A Mesh‐Adjustable Molecular Sieve for General Use in Gas Separation

Gas separation using molecular sieves (MSs) is a environmentally benign, energy-conserving alternative to traditional separation processes, such as distillation and absorption. When using zeolite MSs, an accurate one-on-one match between the mesh size and the separation need is essential. However, when the size disparity of the two gases to be separated is small, a MS with the optimum mesh size is not always readily available. A mismatch inevitably leads to an inefficient separation. Recently, titanosilicate was shown to possess superior flexibility over that of traditional zeolites; a few MSs with discrete mesh sizes were made based on the degree of dehydration of this material at various temperatures. Nevertheless, a MS with more than one mesh size has never been made in the past. Herein, we report the design, synthesis, and application of a novel mesh-adjustable molecular sieve (MAMS-1) that possesses an infinite number of mesh sizes. MAMS-1 is based on a metal–organic framework (MOF), compounds known for their dynamic porous properties. However, the concept of a MAMS has never appeared in the literature prior to the present work. MAMS-1 represents a MOF-based MS whose mesh can be adjusted continuously. The mesh range of MAMS-1 falls between 2.9 and 5.0 5, which covers the size range of almost all commercially important gas separations. When the temperature is precisely controlled, any mesh size within this range can be accurately attained. Gas separations such as those of N2/O2 and N2/CH4, which are normally difficult to achieve, are readily attainable by using MAMS-1. In principle, by precise temperature control, any two gases with a size difference can be separated by a MAMS. MOFs have attracted a great deal of attention because of their unique structures 7] and potential applications in catalysis, separation, and gas storage. In particular, flexible MOFs have caught enormous attention lately. Numerous studies have indicated that the key to constructing a flexible MOF lies in the utilization of weak interactions, such as hydrogen bonding, p–p stacking, and hydrophobic interaction, in addition to strong covalent and coordinative bonding. Flexible MOFs based on hydrogen bonding have been widely studied, but those originating from p–p stacking and hydrophobic interaction have rarely been explored. To make a MAMS, two factors must be taken into account: the material must have permanent porosity to hold gas molecules, and the pores must be flexible. The former usually requires strong bonds, while the latter implies weak interactions in the framework. These two seemingly irreconcilable prerequisites for a MAMS can be met simultaneously by using a graphitic structure, in which atoms in each layer are connected covalently but the layers are held together by weak interactions. One approach to such a graphitic MOF is to apply an amphiphilic ligand that consists of hydrophobic and hydrophilic ends, similar to a surfactant, but with the hydrophilic end functionalized. The functional group at the hydrophilic end of the ligand will bind metal ions/clusters, and the structure will propagate into a 2D layer. Two layers of ligands will sandwich a metal-ion/cluster layer, thus giving rise to a trilayer, and these trilayers will pack through van der Waals interaction. The ligand adopted for the aforementioned purposes is 5tert-butyl-1,3-benzenedicarboxylate (bbdc), which was previously used in our laboratory to build a micelle-like cuboctahedral cage to adjust the solubility of a 24-molybdenum cluster. Recently, it was also used in a zinc microporous MOF. In fact, solvothermal reaction between H2(bbdc) and Ni(NO3)2 in H2O/ethylene glycol in a Teflon-lined autoclave afforded such a graphitic structure, [Ni8(5-bbdc)6(m3-OH)4] (designated MAMS-1 for convenience). Desolvated MAMS1 demonstrates temperature-induced molecular-gating effects in which the size of the gates can be tuned continuously from 2.9 to 5.0 5 for the first time. Commercially relevant gas separations, such as those of H2/N2, H2/CO, N2/O2, N2/CH4, CH4/C2H4, and C2H4/C3H6, can be achieved by MAMS-1. In principle, by precise temperature control, any mesh size within this range can be achieved. In fact, all the pairs of gases listed above have been separated by using MAMS-1. Single-crystal X-ray analysis revealed that MAMS-1 contains an octanickel [Ni8(m3-OH)4] cluster as one of the two secondary building units (SBUs; Figure 1a), the other being the bbdc ligand. The eight octahedral Ni atoms are divided into four pairs by a twofold axis through the center of the cluster. Ni1 binds five carboxylate O atoms from four bbdc ligands and one m3-OH group. Ni2 is coordinated by three carboxylate O atoms and three m3-OH groups. Ni3 is bound to four carboxylate O atoms, one m3-OH group, and an aqua [*] S. Ma, Dr. D. Sun, Dr. X.-S. Wang, Prof. Dr. H.-C. Zhou Department of Chemistry and Biochemistry Miami University Oxford, OH 45056 (USA) Fax: (+01)513-529-8091 E-mail: zhouh@muohio.edu