Control of Al Distribution in the CHA-Type Aluminosilicate Zeolites and Its Impact on the Hydrothermal Stability and Catalytic Properties

The CHA-type aluminosilicate zeolites were synthesized in the presence of the <i>N</i>,<i>N</i>,<i>N</i>-trimethyl-1-adamantammonium cation from different starting materials, including fumed silica, aluminum hydroxide, and the FAU-type zeolite, with their proportions varied. In this work, the proportion of “Q⁴(<i>n</i>Al)”, Si­(OSi)_4‑<i>n</i>(OAl)_<i>n</i> and “Q³(<i>n</i>Al)”, Si­(OSi)_3‑<i>n</i>(OH)­(OAl)_<i>n</i>, in the total framework Si atoms, which can be estimated by the solid-state ²⁹Si MAS NMR technique, has been applied to an index for Al distribution. When the proportion of the Al source derived from the FAU-type zeolite was increased, the proportion of Q⁴(2Al) was increased. Thus, we found a facile method for controlling the Al distribution in the CHA-type zeolite by varying the starting materials. Finally, the impacts of the Al distribution on the hydrothermal stability and catalytic properties in the methanol to olefins (MTO) reaction were investigated

Intramolecular H/D Exchange of Ethanol Catalyzed by Acidic OH Groups on H‑ZSM‑5 Zeolite

IR observation of ethanol adsorption clarified the presence of the apparent intramolecular isotope exchange from CD₃CH₂OH to CHD₂CH₂OD on acidic OH groups of H-ZSM-5 zeolite. This reaction did not proceed with CD₃OH nor CH₃CD₂OH, implying that the β-hydrogen of alcohol had interaction with the lattice oxygen adjacent to Al and that the reaction was mediated by isotope exchange of CD₃ groups of ethanol and OH groups on zeolite

Control of the Al Distribution in the Framework of ZSM‑5 Zeolite and Its Evaluation by Solid-State NMR Technique and Catalytic Properties

The effects of the organic structure-directing agents (OSDAs) and Na cations for the synthesis of ZSM-5 on the location of Al atom in the framework as well as the acidic and catalytic properties were investigated. To achieve these purposes, ZSM-5 zeolites were synthesized by using four kinds of OSDAs including tetrapropylammonium hydroxide cations, dipropylamine, cyclohexylamine, and hexamethylenimine with or without Na cations. In situ FT-IR spectroscopy using CO as probe molecule was applied to the evaluation of the acid property of the ZSM-5 zeolites. The location of Al atoms was examined by high resolution ²⁷Al MAS and MQMAS NMR techniques. The constraint index (CI) has also been used to estimate the distribution of acid sites in the micropores. The location of acid sites was investigated based on the difference in the transition-state shape-selectivity through the cracking of <i>n</i>-hexane and 3-methylpentane. Furthermore, the cracking of various types of paraffins and the conversion of aromatic compounds were conducted to clarify the acid site distributions

Infrared Investigation of Dynamic Behavior of Brønsted Acid Sites on Zeolites at High Temperatures

Temperature-dependent behavior of acidic OH groups on zeolites was observed by infrared (IR) spectroscopy. While the IR band of acidic OH groups appeared the same in frequency and intensity below 300 K, gradual shifts in the peak-top position to lower frequencies and decreases in integrated intensity were recognized when samples were heated at higher temperatures. These changes were completely reversible and only dependent on the temperature. Based on the assumption that there is an equilibrium between undissociated and dissociated states of OH groups, a model is proposed in which the intensity decrease is attributed to the dissociation of OH groups to form IR inactive species at high temperatures. The enthalpy difference between the two states was estimated using the van’t Hoff equation, leading to two different values in two temperature ranges (about 398–548 and 573–773 K) for zeolites with various topologies (<b>MFI</b>, <b>MOR</b>, and <b>CHA</b>). Based on the presence of two different types of enthalpy values, different mechanisms were proposed for these two situations. Liberated protons may move across four lattice oxygen atoms around the Al site at lower temperatures (around 550 K or below). At high temperatures, the protons may move in wider regions over the framework. DFT calculations show that the frequency of the OH band varies depending on which of the four different oxygen atoms around the Al site acts as the proton acceptor. The experimentally observed peak-top shift to the lower frequency side is explained by assuming that the population of protons changed with temperature such that more protons reside at low-frequency sites at higher temperatures. This process is interpreted as localized hopping or limited delocalization. At the higher temperature range (573–773 K), the enthalpy difference was independent of the Al amount but was only dependent on the zeolite topology. This supports the free hopping of protons over the framework. The enthalpy difference at higher temperatures increased in the order <b>CHA</b> < <b>MFI</b> < <b>MOR</b>, indicating that a zeolite with smaller pores tends to generate more protons at the same temperature. In other words, zeolites with small pores function as stronger acid catalysts for reactions at high temperatures. This proposition was supported by experimental results of a monomolecular reaction taking place at a single site, that is, H/D isotope exchange between acidic OD groups and CH₄

Heterogeneous Ni Catalyst for Direct Synthesis of Primary Amines from Alcohols and Ammonia

This paper reports the synthesis of primary amines from alcohols and NH₃ by an Al₂O₃-supported Ni nanoparticle catalyst as the first example of heterogeneous and noble-metal-free catalytic system for this reaction without additional hydrogen sources under relatively mild conditions. Various aliphatic alcohols are tolerated, and turnover numbers were higher than those of Ru-based homogeneous catalysts. The catalyst was recoverable and was reused. The effects of the Ni oxidation states and the acid–base nature of support oxides on the catalytic activity are studied. It is clarified that the surface metallic Ni sites are the catalytically active species, and the copresence of acidic and basic sites on the support surface is also indispensable for this catalytic system

Crystallization of Ti-Rich *BEA Zeolites by the Combined Strategy of Using Ti–Si Mixed Oxide Composites and Intentional Aluminum Addition/Post-Synthesis Dealumination

Titanosilicate zeolites are well-known catalysts for selective oxidation using hydrogen peroxide, an environmentally friendly oxidant. To effectively synthesize these materials with high Ti contents, we have focused on using a Ti–Si mixed oxide composite as the ingredient along with intentional addition of an aluminum source to promote crystallization. Ti-beta, a *BEA-type zeolite containing titanium at the framework sites, was chosen as a model zeolite. First, (Ti, Al)-beta, a *BEA-type zeolite containing both Ti and Al, was prepared; the occluded aluminum inside the product was subsequently removed by an acid treatment. This treatment not only lead to the reduction of the aluminum content to trace levels but also improved the states of the titanium species to the desired tetrahedral coordination state. Thus, Ti-beta zeolites with little extra-framework Ti were successfully obtained with molar compositions up to Ti/(Ti + Si) = 4.0 mol %. As a titanosilicate zeolite catalyst, high functionality was demonstrated based on the oxidation of cyclooctene, confirming the positive impact of having high titanium content with low aluminum content. Finally, investigation of the intermediates during the crystallization process was performed to understand the behavior of titanium species throughout the crystallization and to propose the critical factors for achieving efficient Ti introduction

Rigid-to-Flexible Conformational Transformation: An Efficient Route to Ring-Opening of a Tröger’s Base-Containing Ladder Polymer

The synthesis of ladder polymers is still a big challenge in polymer chemistry, and in particular, there are few examples of conformationally flexible well-defined ladder polymers. Here we report an efficient and convenient route to conformationally flexible ladder polymers, which is based on a postpolymerization reaction of a rigid ladder polymer containing Tröger’s base in its main chain. The postpolymerization reaction involves sequential <i>N</i>-methylation and hydrolysis for the Tröger’s base unit, resulting in a diazacyclooctane skeleton that can exhibit a ring-flipping motion. Molecular dynamics simulations predicted that this motion provides conformational flexibility with the resultant ladder polymer, which was demonstrated by ¹H NMR spectroscopy in solution. The presence of the diazacyclooctane units in the flexible ladder polymer allowed further functionalization through reactions involving its secondary amine moiety. The present synthetic method may lead to the development of a new class of ladder polymers that exhibit both conformational and design flexibility