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. 2020 Oct 14;10(62):37962-37973.
doi: 10.1039/d0ra07329f. eCollection 2020 Oct 12.

Ligand exchange processes between molybdenum and zinc additives in lubricants: evidence from NMR (1H, 13C, 31P) and HPLC-MS analysis

Yu Min Kiw  1   2 , Philippe Schaeffer  1 , Pierre Adam  1 , Benoît Thiébaut  2 , Chantal Boyer  2 , Géraldine Papin  2
Affiliations

Ligand exchange processes between molybdenum and zinc additives in lubricants: evidence from NMR (1H, 13C, 31P) and HPLC-MS analysis

Yu Min Kiw et al. RSC Adv. .

Abstract

The tribological performances of engine oils have been shown to be enhanced by the synergistic interactions between Mo dithiocarbamates (Mo(DTC)2) with other additives, and notably Zn dithiophosphates (Zn(DTP)2). Being two key components in formulated lubricants, a detailed understanding of the mechanisms involved between these two types of additives is needed to develop engine oils with enhanced friction reduction performances, and improved fuel economy. In this context, we report here the investigation at the molecular level of the interactions between Mo and Zn complexes with DTC and DTP ligands using laboratory experiments. Our analytical approach comprised NMR spectroscopy (1H, 13C, 31P) allowing direct investigation of both homoleptic and heteroleptic Mo and Zn complexes as well as a specifically-developed HPLC-MS method for the investigation of the different DTC species formed during lubricant ageing experiments. The results showed that ligand exchange reactions between Mo(DTP)2 and Zn(DTC)2 complexes strongly favor the migration of the DTC ligands from Zn to Mo, illustrating the higher affinity of Mo for DTC ligands. In the case of binary mixtures involving Mo(DTC)2 and Zn(DTP)2 - a combination of additives frequently used in formulated lubricants - the formation of mixed complexes (Mo(DTC)(DTP)) resulting from ligand exchange reactions could be directly evidenced for the first time by the analytical methods used. These species could account, at least to some extent, for the synergistic effect of Mo(DTC)2 and Zn(DTP)2 on the friction reducing properties of engine oils. However, they were formed in significantly lower proportions than those previously reported in the literature using indirect methods.

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Conflict of interest statement

There are no conflicts to declare.

Figures

Fig. 1
Fig. 1. Ligand exchange reactions between Mo(DTC)2 and Zn(DTP)2 (adapted from Jensen et al.).
Fig. 2
Fig. 2. Chemical structures of lubricant additives cited in this work. (a) Molybdenum dithiophosphates Mo(DTP)2; (b) molybdenum dithiocarbamates Mo(DTC)2; (c) zinc dithiocarbamates Zn(DTC)2; (d) primary and secondary alkyl zinc dithiophosphates Zn(DTP)2; (e) Mo complexes with mixed dithiophosphate and dithiocarbamate ligands Mo(DTP)(DTC).
Fig. 3
Fig. 3. Partial 1H-NMR spectra (3.0–5.0 ppm, 500 MHz, CDCl3, 25 °C) of the reference additives 1, 2d, 3a and 4a used to investigate DTC/DTP ligand exchange reactions. *: Impurities.
Fig. 4
Fig. 4. Partial NMR spectra (CDCl3, 25 °C) of a mixture of Zn(DTC)23a and Zn(DTP)24a in a 1 : 1 molar ratio after 1 h reaction. (a) 1H-NMR spectrum (3.0–5.0 ppm, 500 MHz); (b) 31P-NMR spectrum (90–110 ppm, 121 MHz). Green colour: Zn(DTP)2, blue colour: Zn(DTC)2 (cf.Fig. 3). *: Impurities.
Fig. 5
Fig. 5. (a) Monomeric and dimeric forms of Zn(DTP)2 complexes in equilibrium in solution (adapted from Harrison et al.); (b) various monomeric and dimeric complexes in equilibrium in solution formed by reaction of Zn(DTP)2 with Zn(DTC)2. For P atoms, at least 8 types of different chemical environments are potentially distinguishable by 31P NMR. I, II, and III (a) correspond to P atoms with 3 different chemical environments distinguishable by 31P-NMR at low temperature.
Fig. 6
Fig. 6. Partial 31P-NMR spectra (90–110 ppm, 243 MHz, D8-toluene) of a mixture of Zn(DTC)23a and Zn(DTP)24a in a 1 : 1 molar ratio after 24 h at (a) −10 °C; (b) −40 °C; (c) −80 °C.
Fig. 7
Fig. 7. Partial 1H-NMR spectra (3.0–5.0 ppm, 500 MHz, CDCl3, 25 °C) of a mixture of Mo(DTP)21 and Zn(DTC)23a in a 4 : 1 molar ratio after (a) 0.5 h; (b) 18 h; (c) 47 h. *: Impurities.
Fig. 8
Fig. 8. Partial NMR spectra (CDCl3, 25 °C) of a mixture of Mo(DTP)21 and Zn(DTC)23a in a 4 : 1 molar ratio after 47 h. (a) 31P-NMR spectrum (94–115 ppm, 121 MHz); (b) 13C-NMR spectrum (70–74 ppm, 125 MHz).
Fig. 9
Fig. 9. Partial 1H-NMR spectrum (3.0–5.0 ppm, 500 MHz, D8-toluene, 105 °C) of a mixture of Mo(DTP)21 and Zn(DTC)23a in a 4 : 1 molar ratio after 15 min. *: Impurities.
Fig. 10
Fig. 10. Partial NMR spectra (D8-toluene, 105 °C) of a mixture of Zn(DTP)24a and Mo(DTC)22d in a 4 : 1 molar ratio after 15 min. (a) 1H-NMR spectrum (3.0–5.0 ppm, 500 MHz); (b) 31P-NMR spectrum (90–115 ppm, 162 MHz). *: Impurities.
Fig. 11
Fig. 11. Relative concentrations of Mo(DTC)22a–2c in solution in a hydrocarbon base oil at 135 °C under an argon atmosphere over a 6 h time period as determined using HPLC-MS. IS: internal standard. *Y-axis: arbitrary units.
Fig. 12
Fig. 12. Evolution of the relative concentrations of Mo(DTC)2 species over time in the presence of Zn(DTC)23a in lubricant base oil at 135 °C. (a) Mo(DTC)2 substrates 2a–2c; (b) Mo(DTC)22e–2g newly formed by ligand exchange with Zn(DTC)23a. The relative concentrations of Mo(DTC)2 have been determined using HPLC-MS. IS: internal standard. *Y-axis: arbitrary units.
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References

    1. Spikes H. A. Friction modifier additives. Tribol. Lett. 2015;60:5. doi: 10.1007/s11249-015-0589-z. - DOI
    1. De Barros Bouchet M. I. Martina J. M. Le Mogne Th. Bilasa P. Vachera B. Yamada Y. Mechanisms of MoS2 formation by MoDTC in presence of ZnDTP: effect of oxidative degradation. Wear. 2005;258:1643–1650. doi: 10.1016/j.wear.2004.11.019. - DOI
    1. Graham J. Spikes H. Jensen R. The friction reducing properties of molybdenum dialkyldithiocarbamate additives: Part II - durability of friction reducing capability. Tribol. Lett. 2001;44:637–647.
    1. Inoue K. Tominaga E. Akiyama K. Ashida T. Effects of lubricant composition on fuel efficiency in modern engines. SAE Trans. 1995;104:728–736.
    1. Graham J. Spikes H. Korcek S. The friction reducing properties of molybdenum dialkyldithiocarbamate additives: Part I – factors influencing friction reduction. Tribol. Lett. 2001;44:626–636.