Leveraging Adsorption Kinetics and Thermodynamics in a Channel-Pore Interconnected Metal–Organic Framework for Stepwise Splitting Hexanes

Open AccessCCS ChemistryRESEARCH ARTICLES6 Aug 2024Leveraging Adsorption Kinetics and Thermodynamics in a Channel-Pore Interconnected Metal–Organic Framework for Stepwise Splitting Hexanes Xiao-Jing Xie, Zhi-Hao Zhang, Qi-Yun Cao, Min-Yi Zhou, Heng Zeng, Weigang Lu and Dan Li Xiao-Jing Xie , Zhi-Hao Zhang , Qi-Yun Cao , Min-Yi Zhou , Heng Zeng *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] , Weigang Lu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] and Dan Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Citation: CCS Chemistry. 2024;0:1–8https://doi.org/10.31635/ccschem.024.202404180 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Dibranched hexane isomers, particularly 2,3-dimethylbutane (23DMB), are of great significance in upgrading gasoline to high octane numbers. Yet, the separation of 23DMB from 2,2-dimethylbutane (22DMB) has remained challenging, mainly due to their nearly identical kinetic diameters (23DMB: 6.2 Å, 22DMB: 6.3 Å). Herein, we report a channel-pore interconnected metal–organic framework (named JNU-2) exhibiting complete exclusion of 22DMB thanks to a limited opening at the channel-pore junction, while exceptionally high adsorption for n-hexane (nHEX), 2-methylpentane (2MP), 3-methylpentane (3MP), and 23DMB due to the connecting large pores. Further adsorption kinetics studies reveal that the diffusion coefficient of 23DMB in JNU-2 is significantly lower than those of nHEX, 2MP, and 3MP. This difference in diffusion coefficients allowed us to successfully demonstrate the stepwise splitting of hexanes on JNU-2 through column breakthrough experiments. Specifically, a kinetically controlled separation of dibranched hexanes (22DMB and 23DMB) from their linear (nHEX) and monobranched isomers (2MP and 3MP), followed by a thermodynamically controlled sieving of 22DMB from 23DMB. This work presents a robust porous material for its versatility in the separation of hexane isomers by reconciling adsorption kinetics and thermodynamics. Notably, our study represents the first reported material with the ability to separate 22DMB and 23DMB. Download figure Download PowerPoint Introduction Hexanes are important commodities in the petrochemical industry.1,2 Their global market value has surpassed USD 1.34 billion in 2023 and is projected to increase to USD 1.73 billion by 2032, representing an average annual growth rate of 3.2%.3 Hexanes are mainly obtained from the refinery of crude oil,4 and this process often yields a mixture of linear, monobranched, and dibranched isomers. The research octane number (RON) is a key metric for assessing the quality of the gasoline fuel, and the blending of more branched alkanes typically yields gasoline with higher RON values.5 For example, the RON values of n-hexane (nHEX), 2-methylpentane (2MP), 3-methylpentane (3MP), 2,2-dimethylbutane (22DMB), and 2,3-dimethylbutane (23DMB) are 24.8, 74.5, 75.5, 94, and 105, respectively.6 Therefore, to upgrade the RON value of the gasoline fuel, it is preferred to blend in dibranched hexanes.7 However, the separation of hexane isomers poses a great challenge due to their similar physical and chemical properties. Current industrial practice is to use heat-driven distillation, which is not only energy-intensive but also requires huge capital investment in equipment.8 In the pursuit of alternative technologies to replace heat-driven distillation, adsorptive separation using porous solids is considered one of the most promising due to its non-heat-driven selective adsorption. For example, zeolite 5A,4,9 an industrially used porous solid, can exclusively adsorb the linear alkanes and therefore efficiently separate them from their branched counterparts. Yet, zeolite 5A is incapable of discriminating mono- and di-branched isomers, limiting its service in further improving RON values.4,9 Metal–organic frameworks (MOFs)10–17 are porous coordination solids assembled from metal clusters/ions and organic linkers. As opposed to the limited structural tunability of other porous solids, the reticular chemistry enables MOFs with easily adjustable pore dimensions and surface chemistry. As such, extensive research efforts have been devoted to their potential applications in industrially challenging separations, including the separation of hexane isomers.18,19 For example, a microporous MOF material Fe2(BDP)320 featured with triangular channels has been demonstrated to separate all five hexane isomers via their differences in molecular dimensions, yet the adsorption selectivity values are relatively low, resulting in RON values of no more than 92. A bi-based MOF material, UU-200, shows adsorption of both linear and monobranched alkanes but no adsorption of dibranched isomers, with RON values reaching as high as 96.21 Yet, to further upgrade the RON values would require 23DMB instead of a mixture of 23DMB and 22DMB. Although there have been notable progresses in finding MOFs for discriminating dibranched hexanes from their linear and monobranched counterparts.22–30 To the best of our knowledge, no MOF material has been reported with the ability to separate 23DMB and 22DMB, likely due to their nearly identical kinetic diameters (23DMB: 6.2 Å, 22DMB: 6.3 Å) ( Supporting Information Table S1)31 and molecular dimensions (23DMB: 7.8 × 6.7 × 5.3 Å3, 22DMB: 8.0 × 6.7 × 5.9 Å3).32 Herein, we present a channel-pore interconnected MOF (named JNU-2)33 for the efficient separation of dibranched hexanes from their linear and monobranched isomers, and more importantly, the separation of 23DMB from 22DMB for the first time by leveraging the adsorption kinetics and thermodynamics. The slim channel of JNU-2 allows for a complete exclusion of 22DMB, while its connecting large pore facilitates exceptionally high adsorption of nHEX, 2MP, 3MP, and 23DMB, leading to benchmark selectivity values for nHEX/22DMB and 3MP/22DMB. Further kinetic adsorption studies reveal that the diffusion coefficient of 23DMB in JNU-2 is substantially lower than those of nHEX, 2MP, and 3MP. Such a difference in diffusion coefficients allowed us to demonstrate the kinetic separation of dibranched hexanes from their linear and monobranched isomers and the subsequent thermodynamic separation of 23DMB from 22DMB through regulating the breakthrough processes (Figure 1). Figure 1 | Schematic comparison of zeolite 5A (top, industrial practice) and JNU-2 (bottom, this work) in the column breakthrough separation of hexanes. Download figure Download PowerPoint Experimental Methods Vapor-phase adsorption measurements Vapor-phase adsorption isotherms were measured on an automatic volumetric adsorption instrument (BELSORP MAX II, MicrotracBEL Corp., Osaka, Japan). The vapor source was degassed through three cycles of freeze-pump-thaw and JNU-2 was activated in an ultrahigh vacuum (stabilized at >10−3 Pa for at least 60 min) at 473 K for 12 h before measurements. Single-component vapor-phase adsorption isotherms of nHEX, 3MP, 2MP, 22DMB, and 23DMB were collected at 353 K. The manifold of the instrument itself including the vapor dosing bottle was heated to 323 K and kept at this temperature for all measurements. Breakthrough measurements Vapor-phase breakthrough experiments were performed at 353 K using a lab-scale fixed-bed system. About 1.2 g of the activated JNU-2 was packed into a custom-made stainless steel column (3.15 mm ID × 450 mm). The sample was activated under a dynamic vacuum at 473 K and then purged with a helium flow of 100 mL min−1. nHEX/2MP/3MP/22DMB/23DMB (20/20/20/20/20, v/v/v/v/v), 2MP/3MP/22DMB/23DMB (25/25/25/25, v/v/v/v), and 23DMB/22DMB (50/50, v/v) vapor mixtures were produced by helium gas bubbling method. The composition of the liquid mixtures in the bubbler was adjusted until the desired vapor phase ratio was achieved. The vapor mixture carried by helium gas was introduced to the column at a rate of 10 or 3.0 mL min−1. Outlet effluent from the column was continuously monitored using gas chromatography (GC-7890B, Agilent, Santa Clara, California, USA) equipped with a flame ionization detector. After each breakthrough experiment, the sample was regenerated in-situ at 473 K under high vacuum for 8 h. Kinetic adsorption measurements The kinetic adsorption isotherms were collected using a beishide vacuum vapor sorption/dynamic vapor sorption (BSD-VVS/DVS) gas sorption analyzer of Beishide Instrument Technology Co., Ltd. (Beijing, China). Samples were activated under vacuum at 473 K for 12 h prior to each test. The activated sample was placed in a vacuum environment at 353 K, and the hexane vapors were introduced via volatilization and controlled under the specified partial pressure P/P0. To avoid steam condensation, the gas path system was consistently held at a temperature of 333 K. Results and Discussion JNU-2 was synthesized in a 10-g scale according to the previously reported method33 and the phase-purity of JNU-2 was verified by powder X-ray diffraction (PXRD) patterns (see Supporting Information Figure S1). As shown in Figure 2a,b, the crystal structure of JNU-2 can be described as a channel-pore interconnected three-dimensional (3D) network with an opening of 7.4 × 7.4 Å2 at the channel-pore junction. This value is well within the range of molecular dimensions of hexane isomers, which motivated us to investigate JNU-2 for the adsorption of linear, mono-, and di-branched hexanes. Notably, JNU-2 exhibits complete exclusion of 22DMB likely due to its slightly oversized molecular dimension, while exceptionally high adsorption for nHEX, 2MP, 3MP, and 23DMB thanks to the presence of large pores (Figure 2c and Supporting Information Figures S3–S7). The adsorption capacity of nHEX (286 mg g−1, 353 K) on JNU-2 is substantially higher than those on the best-performing MOFs reported for the separation of hexane isomers, such as Al-bttotb (151 mg g−1, 303 K),34 HIAM-302 (166 mg g−1, 298 K),35 Fe2(BDP)3 (114 mg g−1, 403 K),20 UU-200 (146 mg g−1, 303 K),21 HIAM-203 (147 mg g−1, 303 K),36 PTA-MOF (185 mg g−1, 303 K),37 1-NO2 (145 mg g−1, 303 K),38 Ca(H2tcpb) (143 mg g−1, 333 K),39 Zr-bptc (130 mg g−1, 423 K),40 and similar to Zn-tcpt (280 mg g−1, 303 K)41 (Figure 2d and Supporting Information Figure S2). In addition, the uptake ratios of 23DMB/22DMB and nHEX/22DMB on JNU-2 were calculated to be 35 and 75, respectively, which are substantially higher than those on the best-performing MOFs reported so far, such as MFI (Mobil Five, 1.0 and 1.3),42 UU-200 (3.7 and 16.7),21 Fe2(BPD)3 (1 and 1.39),20 Al-bttotb (7 and 29),34 ZU-62 (1.4 and 15.5),43 SIFSIX-2-Cu-i (1.8 and 14.1),43 ZU-72 (1.7 and 14.1),43 and SIFSIX-1-Cu (20.2 and 24.1)43 (Figure 2e). It is noteworthy to point out that the adsorption on JNU-2 exhibits a sharp size cut-off between 22DMB and 23DMB. This is one of the few examples of MOFs that exhibit the potential for separating mixtures of 22DMB and 23DMB.43,44 Figure 2 | (a) Schematic representation of the channel-pore interconnected 3D structure of JNU-2. (b) A close-up view and dimension of the opening at the channel-pore junction. (c) Vapor-phase equilibrium adsorption isotherms of JNU-2 for nHEX, 2MP, 3MP, 23DMB, and 22DMB at 353 K and up to 16 kPa. (d) Comparison of nHEX adsorption capacity on JNU-2 and other high-performance MOFs (see Supporting Information Table S2). (e) Comparison of 23DMB/22DMB and nHEX/22DMB uptake ratios on JNU-2 and other high-performance MOFs (see Supporting Information Table S2). (f) In-situ gravimetric analysis of adsorption kinetics of nHEX, 2MP, 3MP, and 23DMB on JNU-2 at 353 K and 10 kPa. Download figure Download PowerPoint Adsorption kinetics is of great importance in practical applications as it dictates the speed and efficiency of the adsorption process.45 We thus measured the adsorption kinetics of nHEX, 2MP, 3MP, and 23DMB on JNU-2 at 10 kPa and 353 K by in-situ gravimetric analysis on a BSD-VVS/DVS vapor sorption analyzer. As shown in Figure 2f, the adsorption saturations for nHEX, 2MP, 3MP, and 23DMB were reached at 20, 70, 80, and 360 min, with adsorption capacities of 245, 214, 198, and 73 mg g−1, respectively. The kinetic adsorption data are consistent with their equilibrium adsorption data, and the times for hexane isomers to reach adsorption saturation increase with their increasing molecular sizes. Such a correlation supports our assumption that the narrow opening at the channel-pore junction may dictate the adsorption of hexane isomers, resulting in the complete exclusion of 22DMB. To further quantify the adsorption kinetics, the kinetic adsorption curves in Figure 2f were fitted according to the micropore diffusion model,46,47 and the diffusion time constants of nHEX, 2MP, 3MP, and 23DMB in JNU-2 were calculated to be 4.89, 6.6 × 10–3, 5.9 × 10–3, 3.9 × 10–4 min−1 (see Supporting Information Figures S8–S12), respectively. The substantially slow diffusion of 23DMB in JNU-2 points to its potential of separating dibranched hexanes from linear and monobranched isomers through a kinetic separation process. To locate the adsorption sites of hexane isomers inside JNU-2, we collected PXRD patterns of JNU-2 on a Panalytical Empyrean Powder X-ray diffractometer (Malvern, Panalytical, Almelo, Overijssel, Netherlands), and the structures of JNU-2 loaded with hexane isomers were refined using the Rietveld method. As shown in Supporting Information Figure S13 and Table S3, the refined structures show that all four hexane isomers are predominantly located near the apertures interconnecting cage A and cage B ( Supporting Information Figure S14), interacting with oxygen atoms of the apertures. By implementing density functional theory calculations, the interaction energies were estimated to be −53.6, −53.1, −40.6, and −43.1 kJ mol−1 for nHEX, 2MP, 3MP, and 23DMB, respectively ( Supporting Information Figures S15–S23). To verify the feasibility of JNU-2 in the kinetic separation of dibranched hexanes from linear and monobranched isomers, we carried out breakthrough experiments at a relatively high flow rate on JNU-2 and, as a comparison, zeolite 5A ( Supporting Information Figure S24). A five-component equimolar nHEX/2MP/3MP/22DMB/23DMB mixture was introduced into the column filled with zeolite 5A or JNU-2 at a flow rate of 10 mL min−1 and 353 K ( Supporting Information Figure S32). By monitoring the composition of the effluent, the obtained breakthrough curves reveal that zeolite 5A can indeed exclusively adsorb nHEX, but cannot discriminate between the mono- and di-branched isomers (Figure 3a), which limits its potential for further upgrading the RON values of hexanes. For JNU-2, on the other hand, a clean and sharp separation of dibranched hexanes from other isomers was observed. 22DMB breaks through the column first, resulting in the ratio of 22DMB reaching 100% in the outflow gas stream. Upon the breakthrough of 23DMB at 17 min g−1, the ratio of 22DMB drops to around 50%. Subsequently, a slight increase in the ratio of 22DMB was noted, likely due to the release of 22DMB adsorbed in the dead volume (the space between adsorbent particles). 3MP, 2MP, and nHEX break through the column at 52, 67, and 148 min g−1, respectively (Figure 3b). Based on the breakthrough curves, the real-time RON value was calculated to be over 98 prior to the outflow of 3MP (52 min g−1). Additionally, breakthrough experiments were carried out for a four-component equimolar 2MP/3MP/22DMB/23DMB mixture on JNU-2 at a flow rate of 10 mL min−1 and 353 K, demonstrating its potential of achieving RON value over 98 before the outflow of 3MP at 70 min g−1 (Figure 3c). Figure 3 | (a) Breakthrough curves for an equimolar nHEX/2MP/3MP/23DMB/22DMB mixture on zeolite 5A at a flow rate of 10 mL min−1 and 353 K, the orange line, corresponding to the right y-axis, indicates the real-time RON value of the effluent. (b) Breakthrough curves for an equimolar nHEX/2MP/3MP/23DMB/22DMB mixture on JNU-2 at a flow rate of 10 mL min−1 and 353 K, the orange line, corresponding to the right y-axis, indicates the real-time RON value of the effluent. (c) Breakthrough curves for an equimolar 2MP/3MP/23DMB/22DMB mixture on JNU-2 at a flow rate of 10 mL min−1 and 353 K, the orange line, corresponding to the right y-axis, indicates the real-time RON value of the effluent. (d) Breakthrough curves for an equimolar 23DMB/22DMB mixture on JNU-2 at a flow rate of 3.0 mL min−1 and 353 K. (e) Desorption curves following the breakthrough experiment with helium-gas sweeping (20 mL min−1) at 423 K. Grey area, 23DMB output; yellow area, 22DMB output. (f) Five consecutive breakthrough cycles for an equimolar 23DMB/22DMB on JNU-2 at a flow rate of 3.0 mL min−1 and 353 K. Download figure Download PowerPoint To further upgrade the RON values of hexanes, it is imperative to separate 23DMB from 22DMB and use 23DMB as the gasoline blending component rather than 22DMB/23DMB mixtures. Given that JNU-2 shows an unprecedented separation potential of sieving 23DMB from 22DMB, we decided to investigate the feasibility of JNU-2 in the separation of the two dibranched hexane isomers. By taking into consideration the slow diffusion of 23DMB in JNU-2, we performed breakthrough experiments for an equimolar 23DMB/22DMB mixture on JNU-2 at a relatively low flow rate (3.0 mL min−1) and 353 K. As shown in Figure 3d, 22DMB broke through the column almost immediately, indicating no adsorption in the column. 23DMB did not break through the column until ∼34 min g−1, presenting for the first time a clean separation of the two dibranched isomers. Upon reaching breakthrough equilibrium, the 23DMB captured in the column was purged by using a helium-gas sweep at a flow rate of 20 mL min−1 and 423 K, and the purity of the elution was determined to be 99.4% of 23DMB (Figure 3e), corresponding to an RON value of 105. In addition, five consecutive breakthrough experiments under similar conditions showed that the breakthrough times of each component remained almost the same, demonstrating the robustness and reproducibility of JNU-2 in the separation of 23DMB/22DMB mixtures (Figure 3f and see Supporting Information Figure S25). To further evaluate the long-term usability of JNU-2 for potential industrial applications, we carried out continuous in-situ gravimetric analyses of nHEX adsorption/desorption on a BSD-VVS/DVS vapor sorption analyzer. As shown in Figure 4a and Supporting Information Figures S29–S31, JNU-2 can maintain adsorption capacity for nHEX (64 cycles), 2MP (40 cycles), and 3MP (40 cycles) at 353 K. The excellent stability of JNU-2 in nHEX at elevated temperature was further confirmed by comparing the N2 adsorption isotherms and PXRD patterns of the samples before and after the 64-adsorption/desorption cycles (Figures 4b,c). To initially assess the desorption efficiency of JNU-2, we performed a comparative analysis of the desorption on JNU-2 and zeolite 5A via in-situ gravimetric analysis on a BSD-VVS/DVS vapor sorption analyzer at 353 K. As shown in Figure 4d and Supporting Information Figures S26–S28, zeolite 5A can only release about 45% of its adsorbed nHEX within 450 min, the desorption curve was fitted according to the micropore diffusion model, and the desorption time constant was calculated to be –2.3 × 10–4 min−1. On the other hand, although its nHEX adsorption capacity is almost twice that of zeolite 5A, JNU-2 can release about 90% of its adsorbed nHEX within 450 min, and the desorption time constant was calculated to be –5.5 × 10–4 min−1. The results suggest that JNU-2 may have some advantages over zeolite 5A in terms of desorption energy consumption in the separation of hexane isomers. Figure 4 | (a) nHEX adsorption capacity in 64 continuous adsorption/desorption measurements on JNU-2 at 353 K and 10 kPa by in-situ gravimetric analysis. (b) N2 adsorption isotherms of JNU-2 at 77 K: as-synthesized sample (black), and the sample after 64 adsorption/desorption cycles of nHEX (blue). (c) PXRD patterns of JNU-2: as-synthesized sample (black), and the sample after 64 adsorption/desorption cycles of nHEX (blue). (d) Comparison of desorption kinetics of nHEX on JNU-2 and zeolite 5A at 353 K by in-situ gravimetric analysis. Download figure Download PowerPoint Conclusion We report here a channel-pore interconnected MOF (JNU-2) for its stepwise splitting of hexanes by leveraging adsorption kinetics and thermodynamics. Equilibrium adsorption isotherms of JNU-2 show complete exclusion of 22DMB due to the limited opening at the channel-pore junction, while exceptionally high adsorption for nHEX, 2MP, 3MP, and 23DMB that can be attributed to the presence of large pores. Further adsorption kinetics studies reveal that the diffusion coefficient of 23DMB in JNU-2 is significantly lower than those of nHEX, 2MP, and 3MP. Such a difference in diffusion coefficients allowed us to demonstrate a stepwise splitting of hexanes through column breakthrough experiments; specifically, a kinetically controlled separation of dibranched hexanes from their linear and monobranched isomers, followed by a thermodynamically controlled sieving of 22DMB from 23DMB. Overall, this work presents a robust MOF material for its versatility in the separation of hexanes by reconciling adsorption kinetics and thermodynamics. Notably, our study represents the first reported material with the ability to separate 22DMB from 23DMB. Supporting Information Supporting Information is available and includes computational details, and supplementary tables and figures. Conflict of Interest There is no conflict of interest to report. Funding Information This work was financially supported by the National Natural Science Foundation of China (grant nos. 21731002, 21975104, 22150004, 22301102, and 22271120), Guangdong Basic and Applied Basic Research Foundation (grant nos. 2023A1515010952 and 2024A1515012434), and National Postdoctoral Program for Innovative Talents (grant no. BX20220132). References 1. Wang H.; Li J.Microporous Metal−Organic Frameworks for Adsorptive Separation of C5−C6 Alkane Isomers.Acc. Chem. Res.2019, 52, 1968–1978. Google Scholar 2. Zhang Z.; Peh S. B.; Kang C.; Chai K.; Zhao D.Metal-Organic Frameworks for C6–C8 Hydrocarbon Separations.EnergyChem2021, 3, 100057. Google Scholar 3. 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