Volatile alkanes ubiquitous in human activities can rapidly acquire oxygen atoms in a free radical chain reaction, a process significant for fuel combustion and air pollution.

Alkanes are the most common class organic molecules in fuels and have a much more complex relationship with oxygen than previously thought. Researchers from KAUST, University of Helsinki, University of Science and Technology China, and other international collaborators have shown that alkanes participate extensively in autoxidation reactions with oxygen molecules1 . The discovery, which overturns current chemical wisdom, has implications for air quality prediction and efficient fuel combustion in engines.

“alkanes participate extensively in autoxidation reactions with oxygen molecules, this overturns current chemical wisdom”

The Autoxidation process

Autoxidation is a chemical process in which oxygen molecules rapidly and sequentially add to organic molecules in a radical driven chain reaction. These types of reaction mechanisms won Semenov and Hinshelwood the 1956 Nobel Prize in Chemistry. 80 years later, scientists are still developing new insights into autoxidation chemistry. The process is critical for the timing of fuel combustion in engines and is a key step in the atmospheric conversion of volatile organic molecules into air-borne aerosol particles. Improved knowledge of these types of reaction schemes can help design more efficient engines and predict the role of anthropogenic emissions on air quality and climate change.

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“Conventional knowledge suggests atmospheric autoxidation requires precursor molecules with features such as double bonds or oxygen[1]containing moieties,” says Zhandong Wang, now a professor at the University of Science and Technology of China. Alkanes – the primary component of combustion engine fuels and an important class of urban trace gases – do not have these structural features. “Alkanes were thought to have only minor susceptibility to extensive autoxidation,” Wang says.

To overturn this conventional knowledge, Sarathy, Wang and colleagues showed that alkanes do undergo extensive autoxidation under the hot high-pressure conditions of combustion in engines2 . However, the team recently found that this autoxidation process goes further than even what they had originally imagined just a few years ago. The team then set out to explore the possibility that alkane autoxidation also occurs under atmospheric conditions, which is at temperatures and pressure much lower than in engines. It was conventionally thought that such autoxidation of alkanes could not occur at atmospheric conditions because the temperatures are simply too low.

“In 2016, we collaborated with Mikael Ehn and Matti Rissanen at University of Helsinki to win a KAUST Competitive Research Grant,” says Wang. “That was the beginning of this work.”

The team used a state-of-the-art analytical technique, called chemical ionization atmospheric pressure interface time-of-flight mass spectrometry, to detect products of atmospheric alkane autoxidation. “Strikingly, the yield of highly oxygenated organic molecules containing six or more oxygen atoms was much higher than expected,” Wang says.

Air quality

Good air quality is essential for our health, quality of life and the environment. Air becomes polluted when it contains substances which can have a harmful effect on the health of people, animals and vegetation. The main causes of air pollution include transport, domestic combustion and industrial processes.

Many human activities release what are known as greenhouse gases into the atmosphere, these gases include carbon dioxide, methane and low level ozone. The overwhelming body of evidence shows that the level of these gases in the atmosphere is increasing, and that they are starting to warm the planet. In the UK this warming effect is expected to give us warmer, wetter winters and hotter summers with an increased threat of droughts.

The main greenhouse gas of concern is carbon dioxide, or CO2, which is released when we burn fossil fuels such as coal, oil and gas. National and international efforts to reduce CO2 emissions have focused on attempts to set limits on CO2 emissions, and encouragement for technologies that can reduce CO2 emissions.

However, these issues are not unrelated:

• Ozone and black carbon are often considered as ‘local’ pollutants, but are also major climate change drivers. Other local air pollutants can also affect the climate

• Many of the sources of both CO2 and local air pollution are the same, including vehicle exhausts, factory chimneys, energy and heating

Great benefits can be realised if both issues are tackled in an integrated way. However initiatives which focus just on one issue or pollutant, without regard to others, can lead to major increases in pollution.

Under combustion conditions, the team also observed alkanes that had undergone up to five sequential O2 additions, significantly higher than the three additions they observed previously. In engines, these types of reactions result in fuel autoignition, and thereby govern the efficiency of engine operation. By including these new pathways into engine simulations, engineers can discover new fuel formulations offering improved engine efficiency and reduced carbon emissions.

Industry partners

The KAUST team works closely with industry partners to put this science into real world practice. In one partnership, the team works with McLaren Racing to improve fuels for Formula 1 racing vehicles. “By working closely with cutting edge Formula 1 engine technologies, we are able to design new fuel formulations that reduce harmful emissions while also improving engine performance; essentially allowing the car to go faster and further with less fuel being burnt,” says Sarathy. “We are also developing tailor-made carbon neutral e-fuels that are made from renewable hydrogen and CO2 captured from air. The fundamental scientific knowledge gained from our work on autoxidation helps us design these renewable fuels from first principles.”

Ambient air pollution

The following are the main ambient air pollutants and their sources.

Particulate Matter (PM10 AND PM2.5)

Particulate matter (PM) is a complex pollutant as it contains a variety of components in variable concentrations. The principal source of particulate matter in European cities is road traffic emissions, particularly from diesel vehicles. It is also emitted from industrial combustion plants and public power generation, commercial and residential combustion, and some non[1]combustion processes (e.g. quarrying). Natural sources include volcanoes, dust storms and sea salt. Whilst these generally produce only a small percentage of fine particulate matter they can contribute significantly to local breaches of the regulatory limit. Levels of PM are highest in urban areas as they are a traffic-related pollutant. Secondary sources, from material originally in gaseous form have been taken up into the particulate phase and include: sulphuric acid and ammonium sulphate from oxidation of sulphur dioxide; ammonium and other nitrates derived from oxidation of nitrogen oxides; and semi-volatile organic compounds.

“particulate matter (PM) is a complex pollutant as it contains a variety of components in variable concentrations”

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Particulate matter is categorised according to its size in micrometres. PM10 refers to particles under 10 micrometres, sometimes called the ‘coarse fraction’. PM2.5 refers to particles under 2.5 micrometres, sometimes called the ‘fine fraction’. PM2.5 is thought to be more damaging to human health than PM10.

Nitrogen Dioxide (NO2)

Nitric oxide (NO) is produced during high temperature burning of fuel (e.g. road vehicles, heaters and cookers). When this mixes with air, NO2 is formed. Levels are highest in urban areas as it is a traffic-related pollutant.

Ozone (O3)

Ground level ozone is a secondary pollutant; it is formed through a chemical reaction of volatile organic compounds and nitrogen dioxide in the presence of sunlight, so levels are generally higher in the summer. The highest levels tend to be found in rural areas downwind of urban areas or industrial sites.

Sulphur Dioxide (SO2)

Fossil fuel combustion (principally power stations), conversion of wood pulp to paper, manufacture of sulphuric acid, smelting, incineration of refuse. The most common natural source is volcanoes.

Volatile Organic Compounds (VOCS)

Benzene The main source of atmospheric benzene in Europe is petrol vehicles, which accounts for about 70% of emissions. Another 10% comes from the distillation, refining and evaporation of petrol from vehicles.

Other VOCs play a role in the photochemical formation of ozone in the atmosphere.

1,3-Butadiene The main source of 1,3-Butadiene is also principally from road traffic, in the combustion process of petrol and diesel vehicles. Unlike benzene it is not a constituent of fuel but is produced through the combustion of olefins. An additional source is from industrial processes such as synthetic rubber manufacture.

Carbon Monoxide (CO)

CO forms when carbon fuels are burned, either in the presence of too little oxygen or at too high a temperature. One of the main causes is idling vehicle engines and vehicle deceleration. Smaller amounts are released into the atmosphere from organic combustion in waste incineration and power station processes. Levels are highest in urban areas due to its close association with road traffic. However, in the UK levels are generally low being well below the targets set by the Government.

Lead (PB)

As much of the airborne emission of lead originates from road traffic, concentrations have decreased with most cars running on unleaded and lead replacement petrol. Other sources of lead pollutants include waste incineration and metal processing. The largest industrial use is manufacturing batteries.

Toxic Organic Micro-Pollutants (TOMPS)

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PAHs (Polycyclic Aromatic Hydrocarbons), PCBs (Polychlorinated Biphenyls), Dioxins, Furans

Produced by the incomplete combustion of fuels, road transport and industrial plant are the largest source. Open burning is a major source in the UK and comparatively large amounts are released on and around bonfire night, whilst there is increasing concern over domestic wood burning for heating. Tobacco smoke is also a source.

The team is further expanding its reach by developing clean fuels for marine engines and aircrafts. “Our ultimate goal is to minimise the environmental impact of engines by pursuing deep scientific knowledge of combustion processes as well as the fate of emissions in the atmosphere,” says Sarathy

“research findings will allow us to better perform predictive simulations of combustion engines and atmospheric processes that impact air quality”

To this end, a unique finding was enabled by experiments performed at the Leibniz Institute for Tropospheric Research (TROPOS) by Dr. Torsten Berndt. Experiments were conducted on alkanes in the presence of nitric oxide (NO) Conventional knowledge suggested that NO prevents the autoxidation process from proceeding; however, the team found, remarkably, that alkane autoxidation can actually be enhanced in the presence of NO emissions. The retools of important implications of secondary pollutant formation in urban air quality where many chemicals interact with each other.

Summary

All these findings enrich our understanding of autoxidation processes and will allow us to better perform predictive simulations of combustion engines and atmospheric processes that impact air quality and climate.

We are now working with the department of Health, Safety and Environment in KAUST to better understand atmospheric chemical processes using real-world measurements. Using data acquired at the KAUST-based monitoring station, we are attempting to unravel complex atmospheric chemical processes in the western region of Saudi Arabia. Similar studies need to be done in various regions of the world.

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