Gas-Phase Hydrodeoxygenation of Benzaldehyde, Benzyl Alcohol

Article pubs.acs.org/IECR

Gas-Phase Hydrodeoxygenation of Benzaldehyde, Benzyl Alcohol, Phenyl Acetate, and Anisole over Precious Metal Catalysts Celeste González, Pablo Marín, Fernando V. Díez, and Salvador Ordóñez* Department of Chemical and Environmental Engineering, Facultad de Química, University of Oviedo, Julián Clavería 8, Oviedo 33006, Spain S Supporting Information *

ABSTRACT: The aim of this work is to study the gas-phase hydrodeoxygenation (HDO) of bio-oil-related platform molecules over precious metal catalysts (Pt and Pd on alumina) at moderate temperature (325 °C) and low pressure (0.5 MPa). Bio-oils consist of a complex mixture of compounds, many of which are aromatic oxygenates, and have been modeled here by four model compounds with different oxygenated functional groups: benzaldehyde (aldehyde), benzyl alcohol (alcohol), phenyl acetate (ester), and anisole (ether). First, the stability and activity of the catalysts in the HDO of the different compounds have been studied in an isothermal fixed-bed reactor. The performance of the Pt catalyst is better than that of the corresponding Pd catalyst (highest activity and selectivity). The compound more refractory to HDO over Pt was benzyl alcohol. Both catalysts suffer from strong deactivation at the beginning of the tests, mainly by the formation of carbonaceous deposits. The deactivation is particularly strong for the HDO of benzyl alcohol over the Pd catalyst. Second, a kinetic study has been carried out varying the space time (0−1 (kgcat s)/mol). Kinetic models, based on reaction networks derived from the product distributions, have been proposed, and the kinetic constant fit to the experimental data by least-squares regression.

1. INTRODUCTION Producing biofuels from renewable lignocellulosic feedstocks is of great interest in order to increase the yield and reduce the wastes produced in the biofuel production process. The chemical transformation of lignocellulose biomass typically consists of two steps: depolymerization into simpler molecules and upgrading to a biofuel meeting the required specifications. Biomass depolymerization by pyrolysis is a thermal decomposition process that produces pyrolysis oil (bio-oil) consisting of a complex mixture of hydrocarbons with high oxygen content, predominantly aromatics, and water. Some oxygenated functional groups present in bio-oils are carbonyl, aldehyde, ketone, alcohol, and ester.1−3 Oxygen content in biooils is usually high, 35−40%, whereas in heavy petroleum fuel, oil is only around 1%.4−6 The presence of oxygenated compounds deteriorates the properties of bio-oils as biofuels, increasing their viscosity, acidity, and instability and decreasing their volatility and energy density. The key scope of the bio-oil treatments is to reduce its oxygen content. The selected technology will depend on the bio-oil properties and the final product specifications. Thus, the use of liquid−liquid extraction after the pyrolysis separates most of the water, which is enriched in phenols and organic acids that can be purified to be used in the chemical industry. The remaining organic phase is upgraded to adjust the oxygen content by catalytic (hydro)cracking or hydrotreating. The latter is the scope of the present work. © 2016 American Chemical Society

The catalytic hydrotreating leads to bio-oil hydrodeoxygenation (HDO) by catalytic reaction with hydrogen at high temperatures and pressures.7−9 Conventional hydrotreating catalysts, such as CoMo/Al2O3 and NiMo/Al2O3, are used at industrial scale for the upgrading of petroleum fractions (simultaneous removal of sulfur, oxygen, and nitrogen, all present in low amounts). Though the use of these catalysts for hydrotreating of bio-oils has been extensively studied,7,10,11 they present some disadvantages, for example, the requirement of adding an external source of sulfur to maintain the catalyst active phase in sulfide form.7 The use of catalysts based on precious metals as alternative to conventional hydrotreating catalysts has been proposed and studied. The potential advantages, mainly the high activity of precious metals even at low pressure and temperature, would result in smaller reactors and other economic advantages.12−15 The activity and stability of catalysts for bio-oils hydrodeoxygenation can be conveniently studied by means of model compounds representative of aromatic oxygenated compounds typically present in bio-oils, such as aromatic acids, esters, ketones, and alcohols, among others.1−3 In the present work, four model compounds have been studied as representative of Received: Revised: Accepted: Published: 2319

January 4, 2016 February 10, 2016 February 12, 2016 February 12, 2016 DOI: 10.1021/acs.iecr.6b00036 Ind. Eng. Chem. Res. 2016, 55, 2319−2327

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Industrial & Engineering Chemistry Research

abundant in bio-oils. However, HDO of phenyl acetate has been scarcely studied. The primary reaction products generated by hydrolysis of phenyl acetate are phenol and acetic acid. Phenol is further hydrogenated to benzene (Figure 1b).31 It can be observed that anisole and phenyl acetate share phenol as reaction intermediate. For this reason, in this work results for HDO of both reactants have been discussed together. The present work is focused on the HDO of the aforementioned compounds in the gas phase, using Pt and Pd supported on γ-Al2O3 catalysts in a fixed-bed reactor. In a previous paper, we have observed that for acetophenone HDO Pd and Pt are largely more active than other metallic phases.38 The objective of the present work is to extend these findings to other oxygenated compounds of the bio-oil. First, the stability and activity of the catalysts for the HDO of the different compounds has been studied in the midterm (50 h of reaction). Then, reaction kinetic models, based on the corresponding reaction schemes, were proposed and fit to the experimental data. Most published studies on hydrodeoxygenation are focused on optimizing the catalyst formulation, and kinetic models proposed for such reactions are scarce. However, kinetic models are very interesting for assessing and comparing catalyst activity and selectivity. When these models are developed for readily available commercial catalysts, as in the present work, they are a valuable tool for the preliminary evaluation and design of industrial processes.

four oxygenated functional groups: benzaldehyde (aldehyde), benzyl alcohol (alcohol), phenyl acetate (ester), and anisole (or methoxybenzene, ether). HDO of these compounds may lead to complex reaction networks with series and parallel reaction steps, so the use of model compounds facilitates the individual study of the different reactions. The hydrogenation of benzaldehyde has been studied by different authors over both supported precious metal (Pt16 and Pd17−19) and transition metal catalysts (Cu20,21 and Ni21,22). The reaction network proposed for HDO of this compound is shown in Figure 1a. Benzaldehyde hydrogenates to toluene

2. EXPERIMENTAL SECTION 2.1. Materials. The reactants used were high-purity benzaldehyde (98%, Sigma-Aldrich), benzyl alcohol (99%, Merck), phenyl acetate (98%, Merck), and anisole (99%, Merck). Normal heptane (99%, Sigma-Aldrich) and methylcyclohexane (99%, Sigma-Aldrich) were used as solvents. These solvents are adequate to dissolve the oxygenated compounds and vaporize completely at the operating conditions. In addition, neither these compounds nor compounds with similar properties are formed in the HDO reactions. The absence of reactivity has been checked by means of blank tests. Hydrogen and nitrogen (both 99.9%, Praxair) were provided in cylinders. The catalysts used were Pd or Pt supported on γ-Al2O3 with a nominal metal loading of 0.5% (wt.) and an egg-shell configuration supplied by BASF (formerly Engelhard). Morphological properties of the fresh and aged catalysts were measured by nitrogen adsorption at nitrogen boiling point in an ASAP 2020 (Micromeritics) surface area and porosity analyzer. The metal particle size was measured by transmission electron microscopy (TEM) with average values of 4.6 nm for Pt/Al2O3 and 8.2 nm for Pd/Al2O3. Using these values, the exposed metal dispersion was estimated at 22 and 12%, respectively, for Pt/Al2O3 and Pd/Al2O3. Carbonaceous deposits in the aged catalysts were determined by temperature-programed oxidization (TPO) in a TPD/TPR2900 (Micromeritics) device, according to the procedure outlined in previous works.39 The sample (10 × 10−6 kg) was introduced in a U-shaped quartz tube with a flow rate of 1.5 × 10−6 m3/s of 2% O2 (He balance). The temperature was increased at 5 °C/min (up to 900 °C), and effluent was analyzed online in a Pfeiffer Vacuum 300 mass spectrometer. 2.2. Experimental Device. The reaction experiments have been carried out in a packed-bed reactor with 12.7 mm internal diameter and 600 mm total length. The catalyst samples (ground and sieved to 100−250 μm) were mixed with glass particles (350−710 μm) and placed inside the reactor tube.

Figure 1. Reaction schemes: (a) hydrodeoxygenation of benzaldehyde and benzyl alcohol and (b) hydrodeoxygenation of phenyl acetate and anisole.

with benzyl alcohol as intermediate. Depending on the catalyst and the operating conditions, benzene can also be formed as end-product, as evidenced by different authors.16,21,23,24 The hydrogenation of benzyl alcohol has been covered in the literature in a lower extent, except in studies regarding benzaldehyde, where benzyl alcohol is a reaction intermediate. Thus, Vannice et al.16 studied the hydrogenation of benzyl alcohol over Pt catalysts supported on different oxides. As for the case of benzaldehyde, the main products are toluene and benzene. Anisole and similar aromatic ethers (e.g., guaiacols and syringols) are common bio-oil compounds,25 and for this reason, their hydrodeoxygenation has been extensively studied using catalysts such as CuCr,26 Ni,25,27−29 Pt,30−35 or CoMo.36,37 The reaction network for anisole HDO is summarized in Figure 1b,31,35,37 the end-products being benzene and toluene. Phenyl acetate is an ester typically found in bio-oils because it can be formed by reaction between phenol and acetic acid, both 2320

DOI: 10.1021/acs.iecr.6b00036 Ind. Eng. Chem. Res. 2016, 55, 2319−2327

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hydrodeoxygenation.38 Thus, lower temperatures were found to promote the hydrogenation of the aromatic ring, competitive with hydrodeoxygenation and hydrogen consumption. The catalyst stability over time has been studied in terms of reactant conversion. The Pt/Al2O3 catalyst, tested at space time W/F = 0.50 (kgcat s)/moltot, exhibits gradual deactivation for both compounds (Figure 2a). The behavior of the conversion is

The reactor was surrounded by a temperature-controlled electrical oven to maintain isothermal conditions. The studied oxygenated organic compound was fed as liquid (1 mol/L dissolved in methylcyclohexane or n-heptane). This solution was completely vaporized into the hydrogen gas feed inside the reactor tube. The reactor effluent was cooled down and conducted to the sampling cylinders, where the condensed organic liquid was separated from the gas and sampled through a high pressure valve. Further experimental details are given in a previous work.38 Components in the liquid samples were identified by gas chromatography−mass spectrometry in a Shimadzu GCMSQP2010 Ultra gas chromatograph equipped with HP-5MS capillary column and a mass spectrometer detector. Concentrations were quantified using a Shimadzu GC-17A equipped with a HP-5 capillary column and a flame ionization detector (FID). n-Decane (Panreac, > 98%) was used as internal standard in the quantification of reactants and products. 2.3. Reaction Experiments. The stability of the catalysts has been studied by running the reaction under constant operating conditions (pressure, temperature, and space time) for 50 h, with samples being taken every 1−2 h. The kinetic analysis was done by varying the reactor feed and hence space time (W/F = 0−1.5 (kgcat s)/moltot). The catalyst was aged on-stream prior to the analysis, following the same procedure described above for the stability studies, until constant conversion was obtained (around 40 h). To model the reaction kinetics, the catalytic bed has been considered as ideal plug flow, as shown in eq 1.

dCi = ri dτ

(1)

where Ci is the molar concentration of compound i, ri is the reaction rate (per catalyst weight, e.g., mol/s kgcat), τ = C0W/F is the space time, C0 is the molar density (calculated at the operation temperature and pressure using the ideal gas law), W is the weight of catalyst, and F is the feed total mole flow rate. The turnover frequency (TOF) of the reactant, defined as the molconsumed/(molexposed metal s), was calculated under feed conditions (reactant feed concentration) using the conversion data and assuming pseudo-first-order reaction kinetics. The parameters of the kinetic model proposed for ri have been calculated by the least-squares method so that the concentrations predicted by eq 1 give the best fit to the experimental ones. The concentrations of the different compounds have been scaled in the objective function according to their corresponding order of magnitude. The calculations have been carried out with the help of MATLAB using lsqnonlin (trust-region-reflective algorithm), ode15s and lnparci functions, respectively, to solve the least-squares problem and the set of differential equations and to determine the confidence intervals of the fitting.

Figure 2. Catalysts stability for the hydrodeoxygenation of benzaldehyde (blue diamonds in a, cyan diamonds in b) and benzyl alcohol (red squares in a, and orange squares in b) at 325 °C, 0.5 MPa, and H2/O = 23.2. (a) Pt/Al2O3 at W/F = 0.50 (kgcat s)/moltot. (b) Pd/ Al2O3 at W/F = 1.00 (kgcat s)/moltot.

very similar in both cases, decreasing from initial values close to 80% for both compounds to 37% for benzaldehyde and 43% for benzyl alcohol after 47 h of reaction. It should be noted that for t > 37 h the decrease in conversion is much slower and the catalyst activity remains nearly constant. The comparison of nitrogen physisorption characterization results of the fresh and aged catalysts (Table 1) indicates that after the reaction the catalysts suffered an important loss of surface area and pore Table 1. Textural Properties of Fresh and Aged Catalysts Measured by Nitrogen Physisorption

3. RESULTS AND DISCUSSION 3.1. Hydrodeoxygenation of Benzaldehyde and Benzyl Alcohol. The hydrodeoxygenation of benzaldehyde and benzyl alcohol over Pt and Pd supported on γ-Al2O3 has been studied in the gas phase at 325 °C and 0.5 MPa in hydrogen excess (H2/O = 23.2). The reaction scheme of these two compounds is strongly related, as shown in Figure 1a, because benzyl alcohol is the direct product of benzaldehyde hydrogenation. The selected operating conditions were found adequate in a previous work devoted to study acetophenone

catalyst

Pt/ Al2O3

Pd/ Al2O3

2321

substrate

BET area (m2/g)

pore volume (cm3/g)

fresh benzaldehyde benzyl alcohol phenyl acetate anisole fresh benzaldehyde benzyl alcohol phenyl acetate

87 37 14 79 72 82 50 0.5 70

0.46 0.26 0.09 0.35 0.26 0.43 0.31 0.33 0.35

TPO relative CO2 area 0.55 0.32 0.01 0.07 0.21 1.00 0.07


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Table 2. Summary of Conversions and Product Selectivities for the Pt/Al2O3 and Pd/Al2O3 catalysts at 325°C, 0.5 MPa, H2/O = 23.2, and W/F = 1.00 (kgcat s)/moltota Ptb Pd

Pt Pd

benzaldehyde benzyl alcohol benzaldehyde benzyl alcohol phenyl acetate anisole phenyl acetate

X1h (%)

X47h (%)

SBO (%)

79.7 84.6 89 93.7 X1h (%)

36.8 43.3 37.1 19 X47h (%)

19

SP (%)

97.2 93.6 43.1

64.3 53.5 11.2

82.7 55.3 36.8

SBA (%)

ST (%)

SB (%)

total

TOF47h (s−1)

41 SC (%)

32.9 31.6 14 52.9 ST (%)

37.5 29.1 69 3.9 SB (%)

89 81 86 98 total

5.27 3.26 2.59 1.22 TOF47h (s−1)

34.6

1.6

15.8 8.9 12

99 100 49

6.16 4.58 0.69

19.9 3

a

(X = conversion. S = selectivity at 47 h. Compounds: BO = benzyl alcohol, BA = benzaldehyde, P = phenol, C = cresol, T = toluene, and B = benzene). bW/F = 0.50 (kgcat s)/moltot.

reaction conditions. This reaction is undesired from the point of view of hydrodeoxygenation of bio-oils, and its extension can be reduced by operating at high conversion to maximize the selectivity toward the final products (toluene and benzene). The TOF of benzaldehyde and benzyl alcohol HDO was calculated for both catalysts at the conditions existing after 47 h of reaction, as shown in Table 2. The Pt/Al2O3 catalyst was found to present the highest TOF for both reactants. For benzaldehyde, the TOF was 5.27 and 2.59 s−1, respectively, for Pt and Pd/Al2O3. Conversions of these two catalysts were very similar for benzaldehyde, so the differences in TOF are attributed to differences in the atomic mass of the metal and the dispersion. Considering these factors, it can be said that the Pt/Al2O3 catalyst is more active for benzaldehyde HDO. Regarding benzyl alcohol, the TOF was 3.26 and 1.22 s−1, respectively, for Pt and Pd/Al2O3, so a similar conclusion can be obtained: The Pt/Al2O3 catalyst is more active. The TOFs reported in the literature for these reactions are similar to those obtained in the present work. For example, for 0.78% Pt/Al2O3, the TOF was 11 s−1 for benzaldehyde and 7.7 s−1 for benzyl alcohol.16 3.2. Hydrodeoxygenation of Phenyl Acetate and Anisole. Phenyl acetate and anisole are important aromatic oxygenates present in bio-oils, representative of aromatic esters and ethers, respectively. The reaction network of these two compounds is depicted in Figure 1b. As shown, phenol is an intermediate in the hydrogenation of both phenyl acetate and anisole. Phenol is also present in important concentrations in untreated bio-oils. For this reason, HDOs of these three compounds are related to each other and have been studied comparatively. The catalyst stability has been studied at 325 °C, 0.5 MPa, H2/O ratio 23.2, and space time W/F = 1.00 (kgcat s)/moltot. Results (evolution with time of reactant conversion) are summarized in Figure 3. The Pt/Al2O3 catalyst exhibits very high conversions for both reactants at the beginning of the experiment, which decrease markedly during the first 10 h. Then, the loss of activity is slower. For phenyl acetate, conversion is nearly constant (around 64%) from 20 h until the end of the experiment, whereas for anisole conversion decreases slightly more before stabilizing at around 53% at 40 h. The characterization of fresh and aged catalysts (Table 1) revealed losses in surface area of 9−17%, which can at least partially explain the observed loss of catalyst activity. The TPO tests (Figure S2) revels little emissions of CO2 during the analysis as compared to benzaldehyde and benzyl alcohol; the relative CO2

volume. Temperature-programed-oxidation (TPO) of the used catalysts denoted the presence of relevant carbon deposits, which could produce the catalyst deactivation. The stability of the Pd/Al2O3 catalyst for benzaldehyde and benzyl alcohol HDO has been studied in similar experiments, working at W/F = 1.00 (kgcat s)/moltot. The evolution of conversion with time (Figure 2b) indicates that this catalyst is more prone to deactivation, particularly for benzyl alcohol, where conversion decreases from 94 to 19% in 40 h. No region of attenuated deactivation was observed, unlike for Pt/Al2O3. Characterization of the aged catalysts by nitrogen physisorption revealed a strong reduction in surface area with respect to the fresh catalyst. This reduction, which was more marked for benzyl alcohol, may be caused by carbon deposits, as suggested by TPO tests. As reported in the literature,40 benzyl alcohol can polymerize leading to the formation of coke deposits. This causes the strong reduction observed in the surface area and the CO2 release at 600−800 °C during the TPO tests, as shown in Figure S1. The analysis of product distribution for benzaldehyde and benzyl alcohol with both catalysts is summarized in Table 2. The reported selectivity correspond to reaction time 47 h. The sum of the selectivities of all the quantified reaction products is within the 80−100% range both for benzaldehyde and benzyl alcohol. For benzaldehyde, conversion obtained at 47 h with both catalysts is very similar, so selectivity can be compared directly. The quantified products were benzyl alcohol, toluene, and benzene, which are in agreement with the reaction scheme of Figure 1a.16 Thus, the primary hydrogenation product of benzaldehyde is benzyl alcohol, which reacts to form toluene (end product). Benzene is also a final product, generated mainly by benzaldehyde decarbonylation, as reported by different authors.16,21,23 For Pt/Al2O3, the observed high selectivity to benzyl alcohol and toluene suggests that step 1 is faster than step 4. In contrast, selectivity toward benzene was much higher (69%) for Pd/Al2O3, which indicates that this catalyst favors the decarbonylation reaction (step 4) to the detriment of hydrogenation (step 1). The reaction products of benzyl alcohol HDO are benzaldehyde, toluene, and benzene. The presence of benzaldehyde with relatively high selectivity, 20 and 41% respectively for the Pt and Pd/Al2O3 catalysts, is due to the reversibility of the benzaldehyde hydrogenation (steps 1 and 2 in the reaction scheme); the equilibrium constant of the benzyl alcohol reaction to benzaldehyde was calculated from the Gibbs free energy of the reaction at 0.014 MPa (at 325 °C and 0.5 MPa total pressure). The presence of benzaldehyde in benzyl alcohol HDO was also reported by Vannice et al.16 at similar 2322


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manner similar to that in the case of phenyl acetate. The formation of cresol, reported in many works in the literature with different catalysts,29,31,35,41 is produced by methyl transfer from the methoxyl to the phenolic ring. This reaction is catalyzed by the acidic function of the catalyst (Al2O3), as observed in experiments reported in the literature carried out only with the Al2O3 support.37 The further hydrogenation of cresol leads to toluene. We have observed in previous works that alumina acidity tunes the activity of precious metal catalysts used for HDO reactions.42,43 Although benzene can be also formed by hydrogenolysis of toluene (step 7), this reaction can be neglected at these experimental conditions. Direct hydrogenation of anisole to benzene (step 4) is possible but also can be neglected as an important contributor to benzene at the reaction of this study. The analysis of selectivity suggests that the path to phenol (step 3) with total selectivity (phenol and benzene) of 64.2% is faster than the path to cresol (step 5) with total selectivity (cresol and toluene) of 37.2%. It can also be concluded that the hydrogenation of the intermediates, e.g., phenol and cresol, are rate-limiting steps because the selectivity toward the end products (benzene and toluene) is much smaller. The TOF of anisole after 47 h of reaction was determined to be 4.68 s−1. This value is high compared to that of similar Ptbased catalysts studied in the literature for anisole HDO: 0.62 s−1 for 1% (wt) Pt/Al2O3,31 0.35 s−1 for 1% (wt) Pt/HBeta,33 and 0.26 s−1 for 1% (wt) Pt/SiO2.33 The TOFs of the different oxygenated compounds for a given catalyst can be compared to each other. Thus, for Pt/ Al2O3, benzyl alcohol exhibits the lower TOF and hence the lower reaction rate. For the case of Pd/Al2O3, the lower TOF was found for phenyl acetate, followed by that for benzyl alcohol. The knowledge of the most refractory compounds toward HDO is of great importance in the design of commercial scale reactors because the reactor space velocity must be selected according to the reaction rate of these compounds. 3.3. Reaction Kinetics and Modeling. The kinetics of the different HDO reactions of the oxygenated model compounds considered in this work has been studied in the fixed-bed reactor, working under constant operating conditions (temperature 325 °C, pressure 0.5 MPa, and H2/O = 23.2) and changing only space time (0−1 (kgcat s)/mol). To ensure that the activity of the catalyst was constant during the tests, the catalyst was previously aged for 40 h, until constant conversion upon time was obtained. The condition of constant activity was verified by working at a reference space time several times during the experiment and comparing the conversion obtained. The kinetic experiments were performed in the absence of diffusional limitations. This has been confirmed by calculating the intraparticle effectiveness factor and Carberry number at the worst conditions, i.e., highest reaction rate (highest feed concentration and lowest flow rate). Physical properties were evaluated under these conditions using correlations from the literature. The effective diffusion coefficient in the porous catalyst particles was estimated as a contribution of molecular and Knudsen diffusion. The values of the intraparticle effectiveness factor under the worst conditions ranged between 0.94 for benzaldehyde and benzyl alcohol to 0.99 for phenol. Carberry number was used to evaluate the importance of diffusion external to the catalyst particle. The calculations suggested Carberry numbers in the range 0.001−0.006, which are lower than the limit of 0.05.

Figure 3. Catalysts stability for the hydrodeoxygenation of phenyl acetate (green triangles) and anisole (purple circles) at 325 °C, 0.5 MPa, H2/O = 23.2, and W/F = 1.00 (kgcat s)/moltot. (a) Pt/Al2O3 and (b) Pd/Al2O3.

signal areas are less than 0.1, as shown in Table 1. (An area of 1 corresponds to benzyl alcohol over Pd/Al2O3.) The performance of the Pd/Al2O3 catalyst is worst. The initial conversion for phenyl acetate is 43%, markedly lower than that for Pt/Al2O3. Conversion decreases with time until 25 h, when it is around 11%. This value remains approximately constant until the end of the experiment. Selectivity toward the reaction products for both catalysts are shown in Table 2. For the Pt catalyst, the sum of selectivity of the identified compounds is 99−100%. However, for Pd the quantified products correspond to only 49% of the reactant converted. The remaining corresponds to possible products undetected or lost in the sampling process and also to the contribution of carbon deposits. The hydrodeoxygenation of phenyl acetate proceeds through a simple reaction path with two consecutive steps, where phenol is the intermediate and benzene is the final product (Figure 1b). For the Pt catalyst, the selectivity toward phenol at 64.3% conversion is high (82.7%), which suggests that hydrogenation of phenol is rate-limiting in the complete HDO of phenyl acetate. The Pd/Al2O3 catalyst presents a similar behavior in terms of product distribution. The TOF for the phenyl acetate HDO after 47 h of reaction was calculated for both catalysts, as shown in Table 2. The TOF for the Pt/Al2O3 is considerably higher (6.16 s−1) than that of the Pd/Al2O3 catalyst (0.69 s−1). This can be due to the lower deactivation observed for the Pt/Al2O3 catalyst. The reaction network for anisole is more complex, as shown in Figure 1b. The reaction products identified and quantified were phenol, cresol, benzene, and toluene. According to the literature,31 the main reaction paths of anisole over precious metal catalysts are, in order of importance, hydrogenolysis to phenol (step 3), transalkylation to cresol (step 5), and direct hydrodeoxygenation to benzene (step 4). The hydrogenolysis to phenol is followed by hydrodeoxygenation to benzene in a 2323


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Figure 4. Results of the reaction kinetic experiments and fitting of the kinetic models for hydrodeoxygenation of benzaldehyde (a and c) and benzyl alcohol (b and d) over Pt/Al2O3 at 325 °C, 0.5 MPa, and H2/O = 23.

Table 3. Rate Constants Calculated by Fitting of the Kinetic Modelsa benzaldehyde, benzyl alcohol Pt/Al2O3

benzaldehyde Pd/Al2O3

phenyl acetate Pt/Al2O3

rBA = k2CBO − (k1 + k4 + k5)CBA rBO = k1CBA − (k2 + k3 + k5)CBO rB = k6CBO + k4CBA rT = k3CBO + k5CBA

rBA = −(k1 + k4 + k5)CBA

rPA = −k1CPA rP = k1CPA − k2CP rB = k2CP

k1 = 2.5 ± 0.5 k2 = 4.8 ± 0.5 k3 = 5.3 ± 0.8 k4 = 3.6 ± 0.8 k5 = 13 ± 1 k6 = 4.1 ± 0.7 R2 = 0.97

rB = k4CBA rT = (k1 + k5)CBA kinetic constants × 103(m3 kgcat−1 s−1) k1 + k5 = 0.6 ± 0.1 k4 = 2.9 ± 0.5

k1 = 12 ± 1 k2 = 3.8 ± 0.5

R2 = 0.95

R2 = 0.99

a

The subindexes of the rate constants refer to the steps of the reaction scheme in Figure 1. Subindexes indicate compounds: BA = benzaldehyde, BO = benzyl alcohol, PA = phenyl acetate, P = phenol, B = benzene, and T = toluene.

intermediate was detected, and in the HDO of benzyl alcohol, benzaldehyde is formed because the hydrogenation of benzaldehyde is reversible (steps 1 and 2). The concentration of toluene, the main end-product, increases on increasing space time (and hence conversion) for both reactants. The same tendency is observed for benzene, formed by decarboxylation of benzaldehyde. A kinetic model, based on the reaction network in Figure 1, has been developed, assuming that the six reactions present first-order dependences on the organic reactant and zero-order dependences on hydrogen (because of its great excess). The rate constants in this model have been fitted simultaneously to the experimental data obtained in the two experiments (with benzaldehyde and benzyl alcohol as reactants, respectively). Thus, combining the information on both experiments increases the significance of the parameters obtained. Predictions of the model with the fitted parameters (Table 3)

Hence, the influence of diffusional limitation can be considered negligible. Meanwhile, the isothermicity of the system at the catalyst particle level was also evaluated. The maximum temperature difference inside the catalyst particle and in the gas film were estimated to be lower than 0.1 °C for all the reactions (considering the worst case scenario, corresponding to complete conversion). Results obtained for HDO of benzaldehyde and benzyl alcohol with the Pt/Al2O3 catalyst (0.125 × 10−3 kg), operating at 325 °C, 0.5 MPa, and a H2/O ratio 23.2. Two sets of experiments have been carried out at varying space times (0−1 (kgcat s)/mol): one with a gas feed concentration of benzaldehyde of 3.8 mol/m3 and the other with a gas feed concentration of benzyl alcohol of 3.0 mol/m3. Both compounds are discussed together, as shown in Figure 4, because they share a reaction network. Thus, in the HDO of benzaldehyde, a small amount of benzyl alcohol formed as 2324


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benzaldehyde to benzene, with this step being 3 times faster than HDO to toluene. The decarbonylation to benzene contributes to reduce the oxygen content, but usually, it is undesired in the upgrading of bio-oils because the rupture of C−C bonds consumes hydrogen and reduces the energy density of the bio-oil. The reaction kinetics of phenyl acetate HDO was studied for the Pt/Al2O3 catalyst at 325 °C, 0.5 MPa, and H2/O ratio 23.2. As in the previous studies, this ratio was found to be high enough to consider hydrogen concentration constant during the experiments. Concentrations of the different reaction products as a function of space time (0−1 (kgcat s)/mol) are shown in Figure 6. Phenol concentration increases gradually

are depicted as lines in Figure 4. The regression coefficient of the fitting is 0.97. The small value of the 95% confidence intervals indicates the adequacy of the model and that all the reactions considered are statistically significant. Data in Table 3 show that the rate constant of step 3 is higher than that of step 1, in agreement with the low benzyl alcohol selectivity obtained in benzaldehyde hydrogenation. Regarding to benzene, the rates of its formation from benzaldehyde and benzyl alcohol are similar, as evidenced by the similar rate constants of steps 4 and 6 and the similar selectivity obtained in the stability and kinetic experiments. The kinetics of the benzaldehyde reaction has also been studied for the Pd/Al2O3 catalyst (0.25 × 10−3 kg). Results obtained at different space times (0−0.8 (kgcat s)/mol) for constant operating conditions (325 °C, 0.5 MPa, H2/O = 23.2, and gas feed concentration of benzaldehyde of 2.3 mol/m3) are shown in Figure 5. The reaction rate is lower for Pd/Al2O3 than for Pt/Al2O3, e.g., because similar space times conversions are lower. Product distribution is also different.

Figure 6. Results of the reaction kinetic experiments and fitting of the kinetic model for hydrodeoxygenation of phenyl acetate over Pt/Al2O3 at 325 °C, 0.5 MPa, and H2/O = 23.

but with a gradually decreasing slope. Being a reaction intermediate, it is approaching a maximum. Benzene is an end-product, and its concentration is low, indicating that the hydrogenation of phenol is a slow step. As in the previous cases, a kinetic model assuming irreversible first-order kinetics for each step has been developed, and the rate constants were fitted to the experimental data. The fitting is good, as shown in Figure 6 and by the 0.99 regression coefficient. The fitted rate constants are given in Table 3; the small 95% confidence intervals is also an indication of the adequacy of the model. The rate constant for phenyl acetate hydrogenation (k1) is higher than that for phenol hydrogenation (k2), in agreement with the experimental results. Figure 5. Results of the reaction kinetic experiments and fitting of the kinetic model for hydrodeoxygenation of benzaldehyde (a and b) over Pd/Al2O3 at 325 °C, 0.5 MPa, and H2/O = 23.

4. CONCLUSIONS The studies on the gas-phase catalytic hydrodeoxygenation of benzaldehyde, benzyl alcohol, phenyl acetate, and anisole, compounds typically found in bio-oils, on Pt and Pd supported on γ-Al2O3 lead to the following conclusions. (i) Pd/Al2O3 exhibits a marked deactivation for benzyl alcohol and benzaldehyde, caused by coke deposits that lead to a reduction of surface area. For phenyl acetate, there is a plateau of approximately constant activity after 25 h on stream, although with low conversion. (ii) Pt/Al2O3 also deactivates at the beginning of the stability tests and presents a plateau of approximately constant activity after 40 h on stream for the four compounds studied. (iii) The kinetics of the HDO reactions in the activity plateau can be modeled assuming that all the reactions in the corresponding reaction networks are first-order with respect to the organic reactant and zero-order with respect to hydrogen, which is in great excess. (iv) For benzaldehyde and benzyl alcohol hydrodeoxygenation, the model proposed

A kinetic model was proposed on the basis of the same assumptions as those for the Pt/Al2O3 catalyst, e.g., first-order kinetics on the organic compound concentration for each step. However, because of the low concentration measured for benzyl alcohol, the rate constants calculated for steps 2, 3, and 6 are not statistically significant. In addition, the rate constants for step 1 and 5 cannot be evaluated separately, and it is only possible to determine the overall kinetic constant for formation of toluene from benzaldehyde (k1 + k5). The rate constants for the steps found to be significant on the basis of the statistical analysis of the data are shown in Table 3. Model predictions are depicted as lines in Figure 5. The model fits well to the experiments, with a regression coefficient of 0.95. Results for Pd/Al2O3 show that this catalyst favors the decarbonylation of 2325


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consists of six reaction steps. For reaction on Pt/Al2O3, the fastest step is the formation of toluene from benzaldehyde, and the slowest step is the hydrogenation of benzaldehyde to benzyl alcohol. The fastest reaction for benzaldehyde on Pd/Al2O3 is the formation of benzene (undesired). All the reactions can be performed with high selectivity to nonoxygenated compounds. (v) The hydrodeoxygenation of phenyl acetate on Pt/Al2O3 takes place in two steps in series, the first to phenol (fast) and the second to benzene (slow).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b00036. Results of temperature-programmed oxidation (TPO) curves for the aged catalysts. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: 34-985 103 437. Fax: 34-985 103 434. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financed by the Asturian Local Government (Spain, project reference GRUPIN14-078). C.G. also thanks the Asturian Local Government for a Ph.D. Grant (Severo Ochoa Program).

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REFERENCES

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Gas-Phase Hydrodeoxygenation of Benzaldehyde, Benzyl Alcohol

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