Non-corrosive green lubricants: strengthened lignin – [choline][amino acid] ionic liquids interaction via reciprocal hydrogen bonding

A series of novel green lubricants with dissolved lignin in [choline][amino acid] ([CH][AA]) ionic liquids (ILs) have been synthesized in this work. The e ﬀ ect of lignin on the thermal and tribological properties of the lignin/[CH][AA] lubricants was systematically investigated by means of thermogravimetric analysis, di ﬀ erential scanning calorimetry, and a friction and wear tester. The lignin in [CH][AA] has been demonstrated to be an e ﬀ ective additive to improve thermal stability, reduce the wear rates and stabilize the friction coe ﬃ cients of lignin/[CH][AA] lubricants. Density function theory calculations on the electronic structure of [CH][AA] ILs reveal the atomic natural charge of ILs and their hydrogen bonding capability with lignin. Moreover, these green lubricants show excellent anti-corrosive properties against commercial aluminum and iron boards. The strong physical adsorption of [CH][AA] ILs onto the steel surface and the reciprocal hydrogen bonding between [CH][AA] ILs and lignin synergistically contribute to the enhanced lubrication ﬁ lm strength and thus the tribological properties of these new lubricants. This work provides a new perspective on utilizing complete bio-products in advanced tribological lubrication systems. In addition, this will open a new application venue for lignin to improve product value in lignocellulosic biomass utilization.


Introduction
Ionic liquids (ILs) have been dened as molten salts that are entirely ionic in nature, comprising both cationic and anionic species with a melting point below 100 C. 1 ILs are widely used as electrolytes in electrochemistry, solvents, extractants and catalysts in organic synthesis due to their large charge density, excellent electrochemical stability, low/negligible volatility, tunable polarity etc. 2,3 However, the major disadvantages of conventional ILs, such as high cost, potential toxicity, poor biodegradability and corrosivity [4][5][6][7] especially those with imidazolium or pyridinium cations and halogen anions, have been well recognized in recent years which restrict their wider applications considering the economic feasibility and environmental sustainability.ILs synthesized from renewable bio-resources, also named 'Green ILs', are attracting increasing interest from researchers with signicant advantages of biodegradability, non-toxicity, environmentally benign features and more importantly their comparable physicochemical properties with conventional ILs. 4,8holine, an essential nutrient for the synthesis of constructional components in cell membranes, is known to widely exist in nature and is certainly biodegradable. 9Amino acids, composed of amine and carboxylic acid functional groups along with a side-chain specic to each amino acid, are also one of the most abundant organic compounds in nature. 10Both choline and amino acids are important feed-stocks for the synthesis of green ILs. 11,12Recently, different types of [choline][amino acid] ([CH][AA]) have been synthesized 13 and used in biomass pretreatment processes. 14Other similar 'Green ILs' have found applications in the elds of lubrication, 15 catalysis, 16 carbon dioxide capture 17 etc.
Use of ILs as high performance synthetic lubricants started from 2001. 18The major advantages of ILs over petroleum oil based lubricants are their distinct physicochemical characteristics, such as negligible vapour pressure, high polarity and non-ammability. 19Over the past years, the major efforts of exploring IL lubricants in tribological systems are devoted to halogen-containing ILs (such as [BF 4 ] À , [PF 6 ] À ), 20,21 which are easily hydrolysed by moisture from processing uids and generate highly toxic and corrosive hydrogen uoride. 22Besides, ILs meet signicant challenges when operated in severe conditions such as oxidative, high temperature environments and high frequency oscillating movement under high pressure. 7,23ignin, a cross-linked polymer with phenylpropane monomers, is the second most abundant biopolymer in nature.In the pulp and paper industry, lignin is usually considered as a byproduct or even waste, which is directly burnt as a low grade fuel to recover energy. 24However, taking advantage of the unique molecular structure, lignin can be processed into valuable functional additives in composite materials with appropriate surface modication. 25For example, the rigid molecular structure and abundant surface functional groups of lignin well qualies it as a reinforce ller/cross-linker to improve mechanical properties of various polymers including but not limited to epoxy, 26 silicone elastomers, 27 and poly(lactide). 28In addition, lignin has been demonstrated as an excellent antioxidant arising from its phenolic structures. 29,30However, to the best of our knowledge, the usage of lignin as a lubricant additive has rarely been studied especially in 'Green ILs'.
In this work, we synthesized two 'Green ILs', [choline] [glycine] and [choline][L-proline] with choline as the cation and two amino acids, glycine and L-proline, as the anion respectively, and used them as the base to develop high performance lignin promoted green lubricants.Taking advantage of the thermally stable features and anti-oxidative properties of lignin (one of the most important performance indicators for lubricants) as well as its strong bonding with

Synthesis of ILs and lignin/ILs
Choline hydroxide aqueous solution was added dropwise to equimolar glycine or proline amino acid with ice cooling.Then, the mixture was magnetically stirred at room temperature for 48 h.Aer reaction, the water in the mixture was removed using a rotary evaporator at 60 C. Finally, the [choline][glycine] (IL1) and [choline][L-proline] (IL2) were dried in vacuo for 48 h at 70 C, refer to Scheme 1 for their molecular structures.Alkali lignin of different weight fractions (1, 3, 5 and 7 wt%) was added to IL1 at 90 C and stirred for 24 h with N 2 protection, which eventually formed homogeneous solutions which are denoted as IL1-1, IL1-3, IL1-5 and IL1-7.Same procedure was applied to IL2 to prepare the IL2-1, IL2-3, IL2-5 and IL2-7 lubricants.

Characterization
The molecular structures of IL1 and IL2 were analysed by proton nuclear magnetic resonance on the metal board.Upon completing the test, the metal boards were cooled down to room temperature and washed with DI water for corrosion spot detection.The optical images of the corrosion spot were recorded by a microscope (Novel optics Instruments NJF-120A).

DFT calculations
Electronic structure calculations on both IL1 and IL2 were performed with the Gaussian 09 C1 package 31 using density functional theory (DFT) at the B3LYP level of theory. 32,33-31++G* basis sets were used for carbon, nitrogen, oxygen, and hydrogen atoms.Frequency calculations were performed to verify that the geometries were minimal.1.The T onset of IL1 is obviously lower than IL2, which is attributed to the thermally stable pyrrolidine structure in the anion of IL2. 4 Besides, lignin is effective in improving the T onset of both IL1 and IL2 under N 2 and air atmospheres due to the thermally stable phenolic groups in lignin. 34SC results in Fig. 2 and Table 1 reveal that these lignin/[CH]-[AA] ILs do not show melting behavior in the measured temperature range, but exhibit typical glass transition phenomena (T g ) ranging from À47.8 to À13.7 C. For IL1, the T g increases from À47.8 to À13.7 C with the addition of lignin, indicating the stronger cation-anion attraction force aer adding lignin.The lignin provides protons from its hydroxyl groups that form hydrogen bonds with the amine groups from the anions of IL1.Besides, the proton from the hydroxyl group of the cation has the capability to form a hydrogen bond as well with ether groups in lignin.Through this reciprocal interaction between IL1 and lignin, the molecular interactions of cationanion, cation-lignin and anion-lignin could be enhanced and therefore larger T g was observed in lignin/IL1.Similar strengthened cation-anion attractions by substituting smaller volume, lower molecular weight and symmetric cation or anion and thus enhanced T g have been reported in other ILs. 35It is worth mentioning that IL1-7 shows lower T g ¼ À27.4 C than that of IL1-5 probably due to the weakened interactions between IL and lignin with the existence of an excess amount of lignin.Unexpectedly, the addition of lignin in IL2 decreases the T g of lignin/IL2 by 5-9 C probably due to the weaker lignin-IL2 interaction that has been proved by DFT calculations in a later section.

Results and discussion
The wear and friction properties of the lignin/ILs were investigated using a steel ball on steel disc conguration since steel is the most widely used material in industry.Fig. 3 shows the friction coefficient evolution during 1 hour friction test with    the presence of lignin/[CH][AA] lubricants.It is well recognized that the testing pressure of 2.5 GPa is beyond the normal industrial operation pressure, not to mention the 3.0 GPa tested in this work.Apparently, unstable friction coefficients were observed when using pure IL1 and IL2 as lubricants, Fig. 3.The friction coefficient can be stabilized at or below 0.1 by adding 3-7 wt% lignin into both IL1 and IL2, Fig. 3(a-c).Fig. 4 shows the wear volume losses of the discs with different ILs and lignin/IL lubricants.The wear volume losses of the steel discs lubricated by lignin/IL1 are apparently lower than the one lubricated by pure IL1.Specically, the wear volume of the disc lubricated by IL1-7 is only 27% of the one lubricated by pure IL1.For IL2 based lubricants, it is quite clear that the addition of lignin also improves the anti-wear properties.
Comparing the tribological results of the two pure ILs, the antiwear performance of IL2 is relatively better than IL1, which is attributed to the larger adsorption capability of IL2 onto the metal surface. 36To explore the potential of IL2 and lignin/IL2 in extreme pressure conditions, tribological tests were further conducted under the higher pressure conditions at 3.0 GPa.It is observed that the wear volume loss decreases continuously with increasing lignin fraction in the IL2.In terms of friction stabilization and anti-wear protection, [CH][AA] ILs with 3-7 wt% lignin fractions seem excellent lubricants for steel/steel contacts even at high pressure conditions.Fig. 5 presents the three-dimensional (3D) morphology of the corresponding wear tracks on discs aer friction testing.From Fig. 5(a-e), the wear track with IL1 is obviously deeper than the tracks with lignin/IL1.Comparing the 3D images from the rst and second rows of Fig. 5, the IL1 based lubricants show relatively larger and deeper wear tracks than IL2 based lubricants, which further conrms the superior anti-wear properties of IL2 under 2.5 GPa.Fig. 5(k-o) present the 3D images of wear tracks under 3.0 GPa with IL2 based lubricants, which show obviously deeper and larger wear tracks comparing the images obtained at 2.5 GPa.All the above results indicate that: (1) IL2 serves as a better lubricant base than IL1 in the steel/steel contact friction conguration; (2) dissolved lignin in either IL1 or IL2 helps to stabilize the friction coefficient and alleviate wear loss of the contacting metal pairs; (3) higher lignin fraction in ILs seems benecial to the overall performance of the tribological system.
At the molecular scale, the excellent tribological properties of lignin/[CH][AA] green lubricants can be ascribed to two major contributions.Firstly, previous tribological study on amino acids based IL lubricants did not detect nitrogen element on the wear surfaces by XPS technique, 15 which excludes the occurrence of tribochemical reaction between the ionic liquid and metal surfaces.This also means the outstanding tribological properties of the [CH][AA] ILs are most likely attributed to the formation of IL lms by physical adsorption during the friction process.During friction, low-energy electrons on the metal surface are released from contact convex sites, so the negatively charged carboxylic acid group in the amino acid exhibits strong affinity to the positively charged steel surface. 15IL2 has demonstrated even stronger affinity to metal surfaces than IL1 in a previous literature report, 36 which positively contributes to the formation of a mechanically strong liquid lm and thus effectively prevents direct contact between the steel ball and steel disc to reduce the friction coefficient and wear loss.Secondly, looking at the complex molecular structure of lignin, it is not difficult to see that proton donating groups (-OH) and proton accepting groups (-O-) widely exist in the lignin molecule.The nitrogen atom in the anion and the hydroxyl group in the cation tend to accept a proton from lignin (-OH groups) and donate a proton to lignin (-O-groups), respectively, thus reciprocal hydrogen bonds between lignin and [CH][AA] will be formed, Fig. 6.These reciprocal hydrogen bonds help to improve the mechanical strength of the lubrication lm and result in effective interfacial separation between metal/metal contacts to reduce friction and wear. 15o have a better understanding of the hydrogen bonding between lignin and ILs, the electronic structures of the ILs were optimized using DFT calculations and the atomic charges were investigated by natural bond orbital (NBO) analysis at the B3LYP/B3LYP/6-31++G* level of theory, Fig. 7. On the basis of the natural population analysis (NPA), the natural charge of the nitrogen atom in the anion of IL1 is À0.920 e À , which is more negative than that of IL2 (À0.717 e À ).That is to say, the hydrogen bond formation capability between IL1 and lignin is stronger than the one between IL2 and lignin.This also explains

Conclusion
To sum up, a series of novel green lubricants have been developed in this work using [CH][AA] ionic liquids as the lubricant base and strengthened by lignin through reciprocal hydrogen bonding in between.This work also presents a new application of lignin as an effective lubricant additive.The addition of lignin in [CH][AA] not only enhances the thermal stability, but also improves anti-wear properties and friction stability.The overall tribological performance is determined by the affinity of the ionic liquid to the metal surface and the strength of the ionic liquids-lignin interactions by hydrogen bonding.DFT calculations help to identify the bonding strength between lignin and the ionic liquid and explain the property changes with lignin additives.In addition, the lignin/[CH][AA] ILs exhibit excellent non-corrosive features with both aluminum and iron boards.This work successfully demonstrates a new and important application of using lignin in bio-based ionic liquids as advanced non-corrosive green lubricants.
[CH][AA] ILs through reciprocal hydrogen bonding, it is anticipated that the tribological properties of lignin/[CH][AA] green lubricants will be signicantly improved.The anti-corrosive properties of pure [CH][AA] ILs and lignin/[CH][AA] are also investigated in this work.

Fig. 1
Fig. 1 shows the thermal degradation and derivative thermogravimetry (DTG) curves of [CH][AA] ILs with different fractions of lignin in N 2 atmosphere.The onset decomposition temperature (T onset ) of the lignin/[CH][AA] in both N 2 and air are summarized in Table1.The T onset of IL1 is obviously lower than IL2, which is attributed to the thermally stable pyrrolidine structure in the anion of IL2.4 Besides, lignin is effective in improving the T onset of both IL1 and IL2 under N 2 and air

Fig. 6
Fig. 6 Schematic illustration of physical adsorption of ILs onto a steel surface and reciprocal hydrogen bonding between lignin and ILs.

Fig. 7 Fig. 8
Fig.7Optimized structures and natural charges on the nitrogen atoms for IL1 and IL2 by the B3LYP method.(Red, white and blue balls represent oxygen, hydrogen and nitrogen atoms, respectively.)