Effect of MgO promoter on Ru/γ-Al2O3 catalysts for tricyclopentadiene hydrogenation
Graphical abstract
Introduction
Exo-tetrahydrotricyclopentadiene (exo-THTCPD) has drawn keen interest as a type of high-energy-density fuel (HEDF) that features a high density (1.04 g/ml) and high heating value (40∼42 MJ/L) [[1], [2], [3], [4]]. Exo-tetrahydrotricyclopentadiene (exo-THTCPD) goes through the following three steps of production [[5], [6], [7], [8], [9], [10]]: First, as by-products of the naphtha cracking process, cyclopentadiene (CPD) and endo-dicyclopentadiene (endo-DCPD) are produced. In the endo-DCPD and CPD oligomerization step, endo-tricyclopentadiene (endo-TCPD) is produced. Second, endo-tetrahydrotricyclopentadiene (endo-THTCPD) is produced from the hydrogenation reaction of endo-TCPD. Third, isomerization converts endo-THTCPD into exo-THTCPD.
There have been some research findings on catalysts for the DCPD hydrogenation reaction. In contrast, there have been very few studies of heterogeneous catalysts for the TCPD hydrogenation reaction [11]. As a TCPD hydrogenation catalyst, Pd-B/γ-Al2O3 prepared through an impregnation approach has been studied [12]. γ-Alumina has been widely used as a catalyst support for high-temperature hydrogenation reactions owing to its good thermal stability. In addition, catalysts containing Pd, Pt, and Rh metals are known for their outstanding activity during hydrogenation reactions of multicyclic hydrocarbons [[13], [14], [15], [16], [17], [18], [19]]. This study applies catalysts where Ru, which is less expensive than the precious metals discussed above, is deposited onto γ-alumina to realize the TCPD hydrogenation reaction.
During the hydrogenation reaction of multicyclic hydrocarbons, the catalysts are supposed to be regenerated for reuse. One major cause of catalyst deactivation is coke, and combustion of coke deposited at temperatures as high as 600 °C is necessary for catalyst regeneration. Metallic sintering is likely to occur as precious metallic catalysts are regenerated at a high temperature, which is a major cause of catalyst deactivation [[20], [21], [22], [23], [24]]. Although Ru catalysts are known to have higher thermal stability than Pt and Pd catalysts, Ru metals as well often involve sintering at temperatures as high as 600 °C [20].
In order to prevent metallic particle sintering on a catalyst surface, the reaction temperature and regeneration temperature are decreased or a textural promotor is deposited along with the metal. It is known that oxides such as Ba, Zn, La, Si, and Mn serve as a textural promotor in catalysts where a precious metal is deposited onto an alumina support [25,26]. MgO can also be utilized as a textural promotor because it can effectively prevent the sintering of precious metallic materials during a high-temperature regeneration process given that it features high Hüttig and Tamman temperatures [25]. To the best of our knowledge, this study utilizes a catalyst that consists of Ru and MgO in the hydrogenation reaction of multicyclic hydrocarbons for the first time. Particularly, there has been little research on the effects of the catalytic regeneration process on the catalytic performance during the hydrogenation reaction of multicyclic hydrocarbons.
The objective of this study is to determine the effects of MgO promotors on the catalytic physico-chemical properties of Ru/γ-Al2O3 catalysts and the activity during the TCPD hydrogenation reaction. The Ru/γ-Al2O3 and Ru-MgO/γ-Al2O3 catalysts used here were produced by means of a coprecipitation method and realized in a bead form with a binder added to them. After the TCPD hydrogenation reaction with a bead-form catalyst used within a spinning-basket catalyst reactor, the used catalyst was recovered and regenerated in a high-temperature air atmosphere. The regenerated catalyst was applied again to the TCPD hydrogenation reaction. As this procedure was repeated four more times, the catalyst characteristics were analyzed using various methods, in this case thermogravimetric analysis (TG), N2 adsorption, H2-temperature programmed reduction (H2-TPR), X-ray dispersion (XRD), CO-chemisorption, and transmission electron microscopy (TEM).
Section snippets
Synthesis of the catalysts
The Ru/γ-Al2O3 catalyst and Ru-MgO/γ-Al2O3 catalyst were produced by means of a precipitation method. In preliminary research, increasing the Ru loading on the γ-Al2O3 support from 0.5 to 1.5 wt% increased the TCPD hydrogenation activity. However, when the Ru loading amount exceeded 1.5 wt%, the reaction activity no longer increased, which appears to be due to the reduction of the specific surface area and dispersion of Ru. Using a similar approach, the optimal MgO loading level was determined.
Characterization of the catalysts
Fig. 1 shows the results of the thermogravimetric analysis of the recovered catalyst after endo-TCPD hydrogenation using the Ru/γ-Al2O3 catalyst. A weight loss of 1.5 wt% was observed up to approximately 170 °C, most likely due to the volatilization of the residual reactants and products on the catalyst surface. A subsequent weight loss was observed in the temperature range of 200–600 ℃. In this temperature range, a weight loss of about 4.5 wt%, attributed to the degradation of
Conclusion
The specific surface area of the regenerated Ru-MgO/γ-Al2O3 catalyst is much larger than that of the regenerated Ru/γ-Al2O3 catalyst. It was confirmed that the addition of MgO has the effect of alleviating the decrease of the specific surface area during the repetitive regeneration procedure. The addition of MgO had no significant effect on the reducibility of the Ru/γ-Al2O3 catalyst. The results of XRD, CO chemisorption and TEM imagery show that an addition of MgO to the Ru/γ-Al2O3 catalyst
CRediT authorship contribution statement
Youri Park: Conceptualization, Data curation, Formal analysis, Methodology, Writing - original draft. Huiji Ku: Investigation, Data curation. Jae-Yong An: Data curation. Jeongsik Han: Formal analysis, Resources. Chae-Ho Shin: Investigation. Jong-Ki Jeon: Conceptualization, Funding acquisition, Methodology, Project administration, Resources, Supervision, Visualization, Writing - review & editing.
Declaration of Competing Interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was also supported by “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20194010201730).
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2021, International Journal of Hydrogen EnergyCitation Excerpt :Al2O3 is generally used as catalyst for cracking reactions and alcohols dehydrations to olefine due to its high concentration of Lewis acidic sites [30]. MgO is considered a promising heterogeneous catalyst support and promoter for many kinds of chemical reactions including dry reforming and hydrogenation [31,32]. MgO is also used to avoid carbon deposition on the catalyst surface favouring CO2 adsorption and promoting carbon conversion to CO, by the reverse Boudouard reaction [33–35].