Chemistry of hydrocarbon compounds – Aromatic compound synthesis – By ring formation from nonring moiety – e.g. – aromatization,...
Reexamination Certificate
1998-09-14
2001-05-29
Dunn, Tom (Department: 1754)
Chemistry of hydrocarbon compounds
Aromatic compound synthesis
By ring formation from nonring moiety, e.g., aromatization,...
C502S305000, C502S306000, C502S319000, C502S321000, C502S324000, C502S353000
Reexamination Certificate
active
06239323
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a selective catalyst for aromatising aliphatic or alicyclic hydrocarbons with at least 7 carbon atoms in the chain, and to methods of its production and use.
2. The Prior Art
Aromatic compounds such as toluene, ethylbenzene , p-xylene and o-xylene are important aromatic hydrocarbons, which are components of high octane fuels and are used as starting materials for a wide variety of synthetic reactions in the chemical industry. Aromatisation thus belongs to the most important processes of petroleum chemistry for the refining of paraffinic hydrocarbons. Because of the indispensability of catalysts for the technical realisation of the aromatisation process, the development of effective catalysts is a constant demand on catalysis research.
The recovery of aromatic compounds from reactions of hydrocarbon fractions containing mainly C
6
-C
8
paraffins has been carried out for decades according to the known reforming process and its modifications. The successfully used Pt/Al
2
O
3
reforming catalysts, which have meanwhile been modified and improved in many ways, e.g. by the use of bi- or multi-metal additives and by variations of the substrate in different ways, are described for example by B. C. Gates, J. R. Kratzer and G. C. Schuit, Chemistry of Catalytic Processes, McGraw-Hill, N.Y. 1979. In general, a yield of 25-40% C
6
-C
8
aromatic compounds is achieved at 450-550° C. and pressures of 1-2 MPa using a hydrogen/carbon molar ratio of 10/1 to 100/1 with catalysts containing 0.5-1.0% by weight of platinum. Side products of the reforming process are hydrogen formed by dehydrogenation reactions and lower paraffins formed by hydrogenolysis (hydrocracking), which reduce the yield of useful materials.
In spite of the progress achieved, the above named catalyst systems have disadvantages. Catalysts containing noble metals are expensive. The temporal stability of the catalysts under the necessary working conditions is worthy of improvement, meaning that additional expense and/or environmental problems occur in the regeneration or processing of the catalysts. The selectivity for the end-product is diminished by the above named side-reactions; the valuable crude oil carbon carrier is exploited inefficiently.
SUMMARY OF THE INVENTION
The object of the invention is to provide new catalysts with higher selectivity for aromatisation reactions, as well as providing methods for their production and their use in a catalytic aromatisation process.
According to the invention, this object is achieved with a selective aromatisation catalyst, characterised by a porous oxide or mixture of oxides of one or more of the elements Ti, Zr and Hf which, at least at the surface, additionally contains one or more of the elements of the 5th, 6th and 7th sub-group of the periodic system of the elements (PSE) in oxidised form, in amounts from 0.01 to 10% by weight relative to the total mass of the catalyst;
wherein said catalyst has a specific surface area in the range from 30 to 300 m
2
/g;
has the ability to repeatedly absorb and release hydrogen in the temperature range from 400 to 700° C.;
and exhibits no strongly acidic and/or strongly basic centres.
The new catalysts may additionally contain an oxide or several oxides chosen from the elements of main group II, the elements of sub-group III including the lanthanides, the element silicon, the element aluminium and mixtures thereof in an amount ranging from 0.01 to 20% by weight relative to the total mass of the catalyst. Lanthanum, magnesium and calcium are particularly prefer- red.
Among the elements of the 5th, 6th and 7th sub-group those chosen from chromium, tungsten, molybdenum, niobium and tantalum are preferred, particularly Cr, W and Mo.
The catalyst may contain the oxidised form of the elements of the 5th, 6th and 7th sub-group at the surface of the porous structure; it may also contain them at the surface or within the body (in the bulk) of the catalyst.
The specific surface area, determined according to the BET method [(J. Amer. Chem. Soc. 60 (1938) 309], amounts to between 30 and 300 m
2
/g, in particular 50 to 200 m
2
/g.
The catalysts according to the invention prevalently exhibit no strongly acidic or strongly basic centres on their surface. Among others, this was determined by acidity and basicity measurements by means of temperature programmed desorption of ammonia (TPDA) or carbon dioxide (TPDCO
2
).
With regard to the invention, suitable compounds are for example chlorides, oxychlorides, nitrates, alkylates and alcoholates as far as these exist for the individual elements.
The ability of the catalyst to repeatedly absorb hydrogen and—during a later rinsing with an inert gas—to completely release hydrogen is an essential feature of the invention. “Repeatedly absorb” and “completely release” means that after initial saturation with hydrogen and subsequent rinsing with inert gas, a catalyst essentially releases completely the subsequent amount of newly added hydrogen again. This characteristic of the catalyst has not yet been explained in terms of its mechanism and its interrelation with catalytic activity; nevertheless, it is an important precondition for the effectiveness according to the invention.
The invention also relates to production of the aromatisation catalysts carried out such that
i) a suitable compound of Ti, Zr or Hf or a mixture of several of these compounds is transferred from a solution into a suspension of a hydroxide, oxide or mixture thereof, and the solid components of the suspension are separated off, and the product is modified at the surface with a solution of one or more of the compounds of the elements of the 5th, 6th and 7th sub-group, and subsequently calcined at 300 to 750° C. in air or oxygen;
or
ii) a solution of a suitable compound of the elements Ti, Zr or Hf or a mixture thereof together with a solution of one or more compounds of the elements of the 5th, 6th and 7th sub-group is transferred into a suspension of an oxide, hydroxide or mixture thereof, the solid components of the suspension are separated off, and the product is calcined at 300 to 750° C. in air or oxygen.
Preferably, the process is carried in such a way that
i) first of all, a hydroxide, oxide, mixture of oxides or hydroxide/oxide mixture of the elements titanium, zirconium and hafnium is made up of suitable compounds of these elements familiar to persons skilled in the art, preferably salts, by preparation methods familiar to persons skilled in the art, preferably by the steps of precipitation, filtration and drying,
and that
ii) this hydroxide, oxide, mixture of oxides or hydroxide/oxide mixture is subsequently modified at the surface—cumulatively or alternatively—with the elements of the 5th, 6th and 7th sub-group of the PSE using suitable compounds and suitable impregnation techniques, and then calcined in air or in oxygen at temperatures between 300 and 750° C. so that as a result these elements are present in oxidised form.
In special embodiments, one or more elements of main group II and sub-group III of the periodic system, including the lanthanides as well as aluminium or silicon or mixtures of said elements, may be additionally introduced in step i) by the addition of suitable compounds of these elements at suitable points of the preparation process. In other embodiments, step ii) may be evaded by introducing suitable compounds of the elements of the elements of the 5th, 6th and 7th sub-group cumulatively or alternatively in step i), so that as a result these elements are present as oxides both in the bulk and at the surface of the catalyst.
After separation of the solid components from a suspension of hydroxides, oxides or mixtures thereof, annealing may be carried out in every variant of the method until conversion of hydroxides into oxides.
The result of the preparation steps i) and ii) is subjected to thermal treatment in air or oxygen in the range of temperatures from 300 to 750° C. during which a porous solid with a surface area from 30 to
Hoang Dang Lanh
Lieske Heiner
Collard & Roe P.C.
Dunn Tom
Institut für Angewandte Chemie Berlin-Adlershof e.V.
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