Catalyst – solid sorbent – or support therefor: product or process – Zeolite or clay – including gallium analogs – And group viii containing
Reexamination Certificate
1994-01-27
2001-06-12
Achutamurthy, Ponnathapu (Department: 1652)
Catalyst, solid sorbent, or support therefor: product or process
Zeolite or clay, including gallium analogs
And group viii containing
C502S066000, C502S077000, C585S481000
Reexamination Certificate
active
06245704
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is concerned with an alumino-silicate type catalyst comprising an MFI zeolite, the selectivity and/or catalytic properties of which has/have been modified by desalumination of the outer surface of its crystals, and comprising at least one group VIII metal from the periodic classification of elements (Handbook of Chemistry and Physics, 65th edition, 1984-85), and a matrix. Said catalyst is used in isomerization reactions of C
8
aromatic hydrocarbons. The invention is also concerned with the process for the preparation of said catalyst. Usually, the outer surface of the zeolite crystals is modified by at least one treatment of the MFI by at least one solution of a fluorosilicic acid salt. Preferably, the salt selected does not result in the formation of aluminium salts which are not soluble in water.
At the present time, the catalysts used industrially in isomerisation reactions of C
8
aromatic cuts mainly have a base of ZSM-5 zeolite of MFI structural type, alone or mixed with other zeolites. These catalysts are described, in particular, in the patents U.S. Pat. No. 3,856,872, U.S. Pat. No. 4,098,836, U.S. Pat. No. 4,159,282, U.S. Pat. No. 2,267,129 U.S. Pat. No. 4,482,773, European Patent 138,617 and U.S. Pat. No. 5,028,573.
The ZSM-5 zeolite is interesting because of its selectivity of shape which gives an advantageous selectivity with respect to para-xylene and also in its selectivity with respect to undesirable secondary dismutation reactions which remain at a lower level than those recorded on other zeolites with larger pore openings. In fact, 12MR zeolites with larger pore openings (opening with 12 oxygen atoms) have also been used, such as mordenite. The catalysts with a base of mordenite are described, in particular, in the patents. U.S. Pat. No. 4,723,051, U.S. Pat. No. 4,665,258, and French Patent 2,477,903. However, these zeolites do not have special properties of geometric selectivity. This means that, irrespective of their Si/Al ratio, their selectivities with respect to para-xylene are lower than those obtained for ZSM-5 zeolite, and, in particular, there is a very great amount of trimethylbenzenes produced. The production of trimethylbenzenes by dismutation is actually promoted in mordenite whose microporous system is more open than that of ZSM-5: the openings are with 12 oxygens instead of 10 for ZSM-5.
SUMMARY OF THE INVENTION
The Applicant has surprisingly discovered that by carrying out at least one dealumination treatment of a MFI by using at least one fluorosilicic solution of a cation, and, in particular, at least one solution of ammonium hexafluorosilicate, it is possible to obtain active and selective catalysts for the isomerization reaction of C
8
aromatics. An overall or “global” dealumination rate is thus obtained which is less than 5% eon an atomic basis, i.e., “atomic percent” for MFI zeolite thus treated, that is to say that the dealumination treatment draws off a maximum of 5% of the aluminium atoms present in the zeolite framework. Said treatment imparts to the MFI thus treated greatly improved selectivity properties. This is to be seen by a surprising inhibition of undesirable secondary reactions, such as the dismutation reaction, and by an increase in selectivity with respect to para-xylene. The new modified MFI also gives selectivities with respect to desalkylation reactions which are equivalent to those of catalysts with a base of untreated MFI. The solids thus obtained perform better than prior art MFI solids with isomerization of C
8
aromatics.
The MFI used in the catalyst of the present invention is prepared from a MFI which is synthesized just as well in hydroxide medium (U.S. Pat. No. 3,702,886) as in fluoride, in the presence of an organic structurer (for example, U.S. Pat. No. 5,010,048 and Patent Applications European Patent 342,075, European 469,151 and European Patent 472,462 or in the absence of an organic structures (for example, European Application 500,413).
The MFI to which at least one treatment is given with at least one solution of fluorosilicic acid salts is preferably obtained directly on synthesis with the desired Si/Al atomic ratio, or is obtained by dealumination of a MPI which has a lower Si/Al ratio.
The MFI selectivity is modified by at least one treatment with at least one solution of fluorosilicate for MFI with a Si/Al atomic ratio which is between about 5 and 1000, preferably between 5 and 500, and still more preferably between 5 and 250. Said MFI are in the form of Na
+
, H
+
, NH
4
+
, or they contain the organic structurer which was used in their synthesis, or they are all in the form of mixed combinations of the four forms cited hereinabove, preferably in the form of NH
4
+
and/or the form containing the organic structurer.
When the treatment with at least one fluorosilicate solution is carried out on NH4
+
forms of MPI, the following methods are used for placement in NH4
+
form, depending on the case at hand:
a) The MFI has been synthesized in the absence of organic structurer. In this case, the crude MFI from the synthesis operation contains sodium in its microporosity which is removed by several ionic exchanges with concentrated solutions of ammonium nitrate (10N) which allows a content of sodium by weight to be obtained in relation to weight of dry MFI which is usually less than 2000 ppm, preferably less than 1000 ppm, and, more preferably still, less than 500 ppm.
b) The MFI has been synthesised in the presence of an organic structurer. In this case the crude MFI from the synthesis operation contains the organic structurer and sodium in its microporosity. The crude MFI from the synthesis operation is first of all subjected to a calcination operation in air in order to remove from its structure practically all the organic structurer which is found there. Then, it is subjected to a plurality of ionic exchanges by concentrated solutions of ammonium nitrate (10N) which enable a content by weight of sodium to be obtained in relation to the weight of dry MFI which is usually less than 2000 ppm, preferably less than 1000 ppm, and still more preferably less than 500 ppm.
When the treatment with at least one fluorosilicate solution is carried out directly on the MFI forms containing the organic structurer no initial treatment is carried out on the MFI. The residual sodium contained in the microporosity of the zeolite is thus practically eliminated, after said treatment, by carrying out three ionic exchanges with solutions of ammonium nitrate.
The fluorosilicate used as the dealumination agent and as the silicon source which enable silicon atoms to be reinserted into the crystal network of the MFI to replace the aluminium atoms removed can be one of the salts of the following formula: M
2/x
SiF
6
where M is a metal cation or non-metallic cation with the valence x. The M cations can thus be NH
4
+
, an alkylammonium, K
+
, Na
+
, Li
+
, Ba
2+
, Mg
2+
, Cd
2+
, Cu
+
, Cu
2+
, Ca
2+
, Cs
+
, Fe
2+
, Co
2+
, Pb
2+
, Mn
2+
, Rb
+
, Ag
+
, Sr
2+
, Zn
2+
, Tl
+
and H
+
. Preferably, ammonium hexafluorosilicate is used because it results in the formation of aluminium salt (NH
4
)
3
AlF
6
which is soluble in water which can be easily eliminated. Usually, the treatment temperature is between 20 and 100° C., and preferably between 50 and 100° C. The treatment of the MFI is carried out in the presence of ammonium acetate which enables the pH of the reaction medium to be fixed at values of between 4 and 8, preferably between 5.5 and 7, these being pH values for which the zeolite does not suffer any destruction to the framework by direct acid attack.
After the step where fluorosilicate solution is added to the MFI suspension in an ammonium acetate solution, the reaction mixture is left, with vigorous agitation, at the desired temperature for a period of between 30 minutes and 48 hours, but preferably between 1 and 5 hours.
The MFI is th
Benazzi Eric
Da Silva Joao Miguel
Gomes Ribeiro Maria Filipa
Joly Jean-Francois
Achutamurthy Ponnathapu
Institut Francais du Pe'trole
Millen, White, Zelana & Branigan, P.C.
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