The synthesis and tribological properties of small- and large-sized crystals of zeolitic imidazolate framework-71

Abstract

Small- and large-sized crystalline ZIF-71 was synthesized using different strategies and has average diameters of approximately 450 nm, and 1–2 μm. Large-sized ZIF-71 particles with good crystallinity were prepared by a solvothermal synthesis method and small-sized ZIF-71 particles were prepared upon the addition of an acidic additive (formic acid) at room temperature. The two different sizes of particles were used as oil additives in liquid paraffin, and investigated using four-ball tribotesters. The results show that the small-sized particles can improve the anti-wear properties and load-carrying abilities of the base oil. The large-sized particles exhibit good load-carrying capacity. Wear surfaces were analysed by using scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and Zygo Zegage 3D optical profiler after the wear tests.

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1 Introduction

Zeolitic imidazolate frameworks (ZIFs) are a sub-family of metal–organic frameworks (MOFs) in which the networks are connected via divalent metal cations and imidazolate (Im) type linkers with different functionalities.¹ ZIFs are also known to have the zeolite topology and have high chemical and thermal stability.² Due to a combination of the properties of MOFs and zeolites, ZIFs have become promising materials for technological applications such as gas storage,¹ separation,^1,3 catalysis⁴ and increasing numbers of other fields.^5–7

Following two papers,^8,9 we know that some of ZIFs containing organophilic imidazolate linkers have good hydrophobicity, which was confirmed by water adsorption studies. In addition, ZIF-8 and ZIF-71 are highly hydrophobic because they have hydrophobic functional groups such as methyl (–CH₃) and chlorine (–Cl) groups. The organophilic and hydrophobicity of these ZIFs materials may make them easily disperse in oil. Furthermore, Tan Jin-Chong et al.^10–13 reported that some ZIFs are compliant and can afford greater flexibility. Thus, it may promote better wear protection. More recently, Nay Win Khun et al.¹⁴ discovered that fine debris of ZIF-8 nanoparticles encapsulated with Matrimid could facilitate the sliding-rolling contact mechanism, thus offering enhanced protection against unlubricated abrasive wear. Our group have reported that ZIF-8, which has a SOD topology with a methyl group, can be used as an additive in liquid lubricants 100SN, paraffinic neutral oil, and show good anti-wear and load-carrying abilities.¹⁵ Furthermore, the chlorine group is conducive to lubrication,¹⁶ so we have further studied the influence of the chlorine group present in ZIFs materials upon lubrication. This paper has studied the classical ZIF containing the chlorine groups, ZIF-71, used as an oil additive in liquid paraffin. In addition, ZIF-71 has a RHO topology, which is constructed by bridging Zn and 4,5-dichloroimidazole (dcIm) (Fig. 1).

Fig. 1 Crystal structure of ZIF-71: Zn (polyhedral), Cl (green sphere), N (purple sphere), C (white sphere).

Generally, the properties and performance of the materials are highly dependent on the size of the crystal particles.^17,18 Since the size of the wear particle influences the geometry of the contact between the two sliding surfaces, it may be expected that the particle size greatly affects the wear behavior of a sliding system. Some small-sized materials have been proven to have great potential as lubricating materials.^19–22 Therefore, it is important to study the different particle sizes of ZIF-71 as oil additives and discover what effects will occur under friction in order to gain a better understanding of the tribological mechanism of ZIF-71 and thereby placing the synthesis of ZIF-71 on a more rational basis. In addition, most strategies reported so far on the synthesis different sized of ZIF-71 need either alkaline additives (n-butylamine or 1-methylimidazole²³) or excess raw material (an excess of the dcIm linker²⁴), or changing the solvent to less polar n-PrOH²⁵ or mixed solvents^26,27 with the excepting of direct synthesis.^28,29 In this paper, an identical solvent was chosen in order to synthesize small- and large-sized crystals of ZIF-71. In addition, large and small particles were obtained by changing the temperature and adding acid additives, respectively.

The physical characteristics of small- and large-sized crystals of ZIF-71 were characterized and their tribological properties evaluated under the same conditions using a four-ball tribotester. Powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) analyses and white light interferometer analysis were carried out to determine the possible lubrication mechanism of small- and large-sized crystals of ZIF-71.

2 Experimental

2.1 Chemicals

The reagents used in the experiments were zinc acetate [Zn(OAc)₂·2H₂O, 98.0%, Sigma-Aldrich Chemical Co.], 4,5-dichloroimidazole [dcIm, 98.0%, Fluorochem Ltd.], formic acid [HCOOH, ≥98.0%, Aladdin Reagent Chemical Co.], liquid paraffin and petroleum ether [Sinopharm Group Chemical Reagent Co.], ethanol [EtOH, 99.7%, Tianjin No. 3 Chemical Reagent Factory] and methanol [MeOH, 99.5%, Tianjin No. 3 Chemical Reagent Factory].

2.2 Synthesis

Synthesis of large-sized crystals of ZIF-71 via changing the reaction temperature: 219 mg (1.0 mmol) of Zn(OAc)₂·2H₂O was dissolved in 20 mL of MeOH and 548 mg (4.0 mmol) of dcIm was dissolved in 20 mL of MeOH. After complete dissolution, the Zn(OAc)₂·2H₂O MeOH solution was quickly poured into the dcIm MeOH solutions. The two solutions were combined and sealed in a 50 mL vial, heated in an oven at room temperature, 50 °C, 85 °C, respectively, and allowed to react solvothermally for 1 h. The mother liquor was decanted and the crystalline powder washed with EtOH. The formed particles were separated by centrifugation (2000 rpm, 2 min). The large-sized crystals of ZIF-71 were obtained at 85 °C. The yield was 78.2% based on Zn.

Synthesis of small-sized crystals of ZIF-71 via adding acid additives at room temperature: 219 mg (1.0 mmol) of Zn(OAc)₂·2H₂O was dissolved in 5 mL of MeOH. A second solution was prepared by dissolving 548 mg (4.0 mmol) of dcIm and x mmol (x = 0.5, 1.0, 2.0, 3.0 and 4.0, respectively) of formic acid in 15 mL of MeOH. The clear solution was poured into Zn(OAc)₂·2H₂O MeOH solution under stirring with a magnetic bar. Stirring was stopped after 1 h. The gel-like solid was recovered by centrifugation and washing with fresh EtOH. The small-sized crystals of ZIF-71 were obtained with x = 1. The yield was 69.6% based on Zn.

2.3 Friction and wear tests

Liquid paraffin was used as the base oil. Samples of ZIF-71 were added to liquid paraffin at concentrations of 1.0 wt%, 2.0 wt%, 3.0 wt% and 4.0 wt%, respectively. These solutions were dispersed ultrasonically for 60 min. Before each test, all test-section components were cleaned in petroleum ether (b.p. 60–90 °C). Wear tests were carried out on a MS-10S four-ball machine in line with the Chinese National Standard Method SH/T0189 - 92 (speed: 1200 rpm, load: 147 N, time: 60 min, room temperature, which is similar to ASTM D4172 - 94). The test machine used for the maximum nonseizure load (P_B) tests (Method GB/T3142 - 90, which is similar to ASTM D2783 - 03) was a MS-10J four-ball machine with a driving shaft speed of 1450 rpm for 10 s at room temperature. The balls (diameter 12.7 mm) used in the test were made of GCr15 bearing steel (SAE52100 steel) with a hardness of 59–61. At the end of each test, the wear scar diameters (WSD) of the three stationary balls were measured using an optical microscope to an accuracy of ±0.01 mm and the friction coefficients were recorded automatically with a strain gauge equipped with the four-ball tester. After the test, the three lower steel balls were cleaned with petroleum ether for 10 min in an ultrasonic bath.

2.4 Methods of characterization

Powder X-ray diffraction (PXRD) at room temperature was performed with an X-ray diffractometer (Rigaku, MiniFlex II) using monochromatized Cu Kα radiation (λ = 1.5418 Å). Scanning electron microscopy (SEM) was recorded for the powder samples (Hitachi, SU8010). Fourier-transform infrared (FT-IR) spectra of the samples were obtained on a Shimadzu IRAffinity-1 FT-IR spectrometer using the KBr pellet technique. Thermogravimetric analysis (TG-DSC) measurements were performed under an Ar atmosphere on a simultaneous thermal analyzer (Setaram, Labsys evo) at a heating rate of 5 °C min⁻¹.

A 3D non-contact optical surface profiler (Zego, Zegage) was employed to measure the wear volumes of the three lower balls. The worn surfaces were investigated using scanning electron microscopy (Hitachi, TM-3000). An energy dispersive spectrometer (Bruker, QUANTAX 70) was employed to analyze the composition of the chemical elements on the worn surfaces.

3 Results and discussion

In this paper, we first synthesized the different sizes of ZIF-71. The phase purity and crystallinity of the as-synthesized ZIF-71 samples were verified by PXRD (Fig. 2). The PXRD patterns of the samples are in good agreement with the data published by Yaghi et al.,¹ indicating that the products were pure phase and exhibited the expected RHO-type topology. We were able to prepare large-sized crystals of ZIF-71 (around 1–2 μm, see Fig. 3a) by simply combining methanol solutions of Zn(OAc)₂·2H₂O and dcIm followed by work up after 1 h at high temperature (85 °C).²⁹ Adding formic acid as a modulating ligand gave small-sized crystals of ZIF-71 (around 450 nm, see Fig. 3b) under similar conditions at room temperature.

Fig. 2 PXRD pattern of the ZIF-71 product compared with the simulated ZIF-71 pattern.

Fig. 3 SEM images of the (a) large crystals and (b) small crystals of the ZIF-71 products.

FT-IR spectra of the products were recorded in order to provide further evidence that the [Zn(dcIm)₂] framework materials have been obtained. For two products, the absorption bands appear at the same wavenumbers and have almost the same relative intensities as seen for the ZIF-71 reference sample (Fig. S1†). The medium band at 545 cm⁻¹ in the ZIF-71 spectrum can be attributed to the C–Cl stretching vibration of the dcIm ligand. And the characteristic peaks of C–N stretching vibration appear at 1055 cm⁻¹. The spectra are in agreement with the previously reported spectra of ZIF-71.^27,29–31

The TG-DSC analyses carried out under an Ar atmosphere are depicted in Fig. S2.† The curves of small- and lager-sized crystals of ZIF-71 are similar. It showed a negligible weight loss between 25–400 °C, indicating that no guest molecules existed in ZIF-71, and the two samples showed good thermal stability, which is comparable to that in the literature.^25–27 The sharp weight loss takes place after 445 °C that corresponds to the framework structural decomposition. Meanwhile, there was an obvious exothermic peak in the DSC curves.

3.1 The influence of temperature and the amount of solvent on the particle size of ZIF-71

ZIF-71 can be synthesized using methanol as the reaction solvent.^28–32 Following a paper,³³ the synthesis of ZIF-8 upon adding an excess of solvent at room temperature was perform. Firstly, we investigated the effect of increasing the amount of solvent on the synthesis of ZIF-71 at room temperature. A series of three synthesis solutions containing different ratios of methanol as solvent were prepared: Zn/dcIm/MeOH = 1 : 4 : 500, 1 : 4 : 1000 and 1 : 4 : 2000, and their influence on the particle size of ZIF-71 presented in Fig. 4. At room temperature, increasing the volume of methanol from a molar ratio of 500 to 2000 did not lead to an obvious increase in the crystal particle size as shown in Fig. 4a–c. This was more likely caused using of more polar methanol as the solvent. Compared with the use of less polar n-PrOH could synthesize ZIF-71 nanocrystals at room temperature.²⁵ We then, investigated the influence of changing the temperature on the particle size in the same amount of solvent.

Fig. 4 SEM images of the ZIF-71 samples prepared using different amounts of methanol as the solvent at (a–c) room temperature, (d–f) 50 °C and (g–i) 85 °C.

Interestingly, when the molar ratio of Zn/dcIm/MeOH = 1 : 4 : 1000, the ZIF-71 crystals prepared by the solvothermal synthesis method in methanol solvent at 85 °C have a uniform particle size of around 1–2 μm according to SEM (Fig. 4h), which was larger than the as-synthesized ZIF-71 (around 800 nm) obtained from stirring at room temperature (Fig. 4b). The influence of temperature on the reaction process and crystallization were also further studied by measuring the PXRD as shown in Fig. 5, which confirmed that the products were pure phase. In these processes, the key to successfully prepare large-sized crystals of ZIF-71 may be increasing the reaction temperature from room temperature to 85 °C, which thermodynamically drives to the deprotonation of the 4,5-dichloroimidazole linkers at the ZIF-71 crystal surface, resulting in growth occurring in all directions and to yield larger, well-intergrown crystals. Similar observations were made by McCarthy et al.³⁴ during a systematic synthetic study on ZIF-8.

Fig. 5 PXRD of the ZIF-71 products obtained using different reaction temperatures and amounts of solvent.

3.2 The influence of adding acid additives on the particle size of ZIF-71

With the above knowledge, we can synthesize large-sized crystals of ZIF-71 at high temperature (85 °C). In order to further study the effect of particle size on lubrication, we tried to synthesize small particles. The previous method used to prepare nanoparticles, for example, added an excess of the bridging dcIm ligand in the DMF solutions,²⁴ or alkaline additives such as n-butylamine, 1-methylimidazole.²³ The synthesized nano-particles using alkaline additives would appear agglomerated (around 35 μm),²³ which is not suitable for lubrication. Luckily, we first synthesized small-sized crystals of ZIF-71 by adding acid additives (formic acid) at room temperature.

We investigated reaction solutions with a molar ratio of Zn/dcIm/HCOOH/MeOH = 1 : 4 : x : 500 (x = 0, 0.5, 1, 2, 3, 4), which can be seen in Fig. 6 and 7. The products were isolated as described in the Experimental section after 1 h of reaction at room temperature. Fig. 6 demonstrates that all products are pure-phase ZIF-71. As can be seen from the SEM images (see Fig. 7), the reaction performed adding by a modulating ligand (x = 1–4) to the reaction mixture yields ZIF-71 crystals with a particle size of around 450 nm (see Fig. 7c–f), which was smaller than those found upon without the addition of a modulating ligand (x = 0) (see Fig. 7a).

Fig. 6 XRD of the products formed upon adding acid additives (formic acid) at different molar ratios (Zn/dcIm/HCOOH = 1 : 4 : x; x = 0, 0.5, 1, 2, 4).

Fig. 7 SEM of the products formed upon adding an additive (formic acid) at different molar ratios of Zn/dcIm/HCOOH/MeOH = 1 : 4 : x : 500: (a) x = 0, (b) x = 0.5, (c) x = 1 (d) x = 2, (e) x = 3 and (f) x = 4.

As summarized in Fig. 7, increasing the concentration of formic acid modulator (x ≤ 1) unambiguously leads to a decreased mean particle size of the resulting crystals. However, the particle size was about the same upon adding formic acid at x > 1. The pK_a of the solution with formic acid was found to be ∼3.8 from the work of Cravillon et al.³⁵ We speculate that in the presence of formic acid, the 4,5-dichloroimidazole linkers on the ZIF-71 crystals surface are likely to be not well deprotonated due to the decrease in pH, resulting in a reduction in the rate of crystal growth so that the small crystals are obtained (see Fig. 7). Thus, upon adding formic acid in the MeOH solution, in other words, an increase in the concentration of protons in solution, shifts the equilibrium and thereby slows the deprotonation of the surface linkers. Consequently, we hypothesize that one important factor in controlling ZIF-71 crystals was the degree of surface linker deprotonation, which can be influenced by the pH of the growth solution.

3.3 Friction and wear tests

The wear scar diameter (WSD), average friction coefficient (μ) and maximum non-seizure load (P_B value) of the base oil (liquid paraffin) with and without small- and large-sized ZIF-71 additives at different concentrations are shown in Fig. 8 and Table 1. The WSD and μ are used to evaluate the anti-wear and friction-reduction properties of lubricating materials, respectively. Fig. 8 gives the tribological behavior as a function of the additive concentration of ZIF-71 particles under a load of 147 N. The results show that the concentration of both small- and large-sized ZIF-71 additives had little influence on the friction coefficient. The WSD for the large-sized particles at first increases from 0.65 mm to 0.72 mm as the concentration from 0 wt% to 1.0 wt%, and the WSD values fluctuate down slightly in the range of 0.69 to 0.61 mm when the concentration increases from 2.0 wt% or 4.0 wt%. For the small-sized particles, when the concentration increases from 1.0 wt% to 4.0 wt%, the lowest WSD 0.57 mm is obtained for a concentration of 2.0 wt%. Each WSD values of small-sized crystals of ZIF-71 is lower than that large-sized crystals of ZIF-71. So the anti-wear property of small-sized crystals of ZIF-71 is better than large-sized crystals of ZIF-71. Both small- and large-sized crystals of ZIF-71 additives have no effect on friction compared to the base oil.

Fig. 8 The wear scar diameter (WSD, black) and the average friction coefficient (μ, blue) values of base oil (liquid paraffin) containing different concentration of additives (upper curve, large-sized ZIF-71; down curve, small-sized ZIF-71) at 1 wt%, 2 wt%, 3 wt%, 4 wt%.

Table 1 The maximum non-seizure load (P_B value, N) of pure base oil (liquid paraffin, LP) and base oil containing different concentrations of the additives

Additives	LP	+1.0 wt%	+2.0 wt%	+3.0 wt%	+4.0 wt%
Large-sized ZIF-71	431	549	598	598	549
Small-sized ZIF-71	431	510	549	549	510

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The P_B value represents the load-carrying capacity, which is an important parameter in this research study. Table 1 discloses the variation of P_B value with the concentration of additives. It can be seen that both small and large-sized crystals of ZIF-71 can improve the P_B values. When the concentration reached 2.0 wt% or 3.0 wt%, the P_B value of the large-sized additive increased to 598 N. The P_B values of the small-sized additive increased to 549 N at concentrations of 2.0 wt% or 3.0 wt%. These results show that the large-sized particles had better load-carrying capacity than the small-sized particles. Taking into account the WSD, μ and P_B value for small and large-sized crystals of ZIF-71 additives in the entire range of the concentration, 2.0 wt% performs best.

SEM is used to investigate the worn surface. Fig. 9 shows the typical SEM images of the worn steel ball surface lubricated with pure base oil and base oil with 2 wt% of small and large-sized crystals of ZIF-71 additives at 147 N for 60 min. It can be seen that the surface shows a few signs of scuffing. The wear scar of the steel ball lubricated with small-sized crystals of ZIF-71 additive was much smaller than the pure base oil and large-sized crystals of ZIF-71 additive. Therefore, the results further testify that small-sized crystals of ZIF-71 have a good anti-wear property. Fig. 9 also gives the EDS spectrum obtained from the worn scar on the steel balls. The EDS results show that there are no other elements except those of the friction pair itself (C, Fe and Cr) on the worn surface lubricated with pure base oil and base oil with 2 wt% of small- and large-sized crystals of ZIF-71 additives.

Fig. 9 SEM images and EDS of the worn surfaces of the balls lubricated with: (a and b) liquid paraffin, (c and d) 2.0 wt% L-ZIF-71 (large-sized ZIF-71) and (e and f) 2.0 wt% S-ZIF-71 (small-sized ZIF-71).

Table 2 shows the wear volume losses of the lower steel ball in order to evaluate the anti-wear properties. The wear volumes of the pure base oil and base oil with 2 wt% of small- and large-sized crystals of ZIF-71 additives were 8.02 × 10⁻⁴ mm³, 6.97 × 10⁻⁴ mm³ and 8.37 × 10⁻⁴ mm³, respectively. The wear volume of small-sized crystals of ZIF-71 additive was the smallest. Meanwhile, Fig. 10 displays the 3D images of the wear scars of the lower ball. It can be observed the wear of the small-sized additive was obviously small, which is consistent with the results above.

Table 2 The average volume value of pure base oil and the oil + additive lubricants

Lubricant	Average volume (× 10^−4 mm^3)
Pure base oil (liquid paraffin)	8.02
Base oil + 2.0 wt% large-sized ZIF-71	8.37
Base oil + 2.0 wt% small-sized ZIF-71	6.97

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Fig. 10 3D white light interference profilometry images of the samples: (a) the base oil (liquid paraffin), (b) base oil with 2.0 wt% of the large-sized ZIF-71 and (c) base oil with 2.0 wt% of the small-sized ZIF-71.

We also investigated the properties of the particles after the wear test. At end of the wear test, all test-section components were cleaned ultrasonically with petroleum ether and ethanol and dried at room temperature. The obtained samples were characterized by PXRD. Fig. 11 shows that the PXRD patterns of the obtained samples illustrate minimal changes in the diffraction peak positions before and after the wear test. Importantly, the crystalline of the particles decreased rather than becoming amorphous upon being subjected to shear stress and compressive deformation under a tribotester, compared with some ZIFs became amorphous materials after ball-milling.³⁶

Fig. 11 PXRD patterns of the small- and large-sized crystals of ZIF-71 before and after antiwear test.

Based on the results above, it can be concluded that small-sized crystals of ZIF-71 exhibit improved performance, while large-sized crystals of ZIF-71 only shows improved load carrying capacity. The reason why the small-sized crystals of ZIF-71 have better anti-wear performance is that small-sized crystals of ZIF-71 tend to deform and are quickly deposited on the rough metal surfaces, preventing direct contact between the two metal surfaces, where the additive acts as mechanical protection.

4 Conclusions

Large-sized crystals of ZIF-71 with good crystallinity were prepared via a solvothermal synthesis method and small-sized crystals of ZIF-71 were prepared upon the addition of an acidic additive (formic acid) at room temperature. The average particle size was 1–2 μm and 450 nm, respectively. The small- and large-sized crystals of ZIF-71 used as additives exhibited different tribological behavior in the base oil (liquid paraffin). The load-carrying capacity of the base oil in the presence of both small- and large-sized crystals of ZIF-71 were improved. Moreover, small-sized crystals of ZIF-71, used as additives, showed improved anti-wear properties.

Acknowledgements

This work was financially supported by the National Natural Science Funds (Grant Numbers: 51172153, 21306126 and 21136007) and the Natural Science Foundation of Shanxi Province (Grant Numbers: 2013021008-1).

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Footnote

† Electronic supplementary information (ESI) available: The experimental characterizations including FT-IR, TG-DSC and PXRD. See DOI: 10.1039/c5ra22877h

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