Chemical loop systems for biochar liquefaction: Hydrogenation of Naphthalene

Article abstract of DOI:10.1039/c4ra03487b

Liquefaction of biochar from liquid-phase pyrolysis was carried out in the solvent Tetralin. Tetralin is able to act as hydrogen donor during liquefaction of biochar and is itself rearranged into Naphthalene. Naphthalene must be re-hydrogenated to Tetralin to allow for further use in the liquefaction reaction (chemical loop system). Therefore Naphthalene hydrogenation was investigated, applying a full factorial design of experiments approach. The yield of Tetralin was chosen as response variable, while two-level-factors for temperature (150 °C and 200 °C), pressure (20 bar and 50 bar) and Raney-Nickel catalyst load (5 wt% and 10 wt%) were selected. The Design of Experiments approach showed a rising influence of all three factors in the order: temperature < pressure < catalyst load. The reaction kinetics of the hydrogenation of Naphthalene to Tetralin and Decalin was then investigated at 150 °C and 200 °C. The reaction proceeds stepwise and not in consecutive steps. In a first step Naphthalene reacts selectively with 96% yield to Tetralin, while the reaction of Tetralin to Decalin does not start until all Naphthalene is consumed. The rate-constant of the reaction of Naphthalene to Tetralin is one magnitude higher than that for the reaction of Naphthalene to Decalin. This is in agreement with the findings from the design of experiments approach. The results of these investigations indicate that the chemical-loop system Naphthalene-Tetralin is suitable for usage in the liquefaction of biochar.

Full text of DOI:10.1039/c4ra03487b

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Chemical loop systems for biochar liquefaction:
hydrogenation of Naphthalene
ab
Cite this: RSC Adv., 2014, 4, 34955
Nikolaus Schwaiger,ab Hannes Pucher,a Lisa Ellmaier,b
*
Roland Feiner,
Michael Derntl,a Peter Pucherb and Matthaus Siebenhofera
¨
Liquefaction of biochar from liquid-phase pyrolysis was carried out in the solvent Tetralin. Tetralin is
able to act as hydrogen donor during liquefaction of biochar and is itself rearranged into
Naphthalene. Naphthalene must be re-hydrogenated to Tetralin to allow for further use in the
liquefaction reaction (chemical loop system). Therefore Naphthalene hydrogenation was investigated,
applying a full factorial design of experiments approach. The yield of Tetralin was chosen as response
variable, while two-level-factors for temperature (150 ꢀC and 200 ꢀC), pressure (20 bar and 50 bar)
and Raney-Nickel catalyst load (5 wt% and 10 wt%) were selected. The Design of Experiments
approach showed a rising influence of all three factors in the order: temperature < pressure < catalyst
load. The reaction kinetics of the hydrogenation of Naphthalene to Tetralin and Decalin was then
investigated at 150 ꢀC and 200 ꢀC. The reaction proceeds stepwise and not in consecutive steps. In a
first step Naphthalene reacts selectively with 96% yield to Tetralin, while the reaction of Tetralin to
Decalin does not start until all Naphthalene is consumed. The rate-constant of the reaction of
Naphthalene to Tetralin is one magnitude higher than that for the reaction of Naphthalene to
Decalin. This is in agreement with the findings from the design of experiments approach. The results
of these investigations indicate that the chemical-loop system Naphthalene–Tetralin is suitable for
usage in the liquefaction of biochar.
Received 17th April 2014
Accepted 30th July 2014
DOI: 10.1039/c4ra03487b
www.rsc.org/advances
according to the overall balance shown in Fig. 1. Decalin does
form two stereoisomeric molecules, trans-Decalin and cis-Dec-
1. Introduction
alin. Although Decalin has a higher H/C-value than Tetralin it is
unfavourable because it does not have the ability to act as a
hydrogen donor.9 This is due to a higher molecular stability of
Decalin compared with Tetralin. The hydrogenation products of
Naphthalene, dihydro-, hexahydro- and octahydronaphthalene,
are not of technical relevance.8
Naphthalene consists of two aromatic rings and belongs to
the group of polyaromatic hydrocarbons (PAHs). Naphthalene is
found in coal tars and crude oil,8 beside PAHs consisting of
three to ve aromatic rings, like pyrene, anthracene, phenan-
threne and uoranthene etc., Naphthalene at ambient condi-
tions is a crystalline/waxy white solid product with a melting
point of 80 ꢀC, while Tetralin and Decalin are liquids. The
Liquefaction of renewable solid carbon sources is a promising
route for production of second generation biofuels. Due to
rising acceptance of pyrolysis in biofuel production, a signi-
cant amount of the by-product biochar is available.1–3 This
biochar can be liqueed in the presence of Tetralin.4,5 Tetralin
acts as a carrier for the biochar particles, as a heat transfer agent
and most importantly as a hydrogen donor solvent.4,6 The bio-
char particles are thermally decomposed and the radicals of the
breaking fragments need to be saturated instantaneously with
hydrogen. If the availability of hydrogen is low, breaking frag-
ments will fall into repolymerisation reactions to form
unwanted tars.7 The solvent Tetralin provides 2 moles of H2 by
rearranging to Naphthalene. Therefore hydrogenation of
Naphthalene to Tetralin is needed for usage in a chemical loop
system.
A common way to produce Tetralin is to partially hydroge-
nate Naphthalene with 2 moles of H2.8 Further hydrogenation
with 3 moles of H2 leads to formation of fully saturated Decalin,
aGraz University of Technology, Institute of Chemical Engineering and Environmental
Technology, NAWI Graz Central Lab Biobased Products, Inffeldgasse 25/C/II, 8010
Graz, Austria
bBDI-BioEnergy International AG, Parkring 18, 8074 Grambach/Graz, Austria
Fig. 1 Hydrogenation of Naphthalene to Tetralin and to Decalin.
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partial hydrogenation of Naphthalene to Tetralin can be con-
ducted in the liquid-phase and the vapour-phase. Selectivity of
Tetralin over Decalin is higher for liquid-phase reactions.10
Noble metal catalysts like platinum and ruthenium but also
nickel are proposed to have a high selectivity towards the
products.8,11,12 The catalyst Red Mud, a by-product of the
alumina production was investigated with minor success.13
Sulphur compounds14 may act as sulphided catalysts. The
mechanisms of the catalytic hydrogenation of Naphthalene was
investigated in ref. 15 and 16.
Several authors17 propose the hydrogenation of Naphthalene
ꢀ
ꢀ
in a liquid carrier at temperatures between 150 C and 200 C
and a hydrogen pressures of 1 to 20 atm. Cyclohexane, meth-
ylcyclohexane, dimethylcyclohexane, ethylcyclohexane, cyclo-
pentane and methylcyclopenane are preferred liquid carriers.
As the reaction temperature is as low as 260 ꢀC,8 decomposition
of Naphthalene, Tetralin, Decalin or the carrier is not to be
ꢀ
expected. Under biochar liquefaction conditions above 400 C
partial degradation of Tetralin and Naphthalene is observed.
Hydrogenation of Naphthalene to Tetralin was investigated
by applying design of experiments with the program JMP for
data analysis. The aim of design of experiments is to obtain an
accurate mathematical prediction model with as few as possible
physical experiments. A full factorial design was applied to
investigate the hydrogenation for a reaction time of 5 minutes.
Design of experiments needs two sorts of variables: (1) response
variables and (2) factor variables. The yield of the desired
product Tetralin was chosen as the response variable. The
temperature, hydrogen pressure and Raney-Nickel catalyst load
(with respect to Naphthalene feed) were used as factor variables.
All independent factor variables that inuence the response
variable need to be taken into account. Factor constraints
were dened. In the physical experiments a temperature of
either 150 ꢀC or 200 ꢀC, a pressure of either 20 bar or 50 bar and
a Raney-Nickel catalyst load of either 5 wt% or 10 wt% were
used.
Fig. 2 SEM Image of Raney-Nickel catalyst.
Experimental procedure
All design of experiments investigations were performed in a
stirred 450 ml batch reactor. For each experiment 20 g of
Naphthalene was added to the reactor together with 80 g of
MCH and 5 wt% or 10 wt% Raney-Nickel catalyst (with respect
to Naphthalene). The Naphthalene to MCH ratio was 1/4. The
reactor was then sealed tightly. The stirring speed was set to 500
rpm during the experiment as well as the cooling phase. The
reactor was heated with an electric heating jacket to the desired
temperature. When the operation temperature was reached
hydrogen was supplied, pre-set to the desired pressure by an
expansion valve. The exothermic reaction started immediately
and the reactor was cooled during the experiment (5 minutes of
reaction time) in order to keep the reaction isothermal. Upon
completion of the experiment the hydrogen valve was closed,
the heating jacket removed and the reactor was cooled to below
Design of experiment proposes that all combinations of
factor variables should be investigated, which leads to a lower
experimental effort. The number of experiments for the design
of experiments approach is calculated as follows: With two steps
ꢀ
100 C within 3 minutes. When the temperature in the reactor
reached 30 ꢀC the pressure was relieved and the reactor was
opened. The liquid product obtained was ltered through a
syringe lter and analysed.
ꢀ
ꢀ
for all factor variables (temperature of either 150 C or 200 C)
and three different factors (k) the number of experiments n ¼ 2k,
resulting in 8 experiments. For improved accuracy a replica is
recommended.
Reaction kinetics were studied in a 1100 ml reactor at two
temperatures 150 ꢀC and 200 ꢀC over a 60 minutes reaction
period. This bigger reactor allows for taking samples while the
reactor is in operation at elevated temperature and pressure
conditions. The hydrogen pressure applied was 20 bar. For each
experiment 93 g of Naphthalene was added to the reactor
together with 187 g of MCH and 5 wt% Raney-Nickel catalyst
(with respect to Naphthalene). The Naphthalene to MCH ratio
was 1/2. Hydrogenation of Naphthalene to Tetralin is a fast
reaction. Aer loading, the reactor was sealed tightly. The stir-
ring speed was set to 500 rpm during the experiment. The
reactor was heated with an electric heating jacket to the desired
temperature. When the desired temperature was reached the
reactor was supplied with hydrogen, pre-set to the desired
From design of experiments the optimum combination of
operation conditions for investigation of the kinetics of the
hydrogenation of Naphthalene to Tetralin were deduced.
2. Materials and methods
Materials
Reagent-grade Naphthalene (CAS: 91-20-3) with a purity of 99%,
the carrier methylcyclohexane (MCH) (CAS: 108-87-2) with a
purity of 99%, Raney-Nickel as pre-activated catalyst (CAS: 7440-
02-0 from Merck) consisting of 90% Ni and 10% Al on weight
dry basis (SEM image, see Fig. 2) and high grade hydrogen were
used for the experiments.
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pressure by an expansion valve. The exothermic reaction started experimental values which do not t the 95% condence
immediately. The reactor was cooled during the 60 minutes of interval.
reaction time to keep the reaction isothermal. Samples of the
The results in Table 1 were subjected to an effect screening
reaction mixture were taken every minute in the rst 10 minutes through JMP soware. During this effect screening the inu-
and thereaer every 5 minutes. The samples taken were ltered ence of the three factors (temperature, pressure, catalyst
through a syringe lter and subjected to analysis.
amount) as well as two-fold combinations (the combination of
The generic reaction rate equations can be expressed pressure and temperature has an effect) and three-fold combi-
according to:
nation of factors on the response were determined. As a result of
the effect screening the factors temperature, pressure and
catalyst amount have a signicant inuence on the yield of
Tetralin. Two-fold and three-fold interactions showed no
signicance. With the essential effects in mind the mathemat-
ical model can be built. Results of the linear regression model
are shown in Fig. 3.
n
rN ¼ ꢁk1cN
n
n
rT ¼ k1cN ꢁ k2cT
n
rD ¼ k2cT
Fig. 3A shows the experimental Tetralin yields on the y-axis
and the predicted Tetralin yields on the x-axis. If the model
would predict perfectly performed experiments, all data points
would lie on the 45 degree line. The 95% probability con-
dence interval is plotted in red dotted lines The RSME (Root
Mean Square Error) describes the average deviation of the
experimental data from the model with 9.4, which is within
limits. Fig. 3B–D show the inuence of the factor variables on
the yield of Tetralin. The impact is increasing in the following
order: temperature < pressure < catalyst amount, and can be
read from the slope of the solid red line. The x-axis is not
scaled like the observed regressors (temperature, pressure,
catalyst load) because the model doesn't consist of a simple
regression of one regressor but a multiple regression. There-
fore the values of the x-axis are no longer corresponding to
actual data values. Points on the x-axis farther out (+1.5 and
ꢁ1.5) pull on the line of t with greater leverage than the
points near the middle (0.0). The inuential points in all
leverage plots are the ones far out on the x-axis. This makes it
possible to judge if the line of t (red line) on the effect's
leverage plot carries the points signicantly better than the
horizontal line (blue line) does.
where r is for the reaction rate, subscript N is for Naphthalene, T
for Tetralin and D for Decalin (cis- and trans-), k is for the rate-
constants, c is for the concentration and superscript n is for the
order of reaction.
Analytics
The liquid product from ltration was analysed with a GC-FID.
A MXT 2887, 10 m column from Restek and a gas chromato-
graph, type Agilent 7890A, was used. All substances are detec-
ꢀ
ꢀ
tible between 80 C and 150 C. Therefor the heating-rate was
set to 10 ꢀC per minute in this section and to 20 ꢀC per minute in
all other sections. The injection method was “cool on-column”.
ꢀ
The nal temperature was 360 C.
3. Results and discussion
Design of experiments
Detailed experimental data of the conducted experiments are
illustrated in Table 1. The highest yields of Tetralin (96 mol%
and 95 mol%) were observed aer 5 minutes of reaction for
200 ꢀC, 50 bar, 10 wt% catalyst. Decalin production was
observed at the upper temperature level. Experiments marked
grey in Table 1 were not considered in the statistics. These are
The estimated regression parameters and the mean yields
lead to the empirical model equation for the hydrogenation of
Naphthalene to Tetralin (eqn (1)) calculated with JMP.
Empirical model equation.
Table 1 Design of experiments data after 5 minutes of reaction
ꢀ
ꢁ
Temperature ꢁ 175
Yield ¼ 64:74 þ 8:62
25
ꢁ
ꢀ
ꢀ
ꢁ
Pressure ꢁ 35
Catalyst ꢁ 7:5
2:5
þ 11:54
þ 15:04
(1)
15
The graphical display of the model equation is the prediction
variance surface plot, shown in Fig. 4. By setting one factor
constant (temperature, pressure or catalyst load) a surface is
formed in the three-dimensional space. The Tetralin yield is
displayed on the z-axis.
ꢀ
Design of experiment shows that a combination of 200 C,
50 bar and 10 wt% catalyst lead to the best result for the
hydrogenation of Naphthalene to Tetralin. Fig. 4A and B both
show a signicant effect of the pressure on the prediction
variance surface. Fig. 4C and D (catalyst) both show the most
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Fig. 3 Influence of factor variables on the response variable.
ꢀ
signicant shi of the prediction variance surface to higher kinetic experiment at 150 C, 20 bar and 5% catalyst the time
values of Tetralin yield and Fig. 4E and F (temperature) the tN is 9.76 min for the experiment at 200 ꢀC, 20 bar and 5%
least.
catalyst the time tN is 7.92 min. As proposed through design of
experiments the higher reaction temperature increases the
reaction rate from Naphthalene to Tetralin. Kirumakki18
investigated continuous hydrogenation of Naphthalene and
found 200 ꢀC to be the temperature for highest conversions to
Tetralin while further increase of temperature above 200 ꢀC
lead to decrease in conversion because of the exothermic
nature of the reaction.
Investigation of reaction kinetics
Results of the kinetic experiments at 150 ꢀC, 20 bar and 5%
catalyst are shown in Fig. 5 and at 200 ꢀC, 20 bar and 5%
catalyst in Fig. 6. Both gures indicate that the reaction of
Naphthalene (N) to Tetralin (T) to Decalin (D) is not classically
consecutive but rather occurs stepwise. In the rst step
Naphthalene reacts selectively to Tetralin without the forma-
tion of Decalin until 96% of the Naphthalene is consumed. A
small amount of Naphthalene remains in the reaction
mixture. Aer this rst step the Tetralin is hydrogenated to
Decalin, but an order of magnitude slower than the reaction of
The stepwise reaction leads to the following kinetic
equations:
The calculated rate-constants correspond to:
ꢀ
tN|150 C ¼ 9.76 [min]
ꢀ
Naphthalene to Tetralin. Compared to 150 C the reaction of
Naphthalene to Tetralin is faster at 200 ꢀC, which can be seen
by comparing Fig. 5 and 6 or from the rate-constants. This
agrees to the ndings of the temperature dependency through
design of experiments. The reaction of Tetralin to Decalin is
ꢂ
ꢂ
ꢃ
ꢃ
mmol
k10 j150ꢀC ¼ 5:71 ꢂ 10ꢁ5
g ꢂ gcat ꢂ min
ꢀ
slower at 200 C than at 150 ꢀC which can be seen from the
mmol
k20 j150ꢀC ¼ 4:36 ꢂ 10ꢁ6
rate-constants.
g ꢂ gcat ꢂ min
In Fig. 5 and 6 no concentration dependency can be
observed. The reactions are therefore zero-order reactions. The
start concentration of Naphthalene is 2.6 mmol per gram
reaction mixture. When Naphthalene reacts to Tetralin very
fast, there is a time where only a small amount of Naphthalene
is le in the reaction mixture, which is practically zero. The
time where the Naphthalene concentration is zero is herein-
aer dened as tN. Aer the time tN the reaction of Tetralin to
Decalin starts with the rate-constant k2, see Table 2. For the
ꢀ
tN|200 C ¼ 9.76 [min]
ꢂ
ꢃ
mmol
g ꢂ gcat ꢂ min
k10 j200ꢀC ¼ 7:04 ꢂ 10ꢁ5
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Fig. 4 Prediction variance surface plots.
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Fig. 5 Kinetics at T ¼ 150 ꢀC, 20 bar, 5% catalyst.
Fig. 6 Kinetic at T ¼ 200 ꢀC, 20 bar, 5% catalyst.
ꢂ
ꢃ
mmol
k20 j200ꢀC ¼ 3:93 ꢂ 10ꢁ6
Table 2 Kinetic Equations
g ꢂ gcat ꢂ min
cN;0
Naphthalene
Tetralin
rN ¼ ꢁk1
cN ¼ cN,0 ꢁ k1t
rT ¼ k1
N-present
N-present
N-absent
tN \
tN \
k1
cN;0
Analysis of the samples taken during the kinetic experi-
ments at 150 ꢀC and 200 ꢀC and comparison of the results led
cN;0 to a nding concerning the composition of Decalin. It was
k1
cT ¼ k1t
rT ¼ ꢁk2
tN
.
ꢀ
k1
shown that at 150 C reaction temperature the ratio of trans-
to cis-Decalin stays constant with reaction time while the
ratio at 200 ꢀC reaction temperature rose with time. Fig. 7
cT ¼ cT(t¼t ) ꢁ k2(t ꢁ tN)
N
cT ¼ k1tN ꢁ k2(t ꢁ tN)
rD ¼ 0
cN;0
Decalin
N-present
N-absent
tN \
k1
cN;0
cD ¼ 0
shows the ratio of trans-Decalin to cis-Decalin formed with
rD ¼ k2
tN
.
respect to time. The reason for the increase of the ratio is
that at 200 ꢀC the formation of trans-Decalin is thermody-
namically favored but can be inuenced by the used catalyst
k1
cD ¼ k2(t ꢁ tN)
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Fig. 7 Ratio of trans-Decalin to cis-Decalin (20 bar, 5% catalyst).
materials.19,20 While isomerization reaction of cis-Decalin to
trans-Decalin were ruled out during reaction by18 other
groups found isomerization from cis- to trans-Decalin in the
magnitude of trans-Decalin formation in the reaction stage at
Acknowledgements
This work was funded by the FFG - Austrian Research Promo-
tion Agency, A3Plus task [Nr.: 835804], project bioBOOST.
200 C.20
ꢀ
4. Conclusion
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