7 research outputs found
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 āQ<sup>4</sup>(<i>n</i>Al)ā, SiĀ(OSi)<sub>4ā<i>n</i></sub>(OAl)<sub><i>n</i></sub> and āQ<sup>3</sup>(<i>n</i>Al)ā, SiĀ(OSi)<sub>3ā<i>n</i></sub>(OH)Ā(OAl)<sub><i>n</i></sub>, in the total framework Si
atoms, which can be estimated by the solid-state <sup>29</sup>Si 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 Q<sup>4</sup>(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 HāZSMā5 Zeolite
IR
observation of ethanol adsorption clarified the presence of the apparent
intramolecular isotope exchange from CD<sub>3</sub>CH<sub>2</sub>OH
to CHD<sub>2</sub>CH<sub>2</sub>OD on acidic OH groups of H-ZSM-5
zeolite. This reaction did not proceed with CD<sub>3</sub>OH nor CH<sub>3</sub>CD<sub>2</sub>OH, implying that the Ī²-hydrogen of alcohol
had interaction with the lattice oxygen adjacent to Al and that the
reaction was mediated by isotope exchange of CD<sub>3</sub> groups
of ethanol and OH groups on zeolite
Control of the Al Distribution in the Framework of ZSMā5 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 <sup>27</sup>Al 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 BrĆønsted 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
vanāt Hoff equation, leading to two different values in two
temperature ranges (about 398ā548 and 573ā773 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 (573ā773 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 CH<sub>4</sub>
Heterogeneous Ni Catalyst for Direct Synthesis of Primary Amines from Alcohols and Ammonia
This paper reports the synthesis of primary amines from
alcohols
and NH<sub>3</sub> by an Al<sub>2</sub>O<sub>3</sub>-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 acidābase 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 TiāSi 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 TiāSi 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 TroĢgerās 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 TroĢgerās base in its main chain.
The postpolymerization reaction involves sequential <i>N</i>-methylation and hydrolysis for the TroĢgerās 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 <sup>1</sup>H 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