HIGHLIGHTS
• A process has been developed to convert asphaltenes, a byproduct of petroleum refining, to graphene using flash joule heating.
• To show the utility of the asphaltene-derived graphene, this material was combined with carbon black to produce an asphaltene-derived flash graphene composite, which shows potential as a coating that protects metal against corrosion. 
• A life cycle assessment showed that the use of asphaltenes in preparing graphene reduces its carbon footprint compared to the existing use of asphaltenes as a fuel.

In evaluating a process for sustainability, the objective is to figure out how to identify useful applications for not just the main product(s) but also byproducts that may form. A prominent example is the refining of crude oil. Fractional distillation facilitates the preparation of a range of useful product streams ranging from high viscosity fuel oil to low viscosity materials such as butane. 

But refining also generates a significant number of byproducts. One of these materials is asphaltenes, which consist of heavy macromolecules. There are approximately one to two trillion barrels of asphaltenes held in reserve globally. The problem in working with asphaltenes is poor ignitability, reactivity and biodegradability. 

Muhammad Rahman, assistant research professor of materials science and nanoengineering at Rice University in Houston, Texas, says, “Asphaltenes have the potential to be converted into high value added products because of several attractive characteristics. They are rich in carbon (70%-80%), contain a high aromatic content which imparts stability and also include heteroatoms and carbon-carbon double bonds. The latter are reactive under the right processing conditions.” 

Attempts have been made to convert asphaltenes to graphite, carbon fibers, porous carbon foam/scaffolds and carbon nanosheets. But none have been done in a sustainable manner. 

Flash joule heating is a newly developed process that is able to recycle electronic waste, extract heavy metals and leave a residue that is clean enough for use in agricultural applications. In a previous TLT article,¹ this process is described as the rapid heating of ground-up electronic waste mixed with approximately 30% carbon black in a quartz tube squeezed between a porous copper electrode and a graphite electrode. 

Current from a high voltage discharge of a capacitor bank connected to the two electrodes increases the temperature of the mixture by approximately 3,400 K in less than 50 milliseconds. Metal present in the electronic waste is vaporized while carbon containing materials are carbonized. The energy requirement for flash joule heating is found to be much less than other techniques for pyrometallurgical recycling. 

Rahman and his colleagues determined that subjecting asphaltenes to flash joule heating may prove to be an efficient way to sustainably produce graphene, which is under evaluation for many different applications including as a lubricant. 

Asphaltene-derived flash graphene 
Following the flash joule heating procedure, the researchers combined asphaltenes with conductive carbon black (at a minimum of 20% by weight) and produced asphaltene-derived flash graphene (AFG) at a process yield of approximately 45% and an AFG yield above 95%. The product was characterized by Raman spectroscopy and X-ray photoelectron spectroscopy. 

Reactive force-field molecular dynamics simulation was used to determine the mechanism of the process. The conductive carbon black molecules that consist of polycrystalline carbon act as a nucleating agent to stabilize the growth of AFG molecules around them. 

The simulation found that several gases were generated during flash joule heating. Rahman says, “Carbon monoxide and hydrogen gas were determined to be present in the product mixture by the simulation.” 

To demonstrate the performance of AFG, this material was evaluated as a reinforcement in nanocomposites using epoxy resin as the matrix phase. An increase in the mechanical properties of the nanocomposite was realized due to the improvement of interfacial adhesion created by the increased surface area of the AFG. 

The researchers then decided to produce 3D plastic parts with the AFG nanocomposite by using one of the additive manufacturing techniques, direct ink writing. Addition of a common thixotropic agent, fumed silica, was needed to develop an ink that could be processed using this method. Complex architectures such as honeycomb structures were then produced. 

The thermal properties of the AFG nanocomposite were compared to the epoxy polymer resin using a thermal infrared camera. Rahman says, “We heated the samples using a laser source, then employed the thermal infrared camera to determine the peak temperature and then calculated the heat transfer rate. The AFG nanocomposite exhibited superior performance, which means it has better thermal properties.” Corrosion resistance was evaluated by applying a thin layer of the AFG nanocomposite coating to a mild steel (the loading of the AFG in the coating was 10%). After exposure to a 3.5% sodium chloride solution for six hours, extensive corrosion was seen on the bare mild steel, and some rust was observed with the epoxy resin. The AFG nanocomposite only showed slight blemishes under the same conditions. 

A life cycle assessment (LCA) analysis was done to determine the environmental impact of the AFG synthesis. The researchers conducted a gate-to-gate analysis and compared it to the existing use of asphaltene as a fuel. M.A.S.R. Saadi, graduate student at Rice University, says, “We evaluated two electricity generating scenarios, natural gas and hydropower based, and found that AFG synthesis leads to a lower carbon footprint for both. For hydropower, the reduction in carbon footprint is significant.” 

A summary showing this new approach for producing graphene from asphaltene and highlights of the testing done by the researchers is shown in Figure 2. 


Figure 2. A summary of the alternative approach for using asphaltenes to produce graphene and the potential applications for the asphaltene-derived flash graphene composite is shown. Figure courtesy of Rice University.

Rahman says, “We intend to evaluate the AFG nanocomposite for corrosion resistance in applications such as oil and gas where piping will be coated and evaluated under real-world operating conditions.” 

Additional information on this research can be found in a recent article² or by contacting Rahman at maksud@rice.edu. 

REFERENCES
1. Canter, N. (2022), “Electronic waste recycling using new thermal technique,” TLT, 78 (2), pp. 14-15. Available here.
2. Saadi, M.A.S.R., Advincula, P., Thakur, S., Khater, A., Saad, S., Zeraati, A., Nabil, S., Zinke, A., Roy, S., Lou, M., Bheemasetti, S., Bari, A., Zheng, Y., Beckham, J., Gadhamshetty, V., Vashisth, A., Kibria, G., Tour, J., Ajayan, P. and Rahman, M. (2022), “Sustainable valorization of asphaltenes via flash joule heating,” Science Advances, 8 (46), DOI: 10.1126/sciadv.add3555.