares edges with its adjacent FeO₆ octahedra and leads to a chain consisting of edge-sharing FeO₆ octahedron. The parallel chains are linked through MoO₄ tetrahedra. Each MoO₄ tetrahedron shares corners with two adjacent Fe octahedra through the vertical oxygen atoms of each FeO₆, and two adjacent Fe octahedra through an oxygen atom bridging the two adjacent Fe octahedra of its neighboring chain, respectively. A ∞-like 2D layers and pseudo-six-side tunnels are formed and the potassium cations are located in the pseudo-channels.     

CO2加氢合成甲醇用Cu基催化剂的制备及性能研究

分别采用共沉淀法、聚乙二醇（PEG）辅助沉淀法和PEG辅助沉淀一浸渍法制备了3种Cu基催化剂,并于固定床反应器中分别评价了3种催化剂在CO2加氢合成甲醇反应中的催化性能,考察了催化剂的热稳定性和加氢反应温度对催化剂性能的影响,并进行了催化稳定性试验及催化剂的表征。实验结果表明：PEG辅助沉淀-浸渍法制备的cu—zn—Al—La催化剂具有较好的催化性能、热稳定性和催化稳定性。在沉淀过程中引入PEG能够提高还原态催化剂表面cu的比表面积,再通过表面浸渍La后,使催化剂表面有利于CO2吸附,并使表面参与反应的碱性位数量增加、吸附能力增强,进而提高了催化剂的催化性能。     

甲烷直接催化氧化制甲醇催化剂及反应机理

研究甲烷合成甲醇的催化剂及催化机理是甲烷直接催化氧化制甲醇的关键。本文综述了甲烷合成甲醇的Pt系催化剂、Pd系催化剂、Rh系催化剂、Fe改性沸石催化剂、Cu改性沸石催化剂、改性金属有机框架材料催化剂以及相应的催化机理。结果表明,Pt系催化剂中K₂PtCl₄、Pt(bpym)Cl₂、Pt(bipy)Cl₂催化甲烷制甲醇机理为亲电取代反应。Pd系催化剂中Pd(OAc)₂在CF₃COOH水溶液中通过多步电子传递链将甲烷转化为甲醇;Pd/C在乙酸水溶液中催化甲烷合成甲醇是亲电取代和活性氧物种氧化两种机理共同作用的结果。Rh系催化剂中的Rh/ZSM-5、Rh/TiO₂通过锚定在载体孔道内的一价Rh与CO、H₂O和O₂作用将甲烷转化为甲醇。Fe、Cu改性沸石催化剂及改性金属有机框架材料催化剂通过均裂、异裂等自由基反应转化甲烷得到甲醇。指出研究高效活化甲烷分子和抑制甲醇深度氧化催化剂的设计、制备及深入探究其催化机理仍然是今后的研究重点。     

Quasicatalytic and catalytic selective oxidation of methane to methanol over solid materials: a review on the roles of water

ABSTRACT

To date, although no commercial process for the selective oxidation of methane has been realized, various novel processes with effective solid materials operated at low temperature have been proposed. It is found that the addition of water in any processes not only influences the activity, selectivity, and stability of the solid materials but also affects the extraction efficiency of methanol from the product. Herein, the published results on the roles of water in the methanol production via the quasicatalytic and catalytic selective methane oxidation process using various solid materials in gas and liquid phases at low temperatures are critically reviewed.     

Effect of preparation methods on the structure and catalytic performance of Fe–Zn/K catalysts for CO_2 hydrogenation to light olefins

Potassium promoted iron–zinc catalysts prepared by co-precipitation method(C–Fe–Zn/K),solvothermal method(S–Fe–Zn/K)and hydrothermal method(H–Fe–Zn/K)could selectively convert CO_2to light olefins,respectively.The physicochemical properties of the obtained catalysts were determined by SEM,N_2physisorption,XRD,H_2-TPR,CO_2-TPD and XPS measurements.The results demonstrated that preparation methods had great influences on the morphology,phase structures,reduction and adsorption behavior,and hence the catalytic performance of the catalysts.The samples prepared by hydrothermal and co-precipitation method generated small uniform particles and led to lower specific surface area.In contrast,microspheres with larger specific surface area were formed by self-assembly of nanosheets using solvothermal method.ZnFe_2O_4was the only detectable phase in the fresh C–2Fe–1Zn/K,S–3Fe–1Zn/K and S–2Fe–1Zn/K samples.ZnFe_2O_4and ZnO co-existed with increasing Zncontent in S–1Fe–1Zn/K sample,while ZnO and Fe_2O_3could be observed over H–2Fe–1Zn/K sample.All the used samples contained Fe_3O_4,ZnO and Fe_5C_2.The peak intensity of ZnO was strong in the AR-H–2Fe–1Zn/K sample while it was the lowest in the AR-C–2Fe–1Zn/K sample after reaction.The formation of ZnFe_2O_4increased the interaction between iron and zinc for C–2Fe–1Zn/K and S–Fe–Zn/K samples,causing easier reduction of Fe_2O_3to Fe_3O_4.The surface basicity of the sample prepared by co-precipitation method was much more than that of the other two methods.During CO_2hydrogenation,all the catalysts showed good activity and olefin selectivity.The CO selectivity was increased with increasing Zncontent over S–Fe–Zn/K samples.H–2Fe–1Zn/K catalyst preferred to the production of C_5~+hydrocarbons.CO_2conversion of 54.76%and C_2~=–C_4~=contents of 57.38%were obtained on C–2Fe–1Zn/K sample,respectively.     

Effects of dissolved iron and chromium on the performance of direct methanol fuel cell

Effects of Fe³⁺ and Cr³⁺ ions on the performance of direct methanol fuel cell were investigated. The results show that the cell performance decreased remarkably when the concentration of Fe³⁺ or Cr³⁺ exceeded 1 × 10⁻⁴ mol L⁻¹. Fe³⁺ displayed a strong negative effect on the catalytic oxidation of methanol, while Cr³⁺ affected the cell performance primarily by exchanging with protons of the membrane/ionomer and resulted in ionic conductivity losses. Complete recovery of the cell performance was not obtained after flushing the cell with deionized water.     

不同制备工艺的铜基合成甲醇催化剂的性能及结构

探讨不同制备工艺条件对Cu基合成甲醇催化剂性能的影响。分别采用反加共沉淀法、反加共沉淀法+助剂、并流法和并流法+助剂制备了4种催化剂样品,考察催化剂性能的变化,并对助剂在催化剂合成过程中所起的作用进行推测,采用X射线衍射、低温N₂吸附和扫描电子显微镜等对催化剂进行表征。结果表明,Cu基甲醇合成催化剂制备过程中加入助剂可以调整催化剂的孔径分布和表面组分分布,催化剂组成确定后,催化剂制备过程采用并流法+助剂制备的样品,孔径分布更合理,ZnO组分晶粒尺寸较小,Cu和Zn组分间相互接触的几率更大,催化剂性能更好。     

Synthesis and characterization of BZSM-5 and its catalytic performance in the methanol to hydrocarbons reaction

Five kinds of BZSM-5 molecular sieve with different Si/B ratio and a SiZSM-5 molecular sieve were prepared by hydrothermal synthesis method followed by acid exchange and pelletization.The samples were characterized by XRD,SEM,FT-IR,ICP,low temperature N_2 physical adsorption and desorption,NH3-TPD and Py-IR.The catalytic performance in the reaction of methanol to hydrocarbons was evaluated in the fixed bed reactor.Compared with SiZSM-5,the amount and strength of Bronsted(B) acid were enhanced by introducing skeleton boron and the activity of the catalyst was greatly improved.The characterization and evaluation results indicated that the BZSM-5 catalyst synthesized from the gel of SiO_2/B_2 O_3 20 with Si/B ratio 74.48 had modest acidity strength,acid amount of 0.18 mmol NH_3·g~(-1) and large mesopore volume of 0.23 cm~3·g~(-1).The B acid ratio was higher and the acid strength of BZSM-5 was weaker than that of AIZSM-5,which could inhibit the deep coke formation and increase the activity stability.B-2 had the best lifetime which could reach 672 h under the same evaluation reaction conditions,due to the best matching of moderate acidity and good diffusion properties.     

A study of the structural and catalytic effects of sulfation on iron oxide catalysts prepared from goethite and ferrihydrite precursors for methane oxidation

A study of the effects of sulfation on the methane oxidation activity of iron oxide catalysts prepared from goethite and 2-line ferrihydrite precursors is presented. Although catalytic performance is found to be dependent upon the precursor, sulfation produces general effects in both systems. Surface area is increased, oxidation activity <400°C is suppressed, and that at 500°C is enhanced leading to the production of selective products. Despite these similarities, sulfation produces different structural effects in the two systems. In the case of iron oxide prepared from goethite, extensive pitting and an increase in disorder in the cationic arrangement are observed. In contrast, a slight increase in crystallinity of the iron oxide prepared from ferrihydrite occurs. The effect of sulfation on catalytic performance is interpreted in terms of a two-site mechanism, complexation poisoning F_e3+ sites active for the lower-temperature oxidation activity with additional sites active at higher temperature being produced. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of the method of preparing iron orthophosphate catalyst on the structure and the catalytic activity

Catalytic activity of iron orthophosphate [FePO₄] was studied using oxidative dehydrogenation of lactic acid to pyruvic acid as a reaction probe. The structure and surface area are different depending on the differences in the source of starting materials as well as in the preparing methods. However, the specific activity, activity per unit surface area, is little affected with the variation in the structure. By the reduction and reoxidation, the catalytic activity clearly increases.     

Highly active PtRu catalysts supported on carbon nanotubes prepared by modified impregnation method for methanol electro-oxidation

A simple and rapid synthesis method (denoted as modified impregnation method, MI) for PtRu/CNTs (MI) and PtRu/C (MI) was presented. PtRu/CNTs (MI) and PtRu/C (MI) catalysts were characterized by transmission electron microscopy (TEM) and X-ray diffractometry. It was shown that Pt-Ru particles with small average size (2.7 nm) were uniformly dispersed on carbon supports (carbon nanotubes and carbon black) and displayed the characteristic diffraction peaks of Pt face-centered cubic structure. Cyclic voltammetry and chronoamperometry showed that the Pt-Ru/CNTs (MI) catalyst exhibited better methanol oxidation activities than Pt-Ru/C (MI) catalyst and commercial Pt-Ru/C (E-TEK) catalyst. The single cells with Pt-Ru/CNTs (MI) catalyst exhibited a power density of 61 mW/cm², about 27% higher than those single cells with commercial Pt-Ru/C (E-TEK) catalyst.