Silica-supported Iron Catalysts Research Articles

Direct non-oxidative methane coupling on vitreous silica supported iron catalysts

Direct non-oxidative methane coupling (NMC) is one of the promising pathways for methane upgrading into value-added olefin and aromatic hydrocarbons. The silica-supported iron (i.e., Fe/SiO2) catalyst has been reported effective for NMC, but the effects of silica support on the catalyst property and NMC performance have rarely been explored. In this work, we prepared a vitreous silica-supported iron (Fe/SiO2-V) catalyst by flame fusion of a mixture of quartz silica and fayalite. The physicochemical properties and NMC performance of the as-prepared catalysts were measured. Compared to crystalline cristobalite support in Fe/SiO2 catalyst that has been studied previously, vitreous silica support has disordered Si-O bonds and structural defects, enabling better iron dispersion and more vital metal-support interaction. The as-prepared Fe/SiO2-V catalyst had a shorter induction period in methane activation and lower coke yield in NMC. The increase in iron concentration in Fe/SiO2-V catalysts elongated the catalyst induction period and promoted aromatics and coke formation. The coke type of the spent Fe/SiO2-V catalyst is more uniform than the cristobalite-supported iron catalyst.

Active phases for high temperature Fischer-Tropsch synthesis in the silica supported iron catalysts promoted with antimony and tin

Fischer-Tropsch synthesis provides an important opportunity for utilization of biomass and plastic waste. Iron catalysts are the catalysts of choice for light olefin synthesis using Fischer-Tropsch reaction. In this paper, we investigate strong promoting effects of antimony and tin on the catalytic performance of silica supported iron Fischer-Tropsch catalysts using a combination of advanced and in-situ techniques. The catalyst doping with these elements added via impregnation results in a major increase in the reaction rate and much better catalyst stability. No enhancement of iron dispersion was observed after the promotion, while somewhat higher extent of iron carbidization was observed in the antimony promoted catalysts. Iron-bismuth bimetallic nanoparticles are detected by several techniques. In the working catalysts, the promoters are located in close proximity to the iron nanoparticles. The promotion leads to the 7–10 times increase in the intrinsic activity of iron surface sites due to their interaction with the promoters.

Fischer–Tropsch synthesis: The effect of hydrophobicity on silica-supported iron catalysts

This work focuses on the effect of surface hydrophobicity of SiO2 supports on the performance of SiO2-supported Fe catalysts during Fischer–Tropsch synthesis (FTS). Three different hydrophobized SiO2 supports were synthesized by varying the amount of the dichlorodimethylsilane used for the hydrophobization of the hydrophilic mesoporous SiO2 gel. Fe metal of approximately 9wt% was loaded on the hydrophilic SiO2, and three hydrophobized SiO2 supports by homogeneous deposition precipitation. The wholly hydrophobized SiO2 surface improved the catalytic activity of Fe catalysts because the hydrophobized SiO2 increased the reducibility of the iron oxide and inhibited readsorption of water over the surface of the Fe catalyst, therefore creating more available active sites for the adsorption of CO. Also, hydrophobized SiO2 decreased the CH4 selectivity and C2–C4 selectivity and increased C5+ selectivity of the Fe catalysts due to the promotion of chain growth by the hydrophobized SiO2. Furthermore, the hydrophobized SiO2 decreased the CO2 selectivity of the Fe catalysts because the hydrophobized SiO2 hindered the readsorption of water over the surface of the Fe catalysts, which leads to the inhibition of water gas shift reaction and subsequently the hindering of the CO2 formation.

Identification of efficient promoters and selectivity trends in high temperature Fischer-Tropsch synthesis over supported iron catalysts

In this work, 29 elements were evaluated as promoters for silica supported iron catalysts for high temperature Fischer-Tropsch synthesis using a high-throughput experimentation unit. The selected promoters include alkali/alkaline metals, transition metals, precious metals and lanthanides. Several general selectivity trends were observed and discussed. The selectivity enhancement to light olefins requires maintaining low selectivity to methane and light paraffins and at the same time, slowing the chain growth to the C5+ hydrocarbons.A major increase in Fischer-Tropsch rate principally due to higher intrinsic site activity, was observed over the catalysts promoted with metals with low melting points such as tin, antimony, bismuth and lead. These promoted catalysts also exhibited better stability. The effect of the promotion with tin and antimony on the olefin selectivity was not noticeable, while the presence of bismuth and lead results in the major enhancement of the selectivity to light olefins, lower methane and C2-C4 paraffin selectivities.

Heterogeneous Fenton degradation of oxalic acid by using silica supported iron catalysts prepared from raw rice husk

Iron impregnated catalyst (RHSi-Fe) prepared from raw rice husk was used in the degradation of aqueous oxalic acid at acidic pH. The prepared catalyst was characterized using various analytical techniques. X-ray diffraction (XRD) and Energy dispersive X-ray (EDX) analysis were employed to characterize the catalyst which consistently confirmed the presence of silica as a major constituent in amorphous form. Scanning Electron Microscope (SEM)/EDX results indicated that the metal was present as agglomerates and the Fe ions were not homogeneously distributed in RHSi-Fe catalyst. Presence of SiOFe bond was also evident from the FTIR analysis. Percentage degradations of oxalic acid were 20.18 and 59.06 corresponding to the homogeneous (Fe(III)) and heterogeneous (RHSi-10Fe) catalytic degradations, respectively. Influence of different reaction operating parameters like iron loading to silica matrix, mass of prepared solid catalyst, initial concentration of substrate (oxalic acid), oxidant (hydrogen peroxide) and reusability of the catalyst on degradation of oxalic acid was investigated. RHSi-10Fe catalyst caused 59.06% degradation in 60min. 1g/L of RHSi-10Fe dosage was found to be optimum for effective degradation of oxalic acid. Oxalic acid degradation was improved by increasing temperature from room temperature (303K) to 343K. About 69.71% of oxalic acid was degraded within 60min at a temperature of 343K. The present study finally helps to get a biogenic catalyst which act as a heterogeneous catalyst on degradation of oxalic acid.

Thermoanalytical Characterization and Catalytic Conversion of Deoiled Micro Algae and Jatropha Seed Cake

The thermal decomposition of the byproducts of the biodiesel process was studied by thermoanalytical methods. Deoiled algae cake and jatropha seed deoiled cake were pyrolyzed and the catalytic effects of silica supported iron catalysts (Fe/FSM-16 and Fe/SBA-15) and magnetite (Fe3O4) were tested. The evolution profiles of the decomposition products as well as the thermal stability of the samples were determined by thermogravimetry/mass spectrometry (TG/MS). The formation of the volatile products was monitored by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). The composition and the amounts of the gaseous products changed significantly in the presence of the silica supported iron catalysts: the yield of hydrogen and carbon monoxide considerably increased above the decomposition temperature of 400 °C. Both silica supported iron catalysts had important effects on the yield of the products originating from carbohydrates and lignins. The formation of anhydrosugars and phenolic compounds was hindered, while the evolution of aromatic and aliphatic hydrocarbons was enhanced. Fe/FSM-16 proved to be more efficient than Fe/SBA-15 and Fe3O4 catalysts. The thermal decomposition of the protein content of the samples resulted in the formation of 2,5-diketopiperazines and smaller molecules (e.g., ammonia). The silica supported iron catalysts had a special effect: their presence promoted the reaction of fatty acid esters and ammonia resulting in the formation of alkyl nitriles during the thermal decomposition.

Rice husk ash-based silica-supported iron catalyst coupled with Fenton-like process for the abatement of rice mill wastewater

An economically efficient and eco-benign new approach was performed for the treatment of rice mill wastewater by Fenton-like process using rice husk ash (RHA)-based silica-supported iron catalyst. The detailed characterization of the prepared catalyst was described by SEM, XRD, and FT-IR. The maximum COD reduction of 75.5 % was achieved at response surface-optimized conditions that constitutes of catalyst dose 4.12 g/L, H2O2 concentration 4.2 g/L and pH 3.2. The non-toxic nature of the treated rice mill effluent using green catalyst was revealed by phytotoxicity. Meticulous efforts have been made for judicious utilization of one waste (RHA) against another serious waste (rice mill wastewater) abatement of the same industry. Findings clearly established that the proposed designed scheme offered a complete sustainable solution towards minimization of various wastes generated from rapidly growing rice mills.

Efficient removal of Rhodamine B using a rice hull-based silica supported iron catalyst by Fenton-like process

The objective of this study is to evaluate the catalytic performance of a rice hull-based silica supported iron catalyst on heterogeneous Fenton-like degradation of a xanthene dye, Rhodamine B. The catalyst was found to consist of aggregated silica nanospheres with about 3wt.% of iron loaded. The effects of various reaction parameters such as initial dye concentration, catalyst dosage, H2O2 concentration, pH and temperature on the catalytic degradation of Rhodamine B were studied. Almost 100% decolorization was achieved within 10min at an initial pH of 3.0. Chemical oxygen demand measurement shows that considerably high degree of mineralization could be achieved along with the decolorization of Rhodamine B. The degree of mineralization was further enhanced by using stepwise addition strategy in which the impact of adverse reactions could be minimized. In addition, the effects of foreign ions and ionic strength were investigated and both promoting and inhibiting effects on the degradation rate were observed. In particular, common salts found in textile wastewater such as Na2SO4 and NaCl could even enhance the degradation rate in this catalyst system. FTIR and ICP-MS analysis shows that the catalyst exhibits low iron leaching, good structural stability and no loss of performance in at least three times of repeated use.

Epoxide Ring-Opening Reactions with Mesoporous Silica-Supported Fe(III) Catalysts

A mesoporous silica-supported iron(III) catalyst was successfully synthesized by tethering ethylenediamine ligands onto the mesopore channel walls of mesoporous silica and then using the diamine groups to support Fe(III) ions. The resulting material is shown to be an efficient catalyst for the ring-opening of various epoxides by a series of alcohols as well as water under mild reaction conditions giving good-to-high yields of the corresponding ring-opened products. Among the alcohols tested with styrene oxide substrate giving 2-alkoxy-2-phenylethanol products, MeOH was the most reactive. It gave the highest percent conversion and yield (∼100%) within a relatively short reaction time of 6 h, whereas the longer alkyl alcohols gave a relatively lower yield of 93 to ∼100% with longer reaction times of 9−56 h. When the alkyl groups of the linear alkyl alcohols were longer, the rates of the catalytic reaction were slower. For instance, EtOH gave ∼100% yield in 9 h, but n-PrOH produced 93% product in 45 h. Subst...

Catalyst Support and Pretreatment Effects on Carbon Nanotubes Synthesis by Chemical Vapor Deposition of Methane on Iron over SiO2, Al2O3 or MgO

In this study we report the effects of support and pretreatment conditions on growth of carbon nanotubes (CNTs) by chemical vapor deposition of methane on iron catalyst supported on MgO, silica or alumina. The iron was impregnated onto the supports, and then the samples were dried, calcined at 550°C and pretreated in either helium or hydrogen up to 1000°C before exposure to methane as a carbon source for CNTs growth. Temperature programmed reduction (TPR) of the fresh catalysts and the ones pre-treated in He and in H2 shows various interactions of the iron with supports at pretreatment conditions. The CNTs are characterized by SEM, Raman, FTIR, and TEM. The IG/ID of Raman spectroscopy are 6.2, 3.8 and 0.7 for the CNTs grown on the MgO, alumina, and silica-supported iron catalysts pretreated in helium, respectively. When the Fe/MgO catalyst is pretreated in hydrogen the IG/ID ratio dramatically reduces to 0.8. A less significant effect of pretreating of the catalysts in hydrogen is observed for silica- and alumina-supported catalysts. RBM peaks of Raman spectra along with TEM results indicate the formation of bundles of 0.8-1.2 nm single-wall as well as multiwall carbon nanotubes on the Fe/MgO catalyst pre-treated in He.

Selective oxidation of methane to formaldehyde by oxygen over silica-supported iron catalysts

FeO x-SiO 2 catalysts prepared by a sol-gel method were studied for the selective oxidation of methane by oxygen. A single-pass formaldehyde yield of 2.0% was obtained over the FeO x-SiO 2 with an iron content of 0.5 wt% at 898 K. This 0.5 wt% FeO x-SiO 2 catalyst demonstrated significantly higher catalytic performances than the 0.5 wt% FeO x/SiO 2 prepared by an impregnation method. The correlation between the catalytic performances and the characterizations with UV-Vis and H 2-TPR suggested that the higher dispersion of iron species in the catalyst prepared by the sol-gel method was responsible for its higher catalytic activity for formaldehyde formation. The modification of the FeO x-SiO 2 by phosphorus enhanced the formaldehyde selectivity, and a single-pass formaldehyde yield of 2.4% could be attained over a P-FeO x-SiO 2 catalyst (P/Fe = 0.5) at 898 K. Raman spectroscopic measurements indicated the formation of FePO 4 nanoclusters in this catalyst, which were more selective toward formaldehyde formation.

Iron incorporated heterogeneous catalyst from rice husk ash

Silica supported iron catalyst was prepared from rice husk ash (RHA) via the sol–gel technique using an aqueous solution of iron(III) salt in 3.0 M HNO 3. The sample was dried at 110 °C and labeled as RHA-Fe. A sample of RHA-Fe was calcined at 700 °C for 5 h and labeled as RHA-Fe700. X-ray diffraction spectrogram showed that both RHA-Fe and RHA-Fe700 were amorphous. The SEM/EDX results showed that the metal was present as agglomerates and the Fe ions were not homogeneously distributed in RHA-Fe but RHA-Fe700 was shown to be homogeneous. The specific surface areas for RHA-Fe and RHA-Fe700 were determined by BET nitrogen adsorption studies and found to be 87.4 and 55.8 m 2 g −1, respectively. Both catalysts showed high activity in the reaction between toluene and benzyl chloride. The mono-substituted benzyltoluene was the major product and both catalysts yielded more than 92% of the product. The GC showed that both the ortho- and para-substituted monoisomers were present in about equal quantities. The minor products consisting of 16 di-substituted isomers were also observed in the GC–MS spectra of both catalytic products. The catalyst was found to be reusable without loss of activity and with no leaching of the metal.

Temperature-Programmed Hydrogenation (TPH) and in Situ Mössbauer Spectroscopy Studies of Carbonaceous Species on Silica-Supported Iron Fischer−Tropsch Catalysts

Carbonaceous surface species and bulk iron carbides formed under realistic Fischer-Tropsch synthesis (FTS) conditions on moderately dispersed, active silica-supported iron catalysts (Fe/SiO2, FePt/SiO2, and FePtK/SiO2) were characterized. Bulk iron phase compositions were determined by Mössbauer spectroscopy and phase transformations of carbonaceous species during pretreatment with CO, H2, or H2/CO and following reaction were characterized using temperature-programmed hydrogenation (TPH). Isothermal transient rates of FTS were also measured for catalysts after different pretreatments. Six surface and bulk carbonaceous species were quantitatively identified from combined TPH and Mössbauer spectra of the FePtK catalyst. They include, in order of decreasing reactivity, (a) adsorbed, atomic carbon; (b) amorphous, lightly polymerized hydrocarbon or carbon surface species; (c) bulk epsilon' and chi carbides (Fe(2.2)C and Fe(2.5)C); and (d) disordered and moderately ordered graphitic surface carbons. A correlation between the amount of reactive alpha-carbon (C(alpha)) and initial catalytic activity was established. The method of Li et al. for measuring irreversible chemisorption of CO does not appear to provide quantitative measurements of active site densities on silica-supported iron. Models, based on this and previous work, are proposed for iron phase and carbon phase transformations in silica-supported iron during pretreatment, FTS, and postreaction passivation/oxidation.

Design, Synthesis, and Catalytic Properties of Silica-Supported, Pt-Promoted Iron Fischer–Tropsch Catalysts

Silica-supported iron catalysts (Fe/SiO2, FePt/SiO2, and FePtK/SiO2) were prepared using a novel nonaqueous (acetone) evaporative deposition technique. This preparation leads to relatively well-dispersed iron phases at modest (10%) metal loadings. Moreover, catalytic activities of these catalysts for Fischer–Tropsch synthesis are high and comparable to industrially relevant precipitated iron catalysts. Catalyst activities were tested following a nonregular L18 orthogonal array that enabled the number of 150-h activity tests to be reduced from 54 to 18; this statistical design was augmented with five additional runs to provide replication. Primary independent variables affecting catalysts' activity were promoter type, pretreatment gas composition (H2, H2/CO, or CO), pretreatment temperature (250, 280, or 320 °C), and reaction temperature (250 or 265 °C); iron carbide level measured from Mossbauer spectroscopy was correlated with activity in a separate analysis. Activity was found to increase in the order Fe/SiO2, FePt/SiO2, and FePtK/SiO2. For a given catalyst composition, activity increases to a maximum with increasing pretreatment temperature and increasing time. Catalyst activity was also positively correlated with increasing chi-carbide content for Fe/SiO2 and FePt/SiO2 catalysts but not for FePtK/SiO2. While pretreatment atmosphere greatly influences initial activity–time behavior, activity is less dependent on pretreatment after about 150 h of reaction. Steady-state methane and C2+ hydrocarbon selectivities (CO2-free basis) for the FePtK/SiO2 catalyst at 250–265 °C, 10 atm, and H2/CO = 1 are 7–9 and 91–93%, respectively, while its hydrocarbon productivity at 250 °C (normalized to 15 atm, H2/CO = 0.7) of 0.27 g HC/gcat/h is comparable to those reported for unsupported precipitated iron catalysts of high activity and selectivity. These results indicate that preparation of an active, selective, stable, attrition-resistant supported iron catalyst for Fischer–Tropsch synthesis is feasible. Promise for additional improvements in catalyst performance through application of advanced preparation methods and optimization of catalyst chemical and physical properties is also indicated.

Unusually High Selectivity to C2+ Alcohols on Bimetallic CoFe Catalysts During CO Hydrogenation

Silica-supported cobalt and iron catalysts (10% Co and 5% or 1% Fe) were prepared and tested in a flow reactor in the hydrogenation of CO, using H2/CO = 2:1 (molar) ratio in the feed, an overall pressure of 20 bar, and temperatures of 493, 513 and 533 K. Activity and product distribution were found to depend strongly on the composition of the catalysts. Thus, the Fe-free catalyst was selective toward C5+ formation (67% selectivity to C5+) and a low methanation rate, while the Co-free counterpart was less selective toward C5+, with a simultaneous increase in the formation of lighter fractions and alcohols. The behavior of the bimetallic CoFe catalysts was different. In the bimetallic CoFe10/5-c catalyst, selectivity to alcohols increased with respect to the monometallic Co10-c, and this was moderately high (15% to C3+ OH alcohols). In the bimetallic CoFe10/1-c sample, selectivity to alcohols was fairly high (29%), and ethanol reached the highest proportion (17%) among the alcohols. Surface and structural information concerning the activated catalysts, derived from X-ray diffraction, temperature-programmed reduction, Mossbauer, and photoelectron spectroscopy, revealed the appearance of a CoFe phase under the conditions employed during the catalyst activation. In the bimetallic cobalt–iron catalysts, this CoFe phase is suggested to be responsible for the rather high selectivity toward alcohol formation.

Production of bundles of aligned carbon and carbon–nitrogen nanotubes by the pyrolysis of precursors on silica-supported iron and cobalt catalysts

Pyrolysis of acetylene over iron or cobalt nanoparticles well dispersed on silica substrates gives copious yields of aligned carbon nanotube bundles. By carrying out the pyrolysis of pyridine over these catalyst surfaces, good quantities of aligned carbon–nitrogen nanotube bundles have been produced. The composition of the carbon–nitrogen nanotubes varies between C 10N and C 33N, depending on the catalyst.

THE PROMOTER EFFECT OF ALKALI METAL TO SUPPORTED IRON CATALYST IN CARBON MONOXIDE HYDROGENATION, THE INFLUENCE OF ALKALI ON CARBON DEPOSITIONS

The promoter effect of alkali metals on CO hydrogenation was studied on a series of silica supported iron catalysts. Mossbauer spectra of the catalysts show that iron is well dispersed on the silica support. The decrease in chemisorption of carbon monoxide on addition of promoters reveals that alkali covers part of the iron metal surface. The production of olefins, carbon dioxide and long chain hydrocarbons increases with decreasing ionization potentials of the alkali metals added. Hydrogenative decarbidation of catalysts after CO/H2 reaction show that the hydrogenation of the carbon depositions on iron catalysts can be well explained by a series reaction model. The relative reactivity of carbon depositions on well dispersed iron catalysts are strongly affected under the influences of alkali. Addition of alkali metals to iron catalyst may depress the hydrogenation ability of the catalysts.

Activity for ethane hydrogenolysis and carbon gasification of highly-dispersed metallic iron catalysts

The catalytic activities of graphite-, carbon black-, alumina-, and silica-supported metallic iron catalysts for the hydrogenolysis of ethane were measured. Turnover frequencies varied over a range of three orders of magnitude. Carbon black-supported catalysts prepared from ferric nitrate had higher activities than carbon black-supported catalysts prepared from Fe3(CO) 12, the latter having been shown to have more spherical and possibly more widely separated metallic iron particles within the pore structure of the carbon black support. The low catalytic activity of the cluster-derived catalyst may be due to the spherical morphology of the iron particles, giving rise to highly unsaturated iron surface atoms and altering the nature of the catalytic ensembles present, or to a partial coverage of the iron surface by carbon from the support. The iron carbonyl-derived catalyst was also inactive for support gasification in hydrogen, while the conventionally-prepared catalyst almost completely gasified the carbon support during treatment in hydrogen at 773 K for 2 h.

The reduction behavior of silica-supported and alumina-supported iron catalysts: A Mössbauer and infrared spectroscopic study

The reduction behavior of Fe/Al 2O 3 and Fe/SiO 2 catalysts is investigated using in situ Mössbauer spectroscopy and infrared spectra of adsorbed CO. Controlled injection at constant pH (= 6) of an acidified iron(III)nitrate solution into a suspension of the support leads to the formation of highly dispersed supported α-FeOOH particles (2–8 nm). Reduction of the Fe/Al 2O 3 catalyst is found to proceed via a stabilized Fe 1− x O phase according to the sequence α-FeOOH/Fe 3O 4/Fe 1− x O/α-Fe. Reduction for 15 h at 873 K leads to iron crystallites 20–30 nm in diameter. It is shown that these surfaces still contain traces of oxygen. This is in accordance with single crystal investigations which indicate that the reduction of iron surfaces is a surface-sensitive process: full reduction of small particles (consisting of more open surfaces) is virtually impossible. Moreover, it is established that upon high-temperature evacuation following hydrogen reduction the iron surfaces are severely oxidized by water molecules formed by hydrocondensation of hydroxyl groups from the support. Reduction of the Fe/SiO 2 catalyst proceeds via an iron(II)silicate phase, which at higher temperatures is partly reduced to α-Fe. Small iron particles (10 nm) densely and homogeneously distributed over the support are obtained by prolonged reduction (up to 150 h) at 723 K, whereas severe sintering occurs upon reduction for relatively short periods (16 h) at temperatures above 823 K. With the silica-supported iron catalyst the iron crystallites appear to be (partly) encapsulated by an iron(II)silicate layer. The encapsulation of the silica-supported iron particles, the incomplete reduction of the alumina-supported iron particles, and the reoxidation occurring upon high-temperature evacuation presumably account for the small extent of room temperature hydrogen chemisorption usually found with supported iron catalysts.

Preparation and Properties of Small Silica-Supported Iron Catalyst Particles: Influence of Reduction Procedure

The genesis of small metallic iron particles supported on a high-surface area silica support was studied by in situ Mo¨ssbauer spectroscopy, and also by employing applied magnetic fields. α-Fe particles with dimensions of the order of 6–7 nm were found to have a superparamagnetic blocking temperature near 300 K. The particle size was estimated from the magnetic field dependence of the Mo¨ssbauer spectra. The particle size distribution depends critically on the preparation conditions. It was found that relatively large α-Fe particles (d≈10 nm) are formed after reduction under conditions similar to those employed in most earlier studies. After more extended reduction, Fe 2+ ions in the silica (probably in octahedral coordination) are reduced to much smaller α-Fe particles (d≈6 nm). These iron particles behave superparamagnetically at room temperature and are not easily detected in zero-field Mo¨ssbauer spectra.