CA2388992A1 - Crystalline silver catalysts for methanol oxidation to formaldehyde - Google Patents

Abstract

A process for manufacture of a crystalline silver catalyst which includes th e steps of: (a) providing an electrolyte solution having a concentration of at least 10g/l of dissolved silver ions wherein said electrolyte solution has ( i) minimal copper ion concentration; (ii) a pH greater than 4; and (iii) contai ns a complexing agent which generates complex silver cations in solution; (b) subjecting the electrolyte solution to electrolysis in an electrolytic cell having silver anode(s) 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 20 A/m2; and (c) isolating crystalline silver from the electrolytic cell which has a packing density of less than 2.5 g/ml. There is also provided crystalline catalyst for efficient conversion of methanol to formaldehyde, having a packing density of less than 2.5 g/ml.

Description

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 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 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.

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, 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 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 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 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 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 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 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 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, 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 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 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 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-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 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.

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.

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.

A silver catalyst was prepared by the following procedure. A

20 conventional Balbach-Thum electrochemical refining cell comprising of ca.

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.

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.

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.

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 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.

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.

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.

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 used to manufacture the silver catalysts. The packing densities for the resultant silver crystal mesh fractions are displayed in Table 7.

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.

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.

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 parameters used to manufacture the silver catalysts. The packing densities for the resultant silver crystal mesh fractions are displayed in Table 10.

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.

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.

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.

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.

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.

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 of surface areas of the magnitude of those measured for the novel catalysts described in this invention.

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.

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.

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.

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.

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.

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 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.

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 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 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 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 Example Comparative Example.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) Example Number Sieve FractionPacking Packing Packing Density Density Density on Day on Day on Day (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 Example Comparative Example 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 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 1.4 to 1.0 mm 1.72 1.00 to 0.50 1.90 mm 0.5 to 0.25 mm 1.66 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 Example Comparatiue Exampte'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) 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 Example' Comparative Example 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 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 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 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 ':.: ::::':' ::.::::::::::"::::::: :::::::: ':: ::::::::~
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1.5 to 1.0 mm 1.5 2.74 1.Oto0.5mm 1.8 2.99 0.5to0.25mm 2.0 ;:::: ::::::' ::::::: . : :: . - : :. , ; 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 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 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 Tonnes of 37% formaldehyde 100 I 48 produced per kg of silver catalyst TABLE LEGENDS

Production data from a commercial formaldehyde synthesis facility Parameters used to manufacture silver crystals described in example 1 and comparative example 1 Packing densities for silver catalysts prepared in Example 1 and Comparative Example 1, illustrating the effect of copper ions in the electrolyte solution.

Parameters used to manufacture silver crystals described in example 2 and comparative example 2 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 Parameters used to manufacture silver crystals described in example 3 and comparative example 3 Packing densities for silver catalysts prepared in Example 3 and Comparative Example 3, illustrating the effect of electrolyte pH

Packing densities of silver crystals obtained in Example 4 using sodium carbonate to raise the electrolyte pH.

Parameters used to manufacture silver crystals described in example 5 and comparative example 5 Packing densities for silver catalysts prepared in Example 5 and Comparative Example 5, illustrating the effect of silver oxide addition to the electrolyte Parameters used to manufacture silver crystals described in example 6 and comparative example 6 Packing densities for silver catalysts prepared in Example 6 and Comparative Example 6, illustrating the effect of silver oxide addition to the electrolyte Parameters used to manufacture silver crystals described in example 7 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 Packing densities for catalysts A and B which were used for methanol oxidation in a commercial formaldehyde synthesis plant Data obtained from the industrial oxidation of methanol to formaldehyde when using silver catalysts of different bulk packing density Packing densities for catalysts C and D which were used for methanol oxidation in a commercial formaldehyde synthesis plant Plant performance of silver catalyst C and D

Tonnes of 37% formaldehyde produced per kg of silver catalyst in the industrial plant

Claims (35)

1. A process for manufacture of a crystalline silver catalyst which includes the steps of:
(a) providing an electrolyte solution having a concentration of at least 10g/l 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 anode(s) 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 2.5g/ml.

2. A process as claimed in claim 1 wherein the electrolytic solution contains less than 0.5g/l of copper.

3. A process as claimed in claim 2 wherein the electrolyte solution contains less than 0.1 g/l of copper.

4. A process as claimed in claim 1, 2 or 3 wherein the packing density of the crystalline silver is less than 1.8g/ml.

5. A process as claimed in any preceding claim wherein the pH of the electrolyte in step (a) is greater than 5.

6. A process as claimed in claim 5 wherein the pH of the electrolyte in step (i) is greater than 6.

7. A process as claimed in claim 5 or 6 wherein in step (a) a basic solution selected from 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, is added to the electrolyte solution.

8. A process as claimed in any preceding claim wherein the amount of copper in the anode(s) is less than 1%.

9. A process as claimed in claim 8 wherein the amount of copper in the anode(s) is less than 0.1%.

10. A process as claimed in any preceding claim wherein the concentration of dissolved silver ions in the electrolyte solution is between 10-200g/l

11. A process as claimed in any preceding claim wherein the complexing agent is selected from ammonia, aliphatic monamine(s), aromatic monoamine(s), aliphatic diamine(s), aromatic diamine(s) and N-heterocylic compounds.

12. A process as claimed in claim 11 wherein the complexing agent is ammonia.

13. A process as claimed in claim 11 wherein the complex silver ion formed after addition of the complexing agent is selected from silver diamine, [Ag(RNH2)2]+, [Ag(R2NH)2]+, [Ag(R3N)2]+ where R is an aliphatic group {Ag(RNH2)2]+ where R is an aromatic group, silver complexes formed from aliphatic diamines, silver complexes formed from aromatic diamines and silver complexes formed from N-heterocyclic compounds.

14. A process as claimed in claim 11, 12, or 13 wherein the concentration of silver complex ion is at least 1 mol%.

15. A process as claimed in claim 14 wherein the concentration of silver complex ion is at least 10 mol%.

16. A process as claimed in claim 14 wherein the concentration of silver complex ion is at least 20 mol%.

17. A process as claimed in claim 14 wherein the concentration of silver complex ion is less than 80 mol%.

18. A process as claimed in any preceding claim wherein the silver anode is of at least 99.9% purity.

19. A process as claimed in any preceding claim wherein the cathodes of the electrolytic cell are formed from carbon or stainless steel.

20. A process as claimed in claim 19 wherein the electrolytic cell has 1-20 cathodes.

21. A process as claimed in claim 19 wherein the current of the electrolytic cell is from 20-500A.

22. A process as claimed in claim 19 wherein the cell voltage is -3.5 to 10 V.

23. A process as claimed in claim 19 wherein the cathode current density is > 30 A m-2.

24. A process as claimed in claim 19 wherein the cell voltage is from -3.5 to -7.5 V.

25. A process as claimed in claim 19 wherein the average silver concentration is 30-150g/l.

26. A process as claimed in any preceding claim wherein in step (a) the electrolyte solution is formed by addition of silver metal to a solution of concentrated nitric acid to form a silver nitrate solution having 100-500g/l of dissolved silver ions followed by a dilution step to provide a final silver concentration of at least 10g/l.

27. A process as claimed in any preceding claim wherein silver oxide powder in step (a) is added to the electrolyte solution or generated in situ in the electrolyte solution to form a deposit on a bottom cathode.

28. A process as claimed in any preceding claim wherein after preparation of the electrolyte solution , the electrolyte solution is transferred to the electrolytic cell prior to step (b).

29. A process as claimed in any preceding claim wherein in step (c) silver crystals are scraped from the electrolytic cell and washed and dried before use.

30. Crystalline silver catalysts when prepared by the process of any preceding claim.

31. Crystalline silver catalysts for efficient conversion of methanol to formaldehyde having a packing density of less than 2.5g/ml.

32. Silver catalysts as claimed in claim 30 having a packing density of less than 1.8g/ml.

33. Silver catalysts as claimed in claim 30 or 31 having a BET surface area in excess of 200 cm2/g.

34. Silver catalysts as claimed in claim 32 having a BET surface area in excess of 400 cm2/g.

35. Silver catalysts as claimed in any one of claims 30-34 comprising grains having a porous structure.