Development of methods for the determination of pK\u3csub\u3ea\u3c/sub\u3e values

\u3cp\u3eThe acid dissociation constant (pK\u3csub\u3ea\u3c/sub\u3e) is among the most frequently used physicochemical parameters, and its determination is of interest to a wide range of research fields. We present a brief introduction on the conceptual development of pK\u3csub\u3ea\u3c/sub\u3e as a physical parameter and its relationship to the concept of the pH of a solution. This is followed by a general summary of the historical development and current state of the techniques of pK\u3csub\u3ea\u3c/sub\u3e determination and an attempt to develop determination and an attempt to develop insight into future developments. Fourteen methods of determining the acid dissociation constant are placed in context and are critically evaluated to make a fair comparison and to determine their applications in modern chemistry. Additionally, we have studied these techniques in light of present trends in science and technology and attempt to determine how these trends might affect future developments in the field.\u3c/p\u3

Dynamics of silver particles during ethylene epoxidation

\u3cp\u3eStructural changes in α-Al\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e supported silver particles during ethylene oxidation under industrially relevant conditions (20 bar and 225 °C, with and without chlorine) were investigated by quantitative TEM. Identical locations of two powder samples with average silver particle sizes of 41 nm and 127 nm were investigated in the fresh state and after 60 h of ethylene oxidation with and without chlorine. Without chlorine, silver particles were only slightly affected with a tendency for smaller (< 30 nm) particles to shrink. These silver particles even rapidly disappeared in the presence of chlorine. Particles larger than 100 nm tended to break up into smaller ones, indicative of a chlorine-induced re-dispersion process. These results confirm the dynamic nature of silver particles during the EO reaction and emphasize the role of chlorine on the observed substantial morphological changes of the active silver phase.\u3c/p\u3

The Influence and Removability of Colloidal Capping Agents on Carbon Monoxide Hydrogenation by Zirconia-Supported Rhodium Nanoparticles

By using wet-chemical methods, cubic, tetrahedral, and randomly shaped Rh nanoparticles (NPs) with different surface terminations were synthesized and subsequently deposited on ZrO2 supports. To guide the NP shape, three capping agents were used during the synthesis: polyvinylpyrrolidone (PVP), trimethyl(tetradecyl)ammonium bromide (TTAB), and oleylamine (OAm). TTAB and PVP could not be completely removed from the final catalyst, leaving a capping residue as confirmed by X-ray photoelectron spectroscopy (XPS). In contrast, OAm could be fully removed. The influence of the NP shape and the influence of the capping agents were evaluated under CO hydrogenation conditions. Both the PVP and TTAB residues blocked parts of the Rh surface and dominated catalytic activity beyond the effects from NP shape and surface termination. OAm could be successfully removed. The extent of metal surface blocking by the capping residue, probed by chemisorption, is larger than the observed reduction in CO hydrogenation activity. This suggests that a majority of less-active sites are being blocked by the capping residue and that successful removal of the residue from the most active sites is possible.\u3cbr/\u3

A robust Au/ZnCr\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e catalyst with highly dispersed gold nanoparticles for gas-phase selective oxidation of cyclohexanol to cyclohexanone

\u3cp\u3eAchieving uniformly dispersed and stable nanoparticles of gold on oxide supports is a challenge in heterogeneous catalysis. Here, we show that zincochromite (ZnCr\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e) is a promising support for obtaining high and stable gold dispersion. Despite a low surface area of ZnCr\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e, finely dispersed gold (∼3-4 nm particles) could be obtained by a simple deposition-precipitation method, pointing to strong gold-support interactions. Using a combination of XRD, XPS, SEM, TEM, HAADF-STEM, and IR spectroscopy, we confirmed that the calcination temperature of the ZnCr\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e support had a substantial influence on the crystallinity, morphology, and acidic properties of thereof derived Au/ZnCr\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e catalysts. Gold supported on a high-temperature (≥700 °C) calcined ZnCr\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e support displayed the best catalytic performance in gas-phase oxidation of cyclohexanol to cyclohexanone, which is an important intermediate in the chemical industry. When calcined at 800 °C, the material did not show any sign of deactivation in a 90 h stability test, high cyclohexanol conversion (93%) and high cyclohexanone yield (91%) were achieved at 300 °C, with a space-time yield of 250 g\u3csub\u3eproduct\u3c/sub\u3e gAu\u3csup\u3e-1\u3c/sup\u3e h\u3csup\u3e-1\u3c/sup\u3e. On the contrary, Au/ZnCr\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e based on supports calcined at lower temperatures (≤600 °C) catalyzed side reactions such as cyclohexanol dehydration to cyclohexene (Lewis acid sites), which further oligomerized to coke deposits that deactivated the catalyst. The CO oxidation trends with respect to calcination temperature were inversed to those in cyclohexanol oxidation, showing that smaller gold particles and the presence of hydroxyls are favorable for CO oxidation to CO\u3csub\u3e2\u3c/sub\u3e. DFT calculations provided insight into the (electronic) nature of the strong interactions between Au and ZnCr\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e \u3c/p\u3

Structure sensitivity of silver-catalyzed ethylene epoxidation

\u3cp\u3eThe influence of particle size (20-200 nm) of Ag/α-Al \u3csub\u3e2\u3c/sub\u3eO \u3csub\u3e3\u3c/sub\u3e catalysts for epoxidation of ethylene to ethylene oxide (EO) under industrial conditions was investigated. Small silver particles up to 40 nm are predominantly monocrystalline and show a decreasing weight-normalized reaction rate with increasing particle size. Particles larger than 50 nm consist of multiple silver crystallites with a much smaller domain size between 25 and 30 nm. For these polycrystalline silver particles, the weight-normalized reaction rate is independent of particle size. The ethylene conversion rate normalized to the external surface area increases when the silver particles become larger. We attribute this to a specific role of the grain boundaries between silver crystallites in supplying oxygen atoms to the external surface. Oxygen is likely activated at defects of an otherwise low-reactivity silver surface (for oxygen adsorption) followed by diffusion along grain boundaries, dissolution in the bulk, and diffusion to the external surface, where oxygen atoms react with ethylene. The reaction rate normalized to the surface area of the first outer shell of crystallites making up silver particles is independent of size for polycrystalline particles. A higher reaction pressure benefits ethylene oxidation rate and EO selectivity due to a higher oxygen coverage. Adding chlorine further improves the EO selectivity through modification of the active surface. The same particle size dependences are observed at 1 bar and at 20 bar without and with chlorine. The main finding of our work is that for large enough particles the ethylene oxidation rate normalized to the silver weight is independent of size. In addition to the size-independent weight-based activity, the preference for larger particles in industrial catalysts can be attributed to the high silver loadings used to obtain larger silver particles. The resulting high coverage of the α-Al \u3csub\u3e2\u3c/sub\u3eO \u3csub\u3e3\u3c/sub\u3e support with silver decreases undesired consecutive reactions of EO on its hydroxyl groups. \u3c/p\u3

Mordenite nanorods prepared by an inexpensive pyrrolidine-based mesoporogen for alkane hydroisomerization

\u3cp\u3eThe front cover artwork for Issue 12/2019 is provided by researchers from the group of Inorganic Materials and Catalysis, Eindhoven University of Technology (The Netherlands). The image shows how a novel surfactant based on methylpiperidine modified by a hydrophobic tail (C\u3csub\u3e16\u3c/sub\u3eNMP) can downsize the synthesis of mordenite zeolite from massive crystals into more accessible nanorods. See the Full Paper itself at https://doi.org/10.1002/cctc.201900298.\u3c/p\u3

Mordenite nanorods prepared by an inexpensive pyrrolidine-based mesoporogen for alkane hydroisomerization

\u3cp\u3e We report the synthesis of hierarchical mordenite zeolite nanorods in one step using inexpensive mono-quaternary ammonium N-cetyl-N-methylpyrrolidinium (C \u3csub\u3e16\u3c/sub\u3e NMP) as mesoporogen to the synthesis gel. The presence of a small amount of C \u3csub\u3e16\u3c/sub\u3e NMP results in the formation of 0.6–1 μm rods-like crystals oriented along the c-axis with a high mesoporous volume (0.12 cm \u3csup\u3e3\u3c/sup\u3e g \u3csup\u3e−1\u3c/sup\u3e ) and external surface area (∼90 m \u3csup\u3e2\u3c/sup\u3e g \u3csup\u3e−1\u3c/sup\u3e ) compared to bulk mordenite. Acidity characterization shows that the presence of C \u3csub\u3e16\u3c/sub\u3e NMP during mordenite formation leads to a redistribution of aluminum in the zeolite framework: the amount of Brønsted acid sites in the side-pockets (8MR channels) is increased at the expense of those in the 12MR main channels. As these latter acid sites are the ones involved in the conversion of alkene intermediates in bifunctional hydroconversion of alkanes, an optimized hierarchical mordenite prepared with C \u3csub\u3e16\u3c/sub\u3e NMP displays a more ideal hydrocracking selectivity than bulk MOR prepared solely with sodium. \u3c/p\u3

The important role of rubidium hydroxide in the synthesis of hierarchical ZSM-5 zeolite using cetyltrimethylammonium as structure-directing agent

Hierarchical ZSM‐5 zeolite with uniform mesopores was synthesized with a simple cetyltrimethylammonium (CTA+) template, which acted as a bifunctional surfactant in a RbOH‐based alkaline synthesis gel. Rb+ plays a key role in obtaining uniform mesopores within ZSM‐5 crystals. The structural, textural properties and the acidity were characterized by XRD, Ar physisorption, TEM, as well as CO IR and 27Al MAS NMR spectroscopy. These data point to partial retention of the initial mesoscale ordered texture of the precursor in the final zeolite. These textural properties result in a strongly improved catalytic performance in the methanol conversion reaction compared to bulk zeolite

A versatile mono-quaternary ammonium salt as a mesoporogen for the synthesis of hierarchical zeolites

\u3cp\u3eHere we report a versatile method to synthesize hierarchically porous zeolites with FER, CHA and MFI topologies by using inexpensive mono-quaternary ammonium N-cetyl-N-methylpyrrolidinium (C \u3csub\u3e16\u3c/sub\u3eNMP) as a mesoporogen. Extensive characterization revealed that the mesoporous zeolites are crystalline, possess a high mesopore volume and exhibit comparable Brønsted acidity to their bulk counterparts. Due to the improved accessibility of the microporous domains, these hierarchical zeolites display enhanced performance as catalysts in various reactions such as the dehydration-isomerization of 1-butanol to iso-butene (FER) and methanol-to-hydrocarbons reaction (CHA and MFI). \u3c/p\u3

Hierarchically porous (alumino)silicates prepared by an imidazole-based surfactant and their application in acid-catalyzed reactions

In this work, we developed a novel strategy to synthesize porous (alumino)silicate materials using a single structure-directing agent composed of an imidazole unit with a hydrophobic tail, namely 1,2-dimethyl-3-hexadecyl-1H-imidazol-3-ium bromide (C16dMImz). A wide range of products such as ordered mesoporous silicas, layered silica-alumina and hierarchically porous mordenite zeolite were obtained by varying synthesis parameters such as temperature and aluminum concentration. By changing crystallization temperature, we could control the degree of silica condensation and tune the textural and morphological properties of the final materials. By varying the aluminum concentration in the gel, we can obtain mesoporous amorphous silica-alumina or crystalline mordenite zeolite with, respectively, weak and strong Brønsted acid sites. Obtained acidic silica-alumina materials displayed promising performance in catalytic reactions of linear paraffin hydroisomerization and Friedel-Crafts alkylation of benzene with benzyl alcohol