The Underlying Chemistry to the Formation of PO₂ Radicals from Organophosphorus Compounds: A Missing Puzzle Piece in Flame Chemistry

Abstract

Reactive species, such as .PO2 and HOPO, are considered of upmost importance in flame inhibition and catalytic combustion processes of fuels. However, the underlying chemistry of their formation remains speculative due to the unavailability of suitable analytical techniques that can be used to identify the transient species which lead to their formation. This study elucidates the reaction mechanisms of the formation of phosphoryl species from dimethyl methyl phosphonate (DMMP) and dimethyl methyl phosphoramidate (DMPR) under well‐defined oxidative conditions. Photoelectron photoion coincidence techniques that utilized vacuum ultraviolet synchrotron radiation were applied to isomer‐selectively detect the elusive key intermediates and stable products. With the help of in situ recorded spectral fingerprints, different transient species, such as PO2 and triplet O radicals, have been exclusively identified from their isomeric components, which has helped to piece together the formation mechanisms of phosphoryl species under various conditions. It was found that .PO2 formation required oxidative conditions above 1070 K. The combined presence of O2 and H2 led to significant changes in the decomposition chemistry of both model phosphorus compounds, leading to the formation of .PO2. The reaction .PO+O2→.PO2+O: was identified as the key step in the formation of .PO2. Interestingly, the presence of O2 in DMPR thermolysis suppresses the formation of PN‐containing species. In a previous study, PN species were identified as the major species formed during the pyrolysis of DMPR. Thus, the findings of this study has shed light onto the decomposition pathways of organophosphorus compounds, which are beneficial for their fuel additive and fire suppressant applications.

Review of Phosphorus Chemistry in the Thermal Conversion of Biomass: Progress and Perspectives

“…However, in recent work, the intermediates as stated by Korobeinichev,207 shown in Figure 16, could not be evidenced when combusting DMMP in a H 2 /O 2 environment up to about 1000 °C. 274 The addition of H 2 did not greatly affect the decomposition products compared to pyrolysis of DMMP, but the addition of H 2 /O 2 shifted the decomposition products toward PO 2 and HOPO. 274 The observed differences might be a consequence of the lower temperature applied resulting in primarily intramolecular decomposition rather than through reactions with flame radicals.…”

“…274 The addition of H 2 did not greatly affect the decomposition products compared to pyrolysis of DMMP, but the addition of H 2 /O 2 shifted the decomposition products toward PO 2 and HOPO. 274 The observed differences might be a consequence of the lower temperature applied resulting in primarily intramolecular decomposition rather than through reactions with flame radicals.…”

Photoelectron Photoion Coincidence Spectroscopy Provides Mechanistic Insights in Fuel Synthesis and Conversion

“…Also other OPCs such as dimethyl phosphoramidate (DMPR) did not yield PO 2 but rather PO and PN species . Only recently did Liang et al show that oxygen is indeed needed to produce PO 2 in the gas phase, as proven by TPES detection of PO 2 (Figure (b)) and O ( 3 P) . In addition, the hypophosphorous acid (HOPO) signal increased in the mass spectrum when hydrogen and oxygen were co-fed.…”

“…While PO is the unimolecular reaction product of DMMP, PO 2 could only be observed upon addition of oxygen to the reaction mixture. Taken with permission from refs and . Copyright John Wiley and Sons.…”

“…210 Only recently did Liang et al show that oxygen is indeed needed to produce PO 2 in the gas phase, as proven by TPES detection of PO 2 (Figure 27(b)) and O ( 3 P). 211 In addition, the hypophosphorous acid (HOPO) signal increased in the mass spectrum when hydrogen and oxygen were co-fed. The decomposition and oxidation of DMMP to yield PO, PO 2 , and HOPO can thus be summarized according to R26 and reveals mechanisms to control combustion processes.…”