Chemistry of hydrocarbon compounds – Aromatic compound synthesis – From alicyclic
Reexamination Certificate
2001-08-23
2003-07-29
Dang, Thuan D. (Department: 1764)
Chemistry of hydrocarbon compounds
Aromatic compound synthesis
From alicyclic
C585S380000, C585S444000, C585S445000, C585S623000, C585S629000, C585S631000, C585S661000, C585S662000, C585S663000
Reexamination Certificate
active
06600082
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for dehydrogenating organic compounds, in particular paraffins and naphthenes, to produce alkenes and aromatic compounds with optimised yields. The process of the invention is carried out in the presence of a bimetallic supported catalyst comprising a 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.
Aromatic compounds and alkenes constitute the feeds of choice for the petrochemicals industry.
Processes for dehydrogenating light paraffins can upgrade aliphatic hydrocarbons with a low boiling point, such as butanes and isobutanes, pentanes and isopentanes, that can be recovered after extracting the unsaturated compounds from steam cracked or catalytically cracked cuts. The process for dehydrogenating longer paraffins is an important commercial process because of the current demand for monoolefins to prepare biodegradable detergents or pharmaceutical products, for example.
While the principal sources of alkenes are catalytic cracking and steam cracking processes, these two processes also produce by-products and with increasing demand being oriented towards specific alkenes, it would not make economic sense to produce them by cracking.
For this reason, the direct production of alkenes remains in some cases an unavoidable step. This is the case for propylene, isobutene and linear long chain alkenes for the production of polypropylene, MTBE and LAB (linear alkyl benzene) respectively.
The principal features of the paraffin dehydrogenation reaction are because thermodynamic equilibrium limits the degree of conversion per pass and that the reaction is highly endothermic. These two characteristics determine the technological choice regarding the process and also the composition, structure and design of the catalyst.
High temperature operations are necessary to maintain a level of conversion close to thermodynamic equilibrium, but such high temperatures also encourage a certain number of side reactions leading to a lower quality product. Such reactions include reactions resulting in the formation of light products (cracking, hydrogenolysis), of highly unsaturated compounds that are precursors for carbonaceous deposits and thus initiators of deactivation (dehydrocyclisation, deep dehydrogenation) such as aromatic compounds or diolefins and skeletal isomerisation reactions responsible for the formation of branched molecules. Under those particularly severe operating conditions, it is very difficult to maintain high activity for long periods because of those secondary reactions.
PRIOR ART
Means for limiting those secondary reactions can be based on the process and/or on the catalytic formulation. To improve the performance of catalytic systems, in particular their stability, U.S. Pat. No. 4,716,143 describes a catalyst based on supported platinum wherein the distribution of the platinum is limited to the external surface of the support over a maximum depth of 400 &mgr;m The advantage of such a choice resides in the fact that a distribution on the support periphery can limit side reactions and as a result, improve the performance of the catalyst. However, that type of distribution can only rarely produce homogeneous platinum/modifier atomic ratios on the particle scale (nanometer). Further, an over-concentration of active phase can cause diffusional limitations in the catalyst grain (extragranular diffusion) and thus reduce the overall reaction yield.
A vast number of patents and publications demonstrate that adding promoters to a base metal improves the performance of the catalyst. Such elements are added in different forms such as salts or organometallic compounds. In general, more active or more selective catalysts are obtained, which 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 paraffin dehydrogenation have, for example, been described in French patent FR-B-2 031 984 and U.S. Pat. No. 3,531,543.
In particular, catalysts based on PtSn contain different forms of tin. In the reduced state, those 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)). Those catalysts are generally prepared from a solution of tin chloride in an acidic medium (HCl, NHO
3
) and a hexachloroplatinic acid solution. The role of the tin present on the catalyst surface in oxidation state +2 or, more preferably, +4, is to minimise isomerisation and cracking reactions that occur at acidic sites on the support, and to limit coke formation to improve the stability of the catalyst.
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, &Dgr;, 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 Pt
Didillon Blaise
Jumas Jean-Claude
Le Peltier Fabienne
Olivier-Fourcade Josette
Dang Thuan D.
Institut Francais du Pe'trole
Millen White Zelano & Branigan
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