NO994561L - Catalysts based on modified molybdenum oxycarbide prepared from oriented molybdenum and process for the preparation thereof - Google Patents

Description

CATALYSTS BASED ON MODIFIED MOLYBDENOXYCARBIDE

MANUFACTURED FROM ORIENTED MOLYBDENUM AND PROCEDURE

FOR MANUFACTURE THEREOF

TECHNICAL AREA

This invention relates to a catalyst for chemical or petrochemical reactions, containing metaloxycarbides with a large specific surface area, method for its manufacture and use. This catalyst is particularly suitable for the refining and conversion of petroleum products, particularly for the isomerization of saturated hydrocarbons, for the hydrogenation and decyclization of cyclic and aromatic hydrocarbons, and provides improved catalytic properties, particularly specific rate.

STATE OF THE ART

From FR patents 2666520 and 2712587, catalytic reactions are known for the isomerization of linear hydrocarbons having in the range from four to more than twenty carbon atoms and for the hydrogenation and decyclization of aromatic compounds using a molybdenum oxycarbide-based catalyst, the latter being achieved by direct activation of the precursor oxide (MoO3) by means of the reaction mixture itself.

It may be recalled that molybdenum oxycarbide-based catalysts present a major advantage since they always provide high selectivity, especially even at high conversion rates. They are also much cheaper and less sensitive to toxins than conventional Pt-based catalysts.

In petrochemical reactions catalyzed by molybdenum oxycarbide, however, the specific rate of the reaction remains expressed in moles of the converting product per grams of catalyst and per second insufficient.

When this rate is 150 for an ordinary Pt-based catalyst on a zeolite support, it is, as an illustration, generally in the range of 50 for a molybdenum oxycarbide prepared in the usual way.

The inventors therefore proposed the possibility of improving the catalytic activity of this type of catalyst by particularly attempting to increase the specific rate of petrochemical reactions according to the unit weight of the catalyst.

DESCRIPTION OF THE INVENTION

A first aspect of the invention is a modified molybdenum oxycarbide with a high specific surface area which is characterized in that it is produced from a molybdenum oxide MoO 3 which has a structure consisting mainly of plates with a preferred orientation.

The oriented plane of MoO3 (which is best suited for obtaining a more effective oxycarbide, is the plane (010) with preferred development in the direction (001).

The length of the plates in the direction of the preferred orientation is at least 1 µm. It is noteworthy that it can reach or even exceed 10,000/im. The thickness of the plates is generally at least 0.1 µm and their width is at least 0.5 µm, the latter sometimes reaching 6000 µm. They are generally of the monocrystalline type and are more or less closely packed.

The specific surface area BET of this oxycarbide is higher than 5m<2>/g, generally 20m2/g and can even reach 90m<2>/g, and is significantly higher than that of the initial oriented M0O3.

It is difficult to know which microstructural feature it is characterized by and which allows the observed unexpected results to be achieved. However, it can be stated that it is mainly free of the oxide MoO2, which can be considered a harmful by-product from reduction, while this is generally contained in the oxycarbide obtained from polycrystalline MoO3.

A second aspect of the invention is a method for obtaining modified molybdenum oxycarbide which has a high specific surface area, characterized in that the starting point is ordinary polycrystalline molybdenum oxide, or possibly a molybdenum compound (e.g. ammonium heptamolybdate) which is converted into an oxide, and that the molybdenum oxide is subjected to heat treatment by sublimation-condensation at a temperature of between 500 and 800°C in a closed space under an oxidizing gas atmosphere for at least 5 hours to obtain an oriented recrystallized oxide, which is then treated under a stream of a mixture of hydrocarbon and hydrogen to give a modified molybdenum oxycarbide.

The step of sublimation-condensation is usually carried out in a static atmosphere or under a stream of dry or moist oxidizing gas at low speed, the gas possibly being air. In this way, recrystallization of the molybdenum oxide is carried out on site without any significant transfer of material.

To obtain a supported catalyst, the initial oxide or a precursor can be mixed or deposited with or on a material that is inert to the oxide, this material serving as a carrier for the modified oxycarbide catalyst. This material is generally a refractory material, e.g. aluminum, but is preferably a carbide with a high specific surface area such as pure or doped SiC.

The obtained recrystallized oxide is an oxide with an oriented structure which mainly comprises single-crystalline plates with an elongated shape and which has a preferred orientation.

Production of the oxycarbide from the recrystallized oxide is carried out in accordance with the methods described in the above-mentioned patents.

A further aspect of the invention is the use of this modified molybdenum oxycarbide, which has a high specific surface area and is prepared from oriented MoO 3 , as a catalyst in petrochemical reactions, and in particular for the isomerization of hydrocarbons or their hydrogenation, particularly of aromatic compounds, to achieve high selectivity with a high conversion rate.

Such catalysts in accordance with the invention provide greatly improved activity compared to ordinary polycrystalline molybdenum oxycarbide. In particular, their specific speed can reach close to 10 times that of MoOC obtained from polycrystalline MoO3. For the same reaction, in other words, the amount of catalyst needed is more than 10 times less. The specific speed is also higher, in some cases at least 4 times higher than the specific speed achieved with ordinary Pt-based catalysts on zeolite.

The properties of selectivity or degree of conversion especially to MoOC (high conversion selectivity much higher than that of Pt) are usually not modified.

The catalyst according to the invention can be produced as such (ie in bulk) or carried e.g. on a SiC carrier with a high specific surface area, as mentioned above.

DETAILED DESCRIPTION OF THE INVENTION

In the following, various methods are described for producing the oxide phase, MoO 3 in bulk or supported, which has sufficient orientation to obtain a catalyst containing modified molybdenum oxycarbide which is highly active and selective for the isomerization reaction.

A first production method consisted in heating a polycrystalline MoO3 powder in bulk under a slow air flow at a temperature of preferably between 650 and 750°C for a period of 5 to 240 hours. The molybdenum oxide used can be replaced either with a precursor salt (heptamolybdate) or with metal powder or metal plates that have been oxidized. After this treatment, the molybdenum oxide appears as a powder or plates with the desired preferred orientation. The air stream can be replaced by any other gas containing oxygen. The gas flow, generally with a low flow rate, can be used either in the form of dry gas or it can contain steam. Synthesis can also be carried out under a static atmosphere of air or a mixture of oxygen with an inert gas.

A second production method consists in heating not a powder of polycrystalline Mo03, but an intimate mixture of polycrystalline Mo03 and SiC granules. The treatment is identical to that described for the first manufacturing method. The resulting product consists of SiC granules coated with MoO3 plates which have a preferred orientation.

A third production method consists in impregnating a SiC-based carrier with a solution containing a molybdenum salt, e.g. a solution of heptamolybdate. The carrier is then dried in a drying oven at 120°C for 12 hours and then calcined under a stream of air. The temperature is increased from room temperature to calcination temperature (650-750°C) with a gradient of 60°C min"<1> and then maintained at this temperature for a further period of 24 hours. The gas mixtures used are identical to those used for the first two methods.

The general manufacturing conditions for molybdenum oxides, in bulk or supported, with preferred orientation, are the following: the oxidizing gas used is generally air, but may optionally be pure oxygen or oxygen that is more or less diluted with an inert gas. Extensive dilution of the oxygen is not necessary and a gas containing at least 10% oxygen is preferably used. The gas mixture can be used in anhydrous form or

containing steam, as described above, the treatment temperature must be between 500°C and 800°C.

It has been shown that at a temperature below 500°C the treatment must be of unacceptable duration to be effective, while temperatures above 800°C lead to

excessive evaporation of product,

the duration of the treatment is more than 5 hours and it is usually between 24 and 72 hours when performed in the presence of humid air and between 48 and 120 hours in the presence of dry air. However, it can be longer since the performance of the catalyst increases with the duration of the treatment time.

With treatment in the presence of moist air, it is possible to increase the degree of conversion to oriented oxide and therefore obtain larger amounts of recrystallized oxide than in the presence of dry air.

Example 1

This example illustrates, firstly, how bulk molybdenum oxide (MoO3) oriented in accordance with the invention is obtained by using the first method, and secondly, how modified molybdenum oxycarbide in bulk form is obtained for the purpose of catalyzing isomerization of n-heptane.

The oriented oxide was prepared in the following manner: an amount of 1000 mg of polycrystalline MoO 3 was placed in a quartz tube placed in a tube furnace. This was subjected to a stream of dry air (10 cm<3>min"1). The oven temperature was increased from room temperature to 680°C at a rate of 20°C min"<1>. The temperature was maintained for a period of 100 hours. The oven temperature was then lowered to room temperature while maintaining the air flow. At room temperature, the oriented or single-crystalline oxide MoO3 was taken out of the furnace and stored in a box for later conversion to modified oxycarbide in accordance with the invention.

Carburization and application of a portion of the bulk oriented oxide obtained in the presence of dry air was then carried out under the following conditions: An amount of 12 mg of oriented molybdenum oxide (in plate form) prepared above was placed in a stainless steel tubular reactor between two layers of silica wool. The reactor was drained under a stream of hydrogen (purity greater than 99.95%) at room temperature for 30 minutes at atmospheric pressure. The pressure in the system was then increased under a hydrogen stream to 6 bar, the reaction temperature being 350°C. When the temperature and pressure were reached, n-heptane was injected into the hydrogen stream so that the molar ratio between H 2 and n-heptane in the mixture was 30 to obtain molybdenum oxycarbide.

The catalytic performances obtained during reforming of n-heptane were recorded against time to assess any aging of the modified molybdenum oxycarbide catalyst. The results obtained and details of the reaction products obtained are given in table 1.

Furthermore, fig. 1 selectivity for n-heptane isomer (i-C7) as a percentage relative to the conversion of this n-C7.

In table 1, the time that has elapsed since the beginning of the operation is given in the upper part of the columns. The lines indicate in order: the total degree of conversion of the system (% n-heptane-mole coolers converted either by isomerization or by

cracking),

specific rate: number of initial hydrocarbon molecules

converted per g catalyst and per second,

total C7 isomer selectivity (% isomerized molecules in

relative to total converted molecules),

C7isomer yield (product of isomerization selectivity

and transformation, %),

percentages of isomerized chemical types related to total isomers obtained (C7 isomers, %)

DMP = 2, 2-dimethylpentane +

2.3-Dimethylpentane +

2.4-Dimethylpentane

2MHex = 2-methylhexane

3MHex = 3-methylhexane

3EPent = 3-ethylpentane

Cyclic compounds = methylcyclohexane + toluene

percentages of cracked chemical types obtained in relation to total cracked products obtained (cracking products, %).

The specific surface area BET is 80 m<2>/g.

Examination of table 1 gives grounds for the following comments: 1. The number of converted molecules of n-heptane is very low during the first hour of reaction. After about 2 hours under a stream of n-heptane and hydrogen at 350°C, the catalyst begins to become active, and this leads to a significant change in its surface area. This activation corresponds to reduction and carburization of the oriented molybdenum oxide to reactively modified oxycarbide through reaction with the reaction mixture (n-heptane/hydrogen) as in the case of a polycrystalline molybdenum oxide. 2. C7isomer selectivity is very high from the very beginning of the reaction and remains higher than 95%. The formed isomeric products mainly consist of molecules with a single branch (2-methylhexane and 3-methylhexane) and two branches (dimethylpentanes), while the cyclic molecules are formed in a very small amount (< 1%).

The examination of fig. 1 shows that the n-C7 isomer selectivity remains very high, regardless of conversion of the reaction.

Example 2

This example illustrates, firstly, how the molybdenum oxide oriented in accordance with the invention is obtained, supported on a SiC-based support, using the third method, and secondly, how the modified molybdenum oxycarbide is obtained supported on the said SiC to catalyze the isomerization of n -heptane.

SiC granules with a specific surface area BET of 19 m<2>/g were impregnated with an aqueous solution of

ammonium heptamolybdate and calcined at 350°C for 2 hours to decompose the heptamolybdate to MoO 3 oxide.

The SiC granules impregnated with polycrystalline MoO 3 obtained in this way, placed in a ceramic dish, were introduced into a quartz tube placed in a tube furnace. The unit was subjected to a stream of dry air at a rate of 20 cm<3>min"<1>. The rise in temperature and the remainder of the assembly is identical to that used in Example 1. The temperature used was 700°C. The system was maintained at this temperature for a period of 72 hours. The furnace temperature was then lowered to room temperature while maintaining the flow of dry air. At room temperature, the product, in this case oriented or single-crystalline MoO3 supported on SiC, was removed from the furnace and stored in a box for later conversion to modified oxycarbide.

Carburization followed by the use of the oriented supported oxide obtained in the presence of dry air was then carried out as before.

The specific rate and C7 isomer selectivity achieved on the catalyst as a function of time under current are given in Table 2. Details regarding the distribution of

the reaction products are also given in the table.

Such an oxycarbide supported on SiC offers the advantage of higher solidity (abrasion resistance, mechanical resistance), better conditions of use (no fouling of reactors) and improved yield when used (smaller amount used in relation to treated product).

Example 3

This example illustrates the results obtained with bulk molybdenum oxycarbide modified in accordance with the invention using an intermediate oriented oxide obtained in the presence of a moist oxidizing atmosphere. For this purpose, the method according to example 1 was used, where the flow of dry air was replaced by a flow of moist air at 30 cm<3>/min with 3% steam (on a volume basis).

The oriented oxide was obtained after a treatment time of 48 hours and was then converted to oxycarbide and used for the isomerization of n-heptane as in example 1.

The results are given in table 3:

It is seen that the specific velocity is always high, although it is lower than the specific velocity occurred when the oriented molybdenum oxide is prepared in the presence of dry air. On the other hand, the production of the oriented oxide requires a much shorter period of time.

Example 4

In this example, a comparison was made between the specific rates obtained with a modified bulk oxycarbide-based catalyst prepared from the bulk MoO3 precursor oriented in the presence of dry air as in Example 1 and activated under the mixture of n-heptane and hydrogen at 6 bar and 350°C, with the rate obtained with a catalyst prepared from polycrystalline MoO3i bulk according to the state of the art and which is activated under the same conditions.

The specific rates and C7 isomer selectivities achieved with both catalysts and details of the reaction products are given in Table 4.

Examination of this table provides the basis for the following comments: 1. The specific speed for however, the isomerization reaction for n-heptane, obtained on a modified molybdenum oxycarbide prepared from MoO3 oriented in accordance with the invention, is much higher than the specific rate obtained on the same active phase prepared from polycrystalline MoO3 (608 instead of 53 x 10"<7>(mol g "<1>s"1).2. The distribution of the reaction products (isomers and cracked ones) are practically identical between both catalysts. This shows that the active phase formed has an identical chemical nature, i.e. molybdenum oxycarbide.

Example 5

The same type of comparison as in example 4 (modified oxycarbide/oxycarbide according to the state of the art) was carried out.

The specific rates were compared for modified molybdenum oxycarbide supported on SiC, obtained from molybdenum oxide supported and modified in the presence of moist air (as in Example 3) and for an oxycarbide obtained from polycrystalline MoO 3 supported on the same SiC.

The working conditions for activation and isomerization of n-heptane are the same as those in Example 4.

The results are given in table 5:

It can be seen here that again the specific velocity obtained with the supported modified oxycarbide is substantially increased compared to the specific velocity of the oxycarbide obtained from polycrystalline No03.

Example 6

This example illustrates the use of bulk molybdenum oxycarbide, modified in accordance with the invention, in the reaction with the hydrogenation and decyclization of benzene.

In this example, the catalyst is obtained from modified oxycarbide from oriented MoO3 (in plate form) synthesized in the presence of dry air in example 1, and is used for the reaction with hydrogenation and decyclization of benzene. The reaction temperature and working pressure are respectively 350°C and 60 bar. The results obtained are shown in table 6. The catalyst is initially activated at 6 bar under a mixture of n-heptane/hydrogen (cf. example 3), and is then placed under a mixture of benzene/hydrogen at 60 bar.

Two points should be highlighted:

the very high selectivity for products with 6

carbon atoms, which are constantly higher than 97%,

a substantial decyclization rate without cracking in the region of 12%: this value is exceptionally high for such a difficult reaction and leads to assumptions that

this catalyst can be used for

decyclization reactions of aromatic compounds in oil refining, bearing in mind that benzene is the most difficult aromatic compound to decyclize.

Claims (9)

1. Catalyst based on modified molybdenum oxycarbide, characterized in that it is produced from a molybdenum oxide MoO3 which has a structure which mainly comprises monocrystalline plates oriented in a preferred direction, the oxide being then treated in a stream of a mixture of hydrocarbon and hydrogen, and that the has a specific surface area of at least 80 m <2> /g. 2. Catalyst as stated in claim 1, characterized in that the oxycarbide is supported on a carrier, the carrier preferably having a high BET-specific surface. 3. Catalyst as stated in claim 2, characterized in that the carrier is pure or doped silicon carbide. 4. Process for the production of catalyst based on modified molybdenum oxycarbide, characterized by starting from ordinary polycrystalline molybdenum oxide or possibly a precursor molybdenum compound which is converted into oxide, the molybdenum oxide is heat-treated by sublimation/condensation at a temperature between 500 and 800°C in a closed space under an atmosphere of an oxidizing gas for at least 5 hours to obtain an oriented recrystallized oxide, which is then treated in a stream of a mixture of hydrocarbon and hydrogen to give a modified molybdenum oxycarbide. 5. Method as stated in claim 4, characterized in that the polycrystalline molybdenum oxide or the precursor molybdenum compound of the oxide is mixed with or deposited on a carrier before the heat treatment. 6. Method as stated in claim 4 or 5, characterized in that the atmosphere of oxidizing gas is either a slowly moving stream or a static atmosphere. 7. Method as stated in one or more of claims 4-6, characterized in that the oxidizing gas is dry or moist air. 8. Use of catalysts which are based on an oriented molybdenum oxycarbide according to claims 1-3 or which are obtained in accordance with the method according to claims 4-7 as a catalyst for petrochemical reactions. 9. Use as stated in claim 8, where the petrochemical reactions are isomerization, hydrogenation of linear and cyclic hydrocarbons or decyclization of aromatic compounds.