Takahiko Takewaki

Synthesis of ∗BEA-type molecular sieves using mesoporous materials as reagents

Abstract A new method for synthesizing ∗ BEA-type molecular sieves that involves the use of mesoporous materials as reagents is presented. Si-Beta (pure-silica, ∗ BEA-type molecular sieve) is synthesized by heating TEAOH-impregnated Si-MCM-41 to 150°C for several days. The Si-Beta obtained has a small number of Q 3 sites that are mostly siloxy groups that balance the charge of TEA cations. TEA cations can easily be removed by contacting the Si-Beta with acetic acid solution, and as a result, a highly hydrophobic Si-Beta is formed. Ti-Beta is synthesized from Ti-containing mesoporous silica or Ti-impregnated Si-MCM-41 without adding Al 3+ , alkali-metal cations and seeds. The incorporation of Ti into the ∗ BEA structure is confirmed by UV–vis data. ∗ BEA-type materials that contain other species such as Al, B, V, Zr and Zn can also be prepared.

Synthesis of CIT-6, a zincosilicate with the BEA topology

A new, large pore zincosilicate denoted CIT-6 with the framework topology of zeolite beta has been synthesized from reaction mixtures containing Li+, Zn2+ and tetraethylammonium hydroxide (TEAOH) as a structure-directing agent (SDA). The effects of the concentrations of Li+, Zn2+ and TEAOH on the preparation of CIT-6 are investigated and it is shown that there are critical ranges in concentration for all three components for the crystallization of pure CIT-6. The incorporation of Zn2+ into the CIT-6 framework is confirmed by 29Si MAS-NMR and ion exchange results. CIT-6 is the first molecular sieve to contain framework zinc sites accessible to organic molecules.

Tailoring molecular sieve properties during SDA removal via solvent extraction

Abstract Pure-silica molecular sieves with the MFI topology and zincosilicates, aluminosilicates, borosilicates and pure silicates with the *BEA topology are synthesized and subjected to solvent extraction treatments in an effort to remove the organic structure-directing agents (SDAs) from the micropores. For both molecular sieve topologies, the amount of SDA that can be removed by extraction is found to be dependent on the size of the SDA and the strength of interaction of the SDA with the molecular sieve framework. Furthermore, the potential for extraction of SDAs from the micropores of the material is shown to correlate well with the temperature at which the SDA combusts in thermogravimetric analyses experiments. For materials with SDAs that are small relative to the size of the micropores, the fraction of SDA that can be removed is found to correlate well with the fraction of the SDA that decomposes below 400°C in the materials studied here. SDA that burns or decomposes at temperatures exceeding this value is strongly bound to the framework via ionic charge-balancing interactions. The ease of liberation of charge-balancing tetraethylammonium (TEA) cations from the various metallosilicates is shown to be Zn>B>Al, following the reverse trend of known Bronsted acidity of the various types of sites. It is shown that this tightly bound SDA is removed by extraction under conditions that simultaneously hydrolyze part of the framework. For example, TEA cations charge-balancing boron atoms in the silicate framework are removed with concomitant hydrolysis of the B–O–Si bonds, releasing the tightly bound TEA cation with subsequent desorption of the boron and TEA from the molecular sieve pores. A borosilicate with the *BEA topology synthesized with TEA fluoride as an SDA is shown to be a precursor to a variety of molecular sieves as was previously demonstrated for the zincosilicate with the *BEA topology, CIT-6.

Zeolite synthesis using 1,4-diazabicyclo[2,2,2]octane (DABCO) derivatives as structure-directing agents

Abstract A number of 1,4-diazabicyclo[2,2,2]octane (DABCO) derivatives are used as structure-directing agents in high-silica, zeolite syntheses in the presence and absence of Al 3+ , B 3+ and Zn 2+ . Several high-silica products including phases with unknown structure are synthesized. The results suggest that the product selectivity can be influenced by methyl-substitution of the DABCO moiety. However, when using DABCO-polymers, the number of methylene carbon atoms between the DABCO units rather than methyl-substitution of DABCO has the largest effect on the final products formed. 13 C-CP MAS NMR results show that the signal attributed to the carbon atoms in the DABCO unit in both a DABCO polymer and 1,4-diazoniabicyclo[2,2,2]octane,1,4-dimethyl are split into two signals when these organics are guests in zeolites. The data suggest that the ionic interactions between two cation sites of the DABCO unit and the zeolite framework are different.

Characterization of Ti-Beta zeolites and their reactivity for the photocatalytic reduction of CO2 with H2O

A characterization of Ti-Beta zeolites synthesized under various conditions as well as an investigation of their photocatalytic properties for the reduction of CO2 with H2O at 323 K to produce CH4 and CH3OH were carried out. In situ XAFS spectra measurements indicated that a highly dispersed tetrahedral titanium oxide species was present in the zeolite framework and an increase in the coordination number of the titanium oxide species by the addition of H2O and CO2 molecules could be detected. The Ti-Beta zeolite having a hydrophilic property (Ti-Beta(OH)) exhibited a more dramatic increase in the coordination number than the Ti-Beta(F) zeolite which had a hydrophobic property. These results suggest that CO2 and H2O molecules can be adsorbed efficiently onto the highly dispersed tetrahedrally coordinated titanium oxide species. UV irradiation of these Ti-Beta zeolite catalysts in the presence of H2O and CO2 led to the formation of CH4 and CH3OH. Ti-Beta(OH) exhibited a higher reactivity than Ti-Beta(F), while the selectivity for the formation of CH3OH on Ti-Beta(F) was higher than that for Ti-Beta(OH). These results indicated that the reactivity and selectivity of the zeolite catalyst can be determined by the hydrophilic and hydrophobic properties of the zeolites.