High-Performance Materials Based on Lithium-Containing

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High-performance materials based on lithium-containing hydrotalcite-bayerite composites for biogas upgrade Rosalía Torralba-Sánchez, Dulce López-Jurado, José A. Rivera, Geolar Fetter, Rosario Hernández-Huesca, Maria Ana Perez-Cruz, and Pedro Bosch Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00129 • Publication Date (Web): 31 Jul 2016 Downloaded from http://pubs.acs.org on August 2, 2016

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High-performance materials based on lithium-containing hydrotalcite-bayerite composites for biogas upgrade

Rosalía Torralba-Sánchez1, Dulce López-Jurado1, José A. Rivera1, Geolar Fetter1*, Rosario Hernández-Huesca1, María A. Pérez-Cruz1, Pedro Bosch2

1

Universidad Autónoma de Puebla, Facultad de Ciencias Químicas, C. U., Blvd. 14 Sur,

72570 Puebla, PUE, Mexico. 2

Universidad Nacional Autónoma de México, Instituto de Investigaciones en Materiales, C.

U., Circuito Exterior, 04510 México, D. F., Mexico.

* To whom correspondence should be addressed Geolar Fetter:

Universidad Autónoma de Puebla Facultad de Ciencias Químicas Blvd. 14 Sur y Av. San Claudio C.P. 72570 Puebla, PUE, Mexico FAX: (52)2222443106 e-mail: [email protected]

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ABSTRACT To upgrade biogas, carbon dioxide has to be selectively removed from methane. Lithium ceramics have been found to be efficient materials for the retention of CO2 at temperatures as high as 800 °C. However, they do not perform well at low temperatures. In this work, lithium hydrotalcites with various Li:Al ratios were synthesized in the presence of microwave irradiation. The effects of irradiation time, lithium content, and precipitation pH on the structure and textural features were studied. The resulting materials contained bayerite and were tested for the reversible adsorption of CO2 or CH4 at room temperature, resulting in an isosteric heat of 12 kJ/mol for CO2 and an insignificant adsorption of CH4. Furthermore, to prove the efficiency of these materials in biogas purification, a sample was used for the separation of CO2 from synthetic biogas. The results show that Lihydrotalcite/bayerite composites are promising materials for biogas upgrading at room temperature.

KEYWORDS Biogas, carbon dioxide, methane, double layered hydroxides, bayerite, lithium.

INTRODUCTION In recent years, biogas production has received considerable attention because it is a very important source of renewable energy. Biogas is produced from anaerobic biodegradation of biomass. Typical biogas contains 50–65 % methane, 30–45 % carbon dioxide, moisture and traces of hydrogen sulphide.1 The presence of carbon dioxide in biogas considerably reduces the thermal efficiency of energy production; therefore, it has to be removed. The separation of CO2 from biogas can be performed through conventional 2 ACS Paragon Plus Environment

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processes such as the use of membranes, whose main disadvantage is their low resistance to temperature.2 Large-scale gas separation methodologies are divided into “high temperature molecular sieving” and “low temperature reverse molecular sieving”.3 The low temperature reverse molecular sieving mode of separation is used for the removal of CO2 from biogas. Most of the research in this field has focused on using very rigid polymers because these materials have high selectivities due to the large differences in diffusion rates.2,4 However, there are porous inorganic materials that can be substituted for these membranes. The high-temperature CO2 sorbent, Mg/Al hydrotalcite, is able to adsorb CO2 at temperatures between 400 and 500 °C. The adsorption is considerably enhanced by impregnating this hydrotalcite with K2CO3. In the investigated temperature range, the CO2 loading capacity hardly varies.5 Note that hydrotalcite structures are destroyed ca. 300 °C; therefore, these reports are about the resulting metal mixed oxides. Hydrotalcites, also referred as layered double hydroxides or hydrotalcite-like compounds, are anionic clays whose chemical formula is [M2+1-xM3+x(OH)2](Am-)x/m.nH2O where the divalent M2+ cations may be replaced by trivalent M3+ cations, which produce positively charged layers. This charge is neutralized by x/m anions Am-, such as CO32-, SO42-, Cl- or NO3-. The most common divalent metals are those whose ionic radii are between 65 pm (Mg) and 80 pm (Mn), whereas the radii of the trivalent metal has to be between 50 pm (Al) and 69 pm (Cr).6,7 The charge and, therefore, the number and type of anions may be tuned by adjusting the molar ratio M2+/(M2++M3+).8,9 Although hydrotalcite synthesis, which is usually performed via the coprecipitation method, is time consuming, it may be modified by microwave irradiation. Microwave 3 ACS Paragon Plus Environment

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irradiation during the crystallization step not only reduces the preparation time and the amount of water required, it also often provides new materials10-12 by distributing the cations differently via diffusional phenomena.13,14 There is also a preference sequence for some anions in hydrotalcites that is determined by parameters such as the metallic molar ratio, the water content or the calcination temperature. In Mg-Al hydrotalcite, for divalent ions, this order is as follows: SO42− < CO32−, and for monovalent anions, it is I− < NO3− < Br− < Cl− < F− < OH−. Carbonates are always preferred, and inhibiting the formation of CO32− containing hydrotalcites is a difficult task.15 Carbon dioxide may be retained by carbonated Mg-Al hydrotalcites, the amount depends on the structural features of the material determined by thermal treatments.16,17 When treated at 200 °C, the hydrotalcite loses water as shown by the interlayer spacing, which is reduced by 0.6 Å, and carbon dioxide retention at 107 kPa turns out to be ca. half of the total adsorption capacity. Instead, if the hydrotalcite is treated at 400 °C, the material is dehydroxylated, and the interlayered carbonates are decomposed. These results may be explained in terms of the Mg2+ availability. In the first case, Mg2+ is available; thus, MgCO3 may be formed. In the second case, Mg2+ is not available, and physisorption is preferred.18 As previously mentioned, interlayer charge may be tuned by using different cations. At ambient temperatures, carbon dioxide is not sorbed on Zn-Al or Mn-Al hydrotalcites.9 Note that MgO, compared to other metal oxides, such as CaO, Na2ZrO3, Li4SiO4 or Li6Zr2O7, is a better retainer of carbon dioxide, although Li2O is reported to be better.16,19 In this case, a chemical reaction is observed as lithium reacts with carbonates to produce lithium carbonate on the oxide surface. In lithium-hydrotalcites, the adsorption of CO2 could be favored, at the expense of CH4, mainly because i) the intrinsic high affinity of


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CO2 to hydrotalcites and ii) the porosity between layers facilitates CO2 diffusion and its accessibility to structural lithium atoms. In this work, we present the synthesis of hydrotalcites with several Li:Al ratios where the preparation was carried out in the presence of microwave irradiation. The obtained solids were tested for their CO2 and CH4 sorptions at low temperatures; therefore, the hydrotalcite structure must be maintained. Finally, one of the Li-hydrotalcite samples was used to separate CO2 from a synthetic CH4 and CO2 gas mixture, which simulates the composition of a biogas.

EXPERIMENTAL Sample preparation Aqueous solutions of LiNO3 (5 M, 98.1 %, Aldrich) and Al(NO3)3.9H2O (0.24 M, 98 %, Aldrich) were mixed with a 1.85 M aqueous NaOH solution (99 %, Merck) at room temperature. The flow of each solution was adjusted to maintain a constant pH value of 12. The amount of each solution was calculated to obtain a molar ratio Li/Al of 2. The resulting slurry was treated in a microwave autoclave (MIC-I, Sistemas y Equipos de Vidrio, S. A. de C. V.) operating at 200 W and 2.54 GHz. To determine the best irradiation time, fractions of the slurry were irradiated for 10, 30 or 60 minutes: samples HTLi-10MO, HTLi-30MO and HTLi-60MO. An irradiation time of 10 minutes provided a crystalline hydrotalcite; therefore, the next preparations were carried out with an irradiation time of 10 minutes. The autoclave temperature was programmed to 80 °C. Prior to drying at 65 °C for 24 h, the precipitates were washed with deionized water until the pH value of the supernatant reached 10.


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Binary Li:Al-hydrotalcites with molar ratios of 2:1, 5:1 and 10:1 were then synthesized by co-precipitation at a pH of 11. They were thermally treated by microwave irradiation (80 °C, 10 min, 200 W), and the samples were labeled HTLi-2:1, HTLi-5:1, and HTLi-10:1, respectively. Note that the Li/Al molar ratio must be greater than 0.5 to avoid the formation of aluminum hydroxides in the precipitate and to favor an ordered cation distribution in the lamellae.20

CO2 or CH4 retention tests To obtain a clean surface and to desorb water, the samples were placed in glass cells and activated in situ by increasing temperature to 150 °C under vacuum until the pressure was less than 9.0 x 10-3 Torr. Heating was then stopped, and the samples were allowed to return to room temperature. The tests were performed at five different temperatures (0, 20, 60, 100 or 140 °C). The adsorption capacities were measured using a volumetric system. After stabilization at the chosen temperature, the gas pressure was increased to saturation. The amount of adsorbed gas (moles) was estimated as a function of time to obtain kinetic curves.

CO2 separation from synthetic biogas The Li-hydrotalcite sample containing less bayerite was used to separate CO2 from a synthetic CH4 and CO2 gas mixture, which simulates the composition of a biogas. The gas mixture, delivered by PRAXAIR, had the following composition: 61.025 % CH4; 38.01 % CO2; 0.465 % N2 and 0.5 % H2. The gas separation tests were carried out using a Perkin-Elmer AutoSystem XL gas chromatograph with a thermal conductivity detector. A column (160 mm long and 2 mm 6 ACS Paragon Plus Environment

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internal diameter) was filled with a sample. Prior to the adsorption of the gas mixture, the sample was activated by heating the column (1 °C/min) in the presence of a He flow (17.5 mL/min) at 150 °C for 180 minutes and then cooled to room temperature. Using an automatic injection system, 4 mL of the gas mixture was introduced into the column to determine the retention time of the components. Due to the strong interaction of the CO2 with the adsorbents, the desorption process had to be carried out by a thermo-programmed method, in which the temperature increased 1 °C/min up to 150 °C and then held at this temperature for 60 minutes. The results obtained using Li-hydrotalcite material were compared with those of a MgAl hydrotalcite sample that was synthesized in the same way as the lithium samples, i.e., the Mg/Al hydrotalcite precursors were co-precipitated at a constant pH of 11, treated with microwave irradiation (80 °C, 10 min, 200 W), washed and dried for 12 hours at 70 °C. The Mg/Al molar ratio was 3.

Characterization Methods X-ray diffraction (XRD) patterns were recorded with a Bruker AXS D8 advance diffractometer coupled to a copper anode X-ray tube. An inductively coupled plasma mass spectrometer (ICP-OES), 730-ES from Varian, was used to determine the elemental compositions of the samples. The samples (ca. 100 mg) were heated to 300 °C and then dissolved in a HNO3–HCl (1/3 v/v) solution before analysis. N2 adsorption-desorption isotherms were measured using a Micromeritics ASAP 2020 system at -198 °C. Prior to analysis, the samples were pretreated in vacuum at 150 °C for 11 hours. This pretreatment ensures that the hydrotalcite structure is maintained and that 7 ACS Paragon Plus Environment

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it is fully dehydrated. The pore size distributions were determined from the desorption branch of the isotherm using the BJH model. An Excalibur series Digilab FTS3000 MX spectrometer with a wavelength interval of 4000-400 cm-1 was used to measure the FTIR spectra. The DTGS detector had a resolution of 2 cm-1. The powder samples were dispersed in KBr and compressed as pellets.

RESULTS Irradiation time effect Samples with a Li:Al ratio of 2:1 were prepared with different microwave irradiation times: 10, 30 or 60 minutes (samples HTLi-10MO, HTLi-30MO, and HTLi60MO). Figure 1 compares the corresponding XRD patterns. The identified compounds were hydrotalcite and bayerite. Because the peaks are intense and sharp, both solids are fully crystalline. The proportion of hydrotalcite to bayerite is the same in all cases, as shown by the corresponding peak areas; then, in the following experiments, the samples were only irradiated for 10 minutes. It has been already reported that aluminum hydroxide sheets of Mg-Al hydrotalcites (gibbsite-like) present octahedral voids that are large enough to accept small cations such as lithium, while water occupies the interlayer space.21 In Li-hydrotalcite, LiAl2(OH)7.2H2O, the c’ value has been reported to be 7.5 Å;15 this value is in agreement with the 7.6 Å obtained in this work. Therefore, the hydrotalcite is expected to be highly hydroxylated. Indeed, it was prepared at a very high pH. The presence of bayerite must be attributed to an aluminum excess. However, the nominal composition was Li:Al = 2:1; therefore, a certain amount of lithium is missing. Thus, either lithium, which is very light, is either evaporated during the microwave 8 ACS Paragon Plus Environment

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irradiation step or sublimed during the drying step. Lastly, lithium can occupy substitutional positions in bayerite network and remains undetected by X-ray diffraction. The first proposition has to be discarded because the system is hermetic and the same result was obtained for irradiation times of 10, 30 or 60 minutes, Figure 1. The second proposition cannot be true as no lithium deposition was found on the system. Therefore, the only valid explanation is the third one: When a strongly alkaline solution is used, the excess hydroxyl ions are bonded to the aluminum ions and form bayerite. To test these hypotheses, an elemental analysis was performed, Table 1. The atomic ratio Li:Al turns out to be equal to 0.5 in samples HTLi-10MO, and it decreases to 0.2 in the other two. Note that the maximum amount of inserted Li corresponds to a Li:Al ratio of 0.5, but the analysis corresponds to the full sample, i.e., hydrotalcite and bayerite. Thus, in sample HTLi-10MO, both hydrotalcite and bayerite retain lithium as previously suggested. In the two other samples, the excess irradiation time promotes the loss of lithium most likely during washing. It appears that, with increasing irradiation time, lithium diffuses towards the surface so that it is easily lixiviated. The infrared spectra of the three samples irradiated at different times were similar, Figure 2. The bands at 3529, 3623 and 3655 cm-1 and the shoulder at 3184 cm-1 confirm the presence of bayerite.22 The band at 1820 cm-1 is attributed to water bending. The bands at 1365 and 1385 cm-1 most likely correspond to carbonates adsorbed on bayerite or interlayered into hydrotalcite as shown by the X-ray diffraction 003 distance. The bands at 765, 533 and 461 cm-1 may be attributed to Al-O as in Serna et al., 1982.20 Nitrogen isotherm adsorption curves were all similar, Figure 3. They are type IV with H3 hysteresis loops. Such features are typical of lamellar and mesoporous materials.23 The pore size distributions, Figure 4, present a main maximum at D = 30 Å and a broad 9 ACS Paragon Plus Environment

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peak from 200 to 350 Å. These characteristics correspond to hydrotalcite adsorption,23 whereas the isotherm curves and pore size distributions obtained for bayerite are clearly different.24 The corresponding surface areas were 46, 38, and 29 m2/g for the samples irradiated for 10, 30, and 60 minutes, and the specific surface area decreases almost 37 %. Therefore, the irradiation time induces remarkable textural modifications. The structure and content of hydrotalcite and bayerite are maintained. Instead, the lithium content decreases with irradiation time.

Lithium content effect Based on the previous results, the irradiation time was selected to be 10 minutes. Figure 5 compares the X-ray diffraction patterns of the HTLi-2:1, HTLi-5:1 and HTLi-10:1 samples. They all correspond to a mixture of hydrotalcite and bayerite. In the HTLi-2:1 diffractogram, the amount of bayerite is as high as in the previous samples; the peaks are sharp and intense showing that hydrotalcite and bayerite are well crystallized and present a large crystallite size. Instead, sample HTLi-5:1 contains much less bayerite. In sample HTLi-10:1, the hydrotalcite peaks are less intense but broader. Therefore, either the hydrotalcite crystallites are smaller or the arrangement is turbostratic.13,25 The bayerite peaks are again sharp and well defined as in all of the previous samples. The elemental composition, Table 1, reveals that the lithium content is 0.5 for the HTLi-2:1 and HTLi-10:1 samples but 0.7 for the HTLi-5:1 sample. As in the previous samples, lithium is retained by hydrotalcite and bayerite. Only sample HTLi-5:1 contains more lithium than expected. This excess can be explained by assuming that some lithium


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oxide or hydroxide nanoparticles are formed and undetected by X-ray diffraction. The lithium excess is then lost and no other lithium compounds are detected. The infrared spectra, Figure 6, reproduce the principal features of Figure 2. The main difference is the shoulder at 3184 cm-1, which is attributed to OH bonds present in bayerite;22 these bands are smaller in sample HTLi-5:1. Isotherms of these samples are similar and correspond to type IV with H3 hysteresis loops. The pore size distributions reveal that sample HTLi-5:1 reproduces the characteristics of the HTLi-2:1 sample after irradiation for 10 minutes. Instead, if the Li:Al ratio is 10, the pore size distribution presents a broad peak from 100 to 400 Å, revealing that the hydrotalcite is delaminated. The corresponding specific surface areas are 46, 56 and 104 m2/g for the HTLi-2:1, HTLi-5:1, and HTLi-10:1 samples, respectively. Disorder increases with nominal lithium content and, as expected, with the specific surface area. In contrast, the bayerite remains unaltered, most likely saturated with lithium atoms as indicated by the X-ray diffractograms.

Precipitation pH effect The HTLi-10MO and HTLi-2:1 samples only differ in the coprecipitation pH, which was 12 in the first preparation and 11 in the second. They display different lithium and aluminum contents, i.e., 3.46, 25.3 wt % and 2.82, 21.4 wt %. The amount of lithium per aluminum atom is slightly higher if the pH is 12. Still, the difference is not significant if other effects are involved.

CO2 or CH4 adsorption tests as a function of lithium content


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Figure 7 compares the CO2 adsorption curves of the HTLi-2:1, HTLi-5:1 and HTLi10:1 samples at 0 and 20 °C. At both adsorption temperatures, the HTLi-10:1 sample adsorbs almost twice as much as the other two materials. The equilibrium is reached ca. 150 Torr at 0 °C for a maximum retention of 0.25 mmol of CO2/g of solid. The effect of adsorption temperature is not significant enough to justify the 0 °C experiments. Figure 8 shows the adsorption kinetics curves for the HTLi-10:1 sample at temperatures from 0 to 140 °C. If the temperature is below 60 °C, equilibrium is reached after 1.5 hours (0.34 and 0.24 mmol/g for 0 or 20 °C, respectively). Otherwise, the curves retain only 0.18, 0.13 or 0.06 mmol/g at 60, 100 or 140 °C after the same 1.5 hours. As expected, more CO2 was retained at lower temperatures. Another difference is the adsorption rates obtained from the curve slopes and attributed to diffusion effects that are usually observed in microporous materials.26 When the previous experiments were reproduced using CH4, no significant adsorption was observed. Therefore, methane is not retained by the prepared composites of Li-hydrotalcite and bayerite.

CO2 and CH4 separation tests Even though the HTLi-10:1 sample retained more CO2, we chose the HTLi-5:1 sample for the CO2 and CH4 separation test because the Li-hydrotalcite was well crystallized and presented less bayerite in its composition. Figure 9 shows that, for the HTLi-5:1 sample, the CH4 is not retained and elutes at very short retention times (ca. 0.5 minutes). Instead, the CO2 does not elute from the solid, even after 200 minutes. To desorb the CO2 from this sample, it was necessary to employ the thermo-programmed method, i.e., to increase the system temperature (1 °C/min up to 150 12 ACS Paragon Plus Environment

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°C). In this way, Figure 9Ab shows that CO2 elutes in a time of 60 min. For the comparative sample, i.e., MgAl hydrotalcite, the CH4 retention was very low, similar to that of the HTLi-5:1 sample. When using the thermo-programmed method, the CO2 had an elution time of 150 min, Figure 9Bb. When both samples are compared, the lithium hydrotalcite shows greater CO2 retention with a lower desorption temperature. If a material is used in a temperature swing adsorption process (TSA), a low desorption temperature is required.27-29

DISCUSSION The synthesized materials are comprised of a large amount of hydrotalcite with bayerite. As shown by Britto and Kamath (2009),30 two symmetries of hydrotalcite may be obtained: hexagonal resulting from gibbsite or monoclinic from bayerite. Microwave irradiation clearly promoted the formation of Li-hydrotalcite through bayerite. As bayerite is not totally transformed, the nucleation mechanism in the presence of microwave irradiation differs from the usual precipitation or imbibition mechanisms reported elsewhere.25,31 The bayerite-based hydrotalcites do not conserve the stacking sequence of the parent phase; this feature explains the broadening of the X-ray diffraction peaks. It seems that lithium-containing bayerite is initially formed due to microwave irradiation. Microwaves are known to accelerate crystallization.17,32,33 Lithium diffuses into the bayerite network, and a fraction of the crystallized bayerite is transformed into hydrotalcite. Then, the particles present a bayerite core and an external shell of hydrotalcite in agreement with the isotherm shapes. This mechanism is independent of the amount of lithium and the irradiation time, as experimentally shown. Such a configuration should result in a greater exposure of the lithium hydrotalcite to reactants or sorbents than bulk 13 ACS Paragon Plus Environment

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hydrotalcite. This morphology is supported by the Li:Al ratios and the corresponding surface areas, Table 2. The initial sample HTLi-10MO has the highest surface lithium content (Li atoms/m2), most likely due to the synthesis pH and the short crystallization time. As the irradiation time is increased, lithium is lost, and the amount of lithium on the surface decreases. In the HTLi-2:1 sample, which was prepared at a pH 11, the amount of lithium on the surface is lower. Comparing the Li:Al (lithium atoms per bulk aluminum atoms) and the surface lithium (lithium atoms per accessible m2) reported in Table 2, the amount of lithium on the surface is similar for samples HTLi-2:1 and HTLi-5:1, but the Li:Al ratios are different: the HTLi-5:1 sample has more bulk lithium atoms than the HTLi2:1 sample. The HTLi-10:1 sample has the same amount of bulk atoms (Li:Al ratio) but a much lower proportion of surface lithium atoms. Lithium hydrotalcites, whose pore size is ca. 0.40 nm, have two advantages: On the one hand, for adequate CO2 sorption, the ideal pore size must be 0.35-0.45 nm as shown by Krishna and van Baten (2010),34 and on the other hand, lithium compounds are highly efficient for CO2 retention.35,36 The resulting materials could be used to separate CO2 from CH4 at room temperature in biogas streams because only CO2 and not CH4 is retained, as shown in the tests performed using the synthetic biogas. In these tests, the Li-hydrotalcite, exemplified by the HTLi-5:1 sample, was much more efficient than Mg-hydrotalcite, considering that the amount of CO2 retained was higher with a lower desorption temperature, which could save energy if employed in a TSA process. The optimal carbon dioxide sorption was obtained after 1.5 hours at temperatures of 0 or 20 °C. In HTLi-10:1, the best retention was 0.34 mmol/g. For comparison, Li2ZrO3 retains 0.84 mmol/g at 600 °C after 4.6 hours in an isothermal process.36 A covalent organic framework COF-102 is known to retain 26.8 mmol of CO2/g and 11.7 mmol of 14 ACS Paragon Plus Environment

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CH4/g, but it has to be emphasized that those results were obtained at 35 bar and 25 °C.37 The uptake of carbon dioxide by MIL-101 has been shown to occur with a record capacity of 40 mmol/g at 5 MPa and 303 K.38 Although these materials are good adsorbents for CO2, they also retain CH4. Thus, they do not appear to be adequate for selective adsorption in a mixed CO2-CH4 stream. Our new composites are fully selective. The two molecules, methane and carbon dioxide, are very similar as their kinetic diameters and polarizabilities are 0.38 and 0.33 nm and 2.6 and 2.9 C2m2J-1, respectively; they differ in their quadrupole moments, which turn out to be 0.0 and -14.0 x 10-40 C.m2.39 Their diameters are small enough to allow diffusion into the hydrotalcite pore network. The differences in adsorption between CH4 and CO2 have to be attributed to the double bond and the geometry of CO2 molecule. When the CO2 molecule interacts with the hydrotalcite lattice, strong bonds are formed due to the interaction of the quadrupole moment and the π electrons with the positively charged zones of the hydrotalcite. These zones are the locally enriched lithium surfaces. Furthermore, the interaction adsorbent-adsorbate is so strong that in some cases it may be irreversible. Still, no carbonates are formed as in other systems that retain CO2 at high temperatures.36 CH4 is not sorbed because it does not have a quadrupolar moment nor π electrons. From the previous results, the total and reversible adsorption isosteric heats were estimated using the Clausius Clapeyron equation. Figure 10 shows that the isosteric heat increases as a function of the amount of adsorbed gas (a) and that the reversible curve presents a similar profile shifted 0.04 mmol/g. The highest isosteric heat was 12 kJ/mol for 0.23 mmol of CO2/g. This value is much lower than the 43.3 kJ/mol for a hydrotalcite-like compound of the Mg/Al-Fe(CN)6 type reported by Mao et al. (1993).40 This difference guarantees the easy regeneration (loss of 40 %) of our materials. 15 ACS Paragon Plus Environment

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CONCLUSION The fast synthesis using microwave irradiation ensures complex materials that are most likely comprised of Li-bayerite cores and Li-hydrotalcite crusts. Li-hydrotalcite presents an adequate pore size as well as a layer charge that favors CO2 retention. In contrast, CH4 is not captured. This difference is attributed to the corresponding quadrupolar momentum values. The adsorption curves show that the difference between the curves at 0 or 20 °C is not worthwhile and that the process is reversible. The curve shapes correspond to a system with no diffusional problems. The isosteric heat (12 kJ/mol for 0.23 mmol of CO2/g) corresponds to a different adsorption mechanism than that already reported for infinitesimal coverage on oxygen deficient magnetite at 150-300 °C (40.0 kJ/mol).41 Note that the value reported in the present work was obtained at room temperature. Furthermore, in the separation of CO2 from a synthetic biogas, the Li-hydrotalcite/bayerite composite was much efficient because the CO2 was retained in the solid while the CH4 was not. Therefore, this material could be very efficient in the upgrading of biogas at low temperature.

ACKNOWLEDGEMENTS The financial support of CONACYT is gratefully acknowledged. The technical XRD work of Adriana Tejeda from UNAM is recognized.


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REFERENCES (1) Tippayawong, N.; Thanompongchart, P. Energy 2010, 35, 4531-4535. (2) Freemantle, M. Chem. Eng. News 2005, 40, 49-57. (3) Kanellopoulos, N. K., Recent Advances in Gas Separation by Microporous Ceramic Membranes, Membrane Science and Technology, 6. Elsevier: Amsterdam, 2000; pp. 1-510. (4) Saimani, S.; Dal-Cin, M. M.; Kumar, A.; Kingston, D. M. J. Membrane Sci. 2010, 362, 353-358. (5) Reijers, H. T. J.; Valster-Schiermeier, S. E. A.; Cobden, P. D.; van den Brink, R. W. Ind. Eng. Chem. Res. 2006, 45, 2522-2530. (6) Evans, D. G.; Slade, R. C. T. Layered Double Hydroxides, X. Duan & D. G. Evans (Eds.). Structure and Bonding series, 119. Springer: Berlin, 2015; pp. 1-87. (7) He, J.; Wei, M.; Li, B.; Kang, Y.; Evans, D. G.; Duan, X. Layered Double Hydroxides, X. Duan & D. G. Evans (Eds.). Structure and Bonding series, 119. Springer: Berlin, 2015; pp. 89-119. (8) Sommer, A. E.; Fetter, G.; Bosch, P.; Lara, V. H. Clays Clay Miner. 2010, 58, 340-350. (9) Sampieri, A.; Fetter, G.; Pfeiffer, H.; Bosch, P. Solid State Sci. 2007, 9, 394-403. (10) Fetter, G.; Bosch, P. Pillared Clays and Related Catalysts, A. Gil, S.A. Korili, R. Trujillano, M.A. Vicente (Eds.). Springer: New York, 2010, pp. 1-21. 17 ACS Paragon Plus Environment

Page 19 of 36

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(11) Benito, P.; Labajos, F. M.; Rocha, J.; Rives, V. Microporous Mesoporous Mater. 2006, 94, 148-158. (12) Olanrewaju, J.; Newalkar, B. L.; Mancino, C.; Komarneni, S. Mater. Lett. 2000, 45, 307-310. (13) Rivera, J. A.; Fetter, G.; Bosch, P. Microporous Mesoporous Mater. 2006, 89, 306314. (14) Ayala, A.; Fetter, G.; Palomares, E.; Bosch, P. Mater. Lett. 2011, 65, 1663-1665. (15) Cavani, F.; Trifiro, F.; Vaccari, A. Catal. Today 1991, 11, 173-301. (16) Yong, Z.; Rodrigues, A. E. Energy Convers. Manage. 2002, 43, 1865-1876. (17) Bergada, O.; Salagre, P.; Cesteros, Y.; Medina, F.; Sueiras, J. E. Adsorpt. Sci. Technol. 2007, 25, 143-154. (18) Speakman, S. A.; Payzant, E. A. Chem. Mater. 2004, 16, 4135-4143. (19) Mosqueda, H. A.; Vazquez, C.; Bosch, P.; Pfeiffer, H. Chem. Mater. 2006, 18, 23072310. (20) Serna, C. J.; Rendon, J. L.; Iglesias, J. E. Clays Clay Miner. 1982, 30, 180-184. (21) Isupov, V. P. J. Struct. Chem. 1999, 40, 672-685. (22) Balan, E.; Blanchard, M.; Hochepied, J.-F.; Lazzeri, M. Phys. Chem. Miner. 2008, 35, 279-285. (23) Benito, P.; Labajos, F. M.; Rives, V. Pure Appl. Chem. 2009, 81, 1459-1471. (24) Rat’ko, A. I.; Kuznetsova, T. F.; Romanenkov, V. E.; Klevchenya, D. I. Colloid J. 2008, 70, 210-214. 18 ACS Paragon Plus Environment

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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

Page 20 of 36

(25) Britto, S.; Thomas, G. S.; Kamath, P. V.; Kanna, S. J. Phys. Chem. C 2008, 112, 95109515. (26) Hernández-Huesca, R.; Díaz, L.; Aguilar-Armenta, G. Sep. Purif. Technol. 1999, 15, 163-173. (27) Chen, X. Y.; Vinh-Thang, H.; Avalos Ramirez, A.; Rodrigue, D.; Kaliaguine, S. RSC Adv. 2015, 5, 24339-24448. (28) Grande, C. A.; Blom, R. Energy Fuels 2014, 28, 6688-6693. (29) Siriwardane, R. V.; Shen, M.-S.; Fisher, E. P. Energy Fuels, 2005, 19,1153–1159. (30) Britto, S.; Kamath, P. V. Inorg. Chem, 2009, 48, 11646-11654. (31) Hutson, N. D.; Attwood, B. C. Adsorption 2008, 14, 781-789. (32) Rivera, J. A.; Fetter, G.; Giménez, Y.; Xochipa, M. M.; Bosch, P. Appl. Catal. A 2007, 316, 207-211. (33) Tichit, D.; Rolland, A.; Prinetto, F.; Fetter, G.; Martínez-Ortíz, M. J.; Valenzuela M. A.; Bosch, P. J. Mater. Chem. 2002, 12, 3832-3838. (34) Krishna, R.; van Baten, J. M. J. Membrane Sci. 2010, 360, 323-333. (35) Yang, H.; Xu, Z.; Fan, M.; Gupta, R.; Slimane, R. B.; Bland, A. E.; Wright, I. J. Environ. Sci. 2008, 20, 14-27. (36) Pfeiffer, H.; Vazquez, C.; Lara, V. H.; Bosch, P. Chem. Mater. 2007, 19, 922-926. (37) Furukawa, H.; Yaghi, O. M. J. Am. Chem. Soc. 2009, 131, 8875–8883.


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(38) Llewellyn, P. L.; Bourrelly, S.; Serre, C.; Vimont, A.; Daturi, M.; Hamon, L.; De Weireld, G.; Chang, J.-S.; Hong, D.-Y.; Hwang, Y. K.; Jhung, S. H.; Férey, G. Langmuir 2008, 24, 7245-7250. (39) Aguilar-Armenta, G.; Patiño-Iglesias, M. E.; Leyva-Ramos, R. Adsorption Sci. 2003, 21, 81-91. (40) Mao, G.; Tsuji, M.; Tamaura, Y. Clays Clay Miner. 1993, 41, 731-737. (41) Nishizawa, K.; Kodama, T.; Tabata, M.; Yoshida, T.; Tsuji, M.; Tamaura, Y. J. Chem. Soc. Faraday Trans. 1992, 88, 2771-2773.


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Table Captions

Table 1. Elemental composition (Al and Li) determined by ICP as a function of microwave irradiation time and lithium nominal content.

Table 2. Correlation between specific surface areas and Li:Al atomic ratios.


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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

Energy & Fuels

Table 1

Sample

Li

Al

Li:Al

weight content

weight content

(atomic ratio)

(%)

(%)

HTLi-10MO

3.46

25.3

0.5

HTLi-30MO

1.32

26.2

0.2

HTLi-60MO

1.16

26.1

0.2

HTLi-2:1

2.82

21.4

0.5

HTLi-5:1

3.44

20.5

0.7

HTLi-10:1

2.65

21.2

0.5


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Table 2

Specific surface

Li:Al

Surface lithium

area

(experimental

(Li atoms/m2)

(m2/g)

atomic ratio)

HTLi-10MO

46

0.5

6.5 x 1019

HTLi-30MO

38

0.2

3.0 x 1019

HTLi-60MO

29

0.2

3.4 x 1019

HTLi-2:1

46

0.5

5.3 x 1019

HTLi-5:1

56

0.7

5.3 x 1019

HTLi-10:1

104

0.5

2.2 x 1019

Sample


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Figure Captions

Figure 1. X-ray diffraction patterns of samples HTLi-10MO, HTLi-30MO and HTLi60MO. Diffraction peaks corresponding to bayerite are marked with *.

Figure 2. Infrared spectra of samples HTLi-10MO, HTLi-30MO and HTLi-60MO.

Figure 3. Adsorption isotherms of the samples HTLi-10MO, HTLi-30MO and HTLi60MO.

Figure 4. Pore size distributions of the samples HTLi-10MO, HTLi-30MO and HTLi60MO.

Figure 5. X-ray diffraction patterns of the samples HTLi-2:1, HTLi-5:1 and HTLi-10:1. Diffraction peaks corresponding to bayerite are marked with *.

Figure 6. Infrared spectra of the samples HTLi-2:1, HTLi-5:1 and HTLi-10:1.


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Figure 7. CO2 adsorption isotherms of the samples HTLi-2:1, HTLi-5:1 and HTLi-10:1 at 0 °C (A) and 20 °C (B).

Figure 8. Adsorption kinetic curves of the sample HTLi-10:1 at different temperatures.

Figure 9. Retention times of CO2 and CH4 gases for the HTLi-5:1 sample (A) at 25 °C (a) and at increasing temperature (thermo-programmed method) (b), compared with the retention times for a MgAl hydrotalcite sample (B) at 25 °C (a) and at increasing temperature (b).

Figure 10. Total (T) and reversible (R) adsorption isosteric heat for the sample HTLi-10:1, as a function of the amount of adsorbed CO2.


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*

110 113

* *

*

018

*

015

009

* 006

003

*

Intensity (a.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

Energy & Fuels

* *

HTLi-60MO

HTLi-30MO

HTLi-10MO 10

20

30

40

50

60

70

°2θ Figure 1


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100 1621

80 60

HTLi-60MO

40 20

1385 765 1017 974 533

3475

0

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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100 1621

80 60

HTLi-30MO

40 20

765 1384 1011 975 533

3474

0 100 1620

80 60

HTLi-10MO

40 20

765 1385 1013 974 533

3474

0 4000 3600 3200 2800 2400 2000 1600 1200 800 400 -1

Wavenumber (cm ) Figure 2


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120 80 40

HTLi-60MO

0 3

Adsorbed volume (cm /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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120 80 40

HTLi-30MO

0 120 80 40

HTLi-10MO

0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Relative pressure (P/P0) Figure 3


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0.0007 0.0006

Pore volume (cm3/g.Å)

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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0.0005 0.0004 0.0003

HTLi-60MO HTLi-30MO HTLi-10MO

0.0002 0.0001 0.0000 0

200

400

600

800

1000

1200

1400

Pore diameter (Å) Figure 4


110 113

*

018

015

009

* 006

*

Intensity (a.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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003

Page 31 of 36

*

HTLi-10:1

HTLi-5:1

HTLi-2:1 10

20

30

40

50

60

70

°2θ Figure 5


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100 1631

80

HTLi-10:1

60 40

1385

1003 754 535

3463

20 0

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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100 80

1633

HTLi-5:1

60

1005

760

40 533

20 3468

1385

0 100 1633

80

HTLi-2:1 60 40

1005 760 533

1385

3468

20 0 4000 3600 3200 2800 2400 2000 1600 1200 800 400 -1

Wavenumber (cm ) Figure 6


Page 33 of 36

0.30

0.25

HTLi-10:1

HTLi-10:1

A

0.25

B

0.20

0.20

HTLi-5:1

0.15

a (mmol/g)

a (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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HTLi-2:1

0.10

0.15

HTLi-2:1 HTLi-5:1

0.10

0.05

0.05 0.00

0.00 0

50

100

150

200

250

300

0

P (Torr)

50

100

150

200

250

P (Torr)

Figure 7


300

Energy & Fuels

0.4 0 °C

0.3

a (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

Page 34 of 36

20 °C

60 °C 0.2 100 °C 0.1 140 °C 0.0 0

1

2

3

4

5

6

7

8

9

t (h)

Figure 8


Page 35 of 36

A

CH4

a CH4

CO2

B a

Intensity (a.u.)

CH4

Intensity (a.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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CH4

CO2

b 0

0

20 40 60 80 100 120 140 160 180 200

b

20 40 60 80 100 120 140 160 180 200

t (min)

t (min) Figure 9


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20 18 16

Reversible

14 12

Q iso (KJ/mol)

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

Page 36 of 36

Total

10 8 6 4 2 0 0.00

0.05

0.10

0.15

0.20

0.25

a (mmol/g)

Figure 10


High-Performance Materials Based on Lithium-Containing

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