Molecular evidence for sulfurization of molybdenum dithiocarbamates (MoDTC) by zinc dithiophosphates: a key process in their synergetic interactions and the enhanced preservation of MoDTC in formulated lubricants?

Abstract

Molybdenum dithiocarbamates (MoDTC) are widely used in automotive industries as lubricant additives to reduce friction and to enhance fuel economy. Sulfur-containing additives such as zinc dithiophosphates (ZnDTP) are proposed to play a key role in the improvement of friction reducing properties of MoDTC in formulated lubricants by facilitating the formation of MoS₂ tribofilm at the rubbing contacts. This study focuses on the interactions between MoDTC and ZnDTP under conditions comparable with those prevailing in operating engines. The capacity of ZnDTP to sulfurize MoDTC in solution in a hydrocarbon base oil could be demonstrated. Sulfurized Mo complexes bearing one or two additional sulfur atoms (1S-MoDTC and 2S-MoDTC, respectively) which have replaced the genuine oxygen atom(s) from the MoDTC core were detected and quantified using a specifically developed HPLC-MS analytical method. A possible sulfurization mechanism relying on the higher affinity of phosphorus from ZnDTP for oxygen could be proposed. In parallel, the evolution and molecular transformation of the prepared 2S-MoDTC in hydrocarbon base oil under thermal and thermo-oxidative conditions were followed using HPLC-MS and compared with the evolution of their friction coefficients. 2S-MoDTC complexes were shown to exhibit a better retention of friction reducing capability under oxidative conditions than the "classical" MoDTC, although they did not seem to significantly reduce the friction coefficients of lubricants as compared to the "classical" MoDTC. Therefore, sulfurization of MoDTC by ZnDTP might contribute to delaying the progressive consumption of MoDTC and the loss of their friction-reducing efficiency in lubricants under thermo-oxidative conditions.

Fig. 1. Sulfurization of MoDTC by (a) ZnDTP and (b) Lawesson's reagent.

Fig. 2. Chemical structures of lubricant additives cited in this work. (a) Molybdenum dithiocarbamates (MoDTC); (b) zinc dithiophosphates (ZnDTP); (c) Lawesson's reagent; (d) mono-sulfurized molybdenum dithiocarbamates (1S-MoDTC); (e) di-sulfurized molybdenum dithiocarbamates (2S-MoDTC).

Fig. 3. Mass spectra (m/z 900–1250, APPI, positive mode) showing the evolution of the reaction mixture obtained by heating MoDTC 1a–1c with (a) secondary ZnDTP 2b; (b) di-isooctyl ZnDTP 2c; (c) primary ZnDTP 2a in a concentrated medium at 150 °C after 0 h, 3 h and 6 h. % corresponds to the normalized intensity.

Fig. 4. Evolution of the relative abundances of (a and d) MoDTC substrates 1a–1c; (b and e) sulfurized MoDTC 1S-1a–1c and 2S-1a–1c formed during the heating experiment involving MoDTC 1a–1c and ZnDTP 2b and 2c, respectively, in lubricant base oil at 135 °C under argon atmosphere; (c and f) evolution of the proportion of sulfurized MoDTC (sum of 1S-1a–1c and 2S-1b–1c) relative to the sum of the (regular plus sulfurized) MoDTC. IS: Internal standard. *Y-axis: arbitrary units. Error bars (Fig. 4a–e) correspond to triplicate HPLC-MS analyses for each sample.

Fig. 5. Evolution of the relative abundance of MoDTC 2S-1a–1c in a hydrocarbon base oil at 135 °C under argon bubbling over a period of 18 h. IS: internal standard. Error bars correspond to the triplicate HPLC-MS analyses for each sample.

Fig. 6. Evolution of the relative abundance of sulfurized MoDTC 2S-1a–1c and of MoDTC 1a–1c upon heating at 135 °C in a hydrocarbon base oil under (a–c) air; (d–f) NO₂ (2000 ppm in air) bubbling. Error bars correspond to triplicate HPLC-MS analyses for each sample.

Fig. 7. Evolution of the concentration of MoDTC complexes 1S-1a–1c and MoDTC complexes 1a–1c as oxidative degradation products formed during the heating experiment involving 1 wt% of sulfurized MoDTC substrates 2S-1a–1c under (a and b) air bubbling at 135 °C and (c and d) NO₂ bubbling (2000 ppm in air) at 135 °C. IS: internal standard. *Y-axis: arbitrary units. Error bars correspond to triplicate HPLC-MS analyses for each sample.

Fig. 8. Evolution with time of the friction coefficient of MoDTC 1a–1c and di-sulfurized MoDTC 2S-1a–1c in solution in a hydrocarbon base oil subjected to oil ageing experiments under NO₂ (2000 ppm in air).

Fig. 9. Proposed mechanisms of sulfurization of MoDTC by ZnDTP through (a) direct exchange of O and S atoms between MoO and PS bonds; or (b) O and S exchange involving Mo-bound DTP ligands. I, III: ligand exchange reactions, II: rearrangement involving an exchange between a sulfur atom from a DTP ligand and an oxygen atom on the MoDTC core.