Carbon-neutral petrol? Catalysing the future of fuels

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Researchers at the Universidade Federal do Rio de Janeiro, Brazil are taking a cross-disciplinary approach to improve methods for producing biofuels. By converting ‘biocrude’ from sugarcane into hydrocarbons, they can produce the exact same fuels used to power existing transport – and all at net-zero carbon. In their most recent work they describe the merits of conducting this reaction with catalysts mounted on nano-pore materials, which limits the amount of unwanted coke formed.

Heat waves, heavy downpours, and hurricanes are all on the rise according to the National Climate Assessment. And as the climate begins to noticeably change, so does public attitude relating to it: since 2013 the proportion of the UK public ‘expecting climate disaster’ rose from 59% to 78% in 2019 according to IPSOS Mori.

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Politics are beginning to reflect this view: almost all major carbon-emitting countries ratified the Paris Climate Agreement in 2015, and at the heart of the agreement is the aim to limit global temperatures to 1.5 °C above pre-industrial levels, through reducing net emissions as soon as possible. In the EU this has led to ‘net-zero by 2050’ becoming embedded in policy, a commitment the UK will maintain after departing the EU earlier this year. In the USA, President Biden’s recent commitment to rejoin the agreement may mean similar policies are implemented.

But with a problem so systemic, no single change will be enough for any country to meet its carbon targets. A multi-faceted carbon-saving approach is needed across the world in order to reach carbon neutrality. Nowhere is this more true than energy-hungry high-income countries, where meeting these targets will mean a rapid change in infrastructure. To cut down on carbon is, in a very real sense, to break the bond between economic progress and carbon output.

“To cut down on carbon is, in a very real sense, to break the bond between economic progress and carbon output.”

Where does the carbon come from?

Around a quarter of global energy consumption worldwide is from transport. More than half of this is caused by the movement of people, and the remainder from transporting goods. Electrification of these processes would be a major carbon-reduction, and every year the technology to do so makes this prospect seem more viable. However, replacing every car on the planet with an electric one has sizable environmental costs of its own. And whilst electrification allows more efficient production and greater control over carbon output, the amount of electricity required will mean a vast up-scaling in output from power stations around the globe.

Net-zero fuels

In this context, it’s easy to identify the part that biofuels can play in reaching net-zero targets. Yes, carbon dioxide is produced as biofuels are combusted, but the carbon has been pulled out of the air during photosynthesis, making the process inherently carbon-neutral. And whilst the last few decades have seen great progress in harnessing the power of the sun (solar power is now cheaper per megawatt than coal), biology has had billions of years to perfect this specific trade. Lignocellulosic biomass – a broad term which includes various wood materials including wood fuel, sawdust, and feedstock for making paper – is considered the most abundant renewable energy source on earth. This material is composed of carbohydrates bound to lignin, forming large networks of carbon, hydrogen and oxygen atoms.

Bark to biofuel

This energy-dense material has been a promising starting point for creating fuels for a long time. In some senses, it already is a fuel – wood fuel is used around the world to heat homes. But for a world which runs on oil, there is a lot of work to do to convert the complex chemical structure of lignocellulosic biomass into a modern fuel.

Fermentation of biomass to create bioethanol is one way to do this. Techniques to do so have been available since the mid-1900s. More recently, a process called ‘fast pyrolysis’ has become a more common technique. During pyrolysis, the biomass is heated to high temperatures for a few seconds. The crux of this process is its ability to remove heteroatoms like oxygen, and enrich the substance in carbon, making the process similar to the production of charcoal from wood. However, the resulting fuels are not particularly stable, and suffer from high acidity and oxygen content.

From biomass to fuel in two steps: The circular economy for a sustainable carbon neutral future. Shutterstock.com

Creating biocrude

In recent research performed at the Federal University of Rio de Janeiro, a team of researchers has been working on improving methods for catalytically creating hydrocarbons from biomass.

Leandro S. M. Miranda, Associate Professor of Organic Chemistry, and Marcelo Maciel Pereira, Professor of Chemistry, have developed a wealth of experience in biomass conversion using catalysis. Much of their method has been tested on a widely-grown sugar-dense plant: sugarcane. In 2014 they developed the first step of their process: the creation of a black, viscous mixture of carbon-based chemicals called a ‘biocrude’. This process involves a convenient one-pot ketalisation reaction, resulting in a biocrude with isopropyl-ketals as the main component, accounting for 40% of the weight. The overall biocrude material is an important development in biofuel technology, because – unlike biomass – biocrude has properties which allow the researchers to apply and adapt conventional refining techniques.

Cracking and hydroprocessing

One of these techniques, a standard technique in the oil industry, is catalytic cracking. Biomass is not compatible with conventional cracking catalysts due to its high reactivity and low density, but Profs Miranda and Pereira have found their biocrude is a much better candidate material.

The other technique they have developed is the use of hydrotreating processes (HDPs) – using catalysts like platinum, palladium, rhenium or ruthenium – which has been shown to create a higher yield of oil and better de-oxygenation. This process also involves using a porous, high-surface-area support material, like γ-alumina, activated carbon or a zeolite. These act as a surface for the materials in the reaction to adsorb onto and react.

To test and develop these techniques so that biocrude can be treated similarly to crude oil, the researchers used a representative molecule from the isopropyl-ketal class of molecules, called ‘DX’. DX has the chemical name 1,2:3,5-di-O-isopropylidene-α-D-xylofuranose and is composed of three rings of carbon and oxygen, each ring sharing some atoms with another ring.

In their research they are able to demonstrate how biocrude can be processed by both fluidized catalytic cracking (FCC) and hydrotreating processes (HDP). Both of these processes are important in a regular oil refinery, where they act to deoxygenate the crude, stripping the oxygen atoms from molecules like DX, resulting in useful hydrocarbon fuels.

Hydrotreating processes

In their tests of hydrotreating processes, DX was deoxygenated using palladium/acid catalysts mounted on different porous support materials. The support material is crucial to the reaction: it is the place where reactants go to react. The texture of the material – including factors like the size and number of pores, and the way they are connected together via channels – is thought play a crucial role in determining the types of products that can form.

Amongst the resulting liquid product, 25 different substances were found, ranging from small alkanes like methane, all the way up to complex polyaromatic compounds and a small amount of waste material called coke. Between the smallest and the largest molecules are the most desirable: carbon chains with over seven carbons, especially those in long chains.

Along with the removed oxygen atoms go some hydrogen or carbon atoms, resulting in off-gassing of water, carbon dioxide and carbon monoxide. Crucial to maintaining the valuable carbon products is the ability to maintain a high carbon content, so a reaction which avoids producing large amounts of carbon dioxide or monoxide is favourable.

Catalysing carbon-cutting

Of the catalysts tested, one – Pd/HZSM-5 – stood out as a winner for a couple of reasons. It resulted in the least amount of coke deposition, but more importantly it produced the largest amount of hydrocarbons with long chains and more than seven carbons.

“…four different catalysts were tested to compare their ability to strip the oxygen atoms from DX, to create useful hydrocarbon fuels.”

So what is it about the ‘ZSM-5’ support which causes this? Part of the answer may lie in the size of its pores. Unlike the other support materials tested, ZSM-5 is mesoporous, meaning that the pores are very small: between 2 and 50 nm in diameter. In large pores, large intermediate products are thought to react together and form large products, leading to coke accumulation. However, in the nano-sized pores of ZSM-5, this cannot happen as easily, limiting the productions of coke – and maximising the amount of carbon available to create useful hydrocarbons suitable for green fuels.

Under the right conditions, this support yielded an 86% conversion rate of carbon into hydrocarbon products.

Fluidised Catalytic Cracking

In the researchers’ tests of FCC, several typical catalysts were used to determine whether DX – the representative molecule of a major component of the biocrude created by these researchers – is a molecule which can be easily converted by conventional commercial processes. These tests were performed in a standard fluidised catalytic cracking reactor in their laboratory.

Researchers at the Universidade Federal do Rio de Janeiro, Brazil are improving methods for producing biofuels. FOTOGRIN/Shutterstock.com

The researchers found that DX is fully converted by conventional catalysts. This ability was demonstrated for a mixture of DX and hexane with up to 70% by weight of DX. Importantly, this process could deoxygenate DX into useful compounds like GLP, gasoline and diesel – though the majority of compounds formed were aromatics, useful for the petrochemical market.

A catalyst support effect was also seen in the FCC, where beta and ZSM-5 catalysts decreased the coke formation, and yielded 70% conversion of carbon into hydrocarbons.

However, typical process conditions and regular FCC catalysts based on a faujazite zeolite are also effective methods of converting DX and, implicitly, bio-crude. Because the conditions are the same, this unique biocrude can be refined at the same time as regular crude oil.

Powering the future

In the context of creating carbon-neutral fuels for the future, these processes are a real asset. Between HTP and FCC, the researchers can process their unique high-ketalised biocrude to produce a broad range of hydrocarbons. This provides a new two-step refining process for biomass: the first being the all-important transformation of biomass to a DX-containing biocrude; and the second, refining using typical FCC or HTP processes. This can be used to create a range of green fuels, including gasoline, diesel and jet-fuel.


Can you identify where further optimisations might be possible for catalysts like these?

The main point is DX (also bio-crude) is suitable for converting into hydrocarbons or oxygenated compounds and both catalyst and process play key roles. The catalytic development has also played a key role in the development of the oil industry in the last fifty years. Our strategy brings sugar chemistry into the refinery for the first time. As a consequence, we do expect further developments, for example with the catalyst – to increase the yield of target products. The catalytic design (through a combination of active sites, texture, morphology and catalyst preparation procedure) should be done to each type of sugar ketal. We expect this feed to open up a tremendous opportunity for heterogeneous catalysis in the oil industry.

 

References

  • Pereira SC, Souza M, Esteves LM, Batalha N, Lam YL, Pereira MM. (2021). Hydroconversion of xylose derived ketals: A key strategy for producing a broad range of green-hydrocarbons suitable as fuels and petrochemicals. Applied Catalysis A, 609.117911. https://doi.org/10.1016/j.apcata.2020.117911
  • dos Santos DN, Pedrosa IV, Fernandes CRR, Lachgar A, Neli M, Garrett R, Lam YL, Pereira MM. (2020). Catalytic sugarcane bagasse transformation into a proper biocrude for hydrocarbons production in typical refinery processes. Sustainable Energy Fuels 4, 4158–4169. DOI: Pereira SC, Souza M, Pinto J, Esteves LM, Lam YL, Soter de Mariz e Miranda L, Pereira MM. (2020). Sugar ketals as a platform molecule to overcome the limitation of converting biomass into green hydrocarbons in a typical refinery. Sustainable Energy Fuels 4, 1312-1319. https://doi.org/10.1039/D0SE00220H
  • Pereira SC, Souza M, Pinto J, Esteves LM, Lam YL, Soter de Mariz e Miranda L, Pereira MM. (2020). Sugar ketals as a platform molecule to overcome the limitation of converting biomass into green hydrocarbons in a typical refinery. Sustainable Energy Fuels 4, 1312-1319. https://doi.org/10.1039/C9SE00379G

DOI
10.26904/RF-135-1162763179

Research Objectives

The research of Miranda and Pereira focuses on catalysis and sustainable energy production.

Funding

  • Conselho Nacional de
  • Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ)

Collaborators

Ph.D. Yiu Lau Lam; D.Sc. Sergio Pereira; D.Sc. Joana Pinto;
D.Sc. Juliana Carvalho;
M.Sc. Matheus Oliveira;
M.Sc. Alessandra Vieira; B.Sc. Igor Pedrosa; M.Sc. Debora Nobre;
M.Sc. Cristiane Cardoso.

Bio

Leandro S. M. Miranda graduated in Pharmaceutical Sciences from Federal University of Rio de Janeiro (UFRJ), and obtained his PhD in Organic Chemistry in 2007 at the same University. He is associate professor at the Organic Chemistry Department (UFRJ) and has experience in the chemistry of carbohydrates and nucleosides.

Marcelo Maciel Pereira graduated in Chemical Engineering from Federal University of Rio de Janeiro (UFRJ), and obtained his PhD in Chemical Engineering in 1997 at UFRJ. He is full professor at the Chemistry Department (UFRJ). He has expertise in catalysis, zeolites, metal-support catalysis, biomass conversion, hydrocarbons and C1 chemistry.

Contact
Centro de Tecnologia – Av. Athos da Silveira Ramos
149 – bloco A 7° andar – Cidade Universitária da Universidade Federal do Rio de Janeiro
Rio de Janeiro, RJ, 21941-909

Dr Leandro S. M. Miranda
E: leandrosoter@iq.ufrj.br
W: http://lattes.cnpq.br/7387849390654555

Dr Marcelo Maciel Pereira
E: maciel@iq.ufrj.br
W: http://lattes.cnpq.br/1392441615039712

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