Impregnation process for catalysts

Details Impregnation process for catalysts Impregnation process for catalysts

2001-03-30

2002-09-24

Bell, Mark L. (Department: 1755)

Catalyst, solid sorbent, or support therefor: product or process

Catalyst or precursor therefor

Metal, metal oxide or metal hydroxide

C502S326000, C502S327000, C502S355000, C518S715000

Reexamination Certificate

active

06455462

ABSTRACT:

THIS INVENTION relates to catalysts. It relates in particular to a process for preparing a catalyst precursor.
According to a first aspect of the invention, there is provided a process for preparing a catalyst precursor, which process comprises
subjecting, in an initial treatment stage, a slurry comprising a porous oxidic catalyst support or carrier, an active catalyst component or its percursor, and water, to treatment at elevated temperature and at sub-atmospheric pressure such that impregnation of the support or carrier with the active catalyst component or its precursor and partial drying of the impregnated support or carrier occurs, with the initial treatment stage not continuing beyond a point where the impregnated carrier or support has a loss on ignition (LOI) which is less than 1.2 times its loss on ignition at incipient wetness (LOI
iw
);
thereafter, in a subsequent treatment stage, subjecting the partially dried impregnated support or carrier to treatment at elevated temperature and at sub-atmospheric pressure such that the temperature in the subsequent treatment stage exceeds that in the initial treatment stage and/or the sub-atmospheric pressure in the subsequent treatment stage is lower than that in the initial treatment stage, thereby to obtain more vigorous drying of the impregnated support or carrier in the subsequent treatment stage than in the initial treatment stage, with a dried impregnated carrier or support thereby being produced; and
calcining the dried impregnated carrier or support, to obtain the catalyst precursor.
The resultant catalyst precursor is, in practice, subjected to reduction, in order to obtain a catalyst.
The porous oxidic catalyst support may, in particular form. In principle, any commercially available oxidic catalyst support can be used. Examples of catalyst supports that can be used are alumina (Al
2
O
3
) and titania (TiO
2
). The support preferably has an average pore diameter between 8 and 50 nanometers, more preferably between 10 and 15 nanometers. The support pore volume may be between 0.1 and 1 ml/g, preferably between 0.3 and 0.9 ml/g. The average particle size may be between 1 and 500 micrometers, preferably between 10 and 250 micrometers, still more preferably between 45 and 200 micrometers. Alumina is preferred as the support, and the invention is described further hereunder with reference to alumina as the suport.
While the active catalyst component can, at least in principle, be any known Fischer-Tropsch active component such as cobalt (Co), iron (Fe), nickel (Ni) or ruthenium (Ru); however, cobalt (Co) is preferred. In particular, a cobalt precursor can be used. Still more particularly, cobalt nitrate (Co(NO
3
)
2
·6H
2
O) is preferably used.
From 1.18xy to 1.82xy kg Co(NO
3
)
2
·6H
2
O may initially be used in the initial treatment stage, where x is the BET pore volume of the alumina support in ml/g, and y is the mass of alumina support to be impregnated, in kg.
The process may include initially dissolving the Co(NO
3
)
2
·6H
2
O in the water, which is preferably distilled water. Sufficient water may be used such that the volume of the solution is greater than xyl, and preferably is about 2xyl.
In one version of the invention, this solution may be heated to a temperature between 60° C. and 95° C., with the support then being added to the solution at atmospheric pressure, to form the slurry. The slurry may be mixed, preferably on a continuous basis, eg by means of an internal rotating screw in a conical vacuum drier in which the slurry is held.
In the initial treatment stage, vacuum may then gradually be applied to the slurry, preferably under continuous mixing, eg stirring, thereof, at a temperature between 60° C. and 95° C., which may be the same as the temperature to which the solution is initially heated, or different therefrom. This constitutes the initial treatment of the slurry, and it is important that the initial treatment be effected in a gradual manner, ie excessive boiling of the slurry is to be avoided.
The sub-atmospheric pressure or vacuum that is applied during the initial treatment stage may be down to 20 kPa(a), ie between atmospheric pressure and 20 kPa(a). Typically, the vacuum may be about 20 kPa(a) for a slurry temperature of 60° C., and about 83 kPa(a) for a slurry temperature of 95° C.
The initial treatment stage is preferably continued until the loss on ignition (LOI) of the impregnated alumina support is 1.2 times LOI
iw
, ie 1.2 times the LOI at the point of incipient wetness (iw). Incipient wetness occurs when all the pores of the support are filled with liquid and there is no excess moisture, over and above the liquid required to fill the pores, present. Typically, the initial treatment time will be up to 3 hours or more.
Loss on ignition (LOI) is defined as the mass % loss observed during complete calcination, ie during decomposition to Co
3
O
4
/Al
2
O
3
, experimentally to be determined as the mass % loss observed during calcination at 400° C., ie at a temperature sufficiently high to ensure quantitative decomposition of cobalt nitrate to Co
3
O
4
, but too low in order to effect the undesired formation of cobalt aluminates.
The LOI value at the state of incipient wetness, ie LOI
iw
, can be expressed as a function of the pore volume of the support as well as the amount of catalyst active component to be impregnated. The pore volume of the support, prior to impregnation, is as stated hereinbefore, equal to x ml/g. The amount of Co(NO
3
)
2
·6H
2
O to be impregnated is M gram per gram of support material, and will fall within the range: 1.18 x to 1.82 x gram per gram support material. M is thus determined by the amount of Co(NO
3
)
2
·6H
2
O initially used. The LOI value at the state of incipient wetness can be calculated as follows:
LOI
iw
=100 ((0.20
M +x
)/(0.475
M+x+
1))  (1)
This shows that the LOI at the state of incipient wetness is dependent on the pore volume of the support and the amount of Co(NO
3
)
2
·6H
2
O used for the catalyst preparation.
The gradual drying procedure until the LOI is 1.2 times LOI
iw
ensures that about 83% of the cobalt nitrate is quantitatively drawn into the pores of the alumina support without the occurrence of localized saturation, which results in premature crystallization of cobalt nitrate.
At a moisture point somewhat above incipient wetness, ie when LOI of the impregnated alumina support is 1.2 times LOI
iw
, aggressive evacuation, eg increased vacuum pump suction capacity when a vacuum pump is used, may be applied, in the subsequent treatment stage; at the same time, it is ensured that the support temperature is controlled at between 60° C. and 95° C. Thus, when a vacuum drier, in which the impregnated support is contained in the of a bed, is used, an increased setting of the vacuum drier wall temperature is used, thereby ensuring that the bed temperature is controlled between 60° C. and 95° C., under continuous mixing, eg stirring. This constitutes the subsequent treatment in which more forceful drying of the impregnated support takes place. Once the point where LOI=1.2 times LOI
iw
has been reached, the more forceful vacuum drying during the subsequent treatment stage preferably proceeds in an uninterrupted fashion, preferably at the conditions:
>60° C., but not higher than 95° C., and at the minimum pressure which is attainable, with this pressure being <20 kPa(a)
Vacuum drying under these specific conditions should be maintained until a clearly defined maximum required LOI value is reached, which value depends on the need to store the dried material for a certain period of time before calcination can be executed, as hereinafter described, and this maximum required LOI value is smaller than, or equal to, 0.90 times LOI
iw
.
The calcination of this dried impregnated support may be effected in a fluidized bed, or a rotary kiln, calciner at a temperature from 200° C. to 300° C., preferably at about 250° C.
The process thus involves using a slurry, ie an excess of moisture, to achieve impregnation of t

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