08.08.2015   Handbook of biofuels production

Ethanol synthesis via acetate route

In this route methanol is converted into acetic acid, which can be hydrogenated to ethanol. Monsanto (1968) commercialised this process for the production of

Table 17.1 Production of ethanol via synthesis gas based routes

Typical conditions

Yields

Key step

(Mpa)

(°C)

Phase

Catalysts

Ethanol By-products wt%

Synthesis gas to ethanol

4.1­

10.1

240­

370

Gas

Cu-Zn-Co

oxides/halides

28

Methanol, higher alcohols, HCs

Synthesis gas to ethanol

6.1

250

Gas

Cu-Co oxides, alkali metal oxides

35

Methanol,

propanol,

butanol

Synthesis gas to ethanol

0.1

220

Gas

Rh(CO)12 on La2O3

49

Methanol CH4, CO2

Methanol and CO to ethanol

220

Liquid

Fe(CO)5-

Mn2(CO)10.R3N

72

CH4

Methanol and CO to ethanol

27.6­

34.5

200

Liquid

Co-Ru halides

60-80

Higher alcohols, acetates, esters

Methanol to acetaldehyde

24.1

180

Liquid

Group VIII halides

80-90

Higher alcohols, acetates, esters

Methanol to ethanol

220

Liquid

Fe(CO)5-

Co(CO)8.R3N

High

Acetic acid and H2 to ethanol

12.4

250

Liquid

Cu-Co-Mn-Mo

oxides

High

acetic acid from methanol. The hydrogenation of acetic acid is possible but has to take place at high pressures and the mixture is highly corrosive which does not make it an attractive process.

Alternatively Davy McKee has patented the conversion of acetic acid with ethanol to ethyl acetate (temperature 175°C, pressure 7 MPa), which can be hydrogenated to two ethanol molecules thus rendering a net production of ethanol (Bradley et al, 1983). The last reaction can take place at 200°C with a Cu/ZnO catalyst.

The total reaction scheme is:

CH3OH + CO ^ CH3COOH

CH3COOH + C2H5OH ^ CH3COOC2H5 + H2O

CH3COOC2H5 + H2 ^ 2 C2H5OH

The net reaction is:

CH3OH + CO + H2 ^ C2H5OH + H2O

A variation on this concept has been developed by the Halcon SD group (Porcelli and Juran, 1985). In this process methyl acetate is carbonylated instead of methanol. The resulting anhydride forms together with ethanol and methanol two different acetates. After separation the ethyl acetate is hydrogenated to ethanol and the methyl acetate is recycled to be carbonylated again.

CH3COOCH3 + CO ^ (CH3CO)2O

(CH3CO)2O+CH3OH+C2H5OH CH3COOCH3+CH3COOC2H5OH

Alternatively ethanol can be produced via ethylene which can be converted to ethanol via the existing catalytic hydrolysis of ethylene. Overall yields are however not very high and it looks more promising nowadays to produce ethylene from ethanol via sugar fermentation processes making bio-ethylene production possible rather than the other way around.1