Calorimetric investigation of the adsorption of cationic dimeric surfactants with a hydrophilic spacer group onto silica.

6 pagesInternational audienceThe adsorption of dimeric cationic surfactants, made up of two ammonium species linked via a hydrophilic spacer group, i.e. Br−n-C12H25N+Me2-CH2(CH2OCH2)xCH2N+Me2-n-C12H25Br−, referred to as 12-EOx-12, onto silica has been studied in aqueous solution at 298 K and free pH. The effect of the molecular structure on the adsorption of these cationic surfactants onto raw silica (SiNa) and HCl-treated (SiH) silica was investigated. For this purpose, batch microcalorimetry measurements of the differential molar enthalpy of dilution and that of adsorption were performed at 298 K. The enthalpy data were supplemented by the classical observations such as the adsorption isotherms. The microcalorimetry results show that the adsorption mechanism and the aggregation at the solid–liquid interface do not depend on the spacer group length. Then, important quantitative differences are observed between the two silica surfaces. The investigated surfactants provide useful molecular probes for the characterization of the surface active sites onto treated and raw silicas. The amounts adsorbed allow the surface sites present in SiNa and SiH to be quantified

Microcalorimetric studies of cationic gemini surfactant with a hydrophilic spacer group

International audienceThe aim of this work is to investigate the micellization thermodynamics for gemini surfactants with a hydrophilic spacer group, i.e. Br− n-C12H25N+Me2single bondCH2(CH2OCH2)xCH2N+Me2single bondn-C12H25 Br−, referred to as 12-EOx-12, where x = 1, 3, 6, 7. These oligooxaalkanediyl-α,ω-bis(dimethyldodecylammonium bromide) dimeric surfactants were synthesized by a two step reaction. Their physicochemical properties in aqueous solutions have been studied by microcalorimetry at 298 K, electrical conductivity and surface tension measurements. The critical micellar concentration (cmc) and the head-group area (a0) per molecule at the air–water interface increase with increasing number of oxide ethylene moieties in the spacer and, consequently, with the enhanced hydrophilic character of the molecule. The calorimetric studies of the self-assembly process confirm this trend, since the exothermic values of the differential enthalpy of micellization (ΔmicH°) decrease as the ethylene oxide moieties are lengthened. Moreover, the values of the free energy and entropy of micellization indicate an entropy driven phenomenon for all surfactants. These results are compared with those obtained for gemini cationic surfactants with a hydrophobic spacer and the differences observed are discussed in terms of the hydrophilic head-group conformation in the micelle

Exploiting confinement effects to tune selectivity in cyclooctane metathesis

The mechanism of cyclooctane metathesis using confinement effect strategies in mesoporous silica nanoparticles (MSNs) is discussed by catalytic experiments and density functional theory (DFT) calculations. WMe6was immobilized inside the pores of a series of MSNs having the same structure but different pore diameters (60, 30, and 25 Ã\u85). Experiments in cyclooctane metathesis suggest that confinement effects observed in smaller pores (30 and 25 Ã\u85) improve selectivity toward the dimeric cyclohexadecane. In contrast, in larger pores (60 Ã\u85), a broad product distribution dominated by ring contracted cycloalkanes was found. The catalytic cycle and potential side reactions occurring at [(â\u89¡SiO-)WMe5] were examined with DFT calculations. Analysis of the geometries for the key reaction intermediates allowed us to rationalize the impact of a confined environment on the enhanced selectivity toward the dimeric product in smaller pores, while in large pores the ring contracted products are favored

A well-defined mesoporous amine silica surface via a selective treatment of SBA-15 with ammonia

2D double-quantum H-1-H-1 NMR unambiguously shows that the ``isolated'' Si-OH surface silanols of dehydroxylated SBA-15 are converted upon treatment with ammonia into single silylamine surface site Si-NH2. The ``gem'' di-silanols (= Si(OH)(2)) remain intact. Treatment using HMDS produces (= Si(OSiMe3)(2)) but leaves Si-NH2 untouched. The resulting surface is hydrophobic and stable

Contribution of 1H NMR to the investigation of the adsorption of cationic Gemini surfactants with oligooxyethylene spacer group onto silica.

7 pagesInternational audienceThe present study aims to investigate the behavior of a series of cationic Gemini surfactants with a hydrophilic spacer at liquid–gas and solid–liquid interfaces, with particular emphasis on the effect of spacer length. Gemini surfactants containing two quaternary ammonium groups bound by an ethylene oxide spacer chain, referred to as 12-EOx-12 with x=1,3,7 and 12 were synthesized. Surface tension measurements were used to show that the hydrophilic spacer with oxyethylene moieties was not fully extended at the air–water interface. With increasing the spacer group size, it became sufficiently flexible to adopt a particular conformation with a loop at the water side of the interface. A combined study by adsorption isotherm measurements and 1H NMR spectroscopy allowed a detailed description of the adsorption mechanism of these investigated 12-EOx-12 surfactants, with NMR providing more precise information on the conformation of hydrophilic spacer at the solid–liquid interface. Binding to the silica surface involved one cationic headgroup for the surfactants with a short spacer and the two headgroups for the ones with a long spacer. The number of charged surface sites was estimated by considering the dimeric surfactant as a “molecular ruler.” The small density of adsorption sites gave rise to the formation of pinned surface micelles

Single-Site Tetracoordinated Aluminum Hydride Supported on Mesoporous Silica. From Dream to Reality!

The reaction of mesoporous silica (SBA15) dehydroxylated at 700 degrees C with diisobutylaluminum hydride, i-Bu2AlH, gives after thermal treatment a single-site tetrahedral aluminum hydride with high selectivity. The starting aluminum isobutyl and the final aluminum hydride have been fully characterized by FT-IR, advanced SS NMR spectroscopy (H-1, C-13, multiple quanta (MQ) 2D H-1-H-1, and Al-27), and elemental analysis, while DFT calculations provide a rationalization of the occurring reactivity. Trimeric i-Bu2AlH reacts selectively with surface silanols without affecting the siloxane bridges. Its analogous hydride catalyzes ethylene polymerization. Indeed, catalytic tests show that this single aluminum hydride site is active in the production of a high-density polyethylene (HDPE)

C–H and C–C Activation of <i>n</i>‑Butane with Zirconium Hydrides Supported on SBA15 Containing N‑Donor Ligands: [(SiNH−)(SiX−)ZrH₂], [(SiNH−)(SiX−)₂ZrH], and[(SiN)(SiX−)ZrH] (X = −NH–, −O−). A DFT Study

Density functional theory (DFT) was used to elucidate the mechanism of <i>n</i>-butane hydrogenolysis (into propane, ethane, and methane) on well-defined zirconium hydrides supported on SBA15 coordinated to the surface via N-donor surface pincer ligands: [(SiNH−)­(SiO−)­ZrH₂] (<b>A</b>), [(SiNH−)₂ZrH₂] (<b>B</b>), [(SiNH−)­(SiO−)₂ZrH] (<b>C</b>), [(SiNH−)₂(SiO−)­ZrH] (<b>D</b>), [(SiN)­(Si–O−)­ZrH] (<b>E</b>), and [(SiN)­(SiNH−)­ZrH] (<b>F</b>). The roles of these hydrides have been investigated in C–H/C–C bond activation and cleavage. The dihydride <b>A</b> linked via a chelating [N,O] surface ligand was found to be more active than <b>B</b>, linked to the chelating [N,N] surface ligand. Moreover, the dihydride zirconium complexes are also more active than their corresponding monohydrides <b>C</b>–<b>F</b>. The C–C cleavage step occurs preferentially via β-alkyl transfer, which is the rate-limiting step in the alkane hydrogenolysis. The energetics of the comparative pathways over the potential energy surface diagram (PES) reveals the hydrogenolysis of <i>n</i>-butane into propane and ethane

Well-defined silica supported aluminum hydride: another step towards the utopian single site dream?

Reaction of triisobutylaluminum with SBA15(700) at room temperature occurs by two parallel pathways involving either silanol or siloxane bridges. It leads to the formation of a well-defined bipodal [( SiO)(2)Al-CH2CH(CH3) (2)] 1a, silicon isobutyl [ Si-CH2CH(CH3) (2)] 1b and a silicon hydride [ Si-H] 1c. Their structural identity was characterized by FT-IR and advanced solid-state NMR spectroscopies (H-1, C-13, Si-29, Al-27 and 2D multiple quantum), elemental and gas phase analysis, and DFT calculations. The reaction involves the formation of a highly reactive monopodal intermediate: [ SiO-Al[CH2CH(CH3)(2)](2)], with evolution of isobutane. This intermediate undergoes two parallel routes: transfer of either one isobutyl fragment or of one hydride to an adjacent silicon atom. Both processes occur by opening of a strained siloxane bridge, Si-O-Si but with two different mechanisms, showing that the reality of "single site" catalyst may be an utopia: DFT calculations indicate that isobutyl transfer occurs via a simple metathesis between the Al-isobutyl and O-Si bonds, while hydride transfer occurs via a two steps mechanism, the first one is a beta-H elimination to Al with elimination of isobutene, whereas the second is a metathesis step between the formed Al-H bond and a O-Si bond. Thermal treatment of 1a (at 250 degrees C) under high vacuum (10(-5) mbar) generates Al-H through a beta-H elimination of isobutyl fragment. These supported well-defined Al-H which are highly stable with time, are tetra, penta and octa coordinated as demonstrated by IR and Al-27-H-1 J-HMQC NMR spectroscopy. All these observations indicate that surfaces atoms around the site of grafting play a considerable role in the reactivity of a single site system

Bipodal surface organometallic complexes with surface N-donor ligands and application to the catalytic cleavage of C-H and C-C bonds in n-butane

International audienceWe present a new generation of "true vicinal" functions well-distributed on the inner surface of SBA15: [( Si-NH2)( Si-OH)] (1) and [( Si-NH2)(2)] (2). From these amine-modified SBA15s, two new well-defined surface organometallic species [( Si-NH-)( Si-O-)]Zr-(CH(2)tBu)(2) (3) and [( Si-NH-)(2)]Zr(CH(2)tBu)(2) (4) have been obtained by reaction with Zr(CH(2)tBu)(4). The surfaces were characterized with 2D multiple-quantum H-1-H-1 NMR and infrared spectroscopies. Energy-filtered transmission electron microscopy (EFTEM), mass balance, and elemental analysis unambiguously proved that Zr(CH(2)tBu)(4) reacts with these vicinal amine-modified surfaces to give mainly bipodal bis(neopentyl)zirconium complexes (3) and (4), uniformly distributed in the channels of SBA15. (3) and (4) react with hydrogen to give the homologous hydrides (5) and (6). Hydrogenolysis of n-butane catalyzed by these hydrides was carried out at low temperature (100 degrees C) and low pressure (1 atm). While (6) exhibits a bis(silylamido)zirconium bishydride, [( Si-NH-)(2)]Zr(H)(2) (6a) (60%), and a bis(silylamido)silyloxozirconium monohydride, [( Si-NH-)(2)(Si-O-)]ZrH (6b) (40%), (5) displays a new surface organometallic complex characterized by an H-1 NMR signal at 14.46 ppm. The latter is assigned to a (silylimido)(silyloxo)zirconium monohydride, [( Si-N=)( Si-O-)]ZrH (5b) (30%), coexistent with a (silylamido)(silyloxo)zirconium bishydride, [( Si-NH-) ( Si-O-)]Zr(H)(2) (5a) (45%), and a silylamidobis(silyloxo)zirconium monohydride, [( Si-NH-)( Si-O-)(2)]ZrH (5c) (25%). Surprisingly, nitrogen surface ligands possess catalytic properties already encountered with silicon oxide surfaces, but interestingly, catalyst (5) with chelating [N,O] shows better activity than (6) with chelating [N,N]

Bipodal Surface Organometallic Complexes with Surface N-Donor Ligands and Application to the Catalytic Cleavage of C-H and C-C Bonds in n-Butane

We present a new generation of ``true vicinal'' functions well-distributed on the inner surface of SBA15: [( Si-NH2)( Si-OH)] (1) and [( Si-NH2)(2)] (2). From these amine-modified SBA15s, two new well-defined surface organometallic species [( Si-NH-)( Si-O-)]Zr-(CH(2)tBu)(2) (3) and [( Si-NH-)(2)]Zr(CH(2)tBu)(2) (4) have been obtained by reaction with Zr(CH(2)tBu)(4). The surfaces were characterized with 2D multiple-quantum H-1-H-1 NMR and infrared spectroscopies. Energy-filtered transmission electron microscopy (EFTEM), mass balance, and elemental analysis unambiguously proved that Zr(CH(2)tBu)(4) reacts with these vicinal amine-modified surfaces to give mainly bipodal bis(neopentyl)zirconium complexes (3) and (4), uniformly distributed in the channels of SBA15. (3) and (4) react with hydrogen to give the homologous hydrides (5) and (6). Hydrogenolysis of n-butane catalyzed by these hydrides was carried out at low temperature (100 degrees C) and low pressure (1 atm). While (6) exhibits a bis(silylamido)zirconium bishydride, [( Si-NH-)(2)]Zr(H)(2) (6a) (60%), and a bis(silylamido)silyloxozirconium monohydride, [( Si-NH-)(2)(Si-O-)]ZrH (6b) (40%), (5) displays a new surface organometallic complex characterized by an H-1 NMR signal at 14.46 ppm. The latter is assigned to a (silylimido)(silyloxo)zirconium monohydride, [( Si-N=)( Si-O-)]ZrH (5b) (30%), coexistent with a (silylamido)(silyloxo)zirconium bishydride, [( Si-NH-) ( Si-O-)]Zr(H)(2) (5a) (45%), and a silylamidobis(silyloxo)zirconium monohydride, [( Si-NH-)( Si-O-)(2)]ZrH (5c) (25%). Surprisingly, nitrogen surface ligands possess catalytic properties already encountered with silicon oxide surfaces, but interestingly, catalyst (5) with chelating [N,O] shows better activity than (6) with chelating [N,N]