Aromatic aldehydes as tuneable and ppm level potent promoters for zeolite catalysed methanol dehydration to DME

Dimethyl ether (DME) is a valuable chemical intermediate and renewable fuel that can be made, via methanol, from many sources of carbon, including carbon dioxide and biomass. Benzaldehyde and its derivatives have been found to be promoters for zeolite catalysed methanol dehydration to DME at low temperature (110 to 150 oC). For the 3-dimensional medium pore zeolite H-ZSM-5 (MFI) the promotion is readily reversible and the potency of the promoter can be tuned by varying the substituent on the aromatic ring of the aldehyde. The most potent promoters are active at concentrations as low as 1 ppm relative to methanol. High throughput experimentation (HTE) is used to screen and rank potential promoters and catalysts and to collect high quality kinetic data for the most promising candidates discovered. The catalytic data and in-situ FT-IR-MS experiments combined with molecular modelling studies indicate a mechanism involving competitive adsorption of the aldehyde promoter on a Brønsted acid (BA) site, followed by reaction with methanol to give a hemi-acetal intermediate. Loss of water from the hemi-acetal intermediate generates a transient and highly reactive methyl oxonium species, [ArC(H)(=O-Me)]+, which then directly reacts with methanol via a SN2 mechanism to give DME and regenerate the aldehyde promoter and BA site. The methyl oxonium species is stabilized by electron-donating groups on the aromatic ring and the solvent like effect of the zeolite pore walls. Molecular descriptors were calculated by molecular modelling for the 22 aromatic aldehyde promoters tested. Multivariate linear regression analysis was used to build an interpretable model for aldehyde promotional activity in H-ZSM-5 and in another 3-dimensional medium pore zeolite, H-ZSM-11 (MEL)

Volatility and Oxidative Aging of Aqueous Maleic Acid Aerosol Droplets and the Dependence on Relative Humidity

The microphysical structure and heterogeneous oxidation by ozone of single aerosol particles containing maleic acid (MA) has been studied using aerosol optical tweezers and cavity enhanced Raman spectroscopy. The evaporation rate of MA from aqueous droplets has been measured over a range of relative humidities and the pure component vapor pressure determined to be (1.7 ± 0.2) × 10^–3 Pa. Variation in the refractive index (RI) of an aqueous MA droplet with relative humidity (RH) allowed the subcooled liquid RI of MA to be estimated as 1.481 ± 0.001. Measurements of the hygroscopic growth are shown to be consistent with equilibrium model predictions from previous studies. Simultaneous measurements of the droplet composition, size, and refractive index have been made during ozonolysis at RHs in the range 50–80%, providing insight into the volatility of organic products, changes in the droplet hygroscopicity, and optical properties. Exposure of the aqueous droplets to ozone leads to the formation of products with a wide range of volatilities spanning from involatile to volatile. Reactive uptake coefficients show a weak dependence on ozone concentration, but no dependence on RH or salt concentration. The time evolving RI depends significantly on the RH at which the oxidation proceeds and can even show opposing trends; while the RI increases with ozone exposure at low relative humidity, the RI decreases when the oxidation proceeds at high relative humidity. The variations in RI are broadly consistent with a framework for predicting RIs for organic components published by Cappa et al. (J. Geophys. Res. 2011, 116, D15204). Once oxidized, particles are shown to form amorphous phases on drying rather than crystallization, with slow evaporation kinetics of residual water