13th International Colloquium Fuels - September 2021 75 Recent developments in the field of oxymethylene ethers (OMEs) as diesel fuels Ulrich Arnold ulrich.arnold@kit.edu Philipp Haltenort Marius Drexler Jörg Sauer Karlsruhe Institute of Technology (KIT), Institute of Catalysis Research and Technology (IKFT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany Summary With respect to synthetic diesel fuels, so-called oxymethylene ethers (OMEs) are attracting considerable interest. This is mainly due to their diesel-like properties as well as a comparatively clean combustion with extremely low soot and NO x emissions. From a chemical point of view, OMEs are short-chain acetals of the type CH 3 O(CH 2 O) n CH 3 , and especially derivatives with n = 3-5 are predestined for fuel applications. As can be seen from the molecular structure, synthesis of OMEs is based on methanol and thus, OMEs can be produced in sustainable manner from renewable resources if methanol is produced from renewables. Objective of the present study is a comprehensive description of the state of the art of OME fuels, regarding production, properties and evaluation. The work focusses on the synthesis of OMEs from dimethyl ether (DME), transacetalization reactions for the production of OMEs with variable end groups (R 1 O(CH 2 O) n R 2 ), physico-chemical as well as fuel characteristics and evaluation of production processes. 1. Introduction Regarding alternative diesel fuels, current research concentrates on so-called oxymethylene ethers (OMEs) [1]. From a chemical point of view, OMEs are oligomeric acetals with alternating carbon and oxygen atoms. Especially OMEs of the type CH 3 O(CH 2 O) n CH 3 with n = 3-5 (OME 3-5 ) exhibit a diesel-like behavior [2,3]. Despite their favorable fuel characteristics, e.g. extremely low soot and NO x emissions, availability on a larger scale is still limited and an improved production is envisaged [4]. According to the molecular structure, production of OMEs is based on methanol and methanol derivatives [5]. Within this work, innovative strategies for OME production are presented concentrating on the starting materials, catalysts and the resulting product spectra. All synthesis procedures lead to characteristic OME mixtures and strategies to influence the compositions are discussed. As an example, reactions of dimethyl ether (DME) with formaldehyde sources have been investigated and recent developments in this field are presented [6]. Another promising option is the synthesis of OMEs with different end groups, e.g. by transacetalization reactions of OMEs in the presence of acidic catalysts [7]. Thus, properties of OMEs can be tuned to a large extent and adapted to the respective requirements. In addition to OME production, recent progress regarding analytics and physico-chemical as well as fuel characteristics are discussed. In this context, the current status regarding standardization is also addressed. From the perspective of users, practical criteria such as thermal, chemical and oxidation stability as well as compatibility of OMEs with materials and other fuel components are crucial and some aspects referring to this are considered. Furthermore, several studies on the evaluation of OME production are reviewed. 2. Synthesis of OMEs Several synthesis strategies for OMEs have been described and usually, reactions of methanol or methanol derivatives with formaldehyde sources have been employed [1,4,5,8-12]. The classical pathway is based on the reaction of dimethoxymethane (DMM, Methylal, OME 1 ) with trioxane as the formaldehyde source [13]. The reaction offers a high yield of the desired OME 3-5 fraction and low amounts of by-products are formed. However, the starting materials are costly and affect efficiency of the overall process chain. Therefore, current work con- 76 13th International Colloquium Fuels - September 2021 Recent developments in the field of oxymethylene ethers (OMEs) as diesel fuels centrates on alternative procedures such as direct OME synthesis from methanol and formaldehyde sources [14]. This pathway is advantageous but also challenging since water is released during acetalization reactions which is associated with the formation of byproducts. After reaction, water and the by-products have to be separated quantitatively from the product mixtures. Regarding OME production on industrial scale, some plants are operating in China. Capacities around 40 kta - 1 have been reported and OME synthesis from DMM and dewatered formaldehyde is the preferred technology [4]. 2.1 Synthesis of OMEs from DME An option for the anhydrous synthesis of OMEs is the reaction of DME, which can be considered as OME 0 , with non-aqueous formaldehyde sources like trioxane (Figure 1) [6,15]. In own work, efficient catalysts for this reaction have been discovered and meanwhile, several studies on the reaction of DME with trioxane have been carried out. In early attempts, BEA zeolites have been employed as catalysts and exhibited remarkable activity in this reaction. Subsequently, a series of other zeolites has been tested and further promising catalysts, predominantly zeolites, have been identified. Figure 1: Synthesis of OMEs from DME and trioxane. Compared to OME synthesis routes starting from DMM or methanol, the reaction is slow. This enables a kinetic control of product spectra, i.e. in the beginning of the reaction higher OMEs, especially OME 3 , are dominating. These are degraded during the course of reaction and the product distribution is shifted towards short-chain OMEs. Such a behaviour is different from other synthesis procedures which usually proceed via chain growth, i.e. oligomerization reactions from short-chain OMEs to higher OMEs. The resulting mixtures contain mainly OME 1- 6 and concentrations of the OME components decrease with increasing chain length. Regarding by-products, methyl formate has been observed in many cases. However, the formation of methyl formate could be suppressed to a large extent by using improved catalysts. 2.2 Synthesis of (asymmetric) OMEs with higher end groups via transacetalization reactions Very recently, modification of OMEs by transacetalization reactions has been described as a versatile strategy for the preparation of OMEs with variable end groups [7]. By this, asymmetric OMEs with different end groups are also accessible (Scheme 1). Regarding catalysts for this reaction, zeolites exhibited most promising performances. Scheme 1: Synthesis of asymmetric OMEs by acid-catalyzed transacetalization reactions. Current work in this field concentrates on introducing ethyl, propyl, butyl and higher alkyl groups as well as functionalized groups, which enable further chemical modification. Properties of these new OMEs are determined and correlated with their molecular structures. Thus, structure-performance relationships can be determined, which support a rational fuel design according to the respective requirements. Another useful application of transacetalization reactions is the conversion of short-chain OMEs (OME 1 and OME 2 ) or long-chain OMEs (OME ≥5 ) to OMEs with medium chain length. Thus, unwanted OMEs can be reacted to OMEs with appropriate chain length, which meet the respective requirements [16]. 3. Analysis of OMEs All synthesis procedures for oligomeric OMEs usually yield mixtures which contain predominantly OME 1-6 . Since the properties of the individual OMEs vary widely, properties of different OME fractions largely depend on their compositions. Furthermore, impurities can remarkably influence properties and thus, reliable analysis techniques are necessary to separate and characterize the mixtures. Gas chromatography proved to be a powerful tool for the separation of complex mixtures [17,18] and spectrometric as well as spectroscopic methods such as mass spectrometry, FTIR or NMR spectroscopy have been adapted for a detailed characterization [19]. Formaldehyde contents in the reaction and product mixtures are an important aspect in OME synthesis and handling. Usually titration with sodium sulfite is employed but the method fails at low formaldehyde concentrations. Voltammetric determination has been reported to be a suitable method without cross-sensitivity with other components in the mixtures [20]. 4. Properties and standardization of OMEs Within the last years, several studies on OME properties appeared and supplemented previous work [19,21- 23]. Various physico-chemical and fuel-related data are available now regarding single OMEs as well as mixtures containing different OMEs and different quantities of the components. In this context, the synthesis of high purity OME 2 was somewhat challenging since conventional procedures lead to OME 2 contaminated with the starting materials, especially trioxane. This problem could be solved by reacting trioxane with a large excess of OME 1 followed by thorough distillation [23]. OMEs with ethyl end groups instead of the ubiquitous methyl 13th International Colloquium Fuels - September 2021 77 Recent developments in the field of oxymethylene ethers (OMEs) as diesel fuels end groups have also been prepared and characterized in detail [19]. Altogether, properties of OMEs can be tuned to a large extent by tuning compositions or structures and based on comprehensive datasets generated within the last years an OME standard is elaborated currently [24]. Furthermore, the available datasets serve as a base for the development of models for the prediction of physico-chemical as well as fuel properties which supports a rational fuel design. Current activities range from the prediction of simple physico-chemical data like densities to the description of OME properties on a highly sophisticated theoretical level, e.g. by development of a force field for OMEs [25-27]. Regarding OME synthesis, data have been collected for modelling the reaction systems and to improve the procedures. This is particularly beneficial in the case of aqueous reaction mixtures which exhibit a complex phase behavior [28-31]. 5. Stability and compatibility of OMEs From the perspective of the user, criteria such as stability of OMEs and compatibility with materials, especially sealants, as well as other fuel components are crucial. Several studies on these topics have been published recently and a comprehensive overview is given in [32]. To illustrate current challenges, three examples are given in the following: (i) Regarding compatibility with other fuel components, interaction with lubricants is an important aspect, which has been addressed by several research groups, e.g. in a recent study on friction and wear [33]. (ii) In the case of oxygen-containing fuels like OMEs, oxidation stability needs to be investigated since oxygenates are known to be prone to further oxidation initiated by the formation of peroxides. This has been investigated in detail and the use of small quantities of antioxidants has been recommended [34]. (iii) If blending with other diesel fuels is envisaged, miscibility and interaction of the components needs to be addressed and compatibility with current standards must be ensured. In the case of OMEs, blending with conventional diesel fuel is possible up to an OME content of approximately 15%. Several attempts have been made to blend OMEs with HVO. In own work, OME/ HVO blends could be prepared containing low amounts of OMEs (7 and 10%, respectively), but the cold behavior in terms of the CFPP value of such blends was affected [35]. A limited miscibility of OMEs and HVO was also reported by other groups and the preparation of such blends, e.g. by use of suitable additives, still remains a challenge [36-38]. 6. Evaluation of OMEs Regarding the evaluation of OMEs, several comprehensive studies appeared which compare OMEs to various other alternative fuels (Table 1, Ref.s [39-45]). However, analyses have been carried out on a highly generalized level and in many cases recent progress in the field of OME syntheses is not considered adequately. A series of more specialized studies addressed the entire process chain from renewable raw materials to OMEs [46-49]. The process chain comprised production of synthesis gas by gasification of woody biomass, methanol and formaldehyde synthesis and finally OME synthesis via established technologies. The underlying thermodynamics have been analyzed and life-cycle as well as techno-economic assessments have been carried out. Production costs have been estimated for different biomass types. Production costs have also been assessed starting from methanol and considering the classical reaction of DMM with trioxane for OME production [50]. The costs largely depend on the methanol price and assuming a methanol price of 300 US$t - 1 and a capacity of 1 Mio. ta - 1 results in total production costs of about 615 US$t - 1 for OME 3-5 . In many studies, OME synthesis from CO 2 and H 2 has been analyzed [43,51-53]. Such power-to-X concepts, with methanol as the key intermediate, are feasible but the multi-step processes are energy-intensive and largely depend on the availability of CO 2 and renewable energy. OME synthesis directly from CO 2 has also been reported [51,54]. However, yields are low and mainly OME 1 is formed in such highly integrated approaches. Very recent studies focus on the role of hydrogen in the process chain [55]. This topic is crucial since hydrogen is needed for methanol synthesis. The required formaldehyde should be produced by a non-oxidative route, ideally by dehydrogenation of methanol, and thus, hydrogen could be recovered. Optimization of OME production by integration of process steps is also intensely investigated and considerable progress has been made in this field [56]. Currently, different OME synthesis procedures are compared taking also latest technical innovations into account [57]. Thus, identification of the most efficient pathway is envisaged. Table 1: Selected studies on OME evaluation. Study Ref. FVV-Kraftstoffstudie I: Zukünftige Kraftstoffe für Verbrennungsmotoren und Gasturbinen; FVV, Heft 1031 - 2013 [39] FVV-Kraftstoffstudie III, Kurzfassung: Energiepfade für den Straßenverkehr der Zukunft; FVV, Ausgabe R586, 2018 [40] FVV-Kraftstoffstudie III: Defossilisierung des Transportsektors; FVV, 09/ 2018 [41] 78 13th International Colloquium Fuels - September 2021 Recent developments in the field of oxymethylene ethers (OMEs) as diesel fuels 2. Roadmap des Kopernikus-Projektes „Power-to-X“: Flexible Nutzung erneuerbarer Ressourcen (P2X), Optionen für ein nachhaltiges Energiesystem mit Power-to-X Technologien; DE- CHEMA, 31.08.2019, Frankfurt am Main, 1. Auflage, ISBN: 978-3-89746-218-2 [42] Power-to-fuel as a key to sustainable transport systems - An analysis of diesel fuels produced from CO 2 and renewable electricity [43] Biogenous ethers: production and operation in a diesel engine [44] IEA Bioenergy Task 39, Commercialization of conventional and advanced liquid biofuels from biomass: Survey on Advanced Fuels for Advanced Engines; IEA Bioenergy Task 39, October 2018 [45] Biomass-derived oxymethylene ethers as diesel additives: A thermodynamic analysis [46] An optimized process design for oxymethylene ether production from woody-biomass-derived syngas [47] A life cycle assessment of oxymethylene ether synthesis from biomass-derived syngas as a diesel additive [48] The development of the production cost of oxymethylene ethers as diesel additives from biomass [49] From methanol to the oxygenated diesel fuel poly(oxymethylene) dimethyl ether: An assessment of the production costs [50] Cleaner production of cleaner fuels: wind-towheel - environmental assessment of CO 2 -based oxymethylene ether as a drop-in fuel [51] On the energetic efficiency of producing polyoxymethylene dimethyl ethers from CO 2 using electrical energy [52] Comparative well-to-wheel life cycle assessment of OME 3-5 synfuel production via the power-to-liquid pathway [53] Power-to-OME - Processes for the Production of Oxymethylene Dimethyl Ether from Hydrogen and Carbon Dioxide [54] H 2 -based synthetic fuels: A techno-economic comparison of alcohol, ether and hydrocarbon production [55] Optimal Integrated Facility for Oxymethylene Ethers Production from Methanol [56] Economic and life-cycle assessment of OME 3- 5 as transport fuel: a comparison of production pathways [57] 7. Conclusions In recent years, remarkable progress has been made in the field of OMEs regarding production, characterization and application. Promising synthesis procedures have been further developed, e.g. syntheses from methanol and DME employing different formaldehyde sources. Another option are chemical modifications, e.g. by transacetalization reactions which lead to new OMEs with tunable properties. Various properties of OMEs have been determined and an OME standard is currently elaborated. Correlations between molecular structures of OMEs and their performances have been identified which are now implemented in models for the prediction of physico-chemical as well as fuel properties. This will support a rational fuel design according to the respective requirements as well as a targeted optimization. Regarding compatibility of OMEs with materials and other fuel components, new and valuable insights have been gained. These will facilitate the handling of OMEs and also help to develop blending strategies. Several studies on the evaluation of OME fuels are available now, e.g. life-cycle as well as techno-economic assessments. These indicate that sustainable production is possible in principle. One major challenge is certainly the availability of renewable starting materials as well as renewable energies. Ongoing work concentrates on these topics and a largely unexplored potential for further optimization is apparent. Acknowledgement Financial support from Fachagentur Nachwachsende Rohstoffe/ BMEL (Joint research project: “Oxymethylene ethers (OME): Eco-friendly diesel additives from renewables”, funding code 22403814) is gratefully acknowledged. 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