Organocatalysis with carbon nitrides.

Carbon nitrides, a distinguished class of metal-free catalytic materials, have presented a good potential for chemical transformations and are expected to become prominent materials for organocatalysis. This is largely possible due to their low cost, exceptional thermal and chemical stability, non-toxicity, ease of functionalization, porosity development, etc. Especially, the carbon nitrides with increased porosity and nitrogen contents are more versatile than their bulk counterparts for catalysis. These N-rich carbon nitrides are discussed in the earlier parts of the review. Later, the review highlights the role of such carbon nitride materials for the various organic catalytic reactions including Knoevenagel condensation, oxidation, hydrogenation, esterification, transesterification, cycloaddition, and hydrolysis. The recently emerging concepts in carbon nitride-based organocatalysis have been given special attention. In each of the sections, the structure-property relationship of the materials was discussed and related to their catalysis action. Relevant comparisons with other catalytic materials are also discussed to realize their real potential value. The perspective, challenges, and future directions are also discussed. The overall objective of this review is to provide up-to-date information on new developments in carbon nitride-based organic catalysis reactions that could see them rising as prominent catalytic materials in the future.

Tailoring the Pore Size, Basicity, and Binding Energy of Mesoporous C3 N5 for CO2 Capture and Conversion.

We investigated the CO2 adsorption and electrochemical conversion behavior of triazole-based C3 N5 nanorods as a single matrix for consecutive CO2 capture and conversion. The pore size, basicity, and binding energy were tailored to identify critical factors for consecutive CO2 capture and conversion over carbon nitrides. Temperature-programmed desorption (TPD) analysis of CO2 demonstrates that triazole-based C3 N5 shows higher basicity and stronger CO2 binding energy than g-C3 N4 . Triazole-based C3 N5 nanorods with 6.1 nm mesopore channels exhibit better CO2 adsorption than nanorods with 3.5 and 5.4 nm mesopore channels. C3 N5 nanorods with wider mesopore channels are effective in increasing the current density as an electrocatalyst during the CO2 reduction reaction. Triazole-based C3 N5 nanorods with tailored pore sizes exhibit CO2 adsorption abilities of 5.6-9.1 mmol/g at 0 °C and 30 bar. Their Faraday efficiencies for reducing CO2 to CO are 14-38% at a potential of -0.8 V vs. RHE.

Shape-Selective Catalysis in the Alkylation of Naphthalene: Steric Interaction with the Nanospace of Zeolites.

Steric interaction of reagents with nanospace of zeolites was studied in alkylation: isopropylation, sec-butylation, and tert-butylation of naphthalene (NP) over several large-pore zeolites to elucidate the mechanism of shape-selective catalysis. Selectivities for ß,ß- and 2,6-dialkylnaphthalene (DAN) were influenced by the type of zeolite and bulkiness of alkylating agent. Shape-selective formation of ß,ß- and 2,6-diisopropylnaphthalene (DIPN) occurred only over H-mordenite (MOR) in the isopropylation of NP. Bulky α,α- and α,ß-DIPN are excluded because of steric restriction at their transition states by the MOR channels, resulting in the selective formation of ß,ß- and 2,6-DIPN. AFI (SSZ-24) gave also high selectivities for 2,6-DIPN, and CFI (CIT-5) and MSE (MCM-68) gave high selectivities for ß,ß-DIPN. The lower selectivities for 2,6-DIPN were observed over the zeolites, ATS (SSZ-55), LFR (SSZ-42), DON (UTD-1), SFH (SSZ-53), FAU (Y-zeolite), BEA (zeolite ß), and CON (CIT-1). Their channels allow the accommodation of bulky isomers, resulting in the catalysis under kinetic and/or thermodynamic controls. The selectivities for ß,ß- and 2,6-DAN were enhanced with the increase in bulkiness of alkylating agents: 1-butene for sec-butylation and 2-methylpropene for tert-butylation, even over zeolites with large pores and channels: the transition states of the least bulky isomers only fit the channels, and the other bulky isomers are excluded by steric restriction of the channels. However, tert-butylation over FAU, BEA, and CON had selectivities of around 50-60% for 2,6-DTBN, and almost 100% selectivities for ß,ß-DTBN. These zeolites cannot recognize the differences between 2,6- and 2,7-DTBN, but they can differentiate ß,ß-DTBN from the other isomers. The results indicate that the fitting of the least bulky isomers to zeolite channels, resulting in the exclusion of other bulky isomers, is a key for highly shape-selective catalysis.

The beta-zeolite synthesized by dry-gel conversion method without the use of sodium hydroxide: characterization and catalytic behaviors.

The nano-sizedbeta-zeolites were synthesized first time by dry gel conversion (DGC) method without the use of sodium hydroxide. Resultant beta-zeolites had particle size of 35-80 nm with SiO2/Al2O3 ratios in the range of 25-400. They had BET surface areas in the range of 290-750 m2 x g(-1) and the external surface area of 41-283 m2 x g(-1): cumulative surface areas were in the range of 420-1050 m2 x g(-1). Beta-Zeolites modified with alkaline earth and rare earth metal species, such as Ca, La, and Ce, were synthesized by the introduction of these metal nitrates during the dry-gel preparation. To evaluate catalytic properties of these zeolites, they were applied for the isomerization/cracking of hexane and the phenol octylation. The influence of Ca, La, and Ce oxides on the catalytic properties was also examined.

Fe(III) containing MCM-48 type mesoporous molecular sieves with framework zeolite secondary building units.

Fe(III) containing mesoporous materials (FeMMSH) with zeolite building units were first time synthesized. XRD and N2 adsorption/desorption analysis confirm the mesoporous characters similar to MCM-48. FT-IR analysis of FeMMSH showed an additional band at about 530-550 cm(-1), characteristic of double five-membered ring of zeolite (ZSM-5) secondary building unit. The Fe(III) in FeMMSH materials is present in tetrahedral, and distorted tetrahedral framework are evident from EPR and DRUV-VIS spectra.

Pore characteristics of micro and mesoporous materials probed with CO2 adsorption measurement at 273 K.

CO2 adsorption measurement at 273 K was employed to see if the porous material has either micropore or mesopore dominantly. The adsorption isotherm of CO2 on microporous solid such as zeolite A and Y was the typical Langmuir type while on SBA-15, silica gel and polymer, the adsorption isotherm was proportional to the applied pressure, following the Henry's law. Such distinctive difference between the adsorption isotherms provided the reasonable basis to discriminate the pore characteristics otherwise it was difficult to obtain it. Also, it was possible to get the surface property of polymer due to the pretreatment using CO2 adsorption measurement at 273 K while the N2 adsorption isotherm measurement at 77 K did not give the meaningful data.