Infrared studies of silica surfaces.

Abstract

The silanol distributions on "as received" aerosil and precipitated silica have been studied (1) by using time-resolved infrared spectroscopy in order to follow a reaction with AlMe$\sb3$, TiCl$\sb4$, or BCl$\sb3$, (2) by spectroscopically comparing the accessibility of probe molecules having different steric dimensions, that react with surface silanols (SiOH) either by chemisorption or H/D exchange, and (3) by gravimetrically measuring surface silanol densities using vacuum microbalance techniques. The results have been used to compare and characterize these non-porous silicas of similar surface area. Initially, H-bonded silanols are relatively more reactive than isolated silanols in the order of reagents, BCl$\sb3$ $>$ TiCl$\sb4$ $>$ AlMe$\sb3$. In the fully hydroxylated "as received" state, the number of silanol groups on either silica which react with various hydrogen sequestering (HS) agents decreases as the size of the agent increases (ZnMe$\sb2$, BCl$\sb3$, TiCl$\sb4$, AlMe$\sb3$ and Me$\sb3$SiNHSiMe$\sb3$ (HMDS) in increasing size). The number of silanols that undergo H/D exchange also decreases as the size of the probe molecule increases (D$\sb2$O, ND$\sb3$, and deuterated methanol, i-propanol and t-butanol). The reaction between HMDS and H-bonded silanols occurs preferentially with the terminal silanols of a chain, and we propose that these silanols occupy sites which are inherently more accessible to this bulky reactant. By following the evolution of the SiOH bands on both silicas in all three spectral regions for 450, 600, and 800$\sp\circ$C activation, we have assigned bands to two types of isolated silanols. The type I silanols are preferentially eliminated between 450 and 800$\sp\circ$C activation and are more abundant on precipitated silica than on aerosil. Type II silanols (isolated single silanols) dominate the surface of highly activated aerosil or precipitated silica. The vibrational spectra are reported for the physically adsorbed complexes, SiOH...X (X = CO, N$\sb2$, and CH$\sb4$) and the nature of their specific interaction with surface SiOH is discussed. Mechanisms are proposed for the room-temperature reaction between SiOH and TMP to produce DMP, and the SiOH catalyzed isomerization of TMP to DMMP. (Abstract shortened by UMI.)