Mesoporous nanocrystalline sulfated zirconia synthesis and its application for FFA esterification in oils

Applied Catalysis A: General 462–463 (2013) 196–206

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Mesoporous nanocrystalline sulfated zirconia synthesis and its application for FFA esterification in oils Vishwanath Ganpat Deshmane a , Yusuf Gbadebo Adewuyi b,∗ a b

Mechanical Engineering Department, North Carolina Agricultural and Technical State University, Greensboro, NC 27411, USA Chemical, Biological and Bioengineering Department, North Carolina Agricultural and Technical State University, Greensboro, NC 27411, USA

a r t i c l e

i n f o

Article history: Received 20 December 2012 Received in revised form 3 May 2013 Accepted 5 May 2013 Available online xxx Keywords: Sulfated zirconia Esterification Solid acid catalyst Free fatty acids Biodiesel Catalyst characterization

a b s t r a c t Mesoporous nanocrystalline sulfated zirconia catalyst has been prepared from zirconium hydroxide synthesized at different digestion/hydrothermal treatment times. Sulfuric acid and chlorosulfonic acid were used as two different sulfonating agents. The effect of digestion time, sulfonating agent and the calcination temperature on structural, textural and catalytic properties of the prepared catalyst were investigated in details using nitrogen adsorption–desorption (BET), ammonia temperature programmed desorption (NH3 -TPD), X-ray diffraction (XRD), thermogravimetry and differential scanning calorimetry (TGA–DSC), and Fourier transform infrared spectroscopy (FTIR). The sulfated zirconia prepared at various digestion times and two different sulfonating agents were tested for the esterification of free fatty acid (FFAs) in soybean oil (prepared by mixing oleic acid in soybean oil) as model reaction. Sulfated zirconia catalyst prepared with 3 h digestion time and 600 ◦ C calcination temperature using chlorosulfonic acid showed the highest catalytic activity with about 85% conversion in just 80 min of esterification time at 60 ◦ C reaction temperature. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Biodiesel also called fatty acid methyl ester is a clean-burning, renewable fuel produced from vegetable oils, animal fats and recycled cooking oil and greases, etc. It is not only biodegradable but also free of sulfur, making it a cleaner burning fuel than petroleum diesel with reduced emission of SOx, CO, unburnt hydrocarbons and particulate matter [1]. Excellent lubricating properties that extend engine life, superior cetane number, flash point compared to conventional diesel and acceptable cold filter plugging point (CFPP) are some of the attributes that make biodiesel very attractive alternative fuel [2,3]. The major hurdle in the use of biodiesel for replacing conventional petroleum fuels is its higher cost. The two main factors that affect the cost of biodiesel are the cost of raw materials and the processing cost such as catalysts and equipments [4]. The raw materials account for over 60–75% of the biodiesel production expenses. The potential solution to this problem is the utilization of low value alternative feedstocks of varying type, quality and cost. For example, the cost of waste cooking oil is 2–3 times lower than virgin oils. Thus the utilization of less expensive feedstocks such as animal fat, waste cooking oil, yellow and brown grease is expected to

∗ Corresponding author. Tel.: +1 336 334 7564x107; fax: +1 336 334 7417. E-mail address: [email protected] (Y.G. Adewuyi). 0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2013.05.005

appreciably reduce the biodiesel cost [5]. However, many of these alternative feedstocks may contain high levels of free fatty acids (FFA), water, or insoluble matter, which affect biodiesel production [6]. Synthesis of biodiesel via transesterification reaction with feedstocks having higher FFA and moisture is complicated. During the reaction, the feedstocks undergo saponification reaction leading to soaps formation resulting in reduced biodiesel yields, especially when alkaline catalysts are used. Furthermore, the soap formation also leads to the catalyst consumption, lowering catalytic efficiency and increase in the viscosity of reaction mixture and gel formation requiring additional purification steps [5]. These problems could potentially be eliminated via the use of heterogeneous acid catalysts due to their lower susceptibility to FFAs and moisture content in the oil [7]. Also, catalysts can be easily separated from the reaction products with much more simplified product separation steps resulting in high yields of methyl esters and decrease of catalyst cost due to the possibility of catalyst regeneration. To date, several solid acid catalysts have been reported for biodiesel synthesis, including zeolites (e.g. H-ZSM-5, Y and Beta), ion exchange resins (e.g. Amberlist 12, a styrene based sulfonic acid and Nafion-NR-50, a copolymer of tetrafluoroethene and perfluoro-2-(fluorosulphonyle-thoxy) propyl vinyl ether) and metal oxides modified with sulfate ions (SO4 2− /Mx Oy , such as SO4 2− /ZrO2 , SO4 2− /SnO2 , SO4 2− /TiO2 , SO4 2− /WO3 ), etc. [8,9]. Zeolite catalysts with small (micron-sized) pores are not suitable for biodiesel manufacture because of the diffusion limitations induced

V.G. Deshmane, Y.G. Adewuyi / Applied Catalysis A: General 462–463 (2013) 196–206

by the large fatty acid molecules. Ion-exchange resins are active strong acids, but have a low thermal stability which is problematic as the esterification reaction might require high temperatures. Metal oxides modified with sulfate ions and especially sulfated zirconia, has high boiling point, strength, toughness, good corrosion resistant in acidic and alkaline environment. In addition sulfated zirconia has very high activity, selectivity and stability making it a promising candidate not only for the esterification reaction but also for number of industrially important reactions such as hydrocarbon isomerization, methanol conversion to hydrocarbons, alkylation, acylation, etherification, condensation, nitration, cyclization and Fisher–Tropsch synthesis [10–12]. However, the catalytic and structural properties of the sulfated zirconia depends on number of factors including Zr(OH)2 preparation method, precursor used, precipitation pH, precursor concentration, type of sulfonating agent, catalyst pretreatment, and calcination [13–15]. In an earlier work, nanocrystalline mesoporous Zr(OH)2 powder with very high surface area was synthesized using ethylene diamine and zirconyl chloride octahydrate. Ethylene diamine used as precipitating agent also acted as a colloidal protecting agent. The results of the effects of various process parameters such as precipitation pH, precursor concentration, time of hydrothermal treatment and calcination temperature on the structural and textual properties of zirconium oxide were discussed in detail [16]. Yadav and Murkute [17] previously reported the use of chlorosulfonic for the sulfonation of zirconia powder. It was showed that the chorosulfonic acid treated zirconia possesses more sulfate ions, stability and activity compared to the sulfated zirconia synthesized using sulfuric acid. In this study we report the synthesis of mesoporous nanocrystalline sulfated zirconia with high surface area and acidity. Mesoporous zirconium hydroxide prepared at different digestion times of 0, 1, 3, 6, 12, 24 and 48 h were sulfonated by wet impregnation method using sulfuric acid and chlorosulfonic acid as two different sulfonating agents. To the best of our knowledge, no studies have been reported in the literature showing the effect of digestion/hydrothermal treatment on the sulfonation process and acidity of final sulfated zirconia catalyst. The effect of preparation conditions such as digestion time, sulfonating agent and the calcination temperature on the structural phases, textural characteristics and the number and types of available active acidic sites on the surface of the final sulfated zirconia were investigated using nitrogen adsorption–desorption (BET), ammonia temperature programmed desorption (NH3 -TPD), X-ray diffraction (XRD), thermogravimetry and differential scanning calorimetry (TGA–DSC), and Fourier transform infrared spectroscopy (FTIR). The sulfated zirconia prepared at the best synthesis conditions was tested for the esterification of free fatty acid in soybean oil (prepared by mixing oleic acid in soybean oil) as model reaction. 2. Experimental 2.1. Chemicals Zirconyl chloride octahydrate (ZrOCl2 8H2 O), 98 + % pure, ethylenediamine (H2 NCH2 CH2 NH2 ), 99%, extra pure, sulfuric acid, 97%, ethylene dichloride were purchased from Acros Organics, NJ, USA. Chlorosulfonic acid, 99% was obtained from Alfa Aesar, Ward Hill, USA. The water used at all stages of the experiments was purified using a Mill-Q Advantage A10 with Elix 5 system obtained from Millipore Corporation (Bedford, MA, USA). 2.2. Catalyst synthesis The method for the synthesis of zirconium hydroxide (80–90% yield) has been discussed in details previously [16]. Zirconyl

197

chloride octahydrate and ethylene diamine are used as zirconium precursor and precipitating agent, respectively, as reported by D’souza et al. [18]. Sulfated zirconia was prepared from this material using two different methods based upon two different sulfonating agents, i.e., sulfuric acid and chlorosulfonic acid. In the case of sulfuric acid, 1 g of the dried again as prepared zirconium hydroxide power was mixed with 15 ml of 1 N H2 SO4 and then stirred with magnetic stirrer for about 10 min followed by the filtration and air drying. The air dried material was then dried in oven for 24 h at 110 ◦ C. In the case of chlorosulfonic acid, 1 g of dried as prepared zirconium hydroxide power was immersed in 15 ml, 0.5 M solution of chlorosulfonic acid in ethylene dichloride. After about 30 min, ethylene dichloride was evaporated in an oven at 80 ◦ C for 20 h and then dried completely for 24 h at 110 ◦ C. The samples prepared by using both methods were then calcined at 600 ◦ C and 650 ◦ C temperature for 2 h with controlled heating and cooling rates of 0.5 ◦ C/min and 1 ◦ C/min respectively, in the presence of air. The two different samples were denoted as SZ and CSZ for sulfated zirconia prepared using sulfuric acid and chlorosulfonic acid, respectively. Finally, the prepared samples were characterized by using different analytical and instrumentation techniques described in the following section. 2.3. Catalyst characterization The BET surface area, total pore volume and pore size distribution of the catalyst were determined with AUTOSORB-1C, Chemisorption–Physisorption analyzer (Quantachrome Instruments, Boynton Beach, FL, USA). Surface area was calculated by using BET equation from the adsorption branch of the isotherm in a relative pressure range of 0.07–0.3. The pore size distribution was calculated from desorption branches using the Barrett–Joyner–Halenda (BJH) method [19]. The total pore volume was derived based on the amount of N2 adsorbed at a relative pressure close to unity. The acidity of the sulfated zirconia catalyst was measured by using ammonia temperature programmed desorption (NH3 -TPD) experiments. Thermal conductivity detector (TCD) connected to the AUTOSORB-1C was used to measure the ammonia desorption profile. In a typical run, 0.25 g of the catalyst (sandwiched between two small wads of glass wool) was placed into the chemisorptions cell and heated to 140 ◦ C at 20 ◦ C/min under the helium flow for 30 min to remove adsorbed components. Later the sample was cooled to 100 ◦ C and saturated with ammonia by exposing the sample to 100% NH3 for 10 min. Physisorbed ammonia was removed by purging the sample with helium gas for 30 min. Finally, the temperature was ramped to 600 ◦ C at a rate of 20 ◦ C/min and evolved ammonia was quantified by thermal conductivity detector. Thermo-gravimetric (TGA) and differential scanning calorimetry analysis (DSC) were carried out using a SDT Q600 V20.4 Build 14 system (TA Instruments, New Castle, DE, USA). The heating was carried out in an air environment. The air flow rate was maintained at 100 ml/min and the heating rate was 10 ◦ C/min. Infrared absorption–transmission spectra were obtained using an FTIR spectrometer (TENSOR 27, Bruker Optics, Inc., Billerica, MA) with HeNe laser source and a room-temperature deuterated lalanine triglycine sulfate detector (DLATGS detector). The FTIR was equipped with an ATR sampling accessory, MIRacle ATR set with Diamond crystal assembly from PIKE Technologies (PIKE Technologies, Inc., Madison, WI). All spectra were collected at 20 ± 1 ◦ C using an average of 16 scans and with a spectral resolution of 2 cm−1 . The background spectra were obtained using a clean ATR accessory with a continuous dry, CO2 -free air purge from a laboratory generator (Parker-Balston, Haverhill, MA) to remove moisture. During the analysis, a small amount of powdered sample was placed on the crystal and pressured with high pressure clamp to get the intimate contact between the sample and crystal surface.

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Weight (%)

95

90

85

(a) 115

0.0

-0.5

-1.0

48 h 24 h 12 h 6h 3h 1h 0h

110

Heat Flow (W/g)

0h 1h 3h 6h 12 h 24 h 48 h

100

105

Weight (%)

198

100 95

80

90

-1.5 75

85 -2.0 0

200

400

600

800

1000

80

Temperature (°C) 0

200

Fig. 1. TGA–DSC plot for the zirconium hydroxide prepared at different digestion times.

=

0.9 ˇ · cos 

(1)

where  is the crystal size,  is the wavelength of the Cu K␣ radiation, ˇ is the full width half maximum of the respective peak and  is the Braggs angle of diffraction.

600

800

(b)

1000

48 h 24 h 12 h 6h 3h 1h 0h

0.0

Heat Flow (W/g)

The XRD patterns were recorded on D8 DISCOVER X-ray diffractometer from Bruker (Bruker Optics, Inc., Billerica, MA) with a PSD detector using Cu K␣ radiation generated at 40 mA and 40 kV at the scanning rate of 0.01 ◦ /s. The crystal sizes of the samples were determined by using Scherrer equation [20] given by the following equation:

400

Temperature (°C)

-0.5

-1.0

-1.5

2.4. Catalyst testing The catalytic activity/performance of the prepared sulfated zirconia catalyst was determined by testing it for the esterification of oleic acid in soybean oil (10% oleic acid) as model reactant for used oil hereafter mentioned as acid oil. The reaction was carried out in a cylindrical jacketed glass reactor with four necks. It was equipped with condenser to reflux the methanol evaporated during the reaction. A thermocouple with digital temperature indicator was used to measure the temperature of the reaction mixture. The stirring was carried out using a magnetic stirrer (396 W, StableTemp, Cole Parmer) at 1000 rpm. To start the experiments, 42 ml acid oil and 18 ml methanol (methanol/oil: 9/1) were added to the reactor. The mixture was heated to 60 ◦ C by circulating the hot water through a reactor jacket with continuous stirring. Then 2% (wt % of acid oil) of catalyst was added to start the reaction. Samples (2 ml aliquot) were removed from the reaction mixture at specified times during the progress of reaction and immediately cooled to 10–12 ◦ C temperature. The cooled sample was then centrifuged to separate the solid catalyst from the liquid reaction mixture. Approximately, 1 g of the separated liquid phase was dissolved in 5 ml of 2-propanol to make a homogeneous solution which was then titrated against 0.05 N KOH solution in the presence of phenolphthalein indicator to determine the acid value (mg of KOH required to neutralize 1 g of the sample). 3. Results and discussion 3.1. TGA–DSC The TGA–DSC plots for the zirconium hydroxide prepared at different digestion times (0–48 h) are shown in Fig. 1. Two weight

-2.0 0

200

400

600

800

1000

Temperature (°C) Fig. 2. Sulfated zirconia prepared using sulfuric acid with different digestion times (a) TGA (b) DSC.

loss stages were observed in the TGA profile with first one located below 150 ◦ C and is associated with a strong endothermic peak centered at about 80 ◦ C. This weight loss is attributed to the removal of the adsorbed water on the surface. Second stage of weight loss is between 150 ◦ C and ∼500 ◦ C corresponding to the removal of terminal hydroxyl groups bonded to the surface of zirconia. Strong exothermic peaks were observed over the temperature range of 420–830 ◦ C. In this region of the exothermic peaks, no weight loss is observed in the TGA curve, and hence the exothermic peaks are attributed to the transition from an amorphous to a tetragonal metastable phase of zirconia, i.e., a topotatic crystallization of tetragonal zirconia on nuclei present in the amorphous phase. The centers of these exothermic peaks for samples prepared at 0, 1, 3, 6, 12, 24, and 48 h of digestion times were located at 455, 464, 512, 563, 635, 708, and 779 ◦ C, respectively, clearly indicating the increase in crystallization temperature with increase in digestion time. This is attributed to the changes in the as-prepared materials particle sizes which have been observed to decrease with increase in digestion time [16]. It can also be seen that the more the digestion time, the lesser the weight loss (results not shown) due to the removal of hydroxyl groups through the process of polymerization between hydrous zirconia particles [21]. The thermo-gravimetriccalorimetric analysis of the SZ and CSZ are shown in Figs. 2 and 3, respectively. Similar to the hydrous zirconia samples, the weight

V.G. Deshmane, Y.G. Adewuyi / Applied Catalysis A: General 462–463 (2013) 196–206

(a)

48 h 24 h 12 h 6h 3h 1h 0h

120

Weight (%)

100

80

60

in the temperature range of 220–330 ◦ C which is attributed to the removal of organic additives especially ethylene dichloride used as solvent for chlorosulfonic acid. Interestingly, no exotherm was observed for the crystallization whereas an endotherm showing the decomposition of sulfate species was clearly seen. It is believed that the exothermic crystallization of the zirconia may occurs just before or with the endothermic decomposition of the sulfate species and the shape of TGA–DSC profile depends on the relative heat balance between these exothermic–endothermic transformations [24]. Thus, the absence of the exothermic peak in the case of CSZ could be attributed to the more dominant endothermic heat of sulfate ions decomposition than the exothermic heat of crystallization could be due to the presence of more sulfate ions compared to SZ samples. 3.2. X-ray diffraction

40 0

200

400

600

800

1000

Temperature (°C)

(b) 48 h 24 h 12 h 6h 3h 1h 0h

0.5

0.0

Heat Flow (W/g)

199

-0.5

-1.0

-1.5

-2.0

-2.5 0

200

400

600

800

1000

Temperature (°C) Fig. 3. Sulfated zirconia prepared using chlorosulfonic acid with different digestion times (a) TGA (b) DSC.

loss until 500 ◦ C is attributed to the removal of moisture and adsorbed species. The second part of the TGA profile shows that there is a steep decrease in the weight in the temperature range of 580–795 ◦ C. It was also observed that the more the digestion time the higher the temperature needed to start this weight loss. This weight loss is associated with an exotherm along with a small endotherm, whose intensity and the position was also found to be dependent on the time of digestion. The exotherm indicating the crystallization temperature for SZ samples is clearly seen to be shifted toward higher temperature compared to zirconia. The shift in crystallization temperature was observed to be maximum for sample with no digestion (453–624 ◦ C) and then decreased with increase in digestion time (463–649 ◦ C for 1 h, 464–686 ◦ C for 3 h and 635–701 ◦ C for 12 h digestion time), and was almost none for sample digested for 48 h. The intensity of the peak was also seen to be higher for the low digestion time samples compared to the ones digested for longer times. The endotherm associated with the weight loss is attributed to the decomposition of sulfate species on the zirconia surface, which leads to the formation of SO3 moieties [22]. The temperature of sulfates decomposition is also observed to be dependent on the time of digestion, attributed to the change in the sulfate state on the zirconia surface (different co-ordination with ZrO2 sites) [23]. The DSC profile of CSZ in Fig. 3b shows an additional endotherm

The surface acidity and subsequently the catalytic activity of the sulfated zirconia have been found to be dependent on the calcination temperature of sulfated zirconia as well as the pre-sulfonation drying temperature of zirconium hydroxide. The zirconium hydroxide needs to be in the amorphous phase before the sulfonation. Furthermore, the calcinations temperature of sulfated zirconia should yield crystalline tetragonal phase without the degradation of much of the sulfate groups. Comelli et al. [25] found that sulfated zirconia calcined between 530 and 605 ◦ C showed highest catalytic activity for n-hexane isomerization reaction. A high sulfur concentration and co-existence of S4+ and S6+ over amorphous material was observed for the sulfated zirconia calcined at temperature below 500 ◦ C. Whereas for calcination temperature above 500 ◦ C, decreased sulfur concentration and existence of only S6+ on tetragonal zirconia structure was detected, which was attributed to the higher catalytic activity for isomerization reaction. In addition to the S6+ influence, the higher catalytic activity of sulfated zirconia calcined at higher temperature was suggested to be due to the formation of active sites by effectively binding sulfate groups to the zirconia surface and also partial removal of sulfate species from highly uncoordinated sites on ZrO2 , creating strong Lewis acid sites upon calcination at higher temperature [26,27]. Many researchers have suggested that 600–650 ◦ C temperatures are favorable for the formation of highly active sulfated zirconia catalyst [11,28–30]. Fig. 4 shows the XRD patterns of the sulfated zirconia prepared from zirconium hydroxide synthesized with digestion times ranging from 0 h to 48 h, using sulfuric acid and chlorosulfonic acid as two different sulfonating agents; calcined at 600 ◦ C and 650 ◦ C temperature. When sulfuric acid was used, calcination at 600 ◦ C temperature formed crystallites with tetragonal structure for 0 h and 1 h digestion time. For higher digestion times the materials is only partially crystallized to tetragonal structure showing a broad peak at 30.2◦ 2 value. Upon calcination at 650 ◦ C, material was found to be crystallized with 100% tetragonal phase structure, however lower crystallinity was observed for materials digested for 3 h and 6 h. When chlorosulfonic acid was used, 100% tetragonal structure was obtained upon calcination at 600 ◦ C and with further increase in calcination temperature to 650 ◦ C, material retained tetragonal structure. It was also observed that for 0 h digestion sample no monoclinic phase was formed in both the cases even after calcination at 650 ◦ C temperature. However, in an earlier studies we have seen that the zirconia prepared with no digestion and calcined at 600 ◦ C consisted of a mixture of tetragonal (52%) and monoclinic phases (48%) [16]. This indicates that the presence of sulfate groups does stabilize the tetragonal structure of zirconia due to the strong interactions between the sulfate and ZrO2 suppressing or delaying the phase transition [31,32]. The sizes of the crystallites calculated using Scherrer equation were in the range of 6–20 nm depending

200

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2500

48 h 24 h 12 h 6h 3h 1h 0h

Intensity

2000

1500

48 h 24 h 12 h 6h 3h 1h 0h

(b) CSZ

5000 4500 4000 3500

Intensity

(a) SZ

1000

3000 2500 2000 1500 1000

500

500 0 20

30

40

50

60

20

70

30

40

2θ

18000 15000

Intensity

60

12000 9000

10000

70

48 h 24 h 12 h 6h 3h 1h 0h

(d) CSZ 8000

Intensity

48 h 24 h 12 h 6h 3h 1h 0h

(c) SZ

21000

50

2θ

6000

4000

6000 2000

3000 0 20

30

40

50

60

70

20

30

40

2θ

50

60

70

2θ

Fig. 4. X-ray diffraction pattern of sulfated zirconia prepared with different digestion times and calcined at 600 ◦ C (a) SZ (b) CSZ and at 650 ◦ C (c) SZ (d) CSZ.

upon the digestion time, sulfonating agent and the calcination temperature. 3.3. FTIR studies Figs. 5 and 6 show the FTIR spectra for the SZ and CSZ prepared with different digestion times, respectively. FTIR spectra for both SZ and CSZ showed the hydration of all the samples giving rise to a strong, broad, and unresolved band in the 3600–3000 cm−1 region (see Supplemental information), assigned to physisorbed and possibly coordinated water, accompanied by a broad band close to 1620 cm−1 that is ascribed to the bending mode (ıHOH ) of coordinated molecular water associated with the sulfate group [33–35]. The SZ dried at 110 ◦ C showed broad peak in the region of 800–1350 cm−1 with four clear peaks at about 1202 cm−1 , 1123 cm−1 , 1043 cm−1 and 983 cm−1 ; characteristic of inorganic chelating bidentate sulfate ion co-ordinated to metal cation, and which are assigned to asymmetric and symmetric stretching frequencies of S O and S O bonds [27]. Upon calcination at 600 ◦ C and 650 ◦ C, only a broad peak with small shoulders at about 1216 cm−1 , 1133 cm−1 and 1057 cm−1 instead of clear peaks were observed due to the decreased concentration of sulfate species due to the decomposition at high temperature. The shifting of the sulfate vibration band toward higher frequencies upon calcination could be attributed to the formation of strong bonding between sulfates and zirconia atoms. Similar observation was made for the CSZ with 110 ◦ C dried sample showing various peak (at about 1045 cm−1 , 1105 cm−1 , 1170 cm−1 and 1280 cm−1 ) for S O and S O

vibrations as shown in Fig. 6. For 110 ◦ C dried CSZ an intense peak was observed at 1620 cm−1 attributed to HOH bond bending vibration for non digested sample and the intensity was observed to decrease with increase in digestion time. In our earlier studies on zirconia synthesis [16], a strong transmittance band at 935 cm−1 which is attributed to the Si H bending vibrations for non-calcined samples and at 1050 cm−1 attributed to the asymmetric stretching vibrations of the Si O Si bond was observed. The interference of this peak with the sulfate vibration peaks could also be the reason for sulfate IR peaks disappearance. Corma and Garcia [36] observed that the IR bands for sulfates in calcined sulfated zirconia catalysts when exposed to moisture were shown to disappear at certain H2 O:S molar ratio. The changes in IR spectra upon exposure to the moisture were attributed to successive formation of H2 SO4 , HSO4 − and SO4 2− from water sensitive SO3 groups. This could also be one of the reasons for the observed disappearance of sulfate peaks for calcined SZ and CSZ samples in the present study. 3.4. Ammonia TPD studies Temperature programmed desorption (TPD) of ammonia was used to measure the total acid strength and the acid sites distribution on the surface of sulfated zirconia. Fig. 7 shows the acid sites distribution for the SZ prepared from zirconium hydroxide synthesized at 1, 3 and 6 h and calcined at 600 ◦ C and 650 ◦ C. It was observed that for all three digestion times and for both calcination temperatures, SZ exhibit two peaks, at about 300 ◦ C and 600 ◦ C. The CSZ calcined at 600 ◦ C, exhibited 4 different peaks located

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201

Table 1 Textural properties of the SZ and CSZ calcined at 600 ◦ C temperature. Material

Digestion time (h)

Zirconia

Surface area (m2 /g)

Pore volume (cm3 /g)

Avg. pore size (nm)

3

141.6

0.1415

3.996

SZ

1 3 6 12 24 48

114.1 149.0 165.5 158.8 177.6 176.5

0.0965 0.1306 0.1492 0.1546 0.1845 0.1807

3.381 3.507 3.606 3.892 4.157 4.096

CSZ

1 3 6 12 24 48

0.0664 0.0778 0.0881 0.0516 0.0523 0.0478

14.74 14.71 14.85 10.91 9.982 9.528

18.02 20.93 23.73 18.90 20.98 20.07

(b) 1.10

(a) 1.10

1.05

Transmittance (%)

Transmittance (%)

1.05 1.00 0.95 0.90 0.85 0.80

48 h 24 h 12 h 6h 3h 1h 0h

1.00

48 h 24 h 12 h 6h 3h 1h 0h

0.95

0.90

0.75 2400 2200 2000 1800 1600 1400 1200 1000

800

600

2400 2200 2000 1800 1600 1400 1200 1000

-1

Wavenumber (cm )

800

600

-1

Wavenumber (cm )

(c)1.10

Transmittance (%)

1.05

1.00

0.95

0.90

0.85

0.80

48 h 24 h 12 h 6h 3h 1h 0h 2400 2200 2000 1800 1600 1400 1200 1000

800

600

-1

Wavenumber (cm ) Fig. 5. FTIR spectra of sulfated zirconia prepared using sulfuric acid with different digestion times and calcined at (a) 110 ◦ C (b) 600 ◦ C (c) 650 ◦ C.

in the temperature range of 156–200 ◦ C, 280–300 ◦ C, 380–400 ◦ C, and 560–600 ◦ C as shown in Fig. 8. However, upon calcination at 650 ◦ C the two intermediate peaks merged together to give one broad peak. It was also observed that for both SZ and CSZ, the intensity of peaks decreased when calcination temperature increased from 600 ◦ C to 650 ◦ C due to the decomposition of sulfate groups as shown in the TGA–DSC analysis. The ammonia desorption peak at temperature below 200 ◦ C belongs to the physisorption/chemisorptions of ammonia molecules on weak acidic sites.

The peaks at about 300 ◦ C and 480 ◦ C show the existence of intermediate strength acidic sites and finally the peak at 560–600 ◦ C demonstrates the presence of superacidic sites on the surface of zirconia [14]. By comparing Fig. 7 and Fig. 8, it can also be observed that the CSZ possesses more superacidic sites compared to the SZ. The quantitative measurement of total acid sites for the SZ and CSZ prepared at various digestion times and calcination temperature is depicted in Fig. 9. It is observed that, for both SZ and CSZ, the total number of acids sites initially increased when digestion time

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(a)

(b)

1.10

1.2

1.05

Transmittance (%)

Transmittance (%)

1.1

1.0

0.9

0.8

0.7

48 h 24 h 12 h 6h 3h 1h 0h

1.00 0.95

48 h 24 h 12 h 6h 3h 1h 0h

0.90 0.85 0.80 0.75

2400 2200 2000 1800 1600 1400 1200 1000

800

600

2400 2200 2000 1800 1600 1400 1200 1000

(c)

800

600

-1

-1

Wavenumber (cm )

Wavenumber (cm )

1.10

Transmittance (%)

1.05

1.00

0.95

0.90

0.85

0.80

48 h 24 h 12 h 6h 3h 1h 0h 2400 2200 2000 1800 1600 1400 1200 1000

800

600

-1

Wavenumber (cm ) Fig. 6. FTIR spectra of sulfated zirconia prepared using chlorosulfonic acid with different digestion times and calcined at (a) 110 ◦ C (b) 600 ◦ C (c) 650 ◦ C.

increased from 0 to 3 h but then observed to decrease with further increase in digestion time to 24 h. Upon more increase in digestion time to 48 h, the acid sites were seen to increase again by a small number. Similar observation was made about the XRD peak intensities for the sulfated zirconia samples prepared at different digestion times (Fig. 4c and d) wherein lower XRD peak intensities were observed for 3 h and 6 h digested samples. These observations can be explained on the basis of the effect of digestion time on surface area and hydroxyl groups on the surface of hydrous zirconia. Chen et al. [37] demonstrated the importance of surface OH groups for the formation of sulfated zirconia. They suggested that the formation of sulfated zirconia is a two step process. In the first step sulfate group displaces the surface OH group and upon calcination acid sites are formed through oxolation process forming stronger chemical bonds [38]. In an earlier work [16], we demonstrated that the process of digestion leads to an extensive polymerization through the condensation of the hydroxyl groups in the hydrous zirconia to form ordered three-dimensional porous structure with greater thermal stability. Thus an increase in digestion time influences both the surface hydroxyl groups as well as the surface area of the material. As the digestion time increases the surface areas increases at the expense of surface OH groups which are important for the anchoring of sulfate groups during the sulfonation process. On the other hand, more surface area provides more distributed anchoring sites for unhindered attachment of sulfate groups. Thus, with lower digestion time we have more OH groups

but low surface area and at higher digestion times we have less OH groups but higher surface area, which explains the observation made about the variations in crystallinity in the XRD analysis and the existence of optimum number of acid sites for 3 h digestion time. 3.5. Nitrogen adsorption–desorption The nitrogen adsorption–desorption isotherms for the zirconia, SZ and CSZ prepared with 3 h digestion time and calcined at 600 ◦ C temperature are shown in Fig. 10. The isotherms resemble the type IV isotherms with hysteresis loop of type H2 based on IUPAC classification [39]. The hysteresis loop is associated with the capillary condensation taking place in the mesopores signifying existence of mesoporous structure in the calcined sulfated zirconia. The type H2 hysteresis loop is attributed to the “ink-bottle” shaped pores (pores with narrow necks and wide bodies). Table 1 presents the values of surface area, pore volume and average pore sizes of the SZ and CSZ prepared with various digestion times calcined at 600 ◦ C temperature. A slightly higher surface area was observed upon sulfonation using sulfuric acid compared to the non-sulfated zirconia. This supports the observation made in the TGA–DSC studied wherein delay in the crystallization temperature was observed upon sulfonation. The surface area was also observed to increase with increase in digestion time. Several reports have suggested that the introduction of sulfate anions disturbs the transition of amorphous phase to

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203 70

70

(a) SZ-1_600

600

(b) SZ-1_650

600

60

40 30

300 20

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50

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40

o

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o

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30 300 20 200

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800 1000 1200 1400 1600 1800 2000

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TCD Signal (mV)

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60

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Temperature Signal

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2000

0

200

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1000 1200 1400 1600 1800

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Time (sec)

Fig. 7. NH3 TPD of sulfated zirconia prepared using sulfuric acid and zirconia prepared with digestion times and calcination temperature of (a) 1 h, 600 ◦ C (b) 1 h, 650 ◦ C (c) 3 h, 600 ◦ C (d) 3 h, 650 ◦ C (e) 6 h, 600 ◦ C (f) 6 h, 650 ◦ C, respectively.

crystalline phase and the extent of delay in transition depends on the concentration of active sulfate species on the zirconia surface [13]. Thus it can be concluded that the time of digestion certainly has the influence on the number of active sulfate sites on the zirconia surface. A drastic reduction in the surface area and pore volume and increase in the average pore size was observed upon sulfonation using chlorosulfonic acid. The shift of the hysteresis loop toward the higher relative pressures signifies the formation of bigger pores at expense of breaking of smaller pores. This could be due to the destruction of mesoporous structure of hydrous zirconia caused by the highly corrosive action of the chlorosulfonic acid [32,40].

3.6. Catalysts performance for esterification reaction The catalytic activity of the prepared SZ and CSZ catalysts synthesized with digestion time of 1, 3, 6, 12, 24 and 48 h and calcined at 600 ◦ C temperature were tested for the esterification of oleic acid in the soybean oil. The conversion was calculated using the following equation; XA =

AV0 − AV AV0

(2)

where AV0 is initial acid value and AV is acid value at any time t. As the purpose of the study was to investigate the effect of

204

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Fig. 8. NH3 TPD of sulfated zirconia prepared using chlorosulfonic acid and zirconia prepared with digestion time and calcination temperature of (a) 1 h, 600 ◦ C (b) 1 h, 650 ◦ C (c) 3 h, 600 ◦ C (d) 3 h, 650 ◦ C (e) 6 h, 600 ◦ C (f) 6 h, 650 ◦ C, respectively.

catalyst preparation parameters on the rate of reaction and % conversion, all reaction parameters were kept constant at 60 ◦ C reaction temperature, methanol/oil molar ratio of 9:1 and 2% catalyst concentration. The leaching test of the catalyst was carried out in methanol and also in the reaction mixture (without oleic acid) by following the procedure reported by Suwannakarn et al. [41]. It was observed that both SZ and CSZ did not show any leaching into the methanol or reaction mixture. Figs. 11 and 12 summarize the results obtained for catalytic activity of SZ and CSZ in the esterification reaction. The results suggest that the type of sulfonating agent has strong influence on the catalytic activity of the sulfated zirconia. In the case of SZ, maximum conversion of

22.49% was obtained at the end of 80 min whereas for the same reaction time it was 84.33% conversion for CSZ. The significantly higher activity of the CSZ even after having relatively lower surface area compared to the SZ is attributed to existence of more superacidic sites on the surface of CSZ as discussed earlier, hence, resulting in higher conversion. The digestion time of zirconia was also observed to influence the rate of reaction and the final conversion of oleic acid for both SZ and CSZ. The sulfated zirconia prepared from hydrous zirconia synthesized at 3 h digestion time showed higher esterification activity compared to the hydrous zirconia synthesized at other digestion times. These results confirm the observance of more number of total acidic sites for 3 h

V.G. Deshmane, Y.G. Adewuyi / Applied Catalysis A: General 462–463 (2013) 196–206

Fig. 9. Variation of total number of acid sites (␮mol/g) with respect to digestion time and sulfonating agent on the surface of sulfated zirconia calcined at 600 ◦ C and 650 ◦ C temperature.

205

Fig. 12. Conversion vs time plot for CSZ prepared at different digestion times for oleic acid esterification reaction. Methanol/acid oil: 9/1, temperature: 60 ◦ C, catalyst loading: 2% (wt % of acid oil).

digested sample compared to 1 and 6 h which is demonstrated in Fig. 9. 150

4. Conclusions

D A

140 130 120 110

Volume [cc/g]

100

Zirconia

90 80 70

SZ

60 50 40 30 20

CSZ

10 0 0.0

0.2

0.4

0.6

0.8

1.0

P/Po Fig. 10. Nitrogen adsorption–desorption isotherm for zirconia, SZ and CSZ prepared at 3 h digestion time. A: adsorption, D: desorption.

The hydrous zirconia prepared with different hydrothermal treatment times was used to synthesize the sulfated zirconia solid acid catalyst using sulfuric acid and chlorosulfonic acids as sulfonating agents. The digestion time and the type of sulfonating agents were found to have significant influence on structural, textural and catalytic properties of the prepared sulfated zirconia catalyst. The sample prepared with 3 h digestion time was observed to possess the highest number of total acid sites. For the esterification of oleic acid in soybean oil, the CSZ catalyst prepared with 3 h digestion time and 600 ◦ C calcination temperature showed the highest catalytic activity of about 85% conversion in just 80 min at 60 ◦ C reaction temperature. The higher catalytic activity of CSZ though with significantly lower surface area compared to SZ is attributed to the presence of higher surface superacidic sites compared with SZ. Acknowledgements The authors acknowledge the funding received from National Science Foundation (NSF) for the financial assistance via Award CBET 0651811. The authors are also grateful to the College of Engineering at North Carolina Agricultural and Technical State University (NCAT) for partial support for this project. The authors thank the Center for Advanced Materials and Smart Structures (CAMSS) at NCAT for the use of their XRD for material characterization. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.apcata.2013.05.005. References

Fig. 11. Conversion vs time plot for SZ prepared at different digestion times for oleic acid esterification reaction. Methanol/acid oil: 9/1, temperature: 60 ◦ C, catalyst loading: 2% (wt % of acid oil).

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