Dick E. Stobbe

Iron oxide dehydrogenation catalysts supported on magnesium oxide. Part 2.—Reduction behaviour

The reduction behaviour of iron oxide supported on magnesium oxide has been studied using temperature-programmed reduction (TPR), X-ray diffraction (XRD), and high-field magnetic measurements. The catalysts, containing 1–6 wt.% Fe, were prepared by incipient-wetness impregnation of a preshaped magnesium oxide support with organometallic complexes, such as, ammonium iron(III) citrate and ammonium iron(III) EDTA. In the fresh calcined catalyst the iron oxide is present as magnesium ferrite, MgFe2O4. With TPR two broad reduction peaks, reflecting the sequential reduction of Fe3+via Fe2+ to Fe0, were observed. High-field magnetic measurements enabled us to describe the reduction behaviour in detail. In the first reduction step, proceeding at ca. 630 K, MgFe2O4 is reduced to FeO partly via Fe3O4. A second reduction step at 750 K results in the formation of metallic iron. Part of the metallic iron has been found to be present in the form of superparamagnetic particles within the MgO support.

Potassium promotion of iron oxide dehydrogenation catalysts supported on magnesium oxide: 1. Preparation and characterization

Catalysts of iron oxide supported on magnesium oxide and promoted with potassium were prepared by incipient wetness impregnation of preshaped magnesium oxide support pellets with a solution of an iron complex, either ammonium iron (III) citrate or ammonium iron (III) EDTA and potassium carbonate. Iron and potassium were applied wither simultaneously or consecutively. As determined using X-ray diffraction, thermogravimetric analysis, and magnetic measurements, calcination above 923 K results in the formation of a mixed oxide of iron and potassium, viz., KFeO[sub 2]. After calcination at 973 K the average crystallite size of the KFeO[sub 2] phase is about 300 [angstrom]. The formation of KFeO[sub 2] appeared to have a strong retarding effect on the reduction of the iron oxide phase to metallic iron. It was found that the KFeO[sub 2] phase is unstable in atomspheric air due to reaction with carbon dioxide and moisture to form potassium (hydrogen) carbonate and (hydrated) iron oxide.

Iron oxide dehydrogenation catalysts supported on magnesium oxide. Part 1.—Preparation and characterization

Iron oxide catalysts supported on magnesium oxide have been prepared by impregnation of a preshaped magnesium oxide support to incipient wetness with poorly crystallizing organometallic complexes. As organometallic complexes ammonium iron(III) citrate and ammonium iron(III) EDTA have been used. After impregnation the iron has appeared to be homogeneously distributed throughout the magnesium oxide support bodies. Upon calcination in air, the precursors decompose to form magnesium ferrite at temperatures as low as ca. 800 K. Calcination in air at 973 K results in the formation of small MgFe2O4 crystallites with a typical size of ca. 200 A, as determined by X-ray diffraction and electron microscopy.

Iron oxide dehydrogenation catalysts supported on magnesium oxide. Part 3.—But-1-ene dehydrogenation activity

The catalytic behaviour of iron oxide catalysts supported on magnesium oxide in the non-oxidative dehydrogenation of but-1-ene to buta-1,3-diene has been studied. Under dehydrogenation conditions the magnesium ferrite phase of the fresh catalyst was found to be reduced to FeO in the bulk of the ferrite particles, where FeO is strongly stabilized by the formation of a solid solution with MgO. At the surface of the particles magnesium ferrite is reduced to Fe3O4. In the reduced state a catalyst containing ca. 3 wt.% Fe shows a but-1-ene conversion of 26% combined with a buta-1,3-diene selectivity of 65% at 873 K. The activation energy for dehydrogenation was found to be 47 kcal mol–1.The (unpromoted) catalyst gradually deactivates owing to carbon deposition. Under dehydrogenation conditions the catalyst surface is completely covered by a layer of amorphous carbon within 20 h.

Potassium promotion of iron oxide dehydrogenation catalysts supported on magnesium oxide: 2. 1-Butene dehydrogenation activity

Abstract Potassium promotion of iron oxide catalysts supported on magnesium oxide results in considerably more active and selective 1-butene dehydrogenation catalysts. Upon promotion the activation energy was found to decrease from 194 to 156 kJ/mol. KFeO 2 appeared to be the active phase under dehydrogenation conditions. No reduction of KFeO 2 was observed. KFeO 2 shows high 1-butene dehydrogenation activity, yet it is not sufficiently effective to suppress coking entirely. For that purpose the presence of highly dispersed potassium carbonate at the catalyst surface is a prerequisite. Under identical dehydrogenation conditions, a commercial unsupported catalyst, S-105, which contains the more easily reducible KFe 11 O 17 , is reduced to Fe 3 O 4 . Compared with this unsupported S-105 catalyst, the supported catalysts show significantly higher 1,3-butadiene selectivities at comparable conversion levels, which is to be attributed to the different natures of their respective active phases.

Kinetic studies of coke formation and removal on HP40 in cycled atmospheres at high temperatures

An austenitic FeNiCr alloy, HP40Nb, has been preoxidized and subsequently exposed to an alternating carburizing/oxidizing /carburizing atmosphere. During the oxidation at 1000°C a thick Cr 2 O 3 layer was formed which partly spalled off during cooling to room temperature, in this way chromium depleted areas resulted at the surface. The carburizing and reducing condition was established by a C 2 H 6 /C 2 H 4 /H 2 mixture at a temperature of 850 °C while the oxidation for decoking was conducted in air at 800°C. The exposure times were relatively short, respectively 90/30/180 minutes. During the first exposure of the preoxidized alloy to the carburizing atmosphere, coke formation took place, and underneath the coke layer the alloy was carburized, however, only locally. After the decoking in air at 800°C, during the second exposure to the carburizing atmosphere much more catalytic coke formation was observed compared to the first exposure. The coke formation was initiated by the reduction of (Fe,Ni,Cr)-spinels formed in the oxidizing atmosphere. The reduction of the oxides gives rise to the formation of (Fe,Ni)-particles which show strong catalytic activity towards coke formation.

Magnesium oxide as a support material for dehydrogenation catalysts

Two commercial preshaped magnesium oxides, viz. pellet-shaped magnesium oxide (Englhard, Mg-0601) and spherical magnesium oxide (Sud-Chemie, T-4403), have been evaluated for use as carrier material in supported dehydrogenation catalysts. The influences of temperature and steam atmosphere on the textures of the support materials have been studied.The texture of the pellet-shaped support has been found to be well controlled by thermal treatment in air. Upon thermal treatment the B.E.T. surface area of this magnesium oxide decreases, while the average pore radius increases. Typically, a B.E.T. surface area of 6 m2 g–1 and an average pore radius of 1000 A is obtained upon treatment at 1173 K. Spherical magnesium oxide (Sud-Chemie, T-4403) has a very low surface area of 0.18 m2 g–1 and an average pore radius of 40 000 A. Calcination in air at temperatures up to 1473 K does not affect the texture. In 4.8 bar of steam at 423 K both magnesium oxides react completely to magnesium hydroxide. This is accompanied by a drastic increase of the B.E.T. surface area and a decrease of the mechanical strength. At 973 K and a steam pressure of 1 bar no reaction with water is observed.

Supported Dehydrogenation Catalysts Based on Iron Oxide

Abstract A magnesia-supported iron oxide catalyst promoted with potassium was prepared for the dehydrogenation of hydrocarbons, such as, e.g. ethylbenzene or 1-butene. With high-field magnetic measurements it was established that potassium ferrite, KFeO 2 , is the active phase of the catalyst under dehydrogenation conditions.

Assessment of the free surface area of magnesia-supported magnesium ferrite particles using selective oxygen chemisorption

The free surface area of iron oxide catalysts supported on magnesium oxide has been determined using selective oxygen chemisorption at room temperature. The oxygen chemisorption experiments were performed on reduced catalysts. It has appeared to be essential that the iron oxide phase, which is present as MgFe2O4 in the fresh calcined catalysts, is reduced uniquely to FeO, prior to the chemisorption experiment. Crystallite sizes typically in the range of 20–23 nm have been found independent of the catalyst loading and the type of precursors. A higher iron loading does not lead to larger crystallites, but rather to more crystallites of about the same average size. Results are in very good agreement with those obtained by X-ray line broadening and electron microscopy.