catalysts Article Ethanol Dehydrogenation: A Reaction Path Study by Means of Temporal Analysis of Products Joachim Pasel 1,*, Johannes Häusler 1 , Dirk Schmitt 1, Helen Valencia 2,3, Maria Meledina 2,3, Joachim Mayer 2,3 and Ralf Peters 1 1 Institute of Energy and Climate Research, IEK-14: Electrochemical Process Engineering, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany; [email protected] (J.H.); [email protected] (D.S.); [email protected] (R.P.) 2 Central Facility for Electron Microscopy (GFE), RWTH Aachen University, 52074 Aachen, Germany; [email protected] (H.V.); [email protected] (M.M.); [email protected] (J.M.) 3 Ernst Ruska-Centre (ER-C) for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany * Correspondence: [email protected]; Tel.: +49-2461-61-5140 Received: 4 September 2020; Accepted: 4 October 2020; Published: 6 October 2020 Abstract: Conventional fossil fuels such as gasoline or diesel should be substituted in the future by environmentally-friendly alternatives in order to reduce emissions in the transport sector and thus mitigate global warming. In this regard, iso-butanol is very promising as its chemical and physical properties are very similar to those of gasoline. Therefore, ongoing research deals with the development of catalytically-supported synthesis routes to iso-butanol, starting from renewably-generated methanol. This research has already revealed that the dehydrogenation of ethanol plays an important role in the reaction sequence from methanol to iso-butanol. To improve the fundamental understanding of the ethanol dehydrogenation step, the Temporal Analysis of Products (TAP) methodology was applied to illuminate that the catalysts used, Pt/C, Ir/C and Cu/C, are very active in ethanol adsorption. H2 and acetaldehyde are formed on the catalyst surfaces, with the latter quickly decomposing into CO and CH4 under the given reaction conditions. Based on the TAP results, this paper proposes a reaction scheme for ethanol dehydrogenation and acetaldehyde decomposition on the respective catalysts. The samples are characterized by means of N2 sorption and Scanning Transmission Electron Microscopy (STEM). Keywords: renewable fuels; ethanol dehydrogenation; acetaldehyde decomposition; C-supported precious metal catalysts; Temporal Analysis of Products; Scanning Transmission Electron Microscopy 1. Introduction Recently, basic research into new catalysts for the synthesis of higher alcohols on a renewable basis via aldolic condensation has been gaining increasing attention [1–6]. Amongst these alcohols, iso-butanol in particular has been identified as very promising from various perspectives. It has been found that its physical and chemical properties (i.e., density, viscosity, solubility in water, lower heating value, boiling point, flash point, self-ignition temperature, motor octane number, etc.) are similar to those of conventional gasoline and conform to relevant standards for that substance [7]. Additionally, experiments and simulations have demonstrated that the combustion properties of pure iso-butanol and iso-butanol/gasoline blends, respectively, widely resemble the combustion characteristics of conventional gasoline. It has been demonstrated that using pure iso-butanol or iso-butanol/gasoline blends might even enhance the performance of internal combustion engines by reducing the quantities Catalysts 2020, 10, 1151; doi:10.3390/catal10101151 www.mdpi.com/journal/catalysts Catalysts 2020, 10, 1151 2 of 20 of NOx, CO and hydrocarbon emissions [2,8]. Singh and Sung [9] found that butanol isomers are advantageous with respect to soot formation in comparison to the isomers of butane. Thus, iso-butanol clearly offers the potential to become a “green” future fuel for the transport sector if it is synthesized from renewable feedstocks. Against this background, the Forschungszentrum Jülich has been a partner in the project “C3-Mobility”, funded by the German Federal Ministry for Economic Affairs and Energy, since 2018. The fundamental assumption of this project is that in future mobility concepts for the transport sector, renewable methanol will be an important raw material to be refined in different processes, yielding viable transport fuels. It is also assumed in these scenarios that renewable methanol will be produced locally from H2 and CO2 in different venues worldwide, where wind- or solar-driven water electrolysis facilities for H2 production can be economically operated and where CO2 being separated from the exhausts of, e.g., steelworks, cement facilities or even ambient air will be available 3 in suCatalystsfficient 2020 quantities., 10, x FOR PEER In “CREVIEW-Mobility”, Jülich’s specific refining path that starts from renewable3 of 20 methanol, deals with the development of suitable and experimentally-proven synthesis routes to producecomputational iso-butanol. methods This includes such as dens the evaluationity functional of theory proper or reaction a combination conditions of experimental (pressure, methods temperature, etc.) and modelling active catalysts. tools to investigate correlations between catalyst activity and selectivity on the one Figurehand and1 shows material a characteristics reaction scheme such as that crystal constitutes faces, monolayer a possible coverages, solution etc., for on the other synthesis [37– of iso-butanol,41]. starting with methanol and ethanol. It is an adaption of the “hydrogen-borrowing Autthanit et al. [42,43] investigated AgLi/TiO2 and VOx/SBA-15 catalysts under non-oxidative mechanism” proposed by Siddiki et al. [10] that uses a Pt/C catalyst for H uptake and subsequent and oxidative reaction conditions and found significantly higher conversions2 and acetaldehyde release. The original scope of the work of Sidikki et al. was the catalytic methylation of C–H bonds in yields in the case of oxidative dehydrogenation. Ag nanoparticles on SiO2 and SiO2/CeO2 were tested alcohols,by Mamontov ketones andet al. indoles [44], who with observed methanol. increased For thecatalytic synthesis activity of when iso-butanol Ag/SiO displayed2 was modified in Figure by 1, one moleCeO2of nanoparticles. ethanol and Dutov methanol, et al.respectively, [45] tested Ag/OMS-2/SiO react in the presence2 catalysts of for NaOH, the selective and exemplarily, oxidation of a Pt/C catalystethanol at 120 to◦ acetaldehydeC to form acetaldehyde and observed and a positive formaldehyde, effect of whichAg doping in turn on furtherthe oxidation reacts rate to form but also acrolein, and afterundesired hydrogenation, total oxidation, propanol. leading Anotherto CO2. Giannakis comparable et al. C-methylation[46] investigated step Au and on theNiAu Pt /singleC-catalyst atom then yieldsalloys iso-butanol. and found However, that the addition the third of C-methylation Ni increased the step catalytic in Figure activity1 from and iso-butanolreduced the tosintering undesired 2,2-dimethyl-1-propanolprocesses. The best results has were only obtained been demonstrated at 280 °C. The same in a homogeneouseffect of increased catalyst activity thus and stability far [11] and was observed by Shan et al. [47,48] when they doped their Cu nanoparticles with Ni to form a NiCu is not assumed to take place on a Pt/C catalyst. The first step of this reaction sequence, i.e., the alloy. Li et al. [49] used a metal-free catalyst consisting of nitrogen-doped graphenes and found dehydrogenation of ethanol to acetaldehyde, is the focus of this paper. To better comprehend it from acetaldehyde to be the only product of ethanol dehydrogenation at reaction temperatures of between a mechanistic200 °C and point350 °C. of Ob-Eye view (residenceet al. [50–52] time used distributionactivated carbons of products as support and and residual doped them educts, with reactionCo, intermediates,Ce, Cu and undesired Ni in the temperature side reactions, range subsequent of 250 °C to aldol 400 condensation°C. The Cu-containing reactions), catalyst a Temporal showed Analysisthe of Productshighest (TAP)activity investigation and most promising of precious selectivity metal regarding (Pt, Ir, supported acetaldehyde on active (more carbon) than 96%). and Wang base metalet al. (Cu on active[53] tested carbon) Au catalysts supported was on ZnZrO performed.x at temperatures These three between samples 30 were°C and chosen 400 °Cas and it wasobserved demonstrated that the in a recentaddition experimental of Au favors investigation the dehydrogenation by Häusler etroute al. [12over] that the theydehydration are also suitablepath. Mitran for the et synthesisal. [54] of desiredinvestigated iso-butanol mixtures according of ethanol to the and reaction methanol scheme with their from H Figure1−xTi2(PO1.4)3−x(SO4)x-catalyst and observed higher conversions of ethanol (> 95%) compared to those of methanol (55%). FigureFigure 1. Reaction 1. Reaction pathway pathway for for the the synthesissynthesis of iso-butanol iso-butanol from from ethanol ethanol and and methanol. methanol. Only2. Results a few and studies Discussion in the literature focus on the mechanistic aspects of ethanol dehydrogenation. Ashok et al. [13] used Co nanoparticles in a mechanistic study and found aldehyde and acetate 2.1. Catalyst Characterization components, together with H2,H2O, CO2 and CH4. Kim and Ryu [14] worked with V2O5/TiO2 catalysts 2.1.1. N2 Sorption Table 1 summarizes the specific surface areas of the different catalysts from this study. Cu/C shows the largest surface of 1271