Microporous and Mesoporous Materials 71 (2004) 109–115 www.elsevier.com/locate/micromeso

Hydroisomerisation of C6–C7 n-alkanes over Pt loaded zirconium containing Al–MCM-41 molecular sieves I. Eswaramoorthi *, V. Sundaramurthy, N. Lingappan Department of Chemistry, College of Engineering, Anna University, S.P. Road, Chennai 600025, India Received 9 July 2003; received in revised form 8 March 2004; accepted 14 March 2004 Available online 5 May 2004

Abstract Mesoporous Al–MCM-41 and zirconium containing Al–MCM-41 (Si/Zr ratio 200, 100 and 50) molecular sieves were hydrothermally synthesised. The low angle XRD patterns confirm the mesoporous nature of the materials. The higher d-spacing values of Zr–Al–MCM-41 molecular sieves than that of Al–MCM-41 indicate the incorporation of Zr in Al–MCM-41 framework. Further, the SEM picture of Zr–Al–MCM-41 indicates the formation of aggregates without regular shape and size. Also, the nitrogen adsorption studies reflect the mesoporous nature of the materials. The DRS spectrum confirm the presence of Zr in tetrahedral coordination in the framework. The n-hexane and n-heptane hydroisomerisation reactions were carried out over 0.3 wt% Pt loaded Al– MCM-41 and Zr–Al–MCM-41 molecular sieves. Zirconium containing catalysts always show higher activity in both reactions than Al–MCM-41 due to strong Brønsted acid sites strengthened by appropriately positioned Lewis acid sites. Further, the activity of the catalysts increased with increasing Zr content. Also, the Zr containing materials show higher selectivity towards the high octane multibranched isomers in both n-hexane and n-heptane hydriosomerisation than that of Al–MCM-41.
2004 Elsevier Inc. All rights reserved. Keywords: Al–MCM-41; Zr–Al–MCM-41; Hydroisomerisation; n-Hexane; n-Heptane; Platinum

1. Introduction Hydroisomerisation and hydrocracking of linear alkanes over bifunctional catalysts are considered as an efficient method to enhance the octane number of gasoline. Noble metal supported on zeolites such as MOR, beta, Y and mazzite and SAPOs have been considered as new generation commercial isomerisation catalysts. Due to restricted pore size and strong acidity, the isomerisation selectivity is lowered significantly over zeolites. The demand for large pore acidic catalysts has triggered greater efforts in both the academicians and industrialists to synthesise mesoporous molecular sieves. In 1992, Kresge et al. [1] at Mobil Research and Development Corporation synthesised the ordered mesoporous silicate called as M41S with large channels (1.6–10 nm)

*

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2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2004.03.016

ordered in a hexagonal (MCM-41), cubic (MCM-48) and lamellar (MCM-50) arrays. The mesoporous materials are considered as an excellent material for many potential applications due to the larger size of the pores and the ease of adjusting their diameter over a wide
during the synthesis. Among the range (i.e. 16–100 A) different members of M41S family, the so called MCM41 exhibiting a hexagonal array of uniform mesopores, has been the focus of the most of the studies. The mesoporous silicate materials consist of chemically inert silicate framework. In order to induce a specific catalytic activity, researchers have tried to incorporate a variety of metals into the mesostructure by either direct synthesis/ion-exchange or impregnation. The synthesis of mesoporous catalysts with redox properties was carried out by introducing transition metals such as Ti, V, Zr and Cr in the silica and silica-alumina framework [2–4]. Among the transition metals, zirconium is considered as an important one due to the possibility of the strong polarisation of Si–Od
  Zrdþ linkages. Zirconium incorporation in the framework of mesoporous materials was initiated by Tuel and

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co-workers [5] who synthesised a series of zirconium containing mesoporous silicas with Si/Zr ratios in the range of 15–1000. Further they reported that the Zr content does not influence the crystal properties and their activity in liquid phase oxidation increases with increasing zirconium content. Jones et al. [6] reported that the IR spectra of calcined and pyridine adsorbed mesoporous zirconosilicate samples showed IR bands characteristic of Lewis acid sites only, with a linear correlation between the zirconium content and acid site density. Wang et al. [7] prepared zirconium containing MCM-41 by a near neutral route (pH ¼ 8.5) with Si/Zr molar ratios ranging from 20 to 1. All these solids have all characteristics of MCM-41. Addition of zirconium results in a linear increase of both the Lewis and Brønsted acidities. Noble metal supported MCM-41 catalysts are found to be active in transformations involving hydrocarbons such as hydroisomerisation and hydrocracking. Girgis and Tsao [8] studied the metal–acid balance impact of Pt/zeolite, Pt/silica-alumina and Pt/MCM-41, on n-hexadecane hydroisomerisation and hydrocracking and observed two different reaction pathways. Further they hypothesised the changes in pathways in terms of relative concentrations of metal–acid sites. Chaudhari et al. [9] studied n-hexane isomerisation over 0.1–0.5 wt.% Pt impregnated H–Al–MCM-41 with Si/Al ratio 86.3, 43.9, 23.3 and 14.3 and found that the impregnation of Pt increases the activity of H–Al–MCM-41 several fold. The conversion increased rapidly with increasing Pt content up to about 0.2 wt.% and lowered down thereafter. The isomerisation/cracking (I/C) ratio was found to increase with increasing Pt content and reached the maximum at 0.3 wt.% Pt loading. The decrease in I/C ratio above 0.3 wt.% Pt loading was due to the enhanced hydrogenolysis activity. Reports are available in literature on the use of noble metal supported mesoporous materials as catalysts for hydroisomerisation reaction [10,11], an important reaction in petroleum industry. Hence in the present study, it is proposed to carry out the hydroisomerisation of n-hexane and n-heptane reactions over Pt supported Zr–Al–MCM-41 molecular sieves and to study the effect of zirconium incorporation on the activity and selectivity of catalysts.

2. Experimental 2.1. Catalyst preparation-synthesis of Al–MCM-41 and Zr–Al–MCM-41 Using the molar gel composition of SiO2 :0.12CTAB: XZrO2 :0.01Al2 O3 :0.19Na2 O:35H2 O, where X ¼ 0.02– 0.005, Zr–Al–MCM-41 samples with Si/Zr ¼ 200, 100 and 50 were synthesised by hydrothermal method. Zr free Al–MCM-41 (Si/Al ¼ 50) was also synthesised by

adopting the same procedure. In a typical synthesis, 40 ml of ethyl silicate-40 (ES-40) was hydrolysed with 40 ml of water by adding 1 ml H2 SO4 . To the resulting solution 3.42 g of aluminium sulphate in 10 ml of water was added slowly followed by calculated amount of zirconyl oxychloride in water and a 33% aqueous solution containing 3.96 g of cetyl N,N,N-trimethyl ammonium bromide (CTAB). The resulting gel pH was adjusted to 10.5 by adding NaOH solution and the contents were transferred to a 300 ml stainless steel autoclave, sealed and placed in a hot air oven maintained at 175 C for 7– 9 days. At the end of crystallisation, the product was filtered, washed and dried at 120 C. The calcination was carried out at 550 C under nitrogen atmosphere for 3 h and in air for 3 h. H-form of Al–MCM-41 and Zr– Al–MCM-41 (Si/Zr ¼ 200, 100 and 50) samples were obtained by repeated ion-exchange with aqueous ammonium chloride solution followed by calcination at 550 C for 6 h. The above four samples were loaded with 0.3 wt.% Pt by incipient wetness impregnation (IWI) and the resulting materials are designated as catalysts 1–4 respectively. Aqueous solution of chloroplatinic acid (2 · 10
4 g Pt/ml) was used as source for Pt. All the metal loaded catalysts were dried at 120 C for 12 h. The metal loaded catalysts were reduced at 400 C for 6 h under hydrogen flow (30 ml/min/g). 2.2. Characterisation The low angle XRD patterns of Al–MCM-41 (Si/ Al ¼ 50) and Zr–Al–MCM-41 (Si/Zr ¼ 200, 100 and 50) samples were obtained using Rigaku X-ray diffractometer in the scan range of 2h between 1 and 10 using Cu/ Ni 40 kV/30 mA radiation. The size and morphology of Zr–Al–MCM-41 (Si/Zr ¼ 50) sample was examined by scanning electron microscopy using a JEOL 640 instrument. The sample was gold coated using an Instrumental Scientific Instruments PS-2 coating unit. The SEM picture was developed using thin photographic paper. The nature of acidic sites in the synthesised samples was analysed by pyridine adsorbed FT-IR spectroscopy. The experimental procedures and pretreatment conditions for pyridine adsorption were described in our previous report [12]. The FT-IR spectra of the samples were recorded in absorbance mode in a Nicolet (AVATAR) spectrometer using KBr pellet technique. Surface area measurements were carried out on a sorptomatic 1990 CE Instrument following the BET procedure using N2 as adsorbent at liquid nitrogen temperature. Prior to adsorption the calcined samples were degassed at 473 K for 15 h. Helium was the carrier gas and TCD was used as detector. The state of zirconium in Zr–Al–MCM-41 (Si/Zr ¼ 200, 100 and 50) samples was analysed by diffuse reflectance spectroscopy (Shimadzu, UV–VIS spectrophotometer Model 2101 PC) in the wavelength range of 200–240 nm.

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2.3. Catalytic studies Hydroisomerisation of n-hexane and n-heptane reactions were carried out at atmospheric pressure in a fixed bed continuous down flow quartz reactor. Reduced catalyst (1.5 g) was packed in the reactor and placed in a tubular furnace controlled by digital temperature controller. The reactant n-hexane or n-heptane was fed into the reactor by a syringe pump at a predetermined flow rate (LHSV ¼ 1.00 h
1 ), and pure hydrogen gas at a flow rate of 20 ml/min/g was passed along with the reactant and were preheated. The reactions were carried out at 225–375 C in steps of 50 C and the products were passed through the condenser in ice cold condition attached to the end of the reactor and were collected in a trap kept in ice. The products were analysed by a Hewlett Packard 5890A gas chromatograph equipped with FID. The identification of the products was facilitated by GC-MS (SHIMADZU QP5000).

3. Results and discussion 3.1. Characterisation 3.1.1. XRD The low angle XRD patterns of Al–MCM-41 and Zr– Al–MCM-41 with Si/Zr ratio 200, 100 and 50 (Fig. 1) exhibit a major peak at 2h ¼ 2 along with three small peaks due to 1 1 0, 2 0 0 and 2 1 0 plane reflection lines. These three reflection lines are generally indexed for the hexagonal unit cell. In the case of catalysts Al–MCM-41 and Zr–Al–MCM-41 with Si/Zr ratio 200, 100 and 50,

Fig. 1. Low angle XRD patterns of Al–MCM-41 and Zr–Al–MCM-41 molecular. (1) Al–MCM-41 (Si/Al ¼ 50); (2) Zr–Al–MCM-41 (Si/ Zr ¼ 200); (3) Zr–Al–MCM-41 (Si/Zr ¼ 100); (4) Zr–Al–MCM-41 (Si/ Zr ¼ 50).

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the d-spacing values are found to increase with increasing Zr content. Catalysts Al–MCM-41 and Zr– Al–MCM-41 with Si/Zr ratio 200, 100 and 50 are having the d-spacing value of 44.0, 45.3, 46.0 and 47.2 respectively. This observation is in agreement with the general trend that the insertion of a metal ion larger than Si brings about an increase in the unit cell parameters because of the larger M–O bond distance [13,14]. The higher values of d-spacing for Zr–Al–MCM-41 with Si/ Zr ratio 200, 100 and 50 in comparison with that of Al– MCM-41 indicates the presence of zirconium in the Al– MCM-41 framework. 3.1.2. SEM The size and morphology of Zr–Al–MCM-41 (Si/ Zr ¼ 50) was investigated by scanning electron microscopy. The SEM picture of the above said sample is presented in Fig. 2. It can be observed that the material does not have well-defined hexagonal structure. Further, aggregates without regular shape and size are observed in agreement with previous reports [7,15] for metal incorporated mesoporous materials. 3.1.3. Pyridine adsorbed FT-IR spectroscopy The FT-IR spectra of pyridine adsorbed calcined Al– MCM-41 and Zr–Al–MCM-41 with Si/Zr ratio 200, 100 and 50 are shown in Fig. 3. All the samples gave bands similar to that reported by Mokaya et al. [16] claiming the presence of both Brønsted and Lewis acid sites. The peaks at 1547 and 1653 cm
1 are associated with pyridine adsorbed on Brønsted acid sites and pyridine adsorbed on Lewis acid sites is indicated by peaks at 1445 and 1625 cm
1 . A band at 1490 cm
1 is supposed to be due to superimposition of bands of pyridine adsorbed at Lewis and Brønsted acid sites similar to the observation by Miller and Grassian [17] in Zr–MCM-41. The intensity of all the above peaks is found to increase linearly with increasing Zr content indicating that the acidity of the catalysts increases proportionally to Zr content which may be due to the increasing polarisation of Si–Od
  Zrdþ linkages in Zr–Al–MCM-41 [18]. The

Fig. 2. SEM picture of Zr–Al–MCM-41 (Si/Zr ¼ 50) molecular sieve.

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Fig. 3. Pyridine adsorbed FT-IR spectroscopy of Al–MCM-41 and Zr–Al–MCM-41 molecular sieves; (1) Al–MCM-41 (Si/Al ¼ 50); (2) Zr–Al–MCM-41 (Si/Zr ¼ 200); (3) Zr–Al–MCM-41 (Si/Zr ¼ 100); (4) Zr–Al–MCM-41 (Si/Zr ¼ 50).

Fig. 4. Diffuse reflectance spectroscopy of Zr–Al–MCM-41 molecular sieves; (2) Zr–Al–MCM-41 (Si/Zr ¼ 200); (3) Zr–Al–MCM-41 (Si/ Zr ¼ 100); (4) Zr–Al–MCM-41 (Si/Zr ¼ 50).

temperature range 225–375 C in steps of 50 C and the product distributions are presented in Tables 1 and 2 respectively. Product analysis shows that 2-methyl pentane (2MP), 3-methyl pentane (3MP), 2,3-dimethyl butane (23DMB) and 2,2-dimethyl butane (22DMB) in n-hexane hydroisomerisation and 2-methyl hexane (2MH), 3-methylhexane (3MH), 2,3-dimethyl pentane (23DMP), 2,4-dimethyl pentane (24DMP), 2,2-dimethyl pentane (22DMP) and 3,3-dimethyl pentane (33DMP) in n-heptane hydroisomerisation are the major products along with traces of probably cracked, aromatised and cyclised products. The formation of the above products indicates the skeletal rearrangement in both n-hexane and n-heptane carbon chain. The effect of temperature on n-hexane and n-heptane conversion over all the catalytic systems is presented in Fig. 5a and b respectively. It is observed that both nhexane and n-heptane conversion increases with increasing reaction temperature characteristics of a bifunctional catalysts. All the catalysts show their maximum activity in conversion at 375 C. Catalyst 1 shows the n-hexane conversion 13.5 and 25.8 wt% and that of n-heptane 10.5 and 27.0 wt% at 225 and 375 C

possible two types of linkages Si–O  Zr and Si–O. Al play an important role in acid site distribution due to their different polar nature. 3.1.4. Diffuse reflectance spectroscopy and BET surface area The diffuse reflectance spectra (Fig. 4) of Zr–Al– MCM-41 samples exhibit a single narrow band around 210 nm confirming [18] the presence of Zr(IV) in the framework of Al–MCM-41. This type of absorption in UV region may be attributed to the ligand to metal charge transfer involving isolated Zr(IV) atoms in the tetrahedral co-ordination. The BET surface area of Al–MCM-41, Zr–Al–MCM-41 from gels with Si/Zr ratio 200, 100 and 50 is found to be 958, 951, 955 and 948 m2 /g respectively, and they exhibit type IV isotherm with characteristic step at P =P0 0.38 confirming the mesoporous nature of the samples with narrow pore size distribution. 3.2. Catalytic studies Hydroisomerisation of n-hexane and n-heptane reactions were carried out over catalysts 1, 2, 3 and 4 in the

Table 1 Product distribution of n-hexane hydroisomerisation over all the catalysts at different temperatures (LHSV ¼ 1.33 h
1 H2 , flow ¼ 20 ml/min/g, time on stream ¼ 1 h) Products 2MP 3MP 22DMB 23DMB Others Conversion (wt%)

225 C

275 C

325 C

375 C

1

2

3

4

1

2

3

4

1

2

3

4

1

2

3

4

6.0 3.5 1.0 1.0 2.0 13.5

7.2 4.8 2.0 1.5 2.5 18.0

8.0 5.0 2.4 2.2 3.2 20.8

10.2 5.5 3.2 1.8 3.8 24.5

7.3 4.7 1.8 1.5 3.0 18.3

8.5 5.8 2.7 2.2 3.8 23.0

9.0 6.0 3.0 3.0 4.5 25.5

10.5 6.8 5.0 3.2 5.0 30.5

9.0 6.5 2.8 2.0 3.5 23.8

10.3 7.5 4.3 3.2 4.0 29.3

11.5 8.0 5.0 4.1 4.8 33.4

12.0 8.4 6.3 4.8 5.0 36.5

9.5 7.0 2.7 2.3 4.3 25.8

10.5 8.0 4.5 3.7 4.8 31.5

11.2 9.0 7.0 5.5 4.8 37.5

13.5 10.2 9.2 8.0 5.5 46.3

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Table 2 Product distribution of n-heptane hydroisomerisation over all the catalysts at different temperatures (LHSVs ¼ 1.33 h
1 , H2 flow ¼ 20 ml/min/g, time on stream ¼ 1 h) 225 C

Products 2MH 3MH 23DMP 24DMP 22DMP 33DMP Others Conversion (wt%)

275 C

325 C

2

3

4

1

2

3

4

1

2

3

4

1

2

3

4

5.5 2.5 – – – – 2.5 10.5

6.5 3.2 – – – – 3.3 13.0

8.2 5.8 – – – – 3.5 17.5

9.6 6.8 – – – – 4.4 20.8

7.2 5.6 – – – – 3.2 16.0

10.0 6.7 – – – – 4.0 20.7

11.3 8.2 – – – – 5.2 24.7

13.3 9.5 0.5 – – – 5.5 28.8

10.2 8.5 – – – – 4.0 22.7

12.0 8.4 0.5 – – – 4.7 25.6

13.5 9.3 1.3 – – – 5.3 29.4

16.0 11.5 2.0 1.0 – – 5.5 36.0

11.5 9.3 1.0 – – – 5.2 27.0

12.3 9.8 1.5 0.7 – – 5.7 30.0

14.0 11.5 2.3 1.2 1.0 0.5 5.8 36.3

16.2 13.8 3.8 2.2 2.0 1.0 6.2 45.2

1

2

3

4

Conversion (wt%)

60

40

20

0 200

250

(a)

300

350

400

Temperature (˚C) 60

Conversion (wt%)

1 3

(b)

2 4

40

20

0 200

375 C

1

250

300

350

400

Temperature (˚C)

Fig. 5. Effect of temperature on (a) n-hexane and (b) n-heptane conversion over catalysts 1–4.

respectively. The isomerisation in both the reactions is found to decrease marginally with increasing reaction temperature. When the reaction temperature increases, more amount of undesired cracked (sum of cracked, hydrogenolysed, aromatised and cyclised products) products are observed. The comparison of activity of Zr free and Zr containing Al–MCM-41 catalysts leads to study the effect of Zr incorporation on the conversion. It is found that the introduction of Zr in Al–MCM-41 framwork enhances the conversion as well as isomeri-

sation selectivity in both n-hexane and n-heptane significantly at all the temperatures studied. Catalyst 1 shows a maximum n-hexane conversion of 25.8 wt.% with 83.4% isomerisation selectivity at 375 C. Catalysts 2, 3 and 4 show the maximum conversions of 31.5, 37.5 and 46.3 wt.% with isomerisation selectivity 84.7, 87.2 and 88.1% respectively at 375 C. In the case of n-heptane, catalysts 2, 3 and 4 show 13.0, 17.5 and 20.8 wt% and 25.6, 29.4 and 36.0 wt% conversion at 225 and 375 C respectively which are significantly higher than that over catalyst without zirconium content (catalyst 1). The increase in catalytic activity by the incorporation of Zr is similar to the observation made by Rakshe et al. [18], who studied m-xylene isomerisation over Al-b, Zr–Al-b and found an enhanced activity and selectivity on Zr– Al-b than Al-b and concluded that Zr incorporation generates additional acidity due to strong polarisation of Si–Od
  Zrdþ linkages. Generally, isomerisation reactions require stronger Brønsted acid sites. The enhanced activity of Zr–Al–MCM-41 catalysts can be related to the presence of appropriately positioned Lewis acid center with respect to Brønsted acid sites, which may strengthen the acidity of these Brønsted acid sites. Also, zirconium itself is a Lewis acid which seems to enhance the over all Lewis acidity of Al–MCM-41 [19]. The higher isomerisation selectivity of Zr–Al–MCM-41 catalysts is correlated with the stronger Lewis as well as Brønsted acid sites than those present in Al–MCM-41. It is observed from Table 1, the selectivity of 2MP and 3MP is found to decrease and that of 22DMB and 23DMB increases with increasing reaction temperature over all the catalytic systems. Among the hexane isomers, the 2MP is found to appear predominantly relative to 3MP, 23DMB and 22DMB irrespective of the catalysts and reaction conditions. The highest selectivity of 2MP among the isomers indicates that 2MP is likely to be precursor to the other isomers as shown in our previous report [20]. Comparatively the 3MP selectivity is lower than that of 2MP, but considerably higher than that of DMBs (23DMB and 22DMB) over all the catalytic systems at all the temperatures studied. The higher selectivity of both 2MP and 3MP at all the conversion

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levels as well as temperatures indicates that both 2MP and 3MP are primary products of n-hexane isomerisation even though the formation of 3MP is assumed as due to rapid 1,2 alkyl hydride shift in 2MP. The selectivities of DMBs are found to be almost zero at lower temperatures and increases with increasing reaction temperature as well as conversion indicating that DMBs are secondary reaction products of n-hexane isomerisation. Also, when increasing the Zr content, the DMBs formation is favored indicating that strong Brønsted acid sites generated by the incorporation of Zr in the framework are responsible for it. When increasing the reaction temperature as well as the acidity of the catalysts, the DMBs formation is enhanced. In the case of heptane isomers, the selectivity of 2MH and 3MH decreases with increasing temperature as well as conversion over all the catalytic systems (Table 2). The fall in selectivity of 2MH is more than that of 3MH. Also, the selectivity of 2MH is found to be always higher than that of 3MH. The selectivity of dibranched isomers 23DMP, 24DMP, 22DMP and 33DMP increases with increasing temperature. The increasing formation of dibranched isomers with increasing temperature as well

as conversion indicates that the dibranched isomers are secondary products formed from primary monobranched isomers by protonated cyclopropane intermediate formation [21,22]. Also, catalyst 4 shows the maximum slectivity towards the high octane multibranched isomers among the catalysts studied. Fig. 6a and b respectively, show the selectivity of high octane DMBs and DMPs in hexane and heptane hydroisomerisation over all the catalytic systems at 375 C. It is clear that catalyst 4 shows the maximum selectivity towards high octane isomers in both reactions. All the above observations indicate that isomerisation follows the Weisz’s [23] classical bifunctional mechanism, in which the isomerisation proceeds through three consecutive steps viz., dehydrogenation, isomerisation and hydrogenation [24,25]. The sustainability of the catalysts was studied by carrying out time on stream study for a period of 6 h at 375 C. The conversion of n-hexane and n-heptane over catalysts 1–4 at different time is shown in Fig. 7a and b respectively. Generally, all the catalytic systems show fall in activity with time. A slightly faster deactivation rate is observed over catalyst 4, i.e. catalyst with more acidity.

50 Conversion (wt.%)

Catalyst 4 Catalyst 3 Catalyst 2 Catalyst 1

30

10

0

2

(a)

4 Time (h)

6

8

50

Conversion (wt.%)

Catalyst 4 Catalyst 3 Catalyst 2 Catalyst 1

30

10 Fig. 6. Selectivity of high octane multibranched isomers in (a) n-hexane and (b) n-heptane hydroisomerisation over catalysts 1–4; MPs––methyl pentanes; DMBs––dimethyl butanes; MHs––methyl hexanes; DMPs––dimethyl pentanes.

(b)

0

2

4 Time (h)

6

8

Fig. 7. Effect of time on stream on (a) n-hexane and (b) n-heptane conversion over catalysts 1–4 at 375 C.

I. Eswaramoorthi et al. / Microporous and Mesoporous Materials 71 (2004) 109–115

4. Conclusion Al–MCM-41 (Si/Al ¼ 50) and Zr–Al–MCM-41 (Si/Zr ratio 200, 100 and 50) were hydrothermally synthesised and characterised by low angle XRD, SEM, DRS and FT-IR spectroscopy of pyridine adsorbed samples. Hydroisomerisation of n-hexane and n-heptane reactions were carried out over 0.3 wt% Pt loaded Al–MCM-41 and Zr–Al–MCM-41 molecular sieves and found that Zr containing Al–MCM-41 materials always show higher activity and isomerisation selectivity than Al–MCM-41 catalysts. The enhanced activity of Zr–Al–MCM-41 may be related to strong Brønsted acid sites strengthened by appropriately positioned Lewis acid sites. Also, the Zr–Al–MCM-41 catalysts show higher selectivity towards high-octane multibranched isomers than Al– MCM-41 catalyst and isomerisation follows the classical bifunctional mechanism.

Acknowledgements The authors are gratefully acknowledge the financial support from Department of Science and Technology (DST), New Delhi, India.

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