Article pubs.acs.org/EF
Quantitative Nuclear Magnetic Resonance Spectroscopy as a Tool To Evaluate Chemical Modification of Deep Hydrotreated Recycled Lube Oils John V. Muntean, Joseph A. Libera, Seth W. Snyder, Tianpin Wu, and Donald C. Cronauer* Argonne National Laboratory, Argonne, Illinois 60439, United States ABSTRACT: The applications of 1H and 13C nuclear magnetic resonance (NMR) and two-dimensional 1H/13C NMR spectroscopy have been shown to be useful techniques for the qualitative and quantitative characterization of hydrotreated recycled lube oils. The addition of hydrogen to aromatic and alkene hydrocarbons can be quantitatively and selectively measured. The decrease of oxygen/nitrogen/sulfur species can also be inferred from the reduction of specific resonances in the NMR spectra. Treated recycled lube oil was subsequently hydrotreated with Pd catalysts deposited by either atomic layer deposition (ALD) or incipient wetness impregnation (IWI) on a SiO2/Al2O3 support. In both cases, much lower hydrogenation temperatures were required than had been observed with typical NiMo or CoMo on Al2O3. In addition, the ALD-deposited catalyst was more effective for the reduction of aromatics and heteroatom components than the IWI catalyst. The lube oil fractions were of high purity (low aromaticity and low heteroatom content) even at low reaction severity.
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INTRODUCTION Catalytic hydrogenation of used lubrication oil is an important industrial process with economic and environmental impact, in particular, reducing the use of petroleum and the release of waste gases. Improvements to the industrial process that includes caustic washing, stripping, distillation, and initial stages of hydrotreating can be realized by systematic preparation of new and more effective catalysts, followed by detailed physical and chemical analyses. In these investigations, nanostructured catalysts were prepared by atomic layer deposition (ALD) of palladium on SiO2/Al2O3 and incipient wetness impregnation (IWI) and used for the hydrotreating of pretreated used lubricants. The traditional methods of analysis, including viscosity and elemental analysis, were found to be either too slow or insensitive to monitor the subtle modification to the substrate after the initial stages of upgrading had been completed. Nuclear magnetic resonance (NMR) spectroscopy was used to characterize the starting material and resulting oils. The results show that much less severe reaction conditions were required with the above catalysts than with the typical NiMo or CoMo catalysts. Lube Oil Properties. The properties of lubricating oil base stocks currently available on the open market are described by Lillard et al.,1 Wang and Zhang,2 and Duan et al.3 Lubricating oils typically contain paraffins (both normal and iso) and naphthenes. Each supplier uses “additive packages” to modify oil properties, such as viscosity index, lubricity, and antiwear characteristics. Additives include corrosion inhibitors, antioxidants, pour-point depressants, antifoaming agents, and sludge suspending/dispersing agents. For the most part, these additives remain in the recycle oil after use. Many are subsequently removed during the initial steps of spent oil recovery, including stripping and distillation. The properties of recycled oil vary greatly depending upon the location, climate, type and duty of vehicles in use, etc. © 2012 American Chemical Society
Fortuitously, one example typical of the area from which our sample was drawn is summarized in a technical report.4 Their observations included the following: concentrations of alkanes were higher in used oils by about a factor of 2−3 compared to unused oils, and polycyclic aromatic hydrocarbons (PAHs) were generally undetectable in unused oil but were found in all used lubricating oil samples. Hydrotreating Process. Hydrotreating typically refers to the catalytic hydrogenation of refinery streams for the improvement of properties without significant cracking that generates lower molecular weight fractions. As noted by Satterfield,5 “hydrotreating may be used as a finishing process in the manufacture of lubricants and special oils. The catalyst is typically CoMo/Al2O3 or NiMo/Al2O3, which is sulfided before use”. Rana et al.6 reviewed commercial processes for the upgrading of heavy oils and residua using hydroconversion. Additional information is included in a special issue of Applied Catalysis, A: General.7 Hydrotreating of waste and spent or recycled lube oils is generating interest. Pasadakis et al.8 described an evaluation of Ni/Mo, Co/Mo, Ni/W, and Co/W on a variety of supports for the hydrotreating of refinery spent lube oil. They focused on sulfur reduction and the reduction of aromatics content while minimizing hydrogen consumption. Catalysts containing Ni/ Mo on Al2O3 exhibited the highest hydrogenation and hydrodesulfurization (HDS) ability. Co/Mo on Al2O3 was also an effective catalyst. While the Co/Mo and Ni/Mo catalysts were active for HDS and aromatics reduction, they also resulted in high levels of hydrogen consumption. Catalysts supported on ZrO2 were also effective for efficiently removing sulfur and ensuring adequate hydrogenation with low levels of hydrogen consumption. Received: September 13, 2012 Revised: November 19, 2012 Published: December 13, 2012 133
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Article
200 °C and 1 Torr ultrahigh-purity (UHP) N2 carrier gas pressure. A single cycle of deposition was used to provide nominally several weight percentages of Pd. The reduction step (formalin exposure or oxygen) changed the sample color to black, consistent with the presence of reduced Pd metal nanoparticles. Exposure times of 1500 and 3000 s were used for Pd, and the resulting depth of penetration was visualized with optical microscopy. Because Pd(hfac)2 has a relatively low vapor pressure, Pd deposited to only a depth of 50 μm. Use of the ALD technique resulted in 3.14% Pd deposition. For a comparison to the above catalyst, a sample of 3.14% Pd deposited on SiO2−Al2O3 by the IWI technique was prepared as follows: (1) The desired level of Pd(NH3)4(NO3)2 was dissolved in water, and NH4 OH was added to the solution for a total volume equal to the pore volume of the substrate SiO2/Al2O3. (2) This solution was impregnated dropwise into the SiO2/Al2O3 support. (3) The resulting solids were dried at room temperature for 4 h, followed by 125 °C drying overnight, followed by 250 °C calcination for 3 h, and then followed by cooling to room temperature. (4) The resulting catalyst was reduced in 3.5% H2/He at 250 °C for 30 min and cooled to room temperature in He.
EXPERIMENTAL SECTION
Reactor System and Analyses. A continuous-flow catalytic-bed unit was used for the experiments. The liquid oil feed was provided by an ISCO high-pressure syringe pump. The feed oil and gases passed down (i.e., trickle flow) through a preheater/reactor that consisted of a 0.4 cm inner diameter tube of 90 cm in length. This tube consisted of the following zones: (1) an empty (quartz wool) zone of about 46 cm to serve as a preheater, (2) a second zone of about 10 cm of lowsurface α-Al2O3 for radial flow dispersion (the feed oil was introduced in this zone), (3) a third zone of about 4 cm of 1 or 2 g of catalyst mixed with 6 g of α-Al2O3 filler, and (4) a bottom zone of α-Al2O3 packing. The reactor temperature was controlled using a thermocouple at the center of the active catalyst bed. The hydrotreating experiments were carried out at 210−290 °C (410−550 °F), ∼54 MPa (790 psia), 100% H2 feed gas at ∼4.0 L/h, and oil feed rates of 5−15 mL/h. The reactor effluent was periodically recovered in one of two parallel wet ice-cooled traps. The pressure of the off-gas was controlled and was subsequently metered and sampled. Gas composition was determined using a modified HP 6890 series gas chromatograph (GC).9 Elemental analyses (C, H, and S) of selected oil samples were conducted by Galbraith Laboratories, Inc., Knoxville, TN. One-dimensional (1D) and two-dimensional (2D) 13C/1H NMR spectroscopy are well-established analytical techniques.10,11 This characterization is particularly useful for observing and differentiating the fraction of hydrogen attached to sp2-hybridized carbon versus the hydrogen attached to sp3-hybridized carbon ( fa) in the oil samples. Experimental conditions, including pulse-width calibration, recycle delay, receiver gain, and probe tuning, were optimized to ensure quantitative reliability better than 0.3%.12,13 Experiments on standard materials showed integrated signal intensities that were within 0.2% of the stoichiometric values. NMR experiments were performed using a Bruker Avance III 500 MHz NMR spectrometer (11.7 T). With the use of a nitrogen precooler, heater coil, and the variable temperature controller, the temperature was stable at 294.3 ± 0.1 K. The 1H spectra were recorded with a variable temperature probe (two-channel 5 mm direct detection, single-axis gradient, and with 2H lock at 76.773 MHz). In all NMR experiments, 600 μL of oil was added to 400 μL of CDCl3 solvent. 1H NMR data were collected with 256 acquisitions, 5 s recycle delay, 4 μs pulse width (70°), and 6 kHz sweep width. Twodimensional heteronuclear single-quantum coherence (HSQC) data were collected with 512 acquisitions and 1024 points in f2 and 256 slices in f1. HSQC 2D correlation data were recorded “... via double inept transfer using sensitivity improvement phase sensitive using Echo/Antiecho-TPPI gradient selection with decoupling during acquisition using trim pulses in inept transfer with multiplicity editing during selection step using shaped pulses for all 180° pulses on f2 channel for matched sweep adiabatic pulses with gradients in backinept”. This method allows one to clearly assign all resonances as either methylene or methine/methyl based on the sign.14−20 Oil Feed Samples. Selected samples of recycled feed and product oils were provided by ULI/CEP (Universal Lubricants, Inc. and Chemical Engineering Partners). Their oil pretreatment included caustic scrubbing (removal of organic chlorides, acidic sulfur, and heavy metals), flash evaporation (removal of water and naphtha), and wiped film evaporation/distillation (removal of asphaltic residue). The hydrotreating with NiMo and CoMo catalysts by ULI/CEP of the pretreated recycle oil has been described by Libera et al.21 Additional experimentation demonstrated that NiMo and CoMo catalysts with and without promoters needed reaction severity greater than 300 °C and a weight hourly space velocity (WHSV) of 2 h−1 for even low levels of hydrogenation. Catalyst Samples. A sample of ALD-deposited Pd on SiO2Al2O3 (Aldrich grade 135, 343358) was prepared by ALD using palladium(II) hexafluoroacetylacetonate [Pd(hfac)2] as the metalorganic source and formalin as the formaldehyde source, which is used to remove the hfac ligands and reduce Pd to metal22−25 at a reactor temperature of
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RESULTS AND DISCUSSION NMR Characterization of Reaction Products. NMR spectroscopy was used to quantitatively measure the trends from the hydrotreating of used motor oil. The better measure, quantitative 13C NMR spectroscopy, is impractical for a large number of samples. A 37 min 1H NMR spectrum is a costeffective method. The 1H NMR spectra before and after treatment were monitored to evaluate the change in the fraction of aromatic versus aliphatic hydrogen atoms and a variety of lowconcentration constituents. The spectra show very broad resonances in the most intense regions. Broad resonances occur from (1) the superposition of a large number of chemical variants with subtly varying chemical shifts and (2) long correlation times of larger molecular species. The presence of some very narrow line-width species indicates that the line broadening is homogeneous. Also, line broadening from paramagnetic species is probably negligible. The spectra reveal dramatic changes in the middle- and lowfield region of the spectra. In particular, the hydrogen bonded to non-aromatic sp2-hybridized carbon and hydrogen bonds within two bonds of heteronuclei (oxygen, nitrogen, and sulfur) are decreased by catalytic reduction. Nearly complete elimination of these resonances occurs for the most successful reactions. Results are consistent with known reactions of pure starting materials with the investigated catalysts. Although hydrogen directly bonded to oxygen, nitrogen, or sulfur in the form of alcohols, amines, and thiols is certainly present in the raw feed and, therefore, the 1H NMR spectrum, hydrogen bonding will influence the chemical shift and render structural assignment impossible. To provide a more detailed analysis of specific chemical modification, the 2D HSQC 13C/1H NMR spectra were recorded for a subset of samples. From these spectra, one can readily determine the hydrogen bonded to carbon atoms adjacent to heteronuclei and the methylene/methine and methyl ratio (CH2/CH and CH3) by the INEPT subspectra detection method. Moreover, the intense aromatic and aliphatic regions now display changes in two dimensions that were not apparent in the 1D analysis. Specifically, the aromatic resonances appear to be selectively reduced in the low-field hydrogen/high-field carbon region. Although the methyl region appears unchanged in either analysis, one can clearly see a decrease in the methyl groups attached to aromatic rings. The 134
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Table 1. Resonances of Pertinent Peaks in Figure 1a
only other possible source of new methyl resonances would be from the reduction of terminal alkenes (a minor component) or fragmenting of larger molecules (unlikely with the studied catalysts). 1 H NMR spectra were divided into nine regions of interest as shown below. Integrated signal intensity from these distinct regions allows one to evaluate the effectiveness of catalysis. (1) Low-field aromatics, less than 8.00 ppm, were attributed to PAHs, such as anthracene, phenanthrene, and various derivatives. ULI/CEP and Argonne treatments were effective in the reduction of these species. (2) Hydrogen bonded to sp2hybridized aromatic carbon atoms (6.5−8.0 ppm) decreased intensity for the plant-treated oil. Nearly all hydrogen in this region was eliminated in the best effort at Argonne, indicating complete hydrogenation. (3) Likewise, the 5.1−6.5 ppm region (hydrogen bonded to sp2-hybridized non-aromatic carbon) was eliminated by either catalytic process. (4) This NMR method also allows one to distinguish hydrogen bonded to sp2hybridized carbon as terminal alkenes (4.7−5.0 ppm). However, one cannot directly observe alkenes that bear no hydrogen but can assume that these species are similarly reduced. (5) The fifth region can be assigned to methylene and methine hydrogen that are bonded to carbon atoms adjacent to oxygen (3.6−4.3 ppm). Both plant and Argonne catalysis methods are effective in eliminating these structures. (6) Some methylene resonances at ∼2.5 can be assigned to hydrogen attached to carbon adjacent to sulfur or nitrogen. These assignments were made with HSQC data. Although not affected in the plant hydrogenation, the Argonne catalyst removed these species. (7) Methylene and methine resonances from 2.2 to 2.5 ppm were found to increase upon hydrogenation. These data are expected and consistent with the addition of hydrogen to sp2-hybridized carbon. (8) Methyl groups attached to aromatic rings are found at 2.2−2.3 ppm (19−21 ppm in the carbon spectra). These resonances are eliminated in reduced oils not because the methyl groups are eliminated but because the hydrogenation of the aromatic ring causes an upfield shift. (9) The last region is identified with aliphatic hydrogen; hydrogen atoms bonded to sp3-hybridized carbon atoms methine (CH), methylene (CH2), and methyl (CH3). Other than changes to the relative concentration, difference spectra reveal little in the regions between 0.0 and 2.15 ppm. Quaternary carbon is not observed in any case but assumed to be constant in treated and untreated oils. Table 1 displays a list of chemical moieties that are consistent with the 13C and 1H chemical-shift values and are targets of the catalytic reduction. Characterization of Hydrotreating Products. The recycle oil for the hydrotreating process had been washed with caustic, stripped, and distilled using a wiped-film evaporator. This oil was subsequently hydrotreated at both the commercial-scale ULI/CEP facility and relatively mild conditions (
