Scholarly article on topic 'Characterization and catalytic activity of NiO/mesoporous aluminosilicate AlSBA-15 in conversion of some hydrocarbons'

Characterization and catalytic activity of NiO/mesoporous aluminosilicate AlSBA-15 in conversion of some hydrocarbons Academic research paper on "Chemical sciences"

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Abstract of research paper on Chemical sciences, author of scientific article — Heba M. Gobara

Abstract Mesoporous aluminosilicate AlSBA-15 was synthesized and adopted as a support for NiO with 3, 6 and 9wt.% loadings. Characterization of various samples was performed through XRD, FTIR, DSC-TGA, TPR, SEM and TEM techniques. Textural and morphological characteristics were examined using N2 adsorption–desorption isotherms. Catalytic activities were measured in cumene cracking for parent AlSBA-15 and in n-hexane and toluene cracking and cyclohexane dehydrogenation for supported NiO samples. Uniformity of the ordered 2D-hexagonal structure of AlSBA-15 was evident even after loading with NiO. NiO and NiOOH phases could be detected particularly in the sample containing 9wt.% NiO. TPR profile of solid loaded with 3wt.% NiO sample showed negative peaks at 400 and 600°C, related to hydrogen spillover on reduced sample. Selectivity towards n-hexane and toluene cracking increased with both temperature and metal oxide loading, achieving 100% at 350°C. In cyclohexane dehydrogenation, the sample loaded with 3wt.% NiO was the most active and selective one towards benzene formation.

Academic research paper on topic "Characterization and catalytic activity of NiO/mesoporous aluminosilicate AlSBA-15 in conversion of some hydrocarbons"

Egyptian Journal of Petroleum (2012) 21, 1-10

Egyptian Petroleum Research Institute Egyptian Journal of Petroleum

www.elsevier.com/locate/egyjp www.sciencedirect.com

FULL LENGTH ARTICLE

Characterization and catalytic activity of NiO/mesoporous aluminosilicate AlSBA-15 in conversion of some hydrocarbons

Heba M. Gobara

Refining Division, Catalysis Department, Egyptian Petroleum Research Institute (EPRI), Nasr City, Cairo, Egypt

Received 29 December 2010; accepted 16 March 2011

KEYWORDS

Ni catalysts; Mesoporous silica; AlSBA-15; Textural properties; TPR;

N2 physisorption

Abstract Mesoporous aluminosilicate AlSBA-15 was synthesized and adopted as a support for NiO with 3, 6 and 9 wt.% loadings. Characterization of various samples was performed through XRD, FTIR, DSC-TGA, TPR, SEM and TEM techniques. Textural and morphological characteristics were examined using N2 adsorption-desorption isotherms. Catalytic activities were measured in cumene cracking for parent AlSBA-15 and in n-hexane and toluene cracking and cyclohexane dehydrogenation for supported NiO samples. Uniformity of the ordered 2D-hexagonal structure of AlSBA-15 was evident even after loading with NiO. NiO and NiOOH phases could be detected particularly in the sample containing 9 wt.% NiO. TPR profile of solid loaded with 3 wt.% NiO sample showed negative peaks at 400 and 600 °C, related to hydrogen spillover on reduced sample. Selectivity towards n-hexane and toluene cracking increased with both temperature and metal oxide loading, achieving 100% at 350 °C. In cyclohexane dehydrogenation, the sample loaded with 3 wt.% NiO was the most active and selective one towards benzene formation.

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1. Introduction

Highly ordered large pore mesoporous silica molecular sieves SBA-15 with considerably thicker pore walls as compared to MCM-41 have been recently synthesized using an amphiphilic triblock copolymer as the structure directing agent in highly acidic media [1-4]. SBA-15 exhibits improved hydrothermal stability as compared to MCM-41 [4,5]. The incorporation of aluminum into SBA-15 by post synthetic and direct methods has been reported [6-11]. During materials preparation via post synthetic methods often metal oxides are formed in the channels or on the external surface. Metal oxides formed in the mesopores will block the pores partially or fully, thereby

1110-0621 © 2012 Egyptian Petroleum Research Institute. Production and hosting by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ejpe.2012.02.001

Figure 1a Low-angle XRD of Al-SBA-15 support and NiO/Al-SBA-15 catalysts.

reducing surface area, pore volume, and pore diameter, or play a negative role in catalysis [12] Yue et al. [6] have studied the direct synthesis of AlSBA-15 and found that the catalytic activity of AlSBA-15 in cumene cracking is higher as compared to AlMCM-41.

It has been found that the extent of Al incorporation and substitution in the tetrahedral framework [13] of AlMCM-41 and the catalytic activity [14] are greatly dependent on the Al source used in the preparation of these materials [15-20]. Ja-nicke et al. [15] and Reddy and Song [18,20] have reported that Al isopropoxide is the best source for the incorporation of Al (III) in MCM-41. Hartmann et al. [21] have investigated the effect of Al source on the incorporation of MCM-48 and found that a maximum incorporation of Al and large pore diameter on the MCM-48 were achieved by using Al isopropoxide as the Al source. Materials with uniform and tunable pore sizes are expected to play an important role in a number of applications in catalysis, molecular separations and sorption of very bulky molecules [22], and to the fabrication of semiconductors, semiconductor nanowires, and low dielectric devices [23,24].

Ni catalysts have been used for many years for different catalytic processes such as hydrogenation/dehydrogenation, cracking, hydrodesulfurization (HDS) and hydrodechlorina-tion (HDC). However, relatively few papers have been published on applications of nickel supported on mesoporous materials [25]. The most widely used method for the preparation of these catalysts is the use of an incipient wetness impregnation process [26,27]. However, other methods have been reported for preparing mesoporous silica, MCM-41, as a support for nickel catalysts. These methods include ion-exchange with NiCl2 or Ni(NH3)4(NO3)2 solutions, and direct Ni2 +

incorporation during the fabrication of the support using a sol-gel method especially small extend Ni loading (<5 wt.%) [28].

The present work aims at the synthesis of mesoporous AlS-BA-15 as mesoporous support material for nickel catalyst samples prepared by wet impregnation technique. The prepared catalysts were characterized using XRD, N2 physisorption, FT-IR and SEM techniques. The acidity of the employed support was tested through cumene cracking. The prepared catalysts were applied in the catalytic of n-hexane and tolune cracking and cyclohexane dehydrogenation reactions.

2. Experimental

2.1. Synthesis of AlSBA-15 and Ni/AlSBA-15 catalysts

AlSBA-15 was synthesized using a triblock copolymer poly (ethylene glycol)-block-poly (propylene glycol)-block-poly (ethylene glycol) (Pluronic P123, molecular weight) 5800, EO20-PO70EO20, Aldrich) as a structure directing agent. In a typical synthesis, 4 g of Pluronic P123 was added to 30 mL of water. After stirring for few hours, a clear solution was obtained. Thereafter, the required amount of HCl was added, and the solution was stirred for another 2 h. Then, 9 g of tetraethyl orthosilicate and the required amount of the desired Al isopropoxide were added, and the resulting mixture was stirred for 24 h at 40 0C. The sample was prepared by using 70 mL of 0.29 M HCl and a Si/nAl ratio of 7 [29]. The solid product was recovered by filtration, washed several times with water, and dried overnight at 100 0C. The sample was calcined at 540 0C to remove the template.

NiO samples were deposited into the mesoporous AlSBA-15 by wet impregnation method using an aqueous solution of Ni (NO3)2-6H2O. The calculated amount of nickel nitrate precursor was added to 1 g of ALSBA-15, in order to obtain

3, 6 and 9 wt.% Ni in the final catalysts. The solid catalysts samples were dried at 120 0C for 16 h and then calcined at 200 0C in a stream of air. The oxide samples obtained were reduced by heating at 450 0C in a current of pure hydrogen.

2.2. Characterization

The materials were characterized by powder X-ray diffraction recorded on a Brucker D8 advance X-ray diffractogram with Cu Ka radiation (k = 1.5418 AA). IR experiments were performed using AT1 Mattson model Genesis Series (USA) infra red spectrophotometer adopting KBr technique. For all samples, the KBr technique was carried out approximately in a quantitative manner since the weight of the sample and that of KBr were always kept constant.

2 6 /Degree

Table 1 Physicochemical properties of SBA-15 and Ni-Al SBA-15 materials with different Ni contents.

Catalyst samples a0a (nm) Sbet (m2/g) Vt (cm3/g) Smicro (m2/g) Vmicro (cm3/g) Wbjh (nm) Twb (nm)

AlSBA-15 support 12.55 810 1.186 141 0.0340 10.13 2.42

3% NiO 12.34 668 1.025 70 0.0120 10.10 2.24

6%NiO 12.51 551 0.882 23 0.0076 10.16 2.35

9% NiO 12.86 619 0.959 58 0.0076 10.09 2.77

a a0: The length of the hexagonal unit cell a0 = 2d100/(3)1/2. b TW: The wall thickness = a0 — WBJH.

N2 adsorption-desorption isotherms at —196 °C were obtained with a NOVA 3200 apparatus, USA. The samples were previously outgassed under a reduced pressure at 200 °C for 4 h. Specific surface areas (SBET) were calculated from muti-point at relative pressure (P/P0) ranging from 0.05 to 0.30. Pore size distribution curves were obtained from Barrett, Joyner and Halenda (BJH) method using the adsorption branch of the isotherms.

DSC-TGA analyses were carried out for all supported metal oxide samples using simultaneous DSC-TGA SDTQ 600, USA under N2 atmosphere, with a heating rate of 10 °Cmin —\ The morphology of the samples was studied by the aid of scanning electron microscope, Jeol, JXA-840A, Electron Probe Micro Analyzer. The reduced metal catalysts were analyzed by Jeol TEM-1230 electron microscope, 120 K, 600,000 magnification, Japan.

The oxide samples were reduced with hydrogen by temperature programmed reduction (TPR), using Chembet 3000 apparatus, USA. Adopting all the experiments from room temperature to 1000 °C, at a ramping rate of 10 °C/min, in a flow of 10% H2 in nitrogen at atmospheric pressure.

Catalytic activity of pure AlSBA-15 was tested through cu-mene cracking while the other Ni/Al-SBA-15 samples were tested through n-hexane and toluene cracking and cyclohexane dehydrogenation using a micro catalytic pulse flow technique (Fig. 1). The reactor effluent was passed through a chromato-graphic column for separation and determination of products using flame ionization detector connected to computerized data acquisition station. The column was 200 cm long and 0.3 cm internal diameter, containing acid washed PW chromo-sorb (60-80 mesh size) loaded by 15% by weight squalane. The reactions were carried out under atmospheric pressure at temperatures that ranged from 250 to 450 °C. Hydrogen flow rate was kept constant at 50 ml/min. Prior to the catalytic activity test, the reduced catalyst samples were heated in H2 for 2 h at 450 °C. Few doses of reactants were injected first to reach steady state of the activity. The chromatographic column temperature was fixed at 50 °C.

■AL-SBA-15

9% NiO -3% NiO 6% NiO

.S îi

0.4 0.6 P/Po

Figure 2a N2 adsorption- desorption isotherms of Al-SBA-15 and Ni/Al-SBA-15 catalysts.

D (nm)

3. Results and discussion

3.1. X-ray analysis

Figure 2b N2 adsorption-desorption isotherms of Al-SBA-15 and Ni/Al-SBA-15 catalysts.

Fig. 1a illustrates the recorded low-angle X-ray diffractogram of pure AlSBA-15 and NiO/AlSBA-15 samples of different

10 30 50 70 90

26 /Degree

Figure 1b Large-angle XRD of Al-SBA-15 support and NiO/Al-SBA-15 catalysts.

NiO loadings. For Al-SBA-15, three well-resolved peaks at 2h = ~0.9, 1.4 and 2.5 are shown, being characteristic of the planes (100), (110) and (200) characteristic of mesopor-ous material with 2D-hexagonal structure are evident [30]. The shown P6mm hexagonal symmetry of AlSBA-15 seems to be typical of SBA-15 material [31]. For all supported nickel oxide samples, similar peaks at d-spacing (100), (110) and (200) of the hexagonal pore mesostructure of AlSBA-15 material. These findings may point to the good uniformity of the hexagonal arrangement of pores [32], verifying most likely that the original structure of AlSBA-15 has been maintained after the incorporation of NiO and calcination.

The length of the hexagonal unit cell (a0) for AlSBA-15, calculated by the aid of the formula a0 = 2d100/^3, where d spacing is compatible with the hexagonal P6mm space group, was found to be 12.55 nm (Table 1). Compared with typical SBA-15 of a0 = 9.96 nm [33], the expansion in unit cell parameter of AlSBA-15 may be linked with the successful incorporation of Al into silica framework, due to longer bond length of Al-O compared to Si-O bond [34].

Such decrease in intensity of the line at d100, upon loading with NiO, may indicate the penetration of nickel oxide inside the mesopore system of AlSBA-15 support. In the wide - angle region (Fig. 1a), it is clear that aluminosilicate SBA-15 pattern is identical to that typical one of amorphous silica. y-NiOOH and NiO phases could be detected in the patterns of supported NiO/AlSBA-15 samples, where intensities of y-NiOOH (of d = 6.9 A, according to ASTM card No 06-075) and NiO lines (d = 2.09 A, of ASTM card No 22-1149) increase as nickel oxide loading increases up to 9 wt.% NiO (Fig. 1b). y-NiOOH phase seems to form as a result of interaction between OH groups in the framework of ALSBA-15 with NiO phase formed during the decomposition of nickel nitrate hexahydrate and calcination at 200 0C.

3.2. Surface investigation of different samples under study

Low temperature adsorption-desorption isotherms of N2 and BJH pore size distribution curves for parent AlSBA-15 and Ni/ AlSBA-15 samples are illustrated in Fig. 2. Derived surface data are included in Table 1. All the obtained isotherms are of type IV according to IUPAC classification [22], being typical for meso-porous materials with H1 hystersis loops. Such type of loops can be referred to the presence of uniform cylindrical pores of relatively large dimensions (cf., pore size distribution curves, Fig. 2b). The incorporation of NiO in AlSBA-15 could not affect largely the shape of the adsorption isotherm on pure support.

The parent synthesized AlSBA-15 support exhibited markedly high surface area (SBET) with a considerable pore volume (Vt) (Table 1), confirming the expansion of the mesopores upon introducing Al into the SBA-15 framework. Incorporation of NiO in different wt.% into AlSBA-15 led to a gradual decrease in both surface parameters up to 6 wt.% NiO (viz., SBET from 810 m2 g"1 for AlSBA-15 to 551 m2g" and Vt from 1.186 cm3 g"1 for AlSBA-15 to 0.882 cm3 g"1). By increasing Ni loading up to 9 wt.%, a marked increase in both SBET and Vt occurred, accompanied by an increase in wall thickness (Tw) (viz., from 2.42 nm for pure support to

2.77 nm). A portion of supported NiO seemed to be diffused from bulk to top surface of the sample, related most probably to the increase of a0 (or extent of crystallinity, Table 1). The supported NiO particles are evidently located, in this sample, on both external surface and inside channels of the AlSBA-15 support.

3.3. FT-IR analysis

Fig. 3 shows FT-IR spectra in the range, 400-4000 cm"1 for parent AlSBA-15 and various NiO supported samples. The 1079 cm"1 band can be attributed to T-O asymmetric stretching vibrations, while the band at 800 cm"1 seems to be due to that intrinsic vibration of TO4 tetrahedra containing Al and Si. It is worth noting that the incorporation of aluminum leads to a decrease in the intensity of Si-(OH) stretching mode assigned at 960 cm"1. The surface silanol, „Si-OH, stretching mode gives rise to a band at 960 cm"1, of both amorphous and crystalline silicates [35-38]. This band accompanied with a shoulder at 1065 cm"1 band, in all nickel containing AlSBA-15 catalysts, may reflect that more surface silanol groups are consumed through loading with nickel in the preparation step, i.e., surface silanol groups most likely interact with the guest species.

Very similar band could also be observed in titanium silox-ane polymers [39], mixed oxides [40] or TiO2-grafted on silica [41], being attributed to the modification of SiO4 units indirectly in the presence of hetero metals. Accordingly, the obtained band at 960 cm"1 can be suggested to be a characteristic of Si-O-Al. The band at 1386 cm"1 may indicate imperfect decomposition of the remnant nitrate during calcination step [42]. The peaks obtained at 3300-3400 cm"1 and at 1616 cm"1 were attributed to molecular water adsorbed on the oxide surface [43]; the peak at 1620 cm"1 was referred to water angular deformation [44]. Of special interest is the peak at 560 cm"1, which may be assigned to hydrogen bonded O-H and Ni-OH in NiOOH phase [44]. It can be shown specifically in the sample containing higher Ni loading (viz., 9 wt.% Ni).

4000 SSOO 3200 2SOO 2400 2000 1000 I20D SOO 400

wavenumber Cm1

Figure 3 FT-IR of Al-SBA-15 support and NiO/Al-SBA-15 catalysts with different nickel loadings.

Figure 4a DSC of Al-SBA-15 support.

Figure 4b DSC of NiO/Al-SBA-15 catalysts with different Ni concentrations.

3.4. Thermal analysis

0 200 400 600 800 1000 T.oC

Figure 5 TGA of Al-SBA-15 and NiO/Al-SBA-15 catalysts with different Ni concentrations.

the process becomes difficult with loading due to the formation of more nickel oxide on the surface of AlSBA-15 support (surface crowdness increases), and the increase of hydrogen bonding between nickel oxide and adsorbed water molecules, especially for the sample containing 9wt.% NiO. AS value for the sample containing 3 and 6wt.% NiO decreased if it compared with its value of pure support. This is due to the coverage of the adsorption sites of the support by NiO. AH and AS of the second event are decreased with loading up to the sample containing 9 wt.% NiO compared with those of pure support as shown in (Table 2). This means the adsorption power is higher due to the formation of NiOOH on the surface and the closure of adsorption sites of the support.

Figs. 4a and 4b and 5 illustrate the DSC and TGA of AlSBA-15 support and NiO/AlSBA-15 catalysts, respectively. DSC of pure AlSBA-15 support shows only an endothermic peak at Tmax = 63.27 °C. This event is accompanied by mass loss as detected by TGA (Fig. 5). This endothermic peak is attributed to the dehydration of the physisorbed water. Beside the dehydration peak of AlSBA-15, the Ni loaded samples show an endothermic peak which becomes more pronounced as nickel increases at Tmax = 324.24 °C for the sample containing 9 wt.% NiO. This event corresponds to the dehydroxylation of OH groups present in the NiOOH phase resulting from metal support interaction. This confirms the XRD data. The weight loss% increased from 5.45% for pure AlSBA-15 support 7.60% for the higher loaded NiO sample (9 wt.% NiO) for the first dehydration event as seen in TGA curves (Fig. 5).

Also enthalpy and entropy of the first dehydration event are increased as nickel loading increases (Table 2). This means that

3.5. Scanning electron microscope

The SEM reveals that the incorporation of aluminum in Si-SBA-15 pores has no evident effect on the macroscopic morphology of the samples [43]. The SEM image of Al SBA-15 (Fig. 6) sample shows aggregates of regular sphere-shaped particles. For Ni/Al SBA-15 samples there is no evidence for the presence of any particles in different shapes if it is compared with the parent Al-SBA-15. This may be due to the fact that the particles of nickel are too small to be detected or it may be incorporated with the channel of the support.

3.6. TPR analysis

TPR of NiO/SiO2 particles shows that could be reduced at 300-400 °C while that located in the micropores of AlSBA-15 could be reduced at higher temperatures between 400 and

Table 2 DSC results of AlSBA-15 and supported nickel samples.

Catalysts Ti °C Tf °C T °C J max- ^ AT AH (Cal/g) AS (Cal g"1 degree-1) Cp (Cal.g^degree"1) Wt. loss %

ALSBA-15 41.92 115.97 63.27 74.05 23.19 1.268513 0.313167 5.45

3% NiO 35.46 117.87 65.61 82.41 30.84 1.568615 0.374226 6.71

6% NiO 39.67 128.75 64.64 89.08 34.02 1.534149 0.381904 6.77

9% NiO 1 34.333 128.07 63.35 93.70 61.47 2.96 0.66 7.60

2 07.77 350.41 324.24 42.64 3.37 0.30 0.08 3.13

Ti, the temperature at which the DSC peak begins to leave the base line; Tf, the temperature at which the peak lands; AT = Tf — Ti, Cp = AH/AT [r], AS = 2.303 Cp log(Tf/Ti) [59], 1: 1st event, 2: 2nd event.

Figure 6 SEM of (A) A1-SBA15 support and NiO/Al-SBA-15 catalysts with different nickel loadings: (B) 3 wt.% Ni, (C) 6 wt.% Ni and (D) 9 wt.% Ni.

500 0C [44]. A small peak was observed at lower temperature 300-400 0C; (viz., 361 and 367 0C for the catalyst samples containing 6 and 9 wt.% Ni, respectively) (Fig. 7a), which indicate the reduction of NiOOH as supported by DSC, XRD and IR results. From the above mentioned results, it could be confirmed that the large fraction of NiO is included within the micropores so it decreases from 0.034 for the support to

(b) 50

> 40 E

400 600 T.oC

400 600 T.oC

Figure 7a TPR of supported Ni/AlSBA-15 catalysts: (a) 6 wt.% and (b) 9 wt.%.

0 200 400 600 r T.C W 800 10

rc 0 re

(¡5 -20

Figure 7b TPR of supported 3 wt.% Ni oxide/Al-SBA-15 catalyst.

0.0076 cc/g for the samples containing 6 and 9 wt.% NiO as proved from the surface data (cf. Table 1).

Fig. 7b shows a TPR profile of 3 wt.% NiO/AlSBA-15 catalysts. Sharp negative peaks around 400 and 600 0C indicating hydrogen spill over phenomenon which occurred due to the dissolution of hydrogen into the AlSBA-15 support over this samples which may be due to the existence of Ni atoms .The TPR profile of the same sample is repeated several times and gives the same result.

Several authors [34-39] reported the negative peak phenomenon in TPR profile for various catalysts such as mesopor-ous SBA-15 supported Pd-Zn catalysts. These authors also observed a similar TPR behavior while using PdO/ZrO2, PdO/CeO2 and PdO/CexZrt_xO2 (0.2 < x < 0.8) samples prepared by using other surfactants. A sharp negative peaks around 400 and 600 0C indicating the release of molecular hydrogen. This is at the reduction temperature of NiO, some of hydrogen species that adsorbed dissociatively over Ni crystals (reduced nickel atoms) may spill over to AlSBA-15 support on which they are stored in AlSBA-15 pores or defects which did not completely filled with nickel. In this process, the metallic nickel plays a key role in the hydrogen spill over because when Ni metals are absent, hydrogen spill over to AlS-BA-15 support does not take place. It is probably due to the ability of metallic Ni to dissociate molecular hydrogen into atomic hydrogen species that make it possible to migrate to the defects of the AlSBA-15.

There is no TPRs result in the literature for the release of molecular hydrogen over nickel catalysts only over Palladium and Ruthenium. It is the first time for Hydrogen dissolution or spill over nickel metal.

This event does not observe in 6 and 9 wt.% NiO/AlSBA-15 catalysts this may be due to the size of the reduced nickel in the sample containing 3 wt.% Ni is much smaller than its size (few nanometers) in the other samples. This can be confirmed from TEM results in the following section.

3.7. Transmission electron microscope

TEM micrographs of reduced Ni/AlSBA-15 catalysts are shown in Fig. 8. For the sample containing 3 wt.% Ni, TEM micrograph show that Ni metal is existing in a well dispersed state and its size is in nano scale and much lower than 50 nm. The particle size of Ni increased with increasing of Ni loadings up to 9 wt.% Ni. The Ni particles density increased in the sample containing 6 wt.% Ni and Ni particle size increased to 100 nm. For the sample containing 9 wt.% Ni, Ni metal showed spherical like shaped which reaches to 632 nm.

3.8. Catalytic process

The catalytic activity of pure AlSBA-15 (Fig. 9) towards cu-mene cracking was investigated to ascertain the acidic property. The cracking products were only benzene and propene indicating that the active sites are Bronsted type [43]. Each Al atom incorporated into the material framework leads to the formation of one Bronsted acid site. The yield, mole% of benzene increased with reaction temperature whereas that of propene decreases along the reaction temperature. This phenomenon may be due to propene may included in polymerization reaction.

The results of dehydrogenation of cyclohexane and n-hex-ane and toluene cracking experiments over various catalyst samples are expressed in terms of total conversion and% selectivity calculated as:

Figure 8 TEM of micrographs of Ni/AlSBA-15 catalysts (reduced) with different nickel loadings: (A and A') 3 wt.% Ni, (B and B') 6 wt.% Ni and (C and C') 9 wt.% Ni.

400 T.oC

Figure 9 Catalytic activity of cumene cracking over AlSBA-15.

Yield (mole %) = amount of reactant disappeared/amount of reactant x 100

Total Conversion = ^ Yield(mole %)of all products.

%Selectivity = yield of selective product/total conversion x 100.

The catalytic conversion of toluene and n-hexane over Ni/ AlSBA-15 catalysts is illustrated in Figs. 10 and 11. The yield of cracking products increases by increasing both reaction tem-

350 T, oC

250 300 350 T. oC

100 80 60

a 40 20

250 300 350

methane benzene T.Conv. % Selectivity

perature and metal loading up to 9 wt.% Ni. It achieves nearly 100% at 350 0C for catalyst samples containing 6 and 9 wt.% Ni for n-hexane cracking and 90% at 450 0C for toluene. The selectivity of n-hexane cracking increases by temperature and metal loading achieves 100% at 350 0C and remains constant at temperature 350-450 0C. A very negligible amount of C6 isomers are formed in n-hexane conversion over these catalysts. Low yield of benzene is formed in toluene cracking over Ni/AlSBA-15 catalysts.

Ni/AlSBA-15 is a good cracking catalyst for both n-hexane and toluene. This behavior runs in harmony with increasing of the average particle size taking place by increasing metal loading. So the metallic particle size increases (hydrogenolysis increase), metallic dispersion decreases and more surface of cracking support is exposed for the reactants. This is supported by TEM results.

The dehydrogenation of cyclohexane is an endothermic reaction with an energy requirement of about 205.9 kJ mol _1. In addition, catalytic cracking is one of the oldest processes and still among the most important reactions in the refining processes of petroleum distillates.

The total conversion and cracking products (gases) of cyclo-hexane (Fig. 12) increase by temperature and nickel loading

E 60 ■o

80 2 о to

350 T.oC

- 80 ®

350 T.oC

I 60 1 40 ^ 20

gases n-hexane T.Conv. % Selectivity

350 T.oC

Figure 10 Catalytic activity Ni/AlSBA-15 catalysts towards toluene cracking.

Figure 11 Catalytic activity Ni/AlSBA-15 catalysts towards n-hexane cracking.

and reach maximum value at 450 °C. Cyclohexane dehydroge-nation into benzene increase by temperature. The increase of dehydrogenation activity with temperature agrees with the thermodynamic feasibility of dehydrogenation reaction. Benzene formation in the presence of the sample containing 3 wt.% Ni reaches a maximum of 41% at 300 °C then it gently declines. This may be related to the metal dispersion. The latter sample is the most active one towards benzene formation as compared with the others. It is obvious that the selectivity decreases by metal loading (Fig. 11). Catalyst sample containing 3 wt.% Ni is the most selective one compared with the others. However, the selectivity decreases by temperature. The increase of cracking products at expense of benzene formation.

4. Conclusion

Highly ordered large pore mesoporous Al-SBA-15 support was prepared by the incorporation of aluminum into SBA-15 by direct method. Nickel catalysts supported on mesoporous alumi-nosilicate containing 3, 6 and 9 wt.% Ni were synthesized using a witness impregnation method. The prepared samples were investigated by various characterization techniques, viz., N2- adsorption-desorption, XRD, FT-IR, DSC-TGA and SEM. Al-SBA-15 support was thermally stable up to 800 °C. The XRD results showed the uniformity of the ordered hexagonal structure even after loading with nickel by the mentioned method of preparation. Three well-resolved peaks at (100), (110) and (200) observed for the supported samples in low an-gle-XRD were the same as in the parent SBA-15, indicating

good uniformity of the hexagonal arrangement of pores. NiO and NiOOH phases are clearly detected in high angle-XRD pattern especially for the sample containing 9 wt.% NiO. The large fractions of NiO particles are included inside the micropores of AlSBA-15 whereas the small portion of them occupied mesopore system as evidenced from TPR. In higher loading, viz., 9 wt.% NiO, nickel particles were formed with liberating the channels of the support which remain intact leaving the internal mesopores to the external surface of the mesoporous AlSBA-15. Supported nickel samples were investigated through n-hexane and toluene cracking and cyclohex-ane dehydrogenation. The selectivity of n-hexane and toluene cracking increases by temperature and metal loading achieves 100% at 350 °C and remains constant at temperature 350450 °C for n-hexane and 90% at 450 °C for toluene cracking, respectively. For cyclohexane dehydrogenation, 3wt.% NiO sample is the most active one towards benzene formation as compared with the others.

Acknowledgment

The author is greatly indebted to Prof. Dr. Salah A. Hassan, Chemistry Department, Faculty of Science, Ain Shams University, Cairo, Egypt, for helpful advice and revision of this work. The author acknowledges Prof. Maged Samir Ghattas, Refining Division, Catalysis Department, Egyptian Petroleum Research Institute for supplying Plurnic acid material.

The author also would like to appreciate my colleague Dr. Dina Mohamed Refining Division, Physical Separation Department, Egyptian Petroleum Research Institute for TEM measurements.

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