Catalyst, process of making catalyst and process for...

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1996-10-08

2002-07-16

Langel, Wayne A. (Department: 1754)

Chemistry of inorganic compounds

Modifying or removing component of normally gaseous mixture

Nitrogen or nitrogenous component

C502S305000, C502S309000, C502S312000, C502S321000, C502S350000

Reexamination Certificate

active

06419889

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a catalyst, to a process for preparing such catalyst, to a catalyst obtainable by such preparation process and to a process for converting nitrogen oxide compounds with the help of such catalyst.
BACKGROUND OF THE INVENTION
It is known that nitrogen oxide compounds can be removed by selective catalytic reduction with the help of a reducing compound such as NH
3
in the presence of a catalyst.
In EP-B-260614 a process has been described for reduction of nitrogen oxide compounds with the help of a catalyst comprising titanium and vanadium, in which catalyst the concentration of vanadium in the surface layer is at least 1.5 times as much as the concentration of vanadium throughout the catalyst. It is described that such catalyst is resistant to deactivation or poisoning by arsenic compounds contained in waste gases together with nitrogen oxides. No indication has been given of the surface area of the catalysts. In the examples, the catalyst has been prepared in the form of a honeycomb. Honeycombs must have a high crushing strength, which makes that they generally have a relatively low surface area. A typical surface area for honeycombs, measured by nitrogen adsorption, is about 40 to 60 m
2
/g.
EP-A-256359 relates to a catalyst for removing nitrogen oxides which catalyst contains a first group of pores having a diameter of 10 to less than 100 nm and a second group of many pores having a diameter of 100 to 12000 nm. The pore volume of the first group and the pore volume of the second group is at least 10% of the total pore volume of the first and second group. It is described that such catalyst is not easily poisoned by an arsenic compound and/or calcium compound. Surface areas as measured by nitrogen adsorption, have not been given. As the contact angle of the impregnated catalysts is not known, these surface areas can not be derived from the mercury intrusion data.
EP-A-516262 relates to titania based catalyst having such porosity that the catalyst comprises 0.05 to 0.5 cm
3
/cm
3
in pores having a diameter of 60 nm or less, and 0.05 to 0.5 cm
3
/cm
3
in pores having a diameter greater than 60 nm. The catalyst is described to preferably have a surface area of between 25 and 200 m
2
/cm
3
. The catalysts used in the examples of which the surface areas have been given, measured according to BET, have a surface area of 100 m
2
/g or more.
It has now been found that a higher conversion of nitrogen oxides is observed if a catalyst is used having a surface area measured by nitrogen adsorption, of between 70 and 99 m
2
/g.
SUMMARY OF THE INVENTION
The catalyst according to the present invention comprises a titania carrier and one or more metal compounds which metals are selected from the group consisting of vanadium, molybdenum and tungsten, which catalyst has a surface area measured by nitrogen adsorption of between 70 and 99 m
2
/g.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
It is generally thought that a higher surface area will give higher conversions. However, it was unexpectedly found that apparently this rule does not apply to the present catalyst.
The surface area of the catalyst is between 70 and 99 m
2
/g, preferably between 79 and 99 m
2
/g. The surface area is to be measured with the help of nitrogen adsorption as described in ASTM method D 3663-92. As long as the contact angle is not known, the surface area can not be derived from mercury intrusion porosimetry data.
The surface area of the catalyst can be influenced by a large number of circumstances. The surface area of the starting material has been found to be important. Further circumstances influencing the surface area have been found to be the degree of peptization of the titania carrier mix before extrusion, and the calcination temperature. The surface area of the starting material is determined by circumstances such as the reaction conditions during preparation, e.g. reaction time and temperature, the peptization conditions and the drying method.
To make that the catalyst has a high surface area, it is necessary to start with a titania carrier having a relatively high surface area. Such carrier will usually have been prepared by dissolving a titania ore in sulfuric acid to obtain a solution containing titanium sulphate. The solution is neutralized, filtered and washed to obtain a metatitanic acid cake. The metatitanic acid cake is optionally peptized and subsequently dried, calcined and pulverized to obtain titania powder useful for preparing a carrier.
Preferably, the titania present in the catalyst is in the form of anatase, as this form of titania generally has a much higher surface area than other forms of titania. Further, the titania carrier will usually contain a relatively small amount of contaminants such as iron compounds and sulphur oxides. It is preferred that the carrier contains less than 10% by weight of contaminants, based on total amount of catalyst. Preferably, the titania carrier comprises less than 5% by weight of sulphur oxides, based on total amount of catalyst.
The expression metal compounds is used herein to indicate that either the metal per se is present or a derivative of the metal. The metal compounds usually present are metal oxides and/or sulfides.
The metal compounds present on the catalyst can be one or more selected from the group consisting of vanadium, molybdenum and tungsten. Preferably, the catalyst contains vanadium. The amount of metal compound, measured as metal, contained by the catalyst can vary between wide ranges. Suitably, the catalyst contains between 0.5 and 10% by weight of metal compounds, preferably between 2 and 6% by weight.
It is customary to use honeycombs when converting nitrogen oxide compounds. The use of honeycombs has the disadvantage that they must have a high crushing strength in order to withstand the gas flow. A high crushing strength will in most cases lead to a relatively low surface area. In the present invention, it is preferred that the catalyst is in the form of trilobes, rifled trilobes or cylinders. The use in the form of a cylinder has the advantage that a cylinder tends to show a better strength than extrudates of a different shape. The catalyst according to the present invention preferably has a cross-sectional diameter of between 0.5 and 5 mm, more preferably a diameter of between 0.5 and 3 mm.
It is important that the catalyst is strong enough to be able to withstand the pressure drop over the reactor. However, generally a strong catalyst has a low surface area while a high surface area catalyst is weak. It was found that catalysts according to the present invention can have a side crushing strength of at least 70 N/cm, usually between 70 and 300 N/cm, measured with the help of testing apparatus Houndsfield type 5000E (Houndsfield is a trademark). These catalysts are of such strength that they are useful for most fixed bed applications.
In order to obtain catalysts of high activity and selectivity, it is preferred that the catalysts have a bimodal pore distribution. Further, it is preferred that the catalysts have more than 90% of the total pore volume present in pores having a diameter of at most 100 nm, which total pore volume is considered to be the pore volume present in pores having a diameter between 1 and 10
4
nm. A preferred catalyst has a pore volume distribution of the total pore volume present in pores having a diameter between 1 and 10
4
nm which is such that 60-85% of the pore volume is in pores having a diameter of 5-20 nm, 15-40% is in pores having a diameter of 20-60 nm and less than 5% is in pores having a diameter of more than 100 nm. The pore volume distribution is to be measured with the help of mercury intrusion porosimetry according to ASTM D 4284-92.
The total pore volume of the catalyst preferably is between 0.1 and 1.0 ml/g, more preferably between 0.2 and 0.8 ml/g.
The catalyst according to the present invention can be prepared in any way known to be suitable to someone skilled in the art.
A preferred preparation process comprises:
(a) mixing a titania powd

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