Efficient propyne/propadiene separation by microporous crystalline physiadsorbents

Selective separation of propyne/propadiene mixture to obtain pure propadiene (allene), an essential feedstock for organic synthesis, remains an unsolved challenge in the petrochemical industry, thanks mainly to their similar physicochemical properties. We herein introduce a convenient and energy-efficient physisorptive approach to achieve propyne/propadiene separation using microporous metal-organic frameworks (MOFs). Specifically, HKUST-1, one of the most widely studied high surface area MOFs that is available commercially, is found to exhibit benchmark performance (propadiene production up to 69.6 cm3/g, purity > 99.5%) as verified by dynamic breakthrough experiments. Experimental and modeling studies provide insight into the performance of HKUST-1 and indicate that it can be attributed to a synergy between thermodynamics and kinetics that arises from abundant open metal sites and cage-based molecular traps in HKUST-1.


MOFs Synthesis
Synthesis of HKUST-1: HKUST-1 was synthesized based on the previous method. [1] Cu(NO3)·2.5H2O (6.0 g) was dissolved into 250 mL deionized water. It was followed by the addition of 1,3,5-Tricarboxybenzene (4.0 g) in a 250 mL of solvent consisting of equal parts of ethanol and deionized water and mixed thoroughly until it was completely dissolved. The resultant solution mixture was transferred into a 250 mL teflon-lined stainless steel autoclave. It was kept at 110 ℃ for 18 h in oven to yield small crystals. Then the autoclave was cooled down to room temperature naturally and the blue crystals were isolated by filtration. The suspension was washed with the mixture of deionized water and ethanol several times and dried.
Then the gave green, block shaped Crystals was yield.
Then the samples were removed from the oven and allowed to cool to RT. The mother liquor was decanted from the yellow microcrystalline material and replaced with methanol (10 mL per vial). The yellow microcrystalline material was combined into one vial. The methanol was decanted and replenished four times over two days. The solvent was removed under vacuum at 250 °C over 10 hours, yielding the dark yellow microcrystalline, porous material.
Synthesis of MIL-100 (Cr, Fe): MIL-100 (Cr) was synthesized based on the previous method. [5] Metallic chromium (52 mg, 1 mmol) was dispersed into an aqueous solution of 5M hydrofluoric acid (0.4 mL, 2 mmol). After the addition of 1,3,5-benzene tricarboxylic acid (H3BTC) (150 mg, 0.67 mmol) and H2O (4.8 mL, 265 * 10 -3 mol), the mixture was heated in ahydrothermal bomb at a rate of 20 ℃/h to 220 ℃, kept at this temperature during 96 h, then cooled at a rate of 10 ℃/h to room temperature. The S4 resulting green powder was washed with deionized water and acetone and dried in air.
MIL-100 (Fe) was synthesized based on the previous method. [6] This solid was isolated as a polycrystalline powder from a reaction mixture of composition Fe/ H3BTC /HF/HNO3/H2O (1.0/0.66/2.0/1.2/280) that was held at 150 ℃ in a Teflon-lined autoclave for 6 days with a initial heating ramp of 12 h and a final cooling ramp of 24 h. The pH remains acidic (<1) throughout the synthesis. The light-orange solid product was recovered by filtration and washed with deionized water. A treatment in hot deionised water (80 ℃) for 3 h was applied to decrease the amount of residual H3BTC (typically, 1 g of MIL-100(Fe) in 350 ml of water) followed by drying at room temperature.
A treatment in hot deionised water (80 ℃) for 3 h was applied to decrease the amount of residual H3BTC followed by drying at room temperature.
Synthesis of SIFSIX-3-Ni: SIFSIX-3-Ni was synthesized based on the previous S5 method. [9] SIFISIX-3-Ni was synthesized by slurrying 870 mg (3 mmol) of Ni(NO3)2, 534 mg (3 mmol) of (NH4)2SiF6 and 480 mg (6 mmol) of pyrazine in 4 mL of water for 2 days. The resulting suspension was filtered under vacuum and dried in air. This precursor was soaked in methanol for 1 day and then washed twice with two portions (ca. 10 mL) of methanol on a Buchner filter. After drying in air, the solid was heated at 140 ˚C for 1 day to obtain SIFSIX-3-Ni.
Synthesis of ZU-62: ZU-62 was synthesized based on the previous method. [11] A preheated water solution (4.0 mL) of CuNbOF5 (0.0730 g) were dropped into a preheated methanol solution (4.0mL) of 4, 4'-bipyridylacetylene (0.0515 g). Then the mixture was heated at 80 °C for 24 h. The obtained blue power was exchanged with methanol for a day.
Synthesis of UiO-66: UiO-66 was synthesized based on the previous method. [12] Standard synthesis of UiO-66 was performed by dissolving ZrCl4 (0.053 g, 0.227 mmol) and 1,4-benzenedicarboxylic acid (H2BDC) (0.034g, 0.227 mmol) in DMF (24.9 g, 340 mmol) at room temperature. The thus obtained mixture was sealed and placed in a preheated oven at 120 ˚C for 24 hours. Crystallization was carried out under static conditions. After cooling in air to room temperature the resulting solid was filtered, S6 repeatedly washed with DMF and dried at room temperature.
Synthesis of UiO-67: UiO-67 was synthesized based on the previous method. [12] Standard synthesis of UiO-67 was performed by dissolving ZrCl4 (0.053 g, 0.227 mmol) and 1,4-benzenedicarboxylic acid (H2BPDC) (0.227 mmol) in DMF (24.9 g, 340 mmol) at room temperature. The thus obtained mixture was sealed and placed in a pre-heated oven at 120 ˚C for 24 hours. Crystallization was carried out under static conditions.
After cooling in air to room temperature the resulting solid was filtered, repeatedly washed with DMF and dried at room temperature.
Synthesis of ZIF-8: ZIF-8 was synthesized based on the previous method. [13]  The isotherm fit parameters for HKUST-1 are provided in Supplementary Table 2.
The isotherm fit parameters for Mg-MOF-74 are provided in Supplementary Table 3.

Supplementary Equation 2
The isotherm fit parameters for MOF-505 are provided in Supplementary Table 4.
The isotherm fit parameters for NKMOF-1-Ni are provided in Supplementary Table 5.

Supplementary Note 2: Isosteric heat of adsorption
The isosteric heat of adsorption was determined from the unary isotherm by use of the Clausius-Clapeyron equation:

Equation 3
These values were determined using the pure component isotherm fits using the DSL and DSLF equation (Supplementary Fig. 12-14). Qst is the coverage dependent isosteric heat of adsorption and R is the universal gas constant.

Supplementary Note 3: IAST calculations of adsorption selectivity and uptake capacity
IAST calculations were carried out for the following mixture 50/50 propyne/propadiene mixture at 298 K. the adsorption selectivity is defined by:

Supplementary Note 4: Transient breakthrough simulation
The performance of industrial fixed bed adsorbers is dictated by a combination of adsorption selectivity and uptake capacity. Transient breakthrough simulations were carried out for 50/50 propyne/propadiene feed mixture at 298 K and 100 kPa using the methodology described in earlier publications. [14][15][16][17][18] For the breakthrough simulations, (CCDC 112954) [19] and (CCDC 257470), [20] ] respectively. The polarizable force field that was developed for Mg-MOF-74 in previous work was used herein. [21] S10 All atoms of HKUST-1 and MOF-505 were treated with Lennard-Jones (LJ) parameters (ε and σ), point partial charges, [22] and point polarizabilities in order to model repulsion/dispersion, stationary electrostatic, and many-body polarization interactions, respectively. The LJ parameters for all atoms were taken from the Universal Force Field (UFF). [23] The partial charges for the chemically distinct atoms in both MOFs were determined through electronic structure calculations on different gas phase fragments that were selected from the crystal structure of the respective MOFs. These calculations were performed using the NWChem ab initio software with the 6-31G* basis set assigned to C, H, and O and the LANL2DZ ECP basis set assigned to Cu. [24][25][26][27] The exponential damping-type polarizability values for all C, H, and O atoms were taken from a carefully parametrized set provided by the work of van Duijnen and Swart. [28] The polarizability parameter for Cu 2+ was calculated in previous work and used herein. [29] Classical Monte Carlo (MC) simulations of propyne and propadiene adsorption were performed within the unit cell of HKUST-1, 2 × 2 × 1 supercell of MOF-505, and 1 × 1 × 4 supercell of Mg-MOF-74. All MOF atoms were kept fixed at their crystallographic positions. For each MOF, a spherical cut-off distance corresponding to half the shortest system cell dimension length was used for the simulations. Propyne and propadiene were modeled using polarizable potentials of the respective adsorbates that were developed previously. [30] The total potential energy of the MOF-adsorbate system was calculated through the sum of the repulsion / dispersion, stationary electrostatic, and many-body polarization energies. These were calculated using the S11 Lennard-Jones 12-6 potential, [22] partial charges with Ewald summation, [31,32] and a Thole-Applequist type model, [33][34][35][36] respectively. All MC simulations were performed using the Massively Parallel Monte Carlo (MPMC) code. [37,38] In order to identify the global minimum for propyne and propadiene in HKUST-