Tuning of catalytic behaviors in ethanol dehydration with oxygen cofeeding over Pd-HBZ catalyst for ethylene production at low temperature

Highlights

• Different pretreatments on Pd-HBZ and O₂ cofeeding affected ethanol dehydration.

• Enhanced ethylene yield was observed at 200 °C under oxygen cofeeding.

• Pd-HBZ-H exhibited the highest ethylene yield of 62% at 200 °C with O₂ cofeeding.

Abstract

The catalytic ethanol dehydration with oxygen cofeeding over different pretreated Pd-HBZ catalysts was investigated. The catalyst was pretreated with different gas including air, nitrogen, and hydrogen. It was tested in ethanol dehydration in the presence and absence of oxygen cofeeding. It was significantly found that with the presence of oxygen cofeeding, the complete ethanol conversion was obtained at very low temperature (ca. 200 °C) and ethylene was a major product. This can be attributed to increased moderate and strong acid sites of catalyst by oxygen cofeeding. In addition, the different gas pretreatment seemed to have less effect on catalytic behaviors.

Introduction

At present, fossil feedstock has decreased due to the depletion of natural resources. The renewable sources or biomass have been considered as alternative raw materials to produce other hydrocarbons. As biomass-derived compound, ethanol is one of the most important renewable fuels, which can be obtained by fermentation of crops including sugar cane, tapioca, corn, etc. [1,2]. In fact, the conversion of ethanol into other value-added hydrocarbon products is also captivating, especially in catalytic ethanol dehydration to produce diethyl ether (DEE) and ethylene. Ethanol can be converted via the catalytic dehydration using acid catalysts to form two main competitive pathways (180–500 °C) [[3], [4], [5]]. First, the production of DEE is obtained from intermolecular dehydration via exothermic reaction at low reaction temperature. Secondly, ethylene occurs from the endothermic reaction at high reaction temperature, which is called the intramolecular dehydration. For DEE, it is very useful in chemical industries, in which it is usually used as a solvent for waxes, fats, oils, and production of plastics [[5], [6], [7]]. Moreover, the addition of DEE into gasoline and diesel can improve the performance of engines for the ignition property [[7], [8], [9]]. Ethylene is widely used for the synthesis of valuable material, including polyethylene, ethylene oxide, ethylbenzene, ethylene dichloride, etc. [4,10]. However, a byproduct such as acetaldehyde can occur via dehydrogenation or oxidation of ethanol. The product distribution essentially depends on the strength of acidic and basic sites of surface catalysts [11].

As known, the solid acid catalysts commonly used in catalytic dehydration consist of different types of acids including heteropolyacid [3], metal oxides [12,13], phosphoric acid [14] and molecular sieve catalysts [13,[15], [16], [17]]. However, the most effectively used catalysts are zeolites such as HZSM-5, beta zeolite, Si-Al-phosphate (SAPO) zeolite. Due to availability, HZSM-5 is the most used catalyst for catalytic ethanol dehydration. Nevertheless, the properties of HZSM-5 have limitation, especially due to its too strong acidity and small pore size [6,15,18,19]. Therefore, another type of zeolite is more interesting to overcome these drawbacks. Beta zeolite, which is well-known for acid-catalyzed reactions and aliphatic alkylation, exhibits high acidity (Bronsted acid sites; both in the internal as well as on the external surface) and large pore structure. Its specific properties can reduce the coke formation due to higher product diffusivity in pore [2,5,20]. Furthermore, the improvement of solid acid catalysts by adding chemical promoter has been extensively investigated. The chemical promoters including metal oxides, alkali, alkaline earth, halogen group and noble metals such as Rh, Ru, Pd, Pt, Re, Au, and Ir have been reported to enhance catalytic activity [12,21,22]. Especially, these metals can increase ethanol conversion and product yield [5,18]. In previous study [9], we revealed that Pd promotion on beta zeolite catalyst can enhance the catalytic activity in ethanol dehydration at low temperature. This is because the effect of Pd on catalytic activity can be attributed to increased amount of hydroxyl groups with Pd modification based on XPS results [9]. As mentioned in our previous study, the amount of hydroxyl groups is related to the acidity i.e. Bronsted sites. Besides the effect of catalytic properties, some researchers reported that the reaction at low temperature provides low ethanol conversion and low catalytic activity [23,24]. This is because the active sites of catalyst at low reaction temperature are not enough to react with ethanol. They also studied the effect of pretreatment condition with modified catalysts to improve the catalytic activity [25,26]. Some research groups have intensively investigated the improvement of catalytic activity to decrease reaction temperature and enhance product yield [24]. In particular, the use of oxygen cofeeding in the gas-phase ethanol oxidation is interesting because it is able to prevent the catalyst surface from carbon deposition, which counts for preventing the catalyst deactivation by simultaneous burning of coke leading to consistently regenerating of active sites and enhancing the ability to catalyze reaction [27,28]. Furthermore, the presence of oxygen leads to shift the equilibrium toward the reaction products with high catalytic activity under anaerobic conditions. Thus, oxygen is added to enhance ethylene production efficiently.

In our previous work [9], we found that the Pd modification (0.5 wt% of Pd) on H-beta zeolite (HBZ) catalyst is promising to produce significant yield of DEE and ethylene. In this work, we further investigate the effects of different pretreatment of catalyst using N₂, air and H₂ prior to reaction and the oxygen cofeeding during catalytic dehydration reaction of ethanol. This aims to tune the catalytic behaviors, especially on the oxygen cofeeding during catalytic dehydration. The correlation between changes in characteristics and catalytic properties up on the pretreatment and oxygen cofeeding is elucidated.