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. 2024 Mar 13;9(12):14351–14355. doi: 10.1021/acsomega.3c10428

Graphitizable Pitch from Microwave Plasma Pyrolysis of Natural Gas

Akshay Gharpure †,*, Vignesh Viswanathan ‡, Aayush Mantri ‡, George Skoptsov ‡, Randy L Vander Wal †
PMCID: PMC10975635  PMID: 38559986

Abstract

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An innovative approach of microwave plasma was utilized to convert natural gas into tar, from which a highly graphitizable pitch was derived using fractional distillation. The natural gas-derived pitch (NGDP) was thoroughly characterized, and the graphitizability of the carbonized NGDP was assessed using polarized light microscopy. The NGDP and, for comparison, needle coke, petroleum coke, and shot coke were subjected to graphitization heat treatment (GR) at 2500 °C. Results indicate that the graphitizability of the NGDP exceeds those of all industrial standard cokes. The GR-NGDP showed the highest degree of graphitization and crystallite size among all samples.

1. Introduction

Over the past decade, there has been a notable surge in the availability of unconventional oil and gas resources. However, the utilization of unconventional oil presents environmental challenges, particularly in the form of wasteful flaring or venting of associated petroleum gas, contributing significantly to global warming. Additionally, due to the lack of adequate infrastructure in many regions, unconventional gas often remains untapped. Nevertheless, this untapped resource holds the potential to be transformed into value-added chemicals and carbon materials, offering a more practical and efficient means of transport.

Given the rising demand for graphite in energy storage applications, huge potential exists for conversion of natural gas into value-added graphitic carbon precursors. Traditional gas-to-liquid (GTL) technologies, such as the Fischer–Tropsch process, demand high temperatures, high-pressure hydrogen, and the use of catalysts.1−3 Moreover, the resulting products are predominantly aliphatic, making them less than ideal for the synthesis of synthetic graphite materials.

In contrast, microwave (MW) plasma pyrolysis provides an innovative alternative. In our previous work, we have demonstrated the ability of MW plasma to obtain nanographene and graphitic carbon blacks as the carbon products with variable split by tuning process parameters plus hydrogen from natural gas (NG).4,5 In related studies, MW processing of coals yielded liquid chemicals and fuels.6 Importantly, MW plasma pyrolysis operates within atmospheric pressure at low temperatures (below 800 °C), with high energy efficiency and rapid heating of the feedstock. This method eliminates the need for catalysts, minimizing capital investments and reducing retrogressive reactions due to the lower temperature of the bulk media.

This paper introduces a novel application of harnessing MW plasma to derive a high-value pitch from NG, a critical precursor for synthesizing high-quality graphitic carbon materials. The key advantages of this system include its atmospheric pressure operation, low operating temperatures, high energy efficiency, flash heating, and the absence of catalyst requirements. Notably, the proposed approach involves the direct pyrolysis of natural gas, circumventing the conventional synthesis of syngas, which typically involves reactions with catalysts to generate longer hydrocarbons. While traditional GTL methods yield predominantly aliphatic products, MW plasma pyrolysis of NG produces heavier products, such as aromatic tars. These tars can be further converted into pitch through fractional distillation. The resulting NG-derived pitch (NGDP) has been characterized and heat-treated at 2500 °C, and its graphitizability was assessed by comparison with high-quality industrial cokes. This innovative approach not only addresses environmental concerns but also opens new avenues for obtaining valuable materials from untapped natural gas resources.

2. Experimental Methodology

2.1. Materials

A synthetically associated petroleum gas mixture (CH4:C2H6:C3H8 volume ratio = 1:0.167:0.083) was used as the surrogate for real NG. The MW plasma pyrolysis setup of H Quest Vanguard Inc., Pittsburgh, PA, USA that was used to pyrolyze NG was described in our previous publications.4,5,7 A microwave power of 5.75 kW at 2.45 GHz was fed to the steal reactor consisting of a water-cooled isolator, three-stub tuner, dual directional coupler, and waveguide coupling port. The NG mixed with a diluting gas stream (argon:H2 = 3:1) was reacted in the plasma torch. Resulting tars were captured in an impinger condensing system containing solvents, such as dichloromethane. The tar was separated from the condenser solvent using a rotary evaporator and was converted to a pitch (NGDP) by concentrating the heavier portion using fractional distillation at ∼200 °C. This NGDP was compared with a commercial Koppers’ pitch (K174) that has been well analyzed previously.8 For graphitizability comparison, petroleum coke, needle coke, and shot coke were obtained from a local supplier. The images of these benchmark cokes are provided in Figure 1.

Figure 1.

Figure 1

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Images of (a) needle coke, (b) shot coke, and (c) petroleum coke.

2.2. Carbonization and Graphitization

The NG-derived pitch (NGDP) was carbonized in argon at 600 °C for 3 h using a Thermolyne 21100 tube furnace. The benchmark cokes did not require a carbonization step. All samples were graphitized at 2500 °C under inert atmosphere (argon) for 1 h using a Centorr graphite hot zone furnace. The samples subjected to graphitization heat treatment were labeled as “GR-”.

2.3. Materials Characterization

2.3.1. Transmission Electron Microscopy (TEM)

TEM images were acquired at various magnifications ranging from 10 to 500 kX using an FEI Talos F200X microscope operated at 200 kV. The microscope was equipped with a field emission gun source and provided a resolution of 1.2 Å. The TEM samples were prepared by drop-casting a sonicated ethanol-sample solution onto a 300 mesh C/Cu lacey TEM grid.

2.3.2. X-ray Diffraction (XRD)

XRD was used to compare the graphitic quality of the graphitized samples. The XRD spectra were acquired using a Malvern PANalytical Empyrean diffractometer equipped with a copper (Cu) source (wavelength λ = 1.54 Å), para-focusing optics, and a PIXcel 3D detector. Peak deconvolution was performed using MDI Jade software, and the crystallite diameter (La) and stacking height (Lc) were calculated using the Scherrer equation with Kc and Ka of 0.89 and 1.84, respectively.

2.3.3. Fourier Transform Infrared Spectroscopy (FTIR)

FTIR analysis was performed using a Bruker Vertex 70v spectrometer equipped with a liquid nitrogen-cooled mercury–cadmium–telluride (MCT) detector. The spectra were collected in attenuated total reflectance mode, and the aromaticity index of the sample was calculated as Inline graphic.

2.3.4. Solvent Fractionation

The NGDP was further characterized by measuring hexane, toluene, and quinoline soluble fraction, and the wt % of each fraction is reported in the Results and Discussion section. The NGDP was dissolved in the solvents at room temperature while stirring, and the insoluble content was filtered out using filter papers having 1 μm pore size. The insoluble content was measured after drying, and the soluble content was determined by the weight balance.

2.3.5. Polarized Light Microscopy (PLM)

PLM on the carbonized NGDP assessed its graphitizability based on the optical texture using a Nikon Microphot-FXA optical microscope operated at 50× magnification. The carbonized sample was cured in epoxy, and the cured sample was polished using a series of 120, 240, 640, and 1200 grit sandpapers with final polishing performed using alumina and diamond powder slurry (0.3 and 0.05 μm, respectively).

3. Results and Discussion

Table 1 shows various soluble fractions of the NGDP. The characterization of the NGDP showed that the NGDP contained an extremely low quinoline-insoluble (α resins, QI) content, the absence of which is highly beneficial for graphitizability of the pitch because QI hinders mesophase formation. The majority of the pitch comprises toluene-soluble γ resins, containing up to six-ring aromatics with a highly graphitizable nature owing to an optimal H/C ratio. Additionally, the NGDP contained significant (22%) β resins9 (quinoline solubles within toluene insolubles), typically encompassing larger aromatics ranging from 6 to 15 rings. These β resins beneficially contribute binding properties to the pitch.

Table 1. Soluble Fraction Characterization of NGDP.

toluene solubles (γ resins) toluene insolubles quinoline solubles quinoline insolubles
78.2% 21.8% 98.8% 1.2%
hexane solubles in toluene solubles (oils) hexane insolubles in toluene solubles (asphaltenes) quinoline solubles in toluene insolubles (β resins) quinoline insolubles in toluene insolubles (α resins)
36.1% 63.9% 100% 0
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Figure 2 provides the FTIR spectra of the NGDP in comparison to a commercial Koppers’ pitch. The aromaticity index of the NGDP is 0.75 compared to 0.82 of the commercial pitch, indicating a higher H/C content in the NGDP. The substantially lower area of the peak around ∼750 cm–1 compared to other aromatic C–H peaks in the 700–900 cm–1 region suggests that the NGDP contains substantially higher pericondensed aromatics compared to K174. The importance of a higher ratio of pericondensed-to-catacondensed aromatics for forming the graphitic nanostructure has been highlighted in our previous publication.8 Moreover, the distinctive peaks at approximately 3420 cm–1 observed in K174 are notably absent in the spectrum of the NGDP indicating a substantially lower oxygen content in the NGDP. This should be beneficial for the graphitizability of the NGDP as oxygen is known to cause cross-linking of the basic structural units.10−12

Figure 2.

Figure 2

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FTIR comparison of the NGDP versus commercial pitch.

Figure 3 shows the PLM images of the carbonized NGDP. The application of PLM for analyzing mesophase formation has been well studied before on a variety of pitches and cokes.13−16 The color continuity seen within a flake in Figure 3a and flow domain anisotropy in Figure 3b over long range (tens of micrometers) are indicative of continuity of orientation domains in the mesophase and underscore the high graphitizability of the NGDP. The process of graphitization and the graphitic structure of carbons have been studied over many decades.11,12,17−19 Indeed, the GR-NGDP was found to be highly graphitizable by TEM and XRD investigations.

Figure 3.

Figure 3

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(a, b) PLM images of the carbonized NGDP showing a large flow domain anisotropy indicative of high graphitizability.

Figure 4 shows a panel of TEM images of the GR-NGDP. Analogous to the PLM image in Figure 3a, thin continuous flakes larger than tens of micrometers were observed in TEM (Figure 4f). As seen in Figure 4e,g, these flakes were highly graphitic with long contiguous lamellae well stacked with each other over tens of nanometers, consistent with PLM graphitizability assessment. All other particles exhibited thin flakes with a size of less than 1 μm. All surveyed particles regardless of their size and shape contained graphitic nanostructures having long contiguous, well-stacked lamellae. The intergraphene distance in the HRTEM images was measured to be ∼3.36 Å using ImageJ. XRD analysis further confirmed the graphitic nature of the GR-NGDP.

Figure 4.

Figure 4

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(a–g) TEM images of the GR-NGDP showing a highly graphitic nanostructure.

Figure 5 provides the XRD overlay of the GR-NGDP versus GR-benchmark cokes, and Table 2 provides the extracted crystallite parameters. The insets in Figure 5 show magnified XRD peaks in different regions. Notably, the GR-NGDP showed a sharp 002 peak with position at a substantially higher Bragg angle than all GR-benchmark cokes. This indicates the smallest intergraphene distance and the highest degree of graphitization achieved by the GR-NGDP. In addition, the GR-NGDP showed the highest crystallite diameter (La), surpassing that of the GR-shot coke by a substantial 40%. The GR-NGDP also showed a remarkable stacking height (Lc) value larger than the GR-shot coke by 20%. The stacking height of the GR-NGDP was only 10% smaller than those of the GR-petroleum coke and the GR-needle coke.

Figure 5.

Figure 5

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XRD overlay of the GR-NGDP vs GR-benchmark cokes.

Table 2. XRD-Derived Crystallite Parameters for the GR-NGDP versus GR-Benchmark Cokes.

lattice parameter | sample GR-NGDP GR-shot coke GR-needle coke GR-petroleum coke
d002 [Å] 3.358 3.368 3.362 3.362
degree of graphitization 0.86 0.75 0.82 0.82
La (using 110) [nm] 955 671 857 954
Lc (using 002) [nm] 259 214 290 293
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These observations suggest that the graphitizability of the NGDP exceeds that of high-quality benchmark cokes that are well recognized types by industry. This high graphitizability of the NGDP can be attributed to its optimal composition. The NGDP has high aromaticity, low QI, and no ash content, all beneficial for good mesophase formation. The NGDP also has little to no oxygen or other heteroatoms that generally result in lamella cross-linking and crystallite defects. The high aromaticity along with a high toluene-soluble content indicates that a significant amount of hydrogen content in the pitch is present on the aromatics rather than as separate aliphatic molecules. Toluene solubles consist of smaller molecules (typically up to six rings) with a higher hydrogen content compared to the quinoline solubles having 6–15 ring compounds. The presence of hydrogen is critical for maintaining mesophase fluidity and radical stabilization and limiting cross-linking to obtain high graphitic quality.8,14,20,21 This work highlights the potential of natural gas as a source for obtaining high-quality graphitic carbon precursors in light of diminishing conventional coal tar and petroleum pitches.

Conclusions

This research explored an innovative approach for obtaining a graphitizable pitch from NG pyrolysis using microwave plasma. The resulting NG-derived pitch (NGDP) was thoroughly characterized and compared with graphitizable carbon precursors well recognized by industry, namely, petroleum coke, needle coke, and shot coke. The study aimed to assess the graphitizability of the NGDP as a potential precursor to synthetic graphite. The results revealed that the NGDP surpassed industry-standard benchmark cokes in terms of graphitizability. The GR-NGDP exhibited the highest degree of graphitization and crystallite size when compared with industrial benchmark cokes. The exceptional graphitizability of the NGDP was attributed to its optimal composition, characterized by high aromaticity, high toluene solubles, low QI and oxygen contents, and the absence of ash content. These factors collectively led to the high-quality graphitic structure observed in the GR-NGDP. This work shows the potential of obtaining a high-quality pitch from NG for producing high-quality synthetic graphitic carbons for energy storage applications.

Acknowledgments

This work was partially supported by the donors of ACS Petroleum Research Fund under ACS PRF grant 59973-DN4. R.L.V.L. served as principal investigator on ACS PRF 59973-DN4 that provided support for A.G. Partial support for this work is acknowledged through Penn State subaward agreement no. 231217, from H Quest Vanguard Inc., under Prime Award DE-FE0031793.

The authors declare no competing financial interest.

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