Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Thiophene Separation with Silver-Doped Cu-BTC Metal−Organic Framework for Deep Desulfurization Shuai Ban,* Kaiyang Long, Jing Xie, Hui Sun, and Hongjun Zhou Institute of New Energy, State Key Laboratory of Heavy Oil Processing, Beijing Key Laboratory of Biogas Upgrading Utilization, China University of Petroleum (Beijing), Fuxue Road 18, Beijing 102249, China S Supporting Information *
ABSTRACT: Molecular simulations using grand-canonical ensemble Monte Carlo (GCMC) and first-principles calculations were performed to study the silver doped copper(II) benzene-1,3,5-tricarboxylate (Cu-BTC) metal organic framework (MOF) for deep desulfurization. The density function theory (DFT) calculations were conducted to quantify the binding energies of silver dopant with Cu-BTC as well as adsorbates. The computed DFT potentials were taken to parametrize the classical host−guest force fields for GCMC simulations with high accuracy. Adsorption isotherms of thiophene and toluene in isooctane solutions were experimentally measured using pure Cu-BTC samples for the validation of molecular modeling. The simulation results show that the doping of silver leads to significant increase of adsorption uptakes for all components (thiophene, toluene, and isooctane), agreeing with the results of calculated adsorption enthalpies in Ag-Cu-BTC. Simulation snapshots reveal that thiophene is preferentially accommodated in the small tetrahedral cage of Cu-BTC, before intruding into the large octahedral cages to suppress loadings of toluene and isooctane with the assistance of silver dopants. The resulting selectivity of thiophene to toluene in isooctane was as high as 4 in Ag-Cu-BTC. On the basis of the mechanistic understandings, screen criteria and fictionalization schemes of potential MOF materials are proposed for applications of adsorptive desulfurization.
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alumina, 15 zeolites, 11,16 and metal−organic frameworks (MOFs).17,18 As a new class of porous materials, MOFs have great potentials due to their tunable porosity and surface chemistry.19−21 Specifically, the nanopores of MOFs exhibit molecular sieving effect,22,23 and the metal ions (lewis acid sites) can form strong π-complex with sulfur (Lewis alkali sites) of thiophene, BT, DBT, 4,6-DMDBT, thus showing superior separation capacity for desulfurization applications.24,25 Research has been carried out experimentally to design proper MOFs for the removal of sulfur compounds. Wong-Foy et al. investigated adsorption properties of five different MOFs (UMCM-150, Cu-BTC, MOF-505, MOF-177, and MOF-5) for organosulfur compounds in isooctane and toluene solutions and concluded that the surface area and pore volume are not the determinant factors, but the pore size and shape are.19 Yang et al. examined four types of MOFs (Cu-BTC, Cu-BDC, CrBDC, and Cr-BTC) for DBT adsorption, and found Cu-BTC having the highest sulfur uptake since its pore size well matches that of thiophenic sulfur.24 To further enhance the adsorption capacity, a serial of studies have been conducted to functionalize MOFs using metal doping method. The Ni doped MOF-5 was successfully synthesized for hydrogen storage, and
INTRODUCTION Irrespective of the rapid development of sustainable energies during past decades, the major energy resource in the immediate term is still considered to be fossil fuels, which contains inevitable pollutants of sulfur compounds that need to be purified before use. Nowadays, desulfurization of petroleum fuels becomes a major issue in industry owing to serious environmental and healthy impacts caused by sulfur oxides largely produced from engine exhaust of vehicles.1−4 Rigorous environmental protection regulations have been formulated worldwide, requiring ultralow concentrations of sulfur compounds in both diesel and gasoline.5 The conventional industrial process of deep desulfurization is based on hydrodesulfurization, which uses hydrogen to convert thiophenic sulfur compounds including benzothiophene (BT), dibenzothiophene (DBT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT).6 However, this approach simultaneously brings about hydrogenation of olefins that undesirably lowers the octane rating of the fuel. To tackle this issue, alternative techniques have been proposed, including oxidative desulfurization,7 oxidation−extraction desulfurization,8 adsorptive desulfurization,9 biodesulfurization,10 etc. Among them, adsorptive desulfurization with advantages of high energy efficiency, low cost, and simplified operation is being studied intensively in the aspect of developing novel adsorbent materials with high separation efficiency toward sulfur compounds.9,11,12 Typical adsorbents are activated carbon,13 silica gel,14 activated © XXXX American Chemical Society
Received: Revised: Accepted: Published: A
October 30, 2017 January 5, 2018 February 12, 2018 February 12, 2018 DOI: 10.1021/acs.iecr.7b04496 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 1. Frameworks of (a, b) Cu-BTC and (c, d) Ag-doped Cu-BTC. The colors are gray for C, white for H, red for O, coral for Cu, and blue for Ag. The silver atom in the framework of Cu-BTC is highlighted.
calculations and molecular simulations. Doping agents were selected from transition metals (Ni, Zn, Ag, and Cd). Adsorption of thiophene and toluene in isooctane solutions was computed in comparison with experimental measurements. Adsorption of ternary mixtures is simulated with the emphasis of separation selectivity of thiophene in relation to the silver dopant. Mechanistic analysis was performed to justify the feasibility of functionalized MOFs for applications of adsorptive desulfurization.
substantial increase of surface area was observed along with improved H2 loadings.26 Similarly, Cu(I)-doped MOF-5 was developed for desulfurization. It was shown that the proper doping quantity of Cu(I) is crucial to maximize DBT loadings as excessive doping agents may reduce accessible pore volume and hinder mass transfer.27 In addition, metal doping of MOFs is also found effective on improving the separation efficiencies of various gas mixtures. Starting from alkali metals (Li, Na, and K), Gao et al. reported that the selectivity of O2/N2 in KCuBTC increased by 8-fold at 18 bar and 298 K due to the formation of electron donor−acceptor complexes between C atom of CO2 and K cation.28 Adhikari designed two Pd-doped MOF-74(Ni, Co) for the separation of CO2/N2. The selectivities of CO2 for MOF-74(Ni)-Pd and MOF-74(Co)Pd increase 3.3 and 2.7 times at 298 K and 32 bar compared to the original samples, rationalized by the interaction of quadrapole moment of CO2 with Pd atoms of low electronegativity.29 As the fundamental understanding of adsorption and separation mechanisms occurring at the molecular level is crucial to the development of potential MOF materials, computer modeling is employed to study the sorption and diffusion behaviors of adsorbates in various types of MOFs. Through a combined study of experiment and computation, Liu et al. investigated DBT adsorption in copper-based organic frameworks and identified the functionalized organic ligands serving as primary active sites for DBT binding.30 This finding is further confirmed by the work of Mao et al. via the systematical calculation of adsorption enthalpies and isotherms for DBT, BT and thiophene in Cu-BTC.31 Meanwhile, CuBTC was shown to have a high selectivity of thiophene to benzene but significantly lower saturation loadings than IRMOF-1 due to its limited pore volume.32 In addition, metal doping has been investigated theoretically in changing the surface chemistry of MOFs and the nature of interactions with adsorbates. Snurr et al. synthesized and simulated a series of Li-doped MOFs and concluded that the existence of Li cations can significantly enhance the selectivity of CO2 over CH4 due to the strong Li (charge)/CO2 (quadrupole) interaction.33 Density function theory (DFT) study was applied to reveal the doping effect of alkali (Li, Na, and K), alkalineearth (Be, Mg, and Ca), and transition (Sc and Ti) metals in covalent organic frameworks (COF), and substantial improvement of separation efficiency of CO2 is achieved using Li-doped COF.34 In this work, we attempt to study the effect of metal doping in Cu-BTC on deep desulfurization using both first-principles
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EXPERIMENTAL SECTION Simulation Methods. Adsorption isotherms and enthalpies were calculated using configurational-bias Monte Carlo (CBMC) methods coded in RASPA software.35 A typical CBMC simulation consists of 2 × 106 cycles, and the last 106 cycles were used to average thermodynamic properties. For each CBMC cycle, trial moves were attempted to translate, rotate, regrow, or exchange a molecule with reservoir, as well as to switch molecular identities of mixtures.36 The number of trial moves per cycle is equal to the number of molecules with a minimum of 20.35 The isotherms of thiophene, toluene, and isooctane were calculated at the temperature of 293.15 and 313.15 K and the pressures of 10−1−104 Pa. Adsorption equilibrium in liquid phase were computed by converting the concentrations to the fugacities in gas phase, given the identical chemical potentials of two equilibrated phases.37 The partial pressures of gases were calculated using Peng−Robinson equation of state. To characterize the microstructure of MOFs, the pore size distribution (PSD) was calculated using the method of Gelb and Gubbins.38,39 The radial distribution function (RDF) was computed to specify spatial correlations of adsorbate molecules in MOFs. Molecular interactions were modeled using the classical Lennard-Jones (LJ) and Coulombic potentials. The TraPPE force field was adopted, and host−guest interaction parameters were derived using the Lorentz-Berthelot mixing rule as listed in Table S1 in Supporting Information.40−43 In particular, the molecular interactions between sulfur atom of thiophene and silver dopant in Cu-BTC were determined by fitting the DFT potential curves with LJ function. More details are included in the Supporting Information. Thiophene is simulated using a five-site rigid model as shown in Figure S1. The C−C and S−C bonds are fixed at 1.40 and 1.71 Å, and the C−S−C, S−C−C, and C−C−C bond angles are 92.2°, 111.5°, and 112.4°, respectively. The partial charges of thiophene were taken from the work of Siepmann.41 Toluene was modeled as a rigid aromatic molecule with the C−C bond length of 1.40 Å and the B
DOI: 10.1021/acs.iecr.7b04496 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
Figure 2. Calculation of host−guest interactions. (a) DFT results of binding energy between thiophene and Ag-Cu-BTC as a function of Ag−S distance. Highest occupied molecular orbital (HOMO) for the interaction of Ag-Cu-BTC cluster with (b) thiophene and (c) toluene. The colors are gray for C, yellow for S, white for H, red for O, coral for Cu, and blue for Ag.
E binding = Esupermolecule − E host − Eguest
C−CH3 bond length of 1.54 Å. The partial charges of toluene was estimated using the CHelpG method of first-principles calculations as listed in Table S2.41 For validation purpose, vapor−liquid coexistence curve of toluene was computed in Gibbs ensemble, and the result agrees well with the data taken from National Institute of Standards and Technology Database as shown in Figure S2. Flexible model was employed for isooctane.44 The coordinates of of Cu-BTC framework were taken from the Cambridge Crystallographic Data Centre as shown in Figures 1 and S1 and were kept rigid in all simulations. Their partial charges were calculated and shown in Table S4 according to the work of Yang et al.45 Ewald summation method was used to calculate all electrostatic potentials. The LJ interactions were cut off at a distance of 13.0 Å without tail correction. Periodic boundary condition was applied in all directions. Typical transition metals (Ni, Ag, Zn, Cd) were chosen as possible doping agents. Their binding energies with thiophene were computed and shown in Table S3. The DFT calculation was conducted at the theoretical level of second-order Møller− Plesset (MP2) and basis set superposition error (BSSE) correction with 6-31+G** basis set for S, C, H, and O and LANL2DZ basis set for M+ (Ni, Ag, Zn, Cd).46,47 To verify the accuracy of MP2 calculation for dispersive interaction, the binding energies were compared with the results of the doublehybrid function B2PLYP that combines exact HF exchange with a MP2-like correlation.48−52 Geometry optimizations of Ag-Cu-BTC were performed using the hybridized B3LYP/ GenECP method implemented in Gaussian 09 software.34,53,54 The binding energies for silver and Cu-BTC cluster were calculated according to eq 1 using the high-quality PW91/ GenECP method.34,55,56
(1)
Experimental Methods. The adsorption measurement was performed to quantify the loadings capacity of Cu-BTC, as well as to validate the simulation results. Notably, the silver doped Cu-BTC was only studied via computer modeling to estimate the potential of metal doping on separation of sulfur compounds. In detail, commercial Cu-BTC as purchased from Sigma-Aldrich was activated under vacuum at 443.15 K for 24 h before use. Thiophene or toluene solutions in isooctane were prepared with a typical concentration up to 103 ppmw, in accordance with the composite of gasoline prior to deep desulfurization.31 For the measurement of adsorption, dehydrated Cu-BTC powder (60 mg) was mixed rapidly with 6 mL of thiophene/toluene solutions in vials (12 mL). The vials were then sealed and shaken at 120 rpm in a shaker for 24 h at controlled temperatures of 293.15 and 313.15 K, respectively. After equilibration, the solution was filtered and analyzed using an Agilent 7890B gas chromatography system equipped with a HP-5 capillary column. The loading qe of toluene or thiophene follows qe =
(c0 − ce)ρV m
(2)
where c0 and ce are the initial and equilibrated concentration of thiophene or toluene (mmol kg−1), ρ the density of isooctane (6.92 × 10−4 kg mL−3), V the volume of the solution (mL), and m the mass of adsorbent (g).
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RESULTS AND DISCUSSION Structure of Silver Doped Cu-BTC. The binding energies of selected doping agents (Ni, Ag, Zn, Cd) with thiophene were calculated as shown in Figure 2a, and the results are compared
C
DOI: 10.1021/acs.iecr.7b04496 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 3. Calculated adsorption isotherms of (a) thiophene, (b) toluene, (c) isooctane in Cu-BTC and Ag-Cu-BTC at the temperature of 293.15 K. (d) Adsorption and desorption isotherms of thiophene in Cu-BTC.
unsaturated metal sites, particularly through a direct binding between sulfur and copper sites. In Ag-Cu-BTC, the active site for thiophene is changed to silver via the strong Ag−S interaction. The silver dopant is also able to adsorb toluene by means of charge−dipole and charge−quadrupole interactions.61 The interaction of toluene with the organic fragment is of dispersive nature. The quadrupole−quadrupole interaction stabilizes toluene in a perpendicular manner as evidenced in Figure 2c. Additionally, DFT potentials of thiophene and silver cation as a function of Ag−S distance are computed for parametrization of LJ force field in Figure 2a. Three different trajectories were considered as demonstrated in Figure S3. It suggests that the vertical trajectory corresponds to the lowest potential path, namely, the most possible way to approach silver cation. The classical LJ potential well reproduces the firstprinciples data with a minimal energy of −11.25 kcal mol−1 at the Ag−S distance of 2.5 Å. As reported previously that excessive doping agents can reduce accessible pore volume of MOFs and hinder mass transfer,27 the change of Cu-BTC’s microporosity upon silver doping is examined by means of PSD in Figure S4. The computation results show that the total pore volume is reduced from 0.85 to 0.77 cm3 g−1 due to the occupancy of silver atoms, while the pores with sizes of 4 and 9 Å are enlarged slightly. In general, the framework of Cu-BTC consists of two types of nanopores that are the tetrahedral cage (9 Å) and the octahedral cage (12 Å). The tetrahedral cage is constructed of four trimesic acids, while the octahedral cage is composed of eight tetrahedral cages linked by copper cations. The presence
in Table S3. It is seen that all metal atoms have strong affinity with thiophene, following the order of Ni > Zn > Cd > Ag. Typically, thiophene possesses a moderate binding strength of −33.96 kcal mol−1 with silver, corresponding to the level of physical adsorption. As a metal element commonly adopted for functionalization, silver is thus selected for promoting the adsorption capacity of Cu-BTC in this work. The structure of silver doped Cu-BTC optimized by first-principles calculations is shown in Figure 1. In this study, the doping concentration is chosen to be eight silver atoms in each unit cell for the sake of convenience. Starting from an initial configuration of one silver atom above the aromatic ligand of Cu-BTC, the optimized structure shows the bonding of silver with two oxygen atoms with a binding energy of −37.41 kcal mol−1. This value is much lower than the Ag-thiophene binding energy of −28.39 kcal mol−1. As illustrated by the highest occupied molecular orbital shown in Figure 2b, thiophene molecule features a σ binding interaction between sulfur and silver, and a relatively weak π−π stacking with the aromatic ligand. This leads to the parallel configuration of thiophene on the ligand of Cu-BTC. In contrast, toluene tends to orient perpendicularly to the ligand as the binding only exists between methyl and silver species as seen in Figure 2c.57,58 In general, Cu(II) and Ag(I) cations of Cu-BTC serve as Lewis acid sites, and thiophene and toluene are Lewis base rich in π electrons.59 It is known that Cu-BTC features 3 types of adsorption sites, which are the Cu metal site, the O atom site, and the phenyl linker site.60 In the original Cu-BTC, the adsorption of thiophene mainly occurs on the coordinatively D
DOI: 10.1021/acs.iecr.7b04496 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 4. Adsorption enthalpies of pure thiophene, toluene, and isooctane in (a) Cu-BTC and (b) Ag-Cu-BTC as a function of loadings.
Figure 5. Density distribution contours of the center of mass for (a) thiophene and (b) toluene in Cu-BTC and (c) thiophene and (d) toluene in Ag-Cu-BTC at a pressure of 10 Pa. The contour index of molecular density is in the units of molecules per unit cell. (e) Radial distribution functions of S−Ag pair at 10 and 30 Pa.
elevated temperature of 313 K as shown in Figure S6. In addition, the desorption of thiophene from Cu-BTC (Figure 3d) follows the type V isotherm with pronounced sorption hysteresis, indicating the capillary condensation occurring inside Cu-BTC. Even though the related experiments have not been carried out in this work, the simulations are still considered to be instructive in terms of fundamental understanding of sorption behaviors of distinct adsorbate molecules. The isosteric heats of adsorption of thiophene, toluene, and isooctane in Cu-BTC and Ag-Cu-BTC are shown in Figure 4. The calculated adsorption enthalpy of thiophene has the highest value of 11.2 kcal mol−1 at low coverage and increases to 17.0 kcal mol−1 above 10 molecules/unit cell, owing to the condensation in nanopores of Cu-BTC. This again reflects the strong interaction attributed to the affinity of the sulfur of thiophene with metal sites, even though this value is somewhat lower than that in zeolite Cu−Y (Si/Al = 2.43) which is measured at 20.8−24.0 kcal mol−1.11 The heat of adsorption of toluene in Cu-BTC is ranging from 10 to 17 kcal mol−1, higher than the value of 14.3 kcal mol−1 in zeolite Si-MCM-41.64 It is noticed that below the loading of about 1 molecule per unit cell, the adsorption enthalpies of thiophene and toluene remain constant with a difference of 1 kcal mol−1. In the presence of silver dopants, this difference is enlarged by about 2-fold at the loadings below 10 molecules per unit cell. At high loadings,
of silver cations in the window ranges reduces the pore diameter of octahedral cages but contributes to the increase of smaller pore volumes. For the low doping concentration considered in this work, pore blocking by metal dopant is not expected to occur. Adsorption of Pure Component. Adsorption isotherms of single components in Cu-BTC and Ag-Cu-BTC are calculated and shown in Figures 3 and S5. The saturation loadings are different for all three components, which are roughly 80 molecules per unit cell for thiophene, 60 molecules per unit cell for toluene, and 25 molecules per unit cell for isooctane. Given the similar molecular sizes (thiophene 5.3 Å, toluene 6.7 Å, isooctane 5.9 Å), the variation of saturation concentrations implies that the existence of different active sites depends on the type of adsorbate molecules. All adsorption isotherms show sudden increases of molecular uptakes prior to saturation. Such transition takes place at a pressure varying from 10 Pa to 103 Pa, following the order of thiophene ∼ toluene < isooctane. This trend is mainly rationalized by the strength of host−guest interactions. Moreover, silver doping leads to the increase of thiophene loading by 3 times compared to that of pure Cu-BTC at 30 Pa, while uptakes of toluene and isooctane are barely affected. The selective adsorption toward thiophene is rationalized by the strong S−Ag interaction and πconjugated phenyl.19,62,63 This phenomenon remains valid at E
DOI: 10.1021/acs.iecr.7b04496 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 6. Adsorption isotherms of (a) thiophene and (b) toluene in isooctane solutions at 293.15 K and (c) thiophene and (d) toluene in isooctane solutions at 313.15 K in Cu-BTC and Ag-Cu-BTC. The experimental data are taken from Liu et al.31
Figure 7. Density distribution contours of thiophene and toluene in isooctane solutions. The concentrations are 637 ppmw for thiophene and 600 ppmw for toluene. The left figures are (a) thiophene, (b) isooctane in Cu-BTC and (c) thiophene, (d) isooctane in Ag-Cu-BTC. The right figures are (e) toluene, (f) isooctane in Cu-BTC and (g) toluene, (h) isooctane in Ag-Cu-BTC. The contour index of molecular density is in units of molecules per unit cell.
isooctane can only access the large octahedral cages attributed to the effect of steric hindrance. RDF results of thiophene in Ag-Cu-BTC shown in Figure 5e confirm the translocation of thiophene in that sulfur atoms mainly appear in tetrahedral cages at a distance of 6.5−8.5 Å away from silver cations in dilute and then gradually occupy the area close to silver in octahedral cages as shown in Figure S7. This phenomenon suggests the existence of two types of adsorption sites for thiophene, where the primary site is the tetrahedral cage followed by the secondary site of silver dopants in the octahedral cage.
adsorption enthalpies of thiophene become approximately consistent with that of toluene in Cu-BTC but less than that of toluene upon silver doping. Isooctane features the lowest heats of adsorption of 5.2−8.4 kcal mol−1 well below those of thiophene and toluene. Molecular density distribution contours were sampled to trace the locations of thiophene and toluene in Cu-BTC as shown in Figure 5. At low pressures, it is seen that due to the relatively small molecular size (5.3 Å), thiophene is preferentially located in the small tetrahedral cages of CuBTC, irrespective of Ag doping, while bulkier toluene and F
DOI: 10.1021/acs.iecr.7b04496 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
Figure 8. Adsorption isotherms of ternary mixtures in (a) Cu-BTC and (b) Ag-Cu-BTC at 293.15 K. The feed ratio is thiophene/toluene/isooctane = 1:1:98. (c) Adsorption selectivity of thiophene to toluene as a function of total pressure at 293.15 K. The selectivities are compared between two different feed ratios that are thiophene/toluene/isooctane = 1:1:98 and 2.5:2.5:95, respectively.
Adsorption of Binary Mixtures. For validation purpose, adsorption isotherms of binary components in Cu-BTC were calculated and compared with experimental measurements of thiophene/toluene in isooctane solutions as shown in Figure 6. Excellent agreement is achieved for both thiophene and toluene in pure Cu-BTC at various temperatures, reflecting the accuracy of force field parameters and simulation methods. It is noted that according to experimental measurement, the pressure used in simulation is converted to the concentrations of thiophene/toluene solutions in the units of mmol kg−1. The initial concentration of thiophene and toluene in isooctane solution is less than 103 ppmw, corresponding to the partial pressure of around 5 Pa at 293.15 K and 13 Pa at 313.15 K, respectively. The resulting uptakes of adsorbates are less than 10 molecules per unit cell, far below saturation as computed in Figures S5 and S6. The doping of silver leads to pronounced improvement of loadings for all three components. Interestingly, thiophene shows typical Langmuir isotherms that level off after the rapid monolayer adsorption at low concentrations (104 Pa), accompanied by the suppression of toluene and isooctane uptakes. Upon silver doping, the adsorption of thiophene is uniquely enhanced over toluene and isooctane, suggesting the potential of metal doping on improving the desulfurization capability of Cu-BTC. The adsorption selectivity in Figure 8c shows that the both CuBTC and Ag-Cu-BTC do not have sufficient selectivity toward thiophene below the pressures of 5 × 10 3 Pa. This phenomenon can be explained in reference to the adsorption of single components shown in Figures 3 and 5. Thiophene and toluene accommodate tetrahedral cages and octahedral pores, respectively, featuring comparable uptakes under low pressures. Further increasing the pressure or loading results in competitive adsorption of both components inside octahedral pores. This leads to an overwhelming uptake of thiophene over toluene and thus the increase of selectivity toward thiophene. Upon the doping of silver, the strong Ag−S interaction enhances the loading of thiophene, especially at low pressures, while little improvement is gained in the intermediate range of pressures since the adsorption of toluene is also facilitated by silver dopants as seen in Figure 8a and Figure 8b. The selectivities of thiophene as large as 4 are achieved under elevated pressures, particularly in Ag-Cu-BTC with a higher feed ratio of thiophene/toluene/isooctane = 2.5:2.5:95. Apart from the H
DOI: 10.1021/acs.iecr.7b04496 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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is expected that benzene can be simultaneously accommodated into both types of nanopores of Cu-BTC, while other large aromatics will experience competitive adsorption with thiophenics in the octahedral cages. In this manner, Ag-Cu-BTC not only can effectively separate sulfur compounds but also can reduce harmful aromatics to some extent. On the basis of mechanistic understanding of desulfurization process in Ag-CuBTC, metal doping can be used as an effective approach to functionalize other types of MOF materials as well.68−70 The frameworks of synthetic MOFs including ZIF-100, IRMOF-11, and MIL-101(Fe) have similar pore structures with Cu-BTC. For instance, the micropore of ZIF-100 has a diameter of 35.6 Å that can be modified using transition metal to enhance the adsorption and selectivity of thiophenic compounds.71 IRMOF11 represents a series of MOFs with the advantage of adjustable tunnels that can be tailed according to the dimensions of targeted thiophenics. In all, the exposed metal sites and suitable porosity of MOFs are two crucial factors for the practice of adsorptive desulfurization.
Article
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b04496. Table S1, the LJ force field parameters for host−guest interactions; Table S2, the atomic partial charges of toluene; Table S3, binding energies of doped cations with thiophene; Table S4, the atomic partial charges of Cu-BTC and Ag-Cu-BTC; Figure S1, molecular structures of thiophene, toluene, and Ag-Cu-BTC; Figure S2, vapor−liquid coexistence curves for toluene; Figure S3, the binding energies between thiophene and Ag-CuBTC cluster as a function of distance; Figure S4, pore size distributions of Cu-BTC and Ag-Cu-BTC; Figure S5, calculated adsorption isotherms of thiophene, toluene, and isooctane in Cu-BTC and Ag-Cu-BTC at 293.15 K; Figure S6, calculated adsorption isotherms of thiophene, toluene, and isooctane in Cu-BTC and AgCu-BTC at 313.15 K; Figure S7, density distribution contours of the center of mass of thiophene and toluene under 20 and 30 Pa at 298.15 K in Cu-BTC and Ag-CuBTC; Figure S8, the simulated adsorption isotherms of thiophene and toluene in Cu-BTC and Ag-Cu-BTC at 293.15 K and 313.15 K; Figure S9, the simulated adsorption isotherms of thiophene/toluene/isooctane ternary mixtures; Figure S10, radial distribution functions of S atom of thiophene in Cu-BTC; Figure S11, radial distribution functions of S atom of thiophene in Ag-CuBTC (PDF)
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CONCLUSIONS The adsorption of thiophene, toluene, isooctane, and their mixtures in Cu-BTC and metal doped Cu-BTC has been studied by means of experiments and simulations. Firstprinciples calculations show that among the selected transition metals (Ni, Ag, Zn, Cd), silver is an appropriate doping agent that can interact strongly with Cu-BTC (−37.41 kcal mol−1) and thiophene (−28.39 kcal mol−1). Structural optimization of thiophene and toluene in Ag-Cu-BTC indicates that thiophene has a parallel configuration attributed to the σ binding of S−Ag pair and the π−π stacking with aromatic ligand. Toluene tends to orient perpendicularly to the ligand caused by the binding between silver and methyl species as well as the dispersive interaction with organic fragment of Cu-BTC. For single-component adsorption, it is found that silver cations can pronouncedly improve the uptake of thiophene at low pressures, while the adsorption of toluene and isooctane is hardly affected. The calculated adsorption enthalpies show that thiophene features strong interaction strength of 11.2−17.0 kcal mol−1, which is 1.0−2.0 kcal mol−1 higher than that of toluene in relation to silver dopants. Isooctane has heats of adsorption at the scale of 5.0−10.0 kcal mol−1, much lower than those of thiophene and toluene. Simulation snapshots reveal that thiophene is favorably adsorbed in the small tetrahedral cage, and toluene and isooctane can only accommodate in large octahedral cages due to the steric effect. For binary mixtures of thiophene/toluene in isooctane, the calculated adsorption isotherms well reproduce experimental data, ensuring the accuracy of force field parameters and simulation methods. The doping of silver significantly increases the loadings of both thiophene and toluene, even though their concentrations in isooctane solutions are as low as 103 ppmw. For ternary mixtures, predominant uptake of thiophene takes place at total pressures higher than 104 Pa, accompanied by the suppression of toluene and isooctane loadings. This phenomenon corresponds to the process of the simultaneous uptakes of all three components inside the octahedral cages of Cu-BTC after thiophene reaches saturation in tetrahedral cages under low pressures. A selectivity of thiophene as high as 4 can be achieved in silver doped Cu-BTC under elevated pressures. The interplay of micropore sizes and doping agents of MOFs is considered to be of great importance for the efficiency of desulfurization on the basis of physical adsorption.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Shuai Ban: 0000-0002-0954-9572 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the financial support from Science Foundation of China University of Petroleum, Beijing (Grants 2462014YJRC009 and C201604).
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