Production of olefins via oxidative de-hydrogenation of C3‒C4 fraction by CO2 over Cr‒Mo/MCM‒41

Production of olefins via oxidative de-hydrogenation of C₃-C₄ fraction by CO2 over Cr-Mo/MCM–41

A.A. Ijagbuji¹ *, V. V. Schwarzkopf¹, ², I. I. Zakharov¹, ², D. B. Woods⁴, T. C. Philips¹, K. M. Jackson⁴, M. B. Saltzberg³, B. V. Shevchenko³, J. K. Johnson⁴

Abstract

The present study investigates the oxidative de-hydrogenation of propane-butane (C₃-C₄) fraction over mono (Cr, Mo) and bi-metal (Cr-Mo) loaded MCM–41catalysts. The catalysts were prepared by sequential impregnation method at 500oC calcination temperature. Experiments were performed by feeding C₃-C₄ fraction and CO2 into a continuous flow quartz reactor at atmospheric pressure (P = 1 atm.), reaction temperatures between 500 – 650oC, gas hourly space velocity within 100 – 400 h-[1], and at reaction time (t r) = 2h. The physicochemical properties and performance of catalysts were evaluated by BET, XRD, H2–TPR, and NH3–TPD characterization techniques. The major products are ethylene, propylene, isobutylene, butylene. This paper reports that the total yield of olefins (Ʃ C₂-C₄) = 71.6 % was achieved at 89.5 % conversion level of C₃-C₄ at 630oC. The results indicate that the addition of Mo to Cr/MCM–41 modifies its catalytic activity. The Cr/MCM–41 and Mo/MCM–41 catalysts were prepared for comparison purposes. Keywords: oxidative de-hydrogenation of C₃-C₄, C₃-C₄ conversion, selectivity to olefin, olefin yield, olefin production.

1. Introduction

Worldwide production of olefins exceeds that of any other chemicals and constitutes a sizable fraction of total petrochemical production. The chemical industry relies heavily on the low-cost and readily available saturated hydrocarbons as feedstock for many industrially significant processes. ¹ Conventional technologies for olefins production involve catalytic de-hydrogenation of alkanes via either the steam cracking or fluid catalytic cracking method. However, these traditional methods for olefin production require high temperatures, high energy input, and are also limited by coke deposition and thermodynamic considerations.² Consequently, the existing commercial processes for olefins production are very energy-intensive, exhaustive, and not cost-effective. While these two routes are very well developed, increasing the capacity of these processes is only possible to some extent, as changing regulation limits the use of by-products (notably aromatic molecules) in fuels. For these reasons, the present industrial capacity for C₂-C₄ olefins production via these routes is expected to be insufficient, and therefore, cannot meet the fast-growing demand of olefins in the international market.³ There are, however, a number of current challenges preventing oxygen assisted de-hydrogenation from being widely implemented. The difficulties inherent in oxidative dehydrogenation reactions revolve around selectivity control because all equivalent C-H bonds have an equal bonding energy, and therefore an equal chance of reacting.⁴ When two C-H bonds of neighboring carbons are split, a double bond is formed and alkanes are converted to alkenes. Thereby, oxygen addition to alkane feeds exposes the synthesized olefins to further oxidation conditions that results into the formation of environmentally-damaging and economically-useless carbon oxides (CO and CO2), consequently decreasing the yield of alkenes [Eq. (3) ‒ (4)]. Due to the exothermic nature of these reactions, it is expedient to remove heat and avoid the over-oxidation of alkanes to CO2 in order to reach high selectivity to olefins.

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Since producers seek to leverage their existing assets and the available internal streams to find an optimum solution for meeting the demands of olefins, oxidative de-hydrogenation of alkane with mild oxidant such as carbon dioxide, has been widely studied, as a potentially attractive route to circumvent the thermodynamic limitations, eliminate coking, and therefore, extend catalyst lifetime. Therefore, the design of effective catalytic systems that exhibit sufficient activity, high selectivity, periodically-regenerated under severe conditions, and yet operate at temperatures that minimize oxygenation of the desired products, are key performance demands for cost-effective production of olefins.

During the past decade, bi-metallic catalysts have attracted considerable attention because they show multiple functionalities, stability,[⁵ ‒ ⁹ ] and selectivity[¹⁰ ‒ ¹² over mono-metallic catalysts. With respect to redox catalysis, chromium is one of the most important element to be incorporated, whereas, molybdenum exhibits excellent catalyst attrition resistance, facilitates easy products desorption from catalyst surface, maintain optical defect concentration, and blocks non-selective sites. A new class of mesoporous silica designated as MCM–41 has attracted increasing attention due to large surface area (A BET > 1000 m[2]/g), high pore volume (V p = 0.98 cm[3]/g), moderate hydrophobic character, as well as functionalized pore channels of large diameter (d p = 1.5 to 10 nm) without network complications. The large surface area allows the binding of a large number of surface groups, whereas, the functionalized pore channels of large diameter allow an easy reaction with adsorbent. Pure siliceous hexagonal MCM–41 cannot be directly used as catalysts. It may be due to the limited thermal stability and negligible catalytic activity of this MCM–41, because of the neutral framework and the lack of sufficient acidity. Incorporation of hetero–atoms into a silica framework has been reported to increase the mechanical strength,¹³ surface acidity of support, and enhance the overall catalytic performance.¹⁴ Thereby, this new physicochemical properties derived from synergistic effects between bi-metals and the silica frame-work is highly desirable for catalytic applications.

To the best of our knowledge, in-depth investigation of MCM–41 supported mono Cr or Mo, and bi-metal Cr-Mo catalyst has not been previously reported in the literature, hence, this is deemed to be investigation worthy. In this paper, the oxidative de-hydrogenation of C₃-C₄ fraction into olefin, at temperatures (T) between 500 – 650oC, atmospheric pressure (P = 1 atm.), gas hourly space velocity (GHSV) = 100 – 400 h–[1], and reaction time (t r) = 2h in a continuous quartz flow reactor is investigated using synthesized Cr, Mo, and Cr-Mo incorporated MCM–41 catalysts.

2. Experimental

2.1 Materials and Methods:

Tetra-methyl-ammonium hydroxide (TMAOH, 26 wt. %); acetic acid (CH3COOH, 25 wt. %), distilled water (H2O, 1 dm [3]); chromium nitrate nonahydrate [Cr(NO3)3·9H2O, 99 %]; ammonium heptamolybdate ([NH4]6Mo7O24.6H2O, 99 %); cetyltrimethylammonium-bromide (CTAB); and silica (SiO2) were obtained from Sigma-Aldrich. The propane-butane fraction (C₃-C₄, 99.99 %) was supplied by Naftogas, Kiev; carbon dioxide (CO2, 99.9 %) &nitrogen (N2, 99.9 %) were obtained from Azot chemical company.

2.2 Catalysts Preparation:

The parent Si-MCM–41 was synthesized according to the following procedures: Typically, 3.0 g of SiO2 was slowly added to a mixture of 2.63 g of aqueous TMAOH and 0.3 g of NaOH dissolved in 28.0 g of distilled water under vigorous stirring for 30 min. Subsequently, an aqueous suspension of CTAB (4.92 g in 14 g distilled water) was added to the above mixture, and the resultant mixture was stirred for 2 h. The gel mixture had a molar composition of SiO2: 0.27 CTAB: 0.15 TMAOH: 0.15 NaOH: 60 H2O. The pH of the resulting gel was adjusted to 10.3 by the drop-wise addition of acetic acid (CH3COOH) followed by stirring for 3 h. The resulting gel was transferred into Teflon-lined stainless steel autoclaves (ca. 100 mL). The autoclave was placed in an oven and the synthesis gel was aged at 150°C for 24 h. After aging, the reactor was cooled down, the gel was filtered, washed extensively with distilled water in order to remove any unwanted species such as sodium ions, nitrate etc., and then dried at room temperature. Once dry, the gel was then calcined at 550°C (heating rate of 1°C/min) for 6 h to remove the organic template and create hollow porous structure. Prior to its application as a support, the calcined MCM–41 was characterized by low angle X-ray diffraction (XRD), N2 adsorption, and transmission electron microscopy (TEM).

4 wt. % of Cr, V and Cr-Mo catalysts were prepared by incipient wetness impregnation of the powdered MCM–41 support: (i) 0.3 g of chromium nitrate was dissolved in 30 mL de-ionized water, and then added to 1 g of MCM–41. After stirring for 0.5 h under reduced pressure via water aspiration at room temperature, the solid was filtered from the solution and dried at room temperature. Final calcination was carried out at 550°C for 6 h (heating rate of 3°C/min), and the Cr/MCM–41 catalyst was obtained; (ii) 0.092 g of ammonium heptamolybdate was dissolved in 30 mL de-ionized water, and added to 1 g of MCM–41. After stirring for 0.5 h under reduced pressure via water aspiration at room temperature, the solid was filtered from the solution and dried at room temperature. Final calcination was carried out 550°C for 6 h (heating rate of 3°C/min), and the Mo/MCM–41 catalyst was obtained; (iii) For 2 wt. % Cr and 2 wt. % Mo: 0.153 g of chromium nitrate and 0.046 g of ammonium heptamolybdate were dissolved in 30 mL de-ionized water, and added to 1 g of MCM–41. After impregnation, the resulting mixture was stirred for 0.5 h under reduced pressure at room temperature. The solid was filtered from the solution, dried at room temperature, and then calcined at 550°C for 6 h (heating rate of 3°C/min) to remove volatile impurities adsorbed on the catalyst surface. The other samples with different weight percentage loadings: 1.2Cr-2.8Mo/MCM–41 and 2.8Cr-1.2Mo/MCM–41 were prepared by similar way. Thereafter, the Cr/MCM–41, Mo/MCM–41, and Cr-Mo/MCM–41 catalyst samples were pressed and then sieved to appropriate sizes for catalytic evaluations. These catalysts were fully characterized by XRD, Infrared spectroscopy (IR), N2 adsorption, and TEM.

2.3 Catalysts Treatment:

0.15 g of each catalyst was calcined in air at 550oC for 2 h to remove any volatile impurity adsorbed on the surface, followed by reduction in 10 % H2/ 90% Ar at 500oC for 6 h to convert the catalyst into the metallic state. The total flow rate of the H2/Ar mixture was 50cm[3]/min. The samples were then cooled at room temperature for 30 mins, and stored in inert atmosphere to avoid degradation.

2.4 Catalysts Test: The catalytic activity of samples for the decomposition of C₃-C₄ fraction was investigated in a continuous flow quartz fixed-bed reactor (6 L vol.). The catalysts sample (0.15 g) was packed in the reactor and activated with flowing N2 at 550oC for 2 h. After which the flow rate of reactant (C₃-C₄) and N2 was maintained at 20 ml/min and 80 ml/min, respectively. The N2 adsorption isotherms of calcined materials were measured at liquid nitrogen temperature (‒196oC). At this temperature, because of its low cost and inert nature, nitrogen is an ideal adsorbent. The gas carrier was passed through a molecular sieve trap before being saturated with C₃-C₄. The gas product samples were analyzed by gas chromatograph.

2.5 Oxidative De-hydrogenation of C₃-C₄:

The C₃-C₄ feed composition was analyzed by gas chromatography «Chrom‒5». It was established that the total composition equals 100 % by volume: propane = 20, i -butane = 60 and n -butane = 20. The oxidative dehydrogenation experiment of C₃-C₄ fraction was carried out in a continuous flow quartz fixed-bed reactor (5 L vol. ). The experimental set up is shown in Fig. 1

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Fig. 1 Schematic diagram of the pilot unit for dehydrogenation of C₃-C₄ fraction

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¹ Institute of Technology, East Ukrainian National University, Severodonetsk, 93400, Ukraine.

² Boreskov Institute of Catalysis, Novosibirsk, Russia

³ Moscow State University, Moscow, Russia

⁴ University of Melbourne, Parkville, Victoria, Australia e-mail address: dejiijagbuji@yahoo.com

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