Gas-Phase Oxidation of Alcohols: Innovation in Industrial Technologies and Recent Developments - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 205

These processes differ both in the operating conditions and in the
catalyst used.
The oxidehydrogenation of methanol is an exothermal process and is
carried out with an excess of oxygen (air) compared with the stoichiometric
requirement. The catalyst is an Fe-Mo mixed oxide, made of Fe 2 (MoO 4 ) 3
and MoO 3 .
The process of methanol oxidehydrogenation is performed at tempera-
tures around 300 1C (the inlet temperature may be between 260 and 280 1C),
slightly above atmospheric pressure, with air, in a multitubular reactor.
A 98-99% conversion of methanol is obtained, with a yield of formaldehyde
of around 92%; a 50-60% formaldehyde solution is produced, containing
less than 1% methanol. In many cases, a small adiabatic reactor is added
downstream, to improve the methanol conversion by up to 99.8%. At the
highest methanol conversion, the yield of formaldehyde is around 94-95%.
Older processes operate at 6 vol.%methanol in air (just below the lower limit
of flammability, the upper limit is close to 60%). The drawbacks of this
process are relatively low productivity, lack of purity of formaldehyde (as it
contains relatively high amounts of formic acid), limited lifetime of the
catalyst and all the costs associated with the presence of large amounts of
inert gas, i.e. large equipment, costs for compression and large volumes of
exhaust gas.
In recent years, the productivity of many plants has been streamlined by
increasing the concentration of methanol and, at the same time, decreasing
the concentration of oxygen, achieved through recycling part of the spent
gas. Thus a methanol concentration of up to 8.2% can be reached, with a
maximum inlet oxygen concentration of 9.0%. Further decreases in oxygen
concentration or increases in methanol concentration cannot be obtained,
because the catalyst becomes over-reduced and its activity declines.
One disadvantage of operating at higher concentrations of methanol is
an increase in the hot spot temperature, which accelerates catalyst de-
activation. This phenomenon has been partly solved by diluting the
catalyst along the bed, in order to distribute the heat of the reaction better.
Newer generations of catalysts include formulations that differ along the
catalytic bed.
Better control of the bed temperature also improves the selectivity to
formaldehyde, while decreasing undesired decomposition or combustion.
In addition, it is possible to increase the methanol concentration (while
remaining outside the flammability bell) and thus increase throughput and
reduce vent emissions.
Perstorp (now Johnson Matthey) catalyst: Since the implementation
of the first Formox plant in 1959, the core of the process has been the iron-
molybdenum oxide catalyst. The traditional composition comprises an
Mo to Fe ratio that varies from 1 to 5, but is usually higher than 1.5. The Mo
excess is needed to prevent the formation of Fe-rich phases on the catalyst
surface during the reaction, due to the loss of MoO 3 (by sublimation);
however, the stoichiometric iron molybdate Fe 2 (MoO 4 ) 3 is considered to be

d n 4 r 4 n g | 3

.