Method for preparing a catalyst

Details Method for preparing a catalyst Method for preparing a catalyst

2000-09-18

2004-03-23

Griffin, Walter D. (Department: 1764)

Mineral oils: processes and products

Chemical conversion of hydrocarbons

Cracking

C208S109000, C208S110000, C208S111350, C585S477000, C585S481000, C502S063000, C502S071000, C502S074000, C502S077000, C502S232000, C502S439000, C502S514000

Reexamination Certificate

active

06709570

ABSTRACT:

The invention relates to a method for preparing a catalyst comprising a zeolite and a low acidity refractory oxide binder which is essentially free of alumina.
Such a method is described in patent publication U.S. Pat. No. 5,053,374 which illustrates the preparation of a silica-bound USY catalyst. In this preparation 65 weight parts of the zeolite in the form of a powder was mixed with 35 weight parts of silica consisting of various ratios of an amorphous precipitated silica (PPG Industries HiSil 233 EP) and a basic colloidal silica of the type Ludox HS-30. A homogenous mix was obtained by mulling. The moisture content of the mix was adjusted to 42-47 weight percent with deionized water. The mix was extruded to yield an extrudate. The extrudates were subsequently dried and ammonium exchanged to remove sodium and subsequently calcined.
When contents of binder and zeolite are used in the context of the present invention the content on a dry basis is meant. When a pH value is mentioned the pH as measured in water of 18° C. is meant.
There remains room to improve the strength of the catalyst extrudates as obtained by the process as described in U.S. Pat. No. 5,053,374. This is especially true when catalyst extrudates are made having a zeolite content of below 50 wt %. It appears that the strength of the catalyst is negatively influenced when the zeolite content is reduced. For some applications a zeolite content of well below 50 wt % is advantageous in terms of improved selectivity and activity of the catalysed reaction. An example of such an application is the catalytic dewaxing process as described in WO-A-200029511.
The object of the present invention is a process to prepare a low acidity refractory oxide bound zeolite catalyst having a high crushing strength. A further object is to provide a process which enables one to prepare silica bound catalysts of sufficient strength having a zeolite content of below 50 wt %.
The object of the invention is achieved by the following process. A method for preparing a catalyst comprising a zeolite and a low acidity refractory oxide binder which is essentially free of alumina which method comprises:
(a) preparing an extrudable mass comprising a substantially homogenous mixture of zeolite, water, a source of the low acidity refractory oxide binder present as an acid sol and an amine compound,
(b) extruding the extrudable mass resulting from step (a),
(c) drying the extrudate resulting from step (b); and,
(d) calcining the dried extrudate resulting from step (c).
Catalyst particles obtained by the above process have an increased crushing strength. This is advantageous because such catalysts are typically applied in a packed bed reactor. Due to the normally high operating pressure and mass flows in the reactor strong catalyst particles are advantageous.
Examples of the low acidity refractory oxide binder material which is essentially free of alumina are silica, zirconia, titanium dioxide, germanium dioxide, boria and mixtures of two or more of these like for example silica-zirconia and silica-titania. A preferred binder is silica. The description of the invention will further refer to a silica binder only. It will be understood that the below preferred conditions will, when applicable, also apply to other possible binders as here described.
The acid silica sol may be any colloidal silica having a pH lower than 7. An example of a suitable acid silica sol is Nyacol 2034DI which can be obtained from PQ Corp, Valley Forge, Pa. or Ultra-Sol 7H from RESI Inc, Newark. The surface of the acid silica sol particle comprises —OH groups. It is believed that for obtaining a catalyst particle having a sufficient strength it is essential that during the mixing of the components in step (a) some or all of these groups are converted to —O
−
groups. This is achieved by adding the amine compound in step (a). It has further been found that when adding the amine compound just before performing step (b) an even more stronger catalyst particle is obtained. It is believed, although we do not wish to be bound to this theory, that the stronger catalyst is obtained because not all of the —OH groups on the sol particle surface are converted into —O
−
groups. Thus step (a) is preferably performed by first mixing the zeolite and the acid silica sol into a first homogeneous mixture and subsequently adding the amine compound to the first homogeneous mixture such that the pH of the resulting second mixture is raised from below 7 to a value of above 8. It can be easily determined by one skilled in the art, by straightforward experimentation, what the optimal moment in step (a) is for adding the amine compound. As a guideline it is preferred to add the amine compound during the second half of the time and more preferably in the last quarter of the time required to mix the components in step (a). Most preferably the amine compound is added within 20 minutes before performing step (b).
The extrudable mass in step (a) should have a sufficient viscosity in order to be extruded into shapes. One skilled in the art will know how to achieve such a paste like mixtire. For example by adding water in step (a) the viscosity can be lowered. The water content of the sol may be between 60 and 80 wt %. Preferably the water content of the extrudable mass as obtained in step (a) does not exceed 60%, and preferably is at least 35% by weight.
When preparing catalysts having a lower zeolite content, especially lower than 50 wt %, it becomes more difficult to achieve a sufficient high viscosity and preferably some of the acid silica sol is replaced by silica powder as the silica source. To obtain the strongest catalysts it is preferred to maximise the amount of acid silica sol used relative the amount of silica powder used, while still achieving a sufficient viscosity of the extrudable mass. The optimal amount of silica powder to be used will depend on the zeolite content, wherein at a low zeolite content of the catalyst, more silica powder will have to be used. One skilled in the art can easily determine the optimal composition in view of the above teaching. The silica powder may be commercially obtained silica powder, for example Sipernat 22 or 50 (as obtained from Degussa AG), Nasilco Ultrasil VN3SP or HiSil 233 EP from PPG Industries. The solid silica powder particles preferably have a mean diameter of between 10 &mgr;m and 200 &mgr;m.
The amine compound is preferably a compound according to the general formula R
1
R
2
R
3
N in which R
1
-R
3
may be hydrogen and/or an alkyl group having 1-6 carbon atoms. Examples are ammonia, methyl ethyl amine, triethyl amine, of which ammonia, is the most preferred. The amine compound should preferably be added in such an amount in order to raise the pH of the mass to alkaline conditions. Preferred conditions are a pH of the mixture obtained in step (a) of above 8. The pH will be lower than 14.
Step (a) may for example be performed at ambient conditions by first mixing the zeolite, optionally the silica powder and acid silica sol, subsequently adding an amine compound and optionally at the end of step (a) a plasticising agent. The plasticiser agent is used to increase the viscosity of the mixture in order to obtain an extrudable mass. Suitable plasticising agents are for example dextrose, gelatine, glucose, glues, gums, salts, waxes, starch and cellulose ethers. Some typical cellulose ether binders are methylcellulose, ethylhydroxy ethylcellulose, hydroxybutyl methylcellulose, hydroxymethylcellulose, hydroxypropyl methylcellulose,hydroxyethyl methylcellulose, hydroxybutylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, sodium carboxy methylcellulose, and mixtures thereof. Methylcellulose and/or methylcellulose derivatives are especially suited as organic binders in the practice of the present invention with methylcellulose, hydroxypropyl methylcellulose, or combinations of these being preferred. Preferred sources of cellulose ethers are Methocel A4M, F4M, F240, and K75M from Dow Chemical Co.
The extrusion in step (b) may be performed by

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