original proposal summary - university of notre damemjm/nsf_ner_macroporous_catalyst.pdfbeakers...

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Designing nanostructured heterogeneous catalysts to exploit pulsing

in gas-liquid packed bed reactors

M. J. McCready ([email protected])E. J. Maginn ([email protected])University of Notre DameChemical and Biomolecular Engineering

Original Proposal Summary

This proposal describes work that is intended to develop new nanostructured catalysts

for gas-liquid reactions, which have a system of macro pores designed to take

advantage of oscillatory convection that can be created by large scale hydrodynamic

disturbances. Thus we are combining a nanoscale structure that provides the best

reaction environment with a macroscale structure that provides the highest possible

mass transfer rates. The net effect is a new catalyst that can lead to reductions in

volume of gas-liquid packed bed reactors (a.k.a. "trickle" beds) by an order of

magnitude or more because active, useful metal could be placed throughout the pellet

rather than just at the surface as is typical for gas-liquid reactions.

Porous silica structures having both a nanopore network (radius 1.5-10 nm) and a

macropore network (radius ~0.2 mm) will be synthesized. The nanopores will be

created using a surfactant-mediated templating process, while the macropores that

span the entire pellet will be added through use of sacrificial fibers. The silica

material will be impregnated with platinum to make it catalytically active for a test

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reaction of phenylacetylene plus hydrogen going to styrene and then to ethylbenzene.

These pellets will be used in a recycle reactor that uses a novel catalyst configuration

that our previous work has shown can clearly differentiate the mass transfer effects of

pulsing flow compared to trickling flow at the same gas and liquid flowrates.

The spanning network of macro pores will enable convective mass transfer within the

pellet, in the pulsing flow regime, because it will take advantage of Taylor-Aris

oscillatory transport mechanism. That is, the pressure fluctuations associated with the

pulses, will cause an oscillatory flow within the macropores. If the macropore

diameter is chosen correctly for a given pulsing frequency, the time scale for radial

diffusion and axial transport will match, leading to a reduction in diffusion distance

from the pellet radius to just the macropore radius -- about an order of magnitude.

This study will comprise synthesis of this novel catalyst, mass transfer studies in

trickle and pulsing flow to prove the enhancement mechanism and reaction studies to

demonstrate the increased activity per unit reactor volume. The net effect is that the

length of packed bed reactors for gas-liquid reactions could be greatly reduced with

our new catalyst structure. The benefits of such a result would be a considerable

reduction in capital costs for full-scale plant retrofits and new construction, increased

application of gas-liquid packed bed reactors for "closet scale" distributed production

for chemicals that are hazardous to transport and possible use of this reaction scheme

for reforming or other types of fuel conversion on board motor vehicles.

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The proposed work should impact the general problem of how to best exploit the

increasing potential of nano synthesis techniques to produce more active and selective

catalysts by addressing the mass transfer limitations, which are particularly acute in

gas-liquid reactions. It should encourage similar studies to look for links between

nanostructured materials for separation processes and the fluid dynamics of these

devices.

Much of the information below is contained in a thesis by N. S. Martino:

Hydrodynamics of a two-dimensional gas-liquid packed bed. Notre Dame Library.

Activities

Synthesis of Catalyst Support

The synthesis of mesoporous silica as a catalyst support is based on the

techniques used by Nooney et al. (2001). It was scaled up to attempt to produce the

catalyst faster. The following describes the experimental procedure used to produce

both the macroporous and non-macroporous catalyst supports. Several attempts were

made before a useable material was found. A full description of the experiments can

be found in the APPENDIX which is attached below. Only a successful run will be

detailed here.

The chemicals used in this synthesis were purchased and used as received from

Sigma-Aldrich. 4.5 g of cetyltrimethylammonium bromide (CTAB) is divided into

three groups of 1.5 g. Each of these individual groups is then dissolved in separate

beakers containing 83.33 g of deionized water each (total deionized water equals 250

g). A Fisher Scientific sonicater is used to speed up the dissolution. Once dissolved

these solutions are mixed together with 19.3 g of 2 molar NaOH in the reactor. This

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brings the pH of the solution to about 12.7. Then 18.5 g of tetrabutyl orthosilicate

(TBOS) is added drop wise to the solution while it is stirring. The reaction is then

allowed to stir for 24 hours.

The product of this reaction is a viscous gel of silica. This gel is removed from

the reactor and washed with water. It is then broken up and molded into spheres of

approximately 5-6 mm in diameter. Half of these spheres are then stuck with a

minimum of 10 cellulose fibers of approximately 0.2 mm in diameter (sisal fibers

provided by International Fiber Corporation). Then all of the spheres are allowed to

dry in atmosphere for 24 hours before calcination.

It is important to note the chemistry that occurs to form the mesoporous silica.

The silica source, TBOS, forms an emulsion when mixed with water. CTAB is a

surfactant that has a hydrophobic tail and a hydrophilic head. Because of this it

buries its hydrophobic tail inside the TBOS drop leaving its head exposed to the

water. The presence of a base will accelerate this action along the water emulsion

interface creating a long rod. These rods pack hexagonally and form a sphere from

the emulsified drop of TBOS. The silica begins to grow along the TBOS water

interface and eventually grown into the bubble. Spheres form under the proper

reaction conditions but Nooney et al. (2001) showed that by altering the conditions a

multitude of products can be formed.

3.7 Removal of Template and Sacrificial Fibers

The product of the synthesis reaction is not porous until the CTAB template is

removed. This can be done either by chemical extraction or by thermal means. Both

are effective ways to leave a highly porous silica structure. Calcination was chosen

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for this work because it can be used to remove the template and the sacrificial fibers

in one step. After being allowed to dry at atmospheric conditions the spheres were

calcined in a furnace controlled by an Omega temperature controller. The

temperature was ramped up at 1oC per minute to 620oC under flowing oxygen. The

slow ramp rate was used to avoid any cracking of the silica due to stress. It was then

held at that temperature for 10 hours to ensure complete burn out of the CTAB

template and the sacrificial fibers. Finally the samples were brought back down to

room temperature over 6 hours. It should be noted that there was no structural

difference between samples that were calcined after being dried at atmospheric

conditions compared to those that were calcined after being dried in a vacuum

furnace at 100oC to remove water.

3.8 Platinum Impregnation

After the mesoporous silica was made porous by calcination it needed to be

impregnated with platinum to make it an active catalyst. The procedure used was

identical to the one presented by Long and Yang (1998) who were able to impregnate

MCM-41 with up to 5 percent by weight platinum. Incipient wetness impregnation

method was used with hydrogen hexachloroplatinate (IV) hydrate 99.9%. After

initial impregnation by the salt, the samples were dried in atmospheric conditions for

24 hours. To reduce the platinum the samples were then calcined at 400oC for at least

5 hours in 5.34% H2 in N2.

3.9 Hydrogenation of Phenylacetylene

Four experiments were run to test this novel catalyst. For each experiment the

catalyst was placed in the upper or lower packing configurations. Figure 3.6 shows a

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schematic of the packing configurations used. The macroporous catalyst was run in

one time in the upper and one time in the lower configuration. This was repeated for

the non-macroporous catalyst. One liter of tetradecane solvent and 1 ml of decane

GC international standard are added to the reservoir. The liquid is then circulated for

several minutes to assure complete wetting of the catalyst. Nitrogen is passed

through the gas loop for 15 minutes to remove any air that may have become trapped

inside. Once the gas loop was purged and the catalyst were wetted, phenylacetylene

was introduced by syringe and the reactor was sealed. The nitrogen in the gas loop is

then purged with hydrogen followed by opening the gas loop up to the reactor.

Additional hydrogen was added to the system to compensate any losses from

reaction. Samples were taken of the liquid phase at regular intervals and stored for

later analysis. The reaction is terminated by turning off the pump and hydrogen

supply and purging the system with nitrogen.

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Figure 3.6: Representation of different column packing placement used for testing catalyst

4.2.1 Mesoporous Silica Catalyst Support

Each batch of viscous gel from the reactor was rolled into spheres with a diameter

of 0.5 to 1 cm. Half of these spheres were randomly selected to have fibers inserted

in them. Figure 4.34 shows a sample of the dried spheres before calcination. After

calcination the spheres decreased to about half of their original size mostly due to the

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loss of entrapped water. Figure 4.35 shows a group of pellets after calcinations. As it

can be seen the pellets have an outside diameter ranging from 3 to 5 mm. Calcination

was used to remove the CTAB template from the silica spheres leaving a porous

catalyst support. The surface area of the calcined pellets was determined using

nitrogen adsorption and the Brunauer Emmett Teller (BET) equation. It was

determined to be 530 m2/g. The surface area was measured again after the test

reaction and found to be 330 m2/g. This shows the unstable porosity of the

mesoporous silica. An optical microscope with a digital camera attached was used to

take a photograph of the surface of one of the pellets with macropores in it. This can

be seen in Figure 4.36. The photograph was calibrated and a length scale was

attached. Figure 4.36 shows three macropores on the surface of the pellet. The

diameter of these pores ranges from about 0.2 to about 0.4 mm. This shows that the

pore size in the range that enhancement is expected. Only the macropores can be

seen since the mesopores are too small to be viewed with a standard optical

microscope.

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(a) (b)

Figure 4.34: Dried silica spheres used as catalyst support (a) spheres with no fibers inserted (b) spheres with sacrificial fibers inserted to make the macropores

Figure 4.35: Mesoporous silica catalyst support after calcinations

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Figure 4.36: Magnification of the surface of a macroporous silica catalyst support

4.2.2 Platinum Impregnation of Mesoporous Silica Catalyst Support

After calcination, the catalyst was stored under nitrogen until enough was

calcined to run eight reactions (2 each of macropore pulsing, macropore trickling, no

macropore pulsing, and no macropore trickling). Then each group was impregnated

and reduced together. This was stored under nitrogen until it was used for the test

reactions.

Platinum dispersion was measured using hydrogen adsorption. Microbursts of

hydrogen were injected into the sample and the exit concentrations of hydrogen were

measured. This was performed on a random sample twice for reproducibility. The

platinum dispersion was measured at 2.31%. This shows that there are large platinum

crystals on the surface and that much of the platinum on the catalyst was unavailable

for reaction. Even though there was a low platinum dispersion the reaction did

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commence and conversions of up to 50% were accomplished within a reasonable

time. These results will be presented in the following section.

Findings

Support for the catalyst was made out of mesoporous silica. Spherical pellets of

approximately 5 mm to 10 mm diameter were molded out of a viscous gel of

mesoporous silica. Half of the pellets had cellulose fibers inserted in them. After

calcinations the pellets shrunk to about 3 mm to 5 mm in diameter. The loss in size

was mostly due to the loss of water from the pellet. The cellulose fibers left

macropores on the order of 0.2 mm. The surface are of the pellets was 530 m2/g after

calcination but measured 330 m2/g after the test reactions. Platinum was used as the

active material and was placed on the support using incipient wetness. The dispersion

of platinum was measured at 2.31%.

Test of Catalyst Using the Hydrogenation of Phenylacetylene

For each of the four test reactions run, dimensionless concentration profiles as a

function of time were made, where the dimensionless concentration was equal to the

component concentration divided by the initial phenylacetylene concentration. From

the decay of phenylacetylene concentration the initial rates of reaction can be

calculated and compared for each of the four cases. Figure 4.37 shows the

concentration profile for the first reaction run, no macropores in the trickling regime.

This reaction had the best performance of all four test reactions and reached a

conversion of almost 50% in 36 minutes. The reaction results for catalyst with no

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macropores in the pulsing regime can be seen in Figure 4.38. This figure shows an

obvious decrease in reaction performance over the same time period compared to the

trickling reaction. This result is unexpected and was the first sign that the catalyst

used may not be reliable for experimentation. Figure 4.39 shows the reaction using

catalyst with macropores in the trickling flow regime. This reaction is supposed to be

comparable to the other trickling reaction but as it can be seen it more closely

compares to the pulsing reaction run with non-macroporous catalyst. The results of

the fourth reaction can be seen in Figure 4.40. This reaction was run with

macroporous catalyst in the pulsing regime. Large enhancements in reaction rate

were expected but not seen. In fact, this reaction again shows a poorer performance

for the pulsing regime compared to the trickling regime. Degradation of the catalyst

over time could be the major cause behind this type of behavior.

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Figure 4.37: Reaction concentration profile as a function of time for non-macroporous catalyst placed in the trickling flow regime

Figure 4.38: Reaction concentration profile as a function of time for non-macroporous catalyst placed in the pulsing flow regime

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Figure 4.39: Reaction concentration profile as a function of time for macroporous catalyst placed in the trickling flow regime

Figure 4.40: Reaction concentration profile as a function of time for macroporous catalyst placed in the pulsing flow regime

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A comparison of the initial rates of reaction for each of the four experiments can

be seen in Figure 4.41. This shows the concentration profiles for phenylacetylene

with time. It gives a clearer picture of the unpredictable reaction performance of the

catalyst. Table 4.3 shows the initial rates of reaction for each of the four reactions

run. The initial rate of reaction was calculated by fitting a line to the data to get the

rate of decay of phenylacetylene as a function of time.

Figure 4.41: The decay of phenylacetylene as a function of time for each of the four reactions

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Table 4.3: Initial rates of reaction for each of the four test reactions

Packing Set Up Initial Rate of Reaction, mol/(L min)No Macropores, Trickling 4.84 * 10-4

No Macropores, Pulsing 3.16 * 10-4

Macropores, Trickling 2.92 * 10-4

Macropores, Pulsing 2.72 * 10-4

Although the catalyst used for the tests was able to achieve up to 50% conversion

within 1 hour, overall it showed unreliable performance. The trickling regime always

showed increased initial reaction rates when compared to the pulsing regime. The

lowest initial rate of reaction was 0.000272 mol/(L min) and was for the macroporous

catalyst in the pulsing regime. The highest initial rate was seen for the non-

macroporous catalyst run in the trickling regime, it was 0.000484 mol/(L min).

Discussion

A novel macroporous catalyst was created to take advantage of the pressure

fluctuations in the pulsing flow regime. This catalyst uses the pressure fluctuations to

reduce the mass transfer resistance to its internal active sites. It was expected to

exhibit increased reaction rates when compared to a non-macroporous catalyst made

of the same material but it did not.

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Two problems could have contributed to the situation. First a lot of

experimentation went into making a gel phase that could be formed into cm size

pellets and into which fibers could be inserted. Further it was necessary that the

pellets which result after calcinations have sufficient mechanical stability to stand up

to reaction conditions. We were able to accomplish these goals. Unfortunately this

was at the expense of much of the meso-porosity. The low dispersion of Pt indicates

that the final product was not a very active catalyst. A second problem could have

been that the pressure fluctuations that occurred in the pulsing regime were not strong

enough to cause the necessary oscillatory flow within the pellets to achieve the

reaction enhancements. So while no reaction enhancement occurred, we are not

certain that it would not occur in a much larger reactor (perhaps 10 cm in diameter

instead of 2 cm) which could generate larger pressure fluctuations. Unfortunately,

making the catalyst is very labor intensive and producing enough for our small

reactor was already difficult.

References:

Nooney, R.I., M. Kalyanaraman, G. Kennedy, and E.J. Maginn, “Heavy MetalRemediation Using Functionalized Mesoporous Silicas with ControlledMacrostructure,” Langmuir, 17: 528-533, 2001.

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APPENDIX

A.1 Determination of Proper Pore Size

For good enhancement the dimensionless frequency (b) needs to be greater than orequal to 10 (Leighton and McCready Figure 2, 1988)

2/1

˜¯

ˆÁË

Ê=D

aw

b (A.1)

mPoreRadiusa ,=

pw 2/, == sradiansFrequency (corresponds to 1 Hz, typical value in reactor used)

smsmyDiffusivitD /1054.2/, 292 -¥== (Wilhite et al., 2001)

2/1

˙̊˘

ÍÎ

È=

D

aw

b (A.2)

2/1

91054.2

2

10

˙̊˘

ÍÎ

È¥

=

-

pa (A.3)

000201.0=a m = 0.2 mm (A.4)

It should be noted that pore sizes larger than this will also experience transportenhancement but due to the particle size pore sizes around 0.2 mm are the best choice.

A.2 Catalyst Support Reaction Results

The following table shows general results for the variety of attempts to make thecatalyst support. For each attempt the procedure explained in CHAPTER 3, Section3.6 was used.

Tables A.1-A.30: Results for various attempts to make catalyst support sorted by date

Parameter Value Parameter Value

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H2O (g) 250.10 Temperature (oC) 18CTAB (g) 4.50 Stir Rate (rpm) 200NaOH (g) 19.31 pH 12.13TBOS (g) 29.03 Reaction Time (hr) 24

Fibers Added (g) 1.16 Date 11/30/01Results: Too many fibers, light coating formed in needles around fibers

Parameter Value Parameter ValueH2O (g) 250.02 Temperature (oC) 25

CTAB (g) 4.50 Stir Rate (rpm) 275NaOH (g) 19.29 pH 12.20TBOS (g) 28.87 Reaction Time (hr) 24

Fibers Added (g) 0 Date 12/18/01Results: Gel like, did not hold together



Fibers Added (g) 0 Date 1/16/02Results: Wax like material stuck on side of reactor, low yield of irregularly shapedparticles (2-3 cm), very brittle after drying



Fibers Added (g) 0 Date 1/17/02Results: Total yield was wax like structure stuck to the side of the reactor, driedvery brittle


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Fibers Added (g) 0 Date 1/20/02Results: Wide size range of irregularly shaped particles (1 mm – 1 cm), disk like,dried hard



Fibers Added (g) 0 Date 1/27/02Results: Low yield of irregularly shaped spheres (~ 1 cm diameter), mostly gelstuck to reactor, brittle upon drying



Fibers Added (g) 0 Date 1/28/02Results: Wide size range of irregularly shaped particles (1 mm – 1 cm), disk like,dried hard



Fibers Added (g) 0 Date 2/2/02Results: Sludge, some stuck to walls, breaks apart upon drying


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Fibers Added (g) 0 Date 2/3/02Results: Large ball of sludge, much like the sludge used to make the support usedfor experimentation but its use was not realized at the time



Fibers Added (g) 0 2/5/02Results: Extremely low yield of large spheres ~ 1.5 cm diameter



Fibers Added (g) 0 Date 2/6/02Results: Spheres of approximately 3 mm diameter, stuck together as soon as pulledout of solution



Fibers Added (g) 0 Date 2/9/02Results: Attempt to reproduce spheres of 2/6/02 and try to separate them, stucktogether could not be separated


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Fibers Added (g) 0 2/12/02Results: Large amount of viscous sludge



Fibers Added (g) 0 Date 2/15/02Results: Clay-like sludge, stuck to side of reactor



Fibers Added (g) 0 2/16/02Results: Low yield, all stuck to sides of reactor, powder-like



Fibers Added (g) 0 Date 3/27/02Results: Viscous gel, first attempt at molding, did not seem hard enough to use inreactor


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Fibers Added (g) 0 Date 3/29/02Results: Viscous gel, very brittle after drying



Fibers Added (g) 0 4/2/02Results: Approximately 3 mm diameter spheres, dried very hard, shrunk upondrying and calcinations, lost over 1/2 of initial size



Fibers Added (g) 0 Date 4/9/02Results: Large diameter particle, upwards of 1 cm diameter upon drying, much sizevariance, brittle after drying



Fibers Added (g) 0 3/29/02Results: Viscous gel, very brittle after drying


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Fibers Added (g) 0 Date 4/18/02Results: Attempt to make spheres that will be large enough after drying, a uselesslow viscosity gel that could not be separated from solution



Fibers Added (g) 0.08 Date 4/23/02Results: Attempt to get fibers in spheres, some spheres formed with fibersentrapped, became extremely brittle after calcination



Fibers Added (g) 0.11 Date 4/24/02Results: Powdery gel with fibers in it, could not be molded, dried extremely brittle



Fibers Added (g) 0.11 Date 4/30/02Results: Extremely small spheres with no fibers entrapped, fibers in bottom ofreactor with small amounts of gel on them


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Fibers Added (g) 0.10 Date 5/7/02Results: Mostly gel, a few irregularly shaped particles, dried extremely brittle



Fibers Added (g) 0.10 5/9/02Results: Crystal like low viscosity gel, could not be separated from solution



Fibers Added (g) 0.12 Date 5/7/02Results: Viscous gel with fibers entrapped, molded, very brittle after calcination



Fibers Added (g) 0.12 Date 5/15/02Results: Lower viscosity gel than 5/7/02, again extremely brittle after calcination


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Fibers Added (g) 0 Date 5/21/02Results: Attempt to see if leaving fibers out makes a stronger material but stillbrittle after calcinations



Fibers Added (g) 0.10 Date 5/7/02Results: Viscous gel, molded, attempted to use aluminum wire as support to addstrength, still brittle after calcinations

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