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Analyzing Catalyst Deactivation Using a Packed Bed Reactor
Undergraduate Research Thesis
Presented in partial fulfillment of the requirements for Graduation with Research
Distinction in the College of Engineering of The Ohio State University
Brian Diep
William G. Lowrie Department of Chemical and Biomolecular Engineering
The Ohio State University
November 22, 2017
Thesis Committee:
Dr. Nicholas Brunelli, Advisor
Dr. Jeffery Chalmers
ii
© Copyright by
Brian Diep
2017
i
Abstract
As the global demand for industrial and consumer products has increased in recent
years, so too has the need for robust and high-performance catalysts to support this need. In
particular, heterogeneous catalysts have proven to be versatile and economically feasible
solutions to this problem. Organosilane catalysts have been studied for their ability to catalyze
reactions that convert abundant biomolecular feedstocks such as cellulose and glucose to more
valuable products used in the polymer, energy and fine chemical industries. However, one issue
that these catalysts, like many others, encounter is catalyst deactivation, where the active sites
on the catalyst support degrade or leach off over time as a reaction proceeds. My work focuses
on designing a continuous-flow packed bed reactor to study the deactivation of a tertiary amine
functionalized mesoporous catalyst for the isomerization of glucose to fructose. An accurate and
reliable reactor would have to satisfy several design considerations. Namely, the reactor must
maintain a constant temperature with no temperature gradient throughout the reactor, have no
interruption in flow rate of reactants, and must completely immobilize catalyst particles within the
catalyst bed. Several key strategies are implemented to achieve these goals, and the final
design was able to identify several potential deactivation mechanisms that are occurring in the
tertiary amine catalyst. Continued analysis in the future could identify the most impactful
deactivation mechanism(s), and could lead to a more efficient approach to creating better
catalysts by targeting the identified weaknesses.
ii
Acknowledgements
I would like to acknowledge Dr. Nicholas Brunelli for his continuing and unwavering
support for my work and personal aspirations. His mentorship and guidance have been a
cornerstone to my success and confidence in myself as an engineer. This thesis work would not
have been possible without the opportunity he gave me to join the Catalytic Material Design
Group in 2014, and I cannot thank him enough for all of the skills, knowledge, and experiences I
have gained from my time there.
I would also like to acknowledge the support of the graduate and undergraduate
students in the group for their help on developing my laboratory skills and helping me create a
solid skill foundation for my work. In particular, I would like to acknowledge Nitish Deshpande for
his significant support and guidance throughout my time in the group as well as the contribution
of the catalysts tested in this work. I would also like to acknowledge Nora Shaheen and AJ
Wahlstrom for their assistance with catalytic testing and designing the packed bed reactor.
Lastly, I would like to thank my friends and family for their enthusiasm around my
project. Your interest in my work has motivated me to continually strive to perform better and
work harder than I have ever done before. And thank you especially to Cheng Chung for helping
me get through the most stressful parts of the project.
iii
Table of Contents
Abstract…………………………………………………………………………………………………….i
Acknowledgements……………………………………………………………………………………….ii
Table of Contents………………………………………………………………………………………...iii
List of Figures and Tables……………………………………………………………………………….iv
List of Equations…………………………………………………………………………………………..v
Nomenclature……………………………………………………………………………………………..vi
1. Introduction ......................................................................................................................... 1
1.1 Catalyst deactivation mechanisms ............................................................................... 1
1.2 Tertiary amine catalysts and glucose isomerization reaction............................................. 3
2. Experimental methods ........................................................................................................ 4
2.1 Catalyst Synthesis ............................................................................................................ 4
2.2 Material Characterization .................................................................................................. 4
2.3 Packed bed reactor design ............................................................................................... 5
2.4 Pelletizing ......................................................................................................................... 8
2.5 Flow rate identification .....................................................................................................10
2.6 Testing and sampling.......................................................................................................10
3. Results and Discussion ......................................................................................................11
3.1 Reactor temperature distribution .................................................................................11
3.2 Effects of pelletizing ....................................................................................................12
3.3 Material characterization ..................................................................................................14
3.4 Glucose isomerization data .............................................................................................19
4. Future Work .......................................................................................................................21
5. Conclusions .......................................................................................................................22
6. References ........................................................................................................................25
iv
List of Figures and Tables
Figure 1.2.1: Examples of high value products produced from HMF 3
Figure 2.3.1: The packed bed reactor system consisting of reactor column and heating block, nitrogen purge system, piston pump, and PID temperature controller
6
Figure 2.3.2: Experimental setup for reactor column temperature gradient testing 7
Figure 2.3.3: Overall schematic of packed bed reactor system 8
Figure 2.6.1: Interior view of packed bed reactor 11
Figure 3.1.1: Temperature deviation across reactor column at various flow rates 12
Figure 3.2.1: Packed clear column with no flow and during flow 13
Figure 3.3.1: Nitrogen physisorption isotherms of SBA-15 before and after exposure to water or a 2 wt% glucose solution
15
Table 3.3.1: Summary of nitrogen physisorption analysis results for catalysts tested 15
Figure 3.3.2: TGA data for bare SBA-15 run in water and in a glucose solution 16
Figure 3.3.2: TGA data for pre and post reaction C1 catalyst 17
Figure 3.3.3: IR spectrum of bare SBA-15 run in water and in glucose 18
Figure 3.3.4: IR spectrum of bare SBA-15, C1 catalyst prior to reaction, and C1 catalyst post reaction.
18
Figure 3.4.1: Conversion and selectivity over time for the C1 catalyst 19
v
List of Equations
Equation 2.4.1: The Ergun equation 9
Equation 2.4.2: An extension of the Ergun equation 9
vi
Nomenclature
TGA-DSC Thermogravimetric Analysis – Differential
Scanning Calorimetry
HPLC High Performance Liquid Chromatography
ATR FTIR spectroscopy Attenuated Total Reflectance Fourier
Transform Infrared spectroscopy
BET Brunauer-Emmett-Teller Surface area
analysis
BJH Brunauer-Joyner-Helena Pore Size and
Volume analysis
BdB-FHH Broekhoff-de Boer–Frenkel-Halsey-Hill
method for determining pore size distribution
N2 Nitrogen
1
1. Introduction
1.1 Catalyst deactivation mechanisms
Combating deactivation in heterogeneous catalysts is one of the central focal points in the
pursuit of improving efficiency and production capacity of chemical manufacturing plants.
Current estimates place cost of downtime and replacement of deactivated catalysts on the scale
of billions of dollars worldwide1. Deactivation occurs in a variety of mechanisms and at different
rates depending on the catalyst and the process conditions. To reduce, prevent, or even reverse
deactivation, it is necessary for the specific mechanism(s) unique to each catalyst to be
understood.
There are generally six different mechanisms of deactivation: (1) fouling, (2) poisoning, (3)
thermal degradation, (4) volatile compound formation, (5) vapor-solid, liquid/solid, and/or solid-
solid reactions, and (6) attrition/crushing, as described in literature1. Poisoning includes
chemisorption of reactants, products, or byproducts onto the surface of the catalyst. This can
affect the catalyst active sites or neighboring atoms, and may even be able to modify the
surface of the catalyst and cause diffusion limitations as well. Fouling includes physisorption of
reactants, products, or byproducts onto the surface of the catalyst and rendering the active sites
inaccessible. Thermal degradation includes thermally induced collapsing of support material,
thermally induced chemical transformation of active sites, and loss of surface area through
sintering. Vapor compound formation, vapor/solid, liquid/solid (also known as leaching2), and
solid/solid reactions all constitute other chemical deactivation mechanisms where the catalytic
phase undergoes a vapor or solid phase reaction to form a bulk inactive phase or volatile
inactive phase. Attrition/crushing is structural breakdown of the catalyst particles due to
continual wear and abrasion through reaction conditions.
2
Previous work studied deactivation in a variety of ways, both in batch and continuous
systems. While it is undoubtedly important to understand how catalyst deactivation occurs in a
batch process, much of the world’s commodity manufacturing processes are performed in a
continuous setting. Thus, it is logical to focus efforts on studying catalyst deactivation under
continuous flow. The advantage that continuous reactors have over batch reactors is that we
can observe the deactivation of the catalyst occur as a function of product conversion. The use
of a packed bed reactor has been demonstrated in literature as an effective tool for analyzing
reaction kinetics and deactivation mechanisms. Lakhapatri and Abraham were able to study the
mechanism of sulfur poisoning on a Rhodium promoted nickel catalyst for steam reforming
using a fixed bed reactor in the gas phase and found that increased Rhodium concentration on
the surface reduced the catalyst’s susceptibility to sulfur poisoning3. Blasco and associates
studied sintering within a commercial dehydrogenation catalyst in a fixed bed reactor and
observed that deactivation and reactivation of the catalyst is strongly affected by its temperature
history through the reaction and regeneration processes. They also were able to calculate the
sintering kinetics of the catalyst active site at different areas within the reactor and determined
that some portions of the catalyst bed were still active and could be recovered even when the
reaction conversion had fallen below acceptable limits due to sintering4. Perhaps most
relevantly, Tucker and co-workers studied the effectiveness of various supported acid catalysts
for the dehydration of fructose to 5-hydroxymethylfurfural, a key component in the conversion of
biomass to high value products. Deactivation of the catalysts was studied using a flow reactor
and results suggested that the catalyst active sites were being hydrolytically cleaved under
reaction conditions and were leaching into the product solution. Interestingly, it was also
observed that the leached acid groups could be captured to an extent by a silica bed
downstream of the immobilized catalytic bed, which introduces new possibilities in reactor
design and catalyst regeneration.
3
1.2 Tertiary amine catalysts and glucose isomerization reaction
The goal of this work is to design a packed bed reactor to analyze the deactivation
mechanisms that occur in a tertiary amine catalyst for the glucose isomerization reaction to
fructose. The isomerization of glucose to fructose has been shown to be a key step in the
process to produce high value biofuels and additives from biomass (cite). The process involves
the conversion of biomass into glucose, glucose to fructose, fructose into 5-
(hydroxymethyl)furfural (HMF). HMF can then be transformed into various high value products
with uses in biofuels and polymer industries. Figure 1.2.1 below shows some of the possible
product pathways that HMF can travel.
Figure 1.2.1: Examples of high value products produced from HMF
Literature has shown that organic base catalysts such as tertiary amines are selective
catalysts for the isomerization of glucose5. In this work, a packed bed reactor is constructed to
study the deactivation mechanisms that occur in a tertiary amine catalyst. The tertiary amine
catalyst consists of (N,N-diethylaminomethyl) triethoxy silane immobilized on SBA-15, a
mesoporous silica support. SBA-15 was chosen as the support material because of its ease of
synthesis, large surface area, and large accessible pores (~7-10nm). However, organosilane
4
catalysts have been shown to be prone to deactivation, possibly through leaching of the
organosilanes from the surface. Literature also has shown that the linker length of the
organosilane affects the resistance to deactivation. Specifically, organosilanes with methyl
linkers were shown to be bonded tri-podally (3 bonds) to a silica support surface as opposed to
a propyl linker, which was found to be connected bi-podally (2 bonds) to the surface. The
increased number of bonds makes the immobilized methyl-linker organosilanes potentially more
stable and resistant to deactivation by leaching. Deshpande et al. have observed that tertiary
amine catalysts with short linker lengths produce greater activity for the isomerization of glucose
than longer linkers.6 However, it is unlikely that leaching is the only mechanism occurring since
it is common to have several deactivation mechanisms present in a catalyst. Therefore,
continuous improvement can be made to tertiary amine catalysts by identifying the primary
deactivation mechanisms for a more targeted approach on prolonging its effective lifespan.
2. Experimental methods
2.1 Catalyst Synthesis
Pure silica (bare) SBA-15 was synthesized according a procedure reported in literature.
The organosilane catalyst with immobilized tertiary amines were synthesized via grafting
according to methods reported in literature.6 The organosilane selected for grafting was (N,N-
diethylaminomethyl) triethoxy silane. For convenience, the catalyst will be named C1, in
correspondence to the linker length of the grafted tertiary amine. The actual loading for the C1
catalyst was determined to be 0.52 mmol g-1 through elemental analysis.
2.2 Material Characterization
The catalysts were analyzed using a variety of standard characterization techniques,
including nitrogen physisorption, thermogravimetric analysis (TGA), and Fourier transform
infrared spectrometry (FTIR). Nitrogen physisorption analyses were performed on a
5
Micromeritics 3-Flex. Surface area calculations were performed with the BET method. The pore
volume was determined by BJH method, and pore size was determined using BdB-FHH
method. TGA-DSC analysis was performed on a STA 449 F5 Jupiter® (NETZSCH instruments)
under flowing air (20 mL/min), nitrogen (20 mL/min), and at a temperature ramp rate of
10°C/min from 30°C to 900°C followed by a 5 min hold at 900°C. Attenuated Total Reflectance
Fourier Transform Infrared Spectroscopy (ATR-FTIR) analysis was performed on a Nicolet iS50
FTIR. Temperature gradient measurements were performed with an OMEGA Hh806Au
Thermocouple Thermometer equipped with the Artisan Coffee Roaster Scope software for data
collection.
2.3 Packed bed reactor design
A lab-scale packed bed reactor was designed and built for this experiment with several
considerations in mind. Practically speaking, the reactor had to be robust, easy to construct and
use, easy to recover catalyst from, and had to contain a nitrogen purge system. As seen in
Figures 2.3.1 and 2.3.3 below, the packed bed reactor column is constructed of ¼” OD stainless
steel tubing capped on both ends by 15 micrometer in-line filters from Swagelok. The reactor
tube itself is encased in a large aluminum heating block that is furnished with two ½” 150 W
cartridge heaters, which are controlled by an OMEGA PID temperature controller. A ball valve
tee at the bottom of the column allows the reactor to be switched between flow mode and
nitrogen purge mode. The reactant is pumped via a Scientific Systems, Inc. M1 series piston
pump, which pumps the reactant through the column and into a large round bottom flask for
collection and sampling. More importantly however, the packed bed reactor must have three
critical features: constant temperature with no gradients, uninterrupted flow rate of reactants,
and complete immobilization of the catalyst particles within the catalyst bed.
6
Figure 2.3.1: The packed bed reactor system consisting of reactor column and heating
block (red), nitrogen purge system (green), piston pump (blue), and PID temperature controller
(yellow)
Common designs for packed bed reactors utilize clamshell-style ceramic heaters that
are prone to developing internal temperature gradients in the reactor7. This issue is resolved in
this design by using a large aluminum heating block, which has direct contact with the reactor
column surface and allows it to be heated by conduction rather than radiation. Additionally, a
small column external diameter of ¼ in. was chosen to reduce the surface area and volume of
the reactor to be heated, and a large C-clamp was used to press the block tightly against the
reactor tube to maximize contact area. To confirm the reduction of a temperature gradient, an
experiment was designed where to measure the temperature along the axial direction of the
reactor tube. In a typical experiment, a reactor column is packed with glass beads at a certain
bed height ranging from 3 cm to 15 cm, where 3 cm corresponds to 3 cm above the lower
contact boundary of the heating block. A thermocouple is placed inside of the column and
7
allowed to rest on top of the glass beads. The column is then heated to 70°C and 100°C and
water is flowed through the column at various flow rates from 0.4 mL/min to 2 mL/min for 30
min. Figure 2.3.2 illustrates the experimental setup. The results of this experiment are discussed
further in a later section.
Figure 2.3.2: Experimental setup for reactor column temperature gradient testing
Previous work has found that that grafted tertiary amines catalysts are negatively
affected by an air environment. For the isomerization of glucose to fructose, Deshpande and co-
workers demonstrated that for tertiary amine catalysts, conversion of glucose increased by 6%
when the reaction was run under nitrogen rather than under air.6 In light of this observation, a N2
purge system was designed to keep the reactor tube, reactant flask, and product flasks under
nitrogen conditions during the reaction to maximize the conversion. N2 is used to purge the
product vessel, reactant vessel and reactor column prior to a reaction and also is used post
reaction to dry the column for catalyst extraction. An illustration of how the purge system is
integrated into the reactor is show below in Figure 2.3.3
8
Figure 2.3.3: Overall schematic of packed bed reactor system
2.4 Pelletizing
Initial flow testing experiments with SBA-15 used the loose powder form of the
mesoporous silica support material inside the column. Through repeated tests, it was found that
the loose powder had two issues. The first issue observed was that there was a large pressure
drop across the column that prevented the reactor’s piston pump from pushing water through
the catalyst bed. This was surmised to be due to the small particle size of SBA-15, which is on
the scale of 100-400 nm.8 In a packed bed reactor system, the pressure drop across a column
length can be described by the Ergun equation, described below in Equation 2.4.1, where Δp is
the pressure drop across the bed, L is the length of the bed, Dp is the equivalent spherical
2
AD
D1
5
113
34
2
N2 feed
VBAVBA
7
7
5
10
N2 exhaust
11
N2 exhaust
6
9
VBA VBA
6
4
812
9
Reactor column
Heating block
Product vessel
Reactant vessel
9
diameter of the packing, ρ is the density of the fluid, µ is the dynamic viscosity of the fluid, vs is
the superficial velocity, and ε is the void fraction of the bed.
2.4.1
An extension of the Ergun equation can be derived to solve for pressure drop, Δp, and is shown
below in Equation 2.4.2.
2.4.2
From this equation, it is apparent that the pressure drop is inversely proportional to the
particle diameter and explains why there was a large pressure drop, since the particles of SBA-
15 are very small. The second issue that was observed with the loose powder catalyst was that
the small particle size of the material also allowed it to escape the immobilized bed, which is
sandwiched by plugs of quartz wool. During multiple tests, SBA-15 was found to have migrated
out of the immobilized bed area and captured in the inline filters downstream. This is an issue
because it is difficult to account for residence time through a catalyst bed if the bed height is
constantly changing, which will affect the consistency of catalytic testing results. In addition, it is
difficult to remove the catalyst from the inline filters and oftentimes the trapped catalyst is lost to
washing. To resolve both issues, a pelletization procedure was devised to create large pellets
that would reduce the pressure drop and prevent catalyst particles from leaving the bed by
increasing the particle size.
In a typical pelletizing procedure, approximately 200 mg of catalyst is loaded into a ¾”
diameter pellet die and pressed at 6000 pounds of pressure for 1 min. After the desired amount
10
of pellets are made, the large pellets are crushed using a mortar and pestle and subsequently
sieved through a 1.168 mm and 0.590 mm screen mesh. Pellet diameters of between 0.590 mm
and 1.168 mm were retained and used in the experiments. All other pellets sizes were re-
crushed, pressed, and sieved until the required amount of correct sized pellets was
accumulated. These pellets were tested under normal reaction conditions with water and a 2
wt% glucose solution and the results are discussed in a later section.
2.5 Flow rate identification
The appropriate flow rate for the experiments was calculated through a trial and error
process. C1 catalyst was pelletized and tested for 24 h at 0.4 mL/min and 0.2 mL/min with 200
mg and 400 mg loadings in the reactor. The product stream was sampled over time and
analyzed with HPLC analysis to determine glucose conversion and selectivity for fructose.
Based on the conversion results, a flow rate of 0.2 mL/min and 400 mg of pelletized catalyst
recorded the highest conversion was selected for the experiment.
2.6 Testing and sampling
In a typical test, 400 mg of pelletized catalyst (~6 cm bed height) were packed into the reactor.
Figure 2.4.1 shows the packing structure of the column in detail. Reaction conditions are at
100°C with a flow rate of 0.2 mL/min of either pure deionized water or 2 wt % glucose in water.
Bare SBA-15 was tested for 6 h in water and in the glucose solution to establish baseline
conditions. C1 catalyst was tested for 24 h in a glucose solution. Sampling was performed by
diverting the product stream from the product vessel to small HPLC vials every 15 mins for the
first 5 hours. Subsequent samples were taken at 1 hour intervals. At the conclusion of a test,
the column is flushed with water for ~3 h at 100°C, 0.2 mL/min followed by drying with nitrogen
gas at 100°C, 15 psig before recovering the catalyst.
11
Figure 2.6.1: Interior view of packed bed reactor.
Catalyst recovery was performed after the column had been thoroughly dried. The glass bead
supports were removed first, followed by the glass wool and finally the catalyst for
characterization.
3. Results and Discussion
3.1 Reactor temperature distribution
The packed bed reactor column was monitored for temperature using a thermocouple
placed along various heights in the column as described in the experimental procedure in a
previous section. Data collected show that the temperature deviation at low flow rates was on
average below 1°C. Figure 3.1.1 shows the temperature deviation, calculated as the
temperature of the heating block minus the internal reactor temperature, for a range of flow
rates and column heights. A trend observed is that for flow rates of 0.7 and 0.4 mL/min, there
was not a significant temperature difference between the upper and lower limits of the column.
Only higher flow rates of 1 and 2 mL/min show larger temperature deviation greater than 2°C.
12
Even in these cases, the sources of large deviation occur near the bottom of the column, where
water is just entering the heated zone. The catalyst bed, when properly packed, sits in the
region between 6-12 cm. From the data obtained, this critical region generally demonstrates no
significant temperature deviation from the setpoint temperature.
Figure 3.1.1: Temperature deviation across reactor column at various flow rates.
3.2 Effects of pelletizing
Significant system performance improvement was observed when pelletized catalyst
particles were introduced over the powdered form. In subsequent flow tests after introducing
pelletized catalyst, no over-pressurization of the pump was observed, and uninterrupted flow
was achieved for all subsequent tests. However, the issue of catalyst escaping the immobilized
bed still remained. In some of the flow tests, powdered catalyst was found entrained in the
downstream in-line filter during the recovery of the catalyst, even after pelletization. Since the
catalyst particles that were observed in the filters were of the powdered form, it was deduced
that mechanical attrition of the pelletized catalyst particles occurred, causing fine particles to
13
break off from the body of the pelletized particles. However, it was still not clear how the attrition
was occurring, since the flow rate was relatively slow. In order to ascertain the cause of the
attrition, a separate column made of clear vinyl tubing of the same diameter was created and
used to observe the immobilized bed during operation of the reactor without heat. The operation
with clear column showed a surprising behavior of the bed where when under flow, the
immobilized bed shifts upward approximately 2 cm and settles back down. This movement
creates a void space in the packed bed that allows the previously immobilized particles to move
freely and collide with each other. Figure 3.2.1 shows the clear column before and after water
was flowed through.
Figure 3.2.1: Packed clear column with no flow (left) and during flow (right)
To identify the source of the void space, we must turn back to the packing procedure of
the column. The typical process packs the column from the bottom up, where the last
14
component to be attached is the top in-line filter. Due to the design of the top filter, there is a
small pocket of empty volume at the entrance of the filter that does not get filled completely
when the column is packed. The solution devised was to fill the filter with glass beads and glass
wool to occupy the dead volume. Flow tests after the packing adjustment showed little to no
movement of the immobilized catalyst bed.
3.3 Material characterization
Figure 3.3.1 plots the phyisorption isotherms of SBA-15 pre and post reaction. The
overall shape of the isotherm is characteristic of mesoporous silica materials. Nitrogen
physisorption analysis on bare SBA-15 post reaction in both water and glucose solution showed
that there was a loss of surface area and an increase in pore size post reaction. This may be
indicative of structural breakdown of SBA-15 under hydrothermal conditions. Celer and co-
workers noted that SBA-15’s mesopores tend to widen and that the micropore area significantly
decreases when exposed to water and heat in hydrothermal conditions similar to the reaction
conditions of this experiment9. This is corroborated by the characterization results of bare
pelletized SBA-15 that also exhibited a pore size expansion and up to almost 50% surface area
reduction after being exposed to reaction conditions. In addition, the micropore volume shows a
marked decrease from 0.08 cm3/g to 0.03 cm3/g in both post-reaction samples, over 50%
decrease. Table 3.3.1 lists all of the characterization data for SBA-15 tested in both water and in
a 2 wt% glucose solution at reaction conditions. SBA-15 tested in water exhibits a greater
reduction in surface area, micropore volume and a greater increase in pore diameter than SBA-
15 tested in a glucose solution. This data suggest that one potential deactivation mechanism for
the C1 catalyst is the reduction of surface area and access to active sites through closing of
micropores as the SBA-15 undergoes thermal degradation under hydrothermal reaction
conditions.
15
Figure 3.3.1: Nitrogen physisorption isotherms of SBA-15 before and after exposure to
water or a 2 wt% glucose solution
Table 3.3._: Summary of nitrogen physisorption characterization data for SBA-15
Material
Reactant/Solvent
Surface area (m2/g)
Pore diameter
(nm)
Pore volume
(cm3/g)
Micropore volume (cm3/g)
SBA-15 - 740 5.6 0.77 0.08
SBA-15 Water 390 6.0 0.66 0.03
SBA-15 2 wt% glucose 500 5.9 0.74 0.03
Thermogravimetric analysis can determine the presence of organic species on the
surface of the catalyst. Figure 3.3.2 shows the TGA data for bare SBA-15 run in water and in
glucose. Bare SBA-15 run glucose showed no significant difference in mass in the organic
region between pre- and post-reaction samples. This indicates that no reactants, products, or
16
byproducts were chemically bonded to the surface of bare SBA-15. If any adsorption of
chemical species occurs on the catalyst, it will be to the tertiary amine sites. The C1 catalyst
showed a distinct difference in organic mass between the pre-run and post run samples. As
shown in Figure 3.3.2 below, the difference in the mass drop associated with organic region
from 200°C to 800°C between the pre-run and post run C1 catalyst was calculated to be 0.5%.
This difference corresponds to approximately a 0.1 mmol/g loading loss. This suggests that
during the reaction, organosilane groups may have been removed from the surface of the
catalyst through leaching or through mechanical degradation. However, this does not account
for any species that may have chemisorbed onto the amine groups of the catalyst. To further
analyze this possibility, elemental analysis is required.
Figure 3.3.2: TGA data for bare SBA-15 run in water and in a glucose solution
17
Figure 3.3.2: TGA data for pre and post reaction C1 catalyst
Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR)
analysis was performed on the bare SBA-15 and C1 catalysts before and after the reaction.
Figure 3.3.3 shows the IR spectrum for bare SBA-15 run in pure water and in glucose. For bare
SBA-15 run in pure water, there was no discernable difference between the pre- and post-
reaction samples. The same is seen for bare SBA-15 that was run with a glucose solution. This
supports the TGA data for the same material and indicates that bare SBA-15 does not contain
any chemisorbed species. However, this does not rule out the possibility that byproduct species
may be physisorbed on the catalyst during a reaction. For the C1 catalyst, the IR spectrum data
does not show any significant differences in the spectrums of the C1 catalyst before the reaction
and after the reaction, nor does it seem to show any difference from bare SBA-15. This further
indicates that no byproduct species seem to remain on the surface of the catalyst after it has
been washed and dried.
18
Figure 3.3.3: IR spectrum of bare SBA-15 run in water and in glucose
Figure 3.3.4: IR spectrum of bare SBA-15, C1 catalyst prior to reaction, and C1 catalyst post
reaction.
19
3.4 Glucose isomerization data
Bare SBA-15 and C1 catalysts were tested in the isomerization of glucose to fructose.
Bare SBA-15 tests were used as a baseline for measuring the performance of the C1 catalyst.
Bare SBA-15 showed essentially zero conversion of glucose to fructose occurring when bare
SBA-15 is used as the catalyst.
For the C1 catalyst, tests showed that there was a rapid decrease in percent conversion
within the first 5 hours of the reaction, after which the conversion stabilizes near zero, indicating
full deactivation of the catalyst. The selectivity for fructose is lowest when the conversion is the
highest, at the start of the reaction. As the reaction progresses, the selectivity peaks around
55% and decreases as the conversion decreases. Illustrated in Figure 3.4.1, this trend in
selectivity may indicate the rapid formation of a side product in the beginning of the reaction.
Figure 3.4.1: Conversion and selectivity over time for the C1 catalyst
20
It was noted during the HPLC analysis of product samples that a significant peak around
23.5 minutes retention time was observed early in the reaction and continues to persist through
the end. Figure 3.4.2 shows the HPLC chromatographs that show the formation of a mystery
peak that is not glucose or fructose. In the HPLC analysis of bare SBA-15 run with a 2 wt%
glucose solution, the peak does not appear. The presence of such a peak may indicate a
physisorbed or chemisorbed byproduct that adheres itself to the active sites of the catalyst and
prevents access. However, as seen from the IR spectrum data of the C1 catalyst in the previous
section, post-reaction C1 catalyst does not exhibit any unusual peaks that indicate the presence
of a chemisorbed byproduct. This suggest that any byproduct that forms may only physisorb to
the surface and can be removed by flushing with water. Deshpande et al. suggest that an acidic
byproduct species likely form during the isomerization reaction when using a tertiary amine
catalyst. This is evidenced by analyzing pH over time of the reaction solution, where a
significant drop from 9.1 to 6.8 was observed after 5 h6. This mechanism seems to be the most
likely candidate for the primary contributor to deactivation, as it can completely cover the active
sites of the catalyst as the reaction progresses. On the other hand, thermal degradation of the
support material may also play a large role considering the significant reduction in the surface
area and micropore volume, but those effects are not likely to be as pronounced early in the
reaction as deactivation from byproduct formation would be. Leaching is likely the least
significant mechanism of the three identified so far since there was evidence that around 80% of
the immobilized amines still remained on the surface of the catalyst post-reaction. It is unlikely
that a 20% decrease in amine loading would result in a 100% deactivation. Hence, byproduct
formation and subsequent inhibition of the amine sites is the strongest contender for the primary
deactivation mechanism.
21
Figure 3.4.2: HPLC chromatograph of 2 wt% glucose (top) and the product solution 30 mins into
the reaction with the C1 catalyst (bottom)
4. Future Work
Future work with analyzing the deactivation mechanisms would involve continuing to identify
the largest contributors. This would entail a set of experiments that would independently isolate
each mechanism and evaluate its effect on the catalyst activity and physical morphology. For
example, to study the effect of thermal degradation of SBA-15, a modified synthesis of SBA-15
can be performed that reduces the micropore surface area. Minimizing the micropore surface
area could reduce the loss of surface area in SBA-15 under hydrothermal conditions. For
Glucose
(25.3 min)
Glucose
(25.3 min)
Fructose
(31.1 min)
Unknown
(23.5 min)
22
byproduct formation, pH can be monitored over time to confirm if acidic species are generated
during the reaction. If this is the case, a base such as sodium bicarbonate can be introduced
into the reactant solution that immediately neutralizes any acid byproducts that form. To
analyze leaching, elemental analysis of the post-reaction material can confirm if the loading on
the catalyst surface has truly changed. After this analysis is done, catalysts at different loadings
can be tested to see if leaching by hydrolysis is reduced by increased loading, which may
reduce the accessibility to the cleavage sites.
5. Conclusions
With the demand for manufactured products and chemicals on the rise worldwide, the need
for longer lasting and more efficient catalysts has never been higher. One of the many issues
that catalysts have faced is the degradation or deactivation of the catalyst, whether it be through
chemical, physical, or mechanical deactivation. A strategy to combat deactivation involves
identifying and understanding the underlying deactivation mechanisms that occur during the life
of a catalyst.
In this work, I designed and constructed a packed bed reactor to identify the deactivation
mechanisms that occur in an immobilized tertiary amine catalyst for the isomerization of glucose
to fructose reaction. The packed bed reactor was designed to be robust and contains several
innovations such as a nitrogen purge system, a solid aluminum heating block that virtually
eliminates temperature gradient across the reactor length, and a reduced dead volume packing
strategy that aims to minimize negative impacts on the tertiary amine catalyst. In addition, the
high pressure drop across the column was reduced by developing a pelletizing procedure that
produced large catalyst pellets for use instead of loose powder catalyst.
Previous work has proposed leaching of organosilanes via hydrolysis as the leading
deactivation mechanism in organosilane catalysts. Material characterization and reaction data of
23
the amine catalyst revealed that several deactivation mechanisms may be ongoing during the
reaction. First, nitrogen physisorption showed that the surface area of the catalyst support was
reduced and that the pore sizes increased after the catalyst was exposed to reaction conditions
with water and with a 2 wt% glucose solution. This was determined to be an issue with the
structural integrity of the SBA-15 support under hydrothermal conditions, and is corroborated by
reports in literature. TGA data showed a 10% loading loss in the organic content of the catalyst.
Since the catalyst was washed and dried before undergoing the analysis, the mass drop can be
attributed to loss in the immobilized tertiary amines, possibly through leaching or from the
mechanical degradation caused by reaction conditions. However, TGA is not sensitive enough
to quantify any chemisorbed species on the surface of the catalyst that may contribute to the
total organic mass. However, ATR FTIR analysis did not determine that there were any
chemisorbed species on the surface of the C1 catalyst post reaction, indicating that if any
absorption occurred during the reaction, it would have been a physisorbed species or a weakly
chemisorbed species that was removed during washing. Finally, reaction data indicates a low
selectivity for fructose during the first two hours of the reaction that corresponded with a high
initial conversion of glucose. This could indicate byproduct formation early in the reaction that
may contribute to the deactivation of the catalyst. Indeed, further examination of the HPLC
analysis of product samples for the first two hours of the reaction show the presence of an
unknown peak that is not present when bare SBA-15 is used as the catalyst. This is further
supported by work done by Deshpande that indicate the presence of an acidic byproduct that
forms when tertiary amine catalyst reacts with glucose that seems to inhibit the catalyst.
From these characterization results, it is apparent that several mechanisms may be
occurring simultaneously during the life of the C1 catalyst. However, it is likely that the most
influential deactivation mechanism is the formation of byproducts early in the reaction that may
inhibit the active sites of the C1 catalyst. Structural degradation of the support material likely is
24
also major contributor to degradation, but perhaps is more influential with longer reaction times.
Leaching of immobilized amines seems to be the least influential factor in the complete
deactivation of the C1 catalyst, since it is unlikely that a 20% loss in amine loading could result
in a 100% deactivation of the catalyst.
In conclusion, a reliable and accurate packed bed reactor was constructed to analyze the
deactivation of an organosilane catalyst for the isomerization of glucose to fructose. The
possible deactivation mechanisms that were identified in this experiment are thermal
degradation of the support material, leaching of immobilized tertiary amines, and formation of a
reaction byproduct. Future experiments to further narrow down and confirm these mechanisms
would include testing more robust support materials, evaluating the effect of increased
concentration of immobilized species, and identifying and neutralizing byproducts that form
during the reaction.
25
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