Article pubs.acs.org/JPCC
Promotion of Activity and Selectivity by Alkanethiol Monolayers for Pd-Catalyzed Benzyl Alcohol Hydrodeoxygenation Chih-Heng Lien and J. Will Medlin* Department of Chemical and Biological Engineering, University of Colorado, Boulder, Jennie Smoly Caruthers Biotechnology Building, 596 UCB, Boulder, Colorado 80309-0596, United States S Supporting Information *
ABSTRACT: In reactions of aromatic oxygenates, one promising strategy for improving selectivity toward desirable products is to control the ensembles of available surface sites and thus the adsorbed conformations of reactive intermediates. For this study, alkanethiolate self-assembled monolayers with variable surface densities were employed to restrict the conformation of adsorbed benzyl alcohol on Pd for enhancing hydrodeoxygenation selectivity to toluene and reducing decarbonylation selectivity to benzene. Toluene selectivity was dramatically improved on a 1-octadecanethiol-coated catalyst at the cost of a large decrease in reaction rate. On the other hand, deposition of a sparser 1-adamantanethiol (AT) coating improved selectivity to a smaller extent, but resulted in a higher reaction rate than that of the uncoated catalyst. Auger electron spectroscopy and temperature-programmed desorption (TPD) were used to further characterize this chemistry on a Pd(111) surface in ultrahigh vacuum. The TPD results match the selectivity results for the thiol-coated supported catalysts, revealing that increasing the surface density of thiols selectively shuts down decarbonylation while still allowing hydrodeoxygenation. The improved activity and selectivity for the AT-coated surface is attributed to weakened interactions of the phenyl ring with the surface.
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INTRODUCTION
Scheme 1. Benzyl Alcohol Reaction Pathways on Palladium Catalysts
Biomass-derived oxygenates are a promising source for renewable feedstocks for chemicals and fuels.1−3 These oxygenates are more functionalized than typical petroleum-derived building block compounds. During upgrading reactions, this functionality leads to multiple parallel reactions and low selectivity toward desired products.4 The diversity of possible reaction products is associated with adsorption and reaction of oxygenates in multiple conformations on catalyst surfaces.5−7 Benzyl alcohol is a biomass-derived intermediate from the pyrolysis of lignin and a key probe molecule of aromatic oxygenates. Because the alcohol and aromatic functional groups possess lone pairs or π electrons, respectively, to bind with catalytic metals, benzyl alcohol can be converted to a variety of prevalent materials in industry, such as toluene, benzoic acid, benzene, and toluene (Scheme 1).8−12 Controlling selectivity in the reaction of benzyl alcohol is important to avoid purification costs and waste, which are the main challenges in developing biomass-derived products. Development of more selective solid catalysts for reactions of benzaldehyde is an active area of research, with considerable focus on the effects of the support13−19 and the use of bimetallic catalysts19−26 and nanoparticle catalysts having controlled size or shape.17,18,26−29 For the oxidation of benzyl alcohol, nearly 100% selectivity to benzaldehyde has been reached.29,30 However, there are few studies that are focused on enhancing the hydrodeoxygenation (HDO) selectivity to toluene. Such HDO © 2014 American Chemical Society
reactions are important in the upgrading of aromatic oxygenates in pyrolysis oil streams. Recently, self-assembled monolayer (SAM) coatings have been applied to catalysts as selectivity promoters.31−37 In an example particularly relevant to the reaction of benzyl alcohol, the hydrogenation of the aromatic oxygenate furfural has been investigated on supported Pd catalysts modified with alkanethiol SAMs. It was found that modification of the catalyst with 1octadecanethiol (C18) resulted in much higher selectivities to Received: July 17, 2014 Revised: August 21, 2014 Published: September 15, 2014 23783
dx.doi.org/10.1021/jp507114g | J. Phys. Chem. C 2014, 118, 23783−23789
The Journal of Physical Chemistry C
Article
stream (benzyl alcohol:H2:He = 0.0015:0.29:0.71) where the flow rate of benzyl alcohol was 3.8 mmol/min was fed to the reactor. Temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES) experiments were conducted in a stainless steel UHV chamber, kept at a base pressure of ∼1 × 10−10 Torr. The chamber was equipped with a Smart-IQ+ quadrupole mass spectrometer from VG Scienta and a cylindrical mirror analyzer for AES (MINICMA, LK Technologies). The Pd(111) crystal was obtained from Princeton Scientific and cleaned by cycles of heating in 5 × 10−8 Torr of O2 between 500 and 900 K and sputtering with Ar+ ions under room temperature. Sample cleanliness was confirmed through O2 TPD and AES. To deposit SAMs on the Pd(111) crystal, it was withdrawn from the main chamber via a magnetic sample transfer arm to a separate load-lock chamber and removed. After UV-ozone cleaning (Boekel model 135500) for 20 min, the Pd (111) crystal was placed in a 5 mM solution of alkanethiol in ethanol overnight. After removal from the solution, the sample was immersed in ethanol for 2 h and then installed on the magnetic sample transfer arm in the load-lock chamber without further drying. The sample was then reinserted into the UHV chamber via the load-lock system. Benzyl alcohol was directly dosed on the Pd(111) sample through a gas-dosing manifold outside the chamber by opening the valve for 1 s. The exposure was controlled by varying the pressure in the dosing manifold and tracking the pressure change in the chamber after dosing. The Pd sample was cooled to ∼120 K through a liquid nitrogen reservoir, which was in thermal contact with the sample. The temperature was measured by a thermocouple welded onto the stage adjacent to the sample.
hydrogenation and HDO products (furfuryl alcohol and 2methylfuran, respectively) while greatly reducing selectivity to the decarbonylation product furan.31 The selectivity changes were attributed to a selective poisoning effect, where the C18 SAM greatly decreased the availability of contiguous Pd terrace atoms required for C−C bond scission.38 The improved catalytic selectivity was observed to come at the expense of activity, as the coating decreased the overall rate per catalyst mass by up to 2 orders of magnitude. Since furfuryl oxygenates and phenyl oxygenates have been found in surface science studies to proceed by similar reaction mechanisms on Pd,9,38 we hypothesized that the thiolate SAMs could be used to control selectivity in the reaction of benzyl alcohol. In this study, thiol-modified catalysts were prepared with variable surface densities. Because 1-adamantanethiol (AT) has a bulky tail, it is known to form a sparse (7 × 7) adsorbate lattice on (111) facets of the face-centered cubic (fcc) metal surfaces, corresponding to a space between sulfur groups of ∼6.9 A.39 In contrast, C18 possesses a straight chain and forms a dense (√3×√3)R30 adsorption geometry with a space between sulfur groups of ∼4.7 A on Pd(111) surfaces.40,41 The effects of these coatings on the selectivity of benzyl alcohol HDO to toluene were investigated using Pd/Al2O3 as the catalyst substrate under steady-state reaction conditions. To obtain a more in-depth understanding of the mechanism for selectivity promotion, surface science techniques employing thiolate-coated Pd(111) were utilized to investigate the thermal chemistry of benzyl alcohol.
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METHODS 1-Adamantanethiol (95%), 1-octadecanethiol (98%), benzyl alcohol (99.8%), benzyl-2,3,4,5,6-d5 alcohol (98 atom % D), 200-proof HPLC-grade ethanol, and 5 wt % Pd/Al2O3 were obtained from Sigma-Aldrich. The ultra-high-purity H2 and He for the reactor system were obtained from Airgas, and ultra-highpurity O2 and Ar for ultrahigh vacuum (UHV) experiments were obtained from Matheson Trigas. To prepare the SAM-coated Pd/Al2O3 sample, 70 mg of Pd/ Al2O3 (5 wt %) was placed in a 20 mL, 5 mM thiolate/ethanol solution overnight. The catalyst was not reduced prior to coating, since previous studies have found that reduction with flowing hydrogen resulted in no difference in organization of the thiol coating as measured by vibrational spectroscopy.31,37 After removal from solution, the sample was rinsed with ethanol for more than 2 h and dried under air overnight before use. This procedure has been previously found to result in the formation of ordered monolayers of the thiols on Pd catalysts and surfaces, as determined by infrared spectroscopy, water contact angle goniometry, and photoelectron spectroscopy.36,41 Previous work has found that the oxidization of SAMs on Pd is not observable under air for 2 days and furthermore that the SAM remains intact at 190 °C after 15 h of reaction under hydrogenation conditions similar to those reported here.31,41 For reactor studies, 2 mg (uncoated and 1-adamantanethiolcoated cases) or 20 mg (octadecanethiol-coated case) of Pd/ Al2O3 diluted with 25 mg of Al2O3 powder was packed into a gasphase plug flow reactor. Reactions were conducted at 423 K and atmospheric pressure. Feed and product samples were analyzed by an Agilent Technologies 7890A gas chromatograph equipped with a flame ionization detector and a 30 × 0.320 mm2 Agilent HP-5 capillary column (split ratio 25:1, column flow rate 2 mL/ min; the oven temperature was 333 K for 3 min, ramped to 353 K at 10 K/min, and then held for 5 min). A mixed benzyl alcohol
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RESULTS Flow Reactor Studies. Uncoated, AT-coated, and C18coated Pd/Al2O3 catalysts were evaluated in a packed bed reactor using a feed stream containing benzyl alcohol and a large excess of hydrogen at 423 K. As Figure 1 shows, the steady-state
Figure 1. Benzyl alcohol selectivity and turnover frequency for uncoated, AT-coated, and C18-coated Pd/Al2O3 at 423 K. The conversion was 40 ± 4%, and the mass balance was above 93% in all cases.
selectivity to toluene over the uncoated Pd catalyst was around 35%, and the dominant product was benzene. Coating the catalyst with AT resulted in an increase in HDO selectivity and decrease in the decarbonylation (DC) pathway. Suprisingly, the turnover frequency (rate of benzyl alcohol consumption per Pd surface atom) for the AT-coated catalysts was even higher than for the uncoated catalysts. The improvement of both selectivity 23784
dx.doi.org/10.1021/jp507114g | J. Phys. Chem. C 2014, 118, 23783−23789
The Journal of Physical Chemistry C
Article
and activity with thiol coatings is unusual, as thiols are normally considered surface poisons. We attribute the improved activity to the AT monolayer preventing the formation of carbonaceous species on the catalyst surface, which seriously lowered the activity of the uncoated surface.37 The evolution of the selectivity and the turnover frequency with time on stream are shown in Figure S1 in the Supporting Information. These results reveal that there was extensive deactivation on the uncoated catalyst, but deactivation was less severe (and the steady state achieved earlier) on the ATcoated catalyst. Previous kinetic studies of different classes of Pdcatalyzed reactions have suggested that one mechanism by which thiolates could actually improve rates is by reducing the coverage of spectator species that can also block active sites on the surface.37 Thus, the improvement in activity in this case is hypothesized to be caused by AT serving to prevent the accumulation of surface carbonaceous species that are more problematic poisons than the AT modifier itself. When the catalysts were modified by the denser C18 monolayer, there was a remarkable improvement in the selectivity to toluene, which exhibited over 82% selectivity. The production of benzene was decreased to below the detection limit of the gas chromatograph, i.e., < 0.5% selectivity. The dramatic restriction in the generation of benzene indicates that the higher thiolate surface coverage by the octadecanethiol monolayer expected from prior work (measured to be 2.1 times higher by inductively coupled plasma techniques)31 essentially eliminated the active sites required for decarbonylation. The C18 catalyst also showed a different approach to the steady state (Figure S1, Supporting Information) characterized by a longer and slower activation (rather than deactivation) period. This may be due to restructuring or partial decomposition of the monolayer, as discussed in detail below. Prior characterization of the AT-coated and C18-coated Pd/ Al2O3 catalysts with diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) has shown that well-ordered monolayers are formed on the Pd/Al2O3 catalysts.31,35,37 The average Pd particle size on the catalysts used in this study was approximately 8 nm on the basis of CO chemisorption.42 Particles of this size are predominantly terminated by the Pd(111) surface facet,43 but also contain a significant number of edge sites and defects that can influence monolayer packing and also affect reactivity.31,37 Prior studies have suggested that the undesired decarbonylation reaction occurs primarily on (111) terraces;8 in the work reported below, we have used surface science techniques to elucidate this chemistry on Pd(111). However, the role of particle curvature and particle edge sites on the chemistry cannot be ruled out and is currently the subject of detailed investigation. Auger Electron Spectroscopy. For further investigating the chemistry, surface science techniques were employed, including AES for confirming sulfur coverage and TPD for monitoring the thermal chemistry of adsorbates on a Pd(111) crystal. As shown in Figure 2, the uncoated surface had an AES spectrum dominated by the Pd feature around 330 eV. For both thiolate SAM coatings, the sulfur (153 eV) and carbon (273 eV) features indicate the presence of a thiol monolayer on the surface. Furthermore, the Pd feature was dramatically attenuated on the C18-coated surface, consistent with the presence of a thick and relatively dense monolayer film. The peak to peak amplitude and the S:Pd and C:Pd feature ratios for thiol-modified Pd(111) are shown in Table 1. The smaller S/Pd ratio in the AT coating indicates that a lower fraction of the surface was covered by AT
Figure 2. AES spectra of uncoated, AT-coated, and C18-coated Pd(111) surfaces.
Table 1. Peak to Peak Intensities and Ratios of Intensities for AES Spectra Shown in Figure 2a peak to peak intensity (au) coating
S
Pd
C
S/Pd
C/Pd
AT C18
1.89 0.99
3.29 0.77
1.90 4.11
0.57 1.29
0.58 5.34
The error in the value of the peak to peak intensity is ±0.1 on the basis of repeated experiments. a
and a more sparse thiol monolayer structure was formed on the catalyst surface. Previous work has shown that C18 coatings on Pd(111) form a surface with a 0.33 monolayer (ML) coverage.41 Because the S:Pd ratio was approximately half as large on the ATcoated surface, we estimate an approximate coverage of 0.17 ML, very close to the coverage of 0.18 ML determined on Au surfaces.37−39 Temperature-Programmed Desorption. TPD was used to characterize the thermal chemistry of benzyl alcohol on uncoated and SAM-coated Pd(111). Because the thiolate coatings can themselves thermally degrade, TPD spectra were first collected for the coated surfaces to identify decomposition products. Sample spectra are shown in Figure S2 (Supporting Information) and indicate that the decomposition of the SAM is extensive at elevated temperatures. For both coatings, desorption of some ethanol (m/z = 31) was detected, since ethanol was used as the solvent for SAM deposition. On the AT-coated surface, volatile decomposition products were observed beginning at 470 K; these decomposition products include hydrogen, which was hypothetically formed by rate-limiting C−H scission reactions and various carbonaceous products. On the C18-coated surface, decomposition appeared to begin at lower temperature with desorption features having an initial peak near 450 K and significant intensity at somewhat lower temperatures. Higher temperature decomposition peaks were also observed. Note that the earlier onset of decomposition may help explain the activation period observed for C18-coated catalysts during exposure to the reaction temperature of 423 K, as some monolayer decomposition may have enabled exposure of active sites. To prepare surfaces representative of the active catalyst, ATcoated and C18-coated samples were held below 470 and 500 K when the benzyl alcohol TPD experiments described below were conducted. AT-coated and C18-coated samples were first annealed respectively to 470 and 500 K for 30 min before all experiments. After annealing, as shown in Figure S3 (Supporting Information), the TPD spectrum on thiol-coated Pd(111) 23785
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indicates that the thiol-derived monolayers are stable, so that no desorption of decomposition products from thiol monolayers would be convolved with the TPD spectra of benzyl alcohol reported below. Figure 3 shows a typical TPD result from uncoated Pd(111) after adsorption of a saturating dose of benzyl alcohol (m/z =
Figure 4. TPD spectra of hydrogen products (H2, HD, and D2) from C6D5CH2OH decomposition on clean Pd(111) after direct dosing at 130 K.
area ratio of the benzene feature relative to the toluene feature decreased from 1.93 to 0.29. The hydrogen peak at 335 K was also significantly smaller relative to the toluene desorption feature at 345 K as compared to those of the uncoated surface. This indicates a suppression of the DC pathway in preference of the HDO pathway after AT modification. The significantly reduced CO yield compared to that of the uncoated surface (peak area decreased by a factor of approximately 4) is also consistent with less decarbonylation. Furthermore, most of the desorption peaks shifted to lower temperature. We attribute these shifts to the crowded nature of the AT-modified surface, which induced weaker adsorbate−surface interactions. The interaction of benzene with the surface was weakened in particular, resulting in an unusually low temperature desorption feature.46 This large temperature shift suggests that the enhanced activity for AT-coated catalysts may have been due to suppression of benzene decomposition on the catalyst surface. Interestingly, the yield of benzaldehydean intermediate in the DC pathway9was relatively larger on the AT-coated surface, likely due to suppression of the series DC reaction. The TPD results of C18-modified Pd(111) are shown in Figure 5b. The very low desorption yields are consistent with a densely coated surface and help explain the reduced activity for the C18-coated Pd/Al2O3 catalyst. For this denser coating, the benzene and CO signals disappear completely along with the hydrogen peak around 440 K, showing that DC was totally obstructed on the C18-coated surface. In contrast, production of toluene and benzaldehyde was still observed on the C18-coated surface. This result is consistent with the selectivity trends shown in Figure 1 for the coated Pd/Al2O3 catalyst. On the coated catalysts, there were no major variations in peak shapes or product yields as a function of benzyl alcohol exposure, likely because of the highly crowded nature of the surface (by thiolates) even at low benzyl alcohol exposure. Figures S5 and S6 (Supporting Information) show trends in coverage dependence for hydrogen, benzene, and toluene formed from benzyl alcohol decomposition. For the AT coating, all features shifted to slightly lower temperature with exposure, possibly due to intermolecular interactions. The toluene feature shifted slightly to lower temperature with increasing coverage on C18-coated Pd.
Figure 3. TPD spectrum after a saturating dose of benzyl alcohol on uncoated Pd(111) at 123 K.
108) at low temperature (
