Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
2001-08-23
2003-08-12
Bell, Mark L. (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S327000, C502S330000, C502S332000, C502S349000, C502S230000, C585S350000, C585S419000, C585S458000, C585S462000, C585S489000, C585S660000, C585S747000, C208S133000
Reexamination Certificate
active
06605566
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a novel supported catalyst comprising at least one group VIII metal and at least one additional metal constituted by tin, at least a portion of which interacts strongly with said group VIII metal. The invention also relates to the use of said catalyst in the principal hydrocarbon transformation processes using petroleum refining, in particular in catalytic reforming processes.
PRIOR ART
A large number of patents and publications demonstrate that the addition of promoters to a base metal improves the performances of catalysts. Such elements are added in different forms, such as salts or organometallic compounds. In general, more active or more selective catalysts are obtained that are sometimes more stable than the corresponding monometallic catalyst.
The formulation of catalysts used in hydrocarbon transformation processes, in particular catalysts for catalytic reforming and paraffin dehydrogenation, has been the subject of a large number of studies. Of the more frequently used promoters, tin, can increase the selectivity and stability of the catalysts. Catalysts based on PtSn supported on alumina and used in that type of application have, for example, been described in French patent FR-B-2 031 984 and U.S. Pat. No. 3,531,543.
Catalytic reforming catalysts are bifunctional as they combine two functions essential to optimum performance: a hydrodehydrogenating function, which ensures dehydrogenation of naphthenes and hydrogenation of coke precursors, and an acid function that isomerises naphthenes and paraffins, and cyclises long chain paraffins. The hydrodehydrogenation function is generally provided by platinum, which has a hydrogenolysing activity to the detriment of the gasoline and/or aromatic compounds yields desired for catalytic reforming, or in the aromatic compound production process. This hydrogenolysing activity can be substantially reduced by adding tin and the selectivity of the catalyst is thus substantially increased. Further, adding tin can also increase the hydrogenating properties of platinum, which encourages hydrogenation of coke precursors, and thus the stability of the catalyst. Such bimetallic catalysts perform better in terms of activity and/or selectivity than catalysts containing the catalytically active principle alone (palladium, platinum or nickel). The metals in the catalyst are added in different forms, such as mineral salts or organometallic compounds. The manner in which such modifying agents are introduced is important as it has a profound effect on the catalyst's properties.
In particular, catalysts based on PtSn contain different forms of tin. In the reduced state, these catalysts, supported on alumina, essentially contain species of tin in the oxidised state, namely species of divalent tin Sn
II
and tetravalent tin Sn
IV
, and minor quantities of tin in the reduced state Sn
0
(M. C. Hobson et al., J. Catal., 142, 641-654 (1993), L. D. Sharma et al., Appl. Catal. A Genneral., 168, 251-259, (1998)). These catalysts are generally prepared from a solution of tin chloride in an acidic medium (HCl, NHO
3
) and a hexachloroplatinic acid solution.
One technique that can examine the local electronic structure of the tin (oxidation state, environment, chemical bonding) is Mössbauer spectroscopy, which directly provides two fundamental parameters: the isomer shift, &dgr; (IS) and the quadrupolar splitting &Dgr; (QS). The isomer shift &dgr; measures the energy position of the Mössbauer absorption, a function of the density of the nucleus s, directly characterises the oxidation state of the tin. The quadrupolar splitting, A, which defines the form of the absorption, is a function of the distribution of the surrounding charges, and characterizes the degree of coordination and thus the type of chemical bond in which the tin is involved. Each species of tin is characterized by a sub-spectrum defined by the two parameters IS and QS Mössbauer spectroscopy also provides access to the line width LW, by comparison with the natural width of the emission (0.64 mm/s): the line width LW provides information regarding the degree of order and the distribution of the sites occupied by the tin. The relative intensity of the absorption for each species is proportional to the number of tin atoms and to the Mössbauer Lamb factor f, which represents the probability of resonant absorption without recoil and without thermal broadening. The factor f is directly related to the rigidity of the lattice and its value is increased by a reduction in the temperature of measurement. It can be small at ambient temperature (0.06 for the metallic &bgr; phase of tin) and thus requires measurements to be carried out at low temperatures. The proportion of each species is estimated from their contribution to the total absorption, provided that the recoil-free resonant absorption fractions f are not too different.
Characterisations using Mössbauer spectroscopy of reduced catalysts based on PtSn supported on alumina or silica mention the existence of a species Sn
0
contained in a Pt
x
Sn
y
type phase (x and y from 1 to 4) in which the tin is in oxidation state 0 (IS of 1.4 to 1.8 mm/s with respect to BaSnO
3
) in a form that is very close to bulk alloys characterized by a low or zero quadrupolar splitting (M. C. Hobson et al., J. Catal., 142, 641-654 (1993); Z. Huang et al., J. Catal., 159, 340-352 (1993); J. L. Margitfalvi et al., J. Catal., 190, 474-477 (2000); V. I. Kuznetov et al., J. Catal., 99, 159 (1986); R. Bacaud et al., J. Catal., 69, 399 (1981); R. Srinivasan et al., Catal. Today, 21, 83 (1994)). On alumina, the formation of metallic tin in the reduced state, favoured with larger metallic particle sizes of more than 2 nm, is responsible for the loss in performance of PtSn catalysts supported on alumina (Z. Huang et al., J. Catal., 159, 340-352, (1993), F. Yining et al., Stud. Surf. Sci. Catal., 68, 683-690, (1991)). A number of documents describe the use of catalysts containing a PtSn phase dispersed on alumina or tin that is essentially in a higher oxidation state than that of metallic tin (U.S. Pat. No. 3,846,283, U.S. Pat. No. 3,847,794). Under such conditions, the conventional preparation methods used cannot guarantee a close association between tin and platinum, an intimate association between those metals in the catalyst in the reduced state being generally desirable, however, to best exploit the bimetallic effect.
SUMMARY OF THE INVENTION
The invention is based on the discovery of a novel catalyst containing at least one metal from group VIII of the periodic table and at least tin at least a portion of which interacts strongly with the group VIII metal. The supported catalyst of the invention is characterized in that it contains metallic particles, of small size, less than 2 nm, and in that at least 10% of the tin species present in the catalyst in the partially re-oxidised state are in the form of a reduced tin species with oxidation state 0. Said reduced species is in a particular form, as demonstrated by
119
Sn Mössbauer spectroscopy, and is characterized by a very high quadrupolar splitting value of more than 0.65 mm/s and an isomer shift IS in the range 0.8 to 2.6 mm/s with respect to BaSnO
3
. This species is revealed by carrying out perfectly controlled oxidation on the reduced catalyst by discontinuous injections of oxygen. This particular species of tin is very closely associated with the group VIII metal and reveals a very strong interaction between the atoms of said group VIII metal and at least a fraction of the tin in the catalyst in the reduced state. As an example, in the case where the group VIII metal is platinum, a Pt
x
Sn
y
phase is formed in which the tin has set values for IS and QS. The invention also concerns the preparation of said catalyst and its use in hydrocarbon transformation processes, in particular in catalytic reforming processes.
IMPORTANCE OF THE INVENTION
The catalyst of the invention has substantially improved catalytic properties with respect to prior art catalysts, in pa
Didillon Blaise
Jumas Jean-Claude
Le Peltier Fabienne
Olivier-Fourcade Josette
Bell Mark L.
Hailey Patricia L.
Institut Francais du Pe'trole
Millen White Zelano & Branigan P.C.
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