JP2015521094A - Gas phase oxidation catalyst with low charge transfer activation energy - Google Patents

Abstract

A catalyst for vapor phase oxidation of organic hydrocarbons, comprising a multi-element oxide comprising at least one transition metal such as vanadium, said catalyst being less than 0 kJ / mol at a temperature of 375-425 ° C A catalyst having a charge transfer activation energy Ec of Said catalyst serves for the preparation of maleic anhydride.

Description

  Transition metal catalyzed oxygen transfer reactions are the most important reactive species in the chemical industry. The cutting edge is the partial oxidation of hydrocarbons with molecular oxygen. Molecular oxygen in the form of atmospheric oxygen is undoubtedly the cheapest oxidant. In the base state, oxygen exists as a paramagnetic triplet and is relatively non-reactive. As a result of the interaction with the catalyst surface, oxygen is activated by coordination to the catalyst surface, electron transfer, dissociation of oxygen molecules, and incorporation of oxygen atoms into the oxide crystal lattice. Lattice oxygen can be incorporated into activated hydrocarbons by nucleophilic addition, and the oxidized product is desorbed from the catalyst surface. Thereafter, the reduced catalyst surface is reoxidized by gas phase oxygen.

  Oxidation of n-butane to maleic anhydride is an example of a reaction that is used industrially, with the extraction of 8 hydrogen atoms, the incorporation of 3 oxygen atoms, and 14 in a single operation. (See, for example, Millet JMM, Topics on Catalysis 2006, 38, 83-92).

  It is clear from what has been stated that this harsh reaction requires a catalyst with good electronic, oxygen and / or proton conductivity. More specifically, the reaction rate for the selective oxidation of hydrocarbons depends on the electron transport and / or ion transport properties, ie, for example, from the catalyst surface to the substrate (hydrocarbon) (and vice versa). Can be limited by the movement of electrons or holes (in other words, oxidation or reduction of transition metal ions in the catalyst), the movement of hydrogen atoms or oxygen atoms, and therefore imposes high demands on the catalyst.

Millet J.M. M.M. M.M. , Topics on Catalysis 2006, 38, 83-92.

  One of the major challenges is the development of new catalysts with improved selectivity and / or higher conversion. However, the development of new catalysts is very complex. One reason for this is that most industrial catalysts, like catalytically active transition metal oxides, are called promoters that enhance catalysis and / or improve selectivity. It is a multi-element oxide containing. Shortening development time for new catalysts is desired.

  The object of the present invention is to provide a catalyst for the gas phase oxidation of organic hydrocarbons with improved selectivity and / or higher conversion. Another object of the present invention is to provide a method for optimizing the catalyst for the gas phase oxidation of organic hydrocarbons.

The present invention is a catalyst for vapor phase oxidation of organic hydrocarbons, comprising a multi-element oxide comprising at least one transition metal, said catalyst being at a temperature of 375-425 ° C, in particular about 400 ° C. And a charge transfer activation energy E _c (negative charge transfer activation energy) of less than 0 kJ / mol at a temperature of

FIG. 1 shows the schematic structure of a system for non-contact measurement of a powdered catalyst by the MCPT technique. FIG. 2 shows the calibration curve of the system of FIG. 1 with various powdery dielectric constant standards. FIG. 3 shows a plot of the natural logarithm (vertical axis) of microwave conductivity versus reciprocal temperature (horizontal axis) for various catalysts.

In the present invention, it has been found that what is crucial for the advantageous catalytic properties of the gas phase oxidation catalyst is not the absolute value of the electrical conductivity, but rather the charge transfer activation energy E _c of the catalyst. The charge transfer activation energy E _c can be measured based on the temperature dependence of the electrical conductivity of the catalyst. It has been found that the conductivity of the gas phase oxidation catalyst in the relevant temperature range essentially follows Arrhenius behavior. Thus, the charge transfer activation energy E _c can be measured as a plot of the natural logarithm of the conductivity of the catalyst against the inverse of temperature.

  Methods for measuring the direct current (DC) or alternating current (AC) resistance of a sample in contact with two or four electrodes are well known and are already used to measure the conductivity of a catalyst (eg, Hermann, J. M. et al, Journal of Catalysis 1997, 167, 106-117; Sartoni, L. et al, Journal of Materials Chemistry 2006, 16, 4348-4360, Rouvet, F. et al. Soc.Faraday Trans., 1994, 90, 1441-1448). Since contact failure and hence high contact resistance greatly distorts the measured data, the accuracy and reproducibility of these measurements is highly dependent on the quality of contact between the electrode and the catalyst powder. In the case of commonly used powder catalysts, it is difficult to achieve good and reproducible contact. Therefore, for the purposes of the present invention, contactless conductivity measurement by a technique called microwave cavity enclosed perturbation technique (MCPT) is preferred. The measured value by this method is also referred to as microwave conductivity in the specification of the present invention.

Therefore, the charge transfer activation energy E _c is preferably as the slope of the natural logarithm of microwave conductivity at 9.2 GHz measured non-contacting (for the catalyst bed) for the catalyst bed versus the inverse of temperature. Measured.

［式中、ｓは０〜２の範囲であり（普通は、１に近い）、定数Ａは、わずかに温度に依存し、ωは、角速度（angular velocity）である］で説明する。低周波数での導電率は直流伝導率と近似し、それ故に粒子間の導電率と近似するものの、それは高周波数帯域における導電性粒子のバルク導電率に近似する。したがって、マイクロ波周波数での測定は、例えば、不均一の触媒等の多結晶材料の固有の物性を測定するために理想的に適しているだろう。 Conductivity measurements are preferably performed in a non-contact manner by excitation of the catalyst using microwaves and recording of the accompanying resonance spectra. This is because, in a polycrystalline semiconductor composed of a conductive region and an insulating region, the movement of charge carriers is usually limited by an insulating barrier at the particle boundary of the conductive particles. Thus, DC conductivity is measured primarily by the barrier height and width at the grain boundary and not by the intrinsic electronic properties of the grain itself. In contrast, for alternating current measurements, and more specifically for microwave measurements, the particle boundaries are gradually diverted by capacitive coupling between the conductive particles as the frequency increases, so at higher frequencies the particles The actual conductivity of the can be measured. Instead, charge carrier transfer can be explained using a hopping mechanism, in which electronic conduction on the atomic scale is characterized by activated hopping of charge carriers between localized states. It is done. Both barrier and hopping models express the frequency dependence of conductivity at low frequencies as

[Wherein s is in the range of 0 to 2 (usually close to 1), the constant A is slightly temperature dependent, and ω is the angular velocity]. Although the conductivity at low frequencies approximates the direct current conductivity, and hence the conductivity between particles, it approximates the bulk conductivity of the conductive particles in the high frequency band. Therefore, measurements at microwave frequencies would be ideally suited to measure the intrinsic properties of polycrystalline materials such as heterogeneous catalysts.

  A great advantage of measuring at high frequencies is that the signal can be transmitted wirelessly between the transmitter and the receiver and is therefore perfectly suitable for contactless processes. For this purpose, resonator technology is preferred in the specification of the present invention because it has virtually no restrictions on the shape of the sample and is characterized by extremely high sensitivity.

In what is called “endplate perturbation”, the catalyst forms one of the resonator endplates, while being placed in the center of the cavity of the microwave resonator, contact with the catalyst is reduced. Avoided (usually at maximum electric field; see Chen, L.-F. et al, Microwave Electronics, Measurement and Material Characterisation, Wiley 2004, pages 493-530). The contactless conductivity measurement in the present case for heterogeneous catalysts under reaction conditions is based on what is called the microwave cavity encapsulation perturbation technique (MCPT), in which the catalyst and its surrounding reactions The resonator is disposed in the resonator.
MCPT technology has already been successfully applied in a wide frequency range for conductivity and dielectric constant measurements of superconductors, superionic conductors, metals, semiconductors and dielectric materials. It is also possible to qualitatively observe the oxidation state of the three-way catalytic converter from the automobile sector by using the catalyst housing as a microwave resonator and measuring its resonance characteristics (Moos, R. et al. , Topics in Catalysis 2009, 52, 2035-2040).

  In the measurement, the cavity needs to be maintained at room temperature, while the catalyst must be kept at room temperature in order to avoid irreversible deterioration of the cavity wall surface aging and hence the cavity quality factor. It needs to be heated.

Preferably, in the conductivity measurement, the gas flows through the catalyst. Preferably, the gas is preheated to the respective measured temperature. The gas may be an inert gas such as nitrogen. However, preferably the gas is a model reaction gas. The model reaction gas simulates the conditions under which the catalyst is used in production. A suitable model reaction gas of a preferred embodiment for the preparation of maleic anhydride is as follows: 2% by volume n-butene, 20% by volume O ₂ and the balance N ₂ . The mass flow rate of the gas is preferably 1 to 20 ml / min.

  In order to measure the microwave conductivity, a resonator for the microwave is included, in which a reactor having a receiving location for the catalyst sample is arranged, the resonator being connected to a microwave source, the reactor being one of the reactors It is preferred to use a system that includes a resonator that is connectable at one end to a reactive gas source and a gas analyzer at the other end.

  At the same time, here a heatable source is provided for the reaction gas, and the catalyst sample can be heated by the reaction gas so that only the sample is heated and the resonator is not heated. Alternatively, the catalyst sample can also be heated by appropriately selectively introduced microwave power rather than by heating the reaction gas. However, here the microwave field selectively acts on the dipoles in the catalyst, so this changes the properties of the catalyst and this property needs to be considered in the evaluation.

  The reactor may be a sample tube made of quartz, for example, and the storage location is defined by suitable means (eg, setting limits with a glass wool plug) and the catalyst sample is placed in the resonator It is arranged to be stationary at a desired position. Another useful reactor is flat cells (arrays).

  Changes in resonance frequency and Q value due to perturbation by the sample and quartz reactor have electronic properties and linear dependence, which measures the complex permittivity and conductivity of the sample being analyzed. This means that quantitative measurements can be carried out under the reaction conditions, ie more specifically under the high temperatures required for the catalytic reaction. This is supported by the fact that the electric field is concentrated in a reactor made of a dielectric material, for example quartz, and is therefore amplified in the region of the sample.

The resonator can be configured depending on the mode used. For example, the resonator is a cylindrical X-band TM _m0 resonator (n = 0, 1, 2,...; M = 1, 2,...), And the sample storage location of the reactor is: It is preferably arranged in the center of the cavity of the resonator (along the center of gravity axis). Preferably, n = m = 1, so that the sample storage location exists between the local maximums of the electric field. If the sample is stationary between opposite counter-nodes (maximum) at the field maxima, the overall sensitivity of the system is somewhat reduced, but this is even more processed and analyzed. Can easily be offset by the use of larger sample volumes. According to the size (volume) of the cylindrical resonator, in the case of other TM modes, for example, a large resonator (r> 5 cm), the TM _0n0 mode (n = 1, 2, 3) can be used. On the other hand, it is also conceivable here that the TE mode rather than the TM mode uses a rectangular resonator that is used for excitation, although the mode in the center of the resonator can also have a field minimum. However, at the same volume, the quality of the rectangular resonator is lower than that of the cylindrical resonator. The use of split ring resonators is conceivable as well. Although the resonator can in principle be used in the frequency range of 300 MHz to 300 GHz, in practice the range of 1 to 30 GHz, and in particular the range of 8 to 12 GHz (X-band) is preferred for conductivity measurements. Measurements at 9.2 GHz have been found to be particularly useful.

In a preferred embodiment, the system includes a casing with a vacuum jacket disposed around the reactor and capable of being connected to a vacuum source, and a heating device connected to the reactor and introducing hot gas therein. In this way, the catalyst sample can be heated independently, i.e. no heating by the reaction gas is required, the vacuum jacketed casing allows only the sample to be heated at the same time and the resonator is not heated. Secure. The casing is suitably transparent to microwaves and is made of, for example, quartz glass or sapphire glass. A general requirement for the casing is a low dielectric loss, in other words, the imaginary part ε ₂ of its complex dielectric constant needs to be very small. A possible design for the casing is an arrangement known as Dewar.

  The microwave sources used include, for example, klystrons, GaN diodes or vector network analyzers, and these can also detect the microwave spectrum of the activated catalyst.

  In a preferred embodiment, the resonator includes a cooling device that additionally cools the walls of the resonator cavity and maintains them essentially at room temperature. In particular, the surface of the resonator wall has a significant effect on the microwave field that forms in the resonator, so this is particularly effective for resonator wall degradation, which essentially contributes to the function and lifetime of the resonator. Prevent it. In other words, the Q value of the resonator can be kept constant over a very long period allowing a particularly long measurement cycle for the measurement of the physical properties of the electrocatalyst.

  The cooling device includes, for example, a cooling water circulation that removes heat from the resonator, optionally using additional cooling plates. However, alternatively or additionally, air cooling of the resonator is also conceivable, for example using a large radiator, such as a copper mass. Internal ventilation of the resonator is also possible to avoid condensed water. However, the cooling equipment can be used not only for cooling the resonator at high catalyst temperatures, but also for taking measurements at low temperatures, where the heating equipment is adjusted accordingly. Thus, any temperature condition and temperature / time profile can be set under the analysis of the catalytic reaction through the appropriate combination of heating and cooling equipment.

As already mentioned at the outset, the MCPT process is based on a very small perturbation of the resonant condition of the cavity when a sample with a given complex dielectric constant and electrical conductivity is introduced into the cavity. In such a cavity, a standing wave forms a mode characterized by a characteristic resonance frequency ν ₀ (depending on the shape of the cavity) and a resonator Q value Q ₀ (depending on the shape of the resonator wall and conductivity). be able to. The Q value is related by a value called a resonance frequency ν ₀ and a half-value width Γ ₀ .

After the introduction of a small sample into the cavity, the resonant frequency and the cavity Q value change to the values of ν ₁ and Q ₁ , and the accompanying shift in the numerical value is tied to the dielectric constant and the sample volume and the cavity volume It is done.

In the quasi-static approximation, the penetration depth of the microwave into the surface of the sample is much larger than the size of the sample, and the shift in frequency and the shift in Q value are the complex permittivity ε = ε ₁ −iε ₂ of the sample. Directly tied.

  In the above equation, A and B are resonator constants based on the shape and mode of the resonator, and Vs and Vc are the volume of the sample and the volume of the cavity, respectively. The constants A and B can in principle be calculated directly if the electric field distribution of the cavity is known and does not change significantly as a result of the introduction of the sample. However, for practical reasons, the resonator constant is preferably measured by analysis of a suitable calibration material having a known complex dielectric constant, the details of which will be described later. The quasi-static approximation for insulating or low conductive materials is a very good approximation for current related catalysts.

  Suitable dielectric constant standards are, for example, single crystals of sapphire (0001), sapphire (11-20), rutile (001), rutile (100), lanthanum aluminate (100) and titanium available from Crystal GmbH (Berlin). Strontium acid (100).

  A calibration curve is measured for a powder sample based on the linear dependence between the shift in complex dielectric constant and resonant frequency, or shift in Q value. The relationship between the dielectric properties of the single crystal and the powdery sample was calculated using the Landau-Lifschitz equation (Landau / Lifshitz, Electrodynamics of Continuous Media; Pergamon Press, London, 1960).

Where ε _B and ε _P are the complex dielectric constant of the single crystal and the powder of a predetermined packing density δ. This equation can then be used to reorganize equations (2) and (3) as follows:

  Upon completion of calibration, an in-situ measurement of the dielectric constant and electrical conductivity of the gas phase oxidation catalyst can be performed.

式中、σ_０は定数、Ｅ_ｃは電気伝導率の活性化エネルギー、及びｋ_Ｂはボルツマン定数であり、活性化エネルギーＥ_ｃは直線回帰の傾きから計算され得る（Ｈｅｒｒｍａｎｎ Ｊ．Ｍ．，Ｃａｔａｌｙｓｉｓ Ｔｏｄａｙ，２００６，１１２，７３−７７）。任意に、直線の適合は、例えば、３７５及び４２５℃の間の範囲において、最小二乗法によって、達成される。 The logarithm of microwave conductivity σ (σ = ωε ₀ ε ₂ ) is plotted as an Arrhenius plot against the inverse of temperature [K]. By considering the exponential dependence on the reciprocal temperature for electrical conductivity,

Where σ ₀ is a constant, E _c is the electrical conductivity activation energy, and k _B is the Boltzmann constant, and the activation energy E _c can be calculated from the slope of the linear regression (Herrmann JM, Catalysis). Today, 2006, 112, 73-77). Optionally, a linear fit is achieved, for example, by the least squares method in the range between 375 and 425 ° C.

The present invention also simplifies optimization of the gas phase oxidation catalyst. For example, a number of possible catalysts can be prepared by validation of the preparation conditions or by combinatorial chemistry methods. Thereafter, the charge transfer activation energy E _c of the catalyst is measured, thus finding a catalyst with improved catalytic properties.

The present invention is a method for optimizing a catalyst for vapor phase oxidation of an organic hydrocarbon comprising:
a) preparing a first catalyst under a first series of preparation conditions;
b) preparing at least one further catalyst under a further series of preparation conditions;
c) measuring the charge transfer activation energy E _c of the first catalyst and at least one further catalyst; and d) as an optimized catalyst, a charge transfer activation energy E _c of less than 0 kJ / mol. Identifying a catalyst having,
A method comprising:

Preparation conditions that can vary include catalyst composition, catalyst morphology, calcination conditions, formation conditions, and content of adsorbed organic solvent in the VPO precursor at the start of calcination.
In a preferred embodiment, the multi-element oxide of the catalyst of the present invention comprises vanadium, preferably vanadium and phosphorus.

  The catalyst of the present invention is used in a process for partial oxidation of hydrocarbons. In general, a gas stream comprising at least one hydrocarbon and oxygen is passed through the catalyst bed. The conditions for partial oxidation correspond, for example, to those normally used for known catalysts of a particular reactive species and are well known to those skilled in the art.

  A preferred embodiment of the catalyst of the present invention is a catalyst for the preparation of maleic anhydride (MA). It serves for the preparation of MA by gas phase oxidation of hydrocarbons having at least 4 carbon atoms such as n-butane, 1-butene, i-butene, 2-isobutene, 2-transbutene and butadiene.

  The catalytically active composition of the MA catalyst is a phosphorus / vanadium mixed oxide (hence, the catalyst is also hereinafter referred to as a VPO catalyst). The phosphorus / vanadium atomic ratio is generally from 0.9 to 1.5, preferably from 0.9 to 1.2, in particular from 1.0 to 1.1. The average oxidation state of vanadium is preferably +3.9 to +4.4, and more preferably 4.0 to 4.3. Such active compositions are described, for example, in patents US 5,275,996, US 5,641,722, US 5,137,860, US 5,095,125 or US 4,933,312.

  The MA catalyst of the present invention may further contain a cocatalyst. Suitable cocatalysts include the elements of Groups 1-15 of the periodic table and their compounds. Suitable cocatalysts are described, for example, in WO 97/12674 and WO 95/26817, patents US 5,137,860, US 5,296,436, US 5,158,923 and US 4,795,818. Preferred promoters are cobalt, molybdenum, iron, zinc, hafnium, zirconium, lithium, titanium, chromium, manganese, nickel, copper, boron, silicon, antimony, tin, niobium and bismuth element compounds, more preferably niobium. , Molybdenum, iron, zinc, antimony, bismuth and lithium compounds. The promoted catalyst of the present invention may contain one or more promoters. The cocatalyst content is the total amount in the finished catalyst, and in each case is calculated as oxide and is less than about 5% by weight. Preferred catalysts are those that do not contain any promoter and those that contain niobium, molybdenum, and / or iron.

  The essential steps of preparation, shaping, and subsequent calcination to form a preferred catalyst precursor powder are described below.

(A) Reaction while heating in the presence of a phosphorus compound with an organic reducing solvent of a pentavalent vanadium compound. This step can be performed in the presence of an optionally dispersed powdered carrier material.
(B) Separation of the formed vanadium-, phosphorus-, and oxygen-containing catalyst precursor ("VPO precursor"), for example, by filtration or evaporation.
(C) Drying the VPO precursor at a temperature of 50-200 ° C. Optionally, what is called a pore former can be added to the powdered carrier material and / or to the dried and preferably heat treated VPO precursor powder.
(D) Molding by converting to a desired structure. Molding is preferably accomplished by tableting, preferably with prior addition of a lubricant such as graphite.
(E) Firing of the shaped VPO precursor by heating in an atmosphere containing oxygen (O ₂ ), water (H ₂ O) and / or an inert gas.

By changing the above preparation conditions, the catalytic tendency of the catalyst is influenced, and the catalyst of the present invention can be obtained. It has been found that the content of the adsorbed organic solvent in the VPO precursor at the start of calcination constitutes a parameter that has an important influence on the charge transfer activation energy E _c . Adsorbed solvent molecules are converted to oxidation products that can be desorbed only with difficulty, if any, possibly during the calcination process, and / or adsorbed solvent molecules were tentatively present during the calcination process. If not, it causes the formation of vanadium (III) species that can only be reoxidized with difficulty. Thus, it is advantageous to undergo a solvent exchange in the VPO precursor, after its separation, the organic reducing solvent is replaced with a volatile solvent that can be removed from the VPO precursor without substantially remaining prior to calcination. Was found. The solvent exchange is preferably repeated twice or more. As the number of solvent exchange cycles increases, the charge transfer activation energy E _c generally shifts to a negative value.

The pentavalent vanadium compound used may be an oxide, acid, and inorganic and organic salts containing pentavalent vanadium, or a mixture thereof. Vanadium pentoxide (V ₂ O ₅ ), ammonium metavanadate (NH ₄ VO ₃ ), and ammonium polyvanadate ((NH ₄ ) ₂ V ₆ O ₁₆ ) are preferable, and vanadium pentoxide (V ₂ O ₅ ) is particularly preferable. The pentavalent vanadium compound present in a solid form is used in a powder form, preferably in a powder form having a particle range of 50 to 500 μm.

The phosphorus compound used may be, for example, a reducing phosphorus compound such as phosphoric acid, and for example phosphorus pentoxide (P ₂ O ₅ ), orthophosphoric acid (H ₃ PO ₃ ), pyrophosphoric acid (H ₄ P ₂ O ₇ ), pentavalent phosphate compounds such as polyphosphoric acid represented by the general formula H _{n + 2} P _n O _{3n + 1} (where n ≧ 3), or a mixture thereof. The use of pentavalent phosphorus compounds is preferred. Usually, the content of the compounds and mixtures is reported in mass% based on H ₃ PO ₄ . Is preferably used for _80~110% H 3 PO _4, especially preferred use of _95~110% H 3 PO _4, very particularly preferred use of _100~105% H 3 PO _4.

The reducing solvent used is preferably a primary or secondary, acyclic or cyclic, unbranched or branched saturated alcohol having 3 to 6 carbon atoms, or a mixture thereof. Preference is given to the use of primary or secondary, unbranched or branched C _3- to C ₆ -alkanols or the use of cyclopentanol or cyclohexanol.

  Suitable alcohols include n-propanol (1-propanol), isopropanol (2-propanol), n-butanol (1-butanol), sec-butanol (2-butanol), isobutanol (2-methyl-1-propanol) ), 1-pentanol, 2-pentanol, 3-pentanol, 2-methyl-1-butanol, 3-methyl-1-butanol, 3-methyl-2-butanol, 2,2-dimethyl-1-propanol 1-hexanol, 2-hexanol, 3-hexanol, 2-methyl-1-hexanol, 3-methyl-1-pentanol, 4-methyl-1-pentanol, 3-methyl-2-pentanol, 4- Methyl-2-pentanol, 2,2-dimethyl-1-butanol, 2,3-dimethyl-1-butanol 3,3-dimethyl-1-butanol, 3,3-dimethyl-2-butanol, cyclopentanol, cyclohexanol, and mixtures thereof.

  n-propanol (1-propanol), n-butanol (1-butanol), isobutanol (2-methyl-1-propanol), 1-pentanol, 2-methyl-1-butanol 3-methyl-1-butanol, And cyclohexanol are very particularly preferred, especially isobutanol.

  The components can be combined in various ways, for example in a stirred tank. The amount of reducing solvent should exceed the stoichiometric amount required for the reduction of vanadium from the +5 oxidation state to an oxidation state in the range of +3.5 to +4.5. In general, the amount of reducing solvent added is such that it is sufficient for slurrying of the pentavalent vanadium compound, allowing at least vigorous mixing with the added phosphorus compound.

  The slurry is heated for conversion of the compound and formation of the catalyst precursor. The temperature range selected depends on various factors, more specifically the reducing action and boiling point of the above components. In general, temperatures of 50-200 ° C., and preferably temperatures of 100-200 ° C. are observed. High temperature reactions generally take several hours.

  The cocatalyst compound can be added at any time. Suitable promoter compounds are, for example, the aforementioned promoter metal acetates, acetylacetonates, oxalates, oxides or alkoxides, such as cobalt acetate, cobalt (II) acetylacetonate, cobalt chloride (II). ), Molybdenum oxide (VI), molybdenum chloride (III), iron (III) acetylacetonate, iron (III) chloride, zinc (II) oxide, zinc (II) acetylacetonate, lithium chloride, lithium oxide, bismuth chloride (III), bismuth (III) ethylhexanoate, nickel (II) ethylhexanoate, nickel oxalate (II), zirconium oxychloride, zirconyl chloride, zirconium (IV) butoxide, silicon (IV) ethoxide, chloride Niobium (V) and niobium oxide (V).

  After completion of the heat treatment, the formed catalyst precursor is separated, and may optionally be subjected to a cooling step and a storage or aging step of the cooled reaction mixture. In the separation, the solid catalyst precursor is separated from the liquid phase. Suitable methods are, for example, filtration, decantation, or centrifugation. Separation of the catalyst precursor by filtration is preferred.

  The isolated food precursor is subjected to solvent exchange (“washing”). The separated catalyst precursor is washed, for example, with a suitable solvent to remove the reducing agent (eg, alcohol) still attached or its decomposition products. Suitable solvents are, for example, alcohols (eg methanol, ethanol, 1-propanol, 2-propanol), aliphatic and / or aromatic hydrocarbons (eg pentane, hexane, gasoline, benzene, toluene, xylene), ketones (For example, acetone, 2-butanone, 3-pentanone), ether (for example, 1,2-dimethoxyethane, tetrahydrofuran, 1,4-dioxane) or a mixture thereof. When the catalyst precursor is washed, 2-propanone (also referred to as acetone), ethanol and / or methanol is preferably used, and ethanol and / or 2-propanone is particularly preferably used.

  After separation of the catalyst precursor or after washing, the solid is generally dried.

  Drying can be performed under various conditions. Generally, it is carried out under reduced pressure or atmospheric pressure. The drying temperature is generally 30 to 250 ° C. It is preferable to perform drying under a pressure of 1 to 30 kPa (absolute) and a temperature of 50 to 200 ° C. in a residual gas atmosphere with or without oxygen, for example, air or nitrogen.

In a preferred embodiment of molding, the catalyst precursor powder is well mixed with about 2-4% by weight of graphite and pre-compressed. The pre-compressed particles are tableted to obtain a shaped catalyst body. As described in WO2008 / 087116, the use of graphite having a specific surface area of 0.5-5 m ² / g and a particle size d ₅₀ of 40-200 μm is preferred.

  The catalyst precursor powder may be mixed well with the pore former and further processed and shaped as described above. In general, this includes compounds containing carbon, hydrogen, oxygen, and / or nitrogen that are largely removed again by sublimation, decomposition and / or vaporization upon subsequent catalyst activation. Suitable pore formers are, for example, palmitic acid or fatty acids such as stearic acid, dicarboxylic acids such as oxalic acid or malonic acid, cyclodextrins or polyethylene glycols. The use of malonic acid is preferred.

  Molding is preferably accomplished by tableting. Tableting is a method of press agglomeration. This is to introduce the powdered bulk agent into a pressure molding die having a die between two punches (cutting die), and to compress it by uniaxial compression and form it into a solid molded body. Including. This operation is divided into four sections: metering, compression (elastic deformation), plastic deformation, and ejection. Tableting is achieved, for example, with a rotary press or an eccentric press.

The shaped VPO precursor is fired at a temperature in the range of 250 to 600 ° C. by heating in an atmosphere containing oxygen (O ₂ ), hydrogen oxide (H ₂ O) and / or an inert gas. Examples of suitable inert gases include nitrogen, carbon dioxide and noble gases.

  Firing is batch-wise, for example, in a blast furnace, staged furnace, muffle furnace, heating cabinet, or continuously in a rotary tube. Can be performed in a belt calcination furnace or a rotary bulb furnace. It may comprise different sections that are continuous with respect to temperature, for example heating, temperature holding, cooling, etc., or different sections that are continuous with respect to an atmosphere such as, for example, an oxygen-containing, vapor-containing, oxygen-free gas atmosphere. Suitable firing processes are described, for example, in patents US 5,137,860 and US 4,933,312 and published specification WO 95/29006. Particularly preferred is continuous firing in a belt firing furnace having at least two, for example 2 to 10 firing zones, optionally having different gas atmospheres and different temperatures. Appropriate combinations of temperature, processing time and gas atmosphere adapted to the respective catalyst system can affect and be adjusted by the mechanical and catalytic properties of the catalyst.

The catalyst precursor is
(I) in at least one calcining zone under an oxidizing atmosphere having an oxygen content of 2 to 21% by volume, until heated to a temperature of 200 to 350 ° C. and until the desired average oxidation state of vanadium is established And (ii) in at least one further calcination zone under a non-oxidizing atmosphere having an oxygen content of less than 0.5% by volume and a hydrogen oxide content of 20-75% by volume, Heated to a temperature of 300-500 ° C. and maintained under these conditions for more than 0.5 hours,
Firing is preferred.

  In step (i), the catalyst precursor is generally in an oxidizing atmosphere having a molecular oxygen content of 2 to 21% by volume, preferably 5 to 21% by volume, and a temperature of 200 to 350 ° C., and preferably 250 to 350 ° C. And maintained over a period effective to establish the desired average oxidation state of vanadium. Generally, in step (i), oxygen, an inert gas (eg, nitrogen or argon), a mixture of hydrogen oxide (vapor) and / or air, and air are used. From the point of view of the catalyst precursor that is calcined through the calcining zone (s), the temperature during the calcining step (i) may be kept constant and may rise or fall on average. Since step (i) is generally preceded by a heating phase, the temperature will generally increase initially to subsequently settle to the desired final value. Thus, generally the calcining zone of step (i) is preceded by at least one further calcining zone for heating of the catalyst precursor.

  In the method according to the invention, the period during which the heat treatment in step (i) is maintained is preferably from +3.9 to +4.4, more preferably from +4.0 to +4.3. The average oxidation state should be selected to establish.

  Since the measurement of the average oxidation state of vanadium during calcination can only be measured with extreme difficulty for equipment and time reasons, the required period is preferably measured experimentally in a preliminary test. It should be. In general, this objective is met by a series of measurements in which the heat treatment is achieved under defined conditions, the sample is removed from the reaction system after different times, cooled and the average oxidation state of vanadium is analyzed.

  The period required in step (i) is generally determined by the characteristics of the catalyst precursor, the temperature established and the gas atmosphere selected, more specifically the oxygen content. In general, the period in step (i) extends to a duration longer than 0.5 hours, preferably longer than 1 hour. In general, a duration of 4 hours or less, preferably a duration of 2 hours or less, is sufficient to establish the desired average oxidation state. However, periods of more than 6 hours may also be required under appropriately conditioned conditions (eg, low temperature interval range and / or low molecular oxygen content).

  In step (ii), the resulting catalyst intermediate has a molecular oxygen content of less than 0.5% by volume and a hydrogen oxide (steam) content of 20 to 75% by volume, preferably 30 to 60% by volume. Under an oxidizing atmosphere, maintained at a temperature of 300-500 ° C., preferably 350-450 ° C., for a time exceeding 0.5 hours, preferably 2-10 hours, and more preferably 2-4 hours. The A non-oxidizing atmosphere, like the hydrogen oxide described above, should not be understood as implying any limitation, but generally comprises mainly nitrogen and / or a noble gas such as argon. For example, a gas such as carbon dioxide is also suitable in principle. The non-oxidizing atmosphere preferably contains at least 40% by volume nitrogen. From the point of view of the catalyst precursor being calcined through the calcining zone (s), the temperature during the calcining step (ii) may be kept constant and may rise or fall on average. If step (ii) is carried out at a temperature higher or lower than step (i), generally a heating or cooling step, optionally performed in a further calcining zone, during steps (i) and (ii) There is. In order to allow improved separation from the oxidizing atmosphere in step (i), this further calcining zone is purged between steps (i) and (ii) for purging with an inert gas, for example nitrogen. Can be done. It is preferable to perform process (ii) at the temperature 50-150 degreeC higher than process (i).

  In general, the calcination includes a further step (iii) performed after step (ii), wherein the calcined catalyst precursor is brought to a temperature below 300 ° C., preferably 200 ° C., under an inert gas atmosphere. Cooling to a lower temperature, more preferably to a temperature lower than 150 ° C.

  In the calcination according to the method according to the invention, further steps are possible before, during and / or after steps (i) and (ii) or steps (i), (ii) and (iii). Without any limiting effect, further steps include, for example, changes in temperature (heating, cooling), changes in gas atmosphere (replacement of gas atmosphere), additional waiting times, transfer of catalyst intermediate to other equipment, Or the interruption of the whole baking operation is included. Since the catalyst precursor generally has a temperature of <100 ° C. before the start of calcination, it must usually be heated before step (i). Heating is preferably performed in an oxidizing atmosphere as defined in step (i) or in an inert gas atmosphere as defined in step (iii). It is also possible to exchange the gas atmosphere during the heating phase. Heating in an oxidizing atmosphere used also in step (i) is particularly preferred.

  The present invention further provides a method for preparing maleic anhydride, wherein a hydrocarbon having at least 4 carbon atoms is contacted with the bed of the shaped catalyst body of the present invention in the presence of an oxygen-containing gas. provide. The reactor used is generally a tube bundle reactor. A suitable tube bundle reactor is described, for example, in EP-B 1 261 424.

Suitable hydrocarbons in the process according to the invention are aliphatic and aromatic, saturated and unsaturated hydrocarbons having at least 4 carbon atoms, such as 1,3-butadiene, 1-butene, 2-cis- butene, 2-trans - butene, n- butane, _{C 4} mixture, 1,3-pentadiene, 1,4-pentadiene, 1-pentene, 2-cis - pentene, 2-trans - pentene, n- pentane, cyclopentadiene , dicyclopentadiene, cyclopentene, cyclopentane, C ₅ mixture hexenes, hexanes, cyclohexane, and benzene. Propane, 1-butene, 2-cis-butene, 2-trans-butene, n-butane, benzene or a mixture thereof is preferably used, and propane, n-butane or benzene is particularly preferably used. For example, the use of n-butane in the form of pure n-butane or in the form of n-butane-containing gas and liquid is particularly preferred. The n-butane used may originate from, for example, natural gas, steamcrackers, or FCC crackers.

  Hydrocarbons are generally added in a quantitative control, i.e. with a constant supply of a defined amount per unit time. Hydrocarbons can be measured in liquid or gaseous form. Addition measured in liquid form with vaporization prior to subsequent tube bundle reactor unit is preferred.

  The oxidant used is an oxygen-containing gas, for example, air, synthetic air, oxygen-enriched air or, for example, what is called “pure” oxygen originating from a fraction of air. An oxygen-containing gas is also added with quantitative control.

  The process according to the invention is carried out at a temperature of 250-500 ° C. The specified temperature is understood to mean the average temperature of the heat carrier in each case, regardless of the type of reactor. In the case of the use of n-butane as the hydrocarbon reactant, the process according to the invention is preferably carried out at a temperature of 380 to 460 ° C and more preferably 380 to 440 ° C. In the case of the use of propane, the process according to the invention is preferably carried out between 250 and 350 ° C. In the case of the use of benzene, the process according to the invention is preferably carried out at 330-450 ° C.

  The hydrocarbon concentration of the input stream fed to the reactor unit is 0.5-10% by volume, preferably 0.8-10% by volume, more preferably 1-10% by volume, most preferably 2-2%. 10% by volume.

  The conversion of hydrocarbons passed through the reactor is 40-100% of hydrocarbons from the input stream, preferably 50-95%, more preferably 70-95%, and especially 85-95%.

In the method according to the present invention, through the input flow rate into the reactor unit, is preferably 1000～1000H ^-1 and, more preferably of 1500~5000h ^-1 GHSV (gas hourly space velocity (gas hourly space velocity)), 0 Established on the basis of the volume of the input stream normalized and supplied at 0 ° C. and 0.1013 MPa (absolute) and on the basis of the reaction volume whose catalyst or its geometric surface area is filled with the coated catalyst.

  The reaction product or product stream can be optionally diluted so as to obtain a non-explosive product stream that is inert under the reaction conditions. Furthermore, it is possible to advantageously achieve a non-explosive product stream by means of a pressure stage. The product stream can then be processed by a conventional workup unit.

  The present invention is illustrated in detail by the accompanying drawings and the following examples.

  Four VPO catalysts were prepared as follows.

[Preparation of catalyst precursor]
Example A1: Preparation of VPO precursor
First, 2000 ml of isobutanol was placed in a nitrogen-inactivated 3 L stirred tank (1-level, impeller stirrer, d _a = 10 mm with baffle). Subsequently, 287.9 g of polyphosphoric acid (105% (from Thermphos)) was added with stirring (300 rpm). The mixture was homogenized for 10 minutes. Subsequently, 218.3 g vanadium pentoxide (from GfE) and 53.9 g vanadium oxytriethoxide (from Sigma-Aldrich) were added to the mixture. The resulting suspension was heated to a temperature of 100.5-104 ° C. under reflux for 45 minutes and maintained under these conditions for 16 hours. The mixture was cooled to a temperature of 45 ° C. over 40 minutes. Thereafter, the hot suspension was transferred to a suction filter (pore size 4) and filtered at a pressure of 77 MPa (absolute) or less. After 120 minutes, the filter cake was transferred back to the reaction vessel, suspended in 1500 ml of ultra high purity (denatured) ethanol, heated to reflux at 200 rpm (T = 75-78 ° C.) and maintained under these conditions for 1 hour. The mixture was cooled to a temperature of 25 degrees in 60 minutes. Thereafter, the suspension was transferred to a suction filter (pore size 4) and filtered at a pressure of 77 MPa (absolute) or less. After 30 minutes, the filter cake was transferred back to the reactor. This solvent exchange was performed a total of 5 times until the mother liquor (filtrate) had an isobutanol concentration of <2% by weight (detected by GC analysis). Thereafter, the filter cake was transferred to a porcelain dish and finely crushed with a spatula. Subsequently, the product was heated to 150 ° C. in a vacuum dryer and dried at a pressure of 7.7 kPa (77 mbar) (absolute) for 16 hours.

(Comparative Example B1: Preparation of VPO precursor)
First, 2000 ml of isobutanol was placed in a nitrogen-inactivated 3 L stirred tank (1-level, impeller stirrer, d _a = 10 mm with baffle). Subsequently, 287.9 g of polyphosphoric acid (105% (from Thermphos)) was added with stirring (300 rpm). The mixture was homogenized for 10 minutes. Subsequently, 218.3 g vanadium pentoxide (from GfE) and 53.9 g vanadium oxytriethoxide (from Sigma-Aldrich) were added to the mixture. The resulting suspension was heated to a temperature of 100.5-104 ° C. under reflux for 45 minutes and maintained under these conditions for 16 hours. The mixture was cooled to a temperature of 45 ° C. over 40 minutes. Thereafter, the hot suspension was transferred to a suction filter (pore size 4) and filtered at a pressure of 77 MPa (absolute) or less. After 120 minutes, the filter cake was transferred to a porcelain dish and crushed finely with a spatula. Subsequently, the product was heated to 150 ° C. in a vacuum dryer and dried at a pressure of 1 kPa (10 mbar) (absolute) for 16 hours.

Example A2: Preparation of Nb _0.1 VPO precursor
First, 1800 ml of isobutanol was placed in a nitrogen-inactivated 3 liter stirred tank (1-level, impeller stirrer, d _a = 10 mm with baffle). Subsequently, 287.9 g of polyphosphoric acid (105% (from Thermphos)) was added with stirring (300 rpm). The mixture was homogenized for 10 minutes. Subsequently, 242.6 g of vanadium pentoxide (from GfE) and 84.88 g of niobium ethoxide (from Sigma-Aldrich, 95% purity) were added to the mixture. The resulting suspension was further processed as in Example A1.

(Comparative Example B2: Preparation of Nb _0.1 VPO precursor)
First, 1800 ml of isobutanol was placed in a nitrogen-inactivated 3 liter stirred tank (1-level, impeller stirrer, d _a = 10 mm with baffle). Subsequently, 287.9 g of polyphosphoric acid (105% (from Thermphos)) was added with stirring (300 rpm). The mixture was homogenized for 10 minutes. Subsequently, 242.6 g of vanadium pentoxide (from GfE) and 84.88 g of niobium ethoxide (from Sigma-Aldrich, 95% purity) were added to the mixture. The resulting suspension was further processed as in Example B1.

[Preparation of catalyst]
The dry powders obtained from Examples A1 to A2 and Comparative Examples B1 to B2 were powdered to a particle size of <500 μm on a round sieve and transferred to a porcelain dish. Subsequently, the product was heated to 250 ° C. in a forced air oven and heat treated under 800 L (STP (standard temperature, standard pressure)) / h of air for 5 hours. After cooling to room temperature, the catalyst precursor is intimately mixed with 1% by weight of graphite (Timrex T44), compression molding machine (Powertec RCC 100 ^* 20, pressure = 20 MPa (200 bar), sieving mill) 110.7 , Roller 2.4, screw 16). Debris was sieved with a fraction of 0.5-1 mm.

  20 g of debris is introduced into an electrically heated tube, heated to a temperature of 250 ° C. at a heating rate R 1 of 5 ° C./min with air (25 L (STP) / h) and held for 50 minutes. Maintained at temperature. Thereafter, the gas atmosphere was switched to a mixture of 25% by volume air, 25% by volume nitrogen, and 50% by volume steam (the total flow rate of the gas atmosphere G2 was 25 L (STP) / h) and the debris was 1 ° C./min. Heated at a heating rate to a temperature of 370 ° C. and maintained at this temperature for a holding time of 5 minutes. Subsequently, the gas atmosphere was switched to a mixture of 50 volume% nitrogen (12.5 L (STP) / h) and 50 volume% steam (12.5 L (STP) / h), and the debris was heated at 3 ° C./min. Heated to 425 ° C. at a rate and maintained at this temperature for a holding time of 195 minutes. Thereafter, the gas atmosphere was switched to nitrogen (25 L (STP) / h), and the debris was cooled to room temperature.

[Catalyst equilibration]
The catalysts A1 to A2 of the invention and the catalysts B1 to B2 of the comparative examples were equilibrated for 1 ml samples in each case in a 48 tube test reactor as described in DE 198 09 477 A1. The composition of the reaction gas mixture is defined by the concentration of n-butane, air, steam and triethyl phosphate. The difference from 100% by volume consists of argon.

Xn _-butane = butane concentration in the input stream X _air = air concentration in the input stream X _Ar = argon concentration in the input stream X _H2O = vapor concentration in the input stream X _TEP = triethyl phosphate concentration in the input stream GHSV = 0 ° C. and 0 The flow rate of the input stream, based on the volume of the input stream normalized and fed to 1013 MPa (absolute) and based on the reaction volume filled with catalyst T = reaction temperature

[Measurement of conductivity using MCPT technology]
The equilibrated catalyst was transferred to a conductivity measurement system. The conductivity measurement system used is shown schematically in FIG. A TM ₁₁₀ cavity and a cylindrical X-band resonator 2 ′ having a height of 19.5 mm and a diameter of 38.5 mm are used. Such a resonator is available, for example, from Bruker BioSpin.

In the middle of the end plates 2 a ′ and 2 b ′ of the cylindrical resonator 2 ′, the reactor tube 3 is guided radially into the reactor cavity, and the reactor tube 3 is evacuated by the connections 5 a and 5 b (external dewar). Volume) and a quartz dewar 5 connected to a hot gas source (internal dewar volume). A heat source (oven) is provided on the hot gas supply line and brings the introduced nitrogen gas N ₂ and consequently the sample P held in the nitrogen gas stream to the reaction temperature.

  The reactor tube 3 has an inner diameter of 3 mm and an outer diameter of 4 mm, and at its sampling point 4 is held in the position on the longitudinal axis of the resonator with two glass wool plugs (not shown). A catalyst sample P is included. The height of the powdery sample in this example is about 10 mm. The quartz dewar, more specifically its tubular part introduced into the resonator, has an outer diameter of 10 mm and, together with the sample tube, to a waveguide connected to the cavity of the resonator Located at a right angle inside it.

The reactor tube is connected in the upstream direction with a reaction gas supply connection 5c having a mass flow controller (Bronkhorst El-Flow), and in the downstream direction with an online gas chromatograph GC (Agilent 7890A). For quantitative analysis, the gas stream is for a first gas stream for analysis of CO ₂ , H ₂ O, n-butane, N ₂ , O ₂ , and CO, and for the analysis of maleic anhydride (product). Divided into a second gas stream. Individual gases in the first gas stream are separated by Poraplot columns (CO ₂ , H ₂ O, C ₄ H ₁₀ ) and molecular sieves (N ₂ , O ₂ and CO) and analyzed using a thermal conductivity detector. Is done. Maleic anhydride in the second gas stream was separated from the reaction mixture with a DB1 column and detected by a flame ionization detector.

The oven 8 is configured as a resistance oven (Sylvania Tungsten Series I), and 8 L / min of preheated nitrogen is sent to heat the catalyst sample P to a temperature between room temperature and 500 ° C. . All connection lines between the reactor and GC are heated to 150 ° C. to avoid liquid condensation. Quartz dewar evacuation to ^10-5 Pa ( ^10-7 mbar) was achieved using a Pfeiffer HiCube 80 Ecopump. In addition, the cavity was water cooled using a two-circuit cooling system via a copper plate mounted on the resonator endplate.

  To record the resonance spectrum of what is called the S11 parameter in reflection mode (reflected power as a function of frequency), the cavity of the resonator is connected via a coaxial cable 10 to a vector network analyzer 9 (VNA, Agilent PNA-L N5230C- 225, operating range between 10 MHz and 20 GHz) and connected to the waveguide 11 via the opening. The microwave output was 11 dBm and the resonant frequency was measured directly by measuring the frequency at the minimum of the S11 parameter spectrum. The Q value (quality) was measured by measuring the frequency difference at the resonance peak at 3 dB power absorption level (FWHM) and multiplying the reciprocal of the measured bandwidth by the resonance frequency. The critical coupling of the microwave to the resonator was adjusted by a phase shifter 12 and was observed by a Smith chart representation of VNA9. A reference resonator 13 is further connected to the waveguide 11 for independent examination of the frequency of the microwave source in the VNA 9.

  Dielectric constants used were sapphire single crystals (0001), sapphire (11-20), rutile (001), rutile (100), lanthanum aluminate (100) and titanic acid available from Crystal GmbH (Berlin) Strontium (100). The dielectric constant standard was ground into a powdery sample. Thereafter, for the powdery sample, different calibration densities of the calibration sample were measured, and a calibration curve was measured. The results are shown in FIGS. 2a) and 2b) with the shift in resonance frequency (calibration constant Ap = −0.115 ± 0.005) and the shift in Q value (calibration constant Bp = 0.067 + 0.003) in each case. Show.

The catalyst sample P was ground into a powder having a particle size of 100 to 200 μm. For the MCPT measurement, 75 mg of the VPO catalyst 100-200 μm particle size fraction passed through the sieve was introduced into the quartz reactor tube with a sample fill height of 10 mm or less. Thereafter, the temperature dependence of the complex dielectric constant was measured while the catalyst sample was flowed through the reaction gas mixture (2% by volume of n-butane, 20% by volume of O ₂ and the remaining N ₂ ) at 5 ml / min. Measurement was performed by heating from 25 ° C. to 416 ° C. at a heating rate of ° C./min. At the same time, the resonance frequency and Q value were checked at different temperatures, in each case after an isothermal holding time of 10 minutes.

The ε ₂ value was used to calculate the electrical conductivity σ. The logarithmic value of σ was plotted against the inverse of temperature. The results of the three types of catalysts examined at temperatures of 375, 400 and 425 ° C. are shown in FIG. 3 (□ —values measured for catalyst A1, Δ—values measured for catalyst A2, x—about catalyst B1 Measured value, ◯ -value measured for catalyst B2).

  The activation energy measured from them in the temperature range of 375 ° C. to 425 ° C. and the results of the catalyst test for the catalyst examined at a temperature of 400 ° C. are as follows.

Claims (7)

A catalyst for vapor phase oxidation of an organic hydrocarbon comprising a multi-element oxide comprising at least one transition metal, the catalyst having a charge transfer activity of less than 0 kJ / mol at a temperature of 375-425 ° C catalyst characterized by having a activation energy E _c. The charge transfer activation energy E _c is measured as the slope of the natural logarithm of microwave conductivity at 9.2 GHz measured non-contactingly for the catalyst bed, versus the reciprocal of temperature. catalyst.   The catalyst according to claim 1 or 2, wherein the multi-element oxide contains vanadium.   The catalyst according to any one of claims 1 to 3, wherein the multi-element oxide contains vanadium and phosphorus.   A process for the partial oxidation of hydrocarbons comprising the step of passing a gas stream comprising at least one hydrocarbon and molecular oxygen through the catalyst bed of any one of claims 1-4.   A process for preparing maleic anhydride, wherein the gas stream comprises at least one hydrocarbon selected from n-butane, 1-butene, i-butene, 2-isobutene, 2-transbutene and butadiene. 5. The method according to 5. A method for optimizing a catalyst for gas phase oxidation of an organic hydrocarbon, comprising:
a) preparing a first catalyst under a first series of preparation conditions;
b) preparing at least one further catalyst under a further series of preparation conditions;
c) measuring the charge transfer activation energy E _c of the first catalyst and at least one further catalyst; and d) as an optimized catalyst, a charge transfer activation energy E _c of less than 0 kJ / mol. Identifying a catalyst having,
Including methods.

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