Synthesis Of Mesoporous Graphitic Carbon Nitride Materials Using A New Precursor And Their Application In Base Catalysis

Graphitic carbon nitride(g-CN) is a novel metal-free carbon-based material possessing graphitic-analogue structures. Owing to its unique multiple physicochemical advantages, such as optical features, semiconductivity, and abundant basic N-containing species, g-CN has demonstrated promising application in wide fields, including photocatalysis, gas storage, fuel cells, and heterogeneous catalysis. Also, g-CN has been regarded as the most potential candidate to complement the traditional carbon materials. In general, the textural parameters are responsible for the catalytic performance of g-CN. Compared with those typical bulk g-CN samples with low surface areas, mesoporous g-CN materials possess higher surface areas(> 200 m2 g-1) and large pore volumes(> 0.30 cm3 g-1); these features enable g-CN materials to expose higher active sites on their surface. In this context, the design and preparation of mesoporous g-CN materials have therefore become a hot topic in the fields of both catalysis and material.Among the numerous synthetic methods developed, nanocasting is a common and reproducible approach to synthesize mesoporous g-CN. Wherein, N-containing organic compounds and nanosized silica particles(or mesoporous siliceous materials) are employed as precursors and hard templates, respectively. The ordinary precursors reported for the synthesis of mesoporous g-CN consist of carbon tetrachloride(CTC) plus ethylenediamine, cyanamide, melamine, hexamethylenetetramine, guanidinium chloride, etc. However, CTC is hazardous due to its cancer-related toxicity. Cyanamide is highly toxic and explosive. In the case of melamine, the compound is very difficult to solve in water. Moreover, hexamethylenetetramine and guanidinium chloride suffer from low productivity(< 20%) of the final mesoporous g-CN materials. Therein, it is highly desired to explore a low-toxic, environmental-benign and inexpensive precursor for the manufacture of mesoporous g-CN materials.Dicyandiamide(DCDA) is the dimmer of cyanamide; it is low toxic and very cheap. Therein, DCDA is an ideal precursor for the synthesis of mesoporous g-CN. Unfortunately, the main disadvantage associated with the use of DCDA is its poor solubility in most solvents. In the present thesis, we have reported for the first time, a unique and facile approach for dissolving DCDA with the aid of ethylenediamine. After that, a series of ordered mesoporous g-CN materials(CND-SBA15 and CND-FDU12) have been fabricated using 2D hexagonal SBA-15 or 3D cubic FDU-12 as a precursor via a nanocasting route. Several techniques including N2 adsorption–desorption, XRD, SAXS, TEM, FT-IR, XPS, etc, have been applied to characterize the physicochemical properties of the synthesized mesoporous g-CN materials. The corresponding results reveal that the mesoporous g-CN materials have very high surface areas(269–715 m2 g-1) and concentrated pore sizes(4–6 nm) and meanwhile, successfully replicated the mesoporous architecture of the original siliceous templates. Furthermore, the tectonic units of the mesoporous materials are supposed as tri-s-triazine. Due to their abundant basic sites, the mesoporous g-CN materials exhibit highly catalytic activity and stability in a series of Knoevenagel condensation reactions with various substrates.As mentioned above, the mesoporous g-CN materials are widely prepared through nanocasting methods. In these routes, the detemplating procedure is tedious and time-consuming. More importantly, the etching agent(i.e. HF solution) is hazardous to both humankind and environment. Meanwhile, the acidic HF solution would react with the original basic species located on the surface of g-CN, thus sacrificing the overall basic quantity and intensity. Regarding these issues, we have synthesized SBA-15 supported CN materials, namely CND/SBA-15 composites, by facile wet impregnation and calcination. The characterization results confirm that, after the introduction of CN, the mesoporous orderness of SBA-15 has been well retained. Additionally, the whole basic intensity of SBA-15 has been significantly enhanced, which results from the N-containing species of the supported CN, as revealed by FT-IR. As heterogeneous base catalyst, the CND/SBA-15 is very easy to prepare. In addition, the composites demonstrate superior catalytic activity towards Knoevenagel condensation and transesterification reaction.