Simulating Synthesis Of Nano Micro-Porous Materials For Gas Adsorption

Hydrogen is recognized as one of main energy resources in the 21st century, due to its outstanding features, such as clean, zero-emission of greenhouse-gas and pollutants etc. Many countries are deploying and implementing their hydrogen energy strategies urgently. In addition, as a clean and highly-efficient feed stock, natural gas also becomes one of the most promising new energy resources, in which methane is the major component. At present, one of the main problems that impede practical uses of hydrogen and methane is the storage of these two gases. As a result, the development of new materials that can store hydrogen and methane in high capacity and good safety is an important topic. Nowadays, high-pressure compression is still the main storage method of hydrogen and methane, although this method is of some drawbacks, for example, consuming too much energy and requiring expensive equipment. In contrast, owing to the adsorption reversibility and other advantages, adsorption storage is believed as one of the most promising ways for storing hydrogen and methane in future.As is well known, carbon dioxide has the greatest impact on climate changes, and generates about 63% of the total warming effect of green-house gases. In order to reduce the emission of carbon dioxide, people have to capture and store carbon dioxide from industrial gaseous waste and motor vehicle exhaust. Chemical absorption method is the most commonly used technique of capturing carbon dioxide industrially. However, on one hand, this technique needs big equipments generally; on the other hand, the chemical absorbents are corrosive. Therefore, capturing and storing carbon dioxide with adsorption technique have attracted lots of research interest recently.In this work, a "simulating synthesis" approach is adopted to develop several high performance adsorbents for hydrogen, methane and carbon dioxide storage including three-dimensional covalent organic frameworks (3D COFs) and their Li-doped compounds, silicon nanotubes (SiNT) as well as Li-coated silicon fullerene hydride (Li12Si60H60). In this simulating synthesis approach, the design and modification of effective gas adsorbents are carried out by using a multi-scale method, which combines the first principles calculation and molecular simulation method. The first principles calculation is based on the level of electrons, and is widely used for studying electron structures, bonding features, spectral properties (such as infrared, Raman and nuclear magnetic resonance spectroscopy, etc.); while the molecular simulation method is based on the interaction between molecules(atoms), and achieves the macroscopic properties by performing simulations in statistical mechanics. Therefore, a molecular force field that describes the interaction between the molecules of the adsorbate and the atoms of the host material bridges these two levels of theoretical methods. Combining the first principles calculation and molecular force field, the molecular simulation method is then used for obtaining adsorption uptakes. In general, this simulating synthesis approach consists of the following four steps. (1) To design the nano micro-porous materials by using the first principles calculation. (2) To compute the interaction energies between the adsorbate and adsorbent by using the first principles calculation. (3) To fit the interaction energies between the molecules (atoms) of the adsorbate and adsorbent to a force field, and thus the parameters of the force field can be obtained. (4) To obtain the macro-properties by performing molecular simulation. This simulating synthesis approach can be used to predict the adsorption properties of an adsorbent, to screen effective materials for storage and separation of gases, and to provide suggestions for their preparations. To implement the task, this work consists of the following parts.The synthesis of novel crystalline micro-porous materials,3D COFs, has been achieved recently. Due to their unique characteristics, such as extremely low densities and high surface areas,3D COFs have attracted a lot of research interest so far. With the multi-scale method described above, the potential energies between H2 and the COFs were first calculated. Then, the force fields describing the interaction between H2 and the COFs were constructed by the force field fitting. The H2 uptakes of the COFs were predicted by performing grand canonical ensemble Monte Carlo (GCMC) simulation in different thermodynamic conditions. It is found that, at T=77 K, the COFs exhibit high hydrogen uptakes, and belong to the most promising hydrogen adsorbents at present. At T= 77 K and p= 10 MPa, COF-105 and COF-108 show the total hydrogen gravimetric uptakes of 10.05 wt% and 17.80 wt%, and their maximum excess uptakes reach 10.31 wt% and 10.26 wt%, respectively, which are in agreement with those in experiment. Since the hydrogen uptakes of the COFs at room temperature do not meet the storage capacity target of 6 wt% set by the US Department of Energy (DOE) for hydrogen applications, the surface modification of the COFs was carried out to promote their hydrogen uptakes. As is well known, lithium is an ideal element for the surface modification of nano porous materials, because of its unique characteristics, such as light weight and easy to lose its outer valance electron. It is further found that, a single Li atom can be positively charged due to the charge-transfer to the host materials, which can help to improve the affinity of the host materials to hydrogen molecules. In addition, with the fitted force fields that describe the interaction between hydrogen and Li cations doped in the COFs, the hydrogen uptakes of the Li-doped COFs were predicted by performing GCMC simulations. The results show that, at T= 298 K and p= 10 MPa, the Li-doped COF-105 and COF-108 exhibit the total hydrogen uptakes of 6.84 wt% and 6.73 wt%, respectively, which meet the US DOE target for hydrogen storage. Consequently, it is concluded that the Li-doped COFs composites are very promising hydrogen storage materials in future.By using the multi-scale method mentioned above, the storage of methane in the COFs and their Li-doped compounds was then studied. Through the force fields between methane and the non-doped and Li-doped COFs, their methane uptakes were predicted with GCMC simulation. The results show that, the 3D COFs exhibit high methane storage capacities, and exceed the best MOF material reported in the literature, PCN-14. At T= 298 K and p=3.5 MPa, the methane uptakes of COF-102 and COF-103 reach 17.72 wt% and 16.61 wt%, respectively, which are also in good agreement with the experimental results. Moreover, Li atoms loaded in the COFs exhibits strong affinity to methane due to the London dispersion and induced-dipole interaction. The results from GCMC simulation indicate that Li-doping enhances the methane storage performance of the COFs significantly. At T= 298 K and p= 3.5 MPa, the Li-doped COF-102 and COF-103 show the excess methane gravimetric uptakes of 31.35 wt% and 30.98 wt%, while their excess volumetric uptakes are 303 v(STP)/v and 290 v(STP)/v, respectively. Obviously, the volumetric uptakes of methane in the Li-doped COFs significantly exceed the US DOE target of 180 v(STP)/v for methane storage. These results show that the Li-doped COFs in this work are also promising methane storage materials.In addition, novel silicon nanotubes (SiNTs) were synthesized in this work by using the multi-scale method mentioned above. First, the force filed parameters that describe the interaction between hydrogen and the SiNTs were achieved by the first principles calculation and force field fitting. Then the GCMC simulation was performed to predict the hydrogen storage performance of different SiNT arrays at room temperature. For comparison, the adsorption of hydrogen in the carbon nanotubes (CNTs) of the same diameter was also investigated. The results show that, due to the larger density of the extra-nuclear electron cloud of a silicon atom than that of a carbon atom, SiNT shows stronger dispersion interaction to hydrogen than its iso-diameter CNT. The first principles calculation proves that, compared to the (14,14) CNT, the binding energies between a hydrogen molecule and (9,9) SiNT exhibit significant increases of 20% and 26% for inside and outside adsorption, respectively. The results from GCMC simulation show that the hydrogen uptakes of SiNT arrays were significantly higher than the iso-diameter CNT arrays in the same conditions. At p= 2,6 and 10 MPa, the triangular-arranged (9,9) SiNT array shows the hydrogen storage capacities of 1.30,2.33 and 2.88 wt%, respectively, which are 106%,65% and 52% higher than those for the (14,14) CNT array. These results indicate that SiNT exceeds CNT and becomes a more promising candidate for hydrogen storage.Moreover, a novel hydrogen storage material, Li-coated silicon fullerene hydride (Li12Si60H60), was designed. First, the adsorption sites and binding energies of single and multiple Li atoms on Si60H60 were studied with the first principles calculations, so that the reasonable structure of Li12Si60H60 was determined. Then, the force field that describes the interaction between hydrogen and Li12Si60H60 was constructed. Furthermore, the hydrogen uptakes of the Li12Si60H60 array were investigated in different conditions. The results indicate that Li atoms prefer to be adsorbed in the Si60H60 cage by binding with the five-membered rings rather than forming clusters, because of the relatively low Li-Li binding energy and the inhibition of the Si-H bonds. The results from GCMC simulations show that the coated Li atoms outside the Si60H60 cage are positively charged, and can enhance the hydrogen adsorption capacity of Si60H60 cage significantly. At T= 77 K, the maximum excess hydrogen uptake of Li12Si60H60 array reaches 7.46 wt%. This result shows that, Li12Si60H60 is also a very promising adsorbent for hydrogen storage.Carbon dioxide capture is an urgent task for dealing with climate change nowadays. In this work, COFs doped with alkaline metals (Li, Na and K) and alkaline-earth metals (Be, Mg and Ca) as well as transition metals (Sc and Ti) were studied systematically. Their impacts on capturing and storing carbon dioxide were simulated. Our results indicate that Li is an ideal material for modification of COFs due to its light weight, easily losing the valence electron and multiple adsorption sites in COFs. In addition, charged Li can bind with carbon dioxide in a way between physical adsorption and chemical adsorption. On the contrary, Na and K are heavy in weight and unstable when loaded in COFs. Besides, the positively charged Na and K show much weaker affinity to carbon dioxide than Li. It is also found that the transition metals such as Sc and Ti can be adsorbed in COFs very stably, and tend to lose their valence electrons in COFs. However, the binding energies between carbon dioxide and the Sc/Ti cation exceed the lower limt of chemisorption. That is to say, the adsorbed carbon dioxide by Sc/Ti cations has the difficulty of desorption at room temperature. By performing our first-principles calculation and force field fitting, we then obtained the force field parameters between carbon dioxide and the COFs as well as the doped Li cation. Our GCMC simulations show that the Li-doped COF-102 show an excess CO2 uptake of about 1237.6 mg/g at T= 298 K and p= 0.1 MPa, which is among the highest scores of the nanoporous adsorbents before. It indicates that the Li-doped COFs belong to the most promising CO2 adsorbents at present.