WO1993004777A1

WO1993004777A1 - Catalyst carrier

Info

Publication number: WO1993004777A1
Application number: PCT/FI1992/000235
Authority: WO
Inventors: Simo Juhani Salanne; Ahmad Kalantar Neyestanaki; Lars-Eric Lindfors; Pekka Juhani Vapaaoksa
Original assignee: Kemira Oy
Priority date: 1991-09-06
Filing date: 1992-09-04
Publication date: 1993-03-18
Also published as: FI89012C; FI914215A; FI914215A0; FI89012B

Abstract

A combustion catalyst having a silica-based carrier, the catalytic agents being attached to a structure made of a silica-containing viscose fiber.

Description

Catalyst carrier

The present invention relates to a structurally versatile silica-based catalyst carrier material by means of which the efficiency of catalytic units can be increased and the reaction temperature can be lowered, especially in various combustion and oxidation reactions.

The catalyst carriers used in current technology are mostly ceramic granules, or ceramic and recently also metallic so- called monoliths, which are coated with an oxide layer having a large specific surface area, most commonly with aluminum oxide, a so-called wash coat. Both industrial-process catalytic units and exhaust-gas converters are mostly based on this technique.

The use of various fibrous materials as catalyst carriers, especially within the area of so-called flameless combustion, is also described in the literature. The practical applications are still few. The best known is perhaps the catalytic hair curler.

The fibrous materials which are in use or have been proposed include quartz fibers, glass fibers, ceramic fibers, asbestos, etc., which are turned into wool, felt, thread, fabrics or knits for use as carriers. These materials have certain draw¬ backs which have limited their being adopted into use. Their specific surface area is small, usually less than 10 m /g (BET), as compared with conventional carriers. For this reason the actual catalytic material, usually an expensive noble metal, cannot be dispersed into small effective particles be¬ cause of the lack of surface area. In all applications, the low mechanical strength of the fibers limits their usability. Im¬ provement of their strength has a negative effect on the activ¬ ity of the catalyst: the surface area decreases further, or detrimental foreign elements have to be added to the product. Owing to the brittleness and stiffness of the fibers, it is very difficult to turn them into fabrics, knits or felts, and further developed structures, to serve as carriers in catalytic units. It is therefore often necessary to limit their use to small applications for which wool or felt is usable. The best known material is quartz felt.

A silica-based catalytic structure has been described in the literature, EPO Patent No. 377290 (CA 113:196927b).

In the state of the art the principal problems are:

- the difficulty of processing quartz fiber into a structure

- the small surface area obtained in the processing of quartz fiber

- the large amount of noble metal required for catalysis

In the present invention, these problems have been solved in a new manner, i.e. by producing an entirely new catalyst having a ceramic textile carrier the structure of which may be not only a needle felt but also any textile structure, a knit, a non- woven fabric, a woven fabric, wadding, or rope. Such a carrier can be produced if the preliminary stage used is a silica- containing viscose fiber, which is known from, for example, BR patent 1064271. A structure of a suitable shape can be made from the silica-containing viscose fiber, whereafter the vis¬ cose can be removed from the structure by pyrolysis, which means that the structure and shape of the fiber product can be selected freely according to the final use. Before the firing the silica-containing fiber can thus be turned into a woven fabric in the same manner as viscose fiber, or into various textile structures in the same manner as cotton fiber. Thus a new method has been found for producing a catalyst by using a silica-containing viscose textile as a preliminary stage for the carrier and by combining with this a catalytic mixture based on a noble metal. It is also to be noted that the appa- ratus of any viscose process can be used for producing the fabric or knit.

The firing conditions themselves can be used for regulating both the specific surface area (Figure 1) of the fiber and the sintering of the structure. Figure 1 depicts the specific sur- face area BET (m /g) of silica fiber as a function of the firing temperature. If the firing temperature is kept below 1000 degrees, the Si0₂ fiber will remain amorphous, and thus any dust possibly detaching from it will not be as dangerous as, for example, the dust detaching from quartz felt. The firing temperature may thus be 400 - 1500 degrees (preferably 750 - 1050 degrees) .

When the silica according to the invention is used, owing to the intermediate viscose-silica stage it is possible to prepare highly different fabrics and knits and further from these, after the firing, by applying for example the conventional methods used in the technology of assembling filters and metal catalytic units, the catalytic structure best suitable for each given use. The shape of the structure may be based on a cylin¬ der or on cylinders one inside the other, through which the gas will travel in the axial direction, on pack structures of dif¬ ferent shapes, on cylinders rolled together with the support structure, etc. The structure and the required supports are selected according to the projected use, depending on the tem¬ perature, pressure, mechanical stress, impurities, permitted pressure loss, gas amounts, retention periods, etc. The inven¬ tion allows in this respect considerably greater freedom than previously.

A catalytic unit such as this is highly suitable, for example, for flameless combustion and for both large and small appli¬ cations. Some examples to be mentioned are catalytic hair cur¬ lers, stoves for households and for institutional kitchens, hot-air generators, etc. Other uses include the purification of, for example, flue gases and solvent-vapor emissions by oxidation. The purification of diesel engine exhaust gases, in which the use of a silica carrier is an absolute prerequisite for achieving a good purification result also with sulphur- containing gases, is especially worth mentioning. Fabrics and knits produced by the method according to the invention provide new, versatile possibilities for the production of a diesel catalyst.

The adding of the catalytic material, most commonly a metal, onto the carrier surface can be performed by conventional methods by chemisorption, impregnation, evaporation from an alcohol or water solution, etc. Thereafter the product is dried, calcinated, and/or reduced normally.

From the accompanying drawing, Figure 2, it can be seen that a catalyst such as this functions efficiently. With highly suc¬ cessful specimens 1 and 2, a good conversion was achieved with low Pt amounts. The same efficiency is achieved using a tradi¬ tional quartz felt catalyst if the noble metal amount is triple.

This is due to the fact that the fiber has a very large specif¬ ic surface area as compared with quartz fiber. The proportion of noble metal can thus be decreased considerably, and cobalt can be used as an auxiliary metal, which will lower the price of the catalytic structure and, nevertheless, the combustion results achieved will be as good as if pure platinum were used.

The combustion catalyst carrier structure according to the present invention can be produced, for example, as follows:

A fiber of a silica-viscose material is produced by extruding a homogeneous solution which contains silicon oxide monomer and viscose through a spinning nozzle into a polymerization and regeneration solution. The silica-viscose material fiber or filament is thus produced by polymerizing monomeric silica in a cellulose matrix. This is preferably done by utilizing the viscose process.

In a viscose process thus modified, a basic silica solution and viscose are first mixed together to produce a multiple- component solution. The basic solution of silica is added to the viscose at a stage at which the viscose has dissolved and preferably when it has been aged and filtered. In this case, care must be taken that the joint solution contains a suffi¬ cient amount of sodium so that the silica will not regenerate the cellulose too early in its sodium xanthate form.

After the formation of the solution, the solution of silica, a base, and viscose is extruded through a spinning nozzle or other nozzle, directly into an acid bath, which will polymerize the silica and regenerate the cellulose. Thereby a preliminary stage of the fiber product which serves as a carrier, i.e. a silica-viscose material fiber, is produced, which can be washed and dried in the same manner as in the conventional viscose process.

In the following examples, the obtained carrier material is called VISIL.

Examples: l.a Firing of VISIL fabric.

A thread 2 x 25 tex was produced from VISIL fiber, and from the filament a 2 x double knit having a basis weight of 540 g/m . The fabric was washed with an alcohol solution and rinsed with water in order to remove any impurities which had possibly adhered to it during the process. Thereafter the fabric was placed in a kiln having a temperature below 300 °C, the tem¬ perature was allowed to rise to 900 °C, and the specimen was kept at 900 °C for one hour. An analysis showed that the spe- 2 cific surface area of the fiber was approx. -80 m /g, and the structure of the double knit had retained precisely the ap¬ pearance of the original, shrinkage 20 %.

₂ l.b A nonwoven fabric (needle felt) 80 g/m is made from the fiber. By using the carding and needling technique, the proce¬ dure continues as in Example l.a.

l.c As Example l.a, except that the firing temperature is 750 °C.

2.a A piece with a 6 cm diameter was cut out of the fabric according to Example l.a, and it was impregnated with a solu¬ tion of alcohol (20 ml) and cobalt salt CotNO ^ 6H2O. The impregnated fabric was dried at room temperature for 12 hours, whereafter the specimen was heated at 550 °C for one hour. Thereafter the specimen was impregnated with a solution of alcohol and platinum salt (I^ tClg XH₂0) and was dried at room temperature for 12 hours and was heated at 550 °C for 1 hour. The cobalt amount in the specimen was 0.17 g and the platinum amount was 11 mg.

The catalyst was tested, and an efficiency of 99.3 % was ac¬ complished when the fuel flow was 0.1 and the air flow was 2.64 1/min.

2.b As 2.a, except that the platinum amount was 9 mg. The result was the same as in Example 2.a.

2.c As Example 2^*.b, except that the amount of platinum was 7 mg.

2.d As Example 2.a, except that the textile structure was in accordance with l.b so that the weight of the carrier was the same as in Example 2. . 3.a, b and c As in Examples 2.a, b and c, except that the amount of cobalt was 0.08 g. The efficiency accomplished with these was 99 per cent.

4.a, b and c As in Examples 3.a, b and c, except that the amount of cobalt in the catalyst was 0.05 g, efficiency 96.8 %.

The results of Examples 2-4 indicate that the catalysts pre¬ pared functioned excellently.

5. Three catalysts were prepared:

1. 0.18 g of cobalt + 10 mg of platinum

2. 10 mg of platinum

3. 0.18 g of cobalt

The results (Figure 2) show that when the amount of platinum is only one-third as compared with a commercial platinum catalyst, fully satisfactory results are achieved with a catalyst accord¬ ing to the invention. Table 2 shows that the same catalysts also function well as so-called oxidizing exhaust-gas cat¬ alysts. Table 1 shows the composition of the feed gas in a light-off test in which the pulse flow was 8900 ml/min and the constant flow was 187 ml/min.

Table 2 Results of the light-off test

6. 3 catalytic units were prepared from different VISIL mate¬ rials: loose and dense fabric, thread. Any impurities left in the materials during the spinning were washed off with an am¬ monia solution and clean water. Thereafter they were dried, and by raising the temperature in a controlled manner up to 900 °C, the cellulose was burned off.

The obtained silica specimens were impregnated with palladium from an amine solution, and they were dried and calcinated. The obtained specimens were aged for 1 h at 900 °C, whereafter they were tested with a gas mixture representing the composition of diesel exhaust gas, its analysis being: propylene 500 ppm, carbon monoxide 500 ppm, NO 1000 ppm, carbon dioxide 10 %, oxygen 5 %, water vapor 7 %, and the balance was nitrogen.

The size of the specimens was approx. 3 ml and the turnover of the gas was 50,000 1/h. The results are shown in the table below.

The ignition temperatures and conversions are means of the values of CO and hydrocarbon, which were very close to each other.

Claims

1. A hydrocarbonaceous-gas combustion catalyst having a silica-based carrier, characterized in that the carrier mate¬ rial is a silica structure manufactured using as the raw mate¬ rial a silica-containing viscose fiber, the catalytic agents being attached to the silica structure.

2. A catalyst according to Claim 1, characterized in that the carrier used is a ceramic textile structure such as needle felt, rope, knit, fabric or non-woven fabric.

3. A catalyst according to Claim 1, characterized in that before the catalytic agents are attached, the carrier material is fired to produce a substantially amorphous Siθ2 fiber.

4. A catalyst according to Claims 1 and 3, characterized in that the firing temperature is within the range 400 - 1500 °C, preferably 750 - 1050 °C.

5. A catalyst according to Claims 1, 3 and 4, characterized in that the catalytic agents are impregnated in the form of solutions into the structure, whereafter calcination is carried out.

AMENDED CLAIMS

[ received by the International Bureau on 2 February 1993(2.02.93) original claims 1-5 replaced by amended claims 1-6 (1 page )]

1. A hydrocarbonaceous-gas combustion catalyst comprising a silica-based carrier and catalytic agents, characterized in that the carrier material is a silica structure manufactured by using a silica-containing viscose fiber as raw material, the catalytic agents being attached to the silica structure.

2. A catalyst according to Claim 1, characterized in that the silica structure is a substantially amorphous Si0₂ fiber obtained by firing the silica-containing viscose fibre.

3. A catalyst according to Claim 1, characterized in that the carrier is a ceramic textile structure such as needle felt, rope, knit, fabric or non-woven fabric.

4. A catalyst according to Claim 3, characterized in that the ceramic textile structure is obtained by firing a textile structure made of the silica-containing viscose fiber.

5. A catalyst according to Claim 2 or 4, characterized in that the firing temperature is within the range 400 - 1500 °C, preferably 750 - 1050 °C.

6. A catalyst according to any of Claims 1, 3 and 4, characterized in that the catalytic agents are impregnated in the form of solutions into the carrier, whereafter calcination is carried out.