Leader Project Final
Transcript of Leader Project Final
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Kinetics of Gas/Solid Catalytic Reactions: Hydrogenation
of Chloronitrobenzene as a Case Study
2010-2011
Bielser Jean-Marc &
Beswick Oliver
Assistants :Hunt Phil Morten
Fernando Crdenas-Lizana
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0 Table of contents
0 Table of contents ______________________________________________________________ 2
1 The Goals of the Project _______________________________________________________ 3
2 Introduction _________________________________________________________________ 5
3 Experimental _________________________________________________________________ 7
3.1 Characterization ________________________________________________________________ 7
3.1.1 Atomic Absorption Spectrometry (AAS) _____________________________________ 7
3.1.2 Temperature controlled reduction (TCR) _____________________________________ 7
3.1.3 X-ray diffraction (XRD) _________________________________________________ 7
3.1.4 Transmission electron microscopy (TEM) ____________________________________ 7
3.2 Manipulations __________________________________________________________________ 73.3 Blank tests and reproducibility ____________________________________________________ 11
3.4 Turnover Frequency Calculation ___________________________________________________ 11
3.5 Madon-Boudart correlation _______________________________________________________ 13
3.6 Activation energy ______________________________________________________________ 13
3.7 Contact time calculation _________________________________________________________ 14
3.8 Effect of partial pressure (H2 andp-CNB) ___________________________________________ 14
4 Results and Discussion ________________________________________________________ 16
4.1 Characterization _______________________________________________________________ 16
4.1.1 Atomic Absorption Spectrometry (AAS) ____________________________________ 16
4.1.2 Temperature controlled reduction (TCR) ____________________________________ 16
4.1.3 X-ray diffraction (XRD) ________________________________________________ 16
4.1.4 Transmission electron microscopy (TEM) ___________________________________ 17
4.2 Calibration ____________________________________________________________________ 18
4.3 Blank tests ____________________________________________________________________ 20
4.4 Reproducibility ________________________________________________________________ 20
4.5 Kinetic Regime ________________________________________________________________ 21
4.5.1 Madon-Boudart correlation ______________________________________________ 21
4.5.2 Apparent activation energy ______________________________________________ 22
4.6 Time-on -stream effect __________________________________________________________ 24
4.7 Deactivation __________________________________________________________________ 25
4.8 Contact time effect _____________________________________________________________ 27
4.9 Effect of partial pressure _________________________________________________________ 28
5 Conclusion __________________________________________________________________ 30
6 List of symbols ______________________________________________________________ 31
7 References __________________________________________________________________ 33
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1 The Goals of the Project
The aim of this laboratory project was to study in details the kinetics of a gas phase reaction over
(powder) supported catalyst in a fixed-bed (tubular) reactor. The model reaction selected,
hydrogenation ofpara-chloronitrobenzene (p-CNB) catalyzed by a gold-based catalyst, is induced to
producepara-chloroaniline (p-CAN). As the reactant used ispara-chloronitrobenzene, the cleavage
of the carbon-halogen bond is the unwanted step (see Figure 1). The desired product, para-
chloroaniline is a high valuated industrial intermediate product.
Figure 1: Main reaction pathways associated with the hydrogenation ofp-CNB. The targeted route top-CAN is
represented by the solid arrow. [3]
By examining in details different effects on the product distribution like change in contact time,
temperature and partial pressure of the reactants, this project could serve as a basis for a study which
would focus on the feasibility of the selective nitro group reduction as a viable way to produce aniline
compounds.
The first step was to demonstrate that the reaction is exclusively catalytic. Then, the reproducibility of
the system was analyzed. It was also essential to study the optimum reaction at which the intrinsic
kinetics can be approached in order to optimize the reaction rate. Obviously it was necessary to prove
that intrinsic kinetics controlled the reaction. Thus we wanted to study and understand the influence of
those parameters on the reaction kinetics.
Thep-CNB conversion will be calculated as well as the selectivity to p-CAN as the target product in
order to realize this present study.
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The catalyst activity was expressed in terms of turnover frequency (TOF) which is defined as being
the number of produced molecules per active site per time.
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2 Introduction
Functionalized anilines are important intermediates in chemical industry processes. Often used in themanufacture of agrochemicals (herbicides), pharmaceuticals, dyes and also pigments, they are
obtained via hydrogenation of the correspondent functionalized nitrobenzenes. Nevertheless, the
hydrogenation must be highly selective to avoid competitive hydrogenation of a double bond, triple
bond or the cleavage of a carbon-halogen bond [2][3].
Amines in industry are still produced by the so-called Bechamps reaction and some other non-
catalytic reactions [1]. The process based on Bechamp synthesis route consists in using iron filings to
reduce aromatic nitro compounds into their corresponding aniline in an acidic medium. Because of
lower selectivities than catalytic reactions those processes produce large amounts of waste which
sometimes may be toxic or dangerous as iron sludge. The use of catalysts offers a cleaner alternative.
A good catalyst must be highly selective, active and stable. Indeed catalysts undergo deactivation and
this phenomenon should not be limiting for industrial uses. For industries using catalytic reactions the
catalyst regeneration must be easy to realize. Its price should not be limiting for process purposes. A
commercially available catalyst should be used since the preparation of a heterogeneous catalyst is
often tedious [2][3].
Many research groups, at industrial and academic level, have studied the hydrogenation of
halonitrobenzenes. The key challenge in this group of reactions is the exclusive nitro-group reduction,
i.e. high selectivity. In order to achieve this goal a number of parameters have been modified such as
the addition of organic or inorganic modifiers, tailoring the particle size of the metal or the support. Pt
catalysts modified with Pb or H3PO2 (promoted with FeCl2 and V complexes respectively) are
examples of modified catalysts [2].
Pt, Pd and Ni based catalysts have already shown good results in terms of selectivity, conversion and
thus yield in the liquid phase hydrogenation process of chloronitrobenzenes. For example selectivity of
98-99.9% and conversion of 100% were obtained with a bimetallic Pt-Fe/TiO2. Good selectivity has
been obtained even in presence of carbon-carbon double or triple bonds [2]. Nevertheless no catalyst
previously mentioned gave a 100% selectivity response.
In recent literature 100% selectivity was obtained over supported silver and/or gold catalysts on
different oxide supports, e.g. TiO2 and Al2O3 [2][3]. This is a result that confirms the potential of this
metal in terms of selectivity.
Despite on the number of publications in the subject, the selective hydrogenation of halonitrobenzenes
continues gaining attention, the main parameters that have been demonstrated to impact on catalytic
response can be summarized as follow [13]:
Metal catalyst (Pt/Pd/Ru): Noble metals have an excellent activity for the hydrogenation ofhalonitrocompounds. The activity or the hydrodehalogenation are factors that change
depending on the metal used [13].
Metal support (Al2O3/TiO2/Fe2O3): Many studies made with different metal particles.Differences in both activity and selectivity were observed by changing a support. For example,
A tenfold increase in TOF over Pt/TiO2compared to Pt/-Al2O3 was observed [13].
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Metal particle size: Generally the selectivity increases when the particle size is reduced. Onthe other hand the highest selectivity was reached with a large Pt particle (12.3nm). The
dechlorination was suppressed to some extent because p-CAN easily desorbs from large
particle surfaces. It was also shown that the specific activity could increase (15 times withRu/-Al2O3 catalyst with a dispersion decrease from 1 to 2.510
-4[H/Ru]) [13].
Reaction parameters (reaction temperature, hydrogen pressure, concentration of substrate,reaction medium etc.): Interesting studies have been made, for example it has been shown that
the reduction rate of nitro group was higher in the protic solvents than in the non-protic
solvents, following the order: ethanol > methanol > cyclohexane > methylacetate [13].
Regardless on the number of studies on hydrogenation ofp-chloronitrobenzene, no study was found
about the kinetics of this reaction in gas phase over gold supported catalyst. This is the reason why this
particular reaction was studied on gold based catalyst. In gas phase the reaction is run at atmospheric
pressure and often at lower temperatures than in liquid phase. This implies energy savings which arenot negligible for large industrial processes. Furthermore the gas phase allows to synthesis more
complex aromatic compounds than liquid phase. Therefore this study could serve as a basis for other
catalytic synthesis.
Some characterization of the used catalyst will be done for this project. Certain information about the
catalysts size for example must be measured for the calculations.
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3 Experimental
3.1 Characterization
The focus of this project is the kinetics of the studied reaction. Nevertheless some characterization
experiments were made to define clearly the catalyst used. Some parameters must be known in order
to perform the calculation made in this project. Therefore the techniques used will be briefly
explained.
3.1.1 Atomic Absorption Spectrometry (AAS)
The percentage of gold in the catalyst is measured using AAS. The concentration of gold is given in
ppm. The light absorbed by the detector measures the atomic density in the solution. It is then possible
to convert the result in weight percent.
3.1.2 Temperature controlled reduction (TCR)
A known amount of catalyst is contacted to a hydrogen flow. The experiment starts at room
temperature and is heated linearly. A detector will measure the amount of hydrogen and detect either
hydrogen consumption (negative peak) or hydrogen release (positive peak). These peaks indicate for
example the temperature at which the catalyst precursor reduces to metallic gold.
3.1.3 X-ray diffraction (XRD)
X-ray diffraction is used to characterize the crystalline structure of the solid compound of interest or to
determine the chemical composition of the crystalline structures. Libraries of profile are available in
order to relate the results to different compounds. This technique will not be discussed in more details
as it is beyond the scope of this project but it will nevertheless be interesting to comment the profile
obtained.
3.1.4 Transmission electron microscopy (TEM)
An electron beam is passed through a thin layer of catalyst. A special camera will take the picture of
the interactions between the electrons and the catalyst. High resolution pictures could be obtained.
This technique was used to determine the particle size of the catalyst before and after the reaction.
3.2 Manipulations
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The reactions were conducted at atmospheric pressure in gas phase over Au/Al 2O3 in a fixed-bed
(tubular) reactor over the following temperature range: 120C T 170C. This is a vertical tubular
glass reactor containing the fixed catalytic bed. An oven is used to heat up the column. The reaction is
conducted under isothermal conditions, thus the temperature in the catalyst bed is continuouslymonitored using a thermocouple inserted in a thermowell. A co-current flow of hydrogen and organic
compound passes through it. The organic compound is dropping at a constant rate (using an infusion
pump) and the drops will evaporate in the heated column. Different layers are disposed on top of a
fixed bed (see Figure 2). From top to bottom these are:
- Glass beads: They ensure the mixing of the hydrogen with the vaporized organic reactant.- Glass wool: As the glass beads they ensure a good mixing between the reactants.
Moreover it provides a physical separation between the catalyst and the glass beads to
ensure that the catalyst bed remains homogeneously distributed forming a thin layer.
- Catalyst mixed with ground glass (75m): Ground glass is used to maintain the volume ofthe bed constant. To control the activity of the catalyst, the amount of catalyst will be
modified. Therefore, by adjusting the volume of ground glass, and if the volumetric flow
of gases is constant too, the contact time between the reactant and the catalyst is
maintained constant. This combination also ensures the isothermal (1K) condition.
- Glass wool: Physical separation between the catalytic bed and the porous glass frit.
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Figure 2: Schematic diagram of the continuous gas phase reactor [3]
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The effluent is condensed by using a cold trap (liquid nitrogen), collected every 30 minutes and taken
for GC analysis. The temperature is monitored during the whole reaction to ensure isothermal
conditions. After each sampling the gas flow rate will also be monitored using a continuous digital
flowmeter.
The catalyst precursor used was HAuCl4. The catalyst has been previously activated (to formed
metallic gold, active form for the catalytic reaction) and passivated, before each run the passivating
oxygen layer on the catalyst will be removed by leaving it for 30 min in the reactor with 60 cm3min
-1
H2 at the reaction temperature since we have demonstrated elsewhere the removal of this passivation
layer at T > 120C.
The conversion and the selectivity were then calculated using the gas chromatography (GC) results.
As it will be shown in the calibration part, the area is proportional to the amount of moles in the
analyzed sample. Therefore the conversion (Equation 1) and the selectivity (Equation 2) were
calculated using the following equations:
(1)100
%Area100
CNB-p
CNBpCNBpX
CNBoutp
in
outin
CNB-p
(2)%Area100
%Area
CNBpCNBp
CANpS
CNBoutp
CANoutp
outin
out
CAN-p
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How the calculations have been realized is shown in details hereinafter.
In order to calculate total number of gold atoms at the surface NS, Au, the specific surface area of gold
SSAAu (Equation 3) and the dispersion D need to be determined:
(3))g(md
6SSA 12
AuAu
Au AuAu
Where Au is the gold density and dAu is the diameter of the gold particles. The diameter of the
particles is found using transmission electron microscopy (TEM). The dispersion is the ratio of
number of atoms of metal at the surface over the total number of metal atoms (Equation 4).
(4)(-)NA
MSSAD
AAu
AuAu
Where MAu is the atomic weight of gold, AAu is the surface of one atom of gold and NA is the
Avogadro number.
(5)surface)at theatomsgoldof(numberDM
NloadingmN
Au
Acat
surfaceAu,
Where mcat is the total mass of catalyst (including the support) and the loading is the ratio of the mass
of metal in the catalyst over the total mass of catalyst.
The next step is to calculate the number of molecules converted per time which requires the
conversion (Equation 6).
(6)time)ofunitperconvertedCNBof(moleculesNM
QXN
ACNB-p
CNB-pCNB-pCNB
Where Qp-CNB, p-CNB, and Mp-CNB are the volumetric flow rate, the density and the atomic weight ofp-
CNB.
Finally the TOF (Equation 7) is obtained by dividing the reactant converted by the metal atoms on the
surface:
(7)surface)at theatomsAuofnumberperandtimeofunitperconvertedCNB-pof(moleculesN
NTOF
surfaceAu,
CNB
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3.5 Madon-Boudart correlation
To ensure that neither external nor internal mass transfer occurs, a Madon-Boudart correlation was
applied [7]. The measurements were made at two different temperatures changing the gold loading but
keeping the same dispersion. The ln(rate) vs ln(concentration of active sites) was plotted. If a slope of
one is observed it means the reaction is not limited by mass transfer.
The number of gold particles at the surface per gram of catalyst was calculated using Equation 8.
(8)DM
1
100
weight%m(mol)n
Au
AusurfaceAu,
This value was then divided by the total mass of catalyst (0.5g).
The reaction rate was calculated in terms of moles converted per second per gram of catalyst using
Equation 9.
(9)m
1X
3600
Frate
catalyst
CNBp
in
3.6 Activation energy
Another way to know if there is mass transfer limitation is to calculate the apparent activation energy.
If this last is greater than 15 kJ/mol the reaction is not limited by mass transfer limitations. Theapparent activation energy will be calculated using an Arrhenius plot. Therefore it is necessary to
calculate the kinetic rate constant of the reaction (at different temperatures). At this point it is only
possible if one of the reactants (hydrogen) is set in excess (the concentration is assumed to be
constant) and a first order is assumed. From the balance of a plug flow reactor Equation 10 is
obtained:
(10)dwr)()dF(FF AAAA
AAAAA Ck'r:sassumptionthefromanddCQdFandCQF
(11)Ck'dw
dCQ A
A
A
(12)dXCCandX)(1CCC
CCX AAA0A
A0
AA0
(13)dwQ
k'
X)(1
dXA
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(14)Q
wk')
X1
1ln(X)ln(1 A
k is then expressed in moles of reactant per unit of time per moles of gold. Different values of k willbe calculated by changing the amount of catalyst. Equation 14 was expressed with different units
(refer to the list symbols). The slope of the plot of Equation 15 is k.
(15)F
nk')
X1
1ln(
CNB-p
The activation energy was then calculated using a plot of the Arrhenius equation (Equation 16).
(16)T
1
R
Eln(A))ln(k' A
3.7 Contact time calculation
The contact has a similar definition than the residence time in the sense that a volume has to be
divided by a volumetric flow rate to obtain it. In this case the volume of the catalytic bed and the
volumetric flow rate of hydrogen are considered.
The volume of the catalytic bed has been calculated by assuming its height being constant. Therefore
the same value has always been used.
(17)4
hdV reactor
2
reactor
bedcat.
Where dreactor and hreactor are the diameter and the height respectively of the reactor.
(18)Q
VmeContact t i
2H
bedcat.
3.8 Effect of partial pressure (H2 andp-CNB)
The effect of partial pressure was measured to determine the partial reaction orders for both hydrogen
and p-CNB.
To measure the effect of the H2 partial pressure, the reactant was diluted and the concentration fixed.
The hydrogen flowrate was then modified from 1 to 1 stochiometry to far excess of H 2 (up to 129
times more hydrogen than necessary).
Same experiments were made to measure the effect of the partial pressure ofp-CNBusing a constant
hydrogen flow and varying thep-CNB concentration.
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The power rate law relates the reaction rate to the partial pressures of the reactant and can be written:
(19)PPkr m CNBpn
H2
Where n and m are the partial reaction orders of, respectively, H2 andp-CNB.
Nevertheless if one parameter is fixed then the power rate law is dependent on only one parameter
(e.g. if the partial pressure of hydrogen is fixed it will only depend on the partial pressure ofp-CNB).
As the studied reaction is catalytic the reaction rate was always expressed in terms of turnover
frequency. The turnover frequency is proportional to the reaction rate.
(20)PPkTOF m CNBpn
H2
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4 Results and Discussion
4.1 Characterization
4.1.1 Atomic Absorption Spectrometry (AAS)
AAS has confirmed 1.05% wt. Au content in the Au/Al2O3 catalyst.
4.1.2 Temperature controlled reduction (TCR)
The precursor (HAuCl4) put on the alumina support was used for this analysis. Hydrogen consumption
is observed at around 175C where Au3+
is reduced to Au0 (Figure 3). This is the reason why the
catalyst was activated to 333C because the metallic gold is the active form of the catalyst.
Figure 3: Temperature controlled reduction
4.1.3 X-ray diffraction (XRD)
Figure 4 is the profile obtained by x-ray diffraction over the catalyst used in this project. The profileobserved is characteristic of delta Al2O3 (i.e. main peak at 67o, a secondary reflexion at 46o and a
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broad peak over the range 33-41o). No characteristic peak for gold was observed for two possible
reasons. Either not enough gold is present in the sample or the gold particle size is smaller than the
detection limit (~
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Figure 5: Transmission electron microscopy (TEM) was used to take this picture. The gold particle (dark spherical
form) has a diameter of about 7nm
4.2 Calibration
The samples are analyzed by GC (gas chromatography), therefore the first step is to identify the
correlation factor between the number of moles and the area percentage. A calibration curve is
therefore calculated experimentally.
Figure 6 shows the correlation between the mole fraction and the area fraction of p-CAN in a solution
containing both p-CNB and p-CAN. The solvent of this solution is ethanol. Each point corresponds to
a different solution with known concentrations of both compounds.
Figure 6: Calibration curve ofp-CAN.
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The point (0; 0) is obvious and the line was therefore forced through this point. The linear correlation
between the number of moles and the area is hereby proven to be one.
The same method is applied for all the possible product distribution combinations and the two
following figures (Figure 7 and Figure 8) show the results obtained.
Figure 7: Calibration curve of nitrobenzene (NB).
Figure 8: Calibration curve of aniline (AN).
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Graphically the similarity of both results seems obvious. The conversion decreases in the same way
with time and the selectivity is found to be 100% any time of the experiment.
The selectivity top-CAN was observed to be 100% in each reaction made. This property was already
discovered in other studies in liquid phase [2].
4.5 Kinetic Regime
4.5.1 Madon-Boudart correlation
To ensure that no mass transfer limitation occurs and thus that the reaction is under kinetic control, a
Madon-Boudart diagram was used (see Figure 10). The plot is ln(rate) vs. ln(concentration of activesites) and if no mass limitation occurs (nor external nor internal), a slope of one is obtained for two
different temperatures [7]. Figure 10 confirms that the reaction is under kinetic control.
Figure 10: Madon-Boudart diagram
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The reaction is the slowest step of the overall reaction if no mass transport occurs. It means that the
reaction kinetics can be studied in more details to characterize the intrinsic kinetics of this selective
hydrogenation reaction.
4.5.2 Apparent activation energy
As an example the calculation ofk (as explained in 3.6 Activation energy) is shown for a reaction
temperature of 120C. The range of catalyst used is 0.0200.045g.
Figure 11: Treaction=120C and catalyst amount range = 0.020 - 0.045g
The assumption that the reaction follows a first order kinetic is verified in Figure 11 as the
experimental data fits the model derived in the experimental part. Therefore from Figure 11 the rate
constant could be extracted using the slope. The relation used was derived in the experimental part and
the equation obtained was:
(15)F
nk')
X1
1ln(
CNB-p
The Arrhenius equation was then used to plot the three k obtained at different temperatures (range:
120 170C). The activation energy was then calculated using the slop of the line obtained (Figure
12).
CNBpCNBp
Amol
kJ85mol
J84561.710171)(8.314slopeRE
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Figure 12: ln(k') versus 1/T, the activation energy was extracted from the slope
Again the result proves that over the temperature range tested the reaction is under kinetic control as
the apparent activation energy is greater than 15kJ/mol.
Similar values for selective hydrogenation in liquid phase were already obtained in different studies.
One study investigated the hydrogenation of para-nitrophenol to para-aminophenol in a laboratory
scale batch slurry reactor. The apparent activation energy obtained was 61kJ/mol [11]. A second study
investigated the selective hydrogenation of ortho-chloronitrobenzene over Pd/Al2O3 in a downflow
microreactor (liquid phase) at atmospheric pressure and calculated an activation energy of 41kJ/mol
[12].
The values found in this project and in different studies were in the same order of magnitude.
Obviously these values were always found with a kinetic controlled reaction. The fact that these valuesare comparable independently of the phase shows that the interaction between the catalyst and the
molecules is a dominant fact in the kinetics of a reaction. This is also the reason why apparent
activation energies were used. The real activation energy is part of it but there is also the heat of
adsorption that must be considered.
adsorptionrealA,app HEE
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4.6 Time-on -stream effect
The time-on-stream theory has been developed in the purpose of understanding the ageing of catalysts
by relating their activity to the length of time they are used [8]. In the following Figure 13, the
conversion which is related to the activity of the catalyst has been plotted versus the time. Three
different experiments have been realized at same conditions varying the temperature. Indeed three
temperatures have been investigated in order to see its influence on the time-on-stream effect:
Figure 13: Conversion versus time, observation of the effect of time-on-stream for three temperatures
For all three temperatures deactivation is observed and stabilization occurs after around 2 hours. This
confirms what has already been said during the deactivation section. The deactivation rate is more
important for higher temperatures as the conversion at which it stabilizes. As mentioned the
deactivation is linked to the initial conversion, but in order to compare the deactivation between the
different experiments the percentage of deactivation were calculated (Equation 21)
(21)X
XXonDeactivati%
0
1500
Where X150 is the conversion at 150 min (stabilization has occurred) and X0 is the initial conversion
which has been calculated with a polynomial function.
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Table 2: Percentage of deactivation measured for T=120C, 150C, 170C
Temperature X0 X150 % Deactivation
170C 0.183 0.0496 0.729
150C 0.111 0.0295 0.733
120C 0.041 0.0092 0.776
No tendency can be emphasized for the % of deactivation with respect to temperature. The mean value
for the deactivation is 0.746 0.03 (Table 2). The causes of deactivation are discussed in the following
section (4.7).
4.7 Deactivation
Deactivation is an inevitable phenomenon for all catalysts. It seems obvious that catalysts with such
compositions i.e. supported small metal particles undergo thermal, chemical, or mechanical effects [6].
One quality of a catalyst that industries are looking for is stability. Indeed the catalyst should stabilize
after a certain time. This has been observed for the catalyst of interest. Also it could be interesting to
determine the causes of deactivation in order to partly avoid the effects of a part of them. The often
observed reasons of deactivations are thermal sintering (ageing), poisoning, loss of catalyst to gas
phase and deposition.
Some effects are easier to investigate than others. Deactivation due to temperature has already been
reported during hydrogenation ofmeta-chloronitrobenzene into meta-chloroanaline [9]. A manner to
notice thermal sintering is to use transmission electronic microscope (TEM) in order to see if the
particles became bigger. The two following pictures show the catalyst before a reaction and after a
mimicked reaction:
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Figure 14: TEM pictures of the catalyst before and after reaction
The two pictures (Figure 14) show that before the reaction the particles have a size of 7nm but that
after a simulated reaction the particles have increased to 10-50nm particles. Therefore the surface-to-
volume ratio is decreased causing deactivation.
It has been reported that Al2O3-based catalyst, which was the case for this project, are prone to
deactivation [12]. This might be one of the causes of decrease in conversion during the reactions. The
difficulty into proving the deactivation effect of alumina support is that it has a positive effect on the
activity of the catalyst. Therefore substituting it by another support would give completely different
results in terms of conversion versus time. On the other hand adding more alumina instead of a part of
the glass power in the catalyst bed could be a way to observe the effect of this compound on the
catalyst activity.
Poisoning is another source of loss of activity. It occurs when impurities chemisorbs on the active site.
This is an irreversible phenomenon. Inhibition is different because due to physisorption and therefore
is a reversible phenomenon which allows in some cases the regeneration of the catalyst [6]. In the
particular reaction of hydrogenation ofp-CNB chloride is present in the molecule and could be apoisoning compound. Based on Figure 13 where deactivation occurs during about 2 hours it is more
likely that physisorption occurs. If the phenomenon of inhibition truly happens the physisorbed
molecules are in equilibrium at the active sites when stabilization is reached. Nevertheless
chemisorption cannot be excluded as its effects could be slow on the catalyst activity.
It has been proved that presence of chloride increases severely the sintering for Cu-based synthesis [4].
As gold belongs to the same elements group (I B) an assumption could be made that the catalyst used
during the project was partly prone to this type of deactivation. To investigate this negative effect on
the catalyst activity experiments should be made where additional chloride is put in contact to the
catalytic bed. If no further deactivation is observed this would mean that chloride has no inhibition
effect.
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4.8 Contact time effect
The TOF was calculated for several reactions during which two parameters were varied. First the
volumetric flow rate of hydrogen was varied at a fixed temperature in the range of 1.2 l/h to 5.4 l/h.
The temperature range was 120 to 170C. The results of TOF versus the contact time were plotted in
Figure 15.
Figure 15: Turnover frequency (TOF) versus contact time at T=120C, 150C, 170C
The TOF is increasing with the contact time independently of the temperature (see Figure 15). Indeed
bigger is the contact time bigger is the chance that a higher number of molecules of H2 which come in
contact with active sites of the catalyst bed will react with the organic reagent. It can be noticed that
the decrease of TOF tends to diminish when contact time is lowered. Indeed even if the contact time is
decreased by increasing the H2 volumetric flow rate it is slightly compensated by the fact that the
number of H2 molecules that pass through the catalyst per unit of time is more important.
Increasing the temperature leads to obtain a better TOF at a fixed contact time meaning that the rate
follows the Arrhenius law.
The tendency mentioned above is more obvious for higher temperatures where the energy of the
system is more important and therefore the chance of reaction between two reagent molecules is
higher.
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4.9 Effect of partial pressure
The partial orders were calculated for the reaction at 150C. Figure 16 and Figure 17 show the effect
on turnover frequency of the changes of partial pressure of respectively the hydrogen and thep-CNB.
The partial order of hydrogen is 0.18 and the partial order for p-CNB is 0.49 at 150C. Small partial
orders result in the fact that small changes of partial pressure of the reactants do not have a high
influence on the reaction rate. It could be advantageous to have this property because then the reaction
design is not too sensitive. Pressure fluctuations in a system easily occur and if the reaction rate is
highly influenced by this last it might be difficult to design a reproducible and reliable process.
Figure 16: Partial pressure of hydrogen modified with a fixed partial pressure forp-CNB
Figure 17: Partial pressure ofp-CNBmodified with a fixed partial pressure of hydrogen
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Further experiments at different temperature would allow calculating more accurate rate constants
with the real partial orders. The apparent activation energy could then also be calculated more
accurately. This was not done for this project because of lack of time.
A Langmuir-Hinshelwood model could be tested with the different kinetic parameters calculated. The
rate limiting step (either adsorption, reaction or desorption) could be identified.
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5 Conclusion
100% selectivity is one of the major points of the investigated catalytic reaction. This advantage over
the mentioned studies is crucial for further process development. It avoids the formation of unwanted
by-products which could involve more expensive separation steps in the process.
The gold metal catalyst used during the experiment was characterized as being a spherical (diameter =
7nm) particles using transmission electronic microscope (TEM) and X-ray diffraction (XRD). The
active form was determined to be Au0
using temperature controlled reduction (TCR).
In the range of pressure and temperature used for the experiments the reaction was shown to be under
kinetic control by the Madon-Boudart correlation and the calculated apparent activation energy. This
requirement is essential for a further optimization of the Au/Al2O3 catalyst.
Deactivation is observed until stabilization which occurs after around 2 hours of reaction. It was
demonstrated by TEM pictures that the decrease in activity of the catalyst was due to thermal
sintering. Also it is likely that chloride was responsible of a part of the deactivation. The stabilization
of the catalyst activity is really encouraging for further studies which could lead to the process
development based on this particular reaction.
The turnover frequency can be varied by adjusting the contact-time and temperature. Indeed the TOF
increased with higher contact-time and temperature (Arrhenius control).
At the tested temperature of 150C the TOF slightly increased with higher partial pressure of both
reagents (H2,p-CNB).
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6 List of symbols
A0 pre-exponential factor [mol1-n
Ln-1
s-1
] for n order
Ai area taken at the surface by one atom i [m2]
Ci concentration of compound i [molm-3
]
di diameter [m]
D dispersion [-]
Ea activation energy [kJmol-1
]
F molar flow rate [molh-1
]
k rate constant [cm3g
-1cats
-1]
k rate constant [molp-CNBmolAu-1
h-1
]
m mass [g]
M atomic weight [gmol-1
]
Ns number of sites [-]
Ni, Surface number of atoms i at the surface [-]
i molecules of reagent converted per time [-]
Q volumetric flowrate [cm3
h-1
]
density [kgm-3
]
r rate of reaction [molg-1
cats-1
]
Si selectivity with respect to compound i [-]
SSA specific surface area [m2gcat.]
T temperature [K]
TOF turnover frequency [h-1
or s-1
]
xi molar fraction [-]
X conversion [-]
i stoichiometric factor of compound i [-]
Abbreviations
Au gold
cat. catalyst
p-CNB para-chloronitrobenzene
p-CAN para-chloroaniline
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Constants
Gas constant R=8.314 [Jmol-1
K-1
]
Avogadro number NA=61023
[atoms or moleculesmol-1
]
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7 References
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