Kinetics of oxidation of an organic amine with a Cr(V) salen complex in homogeneous aqueous solution and on the surface of mesoporous silica

Abstract

A comparative study of catalytic activity under homogeneous and heterogeneous conditions was carried out using the (salen)Cr(III)-catalyzed oxidation of tetramethylbenzidine (TMB) with iodosobenzene as a model reaction. Amine-functionalized mesoporous silica nanoparticles (MSN) were synthesized in a co-condensation reaction and functionalized with salen via a covalent Si-C bond. A Cr(III) complex of this supported ligand, MSN-(salen)Cr(III), was prepared and characterized. Data from powder XRD, BET isotherms and BJH pore size distribution all showed that MSN-(salen)Cr(III) still had the typical MSN high surface area, narrow pore size distribution, and ordered hexagonal pore structure, which were further confirmed by transmission electron microscopy (TEM) images. (13)C and (29)Si solid-state NMR data provided structural information about the catalyst and verified successful functionalization of the salen ligand and coordination to Cr(III). No unreacted salen or Cr(III) were observed. The loadings of salen and salen-Cr(III) complex were determined via TGA and EDX, respectively. Both measurements indicated that approximately 0.5 mmol/g of catalyst was loaded on the surface of MSN. The oxidation of TMB with iodosobenzene using MSN-(salen)Cr(III) as a heterogeneous catalyst exhibited both similarities and differences with the analogous homogeneous reaction using (salen)Cr(III)(H(2)O)(+) as a catalyst in aqueous acetonitrile. In the presence of 0.10 M HClO(4), the two catalytic reactions proceeded at similar rates and generated the doubly oxidized product TMB(2+). In the absence of acid, the radical cation TMB (+) was produced. The kinetics of the heterogeneous reaction in the absence of added acid responded to concentrations of all three reagents, i.e. (salen)Cr(III), TMB, and PhIO.

Investigating the Interaction of Water Vapour with Aminopropyl Groups on the Surface of Mesoporous Silica Nanoparticles

“…However,ahomogenous distribution of the graftinga gent along the whole length of the channels is unlikely,d ue to diffusion problems and the high reactivity of APTS, which might result in ah igher concentration of aminopropyl groups at the pore entrances. [63] An increasei n the relative intensity of Q 2 is observed in NH 2 -MSN with re- www.chemphyschem.org spect to MSN. The 29 Si MAS NMR spectrum of pristine MSN (Figure 2a)s hows signals for Q 2 (Si(OSi) 2 (OH) 2 ), Q 3 (Si(OSi) 3 OH)a nd Q 4 (Si(OSi) 4 )s ites at À92, À102 and À111 ppm, respectively.I n NH 2 -MSN (Figure 2b), besides the Qs ites (Q 4 at À110, Q 3 at À101 and Q 2 at À92 ppm), Tsilicon sites [T 3 (RSi(OSi) 3 )a tÀ67 and T 2 (RSi(OSi) 2 OH) at À60 ppm;Ri sa na minopropyl group] are also present.…”

“…Szajna-Fuller et al found ar elative concentration of Tsites in their organic-MSN materials of 14 %, which is comparable with that of 13 %f or NH 2 -MSN. [63] An increasei n the relative intensity of Q 2 is observed in NH 2 -MSN with re- www.chemphyschem.org spect to MSN. This can be explained by the amine-assisted partial hydrolysis of siloxane bonds, as observed by Etienne et al [64] The graftingd ensity of the functionalised material, as determined by elemental analysiso fn itrogen, was found to be 0.73 aminopropyl groups per squaren anometre.…”

Facile Strategy for the Synthesis of Gold@Silica Hybrid Nanoparticles with Controlled Porosity and Janus Morphology

“…This was also confirmed by SAXS (Figure 1c,d): the diffractogram reflections at 2 θ = 2.58, 4.40, and 5.10° can be indexed, respectively, to the (100), (110), and (200) planes of a hexagonal pore array. Nevertheless, among the various research groups employing the high-temperature protocol described for system A (no sodium silicate addition), some reported highly crystalline samples [61,62,63,64,65,66]. Silicate salts are a common impurity in sodium hydroxide pellets [66], and we deem their presence to be one of the possible causes of the capricious nature of this method.…”

Chemistry in the Center for Catalytic Hydrocarbon Functionalization: An Energy Frontier Research Center

“…Appending multiple groups to the mesoporous silica nanoparticles (MSN) in addition to the catalyst allows for selectivity to be controlled by physiochemical properties such as polarity and hydrophobicity [42]. Rhodium and chromium catalysts have been supported on MSN and are competent for the desired catalytic transformations [43,44]. For example, bifunctionalized MSN materials with 3-[2-(2-aminoethylamino)ethylamino]propyl and allyl groups attached via siloxy linkages can control the selectivity in competitive nitroaldol condensation reactions (15) [42].…”