GB2109263A - A catalyst and processes for making and using the catalyst - Google Patents

Abstract

A methanol synthesis catalyst contains in the non-reduced state, at least 10% copper, 38-70% zinc, at least 10% chromium, and 1-10% aluminium oxides (by weight), and may be obtained by first preparing an aqueous solution of copper acetate and aluminium acetate, then suspending zinc oxide (and optionally graphite) in the solution which is left to hydrate and is then heated to 75 DEG -85 DEG C, the pH being slightly below 7. The suspension is then reacted with chromic anhydride for a time, until the pH is slightly above 7 then the product is filtered, dried, compressed, heated at 320 DEG -370 DEG C in nitrogen and then in nitrogen and air. The product is then granulated and pelletized. <IMAGE>

Description

SPECIFICATION A catalyst and processes for making and using the catalyst This invention concerns highly active catalysts particularly for the synthesis under favourable energetic conditions (i.e., at low pressure and at a temperature below 280"C), of methanol, the catalysts containing, in the non-reduced state, copper oxide, zinc oxide, chromium oxide and aluminium oxide. The invention also concerns a process for preparing these high-activity catalysts and their application for the synthesis of methanol.

It is known that methanol can be produced by reacting carbon monoxide or carbon dioxide with hydrogen in the presence of catalysts containing, in the non-reduced state, zinc and chromium oxides, or copper, zinc and chromium oxides, or copper, zinc and aluminium oxides; in many cases manganese is also added.

Known catalysts often require high synthesis pressures and may have, besides a non-exceptional activity, a poor selectivity as regards methanol; in their preparation, moreover, it is generally necessary to use compounds such as Na2CO3, NaHCO3, NH40H, NH4CI etc., to maintain the pH above 7 (and thus coprecipitation), and/or precipitants, which may pollute the catalyst or require exceptionally intensive washing. In German Patent No. 2,122,952 we have already described catalysts which are very advantageous compared with known catalysts and particularly effective process for their preparation.These catalysts contain (in the non-reduced state) from 10 to 20% in weight of CuO, from 60 to 80% of ZnO, from 10 to 20% of CrO3 and about 1% of graphite, and are prepared by dissolving copper acetate in water, heating this solution to 70 -80 C, suspending in this solution zinc oxide and graphite, precipitating this suspension, under agitation adding an aqueous solution of chromic anhydride, filtering, washing and drying the precipitate at 100 -110 C, compressing the product so obtained, treating it at about 300"C in the presence of nitrogen and finally peiletizing it.

The catalysts so obtained have a good degree of activity, are long-lasting and can be used at low synthesis pressures.

These advantages notwithstanding, the recent oil crisis has underlined the necessity to produce methanol synthetically in ever-increasing quantities and therefore with ever-increasing yields and ever-diminishing energy consumption.

Continuing our uninterrupted research in the field, we have now developed improved catalysts which precisely satisfy the urgent requirements for methanol production, with more advantageous yields, selectivity and energy consumption.

According to the present invention, there is provided a catalyst which comprises, in the non-reduced state and in weight percent, at least 10% CuO, at least 10% CrO3, from 1 to 10% alumina, and from 38 to 70%, optionally 39 to 70%, ZnO.

The catalyst may be made by a process in which copper acetate and aluminium acetate are dissolved in deionized water at the best solubilization temperature (75"-85"C) and after having suspended zinc oxide under agitation in this solution, the solution is left to allow the zinc oxide to hydrate over a period of 18 to 30 hours, the hydrated suspension is heated to 75"-85"C maintaining the pH only slightly below 7 in all these stages, the hydrated suspension is then reacted and heated with chromic anhydride for a period of time until a pH of slightly higher than 7 is reached, and after having filtered, washed, dried, compressed and heat-treated with nitrogen circulation the product so obtained, the latter undergoes further heat treatment at 300"-400"C with circulation of nitrogen and air, before it is granulated and pelletized. The catalysts according to the invention preferably contain, in the reduced state, from 10 to 40% (by weight) of CuO, from 10 to 20% of CrO3, from 70 to 38% of ZnO and from 10 to 2% of A1203; preferably a small amount (e.g. about 1 - 1.5%) of graphite may be contained by the catalyst.

The various aspects and advantages of the invention will be better illustrated by the following description, the attached drawings and the following examples, given however by way of illustration and not by way of limitation.

The block scheme in Figure 1 shows as an illustration and not as a limitation a process for the preparation of catalysts for the synthesis of methanol which catalysts are highly active at low pressure and at a temperature of about 2200.27000.

In stage 1 an aqueous solution (SOI 1) is prepared with copper acetate (Cu.Ac) and basic aluminium acetate (Al.Ac) by adding these compounds to de-ionised water H20 and by heating it to a temperature of about 50-60"C (for example by passing steam V in a coil R placed at the bottom of the vessel containing the solution). The solution is transferred from stage 1 to stage 2 where, while it is agitated (with agitator 2' for example), zinc oxide (ZnO) and preferably graphite (GRA) also (which, as will be seen, can alteratively be added later to the product in the dry state) are added.

The addition in stage 2 takes place under agitation, and with heat being supplied through coil R' so that the suspension is maintained at a temperature of, for example, 50"C - 60"C for a certain time, for example, for an hour. The suspension SOS so obtained in stage 2 is left to rest for a period (for example 18 to 30 hours, preferably 24 hours) such as to favour, an ambient temperature, the hydration of the zinc oxide. Once the hydration has been completed, the suspension (with hydrated ZnO) again undergoes heat treatment in stage 4 which preferably involves maintaining the suspension at a temperature of between 75"C and 85"C for about one hour.It should be pointed out at this stage that the pH of the solution in stage 1 and of the suspension in stages 2,3 and 4 is always below 7 and is therefore always slightly acid, for example pH=6.

To the suspension (slightly acid) heated in stage 4, is then added chromic anhydride CrO3 in stage 5 where a reaction takes place between the hot suspension SOS and the fresh CrO3 at temperature (preferably between 75 and 85"C, better still 80"C) and for a time until the pH reaches in stage 5 a slightly alkaline value, for example about 7.5. The achievement of this pH value means that the reaction is complete so that heating can be stopped and filtration (stage 6) of the precipitate at ambient temperature can begin.

The filtered product from stage 6 undergoes in stage 7 normal washing with deionized water, it is then dried in stage 8 at a temperature for example of 100-11 0'C, and is then pressed in stage 9; the treatments in 7,8 and 9 are substantially conventional ones.

In contrast, according to an advantageous feature of the invention, the pressed product coming from stage 9 undergoes in stage 10 heat treatment at a high temperature for example about 350"C with nitrogen circulation and then a second heat treatment for example still at 350"C but the circulation of nitrogen containing between 1 and 5% of air in order to produce a catalyst in the oxidized state. The catalyst is then granulated and pelletized in 12. A PFR catalytic final product is obtained, in the non-reduced state.

As already mentioned, graphite may be added to the granulated product (having a particle size, for example, between 0.3 and 1.5 mm) before being pelletized, this graphite replacing totally or in part the graphite (GRA) added in stage 2.

It must be pointed out that the block diagram in Figure 1 indicates the treatments stages rather than treatment stations; the treatments may be, and in effect more often are, carried out at a few stations and in a few vessels.

Example I A solution is prepared by dissolving in deionized water 400.6 g of copper acetate and 81.9 g of aluminium acetate.

The solution SOL 1 is heated (by circulating steam in a coil) at a temperature of about 60"C to facilitate solution of the copper and aluminium acetates (stage 1). To this solution are added under agitation 650 g of zinc oxide and 10 g of graphite (GRA). In stage 2, suspension SOS is obtained, containing 800 cc of deionized water, and kept for an hour at 800C. Suspension SOS is then left to rest (in stage 3) for 24 hours, during which time it falls naturally from the temperature of 80"C it had in stage 2 to ambient temperature. In stage 3, ZnO is fully hydrated. After 24 hours' hydration the suspension is again heated (in stage 4) at 800C under continuous agitation (4').In all treatments in stages 1 to 4, the pH of solution SOL 1 and of hydrated and heated suspension SOS was always below 7, preferably about 6.

The heated (to about 80"C) and agitated suspension from stage 4 is reacted (in stage 5) with a solution consisting of 210.5 g of chromic anhydride in 500 cc of deionized water until its pH is slightly above 7, moving up, for example, to 7.3. Under these conditions full precipitation takes place. The precipitate is filtered in stage 6, washed under normal conditions in stage 7, dried at 100-110 C in stage 8 and compressed in stage 9. The product so obtained is treated for 5 hours with a nitrogen stream at 350"C in stage 10 and then for an hour with a nitrogen stream containing 1 - 5 % air at the same temperature of 350"C (treatment indicated with the reference numeral 11 in the block diagram). The product is then granulated and pelletized in stage 12; its composition after reduction is: CuO 15% ZnO 65% Cr203 16% A1203 3% It should be emphasized that during all treatments in stages 1 to 12 neither pH regulators nor precipitants are used: the absence of these auxiliary chemicals, and in particular of precipitants, is very important since there will not be any trace of pollutant in the catalyst, thus avoiding intensive and expensive washing.

Example 2 Operating as in Example 1, a solution was prepared with 801.2 g of copper acetate and 81.9 aluminium acetate, in which were suspended 500 g of ZnO and 10 g of graphite. After hydrating the ZnO for 24 hours and heating the hydrated suspension to 80"C, the suspension was reacted with 210.5 g of chromic anhydride until the pH reached 7.6.

After filtering, washing, drying, compressing, applying heat treatment at 350"C with circulation first of N2 and afterwards of N2 + 02, a catalytic product was obtained having the following composition: CuO 30% ZnO 50% Cr203 16% A1203 3%.

Examples 3 (comparison) 4 and 5 Utilization of the catalyst produced for the synthesis if methanol The tests are carried out substantially under the same operating conditions as in German Patent No. 21 22 952.

Each time a catalyst sample was taken and reduced at a temperature of 220"C and at ordinary pressure.

When the catalyst could be assumed to be at an advanced state of reduction, its temperature was increased to 290'C, while gas pressure was brought to 100 bar.

The methanol synthesis reactor was fed with synthesis gas during reaction and activity was measured by using a feed mixture having the same composition as the mixture used during reduction and consisting of H2 : 65%, CO: 6.8%, CO2: 11.3%, N2: 16.8%.

Catalyst activity was measured after a period of stabilization lasting 120 hours, that is to say, until two consecutive measurements, taken under the same experimental conditions, yielded the same specific production expressed as weight of pure methanol per hour per litre of catalyst, i.e. Kg of pure CH3OH/H/litre CAT.

The following Table 1 and attached diagram (Figure 2) show the conditions and results obtained with a catalyst according to the German patent (comparative Example 3), with a catalyst according to Example 1 of this invention (Example 4) and with a catalyst from the preceding Example 2 of this invention (Example 5). As can be seen from Figure 2, which is a plot of the rate of production of methanol (measured in kilograms of methanol produced per hour per litre of catalyst) against the space velocity in the reactor (measured in reciprocal hours), the activity of the catalysts according to the present invention (Examples 4 and 5) is greater than that of the comparative catalyst (Example 3), which is the catalyst currently used in industrial operation in a CASALE plant.The two catalysts according to the invention used are both Cu/Cr/Zn/Al but are differejnt in that the catalyst used in Example 4 contains 15% CuO and the catalyst used in Example 5 contains 30% CuO.

TABLE 1 Concen Cat. Type Size Cat. Volume Cat. Weight Pressure Temperature Space Vel. Production tration in m/m in c.c. in g. in bar in C H-1 Kg CH3OH CH3OH litre cat. % weight 1) Accord. German Patent no.2.122952 Cu/Cr/Zn 15% CuO 0.75 ~ 1 6 9.29 100 250 12.500 0.5 87 2) Invention Example 1 Cu/Cr/Zn/Zn 15% CuO 0.75 ~ 1 6 9.29 100 250 12.500 0.7 85 3) Invention Example 2 Cu/Cr/Zn/Al 30% CuO 0.75 ~ 1 6 8.14 100 250 12.500 1 83 The catalyst according to German Patent No. 21 22 952 used as a comparison is a Cu/Cr/Zn catalyst with 15%CuO.

Of all the remarkable features of the invention, only the following are mentioned: I - No precipitant agents are used: II - The variation in pH takes place naturally (without regulators) during reaction of the CuO with ZnO in copper acetate and aluminium acetate; at a temperature of 80"C the pH is about 6 until CrO3 is added and during this reaction after about 3 hours it goes up to about 7.3; lil - Pyrolysis: heat decomposition of the acetate and subsequently oxidation with N2+02 (air); IV - The catalyst with a low CuO content is highly active and inexpensive.

V - The catalyst is selective for methanol synthesis (there are no alkaline ions to form secondary products). It is therefore possible to: Va - Carry out non-intensive filter washing; VI - With the particular fabrication process, a catalyst is obtained with finely dispersed copper oxide.

Claims (12)

1. A catalyst which comprises, the non-reduced state and in weight percent, at least 10% CuO, at least 10%CrO3from 1 to 10% ofalumina and from 38to 70% ZnO.

2. A catalyst according to claim 1, which contains from to CuO 10% 40% Cr03 10% 20% ZnO 70% 38% A1203 10% 2%.

3. A catalyst according to claim 1 or claim 2, which further includes up to 2.5 weight % graphite.

4. A catalyst according to claim 3, which includes from 0.5 to 1.5 weight % graphite.

5. A process for preparing a catalyst, characterised by the fact that a deionized water solution is prepared, containing about 400 to 800 g of copper acetate, 70 to 90g of aluminium acetate, and after heating it to a temperature of between 70 and 80"C, 500 to 650 g of zinc oxide are suspended in it, the zinc oxide is left to hydrate for at least 18 hours, the suspension is heated to 75-85 C, the pH of the solution and subsequent suspensions being a little below 7, then the suspension is reacted with a solution, also in deionized water, containing between 190 and 230 g of chromic anhydride at a temperature and for a period until the pH is slightly above 7, and after filtering, washing, drying, compressing and heat-treating the product obtained by nitr6gen circulation, it is again heat-treated at a temperature of at least 300"C with circulating nitrogen and air, before it is granulated and pelletized.

6. A process for preparing a catalyst, characterised by the fact that a copper acetate and aluminium acetate solution is prepared in deionized water and after zinc oxide has been suspended in it under air agitation, the zinc oxide is left to hydrate for at least 18 hours up to ambient temperature, the suspension is then heated to 75-85"C keeping the pH slightly below 6, after which this suspension is reacted with an amount of chromic anhydride solution in deionized water, at a temperature and for a period until the pH is slightly above 7 (in the absolute absence of pH regulators) and after having filtered, washed, dried, compressed and heat-treated at about 350"C with nitrogen circulation the product obtained, it is again treated at about 350-400"C with nitrogen and air circulation before granulating and pelletizing.

7. A process for preparing a catalyst, which process comprises forming a solution in deionized water of copper acetate and aluminium acetate, suspending zinc oxide in the solution, hydrating the zinc oxide, heating the suspension while keeping the pH below 7, reacting the suspension with chromium an hydride at a temperature and for a length of time sufficient to allow the pH to rise to above 7, filtering the suspension and heat treating the filtered product in a first stage under an inert atmosphere and in a second stage at a temperature of at least 300"C in an atmosphere containing oxygen.

8. A process for making a catalyst substantially as hereinbefore described in connection with, and shown in, Figure 1 or substantially as hereinbefore described in Example 1 or 2.

9. A catalyst made by a process as claimed in any one of claims 5 to 8.

10. A catalyst substantially as hereinbefore described in Examples 1,2,4 or 5.

11. A catalyst which comprises the catalyst as claimed in any one of claims 1 to 4 or 9 or 10, in a reduced state.

12. A process of making methanol, which comprises reacting together carbon monoxide and/or carbon dioxide with hydrogen in the presence of a catalyst as claimed in any one of claims 1 to 4 or 9 to 11.