Applied Catalysis A: General 245 (2003) 119–135

Hydroisomerisation of n-hexane over bimetallic bifunctional silicoaluminophosphate based molecular sieves I. Eswaramoorthi a,∗ , N. Lingappan b a

b

Department of Chemistry, College of Engineering, Anna University, Chennai, India Science and Humanities Division, Madras Institute of Technology, Anna University, Chennai, India

Received 29 September 2002; received in revised form 30 November 2002; accepted 3 December 2002

Abstract Silicoaluminophosphate molecular sieves SAPO-5 and SAPO-11 were synthesised hydrothermally and their purity and structure were checked by X-ray diffraction (XRD). The calcined SAPOs were impregnated with 0.2 wt.% Pt and varying amount (0, 0.2, 0.4 and 0.6 wt.%) of Ni and reduced in hydrogen atmosphere. The TEM analysis shows that the average particle size of the metal particles increases with increase of Ni loading. Acidity by TPD of ammonia and pyridine adsorbed FT-IR spectra studies show that the increasing addition of Ni decreases the total acidity of the catalysts. The Ni-Pt/SAPO-5 and Ni-Pt/SAPO-11 catalysts were subjected to n-hexane hydroisomerisation in hydrogen atmosphere and found that Ni addition up to 0.4 wt.% over 0.2 wt.% Pt loaded SAPO-5 and SAPO-11 increases the n-hexane conversion, isomerisation selectivity, dibranched isomers selectivity and sustainability of the catalyst. Further, Ni addition leads to a fall in activity of the catalysts. The activity of 0.2 wt.% Pt and 0.4 wt.% Ni/SAPOs in n-hexane hydroisomerisation was compared with 0.2 wt.% Pt and 0.4 wt.% Pd/SAPOs and found that Pt-Pd/SAPOs show higher activity and isomerisation selectivity than Pt-Ni/SAPOs. Further, SAPO-5 based catalysts always show higher activity with lesser isomerisation selectivity than SAPO-11 based catalysts. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydroisomerisation; n-Hexane; SAPO-5; SAPO-11; Platinum; Palladium; Nickel

1. Introduction High environmental standards have eliminated the usage of lead based additives for boosting the octane number in gasoline. An interesting alternative to increase the contribution of high octane branched paraffins in the gasoline pool is hydroisomerisation of the corresponding n-paraffins. At present, isomerisation of light naphtha components (n-C6 /n-C7 ) is successfully carried out in the presence of hydrogen and a ∗ Corresponding author. Tel.: +91-44-2351126; fax: +91-44-2200660. E-mail address: [email protected] (I. Eswaramoorthi).

bifunctional catalyst, typically formed by Pt supported on an acidic carrier such as halogen treated alumina or zeolites. The former catalyst is used at lower reaction temperature, affording higher yields of branched paraffins, but with the disadvantage of being highly corrosive and highly sensitive to feed impurities such as water and sulphur. Noble metals supported on zeolites such as MOR [1], ␤ [2], Y [3] and mazzite [4] have been considered as new generation commercial catalysts for C6 /C7 paraffin isomerisation. According to classical bifunctional mechanism of Weisz and Prater [5], the isomerisation of alkanes occurs in three consecutive steps viz. dehydrogenation–isomerisation–hydrogenation, in which the isomerisation of

0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0926-860X(02)00637-3

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olefin is the rate determining step. Different mechanism has been proposed over different isomerisation catalysts for n-alkane hydroisomerisation [6–9]. Generally, enhanced activity and selectivity in isomerisation reactions are observed over bimetallic bifunctional catalysts than monometallic bifunctional catalysts due to higher metal dispersion as well as stability of the former than the latter. An enhanced activity and selectivity in n-heptane isomerisation were observed by the introduction of small amount of Pd to Pt/H-␤ and Pt/USY [10]. Further, it is observed that the introduction of Pd not only increases the metal dispersion, activity, selectivity and stability of the catalyst, but also suppresses the undesired hydrogenolysis and cracking activity. Similarly, Jao et al. [11] used Pt (0.26 wt.%)/H-MOR loaded with various amount (0, 0.2, 0.5 and 1.5 wt.%) of Ni for n-hexane and n-heptane hydroisomerisation and observed an enhanced conversion and dibranched isomers selectivity with suppression of fuel gas formation over Ni (0.5 wt.%)-Pt (0.26 wt.%)/H-MOR than Pt (0.26 wt.%)/H-MOR. Increasing the Ni addition to 1.5 wt.% led to a fall in conversion and dibranched isomers selectivity. The enhanced activity of Ni-Pt/HMOR is attributed to the difference in electronic properties of the Pt particles in Ni-Pt catalyst from those of the Pt catalyst. Also, Ni-Pt/Y-zeolite found to be more active and stable in hydrogenation of acetophenone compared to Ni/Y-zeolite due to strong synergistic effect of Pt in Ni-Pt bimetallic catalyst [12]. Jordao et al. [13] studied n-hexane isomerisation over 1 or 2 wt.% Ni and Pt in different proportions supported on HUSY and found that the bimetallic Ni-Pt catalysts containing 20–30% Pt show higher activity and selectivity for high octane dibranched isomers than Pt/HUSY. Similarly, Ni-Mo/H-Y-zeolite for hydroisomerisation and hydrocracking of n-heptane was used by Vazquez et al. [14]. They observed that the Ni/(Ni + Mo) atomic ratio of the catalysts had a strong influence on activity. The maximum activity was found over catalyst with Ni/Ni + Mo ratio ∼0.5. Lugstein et al. [15] compared Ni impregnated H-ZSM-5 and Co-HZSM-5 for n-heptane hydroconversion and found significantly higher activity on Ni-HZSM-5. Further they observed that catalyst with Ni/Al ratio of 0.3 showed significantly higher activity. Pt and Pd supported silicoaluminophosphates are found to be highly active catalysts for hydroiso-

merisation of long chain alkanes. Parlitz et al. [16] carried out n-heptane isomerisation over 0.1 wt.% Pd loaded various SAPOs and found that the catalytic behaviour of catalysts not only is a function of the number and acidic strength of their active sites, but also influenced by the size of pore apertures and the location of catalytically active hydroxyl groups. They further reported that higher activity and selectivity for branched heptane isomers are achieved with SAPO-11 and SAPO-31 than SAPO-17 and SAPO-5 and accounted that the smallest pore apertures are responsible for the poor activity of SAPO-17. Also, Meriaudeau et al. [17] compared n-octane hydroisomerisation over Pt dispersed SAPO-11, SAPO-31 and SAPO-41 and found higher isomerisation selectivity on Pt-SAPO-11, Pt-SAPO-31 and Pt-SAPO-41, while preferential hydrocracking on large pore Pt-SAPO-5. Further they accounted the differences in isomerisation products selectivities in terms of diffusional restriction and steric constraints. Similarly, Sinha and Sivasanker [18] compared n-hexane hydroisomerisation over Pt impregnated SAPO-11 and SAPO-31 and found that SAPO-31 was more active than SAPO-11. They observed large amount of dibranched isomers over SAPO-11 than SAPO-31 and accounted in terms of small pores of the latter. Further, increasing the Pt content over both the catalysts increase the n-hexane conversion and isomerisation/cracking ratio (I/C) until a maximum in I/C was reached at about 0.5 wt.% Pt. The initial increase in the I/C ratio with increasing Pt content was due to greater availability of Pt sites in the vicinity of acid sites enabling rapid hydrogenation of carbenium ions and desorbing them as alkanes before they undergo cracking. Campelo et al. [19] studied n-heptane hydroisomerisation and hydrocracking on 0.5 wt.% Pt impregnated SAPO-5 and SAPO-11 and reported that Pt/SAPO-5 acts as an excellent n-heptane hydroisomerisation catalyst at lower temperature, while at higher temperature, no isomerisation occurs. On the other hand, Pt/SAPO-11 behaves as an excellent isomerisation catalyst over the temperature ranges 300–450 ◦ C. Further they proposed protonated cyclopropane intermediate mechanism to the branching of the olefinic intermediate. The effect of diffusion coefficients of heptane isomers on reactivity and product distribution of heptane isomerisation was reported by Hochtl et al. [20] from their study of hydroisomerisation

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of heptane isomers over 1 wt.% Pd impregnated SAPO-5 and SAPO-11. The n-heptane conversion was found to increase until a Pd/acid ratio of 0.02–0.03 is reached. An increase in concentration of acid sites results in an enhanced activity and isomerisation selectivity. In our previous study [21], n-hexane hydroisomerisation was carried out over 0.1 wt.% Pt and varying amounts (0, 0.1, 0.3 and 0.5 wt.%) of Ni supported on H-␤ and H-mordenite catalysts. It was observed that the addition of 0.3 and 0.1 wt.% Ni over 0.1 wt.% Pt loaded ␤ and mordenite catalysts, respectively, enhance the conversion, DMBs selectivity and sustainability of the catalyst. Further addition of Ni led to a decreasing trend in activity, DMBs selectivity and stability and enhances the undesired cracking. The improved catalytic activity by the Ni addition was accounted in terms of better metal–acid synergism between the catalytically active bimetallic (Ni-Pt) particles of nanometre scale formed and acidic sites of the supports. Further it was observed that catalysts with larger average particle size showed lesser activity may due to the blockage of pores of zeolites. Silicoaluminophosphate based bimetallic bifunctional catalysts for hydroisomerisation of C6 /C7 alkanes are not widely studied. Hence in the present study, Ni and Pd were introduced to modify the catalytic behaviour of Pt containing SAPO-5 and SAPO-11 molecular sieves and to study the hydroisomerisation of n-hexane. Reports on alloy formation between Ni and Pt along with a common fcc lattice structure suggests that there is a potential for bimetallic interactions between the two metals [22]. Hence, the purpose of the present study involves: (i) comparison of SAPO-5 and SAPO-11 as acidic supports (with different crystal structures, pore size arrangements, number and strength of acid sites) impregnated with Pt, Ni and Pt, Pd; (ii) to study the effect of Ni, Pd addition to Pt on the conversion, isomers selectivity of n-hexane hydroisomerisation at different reaction temperatures. 2. Experimental 2.1. Catalyst preparation Silicoaluminophosphate molecular sieves SAPO-5 (AFI) and SAPO-11 (AEL) were hydrothermally

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synthesised according to the previously reported procedure [23]. Aluminium isopropoxide (Merck), orthophosphoric acid (Ranbaxy) and tetraethylorthosilicate (Acros) were used as sources of Al, P and Si, respectively. Triethylamine (Acros) for SAPO-5 and dipropylamine (Acros) for SAPO-11 were employed as template molecules. At the end of crystallisation, the products were filtered, washed and dried at 120 ◦ C for 12 h. The calcination was carried out at 550 ◦ C for 6 h to remove the template molecules. The calcined and dried SAPO-5 was loaded with 0.1 and 0.2 wt.% Pt by incipient wetness impregnation (IWI) and the resulting catalysts are designated as A1 and A2 , respectively. Similarly, catalysts B1 and B2 were obtained by impregnating 0.1 and 0.2 wt.% Pt over SAPO-11. Parts of the catalyst A2 was impregnated with 0.2, 0.4 and 0.6 wt.% Ni by IWI and are designated as A3 , A4 and A5 , respectively. Catalyst B2 after impregnation with 0.2, 0.4 and 0.6 wt.% Ni resulted in catalysts B3 , B4 and B5 , respectively. For comparison purposes, 0.2 wt.% Pt and 0.4 wt.% Pd impregnated over SAPO-5 and SAPO-11, respectively, and are designated as A6 and B6 . Catalyst A7 was obtained by impregnating 0.6 wt.% Ni over SAPO-5 in which the Ni content is equal to the Ni and Pt content of A4 . Similarly, SAPO-11 impregnated with 0.6 wt.% Ni resulted in catalyst B7 . Metal loadings were done by using aqueous solutions of chloroplatinic acid (Sisco Research Laboratory) (2×10−4 g Pt/ml), nickel nitrate (Central Drug House) (5 × 10−4 g Ni/ml) and tetrammine palladium chloride (Aldrich) (2 × 10−4 g Pd/ml). The metal loaded catalysts were dried at 120 ◦ C. 2.2. Characterisation The purity of hydrothermally synthesised SAPO-5 and SAPO-11 samples was analysed using Rigaku X-ray diffractometer with Ni filtered Cu K␣ radiation (λ = 1.54 Å) in the scan range of 2θ between 5 and 60◦ . Transmission electron microscopy measurements for catalysts A4 , A5 , B4 and B5 , which were reduced at 400 ◦ C for 6–7 h were carried out in JEOL 200 kV electron microscope operating at 200 kV. Catalyst sample powders were dispersed on to “holy carbon” coated grids, which were then introduced to the microscope column, which was evacuated to less than 1 × 10−6 Torr. Specimens were enlarged using thin photographic paper. The size of metal particles

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visible in the photograph is measured manually and averaged. The state of metals in the reduced catalyst A4 , A5 , B4 and B5 were determined by ESCA. The ESCA spectras were acquired with a surface analysis system (ESCALAB-MK11, VG Scientific) by using the Mg K␣ radiation (1253.6 eV) with pass energy of 50 eV. All the catalyst samples were insulators with a very small amount of carbonaceous impurity on their surfaces. The charging effect was corrected by setting the C 1s transition at 284.6 eV. The catalyst powders were placed in a container and was mounted on a sample probe. Calcination and reduction were carried out in the catalyst preparation chamber. So, the catalysts could be moved to the analysis chamber without exposure to air. During the spectral acquisition, the pressure of the analysis chamber was maintained at better than 1 × 10−7 Torr. The total acidity of catalysts was measured by TPD of ammonia. Adsorption of ammonia was carried out on each of the samples in a quartz tube packed with 0.5 g of the catalyst. The initial flushing out was carried out with dry nitrogen for 3 h followed by reduction in flowing hydrogen (30 ml/(min g)) for 6–7 h at 400 ◦ C. Then the system was evacuated to 5 × 10−5 Torr at 550 ◦ C for 5 h and cooled to room temperature. Ammonia adsorption was carried out by passing the ammonia vapours over the catalyst bed. After adsorption, the system was evacuated to remove the physisorbed ammonia and again ammonia was passed. The adsorption and evacuation processes were repeated for five times for saturation of adsorption. The extent of ammonia adsorbed over each catalyst was measured by TGA in a TA3000 Mettler system. Nitrogen as purge gas passed during the desorption of ammonia. The TGA study was conducted at a heating rate of 10 ◦ C/min up to 650 ◦ C. Similarly, instead of ammonia, pyridine vapour was passed over the catalysts A2 , A4 and A5 of A series and B2 , B4 and B5 of B series. Then the pyridine-adsorbed catalyst was mixed with KBr (Merck) (1:3), grinded and made into thin wafers (pellet) by applying pressure 7 × 103 kg/cm2 . The thin wafer was placed in the FT-IR cell and recorded the spectrum in absorbance mode in a Nicolet (AVATAR) spectrometer. Surface area measurements were carried out in Sorptomatic 1990 CE instrument following the BET procedure using N2 as adsorbent at liquid nitrogen temperature.

2.3. Catalytic studies The catalytic reactions were carried out in a fixed bed continuous down flow quartz reactor. 1.5 g of catalyst sample was packed in the reactor and placed in a tubular furnace. The reactor system was flushed with dry nitrogen for 3 h and then metals were reduced at 400 ◦ C under flowing hydrogen (30 ml/(min g)) for 6–7 h. After the reduction process, temperature was lowered to the reaction temperature. The reactant n-hexane was fed into the reactor by a syringe pump at a predetermined flow rate. The LHSV of n-hexane in all the catalytic runs was kept at 1.00 h−1 . Pure hydrogen gas (20 ml/(min g)) was mixed with the feed and the reaction was carried out over each catalyst in the temperature range 225–375 ◦ C in steps of 50 ◦ C. The products were collected for a time interval of 1 h at ice cold condition and were analysed by gas chromatography (HP 5890) equipped with FID. The identification of the products was done by GC–MS (SHIMADZU QP5000). The material balance calculation shows that more than 95% of the feed recovered as products.

3. Results and discussion 3.1. Characterisation 3.1.1. X-ray diffraction (XRD) The powder XRD patterns of calcined SAPO-5 and SAPO-11 are shown in Fig. 1. The position and intensity of XRD patterns of both SAPO-5 and SAPO-11 are found to be in good agreement with the data given in the literature for AFI and AEL structure [24,25]. No additional peaks in any of the XRD patterns were observed indicating that the samples are almost free of impurities. The high intensity of the XRD lines and the low background in the XRD patterns observed indicate high crystallinity of the samples. 3.1.2. TEM analysis The TEM picture of catalysts A4 , A5 , B4 and B5 are shown in Fig. 2a–d, respectively. The black dots appearing on the support matrix are assumed as bimetallic particles consisting of Pt and Ni. Jao et al. [11] reported from the single peak of TPR spectra of Ni (0.5 wt.%)-Pt (0.26 wt.%)/H-MOR that a decrease of

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Fig. 1. XRD patterns of SAPO-5 and SAPO-11.

Ni reduction temperature with increasing Pt concentration and presumed a catalytic reduction of Ni due to mobile platinum oxide particles colliding into each other by thermal migration and the nickel oxide particles are catalytically reduced by pre-reduced Pt particles. The average size of the particles on each support is determined and is presented in Table 1. The average particle sizes of catalysts A4 and B4 are 6.35 and 6.20 nm, respectively. On addition of 0.2 wt.% Ni, the average particle size of the catalysts (A5 and B5 ) is

increased to 11.38 and 10.42 nm, respectively. Such bimetallic particles may still not be smaller in size compared to the pores of acidic supports (Pore size of SAPO-5 and SAPO-11 is 0.8 nm (12MR) and 0.39 × 0.63 nm (10MR), respectively). These particles may have a chance for mobility during reduction and hence may be located outside the pores of SAPO’s similar to the report by Canizares et al. [26] in Ni/H-MOR catalysts. Sachtler and co-workers [27] analysed the TEM pictures of 0.9 and 5 wt.% Pt containing MOR and

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Fig. 2. TEM pictures of catalysts: (a) A4 ; (b) A5 ; (c) B4 ; (d) B5 .

found that the average Pt particle size increases with increasing Pt content as well as increasing reduction temperature. They further reported that the catalysts reduced at higher temperature (500 ◦ C) shows twice the activity in n-heptane isomerisation than catalysts reduced at the lower temperature (350 ◦ C) though the former catalyst having a distinctly lower Pt dispersion. Malyala et al. [12] measured the average particle size of 10 and 20 wt.% Ni containing H-Y by TEM and found that segregates of Ni particles of different size and shapes are formed leading to lower degree of dispersion and highly disordered nature of the catalyst. Further they observed from the TPD and XRD

measurements that the average particle size decreased with increase in metal dispersion and accounted in terms of increase in exposed Ni atoms on the surface. The increasing average particle size with increasing Ni addition is already observed over Pt loaded H-␤ and H-mordenite catalysts in our previous study [21]. 3.1.3. ESCA The ESCA spectra of catalysts A4 , A5 , B4 and B5 are shown in Fig. 3. The Pt in the metallic state is indicated by peaks with binding energy value around 71.2 and 74.3 eV which are corresponding to the core level Pt 4f7/2 and Pt 4f5/2 transition, respectively. The peak

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Table 1 Physiochemical characteristics of A and B series catalysts Catalyst

A1 A2 A3 A4 A5 A6 A7 B1 B2 B3 B4 B5 B6 B7

Pt (wt.%)

0.1 0.2 0.2 0.2 0.2 0.2 – 0.1 0.2 0.2 0.2 0.2 0.2 –

Ni (wt.%)

– – 0.2 0.4 0.6 – 0.6 – – 0.2 0.4 0.6 – 0.6

Surface area (m2 /g)

TPD-NH3 (mmol/g) LT-peak

– 280 264 240 228 245 248 – 268 247 240 236 225 244

0.478 0.463 0.450 0.428 0.405 0.435 0.430 0.408 0.394 0.382 0.364 0.345 0.365 0.370

HT-peak

Total acidity (mmol/g)

Particle size (nm)

0.260 0.256 0.230 0.217 0.195 0.207 0.214 0.224 0.210 0.190 0.174 0.150 0.182 0.194

0.738 0.719 0.680 0.645 0.600 0.642 0.644 0.632 0.604 0.572 0.538 0.495 0.547 0.564

– – – 6.35 11.38 – – – – – 6.20 10.42 – –

LT: low temperature; HT: high temperature.

Fig. 3. ESCA spectra of catalysts A4 , A5 , B4 and B5 .

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observed at the binding energy value 852.5 eV is the indication of presence of nickel in metallic state (Ni metal: 852.3 eV; NiO: 853.3 eV; NiAl2 O4 : 857.2 eV in Phi ESCA data book). Small peak at binding energy value of 854.2 eV is observed over catalysts A5 and B5 and it may be due to the presence of unreduced Ni as NiO. Minchev et al. [28] reported that a remarkable amount of Ni remaining unreduced as NiO from their XPS studies on reduction of Ni supported Y-zeolite. The XPS study on Ni-mordenite catalysts by Narayanan [29] showed that the reduction of Ni in mordenite is difficult and multiple species of nickel are formed during the reduction. The Pt catalysed reduction of Ni2+ to Ni was concluded by Malyala et al. [12] who studied the XPS studies on Ni/Y-zeolite and Ni-Pt/Y-zeolite catalysts. Thus, in the present study,

the added Pt (0.2 wt.%) is supposed to facilitate the reduction of nickel cations upto the range of 0.4 wt.% and above which unreduced Ni2+ is observed over both A and B series catalysts. Similar observation over Ni-Pt/H-␤ and Ni-Pt/H-MOR was already reported in our previous study [21]. 3.1.4. Acidity measurements 3.1.4.1. TPD of ammonia. The information about the number and strength of acid sites can be obtained from TPD of ammonia. The TPD of ammonia was carried out over A and B series of catalysts by TGA method and the amount of ammonia desorbed are presented in Table 1. Fig. 4a and b shows the TGA curves of TPD of ammonia over the typical catalysts A4 and

Fig. 4. TPD-TGA curves of catalysts: (a) A4 ; (b) B4 .

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B4 , respectively. There are two weight losses that occurred at two different temperature ranges due to desorption of adsorbed ammonia on acid sites of the catalyst. The first weight loss occurs from both A and B series catalysts at approximately same temperature region, i.e. 170–200 and 150–180 ◦ C for A and B series, respectively. The higher temperature desorption occurs at 280–310 ◦ C for A series and 325–350 ◦ C for B series catalysts. Parlitz et al. [16] from their TPD-NH3 study of SAPO-5 and SAPO-11 reported that the low temperature (197 ◦ C) NH3 peak is generally attributed to weak (Brønsted or Lewis) acid sites, whereas desorption at higher temperatures (237–252 ◦ C) is attributed to stronger acid (Brønsted) sites, which are generated by the incorporation of silicon into the framework of the AlPO4 molecular sieve. Further Meriaudeau et al. [17] studied the TPD of NH3 for SAPO-5, SAPO-11, SAPO-31 and SAPO-41 and found a low temperature desorption peak at 180–200 ◦ C and a high temperature peak at 300–320 ◦ C corresponding to the desorption of ammonia from weak and strong acid sites, respectively. Further they observed in the case of SAPO-5, that the position of the high temperature peak maximum is approximately 30 ◦ C lower than that observed for SAPO-11, SAPO-31 and SAPO-41. They accounted for their observation in terms of the pore nature of the molecular sieves and their structure, which controls the diffusion of the desorbing ammonia molecules. Choung and Butt [30] measured the total acidity of SAPO-11 and 2 wt.% Pd impregnated SAPO-11 by TPD of NH3 and found that the higher temperature (330 ◦ C) desorption peak is missing for 2 wt.% Pd/SAPO-11 and suggested that a significant number of strong acid sites are lost due to the Pd loading. They further considered that the lower temperature peak at 170 ◦ C is due to the Lewis acid sites of the catalyst, while the two higher temperature peaks (240 and 330 ◦ C) represent weak and strong Brønsted acid sites. On the basis of the above result they concluded that Pd/SAPO-11 has significantly smaller number of Brønsted acid sites than unloaded SAPO-11. In the present study also, it is observed that the amount of ammonia desorbed and hence the total acidity are found to decrease with increasing Ni loading over both A and B series catalysts. Addition of 0.4 wt.% Ni over 0.2 wt.% Pt loaded SAPO-5 (A4 ) decreases the total acidity to 0.645 from 0.719 mmol/g and in

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the case of SAPO-11 same amount of Ni addition decreases the acidity to 0.538 from 0.604 mmol/g. But no significant change in the desorption temperature is observed by the Ni addition indicating the strength of acid sites are not affected by the addition of Ni. Also, the acid strength and number of acid sites are found to be slightly higher for SAPO-5 based catalysts (A series) than SAPO-11 based (B series) catalysts. 3.1.4.2. Pyridine adsorbed FT-IR studies. The pyridine adsorbed FT-IR spectra of A (A2 , A4 , and A5 ) and B (B2 , B4 and B5 ) series catalysts are shown in Fig. 5. A sharp peak between 1455 and 1465 cm−1 appears due to the pyridine adsorbed at Lewis acid sites and the pyridine adsorbed at Brønsted acid sites was indicated by a broad peak appears around 1550 cm−1 and a small peak at 1620 cm−1 . An intense peak around 1490–1510 cm−1 shows the physically adsorbed pyridine on the supports. The intensity of both Brønsted and Lewis acid site peaks are found to be higher for SAPO-5 than for SAPO-11 catalyst indicating the higher acidity of SAPO-5 than SAPO-11. The addition of Ni to Pt loaded SAPOs is not affecting the position of the peaks, but decreases the intensity of the peaks may be due to occupation of acid sites by the added nickel species as observed in TPD-NH3 studies. 3.2. Catalytic studies The n-hexane hydroisomerisation was carried over A and B series catalysts at LHSV = 1.00 h−1 in the temperature range 225–375 ◦ C in steps of 50 ◦ C. The products distribution over A and B series catalysts are presented in Tables 2 and 3, respectively. It is invariably found that 2-methyl pentane (2MP), 3-methyl pentane (3MP), 2,3-dimethyl butane (23DMB) and 2,2-dimethyl butane (22DMB) are the major products indicating the predominance of skeletal rearrangement in n-hexane. Smaller amounts of cyclised, cracked and aromatized products were also observed irrespective of the catalysts and conditions of the catalytic runs. The effect of temperature on n-hexane conversion over A and B series of catalysts are shown in Fig. 6a and b, respectively. It is found that the n-hexane conversion increases with increasing temperature over all the catalytic systems. The increase is rapid in the lower temperature (275 ◦ C) region and moderate at higher temperature (375 ◦ C) region. This is mainly

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Fig. 5. Pyridine adsorbed FT-IR spectra of catalysts.

Table 2 Product distribution (wt.%) and product ratios in n-hexane hydroisomerisation over A series catalysts Products

A1

A2

A3

A4

A5

A6

A7

A1

225 ◦ C 2MP 7.2 3MP 3.7 22DMB 2.1 23DMB 1 Cracked products 2.3 Conversion (wt.%) 16.3 Isomerisation selectivity (%) 85.8 MPs/DMBs 3.5 I/C 6.0

275 ◦ C 10.0 11.8 14.7 13.4 14.7 8.5 10 6.0 6.5 9.5 8.7 9.5 5.0 5.8 1.5 3.0 5.4 4.2 6.0 1.0 3.7 1.0 2.0 3.5 3.0 4.0 1.0 2.3 3.0 3.5 4.1 5.1 3.5 4.5 6.0 21.5 26.8 37.2 34.4 37.7 20.0 27.8 86.0 86.9 88.9 85.1 90.7 77.5 78.4 6.4 3.66 2.71 3.0 2.42 6.75 2.63 6.16 6.65 8.0 5.74 9.77 3.44 3.63

325 ◦ C 2MP 13.5 3MP 7.5 22DMB 5.2 23DMB 3.8 Cracked products 8.5 Conversion (wt.%) 38.5 Isomerisation selectivity (%) 77.9 MPs/DMBs 2.33 I/C 3.52

17.0 11.2 6.5 4.3 8.5 47.5 82.1 2.6 4.58

20.0 12.5 8.0 5.8 9.0 55.3 83.7 2.35 5.14

22.7 16.6 11.3 7.0 10.0 67.6 85.2 2.14 5.76

23.5 14.7 8.5 5.5 11.5 63.7 81.9 2.72 4.53

24.6 9.8 16.5 7.5 11.8 4.3 10.0 3.0 8.0 9.0 70.9 33.6 88.7 73.2 1.88 2.36 7.86 2.73

375 ◦ C 14.2 8.0 5.3 2.5 12.0 42.0 71.4 2.84 2.5

LHSV: 1 h−1 ; H2 flow rate: 20 ml/(min g); weight of catalyst: 1.5 g; time on stream: 1 h.

A2

A3

A4

A5

A6

15.0 8.5 4.3 2.5 6.0 36.3 83.4 3.4 5.0

17.0 11.0 5.7 3.8 6.5 44.0 85.2 2.9 5.76

19.5 12.7 8.0 5.3 7.2 52.7 86.3 2.42 6.31

19.3 12.0 5.0 3.2 8.0 47.5 83.1 3.81 4.93

20.3 9.0 13.0 6.5 9.3 2.0 6.4 1.5 5.5 6.5 54.5 25.5 89.9 74.5 2.12 4.42 8.9 2.92

19.0 12.8 8.2 6.0 10.5 56.5 81.4 2.23 4.38

23.0 15.0 11.2 7.8 11.0 68.0 83.8 2.0 5.18

25.5 17.0 14.0 10.0 12.0 78.5 84.7 1.77 5.54

24.2 18.2 10.5 9.0 13.2 75.1 81.9 2.17 4.68

26.2 19.0 14.2 11.0 9.0 79.4 88.6 1.79 7.8

A7

12.2 9.6 5.0 4.0 11.2 42.0 73.3 2.42 2.75

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Table 3 Product distribution (wt.%) and product ratios in n-hexane hydroisomerisation over B series catalysts Products

B1

B2

B3

B4

B5

B6

B7

225 ◦ C 2MP 3MP 22DMB 23DMB Cracked products Conversion (wt.%) Isomerisation selectivity (%) MPs/DMBs I/C

6.5 3.6 – – 1.7 11.8 85.6 – 5.9

10.2 5.3 – – 2.0 17.5 88.5 – 7.75

2MP 3MP 22DMB 23DMB Cracked products Conversion (wt.%) Isomerisation selectivity (%) MPs/DMBs I/C

325 ◦ C 12 19.5 8.5 12.5 4.5 5.0 3.5 3.5 5.0 5.0 33.5 45.5 85.0 89.0 2.5 3.76 5.7 8.1

B1

B2

B3

B4

B5

B6

B7

275 ◦ C 12.5 6.6 1.0 0.5 2.5 23.1 89.1 12.7 8.24 23 13.8 7.2 5.0 4.8 53.8 91.0 3.0 10.2

15.5 9.4 3.4 2.2 3.5 34.0 89.7 4.4 8.71 26.7 16.5 9.0 7.0 4.8 64.0 92.5 2.7 12.3

14.2 8.0 2.8 2.0 4.7 31.7 85.1 4.62 5.7 25 15.6 6.8 5.3 5.6 58.3 90.4 3.35 9.4

16 9.7 4.5 3.5 3.0 36.7 91.8 3.21 11.2 26.3 18 10.3 8.5 5.2 68.3 92.3 2.35 12.1

8.5 5.5 1.5 – 4.0 19.5 79.4 9.33 3.87

9.0 5.5 2.0 1.0 4.3 22.8 81.1 5.1 4.3

14.3 9.1 2.6 2.0 4.5 32.5 86.1 5.08 6.22

16.9 11 4.0 3.0 4.8 39.7 87.9 3.98 7.27

20.5 13 6.0 3.8 5.7 49 88.3 3.41 7.5

19.2 12.2 4.0 2.6 7.5 45.5 83.5 4.75 5.0

21 15 6.8 4.5 5.5 52.8 89.5 3.18 8.6

9.8 7.5 2.0 1.0 6.0 26.3 77.1 5.76 3.38

12.6 9.7 3.7 2.2 5.5 33.7 83.6 3.77 5.12

375 ◦ C 12.3 9.0 4.5 3.5 9.2 38.5 76.1 2.66 3.18

23.3 14.6 7.2 5.0 5.9 56.0 89.4 3.1 8.49

24.6 17.3 9.6 7.7 6.3 65.5 90.3 2.42 9.39

26.5 18 11.8 10.0 5.0 71.3 92.9 2.0 13.2

26 16.8 9.0 7.7 6.3 65.8 90.4 2.56 9.44

29.2 20.0 12 10.2 6.2 77.6 92.0 2.21 11.5

17.4 12.5 5.5 4.0 6.0 45.4 86.7 3.14 6.56

LHSV: 1 h−1 ; H2 flow rate: 20 ml/(min g); weight of catalyst: 1.5 g; time on stream: 1 h.

due to the near attainment of equilibrium for the isomerisation reaction at higher temperatures. The maximum n-hexane conversion is observed at 375 ◦ C over all the catalytic systems. The 0.1 wt.% Pt loaded catalyst A1 shows 16.3 wt.% conversion with 85.8% isomerisation selectivity when the reaction temperature is 225 ◦ C. On increasing the reaction temperature to 275, 325 and 375 ◦ C, the n-hexane conversion is increased to 27.8, 38.5 and 42.0 wt.% with corresponding isomerisation selectivity 78.4, 77.9 and 71.4%, respectively. Similarly, catalyst B1 shows the n-hexane conversions 11.8, 22.8, 33.5 and 38.5 wt.% when the reaction temperature is 225, 275, 325 and 375 ◦ C, respectively. The isomerisation selectivity is found to decrease with increasing temperature over both series of catalysts. When the reaction temperature is 225 ◦ C, the isomerisation selectivity of catalyst B1 is 85.6% and it is decreased to 76.1% when the reaction temperature is raised to 375 ◦ C. The addition of another 0.1 wt.% of Pt on both A1 and B1 leads to an enhanced conversion at all the reaction temperatures studied. Catalysts A2 and B2 show the conversions 21.5 and 17.5 wt.% at 225 ◦ C and 56.5 and 56.0 wt.% at 375 ◦ C, respectively. Pt/SAPO-5 based

catalysts (A series) always show higher conversion than Pt/SAPO-11 (B series) based catalysts at all the reaction temperatures studied. But the isomerisation selectivity of B series catalysts is found to be more than that of A series catalyst. The enhanced conversion and lower isomerisation selectivity of A series catalysts than B series catalysts are mainly due to the higher total acidity of former as observed from TPD of NH3 and pyridine adsorbed FT-IR spectra studies. The effect of Ni addition on n-hexane conversion is studied by comparing the activity of Ni-Pt and Ni free catalysts. The addition of 0.2 wt.% Ni over catalysts A2 and B2 increases the n-hexane conversion significantly at all the temperatures studied. The increase is good at lower temperature regions and the changes are marginal when the reaction temperature goes up. Here also, all Ni-Pt/SAPO-5 catalysts always show higher conversion than Ni-Pt/SAPO-11 catalysts. Catalysts A3 and B3 show the n-hexane conversions 26.8, 44.0, 55.3 and 68.0 wt.% and 23.1, 39.7, 53.8 and 65.5 wt.% at the reaction temperatures 225, 275, 325 and 375 ◦ C, respectively, which are considerably higher than the conversions observed over catalysts A2 and B2 . The isomerisation selectivity of 86.9, 85.2, 83.7 and 83.8%

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Fig. 6. n-hexane conversion over: (a) A series: (䉬) A1 ; (䊏) A2 ; (䉱) A3 ; (×) A4 ; ( ) A5 ; (䊉) A6 ; (+) A7 ; (b) B series: (䉬) B1 ; (䊏) B2 ; (䉱) B3 ; (×) B4 ; ( ) B5 ; (䊉) B6 ; (+) B7 catalysts.

and 89.1, 87.9, 91.0 and 90.3% was observed at above mentioned temperatures over catalysts A3 and B3 , respectively, which are considerably higher than that observed on A2 and B2 . The increase in n-hexane conversion and isomerisation selectivity steadily continues upto 0.4 wt.% Ni addition over both series catalysts. Maximum conversion of 78.5 and 71.3 wt.% was obtained over catalyst A4 and B4 , respectively, at 375 ◦ C. Also, the isomerisation selectivity of A4 and B4 is found to be maximum at all the temperatures studied. The isomerisation selectivities of A4 are 88.9 and 84.7% and

B4 are 89.7 and 92.9% at 225 and 375 ◦ C, respectively. Further addition of Ni over (catalysts A5 and B5 ) both series catalysts show a decreasing trend in n-hexane conversion. The decreasing trend is much pronounced in B series catalysts may be due to the small pores of SAPO-11. It is similar to the observation made by Jao et al. [11] who used 0.26 wt.% Pt and varying amount (0. 0.2, 0.5 and 1.5 wt.%) of Ni over HMOR for n-hexane and n-heptane hydroisomerisation. They observed an increasing trend in conversion and multibranched isomers selectivity with increasing Ni addition upto 0.5 wt.% and further addition (1.5 wt.%) led to a fall in conversion and multibranched isomers selectivity. Further they accounted that the increase in metallic sites/acid sites (NM /NA ) ratio by Ni addition is responsible for the observed enhanced activity. Our previous study [21] shows that the addition of 0.3 and 0.1 wt.% Ni over 0.1 wt.% Pt loaded ␤ and mordenite, respectively, enhances the activity, DMBs selectivity and sustainability of the catalysts and further addition led to a fall in activity with more cracking. Further, it is observed that the better metal–acid balance between the catalytically active bimetallic (Ni-Pt) nanoparticles formed and acid sites of the supports is responsible for the enhanced activity of the catalysts. The fall in activity due to further Ni addition was accounted in terms of blockage of pores by larger sized bimetallic (Ni-Pt) particles and unreduced nickel species. In the present case also, the maximum activity and selectivity observed over catalysts A4 and B4 is supposed to be due to the better metal–acid synergism between catalytically active bimetallic particles formed as evidenced by TEM analysis and acid sites of the support. These particles are assumed as bimetallic particles consisting of Pt and Ni. Catalysts A4 and B4 show the average particle size 6.35 and 6.20 nm, respectively, by TEM analysis. Also, the ESCA studies on catalysts A4 and B4 shows complete reduction of Pt as well as Ni, which are active in dehydrogenation–hydrogenation steps in isomerisation. Jao et al. [11] reported that the impregnated Ni increased the Pt dispersion over Pt/HMOR. Further they reported that the metallic site/acid site ratio increases with increasing Ni addition. In the present case too, the added Ni species may increase the dispersion and hence increase the number of active metallic sites, i.e. the metallic sites/acid sites ratio may increase towards the optimum value for isomerisation reactions [10]. Isomerisation

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without cracking favours when there are enough metallic sites to generate olefins for feeding all the acid sites. The decrease in n-hexane conversion by increasing Ni addition (catalysts A5 and B5 ) may due to larger sized bimetallic particles formed as evidenced by TEM analysis. Catalysts A5 and B5 show the average particles size of 11.38 and 10.42 nm, respectively, by TEM which may block the pores of both SAPO-5 and SAPO-11. The ESCA spectra of catalysts A5 and B5 show the existence of unreduced Ni as NiO, which is inactive in hydrogenation–dehydrogenation steps of hydroisomerisation reaction. It is observed from Tables 2 and 3 that the cracked products remain under control upto the best synergistic condition and increases thereafter. The initial decrease of cracked products with increasing metal content may due to the greater availability of metallic sites in the vicinity of acid sites enabling rapid hydrogenation of the carbenium ion and desorbing them as alkanes before they undergo cracking reactions as reported by Sinha and Sivasanker [18] from their study of n-hexane hydroisomerisation over Pt loaded SAPO-11 and SAPO-31, i.e. lesser probability for cracking of olefinic intermediates during the migration from one metallic site to another and that would encounter the acidic sites. The 0.2 wt.% Pt, 0.4 wt.% Pd loaded catalysts A6 and B6 show higher n-hexane conversion as well as isomerisation selectivity than that of catalysts A4 and B4 , respectively, at all the reaction temperatures studied. Maximum conversion of 79.4 and 77.6 wt.% with 88.6 and 92.0% isomerisation selectivity were observed over catalysts A6 and B6 , respectively, at 375 ◦ C. The higher activity of Pt-Pd catalysts than Pt-Ni catalysts may due to the better dehydrogenation–hydrogenation property of noble metal Pd than Ni as reported by Lugstein et al. [15] in n-heptane conversion study over NiHZSM-5. The Ni only loaded catalysts A7 and B7 show very low conversion and isomerisation selectivity at all the temperatures. The poor activity of Ni catalysts than Ni-Pt and Pt-Pd catalysts may be due to the poor hydrogenation–dehydrogenation capability of Ni than Pt and Pd. Therefore, more metal sites are necessary in Ni catalysts to reach the same catalytic behaviours of other bimetallic catalysts. The effect of temperature on the selectivity of individual hexane isomers over catalysts A4 and B4 are shown in Fig. 7a and b, respectively. Among the four

131

Fig. 7. Effect of temperature on the selectivity of individual isomers over catalysts: (a) A4 ; (b) B4 ; (䉬) 2MP; (䊏) 3MP; (䉱) 22DMB; (×) 23DMB.

isomers, the selectivity of 2MP is always found to be higher than other isomers irrespective of catalysts and reaction conditions. Comparatively, the 3MP selectivity is lower than that of 2MP, but considerably higher than that of 23DMB and 22DMB. The higher selectivity of both 2MP and 3MP and lower selectivity of 22DMB and 23DMB at all the reaction temperatures indicate that the monobranched isomers are primary reaction products of n-hexane hydroisomerisation even though the formation of 3MP is assumed to be a rapid 1,2-alkyl hydride shift in 2MP. The selectivity of 2MP and 3MP is found to decrease and

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that of 23DMB and 22DMB increase with increasing reaction temperature. The fall in selectivity is more pronounced in 2MP than in 3MP. The selectivity of MPs decreases while that of DMBs increases with increasing temperature suggesting the transformation of the former to latter. Further, the catalyst A4 shows higher selectivity towards DMBs than B4 may be due to larger pore dimensions in SAPO-5 which renders the diffusion of methyl branched isomers to be under less constraint than in the channels of SAPO-11. It is suggested that the lower selectivity in SAPO-11 towards DMBs may due to the difficult and highly constraint formation of DMB isomers as a consequence of the pore sizes, and to the facile diffusion out of the channels of the monobranched isomers. Hence, the extent of secondary reactions to multibranched isomers is considerably lowered. Similar trend in individual isomer selectivity with temperature is observed over other Ni-Pt, Pt-Pd and Ni catalysts. The effect of Ni addition on the ratios among the isomers of hexane over A and B series catalysts are derived from product distributions presented in Tables 2 and 3, respectively. It is observed that the MPs/DMBs ratios over catalyst A2 are 6.4, 2.23 and B2 are 0, 3.0 at 225 and 375 ◦ C, respectively. The thermodynamic equilibrium ratios of hexane isomers reported by Condon [31] and Chen et al. [32] are extrapolated [18] and included to measure the deviations of present experimental results. The Ni addition upto 0.4 wt.% over both catalysts shows a remarkable decrease in MPs/DMBs ratio and it decreases further by increasing reaction temperature. These values are setting an opposite trend with respect to the thermodynamic equilibrium ratio (2.35 at 225 ◦ C and 4.1 at 375 ◦ C) of Condon [31] and Chen et al. [32]. The MPs/DMBs ratios over the catalysts A4 are 2.71 and 1.77 and B4 are 4.4 and 2.0 at 225 and 375 ◦ C, respectively. The opposite trend in MPs/DMBs ratio indicates that the acidity and pore size of supports plays an important role in the inversion of the ratios. Similar deviation was already reported over Pt/SAPO-11 and Pt/SAPO-31 [18] for n-hexane isomerisation and accounted in terms of pore sizes. The increasing addition of Ni upto 0.4 wt.% over both A and B series catalysts decreases the MPs/DMBs ratio and further increase of Ni shows an increasing trend in MPs/DMBs ratio. The increasing selectivity of DMBs upto 0.4 wt.% Ni addition may

due to the better metal–acid synergism and the formation and growth of bimetallic Ni Pt particles of nanometre scale (6.35 and 6.20 nm for A4 and B4 , respectively), which are catalytically very active according to Schepers et al. [33]. Catalysts with still higher average metal particle size (A5 and B5 ) may give hindrance to the movement of bulky intermediates in the pores of SAPOs. The amount of DMBs formed over SAPO-5 based catalysts is always found to be higher than over SAPO-11 based catalysts indicating the acidity and pore structure plays an important role in isomers selectivity. SAPO-5 has one-dimensional pores with a larger diameter (0.73 nm) and SAPO-11 has mono-dimensional pore system consisting of nonintersecting elliptical 10 membered ring pores with 0.39 nm × 0.63 nm diameters. The kinetic diameter of n-hexane, methyl pentanes and dimethyl butanes are 4.8, 5.5 and 6.2 Å, respectively. Hence, more hindrance to the movement of DMBs is expected in SAPO-11 than in SAPO-5. The 2MP/3MP ratios for catalyst A4 are at 1.54 and 1.5 and for B4 are at 1.64 and 1.47 at 225 and 375 ◦ C, respectively. At lower temperatures deviations from thermodynamic equilibrium values (1.70 at 225 ◦ C and 1.45 at 375 ◦ C) are observed. On comparison, the 2MP/3MP ratios over SAPO-11 based catalysts are found to be closer to thermodynamic equilibrium ratios than that over SAPO-5 based catalysts. The coincide of 2MP/3MP ratio with thermodynamic equilibrium ratio indicate that the 1,2-alkyl hydride shift involved in the interconversion of 2MP and 3MP is extremely rapid. The 23DMB/22DMB ratio was found to increase with increasing temperature over both A and B series catalysts. The 23DMB/22DMB ratio over catalysts A4 are 0.64 and 0.71 and B4 are 0.64 and 0.84 at 225 and 375 ◦ C, respectively. These values considerably deviate from thermodynamic equilibrium ratios (0.55 at 225 ◦ C and 1 at 375 ◦ C). The increase of 23DMB/22DMB ratio indicates that there may be some inhibition to the formation of 22DMB in the transition during transformation of more stable tertiary 2,3-dimethyl butyl carbocation to a less stable secondary 2,2-dimethyl butyl carbocation [21,32]. According to Blomsma et al. [10] the 22DMB cannot be formed by the dimerisation-cracking (bimolecular) mechanism. But in the present case, considerable amount of 22DMB is observed over all

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that the 2MP/23DMB ratio decreases with increasing Ni addition upto 0.4 wt.% over both catalytic systems indicating in favour of PCP mechanism working by the addition of Ni. Favouring the formation of 23DMB upto the 0.4 wt.% Ni over both series catalysts is supposed to be due to the better metal–acid synergism offered by the catalysts. According to Weitkamp and Erdgas [36] the better metal–acid balance favours the monomolecular mechanism in which 2MP is converted into 23DMB through a protonated cyclopropane intermediate structure. It is observed from Tables 2 and 3 that the DMBs selectivity of the both A and B series catalysts are found

Fig. 8. Effect of Ni addition on 2MP/23DMB ratio over (a) A series; (b) B series catalysts at different temperatures: (䉬) 225 ◦ C; (䊏) 275 ◦ C; (䉱) 325 ◦ C; (×) 375 ◦ C.

catalytic systems at all the reaction temperatures studied. Hence, in the present case, the n-hexane isomerisation is considered to follow the mechanism of carbenium ion rearrangement and cleavage. According to comprehensive description of rules for carbenium ion rearrangement and cleavage given by Brouwer [34], carbenium ion isomerisations that lead to a change in the degree of branching occur through protonated cyclopropane ring intermediates. Hence the 2MP/23DMB ratio is an indication of rate of branching of hexane by protonated cyclopropane intermediate mechanism, which is slower than alkyl hydride shift mechanism according to Marin and Froment [35]. The effect of Ni addition on the ratios of 2MP/23DMB is studied by comparing the ratios over Ni free and Ni containing catalysts at different reaction temperatures and are shown in Fig. 8a and b for A and B series catalysts, respectively. It is found

Fig. 9. Effect of Ni addition on I/C ratio over (a) A series; (b) B series catalysts at different temperatures: (䉬) 225 ◦ C; (䊏) 275 ◦ C; (䉱) 325 ◦ C; (×) 375 ◦ C.

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to increase with increasing Ni addition upto 0.4 wt.%. Further Ni addition leads to fall in DMBs selectivity. Also the DMBs selectivity is found to increase with increasing reaction temperature over all the catalytic systems. The Pd containing catalysts A6 and B6 show very high DMBs selectivity (26.5 and 21.7% at 225 ◦ C and 31.7 and 28.6% at 375 ◦ C, respectively) among A and B series catalysts. The effect of Ni addition on isomerisation/cracking ratio of A and B series catalysts are shown in Fig. 9a and b, respectively. A good isomerisation catalyst should have very high I/C ratio. It is observed that the I/C ratio is found to increase with increasing Ni addition upto 0.4 wt.% over both series catalysts. Also, the I/C ratio is found to decrease with increasing reaction temperatures. Further Ni addition

beyond 0.4 wt.% leads to a falling trend in I/C ratio. The increasing trend in I/C ratio is supposed to be favoured by monomolecular mechanism due to better metal–acid balance offered by catalysts containing 0.2 wt.% Pt, 0.4 wt.% Ni. Generally the I/C ratio over B series catalysts is found to be more than that on A series catalysts at all temperatures due to the higher total acidity of the former than the latter, which leads to more cracking. The sustainability of the A and B series catalysts was studied by time on stream study for duration of 6 h at 375 ◦ C and are presented in Fig. 10a and b, respectively. It is observed that the n-hexane conversion decreases with time over all the catalytic systems. Pt catalysts A1 , A2 , B1 and B2 and Ni catalysts A7 and B7 show maximum fall in conversion with time. The fall in activity of the catalysts may be accounted in terms of coke formation, which occupy the acid sites of the catalysts leading to deactivation. Among Ni-Pt catalysts, catalyst A4 and B4 show the minimum fall in conversion may due to lesser formation of coke by better acid–metal balance. The Pd containing catalysts A6 and B6 show higher stability (minimum fall in activity with time) than catalysts A4 and B4 , respectively.

4. Conclusions

Fig. 10. Effect of time on stream on n-hexane conversion over (a) A series; (b) B series catalysts at 375 ◦ C.

The XRD patterns of hydrothermally synthesized SAPO-5 and SAPO-11 confirm its AFI and AEL structures, respectively. The particle size measurement by TEM shows that the increase of Ni addition increases the average particle size and brings them into nanometre scale. The ESCA studies of Ni-Pt/SAPO-5 and Ni-Pt/SAPO-11 reveal that completely reduced Ni exists in the bimetallic particle upto 0.4 wt.% Ni additions and beyond which the unreduced Ni as NiO appears. The acidity measurements of Ni-Pt catalysts by TPD of NH3 and pyridine adsorbed FT-IR studies reveal that the total acidity decreases with increasing Ni addition over both SAPO-5 and SAPO-11 based catalysts. The total acidity of SAPO-5 is found to be more than that of SAPO-11 based catalysts. Because of the best synergistic effect between bimetallic particles of nanoscale size and acidic sites, offered by the catalysts containing 0.2 wt.% Pt and 0.4 wt.% Ni, a maximum n-hexane conversion and DMBs selectivity is observed. Further, the I/C ratio and time on stream

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study show that Ni addition upto 0.4 wt.% increases the I/C ratio steadily and sustainability of the catalysts, respectively. SAPO-5 based catalysts are found to be more active and less selective than SAPO-11 based catalysts in n-hexane hydroisomerisation.

Acknowledgements The authors gratefully acknowledge the financial support from Department of Science and Technology (DST), New Delhi, India. References [1] P.J. Kuchar, J.C. Bricker, M.E. Reno, R.S. Haizmann, Fuel Proc. Technol. 35 (1993) 183. [2] L.-J. Leu, L.-Y. Hou, B.-C. Kang, C. Li, S.-T. Wu, J.-C. Wu, Appl. Catal. 69 (1991) 49. [3] G. Boskovic, R. Micic, P. Pavlovic, P. Putanov, Catal. Today 65 (2001) 123. [4] F. Fajula, M. Boulet, B. Cop, V. Rajacfanova, F. Figueras, T. Des Courieres, in: L. Guczi, F. Solymosi, P. Telenyi (Eds.), Proceedings of the 10th International Congress on Catalysis Budapest, Elsevier, Amsterdam, 1993, p. 1007. [5] P.B. Weisz, C.D. Prater, Adv. Catal. 6 (1954) 143. [6] E. Iglisia, S.L. Soled, G.M. Kramer, J. Catal. 144 (1993) 238. [7] S. Tiong Sie, Ind. Eng. Chem. Res. 31 (1992) 1881. [8] E. Blomsma, J.A. Martins, P.A. Jacobs, J. Catal. 155 (1995) 141. [9] H. Yue Chu, M.P. Rosynek, J.H. Lunsford, J. Catal. 178 (1998) 352. [10] E. Blomsma, J.A. Martins, P.A. Jacobs, J. Catal. 165 (1997) 241. [11] R.M. Jao, T.B. Lin, J.-R. Chang, J. Catal. 161 (1996) 222. [12] R.V. Malyala, C.V. Rode, M. Arai, S.G. Hegde, R.V. Chaudhari, Appl. Catal. A: Gen. 193 (2000) 71. [13] M.H. Jordao, V. Simoes, A. Montes, D. Cardoso, Stud. Surf. Sci. Catal. 130 (2000) 2387.

135

[14] M.I. Vazquez, A. Escardino, A. Corma, Ind. Eng. Chem. Res. 26 (1987) 1495. [15] A. Lugstein, A. Jentys, H. Vinek, Appl. Catal. A: Gen. 152 (1997) 241. [16] B. Parlitz, E. Schrier, H. Zubowa, R. Eckelt, C. Lieschke, G. Lieschke, R. Fricke, J. Catal. 155 (1995) 1. [17] P. Meriaudeau, V.A. Tuan, V.T. Nghiem, S.Y. Lai, L.N. Hung, C. Naccache, J. Catal. 169 (1997) 55. [18] A.K. Sinha, S. Sivasanker, Catal. Today 49 (1999) 293. [19] J.M. Campelo, F. Lafont, J.M. Marinas, J. Catal. 156 (1995) 11. [20] M. Hochtl, A. Jentys, H. Vinek, J. Catal. 190 (2000) 419. [21] I. Eswaramoorthi, N. Lingappan, Korean J. Chem. Eng. 20 (3) (in press). [22] J.M. Dominguez, A. Vazquez, A.J. Renouprez, M.J. Yacaman, Surf. Sci. 75 (1982) 101. [23] S.P. Elangovan, Ph.D. Thesis, Anna University, India, 1994. [24] M.M.J. Treacy, J.B. Higgins, Collection of Simulated XRD Powder Patterns for Zeolites, fourth ed., Elsevier, Amsterdam, 2001. [25] B. Parlitz, E. Schrier, H. Zubowa, R. Eckelt, C. Lieschke, G. Lieschke, R. Fricke, J. Catal. 155 (1995) 1. [26] P. Canizares, A. De Lucas, F. Dorado, A. Duran, I. Asencio, Appl. Catal. A: Gen. 169 (1998) 137. [27] D.J. Ostard, L. Kustov, K.R. Poeppelmeier, W.M.H. Sachtler, J. Catal. 133 (1992) 342. [28] C.H. Minchev, V. Knazirev, L. Kosova, V. Pechev, W. Grunsser, F. Schimidt, in: L.V.C. Rees (Ed.), Proceedings of the Fifth International Conference on Zeolites, Heyden, London, 1980, pp. 335. [29] S. Narayanan, Zeolite 4 (3) (1984) 231. [30] S.J. Choung, J.B. Butt, Appl. Catal. 64 (1990) 173. [31] E.F. Condon, Catalysis 6 (1954) 1. [32] J.K. Chen, A.M. Martin, Y.G. Kim, V.T. John, Ind. Eng. Chem. Res. 27 (1988) 401. [33] F.J. Schepers, B. Van, V. Ponec, J. Catal. 96 (1985) 82. [34] D.M. Brouwer, in: R. Prins, G.C.A. Schult (Eds.), Chemistry and Chemical Engineering of Catalytic Processes, Sijthof and Noordhoff, Germantown, MD, 1980. [35] G.B. Marin, G.F. Froment, Chem. Eng. Sci. 37 (5) (1982) 759. [36] J. Weitkamp, E.K. Erdgas, Petrochem 31 (1978) 13.