Desulfurization Efficiency Preserved in a Heterometallic MOF: Synthesis and Thermodynamically Controlled Phase Transition

Abstract Efficient removal of heterocyclic organosulfur compounds from fuels can relieve increasingly serious environmental problems (e.g., gas exhaust contaminants triggering the formation of acid rain that can damage fragile ecological systems). Toward this end, novel metal‐organic frameworks (MOFs)‐based sorbent materials are designed and synthesized with distinct hard and soft metal building units, specifically {[Yb6Cu12(OH)4(PyC)12(H2O)36]·(NO3)14·xS}n (QUST‐81) and {[Yb4O(H2O)4Cu8(OH)8/3(PyC)8(HCOO)4]·(NO3)10/3·xS}n (QUST‐82), where H2PyC = 4‐Pyrazolecarboxylic acid. Exploiting the hard/soft duality, it is shown that the more stable QUST‐82 can preserve desulfurization efficiency in the presence of competing nitrogen‐containing contaminate. In addition, thermodynamically controlled single‐crystal‐to‐single‐crystal (SC–SC) phase transition is uncovered from QUST‐81 to QUST‐82, and in turn, mechanistic features are probed via X‐ray diffraction, inductively coupled plasma atomic emission spectroscopy, and ab initio molecular dynamics simulations.

absorption spectra were examined on Ultrospec 2100 pro spectrophotometer in the wavelength range of 200-320 nm. Scanning electron microscopy (SEM) was acquired on FEI Helios. The N 2 adsorption measurements were performed on Quantachrome automated gas adsorption analyzer.

II. Single-crystal and powder X-ray Crystallography
Single-crystal X-ray Crystallography Single crystal of QUST-81 was selected and mounted on MiTeGen® loops in Paratone oil.
Cooling the sample to below room temperature resulted in a loss of resolution in the diffraction data. So a drop of mother liquor was placed in a MiTeGen® MicroRT® tube which was then placed over the crystal and loop and secured to the base by the means of grease. This was mounted on a Bruker D8 diffractometer equipped with a PHOTONII CPAD detector at 298 K, on Beamline 12.2.1 of the Advanced Light Source at LBNL. A sphere of data were collected using Bruker APEX3 software in shutterless mode with rotations at fixed values at = 0.7749 Å, from a Silicon [111] double crystal monochromator. The intensity data were integrated and correction applied with SAINT v8.38a, absorption and other correction were made using SADABS 2016/2. Dispersion corrections appropriate for this wavelength were calculated using the Brennan method in XDISP with in WinGX. The structures were solved with a dual space method with SHELXT 2014/5 and refined using SHELXL 2014/7. The data were cut at 1.16 angstroms as R(int) jump from 30% to 42% in the next shell.
Experience has shown that this addition data would add more noise than useful data. All nonhydrogen atoms were refined anisotropically, except the partially occupied water molecules.
Hydrogen atoms were found placed geometrical on the carbon atom and refined as a riding model. Hydrogen atoms could neither be found nor placed on the water molecules and were therefore omitted from the refinement but not the chemical formula. Displacement parameter restraints were used globally (RIGU) and same distance restraints were used for Yb1 to O1w and O2w, also for Cu1 to O3w O3w'. Water molecules were used to complete the metal coordination shell as not meaningful electron density could be found in the difference map to such any other option. The same is true of the waters in the void space. CCDC-1835704 (QUST-81) contains the supplementary crystallographic data for this paper.
Data collection for QUST-82 was carried on an Agilent Technologies SuperNova Single Crystal Diffractometer using Cu K radiation ( = 1.54178 Å) at 120 K. The structure was solved using SHELXS-97 and was refined with SHELXL-97. The hydrogen atoms were included in the structure-factor calculations at idealized positions by using a riding model and were refined isotropically. The contributions of guests were removed by using SQUEEZE as implemented in PLATON. CCDC-1827059 (QUST-82) contains the supplementary crystallographic data for this paper.

powder X-ray Crystallography
In order to analyze the air-sensitive colorless intermediate crystals with PXRD, the fresh solution of starting materials for the synthesis of QUST-81 was placed in a 3.0 mm diameter thin walled borosilicate capillary tube for XRD applications (Charles Supper Company). After the capillary was sealed with room temperature vulcanization epoxy, it was placed in a lab oven at 100 °C to generate QUST-81 ( Figure S4) and subsequently heated to 120 °C to   S2c).