Iron Phosphate Catalysts Research Articles

Gas-phase dehydration of glycerol to acrolein over different metal phosphate catalysts

We conducted a comparative study of gas phase dehydration of glycerol to acrolein over aluminium phosphate, iron phosphate and nickel phosphate catalysts prepared by a simple replacement reaction method (AlP, FeP and NiP). The textural properties, acid amounts, acid types, and coke contents of the samples were studied. The results showed that all metal phosphate catalysts remained in an amorphous state. The glycerol conversion was proportional to the acid amount of metal phosphate catalyst in the glycerol dehydration reaction. Higher value of B/L was more likely to produce acrolein. Among the metal phosphate catalysts, FeP showed superior performance due to its suitable textural and acid properties. After 2 h on stream, high glycerol conversion (96%), acrolein selectivity (82%) and acrolein yield (79%) were achieved on the FeP catalyst at 280 °C. The catalyst deactivation was ascribed to carbon deposition on the catalyst surface blocking the active sites during the glycerol dehydration reaction.

Switching Between Oxidation Types Using Molybdenum Phosphate Catalysts for Paraffin Activation Using Doped Fe as Surface Acidity Modifier and MoOx as an Oxygen Insertion Tool

A comparative study of the activation of n-hexane over 12-molybdophopshoric acid (H3PMo12O40 or HPA), Fe3+ doped HPA (Fe0.69H0.93PMo12O40) and an iron phosphate catalyst (P/Fe = 1.22) was carried out. The Fe doped HPA catalyst is thermally more stable and less acidic than the unmodified HPA catalyst. The HPA and the Fe doped HPA catalysts both catalytically produced 2,5-dimethyltetrahydrofuran and 2,5-hexadione (oxygenates), with the Fe doped HPA catalyst selectively producing more oxygenates than the HPA catalyst. The iron phosphate catalyst produced cis-2-hexene and 1-hexene but no oxygenates, which implies that the iron phosphate catalyst promotes dehydrogenation, but not oxygen insertion. At 360 °C, the HPA catalyst initiated benzene formation reactions. At isoconversion of ca. 8% under isothermal conditions, the iron phosphate catalyst gave the highest selectivity (72%) towards benzene, which may have formed through both catalytic and non-catalytic reactions. In contrast, the Fe doped HPA catalyst showed a lower benzene selectivity than the HPA catalyst with a corresponding increase in cyclic oxygenates.

Halogen-Dependent Surface Confinement Governs Selective Alkane Functionalization to Olefins.

The product distribution in direct alkane functionalization by oxyhalogenation strongly depends on the halogen of choice. We demonstrate that the superior selectivity to olefins over an iron phosphate catalyst in oxychlorination is the consequence of a surface-confined reaction. By contrast, in oxybromination alkane activation follows a gas-phase radical-chain mechanism and yields a mixture of alkyl bromide, cracking, and combustion products. Surface-coverage analysis of the catalyst and identification of gas-phase radicals in operando mode are correlated to the catalytic performance by a multi-technique approach, which combines kinetic studies with advanced characterization techniques such as prompt-gamma activation analysis and photoelectron photoion coincidence spectroscopy. Rationalization of gas-phase and surface contributions by density functional theory reveals that the molecular level effects of chlorine are pivotal in determining the stark selectivity differences. These results provide strategies for unraveling detailed mechanisms within complex reaction networks.

Effect of water and methanol in the production of methyl methacrylate over iron phosphate catalysts

This work discusses two production routes to methyl methacrylate (MMA) over an iron phosphate catalyst, namely oxidative dehydrogenation of methyl isobutyrate (MIB) and isobutyric acid (IBA) with methanol as co-feed. In situ studies show that at 400 °C, a mixture of the iron tridymite and quartz like phases are present under air, H2 and N2. The catalyst was tested in a fixed bed stainless steel reactor. The feed was passed over the catalyst with air and methanol; and air, water and methanol. The effects of the water to feed ratio and methanol to feed ratio were studied in terms of maximum conversion, yield and selectivity to MMA. Using methyl isobutyrate as a feed and methanol as a co-feed, the best yield of methylmethacrylate obtained was 40%. This corresponds to a selectivity of 73%, at a conversion of 55%. Feeding isobutyric acid and methanol over the iron phosphate catalyst gave an optimum yield of 15% methyl methacrylate at 400 °C. The yield of the products increased with increasing methanol/IBA ratio due to methanol limiting the hydrolysis reactions. The results obtained in this study show that MMA can be formed by the oxidative dehydrogenation–esterification of IBA and methanol over the iron phosphate catalyst.

An Oxidative Route for the Production of Methyl Methacrylate: A Study Over Iron Phosphate Catalysts

An iron phosphate catalyst prepared using the ammonia gel method was supported on silica (CAT 1) and a commercial iron phosphate catalyst was supported on silica using the wet impregnation method (CAT 2). The XRD patterns of both the catalysts showed the presence of quartz-like iron phosphate and a tridymite like phase. In situ XRD under a reducing environment and TPR showed the formation of iron pyrophosphate for both the catalysts at 500 °C. The Mossbauer spectra of the catalysts were similar and showed the presence of the ferric ion only. NH3-TPD revealed the presence of Lewis and Bronsted acidic sites on both the catalysts. Oxidative dehydrogenation reactions of methyl iso-burate (MIB) were carried out using a continuous flow fixed bed reactor at contact times of 0.4 and 0.8 s. The conversion of MIB was marginally higher at a contact time of 0.8 s. CAT 2 gave an 11 % yield of methyl methacryalate (MMA) compared to 21 % over CAT 1 at a contact time of 0.8 s. The conversion of MIB increased with co-feeding water. A maximum conversion of MIB (82 %) was obtained at 400 °C, giving a yield of 20 % MMA at a contact time of 0.8 s over CAT 1. However, co-feeding water increased the hydrolysis reaction also, which increased the yield towards iso-butyric acid and methacrylic acid.

Fe phase complexes and their thermal stability in iron phosphate catalysts supported on silica

Comparative XRD and Mossbauer spectroscopy studies have been conducted on the effect of temperature on the phase transformations of an iron phosphate catalyst synthesized using the ammonia gel method (CAT1) and a commercial grade FePO 4 catalyst supported on silica using wet impregnation method (CAT2). The XRD patterns of both catalysts showed the presence of iron phosphate and the tridymite phase of aluminum phosphate. Mossbauer spectra of the catalysts show that the phases present in CAT1 are thermally stable up to 500 ∘C, but CAT2 shows significant changes with the tridymite phase of iron phosphate increasing from 6 % to 29 % of the spectral area at a temperature of 500 ∘C.

Model Iron Phosphate Catalysts for the Oxy-bromination of Methane

Three kinds of bulk iron phosphates were prepared via different methods and employed as model catalysts in the oxy-bromination of methane (OBM). Na3Fe2(PO4)3 was obtained via a fluoride route for the first time that showed even superior catalytic performance than FePO4. Phase evolution from Na3Fe2(PO4)3 to Na2Fe3(PO4)3 after the OBM was confirmed both by X-ray diffraction and 57Fe Mossbauer spectroscopy. Temperature-programmed reduction of hydrogen and desorption of CH3Br revealed that the redox capacity of iron phosphates were responsible for the generation of bromine radicals on catalyst surfaces while CH3Br-involved reactions likely belonged to gas-phase reactions. Model iron phosphate catalysts were synthesized by three different methods. It was found that the Na3Fe2(PO4)3 catalyst prepared by a fluoride route showed superior OBM performance than the other two catalysts both with the phase of FePO4, especially for producing higher CH3Br selectivity.

Comparative study on stability and coke deposition over supported Rh and FePO4 catalysts for oxy-bromination of methane

Rhodium- and iron phosphate-based catalysts are by far the most promising catalysts for oxy-bromination of methane (OBM) reaction. However, most literature reported either Rh- or FePO4-based catalysts, and the results were rarely studied in a uniform environmental condition. In this report, comparative study was conducted on silica- and silicon carbide-supported rhodium and iron phosphate catalysts with the main focuses on stability performance and coke deposition. The catalytic results demonstrated that the stability of both Rh- and FePO4-based catalysts was greatly influenced by the supports used, and silicon carbide-supported catalysts showed much better anti-coking ability as compared with silica-supported ones. Temperature-programmed oxidation over the used catalysts further indicated that the coke formation mechanisms were completely different between silica-supported rhodium and iron phosphate catalysts. While cokes might be caused by condensation of CH2Br2 over supported iron phosphate, methane decomposition might be the reason for coke formation over silica-supported rhodium catalyst. These findings might pave the way for designing highly efficient and stable catalysts of the OBM reaction.

Efficient and stable silica-supported iron phosphate catalysts for oxidative bromination of methane

FePO4/SiO2 was employed for the oxidative bromination of methane (OMB) for the first time. A methane conversion of 50% and total selectivity of 96% for CH3Br plus CO (CH3Br:CO ≈ 1) were obtained at 570 °C. Comparison experiments were designed to investigate the roles of O2, HBr and the catalysts. Powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR) and 57Fe Mössbauer spectroscopic studies were conducted on the catalysts. It was confirmed that the FePO4 components of the fresh catalyst transformed quickly to stable active species during the induction period, and the active components of the stable catalyst have been found to consist of near-equimolar α-Fe3(P2O7)2 and Fe2P2O7. The reaction over the FePO4/SiO2 catalyst was proposed to follow a redox route based on the results of comparison experiments and associated characterization.

Effect of method of preparation on the activity and selectivity of iron phosphate catalyst in the ammoxidation of 2-methyl pyrazine

Ammoxidation of methylpyrazine (MP) to cyanopyrazine (CP) was performed at 633–693 K over iron phosphate catalysts having a P/Fe ratio of 1.2, obtained by adopting different methods of catalyst preparation. The catalysts were characterized by XRD, FTIR, UV-DRS, laser-Raman spectroscopy, TPR, TEM, XPS, acidity by Potentiometric titration and TPD of NH 3. XRD and Raman data suggested predominant formation of quartz phase of iron phosphate. The structure and texture of the catalysts varied with the nature of starting material as well as the preparation method. The catalytic activity depended on the extent of quartz phase formation. Precipitation in the basic medium improved quartz phase formation and enhanced the activity. However, the decreased acid strength caused reduction in nitrile selectivity. A judicious combination of the redox and acid strength properties is required for obtaining maximum yield of CP.

Studies on vanadium-doped iron phosphate catalysts for the ammoxidation of methylpyrazine

Bulk and vanadium-doped iron phosphate catalysts were prepared and studied for the vapor phase ammoxidation of 2-methyl pyrazine. The vanadium-doped catalyst showed better activity than bulk catalysts as the vanadium doping leads to formation of quartz-type iron phosphate with additional phases of V 2 O 5 and (VO) 2 P 2 O 7 . Vanadium-doped iron phosphates (with 1, 3 and 5 wt.% V) were prepared and characterized by XRD, FTIR, UV-DRS, TPR, TEM, Laser Raman, XPS, and potentiometric titration techniques. Both undoped and V-doped catalysts displayed predominant formation of quartz type iron phosphate. In addition, the V-doped catalysts indicated formation of two more phases: V 2 O 5 and (VO) 2 P 2 O 7 . The nitrile yield of V-doped catalysts in the ammoxidation of methyl pyrazine to cyanopyrazine were found to be higher than the yield of bulk FePO 4 , V 2 O 5 and (VO) 2 P 2 O 7 . The increase in the redox property of the catalyst, as obtained by benzyl alcohol oxidation to benzaldehyde, was found to be the reason for enhanced activity. The variation of V-content in FePO 4 revealed that the catalyst reached its optimum activity at 3 wt.% V. However, the acid strength of the V-doped catalysts decreased with V-content and this reflected in their cyanopyrazine selectivity.

Characterizations of Unsupported and Supported Rhodium−Iron Phosphate Catalysts Effective for Oxidative Carbonylation of Methane

Unsupported and MCM-41-supported Rh-doped FePO4 catalysts, which are effective for the oxidative carbonylation of methane directly to methyl acetate in the presence of CO and N2O, have been characterized by CO TPR in combination with XPS and FTIR spectroscopy of CO chemisorption. The CO TPR studies provide further evidence that the location of the Rh species is different in catalysts prepared by different methods. The CO TPR results and the correlation between the dispersion of the Rh species and the catalytic performance support the speculation that the dual sites containing RhIII and FeIII connected by phosphate groups are responsible for the conversion of methane to methyl acetate. The CO TPR studies also provide insight into the probable oxidation states of Rh and Fe during the catalytic reaction. We found that the RhIII and FeIII sites undergo simultaneous reductions during the reaction on the Rh−FePO4 sample prepared from a mixed aqueous solution and the MCM-41-supported Rh−FePO4 sample prepared by ...

SBA-15-supported iron phosphate catalyst for partial oxidation of methane to formaldehyde

SBA-15-supported iron phosphate (FePO 4) has been characterized with XRD, TEM, Raman spectroscopy and H 2-TPR, and studied for the partial oxidation of CH 4 with O 2. The characterizations suggest that the FePO 4 species with loading amount lower than 40 wt.% are encapsulated within the mesoporous channels of SBA-15, and these species possibly exist as small FePO 4 clusters with the local coordination environment of iron being similar to that in the crystalline FePO 4. However, these encapsulated FePO 4 clusters could be reduced at remarkably lower temperatures than the crystalline FePO 4. The SBA-15-supported FePO 4 catalysts exhibit higher CH 4 conversion and HCHO selectivity than the unsupported and the MCM-41-supported ones in the partial oxidation of CH 4 with O 2. The catalyst with a loading amount of 5 wt.% shows the highest HCHO selectivity at a given CH 4 conversion and the highest HCHO formation rate based on the amount of FePO 4 in the catalyst. It is likely that the improved catalytic performances of the SBA-15-supported samples are related to the enhanced redox properties of FePO 4 species, the large porous diameter and the high inertness of SBA-15.

Oxidation activity of iron phosphate and its characters

Iron phosphate catalysts possess a unique selectivity to oxidative dehydrogenation. Unlike ordinary catalysts consisting of molybdenum and/or vanadium, they have no double-bond oxygen species (MO) that are considered to be responsible for the oxygen insertion function and, as a result, for the degradation by C–C bond fission. The defect of iron phosphate catalysts is a lack of oxidation activity. Addition of a very small amount of molybdenum into iron phosphate enhances markedly the oxidation activity without modifying the high selectivity originated from iron phosphate. The marked increase in the activity is ascribable to an increase in the reducibility of iron phosphate, because the reoxidizability of iron phosphate is high enough. It is proposed that the reaction over a Mo-doped iron phosphate is promoted by redox cycle of iron phosphate but not by that of molybdenum phosphate.

MCM-41-supported iron phosphate catalyst for partial oxidation of methane to oxygenates with oxygen and nitrous oxide

Iron phosphate supported on MCM-41 has been studied for partial oxidation of methane with both oxygen and nitrous oxide. Characterizations with XRD, Raman spectroscopy, XPS, and H 2-TPR suggest that the supported iron phosphate species with loading amounts lower than 40 wt% are located and dispersed in the mesopores of MCM-41. Such iron phosphate species can be reduced more readily than the unsupported iron phosphate at lower temperatures. Methane is selectively converted to methanol, formaldehyde, and dimethyl ether over the supported and the unsupported iron phosphate with nitrous oxide at milder temperatures (300–500 °C), while formaldehyde is mainly produced along with carbon oxides with oxygen at relatively higher temperatures (400–600 °C). The supporting of iron phosphate onto MCM-41 with loading amounts of ca. 20–40 wt% increases both methane conversion and overall selectivity to useful oxygenates with either oxygen or nitrous oxide. Kinetic studies indicate that the activation of oxygen occurs rapidly, while the activation of nitrous oxide proceeds at a comparable rate with the conversion of methane by the active oxygen species over both the supported and the unsupported catalysts. The supported catalyst, however, enhances the activation of nitrous oxide and thus remarkably inhibits the carbon deposition occurring over the unsupported iron phosphate.

Design of selective catalysts for oxidative dehydrogenation

Iron phosphate catalysts possess a unique selectivity to oxidative dehydrogenation. Unlike catalysts consisting of molybdenum and/or vanadium, they have no double-bond oxygen species (M=O), which are considered to be responsible for the oxygen insertion function and, as a result, for the degradation by C–C bond fission. The defect of iron phosphate catalysts is their lack of oxidation activity. The addition of a very small amount of molybdenum to iron phosphate markedly enhances the oxidation activity without modifying the high selectivity that originates from iron phosphate.

In situ characterization of iron phosphate and bismuth molybdate catalysts prepared by hydrothermal methods: An EDXRD and combined XRD/XAS study

Iron phosphates and bismuth molybdate catalysts have been successfully prepared employing hydrothermal methods. In situ energy dispersive X-ray diffraction (EDXRD) and combined X-ray diffraction/X-ray absorption spectroscopy (XRD/XAS) studies reveal that these solid-state materials are formed from the amorphous precursor gel without the formation of any intermediate phases. While the time-resolved EDXRD measurements allowed us to determine the kinetics of phase formation, the combined XRD/XAS shows that there are local structural modifications taking place during the production of the crystalline active phases. More importantly the materials prepared by this hydrothermal method possess a much higher surface area as compared to the ones prepared by conventional methods.

Following the structural changes in iron phosphate catalysts by in situ combined XRD/QuEXAFS technique

The structure of iron phosphate catalysts prepared by three different synthetic methods has been studied using in situ combined XRD/QuEXAFS. Two of the synthesis procedures have previously been employed to produce these catalysts whilst the third is a new method employing hydrothermal techniques. The combined XRD/QuEXAFS approach supported by ex situ measurements carried out using XRD, TGA and surface area measurements, showed that all three methods produced a catalyst precursor material, having the chemical composition of FePO4·2H2O. The in situ study clearly shows the loss of two water molecules during the initial stages of heating and a transformation to FePO4 having tridymite type structure takes place at temperatures above ca. 100 °C. The crystallinity of the tridymite phase appeared to improve upon heating to ca. 500 °C and above which a further transformation to a quartz phase takes place. Of the three catalysts, the material produced by hydrothermal methods was found to be the most phase pure and to possess a higher surface area than the materials prepared by the other two methods.

Oxidative dehydrogenation of ethane over iron phosphate catalysts

The oxidative dehydrogenation (ODH) of ethane to ethylene over stoichiometric iron phosphate phases, e.g. FePO 4, Fe 2P 2O 7, α-Fe 3(P 2O 7) 2 and β-Fe 3(P 2O 7) 2, and nonstoichiometric (mixed) iron phosphate phases with P:Fe ratios of 1.2:1 and 2:1 was examined at temperatures ranging from 400 to 675 °C. The phosphates tend to be unstable under reaction conditions, and generally convert to the quartz-like FePO 4 and other phases, depending on stoichiometry. Under certain conditions, the nonstoichiometric phases exhibit an unusual activation behavior in that both selectivity and conversion increase over time. Excellent results were achieved over the active forms of the nonstoichiometric catalysts; ethylene yields approach 50% over a material with a P:Fe ratio of 1.2:1. We were unable to correlate a single crystalline or amorphous phase with catalytic performance. Rather, the evidence suggests that the activity and selectivity results from the migration of phosphorus out of the bulk iron phosphate material to the catalyst and reactor surfaces.

Importance of site isolation in the oxidation of isobutyric acid to methacrylic acid on iron phosphate catalysts

Isobutyric acid (IBA) oxidative dehydrogenation to methacrylic acid (MAA) reaction has been studied for a wide variety of iron (hydrogen) phosphates catalysts and mechanical mixtures of several of these phosphates. It was observed that the reaction needs high amounts of water, typically 10–12 mol water per mol IBA, to get stable, active and selective catalysts and follows a Mars and van Krevelen mechanism, as schematised below: $$Fe_3^3+ (PO_3 OH)_3 (PO_4) + (x/2)IBA \to Fe_{3-x}^{3+} Fe_x^{2+} (PO_3 OH)_{3+x} (PO_4)_{1-x} + (x/2)MAA$$ $$Fe_{3 - x}^{3 + } Fe_x^{2 + } (PO_3 OH)_{3 + x} (PO_4 )_{1 - x} + (x/4)O_2 \to Fe_3^{3 + } (PO_3 OH)_3 (PO_4 ) + (x/2)H_2 O$$ The best catalysts were found to be composed of trimers of face sharing FeO6octahedra, isolated one from the others by cationic vacancies and bounded to the next row by PO4tetrahedra. Solids having dimers or long chain octahedra are also active but less selective. Theoretical calculations showed that this corresponds to an electron hopping from one Fe2+to the next Fe3+cation during the redox mechanism for a trimer, while no electron hopping occurs for dimers and whereas it occurs all over the Fe cations for infinite octahedra chains. The presence of such vacancies and, to a lesser extent, of excess phosphorus at the surface, as shown by XPS analysis, exemplifies the site isolation concept in partial oxidation reactions. At variance, the cooperation effect between several phases was not observed.