Dehydrogenation of Propane to Propylene over Cr2O3-Al2O3 Catalyst

Abstract

Dehydrogenation of Propane to Propylene over Cr2O3-Al2O3 Catalyst: A Deactivation and Regeneration Study Seongjun Yoon Dept. of Chemical Engineering The Graduate School Hanyang University This study aims to delve into the laboratory deactivation and subsequent remanufacturing of Cr2O3/Al2O3 catalysts in the context of the PDH reaction. The primary aim is to acquire a comprehensive understanding of the intrinsic factors contributing to the deactivation of Cr2O3/Al2O3 catalysts and to explore viable strategies for the effective reuse of the catalyst. The deactivation study of Cr2O3/Al2O3 catalysts involved subjecting the synthesized catalysts (13 wt% Cr) to high-temperature heat treatment (900◦C). The support and the Cr2O3-containing synthesized catalysts were individually sintered to assess the impact of the support and metal site on catalyst deactivation. Catalytic characteristics were evaluated through Nitrogen physisorption, H2-TPR, X-ray diffraction, TEM, SEM-EDS, and ICP-OES. After long-term use, the catalysts' surface area and pore volume significantly decreased, and the activity substantially reduced as the Cr on the surface of the catalysts migrated into the support. Multiple factors contributed to catalyst deactivation, with a crucial role identified for the phase transition of the support. Notably, the transformation of gamma-phase Al2O3 to alpha phase resulted in the loss of PDH reaction activity. Given the irreversibility of the alpha phase transition, complete reactivation of a deactivated catalyst proved unfeasible. To address this challenge, the study aimed to recover the highly toxic Cr from the deactivated catalyst and convert it into the less toxic Cr3+. Cr extraction was achieved using a mixed acid solution (HNO3, HF) and Microwave treatment. Additionally, a patented technology was used to convert Cr6+ to Cr3+ using oxalic acid. As a result, low-toxicity Cr could be recovered from the used catalysts. The recovered Cr synthesized the Cr2O3/Al2O3 catalyst with freshγ-alumina. The above catalysts had about 65% activity compared to those synthesized through Cr(NO3)3 · 9H2O and γ-Al2O3 in the laboratory. We confirmed the possibility of catalyst remanufacturing through Cr recovery from the used catalyst. Chapter 1 Introduction Propylene can produce various chemical products, such as acrylic acid, polypropylene, propene oxide, etc.[1] Most propylene was produced through cracking in crude oil. However, it has become challenging to meet the demand because of the limitations of crude oil supply and the increasing demand for propylene. This insufficient propylene production is supplemented through the PDH process. According to statistics, nearly 90 % of propene comes from the cracking of naphtha or heavy oil, and the rest is obtained from propane dehydrogenation (PDH), propane oxidative dehydrogenation (ODH) process, and the methanol to olefins (MTO) process.[2] Catofin and Oleflex are the most popular PDH processes. In the case of the Catofin process, a Cr-based catalyst is used. It has the advantage of having high reaction stability and yield. However, high-temperature carbon deposition and agglomeration of Cr species lead to the rapid deactivation of the Cr-based catalysts. [3] Moreover, it is known that Cr3+ is converted to Cr6+ by deactivation of the catalyst. Due to the high toxicity of Cr6+, recovery and treatment of deactivated chromium catalysts is the main problem.[3] The initial segment of this investigation centers on the intentional deactivation of the synthesized catalysts. To achieve this, both the catalyst and its supporting material underwent an elevated- temperature heat treatment (900◦C) to induce forced deactivation. A conceptual model elucidating the deactivation mechanism was formulated, considering catalyst-support sintering and chromium migration. This model was constructed based on a comprehensive analysis involving nitrogen physisorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), and chemical analysis. Furthermore, a proposed methodology for recovering chromium from the spent catalysts and its subsequent application in remanufacturing was introduced. This holistic approach aims to integrate insights from the deactivation studies with practical methods for reactivating and repurposing the catalyst, contributing to the advancement of sustainable practices in catalyst management. Chapter 2 Experimental 2.1. Catalyst preparation Chromia/alumina catalysts with 13 wt% Cr were used to evaluate the effect of alumina and catalyst calcination at high temperatures on propane dehydrogenation activity. First, pure gamma alumina(Thermo Scientific, 047171.36) was stirred in deionized water for 8 hours to rehydrate the support. Then, it was calcined at 600◦C for 6 h. Part of this sample was heat-treated in air at 900◦C for 24 h to induce phase transformation and produce support with different surface areas. The Cr2O3/Al2O3 samples were prepared by wet impregnating hydrated gamma alumina with an aqueous Cr(NO3)3 · 9H2O (Sigma Aldrich, 239259-100G). The whole catalyst was treated at 900◦C for 24 h to induce deactivation. The details about the samples are given in Table 2.1. The CATOFIN process is divided into a reaction process and a regeneration process. The reaction temperature was 600°C. Meanwhile, for the regenerated catalyst, oxygen is supplied, and a higher temperature (about 700°C or more) is applied to remove cokes formed on the catalyst. Typically, the industrial CATOFIN catalysts have a lifespan of 2~3 years. [4] Using the catalyst for over a month (about 50 days more) is required to observe a significant decrease in activity. [5] To shorten this period, post- calcination heat treatment was carried out at 900◦C, much higher than the reaction or regeneration temperature. Through this, the intended forcible deactivation of the catalysts could be achieved.