new component, the first heterogeneous industrial catalyst. Today, industrial
nitrogen fixation—a ‘sledgehammer’ mimic of one of the most gentle and
fundamental reactions of nature—makes roughly the same amount of
ammonia as the sum total of all the world’s biological nitrogen fixation. The
industry now consumes 1–2% of the world’s energy output; two percent of
the world’s natural gas is used to make hydrogen for the process
41
; the world’s
largest plant in Saudi Arabia produces 3300 tonnes of ammonia day
−1
;
world-wide *140 million tonnes of synthetic ammonia are produced each
year and Haber-Bosch is still the only chemical process that uses nitrogen as a
feedstock. Over the years, skilful engineering has made many improvements
and with an efficiency of around 70%, the modern Haber-Bosch is almost at
its theoretical optimum. R&D now concentrates on increasing the capacity
and longevity of ammonia plants and improving the catalyst to operate at
lower temperatures and pressures thereby simplifying the engineering.
But there was once a more fundamental hope, because even the best
optimized ammonia plants consume vast amounts of energy by forcing
nitrogen and hydrogen to combine at searing temperatures and bone-crushing
pressure. Just a few weeks before he died, Robert and his friend Ralph
Chirnside discussed the television film The chemical dream which demonstrated contemporary advances in enzyme chemistry. The implication here
being that a catalyst might one day be found that emulates nitrogenase, the
enzyme found in nitrogen-fixing microbes such as Rhizobia. These microbes
associate themselves with bacteria in the roots of certain crops such as the
legumes and gently ‘unpick’ di-nitrogen one bond at a time to form ammonia
—and at ambient temperature and pressure. But progress in this direction has
been difficult. Even for Mittasch’s much simpler heterogeneous catalyst
perfected in 1910, it took until the 1970–80s for chemists to understand its
behaviour viz, ‘pushing’ electrons into di-nitrogen’s antibonding orbitals to
weaken the molecule’s powerful triple bond. But the natural process has
proved difficult to emulate and neither is biological fixation a paragon of
efficiency; each molecule of nitrogen requires at least 16 molecules of ATP to
make ammonia and the process ‘wastes’ a quarter of the electrons to make
hydrogen gas as a by-product, so researchers today agree that the biggest
challenge to Haber-Bosch will come from plant biology—i.e., genetic engineering—which will engineer cereal crops to associate with nitrogen-fixing
bacteria in the soil in the same way as the legumes, thereby dramatically
reducing the need for artificial fertiliser. Until that happens, it will be
‘business as usual’ for Haber-Bosch, and with today’s plants lasting for
30 years or more, the industry is unlikely to make major changes to the iconic
process in the coming decades. And by-the-way, if you fancy an ammonia
494
D. Sheppard
nitrogen fixation—a ‘sledgehammer’ mimic of one of the most gentle and
fundamental reactions of nature—makes roughly the same amount of
ammonia as the sum total of all the world’s biological nitrogen fixation. The
industry now consumes 1–2% of the world’s energy output; two percent of
the world’s natural gas is used to make hydrogen for the process
41
; the world’s
largest plant in Saudi Arabia produces 3300 tonnes of ammonia day
−1
;
world-wide *140 million tonnes of synthetic ammonia are produced each
year and Haber-Bosch is still the only chemical process that uses nitrogen as a
feedstock. Over the years, skilful engineering has made many improvements
and with an efficiency of around 70%, the modern Haber-Bosch is almost at
its theoretical optimum. R&D now concentrates on increasing the capacity
and longevity of ammonia plants and improving the catalyst to operate at
lower temperatures and pressures thereby simplifying the engineering.
But there was once a more fundamental hope, because even the best
optimized ammonia plants consume vast amounts of energy by forcing
nitrogen and hydrogen to combine at searing temperatures and bone-crushing
pressure. Just a few weeks before he died, Robert and his friend Ralph
Chirnside discussed the television film The chemical dream which demonstrated contemporary advances in enzyme chemistry. The implication here
being that a catalyst might one day be found that emulates nitrogenase, the
enzyme found in nitrogen-fixing microbes such as Rhizobia. These microbes
associate themselves with bacteria in the roots of certain crops such as the
legumes and gently ‘unpick’ di-nitrogen one bond at a time to form ammonia
—and at ambient temperature and pressure. But progress in this direction has
been difficult. Even for Mittasch’s much simpler heterogeneous catalyst
perfected in 1910, it took until the 1970–80s for chemists to understand its
behaviour viz, ‘pushing’ electrons into di-nitrogen’s antibonding orbitals to
weaken the molecule’s powerful triple bond. But the natural process has
proved difficult to emulate and neither is biological fixation a paragon of
efficiency; each molecule of nitrogen requires at least 16 molecules of ATP to
make ammonia and the process ‘wastes’ a quarter of the electrons to make
hydrogen gas as a by-product, so researchers today agree that the biggest
challenge to Haber-Bosch will come from plant biology—i.e., genetic engineering—which will engineer cereal crops to associate with nitrogen-fixing
bacteria in the soil in the same way as the legumes, thereby dramatically
reducing the need for artificial fertiliser. Until that happens, it will be
‘business as usual’ for Haber-Bosch, and with today’s plants lasting for
30 years or more, the industry is unlikely to make major changes to the iconic
process in the coming decades. And by-the-way, if you fancy an ammonia
494
D. Sheppard