Kenta Iyoki

A Working Hypothesis for Broadening Framework Types of Zeolites in Seed-Assisted Synthesis without Organic Structure-Directing Agent

Recent research has demonstrated a new synthesis route to useful zeolites such as beta, RUB-13, and ZSM-12 via seed-assisted, organic structure-directing agent (OSDA)-free synthesis, although it had been believed that these zeolites could be essentially synthesized with OSDAs. These zeolites are obtained by adding seeds to the gels that otherwise yield other zeolites; however, the underlying crystallization mechanism has not been fully understood yet. Without any strategy, it is unavoidable to employ a trial-and-error procedure for broadening zeolite types by using this synthesis method. In this study, the effect of zeolite seeds with different framework structures is investigated to understand the crystallization mechanism of zeolites obtained by the seed-assisted, OSDA-free synthesis method. It has been found that the key factor in the successful synthesis of zeolites in the absence of OSDA is the common composite building unit contained both in the seeds and in the zeolite obtained from the gel after heating without seeds. A new working hypothesis for broadening zeolite types by the seed-assisted synthesis without OSDA is proposed on the basis of the findings of the common composite building units in zeolites. This hypothesis enables us to design the synthesis condition of target zeolites. The validity of the hypothesis is experimentally tested and verified by synthesizing several zeolites including ECR-18 in K-aluminosilicate system.

Seed‐Assisted, One‐Pot Synthesis of Hollow Zeolite Beta without Using Organic Structure‐Directing Agents

Hollow aluminosilicate zeolite beta was successfully synthesized by adding CIT-6, that is, zincosilicate zeolite, which has the same topology as beta, as seeds to the Na-aluminosilicate gel without the need for organic structure-directing agents. One important factor in the successful organic structure-directing agent (OSDA)-free synthesis of hollow beta crystals is the solubility of the seed crystals in alkaline media. CIT-6 was less stable than aluminosilicate zeolite beta in alkaline media and the solubility changed depending on whether the crystals were calcined or not. The hollow beta could be obtained by using the uncalcined CIT-6 seed crystals. The volumes of intra-crystalline voids were tuned by changing the reaction time and the initial gel compositions, such as the SiO2/Al2O3 and Na2O/SiO2 ratios. We estimated that the intra-crystalline voids were formed through the dissolution of the seed crystals, just after the crystal growth of new beta on the outer surface of the seeds. In addition, new crystal growth toward inside of the void was also observed by TEM. On the basis of the characterization data, such as chemical analysis, N2-adsorption/desorption measurements, and TEM observation, a formation mechanism of the intra-crystalline voids is proposed and discussed.

Diol‐Linked Microporous Networks of Cubic Siloxane Cages

A new class of inorganic-organic hybrid porous materials has been synthesized by a reaction between octa(hydridosilsesquioxane) (H(8)Si(8)O(12)), which has a double-four-ring (D4R) structure, and various diols, such as 1,3-propanediol (PD), 1,4-cyclohexanediol (CHD), and 1,3-adamantanediol (AD). Solid-state (29) Si magic-angle-spinning NMR spectroscopic analysis confirmed that most of the corner Si-H groups reacted with diols to form Si-O-C bonds with retention of the D4R cage. Nitrogen adsorption-desorption studies showed that the products are microporous solids with high BET surface areas (up to ≈580 m(2) g(-1) for CHD- and AD-linked products). If n-alkanediols are used as linkers, the surface area becomes smaller as the number of carbon atoms is increased. The thermal and hydrolytic stability of the products strongly depend on the type of diol linkers. The highest stabilities are found for the AD-linked products, which are thermally stable up to around 400 °C and remain intact even after being soaked in water for 1 day. In contrast, the PD-linked product is easily hydrolyzed in water to give microporous silica. These results offer a new route toward a series of silica-based porous materials with unique structures and properties.

Hierarchical porous silica via solid-phase hydrolysis/polycondensation of cubic siloxane-based molecular units

Hierarchical micro–mesoporous silica has been synthesized by solid-phase conversion of molecular crystals of an alkoxy derivative of a cubic siloxane unit (Si8O12) as a molecular building unit. Seven methoxy groups and one adamantoxy group are introduced in a cage by the reaction of octa(hydridosilsesquioxane) (H8Si8O12) with the corresponding alcohols, which are then eliminated in a step-by-step manner. First, the methoxy groups are hydrolyzed by simply dispersing the precursor powder in an acidic aqueous solution. The formation of Si–O–Si linkages between the cages while retaining the bulky adamantoxy groups is confirmed by solid-state NMR. At this stage, broad mesopores (ca. 2 to 7 nm) are formed, as confirmed by nitrogen adsorption–desorption. The adamantoxy groups are then removed by calcination to generate relatively narrow micropores (∼1 nm in diameter). Various control experiments performed suggest that the stepwise solid-phase reaction of bifunctional building blocks is crucial to the formation of such micro–mesoporous silica, providing a new pathway to nanoporous materials with controlled architectures.

Synthesis of New Microporous Zincosilicates with CHA Zeolite Topology as Efficient Platforms for Ion‐Exchange of Divalent Cations

There is growing interest to develop zeolite materials capable of stabilizing divalent cations such as Cu2+ , Fe2+ , and Ni2+ for catalytic applications. Herein the synthesis of a new microporous zincosilicate with CHA zeolite topology is reported for the first time, by particularly focusing on the mixing procedures of the raw materials to prevent the precipitation of zinc oxides/hydroxides and the formation of impurity phases. The obtained zincosilicate CHA products possess remarkably higher ion-exchange ability for catalytically useful, divalent cations, demonstrated here using Ni2+ as an example, compared to that of aluminosilicate and zincoaluminosilicate analogs. It is anticipated that these zincosilicate CHA materials can be an efficient platform for several important catalytic reactions. In addition, the present finding would provide a general guideline for effective substitution of other heteroatoms into the zeolite frameworks.