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Mar 8, 2013 - Hydrodeoxygenation (HDO) of dibenzofuran (DBF) with only a hydrogenation (HYD) reaction route was conducted on Pd/COK-12 catalysts with ...
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Hydrodeoxygenation of Dibenzofuran over Mesoporous Silica COK12 Supported Palladium Catalysts Lei Wang, Mingming Zhang, Miao Zhang, Guangyan Sha, and Changhai Liang* Laboratory of Advanced Materials and Catalytic Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116012, China ABSTRACT: Hydrodeoxygenation (HDO) of dibenzofuran (DBF) with only a hydrogenation (HYD) reaction route was conducted on Pd/COK-12 catalysts with different Pd loadings, which were prepared by metal−organic chemical vapor deposition of [Pd(C3H5)(C5H5)] onto mesoporous silica COK-12. Hydrogenated intermediates with partial or full hydrogenation of the benzene ring were studied in detail. The presence of cycloketones is due to the high selectivity of Pdbased catalysts in the hydrogenation of phenols derived from the C−O bond cleavage of hydrogenated intermediates. The reaction only goes through the HYD reaction route with hardly detected biphenyl, which is further confirmed by HDO of 2phenylphenol. The increase of Pd loading on the catalysts largely promotes the deoxygenation of saturated oxygen-containing species from hydrogenation of DBF, followed by cleavage of the sp3 C−O bond. Finally, a detailed reaction network of HDO of DBF is proposed according to the detected hydrogenated species and cycloketones.

1. INTRODUCTION Dibenzofuran (DBF) is one typical oxygen-containing compound in the nonconventional fuel liquids, such as coal tar and shale oil.1,2 Oxygen removal from fuel liquids is necessary to ensure the stability of oil during transportation and further processing. In addition, DBF is reported as a kind of polycyclic aromatic hydrocarbon (PAH) largely derived from petroleum products, wood treatment processes, and biomass upgrading, and these PAHs share the basic molecular structure of fluorene, which are toxic, carcinogenic, and tend to bioaccumulate in aquatic organisms.3−6 Effective conversion of DBF-like PAHs in the initial hydroprocessing of fuel liquids can significantly reduce the amount of PAHs released to the environment. HDO of oxygen-containing aromatics mainly obtained from transformation of biomass attracts much attention recently.7−11 The selective HDO of these compounds over supported noble metal catalysts can produce alcohol and ketone derivates or deoxygenated products.12−14 Meanwhile, many HDO reaction networks of oxygen-containing compounds are presented.8,11,15,16 A number of studies on the catalytic HDO of DBF over sulfide Ni(Co)−Mo catalysts have been conducted, and several HDO reaction networks of DBF have been proposed.2,17−20 The HDO reaction of DBF over sulfide catalysts followed the HYD reaction route, in which ring hydrogenation occurs, followed by oxygen removal, and the direct deoxygenation (DDO) reaction route through C−O bond hydrogenolysis. Hall and Cawley investigated the HDO of DBF over supported and unsupported Mo catalysts at 10 MPa and 300−450 °C.19 A reaction pathway, that is, benzene ring hydrogenation with 2cyclohexylphenol (CHPOH), was postulated. Biphenyl (BP) and 2-phenylphenol (PPOH) were obtained above 450 °C without requiring ring hydrogenation. With the further development of an HDO reaction network, Krishnamurth and co-workers reported some single ring products (phenol and cyclohexane) cracked from CHPOH.17 Some five-membered ring products isomerized from cyclohexyl-containing hydro© 2013 American Chemical Society

carbons were added into the network by Satterfield in the study of the catalytic HDO of DBF on Ni(Co)−Mo/Al2O3 catalysts.1 Furimsky reviewed the catalytic HDO of oxygen-containing compounds in petroleum, including phenols, naphthols, furans, ethers, and acids.20 Product analysis and the mechanism of HDO of DBF, as a kind of furans, were well summarized based on previous research. However, the HDO of DBF over metal sulfide catalysts shows some drawbacks in the hydrogenation activity compared with metal catalysts, leading to insufficient ring hydrogenation and excessive C−O bond cleavage. Recently, the HDO of DBF over noble metal Pt supported on mesoporous ZSM-5 zeolites was reported.21 An HYD reaction pathway of DBF at mild temperature (200 °C) was proposed with some cycloolefin intermediates detected. Overviewing the reaction network development, more efforts still need to be put to examine the benzene ring hydrogenated intermediates in order to further understand the HYD reaction routes of HDO of DBF. Thus, noble metal Pt and Pd with superior hydrogenation activity are taken into consideration for the HDO of DBF. Throughout the development of the HDO reaction network of DBF, further hydrogenation of phenol intermediates from C−O bond cleavage of hydrogenated DBF to ketones is rarely reported. After comparing the hydrogenation of phenol over Pt and Pd catalysts, it was reported that Pt catalyst showed a better selectivity to cyclohexanol than Pd catalyst on which more hexanone was yielded under the same reaction conditions.22 More results about high selectivity to cycloketone over Pd catalysts in hydrogenation of phenols were reported in the literature.23−25 Therefore, supported Pd catalysts are used in our work to study the transformation of phenols into cycloketones. Received: December 26, 2012 Revised: March 7, 2013 Published: March 8, 2013 2209

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Figure 1. Small-angle (a) and wide-angle (b) XRD patterns of Pd/COK-12 catalysts with different Pd loadings. pressures varying from 80 to 560 mmHg at 30 °C. After completing the initial analysis, the reversibly adsorbed gas was evacuated and the analysis was repeated to determine the chemisorbed molecules alone. A CO/Pd stoichiometry of 1/1 was used to estimate the number of active Pd sites. 2.3. Catalyst Test. The HDO of DBF experiments were performed at 300 °C and 3.0 MPa total pressures in a continuousflow fixed-bed reactor over 60 mg of Pd/COK-12 catalyst diluted with 2.74 g of 60−80 mesh quartz sands. Before the HDO experiments, the palladium catalysts were activated in situ with 40 mL/min H2 at 3.0 MPa and 300 °C for 1 h. The liquid reactants were composed of 3.0 wt % DBF, 1.0 wt % dodecane (as internal standard for gas chromatography (GC) analysis), and 96.0 wt % decane (as inert solvent). The experimental data were collected at different weight times after the fresh catalyst reached steady state for 14 h. The weight time is defined as τ = Wcat/nfeed, where Wcat denotes the catalyst weight and nfeed denotes the total molar flow fed to the reactor. The reaction products after being condensed in a trap at room temperature were collected and analyzed using a GC 7890F with a flame ionization detector and a 0.25 mm × 30 m HP-5 capillary column. Product identifications were conducted on an Agilent 6890N with 5973 MSD and a 0.25 mm × 30 m HP-5MS capillary column. To separate the reaction products, the temperature in the GC oven was heated from 100 to 180 °C with the ramp of 15 °C/min, held at 180 °C for 2.0 min, then heated to 220 °C at a rate of 10 °C/min and kept at 220 °C for 5.0 min. The time for solvent delay was 3.3 min. To verify the full hydrogenation products of DBF, hydrogenation of DBF over a 2.0 wt % Ru/C catalyst was carried out at 100 °C and 4.0 MPa total pressure in the same reactor. To further study the reaction mechanism for the HDO reaction of DBF, the HDO of the PPOH reaction was performed at the same reaction conditions over 0.5% Pd/COK-12 such that the reactant DBF was replaced by the same amounts of PPOH. The conversion (X) and selectivity (S) in the HDO reaction of DBF were used to present the hydrogenation and deoxygenation activity of the prepared Pd/COK-12 catalysts, respectively, which were calculated as

To gain a better insight into HYD reaction routes of HDO of DBF and the intermediates, we prepared a series of Pd catalysts supported on mesoporous silica COK-12 using the MOCVD method. The chosen model compound DBF with a large molecular size could diffuse easily in the mesopores of COK12.26 Deposition of noble metal Pd onto a support by MOCVD is readily available to prepare highly dispersed Pd catalysts with superior hydrogenation activity.27,28 HDO of DBF over asprepared Pd/COK-12 catalysts only proceeds via the HYD reaction route, and a more detailed reaction network is proposed based on the hydrogenated and cycloketone intermediates.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Mesoporous silica COK-12 was prepared by the method from Martens.29−31 The Pd deposition onto the COK-12 support was completed by metal−organic chemical vapor deposition (MOCVD) of palladium metal−organic complex [Pd(C3H5)(C5H5)], which was synthesized according to the literature.27,32 Before MOCVD, the COK-12 was calcined at 500 °C under 30 mL/min O2 for 2 h to remove the physically adsorbed water and impurities, which promotes the loading of the palladium precursor. The loading method of [Pd(C3H5)(C5H5)] onto COK-12 followed the procedure described in our previous work.33 A certain amount of precursor was mixed well with 0.40 g of fresh calcined COK-12 in a Schlenk tube, then sublimed and adsorbed on the COK12 support at 30 °C in vacuum. The COK-12 adsorbed precursor was reduced under 30 mL/min H2, and 300 °C for 2 h. The reactor was heated from room temperature at a rate of 2 °C/min. The Pd/COK12 catalysts with different Pd loadings (0.5, 1.0, 1.5, and 2.0 wt %) were prepared by the above method. 2.2. Catalyst Characterization. X-ray diffraction (XRD) analysis of the samples was carried out using a Rigaku D/Max-RB diffractometer with Cu Kα monochromatized radiation source (λ = 1.54178 Å), operated at 40 KV and 100 mA. Nitrogen adsorption and desorption isotherms at 77 K were measured by using a Quadrasorb SI surface area and pore size analyzer. The specific surface areas of samples were calculated by the BET (Brunauer−Emmett−Teller) method, and pore volumes were calculated from the volume of liquid nitrogen at p/p0 = 0.98. TEM and SEM images were taken on an FEI Tecnai G20 and NOVA NanoSEM 450, respectively. Element analysis was performed using inductively coupled plasma atomic emission spectroscopy (ICP-AES) (PerkinElmer Optima 2000DV). CO chemisorption was performed in a Quadrasorb IQ apparatus under static volumetric conditions. Prior to measurement, the reduced sample was activated in situ in H2 at 300 °C for 2 h and evacuated for 2 h, and then the furnace was cooled to 30 °C. The chemisorption isotherm was obtained by measuring the amount of CO adsorbed for

X = (n0 − nDBF)/n0 × 100%

(1)

S = ni /∑ ni × 100%

(2)

where n0 and nDBF are the number of molecules of DBF in the feed and product, whereas ni represents the number of molecules of a defined HDO product and ∑ni are the total number of molecules of HDO products. Turnover frequency values were calculated from the formula34

TOF = (F /W ) × X /M 2210

(3)

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Table 1. Pd Loadings and Textural Parameters of Pd/COK-12 Catalysts sample COK-12 0.5 wt % 1.0 wt % 1.5 wt % 2.0 wt %

Pd/COK-12 Pd/COK-12 Pd/COK-12 Pd/COK-12

Pd loading (%)

BET surface area (m2/g)

pore volume (cc/g)

pore size (nm)

0.49 0.97 1.31 1.85

481 455 439 426 404

0.48 0.46 0.45 0.40 0.38

5.6 5.6 5.6 5.5 5.0

where F is the molar rate of reactant, W is the catalyst weight, and M is the mole of active sites.

materials. The pore size distribution calculated from the BJH adsorption branch is narrow, with a mean pore diameter of 5.5 ± 0.5 nm for all samples. When increasing the Pd loading, the BET surface area and pore volume of the Pd/COK-12 catalyst decreased from 481 to 404 m2/g and 0.48 to 0.38 cc/g, respectively. These results show that loading of Pd by the MOCVD method has little effect on the mesoporous structure of COK-12 and highly dispersed Pd particles might mostly locate in the mesopores of COK-12. SEM image (Figure 3a) shows that the calcined COK-12 is in a cubic shape with a size of around 500 nm. TEM images of fresh 1.0 and 2.0 wt % Pd/COK-12 and spent 1.0 wt % Pd/ COK-12 catalysts are exhibited in Figure 3b−d. The typical hexagonal structures of COK-12 are well displayed for all prepared samples and are still maintained after loading Pd by MOCVD and after the HDO reaction of DBF. All Pd nanoparticles on the support have a narrow particle size distribution, that is, 1.7 ± 0.3 nm, regardless of the metal loading. Most of the particles are well dispersed in the mesopores of COK-12. No apparent aggregation is seen. The size of Pd particles for the 1.0 wt % Pd/COK-12 catalyst is almost the same before and after HDO reaction of DBF. 3.2. HDO of DBF. Some species detected during the HDO reaction of DBF over the Pd/COK-12 catalysts are tetrahydrodibenzofuran (THDBF), hexahydrodibenzofuran (HHDBF), and dodecahydrodibenzofuran (DHDBF), which can be the products of partial or total hydrogenation of the benzene ring of the reactant DBF. 2-Cyclohexylcyclohexanone (CHCHO), CHPOH, and 2-phenylcyclohexanol (PCHOH) were also detected and are presumed to be the intermediates resulting from C−O cleavage of hydrogenated species. The deoxygenation products with unsaturated double bonds, such as cyclohexylbenzene (CHB), cyclohexenylbenzene (CHEB), and cyclohexenylcyclohexane (CHCHE), were observed as well, while bicyclohexane (BCH) and cyclopentylmethylcyclohexane (iso-BCH) present in the products are the deoxygenated hydrocarbons and isomerization product of BCH, respectively. A typical GC-MS spectrum of HDO products of DBF with the product name abbreviation is shown in Figure 4. On the basis of the products detected in this study, the HYD reaction route could be the only pathway for the HDO reaction of DBF over Pd/COK-12 catalysts; namely, the benzene ring of DBF was hydrogenated first, followed by hydrogenolysis of the C−O bond. This is consistent with the results reported over Ni(Co)−Mo sulfide catalysts.1,17 From the product distribution in the HDO of DBF over the 2.0 wt % Pd/COK-12 catalyst (Figure 5), DBF took 74% amount in the product distribution at the lowest weight time and almost was consumed completely at high weight times. DBF was hydrogenated to THDBF and HHDBF simultaneously, both of which attained maximum values (6% and 14%) at τ = 0.83 g*min/mol. DHDBF was obtained by further hydrogenation of HHDBF, and the maximum amount of DHDBF (20%) was obtained at a high weight time. It can be predicted that DHDBF might transform

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. The small-angle XRD patterns of all Pd/COK-12 catalysts in Figure 1 display wellresolved peaks that represent the characteristic (100) reflections of mesoporous silica COK-12. This suggests that loading 2.0 wt % Pd onto the COK-12 support through MOCVD did not change its structure. Other characteristic reflections of the mesoporous silica, (110) and (200) reflections, are hardly found from the small-angle XRD patterns (Figure 1a) of the prepared catalysts. This could imply that the synthesized mesoporous silica COK-12 was less ordered than that reported in the literature.31 The intensity of the (100) reflection decreased after loading Pd, which is more distinctive for the 2.0 wt % Pd/COK-12 catalyst. This phenomenon might be due to the partial loss of structural order, or the reduced scatter contrasts between the pores and walls after loading metal particles in the pore channels of mesoporous silica as in the case of SBA-15 and MCM-41.35,36 It may suggest that the noble metal Pd was loaded heterogeneously onto the COK-12 support. From the wide-angle XRD patterns of the Pd/COK-12 catalysts shown in Figure 1b, only wide peaks representing SiO2 are observed. No Pd peaks were seen, indicating that the Pd was highly dispersed on the COK-12 support. The Pd loadings for each Pd/COK-12 catalyst were measured by ICP-AES and are summarized in Table 1. It can be seen that the actual Pd loading varies from 0.45 to 1.85 wt %, which is slightly less than the nominal composition. The nitrogen adsorption−desorption isotherms of Pd/COK-12 catalysts are shown in Figure 2. All isotherms are type IV isotherms and display the H1 hysteresis loops with steep branches, which are typical characteristics for mesoporous

Figure 2. Nitrogen adsorption−desorption isotherms and pore size distribution (inset) of Pd/COK-12 catalysts with different Pd loadings. 2211

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Figure 3. SEM image (a) of calcined COK-12 support and typical TEM images of Pd/COK-12 catalysts: (b) fresh 1.0 wt % Pd/COK-12, (c) fresh 2.0 wt % Pd/COK-12, and (d) spent 2.0 wt % Pd/COK-12.

Figure 4. Typical GC-MS chromatogram of HDO products of reactant DBF over 2.0 wt % Pd/COK-12 catalyst.

°C). CHCHOH was reported to be the ring hydrogenation product of CHPOH in the HDO of DBF at 200 °C, where further hydrogenation of HHDBF did not take place, but only

into CHCHOH with the cleavage of the C−O bond due to the detection of CHCHE, which is the product of CHCHOH after eliminating the hydroxyl group at such a high temperature (300 2212

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selectivity is low due to the difficulty in breaking the existing p−π conjugation in benzene ring electrons and O lone-pair electrons. Similar results, that is, it is harder to break the sp2 C− O bond than the sp3 C−O bond in the lignin during the biomass upgrading, were reported.37 The CHPOH was assumed to be further hydrogenated to CHCHO and CHCHOH. The high selectivity to CHCHO may be due to the nature of the Pd catalysts, on which ketone was more selectively formed relative to alcohol in the hydrogenation of phenols.8,22,38 Analyzing the product distribution of HHDBF, CHPOH, and CHCHO, it appears that the hydrogenation and C−O bond cleavage of HHDBF are parallel reactions. Since CHCHE could be rapidly hydrogenated to BCH and iso-BCH, the amount of CHCHE in the products was found to be low. The products BCH and iso-BCH could also be obtained from the hydrogenation of PCHOH. CHEB could be the dehydration product of PCHOH by eliminating the hydroxyl group in the cycloalkanol, and it can be further hydrogenated to form CHB. Therefore, the reaction routes for the HDO of DBF on the Pd catalysts could be that DBF was first hydrogenated to THDBF and HHDBF, and then these intermediate products were transformed to DHDBF and CHCHO, which finally deoxygenated to BCH and iso-BCH. The formation of DHDBF was evident by the hydrogenation of DBF over Ru/C catalyst at a low temperature of 100 °C, at which the C−O bond cleavage rarely occurred. DBF was hydrogenated to THDBF, HHDBF, and DHDBF, and CHCHO and DHDBF were found to be the most abundant products. From the GC spectrogram of DHDBF (Figure 6), three clear peaks representing different structured DHDBF molecules are well exhibited, which is consistent with Eblagon

Figure 5. Product distribution of HDO of DBF over 2.0 wt % Pd/ COK-12 at 300 °C and 3.0 MPa. All detected products with relatively large (a) and small (b) amounts are shown in the figure.

cleavage of the C−O bond occurred over the bifunctional Pt/ ZSM-5 catalysts.21 Meanwhile, HHDBF can also be converted to CHPOH and PCHOH through C−O bond cleavage. The PCHOH is the only species detected by GC-MS, but its

Figure 6. Typical GC chromatogram of HDO products of DBF over Ru/C catalyst. 2213

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more than 90% of total products are chosen to explain the influence of metal loadings on this reaction. At a fixed weight time (τ = 1.37 g*min/mol), the conversion of DBF increased from 11% to 84% when the metal loading was increased from 0.5 to 2.0 wt % (Figure 8a). This could have resulted from more active metal sites on the higher Pd loading catalyst. In Table 2, TOFs for HDO of DBF over Pd/COK-12 catalysts with different Pd loadings were approximately the same with a value of ∼3.2 min−1. This suggests that the loading amount of metal in the catalysts hardly influences the transformation of DBF in the reaction at a fixed reaction condition. As for the 2.0 wt % Pd/COK-12, there might be some metal particles aggregating, which leads to reduced active sites in the reaction. This behavior will result in a slightly higher TOF for HDO of DBF. The selectivities to THDBF and HHDBF decreased with increasing Pd loadings, but the fully deoxygenated product BCH increased dramatically (Figure 8b−e). This suggests that Pd can largely promote the deoxygenation of the hydrogenated intermediates in the HDO of DBF. The relative concentration of BCH in the products increased from 0.4% to 29.4%, indicating that the loading amount of Pd plays an important role on the deoxygenation in the HDO of DBF. DHDBF and CHCHO have a similar trend, which lightly increases at low weight time and then decreases at high weight time for all Pd loading catalysts. These phenomena suggest that DBF was mainly hydrogenated to THDBF and HHDBF, in contrast with DHDBF at low weight time, while the produced DHDBF tends to be deoxygenated at high weight times, resulting in less partially and fully hydrogenated products. CHPOH formed from C−O bond cleavage of HHDBF during the reaction over 0.5 and 1.0 wt % Pd/COK-12 catalysts is high (∼16%), which further suggests that hydrogenation of DBF was a major reaction compared with the deoxygenation over the low Pd loading catalysts. Large amounts of CHPOH detected are evident that CHCHO was formed through the hydrogenation of the phenolic group. 3.5. Reaction Network of HDO of DBF over Pd/COK-12 Catalyst. The HDO of DBF reaction over the Pd/COK-12 catalysts was suggested to proceed via the HYD reaction route (Scheme 1), which is different from the studies over metal sulfide catalysts, in which both HYD and DDO reaction routes were proposed.1,17 Benzene rings in the DBF were hydrogenated first on the surface of Pd nanoparticles, which could be stable in the mesopores of COK-12. This assumption is consistent with the HDS of DBT over supported Pd catalysts.43−45 Aromatic rings being partially or totally hydrogenated were detected in their work, which explained the HYD of DBT in the HDS reaction. Removal of the S atom, followed by the hydrogenation of benzene rings of DBT, was reported. Cleavage of the sp3 C−O bond in the hydrogenated DBF derivates (HHDBF and DHDBF) was assumed to occur in the HDO reaction of DBF since the cleavage of the sp2 C−O bond was energetically unfavorable compared with the sp3 C−O bond.37 However, the cleavages of both bonds can happen together in the HHDBF due to the presence of sp2 C−O and sp3 C−O bonds. CHCHO from the hydrogenation of CHPOH is much more abundant than PCHOH from the sp2 C−O bond cleavage of HHDBF according to our results. This is because the Pd catalysts have high selectivity toward ketone structured products in the hydrogenation of phenols and the HDO reaction mainly took place through the hydrogenolysis of the saturated C−O bond in the hydrogenated DBF intermediates. The formation of CHCHO and DHDBF was proposed through

and co-workers’ study in which similar molecule structured reactants were reviewed in the catalytic hydrogenation of 9ethylcarbazole.39 3.3. HDO of 2-Phenylphenol. PPOH might be an intermediate during the HDO of DBF, which can be obtained from the hydrogenation of the benzene ring, followed by the cleavage of the C−O bond and the dehydrogenation of the benzene ring.20,40 To further verify the HDO reaction route of DBF, HDO of PPOH over 0.5 wt % Pd/COK-12 was carried out at the same reaction conditions as those for the HDO of DBF. The observed products are PCHOH, 2-phenylcyclohexanone (PCHO), CHPOH, and CHCHO. CHCHOH was not detected under the reaction conditions used. CHB, cyclopentylmethylbenzene (iso-CHB), CHEB, BCH, and isoBCH were obtained, which might be originated from further deoxygenation of the hydrogenated species. CHB and BCH are an intermediate and a product, respectively. From the product distribution in the HDO of PPOH over the 0.5 wt % Pd/COK12 catalyst (Figure 7), the high amount of CHB (42%)

Figure 7. Product distribution of HDO of 2-phenylphenol over 0.5% Pd/COK-12 at 300 °C and 3.0 MPa. All detected products with relatively large (a) and small (b) amounts are shown in the figure.

suggested that selective hydrogenation of PPOH over the Pd/ COK-12 catalysts occurred on the benzene ring of the phenol group, which could be due to the nature of the SiO2 support that affected the adsorption modes of PPOH.41,42 Small amounts of BP were detected in the DDO product of PPOH by GC and GC-MS, suggesting that the HDO reaction of PPOH proceeded through both the HYD and the DDO reaction routes, but with more preference to the HYD route. The absence of DBF in the HDO reaction of PPOH may imply that the DDO reaction route of DBF on the Pd/COK-12 catalysts is irreversible. 3.4. Effect of Pd Loadings on HDO of DBF over Pd/ COK-12 Catalyst. HDO reactions of DBF on the prepared Pd/COK-12 catalysts with different Pd loadings show similar product distribution. The products, such as BCH, DHDBF, HHDBF, CHCHO, THDBF, and CHPOH, that contributed to 2214

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Figure 8. Conversion (a) and product selectivity in the HDO of DBF over 0.5 (b), 1.0 (c), 1.5 (d), and 2.0 wt % (e) Pd/COK-12 catalysts at 300 °C and 3.0 MPa.

Table 2. CO Uptake for the Pd/COK-12 Catalysts and TOFs for HDO of DBF at τ = 1.37 g*min/mol under Operating Conditions sample 0.5 1.0 1.5 2.0

wt wt wt wt

% % % %

Pd/COK-12 Pd/COK-12 Pd/COK-12 Pd/COK-12

CO uptake (μmol/g)

conversion (%)

TOF (min−1)

16.7 37.4 66.0 88.6

11 27 49 84

3.2 3.3 3.4 4.4

°C) was used. Finally, the CHEB was proposed to be formed by the dehydration of the hydroxyl group in the cycloalkanol, a substituent of PCHOH, and it can be further hydrogenated to form CHB.

the hydrogenation of the CO bond and sp3 C−O bond cleavage to CHCHOH, respectively. For the formation of BCH and its isomer iso-BCH, it could be that the hydroxyl group in the cycloalkanol dehydrates rapidly to form CHCHE, followed by hydrogenation, because a high reaction temperature (300 2215

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Scheme 1. Proposed HDO Reaction Network of DBF over Pd/COK-12 Catalysts

(3) Shemer, H.; Linden, K. G. Aqueous photodegradation and toxicity of the polycyclic aromatic hydrocarbons fluorene, dibenzofuran, and dibenzothiophene. Water Res. 2007, 41, 853−861. (4) Huelsman, C. M.; Savage, P. E. Intermediates and kinetics for phenol gasification in supercritical water. Phys. Chem. Chem. Phys. 2012, 14, 2900−10. (5) Dickinson, J. G.; Poberezny, J. T.; Savage, P. E. Deoxygenation of benzofuran in supercritical water over a platinum catalyst. Appl. Catal., B 2012, 123−124, 357−366. (6) Jongerius, A. L.; Jastrzebski, R.; Bruijnincx, P. C. A.; Weckhuysen, B. M. CoMo sulfide-catalyzed hydrodeoxygenation of lignin model compounds: An extended reaction network for the conversion of monomeric and dimeric substrates. J. Catal. 2012, 285, 315−323. (7) Zhao, C.; He, J.; Lemonidou, A. A.; Li, X.; Lercher, J. A. Aqueousphase hydrodeoxygenation of bio-derived phenols to cycloalkanes. J. Catal. 2011, 280, 8−16. (8) Liu, C.; Shao, Z.; Xiao, Z.; Williams, C. T.; Liang, C. Hydrodeoxygenation of benzofuran over silica−alumina-supported Pt, Pd, and Pt−Pd catalysts. Energy Fuels 2012, 26, 4205−4211. (9) Alonso, D. M.; Wettstein, S. G.; Dumesic, J. A. Bimetallic catalysts for upgrading of biomass to fuels and chemicals. Chem. Soc. Rev. 2012, 41, 8075−98. (10) Zhu, X. L.; Lobban, L. L.; Mallinson, R. G.; Resasco, D. E. Bifunctional transalkylation and hydrodeoxygenation of anisole over a Pt/HBeta catalyst. J. Catal. 2011, 281, 21−29. (11) Nimmanwudipong, T.; Runnebaum, R. C.; Block, D. E.; Gates, B. C. Catalytic conversion of guaiacol catalyzed by platinum supported on alumina: Reaction network including hydrodeoxygenation reactions. Energy Fuels 2011, 25, 3417−3427. (12) Jiang, Y. J.; Buchel, R.; Huang, J.; Krumeich, F.; Pratsinis, S. E.; Baiker, A. Efficient solvent-free hydrogenation of ketones over flameprepared bimetallic Pt-Pd/ZrO2 catalysts. ChemSusChem 2012, 5, 1190−1194. (13) Lee, C. R.; Yoon, J. S.; Suh, Y.-W.; Choi, J.-W.; Ha, J.-M.; Suh, D. J.; Park, Y.-K. Catalytic roles of metals and supports on hydrodeoxygenation of lignin monomer guaiacol. Catal. Commun. 2012, 17, 54−58. (14) Huang, J.; Jiang, Y.; van Vegten, N.; Hunger, M.; Baiker, A. Tuning the support acidity of flame-made Pd/SiO2−Al2O3 catalysts for chemoselective hydrogenation. J. Catal. 2011, 281, 352−360. (15) Runnebaum, R. C.; Lobo-Lapidus, R. J.; Nimmanwudipong, T.; Block, D. E.; Gates, B. C. Conversion of anisole catalyzed by platinum

4. CONCLUSION Supported Pd/COK-12 catalysts with different Pd loadings were successfully prepared by the metal−organic chemical vapor deposition method, which is a highly efficient way to embed Pd nanoparticles into mesoporous silica COK-12. The HDO of DBF over prepared Pd/COK-12 catalysts is proposed to take place through the HYD reaction route, which requires a fully saturated benzene ring before the removal of heteroatom oxygen. The HDO of 2-phenylphenol with the product of BP further suggested that the HYD reaction of DBF occurred over the Pd/COK-12 catalysts. Increasing the Pd loading largely promoted the deoxygenation of the saturated oxygencontaining species that was formed from the hydrogenation of DBF, followed by the sp3 C−O bond cleavage. Therefore, based on the hydrogenated and cycloketone intermediates detected, the above HDO reaction network of DBF via the HYD reaction route was proposed in this study.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-411-84986056. E-mail: [email protected]. Homepage: http://finechem.dlut.edu.cn/liangchanghai/. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support provided by the National Science Foundation of China (21073023 and 20906008) and the Fundamental Research Funds for the Central Universities (DUT12YQ03).

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