Galarneau Anne, Cangiotti Michela, di Renzo Francesco, Fajula François, Ottaviani M Francesca
Institute of Chemical Sciences, University of Urbino, P.zza Rinascimento 6, 61029 Urbino, Italy.
J Phys Chem B. 2006 Mar 9;110(9):4058-65. doi: 10.1021/jp056186b.
The computer aided analysis of the EPR spectra of radical surfactant probes inserted in cetyltrimethylammonium bromide micelles provided information on the kinetics of formation of micelle templated silico-aluminas (MTSA) at 343 K, obtained by means of silica and alumina alkaline solutions at different Si/Al ratios (from infinity to 4). Mainly two spectral components were analyzed and relatively quantified in the EPR spectra: (1) the micellar component, due to probes inserted in the surfactant aggregates, whose mobility decreases over the synthesis time, thus reporting on the progressive modification of the micelle structure and the solid condensation; (2) the interacting component, mainly arising from the electrostatic interactions between the surfactant heads and the charged surface sites. This last component increases its relative intensity over the synthesis time, informing about condensation and structuration of the silico-alumina at the micelle surface. X-ray diffraction (XRD), nitrogen sorption isotherms at 77 K, thermogravimetric analysis, TEM and chemical analysis were performed to characterize both as-synthesized and calcined MTSA materials. Nitrogen sorption isotherms allowed us to evaluate the pore diameter, the specific surface area and the pore volume. At Si/Al<15 a decrease in pore volume and specific surface area was interpreted as due to the contemporaneous presence of a hexagonal MTSA and an amorphous material, which was ascertained by means of XRD as the only present at Si/Al=4. The amorphous structure at Si/Al<15 used Na+ as contraions, whereas the surfactants are no more needed to neutralize the negatively charged groups at the solid surface. The hypothesis of a "break" at Si/Al=15 was supported by EPR: the interactions between the surfactant probe heads and the negatively charged surface groups are drastically reduced at Si/Al<15. On the contrary, at Si/Al>15, increasing amounts of alumina slow the kinetics of the synthesis but enhance electrostatic interactions between the surfactant heads and the negatively charged surface groups. Dilution of the synthesis mixture decreased the extent of the interactions, due to partial protonation of the silanol groups, and slowed the synthesis process.
对插入十六烷基三甲基溴化铵胶束中的自由基表面活性剂探针的电子顺磁共振（EPR）谱进行计算机辅助分析，提供了在343 K下通过不同Si/Al比（从无穷大到4）的二氧化硅和氧化铝碱性溶液获得的胶束模板硅铝酸盐（MTSA）形成动力学的信息。主要分析了EPR谱中的两个光谱成分并进行了相对定量：（1）胶束成分，归因于插入表面活性剂聚集体中的探针，其迁移率在合成过程中降低，从而反映了胶束结构的逐渐改变和固体缩合；（2）相互作用成分，主要源于表面活性剂头部与带电表面位点之间的静电相互作用。最后这个成分在合成过程中其相对强度增加，说明了硅铝酸盐在胶束表面的缩合和结构化情况。进行了X射线衍射（XRD）、77 K下的氮吸附等温线、热重分析、透射电子显微镜（TEM）和化学分析，以表征合成态和煅烧后的MTSA材料。氮吸附等温线使我们能够评估孔径、比表面积和孔体积。在Si/Al<15时，孔体积和比表面积的降低被解释为由于同时存在六方MTSA和一种无定形材料，通过XRD确定在Si/Al = 4时仅存在无定形材料。Si/Al<15时的无定形结构使用Na⁺作为抗衡离子，而不再需要表面活性剂来中和固体表面带负电的基团。EPR支持了在Si/Al = 15处存在“突变”的假设：在Si/Al<15时，表面活性剂探针头部与带负电表面基团之间的相互作用急剧降低。相反，在Si/Al>15时，氧化铝含量的增加减缓了合成动力学，但增强了表面活性剂头部与带负电表面基团之间的静电相互作用。合成混合物的稀释由于硅醇基团的部分质子化而降低了相互作用的程度，并减缓了合成过程。
