Increasing the Aromatic Selectivity of Quinoline Hydrogenolysis Using Pd/Mox-Al2o3

Increasing the Aromatic Selectivity of Quinoline Hydrogenolysis using Pd/MOx-Al2O3

Mark Bachrach,a Natalia Morlanes-Sanchez,b Christian P. Canlas,b Jeffrey T. Miller,c* Tobin J. Marks,a* and Justin M. Notesteinb*

aNorthwestern University, Department of Chemistry, 2145 Sheridan Road, Tech. G237, Evanston, IL 60208, USA.

bNorthwestern University, Department of Chemical and Biological Engineering, 2145 Sheridan Road, Tech. E136, Evanston, IL 60208, USA.

cChemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, IL 60439, USA.

Corresponding Authors

*Justin M. Notestein, Email:

*Tobin J. Marks, Email:

*Jeffrey T. Miller, Email:

Experimental

Catalysts were prepared using standard Schlenk line or glovebox techniques. TaCl5 (Aldrich), Ta(acac)(OEt)4 (Alfa-Aesar), and p-tert-butylcalix[4]arene (Aldrich) were vacuum sublimed prior to use. Pd(OAc)2 (Strem), TiO(acac)2 (Alfa-Aesar), and MoO2(acac)2 (Alfa-Aesar) were used as received. Tetradecane (Aldrich), toluene (Aldrich), quinoline (Alfa-Aesar), n-propylbenzene (Aldrich), n-propylcyclohexane (Aldrich), and 2-propylaniline (Aldrich) were dried over CaH2 and distilled prior to use. N-propylcyclohexene was synthesized according to the literature procedure.1

1 wt. % Pd / (1.4 wt. % Ta) TaOx-Al2O3 catalyst synthesis: The reagents TaCl5 (139 mg, 0.39 mmol) and p-tert-butylcalix[4]arene (251 mg, 0.39 mmol) were refluxed in 50 mL toluene on a Schlenk line under N2 for 48 h to form the TaCl-calixarene dimer and then added to 5.0 g Al2O3 (Selecto Al2O3-A, BET surface area = 165m2/g, partially dehydroxylated at 120°C) in the glovebox.2-4 Alternatively, a solution of 178 mg (0.39 mmol) Ta(acac)(OEt)4 in 50 mL toluene was added to 5.0 g Al2O3 in the glovebox. The TaOx-Al2O3 suspension was stirred for 24 h at RT in the glovebox, filtered, washed with 100 mL toluene, and dried under vacuum. Pd was then added by impregnation of 105 mg (0.47 mmol) Pd(OAc)2 in 4 mL toluene, to the TaOx-Al2O3 inside the glovebox. The Pd/TaOx-Al2O3 catalyst was left overnight to dry in the glovebox at room temperature and then calcined at 500°C in air for 6 h prior to use.

1 wt. % Pd / (1.4 wt. % Ti) TiOx-Al2O3 and 1 wt. % Pd / (1.4 wt. % Mo) MoOx-Al2O3 catalyst syntheses: A solution of 383 mg (1.46 mmol) TiO(acac)2 or 238 mg (0.73 mmol) MoO2(acac)2 in 50 mL toluene was added to 5.0 g Al2O3 in the glovebox. The suspension was stirred for 24 h at RT in the glovebox, filtered, washed with 100 mL toluene, and dried under vacuum. Pd was then added by impregnation of 105 mg (0.47 mmol) Pd(OAc)2 in 4 mL toluene to the TiOx-Al2O3 or MoOx-Al2O3 inside the glovebox. The resulting Pd/TiOx-Al2O3 or Pd/MoOx-Al2O3 catalyst was allowed to dry overnight in the glovebox at room temperature and then calcined at 500°C in air for 6 h prior to use.

1 wt. % Pd/Al2O3 catalyst synthesis: Pd was added to 5.0 g Al2O3 (Selecto Al2O3-A, BET surface area = 165m2/g, partially dehydroxylated at 120°C) in the glovebox by impregnation of 105 mg (0.47 mmol) Pd(OAc)2 in 4 mL toluene. The Pd/Al2O3 catalyst was left overnight to dry in the glovebox at room temperature and then calcined at 500°C in air for 6 h prior to use.

Product Characterization

GC analysis was performed on an Agilent 7890 GC with a 5975 Triple Axis MS detector, and Agilent Chemstation software. Methods were developed for either a HP-5 MS or a VF-Wax MS column to assay quinoline (Q), 1-tetrahydroquinoline (1-THQ), 5-tetrahydroquinoline (5-THQ), decahydroquinoline (DHQ), 2-n-propylaniline (2-PA), n-propylcyclohexane (PCH), n-propylcyclohexene (PCHE), n-propylbenzene (PB), ethylcyclohexane (ECH), ethylbenzene (EB), methylcyclohexane (MCH), toluene (TOL), cyclohexane, and benzene. Calibration curves were developed for compounds detected in significant quantities: Q, 1-THQ, 5-THQ, DHQ, PB, PCH, ECH, EB, MCH, and TOL. Yields reported here were found to vary with reaction time (6-24 h). The aromatic fractions include any significant cracking products and are defined as: (PB+EB+TOL)/(PB+EB+TOL+PCH+ECH+MCH). Significant cracking to the ethyl- and methyl-branched compounds (up to 38% of the hydrocarbon products) was observed but no significant cracking to benzene and cyclohexane was observed. Cracking/unification of ~1% of the solvent was also detected, primarily to dodecane and hexadecane.

General Catalyst Characterization

13C CPMAS NMR spectroscopy was performed at a spinning rate of 5 KHz on a 400 MHz Varian VNMRS NMR spectrometer. Powder x-ray diffraction data were collected on a Rigaku DMAX PXRD instrument operating with Cu Kα (λ = 1.54059 Å) radiation. Data were recorded with a 1° divergent slit, 1° scattering slit, and a 0.6mm receiving slit with a Ni filter at a dwell time of 7s and 0.1° step size. Diffuse reflectance UV-Vis spectra were obtained on a Shimadzu UV-3600 spectrometer equipped with a Harrick Praying Mantis accessory. Spectra were collected under ambient conditions and polytetrafluoroethylene (PTFE) was used as a blank for calculating Kubelka-Munk pseudoabsorbances. Zeta potential measurements of the Al2O3 support were obtained on a Malvern Instruments Zetasizer Nano ZS at 25°C. Samples were prepared by dispersing 25 mg Al2O3 in 35 mL of H2O (Barnstead Nanopure) and pH treating the dispersion with nitric acid or aqueous NaOH to obtain the desired dispersion pH. Samples were vortexed overnight at room temperature and pH measurements were obtained prior to zeta potential determination. Transmission electron microscopy of the Pd nanoparticles was performed on a JEOL JEM 2100F TEM. The diameter of the Pd nanoparticles was measured using the ImageJ software and the average of 60-140 nanoparticles was taken. TGA measurements were conducted in a TA Instruments Q500 TGA under a helium atmosphere.

X-ray Absorption Spectroscopy (XAS)

In situ XAS was performed at the bending magnet beamline of the Materials Research Collaborative Access Team (MRCAT, Sector 10-BM-B) at the Argonne National Lab Advanced Photon Source. The catalyst was pressed into a 4 mm wafer and loaded into a stainless steel sample holder capable of supporting six samples. The sample holder was inserted into a quartz tube (1 in. O.D., 10 in length) capped with Ultra-Torr Fittings and sealed with Kapton windows. The reactor was equipped with shut off valves as well as an internal thermocouple placed adjacent to the samples to control the clamshell furnace temperature.5

The H2-activated samples were heated to 275°C under He and treated with 3.5% H2/He for 1 h at 275°C in the quartz tube reactor. The reactor was then purged with He and cooled to 25°C. The reactor was sealed off and then placed in the beam. ‘Used’ samples following a typical reaction were prepared by removing the catalyst from the autoclave under air, washing the catalyst with hexanes, and vacuum filtering to remove the hexanes as well as residual tetradecane and reaction products, to yield dry powders which could be pelletized.

Ionization chambers were optimized to the midpoints of the Ta and Pd spectra to yield maximum current with linear response (~1010 photons s-1). For the Ta LIII edge, 100% N2 (10% absorption) was used in the incident X-ray detector and a mixture of 50% Ar in N2 (70% absorption) was used in the transmission X-ray detector. For the Pd K edge, 60% Ar in N2 (10% absorption) was used in the incident X-ray detector and 100 % Ar (15% absorption) was used in the transmission X-ray detector. A third detector in series simultaneously collected a Ta or Pd foil reference spectra, respectively, with each measurement for calibration of the absorption edge. A cryogenically cooled Si(111) double crystal monochromator was used and detuned to 50% in order to minimize the presence of harmonics. Data were recorded at 25°C in transmission mode with an X-ray beam size of 0.5 × 1.5 mm.

XANES energies were determined with the Athena software package by locating the inflection point in the edge, i.e., the maximum in the first derivative spectra or the zero point in the second derivative spectra. EXAFS data were analyzed using WinXAS97 software. The coordination parameters were obtained by a least-squares fit in R-space with a k2 weighting to the Fourier transform. Experimental phase-shift and backscattering amplitudes were obtained for Pd-Pd and Ta-O from Pd foil and Ta2O5 references, respectively.

Scheme S1. Synthesis of highly dispersed Pd/TaOx-Al2O3 materials.

Figure S1. PXRD of Al2O3 support. The reflections at 37°, 46°, and 67° (*) are indicative of γ-Al2O3.6 The broad shoulder from 21-30° (!) shows the presence of an amorphous component to the support. The peak at 39° and 42° (#) are indicative of aluminum oxide hydrate.7

Table S1. Zeta Potential of the Al2O3 Support

pH / Zeta Potential
4.2a / +22.3
4.3b / +28.1
6.8b / +30.6
7.5b / +34.0
9.2b / -0.0677
9.6b / -5.45
11.1b / -28.5
11.9b / -26.8
aNative Al2O3 support dispersed in H2O
bAl2O3 dispersed in H2O and pH adjusted with HNO3/NaOH

Figure S2. TGA of the 1% Pd/TaOx-Al2O3 catalyst.

Figure S3. DRUV-Vis spectra of calcined MOx-Al2O3 catalysts and bulk oxides.

Figure S4. CP-MAS 13C NMR of Ta(acac)(OEt)4 grafted onto Al2O3.

Table S2. EXAFS Fitting Parameters

Catalyst / Edge / Edge Energy (eV) / N / R(Å) / Δσ2 / E0 / Particle Size (nm)
1%Pd/Al2O3 (H2) / Pd / 24349.8 / 8.1 / 2.75 / 0.0005 / -0.51 / 3.1
1%Pd/Al2O3 (Used- H2) / Pd / 24349.8 / 8.6 / 2.76 / 0.0005 / -0.57 / 3.6
1%Pd/TaOx-Al2O3 (H2) / Pd / 24350.0 / 7.7 / 2.75 / 0.0005 / -0.85 / 2.8
1%Pd/TaOx-Al2O3 (Used- H2) / Pd / 24349.8 / 9.2 / 2.75 / 0.0005 / -0.35 / 4.3
Pd Foil Reference (Air) / Pd / 24350.0 / 12.0 / 2.75
1%Pd/TaOx-Al2O3 (Calcined- Air) / Ta / 9883.4 / 4.9 / 1.90 / 0.004 / -2.11
1%Pd/TaOx-Al2O3 (H2) / Ta / 9883.4 / 4.7 / 1.88 / 0.004 / -2.28
1%Pd/TaOx-Al2O3 (Used- Air) / Ta / 9883.4 / 4.9 / 1.89 / 0.004 / -2.15
1%Pd/TaOx-Al2O3 (Used- H2) / Ta / 9883.4 / 4.8 / 1.88 / 0.004 / -2.38
Ta2O5 Reference (Air) / Ta / 9883.3 / 6.0 / 1.98

Figure S5. TEM images of (A) H2-activated 1%Pd/Al2O3, (B) H2-activated 1%Pd/TaOx-Al2O3, (C) Used 1%Pd/Al2O3, (D) Used 1%Pd/TaOx-Al2O3.

Figure S6. TEM Particle size distribution for Pd/Al2O3 and Pd/TaOx-Al2O3 catalysts.

Figure S7. Pd K edge A) XANES and B) EXAFS for H2-activated and used Pd/Al2O3 and Pd/TaOx-Al2O3 compared to a Pd foil reference material.

Figure S8. A) XANES and B) EXAFS of the Ta LIII edge of the calcined, H2-activated, and used Pd/TaOx-Al2O3 catalysts compared to a Ta2O5 reference material.

Table S3. HDN Product Selectivities

Catalyst / Reaction Time (hrs) / Q / 1THQ / 5THQ+ DHQ / Sat’d HC / Arom. HC / Conv. to HC / Aromatic Fraction of HC
Al2O3 / 12 / 20% / 75% / 5% / 1% / <1% / 1% / 0.26
TiOx-Al2O3 / 12 / 85% / 13% / 0% / <1% / 1% / 1% / 0.74
TiOx-Al2O3 / 12 / 68% / 23% / 4% / 1% / 4% / 5% / 0.76
TaOx-Al2O3a / 12 / 24% / 68% / 5% / 1% / 2% / 3% / 0.68
TaOx-Al2O3a / 12 / 22% / 68% / 6% / 1% / 3% / 4% / 0.73
MoOx-Al2O3 / 12 / 56% / 40% / 1% / <1% / 2% / 3% / 0.82
MoOx-Al2O3 / 12 / 65% / 22% / 5% / 1% / 5% / 6% / 0.82
1%Pd/Al2O3b,c / 12 / 2% / 22% / 68% / 3% / 3% / 7% / 0.51
1%Pd/Al2O3 / 6 / 0% / 9% / 68% / 11% / 12% / 23% / 0.54
1%Pd/Al2O3 / 9 / 0% / 10% / 65% / 12% / 13% / 25% / 0.53
1%Pd/Al2O3 / 14 / 0% / 7% / 52% / 21% / 20% / 41% / 0.48
1%Pd/Al2O3 / 12 / 0% / 3% / 52% / 35% / 9% / 45% / 0.21
1%Pd/Al2O3 / 12 / 0% / 0% / 13% / 84% / 3% / 87% / 0.04
1%Pd/Al2O3 / 9 / 0% / 1% / 12% / 68% / 19% / 87% / 0.22
1%Pd/TiOx-Al2O3 / 6 / 2% / 39% / 43% / 4% / 11% / 15% / 0.71
1%Pd/TiOx-Al2O3 / 12 / 1% / 30% / 41% / 4% / 23% / 27% / 0.85
1%Pd/TiOx-Al2O3 / 12 / 1% / 9% / 33% / 9% / 23% / 37% / 0.76
1%Pd/TiOx-Al2O3 / 12 / 4% / 25% / 46% / 29% / 15% / 44% / 0.35
1%Pd/TiOx-Al2O3 / 18 / 1% / 9% / 34% / 37% / 20% / 57% / 0.36
1%Pd/TiOx-Al2O3 / 12 / 1% / 5% / 33% / 48% / 12% / 61% / 0.20
1%Pd/TiOx-Al2O3 / 24 / 3% / 3% / 7% / 68% / 17% / 85% / 0.20
1%Pd/TaOx-Al2O3a,b / 12 / 0% / 16% / 75% / 2% / 7% / 8% / 0.80
1%Pd/TaOx-Al2O3a / 9 / 0% / 12% / 62% / 9% / 17% / 27% / 0.66
1%Pd/TaOx-Al2O3a / 15 / 0% / 12% / 51% / 13% / 24% / 37% / 0.64
1%Pd/TaOx-Al2O3d / 12 / 1% / 5% / 41% / 32% / 22% / 54% / 0.41
1%Pd/TaOx-Al2O3a / 12 / 0% / 4% / 24% / 43% / 29% / 72% / 0.40
3%Pd/TaOx-Al2O3a,b / 12 / 0% / 12% / 53% / 11% / 24% / 35% / 0.69
3%Pd/TaOx-Al2O3a / 12 / 0% / 2% / 34% / 46% / 18% / 64% / 0.28
3%Pd/TaOx-Al2O3a / 6 / 0% / 2% / 26% / 41% / 31% / 72% / 0.43
3%Pd/TaOx-Al2O3d / 12 / 1% / 2% / 9% / 67% / 20% / 88% / 0.23
3%Pd/TaOx-Al2O3d / 12 / 0% / 1% / 6% / 80% / 14% / 93% / 0.15
1%Pd/Al2O3+TaOx-Al2O3a / 6 / 0% / 11% / 64% / 11% / 14% / 25% / 0.55
1%Pd/Al2O3+TaOx-Al2O3d / 8 / 0% / 5% / 34% / 27% / 29% / 56% / 0.52
1%Pd/Al2O3+TaOx-Al2O3a / 12 / 0% / 6% / 38% / 41% / 20% / 61% / 0.32
1%Pd/Al2O3+TaOx-Al2O3d / 12 / 0% / 3% / 18% / 53% / 25% / 78% / 0.33
1%Pd/MoOx-Al2O3 / 12 / 1% / 19% / 66% / 7% / 6% / 13% / 0.49
1%Pd/MoOx-Al2O3 / 24 / 1% / 15% / 55% / 18% / 11% / 28% / 0.38
1%Pd/MoOx-Al2O3 / 12 / 1% / 11% / 58% / 19% / 10% / 29% / 0.34
1%Pd/MoOx-Al2O3 / 12 / 1% / 30% / 26% / 24% / 21% / 35% / 0.59
1%Pd/MoOx-Al2O3 / 24 / 0% / 33% / 18% / 21% / 20% / 42% / 0.49
1%Pd/MoOx-Al2O3 / 12 / 1% / 19% / 28% / 21% / 25% / 46% / 0.54
1%Pd/MoOx-Al2O3 / 12 / 1% / 25% / 3% / 31% / 33% / 65% / 0.52
1%Pd/MoOx-Al2O3 / 24 / 0% / 12% / 15% / 45% / 25% / 71% / 0.36

aDerived fromTa(acac)(OEt)4

bReactor was purged with Ar at 275°C before the quinoline and tetradecane were loaded

c10mg catalyst was used

dDerived from Ta- p-tert-butylcalix[4]arene

Figure S9. H2 consumption vs. HC conversion for Al2O3 supported A) Pd/TiOx, Pd/TaOx, and Pd catalysts and B) Pd/TaOx and Pd catalysts.