Interactions of chlorosilanes with a silica surface catalyzed by amines

Abstract

Experimental and theoretical investigations of trimethylchlorosilane (TMCS) interactions with =SiOH groups catalyzed by NR3 (R = C2H5, CH3) have been conducted using Aerosil samples. The potential energy surface (PES) profiles along the reaction pathway were calculated in a cluster approach by the MNDO/H and AMI methods. The comparison with CIS1H3 reactions showed that the formation of fourcentered stabilized prereaction complexes is possible unlike TMCS reactions with =SiOH groups. The catalytic effect was caused by the enthalpy factor of the rate constants predominantly. Noncatalytic reaction rate constants were calculated and compared with experimental data. interact with silica surfaces at temperatures above 570 K substitute H atoms in silanol groups at catalysts by triethylamine (TEA) practically instantaneously at T « 300 K.* 1 Similar rapid reactions were observed with alcoholization, fluorination, and hydrolysis on silica surface catalyzed by electron-donor compounds: amines, ethers, alcohols, ketones, H2O, HC1.2 The problem of catalytic effect mechanisms by electrondonor compounds at chemosorption processes remains disputable. There exists the opinion that in electrophilic substitution reactions (SeO (for example, haloidsilanes with silica) the formation of hydrogen bonds =Si-OH-XY with electron-donor molecules XY leads to polarization of the 0-H bonds and increases its ability to heterolytic splitting of the Si-Hal bond in adsorbate. At a limiting reaction stage, the sorption complex included a silanol group, haloidsilane, and electron-donor molecule, and it may be considered an intermediate of the Seí reaction.1•2 This reaction was studied on example of the system =SiOH + ClSi(CH3)3 + N(C2H5)3(1) Noncatalytic reactions =SiOH + X"Si(CH3)4_" --SiOSiX^íCHg)^+ HX X = Cl, OCH3; = 1-3 were studied by experimental methods,3-5 but activation energies obtained were strongly different and the investigations of reaction mechanism (1) were scanty.1 45'2 6'7 Experimental SectionMaterials. Dispersed pyrogenic Si02 Aerosil (A-300,99.9%) was obtained from PU "ChloroviniT (Kalush), and its BET surface area of300 m2/g was measured by using nitrogen. Silanol * Author to whom correspondence should be addressed.

Cationic electrolyte adsorption layers on hydrophilic and hydrophobic surfaces

“…Thus, the coatings applied on the HMDS modified capillaries by the above described procedure may be considered stable and hydrophobic. CPE adsorption was studied by the capillary elec trokinetics method [16][17][18][19][20][21][22][23][24][25][26][27][28][29], which makes it possible, based on the measurements of the streaming potential and current in a thin quartz capillary, to calculate the electrokinetic potential (ζ potential) by the Helm holtz-Smoluchowski equation ζ = ηK × ΔE/εε 0 ΔP, where ΔE is the streaming potential, which is deter mined from a plot of the voltage current characteristic of a capillary; ε is the dielectric permittivity of a medium; ε 0 is the dielectric constant; ΔP is the pres [10]; in this case, the intercharge distance is 5.2 Å [30].…”

“…Modification of both porous and nonporous quartz with OSCs has been widely investigated using IR and NMR spectroscopy and other methods [15][16][17][18]. The properties of a modified surface have appeared to depend on the amount of functional groups grafted to it, while the reactivity of residual free silanol groups to be substantially lower than that of the groups on a pure surface.…”

Some peculiarities of the surface chemical modification of the high‐dispersed silica fillers

“…favourable opportunities are provided for the reaction to proceed under mild conditions. More confident results were obtained (Ref 9) by analyses of tlie potential energy surface profiles along tlie reaction pathways for interaction of trimethylclilorosilane with silanol group catalyzed by N(CH3)x. It was established that the reaction rate is provided mainly by an enthalpy factor of the rate constant and by transition state stabilization as a result of threecentred H-bond formation with participation of an organic base molecule.…”

“…4 3 ) but also the whole dynamic picture of the rupture and formation of bonds in a transient complex. During the past few years by adiabatic method of the reaction co-ordinate Gun'ko et al (Refs,8,9) have shown that for the substitution reaction with participation of silanol groups of the silica surface the rate of the chemical process is limited by proton transfer in four-centre complexes postulated for the transient states. It is believed that for systems in lower vibrational conditions such processes proceed by the tunnelling mechanism.…”

Competitive adsorption

“…The reaction temperature plays an important role relative to adsorption in the functionalization of a surface in the case of binary or multicomponent adsorption when one of the adsorbates is a catalyst such as triethylamine since the lifetime of the adsorbed complexes decreases with increasing temperature and the catalytic effect disappears at some critical value [175][176][177]. Similar effects are characteristic for the heterogeneous catalysis of organic synthesis.…”