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PROSIDING KONGGRES DAN SIMPOSIUM NASIONAL KEDUA MKICS 2007

ISSN : 0216 - 4183

MICROMORPHOLOGY, CRYSTALLIZATION BEHAVIOR,

AND CATALYTIC ACTIVITY OF SULFATED

TITANIA-ALUMINA GELS

Silvester Tursiloadi1)

and Hiroshi Hirashima2)

1. Research Center for Chemistry, Indonesian Institute of Sciences, Kawasan

PUSPIPTEK, Serpong, Tangerang 15314, Indonesia, E-mail : [email protected]

2. Department of Applied Chemistry, Faculty of Science and Technology, Keio

University, 3-14-1, Hiyoshi, Kohoku-Ku, Yokohama 223-8522, Japan

Abstract

Two types of 0.2TiO2-0.8Al2O3 gels were prepared by hydrolysis of Al (OC4H9sec

)3 and

Ti(OC3H7iso)4in an n-propanol solution with sulfuric acid or nitric acid catalysts. The solvent in wet

gels were supercritically extracted with CO2. The effects of preparation methods on themicrostructure and catalytic activity of the gels were discussed. The anatase phase in the gels

prepared with sulfuric acid catalyst was stable after calcination at temperatures up to 900C. The

thermal stability of the microstructure of mesoporous titania-alumina was improved by sulfation and

supercritical extraction. The sulfated aerogel had the higher catalytic activity for the esterification

of glycerol with oleic acid than the un-sulfated aerogel.

Key word;Sulfated titania-alumina, glycerol mono oleate, sol-gel, CO2 supercritical extraction.

IntroductionSolid superacids are an important class of catalyst. Acid with acid strength higher than pure sulfuric acid

[1] are classified as superacids. They have many benefits such as the ability to lower reaction temperatures and to

form reaction intermediates unattainable with conventional catalysts. Most superacid currently in use arehomogeneous liquid catalysts, which present many problems. Liquid catalysts are difficult to separate from theproduct stream. Large amounts of catalyst are usually required, often leading to wasted catalyst. Furthermore, theliquid acids are corrosive to the reactive system and the liquid waste is an environmental hazard. A solid superacidcatalyst circumvents many of there problems [1]. Replacement of homogeneous liquid by solid acid catalysts is

highly desired, not only by the easier separation of the catalyst from the reaction products, but also from anenvironmental point of view. After the success of zeolites as acid catalysts with medium-strong acidity, research hasbeen focused on the preparation and characterization of solid catalysts having a superacid character.

Superacid catalysts can be prepared by several approaches: liquid superacids supported on suitable carriers[1], a combination of metal halides with inorganic salts such as AlCl3-Ti (SO4)3, AlCl3-CuCl2etc [2], per fluorinatedresin sulfonic acid such as Nafion-H [3], and sulfate-promoted metal oxides such as SO4

2-/ZrO2, SO42-/TiO2, SO4

2-

/Fe2O3. Among these, sulfate-promoted metal oxides have been found to exhibit excellent catalytic properties for anumber of acid-catalyzed hydrocarbon reactions. These catalysts, especially those of the sulfated zirconia type, are

able to catalyze the isomerization of short linear alkanes at relatively low temperature (below 150 oC) [4, 5]. Eventhough it is accepted that the presence of sulfate species with covalent S=O bonds on the oxide surface is necessaryto obtain superacidity [6], the exact nature of the catalytically active sites remains an open question in the literature.Thus, it is suggested that the superacid centers are Lewis sites associated to the metal cation [7], whose acid strengthis strongly enhanced by an electron induction effect of S=O in the sulfur complex, as is shown in Scheme 1. Othershave suggested that the Lewis and Bronsted sites generated from adsorbed water molecules (Scheme 2) areresponsible for the catalytic activity. These Bronsted sites are easily interconverted to Lewis sites by evacuation at

temperatures above 150oC.

M

O

O

S

O

O

Scheme 1

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O

S

O

O O

Ti Ti

O

O

HH LEWIS

SITEBRONSTED

SITES

Scheme 2

Recently, sulfated zirconia has been one of the most studied solid acids [3, 8]. In a way similar to superacidsolutions, it is able to catalyze low temperature transformations of butane and pentane. It has been confirmed thatdifferent surface species such as sulfate and hydrogen sulfate groups, unshielded zirconium cations or hydroxyls [3,8] are responsible for the catalytic activity of the system in low-temperature alkane transformation. Hino, et al.[4]

have firstly reported in 1979 that the isomerization of n-butane can be done at room temperature by sulfated zirconiacatalyst. Low temperature isomerization is an interest to the petroleum industry as branched alkenes are becoming

an important product for use as octane-enhancing additives to gasoline. A solid superacid catalyst would lower thereaction temperature required for the isomerization. A lower reaction temperature not only represents energysavings, but also favors the desired branched product thermodynamically [9]. Sulfate-promoted zirconia has alsobeen shown to be active for a number of other reactions including cracking, alkylation and esterification [6].

Titania can form stronger covalent surface sulfates such as TiOSO4through sulfuric acid treatment, and itpossesses acid centers of high acid strength in the range -16.04 < H0< -14.52 [3], similar to sulfated zirconia, andredox sites of Ti4+/Ti3+as well as SO4

2-types, where H0 is the Hammett acidity function. However, the relativelylow surface area and the poor stability of the structure at high temperatures are disadvantages. Therefore, much

attention has been paid to the applications of mixed oxides containing TiO 2. Aluminais more stable and has a largesurface area. Alumina which can react with sulfuric acid to form surface sulfates similar to aluminium ionic saltssuch as Al2(SO4)3. The reaction of alumina with sulfuric acid will produces acid sites which are weaker in strengththan in the case of TiO2/SO4

2-; H0 > -14.52 [10], and also oxidizing centers of SO4

2- type. Titania-Alumina is

known as a material that posses Lewis acidity, since it has an alumina and titania structure in its lattice. Through thesulfation of TiO2-Al2O3, during the sol-gel formation of the alcogel, i.e. the addition of sulfuric acid to the mixtureof titanium- and aluminum-alkoxides precursors, sulfate group will be included in the oxide network of the alcogel,

resulting in a titania-alumina-sulfate cogel. It is expected that its acidity would be enhanced and that its catalyticactivity towards the esterification, especially in the esterification of long chain carboxylic acid (fatty acid), would beimproved. Various applications of fatty acid ester can be expected in the industries.

In this study, the effects of the sulfuric acid as catalyst for the sol-gel reaction and CO2 supercriticalextraction on the fine structure of TiO2-Al2O3 powders were investigated. The catalytic activities of resultingmaterials were evaluated for the esterification of oleic acid with glycerol to produce glycerol mono oleate.

Experimental

Monolithic gels of 0.2TiO2-0.8Al2O3 were prepared by hydrolysis of aluminium-sec-butoxide,Al(OC4H9

sec)3(ASB), and titanium isopropoxide, Ti(OC3H7

iso)4 (TIP), in an n-propanol solution with acid catalysts,

H2SO4(sulfated gel) or HNO3(un-sulfated gel). The molar ratios used for the synthesis were [TIP]/[ASB] = 1/4,[H2O]/[total alkoxide] = 2, [H2SO4] or [HNO3]/[total alkoxide] = 0.06 and [n-propanol]/[total alkoxide] = 12. Theappropriate amount of ASB was initially dissolved in n-propanol with vigorous stirring at 65

oC for 1h to complete

dissolution. The ASB solution was cooled to room temperature, and then the appropriate amount of TIP in n-propanol was added and stirred for 1h. After that, a mixture of the remaining n-propanol, H2O and H2SO4or HNO3was added drop wise under continuous stirring. The solution transformed into gel after the addition of the last drop.

The gel time was defined as the time required after mixing for the vortex created by the stirring to disappearcompletely. The wet gel was placed in the flow of supercritical carbon dioxide in a supercritical extraction system,and the solvent was supercritically extracted at 60C and 24MPa for 2h [11]. The supercritically extracted gels wereheated at a heating rate of 10

oC min

-1 and calcined at 500, 600, 800 and 1000

oC for 2h. In this study, the

supercritically extracted gel will be called aerogel(Fig.1).

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Heating n-propano

ng

l at 65 oC with stirring

Al (OC4H9)3

Vigorous stirri at 65o

C for 1 hour

Cooling at om temperature

Ti(OC3H7)4 in n-propanol

Stirring at room mperature for 1 hour

H O + H2SO4 or HNO3 + n-propanol

(drop wise)

(H2O /Akoxides = 2,

H2SO4 or HNO3 / Akoxides = 0.06)

Formation of gel

(gelled after the addition of the last drop )

Supercritically extracted using CO2(60oC and 24MPa for 2h)

Calcination at sev al temperatures for 2 h

Char cterization

ro

te

2

er

a

Figure 1 Flowchart of sample preparation

Changes in the microstructure of the aerogels during heating were evaluated using thermogravimetric- anddifferential thermal analyses (TG-DTA, Seiko, Exstar 6000 TG/DTA 6200) under airflow of 300ml min -1with aheating rate of 10

oC min

-1. Fourier transform infrared (FT-IR) spectra measurements were made with a BIO-RAD

FTS-60A spectrometer. The pore and grain sizes of the samples were estimated from the images observed by

scanning electron microscopy (FESEM, Hitachi, S-4700). Crystallization behaviors of the samples have beenobserved by X-ray diffractometer (Rigaku, RAD-C). The specific surface area, pore volume and pore size

distribution of the aerogels, before and after calcination, were estimated by the Barret-Joyner-Halenda (BJH)method using N2-desorption curves (Quantachrome, Autosorb) [12].

The catalytic activity of both of the aerogels calcined at 500oC for 2h, sulfated and un-sulfated, were

evaluated for the reaction of oleic acid with glycerol to produce glycerol-mono-oleate, when the molar ratio of oleicacid and glycerol was 1:1. The operational temperature was 180oC. During the reaction, sampling was performedevery 30 minutes for 8h. Each sample was analyzed for its acid saponification values and percentage of the

produced esters (yield).

ResultsTG-DTA profiles of the as-extracted aerogels with and without sulfation by H2SO4are given in Figs. 2(a)

and (b). For the sulfated aerogel (Fig. 2a), an endothermic shoulder peak at 80oC and an endothermic peak

accompanied with gradual weight loss about 30% at 125oC, and an exothermic shoulder with weight loss about 5%at 740oC were observed. In addition, a strong endothermic peak with a sharp weight loss, about 15% at 830oC, wasobserved. An endothermic peak with a weight loss about 25% at 80oC and exothermic shoulders at 190 and 250oC,accompanied with weight losses, were observed for the un-sulfated aerogel (Fig. 2b). Above 500oC, the un-sulfated

aerogel sample lost practically no more weight. The small exothermic peaks about at 800 and 1000oC were also

observed.

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Figure 2 TG-DTA profiles for (a) sulfated and (b) un-sulfated alumina-titania aerogel

Figs. 3 and 4 show the FT-IR spectra of the as-extracted- and calcined aerogels with and without sulfation

in the range from 4000cm-1 to 400cm-1. The broad absorption band around 3400cm-1 for all samples can beattributed to the OH group of the occluded water and surface = Ti-OH groups with H-bridging. For the as-extractedsulfated aerogels, two strong peaks were found at 1636cm-1and a broad peak in the range 1200-1150cm-1 attributedto stretching of -OH and SO4

-2, respectively. Infrared spectra of sulfated metal oxides generally show a strong

absorption band at 1390-1375cm-1 and broad band at 1250900cm-1. The peak in the range 1390-1375cm-1 isattributed to the stretching of S=O and the peaks in the range 1250-900cm-1are characteristic for SO4

2-[13]. Aftercalcination at 500oC, the broad absorption band in the range 1200-1150cm-1 increased, and the peaks at 3400cm-1

and at 1635cm-1

ascribed to OH group still existed. Infrared spectra of the sulfated gels calcined at temperatures upto 800

oC (Fig. 3) show a strong and broad absorption band in the range from 1300 to 1050cm

-1, that is a

combination of overlapping peaks with around 1100cm-1

, at 1000cm-1

, and a small peak at 1350cm-1

. Aftercalcination at 800

oC all of the absorption peaks

decreased, and the peaks attributed to SO4

2- disappeared after

calcinations at 900oC. For the un-sulfated gel (Fig.4), after calcination at 800

oC strong peaks at 583 and at 439cm

-1,

attributed to hetero metal-oxygen bonds of -Ti-O-Al-, were found as well as for the sulfated gel.

400900140019002400290034003900

Wavenumber(cm-1)

%T

ransmittance(au)

As-extracted

500oC

800oC

900o

-OH

-OH

SO4-2

-Ti-O-Al-

Figure 3 IR spectra of sulfated alumina-titania gels

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40080012001600200024002800320036004000

Wavenumber(cm-1)

Trasmittance(au)

As-extracted

500

800

-Ti-O-AL-

-OH

-OH

-C-H

Figure 4 IR spectra of un-sulfated alumina-titania gel

After calcination at 500oC, the weak diffraction peak of anatase at d=0.349nm and very weak peaks of

titanium sulfate, Ti2(SO4)3, at d=0.311 and 0.267nm, were found for the sulfated aerogel (Fig. 5a), and these peaksbecame stronger after calcination at 600

oC (Fig. 5b). On the other hand, the un-sulfated gel was still amorphous

after calcinations at temperatures up to 750oC (Fig. 6b and c). After calcination at 800oC, the titanium sulfate peaks

disappeared, but the peaks of anatase were observed (Fig. 5c) for the sulfated gel. The anatase phase of sulfated gelwas stable after calcination at temperatures up to 900C (Fig. 5d). After calcination at 800

oC, the diffraction peaks

of rutile were found for the un-sulfated aerogel (Fig. 6d). Finally, after calcination at 1000oC, the diffraction peaks

of anatase, rutile and -alumina were observed for the sulfated aerogel (Fig. 5e), and those of rutile and -alumina

were observed for the un-sulfated sample (Fig. 6e). The diffraction peaks of - and -alumina phases were notfound for both of the aerogels.

Figure 5 XRD patterns of sulfated alumina-titania gel;

(a) 500oC, (b) 600oC, (c) 800oC, (d) 900oC and (e) 1000oC.

(A; Anatase, R; Rutile, a; a-alumina, TS; titania sulfate)

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Figure 6 XRD patterns of un-sulfated alumina-titania gel;

(a) 500o

C, (b) 700o

C, (c) 750o

C, (d) 800o

C and (e) 1000o

C. (R; Rutile, a; a-alumina)

The SEM images of the aerogels calcined at various temperatures show their highly porous microstructures

(Fig. 7 and Fig. 8). After calcinations at temperatures up to 600oC, large agglomerates, in m order with smoothsurface, were observed for the sulfated aerogel, and fine particles in nm order were not clearly observed (Fig. 7a andb). After calcination at 800oC, the aggregate of fine particles, smaller than 100nm in diameter, were clearlyobserved (Fig.7c). After calcinations at 1000oC, grain growth and sintering were observed for the sulfated gel, andcylindrical particles, about 150nm in length and about 50nm in diameter, were found (Fig. 7d). After calcinations at

temperatures up to 600oC, fine grains, about 30nm in diameter, were found for the un-sulfated TiO2-Al2O3gel (Fig.

8a and b). After calcination at 800oC, the grain diameter of the un-sulfated aerogel increased about to 50 nm (Fig.

8c). After calcination at 1000oC, further grain growth and sintering were observed for the un-sulfated gel, and the

length and diameter of cylindrical grains are about 200nm and 75nm, respectively (Fig. 8d).

Figure 7 SEM images of sulfated alumina-titania gel at different temperatures

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Figure 8 SEM images of un-sulfated alumina-titania gel at different temperatures

Table 1, shows the effect of the sulfation with H2SO4on specific surface area, cumulative pore volume and

average pore diameter of the calcined aerogels. After calcinations at temperatures up to 800oC, specific surface area

and cumulative pore volume of the sulfated TiO2-Al2O3gel are much smaller than those of the un-sulfated TiO2-Al2O3gel. However, the average pore diameter of the sulfated gel is larger than that of the un-sulfated TiO2-Al2O3gel after calcination at temperatures up to 600oC. The specific surface area and cumulative pore volume decreasedwith increasing calcination temperature for the un-sulfated TiO2-Al2O3gel, but the surface area and cumulative porevolume of the sulfated TiO2-Al2O3gel increased after calcinations at temperatures up to 800

oC. After calcination at

1000oC, the pore volume of both of the gels drastically decreased.

Table 1 Specific surface area, cumulative pore volume, and average pore diameter of the

sulfated and un-sulfated titania-alumina gels after calcination at various temperatures.a

15.821.611.27.8Average pore diameter (nm)

0.040.650.650.89Pore volume cm3 -1

13152297434Surface area m2 -1

Un-sulfated TiO2-Al2O3

14.79.914.925.5Average pore diameter (nm)

0.0750.1950.0240.017Pore volume cm3 -1

188294Surface area m2 -1

Sulfated TiO2-Al2O3

1000oC800 oC600 oC500oC

Fig. 9 shows the effect of acid catalysts (H2SO4 and HNO3) in the preparation methods on the catalyticactivity of the aerogels calcined at 500oC for the reaction of oleic acid (C18H34O2) with glycerol (C3H8O3) to produceglycerol mono oleate. Oleic acid (CH3(CH2)7CHCH(CH2)7COOH) react readily with glycerol

a The accuracy of N2adsorption measurements was 0.1%, and the reproducibility of these values for eachsample was within 10%.

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(CH2OHCHOHCH2OH) in the presence of catalytic amount of sulfated TiO2-Al2O3or un-sulfated TiO2-Al2O3acidsto yield compounds called Glycerol mono oleate(CH3(CH2)7CHCH(CH2)7COO CH2OHCHOHCH2).

where R is CH3(CH2)7CHCH(CH2)7 and R is CH2OHCHOHCH2.

The sulfated TiO2-Al2O3 gel shows the higher activity than that of the un-sulfated TiO 2-Al2O3 gel, although thespecific surface area, cumulative pore volume are much smaller than those of the un-sulfated TiO2-Al2O3gel.

Figure 9 Catalytic activities of sulfated and un-sulfated gels,

calcined at 500oC, on production of glycerol mono oleate

DiscussionIntroduction of sulfuric acid onto the oxide carriers substantially changed the TGA and DTA profiles.

Endothermic peaks appeared at 80oC and at 125

oC accompanied with weight losses for the sulfated aerogel may be

attributed to the evaporation of solvent and desorption of water molecules adsorbed on the oxides surface (Fig. 2a).However, the endothermic peak at 125oC was hardly observed for the un-sulfated gel (Fig. 2b). The both of samples(Figs. 3 and 4) after calcination at 500oC indicated the presence of strong IR absorption peak at 1635cm-1, attributed

to -OH stretching mode originated from a high degree of surface hydroxylation. After calcination at 800oC, the

peaks at 3400cm-1for OH group and at 1633cm-1for stretching of -OH still existed (Figs. 3 and 4). These results

suggest that the aerogels has chemisorbed water on its surface.The residual organics in the un-sulfated gel can be eliminated by heating up to 600 oC. Above 600oC,

weight losses were hardly observed. About at 700oC, another weight loss with an endothermic shoulder wasobserved for the sulfated gel (Fig. 2a). This may be caused by desorption or decomposition of chemisorbed organicson the sulfated gel while the IR absorption band attributed to sulfate group is still observed. A strong and broad IRabsorption band in the range 1200 - 1150cm -1, the characteristic frequencies of SO4

-2, and the shoulder peak at

1380cm-1

which is assigned to the stretching of S=O were found for the sulfated aerogel after calcinations attemperatures up to 800oC (Fig. 3). These results show that the surface of the gel is strongly modified with the

sulfation.After calcination at 500

oC, the X-ray diffraction pattern for the sulfated aerogel was anatase phase. It is

indicated that sulfate on the surface of alumina-titania nanoparticles with crystallization (Fig. 5a, Schema 3). Thesulfated aerogel after calcination at 600oC (Fig. 5b) confirmed the presences of sulfate phase, Ti2(SO4)3. A strongendothermic peak with a sharp weight loss about 15% at 830oC for the sulfated gel (Fig. 2a) is attributed to thedecomposition of the specific sulfate, however the anatase phase stable up to 900 oC (Fig. 5d and schema 3). Thatpeak did not found for the un-sulfated gel.

Lewis site

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O

Ti

O

Ti

O

S

OO

O

Al

OO

O O O

O

Ti

O

Ti

O

Al

O

O-

O O O

O

1 0 0 0oC

A n a t a s e R u t i l e

Schema 3

The IR absorption band attributed to SO4-2 was observed after calcination at 800oC and disappeared after

calcinations at 900oC. The decomposition temperature of the specific sulfate phase on the sulfated TiO2-Al2O3

aerogel, 830oC, is much higher than that of the sulfated TiO2, 540

oC [14] and that of the sulfated Al2O3, 600

oC [14].

This result is attractive, and it suggests that the adsorption strength of sulfate on the surface of TiO 2-Al2O3is higherthan Al2O3and TiO2. TiO2-Al2O3is known as a material that posses Lewis acidity, since it has an Al-O-Ti structurein its lattice. The presences of Ti-O and Al-O group were indicated by the presence of the strong, but broad peak

between 400 and 900cm-1. The strong acidity of TiO2-Al2O3makes them to have stronger adsorbed sulfate thanAl2O3and TiO2. After calcination at 800

oC, the strong absorption peaks at 583 and at 439cm-1attributed to hetero

metal-oxygen bonds of -Ti-O-Al- were found (Fig. 3 and Fig. 4).The effect of sulfation on crystallization behaviors can be seen in Fig. 5 and Fig. 6. After calcination at

500oC, the diffraction peaks of anatase and Ti2(SO4)3were found for the sulfated aerogel (Fig. 5a). After calcination

at 800oC, Ti2(SO4)3disappeared (Fig. 5c). Anatase phase was stable at temperatures up to 900

oC (Fig.5d, Schema

3). On the other hand, no diffraction peaks of anatase were found for the calcined aerogel without sulfation (Fig. 6).The amorphous of alumina-titanate gel was stable up to 750 oC (Fig. 6) and decomposed at 800 oC (Fig. 6d). Thediffraction peaks of rutile were found after calcinations at 800

oC. Rutile deposited directly from amorphous of the

un-sulfated gel.

The formation of Ti-sulfate on the surface of the network gel enhanced deposition of anatase at relativelylow temperature (Schema 3). Usually, anatase transforms into rutile around 600oC [11]. However, grain growth of

anatase dispersed in the aerogel is restricted and the transformation may be retarded. The small exothermic peaks at

800oC and at 1000oC without weight changes were observed for the un-sulfated gel (Fig.2b). These peaks areattributed to the crystallization of rutile and -Al2O3, and they are not observed significantly for the sulfated gel (Fig.2a). The DTA peak at 1000

oC for the un-sulfated aerogel may be assigned to the direct crystallization of amorphous

Al2O3into -Al2O3 (Fig. 6e). No diffraction peaks - and - Al2O3were observed for both of the aerogels. Theseresults suggest that the segregation of TiO2in the un-sulfated gel occurred at high temperature, about 800

oC, and

stable form of TiO2, rutile, deposited. The deposition of rutile may induce direct deposition of -Al2O3.The specific surface area and pore volume of the sulfated aerogel were much smaller than those of the un-

sulfated aerogel (Table 1). The SEM images for un-sulfated sample, after calcination at 500oC and 600

oC, show fine

particles about 20nm in diameter or smaller. On the other hand, the SEM image of the sulfated gel calcined at500

oC shows large agglomerates with smooth surface. The agglomerates may consist of fine particles, and include

the sulfate with residual organics in the porous networks. The residual organics trapped in pores of make thesulfated aerogel to have small pore volume and low specific surface area. After calcination at 800oC, residualorganics were removed and the sulfate decomposed, resulting in increases of surface area and pore volume of the

sulfated gel. With increasing calcination temperature, grain growth and sintering were observed for the un-sulfatedgel, resulting in decreases of surface area and pore volume, and increase in pore size. The un-sulfated samples areporous with a large surface area, and its porous structure is stable at temperatures up to 800

oC. The grain growth

was not significant, and the specific surface area was high, more than 150 m 2g-1. The un-sulfated aerogels obtainedfrom translucent gels had a rigid porous framework and good thermal stability.

The sulfated aerogel shows the higher catalytic activity for the reaction of oleic acid with glycerol to

produce glycerol mono oleate than that of the un-sulfated aerogel; although the specific surface area and thecumulative pore volume of the sulfated TiO2-Al2O3aerogel is much smaller than those of the un-sulfated aerogel.

This result also indicates that the sulfated aerogel has the stronger acidity than the un-sulfated aerogel.

Conclusions(1) Using sulfuric acid as catalyst for hydrolysis of metal alkoxides, titania-alumina gel was sulfated and thesulfate phase was formed by calcinations. The decomposition temperature of the specific sulfate phase on the TiO

2-

Al2O3gel is much higher than that of sulfated TiO2and sulfated Al2O3.

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(2) The crystallization behaviors of TiO2-Al2O3aerogels were affected with acid catalyst used for hydrolysis.Anatase phase deposited in the sulfated aerogel was stable up to 900

oC. On the other hand, rutile phase directly

deposited in the un-sulfated aerogel at 800oC.(3) The catalytic activity of the sulfated aerogel for esterification reaction was higher than that of the un-sulfated aerogel, although the specific surface area and cumulative pore volume of the sulfated aerogel were much

smaller than those of the un-sulfated aerogel.

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10. T. Yang, T. Chang and C. Yek,J. Mol. Catal. A: Chemical, 115, (1997), 339,11. H. Hirashima, Y. Yamanaka and S. Tursiloadi, submitted toJ. Sol-Gel Sci Tech.12. I. M. K. Ismail and P. Pfeifer,Langmuir, 10, (1994), 153213. J.P. Chen and R.T. Yang,J. Catal. 139, (1993), 27714. M. Marczewski, A. Jakubiak, H. Marczewska, A. Frydrych, M. Gontarz and A. Sniegula, Phys. Chem.

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