Functionalization Of Nanocarbons For Heterogeneous Catalysis

Nanocarbon materials have shown considerable potential in catalysis as supports or even catalysts,due to their adjustable pore structures,resistance to acid/alkali corrosion,good thermal and electrical conductivity.Based on the various covalent bonding of carbon atom(C-C,C-X,X are other nonmetallic atoms)and the ?-electronic conjugations,nanocarbon can be functionlized with active molecules or sites via convalent bonding on the carbon surface or in the carbon skelecton and non-convalent interaction to fabricate nanocarbon-based catalysts.For a specific catalytic reaction,the design and preparation of nanocarbon-based catalysts should consider the surface chemical properties,pore structures and the interaction styles between active molecules or sites and the carbon matrix,so as to ensure a good catalytic performance.Relying on the diversity of functialized pathways of nanocarbon,herein,we have prepared a series of nanocarbon-based catalysts through convalent grafting,heteroatom doping and non-convalent interaction.The catalytic performances of these catalysts have been fundamentally envaluated.The main research contents are as follows:(1)The covalent-bond grafting on the surface of nanocarbons is an effective way to achieve the heterogenization of chiral molecular catalysts.Herein,we developed a heterogeneous route for immobilization of the cinchona-alkaloid-based catalysts(quinindium-derived urea and quinine-squaramide)by using porous carbon nanosheet(PCN)as the support.Based on diazonium chemistry,the surface of PCN can be chemically modified,on which the two molecular catalysts were then convalently grafted by "click chemistry" and free radical polymerization,respectively.The as-prepared immobilized catalysts were evaluated in the asymmetric Friedel-Crafts reaction,and the vinyl group on the quinine ring was identified as the better grafting site,enabling the immobilized quinine-squaramide catalyst to exhibited homogeneous-like catalytic performance,providing the product with 94%yield and ee value greater than 99%.Furthermore,it showed good stability and still afforded the product in 90%yield with 98%ee after five consecutive recycles.This is the first example for the immobilization of quinine-squaramide catalyst by using carbon support.Due to its relatively chemical inertness and affinity with organic substrates,PCN is preferred over porous silica and activated carbon in this asymmetric catalytic reaction as a porous carbon support.(2)Heteroatom doping is another way to introduce the active sites into the nanocarbon frameworks by covalent bonding.An in-situ doping method by using solid-state ball milling combined with high-temperature pyrolysis was employed for synthesis of B,N co-doped porous carbons(B,N-Cs)derived from glutamic acid(Glu)and boric acid(H3BO3).The ball milling could promote the full contact between Glu and H3BO3,and even enhance their interaction,thus facilitating the uniform dispersion of B and N atoms into the carbon matrix.This method was proved to be efficient towards co-doping B and N with high contents of B(7 at.%)and N(10 at.%).H3BO3 used here also served as a self-template,and as its relative dosage increases,the pore structure of B,N-Cs occurred to change from micropores to mesopores with the specific surface area up to 940 m2 g-1.The B,N-Cs were applied as metal-free catalysts for the cycloaddition of CO2 to epoxides,which outperformed the N-doped carbon catalyst(NC-900)and the mixed catalyst of NC-900/B4C.The better activity is attributed to the cooperative effect between B and N sites.BN3 was identified as a critical active species for catalyzing the reaction.(3)Noncovalent modification is a simple way for the functionalization of nanocarbon.Through this strategy,ketjenblack(KCB),as a highly conductive carbon,was seleted and composited with meso-tetraphenyl metalloporphyrin(Metal-TPP).The ?-? interaction between them could ensure the effective electron transport during an electrocatalysis process.The Metal-TPP/KCB catalysts were employed for electrocatalytic CO2 reduction(CO2RR)in aqueous electrolyte.Compared with Fe-TPP,Ni-TPP/KCB catalysts,Co-TPP/KCB performed better,giving CO product with selectivity of 92%(?=0.486 V),and it also outperformed Co-TPP under a homogeneous condition.Based on the structural diversity of metalloporphyrin,Co-TPP with amino substituents were synthesized,and the corresponding catalyst supported by KCB exhited enhanced activity and stability for CO2RR.The investigation on the effect of pore structure of the carbon support on CO2RR indicated the importance of high specific surface area and open nanopore structure.In addition,compared with the immobilized cobalt porphyrin obtained by convalent grafting,the non-convalent based catalytst showed higher faradic efficiency and current density of CO,implying that non-convalent interaction avoids disturbing the inherent ?-system of the carbon support and is more favourable for electron transport to drive CO2 activation and reduction.Turnover frequency(TOFco)of Co-TPP with one amino substituent was investigated over the CO2 reduction to CO.When the loading amount of Co was 0.04 wt.%,TOFco reached 1841 h-1,and it could reach a higher value of 4990 h-1 at higher potential of-0.896 V(vs RHE).