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Dehydrogenation of methylcyclohexane over Pt supported on Mg–Al mixed oxides catalyst: The effect of promoter Ir

https://doi.org/10.1016/j.cjche.2020.05.026Get rights and content

Abstract

To enhance the hydrogen release during hydrogen storage, several Pt–Ir supported on Mg–Al mixed oxide catalysts were prepared and then applied into the dehydrogenation of methylcyclohexane (MCH) in this study. The effects of iridium content, reduction temperature on the activity and stability of the catalysts were studied in detail. In the presence of Ir, metal particle size was decreased and electron transfer between Ir and Pt was observed. High reduction temperature increased the metallic Ir content but enlarged the particle size of active sites. During the dehydrogenation reaction on Pt–Ir bimetallic catalyst, MCH was efficiently converted into toluene and PtIr-5/Mg–Al-275 exhibited the highest activity. After prolonging the residence time and raising the reaction temperature to 350 °C, the conversion and hydrogen evolution rate were increased to 99.9% and 578.7 mmol·(g Pt)−1·min−1, respectively. Moreover, no carbon deposition was observed in the spent catalyst, presenting a high anti-coking ability and good potential for industrial application.

Graphical abstract

Hydrogen evolution rate reached up to 578.7 mmol·gPt−1·min−1 in the dehydrogenation of methylcyclohexane on Pt–Ir supported on Mg–Al mixed oxide catalyst and no carbon deposition on the catalyst surface was observed after dehydrogenation reaction.

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Introduction

Due to the declining fossil fuel reserves and environmental pollution problems, the exploitation and utilization of renewable and sustainable alternative energy to substitute or supplement fossil fuels is extremely urgent [1]. Hydrogen energy, derived from renewable wind or solar power via electrolysis, is characterized by large quantity, pollution-free and high gravimetric energy storage density, which is considered as an efficient and clean energy for various industrial applications [2]. However, for utilizing hydrogen energy in large scale, reversible hydrogenation–dehydrogenation cycles are widely accepted as a promising method for transportation and delivery of hydrogen, where hydrogen can be easily stored and released on demand via a catalytic process. Consequently, liquid organic hydrides are usually selected as hydrogen carries because of their advantages on hydrogen storage density and convenient transportation under relatively mild conditions [[3], [4], [5], [6]].

From an environmental point of view, methylcyclohexane (MCH) is one of suitable candidates as an organic hydride because both MCH and toluene exist as liquid state in the temperature range of − 50–100 °C, which is widely employed to study the storage and release of hydrogen, but the technology bottleneck is to develop an efficient dehydrogenation catalyst with high activity and stability. Pt based catalysts can selectively activate Csingle bondH bond and decrease its activation energy but not break Csingle bondC bond, presenting a promising application potential in dehydrogenation reaction. For example, Kustov et al. [7] had concluded that Pt/C had excellent performance in dehydrogenation of cyclic naphthenes. Biniwale et al. [8] had studied the dehydrogenation of MCH over Pt supported on metal oxides and obtained a hydrogen evolution rate over 21.1 mmol·(g met)−1·min−1. To further enhance the dehydrogenation activity, several strategies have been proposed, and adding promoter was an easy way to reach a satisfactory hydrogen evolution rate [[9], [10], [11], [12], [13], [14], [15], [16]]. Until now, some components such as Ca, Sn, Mn and Mo have been tested as a promoter [10,[16], [17], [18]], and the results showed that both the dehydrogenation activity and carbon deposition resistance were improved [19].

In addition, the support also has a great effect on the activity and stability of Pt-based catalysts. Li et al. [20] had reported that carbon materials with distinct microstructures could adjust the Pt particle size from 1 to 9 nm and found that high dehydrogenation activity of Pt/CNFs depended on a large fraction of boundary Pt atoms. Because the confinement effect of ordered mesochannels restricted the growth of Pt nanoparticles, Pt-SBA-15 showed higher stability than Pt-SiO2 in the dehydrogenation reaction [21]. Recently, to lower the acidity of Al2O3 support and then enhance the resistance for coke formation, some alkaline or alkaline earth metals were added into Al2O3. Zhou et al. [22] had reported that the agglomeration of metallic particles was suppressed while both the dehydrogenation activity and stability were enhanced when mesoporous Al2O3 was modified by magnesium. Our previous investigation had also indicated that Mg–Al mixed oxides were good supports for Pt based catalysts in the dehydrogenation of MCH and the presence of Sn could enhance the activity, but the hydrogen evolution rate required to be further improved [14]. Hence, this study employed iridium (Ir) as a promoter for Pt supported on Mg–Al mixed oxides catalyst and concentrated on the effects of iridium content, catalyst reduction temperature, reaction temperature, flow rate and residence time on the hydrogen evolution rate in the dehydrogenation of MCH.

Section snippets

Experimental

Mg–Al hydrotalcite was prepared by a constant-pH co-precipitation method as reported in the previous literatures [23,24]. Pt–Ir supported on Mg–Al mixed oxides catalysts were prepared by an incipient wetness impregnation of Mg–Al hydrotalcite with chloroplatinic acid and chloroiridic acid mixed solution and then reduced by hydrogen. The content of Pt in the catalyst was fixed to 2.0 wt%. The resultant catalysts were denoted as PtIr-X/Mg–Al-T, where X and T represented the mass percentage of Ir

Characterization of Pt–Ir supported on Mg–Al mixed oxides catalysts

Fig. 1 shows the XRD patterns of Pt–Ir supported on Mg–Al mixed oxides catalysts. Three peaks appeared at 2θ = 35.1°, 43.0° and 62.8° in Fig. 1(a) and (b) were attributed to the crystalline structure of MgO-Al2O3 [25]. In comparison with the XRD pattern of Mg–Al hydrotalcite in previous literatures [26,27], it was obvious that the typical hydrotalcite structure was destroyed and transformed into mixed oxides after calcination at 500 °C. Due to the low loading amount and the high dispersion,

Conclusions

The addition of Ir enhanced the dispersion of Pt and decreased the average metal particle size and facilitated the electronegativity of Pt. The metal particle size was enlarged with the raising of reduction temperature. During the dehydrogenation of MCH, toluene selectivity reached up to 99.9%. After optimizing the Ir content and reduction temperature, MCH conversion and hydrogen evolution rate was up to 91.1% and 263.9 mmol·(g Pt)−1·min−1 on PtIr-5/Mg–Al-275 at 300 °C, respectively, which was

Acknowledgements

This research was supported by the National Natural Science Foundation of China (Nos. 21676225 and 21776236), Natural Science Foundation of Hunan Province (2018JJ2384) and Scientific Research Fund of Hunan Provincial Education Department (19A478), Collaborative Innovation Centre of New Chemical Technologies for Environmental Benignity and Efficient Resource Utilization, and Engineering Research Centre of Chemical Process Simulation and Optimization of Ministry of Education.

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