Germanosilicaat zeolieten voor het genereren van hiërarchische, poreuze roosters

Each year, zeolites are used in millions of tons and find applications in a wide range of fields including separation, adsorption, ion exchange, catalysis, and more advanced techniques such as microelectronics, or medical diagnostics. Zeolite frameworks typically consist of silicon and aluminum tetrahedrally coordinated by oxygen atoms, which are shared between two tetrahedra. Owing to their interconnected pore systems in molecular dimensions, they exhibit large surface area and show high stabilities. Nowadays over 200 zeolite framework types have been acknowledged by the International Zeolite Association. Although the existing number of zeolites is huge, synthesis of novel zeolites and studies of the existing ones is necessary to meet the challenges involving more efficient conversion of fossil fuels into cleaner energy sources, abatement of environmental pollution, and development of more greener and cost-efficient chemical processes. The structural diversity of zeolites is further broadened by the option to introduce heteroelements such as Ti, Ge, B and P into the framework. By introducing germanium to reactant mixtures, over 20 novel zeolite topologies have been discovered in the last two decades. Many germanium-based zeolites contain double 4-ring units where germanium preferably is located. Germanium promotes and stabilizes these small units in zeolite frameworks which would be highly strained in siliceous form. Additionally, zeolite building units composed by small four-membered rings and three-membered rings often lead to porous materials with low framework densities, as these units can serve as stabilizing structures in large rings of the structure. Therefore, a large number of germanosilicate zeolites exhibit interesting large pores in their frameworks due to the presence of double 4-rings. Aside from facilitating the formation of double four-ring units, Ge-O bonds in zeolite frameworks exhibit high sensitivity to moisture and can undergo hydrolysis even in ambient atmosphere. This instability of Ge-O bonds renders them potential candidates for chemical selective post-modifications. As documented in literature, new zeolites such as COK-14, IPC-2 and IPC-4, or intergrowths IPC-6 and IPC-7 have been prepared via post modifications of UTL type germanosilicate zeolites by removal of the hydrolysis sensitive Ge atoms. The objective of this doctoral project was to investigate the templating effect of azonia-spiro organic compounds, more specifically 5-azonia-spiro[4,4]nonane, 5-azonia-spiro[4,5]decane, 6-azoniaspiro[5,5] undecane, 5-azoniaspiro[4,6]undecane, 6-azoniaspiro[5,6]dodecane, and 7-azoniaspiro[6,6] tridecane, on the syntheses of zeolites in the presence of germanium and how the specific structures of germanosilicate zeolites impact on their performances upon post treatment by acid leaching. Pure germanosilicate zeolite with IWW topology has been synthesized by using 5-azonia-spiro[4,4]nonane hydroxide as template in fluoride free medium. Literature documented that 1,5-bis(methylpyrrolidinium)pentane and hexamethonium can be used for IWW type zeolite synthesis resulting in the discovery of ITQ-22 material. In this project 5-azonia-spiro[4,4]nonane hydroxide was discovered to template IWW zeolite as well. Crystallized IWW was already observed in 1 day and full crystallization was achieved in 3 days. Comparing to ITQ-22, this IWW-type zeolite has a slightly higher Si/Ge (4.9 vs. 3.2). Germanosilicate zeolites generally degrade in very short periods of time in ambient conditions after removal of organic templates. This IWW zeolite was observed to be stable for at least 6 months after calcination. Moreover, the Si/Ge ratio of the obtained IWW zeolite can be further improved from 5 to 10 without noticeable loss of crystallinity by acid treatment. In contrast, 5-azonia-spiro[4,5]decane with similar shape and size as 5-azonia-spiro[4,4]nonane directs the crystallization towards the formation of a mixture of IWW, NON (nonasil) and a dense sodium germanate phase under similar hydrothermal conditions. Azonia-spiro compounds are known for their low selectivity as templates for zeolites, and it may be assumed these azonia-spiro compounds have been acting mostly as pore filling agents. Observation of a dense sodium germanate phase with 5-azonia-spiro[4,5]decane was surprising, as no sodium was added to the synthesis. It turned out the sodium was a contaminant of the template solution, a fact which has been overlooked in literature. Preparation of azonia-spiro compounds typically involves reaction of a cyclic amine and a alkyl halide, followed by purification with a large amount of concentrated sodium hydroxide solutions and sometimes involves even solid sodium hydroxide. Extraction by chloroform should remove most of the inorganic impurities including sodium hydroxide, but this is not always the case. Measurement of the sodium content in a number of azonia-spiro solutions showed that the presence of sodium during zeolite synthesis cannot be neglected. Residual sodium cations have great impacts on zeolite crystallization which may explain some controversial observations in literature. Aluminosilicate zeolites MTW, MOR, etc. have been reported to be generated using azonia-spiro compounds as structure directing agents and sodium as mineralization agent. However, the synthesis of germanosilicates is significantly affected by the residual sodium hydroxide in the template solutions since germanium is consumed as dense sodium germanate in the reactant mixture. Besides, due to the presence of excess sodium hydroxide, the amount of organic azonia-spiro hydroxide for synthesis is systematically overestimated. This issue has been studied for several different azonia-spiro compounds (6-azoniaspiro[5,5]undecane, 5-azoniaspiro[4,6]undecane, 6-azoniaspiro[5,6]- dodecane, and 7-azoniaspiro[6,6]tridecane). These compounds were synthesized using a standard procedure and different amounts of residue sodium hydroxide have been observed after purifications. These template solutions led to pure zeolite MTW phases, dense germanates or a mixture of both depending on the amounts of contaminating sodium in the solutions. Pure MTW zeolite were also generated when germanium was absent from synthesis mixture. Sodium germanate formed together with MTW zeolite can possibly be removed by acid leaching, resulted in almost pure MTW and secondary microporosity in MTW. Recently, researchers have reported various post-modifications of germanosilicate zeolites which resulted in new zeolite topologies, created extra mesopores in the micropore zeolite domains or allowed modification of the chemical compositions. Two representatives of post-treatments which alter the parent germanosilicate topology are the inverse sigma transformation and the Assembly-Disassembly-Organization-Reassembly procedure, leading to the discovery of OKO, PCR and also several intergrowth phases. Both approaches involve disassembly of the UTL structure into a layered phase, followed by layer condensation. Sometimes organics were utilized to align the layers to a desired stacking sequence. The instances for generating extra mesopores and modifying chemical compositions included IWW, ITH, IWR and ITR zeolites. Removal of germanium species in IWW zeolite created mesopores and modified germanosilicate IWW into an almost siliceous zeolite with preservation of the IWW topology. Similarly, disassembly of ITH, IWR and ITR into their layered analogues was attempted but only led to significant removal of germanium species from the frameworks. Up to now, UTL zeolite is the unique germanosilicate zeolite to undergo transformation into novel topologies. This raised the question why UTL behaves so different compared to other germanosilicates since they all possess dense silicate layers connected by germanium-enriched D4R units. Detailed localization of the Ge species in germanosilicate zeolites is necessary to evaluate the potential of a framework for its use to generate new topologies by sacrifice of specific building units. Investigation of the global germanium distribution in UTL zeolite has been performed by combination of Rietveld refinement of X-ray diffraction patterns, 19F-NMR, Ge K-edge EXAFS and theoretic modelling. These characterizations indicated that germanium atoms in UTL zeolites, more specifically in D4R units, are observed as S4R oriented parallel to the silicate layers they are connecting. This unit can be eliminated by hydrolysis with preservation of the silicate layers which can be condensed into new crystalline zeolite frameworks. The ease with which germanosilicates with UTL topology undergo this arrangement can be based on the unique arrangement of the germanate-oligomer. This detailed Ge location readily explains why UTL zeolite is so susceptible to structural breakdown and reassembling into denser OKO and PCR type zeolite phases by elimination of 4-rings.nrpages: 107status: publishe