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Kinetics of low temperature aqueous-phase hydrogenation of model bio-oil
compounds
Ankush B. Bindwal, Atul H. Bari, Prakash D. Vaidya
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http://dx.doi.org/10.1016/j.cej.2012.07.043
CEJ 9558
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Please cite this article as: A.B. Bindwal, A.H. Bari, P.D. Vaidya, Kinetics of low temperature aqueous-phase
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1
Kinetics of low temperature aqueous-phase hydrogenation of model bio-oil compounds
Ankush B. Bindwal, Atul H. Bari and Prakash D. Vaidya*
Department of Chemical Engineering, Institute of Chemical Technology, Mumbai-400 019, India
* Author to whom correspondence should be addressed
(Fax: +91-22-33611020; Tel.: +91-22-33612014; Email: [email protected])
2
Abstract
Chemically complex bio-oils from biomass fast pyrolysis are promising intermediate renewable
energy carriers, which can be transformed into hydrogen (H2) or alkanes (C1-C6) by aqueousphase processing. Bio-oil transformation can be accomplished in three steps: water extraction,
low temperature hydrogenation of the water-soluble portion, and aqueous-phase reformation (to
H2) or dehydration/hydrogenation (to alkanes). In this work, the reaction kinetics of mild
aqueous-phase hydrogenation (T≤423 K) of four model compounds of the bio-oil aqueous
fraction, viz. hydroxyacetone, hydroxyacetaldehyde, guaiacol and 2-furanone was studied in a
slurry reactor using Ru/C catalyst. The investigated compounds were converted to 1,2propanediol, ethylene glycol, 1,2-cyclohexanediol and γ–butyrolactone, respectively. Wide
ranges of temperature (323-423 K), H2 partial pressure (0.69-2.76 MPa) and catalyst loading
(0.2-2 kg/m3) were examined. To deduce the mechanistic features of reaction kinetics,
Langmuir-Hinshelwood-Hougen-Watson (LHHW) type models were considered.
Keywords: catalysis, kinetics, mass transfer, reaction engineering, bio-oil, hydrogen.
3
1.
Introduction
Fast pyrolysis technology is a promising option for the effective conversion of biomass to
liquid fuels. In this process, biomass is rapidly heated to intermediate temperatures in the
deficiency of oxygen; by this way, it is converted into bio-oil (or pyrolysis oil). This intermediate
renewable energy carrier can be transformed into hydrogen (H2) or alkanes (C1-C6) by aqueousphase processing [1], an approach earlier proposed by Huber et al. [2] and Cortright et al. [3] for
the selective conversion of sugars and polyols to targeted products. Bio-oil transformation can be
accomplished in three stages: water extraction, low temperature hydrogenation of the watersoluble portion, and aqueous-phase reformation (to H2) or dehydration/hydrogenation (to
alkanes).
In the first stage, after water addition, bio-oil can be separated into a water-insoluble portion
(also known as pyrolytic lignin) and an aqueous portion. Pyrolytic lignin, whose energy content
is high, can be upgraded to fuels by hydrotreatment. In the second stage, the bio-oil aqueous
fraction, a complex mixture of several constituents (such as alcohols, sugars, aldehydes, ketones,
acids, guaiacols, syringols, furans, furfurals and water), can be hydrogenated at low temperature.
By this way, thermally unstable compounds (e.g., aldehydes, acids and sugars) which decompose
at high temperature and cause catalyst coking during subsequent processing can be converted
into stable compounds (e.g., alcohols and diols). Consequently, the water-soluble portion of biooil becomes suitable for further treatment. The third stage involves selective conversion to H2
(by aqueous-phase reforming) or alkanes (by aqueous-phase dehydration/hydrogenation).
Vispute and Huber [1] reported the formation of C2-C4 diols and sorbitol during low
temperature (i.e. 398-448 K) catalytic hydrogenation of the bio-oil aqueous fraction. Up to now,
this is the only investigation on the intermediate stage of the bio-oil transformation process
4
available. In the present work, hydroxyacetone (HA), hydroxyacetaldehyde (HAL), guaiacol
(GL) and 2-furanone (FU) were selected as model compounds of the water-soluble fraction of
bio-oil, and the kinetics of their mild aqueous-phase hydrogenation reactions was investigated in
a three-phase slurry reactor using a commercial Ru/C catalyst. Interestingly, HA, HAL, GL and
FU are oxygenated species representing ketones, aldehydes, mono- and dimethoxyphenols, and
furan-based compounds present in biomass pyrolysis liquids [1].
Certainly, model compound studies are useful, due to the fact that they highlight structurereactivity of bio-oil components. They provide a systematic approach for determining how best
the targeted C-O bonds can be selectively hydrogenated while minimizing the cleavage of C-C
and C-O bonds (which results in the formation of undesired methane). To assist the design and
operation of hydrogenation reactors in a fast pyrolysis-based bio-refinery, we investigated
reaction mechanism and kinetics. In all the investigated reaction systems, we examined wide
ranges of the reaction variables such as temperature, H2 partial pressure, reactant concentration
and catalyst loading. Because Ru/C has high efficacy for hydrogenation of the bio-oil aqueous
fraction [1], we selected this catalyst. Besides, the performance of Ru/C in the catalytic
hydroprocessing of bio-oils to liquid fuels appears encouraging, too [4,5].
2.
Experimental
2.1 Materials
Hydroxyacetone (purity 90%) and guaiacol (purity 99%) were purchased from S. D. Fine
Chemicals Pvt. Ltd., Mumbai. Hydroxyacetaldehyde dimer (purity 98%) and 2-furanone (purity
99%) were acquired from Sigma Aldrich Pvt. Ltd., Mumbai. H2 and nitrogen (N2) cylinders
5
(purity 99.9 %) were purchased from Inox Air Products, Mumbai. A commercial 5% Ru/C
catalyst was supplied by Arora-Matthey Ltd., Kolkata, India.
2.2 Experimental Setup
All experiments were conducted in a 0.1 dm3 SS-316 high pressure reactor (Parr Instruments
Company, Illinois, USA). This experimental device was supplied with a variable speed magnetic
drive, turbine agitator (diameter 35 mm, four 45° pitched-blades) and a cooling coil. Besides, it
was also equipped with inlet and outlet ports for the gas, a rupture disk, a liquid outlet port and a
chilled water condenser. The entire assembly was proven to have no leak. A pressure gauge
enabled measurement of the total pressure inside the reactor. A temperature sensor, immersed in
the reactor content, was used to measure the liquid temperature. The setup was supplied by an
electrically heated jacket to ensure isothermal conditions. The temperature and speed of agitation
were controlled by using a Parr 4842 controller.
2.3 Experimental Procedure
In each experiment, the reactor was charged with 0.05 dm3 of an aqueous solution of the
reactant and a fixed amount of the fresh catalyst. The gas inside the reactor was then purged with
N2 to ensure an inert atmosphere and leak-proof system. Thereafter, N2 was released through the
gas outlet port. All the lines were closed and the reactor was heated to the desired temperature.
H2 from the gas cylinder was then charged in excess inside the reactor, this being considered as
the starting point for the reaction. The reactor content was stirred at the desired speed of
agitation. The reaction temperature was maintained at its desired value with an accuracy of ±1 K
by circulating cold water through the cooling coil. Additional H2 was charged to offset the
6
amount consumed during reaction, thereby resulting in constant total pressure. The partial
pressure of water at the operating conditions was considered while determining H2 partial
pressure. Liquid samples were collected at numerous time intervals. Every time, a sample
volume equal to 5×10-4 dm3 was collected. After each experiment, the reactor was cooled and the
catalyst was recovered. The change in catalyst concentration during the course of the reaction
was neglected. The residual concentrations of the reactants/intermediates in solution were
analyzed thereafter. The reproducibility of experiments was checked and the error in all
experimental measurements was found to be less than 3%.
2.4 Product Analysis
The residual concentrations of HAL, GL and FU in the aqueous solutions were determined
by HPLC technique (Knauer Instruments, Berlin, Germany) using a UV K-2501 detector and a
C-18 reverse phase column (Merck, Darmstadt, Germany). On the other hand, HA
concentrations in the solutions were determined by GC technique (GC-8610, Thermo Fischer
Scientific, Mumbai) using a flame ionization detector with SE-30 column (length 4 m, diameter
0.22 mm). HA, HAL, GL and FU were converted to 1,2-propanediol (or propylene glycol, PG),
ethylene glycol (EG), 1,2-cyclohexanediol (CHD) and γ–butyrolactone (GBL), respectively. The
formation of these stable hydrogenation products was confirmed by using GC-MS (QP-2010
Shimadzu) and LC-MS (QP-2010 Shimadzu) techniques.
2.5 Catalyst Characterization
BET specific surface area (793 m2/g), micropore volume (0.4 cm3/g) and average pore
diameter (2.2 nm) were determined using a SMART SORB 93 device. To accomplish this, N2
7
adsorption-desorption isotherms were obtained at 77.5 K over a wide range of relative pressures
on samples previously out gassed at 573 K for 1 h. The surface morphology of the catalyst was
studied by using the scanning electron microscopy technique (JEOL-JSM 6380 LA Scanning
Electron Microscope). A SEM image of the unused catalyst is shown in Fig. 1. The monolithic
shape is distinctly seen. Powder X-ray diffraction patterns of the unused and used Ru/C catalyst
were obtained using Rigaku Miniflex D500 diffractometer and monochromic Cu Kα radiation
(see Figs. 2a and 2b). The high peak at 26.8o in Fig. 2 (a) was attributed to C; in Fig. 2 (b), this
peak was broadened. No intensive peaks characteristic of the metal crystal at 38.4o, 42.2o, 44o,
58.3o, 69.4o, 78.4o, 84.7o and 86o were seen in any of the two figures. Low intensity peaks for
RuO2 crystal of the rutile type at 28.3o, 35.3o and 54.3o were observed in Fig. 2 (b) [6,7].
2.6 Mass Transfer Considerations
Heterogeneous catalytic hydrogenation is a gas-liquid-solid reaction system that involves the
following processes [8]: transfer of H2 from bulk gas phase to the gas-liquid interface;
instantaneous saturation of the interface with H2; diffusion of H2 through the interface into the
bulk liquid; transfer of H2 to the external surface of the catalyst particle; transfer of the other
reactant in liquid phase to the external surface; intra-particle diffusion followed by chemical
reaction at the active sites; and diffusion of the products. Any of these mass transfer processes
(external or internal) can influence the rates of reaction. To determine the kinetic parameters, it is
essential to ensure the absence of mass transfer limitations.
The resistance to mass transfer of H2 on the gas-side was negligible, due to its high
diffusivity in the gas phase and low solubility in the liquid. The extent of the liquid-phase and
liquid-solid mass transfer resistances is determined by the level of turbulence in the liquid phase.
8
In the kinetically controlled regime, the reaction rate is independent of the mass transfer
coefficients kL and kSL, and hence it should not depend on the speed of agitation. We studied this
effect experimentally by varying the stirring speed in the range 300-1400 rpm. From our results,
we deduced that there is practically no change in reaction rates above 900 rpm. In all
experiments, a stirring speed of 1200 rpm was used. To investigate any possible influence of
intra-particle diffusion on the reaction rates, the Weisz and Prater criterion was used [9]. The
parameter ηϕi2 was calculated by using the following relationship:
(1)
where r denotes reaction rate, ω denotes catalyst loading and L denotes the characteristic length
of the spherical catalyst particle (=Dp/6, where DP=50 µm). The value of
(kmol/m3) was
estimated by using the following correlation suggested by Pintar et al. [10]:
where the dimensionless mole fraction solubility xg is given by the relationship [11]:
The liquid-phase effective diffusivities of hydrogen (H2) and bio-oil model compounds (B) were
estimated using the Wilke-Chang equation [12]. It was found that the value of ηϕi2 for all
reactants was much less than unity at the conditions used for this study. Therefore, the intraparticle diffusion resistance was neglected.
9
Reactant concentration vs. time data were plotted and fitted to a third degree polynomial
using least square regression. Rates of disappearance of the reactant (B) were calculated from the
values of the slope –dCB/dt.
3.
Results and Discussion
3.1 Reactions
The hydrogenation reactions of HA, HAL, GL and FU are represented as:
(4)
(5)
(6)
(7)
The ranges of temperature (T), H2 partial pressure (
), feed concentration (CB) of the
oxygenated species and catalyst loading (ω) used in this work are listed in Table 1. Because the
reaction velocities for different substrates differed, we examined a wide range of CB values.
3.2 Effect of Temperature
The “reactant concentration vs. time” data at various temperatures and comparison with the
results reported by Vispute and Huber [1] is shown in Table 2. We investigated HA
disappearance at T=373, 398 and 423 K at constant H2 partial pressure (
=0.69 MPa) using
CB=1.35 M and ω=1 kg/m3. The increase in T resulted in increased catalytic activity. For
example, HA conversion after 3 h increased from 50 to 100% when T was raised from 373 to
423 K. Vispute and Huber [1] reported complete conversion to PG within 2.5 h at T=398 K,
10
=6.9 MPa and CB=135.5 mmol-C/L using Ru/C catalyst (see Table 2). Using homogeneous
Ru catalyst in a biphasic water/toluene system at T=333 K,
=4 MPa and CB=90 mM, Mahfud
et al. [13] reported just 7.5% conversion in a previous work.
The reaction with HAL (CB=67 mM) was fast and this substrate was completely converted
within 3 h even at low temperature (T=348 K) and H2 pressure (
=0.69 MPa). As seen in
Table 2, Vispute and Huber [1] reported 100% conversion to EG at T=398 K, CB=28.1 mmolC/L and
T=363 K,
=6.9 MPa in 0.5 h. On the other hand, Mahfud et al. [13] reported 54% conversion at
=4 MPa and CB=90 mM using homogeneous Ru catalyst.
At T=348 K and
=0.69 MPa, GL conversion to CHD was complete within 4 h. The time
required for complete conversion was lesser (1 h) at high temperature (398 K) and pressure (6.9
MPa) [1]. Notwithstanding the mild reaction conditions used in our work, carbon loss in the form
of methane was observed. Finally, it was found that FU was completely converted to GBL in 3 h
at T=348 K and
=0.69 MPa.
3.3 Effect of Catalyst Loading
The effect of catalyst loading (ω) on the HA disappearance rates was studied at T=423 K
and
=0.69 MPa. The initial HA concentration (CB) was kept constant at 1.35 M, whereas ω
was varied in the range, 0.5-2 kg/m3. The results are represented in Fig. 3 (a). Since the data in
Fig. 3 (a) fall on a straight line passing through the origin, we concluded that the initial reaction
rate varies linearly with catalyst concentration. Analogously, rates of HAL hydrogenation
showed similar behavior at T=348 K (see Fig. 3 (b)). The effect of catalyst loading on initial
rates during the reactions with GL and FU at the selected temperatures is shown in Fig. 4 (a) and
11
(b), respectively. Certainly, the reaction exhibits first order kinetics with catalyst concentration at
all temperatures.
3.4 Effect of H2 Partial Pressure
The effect of H2 partial pressure on HA disappearance rate was studied in the range, 0.692.76 MPa, at 373, 398 and 423 K (see Table 3). These results suggest first-order kinetics with
H2. In an analogous manner, the rates of disappearance of HAL at 323, 338 and 348 K increased
by a factor of four when
rates on
was increased from 0.69 to 2.76 MPa. Clearly, the dependence of
was linear.
On the other hand, the dependence of GL hydrogenation rates on H2 partial pressure was
much less than linear (see Table 3). The rate vs.
data at various temperatures for FU
hydrogenation is shown in Table 3. Clearly, a low reaction order (<1) was observed; this
behavior was similar to that of GL. From our results, we concluded that H2 is strongly adsorbed
on the catalyst surface during the reactions with GL and FU.
3.5 Effect of Initial Reactant Concentration
The effect of initial HA concentration was studied, too (see Fig. 5 (a)). From our results in
the concentration range, 0.34-1.35 M, we found that the reaction exhibits fractional-order
kinetics between 0 and 1. Conversely, the dependence of rate on the concentration of HAL was
linear (see Fig. 5 (b)). The rate vs. concentration data for GL hydrogenation is shown in Fig. 6
(a). The dependence of the rate on GL concentration is more than linear. As FU concentration
12
increased from 23 to 59 mM, the rate at 348 K was almost doubled. These results are shown in
Fig. 6 (b).
3.6 Reaction Mechanism and Kinetics
The initial rate data presented in the previous sections suggest complex kinetic behavior;
thus, mechanistic models (rather than power law relationship) are essential for studying reaction
kinetics. Among the compounds selected for this work, HAL is an aldehyde; HA and FU are
ketones, whereas GL is a methoxyphenol. The aldehyde (i.e. HAL) is the easiest to hydrogenate;
contrarily, the reaction with GL is most difficult because it involves cleavage of the ether bond.
Commonly, aldehydes and ketones follow a similar mechanism of hydrogenation, and thus,
should be governed by the same or similar models. There are some useful papers on kinetics and
mechanism of hydrogenation of aldehydes [14] and ketones [15] available. According to Chang
et al. [15], the sequence of elementary steps for dissociative adsorption of H2 and competitive
adsorption of the reactant B on the same active sites is represented as:
(8)
(9)
(10)
(11)
(12)
If steps represented by Eqs. 10 and 11 are combined, the surface reaction may be represented as:
(13)
13
For the case of non-dissociative adsorption of H2, Eq. 8 is replaced by:
(14)
Using Langmuir-Hinshelwood kinetics and assuming different rate-determining steps, several
kinetic models result. These models were simplified to the initial rate equations (see models I to
V in Table 4).
A model discrimination technique was used for selecting the most appropriate model. The
model parameters were estimated by optimization program POLYMATH utilizing the
Levenberg-Marquardt method for minimization of residual sum of squares (RSS). Model III,
which follows the Langmuir mechanism with the assumption that the surface reaction of
adsorbed reactant and adsorbed H2 (i.e. Eq. 13) is the rate-controlling step, best represented the
data for the reaction with HAL and HA. On the other hand, model IV which assumes that Eq. 10
is rate-determining provided the best fit for the experimental data for FU and GL hydrogenation.
Clearly, the GL reaction pathway is different from that of HAL and HA. GL hydrogenation
proceeds through the sequential addition of H2 to the partially hydrogenated intermediate species
(see Eq. 11) in consecutive surface reaction steps of equal velocities. Interestingly, model III
suggests a first-order dependence of the initial rates on
(as is the case for HA and HAL),
whereas model IV proposes a low reaction order with H2 (as is the case for GL and FU). Further,
model IV predicts higher-order kinetics when the values of
and CB are comparable; this
observation is in line with the kinetic behavior shown in Fig. 6 (a).
All adsorption and surface reaction rate constants are given in Table 5. Parity plots for all
the investigated reaction systems are shown in Figs. 7 (a) and (b). Certainly, there is good
agreement between the predicted and experimental rates. The values of RSS and variance for the
proposed models are shown in Table 6. From the temperature dependence of the reaction rate
14
constant, the activation energy was determined. The heats of adsorption of H2 and the selected
compounds were estimated by using the Van’t Hoff isochore. These values are given in Table 6.
To ascertain the quality of predictions, concentration vs. time data for few experiments showing
both experimental data points and fitted curves based on kinetic parameters were shown in the
Supplementary Information (see Figs. S1 to S4). There exists good agreement between the two
sets of data.
In Table 4, we considered competitive adsorption of H2 and the reactant B on the same
active sites only. When other models following Langmuir mechanism with non-competitive
adsorption or Eley-Rideal kinetics were considered, statistical analysis of the kinetic data proved
to be unsatisfactory. Therefore, such models were excluded from Table 4.
4
Conclusions
In this work, we investigated the aqueous-phase hydrogenation of hydroxyacetone to 1,2-
propanediol, hydroxyacetaldehyde to ethylene glycol, guaiacol to 1,2-cyclohexanediol, and 2furanone to γ–butyrolactone using Ru/C in a slurry reactor. We considered wide ranges of
temperature (323-423 K), H2 partial pressure (0.69-2.76 MPa) and catalyst loading (0.2-2 kg/m3).
To deduce the mechanistic features of reaction kinetics, Langmuir-Hinshelwood-HougenWatson (LHHW) type models were considered. By this way, we managed to provide
comprehensive information on the reaction mechanism and kinetics.
Acknowledgements
Ankush B. Bindwal and Atul H. Bari are grateful to University Grants Commission, New Delhi,
for the financial assistance.
15
Nomenclature
A1
pre-exponential factor in Arrhenius law, kmol/(kgcat min)
A2
pre-exponential factor for the adsorption constant of H2, m3/kmol
A3
pre-exponential factor for the adsorption constant of reactant, m3/kmol
B
reactant in liquid phase
CB
concentration of reactant in the liquid phase, kmol/m3
CH2
concentration of H2 in the liquid phase, kmol/m3
Dei
effective diffusivity of species i in eq. 1, (m2/s)
DP
catalyst particle diameter, m
Eact
energy of activation, kJ/mol
∆HH2
adsorption enthalpy of H2, kJ/mol
∆HB
adsorption enthalpy of reactant, kJ/mol
KB
adsorption equilibrium constant for reactant, m3/kmol
KH2
adsorption equilibrium constant for H2, m3/kmol
k3, k3a
surface reaction rate constant in Eq. 13 and 10, kmol/(kgcat min)
kL
liquid-side mass transfer coefficient, m/s
kSL
solid-liquid mass transfer coefficient, m/s
L
characteristic length of catalyst particle (=DP/6)
molecular weight of water, kg/kmol
PH2
H2 partial pressure, MPa
Ptotal
total operating pressure, bar
16
r
initial rate of reaction, kmol/(kgcat min)
t
time, min
T
temperature, K
xg
dimensionless mole fraction solubility of H2 in water
yi
mole fraction of H2 in gas phase
Greek symbols
density of water, kg/m3
ω
catalyst loading, kg/m3
η
effectiveness factor
ϕ
Thiele modulus
ηϕi2
observable modulus for species i in Eq. 1
17
References
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T. P. Vispute, G. W. Huber, Production of hydrogen, alkanes and polyols by aqueous phase
processing of wood-derived pyrolysis oils, Green Chem. 11 (2009) 1433-1445.
[2]
G. W. Huber, R. D. Cortright, J. A. Dumesic, Renewable alkanes by aqueous-phase
reforming of biomass-derived oxygenates, Angew. Chem. Int. Ed. 43 (2004) 1549-1551.
[3]
R. D. Cortright, R. R. Davda, J. A. Dumesic, Hydrogen from catalytic reforming of
biomass derived hydrocarbons in liquid water, Nature 418 (2002) 964-967.
[4]
J. Wildschut, I. Melián-Cabrera, H. J. Heeres, Catalyst studies on the hydrotreatment of fast
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J. Wildschut, M. Iqbal, F. H. Mahfud, I. Melián-Cabrera, R. H. Venderbosch, H. J. Heeres,
Insights in the hydrotreatment of fast pyrolysis oil using a ruthenium on carbon catalyst,
Energy Environ. Sci. 3 (2010) 962-970.
[6]
T.
Yoneda,
T.
Takido,
K.
Konuma,
Hydrodechlorination
of
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chlorobenzenes over a ruthenium/carbon catalyst, Appl. Catal. B: Environ. 84 (2008) 667677.
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V. I. Zailkovskii, K. S. Nagabhushana, V. Kriventov, K. N. Loponov, S. V. Cherepanova,
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characterization of Se-modified carbon-supported Ru nanoparticles for the oxygen
reduction reaction, J. Phys. Chem. B 110 (2006) 6881-6890.
[8]
L. K. Doraiswamy, M. M. Sharma, Heterogeneous Reactions: Analysis, Examples and
Reactor Design, Vol. 2, John Wiley and Sons, New York, USA, 1984.
18
[9]
H. S. Fogler, Elements of Chemical Reaction Engineering, Prentice-Hall of India, New
Delhi, India, 2008.
[10] A. Pintar, G. Bercic, J. Levec, Catalytic liquid-phase nitrite reduction: kinetics and catalyst
deactivation, AIChE J. 44 (1998) 2280-2292.
[11] P. G. T. Fogg, W. Gerrard, Solubility of Gases in Liquids, Wiley Sons, Chichester,
England, 1991.
[12] C. R. Wilke, P. Chang, Correlation of diffusion coefficients in dilute solutions, AIChE J. 1
(1955) 264-270.
[13] F. H. Mahfud, F. Ghijsen, H. J. Heeres, Hydrogenation of fast pyrolysis oil and model
compounds in a two-phase aqueous organic system using homogeneous ruthenium
catalysts, J. Mol. Catal. A: Chem. 264 (2007) 227-236.
[14] P. D. Vaidya, V. V. Mahajani, Kinetics of liquid-phase hydrogenation of furfuraldehyde to
furfuryl alcohol over a Pt/C catalyst, Ind. Eng. Chem. Res. 42 (2003) 3881-3885.
[15] N. S. Chang, S. Aldrett, M. T. Holtzapple, R. R. Davison, Kinetic studies of ketone
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19
List of Figures
Fig. 1. SEM image of unused Ru/C catalyst
Fig. 2. XRD images of (a) unused and (b) used Ru/C catalyst
Fig. 3 (a). Effect of catalyst loading on initial rate of disappearance of HA (T=423 K,
=0.69
MPa, CB=1.35 kmol/m3).
Fig. 3 (b). Effect of catalyst loading on initial rate of disappearance of HAL (T=348 K,
=0.69
MPa, CB=17 mM).
Fig. 4 (a). Effect of catalyst loading on initial rate of disappearance of GL (
=0.69 MPa,
CB=40 mM).
Fig. 4 (b). Effect of catalyst loading on initial rate of disappearance of FU (
=0.69 MPa,
CB=59 mM).
Fig. 5 (a). Effect of initial HA concentration on initial rate of disappearance of HA (
=0.69
MPa, ω=1 kg/m3).
Fig. 5 (b). Effect of initial HAL concentration on initial rate of disappearance of HAL (
=0.69
MPa, ω=1 kg/m3).
Fig. 6 (a). Effect of initial GL concentration on initial rate of disappearance of GL (
=0.69
MPa, ω=0.5 kg/m3).
Fig. 6 (b). Effect of initial FU concentration on initial rate of disappearance of FU (
MPa, ω=0.5 kg/m3).
=0.69
20
Fig. 7 (a). Comparison of experimental and predicted rates for HAL, GL and FU at all
temperatures.
Fig. 7 (b). Comparison of experimental and predicted rates for HA at all temperatures.
List of Tables
Table 1. Reaction conditions used in the present work
Table 2. The “reactant concentration vs. time” data at various temperatures and comparison with
the results reported by Vispute and Huber [1]
Table 3. Dependence of initial rate of hydrogenation on H2 partial pressure
Table 4. Initial rate equations for competitive adsorption of reactants (see Chang et al. [15])
Table 5. Rate parameters with 95% confidence intervals for proposed models
Table 6. Estimation of the values of pre-exponential factor, activation energy, adsorption
enthalpy of H2 and bio-oil model compounds, RSS and variance for proposed models
21
Fig. 1
22
Fig. 2 (a)
Fig. 2 (b)
23
Fig. 3 (a)
Fig. 3 (b)
24
Fig. 4 (a)
Fig. 4 (b)
25
120
Initial rates × 104 (kmol/(kgcat min))
373 K
398 K
100
423 K
80
60
40
20
0
0
0.3
0.6
0.9
1.2
1.5
Initial hydroxyacetone concentration (kmol/m3 )
Fig. 5 (a)
Fig. 5 (b)
26
Fig. 6 (a)
70
Initial rates × 104 (kmol/(kgcat min))
323 K
60
333 K
348 K
50
40
30
20
10
0
0
0.015
0.03
0.045
0.06
0.075
Initial 2-furanone concentration (kmol/m3 )
Fig. 6 (b)
27
Fig. 7 (a)
Fig. 7 (b)
29
Table 1. Reaction conditions used in the present work
Investigated
Temp. (K)
compound
H2 pressure
Reactant conc.
Catalyst loading
(MPa)
mmol/L
kg/m3
Hydroxyacetone
373-423
0.69-2.76
0.34-1.35 M
0.5-2.0
Hydroxyacetaldehyde
323-348
0.69-2.76
17-67
0.5-2.0
Guaiacol
323-348
0.69-2.76
16-40
0.2-0.8
2-Furanone
323-348
0.69-2.76
23-59
0.2-0.8
30
Table 2. The “reactant concentration vs. time” data at various temperatures and comparison with
the results reported by Vispute and Huber [1]
Reaction
Conc.
Conc.
Conc.
Conc.
Conc.
Conc.
Cond.
(mM)
(mM)
(mM)
(mM)
(mM)
(mM)
t=0
t=0.5 h
t=1 h
t=2 h
t=3 h
t=4 h
P=0.69MPa
Ref.
This
HA
T=423 K
1.35 M
1.04 M
0.80 M
0.35 M
0
0
HAL
T=348 K
67
51
32
11
0
0
GL
T=348 K
40
31
17
6
2
0
FU
T=348 K
59
39
20
7
0
0
P=6.9 MPa
mmol-
mmol-
mmol-
mmol-
mmol-
mmol-
T=398 K
C/L
C/L
C/L
C/L
C/L
C/L
HA
135.5
55.7
24.5
-
0
0
HAL
28.1
0
0
0
0
0
GL
30.8
28.8
0
0
0
0
work
1
31
Table 3. Dependence of initial rate of hydrogenation on H2 partial pressure
H2 partial pressure (MPa)
0.69
Compound
HA
HAL
GL
FU
1.38
2.07
CB
ω
T
r × 103
(M)
(kg/m3)
(K)
(kmol/(kgcat min))
1.35
0.017
0.04
0.059
1.0
1.0
0.5
0.5
2.76
373
3.39
6.36
9.85
11.34
398
6.86
12.21
20.81
23.43
423
11.28
22.43
29.41
39.51
323
0.04
0.07
0.12
0.15
338
0.12
0.21
0.31
0.42
348
0.21
0.41
0.59
0.76
323
2.16
2.64
3.12
3.60
333
2.84
3.53
4.19
4.77
348
3.68
5.22
6.06
6.50
323
2.62
3.04
3.63
3.81
333
3.82
4.63
5.41
5.41
348
6.24
7.04
8.22
8.63
32
Table 4. Initial rate equations for competitive adsorption of reactants (see Chang et al. [15])
Model
Rate-controlling step
Initial-rate expression
Dissociative adsorption of H2
I
II
III
IV
Eq. 8 (adsorption of H2)
Eq. 9 (adsorption of B)
Eq. 13 (surface reaction)
Eq. 10 (surface reaction)
r=
r=
r=
r=
k1 C H2
(1 + K B C B ) 2
k 2 CB
(1 + K 1/2
H2 C H2
1/2
)
k 3 K H2 K B C H2 C B
(1 + K 1/2
H2 C H2
1/2
+ K B CB )3
k 3,a K 1/2
H2 K B C H2
(1 + K 1/2
H2 C H2
1/2
1/2
CB
+ K B CB )2
Non-dissociative adsorption of H2
V
Eq. 13 (surface reaction)
r=
k 3 K H2 K B C H2 C B
(1 + K H 2 C H 2 + K B C B ) 3
33
Table 5. Rate parameters with 95% confidence intervals for proposed models
Compound
HA
HAL
GL
FU
T
k3 or k3a
KH2
KB
(K)
(kmol/(kgcat min))
(m3/kmol)
(m3/kmol)
373
3.54 ± 0.61
1.92 ± 0.41
0.20 ± 0.02
398
13.15 ± 0.48
1.07 ± 0.44
0.18 ± 0.03
423
24.91 ± 0.11
0.84 ± 0.44
0.17 ± 0.04
323
0.80 ± 0.31
0.72 ± 0.02
1.49 ± 0.01
338
3.78 ± 0.52
0.56 ± 0.06
1.07 ± 0.06
348
9.28 ± 0.18
0.22 ± 0.03
0.73 ± 0.06
323
3.06 ± 0.69
0.30 ± 0.12
0.51 ± 0.09
333
3.98 ± 0.72
0.24 ± 0.84
0.47 ± 0.10
348
6.38 ± 0.14
0.21 ± 0.09
0.44 ± 0.10
323
0.15 ± 0.07
12.78 ± 0.16
2.92 ± 0.18
333
0.24 ± 0.06
12.29 ± 0.13
2.29 ± 0.35
348
0.28 ± 0.01
12.06 ± 0.12
2.03 ± 0.26
34
Table 6. Estimation of the values of pre-exponential factor, activation energy, adsorption
enthalpy of H2 and bio-oil model compounds, RSS and variance for proposed models
Model III
Model III
Model IV
Model IV
Parameter
HA
HAL
GL
FU
A1 (kmol/(kgcat min))
1.3 × 1012
2.9 × 1014
9.1 × 104
6.2 × 102
Eact (kJ/mol)
71.0
89.5
27.7
22.1
A2 (m3/kmol)
5.5 × 104
3.2 × 107
3.7 × 102
5.7
∆HH2 (kJ/mol)
30.7
45.7
12.5
2.1
A3 (m3/kmol)
30.3
1.2 × 104
11.9
47.6
∆HB (kJ/mol)
4.8
26.4
4.8
13.1
RSS
1.0 × 10-4
1.6 × 10-6
5.8 × 10-5
5.0 × 10-5
Variance
1.3 × 10-7
3.4 × 10-11
4.1 × 10-8
3.0 × 10-8
35
36
Highlights
•
Using Ru/C, hydrogenation of hydroxyacetone, hydroxyacetaldehyde, guaiacol and 2furanone was studied.
•
Investigated reactions belong to the kinetically controlled reaction regime systems.
•
LHHW models were proposed to describe reaction kinetics.