Industrial manufacture of alcohols

Methanol — the Bosch (high-pressure) process. Synthesis gas, a mixture of carbon monoxide and hydrogen obtained by reacting steam with natural gas or coal, is compressed and passed over a zinc oxide–chromium(III) oxide catalyst. CO and H2 combine directly:

where: the numbers over the arrow are the operating pressure and temperature, and the catalyst is written under the arrow — both must be stated for full credit, even though the syllabus does not require plant-engineering detail.

Ethanol — fermentation. Starch-containing raw material (grain, molasses) is converted to ethanol by yeast in two linked steps. First, the enzyme diastase hydrolyses starch to glucose:

then the yeast enzyme zymase ferments glucose anaerobically to ethanol and carbon dioxide:

where: $n$ is the (large, variable) number of glucose units in the starch polymer, and the two enzymes act in strict sequence — diastase only breaks the polymer bonds, zymase only ferments the free sugar. The dilute aqueous product of fermentation (the "wash") is finally concentrated by fractional distillation, which — because of the ethanol–water azeotrope — can never push past roughly 95% ethanol by ordinary distillation alone.

Both routes exploit basic principles of equilibrium and biochemistry rather than exotic chemistry, and seeing why makes the memorised conditions stick.

Why high pressure for methanol. Look at the mole count in $CO + 2H_2 \rightleftharpoons CH_3OH$: three moles of gas on the left become one mole on the right. By Le Chatelier's principle, squeezing the system to higher pressure shifts the equilibrium toward the side with fewer gas moles — that is, toward methanol — so $300$–$400\ \text{atm}$ is chosen specifically to push the yield up, not merely to speed the reaction along.

Why only a moderate temperature, not a very high one. The forward reaction is exothermic, so by the same Le Chatelier reasoning a lower temperature would favour methanol at equilibrium. But at low temperature the reaction is too slow to be commercially useful, since molecules simply do not collide with enough energy often enough. The $300$–$400\,^\circ C$ operating range, together with the $ZnO/Cr_2O_3$ catalyst, is the industrial compromise: the catalyst lowers the activation energy so a usable rate is reached without needing the very high temperature that would drag the equilibrium the wrong way and destroy yield.

Why fermentation needs two enzymes in sequence, not one. Diastase and zymase are biological catalysts, and each is specific to one substrate: diastase's active site fits the glycosidic bonds of starch and cannot act on the small glucose molecule, while zymase's machinery is built to ferment simple sugars and cannot touch the starch polymer. This lock-and-key specificity is exactly why the two steps cannot be merged or reordered — the polymer must first be cut down to monomer sugar before the fermentation enzyme has anything it can act on.

A brewery ferments $100\ \text{kg}$ of glucose completely by the zymase reaction. Calculate the theoretical mass of ethanol produced, and explain why the fermenting liquid never actually reaches this ethanol concentration undiluted.

Step 1. Write the balanced equation and identify the mole ratio. $C_6H_{12}O_6 \xrightarrow{\text{zymase}} 2C_2H_5OH + 2CO_2$. One mole of glucose (molar mass $180\ \text{g/mol}$) gives two moles of ethanol (molar mass $46\ \text{g/mol}$ each).

Step 2. Convert the given mass of glucose to moles. Moles of glucose $= \dfrac{100{,}000\ \text{g}}{180\ \text{g/mol}} = 555.6\ \text{mol}$.

Step 3. Use the mole ratio to find moles, then mass, of ethanol. Moles of ethanol $= 2 \times 555.6 = 1111\ \text{mol}$. Mass of ethanol $= 1111\ \text{mol} \times 46\ \text{g/mol} = 51{,}100\ \text{g} \approx 51.1\ \text{kg}$.

Step 4. Explain the gap between theory and practice. This $51.1\ \text{kg}$ is the maximum yield if every glucose molecule reacted with no losses — but fermentation is a living process, and yeast can only survive in a solution up to about $12$–$15\%$ ethanol by volume before the alcohol itself poisons the enzyme and halts the reaction. The wash is therefore always a dilute mixture, well short of complete theoretical conversion, and must be concentrated afterwards by fractional distillation. Answer: theoretical maximum yield $\approx 51.1\ \text{kg}$ ethanol from $100\ \text{kg}$ glucose, though the actual fermenting wash stays dilute (12–15% by volume) because yeast is killed by higher ethanol concentrations.

• Confusing the Bosch process (the catalytic $CO + 2H_2 \rightarrow CH_3OH$ step) with the earlier steam-reforming step that only generates the syngas feedstock from methane or coal.
• Explaining the high pressure in the Bosch process as "just for speed" — it is chosen chiefly to shift the equilibrium toward methanol via Le Chatelier's principle, since the forward reaction reduces gas moles.
• Skipping the diastase step and writing fermentation as a single leap from starch straight to ethanol — the enzyme-catalysed hydrolysis to glucose is a separate, examinable step.
• Assuming fermentation alone produces concentrated, pure ethanol — it only ever gives a dilute aqueous wash (12–15%), and even distillation is capped near 95% by the ethanol–water azeotrope.

Ask the tutor: want to see how the same Le Chatelier reasoning applies to other industrial gas-phase processes like the Haber process for ammonia?