8852
Ind. Eng. Chem. Res. 2006, 45, 8852-8859
Liquid-Phase Hydroisomerization of n-Octane over Platinum-Containing Zeolite-Based Catalysts with and without Binder Antonio de Lucas, Paula Sa´ nchez,* Antonia Fu´ nez, Marı´a Jesu´ s Ramos, and Jose´ Luis Valverde Departamento de Ingenierı´a Quı´mica, Facultad de Ciencias Quı´micas, UniVersidad de Castilla-La Mancha, AVd. Camilo Jose´ Cela s/n, 13071 Ciudad Real, Spain
Liquid-phase hydroisomerization of n-octane with three different framework zeolites catalysts with similar Si/Al ratios, USY, mordenite, and beta, was performed in a stirred batch autoclave microcatalytic reactor. As the hydrogenating-dehydrogenating function, platinum supported by impregnation (1 wt %) was used. Additionally, these zeolites were agglomerated with bentonite, the catalytic performance being compared with that obtained for the nonagglomerated samples. To characterize the catalysts, surface area measurements, temperature-programmed desorption of ammonia, atomic absorption spectroscopy, inductively coupled plasma emission spectrophotometry, hydrogen chemisorption, and solid-state 27Al NMR were used. In all cases, a decrease in the catalytic activity of agglomerated samples, because of the modification of the acidity and the porosity of the zeolites by the binder was observed. The decrease in the acidity after the agglomeration could be attributed to solid-state ion exchange between zeolite protons and clay sodium during the calcination of the catalyst. The binder also modified the porosity of the zeolite providing meso- and macropores, causing a partial blocking of the micropore mouths, which would lead to an increase in the length of the effective diffusional pathway. The presence of aluminium extraframework (EFAL) species in the agglomerated samples could also affect the catalytic performance. Due to the high pressure in the liquid phase, a significantly higher yield of isomers was obtained, and cracking became important only at relatively high average conversions. The maximum n-octane isomers yield obtained with all the nonagglomerated catalysts was very similar (about 75 mol %). This value was reached in 7.5 h for the beta-zeolite-based catalyst and in 10.5 and 23 h for the mordenite- and USY-zeolite ones, respectively. 1. Introduction Due to the current environmental requirements, gasolines must contain the minimum content in compounds considered harmful to the environment or the public health such as lead, aromatics, and oxygenated compounds. In the production of gasoline from naphtha, C5 and C6 n-paraffins are typically isomerized to their branched isomers, which have a higher octane rating.1-3 Larger paraffins (C8C14) are present in streams used for catalytic reforming. Although reformates are high-octane products, they typically contain over 60% aromatics, and their use in reformulated gasoline should therefore be limited. The technologically more interesting alternative is the utilization of these paraffins for the production of branched paraffins with high octane numbers. The hydroisomerization of C7 and C8 has been widely studied.4-9 The multibranched C8 alkanes obtained from this process are the most useful isomers because of their high octane number. However, no industrial hydroisomerization process exists for C7 and C8 paraffins due to their high tendency to crack.10,11 The hydroisomerization processes of n-paraffins require the presence of bifunctional catalysts. These catalysts consist of a noble metal supported on an acid zeolite. Hydrocarbon molecules from the feed are dehydrogenated on the noble metal producing unsaturated hydrocarbons which in turn undergo protonation on the acid centers of the zeolite with formation of carbenium ions as reaction intermediates. These carbenium ions suffer skeletal rearrangements and β-scission reactions followed by deprotonation and hydrogenation of the resulting olefins.12 * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: +34-926-29 54 37. Fax: +34-92629 53 18.
Bifunctional noble metal-zeolite catalysts especially Pt or Pd loaded beta, mordenite, and Y possess a high activity and selectivity for hydroisomerization of n-alkanes.13 Zeolite beta presents a three-dimensional 12-MR structure and has shown a better catalytic performance than one-dimensional 12-MR mordenite.13,14 Pt-containing Y zeolite catalysts have been used because this zeolite has the largest pore system among the crystalline alumino-silicates.15-17 Both acid and metal site densities are important and their proper balance is critical in determining the activity, stability, and product selectivity of the catalyst.18 On the other hand, most industrial zeolite-based catalysts require the zeolite to be pelletized with a binder in order to improve the mechanical properties of the catalyst particles and avoid extremely high-pressure drops in fixed-bed reactors.19 It has been demonstrated that the binder is not active as a catalyst but could change the acid properties of a zeolite as a result of changes in the proton-exchange efficiency, trapping by the binder of coke precursors and/or blocking of zeolite channels during the pelletization process.19,20 Some authors have studied the influence of different binders on the catalytic performance of zeolites,21-24 even though bentonite has not been widely studied. Bentonite is a laminar and expandable clay with wet binding properties and is widely available throughout the world. The scientific interest in bentonite is due to its physical and chemical properties as well as its low price. Consequently, the industrial application of bentonite is an attractive process. The dispersability of clays in aqueous suspensions is the reason for their agglomeration properties: zeolite particles are surrounded by clay laminae. When the water is removed, a solid phase is achieved with the zeolite particles are agglomerated by the clay. It has been shown that the acid form of clays does not have
10.1021/ie060388s CCC: $33.50 © 2006 American Chemical Society Published on Web 11/15/2006
Ind. Eng. Chem. Res., Vol. 45, No. 26, 2006 8853
binding properties whereas its sodium form exhibits better properties.25 Because zeolites are mainly used as acid catalysts, further transformation of the agglomerated zeolite to the acid form is required.19 Furthermore, most of academic studies on hydroisomerization are performed in the vapor phase using a hydrogen pressure below 10 bar,26 the existing literature about the process in the liquid phase being very scarce. Pope et al.1 studied the n-heptane hydroisomerization in the liquid phase over different catalysts formulations based on zeolites and amorphous supports. Pt-based formulations provided the greatest selectivity to isomers over cracked products, with up to 94% selectivity to isomers at 72% conversion. Recently, our research group27-30 has reported that, depending on the zeolite studied, the binder (bentonite) influenced the activity for the hydroisomerization of n-octane. The agglomeration process caused a decrease in the activity of ZSM-5 and mordenite catalysts. However, beta improved its catalytic activity when it was agglomerated. All these experiments were performed in the vapor phase. The good results obtained with the agglomerated catalysts in the vapor phase lead us to test these kinds of catalysts in the liquid phase. Therefore, the aim of this work was to investigate the influence of the binder (sodium bentonite) on the catalytic activity and selectivity in the hydroisomerization of n-octane in the liquid phase over USY, mordenite, and beta zeolites using platinum as the hydrogenating-dehydrogenating function. A pressure of 90 bar was enough to achieve liquid-phase conditions at a temperature of 270 °C. 2. Experimental Section 2.1. Catalyst Preparation. The parent zeolites USY (Si/Al ) 13.0), mordenite (Si/Al ) 10.4), and beta (Si/Al ) 13.0) were supplied in the ammonium form by Zeolyst International. To obtain the protonic form, zeolites were calcined at 550 °C for 15 h. Bentonite (sodium form) was supplied by Aldrich Chemical Co. USY, mordenite, and beta samples on the protonic form were named as HUSY, HMOR, and HBETA. Zeolite (35 wt %) and bentonite (65 wt %) were mixed and suspended in water at 60 °C for 2 h. The suspension was then dried at 120 °C for 12 h. After grinding and sieving, particles with an average particle size of 0.75 mm were obtained. With this size, no internal diffusion limitations in the catalytic runs were detected. Finally, the agglomerated zeolites were calcined at 550 °C for 15 h. After the agglomeration process, an ionexchange process with a mineral acid must be carried out in order to re-incorporate the acid function to the zeolite. In the case of mordenite, the agglomerated catalyst was ion-exchanged with 0.6 N HCl (35 mL‚g-1). The ion-exchange for the USY and beta zeolites was carried out three times with 1 M NH4Cl (30 mL‚g-1). Ion-exchanged samples were subsequently calcined again at 550 °C for 15 h in order to obtain the acid form of the zeolites. A known volume of an aqueous H2PtCl6 solution was poured over all the catalysts. The solvent was removed by evaporation under vacuum. The metal concentration of the impregnating solution was calculated to yield a final Pt content in the resulting catalysts of 1 wt %. After the impregnation process, the catalysts were calcined at 400 °C for 4 h and reduced in situ under a hydrogen flow of 190 mL‚min-1‚g-1 at 410 °C for 4 h. Nonagglomerated USY, mordenite, and beta samples containing both acid and metallic functions were named as PtUSY, PtMOR, and PtBETA, respectively. Agglomerated catalysts were named as with “Bent” following the name of the
nonagglomerated zeolite (PtUSY, PtMOR, and PtBETA). For example, PtUSY/Bent corresponds to a HUSY zeolite agglomerated with bentonite. 2.2. Catalyst Characterization. The pore size distribution and BET surface area were determined by adsorption and desorption data acquired on a Micromeritics ASAP 2010 adsorptive and desorptive apparatus. The samples were evacuated under vacuum of 5 × 10-3 torr at 350 °C for 15 h. Specific total surface areas were calculated using the BET equation, whereas specific total pore volumes were evaluated from N2 uptake at a relative pressure (P/Po) of N2 equal to 0.99. The Horvath-Kawazoe method was used to determine the microporous surface area and micropore volume.31 The Barret, Joyner, and Halenda (BJH) method was used to determine the distribution of the mesopores.32 Surface area measurements had an error of (3%. The concentration of the acid sites was measured by temperature programmed desorption of ammonia (TPDA) using a Micromeritics TPD/TPR 2900 analyzer. The sample was first heated from room temperature to the calcination temperature at 15 °C‚min-1 under a flow of helium, holding this temperature for 30 min. After reducing the catalyst under a hydrogen flow, the system was cooled to 180 °C. Ammonia was then passed over the sample for 15 min. Later, the sample was purged with helium for 1 h in order to eliminate physisorbed species. The temperature was ramped at 15 °C‚min-1 from 180 to 560 °C, and TPDA data were acquired. Weak and strong acidities are defined as the concentration of weak and strong acid sites, respectively, obtained by integration of the area under the peaks at the lowest and the highest temperatures, respectively.20,33 The average relative error in the acidity determination was lower than 3%. To quantify the platinum content in the catalysts, atomic absorption spectroscopy (AA) measurements were performed using a SpectrAA 220FS spectrophotometer. The aluminum and silicon content were measured using an inductively coupled plasma emission spectrophotometer LIBERTY RL sequential ICP-AES. Prior to measurement, the samples were dissolved in hydrofluoric acid and diluted to the interval measurement. The error of these measurements was of (1%. The platinum dispersion was determined from chemisorption measurements. The apparatus used was the same as that described for the TPDA runs. The experiments were carried out using the dynamic pulse technique with an argon (99.9990%) flow of 50 mL‚min-1 and pulses of hydrogen. To calculate the metal dispersion, an adsorption stoichiometry of Pt/H ) 1 was assumed.34 The chemisorption experiments with hydrogen pulses were carried out at 60 °C to avoid the spillover phenomenon.35 Previously, the sample was pretreated by heating at 15 °C‚min-1 in flowing helium up to 250 °C and kept constant at this temperature for 20 min. Then, the sample was reduced in situ. Afterwards, the hydrogen was removed by flowing inert gas for 30 min, the temperature being 10 °C higher than the reduction temperature. Finally, the sample was cooled to the experimental temperature in an inert gas flow. The dispersion measurements with H2 pulses had an error of (5%. Solid-state 27Al NMR spectra were collected in a BRUKER Avance WB 400 spectrometer. The 27Al NMR spectra were obtained at 12.5 kHz using 15° pulses and a 1 s delay, a total of 5000 pulses being accumulated. 2.3. Reaction Tests. The n-octane hydroisomerization runs were carried out in a 50 mL batch microreactor (Autoclave Engineers). First, the catalyst was reduced in a fixed-bed reactor under a hydrogen flow of 190 mL‚min-1‚g-1 at 410 °C for 4 h.
8854
Ind. Eng. Chem. Res., Vol. 45, No. 26, 2006
Table 1. Metal Dispersion, Surface Area and Pore Volume Measurements pore volume (µL‚gcat-1) catalyst
surface areaa (m2‚gcat-1)
bentonite HUSY HMOR HBETA PtUSY PtMOR PtBETA PtUSY/Bent PtMOR/Bent PtBETA/Bent
37 868 562 636 799 556 614 293 202 232
a
dev from the DH2 microtheoretical value (%) (%) pores
-3.6 -8.3 -3.0
69 92 65 96 80 71
meso- and macropores
4
88
273 193 159 95 70 67
164 56 409 131 158 324
Experimental value.
Then, the batch microreactor was filled with 17 mL of n-octane, and the catalytic basket, with 0.75 g (related to zeolite) of catalyst; then, it was pressurized at 80 bar with hydrogen and heated gradually at 270 °C. Once this temperature was reached, the total pressure was finally set at 90 bar. No diffusion control mechanism was detected when the stirring rate was over 370 rpm. Blanks runs demonstrated that n-octane conversion was negligible. The reaction time was elapsed up to 24 h. Samples of the vapor and liquid (20 µL) products were taken at regular intervals. Gas products were analyzed in a gas chromatograph (GC; HP 5890 Series II) equipped with a flame ionization detector and a capillary column (SUPELCO Petrocol DH 50.2, 0.2 mm i.d. and 50 m length). Liquid products were analyzed in a gas chromatograph (GC-17A SHIMADZU) coupled to a mass spectrometer (QP-5000 SHIMADZU). A capillary column (SUPELCO Petrocol DH, 100 m length with a 0.25 mm i.d.) was used in this GC. Results from a reproduced experiment showed that conversion and isomer selectivity had an error of (3%. 3. Results and Discussion 3.1. Characterization of Catalysts. Surface area and platinum dispersion data for all the catalysts are summarized in Table 1. The deviation of the surface area for the agglomerated catalysts from the theoretical one (calculated from the contribution of the bentonite and the nonagglomerated zeolites) is also included. A slight decrease in the experimental values of the surface area of the agglomerated catalysts in comparison to those expected was observed. This result suggests that there was not a total blocking of zeolite channels by the binder. However, it is not possible to claim that a partial blocking of the zeolite micropore mouths by the binder did not take place,20 especially for the mordenite-zeolite-based catalyst. This fact cannot be supported by the measurements of surface area and pore volume achieved, which were evaluated using N2. It can be observed, for the mordenite-zeolite-based catalyst, that the agglomerated sample presented a higher volume of meso- and macroporos than the nonagglomerated one. It is obvious that these meso- and macropores were provided by the binder. However, the contrary effect was observed for the USY and beta samples. USY zeolite is a highly dealuminated zeolite which is obtained by hydrothermal treatment in the presence of steam followed by leaching with a mineral acid.36 During these processes, the aluminum is expelled from the framework causing a vacancy that can grow to form mesopores.37 Beta zeolite crystallizes with many stacking faults,38 suggesting that most of their mesoporosity is associated with intracrystalline voids.39 When both zeolites are agglomerated, binder fills part of these mesopores, the mesoporosity lost due to the filling up
of the voids being more important than the mesoporosity provided by the binder. Table 1 also includes the surface area values of the acid form of the zeolites (HUSY, HMOR, and HBETA). Slight differences in the surface area values were observed when the zeolites with and without metal were compared. These results would suggest that platinum particles were located mainly on the external surface of the zeolite crystals. Otherwise, a considerable lost in the surface area value for the metal-containing samples would be expected. Platinum dispersion (DH2) values were coherent with this supposition. The average diameter of the metal particles, calculated from the dispersion data and a theoretical expression,33,40 would be about 12-18 Å, too big to think that the platinum particles could be located in the zeolite main channels. The platinum dispersion decreased when mordenite was agglomerated with bentonite, the opposite effect being observed in USY- and beta-zeolite-based catalysts. This effect was also observed by other authors.28,41 Acidity data of the different catalysts are summarized in Table 2. All the samples showed two peaks corresponding to weak and medium-strong acidity. Samples based on mordenite zeolite showed the highest acid site density; additionally, the peak in the TPDA related to strong acid sites was obtained at higher temperatures. It should be noted that for the three nonagglomerated zeolites similar acidity values were found regardless the incorporation of platinum to the catalysts. This fact shows the scarce influence of the metal on the acidity. Bentonite acidity was very low, even not observing strong acidity. Therefore, the contribution of the bentonite to the acidity of the agglomerated catalysts should be considered negligible. Table 2 also lists the acidity of agglomerated catalysts. Differences between the experimental acidity of the agglomerated catalysts and the predicted one (calculated from the contribution of the raw materials: zeolite and bentonite) can be observed: the experimental values of weak acidity of the agglomerated catalysts were always higher than the predicted one, the opposite effect being observed for strong acidity. According to different authors,42-44 the decrease in the number of expected strong acid sites could be attributed to solidstate ion-exchange between zeolite protons and clay sodium during the calcination of the catalyst. Bentonite is a rich source of Na+ cations, and they are also weak acid sites.45 Although part of them are removed and substituted by H+ during the reincorporation of the acid function, the Na+ cations that remain in the sample alter the density of the acid sites because at least some of the weak sites can be attached to them. Figure 1 shows the 27Al NMR spectra for different samples. The signal at 0 ppm was assigned to octahedral Al (EFAL).39 It can be observed that this signal was significantly higher when the samples were agglomerated. The formation of these species in the agglomerated catalysts could be due to the migration of reasonable mobile cations (Al in this case) from the interlayer space of the clay structure to the zeolite.46 27Al NMR analysis of bentonite clay (not shown) showed the presence of a main signal at 0 ppm that could support the above suppositions. These aluminum extraframework species could influence the catalytic behavior of the catalysts. In fact, some authors11,39,47 have suggested that, for mordenite zeolite, these extraframework species could block the zeolite pores and neutralize part of the acid sites. Fernandes et al.39 suggested that certain EFAL species could even block the acid sites of beta zeolite and probably also neutralize part of the framework charge. On the other hand, Remy et al.36 observed a lower catalytic activity for the highly
Ind. Eng. Chem. Res., Vol. 45, No. 26, 2006 8855 Table 2. Acidity Data for the Nonagglomerated and Agglomerated Samples sample bentonite HUSY HMOR HBETA PtUSY PtMOR PtBETA PtUSY/Bent PtMOR/Bent PtBETA/Bent a
total acidity (mmol NH3‚gcat-1)
weak acidity (mmol NH3‚gcat-1)
0.038 0.477 0.996 0.626 0.449 0.918 0.799 0.184a 0.192b 0.412a 0.373b 0.298a 0.305b
0.038 0.089 0.164 0.149 0.050 0.123 0.187 0.061a 0.060b 0.143a 0.082b 0.123a 0.090b
Td (°C) 274 282 312 273 275 300 277 284 305 300
strong acidity (mmol NH3‚gcat-1) 0 0.388 0.832 0.477 0.399 0.795 0.612 0.123a 0.136b 0.269a 0.291b 0.174a 0.214b
Td (°C) 392 479 353 390 480 387 388 432 388
Experimental value. b Predicted value calculated from the contribution of the nonagglomerated zeolite and the binder.
Figure 2. Temporal conversion of n-octane over the nonagglomerated (b) PtUSY, (2) PtMOR, (9) PtBETA, and (*) PtBETA-Na catalysts and the agglomerated (O) PtUSY/Bent, (4) PtMOR/Bent, and (0) PtBETA/Bent catalysts.
Figure 1. 27Al NMR spectra of (a) USY samples; (b) mordenite samples; and (c) beta samples.
dealuminated Y zeolites in the heptane and decane hydroisomerization. They attributed the decline in the activity to the lower reactant concentration within the pores. 3.2. Influence of the Binder in the Hydroisomerization of n-Octane on Pt Catalysts. 3.2.1. n-Octane Conversion. Figure 2 shows the n-octane conversion versus the reaction time for
the agglomerated and nonagglomerated catalysts. It can be seen that, at each reaction time, the conversion obtained with the agglomerated samples was lower than that corresponding to the nonagglomerated samples. The catalytic activity for n-octane conversion for the nonagglomerated samples decreased in the following order: beta (Si/Al ) 13.0) > mordenite (Si/Al ) 10.4) > USY (Si/Al ) 13.0). It is believed that differences in activities of various zeolite catalysts could be attributed to intrinsic differences in zeolite acidity.11,48 It should be noted that all the catalysts used are bifunctional. It is assumed that, with 1 wt % of platinum in a specific catalyst, the hydroisomerization reaction is ideal because the metal component is present in a sufficient excess amount, to consider that the reactions in the acid sites are the rate-limiting steps.49 As has been already discussed, mordenite zeolite presents the highest acidity value. However, a higher temporal n-octane conversion was obtained with beta zeolite. This fact can be explained because the mordenite zeolite presents a high steric hindrance that did not allow accessibility to all its acid sites.48,50,51 According to literature data, only one-third to twothirds of the acid sites of this zeolite can be accessible to alkanes.50,51 Regarding the nonagglomerated samples, the conversion obtained for USY zeolite was the lowest, presumably due to its low acidity. It is worth noting that although USY and beta zeolites have almost the same pore size and metal dispersion, the catalytic activity of beta is higher due to its higher acidity.52 According to Gopal and Smirniotis,11 the conversion can be affected not only by the acidity but also by other
8856
Ind. Eng. Chem. Res., Vol. 45, No. 26, 2006
parameters, such as the generation of mesopores by acid leaching and the accessibility of the acid sites. In a previous work, the effect of the binder (bentonite) in the n-octane hydroisomerization in the vapor phase over mordenite, beta, and ZSM-5 using Pt as the hydrogenating-dehydrogenating function was studied.28 A decrease in the n-octane conversion for the agglomerated catalysts was observed, except for the catalyst based on beta zeolite. This zeolite improved its catalytic activity when it was agglomerated. In the liquid phase, however, no increase of the catalytic activity was found for this catalyst. Dorado et al.20 also observed a decrease in the activity for the hydroisomerization of n-butane when the catalyst based on zeolite beta impregnated with Pd was agglomerated with bentonite. The decrease in the catalytic activity of the agglomerated catalysts could be attributed to the decrease in the number of strong acid sites (responsible for the isomerization reaction), which took place during the agglomeration process due to solidstate ion-exchange between the Na+ cations of the clay and the H+ from the zeolite. Jasra et al.46 studied the effects of bentonite on the sorption and catalytic properties of X, Y, and mordenite zeolites, observing a decrease in the acidity of the zeolite upon agglomeration. They also suggested that such a decrease could be due to the replacement of acidic H+ ions of the zeolites with exchangeable clay cations such as Na+, Mg2+, Ca2+ which would migrate to the zeolite pores during the agglomeration process. Others authors have also observed this effect.20,27,28 Moreover, it could also be possible that during the agglomeration process a partial blocking of the micropore mouths of the zeolite by the binder or by the EFAL species takes place, which could hinder the access to the acid sites. The experimental values of surface area of the agglomerated catalysts were close to the theoretical ones. However, as noted earlier, a deviation of 3% or higher was observed, being close to 10% for the mordenite-zeolite-based catalyst. The conclusion of this behavior is that the binder may block partially the zeolite micropore mouths.20 On the other hand, some authors11,39,47 suggested that in some mordenite samples a channel blockage by the extraframework species in the pores could be produced. Fernandes et al.39 also suggested that certain EFAL species could block the acid sites of the beta zeolite. Finally, Remy et al.36 observed a lower catalytic activity for the highly dealuminated Y zeolite. Figure 1 shows the 27Al NMR spectra for different samples. The presence of EFAL species can be observed in all the agglomerated samples. The EFAL signal was significantly higher in these samples as compared to that of nonagglomerated ones. Therefore, the EFAL species created after the agglomeration process could also hinder the accessibility of the acid sites, especially in the mordenite-zeolite-based catalyst, and, consequently, the diffusion of the products. Thus, the decrease of the number of strong acid sites evaluated by TPDA, due to the ion-exchange among the cations of the clay and the H+ from the zeolite together with the possible partial blocking of the micropore mouths of the zeolites by the binder and the EFAL species, allows one to believe that the catalytic performance could drop after the agglomeration process. To verify the influence of the loss of the strong acidity of the agglomerated catalysts on the hydroisomerization activity and, then, the diffusional restrictions provoked by the binder and the EFAL species, a catalyst based in beta zeolite without binder but with a lower strong acidity value than that corresponding to the agglomerated catalyst was prepared. For this
Figure 3. n-Octane conversion, n-octane isomer selectivity, and n-octane isomer yield versus the reaction time over PtBETA.
purpose, beta zeolite was consecutively ion-exchanged with a 1 M NaNO3 solution before the metal incorporation. This catalyst was named as PtBETA-Na. The strong acidity obtained with this catalyst was 0.377 mmol NH3/gzeolite, the corresponding value for the sample PtBETA/Bent being 0.497 mmol NH3/ gzeolite. The temporal conversion obtained with this catalyst has been included in Figure 2. It can be noted that the catalytic activity of this sample (PtBETA-Na) was very similar to that corresponding to the agglomerated one (PtBETA/Bent) even though the former presented a lower strong acidity value. This would explain the fact that the catalytic performance was affected by the binder presence not only by the neutralization of the acid sites by the Na+ but also, probably, by the diffusional constraint imposed by EFAL species created after the agglomeration process and by the possible partial blocking of the micropore mouths of the zeolite. Under catalytic conditions, the pores of the zeolite contain not only n-octane but also dissolved hydrogen, alkane isomers, and light products from cracking. This situation should certainly influence the adsorption behavior.26 Denayer et al.53 observed lower reaction rates in the liquid phase than in the vapor phase. They explained this fact by the higher concentration of dissolved hydrogen in the zeolite pores compared to that observed in the vapor-phase experiments. It can be seen in Figure 2 that the catalytic activity in terms of n-octane conversion of the agglomerated catalysts decreased in the following order: beta (Si/Al ) 13.0) > USY (Si/Al ) 13.0) > mordenite (Si/Al ) 10.4). Despite the trends observed with the nonagglomerated catalysts, the conversion obtained with the agglomerated USY-based catalyst was higher than that obtained with the agglomerated mordenite one. This fact can be explained because the diffusional constraint imposed by the binder and the EFAL species affected to a greater extent the mordenite zeolite due to its narrower pore system. Moreover, the platinum dispersion for the agglomerated mordenite-based catalyst decreased due to the creation of mesopores during the agglomeration process. In this way, there were less metal sites available for the dehydrogenation-hydrogenation reaction leading to a lower n-octane conversion. 3.2.2. Isomer Yield. Figure 3 shows the evolution of the main reaction parameters (n-octane conversion, isomer selectivity, and isomer yield) for the sample PtBETA. All the catalysts followed a similar pattern. Isomer yield passed through a maximum, which is consistent with the existence of consecutive reactions. It is well established that the hydroisomerization reactions proceed through successive mono-, di-, and tribranched intermediates formed by type A (methyl shift) and B [via protonated cyclopropane (PCP)] isomerization mechanisms. The former
Ind. Eng. Chem. Res., Vol. 45, No. 26, 2006 8857
Figure 4. n-Octane isomer distribution as a function of the reaction time over PtBETA.
Figure 5. n-Octane isomer yield as a function of the reaction time over (b) PtUSY, (2) PtMOR, (9) PtBETA, (O) PtUSY/Bent, (4) PtMOR/Bent, and (0) PtBETA/Bent.
type of isomerization results in products with the same number of branches as the reactant. Isomerization via PCP intermediates is responsible for the net increase or decrease in the extent of branching of the reaction products. At the same time, hydrocracking follows type A (tertiary carbenium ion to tertiary), B1 (secondary carbenium ion to primary), B2 (tertiary to secondary), C (secondary carbenium ion to secondary), and D (secondary carbenium ion to primary) β-scissions.54 Our experiments agree with these reaction mechanisms. As shown in Figure 4, at very short reaction times, the monobranched isomers were predominant over PtBETA. They were clearly primary products of the reaction. When the reaction time increased, the monobranched yield reached a maximum and then decreased. The multibranched yield, which was very low at short reaction times, also increased with the reaction time, went through a maximum, and then decreased. Cracked products appeared at higher reaction times and increased dramatically at high conversion values. They were clearly secondary products of the reaction.6 As mentioned above, these results confirm that the scheme of the reaction on PtBETA is a consecutive one, indicating that
the hydrogenating function is probably not limiting.6 The same trend was obtained for the rest of the catalysts. The n-octane isomer yield versus reaction time for all the catalysts is shown in Figure 5. It can be clearly noted that for the nonagglomerated catalysts, a maximum of the n-octane isomer yield was reached. In the case of the agglomerated catalysts, longer reaction times than 25 h were needed to reach the maximum n-octane isomer yield. The maximum n-octane isomer yield obtained with each nonagglomerated catalyst was very similar. It is noteworthy that, for beta-zeolite-based catalyst, this maximum was reached in the shortest time (79 mol % at 7.5 h), followed by the time for the mordenite one (71 mol % at 10.5 h). For USY-zeolite-based catalyst, a reaction time of 23 h was needed to reach the maximum of n-octane isomer yield (75 mol %). The isomer selectivity over the zeolite-based catalysts studied at comparable n-octane conversion in the vicinity of about 40% is shown in Table 3. As expected, nonagglomerated catalysts based on USY and beta zeolites, due to their relatively large pore sizes, led to a higher extent of C8 at the same level of n-octane conversion in comparison with mordenite-based catalyst. However, for these three zeolites, differences in isomerization-cracking selectivity could be explained on the basis of differences in not only the pore size but also the acidity. This would explain the highest multibranched isomer selectivity obtained with mordenite-zeolite-based catalyst. Since the isomerization proceeds through successive branching from normal to mono- to di-, and possibly to tribranched alkanes, high acid strength favors more branching. The reason for this behavior is that high acid strengths allow relatively longer residence times of the intermediate carbenium ions on the acid sites, thus providing sufficient time for the latter intermediates to be isomerized.28,54 Similarly, the n-octane isomer total selectivity was obtained with the samples based on mordenite zeolite. When this zeolite was agglomerated, the selectivity toward monobranched isomers was increased while the multibranched isomer selectivity decreased (Table 3). In the agglomerated mordenite, meso- and macropores were provided by the binder, allowing a better accommodation of the metal, likely located in the meso- and macropores of the catalyst. This fact would decrease the diffusional constraints. Nevertheless, since the isomerization proceeded through successive branching from normal to mono- to di-, and possibly to tribranched alkanes, high acid strength should favor more branching. It should be noted that during the agglomeration process part of the acid sites of the zeolite were neutralized, reducing the possibility for latter intermediates to be isomerized.10 On the other hand, a decrease in the selectivity was observed when the USY zeolite was agglomerated. This fact would be a consequence of the loss of mesoporosity that, as noted earlier, occurred during the agglomeration process leading to the filling of the mesopores by the binder. This effect was not remarkable in sample PtBETA/Bent, since this catalyst presented a relatively higher mesoporosity after being agglomerated.
Table 3. Isomer Selectivities (mol %) over Different Catalysts at Aproximately 40 mol % Conversiona product %)b
Smono (mol Smulti (mol %)c SC5+ (mol %)d Stotal (mol %)
PtUSY (6 h)
PtMOR (5 h)
PtBETA (2 h)
PtY/Bent (22 h)
PtMOR/Bent (27 h)
PtBETA/Bent (9 h)
84.78 14.31 99.59 99.09
67.34 26.21 96.15 93.55
80.18 18.48 99.20 98.66
67.76 11.86 83.25 79.62
78.14 17.43 96.73 95.57
82.76 16.91 99.90 99.67
a W/M,7 g b zeolite/moln-C8; temperature, 270 °C; total pressure, 90 bar. The reaction time is shown in brackets. Monobranched isomers selectivity obtained at 40 mol % conversion. c Multibranched isomers selectivity obtained at 40 mol % conversion. d C5-C8 isomer yield at 40 mol % conversion.
8858
Ind. Eng. Chem. Res., Vol. 45, No. 26, 2006
Finally, because the aim of the hydroisomerization is to obtain branched isomers in order to increase the octane number of gasolines, it was considered very interesting to observe the selectivity to C5+ products. Isomer products from C5 to C8 contributed and increased the degree in the octane number of the gasoline. It is clear that for all the catalysts (agglomerated and nonagglomerated) this purpose was reached since a high C5+ selectivity was obtained.27,28 4. Conclusions Catalytic performance was strongly influenced by the binder due to the fact that zeolite hydrogen transfer activity, metal/ acid site balance, acid site density, and diffusional limitations were altered after agglomeration. A lower catalytic activity was obtained for all the agglomerated catalysts, due to the decrease in the zeolite strong acid sites as a consequence of the solid ion-exchange between zeolite protons and clay sodium. Additionally, a possible partial blocking of the micropore mouths by the binder and the EFAL species created during the agglomeration process could take place. Due to the high pressure, a significantly higher yield of isomers was obtained as compared to vapor-phase conditions and cracking becomes important only at relatively high average conversions (90 mol %). These cracked products would appear at longer reaction times in the case of the agglomerated catalysts. A similar maximum n-octane isomer yield was reached (75 mol %) for the nonagglomerated catalysts. For the beta zeolite, this maximum was reached in a shorter time. Similar n-octane isomer selectivity was obtained with the samples based on mordenite zeolite. However, a decrease in the selectivity was observed when the USY and beta zeolites were agglomerated. This fact was fundamentally due to a mesoporosity loss occurring during the agglomeration process as a consequence of the filling of the mesopores by the binder. With all the catalysts, a high selectivity to C5+ products was obtained, which could contribute and increase the degree in the octane number of the gasolines. Acknowledgment Financial support from the Ministerio de Ciencia y Tecnologı´a of Spain (Project CTQ-2004-07350-C02-O) and the Consejerı´a de Ciencia y Tecnologı´a de la Junta de Comunidades de CastillaLa Mancha (Proyect PBI-05-038) are gratefully acknowledged. Literature Cited (1) Pope, T. D.; Kriz, J. F.; Stanciulescu, M.; Monnier, J. A study of catalyst formulations for isomerization of C7 hydrocarbons. Appl. Catal., A 2002, 233, 45. (2) Maxwell, I. Zeolite catalysis in hydroprocessing technology. Catal. Today 1987, 1, 385. (3) Raghuram, S.; Haizmann, R. S.; Lowry, D. R.; Schieferli, W. J. Isomerization with separation/recycle brings big octane boost. Oil Gas J. 1990, 88, 66. (4) Blomsma, E.; Martens, J. A.; Jacobs, P. A. Mechanisms of heptane isomerization on bifunctional Pd/H-Beta zeolites. J. Catal. 1996, 159, 323. (5) Blomsma, E.; Martens, J. A.; Jacobs, P. A. Reaction mechanisms of isomerization and cracking of heptane on Pd/H-Beta zeolite. J. Catal. 1995, 155, 141. (6) Patrigeon, A.; Benazzi, E.; Travers, Ch.; Bernhard, J. Y. Influence of the zeolite structure and acidity on the hydroisomerization of n-heptane. Catal. Today 2001, 65, 149. (7) Blomsma, E.; Martens, J. A.; Jacobs, P. A. Isomerization and hydrocracking of heptane over bimetallic bifunctional PtPd/H-Beta and PtPd/ USY zeolite catalysts. J. Catal. 1997, 165, 241. (8) Me´riaudeau, P.; Tuan, V. A.; Nghiem, V. T.; Lai, S. Y.; Hung, L. N.; Naccache, C. SAPO-11, SAPO-31, and SAPO-41 molecular sieves:
synthesis, characterization, and catalytic properties in n-octane hydroisomerization. J. Catal. 1997, 169, 55. (9) Me´riaudeau, P.; Tuan, V. A.; Nghiem, V. T.; Sapaly, V.; Naccache, C. Comparative evaluation of the catalytic properties of SAPO-31 and ZSM48 for the hydroisomerization of n-octane: effect of the acidity. J. Catal. 1999, 185, 435. (10) Raybaud, P.; Patrigeon, A.; Toulhoat, H. The origin of the C7hydroconversion selectivities on Y, β, ZSM-22, ZSM-23, and EU-1 zeolites. J. Catal. 2001, 197, 98. (11) Gopal, S.; Smirniotis, P. G. Factors affecting isomer yield for n-heptane hydroisomerization over as-synthesized and dealuminated zeolite catalysts loaded with platinum. J. Catal. 2004, 225, 278. (12) Mun˜oz Arroyo, J. A.; Martens, G. G.; Froment, G. F.; Marin, G. B.; Jacobs, P. A.; Martens, J. A. Hydrocracking and isomerization of n-paraffin mixtures and a hydrotreated gasoil on Pt/ZSM-22: confirmation of pore mouth and key-lock catalysis in liquid phase. Appl. Catal., A 2000, 192, 9. (13) Kinger, G.; Majda, D.; Vinek, H. n-Heptane hydroisomerization over Pt-containing mixtures of zeolites with inert materials. Appl. Catal., A 2002, 225, 301. (14) Chao, K. J.; Wu, H. C.; Leu, L. J. Hydroisomerization of light normal paraffins over series of platinum-loaded mordenite and beta catalysts. Appl. Catal., A 1996, 143, 223. (15) Giannetto, G.; Alvarez, F.; Ramoa Ribeiro, F.; Perot, G.; Guisnet, M. Guidelines for Mastering the Properties of Molecular SieVes; Plenum Press: New York, 1990; p 355. (16) Martens, J. A.; Tielen, M.; Jacobs, P. A. Zeolites as Catalysts, Sorbents and Detergent Builders; Elsevier: Amsterdam, 1989; p 49. (17) Martens, J. A.; Jacobs, P. A.; Weitkamp, J. Attempts to rationalize the distribution of hydrocracked products. I. Qualitative description of the primary hydrocracking modes of long chain paraffins in open zeolites. Appl. Catal., A 1986, 20, 239. (18) Lugstein, A.; Jentys, A.; Vinek, H. Hydroisomerization and cracking of n-octane and C8 isomers on Ni-containing zeolites. Appl. Catal., A 1999, 176, 119. (19) Dorado, F.; Romero, R.; Can˜izares, P. Influence of clay binders on the performance of Pd/HZSM-5 catalysts for the hydroisomerization of n-butane. Ind. Eng. Chem. Res. 2001, 40, 3428. (20) Dorado, F.; Romero, R.; Can˜izares, P. Hydroisomerization of n-butane over Pd/HZSM-5 and Pd/Hβ with and without binder. Appl. Catal., A 2002, 236, 235. (21) Rough, S. L.; Wilson, D. I.; Bayly, A.; York, D. Tapping characterization of high shear mixer agglomerates made with ultrahigh viscosity binders. Powder Technol. 2003, 132, 249. (22) Devadas, P.; Kinage, A. K.; Choudhary, V. R. Effect of silica binder on acidity, catalytic activity and deactivation due to coking in propane aromatization over H-gallosilicate (MFI). Stud. Surf. Sci. Catal. 1998, 113, 425. (23) Wu, X.; Alkhawaldeh, A.; Anthony,R. G. Investigation on acidity of zeolites bound with silica and alumina. Stud. Surf. Sci. Catal. 2002, 143, 217. (24) Chen, W. H.; Huang, S. J.; Lai, C. S.; Tsai, T. C.; Lee, H. K.; Liu, S. B. Effect of binder, coking and regeneration on acid properties of H-mordenite during TPD reaction. Res. Chem. Intermed. 2003, 29, 761. (25) Uguina, M. A.; Sotelo, J. L.; Serrano, D P. Toluene disproportionation over ZSM-5 zeolite: effects of crystal size, silicon-to-aluminium ratio, activation method and pelletization. Appl. Catal., A 1991, 76, 183. (26) Denayer, J. F. M.; Ocakoglu, R. A.; Hybrechts, W.; Dejonckheere, B.; Jacobs, P.; Calero, S.; Krishna, R.; Smit, B.; Baron, G. B.; Martens, J. A. High-pressure liquid-phase hydroconversion of heptane/nonane mixtures on Pt/H-Y zeolite catalyst. J. Catal. 2003, 220, 66. (27) De Lucas, A.; Valverde, J. L.; Sa´nchez, P.; Dorado, F.; Ramos, M. J. Influence of the binder on the n-octane hydroisomerization over palladium-containing zeolite catalysts. Ind. Eng. Chem. Res. 2004, 43, 8217. (28) De Lucas, A.; Valverde, J. L.; Sa´nchez, P.; Dorado, F.; Ramos, M. J. Hydroisomerization of n-octane over platinum catalysts with or without binder. Appl. Catal., A 2005, 282, 15. (29) De Lucas, A.; Ramos, M. J.; Dorado, F.; Sa´nchez, P.; Valverde, J. L. Influence of the Si/Al ratio in the hydroisomerization of n-octane over platinum and palladium beta zeolite-based catalysts with or without binder. Appl. Catal., A 2005, 289, 205. (30) De Lucas, A.; Sa´nchez, P.; Dorado, F.; Ramos, M. J.; Valverde, J. L. Effect of the metal loading in the hydroisomerization of n-octane over beta agglomerated zeolite based catalysts. Appl. Catal., A 2005, 294, 215. (31) Horvath, R.; Kawazoe, K. J. Method for the calculation of effective pore size distribution in molecular sieve carbon. Chem. Eng. Jpn. 1983, 16, 470.
Ind. Eng. Chem. Res., Vol. 45, No. 26, 2006 8859 (32) Barret, R. M.; Joyner, L. G.; Halenda, P. P. The determination of pore volume and area distribution in porous substances. J. Am. Chem. Soc. 1951, 73, 373. (33) Can˜izares, P.; De Lucas, A.; Valverde, J. L.; Dorado, F. n-Butane hydroisomerization over Pt/HZSM-5 catalysts. Ind. Eng. Chem. Res. 1997, 36, 4797. (34) Rodrı´guez, F.; Rodrı´guez, I.; Moreno, C.; Guerrero, A.; Lo´pez, J. D. Platinum catalysts supported on activated carbons: I. Preparation and characterization. J. Catal. 1986, 99, 171. (35) Bond, G. C.; Mallat, T. Studies of Hydrogen Spill over. J. Chem. Soc., Faraday Trans. 1981, 77, 1743. (36) Remy, M. J.; Stanica, D.; Poncelet, G.; Feijen, E. J. P.; Grobet, P. J.; Martens, J. A.; Jacobs, P. A. Dealuminated H-Y zeolites: relation between physicochemical properties and catalytic activity in heptane and decane isomerization. J. Phys. Chem. 1996, 100, 12440. (37) Donk, S.; Janssen, A. H.; Bitter, J. H.; Jong, K. P. Generation, characterization and impact of mesopores in zeolite catalysts. Catal. ReV. 2003, 45, 297. (38) Mu¨ller, M.; Harvey, G.; Prins, R. Comparison of the dealumination of zeolites beta, mordenite, ZSM-5 and ferrierite by thermal treatment, leaching with oxalic acid and treatment with SiCl4 by 1H, 29Si and 27Al MAS NMR. Microporous Mesoporous Mater. 2000, 34, 135. (39) Fernandes, L. D.; Monteiro, J. L. F.; Sousa-Aguiar, E. F.; Martı´nez, S. A.; Corma, A. Ethylbencene hydroisomerization over bifunctional zeolite based catalysts: the influence of framework and extraframework composition and zeolite structure. J. Catal. 1998, 177, 363. (40) Can˜izares, P.; De Lucas, A.; Dorado, F.; Dura´n, A.; Asencio, I. Characterization of Ni and Pd supported on H-mordenite catalysts: Influence of the metal loading method. Appl. Catal., A 1998, 169, 137. (41) Gille, W.; Enke, D.; Janowski, F.; Hahn, T. Platinum dispersion analysis depending on the pore geometry of the support. Catal. Lett. 2004, 93, 13. (42) Choudhary, V. R.; Devadas, P.; Kinage, A. K.; Guisnet, M. Influence of binder on the acidity and performance of H-Gallosilicate (MFI) zeolite in propane aromatization. Appl. Catal., A 1997, 162, 223. (43) Romero, M. D.; Calles, J. A.; Rodrı´guez, A.; De Lucas, A. Acidity modification during the agglomeration of ZSM-5 with montmorillonite. Microporous Mesoporous Mater. 1997, 9, 221.
(44) Yoon, K. B.; Park, Y. S. Visual observation of salt migration between zeolite and clay in the solid state. J. Chem. Soc., Chem. Commun. 1993, 522. (45) Choudhary, V. R.; Nayak, V. S. Effect of degree of proton exchange on the acidity distribution of HNa-ZSM-5. Zeolites 1985, 5, 15. (46) Jasra, R. V.; Tyagi, B.; Badheka, Y. M.; Choudary, V. N.; Bhat, T. S. G. Effect of clay binder on sorption and catalytic properties of zeolite pellets. Ind. Eng. Chem. Res. 2003, 42, 3263. (47) Olken, M. M.; Garces, J. M. In Proceedings, 9th International Zeolite Conference; van Ballmoos, R., Higgins, J. B., Treacy, M. M. J., Eds.; Butterworth-Heineman: Stoneham, MA, 1993; Vol. 2, p 559. (48) Kinger, G.; Vinek, H. n-Nonane hydroconversion on Ni and Pt containing HMFI, HMOR and HBEA. Appl. Catal., A 2001, 218, 139. (49) Thybaut, J. W.; Narasimhan, C. S. L.; Denayer, J. F.; Baron, G. V.; Jacobs, P. A.; Martens, J. A.; Marin, G. B. Acid-metal balance of a hydrocracking catalyst: ideal versus nonideal behavior. Ind. Eng. Chem. Res. 2005, 44, 5159. (50) Makarova, M. A.; Wilson, A. E.; van Liemt, B. J.; Mesters, C. M. A. M.; de Winter, A. W.; Williams, C. Quantification of Brønsted Acidity in Mordenites. J. Catal. 1997, 172, 170. (51) Eder, F.; Stockenhuber, M.; Lercher, J. A. Sorption of light alkanes on H-ZSM5 and H-mordenite. Stud. Surf. Sci. Catal. 1995, 97, 495. (52) Park, K.; Ihm, S. Comparison of Pt/zeolite catalysts for nhexadecane hydroisomerization. Appl. Catal., A 2000, 203, 201. (53) Denayer, J. F. M.; Jonckheere, B.; Hloch, M.; Marin, G. B.; Vanbutsele, G.; Martens, J. A.; Baron, G. V. Molecular competition of C7 and C9 n-alkanes in vapor- and liquid-phase hydroconversion over bifunctional Pt-USY zeolite catalysts. J. Catal. 2002, 210, 445. (54) Zhang, W.; Smirniotis, P. G. Effect of zeolite structure and acidity on the product selectivity and reaction mechanism for n-octane hydroisomerization and hydrocracking. J. Catal. 1999, 182, 400.
ReceiVed for reView March 28, 2006 ReVised manuscript receiVed September 18, 2006 Accepted October 4, 2006 IE060388S
