zeolites and negative thermal expansionntpl.me.cmu.edu/documents/madhurtalk.pdf · zeolites and...

Zeolites and Negative Thermal Expansion

Madhur Paharia

Carnegie Mellon UniversityDepartment of Mechanical Engineering

Nanoscale Transport Phenomena Laboratory

Outline

MotivationOverview of ZeolitesData CollectionResults and AnalysisComparison to other ResearchZeolites Summary

Motivation

To better understand the reason for a negative coefficient of thermal expansion in certain zeolite structures.Negative Thermal Expansion: structure contracts upon heating.

Brief Overview of ProjectZeolites are made up of one silicon and two oxygen atoms. Frameworks may contain linked cages, cavities or channels. These gaps in the structure are used as filters for separating molecules of different sizes, shapes, and polarity. This process is called molecular sieving.Some zeolite applications include things like generating medical grade oxygen, water purification, and laundry detergent.

More Overview

-3.98E-06RHO

-3.24E-06Faujasite (FAU)

-4.98E-06Zeolite-A (LTA)

2.26E-05Sodalite (SOD)

Coefficient of Thermal Expansion

(1/K)

Structure

Interested in four different zeolite structures. Sodalite (SOD), Zeolite-A (LTA), Faujasite (FAU), RHO. Used pure zeolite structures for data collection and analysis.Sodalite has a positive coefficient of thermal expansion, and the other three zeolite structures all show negative thermal expansion. The reason behind this behavior is not properly understood.

Zeolite Structure PropertiesDistinct Oxygen Positions

SOD – OneLTA – ThreeFAU – FourRHO – Two

Secondary Building Units (SBU’s)Cage and Bridge

SOD LTA FAU RHO

Data CollectionSi-O Bond distance (for each distinct oxygen position)O-Si-O Bond anglesSi-O-Si Bond anglesSi-Si Bond distance

Si-O BondlengthsAverage Bondlength

1.14

1.145

1.15

1.155

1.16

1.165

1.17

150 250 350 450 550 650

Temperature (K)

Bon

dlen

gth

()

SOD DynamicLTA Dynamic O1 (O)LTA Dynamic O2 (B)LTA Dynamic O3 (N)FAU Dynamic O1 (C)FAU Dynamic O2 (O)FAU Dynamic O3 (N)FAU Dynamic O4 (B)RHO Dynamic O1 (O)RHO Dynamic O2 (B)

Si-O Bond distance shows a trend of increasing with temperature in all of the oxygen positions.The rate of change of the bond distance increases as the initial bond distance is increased

dBondlength/Temperature

5.00E-06

5.20E-06

5.40E-06

5.60E-06

5.80E-06

6.00E-06

6.20E-06

6.40E-06

6.60E-06

6.80E-06

1.145 1.15 1.155 1.16 1.165Bondlength()

1/B

*dB

/dT

Dynamic SODDynamic RHO O1 (O)Dynamic RHO O2 (B)Dynamic LTA O1 (O)Dynamic LTA O2 (B)Dynamic LTA O3 (N)Dynamic FAU O1 (C)Dynamic FAU O2 (O)Dynamic FAU O3 (N)Dynamic FAU O4 (B)

Silicon AnglesAverage Si Angles

105

106

107

108

109

110

111

112

113

114

115

150 250 350 450 550 650Temperature (K)

Ang

le (i

n de

gree

s)

FAU NO (O3-O2)FAU NB (O3-O4)FAU CO (O1-O2)FAU CN (O1-O3)FAU CB (O1-O4)FAU BO (O4-O2)SOD OO (O1-O1)RHO OO (O1-O1)RHO BO (O2-O1)RHO BB (O2-O2)LTA OB (O1-O2)LTA BN (O2-O3)LTA NN (O3-O3)LTA NO (O3-O1)

The silicon angles show almost no change in all four of the zeolites that are of interest.

Implies that the O-Si-O angles has negligible impact on negative thermal expansion.

Oxygen AnglesAverage O Angles

140

142

144

146

148

150

152

154

156

158

160

162

164

166

168

150 250 350 450 550 650Temperature (K)

Ang

le ()

FAU O1 (C)FAU O2 (O)FAU O3 (N)FAU O4 (B)LTA O1 (O)LTA O2 (B)LTA O3 (N)RHO O1 (O)RHO O2 (B)SOD

The oxygen angles in all the structures, except SOD, exhibit a trend of getting smaller as the temperature increases.The slope of the oxygen angles show that the bigger the initial oxygen angle, the bigger the rate of change according to temperature.The Si-O2-Si angle in LTA shows the greatest change in oxygen angles and LTA also has the highest negative coefficient of thermal expansion.

Slope of Oxygen Angles

-4.00E-05

-3.00E-05

-2.00E-05

-1.00E-05

0.00E+00

1.00E-05

2.00E-05

135 140 145 150 155 160 165 170Angle ()

1/A

*Slo

pe o

f Oxy

gen

Ang

les(

)

Dynamic SODDynamic RHO O1 (O)Dynamic RHO O2 (B)Dynamic LTA O1 (O)Dynamic LTA O2 (B)Dynamic LTA O3 (N)Dynamic FAU O1 (C)Dynamic FAU O2 (O)Dynamic FAU O3 (N)Dyanmic FAU O4 (B)

Si-Si Bond DistanceAverage Si-Si Bondlengths

2.18

2.2

2.22

2.24

2.26

2.28

2.3

150 250 350 450 550 650Temperature (K)

Bon

dlen

gth

()

FAU O1(C)FAU O2(O)FAU O3(N)FAU O4(B)SOD RHO O1 (O)RHO O2 (B)LTA O1 (O)LTA O2 (B)LTA O3 (N)

Shows a negative trend in the zeolites with NTE. Is probably the best way to measure the NTE in these zeolite structures.Shows the combined effect of the oxygen angles and the Si-O bond distance.The Bridge oxygen atoms in the structures are the ones with the most negative slope.

Slope of Dynamic Si-Si Bond Distance

-1.00E-05

-8.00E-06

-6.00E-06

-4.00E-06

-2.00E-06

0.00E+00

2.00E-06

4.00E-06

6.00E-06

8.00E-06

2.18 2.2 2.22 2.24 2.26 2.28 2.3Si-Si distance ()

1/di

stan

ce*S

lope

SOD DynamicRHO Dynamic O1 (O)RHO Dynamic O2 (B)LTA Dynamic O1 (O)LTA Dynamic O2 (B)LTA Dynamic O3 (N)FAU Dynamic O1 (C)FAU Dynamic O2 (O)FAU Dynamic O3 (N)FAU Dynamic O4 (B)

Static versus Dynamic

Average Static Bondlength

1.14

1.145

1.15

1.155

1.16

1.165

1.17

150 250 350 450 550 650Temperature (K)

Bon

dlen

gth

()

SOD StaticLTA Static O1 (O)LTA Static O2 (B)LTA Static O3 (N)FAU Static O1 (C)FAU Static O2 (O)FAU Static O3 (N)FAU Static O4 (B)RHO Static O1 (O)RHO Static O2 (B)

Static: Uses an average position of atoms to generate data. Is what is measured experimentally.Dynamic: Uses MD to simulate the actual position of the atoms at every time step. All the data shown before has been dynamic data.The static trends for the Si-O bond length and the oxygen angles show the opposite trends when compared to their dynamic counterparts.Could be the reason for some disagreements within other studies.

Average Static O Angles

140

142

144

146

148

150

152

154

156

158

160

162

164

166

168

150 250 350 450 550 650Temperature (K)

Ang

le (i

n de

gree

s)

FAU O1 (C)FAU O2 (O)FAU O3 (N)FAU O4 (B)SODRHO O1 (O)RHO O2 (B)LTA O1 (O)LTA O2 (B)LTA O3 (N)

Comparisons to other results

Villaescusa et al (2001) Studied negative thermal expansion in pure silica zeolite. Came up with two main reasons for the NTE: Rigid Unit Modes (RUM’s) and transverse vibrations of two-coordinated oxygen atoms.

Similarities: Si-O Bond distance increases with temperature and O-Si-O angles show no significant trend. The Si-Si bond distance decreases.Differences: The Si-O-Si angles don't decrease significantly with temperature. The structure they studied (zeolite IFR) has multiple distinct silicon atoms.

More Other Research

Woodcock et al (1999) discusses the properties of zeolite faujasite and A1PO-17. Suggests that changes in Si-O-Si angles are the driving force for NTE. They also tried to relate the unit cell volume and thermal expansion, which turned out to be inconclusive.

Similarities: The Si-Si bond distance generally decreased with increasing temperature.Differences: O-Si-O angles show a great change. I found the opposite to be true.

Summary of ZeolitesThe Si-O-Si angles decrease in LTA, FAU, and RHO. It increases in SOD.The Si-Si bond distance decreases when NTE is true.The bond distances and silicon bond angles don’t seem to have direct affect on the thermal expansion. The bond distance does have an indirect effect on the Si-Si bond distance.It appears that the NTE trend in zeolites is due to the oxygen angles and that effect can be seen in the Si-Si bond distances.