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Dehydroisomerization of n-butane over H-Y zeolite supported Pt and Pt,Sn catalysts
Applied Catalysis A: General 274 (2004) 151–157
Dehydroisomerization of n-butane over H-Y zeolite supported Pt and Pt,Sn catalysts S. Scirè∗ , G. Burgio, C. Crisafulli, S. Minicò Dipartimento di Scienze Chimiche, Università di Catania, Viale Andrea Doria 6, 95125 Catania, Italy Received in revised form 4 June 2004; accepted 11 June 2004 Available online 4 July 2004
Abstract The direct one-step conversion of n-butane to isobutene was investigated over H-Y zeolite supported platinum and platinum-tin catalysts. Catalytic results over monometallic Pt catalysts showed that dispersion of Pt mainly affects the selectivity towards reaction products, larger Pt ensembles resulting in a higher degree of cracking/hydrogenolysis reactions and consequently to a lower n-butenes and isobutene formation. Addition of tin has been found to significantly improve both the selectivity to isobutene and the resistance to deactivation of the catalytic system. On the basis of characterization results (H2 chemisorption, NH3 -TPD, FT-IR of adsorbed CO) the higher isobutene selectivity of Pt,Sn catalysts have been accounted for a dilution effect of Pt ensembles induced by the second metal which inhibits hydrogenolysis reactions, requiring several contiguous Pt sites, and enhances the selectivity for the dehydrogenation reaction that can instead proceed over smaller metal ensembles. The improved stability of Pt,Sn catalysts has been related to the lower surface acidity of the system in the presence of Sn, which results in a reduced formation of carbonaceous deposits responsible for catalysts deactivation. © 2004 Elsevier B.V. All rights reserved. Keywords: Dehydroisomerization; n-Butane; Isobutene; Zeolite; Platinum; Tin; Bimetallic
1. Introduction Isobutene, which is a key intermediate in the petrochemical industry, is currently produced from butanes in a twin reactor system technology where an isomerization step is followed by a dehydrogenation step or vice versa [1]. Recently, the direct one-step conversion of n-butane to isobutene has been reported as an interesting alternative [2–9]. For this reaction two options has been taken into consideration, the first one involves the use of a two-bed system with a dehydrogenation catalyst in combination with an isomerization catalyst [2,3]. The other approach uses a bifunctional system, usually a zeolite supported Pt catalyst [4–9], where Pt provides the dehydrogenation activity while Bronsted acid sites, responsible for isomerization, are supplied by the zeolite. In this context several Pt catalysts have been upto now explored, e.g. Pt-MOR [4], Pt-ZSM5 [5,6], Pt-ZSM11 [7], Pt-MCM22 [8], Pt-FER [6], Pt-TON
∗
Corresponding author. Tel.: +39 095 7385034; fax: +39 095 580138. E-mail address: [email protected] (S. Scir`e).
0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.06.019
[6,9], with the aim to maximize isobutene yield, minimizing undesired side reactions (cracking and oligomerization). Bimetallic catalysts have been extensively investigated in recent years due to the fact that they may exhibit better catalytic performances than corresponding monometallic samples [10–12]. The first application of a bimetallic system in the direct conversion of n-butane to isobutene has been recently reported by Derouane et al. [9]. They found that addition of copper to Pt/H-TON has a positive effect for dehydroisomerization, reducing hydrogenolysis side-reactions. Following these considerations we hereby report a study on the dehydroisomerization of n-butane over Pt,Sn catalysts supported on H-Y zeolites. Pt,Sn catalysts are, in fact, among the most studied bimetallic systems, their use being reported in several industrial processes such as naphta reforming and dehydrogenation of heavy and light paraffins [13–15]. In particular the purpose of our work was to evaluate the influence that the addition of different amounts of Sn to Pt may have on activity, selectivity and stability of the catalytic system. The effect of both SiO2 /Al2 O3 ratio of the zeolitic support and Pt dispersion on the performance of catalysts was also addressed.
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2. Experimental
Table 1 Code and physicochemical properties of supported Pt and Pt,Sn catalysts
Pt and Pt,Sn catalysts were prepared by (co)impregnation of the support with aqueous solutions of precursors (Pt = 0.5 wt.%, M/Pt atomic ratio ranging from 0.25 to 1). H2 PtCl6 ·6H2 O (Alfa Aesar), and SnCl2 ·2H2 O (Fluka) were used as precursor salts. Two H-Y zeolites (faujasites), provided by Zeolysts, with SiO2 /Al2 O3 ratios, respectively of 5 (FAU5) and 80 (FAU80) were used as support. Catalytic activity tests were carried out in a fixed bed reactor operating at atmospheric pressure and at 550 ◦ C, using 50–200 mg of catalyst (80–140 mesh) mixed with 400 mg of quartz of the same grain size. The reactant mixture was 10% n-butane, 20% H2 and 70% He. The total gas flow was varied from 15 to 60 ml min−1 (WHSV ranging from 1.2 to 18.6 h−1 for n-butane). Before catalytic tests all samples were reduced at 550 ◦ C in H2 for 1 h. Reaction products were analyzed with an online gas chromatograph equipped with FID detector. Under experimental conditions used isobutene, n-butenes, isobutane and C1 –C3 hydrocarbons were major products formed. Small amounts of butadiene and pentenes were also detected. Preliminary runs carried out at different flow-rates showed the absence of external diffusional limitations. The absence of internal diffusion limitations was verified by running experiments with crushed pellets at different grain size. H2 uptake was measured in a static system operating at room temperature. Before the measurements the samples have been reduced in H2 at 550 ◦ C for 1 h, outgassed at 550 ◦ C for 1 h and then cooled at room temperature. For IR studies the powdered samples were compressed (using a pressure of 15 × 103 bar) into thin self-supporting discs of about 25 mg cm−2 and 0.1 mm thick. The disc was placed in an IR cell which allows thermal treatments in vacuum or in a controlled atmosphere. In the cell all samples were evacuated by slowly increasing the temperature upto 550 ◦ C and then reduced in pure H2 at this temperature for 1 h. The sample was then evacuated at this temperature for 1 h and finally cooled at room temperature. CO (2 mbar) was then adsorbed at room temperature. Subsequent evacuations were then performed at room temperature. Spectra were recorded with a Perkin Elmer System 2000 FT-IR spectrophotometer with a resolution of 2 cm−1 . Data are reported as difference spectra obtained by subtracting the spectrum of the sample before the admission of the adsorbate and are normalized to the same amount of catalysts per cm2 . Temperature-programmed desorption of ammonia (NH3 -TPD) was carried out in a quartz U-shape reactor in a flow of He with a constant heating rate of 10 ◦ C min−1 . The desorbed products were detected by a quadrupole mass spectrometer (Sensorlab VG Quadrupoles). Before TPD all samples were reduced in flowing H2 for 1 h at 550 ◦ C, maintained at this temperature for 1 h in flowing He and then cooled at 30 ◦ C always in a flow of He. Temperature-programmed oxidation (TPO) of carbon was carried out in a quartz U-shape reactor in flowing pure O2
Code
Support
Sn/Pt ratio
H/Pt ratio
NH3 adsorbed (mmol g−1 )a
Pt5 Pt5LD 0.25SnPt5 0.5SnPt5 1.0SnPt5 Pt80
FAU5 FAU5 FAU5 FAU5 FAU5 FAU80
0 0 0.25 0.5 1.0 0
0.60 0.20 0.50 0.40 0.10 0.25
0.73 0.69 0.54 0.50 0.48 0.02
a Estimated by integrating the area of NH desorption peaks reported 3 in Fig. 3.
with a constant heating rate of 10 ◦ C min−1 . A quadrupole mass spectrometer (Sensorlab VG Quadrupoles) was used to analyze the composition of the effluent stream. The surface carbon was previously produced during deactivation tests carried out at 550 ◦ C for 8 h. Code of all samples together with support used, H/Pt ratio and total amount of NH3 adsorbed are reported in Table 1. It must be noted that the sample coded Pt5LD has been obtained by treating the Pt5 sample in air at 700 ◦ C for 2 h.
3. Results Fig. 1 reports the selectivity to the major reaction products as a function of conversion levels (obtained by varying contact times) over monometallic Pt catalysts (Pt5, Pt5LD and Pt80 samples). It must be reminded that, under experimental conditions of this work, Pt-free H-Y samples exhibited no significant n-butane conversion. Results of Fig. 1 shows that product selectivity curves have a similar trend for all three catalysts. In particular, the selectivity to n-butenes is very high at low conversion (a value of 75–95% selectivity can be extrapolated at zero conversion) decreasing continuously with increasing conversion levels. On the contrary the selectivity to C1 –C3 hydrocarbons extrapolates to a value between 5 and 20% at zero conversion undergoing a rapid and continuous growth with conversion. The initial selectivities both to isobutene and isobutane are close to zero. On increasing conversions isobutene selectivity rises moderately reaching a maximum at ca. 40% conversion then decreasing at higher contact times; isobutane becomes detectable only at ca. 30% conversion, then increases very slightly with conversion, however remaining very low (selectivity < 4%) even at high conversion. A closer inspection of data of Fig. 1 allows to point out some interesting differences in the behaviour of investigated monometallic Pt samples. In fact, by comparing selectivity curves of the two FAU5 supported Pt samples (Pt5 and Pt5LD, respectively in Fig. 1a and b), which differs with each other substantially in the dispersion of platinum (see Table 1), it can be clearly noted that the catalyst with higher Pt dispersion (Pt5 sample) produces smaller amounts of C1 –C3 hydrocarbons (the related selectivity
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Table 2 Catalytic activity data of supported Pt and Pt,Sn catalysts Code
V × 105 (mol gcat −1 s−1 )a
Activity loss (%)b
Pt5 Pt5LD 0.25SnPt5 0.5SnPt5 1.0SnPt5 Pt80
2.73 2.34 2.72 2.78 1.20 1.46
40 45 22 20 21 28
a b
Fig. 1. Selectivities to the major reaction products as a function of n-butane conversion levels over monometallic Pt samples: (a) Pt5; (b) Pt5LD; (c) Pt80. Selectivity to (䊊) isobutene; (䊏) C1 –C3 hydrocarbons; (䉱) isobutane; (×) n-butenes.
Reaction rate calculated at 550 ◦ C. After 8 h of run.
versus conversion curve is downshifted from 10 to 20%) with respect to the less disperse Pt sample (Pt5LD). Correspondingly, a sensibly higher selectivity to n-butenes is shown by the Pt5 sample. In terms of isobutene selectivities the difference between the two samples is still existing but less remarkable (peak selectivity reaches values of 27 and 23%, respectively on Pt5 and Pt5LD). From the figure it can be also seen that the product selectivity curves (Fig. 1c) of the FAU80 supported Pt sample (Pt80) are between the corresponding ones of the two FAU5 supported Pt samples, however being quite similar to those of Pt5, notwithstanding the dispersion of the Pt80 sample is much closer to that of the Pt5LD sample (Table 1). Catalytic activity data (reaction rates, expressed as moles of n-butane transformed per gram of catalyst per second, and deactivation rates as percent activity loss after 8 h of run) of supported Pt and Pt,Sn catalysts are shown in Table 2. Concerning the activity of monometallic Pt samples, it can be seen that the reaction rate of the Pt5 sample is about 20% higher than that of the Pt5LD sample. The Pt80 sample is further less active (the reaction rate is about half) than both FAU5 supported monometallic Pt samples. The observed order of stability of monometallic Pt samples is: Pt80 Pt5 > Pt5LD. With regard to the catalytic behaviour of bimetallic Pt,Sn catalysts results of Table 2 shows that addition of tin does not substantially affect the activity of FAU5 supported Pt catalysts upto a Sn/Pt ratio of 0.5. At higher Sn content (Sn/Pt = 1.0) a marked decrease in the reaction rate is instead observed. In terms of catalysts stability, results of Table 2 shows that all Pt,Sn catalysts exhibit a lower deactivation rate compared to all monometallic Pt samples, the 0.5SnPt5 sample being the most stable. It must be underlined that the difference in the stability among investigated Pt,Sn samples is indeed small. Fig. 2 reports the products selectivities, compared at the same conversion level (40%), over FAU5 supported Pt and Pt,Sn samples. An evident change in the products distribution of supported Pt samples is observed in the presence of tin. On bimetallic Pt,Sn catalysts, in fact, selectivities to isobutene and n-butenes are sensibly higher than those observed on the corresponding monometallic Pt samples (Pt5 and Pt5LD). It must be noted that the positive effect of tin towards unsaturated C4 formation is evident since
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Fig. 2. Comparison among products selectivities at the same conversion level (40%) over FAU5 supported Pt and Pt,Sn samples.
low Sn concentrations, then growing moderately with the concentration of this metal. Correspondingly, on Sn-doped catalysts the formation of C1 –C3 products is strongly reduced. It is important to underline that, within C1 –C3 hydrocarbons, saturated ones resulted to be those preferentially suppressed. NH3 -TPD profiles for Pt and Pt,Sn samples, normalized to the mass of the catalyst, are reported in Fig. 3. Total amounts of desorbed ammonia, expressed as mmol of adsorbed NH3 per gram of catalyst and estimated by integrat-
Fig. 3. NH3 -TPD profiles of FAU5 supported Pt and Pt,Sn samples.
ing the area of desorption peaks, are listed in Table 1. On all FAU5 supported Pt samples a broad desorption peak with a maximum at ca. 150 ◦ C and a shoulder at higher temperature (above 350 ◦ C) can be clearly visible. According to the literature [17] the low temperature peak can be ascribed to weak and medium strength acid sites, whereas the peak above 350 ◦ C is typical of strong acidity. It is noteworthy that dispersion of Pt does not affect substantially the acidity of the system (compare curves of Pt5 and Pt5LD samples). This is quite reasonable considering that platinum should be present almost exclusively as Pt◦ (samples are in fact pre-reduced at 550 ◦ C), thus making improbable that protons of the zeolite are replaced by Pt. This is also in accordance with the fact that Pt-free H-Y zeolites were found to show the same acidity compared to Pt/zeolite samples. It must also be reminded that the TPD-NH3 profile of the FAU80 supported Pt sample (not reported in the figure) was almost flat indicating that the amount of acid sites of this sample is very low (Table 2), accordingly with the high SiO2 /Al2 O3 ratio of the zeolite. Addition of tin leads to a moderate decrease in the total acidity of the system, both weak-medium and strong acid sites being affected by the presence of Sn. It is noteworthy that the negative influence of Sn on the acidity is evident already at low Sn/Pt ratio (Sn/Pt = 0.25), the presence of higher Sn contents being almost ineffective. FT-IR spectra after CO adsorption at room temperature (spectrum a) and following outgassing treatments (spectra b–e) on the monometallic Pt5 sample are reported in Fig. 4. After admission of CO two main bands can be observed, respectively centred at 2080 cm−1 (HF band) and 1840 cm−1
Fig. 4. FT-IR spectra of CO adsorbed on the Pt5 sample: (a) admission of 2 mbar of CO; (b) 5 s outgassing at RT; (c) 1 min outgassing at RT; (d) 5 min outgassing at RT; (e) 10 min outgassing at RT.
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Fig. 6. Correlation between catalysts deactivation (expressed as percent activity loss after 8 h of time of stream) and amount of carbonaceous deposits (estimated by TPO experiments) over FAU5 supported Pt and Pt,Sn samples.
Fig. 5. FT-IR spectra of CO adsorbed on FAU5 supported Pt and Pt,Sn catalysts (compared after 10 min outgassing at RT).
(LF band). In accordance to the literature the HF band is assigned to CO linearly adsorbed on Pt◦ , whereas the broader and weaker LF band can be ascribed to bridged CO species on Pt◦ [18,19]. The intensity of both HF and LF bands remains almost unchanged upon outgassing at room temperature. It must be also noted that the frequency of the HF band undergoes an evident downshift by RT evacuation, as a consequence of the decrease in the dipole–dipole coupling with decreasing CO coverage [18,19]. The FT-IR spectra of CO adsorbed at room temperature on Pt and Pt,Sn samples after 10 min outgassing at RT are compared in Fig. 5. It can be seen that in the case of 0.25SnPt5 and 0.5SnPt5 samples the presence of Sn results in the almost complete suppression of the LF band, whereas the HF band is affected only to a small extent, being moderately less intense and very slightly downshifted (<5 cm−1 ) with respect to the monometallic Pt sample. This behaviour can be rationalized on the basis of a dilution of Pt atoms by Sn on the catalyst surface layer [20,21]. In fact an ensemble effect should account for the preferential suppression of bridged CO species which requires the presence of adjacent Pt atoms. The concomitant occurrence of a ligand effect induced by Sn cannot be completely ruled out but appears not consistent with FT-IR data. In fact, an electronic Pt–Sn interaction should affect mainly the frequency of the HF band [22], which is instead almost unchanged. Fig. 5 shows that, together with the disappearance of the LF band, also a strong reduction in the intensity of the HF band can be observed in the case of the sample with the highest Sn/Pt ratio (1.0SnPt5). This seems to indicate that large amounts of tin suppress almost completely CO adsorption, probably
as a result of a very high extent of coverage of surface Pt by Sn atoms. This fits with the very low H2 uptake and reaction rate shown by the 1.0SnPt5 sample. Fig. 6 displays deactivation rates versus amounts of carbonaceous deposits (estimated by TPO experiments) over Pt and Pt,Sn samples. It must be reminded that the surface carbon was produced during deactivation tests carried out at 550 ◦ C for 8 h. From the figure it can be clearly observed that a good correlation between deactivation and coke formation exists, the higher the amount of carbonaceous products formed the higher the deactivation rate of the catalysts. This suggests, in accordance with the literature [5,9] that the decrease in the activity of Pt and Pt,Sn catalysts with time on stream is essentially related to coke deposition on the catalytic surface.
4. Discussion The curves of products selectivity as a function of conversion in the dehydroisomerization of n-butane over H-Y supported Pt catalysts (Fig. 1) are consistent with the reaction mechanism previously reported in the literature for a Pt/H-ZSM5 catalyst [5]. On the basis of this mechanism, dehydrogenation of n-butane to n-butenes is the primary reaction step which occurs on Pt metal sites. n-Butenes, which are more reactive than n-butane, are successively isomerized to isobutenes or oligomerized/cracked to form other by-products (mainly C1 –C3 hydrocarbons) over acid sites of the zeolite. Hydrogenolysis (over Pt sites) and protolytic cracking (over support acid sites) of n-butane to C1 –C3 hydrocarbons are also primary parallel reactions competing with dehydrogenation. In particular cracking leads to olefins (or mixture of olefins and alkanes), whereas hydrogenolysis
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gives alkanes exclusively. Finally the formation of isobutane presumably occurs as a consequence of the rehydrogenation of isobutene over Pt sites [5]. Catalytic data (Fig. 1 and Table 2) points out that Pt dispersion plays a certain role in directing the catalytic behaviour of monometallic Pt samples, affecting both activity and selectivity towards reaction products. In particular it was found that larger Pt ensembles lead to a higher C1 –C3 hydrocarbons formation with consequent lower selectivities to n-butenes and isobutene. This behaviour is reasonable considering that it is generally accepted that hydrogenolysis is a structure sensitive reaction, being favoured by larger Pt ensembles [9,16]. With concern to reaction rates (in terms of moles of n-butane transformed per gram of catalyst per second) it was observed that they depend on Pt dispersion. It is worth noting, however that to a three times drop in the Pt dispersion corresponded just to a moderate decrease (20%) of the reaction rate. This could be rationalized, considering the fact that when dispersion decreases the lower number of active Pt sites exposed on the catalytic surface is partially counterbalanced by the higher activity of these larger sites towards hydrogenolysis reactions [16]. Activity and selectivity of H-Y supported monometallic Pt samples were also influenced by the acidity of the support. A lower SiO2 /Al2 O3 ratio of the zeolite (i.e. a higher number of acid sites) resulted in a more active catalyst, however giving rise to a higher amount of C1 –C3 products (Fig. 1) and to a higher deactivation rate (Table 2). This behaviour is in accordance with the reaction mechanism above discussed, a higher acidity resulting in an improved isomerization efficiency, however accompanied by a larger extent of cracking reactions. The lower stability of the Pt samples supported on the more acidic zeolite (FAU5) is accounted for the fact that a higher acidity also favours oligomerization reactions which leads to carbonaceous residues, considered responsible of catalyst deactivation [5,9]. Effectively, this last statement is confirmed by the results reported in Fig. 6 which shows the existence of a good correlation between deactivation rate of catalysts and the amount of coke deposited on the catalyst surface during the dehydroisomerization reaction. Addition of tin to platinum upto a Sn/Pt ratio of 0.5 does not substantially affect the catalytic activity, whereas a higher Sn content (Sn/Pt = 1.0) caused a marked decrease in the reaction rate (Table 2). On the contrary an evident change in the products distribution of FAU5 supported Pt samples was determined by the presence of tin, since the lowest Sn concentrations (Fig. 2). In particular on Pt,Sn samples the selectivity to isobutene and n-butenes is sensibly higher compared to the monometallic Pt sample, while the formation to C1 –C3 products is strongly reduced. A similar result has been observed by addition of copper to Pt/H-TON catalysts [9]. It is noteworthy that isobutene yield obtained on the most selective samples (0.5SnPt5 and 1.0SnPt5) is about 14%, value which is sensibly higher than that of monometallic Pt samples (10% maximum). From a practical point of
view this value can be considered quite satisfactory, taking into account that the thermodynamic limit under experimental conditions of this work is about 20%. In order to explain how tin affects the performances of H-Y supported Pt catalysts let us consider characterization experiments (H2 chemisorption, FT-IR of adsorbed CO and NH3 -TPD) carried out on Pt,Sn/FAU5 catalysts. H2 chemisorption data showed that addition of tin to Pt/FAU5 samples causes a remarkable decrease in the total uptake of H2 , this effect being dramatic at high Sn/Pt ratio. This result, which agrees with the observed intensities of the FT-IR bands of adsorbed CO over Pt,Sn samples (Fig. 5), suggests that a lower number of active Pt sites is exposed on the catalytic surface of bimetallic Pt,Sn systems. On the 1.0SnPt5 sample the amount of Pt available on the catalytic surface is dramatically suppressed, thus accounting for the very low reaction rate observed on this sample. FT-IR spectra of adsorbed CO pointed out that tin results in a dilution of surface Pt clusters. A dilution of Pt caused by the addition of a second metal (Sn, Pb, Cu) has been frequently reported in the literature [11]. On the basis of the above reported results it seems possible to suggest that bimetallic Pt-Sn particles are formed having a surface mainly covered by Sn. This agrees with the higher volatility of Sn with respect to Pt, which would favour a segregation of tin on the surface. A similar enrichment of the catalyst surface in Ni has been reported by Rinkowski et al. in the case of Pt,Ni bimetallic systems [23]. Following these considerations we suggest that the improved isobutene selectivities of Pt,Sn catalysts in the n-butane dehydroisomerization can been essentially related to a dilution effect of Pt ensembles induced by the second metal, leading to a strongly reduced probability that adjacent Pt atoms are present on the catalyst surface. This should inhibit preferentially hydrogenolysis reactions, which requires several contiguous Pt sites, consequently enhancing the selectivity for the dehydrogenation reaction that can proceed over smaller metal ensembles [4]. On the other hand it must be also noted that addition of tin was found to cause a slight decrease in the acidity of the system (Fig. 3 and Table 2). This could be ascribed to a partial exchange of zeolitic protons by the fraction of tin which did not interact with platinum and therefore, was not converted to Sn◦ during the reduction step. Therefore, it cannot be ruled out that the decreased acidity can contribute to the minor C1 –C3 formation over Pt,Sn catalysts. On the basis of data reported in this paper we believe, however that the ensemble effect overcomes that of acidity change in directing selectivity towards reaction products. In fact, whereas the effect of Sn on the acidity is substantially moderate and almost independent from Sn content, product selectivities undergo drastic variations, which are moreover strongly dependent on Sn content. A further confirm to this hypothesis derives from the fact that over Pt,Sn the formation of saturated C1 –C3 hydrocarbons (due to hydrogenolysis) is preferentially suppressed with respect to that of unsaturated products (due to protolytic cracking). Nevertheless, it must be noted that the trend of
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deactivation observed fits well with that of catalysts acidity. This leads to the suggestion that the improved resistance to the deactivation of Pt,Sn catalysts, compared to that of the corresponding monometallic Pt sample, should be related to the lower surface acidity of the system in the presence of Sn, resulting in a reduced formation of carbonaceous deposits which, as before discussed, are responsible for catalysts deactivation.
5. Conclusions On the basis of the results reported in this paper the following conclusions can be drawn concerning the one step dehydroisomerization of n-butane over H-Y zeolite supported Pt and Pt,Sn catalysts: (a) Over monometallic Pt catalysts, dispersion of Pt mainly affects the selectivity towards reaction products, larger Pt ensembles resulting in a higher degree of hydrogenolysis reactions and consequently to a lower n-butene and isobutene formation. Acidity of the zeolitic support influence both activity and selectivity of monometallic Pt samples. A more acidic support results in a more active catalyst, giving also rise to a higher amount of C1 –C3 products and to a lower catalyst stability. (b) Addition of tin has been found to significantly improve both the selectivity to isobutene and the resistance to deactivation of the catalytic system. The higher isobutene selectivity of Pt,Sn catalysts have been accounted for a dilution effect of Pt ensembles induced by the second metal which inhibits hydrogenolysis reactions, requiring several contiguous Pt sites, and enhances the selectivity for the dehydrogenation reaction that can instead proceed over smaller metal ensembles. The improved stability of Pt,Sn catalysts has been related to the lower surface acidity of the system in the presence of Sn, which
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Dehydroisomerization of n-butane over H-Y zeolite supported Pt and Pt,Sn catalysts S. Scirè∗ , G. Burgio, C. Crisafulli, S. Minicò Dipartimento di Scienze Chimiche, Università di Catania, Viale Andrea Doria 6, 95125 Catania, Italy Received in revised form 4 June 2004; accepted 11 June 2004 Available online 4 July 2004
Abstract The direct one-step conversion of n-butane to isobutene was investigated over H-Y zeolite supported platinum and platinum-tin catalysts. Catalytic results over monometallic Pt catalysts showed that dispersion of Pt mainly affects the selectivity towards reaction products, larger Pt ensembles resulting in a higher degree of cracking/hydrogenolysis reactions and consequently to a lower n-butenes and isobutene formation. Addition of tin has been found to significantly improve both the selectivity to isobutene and the resistance to deactivation of the catalytic system. On the basis of characterization results (H2 chemisorption, NH3 -TPD, FT-IR of adsorbed CO) the higher isobutene selectivity of Pt,Sn catalysts have been accounted for a dilution effect of Pt ensembles induced by the second metal which inhibits hydrogenolysis reactions, requiring several contiguous Pt sites, and enhances the selectivity for the dehydrogenation reaction that can instead proceed over smaller metal ensembles. The improved stability of Pt,Sn catalysts has been related to the lower surface acidity of the system in the presence of Sn, which results in a reduced formation of carbonaceous deposits responsible for catalysts deactivation. © 2004 Elsevier B.V. All rights reserved. Keywords: Dehydroisomerization; n-Butane; Isobutene; Zeolite; Platinum; Tin; Bimetallic
1. Introduction Isobutene, which is a key intermediate in the petrochemical industry, is currently produced from butanes in a twin reactor system technology where an isomerization step is followed by a dehydrogenation step or vice versa [1]. Recently, the direct one-step conversion of n-butane to isobutene has been reported as an interesting alternative [2–9]. For this reaction two options has been taken into consideration, the first one involves the use of a two-bed system with a dehydrogenation catalyst in combination with an isomerization catalyst [2,3]. The other approach uses a bifunctional system, usually a zeolite supported Pt catalyst [4–9], where Pt provides the dehydrogenation activity while Bronsted acid sites, responsible for isomerization, are supplied by the zeolite. In this context several Pt catalysts have been upto now explored, e.g. Pt-MOR [4], Pt-ZSM5 [5,6], Pt-ZSM11 [7], Pt-MCM22 [8], Pt-FER [6], Pt-TON
∗
Corresponding author. Tel.: +39 095 7385034; fax: +39 095 580138. E-mail address: [email protected] (S. Scir`e).
0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.06.019
[6,9], with the aim to maximize isobutene yield, minimizing undesired side reactions (cracking and oligomerization). Bimetallic catalysts have been extensively investigated in recent years due to the fact that they may exhibit better catalytic performances than corresponding monometallic samples [10–12]. The first application of a bimetallic system in the direct conversion of n-butane to isobutene has been recently reported by Derouane et al. [9]. They found that addition of copper to Pt/H-TON has a positive effect for dehydroisomerization, reducing hydrogenolysis side-reactions. Following these considerations we hereby report a study on the dehydroisomerization of n-butane over Pt,Sn catalysts supported on H-Y zeolites. Pt,Sn catalysts are, in fact, among the most studied bimetallic systems, their use being reported in several industrial processes such as naphta reforming and dehydrogenation of heavy and light paraffins [13–15]. In particular the purpose of our work was to evaluate the influence that the addition of different amounts of Sn to Pt may have on activity, selectivity and stability of the catalytic system. The effect of both SiO2 /Al2 O3 ratio of the zeolitic support and Pt dispersion on the performance of catalysts was also addressed.
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2. Experimental
Table 1 Code and physicochemical properties of supported Pt and Pt,Sn catalysts
Pt and Pt,Sn catalysts were prepared by (co)impregnation of the support with aqueous solutions of precursors (Pt = 0.5 wt.%, M/Pt atomic ratio ranging from 0.25 to 1). H2 PtCl6 ·6H2 O (Alfa Aesar), and SnCl2 ·2H2 O (Fluka) were used as precursor salts. Two H-Y zeolites (faujasites), provided by Zeolysts, with SiO2 /Al2 O3 ratios, respectively of 5 (FAU5) and 80 (FAU80) were used as support. Catalytic activity tests were carried out in a fixed bed reactor operating at atmospheric pressure and at 550 ◦ C, using 50–200 mg of catalyst (80–140 mesh) mixed with 400 mg of quartz of the same grain size. The reactant mixture was 10% n-butane, 20% H2 and 70% He. The total gas flow was varied from 15 to 60 ml min−1 (WHSV ranging from 1.2 to 18.6 h−1 for n-butane). Before catalytic tests all samples were reduced at 550 ◦ C in H2 for 1 h. Reaction products were analyzed with an online gas chromatograph equipped with FID detector. Under experimental conditions used isobutene, n-butenes, isobutane and C1 –C3 hydrocarbons were major products formed. Small amounts of butadiene and pentenes were also detected. Preliminary runs carried out at different flow-rates showed the absence of external diffusional limitations. The absence of internal diffusion limitations was verified by running experiments with crushed pellets at different grain size. H2 uptake was measured in a static system operating at room temperature. Before the measurements the samples have been reduced in H2 at 550 ◦ C for 1 h, outgassed at 550 ◦ C for 1 h and then cooled at room temperature. For IR studies the powdered samples were compressed (using a pressure of 15 × 103 bar) into thin self-supporting discs of about 25 mg cm−2 and 0.1 mm thick. The disc was placed in an IR cell which allows thermal treatments in vacuum or in a controlled atmosphere. In the cell all samples were evacuated by slowly increasing the temperature upto 550 ◦ C and then reduced in pure H2 at this temperature for 1 h. The sample was then evacuated at this temperature for 1 h and finally cooled at room temperature. CO (2 mbar) was then adsorbed at room temperature. Subsequent evacuations were then performed at room temperature. Spectra were recorded with a Perkin Elmer System 2000 FT-IR spectrophotometer with a resolution of 2 cm−1 . Data are reported as difference spectra obtained by subtracting the spectrum of the sample before the admission of the adsorbate and are normalized to the same amount of catalysts per cm2 . Temperature-programmed desorption of ammonia (NH3 -TPD) was carried out in a quartz U-shape reactor in a flow of He with a constant heating rate of 10 ◦ C min−1 . The desorbed products were detected by a quadrupole mass spectrometer (Sensorlab VG Quadrupoles). Before TPD all samples were reduced in flowing H2 for 1 h at 550 ◦ C, maintained at this temperature for 1 h in flowing He and then cooled at 30 ◦ C always in a flow of He. Temperature-programmed oxidation (TPO) of carbon was carried out in a quartz U-shape reactor in flowing pure O2
Code
Support
Sn/Pt ratio
H/Pt ratio
NH3 adsorbed (mmol g−1 )a
Pt5 Pt5LD 0.25SnPt5 0.5SnPt5 1.0SnPt5 Pt80
FAU5 FAU5 FAU5 FAU5 FAU5 FAU80
0 0 0.25 0.5 1.0 0
0.60 0.20 0.50 0.40 0.10 0.25
0.73 0.69 0.54 0.50 0.48 0.02
a Estimated by integrating the area of NH desorption peaks reported 3 in Fig. 3.
with a constant heating rate of 10 ◦ C min−1 . A quadrupole mass spectrometer (Sensorlab VG Quadrupoles) was used to analyze the composition of the effluent stream. The surface carbon was previously produced during deactivation tests carried out at 550 ◦ C for 8 h. Code of all samples together with support used, H/Pt ratio and total amount of NH3 adsorbed are reported in Table 1. It must be noted that the sample coded Pt5LD has been obtained by treating the Pt5 sample in air at 700 ◦ C for 2 h.
3. Results Fig. 1 reports the selectivity to the major reaction products as a function of conversion levels (obtained by varying contact times) over monometallic Pt catalysts (Pt5, Pt5LD and Pt80 samples). It must be reminded that, under experimental conditions of this work, Pt-free H-Y samples exhibited no significant n-butane conversion. Results of Fig. 1 shows that product selectivity curves have a similar trend for all three catalysts. In particular, the selectivity to n-butenes is very high at low conversion (a value of 75–95% selectivity can be extrapolated at zero conversion) decreasing continuously with increasing conversion levels. On the contrary the selectivity to C1 –C3 hydrocarbons extrapolates to a value between 5 and 20% at zero conversion undergoing a rapid and continuous growth with conversion. The initial selectivities both to isobutene and isobutane are close to zero. On increasing conversions isobutene selectivity rises moderately reaching a maximum at ca. 40% conversion then decreasing at higher contact times; isobutane becomes detectable only at ca. 30% conversion, then increases very slightly with conversion, however remaining very low (selectivity < 4%) even at high conversion. A closer inspection of data of Fig. 1 allows to point out some interesting differences in the behaviour of investigated monometallic Pt samples. In fact, by comparing selectivity curves of the two FAU5 supported Pt samples (Pt5 and Pt5LD, respectively in Fig. 1a and b), which differs with each other substantially in the dispersion of platinum (see Table 1), it can be clearly noted that the catalyst with higher Pt dispersion (Pt5 sample) produces smaller amounts of C1 –C3 hydrocarbons (the related selectivity
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Table 2 Catalytic activity data of supported Pt and Pt,Sn catalysts Code
V × 105 (mol gcat −1 s−1 )a
Activity loss (%)b
Pt5 Pt5LD 0.25SnPt5 0.5SnPt5 1.0SnPt5 Pt80
2.73 2.34 2.72 2.78 1.20 1.46
40 45 22 20 21 28
a b
Fig. 1. Selectivities to the major reaction products as a function of n-butane conversion levels over monometallic Pt samples: (a) Pt5; (b) Pt5LD; (c) Pt80. Selectivity to (䊊) isobutene; (䊏) C1 –C3 hydrocarbons; (䉱) isobutane; (×) n-butenes.
Reaction rate calculated at 550 ◦ C. After 8 h of run.
versus conversion curve is downshifted from 10 to 20%) with respect to the less disperse Pt sample (Pt5LD). Correspondingly, a sensibly higher selectivity to n-butenes is shown by the Pt5 sample. In terms of isobutene selectivities the difference between the two samples is still existing but less remarkable (peak selectivity reaches values of 27 and 23%, respectively on Pt5 and Pt5LD). From the figure it can be also seen that the product selectivity curves (Fig. 1c) of the FAU80 supported Pt sample (Pt80) are between the corresponding ones of the two FAU5 supported Pt samples, however being quite similar to those of Pt5, notwithstanding the dispersion of the Pt80 sample is much closer to that of the Pt5LD sample (Table 1). Catalytic activity data (reaction rates, expressed as moles of n-butane transformed per gram of catalyst per second, and deactivation rates as percent activity loss after 8 h of run) of supported Pt and Pt,Sn catalysts are shown in Table 2. Concerning the activity of monometallic Pt samples, it can be seen that the reaction rate of the Pt5 sample is about 20% higher than that of the Pt5LD sample. The Pt80 sample is further less active (the reaction rate is about half) than both FAU5 supported monometallic Pt samples. The observed order of stability of monometallic Pt samples is: Pt80 Pt5 > Pt5LD. With regard to the catalytic behaviour of bimetallic Pt,Sn catalysts results of Table 2 shows that addition of tin does not substantially affect the activity of FAU5 supported Pt catalysts upto a Sn/Pt ratio of 0.5. At higher Sn content (Sn/Pt = 1.0) a marked decrease in the reaction rate is instead observed. In terms of catalysts stability, results of Table 2 shows that all Pt,Sn catalysts exhibit a lower deactivation rate compared to all monometallic Pt samples, the 0.5SnPt5 sample being the most stable. It must be underlined that the difference in the stability among investigated Pt,Sn samples is indeed small. Fig. 2 reports the products selectivities, compared at the same conversion level (40%), over FAU5 supported Pt and Pt,Sn samples. An evident change in the products distribution of supported Pt samples is observed in the presence of tin. On bimetallic Pt,Sn catalysts, in fact, selectivities to isobutene and n-butenes are sensibly higher than those observed on the corresponding monometallic Pt samples (Pt5 and Pt5LD). It must be noted that the positive effect of tin towards unsaturated C4 formation is evident since
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Fig. 2. Comparison among products selectivities at the same conversion level (40%) over FAU5 supported Pt and Pt,Sn samples.
low Sn concentrations, then growing moderately with the concentration of this metal. Correspondingly, on Sn-doped catalysts the formation of C1 –C3 products is strongly reduced. It is important to underline that, within C1 –C3 hydrocarbons, saturated ones resulted to be those preferentially suppressed. NH3 -TPD profiles for Pt and Pt,Sn samples, normalized to the mass of the catalyst, are reported in Fig. 3. Total amounts of desorbed ammonia, expressed as mmol of adsorbed NH3 per gram of catalyst and estimated by integrat-
Fig. 3. NH3 -TPD profiles of FAU5 supported Pt and Pt,Sn samples.
ing the area of desorption peaks, are listed in Table 1. On all FAU5 supported Pt samples a broad desorption peak with a maximum at ca. 150 ◦ C and a shoulder at higher temperature (above 350 ◦ C) can be clearly visible. According to the literature [17] the low temperature peak can be ascribed to weak and medium strength acid sites, whereas the peak above 350 ◦ C is typical of strong acidity. It is noteworthy that dispersion of Pt does not affect substantially the acidity of the system (compare curves of Pt5 and Pt5LD samples). This is quite reasonable considering that platinum should be present almost exclusively as Pt◦ (samples are in fact pre-reduced at 550 ◦ C), thus making improbable that protons of the zeolite are replaced by Pt. This is also in accordance with the fact that Pt-free H-Y zeolites were found to show the same acidity compared to Pt/zeolite samples. It must also be reminded that the TPD-NH3 profile of the FAU80 supported Pt sample (not reported in the figure) was almost flat indicating that the amount of acid sites of this sample is very low (Table 2), accordingly with the high SiO2 /Al2 O3 ratio of the zeolite. Addition of tin leads to a moderate decrease in the total acidity of the system, both weak-medium and strong acid sites being affected by the presence of Sn. It is noteworthy that the negative influence of Sn on the acidity is evident already at low Sn/Pt ratio (Sn/Pt = 0.25), the presence of higher Sn contents being almost ineffective. FT-IR spectra after CO adsorption at room temperature (spectrum a) and following outgassing treatments (spectra b–e) on the monometallic Pt5 sample are reported in Fig. 4. After admission of CO two main bands can be observed, respectively centred at 2080 cm−1 (HF band) and 1840 cm−1
Fig. 4. FT-IR spectra of CO adsorbed on the Pt5 sample: (a) admission of 2 mbar of CO; (b) 5 s outgassing at RT; (c) 1 min outgassing at RT; (d) 5 min outgassing at RT; (e) 10 min outgassing at RT.
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Fig. 6. Correlation between catalysts deactivation (expressed as percent activity loss after 8 h of time of stream) and amount of carbonaceous deposits (estimated by TPO experiments) over FAU5 supported Pt and Pt,Sn samples.
Fig. 5. FT-IR spectra of CO adsorbed on FAU5 supported Pt and Pt,Sn catalysts (compared after 10 min outgassing at RT).
(LF band). In accordance to the literature the HF band is assigned to CO linearly adsorbed on Pt◦ , whereas the broader and weaker LF band can be ascribed to bridged CO species on Pt◦ [18,19]. The intensity of both HF and LF bands remains almost unchanged upon outgassing at room temperature. It must be also noted that the frequency of the HF band undergoes an evident downshift by RT evacuation, as a consequence of the decrease in the dipole–dipole coupling with decreasing CO coverage [18,19]. The FT-IR spectra of CO adsorbed at room temperature on Pt and Pt,Sn samples after 10 min outgassing at RT are compared in Fig. 5. It can be seen that in the case of 0.25SnPt5 and 0.5SnPt5 samples the presence of Sn results in the almost complete suppression of the LF band, whereas the HF band is affected only to a small extent, being moderately less intense and very slightly downshifted (<5 cm−1 ) with respect to the monometallic Pt sample. This behaviour can be rationalized on the basis of a dilution of Pt atoms by Sn on the catalyst surface layer [20,21]. In fact an ensemble effect should account for the preferential suppression of bridged CO species which requires the presence of adjacent Pt atoms. The concomitant occurrence of a ligand effect induced by Sn cannot be completely ruled out but appears not consistent with FT-IR data. In fact, an electronic Pt–Sn interaction should affect mainly the frequency of the HF band [22], which is instead almost unchanged. Fig. 5 shows that, together with the disappearance of the LF band, also a strong reduction in the intensity of the HF band can be observed in the case of the sample with the highest Sn/Pt ratio (1.0SnPt5). This seems to indicate that large amounts of tin suppress almost completely CO adsorption, probably
as a result of a very high extent of coverage of surface Pt by Sn atoms. This fits with the very low H2 uptake and reaction rate shown by the 1.0SnPt5 sample. Fig. 6 displays deactivation rates versus amounts of carbonaceous deposits (estimated by TPO experiments) over Pt and Pt,Sn samples. It must be reminded that the surface carbon was produced during deactivation tests carried out at 550 ◦ C for 8 h. From the figure it can be clearly observed that a good correlation between deactivation and coke formation exists, the higher the amount of carbonaceous products formed the higher the deactivation rate of the catalysts. This suggests, in accordance with the literature [5,9] that the decrease in the activity of Pt and Pt,Sn catalysts with time on stream is essentially related to coke deposition on the catalytic surface.
4. Discussion The curves of products selectivity as a function of conversion in the dehydroisomerization of n-butane over H-Y supported Pt catalysts (Fig. 1) are consistent with the reaction mechanism previously reported in the literature for a Pt/H-ZSM5 catalyst [5]. On the basis of this mechanism, dehydrogenation of n-butane to n-butenes is the primary reaction step which occurs on Pt metal sites. n-Butenes, which are more reactive than n-butane, are successively isomerized to isobutenes or oligomerized/cracked to form other by-products (mainly C1 –C3 hydrocarbons) over acid sites of the zeolite. Hydrogenolysis (over Pt sites) and protolytic cracking (over support acid sites) of n-butane to C1 –C3 hydrocarbons are also primary parallel reactions competing with dehydrogenation. In particular cracking leads to olefins (or mixture of olefins and alkanes), whereas hydrogenolysis
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gives alkanes exclusively. Finally the formation of isobutane presumably occurs as a consequence of the rehydrogenation of isobutene over Pt sites [5]. Catalytic data (Fig. 1 and Table 2) points out that Pt dispersion plays a certain role in directing the catalytic behaviour of monometallic Pt samples, affecting both activity and selectivity towards reaction products. In particular it was found that larger Pt ensembles lead to a higher C1 –C3 hydrocarbons formation with consequent lower selectivities to n-butenes and isobutene. This behaviour is reasonable considering that it is generally accepted that hydrogenolysis is a structure sensitive reaction, being favoured by larger Pt ensembles [9,16]. With concern to reaction rates (in terms of moles of n-butane transformed per gram of catalyst per second) it was observed that they depend on Pt dispersion. It is worth noting, however that to a three times drop in the Pt dispersion corresponded just to a moderate decrease (20%) of the reaction rate. This could be rationalized, considering the fact that when dispersion decreases the lower number of active Pt sites exposed on the catalytic surface is partially counterbalanced by the higher activity of these larger sites towards hydrogenolysis reactions [16]. Activity and selectivity of H-Y supported monometallic Pt samples were also influenced by the acidity of the support. A lower SiO2 /Al2 O3 ratio of the zeolite (i.e. a higher number of acid sites) resulted in a more active catalyst, however giving rise to a higher amount of C1 –C3 products (Fig. 1) and to a higher deactivation rate (Table 2). This behaviour is in accordance with the reaction mechanism above discussed, a higher acidity resulting in an improved isomerization efficiency, however accompanied by a larger extent of cracking reactions. The lower stability of the Pt samples supported on the more acidic zeolite (FAU5) is accounted for the fact that a higher acidity also favours oligomerization reactions which leads to carbonaceous residues, considered responsible of catalyst deactivation [5,9]. Effectively, this last statement is confirmed by the results reported in Fig. 6 which shows the existence of a good correlation between deactivation rate of catalysts and the amount of coke deposited on the catalyst surface during the dehydroisomerization reaction. Addition of tin to platinum upto a Sn/Pt ratio of 0.5 does not substantially affect the catalytic activity, whereas a higher Sn content (Sn/Pt = 1.0) caused a marked decrease in the reaction rate (Table 2). On the contrary an evident change in the products distribution of FAU5 supported Pt samples was determined by the presence of tin, since the lowest Sn concentrations (Fig. 2). In particular on Pt,Sn samples the selectivity to isobutene and n-butenes is sensibly higher compared to the monometallic Pt sample, while the formation to C1 –C3 products is strongly reduced. A similar result has been observed by addition of copper to Pt/H-TON catalysts [9]. It is noteworthy that isobutene yield obtained on the most selective samples (0.5SnPt5 and 1.0SnPt5) is about 14%, value which is sensibly higher than that of monometallic Pt samples (10% maximum). From a practical point of
view this value can be considered quite satisfactory, taking into account that the thermodynamic limit under experimental conditions of this work is about 20%. In order to explain how tin affects the performances of H-Y supported Pt catalysts let us consider characterization experiments (H2 chemisorption, FT-IR of adsorbed CO and NH3 -TPD) carried out on Pt,Sn/FAU5 catalysts. H2 chemisorption data showed that addition of tin to Pt/FAU5 samples causes a remarkable decrease in the total uptake of H2 , this effect being dramatic at high Sn/Pt ratio. This result, which agrees with the observed intensities of the FT-IR bands of adsorbed CO over Pt,Sn samples (Fig. 5), suggests that a lower number of active Pt sites is exposed on the catalytic surface of bimetallic Pt,Sn systems. On the 1.0SnPt5 sample the amount of Pt available on the catalytic surface is dramatically suppressed, thus accounting for the very low reaction rate observed on this sample. FT-IR spectra of adsorbed CO pointed out that tin results in a dilution of surface Pt clusters. A dilution of Pt caused by the addition of a second metal (Sn, Pb, Cu) has been frequently reported in the literature [11]. On the basis of the above reported results it seems possible to suggest that bimetallic Pt-Sn particles are formed having a surface mainly covered by Sn. This agrees with the higher volatility of Sn with respect to Pt, which would favour a segregation of tin on the surface. A similar enrichment of the catalyst surface in Ni has been reported by Rinkowski et al. in the case of Pt,Ni bimetallic systems [23]. Following these considerations we suggest that the improved isobutene selectivities of Pt,Sn catalysts in the n-butane dehydroisomerization can been essentially related to a dilution effect of Pt ensembles induced by the second metal, leading to a strongly reduced probability that adjacent Pt atoms are present on the catalyst surface. This should inhibit preferentially hydrogenolysis reactions, which requires several contiguous Pt sites, consequently enhancing the selectivity for the dehydrogenation reaction that can proceed over smaller metal ensembles [4]. On the other hand it must be also noted that addition of tin was found to cause a slight decrease in the acidity of the system (Fig. 3 and Table 2). This could be ascribed to a partial exchange of zeolitic protons by the fraction of tin which did not interact with platinum and therefore, was not converted to Sn◦ during the reduction step. Therefore, it cannot be ruled out that the decreased acidity can contribute to the minor C1 –C3 formation over Pt,Sn catalysts. On the basis of data reported in this paper we believe, however that the ensemble effect overcomes that of acidity change in directing selectivity towards reaction products. In fact, whereas the effect of Sn on the acidity is substantially moderate and almost independent from Sn content, product selectivities undergo drastic variations, which are moreover strongly dependent on Sn content. A further confirm to this hypothesis derives from the fact that over Pt,Sn the formation of saturated C1 –C3 hydrocarbons (due to hydrogenolysis) is preferentially suppressed with respect to that of unsaturated products (due to protolytic cracking). Nevertheless, it must be noted that the trend of
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deactivation observed fits well with that of catalysts acidity. This leads to the suggestion that the improved resistance to the deactivation of Pt,Sn catalysts, compared to that of the corresponding monometallic Pt sample, should be related to the lower surface acidity of the system in the presence of Sn, resulting in a reduced formation of carbonaceous deposits which, as before discussed, are responsible for catalysts deactivation.
5. Conclusions On the basis of the results reported in this paper the following conclusions can be drawn concerning the one step dehydroisomerization of n-butane over H-Y zeolite supported Pt and Pt,Sn catalysts: (a) Over monometallic Pt catalysts, dispersion of Pt mainly affects the selectivity towards reaction products, larger Pt ensembles resulting in a higher degree of hydrogenolysis reactions and consequently to a lower n-butene and isobutene formation. Acidity of the zeolitic support influence both activity and selectivity of monometallic Pt samples. A more acidic support results in a more active catalyst, giving also rise to a higher amount of C1 –C3 products and to a lower catalyst stability. (b) Addition of tin has been found to significantly improve both the selectivity to isobutene and the resistance to deactivation of the catalytic system. The higher isobutene selectivity of Pt,Sn catalysts have been accounted for a dilution effect of Pt ensembles induced by the second metal which inhibits hydrogenolysis reactions, requiring several contiguous Pt sites, and enhances the selectivity for the dehydrogenation reaction that can instead proceed over smaller metal ensembles. The improved stability of Pt,Sn catalysts has been related to the lower surface acidity of the system in the presence of Sn, which
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