Langmuir 1991, 7, 1675-1688
1675
Surface Activation of Air Oxidation of Hydrazine on Kaolinite. 2. Consideration of Oxidizing/Reducing Entities in Relationship to Other Compositional, Structural, and Energetic Factors Lelia M. Cope**+ Department of Chemistry, San Jose State University, San Jose, California 95192
David P. Summers Planetary Biology Branch, NASA-Ames Research Center, Mail Stop 239-4, Moffett Field, California 94035 Received May 4,1990. In Final Form: February 26,1991
The rates (previouslyreported) for the air oxidation of hydrazine on kaolinite and substituent oxides of kaolinite showed a complexdependenceon the relative amounts of severalstructural oxidizing/reducing entities within the reaction-promotingsolids. The rates indicated an important role of the clay but no dominant role of any one of the oxidizing/reducing entities. In this paper we review (a) the reactionpromoting activity of these centers as studied in other systems, (b) various spectroscopicresults showing interaction between these entities in clays, and (c) reported spectroscopic studies of the complexation between hydrazine and aluminosilicate surfaces as a whole, in an effort to propose a mechanism for the reaction. Whereassome uncertaintiesremain,the present synthesisconcludesthat a mechanismoperating through single electron/hole transfers and hydrogen atom transfers by discrete centers is adequate to explain the observed rate behaviors including the observed second order dependence of the oxidation rate on catalyst amount. The effects of these operations on the catalyst can result in no alteration of, or complete or partial electronic relaxation of ita contingent of trapped separated charge pairs. The degree to which surface complexation as a whole, intercalation, or luminescent processes may also be associated with the reaction cannot be adequately assessed with the information in hand.
Introduction The importance of clay minerals as natural adsorbent and reaction-promotingsurfaces is undisputed. A number of references of special pertinence to this discussion are summarized in ref 1, but many others could easily be assembled. The structural complexity and variability of clays are equally undisputed and prolifically documented. Therefore it is of utility to study, in some detail, the course and, possibly, mechanisms of a few generic reaction types on well-characterized materials. We selected hydrazine oxidation as a probe reaction for clay reaction mechanisms for a number of reasons. Some of these reasons are general to clay reactivity, regardless of specific reaction (even genericreaction type)and someare specificto surface redox reactivity, but not confined to clays. Other reasons must be considered in terms of the specific, curious nature of the kaolinite hydrazine interaction itself. These reasons are detailed in the remainder of this introduction. An in-depth understanding of the mechanism of oxidation of any given reaction on a soil mineral will give insights into the determinants of redox catalysis of other reactions and on other minerals, as many of the reactionpromoting centers are in common. In addition, hydrazine oxidation is highly unusual in that there are two processes (interesting in themselves with regard to the kaolinite/hydrazine interaction) (1) clay intercalationz-5 and (2) light emission.6 These’processes appear to be f To whom correspondence ehould be addressed at NASA-Amee Research Center, Mail Stop 239-4, Moffett Field, CA 94035. (1) Coyne, L.M.; Mariner, FL;Rice, A. Air Oxidation of Hydrazine. 1. Reaction Kinetica on Natural Kaolinitas, Halloysitan, and Model Substituent Layen with V Iron and Titanium Oxide and 0- Center Contento. w m u i r , p x i n g paper in w issue. (2) Van Olphen, H. An Introduction to Clay Colloid Chemistry, 2nd ed.; John Wiley and Sone: New York, 1963; 318 pp.
involved in complex interrelationships with the proposed catalytic centers (iron, hole centers). Luminescence is affected by water which is one of the products and intercalation might be, although there is no evidence to support this supposition for this intercalating agent. In addition, all of these factors (luminescence,intercalation, water, iron, and hole centers) are involved with each other in additional ways that will be discussed in further detail later. That hydrazine spills are arecurrent environmental problem is a practical reason. Air oxidation of hydrazine occurs according to the equation
-
N,H, + 0, 2H,O
+ N,
Heterogeneously promoted hydrazine oxidation can be accurately followed by observing free elemental nitrogen. Because of the weak physisorption of dinitrogen on clay surfaces, discrepancies in stoichiometry can be safely attributed to alternate reaction paths rather than to product sequestration (often not the case in product analysis of clay reactions because of clay adsorptive properties). The reaction is known to be highly exothermic, yet shows little tendency to proceed in the absence (3) Weiea,A.;Thielpnpe,W.;Goring,R.;Ritt6r,W.;Schafer,H.Kaolinit. EmlageNngs-Vergindungen. Roc. Int. Clay Conj. Stockholm Vol. I; benqvist, Th.,Graff-Petenon,P., Eds.;Pergamon Press: Oxford,1989; pp 287-305. (4) Ledoux, R. L.;White! J: L.Infrared Studied of Hydr en Bonding Interaction Between ~ a o l u u t esurfncea and ~ n p r c a d Potauium Acetate, Hydrazine, Fonnamide, and Urea. J. C o l h d Interface Sei. 1968, 21, 127-162. (5) Thompson, J. G. Interpretation of Solid State IFand nOgi Nuclear Magnetic Resonance Spectra of KaoliniteIntercalates.Clays Clay Miner. 1986,33, 173-180. (6) Coyne, L.;Pollock, G.; Kloepping, R. Room Temperature Lumineecencefrom Kaolin Induced by OrganicAmines. Clays and Clayhiiner. 1984, 32, 58-67.
0743-7463/91/2407-l675$02.50/0 0 1991 American Chemical Society
1676 Langmuir, Vol. 7, No. 8, 1991 of a surface. Since a catalyst is required for reaction, competinghomogeneousmechanismscan be, for most part, ignored. In a preceding paper1, the rate of air oxidation of hydrazine was studied by using a variety of natural kaolinite and halloysitecatalystsand somemodel substituent oxides, both natural and synthetic. These materials are known to contain a number of structural oxidizing/reducing entities that might be expected to be catalytic for the reaction. By selection and treatment of materials, the absolute and relative concentrations of these entities can be varied over a wide range. The reaction rate was considerably accelerated, in the presence of the materials selected for study, relative to the uncatalyzed reaction. The amount of nitrogen formed was nearly equal to the amount of oxygen disappearing, indicating simple oxidation to be the primary reaction pathway. The study revealed a remarkable sensitivity of the rate of the reaction of a predetermined volume of hydrazine to variation of the amount of kaolinite (over a 2 order of magnitude variation of the catalyst weight). The stoichiometry of the relationship between oxygen disappearance and nitrogen appearance,coupled with this sensitivity to catalyst weight, showed that oxygen was the oxidant and that the role of the solid was catalytic, rather than stoichiometric. By contrast, the measured rate was remarkably insensitive to the amounts of any of the individual putative oxidizing centers associated with the catalytic materials. Apparently, either these centers are not the only, even the dominant, determinants of catalytic activity or they interact with each other in such a manner that a composite parameter relating them needs to be developed. Because of the contrast between sensitivity of the rate to weight of catalyst, and insensitivity to the numbers of various individualputative centers,this paper will consider the possible mechanistic implications of our results in the broader context of what is generally known about: the mechanismsof hydrazine reactions;promotion of reactions by clay surfaces; a number of highly unusual aspects of the specificnature of the hydrazine/ kaolinite interaction; our own previous spectroscopic studies indicating a high degree of interaction between some of the putative centers; our observations of influence of some of the key reaction conditions on properties of the putative centers. We propose that "catalysis" by naturally occurring,nonstoichiometric materials is inadequate terminology for a number of reasons. Among these reasons are defect centers may serve multiple roles in surface activation, some steps in the reaction may actually be energized by stored electronic energy in the reaction-promoting material, and simultaneous reaction pathways, even reactions, may be supported by the same centers acting in different capacities. It is difficult, with clay minerals, to separate aspects of activation that are produced by the periodic crystal structure of the surface, as a whole (e.g. OH groups, octahedrally coordinated A1atoms,etc.), from those produced by the action of discrete defect sites (e.g. A1 for Si substitutions in the tetrahedral sheet); discrete centers within the catalytic material may interact significantly with each other, as well as with the reactive adsorbate. The intent of the discussion is that it may serve as a frameworkfor devising a general model for clay reactivity if certain key features of clay structure, environmental history, and reaction protocol can be considered in ensemble, rather than individually. Such a model may have a greater degree of both qualitative and quantitative
Coyne and Summers
predictive value than has been attainable by independent consideration of the same factors. In the following discussion, we will refer to normal structural entities as "sites" or as "the surface as a whole". Defect centers produced by cation substitutions,dangling bonds on edges of the crystal, etc., will be refered to as discrete sites. The paper will consist of two parts. The first will present a general mechanistic background and will discuss issues involved with catalysis (involving both normal and discrete sites) by clay minerals. The second will involve the development of a general model of the oxidation reactions,mediated by discreteoxidizingentities, that are important on clays.
General Mechanistic Background The general mechanistic background will focus first on the reactivity of hydrazine in general and then on how clay minerals might promote such reactions. I. H y d r a z i n e Reactivity. A. H y d r a z i n e Decomposition-General Comments. Vapor-phase reactions of hydrazine with substances other than 02 as reactant have indicated the processes to be multistep with several free radical intermediate^.^^^ Diimide is a frequently observed intermediate in hydrazine reactions in gas-phasereaction systems(ref 9, p 310). In solution,metal ions are essential catalysts in the air oxidation of hydrazine. HzOz has been posed as an 0 2 intermediate (ref 9, p 315). Free radical intermediates typicallyare postulated for reactions at surfaces as well, since it is known that mechanisms of vapor-phase reactions frequently require an active surface or p h ~ t o l y s i s . ~ Diimide ~ has been observed in at least one of the surface-inducedair-oxidation studies.1° B. Hydrazine Oxidation. In spite of its reluctance to proceed in the absence of a catalyst, hydrazine is stable in the presence of only a few surfaces. It has been reported to be catalyzed by diverse materials."ll For instance, in ref 10 complete conversion is reported using a variety of metals and metal oxides, including stainless steel, Fe, Al, AlzOs, Zn, Cr,Ni, Ti, sand, concrete, and powdered cinder block. Hayes et al.12have shown oxidation on homoionic montmorillonites, kaolinites, and NaOH-treated kaolinites. A few highly unreactive surfaces can be found. It was shown, for instance, in ref 7 that the appreciable loss of hydrazine on Teflon surfaces can be entirely attributed to permeation into the Teflon. This permeation is enhanced by water, whereas the reaction is seriously inhibited by it.s Similarly,in ref 10,89% recovery is reported from the (7) Stone D. A.; Wiseman, F. L.; Kilduff, J. E.; Koontz, S. L.; Davis, D. D. The Disappearance of Fuel Hydrazine Vapom in FluorocarbonFilm EnvironmentalChambers. ExperimentalObaervationeand Kinetic Modeling. Environ. Sci. Technol. 1989,23,328-333. (8) Schmidt, E. W. Hydrazine and Its Derivatives, Reparation, Properties, Applicatrona; John Wiley and Sons: New York, 1984. (9) Back, R. A. T h e Preparation, Properties and Reactione of Diiide. Rev. Chem. Intermed. 1984,4293-323. (10) Kilduff, J. E.; Davis, D. D.; Koontz, S. L. Surface-catalyzd Air Oxidation of Hydrazines: Environmental Chamber Studies. The Third Conference on the EnvironmentalChemistry of HydrazineFueb; Stone, D. A,, Wiseeman, F. L., Ede.; National Technical Information Service: Springfield, VA, 1988; pp 164-167, 38-49, (11) Kilduff, J. E.; Davis, D. D.; Koontz, 5. L. Surface Catalyzed Air OxidationReactionsof Hydrazines: TubularReactor Studies. The Third Conferenceon the EnvironmentalChemistry of Hydminelibeb; Stone, D. A., Wieeman, F. L., Ede.; National Technical Information Service: Springfield,VA, 1988, pp 128-137. (12) Hayes, M. H. B.; Chia, K. Y.; Yormah, T. B. R.Interactione of Hydrazines with Colloidal Conetituenta of Soi. The Third Conference on the Environmental Chemistry of HydrazineFuels; Stone, D. A., Wineman, F. L., E&.; National Technical Information Service: Springfield, VA, 1988, pp 94-107.
Langmuir, Vol. 7, No. 8, 1991 1677
Mechanism of Kaolinite-Catalyzed Hydrazine
Kaolinite
N2H4+02
SITES CONSIDERED Surface T.M. oxides * Substltutionalcations Multiple charge state T.M. - Charge deficient non T.M. Coordinatively unsaturated AI * OHExchangeable H'
.
-
-
+N2+2H20
INHIBITION BY WATER ( 2 0 2 - + H20 -+ HOC + 0 2
+ OH)
FINAL STEPS OF REACTION H3N2 * + 0 2 NH2NO + HO NH2NO+ N2 + H2O
+
.
MINIMAL REACTION SYSTEM CONSIDERABLE
ACTIVE INTERMEDIATES CONSIDERED
Non-Central Species Considered H' OHOH OIC
PROCESSES CONSIDERED Electron transfer Hole transfer Hydrogen atom abstraction (Proton transferlexchange) PROCESSES ASSOCIATED WITH INTERMEDIATE FORMATION
Figure 1. Schematic summary of proposed mechanism of hydrazine oxidation on kaolinite. The kaolinite structureis labeled with the key structural substituents, contaminants, and proposed discrete active centers. Proposed intermediates are shown for both oxygen and hydrazine. Primary processes producing these are shown on the upper left. Some lesser intermediatesare shown on the upper right. Active centers considered are listed on the lower left. The final stages of the reaction are shown on the lower right.
fluorinated ethylene propylene reaction tubes used to contain their catalysts. In some cases the "catalyst" serves a stoichiometric role as oxidant, in preference to, or preliminary to,air oxidation. For instance, on Fez03 the oxidation proceeds even in the absence of oxygen. A proposed product is FeO," although our own qualitative control reaction showed the formation of magnetite (the black product oxide adhering ad perpetuum to a magnetic stirring bar). The reaction with heavily iron-bearing clays, nontronites, also shows the reaction to be initially mediated by the iron.13J4 For nontronite it has been shown that, in time, the portion of the iron that is located in the octahedral sheet is reoxidized, whereas that in the tetrahedral sheet solubilized. Not all of the considerable iron in nontronites is even susceptible to hydrazine, and the oxidation state of the iron in clays is subject to factors other than the presence of a reducing adsorbate. However, as will become increasingly clear, the clay-mediated oxidation is even more multifaceted. The numerous loci in which iron occurs in clays (tetrahedral and octahedral sites both in the structure and on the surface) and multiple oxidation states (di- and trivalent) constitute only a part of the explanation. ~
~~~
(13) Rumell, J. D.; Goodman, B. A.; kwr,A. R. Infrared and MOMbauer Studies of Reduced Nontronite. Clays and Clay Miner.1979,27,
63-71. (14) Stucki, J. W.; bar P.R.Variable Oxidation States of Iron in the Crystal Structure of Smectite Minerals. In Spectroscopic Characterization of Minerale and Their Surfaces; ACS Symposium Series 415; Coyne, L.M., McKeever, S. W. S.,Blake, D., Us.; American Chemical Society: Washington, DC, 1989; Chapter 17, pp 330-360.
11. Reaction Promotion by Clay Minerals. The promotion of reactions by clay surface includes two important issues, catalysis at discrete centers and interactions of the reactant with the general surface of the clay. A. Discrete Centers of Possible Importance in Hydrazine Oxidation. Natural clays contain catalytic nontransition-metal oxide (silica) and hydroxide (gibbsite) sheets with transition and non-transition-metal substitutional cations. Often times the substitutional cations are positive charge-deficient, these deficiencies being compensatedby exchangeable cations, which, in turn, are hydrated by dissociable water. There are other discrete sites as well, such as coordinatively unsaturated aluminum on the edges of the gibbsite sheets, and trapped separated charge pairs, e.g. centers which are produced near the substitutional cations as the result of ionizing radiation. The complexity of clay composition thus yields many potentially reaction-promoting centers, any one or all of which might assist in the reaction. In Figure 1,a schematic summaryis presented for use in the subsequentdiscussion. The figure compiles the reaction, our proposed reaction intermediates, the kaolinite structure with ita proposed prominent active centers, and the processes by which we postulate these to interact with hydrazine. The important discrete centers have been divided into two categories, oxidizing/reducing entities and acidic centers. We will deal with oxidizing/reducing entities fmt and acidic centers second. 1. Oxidizing/Reducing Entities. i. Surface and Structural Transition-Metal Centers. Clay minerals
Coyne and Summers
1678 Langmuir, Vol. 7, No. 8, 1991
frequently are contaminated by admixed transition-metal oxides, most particularly of iron and titanium, and they contain structural transition-metal cationic substitutions as well, also commonly of iron and titanium. Although the oxides admixed with clays are known to be catalytic, our own studies' showed that they are not the dominant catalytic agents for kaolinite-mediated hydrazine oxidation. Thus they will not be further considered in this discussion. Structural ferric iron, the most prevalent substitutional transition-metal ion in clays, occurs in numerous forms and is identifiable by its electron paramagnetic resonance (EPR) signal as detailed in ref 1and referencestherein. Structural iron centers will be focused on in this discussion. For catalysts containing discrete transition-metal centers (intrinsic to the clay), it seems plausible, and is supported by our data, that the transition metal can serve as catalyst as well as oxidant. A catalytic cycle could proceed through anumber of singleelectron transfers from hydrazine (and intermediates) to metal, ultimately to be closed by reciprocal transfers from metal to oxygen. The means by which these substitutional defects work should be dependent on the electronic structure of the metal center and should be able to be, to some extent, modeled for clays by consideration of the effects of such metal centers on catalysis by silica, alumina, and silica/alumina. Reported studies of this type are concerned with protic and aprotic acid sites. These studies will be summarized in a later paragraph. ii. Trapped Hole Centers. There exists, in addition to Fe(II1) centers, another structural oxidizing entity in clay minerals.15 These centers, often called 0-centers, are one trapping locus for electron vacancies (holes). In clays, these typically are located near isomorphically substituted sites in which Mg or Fe(I1) (positive charge deficient with respect to the predominant structural cations) are substituted for A1 in the octahedral sheet, or Al, or possibly Fe(III), or Si in the tetrahedral sheet of the ~ 1 a y . lHole ~ centers in clays typically are produced, along with "free" electrons, by ionizing radiation (either from natural radionucleotidesor laboratory sources). Thus they can be considered to represent a form of electronic energy storage, as they are the positive moiety of the trapped separated charge pair produced by electronic excitation. Trapped hole centers were argued in ref 1to be present in numbers potentially comparable to those of iron, if all charge-deficientstructural cations were to be compensated by trapping a hole. Many natural kaolinites have an EPR signal attributable to 0- centers of comparable intensity to that of Fe(III).lJ6 Depending on the relative concentrations of structural iron substitutions and holes, and to the extent that the reactivity of kaolinite for hydrazine oxidation is dependent on the presence of structural or surface iron centers, it is postulated that it should also be dependent on the hole center population as well. The influence of stored energy on clay catalytic activity has not been extensively studied, although the existence of significant concentrations of 0-centers in kaolinites has long been known15 and the possibility of their influence on clay reactivity has been ~uggested.l~-~l Hydrazine (15) Angel, B.R.;Jonea, J. P. E.; Hall, P. L. Electron Spin h n a n c e Studies of Doped Synthetic Kaolinites. Cloy Miner. 1974,10,247-266. (16) Coyne, L. M.; Coatanzo,P. M.; Thew,B. K. G. Luminescence and Electron Spin Resonance Studies of Relatio~bipeBetween O--Centera and Structural Iron in Natural and Synthetically Hydrated Kaolinites. Clay Miner. 1989,24,671-693. (17) Coyne, M.L.; McKeever, S. W. S. Overview. In Spectroecopic CharacterizationofMinerab and Their Surface8;ACS SymposiumSeries 415;Coyne,L.M., McKwver,S.W. S.,Blake,D.,Ede.;AmericanChemical Society: Washington, DC, 198%Chapter 1, pp 1-29.
HYDROXYL P A R T I C I P A T I O N
- HD +
H p + SlOD D I
SlOH H
TERMINAL
0.7 x 1 0 ' ~ moleculesigm.hr.'
I
BRIDGING
40 x 10l6
molecules/gm.hr.'
0- C E N T E R P A R T I C I P A T I O N ( p r o p o s e d )
il/
AI
0\
Si
/o\
SI
+
+
hv
H2
- AI - AI
/o\ H I /O\
SI
+
SI
+ H*
e-
D
I
AI
/o\
/O\
SI+Dp-Al
S l + D *
12,000 X H * + D.
10l6 moleculeo/gm.hr.'
- HD
*Taylor. E. H . , ADVANCES IN CATALYSIS
Figure 2. Summary of critical review of past studies of H/D exchange on y-irradiated silica."
reactivity seems an appropriate system in which to address the question. 0- centers have been associated with catalytic activity in several systems, as reviewed in refs 22-24. However, in these materials the prominent mode of hole production is by defects in crystallization or surface adsorption of simple gases, not via y irradiation, as is the case for clays, and therefore is not accompanied by conjugateproduction of electrons. By analogy with iron, 0- center produced reactivity may proceed through catalysis or by direct electron transfer to the hole. By another mechanism, catalysis by clays may be mediated by hole centers. They conceivably can also participate in hydrogen abstraction reactions. Support for this postulate is taken from a critical review17of the literature, summarized in ref 23, surrounding H2 exchange in y-irradiated silica. Routes to H2/D2 exchangeon y-irradiated silica are based on miscellaneous reviewsl7t23and are summarized in Figure 2. It was concluded in ref 17 that hydrogen atom abstraction is the likely source of Hz bond breaking and, as such, stoichiometrically related to the exchange. NH bonds are considerably lower in strength than are HH bonds. Therefore,it is plausiblethat hole centers canalso abstract hydrogen atoms from hydrazine. iii. Interactions between Structural Oxidizing Entities. It is not to be expected that any relationship (la) Coyne, L.; Lawless, J.; Lahav, N.; Sutton, S.;Sweeney, M. Clryl as Prebiotic Catalysts. In O r i g i oflife, ~ ProceedingsGthZnternational Conference of the Znternotionol Society for the Study of the Origin of Life, Jerusalem, June 22-26, la@ Wolman, Ed.; Reidel: Dordrecht, Holland, 1980; pp 115-124. (19) Coyne, L.; Sweeney, M.; Hovatter, W. Luminescence I n d u d by Dehydrationof Kaolin-Aaeociation with ElectronSpinh n a u c a Active Centere and with Surface Activity for Dehydration-Polymerization of Glycine. J. Lumin. 1983,!28,396409. (20) Coyne, L. A. P d b l e Energetic Role of Mineral Surfaces in Chemical Evolution. O r i g i Life ~ 1986,16, 162-206. (21)Aronowitz, S.;Coyne, L.; Lawless, J.; R i p o n , J. Quantum Chemical Modelliing of Smectite Clays. Znorg. Chem. 1982,21,sM)g3593. (22) Che, M.; Tench, A. J. Characterizationand Fteactivity of Monuclear Oxygen Specieson OxideSurfaces. In Advances in Catalyse8;Eley. D. D., Pines, H., Weka, P., Ed.;Academic F%aw New York, 1982; Vol. 31, Chapter 2, pp 78-128. (23) Taylor,E.H. TheEff~ofIonizingRadiationonSolidCatrlyrtr. In Advance8 in Catalysk, Eley, D. D., Pinee, H., W e b , P. B., Eda.; Academic Press: New York, 1968, Vol. 18, pp 11-265. (24) Boudart, M.; Delboulle, A.; Derouane, E. C.; Indovina, V.; Waltere, A. Activation of Hydrogen at 78 OK on Paramagnetic Centan of Magnesium Oxide. J. Am. Chem. SOC. 1972, M,6622+6!3(l.
Mechanism of Kaolinite- Catalyzed Hydrazine
between kaolinite surface reactivity and either iron or hole centers will be a straightforward one, because two studies have shown that hole centers may interact with structural iron in kaolinites to alter both hole center population and the oxidation state of the iron. In the first study, Coynem has reported an increase in the intensity of the EPR signal for structural ferric iron on heating to temperatures which annealed out the hole center signal,implying the possibility that Fez+ transfers an electron to 0-upon heating. A more recent study by Coyne et al.16 has shown that hole center/Fe interactions may also be mediated, in some undetermined manner, via intercalation of water between kaolinite layers. In summary,the isotropic signal for Fe3+ near clusters of Fe2+was found to be increased,while those of both isolated Fe3+ and 0-were decreased over time, after hydration of the interlayer. This work indicates either that charge transfer takes place at ambient temperatures between structural iron and hole centers in hydrated kaolinites and halloysites or that these centers are mutually subject to surface-to-bulk charge transfer mediated by interlayer water. Other examples of interactions between structural and surface oxidizing entities can be found. In montmorillonites it has been reported previously that transport of electronsfrom surfaceadsorbates to structuraliron centers and vice versa13@may be involved in surface reactions. The interlayer separation in a "swelling" clay (a 2:l clay) is only a few angstroms, so such transfer, 'bulk" to surface, would seem plausible. The interlayer surface of a kaolinite (a nonswelling clay, 1:l) typically is not accessible for similar processes. Also, the number of structural oxidizing entities is much smaller. However, the interlayer is available in a kaolinite swollen by hydrazine intercalation, although the distribution of the substitutional cations is not well-known. The possibility of analogous surface to adsorbate charge transfer processes in intercalated kaolinites, therefore, is not out of the question. Given the (a) proximity of adsorbate and clay structural centers to each other, (b) number of potential donor/acceptor sites on the surface and in the structure, and (c) experimental evidence for charge transfer both laterally within the structure (kaolinites)and from interior to exterior (montmorillonites), we postulate that charges transported during interlayer and external surface reactions may be intercepted at centers other than iron and that all charge accepting/donating entities must be considered in formulating catalytic mechanisms for all clay minerals, which plausibly might proceed via electron or hole transfer. 2. Acidic Centers. The surfaces of alumina and silica have themselves been shown to be catalytic2Btmand represent models for the activity of the gibsite (Al(OH3)) and silica sheets of the clay. Catalytic materials such as silica, alumina, and aluminum hydroxide contain no, or very few, transition-metal centers. For these materials it is of interest to consider possible reactive-surface-adsorbed intermediates produced at intrinsic defect sites, such as coordinatively unsaturated sites at the termination of the crystal structure, or extrinsic defects resulting from substitutions by non-transition-metal ions. Silica/alumina has both protic and aprotic sites.M Alumina is known to (25) Tennakoon, D. T. B.; Thomas, J. M.; Tricker, M. J. Surface and Intercalation Chemistry of Layered Silicates, 11. J. Chem. SOC.,Dolton Trona. 1974,2211-2215. (26) Connell, G.; Dumeaic, J. A. The Generation of Broneted and Lewis Acid Siten on the Surface of Silica by Addition of Dopant Catione. J. Cotol. 1987, 105, 285-198. (27) Connell, G.; Dumesic, J. A. Acidic Properties of Binary Oxide Catalyeta 11. J. Cotol. 1986, 102, 216-233.
Langmuir, Vol. 7, No. 8, 1991 1679
have aprotic acidlbase sites.n*m Numbers of these have been calculatedm and their activityBTBhas been reported for zeolites. The actual intermediates produced by interaction with the surface as a whole will be discussed later. i. Silica. The acidity of silica, protic or aprotic, is not an intrinsic property but derives from two sources: (1) surface metal oxide impurities, for which it serves as a s u p p o (2) ~ ~aluminum ~ substitution in tetrahedral structural positions. The second kind, tetrahedral aluminum substitutions, result in protic acidity when the charge compensating cation is a proton. To some extent, protic acidity also results from dissociation of the waters of hydration of hydrated, charge-compensatingmetal ions. Positive charge deficient sites from aluminumsubstitution are comparable to the more numerous ones in clays resulting from tetrahedral substitutions in the silica sheet. They are analogous to sites formed by octahedral substitutions in the gibbsite sheet of the clay. Aprotic sites arise from aluminum substitutions which occur on the edges. These may be coordinatively unsaturated, rather than simply positive charge deficient, thus yielding the aprotic acid sites. ii. Alumina and Silica/Alumina. Admixed oxide impurities or charge-deficientcation substitutions increase the acidity of alumina as well as that of silica. However, some additional aprotic acid sites are produced by oxygen vacancies on alumina, i.e. defects intrinsic to the pure but finite crystal a t ita terminati~n.~'Thus for alumina, catalytic sites can be either intrinsic or extrinsic. Magnesium-substituted aluminumhydroxide, which is a better model for the octahedral sheet of kaolinite, would be expected to provide some protic acidity, and unsaturated edge sites will contribute some Lewis acidity. These sites are stronger acids after thermal dehydration treatments.% iii. Aluminosilicate Structural Factors. Clays will contain all of the discrete site types possessed by their substituent oxides. Those resulting from substitutions will contribute to their redox as well as acidic functionality, via both hole traps and, if the substitutions are by transition metals, multiple valence states. The effects of coordinatively unsaturated edge site^^^^^ similar to those in aluminum hydroxide, mentioned above, are difficult to evaluate in clays. Past studies of the models have not included enough compositional detail to ensure that extrinsic defect structure is not also contributory to observed effecte and, in clays themselves, intrinsic defects are a sine qua non of the structure. Enumerating aprotic edge sites in clays is problematic. The proportion of edge aluminums (tetrahedral or octahedral) relative to the facial ones in a clay will be a function not only of the condition of the edges but also of the aspect ratio (ratio of particle diameter to particle thickness). For the stacks of hexagonal platelets (characteristic of the kaolinites used here) an estimate of this ratio is 6:l for the Georgia providing for roughly 5 times the area on the external planar faces as on the edges. For the Cornish (28) Uyttarhoeven, J. B.;Christner, L. G.;Hall,W.K.Studies of the Hydrogen Held by Solide VIII. The Decationatad k l i t e c l . J. Phys. Chem. 1966,69,2117-2126. (29)Vit, Z;Vala, J.; Jalek,J. Acid-BaseProperties of Aluminum Oxide Appl. Cotol. 1988, 7,169-168. (30)Solomon, D. H.Clay Minerah an Electron Accepton and/or Electron Donors in Organic Reactions. Clays Cloy Miner. 1968,16,3139. (31) Raurell-Colom, J. A.; Serratoea, J. M.Reactions of Clap with Organic subatancee. ~hemietryof Chy8 ond C h y Minerob; Newman, A. C. D., Ed.; Chapter 8,371-423. (32) Card, J. A., Ed. The Electron-Optical Investigation of Clays
MineralogicolSociety Monogmph S;MinerdOgid society (clay Mh" Group): London, 1971; p 128.
1680 Langmuir, Vol. 7, No. 8, 1991
kaolinite, it is more like 2 5 1 (private communication from Len Gate of ECC Int., based on characteristics of typical Cornish deposita). As mentioned, an estimation of the number of hydroxyls required to terminate coordinatively unsaturated edge aluminums has been made for zeolites.28 However we are currently unaware of such calculations for kaolinites. Assuming a similar frequency per edge surface area as calculated for zeolites would place edge sites in kaolinite at a population lowered relative to zeolites in proportion to the fraction of the surface area of the kaolinite which is attributable to the edges. Even if coordinately unsaturated edge aluminum sites were comparablein frequency/unit surface area to charge deficient facial acid sites resulting from structural cation substitutions (or to hole centers produced by trapping excitations near them) they almost certainly will not dominate these in number, because of the relatively low surface area associated with edges. These they do not dominate in strength either, for clays, as is indicated by data (ref 1,Figure 9) that show product inhibition on clays but not on their substituent oxides. This observation can be understood in terms of our proposed mechanism. Hayes et a1.12attributes the adsorption mechanism and catalytic mechanism of hydrazine oxidation on montmorillonites primarily to processes involving hydrazinium ions formed by protons originating from dissociation of the waters of hydration of the exchangeable cations. It is not expected that hydrazinium ion concentration would be of importance for kaolinites, as the exchange capacity for these is of the order of 1-10 mequiv 100g of clay compared to 50-100 mequiv 100g of montmorillonite. In kaolinite, instead, hydrogen bonding with numerous OH groups exposed by intercalation would be expected to be the dominant mode of surface complexation. Even those hydraziniums present would not seem to be likely intermediates for the oxidation, since Schmidt8 reports that the oxidation potential of hydrazinium ion is considerably less than that of hydrazine itself. It would seem that protonation might deactivate rather than activate hydrazine. We therefore postulate that protic acidity will not contribute significantly to the reaction but that aprotic acidity may influence the reaction at the clay to a variable but small extent and dominate it on silica and alumina. B. Overall Surface Interactions i n Reaction Promotion. It was well-demonstrated by our results1that hydrazine oxidation on the materials investigated is being promoted by a number of centers, no one of which appears dominant and for which, as yet, a useful compositevariable has not yet been defined which blends the individual and interactive influences of the various discrete centers which were just discussed. Therefore it is worthwhileto consider further the possible participation of overall surface complexation, in addition to these discrete centers. The kaolinite/hydrazine interaction is intricate in itself, as clay/ hydrazine complex formation is sufficiently disruptive to kaolinite self-hydrogen bonding as to produce intercalation between the aluminosilicate layers and is sufficiently specific as to show pressure-dependent stoichiometries.a* Complex formation via hydrogen bonding to the OH moieties of the gibbsite surface suggests the possibility of a hydrogen-bonding interaction between hydrazine and the kaolinite surface (both external and in(33) Johnston, C. T. Raman and FT-IR Spectra of the Kaolinite-Hydrazine Intercalate. In Spectroscopic Characterization of Minerale and Their Surfaces; ACS Symposium Seriea 415; Coyne, L. M.,McKeever, S.W. S., Blake,D., Ede.;American Chemical Society: Wae.hington, DC, 1989; Chapter 22, pp 432-465. (34) Johnston, C. T.; Stone, D. A. Influencee of Hydrazine on the Vibrational Modes of Hydrazine. Clays Clay Miner. 1990,38,121-128.
Coyne and Summers
terlayer) as a mechanism of hydrazine activation, in addition to hydrazine interaction with admixed impurity or discrete crystal defect centers. Therefore, it is necessary to consider catalytic mechanisms mediated by the aluminosilicate surface as a catalytic entity in itself, in addition to catalysis via sundry discrete centers. 1. Surface Complexation. In order to assess the possible contribution of surface complexation to the reaction mechanism, catalytic and IR studies of hydrazine oxidation on alumina, IR studies of hydrazine adsorption on silica, and IR and Raman studies of hydrazine adsorption on kaolinite will be summarized and the implicationsfor our studies evaluated. This summary will focus on hydrazine complexation on Al2O3, Si02, and clays. i. Hydrazine Surface Complexation with &Os. On the basis of the appearance of diimide and a square dependenceof the rate on the nominal surface area of the catalyst, a skeletal mechanistic scheme was proposed1° for oxidation by water-damagedaluminum,i.e. aluminum coated with aluminum oxide. These authors proposed the formation of a six-membered ring intermediate wherein two hydrogens (initially of hydrazine itself, subsequently of diimide) are bonded to the surface, one to the oxygen and the other to the adjacent aluminum. Two hydrogen atoms supposedly are abstracted in each of two steps, freeing the intermediate diimide (a diradical, H2N2) subsequent to the first binding, or nitrogen, the final product, after the second step. Water supposedly is released from the surface and the remaining metal cation is then reoxidized by the molecular oxygen. The diimide was detected by IR. There is no direct evidence for the proposed intermediate state of the catalyst or any intermediates for the oxygen. ii. Hydrazine Surface Complexation with Si02. Silica is also known to be catalytic for hydrazine oxidation. The authors reportingon the interaction of hydrazinewith water-damagealumina have also reported diffuse reflectance infrared Fourier transform (DRIFT) spectra of hydrazine adsorbed on Cabosil and have found slight shifts of the NH2 deformation frequencies and evidence of H/D exchange which they attribute to hydrogen bonding of hydrazine to silica surface hydroxyl group^.^ They do not positively conclude that a mechanism similar to that proposed for alumina is operative on silica but do express the possibility that the hydrogen bonding tendency may playa role in the reaction mechanism. They do not report seeing a diimide intermediate or any modifications of the silica frequencies that would indicate further hydroxylation of the surface by hydrazine, if indeed the steadystate concentration of this intermediate would be detectable. Therefore there is no direct evidence for the applicability of the scheme proposed for alumina. iii. Potential Contributions of Surface Complexation to Clay Activity. The scheme offered to explain catalysis on alumina,1° which implicates specific surface complex formation, cannot be simply reapplied to kaolinites. For the silica sheet of clay, the same problems mentioned in the previous paragraph apply. Infrared showed only minimal perturbation of hydrazine frequencies on adsorption onto silica and no indication of formation of a silica intermediate, so there simply is no supporting experimental evidence for the extension of the surface adduct mechanism proposed for alumina to silica. Nor is (35) Davis, D. D.; Kilduff, J. E.; Kwntz, 5.L. The Third Conference on the Environmental Chemistry of Hydrazinehteb. Stone,D. A., W b man,F. L., Eds.; National Technical Information Service: Springfield, VA, 1988; pp 154-167.
Mechanism of Kaolinite- Catalyzed Hydrazine
this mechanism for alumina applicable to kaolinite either, because the octahedral gibbsite sheet is already fully hydroxylated. More directly to the point of interactions of kaolinite, overall,with hydrazine are the recent detailed IR/Raman studies of Johnston.38*MWhen excesshydrazine is present, most of this hydrazine is trapped in the interpore space or adsorbed on the external surface of kaolinite. However an intercalation complex is formed which has one of two stoichiometries: (a) For total pressures from ambient pressure to atm (referred to as phase 1 in refs 33 and 34) hydrazine serves as a prop between the layers, interacting primarily with the exposed interlayer hydroxyls of the gibbsite sheet, and is characterized by an IR frequency of 3620 cm-l; the composition was reported to range between 2 and 4 molecules/unit cell. The results for phase 1 compare favorably with earlier ones determined by Barrios et al.% at ambient pressures. The system at atmosphericpressure (corresponding to phase 1) was shown by these investigators to contain at least three hydrazines/unit cell (which itself contains three hydroxyls)in the interlamellar space. (b)When the system is pumped below lo4 atm (referred to in refs 33 and 34 as phase 2) the observed hydrazine is keyed into the ditrigonal holes of the silica sheet and interacts primarily with the inner recessed hydroxyls of the gibbsite sheet. It is characterized by an IR frequency of 3628 cm-l. The composition of this phase down to atm appears constant at 0.5 hydrazine/unit cell determined gravimetrically. The constancy of this compositionis more certain than that determined at ambient pressures. Associationbetween hydrazine and kaolinite is retained at least down to atm. The changing ratio of the 3628 to 3620 cm-l hydrazine bands does not appear to increase to more than 2:1, at atm. At atm the peaks are of nearly equal height. The IR data support a mixture of discrete phase 1 and 2 complexes at low pressures, rather than an average composition. Thus, the molar ratios of hydrazine to kaolinite are welldefined and are altered distinctly (from -3 to -0.5) by diminishing the pressure after initial preparation of the intercalated sample. It appears from these molar ratios that intercalation, at ambient pressures, resulta in stoichiometric complex formation rather than simply taking up hydrazine, a conclusion which is supported by consideration of the available surface area per hydrazine molecule, to be discussed later. The primary interactions of hydrazine with kaolinite are shown by to be via interaction of the lone pair electrons of nitrogen with the inner surface hydroxyls for phase 1 or with inner hydroxyls and the siloxane ring in phase 2. The complex is such that the hydrogen bonds of kaolinite are stronger with hydrazine than with another kaolinite layer, but those of the hydrazine with kaolinite are weaker than those with other hydrazines in the liquid. A red shift of the NN band and a diminution of the intensity of the NH2 rocking band attest to a conformational change of the hydrazine upon complexation. A pronounced narrowing of the hydrazine bands is interpreted to imply ordering of the hydrazine in the interlayer relative to that in liquid. Such conformational changescould, to some degree,activate for decomposition. Only weak hydrogen bonding to the silicasheet is evidenced (36)Barrios, J.; Plancon,A.; Cruz,M. I.; Tchoubar, C. Qualitative and Quantitative Study of Stacking Faulta in a Hydrazine Treated Kaolinite-&lationehip with the Infrared Spectra. Clays Clay Miner. 1977,26,422-429.
Langmuir, Vol. 7, No. 8, 1991 1681
by small shifts of the silica frequencies,parallelling results mentioned earlier% for silica.
Model Development In development of our model of kaolinite catalytic activity we will focus first on the distribution of reactive species on the external and internal surfaces of the clay. Then we will consider the actual mechanism. The mechanism will include consideration of the possible role of kaolinite luminescence in catalytic activity, the role of water (perhaps in autoinhibition),specific conclusions we can draw with respect to kaolinite catalytic activity, and what reactions are expected to occur in hydrazine oxidation. Finally, we will considerthe conditions under which a reactionmechanism,dominated by the activityof discrete oxidizingcenters, might be expected to produce a secondorder dependence of rate on the catalyst amount (under the conditions of our reaction). I. Surface Distribution of Reactants. First we will discuss general issues involved in determining how species are distributed on the surface. Second, the issue of the availability of sites located on the surface of the internal layers sandwiched between the outer layers will be considered. Lastly, we will consider how hydrazine and oxygen species are distributed on the surface. A. General Comments. Vibrational spectroscopy is not likely to be sensitiveenough to detect the intermediates directly,although some unidentified hydrazine bands were observed in the Johnston studies. Even so, IR spectroscopy does assist in two important ways to discriminate between discrete site-mediated and overall surface-mediated reaction mechanisms. First of all, the modest changes in molecular conformation of hydrazine resulting from the complexation do not provide compelling positive evidence for exclusive activation of the hydrazine by complexation. If surface complexation is of importance, its effect is likely to be entropic (holding the reactant fixed in a favorable configuration for reactive collision) rather than enthalpic. Secondly, the recent spectroscopic studies allow us to establish more firmly than did the previous crystallographic studiesw a model distributing hydrazine between external and internal surfaces of the clay (see below). This model is important in ascertaining the surface area, knowledge of which is imperative if the rate dependence on catalyst weight is to be interpreted. Given the available surface area for the reaction, in conjunction with independent data describing the distribution of active centers between external and internal surface, the dependence of the rate on catalyst weight can be used to differentiate between dominance of an overall surface-mediated versus a discrete center-mediated reaction mechanism. B. Site Availability. Collapsed kaolinite comprises attacks of identical monomolecular layers bound together by hydrogen bonds. By this perspective, the surfaces of the internal layers of the intercalated clay might be thought to be comparable to that of the external surfaces in terms of the types and density of intrinsic, normal,and structural sites. If the kaolinite active sites are similarly distributed on exterior and interlayer surfaces (as they would be expected to be in a 2:l clay),site availability (both normal and defect-associated),as well as surface coverage, would be affected by the degree of intercalation. Unfortunately, it is not altogether safe to assume, for kaolinites (1:l) that discrete site types, even isomorphic substitutions, are uniformly distributed between exterior and interior locations. If discrete sites are not uniformly distributed, intercalation could raise or lower the rate (depending on
Coyne and Summers
1682 Langmuir, Vol. 7, No. 8,1991
which region is more densely populated with active sites), unless there is very high mobility of the intermediates between interlayer and exterior surfaces and between grains. If discrete sites were located mostly on external surfaces,intercalationmight actuallyslow the rate, because some of the hydrazine will be sequestered on the less reactive internal surface and be less able to reach reactive sites. More information regarding distribution of substitutional cations would be welcome. Even so, keeping the above comments in mind, the system will be viewed as a multilayered one with active centers associated with both external and internal planar faces of the clay interacting with the fluid reactants. Were the reaction dependent unspecifically on hydrazine residence on the aluminosilicate surface as a whole, or even on specific complexation of the type reported in refs 33 and 34, the rate of disappearanceof a predetermined amount of hydrazine should increase with increasing catalyst amount until there is not enough hydrazine to completely cover the availablesurface of the catalyst. When the amount of catalyst reaches a level whereby the thickness of the hydrazine is only a monolayer, the initial rate, as a function of increasing catalyst weight, will level off as the coverage drops to less than a monolayer and all the hydrazine can be accommodated on the surface (unless there is some difficulty of access of the oxygen to the interlayer). Oxygen diffusion time$' are expected to be rapid, as is also indicated by our observed rates. Under ambient conditions, the hydrazine is readily intercalated, so access difficulties are not anticipated. On the other hand, limitation of the rate because of limited numbers of discrete reactive sites (or deactivation of reactive sites) should be manifested by a marked increase of the initial rate both with an increasing density of these sites in the particular catalytic material and also over a catalyst weight range far greater than that required to provide monolayer coverage for the reactant. It is important also to realize that, since there are several site types, several types of intermediates could be simultaneously formed. To the extent that the intermediates so formed are mobile on the extended surface (i.e. external, internal,and between particles), intermediates formed by reactant interaction with diverse site types may have opportunity to interact with each other and with other site types. If the distribution is nonuniform, or if there are multiple pathways produced by the available reactive centers, the composite rate equation will be made up of contributions from numerous terms. C. Surface Coverage in our Reaction System. The most striking observation from the kinetic studies is the remarkable dependence of the oxidation rate on clay weight (ref 1, Figures 7 and 8 and Table V). A marked increase of rate with increasing weight of catalyst was evident over a range of from 3 to 300 mg of clay, using a constant 10-pL aliquot of hydrazine. The data in Table V indicate the possibility that the dependence of the rate on catalyst weight actually exceeds linearity. As discussed above, the implications of a weight dependence depend on numerous factors: surface coverage (itself determined by whether the interlayer surface is available or not); whether the sites are similarly distributed on external and interlayer surfaces; whether the sites are intrinsic to the perfect crystal or intrinsic to discrete defects within it; mobility of intermediates between interlayer surfaces, external surfaces, between grains, and in the solution bulk. ~
~~~
~
(37) Barrer, R.M.Sorption and Molecular Sieve Pro rtien of Clays and Their Importance an Catalysta. P h i h . Trans. R. London, A 1984,311,333-352.
gc.,
Estimates of the degree to which the hydrazine is covering our clay "surface" were made in three ways. Two difference estimations of the area required for the hydrazine molecule were used to estimate the minimal requirements for hydrazine occupancy, assuming no specific interaction between hydrazine and kaolinite, i.e. that kaolinite simply takes up hydrazine, rather than complexing with it. In the third, the actual crystallographically observed kaolinite unit cell occupancy (ca. 3) and the kaolinite unit cell dimensions were used to determine an actual required area in this system, thus allowing assessmentof whether the hydrazine is taken up or complexed by the clay. (1) Hydrazine molecular dimensionstaken from refs 8, and 38 were used to provide a planar approximationof the surface area subtended by hydrazine in several known conformations. By this approximation, the area of the m2. hydrazine molecule is roughly 3 X (2) The crystallographic dimensions of the hydrazine unit cell measured by Yamaguchim and reported in Schmidt8are divided by 2, because two molecules of hydrazine occupy the cell. By this approximation, the area of the hydrazine is larger, roughly 8 X 10-20m2. (3) The area of the basal plane of the kaolinite unit cell m) X b (8.95 X 1 V 0 m) = 45.6 X is a (5.15 X m2 is much larger than that required by even the more conservative second estimate of the area of a single hydrazine molecule. Even with the observed occupancy of three, an area of 15.2 X 10-20 m2 is provided for each of them. That the available space is not fully occupied lends further suggestion to the possibility that some specific complex formation with the surface seems likely to have occurred rather than the interlayer simply taking up hydrazine. The molecular, or geometric, requirements for complex formation limits the amount intercalated to no more than 3 hydrazines/unit cell there are three hydroxyls/ unit cell of kaolinite. Since there are 3 or 0.5 hydrazines/ unit cell observed (as reported for phases 1 and 2, respectively, see above), it appeara likely that a specific interaction occurs between a kaolinite OH group and hydrazine molecule. For our reaction conditione, the above considerations allow us to postulate where the hydrazine is located. The degree of coverage in excess of monolayer is estimated for the intercalated and unintercalated samples. The external surface area of our unexpanded kaolinites is 15m2/gwhereas the fully expanded kaolinite hydrate has an overall area (mostly 'internal") of nearly lo00 m2/ g." This latter figure is a reasonable estimate for the area of the hydrazine intercalate, as well. Thus the amount of hydrazine used in our experiments, 10 pL of hydrazine ( 2 X lP molecules)is adequate to cover a 100-mgsample of the external surface of unintercalated kaolinite to a depth of about: (1) four molecular layers using the data in refs 8 and 38 (estimate 1)based on the estimated area of the hydrazine molecule and (2) 10 layers using the hydrazine unit cell d a h s (estimate 2). By estimate 1,both external and interlayer surface area of intercalated 100mg samples would be -3% covered and by estimate 2, 16 7% covered. By use of the crystallographic dataa for the kaolinite unit cell dimensions and the experimentally
-
-
-
(38)A b , Y.; Nuclear Quadrupole Rehation of Nitrogen in Hydrazine. J. phy8. h ~ Jpn. . 1977,23, 61-58. (39)Yameguchi,A.; Ichiahima, I.; Shimanouchi, T.;Mimuhima, 8.Far Infrared Spectra of Hydrazine. Spectrochim.Acta 1960,16,1471-1486. (40)Brown,G. X-Ray Identification and Cfy8td Structure8 of Clay Minerale; Clay Minerals Group, Mineralogical Society: London, 1981. (41) Cwtanzo, P. M.; Gieee,R.F., Jr.; Lipeicae, M.Static and Dynnmic Structure of Water in Hydratsd Kaolinites. I. The Static Structure. Clays Clay Miner. 1984,31, 419-428.
Mechanism of Kaolinite- Catalyzed Hydrazine
determined cell occupancy for the kaolinite/ hydrazine intercalate (estimate 3) the internal surface area of the intercalated sample would be 100 O C and EPR signal intensity ascribed to these centers. A low-temperatureTL peak of unexplained origin was seen in freshly irradiated samples and another broad one at -350 "C for all materials. According to ref 15 tetrahedral 0-centers are stable to -250 O C and octahedral ones to -400 O C . Also, the short-term light emission resulting from fresh y irradiation of kaolinites decreases monotonically with time after irradiation and does not alter the intensity of the superposeddehydrationinduced luminescence peak of wetted samples prepared after irradiation. It seems plausible that in clay minerals, trapped charge pairs, photons, and chemically reactive centers may be readily interconvertable17*2'Jby virtue of the nature of excitation, energy transfer, trapping, detrapping, and recombination in solid inorganicmedia. The consequences ~~
~~
(42) McDougall, D. J., Ed. Thermoluminescence of Geological Materials, Proceedings of NATO Advanced Reaearch Institute on Applications of Thermoluminescence in GeologicalProblems;Academic Press: London, 1968,678 pp, See in particular Townsend,P. D., Chapter 2.5; Roach, C. H., Chapter 1. (43) Coyne, L.; Lahav, N.; Lawlean, J. Dehydration-induced Luminescence in Clay Minerals. Nature 1981,292,819-821. (44)Lahav, N.; Coyne, L.; Lawless, J. Prolonged Triboluminescence in Clays and Other Minerals. Clays Clay Miner. 1982,30,73-79.
Coyne and Summers
1684 Langmuir, Vol. 7,No. 8,1991 to surface reactivity are unexplored, particularly for exothermic reactions. In these, the presence of electronic energymay impede establishment of thermodynamicequilibrium, rather than assist it, as would heating. For example, viewing photons as a stoichiometric reagent in exothermic reactions, the electronic energy storage might conceivably stabilize, rather than destabilize surface reactants. It has been previously suggestedm that both wetting and dewetting-induced light release is associated with trapped charge mobilizationoccurringupon change of the liquid/solid interfacial fluid. When the fluid at the liquid solid interface is changed, it is to be expected that there will be a transient charge mobilization resulting from readjustment of the surface energy states of the clay to match those of the replacing interfacial fluid. Recombination of mobilizedcharges intercepted by charge-deficient centers constitutes electronic relaxation, which frequently is accompanied by luminescence. Interception of the same mobilized charges by a reactive adsorbate, however, represents a different fate for the excitation and may thus proceed independent of the luminescence, even at its expense. Of the various room-temperature luminescentprocesses that have been reported, that one releasing the most light over the longest period of time is triggered by wetting with hydrazine. Hydrazine may simply be more efficient in releasing prior-stored energy than are other wetting agents. However,despite the associationbetween emission and hole centers, in the case of hydrazine, it is also possible that at least some portion of the light release results from additional luminescent processes, perhaps differently or not at all related to clay-stored energy, in contrast to light release stimulated by other wetting agents in which the emission profile is less complex. For instance, light likely is released as the result of solidstate phenomena occurring on intercalation of hydrazine in the interlayer. The rate of intercalation is itself a complex resultant of many interacting factors.46 Or perhaps, the energetics of the intrinsically exothermic hydrazinereaction might permit production of some chemiluminescent intermediate or product when hydrazine is oriented by the clay surface. The possibility that the clay surface orients the hydrazine for a chemiluminescent decomposition could not be controlled by these experiments, but work described in ref 16 bears on this issue. At this time it is not thought that chemiluminescence plays a major role in the processes described. The amount of light and the trigger producing its release are of importance to the hydrazine oxidation,because light can be (i) an energy source to aid in promotion/inhibition of the reaction, (ii) a sink for excess energy produced by it and thus an energy source for other reactions, or (iii) an indication of the loss of active centers in the clay. Therefore, it is of interest to know if, and if so, how, luminescence,intercalation, and chemicalreactivity of hydrazine-treated kaolinites are interrelated. An additional caveat must be introduced at this point, which adds yet another dimension to the complexity of this highly interacting system. It was previously stated that structural and surface iron centers can serve independently as (1)catalytic sites or as (2) oxidants, and that (3) structural iron can also engage in internal redox interchanges with hole centers.16*mAdditionally,iron can alter the available level of electronic excitation of the clay ~~~
(45) Churchman, G. J.; Theng, B. K. G. Interactions of Halloyeites with Amidae: MineralogicalFactorsAffecting ComplexFormation. Clay Miner. 1984, 19, 161-175.
in other ways as well. It can (4) serve as a reabsorbing chromophorefor luminescence and (5)promote radiationless relaxationin solids. The roles of structural and surface iron centers in deactivating hole centers or dissipating available photons must be separated and delineated from their simultaneous role as discrete catalytic centers before the roles of energy storage, intercalation, luminescence, or chemiluminescence can be assessed as energy sources for chemical reactivity. Whereas increasing iron will enhance the catalytic contribution due to iron itself, the contributions of these other factors may be diminished by increasing iron. B. Water-Relationship to Surface Activity. The roles of water in the reaction system under study are as numerous and pivotal as those of iron: (1) water is a product of the reaction under study; (2) water strongly inhibits its rate;l*B(3) it mediates the interaction of structural oxidizing entities more generally, as shown by numerous moisture-dependent changesin the spectra and reactivity of clays;6@*31(4) water is one member of a small class of hydrogen-bonding substances that can be intercalated into kaolinite, albeit only if the interlayer is first opened by a primary intercalating agent like hydrazine;& ( 5 ) water strongly alters the intensity and time profile of light release from kao1inite;e(6) water has been implicated in the intercalating efficiency of another intercalating agent, DMSO:' which has a very high strength of selfhydrogen-bonding. There are no published data nor measurements made by us to imply similar importance of water in intercalation by hydrazine which has a considerably lower dipole moment than does DMSO. Water is involved as a product in two chemical reactions that are known to occur under conditions in which light is being released from the catalyst: hydrazine oxidation and peptide bond formation. This raises the question as to what the role of water might be in altering the electronic properties of reaction systems baaed on kaolinite catalysts. Admittedly the two reactions are very different in that peptide bond formation is endothermic, but hydrazine oxidation is exothermic,and in that dehydration-induced luminescence occurs aa the result of dewatering the clay, independent of the amino acid reactant, whereas hydrazine-induced luminescence occurs upon wetting with hydrazine and therefore involves the reactant directly. They are comparable in that the light release occurs for both reactions during the period in which the reaction is presumed to occur and in that water is a product of both reactions. Early experimentation indicated a possible effect of stored electronic energy on peptide bond formation in a wet/dry cycling reaction protocol.'@ It was found that if energystorageis a factor in this latter reaction, the remnant trapped charges, rather than those recombining to release light during the dehydration, must be the operative fraction. C. General Experimental Insights Bearing on Reaction Mechanism. Hydrazine reactivity on kaolinite surfaces clearly is extremely complex. It is plausible to consider activation by both discrete sites and the surface as a whole. However, IR studies of surface complexes, discussed earlier, showed that complex formation affects kaolinite vibrational frequenciesmore than hydrazineones, implying that if complexationactivates hydrazine, it does so via an entropic, rather than enthalpic, effect. The lack of compelling evidence for mediation by the surface as a (46)Costa", P. M.; G i w , R. F.; Clemency, C. V. Synthesia of a lOA Hydrated Kaolinite. Clays Clay Miner. 1984,32, 29-36. (47) Olejnic, S.; Aylmore, L. A. G.; Poener, A. M.; Quirk, J. P. Infrared Spectraof KaolinMineral-DmethylSulfoxideComplexea. J.Phys. Chem. 1968, 72, 241-249.
Mechanism of Kaolinite-Catalyzed Hydrazine
whole, coupled with the observedweight dependence,leads us to develop a discrete site-mediated mechanism in spite of the complicationsof multiple interacting sites, some of which are associated with electronic energy storage. All of the oxidizing site types occurring are of comparable number and all have shown documented reactivity in other systems. The discrete sites that must be considered include iron in various locations and with various geometrical configurations of ligands, and conjugate trapped separated charge pairs produced by prior electronic excitation of the clay. Not one of these classes of discrete sites can be eliminated on the basis of logic or the data in ref 1. For instance, our studies' show some tendency for the rate to be altered by reduction of surface iron prior to the reaction, though by no means proportionally to the diminution of ferric iron centers. In addition, the preceding discussion has shown pointedly that reactivity dominantly mediated by even a constellationof discrete centers cannot be fully understood independently of a detailed understanding of the interrelationships between luminescence,structural oxidizing entities, surface acidity, water, and ease of intercalation, all of which variables can exert than one mode of influence on the others. The necessary detailed structural, spectroscopic, and relative data base does not exist to specify a rate equation in a system as multiparametered as this one. Sorting out the actual mechanism of the reaction will need to be done (a) using a spectroscopicmethod that detects intermediates, or (b) a chemical method that traps them, (c) irradiating the catalysts prior to the reaction to increase the hole center population, or (d) using treated or synthetic materials that eliminate certain sites in favor of others, if not a combination of these methods. We need to better understand the types, relative numbers, and distribution of active centers, and types and mobility of intermediates. We can, however, indulge in limited speculation based on a small number of data-based assumptions. The proposed intermediate steps (see below) are independent of the degree of intercalation of the kaolinite or the distribution of sites between internal and external surfaces or degree of reactant and intermediate mobility. The actual rate would, however, be keenly sensitive to these details. These intermediates steps are consistent with all of the observed data, if it is assumed that some portion of the A1 substitutions in silica (evidenced by the appearance of hole centers upon y irradiation) are coordinatively unsaturated, and thus exhibit aprotic acidity similar to the edge sites of gibbsite. The following assessment of the dependence of the reaction on type of clay seems a plausible starting point for predicting likely intermediates in terms of the nature of the special sites intrinsic to the clay itself. Even this simple compilation of initial steps and the expected interactions among the intermediatesproduced by them, brings considerable clarity to interpreting the salient features of our observations: the insensitivity of the rate to increasing numbers of any of the proposed important site types; the inhibition of the rate by water on clay, but not substituent oxides;the apparent square dependence of rate on catalyst weight; the expected fate of clay sites consequent to the reaction. D. Specific Mechanistic Implications of Our Kinetic Measurements. (1) The 02 stoichiometry of the reaction shows that oxygen, not some oxidizing entity of the catalyst, is the oxidant, thus implying a catalytic function of the clay. (2) The strong clay weight dependence of the reaction,
Langmuir, Vol. 7, No. 8, 1991 1685
even as clay is increased beyond amounts adequate to provide room for monolayer coverage of both external and internal surfaces impliesa highly limited number of active centers, providing further evidence that the role of the surface is catalytic. (3) The dependence seen in ref 1 of the rate of the reaction on the presence of clays containing different amounts of Ti02 and Fez03 is more or less independent of the total amounts of Ti02 and FezO3. These observations indicate that the role of the clay is catalytic in its own right, not primarily as an oxide support for trace catalytic transition-metal ions, as has been observed for silica. Nor do the impurity phases provide most of the reaction-promoting activity. (4)The apparent sensitivityof the rate to prior reduction of the surface Fez03 in the clay implies that surface ferric iron does play some role, but the relative insensitivity of the initial rate to a 17.6 change in this contaminant proves that it is not the only, nor even the dominant, catalytic center. ( 5 ) Similarly the lack of sensitivity of the rate to a 4.8 variation in the amount of structural iron, 4.4 variation in total iron, 8.7 variation in total ferric iron, 8.9 variation in total titanium, and 4.1 variation in 0-centers implies that none of these is the dominant catalytic center. (The rates, not including the bleached clays, vary by a factor of 1.7-fold and those including them vary by 3.7-fold.) Whereas some of the above-listed parameters also vary over a similar range, as seen in Table VI11 and in Figures 10-12 of ref 1,there is no apparent correspondencebetween individual parameter amount and rate. (6)The major diminution of the rate on the clay, relative to that on silica and alumina, as the reaction proceeds implies that (a) one of the products inhibits the clay reaction or (b) the more active sites are deactivated during the course of the reaction, the initial sites being replaced by less vigorous ones as catalyst deactivation occurs. (7) In contrast to the clay itself (Figure 9 of ref l),the apparent lack of product inhibition on the reactions promoted by model silica and gibbsite sheets imply dominance of a different reaction pathway for reaction catalyzed by these model sheets relative to that catalyzed by clay. The transitional behavior shown by the actual clay-derived silica layer, between that of clay and model layers, shows that dominance of the alternative pathways is highly materials dependent. (8) The apparently (greater than) square dependence of the initial rate on the catalyst weight (over the range of weights where even the initial rate is not strongly distorted by product inhibition) is compatible with a role of the catalyst in activating both the hydrazine and oxygen reactants (see below). Interestingly, activation of the catalyst by ionizing radiation produces conjugate oxidizing/reducing charge pairs and thus is expected to be able to serve as activator for both oxidizingand reducing agents. (9) That the number of active sites is limited (from 2 above) is compatible with, but by no means compels, the notion that the active sites are at places where trapped electrical charges are stored, in addition to the more conventionalredox centers involvingtransition-metal ions. E. Operation of Discrete Centers in KaoliniteMediated Air Oxidation of Hydrazine. 1 . Formulation of a Mechanism. In the following groups of reactions (Scheme I), a limited number of intermediates will be presented by which the oxidation could be initiated using the structural redox entities discussed earlier and Lewis acid sites on the edges. These intermediates seem plausible within the constraints of reported hydrazine
Coyne and Summers
1686 Langmuir, Vol. 7, No. 8, 1991 Scheme I CATALYSIS BY STRUCTURAL IRON
HYDRAZINE ACTIVATION
+
(c) O-,.t
:
-
OH*Cel
N-N
+
hole losa by Hatom abalrsctlon
N-N
(b)
o--~
+
-
0 2 - * ~ ~ t~
N-N
:
N-
holeloaabye- tranarer
OXYGEN "ACTIVATION" 02
(I) 02-*ut
(e)
cat-
+ 02
(1)
02-'-t
+
02
(9)
F&
+
02
cat
__t
__*
cat
+
02-
tfappd electron removal
O-cat
02-
hole fanewal by a- lransler
Fe*3cat +
02-
oxldallon 01 susu Iron
FULL RELAXATION (b) O-cat
t
(.)cat-
+
N-N
__+
:
PROGRESS OF HYDRAZINE OXIDATION
(h)
!N-N
:
+ 02-
(I)
N-
+ 02-
U)
N-N*!
t
(k)
N-N';
(I)
N-N
(m)
NHpNO
t
02-*cat
__*
-
~ X M * ~ , _* ~
+ 02
-
HN-N
+
HO2-
N-N
+ HO2
H * trenahr
N-N
+
H*MNln
OH-*ca1
02
H atom abslractlon
PARTIAL RELAXATION
!
N-N
H
+ ex^*,,^ +
(C)
M * Ionexchange
O-cat
N-N
H
+ OH.
Np
+ HzO
reactivity and our observations. Only a limited attempt is made to follow the mechanism in stepwise fashion to ita conclusion,to comment on the relative probability of the various hypothesized intermediates, or to assert that the processes proposed come even close to exhaustively limiting the possibilities for this very complex system and reaction. The fate of the catalyst as the result of these steps will be sketched just far enough to illustrate the fact that the clay may be either restored to its initial state or fullyor partially electronically deexcited by the reaction, depending on which of the proposed possibilities dominate in the actual system. That these varied fates of the clay sites arise so naturally from the nature of the postulated sites underscores the previous assertion' that one should not expect to find a simple relationship between site number and reaction rate. In the set of equations e-g, activation for oxygen was placed in quotation marks because AHfomation for 02-from 02 in the gas phase is positive (ref 48, p 9). It is difficult to predict the energetics of the surface reaction. This
op-
02-*,,1
+
1
CUI
+ 02-
Irapped electron removal
+
holaloUbyHltomab.trectlon. Conjugatea k i m n can ba ntalwd on a u r l w , as oxywn actlvatlon Is not requlnd lor lurlhu h y d W l M mellon.
-
or NH2NO
+
o,-
OH*,t
N-
