To evaluate the relative stability of different intramolecular N-H…OMe hydrogen bonds of aromatic amide-based foldamers, 3-, 5-, and 7-mer aromatic amide foldamers F-3, F-5 and F-7, which possess one, two, and three different amide units, have been constructed from benzene-1,3-diamine and isophthalic acid derivatives.1 H NMR experiments in CDCl 2 CDCl 2 and DMSO-d 6 showed that the hydrogen bonds formed in the central area of the foldamer backbones are least stable, whereas the hydrogen bonds formed at the two ends are most stable.1 H NMR hydrogen-deuterium exchange experiments for F-3, F-5 and F-7 in CDCl 2 CDCl 2 -CD 3 OD (19∶1, V/V) and DMSO-d 6 -CD 3 OD (19∶1, V/V) were performed. In the former less polar solvent mixture, the half-life values of the process, corresponding to amides from the central area to the end areas, were determined to be 140 h for F-3, 71.8 and 405 h for F-5, and 36.3, 216 and 314 h for F-7, respectively. In the latter more polar solvent mixture, the related values were evaluated to be 97.1 h for F-3, 69.0 and 300 h for F-5, and 13.5, 38.3 and 57.5 h for F-7, respectively. These quantitative results are consistent with the above 1 H NMR observation. To further assess the strength of the intramolecular hydrogen bonds, the three folded aromatic amide segments have also been incorporated into the main chains of dodecane-1,12-diamine-derived amide polymers to afford macromolecules P-3, P-5 and P-7. The degree of polymerization of the macromolecules was determined by GPC to be 22, 14 and 13, respectively. Force-extension curves obtained from single molecular force spectroscopy (SMFS) revealed that, in tetrachloroethane, all the three macromolecules exhibited saw-tooth force peaks, which had been attributed to the step-by-step breaking of the intramolecular hydrogen bonds of the foldamer segments. P-3 exhibited 4 peaks at ca. 83, 121, 181 and 236 pN, P-5 displayed 7 peaks at ca. 20,44, 73, 101, 130, 171 and 278 pN, and P-7 generated 8 peaks at ca. 31, 43, 50, 60, 90, 152, 173 and 221 pN. The increasing number of the force peaks observed from P-3 to P-5 and P-7 was ascribed to the increasing number of the intramolecular hydrogen bonds. It was proposed that the peaks at lower forces corresponded to the less stable hydrogen bonds, whereas those observed at higher forces were produced by the breaking of the more stable ones. The fact that the first peaks of P-3 was higher than that of P-5 and P-7 indicated that the intramolecular hydrogen bonds of P-3 were pronouncedly more stable than some of the intramolecular hydrogen bonds of P-5 and P-7, which is consistent with the above 1 H NMR and hydrogen-deuterium exchange observations. Similar results were also observed for P-5 and P-7 in hexadecane, whereas P-3 did not generate measurable force peaks possibly due to the strong absorption of its short, but more planar foldamer segments to the surface of the slide. Simulated stretching curves of the three macromolecules were also consistent with the SMFS results.

Propylene is widely used as a raw material for producing polypropylene, acrylonitrile, propylene oxide, etc. Typical manufacturing processes for propylene (steam cracking and FCC process) are over-reliant on petroleum resources and cannot meet the rapidly growing global demands. New routes for producing propylene from non-oil resources, particularly methanol-to-propylene (MTP) technology, have attracted increasingly more attention, where a fixed-bed reactor is used and ZSM-5 zeolite is the best alternative catalyst. However, structural optimization of ZSM-5 to enhance the lifetime and propylene selectivity and a deep understanding of the mechanism of the MTP reaction are still considerable challenges. For the conventional ZSM-5 zeolite, carbon deposition preferentially occurs near the outer surface of the zeolite particles because of the high acid density on the external surface, which accelerates the deactivation by blocking the outer pore openings, especially in a long-term MTP reaction. Large amounts of external strong acids also promote secondary reactions, such as hydrogen transfer reactions, resulting in a decrease in propylene selectivity. To study the effects of strong and weak acid distributions of ZSM-5 zeolite on the MTP reaction, two series of boron-modified ZSM-5 zeolites were designed: B-Al-ZSM-5 zeolites by one-step synthesis and Al-ZSM-5@B-ZSM-5 core-shell zeolites by two-step synthesis. These were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM) mapping, N2 physical adsorption-desorption, temperature-programmed desorption of ammonia (NH3-TPD) and 1,3,5-triisopropylbenzene (TIPB) cracking, and B1-Al-ZSM-5 and Al@B1-ZSM-5, B2-Al-ZSM-5 and Al@B2-ZSM-5, and B3-Al-ZSM-5 and Al@B3-ZSM-5 samples in the two series were found to have similar texture properties, acid amounts and acid strengths, but different B and Al elemental distributions and acid distributions. We used these two sets of samples to compare the effect of different strong and weak acid distributions-a uniform distribution and a gradient distribution of strong and weak acids on the performance of the MTP reaction. The results showed that samples with a uniform distribution of strong and weak acids have higher propylene selectivity due to lower strong and weak acid densities, whereas samples with a gradient acid distribution have a longer catalytic lifetime in the MTP reaction due to the absence of strong acid density and higher weak acid density on the outer surface. The different acid distributions lead to two different carbon deposition modes. Carbon deposition of the sample with the uniform acid distribution preferentially formed on the outer surface, resulting in rapid deactivation by blocking external micropores and leaving the internal active centers not fully utilized. However, for the sample with the gradient acid distribution, the carbonblocking rate of the external surface considerably decreased, which increased the time that the reactant molecules had t...