_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Chemistry based on synthesis gas
at 1000°C and 30 atm. In Britain steam reforming of naphtha preceded that of
natural gas but since the development ofNorth Sea supplies the latter has become
the preferred feedstock.
There is not space to discuss the details of syn. gas production here. The first
reaction (steam reforming) is highly endothermic and involves an increase in
entropy. It is carried out at high temperatures and moderate pressures
( 700-8 30°C/15-40 atm) over a nickel catalyst supported on o:-alumina or
calcium aluminate and promoted by alkali metal oxides, which help to prevent
the deposition of carbon.
CH 4 (g) + H 2 0(g)~CO(g) + 3H 2 (g) 11H+-> = + 206 kJ mot·· 1
11S'c" = + 2l4JK- 1 mol 1
It is important tobe able to control the ratio of carbon monoxide to hydrogen. This
may be done via the 'water gas shift' equilibrium
CO(g) + H 2 0(g)~C0 2 (g) + H 2 (g) L1W' = - 41 kJ mol- 1
L1SB = - 31 JK. 1 mol- 1
This moves to the right as the tempera ture is lowered. Catalysts based on iron and
chromium oxides (Fe 3 0 4 on Cr 2 0 3 ) require temperatures of 400-500°C, leaving
2-4% CO in the reactant gas. More active catalysts (copper and zinc oxides) can
be operated at 190-2 60oC but are easily poisoned.
A summary of some of the chemicals which can be manufactured from syn. gas
is given in Fig. 12.14.
12.4.1 The Monsanto acetic acid process
Methanol is manufactured from syn. gas by passing the latter at
230-270oC/50-100atm over a copper oxide catalyst (CuO.ZnO or
CuO.ZnO.Al 2 0J (ICI Low Pressure Methanol process). (The copper oxide is
reduced to metal.)
CO(g) + 2H 2 (g)~CH,OH(g) 11H'' =- 92 kJmol- 1
When syn. gas rich in hydrogen is used ( the ratio H 2 : CO from the steam
reforming of methane is 3: 1 ), the balance is restored by adding carbon dioxide.
C0 2 (g) + 3H 2 (g)~CH,OH(g) + H 2 0(g) 11H . = - 50 kJ mol 1
Alternatively C0 2 can be introduced, along with steam, at the reforming stage.
CH 4 + C0 2 ~2CO + 2H 2 •
Methanol is now the feedstock for production of acetic acid. The reaction
CHpH + CO--+ CH 3 COOH is homogeneously catalysed by soluble rhodium
complexes. The development of the process from the laboratory bench to full
industrial production was amazingly rapid. Only three years, from 196 7 to 19 70.
were required. Very thorough studies were made of the reaction mechanism
during this time and the information thus gleaned was used to optimize
385