3D Printing of Zeolite-Cellulose NanofibrilComposites

Henny Hauggaardh.hauggaard@gmail.com

under the direction ofProf. Lennart BergströmPh.D. Neda Keshavarzi

Department of Materials and Environmental ChemistryStockholm University

Research Academy for Young ScientistsJuly 10, 2014

Abstract

The aim of the study was to investigate whether a 3D printed composite of cellulose

nanofibrils and zeolite 13X, in the form of a self-standing film, has potential to replace

today’s inefficient gas adsorbents. The method included creating a cellulose and zeolite

dispersion, printing the film, a SEM analysis, a BET test and a CO2 adsorption test. The

tests were performed using the composite and zeolite 13X powder as samples. The results

indicated that the composite was surpassingly homogeneous, however additional mixing

or the use of smaller zeolite particles could improve this aspect. The results also showed

that the composite had a relatively large surface area and high CO2 uptake even after

processing and 3D printing. Further research could include breakthrough measurements,

that examine the diffusion path, cycle time, pressure drop and mass transfer of the sys-

tem. The results of this paper together with the breakthrough measurements could have

significant implications, regarding energy consumption, when choosing an adsorbent for

a gas separation system.

Contents

1 Introduction 1

1.1 Zeolite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Gas Separation Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.3 Cellulose Nanofibrils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.4 Cross linking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.5 3D Printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.6 The Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Method 6

2.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2 Dispersion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.3 Method Characterisation . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3 Results 9

3.1 Rheology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.2 Scanning Electron Microscopy . . . . . . . . . . . . . . . . . . . . . . . . 9

3.3 BET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.4 CO2 Capture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4 Discussion 12

4.1 Rheology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.2 Scanning Electron Microscopy . . . . . . . . . . . . . . . . . . . . . . . . 12

4.3 BET and CO2 Capture . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5 Conclusion 13

1 Introduction

The world’s main energy resources are crude oil, coal and gas. The question “When will

fossil fuels reserves be diminished?” has been asked many times. A study carried out by

Shafiee, S. et al. has shown that coal reserves are available up to 2112, and will be the only

fossil fuel remaining after 2042 [1]. When fossil fuels are burned to obtain energy, carbon

dioxide (CO2) is produced instantaneously [2]. Due to human activity, the concentration

of carbon dioxide in the atmosphere has increased, which is generally believed to increase

the temperature of the Earth [4]. It is widely feared that future generations will suffer

unless today’s scientists manage to devise a means of reducing the release of carbon

dioxide into the atmosphere. The capture of carbon dioxide from large point sources, like

power plants, is one possible way to decrease carbon dioxide emissions. However, the CO2

capture alone currently heightens a plant’s energy consumption by 25–40% [5].

1.1 Zeolite

Zeolites are inorganic crystalline structures with multidimensional channel systems, that

can be found naturally. They consist of tetrahedrally coordinated atoms, such as alu-

minium or silicon, that are bound to oxygen atoms. The aluminium atom contributes

with a negative charge to the framework, therefore cations such as Na+, K+ or Ca2+, are

required to balance the total charge. Zeolites yield a strong electrical field gradient due

to the negatively charged framework and charge balancing cations. Electrostatic inter-

actions between the gas molecules and the electrical field gradient of the zeolite is the

driving force for CO2 uptake. The CO2 molecule has a quadrupole moment, leading to

uneven charge distribution for short periods of time, comparable to van der Waals forces.

When one side is slightly positive, an attraction between the positive charge on the gas

molecule and electrical field gradient of the zeolite will occur. Zeolites also have high

porosity, meaning that they have a large surface area that can accommodate high capaci-

ties of CO2 [2]. Some zeolites are highly hydrophilic due to the presence of aluminium and

1

exchangeable metal cations, this is another disadvantage since a zeolite molecule that has

bound water has a lower uptake of CO2 [2]. In this study zeolite 13X is used, see Figure

1. The chemical formula for zeolite 13X is Na86[(AlO2)86(SiO2)106]. 13X represents that

the pore size is 8Å [3].

Figure 1: The structure of zeolite 13X.1

1.2 Gas Separation Systems

One method for separation of gas mixtures is using a solid sorbent to adsorb a specific

gas molecule. This can be done by pressure swing adsorption (PSA), which depends on

temperature, pressure, surface forces, surface area and pore size [6]. When the pressure

is increased, the gas molecules are adsorbed whereas when the pressure is decreased,

desorption commences and the gas is desorbed from the sorbent so it can be reused [7].

Aside from PSA, there is a method called temperature swing adsorption (TSA), which

exploits the fact that adsorption is exothermic. During TSA, when the gas molecules

have been adsorbed, the temperature can be increased so that the adsorbed gas can

desorb from the zeolite surface so that the adsorbent becomes reusable. There are physical

parameters that affect the efficiency and energy consumption of the separation of gas

mixtures, such as volumetric capacity, pressure drop, mass transfer characteristics and1Xinyuan Molecular Sieve, http://www.molecularsieve.org/Zeolite_Molecular_Sieve.htm

2

thermal management. The structure of the adsorbent controls these parameters. Present

gas separation systems use adsorbents like zeolite in the form of beads or granules packed

in beds, i.e. tablets. A disadvantage when using tablets, is that the tablet wall is thick

which increases the diffusion path [8]. Tablets only containing zeolite are weak therefore, a

binder must be added to strengthen the structure. The binders that are used today include

aluminium oxide, silicon dioxide or clay, which are inorganic and nonporous particles.

There are drawbacks when using non-zeolitic binders, because they are occupying the

efficient volume in the tablet. In other words, the binder is reducing CO2 uptake capacity

and volume efficiency of the adsorbent [9].

1.3 Cellulose Nanofibrils

Cellulose is the planet’s most abundant natural polymer. It is composed of unbranched

polymer chains of β-glucose. There is a hierarchical structure of cellulose because of the

hydrogen bond network, which leads to the extraordinary properties of natural composite

materials like wood, cotton, flax and hemp [10]. Nanoparticles can be extracted from cel-

lulose, by TEMPO-oxidation, to be utilised in composites. The individual nanofibres are

3–4 nm wide and are several microns in length. Cellulose nanocrystals (CNC), cellulose

nanofibrils (CNF) and bacterial nanocellulose (BNC) are different forms of nanocellu-

lose that can be produced [11]. Figure 2 shows nanofibres forming a larger bundle of

20 nm in diameter. Nanocellulose has many attractive mechanical properties like abun-

dance, low density, biodegradability [12], high crystallinity and high aspect ratios [11].

Futhermore, cellulose is both inexpensive and nontoxic [13]. However, there are some dis-

advantages with nanocellulose, such as the lack of satisfactory interfacial adhesion, low

melting point and water sensitivity. To avoid these undesirable properties, the surface

of cellulose nanofibrils can be pretreated to reduce moisture absorption and increase the

surface roughness [13].

The use of cellulose nanofibrils for the creation of sustainable functional materials or

in polymer reinforcement is a relatively novel practice. The reason why CNF is a such

3

Figure 2: Photo of cellulose nanofibrils using a scanning electron microscope.

an adequate candidate to use in composites, is because of the fibrils high rigidity level.

Nanocellulose can be utilised in a large number of fields, such as in paper and packaging

products, construction, automotive, furniture, electronics, pharmacy and cosmetics [13].

1.4 Cross linking

The definition of cross linking is when a small area in a macromolecule interacts with

other existing macromolecules, where the cross link is predominantly a covalent structure

[14]. Cross linkers are used to bond polymer chains to one another, among many other

applications. Polymer chains lose part of their free movement when they are transformed

from a liquid polymer to a solid or gel [15]. The cross linker used in this study was CaCl2.

Calcium ions are positively charged and they will attract the negatively charged cellulose

and the negatively charged zeolite, forcing the surfaces to link. It is important that the

dispersion gels in the presence of Ca2+, since a high viscosity is required to ensure that

the dispersion has the right properties for being printed.

1.5 3D Printing

3D printing, also known as additive manufacturing, creates 3D solid objects of virtually

any shape from a digital model. 3D printing has existed for 30 years, but during the last

4

decade the printer has been developed significantly and can be bought much cheaper.

The 3D printer may revolutionise several fields of human activity. An example is the

development of 3D printed implants that precisely match the patient [16].

Using a 3D printer for creating composites of nanocellulose and zeolite for carbon

dioxide capture is something that has never been tested before. The following question

may arise: why use a 3D printer to print a self-standing film instead of using traditional

methods, such as tape casting or coating? The 3D printer is very accurate as it has the

ability to control the architecture of each layer that is printed. Porous scaffold structures

can be built precisely with a layer-by-layer method, meanwhile tape casting and coating

have difficulty producing specific structures [17].

1.6 The Purpose

Gas separation systems of today still need to be optimised since the inorganic binders

used in zeolites and the structure of the sorbent, leads to reduced CO2 uptake. Therefore

a printed composite of zeolite and nanocellulose could potentially improve the system

efficiency. Nanocellulose would act as an organic and porous binder and it would con-

tribute with many advantageous mechanical properties. If the composite is printed in

the form of a thin self-standing film with a special structure, the diffusion path would

be much shorter than it is in a tablet of zeolite. The self-standing film could potentially

lead to a lower pressure drop and a better mass transfer. The aim of this paper was to

print a structured composite of zeolite and cellulose nanofibrils using a 3D printer. The

surface area and carbon dioxide adsorption capacity of the composite and zeolite powder

were measured, respectively. This might determine the composite’s potential to replace

inefficient adsorbents used in today’s gas separation systems.

5

2 Method

2.1 Materials

Commercially available zeolite powder 13X with the particle size of ≤2 µm was used.

CNF prepared by TEMPO-oxidation of wood fibre (1 wt%) at KTH Royal Institute of

Technology according to the method presented in the paper TEMPO-oxidized cellulose

nanofibers written by Isogai, A. et al. [11]. Polyethylene glycol was used to increase the

flexibility of the composite and calcium chloride was used as a cross linker.

2.2 Dispersion

The following method was developed by Grégoire Vauché at the Department of Materials

and Environmental Chemistry at Stockholm University during the spring of 2014. The

method was optimised, taking the rheological properties into account, to fit the universal

paste extruder. The ratio between zeolite and cellulose has been determined to 90 w% to

10 w%.

A solution of 17.64 g CNF gel and 20ml Millipore water was mixed with a magnetic

stirrer for 15 minutes. The beaker with the solution was then placed in a bath for ultra-

sound sonification for 30 minutes. 1.50 g of zeolite 13X was added to the cellulose and

was stirred with a magnetic stirrer. 0.03 g of polyethylene glycol (PEG) and 0.44 g g of

CaCl2 was weighed and mixed with 5ml of Millipore water until homogeneity was at-

tained. When the mixture of PEG and CaCl2 and the mixture of zeolite and cellulose

were homogeneous, the two were mixed with each other. A volume of 10ml Millipore

water was added and the dispersion was mixed for 10 min. The last step was done prior

to 3D printer.

6

2.3 Method Characterisation

For this experiment, a commercial 3D printer with a so called "universal paste extruder"

was used, see Figure 3. Self-standing films, with the dimensions of 20 cm × 15 cm, were

printed directly on the 3D printer’s plate. A program was used to control the 3D printer,

which had the ability to maintain the appropriate extrusion temperature, extrusion speed

and the distance between the head of the extrusion and the plate. The self-standing film

is designed to have a special scaffold structure, in order for the film to undergo fast uptake

kinetics. The printed film was left at room temperature until all water evaporated, which

took approximately 24 hours.

Figure 3: A commercial 3D printer and a printed self standing film of the composite.

In order to establish that the dispersion had the right viscosity, before printing, the

rheological properties of the dispersion were tested with the rheometer Paar Physica

MCR301. The measurements were performed in concentric cylinder geometry at 25 ◦C

over a shear rate from 1–4000 s−1.

7

Scanning electron microscopy (SEM) was used to observe the structural morphology

of the 3D printed composites. A field-emission gun scanning electron microscope (FEG-

SEM), JSM-7000F was used. It was operated at an acceleration voltage of 2 kV and had

a working distance of 10mm and 6mm.

The specific surface area was calculated with help of the BET model, which was cre-

ated by Brunauer, Emmet and Teller. The calculation is carried out by physical adsorption

of N2 on the surface of the sample. The experiment is performed at the temperature of

liquid nitrogen 77.4K. An adsorption isotherm was plotted and represented the equi-

librium adsorption states at the relative pressure, which is the pressure divided by the

vapour pressure for the adsorbate, denoted as p/p0. Microporous solids, like zeolites, have

isotherms that show that at low relative pressures, often below 0.05 p/p0, the amount of

nitrogen adsorbed increases rapidly. This increase represents the micropores being filled.

After the micropores are filled the surface adsorbs the gas molecules in a constant pace

[18]. An Accelerated Surface Area and Porosimetry (ASAP) 2020 analyzer was used to

measure the surface area. CO2 capture was measured by the same adsorption instrument,

ASAP 2020 however, CO2 was used instead of N2 and it was performed at room temper-

ature. The isotherm showed the amount adsorbed as a function of absolute pressure and

the highest value was determined. Before the BET and CO2 capture tests were performed,

the zeolite 13X powder and the composite were degassed. The surfaces of the powder and

composite have impurities, therefore, degassing must occur to remove moisture and cre-

ate a clean surface. The composite was degased at 80 ◦C for 5 hours and the powder was

degased at 300 ◦C for 10 hours. Degassing implies the establishment of a vacuum and the

application of heat to the sample. The reason why the composite was degassed at a lower

temperature and for a shorter period of time, is the risk of destruction of the cellulose

nanofibrils.

8

3 Results

3.1 Rheology

0 500 1000 1500 2000 2500 3000 3500 400010

−2

10−1

100

Shear Rate (s−1)

Visco

sity

(Pa·s)

Figure 4: Rheological properties of the dispersion of cellulose nanofibrils and zeolite 13X.

The rheology test, see Figure 4, shows that the dispersion has a shear thinning be-

haviour. The dispersion has a high viscosity at low pressures and a low viscosity at high

pressures.

3.2 Scanning Electron Microscopy

The particle size of zeolite 13X powder is ≤2 µm according to Figure 5. The structure

of the composite is revealed in Figure 6, and some holes in the cellulose network are

observed. Figure 7 shows that the zeolite particles have a relatively even distribution in

the cellulose network.

9

Figure 5: SEM photo of zeolite 13X particle.

Figure 6: SEM photo of composite with the magnification 10,000×.

Figure 7: SEM photo of composite with the magnification 1,000×.

10

3.3 BET

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

1

2

3

4

5

6

7

8

9

10

11

Relative Pressure (p/p0)

AmountAdsorb

ed(m

mol/g)

Zeolite 13x powder

CNF−13x composite

Figure 8: The N2 adsorption isotherm of zeolite 13X and CNF-13X composite.

The N2 adsorption isotherm for zeolite 13X and the CNF-13X composite is shown in

Figure 8. The BET test provided the following results: the composite has a specific surface

area of 296m2/g and zeolite has a specific surface area of 743m2/g. The composite’s

surface area constitutes 40% of the powder’s surface area.

3.4 CO2 Capture

0 100 200 300 400 500 600 700 8000

1

2

3

4

5

6

7

Absolute Pressure(Torr)

AmountAdsorb

ed(m

mol/g)

Zeolite 13X powder

CNF−13X composite

Figure 9: The CO2 adsorption isotherm of zeolite 13X and CNF-13X composite.

The graph, see Figure 9, shows that zeolite 13X powder has a higher CO2 uptake

than the composite. The samples adsorb a larger amount of CO2 at lower pressures,

11

then the amount adsorbed subsides at higher pressures. Zeolite can adsorb 6.9mmol/g

meanwhile the composite can adsorb 1.4mmol/g, which means that the composite adsorbs

approximately 1/5 as much CO2 as zeolite.

4 Discussion

4.1 Rheology

The rheology test, see Figure 4, shows that the dispersion has a shear thinning behaviour.

This behaviour is beneficial because, when the dispersion is located in the syringe and

no pressure is applied, it has a high viscosity preventing leakages from the syringe. When

pressure is applied, the dispersion has a lower viscosity and will easily be extruded from

the tip of the syringe. The results of the rheology test show that the dispersion has the

right requirements for 3D printing, which guarantees that the structure of the film is

precise.

4.2 Scanning Electron Microscopy

The SEM photo, Figure 6, shows that the cellulose network is destroyed in some areas,

which may have either been caused by mechanical destruction or a too big zeolite particle

size. Figure 7 shows that the composite is relatively homogeneous, however there are

tendencies to an uneven distribution of zeolite. One way in which the method could be

improved is to mix the dispersion more thoroughly to ensure that it is homogeneous.

Another possible improvement would be to use smaller-sized zeolite particles in order to

increase the homogeneity of the zeolite-cellulose network.

4.3 BET and CO2 Capture

The BET test reveals that zeolite 13X has more than twice the surface area than the com-

posite, and that the composite adsorbs much less N2 in comparison to zeolite powder. The

12

results show, see Figure 9, that zeolite has a much higher CO2 uptake than the composite.

The given values may be due to two factors, one might be that the composite was only

degassed at 80 ◦C for 5 hours to prevent the destruction of the cellulose, therefore there

may still have been moisture on the surface which reduces adsorption. An alternative is

that the cellulose blocks some zeolite pores which decreases the surface area of zeolite,

thus decreasing CO2 capacity. It is hard to compare the surface area and CO2 capture

of the composite with other adsorbents because they have been degassed with different

conditions. However, 296 m2/g is a relatively large surface area and 1.4mmol/g is a rel-

atively high CO2 adsorption. The BET and CO2 capture test show that the composite

nonetheless has its adsorptions properties after processing and 3D printing. An improve-

ment is to alter the conditions when degassing, to reduce the amount of impurities on the

surface of the composite without destroying the cellulose. The degassing process for the

composite could be performed by having the same temperature, 80 ◦C, but applying more

vacuum for a longer period of time than 5 hours. It is clear that the degassing process is a

limitation within the method, as it has large affects on the results. However, the ultimate

goal is for the composite to attain the same surface area and CO2 adsorption as zeolite.

5 Conclusion

A composite of cellulose nanofibrils and zeolite 13X has been successfully created with a

3D printer in the form of a self standing film. The BET and CO2 capture test show that

even after processing and 3D printing the composite has a large surface area and high

CO2 uptake. Even though cellulose has its drawbacks when used as a binder, cellulose

can contribute with many attractive mechanical properties which other inorganic binders

cannot. In theory, the composite of cellulose and zeolite printed with a 3D printer is more

efficient than using zeolite tablets in gas separation systems, however break through tests

must be performed to further prove this. Breakthrough tests measure the diffusion path,

cycle time, pressure drop and mass transfer. The results of this paper together with the

13

breakthrough measurements could have significant implications when choosing a binder

for zeolite and a specific structure to improve system efficiency. The composite with this

structure will be attractive on the market, since it is better for the environment compared

to other sorbents. A potential problem when using a composite of zeolite and cellulose,

is that the components are water sensitive. To avoid water capture and the reduction

of CO2 capture, another hydrophilic adsorbent could be used first to separate the water

vapour in the gas mixture.

Acknowledgements

I would like to show my appreciation to Lennart Bergström for giving me the opportunity

to work on such an interesting project. My mentor Neda Keshavarzi who has been very

helpful and so kind to me during my time at MMK. Gregroire Vauche for teaching how the

3D printer works. I would like to extend my gratitude to Stockholm university and MMK

for the chance to be exposed to an interesting research field. Thank you to everyone

at the department for helping me with my work as well as providing me with insight

of what life is like as a researcher. Many thanks to the Research Academy for Young

Scientists, the counsellors, Mariam Andersson, Agnes Nordquist, Dennis Alp and the

director Philip Frick. Last but not least, thanks to ABB, Teknikföretagen and Kungliga

Patriotiska Sällskapet for the opportunity to spend my summer in the best way possible,

researching!

14

References[1] Shafiee, S., Topal, E., When will fossil fuel reserves be diminished?, Energy Policy,

37(1), 2009, P. 181-189.

[2] Cheung, O., Narrow-pore zeolites and zeolite-like adsorbents for CO2 separation,Stockholm University, 2014, P. 19, 31-34.

[3] Magee, H.M., Sullivan, N.S., Nitrogen Gas Adsorption in Zeolites 13X and 5A, WallaWalla University.

[4] Vitousek, P. M., Beyond Global Warming: Ecology and Global Change, Ecology,75(7), 1994, P. 1861-1876.

[5] D’Alessandra, D.M., Smit, B., Long, J.R., Carbon Dioxide Capture: Prospects forNew Materials, Angewandte Chemie, 49(35), 2010, P. 6058-6082.

[6] Aaron, D., Tsouris, C., Separation of CO2 from Flue Gas: A Review, SeparationScience and Technology, 40(1-3), 2005, P. 321-348.

[7] Siriwardane, R., Shen, M., Fisher, E., Poston, J., Shamsi, A., Adsorption and Des-orption of CO2 on Solid Sorbents, U.S. Department of Energy, National EnergyTechnology Laboratory.

[8] Rezaei, F., Webley, P., Structured adsorbents in gas separation systems, Separationand Purification Technology, 70(3), 2010, P. 243-256.

[9] Akhtar, F., Bergström, L., Colloidal Processing and Thermal Treatment of BinderlessHierarchically Porous Zeolite 13X Monoliths for CO2 capture, The American CeramicSociety Journal, 94(1), 2011, P. 92-98.

[10] Klemm, D., Heublein, B., Fink, H., Bohn, A., Cellulose: Fascinating Biopolymer andSustainable Raw Material, Angewandte Chemie, 44, 2005, P. 3358-3393.

[11] Isogai, A., Saito, T., Fukuzumi, H., TEMPO-oxidized cellulose nanofibers, Nanoscale,3(71), 2011.

[12] Dufresne, A., Nanocellulose: a new ageless bionanomaterial, Materials today, 16(6),2013, P. 220-227.

[13] Kalia, S., Dufresne, A., Cherian, B.M., Kaith, B.S., Avérous, L., Njuguna, J., Nas-siopoulos, E., Cellulose-Based Bio- and Nanocomposites: A Review. InternationalJournal of Polymer Science, 2011, 2011, P. 1–35.

[14] Jenkins, A.D., Stepto, R.F.T., Glossary of Basic Terms in Polymer Science, Inter-national Union of Pure and Applied Chemistry, Pure & Applied Chemistry, 68(12),1996, P. 2287–2311.

[15] Moro, D.F., Guglielmi, M., Salazar-Alvarez, G., Wicklein, B., BionanocompositeMembranes Based on Cellulose Nanofibrils: Effects of Graphene Oxide and CalciumIons, Stockholm university, 2014.

15

[16] Chen, T-H., Lee, A., Flood, A-H., Miljanic, O., How to print a crystal structuremodel in 3D. CrystEngComm, 16, 2014, P. 5488–5493.

[17] Wu, C., Fan, W., Zhou, Y, Lou, Y., Gelinsky, M., Chang, J, Xiao, Y., 3D-printing ofhighly uniform CaSiO3 ceramic scaffolds: preparation, characterization and in vivoosteogenesis. Journal of Materials Chemistry, 22, 2012, P. 12288–12295.

[18] Cejka, J., van Bekkum, H., Corma, A., Schüth, F., Introduction to Zeolite Scienceand Practice. 3rd revided edition. Studies in Surface Science and Catalysis, 168,2007, P. 555–587.

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