Akademik Veri Yönetim Sistemi
7th International Hydrogen Technologies Congress, Elazığ, Türkiye, 10 - 12 Mayıs 2023, ss.1
Effect of Ce and Ca Metals on Light Olefins Production in
Methanol to Olefins Reaction
Ezgi Cigdem Arpaci, Pinar Degirmencioglu, Nuray Oktar*
Gazi University, Faculty of Engineering, Chemical Engineering Department, Ankara, 06810, Turkey
* nurayoktar@gazi.edu.tr; oktarnuray@gmail.com
Abstract
Olefins are hydrocarbons that contain at least one carbon-carbon double bond. Light olefins are a subgroup of olefins that contain 2-4 carbons. Ethylene and propylene, which are the most known light olefins, are important monomers for the production of plastics. Light olefins are obtained from oil-dependent methods such as the thermal cracking of naphtha. Due to the decreasing oil resources and increasing energy demand, different production ways have gained importance. One of them is the “Methanol to Olefins (MTO)” reaction, which provides more environmentally friendly conditions and high olefins selectivity. In this study, Ca (3 wt%) and Ce (3 wt %) incorporated commercial HZSM-5 catalysts were synthesized using wet impregnation technique. XRD and N2 Adsorption/Desorption analyses were performed to the fresh catalysts. N2 Adsorption/Desorption isotherms of the synthesized catalysts were compatible with the Type IV isotherm and H4 hysteresis indicating mesoporous structure. Activity tests of the catalysts were performed under atmospheric pressure at 450°C in a continuous flow-packed bed reactor system. The most active and highly selective catalyst was found as 3-Ce-HZSM-5 with complete methanol conversion and 53% light olefins selectivity.
Keywords: Methanol to Olefins (MTO), Olefin, HZSM-5
I. Introduction
Light olefins, which are transformed into polyethylene and polypropylene via polymerization reactions, are among the most important raw materials of the plastics industry. Ethylene and propylene are generally produced in an oil- dependent manner. Excess energy consumption and high CO2 emissions in the production of these monomers are important problems in the production of light olefins. For this reason, more environmentally-friendly and economical alternative methods have been applied. Light olefins are the products obtained from methanol in a non-oil based route, obtaining olefins from methanol, known as 'Methanol to Olefins' (MTO), without depleting oil. In addition, the environmentally-friendly production of methanol, which is the source of this process, from sources such as natural gas, coal and biomass is important for the sustainability of this process. MTO has the advantage of product flexibility thanks to the Ethylene/Propylene ratio that can be adjusted by the acidity of the catalyst. Considering the advantages and working conditions it provides, the MTO process seems to be a more preferable option for the production of light olefins (Aghaei and Haghighi., 2018) (Ghaedı et.al., 2021) compared to other routes such as DTO, FTO, etc. Zeolite catalysts with acidic characteristics are generally used in the MTO reaction. HZSM-5 is often preferred in MTO. However, since the product distribution in MTO is directly affected by the acidity of the catalyst, catalysts must contain mild Brønsted acid sites to increase the light olefins selectivity. Adjusting the acidity of HZSM-5 and the Brønsted acid sites is crucial for olefin selectivity, increasing the catalytic lifetime and determining the product distribution (Khezri et.al., 2020). For this purpose, metal loading is applied to the structure of the catalyst affecting the selectivity positively by adjusting the acidity, morphological structure and the pore size distribution (Aghaei and Haghighi, 2018). The addition of Ce reduces the acidity, increases the light olefins selectivity, reduces the formation of side products and contributes positively to the catalyst life (Mirza et.al., 2018). Ca content, on the other hand, affects the activity positively by modifying the density and intensity of the Brønsted and Lewis acid sites, and increases the propylene selectivity thanks to its alkali metal structure (Ghaedı et.al., 2021), (Mirza et.al., 2018).
In this study, the performance of commercial HZSM-5 at different temperatures (400-500℃) was investigated together with the effect of cerium and calcium on light olefins selectivity and methanol conversion.
II. Experimental Set-up and Procedure
Ca and Ce metals were added to commercial HZSM-5 (Si/Al: 23) by wet impregnation method. 50 mL of deionized water was added to 1 g of HZSM-5 and the resulting mixture was stirred at 40⁰C. 3 wt % metal salt was dissolved in water and added dropwise to this mixture. The resulting mixture was then stirred at 40⁰C for 24 hours. The solid sample obtained after 24 hours was calcined at 750⁰C for 6 hours (heating rate 1°C/min) under a stream of dry air. At the end of the synthesis, metal-loaded HZSM-5 was obtained.
The activity tests of the catalysts were carried out in continuous flow packed-bed reaction system. Prior to MTO reaction, the catalysts were activated by passing He over (at a flow rate of 35 mL/min for 1 hour at 500°C) the catalysts. 0.1 grams of catalyst was placed inside a quartz reactor and placed in a tubular furnace. During the activation process, the catalyst was heated to 500°C at a rate of 10°C/min. It was kept at this temperature for 1 hour and then cooled to the reaction temperature. The methanol in the liquid phase was sent to the evaporator with a syringe pump (0.008 mL/min), where it turns into the gas phase and carried through the reactor via heated lines in which the carrier gas helium (He) flows. The gas mixture formed after the reaction was fed to the gas chromatography (GC) device through the heated lines for analysis. To determine the reaction temperature, experiments were carried out using commercial HZSM-5 at 400, 450 and 500 °C, and then the activity tests of the metal-loaded catalysts were carried out.
III. Analysis
In order to determine the pore structure and the surface area of the synthesized catalysts, N2 adsorption/desorption analysis was performed. The crystal structures of the materials were investigated by X-ray diffraction (XRD) analysis. To determine the morphological structures of fresh catalysts, scanning electron microscopy (SEM) technique was performed.
IV. Results and discussions
Table 1. shows the physical properties of the parent and metal-loaded catalysts prepared by impregnation. The synthesized 3-Ce-HZSM-5 had the highest surface area compared to other catalysts. After Ce and Ca incorporation into HZSM-5, a slight decrease in the surface area was observed due to the possible clogging of the pores by Ce and Ca.
Table. 1: Physical Properties of the Synthesized Catalysts (3 wt % Ce and 3 wt % Ca in synthesis solution)
Catalysts |
Synthesis Method |
BET Surface Area, m2/g |
BJH Desoption average pore size, nm |
Pore volume, cm3/g |
HZSM-5 | commercial | 458 | 3.77 | 0.056 |
3-Ca-HZSM-5 | Wet impregnation | 412 | 3.76 | 0.083 |
3-Ce-HZSM-5 | Wet impregranion | 406 | 3.76 | 0.073 |
Activity test results of all catalysts were given in Fig 1. First of all, the activity tests of commercial HZSM-5 were performed at 400, 450 and 500℃ and 100% conversion was achieved at all temperatures. The highest selectivity towards light olefins was observed at 450℃ with 51%. The activity tests of the metal-incorparated catalysts were carried out at the optimum reaction temperature, 450℃, and according to the time on stream results (Fig 2), the catalysts almost preserved their activities through the reaction. The highest light olefins selectivity was observed with the 3-Ce –HZSM-5 catalyst with as 54%, exceeding that of the parent HZSM-5.
Fig 1. Conversion and light olefins selectivity of the catalysts at 450℃
Fig 2. Light olefins selectivity of the catalysts of with time on stream at 450℃
V. Conclusions
The catalytic performances of HZSM-5, 3-Ce-HZSM-5, 3-Ca-HZSM-5 catalysts prepared with wet impregnation method were investigated in MTO reaction at different temperatures (400,450 and 500℃). The aim of this study was to determine the effect of metals on the physicochemical properties and the catalytic activity of the catalyst in MTO reaction. N2 adsorption/desorption isotherms of the synthesized catalysts were compatible with the Type IV isotherm and H4 hysteresis indicating mesoporous structure. In the activity test performed at 450℃, 3-Ce-HZSM-5 had the highest light olefins selectivity (54%) and 100% methanol conversion, showing an improved catalytic activity compared to the parent HZSM-5.
References
Aghaei E., Haghighi M., Hydrothermal synthesis of nanostructured Ce-SAPO-34: High-performance and long-lifetime catalyst with various ceria contents for methanol to light olefins conversion, Microporous and Mesoporous Metarials, 270, 227-240 (2018).
Ghaedı M., Izadbakhsh A., Effects of Ca content on the activity of HZSM-5 nanoparticles in the conversion of methanol to olefins and coke formation, Journal of Fuel Chemıstry and Technology, 49, 1468-1486 (2021).
Khezri H., Izadbakhsh A., Izadpanah A., Promotion of the performance of La, Ce and Ca impregnated HZSM-5 nanoparticles in the MTO reaction, Fuel Processing Technology, 199 (2020).
Mirza K., Ghadiri M., Haghighi M., Afghan A., Hydrothermal synthesize of modified Fe, Ag, and K-SAPO-34 nanostuctured catalysts used in methanol conversion to light olefins, Microporous and Mesoporous Metarials, 260, 155-165 (2018)