i

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.

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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.

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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

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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.

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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.

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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

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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.

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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.

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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

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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

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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.

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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

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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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