Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION
"CRYSTALLINE SILVER CATALYSTS FOR
METHANOL OXIDATION TO FORMALDEHYDE"
FIELD OF THE INVENTION
This invention relates generally to the field of industrial catalysis and
more particularly to crystalline silver catalysts for methanol oxidation to
formaldehyde conversion. This invention describes the formation of novel
forms of crystalline silver which have catalytic properties that are superior
to
conventional silver materials prepared by known electrochemical methods.
BACKGROUND OF THE INVENTION
Formaldehyde is a highly versatile chemical that finds widespread
application in industry, particularly in the resins sector. Commercially, it
is
synthesised via either the partial oxidation and dehydrogenation of methanol
over
crystalline silver (US Patent 4,594,457, US Patent 4,584,412) or in a uniquely
oxidative process in conditions of excess air in the presence of a mixed iron
oxide-molybdenum oxide catalyst (US Patent 3,843,562 and US Patent
3,855,153). The metal oxide system requires a substantial volume of gas which
is 3.0 to 3.5 times greater than the gas flow of a conventional silver
catalysed
process (Kirk-Othmer, Encyclopedia of Chemical Technology, 4t" Edition). This
factor results in additional plant costs for air compression as well as energy
input
for cooling systems to dissipate the heat of reaction. Consequently, despite
superior conversion and good selectivity the iron oxide-molybdenum oxide
process demands higher capital investment and operating costs and thus the
more economical silver process still demands attention and indeed is
extensively
used worldwide.
Silver catalysts have been used since 1908 to convert methanol to
formaldehyde by means of two simultaneous reactions, the partial oxidation of
methanol
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CH30H + 0.5 02 ~ CH20 + H20 OH = - 154.88 kJ/g-mol at 923 K
and the dehydrogenation of methanol
CH30H ~ CH20 + HZ DH = + 92.09 kJ/g-mol at 923 K
Important aspects for formaldehyde production are; firstly, the need to
convert the maximum amount of methanol in the feed per pass ; secondly, the
necessity of producing formaldehyde in high selectivity thus achieving a high
formaldehyde yield; thirdly, the requirement of the catalyst to minimise the
amount of by-products formed; fourthly, the achievement of rapid reaction
"light-ofP' to avoid downtime costs and fifthly, the desire to operate the
catalyst
for a life in the industrial plant of at least several months without loss of
performance and lastly, the ability of the catalyst to increase plant
throughput.
Quantities of both methanol and formaldehyde are lost to competing and/or
consecutive reaction pathways. The major by-products formed over the
catalyst are carbon dioxide, carbon monoxide, formic acid and methyl formate.
Formic acid is especially distressing as it interferes with any subsequent use
of
formaldehyde for polymerisation processes. The formation of COZ and CO is
attributed to both combustion and thermal decomposition processes.
Correspondingly, efforts must be made to maximise the methanol conversion.
Another significant factor is the catalyst lifetime which has been reported to
be
approximately 70 to 130 days (British Patent 1,217,717 (Dec. 31, 1970) and
German Offen. 2,520,219 (Nov. 18, 1976) and JP 48-16892 (Mitsubishi Gas
Chemical, May 25, 1973). Speed of catalyst light-off in the industrial plant
is
also of relevance in order to minimise downtime periods which are
economically unfavourable.
PRIOR ART
In general, crystalline silver can be obtained by operation of an
electrochemical cell, for example the conventional Moebius, Thum or Prior
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cells, using a silver nitrate electrolyte in the pH range 1-4 containing
between 5
and 100 g/L dissolved silver, a cell temperature of 10-80°C, a current
density
between 100 and 3000 A/m2 and a cell voltage between 0.2 and 9 volts(US
Patent 5,135,624). In particular, preparation methods specifically related to
crystalline silver catalysts which are well known to those skilled in the art
include the continuous electrolytic refining of silver in an aqueous solution
of
silver nitrate and nitric acid at 24 °C, 3.1 volts and a current
density of 1.2
amp/dm2 (120 A/m2) (Graefen et al., French Patent 2,141,893) wherein silver
grains of 0.2 to 2.5 mm in size are stripped from a slowly rotating
polypropylene
anode. In addition, Graefen et al.(German Patent DE 2129776) have claimed
that silver catalysts are best electrochemically synthesized with a current
density in the range 0.3 to 3 A/dm2 (30 to 300 A/m2) and by use of a rotating
cathode with turns between 0.1 and 3 times per day.
Also known is the addition of organic inhibitors to the electrochemical
cell to modify the structure of the silver crystals deposited, albeit, not in
the
context of the use of silver as a catalyst. For example, the addition of
thiourea
at the solubility limit produces crystals of the unorientated dispersion type
(R.
Winand, Application of Polarization Measurements in the Control of Metal
Deposition (1.H. Warren, Ed.), Elsevier, 1984).
Szustakowski et al. (M. Szustakowski, J. Schroeder, A. Jakubowics, T.
Kelm. I. Cieslik and E. Francman, Polish Patent PL 122783) disclosed the
doping of silver catalysts by < 1 % of activators such as AI, Be, Zr, Mg, Si,
V,
Mo, Se, Cd, Cr, As or Sb. These latter additives were typically introduced by
addition of corresponding ions into the electrolytic refining procedure. These
inventors demonstrated that the formaldehyde yields and methanol conversion
efficiency could be enhanced by such additives. However, the efficiencies
reported are significantly less than the comparable figures presented in this
invention.
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Similarly, Szustakowski et al. (M. Szustakowski, J. Baron and J.
Jarmakowicz, React. Kinet. Catal. Lett., Vol 39, 351-356 (1989)) have revealed
that the presence of 250 mg/L of Pd in the electrolyte solution results in the
formation of silver crystals doped with small amounts of palladium. Notably,
this procedure resulted in a higher methanol conversion efficiency, albeit, at
the
expense of decreased formaldehyde selectivity. In contrast, this invention
discloses catalysts which not only exhibit higher methanol conversion but also
are characterized by improved formaldehyde yield.
Silver catalysts modified by the presence of other inorganic elements
have also been previously disclosed such as in US Patent 4,045,369 which
reveals that the addition of barium, strontium, calcium and/or indium may be
beneficial for oxidation reactions. Also known is the use of silver-gold
alloys
(EP 104,666 and EP 003,348), silver-cadmium alloys (US 3,334,143) and silver
oxide (JP 46-20693). Japanese patent 08117599 advocates the immersion of
silver crystals in a platinic chloride solution to produce a catalyst, which
consists of 5 ppm to 3 wt% platinum on silver. European patent 0 486 777 A1
suggests that the addition of either magnesium oxide, zirconium oxide, silica,
yttrium oxide or aluminium oxide in amounts of 4 wt% or less can be beneficial
for methanol oxidation. Supported catalysts have also been reported such as
silver on Kellundite (US Patent 4,330,437) and silver on porcelain (US Patent
4,126,582). Additionally, the application of silver supported on pumice stone
has been reported (Sacharov et al., Khimicheskaya Promyshiennost, 2, 75-76,
1991 ). Japanese patent 60-89441 suggests that the use of catalysts
comprising of silver and zinc dispersed on a silica support may be useful.
Furthermore, it has been disclosed that addition of nitrous oxide to the
feedstream can enhance the formaldehyde yield (US Patent 4,233,248 and EP
0 624 565 A1 ). Belgian patent BE 847775-A reveals that continuous supply of
volatile halide compounds may increase formaldehyde yield. Similarly, EP 0
104 666 reveals that continuous incorporation of trimethylphosphite,
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triethylphosphite, or tri-n-butylphosphite moderator may increase the
formaldehyde yield albeit typically at the expense of methanol conversion.
Recently, in a patent assigned to BASF (WO 97/30014) it has been suggested
that doping the silver catalyst bed with a finely divided phosphorous compound
5 having a melting or decomposition point higher than 500°C can be
beneficial in
attaining a higher formaldehyde yield.
Previous inventions to alleviate the problem of slow reaction light-off
include construction of elaborate multilayered beds of silver catalyst of
different
particle sizes (German patent 2,322,757). Using this latter invention it is
claimed that light-off can be achieved at temperatures of 553-573 K. Very fine
silver powder (0.1-1 micron) can also be sprinkled on top of the silver bed
and
this procedure permits reaction to start at 478-503 K (German Offen.
2,520,219). In general, smaller silver grains are found to exhibit greater
activity, however, due to sintering and plugging effects a bed cannot be
entirely
constructed of these fine particles.
A patent assigned to Koei Chem Co Ltd (JP 06 172248) discloses that
the pressure drop over a silver catalyst bed can be minimized (and thus the
useful catalyst lifetime increased) by ensuring that the uppermost 1.5 mm of a
silver catalyst bed is composed of particles, at least 10% < 0.38 mm in
diameter and at least 50% > than 0.38 mm in diameter, which have a packing
density in the range 3.5 to 4.5 g/mL. These latter inventors revealed that
silver
particle densities less than 3.5 g/mL resulted in a reduction in catalyst
efficiency, whereas catalyst densities in excess of 4.5 g/mL resulted in
pressure
rises over the catalyst bed which made for inefficient use of the air
compressor.
DEFICIENCIES IN PRIOR ART
Due to the complexities of the silver catalyzed oxidation of methanol to
formaldehyde it is often found that the supplied catalyst does not perform
adequately. Indeed, catalyst performance is known by practitioners in the area
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of formaldehyde production to vary considerably not only between the different
commercial suppliers but also within batches taken from the same source. For
instance, Table 1 illustrates data from a commercial methanol oxidation plant
wherein catalyst used from the same supplier was employed. The production
results clearly illustrate that there still exist serious problems to be
overcome
with respect to the performance of silver catalysts, which prior art has
failed to
address.
Of general concern is; (1 ) the variability in the formaldehyde selectivity,
(2) the high level of residual methanol in the product formaldehyde, (3) the
fluctuation in formaldehyde yield, (4) the inconsistency of the catalyst
lifetime,
(4) the changeable plant capacity and (5) unstable formic acid concentrations.
Practitioners in this area will be aware that aspects such as incomplete
methanol conversion represent significant financial penalties to the
industrial
formaldehyde user since methanol is the major raw material cost in the process
and as such should be used as efficiently as possible. Therefore, even a one
percent increase in methanol conversion has substantial commercial
implications for the formaldehyde producer. Accordingly, an issue such as
formaldehyde yield is also of importance as it is not only desirable to
convert
the valuable methanol but it is also necessary to convert this methanol to
formaldehyde rather than to carbon dioxide. Again, even small gains in
formaldehyde yield in the order of 0.5% or greater represent significant
financial
gain to the formaldehyde producer and may result in lower levels of carbon
dioxide emissions from the production plant.
Also of concern are the long "light-off' periods, which can occur during
the initial few days that the catalyst is introduced to the plant. The "light-
off'
period is defined, as the reaction time following introduction of the catalyst
to
the industrial reactor, required for maximum formaldehyde yield to be
attained.
Fig. 1 shows data acquired from an industrial formaldehyde synthesis plant
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using a commercially available silver catalyst. Notably, the maximum level of
formaldehyde production is not achieved until after seven days of reaction.
Therefore, the diminished formaldehyde production capacity during this latter
period represents a significant financial penalty to the commercial producer.
The reduced production capacity is not the only distressing feature of the
current generation of commercial silver catalysts; Fig. 2 displays data
regarding
the formation of formic acid by-product during a commercial methanol oxidation
process which shows that formic acid levels are concomitantly undesirably high
during the initial "light-off' period.
Consequently, there exists a commercial need to discover new silver
catalysts and related technology to alleviate the problems described above. An
invention, which allows reliable and consistent production of silver catalysts
that
either exhibit rapid reaction "light-off', and/or produce low formic acid
levels,
and/or give high formaldehyde yield and/or give long useful lifetime, would be
extremely valuable. However, before the time of this present discovery no such
insight exists.
The crystalline silver catalysts previously made have catalytic properties
which are inferior to those of the silver catalysts produced in this
invention.
This invention discloses methods that can be used to modify crystalline silver
catalysts in a manner which, reduces detrimental effects in the industrial
plant.
OBJECTS OF THE INVENTION
The primary object of the invention is to produce a silver catalyst, which
exhibits superior formaldehyde yield during methanol oxidation conditions.
Another preferred object of the invention is to achieve faster reaction light
off
during industrial plant start-up, thus minimising financial penalties
accumulated
during plant downtime. Another preferred object of the invention is to produce
a silver catalyst, which exhibits minimal production of formic acid by-product
that inhibits the ability of formaldehyde to polymerise in downstream
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applications. Yet a further preferred object of this invention is to produce a
catalyst which enhances the conversion of methanol. Yet another preferred
object of this invention is to increase the plant throughput. Still yet
another
preferred object of the invention is to provide means to achieve quality
control
on catalyst production.
Other objects and advantages of the present invention will become
apparent from the following descriptions, taken in connection with the
accompanying drawings, wherein, by way of illustration and example, an
embodiment of the present invention is disclosed.
SUMMARY OF THE INVENTION
The primary criteria by which to achieve not only enhanced
formaldehyde yield, but also increased methanol conversion and minimal by
product formation in contrast to the previous disclosure by Koei Chem Co Ltd
(JP 06 172248) has been surprisingly discovered to be the apparent bulk
density of the silver catalyst. Crystalline silver catalysts with packing
densities
< 2.5 g/mL and more preferably with packing densities < 1.8 g/mL have been
unexpectedly found to exhibit superior catalytic properties than comparable
silver catalysts with densities in excess of 3.5 g/mL as proposed by Koei Chem
Co Ltd (JP 06 172248).
A secondary criterion for optimal catalyst performance in addition to
reduced packing density has been found to be associated with the surface area
of the silver catalyst particle. Silver crystals with a BET surface area in
excess
of 200 cm2/g and more desirably in excess of 400 cm2lg have been discovered
to provide increased formaldehyde yield which is unexpected as those of
average skill in the art will know that catalyst selectivity does not
necessarily
relate to increased surface area. Similarly, the fact that we have found that
the
level of formic acid by-products produced during industrial methanol oxidation
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conditions when using catalysts of the described relatively high surface area
cannot be explained by practitioners in this area from existing knowledge.
A third criterion, in addition to the other features described above, for
enhanced catalyst performance has been discovered to be the morphology and
shape of the silver grains as identified by electron microscopy. Specifically,
silver grains of a porous morphology have been surprisingly established to
provide enhanced useful catalytic activity.
The invention also provides a method of manufacture of the catalyst of
the invention which includes the steps of:
(a) providing an electrolyte solution having a concentration of at least
10 g/1 of dissolved silver ions wherein said electrolyte solution has
(i) minimal copper ion concentration;
(ii) a pH greater than 4; and
(iii) contains a complexing agent which generates complex
silver cations in solution;
(b) subjecting the electrolyte solution to electrolysis in an electrolytic
cell having silver anodes) of at least 90% purity and having a concentration
of
copper of less than 10% and cathodes which are formed from conductive but
chemically inert material wherein said electrolysis is conducted at a
temperature of 10°C - 40°C and at a current density of greater
than 20A/m2;
and
(c) isolating crystalline silver from the electrolytic cell which has a
packing density of less than 2.5 g/ml.
The concentration of copper species in the electrolyte has been found to
be important with respect to synthesis of an active catalyst. Indeed, in
contrast
to conventional electrorefining practices employed by those of average skill
in
the art who routinely add copper ions to electrolyte solutions, it has been
discovered that it is beneficial to have a concentration of less than 0.5 g/L
and
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more preferably less than 0.1 g/L of copper in the electrolyte solution to aid
formation of the desirable low density silver structure.
In addition, it has been unexpectedly found that it is important to use an
5 anode material, which contains minimal amounts of copper impurities. In
particular, the concentration of copper in the silver anode should be less
than
10% and more preferably less than 1 % and even more preferably less than
0.1 %. Use of anode materials comprising of less than 99.9% silver has been
found to result in silver crystals of relatively high packing density, and
10 comparatively low surface area. This discovery is surprisingly with respect
to
prior art which does not indicate any requirement for silver anode materials
comprising of relatively low amounts of contaminants when synthesizing
polycrystalline silver catalysts.
The pH of the electrolyte has been unexpectedly found to be of critical
importance in preparing silver crystals with enhanced catalytic properties.
Raising the pH of the solution by addition of a base such as, but by no means
limited to, ammonium hydroxide, sodium hydroxide, potassium hydroxide,
caesium hydroxide, calcium hydroxide, magnesium hydroxide, strontium
hydroxide, barium hydroxide, sodium bicarbonate, sodium carbonate, disodium
tetraborate and organic amines, can unexpectedly produce silver catalysts of
not only preferable low packing density but also of relatively high surface
area.
The pH of the solution should preferably be at least 4 and more preferably at
least 5 and even more preferably greater than 6. This discovery is novel in
that
conventional wisdom indicates that silver should only be refined at a pH of
less
than 4.
One method for enhancing the production of silver catalysts of packing
density less than 2.5 g/mL has been discovered to be the addition of a silver
oxide material to the electrochemical bath. The silver oxide material deposits
itself over the cathode plate and without wishing to be bound by theory
appears
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to inhibit the silver grain growth in a manner which favours formation of the
desired low packing density crystals.
The formation of a complex with silver in the electrolyte solution has
been discovered to play a vital role in the production of very low density
silver
catalyst with packing densities less than 1.8 g/mL. In particular, the
presence
of a silver diammine complex ion in the electrolyte solution has been
demonstrated to exhibit a desirable effect and the concentration of this
complex
is preferably at least 1 % and more preferably at least 10% and even more
preferably at least 20% and more preferably less than 80% with respect to the
concentration of silver in the electrolyte. Alternatively, other soluble
complexes
of silver may be formed. Examples include but are by no means limited to,
aliphatic monoamines such as tert-butyl, tert-octyl, dibutyl, triethyl and
tributyl-
amine which give complexes with silver of general formulae [Ag(RNH2)2]+,
[Ag(R2NH)2]+ and [Ag(R3N)2J+ ; to aromatic monoamines of which give silver
complexes of general formula [Ag(RNH2)2]+ where R = an aromatic group ; to
aliphatic diamines such as ethylene diammine which can give silver complexes
of general formula [Ag en]+, [Ag2 en]2+ or [Ag2 en2]2+, where en = aliphatic
diamine ; to aromatic diamines and N-Heterocycles. However, the addition of
ammonia to a silver ion solution is the most preferable method due to the ease
of silver diammine formation and the low cost of the ammonia solution.
Surprisingly, the best silver catalysts in terms of low bulk density can
only be synthesized by control of a multiple of the critical variables
described
above. In particular, optimum crystals are prepared when using anode of
>99.9% silver, an electrolyte with less than 0.1 % copper present, at least 10
g/L
of dissolved silver ions which comprise of silver in the form of a complex
such
as silver diammine and a solution pH > 4.
The drawings constitute a part of this specification and include
exemplary embodiments to the invention, which may be embodied in various
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forms. It is to be understood that in some instances various aspects of the
invention may be shown exaggerated or enlarged to facilitate an understanding
of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1: Production level of formaldehyde as a function of time in a
industrial methanol oxidation facility using a crystalline silver catalyst.
FIG. 2: Concentration of formic acid as a function of time in a industrial
methanol oxidation facility using a crystalline silver catalyst.
FIG. 3: Scanning Electron Microscopy (SEM) image of silver catalyst of
0.64 g/mL bulk packing density.
FIG. 4: Scanning Electron Microscopy (SEM) image of silver catalyst of
3.70 g/mL bulk packing density
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Detailed descriptions of the preferred embodiment are provided herein.
It is to be understood, however, that the present invention may be embodied in
various forms. Therefore, specific details disclosed herein are not to be
interpreted as limiting, but rather as a basis for the claims and as a
representative basis for teaching one skilled in the art to employ the present
invention in virtually any appropriately detailed system, structure or manner.
In general, crystalline silver catalysts can be synthesized by use of either
a Balbach-Thum, Prior or Moebius electrochemical cell or modifications of each
type as known to those skilled in the art. The basic concept of the Moenius
cell
is to attach anodes of cast silver, which can be obtained from any convenient
source, to hanger bars which are in turn surrounded by a woven cloth or
polymer bag to catch slime. The cathodes are usually made of stainless steel,
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which are convenient for removal of silver crystals by scraping, thus causing
the silver crystals to collect on the bottom of the tank. Modifications to the
standard design include incorporation of catchment trays to the cell to
facilitate
silver recovery. A typical range of operating parameters would be ; (1 )
between
four and twenty cathodes, (2) a current of 100 - 500 A, (3) a cell voltage of -
1.5
to -2.8 V, (4) a temperature close to ambient, (5) a cathode current density
of
20-40 mAcrri 2.
In contrast, the Balbach-Thum cell is designed around a rectangular
trough containing either a carbon plate or stainless steel cathode on the
bottom
of the cell and a group of silver anodes suspended in a basket in the upper
portion of the cell. Again, woven cloth or polymer material may envelop the
basket to contain anode slime. Due to the increased separation of the anode
and cathode the cell voltage is normally significantly higher than the value
found in a Moebius cell. Typical cell voltages in a Balbach-Thum cell may be
from -3.5 to -5.5 V. In both cells the average silver concentration is
approximately 30-150 gL'~ and the silver nitrate electrolyte often contains
free
nitric acid and traces of copper nitrate.
Of course there have been many improvements to the fundamental
Moebius and Balbach-Thum cells. For example, Claessens et al.(US
5,100,528) describe the use of a continuous silver refining cell which
comprises
of a tank containing an electrolyte, and at least one vertical cathode disk
mounted on a rotating horizontal shaft and a means for continuously removing
silver from the rotating cathode. Prior (A. Prior, Precious Metals, 22, 163
(1998)) developed a variation on the Moebius Cell which featured not only
automation of the silver electrolysis but also removal of the anode slime and
silver crystals. The essential element of construction, which allowed
throughput
to be increased, was the creation of an anode in the form of a basket in which
was placed silver grains (instead of the conventional cast anode). As a
result,
a higher current can be applied to the system without inducing a high anodic
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current density due to the relatively high surface area of the silver grains
compared to cast anodes. The anode basket is composed of titanium and a
non-conducting plastic material and is designed in two distinct compartments.
The upper compartment contains the silver grains whereas the lower
compartment allows collection of anode slime to occur. Anode slime can be
removed by application of an appropriate suction system.
Silver crystals are scraped from the stainless steel cathodes and
subsequently collected at the bottom of the cell wherein they are removed by
use of spiral conveyor. The conveyor itself is equipped with a washing system
and subsequently transfers the washed silver crystals to a centrifugal drier
and
finally directly to the melting furnace if required.
Nevertheless, fundamentally any Moebius or Balbach-Thum cell of any
known configuration can be used to produce polycrystalline silver catalysts
disclosed in this invention. No particular limitation is placed on the
electrochemical cell to be employed in the present invention.
The electrochemical synthesis of crystalline silver by methods already
known to those skilled in the art invariably leads to the production of dense
silver crystals, that is crystals with bulk density > 3.5 g/L. It has been
surprisingly discovered that a set of novel experimental conditions are
required
to form the desired silver crystals of low packing density, < 2.5 g/mL.
Firstly, it has been unexpectedly found that the presence of impurities in
the electrolyte solution, i.e. metal ions other than silver, has a deleterious
effect
upon the silver density. In particular, those skilled in the art would be
aware of
the practice of adding copper ions to the electrolyte solution to enhance the
yield of silver metal obtained from a cell on a daily basis. In this
invention, as
described above, it has been discovered that the presence of copper ions in
the
electrolyte solution results in the production of silver crystals with
inferior
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catalytic properties. Thus, the initial preparation of the electrolyte
solution
should be performed as follows. Typically, silver metal is added to a solution
of
concentrated nitric acid to form a silver nitrate solution. This initial
solution
normally comprises of relatively high concentrations of silver ions in the
order of
5 several hundred grams (e.g. 100 - 400) of silver per litre of electrolyte.
Therefore, dilution of the initial silver solution with purified water should
then
occur to create an electrolyte with the desired final concentration of silver
ions.
Of course, alternative means can be used to synthesise an electrolyte solution
comprising of silver ions, albeit, the outlined method is simple and
economical.
10 The concentration of silver should be 10 g/L or more as when lesser
concentrations of silver are used the current stability in the cell becomes
problematic and in turn crystals of inferior catalytic properties are
produced.
Normally, the silver nitrate electrolyte solution comprises of some free
15 nitric acid, thus the initial pH is in the order of 0.1 to 2. It has been
found in this
invention that the best catalytic silver material is made when the electrolyte
pH
is higher than 4. Notably, prior art has taught practitioners to refine silver
at a
pH between 1 and 4, Consequently, our discovery that a pH in excess of 4 is
beneficial for the synthesis of optimal silver catalyst is indeed surprising.
In
essence, any basic solution, as described above, may be added to the
electrolyte solution to raise the pH from the initial value to a pH in excess
of 4.
At this stage, as referred to above, it is has been discovered that the
addition of silver oxide powder to the electrochemical cell can enhance the
production of silver crystals with the disclosed low packing density. The
silver
oxide material may comprise of silver in more than one oxidation state such as
Age, Ag~~ and Ag~~~, the identity of the silver oxide not being particularly
limited.
There exist several means by which to add the silver oxide material. Firstly,
silver oxide powder may be purchased from any commercial supplier and
simply weighed to a prescribed amount and added to the electrolyte solution.
Stirring of the electrolyte solution aids the dispersion of the silver oxide
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material, and after an appropriate settling time the cathode should now
comprise of a thin layer of well dispersed silver oxide deposit.
Alternatively,
silver oxide may be freshly made in a container which comprises of a solution
of silver ions to which a suitable quantity of base is added to raise the pH
to a
point where silver oxide begins to precipitate. This solution can then be
decanted until a silver oxide slurry remains. This slurry can be directly
added to
the electrolyte bath or initially dried in an oven at a temperature sufficient
to
remove the water content and then the resultant silver oxide powder added.
Thirdly, during the pH addition procedure disclosed above it is possible to
generate silver oxide material due either to localised pH conditions or by
deliberate raising of the solution pH to values sufficient to precipitate
silver
oxide material. Again, it is appropriate to agitate the solution to obtain a
more
even coverage of the cathode plate with silver oxide.
To produce silver catalysts with the best properties for catalysis, i.e.
those of exceptionally low bulk packing density, e.g. < 2.0 g/mL it has been
unexpectedly discovered that it is necessary to further convert the silver
ions in
the electrolyte solution to a complex such as silver diammine.
The addition of ammonia to a solution containing Ag+ ions should result
in the formation of the linear complex (Ag(NH3)2]+ which is thermodynamically
very favorable.
(1) Ag+ (aq) + 2 NH3 (aq) ~ [Ag(NH3)2]+
The silver diammine complex can also be formed by addition of
ammonia solution to silver oxide.
(2) 3 Ag20 + 12 NH3 + 3 H20 "~ 6 [Ag(NH3)2]+ OH-
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and if the silver is initially present in the electrolyte solution as silver
nitrate then
addition of ammonia solution will convert the silver nitrate to silver
diammine
nitrate as follows:
(3) AgN03 + 2 NH3 --~ [Ag(NHs)2]+ N03
Without wishing to be bound by theory, it appears that it is the reduction
of this latter complex or other related silver complexes at the cathode which
results in the synthesis of exceptionally low density silver crystallites.
The simplest way for a practitioner to make such a silver diammine
complex is to add concentrated ammonia solution to the electrolyte described
above. The precise amount of ammonia to add can readily be calculated from
a knowledge of the silver concentration in the electrolyte and the molarity of
the
ammonia solution employed. Notably, use of ammonia to complex the silver
ions in solution has the dual advantage of also concomitantly raising the
solution pH to the disclosed value of > 4.
Naturally, it will now be obvious to those skilled in the art that other
ligands
other than ammonia can be used to complex the silver ions in solution and
these have been described previously.
The fresh electrolyte material is now ready for transfer to the
electrochemical cell, the identity of which is not particularly limited in
this
invention. One important aspect of the cell is the composition of the anode
material. Typically, silver refiners obtain silver from either gold mines or
the
photographic industry. Consequently, the silver material which is cast into
the
required shape for the anode may comprise of gold and copper impurities.
Therefore, once electrorefining is underway the electrolyte solution becomes
contaminated with copper ions in particular and to a lesser extent gold ions.
As
already disclosed in this invention the presence of metal ions other than
silver
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in the electrolyte solution has a deleterious effect upon the formation of
silver
crystals with optimal catalytic properties.
It is now disclosed that it is beneficial to use anode material which is at
least 99.9% silver. The production of such anode compositions is best
achieved by initially electrorefining the silver stock material in any type of
electrochemical cell to purify the raw silver material to > 99.9% purity and
then
subsequently recasting this pure silver into the required anode shapes.
The identity of the cathode material is not particularly limited with the
main criteria being that the cathode material is not only conductive but also
chemically inert under the applied cell conditions. Consequently, stainless
steel
or carbon make good cathode materials.
The electrochemical cell is now full of the electrolyte composition
disclosed in this invention and comprises of a precise anode composition as
revealed in this patent application. It is now that the electrochemical cell
can be
connected to the rectifier and current supplied to initiate the
electrocrystallization of silver.
The current density is another parameter with respect to synthesis of an
active silver catalyst. In particular, the value of the current density should
be
greater than 30 A/m2 to enhance the yield of silver metal obtained.
The temperature of the electrolyte also appears to have an effect upon
the silver catalytic properties. In particular, temperatures are best
maintained in
the range 10 to 40°C . Placement of a heating/cooling coil in the
electrochemical bath provides a simple means of controlling the bath
temperature, and if desired a stirrer can also be located in the electrolyte
solution to circulate the fluid and maintain a more even temperature profile
within the solution.
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The time allowed for electrorefining of silver has also been found to be
critical with respect to obtaining silver crystals of the desired low packing
density which has been disclosed in this invention. The synthesis time can be
from as little as one hour to over one hundred hours if so desired, before the
current is switched off and the silver crystals removed from the
electrochemical
cell. In general, the longer the time of electrorefining is the greater the
amount
of silver crystal obtained. The only practical limitation is the capacity of
the cell
to hold silver crystal. However, it has been discovered that the run time
should
not exceed such a period where it is found that the silver crystal density has
increased beyond the point where the values are not optimal for catalytic
performance.
Once the electrochemical cell is turned off, the silver crystals should be
removed with a scraper and then comprehensively washed with purified water.
Finally, the silver crystals should be dried in an oven at a temperature of
>80°C.
The packing density of the crystals can then be measured, wherein the
packing density is defined as the mass of silver crystals per unit volume. For
example, a simple procedure known to those of average skill in the art would
be
to weigh a known mass of silver crystals (e.g. 100 g) and then to pour this
amount of silver into a measuring cylinder which comprises of calibrated
markings which allow the volume to be calculated. To obtain an accurate value
for the packing density it is usually necessary to tap the measuring container
to
ensure that optimum packing of the crystals occurs. Practically, practitioners
in
this area would be aware that the point of optimum packing can easily be
determined by observation of the changes in the volume of the material
recorded as a function of increased tapping. In particular, when increased
periods of tapping do not result in further reduction in the volume of
catalyst
measured then the point of optimum packing has been reached.
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Once formed the silver catalysts are ready for placement in an industrial
reactor. Conventionally, the silver crystals are placed in distinct layers of
prescribed grain sizes on top of a copper gauze which is itself located on a
base plate which provides mechanical support for the weight of the catalyst
5 bed.
While the invention has been described in connection with a preferred
embodiment, it is not intended to limit the scope of the invention to the
particular form set forth, but on the contrary, it is intended to cover such
10 alternatives, modifications, and equivalents as may be included within the
spirit
and scope of the invention as defined by the appended claims.
EXAMPLES
The first series of examples will address the electrochemical processing
15 of polycrystalline silver catalyst to produce crystals of the combined
attributes of
low packing density and high surface area as disclosed in this invention.
EXAMPLE 1
A silver catalyst was prepared by the following procedure. A
20 conventional Balbach-Thum electrochemical refining cell comprising of ca.
250
kg of an anode composed of a silver dore material obtained from a goldfield
and a cathode made of stainless steel was used to synthesize silver catalyst
crystals. The area of the cathode employed was ca. 1.5 m2. The basic
procedure was to operate the electrochemical cell over a time frame of 24
hours wherein at the end of that 24-hour period the silver catalyst crystals
were
removed from the cathode surface. Once the silver crystals were collected they
were then thoroughly washed with deionized water and then separated into
distinct particle sizes, by means of pouring into a series of meshes of well
defined aperture dimensions. Table 2 shows the parameters used to
manufacture the silver catalysts.
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Significantly, the electrolyte used for this synthesis experiment
comprised of only 0:16 g/L copper. The packing densities for the resultant
silver crystal mesh fractions are displayed in Table 3.
COMPARATIVE EXAMPLE 1
By similar methods to those described in example 1, silver crystals were
produced by electrochemical techniques. However, in this instance copper
nitrate crystals were added to the electrolyte solution to give a copper
concentration of 21.37 g/L, thus turning the electrolyte solution a deep blue
color. The conditions in this experiment would be recognizable to those of
average skill in the art as being characteristic for conventional refining of
silver
metal which is currently practiced industrially (Table 2).
Notably, the packing densities for the silver catalyst mesh fractions in
this example were all in excess of 2.5 g/L (Table 3). Consequently, this
catalyst
is not claimed as part of this invention.
EXAMPLE 2
A silver catalyst was prepared by the following procedure. A
conventional Balbach-Thum electrochemical refining cell comprising a cathode
made of stainless steel was used to synthesize silver catalyst crystals. The
area of the cathode employed was ca. 1.54 m2. The anode material was a
silver dote material which comprised of between 7 and 15% gold and 200 to
1000 ppm copper (0.02 to 0.1 %). An anode cloth was used to trap the gold
impurity, however, this material did not prevent passage of copper into the
electrolyte solution.
The basic procedure was to operate the electrochemical cell over a time
frame of three days wherein at the end of each 24-hour period the silver
catalyst crystals were removed from the cathode surface and the process
restarted. Table 4 shows the parameters used to manufacture the silver
catalysts as recorded each day of the experiment. Once the silver crystals
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were collected they were then thoroughly washed with deionized water and
then separated into distinct particle sizes, by means of a pouring into a
series of
meshes of well-defined aperture dimensions.
Finally, the various mesh sizes acquired on differing days of production
were characterized in terms of the silver packing density and this data is
recorded in Table 5. The main observation is that the packing density of the
silver crystals steadily increased as a function of the electrolyte age.
COMPARATIVE EXAMPLE 2
A silver catalyst was prepared by the following procedure. A
conventional Balbach-Thum electrochemical refining cell comprising a cathode
made of stainless steel was used to synthesize silver catalyst crystals. The
area of the cathode employed was ca. 1.54 m2. In contrast to the anode in
example 2, the anode material was a silver sponge material which comprised of
between 1 and 2% gold and up to 1 % copper. As in example 2, an anode cloth
trapped the gold mud but allowed the copper to pass through into the
electrolyte solution.
The basic procedure was to operate the electrochemical cell over a time
frame of three days wherein at the end of each 24-hour period the silver
catalyst crystals were removed from the cathode surface and the process
restarted. Table 4 shows the parameters used to manufacture the silver
catalysts as recorded each day of the experiment. Once the silver crystals
were collected they were then thoroughly washed with deionized water and
then separated into distinct particle sizes, by means of a pouring into a
series of
meshes of well-defined aperture dimensions.
A visual aspect of the silver crystal production in this example showed
that the electrolyte solution increasingly became a blue color characteristic
of
Cu2+ ions as the experimental time extended.
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Finally, the various mesh sizes acquired on differing days of production
were characterized in terms of the silver packing density and this data is
recorded in Table 5. The main observation is that the packing density of the
silver crystals made increased as a function of the electrolyte age.
Inspection of the calculated packing densities for the silver crystals
produced (Table 5) reveals that the silver crystals produced when using an
anode of relatively high copper content densified at a faster rate than those
crystals made using an anode of comparatively low copper content. Therefore,
in accord with this invention is best to use anode materials which are
substantially free of copper impurities if low density silver crystal is to be
produced.
EXAMPLE 3
A silver catalyst was prepared by the following procedure. A
conventional Balbach-Thum electrochemical refining cell comprising of ca. 250
kg of an anode composed of purified silver material of purity in excess of
99.9%
and a cathode made of stainless steel was used to synthesize silver catalyst
crystals. The area of the cathode employed was ca. 1.5 m2. Importantly, to
the electrolyte which was initially at a pH of ca. 1, was added sodium
hydroxide
solution until the pH of the electrolyte attained a value of 4.3. Also,
significantly, the electrolyte used for this synthesis experiment comprised of
less than 100 ppm copper species.
The basic procedure was to operate the electrochemical cell over a time
frame of 24 hours wherein at the end of that 24-hour period the silver
catalyst
crystals were removed from the cathode surface. Once the silver crystals were
collected they were then thoroughly washed with deionized water and then
separated into distinct particle sizes, by means of pouring into a series of
meshes of well-defined aperture dimensions. Table 6 shows the parameters
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used to manufacture the silver catalysts. The packing densities for the
resultant silver crystal mesh fractions are displayed in Table 7.
COMPARATIVE EXAMPLE 3
A similar experiment to that described in example 3 was performed with
the same cell conditions apart from the fact that the pH of the electrolyte
was
not adjusted by addition of a basic solution. Thus, the electrolyte pH was
1.95
instead of the value of 4.3 used in example 3 (Table 6). Importantly, the
packing densities measured for the silver crystals in this example relative to
the
crystals formed in example 3, were slightly higher (Table 7). Therefore, the
benefit of employing an electrolyte pH in excess of 4 with regards to the
synthesis of silver crystals of relatively low packing density is disclosed
here.
Moreover, comparison of the silver crystal packing densities for the
catalyst made in this example with those crystals manufactured in example 1,
example 2 and comparative example 2, where impure anode materials were
employed reveals that the employment of anode materials of > 99.9% purity
does indeed result in the production of silver crystals of comparatively low
density.
EXAMPLE 4
Silver crystals were synthesized in an electrochemical cell in the same
manner as described in example 3, except that the pH was raised to an initial
value of 5.12 by addition of sodium carbonate to the electrolyte instead of
sodium hydroxide. Table 8 illustrates the packing density of the silver
crystals
obtained. Significantly, all the packing densities recorded are lower than the
value of 2.5 g/mL discovered in this invention to be important with relevance
to
obtaining silver catalysts of good catalytic properties.
Notably, this example indicates that the pH of the electrolyte solution can
be raised by a variety of basic chemicals.
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EXAMPLE 5
A silver catalyst was prepared by the following procedure. A
conventional Balbach-Thum electrochemical refining cell comprising of ca. 250
kg of an anode composed of purified silver material of purity in excess of
99.9%
5 and a cathode made of stainless steel was used to synthesize silver catalyst
crystals. The area of the cathode employed was ca. 1.5 m2. To the electrolyte
which was initially at a pH of ca. 1, was added sodium hydroxide solution
until
the pH of the electrolyte attained a value of 4. Also, significantly, the
electrolyte
used for this synthesis experiment comprised of no copper species.
In this example, silver oxide powder was added to the electrolyte
solution before commencement of the electrorefining process. Silver oxide was
prepared by the addition of a solution of sodium hydroxide to an aqueous
solution comprising of 50 g/L silver ions. Raising the pH to a value in excess
of
5 was sufficiently high to promote the precipitation of brown/black silver
oxide
material. After settling for a period of several hours the aqueous solution
was
decanted and the resultant silver oxide slurry allowed to dry at 150
°C.
To the Balbach-Thum electrochemical cell was added a measured
quantity of silver oxide material, in this case 6000 g, and subsequently the
solution was stirred to enhance the dispersion of the silver oxide material.
Upon settling, a uniform deposit of silver oxide material was observed over
the
entire surface of the cathode plate.
As before, the basic procedure was to operate the electrochemical cell
over a time frame of 24 hours wherein at the end of that 24-hour period the
silver catalyst crystals were removed from the cathode surface. Once the
silver
crystals were collected they were then thoroughly washed with deionized water
and then separated into distinct particle sizes, by means of a pouring into a
series of meshes of well-defined aperture dimensions. Table 9 shows the
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parameters used to manufacture the silver catalysts. The packing densities for
the resultant silver crystal mesh fractions are displayed in Table 10.
COMPARATIVE EXAMPLE 5
A similar experiment to that performed in example 5 was performed
except that no silver oxide material was added to the electrochemical bath
(Table 9). The product silver crystals were analyzed in terms of their packing
densities and the results are shown in Table 10. Notably, the silver crystal
packing densities for the catalyst prepared in the absence of silver oxide
powder are higher than those values recorded for the silver crystals produced
in example 5.
EXAMPLE 6
An alternative method of silver crystal formation involved the conversion
of the silver ions in the electrolyte solution to a complex between ammonia
and
silver which was probably of the form [Ag(NH3)2]+. Aqueous ammonia was
carefully added to a solution comprising of ca. 50 g/L of Ag+ ions until a
point
where theoretical calculations indicated that the a significant fraction of
silver
ions had been converted to the form [Ag(NH3)2]+. This prepared solution was
then used as an electrolyte in an electrochemical cell as employed in previous
examples. The detailed cell conditions used are displayed in Table 11.
Notably, the silver crystals produced in this example were of exceptionally
low
packing density (Table 12). Therefore, it has been unexpectedly discovered
that it is beneficial to use not only pure silver anode material, but also
electrolyte solutions comprising of minimal concentrations of copper and
moreover electrolyte solutions of pH in excess of four, in conjunction with
the
complexation of the silver ions in solution, e.g. in the form of [Ag(NH3)2]+
species.
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COMPARATIVE EXAMPLE 6
An electrochemical experiment using the similar conditions to those
described in example 6 was performed with the only exception being that no
complexation of the silver ions in the electrolyte with ammonia was performed
prior to initiating the silver production (Table 11 ). Notably, Table 12
indicates
that silver crystals of a lower packing density relative to those in example 6
were made. Hence, the value of complexing the silver ions with ammonia or
similar ligand has been demonstrated.
EXAMPLE 7
An electrochemical experiment using the similar conditions to those
described in example 6 was performed with the only exception being that a
lesser degree of complexation of the silver ions in the electrolyte with
ammonia
was performed prior to initiating the silver production (Table 13). Notably,
Table 14 indicates that silver crystals of a higher packing density relative
to
those in example 6 were made. Hence, the value of complexing a greater
fraction of the silver ions with ammonia or similar ligand has been
demonstrated.
EXAMPLE 8
A silver catalyst synthesized according to the methodology described in
Example 4 was subjected to a BET surface area measurement using Krypton
as the adsorption gas. The surface area was calculated to be 878 cm2/g.
COMPARATIVE EXAMPLE 7
Silver catalysts were obtained from three commercial suppliers who
employ traditional electrochemical synthesis procedures known to those of
average skill in the art. BET surface area measurements of these crystals was
again performed using Krypton as the adsorption gas. The surface areas were
calculated to be 70, 141 and 186 cm2/g, respectively. Therefore, it can be
concluded that conventional refining techniques do not produce silver
catalysts
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of surface areas of the magnitude of those measured for the novel catalysts
described in this invention.
EXAMPLE 9
Samples of silver crystals formed using the methodology disclosed in
this invention with a packing density of 0.64 g/mL were subjected to analysis
by
Scanning Electron Microscopy (SEM) in order to investigate the shape of the
material to ascertain if there existed any surprising morphological attributes
of
the catalysts. Inspection of Figure 3 reveals that the silver grains were
characterized by a distinctive layer structure. More remarkable was the fact
that each silver grain was actually a porous network of agglomerated strands
of
the layer structure which is a structure which previously has not been
disclosed.
The term porosity in this instance is interpreted in terms of the presence of
open space in a certain area of silver crystal. To allow effective comparison
between samples this may be defined as an area of 200 microns by 200
microns as recorded in an SEM image. In the case of Figure 3 it can be seen
that in excess of 10% of the area occupied any of the silver agglomerates is
comprised of open space.
COMPARATIVE EXAMPLE 8
Silver crystals obtained from a commercial silver catalyst supplier were
analyzed by means of SEM in the same manner as outlined in example 9.
Notably, Figure 4 displays the fact that these particles of packing density
3.70
g/mL were composed of individual grains which exhibited relatively large,
angular faces. Comparison of the SEM micrograph for the silver crystals of
density 3.70 g/mL as shown in Figure 4 with the SEM micrograph for the silver
crystals of density 0.64 g/mL as depicted in Figure 3 highlights the novel
morphology of the silver catalysts disclosed in this invention. Indeed,
inspection of the porosity of the crystals in Figure 4 reveals that typically
less
than 10% of the area occupied by the crystal is open space.
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The examples which follow disclose the benefits achieved by employing
the silver catalysts of packing density < 2.5 g/ml in industrial formaldehyde
plants. In particular, the enhanced benefits of using silver crystals of low
packing density, < 1.8 g/mL, is demonstrated.
EXAMPLE 10
A silver catalyst with the relatively low packing density described in this
invention (catalyst A) was tested in a commercial formaldehyde plant with
capacity of 31.9 tonnes per day of 100% formaldehyde. The feedstock was a
mixture of air and methanol in the ratio 1.25 which also contained 5 mol%
water
as ballast. A silver catalyst with a comparatively high packing density
(catalyst
B, obtainable from Borden Chemicals Inc, USA) was also evaluated under
similar conditions in the same formaldehyde plant to ascertain the effect of
using silver crystals typified by relatively low packing densities. The
densities
of the mesh sizes employed are described in Table 15 for both types of
catalytic material.
Table 16 illustrates the industrial plant data obtained for both catalysts A
and B. Significantly, it was surprisingly found that a correlation existed
between
lower silver packing density and better catalytic activity. Firstly, the light-
off
period was decreased from 2 hours to 0.5 hours. Notably, under the same
plant conditions not only did the degree of methanol conversion increase
markedly, but also the formaldehyde yield concomitantly increased by 2.5%
which represents considerable financial benefit to the formaldehyde producer.
Importantly, the level of formic acid by-product formation was also diminished
by use of silver crystals of lower packing density. In this case the
concentration
of formic acid was reduced by 50%. Yet another benefit to the formaldehyde
producer was the ability to operate the plant at substantially higher rates
without any reduction in catalyst performance.
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Finally, it is significant to note that all the accrued benefits of using a
relatively low density silver catalyst were obtained by using only 36 kg of
catalyst A, in contrast to the 88 kg of catalyst B required. Thus, the
formaldehyde producer needs to use less silver catalyst in the reactor which
in
5 turn may lead to further financial benefits.
EXAMPLE 11
Two silver catalysts of differing packing densities were also evaluated in
an industrial formaldehyde synthesis plant wherein the catalyst temperature
10 was 670°C, instead of the lower value of 540°C illustrated in
example 10. In
this instance, the feedstock comprised of not only air and methanol but also a
substantial amount of steam, typically between 20 and 30 mol%, to moderate
the reactor temperature. The densities of the mesh sizes employed are
described in Table 17 for both types of catalytic material.
Table 18 illustrates the industrial performance data obtained for both
catalysts C and D. Significantly, it was again surprisingly found that even
under
high temperature industrial plant conditions, a correlation existed between
lower silver packing density and better catalytic activity. Notably, under the
same plant conditions not only did the degree of methanol conversion increase
markedly, but also the formaldehyde yield concomitantly increased by 4%
which represents considerable financial benefit to the formaldehyde producer.
Importantly, the level of formic acid by-product formation was also diminished
by use of silver crystals of lower packing density. In this case the
concentration
of formic acid was reduced by 51 %. Yet another benefit to the formaldehyde
producer was the ability to operate the plant at substantially higher rates
without any reduction in catalyst performance.
Finally, it is significant to note that all the accrued benefits of using a
relatively low density silver catalyst were obtained by using only 33 kg of
catalyst A, in contrast to the 50 kg of catalyst B required. Thus, the
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formaldehyde producer needs to use less silver catalyst in the reactor which
in
turn may lead to further financial benefits.
Another aspect of the commercial value for the silver catalysts of
comparatively low packing density described in this invention is the ability
of
these latter catalysts to produce larger quantities of formaldehyde per kg of
catalyst used in the catalytic reactor (Table 19).
Yet again, the surprising benefits of using silver catalysts of relatively low
packing density, comparatively large surface area and novel shape and
morphology, as discovered in this invention are illustrated.
In summary, examples 1 to 10 have demonstrated that silver catalyst
prepared according to the novel processes described in this invention does
indeed exhibit superior formaldehyde yield during methanol oxidation
conditions
relative to catalysts typified by packing densities in excess of 2.5g/mL.
Additionally, faster reaction light off during industrial plant start-up has
been
observed while reduce formation of formic acid has been recorded.
Simultaneously, the silver catalyst of this invention enhanced the conversion
of
methanol and resultantly increased the plant throughput.
Importantly, the discovery that the packing density of the silver catalyst is
significant with respect to achieving good plant performance allows a means to
monitor quality control on catalyst production.
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Period FormaldehydeFormicMethanol' ProductCatalystHZCO Methanol'.HzCO
Content Acid ContentYield Lifetime' SelectivityConversionYield
'
(!9) COnfent(!) 'ltonlday)c~iaY){!)' (!o) ~!)
cnp>
r
Jun-Ju19837.27 239 3.78 67.57 28 89.7 92.14 82.68
Jul-Aug37.29 347 4.15 69.33 22 87.6 91.62 80.27
Aug-Sept37.28 277 4.22 65.55 27 87.4 91.51 79.98
Oct-Nov37.23 253 4.15 67.27 46 90.0 91.40 82.30
Nov-Dec37.34 189 3.86 69.93 34 91.1 91.88 83.75
Feb-Mar37.39 305 3.38 65.20 30 85.8 93.22 79.96
99
Mar-Apr37.34 419 3.83 61.12 21 87.3 92.25 80.58
May-Jun37.18 320 3.56 66.00 40 89.5 92.56 82.86
TABLE 1
Example Comparative Example
'1 1.
Cell Current (A) 150 150
Cathode Area (m') '1.5 1.5
Current Density (A/m') 100 100
Anode Material Ag dore Ag dore
Electrolyte pH 1.01 1.01
Silver Concentration (g/L)75 100
Copper Concentration (g/L)0.16 21.37
TABLE 2
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Example..Number' Sieve Fraction Pawrking Density'
(gym L)
Example 1 2.80 to 2.36 1.44
mm
2.36 to 1.0 1.60
mm
1.00 to 0.85 2.31
mm
Comparative Example 2.80 to 2.36 3.61
1 mm
2.36 to 1.0 4.06
mm
1.00 to 0.85 5.01
mm
TABLE 3
Example Comparative Example.2
2
Cell Current (A) 150 150
Cathode Area (m') 1.5 1.5
Current Density (A/m') 100 100
Anode Material Ag dore Sponge Ag
Electrolyte pH 1.01 1.01
Silver Concentration (g/L) 75 100
Initial Copper Concentration 0 0
(g/L)
TABLE 4
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Example Number Sieve FractionPacking Packing Packing
Density Density Density
on Day on Day on Day
1 2 3
(J/mL)' (glmL)' ~9/mL)'
Example 2 2.80 to 2.36 0.641 0.794 1.082
mm
2.36 to 1.0 1.087 1.370 1.980
mm
Comparative Example2.80 to 2.36 1.905 2.128 2.058
2 mm
2.36 to 1.0 2.469 2.778 3.175
mm
TABLE 5
Example Comparative Example
3 3
Cell Current (A) 50 50
Cathode Area (m ) 1.5 1.5
Current Density (A/mz) 33.3 33.3
Anode Material Purified Purified Ag
Ag
Electrolyte pH 4.3 1.95
Silver Concentration (g/L)50 50
Copper Concentration (g~L)0 0
TABLE 6
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Example-Number Sieve Fraction Packing. Density
(glmL)
Example 3 2.00 to 1.4 mm 1.39
1.4to1.Omm 1.22
1.00 to 0.50 1.41
mm
0.5 to 0.25 mm 1.95
Comparative Example 2.00 to 1.4 mm 1.57
3
1.4 to 1.0 mm 1.72
1.00 to 0.50 1.90
mm
0.5 to 0.25 mm 1.66
TABLE 7
Exampie Number Sieve Fraction Packing Density
(glmL)
Example 4 2.00 to 1.4 mm 1.41
1.4 to 1.0 mm 1.53
1.00 to 0.50 mm 2.50
0.5 to 0.25 mm 2.28
TABLE 8
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Example Comparatiue Exampte'5
5
Cell Current (A) 50 50
Cathode Area (m') 1.5 1.5
Current Density (A/m') 33.3 33.3
Anode Material Purified Purified Ag
Ag
Electrolyte pH 4 4
Silver Concentration (g/L) 50 50
Copper Concentration (g/L) 0 0
Amount of Silver Oxide Added6000 0
(g)
TABLE 9
Example Number Sieve Fraction Packing Density
~glmL)-
Example 5 2.00 to 1.4 mm 1.18
1.4 to 1.0 mm 1.07
1.00 to 0.50 1.24
mm
0.5to0.25mm 1.61
Comparative Example 2.00 to 1.4 mm 1.47
1.4 to 1.0 mm 1.55
1.00 to 0.50 2.01
mm
0.5 to 0.25 mm 2.85
TABLE 10
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Example' Comparative Example
6 6
Cell Current (A) 100 50
Cathode Area (m') 1.5 1.5
Current Density (A/m') 66.6 33.3
Anode Material Purified Purified Ag
Ag
Electrolyte pH 4.5 4
Silver Concentration (g/L) 50 50
Copper Concentration (g/L) 0 0
Amount of Silver Oxide Added0 0
(g)
of Ag converted to [Ag(NH3)2]t73.9 0
TABLE 11
Example Number Sieve Fraction Packing Density
~9~mL)
Example 6 2.0O to 1.4 1.03
mm
1.4 to 1.0 mm 1.05
1.00 to 0.50 1.37
mm
0.5 to 0.25 1.85
mm
Comparative Example 6 2.00 to 1.4 1.47
mm
1.4 to 1.0 mm 1.55
1.00 to 0.50 2.01
mm
0.5 to 0.25 2.85
mm
TABLE 12
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Example 7
Cell Current (A) 100
Cathode Area (m') 1.5
Current Density (A/m') 66.6
Anode Material Purified Ag
Electrolyte pH 6.4
Silver Concentration (g/L) 50
Copper Concentration (g/L) 0
Amount of Silver Oxide Added0
(g)
of Ag converted to [Ag(NH3)2]T25.7
TABLE 13
Example Number Sieve'Praction Packing Density
~glm L)
Example 7 2.00 to 1.4 1.63
mm
1.4 to 1.0 mm 1.73
1.00 to 0.50 1.99
mm
TABLE 14
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':.: ::::':' ::.::::::::::"::::::: :::::::: ':: ::::::::~
".:::::!!::'::::i:::::v:v::v::a'::::::::::.:::::::::::i::::::::f::a.::::::.::::
:::.:::':::::::.'::::::::::::~
:'.:;::::::::: _:: : ~'
. :: :::: i: :: :::::'.:::::: :.: ::::::~ ::: :::
' ~ ~ ~. ' ~ :: ' : : .: ... : ~~~ ~ ' : : .
:::::!:::::. : .. '.~ .~ W ~' ::~r : . :
..........................................................:' : .::::::::::::::
': .::::
.:._: .: ::::::::::::::::::::::::::::::: .::::::::::::::::::
:::::::::::::::::::::::::::::::::::::::.:::::::::::: :.........
.....................
:.:.:....:.. :.::::::. :::::::::.........
...............................................................................
.......:.:::::
:.: ::::::::::::::::::::.
:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
::::::::::::::~~:::::::::::::::::::::::::::::;::::::::::
.._:_:..'_'.......'~:,~~~~:::::::::::::':::::::::::::::::::::r:::::v
~::::::: : : :: ~:afa~. ~~.8 i:.:::..;
:.;...:':::::...:::;..:..'.;<::::::.::::::::::..::.:::'~'~~xal s~'A...'.
...:...............:...
..................................................................
::............
. :...... :.....:.......:................. :......
... ........................:.:... :..
.... :
; : :,: : :1~: "::;.;::.:::: lL:::::
<:..: ,.,;.:: ' ~9 , ~9::,.:'..: ..~:...
....I .. ;
1.5 to 1.0 mm 1.5 2.74
1.Oto0.5mm 1.8 2.99
0.5to0.25mm 2.0
TABLE 15
;:::: ::::::' ::::::: . : :: . - : :. , ; dal:. a ~:::~.'
: : . :: ::: : ; :.::.:: alver ~at~l .5uu~r :Ca ,
.. ete~,:::::: ,:::::::,: t: ~.:::, ..
. . 5 , .
Plantt.P~ram, ~............",...", . ~. ..,:...._:,
Catalyst Temperature (K) 813 813
Catalyst Loading (kg) 36 88
Light-off period (hours) 0.5 2
Methanol Conversion (%) 79 74
Formaldehyde Yield (%) 90.5 88
Formic Acid Concentration 100 200
(ppm)
Plant Rate (% of Full Capacity)95.8 90
TABLE 16
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Mesh .fraction Packing Density Packing Density
of .Catalyst C of Catalyst D
(glmL) tglmL)
1.5 to 1.O mm 1.35 1.4
1.Oto0.5mm 1.2 1.8
0.5 to 0.25 mm 1.7 2.4
TABLE 17
5
Plant Parameter; Silver Catalyst'Silver Catalyst
C D
Catalyst Temperature (K) 943 943
Catalyst Loading (kg) 33 50
Methanol Conversion (%) 98 94
Formaldehyde Yield (%) 86 82
Formic Acid Concentration 200 390
(ppm)
Plant Rate (% of Full Capacity)90 75
TABLE 18
Tonnes of 37% formaldehyde 100 I 48
produced per kg of silver catalyst
TABLE 19
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TABLE LEGENDS
TABLE 1
Production data from a commercial formaldehyde synthesis facility
TABLE 2
Parameters used to manufacture silver crystals described in example 1 and
comparative example 1
TABLE 3
Packing densities for silver catalysts prepared in Example 1 and Comparative
Example 1, illustrating the effect of copper ions in the electrolyte solution.
TABLE 4
Parameters used to manufacture silver crystals described in example 2 and
comparative example 2
TABLE 5
Packing densities for silver catalysts prepared in Example 2 and Comparative
Example 2, illustrating the effect of purity of the silver anode employed in
the
electrochemical cell
TABLE 6
Parameters used to manufacture silver crystals described in example 3 and
comparative example 3
TABLE 7
Packing densities for silver catalysts prepared in Example 3 and Comparative
Example 3, illustrating the effect of electrolyte pH
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TABLE 8
Packing densities of silver crystals obtained in Example 4 using sodium
carbonate to raise the electrolyte pH.
TABLE 9
Parameters used to manufacture silver crystals described in example 5 and
comparative example 5
TABLE 10
Packing densities for silver catalysts prepared in Example 5 and Comparative
Example 5, illustrating the effect of silver oxide addition to the electrolyte
TABLE 11
Parameters used to manufacture silver crystals described in example 6 and
comparative example 6
TABLE 12
Packing densities for silver catalysts prepared in Example 6 and Comparative
Example 6, illustrating the effect of silver oxide addition to the electrolyte
TABLE 13
Parameters used to manufacture silver crystals described in example 7
TABLE 14
Packing densities for silver catalysts prepared in Example 7, illustrating the
effect of adding sufficient ammonia to the electrolyte solution to only
complex
25.7% of the Ag ions
TABLE 15
Packing densities for catalysts A and B which were used for methanol oxidation
in a commercial formaldehyde synthesis plant
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TABLE 16
Data obtained from the industrial oxidation of methanol to formaldehyde when
using silver catalysts of different bulk packing density
TABLE 17
Packing densities for catalysts C and D which were used for methanol oxidation
in a commercial formaldehyde synthesis plant
TABLE 18
Plant performance of silver catalyst C and D
TABLE 19
Tonnes of 37% formaldehyde produced per kg of silver catalyst in the
industrial
plant
