Lubricant antioxidants traditionally have attempted to halt the chain-reaction mechanism of oxidation by targeting two key points — radical scavenging and peroxide decomposition. With increasing requirements for performance, lower costs and strict environmental demands, there is an increasing interest in environment-friendly oil formulations. While using traditional antioxidant additives in lubricants leads to an improved overall performance, it comes at the expense of higher costs and environmental concerns.

Materials based on molecular sieves are envisioned as effective, environmentally safe, economical and reusable alternatives to traditional antioxidant additives. These nanoporous materials can be engineered to capture oxidized byproducts and residual water produced during the early and late stages of lubricant degradation.These materials possess tailored porosity along with a variable morphology. Therefore, they can be considered as a trapping system during lubricant oxidation by means of physical adsorption, thus effectively halting the chain reaction of lubricant degradation.

In this case study, two commercial VDL-46 type oil formulations at two different stages of oxidation were monitored for adsorption kinetics of microporous materials, thus establishing the most favorable operation conditions to adsorb lubricant degradation products.

Oil Purification and Characterization

Two commercial, fully formulated VDL-46 oil samples after different times of operation in screw compressors were treated with four types of nanoporous sorbents, two hydrophobic (C1 and C2) and two hydrophilic (W1 and W2) materials. The sorbents were used either as powders or pre-shaped with a binder. The nanoporous materials were activated at 150 degrees C for one hour and introduced into the oil samples. The lubricants were treated with a variety of techniques, including the use of one type of nanoporous sorbent, a one-step treatment with combined hydrophilic/hydrophobic nanoporous sorbents, and a two-step treatment (hydrophilic/hydrophobic and vice versa).

Particle size, degree of crystallinity, chemical composition and porosity of all nanoporous samples were characterized prior to preparation of the active sorption composites. The porosity of the composites based on the sorption of water and nitrogen were determined and compared with the parent powder nanoporous sorbent samples. After separation of the lubricant and the porous material, both were analyzed for trapped molecules, antioxidants, water content and color according to ASTM standard methods. For quantitative water measurements, FTIR spectroscopy was combined with Karl Fischer measurements.

Purification Monitoring

In order to purify oxidized oil samples, four types of nanoporous materials with variable framework structures and different degrees of hydrophobicity/hydrophilicity were used. The most obvious changes in the used oil after purification were coloration and viscosity (Figure 1). FTIR spectroscopy and colorimetry measurements were taken on oil samples before and after treatment with the porous materials. A remarkable decrease in the intensity of the carbonyl (C=O) absorption band around 1770-1700 cm-1 in the FTIR spectra corresponding to the concentration of oxidation products was observed (Figure 2). This corresponded to an amount of adsorbed oxidation products up to 56 and 72 percent by successive treatment.

Figure 1. Photographs of Oil 1 (left) and Oil 2 (right) before and after treatment with nanoporous materials and calculated adsorbed amount of oxidized products by colorimetry (verified by IR spectroscopy).

A similar behavior in the OH vibration at 3350 cm-1 originating from the O-H stretching vibration of water was seen in the IR spectra obtained for oils treated with hydrophilic samples. Additionally, the water content in the lubricant oil before and after purification with nanoporous sorbents was also measured with Karl Fischer titration. It was observed that the water content up to 67 percent could be easily removed using the adsorption technique. No significant change of peak intensity for the other IR absorption regions, showing a negligible loss in the content of additives after purification, was observed.

From the wide variety of oxidation compounds in the used oil samples, previously identified by GC/MS and simulated distillation measurements, the prevalent compounds belong to the octanoic acid and oleic acid families. These raise the viscosity, the content of polymerization residues and water, thus increasing the acid number (AN) and contributing to corrosion of metal alloys.

Figure 2.IR spectra of Oil 1 sample before and after treatment with microporous materials: (a) hydroxyl (3600-3200 cm-1) and (b) carbonyl (1600-1800 cm-1) regions.

As expected, oils treated with highly hydrophobic nanoporous sorbent C2 performed better in relation to content of oxidation products, while the ones treated with hydrophilic materials H1 and H2 had a lower content of water. Oil 2 showed almost the same characteristics as Oil 1 after treatment. Nonetheless, the performance of the nanoporous sorbent was the opposite, with C2 performing better for the second type of oil in comparison to nanoporous sorbent C1, which most likely was due to differences in oil compositions (Figure 3).

Figure 3. Summary of the results from the purification of Oil 1 sample with different microporous materials: (a) water and (b) oxidation products based on the IR and Karl Fischer measurements.

Variation in treatment time showed little to no increase in contaminant removal. Both water and oxidation product removal reached a peak after only five hours. Concerning treatment temperature, a slight increase in performance of nanoporous materials was observed with temperatures above 50 degrees C. The most remarkable change was seen by modifying the sorbent loading, which showed a proportional increase in purification performance. In order to simultaneously remove contaminants from the oils, different approaches for oil treatment were investigated. For example, the oil was treated successively by sorbents C1 and W2 in a two-step treatment or by a combination of both nanoporous sorbents (C1W2). The first approach had one of the highest performances for oxidation product adsorption, while the second showed similar results compared to the individual nanoporous sorbents.

In order to limit the amount of free material residue in the oil, different solid presentations were also used for treatment. These shapes physically attach to the material with the help of a binder, which also helps to increase the size of possible residues for an effective removal by normal filters. The used pre-shaped sorbents in extrudate form and pellets showed high mechanical stability during oil treatment. However, it was observed that the removal of oxidation products and water was more efficient in the case of oil treatment with powder-form materials than in extrudates due to higher contact surfaces.

In summary, oil purification of water and oxidation products with the help of nanoporous materials was performed successfully. The obtained oils showed characteristics of a high degree of purification. The control of temperature and time of treatment, along with quantity of sorbent used were investigated. Accurate tuning of these parameters along with the sorbent chemical properties made possible the individual and also simultaneous removal of both oxidation products and residual water in any state of oxidation without influencing the content of additives in oil formulations. In addition, it was possible to control the degree of oxidation during the process of oil oxidation itself. All these characteristics along with the chemical inertness and ability to be regenerated by the used materials make lubricant purification by use of nanoporous materials an effective and environment-friendly process.

A new filtration system composed of two types of active compounds, i.e. hydrophobic and hydrophilic sorbents, capable of trapping oxidation molecules and water at the same time is currently under investigation.