Ind. Eng. Chem. Res. 1998, 37, 3869-3878
3869
Synthesis and Characterization of Sulfated Titania Solid Acid Catalysts Ajay K. Dalai,* R. Sethuraman, Sai P. R. Katikaneni, and R. O. Idem Catalysis and Chemical Reaction Engineering Laboratory, Department of Chemical Engineering, 110 Science Place, University of Saskatchewan, Saskatoon, SK S7N 5C9, Canada
Sulfated titania catalysts were prepared by precipitation of titanic acid from titanium tetrachloride using aqueous ammonia, followed by impregnation with sulfuric acid. The sulfate concentrations in the catalysts were in the range 5-15 wt %. These sulfated catalysts were calcined at temperatures in the range 450-650 °C. The characteristics of these sulfated titania catalysts were examined using BET, SEM, powder XRD, ICP-AES elemental analysis, FT-IR, TPD of NH3, TPD of pyridine, and 1H MAS solid state NMR measurements. The activities of these catalysts were evaluated using n-butane isomerization studies to produce isobutane in a fixed-bed microreactor operated at 1 atm, 100-250 °C, and a space velocity (WHSV) of 5-15 h-1. The n-butane conversion and isobutane selectivity were at a maximum with a sulfate concentration of 12.5 wt % and a reaction temperature of 150 °C. Results showed a linear relationship between the acidity of the catalysts and their isomerization activity. The isomerization activity of sulfated titania catalysts were compared with those of other solid acid catalysts such as sulfated zirconia and Pt-HZSM-5 catalysts. This study indicated that isobutane selectivity as well as the acidity increased in the order sulfated zirconia . sulfated titania > Pt-HZSM-5. Introduction There is a considerable increase in the demand for i-C4 hydrocarbons such as isobutane and isobutylene because of the growing interest in their applications for the manufacture of highly desirable products such as methyl tert-butyl ether (MTBE) and ethyl tert-butyl ether (ETBE). Both MTBE and ETBE are produced from methanol and ethanol, respectively, each reacting with isobutylene. They are used as oxygenate additives in producing reformulated gasoline. Reformulated gasoline is in high demand since it is one of the fuels designed to meet the stringent requirements of the US Clean Air Act of 1990.1,2 Traditionally, straight chain C4 hydrocarbons (n-C4) are obtained from petroleum sources such as natural gas and the steam cracking of naphtha and gas oil. The branched chain C4 hydrocarbons (i-C4) are then produced from n-C4 by isomerization over acid catalysts such as hydrofluoric acid and chlorinated Pt-Al2O3 catalysts.3 On the other hand, the recent literature4,5 shows that solid acid catalysts such as sulfated zirconia are effective for the isomerization of n-butane at low temperatures ((150 °C). However, these catalysts undergo fast deactivation. A recent study6 using canola oil as feed has shown that Pt-HZSM-5 is effective for n-butane isomerization. However, unlike sulfated zirconia, Pt-HZSM-5 is effective for n-butane isomerization at temperatures higher than 250 °C but does not undergo any significant deactivation. Furthermore, it has been shown4-7 that, unlike hydrofluoric acid and chlorinated Pt-Al2O3, both the sulfated zirconia and Pt-HZSM-5 catalysts are noncarcinogenic and noncorrosive. In addition, they are quite convenient to handle. The current literature8,9 indicates that sulfated titania catalysts may have acid characteristics similar to * Corresponding author.
those of sulfated zirconia. Consequently, sulfated titania may also be effective for n-butane isomerization. However, to our knowledge no studies have been reported using sulfated titania catalysts to evaluate their isomerization as well as deactivation characteristics. In this work, various sulfated titania catalysts have been prepared with sulfate concentration in the range 0-15 wt %. Also, these catalysts have been extensively characterized. A performance evaluation of these catalysts has been conducted in order to develop an understanding of the relationship between their performances and characteristics. Furthermore, a comparison has been made on both acidity and activity of sulfated titania catalyst with those of other solid acid catalysts such as sulfated zirconia and Pt-HZSM-5. Experimental Section Preparation of Catalysts. The sulfated titania catalysts were prepared by incorporation of sulfate ions in a dried titanic acid precipitate by impregnation with 0.5 M sulfuric acid. The dried titanic acid was prepared by precipitation from titanium tetrachloride (obtained from BDH, Poole, England) using aqueous ammonia (obtained from BDH Inc., Toronto, ON, Canada) as the precipitating agent. Prior to precipitation, titanium tetrachloride and aqueous ammonia were each diluted with water and equilibrated in an ice bath at a temperature of 9 °C to mitigate the high reactivity and vapor pressure of titanium tetrachloride solution and ammonia solution, respectively. The titanic acid precipitate (T-D) was dried overnight at 60 °C. Altogether, sulfated titania catalysts with five levels of sulfate loading were prepared. The sulfate loadings were 5, 7.5, 10, 12.5, and 15 wt % and were obtained by impregnating the dried titanic acid powder (T-D) with appropriate amounts of 0.5 M sulfuric acid. For example, 5 g of sulfated titania catalyst with 15 wt %
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3870 Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998
sulfate loading was obtained by impregnating 6.1 g of T-D with 25 mL of 0.5 M sulfuric acid. After sulfation, each catalyst was washed twice with cold distilled water to remove any physisorbed sulfate ions and then dried overnight at 110 °C. All the catalysts were calcined at 450 °C for 4 h. These are referred to as 450ST5, 450ST7, 450ST10, 450ST12, and 450ST15 for a calcination temperature of 450 °C and for sulfate treatments of 5, 7.5, 10, 12.5, and 15 wt %, respectively. The dried sulfated titania catalyst treated with 15 wt % SO42- was also calcined at 500, 550, and 650 °C. These sulfated titania catalysts are referred to as 500ST15, 550ST15 and 650ST15, respectively. To compare the nature of acidity of sulfated titania with those of typical solid acid catalysts, a sulfated zirconia catalyst and a Pt-HZSM-5 catalyst were prepared. The sulfated zirconia catalyst treated with 15 wt % sulfate was prepared according to the procedure reported by Sethuraman et al.10 The Pt-HZSM-5 catalyst containing 0.5 wt % platinum and a Si/Al ratio of 56 was prepared according to the procedure reported by Katikaneni et al.6 Catalyst Characterization. Details of the characterization studies of Pt-HZSM-5 and sulfated zirconia have been reported earlier.6,10 This work focuses on the characterization study (both physical and chemical) of sulfated titania catalysts. Physical Property Characterization. (a) N2 Adsorption Measurements. The BET surface area and pore volume of all the titania catalysts were determined using Micromeritics adsorption equipment (Model ASAP 2000, manufactured by Micromeritics Instruments Inc., Norcross, GA) using nitrogen (99.995% purity; obtained from Linde, Calgary, AB, Canada) as the analysis gas. Prior to analysis, each catalyst was evacuated overnight at 200 °C and a vacuum of 500 µmHg. Average pore diameter and pore size distributions were estimated using d ) [4(pore volume)/(surface area)]. These calculations were done automatically using software provided by Micromeritics Instrument Inc. (b) Scanning Electron Microscopy (SEM). SEM studies were conducted to determine the average particle size in sulfated titania catalysts as a function of sulfate loading and calcination temperature. These measurements were made using a Phillips SEM-505 scanning electron microscope. In this case, the average particle size has been defined in terms of a median dimension, which is the maximum dimension of the particle perpendicular to the direction of maximum length of the particle. (c) Powder X-ray Diffraction (XRD) Measurements. Powder XRD measurements were performed to identify the component phases as well as to determine the degree of crystallinity of sulfated titania catalysts as a function of sulfur loading and calcination temperature. The XRD measurements were made with a Rigaku diffractometer (Rigaku, Japan) using Fe KR radiation in the range 6-60° (2θ) at a scanning speed of 10 deg/min. Chemical Property Characterization. The chemical property characterization techniques were employed for the determination of acidity as well as sulfur content of the calcined catalysts. (a) Sulfur Content. The sulfur content in calcined sulfated titania catalysts was determined using a Horiba Carbon/Sulfur analyzer EMIA-520 at Saskatchewan Research Council, Saskatoon, Canada. Each
sample was prepared for analysis by fusing 200 mg of the sample together with 200 mg of a standard, tungsten and iron chips, in a Leco induction furnace. The reported accuracy of the analysis was 0.01 wt % sulfur. (b) Temperature-Programmed Desorption of Ammonia. The temperature-programmed desorption (TPD) of NH3 was performed in order to determine the distribution of both the strong and weak acid sites on sulfated titania catalysts. The analysis was conducted in a conventional flow system similar to the one described by Katikaneni et al.11 The carrier gas used was N2 (99.995% purity; obtained from Linde) at a flow rate of 60 mL/min while a mixture of 1 mol % NH3 in N2 (high purity and also obtained from Linde, Calgary, Canada) was used as the adsorbing gas. About 0.2 g of the calcined catalyst sample was used for each experiment. NH3 adsorption was performed by flowing 1% NH3 in N2 gas over the catalyst for 45 min. Adsorption was carried out at 100 °C in order to eliminate physically adsorbed NH3. After adsorption of NH3 on each sample, the carrier gas was allowed to flow over the sample at 100 °C for 30 min. The TPD of NH3 was started at 100 °C and was continued until 800 °C at a programmed rate of 8 °C /min. (c) Temperature Programmed Desorption (TPD) of Pyridine. TPD of pyridine was performed in order to determine the distribution of the strong acid sites on the sulfated titania catalysts. The procedure and apparatus used were similar to those for TPD of NH3. Also, to eliminate physical adsorption pyridine, pyridine adsorption was performed by passing pyridine vapor over the catalyst sample at 100 °C. (d) FT-IR Measurements. The FT-IR technique was employed to identify the nature of acid sites present on the catalyst samples. The pyridine (1400-1650 cm-1) region of the IR spectra was explored. Initially, spectra were obtained for both fresh and pyridine chemisorbed catalyst samples. Pyridine chemisorbed samples were obtained by passing pyridine vapor over the catalysts at 100 °C for 1 h in the flow system, which was used previously for NH3 adsorption. After the pyridine adsorption, each sample was stabilized in a N2 flow for 30 min, then allowed to cool to room temperature, and subsequently used for IR analysis. All the IR measurements were made on powdered catalyst samples using a Biorad infrared spectrometer (Model FTS 40, Digilab Division, Bio-Rad Laboratories, Cambridge, MA). The spectra thus obtained were corrected by subtracting the spectra for the fresh samples from those for the corresponding pyridine adsorbed samples. (e) Solid-State NMR Studies. 1H MAS solid-state NMR studies were conducted to determine the number of hydroxyl groups present on the catalyst. These experiments were performed at the National Research Council (NRC) Laboratory, Saskatoon, Canada, using Bruker AM 360 WB instrument equipped with a CP/ MAS facility. The instrument was operated at a frequency of 360.13 MHz and a magnetic field of 7.05 T. Other operational parameters were a pulse width and frequency of 6.5 µs and 4000 Hz, respectively. All operations were performed at room temperature, whereas all the chemical shifts were measured relative to that of tetramethylsilane (TMS). Catalyst Performance Studies. Reactor Setup and Experimental Procedure. The isomerization of n-butane to isobutane was performed on sulfated titania and on sulfated zirconia and Pt-HZSM-5 catalysts in
Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 3871 Table 1. BET Surface Area, Average Pore Diameter, and Total Pore Volume of Various Titania Catalysts catalysts
sulfur loading (wt %)
BET surface area (m2/g)
av pore diam (Å)
tot. pore vol (cm3/g)
T-D T-C 450ST5 450ST7 450ST10 450ST12 450ST15 500ST15 550ST15 650ST15
0.0 0.0 0.36 0.42 1.2 1.6 1.75 1.55 1.3 0.30
334 ( 10 61 ( 2 84 ( 2 114 ( 3 154 ( 4 173 ( 4 135 ( 3 90 ( 2 97 ( 2 57 ( 1
33 ( 2 95 ( 7 87 ( 6 70 ( 5 52 ( 4 46 ( 3 53 ( 4 77 ( 5 85 ( 6 141 ( 9
0.27 ( 0.01 0.14 ( 0.01 0.18 ( 0.01 0.20 ( 0.01 0.20 ( 0.01 0.20 ( 0.01 0.18 ( 0.01 0.17 ( 0.01 0.21 ( 0.01 0.20 ( 0.01
a fixed-bed SS microreactor (4 mm i.d. and 230 mm long) at atmospheric pressure. The reactor was loaded with 0.1 g of catalyst. In the case of sulfated titania and zirconia catalysts, the reaction was carried out at n-butane weight hourly space velocities (WHSV) and reaction temperatures in the range 2.5-10 h-1 and 100-250 °C. In the case of Pt-HZSM-5, runs were performed at atmospheric pressure, n-butane WHSV of 2.5 h-1, and reaction temperatures in the range 200300 °C. The reactor temperature was controlled using a temperature controller (Model No. SWT-100018, ThermoElectric, East Rutherford, NJ). The feed gas consisted of 10 mol % n-butane in nitrogen (high purity; supplied by Linde, Calgary, Canada). The flow of the feed gas was monitored using a Matheson mass flow meter (Model 8141, Matheson, East Rutherford, NJ). Results and Discussion Characterization of Sulfated Titania Catalysts. BET Surface Area, Average Pore Diameter, Crystallite Size, and Sulfur Content Determinations. Table 1 shows that the sulfur content in the sulfated titania catalysts calcined at 450 °C was in the range 0-1.75 wt %. It may be noted that these sulfur contents in the catalysts were achieved due to sulfate treatments in the range of 5 to 15 wt %. Table 1 also shows the BET surface area, average pore diameter, and total pore volume data with their standard deviations for both calcined and uncalcined as well as sulfated and unsulfated titania catalysts. The results show that uncalcined unsulfated titania has a surface area of 334 m2/g. Upon calcination at 450 °C the surface area decreased to 61 m2/g. This reduction in surface area can be attributed to sintering of the catalyst on calcination. The sintering was also responsible for reducing the pore volume from 0.27 to 0.14 cm3/g. However, it is seen in the table that when titania is sulfated and then calcined at 450 °C, the decrease in surface area of the resulting calcined sulfated titania catalyst is not as drastic as that for the unsulfated calcined catalyst. For example, 450ST5 catalyst has a surface area of 84 m2/g as compared to 61 m2/g for the calcined unsulfated titania (T-C). Higher sulfur loading on titania catalyst in general increased the BET surface area for these catalysts. For example, the surface area increased from 61 to 173 m2/g with an increase in sulfate loading from 0 to 12.5 wt %. Further increase in sulfate loading to 15 wt % resulted in a decrease in surface area to 135 m2/g (see Table 1). However, the total pore volume of sulfated titania catalysts was unaffected on sulfate treatments at different concentrations. These observations can be explained on the basis of a two-step process which might be taking place during
sulfation followed by calcination of titania. The first step is the incorporation of sulfate ions, causing the plugging of the pores present in the catalyst.5 Therefore, there was a net decrease in the BET surface area upon sulfation.12 The second step is that the incorporation of sulfate ions might retard the loss in surface area as well as pore volume during calcination. This step explains the higher surface area and pore volume values for sulfated calcined titania catalysts compared to unsulfated and calcined titania catalyst (see Table 1). It has been shown13 that sulfation results in the replacement of some of the hydroxyl bridges originally present in dried uncalcined and unsulfated titania with sulfate ions. Subsequently, calcination of sulfated titania favors the formation of oxy bonds and also results in changes in the Ti-O-Ti bond strength due to attachment of the sulfate bridges. These inhibit sintering and hence stabilize the surface area of the resulting catalyst upon calcination. Consequently, the increase in surface area with an increase in sulfate loading up to 12.5 wt % sulfate appears to be due to the stabilizing effect of the sulfate ions. This stabilization effect can be seen more clearly from the SEM pictures (Figure 1), which indicate that the particle size in these calcined catalysts decreased with an increase in sulfate loading. For example, with an increase in sulfate loading from 0 to 10 wt %, the average particle size decreased from 90 to 25 µm (Figure 1a-c). This implies that sintering (which results from agglomeration of the catalyst particles) was retarded because of sulfation up to 12.5 wt %. As a result, the loss in surface area was also retarded. Moreover, Table 1 shows that the average pore diameter decreased from 95 Å in calcined unsulfated titania (T-C) to 46 Å in 450ST12 (i.e., catalyst treated with 12.5 wt % sulfate) due to plugging of larger pores by sulfation. However, the total pore volume remained more or less constant. Therefore, the formation of additional pores must be occurring. This phenomenon and the shift toward smaller pores resulted in an increase in the BET surface area of the sulfated catalyst. In the case of the catalyst with sulfate loading of 15 wt %, the plugging of more pores may have occurred resulting in a reduction of total pore volume to some extent and a decrease in BET surface area from 173 to 135 m2/g. However, statistically there was no significant change in average pore diameter (see Table 1). In this case, the effect of pore plugging superseded the stabilizing effect due to oxy-bond formation. This can also be confirmed from SEM results (Figure 1), which show that the particle size did not decrease to a large extent on increasing the sulfate concentration from 10 wt % (25 ( 2 µm) to 15 wt % (22 ( 2 µm). The effect of calcination temperature on BET surface area for the sulfated titania catalysts with 15 wt % sulfate is also shown in Table 1. The surface area decreased from 135 to 57 m2/g with an increase in calcination temperature from 450 to 650 °C. The calcination at high temperatures may have resulted in an increase in sintering and agglomeration without affecting much the pore volume. The agglomeration of the catalyst particles is also evident from the SEM results shown in Figure 1. When the calcination temperature is increased from 450 to 650 °C, the average crystallite size increased from 22 to 150 µm. An increase in calcination temperature also resulted in an increase in the average pore diameter from 53 to 141 Å
3872 Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998
Figure 1. Scanning electron micrographs of sulfated titania catalysts: (a) T-C calcined at 450 °C (70×), (b) 450ST10 (70×), (c) 450ST15 (70×), (d) 550ST15 (145×), and (e) 650ST15 (145×).
(see Table 1). This increase in average pore size was attributed to collapsing of the sulfate bridges reducing the sulfur content from 1.75 to 0.3 wt % when calcination temperature was increased from 450 to 650 °C. This extreme calcination temperature of 650 °C led to a situation similar to unsulfated titania. X-ray Diffraction Measurements. The X-ray diffraction (XRD) patterns of the calcined unsulfated and sulfated catalysts are presented in Figure 2. The
anatase phase was the only crystalline phase observed in these catalysts. Due incorporation of sulfate, the intensity of the XRD peaks decreased slightly showing that there was a slight decrease in the crystallinity of sulfated titania as compared to unsulfated titania (spectra 1 and 2 in Figure 2). On the other hand, a comparison of the spectra in the case of sulfate loading of 7.5 and 15 wt % (spectra 2 and 3 in Figure 2) shows that there was no change in intensity of the XRD peaks
Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 3873
Figure 3. Temperature programmed desorption profiles of NH3 over sulfated titania catalysts with different sulfate loadings: (1) 450ST5, (2) 450ST7, (3) 450ST10, (4) 450ST12, and (5) 450ST15. Table 2. Areas under the Curves from TPD Profiles of NH3 and Pyridine for Various Sulfated TiO2 Catalysts area under the curves, arbitrary units
Figure 2. X-ray diffraction spectra for various sulfated titania catalysts: (1) T-C calcined at 450 °C, (2) 450ST7, (3) 450ST15, (4) 550ST15, and (5) 650ST15.
for an increase in the sulfate loading from 7.5 to 15 wt %. This shows that crystallinity is more or less independent of sulfate loading in the range 5-15 wt %. These XRD results showed no phase transformation in sulfated titania catalysts calcined at different temperatures. These substantiate our earlier BET and SEM measurements which showed that loss in BET surface area was due to crystallite size growth and/or plugging of pores with sulfate ions but was not due to phase transformation as determined from XRD studies. On the other hand, quite unlike the case of the effect of sulfate concentration, there was a change in the intensities of XRD peaks for the anatase phase due to a variation in calcination temperature (spectra 3-5 in Figure 2). This indicates that there is an increase in the fraction of the crystalline phase with an increase in calcination temperature. A similar effect of the calcination temperature on the particle size was also observed from SEM studies. These results corroborate the observed decrease in BET surface area with calcination temperature. Acidity Characteristics. The sulfated titania catalysts were used for the acid-catalyzed n-butane isomerization reaction. Thus, it was desirable to determine how acidity, nature and strength of acid sites present in the catalyst were affected by sulfate concentration and calcination temperature. (a) Temperature-Programmed Desorption of NH3. Figure 3 shows the temperature-programmed desorption (TPD) of NH3 profile for sulfated titania
catalysts
from TPD profiles for NH3
from TPD profiles for pyridine
450ST5 450ST7 450ST10 450ST12 450ST15 550ST15 650ST15
2560 3000 3565 4330 2475 2030 1950
2230 2240 2420 2980 1441 1055 430
catalysts with different sulfate loadings. This figure shows that each catalyst exhibits a broad NH3 desorption peak starting from 140 °C and extending beyond 800 °C. For TPD anylysis, the peak temperature is a measure of the strength of acid sites while the area under the peak represents the total amount of acid sites present on the catalyst. The temperature range of this broad peak indicates that these catalysts contained moderate to strong acid sites.12 In addition, it is seen that as the sulfate concentration increased, the TPD peak temperature shifted slightly toward a higher temperature. This implies that there was a slight monotonic increase in the average acid strength of the catalyst with an increase in sulfate concentration/sulfur loading. Figure 3 and Table 2 also show that the total area under the broad TPD peak for each catalyst increased with an increase in sulfate concentration up to 12.5 wt % sulfate. A further increase in sulfate concentration to 15 wt % lead to a decrease in the area under the TPD profile. In all cases except for 450ST15, the acidity was found to be propertional to the amount of sulfate treatment/sulfur loading on these catalysts, and was, therefore, due to true “sulfated catalyst sites”. This suggests that 450ST12 (12.5 wt % sulfate and calcined at 450 °C) may have more number of acid acid sites as compared to 450ST15 (15 wt % sulfate and calcined at 450 °C). Figure 4 shows the TPD profile of NH3 for the titania catalysts containing 15 wt % sulfate and calcined at 450, 550, and 650 °C for 4 h. In this case also, a broad NH3 desorption peak was observed which started from 140 °C and extended beyond 800 °C. However, as the calcination temperature increased, the area under the ammonia desorption peaks decreased, indicating a decrease in the acidity of the catalysts. This can be explained on the basis that an increase in calcination temperature leads to a disruption of oxy bonds present in the catalyst, and consequently to a loss in acidity. This decrease in acidity is also related to decrease in
3874 Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998
Figure 4. Temperature programmed desorption profiles of NH3 over sulfated titania catalysts calcined at different temperatures: (1) 650ST15, (2) 550ST15, and (3) 450ST15.
Figure 6. Temperature programmed desorption profiles of pyridine over sulfated titania catalysts calcined at different temperatures: (1) 650ST15, (2) 550ST15, and (3) 450ST15.
Figure 7. FT-IR spectra of sulfated titania catalysts with different sulfate loadings: (1) 450ST5, (2) 450ST7, (3) 450ST10, (4) 450ST12, and (5) 450ST15. Figure 5. Temperature programmed desorption profiles of pyridine over sulfated titania catalysts with different sulfate loading: (1) 450ST5, (2) 450ST7, (3) 450ST10, (4) 450ST12, and (5) 450ST15.
sulfur content in catalysts upon calcination at higher temperatures (see Table 1). (b) Temperature-Programmed Desorption of Pyridine. Figure 5 shows the TPD profile of pyridine for sulfated titania catalysts treated with different sulfate concentrations. Pyridine is a weak base and adsorbs mostly on strong acid sites. The TPD of pyridine was used to show the distribution of strong acid sites on the catalysts. Pyridine desorption temperature represents the strength of the acid sites present on the sulfated titania catalysts. Higher pyridine desorption temperature implies stronger acid sites. Figure 5 shows that for sulfated titania catalysts, pyridine molecules were desorbed at a temperature higher than 575 °C. This shows that sulfated titania contained strong acid sites. Also, there is a slight increase in the pyridine desorption temperature with an increase in sulfate concentration up to 12.5 wt %. Further increase in sulfate concentration to 15 wt % resulted in a decrease in the pyridine desorption temperature. This result suggests that the 450ST12 catalyst contained the strongest acid sites of all the sulfated titania catalysts under consideration. In addition, the area under the pyridine desorption peaks increased with sulfate treatment up to 12.5 wt % sulfate, and a further increase in sulfate loading resulted in a decrease in the area (see Figure 5 and Table 2). This observation indicates that the amount of strong acid sites on the sulfated titania catalyst also increased with sulfate loading and reached a maximum at 12.5 wt % sulfate. Figure 6 shows the TPD profiles of pyridine obtained for the sulfated titania catalysts treated with 15 wt % sulfate and calcined at different temperatures. This
figure shows that pyridine molecules were also desorbed at a temperature higher than 575 °C. However, it is seen from Table 2 that as the calcination temperature increased from 450 °C, the area under the TPD profile of pyridine decreased, which was due to a decrease in the amount of strong acid sites. (c) FT-IR Studies. In addition to determining the strength and total number of acid sites using the TPD of NH3, it was also necessary to determine the types of acid sites present (i.e., Lewis or Bronsted acid sites) as well as their distribution on the catalyst surface as a function of sulfate concentration. As suggested by Vedrine et al.,14 information regarding the types of acid sites present on the catalyst was obtained from the IR spectra of the samples in the pyridine region (wavenumbers in the range 1425-1775 cm-1). The IR spectra of pyridine adsorbed samples showing this region are presented in Figure 7 for the five sulfated titania catalysts. The figure shows that the catalysts exhibited six bands at frequencies of 1640, 1610, 1575, 1540, 1490, and 1445 cm-1. However, only those at 1640, 1540, 1490, and 1445 represent the various acid sites. According to Rahman et al.15 and Borade and Clearfield,16 the band at a frequency of 1540 cm-1 (B) represents the Bronsted acid sites whereas the one at 1490 cm-1 (BL) represents the pyridine chemisorbed either on Lewis or Bronsted acid sites. On the other hand, the absorption bands at 1445 (L1) and 1640 cm-1 (L2) correspond to different vibrations of pyridine chemisorbed on Lewis acid sites. It was observed that there was no significant change in the wavenumbers of each of these peaks as a result of changes in sulfate concentration. Figure 7 shows that the intensity of the peak at 1640 cm-1 (L2) was more or less the same for the sulfated titania catalysts with sulfate treatments from 5 to 12.5
Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 3875 Table 3. Chemical Shifts and Peak Areas from 1H MAS NMR Spectra of Various Sulfated Titania Catalysts
Figure 8. 1H MAS solid-state NMR spectra of sulfated titania catalysts with different sulfate loadings: (1) 450ST5, (2) 450ST7, (3) 450ST10, (4) 450ST12, and (5) 450ST15.
wt %. However, the intensity of the peak at 1640 cm-1 (L2) for 450ST15 was much lower. Also, this figure shows that there was a monotonic increase in the intensity of the band at 1445 cm-1 (L1) as the sulfate concentration in the catalyst increased. The intensity of the peaks represents the amount of that particular type of acid site present. Hence, these results indicate an increase in the number of Lewis acid sites with an increase in sulfate concentration. Figure 7 indicates that the intensities of the bands at 1540 cm-1 (B) representing Bronsted sites increased with an increase in sulfate concentration up to a maximum for 450ST10 or 450ST12. Also, Figure 7 shows that the intensities of the bands at 1490 cm-1 (BL) representing pyridine chemisorbed either on Lewis or Bronsted acid sites were maximum in the case of 450ST12 catalyst. In summary, FT-IR results show that on the whole, 450ST12 contains the maximum number of acid sites. These results are consistent with those obtained using the TPD of NH3 and pyridine. (d) 1H MAS Solid-State NMR Studies. 1H MAS solid-state NMR was used principally to provide quantitative information regarding the amount of the structural hydroxyl groups present on the catalyst. The 1H MAS NMR spectra of sulfated titania catalysts with different sulfate loadings are shown in Figure 8. It is known17 that the chemical shift (δ) assigned to 6.1 ppm is due to structural hydroxyl groups (responsible for generation of Bronsted acid sites) whereas that at 2.7 ppm is due to terminal OH groups. It is seen in Figure 8 that the chemical shift (δ) relative to the standard TMS for all the sulfated titania catalysts were greater than 6.1 ppm except for 450ST5 (5.9 ppm). On the other hand, the unsulfated titania (T-C) had a chemical shift (δ) of 2.0 ppm (Table 3). The chemical shift for unsulfated titania also appears as the smaller peak on the right of the main peak for sulfated titania catalysts. Therefore, our results suggest that the incorporation of sulfate ions induces a significant downfield shift from 2.0 ppm to values greater than 6.1 ppm. This dramatic
catalysts
chem shift (ppm)
peak area (arbitrary units)
T-C 450ST5 450ST7 450ST10 450ST12 450ST15 500ST15 550ST15 650ST15
2.0 5.9 6.5 6.3 6.4 6.3 5.5 7.8 6.3
485 750 840 955 1470 970 925 885 750
increase in the chemical shift is attributed to the enhancement of the Bronsted acid strength in sulfated titania catalysts due to the incorporation of sulfate ions. It is seen in Figure 8 that the chemical shift for sulfated titania catalysts do not exactly increase monotonically with the sulfate concentration, implying that the acid strength may not be increasing monotonically with sulfate concentration. This observation appears to be contrary to the results obtained using the TPD techniques, which showed a slight but definite increase in acid strength with increase in sulfate concentration. According to Semmer et al.18 an apparent nonmonotonic behavior was observed when NMR technique was used to determine the Bronsted acid strength of sulfated zirconia catalysts as a function of sulfate concentration. This can be explained on the basis that the exact value of the chemical shift (i.e., Bronsted acid strength) was determined not only by the sulfate concentration but also by the ratio of the concentrations of adsorbed water to sulfate ions in the catalyst. In the present work, the sulfated titania catalysts were used for NMR measurements without any pretreatment. Consequently, it was not ensured that the moisture adsorbed by the catalyst from the atmosphere would result in a constant water/ sulfate ratio. Therefore, these Bronsted acid strength results measured for nonpretreated sulfated titania catalysts using 1H MAS NMR technique are estimates for sulfated titania catalysts under conditions close to those used for reaction studies. An estimate of the number of hydroxyl groups present on the surface of each catalysts was obtained as the area under the main NMR spectrum (Figure 8 and Table 3). The number of hydroxyl groups gives an indication of the amount of Bronsted acid sites present in the catalysts. It is seen on the basis of peak areas (see Table 3) that the number of Bronsted acid sites increased with sulfate loading up to 12.5 wt % and decreased on a further increase in concentration to 15 wt % sulfate in the catalyst. Thus, the results for the amount of acid sites obtained using 1H MAS NMR measurements are in agreement with our earlier results obtained using TPD of NH3, TPD of pyridine, and FT-IR. Figure 9 shows the NMR spectra of the sulfated titania catalysts with 15 wt % sulfate which were calcined at different temperatures. It is seen that 450ST15 had a peak area of 970 (arbitrary units) where as 650ST15 had an area of 750 (see Table 3). There was a slight decrease in the area with an increase in calcination temperature indicating that 450ST15 had relatively higher amount of Bronsted acid sites. Comparison of Acidity Characteristics of Sulfated Titania, Sulfated Zirconia, and Pt-HZSM-5 Catalysts. A comparison of acidity characteristics of optimum catalysts for sulfated titania, sulfated zirconia and Pt-HZSM-5 was made using FT-IR, 1H MAS NMR,
3876 Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998
Figure 9. 1H MAS solid-state NMR spectra of sulfated titania catalysts calcined at different temperatures: (1) 650ST15, (2) 550ST15, (3) 500ST15, and (4) 450ST15.
and TPD of NH3 measurements. An earlier work from our laboratory10 has shown that sulfated zirconia treated with 15 wt % sulfate solution when calcined at 650 °C, produced a catalyst with optimum acid strength. This catalyst also gave higher selectivity to i-C4 hydrocarbons in a modified Fischer-Tropsch synthesis process. In the case of Pt-HZSM-5,6 the catalyst with maximum activity had a Si/Al ratio of 56 and a platinum concentration of 0.5 wt %. A comparison of the present results for sulfated titania (450ST12) with those of earlier work with sulfated zirconia10 and Pt-HZSM-56 showed that the amounts and strengths of both Bronsted and total acid sites increased in the order sulfated zirconia . sulfated titania > Pt-HZSM-5. Isomerization of n-Butane to Isobutane over Sulfated Titania Catalysts. Isomerization of n-butane to isobutane (see eq 1) was chosen as a model reaction to test the activity of the sulfated titania catalysts. The isomerization reaction is promoted due to the acid sites present on the catalyst. Thus, the nature and strength of acid sites present are expected to alter the product distribution obtained from this isomerization study. The operating variables studied were reaction temperature, space velocity, and sulfate treatment. The activities of sulfated zirconia and PtHZSM-5 were evaluated at specified reaction temperatures. The performance of sulfated titania as well as sulfated zirconia and Pt-HZSM-5 catalysts was evaluated in terms of n-butane conversion (eq 2) and isobutane product selectivity (eq 3), where X ) n-butane acid catalyst
n-C4H10 y\z i-C4H10 X)
(1)
mn,f - mn,p × 100 mn,f
(2)
mi,p mn,f - mn,p
(3)
S)
conversion (wt %), S ) selectivity for isobutane formation (wt %), mn,f ) mass flow rate of n-butane in feed (g/h), mn,p ) mass flow rate of n-butane in products (g/ h), and mi,p ) mass flow rate of isobutane in products (g/h).
Figure 10. Effect of sulfate loading in titania catalyst on n-butane conversion and isobutane selectivity at 150 °C, 1 atm, and 2.5 h-1 in a fixed bed microreactor.
Figure 11. Effect of temperature on n-butane conversion and isobutane selectivity using 450ST12 catalyst at 1 atm and 2.5 h-1 in a fixed bed microreactor.
Effect of Sulfate Loading in Titania Catalyst on n-Butane Isomerization. Figure 10 shows the typical effect of titania catalyst treated with various concentrations of sulfate on n-butane isomerization at 150 °C and 2.5 h-1. The figure shows that n-butane conversion increased with sulfate loading and reached a maximum of 10.0 wt % at a sulfate loading of 12.5 wt %. A further increase in sulfate loading to 15 wt % resulted in a decrease in n-butane conversion. The figure also shows that the selectivity for isobutane formation increased with sulfate loading and reached a maximum of 55% at a sulfate loading of 12.5 wt %. A further increase in sulfate loading to 15 wt % did not result in an increase in isobutane selectivity. These observations can be explained on the basis of the characterization results discussed earlier. The characterization study showed that the strengths and number of acid sites were maximized at 12.5 wt % sulfate loading. Therefore, there is a direct correlation between the amount of acid sites present on the catalysts and n-butane isomerization for sulfated titania catalysts. Since high n-butane conversion and isobutane selectivity were obtained for the 450ST12 catalyst, further investigations were conducted using this catalyst. Effect of Temperature. Figure 11 shows the effect of temperature on n-butane conversion and isobutane selectivity over 450ST12 catalyst at 2.5 h-1 and 1 atm.
Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 3877
Figure 12. Effect of space velocity on n-butane conversion and isobutane selectivity using 450ST12 catalyst at 150 °C and 1 atm in a fixed bed microreactor.
It is seen that n-butane conversion was maximized (10.0 wt %) at 150 °C. This observation can be explained on the basis that n-butane isomerization is a reversible and net exothermic reaction. Under the experimental conditions, the equilibrium conversion of n-butane to isobutane was calculated and was found to decrease from 73.3 to 55.8% with increase in temperature from 100 to 250 °C. A similar trend in conversion with temperature for 450ST12 catalyst was observed from experiments except at 100 °C (see Figure 11). Low conversion at 100 °C compared to 150 °C could be due to low catalytic activity at such low temperature. The selectivity to isobutane was maximum (55.5 wt %) at 200 °C (see Figure 11). The major byproducts obtained from this process were propylene and methane. This was due to cracking of both n-butane and isobutane (eqs 4 and 5). It is known3
n-C4H10 f C3H6 + CH4
(4)
i-C4H10 f C3H6 + CH4
(5)
that an increase in temperature favors cracking at the expense of isomerization. Therefore, it may be concluded for the 450ST12 catalyst that the rate of cracking increases at a faster rate than the isomerization rate above 200 °C. Effect of Space Velocity on the Conversion of n-Butane. Figure 12 shows that the conversion of n-butane decreased with an increase in space velocity. This was due to a decrease in the residence time at higher space velocities. In contrast, the selectivity for isobutane increased with an increase in space velocity. The isobutane yield was maximum at WHSV of 5 h-1. As mentioned earlier, the products such as isobutane, propylene, and methane obtained in the isomerization runs imply that both isomerization and cracking reactions take place in the reactor. Garwood19 had indicated that cracking requires a longer residence time than isomerization. Therefore, short residence times (i.e., high space velocity) favor isomerization and increase isobutane selectivity (see Figure 12). On the other hand, at long residence times (i.e., low space velocity), cracking reactions may become significant, producing more methane and propylene and less isobutane. This study indicated that n-butane conversion and isobutane yield are optimum for 450ST12 titania catalyst when the reactor was operated at 150 °C and 2.5 h-1.
Figure 13. Effect of temperature on n-butane conversion and isobutane selectivity for 450ST12 titania and other solid acid catalysts at 1 atm and 2.5 h-1 in a fixed bed microreactor.
Comparison of the Performance of Sulfated Titania, Sulfated Zirconia, and Pt-HZSM-5 Catalysts. Figure 13 shows the performances of sulfated titania, sulfated zirconia, and Pt-HZSM-5 catalysts in terms of n-butane conversion and isobutane selectivity as functions of reaction temperature. The reactor pressure and WHSV were maintained at 1 atm and 2.5 h-1, respectively. As was discussed earlier, the n-butane conversion reached a maximum of 12.5 wt % at a temperature of 150 °C with 450ST12 catalyst. However, an increase in temperature led to an increase in nbutane conversion for the sulfated zirconia catalyst. The trend for Pt-HZSM-5 catalyst was similar to that for sulfated zirconia. In this case, higher temperatures were required to effect n-butane conversion. Since the n-butane isomerization is reversible and exothermic, the increase in conversion with temperature is contrary to thermodynamic equilibrium considerations (see Figure 13). This trend could be explained on the basis that these catalysts are highly acidic and participate in both isomerization (to produce isobutane) and cracking reactions (to produce methane and propylene). Cracking reactions are primarily endothermic in nature. For these two catalysts there is an increase in the overall conversion of n-butane with temperature indicating domination of cracking over isomerization reaction. The n-butane conversion data in Figure 13 show that the performance of sulfated zirconia was superior. Also, a comparison in terms of n-butane conversionon for these three catalysts shows that the sulfated zirconia performed better than both sulfated titania and PtHZSM-5 under similar temperature conditions. As was discussed earlier, the isobutane selectivity (Figure 13) using 450ST12 catalyst reached a maximum at 200 °C. On the other hand, it is seen from this figure that there was a decrease in isobutane selectivity with an increase in temperature for sulfated zirconia. In the case of Pt-HZSM-5, the isobutane selectivity increased with an increase in temperature from 200 to 250 °C. Further increase in temperature to 300 °C did not result in any significant change in isobutane selectivity. A comparison of sulfated titania and sulfated zirconia in terms of isobutane selectivity shows that sulfated zirconia is superior. Using the selectivity data at 200 °C, it is clear that the sulfated zirconia is the best, followed by sulfated titania, and, finally, Pt-HZSM-5. The fact that Pt-HZSM-5 requires higher temperatures for the
3878 Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998
isomerization reaction implies that the acid sites on this catalyst are weaker than those on either sulfated zirconia or sulfated titania. Conclusions 1. BET results for calcined sulfated and unsulfated titania catalysts indicated that the sulfation retards the loss in surface area. The retardation in loss is due to the oxy bond created on sulfation. 2. The acid sites present in the sulfated titania catalysts were a mixture of Lewis and Bronsted acid sites. 3. TPD of NH3, TPD of pyridine, FT-IR, 1H MAS NMR, and n-butane isomerization studies indicated that a maximum in the amounts and strengths of acid sites was observed for the sulfated titania catalyst obtained after treatment with 12.5 wt % sulfate solution followed by calcination at 450 °C. 4. An increase in calcination temperature of the sulfated titania catalysts above 450 °C increased the crystallite size and reduced the acidity as well as the BET surface area of the catalyst. 5. The optimum operating conditions for the isomerization of n-butane were 2.5 h-1 and 150 °C for the 450ST12 catalyst at atmospheric pressure. A direct correlation was observed between the acidity and the isomerization activity of the solid acid catalysts. 6. A comparison of the performance of sulfated titania, sulfated zirconia and Pt-HZSM-5 showed that their acidity as well as their activity were in the order of sulfated zirconia . sulfated titania > Pt-HZSM-5. Literature Cited (1) Parkinson, G. Refining’s Clean New Gingle. Chem. Eng. 1992, 35-39. (2) Unzelman, G. H. Options to Meet 1990s Fuel Compositions Rules Timited. Oil Gas J. 1990, April 23, 91-93. (3) Gary, J. H.; Handwerk. G. E. Petroelum Refining Technology and Economics; Marcel Dekker Inc.: New York, 1994. (4) Comelli, R. A.; Canavese, S. A.; Vaudagna, S. R.; Figoli, N. S. Pt/SO42-/ZrO2: Characterization and Influence of Pretreatments on Hexane Isomerization. J. Catal. 1996, 135, 287-299. (5) Song, X.; Sayari, A. Sulfated Zirconia-Based Strong SolidAcid Catalysts: Recent Progress. Catal. Rev.-Sci. Eng. 1996, 38, 329-411.
(6) Katikaneni, S. P. R.; Adjaye, J. D.; Bakhshi, N. N. Conversion of Canola Oil To Various Hydrocarbons Over Pt/HZSM-5 Bifunctional Catalyst. Can. J. Chem. Eng. 1997, 75, 391-401. (7) Song, X.; Sayari, A. Direct Synthesis of Isoalkanes Through Fischer-Tropsch Reaction on Hybrid Catalysts. Appl. Catal. A: Gen. 1994, 110, 121-136. (8) Guo, C.; Yao, J.; Cao, J.; Qian, Z. Alkylation of Isobutane with Butenes Over Solid Superacids, SO42-/ZrO2 and SO42-/TiO2. Appl. Catal. A: Gen. 1994, 107, 229-238. (9) Guo, C.; Liao, S.; Qian, Z.; Tanabe, K. Alkylation of Isobutane with Butenes Over Solid Acid Catalysts. Appl. Catal. A: Gen. 1994, 107, 239-248. (10) Sethuraman, R.; Katikaneni, S. P. R.; Idem, R. O.; Bakhshi. N. N. Selective Production of C4 Hydrocarbons for FischerTropsch Synthesis in a Dual Bed Reactor Consisting of Co-Ni/ ZrO2 and Sulfated Zirconia Catalyst Beds. Submitted for publication in Can. J. Chem. Eng. (11) Katikaneni, S. P. R.; Adjaye, J. D.; Idem, R. O.; Bakhshi, N. N. Catalytic conversion of Canola oil over Potassium-Impregnated HZSM-5 Catalysts: C2-C4 Olefin Production and Model Reaction Studies. Ind. Eng. Chem. Res. 1996, 35, 3332-3346. (12) Sethuraman R. Selective Production of C4 Hydrocarbons From Syngas. M.Sc. Thesis, University of Saskatchewan, Saskatoon, Canada, 1996. (13) Davis, B. H.; Keogh, R. A.; Srinivasan, R. Sulfated Zirconia as a Hydrocarbon Conversion Catalyst. Catal. Today 1994, 20, 219-256. (14) Vedrine, J. C.; Auroux, A.; Bolis, V.; Dejaifve, P.; Naccache, C.; Wierzchowski, P.; Derouane, E. G.; Nagy, J. R.; Gilson, J. P.; Jan, H. C.; van Hooff, J. P. V. D.; Wlthuizen, J. Infrared, Microcalorimetric and Electron Spin Resonance Investigations of the Acidic Properties of the H-ZSM-5 Zeolite. J. Catal. 1979, 59, 248-262. (15) Rahman, A.; Lemay, G.; Adnot, A.; Kaliaguine, S. Spectroscopic and Catalytic Study of P-Modified ZSM-5. J. Catal. 1988, 112, 453-463. (16) Borade, R. B.; Clearfield, A. A Comparative Study of Acidic Properties of SAPO-5, -11, -34, and -37 Molecular Sieves. J. Mol. Catal. 1994, 88, 249-266. (17) Thomas, J. M.; Klinowski, J. The Study of Aluminosilicate and Catalysts by High-Resolution Solid-State NMR Spectroscopy. Adv. Catal. 1985, 33, 200-374. (18) Semmer, V.; Batamacck, P.; Doremieux, C.; Vincient, R.; Fraissard, J. The Acid Strength of Sulfated Zirconia Measured By Two 1H NMR Techniques In Presence of Water: 4K Broad-Line and 300 K High-Resolution MAS. J. Catal. 1996, 161, 186-193. (19) Garwood, W. E. Conversion of C2-C10 to Higher Olefins over Synthetic Zeolite ZSM-5. Am. Chem. Soc. Symp. Ser. 1983, 218, 383-396.
Received for review February 16, 1998 Revised manuscript received July 6, 1998 Accepted July 9, 1998 IE980091X
