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I n d . E n g . Chem. Res. 1996,34, 421-433
REVIEWS Preparing Catalytic Materials by the Sol-Gel Method David k Ward and Edmond I.
KO*
Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213-3890
Sol-gel chemistry is a versatile tool for both the preparation and understanding of catalytic materials. The large variety of available synthetic parameters provides exceptional control of the sol-gel product’s structural and chemical properties, and this control allows the catalytic researcher to design for and systematically study the effects of composition, homogeneity, metastable phases, and pore structure on catalytic performance. This review describes the basic relevant issues in the sol-gel preparation of catalytic materials by looking at both singlecomponent and multicomponent systems as well as different products such as aerogels and catalytic membranes. In doing so, we highlight the advantages of sol-gel preparation and illustrate new ideas about and understandings of catalytic materials that it has brought forth.
What Is This Paper About? We start by telling you what this paper is not about. It is not about sol-gel chemistry per se because that topic has been extensively reviewed (1-4). It is also not about surveying the literature and inundating you with all the relevant information. Instead, we try to provide a critical analysis of the current status of catalytic applications of sol-gel materials and its future direction. We have a basic premise: We believe solgel preparation to be a versatile means in developing catalytic materials and an important experimental tool in understanding their physical and chemical properties. To defend this premise, we organize the rest of the paper in three sections: 1. We describe the versatility of sol-gel preparation by highlighting its advantages over other preparation methods. 2. We consider three areas (single-component systems, multicomponent systems, and catalytic membranes) in which the advantages of sol-gel preparation have been realized and cite relevant results, from our own work and others, to illustrate what the key findings have been. 3. We speculate on where the field is heading by identifying several research challenges. Our intended audience is catalytic researchers who may be using sol-gel preparation in their work or who may benefit from knowing the latest results of sol-gel catalysts. At the same time, we include sufficient background so that this paper should also appeal to people working in the areas of solid-state chemistry and chemical processing. We encourage those who are interested in learning more after reading this paper to consult the references listed in Table 1.
Why is Sol-Gel Chemistry Important in Catalyst Preparation? Sol-gel preparation, which involves the formation of a sol followed by formation of a gel, typically uses either colloidal dispersions or inorganic precursors as the starting material. Because most literature results are
* To whom correspondence should be addressed.
Table 1. Recommended Reading Materials for background: Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic: New York, 1990 for sol-gel chemistry: Livage, J.; Henry, M.; Sanchez, C. Prog. Solid State Chem. 1988,18,259 Mehrota, R. C. J.Non-Cryst. Solids 1992,145,1 for metal alkoxides: Bradley, D.C.; Mehrota, R. C.; Gaur, P. D. Metal Alkorides; Academic: New York, 1978 Bradley, D. C. Chem. Reu. 1989,89,1317 for catalytic materials: Pajonk, G.M. Appl. Catal. 1991,72,217 Cauqui, M. A.; Rodriguez-Izquierdo, J. M. J.Non-Cryst. Solids 1992,1471148,724 for sol-gel membranes: Burggraaf, A. J.; et al. In Inorganic Membranes: Synthesis, Characteristics, and Applications; Bhave, R. R., Ed.; Van Nostrand Reinhold: New York, 1991;Chapters 2,6,and 7 for general information: Materials Research Society Symposium Proceedings: Better Ceramics Through Chemistry ZZ (Vol. 73,1986);ZZZ (Vol. 121,1988);N(Vo1. 180,1990);V(Vo1. 271,1992);and VZ (Vol. 346,1994)
based on the latter approach, especially on the alkoxide route, that will be our primary focus. However, it is important to realize that sol-gel preparation can be done with a wide variety of precursors. With an alkoxide (M(OR),) as a precursor, sol-gel chemistry can be described in terms of two classes of reactions: hydrolysis: condensation:
-MOR
+ ROM- -..-MOM- + ROH + HOM- -MOM- + H20
-MOH
or
-MOH
+ H 2 0-. -MOH + ROH
-
Despite its oversimplification, this description of solgel chemistry identifies two key ideas. First, a gel forms because of the condensation of partially hydrolyzed species into a three-dimensional polymeric network. Second, any factors that affect either or both of these
0888-5885/95/2634-0421$09.00/0 0 1995 American Chemical Society
422 Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995
reactions are likely to impact on the properties of the gel. In fact, it is the control of many of these factors, generally referred to as sol-gel parameters, that separates sol-gel preparation from other methods. A representative but not exhaustive list of these parameters includes type of precursor, type of solvent, water content, acid or base content, precursor concentration, and temperature. These parameters affect the structure of the initial gel and, in turn, the properties of the material at all subsequent processing steps. A gel, which is a solid matrix encapsulating a solvent, needs to be dried to remove the solvent. The time between the formation of a gel and its drying, known as aging, is also an important parameter. As Scherer (5)pointed out, a gel is not static during aging but can continue t o undergo hydrolysis and condensation. Furthermore, syneresis, which is the expulsion of solvent due t o gel shrinkage, and coarsening, which is the dissolution and reprecipitation of particles, can occur. These phenomena can affect both the chemical and structural properties of the gel after its initial formation. One other parameter that affects a sol-gel product is the drying condition. Conventional evaporative drying, such as heating a gel in an oven, induces capillary pressure associated with the liquid-vapor interface within a pore. In a sample with a distribution of pore sizes, the resultant differential capillary pressure often collapses the porous network during drying. The dried sample, known as a xerogel, thus often has a surface area and pore volume that are too low to be of catalytic interest. There are several ways to minimize the deleterious effect of conventional drying, and xerogels with high surface areas and pore volumes can be prepared with care. One other approach is that of Kistler, who used supercritical drying t o bypass the problem of differential capillary pressure (6). The resultant materials, known as aerogels, have high surface area, porous structure, and low density. In a way drying can be viewed as part of the overall aging process because the material can, and often does, undergo physical and chemical changes during this stage. Let us return t o the issue of control and see how it is relevant t o the preparation of catalytic materials. We simply make a few claims here and substantiate them with actual examples in the next section. In the preparation of single-component materials, sol-gel preparation can achieve very high purity because of the quality of available precursors. Furthermore, the textural properties of the product, most notably surface area and pore size distribution, can be tailored. However, we believe the area in which sol-gel preparation is going to make the biggest impact is multicomponent systems. We see the following specific advantages: (i) the ability to control structure and composition at a molecular level, (ii) the ability to introduce several components in a single step, (iii) the ability to impose kinetic constraints on a system and thereby stabilize metastable phases, and (iv) the ability to fine tune the activation behavior of a sample and thereby trace the genesis of active species. Finally, with either single-component or multicomponent systems, sol-gel preparation allows different product forms to be made. As we shall see, one particular interesting class of materials is inorganic membranes that can perform both catalytic and separation functions.
What Has Been Done? Single-ComponentSystems. A. Control of Pore Structure. The classic example of using a sol-gel parameter to control the pore structure (surface area, pore volume, and pore size distribution) of products is the effect of pH on the properties of silica (I,7). Under acidic conditions hydrolysis occurs at a faster rate than condensation and the resulting gel is weakly branched. Condensation is accelerated relative to hydrolysis with increasing pH. Thus, a base-catalyzed gel is highly branched and contains colloidal aggregates. Handy et al. (8) found that because of the different extent of branching, acid-catalyzed gels contain mostly micropores whereas base-catalyzed ones contain mesopores. There are also chemical differences. Monitoring the structural evolution of silica gels with in situ photoacoustic Fourier-transform infrared spectroscopy, Ying et al. (9, IO) have recently shown that acidcatalyzed gels contain higher concentrations of adsorbed water, silanol groups, and unreacted alkoxy groups than base-catalyzed ones (see Figure 1). The densification behavior of these gels can be understood in terms of these surface functionalities. The implication of these results on catalyst preparation is that, after heat treatment, samples of different physical (pore structure) and chemical (hydroxyl content) characteristics can be prepared for different applications. The type of precursor is another important parameter because the size of the alkoxy ligands changes the rates of both hydrolysis and condensation due to steric and inductive effects (I). Fahrenholtz et al. (21)prepared a series of base-catalyzed silica gels by using as precursors a mixture of tetraethylorthosilicate (TEOS, also known as tetraethoxysilane) and methyltriethoxysilane (MTEOS). They found that when the mole fraction of MTEOS exceeds about 0.50, there is a significant change in the physical morphology of the gel that translates into a large decrease in surface area and pore volume of the dried sample. Adsorption data of carbon dioxide, methane, and water further reveal an alteration of the surface in terms of its polarity and hydrophobicity. Even though most research results regarding sol-gel chemistry thus far are on silica, many concepts and trends are generic and in principle can be applied to non-silicate systems ( 2 , 12). For example, we have demonstrated that changing the relative rates of hydrolysis and condensation, by using different amounts of water and acid in the preparation, affected the pore structure of titania (13) and zirconia (14)aerogels significantly. Figure 2 shows the variation of BET surface area as a function of water and acid content for a series of zirconia aerogels calcined at 773 K for 2 h. We believe that these results actually reflect a variation of gel time, defined as the time required for the solution to undergo a significant viscosity increase during the sol-to-geltransition such that we can no longer maintain a vortex in the solution with a magnetic stir bar. This point corresponds to the incipient formation of a threedimensional network. Figure 3 shows explicitly the variation of surface area and pore volume with gel time, for which we offer the following explanation (14).A gel time of zero corresponds to the formation of a precipitate instead of a gel. This happens when no acid is used in the preparation and rapid condensation leads t o particle growth. The resultant calcined product thus has very little surface area and pore volume. With the use of acid which protonates the functional group, we slow down
Ind. Eng. Chem. Res., Vol. 34, No. 2,1995 423
(B) Figure 1. Schematics of silica gel network derived from the hydrolysis and condensation of tetraethylorthosilicate: (A) acid-catalyzed gel; (B)base-catalyzed gel. (Reproduced from ref 10 with permission. Copyright 1993 American Ceramic Society.)
1150 I CI
0) ln
E
130-
5 H 2 0 I Zr4+ = 2.0
O
0.4
h
H 2 0 I Zr4’
--*
100
4.0
- 0.2
Q)
0
U J
3I-
- 0.3
50
701
-0.1 Filled Circle
I
504 . . . u ’ . ’ . . ’ 1.60 1.75 1.90 2.05 2.20 2.35 2.50 2.65 8
I
8
8
HNOJ amount (ml)
Figure 2. Effect of acid and water content on the surface area of zirconia aerogels after calcination at 773 K for 2 h. (Reproduced from ref 14 with permission. Copyright 1993 American Chemical Society.)
condensation to allow branching t o occur before particle growth. Hence the product gives large surface area and pore volume upon calcination. However, with further lengthening of the gel time (by increasing the acid content), condensation slows down to form a weakly branched network. The collapse of this network during calcination leads t o a decrease in surface area and pore volume. In addition to macroscopic physical properties, gel time affects the microscopic “quality” of the material in terms of defect density. We found that aerogels prepared at different gel times exhibit different behavior with respect to their tetragonal-to-monoclinic phase transformation (14). In the sol-gel preparation of zirconia, Yoldas (15) reported that changing the precursor (from zirconium ethoxide to isopropoxide to n-propoxide to n-butoxide) changes both the particle size and morphology of the
0
10
I
BET of Gel
I00
No . 82
. 0.0 1000
Gel Time (sec) Figure 3. Effect of gel time on the surface area and pore volume of zirconia aerogels aRer calcination a t 773 K for 2 h. (Reproduced from ref 14 with permission. Copyright 1993 American Chemical Society.)
resulting oxide. The larger the alkyl group of the alkoxide, the coarser the texture of the material as shown by scanning electron micrographs. This observed trend, though not fully understood, serves as another example of the effect of the precursor. Results in this section teach us the following. Any of the commonly used sol-gel parameters is effective in changing the product properties as long as it alters rates of hydrolysis and condensation. In fact, since the quality of the gel is sensitive to not only the absolute but also the relative rates (see Table 21, a parameter that changes either hydrolysis or condensation rate can potentially impact the product. Along this line, because gel time approximately varies inversely with the condensation rate, it can be a useful observable in screening the effects of various parameters during preparation.
424 Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 Table 3. Physical Characteristics of Niobia Samplesa (DataTaken from Reference 26)
Table 2. Effect of Relative Rates of Hydrolysis and Condensation on Gel Quality (Information Taken from Reference 2) ~
hydrolysis rate slow fast fast
condensation rate slow slow fast
slow
fast
result colloids/sols polymeric gels colloidal gel or gelatinous precipitate controlled precipitation
B. Effect of Aging Conditions. The study of aging poses an experimental difficulty because solvent remains in a wet gel during the process. Smith and coworkers have overcome this difficulty by developing an in situ nuclear magnetic resonance (NMR) technique that is based on relaxation measurements (16-18). The principle is that molecules in a fluid near a surface undergo spin-lattice and spin-spin relaxation at a faster rate than those far away from the surface. The measured relaxation time can thus be related to a pore size defined as the hydraulic radius. A distribution of pore sizes can be obtained from a distribution of relaxation times. Using this technique, Smith and co-workers have shown that the pore size distribution of a base-catalyzed silica gel is a function of the aging time, aging temperature, and the pH and nature of the aging fluid (17,181. For example, when a sample aged in pure ethanol is compared with one aged in an ethanol-KOH mixture, the latter has a higher initial surface area but the trend is reversed upon complete solvent removal. The mechanistic details of all these observations are not fully understood at present. However, it is important to recognize that a gel is not static during aging and it may be fruitful to use aging conditions to change material properties. This is an area of research that merits more attention, especially for nonsilicate systems. Smith et al. have also developed a series of aging and derivatization steps t o prepare low-density aerogels at ambient pressure (19). We discuss these results within the context of drying in the next section. C. Effect of Drying Conditions. Strategies that are effective in maintaining the integrity of a gel network during drying are based on minimizing the capillary pressure or eliminating it altogether. The capillary pressure (PI associated with the liquid-vapor interface within a pore is given by
where 0 is surface tension, 8 is the contact angle between liquid and solid, and r is the pore radius. Obviously, using a solvent with a lower surface tension will reduce the capillary pressure. Smith and coworkers demonstrated this approach by aging silica gels in either ethanol or water and subsequently washing them in various aprotic solvents with a range of surface tensions (19, 20). They found that for base-catalyzed gels, an increase in surface tension leads t o a linear decrease in the surface area, pore volume, and pore size of dried samples. For acid-catalyzed gels that are less highly cross-linked,the micropore surface area and pore volume increase with increasing surface tension, whereas total surface area and pore volume show an opposite trend. These results clearly show the effects of network rigidity and capillary pressure on the microstructure of xerogels. Furthermore, they establish the feasibility of preparing high-surface-area, low-density materials at
pore BET surface volume, phase identified by sample area, m2/g cm3/g X-ray diffraction amorphous A-Nb205 (aerogel) 190 1.28 100 0.184 TT X-Nb205 a
After calcination at 773 K for 2 h.
ambient pressure, in contrast with the preparation of aerogels with supercritical drying. The main idea behind supercritical drying is to eliminate the liquid-vapor interface altogether and thereby remove the accompanying capillary pressure. Since the pioneering work of Kistler (211, many researchers have explored the catalytic applications of aerogels (22-25). The advantages of supercritical drying on the physical and chemical properties of aerogels can be appreciated with our recent work on niobia (26). We prepared niobia both as a xerogel and as an aerogel under otherwise identical conditions. Table 3 shows that, after the same heat treatment, the aerogel has a surface area that is about twice as large and a pore volume that is almost an order of magnitude higher. Furthermore, the aerogel is X-ray amorphous after heating at 773 K, whereas the xerogel crystallizes into a low-temperature phase of niobia, the M ' ' phase. This difference illustrates how the more rigid porous network of the aerogel kinetically constrains the crystallization of niobia. While this kind of stabilization is often achieved by the introduction of a second component, our aerogel preparation stabilizes a pure niobia that also has a high surface area. The catalytic implication is that the X-ray amorphous form of niobia is critical to maintaining its high acid strength (26). Most supercritical drying is done with alcohol because of its use as a solvent in the sol-gel step. However, Tewari and Hunt (27) showed that carbon dioxide can be used as a drying agent after the displacement of alcohol. There have been recent reports on the effect of drying agent (i.e., alcohol versus carbon dioxide) on the properties of silica (19, 20) and titania-silica (28) aerogels. Specifically, Smith et al. (19) found that the surface area of a base-catalyzed silica gel dried with carbon dioxide is about 25% higher than that of a sample dried with ethanol. Because ethanol has a higher critical temperature than carbon dioxide, these authors ascribed this difference in surface area to accelerated aging at the higher temperature. In the work of Beghi et al. (281, a higher drying temperature with alcohol facilitates the crystallization of titania in titania-silica gels. Since these results suggest that a gel network does not stay intact during supercritical drying, we recently studied the effect of varying the temperature while using a single drying agent, carbon dioxide, on the properties of titania aerogels (29). Figure 4 shows the surface area and pore volume of four titania aerogels, supercritically dried at 343, 383, 423, and 473 K and subsequently calcined at 773 K for 2 h. The drying temperature does not change the surface area appreciably but does increase the pore volume by more than 50% from 343 to 473 K. The change in pore volume but not in surface area suggests that the pore size distributions of these samples are different. In fact, we found that between the two extremes, 343 and 473 K, the higher drying temperature broadens the pore size distribution and shifts the peak maximum to a higher
Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 425 encapsulated in the gel network of the major component.
I As we shall see, in both cases sol-gel preparation leads
"O
t
0.60 1 300
400
5
Drying Temperature (K)
Figure 4. Effect of supercritical drying temperature on the surface area and pore volume of titania aerogels after calcination at 773 K for 2 h. (Reproduced from ref 29 with permission. Copyright 1994 Royal Society of Chemistry.)
value. The drying temperature also affects the crystallization of titania. After heating a t 573 K, the aerogel dried at 343 K remains X-ray amorphous, whereas the sample dried at 473 K crystallizes into the anatase phase (29). These results clearly establish drying temperature as yet another parameter that can be used to alter the properties of aerogels. In addition t o the drying temperature, other experimental variables such as the path to the critical point and depressurization can affect the properties of aerogels (30, 31). In general, a good amount of attention has been paid to the effect of drying conditions on the preparation of glasses and ceramics ( 1 , 321, but less so on catalytic materials which have very different enduse properties. For example, the effect of different drying agents (e.g., alcohol vs carbon dioxide) on the surface functionalities of the dried material has not been systematically studied. This is thus an area that offers many research challenges and opportunities. Multicomponent Systems. A. Overview. This is the longest section of the paper because we believe the topic to be the most important. Many conventional methods of catalyst preparation involve several steps because most active catalysts contain more than one component. For example, the preparation of supported metals or oxides consists of first forming the support, followed by the introduction of a precursor. Sol-gel preparation allows the introduction of several components in a single step. In a two-component system, the minor component can either participate directly in the sol-gel chemistry, in which case there is control over the structural and compositional homogeneity of the product, or not participate, in which case it is simply
t o unique behavior in terms of the activation and deactivation of the sample. A comparison of samples prepared by different methods thus enables us t o better understand the structure-activity relationship in catalytic materials. B. Oxide-Oxide. Two oxides AO, and BO, can be brought together to form either a mixed oxide (A0,BO,, both components in the bulk) or a surface oxide (AOJBO,, the former is supported on the surface of the latter). These materials are of catalytic interest because they often display acid strengths that are significantly higher than either of the component oxides (33-36). In a mixed oxide, the acidic properties are related to the homogeneous mixing of the two component oxides in terms of the number of available A-0-B linkages. However, conventional mixed oxide preparation techniques do not always produce molecularly homogeneous, high-surface-area materials. For example, heating a mechanical mixture of two oxides to high temperatures often results in a sample that has low surface area and pore volume. Coprecipitation, a commonly used technique, does not favor molecular homogeneity because hydroxides of different metallic cations generally do not precipitate at the same pH (37). In contrast, the sol-gel preparation of mixed oxides offers excellent control of mixing because of its ability to alter relative precursor reactivity. In a qualitative manner, we expect good mixing when two precursors have similar reactivities and poor mixing when they do not. Because both hydrolysis and condensation are nucleophilic substitution reactions (2),the matching of precursor reactivity can be accomplished by the following four strategies. (i) Using a precursor containing a different alkoxy group. (ii) Giving a less reactive precursor a head start by "prereacting" it with water. This approach, known as prehydrolysis, is the most commonly used since Yoldas demonstrated its effectiveness in promoting homogeneity in titania-silica glasses (38, 39). (iii) Slowing down a more reactive precursor by replacing some of its alkoxy groups with different ligands in an approach known as chemical modification. Acetic acid and acetylacetone are two common chemical modifiers (40,41). (iv) Performing the preparation at a different temperature. Let us use the preparation of titania-silica mixed gels to illustrate the role of prehydrolysis. In this combination it is the silicon alkoxide that needs to be prehydrolyzed because of its much lower reactivity (2). Schraml-Marth et al. (42)developed a two-stage process which involves the partial hydrolysis of the silicon alkoxide, the addition of the titanium alkoxide, and the subsequent complete hydrolysis. For comparison, they prepared another series of samples by completely hydrolyzing each of the two alkoxides before mixing. The microstructure of all the samples was characterized by spectroscopic techniques including IR, Raman, and NMR (42,43).Figure 5 schematically summarizes their key findings, with the leR and right panels representing samples prepared with the single-stage and two-stage hydrolysis, respectively. The major lesson is that the one-stage samples contain domains of titania crystallites embedded in an amorphous silica matrix, whereas the two-stage samples contain no such domains and are
426 Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995
-
106m2/g
,
I
1
acac 64 sec
PH 67 sec 112 *2/g
TMOS 70 sec 109 m */g
-
T
I
L
I
114
-
I
1
II
-
103
I I-
25
I
I
1
1
115
--
Figure 5. Schematics of titania-silica mixed oxides. Panels on the leR represent samples that are prepared without prehydrolysis (i.e., the one-stage method); on the right, those prepared with prehydrolysis k e . , the two-stage method). The number at the upper-right-hand corner of each box is the sample number. (Reproduced from ref 42 with permission. Copyright 1992 Elsevier.)
better mixed at the molecular level. This observation can be understood in terms of a better match in precursor reactivity from the two-stage prehydrolysis. Differences in the distribution of two components in a mixed oxide lead to differences in catalytic properties. When the above titania-silicas are used to support vanadia, Handy et al. (44) found that the one-stage sample is active for the selective catalytic reduction (SCR) of NO with NH3 and the two-stage sample is not. These authors suggested that the domains of titania crystallite that are present in the former sample interact strongly with vanadia, thereby stabilizing a twodimensional active overlayer of vanadia. On the other hand, the two-stage sample is more silica-like due to its better mixing and, as such, interacts too weakly with vanadia t o stabilize it in an active form. Good mixing turns out not t o be desirable for this particular application. On the other hand, Miller et al.(45) recently found that titania-silica aerogels prepared with prehydrolysis are about a factor of 3 more active in isomerizing 1-butene than those prepared without prehydrolysis. In this case prehydrolysis gives a more homogeneous sample containing more acid sites. The important point is that with prehydrolysis, the homogeneity of a sample can be tailored to meet specific needs-a level of control that is inaccessible with many other preparation methods. Results that are similar to titania-silica have been reported for zirconia-silica aerogels in terms of the effect of prehydrolysis on sample homogeneity (46). In
35/25
35/25
35
28 Figure 6. Effect of different precursor reactivity matching techniques on the crystallization of zirconia in zirconia-silica aerogels after calcination at 773 K for 2 h. Left panel: modification of zirconium n-propoxide with acetylacetone; middle panel: prehydrolysis (PHI of TEOS; right panel: TMOS instead of TEOS. The gel time and surface area of each sample are shown within the panel. (Reproduced from ref 46 with permission. Copyright 1994 Academic Press.)
general, in the co-gelling of two alkoxides, the reactivities of the two alkoxides must be fairly well matched to achieve a homogeneous distribution. If this is not accomplished, then the more reactive component tends to form a 'core" onto which the less reactive component attaches. In the case of non-prehydrolyzed zirconiasilica aerogels in which the silica precursor is less reactive than the zirconia precursor, the segregation of silica on the surface of a zirconia core is evidenced by a higher silanol signal detected by IR spectroscopy (46). Again, such a segregation is not necessarily undesirable as shown earlier for titania-silica. In fact, a poor match of precursor reactivity may even be a convenient way to prepare a supported-oxide sample in a single step. For example, we have observed that a titania-vanadia aerogel, prepared by co-gelling the titanium and vanadium precursors without prehydrolysis, has similar SCR activity to a titania-supported vanadia prepared by the conventional impregnation method (47). Because the titanium alkoxide is more reactive than the vanadium precursor, we expect the co-gelled sample to contain a core of titania crystallites after calcination. Apparently an active vanadia species only needs to be in close proximity and interacting with, and not necessarily deposited on the surface of, crystalline titania. In their study of zirconia-silica aerogels, Miller et al. (46) also examined other strategies in matching precursor reactivity. Specifically,they used tetramethylorthosilicate (TMOS)in place of tetraethylorthosilicate (TEOS), modified zirconium n-propoxide with acetylacetone, and performed the preparation in an ice bath instead of at room temperature. The first two of these three strategies are as effective as prehydrolysis in enhancing the activity of a 95 mol % zirconia-5 mol % silica sample in 1-butene isomerization. At the same time, there are subtle differences in the details, or quality, of the reactivity match. One difference is the extent to which silica affects the amorphous-to-tetragonal transformation of zirconia upon heating as shown in Figure 6. Other issues, such as pore size distribution and the dispersion of surface-segregated silica, are also important. Toba et al. (48) recently prepared alumina-titania with three different methods: complexing-agent-assisted sol-gel, coprecipitation, and hydrogel kneading.
Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 427 Their findings are qualitatively very similar to those of Miller et al. (46)in that the sol-gel sample is the most homogeneous, i.e., well mixed. Together these results show a lot of promise in using various strategies in solgel preparation to control the homogeneity of mixed oxides, but much more work is needed in order to realize its full potential. The work of Uguina et al. (49) demonstrated another exciting possibility of using co-gelled mixed oxides as precursors in the preparation of other catalytic materials. These authors prepared a silica-titania co-gel with a two-step (acid-base) process, impregnated it with a tetrapropylammonium hydroxide solution, and calcined it to obtain titanium silicate-1 (TS-l), a molecular sieve. This approach is superior to the conventional method of preparation because (i) there is control over the properties of the starting material, the silica-titania co-gel, and (ii) it is relatively easy to prevent the formation of titania precipitates. Even though we have intentionally limited our discussion t o a few mixed oxides, the general principles governing their preparation are applicable to other systems. To date there are over 20 mixed oxides containing 2 or 3 components that have been prepared with the sol-gel method (23, 50). Three-component systems pose an interesting challenge from the viewpoint of controlling molecular homogeneity, an area that has received relatively little attention thus far. C. Oxide-Mea. The sol-gel preparation of oxidemetal systems can mean one of two things: the introduction of a metal precursor by conventional methods (such as impregnation and ion exchange) into a support that has been prepared by the sol-gel method, or the direct addition of a metal precursor during the sol-gel process. The first case does not require further mention because we have already discussed the preparation of a support, be it a single or mixed oxide. The second case is important because it again represents a one-step synthetic approach. Unlike oxide precursors which are usually alkoxides, metal precursors do not always participate directly in the sol-gel chemistry. Instead, the metal precursor is often simply encapsulated in a growing gel network, but its presence can still indirectly influence the sol-gel process. The catalytic implications are that sol-gel prepared oxide-metal systems can have very different activation and deactivation behavior than conventionally prepared supported-metal systems, as illustrated by the several examples below. Azomoza et al. (51)prepared a series of WSi02 catalysts by the sol-gel (using HzPtCls-6H20 and tetraethoxysilane) and impregnation methods. They found that the sol-gel samples have higher BET surface areas (from 450 t o 1134 m2/g depending on the metal loading) due to their highly microporous structure. However, for these samples, Pt particle sizes determined by chemisorption and transmission electron microscopy do not agree, suggesting that some surface R atoms do not chemisorb hydrogen. These authors ruled out electronic effects as being responsible for this observation because turnover frequencies for the hydrogenation of phenylacetylene to styrene, calculated on the basis of exposed F't atoms, turn out t o be independent of preparation method. They believe that the Pt particles in the sol-gel samples are a t the surface but are partially buried. These more strongly anchored particles can better resist sintering, maintain a high dispersion and, for this particular reaction, resist deactivation by surface carbon deposition.
Lopez et al. (52)found similar results for RdSiO2 samples prepared by the sol-gel method in that (i)these samples are microporous and have high surface areas, (ii) Ru particles are incorporated into the support framework leading to suppressed chemisorption, and (iii) Ru particles are probably occluded. These characteristics actually give a more mechanically stable catalyst in that the Ru particles do not sinter or volatilize following oxygen treatment a t 450 "C. By comparison a Ru/SiOz sample prepared by ion exchange is not stable under the same thermal treatment. Lopez and coworkers have further demonstrated the effect of structural differences on the catalytic behavior of sol-gel and impregnated Ru/SiOz catalysts, in terms of activity, selectivity, and deactivation, by studying the hydrogenation of benzene (53, 54) and o-xylene (55) and the hydrogenolysis of n-pentane (54). The recent work of Gomez et al. showed that the occlusion (or immersion) of metal particles in a support in sol-gel samples is not limited to silica (56). These workers found that a P t / T i 0 2 catalyst prepared by the sol-gel method shows high metal dispersion and high resistance to sintering that would be consistent with the presence of better anchored particles. Of course, with Ti02 as a support, strong metal-support interaction could be a complicating factor, especially when samples of different particle sizes are compared (57). The examples discussed so far clearly establish that changing the preparation method, from sol-gel to impregnation or ion exchange, is an effective way to alter the morphology and structure of oxide-metal systems and thereby their catalytic performance. Another level of control is that within the sol-gel process itself where different preparation and drying conditions can lead t o different products. In fact, we expect this possibility because of our earlier discussion on the role of the same parameters in the preparation of singlecomponent and multicomponent oxides. In the following examples, we simply have to focus on how different support properties affect the distribution and dispersion of metal particles. Lopez et al. (58)recently prepared sol-gel PdSiOz catalysts in both acidic (pH = 3) and basic media (pH = 9). They found a stronger metal-support interaction in the sol-gel sample than in an impregnated sample and ascribed this difference t o the square planar palladium precursor (t[Pd(NH&C121) used in the former preparation. Again, in the sol-gel samples a fraction of Pd is incorporated in the support network with the rest anchored to the surface. Finally, the effect of pH manifests itself in the distribution of Pd particle sizes and in turn affects the selectivity and deactivation of these samples in the hydrogenation of phenylacetylene. The work of Zou and Gonzalez (59)shows a clever use of another sol-gel parameter, the water to tetraethoxysilane (TEOS) ratio, in stabilizing Pt/SiOz catalysts. In a series of samples prepared with TEOS and platinum acetylacetonate, they found that the H20/"EOS ratio has a large effect on the pore size distribution. Figure 7 illustrates this trend as well as the fact that all sol-gel samples have a narrower pore size distribution and a smaller average pore diameter than a sample prepared by ion exchange. The most interesting observation is that WSi02 catalysts in which the metal particle size matches the average pore diameter resist sintering in oxygen at temperatures up to 675 "C as long as the metal loading is kept between 0.2 and 0.3 wt %. Furthermore, there is no evidence of metal occlusion in
428 Ind. Eng. Chem. Res., Vol. 34,No. 2, 1995
Ion-exchange
-f 8
>
n
20
40
6.0 R O 100 20 0 Pore Diameter ( n n i )
L400
YO0
Figure 7. Effect of HzOmEOS ratio (the numerical value of which is shown on the top of each peak) on the pore size distribution of PffSiOz. (Reproduced from ref 59 with permission. Copyright 1993 Elsevier.)
this study because Pt particle sizes determined from chemisorption agree with those from transmission electron microscopy. Clearly, in the characterization of oxide-metal samples prepared by the sol-gel method, there is a need to quantify the metal content and to perform careful particle size measurements with multiple techniques. In another study Balakrishnan and Gonzalez varied a host of sol-gel parameters and characterized the resulting PUN203 samples in terms of surface area, pore structure, metal dispersion, and catalytic reactivities of n-hexane (60). Specifically they used different precursors (for both alumina and platinum), watedalkoxide ratios, Pt loadings, and drying conditions (conventional drying versus supercritical drying with methanol). They also examined the effect of adding 5 and 10 wt % silica to the support, with and without the prehydrolysis of the silicon precursor. Their results can best be summarized as follows: Sol-gel preparation allows the preparation of PtIAl203 catalysts covering a wide range of support and metal particle morphologies. These samples show a wide range of activity and selectivity, depending on the preparative parameters, but as a group they are all more resistant to coke formation than conventionally prepared catalysts. The effect of drying condition brings us to the discussion of oxide-metal aerogels. The preparation of these materials can take advantage of both the excellent control in sol-gel chemistry and the structural preservation offered in supercritical drying. In fact, setting aside the structural differences, we can identify many common features between xerogels and aerogels. For example, Mizushima and Hori (61)found that a WAlzO3 aerogel has a more uniform distribution and a higher dispersion of Pt particles than a conventionally prepared sample. However, the aerogel has a low activity in methane combustion due t o the possible encapsulation of Pt particles in the support. One feature that is unique to aerogels is that the reduction of a metal precursor can actually take place during the drylng step, especially when high-temperature solvents are used or when the autoclave is filled with hydrogen (23). Evidence of such an in situ reduction has been provided by Armor et al. (62) and more recently by Schneider et al. (63). Schneider and co-workers(63,64) further showed
that p t p T i 0 2 and Pd/Ti02 aerogels, with their meso- t o macroporosity arising from supercritical drying, are particularly attractive in catalyzing liquid-phase reactions. For example, an optimized Pd/TiO2 aerogel has superior activity and selectivity than a conventionally impregnated sample for the hydrogenation of 4-methylbenzaldehyde. The flexibility of sol-gel preparation also makes this method an excellent approach to study supported bimetallic catalysts because (i) both metal precursors can be added directly in the gelling step or (ii) one metal can be added in the gelling step followed by the introduction of a second metal by a conventional method. Even though this area of research has yet t o receive a lot of attention, results t o date have been promising in both establishing feasibility and identifying follow-up work (65-67). D. Other Systems. Examples in this section show that the idea of a one-step sol-gel preparation is completely general in that it can also be used t o introduce dopants into oxides and to prepare other catalytic materials. Ward and KO(68) recently prepared zirconia-sulfate aerogels by mixing sulfuric acid directly with the zirconium alkoxide in the sol-gel step. This approach offers a convenient way to change sulfate content and a systematic way t o study the activation behavior of these materials. From X-ray diffraction and infrared measurements, these authors found that sulfate ions are initially trapped in the bulk of the aerogel. Coincident with crystallization of the zirconia support upon calcination, sulfate is expelled onto the surface and transformed into an active surface species that promotes strong Bransted acidity. A minimum density of sulfate groups is required to create this Bransted acidity which is necessary for n-butane isomerization. These results establish the relationship between crystallinity, sulfate structure and content, and the acidity and activity of this important class of solid superacids. Furthermore, this work illustrates the important effects of preparation method on the sample’s chemical properties. Lopez et al. (69) reported a new way to prepare Li/ MgO catalysts by the homogeneous hydrolysis of magnesium alkoxide in a solution containing ethanol, water, and LiC1, i.e., a one-step sol-gel process. For comparison they also prepared a sample by the conventional method of impregnating MgO with an aqueous solution of LiC1. The major differences between the two samples are that the sol-gel catalyst is (i) more highly hydroxylated, (ii)less active in the oxidative coupling of methane, and (iii) more selective toward olefin formation. Lopez et al. speculated that the way by which Li is introduced affects the nature of active centers in these samples. Apparently in the sol-gel sample, the exchange of a Mg ion by a Li ion forms active centers that produce the successive oxidative dehydrogenation, resulting in high selectivity to ethylene. Satek and co-workers (70, 71)prepared copper aluminum borate by reacting copper nitrate hexahydrate, alumina sols, and boric acid together at a pH of 4-10. The resulting gel is air dried, vacuum dried, and then calcined at 800 “C to give a surface area of about 200 m2/g. These materials are active for the dehydrogenation and dehydrocyclization of alkyl aromatics. A final example is the preparation of inorganic phosphate aerogels. Iacobucci (72) showed that reacting an inorganic alkoxide or metal salt with a phosphate source (e.g., phosphoric acid) forms a gelled or colloidal mate-
Ind. Eng. Chem. Res., Vol. 34,No. 2, 1995 429 rial. Further treatment of such a material by supercritical drying yields an inorganic phosphate aerogel that has high surface area and pore volume. Catalytic Membranes. A Overview. Another advantage of sol-gel preparation is that it can produce materials in a wide variety of physical forms such as powders, monoliths, and membranes. Up to this point, we have been discussing primarily the preparation of powders. However, inorganic or ceramic membranes, due to their dual ability in catalysis and separation, have also been extensively studied. By separating the products from reactants in the reactor instead of downstream, a catalytic membrane could increase the yield of an equilibrium reaction (73).In addition, a membrane can be self-supportingand its physical nature can be changed to control important properties such as pore size, surface area, and dispersion. The inorganic or ceramic membranes prepared by the sol-gel method have much greater chemical and thermal stability than organic membranes (74). These advantages allow their use in the harsher environments typical of catalytic reactors. Ceramic membranes also have a higher resistance to microbial degradation and are mechanically more stable when placed on a robust support (74).While organic membranes are limited to carbon-containingcompounds, ceramic membranes can contain a much larger range of materials allowing for their use in a greater variety of applications (74).The sol-gel method itself provides additional advantages over conventional ceramic processing of membranes. Small colloidal particles, found in the sol-gel method, are required to form the small pores (
