SURFACES Of NANOPARTICLES AND POROUS MATERIALS ppt

HUBBARD Department of Chemistry University of Cincinnati Cincinnati, Ohio ADVISORY BOARD Department of Chemical Engineering Department of Chemical Engineering Massachusetts Institute of

URFACTANT S IENCE SERIES

FOUNDING EDITOR

MARTIN J SCHICK

Consultant New York, New York

SENIOR ADVISOR

ARTHUR T HUBBARD

Department of Chemistry University of Cincinnati Cincinnati, Ohio

ADVISORY BOARD

Department of Chemical Engineering Department of Chemical Engineering Massachusetts Institute of Technology University of Delaware

Cambridge, Massachusetts Newark, Delaware

Institute for Applied Surfactant Shell International Oil Products B V

University of Oklahoma Norman, Oklahoma

P SOMASUNDARAN

Henry School of Mines Columbia University New York, New York

Nonionic Surfactants, edited by Martin Schick (see also Volumes 19, 23, and 60)

2 Solvent Properties of Surfactant Solutions, by Kozo Shinoda (see Volume 55)

3 Surfactant Biodegradation, Swisher (see Volume 18)

4 Surfactants, edited by Eric (see also Volumes 34, 37, and 53)

5 Detergency: Theory and Test Methods (in three parts), edited by G Cutler and Davis (see also Volume 20)

6 Emulsions and Emulsion Technology (in three parts), edited by Kenneth

7 Anionic Surfactants (in two parts), edited by Wamer M Linfield (see Volume

8 Anionic Surfactants: Chemical Analysis, edited by John Cross (out of print) Stabilization of Colloidal Dispersions by Polymer Adsorption, Sato and Richard (out of print)

10 Anionic Surfactants: Biochemistry, Toxicology, Dermatology, edited by Christian Gloxhuber (see Volume 43)

Anionic Surfactants: Physical Chemistry of Surfactant Action, edited by H.

(out of print)

12 Amphoteric Surfactants, edited by and Clifford L (see Volume 59)

13 Demulsification: Industrial Applications, Kenneth Lissant (out of print)

14 Surfactants in Textile Processing, Datyner

15 Electrical Phenomena at Interfaces: Fundamentals, Measurements, and Applications, edited by Ayao Kitahara and Akira Watanabe

16 Surfactants in Cosmetics, edited by Martin Rieger (seeVolume 68)

17 Interfacial Phenomena: Equilibrium and Dynamic Effects, Clarence A Miller and P Neogi

18 Surfactant Biodegradation: Second Edition, Revised and Expanded,

Swisher

19 Nonionic Surfactants: Chemical Analysis, edited by John Cross

20 Detergency: Theory and Technology, edited by Gale Cutler and Erik

21 Interfacial Phenomena in Apolar Media, edited by Hans-Friedrich Eicke and Geoffrey

22 Surfactant Solutions: New Methods of Investigation, edited by Zana

23 Nonionic Surfactants: Physical Chemistry, edited by Martin J Schick

24 Microemulsion Systems, edited by Henri L Rosano and Marc

25 Biosurfactants and Biotechnology, by W Cairns, and Neil C Gray

26 Surfactants in Emerging Technologies, edited by Milton Rosen

27 Reagents in Mineral Technology, edited by P Somasundaran and Moudgil

28 Surfactants in Engineering, edited by Martin and Dinesh 0 Shah

29 Thin Liquid Films, edited by 1 Ivanov

30. Microemulsions and Related Systems: Formulation, Solvency, and Physical Properties, edited by Maurice and Robert Schechter

31. Crystallization and Polymorphism of Fats and Fatty Acids, edited by

and Kiyotaka Sato

32. Interfacial Phenomena in Coal Technology, edited by Gregory Botsaris

33. Surfactant-Based Separation Processes, edited by John F Scamehom and Jeffrey Harwell

34. Surfactants: Organic Chemistry, edited by James M Richmond

35. Alkylene Oxides and Their Polymers, Bailey, and Joseph V Koleske

36. Interfacial Phenomena in Petroleum Recovery, edited by Norman Morrow

37. Surfactants: Physical Chemistry, edited by Donn and Paul Holland

38. Kinetics and Catalysis in Microheterogeneous Systems, edited by and K Kalyanasundaram

39. Interfacial Phenomena in Biological Systems, edited by Max Bender

40. Analysis of Surfactants, Thomas M.

Light Scattering by Liquid Surfaces and Complementary Techniques, edited

42. Polymeric Surfactants,

43. Anionic Surfactants: Biochemistry, Toxicology, Dermatology Second Edition, Revised and Expanded, edited by Christian Gloxhuber and Klaus Kunstler

44. Organized Solutions: Surfactants in Science and Technology, edited by Stig

Friberg and Bjom Lindman

45. Defoaming: Theory and Industrial Applications, edited by P Garrett

46 Mixed Surfactant Systems, edited by Keizo Ogino and Masahiko Abe

47. Coagulation and Flocculation: Theory and Applications, edited by

48. Biosurfactants: Production Properties Applications, edited by Naim

49. Wettability, edited by John Berg

50. Fluorinated Surfactants: Synthesis Properties Applications, Erik Kissa

51. Surface and Colloid Chemistry in Advanced Ceramics Processing, edited by Robert J Pugh and Lennart Bergsfrom

52. Technological Applications of Dispersions, edited by Robert

53. Surfactants: Analytical and Biological Evaluation, edited John Cross and Edward Singer

54, Surfactants in Agrochemicals, Tharwat Tadros

55. Solubilization in Surfactant Aggregates, edited by Sherril D Christian and John Scamehom

56. Anionic Surfactants: Organic Chemistry, edited by Helmut

57. Foams: Theory, Measurements, and Applications, edited by Robert K

homme and Saad A Khan

58. The Preparation of Dispersions in Liquids, H Stein

59. Amphoteric Surfactants: Second Edition, edited by Eric

60. Nonionic Surfactants: Polyoxyalkylene Block Copolymers, edited by Vaughn

Nace

Emulsions and Emulsion Stability, edited by Johan Sjoblom

62 Vesicles, edited by Morton Rosoff

63 Applied Surface Thermodynamics, edited by A Neumann and Jan K Spelt

64 Surfactants in Solution, edited by Arun K Chaifopadhyay and K.

65 Detergents in the Environment, edited by Milan Johann Schwuger

66 Industrial Applications of Microemulsions, edited by Conxita Solans and Hironobu Kunieda

67 Liquid Detergents, edited by Kuo-Yann Lai

68 Surfactants in Cosmetics: Second Edition, Revised and Expanded, edited by Martin Rieger and Linda Rhein

69 Enzymes in Detergency, edited by Jan H van Ee, and Erik Baas

70 Structure-Performance Relationships in Surfactants, edited by Kunio

Powdered Detergents, edited by Michael

72 Nonionic Surfactants: Organic Chemistry, edited by M van

73 Anionic Surfactants: Analytical Chemistry, Second Edition, Revised and Expanded, edited by John Cross

74 Novel Surfactants: Preparation, Applications, and Biodegradability, edited by Krister

75 Biopolymers at Interfaces, edited by Martin Malmsten

76 Electrical Phenomena at Interfaces: Fundamentals, Measurements, and plications, Second Edition, Revised and Expanded, edited by Hiroyuki and Kunio

Ap-77 Polymer-Surfactant Systems, edited by Jan T Kwak

78 Surfaces of Nanoparticles and Porous Materials, edited by James A.

79 Surface Chemistry and Electrochemistry of Membranes, edited by Torben Smith

ADDITIONAL VOLUMES IN PREPARATION

Interfacial Phenomena in Chromatography, edited by Pefferkom Solid-Liquid Dispersions, Xueping and Wolfgang von Rybinski

Modern Characterization Methods of Surfactant Systems, edited by P.

Interfacial Forces and Fields, edited by Jyh-Ping

Silicone Surfactants,edited by Randal M Hill

Surface Characterization Methods: Principles, Techniques, and Applications,

edited by Andrew Milling

edited by

Copyright 1999 by Marcel Dekker All Rights Reserved.

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PRINTED IN THE UNITED STATES OF AMERICA

They knew each other, they liked each other, and they departed from their families and us almost at the same

time, in the last 1997.

They were our mentors W e dedicate this volume to

assurance that they knew the roles they have

played in our lives.

as it goes, a small may give analogy o f great things, and show the tracks

of knowledge.

from De (On The Nature of Things) BC)

In the world of nanoparticles and microporous materials, the interaction forcesbetween nanosized particles and molecules from the surrounding medium, or theforces between particles themselves, may exceed the mechanical forces between bodies of the macroscopic world This is caused by the high surface-to-volumeratio of nanoparticles and microporous materials When familiar materials becomemainly surface, they acquire new optical, magnetic, electrical, chemical, and trans-port properties Thus, dispersions tend to agglomerate, fine particles show increasedmechanical strength, and microporous solids develop tremendous sorption and molecular sieving properties

Materials with a high surface-to-volume ratio have played important roles inevolution and in our own lives Extensive surface area provides optimal conditions for chemical transformations to proceed with high reaction rates and high productselectivity The organization and stability of nanosized structures are controlled byinteractions at the molecular electrical or magnetic-rather than

by the mechanical forces that shape the macroscopic world Our ability to controland use for our benefit all kinds of special properties developed at the extended surface that characterizes the objects of the nanoworld depends on our understand-ing of phenomena at and across the interfaces

Nanoparticles and porous materials exist essentially as an extended surface The line of structural similitude, however, ends at this point Nanoparticles derive theirproperties from their small solid size in their condensed phase, while porous mate-rials derive their properties from the absence of solid material in their condensed state In other words, the shape of the is either convex or concave,

depending on whether the object is a collection of microscopic particles or a tion of microscopic voids

collec-In either case, the surface can strongly influence the chemistry that occurs within the surrounding media The gas-solid and liquid-solid interfaces of porous materi-als have been the focus of many studies, over decades In recent years,

materials have become the focus of many researchers Such materials haveinteresting properties, but they d o not definitively promise to change the state ofscience or technology What might change is our use of design strategies for con-verting theoretical materials into practical materials for use

The purpose of this volume, therefore, is to collect state-of-the-art procedures for construction and design of nanoparticles and porous materials, where their applica-tions might be most appropriate To that end, synthesis and characterization pro-cedures are presented The ultimate test is their practical utilization in world"environments that exist at the gas and liquid interfaces of these materials Casestudies are presented and, in some instances, conclusions and projections for opti-mal design procedures of nanoparticles and porous materials are offered The scope

of this volume is inherently multidisciplinary from the viewpoint of usage of rials The common factor, however, is that their surface chemical behavior

mate-mates, and thus, unification of purpose and scope becomes a reality

The volume is organized into three sections, each of which addresses tal and practical realization of the production of nanostructured materials The firstsection deals with the preparation, characterization, and transport properties of thisunique class of materials Structural and chemical heterogeneity are the result ofpreparation protocols, and various spectroscopies can be used to characterize theseproperties Transport of adsorbates is affected by both intraparticle and interpar-ticle resistance, which can greatly influence applications in practical processes Each

fundamen-of these topics is represented as a case study that is general enough in scope thatcautious application of the reported results can be extended to other systems oftechnological importance

The remaining two sections deal with the fundamental and practical utilization

of nanostructural materials in gaseous and liquid environments The second section deals with the former case A balanced blend of theoretical and practical applica- tions is presented Both theory and application emphasize the importance of struc-tural and energetic heterogeneity and its influence on the performance of three-dimensional materials that are essentially only two-dimensional due to their highsurface-to-volume ratio

The final section presents case studies of the adsorption properties of surfacesunder the influence of a solvent that can alter the surface and chemical heteroge- neity by mechanisms that have been studied by colloidal scientists for many years.Again, in this section, there is a complimentary blend of theory and application.Applications involving both inorganic and organic adsorbates are considered.The subject areas in all three sections are quite diverse, which is a reflection of theversatility of nanoparticles and porous materials in technologies that demand super- ior performance, yet still require latitude for optimization strategies Each of thecontributions has been peer-reviewed and we feel confident that the information contained is complete and can be used by seasoned researchers and newcomers

to the field to condition their own research objectives in alignment with their ownexpertise.

We wish to first acknowledge Professor Arthur for the suggestion that

a volume of this type was appropriate at this time Each of the authors hasresponded under undue pressure from both of us to submit their contributions in

a timely fashion This has resulted in a final document that many should find ofinterest We appreciate the assistance of Dr Adriana Contescu in maintaining up-to-date folders of our correspondence with the contributors Finally, the attention

to detail and the electronic communication to get everything in place for publicationcame from Ms Dawn Long She has been responsible for helping us both remainhonest and on time, so that the final submission to the publisher occurred onscheduleand with minimum confusion

James A

I.

Part I Preparation, Characterization, and Transport Properties of Nanoparticles

and Porous Solids

Synthesis of a Polysilazane Coating on a Silica Gel via Chemical SurfaceCoating Comparing Liquid- and Gas-Phase Chlorosilylations 1

Nathalie R E N Ztnpens and Etienne F Vansant

Preparation of Molecular Sieves by Pillaring of Synthetic Clays 15

Soon- Yong Jeong

Engineering of Nanosize Superparamagnetic Particles for Use in MagneticCarrier Technology 31

Zhenghe Xu, Qingxia Liu , and James A. Finch

Acid-Base Behavior of Surfaces of Porous Materials 51

Cristian I Contescu and James A Schirarz

Electro-Optical Spectroscopy of Colloidal Systems 103

Maria Stoimenova and Tsuneo Okuho

N M R Studies of Colloidal Oxides 125

Edisson Morgado, Jr., Sonia Maria Cabral de Menezes, and Carlos Roberto Nogueira Pacheco

Polymer Surface Dynamics Using Surface-Modified Glasses via DynamicContact Angle Measurements 169

Joung-Mafz Park

8 Microporous Structure of Collagen Fibers 185

Keito Boki

9. Adsorption onto Oxides: The Role of Diffusion 199

Lisa Axe and Paul R Anderson

10. Electrokinetic Phenomena in Porous Media and Around Aggregates 211

Pierre M Adler, David Coelho, Jean-Francois Thoven, and Michael Shapu'o

1 1 Transport Processes in Microemulsions 259

Satya P Moulik and Bidyut K Paul

12 Structural Effects on Diffusivity Within Aggregates of Colloidal Zirconia 281

David H Reeder, Alon V McCormick, and Peter W Carr

Part I I Adsorption from the Vapor/Gas Phase onto Nanoparticles and Porous

15 Surface Heterogeneity Effects o n Adsorption Equilibria and Kinetics:

Rationalizations of the Elovich Equation 355

WladyslawRudzinski and Tornasz Panczyk

16 Single- and Multicomponent Adsorption Equilibria of Hydrocarbons onActivated Carbon: The Role of Micropore Size Distribution 391

17 Surface and Structural Properties of Modified Porous Silicas 443

Michal Kruk and Mietek Jaroniec

18 Nanodimensional Magnetic Assembly of Confined O2 473

Katsumi Kaneko

19. Heat of Adsorption of Pure Gas and Multicomponent Gas Mixtures onMicroporous Adsorbents 501

Shivaji Sircar and Madhukar B Rao

art 111 Adsorption from the Liquid Phase onto Nanoparticles and Porous Solids

20 Surface Chemistry of Activated Carbon Materials: State of the Art andImplications for Adsorption 529

Ljubisa R Radovic

21 Charge Regulation at the Surface of a Porous Solid 567

Boris V Zhrnud and Lennart Bergstrom

22 Surface Ionization and Complexation 593

Zhenghe Xu, Qingsong Zhang, and James A Finch

23 The Surface Charge of Alkali Halides in Their Saturated Solutions 613

Jan D Miller and Srinivas Veeraimsuneni

24 Ionic Adsorbates on Hydrophobic Surfaces 645

Richard L Zollars

25 Adsorption of Metal Ions onto Humic Acid 661

Hideshi Seki and Akira Suzuki

26. Hydrous Metal Oxides as Adsorbents for Aqueous Heavy Metals 675

Russell Crawford, David E Maimvaring, and Ian H Hording

27 Adsorption of Ions onto Alumina 711

Pierre M Adler Equipe Milieux Poreux, Institut de Physique du Globe de Paris,Paris, France

Paul R. Anderson Department of Chemical and Environmental Engineering,Illinois Institute of Technology, Chicago, Illinois

Lisa Axe Department of Civil and Environmental Engineering, New Jersey Institute of Technology, Newark, New Jersey

aumgarten Institut fur Physikalische Chemie und Elektrochemie,Heinrich-Heine-Universitat, Dusseldorf, Germany

ergstrom Institute for Surface Chemistry, Stockholm, Sweden

oki Department of Pharmaceutical Sciences, Kinki University, Osaka,Japan

. Carr Department of Chemistry, University of Minnesota, Minneapolis,Minnesota

oelho Equipe Milieux Poreux, Institut de Physique du Globe de Paris,Paris, France

Cristian I Contescu Department of Chemical Engineering and MaterialsScience, Syracuse University, Syracuse, New York

xiii

Mietek daroniec Department of Chemistry, Kent State University, Kent, Ohio

Soon-Yong deong Chemical Process and Engineering Center, Korea ResearchInstitute of Chemical Technology, Taejeon, Korea

Katsumi Kaneko Department of Chemistry, Chiba University, Chiba, Japan

Michal Kruk Department of Chemistry, Kent State University, Kent, Ohio

Jeffrey Leaver Molecular Recognition Group, Hannah Research Institute, Ayr, Scotland

xia Liu Cominco Research Cominco Ltd., Trail, British Columbia, Canada

Department of Applied Chemistry, Royal Melbourne Institute of Technology, Melbourne, Australia

Ion V McCormick Department of Chemical Engineering and MaterialsScience, University of Minnesota, Minneapolis, Minnesota

onia Maria Cabral e Menezes Chemistry Division, Petrobras Research andDevelopment Center, Rio de Janeiro, Brazil

.Miller Department of Metallurgical Engineering, University of Utah, SaltLake City, Utah

disson Morgado, Jr. Catalysts Division, Petrobras Research and DevelopmentCenter, Rio de Janeiro, Brazil

atya P Moulik Center for Surface Science, Department of Chemistry, JadavpurUniversity, Calcutta, India

ontributors xv

konogi Department of Pharmaceutical Technology, Chiang MaiUniversity, Chiang Mai, Thailand

Tsuneo Okubo Department of Applied Chemistry, Gifu University, Gifu, Japan

livier Micromeritics Instrument Corporation, Inc., Norcross,Georgia

Carlos Roberta Nogueira Pacheco Chemistry Division, Petrobras Researchand Development Center, Rio de Janeiro, Brazil

anczyk Department of Theoretical Chemistry, Sklodowska University, Lublin, Poland

Marie-Curie-Joung-Man Park Department of Polymer Science and Engineering, Gyeongsang National University, Chinju, Korea

idyut K Paul Geological Studies Unit, Indian Statistical Institute, Calcutta, India

Ljubisa R Radovic Department of Materials Science and Engineering, ThePennsylvania State University, University Park, Pennsylvania

ao Corporate Science and Technology Center, Air Products and Chemicals, Inc., Allentown, Pennsylvania

er Department of Chemical Engineering and Materials Science,University of Minnesota, Minneapolis, Minnesota

Wiadyslaw Rudzinski Department of Theoretical Chemistry Sklodowska University, Lublin, Poland

Marie-Curie-chwarz Department of Chemical Engineering and Materials Science,Syracuse University, Syracuse, New York

ideshi Seki Department of Marine Bioresources Chemistry, HokkaidoUniversity Hakodate, Japan

hapiro Department of Mechanical Engineering, Technion-IsraelInstitute of Technology, Haifa, Israel

Shivaji Sircar Corporate Science and Technology Center, Air Products andChemicals, Inc., Allentown, Pennsylvania

teele Department of Chemistry, The Pennsylvania State University,University Park, Pennsylvania

Maria Stoimenova Institute of Physical Chemistry Bulgarian Academy ofSciences, Sofia, Bulgaria

University, Hakodate, Japan

Thovert Laboratoire de Combustion et de Detonique, Centre National de la Recherche Scientifique, Futuroscope, France

Etienne F. Vansant Chemistry Laboratory of Adsorption and Catalysis,University of Antwerp, Antwerp, Belgium

rinivas Veeramasuneni Department of Metallurgical Engineering, University

of Utah, Salt Lake City, Utah

. Wang Department of Chemical Engineering, University of Queensland, St Lucia, Australia

Zhenghe Xu Department of Chemical and Materials Engineering, University ofAlberta, Edmonton, Alberta, Canada

Keiji Yamamoto Department of Pharmaceutical Technology, Chiba University,Chiba, Japan

Zhang Hewlett Packard, Palo Alto, California

Institute for Surface Chemistry, Stockholm, Sweden

ollars Department of Chemical Engineering, Washington State University, Pullman, Washington

NATHALIE and ETIENNE

Chemistry Laboratory of Adsorption and Catalysis, University of

Belgium

I

I I III

IV

IntroductionExperimentalResults and Discussion

A Liquid-phase modification of silica gel with tetrachlorosilaneand the first ammoniation

B Second and higher reaction cycles

C Porosity studyConclusions

References

I INTRODUCTION

Due to the widespread use of ceramic coatings, several synthesis techniques have

che-mical vapor deposition (CVD) and physical vapor deposition (PVD) and theirvariants, are focused on the synthesis of flat coatings Recently, the preceramicpolymeric synthesis route has offered the possibility to impregnate preceramicmaterials into porous matrices prior to pyrolysis in order to create coated or

altera-tions of the original substrate texture The impregnation does not guarantee achemical bond between the substrate and the preceramic material This can lead

to an unstable composite material after pyrolysis T o avoid this another technique,

chemisorption of gaseous reagents in a cyclic way at relatively low temperatures,

coating is chemically bound to the substrate surface and that is has grown in a veryhomogeneous way in spite of the very irregular substrate texture The so-calledcoating precursor is afterwards pyrolyzed, and a chemically bound ceramic coating, e.g., or BN; is formed on silica gel Characterization of the thermally con-verted Si- N or B-N polymer shows that a very thin homogeneous siliconoxynitride or boron nitride coating, respectively, is formed, whereas the original substrate texture is practically unchanged [7,8]

In the procedure described in this chapter, a preceramic polysilazane coating issynthesized on silica gel via CSC using S i c &and NH3, but the chlorosilylations areperformed in the liquid phase In this way, a very small amount of the hazardous reagent is used in comparison with gas-phase reactions, and the total precursorsynthesis can be performed at room temperature

The solid-gas reaction of with thermally pretreated silica gel has beenoptimized, and the reaction mechanisms were studied earlier [4]; a reaction tem-perature of 663K is needed to modify all the surface hydroxyl groups T o reducethe reaction temperature, and Hair [9] studied the effect of the presence of

very reaction yield was obtained The reaction mechanism was deduced from

The nitrogen-containing base promotes the reaction by rendering thegroup more nucleophilic When the base strength is high enough (as in

substrate surface and must be removed by sublimation or dissolution in a

methanol which results in the hydrolysis of groups This has to beavoided in the case of the CSC polysilazane synthesis, as these groups are especiallycreated to enable a reaction with that creates amine groups

The effects of solvent and amine base on the solid-liquid reactions ofchlorosilanes with the silica surface hydroxyls were throroughly investigated by

obtained by using strong bases together with solvents with high Lewis donatingand accepting properties T o our knowledge, none of the authors reported a pure

with the formation and conservation of groups without theformation of ammonium salt on the silica surface

In this study, the reaction parameters were chosen in such a way that the salt

to that obtained with the gas-solid reactions at high temperature For the reaction

that would not affect the precursor A comparison of the chemical and gical composition of the coating precursor using liquid- and gas-phase

morpholo-synthesized in solid-gas and solid-liquid conditions was conducted usingelemental analysis, Fourier transform infrared spectroscopy with photoacousticdetection (FTIR-PAS), and nitrogen

glove box to avoid any possible rehydration of the sample This

methane (99.8%) were zeolite-dried prior to use 0.5 g of silica, and excess of 0.8

solvent After 3 h of stirring, the silica was filtered, then washed four times with

of pure solvent Afterwards, the silica was dried using a cryogenic trap and avacuum pump

Ammoniation was carried out in a dynamic volumetric adsorption apparatus, as

whereas the surfce chlorine concentration was determined

Extreme care was taken to prevent the samples from hydrolysis, by handling

reaction

with photoacoustic detection The PA detector is a prototype of the MTEC-100 cell

combination mode

Nitrogen adsorption-desorption isotherms at 7 7 K were measured with a

plot method was used to check the microporosity

Modification of Ammoniation

The creation of chlorosilyl groups on the substrate surface is very important in the synthesis of a chemically bound preceramic polysilazane coating, because theamounts and types of chlorosilyl species present influence the reaction behavior

of the reactions In Table 1 , a comparison of the elemental analysis data is shownfor the gas- and liquid-phase synthesis routes as a function of the number ofreaction cycles Infrared spectroscopy of the modified silica gel was used to study the reaction yield In Figure the infrared spectra of pure silica (973 spectruma) and silica modified with in both the gas and liquid phases (spectra areshown When toluene (spectrum c) or cyclohexane were used as solvents, all thehuge bands between 3000 and 2000 c m ' and the sharp bands superimposed o n the

Si 0 vibrations between 1500 and 800 exactly correspond to

chloride, which was formed o n the silica surface and could not be removed bysevere washing When dichloromethane was used (lines d and e) no salt formationwas detected, and very fast removal of the solvent was possible due to its volatility.This solvent's high Lewis accepting property is shown to promote the reaction ofsilica gel with octadecyldimethylchlorosilane a t room temperature The nitro-gen-containing base used in this study has to be a tertiary amine, avoiding anypolymerization reactions with Triethylamine was chosen, as its conjugatedbase has a relatively high value (10.8) When the base was added in a catalyticamount (5 1/20 relative to the OH groups present), the reaction amountwas very low (Fig 1, spectrum e) The reason is that after protonation the base has

no further influence on the reaction, which implies that it does not act as a catalyst,

as was erroneously reported in the literature An excess of base (800 resulted

in almost complete modification of the surface hydroxyl groups (Fig 1, spectrumd) As a blank, the reaction was also carried out under similar conditions but

1 Nitrogen and Chlorine Uptake (in Unmodified Silica Gel) and Ratio as a Function of the Number of Reaction Cycles for the Liquid- and Gas-PhaseSynthesis Routes

. FTIR-PA spectra of KG 60 (a) pretreated at 973 unmodified, and modified with (b) in gas phase, at optimal reaction conditions; (c) in liquid phase,using toluene and in excess; (d) in liquid phase, using and in excess;(e) in liquid phase using and in catalytic amounts; (f) in liquid phase, using without Relevant peak identification of silica gel: 3747 free

stretch

without triethylamine, resulting in no modification at all, as shown in Fig 1, line

In the liquid phase the highest yield is obtained with a high amine concentration incombination with a (spectrum d) The vibration is positioned at thesame frequency ( 6 2 0 c m1) for both a gas-phase reaction a t 633K(spectrum a) and

a liquid-phase reaction a t 298K (spectrum d), indicating that monodentate binding

of the liquid-phase modification (spectrum d) represents 25% of the total amount

of hydroxyl groups at the surface (0.53 x of O H groups per gram of

These two independently calculated values are in good agreement Longer reaction times or higher amounts of reactants or amine base did not improve the reaction,

neighborhood in order to have any reaction

Similar to the gas-phase chlorosilylated silica gel, the ammoniation was

reaction conditions, resulting in the reaction [5]

liquid synthesis routes However, during the liquid-phase route, the nitrogen uptake

is twice the chlorine uptake within the limit of experimental error whereas only 90% of the chlorines are ammoniated after a gas-phase chlorosilylation

1.8) From a steric point of view, this is a logical result, as in the liquid synthesis

smaller amount of chlorosilyl groups is formed in the liquid-phase synthesis route,situated at the most easily reached hydroxyls After the liquid-phase reaction,

and higher proton mobility is expected at the high temperature, resulting

in silylation of O H groups at sterically hindered sites Anyway a significant tion of these chlorosilyl groups synthesized in the liquid phase could not be mod-ified by ainmoniation due to steric reasons

frac-In Fig 2, the spectra of the samples after the first (spectruma) andthe second liquid chlorosilylation are shown for different reaction conditions

the second chlorosilylation on, dichloromethane could not be used and wasreplaced by cyclohexane, because dichloromethane reacts with the amino groups

deposition of triethylammonium chloride a t the sample surface, which could not beremoved with the solvent used or by sublimation, because thermal treatment also

that of the preceramic precursor after the second chlorosilylation In the

modification of these groups Therefore, an estimation of the ratio of mono

by monodentate binding

FTIR-PA spectra of KG 60 pretreated at 973 (a) Reacted with inand excess and afterwards reacted with in the gas phase.(b.c)

as (a) but modified with (b) in the presence of using cyclohexane, and (c)without using cyclohexane

1.33 is reached A similar phenomenon was observed in the gas-phase synthesisroute, where an ratio of 1.5 was found In the gas-phase synthesis route, ahigher uptake of Cl is observed during the third and fourth chlorosilylations than inthe liquid phase This is certainly due to the presence of the solvent in the liquidcase, which leads to a sterically unfavorable situation Therefore, chlorosilylation isexpected only at the most easily reached sites From the third reaction cycle on, theammoniation leads to a complete modification of the groups in the

phase synthesis route, whereas the gas-phase synthesis route never leads to a n

ratio of 2.After the solvent evacuation in the liquid case, there is enough space leftfor to reach and react with all the groups The general trend of

nitrogen, chlorine, and weight uptake during the four reaction cycles for both the

are linear except for the chlorine uptake in the liquid-phase route where a tion is observed at two cycles However, this trend is not followed by the

deflec-uptake, as the ammoniation is not disturbed by solvent hindrances From this

on the silica surface, whereas all the gas-phase slopes are higher than in the case ofliquid-phase modification The total increase in weight gain after four reactioncycles if 0.55 and 0.8 g per gram of unmodified silica in the liquid and gas synthesisroutes, respectively

roughly the ratio between the increasing weight of the two synthesis routes (weight:

groups is relatively lower in the liquid-phase synthesis route, even when the

whereas 36% is unreacted in the liquid-phase synthesis route This means that alower cross-linking efficiency is reached in the liquid-phase route This value, incombination with the lower uptake of nitrogen in the liquid-phase synthesis route,

cross-linked precursor

amount of product left at the silica pore walls increases To study the effect on the

Therefore, both the substrate and the coated material are mesoporous materials.The total pore volume decreases with increasing number of reaction cycles The

type indicates ink-bottle or irregularly shaped pores The evolution in the hysteresisshows the occurrence of pore shape modification during the formation of thepolymer This phenomenon earlier observed was for the gas-phase synthesis

decrease is observed, indicating total monolayer coverage

between the theoretically expected and experimentally observed pore volumedecreases can be used to calculate the amounts of pore blocking and regular

"'Values in parantheses represent the absolute experimental errors

pore narrowing This is independent of the pore shape In Figs 5a andthe theoretical a n d experimental pore volumes are shown a s a function o f the

a m o u n t of chemisorption for both synthesis routes A s a n example, the

a m o u n t of chemisorbed is highlighted in the figures when a pore volume of

0.2 is left In the liquid-phase synthesis route (Fig only 13

is chemisorbed In the gas-phase synthesis route (Fig the chemisorbed is

already 15 This indicates a higher a m o u n t of pore blocking in the

phase synthesis route

.4 Nitrogen adsorption-desorption isotherms a t 77 K for the polysilazane-coated

silica gel after zero, one, three and four reaction cycles of the liquid-phase synthesisroute The experimental error o n the amount of adsorbed is 0.1

FIG. 5 Pore volume decreases as a function of the of reacted (Theoretical and (+) experimental values, (a) Liquid-phase synthesis route; (b)phase synthesis route The experimental errors are smaller than the symbols used The linear regressions serve only as a guide for the eyes

A similar trend was observed when the decreases in pore length were calculated using integration of the pore volume distributions obtained by the BJH method

as explained earlier However, this method is valid exclusively for rical pores T h e results are shown in Fig 6 A systematically pore lengthvalue is obtained for the liquid-phase route These d a t a show that more poreblocking occurs in this synthesis route

cylind-T h e calculated amounts of pore-blocking and pore narrowing a n d their bution to the total pore volume decrease a r e listed in Table 3.The results are based

contri-o n the d a t a contri-of Fig 5 Systematically higher abscontri-olute and relative amcontri-ounts contri-of pcontri-ore

blocking are seen for the liquid-phase synthesis route in spite of the lower uptake of

N and per reaction cycle, resulting in a lower coating homogeneity Moreover, from the higher absolute and relative narrowing values in the gas-phase synthesisroute, it can be concluded that a higher coating efficiency is obtained using thephase synthesis route

Successive reactions of in the liquid phase a n d in the gas phase wereperformed o n silica gel T h e reaction parameters for the

solvent choice and base promotion) were optimized t o get as much uptake

as possible T h e chemical composition of the polysilazane layer as a function of thenumber of reaction cycles is obtained by using a combination of elemental andinfrared analysis

A comparison of these results with the composition of a polysilazane coatingfully synthesized in the gas phase showed less uptake of N and and a greaternumber of groups due t o the presence of the solvent This implies that the

0 2 4 6 8 10 12 14 16

uptake

FIG. 6 Pore length decreases for (+) gas and liquid synthesis routes, withrespective linear regressions The experimental errors are smaller than the symbols used The linear regressions serve only as a guide for the eyes

Contribution of Pore Narrowing and Pore Blocking to the Decrease in Pore

gas-phase synthesis route is more suitable for the creation of a dense, cross-linkedpolysilazane polymer o n the silica surface.

The porosity was studied by using adsorption- desorption isotherms recorded

a t 77 A higher a m o u n t of pore blocking in the liquid-phase synthesis route wasobserved, although a smaller a m o u n t of product was A combination

of the pore blocking and pore narrowing results shows that the gas-phase sized polysilazane coating is more homogeneously spread o n the silica surface

synthe-1 Seyferth, Adv Chem 224: 565

2 D Seyferth and H Weiseman, Polymer Prepr (1984)

3 Vansant, P Van Der Voort and Vrancken Characterization and Chemical o f the Silica Surface, Elsevier Amsterdam 1995

Vansant, Porous Mater 4121 (1997)

10 J Blitz, S Murthy, and Leyden, Colloid Interface

(1988)

62:1723 (1940)

17 and Sing, Adsorption, Surface Area and Porosity, Academic,New York, 1976, p 287

Chemical Process and Engineering Center, KoreaResearch Institute of Chemical Technology, Taejeon, Korea

I Introduction

I I Experimental Methods

A Synthesis of aluminum-free clays

B Synthesis of H-magadiite and H-kenyaite

C Silica-intercalated layered silicate

D Analytical methods

III Results and discussion

A Synthesis of aluminum-free clays

B Octylamine/octylammonium layered silicate gel

C Reactions with TEOS

D Gelation effects

IV Conclusions

References

In recent years, the synthesis of aluminum-free clays such as magadiite

has been of increasing interest due to their catalytic, absorptive, and

sili-cates because they have a layered structure They have the characteristic of being free

of aluminum, unlike zeolites, and exhibit acid-resistance and thermal stability

Magadi in Kenya Since then they have been reported to occur in various regions

Most of these deposits were found in sodium carbonate-rich alkaline lake waters Also they have been successfully synthesized under hydrothermal

conditions Their basic structures are composed of duplicated tetrahedral

reaction time for the formation of kenyaite was much decreased, but quartz was

found that the pillaring of magadiite could be facilitated by using a preswelling step

TEOS with EtOH suspension After calcining to remove organic compounds,

amount of gelled TEOS, was formed However, the pillaring of kenyaite has rarely

exhibited a high surface area,

In the sol-gel process, a solvent such as EtOH is added to prevent liquid-liquidseparation during the initial stage of the hydrolysis reaction and to control the

of TEOS was influenced by the strength and concentration of the acid and base

release of TEOS from the layered phases during gelation Generally, TEOS

by acid- or base-catalyzed hydrolysis could diversify the interfacial properties ofproducts and results in such products as bulk gel, film, fiber, powder, and catalyst

bring out the pillaring effect as well as the diversity of interfacial properties of

gallery height can be increased

We synthesized pure and well-crystallized Na-magadiite and Na-kenyaite under

these clays in the sodium carbonate system Also, I report the effects of acid andbase catalysts on the hydrolysis and condensation polymerization of intercalatedTEOS in layered silicates

Na-magadiite and Na-kenyaite were prepared by the reaction of the

system under hydrothermal conditions The materials used were silica gel

were carried out in a stainless steel autoclave without stirring for the various

then dried at

of deonized water was stirred for 1 h The suspension was then

week in a refrigerator H-magadiite was recovered by filtering, washed with

H-kenyaite was the same as that of

magadiite gel was reacted for 24 h at room

organic pillar precursor, 40 g of TEOS, was added to

magadiite gel and then stirred for 24 h at room temperature TEOS was then absorbed into the organophilic interlayer region TEOS-intercalated magadiite

forming a gelatinous mixture that will not flow Silica-intercalated derivatives of

mother liquid

of the intercalated TEOS without catalyst was carried out by drying

effects of base and acid catalysts during the gelation The compositions of acid and

TEOS are well known, and the physical characterization of gelled silicate has been

Identification of samples was carried out by X-ray poweder diffraction (Rigaku

and kenyaite were determined with an energy-dispersive X-ray spectrometer (Link

carried out using an electron beam with the sample of carbon-coated pellets For thequantitative analysis of silica and sodium, quartz and sodium chloride were used as

analyzer Basal spacings of samples were determined from the

TEOS-intercalated layered silicate gels were prepared by smearing a thin film across amicroscope slide and then recording diffraction patterns of wet samples The pro-ducts gelled by EtOH suspension base-catalyzed reaction and acid-catalyzed reac-tion were prepared by depositing a gelled suspension on a glass plate and allowing

for X-ray diffraction analysis were prepared by calcining the gelled products on glass

Emmett-Teller (BET) equation The pore size distributions of silica-pillared

1

Figure 1 shows the development of crystalline magadiite with increasing reaction

FIG 1 X-ray diffraction patterns of synthetic magadiites at according to

=100 (a) 15 (b)36 (c) 72 Magadiite

magadiite exhibited 001 X-ray reflections of the film sample corresponding to a basal spacing of The peak positions for synthetic magadiite agree well with values reported previously for synthetic and natural magadiite Pure magadiite is obtained after 36 h of reaction, and then the intensity of the peak increases as the reaction time increases from 36 to 72 h Magadiite is synthesizeddirectly from an amorphous form of silica, and the pure magadiite can be synthe-sized easily by adjusting reaction time and temperature Figure 2 shows XRD

patterns of synthetic samples a t at fixed reactant compositions of

reaction times As reaction time increases from 20 to 72 magadiite is transformedinto kenyaite, and then kenyaite is transformed into quartz Beneke and

suggested that at amorphous silica is transformed into magadiite and theninto kenyaite Above the rate of kenyaite formation is much increased, andthe rate of quartz formation is increased simultaneously After 55 pure crystal-lized kenyaite is formed The XRD pattern of synthetic kenyaite exhibits 001 X-rayreflections of the film sample corresponding to a basal spacing of 2.03 nm The peakpositions for synthetic kenyaite agree with values reported previously forsynthetic kenyaite After 72 however, the quartz phase appears This resultindicates that it is very difficult to obtain pure kenyaite by controlling reaction

2 X-ray diffraction patterns of synthetic samples at according to

= 100 (a) 20 h; (b) 45 (c) 50 (d) 55 (e) 72 h

kenyaite; quartz (From Ref 30.)

time because pure kenyaite can be synthesized within only a short range of reaction time

= with reaction time can be synthesized directly fromamorphous silica, a n d magadiite was not observed a s a n intermediate

The higher molar ratio accelerates the formation of kenyaite Silica is first dissolved in alkali solution a t elelvated temperature, and then the nucleation ofkenyaite occurs gradually Therefore, the ratio in the solution is a veryimportant factor in the formation of kenyaite Also, the higher content o f silica

in the composition o f starting materials may cause a large ratio in thesolution These conditions could accelerate the nucleation of kenyaite that has a

of Synthetic

kenyaite is capable of forming directly from amorphous silica without the

synthesis without an intermediate could be taken advantage of for obtaining purekenyaite industrially

Figure 4 shows scanning electron micrographs of the synthetic magadiite andkenyaite Na-magadiite shows a particle morphology composed of silicate layers intergrown to form spherical nodules resembling rosettes The shape of kenyaite isvery similar to that of magadiite

The chemical compositions of Na-magadiite and Na-kenyaite were obtained by

T G A and EDS analyses for silica and sodium T G A and EDS data obtained from

(The degree of hydration can differ due to differences in sample treatment.)