Kinetic and thermodynamic issues in the early stages of sol-gel processes using silicon alkoxides

Kinetic and thermodynamic issues in the early stages of sol-gel processes using...

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ELSEVIER Catalysis Today 35 (1997) 205-223

Kinetic and thermodynamic issues in the early stages of sol-gel processes using silicon alkoxides

J. Sef~a'k, A.V. McCormick *

Department of Chemical Engineering and Materials Science, and Center for Interfacial Engineering, University of Minnesota, Minneapolis, MN 55455, USA

Abstract

An understanding of the chemical processes that take place in the earliest stages of a sol-gel preparation can provide the potential to better control microstructural evolution of a catalyst. While the desired catalyst properties depend on specific details of a catalytic application, in general one wants at least to control textural and chemical homogeneity. Silica provides an excellent test system for the study of sol-gel processes starting from alkoxide precursors as it can exhibit a wide variety of structure and has been extensively studied. In this review the features of tetraethoxysilane (TEOS) polymerization as observed by 29Si-NMR spectroscopy are summarized. Trends in hydrolysis and condensation with increasing oligomer size are identified. The kinetics and equilibrium of these reactions, metastability and phase separation are reviewed. Finally we suggest a comprehensive reaction engineering picture of TEOS polymerization with special focus on the crossover between gelation and precipitation. Selected comments on other alkoxides, non-alkoxides, and on multicomponent formulations are also offered.

1. Introduction acid/base reactivity and to manipulate the num- bers of such sites. In the extreme case, corn- One of the great promises of sol-gel synthesis pletely random structures might be sought. To is that one might hope to make solid surfaces direct such a random and homogeneous assem- with new catalytic properties by connecting dif- bly, one would ideally like the dissolved oxide ferent dissolved oxide precursors in proportions components and their assembling networks to and with local order chosen at will. We believe remain in solution until solidifying by 'gelation', this hope is fundamentally well founded. In i.e., by forming an infinite polymeric network, many instances one might hope for a metastable avoiding (to the degree possible) precipitation oxide structure and composition that is quite of the evolving intermediates. On the other hand, different from that presented by stable materials if a certain degree of order is required, one so as to create surface reactive sites of desired would like at least to control phase separation and other ordering processes.

The degree of molecular level randomness * Corresponding author, and homogeneity should have a profound influ-

0920-5861/97/$32.0 © 1997 Elsevier Science B.V. All rights reserved. PIIS0920-5861(96)00158-7

206 J. Sefdfk, A.V. McCormick/Catalysis Today 35 (1997) 205-223 ence on the catalytic properties of a site. It can the scope of this review. In practice, of course, also have a dramatic effect on aging, energetics the drying of the gel and exposure to catalyti- and thermal behavior of gels (which, naturally, cally useful conditions will no doubt cause reor- will also affect catalytic properties). For exam- ganization of the solid to structures more stable pie, gels prepared from tetraethoxysilane than a random assembly, but those structures (TEOS) at pH less than 3.5 are microporous and may well be metastable and can provide at least the time to gel and time to syneresis are sensi- some catalytically useful sites not ordinarily five functions of the synthesis pH. On the other available unless one had started from a random hand, gels prepared at pH higher than 3.5 are assembly of high free energy precursors. mesoporous with microstructure that is highly This review is also limited to the behavior of pH dependent. These gels do not exhibit synere- SiO 2 systems, particularly those from TEOS. sis \[1\]. Ying et al. indicate that the acid-pre- TEOS provides an excellent test system for the pared silica gels are energetically more stable, study of homogeneous vs. heterogeneous sol-gel retain much more water and unreacted ethoxy processes starting from alkoxide precursors. It is and hydroxy groups and densify at significantly exceptionally easy to force TEOS to evolve into lower temperatures than alkaline-prepared gels a homogeneous fairly randomly structured gel \[2\]. In this review, we will seek kinetic and over a wide range of preparation conditions. In thermodynamic reasons for the different chemi- other conditions, though, it can form dense col- cal and textural properties, loidal precipitates; in the presence of templates

Inquiring what causes chemical and structural or cations, TEOS derivatives can even nucleate differences in SiO 2 can serve as an entry into and grow zeolites \[1\]. Thus we have the oppor- the exploration of homogeneous multicompo- tunity to learn what kinetics and intermediate nent gels containing SiO 2 and can provide a structures are required to obtain homogeneous paradigm for understanding the chemistry and vs. heterogeneous structures. Acquiring a funda- structure of non-SiO 2 materials. Recent work in mental understanding of TEOS polymerization

the synthesis of A1203-SiO 2, ZrO2-SiO 2, under a variety of experimental conditions pro- V2Os-SiO 2 and TiO2-SiO 2 gels has demon- vides important insights into sol-gel processes strated distinct advantages of sol-gel 'prehydrol- in general. Moreover, the nuclear magnetic res- ysis' techniques in producing a controlled de- onance (NMR) properties of the 29Si nucleus are gree of component mixing (e.g., \[3-7\]). It has sufficiently favorable and TEOS sol-gel pro- been pointed out that the aluminosilicate system cesses can be sufficiently slow that these sys- is notoriously difficult to process by conven- tems can be quantitatively monitored with tional techniques because of the existence of now-standard NMR instrumentation. We will wide compositional regions prone to phase sep- note that the kinetic behavior of the TEOS arate into stable, stoichiometric aluminates, sili- system is simpler than that of other alkoxides. cates, and aluminosilicates \[8\]. Yoldas and Part- The behavior of TEOS in water/ethanol so- low have shown that the homogeneity of sol-gel lutions results from an interplay of three phe- derived aluminosilicates can be vastly superior nomena typically encountered in sol-gel pro- to sintered colloids \[9\]. It has been demonstrated cesses: hydrolysis, condensation and phase sep- that increased homogeneity led to enhanced re- aration. TEOS condensation polymerization ki- sistance to low-temperature densification of alu- netics are strongly non-ideal (cf. random step minosilicate gels \[10\]. polymerization), and this system has even pro-

This review, however, is limited to early vided a convenient benchmark for testing non- stage aspects of a sol-gel process, i.e., reactions ideal polymer structure development models. in the synthesis reactor. The restructuring of TEOS precipitation to colloidal particles has gels upon aging, drying and sintering is beyond also been studied a great deal, particularly in

J. Se3~ik, A. V. McCormick/Catalysis Today 35 (1997) 205-223 207 conditions providing monodispersely sized col- neous polymerization. We also discuss impor- loids, tant effects such as cyclization and decreased

Phase separation during TEOS polymeriza- group mobility that cannot be treated using tion is related both to kinetics (and hence struc- simple lumping procedures. In the fourth sec- ture evolution) and to the solvent quality for tion we pull together the kinetic, thermody- hydrolyzed TEOS and its condensation prod- namic and structural trends identified in the ucts. Water and ethanol play the role of solvents previous sections. We suggest a consistent pic- and, at the same time, the role of reactants (in ture of TEOS polymerization across the pH hydrolysis/reesterification of ethoxy/hydroxy range, with special focus on the crossover be- groups and in hydrolysis of siloxane bonds). .tween gelation and precipitation. TEOS hydrolysis and condensation are influ- Sol-gel science up to 1990 was thoroughly enced by concentration and type of acidic or reviewed in the book Sol-Gel Science \[12\], fol- basic catalyst. The kinetics and thermochem- lowed by the recent progress reports on sol-gel istry of these processes vary in a complicated silica synthesis reaction kinetics (e.g., \[13\]) and fashion as composition changes, and this gives structure development \[1\]; references contained rise to the intriguing composition dependence of therein provide a crucial reading list on sol-gel TEOS polymerization/precipitation behavior, topics. This paper continues from that point. We also note that the 'solvent' composition As this review mostly deals with TEOS poly- influences solubility of the evolving intermedi- merization, we do not do justice to much of ates, as they bear polar and charged groups, interesting literature on systems starting from Previous studies have revealed some qualitative colloids and salt solutions (e.g., sodium silicate kinetic trends, but true materials reaction engi- or waterglass). However, at times we find it neering will require a quantitative understanding instructive to compare some features of TEOS of the competing processes. In this paper we systems with those of similar overall composi- identify kinetic and thermodynamic trends of tion starting from other silicon alkoxides in their the competing processes involved in TEOS parent alcohols (e.g., we will refer to tetram- polymerization and we use these trends to de- ethoxysilane (TMOS)in methanol simply as the duce reasons responsible for the structure pro- TMOS system) and aqueous silica solutions. duced (gel vs. precipitate) in various composi- We are forced to focus on ambient temperature tion regions, preparations, since few kinetic studies at differ-

In the first section we summarize the struc- ent temperatures have been made. tural features of TEOS polymerization as ob- This review is also largely limited to single served by 29Si-NMR spectroscopy across a range step batch reactors. Silica sol-gel kinetic studies of pH -- the single most important composition have most frequently been carded out in batch parameter. In the second section we describe the reactors, although this by no means has to be hydrolysis of TEOS and condensation of par- so! In fact, the most successful multicomponent tially hydrolyzed monomers to dimers during sol-gel processes involve multistep batch pro- the earliest stages of TEOS polymerization, cesses; semi-continuous and continuous steady

These processes can be conveniently and un- state processes are becoming more common, equivocally followed by 29Si-NMR spec- particularly in industrial settings. Even for sin- troscopy. We also discuss metastability and gle component silica synthesis, multistep and phase separation, continuous coating operations offer advantages

In the third section we identify and quantify in microstructure control. We will focus, though, trends in hydrolysis and condensation (com- on single step batch reactors here since most of menting on both kinetics and thermodynamics) the reported experimental data correspond to with increasing oligomer size during homoge- these systems.

208 J. Sef6fk, A. V. McCormick/Catalysis Today 35 (1997) 205-223

2. 29Si-NMR of structural evolution 29Si-NMR spectroscopy has been an ex- tremely useful tool in sol-gel research because it

In the course of a typical SiO 2 sol-gel pro- allows one to monitor concentrations of certain cess, TEOS is introduced to a batch reactor with specific silicon sites quanritatively over the ethanol, water and 'catalyst' -- usually acid course of polymerization without disturbing the (e.g., HC1) or base (e.g., NH3). Water is neces- evolving system (e.g., \[14\]). 29Si-NMR spectra sary to hydrolyze TEOS to allow for poly- show silicon sites with distinct first shell chemi- merization and/or phase separation. Alcohol cal environments (i.e., combinations of groups acts as a common solvent to regulate the misci- immediately attached to silicon) \[15\]. Following bility of TEOS, its polymerization products, and nomenclature introduced in 70's \[16\] these sites water; we will see that it also acts as a reagent, can be labeled Q/, where Q denotes the poten- Acids or bases influence the kinetics of various tially four(quad)-functionality for polymeriza- steps in the sol-gel process, can change solubil- tion, and i and j denote the number of siloxy ity, and can shift polymerization equilibrium and hydroxy groups on the given silicon, re- through coupled ionization reactions, specrively; the number of ethoxy groups on the

The most important feature of TEOS poly- silicon is 4-i-j. QO corresponds to TEOS merization is the competition between the ongo- monomer, Q~ (j = 1,2,3) correspond to partially ing processes of hydrolysis, condensation and hydrolyzed monomers with j hydroxy groups phase separation. For example, when TEOS un- and 4 -j ethoxy groups, and Q4 corresponds to dergoes its first hydrolysis, it may then either: fully hydrolyzed monomer, i.e., silicic acid. Qi (1) undergo the second hydrolysis, (2) react (j = 0,1,2,3)correspond to chain end sites, Q~ with another monomer to produce a dimer, (j=0,1,2) correspond to middle sites, and Q~ and/or (3) precipitate to form a separate phase (j = 0,1) correspond to branching sites. QO cor- (generally in the form of colloidal particles), responds to fully condensed silicon sites.

The fate of a single step batch TEOS system is Si-NMR resonances from fully hydrolyzed governed by the kinetics and thermodynamics silicon sites Q/= 4-i are well separated into five of the competing processes. As we will review bands according to their connectivity i, i.e., shortly, hydroxy (silanol)groups might be barely number of siloxy groups connected to silicon soluble in an alcohol/water solvent, while \[17\]. The presence of ethoxy groups (varying j) ethoxy groups are quite soluble. If silanol groups substantially enriches the NMR spectra by creat- can condense quickly enough, the polymers ing fine structure. In addition, it is a fortunate might stay in the solution. On the other hand, if artifact of 29Si chemical shift sensitivity that it the condensation rate is slow, hydrolyzed species is possible to distinguish Q2 sites belonging to might accumulate and phase separate. (Multi- several specific environments with unusually step batch, semi-continuous and continuous pro- small siloxane bond angles, such as cyclic trimer cesses can be used to manipulate this race!) and cyclic tetramer (Fig. 1; cf. \[18,19\]). How-

We will also review situations where phase ever, using ordinary single pulse NMR it is separation fails to occur even though the silanol difficult to distinguish branched Q3 sites partici- concentration is well above saturation levels, paring in 3-membered rings due to proximity of When phase separation does not interfere, TEOS their chemical shifts to chain middle sites Q2. undergoes homogeneous step growth poly- We have assembled and analyzed a collection merization, although with unusually complex of solution 29Si-NMR spectra monitoring the kinetics. Concurrent hydrolysis and condensa- one step batch polymerization of TEOS with a tion of TEOS then leads to an array of oligomers wide range of initial compositions (e.g., \[18- and polymers of various size, architecture and 28\]). This allows us to qualitatively identify degree of hydrolysis, accessible reaction trajectories in such systems

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