Search

In the field of alternative diesel fuels, so-called oxymethylene ethers (OMEs) are currently intensely investigated. Particularly OMEs of the type CH$_{3}$O(CH$_{2}$O)$_{n}$CH$_{3}$ with n = 3–5 exhibit promising fuel properties and combustion characteristics with strongly reduced particle and NO$_{x}$ emissions. According to their molecular structure, OMEs can be produced from methanol thus enabling sustainable production strategies from CO$_{2}$ and renewable resources. Compared to the methyl derivatives, analogous compounds with higher alkyl groups (oxymethylene dialkyl ethers, OMDAEs) have been investigated to a much lesser extent. Thus, commercially available OMDAEs, i.e. compounds of the type ROCH$_{2}$OR bearing ethyl, propyl, butyl and 2-ethylhexyl groups, have been studied. Furthermore, asymmetric compounds of the type R$^{1}$OCH$_{2}$OR$^{2}$ have been synthesized from the symmetric compounds employing transacetalization reactions catalyzed by zeolite BEA-25. The OMDAEs have been characterized by spectroscopic and spectrometric methods and several physico-chemical, thermodynamic and fuel-related data have been determined and compared. Despite their structural peculiarities, such as the oxygen-containing acetal moiety in the molecular backbone, all OMDAEs exhibit properties similar to conventional diesel fuels. Based on experimental and analytical data, the development of tools for the prediction of properties by a simple regression method is described. Furthermore, the suitability of group contribution modelling is investigated for OMDAE compounds.

Catalyst systems for the conversion of synthesis gas, which are tolerant to fluctuating CO/CO$_{2}$ gas compositions, have great potential for process-technical applications, related to the expected changes in the supply of synthesis gas. Copper-based catalysts usually used in the synthesis of methanol play an important role in this context. We investigated the productivity characteristics for their application in direct dimethyl ether (DME) synthesis as a function of the CO$_{2}$/CO$_{x}$ ratio over the complete range from 0 to 1. For this purpose, we compared an industrial Cu/ZnO/Al$_{2}$O$_{3}$ methanol catalyst with a self-developed Cu/ZnO/ZrO$_{2}$ catalyst prepared by a continuous coprecipitation approach. For DME synthesis, catalysts were combined with two commercial dehydration catalysts, H-FER 20 and γ-Al$_{2}$O$_{3}$, respectively. Using a standard testing procedure, we determined the productivity characteristics in a temperature range between 483 K and 523 K in a fixed bed reactor. The combination of Cu/ZnO/ZrO$_{2}$ and H-FER 20 provided the highest DME productivity with up to 1017 g$_{DME}$ (kg$_{Cu}$ h)$^{-1}$ at 523 K, 50 bar and 36 000 ml$_{N}$ (g h)$^{-1}$ and achieved DME productivities higher than 689 g$_{DME}$ (kg$_{Cu}$ h)$^{-1}$ at all investigated CO$_{2}$/CO$_{x}$ ratios under the mentioned conditions. With the use of Cu/ZnO/ZrO$_{2}$//H-FER 20 a promising operating range between CO$_{2}$/CO$_{x}$ 0.47 and 0.8 was found where CO as well as CO$_{2}$ can be converted with high DME selectivity. First results on the long-term stability of the system Cu/ZnO/ZrO$_{2}$//H-FER 20 showed an overall reduction of 27.0% over 545 h time on stream and 14.6% between 200 h and 545 h under variable feed conditions with a consistently high DME selectivity.

The manufacturing of technical catalysts generally involves a sequence of different process steps, of which co-precipitation is one of the most important. In this study, we investigate how continuous co-precipitation influences the properties of Cu/ZnO/ZrO2 (CZZ) catalysts and their application in the direct synthesis of dimethyl ether (DME) from CO2/CO/H2 feeds. We compare material characteristics investigated by means of XRF, XRD, N2 physisorption, H2-TPR, N2O-RFC, TEM and EDXS as well as the catalytic properties to those of CZZ catalysts prepared by a semi-batch co-precipitation method. Ultra-fast mixing in continuous co-precipitation results in high BET and copper surface areas as well as in improved metal dispersion. DME synthesis performed in combination with a ferrierite-type co-catalyst shows correspondingly improved productivity for CZZ catalysts prepared by the continuous co-precipitation method, using CO2-rich as well as CO-rich syngas feeds. Our continuous co-precipitation approach allows for improved material homogeneity due to faster and more homogeneous solid formation. The so-called &ldquo, chemical memory&rdquo, stamped during initial co-precipitation is kept through all process steps and is reflected in the final catalytic properties. Furthermore, our continuous co-precipitation approach may be easily scaled-up to industrial production rates by numbering-up. Hence, we believe that our approach represents a promising contribution to improve catalysts for direct DME synthesis