Comparison of four catalysts in the catalytic dehydration of ethanol to ethylene

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Abstract

The aim of this study was to compare the activity and stability of γ-Al2O3, HZSM-5 (Si/Al = 25), silicoaluminophosphate (SAPO-34) and Ni-substituted SAPO-34 (NiAPSO-34) as catalysts in the dehydration of ethanol to ethylene. γ-Al2O3- and HZSM-5 were commercial catalysts. SAPO-34 and NiAPSO-34 molecular sieves had been synthesized with hydrothermal method in the laboratory, characterized with X-ray powder diffraction (XRD), Infrared Spectroscopy (FT-IR), H2 temperature-programmed reduction (H2–TPR) technique and NH3 temperature-programmed desorption (NH3–TPD) technique. The incorporation of Ni2+ into the SAPO-34 framework generated in NiAPSO-34 sample was proved by XRD, FT-IR and H2–TPR techniques. NH3–TPD study had revealed that substitution of Ni2+ for Al3+ in the SAPO-34 framework led to increase the weak and moderately strong acid strength and give rise to weak acid sites. Dehydration of ethanol was carried out over four catalysts and the results showed that conversion of ethanol and selectivity to ethylene decreased in the order HZSM-5 > NiAPSO-34 > SAPO-34 > γ-Al2O3. As to the stability of catalyst, NiAPSO-34 and SAPO-34 were better than other two catalysts. Considering the activity and stability of the four catalysts comprehensively, NiAPSO-34 was the suitable catalyst in the dehydration of ethanol.

Introduction

Ethylene is an important material for the organic chemistry industry used in the preparation of polyethylene, ethylene oxide, ethylene dichloride, etc. Conventionally, it has been commercially produced by the thermal cracking of liquefied petroleum gas (LPG) or naphtha [1]. This process is an endothermic reaction requiring high temperature of 600–1000 °C. Compared to the conventional route, catalytic dehydration of ethanol to ethylene is proving attractive as it requires lower temperature and offers higher ethylene yield [2], [3]. Moreover, ethanol can be produced from renewable sources, production of ethylene, therefore, does not depend on petroleum source.

Dehydration of ethanol can take place by two competitive paths. One is the intramolecular dehydration of ethanol to ethylene and the other is intermolecular dehydration of ethanol to diethyl ether. At lower temperature, diethyl ether is produced in significant quantities, while, at the higher temperature, ethylene is the dominant product. Ethanol dehydrogenation to produce acetaldehyde can also occur as a side reaction at the higher temperature [4].

One of the early catalysts employed for the dehydration of ethanol was γ-alumina (Al2O3) [5]. With Al2O3 catalyst, dehydration of ethanol required higher reaction temperature (450 °C) and offered lower ethylene yield (80%). At present, researchers studied on dehydration of ethanol over HZSM-5 zeolite. Numerous papers had already been published on the subject [6], [7]. With HZSM-5 catalyst, at lower temperature (300 °C), 95% of ethylene was reported in the reaction produce at a conversion level of 98% of ethanol. But because of strong acidity of HZSM-5, catalyst easily occurred on coking deactivation. Therefore, stability of HZSM-5 was worse.

In 1982, SAPO-34 had been synthesized by UCC [8]. Due to narrow pores, extending in three dimensions and mild acidity, SAPO-34 was successfully applied in the MTO (Methanol to Olefins) process [9], [10], [11]. In previous studies, SAPO-34 was believed to be the best catalyst in terms of activity and selectivity to light olefins [12]. Detailed research revealed that Ni-substituted SAPO-34 (NiAPSO-34) showed excellent performance for ethylene production from methanol [13], [14], [15]. However, for catalytic dehydration of ethanol to ethylene, application of SAPO-34 and NiAPSO-34 was no reported.

In this work, we had devoted ourselves to investigations to look for a catalyst capable of exhibiting high efficiency as well as stability in the formation of the final product ethylene. The selectivity for catalytic dehydration of ethanol to ethylene of four catalysts, Al2O3, HZSM-5, SAPO-34 and NiAPSO-34, was studied to find a suited catalyst in the activity and stability.

Section snippets

Preparation of catalyst

Commercial Al2O3 and HZSM-5 (Si/Al = 25) were obtained from Tianjin Research and Design Institute of Chemical Industry and the Catalyst Plant of Naikai University, respectively. SAPO-34 and NiAPSO-34 samples were prepared with hydrothermal method in the laboratory. Pseudoboehmite (Tianjin Research and Design Institute of chemical Industry), orthophoshporic acid (85wt%, Tianjin Benchmark Chemical Reagent Co. Ltd.), colloidal silica (Beijing Hongxingguangsha Chemical Architectural Material Co.

Characterization of SAPO-34 and NiAPSO-34 samples

The elemental analyses of SAPO-34 and NiAPSO-34 samples were measured by XRF spectrometer as given in Table 2. The observed framework chemical compositions for SAPO-34 and NiAPOS-34 samples suggested Si substitution for P. In NiAPOS-34 sample, a portion of Ni2+ in gel mixtures was incorporated into the SAPO-34 framework. This result showed that incorporation of Ni into SAPO-34 framework was very difficult.

XRD patterns of as-synthesized SAPO-34 and NiAPSO-34 samples are shown in Fig. 2. The

Conclusions

Dehydration of ethanol to ethylene was studied in this work. Al2O3, HZSM-5, SAPO-34 and NiAPSO-34 were employed as catalyst in the dehydration of ethanol. Al2O3 and HZSM-5 were commercial catalysts. SAPO-34 and NiAPSO-34 were synthesized in the laboratory. For NiAPSO-34 sample, unit cell parameters and volume increased and the framework vibration frequencies shifted to position of lower wavenumber. In NH3–TPD profiles of samples, NiAPSO-34 sample exhibited higher desorption temperature of both

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