Hydrodeoxygenation of Acetophenone over Supported Precious Metal

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Hydrodeoxygenation of acetophenone over supported precious metal catalysts at mild conditions: process optimization and reaction kinetics Celeste Gonzalez, Pablo Marin, Fernando V. Díez, and Salvador Ordonez Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02112 • Publication Date (Web): 23 Nov 2015 Downloaded from http://pubs.acs.org on November 24, 2015

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Hydrodeoxygenation of acetophenone over supported precious metal catalysts at mild conditions: process optimization and reaction kinetics

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Celeste González, Pablo Marín, Fernando V. Díez, Salvador Ordóñez*

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Department of Chemical and Environmental Engineering, University of Oviedo, Facultad de Química,

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Julián Clavería 8, Oviedo 33006, SPAIN

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* Phone: 34-985 103 437, FAX: 34-985 103 434, e-mail: [email protected]

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Abstract

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Bio-oils obtained by pyrolysis of lignocellulose feedstocks must be upgraded to reduce the oxygen

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content, improving their quality as bio-fuels. Catalytic hydrotreatment has been proposed to reduce

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the oxygen content of bio-fuels and meet the standard requirements. Acetophenone is interesting as

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a model compound for the study of hydrodeoxygenation of pyrolysis bio-oils, which contain aromatic

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ketones. In this work, acetophenone gas phase hydrodeoxygenation over precious metal (Pt, Pd, Ru

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and Rh) supported catalysts has been studied in a fixed-bed reactor (space time, W/F = 0.75-1.0 kgcat

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s/moltot). The influence of catalyst active phase and operating conditions (pressure 0.5-1.5 MPa, and

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temperature 275-375ºC) on the catalyst stability and activity, and product distribution was studied.

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For the optimum pressure (1.0 MPa), and 275 and 375ºC, a reaction kinetic model based on the

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reaction scheme has been proposed and fitted to the experimental data obtained at different space

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velocities (W/F = 0-1.5 kgcat s/moltot).

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Keywords: bio-oil; hydrotreating; palladium catalyst; fixed-bed reactor; kinetic modelling.

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1. Introduction

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The development of cleaner and renewable fuels is growing due to the environmental hazards and

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possible shortage of traditional fossil fuels. In this context, biomass constitutes an energy resource

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widely available around the world. The use of lignocellulose materials as feedstock to produce bio-

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fuels increases the yield of the biomass conversion, in comparison to traditional bio-fuels (bioethanol

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and lipid biodiesel), whose manufacture requires the use of a very small, and edible (starches, lipids),

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fraction of this biomass.1

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One of the proposed schemes for upgrading lignocellulosic feedstocks is based on biomass pyrolysis

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followed by a chemical upgrading of the resulting oil. Pyrolysis is a thermal decomposition of the

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biomass that produces a pyrolysis oil (bio-oil), formed by a complex mixture of hydrocarbons, with

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high oxygen content. Some oxygenated functional groups appearing in bio-oils are carboxyl, carbonyl

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(acetone and aldehyde), alcohol and ester.2-4 Oxygen content in bio-oils is usually 35-40%,5 while in

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heavy petroleum fuel oil is only 1%.6 The presence of oxygenated compounds deteriorates the

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properties of bio-oils as bio-fuels, increasing viscosity, acidity and instability, decreasing volatility and

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energy density, etc.7 Upgrading of bio-oils is aimed to decrease their oxygen content, improving the

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properties of the resulting bio-fuel. Two types of upgrading catalytic processes have been proposed:

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cracking and hydrogenation.

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Catalytic hydrogenation, and most specifically hydrodeoxygenation (HDO), is based on the reaction

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with hydrogen at high temperature and pressure in presence of a catalyst, leading to bio-oil

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constituents.8-10 Conventional hydrotreating catalysts, such as CoMo/Al2O3 and NiMo/Al2O3, are used

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at industrial scale in hydrotreating petroleum fractions, for the simultaneous elimination of sulphur,

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oxygen and nitrogen.10 The use of these catalysts for hydrotreating of bio-oils has been extensively

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studied: optimization of catalyst formulation and operating conditions (typically 250-400ºC and 3.0-

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20.0 MPa)8, 11, 12 and also kinetic studies.13-17 These catalysts are used in sulphided form, which

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considerably outperforms the oxidic form in HDO of bio-oils, and hence the reaction must be carried


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out in presence of sulphur. Sulphur content of bio-oil is too small to produce enough H2S during

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hydrotreating, and for this reason an external source of H2S is required to maintain the catalyst in a

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stable sulphided form.8 This is an important drawback of hydrotreating catalysts for bio-oil HDO.

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Recently, precious metal-based catalysts, e.g. Pt18 and Ru,19-21 have been proposed and studied as

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alternative to conventional hydrotreating catalysts.22, 23 The most important advantage of precious

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metal catalysts is the high activity, even at low pressure and temperature, which would result in

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smaller reactors and economic advantages.

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Studies with model compounds are useful for determining the activity and stability of catalysts used

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in hydrodeoxygenation of bio-oils, and also for understanding the reaction kinetics. The composition

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of pyrolysis bio-oils is rather complex, formed by different families of compounds with concentration

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varying according to the biomass feedstock or the processing conditions. Compounds often used as

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models for bio-oils are guaiacol (2-methoxyphenol), ethyl phenol, or anisole (methoxybenzene),

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among others. These compounds are representative of lignin precursor molecules.8 The composition

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of bio-oils also accounts for acids, esters, ketones, etc. These compounds, with a higher oxidation

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state, require a higher degree of hydrodeoxygenation.

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In this work, the focus is set on ketones (around 27 wt% of the crude bio-oil24), and particularly

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acetophenone, which is selected as model compound.25, 26 Acetophenone is a simple aromatic

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ketone, representative of other more complex ketones and very interesting as model compound,

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because contains two functional groups frequently present in bio-oils:4, 27 carbonyl and aromatic ring.

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Hydrogenation of these functional groups leads to a complex reaction network formed by

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competitive and consecutive steps. In bio-oils HDO, hydrogenation of the aromatic ring is not

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desired, because the saturated product has a lower octane number, and the reaction consumes

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valuable hydrogen.28 Hence, the optimization of catalyst formulation and reaction conditions is

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critical to achieve high de-oxygenation with low hydrogen consumption. Although acetophenone

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hydrogenation has been widely studied over different catalysts, such as Pt,29-34 Pd,28, 34-38 Rh,39 Ni,40, 41


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Co42 and Cu,43 most studies were carried out in liquid phase and focused on catalyst screening and

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optimization to maximize the yield of 1-phenylethanol, an alcohol extensively used in perfumery and

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pharmaceuticals.33 But for bio-oil upgrading, the catalyst and operating conditions should be

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optimized to maximize hydrodeoxygenation of the carbonyl group in detriment of hydrogenation of

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the aromatic ring.

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The stability and activity of different commercial precious metal (Pt, Pd, Ru and Rh) supported

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catalysts was firstly studied in a laboratory fixed-bed reactor. Then, the operating conditions were

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optimized for the most promising catalyst. Finally, reaction kinetic models, based on a reaction

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scheme, were proposed and fit to the experimental data. Most published studies on

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hydrodeoxygenation are only focused on optimizing the catalyst formulation. However, kinetic

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models for readily available commercial catalysts are very interesting, particularly in the scale-up of

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the process to industrial scale.

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2. Experimental section

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2.1. Materials

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The organic compounds used were acetophenone (99%, Merck), n-heptane (99%, Sigma–Aldrich),

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used as solvent, and n-decane (>98%, Panreac), used as internal standard in the analysis. Hydrogen

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(99.9%, Praxair) and nitrogen (99.9%, Praxair) were provided in gas cylinders.

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Catalysts used in this work were precious metals (Pd, Pt, Ru and Rh) supported on γ-Al2O3, provided

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by BASF (formerly Engelhard). For all the catalysts, the metal loading is 0.5% (wt.) with an egg-shell

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impregnation pattern.

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The textural properties of the fresh and aged catalysts were measured by nitrogen adsorption at 77 K

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in a MICROMERITICS ASAP 2020 surface area and porosity analyzer.


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The aged catalysts were also characterized by temperature programed oxidization (TPO) in a

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MICROMERITICS TPD/TPR-2900 device, according to the procedure outlined in previous works44. The

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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%

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O2 (He balance). The temperature was increased at 5ºC/min (up to 900ºC) and effluent was analyzed

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on-line in a PFEIFFER VACCUM 300 mass spectrometer.

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2.2. Experimental device

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The experimental device for the reaction experiments is depicted in Figure 1. Hydrogen and nitrogen

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flow rates were set by BRONKHORST mass flow controllers (respectively, FIC-01 and 02). Nitrogen

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was used for inertization and for leak testing in the apparatus. During reaction experiments, pure

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hydrogen (flow rate 0-5·10-5 m3/s n.t.p) was used as gas feed. The organic feed (acetophenone

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dissolved in n-heptane) was pumped by a piston pump (ALLTECH model 452, flow rate 0-5·10-8 m3/s).

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Both streams were mixed in a “T-piece” in the reactor feed line, as shown in Figure 1. The reactor

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consisted of a stainless steel tube with 12.7·10-3 m internal diameter and 0.600 m length, surrounded

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by a temperature-controlled electrical oven. The catalyst sample (0.188-0.25·10-3 kg), ground and

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sieved to 100-250 µm, was mixed with 1·10-3 kg of glass particles of 350-710 µm diameter and placed

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inside the reactor tube. The catalyst bed height (8.5-9.3·10-3 m) was high enough to assume plug flow

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behavior (bed height/particle size > 50). The resulting packed bed was hold in position with a

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stainless steel mesh of 60 µm. Upstream the packed bed, the reactor tube was filled with glass

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spheres with 10-3 m diameter, in order to ensure a flat radial velocity profile (plug flow) and pre-heat

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the feed. In all the experiments, the reaction mixture was in gas phase at reaction conditions.

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The reactor effluent was cooled down and conducted to the sampling section, formed by two

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cylinders connected in parallel that were used alternatively to collect the samples. Thus, the

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condensate organic liquid was separated from the gas and accumulated in one of the cylinders; when


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the amount of liquid accumulated was adequate, the valves were switched in order to accumulate

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liquid in the other cylinder. Then, the first cylinder (now off-line) was emptied through an

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AUTOCLAVE ENGINEERS valve situated in the bottom. When the second cylinder is ready to be

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sampled, the valves are switched another time. Hence, samples are collected alternatively from the

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cylinders. This way, the decrease in pressure during the sampling does not affect the reactor,

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because it is local to the cylinder. The reactor outlet is always connected to a backpressure regulator

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through the gas outlet of the corresponding cylinder. Pressure in the reactor is maintained constant

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at the set value (in the range 0.5-1.5 MPa).

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Liquid samples were analyzed by gas chromatography. A Shimadzu GCMS-QP2010 Ultra gas

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chromatograph equipped with HP-5MS capillary column and a mass spectrometer detector was used

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to identify the components in the sample. Quantitative concentration measurements were done with

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a Shimadzu GC-17A chromatograph, equipped with a HP-5 capillary column and a flame ionization

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detector (FID), using n-decane as internal standard.

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2.3. Reaction experiments

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Catalyst stability and reaction kinetics were studied in this work. Catalyst stability studies were

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carried out running the reaction for long periods of time (60 h) at constant operating conditions,

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taking effluent samples every 1-2 h. Kinetic experiments were performed running the reaction at

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different space times (W/F in the range 0-1.5 kgcat s/moltot) and constant pressure and temperature

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during the period of constant catalytic activity. For this purpose, the kinetic experiments were done

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with a new catalyst sample, which was initially stabilized, following the same procedure as in the

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stability study, until constant conversion was obtained (40 h). Then, flow rates were varied randomly

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(and hence space time), repeating the reference space time (0.75 kgcat s/moltot) every three flow

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rates, in order to monitor any possible change in the catalyst activity during the kinetic test.


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In both types of studies, the liquid feed consisted of a solution of 1000 mol/m3 acetophenone in n-

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heptane. This solvent has an appropriate boiling point that ensures complete vaporization at reaction

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conditions and almost quantitative condensation in the sampling cylinders. The blank experiments, in

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which n-heptane with no acetophenone was fed to the reactor, showed negligible n-heptane

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conversion at reaction conditions, which is very important to study the HDO of acetophenone.

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In the kinetic analysis, the catalytic bed has been modelled as an ideal plug flow reactor, as shown in

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eq. (1). = /

(1)

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Where is the molar concentration of compound (acetophenone or reaction products), is the

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reaction rate per catalyst weight, / is the space time, is the weight of catalyst in the reactor,

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is the feed total molar flow rate, and is the feed total molar density (calculated at the reaction

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temperature and pressure, using the ideal gas law).

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Kinetic parameters have been calculated by fitting eq. (1) to the experimental results by the least-

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square method. The concentrations of the different compounds have been appropriately scaled in

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the objective function. Calculations have been carried out with the help of MATLAB using lsqnonlin

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(trust-region-reflective algorithm), ode15 sand lnparci functions, respectively, to solve the least-

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square problem, the set of differential equations and determine the confidence intervals.

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3. Results and Discussion

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3.1. Selection of precious metal catalyst

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The performance (activity and stability) of different precious-metal alumina-supported catalysts for

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the gas phase hydrodeoxygenation of acetophenone has been studied in a fixed-bed reactor. As


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described in the experimental section, experiments consisted of reactions carried out for 60 h at

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constant operating conditions: temperature 325ºC, pressure 0.5 MPa, space time W/F = 1.00 kgcat

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s/moltot and feed hydrogen to oxygen in acetophenone molar ratio, H2/O = 23. Results are depicted in

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Figure 2 in terms of acetophenone conversion, defined as X = 1-CA/CA0 (where CA is concentration of

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acetophenone and sub-index 0 indicates at reactor inlet). As observed, all the catalysts deactivate

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markedly during the first 15 h of reaction. Conversion for Pd, Ru and Rh decreases from 25-50%

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initially to 1-5% by the end of the experiment, while for Pt, conversion decreases from the initial 97%

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to around 30% at 40 h on stream, remaining then nearly constant till the end of the experiment.

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Analysis of the reactor effluent revealed that the main reaction products were ethylbenzene and

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styrene. The presence of these compounds is in agreement with the reaction scheme proposed for

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this reaction in the literature, Figure 3.31, 39, 40 Precious metals catalyze the hydrogenation of

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acetophenone to 1-phenyl ethanol, then alcohol dehydration takes place in the presence of an acid

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catalyst (the Al2O3 support), and finally styrene is hydrogenated to ethylbenzene. 1-Phenyl ethanol

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was not identified among the reaction products, because the alcohol dehydration is very fast in

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comparison with the other reactions.32 Hydrogenation of the aromatic ring is also possible according

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to the reaction scheme, but the corresponding reactions products were not identified at our

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experimental conditions.

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Catalysts tested can be classified in two groups, according to the product selectivity of the aged

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catalysts (after 55 h on stream). Selectivity to product “i” is defined as Si = Ci/(CA0-CA), where Ci is

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concentration of product “i”, CA is concentration of acetophenone, and sub-index 0 indicates at

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reactor inlet. On one hand, Pt and Pd exhibit 80-85 % selectivity to ethylbenzene, 12-14% selectivity

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to styrene, and very small selectivities towards benzene and toluene. On the other hand, Ru and Rh

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show lower ethylbenzene selectivity (41-58%), slightly higher styrene selectivity (19-20%), and an

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important selectivity towards benzene (10-23%), and toluene (10-15%). Benzene and toluene are

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associated to hydrogenolysis of C-C bonds in styrene and ethylbenzene products. These cracking


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products can be formed through a bifunctional mechanism which requires both acid and metallic

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centers on the catalyst surface, as evidenced in studies regarding ethylbenzene hydrogenation and

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dehydrogenation.45, 46 It has been checked that most reaction products were identified and

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quantified by means of the total mass balance, with a maximum error of 6% for Pd. Concerning to

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the support, it must be noted that alumina has a low-medium strength acidity, which present a good

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balance between the acid sites needed for the main reactions and the low concentration of the

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strong acids leading to by-products by C-C bond cleavage.47,48

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The aged catalysts have been characterized by nitrogen adsorption and temperature programmed

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oxidation (TPO). The BET surface area of the aged catalysts is lower than the corresponding surface

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area of the fresh catalysts. The surface area decreased from 82 to 72 m2/g for Pd/Al2O3, from 106 to

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104 m2/g for Ru/Al2O3, from 107 to 98 m2/g for Rh/Al2O3 and from 87 to 75 m2/g for Pt/Al2O3. In

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general terms, a decrease in the surface area is observed, although it is not conclusive. This fact is

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expected for mesoporous catalysts where porous structure is unlikely to be blocked by the

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carbonaceous deposits.

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The CO2 release profiles obtained for the TPO of the spent catalysts, shown in Figure 4, provide more

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evidences on coke formation. On increasing temperature, carbon dioxide evolves from the catalysts

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in the form of two marked peaks. The high temperature peak, generated in all the catalysts at 520-

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540ºC, suggests the presence of carbon deposits on the catalyst surface. The low temperature peak

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appears at different temperatures depending on the catalyst: for Pt and Pd at around 240ºC, while

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for Ru and Rh at 350ºC. The low temperature peaks can be associated to the presence of adsorbed

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heavier products, which may differ on nature for the different catalysts, and hence the difference

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oxidation temperature observed. It should be noted that there is a correlation between the low

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temperature peak and the product distribution, as discussed previously for the different catalysts.

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Thus, Ru and Rh catalysts presented important selectivity towards benzene and toluene, which was

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negligible for Pd and Pt catalysts. Also the selectivity towards styrene was higher. In fact, styrene is


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known to generate coke precursors (in hydrogen media) that cause catalyst deactivation and would

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explain the observed behavior.45, 49 For example, in the commercial process of styrene production

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from dehydrogenation of ethylbenzene, the catalyst deactivation is controlled using an excess of

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steam that reacts with the coke precursors. In the present case, acetophenone hydrodeoxygenation,

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steam is generated as the product of deoxygenation, but the concentration is low.

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The adsorbed products explain the deactivation of the catalysts observed in the stability test. The

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size of the peaks is higher for Ru and Rh, which are the catalysts with faster deactivation. On the

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contrary, Pt, which was found to be stable after 40 h of stabilization, presents the smallest peaks.

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3.2. Optimization of operating conditions for Pt/Al2O3

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In the previous section, it was determined that Pt/Al2O3 is the most stable and active catalyst for

227

acetophenone HDO in gas phase. Now, the influence of operating conditions (pressure and

228

temperature) in the process with this catalyst is studied. For this purpose, stability experiments with

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60 h duration, and using fresh catalyst in each one, have been carried out.

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The influence of pressure in the range 0.5-1.5 MPa on the catalyst stability has been determined at

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325ºC, H2/O molar ratio 23 and a space time 1.00 kgcat s/moltot. Results are summarized in Figure 5a

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and Table 1. As observed, initial acetophenone conversion was high (95-97%) in all experiments.

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Acetophenone conversion decreases upon time, very markedly in the experiment at 0.5 MPa, and

234

more gradually at 1.0 and 1.5 MPa. For 1.0 and 1.5 MPa, the behavior is very similar, with conversion

235

at the end of experiment (t > 45 h) around 64%.

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Regarding product distribution, the main product was ethylbenzene, with selectivity 85-92% for all

237

pressures. Styrene was only detected at 0.5 MPa, with 12% selectivity at 55 h. Styrene is an

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intermediate in the hydrodeoxygenation of acetophenone to ethylbenzene, and hence it is only

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detected at low conversion. Benzene and toluene, generated by hydrogenolysis of C-C bonds as


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explained in section 3.1, were detected at all pressures, with similar low selectivities (1-3%).

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Ethylcyclohexane was detected at the higher pressures, with selectivity increasing as pressure

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increases (1% selectivity at 1.0 MPa and 4% at 1.5 MPa). Ethylcyclohexane is produced by

243

hydrogenation of the aromatic ring, favored at high pressure. Hydrogenation of the aromatic ring is

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usually undesired in bio-oil de-oxygenation, as it consumes valuable hydrogen. For this reason, and

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given the similar acetophenone conversions obtained at 1.0 and 1.5 MPa, the recommended

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operating pressure in the range studied is 1.0 MPa.

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The influence of temperature on the hydrodeoxygenation of acetophenone was studied at 275 and

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375ºC for pressure 1.0 MPa (found optimal, as explained previously), H2/O molar ratio 23 and space

249

time 0.75 kgcat s/moltot. It should be noted that in this set of experiments, space time is 25% lower

250

than in the pressure dependence experiments. This value was selected in order to avoid too high

251

conversions, which could mask catalyst deactivation. Results in Figure 5b indicate that acetophenone

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conversion decreases upon time during the first 20 h, and then remains approximately constant for

253

both temperatures. At 375ºC, conversion decreases faster than at 275ºC. Conversion achieved after

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55 h is slightly higher at 375ºC: 27% at 275ºC and 36% at 375ºC.

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Results of products distribution are summarized in Table 1. At 275ºC, the main product is

256

ethylbenzene with 83% selectivity, followed by cyclohexyl methyl ketone with 7% selectivity and

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ethylcyclohexane with 1% selectivity. The presence of cyclohexyl methyl ketone evidences that at

258

275ºC the hydrogenation the aromatic ring of acetophenone is hydrogenated to some extent.33 This

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compound was not detected at 375ºC, neither at 325ºC (in the previous set of experiments).

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Formation of cyclohexyl methyl ketone is undesired from the point of view of bio-oil upgrading,

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because the oxygen content of the molecule does not decrease, and its formation is hydrogen

262

consuming. Ethyl cyclohexane found among the reaction products can be produced by

263

hydrogenation of ethylbenzene or hydrogenation followed by dehydration-hydrogenation of the

264

resulting cyclohexyl methyl ketone, as shown in the reaction scheme of Figure 3. At 375ºC, ethyl


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benzene is also the main product, with 80% selectivity, but styrene is formed with 11% selectivity. As

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explained before, styrene is a reaction intermediate in the formation of ethylbenzene.

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As shown in Table 1, the presence of an important selectivity towards styrene (experiment at 0.5

268

MPa and 325ºC, and experiment 1.0 MPa and 375ºC) is correlated to a stronger loss of activity during

269

the catalyst stabilization experiment. As explained in section 3.1, the characterization of the aged

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catalysts revealed that the presence of high selectivity towards styrene is correlated to a higher

271

carbon dioxide emission during the temperature programmed oxidation tests. Thus, styrene is known

272

to be responsible of the formation of coke precursors at hydrogen media. The adsorption of these

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higher molecular weight products is the most likely responsible of observed catalyst deactivation.

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3.3. Reaction kinetics

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The kinetics of the hydrodeoxygenation of acetophenone on the Pt/Al2O3 catalyst has been studied

277

at 1.0 MPa, H2/O molar ratio 23, and 275 and 375ºC. Hydrogen/oxygen ratio is high enough as to

278

assume constant H2 concentration inside the reactor. Experiments have been done using 0.1875·10-3

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kg of catalyst, previously aged for 40 h at the corresponding temperature. This way, the catalyst

280

activity is constant during the experiments, as previously found in the stability tests. Space velocities

281

ranged from 0 to 1.5 kgcat s/moltot, obtained by varying the gas and liquid flow rates while

282

maintaining constant the H2/O ratio. Experiments have been done in absence of diffusional

283

limitations, as indicated below.

284

Experimental results are depicted as symbols in Figure 6 and 7. Product distributions obtained agree

285

with the reaction scheme in Figure 3. Cyclohexyl methyl ketone is a primary product from

286

acetophenone, because important amounts are formed at low space times, when acetophenone

287

conversion is low. On the contrary, the concentration of ethyl cyclohexane is very low at low space

288

times, and increases only when acetophenone conversion increases at higher space times, which


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289

suggests that it is a secondary product. Styrene was only detected at 375ºC, with very low

290

concentration, that first increases and then decreases as space time increases, behavior expected for

291

an intermediate product. Finally, the main product, ethylbenzene, behaves as an end-product, with

292

concentration increasing with space time. Benzene and toluene have been excluded from this

293

analysis, because their selectivity is always very low (< 1%). According to these results, at 275ºC the

294

kinetically relevant steps are reactions 1, 4 and 5 (reactions 2, 3 and 6 are very fast, so the products

295

are not detected, and reactions 3 and 8 very slow). At 375ºC, the kinetically relevant steps are

296

reactions 1 and 3, since hydrogenation of the aromatic ring does not take place.

297

Kinetic models, based on the reaction scheme in Figure 3, have been fitted to the experimental

298

results. The kinetic models consider only the reactions kinetically relevant at the corresponding

299

temperature, as discussed before. The reactions are supposed pseudo-first order with respect to the

300

corresponding organic reactant, as proposed in the literature.43 Hydrogen concentration is the same

301

in the different kinetic tests, and in great excess. Hence, it is found constant along the reactor length,

302

and its influence on the reaction kinetics can be included in the apparent rate constants. As

303

mentioned in section 2.3, the catalytic bed has been modelled as a plug flow reactor. Results of the

304

least-square fitting of the models to the experimental data are summarized in Table 2. The regression

305

coefficients obtained for the fittings are R2 = 0.98 and 0.97, respectively, for 275ºC and 375ºC.

306

Model predictions for 275ºC are depicted as lines in Figure 6, showing that the model fits fairly well

307

the experimental data. The reaction step with the higher rate constant (1.7·10-3 m3/kgcat s) is reaction

308

1, hydrodeoxygenation of the acetophenone carbonyl group.

309

At 375ºC, the model also fits the experiment data well, see Figure 7. At this temperature, reaction 1 is

310

faster than at 275ºC (kinetic constant 3.7·10-3 m3/kgcat s), and for this reason reaction 3 is kinetically

311

relevant and styrene was detected in the reaction products. Hydrogenation of styrene (rate constant

312

130·10-3 m3/kgcat s) is much faster than hydrodeoxygenation of acetophenone, hence the

313

concentration of styrene is maintained very low.


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Page 14 of 30

314

The experiments were performed in absence of diffusional limitations. This has been confirmed by

315

calculating the intraparticle effectiveness factor and Carberry number at the worst conditions, i.e.

316

highest reaction rate (highest temperature, feed concentration and lowest flow rate). Physical

317

properties were evaluated at these conditions using correlations from the literature.50 The effective

318

diffusion coefficient in the porous catalyst particles was estimated as a contribution of molecular and

319

Knudsen diffusion. The values obtained are 0.91 for the intraparticle effectiveness factor and 0.0022

320

< 0.05 for the Carberry number. The isothermicity of the system at the catalyst particle level was also

321

evaluated. The maximum temperature difference inside the catalyst particle is estimated as 0.28ºC

322

(considering the worst case scenario, corresponding to complete conversion), and the maximum

323

temperature difference in the gas film outside the catalyst particle is 0.04ºC.

324

325

4. Conclusions

326

Hydrodeoxygenation of oxygenated aromatics present in biomass-derived bio-oils, to produce de-

327

oxygenated compounds is of interest for the upgrading of biofuels. Acetophenone is an interesting

328

model compound for the study of catalysts activity and optimization of operating conditions, because

329

of the carbonyl group and aromatic ring are chemical structures representative of most bio-oils.

330

Thus, the reactivity of the carbonyl group and aromatic ring in acetophenone leads to a rather

331

complex reaction scheme with competitive parallel and series steps. Experiments of acetophenone

332

hydrodeoxigenation, carried out in gas phase in a fixed-bed reactor with different precious metal

333

catalysts and operating conditions lead to the following conclusions:

334

-

Pt/Al2O3 was found the most active and stable catalyst at 0.5 MPa and 325ºC, the main

335

reaction products being ethylbenzene and styrene. Pd, Ru and Rh on Al2O3 catalysts

336

deactivates drastically in the first 15 h.


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337

-

In the range 0.5-1.5 MPa and at 325ºC, the optimum pressure for hydrodoexygenation of

338

acetophenone on Pt/Al2O3 was 1.0 MPa. At 0.5 MPa, the activity and stability of the catalyst

339

is lower, while at 1.5 MPa the selectivity towards ethylcyclohexane increases.

340

-

341

Operation at 375ºC (Pt/Al2O3 catalyst, 1.0 MPa) is preferred in order to avoid hydrogenation of the aromatic ring, observed at 275ºC. In addition, at 375ºC the reaction is faster.

342

-

The reaction kinetics for the Pt/Al2O3 catalyst at 1.0 MPa and temperature 275ºC and 375ºC

343

fits well to a pseudo-first order kinetic model based on the kinetically relevant steps of the

344

reaction scheme.

345

346

Acknowledgements

347

This work was financed by the Asturian Local Government (Spain, project reference GRUPIN14-078).

348

C. González also thanks the Asturian Local Government for a Ph.D Grant (Severo Ochoa Program).

349

350

List of abbreviations

351

B

352

CHMK Cyclohexyl methyl ketone

353

EB

Ethylbenzene

354

ECH

Ethylcyclohexane

355

S

Styrene

356

T

Toluene

Benzene

357


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References

359 360 361

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362 363

2. Guo, Y. W., Y. Wei, F., Research progress in biomass flash pyrolysis technology for liquids production. Chemical industry engineering progress 2001, 8, 13-17.

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3. French, R.; Czernik, S., Catalytic pyrolysis of biomass for biofuels production. Fuel Processing Technology 2010, 91, (1), 25-32.

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4. Moraes, M. S. A.; Georges, F.; Almeida, S. R.; Damasceno, F. C.; Maciel, G. P. d. S.; Zini, C. A.; Jacques, R. A.; Caramão, E. B., Analysis of products from pyrolysis of Brazilian sugar cane straw. Fuel Processing Technology 2012, 101, (0), 35-43.

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5. Zhang, Q.; Chang, J.; Wang, T.; Xu, Y., Review of biomass pyrolysis oil properties and upgrading research. Energy Conversion and Management 2007, 48, (1), 87-92.

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6. Zhang, L.; Liu, R.; Yin, R.; Mei, Y., Upgrading of bio-oil from biomass fast pyrolysis in China: A review. Renewable and Sustainable Energy Reviews 2013, 24, (0), 66-72.

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7. Imran, A.; Bramer, E. A.; Seshan, K.; Brem, G., High quality bio-oil from catalytic flash pyrolysis of lignocellulosic biomass over alumina-supported sodium carbonate. Fuel Processing Technology 2014, 127, (0), 72-79.

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8. Choudhary, T. V.; Phillips, C. B., Renewable fuels via catalytic hydrodeoxygenation. Applied Catalysis A: General 2011, 397, (1–2), 1-12.

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9. Adjaye, J. D.; Bakhshi, N. N., Production of hydrocarbons by catalytic upgrading of a fast pyrolysis bio-oil. Part I: Conversion over various catalysts. Fuel Processing Technology 1995, 45, (3), 161-183.

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10. Furimsky, E., Catalytic hydrodeoxygenation. Applied Catalysis A: General 2000, 199, (2), 147-190.

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11. Elliott, D. C., Historical Developments in Hydroprocessing Bio-oils. Energy & Fuels 2007, 21, (3), 1792-1815.

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12. French, R. J.; Hrdlicka, J.; Baldwin, R., Mild hydrotreating of biomass pyrolysis oils to produce a suitable refinery feedstock. Environmental Progress and Sustainable Energy 2010, 29, (2), 142-150.

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13. Grilc, M.; Likozar, B.; Levec, J., Hydrotreatment of solvolytically liquefied lignocellulosic biomass over NiMo/Al2O3 catalyst: Reaction mechanism, hydrodeoxygenation kinetics and mass transfer model based on FTIR. Biomass and Bioenergy 2014, 63, 300-312.

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14. Grilc, M.; Likozar, B.; Levec, J., Hydrodeoxygenation and hydrocracking of solvolysed lignocellulosic biomass by oxide, reduced and sulphide form of NiMo, Ni, Mo and Pd catalysts. Applied Catalysis B: Environmental 2014, 150–151, 275-287.

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15. Grilc, M.; Veryasov, G.; Likozar, B.; Jesih, A.; Levec, J., Hydrodeoxygenation of solvolysed lignocellulosic biomass by unsupported MoS2, MoO2, Mo2C and WS2 catalysts. Applied Catalysis B: Environmental 2015, 163, 467-477.


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16. Brillouet, S.; Baltag, E.; Brunet, S.; Richard, F., Deoxygenation of decanoic acid and its main intermediates over unpromoted and promoted sulfided catalysts. Applied Catalysis B: Environmental 2014, 148–149, 201-211.

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17. Bykova, M. V.; Ermakov, D. Y.; Kaichev, V. V.; Bulavchenko, O. A.; Saraev, A. A.; Lebedev, M. Y.; Yakovlev, V. А., Ni-based sol–gel catalysts as promising systems for crude bio-oil upgrading: Guaiacol hydrodeoxygenation study. Applied Catalysis B: Environmental 2012, 113–114, 296-307.

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18. Fisk, C. A.; Morgan, T.; Ji, Y.; Crocker, M.; Crofcheck, C.; Lewis, S. A., Bio-oil upgrading over platinum catalysts using in situ generated hydrogen. Applied Catalysis A: General 2009, 358, (2), 150156.

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19. de Wild, P.; Van der Laan, R.; Kloekhorst, A.; Heeres, E., Lignin valorisation for chemicals and (transportation) fuels via (catalytic) pyrolysis and hydrodeoxygenation. Environmental Progress & Sustainable Energy 2009, 28, (3), 461-469.

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20. Wildschut, J.; Melián-Cabrera, I.; Heeres, H. J., Catalyst studies on the hydrotreatment of fast pyrolysis oil. Applied Catalysis B: Environmental 2010, 99, (1–2), 298-306.

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21. Wildschut, J.; Iqbal, M.; Mahfud, F. H.; Cabrera, I. M.; Venderbosch, R. H.; Heeres, H. J., Insights in the hydrotreatment of fast pyrolysis oil using a ruthenium on carbon catalyst. Energy & Environmental Science 2010, 3, (7), 962-970.

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22. Chen, C.; Chen, G.; Yang, F.; Wang, H.; Han, J.; Ge, Q.; Zhu, X., Vapor phase hydrodeoxygenation and hydrogenation of m-cresol on silica supported Ni, Pd and Pt catalysts. Chemical Engineering Science 2015, 135, 145-154.

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23. Yao, G.; Wu, G.; Dai, W.; Guan, N.; Li, L., Hydrodeoxygenation of lignin-derived phenolic compounds over bi-functional Ru/H-Beta under mild conditions. Fuel 2015, 150, 175-183.

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24. Valle, B.; Gayubo, A. G.; Aguayo, A. T.; Olazar, M.; Bilbao, J., Selective Production of Aromatics by Crude Bio-oil Valorization with a Nickel-Modified HZSM-5 Zeolite Catalyst. Energy & Fuels 2010, 24, (3), 2060-2070.

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25. Yang, H.-M.; Zhao, W.; Norinaga, K.; Fang, J.-J.; Wang, Y.-G.; Zong, Z.-M.; Wei, X.-Y., Separation of phenols and ketones from bio-oil produced from ethanolysis of wheat stalk. Separation and Purification Technology 2015, 152, 238-245.

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26. Undri, A.; Abou-Zaid, M.; Briens, C.; Berruti, F.; Rosi, L.; Bartoli, M.; Frediani, M.; Frediani, P., A simple procedure for chromatographic analysis of bio-oils from pyrolysis. Journal of Analytical and Applied Pyrolysis 2015, 114, 208-221.

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27. Demirbas, A., The influence of temperature on the yields of compounds existing in bio-oils obtained from biomass samples via pyrolysis. Fuel Processing Technology 2007, 88, (6), 591-597.

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28. Huang, J.; Jiang, Y.; Van Vegten, N.; Hunger, M.; Baiker, A., Tuning the support acidity of flamemade Pd/SiO2-Al 2O3 catalysts for chemoselective hydrogenation. Journal of Catalysis 2011, 281, (2), 352-360.

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29. Lin, S. D.; Sanders, D. K.; Albert Vannice, M., Influence of metal-support effects on acetophenone hydrogenation over platinum. Applied Catalysis A: General 1994, 113, (1), 59-73.


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30. Schimmoeller, B.; Hoxha, F.; Mallat, T.; Krumeich, F.; Pratsinis, S. E.; Baiker, A., Fine tuning the surface acid/base properties of single step flame-made Pt/alumina. Applied Catalysis A: General 2010, 374, (1–2), 48-57.

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31. Chen, C.-S.; Chen, H.-W.; Cheng, W.-H., Study of selective hydrogenation of acetophenone on Pt/SiO2. Applied Catalysis A: General 2003, 248, (1–2), 117-128.

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32. Alharbi, K.; Kozhevnikova, E. F.; Kozhevnikov, I. V., Hydrogenation of ketones over bifunctional Ptheteropoly acid catalyst in the gas phase. Applied Catalysis A: General, (0).

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33. Chen, M.; Maeda, N.; Baiker, A.; Huang, J., Molecular insight into Pt-catalyzed chemoselective hydrogenation of an aromatic ketone by in situ modulation-excitation IR spectroscopy. ACS Catalysis 2012, 2, (9), 2007-2013.

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34. Jiang, Y.; Büchel, R.; Huang, J.; Krumeich, F.; Pratsinis, S. E.; Baiker, A., Solvent-free Hydrodeoxygenation of Bio-oil Model Compounds Cyclopentanone and Acetophenone over Flamemade Bimetallic Pt-Pd/ZrO(2) Catalysts. ChemSusChem 2012, 5, (7), 1190-1194.

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35. Chen, C.-S.; Chen, H.-W., Enhanced selectivity and formation of ethylbenzene for acetophenone hydrogenation by adsorbed oxygen on Pd/SiO2. Applied Catalysis A: General 2004, 260, (2), 207-213.

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36. Wang, Y.; Su, N.; Ye, L.; Ren, Y.; Chen, X.; Du, Y.; Li, Z.; Yue, B.; Tsang, S. C. E.; Chen, Q.; He, H., Tuning enantioselectivity in asymmetric hydrogenation of acetophenone and its derivatives via confinement effect over free-standing mesoporous palladium network catalysts. Journal of Catalysis 2014, 313, (0), 113-126.

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37. Xiang, Y.-z.; Lv, Y.-a.; Xu, T.-y.; Li, X.-n.; Wang, J.-g., Selectivity difference between hydrogenation of acetophenone over CNTs and ACs supported Pd catalysts. Journal of Molecular Catalysis A: Chemical 2011, 351, (0), 70-75.

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38. Sumner, C., Jr.; Burchett, W., Developments in the Pd Catalyzed Hydrogenation of Oxygenated Organic Compounds. Topics in Catalysis 2012, 55, (7-10), 480-485.

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39. Bergault, I.; Fouilloux, P.; Joly-Vuillemin, C.; Delmas, H., Kinetics and Intraparticle Diffusion Modelling of a Complex Multistep Reaction: Hydrogenation of Acetophenone over a Rhodium Catalyst. Journal of Catalysis 1998, 175, (2), 328-337.

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40. Malyala, R. V.; Rode, C. V.; Arai, M.; Hegde, S. G.; Chaudhari, R. V., Activity, selectivity and stability of Ni and bimetallic Ni–Pt supported on zeolite Y catalysts for hydrogenation of acetophenone and its substituted derivatives. Applied Catalysis A: General 2000, 193, (1–2), 71-86.

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41. Rajashekharam, M. V.; Bergault, I.; Fouilloux, P.; Schweich, D.; Delmas, H.; Chaudhari, R. V., Hydrogenation of acetophenone using a 10% Ni supported on zeolite Y catalyst: kinetics and reaction mechanism. Catalysis Today 1999, 48, (1–4), 83-92.

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42. Wang, W.; Yang, Y.; Luo, H.; Hu, T.; Liu, W., Amorphous Co–Mo–B catalyst with high activity for the hydrodeoxygenation of bio-oil. Catalysis Communications 2011, 12, (6), 436-440.

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43. Trasarti, A. F.; Bertero, N. M.; Apesteguía, C. R.; Marchi, A. J., Liquid-phase hydrogenation of acetophenone over silica-supported Ni, Co and Cu catalysts: Influence of metal and solvent. Applied Catalysis A: General 2014, 475, (0), 282-291.


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44. Ordóñez, S.; Sastre, H.; Díez; F.V.; Thermogravimetric determination of coke deposits on alumina-supported noble metal catalysts used as hydrodechlorination catalysts. Thermochimica Acta 2001, 379, 25-34.

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45. Cavani, F.; Trifirò, F., Alternative processes for the production of styrene. Applied Catalysis A: General 1995, 133, (2), 219-239.

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46. Seoane, X. L.; Arcoya, A.; Gonzalez, J. A.; Travieso, N., Hydrogenation of ethylbenzene over a nickel/mordenite catalyst. Catalytic decay by thiophene poisoning. Industrial & Engineering Chemistry Research 1989, 28, (3), 260-264.

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47. Faba, L.; Díaz E.; Ordóñez, S.; Role of the support on the performance and stability of Pt-based catalysts for furfural-acetone adduct hydrodeoxygenation. Catalysis Science and Technology 2015, 5, 1473-1484.

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48. Faba, L.; Díaz E.; Ordóñez, S.; Hydrodeoxygenation of acetone-furfural condensation adducts over alumina-supported noble metal catalysts. Applied Catalysis B: Environmental. 2014, 160-161, 436444.

484 485 486

49.Irún, O.; Sadosche, S. A.; Lasobras, J.; Soler, J.; Francés, E.; Herguido, J.; Menéndez, M., Catalysts for the production of styrene from ethylbenzene: Redox and deactivation study. Catalysis Today 2013, 203, (0), 53-59.

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50. Polling, B. E.; Prausnitz, J. M.; O'Connell, J. P., The properties of gases and liquids. 5th ed.; McGraw-Hill: Boston MA, 2001.

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491 492

Caption to figures

493 494

Figure 1

Diagram of the experimental device.

495

Figure 2

Stability of alumina supported precious metal catalysts for the hydrodeoxygenation

496

of acetophenone: Pd (), Ru (), Rh () and Pt (). Conditions: 325ºC, 0.5 MPa,

497

H2/O = 23.2 molar, W/F =1.00 kgcat s/moltot.

498

Figure 3

General scheme of the hydrodeoxygenation of acetophenone.

499

Figure 4

Temperature programmed oxidation (TPO) of aged catalysts (temperature ramp

500 501

5ºC/min): Pd (▬), Ru (▬), Rh (▬) and Pt (▬). Figure 5

Stability of Pt/Al2O3 catalyst for the hydrodeoxygenation of acetophenone.

502

a) Influence of pressure: 0.5 MPa (), 1.0 MPa () and 1.5 MPa (). Conditions:

503

325ºC, H2/O = 23.2 molar, W/F = 1.00 kgcat s/moltot.

504

b) Influence of temperature: 275ºC () and 375ºC (). Conditions: 1.0 MPa, H2/O =

505

23.2 molar, W/F = 0.75 kgcat s/moltot.

506

Figure 6

507 508 509

Results of the reaction kinetic experiments and fitting of the kinetic model for the Pt/Al2O3 catalyst at 275ºC, 1.0 MPa and H2/O = 23 molar.

Figure 7

Results of the reaction kinetic experiments and fitting of kinetic model for the Pt/Al2O3 catalyst at 375ºC, 1.0 MPa and H2/O = 23 molar.

510


Page 21 of 30

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Energy & Fuels

511 512

List of tables

513 514

Table 1

Summary of acetophenone conversion and product selectivities for the Pt/Al2O3

515

catalyst at different operating conditions (X = conversion. S = selectivity at 55 h. Sub-

516

index indicates compounds: S = styrene, EB = ethylbenzene, ECH = ethylcyclohexane,

517

CHMK = cyclohexyl methyl ketone, B = benzene, T = toluene)

518 519

Table 2

Results of the fitting of the kinetic models for the Pt/Al2O3 catalyst at 1.0 MPa and

520

temperature 275ºC and 375ºC (Sub-indexes of the rate constants refers to the steps

521

of the reaction scheme in Figure 3. Sub-indexes of compounds: S = styrene, EB =

522

ethylbenzene, ECH = ethylcyclohexane, CHMK = cyclohexyl methyl ketone)

523 524


Energy & Fuels

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Page 22 of 30

525 526

Table 1

527 p T (ºC) (MPa)

W/F (kg s/mol)

X1h

X55h

SS

SEB

SECH

SCHMK

SB

ST

Total

0.5

325

1.00

97%

30%

12%

85%

-

-

3%

-

100%

1.0

325

1.00

97%

64%

-

85%

1%

-

1.2%

0.3%

88%

1.5

325

1.00

95%

64%

-

92%

4%

-

1.5%

0.4%

98%

1.0

275

0.75

97%

27%

-

83%

1%

7%

-

-

91%

1.0

375

0.75

79%

36%

11%

80%

-

-

1%

1%

93%

528 529 530


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Energy & Fuels

531 532

Table 2

533 275ºC

375ºC

= − −

= −

= −

= −

=

=

= Kinetic constants (m3/kgcat s) · 103 k1 = 1.7 ± 0.2

k1 = 3.7 ± 0.3

k4 = 0.22 ± 0.02

k3 = 130 ± 10

k5 = 1.4 ± 0.3 R2 = 0.98

R2 = 0.97

534 535


Energy & Fuels

536 537

Figure 1

Cylinder 2

538

Cylinder 1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

539 540 541 542


Page 25 of 30

543 544

Figure 2

545 100

Pd Ru Rh Pt

80

Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

60 40 20 0 0

546

10

20

30

40

50

60

Time-on-steam (h)

547 548


Energy & Fuels

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Page 26 of 30

549 550

Figure 3

551 552 553 554


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Energy & Fuels

555 556

Figure 4

557

558 559 560 561 562


Energy & Fuels

563 564

Figure 5

565

100

Conversion (%)

80 60 40 0.5 MPa 1.0 MPa 1.5 MPa

20 0 0 566

10

20

30

40

50

60

Time-on-stream (h)

a) 100

275ºC

Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 30

80

375ºC

60 40 20 0 0

567

b)

10

20

30

40

50

60

Time-on-stream (h)

568 569 570 571


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572 573

Figure 6

574

a)

575

b)

576


Energy & Fuels

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Page 30 of 30

577 578

Figure 7

579

580 581 582 583 584


Hydrodeoxygenation of Acetophenone over Supported Precious Metal

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