Continuous Anhydrous Synthesis of Oxymethylene Dimethyl Ethers by Reaction of Dimethyl Ether with Molecular Formaldehyde.

A novel gaseous synthesis route to oxymethylene dimethyl ethers (OMEn, n = 3-5) starting from CO2 and green H2 by using molecular formaldehyde (FA) and dimethyl ether (DME) is presented. The anhydrous reaction runs in a pressure free, gaseous, and continuous reaction setup. Hetero-geneous cata-lysts including zeolites and ion exchange resins (IER) are investigated, if they catalyze this reaction. While IER is almost inactive, zeolites with a 3D pore structure and an acidity exceeding ρm,H+ (NH3,ads ) = 250 µmol·gcat.-1 proved to be catalytically active. DME conversions of up to 2.76 mol-% are observed. The observed product gas stream compositions confirm thermo-dynamic considerations with back reactions / OMEn decomposition occurring as part of the equilibria under the investigated reaction conditions (90…180 °C). However, feed gas ratio variations (FA:DME = 1:2 to 1:9.5) highlighted the possibility to shift the product selectivity in favor of OMEn and suppress FA disproportionation to methyl formate. FA trimerization to trioxane is almost completely suppressed by running the reaction at 120 °C. The results presented here provide an important and unprecedented contribution to understand the complex reaction network in the OMEn synthesis reaction necessary to establish an energy efficient sustainable OMEn production process.

Critical role of H-aggregation for high-efficiency photoinduced charge generation in pristine pentamethine cyanine salts.

The mechanism of photoinduced symmetry-breaking charge separation in solid cyanine salts at the base of organic photovoltaic and optoelectronic devices is still debated. Here, we employ femtosecond transient absorption spectroscopy (TAS) to monitor the charge transfer processes occurring in thin films of pristine pentamethine cyanine (Cy5). Oxidized dye species are observed in Cy5-hexafluorophosphate salts upon photoexcitation, resulting from electron transfer from monomer excited states to H-aggregates. The charge separation proceeds with a quantum yield of 86%, providing the first direct proof of high efficiency intrinsic charge generation in organic salt semiconductors. The impact of the size of weakly coordinating anions on charge separation and transport is studied using TAS alongside electroabsorption spectroscopy and time-of-flight techniques. The degree of H-aggregation decreases with increasing anion size, resulting in reduced charge transfer. However, there is little change in carrier mobility, as despite the interchromophore distance increasing, the decrease in energetic disorder helps to alleviate the trapping of charges by H-aggregates.

The perfluoroadamantoxy aluminate as an ideal weakly coordinating anion? - synthesis and first applications.

Weakly coordinating anions (WCAs) facilitate the stabilization and isolation of highly reactive and almost "naked" cations. Alkoxyaluminate-based WCAs such as [Al(OC(CF3)3)4]- ([pf]-) are widely used due to their synthetic accessibility and their high stability. However, small cations are still able to coordinate the oxygen atoms of the [pf]- anion or even to abstract an alkoxy ligand. The novel WCA [Al(OC10F15)4]- ([pfAd]-; OC10F15 = perfluoro-1-adamantoxy) is characterized by a very rigid core framework, thus indicating a higher stability towards fluoride-ion abstraction (DFT calculations) and providing hope to generate less disordered crystal structures. The [pfAd]- anion was generated by the reaction of the highly acidic alcohol perfluoro-1-adamantanol C10F15OH with LiAlH4 in o-DFB. Li[pfAd] could not be synthesized free of impurities (and still contains unreacted alcohol). Yet, starting from contaminated Li[pfAd], the very useful pure salts Ag[pfAd], [Ph3C][pfAd] and [H(OEt2)2][pfAd] could be synthesized. The salts were characterized by NMR spectroscopy, single-crystal X-ray diffraction and IR spectroscopy. Additionally, [NO][pfAd] could be synthesized containing alcohol impurities but nonetheless enabled the synthesis of the salt P9+[pfAd]-. The synthesis of Tl[pfAd] in a mixture of H2O/acetone/o-DFB demonstrated the water stability of the [pfAd]- anion.