Pyrolysis of Hexamethyldisiloxane

The pyrolysis of hexamethyldisiloxane (HMDSO) was studied behind reflected shock waves (1400–1500 K, 0.28–12.0 atm) with molecular beam sampling and time-of-flight mass spectrometric (TOF-MS) detection. The higher-pressure experiments used tunable synchrotron vacuum ultraviolet (SVUV) photoionization while the lower-pressure experiments employed electron-impact ionization. The results yielded complementary time-resolved information about the species formed during HMDSO pyrolysis, allowing improved understanding of the dominant reaction pathways. Additional SVUV-TOF-MS experiments, without time resolution and using a SiC microreactor (290 and 1273 K), yielded higher resolution photoionization mass spectra and improved signal-to-noise for small but important mass spectral features. The observed pyrolysis products included H2, SiC, methane, acetylene, ethylene, trimethylsilanol and several silicon-containing molecules and radicals smaller than HMDSO. H2 and SiC were observed for the first time during HMDSO pyrolysis. Almost negligible peaks were observed for species larger than HMDSO indicating that growth pathways analogous to polyaromatic hydrocarbon formation were unfavorable in the shock tube and microreactor experiments. This is contrary to literature studies at lower temperatures and longer residence times. The experimental work was complemented by a theoretical study of the initiation pathways in HMDSO pyrolysis. Over the range of conditions studied here, the dominant dissociation channel (∼95%) is Si–C bond fission to make CH3 radicals. Minor initiation reactions include a molecular channel yielding 2-silaisobutene + trimethylsilanol and C–H bond fission yielding H + (CH3)3SiOSi(CH3)2CH2. Pressure falloff in these unimolecular initiation reactions was negligible at T < 1500 K but persists at higher temperatures, even in this large molecule. Mass spectral features consistent with tetramethylsilane and trimethylsilanol were observed, but not their more reactive coproducts. The present experimental and theoretical studies provide considerable insights into the initial pyrolysis mechanism of an important silica nanoparticle precursor and provide a new foundation for extrapolation to other fully methylated siloxanes.