Chemistry: fischer-tropsch processes; or purification or recover – Group viii metal containing catalyst utilized for the...
Reexamination Certificate
2002-03-04
2003-07-01
Silverman, Stanley S. (Department: 1754)
Chemistry: fischer-tropsch processes; or purification or recover
Group viii metal containing catalyst utilized for the...
C502S327000, C502S332000, C502S355000, C502S415000, C502S439000
Reexamination Certificate
active
06586481
ABSTRACT:
The present invention relates to a catalyst based on cobalt, its preparation and its use in the Fischer-Tropsch process.
The Fischer-Tropsch process is a process well-known to experts in the field, which essentially consists in the hydrogenation of CO to give hydrocarbons. The reaction conditions are also described in literature.
Catalysts which can be used in the Fischer-Tropsch process generally consist of metals of group VIII supported on a carrier, preferably selected from alumina, silica, titania and relative mixtures.
All research projects on the Fischer-Tropsch process are being increasingly orientated towards a greater selectivity to C
9+
, particularly C
22+
hydrocarbons, the latter also known as Fischer-Tropsch waxes.
Among these Fischer-Tropsch catalysts, Cobalt, which is particularly effective in directing the reaction towards the formation of waxes, is becoming more and more widely used.
A particular catalyst based on cobalt supported on alumina has now been found, which is more selective, with respect to normal cobalt catalysts, towards the formation of waxes in the Fischer-Tropsch process.
In accordance with this, the present invention relates to a catalyst which can be used in the Fischer-Tropsch process, essentially consisting of cobalt oxide supported on an inert carrier essentially consisting of alumina, characterized in that the above cobalt oxide essentially consists of crystals having an average size ranging from 20 to 80 Å, preferably from 25 to 60 Å, even more preferably from 30 to 40 Å.
The above crystals of cobalt oxide can be optionally partly doped with Al atoms.
With respect to catalysts known in the art, the catalyst of the present invention has the characteristic of consisting of crystals with much lower dimensions, 20-80 Å for the catalyst of the present invention against the 120-180 Å of a catalyst prepared according to the conventional techniques. This allows a better dispersion of the cobalt on the carrier, with a consequent better contact, in reaction phase, between the catalyst and reagents.
In addition to cobalt oxide, the catalyst of the present invention may optionally also contain, in a much lower quantity than the cobalt, metals normally known as promoters, such as Si, Zr, Ta, Zn, Sn, Mn, Ba, Ca, La, Ve, W. Promoters are used for improving the structural stability of the carrier itself.
One or more activity promoters with a different effect on the catalytic performances as described in the art (see for example B. Jager, R. Espinoza in “Catalysis Today”, 23, 1995, 21-22), can also be optionally present together with the cobalt. For example, promoters such as K, Na, Mg, Sr, Cu, Mo, Ta, W and metals of group VIII essentially increase the activity. Ru, Zr, rare-earth oxides (REO), Ti increase the selectivity to high molecular weight hydrocarbons. Ru, REO, Re, Hf, Ce, U, Th favour the regenerability of cobalt catalysts.
As far as the alumina is concerned, this can have any phase form selected from eta, gamma, delta, theta, alpha and relative mixtures, in the presence of or without one or more structural stability promoters selected from those described above. In the preferred embodiment, the alumina is in &ggr; or &dgr; form, and relative mixtures.
The surface area of the alumina is that which is normal in catalytic carriers, i.e. from 20 to 300 m
2
/g, preferably from 50 to 200 m
2
/g (BET), whereas the average dimensions of the alumina itself range from 1 to 300 &mgr;m.
The cobalt content of the catalyst of the present invention ranges from 2 to 50% by weight, preferably from 5 to 20% by weight, 100 being the total weight of the carrier and cobalt (plus possible promoters). When present, the promoters are in a quantity not higher than 20% by weight with respect to the cobalt, preferably 10% by weight.
Before being used in the Fischer-Tropsch process, the catalyst of the present invention should be activated by means of the usual procedures, for example by reduction of cobalt oxide to metallic cobalt in the presence of hydrogen.
In accordance with this, the present invention relates to a process for obtaining cobalt oxide supported on an inert carrier essentially consisting of alumina, the above cobalt oxide essentially consisting of crystals having an average size ranging from 20 to 80 Å, which comprises the following steps:
1) preparation of an intermediate, supported on alumina, having general formula (I)
[Co
2+
1−x
Al
+3
x
(OH)
2
]
x+
[A
n−
x
].m
H
2
O (I)
wherein x ranges from 0.2 to 0.4, preferably from 0.25 to 0.35, A is an anion, x
is the number of anions necessary for neutralizing the positive charge, m ranges from 0 to 6, and is preferably 4;
2) calcination of the intermediate having general formula (I) with the formation of crystalline cobalt oxide.
With respect to the anion A
n−
, this can be indifferently selected from inorganic anions (for example F
−
, Cl
−
, Br
−
, I
−
, ClO
4
−
, NO
3
−
, OH
−
, IO
3
−
, CO
3
2−
, SO
4
2−
, WO
4
2−
), hetero-polyacids (for example PMo
12
O
40
3−
, PW
12
O
40
3−
) organic acids (for example adipic, oxalic, succinic, malonic acid). In the preferred embodiment the anion A
n−
is chosen from NO
3
−
, OH
−
, CO
3
2−
. In an even more preferred embodiment A
n−
is equal to CO
3
−−
.
The compound having general formula (I) can be prepared according to various techniques known to experts in the field.
For example, the so-called precipitation technique can be used, according to which Co
2+
and Al
3+
are co-precipitated on alumina in the form of hydroxides. According to this technique, a solution of an Aluminum salt and a Cobalt salt, preferably an aqueous solution of the above salts, is dripped onto a suspension, preferably aqueous, of alumina. This operation must be effected maintaining the pH within a range of 6.6 to 7.2, preferably from 6.8 to 7.1, for example by the use of an aqueous solution of bicarbonate or soda. Alternatively, two separate solutions can be added; however, for the sake of simplicity, it is obviously preferable to use a single solution of the two salts. The compound having general formula (I) is recovered by means of filtration.
According to another less preferred embodiment, the so-called hydro-thermal technique can be used, which consists in treating freshly precipitated mixed cobalt and aluminum hydroxides, or mechanical mixtures of the oxides, with water.
The compound having general formula (I) can be amorphous or crystalline. The ratio between the amorphous part and the crystalline part can be modified using known techniques (for example by annealing). The crystalline part of the compound having general formula (I) has a structure similar to that of hydrotalcite.
Hydrotalcite is a mineral existing in nature and consists of an Al and Mg hydroxy-carbonate. A hydrotalcite-type system has an analogous structure, but contains different elements.
The group of hydrotalcites can be represented by the following formula:
[M
2+
1−x
M
+3
x
(OH)
2
]
x+
[A
n−
x
].m
H
2
O
in the case of actual hydrotalcite M
2+
=Mg
2+
, M
3+
=Al
3+
and A
n−
=CO
3
2−
.
One of the main characteristics of this group of compounds is the layered structure: layers of the brucite type, Mg(OH)
2
, or [M
2+
1−x
M
+3
x
(OH)
2
]
x+
, in which a part of the bivalent M
2+
ions is substituted by trivalent M
3+
ions, alternate with anionic layers associated with a varying content of water (A
n−
x
].mH
2
O. The anionic layers balance the positive charge of the hydroxide layers, the latter linked to the presence of trivalent ions.
In general, M
2+
and M
3+
can be ions of a varying nature, the only requisite is that they are able to insert themselves in the cavities left by the hydroxyls in a compact
Del Piero Gastone
Morselli Sonia
Pederzani Giovanni
Zennaro Roberto
ENI S.p.A.
Nguyen Cam N.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Silverman Stanley S.
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