Scholarly article on topic 'Optimization of silica content in alumina-silica nanocomposites to achieve high catalytic dehydrogenation activity of supported Pt catalyst'

Optimization of silica content in alumina-silica nanocomposites to achieve high catalytic dehydrogenation activity of supported Pt catalyst Academic research paper on "Chemical sciences"

CC BY-NC-ND
0
0
Share paper
Academic journal
Egyptian Journal of Petroleum
OECD Field of science
Keywords
{"Silica-containing alumina nanocomposites" / "Pt nanocatalyst" / "Cyclohexane dehydrogenation" / "Scanning and transmission electron microscopy" / "Surface characterization"}

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

Abstract The present work aims at obtaining a suitable and selective catalyst for catalytic dehydrogenation reactions through designing pore structures of silica-containing alumina nanocomposites by optimizing silica content in the structure. In this trend, series of silica-containing alumina nanocomposites with different molar ratios Al2O3/SiO2 were prepared by the solvothermal method. According to surface characterization of silica-containing alumina nanocomposites, the sample with the highest molar ratio of Al2O3/SiO2 (2.06) showed mesoporous structure with selective pore sizes of 3.7 and 4.6nm. In addition, it had a high surface area value of 308m2/g. Furthermore, SEM and TEM images of the same sample showed ultra fine sized particles in the nano size (7–17nm). Dehydrogenation catalysts, as developed structures, were then achieved by loading 0.6wt.% platinum metal over the prepared nanocomposites. Performances of the prepared nanocatalysts were investigated via the dehydrogenation of a model compound namely; cyclohexane. Experimental results showed that the Pt catalyst supported on the silica-containing alumina nanocomposites with the highest molar ratio of Al2O3/SiO2, is an efficient and selective catalyst toward the dehydrogenation reaction. This was revealed in terms of 100% selectivity of this catalyst toward the conversion of cyclohexane at all ranges of temperatures with the conversion reaction being temperature dependent. Practically, the total conversion of cyclohexane increased with increasing reaction temperature and reached 100% at 450°C while the prepared catalyst demonstrated absolute selectivity.

Academic research paper on topic "Optimization of silica content in alumina-silica nanocomposites to achieve high catalytic dehydrogenation activity of supported Pt catalyst"

Egyptian Journal of Petroleum (2014) 23, 445-454

HOSTED BY

.^.laaBtMB

ELSEVIER

Egyptian Petroleum Research Institute Egyptian Journal of Petroleum

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

FULL LENGTH ARTICLE

Optimization of silica content in alumina-silica c^ark

nanocomposites to achieve high catalytic dehydrogenation activity of supported Pt catalyst

Osama Saber a,b1, Heba M. Gobara b *

a Physics Dept, Faculty of Science, King Faisal University, P.O. 400, Al-Hassa 31982, Saudi Arabia b Egyptian Petroleum Research Institute, Nasr City, P.O. 11727, Cairo, Egypt

Received 8 December 2013; accepted 5 February 2014 Available online 4 December 2014

keywords

Silica-containing alumina nanocomposites; Pt nanocatalyst; Cyclohexane dehydrogenation;

Scanning and transmission electron microscopy; Surface characterization

Abstract The present work aims at obtaining a suitable and selective catalyst for catalytic dehydrogenation reactions through designing pore structures of silica-containing alumina nanocomposites by optimizing silica content in the structure. In this trend, series of silica-containing alumina nanocomposites with different molar ratios Al2O3/SiO2 were prepared by the solvothermal method. According to surface characterization of silica-containing alumina nanocomposites, the sample with the highest molar ratio of Al2O3/SiO2 (2.06) showed mesoporous structure with selective pore sizes of 3.7 and 4.6 nm. In addition, it had a high surface area value of 308 m2/g. Furthermore, SEM and TEM images of the same sample showed ultra fine sized particles in the nano size (7-17 nm). Dehydrogenation catalysts, as developed structures, were then achieved by loading 0.6 wt.% platinum metal over the prepared nanocomposites. Performances of the prepared nanocatalysts were investigated via the dehydrogenation of a model compound namely; cyclohexane. Experimental results showed that the Pt catalyst supported on the silica-containing alumina nanocomposites with the highest molar ratio of Al2O3/SiO2, is an efficient and selective catalyst toward the dehydrogenation reaction. This was revealed in terms of 100% selectivity of this catalyst toward the conversion of cyclohexane at all ranges of temperatures with the conversion reaction being temperature dependent. Practically, the total conversion of cyclohexane increased with increasing reaction temperature and reached 100% at 450 °C while the prepared catalyst demonstrated absolute selectivity.

© 2014 Production and hosting by Elsevier B.V. on behalf of Egyptian Petroleum Research Institute.

Open access under CC BY-NC-ND license.

* Corresponding author. Tel.: +2 (02) 01223898196, +2 (02) 01091446345; fax: +2 (02) 35724559, +2(02) 27727433. E-mail addresses: osamasy@yahoo.com, osmohamed@kfu.edu.sa (O. Saber), hebagobara@ymail.com, hebagobara@yahoo.com (h.M. Gobara).

1 Tel.: +966 3 5899440; fax: +966 3 5886437. Peer review under responsibility of Egyptian Petroleum Research Institute.

1. Introduction

Development of a new catalyst supports the fact that dehydro-genation reaction has been a great challenge since it can be used as a source of hydrogen (clear energy). It is well-known that many catalytic reactions depend on the structure of the

http://dx.doi.org/10.1016/j.ejpe.2014.11.001

1110-0621 © 2014 Production and hosting by Elsevier B.V. on behalf of Egyptian Petroleum Research Institute.

Open access under CC BY-NC-ND license.

catalyst such as its particle size, amount and types of internal defects, surface structure, etc.

Also, the importance of the catalytic support as well as the amount of catalytic material has been reported in many studies. Many heterogeneous catalysts [1] were synthesized by using different support materials like SiO2, Al2O3, TiO2, ZrO2, MCM-41 and were tested for the dehydrogenation of cyclohexane.

Mixed oxide catalysts are usually characterized by higher specific surface area, stronger surface acidity, basicity and favourable thermal stability compared to respective single metal oxides [2,3]. Mixed oxide surface characterization could be controlled through the choice of suitable preparation conditions. A sol-gel technique as a controllable process has been recognized for the preparation of Al2O3/SiO2 supports [4-7]. Amorphous alumina-silica material was extensively used as a catalyst and support, if it compared to the previous zeolites with its limited function were identified and introduced. Increased attention towards alumina-silica was recognized once more upon preparation of an active mesoporous structure [8]. While alkylation of aromatic hydrocarbons by olefins was convincingly reported [9], Corma et al. [10], Calemma et al. [11] and Peratello et al. [12] found that MSA (mesoporous alumina-silica) catalysts are highly suitable acidic components for the preparation of supported metal bi-functional catalysts applied in hydroisomer-ization, hydrocracking and olefins oligomerization. Among the reported MSA mesoporous syntheses by sol-gel and reaction conditions controlling parameters such as pore size, structure, distribution, hydrothermal stability and the density of acid sites [13,14] various templates and pore regulating agents such as tetra alkyl ammonium cation and tetra alkyl ammonium hydroxide [15-17] were reported as effective for such tasks.

Within the last ten years, unique properties of nano-struc-tured materials have been revealed and many technological areas clearly stand to benefit from the development of materials on the nano-scale [18,19]. Dispersion and arrangement of nanoparticles in the alumina/silica oxide composites were important factors governing the hydrodesulphurization (HDS) reaction [20]. Jien et al. [21] used carbon nanoparticles (with different structures), carbon nanotubes and traditional carbon materials as supports to prepare platinum catalysts and measure their activity in cyclohexane dehydrogenation.

In the present work, in the absence of templates or pore-regulating agents, series of silica containing alumina nanocom-posites at different ratios Al2O3/SiO2 are to be prepared using the solvothermal method, employing likely precursors, for the aim of modifying the surface structure of alumina while in the presence of silica to attain catalyst supports with excellent capability. Alumina nanocomposites, as prepared, were characterized geometrically and structurally using X-ray diffractions, scanning electron microscopy (SEM), transmission electron microscopy (TEM) studies. In addition, thermal stability of the nanocomposites was monitored using thermo-gravimetric analysis (TG), differential thermal analysis (DTA) and differential scanning calorimetry (DSC).

2. Material and methods

Aluminum tri-sec-butoxide, tetra ethyl ortho-silicate, ethanol (precursors and solvent, respectively), chloroplatinic acid; H2PtCl66H2O (source of platinum metal), ethylene glycols

(complexing agents of platinum) and hydrazine monohydrate (reducing agent of platinum) were all from Merck, Germany. Pure sodium hydroxide (99.9% purity) and acetone (redistilled) were supplied by a local manufacturer. Deionized water was used throughout the work as a solvent or for washing solids.

2.1. Preparation of silica-containing alumina nano-composites

A series of silica-containing alumina nanocomposites with different molar ratios was prepared via the solvothermal method. Initially, appropriate amounts of aluminum tri-sec-butoxide and tetra ethyl ortho silicate were dissolved in excess amounts of solvent until the Al2O3/SiO2 M ratios were obtained to be 2.09, 0.698 and 0.023, respectively and were denoted as AS1, AS2 and AS3. Stirring continued for 2 h where the solution was then placed in a pressurized vessel with a temperature controller unit (Autoclave) as shown in Fig. 1. The reaction was achieved under supercritical conditions in the presence of eth-anol (temperature 250 0C and pressure 70 bar).

2.2. Platinum loading

For the synthesis of platinum nanoparticles, chloroplatinic acid; H2PtCl66H2O was dissolved into 50 mL of ethylene glycol. The solution was heated and stirred at 60 0C for 30 min. A certain amount of sodium hydroxide was afterward added until the solution became alkaline. As soon as the color of the solution started to change from orange to grey-black, the temperature of the water bath was raised to 65 0C and kept at that temperature for a time of 30 min. 5 mL of hydrazine hydrate was subsequently added dropwise and the mixture was then kept at the same temperature for 2 h. The color of the mixture became completely grey-black.

For platinum loading, silica-containing alumina nanocom-posites were impregnated with the prepared mixture of platinum. An appropriate volume of impregnating solution was added to the powder sample to obtain 0.6 wt.% Pt in the final catalysts. The mixture was stirred for a long time under bubbling by an inert gas. The precipitate was washed by deionized water and acetone several times, and then finally it was washed by absolute ethyl alcohol. The washed precipitate was dried under vacuum for approximately 3 h. For simplicity, samples were denoted as Pt/AS1, Pt/AS2 and Pt/AS3.

2.3. Structural, morphological and thermal characterization

Solid catalysts, as prepared were characterized using powder X-ray diffraction (XRD) spectra registered between 1.80 and

Figure 1 A schematic illustration of the set up used for preparation.

70° by Rigaku RINT 2200 using CuK<alpha>(filtered) radiation (<lambda> = 0.154 nm) at 40 kV and 20 mA. SEM investigation was performed using a JEOL, JSM-6330, while TEM examination was carried out using JEOL - JEM-1230. DSC-TGA analyses were carried out by using simultaneous DSC-TGA instrument, SDTQ 600, under N2 atmosphere with a heating rate of 10 °C/min. All measurements were carried out at the Central laboratory of the Egyptian Petroleum Research Institute (EPRI), Nasr city, Cairo.

2.4. Surface characteristics

Nitrogen adsorption-desorption isotherms at 77 K (—196 °C) were obtained with a NOVA 3200 apparatus (USA). Samples, before being subjected to surface area measurements, were degassed under vacuum at 200 °C for 4 h. Adsorption isotherms, specific surface area, pore volume and average pore radius were calculated by using Brunauer, Emmett and Teller (BET) equation [22]. In addition, surface area and porosity were detected by applying the t-method [23]. The analyses of desorption data to obtain pore size distribution and surface areas, SBJH and SDH were proposed by: Barrett, Joyner and Halenda (BJH-method) [24] and Dollimore and Heal (DH-method) [25] respectively.

2.5. Catalytic processes

Catalytic activities of each of the prepared solid catalysts were tested using cyclohexane dehydrogenation as model compounds using a micro-catalytic pulse unit. In the micro catalytic reactor, 0.20 g of the dried catalyst was placed between two thin quartz layers. Prior to the catalytic activity test (all the investigated samples in oxide form were reduced by heating at 350 °C in a stream of dry and clean H2 gas at a flow rate of 100 cm3 min—1 for 4 h). The reduced catalyst was activated at 450 °C for 2 h in a stream of H2 gas. Two microliters of the hydrocarbon reactants were injected over the catalyst at a flow rate of 50 cm3 H2 min—1 The reaction was carried out under atmospheric pressure in the temperature range of 300450 °C. The reactor effluent was passed through a chromato-graphic column for separation, identification and determination of products using a flame ionization detector (FID) connected to a computerized data acquisition station. The column devised was 200 cm long and had 0.3 cm of internal diameter, containing acid washed Chromosorb AW (60-80 mesh size) loaded by 15 wt.% squalane. GLC chamber temperature was adjusted and controlled at 40 °C.

3. Results and discussion

3.1. X-ray diffraction

XRD patterns of silica-containing alumina samples with different Al2O3/SiO2 molar ratios are shown in Fig. 2. The broad XRD peak detected around 2<theta> = 23° for all samples indicates an amorphous silica-alumina structure. For the sample containing a low concentration of silicon oxide (AS1), the characteristic peaks of the <gamma>-alumina phase at 2<theta> =39.4° for (222) reflection and 2<theta> =66° for (440) reflection are detected for the calcined material in

addition to the broad weak peak at 2<theta> = 23. The characteristic peaks of <gamma>-alumina are observed to disappear upon increasing the silicon oxide (AS2 and AS3) content, which is an indication of aluminium incorporation. It is then concluded that silica-alumina materials prepared are mainly of amorphous nature of a random array of silica and alumina tetrahedral structures interconnected over three dimensions.

3.2. Thermal analysis

Thermal profiles of silica-containing alumina samples investigated using DSC and TGA are exhibited in Fig. 3a, for AS2. As observed, at 93 °C the loss of physically adsorbed water is observed at 14 wt.% loss. At higher temperatures, however, the DSC curve showed one endothermic peak at 329 °C and 540 °C, associated with a weight loss of 9% which could be assigned to dehydroxylation reactions. Upon further heating, an exothermic peak was observed above 500 °C accompanied with a small weight loss of 2 wt.% due to the probable recrys-tallization process [26]. This is usually a result of removal of chemisorbed water. For structural stability studies, the same sample heated at 600 °C was once again subjected to DSC and TGA analyses. The results evidenced no phase transitions, Fig. 3b. Such observation indicates clearly the higher stability the oxide which attains once the physically chemisorbed water is removed in the range of ambient temperature to 600 °C.

3.3. Surface characterization of silica-containing alumina

Full adsorption-desorption isotherms of nitrogen for the series of silica-containing alumina samples prepared via the

4 10 20 30 40 50 60 70 80 2 0 / degrees

Figure 2 X-ray diffraction patterns of the prepared silica-containing alumina with different Al2O3/SiO2 M ratios.

Temperature, C

Figure 3 Thermal analyses (TG and DSC) of the prepared silica-containing alumina (AS2) at (a) critical conditions and (b) 600 °C.

solvothermal method and calcined at 600 0C are exhibited in Fig. 4. In the case of silica-containing alumina (AS1), the isotherm is observed similar to the type IV isotherm, and no plateau at high p/po is recorded. Therefore, this isotherm will belong to pseudo-type II or sometimes denoted as in between types II and IV. In addition, the hysteresis loop exhibited allows assigning this isotherm to the H3 type of the IUPAC classification. The hysteresis loop is usually associated with filling and emptying of mesopores through capillary condensation. Upon increasing the silica content, sample AS2, the isotherm is found initially convex to the p/po axis similar to type IV, Fig. 4. This is indicative of weak adsorbent-adsorbate interactions. At a relatively high pressure, it exhibits a small hysteresis loop which is associated with the mechanism of pore filling and emptying. In the same trend, by increasing the percentage of silica higher than that of alumina. The isotherm and hystersis loop are similar to that of Sample (AS3); the silica containing alumina were observed, Fig. 4.

Pore size distribution study obtained from the BJH method of silica-containing alumina sample, AS1 is illustrated in Fig. 5, where narrow pore size distribution with two maximum peaks at 3.7 and 4.6 nm is distinguished, providing a mesopor-ous nature that strongly supports the apt scheme of preparation adopted in the present work. Such observation is also confirmed via the V-t plot, Fig. 6a, which is obtained using the de Boer method. The plot exhibits an upward deviation at t >1 nm that is associated with mesoporous texture phenomenon.

As the percentage of silica is increased within the alumina, a larger pore size (macropores) in the range of 3.6 nm to 80 nm was also observed, as shown by the AS2 sample; Fig. 5. In addition, the corresponding V-t plot, Fig. 6b, exhibits an upward deviation at t > 1.5 nm indicating a mesoporous structure. Upon increasing the silica content further, pore size distribution in the range of 1-13 nm is observed indicating a mesoporous structure, Fig. 5 confirmed by its V-t plot, Fig. 6c.

0.4 O.G

Figure 4 Nitrogen adsorption-desorption isotherm of the prepared silica-containing alumina with different Al2O3/SiO2 molar ratios.

Table 1 summarizes specific surface area, pore size distribution and total pore volume results for silica-containing alumina designated as AS1, AS2, and AS3 calcined at 600 0C.

The results indicate that AS1 is characterized by a high specific surface area of 308 m2/g with a total pore volume of 1.08 cc/g and consequently a small average pore size of 4.6 nm. Upon

Figure 5 Pore size distribution of the prepared silica-containing alumina with different Al2O3/SiO2 molar ratios.

increasing the percentage of silica in sample AS2, a big reduction in total pore volume is observed; leading to a sharp decrease in surface area. Such indication discloses pore blocking as the percentage of silica is increased. Silica precipitation in the alumina matrix could have resulted in the formation of free silica which can cover, partially, the alumina network or precipitate in the cavities of the alumina network. This may throw some light on the silica coating for alumina surface leading to blocking of pores and subsequently, hindering the internal surface for nitrogen gas adsorption. A further increase in the concentration of silica, AS3, has led to a surface area increase to 456 m2/g with an appreciable increase in the pore radius to a near 40% of that of AS1.

3.4. Particle size identification

Fig. 7, illustrates the TEM images of silica-containing alumina designated as AS1 calcined at 600 °C. AS1 composite exhibits groups of ultra-fine particles assembled into a larger agglomeration with constructing mesopores (dark spots), Fig. 7. The image reveals spherical fine sized particles in a range from 6.6 to 16.7 nm. Spherical particles of silica-containing alumina adhere to two or three other particles and the pores are the cavities between the globules. The size of the globules determines the specific area, pore volume and diameter of the particles. The morphology of the AS2 composite using SEM is illustrated in Fig. 8, where nanoparticles of silica-containing alumina are identified in large aggregates. Fig. 9, however, displays the TEM images of the composite sample AS3 at different locations and magnifications. It is obvious that the sample is in nano particle forms of 2.9-5.8 nm and 5.7-9.6 nm. TEM images confirm the domination of the mesoporous structures is obvious from large aggregates of nano composite silica-containing alumina.

4. Platinum-nano catalysts based on silica-containing alumina nanoparticles

Dehydrogenation of cyclohexane is one of the most promising methods to store, transport and supply with in situ generation of hydrogen. Cyclohexane is of 7.2 wt.% hydrogen content, and it can be dehydrogenated to produce gaseous hydrogen and benzene as shown in the following equation:

C6H12 $ C6H6 + 3H2 AH = +205 : 9 kJ mol—1

Dehydrogenation of cyclohexane is an endothermic reaction with an energy requirement of ~205.9 kJ mol— 1 Results of dehydrogenation experiments over various catalysts were expressed in terms of total conversion and % selectivity was calculated as in the following:

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

Total Conversion = ^ Yield(mol%)of all products

%Selectivity = yield of benzene/total conversion x 100

As-prepared silica-alumina nanocomposites AS1, AS2, AS3 and those supported by 0.6 wt.% platinum were subjected to heterogeneous catalytic dehydrogenation of cyclohexane, results of which are illustrated in Fig. 10 and Table 2. Result verification indicates that only benzene and hydrogen were the main products indicating that selectivity and conversion toward the dehydrogenation reaction is 100% at elevated temperatures of 450 °C, and as previously reported when catalysts based on layered structure of Zn-Al-Si were utilized [18]. At lower temperatures of 250 °C and 350 °C, however, Pt/AS1 did not prove active towards the dehydrogenation reaction.

Figure 6 Vl-t plot of the prepared silica-containing alumina with different Al2O3/SiO2 molar ratios.

Table 1 Adsorption data derived from adsorption-desorption isotherms for silica-containing alumina using BET and BJH methods.

Sample Molar ratios Al2O3/SiO2 SBET (m2/g) Vp BJH (cc/g) Rp BJH (nm)

AS1 2.09 308 1.0820 4.606

AS2 0.698 71 0.0779 6.24

AS3 0.023 456 1.022 6.24

Therefore, the relation between reaction temperatures, total conversion of cyclohexane and selectivity towards dehydroge-nation reaction is not only a catalyst controlling process but also temperature dependent.

Although various metal oxides may be good supports for Pt to design dehydrogenation catalysts, selectivity towards hydrogen formation is an important issue. It was reported that

on well dispersed Pt catalysts dehydrogenation reaction prevails whereas a relatively high grain size leads to hydrogenoly-sis reaction in addition to dehydrogenation reaction. Shukla et al. [27] reported that when metal surface areas for various catalysts were compared, no direct correlation was found with catalytic activity. This indicates that metal support interaction has the major effect on catalytic activity. In general, the

Figure 7 TEM images of the prepared sample of silica-containing alumina (AS1) calcined at 600 °C.

activity of platinum catalysts depends on the surface area of the support, dispersion nature and size of metal particles, therefore, catalyst activity should be attributed to its respective structure and support where the well nanosized Pt atoms are being interacted with the subsurface octahedral sitting of alumina support.

The exhibited high catalytic activity of Pt/AS1 is related the texture of silica containing alumina. The short pore channel which is produced from packing of nanoparticles is favourable for the diffusion of benzene and then may also play a major role in the improvement of its catalytic performance. Also, the pore radius distribution exhibited by AS1 (4.6 nm) is then considered convenient for platinum assembly enhancement towards dehy-drogenation of cyclohexane reaction selectivity. Such pore size attained seems to allow cyclohexane molecules, smaller than the pore opening, to gain access to platinum sites and/or allow benzene molecule evolution. The increase in the silica content as presented by the sample designated Pt/AS2, however, exhibited deteriorated catalytic activity toward the dehydrogenation reaction due mainly to pores blocking the alumina porous structure through covering of the alumina surface (core-shell system). This is indicated by the sharp decrease in the surface area and total pore volume. A further increase in the silica content, dilutes the active alumina and in turn does not support the elevation of the dehydrogenation process as was evident from the slim activity exhibited. Nakano et al. [20] proposed a series

- pi ' s»

Jp» J

SUHOUO 0 SkV X1SUk Sfc(U) 1 1 ■ ■ I I 1 1 1 1 1 30Unm

Figure 8 SEM images of the prepared sample of silica-containing alumina (AS2) calcined at 600 °C.

of alumina-silica supports for deep hydrodesulphurization (HDS) reporting that the SiO2 content was of an influential effect on the catalytic performance, revealing a better performance by around 27% regardless of preparation conditions.

5. Catalyst/support microstructure influence and the reaction mechanism

The pore structure of the support does contribute with no doubt to the selectivity of a catalytic reaction when the approach of the solid pore opening geometry and size is in relation to the molecular forms of the reactants and products. Upon interpretation of the results obtained, hereafter, the dehydrogenation reaction of the cyclohexane mechanism could be visualized via the adsorption of the reactant species on two active sites followed by the removal of hydrogen to form adsorbed cyclohexene which is followed by the stepwise removal of hydrogen atoms to form benzene as the final product. Therefore, the Pt/AS1, proven with high conversion and selectivity of cyclohexane to benzene 100%, may be correlated with the aggregates of the nano-size of crystallites of the support which creates nanopores suitable for Pt-active sites to function efficiently. In addition, the morphological network formed by the presence of Pt-sites within the pores at 47 nm in radius may have increased ultimately the desorption

Table 2 Product distribution of cyclohexane dehydrogenation with respect to reaction temperature by using Pt/AS1.

Figure 9 TEM images of the prepared sample of silica-containing alumina (AS3) calcined at 600 oc.

ability of the surface adsorbed benzene molecules avoiding further undesirable reactions of degradation or catalyst poisoning.

Components (mol.%) Temperature (oc)

250 300 350 400 450

Benzene 0 30 50 66 100

Cyclohexane 100 70 50 34 0

Total conversion - 30 50 66 100

% Selectivity - 100 100 100 100

6. Conclusions

The performance of Pt catalyst supported on different kinds of silica-containing alumina in the catalytic conversion dehydro-genation conversion of cyclohexane to benzene has been confirmed to be microstructure dependable process. Series of silica-alumina nanocomposites were prepared using the solvo-thermal technique for the aim of inducing high surface area with a selective nano-porosity envisaging more selective dehy-drogenation platinum/supported catalyst. Silica-containing alumina support containing low concentrations of silicon oxide designated as AS1, provided a high specific surface area of 308 m2/g at 1.08 cc/g total pore volume and an average pore size of 4.6 nm. Accordingly, Pt nanocatalyst deposited on such nano silica/alumina composites, designated as Pt/AS1, has exhibited excellent activity at 450 0C for the conversion of cyclohexane to benzene with recognized selectivity and a conversion yield of 100%. Upon increasing the percentage of silica, the dehydrogenation activity sharply decreased suggesting the dilution of the active species of alumina and its subsequent effect on the microstructure of the support. This study concludes that the Pt support on silica-alumina nanocompos-ite with a low percentage of silica is an effective and selective catalyst for cyclohexane dehydrogenation particularly at nano sized particles around 10 nm emphasizing the nanotechnology

Figure 10 Effect of temperature on cyclohexane dehydrogenation using Pt/AS1.

impact of the porous structure of alumina on its performance towards the dehydrogenation process of cyclohexane. The main emphases were regarded as firstly due to the penetration of the low concentration of silica into the texture of alumina to become more convenient for the dehydrogenation reaction while avoiding the undesirable reactions and secondly, to the production of short pore channels from the packing of nanoparticles within the composite which is favourable for the diffusion of benzene.

Acknowledgement

The authors acknowledge Egyptian Petroleum Research Institute (EPRI) for providing all facilities, chemicals and instruments. This work was financially supported through Annual Research Grants Program (ARP-29-111) by King Abdulaziz City for Science and Technology (KACST).

References

[1] S. Taubmann, H.G. Alt, J. Mol. Catal. A: Chem. 287 (2008) 102-109.

[2] M. Khatamian, A.A. Khandar, M. Haghighi, M. Ghadiri, Appl. Surf. Sci. 258 (2011) 865-872.

[3] D.B. Adriana, G.R. Claudia, R.M. Sergio, A.S. Osvaldo, Catal. Today 133 (2008) 28-34.

[4] J.M. Miller, L.J. Lakshmi, Appl. Catal. A 190 (2000) 197-206.

[5] P. Padmaja, K.G.K. Warrier, M. Padmanabhan, W. Wunderlich, F.J. Berry, M. Mortimer, N. Creamer, Mater. Chem. Phys. 95 (2006) 56.

[6] M. Crisan, M. Zaharescu, V.D. Kumari, M. Subrahmanyam, D. Crisan, N. Dragan, M. Raileanu, M. Jitianu, A. Rusu, G. Sadanandam, J.K. Reddy, Appl. Surf. Sci. 258 (2011) 448-455.

[7] V.M. Gun'ko, V.M. Bogatyrev, M.V. Borysenko, M.V. Galaburda, I.Y. Sulim, L.V. Petrus, O.M. Korduban, E.V. Polshin, Y.a.V. Zaulychnyy, M.V. Karpets, O.O. Foya, I.F. Myronyuk, V.L. Chelyadyn, U.Ya. Dzhura, R. Leboda, J. Skubiszewska-Zieba, J.P. Blitz, Appl. Surf. Sci. 256 (2010) 5263-5269.

[8] A. Corma, J. Perez-Pariente, V. Fornes, F. Rey, D. Rawlence, Appl. Catal. 63 (1990) 145-164.

[9] C. Perego, S. Amarilli, A. Carati, C. Flego, G. Pazzuconi, C. Rizzo, G. Bellussi, Microporous Mesoporous Mater. 27 (1999) 345-354.

[10] A. Corma, A. Martinez, S. Pergher, S. Peratello, C. Perego, G. Bellusi, Appl. Catal. A 152 (1997) 107-125.

[11] V. Calemma, S. Peratello, Perego, Appl. Catal. A 190 (2000) 207-218.

[12] S. Peratello, M. Molinari, G. Bellussi, C. Perego, Catal. Today 52 (1999) 271-277.

[13] B. De Witte, J. Uytterhoeven, J. Colloid Interface Sci. 181 (1996) 200-207.

[14] J.P. Reymond, G. Dessalces, F. Kolenda, Stud. Surf. Sci. Catal. 91 (1995) 453.

[15] G. Bellussi, C. Perego, A. Carati, S. Peratello, E.P. Massara, G. Perego, Stud. Surf. Sci. Catal. 84 (1994) 85.

[16] G. Perego, R. Millini, C. Perego, A. Carati, G. Pazzuconi, G. Bellussi, Stud. Surf. Sci. Catal. 105 (1997) 205-212.

[17] G. Bellussi, M.G. Clerici, A. Carati, F. Cavani, 1991; US Patent 5 049 536.

[18] O. Saber, H.M. Gobara, A.A. Al Jaafari, Appl. Clay Sci. 53 (2011) 317-325.

[19] O. Saber, Heba M. Gobara, Curr. Nanosci. 9 (2013) 394-400.

[20] K. Nakano, W. Pang, J. Lee, J. Park, S. Yoon, I. Mochida, Fuel Process. Technol. 92 (2011) 1012-1018.

[21] D. Jian-ping, S. Chang, S. Jin-ling, Z. Jiang-hong, Z. Zhen-ping, J. Fuel Chem. Technol. 37 (2009) 468-472.

[22] S. Brunauer, P.H. Emmett, E. Teller, J. Am. Chem. Soc. 60 (1938) 309-319.

[23] J.H. De Boer, B.G. Linsen, Th.J. Osinga, J. Catal. 4 (1965) 643648.

[24] E.P. Barrett, L.G. Joyner, P.H. Halenda, J. Am. Chem. Soc. 73 (1951) 373-380.

[25] O. Saber, Heba M. Gobara, Curr. Nanosci. 9 (2013) 654-661.

[26] J.G. Yu, X.J. Zhao, Q.N. Zhao, Thin Solid Films 379 (2000) 714.

[27] A. Shukla, P. Gosavi, J. Pande, V. Kumar, K. Chary, R. Biniwale, Int. J. Hydrogen Energy 35 (2010) 4020-4026.