Silica Surface Modification Reactions: Kinetics, Mechanisms, and Surface Structures

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

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

4. The SiO2 Surface H H H H Surface O O O O Si Si Si Si O O O Interior

5. Diffuse reflectance FTIR spectrum of silica gel

6. Specular and DiffuseReflection Sample ~10% (w/w) SiO2 dispersion in KCl

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

8. HMDS HMDS (CH3)3SiNHSi(CH3)3

9. 600oC

11. TiCl4 Reactions Si(s)-OH + TiCl4 Si(s)-O-TiCl3 + HCl

14. Aluminum Alkyl Reactions Si(s)-OH + Al(Et)2Cl Si(s)-O-Al(Et)2 + HCl ? Si(s)-OH + Al(Et)2Cl Si(s)-O-Al(Et)Cl + C2H6 ? Si(s)-OH + Al(Et)3 Si(s)-O-Al(Et)2 + C2H6 Another Complication

15. Extent of Aluminum Alkyl Reactions

16. Extent of Aluminum Alkyl Reactions

17. Et Si(CH3)3 Al O O O Cl Si(CH3)3 Al O O O Cl Cl Cl Cl Al Al Si(CH3)3 O O O Possible Explanation of Analytical Data H H Si(CH3)3 Al(Et)3 O O O H H Si(CH3)3 Al(Et)2Cl O O O H H Si(CH3)3 AlEtCl2 O O O

18. ab initio computational chemistry + Adsorbed or pre-reaction complex

19. Transition state 1 Transition state 2 Adsorbed complex

20. + Products 1 + Products 2 + +

23. 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)

24. HMDS Reaction with silica nanoparticles 1 h reacted 12 s reacted unreacted

25. 45h reacted 4h reacted

27. 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.

28. Main Conclusions … Slow Reaction Kinetics Fast Reaction Kinetics Slow Reaction Kinetics Fast Reaction Kinetics …obtained indirectly

29. …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

30. Reaction rate = 1.5 x 10-5 Ms-1 Direct measurements of relative silanediol reactivity supports conclusions from silica studies Reaction rate = 1.9 x 10-6 Ms-1 Internal standard

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

32. Silanol Loss from Aminopropylsilane Reaction in Hexane Solution

36. An increase in temperature results in an increased rate constant …

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

40. Arrhenius Plot

41. 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

42. 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