Fluid bed vinyl acetate catalyst

Details Fluid bed vinyl acetate catalyst Fluid bed vinyl acetate catalyst

2001-12-04

2003-03-18

Bell, Mark L. (Department: 1755)

Organic compounds -- part of the class 532-570 series

Organic compounds

Carboxylic acid esters

C560S241100, C560S244000, C560S245000

Reexamination Certificate

active

06534672

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a catalyst and catalyst support useful in producing vinyl acetate monomer (VAM) in a fluid bed process and more particularly relates to an active and selective fluid-bed VAM catalyst suitable for use in a fluid bed reactor in which catalytically active metal is well dispersed within a support.
Conventionally, vinyl acetate monomer is produced in the gas phase by reacting ethylene, acetic acid, and oxygen in the presence of a supported catalyst in a fixed bed reactor. In this type of reactor, a support material such as silica or alumina is impregnated with a catalytic metal such as palladium in combination with gold and an alkali metal salt, typically in the form of an acetate. A requirement of a fixed bed reactor process is that the supported catalyst formed into relatively large structural shapes such as balls and may be 2 to 50 mm in diameter or more.
In early examples of fixed-bed catalysts, palladium and gold are distributed more or less uniformly throughout the carrier, e.g., U.S. Pat. Nos. 3,275,680, 3,743,607 and 3,950,400 and Great Britain Patent No. 1,333,449 and South African Patent No. 687,990. Since gaseous reactants do not diffuse significantly into the large fixed-bed catalyst structure, much of the expensive catalytic metal components in the interior of the catalyst were not useful. Subsequently, fixed-bed catalysts were developed in which most of the catalyst metals were deposited onto the outer shell of the supported catalyst. For example, Great Britain Patent No. 1,500,167 describes a catalyst structure in which at least ninety percent of the palladium and gold is distributed in that part of the carrier particle which is not more than thirty percent of the particle radius from the surface. In addition, Great Britain Patent No. 1,283,737 teaches that the degree of penetration into the porous carrier can be controlled by pretreating the porous carrier with an alkaline solution of, for example, sodium carbonate or sodium hydroxide. Another approach to produce an active catalyst is described in U.S. Pat. No. 4,048,096 and other methods of producing shell-impregnated catalyst are disclosed in U.S. Pat. Nos. 4,087,622 and 5,185,308. Shell impregnated catalysts containing elements in addition to palladium and gold such as lanthanide compounds include U.S. Pat. No. 5,859,287 and WO 99/29418. In other fixed bed catalysts described in EP-A-0723810, a silica support may be impregnated with a Group IA, IIA, IIIA, or IVB metal salt and then calcined before addition of palladium and gold. Each of these patents primarily is concerned with the manufacture of fixed bed catalyst useful for the manufacture of vinyl acetate.
A new approach to produce vinyl acetate monomer is to use a fluid-bed process in which gaseous reactants are contacted continuously with small supported catalyst particles under fluidized bed conditions. Expected benefits of a fluidized bed VAM process include a simpler fluid bed reactor design than a multi-tubular fixed bed reactor and increased catalyst life due to decreased hot spots which are typical of a fixed bed reactor. Further, continuous addition of make-up catalyst maintains catalyst performance and eliminates complete catalyst change-out and shut-downs. Higher production rates may be achieved because higher oxygen levels safely may be fed into a fluid-bed reactor without producing a flammable mixture. Recently-issued U.S. Pat. Nos. 5,591,688, 5,665,667, and 5,710,318, assigned to the assignee of the present invention and incorporated by reference herein, are directed to the production of fluid bed vinyl acetate catalyst, or a fluid bed process for the manufacture of vinyl acetate.
In any regard, conventional commercially acceptable VAM catalyst, whether used in fixed or fluid-bed reactor systems, uses gold in combination with the palladium metal species, such as described in U.S. Pat. No. 5,859,287 and European Published Application EP 0 723 810, incorporated by reference herein. It is believed that gold forms an alloy with the palladium and inhibits agglomeration or sintering of palladium particles during the life of the catalyst under process conditions. Although other metals have been suggested as substitutes for gold in catalyst systems, gold has been found to be required for commercially practicable catalyst in terms of activity and selectivity. However, gold is an expensive component in the catalyst preparation. Therefore, there is a need for a commercially useful catalyst that does not require, or minimizes, the presence of gold.
Further, there is a continuing need for VAM catalysts, especially fluid bed catalysts, which have more advantageous activity/selectivity characteristics and which are more resistant to attrition. As described in this specification, the catalyst and catalyst support of this invention show commercially significant activity/selectivity properties without a necessity of gold as a catalyst component. Further, catalyst particles of this invention typically show improved attriton resistance under normal fluid bed conditions.
SUMMARY OF THE INVENTION
A catalytically active material useful to produce vinyl acetate monomer from ethylene, acetic acid, and an oxygen-containing gas under fluid bed conditions comprises a porous microspheroidal support containing catalytically active palladium crystallites finely dispersed within the support. The catalyst material does not require incorporation of gold to maintain activity and selectivity.
A process to produce a vinyl acetate fluid bed catalyst in which catalytically active small palladium crystallites are finely dispersed within the support comprises dispersing selected metal species within the support which have an affinity to palladium to form very fine crystallites of palladium. The affinity metal species may be dispersed by impregnation onto a preformed microspheroidal support or may be intimately incorporated within the support before impregnation with a soluble palladium species.
DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
The vinyl acetate catalysts of this invention, suitable for use in a fluid bed reactor system, contain catalytically active palladium crystallites finely dispersed within microspheroidal supports. It is believed that small, finely dispersed crystallites maintain catalytic activity and selectivity without a necessity of incorporation of gold in the catalytic material.
In one aspect of this invention, catalytically-active palladium crystallites are incorporated within a support particle, suitable for use in a fluid bed reactor system, such that the palladium crystallites are well dispersed in an interior region of the particle. In preferable catalysts, palladium crystallites are contained in an interior region of the catalyst particle and not concentrated at the surface. Although there may be a gradation of palladium crystallite concentration from below the surface to the particle center, the palladium crystallites appear finely dispersed in TEM photomicrographs, i.e., the palladium crystallites are substantially evenly distributed within the region without prominent agglomerations. In comparison to similarly produced particles using palladium and gold, which show significant numbers of agglomerated Pd/Au crystallites, a preferable catalyst particle of this invention shows few, if any, agglomerated palladium crystallites.
Typically, palladium crystallites in catalyst particles of this invention are no more than about 20 nanometers (nm) in mean diameter. In preferred catalysts of this invention, reduced metal crystallites in the catalyst particle including palladium crystallites are less than about 15 nm and more preferably less than about 10 nm. Typical crystallites are between about 5 to about 15 nm.
In order to form the fine palladium crystallites within a microspheroidal support according to this invention, a metal species that binds or has an affinity to palladium must be well dispersed with in the support particle. These affinity metal species include lanthanides such as lanthanum and cerium and G

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