Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Silicon containing or process of making
Reexamination Certificate
2001-06-13
2004-01-27
Nguyen, Cam N. (Department: 1764)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Silicon containing or process of making
C502S254000, C502S255000
Reexamination Certificate
active
06683019
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a supported catalyst for the metathesis, or disproportionation, of olefin(s), and to a metathesis process employing the catalyst.
2. Description of the Related Art
The metathesis, or disproportionation, of olefin(s) is a reaction in which one or more olefinic compounds are transformed into other olefins of different molecular weights. The disproportionation of an olefin with itself to produce an olefin of a higher molecular weight and an olefin of a lower molecular weight is also referred to as self-disproportionation. For example, propylene can be disproportionated to ethylene and cis- and trans-2-butene. Another type of disproportionation involves the cross-disproportionation of two different olefins to form still other olefins. An example would be the reaction of one molecule of 2-butene with one molecule of 3-hexene to produce two molecules of 2-pentene.
When olefins are contacted with metathesis catalysts, the reactions proceed according to a specific structural relationship depending upon the character of the feedstock. The reaction is generally considered to proceed using a four-centered active site on the catalyst. The olefinic double bonds line up on opposite sides of the four-centered site. The reaction proceeds under equilibrium conditions with the bonds exchanging sides of the four-centered site and thusly exchanging the hydrocarbon groups attached to one end of the double bond with the groups attached to the other olefin. For example, 2-butene if reacted with ethylene can form two propylene molecules as shown by equation (1) where each corner of each box in equation (1) represents one of the four active sites on the catalyst:
Extending this concept to any number of olefins, one can see that depending upon the nature of the R group attached to the double bonds, different olefins are formed with strict adherence to the exchange of R groups around the double bond. Thus, olefin R1-C═C—R2 when reacted with olefin R3-C═C—R4 forms an olefin of R1-C═C—R3 and an olefin of R2-C═C═—R4 This is illustrated in equation (2) where each corner of each box in equation (2) represents one of the four active sites on the catalyst:
One skilled in the art can imagine many potential reactions over the entire range of possible olefin pairs.
In addition to the metathesis reactions, however, it is not uncommon for various side reactions to occur. One such reaction is an oligomerization reaction where olefins combine to form larger olefins. This reaction, if the olefin grows large enough, leads to fouling of the catalyst as the active sites are blocked. Another reaction that could occur is the double bond isomerization of the olefin. In this case, the position of the double bond shifts within the hydrocarbon chain. Examples are the isomerization of 1-butene to 2-butene and 3-hexene to 2-hexene. If this occurs, the number and character of the olefins available for metathesis changes. With olefins having different R groups available, different reaction products can be formed. The isomerization side reaction leads to a loss in the selectivity of the metathesis reaction to the products defined by the structure of the feedstock olefins.
For example, if the feedstock to the metathesis reaction was essentially pure 1-butene, the primary products of that reaction would be ethylene and 3-hexene. No other products would form. If, however, some portion of the 1-butene was isomerized to 2-butene, then 1-butene could react with 2-butene to form propylene and 2-pentene. The propylene and pentene represent non-selective products.
The ability to control unwanted side reactions allows the process designer to selectively produce specific products based upon the purity and character of the feedstocks. In many cases this is important to maximize the value of a particular reaction. An example of such a process where selectivity is critical is the production of linear alpha olefins as described in commonly assigned, co-pending U.S. patent application No. 60/263,924, filed Jan. 25, 2001, incorporated by reference herein. That process requires a catalyst with low isomerization activity as described therein.
Many catalysts have been developed for metathesis. For example, those comprising inorganic oxides containing a catalytic amount of a metal or metal oxide have been employed widely for continuous, fixed-bed conversion of olefins. One such catalyst comprises a silica support and an oxide of tungsten. The present invention is based on the discovery of a way to improve the selectivity of metathesis catalysts to specific products.
SUMMARY OF THE INVENTION
In accordance with the present invention, a metathesis catalyst is provided which consists essentially of a transition metal or oxide thereof supported on a high purity silica support. “High purity silica” is defined as silica possessing low amounts of acidic or basic sites such that in a reaction of pure 1-butene over said catalyst under metathesis conditions the reaction possesses a weight selectivity to hexene-3 of at least 55 wt %. Specifically, the high purity silica support contains less than about 150 ppm magnesium, less than about 900 ppm calcium, less than about 900 ppm sodium, less than about 200 ppm aluminum and less than about 40 ppm iron.
A critical feature of the catalyst of this invention is the purity of the silica support. Certain impurities adversely affect the activity and selectivity of metathesis catalysts. Activity-affecting and selectivity-affecting impurities such as aluminum and iron form acidic sites that will act as sites for olefin isomerization. Alkali metal impurities such as sodium and alkaline earth metal impurities such as calcium and magnesium form basic sites that also act as double bond isomerization catalysts at temperatures employed in metathesis reactions. The amounts of activity-affecting impurities in the catalyst of the invention are substantially below the amounts of such impurities present in conventional silica supports currently employed in the preparation of metathesis catalysts. As a result, the catalyst of the invention exhibits superior selectivities to the desired metathesis reaction products, and minimizes the production of undesired double bond isomerization reaction products.
The lower impurities levels of the catalysts of this invention also lead to a more environmentally friendly catalyst in which the trace elements leachability rates upon landfilling of the fully spent catalysts will be lower than those of the commercial silica formulations.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The high purity silica support utilized in the preparation of the metathesis catalyst of the invention possesses low amounts of both acidic and basic sites (preferably essentially no acidic and basic sites) and thereby improves the selectivity of the metathesis reaction and minimizes undesirable double bond isomerization.
By “low amount” of acidic and basic sites on the support, it is meant that the silica support possesses less than about 150 ppm magnesium (measured as the element), less than about 900 ppm calcium (measured as the element), less than about 900 ppm sodium (measured as the element), less than about 200 ppm aluminum (measured as the element) and less than about 40 ppm iron (measured as the element). Preferably, the high purity support possesses less than about 100 ppm magnesium, less than about 500 ppm calcium, less than about 500 ppm sodium, less than about 150 ppm aluminum and less than about 30 ppm iron. More preferably, the high purity support possesses less than about 75 ppm magnesium, less than about 300 ppm calcium, less than about 300 ppm sodium, less than about 100 ppm aluminum and less than about 20 ppm iron. High purity silica within the scope of this invention can be commercially obtained as chromatographic grade silica.
Transition metals and oxides thereof that can be employed herein are known and include, but are not limited to, tungsten, molybdenum, rhenium, oxides thereof and mixt
Gartside Robert J.
Greene Marvin I.
Khonsari Ali M.
Murrell Lawrence L.
ABB Lummus Global Inc.
Dilworth & Barrese LLP
Nguyen Cam N.
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