Post on 05-Feb-2016
description
Silica Surface Modification Reactions: Kinetics, Mechanisms, and Surface Structures
Jonathan BlitzDepartment of Chemistry
Applications of Surface Modified Silicas
• Composites• Adhesives/Sealants• Paints/Coatings• Chromatographic stationary phases• Catalyst supports/Catalysts• Adsorbents
The SiO2 Surface
Si Si Si Si
O O O
Interior
Surface O O O O
H H H H
Diffuse reflectance FTIR spectrum of silica gel
Specular and Diffuse Reflection
Sample
~10% (w/w) SiO2 dispersion in KCl
The Silica SurfaceAn FTIR Perspective
Si
OH
Si
O
Si
O
Si
HO
Si
H
HMDS
(CH3)3SiNHSi(CH3)3
Si
O
H
Si
O
Si Si
O
H
Si
O
H
Si
O
Si
O
Si Si
O
H
Si
O
HSi(CH3)3
HM
DS
Si
O
H
Si
O
Si Si
O
H
Si
O
H
Si
O
H
Si
O
Si Si Si
O
+ H2O(g)
600oC
TiCl4 Reactions
Si(s)-OH + TiCl4
Si(s)-O-TiCl3 + HCl
Aluminum Alkyl Reactions
Si(s)-OH + Al(Et)3 Si(s)-O-Al(Et)2 + C2H6
Si(s)-OH + Al(Et)2Cl Si(s)-O-Al(Et)Cl + C2H6 ?
Si(s)-OH + Al(Et)2Cl Si(s)-O-Al(Et)2 + HCl ?
Another Complication
Silica Gel Type
Al(Et)3(mmol Al/g SiO2)
Al(Et)2Cl(mmol Al/g SiO2)
AlEtCl2 (mmol Al/g SiO2)
AlMe3
(mmol Al/g SiO2)
Unmodified 1.80 1.70 1.96 2.18
600oC 1.20 1.20 1.26 1.40
HMDS 0.57 0.48 1.07 0.73
600oC/HMDS
0.41 0.33 0.57 0.47
Extent of Aluminum Alkyl Reactions
Silica Gel Type
Al(Et)3(mmol Al/g SiO2)
Al(Et)2Cl(mmol Al/g SiO2)
AlEtCl2 (mmol Al/g SiO2)
AlMe3
(mmol Al/g SiO2)
Unmodified 1.80 1.70 1.96 2.18
600oC 1.20 1.20 1.26 1.40
HMDS 0.57 0.48 1.07 0.73
600oC/HMDS
0.41 0.33 0.57 0.47
Extent of Aluminum Alkyl Reactions
Possible Explanation of Analytical Data
AlEtCl2O O O
H H Si(CH3)3
Al(Et)3O O O
H H Si(CH3)3
Et
O O O
Si(CH3)3Al
Al(Et)2ClO O O
H H Si(CH3)3
O O O
Si(CH3)3Al Al
Cl Cl ClCl
O O O
Si(CH3)3Al
Cl
+
Adsorbed or pre-reaction complex
ab initio computational chemistry
Transition state 2
Transition state 1
Adsorbed complex
+
++ Products 2
+ Products 1
-40
-35
-30
-25
-20
-15
-10
-5
0
5
10D
elta
G (k
J/m
ol)
Reactants
Adsorbed Complex
Transition State 1 (-HCl)
Transition State 2 (-CH4)
Si
O
Al
ClH3C
OO
O
H
Cl
Si
O
Al
ClH3C
OO
O
+ HCl
Si
O
Al
OO
O
ClCl
+ CH4
Si
O
HAl
Cl
Cl
H3C
OO
O
SiOH Type IR band position
29Si CPMAS NMR
3745 cm-1 -100 ppm
3660 cm-1 -100 ppm
3745cm-1 -94 ppm
Si O
H
Si O
HO
Si O
H
Si
OH
OH
Time is Money
•Industrial scale synthesis is aided by reaction kinetics information
•Empirical kinetics data can be obtained, fundamental information is much more difficult to come by
1) Heterogeneous system (adsorption vs. reaction rates)
2) Silica surface chemistry is complex (more than one type of reactive group)
HMDS Reaction with silica nanoparticles
unreacted
12 s reacted
1 h reacted
15001750200022502500275030003250350037504000Wavenumber
Abs
orba
nce
(Arb
itrar
y U
nits
)
4h reacted
45h reacted
15001750200022502500275030003250350037504000Wavenumber
Abs
orba
nce
(Arb
itrar
y U
nits
)
Analysis of Kinetics Data
• Knowing the reaction mechanism (determined by ab initio calculation)…
• Knowing the initial starting conditions, including different silanol concentrations…
Differential rate equations for all reactants, transient species, and products obtained.
Numerical integration giving best fit to data provides rate constants for various reactions.
Main Conclusions …
Fast Reaction KineticsSlow Reaction Kinetics
Si Si Si Si
HO OH
O
H
O
H
O
H
Fast Reaction Kinetics
Slow Reaction Kinetics
…obtained indirectly
…Empirical kinetics data can be obtained, fundamental information is much more difficult to come by
1) Heterogeneous system (adsorption vs. reaction rates)
2) Silica surface chemistry is complex (more than one type of reactive group)
Model Compound Solution Studies• Diphenylsilanediol/HMDS reactions
• Silsesquioxane/aminopropylsilane reactions
Si
OH
HMDS
Si
OH
OH
O
Si(CH3)3Si
O
Si(CH3)3
HMDS Si
OH
O
Si(CH3)3
O
Si(CH3)3 Reaction rate = 1.9 x 10-6 Ms-1
Reaction rate = 1.5 x 10-5 Ms-1
Internal standard
Direct measurements of relative silanediol reactivity supports conclusions from silica studies
Si
R O
Si
RO
OH
O
Si
R
O
SiO
Si
O
Si
R
O
R
O
Si
O
Si
OR
O
R
O
R =Si
CH3
CH3
+Hexane
CH3O NH2
Si
R OSi
RO
O
Si
R
OSi
O
Si
O
Si
R
O
RO
SiO
Si
O R
O
R
O
O Si NH2
CH3
CH3
+ CH3OH
Using solution FTIR to monitor SiOH loss provides kinetics data on this reaction…
Silsesquioxane/Aminosilane Studies
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
3650367537003725375037753800
Wavenumber (cm-1)
Abs
orba
nce
1 minute
30 minutes
75 minutes
125 minutes
300 minutes
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Silanol Loss from Aminopropylsilane Reaction in Hexane Solution
0.05
0.1
0.15
0.2
0.25
0.3
0 50 100 150 200 250 300 350
Time (min)
1/[S
iOH
] (m
Mol
ar)
Aminosilane ReactionR2 = 0.996k = 6.27 x 10-4 L/mmol.s
Blank Reaction
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
3100320033003400350036003700380039004000
Wavenumber (cm-1)
Abs
orba
nce
Silsesquioxane/Hexane
Silsesquioxane/THF
Nanoparticulate silica
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
0.08
0.1
0.12
0.14
0.16
0.18
0.2
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
Time (min)
1/[S
iOH
] (m
Mol
ar)
Aminosilane ReactionR2 = 0.997k = 1.06 x 10-5 L/mmol.s
Blank Reaction
Solvent Rate constant 95% Confidence Interval
Hexane 6.27 x 10-4 L/mmol.s ± 3.2 x 10-5 L/mmol.s
THF 1.06 x 10-6 L/mmol.s ± 6.1 x 10-7 L/mmol.s
Reaction Temperature (oC) Rate Constant (x 104 mM/min)
25 5.86
20 7.36
15 9.93
An increase in temperature results in an increased rate constant …
Temperature vs. k -values
0
50
100
150
200
250
300
350
-78 -58 -38 -18 2 22
Temperature (ºC)
Rat
e co
nsta
nt (x
10-5
mM
-1 m
in-1
)
Si
CH3
CH3
+ CH3O NH2Si
R OSi
RO
O
Si
R
OSi
O
Si
O
Si
R
O
RO
SiO
Si
O R
O
R
O
O H
Si
R OSi
RO
O
Si
R
OSi
O
Si
O
Si
R
O
RO
SiO
Si
O R
O
R
O
O H N Si
H
H
CH3
CH3
OCH3
H-bond complex formation is rate limiting at higher temperatures
Si
R OSi
RO
O
Si
R
OSi
O
Si
O
Si
R
O
RO
SiO
Si
O R
O
R
O
O Si NH2
CH3
CH3
+ CH3OH
Si
R OSi
RO
O
Si
R
OSi
O
Si
O
Si
R
O
RO
SiO
Si
O R
O
R
O
O H NH2
SiOCH3
CH3
CH3
Activation energy is rate limiting at lower temperatures
Temperature vs. k -values
0
50
100
150
200
250
300
350
-78 -58 -38 -18 2 22
Temperature (ºC)
Rat
e co
nsta
nt (x
10-5
mM
-1 m
in-1
)
-10
-9.5
-9
-8.5
-8
-7.5
-7
-6.5
-6
0.0044 0.0045 0.0046 0.0047 0.0048 0.0049 0.005 0.0051 0.0052
1/T (K)
ln k
Ea = 45 kJ/mol
Arrhenius Plot
Summary• Surface structures can be controlled by silica pretreatments prior to chemical modification
• Computational studies provide insight into mechanisms and driving forces
• Kinetics data can be obtained on silica – but it is difficult and the information is indirect
• Model solution studies provide confirmation and additional information otherwise unobtainable
Acknowledgments
Funding:
• ACS Petroleum Research Fund
• Cabot Corporation
• Equistar Chemicals
• Eastern Illinois University
Students & Collaborators:
• Carol Deakyne
• Reto Frei
• Vlad Gun’ko
• Giles Henderson
• John Sipple
• Mary Vedamuthu