Ind. Eng. Chem. Res. 1995,34, 2955-2963
2955
The Hydrogenation of Naphthalene with Platinum/ Alumina-Aluminum Phosphate Catalysts Ting-Chia Huang* and Ben-Chang Kang Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan, Republic of China 701
Alumina-aluminum phosphate (AAP) was used as a noble metal support, and PtlAAP was used as a hydrogenation catalyst to reduce the aromatic content in diesel fuels. Naphthalene dissolved in n-hexadecane was used as a model compound to simulate the aromatics in diesel fuels for the hydrogenation activity test. The experimental results indicated that the Pvaluminaaluminum phosphate catalyst had a better hydrogenation activity and lower &-Decalin selectivity than Ptly-Al~Oscatalyst due to the higher acidity of the AAP support than that of the y-Al203 support. The catalysts PtlAAP, a t 6 < AVP < 10, have the highest hydrogenation activity and the lowest &-Decalin selectivity of all PtIAApx catalysts. By controlling the AVP ratio, variable pore size and surface area of AAP supports have been obtained. The high surface area and large pore size of Pt/AAP catalyst can be used for aromatic hydrogenation with good activity and an effective diffusion coefficient. When phosphorus was added to Wy-AlnOs catalyst, it improved the hydrogenation activity but reduced the cis-Decalin selectivity for the naphthalene hydrogenation reaction system. However, the improvement of activity was not as good as that for Pt/AAP catalyst and too high a phosphorus content (AVP < 20) of Ptly-AlaOa was poisonous to the hydrogenation activity and cis-Decalin selectivity.
Introduction High aromatics content in diesel fuels lowers fuel quality and contributes significantly to the formation of undesired emissions in exhaust gases (Barry et al., 1985; Ullman, 1989). Because of the health hazards associated with these emissions, the environmental regulations governing the composition of diesel fuels and the limitations of aromatics are being tightened in the developed nations. In recent years, considerable attention has been paid to develop new catalysts and processes for aromatics saturation in diesel fuel. The most famous process is one in which the conventional hydrotreating catalysts (sulfided CoMoly-AlzOs or NiWly-AlzOs) are used for deep hydrodesulfurization in first-stage operation and the noble metal catalysts are used for deep aromatic hydrogenation in second-stage operation (van der Berg et al., 1993; Stanislaus and Cooper, 1994; Suckanek, 1990). Noble metal catalysts used in hydrogenation reactions are known t o be deactivated rapidly by the adsorption of sulfur-containing compounds which are present in industrial feedstocks (Stanislaus and Cooper, 1994). A severe pretreatment is necessary to reduce the sulfur content to a level that does not affect the performance of the noble metal catalysts (Barbier et al., 1990). Aromatics saturation is a reversible reaction favored at low temperature. The hydrogenation reactions may be carried out at relatively low temperatures by using high-activity noble metal catalysts. The boiling point range of diesel blending components generally falls between 477 and 633 K. Severe diffusion problems may be encountered for such processes. The overall catalyst activity may be limited due t o the rate of diffusion of liquid reactants into the pores. Therefore, large pore catalysts will be chosen to minimize diffusion resistance. The alumina-aluminum phosphate (AAP) support has received only a little attention in the catalytic
* Author t o whom correspondence should be addressed. 0888-5885/95/2634-2955$09.00/0
reaction literature. By controlling the aluminum to phosphorus ratio, AAF' supports can exhibit variable pore size, pore volume, and surface area properties (Marcelin et al., 1983). Within certain stoichiometric compositions of AVP ratio, high surface areas and large pore sizes of AAP can be obtained. Campelo et al. (1982) and Marcelin et al. (1983, 1984) have shown that AAP could be used as a support for nickel catalyst in liquidphase hydrogenation and the N i / M catalysts was superior to commercial Ni-catalysts. Chen et al. (1990) have used AAP as a support for Co-Mo catalysts in the hydrodesulfurization of residual oil, and they have obtained better activity than the conventional CoMol A1203 catalysts. The AAP supports have not been successfully deposited by noble metals and compared with conventional supports, such as Si02 or A1203 for the hydrogenation activity. In this study, AAP was used as the support for Pt catalysts to minimize the diffusion resistance for aromatics saturation in diesel fuel. Naphthalene dissolved in n-hexadecane was used as a model compound to simulate the aromatics in diesel fuels for the hydrogenation activity tests. The W M ,Wy-AlzO3 and PtP/y-A1203catalysts were prepared and characterized. The naphthalene hydrogenation activity and the cisDecalin selectivity of WAAPand Pt-Ply-Al~Oscatalysts were compared with the Ptly-AlzOa catalysts. The influence of the Avp atomic ratio of Pt/AAPx and PtPly-Al203catalysts on the activity and the cis-Decalin selectivity for naphthalene hydrogenation was studied.
Experimental Section 1. Preparation of AAP Supports. The aluminaaluminum phosphate supports were prepared by neutralizing mixed solutions containing appropriate ratios of aluminum nitrate, ~(N03)3*9H20 (Merck),and phosphoric acid (Shimakyu, 85%), with an ammonium hydroxide (Merck) solution. Both the acid solution and the base solution were added to a vessel containing wellstirred distilled water. The pH value was maintained at 8.0 & 0.1 throughout the precipitation process.
0 1995 American Chemical Society
2956 Ind. Eng. Chem. Res., Vol. 34, No. 9, 1995
The resulting precipitates were filtered, washed with distilled water, and dried a t 383 K overnight in an oven. By altering the relative amount of aluminum nitrate and phosphoric acid, a desired alumina-aluminum phosphate composite could be obtained. The oven-dried uncalcined AAP material was ground into powder, and then an appropriate amount of distilled water was added. The mixture was kneaded and then extruded. The wet extruded AAP cylinders were dried in air at room temperature overnight, then dried at 383 K for 4 h, and calcined at 823 K for 6 h. The extruded AAP supports were 1.6 mm in diameter and 3-5 mm in length. 2. Preparation and Characterization of Catalysts. The homemade AAP (alumina-aluminum phosphate), AB (aluminum borate), and AlZn (Al&-Zno) and commercial y-alumina Cs-331-4 (United Catalysts Inc.) and Cond-B (supplied by Condea Co.) were used as the supports for the Pt catalysts. The supports were impregnated with appropriate amounts of chloroplatinic acid aqueous solution (Merck products). Impregnated supports were kept in air overnight and calcined at 383 K for 4 h and at 673 K for 4 h. The metal content of the catalysts was 0.6 wt %. For convenience, Pt/ alumina-aluminum phosphate catalysts were denoted as P t / M x , where x represented the AVP atomic ratio. If AAP was kneaded with methylcellulose for extrusion, it was denoted as P t J M x c . Pt-P/Al203 catalysts, loaded with 0.6 wt % Pt and different contents of phosphorus, were obtained by impregnation of various amounts of phosphoric acid on the alumina support, Cs-331-4, followed by drying and calcining, and then by impregnation of chloroplatinic acid, drying, and calcining. For convenience, these catalysts were denoted as Pt-PxlAlzOa, where x represented the AVP atomic ratio. The surface area (BET) and pore volume of the supports and catalysts were determined from the nitrogen adsorption-desorption isotherms at 77 K, using a Micromeritics 2400 apparatus. The measurements were performed on the oxidized form of the catalyst samples. Catalysts were characterized in terms of exposed metallic surface area and metal dispersion by selective chemisorption with carbon monoxide. Carbon monoxide uptake was determined in a constant volume, highvacuum apparatus with a Micrometrics Chemisorb 2800. The CO to Pt surface atom stoichiometric ratio was supposed to be 1:l (Wells, 1985). The volume of CO adsorbed for monolayer coverage was obtained by the difference between total adsorption and physical adsorption (Delannay, 1984). The actual Pt concentration of Pt/Al2O3 catalyst, 0.63 wt %, was determined by inductively coupled plasmaatomic emission spectrometry (Jarrell-Ash, Model 1100). The Pt metal profiles of the catalyst pellets along the diameter were determined by using an electron probe microanalyzer (JEOL JSMr840 Multi-Function SEM) with a wavelength dispersion spectrometer. 3. Reactants Used. The model compound, naphthalene (Merck products, purity > 99%), was used to represent the aromatic compounds in diesel fuel. The liquid feed including 10 w t % naphthalene and 90 wt % n-hexadecane (Merck Products, purity > 99%)was used in all reactions. Hydrogen used in this reaction system was supplied by Sanfu Co. (Taiwan), with a purity of 99.99%.
Table 1. Characteristics of Platinum/ Alumina-Aluminum Phosphate Catalysts catalyst
WAAP1 WAAP3 WAAP5 Pt/AAp8 WAAP10 WAAP12 WAAPBO PtJAAF”
surface pore volume average pore medium pore radius (nm) radius (nm) area (m2/g) (cm3/g) 11.20 26.50 74.8 0.614 10.23 20.00 105.6 0.710 11.00 8.14 112.8 0.872 6.50 187.5 0.615 6.10 3.74 245.0 0.550 4.50 2.50 3.40 247.5 0.510 2.50 303.9 0.558 2.89 3.10 3.29 220.7 0.485
301 0
Pt AAPl
0
PLNAAP3
a 0 C
Pt/AAPS Pt AAP8 P t , AAP20
pore radius ( A )
Figure 1. Pore size distributions of different WAAPx catalysts.
4. Reaction System. A cocurrent downflow trickle bed reactor, similar to that used in our previous work (Huang and Kang, 1995a), was used in this study. Catalysts were reduced in situ by H2 a t 673 K for 4 h, the reactor was cooled to the desired temperature, and then the reactants were introduced. All the reactions proceeded at the same LHSV (liquid hourly space velocity),2.8 h-l, and the same hydrogedoil ratio, 1000 mUmL (in excess). At an appropriate time, the liquid samples were withdrawn periodically with a sampling valve from the separator and analyzed by using a gas chromatograph (Carlo Erba, Model 60001, equipped with a flame ionization detector (FID) and an electronic integrator (Carle Erba, Model DP 700). A 10 m by 0.53 mm fused-silica wide-bore column, type WCOT, with a liquid phase of CP-SIL-5CB, was used to separate reactants and products. The chromatographic analysis was conducted under temperature-programed conditions. Results and Discussion 1. Characterization of Catalyst. The surface areas, pore volumes, and pore sizes of WAAPx catalysts with different AVP atomic ratios are listed in Table 1, and the pore size distribution of WAAPx catalysts are shown in Figure 1. It is shown that the surface area of catalysts increases and the average pore diameter of catalysts decreases as the AVP atomic ratio increases. The A A P x support in the literature (Vogel et al., 1983; Marcelin et al., 1983; Chen et al., 1990) had the same trend. A desired surface area and pore diameter of W A A P x catalyst can be obtained by varying the AVP atomic ratio.
Ind. Eng. Chem. Res.,Vol. 34, No. 9, 1995 2957 1 0 ,
I
I
1
'!
Table 3. Characteristics of Different Supports and Prepared Catalysts surface pore average area volume pore radius metal Pt (m2/g) (cm3/g) (nm) dispersion
support
A. CS-331-4
f 00' 0
1
I
10
20
..
co
AL/P atomic ratio
Figure 2. Effect of Al/P ratio on the metal dispersion of PtIAAPx catalyst. Table 2. Characteristics of Pt-Pzly-Al209 Catalysts
suppodcatalyst 1.fresh support 2. prepared catalyst P t -P-/y -A1203 Pt-P80/y-A1203 Pt-P20/y-Al203
Pt-PJ.O/y-A1203
surface pore average area volume pore radius metal Pt (nm) dispersion (m2/g) (cm3/g) 6.58 198.9 0.658 195.0 181.7 173.6 184.4
0.668 0.705 0.677 0.645
6.62 6.98 7.02 5.93
0.924 0.804 0.861 0.553
The metal dispersions of PtJAAPx catalysts are shown in Figure 2. The low AVP atomic ratio (AVPI3) of PtJ AAPx catalysts exhibits low metal dispersion due to two reasons. One is the low surface area and large pore size of the support. According to the result of Anderson (1975), the average particle diameter of Pt metal increased by increasing the average pore diameter of alumina supports. The same results were obtained for the silica gel supports and the AAP supports in this study. The other reason for low metal dispersion is the basicity or increased amount of aluminum phosphate in the low AVP ratio of AAF' supports (Campelo et al., 1984), which will decrease the absorption strength between HPtC16- and the support (Mang et al., 1993; Brunelle, 1978). The surface areas, pore volumes, pore sizes, and Pt metal dispersions of Pt-PxlAl203 catalysts with different AI/P ratios are shown in Table 2. When the phosphorus was added to WAl2O3 catalysts, the surface area and the metal dispersion of catalysts were decreased. Morales et al. (1988) had the same results, namely, that the surface area of the alumina support was decreased by addition of phosphorus. When the phosphorus content was high (AYP = lo), the metal dispersion was low due to the formation of a multilayer of phosphate. The surface areas, pore volumes, porq sizes, and metal dispersions of different supports and prepared catalysts are shown in Table 3. The combined effect of varying the characteristics associated with these different AAP and other supports leads to the widely varying metal dispersion and hydrogenation activities of the supported catalysts, as will be discussed in the following. In Table 3, the A B l O (aluminum borate, AVB = 10) and AlZn9 (alumina-zinc oxide, AVZn = 9) supports were homemade with the same method as the preparation of AAF',
1.fresh support 2. prepared catalyst B. Cond-B 1.fresh support 2. prepared catalyst c. AAP3 1.fresh support 2. prepared catalyst D.AAP5 1.fresh support 2. prepared catalyst E.AAP8c 1.fresh support 2. preparedcatalyst F.AB10 1.fresh support 2. prepared catalyst G . AAZn9 1. fresh support 2. prepared catalyst
198.9 195.0
0.658 0.668
6.58 6.62
0.924
117.7 121.3
0.537 0.554
8.93 9.00
0.636
147.4 115.6
0.913 0.740
8.6 9.83
0.250
257.7 172.8
0.916 0.872
8.17 8.75
0.709
201.8 198.0
0.876 0.903
7.31 7.40
0.710
289.6 285.4
0.465 0.435
2.61 2.50
0.810
195.0 179.8
0.487 0.509
4.56 4.67
0.639
except that phosphoric acid was replaced by boric acid (Huang and Kang, 1995b) and zinc nitrate (Tanabe et al., 1979), respectively. According to the results of Tanabe et al., the acidity of alumina decreased and the basicity increased upon mixing with ZnO. The strength and amount of absorption between HPtCl6- and AlZn9 supports would decrease (Brunelle, 1978). Therefore, the Pt metal dispersion of WAlZn9 was lower than that of PtJCs-331-4, although both supports had the same surface area and AlZn9 had smaller pore size than Cs331-4. Although the surface area of support ABlO was larger than that of Cs-331-4 and the pore size of ABlO was smaller than that of Cs-331-4,the metal dispersion of PtJABlO was smaller than that of PtJCs-331-4. It might be due to the too small pore size of support ABlO to diffusion of HPtC16- ion into the interal area of the support. Although the catalysts Pt/AAP5 and PtJCond-B had the same pore radius, as shown in Table 3, the metal dispersion of PtJAAF'5 was higher than that of PtJ Cond-B due t o the larger surface area of AAP5. Although WAAP8c and WCs-331-4 had the same surface area, the metal dispersion of AAP8c was lower than that of Cs-331-4 due t o the larger pore size of AAP8c. The supports considered in this study mainly consisted of monodispersed pores, as shown in Figure 3. At the onset of impregnation, the pressure inside the pellet is similar to that outside, and the impregnating liquid penetrates with capillary pressure as the driving force. The capillary pressure, P,, is given by
P, = (27 COS @/a
(1)
where y is the surface tension of the liquid, 8 is the contact angle, and a is the representative pore radius. When a pellet with a small pore radius is employed, the capillary pressure is large and the pore volume occupied by the entrapped air is small (Komiyama, 1985; Lee et al., 1985). The results of the Pt metal profile of the catalysts with the electron probe microanalyzer are shown in Figure 4. For the support Cond-B, which contains the largest pore radius, the pore volume occupied by the entrapped air is the largest. The Pt metal content of PtJCond-B catalyst is mostly deposited at the periphery of pellet;
2968 Ind. Eng. Chem. Res., Vol. 34, No. 9, 1995
3.0
- 1 5100 0
1
0
AAP8c cs-331-4
a
Cond-B
0
1 u
I
d
u
2
i
i
60.0
001
-'
10
0
20
m
AL/P a t o m i c ratio
Figure 5. Effect of PUAAPx catalysts.
Figure 3. Pore size distribution of different supports.
AI/p
ratio on the hydrogenation activity of
Table 4. Effect of AVP Atomic Ratio on the Selectivity of cis-Decdin for Pt/AAPx Catalysts. Temperature = 493 K, Pressure = 5.17 MPa, HdOil = 1000 20 m 10 12 ALT 3 5 8 0.66 1.02 0.33 0.54 Xcd/Xtd" 0.44 0.52 0.34 a
L I 1
0
1
r/ R (C) P t / C o n d - B h
:: Y
L
l
I
I
0
1
r/R ( 6 ) Pt/AA P8 c
l 1
alumina due to the greater acidity of the AAP support than the alumina support. Therefore, only a little HPtC16- moved to the pore mouth during drying, and the metal profile of the PtJAAP8c catalyst is more uniform than the other two Pt/Al203 catalysts. 2. Activity and Selectivity of PtIAApx Catalysts, All the activity tests of catalysts used the full extruded cylinder except when otherwise metioned. Since the heat combustion of cis-Decalin is higher than transDecalin (Weitkamp, 19681, the cis-Decalin selectivity of naphthalene hydrogenation was also studied in this work. There are three different kinds of hydrogenated products of naphthalene in this study for all catalysts. The reaction can be described as follows (Huang and Kang, 1995a):
- trans-Decalin
center L
Xcd = mole fraction of &-Decalin; Xa = mole fraction of truns-
Decalin.
I
1
0
1
r/ R (a) Pt/Cs-33!-4
Figure 4. Pt metal profile of the catalysts
therefore, Pt/Cond-B has the least uniform metal distribution, as shown in Figure 4. For the cs-331-4 support, which contains small pore radius, the Pt metal distribution is more uniform than that of Cond-B. The AAP8c support contains a broad range of pore size distributions, including large pore and small pore, and may have a smaller contact angle than the Cs-331-4 support. The bond between HPtCls- and the AAP surface is stronger than that between HPtC16- and
In this study, the naphthalene was hydrogenated completely to Tetralin for all catalyst activity tests, except for catalyst PUAAP1. To check the influence of the Avp ratio on the hydrogenation activity of PtIAAPx catalysts, a series of experimental tests were performed in the continuous reaction system by using different PtIAAPx catalysts, respectively. The activity and selectivity of Pt/AAPx catalysts were compared at the steady state (800 min after the beginning of reaction), and the experimental results are shown in Figure 5 and Table 4. It is shown that W A A P x catalysts have completely saturated naphthalene at 493 K, 5.17 MPa, except for Pt/AAPl, WAAP3, and WAAP- catalysts. For WAAPl catalyst, the activity of hydrogenation was particularly low,
which might be due to the basic site of AAPl support (Campele et al., 1984; Yang et al., 1986), low Pt metal dispersion, and high phosphate content. For PffA4F'- (PffAl2O3) catalyst, support was made by the same method as AAP except that phosphoric acid was not added. The hydrogenation activity of Pt/AAF+= catalyst was lower than the other Pt/AAPx catalysts except WAAP1 and WAAPS. From the results of Yang et al. (19861, the AAPx supports were more acidic than AAPm and the acid amount per gram of support reaches a maximum a t AVP = 7. From Table 1 and Figure 2, Pt/AAP12 and PffAAP20 had smaller pore sizes and a little smaller metal dispersion than PffAAPm. From Figure 5, the hydrogenation activities of Pt/AAP12 and Pt/AAP20 were higher than PffAAP-; therefore their specific activities were higher than WAAP- due to the high acidity of their supports. Because the catalysts Pt/ AAP5, Pt/AAPS, and PtlAAP10 had higher specific activities (due t o more acidity) and higher effective diffusivities (due t o large pore size) than WAAPm,the former had a higher hydrogenation activity than the latter, as shown in Figure 5. Although WAAP3 had the low metal dispersion, its specific activity was high due t o its high acidity, and its particle diffusion coefficient was also high due to its large pore size. Therefore, it still had good hydrogenation activity, as shown in Figure 5, and it can be used in a diffusion limited hydrogenation system. The influence of the AVP ratio of Pt/AAPx catalysts on the selectivity of cis-Decalin was obtained by the same experiments, as shown in Table 4. According to the reaction sequence (eq 2), trans-Decalin can be obtained by the direct hydrogenation of Tetralin or isomerization of &-Decalin. At complete saturation, &-Decalin can continue t o isomerize to trans-Decalin until it approaches equilibrium. The metal distribution and diffusion coefficients also influence the selectivity. Therefore, it is not easy to determine the influence of the AVP ratio of Pt/AAPx catalysts on the Selectivity. From Table 4, the high acidity of PtIAAPx catalysts is unfavorable to the selectivity of cis-Decalin. The cisDecalin selectivity of W A A P 8 is the least of all PvAAF'x catalysts, and its support has the maximum acid amount per gram of all AAP supports at the same time (Yang et al., 1986). Lin and Vannice (1993) and Chou and Vannice (1987) proposed a model in which aromatics adsorbed on acid sites in the metal oxide interfacial region could react with hydrogen spilled over from the metal surface; consequently, more acid supports would give an additional contribution to the overall activity. Aromatics adsorbed on an oxide surface appears to have a n-bond interaction similar to that on a metal (Pt) surface; however, the bond strength is lower than that on the metal surface. According t o the reaction steps suggested by Weitkamp (1968), Tetralin can react directly with hydrogen to form cis-Decalin or through the intermediate octalins. But Tetralin cannot convert directly into trans-Decalin, it needs to go through the intermidiate, A1,9-octalin, and then t o trans-Decalin. For the greater acidity of Pt/AAF'x, more reactions were through the metal oxide interfacial region due to the high acidity of the support. Because of the low bond strength between octalin and interfacial sites, more octalin was desorbed and adsorbed again, and then more trans-Decalin was produced. Therefore, the &-Decalin selectivity to the more acidic P t I M x catalysts was
-EQ.
Ind. Eng. Chem. Res., Vol. 34, No. 9, 1995 2959 1001
I
0
0
I
I
60
"
I
m
120
A l / P atomic ratio
Figure 6. Effect of AL/p on the hydrogenation activity of Pt-Pxl &03.
L
i 1
I
03
I 0
1
1
60
120
..
m
A l / P atomic ratao
Figure 7. Effect of Al/P on the selectivity of &-Decalin for PtPxIA1203.
lower than the that to the less acidic PffAAPx (AVP=. 10). Because different AApx supports have different pore sizes and acidities, the effect of the AVP ratio of PtJA4F'x catalysts on the selectivity of cis-Decalin is more complex than that for R-P/Al203 catalysts. 3. Activity and Selectivity of Pt-PdAl203 Catalysts. A series of Pt-PxlAlzOs catalysts with different AVP compositions were prepared to compare the effect of phosphorus on the PffAAPx and PffAl203 catalysts. The activity and selectivity of catalysts were compared at a temperature of 493 K, a pressure of 5.17 MPa, and stable activity state (800 min after the starting reaction), as shown in Figures 6 and 7. When the phosphorus was inserted into Pt/Al203 catalysts, the hydrogenation activity of the catalyst was increased. But too much phosphorus (AVP = 20) was poisonous to the hydrogenation activity of the catalyst. The optimal phosphorus amount of Pt-P/Al203 catalysts for the hydrogenation activity was about AVP = 70, as shown in Figure 6. According to the results of Lopez Corder0 et al. (1989) and Morales et al. (1988), at low phosphorus contents (below 2.5 wt % P205, AVP = 551, phosphorus was
2960 Ind. Eng. Chem. Res., Vol. 34, No. 9, 1995
\
C
"
0
200
400
-\
1"" > I
\
-
-
-
6 0:
10 0
200
400
600
800
1000
time (min)
tzme(mzn) Figure 8. Effect of the support's acidity on the hydrogenation activity.
Figure 9. Activity of WAAF'x and WAlzO3 catalysts for different textures.
homogeneously distributed on the alumina surface. But at high phosphorus contents inhomogeneity gradually increased due to the formation of phosphate multilayers. The acid amount (weak acid sites only) of alumina increased as the phosphorus content increased. When the acidity of the support increased the rate of hydrogenation increased, too. But, the hydrogenation activity of Pt-PxlAlzOa catalysts is not as good as that of PtJ AApx catalysts. When too much phosphorus was added to alumina, the hydrogenation activity decreased due to the formation of a multilayer of phosphate and low metal dispersion. It was unfavorable to the selectivity of cis-Decalin for naphthalene hydrogenation when phosphorus was inserted into WAl203 catalyst, as shown in Figure 7. The more phosphorus that was added to PtJAl2O3 catalyst, the lower the selectivity of cis-Decalin that was obtained. For Pt-PdAl203 catalyst, the reason for decreasing the selectivity of cis-Decalin was similar to that for the Pt/AAPx catalyst due to the increasing support acidity. Therefore, the AAPx supports are greatly different from the supports made by impregnating y-Al203 with phosphorus. AAPz may be considered similar to silicaalumina, since aluminum phosphate and silica are very similar structurally (Marcelin et al., 1984). The alumina and aluminum phosphate are not merely a physical admixture but in fact form a new composition in which crystallization of individual phases is mutually inhibited (Marcelin et al., 1983). 4. Influence of Support's Acidity on the Activity of Catalysts. To neglect the particle diffusion resistance, small sizes of the supports (25-45 mesh) were used to prepare the catalysts (Huang and Kang, 1995a). The catalyst hydrogenation activities were tested a t a temperature of 533 K and a pressure of 5.17 MPa, and the results are shown in Figure 8. According to the results of Tanabe et al. (1979) and Yang et al. (19861, the sequence of support acidity was AAP8c > y - A l 2 0 3 (Cs-331-4)> AlZn9. From the results of Figure 8, the hydrogenation activity of PtJAlZn9 was much lower than W A A P 8 c and Ptly-AlzO3. The textures of these three supports are only little different, as shown in Table 3. Because the diffusion resistance could be neglected, the reason for the low hydrogenation activity of PtJAlZn9 was the low acidity of the AlZn9
support. From Figure 8, the catalysts Pt/ y-Al203 and PtJAAP8c had completely saturated naphthalene, but PtJAAP8c had low selectivity of cisDecalin. Because a more homogeneous metal distribution was obtained by impregrating with small particles and these two catalysts had similar pore sizes, the reason for low cisDecalin selectivity is the high acidity and high activity of PtJAAP8c catalyst. 5. Activity and Selectivity of Pt/AAF%and Pt/ y-Al~Osfor Different Textures of Supports. To compare the AAPx and commerical alumina supports for the application to hydrogenation, two sets of WAAPx and Wy-Al203 catalysts were prepared and their activities and selectivities were compared. The results are shown in Figure 9. From Tables 1 and 3, the surface area, pore volume, and average pore size of PtJAAP8 are a little smaller than those of PtJCs-331-4. Therefore, PtJCs-331-4catalyst has a slightly lower diffusion resistance. Although the metal dispersion of PtJAAP8 catalyst is lower than that of Pt/Cs-331-4, the hydrogenation activity of Pt/ AAP8 catalyst is higher than that of PtJCs-331-4. Therefore, WAAP8 catalyst has a higher specific activity than WCs-331-4. Lin and Vannice (1993) had the same result that the more the acidic the support, the higher the specific hydrogenation activity of the catalyst. The cis-Decalin selectivity of PtJAAP8 catalyst was lower than that of Pt/Cs-331-4. Because the PtJAAP8 has more acidity and more reactions were contributed to by the metal oxide interfacial region of the catalyst, low cis-Decalin selectivity of PUMP8 was obtained, as shown in Figure 9. The high activity of PtJAAP8 and high conversion in this case might make more cisDecalin isomerize to truns-Decalin. This was another reason which might make the cis-Decalin selectivity of PtJAAP8 smaller than that of Pt/Cs-331-4. According to Table 3, the pore size of PtfA4P5 catalyst is similar to that of PtJCond-B, but its surface area is larger than that of PtJCond-B. The two catalysts have only slightly different metal dispersions. Because the pore sizes of these two catalysts are larger than that of PtJCs-331-4, the diffusion resistance of catalysts can be neglected, as will be shown in the flollowing paragraph. Because PtJAAP5 has a high hydrogenation activity, as shown in Figure 9, the turnover frequency of PtJAAP5 is higher than that of PtJCond-B. Therefore, the higher
Ind. Eng. Chem. Res., Vol. 34, No. 9, 1995 2961
T
-
looE
5
Y
d
-80
0 0 A
“0
f
- $
0
-60
A
0
::
Ilj
Pt/Cs-331-4 Pt/AAPBc Pt/ABIO
c
Pt/Cs-331-4 Pt/AAPBc Pt/AB 10
200
600
400
800
1000
70 0
12lh
400
200
600
1000
800
1200
t i m e (min)
tame(min)
Figure 10. Comparison of the activity and selectivity of PffAAPx with Pt/ABlO and PffA12O3 catalysts a t 473 K and 5.17 MPa.
Figure 12. Comparison of the activity and selectivity of PtJAAPx with WABlO and WA1203 catalysts a t 533 K and 5.17 MPa. Table 5. Kinetics of Tetralin Hydrogenation over Pt Catalyst at 5.17 MPa
-
3 1 looL
s
w
B
80 O 0 A
2
Pt, C S - 3 3 1 - 4 Pt/AAPBc Pt/AB I O
2
zoc
0- -0 0
-120
200
400
600
800
:
1000
t tme (min)
Figure 11. Comparison of the activity and selectivity of WAAPx with PtJAB10 and WA1203 catalysts a t 493 K and 5.17 MPa.
specific activity of WAAP5 is obtained due to the higher acidity of the AAP5 support, matching the results of Lin and Vannice (1993) and Chou and Vannice (1987). The hydrogenation activity of PffCs-331-4 was higher than PUCond-B due to the higher metal dispersion of PffCs331-4. Because AAP has only one type of acid site with medium acid strength, Pt/AAPx catalysts not only have a high hydrogenation activity but also have a low hydrogenolysis activity. Therefore, using WAAPx catalysts t o hydrogenate aromatics in fuel oil will result in a good liquid product yield and reasonable hydrogen consumption. 6. Activity and Selectivity of P f f M x ,PffABlO, and Pt/AI203Catalysts. The W A A P S c , PUAB10, and WCs-331-4 catalysts were tested at 5.17 MPa and three different temperatures, 473, 493, and 533 K, and the results are shown in Figures 10-12, respectively. According to the result of Wang and Chen (19911, the acidity of support AB10 was higher than that of AAP8c. The hydrogenation activity of catalyst PffAAP8c was lower than PffCs-331-4 at a temperature of 473 K but higher than Pt/Cs-331-4 at a temperature of 493 K. The
-
catalyst
473 K
493K
(kcallmol)
473K
493 K
Pt/Cs-331-4 PUAAP8c PffAB 10 PtJCond-B
3.17 2.79 6.92
5.73 9.48 15.81 2.48
13.6 28.1 19.3
3.42 3.93 8.54
6.18 13.40 19.51 3.97
hydrogenation activity of PUB10 was higher than that of W A A P S c at all temperatures due t o the high acidity of support AB10, as shown in Figures 10-12. If the diffusion resistance could be neglected, according to the result of Huang and Kang (1995a))
X , = exp[-(k,’
+ k3’)zl
(3)
where Xt is the mole fraction of Tetralin, z is the space time, k2’ = k&-12‘, k3‘ = k 3 P ~ 2 and ~ , k2 and k3 are the rate constants, as shown in eq 2. The sums of the rate constants (k2’ 123’) were obtained by applying eq 3, space time, and conversion in Figures 10 and 11, and the results are shown in Table 5. Table 5 shows that the specific hydrogenation activities of catalysts, a t temperatures between 473 and 493 K, follow the sequence: PffABlO > PWAAP8c > PffCs331-4. The activation energy of Tetralin hydrogenation for PffCs-331-4, 13.6 kcavmol, is similar t o the results of Lin and Vannice (1993) and Basset et al. (1975). However, the activation energies of Tetralin hydrogenation for PffAAP8c and PffAB10 catalysts are 28.1 and 19.3 kcal/mol, which are much higher than that of Pt/ Cs-331-4. If the activation energy is larger than 6 kcal/ mol, it may imply that the operation is in a reactioncontrolled region (Bond, 1987). The data confirmed that our assumption was correct. For P t J U 8 c and Pt/ABlO catalysts, there are two reaction paths for aromatic hydrogenation. One is the same as that for PffCs-331-4 catalyst, namely, that aromatics absorb on the metal surface and react with hydrogen, and the other is that aromatics adsorb on acid sites in the metal oxide interfacial region and then react with hydrogen spilled over from the Pt metal surface. The latter path has a higher energy barrier than the former. Because Tetralin is a larger molecule than benzene and the aromatic ring of naphthalene near the
+
2962 Ind. Eng. Chem. Res., Vol. 34,No. 9, 1995
metal has first been hydrogenated, the distance of hydrogen spilled over in Tetralin hydrogenation is longer than that in benzene hydrogenation. Therefore, the activation energes of WAAP8c and WAB10 catalysts in this study are higher than the result of Lin and Vannice (1993) and PUCs-331-4. Because the acidity of support AI310 is higher than that of AAF'8c, the activation energy of PVAl310 is lower than that of W AAP8c. The PUAAP8c catalyst has a large pore size and a high particle diffusion coefficient. Therefore, the W AAP8c catalyst will have a much higher hydrogenation activity than PKs-331-4 when more larger aromatic molecules have to be hydrogenated. The PUAAP8c had a more uniform metal distribution that made more reactions proceed in the internal area of the pellet, and A A P x supports contained more acidity than y-AlzO3 supports. All the factors made the selectivity of cisDecalin of WAAP8c lower than that of WCs-331-4, as shown in Figures 10-12. All catalysts had completely saturated naphthalene at a temperature of 533 K, and due to the different acidities of the supports, the selectivity to cis-Decalin of the catalysts follows the sequence PUCs-331-4 > PUAAP8c > WAl38, as shown in Figure 12. Because the acidity of support AI310 is higher than that of AAP~c,the cis-Decalin selectivity of catalyst P U B 1 0 is lower than that of PUAAP~C, as shown in Figures 10 and 11.
Conclusion AAF'x supports with desired surface areas and pore sizes can be obtained by varying the atomic ratio of AI/ P. The W M x catalyst, used as an aromatic hydrogenation catalyst, has a better activity but a lower cisDecalin selectivity than Wy-Alz03 due to the higher acidity of the AAPx support than y-&03. The WAAPx catalyst, at 6 < AVP 10, has the highest hydrogenation activity and the lowest cis-Decalin selectivity of all WAAF'x catalysts. At a low AI/p ratio (AVP= 31, a large pore size PUAAP catalyst can be obtained with good hydrogenation activity. Because the acid strength of Pt/ A A P x is medium, a good liquid product yield of aromatic hydrogenation for fuel oil can be obtained with reasonable hydrogen consumption. Adding phosphorus to a y-Al203 support can improve the aromatic hydrogenation activity of Wy-Alz03 catalyst, but its activity is not as good as that of PtJAAPx catalyst, and its cis-Decalin selectivity is decreased. The reason for the high hydrogenation activity and low cisDecalin selectivity of PUAAPx and Pt-Pxly-AlzO3 catalysts can be explained by the model suggested by Lin and Vannice (1993).
Acknowledgment The authors wish to express their gratitude for the support of this work by the Refining and Manufacturing Research Center of Chinese Petroleum Coorporation.
Nomenclature ki = k,'lPHal ki' = rate constant, h-'
P, = capillary pressure PH?= partial pressure of hydrogen, MPa XT = mole fraction of Tetralin in product t=
1LHSV = space time, h
y = surface tension of the liquid e = contact angle a = representative pore radius
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Received for review September 27, 1994 Accepted May 15, 1995@ I39405652
Abstract published in Advance ACS Abstracts, August 1, 1995. @
