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Letter
Direct Deoxygenation of Phenylmethanol to Methylbenzene and Benzyl Radicals on Rutile TiO2(110) Long Chen, R. Scott Smith, Bruce D. Kay, and Zdenek Dohnalek ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03225 • Publication Date (Web): 10 Feb 2017 Downloaded from http://pubs.acs.org on February 10, 2017
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ACS Catalysis
Direct Deoxygenation of Phenylmethanol to Methylbenzene and Benzyl Radicals on Rutile TiO2(110) Long Chen, R. Scott Smith, Bruce D. Kay, and Zdenek Dohnalek* Physical and Computational Sciences Directorate and Institute for Integrated Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA, 99352, USA. ABSTRACT: Understanding the deoxygenation of biomass-derived alcohols is of great importance for the conversion of renewable biomass to energy carriers. In this work, we present unique reaction pathways for phenylmethanol on a rutile TiO2(110) by using a combination of molecular beam dosing and temperature programmed desorption. The results from both regular and OD-labelled phenylmethanol demonstrate that hydroxyl hydrogen is transferred to the benzyl group to yield methylbenzene between 300 K and 480 K. In the competing reaction, the hydroxyl hydrogen is also converted to water in the same temperature range. Once the hydroxyl hydrogen is depleted above 480 K, the remaining phenylmethoxy surface species undergo C-O bond cleavage yielding gas-phase benzyl radical species. These findings reveal the formation of free radical species from the interaction of phenylmethanol with TiO2(110) and demonstrate a direct mechanism for deoxygenation of lignin-derived benzylic alcohols to aromatics on TiO2.
KEYWORDS: Reaction mechanisms • Radicals • Biomass • Deoxygenation • Temperature programmed desorption
The conversion of renewable biomass to energy carriers has received increased attention with the growing demand for sustainability.1 However, biomass-derived bio-oils contain large amounts of oxygen-containing functional groups that are generally not desired in the current energy infrastructure due to corrosion issues and a lower energy density than conventional hydrocarbon fuels.2-4 Consequently, the upgrading of bio-oils by catalytic deoxygenation is necessary to make them useful as fuels. Over the past decades, extensive effort has been devoted to developing efficient catalysts,5-7 as well as to the fundamental understanding of deoxygenation mechanisms with the aim of guiding the rational design of new catalytic materials.8-10 Despite extensive studies, a molecular-level understanding of complex deoxygenation mechanisms is far from complete. The structural complexity of lignocellulosic biomass has motivated studies using simple model compounds, such as alcohols,11-13 diols,10,14,15 aldehydes,8,16 and ethers17,18 as biomass surrogates to gain a mechanistic understanding of the underlying chemical transformations. Phenylmethanol (or benzyl alcohol) can be considered as one of the most abundant structural units in lignin,19-21 and often serves as the reaction product from lignin model compounds.17,18 Further, recent studies demonstrate that chemo-selective oxidation of the hydroxyls in the benzylic alcohol fragment in lignin could facilitate its depolymerization, leading to enhanced yields of low-molecular-mass aromatics.19-21 Despite the important role played by benzylic alcohols in lignin conversion to useful biofuels, their surface chemistries on well-defined metal or metal oxide surfaces are not well understood.
In this communication, we present evidence for a new thermally-driven deoxygenation pathway for phenylmethanol on rutile TiO2(110) which often serves as a prototypical model oxide surface.22,23 Employing a combination of molecular beam dosing with temperature programmed desorption (TPD), we show that the amount of reacted phenylmethanol far exceeds the concentration of bridge bonded oxygen (Ob) vacancy (VO) defects on the surface. This is in contrast to prior studies of aliphatic alcohols on TiO2(110), where VO defects represent the primary reaction sites.11-13,24 Our studies also reveal that the hydroxyl hydrogen in phenylmethanol can be transferred to the benzyl group to yield methylbenzene on a slightly reduced TiO2(110) (further referred to as r-TiO2(110)) under mild reaction conditions (300 – 480 K). Simultaneously, a fraction of the hydroxyl hydrogen is converted to water. After the hydroxyl hydrogen in phenylmethanol is consumed by methylbenzene and water formation (> 480 K), the phenylmethoxy species remaining on the surface dissociate via CO bond cleavage yielding benzyl radicals that desorb into the gas phase. Results on the hydroxylated and oxidized TiO2(110) (referred to as h- and o-TiO2(110), respectively) where the VO sites were reacted away by H2O and O2, respectively,25,26 provide further insight into the reaction. The experiments were performed in an ultrahigh vacuum (UHV) molecular beam surface scattering apparatus described previously.14,15 A rutile TiO2(110) crystal (10 × 10 × 1 mm3, Princeton Scientific) was cleaned by cycles of Ne+ sputtering and annealing to 850 − 900 K in UHV until impurities were undetectable in Auger electron spectra. The
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sample prepared in this manner contained ∼0.05 monolayer (ML, defined relative to Ti4+ sites on the surface, 1 ML ≡ 5.2 x 1014 cm-2) VO’s on the surface as gauged by H2O TPD and is referred to as r-TiO2(110).27,28 The h- and o-TiO2(110) surfaces were prepared by exposing r-TiO2(110) to H2O at 400 K and to O2 at 70 K followed by annealing to 300 K, respectively.25,26 Phenylmethanol (C6H5CH2OH, Sigma-Aldrich, 99.8%) was purified by three freeze-pump-thaw cycles before use and dosed on the substrate using a flux-calibrated effusive molecular beam (Figure S1 in Supporting Information, SI). All TPD measurements (heating rate of 1 K/s) were performed using a quadrupole mass spectrometer (QMS, UTI) in a line-of-sight geometry unless stated otherwise.
Figure 1. (a) Coverage dependent TPD spectra of phenylmethanol, obtained using the C6H7+ mass fragment (m/z = 79 amu) following its adsorption on r-TiO2(110) at 100 K. The coverages for phenylmethanol are defined relative to Ti5c sites (1 ML ≡ 5.2 × 1014 cm-2). (b) Integrated desorption peak area in (a) as a function of phenylmethanol coverage.
The coverage dependent TPD spectra of phenylmethanol following its dose on r-TiO2(110) at 100 K are shown in Figure 1a. No desorption is observed at low phenylmethanol coverages (< 0.15 ML), indicating that it has been completely converted to other products. As the coverage is increased to ~0.17 ML, a single desorption peak at ∼420 K is observed. This desorption peak continues to grow with increasing dose and saturates at a coverage of ~0.5 ML while the peak gradually shifts to ∼350 K. Further increase in the phenylmethanol coverage above 0.5 ML leads to a new TPD peak which maximizes at ~200 K and grows indefinitely with exposure. By analogy with other adsorbates (e.g. H2O, alcohols, diols and small hydrocarbons) studied previously on TiO2(110),11-15,24,28,29 we assign the ~350 K peak to the desorption of phenylmethanol from the 5-fold coordinated titanium sites (Ti5c), and the ~200 K low temperature peak to desorption from Ob sites and multilayers. The total integrated desorption peak area from Figure 1a as a function of phenylmethanol coverage is shown in Figure 1b. The linear relationship between the peak area and the coverage above ~0.3 ML indicates that the amount of phenylmethanol converted saturates in this coverage regime. The x-axis intercept at ~0.2 ML represents the saturation amount of phenylmethanol that is converted to other products. The value far exceeds the concentration of VO’s on the surface (~0.05 ML), suggesting that the majority of the reaction occurs on the Ti5c sites. This is in contrast
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to aliphatic alcohols where the reactions were shown to proceed only on VO’s.11-13,24
Figure 2. TPD spectra of various mass fragments (m/z) obtained from r-TiO2(110) exposed to 0.08 ML (a) and 0.41 ML (b) of phenylmethanol at 100 K. Both OH-labelled (C6H5CH2OH, black traces) and OD-labelled (C6H5CH2OD, red traces) phenylmethanol molecules were employed in order to conclusively identify the reaction products. At low coverage (a), a shift in the peak maxima can be observed between larger (m/z = 91, 92 and 93 amu) and smaller (m/z = 39 and 65 amu) fragments, suggesting that a second product also contributes to these fragments. For (b), the contribution of phenylmethanol to masses 65, 91 and 92 amu was not subtracted from the raw spectra due to the unknown fragmentation pattern of C6H5CH2OD in our system (multilayer C6H5CH2OD cannot be obtained on r-TiO2(110) with an H-D exchange procedure).
To conclusively identify the reaction products, we compare TPD spectra from OH-labeled (C6H5CH2OH) and OD-labelled (C6H5CH2OD) phenylmethanol. The latter was directly prepared on the surface at 300 K using an H/D exchange procedure with D2O as described previously.25 Figures 2a and 2b display a set of selected mass fragments obtained from both reactants at a low (0.08 ML) and a high (0.41 ML) coverage, respectively. A survey TPD spectra containing a complete set of mass fragments is displayed in Figure S2, SI. The lowest temperature desorption product detected at all coverages is water which is monitored by masses 18 (H2O+ + 35% DO+ from D2O) and 20 amu (D2O+). Water desorption extends from ~300 to ~450 K with a peak maximum at ~350 K. This temperature range is consistent with water formation via the recombination of terminal (HOt) and bridging (HOb) hydroxyls formed on the surface.30,31 For the C-containing products, at low phenylmethanol coverage (Figure 2a), methylbenzene is identified at ~480 K (peak labeled as α) by fragments at m/z = 91, 92, and 93 amu. Changes in the intensities of these fragments from C6H5CH2OD relative to those from C6H5CH2OH clearly show that deuterium from OD has been incorporated into the product. The intense α peak from C6H5CH2OD at m/z = 93 amu (red trace, C6H5CH2D+) demonstrates that the hydroxyl hydrogen has been abstracted by the benzyl group. Additionally, the absence of intensity changes in fragments at m/z = 65 amu (C5H5+) and 39 amu (C3H3+), demonstrates that the phenyl ring does not undergo deuteration. Note
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that methylbenzene is not the only product as evidenced by a shift in the peak maxima between the larger (m/z = 91, 92 and 93 amu) and smaller (m/z = 39 and 65 amu) fragments. Importantly, the relative intensity between them is also not consistent with the measured fragmentation pattern of methylbenzene in our system (see Figure S3 in SI and further discussion below). The desorption of a second C-containing product is directly supported by the TPD experiments at higher phenylmethanol coverage (Figure 2b). Two desorption peaks (labeled α and β, respectively) are clearly observed for the Ccontaining products monitored at m/z = 39, 65, 91, and 92 amu. As for the low coverage, significant changes in the α peak for mass 91, 92 and 93 amu can be observed when the reactant is changed from C6H5CH2OH to C6H5CH2OD. This suggests that the α peak is due to methylbenzene desorption. In contrast, the absence of changes in the β peak with isotopic labeling indicates that hydroxyl hydrogen has not been incorporated into this product, excluding methylbenzene as a candidate. We also considered the coupling products such as phenylmethoxymethylbenzene (C6H5CH2OCH2C6H5) and 1,1’-Ethane-1,2-diyldibenzene (C6H5CH2CH2C6H5) as other possibilities. In separate experiments, we adsorbed multilayers of these molecules and determined their fragmentation patterns (see Figure S4 in SI). We found that m/z = 65 amu intensity is about a factor of three (for C6H5CH2CH2C6H5) or four (for C6H5CH2OCH2C6H5) lower than that of 91 amu. In contrast, the product desorbing in the β peak exhibits an intensity ratio close to unity at all coverages (See Figure S5 in SI), arguing strongly against formation of these molecules as reaction products. After excluding a range of possible products, we further consider the possibility of benzyl radical formation and desorption in the interpretation of the β TPD peak. While we do not have a reference fragmentation pattern of benzyl radicals for our experimental setup, the fragments observed in β TPD peak such as m/z = 91, 89, 65, 63, 51 and 39 amu (see survey TPD spectra in Figure S2, SI) are consistent with those of benzyl radicals published previously.32 In addition, the formation of free radicals has been extensively proposed for catalytic transformation of lignin model compounds to functionalized aromatics,33 and firmly established for oxidative coupling of methane and selective oxidation of propylene on high surface area metal oxide catalysts.34-36 Furthermore, our prior theoretical studies of diols on TiO2(110) show that the energy required for homolytic C-O bond scission of the alkoxy species is on the order of only 1 eV,10 which is consistent with the radical desorption temperature observed in these studies. To assess the feasibility of benzyl radical formation, we compared the TPD results from two QMS detection configurations. Generally, most of our TPD experiments were carried out in the line-of-sight configuration (Figures 1 and 2) where the sample is positioned in close proximity and faces the mass spectrometer to maximize the signal intensities. In the alternate configuration (“background”), the sample was rotated and moved away from the mass spec-
trometer. In this configuration, the desorbing species reflect multiple times from the walls of the UHV chamber before entering the mass spectrometer. Consequently, while the intensities for all of the masses are significantly lower, the proportional decrease for highly reactive species, such as benzyl radicals, is even greater. Thus changes in the fragmentation pattern are expected due to reactions with the chamber walls.
Figure 3. TPD spectra of selected mass fragments acquired in line-of-sight (black traces) and background (red traces) configurations, following 0.41 ML of C6H5CH2OH dose on rTiO2(110) at 100 K. The contribution of phenylmethanol to masses 91 and 65 amu was subtracted based on the measured fragmentation pattern of phenylmethanol in our system (see Figure S6 in SI). All spectra acquired in the background configuration were scaled by a factor of 6.3.
The comparison of the TPD spectra acquired in line-ofsight (black) and in background (red) configurations are shown in Figure 3. The signal in the background configuration is reduced by factor of ~6.3 as determined by adsorbing methylbenzene on r-TiO2(110) and performing TPD experiments in both configurations (Figure S7 in SI). The fragmentation pattern of the α peak remains identical, in accord with the results for adsorbed methylbenzene on rTiO2(110) (Figure S7 in SI). In contrast, the fragmentation pattern of the β peak changes dramatically with a concomitant intensity decrease in the background configuration, indicating that the desorbing species is highly reactive, supporting our hypothesis of benzyl radical desorption.
Figure 4. TPD spectra of various mass fragments (m/z) obtained from (a) h-TiO2(110) (red traces) and (b) o-TiO2(110) (blue traces). Both surfaces were exposed to 0.08 ML of
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C6H5CH2OH at 100 K. For comparison, the TPD results from r-TiO2(110) (black traces in both figures) exposed to the same coverage of C6H5CH2OH at 100 K are also included. On all three surfaces, the adsorbate is completely converted, as evidenced by the absence of a desorption peak at m/z = 79 amu.
Further insights into the product formation can be obtained by comparing the product distribution on rTiO2(110) with that on h- (Figure 4a) and o-TiO2(110) (Figure 4b). For h-TiO2(110), the only difference observed is the increased water desorption due to the initial surface hydroxylation.25,26 These results also establish that the reaction proceeds predominantly on Ti5c sites rather than on VO’s which are regenerated by HOb recombination at ~500 K,23,27,37 far exceeding the temperature for methylbenzene formation (Figure 2). In contrast, dramatic changes are observed on oTiO2(110). As expected based on prior studies,24,38 water desorption intensifies, shifts to lower temperature, and maximizes at ~350 K. This is a consequence of the presence of oxygen adatoms (Oa’s) on the Ti5c rows, which readily abstract hydroxyl hydrogen from alcohols to form water.24,38 As a result, hydroxyl hydrogen is not available for the formation of methylbenzene. This is clearly revealed in the Ccontaining product distribution which shifts from methylbenzene to benzyl radicals desorbing in a single peak at ~550 K. Note that in addition to benzyl radicals, a minor disproportionation channel at ~550 K leading to phenylmethanal and phenylmethanol is also observed on o-TiO2(110) (see TPD spectra in Figure S8 in SI). This is consistent with the disproportionation reaction of methanol on o-TiO2(110) observed in prior studies.24 Since the amount of Oa’s is limited by VO’s on the surface,25,26 this effect is most dramatic at low phenylmethanol coverages.
Figure 5. Proposed reaction pathways for the conversion of phenylmethanol on TiO2(110).
Taken together, the conversion of phenylmethanol proceeds sequentially on TiO2(110) as shown schematically in Figure 5. The overall reaction stoichiometries can be written as follows:
C6H5CH2OD + Ti5c + Ob → C6H5CH2Ot + DOb (1) C6H5CH2OD + Ti5c + Ob → C6H5CH2Ob + DOt (2) C6H5CH2Ob/t + DOb/t → C6H5CH2D +Ob/a (3) DOb + DOt → D2O + Oa (4) C6H5CH2Ob/t → C6H5CH2● + Ob/a (5) Similar to aliphatic alcohols in earlier work,11-13,24 phenylmethanol (take C6H5CH2OD as an example) initially undergoes O-D bond cleavage on Ti5c sites, yielding phenylmethoxys bound to Ti5c sites (C6H5CH2Ot) and DOb (eq. 1). Alternatively, it may also undergo C-O bond cleavage, yielding phenylmethoxys on Ob rows (C6H5CH2Ob)
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and DOt (eq. 2), as is seen for diols on TiO2(110).10 Upon heating to 300 K – 480 K, the benzyl group in phenylmethoxys (C6H5CH2Ob/t) abstracts hydroxyl hydrogen (DOb/t) to form methylbenzene (eq. 3). Simultaneously, the hydroxyls (DOb and DOt) can also recombine to form D2O (eq. 4). Following these steps, all the hydroxyl hydrogens are consumed by methylbenzene and water formation, and the surface is covered exclusively by phenylmethoxys. Further heating results in dissociation of the phenylmethoxys via C-O bond scission, yielding gasphase benzyl radicals and surface adsorbed Oa’s (eq. 5). Interestingly, if hydroxyls are intentionally introduced to the surface prior to the second step, methylbenzene is formed again (see Figure S9 in SI). The amount of methylbenzene and benzyl radicals desorbed can be quantitatively determined by the mass balance of hydroxyl hydrogen and carbon. Specifically, the amount of D2O desorbed is ~0.05 ML at a C6H5CH2OD coverage of 0.41 ML based on the TPD integrals (Figure 2b). The amount of methylbenzene desorbed is determined to be ~0.1 ML by considering that the total amount of C6H5CH2OD converted is ~0.2 ML at this coverage (Figure 1). As a result, an approximately equal amount of benzyl radicals (~0.1 ML) can be determined at this coverage on the basis of carbon mass balance. In a similar way, the amount of Oa’s left on the surface can be determined by oxygen mass balance. For example, the 0.05 ML of D2O (Figure 2b) combined with the ~0.2 ML of converted C6H5CH2OD (Figure 1), allows us to conclude that ~0.15 ML of Oa’s were left on the surface at phenylmethanol coverages above 0.3 ML. Regarding the fate of these Oa’s, we speculate that they probably react with Ti interstitials which diffuse from the bulk of TiO2 above ~360 K, as demonstrated in previous studies.39,40 While the Oa’s left on the surface likely result in sample oxidation, the surface is fully restored before each experiment as the TPD extends to 850 K, close to the temperature used to order the surface following the sputter−anneal cycles. The different surface chemistry between phenylmethanol reported herein and aliphatic alcohols in the previous studies on TiO2(110) could be explained by their structural differences. Except for methanol, previous studies of aliphatic alcohols have shown that their dehydration on TiO2(110) proceeds only on VO’s and that the detailed mechanism involves a simple concerted heterolytic C-O bond breaking/β-hydrogen transfer step of Ob-bound alkoxy species to form alkenes.11-13 Clearly, this reaction mechanism is not applicable to phenylmethanol (and methanol) due to the lack of β-hydrogen in the molecule. For methanol, the dissociation on r-TiO2(110) occurs on VO sites and yields CH3Ob and HOb species which recombine back to methanol at ~480 K.24,41 On o-TiO2(110), the hydroxy hydrogen is consumed in the reactions with O adatoms yielding water at ~350 K.24,38 The remaining methoxy species on the Ti5c sites (CH3Ot) undergo disproportionation to form methanol and formaldehyde above 600 K.24 As already mentioned above, this channel is a minority channel for phenylmethanol (see Figure S8). Instead, hydrogenation to methylbenzene occurs already slightly
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above room temperature indicating a facile C-O bond cleavage on Ti5c rows. It is likely that this is a consequence of the presence of the delocalized π system on the phenyl ring that can effectively stabilize the ionic or radical transition states yielding methylbenzene and at higher temperatures to benzyl radicals as products. In summary, we have demonstrated that the conversion of phenylmethanol on TiO2(110) proceeds sequentially with the first step being the transfer of hydroxyl hydrogen to the benzyl group to form methylbenzene and the simultaneous recombination of hydroxyls to form water. This is followed by desorption of benzyl radicals from the surface now covered exclusively by phenylmethoxys following the first step. These findings provide a hitherto overlooked reaction pathway for alcohols on TiO2(110), and may also open up a new way to convert lignin-derived benzylic alcohols directly to aromatics on TiO2.
ASSOCIATED CONTENT Supporting Information. The calibration of beam flux for phenylmethanol. Survey TPD spectra with a complete set of mass fragments. Fragmentation patterns of methylbenzene, phenylmethanol, phenylmethoxymethylbenzene and 1,1’ethane-1,2-diyldibenzene in our system. The evolution of 91 and 65 amu fragments as a function of phenylmethanol coverage. Methylbenzene TPD spectra in both line-of-sight and background configurations. Survey TPD spectra acquired from o-TiO2(110). TPD spectra obtained from TiO2(110) covered only by phenylmethoxys and by a combination of phenylmethoxys and 1 ML of water (H2O and D2O). This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author *
[email protected] Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT This work was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences, and performed in EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is a multi-program national laboratory operated for the DOE by Battelle. We thank N. G. Petrik, M. Henderson, J. Szanyi and R. Rousseau for numerous stimulating discussions.
ABBREVIATIONS
TPD, temperature programmed desorption; Ob, bridge bonded oxygen; VO, bridge bonded oxygen vacancy; UHV, ultrahigh vacuum; ML, monolayer; SI, supporting information; QMS, quadrupole mass spectrometer; Ti5c, 5-fold coordinated titanium sites; HOt, terminal hydroxyl; HOb, bridging hydroxyl; Oa, oxygen adatom; C6H5CH2Ot, phenylmethoxys bound to Ti5c sites; C6H5CH2Ob, phenylmethoxys on Ob rows.
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