Anton De Vylder modeling chemical building blocks via experiments and kinetic Unraveling the ...

160 Pages·2015·7.76 MB·English

Unraveling the mechanism of bioethanol conversion to
chemical building blocks via experiments and kinetic
modeling
Anton De Vylder
Supervisors: Dr. Vladimir Galvita, Prof. dr. ir. Joris Thybaut
Counsellor: Kristof Van der Borght
Master's dissertation submitted in order to obtain the academic degree of
Master of Science in Chemical Engineering
Department of Chemical Engineering and Technical Chemistry
Chairman: Prof. dr. ir. Guy Marin
Faculty of Engineering and Architecture
Academic year 2014-2015

Unraveling the mechanism of bioethanol conversion to
chemical building blocks via experiments and kinetic
modeling
Anton De Vylder
Supervisors: Dr. Vladimir Galvita, Prof. dr. ir. Joris Thybaut
Counsellor: Kristof Van der Borght
Master's dissertation submitted in order to obtain the academic degree of
Master of Science in Chemical Engineering
Department of Chemical Engineering and Technical Chemistry
Chairman: Prof. dr. ir. Guy Marin
Faculty of Engineering and Architecture
Academic year 2014-2015
FACULTY OF ENGINEERING AND ARCHITECTURE

Department of Chemical Engineering and Technical Chemistry
Laboratory for Chemical Technology
Director: Prof. Dr. Ir. Guy B. Marin

Laboratory for Chemical Technology

Declaration concerning the accessibility of the master thesis

Undersigned,

Anton De Vylder

Graduated from Ghent University, academic year 2014-2015 and is author of the
master thesis with title:

Unraveling the mechanism of bioethanol conversion to chemical building blocks via
experiments and kinetic modeling

The author gives permission to make this master dissertation available for consultation
and to copy parts of this master dissertation for personal use.
In the case of any other use, the copyright terms have to be respected, in particular with
regard to the obligation to state expressly the source when quoting results from this master
dissertation.

22/05/2015

Anton De Vylder

Laboratory for Chemical Technology • Technologiepark 914, B-9052 Gent • www.lct.ugent.be
Secretariat : T [phone] • F [phone] • [email protected]
Acknowledgement
Now that my master thesis year is coming to an end, I would like to express my gratitude to a
number of people. First and foremost, I want to thank prof. Guy B. Marin for providing the
facilities at the Laboratory for Chemical Technology and the opportunity to be at the forefront
of academic research for a year.
I want to thank my supervisors prof. Joris W. Thybaut and dr. Vladimir V. Galvita for the
follow-up of my work and the many interesting discussions we had had regarding it.
A special thanks goes to my supervisor Kristof Van der Borght, whose enthusiasm always
kept the spirits up, even when many challenges were thrown at us! Without your superb
guidance, I would never have been able to do this thesis. Thanks also for the opportunity I got
to join you in an EXAFS campaign to the Synchrotron in Grenoble, it was an experience to
never forget.
Jolien De Waele and Tapas Rajkhowa, I owe you both a sincere thanks for helping me with
the small and big problems that I faced doing experiments on the High-Throughput setup.
Your expert knowledge on the quirky sides of this setup helped me greatly in achieving the
results for this thesis. In this regard, I also want to thank the technical staff for helping me in
the modification of a part of the setup.
A thanks also goes to all the thesis students with whom I undertook this journey. Specifically
I want to thank Lies, Jenoff, Jonathan and Yoshi for the late nights and early mornings fueled
with coffee!
In particular I want to thank Alexandra, Lies, Florence, Timothy and Jasper from the MaChT
team. Together we made it an incredible year to never forget!
Last but not least I want to thank my parents who made it possible for me to do these studies.
Unraveling the mechanism of
bioethanol conversion to chemical
building blocks via experiments
and kinetic modeling

Anton De Vylder

Coach: ir. Kristof Van der Borght
Promotors: prof. dr. ir. Joris W. Thybaut, dr. Vladimir V. Galvita

Master’s dissertation submitted in order to obtain the academic degree of Master
of Science in Chemical Engineering
Department of Chemical Engineering and Technical Chemistry
Chairman: Prof. dr. ir. Guy Marin
Faculty of Engineering and Architecture
Academic year 2014-2015
Abstract: The reaction mechanism governing the conversion of ethanol to hydrocarbons has been
further elucidated using steady-state experiments on HZSM-5 and microkinetic modeling. The
experimental results show that there is no difference in reaction mechanism between ethanol or
ethylene as a reactant. An induction period in the catalyst activity indicates that there is a contribution
of an autocatalytic mechanism, which has been identified to be related to the hydrocarbon pool
mechanism in MTO. A microkinetic model is constructed including both acid catalyzed steps and a
modified version of the hydrocarbon pool mechanism which shows that the importance of the latter is
primarily for the first olefin production.
Keywords: bioethanol, HZSM-5, microkinetic modeling
Unraveling the mechanism of bioethanol conversion
to chemical building blocks via experiments and
kinetic modeling
Anton De Vylder
Coach: ir. Kristof Van der Borght
Promotors: prof. dr. ir. Joris W. Thybaut, dr. Vladimir V. Galvita

 of multiple tubular reactors. Ethanol (Sigma Aldrich, 99%,
Abstract: The reaction mechanism governing the conversion of absolute) was fed through Bronkhorst Liqui-Flow controllers
ethanol to hydrocarbons has been further elucidated using and diluted with nitrogen (Air Liquide, Alphagaz 1) using
steady-state experiments on HZSM-5 and microkinetic modeling. Bronkhorst El-Flow controllers. The catalyst has been tested
The experimental results show that there is no difference in in a temperature range of 573 K to 623 K, an ethanol partial
reaction mechanism between ethanol or ethylene as a reactant. pressure between 20 kPa and 60 kPa, and space times between
An induction period in the catalyst activity indicates that there is 1 kg s mol-1 and 40 kg s mol-1. The absence of internal and
a contribution of an autocatalytic mechanism, which has been
external transport phenomena and temperature gradients has
identified to be related to the hydrocarbon pool mechanism in
been verified for these reaction conditions.
MTO. A microkinetic model is constructed including both acid
catalyzed steps and a modified version of the hydrocarbon pool Product analysis is performed on an online gas
mechanism which shows that the importance of the latter is chromatograph with a PONA column and flame ionizing
primarily for the first olefin production. detector. Methane was used as internal standard to confirm
Keywords: bioethanol, HZSM-5, microkinetic modeling that the mass balance is closed within 5%.
A C conversion is defined to describe the combined
2
conversion of ethanol and ethylene. This is defined as follows:
I. INTRODUCTION X = FE0tOH+FC02H4−(FEtOH+FC2H4) (1)
Ethanol has the potential to replace crude oil in a variety of C2 F0 +F0
EtOH C2H4
applications, ranging from renewable fuel to feedstock for
Correspondingly, an oxygenates conversion is defined to
chemical building blocks [1]. Bioethanol can be produced
describe the combined conversion of methanol and dimethyl
from the fermentation of sugars or lignocellulosic biomass,
ether.
whereby the latter is favored due to its independence from the
food chain. A major drawback for its direct application as a X = FM0eOH−(FMeOH+2FDME) (2)
fuel is the high water content obtained in crude bioethanol. oxy F0
MeOH
Hence the focus shifts to production of hydrocarbons from
For a propylene and ethylene feed, the following definition
bioethanol, using solid acid catalysts [2, 3].
of conversion is used:
Dehydration of ethanol to ethylene on zeolite catalysts has
been the topic of much research already [4], yet the X =Fi0−Fi (3)
conversion to higher hydrocarbons has received less attention. i Fi0
It has been noted that similarities exist between the conversion With F0 the molar inlet flow rate of component i (mol s-1)
i
of methanol to olefins and ethanol to olefins [5], but hitherto and F the molar outlet flow rate of component i (mol s-1).
i
no systematic reaction mechanism elucidation has been
performed yet.
B. Kinetic Modeling
A 1-dimensional ideal plug flow reactor is modeled using
II. PROCEDURES
the mass balance equation for each different component i:
A. Experimental dFi =R (4)
dW i
The zeolite in this work is a commercially available NH -
4
ZSM-5 (Zeolyst, CBV8014) with a Si/Al = 40. The acidic Where W is the catalyst weight, Fi the molar flowrate of
form HZSM-5 is obtained by calcination at 823 K for 4 h with component i and Ri the net production rate of component i.
a ramp of 1 K min-1. The concentration of acid sites is 0.36 An in-house developed Reaction Network Generation
mol kg-1 as determined by NH -TPD. Experiments are Program (ReNGeP) is used to create a reaction network with a
3
performed on the High-Throughput Kinetic setup (HTK-1) at maximum of 10 carbon atoms for alkylation and β-scission
the Laboratory for Chemical Technology. This set-up consists reactions with olefins and carbenium ions. More than 800
reactive species and 1500 reactions are considered in this
network.
E-mail: [email protected]
To reduce the amount of parameters significantly, the product in the conversion of ethanol. The feed is hence
Single-Event MicroKinetic (SEMK) modeling theory compared to a methanol feed, which exhibits an induction
developed within the Laboratory of Chemical Technology is period related to the slow formation of hydrocarbon pool
applied: species that are used to produce lower olefins. These lower
olefins, such as propylene, can then alkylate further on solely
k=kBTn exp(ΔS̃0,‡)exp(−Ea) (5) the acid sites. This is clear from the higher reactivity for a
h e R RT
propylene feed, depicted in Figure 1.
This methodology uses structure independent kinetic
To further investigate the nature of the induction period,
descriptors (Ea) complemented with transition state theory UV-VIS spectroscopy is performed on the catalyst bed. Figure
based entropy (ΔS̃0,‡) and single event numbers (ne), allowing 2 shows a contour plot of the observed UV-VIS intensity as
to reduce the amount of parameters to six: three protonation function of the bed length and excitation wavelength.
enthalpies, for primary, secondary and tertiary carbenium ion
formation, and three activation energies, for the ethylation
reaction to primary, secondary or tertiary carbenium ions.
III. RESULTS AND DISCUSSION
A. Experimental reaction mechanism elucidation
The catalyst is found to be stable in activity as well as
selectivity for up to 12 hours on stream. This has been
observed with both an ethanol feed as well as an ethylene
feed, which proves that the water formed from the
dehydration reaction does not have an influence on the
catalyst stability.
The catalyst activity curve for an ethanol feed at 573K and
30 kPa, depicted in Figure 1, shows an induction period. This
indicates that part of the active sites are not used for the
production of C hydrocarbons.
3+
Figure 2: UV-VIS spectroscopy of the catalyst bed after reaction (T =
573 K; p = 30 kPa; X = 0.4).
EtOH C2
In the top part of the catalyst bed no retained species are
found. This region can hence be related to the part which is
used for the dehydration reaction to ethylene. The second part
of the catalyst bed clearly contains species with an excitation
wavelength between 390 and 420 nm, which are identified as
polysubstituted aromatic cations. In the last part of the catalyst
bed, polyaromatic cations with an excitation wavelength of up
to 600 nm appear. The same species have been observed in
MTO, which could indicate a similar autocatalytic
hydrocarbon pool mechanism [6].
B. Kinetic modeling results

A first model is constructed using solely acid catalyzed
Figure 1: C conversion for an ethanol feed (●), ethylene conversion
2 steps, including the formation of a primary butyl carbenium
for an ethylene feed (○), propylene conversion for a propylene feed
ion from the ethylation reaction of ethylene. The parameter
(■) and oxygenates conversion for a methanol feed (►) as a function
estimates in Table 1 are obtained for this model.
of site time.
Experiments with an ethylene feed, also depicted in Figure Table 1: Estimated parameters for the first model
1, show that part of this induction period can be related to the
Parameter Estimated value
dehydration of ethanol to ethylene. Additionally, the curve for [kJ mol-1]
ethanol and ethylene exhibit the same slope, indicating that
ΔH -68.28 ± 0.18
the reaction rate is the same for the production of C prot,p
3+ ΔH -74.06 ± 1.77
hydrocarbons from both reactants. Hence, ethylene, which is a prot,s
ΔH , -95.63 ± 1.17
product of the ethanol dehydration, can be considered the prott
E 104.02 ± 2.1
actual reactant for the formation of C hydrocarbons. a,(p,p)
3+
E 25.69 ± 0.19
Experiments with additional water in the ethanol feed shows a,(p,s)
E 20.28 ± 0.76
no difference in C conversion or product selectivity, when a,(p,t)
2
comparing at same ethanol partial pressure. This indicates that
water has no influence on the reaction mechanism and proves The activation energy for the ethylation reactions to a
the viability of crude bioethanol. secondary and tertiary carbenium ion are estimated at 25.7 kJ
Interestingly, an induction period is still visible in the mol-1 and 20.3 kJ mol-1 respectively, which are physically
activity curve for the reaction with ethylene. Moreover, it was quite low values. Moreover, the primary character of
found from a delplot analysis that propylene is a primary propylene is not simulated correctly.
A second model is constructed where the ethylation reaction V. FUTURE WORK
to a primary butyl carbenium ion is replaced by a modified
Using the hypothesis above, the induction period should
version of the hydrocarbon pool mechanism. The intrinsic
decrease when a small amount of propylene is added to the
reactivity of this hydrocarbon pool mechanism is fixed using
ethylene feed. A feed with both ethylene and propylene
the kinetic descriptors obtained by Kumar et al. for the
should be used to elucidate the nature of the induction period
reaction of methanol to olefins on HZSM-5 [7].
further.

The simulation with the hydrocarbon pool is performed
whereby the catalyst descriptors are assumed to be equal to
the model without the hydrocarbon pool. To more accurately
estimate parameters, regression should be performed with the
hydrocarbon pool model. Combined regression with the direct
ethylation reaction with ethylene and the hydrocarbon pool
should then be performed in order to assess the relative
importance of both mechanisms.
REFERENCES
[1] J. D. McMillan, "Bioethanol production: status and prospects,"
Renewable energy, vol. 10, pp. 295-302, 1997.
[2] V. Tret’yakov, Y. I. Makarfi, K. Tret’yakov, N. Frantsuzova, and
R. Talyshinskii, "The catalytic conversion of bioethanol to

hydrocarbon fuel: A review and study," Catalysis in Industry, vol.
Figure 3: Logarithm of the production rate of propylene via β- 2, pp. 402-420, 2010.
scission (- - -) and via the hydrocarbon pool mechanism (─) as a [3] A. T. Aguayo, A. G. Gayubo, A. M. Tarrío, A. Atutxa, and J.
Bilbao, "Study of operating variables in the transformation of
function of C conversion.
2 aqueous ethanol into hydrocarbons on an HZSM‐5 zeolite,"
The primary character of propylene can now be adequately Journal of chemical technology and biotechnology, vol. 77, pp.
211-216, 2002.
predicted. Furthermore it is seen in Figure 3 that the activity
[4] M.-F. Reyniers and G. B. Marin, "Experimental and theoretical
of the hydrocarbon pool is the highest at a C conversion level
2 methods in kinetic studies of heterogeneously catalyzed reactions,"
below 0.1. After this, the produced propylene and butylene Annual review of chemical and biomolecular engineering, vol. 5,
undergo direct ethylation reactions and the net production of pp. 563-594, 2014.
[5] R. Johansson, S. L. Hruby, J. Rass-Hansen, and C. H. Christensen,
propylene is mainly from the β-scission reactions of higher
"The hydrocarbon pool in ethanol-to-gasoline over HZSM-5
hydrocarbons.
Catalysts," Catalysis letters, vol. 127, pp. 1-6, 2009.
[6] K. Hemelsoet, Q. Qian, T. De Meyer, K. De Wispelaere, B. De
IV. CONCLUSIONS Sterck, B. M. Weckhuysen, et al., "Identification of Intermediates
in Zeolite‐Catalyzed Reactions by In Situ UV/Vis
A systematic experimental investigation has been performed Microspectroscopy and a Complementary Set of Molecular
for the reaction of ethanol to hydrocarbons on HZSM-5. No Simulations," Chemistry-A European Journal, vol. 19, pp. 16595-
16606, 2013.
difference in catalyst activity or selectivity to C
3+ [7] P. Kumar, J. W. Thybaut, S. Svelle, U. Olsbye, and G. B. Marin,
hydrocarbons is noticed between an ethanol and an ethylene
"Single-Event Microkinetics for Methanol to Olefins on H-ZSM-
feed. Extra water in the feed also has no influence on the 5," Industrial & Engineering Chemistry Research, vol. 52, pp.
activity or selectivity, when comparing at the same ethanol 1491-1507, 2013/01/30 2012.

partial pressure. Hence, the actual reactant for the production
of C hydrocarbons from ethanol is ethylene, which is
3+
obtained after full ethanol dehydration.
An induction period in the catalyst activity for ethylene
points towards an autocatalytic mechanism, which has been
identified with UV-VIS spectroscopy to be related to the
hydrocarbon pool mechanism in methanol-to-olefins on
HZSM-5. Simulation results indicate that a hydrocarbon pool
mechanism can indeed properly explain the primary character
of propylene, but its influence is limited to low C conversion.
2
After this, ethylation reactions with the produced lower
olefins take over and the conversion of ethylene increases
rapidly.
Table of contents
Table of contents i
List of figures v
List of tables x
List of symbols and abbreviations xii
Chapter 1 Introduction 1
1.1 Aim of the thesis 3
1.2 Structure of the work 3
1.3 References 4
Chapter 2 Chemical building blocks from ethanol 4
2.1 Production of ethanol 6
2.1.1 Ethanol from biomass fermentation 6
2.1.2 Ethanol from ethylene hydration 8
2.1.3 Ethanol from syngas 8
2.2 Acid catalyzed dehydration of ethanol 9
2.2.1 Alumina catalyzed 10
2.2.2 Zeolite catalyzed 10
2.3 Acid catalyzed reaction for methanol to hydrocarbons 13
2.4 Reaction mechanism for methanol to hydrocarbons 14
2.4.1 Direct coupling of methanol 14
2.4.2 Autocatalytic nature of methanol conversion 14
2.4.3 Dual cycle hydrocarbon pool mechanism 15
2.5 Reaction mechanisms for ethanol conversion over zeolite catalysts 17
2.5.1 Ethanol dehydration 17
2.5.2 Higher hydrocarbons 20
2.5.3 The role of water 22
2.6 Kinetic modeling of ethanol conversion to hydrocarbons 23
2.7 References 25
Chapter 3 Procedures 32
Table of contents i