Immobilization of a selective Ru-pincer complex for low temperature methanol reforming–Material and process improvements
Graphical abstract
Introduction
Recently, a range of homogeneous catalysts for aqueous phase methanol reforming have successfully been employed. They facilitate the release of hydrogen and the concomitantly carbon dioxide in the ratio 3 to 1 from liquid mixtures of methanol and water according to Scheme 1 already at temperatures below 100 °C. Remarkable activities of up to 4719 h−1 are achievable while the amounts of carbon monoxide as an undesired side product are minimal. [1]
Most of the published catalysts are Ruthenium-based and need high amounts of base, commonly KOH, to catalyze the reaction efficiently [[1], [2], [3]]. Currently one of the most promising systems is the Ruthenium-based PNP-Pincer catalyst [RuH(CO)Cl(HN(C2H4Pi-Pr2)2)] (1 in Fig. 1) published by Beller and co-workers. Mechanistic [4,5] and DFTbased [[6], [7], [8]] investigations on this specific molecular catalyst revealed the vital role of the ligand and basic additive, and thereby proposing an overall mechanistic picture. Notably a bi-catalytic Ru-based system can catalyze the methanol reforming reaction without the need of a basic additive, albeit at highly reduced activities of only 90 h−1 [9]. A different approach is the use of a Lewis-acidic additive and thereby circumventing the implication of the basic additive like solids (carbonates) formation [10]. Other catalysts based on iron, manganese and iridium have been investigated, although not reaching the activities of the [RuH(CO)Cl(HN(C2H4Pi-Pr2)2)] while also being more prone to oxidation (or sensitive to oxygen) [[11], [12], [13]]. Comprehensive reviews of homogeneous catalysts for CO2 hydrogenation and methanol reforming can be found in the literature [[14], [15], [16]].
Applying 1 the aqueous-phase methanol dehydrogenation and reforming was performed at low temperature around 90 °C. The reaction proceeds in three steps from methanol via formaldehyde/gem-diolate and formic acid to carbon dioxide and hydrogen in a 1:3 ratio (Scheme 1). Thus, full conversion of all hydrogen atoms present in the substrates to hydrogen is achieved. Besides an excellent productivity (> 350,000 turn overs) and stability (> 3 weeks) also a very good selectivity was obtained as only less than 10 ppm CO and CH4 were detected. Detailed mechanistic studies revealed that the C-H cleavage steps occur via an inner-sphere coordination of the methanolate, the gem-diolate and the formate. The presence of the base turned out to be essential for the reaction. The main role of the base is to enable the energetically favorable anionic, inner-sphere pathway. Further roles are the dehydrochlorination of the precatalyst 1 and the sequestration of the dehydrogenation products. The latter renders the key dehydrogenation step thermodynamically more feasible by providing a significant driving force (Fig. 1). The dehydrogenation of the formate was identified to be the least facile step of the overall process This is in agreement with the previously reported accumulation of formate during the reaction [4,5].
Aiming at the combination of methanol steam reforming for hydrogen generation and its use in PEM fuel cells (see Fig. 2), we became interested in the immobilization of such homogeneous catalyst complexes for continuous operation. Liquid-phase operation is not feasible, since the high amount of formed carbonates and the corrosive nature of the KOH constitute major obstacles. Ideally, a continuous vapor-phase process is envisioned, allowing direct use of the hydrogen in the fuel cell. A promising technology to enable homogeneous catalysis in vaporphase mode is the so called supported liquid phase (SLP) approach. The technology involves surface modification of a porous solid by dispersing a thin film of liquid onto the large inner surface area. Dedicated transition metal complexes dissolved in supported organic films (SLP) have been studied by Schoulten in the 1980s in great detail, mainly for the application in gas-phase hydroformylation reactions [[17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]]. Supported aqueous phase (SAP) materials have been applied as suspended catalysts by the groups of Davis and Hanson in the 1990s [[33], [34], [35], [36], [37]]. Ionic liquids were independently introduced as novel coating materials by Mehnert et al. and Riisager et al. in 2003 [38,39]. Due to the extremely low vapor pressure of ionic liquids under the conditions of commercial gas-phase reactions, these supported ionic liquid phase (SILP) materials represent a fundamentally new approach to realize long-term stable, surface coated catalytic materials and absorbents [40].
Here we immobilize the homogeneous Ru-pincer complex RuH(CO)Cl(HN(C2H4Pi-Pr2)2) together with KOH in two ionic liquids dispersed onto alumina. In addition, a pure KOH deposited on alumina was also tested. The latter catalyst became a SLP system under reaction conditions, when the substrates water and methanol formed a liquid layer inside the pore system of alumina. Process parameters were identified and a preliminary optimization was carried out in order to achieve the highest catalyst activity. These data will be the basis for the implementation of continuous vapor- phase methanol reforming in direct combination with a PEM-fuel cell as depicted in Fig. 2.
Section snippets
Experimental
Chemicals were purchased from Strem and VWR and used without further purification. [P1444][MeSO4] and [P1444][NTf2] were purchased from IoLiTec. Nitrogen (99.9990 vol.-%) was purchased from Linde Gas.
Results and discussion
The Ru-pincer complex reported by Beller and co-workers is highly active in low temperature methanol reforming at 90 °C, but requires rather high concentrations of base, in this case KOH. Particularly challenging for this reaction is the presence of water and a strong base at elevated temperatures. Most if not all ionic liquids do not cope well with these harsh conditions and quickly decompose. Numerous reviews concerning IL stability were published over the last two decades [[41], [42], [43]].
Conclusion
In the present work we have for the first time immobilized an active Ru-pincer complex in the supported liquid phase for the continuous vapor phase methanol steam reforming using fixed bed reactors. By employing a basic and hygroscopic coating in the form of KOH deposited onto alumina support, the ruthenium-based PNPPincer complexes can efficiently be immobilized and catalyze the selective decomposition of methanol and water to hydrogen and carbon dioxide, with only trace amounts of CO under
Acknowledgements
The authors gratefully acknowledge funding from the German Federal Ministry of Economic Affairs and Energy (BMWi) within the project Metha-Cycle (grant number 03ET6071F).
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