Scholarly article on topic 'Influence of Pt nanoparticles modified by La and Ce oxides on catalytic dehydrocyclization of n-alkanes'

Influence of Pt nanoparticles modified by La and Ce oxides on catalytic dehydrocyclization of n-alkanes Academic research paper on "Chemical sciences"

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Abstract of research paper on Chemical sciences, author of scientific article — A.H. Samia, M.S. Mohammed, S. Faramawy, S.A. Ahmed, H.B. Ahmed

Abstract Catalytic reforming accounts for a large share of the world’s gasoline production, it is the most important source of aromatics for the petrochemical industry. In addition, reforming of hydrocarbon on the dual-function catalysts has been found to form fundamentally different products in hydrogen diluents. Typical catalysts employed for this reforming process are Pt/Al2O3 and Pt-M/Al2O3, M being the promoter. These solids are characterized by both acid and metal functions which catalyze dehydrocyclization, dehydrogenation, isomerization and cracking processes. In this regard, information about cerium and lanthanum, as promoters, is hardly revealed. The present work aims to study the performance of Pt/Al2O3 catalysts modified by lanthanum or cerium during the conversion of cyclohexane, n-hexane and n-heptane. Catalytic activities of the prepared catalysts were tested using a micro catalytic pulse technique. Physicochemical characterization of the solid catalysts such as, surface area (SBET), Fourier transform infrared (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), hydrogen-temperature programed reduction (H2-TPR), hydrogen-temperature-programed desorption (H2-TPD), CO2-TPD, NH3-TPD, high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) were depicted. Results indicated clearly that Pt/Al2O3 catalyst is selective toward dehydrogenation to benzene which could be explained as due to the decrease in the active acid sites and the comparative segregation of the alumina support especially at 3% load of CeO. The presence of La2O3 in the Pt/Al2O3 catalyst promotes aromatization of n-hexane and n-heptane, also the dehydrocyclization of n-hexane is more difficult than that of n-heptane. Thus, modification of the Pt/Al2O3 catalyst by La, resulted in a more active and selective reforming catalyst.

Academic research paper on topic "Influence of Pt nanoparticles modified by La and Ce oxides on catalytic dehydrocyclization of n-alkanes"

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Influence of Pt nanoparticles modified by La and Ce ^ oxides on catalytic dehydrocyclization of w-alkanes

A.H. Samia a, M.S. Mohammed a'*, S. Faramawy a, S.A. Ahmed a, H.B. Ahmed

a Egyptian Petroleum Research Institute, Nasr City 11727, Cairo, Egypt b Faculty of Science, AL-Azhar University, Cairo, Egypt

Received 25 January 2014; accepted 6 April 2014 Available online 15 June 2015

KEYWORDS

Dehydrogenation; Dehydrocyclization; La, Ce promotion; Pt/Al2Ö3; Characterization

Abstract Catalytic reforming accounts for a large share of the world's gasoline production, it is the most important source of aromatics for the petrochemical industry. In addition, reforming of hydrocarbon on the dual-function catalysts has been found to form fundamentally different products in hydrogen diluents. Typical catalysts employed for this reforming process are Pt/Al2O3 and Pt-M/Al2O3, M being the promoter. These solids are characterized by both acid and metal functions which catalyze dehydrocyclization, dehydrogenation, isomerization and cracking processes. In this regard, information about cerium and lanthanum, as promoters, is hardly revealed. The present work aims to study the performance of Pt/Al2O3 catalysts modified by lanthanum or cerium during the conversion of cyclohexane, n-hexane and n-heptane. Catalytic activities of the prepared catalysts were tested using a micro catalytic pulse technique. Physicochemical characterization of the solid catalysts such as, surface area (SBET), Fourier transform infrared (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), hydrogen-temperature programed reduction (H2-TPR), hydrogen-temperature-programed desorption (H2-TPD), CO2-TPD, NH3-TPD, high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) were depicted. Results indicated clearly that Pt/Al2O3 catalyst is selective toward dehydrogenation to benzene which could be explained as due to the decrease in the active acid sites and the comparative segregation of the alumina support especially at 3% load of CeO. The presence of La2O3 in the Pt/Al2O3 catalyst promotes aromatization of n-hexane and n-heptane, also the dehydrocyclization of n-hexane is more difficult than that of n-heptane. Thus, modification of the Pt/Al2O3 catalyst by La, resulted in a more active and selective reforming catalyst.

© 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of Egyptian Petroleum Research Institute. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/

licenses/by-nc-nd/4.0/).

1. Introduction

* Corresponding author.

Peer review under responsibility of Egyptian Petroleum Research Institute.

Many feedstock sources, including condensate or natural gasoline, are attractive reforming feed [1]. Condensate is a large source of potential feedstock for aromatics production and may contain alkanes, naphthenes and aromatics. Although

http://dx.doi.org/10.1016/j.ejpe.2015.05.007

1110-0621 © 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of Egyptian Petroleum Research Institute. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

most condensate is currently used as cracker feedstock to produce ethylene, condensate will likely play an increasingly important role in aromatics production in the future.

Many efforts have been made to improve the catalytic performance of platinum-based catalysts. The addition of a second specific metal or metal oxide can improve the activity and/or selectivity toward the desired product [2]. The rare earth elements (RE), due to their excellent electronic properties, are widely used in the form of oxides for the modification and promotion of catalyst activity [3].

In the case of metal based catalysts the effect of RE elements was promoters that lead to metal dispersion and thermal stability elevator of the support [4]. In fact, the goal of reducing the amount of noble metals present in industrial catalysts, appeals for the use of RE on metal loaded catalyst. Automotive exhaust catalysts are an example of catalyst improvement upon the introduction of small amounts of RE elements and their influence as on bi-functional catalysts [5]. The effect of RE addition on platinum supported KL Zeolite, during catalytic reforming processes, was also reported as effective against sulfur poisoning resistance and as reported by Fang et al. [6]. Pt is rarely used in the pure state but is more often used in modern catalysts in the presence of the so-called "promoters". Promoted catalysts are considered more selective and stable than pure Pt supported catalyst when operated under less severe reaction conditions (lower pressure, lower hydrogen-to hydrocarbon ratio). The catalytic properties of the Pt M supported catalysts, (M = promoter), strongly depend on the interaction between Pt and M. Rare earth elements, such as lanthanum and cerium, who acquire unique thermal stabilization properties such as that described for alumina support due to strong metal-rare earth oxide interactions [7]. Accordingly, the promotion effects of cerium or lanthanum on the catalytic performance of Pt/Al2O3 catalyst are expected to motivate the catalytic behavior of noble metal supported catalysts. The present work aims at studying the effect of La and Ce addition, as promoters, to Pt/Al2O3 acidity and textural alterations and therefore their direct effect on the catalytic conversion and selectivity of dehydrocyclization of w-alkanes at different load contents.

2. Experimental

2.1. Chemicals

Boehmite [Chemical Formula: AlO(OH)] from Prolabo, La(NO3)-6H2O, Ce(NO3)-6H2O, H2PtCl6-6H2O and citric acid all from Merck., cyclohexane, w-hexane and w-heptane; all from Aldrich.

The y-Al2O3 support was prepared by drying Boehmite at 120 0C overnight and then calcination at 550 0C, in air for 4 h.

2.2. Catalyst preparation

The reference Pt (0.35 wt.%)/Al2O3 catalyst was prepared by wet impregnation of the y-Al2O3 support with appropriate concentration of Pt precursor (H2PtCl6-6H2O) aqueous solution to achieve 0.35wt.%/Pt. A small amount of citric acid was added to the Pt precursor solution to improve the penetration of the Pt into the alumina support particles to achieve optimum dispersion of the metal and as reported by Ahmed

et al. [8]. The sample was then dried at 120 0C overnight and finally calcined at 550 0C in air for 4 h.

Promoters, i.e., La and Ce, were added to Pt/y-Al2O3 by successive impregnation at appropriate amounts of La, or Ce precursor aqueous solution to acquire loads of 0.25, 1.5, and 3 wt.%. The final product samples were dried and calcined as described earlier. All the prepared catalysts were at fixed Pt content of 0.35 wt.%. Catalyst samples designation is referred to as in Table 1.

Prior to catalytic reaction, an ex-situ reduction in flowing H2 (20 ml min-1) at 500 0C for 4h and an in situ reduction at 500 0C for 1 h in 20 ml min-1 of H2 flow was performed. During reduction, the temperature increase rate was 8 0C min-1 in each case.

2.3. Support and catalysts characterization

The elemental analysis of La, Ce, and Pt contents was performed through X-ray fluorescence (XRF) using channel control PW 1390-Philips and spectrometer PW1410 whereby the content of each element is ensured according to desired permutation prior to use.

X-ray powder diffraction (XRD) patterns using Brucker AXSD 8 Advance (Germany) employing nickel-filtered copper radiation (e = 1.5405 A) at 60 kv and 25 mA with a scanning speed of 8 0C over 20 min-1 were considered for crystallinity studies. Surface area and pore volumes of the support and catalyst materials were determined using USA Nova 3200 apparatus. The chemical structure was elucidated through the use of ATI Mattson Genesis, 960 M009 series FT-IR (KBr disk method) Thermal behavior (DSC and TGA) of the solid samples was examined using SDT-Q600 thermogravimetic apparatus. Hydrogen-temperature-programed reduction (H2-TPR) for precursor catalysts were carried out from ambient to 1000 0C, with a flow of 10% H2/N2 (85 ml min-1) at a heating rate of 10 0C min-1 using Chembet 3000 analyzer and were carried out in the same setup as TPR. Before H2-TPD experiments are carried out, the solid samples of 0.2 g were reduced under flowing pure H2 at 576 0C for 2.5 h. After reduction, the solid catalyst was cooled to 25 0C in the flowing H2. The gas stream was replaced by Argon gas and subsequently a TPD experiment was performed (10 0C min-1) up to 540 0C. Basicity assessment of the solid samples was also carried out in order to relate the catalytic activity and its basic properties of the catalyst samples. TPD of CO2 is frequently used to measure the basicity of catalysts. The number and strength of basic sites are evaluated from the intensity and position temperature

Table 1 Chemical and textural characteristics of the catalysts.

Catalyst Loading (wt.%) Surface area Pore volume

Pt La Ce (m2/g) (cm3/g)

Pt/A* 0.35 — — 260 0.60

PtLa(0.25)/A* 0.35 0.25 — 261 0.28

PtLa(1.5)/A* 0.35 1.5 — 243 0.26

PtLa(3)/A* 0.35 3 — 227 0.25

Pt Ce(0.25)/A* 0.35 — 0.25 270 0.26

Pt Ce(l.5)/A* Pt — 1.5 226 0.24

Pt Ce(3)/A* 0.35 — 3 221 0.23

* y-Al2Ö3

of TPD signals. To assess the acidity of the different catalysts NH3-TPD adsorption/desorption was followed using the same apparatus. Solid catalyst particle size and features were conducted through high resolution transmission electron microscopy (HR-TEM) using Jeol/JEM 2100, 200 kv.

2.4. Catalytic activity measurement

Catalytic activity assessment of the prepared catalyst samples (0.20 g) was revealed through its application of cyclohexane, n-hexane and n-heptane conversion, as model compounds of cyclo-paraffins and n-paraffins, using the micro catalytic pulse technique. The reactants were injected in micro quantities (2 il) by micro syringe in the form of pulses. Reactants and products were analyzed with an online gas chromatograph. Computerized data acquisition system, model 201 24-Bit (Italy) was used for integrating and recording the effluent yield. The reaction was carried out under atmospheric pressure and a temperature range of 300-450 0C at a constant hydrogen flow of 20ml/min~1. Prior to the catalytic activity test, the reduced catalyst samples were heated in H2 flow up to 500 0C with a heating rate of 8 0C min-1 and kept at 1 h at 500 0C for their activation. Few doses of reactants were injected first to reach a steady state of activity. The gas chromatograph column used was 200 cm in length and 0.3 cm in diameter, packed with Chromosorb AW (80-100 mesh size) from Merck, loaded with 15 wt.% Squalan (Merck). The chro-matographic column temperature was adjusted and controlled at 70 0C.

3. Results and discussion

3.1. Catalyst characterization

3.1.1. XRF-XRD

X-ray fluorescence (XRF) was used to monitor the required wt.% of La, Ce, and Pt of catalysts. XRD analysis is presented in Fig. 1. As revealed only the diffraction peaks that correspond to y-Al2O3 were observed but distinct peaks that correspond to either the metallic platinum or the rare earth were not observed may be due to their low concentration and their diffusion and inclusion in the alumina matrix without affecting the alumina matrix agglomeration mode or incapability to rearrange its characteristic agglomeration phase [13]. Such elucidation is supported by HRTEM images, where particles grain size of alumina support seems unchanged upon loading with the noble metal or the rare earth elements, Fig. 9, indicating alumina structural sustainability even upon loading of limited loading of sizable foreign elements.

3.1.2. Structural characterization

Surface area and pore volume of Pt/Al2O3, La, and Ce promoted catalysts after calcinations at 550 0C are summarized in Table 1. The surface area of samples with lower La2O3 and CeO2 content (0.25 wt.%) is observed as slightly higher in value than those of the unpromoted catalyst. However, higher La2O3 and CeO2 loadings (>0.25%) lead to a significant decrease with regard to the surface area and pore volume may be due to blocking of the pores by promoter oxide species, thus influencing the thermal stability of alumina [9]. It has

Figure 1 XRD patterns of (a) the support Al2O3, (b) Pt(0.35)/ Al2O3, (c) Pt (0.35) Ce (3)/Al2O3 and (d) Pt (0.35)La(3)/Al2O3 samples.

been reported that low rare earth oxide loading could stabilize y-Al2O3 against surface area loss by preventing the agglomeration of Al2O3 particles, the transformation of y-Al2O3 to k-Al2O3 and hinders platinum sintering upon calcination treatment of the catalyst [10]. Over Pt-Ce/y-Al2O3 catalyst, the strong interaction between Ce and alumina results in the formation of surface Ce Al2O3 by Ce3 + cation occupying the vacant octahedral sites on the alumina surface. This initial occupation of octahedral sites by cerium cations blocks the transition of Al3+ cations from tetrahedral to octahedral sites during high temperature treatment which causes the loss of the surface area of alumina [9,10].

3.1.3. Thermal characterization

Evaluation of weight loss, rate of weight loss as well as the heat effects associated with heating the sample from room temperature up to 1000 0C (with a heating rate of 10 0Cmin-1) is important, because it enables us to determine the maximum temperature after which the catalyst weight loss is negligible (i.e., complete decomposition). This, in turn, allows establishing the minimum temperature at which the catalyst becomes thermally stable, and as such, the minimum temperature for catalyst calcinations. TGA profiles for the dried samples are shown in Figs. 2 and 3. The profiles indicates a two-stage weight loss at 90 0C which is attributed to the loss of physi-sorbed water and that at 550 0C is attributed to the elimination of the hydroxylic water and precursor salt oxidation to the oxide compound formation. At temperatures above 550 0C a slow and gradual loss of mass was observed which can be attributed to the continuous release of residual traces of hydroxylic H2O and that of the metal oxide precursor salt. DSC profile of parent alumina, Fig. 2a, reflects such behavior upon thermal treatment exhibiting two types of endothermic and exothermic features. The first endothermic appeared at 83 0C, this peak is related to the removal of physisorbed water covering the surface. The second endothermic peak that appeared at 275 0C is related to various endothermic transitions, i.e., the losses of chemically bound water and departure of the hydroxide precursor may still be contaminated with the precipitated materials [11]. The two exothermic behaviors around 450 and 875 0C correspond to crystallization of y-Al2O3 and k-Al2O3, respectively [12].

The thermal behavior profile of Pt/Al2O3 catalyst shown in Fig. 2b reveals that no variation compared to y-Al2O3 occurs due may be to the low platinum loading. Upon addition of

Figure 2 TGA and DSC profiles of: (a) y-Al2O3 support; (b) Pt/Al2O3 catalyst; (c) Pt La (0.25 wt.%)/Al2O3 catalyst; (d) Pt La (3 wt.%)/ Al2O3 catalyst.

La2O3, the thermal stability of y-Al2O3 was noticed to increase may be due to the dispersion of the promoter particles within the alumina and its direct impact on its thermal properties, Fig. 2a and b. Cerium promoted catalyst, Fig. 3b, exhibits an emerging new exothermic peak around 550 0C in addition to the original exothermic peak characterizing Al2O3 oxide. The new exothermic peaks could be due to a segregation process of CeO2 (3 wt.%). The elevated thermal stability of y-Al2O3, upon the presence of La and Ce, is explained as due to the hindering of transformation to the less stable k-Al2O3 phase [10].

3.1.4. FT-IR

FT-IR examination was utilized to monitor any possible structural changes that could occur to the alumina support, Pt/Al2O3 and upon loading by of La, and Ce promoters,

(Fig. 4). Pure y-Al2O3 structure was regarded as hydrogen spinal and the complex Al-O IR absorption band between 1100 and 500 cm-1, (Fig. 4a) and as reported [14]. The low frequency areas (500-680 cm-1) should be dominated by the stretching vibration band of AlO6 octahedral. The high frequency areas (680-1000 cm-1) are associated due to the stretching vibration band of AlO4 tetrahedral [15]. In addition, the y-Al2O3 exhibits a new absorption band at 1406 cm-1 which could be assigned as due to the longitudinal acoustical mode vibration of the nano-y-Al2O3 [16]. Alumina possess well-defined hydroxyl bands whose nature and intensity are affected by alumina impurity content [17]. Broad intense bands between 3600 and 3300 cm-1 are mainly due to the -OH stretching mode of layer hydroxyl groups and interlayer water molecules. A shoulder band that appears at 3790 cm-1 is related to the terminal -OH groups situated over one

Figure 3 TGA and DSC profiles of: (a) Pt Ce (0.25 wt.%)/Al2O3 catalyst; (b) Pt Ce (3 wt.%)/Al2O3 catalyst.

Figure 4 FTIR of: (a) Al2O3; (b) Pt/Al2O3; (c) Pt-La

(0.25 wt.%)/Al2O3; (d) Pt-La (1.5 wt.%)/Al2O3; (e) Pt-La

(3 wt.%)/Al2O3; (f) Pt-Ce (0.25 wt.%)/Al2O3; (g) Pt-Ce (1.5 wt.%)/Al2O3; (h) Pt-Ce (3 wt.%)/Al2O3.

tetrahedral coordinated alumina ion in a non vacant environment. The spectra of Pt/Al2O3 catalyst (Fig. 4b) exhibit the presence of a broad band in the range 1500-1000 cm-1 which are related to different electronic Pt species [18], indicating that Pt exists in several oxides species [19]. The vibration band at 3790 cm-1 has almost disappeared, after Pt loading, which indicates that loading of Pt has lowered the acidity of the alumina sample as some Pt particles fit into the Al2O3 pores and others present on the outer surface of the catalyst. However, the intensity of the bands associated to Al2O3 in the area of 1100-500 cm-1 has decreased upon loading by La and Ce (Fig. 4c-h). As Ce content increases, additional new bands appear in the region 1000-500 cm-1(Fig. 4h) which indicate the presence of CeO2 particles on the external surface of Al2O3 in accordance with DSC results verification (Fig. 2b). Accordingly, loading alumina by La2O3 has proved a strong interaction occurrence between the two elements that lead to a higher degree of La2O3 dispersion (Fig. 3c, d and e) that could be reflected by an increased basicity according to Kubota et al. [20]. Fig. 4 indicates changes in the frequencies of the bands 1500-1000 cm-1 which might lead to a change in electronic properties of Pt species leading to the formation of new Pt species.

3.1.5. H2-TPR

Effects of the addition of La and Ce promoters on the reducibility of Pt/y-Al2O3 catalyst were studied by H2-TPR. (Fig. 5) As observed, all catalysts exhibit a sharp reduction peak around 730-700 0C which might be related to the reduction of contaminants in alumina support (Fig. 5a) [21]. The profile of the undoped monometallic catalyst Pt/Al2O3 (Fig. 5b) displays two reduction peaks, one near 314 0C which is explained as due to the reduction of Pt oxides that interact weakly with Al2O3 support and a second smaller one at about 500 0C that is due to the reduction of Pt oxides which interact strongly with the support and as reported by Damyanova and Bueno [23]. The TPR profiles of the ternary system (Pt CeO2/Al2O3) are shown in (Fig. 5c-e). As denoted, the position of TPR peaks depends on the amount of CeO2 and therefore on the type and strength of the interaction between supported Pt-oxide species and the alumina support. Previous studies concluded that the presence of ceria can strongly influence the oxidation and reduction properties of

supported noble metals [22]. While a peak shoulder appears near the first reduction peak of Pt-oxide species in the lower temperature region 225-250 0C (Fig. 5d and e) another peak appears at 475 and 450 0C at CeO2 loading of 1.5 and 3 wt.%, respectively. The intensity of this peak increases with increasing of CeO2 loading, confirming the presence of reduced surface ceria presence, indicating the influence of Pt on the surface of CeO2-Al2O3 toward easier reduction of CeO2. TPR profiles of the Pt La2O3/Al2O3 catalysts are shown in (Fig. 5f-h) indicating the addition influence of La2O3 on the state of pt and as revealed by FT-IR analysis.

3.1.6. H2-TPD

Hydrogen is the one of the products of dehydrogenation reaction; accordingly its chemisorptions to the solid catalyst can strongly affect its performance. Utilizing H2-TPD analytical tool would then shed some light on the disparity of metal/support interaction and electronic properties of supported metal particles [24]. Usually, low temperature desorption peaks are assigned to hydrogen by metallic Pt [25] and the high temperature desorption peaks are assigned to spillover hydrogen, strong chemisorbed hydrogen and hydrogen in subsurface layers of the platinum [26]. TPD of hydrogen adsorbed during reduction and cooling in flowing H2 (Fig. 6) exhibits the performance of the promoter free and that added to Pt/Al2O3. As noted, Pt/Al2O3 catalyst exhibits two peaks at a maximum of 140 and 180 0C; which are attributed to chemisorbed hydrogen desorbing from the platinum particles [25]; two Gaussian

peaks at 480 0C and a smaller one with a maximum at 380 0C. Upon addition of La and Ce promoter to the Pt/Al2O3 catalyst, the ability to absorb hydrogen was found to be increased especially in the high temperature range. Previous studies by Odd et al. [21] indicated that over Pt-Sn/Al2O3 catalyst, the high temperature adsorbed hydrogen aids in maintaining catalyst activity, most probably by reducing coking on the metal. The appearance of multiple TPD peaks is attributed to desorption of ''re-adsorbed hydrogen'' [27]. Fig. 6 shows a change in intensity of the lower temperature desorption peaks indicating that addition of Ce leads to changes in the electronic properties of Pt supported on Al2O3.

3.1.7. CO2-TPD

In order to explore such a relationship between the catalytic and basic properties of the catalyst samples a detailed investigation of basicity of these samples was carried out using TPD of CO2. The number and strength of basic sites are evaluated from the intensity and position temperature of TPD signals, respectively, results of which are illustrated in Fig. 7. As can be seen, the strength of the basic sites decreases in the order Pt Ce/Al2O3>Pt La/Al2O3, in addition to the increase of the basic strength as additive content is increased.

3.1.8. NH3-TPD

To assess the acidity of the different catalysts NH3-TPD adsorption/desorption behavior was followed. NH3 desorption from Al2O3-based materials is usually reported in the range between 100 and 800 0C for NH3 that is adsorbed on the acid sites (OH groups). Fig. 8 illustrates the influence of La2O3 and CeO2 promoting results on the population of acidic sites and consequently on the amount of adsorbed NH3. Pt/Al2O3

Figure 5 H2-TPR profiles of: (a) Al2O3; (b) Pt/Al2O3; (c) Pt-Ce (0.25 wt.%)/Al2O3; (d) Pt-Ce (1.5 wt.%)/Al2O3; (e) Pt-Ce (1.5 wt.%)/Al2O3; (f) Pt-La (0.25 wt.%)/Al2O3; (g) Pt-La (1.5 wt.%)/Al2O3; (h) Pt-La (3 wt.%)/Al2O3.

Figure 6 H2-TPD profile of: (a) Pt/Al2O3; (b) Pt-La (0.25 wt.%)/ Al2O3; (c) Pt-La (3% wt.%)/Al2O3; (d) Pt-Ce (0.25 wt.%)/Al2O3; (e) Pt-Ce (3% wt.%)/Al2O3.

presents the highest amount of NH3 adsorbed on the catalyst, with two superimposed desorption peaks identified (~220 and ~325 0C), related to weak and medium/strong adsorbed NH3. Upon introduction of La2O3 and CeO2 a clear reduction of the desorbed NH3 together with a significant shift to lower temperature for both the desorption maxima are observed indicating a reduction in the number and strength of the acidic sites of the catalysts aided by the very high dispersion of the promoting oxide on the alumina surface. The reduced acidity of the materials promoted by La2O3 and CeO2 justify the performances of solid catalysts. Fig. 8 illustrate the NH3-TPD profiles Pt/Al2O3 (a), Pt-La (3 wt.%)/Al2O3 (b) and Pt-Ce(3 wt.%)/Al2O3 (c) reduced at 500 0C.

3.1.9. HRTEM

To construct an overall preview of the effect of La and Ce promoters on the performance of Pt/Al2O3 during catalytic conversion of w-alkanes TEM studies of the solid particles were carried out. Fig. 9 illustrates the TEM images of the different solid catalysts. The TEM images indicate that the Pt particles appear as darker points in the lower nanosize range of 15.6 nm, while when the Pt particles supported over the alumina support their size increase to 3-5.6 nm. Nevertheless, a significant observation was noted when promoters are added where Pt particles are measured at 1.9-4.3 nm in the presence of either promoter, namely; La and Ce. It is also noted that the increase of La from 0.25 to 3 wt.% aided to decrease the particles size from 1.9-4.3 nm to 1.5-3 nm.

3.2. Catalytic conversion and selectivity

In the present work, the influence of the La, and Ce, as promoters, to the activity of Pt/Al2O3 during cyclohexane dehy-drogenation, n-hexane and n-heptane dehydrocyclization reactions is analyzed using a pulsed catalytic microreactor.

Figure 8 NH3-TPD profiles recorded for the samples reduced at 500 0C: Pt/Al2O3 (a), Pt-La (3 wt.%)/Al2O3 (b) and Pt-Ce(3 wt.%)/Al2O3 (c).

The conversion (X) either by dehydrogenation or dehydrocyclization and selectivity (S) were calculated from the corrected integrated signals by:

X(%) =

■X 100

S(%) = /produfct X 100

hIn hout

Figure 7 CO2-TPD profile of: (a) Pt-La (0.25 wt.%)/Al2O3; (b) Pt-Ce (0.25 wt.%)/Al2O3/; (c) Pt-La (3 wt.%)/Al2O3; (d) Pt-Ce (3 wt.%)/Al2O3.

where, the subscripts 1In and Iout represent the composition of components in the feed stream and in the product stream, respectively [30].

3.2.1. Dehydrogenation of cyclohexane

The catalytic conversion results are represented in Tables 2-4. The experimental results show that the conversion of cyclohex-ane is affected greatly by the reaction temperature, and that the activity of Pt/Al2O3 catalyst is higher than that shown by the promoted alumina (Table 4). The order of catalytic activities at all reaction temperatures for cyclohexane dehydrogena-tion to benzene with respect to the catalyst type is as follows: Pt/Al2O3 >Pt La/Al2O3 <Pt Ce/Al2O3. Table 3 shows that catalysts containing La2O3 exhibit higher activity for cyclohexane dehydrogenation than those containing CeO2 (Table 4). As indicated by DSC results, the Pt Ce/Al2O3 samples show two crystalline structures of both y-Al2O3 and ceria (Fig. 3). On the other hand we can see that for Pt La/Al2O3 samples < y-Al2O3 phase is the only structure present. The formation of the y-Al2O3 and CeO2 structures in the ceria containing catalysts is an indication for the segregation of ceria. La2O3 segregation phenomenon is not observed, however, within the La containing samples. The formation of tetrahedral alumina when doped by La2O3 is considered as an additional support that the lanthanum is responsible for the modification of the alumina environment due to its incorporation and stabilization into the alumina network as revealed by Vazquez et al. [28]. FTIR and CO2 analysis indicated that ceria containing catalysts are considered as basic oxides, which is considered as a disadvantage to dehydrogenation.

Figure 9 TEM images of: (a) Pt (0.35)La(0.25)/Al2O3; (b) Pt (0.35)La(3)/Al2O3; (c) Pt (0.35)La(0.25)/Al2O3; (d) Pt(0.35)/Al203.

3.2.2. Dehydrocyclization of n-heptane

Comparing cyclohexane dehydrogenation data, it can be seen that the Pt La/Al203 catalysts, possess higher dehydrogenation activity than the other promoted catalysts. These catalysts appeared to be more resistant to thermal treatment than the base Pt/Al203 catalyst as indicated by DSC analysis, Fig. 2. With regard to product distribution as a function of catalyst type, toluene was found as the major aromatic product in addition to heptane, methylcyclohexane and ethylcyclopentane. Benzene and methane are produced as probably due to hydrogenolysis by product. Table 5 exhibits the appearance of toluene which is formed probably through a consecutive process in which the intermediates are adsorbed species that quickly transform on the same active site of the catalyst surface from n-heptane to toluene as indicated by Davis [29]. Benzene and methane yield continuously grows up by increasing La content which might be formed as a secondary and final

product from toluene by hydrogenolysis of its lateral chain as indicated Adolfo et al. [30]. In addition, formation of methyl-cyclohexane and ethylcyclopentane are might formed via direct cyclization of n-heptane in two possible parallel reactions, namely; C1-C6 and C1-C5 ring closure, respectively. Both reactions are claimed, although the 1,6-cyclization is more favored, at least over platinum [31] on non acid supports (non chlorinated alumina) at the temperature followed in the present work. Direct cyclization of C1-C6 might have occurred via ring closure of heptatrienes to methylcyclohexandiene. Skeletal iso-merization takes place by hydrogenolysis of ethylcyclopentane and to a smaller extent by a bond-shift process [32]. In all experiments, the reaction product included a small concentration of n-heptene, which is considered as an intermediate during the formation of toluene from n-heptane by dehydrogenation on Pt metal and as reported by Lafyatis et al. [33].

3.2.2.1. n-Heptane dehydrocyclization mechanism. According to the above results and mechanistic elucidation, n-heptane is transformed over the Pt/Al2O3 and La promoted catalysts through a complex network of parallel and consecutive interconnected reactions as suggested in Scheme 1, with intermediate compounds [30], the overall chemical process occurs on the catalyst surface, involving adsorbed species (ri). The pathway includes reaction of cyclization (CY), dehydrogenation (DH), isomerization (IS) and hydrogenolysis (HL).The initial stage of the mechanism is the adsorption of n-heptane through its terminal carbon atom (C1) on a metallic center, with abstraction of a hydrogen atom by other neighboring metallic centers, giving the first reactive intermediate species (rC7). Heptenes and methylcyclohexane have been identified as precursors of

Table 2 Cyclohexane dehydrogenation by using Pt/Al203

catalysts as a function of reaction temperature

Catalyst Pt/Al203

Temperature (°C) 300 350 400 450

Conversion (wt.%) 47 60 72 78

Benzene 47 60 64 66

n-Hexane - - 8 12

Selectivity (wt.%)

Benzene 100 100 88.9 91.7

n-Hexane - - 11.1 8.3

Table 3 Cyclohexane dehydrogenation by using PtLa/Al2O3 catalysts as a function of reaction temperature.

Catalyst Pt La(0.25 wt.%)/Al2O3 Pt La(1.5 wt.%)/Al2O3 Pt La(3 wt.%)/Al2O3

Temperature (°C) 300 350 400 450 300 350 400 450 300 350 400 450

Conversion, wt.% 34 47 58 63 39 51 60 64 43 55 62 65

Benzene 34 47 53 55 39 51 57 59 43 55 62 65

n-Hexane - - 5 8 - - 3 5 - - - -

Selectivity, wt.%

Benzene 100 100 91.4 87.3 100 100 95 92.2 100 100 100 100

w-Hexane - - 8.6 12.7 - - 5 7.8 - - - -

Table 4 Cyclohexane dehydrogenation by using Pt Ce/Al2O3 catalysts as a function of reaction temperature

Catalyst Pt Ce(0.25 wt.%)/Al2O3 Pt Ce(1.5 wt.%)/Al2O3 Pt Ce(3 wt.%)/Al2O3

Temperature °C 300 350 400 450 300 350 400 450 300 350 400 450

Conversion, wt.% 24 40 46 52 32 44 55 58 36 48 60 66

Benzene 24 36 40 42 32 44 52 52 36 48 60 66

w-Hexane - 4 6 10 - - 3 6 - - - -

Selectivity, wt.%

Benzene 100 90 87 80.8 100 100 94.5 89.7 100 100 100 100

w-Hexane - 10 13 19.2 - - 5.5 10.3 - - - -

Table 5 «-Heptane dehydrocyclization at 450 °C by using Pt La/Al2O3 as a function of La content and Pt /Al2O3 catalysts.

w-Heptane dehydrocyclization Catalyst type

Pt La(0.25 wt.%)/Al2O Pt La(1.5 wt.%)/Al2O3 Pt La(3 wt.%)/Al2O3 Pt/Al2O3

Conversion, wt.% 33 63 97 91

Methane -- 1 2 4

w-Heptene 2 5 9 5

Ethylcyclopentane 3 4 4 4

Methylcyclohexane 18 23 28 18

Benzene -- 7 15 27

Toluene 10 23 39 33

Selectivity, wt.%

Methane -- 1.6 2 4.4

w-Heptene 6 8 9.3 5.5

Ethylcyclopentane 9 6.4 4.0 4.4

Methylcyclohexane 54.5 36.5 29 19.8

Benzene -- 11 15.5 29.7

Toluene 30.5 36.5 40.2 36.2

toluene and they are transformed faster than w-heptane on the catalyst surface. From this C1 adsorbed entity, the different paths of the network are determined by the position of the second adsorbed carbon atom in the chain of the rC7. Results revealed in Table 5 show that heptene, appears as a precursor of methylcyclohexane. Thus, it is reasonable to assume that the subsequent adsorption of the C2 carbon atom on Pt with the formation of the first alkene-like intermediate (rC=) should be highly favored, in agreement with the most published results [29].

This species may be transformed following different routes, namely:

i- Desorption to the gas phase as heptane.

ii- C1-C6 ring closure with formation of methylcyclohex-ane, which may desorb to the gas phase or successively dehydrogenate on the surface to give finally toluene (Path I). This path is very feasible due to the high aromatizing selectivity of the Pt/Al2O3 catalyst, which could have be enhanced by the addition of La2O3, which increases the reducibility of platinum as evidenced by TPR analysis (Fig. 5).

iii- The species rC= may be successively dehydrogenated to heptatriene (rC=) (Path II) and then undergo cycliza-tion to toluene. Eventually, cyclization of the alkene and/or alkadiene adsorbed species may give the

corresponding adsorbed cycle of the path I. It should be noted that neither heptadienes nor heptatriene were detected in the product, indicating that the reactivity of these intermediates is so high that they react before they desorb, in accordance to the literature data [33]. Q-C5 closure with formation of ethylcyclopentane, which may desorb to the gas phase or give rise to different types of compounds and, indirectly, can also lead to toluene (Paths III of the scheme). To meet the Tee principle [34] the appearance of toluene as a primary product can be explained assuming that the successive transformations, of the intermediates in paths I and II take place in the adsorbed phase, on the same active site, following the mechanism in the literature data [34]. In this mechanism, the intermediates methylcyclohexane and heptenes formed are stable molecules, which can desorb to the gas phase. In contrast, the highly unsaturated adsorbed intermediates are too reactive to be desorbed and

appearing between reaction products. Experimental evidences of the dehydrocyclization mechanism via unsatu-rated intermediates have been also discussed in earlier publications [34]. It is important to point out, on the other hand, that the mechanism in Scheme 1 is proposed on the basis of results obtained at atmospheric pressure, because at higher pressures it may be different.

3.2.3. Dehydrocyclization of n-hexane

It has been demonstrated that Pt La (3 wt.%)/Al203 was the best catalyst for n-heptane dehydrocyclization. Aromatization of n-hexane is more difficult than that of higher hydrocarbons. [35] An attempt to explain this property is made here, by a comparative study on the mentioned catalyst and Pt/Al203. Two catalysts have been compared in a pulse system at 450 °C as in n-heptane aromatization reaction. Inspecting Table 6 we observe that the presence of La203 leads to double

Scheme 1 Reaction pathway for dehydrocyclization of n-heptane [30].

3-ineihylpcniane

Scheme 2 n-Hexane dehydrocyclization mechanism [36].

Table 6 n-Hexane dehydrocyclization at 450 °C by using Pt/ Al203 and Pt La(3 wt.%)/Al203 catalyst.

n-Hexane dehydrocyclization Catalyst

Pt/Al203 Pt La(3 wt.%)/Al203

Conversion, wt.% 64.5 71

Methylcyclopentane 34.5 12

Benzene 30 59

Selectivity, wt.%

Methylcyclopentane 53.5 17

Benzene 46.5 83

benzene formation from 30 to 59% and selectivity was also modified from 46.5 to 83%. The aromatization of n-hexane to benzene is accompanied by the formation of methylcy-clopentane. Table 6 shows the absence of cyclohexane as a product, explaining that it is completely converted into benzene. It is reasonable to expect that the effect of La203 during aroma-tization is different for the two reactions studied here: n-hexane to benzene and cyclohexane to benzene, although the end-product is the same. This indicates that the promotion of aromatization had no effect on the dehydrogenation of cyclo-alkane, but rather an effect on the ring closure. Dehydrogenation of cyclohexane to benzene is a reaction which occurs on the freely exposed platinum surface. Suppression of the activity in dehydrogenation of cyclohexane on Pt/Al203 promoted catalysts points to a blocking and ensemble size effects, respectively. The effect on aromatization is then clearly a promotion effect which prevails over the blocking effect as indicated by Fukunaga and Ponec [36].

3.2.3.1. n-Hexane dehydrocyclization mechanism. The aromati-zation of n-hexane to benzene goes by the ring closure, which proceeds either via a stepwise dehydrogenation (n-hexane-hex ene-hexadiene-hexatriene) or via intermediates bound to the surface by a carbon-like or carbine-like bonds, possibly in combination with a p-complexing of the olefinic bond in the intermediate [37]. Manninger et al. [38] expressed a very interesting view that the presence of alkali cations can influence the formation of the intermediates adsorbed on Pt; considered as a sterical or electronic field effect. The aromatization of n-hexane to benzene is accompanied by the formation of methyl-cyclopentane, which is then dehydroisomerized into benzene (Scheme 2). The methylcyclopentane is formed directly on the metal sites by a 1,5 ring closure rather than by an acid catalyzed isomerization of cyclohexane as indicated by Shum et al. [39] Also cyclohexane was not detected in the product indicating that it is converted completely into benzene.

4. Conclusions

In this study several catalysts of platinum supported on y-Al203 promoted with Ce, and La oxides were prepared as functional catalysts for the dehydrocyclization of alkanes. Catalytic cyclohexane dehydrogenation, n-hexane and n-heptane dehydrocyclization were carried out with regard to the type and concentration of the promoter. Results of cyclo-hexane dehydrogenation indicate that the reaction selectivity decreases upon the addition of the promoters to Pt/Al203 catalyst that may be related to a geometric or electronic

modification of the active Pt particles, as indicated by FTIR, H2-TPD, and H2-TPR analysis.

Moreover, the activity loss of La and Ce containing catalysts, related to the reduction of the number and strength of the acidic sites present on the catalyst surface as indicated by NH3-TPD analysis, which is one of the disadvantages of the dehydrogenation. The significantly lower acidity of the La203-based system can be associated with the very high dispersion of the promoting oxide on the alumina surface as indicated by DSC and FTIR. Segregation phenomena are not observed employing La203 containing catalyst confirming y-Al203 as the stabilized structure presence while segregation in presence of cerium oxide was observed.

The effect of the presence of promoters on aromatization is very different for the two reactions studied, namely; cyclohex-ane to benzene and n-hexane to benzene, although the end-product was found to be the same. Such observation indicates that the promotion of the aromatization is not an effect of the dehydrogenation of cyclo-alkane, but rather an effect of ring closure. The presence of La203 within the Pt/Al203 catalyst proved promotion of aromatization of n-hexane and n-heptane. Aromatization of n-hexane is more difficult than that of n-heptane. Pt La (3 wt.%)/Al203 catalyst appeared to be more active, more selective toward aromatization and more thermally resistant than the basic Pt/Al203 catalyst. Thus modification of the Pt/Al203 catalyst by La203 results in a catalyst with altered activity and selectivity being more active and selective reforming of catalyst.

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