NH3-SCR catalysts for heavy-duty diesel vehicles: Preparation of CHA-type zeolites with low-cost templates

Computer-assistance allows selecting the most adequate low-cost organic structure directing agents (OSDAs) for the crystallization of Al-rich CHA-type zeolites. The host-guest stabilization energies of tetraethylammonium (TEA), methyltriethylammonium (MTEA) and dimethyldiethylammonium (DMDEA), in combination with Na, were first theoretically evaluated. This “ab-initio” analysis reveals that two TEA show a serious steric hindrance in a cha cavity, whereas two MTEA would present excellent host-guest confinements. The synthesis of Al-rich CHA-type zeolites has been accomplished using TEA and MTEA. Electron diffraction and high-resolution transmission electron microscopy reveal large CHA-domains with narrow faulted GME-domains in the CHA-type material synthesized with TEA, confirming the better OSDA-directing roles of MTEA cations towards the cha cavity, in good agreement with DFT calculations. Cu-exchanged Al-rich CHA-type samples achieved with MTEA and TEA show excellent catalytic activity and hydrothermal stability for the selective catalytic reduction (SCR) of NOx with ammonia under conditions relevant for future heavy duty diesel conditions.This work has been supported by Umicore and by the Spanish Government-MCIU through RTI2018-101033-B-I00 (MCIU/AEI/FEDER, UE) and PID2020-112590GB-C21 (AEI/FEDER, UE). T.W. acknowledges financial support by the Swedish Research Council (Grant No. 2019-05465). E.B. acknowledges the Spanish Government-MCIU for a FPI scholarship (PRE2019-088360). P.F. thanks ITQ for a contract. The Electron Microscopy Service of the UPV is acknowledged for their help in sample characterization. The computations were performed on the Tirant III cluster of the Servei d'Informàtica of the University of Valencia

Stability and ion-exchange properties of zeolites in hyper-alkaline media

At present zeolite synthesis is still more an art than a science, especially these syntheses that are exclusively depending on alkali metal cations as structure directors. Much remains unclear about zeolite crystallisation, as is often evidenced by the dramatic impact of slight modifications of the synthesis conditions on the products formed. Today, less than 10 zeolite types are commercially exploited, while more than 200 framework types have been synthesized by many research groups. One reason is that the majority of these frameworks can only be produced with expensive organic templates, and/or via complex procedures, hindering commercial applications. Therefore, it is of high interest to increase the number of zeolite types that can be synthesized using inorganic cations only. Moreover, there is a need to gain control over zeolite formation processes, not only to control the framework type obtained, but also to tailor the zeolite crystal properties for specific applications. To this end, this PhD focusses on three distinct, but related synthesis systems yielding insight in the formation and stability of high alumina zeolites in presence of alkali metal cations. Initially, the recrystallization of one zeolite framework into another upon contact with alkali metal hydroxide solutions was investigated. By conversion of readily available frameworks, zeolite products often can be obtained faster and in an easier way compared to their synthesis from amorphous, complex aluminosilicate gels. It is generally assumed that alkali metal cations act as templating species in zeolite framework formation, playing a crucial role in controlling the zeolite type formed, in combination with other important parameters such as the water content, Si/Al ratio or synthesis temperature. However, this research demonstrated for the first time that under identical conditions and at an identical hydroxide concentration, the type of alkali metal cation present determines the topology obtained by transformation of zeolite Y (FAU) at high pH. By transformation of zeolite Y in 1 M LiOH, NaOH, KOH, RbOH and CsOH, zeolites with ABW, FAU (GIS after longer exposure times), CHA, MER and ANA topology were obtained respectively. In addition, the dissolution and transformation of zeolite Y in different alkali metal hydroxides was followed in time with different characterization techniques (XRD, TEM, SEM, chemical analysis, 29Si and 27Al MAS NMR), which permitted to attain a full view of the transformation processes. This research also confirmed the crucial role of the K+ cation in the formation and stability of the chabazite framework (CHA). Finally, to evaluate the role of the connectivity of the FAU type starting material in these conversions, other zeolites with a composition identical to zeolite Y were synthesized, including LTA, EMT, HEU, MAZ and KFI. Exposure of these phases to KOH solution demonstrated the dramatic impact of the alkali cation on zeolite conversions. It was illustrated that next to zeolite conversions, hyper-alkaline solutions can be used for post-synthetic crystal engineering of high alumina zeolites. The research on the stabilizing impact of alkaline cations on zeolite structures was subsequently transferred and applied to a second system: zeolitic ion exchangers in contact with highly alkaline cementitious pore water originating from water-exposed concrete. Ion-exchangers operating in highly alkaline conditions are needed in concrete based waste disposal facilities, for example to contain toxic or radioactive waste. Contact of rainwater with Portland cement initially generates an alkaline fluid dominated by K+ (0.25 M, K/Na = 3, traces of Ca), which is comparable to the highly alkaline (but pure) alkali metal hydroxide solutions described above. Zeolites are being investigated for application in such environment, requiring excellent sorption properties as well as long-term stability under hyper-alkaline conditions. In literature, studies reporting on cation exchange properties of zeolites at high pH or as function of pH are scarce, as one assumes that zeolites do not carry pH-dependent ion-exchange sites. Choosing 137Cs as a benchmark element, the research presented in this PhD thesis revealed an increase in the cation exchange capacity of synthetic and natural zeolites at high pH, while the selectivity for the 137Cs cation remained unaffected. The hypothesis of hydroxyl induced structure alteration due to hydrolysis of siloxane bridges bonds is currently under more detailed investigation. Zeolite frameworks such as HEU and CHA clearly exhibit a high selectivity for Cs+, even in hyper-alkaline, saline media. Assessing their stability in concrete pore water is less straightforward, as it is not feasible to expose the zeolites to these fluids for the lifespan of the application (i.e. many decades to several centuries). Consequently, a profound insight in mechanisms determining zeolite formation and transformations is essential. It was assumed that zeolites dissolve in base and therefore are not applicable in alkaline conditions. The extent and kinetics of framework desilication and/or subsequent transformation are, however, strongly dependent on the framework and the presence of (stabilizing) cations. This work demonstrates that chabazite (CHA) forms, together with a smaller fraction of merlinoite (MER), in simulated concrete pore water when zeolite Y is used as the aluminosilicate source. The transformation was investigated both at 60°C and 85°C, and after 1 year, the products remained unchanged. The standard synthesis of CHA from zeolite Y in KOH (pH 14) and its formation, together with MER, in the simulated concrete pore water make CHA a strong candidate for application in K+-rich alkaline environment. To investigate direct nucleation and growth of zeolites in presence of alkali metal cations, syntheses in hydrated silicate ionic liquids (HSILs) were selected. These liquids being clear zeolite precursor solutions were developed to overcome the experimental difficulties experienced when studying zeolite formation in Al containing syntheses mixtures typically forming complex gel systems. The advantage of such clear precursors is their potential for full characterisation by NMR and scattering techniques (DLS, SAXS). Using extremely low water content (12 H2O/Si or 6 H2O/KOH) to avoid presence of free water, hyposolvated silicate ionic liquids (HSIL) are obtained. In these HSILs so-called ‘nanoaggregates’ (1-2 nm radius) are observed and considered to consist of molecular zeolite precursors. Crystallization of the K+ containing precursor solutions at different temperatures (60-170 °C) resulted in the formation of MER or mixtures of CHA and MER. Studying the products as a function of time with XRD and SEM, contributed to an increased understanding of their stability in presence of K+ and the competition between both framework types.nrpages: 157status: publishe

ESRF Experiment - 26-01-1003 - Rb+ cations as templating species in zeolite formation

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Invited: Impact of Cations on (Un)desirable Zeolite Transformations at High pH

High alumina zeolites are typically synthesized hydrothermally in hyperalkaline conditions. Postsynthetic alkaline treatment induces partial or complete dissolution, followed by nucleation and growth of new (zeolite) products. Major factors controlling such zeolite transformations are temperature, alkalinity and the presence of specific cations in the zeolite and the alkaline medium. While material scientists exploit zeolite transformations to design novel zeolite materials, zeolite stability in hyperalkaline media is of high interest to geoscientists, evaluating long-term stability of natural zeolites used as reactive barrier in concrete based disposal strategies for nuclear waste. This contribution discusses the role of alkali metal cations on FAU (faujasite) type zeolite transformations in 1 M hydroxide solutions under autogeneous pressure at 95°C or lower. Liquid and solid phase analysis was performed as function of contact time, with ICP-AES, NMR, SEM and XRD. Although exposure of FAU to KOH nowadays is a standard recipe to synthesize chabazite (CHA), few studies dealing with the transformation mechanism and kinetics are available. Our systematic study revealed, among others, that the presence of K+ ions is crucial for the conversion of FAU to CHA. Identical transformation conditions yielded ABW, FAU, MER and ANA frameworks by substituting KOH with LiOH, NaOH, RbOH, and CsOH respectively. [1] In addition to cation identity, other important variables determining the outcome of the transformations were the Si/Al ratio of the initial FAU framework, and the solid/liquid ratio. Furthermore, CHA was obtained by contacting FAU with K+ rich young cement water (YPW, pH 13.5) at 60°C, illustrating its potential use in such hyperalkaline conditions. In these conditions, a minor fraction of zeolite with MER (merlinoite) topology was detected in addition to CHA. The MER framework has buckled 8-rings of MER that form a perfect nest for K+ .[2] Currently, the relationship between CHA and MER formation, starting from FAU, is investigated and compared to direct syntheses from amorphous sources. The hypothesis is explored that the FAU framework structure directs the CHA formation, as CHA was observed to nucleate on the FAU (111) crystal faces. [1] Van Tendeloo et al (2013) Chem. Commun. 49, 11737- 11739 [2] Skofteland et al (2001) Microporous Mesoporous Mater. 43, 61-71status: publishe

Zeolites as Sorption Sink for Cs+ in ultra-alkaline conditions

Cement based waste disposal is the most important option to provide safe storage of the non-recyclable often highly toxic waste fraction remaining at the end of today’s waste processing chain. Despite all engineering efforts and safety investments ultra-alkaline, concrete derived, pore waters (pH 12 – 14) will always remain associated with cement based waste disposal. To avoid contamination of the environment due to slow leeching of the metals from these waste forms, engineered barriers are built around the waste disposal sites in order to provide containment of the metals leeching out. Upon consideration of a concrete-based storage scenario for 137Cs containing radioactive waste, care must be taken to provide buffer materials that ensure the retention of this cation in presence of concrete pore water with changing composition. Since the applicability of organic ion exchangers in ultra-alkaline, radioactive conditions is limited to a fairly short timeframe, such a buffer sink should include inorganic materials combining long-term stability in such conditions with a high selectivity for the monovalent Cs+ cation, thereby enabling its efficient sorption in the saline, alkaline conditions imposed by the concrete. Zeolites (natural and synthetic) represent a family of crystalline framework materials typically formed at high pH from different starting materials such as clays, fly ash, high purity silicon sources (TEOS, aerosil,…), etc. The combination of their stability in alkaline aqueous conditions with their cation exchange capacities makes them highly promising candidates for the application as a buffer material in a cement based storage scenario for radioactive waste.status: publishe

Stability and ion exchange properties of zeolites in cementitious aqueous solutions

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Framework transformation of FAU-type zeolite in high alkalinity

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Framework transformation of FAU-type zeolite in high alkalinity

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Keynote: A Unifed NMR View of Silicates from Zeolites to Ionic Liquids

Silicates represent a considerable technological and scientific issue from geology to nanomaterials. Among them zeolites are an essential societal issue, impacting about 20% or the mondial economy, directly or indirectly. Formation of nanoporous or dense silicates phases undergoes different chemical or physical states, and are of utmost importance. NMR is a spectroscopic method allowing to assess at a very local scale, chemical or physical state of matter, structure of liquids or of crystalline or amorphous solids and allows to investigate crystalization processes in an unique way. Nucleation, growth of crystals are challenging issues to understand most of geochemical, chemical or crystallogenesis formation of solids from the speciation of molecular state in liquids. NMR can access to structural, topological aspects of crystals, without the constraints of periodic boudaries conditions, liquids an solids state stuctural organization. This represents a unique way to access analytical, speciation or molecular formation of crystals. NMR represents a unique way to link the differents stages of organization from oligomerization to crystal formation. Insitu, ex-situ NMR characterization bring structural and dynamical organization of density flucutuations. As an archetype of silicate chemistry, zeolites formation will be reformulated. Elementary condensation steps will be identified and distinguished carefully, avoiding technical jargon, and favouring phsyco-chemical general concepts. As NMR can access solid state, crystalline or amorphous order, from oligomerization to nanoaggregation and crystalline order, an unified view of the successive condensation elementary steps of silicates formation will be presented. Such a enumeration of the progressive organization of condensed matter from silicic acid to the most complex zeolites may give rise to enumerate many different processes with a common knowledge and language between vastly different scientific and technological disciplines.status: publishe