Lin-Bing Sun

Cooperative template-directed assembly of mesoporous metal-organic frameworks.

Despite great efforts, the development of a reliable way to assemble mesoporous metal-organic frameworks (mesoMOFs) remains a challenge. In this work, we have designed a cooperative template system, comprising a surfactant (cetyltrimethylammonium bromide) and a chelating agent (citric acid), for the generation of a mesoMOF containing a hierarchical system of mesopores interconnected with microspores. The surfactant molecules form micelles and the chelating agent bridges the MOF and the micelles, making self-assembly and crystal growth proceed under the direction of the cooperative template. However, when the surfactant or the chelating agent was applied individually, no mesoMOF was obtained.

Introduction of Functionalized Mesopores to Metal–Organic Frameworks via Metal–Ligand–Fragment Coassembly

Introduction of functionalized mesopores into microporous metal-organic frameworks (MOFs) can endow them with suitable properties for applications in gas storage, separation, catalysis, and drug delivery. However, common methods for functionalization (including pre- and post-synthetic modifications) of the internal surface of a MOF reduce the pore size of the MOF because the additional functional groups fill up the pores. We present a metal-ligand-fragment coassembly strategy for the introduction of (meso)pores functionalized with various substituent groups on the ligand fragments. Astonishingly, this new functionalization strategy increases the pore volume of a MOF instead of reducing it. Since the ligand fragments are often readily available or easily prepared, the new procedure for synthesis of the modified MOFs becomes much easier and more applicable than existing approaches. Remarkably, mesopores can be generated conveniently and controllably by the coassembly of a ligand and its fragment containing the desired functional groups. The fragment/ligand ratio has been optimized to preserve the parent structure and to promote maximum mesopore introduction, which has led to a systematic evaluation of the effectiveness of a series of functional groups for the adsorption of guest molecules.

Adsorptive Desulfurization by Copper Species within Confined Space

Copper species were incorporated into SBA-15 by solid-state grinding precursor with as-prepared mesoporous silica (SPA). The obtained materials (CuAS) were well-characterized by XRD, TEM, N(2) adsorption, H(2)-TPR, IR, and TG and compared with the material derived from calcined SBA-15 (CuCS). Surprisingly, CuO up to 6.7 mmol·g(-1) can be highly dispersed on SBA-15 by use of SPA strategy. Such CuO forms a smooth layer coated on the internal walls of SBA-15, which contributes to the spatial order and results in less-blocked mesopores. However, the aggregation of CuO takes place in CuCS material containing 6.7 mmol·g(-1) copper, which generates large CuO particles of 21.4 nm outside the mesopores. We reveal that the high dispersion extent of CuO is ascribed to the abundant silanols, as well as the confined space between template and silica walls provided by as-prepared SBA-15. The SPA strategy allows template removal and precursor conversion in one step, avoids the repeated calcination in conventional modification process, and saves time and energy. We also demonstrate that the CuAS material after autoreduction exhibits much better adsorptive desulfurization capacity than CuCS. Moreover, the adsorption capacity of regenerated adsorbent can be recovered completely.

Facile fabrication of cost-effective porous polymer networks for highly selective CO2 capture

Due to their synthetic diversification, low skeletal density, and high physicochemical stability, porous polymer networks (PPNs) are highly promising in a variety of applications such as carbon capture. Nevertheless, complicated monomers and/or expensive catalysts are normally utilized for their synthesis, which makes the process tedious, costly, and hard to scale up. In this study, a facile nucleophilic substitution reaction was designed to fabricate PPNs from low-cost monomers, namely chloromethyl benzene and ethylene diamine. A surfactant template was also used to direct the assembly, leading to the formation of PPN with enhanced porosity. It is fascinating that the polymerization reactions can occur at the low temperature of 63 °C in the absence of any catalyst. The obtained PPNs contain abundant secondary amines, which offer appropriate adsorbate–adsorbent interactions from the viewpoints of selective CO2 capture and energy-efficient regeneration of the adsorbents. Hence, these PPNs are highly active in selective adsorption of CO2, and unusually high CO2/N2 and CO2/CH4 selectivity was obtained. Moreover, the PPN adsorbents can be completely regenerated under mild conditions.

Dispersion of copper species in a confined space and their application in thiophene capture

Deep desulfurization via π-complexation adsorption is an effective method for the selective capture of thiophenic sulfur compounds. The adsorptive desulfurization capacity of an adsorbent has been demonstrated to strongly depend on the dispersion degree of active species. In the present study, a strategy was developed to promote the dispersion of copper species by directly using as-synthesized mesoporous silica SBA-15 as a support. The results show that the confined space between template and silica walls is highly efficient in dispersing the resultant guest oxide, and unusual CuO dispersion is realized. However, severe CuO aggregation occurs on the material prepared through the conventional method based on template-free SBA-15. Interestingly, copper precursors have a significant effect on the dispersion degree of the oxide, which decreases in the order nitrate > acetate > chloride. After autoreduction, the materials are active in adsorptive desulfurization, and the desulfurization performance relates well to the dispersion degree of the oxide. The present strategy allows template removal and precursor conversion in one step, avoids the repeated calcination in the conventional modification process, and saves time and energy. This strategy may open up an avenue for the design and synthesis of new functional materials by use of some particular micro environments.

Fabrication of Supported Cuprous Sites at Low Temperatures: An Efficient, Controllable Strategy Using Vapor-Induced Reduction

Selective reduction of supported CuO to Cu2O was realized using the strategy of vapor-induced reduction, in which HCHO/H2O vapor diffuses into the pores of the support and interacts with predispersed CuO. This new strategy allows the fabrication of supported cuprous sites at much lower temperatures within a short time, avoids the formation of Cu(0) with a Cu(I) yield of nearly 100%, and results in materials with good adsorption performance, which is impossible to achieve by conventional methods.

Molecular template-directed synthesis of microporous polymer networks for highly selective CO2 capture.

Porous polymer networks have great potential in various applications including carbon capture. However, complex monomers and/or expensive catalysts are commonly used for their synthesis, which makes the process complicated, costly, and hard to scale up. Herein, we develop a molecular template strategy to fabricate new porous polymer networks by a simple nucleophilic substitution reaction of two low-cost monomers (i.e., chloromethylbenzene and ethylene diamine). The polymerization reactions can take place under mild conditions in the absence of any catalysts. The resultant materials are interconnected with secondary amines and show well-defined micropores due to the structure-directing role of solvent molecules. These properties make our materials highly efficient for selective CO2 capture, and unusually high CO2/N2 and CO2/CH4 selectivities are obtained. Furthermore, the adsorbents can be completely regenerated under mild conditions. Our materials may provide promising candidates for selective capture of CO2 from mixtures such as flue gas and natural gas.

Unusual ceria dispersion formed in confined space: a stable and reusable adsorbent for aromatic sulfur capture

An unusual ceria dispersion was achieved by using the confined space between template and silica walls in as-prepared mesoporous silica, for the first time. The new adsorbents exhibit high adsorptive desulfurization activity and, more importantly, excellent stability and reusability, which is impossible to realize with conventional adsorbents.

Low-temperature fabrication of mesoporous solid strong bases by using multifunction of a carbon interlayer.

Mesoporous solid strong bases are highly promising for applications as environmentally benign catalysts in various reactions. Their preparation attracts increasing attention for the demand of sustainable chemistry. In the present study, a new strategy was designed to fabricate strong basicity on mesoporous silica by using multifunction of a carbon interlayer. A typical mesoporous silica, SBA-15, was precoated with a layer of carbon prior to the introduction of base precursor LiNO3. The carbon interlayer performs two functions by promoting the conversion of LiNO3 at low temperatures and by improving the alkali-resistant ability of siliceous host. Only a tiny amount of LiNO3 was decomposed on pristine SBA-15 at 400 °C; for the samples containing >8 wt % of carbon, however, LiNO3 can be entirely converted to strongly basic sites Li2O under the same conditions. The guest-host redox reaction was proven to be the answer for the conversion of LiNO3, which breaks the tradition of thermally induced decomposition. More importantly, the residual carbon layer can prevent the siliceous frameworks from corroding by the newly formed strongly basic species, which is different from the complete destruction of mesostructure in the absence of carbon. Therefore, materials possessing both ordered mesostructure and strong basicity were successfully fabricated, which is extremely desirable for catalysis and impossible to realize by conventional methods. We also demonstrated that the resultant mesoporous basic materials are active in heterogeneous synthesis of dimethyl carbonate (DMC) and the yield of DMC can reach 32.4%, which is apparently higher than that over the catalysts without a carbon interlayer (<12.9%) despite the same lithium content. The strong basicity, in combination with the uniform mesopores, is believed to be responsible for such a high activity.

Constructing a confined space in silica nanopores: an ideal platform for the formation and dispersion of cuprous sites

Due to their versatility, nontoxicity, and low cost, much attention has been paid to the fabrication of Cu(I) sites on various supports. High-temperature autoreduction is a widely used method for selective conversion of supported Cu(II) to Cu(I). However, aggregation of copper species usually takes place during autoreduction, which seriously decreases the yield of Cu(I) and compromises the activity of resultant materials. In the present study, a strategy was developed for the effective formation and dispersion of Cu(I) sites by constructing a confined space in silica nanopores, for the first time. A layer of porous silica is coated on the precursor (namely CuO-modified SBA-15) before autoreduction, and CuO is thus confined between original pore walls and newly formed silica layers. Owing to the energy barrier offered by the confined space, the dispersion degree of copper species after autoreduction is well improved. Furthermore, the yield of Cu(I) can reach ∼82% for the samples coated with silica, which is obviously higher than that for the sample without a silica layer (∼54%). More importantly, abundant pores with a uniform size of ∼2.5 nm are successfully generated on the silica layer under the direction of cetyltrimethylammonium bromide (CTAB). This endows the resultant materials with active sites highly accessible to guest molecules. The materials were also applied to the adsorption of CO by using π-complexation between CO and Cu(I) sites. The results show that these materials exhibit good performance for selective adsorption of CO from H2, CH4, and N2, which is apparently better than the material without a silica layer with regard to both capacity and selectivity.