Abstract
Pyrolysis and low-temperature oxidation of dimethoxymethane (methylal, MeOCH2OMe) play an important role in the ignition of blended diesel fuels, but the underlying mechanisms are still debated. In these kinetic models, bimolecular hydrogen abstraction or unimolecular C–O bond fission are considered as the primary initial steps, while MeOCH2OMe isomerization is sometimes disregarded. In this work, we investigate the pyrolysis of MeOCH2OMe combining imaging photoelectron photoion coincidence spectroscopy with vacuum ultraviolet (VUV) synchrotron radiation and CBS-QB3 theoretical calculations to unveil reaction paths and energetics. In the mass spectrum of MeOCH2OMe, pyrolysis products and radical intermediates were observed at m/z 15 (CH3), 28 (CO), 29 (HCO), 30 (H2CO), 31 (CH2OH), 32 (CH3OH), 45 (CH3OCH2), and 75 (H-loss from methylal). Only the m/z 45 and 75 ions are found to be dissociative photoionization products of MeOCH2OMe, the other mass spectral peaks are attributed to ionization of the neutral MeOCH2OMe pyrolysis products. The m/z 31 peak was assigned to the methoxy radical in the previous studies. However, our photoion mass-selected threshold photoelectron spectrum (ms-TPES) confirms that it originates from dissociative photoionization of the primary pyrolysis fragment methanol. Based on the experimental and computational results, a thermal decomposition mechanism of MeOCH2OMe is proposed. Here, H-migration precedes the production of methoxymethylene (CH3OCH) and methanol, while dimethyl ether and formaldehyde are probably formed in multi-step processes, too. The sequential dissociation of CH3OCH and of dimethyl ether yields enhanced m/z 15, 28 and 29 signals at high temperature. Rate constants have been calculated to confirm the dominant role of MeOCH2OMe isomerization and to help improve predictive combustion models.
Highlights
Oxygenated hydrocarbon additives are blended into gasoline and diesel fuels to reduce carbon monoxide and unburned hydrocarbon emissions [1,2,3,4,5,6]
The most stable products and some radical intermediates produced in pyrolysis of MeOCH2OMe were observed, e.g. m/z 15, 28, 29, 30, 31, 32, 45 and 75, and their temperature dependence was analyzed
The photoionization mass spectra of MeOCH2OMe with and without pyrolysis confirm that the observed m/z 45 and 75 ions originate from dissociative ionization (DPI) of MeOCH2OMe
Summary
Oxygenated hydrocarbon additives are blended into gasoline and diesel fuels to reduce carbon monoxide and unburned hydrocarbon emissions [1,2,3,4,5,6]. Unimolecular processes may play a relatively subdued role in comparison with bimolecular hydrogen abstraction by fuel radicals [24] It has been seen in the unimolecular decomposition of numerous, mid-sized organic species, such as dimethyl methylphosphonate [25] and resorcinol [26] that key products are formed in dissociation processes following isomerization steps, while simple bond scissions are often unfavorable. With the aid of the high-level quantum chemical calculations and kinetic modeling, a thermal decomposition mechanism is proposed for MeOCH2OMe. Golka et al [20], adressed this question further by theoretical analysis on the branching ratio of C–O bond fissions. Neither discussed the mechanism and energetics of the CH3OCH3 + H2CO and
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