Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
1997-04-14
2002-12-31
Griffin, Steven P. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S350000, C502S305000
Reexamination Certificate
active
06500780
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for preparing a heterogeneous catalyst comprised of a support material and at least one catalytically active species.
According to the present method, the catalytically active species or a reagent containing its precursor is transferred into a reaction space where it is reacted in vapour phase with the surface of the support material.
2. Description of the Related Arts
When preparing heterogeneous catalysts in the traditional manner, the catalytically active species are bonded to the surface of the support material using, e.g., impregnation, precipitation or ion exchange techniques. The initial reagents here are chemical compounds, generally salts, that are soluble in conventional solvents. The most common solvents used are water and alcohols.
The metal content of the catalysts being prepared is controlled in the impregnation technique by altering the metal compound concentration in the solution and using a certain precalculated volume of the solvent which is then used in toto to impregnate a porous support material.
This traditional method of catalyst preparation is hindered by the great number of different work phases required, whereby the risk of preparation errors increases. The catalyst preparation process is very sensitive to ambient conditions, thus necessitating very accurate control. Another disadvantage is related to the use of solvents. Namely, solvents can often react with the support material causing changes in its surface structure. Furthermore, solvents frequently contain impurities, which may affect the activity of the prepared catalyst. To avoid the disadvantages of liquid-phase techniques, several different gas-phase techniques have been developed in which the support materials are reacted with gas- or vapour-phase reagents containing the catalytically active species or its precursor. When using gas-phase techniques, the content of the metal compound in the final product is conventionally controlled by metering a certain amount of the gas into the reaction space.
Such gas-phase techniques known in the art achieve the control of the average concentration of the metal compound but fail in simultaneously achieving the control of the active species distribution on the support material. A frequently occurring phenomenon is the aggregation of the active species into clusters, so all molecules of the metal species cannot act as catalytically active points.
SUMMARY OF THE INVENTION
It is an object of the present invention to overcome the drawbacks of conventional technology and achieve a method suited to the preparation of heterogeneous catalysts having a desired content of the active species.
The invention is based on two basic ideas. Firstly, the method aims to achieve a situation in which the bonding of the gas-phase materials onto the support material surface is primarily determined by the properties of the support material surface. In the context of the present invention, this property is called the “surface-bond selectivity”. Namely, the goal is to achieve such process conditions in which the constituents of the reagent are selectively bonded to the bonding sites available on the support material surface, thus forming permanent surface bonds. Which bonding sites under certain conditions are available for achieving a stable end product is determined by, among other factors, the surface structure of the support material, the reaction temperature and other reaction parameters, as well as the reactivity of the reagent and its bonding energy in the reaction. The principal properties of the support material surface affecting the end result are the structural geometry of the atoms in the support material surface and their electron configuration (that is, the energy potential function of the surface).
The reaction temperature and time applied in the method, as well as other similar conditions, are determined by the support material/reagent pair. Independently of the support material and the reagent, the method according to the invention is, however, characterized in that the surface-bonding selectivity is ensured by maintaining the vapour pressure of the reagent sufficiently high and the reaction time sufficiently long to keep the amount of the reagent at least to the number of surface-bond sites available at a time.
According to the present invention, through the fulfillment of the surface-bonding selectivity requirement, a homogeneous distribution of the active species is achieved by virtue of saturating surface-bond reactions. The utilization of the saturation principle of the surface-bond reactions yields a homogeneous distribution of the active species and simultaneously controls the active species content at a saturation level which is determined by the number of surface-bond sites participating in the reaction. Consequently, the second basic idea of the invention requires that the number of those surface-bond sites which, under the predetermined conditions of set temperature, introduced reagent, and chemical structure of the support material surface, are available to form a stable surface-bond reacted product must at least essentially correspond to the desired content of the catalytically active species in the catalyst being prepared. For this purpose, according to the invention, the number of surface-bond sites is predetermined through two major variables, namely, control of the reaction temperature and/or proper selection of the reagent.
DETAILED DESCRIPTION OF THE INVENTION
The invention combines the benefits of surface-bonding selectivity and reaction controllability. Hence, the invention makes it possible to achieve a heterogeneous catalyst whose activity even at a low content of the catalytic metal is as high as that of a catalyst of higher metal content prepared in a conventional manner. Moreover, the metal content of the end product can be accurately controlled at a predetermined level.
The definitions used in the context of the present invention are as follows:
Catalyst reagent refers to an initial reagent which is capable of being converted into gaseous form and then reacting on the support material surface so as to form a catalytically active site or a precursor necessary for generating such a site. The catalyst reagent can be any vapourizable or gaseous compound conventionally used in the preparation of heterogeneous catalysts. Thus, applicable reagent materials include, for example, elemental metals such as zinc, metal compounds such as rhenium oxides, metal halides such as halogenated chromium compounds, tungsten chlorides and oxychlorides, and metal complex compounds such as Cr(acac)
3
and Mg(thd)
2
.
Precursor refers to such available (inactive) initial forms of the catalytically active constituent from which the active species can be obtained by means of an appropriate treatment.
Active species refers to a catalytically active component on the support material surface, whereby the active species can be in the form of, e.g.; an atom, ion, molecule, chemical compound or complex compound. Conventionally, the active species is comprised of a metal ion or atom or metal compound bonded to the support material surface.
Support material refers to a material in solid state that provides a surface of at least a relatively large area, capable of bonding the catalytically active species or its compound. The surface area of the support material determined by the BET method typically is in the range from 10 to 1000 m
2
/g. The support material can be comprised of an inorganic oxide such as silicon oxide (silica gel), aluminium oxide, thorium oxide, zirconium oxide, magnesium oxide, or any of their mixtures. In their inherent form, these support materials are essentially inactive as catalysts. Alternatively, support materials can be employed that inherently act as catalysts in the chemical reaction to be catalyzed. Examples of such support materials are natural and synthetic zeolites. Also inactive support materials having species of a catalytically active materia
Haukka Suvi
Hietala Jukka
Hokkanen Harri
Knuuttila Hilkka
Knuuttila Pekka
Birch & Stewart Kolasch & Birch, LLP
Griffin Steven P.
Ildebrando Christina
Neste Oy
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