Infrared and solid state NMR studies of the adsorption of phosphorus compounds on oxides.

Abstract

Infrared and Solid State NMR spectroscopies have been used to study the reactions of $\rm PMe\sb{n}Cl\sb{3-n}$ (n = 0 to 2) with silica, the reactions of P(OMe)$\sb3$ and related compounds with alumina, as well as the adsorption of PMe$\sb3$ on a variety of oxides. PMe$\sb3$ interacts with silica activated at moderate temperatures via hydrogen bonding or physisorption only. PCl$\sb3$ interacts with silica at room temperature via a weak hydrogen bonding interaction. Previous assignments in the literature of two kinds of hydrogen bonded complex are shown to be in error. OPCl$\sb3$, often present as an impurity in PCl$\sb3$, is shown to be responsible for the infrared peaks that had previously been incorrectly assigned to a second type of adsorbed PCl$\sb3$. Prolonged room temperature reaction led to the formation of small amounts of $\equiv$Si-O-PCl$\sb2$, ($\equiv$SiO)$\sb2$P(H)=O, $\equiv$SiOP(H)OH, and another unidentified product. vskip18pt PMe$\sb2$Cl was much more reactive. It reacts to form $\equiv$Si-O-P(O)Me$\sb2$, removing most of the surface silanol groups in the process. The identity of this species could be verified by both IR and NMR. IR spectra of a thin film of $\sp $O exchanged silica showed that this compound definitely does extract oxygen from the silica surface. PMeCl$\sb2$ shows intermediate reactivity towards silica. Most of the surface silanol groups are consumed after four or five hours reaction time. The dominant phosphorus containing surface species is $\equiv$Si-O-P(O)(H)Me. Dimethyl methylphosphonate (DMMP) reacts with $\rm Al\sb2O\sb3$ by losing its methoxy groups in a sequential manner. For room temperature interaction molecular DMMP, either hydrogen bonded to the surface or coordinated to Lewis acidic aluminium atoms, is the predominant species. After reaction at 100$\sp\circ$C most of the DMMP molecules have lost one methoxy group. After 300$\sp\circ$C reaction all of the DMMP has lost both of its methoxy groups. Dimethyl phosphite (DMP) reacts in a similar manner to DMMP, although some P-Me groups form during the reaction, indicating that $\rm Al\sb2O\sb3$ can cleave the P-H bond of dialkyl phosphites. Trimethyl phosphite reacts with $\rm Al\sb2O\sb3$ to give all of the products observed in the reactions of DMMP and DMP with $\rm Al\sb2O\sb3$. Many of the magic-angle-spinning NMR spectra presented in this thesis were acquired with pulse sequences that result in the Total Suppression of Sidebands (TOSS). For completeness, a similar mathematical treatment of pulse sequences that produce Phase Adjusted Spinning Sidebands (PASS) is also presented. Two interesting new pulse sequences that produce Hahn echoes in rotating samples are demonstrated. (Abstract shortened by UMI.)