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Biofuels and Biomass
Spectroscopic Studies on Formation and Guest Behaviors of Hydroquinone Clathrate with Binary CO and H2 Gas Mixtures Jong-Won Lee, Seo Hee Lee, Sang Jun Yoon, and Ji-Ho Yoon Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00837 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 19, 2018
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Energy & Fuels
Spectroscopic Studies on Formation and Guest Behaviors of Hydroquinone Clathrate with Binary CO and H2 Gas Mixtures Jong-Won Lee1, Seo Hee Lee1, Sang Jun Yoon2*, Ji-Ho Yoon3* 1
Department of Environmental Engineering, Kongju National University, 1223-24 Cheonandaero, Cheonan-si, Chungnam 31080 Republic of Korea
2
Clean Fuel Laboratory, Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea
3
Department of Energy and Resources Engineering, Korea Maritime and Ocean University, 727 Taejong-ro, Yeongdo-gu, Busan 49112, Republic of Korea
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ABSTRACT Syngas clathrate compounds were for the first time investigated using hydroquinone (HQ) with binary CO and H2 gas mixtures as syngas replicas. Combined with X-ray diffraction and Raman spectroscopy, solid-state 13C magic-angle spinning NMR measurements revealed that HQ molecules with the syngas mixtures were converted into the clathrate form with selective enclathration of CO partially or completely, depending on the experimental pressure and the CO concentration. When formed with gas mixtures of 50% and higher CO concentrations at 80 bar, more than 95% HQ molecules were converted to the HQ clathrates. The cage occupancy and stored amounts of CO molecules in the solid clathrates were estimated to be 20–50% and 13.6–33.9 L-CO/kg-HQ under STP conditions, respectively, when crosschecked with solid-state NMR and elemental analysis methods.
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Energy & Fuels
1. INTRODUCTION Syngas, or synthesis gas is a fuel gas mixture mainly comprising hydrogen (H2), carbon monoxide (CO), and some carbon dioxide (CO2). The basic feedstocks for production of synthesis gas are natural gas, methanol, LPG, naphtha, heavy oil, bitumen, and petroleum coke.1 In addition, gasification of coal and biomass was recently developed to produce synthesis gas instead of expensive petroleum resources. Although synthesis gas can be utilized as a fuel by itself, the separation and purification of H2 and CO from the synthesis gas are of great significance because CO is important as a starting material for the synthesis of various basic chemicals in C1 chemistry, and because H2 is expected to be a major energy resource in the future.1-2 Because the synthesis gas is a mixture of gas components, a variety of gas separation technologies are currently being used. Such technologies include: 1) cryogenic processes consisting of liquefaction of at least a part of the fuel stream, and phase separation and distillation of the remaining liquid components, 2) partial condensation cycles that involve liquefying CO in several cooling steps to leave a residual gas stream containing almost pure H2, 3) methane wash cycles using liquid CH4 to absorb CO from the feed to produce H2 product requiring pressures less than 10 bar, 4) liquid absorption processes adapting various organic and inorganic absorbents, and 5) physical adsorption processes by various adsorbents with the pressure swing adsorption (PSA) method requiring pressures up to tens of bar to produce high-purity product.1,3 Since the cost of the separation process contributes significantly to the overall production cost of H2 and CO from the synthesis gas, it is essential to achieve a cost-effective separation process to reduce the overall cost. In this regard, a water-gas shift reaction is a cost-effective way to convert CO into CO2, to separate it from the other components, and to easily produce high-purity product with high separation selectivity. In addition to these conventional technologies, emerging membrane processes 3 ACS Paragon Plus Environment
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have potential for these uses. Since they were first introduced in the mid-1970s, membrane processes have been expected to be good alternatives to conventional technologies due to their high selectivity and versatility.1,4 Solid and liquid membrane technologies are currently being investigated for application to gas separation processes of various gas mixtures, including synthesis gas.5-6 Another novel approach involving the use of clathrate compounds has also been reported for gas separation or storage. The clathrate compounds are crystalline solids formed by enclathration of guest species into three-dimensional cages in frameworks of hydrogenbonded host molecules.7 Since its identification for the first time in the 19th century, hundreds of guest species including light hydrocarbons, acid gases, and organic compounds, as well as inert and rare gases, have been known to form clathrate compounds.8 Many host species having the hydroxyl group for hydrogen bonds such as water, phenol, p-cresol, and hydroquinone (HQ) have also been known to form such compounds.9 The guest species are mainly gaseous materials with low molecular weights and the thermodynamic equilibria of the clathrate compounds vary depending on the guest species. This means that the clathrate compounds could be used to separate, concentrate, or recover specific components from mixtures of various gases by capturing them in the solid phase. Among the host materials forming gas hydrates, water is the most investigated compound for applications of gas separation/recovery so far.8 Kang et al. reported phase equilibrium measurements and suggested CO2 recovery from flue gas by forming gas hydrates.10 Many researchers have subsequently reported applications of gas hydrates for separation of various gas mixtures.11-14 Recently, an organic material, hydroquinone was proposed as an alternative host to overcome some disadvantages of gas hydrates (high energy consumption for cooling, and difficulties of mass and energy transfer during the gas-liquid reactions for hydrate formation). Lee et al. 4 ACS Paragon Plus Environment
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Energy & Fuels
reported dry synthesis of hydroquinone clathrates with methane (CH4) gas and guest behaviors, by performing spectroscopic measurements.15 Moreover, the use of hydroquinone clathrates has also been reported as a potential gas separation technology for other mixed gas systems including CO2, C2H4, CH4, and H2.16-19 Because both CO and H2 are known to form gas hydrate and organic clathrate compounds with water and hydroquinone; thus, a clathratebased process has potential for use in the separation of synthesis gas or binary (CO+H2) gas.20-23 However, the conditions favoring formation of CO and H2 clathrate hydrates are known to be very extreme (that is, requiring very high pressure at a given temperature or very low temperature at a given pressure).20,23-24 Therefore, a limited number of relevant reports occur in the literature due to the experimental difficulties involved. Moreover, this work has been done from an academic point of view (for example, phases of CO hydrates in the galaxy or in interstellar gas clouds) rather than from an engineering point of view. Furthermore, there are few reports on clathrate compounds with (CO+H2) gas mixtures so far.25 For this report, HQ samples were prepared using binary (CO+H2) gas mixtures of various compositions. Then, a series of spectroscopic measurements were performed to provide both qualitative and quantitative information. Guest storage capacities in the solid clathrates were cross-checked using elemental analysis and solid-state NMR spectroscopy. The experimental and calculated results from this study can provide fundamental information as well as helping in development of a clathrate-based separation technology for synthesis gas or binary (CO+H2) gas mixtures.
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2. EXPERIMENTAL SECTION Pure HQ (α-form crystal) with a minimum purity of 99 mol% was supplied by Sigma– Aldrich Chemicals Co. Pure CO and H2 gases with nominal purities of 99.9% and 99.5%, respectively, were purchased from Daemyoung Special Gas Co. The CO+H2 gas mixtures were also supplied by Daemyoung Special Gas Co. The nominal compositions of the gas mixtures were (9.85, 29.1, 49.3, 70.0 and 90.1) mol% CO, all balanced with H2 (hereafter, they will be referred to simply as (10, 30, 50, 70, and 90) mol% CO). These materials were used without further purification or treatment. To prepare the guest-loaded HQ clathrate (βform crystal), pure HQ was charged into a high-pressure reactor (made from 316 stainless steel, with an internal volume of ~200 cm3) connected with an external reservoir cell (~500 cm3) and allowed to react with the purchased pure and mixed gases. Before loading into the reactor, the HQ particles were ground into fine powder (particle size
