Note: Descriptions are shown in the official language in which they were submitted.
ao81942
PRODUCTION OF ALKENYL ALKANOATE CATALYSTS
FIELD OF THE lNv~NlION
This invention relates to a process for
producing catalysts (hereinafter referred to as
~alkenyl alkanoate catalysts") for the production
of alkenyl alkanoates from alkenes, alkanoic acids
and an oxygen-containing gas.
DESCRIPTION OF THE RELATED ART
Processes for producing alkenyl alkanoate
catalysts are known. By way of illustration,
British Patent 1,215,210 (National Distillers)
discloses a process for the production of
olefinically unsaturated carboxylic esters (e.g.,
vinyl acetate) comprising reacting an olefinically
unsaturated compound, a carboxylic acid and oxygen
in the presence of a catalyst containing palladium
metal and platinum metal and activated with at
least one alkali metal or alkaline earth metal
hydroxide or organic acid salt or inorganic acid
salt.
The alkenyl alkanoate catalysts of
National Distillers are produced by: (1)
dissolving salts of the metals in conventional
solvents, (2) spraying the solutions on an inert
carrier or soaking the inert carrier in the
solutions, (3) removing the solvent, (4) converting
the salts so deposited on the carrier to the free
metals: (a) by thermal decomposition,
D-16818-2
~.
208I942
(b) by reduction with hydrogen or (c) by reduction in
suspension using aqueous alkaline formaldehyde,
aqueous hydrazine or aqueous or alcoholic sodium
borohydride, (S) washing the catalyst with water to
remove chlorides (see the National Distillers
Examples) and (6) activating the catalyst. In the
National Distillers catalyst preparation procedure,
there is no precipitation of the metal salts on the
carrier prior to their conversion to the free metals.
The National Distillers' alkenyl alkanoate
catalysts are activated with a minor amount of at
least one alkali metal or alkaline earth metal
hydroxide or organic acid salt or inorganic acid
salt. The alkali metal or alkaline earth metal salts
of weak acids, both organic and inorganic acids are
stated to be especially useful as activators.
Sodium, lithium, potassium, rubidium and cesium salts
and mi~tures thereof are stated to be most effective
as activator and the use of sodium and potassium
salts, e.g., sodium and potassium acetates is
especially preferred. The salts may have such anions
as citrate, acetate, borate, phosphate, tartrate,
benzoate or aluminate. National Distillers discloses
that alkali metal and alkaline earth metal hydroxides
are also effective activators and that the use of
halide anions should preferably be avoided, since the
presence of halides is stated to deleteriously affect
the synthesis reaction.
In both of the National Distillers' Examples
the salts were reduced with hydrogen and washed with
water to remove chlorides. Then the-catalysts were
treated with aqueous solutions containing both sodium
acetate and potassium acetate and dried in a rotary
D-16818-2
2o~l9~2
evaporator and then under vacuum. Based on the
amount of sodium acetate used, the resulting
catalysts contained about 0.23 weight percent
sodium. The most active catalyst of the National
Distillers' E~amples (i.e., the catalyst of E~ample
2H) is disclosed as having an activity of 8.3 grams
of vinyl acetate per gram of palladium per hour
(equivalent to about 165 grams of vinyl acetate per
liter of catalyst per hour, assuming a catalyst
density of one gram per milliliter).
As another illustration, Journal of
Catalysis, volume 17, pages 366 to 374, 1970
(Nakamura et al.) discloses vinyl acetate catalysts
produced by impregnating a carrier (calcined alumina)
with an aqueous solution of palladium chloride,
evaporating to dryness, reducing with an alkaline
hydrazine hydrate solution, washing with distilled
water to remove chloride ions, impregnating with a
metal salt solution (e.g., a potassium or sodium
acetate solution) and drying. Nakamura et al.
reports that impregnating with potassium acetate
results in a catalyst activity of 25 grams of vinyl
acetate per hour per liter of catalyst while
impregnating with sodium acetate results in a
catalyst activity of 19 grams of vinyl acetate per
hour per liter of catalyst.
As a further illustration, U. S. Patent
3,743,607 (Sennewald et al.) discloses a process for
making vinyl acetate from ethylene, acetic acid, and
molecular o~ygen or air in the gas phase. A mi~ture
of these reactants is passed in contact with a
supported catalyst containing metallic palladium, an
alkali metal formate or acetate, and metallic gold.
D-16818-2
2081942
-- 4
,
The vinyl acetate catalysts of Sennewald et
al. are produced by impregnating a catalyst carrier
with an aqueous solution of a palladium salt and a
gold salt and the resulting mixture is evaporated to
dryness. The mass so obtained is introduced into an
aqueous solution containing an appropriate reducing
agent (e.g., hydrazine) that is capable of reducing
both the palladium and gold salts to the metallic
state. In the Sennewald et al. catalyst preparation
procedure, there is no precipitation of the salts on
the carrier prior to the reduction. Once the
reduction is complete, the catalyst mass is removed
from the liquid by filtration and washed with water.
When the reduction is achieved by means of a reducing
agent free of alkali (e.g., hydrazine), the catalyst
is conveniently impregnated with an about 10%
solution of sodium acetate. The formates or acetates
of lithium or potassium can also be used. The
catalyst is then dried and is ready for use.
Sennewald et al. discloses that, in the absence of
such treatment, despite the gold it contains, the
catalyst is found to have a substantially lower
activity (e.g. an activity of only 15 grams vinyl
acetate per liter of catalyst per hour) instead of
the activity of 50 to 120 grams vinyl acetate yield
per liter of catalyst per hour disclosed in Sennewald
et al. for the Sennewald et al. catalysts. Catalysts
which have been reduced by means of a composition
comprising sodium formate and formic acid are
disclosed to be active, even if no sodium acetate has
been added thereto.
Sennewald et al. states that it has
unexpectedly been found that the vinyl acetate
D-16818-2
_ ~ 5 ~ ~I9
space/time yields and, more particularly, the
lifetime of the supported catalyst until regeneration
thereof, can be substantially increased when the
catalyst is impregnated with a solution prepared from
a mi~ture of various acetates of sodium, potassium,
rhodium or cesium instead of impregnation with a
solution of a single alkali metal acetate.
The highest activity reported in the
Sennewald et al. E~amples is in Example 11 where a
catalyst was impregnated with sodium and potassium
acetate as described in E~ample 9 of Sennewald et
al. To effect the impregnation, the catalyst was
introduced into a solution of the acetates, the
supernatant solution was decanted and the catalyst
was dried. The catalyst so obtained in Example 11 of
Sennewald et al. is reported to contain about 0.8 %
sodium and to have an activity of 146 grams of vinyl
acetate per liter of catalyst per hour. The other
Sennewald et al. Examples (including E~amples 4, 5
and 12(e) where the catalysts were apparently
substantially free of sodium) reported even lower
catalyst activities than E~ample 11. The catalysts
of Sennewald et al. Examples 4 and 9 were reported to
have the same palladium and gold contents and
appro~imately the same activities. The catalyst of
Example 4 is reported to contain 2.5% potassium (as
potassium acetate) while the catalyst of E~ample 9 is
reported to contain about 1.8% sodium (as sodium
acetate).
U.S. Patent 3,822,308 (Kronig et al.)
discloses that particularly active supported
catalysts containing palladium and gold for the
production of vinyl esters from ethylene, lower
D-16818-2
_ - 6 - 20819~2
carbo~ylic acids with 2 to 4 carbon atoms and oxygen
in the gas phase at elevated temperature and at
normal or elevated pressure are obtained by the
following procedure: The catalyst support is
treated, simultaneously or successively, with or
without intermediate drying, with a solution
("Solution A") containing dissolved salts of
palladium and gold and, optionally, salts of other
metals, and another solution ("Solution B")
containing compounds (hereinafter referred to as
precipitating agents") which are able to react on
the catalyst support with the noble metal salts of
the Solution A to form water-insoluble noble metal
compounds which are practically free from halogen,
sulphur and nitrogen. Solutions A and B (separately
or in combination) are used to impregnate the
catalyst support in quantities which correspond to
from 10 to 110% of the absorptive capacity of the
catalyst support for these solutions. The catalyst
support impregnated with Solutions A and B is
subjected to a time/temperature treatment such that
at least 9S% of the impregnated palladium and at
least 95% of the impregnated gold are transformed
into water-insoluble noble metal compounds. The
water-insoluble noble metal compounds are largely
transformed by treatment with reducing agents into
the noble metals and the water-soluble compounds
which are contained in the catalyst are removed by
washing, before or after the reduction.
In a preferred embodiment of the Kronig et
al. process, alkali metal carbo~ylates (especially
alkali metal acetates) are applied on the catalyst
D-16818-2
- 7 - 20~1 9~2
-
before or after the treatment with reducing agents in
such quantities that the catalyst, after being dried,
contains rom 1 to 30% by weight of alkali metal
carboxylate. Examples of the alkali metal
carbo~ylates disclosed in Kronig et al. include
sodium formate, potassium acetate, sodium acetate,
lithium acetate, potassium propionate and potassium
butyrate.
The Kronig et al. Examples employing
precipitating agents report catalyst activities
markedly higher than the activities reported by
National Distillers and Sennewald et al. where no
precipitating agents are used. Thus, Kronig et al.
Example 1 employs sodium hydro~ide as a precipitating
agent and potassium acetate as a promoter and reports
a activity of 952 grams of vinyl acetate per hour per
liter of catalyst. Kronig et al. E~ample 3 employs
potassium carbonate as a precipitating agent and an
"alkali metal acetate" as a promoter and reports that
the results obtained with the catalyst were
comparable to those of Kronig et al. Example 1.
U. S. Patent 4,048,096 (Bissot) discloses a
catalyst having a specific activity of at least about
83 grams of vinyl acetate per gram of precious metal
per hour measured at 150C. The ~issot vinyl acetate
catalyst consists essentially of: (1) a catalyst
support having a particle diameter of from about 3 to
about 7 mm and a pore volume of from about 0.2 to
about 1.5 ml.~g., a 10% by weight water suspension of
the catalyst support having a pH of from about 3.0 to
about 9.0; (Z) a palladium-gold alloy distributed in
a surface layer of the catalyst support, the surface
D-16818-2
- 8 - 2 081 9~2
layer e~tending less than about 0.5 mm from the
surface of the support, the palladium in the alloy
being present in an amount of from about 1.5 to about
5.0 grams per liter of catalyst, and the gold being
present in an amount of from about 0.5 to about 2.25
grams per liter of catalyst, and (3) from about 5 to
about 60 grams per liter of catalyst of alkali metal
acetate. Bissot discloses that the palladium is the
active catalyst metal and the gold is a catalyst
promoter.
Bissot also discloses a process for
preparing the Bissot catalyst. Like ~ronig et al.,
the Bissot process involves precipitation of the
metal salts on the catalyst support. The Bissot
process comprises: (1) impregnating the catalyst
support with aqueous solution of water-soluble
palladium and gold compounds, (2) precipitating
water-insoluble palladium and gold compounds on the
catalyst support by contacting the impregnated
catalyst support with a solution of compounds
(preferably sodium metasilicate) capable of reacting
with the water-soluble palladium and gold compounds
to form water-insoluble palladium and gold compounds,
(3) converting the water-insoluble palladium and gold
compounds into palladium and gold metal on the
support by treatment with a reducing agent, (4)
washing the catalyst with water, (5) drying the
catalyst (see E~ample 1 of Bissot), (6) impregnating
the catalyst with alkali metal acetate promoter
(e.g., a potassium promoter), and (7) drying the
catalyst.
D-16818-Z
- 20819~2
The improvement disclosed in Bissot involves
distributing the palladium and gold as an alloy in a
surface layer of the catalyst support, the surface
layer e~tending less than about 0.5 millimeter from
the surface of the support. The impregnating step is
carried out with an aqueous solution of palladium and
gold compounds and the total volume of the solution
is from about 95 to about 100% of the absorptive
capacity of the catalyst support. The precipitating
step in Bissot is carried out by soaking the wet
catalyst support with a solution of an alkali metal
silicate, the amount of alkali silicate being such
that, after the alkali metal silicate solution has
been in contact with the catalyst support for about
12 to 24 hours, the pH of said solution is from about
6.5 to about 9.5.
Bissot does not report the sodium content of
the catalysts of the Bissot Examples. Bissot Example
1 reports that the catalyst of that Example had an
activity of 560 grams of vinyl acetate per liter of
catalyst per hour. In Example III below, two
catalysts produced following the disclosure of
Example 1 of Bissot were found to have sodium
contents of 0.32 and 0.38 weight percent and
activities of 551 and 535 grams of vinyl acetate per
liter of catalyst per hour.
Despite the foregoing prior art processes,
it is desirable to further improve the activity of
alkenyl alkanoate catalysts.
D-16818-2
- 10 - ' 2~8~9~'2
SUMMARY OF THE INVENTION
This invention is based, in part, on the
discovery that the activity of alkenyl alkanoate
catalysts is increased if their sodium content is
decreased by employing essentially sodium free
starting materials in producing the catalyst.
More specifically, this invention provides a
process for producing a catalyst that is useful in
catalyzing the reaction of an alkene, an alkanoic
acid and an o~ygen-containing gas to produce an
alkenyl alkanoate and that comprises support
particles which are capable of e~changing cations and
which are impregnated with palladium, gold and
potassium acetate:
said process comprising the steps of:
(a) impregnating the support particles
with aqueous solutions of water-soluble palladium and
gold compounds;
(b) precipitating water-insoluble
palladium and gold compounds onto the support
particles from such solutions using a precipitating
agent;
(c) converting the precipitated
water-insoluble palladium and gold compounds to
palladium and gold on the support particles using a
reducing agent;
(d) washing the support particles with
water;
(e) drying the support particles;
(f) further impregnating the support
particles with a potassium promoter; and
(g) drying the impregnated particles
to produce the catalyst;
D-16818-2
20819~2
and said process being conducted by employing
essentially sodium-free starting materials in steps
(b)and (c) so as to reduce the amount of sodium in
the catalyst and thereby to increase the activity of
the catalyst.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows the predicted effect of
sodium on the performance of vinyl acetate catalysts
produced in accordance with this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the practice of the process of the
present invention, sodium-containing, water-soluble
palladium and/or gold compounds can usually be
employed since they are usually not used in amounts
that result in catalysts having substantial amounts
of sodium. The principal sources of sodium in
alkenyl alkanoate catalysts are sodium-containing
precipitating agents (e.g., sodium metasilicate)
and/or sodium-containing promoters or activators
(e.g., sodium acetate). To a lesser e~tent, some
supports and some reducing agents (e.g., sodium
borohydride) can introduce substantial amounts of
sodium into the catalyst. Accordingly, in the
practice of this invention, essentially sodium-free
precipitating agents (e.g., potassium hydro~ide),
promoters (e.g., potassium acetate), reducing agents
(e.g., hydrazine) and carriers are employed. When
using potassium hydro~ide as the precipitating agent,
a suitable potassium salt (e.g., potassium acetate)
can also be used in the precipitating step to aid in
displacement by potassium of any sodium bound on the
D-16818-Z
- 12 - 20319~2
carrier. Preferably the potassium hydroxide and the
potassium salt are employed in an aqueous solution.
The salt is used in an amount that provides from 1 to
10 weight percent potassium based on the total weight
of the solution. Care should be e~ercised to ensure
that the resulting catalyst does not contain so much
potassium that catalyst activity is less than
desired.
The support particles used in the process of
this invention are solid particulate materials that
are capable of e~changing cations (e.g., due to the
presence of SiOH or AlOH groups), that are capable of
being impregnated with palladium, gold and a
potassium promoter and that are inert under the
conditions used to produce alkenyl alkanoates.
Illustrative of such support particles are
particulate silica, alumina, and silica-aluminas.
Silica is the preferred support. The support
preferably has a surface area from 100 to 800 square
meters per gram.
The aqueous solutions of water-soluble
palladium and gold compounds used in the process of
this invention include aqueous solutions of any
suitable palladium or gold compound such as palladium
(II) chloride, sodium tetrachloropalladium (II)
(Na2PdC14), palladium (II) nitrate, palladium (II)
sulfate, gold (III) chloride or auric (III) acid
(HAuC14). The volume of the solution preferably
corresponds to from 95 to 100% (more preferably from
98 to 99%) of the pore volume of the support.
The precipitating agents used in the process
of the present invention include lithium and
D-16818-2
- 13 _ i 2 0~1 9q 2
potassium silicates and hydroxides. The
precipitating agents are preferably employed in the
form of aqueous solutions containing a 1.6 to 1.8
molar excess of the precipitating agents. The volume
of such solution used is preferably just sufficient
to cover the support particles. To avoid possible
degradation of the support, the weight ratio of the
precipitating agent to the support should not be too
high. By way of illustration, a weight ratio of
potassium hydro~ide to support of about 0.08:1
resulted in no noticeable degradation of the support.
The reducing agents used in the process of
this invention include ethylene, hydrazine,
formaldehyde and hydrogen. The reducing agents are
preferably employed in the form of aqueous solutions
containing a 50:1 (or more preferably a 10:1) molar
e~cess of the reducing agents. If hydrogen is used,
it is usually necessary to heat the catalyst to 100
to 300C to complete the reduction.
The potassium promoters used in the process
of this invention include potassium alkanoates and
any potassium compound that is converted to a
potassium alkanoate during the alkenyl
alkanoate-forming reaction (i.e., the reaction of
ethylene, an alkanoic acid and an oxygen-containing
gas in the presence of the catalyst to produce an
alkenyl alkanoate). Suitable potassium compounds
include potassium acetate, bicarbonate, nitrate and
(when a stable support is used) hydroxide. The
promoters are preferably applied in the form of
aqueous solutions.
The washing step in the process of this
invention can be conducted batchwise or
~-16818-2
` - 14 - 20~19~2
continuously. Continuous washing is more efficient
but may not be most suitable for large scale (e.g.,
plant scale) catalyst production. In continuous
washing, the wash water is slowly and continuously
passed through the catalyst over a period of time
(e.g., from 8 to 24 hours). In batch washing, the
catalyst is contacted with the wash water, the
mi~ture is allowed to stand (e g., for from 0.5 to
2.0 hours) and the water and catalyst are separated.
In batch washing, several such washes (e.g., from 2
to 10, or preferably from 4 to 6 washes) are often
required to reduce the impurity (e.g., halide)
content of the catalyst to the desired level.
Temperatures from 20C to 80 and volume ratios of
wash water to catalyst of from 2:1 to 100:1 can be
used in either batch or continuous washing. The
washing step removes certain impurities, particularly
chlorides, from the catalyst.
The drying of the catalyst in accordance
with step (e) and step (g) of the process of this
invention for producing alkenyl alkanoate catalysts
can be conducted in any convenient manner. By way of
illustration, drying can be conducted at 40C to
120C in a forced air oven for 15 to 30 hours.
The catalyst produced by the process of this
invention are useful in catalyzing the reaction of an
alkene, an alkanoic acid and an oxygen-containing gas
to produce an alkenyl alkanoate and comprise support
particles which are capable of exchanging cations and
which are impregnat~ed with precipitated and reduced
palladium and gold and a potassium promoter, any
sodium in the catalyst being preferably present in an
D-16818-2
_ - 15 _ 20819~2
amount no more than 0.3 weight percent based on the
total weight of the catalyst.
The catalysts produced by the process of
this invention preferably have a palladium content of
greater than 0.25 weight percent based on the total
weight of the catalyst, more preferably greater than
0.5 weight percent based on the total weight of the
catalyst and most preferably from 0.5 to 1.7 weight
percent based on the total weight of the catalyst.
It is preferred that the gold to palladium weight
ratio of the catalyst is from 0.2 to 1.5 and, most
preferably, from 0.4 to 1.2
The catalysts produced by the process of
this invention desirably contain no more than 0.3
weight percent sodium based on the weight of the
catalyst. Preferably the catalysts contain no more
than 0.2 weight percent sodium and, most preferably,
the catalysts contain no more than about 0.1 weight
percent sodium based on the weight of the catalyst.
The amount of sodium in the catalysts of this
invention will depend primarily on the particular
starting materials used and, to a lesser extent, on
the number of washes, the volume of wash water and
the total washing time.
It is preferred that the catalysts produced
by the process of this invention are shell
impregnated catalysts wherein a catalyst support has
a particle diameter from about 3 to about 7
millimeters and a pore volume of 0.2 to 1.5
milliliters per gram. The palladium and gold are
preferably distributed in the outermost 1.0
millimeter thick layer of the catalyst support
particles. The catalysts preferably contain from
D-16818-2
- 16 - 2~ 21 9~ 2
about 1.4 to about 3.8 weight percent (and more
preferably from 2 to 3.6 weight percent) of the
potassium derived from the potassium promoter.
The catalysts produced by the process of
this invention are characterized by their increased
catalytic activity. Typically the activity of the
catalysts is 5% to 25% greater (in terms of quantity
of alkenyl alkanoate produced per unit of catalyst
per unit time) than otherwise identical catalysts
containing from over 0.3 to about 1.0 weight percent
sodium. Although catalyst selectivity (i.e., the
tendency to produce alkenyl alkanoates rather than
by-products such as carbon dioxide) declines somewhat
with decreasing sodium content, that disadvantage is
more than offset by increased catalyst activity,
particularly in the range of sodium contents found in
commercial alkenyl alkanoate catalysts (e.g., up to
about 1.0 weight percent sodium).
The process for producing alkenyl alkanoates
using the above-described catalysts ("alkenyl
alkanoate process") comprises reacting an alkene, an
alkanoic acid, and an oxygen-containing gas in the
presence of a catalytic amount of catalyst of this
invention as described above. The alkenyl alkanoate
process is preferably conducted at a temperature from
100C to 250C (and most preferably at a temperature
from 140C to 200C) and at a pressure from 15 psi to
300 psi (most preferably at a pressure from 90 pounds
per square inch to 150 pounds per square inch.) The
alkenyl alkanoate plrocess is preferably conducted
continuously in the vapor phase.
Preferred alkanoic acid starting materials
used in the alkenyl alkanoate process contain from
D-16818-2
- 17 - a O ~ 2
two to four carbon atom~ (e.g. ethylene,
propylene and n-butene). Preferred products
of the alkenyl alkanoate process are vinyl
acetate, vinyl propionate, vinyl butyrate,
and allyl acetate.
The alkenyl alkanoates produced
by the alkenyl alkanoate process are known
compounds having known utilities (e.g.,
vinyl acetate is useful in producing
polyvinyl acetate).
D-16818-2
, . ~
-L~
2~1942
- 18 -
EXAMPLES
In the following E~ample, the following
abbreviations are used:
Abbreviation Meaning
Support I Silica beads having an
average diameter of 5 to 6
millimeters and containing
about 0.1 weight percent
sodium. The beads have a
surface area from lS0 to 200
square meters per gram and a
pore volume from 0.6 to 0.7
milliliters per gram.
Support I contains SiOH
groups that are capable of
e~changing cations. Support
I is sold by Sud Chemie AG
as "KA-160"
STY* Space Time Yield (a measure
of catalyst activity)
expressed as grams of vinyl
acetate per liter of
catalyst per hour.
% Selectivity* Selectivity was calculated
as follows: Selectivity =
100 X (moles vinyl
acetate)/(moles vinyl
acetate + 1/2 X moles CO2)
AA Analysis Atomic Adsorption
Spectroscopy
ICP Inductively Coupled Plasma
Optical Emission Spectrometry
g VA/l cat/hr Grams of vinyl acetate
produced per liter of
catalyst per hour
*All the values for activities and selectivities
reported in the E~amples appearing below are based on
the activities and selectivities measured twenty-six
hours after full oxygen feed was reached in the
Catalyst Test Method described below.
D-16818-2
20~1942
-- 19 --
.
VA vinyl acetate
KOAc potassium acetate
EcOAc ethyl acetate
NaOAc sodium acetate
% percent by weight
g grams
ml milliliter
mm millimeter
hrs hours
min minute
In the following Examples, the following
procedures were used:
Catalyst PreParation Procedure
Support I (15 g) was added to a solution of
Na2PdC14 (35.86% Pd, 0.258 9) and HAuC14 (48.95% Au,
0.094 9) dissolved in 9.0 ml of deionized water. The
mixture so formed was gently agitated until all of
the moisture was absorbed into the support and then
was allowed to stand in a sealed flask for about one
hour at room temperature so as to impregnate the
support with the palladium and gold salts. The damp
catalyst was covered with a solution of potassium
hydroxide (0.371 g in 28 ml water) as a precipitating
agent. Potassium acetate is used in conjunction with
potassium hydroxide in the precipitation step in a
preferred embodiment of this invention. After mi~ing
for a few seconds, the mi~ture was allowed to stand
covered and undisturbed for 23 hours at room
D-16818-2
2 0 8 ~ 9 ~ 2
- 20 -
-
temperature to deposit water-insoluble palladium and
gold compounds on the support. The palladium and
gold were then reduced by the addition of 1.0 g of
85% hydrazine hydrate to the above mixture. The
mixture was agitated for a few seconds and allowed to
stand covered and undisturbed at room temperature for
another 23 hours. The supernatant liquid was
decanted from the catalyst and the catalyst was
rinsed four times with water to remove the small
amount of metal sludge present. The catalyst was
washed thoroughly by the Column Washing Procedure
described below to remove chlorides and residual
reagents. The catalyst was dried on a stainless
steel screen at 60C in a forced air oven for 20 to
24 hours. The catalyst was analyzed for potassium
using AA Analysis. Then the catalyst was impregnated
with the desired amount of potassium acetate in water
using the impregnation technique described above for
the palladium and gold salts. Then the impregnated
catalyst was dried at 60C for 20 to 24 hours. The
palladium, gold, sodium, and potassium contents in
the finished catalysts were determined by ICP
analyses and the sodium and potassium contents were
also determined by AA Analyses for greater accuracy.
The catalysts so produced were shell-impregnated
(i.e., substantially all of the palladium and gold
was present in a shell within 0.5 mm of the surface
of the beads of Support I).
Column Washinq Procedure
Catalysts were washed or rewashed in a
1.24 inch o.d. g 24 inches glass chromatography
~-16818-2
~ - 20819~2
- 21 -
column fitted with a Teflon~ stopcock. Typically 15
g of catalyst was added to the column which was then
filled with water. The stopcock was adjusted to
allow the liquid to flow from the column such that
about one liter passed through the catalyst at room
temperature over a period of about 24 hours. After
this period, the e~cess liquid was drained from the
column and the catalyst removed and dried as
described above in the Catalyst Preparation Procedure.
Catalyst Test Method
The catalyst ~2.5 g samples of 5 to 6 mm
catalyst spheres) was diluted with 10.5 ml of 0.5 mm
glass beads and the mi~ture was uniformly distributed
in both legs of a 316-stainless steel U-tube
reactor. The reactor had an outside diameter of 3/8
inch and an overall height of about 6 inches. An
ethylene flow of 151 ml/min. was started through the
reactor after which the catalyst was heated in an
oven maintained at 150C while allowing the system to
pressurize to 115 psig. After maintaining at these
conditions for 1.5 hours, acetic acid vapor was added
to the ethylene and the mi~ture was passed over the
catalyst for 45 minutes. Air was added to the feed
gas at a slowly increasing rate over a 45-minute
period until a total flow of 105 ml./min. was
reached. The catalyst was allowed to stabilize for
two hours before beginning data collection. The
final gas composition was ethylene:acetic
acid:o~ygen:nitrogen e 52.9:10.7:7.7:28.7, the total
gas hourly space velocity was about 3800 hrl, and the
acetic acid liquid hourly space velocity was about 1
D-16818-2
20~1942
- 22 -
-
hr~l. The product was analyzed by gas
chromatography. The run-to-run reproducibility of
the microreactors used in these experiments is about
~10 STY units.
EXAMPLE I
Support I contains about 0.1 weight percent
sodium as received from the manufacturer. An
additional 0.4 to 0.8 weight percent of sodium is
introduced during the precipitation step when sodium
hydro~ide or sodium metasilicate is used as the
precipitating agent. Three catalysts were prepared
using potassium hydroxide as the precipitating agent
to reduce the sodium level in the final catalyst.
Varying concentrations of potassium acetate were
added to the precipitating solution to further shift
the ion-e~change equilibrium toward potassium. The
catalysts were prepared from the same master batch
having a nominal palladium loading of 0.58 weight
percent and an Au/Pd ratio of 0.46. After reduction
with hydrazine and washing using the Column Washing
Procedure, the catalysts were analyzed for sodium and
potassium. Then additional potassium acetate was
added as required to give a final potassium acetate
content of about 5.3 weight percent. The results
given in Table A show that the sodium content of the
catalysts was reduced. The sodium content in the
catalysts dropped uniformly and catalytic activity
improved as the potassium concentration of the
precipitating solution was increased.
D-16818-2
23 20~19~2
Table A
Low-Sodium Catalyst PreParations
% KOAc (a) % Na (b) ~ % Selectivity
0 0.186 576 93.4
2.5 0.121 603 93.2
5.0 0.091 597 93.4
(a) Wt % potassium acetate added to the precipitating
solution.
(b) Wt % sodium calculated to be in the finished
catalyst. Based on AA Analyses prior to KOAc
impregnation and the quantity of KOAc added.
A similar catalyst, prepared using sodium
metasilicate as the precipitating agent (in lieu of
potassium hydro~ide) had an STY of 544, a selectivity
of 93.6 and a sodium content of 0.44 weight percent.
No potassium acetate was added in the precipitation
step.
E~ample II
The effect of sodium on the performance of
vinyl acetate catalysts was studied using
statistically designed e~periments and models were
D-16818-2
_ - 24 - 20819~2
obtained which are useful in predicting the
performance of the vinyl acetate catalysts produced
by the process of this invention as well as vinyl
acetate catalysts produced by the processes of the
two above-mentioned U.S. patent applications filed
concurrently herewith. The models predict catalyst
activity and selectivity as a function of sodium
content, palladium loading, gold to palladium weight
ratio, potassium content and catalyst weight. These
models and the data from which they were generated
are shown in Tables B and C respectively.
Because the degree of conversion has a major
effect on both catalyst productivity and selectivity,
meaningful comparisons of catalyst variables can only
be done at constant conversion. In order to predict
the effects of catalyst composition at constant
conversion, the Oxygen Conversion Model in Table B
was rearranged to e~press catalyst weight as a
function of the palladium content, gold~palladium
ratio, potassium content, sodium content and
conversion. This catalyst weight term was then used
to replace the catalyst weight terms in the STY and
Selectivity models. The predicted effects of
increasing sodium content on vinyl acetate catalyst
activity and selectivity are plotted in Figure 1.
D-16818-2
2081942
- 25 -
The abbreviations used in Tables B and C
have the following meanings:
Pd Weight percent palladium in the catalyst
Au/Pd Weight ratio of gold to palladium in
the catalyst
Cat.Wt Catalyst weight in grams
K Weight percent potassium in the catalyst
Na Weight percent sodium in the catalyst
STY Space time yield in grams of vinyl
acetate per liter of catalyst per hour
R2 Correlation coefficients which are
indicative of the quality of fit of the
data to the models
RSD Relative standard derivation
EtOAc By- Production of ethyl acetate in moles/
Product Rate kilogram of catalyst/hour
~ Heavies Heavy by-products expressed as a weight
By-Products percent of the vinyl acetate produced.
in VA Heavies by-products are defined as all
products which elute after acetic acid
in the gas chromatographic analytical
procedure.
Table D shows the effect of varying sodium
on catalyst activity as predicted by the models in
Table B.
D-16818-2
2~819~2
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TABLE D
Predicted Effect of Sodium on Catalyst* Activity
% Na STY % Improvement**
0.1 665 0.0
0.2 649 2.5
0.3 633 5.1
0.4 618 7.6
0.5 603 10.3
0.6 581 14.5
0.7 574 15.9
0.8 560 18.8
0.9 546 21.8
* For a catalyst composition set at: O. 58% Pd, Au/Pd =
0.45 and 2.2% K and an o~ygen conversion set at 35%.
*~ Predicted percent change in STY resulting from
decrease in sodium content from amount shown in
first column to 0.1%.
D-16818-2
- 31 - Z 081 9~ 2
EXAMPLE III
The procedure of E~ample 1 of U.S. Patent
4,098,096 (Bissot) was repeated as follows: Two
preparations (Runs 1 and 2) were made, each employing
159 of Support I, 0.315 g of Na2PdC14, 0.085 g of
HAuC14, 0.585 g of Na2SiO3.9H2O, 0.59 g of 85%
hydrazine hydrate, and 0.823 g of potassium acetate.
Since the e~act washing procedure is not disclosed in
E~ample 1 of Bissot the catalyst of Run 1 was washed
using the Column Washing Procedure for 16 hours using
23 ml of H2O per gram of catalyst whereas the
catalyst of Run 2 was similarly washed but with 31 ml
of H2O per gram of catalyst. The catalysts of Runs 1
and 2 were analyzed by ICP for palladium and gold and
by AA Analysis for potassium and sodium. The
e~perimental error of the sodium determination is
estimated to be about + 0.01 relative percent. The
catalyst of Run 1 was analyzed in duplicate. The
results are shown in Table E.
TABLE E
%Pd %Au %K %Na ~Y Selectivity
Run 1 0.544 0.201 2.34 0.32 550 93.9
Run 1 0.556 0.204 2.35 0.32
Run 2 0.552 0.195 2.34 0.38 535 93.7
Bissot 0.578* 0.242* 2.08* ---- 560** 93**
* Calculated based on the data in E~ample 1 of Bissot
** Disclosed in Example 1 of Bissot
D-16818-2
- 32 - 2081942
EXAMPLE IV
The measured activities of three commercial
catalysts (Catalysts X, Y and Z) are shown in Table
F. In Table F, the activities of Catalysts ~ and Y
are compared to the predicted activities of catalysts
of this invention having the same composition (~Model
Catalysts"). The predicted activities were
determined from the models of Table B and assuming a
0.15% level of sodium. The Model Catalysts had
markedly higher predicted activities. A similar
comparison could not be made for Catalyst Z because
its composition is outside the range of the models of
Table B.
Catalyst X was prepared using the Catalyst
Preparation Procedure. The preparation of Catalyst X
differed from the preparation of Catalyst Y in that,
in the preparation of Catalyst Y: (1) the catalyst
was dried before precipitation and (2) the
precipitating agent used was sodium hydro~ide rather
than sodium metasilicate. The reduction, washing,
drying and potassium acetate impregnation steps were
the sa~e for both catalyst preparations.
With reference to Catalyst Y, the suffi~es
A, B and C in Table F denote different preparations
("lotsn) of nominally the same catalyst and, with
respect to Catalysts Y and Z, the suffixes 1 and 2
denote duplicate analyses of different samples from
the same lot of the catalysts.
Catalyst Z has a high palladium content and
uses a cadmium co-catalyst rather than gold. Cadmium
is significantly more toxic than gold. In addition,
Catalyst Z is prepared by a process substantially
D-16818-2
33 20819~2
different from the process of this invention, that
is, Catalyst Z is prepared by impregnating a support
with a solution of palladium, cadmium and potassium
acetates and drying. There are no precipitation,
reduction or washing steps in the process used to
produce Catalyst Z.
TABLE F
%Pd %Au ~Ç~ %K %Na STY
Catalyst X~ 0.530.22 0 2.36 0.54 272
Model Catalyst 0.53 0.22 0 2.36 0.15 589
Catalyst YA-l* 0.490.19 0 2.29 0.60 360
Model Catalyst 0.49 0.19 0 2.29 0.15 546
Catalyst YA-2* 0.490.19 0 2.31 0.60 360
Model Catalyst 0.49 0.19 0 2.31 0.15 545
Catalyst YB* 0.630.24 0 2.27 0.70 386
Model Catalyst 0.63 0.24 0 2.27 0.15 669
Catalyst YC-l* 0.610.29 0 2.24 0.69 395
Model Catalyst 0.61 0.24 0 2.24 0.15 653
Catalyst YC-2* 0.610.26 0 2.18 0.70 395
Model Catalyst 0.61 0.26 0 2.18 0.15 658
Catalyst Z-l* 2.16 0 1.88 1.89 0.08 685
Model Catalyst Outside range of models
Catalyst Z-2* 2.16 0 1.89 1.92 0.09 685
Model Catalyst Outside range of models
*Comparative Catalysts
D-16818-2
