Prussian Blue Analogues for the Separation of Hydrocarbons in

Communication pubs.acs.org/IC

Prussian Blue Analogues for the Separation of Hydrocarbons in Humid Conditions Lotfi Boudjema,† Ekaterina Mamontova,† Jérôme Long,*,‡ Joulia Larionova,‡ Yannick Guari,‡ and Philippe Trens*,† †

UMR 5253, Matériaux Avancés pour la Catalyse et la Santé, ENSCM/CNRS/UM, Institut Charles Gerhardt Montpellier, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France ‡ UMR 5253, Ingénierie Moléculaire et Nano-Objets, Université de Montpellier, ENSCM, CNRS, Institut Charles Gerhardt Montpellier, Place E. Bataillon, 34095 Montpellier Cedex 5, France S Supporting Information *

formula AaM[M′(CN)6]b□c·xH2O (where A is an alkali ion, M and M′ are transition-metal ions, and □ represents the cyanometallate vacancies that may be present to ensure the electroneutrality), and they crystallize in a face-centered-cubic (fcc) structure of the NaCl type. In the crystal structure, octahedral [M(CN)6]x− complexes are linked through a cyano bridge to octahedral Mn+ ions, creating a 3D framework. The alkaline ion may occupy the tetrahedral sites of the structure, and the nonstoichiometry leads to two extreme chemical compositions: lacunary and nonlacunary structures of the respective formulas M[M′(CN)6]2/3 (or Fe[Fe(CN)6]3/4 for PB) and AM[M′(CN)6] (Figure 1). Zeolitic water molecules occupy the

ABSTRACT: The relative affinity of a series of Prussian Blue analogues presenting various chemical compositions for the adsorption of different gases, such as water vapor and hydrocarbons, was studied. The ability of Co[CoIII(CN)6]0.66·5.2H2O for the separation of hydrocarbons in a humid atmosphere was demonstrated.

T

he design and development of porous materials are essential for industrial applications such as sensing, catalysis, gas storage, and separation.1 However, it is important to bear in mind that water stability is a key property for any materials to be industrially applicable because water is abundant in the preparation, storage, transportation, and application processes. For hydrocarbon separation, water vapor is always present in gas, and it is considered as a poison for the adsorbent materials usually employed.2 While moisture can be removed prior to storage and separation, it is impossible to avoid corrosion from trace moisture during sample loading, activation, and regeneration processes for long-term usage. Even though in very few cases, water was shown to improve the performance of separation,3,4 it usually makes the separation processes rapidly ineffective and adsorbent regeneration cycles very frequent. For all of these reasons, porous materials presenting a good stability against water and are able to efficiently work in a humid atmosphere are challenging. Metal−organic frameworks or porous coordination polymers are a class of material made by the assembly of transition-metal ions and organic or inorganic ligands that benefit from many advantages of molecule-based materials, such as soft chemistry routes, low density, optical transparency, stability, and structural diversity.5−7 In this line of thought, they often compete with zeolites in terms of their large internal surface area, high porosity, and crystallinity. Consequently, this class of porous material has been tested for many applications where zeolites are not fully satisfactory such as highly selective separation.8 However, they often exhibit water instability, which prevents them from being considered for several applications, such as gas separation in a humid atmosphere.2,9 For this reason, different strategies have recently been developed to circumvent this weakness.10−12 Among these molecule-based porous materials, Prussian Blue and its analogues (PBAs) were shown to be stable in both hightemperature and -humidity conditions.13 They have the general © 2017 American Chemical Society

Figure 1. Typical structures of lacunary (left) and nonlacunary (right) PBAs. Color code: orange, MIII; purple, MII; green, AI; blue, N; gray, C; red, O.

porosity of the framework. Furthermore, in the case of the lacunary structure, water ligands complete the coordination sphere of the Mn+ cations. Several PBAs have already been used for the capture of gaseous molecules, such as H2 or CO2,14−16 or for propane/propylene separation.17 However, as far as we know, the separation capacity of PBAs with various hydrocarbons and their efficiency in humid conditions have never been investigated. In this Communication, we report on an investigation of a series of PBAs presenting various chemical compositions in terms of their relative affinity toward water vapor. We demonstrate for the first time that Co[CoIII(CN)6]0.66·5.2H2O is able to efficiently separate a hydrocarbon mixture (n-pentane, n-hexane, cyclohexane, and cyclohexene) in a humid atmosphere. Received: March 4, 2017 Published: June 28, 2017 7598

DOI: 10.1021/acs.inorgchem.7b00563 Inorg. Chem. 2017, 56, 7598−7601

Communication

Inorganic Chemistry

microporosity. The uptake at high relative pressure (p/p° > 0.6) in the case of PBAs 4 and 6 can be attributed to some interparticular adsorption, which suggests that these PBAs are more aggregated than the other materials. It is therefore more appropriate to compare the adsorption capacities at p/p° = 0.5, for which all adsorption isotherms are flat. The influence of the nature of both transition-metal ions, M and M′, as well as the vacancy content can explain the difference in the vapor adsorption capacities. First, the change in the nature of the divalent metal ion in samples 1 and 2 having the same hexacyanocobaltate moiety and exhibiting identical vacancy concentrations leads to dramatic changes in their adsorption capacities (Figure 2). Indeed, 1 exhibits a much higher capacity than 2. This can be ascribed to differences in (i) the coordination geometry of the divalent metal ion (while Co2+ usually adopts an octahedral geometry, Cu2+ often exhibits a pentacoordinated environment) and (ii) in the strength of the cyano−metal ion interaction as well as in the polarizing power.13 The strong CuII− N coordination bond induces a decrease of the divalent ion acidity with respect to CoII. Second, the use of [Fe(CN)6]3− (3) instead of [Co(CN)6]3− (1) with bivalent CoII also induces a strong decrease of the adsorption capacity, reflecting a modification in the electronic density. This may originate from the variation in the back-bonding donation of the cyanometallate which affects the acidity of the CoII ions. Note that the employment of [Fe(CN)6]4− (5) leads to a capacity similar to that of 3. Third, a comparison between 1 and 4, having the same transition-metal ions, clearly confirms the role of the vacancies in the adsorption capacities of water, with a 4-fold weaker adsorption for 4, for which the number of vacancies is rather limited. It can be noticed that, however, CuII-containing sample 6, which exhibits a higher vacancy concentration in comparison with 5 and shows higher adsorption capacity. Consequently, it turns out that the number of vacancies and the nature of both the cyanometallate moiety and the divalent metal ions have an influence on the adsorption isotherms. Note that the high sorption capacities (up to 35 wt %) observed for 1 may be favorably compared with previously published results. For instance, Thallapally et al. found only 20% weight uptake at saturation.15 The discrepancy could be explained by a higher degree of crystallinity for 1 and/or by a better activation procedure. Note also that the adsorption capacity of 1 is higher than the ones observed for other microporous or mesoporous materials, such as zeolites or MCM-41.18 Interestingly, the adsorption isotherms exhibit weak slopes at low relative pressure, which is an indication of a rather low affinity of water. In general terms, the affinity is defined at low relative pressure and corresponds to the Henry’s constant of the sorbate/sorbent system, namely, KH. It has been calculated (Table 1), and among those, it can be noticed that 1 exhibits a better affinity for water. Despite this feature, 1 appears as a promising candidate for separating hydrocarbon vapors’ mixtures in humid conditions thanks to its high sorption capacity, high specific surface area, and very high thermal stability. The vapors used were n-pentane, nhexane, cyclohexane, and cyclohexene, which have different van der Waals (vdW) or π interactions. The corresponding adsorption isotherms are presented in Figure 3. The sorbate sizes are also compared to those of the PBAs’ pore sizes along with the Henry’s constants in Table 2. The latter are true type I compounds, apart from the one obtained with water. The affinity of 1 for hydrocarbons is higher (Table 2) than that found for 1, making this PBA a rather lipophilic material. This can be seen by the delayed adsorption process at low relative pressure.

The different PBA frameworks 1−6 were obtained following the self-assembly reaction between the hexacyanometallate precursors [M(CN)6]x− (M = CoIII, FeII, FeIII) and the divalent transition-metal ions [M′(H2O)6]2+ (M = CoII, CuII) (Table 1 Table 1. Some Characteristics of PBAs

compound 1 2 3 4 5 6

formula Co[CoIII(CN)6]0.66· 5.2H2O Cu[CoIII (CN)6]0.66· 5.1H2O Co[FeIII(CN)6]0.66· 4.65H2O Cs0.74Co[CoIII(CN)6]0.91· 3.6H2O K0.96Co[FeII(CN)6]0.74· 3.0H2O K0.17Cu[FeII(CN)6]0.54· 5.66H2O

adsorbed amount at p/p° = 0.5/mg·g−1

SSAa/ m2· g−1

KHb/ cm3· g−1· Torr−1

325

637

29

135

172

12

110

387

6

75

273

18

75

520

5

225

217

8

a Equivalent specific surface area determined from the water vapor adsorption isotherms at 313 K, using the Brunauer−Emmett−Teller method and taking 0.105 nm2 as the cross-sectional area of adsorbed water. bHenry’s constants determined from the slope of the adsorption isotherms at zero coverage plotted as a function of the absolute pressure.

and the Supporting Information for syntheses details). 4 was prepared in the presence of CsCl to favor an alkaline insertion and to reduce the number of vacancies. The powder X-ray diffraction (PXRD) patterns confirm the PBA fcc structure for all compounds (Figure 1, Table S1). Thermogravimetric analysis (TGA) allows estimation of the water content and shows various decomposition temperatures depending on the PBA composition (Figure S2 and Table S1). As previously evidenced, the most thermostable frameworks are cobalt hexacyanocobaltate-based PBAs 1 and 4 (up to 573 and 583 K, respectively).1,14 The solids were activated for sorption experiments by heating at 423 K for 12 h under a secondary vaccum. Their PXRD patterns after a cycle (activation + adsorption) match with the pristine ones (Figure S1). The water adsorption isotherms are presented in Figure 2. For all samples, they are close to the type I shape, which unsurprisingly corresponds to microporous materials, which is characterized by strong uptake at low relative pressure, followed by a flat plateau. The latter corresponds to saturation of the

Figure 2. Water adsorption by the PBAs 1−6 at 313 K. Solid lines are guides for the eyes. 7599

DOI: 10.1021/acs.inorgchem.7b00563 Inorg. Chem. 2017, 56, 7598−7601

Communication

Inorganic Chemistry

Figure 4. Chromatographic separation of hydrocarbons by 1 in dry or humid conditions. Representative chromatograms after 30 separation tests. Separation temperature: 523 K.

Figure 3. Vapor adsorption isotherms for 1 for different hydrocarbon vapors’ mixtures at 303 K. Solid lines are guides for the eyes.

On the other hand, there is a clear difference between the sorbates, observed at saturation (say, p/p° = 0.5). Water, which has a lower affinity for 1, is, nevertheless, adsorbed more than the hydrocarbons at saturation, which can be interpreted in terms of the molecular size (Table 2). Water is the smallest sorbate for the series studied; the pores can accommodate a larger amount. The slight differences at saturation between the hydrocarbons can be discussed in terms of packing. For the C6 series, cyclohexene adsorbs slightly more than cyclohexane and n-hexane, which can be attributed to a better packing due to π stacking.20 The lipophilic character of 1 can be very useful when separation applications are envisaged. Indeed, the previous results may indicate that hydrocarbons should be preferentially and efficiently adsorbed by 1 even in the presence of water. In order to prove this, a chromatographic separation of hydrocarbons’ vapors was realized, using a filled column in a homemade separation device.21 The single components were first eluted in order to precisely determine the elution times obtained upon separation of the hydrocarbon mixture. This separation was performed both with and without the presence of water. The results are shown in Figure 4. The mixture of the four components was separated into four peaks corresponding to each constituent. Expectedly, the order of elution corresponded to the opposite order of the Henry’s constants (Table 2), which means that low Henry’s constants (weak affinity) led to short retention times. The elution times with or without water are very similar, indicating that the separation occurs without particular hindrance of the pores. Such a feature is very encouraging for humid gas stream purification. Additionally, the separation in humid conditions could be repeated several times (30 separations), which highlights both the exceptional hydrothermal stability (50% humidity at 523 K) and the rather low affinity of water for 1. Indeed, water does not poison 1 upon adsorption, as was already suggested by its rather low Henry’s constant. This is further evidenced by (i) the PXRD patterns of 1 before and after a series of 30 hydrocarbon separations showing no loss of crystallinity (Figure S3) and (ii)

the different chromatograms with various water contents (Figure S4). In summary, we have synthesized a series of PBA compounds with various chemical compositions by varying the bivalent transition-metal ions, hexacyanometallate moiety, and number of vacancies in order to study the adsorption of water vapor. We demonstrated that among the investigated PBAs, 1 exhibits the highest sorption capacity, specific surface area, and thermal stability, which makes it an excellent candidate for the separation of different hydrocarbons with and without water. Thus, this compound possesses an exceptional hydrothermal stability allowing hydrocarbon separation as efficient as that in a dry atmosphere. These results emphasize the remarkable potential of the Prussian Blue family for gas separation.

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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/acs.inorgchem.7b00563. Experimental details and PXRD and TGA data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ekaterina Mamontova: 0000-0002-4959-3332 Jérôme Long: 0000-0002-2601-4289 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the University of Montpellier, CNRS, and PAC Balard ICGM.

Table 2. Sorbate Diameters19 Compared to the PBA 1 Pore Sizes [[1 0 0] Direction] and the Henry’s constants for the PBA 1/ Hydrocarbons Derived as the Slopes of the Adsorption Isotherms, Where the Adsorbed Amount Is Plotted as a Function of the Absolute Pressure of the Different Sorbates

vdW diameter/Å PBA 1 KH at 303 K/mg·g−1·Torr−1

1

water

n-pentane

n-hexane

cyclohexane

cyclohexene

∼7.5

∼3.6 9

∼4.5 16

∼4.5 30

∼5.8 49

∼5.8 55

7600

DOI: 10.1021/acs.inorgchem.7b00563 Inorg. Chem. 2017, 56, 7598−7601

Communication

Inorganic Chemistry

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(21) Trens, P.; Belarbi, H.; Shepherd, C.; Gonzalez, P.; Ramsahye, N. A.; Lee, U.-H.; Seo, Y.-K.; Chang, J.-S. Adsorption and separation of xylene isomers vapors onto the chromium terephthalate-based porous material MIL-101(Cr): An experimental and computational study. Microporous Mesoporous Mater. 2014, 183, 17−22.

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DOI: 10.1021/acs.inorgchem.7b00563 Inorg. Chem. 2017, 56, 7598−7601