Silica Surface Modification Reactions: Kinetics, Mechanisms, and Surface Structures Jonathan Blitz Department of Chemistry

Applications of Surface Modified Silicas Composites Adhesives/Sealants Paints/Coatings Chromatographic stationary phases Catalyst supports/Catalysts Adsorbents

The SiO 2 Surface 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) SiO 2 dispersion in KCl

The Silica Surface An FTIR Perspective Si O H O O H O H

HMDS (CH 3 ) 3 SiNHSi(CH 3 ) 3 HMDS

600 o C

TiCl 4 Reactions Si(s)-OH + TiCl 4 Si(s)-O-TiCl 3 + HCl

Aluminum Alkyl Reactions Si(s)-OH + Al(Et) 3 Si(s)-O-Al(Et) 2 + C 2 H 6 Si(s)-OH + Al(Et) 2 Cl Si(s)-O-Al(Et)Cl + C 2 H 6 ? Si(s)-OH + Al(Et) 2 Cl Si(s)-O-Al(Et) 2 + HCl ? Another Complication

Silica Gel Type Al(Et) 3 (mmol Al/g SiO 2 ) Al(Et) 2 Cl (mmol Al/g SiO 2 ) AlEtCl 2 (mmol Al/g SiO 2 ) AlMe 3 (mmol Al/g SiO 2 ) Unmodified o C HMDS o C/ HMDS Extent of Aluminum Alkyl Reactions

Silica Gel Type Al(Et) 3 (mmol Al/g SiO 2 ) Al(Et) 2 Cl (mmol Al/g SiO 2 ) AlEtCl 2 (mmol Al/g SiO 2 ) AlMe 3 (mmol Al/g SiO 2 ) Unmodified o C HMDS o C/ HMDS Extent of Aluminum Alkyl Reactions

Possible Explanation of Analytical Data AlEtCl 2 OOO HHSi(CH 3 ) 3 Al(Et) 3 OOO HHSi(CH 3 ) 3 Et OOO Si(CH 3 ) 3 Al Al(Et) 2 Cl OOO HHSi(CH 3 ) 3 OOO Al Cl OOO Si(CH 3 ) 3 Al Cl

+ Adsorbed or pre-reaction complex ab initio computational chemistry

Transition state 2 Transition state 1 Adsorbed complex

+ + + Products 2 + Products 1

SiOH TypeIR band position 29 Si CPMAS NMR 3745 cm ppm 3660 cm ppm 3745cm ppm

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

4h reacted 45h reacted

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 Kinetics Slow Reaction Kinetics 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

Reaction rate = 1.9 x Ms -1 Reaction rate = 1.5 x Ms -1 Internal standard Direct measurements of relative silanediol reactivity supports conclusions from silica studies

Using solution FTIR to monitor SiOH loss provides kinetics data on this reaction… Silsesquioxane/Aminosilane Studies

Silanol Loss from Aminopropylsilane Reaction in Hexane Solution

SolventRate constant95% Confidence Interval Hexane6.27 x L/mmol.s± 3.2 x L/mmol.s THF1.06 x L/mmol.s± 6.1 x L/mmol.s

Reaction Temperature ( o C)Rate Constant (x 10 4 mM/min) An increase in temperature results in an increased rate constant …

H-bond complex formation is rate limiting at higher temperatures Activation energy is rate limiting at lower temperatures

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