Cu-SSZ-13 has been characterized by different spectroscopic techniques and compared with Cu-ZSM-5 and Cu-β with similar Si/Al and Cu/Al ratios and prepared by the same ion exchange procedure. On vacuum activated samples, low temperature FTIR spectroscopy allowed us to appreciate a high concentration of reduced copper centres, i.e. isolated Cu+ ions located in different environments, able to form Cu+(N2), Cu+(CO)n (n = 1, 2, 3), and Cu+(NO)n (n = 1, 2) upon interaction with N2, CO and NO probe molecules, respectively. Low temperature FTIR, DRUV-Vis and EPR analysis on O2 activated samples revealed the presence of different Cu2+ species. New data and discussion are devoted to (i) [Cu–OH]+ species likely balanced by one framework Al atom; (ii) mono(μ-oxo)dicopper [Cu2(μ-O)]2+ dimers observed in Cu-ZSM-5 and Cu-β, but not in Cu-SSZ-13. UV-Vis-NIR spectra of O2 activated samples reveal an intense and finely structured d–d quadruplet, unique to Cu-SSZ-13, which is persistent under SCR conditions. This differs from the 22 700 cm−1 band of the mono(μ-oxo)dicopper species of the O2 activated Cu-ZSM-5, which disappears under SCR conditions. The EPR signal intensity sets Cu-β apart from the others.

Characterization of Cu-exchanged SSZ-13: a comparative FTIR, UV-Vis, and EPR study with Cu-ZSM-5 and Cu-β with similar Si/Al and Cu/Al ratios

Location and nature of Cu species in Cu/SAPO-34 for selective catalytic reduction of NO with NH3

Metal–organic frameworks have recently attracted attention as heterogeneous catalysts due to their high content of metal centres and large surface area and pore volume, along with their impressive topological richness. Therefore, many studies describing the use of MOFs as heterogeneous Lewis acid catalysts have been published. In this regard, these efforts have been directed towards probing the catalytic activity. Further information is required in terms of kinetic parameters and comparison of performance with their homogeneous counterparts, or other conventional heterogeneous catalysts. Here we have attempted to put MOFs into perspective with respect to their homogeneous counterparts and more conventional heterogeneous catalysts to show their advantages and limitations. We have exemplified a number of reactions reported in the literature wherein MOFs have been used as catalysts, and we have carried them out using homogeneous counterparts, i.e. benzoates and acetates, and other well defined conventional solid catalysts. The activities and selectivities of the catalysts are compared and then put into perspective on the basis of kinetic parameters, such as turnover numbers and turnover frequencies. Additionally, we illustrate using selected examples what the potential advantages of MOF catalysts could be, and how they may outperform the potential of other solid catalysts.

MOF catalysis in relation to their homogeneous counterparts and conventional solid catalysts

Migration of Cu species in Cu/SAPO-34 during hydrothermal aging

A combination of broadband dielectric $({10}^{\ensuremath{-}2}\phantom{\rule{0.3em}{0ex}}\mathrm{Hz}--{10}^{9}\phantom{\rule{0.3em}{0ex}}\mathrm{Hz})$ and specific heat $({10}^{\ensuremath{-}3}\phantom{\rule{0.3em}{0ex}}\mathrm{Hz}--2\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{0.3em}{0ex}}\mathrm{Hz})$ spectroscopy is employed to study the molecular dynamics of the glass-forming nematic liquid crystal $E7$ in a wide temperature range. In the region of the nematic phase the dielectric spectra show two relaxation processes which are expected theoretically: The $\ensuremath{\delta}$ relaxation which corresponds to rotational fluctuations of the molecules around its short axis and the tumbling mode at higher frequencies than the former one. For both processes the temperature dependence of the relaxation rates follows the Vogel-Fulcher-Tammann formula which is characteristic for glassy dynamics. By applying a detailed data analysis, it is shown that close to the glass transition the tumbling mode has a much steeper temperature dependence than the $\ensuremath{\delta}$ process. The former has a Vogel temperature which is by $30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ higher than that of the $\ensuremath{\delta}$ relaxation. Specific heat spectroscopy gives one relaxation process in its temperature and frequency dependence which has to be assigned to the $\ensuremath{\alpha}$ relaxation (dynamic glass transition). The unique and detailed comparison of the temperature dependence of the dielectric and the thermal relaxation rates delivers unambiguously that the dielectric tumbling mode has to be related to the dynamic glass transition.

H.-L. Zubowa

Papers

Microwave-assisted synthesis and characterization of Cu-containing AlPO4-5 and SAPO-5

The gallophosphate molecular sieve cloverite as a host for liquid crystals

On the Dielectric Behavior of Unaligned Samples of 4‐n‐Octyl‐4′‐cyanobiphenyl (8CB)