Research ArticleORGANIC CHEMISTRY

Renewable lubricants with tailored molecular architecture

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Vol. 5, no. 2, eaav5487

Abstract

We present a strategy to synthesize three types of renewable lubricant base oils with up to 90% yield using 2-alkylfurans, derived from nonfood biomass, and aldehydes, produced from natural oils or biomass through three chemistries: hydroxyalkylation/alkylation (HAA), HAA followed by hydrogenation, and HAA followed by hydrodeoxygenation. These molecules consist of (i) furan rings, (ii) saturated furan rings, and (iii) deoxygenated branched alkanes. The structures of these molecules can be tailored in terms of carbon number, branching length, distance between branches, and functional groups. The site-specific, energy-efficient C–C coupling chemistry in oxygenated biomass compounds, unmatched in current refineries, provides tailored structure and tunable properties. Molecular simulation demonstrates the ability to predict properties in agreement with experiments, proving the potential for molecular design.

DISCUSSION

We demonstrated a strategy to synthesize new and existing lubricant base oils with structural diversity and excellent control over molecular weight, size distribution, and branching using energy-efficient C–C coupling without complex separations needed for current petroleum-based base oils. Nonfood biomass and natural or WCO can be harnessed to obtain synthons (alkylfurans and aldehydes) of varying carbon lengths and branching for various targeted applications. Efficient and easily separable heterogeneous catalysts enable high product selectivity and yield (up to 95%). In addition, low-temperature processing compared with the current refinery processing (cracking, isomerization, and distillation for synthetic and mineral base oils) and use of sustainable, abundant feedstock provide a sustainable route to biolubricants. Structural control enables tuning of product properties, and use of one-step chemistry (HAA) or two-step chemistry (HAA followed by HDO) leads to diverse classes of products with aromatic rings (FL), oxygen-containing cyclic rings (SFL), or branched alkanes (BPAOL). C30 products, chosen for demonstration, have comparable or better properties than the current commercial mineral or synthetic base oils. The properties can be predicted by molecular simulation to inform design of molecules, an approach previously unavailable for petroleum-derived base oils that cannot be synthesized with molecular specificity.

MATERIALS AND METHODS

Materials

Aquivion PW98 [coarse powder, Brunauer-Emmett-Teller (BET) surface area <1 m2/g, and 1.0 mmol H+/g) (36), Nafion NR50 (pellets, BET surface area <1 m2/g, and 0.89 mmol H+/g) (37), Amberlyst-15 (dry hydrogen form; pore size, 34.3 nm; BET surface area, 42 m2/g; and 4.8 mmol H+/g) (38), amorphous silica alumina (ASA; catalyst support grade 135; 12 wt % Al2O3; >90% AS-100 mesh; pore size, 5.4 nm; BET surface area, 569 m2/g; and 0.34 mmol H+/g) (39), methanesulfonic acid (≥99.0%), acetic acid, silica gel (high-purity grade; pore size, 6 nm; and 70 to 230 mesh), 2-pentylfuran (≥98.0%), 2MF (99%), 2-ethylfuran (≥99.0%), lauraldehyde (≥95%), hexanal (98%), octanal (99%), decanal (≥98.0%), 2-ethylhexanal (96%), eicosane (99%), Pd/C (10 wt % Pd loading), and H2IrCl6 (99.98%, trace metal basis) were purchased from Sigma-Aldrich. 2-Propylfruan (>98%) and 2-butylfruan (>98%) were purchased from Tokyo Chemical Industry Co. Ltd. 2-Hexylfuran (97%) and 2-heptylfuran (97%) were purchased from Alfa Aesar. Amberlyst-36 dry resin (pore size, 32.9 nm; BET surface area, 33 m2/g; and 5.4 mmol H+/g) (38) was purchased from the Rohm and Haas Company. H2SO4 (5 M) was purchased from Fluka. o-Phosphoric acid (85%) and cyclohexane (99.9%) were purchased from Fisher Chemical. Ammonium perrhenate(VII) (99.999%, metals basis) was purchased from Alfa Aesar. Silica gel G6 (BET surface area, 535 m2/g) was provided by Fuji Silysia Chemical Ltd. The commercial ZSM-5 (CBV2314; Si/Al = 11.5; pore size, ~0.5 nm; BET surface area, 425 m2/g; and 0.65 mmol H+/g) (40) and HY (CBV720; Si/Al = 15; pore size, ~0.7 nm; BET surface area, 780 m2/g; and 0.31 mmol H+/g) (39) were purchased from Zeolyst.

Material pretreatment

2-Alkylfurans (2MF, 2-ethylfuran, 2-propylfuran, 2-butylfruan, 2-pentylfuran, 2-hexylfuran, and 2-heptylfuran) were purified by vacuum distillation. The zeolites ZSM-5 and HY were calcined at 550°C for 4 hours at a heating rate of 2°C/min. The ASA was calcined before use for 10 hours at 500°C at a heating rate of 2°C/min under static air. The silica gels were calcined in air at 700°C for 1 hour at a heating rate of 10°C/min before catalyst impregnation.

Catalyst preparation

The P-SiO2 catalyst (H3PO4, 10 wt % loading) was prepared by impregnation. First, SiO2 (Sigma-Aldrich) was impregnated by aqueous H3PO4 solution. After evaporating the solvent at 75°C on a hotplate and subsequently drying at 110°C for 12 hours in an oven, the catalyst was calcined in a crucible in air at 500°C for 3 hours with a 2°C/min temperature ramp. The Ir-ReOx/SiO2 (Ir, 4 wt % loading; Re/Ir = 2 M) catalyst was prepared using sequential impregnation. First, Ir/SiO2 was prepared by impregnating Ir on SiO2 (Fuji Silysia G-6) using an aqueous solution of H2IrCl6. After evaporating the solvent at 75°C on a hotplate and drying at 110°C for 12 hours in an oven, the resulting Ir/SiO2 was impregnated with ReOx using an aqueous solution of NH4ReO4. The catalysts were calcined in a crucible in air at 500°C for 3 hours with a 10°C/min temperature ramp. The reported metal loadings in the catalysts were based on the theoretical amount of metals used in impregnation.

Reaction procedures

HAA reaction. In a typical reaction, 10 mmol 2-alkylfuran and 5 mmol aldehyde without any solvent were mixed in a 20-ml glass vial. The vial was placed in a preheated oil bath and stirred at 500 rpm using a magnetic bar on a stirring cum hotplate. Last, the catalyst was added into the vial, and the reaction continued at the desired temperature (65°C unless otherwise mentioned) for 6 hours. After the reaction, the solution was diluted using 10 ml of cylcohexane solvent. A small amount of eicosane was added as an internal standard.

HAA rate measurements. Because different 2-alkylfurans and aldehydes have different densities and their mixing without a solvent changes the volume for different reactions, we used cyclohexane as the solvent for the measurement of reaction rates. Typically, 10 mmol 2-alkylfuran and 5 mmol aldehyde were dissolved in cylcohexane, and their concentrations were kept at 1 and 0.5 mol/liter, respectively. The reactions were performed in the same way as HAA reactions described above. After the reaction, the solution was diluted with 10 ml of cyclohexane containing a small amount of eicosane as an internal standard.

Hydrogenation reaction. Hydrogenation of HAA condensation products, hereto referred to as unsaturated FLs, was carried out in a 50-ml Parr reactor with an inserted Teflon liner and a magnetic stirrer. First, Pd/C catalyst was pretreated at 200°C with a temperature ramp of 10°C/min for 1 hour with H2 (50 ml/min). Then, Pd/C catalyst (0.03 g), FL (0.5 g), and 10 ml of cyclohexane were added to the reactor, and the mixture was heated at 60°C. Upon completing the reaction, the reactor was immediately transferred to an ice-water bath. Upon cooling down the solution to room temperature, the reactor was opened, the solution was diluted using 10 ml of cyclohexane containing a small amount of eicosane as an internal standard, and the catalyst was separated from the solution by filtration.

HDO reaction. HDO of FLs over Ir-ReOx/SiO2 was performed in a 50-ml Parr reactor with an inserted Teflon liner and a magnetic stirrer. First, the catalyst (0.15 g) and solvent (10 ml of cyclohexane) were added to the reactor for catalyst prereduction, and the reactor was sealed with the reactor head equipped with a thermocouple, a rupture disk, a pressure gauge, and a gas release valve. The mixture was heated at 200°C and 5 MPa H2 for 1 hour at 240 rpm. Upon prereduction, the reactor was cooled to room temperature and H2 was released. Then, we added FLs (0.3 g), closed the reactor head immediately, purged the reactor with 1 MPa H2 for three times, pressurized to 5 MPa H2, and heated the reaction mixture to the desired temperature with continuous stirring at 500 rpm. The heating time to reach the set temperature was about 25 min. Upon reaction, the reactor was immediately transferred to a water bath. The reaction solution was diluted using 15 ml of cyclohexane with a small amount of eicosane as an internal standard, and the catalyst was separated from the solution by centrifugation or filtration.

Analysis of products. The products were analyzed using a gas chromatograph (GC, Agilent 7890A) equipped with an HP-1 column and a flame ionization detector using eicosane (C20) as an internal standard. The products were identified by a GC (Agilent 7890B) mass spectrometer (MS) (Agilent 5977A with a triple-axis detector) equipped with a DB-5 column, high-resolution MS with liquid injection field desorption ionization, 1H nuclear magnetic resonance (NMR), and 13C NMR (Bruker AV400, CDCl3 solvent).

The conversion and the yield of all products from the HAA, hydrogenation, and HDO reactions were calculated on a carbon basis using the following equations

Lubricant properties

The lubricant properties of select C30 compounds (C30-FL1, C30-SFL1, and C30-BPAOL1) were evaluated according to American Society for Testing and Materials (ASTM) methods. The kinematic viscosities at 100° and 40°C (KV100 and KV40) were determined using the ASTM D445 method. The VI was calculated using the KV100 and KV40 following the ASTM D2270 method. The PP tests were carried out according to ASTM D97. The kinematic viscosities and PP measurements were performed at Southwest Research Institute in San Antonio, Texas, USA. The differential scanning calorimetry (DSC) oxidation onset temperature and Noack volatility were measured according to ASTM E2009 (method B, 500 pis O2) and ASTM D6375, respectively, at Petro-lubricant Testing Laboratories Inc. in Lafayette, NJ, USA.

Characterizations

The TGA of the fresh and used P-SiO2 catalysts was performed on a TA Q600 HT TGA/DSC. The microstructures of the fresh and used catalysts (Pd/C and Ir-ReOx/SiO2) were examined using the field emission transmission electron microscope JEM-2010F FasTEM at 200 kV.

MD simulations

MD simulations in the NVT ensemble were performed using the GROMACS 5.0.0 software (41, 42). The lubricants were modeled using the TraPPE-UA force fields (43, 44), with a carbon-carbon bond force constant of 376,500 kJ mol−1 nm−2. A 14-Å cutoff was used with the analytical tail correction (45). The number of molecules was N = 170 for all lubricants studied. The volume V and initial structures were generated using Monte Carlo (MC) simulations in the NpT ensemble for 2 × 105 MC cycles (one MC cycle consisting of N randomly selected MC steps). MC simulations were run using the MC for Complex Chemical Systems–Minnesota software program (46) with the k-d tree data structure (47). Center-of-mass translation, center-of-mass rotation, conformational (44), and volume moves were used to equilibrate the system at desired T and p = 1 atm. Four independent MD simulations were performed starting from different initial structures. A time step of 2 fs was used, and the pressure tensor was output every two time steps. A Nosé-Hoover thermostat (48, 49), with a coupling time constant of 5 ps, was used to control temperature. Each simulation lasted for at least 1500 ns. The initial part (first 100 ns) of each trajectory was discarded, and the remainder was used for analysis. The method developed by Zhang et al. (50) was used to obtain the viscosity for each independent run using the Green-Kubo integral (51) with a block size of 10 ns. Uncertainties were calculated as SEM from eight independent simulations and were reported as 95% confidence intervals. Viscosities obtained at 313 and 373 K were used to calculate the VI using ASTM D2270. Simulation snapshots were visualized using visual molecular dynamics (52) and Tachyon (53).

Technoeconomic analysis

Using the available data, a simulation of the production processes was performed using Aspen Plus version 8.8.2. The nonrandom two-liquid method was used to predict the liquid-liquid and liquid-vapor behaviors. Most of the components involved in the reactions were directly selected from the Aspen database. The components not found in the database (i.e., synthesized furan compounds and lubricants) were defined by their structures. All missing parameters were estimated by the molecular structures using the UNIFAC Model and Thermo Data Engine (TDE). TDE is a thermodynamic data correlation, evaluation, and prediction tool developed by the collaboration of Aspen Plus and the National Institute of Standard and Technology (54). The detailed analysis, process block flow diagram, and assumptions (5562) are given in the Supplementary Materials.

SUPPLEMENTARY MATERIALS

Scheme S1. Strategies for the synthesis of 2-alkylfurans.

Scheme S2. Reaction pathway for HAA of 2-alkylfuran with an aldehyde over acid catalyst.

Table S1. Properties of commercial solid acid catalysts.

Table S2. HAA reaction of different 2-alkylfurans with aldehydes over P-SiO2.

Table S3. Simulated kinematic viscosities and viscosity index at 40° and 100°C of C30-BPAOL lubricant base oils.

Table S4. Reaction specifications.

Table S5. Summary of capital and operating cost for combined production of p-xylene and lubricants.

Table S6. Summary of capital and operating cost.

Fig. S1. Catalysts screening for the synthesis of C30-FL1.

Fig. S2. Effect of catalyst (Aquivion PW98) amount on the yield of C30-FL1 at low and high conversions of reactants.

Fig. S3. Effect of reaction temperature on the production of C30-FL1.

Fig. S4. Arrhenius plot for HAA of 2-pentylfuran with lauraldehyde over Aquivion PW98.

Fig. S5. Time course of the HAA reaction over Aquivion PW98 catalyst.

Fig. S6. Recyclability of Aquivion PW98.

Fig. S7. Time course of the HAA reaction for C30-FL1 synthesis over the P-SiO2 catalyst.

Fig. S8. Recyclability of P-SiO2 for the synthesis of C30-FL1.

Fig. S9. Thermogravimetric profiles of P-SiO2 before and after reaction.

Fig. S10. Hydrogenation of C30-FL1 over pretreated Pd/C.

Fig. S11. TEM images of Pd/C before and after reaction.

Fig. S12. Recyclability of Ir-ReOx/SiO2 for the HDO of C30-FL1.

Fig. S13. TEM images of Ir-ReOx/SiO2 before reaction and after 5 cycles.

Fig. S14. Effect of chain length of 2-alkylfurans on the HAA reaction rate at <25% conversion.

Fig. S15. Effect of chain length of linear aldehydes on the formation rates of the HAA condensation products at <25% conversion.

Fig. S16. Effect of aldehyde branching on the formation rates of the HAA condensation products at <25% conversion.

Fig. S17. Gas chromatogram trace of C30-BPAOL1 and ExxonMobil PAO4.

Fig. S18. Flow diagram for the production of lubricants from furfural.

Fig. S19. Overview of cost and impact of raw materials on selling price of lubricants.

Fig. S20. Overview of cost and impact of raw materials on selling price of lubricants.

Fig. S21. Distribution of various costs by process.

This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited.

REFERENCES AND NOTES

Acknowledgments: We acknowledge the Advanced Materials Characterization Laboratory at the University of Delaware for providing the TGA facility and W. Zheng for the help with the HR-TEM. Funding: This work was supported as part of the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award number DE-SC0001004. Author contributions: S.L., B.S., and D.G.V. conceived the project and designed the experiments. S.L. executed all the experiments. A.N. reproduced some experiments. T.R.J., Q.P.C., and J.I.S. designed and performed the molecular simulations. A.A. and M.I. performed the technoeconomic analysis. S.L., T.R.J., A.A., B.S., and D.G.V. wrote the article. All the authors proofread the manuscript. Competing interests: S.L., B.S., and D.G.V. have submitted a Patent Cooperation Treaty patent application as inventors. All the other authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.
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