I. Shape Selectivity of Small-pore Molecular Sieves for the Methanol-to-Olefins Reaction and II. Synthesis and Topotactic Transformation of Germanosilicate CIT-13

This thesis presents research results from two projects involving molecular sieves. These investigations concern their synthesis, characterization and use as heterogeneous catalysis. In part I, the shape selectivity in the methanol-to-olefins (MTO) reaction is studied, and a new molecular sieve structure – MTO reaction selectivity relationship is developed. 17 zeolites and 13 phosphate-based molecular sieves having 14 selected cage-type/small-pore topologies (CHA, AFX, SFW, LEV, ERI, DDR, AEI, RTH, ITE, SAV, LTA, RHO, KFI, and UFI) are synthesized. The MTO reaction is performed using these catalysts at the same reaction conditions. The reaction results lead to the conclusion that the molecular sieve cage topology is the most important structural factor that primarily determines the olefin product distribution. For example, AEI and CHA are synthesized with four different elemental compositions (zeolite, SAPO, CoAPO, MgAPO). Regardless of differences in elemental compositions, very similar product distribution patterns are observed in each of the isostructural groups of molecular sieves. Additionally, other isostructural pairs of SAPOs and zeolites show similar product distributions. The reaction results from 14 topologies are grouped into four categories. Category I consists of CHA, AFX, SFW, and other GME-related topologies. Catalysts having these topologies show ethylene-to-propylene ratios close to one. Category II is a group of ERI and LEV which generate more ethylene than propylene. Category III is a group of DDR, AEI, RTH, ITE, and SAV which shows propylene selectivities higher than those of ethylene. Category IV is a group of LTA, RHO, KFI, and UFI which possess LTA-cages. These types of catalysts give high butylene selectivities. The concept of cage-defining ring and its size is introduced as a reliable geometric indicator on the basis of a hypothetical ellipsoid cage model. The cage-defining ring size can be easily calculated from crystallographic information which is available online. A strong correlation is found between the cage-defining ring sizes and the four categories of reaction behavior. In part II, an extra-large-pore germanosilicate molecular sieve CIT-13 with 14- and 10-ring pores is synthesized using monoquaternary, methylbenzylimidazolium-derivative OSDAs, and the synthesis conditions are optimized. Fluoride-free synthetic pathways for pure germanosilicate CIT-13 and isomorphous aluminum substitution in synthesis of aluminogermanosilicate CIT-13 are also described. The nature of disorder in the arrangement within CIT-13 framework is discussed, and its physicochemical properties compared to a UTL-type germanosilicate IM-12. A comprehensive network of topotactic transformation and postsynthetic modification pathways starting from germanosilicate CIT-13 (Ge-CIT-13) is described. The moisture-mediated transformation of Ge-CIT-13 into another extra-large-pore CFI-type germanosilicate (Ge-CIT-5) is discovered, and the role of sorbed water in the transformation kinetics studied. The resultant Ge-CIT-5 is the first germanosilicate molecular sieve having a CFI topology, and the corresponding transformation is also the first inter-germanosilicate transformation occurring at room temperature. The microporosity of Ge-CIT-5 matched well with the reference pure-silica CIT-5 synthesized using the sparteine-type OSDA. The acid-delamination processes of Ge-CIT-13 and Ge-CIT-5 are investigated. Ge-CIT-13 can be transformed into two new frameworks, CIT-14 with 12- and 8-ring pores and CIT-15 with 10-ring pores, on the basis of an ADOR-type topotactic transformation. The inverse sigma transformation of Ge-CIT-13 directly into CIT-14 is also firstly described. The conventional acid-delamination of Ge-CIT-13 does not yield Ge-CIT-5. However, the CIT-15-type material is obtained via the base-delamination pathway from Ge-CIT-5. The postsynthetic alumination of Ge-CIT-13 and Ge-CIT-5 is also achievable.</p

CIT-9: a fault-free, gmelinite zeolite

A synthetic, fault-free gmelinite (GME) zeolite is prepared using a specific organic structure-directing agent (OSDA), cis-3,5-dimethylpiperidinium. The cis-isomers align in the main 12-membered ring (MR) channel of GME. Trans-isomer OSDA leads to the small-pore zeolite SSZ-39 with the OSDA in its cages. Data from N_2-physisorption and rotation electron diffraction provide evidence for the openness of the 12 MR channel in the GME 12×8×8 pore architecture and the absence of stacking faults, respectively. CIT-9 is hydrothermally stable when K^+-exchanged, while in the absence of exchange, the material transforms into an aluminous AFI-zeolite. The process of this phase-change was followed by in situ variable temperature powder X-ray diffraction. CIT-9 has the highest Si/Al ratio reported for GME, and along with its good porosity, opens the possibility of using GME in a variety of applications including catalysis

Transformation of Extra-Large Pore Germanosilicate CIT-13 Molecular Sieve into Extra-Large Pore CIT-5 Molecular Sieve

The 14- and 10-membered ring germanosilicate Ge-CIT-13 (*CTH) is transformed into the 14-membered ring germanosilicate Ge-CIT-5 (CFI). The transformation can occur at room temperature but requires the presence of adsorbed water. The *CTH-to-CFI transformation involves rearrangement of germanium-rich double-4-ring units in *CTH to form double-zigzag chains in CFI. The rate of transformation is dependent on the germanium content of the starting Ge-CIT-13 and the humidity of the transforming atmosphere. Other germanosilicates—UTL, IWW, and ITH—do not show this type of transformation because of arrangements of Ge-sites within their d4r units and/or to spatial restrictions regarding the d4r unit arrangement within their interlayer regions. Ge-CIT-5 can be further transformed into 10-membered-ring CIT-15 using ammonium hydroxide solution as the delaminating agent. Postsynthetic alumination of Ge-CIT-5 yielded high-silica CFI-type aluminogermanosilicates having molar Si/Al ratios in the range of 14–230, primarily depending on the acidity of the solution phase

Cage-defining Ring: A Molecular Sieve Structural Indicator for Light Olefin Product Distribution from the Methanol-to-Olefins Reaction

The methanol-to-olefins (MTO) process produces high-value-added light olefins from nonpetroleum sources. Acidic zeotypes containing cages bounded by 8-ring (small-pore) windows can effectively catalyze the MTO reaction, since their cages can accommodate the necessary aromatic intermediates that produce the light olefin products that escape. While progress on the mechanisms of the MTO reaction continues, zeotype structure–reaction property relationships have yet to be elucidated. Here, we report MTO reaction results from various small-pore, cage-containing silicoaluminophosphate/metalloaluminophosphates (SAPO/MAPOs) and zeolites under the same reaction conditions. The MTO behaviors of microporous materials having the following topologies are investigated: LEV, ERI, CHA, AFX, SFW, AEI, DDR, RTH, ITE, SAV, LTA, RHO, KFI, and UFI. The previous observation that light olefin product distributions from a series of small-pore, cage-containing zeolites can be classified into four structural categories is further supported by the results shown here from zeolite structures not investigated in the previous study and SAPO and MAPO materials with isostructural frameworks to all the zeolites. Additionally, these data reveal that light olefin product distributions are very similar over a given topology independent of framework composition. To develop a structure–property relationship between the framework topology and the MTO light olefin product distribution, the concept of the cage-defining ring size is introduced. The cage-defining ring size is defined as the minimum number of tetrahedral atoms of the ring encircling the center of the framework cages in the molecular sieve topology. It is shown that the cage-defining ring size correlates with MTO light olefin product distribution

CIT-9: a fault-free, gmelinite zeolite

A synthetic, fault-free gmelinite (GME) zeolite is prepared using a specific organic structure-directing agent (OSDA), cis-3,5-dimethylpiperidinium. The cis-isomers align in the main 12-membered ring (MR) channel of GME. Trans-isomer OSDA leads to the small-pore zeolite SSZ-39 with the OSDA in its cages. Data from N_2-physisorption and rotation electron diffraction provide evidence for the openness of the 12 MR channel in the GME 12×8×8 pore architecture and the absence of stacking faults, respectively. CIT-9 is hydrothermally stable when K^+-exchanged, while in the absence of exchange, the material transforms into an aluminous AFI-zeolite. The process of this phase-change was followed by in situ variable temperature powder X-ray diffraction. CIT-9 has the highest Si/Al ratio reported for GME, and along with its good porosity, opens the possibility of using GME in a variety of applications including catalysis

Surgical management of pilon fractures with large segmental bone defects using fibular strut allografts: a report of two cases

We present two patients with open pilon fractures with large bone defects treated successfully with fibular strut allografts. The patients were initially treated by massive irrigation, wound debridement, and temporary external fixation. After complete wound healing, the bone defects were managed. Because autologous iliac crest or fibular bone grafts were impossible to be harvested due to multiple fractures, the bone defects were reconstructed with fibular strut allografts. Fixation was performed with a periarticular distal tibia locking plate. At 2 months postoperatively, the patients ambulated with partial weight-bearing; at 6 months, they had full range of motion of the ankle joint and full weight-bearing