Asphaltene-Derived Activated Carbon and Carbon Nanotube

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Environmental and Carbon Dioxide Issues

Asphaltene-Derived Activated Carbon and Carbon Nanotube Membranes for CO2 Separation Benjamin Kueh, Maria Kapsi, Charitomeni Veziri, Chrysa Athanasekou, George Pilatos, K. Suresh Kumar Reddy, Abhijeet Raj, and Georgios N. Karanikolos Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02913 • Publication Date (Web): 18 Oct 2018 Downloaded from http://pubs.acs.org on October 19, 2018

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Asphaltene-Derived Activated Carbon and Carbon Nanotube Membranes for CO2 Separation Benjamin Kueh1, Maria Kapsi2, Charitomeni M. Veziri2, Chrysa Athanasekou2, George Pilatos2, K. Suresh Kumar Reddy1, Abhijeet Raj1,3*, Georgios Karanikolos1,3* 1Department

of Chemical Engineering, The Petroleum Institute, Khalifa University of Science & Technology, P.O. Box 2533, Abu Dhabi, UAE

2Institute

of Nanoscience and Nanotechnology (INN), Demokritos National Research Center, Athens, 153 10, Greece

3Center

for Catalysis and Separation, Khalifa University of Science & Technology, P.O. Box 127788, Abu Dhabi, UAE

*Corresponding

authors email: [email protected], [email protected]

Keywords: Activated carbon, carbon nanotubes, asphaltene, waste, petroleum, carbon dioxide, adsorption, membranes, gas separation

Abstract Due to large energy requirements of the traditional gas separation processes, novel and less energy-intensive technologies, such as adsorption- and membrane-based ones, are anticipated to play major role in future industrial separations. Thus, finding new means for economical fabrication of materials related to these processes is of significant importance to facilitate their implementation in large-scale operations. In this work, we synthesized high-quality activated porous carbons (AC) and carbon nanotube (CNT) membranes using asphaltene, an abundant waste of the petroleum industry. The resulting materials were tested for CO2 separation in adsorption and membrane modes. Among the various porous carbons produced, AC from raw asphaltene reached a CO2 sorption capacity of 7.56 mmol/g at 4 bar and 25°C with a relatively low heat of adsorption (up to

ACS Paragon Plus Environment

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23 kJ/mol) implying low energy requirement for regeneration. The versatility of the asphaltene precursors in the formation of carbon nanomaterials was also demonstrated by growing, for the first time, CNT membranes via template-based, catalyst-free carbonization of asphaltene inside the pores of anodized alumina. The resulting CNT membranes attained a promising separation performance with permeability ratios exceeding the respective Knudsen values for H2/CO2, N2/CO2, N2/CH4, and H2/CH4 gas pairs.

1. Introduction Porous activated carbon (AC) adsorbents are currently utilized in several gas separation/capture technologies. The quality and the performance of a carbon adsorbent mainly depend on the precursor used for its preparation. Asphaltene is rich in aromatics and consists of carbon, hydrogen, sulfur, nitrogen, oxygen, and small amounts of other heteroatoms such as vanadium and nickel, thus making it an ideal precursor for developing heteroatom-rich and highly-porous adsorbents1. On the other hand, membranes consisting of carbon nanotubes (CNTs) as pores have attracted significant attention because of theoretical

2-3

and experimental studies

4-9

that indicate high fluxes,

potentially higher by several orders of magnitude compared to zeolites with comparable pore sizes. This is mainly due to the inherent smoothness of the nanotubes and the interaction of their surfaces with the permeating molecules. Asphaltene is an abundant waste resulting from oil extraction, processing, and transportation operations in the oil industry. It exists in suspended form in the crude oil, and tends to deposit as fouling layers that adhere to formation grains, pumps, valves, and flow lines. These deposits reduce the oil production rate, increase the energy


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consumption, and involve significant cost for cleaning and equipment maintenance and replacement. The clogging phenomenon is usually caused by fluid property variations when there are changes in pressure, temperature, composition, or shear rate during oil production and/or transportation. Asphaltene is usually recovered from the deposits in the oil wells and as the bottom product during distillation of the crude oil. It is a highly aromatic material and has been mostly employed as a secondary fuel in many coal-fired power plants, as a binder in road construction and roofing field, and for waterproofing due to its stable and complex structure. These applications, however, constitute a rather low-level utilization of asphaltene. Such carbon-rich material would release molecules such us CH4, CO, H2, N2, CO2, H2S and low molecular weight hydrocarbons upon thermal treatment, and thus could have great potential as an inexpensive and readily available carbon feedstock in making novel carbon materials including activated carbon, vapor-grown carbon fibers, and carbon nanotubes (CNTs) and graphene through aromatization and carbonization 10. Converting such a cheap and abundant byproduct of the petroleum industry into a novel nanostructured carbon product could help in fulfilling the demand for such materials at large-scale production, and could provide a new route for high-value utilization of asphaltene, which is generally considered a waste 11-12. Several experimental studies have been performed on the synthesis on AC with highly porous structure and desired physicochemical characteristics using precursors based on coal, char, anthracites, cokes, and biomass such as coconut shell

13

, wood

14,

peanut shell 15, and date palm seed 16, via various activation procedures (physical and/or chemical). For instance, chemical activation of Spanish anthracite with NaOH produced an AC with high BET surface area (2700 m2/g) and micro-pore volume (1 cc/g). Physical


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mixing of NaOH and anthracite coal yielded better performance than the impregnation method, and also had the advantage of easiness of preparation

17.

Physical mixing of

Mongolian anthracite precursor with KOH as an activating agent in the ratio of 1:4 produced AC with a BET surface area of 2784 m2/g and a pore volume of 2.27 cc/g

18.

Rice husk char as precursor and KOH as activating agent were used to prepare AC for low pressure CO2 capture 19. It was shown that a KOH/Char ratio of 1:1 favoured the CO2 uptake (2.1 mmol/g at 0.1 bar and 0°C), which was attributed to the micropores formed during the applied chemical activation treatment. For a KOH/Char ratio of 3:1, the CO2 uptake at 1 bar and 0°C also reached a high value (6.24 mmol/g). The effect of oxidative pre-treatment of the carbonaceous material bituminous coal has been studied by Verheyen et. al.

20.

Pre-treatment of bituminous coal by nitric acid produced hard AC

with a high surface area (1602 m2/g) and a micropore volume of 0.74 cc/g in the presence of KOH. Up to date, only few studies have reported the application of petroleum pitchderived AC using alkali additives for CO2 adsorption 21. In one such study, two types of petroleum pitch were selected in order to prepare a series of carbon molecular sieves using NaOH as activating agent. The resulting product exhibited high CO2 adsorption capacity of 8.6 mmol/g at 1 bar and 0°C, which is one of the best results reported for carbon-based materials 22. Formation of various carbon microstructures using asphaltene as carbon source has been investigated. Recently, the conversion of asphaltene through thermal decomposition to large sheets of multi-layered graphene was reported

23.

Preliminary

investigations have shown transformation of a thin film of asphaltene cast to carbon nanoparticles upon heating under both nitrogen and oxygen atmosphere 24 and production


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of carbon microspheres by chemical vapor deposition (CVD) using asphaltene as carbon precursor

25.

Carbon fibers and multi-walled carbon nanotubes (MWCNTs) were

synthesized by CVD using de-oiled asphalt as carbon source and ferrocene as catalyst, 26 while controllable growth of single-walled CNTs from heavy oil residue was achieved by CVD after selecting different metals as catalysts 12. In this work, various AC adsorbents for CO2 capture were prepared from asphaltene precursors. The surface, chemical, and textural properties of the produced ACs from raw and purified asphaltenes as well as their oxidized derivatives were prepared and examined, and their CO2 adsorption capacities and heats of adsorption were evaluated. CNT arrays and membranes were also produced via templated thermal cracking of asphaltene inside the pores of anodized alumina. Permeability measurements with various gases were performed on the resulting CNT membranes in order to investigate their separation potential. Pure CNTs from raw and purified asphaltene were also tested for their CO2 sorption capacities after removing the anodized alumina template. This work serves the dual goal of (1) discovery of new applications and ways for adding value to waste asphaltene and (2) economical production of adsorbents and membranes for energy-efficient gas separations.

2. Materials and methods 2.1. Materials Raw asphaltene (RA), used as a carbon precursor in this work, was obtained from an oil field in Abu Dhabi. Reagent-grade toluene and n-pentane were purchased from


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Sigma-Aldrich. Anodized alumina membranes (Anodisc-13) with unidirectional straight pores of 200-nm diameter were purchased from Whatman. 2.2. Treatment of asphaltene samples As received raw asphaltene was crushed to fine powder, and 10 g of the powder was suspended in 5 L of toluene. The asphaltene-in-toluene suspension was then stirred for 2 h and kept static for 2 days. After aging, stirring of the suspension was repeated for 6 h. The slurry was then centrifuged in order to precipitate undissolved solids or impurities. The supernatant toluene, which contained the dissolved asphaltene, was then collected and concentrated using Rota vapor to 200 mL. To the concentrated toluene extract, 2 L of n-pentane was added slowly. The mixture was kept under stirring for 3-4 h and was subsequently kept static at room temperature for 16-18 h. The n-pentane blend was centrifuged in order to separate the precipitated asphaltene. The asphaltene was then cleaned to remove any traces of impurities using a Soxhlet extractor. The sample was subsequently dried at room temperature for 2 days to obtain the purified asphaltene (PA). It should be noted that most of the toluene used initially to dissolve the asphaltenes was recovered, as a rotavapor was used to reduce the solution volume to 200 mL from 5 L. Thus, most of toluene can be re-used contributing to minimization of solvent waste. After mixing the concentrated solution of toluene and asphaltene (200 mL) with pentane (2L) in order to precipitate the asphaltene, the resulting mixture of toluene and pentane can also be easily separated through distillation if the purification process is to be carried out on a large scale and/or as a continuous process. This mixture can also be reused as it is for purifying more than one samples as it contains only a small amount of toluene.


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The raw and purified asphaltenes (denoted as RA and PA, respectively) were ground to small particle size before treating. A 30 mL of concentrated nitric acid (65%) was added to both RA and PA for oxidation, and the mixtures were stirred at 70°C for 3 h. The mixtures were then filtered and the collected asphaltenes were dissolved in NaOH solution by stirring for about 30 min to neutralize the samples. Then, the mixtures were filtered and washed with 5 mL of 5% HCl and then with distilled water for neutralisation of the alkaline samples until pH value of 7. The samples were then dried in an oven at 110°C for 12 h to obtain raw oxidized asphaltene (ROA) and purified oxidized asphaltene (POA). 2.3. Synthesis of activated carbon from asphaltenes Activation of asphaltene precursor using KOH as chemical activating agent was carried out by physical mixing of asphaltene with KOH. The KOH pellets were first crushed and ground into fine powder and then mixed with the asphaltene in a weight ratio of 3:1 for KOH:asphaltene. One gram of each sample, i.e. RA, PA, ROA or POA, was added to KOH powder in separate crucibles in a horizontal furnace (CARBOLITE). The activation process was carried out at 600 °C for 90 min with a heating rate of 20 °C/min under nitrogen purge. After activation, the samples were cooled to room temperature under the same nitrogen flow. The samples were then washed and filtered with dilute HCl solution (5%) and subsequently with distilled water until neutral pH. The samples were then dried at 110°C for 12 h. The activated RA, PA, ROA, and POA are denoted as ARA, APA, AROA and APOA, respectively. 2.4. Growth of CNTs and CNT membranes from asphaltenes


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Solutions of RA and PA in toluene were prepared by dissolving 66.7 mg of each asphaltene precursor in 1 ml toluene. Then, one drop (~0.022 ml) of each solution was cast onto an anodisc (13 mm in diameter) and was allowed to settle for a few minutes before thermal processing. The impregnated samples were then heated under nitrogen atmosphere to a maximum temperature of 750°C with a heating rate of 5°C/min and the temperature was maintained at this level for 2 h. The samples were then cooled down to room temperature with a cooling rate of 2°C/min under nitrogen flow. The anodized alumina templates after pyrolytic growth of CNTs inside their channels are denoted as RA- and PA-CNT membranes. Pure RA- and PA-CNTs were also obtained by dissolving the alumina templates in 6 M aqueous NaOH solution for 6 h followed by repeated washing of the extracted CNTs with DI water. 2.5. Characterization The morphological characteristics and elemental distribution of the prepared samples were evaluated by scanning electron microscope (SEM, Field Emission Gun, FEG-250, FEI Holland) equipped with an energy dispersion X-ray spectroscopy (EDS) detector. The surface area and porosity were determined by N2 adsorption-desorption at 77 K using a Quantachrome AUTOSORB-6 system. The samples were first degassed at 130 °C in vacuum for 5h. The Brunauer-Emmett-Teller (BET) method was used to calculate the surface area of the samples from the isotherms. The total pore volume (Vt) was examined from single point adsorption at a relative pressure, P/Po, of 1, whereas the micropore volumes were calculated from the adsorption isotherms applying the DubininRadus-hkevich (DR) equation. Inductively coupled plasma mass spectrometry (ICP-MS) was performed on the raw and purified asphaltene precursors to evaluate their metal


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content. The samples were analyzed with a Perkin Elmer SCIEX DRC-e ICP-MS (Ontario, Canada) fitted with a New Wave UP-213 laser ablation system. Fourier transform infrared (FTIR) spectroscopy was performed to identify the surface functional groups on a PerkinElmer FT-IR Spectrometer using KBr as background standard. The produced CNT membranes were inspected using a JEOL JSM-7401F fieldemission gun (FEG) SEM at typical conditions of 10 mA emission current and 2 kV operating voltage. The CNT arrays were observed after dissolution of the alumina template in a 6M NaOH solution for 6h. RA- and PA-CNT arrays were stuck on epoxy resin to perform the SEM analysis. Raman spectroscopy of the templated CNTs was performed using an inVia Reflex (Renishaw) micro-Raman spectrometer with a focal point of 1–2 μm2 and an Ar+ ion laser (514.5 nm) operating at 10 mW (~10% of full power). 2.6. CO2 Sorption measurements CO2 adsorption on the AC samples was carried out using a gravimetric sorption analyzer (IsoSORP STATIC 3xV-MP) from Rubotherm, Germany. The magnetic suspension balance (MSB) accurately measures sample weight and gas dosing in order to determine the adsorption equilibrium. Samples were first heated under vacuum at 150°C for 3 h in order to remove moisture and other remaining volatile substances until a constant sample mass was obtained. The three positions of the MSB allow the measurement of the zero point (position one), the sample weight plus the sample holder (position two), while in position three a titanium sinker with a known volume and weight is added to the sample-containing holder. By measuring the weight change of the sinker, the density and the volume of the sample was measured in situ, and buoyancy correction


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was applied to the sample weight. The sample on the stainless steel holder was pressurized up to 4 bar. A continuous measurement of sample mass gain during CO2 adsorption was determined under equilibrium/saturation conditions. 2.7. Gas sorption and transport measurements on the CNT membranes Gas sorption experiments of the as-grown anodisk-templated CNTs were performed in a custom-made low pressure gravimetric apparatus based on a CI electronic microbalance, described in our previous work

27.

To conduct the sorption experiments,

small amounts (~15 mg) of the two CNT membrane samples (RA-and PA-CNT) were introduced in the weighing pan and outgassed at 190°C under vacuum (10–4 mbar). All the experiments were performed at 25°C. Single gas permeability measurements were conducted at 25°C, in a home-made stainless-steel permeability rig involving the dead-end technique described elsewhere 28. Due to the limited mechanical strength of the anodisc templates, gas permeance experiments were conducted for head pressures up to 200 mbar. 3. Results and Discussion

3.1. AC physicochemical and textural characteristics Physicochemical characterization of the treated asphaltene precursors and produced AC samples was first performed. Figures 1 and 2 show SEM images and the N2 adsorption isotherms of RA, PA and the produced porous carbons, namely APA, ARA, APOA, and AROA, as defined in the experimental section. According to the SEM micrographs of Figures 1a and 2a, PA and RA samples consist of solid dense particles in random dispersion, with no evident porosity as confirmed by N2 adsorption (Figure 1e).


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However, in APA, an open structure including near-spherical particle formations adhering to each other is observed (Figure 1c). Porosity is more obvious in APOA (Figure 1b). A porous layered structure was formed on ARA and AROA with porosity being evident in the form of sponge-like pores of different sizes on the surface (Figures 2c and 2b). The morphologies and the porosity patterns in the current asphalteneproduced carbons are qualitatively similar to the morphologies of ACs synthesized from other precursors in the literature 29-30. Nitrogen adsorption isotherms (Figures 1d, 2d), illustrate micropore filling up to a relative pressure of 0.2 with high uptakes observed due to narrow pore width and high adsorption potential. In all ACs, a hysteresis loop was observed at higher pressures, which infers a mesoporous component of the sorbents associated with the occurrence of pore condensation. The limiting uptake over the range of high P/Po resulted in a plateau of the isotherm indicating complete pore filling. The fact that the activated carbons produced in this study had both Type-I and Type-IV isotherm characteristics also indicates the formation of both micro-pores and meso-pores. The physical properties of all the activated carbons are listed in Table 1. The highest BET surface areas values of 2358 m2/g and 2894 m2/g were observed for APOA and AROA, respectively. These samples had average pore diameters of 8.6 Å and 9.6 Å, respectively, which are lower than the pore diameters of the corresponding APA and ARA samples. This indicates that the oxidation of the samples by HNO3 assisted in the formation of a large number of micro-pores in the carbons. A similar finding was reported for bituminous coal, where its pre-oxidation (before KOH activation) introduced sufficient acidic functionalities on the material to enable dissolution in KOH solution

20.

Thus, the pre-oxidation treatment


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increased the ability of KOH to generate micro-porosity in activated carbon that resulted in higher BET surface area 20. For ARA and APA, BET surface areas were observed to be 1590 and 1605 m2/g, with average pore diameters of 10.9 and 9.8 Å, respectively. The increase in the slope of adsorption isotherm for ARA at high relative pressures (P/Po > 0.9) can be explained by the formation of wider pores and the possibility of capillary condensation in the mesopores 31. Also it was observed that, due to the oxidation process, the pore volumes increased in all cases without negatively impacting the average pore diameter, which is desired for ACs for gas sorption applications. The above results indicate that the pre-oxidation and the activation process result in extended porosity formation in the RA and PA sources with high surface area, and create pores with different sizes that can facilitate adsorption on the surface of the developed ACs. Table 2 provides the elemental composition of the asphaltenes and the resulting porous carbons. Before activation, RA and PA were carbon-rich, i.e. with carbon content reaching the values of 93.0 and 82.4 wt%, respectively. The carbon content decreased upon formation of the porous carbons as carbon atoms were released in the form of CO2, and new oxygen atoms were added during pre-oxidation and activation. The most notable increase in oxygen content took place in the conversion of RA to ARA. In addition, for PA, the pre-oxidized and activated sample (APOA) had a higher oxygen content than the activated carbon one (APA). This was not the case with RA, where the pre-oxidized and activated sample (AROA) contained less oxygen than ARA. FT-IR analysis was used to investigate the differences in the functional groups of asphaltenes and the produced activated carbons. Figure 3 contains the FT-IR spectra of RA and PA before and after activation and/or oxidation. A strong and broad adsorption


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peak appeared near 3400 cm-1 in RA, which corresponds to the stretching of O–H functional group. However, the intensity of this peak was weak in the PA sample. A distinct aromatic C-H stretching peak appeared in between 3000 - 3100 cm-1, while an aliphatic C-H peak appeared in between 2800 - 3000 cm-1. A drastic decrease in aliphatic and aromatic hydrogen content of asphaltene upon purification is observed in the aforementioned range for PA

32-33.

Multiple absorbance bands were also found in the

region from 1800 to 700 cm-1. The band near 1750 cm-1 is the characteristic band of C=O stretching vibration in carboxylic groups, the band near 1600 cm-1 is attributed to C=C stretching vibration in aromatic rings or ketone groups, while the band at 1250 cm-1 is ascribed to C-O stretching in carboxylic groups, phenols, and ethers. RA also exhibited two distinct modes in the region of 1300-1500 cm-1, which are mainly due to CH2 or CH3 bending modes 34. The region in between 700 - 900 cm-1 in RA and PA contained various bands related to aromatic ring and out of plane C-H bending with different degrees of substitution

35.

The last band at 720 cm-1 is usually considered to represent the alkyl

structure. While RA exhibited four peaks in the aforementioned region, i.e., near 880, 830, 780, and 720 cm-1, PA exhibited only two weak bands indicating a lower aromaticity of PA as compared to RA. The spectra indicate that RA had aromatic clusters substituted to a high degree by aliphatic or naphthenic groups 36. It should be noted that the degree of mesophase development during the carbonization of the asphaltene strongly depends on the chemical constituents of the feedstock, i.e., it is increased with increasing hydrogen aromaticity of the asphaltene

37.

A possible explanation is a relation between the high

hydrogen aromaticity and the low carbonization rate, which enhance mesophase production

37.

The above observations explain the creation of mesopores in RA and PA


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upon activation, which have been confirmed by N2 adsorption isotherms (Figures 1(d), 2(d)). After activation and/or pre-oxidation, some peaks disappeared and some pronounced peaks became less intense. The peaks in the range of 1800-1100 cm-1, corresponding to the oxygenated functional groups, were found in all four AC samples. These groups determined the chemical nature of the porous carbons produced, and are anticipated to have a great impact on their adsorption capability 38. The CH peaks in the range 2900-3000 cm-1 and 700 - 900 cm-1 disappeared upon activation, most possibly due to the loss of carbon chains and dehydrogenation. A more pronounced O-H band at 3400 cm-1 was observed for ARA as compared to the pre-oxidized samples, APOA and AROA, which indicates a lower carbonization degree in ARA than in the rest of the samples. The higher aromaticity of ARA gave a basic character to it, which arises from the delocalized π-electrons in the aromatic rings. These π-electrons could act as Lewis bases, and together with -OH basic sites on the surface of the ARA, can favor the adsorption of carbon dioxide, which is acidic. The RA and PA asphaltenes were also used as precursors loaded in the onedimensional, parallel channels of anodized alumina template for growth of CNTs by pyrolysis inside the template channels. Template-based growth of CNT arrays and membranes has been reported previously by our group using a variety of CVD-based techniques, precursors, templates, and pore sizes

4-5, 39-40.

Figures 4 and 5 show SEM

images of the as-synthesized CNT arrays produced by asphaltene carbonization after removal of the anodized alumina template. The CNTs grow concentrically within the channels of the anodized alumina template, as their diameter and orientation is similar to those of the anodisk pores and do not extend beyond the surface of the template. Figures


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4(c) and 5(c) show open tips indicating that there was no significant unreacted asphaltene deposits on the outer surface. Figures 4(d) and 5(d) show the Raman spectra for the arrays produced by the pyrolysis of PA and RA, respectively. The Raman spectra reported were taken from the top surface of the arrays. All the spectra display the main Raman features of MWCNTs, namely, the tangential stretching mode (G-band) at 1600 cm-1 and the lattice defects and amorphous carbon deposition mode (D-band) at 1300 cm1.

Analysis of the opposite side of the arrays yielded analogous Raman features, which

demonstrates that CNTs as grown here by the template technique, extended along the internal surfaces of the aligned channels and spanned the whole thickness of the template. The ratio of the intensity of the D peak (ID) to the intensity of the G peak (IG) is a measure of the amount of disorder in the CNTs

41.

The ID/IG ratio of the asphaltene-

derived CNTs was around 0.7, which is in good agreement with the literature-based values for MWNTs and indicates CNT materials with high graphitic quality and low amount of defects. Similar relative intensity values were collected from the Raman spectra taken at the bottom surface of the arrays, which indicates that the produced CNTs were well graphitized from the one end to the other. Compared to other CNT structures produced by using bare pores of anodized alumina, the ID/IG ratios of asphaltene-derived CNTs were quite lower and almost comparable to those produced by involving catalysts to enable CNT growth 42. Notably, the growth reported here was a one-step, catalyst-free procedure involving only the asphaltene precursors, thus proving that asphaltene, which is rich in aromatic clusters and aliphatic groups, is a promising raw material for the production of highly ordered carbon nanotubes and related graphitic nanostructures.


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Concerning the fact that an external catalyst was not required in this work to produce good quality CNTs in high yield, we proceeded to investigate possible metal content in the used asphaltene precursors. It is known that growth of CNTs, in addition to carbon precursor, requires also the presence of catalyst to nucleate graphitic carbon species formation

39, 43-44.

ICP-MS analysis was performed in the asphaltene precursors

used in this work and the results are shown in Table 3. It is evident that both the raw (RA) and the purified (PA) asphaltene contain significant amounts of metal species, in particular Fe, Ni, and Cu that are known for their catalytic activity towards formation of graphitic structures such as CNTs and graphene

43, 45.

This fact provides an additional

advantage to asphaltene as precursor for growth of graphitic nanostructures, i.e. containing metal heteroatoms able to provide catalytic action upon growth thus acting as a “dual-action” carbon and catalyst precursor and eliminating the need for additional externally provided catalysts for the growth.

3.2. CO2 adsorption studies The CO2 adsorption isotherms of APA, APOA, ARA, and AROA activated carbons were obtained at 25, 50, and 100 °C for a pressure range up to 4 bar, and are shown in Figure 6. As the temperature increased, the adsorption capacity decreased for all the ACs, which is indicative of a physisorption mechanism 22. Moreover, all the ACs exhibited high CO2 adsorption capacities with an increase in CO2 uptake with increasing pressure. For instance, at 25°C and 1 bar, CO2 sorption capacities of ARA, AROA, APA, and APOA were 3.01, 2.46, 2.33, 2.40 mmol/g, respectively, which increased to above 5 mmol/g for all samples as the pressure was increased to 3.8 bar. Base on literature, the


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maximum CO2 sorption capacity reported for asphalt-based activated carbon with a surface area of 2860 m2/g after activation with KOH is 26 mmol/g at 23°C and 30 bar upon treatment with NH3 and H2 at high temperatures 46. The same group reported a CO2 uptake value of 35 mmol/g at 54 bar for an ultra high surface area porous carbon (4200 m2/g) obtained by pretreating asphalt at 400°C for 3h prior to KOH activation46. Our study however focuses on low-pressure capture, which entails a higher potential for lessenergy demanding capture, and the amount of CO2 adsorbed on the ARA AC reached a value of 7.56 mmol/g at 25°C and 3.8 bar, which is one of the best reported values for porous carbon materials at these conditions irrespectively of the production method. The ARA sample exhibited the best performance in terms of sorption capacity at all conditions even though it has a relatively low surface area (1590 m2/g) and a pore volume of 0.87 cc/g. Interestingly, in the present study, despite the fact that acid treatment and subsequent chemical activation resulted in activated carbons (APOA and AROA) with very high surface areas (2358 and 2894 m2/g, respectively) and average pore diameters of less than 1 nm, their CO2 sorption capacity was lower compared to APA and ARA. This indicates that the amount of CO2 adsorbed depends significantly on the attractive interactions between CO2 and the basic sites of ARA, the existence of which was confirmed by FTIR analysis, in addition to the physical/textural properties of the adsorbents such as BET surface area, total pore volume, or micropore volume 47. Regarding the asphaltene-derived CNTs, the CO2 adsorption capacity of RA- and PA-CNTs after the removal of the anodized alumina template in the pressure range up to 4 bar at 25 and 50 °C is shown in Figure 6e. Evidently, the amount of CO2 adsorbed on RA- and PA-CNTs was significantly lower than that on the ACs, which was anticipated


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given the large size of the CNTs compared to the microporous nature of the AC samples. A CO2 sorption capacity of 2.5 mmol/g was observed in RA-CNTs at 25°C and 4 bar, which is higher than the value for PA-CNTs under the same conditions. The trend of the raw-based CNT samples adsorbing higher amount of CO2 compared to the purified-based ones is similar to the observations for the AC samples above and is attributed to the surface functional groups of the raw asphaltene-based carbons. In addition, the adsorption isotherms of CNTs increased more linearly as compared to those of ACs. It appears therefore that the CO2 uptake could be continued by increasing pressure, whereas the AC isotherms had a tendency to lead to saturation at relatively low pressures. This is attributed to the large diameter and large pore volume of the CNTs, in contrast to the microporous ACs, available to accommodate CO2 in the bore CNT space as pressure increases. Interestingly, in the PA-CNTs the adsorption capacity at the low pressure range is higher at 50 °C that at 25 °C, with the phenomenon being revered as pressure increases above 1.5 bar. This indicates that CO2 interactions with the CNT surface is dominant at lower pressures while accommodation and multi-layer adsorption of CO2 into the bore space of the CNTs is enhanced as pressure increases. The heat of adsorption was calculated to determine the affinity of CO2 with the asphaltene-produced carbon materials (ARA, AROA, and CNTs). The CO2 adsorption isotherms were collected at three different temperatures and used to calculate the isosteric heat of the carbon materials. The isotherms were used to extract the pressure, temperature, and corresponding CO2 loadings, and the Clausius-Clapeyron equation, given below, was used to calculate the isosteric heat of adsorption (ΔH). ln (p) = -ΔH/RT + C


(1)

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Where, p (bar) is the pressure, ΔH (J/mol) is the isosteric heat of adsorption for a specific amount of CO2 adsorbed, T (K) is temperature, R is the universal gas constant, and C is an integration constant. The CO2 isotherms were converted to CO2 adsorption isosteres, and lnp vs 1/T was plotted for a given adsorbed CO2 amount. On the basis of Eq. (1), the slope provided –ΔH/R, which was used to calculate the isosteric heats of adsorption for ARA, AROA, and CNT (Fig. 7). The isosteric heat of adsorption provides a measure of the interaction strength and affinity between CO2 and the adsorbent surface 48. The heats of adsorption at low values of CO2 loading in all tested carbon materials varied compared to those at higher pressures, with the trend to become nearly constant as the CO2 loading increased. Among the carbon materials, ARA and AROA showed higher heats of adsorption than the CNTs, i.e. about 21-23 kJ/mol at high CO2 loadings. In the case of ARA, the isosteric heat of adsorption increased with increasing CO2 loading, with the most notable increase observed at 1 mmol/g indicating accessibility to additional porosity as relative pressure and CO2 loading increase. At high pressures, the affinity of ARA towards CO2 became almost constant. In the case of AROA, the heat of adsorption continuously increased with CO2 loading. Overall, the reported heats of adsorption are relatively low and indicative of weak binding on the surface mainly by physisorption implying that the asphaltenederived carbons can be easily regenerated, which is advantageous for low-energy regeneration. Notably, in the case of CNTs, the heat of adsorption was higher at low CO2 loading, which is indicative of stronger interaction of the adsorbed CO2 molecules with the CNT surface upon monolayer formation. It should be noted that the treatment applied to remove the anodized alumina template might have introduced additional functionalities


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on the CNTs that enhance a stronger CO2 binding. As loading increases the heat of adsorption drops significantly from 27 to 12 kJ/mol owing to physical adsorption of additional CO2 in the bore space of the large-diameter CNTs and multilayer adsorption.

3.3. Gas transport behavior of the CNT membranes In order to examine the potential of the anodisc-templated CNT membranes produced from asphaltene for gas separation applications, combined single gas permeability and adsorption measurements for a variety of gases, namely CO2, N2, H2, and CH4, were made on the membranes. Gas transport phenomena through ceramic-based porous membranes typically combine the mechanisms of adsorption and diffusion. Aspects such as pore width and geometry, pore functionalities, thermodynamic conditions (T, P), and physicochemical properties of the transported gases (e.g., kinetic diameter) are crucial for their description. Figure 8 shows the evolution of the permeance values for the RA-CNT membranes and PA-CNT membranes with the increase in the differential pressure applied. The mean permeance values for all gases and for both membranes are provided in Table 4 and in Figure 9. It is evident that the order of permeance values for both membranes is H2 > CH4 > N2 > CO2, with the RA-CNT membrane being significantly more permeable than the PA-CNT one for all gases tested. In addition, permeance is almost independent of pressure for all gases in both membranes, which is indicative of a dominant Knudsen diffusion mechanism. Knudsen diffusion can describe the gas molecule movement inside a CNT pore, particularly at relatively low pressures. It occurs within pores whose diameter is smaller than the mean free path in the gas phase, and where the frequency of the collision of gas


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molecules with the pore walls is higher than that with each other 4. Its main characteristic is that the gas permeance value is independent of pressure, and is inversely proportional to the gas molecular weight. The gas separation is achieved due to the difference in the velocities of various gas molecules. The perm-selectivity (the theoretical selectivity value) of a membrane for a binary gas mixture within the Knudsen diffusion regime is equal to the square root of the inverse of the ratio of molecular weights of the two gases. In cases where the ratio of permeability values exceeds that of the Knudsen ratio, the membrane is considered to be capable of selectively separating the binary gas mixture. Table 5 shows the mean permeability ratios in relation to the Knudsen ratio of each gas pair for both RA- and PA-CNT membranes. The PA-CNT membrane seemed capable of selectively separating more gas pairs (N2/CH4, H2/CH4, H2/CO2, N2/CO2) than the RACNT membrane (CO2/CH4, N2/CH4, N2/CO2). In addition, for two gas pairs (H2/CH4 and H2/CO2), the RA membrane permeability ratios were much lower than the theoretical ratio, which may indicate a change in the RA membrane flow regime from Knudsendominated flow (permeance independent of the gas pressure) to Poisseuille (permeance proportional to the gas pressure). This behavior may be attributed to the presence of larger channels in that membrane. In order to further understand the diffusion mechanism within the asphaltenederived CNTs, sorption isotherms for the as-prepared RA- and PA-CNT membranes without removing the anodized alumina template were collected for gas pressures up to 1 bar (Figure 10). The gas sorption trend is as follows: CO2 > N2 > H2 for both membrane types, with CO2 being highly adsorbed, while H2 being very poorly adsorbed. The N2 sorption isotherm was nearly identical for both membranes. The CH4 molecules seemed


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to interact more with the PA-CNT membrane as compared to RA-CNT, as the sorption capacity was higher with the former one than with the latter membrane. By comparing the sorption with the permeance profiles (Figure 9) the gas sorption trend was opposite to the order followed by the permeability values. Indeed, CO2 being the most highly adsorbed gas, exhibited the lowest permeability, while the poorly adsorbed H2 was the one with the highest permeability. This implies that the surface diffusion did not contribute significantly to the overall diffusion mechanism under the conditions tested. PA may have formed thicker CNT walls and/or walls with higher amount of internal surface imperfections/narrowings similar to our previous observations in CNT membranes produced from ferrofluid precursors 5, leading to a membrane with a higher variability in internal diameter and increased selectivity than the membrane formed from RA.

4. Conclusions The potential of asphaltene, an abundant crude oil byproduct of low economic value, for the formation of high quality carbon nanomaterials for gas separation applications was demonstrated. It was shown that asphaltene is a favorable precursor for the production of a series of microporous activated carbon adsorbents for CO2 capture, with CO2 uptakes above 5 mmol/g at 25°C and 4 bar. Among the various materials formed, the AC developed by activating raw asphaltene using KOH (ARA) exhibited a CO2 uptake of 7.56 mmol/g at the above conditions, which is one of the highest values reported for activated carbon materials. The produced adsorbents exhibit relatively low heats of adsorption providing the potential for low-energy regeneration, with the highest


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value being observed for ARA (23 kJ/mol) due to its higher affinity towards CO2. Despite its moderate surface area (1590 m2/g), ARA performed superiorly owing to the high concentration of basic functional groups on its surface originated from the asphaltene precursor and the applied pre-treatment. The asphaltene precursors were also employed to produce CNT membranes by carbonization in anodized alumina template pores. The spatial regulation of the carbonization process as directed by the nanospace of the utilized alumina template made it possible to control structural parameters of the produced CNT pores such us diameter, length, and cap opening. The CNT growth took place by asphaltene pyrolysis in the template channels without involvement of any external metal catalyst, owing to the inherent existence of metal heteroatoms in the asphaltene that acted catalytically and nucleated graphitization and CNT growth. The produced CNT membranes exhibited high permeance values for a variety of gases and good selectivities, i.e. approximately 5 for the H2/CO2 pair. The purified asphaltene (PA) may have yielded thicker and non-uniform internal CNT walls contributing to attaining higher selectivity than the one produced from raw asphaltene (RA). Gas separation using the produced CNT membranes was achieved due to difference in diffusivities of the gas molecules through the CNT pores for H2/CO2, N2/CO2, N2/CH4, and H2/CH4 gas pairs. The potential of the asphaltene precursor was therefore demonstrated towards an economical, single-step, and catalyst-free route for the production of monodispersed CNT arrays that, after tuning of the CNT diameter by selecting the proper template, could be further exploited towards use in membrane separation units, catalytic reactors, and electronic devices. Overall, the reported results indicate that the abundant asphaltene from the petroleum industry could be a potentially inexpensive yet valuable source for


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producing a variety of carbon nanomaterials for capture and separation of gases of industrial interest and for meeting the anticipated increasing demand for large amounts of low-cost adsorbents and membranes.

Acknowledgements Financial support by the Abu Dhabi National Oil Company R&D division (project RDProj.018-GP) and the Gas Research Center (project GRC16002) is greatly appreciated. We thank Ms. Sudha Prasad and Dr. Eirini Siranidi for assistance with material characterization.

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Figures: 50 μm

50 μm

(b)

(a)

(d) 50 μm

(c)

(e)


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Figure 1: SEM micrographs of (a) PA (b) APOA, and (c) APA. (d) N2 adsorption isotherms of APA and APOA. (e) N2 adsorption isotherm of PA (without activation) showing no evident porosity.

50 μm

50 μm

(b)

(a)

50 μm

(c)

(d)


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Figure 2: SEM micrographs of (a) RA, (b) AROA, and (c) ARA. (d) N2 adsorption isotherms of ARA and AROA.

FTIR

100,9 100,4 99,9

% Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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99,4 98,9 98,4 97,9 97,4 96,9 96,4 95,9 700

1000

1300

1600

1900

2200

2500

Wavenumber

2800

3100

3400

3700

4000

(cm-1 )

Activated Purified Asphaltene (APA)

Activated Raw Asphaltene (ARA)

Activated Purified Oxidized Asphaltene (APOA)

Activated Raw Oxidized Asphaltene (AROA)

Raw Asphaltene (RA)

Purified Asphaltene (PA)

Figure 3: FTIR spectra of RA and PA asphaltenes and the resulting activated carbons.


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10 μm

1 μm

(a)

(b)

100 nm

3000

(d)

(c) Raman Intensity (a.u)

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2000

1000

ACS Paragon Plus Environment 0

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Figure 4: SEM images of (a), (b) cross section view, and (c) top-view of PA-derived CNT arrays after template removal. (d) Raman spectra of the PA-CNT arrays taken at the top side.

1 μm

1 μm

(a)

(b)

100 nm

(c) ACS Paragon Plus Environment 2000 u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(d)

Figure 5: SEM images of (a), (b) cross section view and (c) top-vew of RA-derived CNT arrays after template removal. (d) Raman spectra of RA-CNT taken from the top side.

(a)

APA

APA-CO2-25°C APA-CO2-50°C APA-CO2-100°C

6 CO2 Sorption (mmol/g)

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5 4 3 2 1 0 0

1

2 Pressure/bar

3

(b)


4

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(c)


Energy & Fuels

(c) 8

ARA

ARA-CO2-25°C ARA-CO2-50°C ARA-CO2-100°C

7

CO2 Sorption (mmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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6 5 4 3 2 1 0 0

1

2 Pressur/bar

3

(d)


4

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(e) 3 CO2 Sorption (mmol/g)

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CNT-C5-CO2-25°C CNT-C5-CO2-50°C CNT-Raw-CO2-25°C CNT-Raw-CO2-50°C

2

CNT

1

0 0

1

2 Pressure/bar

3

4

Figure 6: CO2 adsorption isotherms of (a) APA, (b) APOA, (c) ARA, and (d) AROA at 25, 50 and 100°C, and (e) RA- and PA-CNTs at 25 and 50°C after removal of the anodized alumina template.


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30

ARA

AROA

CNT

25

ΔH/(KJ.mol-1 )

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20 15 10 5 0 0

0,5

1

1,5

2

2,5

Q/mmol.g-1

Figure 7: Isosteric heats of CO2 adsorption on asphaltene-derived activated carbons and CNTs.


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(a)

(b)


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 8: Permeances of various gases as a function of the differential pressure for (a) RA-CNT membranes, and (b) PA-CNT membranes.

Figure 9: Mean permeance values of different gases through RA- and PA-CNT membranes.


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(a)

(b)


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 10: Gas adsorption isotherms of a) RA-CNT membranes, and b) PA-CNT membranes.

Tables: Table 1: Porosity characteristics of the produced activated carbons. Sample

BET Surface

Total pore volume

Average pore diameter

area (m2/g)

(cc/g)

(°A)

APA

1605

0.79

9.85

APOA

2358

1.02

8.63

ARA

1590

0.87

10.92

AROA

2894

1.39

9.59

Table 2: EDS analysis of asphaltene and activated carbon samples. Sample

Elemental Report (wt %) C

N


O

S

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APA

76.63

--

19.80

2.39

APOA

76.19

--

22.66

1.09

RA

93.09

0.6

3.32

1.98

PA

82.68

0.84

13.41

2.19

ARA

77.67

--

20.34

0.11

AROA

80.16

--

17.98

---

Table 3: Heavy metal ICP-MS analysis of the asphaltene precursors RA

PA

(ppb)

(ppb)

Element

Li

3514.4

3150.8

B

4939.8

6266.6

Mg

210307.1

119487.9

P

116240.2

101922.4

Ca

533914.6

263606.3

V

22817.0

36782.2

Mn

21482.0

3491.9

Fe

294620.7

95127.4


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Co

534.7

182.3

Ni

146952.2

214206.1

Cu

191183.8

32086.7

Sr

119015.4

105081.5

Zr

267.7

83.8

Ag

3291.1

629.7

Cd

520.9

76.1

La

259.4

1.6

Pb

14547.7

3502.1

Bi

949.4

213.8

Table 4: Mean gas permeance values for RA- and PA-CNT membranes.

Permeance (mol/m2/sec/Pa) H2

CH4

N2

CO2

-7 -7 -7 -7 RA 8.41 x 10 5.42 x 10 4.66 x 10 3.34 x 10

PA 3.30 x 10-7 1.15 x 10-7 9.11 x 10-8 6.79 x 10-8

Table 5: Mean permeability value ratios in relation to the respective Knudsen ratios for each gas pair for RA- and PA-CNT membranes.


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Energy & Fuels

Knudsen

P1/P2

P1/P2

√(M2/M1)

RA

PA

CO2/CH4

0.60

0.62

0.59

N2/CH4

0.76

0.86

0.79

H2/CH4

2.83

1.55

2.88

H2/CO2

4.69

2.52

4.85

N2/CO2

1.25

1.40

1.34

H2/N2

3.74

1.80

3.62

Graphical Image:


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Asphaltene-Derived Activated Carbon and Carbon Nanotube

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