formaldehyde - Sciencemadness Dot Org

FORMALDEHYDE PRODUCTION
believed to keep the catalyst active and displace the dehydrogenation
equilibrium to the right.
Undesirable reactions, which must be avoided by proper control if high
yields are to be obtained, include: (a) pyrolytic decomposition of formaldehyde
and (b) further oxidation of formaldehyde to formic acid, carbon
oxides., and water.
(a) CHaO -• CO 4- H2
(b) CH30 + K>3 -* HGOOH or CO + H*C
CH20 + 02 -* C02 -f H30
Decomposition of formaldehyde by heat proceeds at a slow but measurable
rate at 300°C3 increasing rapidly at temperatures above 400°C (page 000).
Formation of formaldehyde by the dehydrogenation of methanol was
studied in 1910 by Sabatier and Mailhe 33 , who showed it to be a reversible
reaction. More recently, Newton and Dodge 29 quantitatively determined
the equilibrium relations for this reaction and found that
v P CH30PHS , - T 4600 I
From this equation it is possible to calculate that, for equilibrium conditions
at atmospheric pressure, the dehydrogenation of methanol vapors to
formaldehyde and hydrogen would be about 50 per cent at 400°C, 90 per
cent at 500°C: and 99 per cent at 700°C. These yields are not readily
obtained by dehydrogenation alone, since in the absence of air this reaction
is relatively slow compared with the rate of decomposition of formaldehyde
to carbon monoxide and water.
Development of the Methanol Process, In the original method by which
Hofmann 18 first produced formaldehyde, a mixture of air and methanol
vapors obtained by drawing air through a reservoir of liquid methanol
was passed into a flask containing a hot platinum spiral. Once initiated,
the reaction was self-sustaining, as manifested by the glowing spiral. The
product was dissolved in water contained in a series of gas-washing bottles.
Because of difficulties with explosions in carrying out this procedure, Volhard
39 devised a scheme employing a spirit lamp filled with methanol and
provided with a wick contiguous to the platinum spiral. Koblukov 19
subsequently found that the platinum could be replaced by platinized
asbestos placed in a heated tube.
By heating methanol in a reservoir over a water bath, Tollens 34 was
able to regulate the air-methanol vapor ratio, which was found to have a
direct effect on the formaldehyde yield. This and minor refinements 35
were later applied to Loew's 24 procedure which replaced the platinum
catalyst by copper gauze. Loew drew a rapid current of dry air through
cold methanol (at about 1S°C) and passed the resultant mixture through a