Catalyst – solid sorbent – or support therefor: product or process – Zeolite or clay – including gallium analogs – And additional al or si containing component
Reexamination Certificate
1998-09-24
2003-02-25
Silverman, Stanley S. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Zeolite or clay, including gallium analogs
And additional al or si containing component
C502S063000, C502S071000, C423S713000
Reexamination Certificate
active
06524984
ABSTRACT:
The present invention relates to a new process for preparing zeolitic catalysts in the form of microspheres, comprising zeolite and oligomeric silica, characterized by a high mechanical resistance.
It is known that zeolites, synthetic porous crystalline materials having a catalytic activity, can be bound by means of inorganic binders to increase the dimensions and enable them to be used in industrial plants. In fact, whereas on the one hand small dimensions of the zeolite crystallites favour the intraparticle diffusion of the reagents and reaction products and allow good catalytic performances to be obtained, similar dimensions on the other hand make interparticle diffusion impossible in fixed-bed reactors or the separation of the zeolitic catalyst from the reaction medium in mixing reactors. To overcome this, the zeolites are bound with suitable binders. The preparation methods of bound zeolites must be such as to prevent the blocking of the zeolitic cavities which would obviously cause a reduction in the catalytic activity. For reactions in which a catalytic contribution on the part of the binder should be avoided, such as for example many oxidations and acid-catalyzed reactions, the use of a catalytically inert material such as silica as binder is of particular interest. Silica however has poor binding properties and is therefore not normally used in extrusion as it is not capable of providing the extruded product with sufficient hardness. EP 265018 describes a process for preparing catalysts based on zeolites and oligomeric silica. The zeolitic catalysts bound in microspheric form according to this method are characterized by a high mechanical resistance and are very suitable for reactions which take place in a fluid bed or in slurry reactors. This process comprises at least the following steps:
a) preparation of the zeolite to be bound by crystallization under hydrothermal conditions of the appropriate mixture of reagents;
b) separation of the zeolite crystals obtained in phase (a);
c) possible washing of the zeolite by redispersion in water and new separation of the crystalline phase;
d) preparation of an aqueous solution of oligomeric silica and tetra-alkylammonium hydroxide by hydrolysis in liquid phase of a tetra-alkylorthosilicate in an aqueous solution of tetra-alkylammonium hydroxide, at a temperature ranging from 2 to 120° C., for a time ranging from 0.2 to 24 hours;
e) dispersion of the crystalline zeolitic phase obtained under point (c) in the solution described under point (d);
f) rapid drying of this dispersion by feeding to a spray-drier;
g) calcination of the microspheres of the product obtained in the drying step.
We have now found a considerably simplified process for preparing catalysts comprising zeolites and oligomeric silica having a high mechanical resistance. This simplified process is capable of giving the binding phase particular properties such as pore distribution essentially in the mesoporous region and a high surface area, which guarantee the absence of diffusional barriers on the part of the binder while the catalytic properties of the zeolite remain unaltered. This new process at the same time allows both a reduction in the number of unitary operations required and also a reduction in the quantity of reagents used. In particular, unlike the prior art, the process of the present invention no longer requires a separation passage of the zeolitic crystalline phase from the suspension obtained at the end of the synthesis: it has in fact been unexpectedly found that, for zeolites whose synthesis require the presence of tetra-alkylamonium hydroxide (TAAOH) as templating agent, i.e. as a compound capable of inducing a regular and constant porosity in the structure of the zeolite, the resulting suspension at the end of the synthesis, in which zeolite crystals and tetra-alkylammoniumhydroxide remaining in solution, are present, can be used as such, without further purification and/or filtration passages, for the preparation of zeolites bound with oligomeric silica in the form of microspheres. The possibility of avoiding the separation step of the zeolite is, from an industrial point of view, a great advantage, particularly when operating with zeolite crystals having dimensions of less than 0.5&mgr;, which cannot be separated from the synthesis medium with the usual techniques, for example by filtration or continuous centrifuges, but require the use of more costly techniques operating batchwise.
The present invention therefore relates to a process for the preparation of zeolitic catalysts in the form of microspheres comprising zeolite and oligomeric silica which consists in subjecting to rapid drying the suspension, to which tetra-alkylorthosilicate is optionally added, resulting from the synthesis of zeolite by hydrothermal treatment at autogenous pressure of the reagent mixture containing tetra-alkylammonium hydroxide as templating agent, and subjecting the product resulting from the drying to calcination.
In the suspension obtained at the end of the synthesis of the zeolite by hydrothermal treatment in the presence of tetra-alkylammonium hydroxide (TAAOH) as templating agent, which is subjected to rapid drying according to the present invention, zeolite crystals, the fraction of tetra-alkylammonium hydroxide which has not been englobed inside the pores of the zeolite and possibly oxides of silicon and other heteroatoms, will be present. It is in fact known that in the synthesis of zeolites, it is necessary to use an excess of tetra-alkylammonium hydroxide and consequently part of the TAAOH is in solution in the reaction medium at the end of the synthesis. The use of this suspension resulting from the synthesis of the zeolite, in addition to the advantages described above, therefore enables a reduction in the total quantity of TAAOH used in the process for the preparation of zeolites bound with oligomeric silica, with respect to that necessary in the process of the prior art EP 265018 in which it was necessary to prepare separately a mixture of TAAOH and oligomeric silica and add it to the zeolite crystals, separated by filtration or centrifugation, before the rapid drying step.
When, according to the present invention, tetra-alkylorthosilicate is added to the suspension resulting from the syntheis of zeolite, before this is subjected to rapid drying, it will be added in a quantity ranging from 0.08 to 0.50 moles per 100 grams of zeolite contained in the suspension. The tetra-alkylorthosilicate is selected from compounds having the formula Si(OR)
4
wherein the R substituents, which can be equal to or different from each other, are alkyl chains containing from 1 to 4 carbon atoms.
The rapid drying of the suspension is preferably carried out by feeding to a spray-drier. At the end of the rapid drying treatment microspheres are obtained which are calcined at a temperature ranging from 400 to 800° C.
The zeolitic catalysts obtained according to the process of the present invention are characterized by a high mechanical resistance, they comprise oligomeric silica and zeolite in a weight ratio ranging from 0.05 to 0.30 and are in the form of microspheres having a diameter ranging from 5 to 300 &mgr;m in which the zeolite crystals are encaged by Si—O—Si bridges. The zeolites which can be bound with oligomeric silica by means of the process of the present invention are those whose preparation comprises the use of tetra-alkylammonium hydroxide as templating agent, in particular zeolites which can be well used can be selected from those with an MFI, MEL, MFI/MEL, MOR, FAU, FAU/EMT and BEA structure.
Suspensions which can be subjected to rapid drying and subsequent calcination to obtain zeolites bound with oligomeric silica, according to the present invention, are those resulting from the synthesis of zeolite under hydrothermal conditions, at autogenous pressure, and in the presence of TAAOH, effected with the methods described in the prior art and known to experts in the field: in particular, according to a preferred aspect of the present invention, the zeoli
Bellussi Giuseppe
Carati Angela
Mantegazza Maria Angela
Petrini Giudo
Enichem S.p.A.
Ildebrando Christina
Silverman Stanley S.
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