Effects of Open Metal Site Availability on Adsorption Capacity and

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Effects of Open Metal Site Availability on Adsorption Capacity and Olefin/Paraffin Selectivity in the Metal-Organic Framework Cu3(BTC)2 Jason Bentley, Guo Shiou Foo, Meha Rungta, Neeraj Sangar, Carsten Sievers, David S. Sholl, and Sankar Nair Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00774 • Publication Date (Web): 13 Apr 2016 Downloaded from http://pubs.acs.org on April 18, 2016

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Industrial & Engineering Chemistry Research

Effects of Open Metal Site Availability on Adsorption Capacity and Olefin/Paraffin Selectivity in the Metal-Organic Framework Cu3(BTC)2 Jason Bentleya, Guo Shiou Fooa, Meha Rungtab, Neeraj Sangarb, Carsten Sieversa, David S. Sholla, Sankar Naira,* a

School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta GA, 30332-0100 b

Global Chemical Research, ExxonMobil Chemical Company, Baytown TX, 77520 *E-mail: [email protected]

Abstract We report a detailed investigation of the factors influencing olefin/paraffin separation in the metal-organic framework (MOF) adsorbent Cu3(BTC)2. The effects of synthesis and activation parameters on the availability of open Cu metal sites in the MOF were studied via measurements of crystallinity, textural properties, structural defects, water desorption, as well as binary 1-hexene/hexane vapor breakthrough chracteristics. A reduced crystallization temperature and improved solvent exchange with methanol were found to lead to CuBTC adsorbents with higher olefin selectivity and capacity. In situ FTIR and mass spectrometry measurements were used to quantify the structural defects and availability of open metal sites in the MOF as a function of the activation conditions. Based upon these measurements, we identify the relationships between the defects, activation conditions, open metal site availability, and consequent separation performance. Recommendations are made for appropriate activation and desorption conditions that improve olefin selectivity while maintaining total capacity. This investigation led to the identification of a functional CuBTC adsorbent with a selectivity of ~5 for a 10%/90% 1-hexene/hexane liquid feed mixture at 50°C, and also suggests a systematic methodology to determine operating conditions for MOF adsorbents with open metal sites. 1 ACS Paragon Plus Environment


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Introduction Olefin/paraffin separations are important yet expensive processes in the petrochemical sector, and are currently performed using distillation to produce polymer-grade olefins1. Metalorganic frameworks (MOFs) are a promising class of materials that may be used in olefinselective adsorption processes to augment or replace distillation2, 3. In particular, MOFs with coordinatively unsaturated sites are of interest because of their strong π-electron interactions with olefins4-6. Such binding sites (often called ‘open metal sites’) result from the incompletion of the coordination sphere of the metal atoms in the MOF by the electron-donating atoms of the linker molecules due to the geometrical constraints of the MOF crystal structure7. Our investigation focuses on Copper (II) benzene-1,3,5-tricarboxylate (CuBTC), also known as HKUST-1, a wellknown model of a MOF with open metal sites. Its synthesis was first reported by Chui et al. (1999)8. Several researchers have studied CuBTC for applications in catalysis, gas storage, and adsorption-based separations. Wang et al. (2002)9 were the first to report gas adsorption isotherms on CuBTC and study the thermodynamics of common adsorbates. Schlichte et al. (2004)10 improved upon the solvothermal synthesis method for CuBTC and studied the Lewis acidity of its open Cu (II) sites for catalysis of cyanosilylation of aldehydes and ketones. Yoon et al. (2010) 11 evaluated propane and propylene adsorption in CuBTC in more detail. They showed that the isosteric heats of adsorption for propane and propylene on CuBTC are significantly different (about -35 kJ/mol and -49 kJ/mol, respectively, at a loading of 1 mmol/g for the pure gases). The concentration of copper atoms in the framework is 4.96 mmol/g based on the unit cell. Therefore, at 5 mmol/g of adsorbate loading, the heat of adsorption for propylene was found to decrease to -43 kJ/mol because all the available open metal sites are occupied at this concentration. They reported an equilibrium selectivity of 3.3 for propylene/propane based upon

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breakthrough adsorption data at 313 K and 2.5 kPa partial pressure of each component. This separation was also studied by Plaza et al. (2012)12 who evaluated small tablets of CuBTC, and it was concluded that the material may be feasible for use in a simulated moving bed process. Small-molecule adsorption on CuBTC has also been studied more recently using molecular modeling to understand the mechanism of adsorption for light paraffins and olefins13.

There is much less information available regarding the use of CuBTC for separation of heavier olefins such as hexenes from hexane. These are also important separations of interest in the recovery of useful feedstock molecules from naphtha streams4. In related work, Maes et al. (2010)14 studied competitive adsorption of pentene and pentane on CuBTC using liquid phase batch experiments, and observed selectivity between 2.8-3.5. Hartmann et al. (2008)15 studied competitive adsorption of isobutene and isobutane using vapor breakthrough experiments, and observed a selectivity of about 2.1. Münch and Martens et al. (2012)16 prepared a GC capillary column with a coating of CuBTC to separate hydrocarbons and electron-donating components. It is known that the properties of CuBTC can be strongly affected by its synthesis and activation parameters. For example, Wang et al. (2002)9 investigated the difference between static and agitated solvothermal synthesis of CuBTC. They found that the agitated synthesis produced needle-like particles instead of the octahedral cubic particles obtained from the solvothermal method, although the powder XRD crystal structure was shown to be the same for both products. Chowdhury et al. (2009)17 compared the adsorption isotherms of several gases on two CuBTC materials synthesized different temperatures to reveal differences in adsorption capacities. The sample synthesized at lower temperature showed increased surface area and pore volume, and also had increased adsorption capacity for several gases. Hartmann et al. (2008)15 compared



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textural properties and adsorption of isobutene and isobutane on CuBTC materials synthesized under different conditions. They found that the best synthesis method involved crystallization at ambient pressure in refluxing ethanol, but only measured a pore volume of 0.62 cm3/g by nitrogen physisorption. An improved solvent exchange during sample activation was demonstrated by Liu et al. (2007)18, resulting in increased surface areas for CuBTC samples. This report was recently cited by others as demonstrating stable CuBTC samples with the highest surface areas and pore volumes as measured by nitrogen physisorption13. Alaerts et al. (2006)19 studied various washing and drying treatments of CuBTC and their effects on catalytic activity for isomerization of α-pinene oxide. In this report, they did not include a detailed analysis of the difference in activation procedures. Other coordinatively unsaturated MOFs have been studied in some detail to observe the effect of activation conditions on the availability and oxidation state of open metal sites.20

Despite this previous work, there is still not a clear understanding of how the synthesis and activation parameters affect the open metal site availability of MOFs such as CuBTC and consequently its performance in olefin separation. It is well known that the open metal sites of CuBTC are made available for adsorption upon removal of water or any other guest molecule in the framework, but the number, availability, and behavior of these open metal sites as a function of two key aspects, viz. the synthesis and activation conditions, have not been quantified. Here we investigate the availability of open metal sites in CuBTC and the effects on the adsorption and selectivity of olefins and paraffins, focusing specifically on the separation of heavier olefins (particularly 1-hexene from hexane). To our knowledge, this is the first detailed experimental study of hexane and 1-hexene competitive adsorption on CuBTC in the context of its structural


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and activation characteristics. Two different preparation methods are used to produce CuBTC crystals, and these two types of CuBTC crystals are characterized to understand the effect of structural properties of CuBTC on the olefin/paraffin separation performance. The material properties such as crystallinity, particle morphology, and surface area, and pore volume are determined by powder XRD, SEM, and N2 physisorption, respectively. Several thermal activation conditions are used to treat the CuBTC samples, and the effect of activation conditions on the material properties such as crystallinity and surface area and pore volume are studied. The removal of guest molecules is observed in more detail by in situ FTIR and in situ mass spectrometry. The effect of activation conditions and feed compositions on the separation performance is then studied in detail by vapor-phase binary breakthrough experiments. Based upon this information, we analyze the effects of both synthesis and activation parameters on the availability of open metal sites in CuBTC, thereby suggesting a methodology for determining suitable operating conditions for open metal site MOF adsorbents.

Experimental Section Synthesis CuBTC materials were synthesized using the following procedures. One procedure (S1) follows Schlichte et al. (2004)10, and the other procedure (S2) follows He et al. (2012)21. For S1, copper (II) nitrate trihydrate (0.875 g, 3.6 mmol, Sigma-Aldrich) was dissolved in water (12 mL) in a scintillation vial (20 mL capacity), whereas 1,3,5-benzenetricarboxylic acid (0.420 g, 2.0 mmol, Sigma-Aldrich) was dissolved in ethanol (12 mL) in a scintillation vial and sonicated until homogeneous. The contents of these vials were transferred to a Teflon-lined autoclave (40 mL capacity), and sealed tightly. The autoclave was placed in an oven set at 120 °C for 12 hr, and



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then cooled to room temperature (RT). The contents of the autoclave were transferred to a centrifuge tube using ethanol/water (v/v, 1:1) to rinse the Teflon liner. After centrifugation, the liquid phase was removed and replaced with ethanol/water twice more to wash the sample. Then, the sample was washed once with deionized water. After removing the water, the sample was dried at 70 °C for 2 hr in a drying oven, and then it was evacuated at RT for 24 hr in a vacuum oven at 10-2 psi. For S2, copper (II) nitrate trihydrate (1.240 g, 5.33 mmol, Sigma-Aldrich) and 1,3,5-benzenetricarboxylic acid (0.740 g, 3.52 mmol, Sigma-Aldrich) were mixed with N,Ndimethylformamide/ethanol/water (v/v/v, 1:1:1, 180 mL) in a 250 mL screw cap jar and sonicated until homogeneous. Then, 18 mL of the reaction solution was dispensed into each of ten scintillation vials (20 mL capacity) and sealed tightly. The vials were placed in an oven set at 75 °C for 24 hr and then cooled to RT. The contents of each vial were transferred to centrifuge tubes, using dry acetone to rinse the vials. After centrifugation, the liquid phase was removed and replaced by dry acetone for solvent exchange. The dry acetone was replaced three times over a period of two days. After removing the acetone, the sample was dried at 70 °C for 1 hr in a drying oven, and then it was evacuated at RT for 24 hr in a vacuum oven.

Characterization Methods Powder X-ray diffraction (XRD) measurements were performed using a PAnalytical XPert Pro instrument operating with CuKα radiation. The as-made samples were measured in air at RT for the initial XRD measurements. A TTK-450 sample chamber (Anton Paar) was used for temperature-dependent XRD. In these measurements, scans were performed at various temperature set points in an inert (nitrogen) environment. Scanning electron microscopy (SEM) images were taken using a Zeiss LEO 1553 SEM instrument. Prior to testing, 1 mg of as-made


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sample was placed on a flat sample stage with carbon adhesive, and coated with gold for 60 sec using a spin coater. Nitrogen physisorption measurements were performed at 77 K on a Tristar II 3020 instrument (Micromeritics). Prior to testing, the as-made samples were thermally activated under dynamic vacuum with a turbomolecular pump using various temperature-time programs. In situ infrared (FTIR) spectra were collected on a Nicolet 8700 FTIR spectrometer with a MCT/A detector. For each spectrum, 64 scans were recorded at a resolution of 4 cm-1. A small amount of MOF sample (2 mg) was spread and pressed on top of a KBr disk. The sample was loaded into an FTIR transmission vacuum chamber and spectra were collected before and after the evacuation of the chamber. Subsequently, spectra were recorded following a temperaturetime program.

Vapor Breakthrough Measurements The vapor breakthrough unit used in this work is shown schematically in Figure 1. The unit was designed and assembled in-house. A gas washer (Ace Glass, MA) was used to provide vapors of a liquid mixture together with a He/N2 mixture as a carrier gas. Helium acts as a tracer, since it was negligibly adsorbed on CuBTC at the experimental conditions. A second N2 stream was used to control the feed concentration by dilution. Both gas streams were controlled by mass flow controllers (MFCs). The composition of the gas outlet was measured online using an OmniStar mass spectrometer (MS, Pfeiffer Vacuum). The MS was calibrated using individual gases and the feed mixture to obtain reliable online measurements of mole fractions for each component in the outlet vapor. Several thermocouples and temperature controllers were used to maintain experimental conditions in the column oven, around the sample bed, and in the capillary tubing leading to the MS. Electrical heating tape was used to control the temperature of



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the sample bed. The sample bed was a quartz tube, 30 cm long and 1 cm outer diameter, with a frit in the middle to hold the sample. For each experiment, between 100 to 200 mg of CuBTC was packed together with sand to disperse the sample in a bed of 2 cm height. For comparative breakthrough experiments, the difference in sample masses was less than 1%. Before each binary breakthrough measurement, the sample was thermally activated in situ under N2 flow at specific temperatures and holding times. To generate the vapor feed mixture to the column, a 10% He90% N2 mixture was used as a carrier gas and was bubbled into the feed liquid mixture. The saturated vapor/carrier gas mixture emerging from the bubbler was fed into the sample bed. The volumetric flow rate of the feed vapor was measured to be 10.2 cm3/min, and the flow rate of the purge gas was 10.1 cm3/min. The purge gas was always pure N2.

Four types of breakthrough measurements were performed using the vapor breakthrough unit: in situ activation, binary vapor breakthrough, initial desorption, and thermal desorption. Each measurement and the corresponding analysis method is explained below. For in situ activation, N2 was passed through the sample bed at 10 cm3/min (STP) while maintaining the sample bed at a constant temperature, and the exit N2 was monitored by MS over time. To calculate the total mass of component i removed from the sample during activation, mi,act, we used:

mi ,act =

FT Ai ,act (t )PT MWi RTave

(1)

where FT is the total volumetric flow rate of the carrier gas, Ai,act(t) is the integrated area under the mole fraction signal for component i in the MS over time, PT is the total pressure, MWi is the molecular weight of component i, R is the gas constant, Tave is the average between RT and the activation temperature of the sample bed. The area, Ai,act(t), was calculated using the trapezoidal 8 ACS Paragon Plus Environment

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rule. The average temperature was used because the vapors were supplied to the sample bed at RT and were heated during the experiment to the activation temperature at the gas outlet. This in situ activation method can be used repeatedly to quantify the number of open metal sites that are made available for adsorption on the CuBTC sample as a function of activation temperature. In the case of CuBTC, the only significant component removed during activation is water, since the sample is strongly hydrophilic. The steps for this analysis are: (1) Measure the hydrated sample mass, (2) Pack the sample bed and activate at a low temperature, such as 50 °C, (3) Calculate the mass of water desorbed from the sample using Equation 1, (4) Increase the activation temperature by about 30 °C and again calculate the mass of water desorbed, (5) Repeat Step 4 until the mass of water desorbed is less than a minimum threshold, (6) Subtract the total mass of water desorbed from the initial hydrated sample mass—this gives the final dry mass of the sample, (7) Using the dry sample mass, calculate the theoretical number of Cu atoms in the framework using the CuBTC unit cell, (8) Assuming that the Cu atoms form open metal sites facing the pore interior, and that each of these has one water molecule coordinated to it in the hydrated form, calculate the total mass of water that needs to be removed in order to have 100% availability of open metal sites 8, 10, and (9) Back-calculate the fraction of open metal sites made available at each activation temperature, starting from the last and highest temperature and work backwards subtracting the cumulative amount from 100% at each step.

For binary vapor breakthrough, the sample bed temperature was fixed and the feed stream was sent to the column in addition to the N2 stream. The MS was calibrated to measure the mole fraction of each component in the gas streams. Helium was used as a non-adsorbing tracer in the



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feed stream. To calculate the mass of component i adsorbed, mi,ads, the following component mass balance was used: mi ,ads = ∫

t =t f

t =t 0

(m&

i , IN

− m& i ,OUT (t ))dt

(2)

where t0 is the initial time, tf is the final time for the breakthrough experiment. The mass of component i sent into the column per time, and the mass of component i coming out of the column per time are defined by Equations 3 and 4, respectively.

m& i ,IN

FT PT yifeed MWi = RTave

m& i ,OUT (t ) =

FT PT MWi yi (t ) RTave

(3)

(4)

where yifeed is the equilibrium mole fraction of component i in the feed stream, yi(t) is the mole fraction of component i in the gas stream analyzed by the MS, and all other terms were defined in Equation 1. The value of t0 in Equation 2 is the breakthrough time for the He tracer and is determined by the inflection point of the He signal, which is located approximately halfway between the minimum and maximum He concentrations. The value of tf was selected to be about 2tb,j, where tb,j is the breakthrough time of component j, which is the most-retained component of the mixture. It is important that tf is long enough so that y i (t ) ≈ y ifeed , ∀t ≥ t f , and the solution to Equation 2 is approximately independent of tf. Using Equation 2, the mass adsorbed for each component was calculated, and the total gravimetric capacity was calculated by summing the adsorbed amounts and dividing by the dry mass of the sample. The total volumetric capacity was calculated by multiplying the gravimetric capacity by the framework density of the sample. These calculations are typical of those seen in the gas adsorption and liquid chromatography literature 22, 23. The equilibrium selectivity was calculated by:


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S BA =

(y

(q B feed B

qA ) y Afeed

)

(5)

where qi is the adsorbed concentration of component i on a molar basis. The selectivity depends on the feed composition.

For initial desorption measurements, the carrier gas was turned off, the temperature was kept constant, the nitrogen purge stream remained on, and after the helium tracer signal reduced by 95%, Equation 1 was used to calculate the masses of paraffin, olefin, and water desorbed from the sample. In this type of measurement, the components were desorbed by convection and mass transfer to approach a new equilibrium. For thermal desorption measurements, the temperature was raised to some increased set point and Equation 1 was used again to calculate the mass removed from the sample. For this step, the components are desorbed by convection and an increased mass transfer rate, because the adsorption capacity is dramatically reduced at increased temperatures. The mass desorbed for each component was calculated by summing the masses for both the initial and the thermal desorption steps. The total gravimetric capacity was calculated by summing the desorbed amounts and dividing by the dry mass of the sample. Similarly, desorption selectivity was calculated using Equation 5 except substituting qi using the desorbed concentration of component i on a molar basis.

Results and Discussion Crystallinity and textural properties

Figure 2a shows powder XRD patterns obtained from CuBTC materials S1 and S2. Although the synthesis procedures use different solvents, reaction vessels, reaction temperatures and workup methods, the formation of CuBTC crystals is apparent for both S1 and S2, and the 11 ACS Paragon Plus Environment


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XRD patterns match the literature reports21. The diffraction peaks of S1 are noticeably broader than those of S2, which may indicate the presence of defects related to incomplete coordination of the BTC linkers in the crystal structure as discussed later. Figures 2b and 2c show SEM images of these samples. Both samples contain octahedrally shaped CuBTC crystals. Based on inspection of several SEM images, S1 shows a larger mean crystal size of ~20 µm with a standard deviation of ~5 µm, whereas S2 has a mean crystal size of ~5 µm but a relatively large standard deviation of ~5 µm (some larger crystals skew the distribution). The larger particles obtained in S1 are mainly due to the synthesis temperature, which is 45°C higher than that of S2. The surface area and pore volume of each sample was estimated from N2 isotherms (Figure 2d), and the experimental values, along with theoretically calculated values based on the ideal crystal structure of CuBTC, are shown in Table 1. Sample S2 has 29% greater surface area than S1, confirming that S2 is an improved sample preparation. The workup for S2 includes a more extensive solvent exchange than S1, using several rinses of acetone over a few days to remove any residual DMF from the pores. However, there are several variables in the synthesis and workup procedures that differ for S1 and S2, of which the solvent exchange step was found to have important effects in this work.

The CuBTC material synthesized using S1 was studied using temperature-dependent XRD under an N2 atmosphere. The sample was heated from room temperature to 350°C, and the temperature was held for 4 hr at each step for equilibration before collecting XRD data. It should be noted that the temperature program used in this experiment reflects the in situ thermal activation procedure used during the breakthrough adsorption experiments. Figure 3 shows a disappearance of the peak at 2θ = 6.5° after heating S1 from RT to 100°C. Prior to thermal


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activation, water is adsorbed on the Cu metal sites in the pores of the sample, and when much of this water is removed during the first activation step, the peak intensity at 2θ = 6.5° reduced significantly. The overall crystallinity of the material remains stable up to 300°C. At 350°C, crystallinity is lost due to degradation of the framework, and this process is irreversible as confirmed from an XRD pattern taken after cooling back down to 100°C. Several CuBTC S1 samples were examined by N2 physisorption after activating at different temperatures, as shown in Figure 4. Each sample was activated under dynamic vacuum for 12 hr at the specified temperature. The surface area and pore volume increased dramatically after activation at 110°C, due to removal of water. The surface area and pore volume continue to increase slightly up to an activation temperature of 230°C, after which the surface area and pore volume both collapse. Based upon the observations in Figures 3 and 4, it is clear that CuBTC should be activated in the temperature range of 110-230°C, wherein the crystallinity and textural properties of the material remain stable. This information is used in subsequent measurements described in this paper.

Breakthrough measurements to initially assess the separation performance were carried out on CuBTC samples S1 and S2 at 50°C column temperature (Figure 5), and the corresponding capacity and olefin selectivity are shown in Table 2. In these measurements, the sample was thermally activated in situ under N2 flow at 160°C for 12 hr. The composition of the liquid in the bubbler was 10% 1-hexene/90% hexane by volume, from which a vapor feed containing 12% 1hexene/88% hexane (on a carrier-free basis) was obtained at ambient temperature of 20±1°C. Table 2 shows large differences in adsorption behavior between the two materials, most notably a large decrease in the adsorption capacity in S1, with the total adsorption capacity being 44-55% greater for S2. The two materials were activated in the same way, and hence the difference in



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capacity clearly originates from the differences in synthesis and work-up procedures. The equilibrium olefin selectivity (average of adsorption and desorption measurements) of S1 is only about 10% lower than that of S2. Hence, the adsorption capacity of S2 is significantly greater than S1, but the olefin selectivity is nearly the same (about 4.5) for these samples. The results of Table 2, when taken together with the textural properties shown in Table 1, lead to the questions of why the improved S2 procedure allows a much greater pore volume of the CuBTC material to be activated, and how this affects the number of open metal sites available for olefin adsorption. These questions are investigated in detail below.

Structural changes upon activation

To further study the evolution of the CuBTC structure upon heat treatment, we conducted in situ FTIR measurements on both S1 and S2 materials. For temperatures up to 100°C, the

holding time at a specific temperature was 1 hr before a spectrum was collected at that temperature. Between 150 and 300°C, the temperature was held for 12 hr at each step in order to replicate the activation procedure as closely as possible. Figure 6 shows the FTIR spectra obtained from S1 and S2 at different activation temperatures. For both samples, a band is observed at 1650 cm-1 which is due to the νasym vibration of the COO- groups25-28. Bands at 1453 and 1378 cm-1 are assigned as νsym COO- vibrations. Other peaks are assigned as δC-H vibrations (1110 and 939 cm-1) and νC-C vibrations (757 and 730 cm-1) of the aromatic ring of the BTC linkers. For sample S1 (Figure 6a), bands at 1708 and 1234 cm-1 were also observed. These bands respectively correspond to νC=O and νC-OH vibrations of free protonated carboxylic acid groups25-28. The presence of these bands signifies that there are significant numbers of free carboxylic acid (-COOH) groups in the framework of S1, most likely in the form of BTC linker


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species occluded in the pores due to incomplete coordination with the Cu centers. As the temperature increased from 25 to 280°C, the intensity of all the bands decreased only slightly, except for the intensity of the peaks at 1708 and 1234 cm-1 (free carboxylic acid) which decreased greatly (e.g., see zoomed plot of the 1708 cm-1 band on the right-hand side of Figure 6a). For S2 (Figure 6b), the bands corresponding to free protonated carboxylic acids are much less intense than in S1.This result signifies that there are very few free carboxyl groups in material S2, further corroborating its more defect-free structure. For both materials, the IR spectra do not show significant removal of organic linkers from the framework upon increase in temperature, since the intensities of the framework BTC peaks (1110, 939, 757, and 730 cm-1) remained nearly constant. This means that the CuBTC structure is generally intact over this range of activation temperatures. However, the intensities of the free carboxylic acid bands (1708 and 1234 cm-1) decrease strongly with heat treatment in both materials, signifying the near-complete removal of free BTC linkers from the pores upon activation at higher temperatures. The additional peak at 1678 cm-1 in the S2 material (see zoomed plot on the righthand side of Figure 6b) arises from the νC=O vibration of residual acetone or DMF which are present in the pores due to their use in the synthesis of S2 (but not in S1). This peak was also observed by Schlichte et al. upon chemisorption of benzaldehyde to CuBTC10. As the temperature increased from 25 to 300°C, the intensity of this peak also decreased greatly, signifying the near-complete removal of acetone/DMF from the structure of S2 upon activation.

Concurrently, Figures 7a-7b show desorption of water from CuBTC as obtained from the decreasing intensities of the broad νOH vibration region (3680 to 2690 cm-1) with increasing temperature. Water desorption is expressed quantitatively by integrating the peak intensity of this



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region from the difference spectra. Figure 7c shows the normalized integrated area of the vOH band, where the area calculated at each temperature was normalized by the area calculated at 25°C (which was the temperature at the start of the thermal activation process). Material S1 displays a gradual decrease in water content as a function of activation temperature, and still shows significant peak intensity even at 250°C. On the other hand, S2 shows a rapid decrease in water content and appears to be essentially dehydrated after 150°C. It is also seen from Figures 7a-7b that S1 has higher water content than S2. All the above observations are consistent with the presence of defects in S1 caused by incomplete coordination of Cu centers with BTC linkers. These likely involve strong coordination/chemisorption of water species with the defective Cu centers, which leads to the incomplete desorption of water even at high temperature. On the other hand, S2 is relatively free of such defects and loses its more weakly bound water molecules more easily upon heating.

Availability of open metal sites

In conjunction with the results of the in situ FTIR measurements, the breakthrough unit was used along with the MS in the in situ activation mode to quantify the desorption of water and hence the fraction of available open metal sites. The hydrated CuBTC samples initially have two types of water, i.e. water coordinated to the open-metal sites (whether defect-free or defective) and water that is weakly adsorbed in the pores.10 Based upon Figure 7c, we reasonably assume that essentially all the open metal sites are dehydrated after the final activation step at 300°C, even though the crystallinity of the sample may be degraded at that point. It is also reliable to assume that all weakly adsorbed water is removed from the material well before it reaches 300°C, and hence the determination of the amount of open metal site-coordinated water at each


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intermediate temperature can be performed in reference to the highest temperature of 300°C. By measuring the amount of water desorbed (Equation 1) from a known mass of completely dehydrated CuBTC material (see Experimental section for detailed protocol), we can estimate the percentage of open metal sites that have been activated. In Figures 8a and 8b, the resulting percentages of open metal sites available for adsorption at each activation temperature (as estimated from the breakthrough MS data) are shown in comparison to the normalized amount of water removed (as estimated earlier by FTIR). In the case of S1, about 69% of the open metal sites are made available after treating the material at 70°C for 12 hr. As the activation temperature increases, the availability of open metal sites increases slowly. Based upon N2 physisorption experiments, it was shown earlier in this work that the material should not be activated beyond 230°C. Figure 8a shows that about 93% of the open metal sites can be made available for adsorption by thermal activation at this temperature before the framework begins to collapse. This data independently corroborates the in situ FTIR results for S1, in particular the linearly decreasing amount of water at each activation step that closely matches the linearly increasing fraction of open metal sites. This linear trend is clearly due to the presence of defects in the material owing to incomplete coordination of BTC linkers to the metal centers, thus leading to stronger chemisorption of water on the defective open metal sites. In the case of S2, about 78% of the open metal sites are made available after treating the sample at 50°C for 12 hr (Figure 8b). The open metal site availability increases quickly to 98% at 200°C, signifying nearcomplete activation of S2 well before the framework begins to degrade around 230°C. This data also agrees well with the in situ FTIR measurements which show a near-complete removal of water at 200°C.



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Olefin/paraffin separation performance

Since the foregoing investigations have established that material S2 is superior to material S1 in structural, textural, and activation properties, S2 was used to investigate the 1hexene/hexane separation performance in more detail as a function of activation temperature via vapor breakthrough measurements. The composition of the liquid in the feed bubbler was 40% 1hexene/60% hexane by volume, from which a vapor feed containing 45% 1-hexene/55% hexane (on a carrier-free basis) was obtained at ambient temperature of 20±1°C. The vapor feed composition remained constant during the breakthrough measurement, which typically was performed over a few hours. In the first series of breakthrough experiments (Case A), a fresh sample of material S2 was packed in a column, activated at a specific temperature, and its separation performance was then measured at 50°C. Five fresh S2 samples were measured in this manner using activation temperatures of 50, 100, 150, 200, and 250°C. After the breakthrough measurement, each sample was regenerated using 100, 150, 150, 200, or 250°C as the desorption temperature respectively. N2 purge gas was passed through the column during activation and desorption steps. Figure 9 shows example activation, breakthrough, and desorption traces obtained from the material activated (and desorbed) at 150°C. From Figure 9a, the water signal was integrated to determine the sample mass lost by desorption of water (Equation 1). For this sample, 8.8 mg (5.3 wt%) of water was removed during activation, corresponding to about 96% of the open metal sites being available (Figure 8b). In Figure 9b the He tracer eluted first, thereby allowing the calculation of the overall dead volume of the system, which was found to be 79.5 cm3. The hexane eluted next because of its weak affinity for CuBTC, and it was competitively desorbed by 1-hexene resulting in a “rollup” effect29. The 1-hexene eluted relatively slowly because of its strong adsorption on the open metal sites via π-electron


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interactions. In Figure 9c the sample was desorbed at 150°C and the amounts of adsorbed 1hexene and hexane were quantified by integrating the areas under each desorption curve. The results of all the Case A measurements are shown in Figures 10a and 10b. The adsorption capacity and olefin selectivity both increased as the activation temperature increased up to 150°C. Beyond this temperature, the capacity and selectivity both decreased, with a sharp decrease beyond 200°C. These results are broadly consistent with the dependence of the textural properties on the activation temperature (Figure 4). A quantitative discrepancy is that the pore volume and surface area measured by N2 physisorption only dropped upon activation at 250°C or higher (Figure 4) whereas the separation performance began to drop around an activation temperature of 150-200°C. Overall, it is clear that there is no advantage in activating the CuBTC material at the higher temperatures. It is recommended that the activation temperature be around 150°C for the best initial performance. Under these conditions the total (olefin+paraffin) capacity of the material at 50°C is around 0.45 g/g (0.40 g/cm3) with an olefin/paraffin equilibrium selectivity of ~2.6 (Figure 10b). Figure 10b also shows a general agreement (within 10% error) between the results obtained from the equilibrium (adsorption) and desorption measurements.

In a second series of breakthrough experiments (Case B), a single sample of CuBTC S2 was packed in a column and cycled through activation and desorption at successively higher temperatures, with breakthrough measurements being conducted at 50°C between each activation and desorption. In other words, the sample was first activated at 100°C, then a breakthrough experiment was performed at 50°C, then the sample was desorbed at 150°C, then another breakthrough experiment was performed at 50°C, and so on. This was continued until the sample degraded after desorption at 250°C. The results of all breakthrough measurements for Case B are



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shown in Figures 10c and 10d. The S2 material showed good thermal stability after cycling the breakthrough and desorption at progressively higher temperatures up to 200°C, with a good 1hexene/hexane selectivities (~3) and high volumetric adsorption capacity (~0.35 g/cm3) after activation at 200°C. A comparison of the olefin selectivity from experiments carried out with different liquid feed mixtures (40% 1-hexene from Figure 10 and 10% 1-hexene from Table 2) shows the interesting observation that the olefin/paraffin adsorption selectivity increases significantly (from 3 to 4.9) as the olefin concentration in the feed mixture was decreased from 40% to 10%. The reason for this effect is that the olefin has a much stronger affinity for the open metal sites than the paraffin. Therefore, the open metal sites are saturated with olefin molecules even at low concentrations of olefin in the feed, leading to an increase in the selectivity as calculated from Equation 5.

Conclusions We conducted a detailed study on the applicability of CuBTC as a selective adsorbent for the separation of 1-hexene from hexane. First, the effects of synthesis and activation parameters on the CuBTC structure (in particular the changes in crystallinity, pore surface area, pore volume, and open metal site availability) were determined. It is clearly shown that defects such as missing/disconnected framework linkers (which manifest as free BTC linker vibrational bands in FTIR spectra) have a significant effect on the open metal site availability. The competitive adsorption properties of hexane and 1-hexene on CuBTC were then measured and compared based on the different synthesis and activation parameters as well as different feed compositions. It is shown that the synthesis and solvent exchange method used by He et al. (2012)21 leads to a more useful CuBTC adsorbent, because of its relatively defect-free structure in which nearly all


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the crystallographically allowed connections between the Cu centers and the BTC linkers are in place. As a result, the activation procedures allow near-complete removal of chemisorbed water as opposed to the case of defective Cu centers that can bind water molecules more strongly. Our in situ activation study shows that CuBTC should be activated at a temperature between 150-

200°C for optimal separation performance, and that a desorption temperature between 150200°C is needed to efficiently regenerate the adsorbent. Finally, the olefin/paraffin separation has better performance at reduced olefin concentrations due to saturation of the open metal sites at low olefin feed concentrations. The equilibrium selectivity was found to be about 4.9 for a 10%/90% 1-hexene/hexane liquid feed mixture at a breakthrough temperature of 50°C.

Acknowledgments This work was supported by ExxonMobil Chemical Company.

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Eldridge, R. B., Olefin/paraffin separation technology: a review. Ind. Eng. Chem. Res.

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Hartmann, M., Ethene/ethane and propene/propane separation via the olefin and paraffin selective metal–organic framework adsorbents CPO-27 and ZIF-8. Langmuir 2013, 29, (27), 8592-8600. 7.

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Wang, Q. M.; Shen, D.; Bülow, M.; Lau, M. L.; Deng, S.; Fitch, F. R.; Lemcoff, N. O.;

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Schlichte, K.; Kratzke, T.; Kaskel, S., Improved synthesis, thermal stability and catalytic

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Yoon, J. W.; Jang, I. T.; Lee, K.-Y.; Hwang, Y. K.; Chang, J.-S., Adsorptive separation

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Plaza, M.; Ferreira, A.; Santos, J.; Ribeiro, A.; Müller, U.; Trukhan, N.; Loureiro, J.;

Rodrigues, A., Propane/propylene separation by adsorption using shaped copper trimesate MOF. Microporous and Mesoporous Mater. 2012, 157, 101-111.

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De Vos, D. E., Separation of C5-hydrocarbons on microporous materials: complementary performance of MOFs and zeolites. J. Am. Chem. Soc. 2010, 132, (7), 2284-2292. 15.

Hartmann, M.; Kunz, S.; Himsl, D.; Tangermann, O.; Ernst, S.; Wagener, A., Adsorptive

separation of isobutene and isobutane on Cu3(BTC)2. Langmuir 2008, 24, (16), 8634-8642. 16.

Münch, A. S.; Mertens, F. O., HKUST-1 as an open metal site gas chromatographic

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Vimont, A.; Daturi, M.; Bloch, E.; Llewellyn, P. L.; Serre, C.; Horcajada, P.; Grenèche, J.-M.; Rodrigues, A. E.; Férey, G., Controlled Reducibility of a Metal–Organic Framework with Coordinatively Unsaturated Sites for Preferential Gas Sorption. Angew. Chem., Int. Ed. 2010, 49, (34), 5949-5952. 21.

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Figure Captions

Figure 1. Schematic of vapor breakthrough unit.

Figure 2. (A) Powder XRD diffractograms, (B) N2 uptake curves at 77 K, (C) and (D) SEM images, for as-made CuBTC samples S1 and S2.

Figure 3. Powder XRD diffractograms for S1 during activation at various temperatures.

Figure 4. (A) Surface areas, and (B) pore volumes based on N2 physisorption at 77 K for several CuBTC S1 samples after activation at various temperatures.

Figure 5. Breakthrough curves for hexane and 1-hexene on CuBTC samples S1 and S2 at 50 °C.

Figure 6. FTIR spectra of (A) S1 and (B) S2 between 1800 cm-1 and 650 cm-1. Right-hand plots show zoomed areas around 1708 and 1678 cm-1.



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Figure 7. FTIR spectra of (A) S1 and (B) S2 between 3800 cm-1 and 2200 cm-1. (C) Normalized integrated areas of vOH band as a function of activation temperature from difference spectra for S1 and S2.

Figure 8. Normalized mass of water lost from sample and % of open metal sites available for (A) S1 and (B) S2 at each activation step.

Figure 9. (A) Activation at 150 °C, (B) breakthrough at 50 °C, and (C) desorption at 150 °C, on CuBTC S2 sample.

Figure 10. Gravimetric capacity and olefin selectivity for: (A), (B) Case A vapor breakthrough measurement on CuBTC S2 samples activated at different temperatures, and (C), (D): Case B vapor breakthrough measurement on a CuBTC S2 sample activated at successively increasing temperatures.


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Figures

Figure 1



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Page 28 of 39

Figure 2


Page 29 of 39

350 °C

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


300 °C 250 °C 200 °C 150 °C 100 °C RT 5

10

15

20

25

30

35

40

2 theta 2θ(degrees) (°) Figure 3



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Page 30 of 39

Figure 4


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Figure 5



(A) 1378 1650

730

1708

vC=O S1

1708

1453

Absorbance (a.u.)

757

25 °C 70 °C 100 °C 130 °C 160 °C 190 °C 220 °C 250 °C 280 °C

1110

1708

1234

1800

1600

1400

939

1200

1000

800

1730

Wavenumber (cm-1)

1720

1710

1700

1690

Wavenumber (cm-1)

vC=O S2

(B) 1378

1678

1678

1650

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 39

25 °C 50 °C 100 °C 150 °C 200 °C 250 °C 300 °C

730

1453 1678

757

1110

939

1800

1600

1400

1200

1000

800

1700

Wavenumber (cm-1)

(C)

1690

1680

1670

1660

Wavenumber (cm-1)

Figure 6


Page 33 of 39

(A) Absorbance (a.u.)

vOH S1

25 °C

280 °C

3600

3300

3000

2700

2400

-1

Wavenumber (cm )

(B)

Absorbance (a.u.)

vOH S2

25 °C

300 °C

3600

3300

3000

2700

2400

-1

Wavenumber (cm )

(C) Normalized integrated area -1 (a.u. cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


S1 S2

1.0 0.8 0.6 0.4

vOH

0.2 0.0 0

50

100

150

200

250

300

Activation temperature (C)

Figure 7 33 ACS Paragon Plus Environment


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(A)

(B)

Figure 8


Page 35 of 39

(A)

water He hexane 1-hexene

10

Mol % of vapor

8

6

4

2

0 0

2000

4000

6000

8000

Time (sec)

(B)

10

Mol % of vapor

8

6

4

2

0 0

500

1000

1500

2000

2500

3000

3500

Time (sec)

(C)

20

16

Mol % of vapor

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


12

8

4

0 0

2000

4000

6000

8000

Time (sec)

Figure 9



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Figure 10


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Tables

Table 1: Surface area and pore volume of CuBTC materials S1 and S2 and comparison to theoretically calculated values for the ideal CuBTC structure. Sample

Surface areas (m2/g) Pore volumes (cm3/g) BET Langmuir Theoretical Single-point t-plot Theoretical

S1

1240

1500

2280

0.55

0.48

0.85

S2

1600

1930

2280

0.71

0.61

0.85

Table 2: Comparison of 1-hexene (C6=) adsorbed/desorbed amounts, 1-hexene/hexane selectivities, and total (1-hexene+hexane) capacities for CuBTC materials S1 and S2 determined by vapor breakthrough measurements at 50°C and a 10% 1-hexene/90% hexane liquid feed mixture. Sample

C6= Amounts (g/g-adsorbent)

C6=/C6 Selectivities

Total Capacities (g/g-adsorbent or g/cm3-adsorbent) Gravimetric Volumetric

Adsorbed

Desorbed

Equilibrium

Desorption

S1

0.13

0.12

4.1

4.7

0.31-0.36

0.28-0.32

S2

0.20

0.17

4.9

4.3

0.48-0.52

0.42-0.46



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TOC Graphic


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Open metal site availability in CuBTC as a function of activation conditions, and its effect on olefin/paraffin separation. 237x82mm (150 x 150 DPI)

ACS Paragon Plus Environment

Effects of Open Metal Site Availability on Adsorption Capacity and

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