Experimental kinetic study of transesterification of ethyl acetate with methanol catalyzed by gel and macroporous acidic ion exchange resins

The reaction kinetics of the liquid-phase transesterification of ethyl acetate with methanol have been investigated over a series of commercially available ion-exchange resins. Two morphology types of cross-linked polymer resins have been considered, i.e., a gel type (Lewatit K1221) and a macroporous type (Lewatit K2640, Lewatit K2629 and Amberlyst 15). The effect of the swelling of the resin, the initial reactant molar ratio (1:1 – 10:1) and the temperature (303.15 – 333.15 K) on the reaction kinetics was experimentally assessed. Lewatit K1221, the gel-type resin, outperformed the macroporous-type resins, despite its similar number of sulfonic acid sites. The resin’s swelling behavior, which can be related to its degree of cross-linking with divinylbenzene, was identified as the key parameter to explain differences in acid site accessibility between the considered resins and, hence, the observed transesterification kinetics. A fundamental kinetic model, accounting for the chemical elementary steps as well as for the physical swelling due to solvent absorption, was constructed to quantitatively assess the experimental observations. According to this model (1) all active sites are occupied by methanol in protonated form, (2) the esters undergo a proton exchange with the protonated methanol and (3) the reaction occurs through an Eley-Rideal mechanism with the surface reaction of protonated ethyl acetate with methanol from the bulk as the rate-determining step. The kinetic model was able to adequately describe the entire experimental data set. An activation energy amounting to 49 kJ mol-1 was obtained, irrespective of the resin. Also the affinity of each of the resins for the esters was found to be similar. The differences in catalytic activity between the considered resins are found back in the values for the rate coefficients and, hence, can be brought into relation with the active site accessibility. The latter is a factor 3 to 4 higher for gel-type resins compared to macroporous-type resins. An independent experimental assessment of the resins’ swelling behavior confirmed the more pronounced swelling of the gel-type compared to the macroporous-type resins

Experimental and kinetic study of (trans)esterification reactions on Lewatit K1221

(Trans)esterification reactions play a key role in today’s biorefineries. Conventionally, these reactions are performed using an acid and a base homogeneous catalyst, respectively for esterification and transesterification. Heterogeneous catalysis offers a competitive alternative for these reactions by not requiring dedicated catalyst separation and purification. A single, fixed bed catalytic reactor, comprising a top layer of acid catalyst for esterification of free fatty acids followed by a base catalyst bed for triglyceride transesterification, would allow intensifying present-day biorefinery conversion processes. In the present work, the attention is focused on the acid top layer, for which an acid ion-exchange resin has been considered. Apart from esterification, also acid catalyzed transesterification has been investigated on this resin. A fundamental kinetic model has been constructed to gain detailed insight in the (trans)esterification reactions that will be essential in the intensification of the corresponding biorefinery processes. Acid ion exchange resins have been selected as they are ecofriendly, noncorrosive and have a good stability and reusability. The investigated resin consists of a cross-linked polystyrene matrix with sulfonic acid groups as active sites. Due to its particular structure, the resin exhibits a remarkable swelling phenomenon when in contact with a polar solvent. Since an excess of methanol is used as (trans)esterification agent and water is formed by esterification, the resin is swollen during the reaction. The resin’s swelling determines the accessibility of its active sites and, hence, plays a critical role in the finally observed reaction kinetics. An experimental investigation of the temperature effect and that of the initial molar ratio of acetic acid and methanol for esterification and of ethyl acetate and methanol for transesterification, both catalyzed by Lewatit K1221, was performed. The experimental data have been adequately modelled using an Eley-Rideal reaction mechanism. Exchange between protonated methanol, which was assumed to, initially, occupy all active sites, and the acid or ester in esterification and transesterification, respectively, was explicitly accounted for. While the activation energy was determined at 49 kJ mol-1, irrespective of the reaction type, the difference in catalytic activity, is found back in the value for the rate coefficient, which is about 1 order of magnitude higher for esterification than for transesterification. A unique set of exchange coefficients was obtained, irrespective of the considered reaction, which indicates the model’s adequacy. Throughout the experimentation at 333 K and using an initial molar ratio 10:1, the catalyst’s active sites were found to be occupied by methanol for at least 60%

Radio Observations of New Galactic Bulge Planetary Nebulae

We observed 64 newly identified galactic bulge planetary nebulae in the radio continuum at 3 and 6 cm with the Australia Telescope Compact Array. We present their radio images, positions, flux densities, and angular sizes. The survey appears to have detected a larger ratio of more extended planetary nebulae with low surface brightness than in previous surveys. We calculated their distances according to Van de Steene & Zijlstra (1995). We find that most of the new sample is located on the near side around the galactic center and closer in than the previously known bulge PNe. Based on H-alpha images and spectroscopic data, we calculated the total H-alpha flux. We compare this flux value with the radio flux density and derive the extinction. We confirm that the distribution of the extinction values around the galactic center rises toward the center, as expected.Comment: accepted for publication in A&

Photo-ionization modelling of planetary nebulae -- II. Galactic bulge nebulae, a comparison with literature results

We have constructed photo-ionization models of five galactic bulge planetary nebulae using our automatic method which enables a fully self-consistent determination of the physical parameters of a planetary nebula. The models are constrained using the spectrum, the IRAS and radio fluxes and the angular diameter of the nebula. We also conducted a literature search for physical parameters determined with classical methods for these nebulae. Comparison of the distance independent physical parameters with published data shows that the stellar temperatures generally are in good agreement and can be considered reliable. The literature data for the electron temperature, electron density and also for the abundances show a large spread, indicating that the use of line diagnostics is not reliable and that the accuracy of these methods needs to be improved. Comparison of the various abundance determinations indicates that the uncertainty in the electron temperature is the main source of uncertainty in the abundance determination. The stellar magnitudes predicted by the photo-ionization models are in good agreement with observed values.Comment: Accepted for publication in MNRA

Kinetic study of the (trans)esterification catalyzed by gel and macroporous resins

(Trans)esterification reactions with typical production volumes ranging from a few hundred to thousand tonnes per year are commercially important and are used to make esters which serve as precursors or additives for a variety of perfumes and flavours, pharmaceuticals, agrochemicals and polymers. The homogeneous Lewis or BrØnsted acids and bases that are commonly used for (trans)esterification purposes exhibit several well-known disadvantages, such as additional separation and neutralisation steps. A heterogeneously catalysed implementation of the reaction making use of acid ion exchange resins is promising as they are ecofriendly, noncorrosive and reusable. Acid ion exchange resins are promising catalysts for (trans)esterification. Thanks to their pronounced swelling when in contact with polar media, the highest conversions are obtained with a gel type resin, Lewatit K1221. The resins’ swelling significantly affects the catalytic activity and, hence, should also be reflected in the kinetic model. A novel kinetic model, implicitly accounting for resin swelling, was developed based on the following assumptions: (1) all active sites are occupied, (2) the exchange reactions between reagents or products are quasi equilibrated and (3) an Eley-Rideal type surface reaction between protonated (ester)acid with methanol from the bulk occurs as the rate-determining step. This kinetic model describes accurately the (trans)esterification reaction kinetics on different type of resins. Parameter estimates generally adopted physically meaningful values although the exchange coefficients obtained for acetic acid esterification indicated the need for further model refinement, in particular with respect to the description of the thermodynamics inside the resin. The kinetic model can be used as a helpful tool for resin selection. While resin selection and design frequently occurs on a trial and error base, the insight gained by the model, may provide clear guidelines about the desired resin properties. This knowledge may further lead to an enlargement of the use of ion exchange resins as catalysts, and, hence, also be exploited for industrial reactor design and optimization

IRAS08281-4850 and IRAS14325-6428: two A-type post-AGB stars with s-process enrichment

One of the puzzling findings in the study of the chemical evolution of (post-)AGB stars is why very similar stars (in terms of metallicity, spectral type, infrared properties, etc...) show a very different photospheric composition. We aim at extending the still limited sample of s-process enriched post-AGB stars, in order to obtain a statistically large enough sample that allows us to formulate conclusions concerning the 3rd dredge-up occurrence. We selected two post-AGB stars on the basis of IR colours indicative of a past history of heavy mass loss: IRAS08281-4850 and IRAS14325-6428. They are cool sources in the locus of the Planetary Nebulae (PNe) in the IRAS colour-colour diagram. Abundances of both objects were derived for the first time on the basis of high-quality UVES and EMMI spectra, using a critically compiled line list with accurate log(gf) values, together with the latest Kurucz model atmospheres. Both objects have very similar spectroscopically defined effective temperatures of 7750-8000K. They are strongly carbon and s-process enriched, with a C/O ratio of 1.9 and 1.6, and an [ls/Fe] of +1.7 and +1.2, for IRAS08281 and IRAS14325 resp. Moreover, the spectral energy distributions (SEDs) point to heavy mass-loss during the preceding AGB phase. IRAS08281 and IRAS14325 are prototypical post-AGB objects in the sense that they show strong post 3rd dredge-up chemical enrichments. The neutron irradiation has been extremely efficient, despite the only mild sub-solar metallicity. This is not conform with the recent chemical models. The existence of very similar post-AGB stars without any enrichment emphasizes our poor knowledge of the details of the AGB nucleosynthesis and dredge-up phenomena. We call for a very systematic chemical study of all cool sources in the PN region of the IRAS colour-colour diagram.Comment: 8 pages, 6 figures, accepted by A&

An Infrared Imaging Study of the Bipolar Proto-Planetary Nebula IRAS 16594-4656

High-resolution mid-infrared images have been obtained in N-band and Q-band for the proto-planetary nebula IRAS 16594-4656. A bright equatorial torus and a pair of bipolar lobes can clearly be seen in the infrared images. The torus appears thinner at the center than at the edges, suggesting that it is viewed nearly edge-on. The infrared lobes correspond to the brightest lobes of the reflection nebula seen in the Hubble Space Telescope (HST) optical image, but with no sign of the point-symmetric structure seen in the visible image. The lobe structure shows a close correspondence with a molecular hydrogen map obtained with HST, suggesting that the dust emission in the lobes traces the distribution of the shocked gas. The shape of the bipolar lobes shows clearly that the fast outflow is still confined by the remnant circumstellar envelope of the progenitor asymptotic giant branch (AGB) star. However, the non-detection of the dust outside of the lobes suggests that the temperature of the dust in the AGB envelope is too low for it to be detected at 20 microns.Comment: Accepted by the Astrophysical Journa