Amphiphilic-Polymer-Coated Carbon Nanotubes as Promoters for

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Research Article pubs.acs.org/journal/ascecg

Amphiphilic-Polymer-Coated Carbon Nanotubes as Promoters for Methane Hydrate Formation Yuan-Mei Song,†,‡,§ Fei Wang,†,‡,§ Gang Guo,† Sheng-Jun Luo,*,† and Rong-Bo Guo*,† †

Shandong Industrial Engineering Laboratory of Biogas Production & Utilization, Key Laboratory of Biofuels, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, No. 189 Songling Road, Qingdao 266101, Shandong, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 10049, P. R. China S Supporting Information *

ABSTRACT: Despite being a good promoter for methane hydrate formation, carbon nanotubes (CNTs) aggregate easily, which weakens their promotion effect. In this work, highly dispersed carbon nanotubes (CNTs) were prepared by coating the nanotube surface with an amphiphilic copolymer, namely, poly(styrene-co-sodium styrenesulfonate) (PSCS), to accelerate methane hydrate formation. In the presence of functionalized CNTs (f-CNTs), the hydrate storage capacity increased from 46 ± 8 to 138 ± 8 v/v when the PSCS/CNT mass ratio (R) was increased from 0 to 20. In addition, the hydrate formation period gradually shortened as the R value was increased from 5 to 20. The methane uptake was also enhanced from 68 to 135 mmol of gas/(mol of water) as the f-CNT loading was changed from 10 to 150 ppm. Meanwhile, hydrate formation could be completed within 100 min in f-CNT dispersions with high concentrations. Compared with those formed using sodium dodecyl sulfate (SDS) as the promoter, the hydrates formed with f-CNTs as the promoter mainly aggregated at the reactor bottom and generated no foams during the dissociation process. Moreover, the f-CNTs showed excellent recycling performance in eight repeated hydrate formation−dissociation processes with high gas storage capacities of 120−132 v/v, which therefore is of great significance for hydrate-based gas storage and transportation. KEYWORDS: Methane hydrates, Promoters, Polymer-coated CNTs, Formation rate, Recycling

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INTRODUCTION Gas hydrates are icelike clathrate compounds that are generated when small-sized gas molecules, such as methane, carbon dioxide, and ethane, interact with water molecules at high pressure and low temperature. The guest gases are trapped in water-formed cages by van der Waals forces, and the cage structures are linked by hydrogen bonds.1 With distinguishing features of economic feasibility, mild production conditions, flexibility, and high gas storage capacities, hydrate-based technologies have provided significant prospects in a variety of applications such as natural gas storage and transportation, gas separation, and CO2 capture,2−4 among which the storage and transportation of natural gas in hydrates is assumed to be a promising method for replacing the traditional approaches using liquefied and compressed gases. However, the industrial scaleup and commercial use of hydrated-based technology are challenged by the long induction time and low growth rate of hydrate formation, as well as the difficulties of separating and packaging large amounts of hydrates.5 Researchers have proposed that the introduction of surfactants could effectively mitigate such challenges. Sodium dodecyl sulfate (SDS) ranks as the most efficient kinetic © 2017 American Chemical Society

promoter for enhancing the hydrate formation process. Nevertheless, in SDS solutions, hydrates tend to grow along the reactor walls in the form of porous structures because of capillary effects,6 resulting in the inconvenient separating and packaging of hydrates. In addition, large amounts of foams are always generated during the methane hydrate dissociation process in the presence of SDS, which adversely affects methane recovery and causes surfactant loss.7 As an alternative, nonsurfactant-based means for enhancing the hydrate formation rate have evoked extensive interest over the past 10 years, for instance, the addition of amino acids or tetrahydrofuran (THF) and the use of such fixed-bed supports as silica gel, activated carbon, and dry water.8−10 Nanoparticles have also been found to improve hydrate formation by increasing either mass transfer or heat transfer. Silver, copper, aluminum, and polystyrene nanospheres were all found to show good performance in increasing the gas uptake and hydrate formation rate.7,11,12 The emergent material of carbon nanoReceived: July 5, 2017 Revised: August 4, 2017 Published: August 29, 2017 9271

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exist as soluble macromolecules.20 Thus, the mass ratio of 1:1 was selected to ensure the formation of amphiphilic polymer molecules rather than nanospheres. First, SS solution was prepared by dissolving 1 g of SS in 94 mL of deionized water and then transferred into a flask that had been equipped with a heating device and a Teflon stirrer. Then, the reaction temperature was set to 343.1 K, and the stirring rate was set to 300 rpm. After styrene had been added to the solution drop by drop to form a homogeneous mixture, three doses of KPS solution (0.1 g in 2 mL of water) were added dropwise into the reaction mixture in intervals of 30 min to initiate polymerization and to avoid explosive polymerization in the case of the one-time addition of KPS. With N2 protection and refluxing, the polymerization process was continued for 6 h until a transparent yellow solution was obtained. The yellow product indicated the successful synthesis of poly(styreneco-sodium styrenesulfonate), abbreviated as PSCS. Preparation of Functionalized Carbon Nanotubes. Liquid dispersions of carbon nanotubes were prepared by dispersing 30 mg of carbon nanotubes into 30 mL of poly(styrene-co-sodium styrenesulfonate) solution with the aid of an ultrasonic apparatus (Skymen, JP040ST) at 40 W for 3 h. The amount of copolymer was adjusted to obtain various mass ratios (R) of CNTs to PSCS (0, 0.5, 1, 2, 5, 10, 20), whereas the CNT concentration was kept constant at 1 mg/mL. The CNTs gradually became homogeneously distributed in the aqueous phase upon continuous sonication. Once sonication had finished, the CNTs were recovered by filtration through a 0.2-μmpore-size polytetrafluoroethylene (PTFE) membrane and washed with ethanol and deionized water three times each to eliminate the free copolymer. The filtered residues were collected and vacuum-dried at 55 °C overnight to obtain the amphiphilic-polymer-functionalized carbon nanotubes (denoted as f-CNTs). f-CNT dispersions were formed when the f-CNTs were dissolved in deionized water under ultrasound. Characterization Methods. The obtained CNT dispersions were diluted 40-fold for dispersability testing. Specifically, the intensity of absorbance at a wavelength of 260 nm was measured with a UV−vis spectrophotometer (UV2000, UNICO), and the average diameter of the carbon nanotubes was obtained using a Malvern Nano-s90 laser particle size analyzer from Malvern Instruments (Malvern, U.K.). The morphologies of the carbon nanotubes were observed by transmission electron microscopy (TEM) with a JEM-1200EX instrument from Japan Electronics Co., Ltd. In addition, the functional groups attached to the surface of the CNTs were determined by infrared spectroscopy of the materials using a Nicolet iN 10 infrared spectrometer (Thermo Fisher Scientific). Methane Hydrate Formation. The setup designed for methane hydrate formation is shown in Figure 2. It included four main parts:

tubes (CNTs), as one-dimensional tubular nanostructures, have excellent prospects for accelerating hydrate formation, considering their high thermal conductivity and the microstirring effects of nanotubes in hydrate-forming systems.13 Park et al.14 found that methane consumption could be increased by 4.5 times when multiwalled carbon nanotubes were used. Kim et al.15 concluded that short carbon nanotubes were more effective in decreasing the hydrate formation time than longer CNTs. In contrast, CNTs that agglomerate easily can dramatically weaken their promotion effect, taking the reduction of the available surface area of the nanotubes into account.16 Some modifications of the CNT surface have been carried out to obtain highly dispersed nanotubes, with the surfactants SDS and sodium dodecyl benzenesulfonate (SDBS) exhibiting great stabilization abilities by encapsulating nanotubes in the form of micelles or semimicelles.17,18 Moreover, nonsurfactant amphiphilic polymers such as poly(sodium 4styrenesulfonate) (PSS) containing hydrophilic SO3− groups were confirmed to successfully coat the surfaces of nanotubes, which also enabled the nanotubes to be more soluble in water.19 According to Wang and co-workers,20 who studied the impacts of functional groups on hydrate formation, the presence of the SO3− group (the same group as contained in SDS) has a significant influence on methane hydrate formation, which thus suggests a potential use of amphiphilic-polymercoated CNTs in hydrate formation systems. In the present study, poly(styrene-co-sodium styrenesulfonate) was chemically synthesized and used to coat the surface of carbon nanotubes. Afterward, the amphiphilic-polymerfunctionalized carbon nanotubes were applied for the first time in methane hydrate formation to explore the promotion effect of the highly dispersed CNTs on the methane hydrate formation kinetics and gas storage capacity.

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EXPERIMENTAL SECTION

Materials. Multiwalled carbon nanotubes (purity > 99.8%, diameter ≈ 8 nm, length ≈ 2 μm) that had been synthesized by chemical vapor deposition were purchased from Xfnano, Inc. Sodium p-styrenesulfonate (SS, AR, 90%) was obtained from Aladdin. Styrene (St, AR, 98%) was purchased from Tianjin Guangcheng Chemical Reagent Co. Ltd. (Tianjin, China). Potassium persulfate (KPS, AR, 99.5%) was obtained from Hengxing Chemical Reagent Co. Ltd. (Tianjin, China). The deionized water used in the experiments was laboratory-made and had a conductivity of 0.014 μs/cm at 293.15 K. Preparation of Poly(styrene-co-sodium styrenesulfonate). The synthesis of the amphiphilic copolymer was carried out in a 250 mL round-bottom flask by a free-radical polymerization process in which sodium styrenesulfonate acted as the hydrophilic monomer, styrene acted as the hydrophobic monomer, and KPS functioned as an initiator. The formation process is shown in Figure 1. Based on our previous studies on copolymer synthesis, recipes using St/SS mass ratios of less than 2:1 are more likely to form long-chain polymers that

Figure 2. Schematic diagram of the reactor system for methane hydrate formation. gas cylinder, reactor, cooling system, and data collection system. The 80 mL reactor, made of 316L stainless steel, was equipped with one BP801 pressure transducer with an uncertainty of 0.01 MPa and a PT100 temperature transducer with an uncertainty of 0.01 K, with which live data were transmitted to a computer automatically. To maintain a low temperature for hydrate formation, the whole reactor was immersed in a thermostatic water bath composed of glycol and

Figure 1. Formation process for poly(styrene-co-sodium styrenesulfonate). 9272

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ACS Sustainable Chemistry & Engineering water in a volume ratio of 1:2. Moreover, the agitation of the reagent was realized with a rotator controlled by a magnetic apparatus installed under the reactor. Before each reaction started, the reactor was rinsed with deionized water three times to remove any residues and then charged with 10 mL of reaction solution. Afterward, the sealed reactor was cooled until the temperature inside the reactor decreased to the reaction temperature of 275.15 K with a stirring rate of 300 rpm. Air in the reactor was evacuated by three cycles of methane flushing, and then 6 MPa of methane was pressurized into the reactor. The evolutions of the temperature and pressure throughout the whole hydrate formation process were recorded on the computer. The methane consumption (in millimoles of gas per mole of water), denoted as nt, which also refers to the methane contained in the hydrates during hydrate formation, was calculated according to the following equation derived by Wang et al.7

⎛ PV PV ⎞ nt = ⎜ 0 0 − t 0 ⎟ zt RTt ⎠ ⎝ z 0RT0

⎡ ⎛ P ΔVm ⎞⎤ ⎢n w ⎜1 − t ⎟⎥ ⎢⎣ ⎝ ZtRTt ⎠⎥⎦

(1)

where P, V, and T are the pressure, gas volume, and temperature, respectively, in the reactor; R is the universal gas constant; m is the hydration number, whose value is 5.75 for methane;1 ΔV is the difference in molar volume between water and methane hydrate, whose value is 4.6 cm3 per mole of water;21 nw is the number of moles of water consumed during hydrate formation; z represents the compressibility factor, which was derived from Pitzer correlations;22 and the subscripts 0 and t represent the initial time and any time point during the hydrate formation process, respectively. The gas storage capacity (i.e., the amount of methane stored per unit volume of hydrate, v/v) of the formed hydrate was obtained according to the equation cs =

Figure 3. (A) Average diameter and absorbance intensity of carbon nanotubes with different R values. (B) Photographs of CNT dispersions after 1 month.

nVmgVmw Vw(Vmw + ΔV )

after which it decreased quite slowly as R was increased further, whereas the absorbance intensity (blue line) exhibited a completely opposite trend. These results demonstrate that the polymers played a key role in dispersing the aggregated carbon nanotubes. Higher amounts of amphiphilic polymers contributed more to exfoliated CNT bundles, especially when the amount of polymer added was 5 times higher than the amount of CNTs. In view of the macroscopic observations in Figure 3B, the f-CNT dispersions behaved differently when the samples were kept still for 1 month after sonication, with the dispersions with R values greater than 5 maintained high stability without precipitation, whereas poorer stability was presented by dispersions with R values less than 5. The microscopic morphology and structure of the functionalized carbon nanotubes can be observed from Figure 4A−E, where the original CNTs existed in the form of entwined aggregates whereas the aggregates were gradually dispersed into numerous individuals at higher R values. As shown in Figure 4B,C, for smaller amounts of polymer, there were still some overlapped nanotubes, possibly because of the insufficient wrapping of polymers on the surface of the nanotubes. Evidence for this conclusion is further provided by Figure 4C, where some nanotubes (as indicated by the black arrow) resembled irregular cylinders without clear hollow tubes because of the polymer coverage whereas others appeared as thin tubes (as indicated by white arrow). When R was increased to 10, the uniform distribution of nanotubes (shown in Figure 4D) illustrates that the nanotubes could be effectively dispersed with a sufficient polymer coating. An amorphous shell was coated conformally along the surface of the CNTs (shown in Figure 4E). Thus, by means of polymer wrapping, the dispersability of the CNTs in polar solvents was improved dramatically.

(2)

where n represents the number of moles of gas consumed during hydrate formation; Vmg and Vmw are the molar volumes of the gas and water under standard conditions; Vw is the volume of water used in gas hydrate formation; and ΔV is the difference in molar volume between water and the methane hydrate. The recycling performance of the functionalized CNTs in hydrate formation was also studied by the following process: After the reaction had finished, the reactor was depressurized and kept at room temperature for 2 h to allow the methane hydrate to dissociate completely. Then, when the temperature inside the reactor was decreased to 275.15 K with the help of the cooling system, methane gas was again charged into the reactor to 6 MPa, and a second cycle of hydrate formation was performed. Additional cycles were achieved by repeating the above procedures. Before the next cycle of methane hydrate formation, the f-CNT dispersion was sampled, and the corresponding absorbance intensity was measured.

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RESULTS AND DISCUSSION Characterization of Functionalized Carbon Nanotubes. The dispersities and stabilities of the obtained dispersions of functionalized carbon nanotubes with different R values are shown in Figure 3. The diameter measured using the laser particle size analyzer was a rough average value at a fixed position, which can indirectly reflect the extent of exfoliation of CNTs bundled by the copolymer. The absorbance intensity is an important indicator of the individual nanotubes suspended in an aqueous dispersion.23 In Figure 3A, for the dispersion of pristine CNTs, the absorbance intensity was close to zero, and the diameter was up to 4400 nm, indicating serious aggregation of the CNTs into large masses through strong hydrophobic interactions. The average diameter decreased sharply when the R value was increased from 0 to 5, 9273

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Figure 4. (A−E) TEM images of functionalized CNTs with different R values. (F) Proposed mechanism for polymer-assisted dispersion of nanotubes.

suspended stably in water, further confirming the above results for TEM and the dispersability tests. Methane Hydrate Formation. The amphiphilic-blockcopolymer-functionalized CNTs were dissolved in water under ultrasound to form 50 ppm dispersions, which were then used in the methane hydrate formation process. Given the stochastic nature of hydrate formation,1 each experiment was repeated three times, and the results were processed by an averaging method. The evolution of the methane consumption during the hydrate formation process, with error bars, is shown in Figure S1 of the Supporting Information. To avoid the overlapping of error bars, Figure 6 presents only the average methane consumption for the three repetitions, where the mass ratio of PSCS to CNTs was varied from 0 to 20 and the deionized water system served as the control group. Without promoters, slow methane consumption proceeded under stirring until the amount of methane consumed reached a small value of 32 mmol of gas/(mol of water). At R0 and R1, the final average methane uptake of about 45 mmol of gas/(mol of water) was slightly higher than that obtained for the pure water system. Further increasing R to values higher than 5 increased the amount of gas consumed to about 140 mmol of gas/(mol of water). The corresponding gas storage capacity reached about 138 ± 8 v/v, much higher than the values of 46 ± 8 v/v for R0 and R1 and 28 ± 8 v/v for deionized water. Moreover, observations of the hydrate formation process for R > 5 revealed that the hydrate formation period was shortened with increasing R, as the time required for the reaction to be completed was reduced from 200 min at R5 to 100 min at R20. Therefore, the polymer coated on the surface of the CNTs had

On the basis of the above analysis, we propose that the synthesized amphiphilic copolymer, poly(styrene-co-sodium styrenesulfonate), can enable CNTs to be soluble in water through the electrostatic repulsive force of SO3− groups attached to the nanotube surface, by analogy with the stabilization of colloidal particle, as described in Figure 4F. On the other hand, supported by sonication, the hydrophobic blocks (aromatic rings) of the polymer could also exert an “unzippering force” to exfoliate the CNT bundles by anchoring on the surface of CNTs through π−π stacking interactions while the hydrophilic parts were stretched out to prevent the CNTs from aggregating.24 To confirm the successful grafting of copolymer on the surface of the CNTs, the infrared spectra of the dried f-CNTs were obtained, as shown in Figure 5. In the spectrum of the copolymer, the characteristic band of SO3− mainly appears in the range from 1000 to 1100 cm−1. The peaks at about 1041 and 1150 cm−1 represent the symmetric stretching vibration and asymmetric vibration, respectively, of the SO3− group.25 The spectral range of 800−1300 cm−1 was magnified so that the differences in the peaks could be more easily distinguished, as shown in Figure 5B. The pristine CNTs barely showed any obvious peaks, whereas the peaks at 1041 and 1150 cm−1 became more distinct with increasing R, especially for samples R10 and R20. This reveals that low dosages of polymers were not sufficient to coat the CNT surfaces successfully or that the amount of grafted polymer was too low to be detected. When the amount of PSCS was increased, the successful grafting of a sufficient amount of polymer on the surface would strengthen the characteristic peaks. Therefore, functionalization using amphiphilic polymer could be helpful for CNTs to be 9274

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grow downward,26 a thin icelike layer forms to insulate the diffusing gas when hydrate reacts with deionized water, which thus causes a low extent of the formation process and a small gas storage capacity. Carbon nanotubes have been reported to be effective microstirrers with continuous Brownian motion in the reaction system to increase the interfacial area between the gas and the liquid phase.27,28 Then, the thin hydrate layer formed would be broken down by those numerous stirrers, and the hydrates would further grow with the enhanced gas diffusion. On the other hand, the nanofluid is helpful for removing the heat generated during the exothermic hydrate formation process and thereby enhancing the hydrate growth. However, the promotion effect of the carbon nanotubes is restricted by their aggregation due to their intrinsic hydrophobicity, as discussed above in terms of the fact that the CNTs with R0 and R1 had only slightly better effects on hydrate formation than deionized water. When the CNTs had a high dispersability (at R > 5), there were more effective individual nanotubes to take part in enhancing mass transfer, which accounted for the rapid hydrate formation at higher R values. Afterward, a series of concentrations of f-CNTs with a constant ratio of R10 was investigated in methane hydrate formation. The corresponding evolutions of the average gas consumption are summarized in Figure 7 (more details for

Figure 5. (A) Infrared spectra of PSCS and f-CNTs with different R values. (B) Magnification of the spectra l range in the rectangular area in panel A.

Figure 7. Evolution of the methane consumption at various CNT loadings (R10; 6 MPa, 275.15 K, and 300 rpm).

three repeated results are shown in Figure S2 of the Supporting Information). When the concentration was as low as 10 ppm, the final amount of methane consumed was 68 mmol/(mol of water), also slightly higher than for the deionized water system. Then, the amount of methane uptake rose to 135 mmol/(mol of water) when the f-CNT concentration was increased to 20 ppm. This shows that the gas consumption was enhanced by concentrated CNTs, probably because enough gas diffused into the reaction system with improved mass transfer by more nanoparticles. Despite the equal gas consumptions at the end of each reaction with 20−150 ppm f-CNTs, the formation period was gradually reduced from 200 to 80 min. As the difference between 100 and 150 ppm was quite small taking the error bars into consideration, it can be inferred that a concentration of fCNTs higher than 100 ppm had almost similar excellent promotion effect on the methane hydrate formation. Based on the report that the CNT surface with numbers of defects could potentially act as nucleation sites, the methane hydrate nuclei

Figure 6. Evolution of the methane consumption during methane hydrate formation at different R values (6 MPa, 275.15 K, and 300 rpm).

a positive influence on both the gas storage capacity and the hydrate formation rate. As hydrate formation is an interface phenomenon in which hydrates are mainly formed at the gas−liquid interface and 9275

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broader with increasing number of cycles, possibly because of the aggregation of some nanotubes with the coated polymers gradually peeling off from CNT surface. As a result, the final gas storage capacities of samples C7 and C8 were slightly lower than those of samples C1−C6. Without regard for the slight differences among the eight hydrate formation processes, the functionalized CNTs were still equipped with excellent recycling performance, which is of great significance for the utilization of f-CNTs in hydrate formation. Considering the structural properties of SDS, we have successfully grafted the CNT surface with a certain amount of SO3− groups by coating the amphiphilic copolymer. For the same loading of 50 ppm for SDS and f-CNTs, the hydrate formation processes in these two promoter systems are compared in Figure 10. For the whole reaction process, the

could formed rapidly in the liquid water phase with the aid of these nanotubes.16 Therefore, the existence of more polymerwrapped nanotubes in the reaction system would contribute more to rapid methane hydrate formation. The recycling performance of the f-CNTs in the hydrate formation was then examined. The methane gas storage capacity during eight hydrate formation−dissociation cycles was obtained in 50 ppm f-CNT (R10) dispersions. As shown in Figure 8, each hydrate formation process went through a similar

Figure 8. Changes in storage capacity during eight cycles of the hydrate formation−dissociation process in a 50 ppm f-CNT dispersion (R10). (Ci indicates the ith hydrate formation−dissociation cycle.) Figure 10. Comparison between SDS and f-CNTs: (A) Evolution of the methane consumption during hydrate formation. (B,C) Hydrate morphologies in the reactor with (B) SDS and (C) f-CNTs as the promoter.

period of about 100 min, illustrating that the promotion effect of f-CNTs on the hydrate formation rate was barely affected by recycling usage. The gas storage capacities in these cycles all showed high values of 120−132 v/v except for slight variations. To determine the property changes of the used f-CNTs, the absorbance intensities of f-CNT dispersions after each cycling process were measured. Figure 9 presents the relative absorbances of the used dispersions after each cycle based on the fresh f-CNT dispersion. The negative ΔA values represent the poorer dispersability of CNTs compared with the fresh sample. It is clear that the ΔA gap tends to become a little

hydrate formation in SDS solution completed earlier than that in f-CNT dispersions. However, in the initial 30 min, the methane uptake in f-CNT solution was higher because the hydrates began to grow rapidly without an induction period once the methane was charged, whereas an occasional induction period always preceded the hydrate growth stage when SDS was used. However, after the induction period, the hydrates grew rapidly in SDS solutions because of the excellent promotion effect caused by the capillary effect. Unlike in SDS solution, a thin hydrate layer would form in the CNT suspension because of the initial rapidly formed hydrate layer that limited the gas diffusion into the liquid bulk and further limited the rapid hydrate formation, which explains the lower hydrate growth rate after 30 min. However, with the capillary effect, a hydrate shell covered the sidewall of the reactor with SDS as the promoter (Figure 10C), causing difficulties in separating the adhered hydrates and compacting in the transportation and storage processes. In contrast, when fCNTs were used, the aggregation of the hydrates at the reactor bottom (Figure 10B) could improve the maneuverability of methane hydrates. Furthermore, the SDS-containing hydrates generated considerable foams during the dissociation process, as shown in Video S1. This phenomenon is adverse to the actual hydrate-based application and causes chemical loss. When f-CNTs were applied, the hydrates dissociation were mild without noticeable foaming, as shown in Video S2.

Figure 9. Changes in absorbance of used CNT dispersions compared with pristine f-CNT dispersions as a function of the number of hydrate formation−dissociation cycles. 9276

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ACKNOWLEDGMENTS This work was supported by the China Postdoctoral Science Foundation funded project (2017M612370) and Shandong Province Key Research and Development Plan (2017GSF16106).

Associated with the above results, f-CNTs can be recycled and reused as excellent promoters without a weakened promotion effect on hydrate formation, whereas SDS gradually runs out with repeated cycles of the hydrate formation−dissociation process, causing serious waste. Thus, in view of both economic and practical feasibility in the hydrate-based storage and transportation, f-CNTs are more suitable as efficient promoters for natural gas hydrate formation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02239. Video of the dissociation morphology of methane hydrates formed with SDS (AVI) Video of the dissociation morphology of methane hydrates formed with f-CNTs at a concentration of 50 ppm (AVI) Results for repeated experiments (Figures S1 and S2) (PDF)

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REFERENCES

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CONCLUSIONS Highly dispersed carbon nanotubes were obtained through surface functionalization with a certain amount of synthesized poly(styrene-co-sodium styrenesulfonate). On the basis of TEM, particle size, and absorbance analyses, the f-CNTs were endowed with excellent dispersability in water when the PSCS/ CNT mass ratio was greater than 5. When applied in methane hydrate formation, the f-CNTs showed an efficient promotion effect compared with pristine CNTs. The gas storage capacity was up to 138 ± 8 v/v, and the hydrate formation period was shortened to 100 min with the aid of f-CNTs especially when R higher 5. Moreover, f-CNTs at higher concentrations of 20− 150 ppm were more beneficial in shortening the hydrate formation process and enhancing the gas consumption compared with lower f-CNT concentrations owing to the numerous effective microstirrers in the reaction system. The eight-cycle testing of the f-CNTs demonstrated the excellent recycling performance in promoting the hydrate formation process with a similar gas storage capacity of 120−132 v/v. Moreover, compared with SDS solutions, no foams were generated in f-CNT dispersions during hydrate dissociation, and the hydrate formed at the bottom of the reactor, both of which facilitate actual hydrate applications. In spite of weak noncovalently attached polymers on CNT surfaces, the polymer-modified nanomaterials still provided great potential in further improving hydrate-based technologies for gas storage and transportation.

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Corresponding Authors

*E-mail: [email protected]. Tel.: +86-532-80662750. *E-mail: [email protected]. Tel.: +86-532-80662708. ORCID

Fei Wang: 0000-0002-2811-7636 Rong-Bo Guo: 0000-0003-1880-2960 Author Contributions §

Y.-M.S. and F.W. contributed equally.

Notes

The authors declare no competing financial interest. 9277

DOI: 10.1021/acssuschemeng.7b02239 ACS Sustainable Chem. Eng. 2017, 5, 9271−9278

Research Article

ACS Sustainable Chemistry & Engineering (20) Wang, F.; Guo, G.; Luo, S. J.; Guo, R. B. Preparation of − SO3−coated nanopromoters for methane hydrate formation: effects of the existence pattern of − SO3− groups on the promotion efficiency. J. Mater. Chem. A 2017, 5, 2640−2648. (21) Makogon, Y. F. Hydrates of Hydrocarbons; PennWell Publishing Company: Tulsa, OK, 1997; pp 35−37. (22) Smith, J. M.; Van Ness, H. C.; Abbott, M. M. Introduction to Chemical Engineering Thermodynamics, 6th ed.; The McGraw-Hill Chemical Engineering Series; McGraw-Hill: New York, 2001; pp 96− 98. (23) Njuguna, J.; Vanli, O. A.; Liang, R. A review of spectral methods for dispersion characterization of carbon nanotubes in aqueous suspensions. J. Spectrosc. 2015, 2015, 1−11. (24) Yao, X. P.; Li, J.; Kong, L.; Wang, Y. Surface functionalization of carbon nanotubes by direct encapsulation with varying dosages of amphiphilic block copolymers. Nanotechnology 2015, 26, 325601. (25) Arunbabu, D.; Sanga, Z.; Seenimeera, K. M.; Jana, T. Emulsion copolymerization of styrene and sodium styrene sulfonate: kinetics, monomer reactivity ratios and copolymer properties. Polym. Int. 2009, 58, 88−96. (26) Lee, J. D.; Song, M.; Susilo, R.; Englezos, P. Dynamics of methane-propane clathrate hydrate crystal growth from liquid water with or without the presence of n-heptane. Cryst. Growth Des. 2006, 6, 1428−1439. (27) Pasieka, J.; Jorge, L.; Coulombe, S.; Servio, P. Effects of asproduced and amine-functionalized multi-wall carbon nanotubes on carbon dioxide hydrate formation. Energy Fuels 2015, 29, 5259−5266. (28) Pasieka, J.; Coulombe, S.; Servio, P. The effect of hydrophilic and hydrophobic multi-wall carbon nanotubes on methane dissolution rates in water at three phase equilibrium (V-L-w-H) conditions. Ind. Eng. Chem. Res. 2014, 53, 14519−14525.

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DOI: 10.1021/acssuschemeng.7b02239 ACS Sustainable Chem. Eng. 2017, 5, 9271−9278