Research articleEfficient synthesis of oxymethylene dimethyl ethers (OME) from dimethoxymethane and trioxane over zeolites
Graphical abstract
Oligomeric oxymethylene dimethyl ethers (OMEn, CH3O(CH2O)nCH3) bearing three to five CH2O units are promising diesel fuels for the reduction of soot and NOx emissions. This work describes efficient synthesis of OMEs from dimethoxymethane and trioxane under mild reaction conditions employing zeolite catalysts. Zeolite BEA25 exhibited highest activity and the reaction was completed within 12 min at 25 °C.
Introduction
The reduction of harmful exhaust gases is indispensable, notably in the transportation sector. Thus, a series of strict emission standards for combustion engines is defined. It is well known, that special fuel additives can lower the formation of pollutants during fuel combustion [1], [2], [3]. In particular oxygen-containing compounds without carbon-carbon bonds are promising additives for diesel fuel to meet current emission standards. In contrast to methanol and dimethyl ether (DME), the related oligomeric oxymethylene dimethyl ethers (OMEs) with the general formula CH3O(CH2O)nCH3, are suitable as soot reducing diesel fuels due to their favorable physico-chemical properties [4], [5]. OMEs with n = 3, 4 and 5 (OME3, OME4 and OME5) are of particular interest [6], [7]. They exhibit suitable fuel properties with enhanced cetane numbers as well as excellent miscibility with diesel [8], [9]. Because of the matching properties, they can be added directly to common diesel with only slight changes in the fuel supply and engine control system. OMEs can be synthesized on large scale from methanol, which can be obtained from synthesis gas [10]. Regarding the raw materials for synthesis gas production, coal and natural gas exhibit economic benefits while CO2 and biomass offer eco-friendly strategies for OME production [11], [12].
Within the production of OMEs, two main synthesis routes have been described in the literature. According to the first route, already studied by Staudinger in the 1920s [13], OMEs are synthesized from methanol and various formaldehyde sources such as formalin, p-formaldehyde or trioxane in the presence of acidic catalysts [14], [15], [16], [17], [18], [19], [20], [21]. Unwanted byproducts such as hemiformals and glycols are always formed, decreasing the yields of OMEs significantly and leading to a high degree of complexity regarding product separation and purification. As a result several distillation steps are needed [20], [21], [22], [23]. In 1948, Gresham and Brooks introduced a second synthesis route for OMEs using dimethoxymethane (DMM, OME1), instead of methanol (Scheme 1) [24]. Similar to the previous synthesis method, OME formation from DMM and different formaldehyde sources, preferably trioxane, was carried out using acidic catalysts [8], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35]. It was assumed that trioxane decomposes before reaction to three monomeric formaldehyde units, which react subsequently with DMM to OMEs [31]. Due to the low methanol and water content in the reaction mixtures, formation of unwanted byproducts, such as hemiformals, is suppressed, so that higher selectivities and OME yields compared to the first route can be obtained [26].
In recent studies, OME synthesis from DMM and trioxane was investigated in detail by Burger et al. [26], [35], [36]. These activities included the determination of chemical equilibrium, reaction kinetics, development of a process model, product separation and cost estimation. An adsorption-based reaction mechanism was proposed and adsorption-desorption processes on catalyst surfaces were considered as rate limiting steps. The ion exchange resin Amberlyst46 was used as catalyst between 50 and 90 °C. Yields of OME3–5 up to 29 wt% and trioxane conversion up to 95% were reported [26], [36]. Wang et al. investigated further acidic ion exchange resins with respect to activity, long term stability and selectivity [29]. An OME3–8 selectivity of 64.2 wt% coming along with a trioxane conversion of 89.0% were reached, running the reactions at 90 °C.
Another possibility to obtain OMEs is the reaction of DMM with p-formaldehyde, instead of trioxane. Zheng et al. showed that OME synthesis from DMM and p-formaldehyde can be realized using acidic ion exchange resins similar to the reaction mentioned above [27], [33]. In contrast to the adsorption-based reaction mechanism, they assumed a mechanism based on subsequent reactions of OMEn with formaldehyde to give OMEn + 1. Reactions were carried out at 80 °C with OME3–5 yields up to 36.6 wt% and formaldehyde conversion up to 85.1%.
Several attempts have been made to optimize OME synthesis from DMM and trioxane by employing a series of catalysts. Several studies concentrated on strongly acidic ion exchange resins [26], [29]. Alternatively, acidic zeolites can be used as catalysts. Wu et al. showed that different ZSM5 zeolites catalyze the reaction of DMM with trioxane [31]. However, low catalyst activities were observed, so that reaction temperatures up to 120 °C were necessary. The best catalyst was found to be a ZSM5 zeolite with a Si/Al ratio of 580 containing predominantly Brønsted acidic sites of medium strength. Due to the comparatively high reaction temperatures, byproducts such as formaldehyde (7.3 wt%), methanol (3.7 wt%) and methyl formate (0.5 wt%) were formed within 45 min reaction time.
Objective of this work was the optimization of OME synthesis from DMM and trioxane to establish an efficient batch process, which operates at room temperature without any overpressure. To realize this, several zeolites with wide pore sizes were tested as catalysts. Furthermore, the influence of water on reactivity was studied in detail.
Section snippets
Materials
The reagents DMM (ReagentPlus, 99.9% GC-area, 0.40 wt% H2O-content) and trioxane (99.98% GC-area) were purchased from Sigma-Aldrich. To lower the water content of DMM, it was pre-dried with molecular sieve 3 Å, then dried with CaH2 (Sigma-Aldrich) and finally distilled. The acidic ion exchange resin Amberlyst36wet (A36bead, concentration of acid sites: ≥ 1.95 eq l− 1, ≥ 5.40 eq kg− 1) was obtained from Rohm and Haas [37]. It exhibits a spherical bead form with different particle sizes. For comparison,
OME synthesis catalyzed by the ion exchange resin A36bead
In OME synthesis, product yields are strongly limited due to chemical equilibrium and significant amounts of DMM and trioxane are still present after reaction, besides the oligomeric products. The OME distribution primarily depends on the trioxane/DMM ratio and neither pressure nor temperature have significant influence on equilibrium [26]. Typical catalysts for OME synthesis are solid Brønsted acids like strongly acidic ion exchange resins, which were already successfully tested by Burger et
Conclusion
The ion exchange resins A36bead and A36powder as well as the zeolites BEA25, MFI27, MOR30 and Y30 were tested as catalysts in the synthesis of OMEs from DMM and trioxane. Product spectra corresponded to the Schulz-Flory distribution in all experiments. Initial catalytic tests with A36bead showed that the water content of the starting materials significantly decreases the reaction rate. This crucial aspect was inadequately considered until now and therefore, starting materials and catalysts were
Acknowledgement
The authors would like to thank Clariant AG and Zeolyst International for providing catalysts. Financial support from the Helmholtz Association (Research programme: Storage and cross-linked infrastructures) and Fachagentur Nachwachsende Rohstoffe (Joint research project: Oxymethylene ethers (OME): Eco-friendly diesel additives from renewables, FKZ 22403814) is gratefully acknowledged.
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