Separation of 1-Butene from 2-Butene Using Framework Open Metal Sites Inventors: Brandon Barnett, Jeffrey R. Long, all of Berkeley, CA Applicant/Assignee: The Regents of the University of California Priority: [001] This invention was made with government support under Grant Number DE- SC0001015 awarded by the U.S. Department of Energy. The government has certain rights in the invention. [002] Introduction [003] Isomers of butene are difficult to separate, and industry often employs cryogenic distillation to isolate 1-butene. Some adsorbent-based separation schemes have been developed and are employed on large scales (i.e. Sorbutene process of UOP; Olefinsiv process of Union Carbide), although these processes still require a distillation step, and are hampered by some isomerization of 1-butene to 2-butene. This invention provides this separation while obviating both distillation and isomerization. [004] Summary of the Invention [005] The invention provides systems and methods for separating 1-butene from 2-butene using framework open metal sites. [006] In an aspect the invention provides a method of separating hydrocarbons comprising contacting a mixture of 1-butene and 2-butene with a MOF of M ₂ (4,6-dioxido-1,3- benzenedicarboxylate(m-dobdc)), wherein the M ₂ is Co ₂ or Ni ₂ , under conditions wherein the 1- butene is selectively adsorbed to the MOF over the 2-butene. [007] In embodiments: [008] the 2-butene is cis-2-butene; [009] the 2-butene is trans-2-butene; [010] the mixture is gaseous; [011] the mixture is liquid;. [012] M ₂ is Co ₂ ; ^{[013] M} ₂ ^{is Ni} ₂ ^. [014] In an aspect the invention provides corresponding hydrocarbon separation systems comprising a metal–organic framework (MOF) of M ₂ (4,6-dioxido-1,3-benzenedicarboxylate(m- dobdc)), 1-butene and 2-butene, wherein the M ₂ is Co ₂ or Ni ₂ , and the 1-butene is selectively adsorbed to the MOF over the 2-butene. [015] In embodiments: [016] the 2-butene is cis-2-butene; [017] the 2-butene is trans-2-butene; [018] the mixture is gaseous; [019] the mixture is liquid;. [020] M ₂ is Co ₂ ; [021] M ₂ is Ni ₂ . [022] The invention encompasses all combinations of the particular embodiments recited herein, as if each combination had been laboriously recited. [023] Description of Particular Embodiments of the Invention [024] Unless contraindicated or noted otherwise, in these descriptions and throughout this specification, the terms “a” and “an” mean one or more, the term “or” means and/or. The examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims. All publications, patents, and patent applications cited herein, including citations therein, are hereby incorporated by reference in their entirety for all purposes. [025] Example: Thermodynamic Separation of 1-Butene from 2-Butene in Metal– Organic Frameworks Bearing Open Metal Sites [026] Most C ₄ hydrocarbons are obtained as byproducts of ethylene production or oil refining, and complex and energy-intensive separation schemes are required for their isolation. Substantial industrial and academic effort has been expended to develop more cost-effective adsorbent- or membrane-based approaches to purify commodity chemicals such as 1,3- butadiene, isobutene, and 1-butene, although the very similar physical properties of these C ₄ hydrocarbons makes this a challenging task. Herein, we examine the adsorption behavior of 1- butene, cis-2-butene and trans-2-butene in the metal–organic frameworks M ₂ (dobdc) and M ₂ (m- dobdc) (M = Mn, Fe, Co, Ni; dobdc ^2- = 2,5-dioxidobenzene-1,4-dicarboxylate and m-dobdc ^4- = 4,6-dioxido-1,3-benzenedicarboxylate), which both contain a high density of coordinatively- unsaturated metal sites. We find that both Co ₂ (m-dobdc) and Ni ₂ (m-dobdc) are able to separate 1-butene from the 2-butene isomers, a critical industrial process that relies largely on energetically demanding cryogenic distillation. The origin of 1-butene selectivity is traced to the large charge density retained by the M ²⁺ cations in the M ₂ (m-dobdc) family, which results in a reversal of the cis-2-butene selectivity typically observed at framework open metal sites. Selectivity for 1-butene adsorption under multicomponent conditions is demonstrated for Ni ₂ (m- dobdc) in both the gaseous and liquid phases via breakthrough and batch adsorption experiments. [027] Introduction [028] Separation of hydrocarbon mixtures is carried out on an immense scale in industry, where a heavy reliance on thermal separation methods results in substantial and costly energy expenditure. ^1-3 Accordingly, there is great interest in incorporating more energy-efficient adsorbent- or membrane-based technologies into existing hydrocarbon separation schemes. ⁴ Much of this focus has centered on the separation of C2 or C3 hydrocarbon mixtures, given substantial global demand for ethylene and propylene, ^5-10 while alternative separation schemes for C ₄ hydrocarbon mixtures have received far less attention. Most C ₄ hydrocarbons are obtained as byproducts from steam cracking in ethylene plants and refinery fluid catalytic cracking, and their fractionation from complex mixtures is necessary for isolation of individual components. ^{11- 13} The C ₄ cut from steam crackers contains significant quantities of 1,3-butadiene, which can be isolated using extractive distillation or removed via selective hydrogenation. This produces a mixture of mono-olefin (1-butene, cis-2-butene, trans-2-butene, isobutene) and paraffin (n- butane, isobutane) isomers, known as Raffinate I. Isobutene is most commonly oligomerized or converted to tert-butanol or tert-butyl ethers and then separated, leaving a mixture of the n- butenes and C ₄ paraffins known as Raffinate II. While much of Raffinate II is used as a feedstock for alkylate gasoline production, this mixture is also used to produce high-grade 1- butene, which is in wide demand as a monomer for the production of poly-1-butene, high- density polyethylene and linear low-density polyethylene. ^12,14,15 These processes require 1- butene purities in excess of 99.5%, and thus the separation of 1-butene from Raffinate II typically requires multiple separation stages. The high-purity separation of 1-butene from the 2- butenes is particularly challenging, and often relies on energy-intensive cryogenic distillation. ¹¹ [029] Table 1. Relevant physical properties of C ₄ olefins and paraffins. ^16,17 [030] Given their similar boiling points and comparable sizes and physical properties (Table 1), the C4 hydrocarbons are particularly challenging to separate. In contrast to lighter C2-3 hydrocarbon mixtures, where the components differ in their degree of unsaturation, the presence of olefin and paraffin isomers leads to diverse mixtures of components bearing few chemical handles that can be exploited in viable separation schemes. The fact that only 1,3-butadiene, isobutene and 1- butene are currently marketed with standardized product purities underscores this difficulty, ¹⁶ but also points toward the need for novel, low-cost separations to diversify the availability of high-grade C ₄ hydrocarbons. The Sorbutene process of UOP uses molecular sieves to produce 1- butene in 99.2% purity from Raffinate II, although the yield suffers somewhat from in situ isomerization. ¹¹ Various studies have explored the potential of zeolites to effect useful C ₄ hydrocarbon separations, ^16,18 with particularly notable success in the selective removal of 1,3- butadiene from C ₄ mixtures. ^19-21 Separation of n-butene mixtures has also been studied using several different zeolitic frameworks. ¹⁶ Notably, all-silica RUB-41 was demonstrated to selectively adsorb both cis- and trans-2-butene over 1-butene in liquid-phase adsorption and breakthrough experiments. ²² Unfortunately, the saturation capacities of this zeolite are quite low (£ 1.0 mmol/g for the 2-butenes) and the origin of its selectivity is not well understood. [031] Metal–organic frameworks have only been sparingly studied for C ₄ hydrocarbon separations. ^23-32 Two recent studies leveraged selectivity based on adsorbate size ²³ or shape ²⁴ to separate various C ₄ components, although the necessarily compact framework pores preclude high capacities. One approach to engender higher capacities is to utilize frameworks containing open metal sites, which can act as strong binding sites for hydrocarbon adsorbates. To our knowledge, the only framework featuring open metal sites that has been investigated for C ₄ hydrocarbon separations is HKUST-1 (Cu ₃ (btc) ₂ , btc ^3– = benzene-1,3,5-tricarboxylate). ^26,27 The unsaturated Cu ²⁺ sites of this material were shown to be capable of discriminating between the n-butenes in liquid-phase multicomponent adsorption experiments, albeit with modest selectivities. ²⁶ The relative adsorption strengths of cis-2-butene > 1-butene > trans-2-butene found for HKUST-1 mirror those seen in alkali cation-substituted Faujasite, ^33,34 where butene adsorption occurs at exposed M ⁺ sites. In contrast, a computational study of butene adsorption in Fe ₂ (dobdc) (Fe-MOF-74, dobdc ^2- = 2,5-dioxidobenzene-1,4-dicarboxylate) ³⁵ predicted preferential adsorption of 1-butene over both 2-butenes at the open Fe ²⁺ sites. ³⁶ [032] The high density of open metal sites within the M ₂ (dobdc) (M = Mg, Mn, Fe, Co, Ni, Cu, Zn) family of frameworks has previously been shown to endow these materials with high capacities for light hydrocarbons, ^37-42 and preferential adsorption of unsaturated hydrocarbons at these metal sites leads to excellent performance in ethylene/ethane and propylene/propane separations. ^37-39,42 Notably, the isomeric series of frameworks M ₂ (m-dobdc) (m-dobdc ^4- = 4,6- dioxido-1,3-benzenedicarboxylate, M = Mg, Mn, Fe, Co, Ni) ⁴³ was found to exhibit superior performance in the separation of ethylene/ethane and propylene/propane mixtures, a result of increased open metal site charge density afforded by the m-dobdc ^4- ligand. ⁴⁴ Finally, Co ₂ (dobdc) and Co ₂ (m-dobdc) frameworks have been shown to separate mixtures of C ₈ aromatic hydrocarbons as a result of unique synergistic interactions between adjacent metal centers in each material. ⁴⁵ [033] Here, we disclose the ability of M ₂ (dobdc) and M ₂ (m-dobdc) (M = Mn, Fe, Co, Ni) to separate mixtures of 1-butene, cis-2-butene and trans-2-butene. Crystallographic studies provide insight into the metal–butene interactions and rationalize the observed adsorption profiles in both classes of adsorbents. We find that Co ₂ (m-dobdc) and Ni ₂ (m-dobdc) show a remarkable selectivity for adsorption of 1-butene over the 2-butenes, and the observed selectivity trends allow for correlation of separation performance with the electronic character of the unsaturated metal site. Breakthrough experiments and liquid-phase batch adsorption verify that this selectivity is retained under multicomponent conditions, enabling the separation of valuable 1- butene from the internal olefins. [034] C ₄ Olefin Single-Component Adsorption Isotherms [035] Single-component gas adsorption isotherms were collected at 328 K to investigate the ability of the M ₂ (dobdc) and M ₂ (m-dobdc) (M = Mn, Fe, Co, Ni) frameworks to discriminate between the n-butene isomers. As previously observed for ethylene and propylene adsorption in these materials, ^39,44 the isotherms display steep uptake at low pressures and approach saturation at loadings corresponding to one olefin per metal site (approximately 6.3 mmol/g). A comparison of the adsorption isotherms within each family of frameworks revealed that, overall, trans-2-butene adsorbs more weakly than 1-butene and cis-2-butene, as indicated by the higher onset adsorption pressure of trans-2-butene adsorption. This finding is consistent with other studies of butene adsorption at framework open metal sites. ^26,33,34 In particular, the trans arrangement of the methyl groups results in steric hindrance to metal coordination, as there is no possible coordination geometry that can orient both methyl groups away from the metal center. While this effect is somewhat mitigated due to the long M–C distances observed for all butene isomers studied here, the absence of a permanent dipole moment in trans-2-butene further attenuates its interaction with the positively charged metal centers. [036] In general, the uptake of each isomer at low pressures is higher in the M ₂ (m-dobdc) series than in M ₂ (dobdc). This effect is most pronounced for 1-butene, which has very similar low-pressure uptake to cis-2-butene in all of the studied M ₂ (dobdc) materials. For example, Ni ₂ (m-dobdc) achieves a 1-butene loading of 1.0 mmol/g (~0.15 olefin/M ²⁺ ) at 0.10 mbar, whereas a pressure of 0.34 mbar is required to achieve the same loading in Ni ₂ (dobdc) (Table 2). In contrast, the pressures required to reach the same loading of cis-2-butene in Ni ₂ (dobdc) and Ni ₂ (m-dobdc) differ by a smaller factor of 2.4. While all materials display 1-butene uptakes that exceed those of cis-2-butene at intermediate pressures, this is likely due in part to more significant adsorbate–adsorbate interactions in the case of 1-butene (see below). Only Fe ₂ (m- dobdc), Co ₂ (m-dobdc), and Ni ₂ (m-dobdc) show higher 1-butene uptakes at even the lowest pressure points, where uptake correlates the most strongly with metal–olefin adsorption strength. Accordingly, it appears as though the M ₂ (m-dobdc) frameworks are uniquely primed to achieve discrimination of 1-butene and cis-2-butene. Only the soft Mn ²⁺ open metal sites in Mn ₂ (m- dobdc) display poor performance in this regard, suggesting that harder and more charge-dense metal centers represent ideal candidates for achieving 1-butene selectivity. Indeed, a prior theoretical study predicted that 1-butene should adsorb preferentially over cis-2-butene at compact, highly-charged metal centers, while softer cations should interact more strongly with cis-2-butene. ³⁴ Comparing the two families of frameworks, it is the increased charge density ⁴³ at the open metal sites in M ₂ (m-dobdc) versus M ₂ (dobdc) that is likely a dominant factor leading to the greater selectivity for 1-butene over cis-2-butene. It is unlikely that charge transfer, such as p-backbonding, ³⁶ plays a significant role, given the high-spin electronic configurations of the metal centers and long M–C _olefin distances evident from X-ray diffraction (see below). [037] Table 2. Pressures (mbar) at which each framework reaches a loading of 1.00 mmol/g for the indicated adsorbate at 328 K. ^a [038] Using the Clausius–Clapeyron relationship, differential enthalpies of adsorption were calculated from Langmuir-Freundlich fits of the single-component isotherm data collected at 308, 318 and 328 K. The differential enthalpy values calculated for all three isomers on each adsorbent are approximately 50-60 kJ/mol at low coverage, and in all cases are seen to become larger with increasing loadings. This trend is likely the result of attractive interactions between adsorbed butene molecules, which should become more prominent at higher coverages. ⁴⁶ The differential enthalpy values for 1-butene generally show the largest degree of increase at higher loadings, with maximum values being ~20-25 kJ/mol larger in magnitude than at zero coverage. The presence of more significant adsorbate-adsorbate interactions for 1-butene is not surprising, as its ethyl group can extend further into the framework one-dimensional channels than the methyl groups of the 2-butenes. The more modest loading dependencies for the 2-butene isomers are similar to those previously characterized for propylene adsorption in M ₂ (m-dobdc), ⁴⁴ which features a single methyl substituent. [039] Structural Characterization of butene adsorption [040] Structural characterization of Co ₂ (dobdc) and Co ₂ (m-dobdc) loaded with each n-butene isomer was first carried out to facilitate comparison of the adsorption profiles observed for both framework families. The cobalt(II) variants of each material were chosen given the availability of single crystals of Co ₂ (dobdc), which allows for the determination of very precise structural parameters. Single-crystal X-ray diffraction data were collected at 100 K on butene-loaded single crystals of Co ₂ (dobdc). Unlike most molecular transition metal–olefin complexes, in which the olefin binds in symmetrical fashion with its p-cloud pointing toward an empty metal valence orbital, ⁴⁷ the Co ₂ (dobdc)(C ₄ H ₈ ) _x structures exhibit hydrocarbon binding geometries that reflect both a largely electrostatic interaction with Co ²⁺ and the effects of the surrounding pore environment. Consistent with its preferential adsorption determined from single-component isotherms, Co ₂ (dobdc)(1-butene) _1.59 exhibits the shortest average Co–C distance of all the butene loaded samples (2.789(4) Å). In both Co ₂ (dobdc)(1-butene) _1.59 and Co ₂ (dobdc)(trans-2- butene) _1.48 , the olefins are bound to Co ²⁺ in an asymmetric fashion, although the average Co–C bond length in Co ₂ (dobdc)(trans-2-butene) _1.48 is much larger at 2.992(9) Å. [041] While coordination of the C=C double bond in Co ₂ (cis-2-butene) _1.58 is rather symmetric (average d(Co–C) = 2.888(4)), the olefinic C–H bonds are canted downward and situated directly over two ligating O atoms. Thus, while the presence of H···O interactions may be a contributor to the olefin binding energy, this geometry also orients the dipole moment of cis-2- butene toward the charged Co ²⁺ center, further enhancing the framework–adsorbate interaction. [042] In general, the butene alkyl groups point outward into the framework pores, allowing for increased dispersion interactions between adsorbed butene molecules. Indeed, examination of the extended lattice for each adsorbed species reveals close contacts between alkyl groups both within the ab plane and extending down the pores in the c direction. [043] Structures of Co ₂ (m-dobdc) loaded with 1-butene and cis-2-butene were obtained through Rietveld refinement of synchrotron powder X-ray diffraction data. In a fashion similar to the binding of cis-2-butene in Co ₂ (dobdc), the structures of both Co ₂ (m-dobdc)(1-butene) _1.64 , and Co ₂ (m-dobdc)(cis-2-butene) _2.88 show the olefinic C–H bonds canted downward toward the coordination sphere of cobalt, placing the H atoms in close proximity to the ligating O centers. In both structures, the Co–C distances are slightly elongated compared to those seen for Co ₂ (dobdc) (average = 2.92(4) Å for 1-butene; 3.02(8) for cis-2-butene). The occupation of a secondary adsorption site near the center of the pore was also located for cis-2-butene. [044] Ideal Adsorbed Solution Theory. [045] We used Ideal Adsorbed Solution Theory (IAST) to calculate multicomponent equilibrium selectivities for these C ₄ hydrocarbons in M ₂ (m-dobdc) and M ₂ (dobdc). ^48,49 [046] Selectivity values were calculated using IAST for two-component 1-butene/cis-2-butene and 1-butene/trans-2-butene mixtures. For all M ₂ (dobdc) frameworks as well as Mn ₂ (m-dobdc) and Fe ₂ (m-dobdc), the 1-butene/cis-2-butene selectivities are less than or equal to 2.0, while the selectivities for Co ₂ (m-dobdc) and Ni ₂ (m-dobdc) are larger at 2.9 and 2.4, respectively. As discussed above, the superior selectivities for these two frameworks can be traced to the remarkable affinities of their open Co ²⁺ and Ni ²⁺ sites for 1-butene. As a result, Co ₂ (m-dobdc) and Ni ₂ (m-dobdc) also show the largest 1-butene/trans-2-butene selectivities among the materials studied here (5.7 and 6.0, respectively). These values are quite large for an equilibrium process where each olefin has unimpeded access to the unsaturated metal, and demonstrate the potential of charge-dense open metal sites to discriminate between components that share very similar physical properties. [047] The extremely challenging separation of cis-2-butene and trans-2-butene is usually neglected in industry, given that both isomers behave identically in most reactions of interest. ^11,16 However, economical separation schemes could potentially open up new opportunities for access to purified supplies of these isomers. The two-component selectivity values measured here cis-2- butene/trans-2-butene range from 1.5-2.7. Unlike the two-component selectivities involving 1- butene, no obvious trends exist between the metal center, framework family, and cis-2- butene/trans-2-butene selectivities. We note that these selectivity values are similar to that measured for the same mixture in HKUST-1 (1.9), ²⁶ which contains open Cu ²⁺ sites as part of its Cu ₂ (COO) ₄ paddlewheel motifs. Owing to Jahn-Teller effects, the square pyramidal Cu ²⁺ sites in HKUST-1 are unlikely to engage in particularly strong interactions with the olefin adsorbates compared to the frameworks studied in this work. ^40,50 Thus, the thermodynamic selectivity for cis-2-butene versus trans-2-butene appears less sensitive to the polarizing power of the open metal site compared to selectivity of 1-butene over the 2-butenes. [048] Multicomponent column breakthrough and batch adsorption experiments [049] Column breakthrough experiments were carried out using Ni ₂ (m-dobdc) to more accurately assess the separation performance attainable under multicomponent conditions. In these experiments, a pre-mixed gaseous mixture of n-butene isomers (1:1:1 ratio diluted in helium) was passed through a column packed with pelletized metal–organic framework and heated to 328 K, and the eluent was analyzed by GC-FID. Given the inability of our gas chromatograph to achieve complete resolution of cis- and trans-2-butene, these isomers were analyzed and integrated together to enable calculation of precise selectivity values for 1-butene over the less desirable internal olefins. Consistent with the ordering of low-pressure butene uptake determined from the single-component isotherm data, the 2-butenes break through the adsorption bed first. The sharp profile of the 2-butene breakthrough suggests that this event is an equilibrium rather than a diffusion-limited process. ⁵¹ It should be noted that the bifurcation in this curve at ~47 min is due to slightly different breakthrough times for the cis and trans isomers. The initial elution corresponds to breakthrough of cis-2-butene, a surprising finding given the weaker adsorption of trans-2-butene seen in the single-component isotherm data. However, this multicomponent selectivity ordering is consistent with that seen in batch liquid adsorption experiments (see below) and suggests that multicomponent conditions result in an inversion of cis-2-butene/trans-2-butene selectivity. Once the adsorbent bed approaches saturation, 1-butene begins to elute, with the outlet concentrations for each isomer promptly returning to those in the feed gas. Integration of the breakthrough curves yields adsorption capacities of 2.58 and 1.87 mmol/g for 1-butene and combined 2-butenes, respectively. These capacities equate to a 1-butene selectivity of 2.8. Assuming a 1-butene/2-butenes feed ratio of 1.4, representative of steam cracker-derived Raffinate II, ¹² passage through four consecutive adsorption beds would yield 1-butene in 99% purity. Importantly, such a purification scheme would obviate the final distillation steps required for 1-butene isolation using the UOP Sorbutene process and the competing OlefinSiv procedure from Union Carbide. ¹¹ In tandem with the high capacity of Ni ₂ (m-dobdc) for 1-butene, the breakthrough performance indicates that this framework enables alternative C ₄ olefin separation schemes. [050] Table 3. Comparison of selectivities calculated for adsorption in Ni ₂ (m-dobdc). [051] In contrast to the gaseous conditions employed in our breakthrough measurements, adsorptive butene isomer separations in the chemical industry would likely be performed at higher pressures with liquefied C ₄ streams. ¹⁶ To more closely model these conditions, we also performed batch adsorption experiments using solutions containing all three n-butene isomers in cyclohexane-d ₁₂ (~0.5 M total butenes). Proton NMR spectra were collected for these solutions before and after exposure to Ni ₂ (m-dobdc) at 306 K for 24 h, and the amount of each adsorbed butene was determined from its concentration difference between the samples. The calculated 1- butene/cis-2-butene and 1-butene/trans-2-butene selectivity values determined from these competitive adsorption experiments are shown in Table 3. While the 1-butene/cis-2-butene selectivity is comparable to that determined from single-component isotherms using IAST (2.3 ± 0.1 versus 2.4, respectively), the 1-butene/trans-2-butene selectivity decreases substantially in the batch adsorption experiment (2.1 ± 0.1 versus 6.0 for IAST). As implicated in the gas-phase breakthrough experiments, the two-component 1-butene/trans-2-butene selectivity is also slightly smaller than that for 1-butene/cis-2-butene. This result indicates that adsorbate– adsorbate interactions are more pronounced between 1-butene and trans-2-butene relative to 1- butene and cis-2-butene and ultimately facilitate increased adsorption of trans-2-butene. We note that it is not possible to completely rule out a small amount of cyclohexane-d ₁₂ adsorption (solution concentration of approximately 9 M), which could play a minor role in altering selectivity. Nonetheless, the retention of 1-butene selectivity in a mixture containing both olefins and paraffins is significant, given that Raffinate II contains substantial quantities of both n- butane and i-butane. In combination with the established performance of the M ₂ (m-dobdc) frameworks for separation of light olefin/paraffin mixtures, ⁴⁴ these results further attest to the utility of Ni ₂ (m-dobdc) to isolate 1-butene from industrially relevant C ₄ mixtures. 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(46) We note that the derivation of the Clausius-Clapeyron equation does not account for the adsorbed phase volumes, which can lead to artificial correlations between enthalpy and loading. However, any such correlation should be manifested only at very high loadings,. See: Pan, H ; Ritter, J. A.; Balbuena, P. B. Examination of the Approximations Used in Determining the Isosteric Heat of Adsorption From the Clausius-Clapeyron Equation. Langmuir 1998, 14, 6323- 6327.

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(49) Yaghi, O. M.; Kalmutzki, M. J.; Diercks, C. S. Introduction to Reticular Chemistry: Metal- Organic Frameworks and Covalent Organic Frameworks, Wiley-VCH Verlag & Co. KGaA, 2019. (50) Queen, W. L.; Hudson, M. R.; Bloch, E. D.; Mason, J. A.; Gonzalez, M. I.; Lee, J. S.; Gygi, D.; Howe, J. D.; Lee, K.; Darwish, T. A.; James, M.; Peterson, V. K.; Teat, S. J.; Smit, B.; Neaton, J. B.; Long, J. R.; Brown, C. M. Comprehensive Study of Carbon Dioxide Adsorption in the Metal–Organic Frameworks M ₂ (dobdc) (M = Mg, Mn, Fe, Co, Ni, Cu, Zn). Chem. Sci. 2014, 5, 4569–4581. (51) Peralta, D.; Chaplais, G.; Simon-Masseron, A.; Barthelet, K.; Pirngruber, G. D. Separation of C ₆ Paraffins Using Zeolitic Imidazolate Frameworks: Comparison with Zeolite 5A. Ind. Eng. Chem. Res. 2012, 51, 4692–4702. [054] General Procedures. Unless otherwise stated, all reagents and solvents were purchased from commercial sources and either used as received or purified using standard procedures. Cyclohexane-d ₁₂ was degassed via successive freeze–pump–thaw cycles and stored in a glovebox over activated 3 Å molecular sieves prior to use. The metal–organic frameworks M ₂ (dobdc) (M = Mg, Mn, Fe, Co), ^1-3 M ₂ (m-dobdc) (M = Mn, Fe), ⁴ and M ₂ (m-dobdc) (M = Co, Ni) ⁵ were synthesized according to published procedures. The framework Ni ₂ (dobdc) was synthesized according to a modified published procedure ¹ as detailed below. [055] Synthesis of Ni ₂ (dobdc). A 250 mL Schlenk flask equipped with a reflux condenser was charged with a magnetic stirbar, H ₄ (dobdc) (0.30 g, 1.5 mmol), Ni(NO ₃ ) ₂ •6 H ₂ O (1.34 g, 4.60 mmol, 3.0 equiv), and a 1:1:1 DMF/EtOH/H ₂ O mixture (120 mL). The solution was sparged with Ar for 1 h, and then heated under Ar in a 100 ˚C oil bath with gentle stirring for 3 d. The reaction mixture was cooled to room temperature and then opened to the atmosphere. Following decantation of the mother liquor, 40 mL of fresh DMF was added, and the flask was heated to 80 ˚C for 1 d. Following isolation via filtration, the material was soaked four times in MeOH at 60 ˚C, each for 12 h. Yellow Ni ₂ (dobdc)•(MeOH) _x was isolated via filtration, transferred to a pre-weighed glass tube capped with a septum, and heated in vacuo on a Schlenk line at 80 ˚C for 1 d. The septum was exchanged for a Transeal, and the material was then fully activated by heating in vacuo (< 4 µbar) on a Micromeritics 2420 gas sorption analyzer at 180 ˚C for 1 d, affording Ni ₂ (dobdc) as a dark yellow powder (0.12 g, 0.38 mmol, 25 % yield). [056] Multicomponent Gas-Phase Breakthrough. Breakthrough experiments were performed using a custom-built apparatus, composed of 1/8” copper and stainless-steel tubing fitted with Swagelok fittings and valves to control the gas flow. The system allows for gas flow through the breakthrough column, used to house the adsorbent material, or through a bypass. In either case, the outlet stream is directed to a gas chromatograph used to monitor the outlet composition. A premixed gas comprised of 3.33% each of 1-butene, cis-2-butene, and trans-2- butene with a helium balance was attached to the system. This mixture was diluted by flow from a helium cylinder (99.999%), with both flows monitored by Parker Porter mass flow controllers. After filtering off from an MeOH slurry, the Ni ₂ (m-dobdc) material was pelletized by mechanical press and subsequently broken over 25-45 mesh sieves yielding particles of 350-700 mm diameter. The breakthrough column (6 in of ¼” stainless steel tubing with an inserted internal K type thermocouple), was then loaded with 0.418 mg of the pelletized MOF. The column was connected to the breakthrough apparatus through Swagelok fittings and kept in place with glass wool. Heat tape along with the internal thermocouple were used to control the temperature of the breakthrough column. The column was initially heated to 180 °C under flowing helium for activation and subsequently cooled to 55 °C for breakthrough testing. A 10 SCCM flowrate for the butene mixture and 10 SCCM flowrate of pure helium were used to achieve an inlet concentration of 1.67% of each butene for breakthrough measurements (approximate partial pressure of each butene isomer = 17 mbar). The breakthrough tests were comprised of flowing the aforementioned mixture of butene isomers in helium over the column at 20 SCCM and measuring the outlet concentration on a SRI Instruments 8610C gas chromatograph equipped with a C8 column and FID detector. The effluent gas stream was sampled at 3 min intervals. The C8 column was unable to separate the 2-butene isomers at a reasonable temperature or flowrate, thus the 2-butene isomers were treated as one species and compared to 1-butene. Peak integration and analysis were carried out on PeakSimple software. [057] Liquid Phase Multicomponent Adsorption Experiments. Analysis of butene concentrations was performed using ¹ H NMR spectroscopy data obtained at 298 K using a Bruker Avance 600 MHz spectrometer equipped with a Prodigy Cryoprobe. Solutions of butenes in C ₆ D ₁₂ were prepared by condensing the butene isomers in a glovebox cold well at approximately 87 K and adding them to thawing C ₆ D ₁₂ . The concentration of each isomer was determined by ¹ H NMR via integration of the olefin –CH ₃ multiplets against the –CH ₃ singlet of a toluene internal standard. Prior to adsorption, these concentrations were determined to be 0.184 M, 0.212 M, and 0.146 M for 1-butene, cis-2-butene, and trans-2-butene, respectively. Since the –CH ₃ multiplets of cis-2-butene and trans-2-butene were found overlap slightly, constant integration limits were employed for all spectra to minimize errors in integral values. Global Spectral Deconvolution as implemented by the Mestrenova software package was explored but yielded inconsistent results for integration of these peaks and was therefore not utilized. In the adsorption experiments, 0.90 mL of the above solution was added to a known quantity of activated Ni ₂ (m-dobdc) (~ 0.025 g) in an argon-filled glovebox. The vials were allowed to stand at ambient temperature (306 K) for 24 h, whereupon the solution was filtered off from the metal–organic framework. Toluene was subsequently added as an internal standard to an aliquot of known volume. The solutions were then analyzed by ¹ H NMR, and the amount of each butene adsorbed was determined from the difference in concentration before and after adsorption. Two component selectivities S were calculated according to the equation: where x is the amount adsorbed, y is the equilibrium concentration in solution, and the subscripts a and b refer to the individual mixture components. Each reported selectivity value in the main text corresponds to the average value from three independent experiments. [058] Single-Component Gas Adsorption Isotherm Measurements. The olefins 1-butene, cis-2-butene and trans-2-butene were purchased from Sigma-Aldrich (³ 99% purity) as compressed liquids in Sure/Pac™ steel cylinders. In a typical experiment, a pre-activated framework (approximately 0.1 g) was loaded into a pre-weighed sample tube in an Ar-filled glovebox, which was then capped with a Transeal equipped with Kalrez O-rings (Viton O-rings were found to adsorb significant quantities of butenes). The samples were then transferred to a Micromeritics 3Flex gas adsorption analyzer, the seal between the tube and the instrument analysis port being formed by a Kalrez O-ring. The samples were heated in a sand bath to 433 K for at least 1 h until the outgas rate was < 1 µbar/min. The samples were then immersed in either a temperature-controlled bath comprised of Dow Syltherm fluid or a circulating Julabo F-25 water bath. Adsorption isotherm measurements were performed between 0–1.0 bar. Between isotherms, the samples were re-activated in a sand bath at 160 ˚C in vacuo for at least 3 h. [059] Isotherm Fitting, Differential Enthalpies of Adsorption, and IAST Calculations. Single-component adsorption isotherms were fit individually using a three-site Langmuir Freundlich equation, given by: where n is the amount adsorbed in mmol/g, q _sat is the amount adsorbed when saturated with the gas in mmol/g, b is the Langmuir parameter in bar ^–1 , P is the gas pressure in bar, v is the dimensionless Freundlich parameter, and subscripts a, b, and c correspond to three different site identities. These parameters were determined using a least-squares method. Note that limiting the fitting to fewer than three distinct sites consistently led to inferior fits as judged by both the overall sum of squared variance values and visual inspection. Use of three-site Langmuir- Freundlich fits have previously been shown to yield excellent fits for H ₂ adsorption isotherm data in the M ₂ (dobdc) and M ₂ (m-dobdc) frameworks that exhibit very steep uptake in the low pressure region. ⁴ The differential enthalpies of adsorption as a function of gas loading were extracted from the fits of single-component isotherms at 308, 318 and 328 K. The isotherm fits were numerically inverted and solved as P(n). The enthalpy h can then be determined at constant loadings using the Clausius–Clapeyron relationship: where R is the ideal gas constant, P is the pressure at a given loading, and T is the temperature at which the isotherm data were collected. [060] Ideal Adsorbed Solution Theory (IAST) was used to determine adsorbent selectivities from single-component gas adsorption isotherms. ^6,7 This approach involves numerically solving for the spreading pressure and subsequently determining the composition of the adsorbed phase at a given gas phase composition. The selectivity S for adsorption of a from a two-component mixture comprised of a and b is then given by: where x is the mole fraction in the adsorbed phase, y is the mole fraction in the gas phase, and the subscripts a and b refer to the mixture components. [061] Single-Crystal X-ray Diffraction Experiments. Single-crystal X-ray diffraction data were collected at Beamline 11.3.1 at the Advanced Light Source at Lawrence Berkeley National Laboratory using synchrotron radiation (see Table 4 for X-ray wavelengths) and a Bruker PHOTON100 CMOS detector mounted on a D8 diffractometer. The samples were cooled to 100 K using an Oxford Cryosystems cryostream for data collection. Data for 1-butene and trans-2- butene were collected on single crystals in gas-dosed capillaries, which were prepared according to a previously reported procedure. ⁸ Briefly, a methanol-solvated crystal of Co ₂ (dobdc) was mounted onto a borosilicate glass fiber using a minimal amount of epoxy, ensuring accessibility of the crystal pores. The glass fiber was then inserted into a 1.0 mm borosilicate glass capillary, which was connected to a capillary-dosing assembly attached to a port on a Micromeritics ASAP 2020 instrument. The sample was evacuated under reduced pressure at 180 °C for 24 h to remove all solvent molecules within the crystal. The capillary was then dosed with either 1- butene (800 mbar) or trans-2-butene (400 mbar) and flame-sealed with a methane/oxygen torch. A single crystal of Co ₂ (dobdc)(cis-2-butene) _1.58 was prepared by soaking an activated crystal in liquefied cis-2-butene in a glovebox freezer at –35 ˚C for 1 day. After evaporation of the butene, the crystal was coated in paratone oil, removed from the glovebox, and brought immediately to the beamline for data collection. The crystals for all structures were found to be an obverse/reverse twin based on analysis of their diffraction patterns, and CELL_NOW ⁹ was used to determine their orientation matrices. Raw data for each structure were integrated and corrected for Lorentz and polarization effects using Bruker AXS SAINT ¹⁰ software and corrected for absorption using TWINABS. ¹¹ For all structures, TWINABS was used to produce a merged HKLF4 file for structure solution and initial refinement and an HKLF5 file for final structure refinement. The structures were solved using either direct methods with SHELXS ^12,13 and refined using SHELXL ^12,14 operated in the OLEX2 interface. ¹⁵ Thermal parameters were refined anisotropically for all non-hydrogen atoms. Disorder and thermal motion of trans-2- butene in Co ₂ (dobdc)(trans-2-butene) _1.48 required the use of displacement parameter and distance restraints. All framework hydrogen atoms were refined using the riding model. [062] Powder X-ray Diffraction Measurements. High-resolution synchrotron X-ray powder diffraction data for all Co ₂ (m-dobdc) samples were collected at beamline 17-BM at the Advanced Photon Source at Argonne National Laboratory with a Perkin-Elmer a-Si flat panel detector. Diffraction patterns for Co ₂ (m-dobdc) dosed with 1-butene and trans-butene were collected at a wavelength of 0.45415 Å, while the temperature was held constant at 250 K with an Oxford Cryosystems Cryostream 800. Diffraction patterns for cis-butene dosed Co ₂ (m-dobdc) were collected at 125 K with a wavelength of 0.45411 Å. The crystal structure analyses (indexing, Pawley and Rietveld refinement) were performed with the program TOPAS- Academic v4.1. ¹⁶ [063] To obtain patterns of Co ₂ (m-dobdc) dosed with 1-butene and trans-butene, the activated framework was loaded into 1.0 mm borosilicate capillaries inside an argon glovebox. The capillaries were then placed in a gas-dosing cell which was attached to a Micromeritics 3Flex gas adsorption analyzer and dosed with 250 mbar of 1-butene and 200 mbar of trans-butene, respectively, and flame sealed. For measurements of cis-butene dosed onto Co ₂ (m-dobdc), the activated framework was loaded into a 1.0 mm borosilicate capillary inside a nitrogen glovebox, attached to a gas-dosing cell connected to a custom gas-dosing manifold, turbomolecular pump, and fixed to a goniometer. The framework was first evacuated, then was dosed in situ with 4 mbar of cis-butene at 298 K, and cooled slowly to 125 K at a rate of 3 K/min. [064] Upon inspection of the diffraction patterns, it was found that Co ₂ (m-dobdc) exhibits noticeable changes in peak intensity upon adsorption of 1-butene and trans-butene, and more pronounced changes upon adsorption of cis-butene. These changes in intensity are particularly apparent upon cooling and is indicative of gas molecules occupying the pores of the framework. Precise unit cell parameters of all dosed samples were first obtained by Pawley refinement. [065] Using the refined structure of activated Co ₂ (m-dobdc) as a starting reference, a Fourier difference map was calculated after Rietveld refinement. Excess electron density was observed most clearly above the Co open metal site with all three butenes. When atoms were placed in the region with the highest concentration of electron density and their atomic coordinates and occupancies were refined the goodness-of-fit parameters showed a marked improvement. In the initial stages of the refinement, C–C bond distances were refined with soft constraints. In the final stages of the refinement, the adsorbed gas occupancies and thermal and unit cell parameters were fully refined and convoluted with the sample and instrument parameters and Chebyshev background polynomials. The calculated diffraction pattern for the final structural models of Co ₂ (m-dobdc) dosed with 1-butene and cis-2-butene agree well with the experimental data (Rietveld plots, and crystallographic details in Table 5). [066] In the refinement of Co ₂ (m-dobdc) dosed with 1-butene, the olefinic carbons C6 and C7 were easily located over the Co open metal site and refined to a reasonable C–C double bond distance of 1.26(9) Å. The alkyl groups were challenging to model due to the disorder imposed by the mirror plane characteristic to the R3m space group which bisects the framework ligand. This mirror plane places the terminal carbon C8 of the alkyl chain close to the neighboring butene, however the large atomic displacement parameter of 0.13(3) suggests that there is significant disorder at this position caused by the flexibility of the alkyl chain and steric hinderance of the neighboring butene, in addition to the crystallographic mirror plane. [067] In the refinement of Co ₂ (m-dobdc) dosed with cis-butene, the olefinic carbons C6 and C7 of the butene were located above the Co centers without difficulty. However, only one other carbon atom belonging to the butene, C9, could be found using the Fourier difference map. The final carbon of the butene, C10 could not be located using this method and was instead attached to the partial molecule using Mercury CSD v4.1.0, based on the expected structural parameters of cis-butene determined from single-crystal x-ray diffraction experiments of Co ₂ (dobdc). As a result, this carbon’s atomic positions were not freely refined. [068] The Fourier difference map indicated there was unmodeled electron density in the middle of the MOF pore. This led to the addition of C8 to the structure, which greatly improved the statistics of the refinement (reduction of ~2% in the R _wp to 7.69%) and is assigned as an additional molecule of cis-butene at the center of the pore which is highly disordered, as evidenced by the high value of the thermal displacement parameter and inability to resolve all carbons of the butene molecule. The close C and H distances between neighboring butenes should not be regarded as accurate, due to significant disorder imposed by the ligand-bisecting mirror plane. [069] Finally, attempts to conduct a Rietveld refinement to obtain the trans-butene-dosed Co ₂ (m-dobdc) structural model were unsuccessful. 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