ZrO2

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Impregnation Synthesis of Cr/ZrO2 Nanocatalyst Used in Oxidative ... oxidative dehydrogenation of ethane in the presence of CO2 with the aim of obtaining ...

Proceedings of 5th International Congress on Nanoscience & Nanotechnology (ICNN2014) 22-24 October 2014, Tehran, Iran

Combustion vs. Impregnation Synthesis of Cr/ZrO2 Nanocatalyst Used in Oxidative Dehydrogenation of Ethane to Ethylene by Carbon Dioxide M. Aminia,b, M. Haghighi*,a,b, F. Rahmania,b a

b

Chemical Engineering Faculty, Sahand University of Technology, Sahand New Town, Tabriz, Iran Reactor and Catalysis Research Center (RCRC), Sahand University of Technology, Sahand New Town, Tabriz, Iran *[email protected]

Abstract: The Cr2O3-ZrO2 nanocatalyst was prepared via combustion and co-impregnation methods and characterized by XRD, FESEM, EDX and BET techniques. The present research deals with nano-sized catalyst development for the oxidative dehydrogenation of ethane in the presence of CO2 with the aim of obtaining higher ethylene yield. The XRD results confirm existence of ZrO2 as crystalline phase in the catalyst structure. FESEM analysis confirmed that impregnation prepared sample is nano scale. Furthermore, it reveals small particle size, uniform morphology and narrow particle size distribution. EDX dot-mapping indicated homogenous dispersion of elements, especially for the impregnated one. Also, according to the BET analysis, the impregnated nanocatalyst represented the higher surface area. Based on the excellent characterization, Cr-based nanocatalysts synthesized via impregnation exhibited the superior product yields through all of the investigated temperature. However, the ethylene yield considerably declined during the reaction time. During the 10 h stability test, ethylene yield of combustion made catalyst were remained at same level throughout the 600 min of time on stream and the deactivation was not observed. Keywords: Impregnation, Combustion, Cr/ZrO2, Dehydrogenation, Ethane, Ethylene.

Introduction In the last decade, the dehydrogenation of ethane by carbon dioxide has received much attention [1, 2]. Carbon dioxide is a promising oxidant for dehydrogenation of ethane. However, a number of challenges like controlling selectivity have prevented oxidative dehydrogenation (ODH) from being widely implemented that can be solved by achieving catalysts with good performance [3]. Therefore, attaining the promising catalyst is the most significant obstacle facing ODH commercialization Among all the studied oxide catalysts, the supported chromium oxide-based nanocatalysts have been most promising for ODH due to their high catalytic activity [1, 4]. Activity and selectivity of these catalysts strongly depend on the degree of Cr species dispersion, method of catalyst preparation and also the type of support [5, 6]. Synthesis method has significant effect on properties and performance of catalyst by influence on the catalyst structure, location and type of active phases. Among common synthesis methods, the combustion synthesis is a novel, one step method recently used for fabricating nanosized composites and nanocatalysts. In the present work, Cr2O3-ZrO2 mixed oxide has been prepared using combustion and impregnation procedures with the aim of investigating the influence of the preparation method on the catalytic properties in the oxidative dehydrogenation of ethane with CO2. In this regard, the structural evaluation of the catalysts was thoroughly studied using various physiochemical techniques namely, XRD, FESEM, EDX, and BET. The catalytic performance was performed under atmospheric pressure.

Materials and method zirconyl (IV) nitrate hydrate (Aldrich, Germany, 99%) and chromium (ІІІ) nitrate (Chem-Lab, Belgium, 96%) as metal precursors and sorbitol (Aldrich, Germany, 98%) as fuel were employed in catalyst preparation methods.

Cr2O3-ZrO2 nanocatalyst was synthesized by impregnation and combustion techniques. For the impregnation method, the required amounts of zirconyl (IV) nitrate hydrate and chromium (ІІІ) nitrate corresponding to 5 wt% Cr2O3 in the final product were dissolved separately in the de-ionized water and mixed together under mechanical stirring. Next, the solution was dried at 110 °C for 24 h and calcined for 5 h at 600 °C in static air atmosphere. In the combustion method, the homogeneous solution of metal nitrates and fuel with molar ratio of 1/2 is heated over a hot plate until it reaches the ignition temperature. Once ignited, the reaction can proceed very fast and ultimately, puffy and porous powders were obtained. Catalytic measurements were carried out in a U-type fixed-bed reactor made of quartz (i.d.=6mm) at atmospheric pressure with 500 mg catalyst loading. The reaction temperature ranged between 500 to 700 °C. The reactant stream was constituted of 10% ethane, 50% CO2 and 40% N2 introduced into the reactor at a total gas flow of 50 ml min-1. The feed and product gases were analyzed on-line with a gas chromatograph (GC Chrom, Teif Gostar Faraz, Iran) equipped with a FID and TCD in series. The Carboxen-1000 was used for the analysis of ethane, ethylene, CO, CO2, CH4 and H2. Carbon balance was closed within 3% error in all experiments.

Results and Discussion Figure 1 shows the XRD patterns of Cr2O3-ZrO2 mixed oxides prepared by combustion and impregnation methods. The diffraction peaks corresponding to monoclinic and tetragonal ZrO2 are observed over the both Cr-based catalysts. It reveals that more intense peaks appeared in the XRD pattern of combustion made sample comparing to impregnation synthesized catalyst. It is indicated that impregnation method is capable of reaching uniform and nano-sized particles which can be confirmed

Proceedings of 5th International Congress on Nanoscience & Nanotechnology (ICNN2014) 22-24 October 2014, Tehran, Iran

by FESEM analysis. A detailed examination of XRD patterns revealed that no crystalline Cr2O3 was observed which can denote the high metal dispersion in the obtained nanocatalyst which is going to be supported by EDX analysis. It is known that high Cr species dispersion leads to better reactivity of the nanocatalyst used.

(a) Combustion

Cr2O3 (Rhombohedral, 00-006-0504) ZrO2 (Tetragonal, 01-080-0784) ZrO2 (Monoclinic, 00-013-0307)

Cr Zr

O

25.0µm

Intensity (a.u.)

keV (a) Combustion

(b) Impregnation

(b) Impregnation Reference patterns Cr2O3 (Rhombohedral, 00-006-0504)

ZrO2 (Tetragonal, 01-080-0784)

Cr Zr

O

25.0µm

ZrO2 (Monoclinic, 00-013-0307)

10

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2θ (degree)

Fig. 1: XRD patterns of the synthesized nanocatalysts.

Fig. 2 displays the surface morphology of impregnation synthesized Cr 2O3-ZrO2 nanocatalyst. According to the image, the morphology of surface material is homogeneous and contains spherical type agglomerates with small sizes within the nanometer scale which this can provide very good surface for active phase dispersion and accessibility. Therefore, it is expected that the nanocatalyst exhibits superior catalytic activity.

Impregnation

Fig. 2: FESEM image of the synthesized nanocatalyst.

Based on the so far reported data, the Cr species dispersion is a major factor affecting the intrinsic activity of oxidative dehydrogenation of ethane. Figure 3 shows the EDX analysis of the as-synthesized Cr-based nanocatalysts. The result represented the good existence and superior dispersion of the desirable elements in the impregnated catalyst structure which is going to affect the catalytic performance.

keV

Fig. 3: EDX analysis of the synthesized nanocatalysts.

In heterogeneous catalysis, adsorption is one of the most essential steps because it activated the decisive chemical bonds of the adsorbed reactants. Higher surface area not only facilitates these reactions but also enhances the Cr species dispersion as decisive parameter in the catalytic performance of ODH process, leading to the enhancement of catalytic performance of Cr-based catalyst. Due to the mentioned issue, the measured surface area of the impregnation and combustion prepared catalysts are 41 and 23 m2/gr, respectively. The synthesized catalyst via impregnation exhibited relatively high surface area, representing the ability of the synthesis method for better Cr dispersion over zirconia compared to combustion method. This is supported the claim of better Cr species dispersion by the XRD analysis. The C2H6 conversion and ethylene yield as a function of temperature are depicted in figures 4 and 5, respectively. This evolution was carried out from 500 up to 700 °C. Blank test results indicate that the homogeneous reaction will not make significant contribution to the heterogeneous reaction. The C2H6 conversion and ethylene yield was dramatically enhanced by increasing operation temperature due to the endothermicity of the ODH reaction. Moreover, preparation method exerts an important influence on the catalytic activity of the nanocatalysts. Over all the examined temperatures, the impregnation synthesized nanocatalyst exhibits superior catalytic activity for the oxidative dehydrogenation of

Proceedings of 5th International Congress on Nanoscience & Nanotechnology (ICNN2014) 22-24 October 2014, Tehran, Iran

Besides the catalytic activity, time on stream performance of product yield is significant issues for ODH reaction. The catalytic activity of Cr-based samples was tested for 600 min time on stream at constant temperature (700 ºC), feed ratio and GHSV and the results are shown in Figure 6. The result depicts the high catalytic activity during the reaction up to 10 h for the impregnated sample. However, the ethylene yield reduced gradually by the time. This decrease in yield may be due to deactivation of active phases by coke deposition or reducing to metal form which would expedite other side reactions. Moreover, it can be seen that the yields for combustion sample were remained at same level throughout the 600 min of time on stream and the deactivation was not observed for the nanocatalyst. 60

C2H6 Conversion (%)

C2H6/CO2/N2 = 10/50/40 GHSV = 6000 h-1

Impregnation

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Time on stream (min)

Fig. 6: Stability of the nanocatalysts.

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References 20 10 0 550

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Temperature (°C)

Fig. 4: Effect of temperature on ethane conversion. 50 Blank

40

Combustion Impregnation

C2H4 Yield (%)

C2H6/CO2/N2 = 10/50/40 T = 700ºC GHSV = 6000 h-1

Combustion

The authors gratefully acknowledge Sahand University of Technology for the financial support of the research as well as Iran Nanotechnology Initiative Council for complementary financial supports.

Combustion Impregnation

40

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Acknowledgment

Blank

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the best activity over the course of reaction. However, the ethylene yield considerably declined during the reaction time. In spite of lower catalytic activity, combustion prepared nanocatalyst exhibited the stable performance during the 600 min of time on stream.

C2H6 Conversion (%)

ethane in the presence of CO2 which can be ascribed to its enhanced physiochemical properties like homogeneous morphology and small particle size; higher metal dispersion and surface area evidenced by FESEM, EDX dot-mapping and BET analysis, respectively.

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C2H6/CO2/N2 = 10/50/40 GHSV = 6000 h-1

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0 550

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Fig. 5: Effect of temperature on ethylene yield.

Conclusions Based on the characterization results, the impregnated Cr2O3-ZrO2 nanocatalyst exhibited the nano scale particles, uniform particle size distribution, higher surface area and enhanced metals dispersion compared to combustion made one. Catalytic performance tests of synthesized samples showed that impregnated sample had

[1] X. Shi, S. Ji, K. Wang, "Oxidative Dehydrogenation of Ethane to Ethylene with Carbon dioxide over Cr– Ce/SBA-15 Catalysts", Catalysis Letters, 125 (2008) 331. [2] R. Wu, P. Xie, Y. Cheng, Y. Yue, S. Gu, W. Yang, C. Miao, W. Hua, Z. Gao, "Hydrothermally prepared Cr2O3–ZrO2 as a novel efficient catalyst for dehydrogenation of propane with CO2", Catalysis Communications, 39 (2013) 20. [3] J.-H. Li, C.-C. Wang, C.-J. Huang, Y.-F. Sun, W.-Z. Weng, H.-L. Wan, "Mesoporous nickel oxides as effective catalysts for oxidative dehydrogenation of propane to propene", Applied Catalysis A: General, 382 (2010) 99. [4] A.J.R. Castro, S.P.D. Marques, J.M. Soares, J.M. Filho, G.D. Saraiva, A.C. Oliveira, "Nanosized aluminum derived oxides catalysts prepared with different methods for styrene production", Chemical Engineering Journal, 209 (2012) 345. [5] P. Michorczyk, J. Ogonowski, M. Niemczyk, "Investigation of catalytic activity of CrSBA-1 materials obtained by direct method in the dehydrogenation of propane with CO2", Applied Catalysis A: General, 374 (2010) 142. [6] P. Michorczyk, P. Pietrzyk, J. Ogonowski, "Preparation and characterization of SBA-1– supported chromium oxide catalysts for CO2 assisted dehydrogenation of propane", Microporous and Mesoporous Materials, 161 (2012) 56.