6 research outputs found
Nonaqueous Fluoride/Chloride Anion-Promoted Delamination of Layered Zeolite Precursors: Synthesis and Characterization of UCB-2
The delamination of layered zeolite precursor PREFER is demonstrated under mild nonaqueous conditions using a mixture of cetyltrimethylammonium bromide, tetrabutylammonium fluoride, and tetrabutylammonium chloride in N,N-dimethylformamide (DMF) as solvent. The delamination proceeds through a swollen material intermediate which is characterized using powder X-ray diffraction (PXRD). Subsequent addition of concentrated HCl at room temperature leads to synthesis of UCB-2 via delamination of the swollen PREFER material and is characterized using PXRD, transmission electron microscopy (TEM), and argon gas physisorption, which shows lack of microporosity in UCB-2. ^(29)Si magic angle spinning (MAS) NMR spectroscopy indicates lack of amorphization during delamination, as indicated by the entire absence of Q^2 resonances, and ^(27)Al MAS NMR spectroscopy shows exclusively tetrahedral aluminum in the framework following delamination. The delamination process requires both chloride and fluoride anions and is sensitive to solvent, working well in DMF. Experiments aimed at synthesizing UCB-2 using aqueous conditions previously used for UCB-1 synthesis leads to partial swelling and lack of delamination upon acidification. A similar lack of delamination is observed upon attempting synthesis of UCB-1 under conditions used for UCB-2 synthesis. The delamination of PREFER is reversible between delaminated and swollen states in the following manner. Treatment of as-made UCB-2 with the same reagents as used here for the swelling of PREFER causes the delaminated UCB-2 material to revert back to swollen PREFER. This causes the delaminated UCB-2 material to revert back to swollen PREFER. Altogether, these results highlight delamination as the reverse of zeolite synthesis and demonstrate the crucial role of noncovalent self-assembly involving the zeolitic framework and cations/anions/structure-directing agent and solvent during the delamination process
Heteroatom-Tolerant Delamination of Layered Zeolite Precursor Materials
The synthesis of the first delaminated borosilicate layered zeolite precursor is described, along with its aluminosilicate analogue, which consists of Al-containing UCB-3 and B-containing UCB-4 from as-made SSZ-70. In addition, the delamination of PREFER (which is the precursor to ferrierite zeolite) under similar conditions yields delaminated layered zeolite precursors consisting of Al-containing UCB-5 and Ti-containing UCB-6. Multinuclear solid-state NMR spectroscopy (^(11)B and ^(27)Al), diffuse-reflectance UV-vis spectroscopy, and heteroatom/Si ratios measured via elemental analysis are consistent with a lack of heteroatom leaching from the framework following delamination. Such mild delamination conditions are achieved by swelling the zeolite precursor in a fluoride/chloride surfactant mixture in DMF solvent, followed by sonication. Powder X-ray diffraction, argon gas physisorption, and chemisorption of bulky base probes strongly suggest delamination, and demonstrate a 1.5-fold increase in the number density of external acid sites and surface area of calcined UCB-3, relative to calcined Al-SSZ-70. The synthesis of microporous pockets in materials UCB-3–UCB-5 suggests the possibility of interlayer porosity in SSZ-70, which is a layered zeolite precursor material whose structure remains currently unknown. The mildness of the delamination method presented here, as well as the lack of need for acidification in the synthesis procedure, enables the delamination of heteroatom-containing zeolites while preserving the framework integrity of labile heteroatoms, which could otherwise be leached under harsher conditions
Nonaqueous Fluoride/Chloride Anion-Promoted Delamination of Layered Zeolite Precursors: Synthesis and Characterization of UCB-2
The delamination of layered zeolite precursor PREFER is demonstrated under mild nonaqueous conditions using a mixture of cetyltrimethylammonium bromide, tetrabutylammonium fluoride, and tetrabutylammonium chloride in <i>N</i>,<i>N</i>-dimethylformamide (DMF) as solvent. The delamination proceeds through a swollen material intermediate which is characterized using powder X-ray diffraction (PXRD). Subsequent addition of concentrated HCl at room temperature leads to synthesis of UCB-2 via delamination of the swollen PREFER material and is characterized using PXRD, transmission electron microscopy (TEM), and argon gas physisorption, which shows lack of microporosity in UCB-2. <sup>29</sup>Si magic angle spinning (MAS) NMR spectroscopy indicates lack of amorphization during delamination, as indicated by the entire absence of Q<sup>2</sup> resonances, and <sup>27</sup>Al MAS NMR spectroscopy shows exclusively tetrahedral aluminum in the framework following delamination. The delamination process requires both chloride and fluoride anions and is sensitive to solvent, working well in DMF. Experiments aimed at synthesizing UCB-2 using aqueous conditions previously used for UCB-1 synthesis leads to partial swelling and lack of delamination upon acidification. A similar lack of delamination is observed upon attempting synthesis of UCB-1 under conditions used for UCB-2 synthesis. The delamination of PREFER is reversible between delaminated and swollen states in the following manner. Treatment of as-made UCB-2 with the same reagents as used here for the swelling of PREFER causes the delaminated UCB-2 material to revert back to swollen PREFER. This causes the delaminated UCB-2 material to revert back to swollen PREFER. Altogether, these results highlight delamination as the reverse of zeolite synthesis and demonstrate the crucial role of noncovalent self-assembly involving the zeolitic framework and cations/anions/structure-directing agent and solvent during the delamination process
NMR and SAXS Analysis of Connectivity of Aluminum and Silicon Atoms in the Clear Sol Precursor of SSZ-13 Zeolite
We report the first study of the hydrolysis of tetraethyl
orthosilicate
(TEOS) in an aqueous solution of <i>N</i>,<i>N</i>,<i>N</i>-trimethyl-1-adamantammonium (TMAda) hydroxide,
the clear sol precursor for the preparation of the high-silica zeolite
SSZ-13 (<b>CHA</b>). The initial stages of the hydrolysis of
TEOS were monitored by quantitative <sup>29</sup>Si and <sup>27</sup>Al nuclear magnetic resonance (NMR) and small-angle X-ray scattering
(SAXS). <sup>29</sup>Si NMR allowed quantitative characterization
of Si in nanoparticles and dissolved oligomers and measuring the average
Si–O–Si connectivity. The average Si connectivity increases
when hydrolysis advances, and at a [Si]/[TMAdaOH] ratio of one, nanoparticles
are detected. The average connectivity of nanoparticles reached 3.1.
This is similar to what has been observed during TEOS hydrolysis with
other organic bases, i.e., tetrapropylammonim hydroxide (TPAOH) and
tetrabutylammonium hydroxide (TBAOH) used for silicalite-1 and silicalite-2
syntheses, confirming that it is a general phenomenon independent
of the structure of the organocation. <sup>27</sup>Al NMR shows that
the connectivity of Al increases as well with increasing [Si]/[TMAdaOH]
ratio. Aluminum atoms are in tetrahedral coordination to four silicate
units SiO<sub>4</sub><sup>4–</sup> and are located exclusively
in the nanoparticles
Heteroatom-Tolerant Delamination of Layered Zeolite Precursor Materials
The synthesis of the first delaminated
borosilicate layered zeolite
precursor is described, along with its aluminosilicate analogue, which
consists of Al-containing UCB-3 and B-containing UCB-4 from as-made
SSZ-70. In addition, the delamination of PREFER (which is the precursor
to ferrierite zeolite) under similar conditions yields delaminated
layered zeolite precursors consisting of Al-containing UCB-5 and Ti-containing
UCB-6. Multinuclear solid-state NMR spectroscopy (<sup>11</sup>B and <sup>27</sup>Al), diffuse-reflectance UV-vis spectroscopy, and heteroatom/Si
ratios measured via elemental analysis are consistent with a lack
of heteroatom leaching from the framework following delamination.
Such mild delamination conditions are achieved by swelling the zeolite
precursor in a fluoride/chloride surfactant mixture in DMF solvent,
followed by sonication. Powder X-ray diffraction, argon gas physisorption,
and chemisorption of bulky base probes strongly suggest delamination,
and demonstrate a 1.5-fold increase in the number density of external
acid sites and surface area of calcined UCB-3, relative to calcined
Al-SSZ-70. The synthesis of microporous pockets in materials UCB-3–UCB-5
suggests the possibility of interlayer porosity in SSZ-70, which is
a layered zeolite precursor material whose structure remains currently
unknown. The mildness of the delamination method presented here, as
well as the lack of need for acidification in the synthesis procedure,
enables the delamination of heteroatom-containing zeolites while preserving
the framework integrity of labile heteroatoms, which could otherwise
be leached under harsher conditions
Confirmation of isolated Cu 2+ ions in SSZ-13 zeolite as active sites in NH 3-selective catalytic reduction
NH 3-Selective Catalytic Reduction (NH 3-SCR) is a widely used technology for NO x reduction in the emission control systems of heavy duty diesel vehicles. Copper-based ion exchanged zeolites and in particular Cu-SSZ-13 (CHA framework) catalysts show both exceptional activity and hydrothermal stability for this reaction. In this work, we have studied the origin of the SCR activity of Cu-SSZ-13 as evidenced from a combination of synchrotron-based and laboratory techniques. Synchrotron-based in situ XAFS/XRD measurements were used to provide complementary information on the local copper environment under realistic NH 3-SCR conditions. Crucial then to the catalytic activity of Cu-SSZ-13 is the local environment of the copper species, particularly in the zeolite. Cu-SSZ-13 contains mononuclear Cu 2+ species, located in the face of the double-6-ring subunit of the zeolite after calcination where it remains under reaction conditions. At lower temperatures (with low activity), XAFS and XRD data revealed a conformational change in the local geometry of the copper from a planar form toward a distorted tetrahedron as a result of a preferential interaction with NH 3. This process appears necessary for activity, but results in a stymieing of activity at low temperatures. At higher temperatures, the Cu 2+ possess a local coordination state akin to that seen after calcination. © 2012 American Chemical Society