Synthesis of High-Surface-Area SiC through a Modified Sol− Gel

A modified sol-gel route was chosen to synthesize SiC, where the precursor ... enhance reaction rates and control the pore structure of the SiC formed...
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Ind. Eng. Chem. Res. 2004, 43, 4732-4739

Synthesis of High-Surface-Area SiC through a Modified Sol-Gel Route: Control of the Pore Structure Puneet Gupta, William Wang, and Liang-Shih Fan* Department of Chemical Engineering, 121 Koffolt Laboratories, The Ohio State University, 140 West 19th Avenue, Columbus, Ohio 43210

High-surface-area SiC has unique mechanical and thermal properties for use as a catalyst and sorbent support. A modified sol-gel route was chosen to synthesize SiC, where the precursor (phenyltrimethoxysilane) was hydrolyzed in the presence of a solvent (methanol) in order to enhance reaction rates and control the pore structure of the SiC formed on firing. High surface areas in the range of 450-620 m2/g and pore volumes in the range of 0.37-0.45 cm3/g were obtained. Use of NaOH instead of NH4OH in the final stage of gel formation resulted in a higher proportion of pores bigger than 50 Å in the pore structure. Similar changes in pore structure were observed when a surfactant (sodium dodecyl sulfate) was added along with methanol to enhance the contact between the precursor and water. Introduction

SiC. Vannice et al.19 synthesized SiC of surface area 50 m2/g by vapor-phase decomposition of tetramethylsilane. White et al.20,21 studied the formation of high-surfacearea SiC using hydrolysis of organosilicon precursors resulting in gel formation, followed by pyrolysis of the gel at 1500 °C in an inert atmosphere of argon. Various precursors and hydrolysis catalysts were tested. The surface areas achieved were in the range of 200-800 m2/g depending upon the method used. The prepared SiC was not found to be in pure form, and HF treatment or carbon burnoff were important to achieve high purity. Phenyltrimethoxysilane was found as the best precursor. Liu et al.22 used a similar method with a mixture of precursors to synthesize SiC/C/silicon oxycarbide composites having surface areas in the range of 400500 m2/g. The hydrolysis of phenyltrimethoxysilane is not much different from standard sol-gel alkoxide precursors. Three water molecules can react with phenyltrimethoxysilane, as shown in reactions (1)-(3) to form phenyltrihydroxysilane with removal of three methanol molecules. As illustrated in reactions (4) and (5), the hydrolyzed molecules may combine and eliminate a water/methanol molecule in condensation reactions. The condensation reactions can continue on, with many molecules combining to form a three-dimensional network of silane molecules having a Si-O-Si linkage. The general formula for such polymers can be written as [PhSiO1.5]n, where Ph is the phenyl group. In the overall reaction, to form one Si-O-Si link, one water molecule is consumed. This fact may be determinant for the final products formed if less than three water molecules are added per precursor molecule. The hydrolysis reactions can be accelerated using an acid catalyst.

The use of SiC in the ceramic and abrasive industry1 because of its good mechanical and thermal properties is well-known. Increasingly, SiC is also being studied for other applications such as catalyst2-7 and sorbent support8,9 and membrane support10 where particle strength, attrition resistance, and high heat conduction are of prime importance. Commercially available SiC (produced by the Acheson process) tends to have a very low specific surface area (0.1-15 m2/g) as a result of the extremely high temperatures (1800-2200 °C) of production, which leads to excessive sintering. A process by which high-surface-area, high-porosity SiC can be made is highly desirable. Numerous studies that report new approaches to produce high-surface-area SiC are reported in the literature. Most traditional approaches utilize the reaction of silica with an external source of carbon, followed by firing at high temperatures to form SiC. Parmentier et al.11 used chemical vapor deposition of carbon using propylene into mesoporous MCM-48 silica material, followed by firing at 1250-1450 °C to form SiC of a specific surface area of 120 m2/g. Singh et al.12 have reported reacting charred rice husk with silica in a plasma furnace and obtained SiC with a specific surface area of 150 m2/g. Other studies used a templating approach using highsurface-area activated carbon to achieve higher area SiC. Moene et al.3,4,13-15 produced SiC of surface areas ranging from 30 to 80 m2/g and a pore volume of 0.2 cm3/g on reacting activated carbon with tetrachlorosilane and hydrogen in the presence of Ni. In other studies,16,17 reaction of SiO vapors and activated charcoal was used to form SiC having a surface area of around 50 m2/g. In yet another set of studies, Atwater et al.18 deposited a SiC film on high-surface-area activated carbon by reaction with SiO vapors. They prepared Pt-Ru/SiC/C catalysts and found that the oxidative resistance was greater if the activated carbon was coated with SiC than without. Another approach to make high-surface-area SiC utilizes an internal source of carbon and silicon to form

PhSi(OCH3)2OH + H2O w PhSi(OCH3)1(OH)2 + CH3OH (2)

* To whom correspondence should be addressed. Tel.: (614) 688-3262. Fax: (614) 292-3769. E-mail: [email protected].

PhSi(OCH3)1(OH)2 + H2O w PhSi(OH)3 + CH3OH (3)

Hydrolysis reactions PhSi(OCH3)3 + H2O w PhSi(OCH3)2OH + CH3OH (1)

10.1021/ie034244e CCC: $27.50 © 2004 American Chemical Society Published on Web 05/08/2004

Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4733 Table 1. Reagents Used in Gel Preparation precursor solvent water base base acid surfactant

chemical

concentration/purity

supplier

phenyltrimethoxysilane methanol, anhydrous water sodium hydroxide ammonium hydroxide hydrochloric acid sodium dodecyl sulfate

97%, reagent grade reagent grade double-distilled demineralized 0.5 M, reagent grade 7.8 M, reagent grade 1 M, reagent grade 70%

Sigma Aldrich Mallinckrodt Mallinckrodt Mallinckrodt Mallinckrodt Sigma Aldrich

Figure 1. Overall gel preparation procedure when (a) NH4OH was used as a base, (b) NaOH was used as a base, and (c) a surfactant was added along with methanol to enhance the reaction rates.

Condensation reactions PhSi(OCH3)2OH + PhSi(OCH3)2OH w Ph(OCH3)2Si-O-SiPh(OCH3)2 + H2O (4) PhSi(OCH3)2OH + PhSi(OCH3)3 w Ph(OCH3)2Si-O-SiPh(OCH3)2 + CH3OH (5) As these molecules (or sol particles) grow in size to dimensions on the order of the wavelength of visible light, they scatter light and cause translucence (cloudiness) in the solution. Upon addition of a base to the solution, the charged double layer shrinks around the particle, making them more neutral and causing them to agglomerate and form a bigger three-dimensional structure called the gel. The kind of pore structure in a material is of high importance for its use as catalyst and sorbent supports. Though high-surface-area SiC has already been created using a variety of techniques, studies related to the effect of various preparation parameters on the pore structure are lacking. For use as a catalyst support, an even higher surface area may be desirable. For use as a sorbent support, higher mesoporous character and a higher total pore volume may be desired. The research discussed in this paper extends the work done by White et al.20,21 to have a better understanding of the effect of sol-gel synthesis parameters on the surface area and pore structure of the prepared SiC. The

preparation scheme was altered in order to achieve different pore-size distributions and higher surface areas. The studies utilized methanol as a solvent to promote the hydrolysis reactions between the otherwise immiscible organosilicon precursor and water. Methanol is soluble in both the organic and inorganic phases and hence enhances the contact between them, which leads to higher rates of reaction. The effect of a strong base and a weak base to form the gel was studied. A surfactant (sodium dodecyl sulfate) was also used to enhance the mixing of the precursor and water phases. The technique also utilized vacuum instead of a flowing stream of argon during the gel pyrolysis step to enhance the surface areas. Though a high-purity SiC may be desired, the use of phenyltrimethoxysilane is known to leave carbon as a residue. White et al.20,21 removed the excess carbon by oxidation in air, followed by HF leaching to remove any SiO2 phase formed. This led to major changes in the surface area of SiC. For the purpose of the present studies, that was not attempted for the following reasons. Because excess carbon removal led to significant pore structure changes, it is difficult to estimate the effect of the initial sol-gel parameters on the pore structure formed after pyrolysis. Also the excess carbon can be removed once the pyrolyzed mass has been impregnated with active ingredients for sorbents/ catalysts rather than before impregnation. For the case of gas-solid reaction-based sorbents, this is highly

4734 Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004

Figure 2. Miscibility of methanol as a function of the water-to-precursor molar ratio.

beneficial because an even greater pore volume is generated upon carbon removal, allowing the reacting gas to easily diffuse into the particle and preventing pore pluggage due to expansions/contractions related with formation of a higher molar volume product. In the case of supported catalysts, excess carbon may be beneficial if a carbide catalyst is desired or a phase change (reduction) of a transition-metal oxide is desired. Depending upon application, the excess carbon may not prove to be detrimental if the catalyst is to be used in a reducing environment. For use in an oxidizing environment, carbon removal methods outlined by While et al.20,21 may be used before impregnation with the active phase. Experimental Procedures Gel Preparation. Table 1 shows the various reagents that were used in the gel preparation. Figure 1a shows the general steps followed to make highsurface-area SiC. The precursor, water, and methanol were added to a clean 50 mL beaker in predetermined ratios and sequence. Mass was taken as a measure of quantity rather than volume because it can be directly correlated to the moles of the substance. A total of 10 g of the precursor (phenyltrimethoxysilane) was always taken. The contents of all of the preparations were stirred at the same speed using similar magnetic stirrers. A total of 1 mL of 1 M HCl was added (always within 30 s) to the beaker. The hydrolysis reaction was allowed to progress for 30 min, followed by the addition of 3 mL of a base (generally NH4OH) to coagulate the growing hydrolyzed precursor particles in the solution. An effort was made with initial gels to monitor the pH as the base was added to the solution. However, the resulting gel formation soon coated the pH probe tip, resulting in unreliable pH values. As a result, the pH measurement was removed from successive gel preparation procedures. Stirring was continued until the gel solidified. For other studies, a different base (NaOH) was used, as shown in Figure 1b, in order to study the change in the pore-size distribution of the high-surface-area SiC. In yet another series of experiments, as depicted in Figure 1c, a surfactant (sodium dodecyl sulfate) was

added in the initial mixture to further aid in increasing the contact between the organic and inorganic phases. Once the gel was formed, the supernatant solution was drained off. The gel was then rinsed with 10 mL of water five times, to remove the excess acid/base/ methanol. The purified gel was then dried in a vacuum oven, under 0.41 atm absolute vacuum, for a minimum of 17 h at 80 °C. The dried gel was transferred to glass vials and stored until pyrolysis. A total of about 40 gels were prepared to conduct the studies. Gel Pyrolysis/Firing. A high-temperature vacuum furnace was used for gel pyrolysis. Graphite crucibles, 0.025 m (1 in.) in diameter and 0.025 m (1 in.) in height, were used to hold the various gels. A total of 11-15 gels could be fired at one time. The crucibles were arranged in special patterns to ensure that each crucible was exposed to the same heating environment. The gels were also randomized with respect to crucibles to further aid in the cancellation of random errors. A 10-5 Torr vacuum was generated inside the furnace. A similar temperature ramping program was used for all firings. The program consisted of raising the furnace at 20 °C/ min to 700 °C, followed by 10 °C/min to 1100 °C and then 5 °C/min to 1500 °C. The gels were kept at 1500 °C for 2 h, after which the furnace was allowed to cool. When the temperature in the furnace was close to ambient, the vacuum was released by introducing fresh air into the furnace. One firing of the furnace required 1 day to complete. The residual mass was removed from the graphite crucibles, ground up, and stored in vials. Characterization tests then followed. Miscibility Curves. Methanol is completely miscible with both water and the precursor. However, water and the precursor are not soluble in each other and form a two-phase solution. Upon addition of a sufficient amount of methanol to a water/precursor mixture, a one-phase solution exists. This is referred to as the miscibility point. The miscibility curve for methanol as a function of the water-to-precursor ratio was obtained by adding methanol drop by drop to emulsions of water and the precursor. The emulsion appeared white as a result of the formation of tiny droplets. Upon addition of sufficient methanol (miscibility point), the white color disappeared into a one-phase solution. Figure 2 shows

Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4735

Figure 3. Phase separation as seen on (a) addition of precursor and water and (b) further addition of methanol below the miscibility point.

the miscibility curve of methanol as a function of the water-to-precursor molar ratio. This can be taken as a two-dimensional representation of the ternary solubility diagram in the region of interest for the studies. Characterization Techniques. The X-ray diffraction (XRD) technique using Cu KR radiation in a Scintag XDS2000 X-ray diffractometer was used to identify the compounds in the pyrolyzed mass. Low-temperature (77 K) nitrogen adsorption was carried out in a Quantachrome Nova 2200 Brunauer-Emmett-Teller (BET) apparatus to measure the surface area and pore structure of the prepared gels and silicon carbide. All samples were degassed in vacuum at 300 °C for 1 h. The linear range of the adsorption isotherms spanning a range of P/P0 (P0 ) 1 atm) from 0 to 0.35 was used to calculate the specific surface area of the solid. The total pore volume was calculated based on the total adsorption at a relative P/P0 of 0.95. The Barrett-Joyner-Halenda (BJH) pore-size distribution was calculated based on the desorption isotherm in the range of P/P0 ) 0.05-0.95. Results and Discussion Gel Preparation. The addition of the precursor and water resulted in a two-phase solution. However, because of the similarity of densities of the precursor and water (1.06 and 1 g/cm3, respectively), separate top and bottom layers were not observed. The phase separation is depicted in Figure 3a. Upon addition of methanol below the miscibility limit, the two separate phases formed distinguishable layers, as shown in Figure 3b. The top layer is expected to be methanol-rich (because its density is 0.79 g/cm3), saturated with the precursor and water, while the bottom layer can be either waterrich or precursor-rich depending on the starting composition. On standing, the top layer turned milky. This phenomenon corresponds to the growth of sol particles via the hydrolysis of the precursor. Once the dimension of these particles is similar to the wavelength of visible light, they start scattering light and the solution appears milky. It can be concluded that a solvent (methanol, as in the top layer) is able to enhance the hydrolysis reaction by bringing the precursor and water molecules in proximity to each other. If the initial beaker contents contained methanol in excess of the miscibility point, a clear one-phase solution was observed. This solution turned milky over longer periods of standing. An acid (HCl) was always used to catalyze the hydrolysis reaction. The two phases disappeared when HCl was added, giving way to a one-phase clear solution. The addition of acid seems to show phase-altering properties similar to those of the addition of methanol. The effect of acid concentration was not studied for the purpose of this paper, and fixed amounts were always added.

Once the designated gelation time of 30 min was over, a base was added to the solution to create the gel. Upon addition of NaOH, almost instant gel formation was seen. However, upon addition of NH4OH, gelation occurred over a longer period of time, ranging from a few minutes to about half an hour depending on the composition of the solution. The gel was vacuum-dried for a minimum of 17 h. The final gel yield was found to average around 0.61 g of gel/g of precursor. The gel was easily broken and ground into a fine powder. No significant peaks were found in the XRD patterns corresponding to the gels, showing an absence of crystallinity in the gel. Though short-range crystallinity may exist in the sample because of the formation of polymerlike chains, long-order crystallinity was essentially missing. Surface Areas of Gels. The surface areas of all of the prepared gels were found to be below 3.5 m2/g. Though not a strong influence, there was a general tendency for the surface areas to be higher when methanol was added as a solvent compared to when it was not. No major correlation could be found with respect to other experimental parameters such as the precursor-to-water ratio, the base used, etc. This is probably due to the fact that the surface area of the gel is governed by other parameters as well, which include the rate of nucleation and growth of gel particles in the solution and time provided after base addition for gel formation and consolidation, filtration, gel rinsing, and drying. The latter three factors were more accurately controlled in a similar way for all of the gels. However, nucleation and growth rates for gel particles were not adequately controlled and hence may have led to the observed gel surface areas. Gel Firing. The residual mass left after pyrolyzing the gel in the vacuum furnace was a porous, black, shiny solid. The black mass was quite brittle when it was a few millimeters in size but became progressively more difficult to break as the size became smaller, on the order of 500 µm. After the pyrolysis, there was a coating found on the walls of the vacuum furnace. It is speculated that the coating is composed of the volatile pyrolysis products that deposited on the much cooler furnace wall. The coating was easily removed using a stainless steel cleaning solution. The final pyrolyzed mass yield was found to be about 0.24 g/g of precursor taken. XRD (Figure 4) conducted on the pyrolyzed mass found peaks corresponding to only SiC regardless of the starting gel. A similar observation was reported by White et al.20,21 They also reported the presence of unreacted carbon. The pyrolyzed mass was treated with air in a thermogravimetric analysis apparatus. About 32% weight drop was observed, attributed mostly to the excess carbon. Combined with the pyrolyzed mass yield, this

4736 Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004

Figure 4. XRD pattern of the fired gel showing peaks corresponding to only β-SiC.

Figure 5. Comparison of BET surface areas achieved as a result of vacuum pyrolysis (present work) and pyrolysis in an argon environment (as reported by White et al.20,21 without excess carbon removal) for phenyltrimethoxysilane-derived gels with no methanol addition.

accounted for about 0.163 g of SiC/g of precursor taken. For a 100% conversion of the precursor to SiC, a yield equal to 0.202 g of SiC/g of precursor is predicted. Hence, about 81% conversion of the precursor to SiC is obtained. As discussed before, because this paper studies the effect of nontraditional sol-gel synthesis parameters on the pore structure of the pyrolyzed mass, carbon removal and subsequent changes in the pore structure have not been attempted here. Also, from an application point of view, it may not be necessary to remove the excess carbon at this stage. White et al. have studied the issues related to carbon removal in great detail, and their method should be followed in case carbon removal is absolutely necessary before impregnation of active catalyst/sorbent ingredients. For the ease of reference, the pyrolyzed mass will be referred to as SiC henceforth. Surface Areas of SiC. The specific surface areas of the various SiC prepared were found to lie in the range of 450-620 m2/g. White et al.20,21 had reported surface areas in the range of 220-590 m2/g for acid-catalyzed hydrolysis of the precursor. The modified technique used in the current study was found to provide a higher surface area corresponding to that used by White et al.20,21 Figure 5 shows the comparison of surface areas for SiC prepared without any use of solvent (methanol) using vacuum pyrolysis (present study) and pyrolyzing in an argon environment

(White et al.,20,21 without excess carbon removal). It can be clearly seen that the use of vacuum pyrolysis led to higher surface areas rather than an argon environment pyrolysis. The gels made in the current study also had much larger amounts of acid catalyst than those used in the literature.20,21 A few possible arguments may be used to explain the higher surface areas achieved. Pyrolysis products easily escape the particles during vacuum pyrolysis, which may lead to further opening of pores. To accelerate the pyrolysis, the rate of heating was kept at 5 °C/min for the pyrolysis step, higher than the heating rate of 2 °C/min recommended in the literature.20,21 Faster pyrolysis would lead to higher porosity structure generation because of the higher velocity of the issuing gases. Also, with higher acid catalyst present, the hydrolysis reaction may proceed to higher conversions and to higher surface areas in the pyrolyzed product. An additional advantage of using vacuum pyrolysis is that it eliminates the chance of oxidation of the gel as in the case of the argon environment, where trace quantities of oxygen are usually present. The trend observed for argon environment pyrolysis, where the surface area decreased for a higher waterto-precursor ratio, was also observed for SiC made through vacuum pyrolysis. However, the trend was not as sharp for vacuum pyrolysis. Effect of the Composition. Surface Area. Various starting feed compositions were investigated to study its effect on the surface area of SiC produced. Figure 6 shows the variation in the surface area as a function of different water/precursor and methanol/precursor molar ratios. NH4OH was used as the base for these experiments. As discussed before, in the absence of methanol, a decrease in the specific surface area was observed upon the addition of additional quantities of water. For methanol additions below the miscibility limit, water is not observed to have any significant impact on the surface area for water/precursor ratios above 3. Further, it is clearly observed that, in this range, further addition of methanol led to higher surface areas. However, below a water/precursor ratio of 3, the surface area is observed to increase upon the addition of additional amounts of water and to decrease with the addition of additional amounts of methanol. A possible explanation may be given as follows: White et al.20,21 had postulated that having a higher degree of hydrolysis led to higher surface areas. Because every molecule of phenyltrimethoxysilane needs three molecules of water to completely hydrolyze, any excess water molecules added

Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4737

Figure 6. Variation in the BET specific surface area of SiC as a function of the water-to-precursor molar ratio at methanol-toprecursor molar ratios of 0 (b), 1 (9), and 2 (2) and at the miscibility limit ([).

stay in solution even on complete hydrolysis. From an overall reaction point of view, to create a single Si-OSi bond, only one water molecule is needed. In excess water, such linkages are easily formed as a result of reactions (4) and (5), leading to long-chain threedimensional structures. However, when water molecules are deficient, fewer number of Si-O-Si linkages form, resulting in smaller molecular chains and unhydrolyzed precursor molecules. Such unhydrolyzed molecules would probably lead to lower surface areas and hence would lead to the observed trends. The observed increase in the surface area as a result of higher methanol concentration can be explained on the basis of two factors. First it leads to higher mixing of precursor and water, enhancing the rates of reaction and leading to a higher degree of hydrolysis. The second factor can be due to the increased distance between different nucleation sites and hence reduced steric hindrance toward the approach of water molecules toward partially hydrolyzed precursor molecules. Both factors will lead to higher Si-O-Si bonding. However, upon an increase of the methanol content of the starting mixture to just above the miscibility point, the surface areas of SiC formed are found to decrease considerably below the surface areas obtained with methanol composition below the miscibility point. It is to be noted here that, for increasing water/precursor ratios, a higher amount of methanol is required to obtain miscibility, as depicted in Figure 2. The possible explanation of the lowered surface areas is as follows. Though methanol aids in bringing the precursor and water together and enhancing the reaction rates, it is also a product of the hydrolysis reaction. If added in excess to gain complete miscibility, which is the case in this experiment, methanol may prevent the hydrolysis reaction from going to completion. Incomplete hydrolysis may lead to the lowered surface areas observed. Sequence of Methanol Addition. The hydrolysis reactions first involve the formation of active nuclei sites through either initial hydrolysis reaction or the presence of impurities and dirt particles. As the hydrolysis reactions proceed, these nuclei grow in size to form the sol particles, and finally upon addition of a base, these sol particles come together to form the gel. The sequence in which the precursor, water, and methanol are added may have an important impact on the rate of nuclei formation.

Figure 7. BJH pore-size distribution of a typical SiC formed using a submiscibility amount of methanol and NH4OH as the base.

The sequence of the addition of methanol was studied in order to understand if it has an influence on the surface areas of SiC formed. Experiments were conducted for a methanol-to-precursor ratio of 1, at different water-to-precursor ratios, where methanol was either added first or last to the reaction beaker. The corresponding surface areas obtained were then analyzed using the matched pair analysis to see if there is any difference due to the order of the addition of reagents. The probability of the sequence of adding reagents affecting the surface areas of SiC formed using a simple t test23 on the differences of matched pairs was found to be 40.9%, much below the desired confidence level of 95%. Hence, it was concluded that the order of reagent addition to the beaker was not significant. Pore Structure. The BJH pore-size distribution of a typical SiC produced using NH4OH as the base below the miscibility point of methanol is shown in Figure 7. It is readily seen that most of the pores are found in the 30-50 Å range. The average total pore volume for the SiC particles was found to be 0.42 cm3/g. Given that the ultimate specific gravity of SiC is 3.1, the high pore volume corresponds to about 57% porosity within the SiC particle. Such a pore volume is highly desirable if SiC has to be impregnated with active catalytic or sorbent ingredients. Effect of the Base. Though NH4OH was predominantly used to coagulate the growing sol particles, NaOH was also used to study its influence on the pore structure of SiC formed. NaOH led to immediate coagulation and formation of gels compared to NH4OH. This can be expected because NaOH is a strong base while NH4OH is a weak base. NaOH is able to shrink the charged double layer around the sol particles more strongly, making them more neutral and thereby gelling them easily. NH4OH, on the other hand, shrank the charged double layer slowly, resulting in a longer gelation time in which probably the hydrolysis reaction may also go to higher conversion. SiC was prepared for compositions corresponding to methanol/precursor molar ratios of 0 and 1 and water-to-precursor ratios of 2 and 5. Both NaOH and NH4OH were used as bases with replicates. Table 2 gives a comparison for the different surface areas and pore volumes achieved using NaOH and NH4OH. It is clearly observed that higher surface areas and pore volumes were achieved when NH4OH was used. Figure 8 details the pore-size distribution of two typical SiC samples made using the two bases. It is seen that there is still a maximum seen in the pore-

4738 Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 Table 2. Variation of the Specific Surface Area and Total Pore Volume as a Function of the Initial Composition and Type of Base Added To Form the Gel NH4OH

NaOH

precursor/ methanol/water molar ratio

surface area (m2/g)

total pore volume (cm3/g)

surface area (m2/g)

total pore volume (cm3/g)

1:0:2 1:0:5 1:1:5 1:1:2

577 557 575 564

0.40 0.42 0.43 0.42

461 483 490 491

0.37 0.38 0.37 0.39

Figure 8. Variation in the BJH pore-size distribution for a precursor/methanol/water molar ratio of 1:1:5 as a result of different base additions.

Figure 9. Variation in the BJH cumulative pore-size distribution for a precursor/methanol/water molar ratio of 1:1:5 as a result of different base additions.

size distribution around 30-40 Å for SiC prepared using NaOH. However, the volume of the pores in this region is much diminished compared to SiC prepared using NH4OH. However, the pore volume in the range of 40200 Å is higher for the NaOH case. A more thorough understanding can be obtained by studying the cumulative pore-size distribution for the two bases, as in Figure 9. It is readily seen that most (94%) of the pore volume for SiC made using NH4OH resides in pores of size less than 50 Å. However, only 73% of the total pore volume in SiC prepared using NaOH is occupied by pores smaller than 50 Å. The rest, 27% of the pore volume, is occupied by pores in the size range of 50-200 Å. Table 3 lists the fraction of the volume occupied by pores bigger and smaller than 50 Å on the use of NH4OH and NaOH as the base. It is clearly observed that SiC prepared using NaOH has a higher

Table 3. Percentage of the Total Pore Volume Occupied by Pores Smaller and Bigger than 50 Å for SiC Made Using NH4OH and NaOH, Based on the BJH Cumulative Pore-Size Distribution precursor/ methanol/water molar ratio

50 Å

50 Å

1:0:2 1:0:5 1:1:5 1:1:2

90 94 94 95

10 6 6 5

71 68 73 69

29 32 27 31

NH4OH

NaOH

Figure 10. Variation in the BJH cumulative pore-size distribution for a precursor/methanol/water molar ratio of 1:1:5 as a result of different base additions and use of a surfactant.

proportion of pores bigger than 50 Å compared to SiC prepared using NH4OH. One of the possible reasons could be in the way the gel is created upon addition of NaOH. Because it is a strong base, it causes the sol particles to coagulate at a very fast rate, tending them to pack loosely. This will result in voids in the gel, leading to higher mesoporous character. However, more experiments and characterization are required to fully determine the reasons for the pore structure change. The presence of pores greater than 50 Å can be important if the prepared SiC is to be used as a catalyst or sorbent support. Mesopores will allow the reaction gas molecules to freely enter the particles, enhancing the rates of reaction. A higher surface area, on the other hand, as in the case of NH4OH, will provide enhanced reaction rates. The use of different bases allows for control of the pore structure of the high-surface-area SiC. Effect of Using a Surfactant. In an effort to improve the contact between the precursor and water, the use of a surfactant was also considered along with methanol. Sodium dodecyl sulfate was chosen as the surfactant. Compositions corresponding to a precursor/ methanol/water/surfactant molar ratio of 1:1:5:0.137 were tested, with the surfactant being added last to the preparation beaker, followed by 30 min of stirring and addition of NH4OH to form the gel. The surfactant was extracted into the water on rinsing of the gel. The XRD pattern on the pyrolyzed mass revealed peaks corresponding to only SiC. The surface area of SiC was found to be around 520 m2/g. Figure 10 shows the cumulative BJH pore-size distribution for SiC made using NH4OH as the base and NaOH as the base and finally with the use of surfactant. With surfactant, it is seen that about 74% of the volume was occupied by pores smaller than 50 Å, while the rest, 26%, was occupied by pores bigger

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than 50 Å. The total pore volume was found to be 0.42 cm3/g. The difference in the pore structure can be due to the large surfactant molecules situated in the gel matrix. As the gel is pyrolyzed, these surfactant molecules dissociate, leaving a mesoporous cavity behind in the SiC structure. Because the pore structure created on using a surfactant is still different from that obtained by using only NH4OH or NaOH as the base, there is a unique possibility of controlling the pore-size distribution by changing these parameters in a coordinated manner.

(7) Keller, N.; Pham-Huu, C.; Estournes, C.; Ledoux, M. J. Lowtemperature selective oxidation of hydrogen sulfide into elemental sulfur on a NiS2/SiC catalyst. Catal. Lett. 1999, 61, 151-155.

Conclusions

(10) Suwanmethanond, V.; Goo, E.; Liu, P. K. T.; Johnston, G.; Sahimi, M.; Tsotsis, T. T. Porous Silicon Carbide Sintered Substrates for High-Temperature Membranes. Ind. Eng. Chem. Res. 2000, 39, 3264-3271.

SiC with a high surface area ranging from 450 to 620 m2/g and a high pore volume range of 0.37-0.45 cm3/g was prepared using the modified sol-gel route. Firing the gel in vacuum instead of an argon environment produced higher surface areas. The methanol addition to enhance the contact between precursor and water led to a higher surface area below the miscibility limit. However, the addition of methanol beyond the miscibility limit led to a reduction in the SiC surface areas. The sequence of methanol addition was not found to affect the SiC surface areas. An increase in the water content beyond a water-to-precursor ratio of 3 did not show significant improvements in the SiC surface areas. The majority of pores were found to be smaller than 50 Å when NH4OH was used as the base. However, upon using NaOH as the base, a greater proportion of pores greater than 50 Å were observed. The addition of a surfactant (sodium dodecyl sulfate) showed similar results. Hence, it is possible to tailor the pore-size distribution of a high-surface-area SiC in a desired manner, using altered preparation techniques. Acknowledgment We thank Gary Dodge for his help with firing of gels in the high-temperature vacuum furnace and the Ohio Coal Development Office of the Ohio Air Quality Development Authority for providing financial support for the project. Literature Cited (1) Jiang, D. Research on the High Performance Silicon Carbide Ceramics and Silicon Carbide Based Composites. Ceram. Trans. 2002, 144, 39-54. (2) Pham-Huu, C.; Gallo, P. D.; Peschiera, E.; Ledoux, M. J. n-Hexane and n-heptane isomerization at atmospheric and medium pressure on MnO3-carbon-modified supported on SiC and γ-Al2O3. Appl. Catal. A 1995, 132, 77-96. (3) Moene, R.; Tijsen, E. P. A. M.; Makkee, M.; Moulijn, J. A. Synthesis and Thermal Stability of Ni, Cu, Co, and Mo Catalysts Based on High Surface Area Silicon Carbide. Appl. Catal. A 1999, 184, 127-141. (4) Moene, R.; Makkee, M.; Moulijn, J. A. High Surface Area Silicon Carbide as Catalyst Support Characterization and Stability. Appl. Catal. A 1998, 167, 321-330. (5) Keller, N.; Pham-Huu, C.; Crouzet, C.; Ledoux, M. J.; SavinPoncet, S.; Nougayrede, J. B.; Bousquet, J. Direct oxidation of H2S into S. New catalysts and processes based on SiC support. Catal. Today 1992, 53, 535-542. (6) Keller, N.; Pham-Huu, C.; Ledoux, M. J. Continuous process for selective oxidation of H2S over SiC-supported iron catalysts into elemental sulfur above its dewpoint. Appl. Catal. A 2001, 217, 205-217.

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Received for review November 11, 2003 Revised manuscript received March 4, 2004 Accepted March 9, 2004 IE034244E