Process for the electrooxidation of methanol to formaldehyde and methylal

Formaldehyde and/or methylal are made by the electrooxidation of methanol in an apparatus which is divided into the anode compartment and the cathode compartment by a cationic membrane made of a fluoropolymer and having pendant fluorosulfonyl or sulfonic acid groups, which is coated on one side with platinum applied by the impregnation-reduction method, said coating serving as the anode. The cathode may be a metal layer on the opposite side of the membrane or a separate metal cathode. When the separate metal cathode is employed, the cathode compartment contains a liquid electrolyte, which is a concentrated inorganic acid, preferably phosphoric acid. Within specifically identified temperature ranges, the yield of formaldehyde and of methylal can be optimized by controlling the mole fraction or partial pressure of methanol feed.

BACKGROUND OF THE INVENTION 
This invention relates to a process for the electrooxidation of methanol to 
formaldehyde and methylal. 
Formaldehyde is an important starting material for the production of 
certain polyacetal resins. It can be produced by the partial oxidation of 
methanol with air or oxygen in the presence of a variety of catalysts and 
under a variety of conditions. In some cases, methanol and formaldehyde 
formed in the process react further to give methylal, CH.sub.2 
(OCH.sub.3).sub.2 ; furthermore, formaldehyde often is in the same process 
oxidized in part to formic acid, which may again react with methanol to 
give methyl formate. Formation of methylal is not considered to be 
particularly detrimental to the production of formaldehyde because 
methylal can be readily hydrolyzed back to methanol and formaldehyde, as 
is well known in general for acetals and ketals. This catalytic process, 
in its various forms, is currently used to produce formaldehyde 
commercially. Nevertheless, it has the drawback of producing one mole of 
water for each mole of formaldehyde formed. This requires a rather complex 
and costly procedure for removing water from the formaldehyde produced. In 
addition, residual formaldehyde still present in the waste water must be 
removed before this water can be safely discharged. 
Electrochemical oxidation of ethanol to acetaldehyde in a fuel cell 
arrangement is described in U.S. Pat. No. 4,457,809 (to Meshbesher). 
Simultaneous electrochemical reduction of a mixture of benzene and methanol 
in the presence of tetrabutylammonium perchlorate to phenol in the cathode 
compartment and oxidation to methylal in the anode compartment in a cell 
equipped with an ionic transfer membrane is reported in U.S. Pat. No. 
4,732,655 (to Morduchowitz et al.). 
Electrocatalytic oxidation of methanol to methyl formate and methylal on a 
platinized solid polymer electrolyte membrane was reported by Otsuka et 
al. in Applied Catalysis, 26(1986) 401-404. The authors describe a process 
that occurs without either a solvent or a liquid electrolyte. 
It is desirable to be able to produce formaldehyde and/or methylal in a 
selective manner and in an industrially satisfactory yield in a process 
that requires or produces a smaller amount of water than is produced in 
the conventional catalytic oxidation processes. 
SUMMARY OF THE INVENTION 
According to the present invention, there is now provided a process for the 
electrooxidation of methanol to a mixture of products comprising as the 
largest component a mixture of formaldehyde and methylal in a ratio that 
can be optimized with respect to either one, together with small amounts 
of methyl formate and any other products of oxidation or of side 
reactions, said process comprising: 
(1) providing an electrolysis apparatus comprising a hydrated membrane made 
of a fluorocarbon resin having pendant sulfonic acid groups but no pendant 
carboxylic acid groups, said membrane having a platinum layer attached to 
at least one side thereof by impregnation of the membrane with a cationic 
salt of platinum and reduction with an anionic reducing agent, and being 
placed in the apparatus so as to divide the apparatus into two 
compartments, said platinum layer serving as the anode and the compartment 
which the anode is facing being the anode compartment, the opposite side 
of the membrane facing the cathode compartment, the cathode being a metal 
layer on the opposite side of the membrane when such layer is present but 
otherwise being a separate metal cathode located in the cathode 
compartment; 
when a metal layer on the side of the membrane facing the cathode 
compartment is not present, and the separate metal electrode is not in 
direct contact with the membrane, the membrane being maintained wet with 
an electrolyte selected from the group consisting of aqueous solutions of 
at least 1M sulfuric acid and of at least about 85 weight percent 
phosphoric acid, said electrolyte also being in contact with the separate 
cathode; 
(2) introducing a gaseous stream of methanol vapor plus any diluent gas 
continuously into the anode compartment at a pressure of about 101.3-1013 
kPa and, when formaldehyde is the desired main product, at a temperature 
of about 75.degree.-125.degree. C., the mole fraction of methanol in the 
gaseous stream being about 0.005-0.02, and, when methylal is the desired 
main product, at a temperature of 25.degree.-125.degree. C., the mole 
fraction of methanol in the gaseous stream being about 0.02-1, while 
applying to the electrodes a voltage of about 0.6-1.2 V relative to a 
reference hydrogen electrode; and 
(3) recovering from the anode compartment the desired product mixture.

DETAILED DESCRIPTION OF THE INVENTION 
As shown by Otsuka et al., supra, the two-electron methanol oxidation 
process leading to formaldehyde and the four-electron reaction leading to 
formic acid can be represented by the following equations (1) and (2): 
EQU CH.sub.3 OH.fwdarw.HCHO+2H.sup.+ +2e.sup.- (1) 
EQU HCHO+H.sub.2 O.fwdarw.HCOOH+2H.sup.+ +2e.sup.- (2) 
The protons formed at the anode are transported through the membrane into 
the cathode compartment, where they may combine to form molecular 
hydrogen, as shown in equation (3). 
EQU 2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 (3) 
Formaldehyde and/or formic acid, which are generated electrochemically, may 
be in the acidic environment of the membrane react further with methanol 
to form either methylal or methyl formate. These are reversible reactions 
illustrated by equations (4) and (5), below: 
EQU HCHO+2CH.sub.3 OH.revreaction.CH.sub.2 (OCH.sub.3).sub.2 +H.sub.2 O(4) 
EQU HCOOH+CH.sub.3 OH.revreaction.HCOOCH.sub.3 +H.sub.2 O (5) 
The process conditions can be adjusted so as to obtain high selectivity of 
product formation. However, under the conditions of the present process, 
formation of methyl formate will be at most a minor side reaction. 
According to the process of this invention, the anodic platinum coating is 
attached to the membrane by the impregnation-reduction method, sometimes 
referred to herein as the I-R method, which is described in U.S. Pat. No. 
4,959,132 to Fedkiw. The I-R method is a two-step chemical process which 
produces a platinum deposit predominantly within the membrane and can be 
confined to within about 0.1 .mu.m from the surface. The electroactive 
platinum is adjacent to the sulfonic acid sites on the membrane. 
In the first step, the membrane surface is contacted for a controlled time 
(about 20-60 minutes) with a cationic salt solution of the metal to be 
deposited (e.g., Pt[(NH.sub.3).sub.4 ]Cl.sub.2). In the second step, which 
follows immediately the metal salt impregnation, the same membrane surface 
is exposed to an anionic reducing agent solution (e.g., aqueous, about 1 
mM sodium borohydride at a pH of about 13). This treatment reduces Pt (II) 
to Pt(0). 
The prior art also reports another process for the platinization of ionic 
membranes known as the diffusion-reduction method, described by Takenaka 
and Torikai in Japanese Patent 55/38934 (1980), sometimes referred to 
herein as the T--T method. It has been found, however, that membranes 
platinized by the T--T method are not well suited for the production of 
formaldehyde, although under proper conditions they can be satisfactorily 
used for the oxidation of methanol to methylal. Using a membrane 
platinized by the I-R method permits good process control to produce 
either formaldehyde or methylal with a good degree of selectivity. 
While protons adsorbed on the cathode may combine to molecular hydrogen, 
which can be allowed to escape as such, this may not be a desirable 
alternative. It is energetically more attractive to electrochemically 
reduce oxygen with the protons at the cathode with formation of water, as 
shown in equation (6), below, 
EQU O.sub.2 +4H.sup.+ +4e.sup.- .fwdarw.2H.sub.2 O (6). 
Water formed in this reaction is separated from the product stream by the 
membrane barrier. While evaporative water loss from the hydrated membrane 
into the product stream will occur to some extent, the water content there 
is lower than would be the case if the entire water produced entered the 
product stream; consequently, water separation energy costs are lowered by 
comparison with conventional processes and waste generation is minimized. 
Alternatively, hydrogen produced in the cathode compartment can be used as 
a reactant in a parallel process, e.g., for the hydrogenation of an alkene 
to an alkane, as described by Fedkiw et al. in J. Electrochem. Soc., 137, 
1451 (1990). 
The process of the present invention, as can be seen, can be run within a 
rather broad range of gas pressures, from atmospheric to superatmospheric. 
The recited range is believed to be the most practical from the standpoint 
of economics, although it is not considered critical. In fact, these 
electrooxidation reactions would most likely be carried out in practice at 
atmospheric pressure (101.3 kPa). When these reactions are run at 
atmospheric pressure, the corresponding partial pressures of methanol will 
be 507-2026 Pa when formaldehyde is the desired main product and 
2.03-101.3 kPa, when methylal is the desired main product. As can be seen, 
a diluent or carrier gas will always be employed when formaldehyde is the 
desired product but not necessarily when methylal is the desired product. 
In fact, in the latter case it may be preferred to operate without a 
diluent gas. When such gas is employed, it normally is helium, although 
another inert gas, e.g., argon or nitrogen, could be used instead. 
An electrolysis apparatus that can be used in one embodiment of the process 
of this invention is schematically represented in FIG. 1. Here, the 
membrane side facing the cathode compartment does not carry a metal layer; 
a separate electrode not in contact with the membrane is used; and liquid 
electrolyte in the cathode compartment assures ion transport between the 
membrane and the separate electrode. In this drawing, 1 is the housing; 2 
is the membrane, which has a platinized, anode-side A and a platinum-free 
side B; 3 is a platinum cathode. The apparatus is divided into the anode 
compartment AA and the cathode compartment CC. Hydrogen reference 
electrode 4 and nitrogen gas purging tube 5 are placed in the cathode 
compartment. The cathode compartment rests on a polytetrafluoroethylene 
plate, 6, from which it is sealed by means of an O-ring seal, 7. Catholyte 
(sulfuric or phosphoric acid) in the cathode compartment reaches level L. 
Opening 6A in plate 6 is closed by means of an assembly of membrane 2, 
carbon fiber cloth 8, and platinum wire gauze 9, with 8 and 9 serving as 
the anode current collector. This assembly is thus sandwiched between 
plate 6 and a second plate, 10, substantially parallel to plate 6 and is 
sealed from plate 10 by means of an O-ring seal 11. Although the drawing 
represents the membrane assembly parts as being separate from one another 
and separate from both plates, this representation is only for the purpose 
of better understanding of the construction of the apparatus; in fact, 
both plates and the assembly between them are held tightly together by 
means of adjustment bolts 12. Plate 10 has two holes drilled in it, 13, 
ending with a pipe 13A, which delivers to the anode compartment AA a 
stream of methanol vapor-carrying helium gas, and 14, through which the 
anodic reaction products are carried away. Methanol vapor impinges upon 
membrane 2, where the electrochemical oxidation of methanol takes place. 
Formaldehyde, which is a gas boiling at atmospheric pressure at -19.degree. 
C., can be separated from methylal, which is a liquid boiling at 
atmospheric pressure at about 42.degree.-43.degree. C., and recovered from 
the anodic gas stream in any convenient manner, including the distillation 
process employing a partial condenser described in U.S. Pat. No. 3,321,527 
to Funderson et al. and the distillation process of U.S. Pat. No. 
4,962,235 to Morishita et al., where crude formaldehyde is fed to the 
middle or lower section of a column, and a polyethylene oxide dimethyl 
ether is fed to the top of the column; formaldehyde is recovered overhead, 
and a solution containing polymethylene oxide dimethyl ether, water, and 
methanol is recovered from the bottom of the column. Under the conditions 
of the Morishita patent, a major portion of methylal is expected to be 
found in the liquid phase recovered from the bottom of the column. In the 
situation where the concentration of formaldehyde in the vapor phase 
exceeds approximately 60 weight percent, the Funderson et al. method is 
more practical. When the concentration of formaldehyde is less than about 
60 weight percent, the Morishita process can be used to concentrate 
formaldehyde to at least 60 weight percent, and then the distillation is 
continued according to the Funderson et al. process. 
The cathodic reaction products can be vented through an opening in the 
housing of the cathode compartment, e.g., the hole through which cathode 3 
is inserted, or can be recovered through an exit tube, not shown. 
It is to be kept in mind that other, equally good or better cell designs 
would also be suitable for use in the process of the present invention. 
For example, FIG. 1 shows the cathode as a separate metal electrode placed 
in the cathode compartment. This can be a rod, a wire, or a foil, and 
preferably is made of platinum when oxygen reduction is to take place at 
the cathode. However, the cathode could be made of another metal, for 
example, palladium, rhodium, or nickel. When it is proposed to allow 
molecular hydrogen to form at the cathode, platinum normally would not be 
recommended; a better cathode material in such a case would be nickel. 
An alternative embodiment of an electrolytic apparatus suitable in the 
process of this invention is shown schematically in FIG. 2, where the 
cathode is a metal, preferably platinum, coating on the opposite side of 
the membrane from the anode coating. Such a second coating does not 
necessarily have to be made of platinum but may be made of a different 
metal, such as, e.g., palladium, rhodium, or nickel and does not have to 
be applied to the membrane by the same method as the anodic coating. In 
FIG. 2, M is a cationic membrane, which has a surface A' platinized by the 
above-described I-R method. Surface C" of the membrane is coated with any 
acceptable metal and by any suitable method. The T--T platinization method 
can be used here with good results. Further, the cathode can be a separate 
electrode in close contact with the membrane, for example, a wire gauze 
made of any one of the above-named metals, including nickel-plated copper 
or brass or, for that matter, of any conductive metal chemically stable in 
the cathode compartment environment or can even be a metallized plastic 
mesh. The anode compartment is designated AA' and the cathode compartment 
CC'. In this embodiment, no liquid electrolyte is needed in the cathode 
compartment. However, it may be desirable to saturate the membrane with 
acid electrolyte prior to use. Direct current is applied between both 
electrodes. Methanol vapor is introduced into the anode compartment, while 
humidified air is introduced into the cathode compartment. Hydrogen ions 
which are transported through the membrane are used to reduce oxygen on 
the cathode to water. 
The membrane itself can be a commerical cationic membrane made of a 
fluoro-or perfluoropolymer, preferably a copolymer of two or more fluoro- 
or perfluoromonomers, at least one of which has pendant sulfonic acid 
groups. The presence of carboxylic groups is not desirable because those 
groups tend to decrease the conductivity of the membrane when they are 
protonated. Various suitable resin materials are available commerically or 
can be made according to patent literature. They include fluorinated 
polymers with side chains of the type --CF.sub.2 CFRSO.sub.3 H and 
--OCF.sub.2 CF.sub.2 CF.sub.2 SO.sub.3 H, where R is a F, Cl, CF.sub.2 Cl, 
or a C.sub.1 to C.sub.10 perfluoroalkyl radical. The membrane resin may 
be, for example, a copolymer of tetrafluoroethylene with CF.sub.2 
.dbd.CFOCF.sub.2 CF(CF.sub.3)OCF.sub.2 CF.sub.2 SO.sub.3 H. Sometimes 
those resins may be in the form that has pendant --SO.sub.2 F groups, 
rather than --SO.sub.3 H groups. Such resins are easier to fabricate than 
those containing --SO.sub.3 H groups. The sulfonyl fluoride groups can be 
hydrolyzed with potassium hydroxide to --SO.sub.3 K groups, which then are 
exchanged with an acid to--SO.sub.3 H groups. Suitable cationic membranes 
are offered by E. I. du Pont de Nemours and Company, Wilmington, Del., 
under the name NAFION.RTM.. In particular, NAFION.RTM. membranes 
containing pendant sulfonic acid groups include NAFION.RTM. 117, 
NAFION.RTM. 324, and NAFION.RTM. 417 . The first one is unsupported and 
has an equivalent weight of 1100 g, equivalent weight being defined as the 
amount of resin required to neutralize one liter of a 1M sodium hydroxide 
solution. The other two types are both supported on a fluorocarbon fabric, 
the equivalent weight of NAFION.RTM. 417 also being 1100 g. NAFION.RTM. 
324 has a two-layer structure, a 125 .mu.m thick membrane having an 
equivalent weight of 1100 g, and a 25 .mu.m thick membrane having an 
equivalent weight of 1500 g. There also is offered a NAFION.RTM. 117F 
grade, which is a precursor membrane having pendant --SO.sub.2 F groups 
that can be converted to sulfonic acid groups. As is well known to those 
skilled in the art, a cationic membrane used in an electrooxidation 
reaction should be solvated or hydrated to assure sufficient conductivity. 
These resins are to some extent hygroscopic and the membranes are packed 
for shipping in a constant humidity environment. Further, the 
impregnation-reduction reaction, which is conducted in an aqueous medium, 
assures a sufficient level of hydration in the membrane, which can be, for 
example, 20 weight percent or higher. 
The heterogeneous reaction shown by equation (1), above, takes place on the 
platinized membrane surface, which is the anode. The membrane also serves 
to transport H.sup.+ ions and to separate water which may be formed on the 
cathode from the formaldehyde formed on the anode. By the proper choice of 
reaction temperature, methanol feed partial pressure, cathode electrolyte 
water content, and electrical potential driving force, it is possible with 
this particular metallized membrane morphology to selectively control the 
distribution of the products formed. Specifically, the selective 
conversion of methanol to formaldehyde and methylal can be varied 
significantly by manipulation of those variables. 
The process of the present invention illustrated in FIG. 1, which employs a 
separate metal cathode, requires a liquid electrolyte in the cathode 
compartment. For the purpose of the present invention, the most practical 
acid electrolyte is phosphoric acid having a concentration of about 85 
weight % or higher, especially 85 weight %, which is readily available 
commercially. Another useful electrolyte is sulfuric acid, preferably 
having a 3 M (about 25 weight %) concentration or higher. Dilute aqueous 
acid solutions are not recommended, i.a., because a water-rich electrolyte 
favors the formation of methyl formate, rather than of formaldehyde or 
methylal. Besides, the more concentrated the acid is, the less water needs 
to be separated from the electrooxidation products. 
The conditions employed for the electrooxidation of the present invention 
can be optimized for either desired main product as shown below. Thus, in 
a reaction run at atmospheric pressure, the conditions most favorable for 
the formation of formaldehyde are as follows: 
temperature: 75.degree.-125.degree. C. 
electrolyte: 85% phosphoric acid 
mole fraction of methanol: 0.005-0.02 
(partial pressure of methanol: 507-2026 Pa) 
voltage: 0.9-1.1 V relative to hydrogen reference electrode, hereinafter 
sometimes abbreviated to RHE. 
membrane anode Pt loading of about 0.05-0.50 mg Pt/cm.sup.2. 
For the formation at atmospheric pressure of methylal as the principal 
product, the preferred conditions are as follows: 
temperature: 75.degree.-125.degree. C. 
electrolyte: 85% phosphoric acid 
mole fraction of methanol: 0.1-0.8 
(partial pressure of methanol: 10.13-81.04 kPa) 
voltage: 0.9-1.1 V RHE 
membrane anode Pt loading of about 0.05-0.50 mg Pt/cm.sup.2. 
On the other hand, formation of methyl formate is favored by low 
temperature, for example, 0.degree.-25.degree. C.; a water-rich 
electrolyte, for example, 0.5M sulfuric acid; and a membrane platinized by 
the T--T technique, unlike the membrane platinized by the I-R method used 
in the present invention. Variants of that earlier process have been 
described in the art. 
While the temperature and mole fraction or partial pressure of methanol are 
important parameters of the process of this invention, the morphology of 
the platinized membrane surface is also considered to be critical. The 
deposit must be within the membrane, so that the water content of the 
environment of the reaction site is lowered. In the case of a membrane 
platinized by the I-R method, this is the case, while with a membrane 
platinized by the T--T method the electrode is flooded with water. 
The preferred apparatus is that illustrated in FIG. 2, where the cationic 
membrane carries a metal layer on the side opposite to the anode side. 
However, it is understood that neither the construction details of the 
apparatus nor the preferred process parameters are critical to a 
successful operation. For example, although operation at atmospheric 
pressure has been found to be most convenient in the laboratory, 
industrial operations may preferably be run at a superatmospheric 
pressure, which would permit a higher throughput and thus would result in 
a better utilization of equipment. Further, one skilled in the art would 
be able to devise a different apparatus capable of operating according to 
the above general principles or even occasionally go outside one of the 
indicated process parameters and still be able to make the desired 
electrooxidation products in a satisfactory manner. All such reasonable 
equivalents are intended to be within the scope of the present invention. 
This invention is now illustrated by the following examples of certain 
preferred embodiments thereof, where all parts, proportions, and 
percentages are by weight, unless otherwise indicated. All experiments 
were run at atmospheric pressure. 
Cell Assembly and Instrumentation 
The cell assembly, shown in FIG. 1, consists of a cathode, (3), which often 
also is called the counterelectrode, and the anode, (A), or the working 
electrode. The cathode compartment (CC) was a glass reservoir capable of 
holding 120 ml of liquid electrolyte, and the anode compartment (AA) was a 
small cylindrical hole in a polytetrafluoroethylene plate with provision 
for gas inlet and outlet. The cathode (3) was a platinum foil connected to 
a platinum wire. The cathode compartment was filled with either 0.5M 
sulfuric acid or 85% phosphoric acid. The reference electrode (4) was a 
self-contained miniature hydrogen reference electrode made according to 
the procedure of Will described in J. Electrochem Soc., 133, 454 (1986) 
filled with the same electrolyte. A platinized membrane, (2), made of a 
fluoropolymer having pendant --SO.sub.3 H groups (Du Pont Nafion.RTM. 117) 
was placed so that the Pt surface was facing the incoming methanol vapor. 
Membrane platinization was obtained either by the I-R method or by the 
T--T method. The projected surface area of the electrode, i.e., the area 
defined by the opening in the polytetrafluoroethylene plate, was 0.78 
cm.sup.2. 
Helium purged through liquid methanol at a rate of 10 ml/min so as to 
become saturated with methanol vapor served as the methanol feed. The 
partial pressure of feed methanol was controlled by controlling the 
temperature of the helium purge. Methanol partial pressures were 533, 
1067, 2133, 4000, 12932, and 54662 Pa, while the total gas pressure in the 
electrolytic cell was 101.3 kPa. The electrooxidation temperature was 
controlled at 25.degree., 50.degree., 75.degree., and 100.degree. C. by 
placing the electrolytic cell in an electric oven. 
The electrolytic cell exit gas was analyzed by gas chromatography in a 
Perkin-Elmer instrument equipped with a thermal conductivity detector. 
EXAMPLE 1 
Repeated runs at different temperatures and partial pressures of methanol 
showed that best formaldehyde selectivity was obtained at a temperature of 
100.degree. C., with a membrane platinized by the I-R technique, at either 
533 or 1067 Pa, but at the lower partial pressure the limiting current was 
lower. The overall best reaction conditions with best selectivity were: 
membrane anode platinized by the I-R technique, a temperature of 
100.degree. C., methanol partial pressure of 1067 Pa, and 85% phosphoric 
acid electrolyte. Under those conditions, the selectivity of formaldehyde 
formation was about 75 mole % at a current density of 5 mA/cm.sup.2 (998 
mV RHE), and the rate of formaldehyde formation was 0.65 
.mu.mole/min/cm.sup.2. 
By contrast, a membrane platinized by the T--T method, reaction temperature 
of 25.degree. C., and a high water content environment favored the 
formation of a four-electron oxidation product, methyl formate, rather 
than of a two-electron oxidation product, formaldehyde or methylal. 
Further, under these conditions, high partial pressure of methanol favored 
the formation of methylal over that of formaldehyde. 
Thus, at a temperature of 25.degree. C. and methanol partial pressure of 
12932 Pa, with 0.5M sulfuric acid electrolyte, the selectivity of 
formaldehyde formation with a T--T platinized membrane was about 5% at a 
current density of 10 mA/cm.sup.2 (996 mV RHE), and the rate of 
formaldehyde formation was 0.08 .mu.mole/min/cm.sup.2. 
EXAMPLE 2 
Methylal formation was favored by the use of a membrane platinized by the 
I-R method, with high temperature, high partial pressure of methanol, and 
low water content environment. The best conditions were observed at: 
temperature of 100.degree. C., partial pressure of methanol of 54662 Pa, 
and 85% phosphoric acid electrolyte. The selectivity of methylal formation 
under these conditions was 80 mole % at a current density of 25 
mA/cm.sup.2 (1.052 V RHE), and the rate of methylal formation was 3.9 
.mu.mole/min/cm.sup.2. 
Again, by contrast, when a T--T platinized membrane was used, and the 
experiments were run at room temperature and in a high water content 
environment, methylal formation selectivity was low, due to the tendency 
of forming a four-electron oxidation product. Thus, when the partial 
pressure of methanol was lowered to 12932 Pa, and the reaction was run at 
25.degree. C., using 0.5M sulfuric acid as the electrolyte, the methylal 
formation selectivity was about 15 mole % at a current density of 10 
mA/cm.sup.2 (996 mV RHE), and the methylal formation rate was 0.18 
.mu.mole/min/cm.sup.2. 
EXAMPLE 3 
Comparative 
It also was found that methyl formate formation is favored by the use of a 
membrane platinized by the T--T method, low reaction temperature, high 
partial pressure of methanol, and high water content electrolyte. At 
25.degree. C., methanol partial pressure of 12932 Pa, and 0.5M sulfuric 
acid electrolyte, the selectivity of methyl formate formation was about 75 
mole % at a current density of 15 mA/cm.sup.2 (1275 mV RHE), and its rate 
of formation was 1.6 .mu.mole/min/cm.sup.2. 
FIG. 3 is a semilogarithmic plot of product distribution, in a reaction run 
at atmospheric pressure, as mole percent of the total of formaldehyde, 
methylal, and methyl formate, as a function of partial pressure of 
methanol feed under the following conditions: 
membrane platinized by the I-R method; 
current density=4 mA/cm.sup.2 ; 
temperature=100.degree. C. 
catholyte: 85% phosphoric acid. 
It can be very well seen from this graph that under these conditions, the 
formation of either formaldehyde or methylal can be optimized by adjusting 
the partial pressure of methanol feed while at the same time minimizing 
the formation of methyl formate. The graph suggests that although the 
optimum methanol feed pressure for making formaldehyde lies at this 
temperature within the range of about 0.5-2 kPa, operation within the 
range of about 0.5-1.3 kPa produces a fair amount of methyl formate, while 
within the range of about 1.3-2 kPa, it produces less methyl formate but 
increasing amounts of methylal. From the practical standpoint, it is 
believed that operation with the methanol feed pressure within the range 
of about 1.3-2 kPa is the most practical because the yield of methyl 
formate is at its minimum, while the yield of formaldehyde is near its 
optimum, the material throughput being at the upper end of the operable 
range.