The synthesis of titanium–carboxylate metal–organic frameworks (MOFs) is hampered by the high reactivity of the commonly employed alkoxide precursors. Herein, we present an innovative approach to ...titanium‐based MOFs by the use of titanocene dichloride to synthesize COK‐69, the first breathing Ti MOF, which is built up from trans‐1,4‐cyclohexanedicarboxylate linkers and an unprecedented TiIV3(μ3‐O)(O)2(COO)6 cluster. The photoactive properties of COK‐69 were investigated in depth by proton‐coupled electron‐transfer experiments, which revealed that up to one TiIV center per cluster can be photoreduced to TiIII while preserving the structural integrity of the framework. The electronic structure of COK‐69 was determined by molecular modeling, and a band gap of 3.77 eV was found.
A breath of fresh air: Titanocene dichloride was used as the metal source in an innovative synthetic approach to titanium‐based metal–organic frameworks (MOFs). In this way, a breathing Ti MOF (designated COK‐69) featuring a photoactive trinuclear cluster was built up from trans‐1,4‐cyclohexanedicarboxylate and an unprecedented, photoactive TiIV3(μ3‐O)(COO)6 inorganic building unit (see picture).
n-Decane is shape-selectively converted inside the short straight channels crossing MFI nanosheets even when only one unit cell thick. Display omitted
► MFI zeolite nanosheets were transformed into ...bifunctional catalysts. ► MFI nanosheets with one unit cell thickness behave zeolite like. ► MFI nanosheets display transition-state shape-selectivity similar to bulk zeolite. ► As expected, in MFI nanosheets diffusion-based selectivity is less pronounced.
MFI zeolite nanosheets with thickness of 2 and 8nm were synthesized, transformed into bifunctional catalysts by loading with platinum and tested in n-decane isomerization and hydrocracking. Detailed analysis of skeletal isomers and hydrocracked products revealed that the MFI nanosheets display transition-state shape-selectivity similar to bulk MFI zeolite crystals. The suppressed formation of bulky skeletal isomers and C5 cracking products are observed both in the nanosheets and the bulk crystals grown in three dimensions. This is typical for restricted transition-state shape-selectivity, characteristic for the MFI type pores. It is a first clear example of transition-state shape-selectivity inside a zeolitic nanosheet. Owing to the short diffusion path across the sheets, expression of diffusion-based discrimination of reaction products in the MFI nanosheets was limited. The 2-methylnonane formation among monobranched C10 isomers and 2,7-dimethyloctane among dibranched C10 isomers, which in MFI zeolite are favored by product diffusion, was much less favored on the nanosheets compared to the reference bulk ZSM-5 material.
A new hierarchical MOF consisting of Cu(II) centers connected by benzene-tricarboxylates (BTC) is prepared by thermoinduced solid transformation of a dense CuBTC precursor phase. The mechanism of ...the material formation has been thoroughly elucidated and revealed a transformation of a ribbon-like 1D building unit into 2D layers and finally a 3D network. The new phase contains excess copper, charge compensated by systematic hydroxyl groups, which leads to an open microporous framework with tunable permanent mesoporosity. The new phase is particularly attractive for molecular separation. Energy consumption of adsorptive separation processes can be lowered by using adsorbents that discriminate molecules based on adsorption entropy rather than enthalpy differences. In separation of a 11-component mixture of C1–C6 alkanes, the hierarchical phase outperforms the structurally related microporous HKUST-1 as well as silicate-based hierarchical materials. Grand canonical Monte Carlo (GCMC) simulation provides microscopic insight into the structural host–guest interaction, confirming low adsorption enthalpies and significant entropic contributions to the molecular separation. The unique three-dimensional hierarchical structure as well as the systematic presence of Cu(II) unsaturated coordination sites cause this exceptional behavior.
A new ordered mesoporous silica material (COK-19) with cubic symmetry is synthesized by silicate polycondensation in a citric acid/citrate buffered micellar solution of Pluronic F127 triblock ...copolymer near neutral pH. SAXS, nitrogen adsorption, TEM, and electron tomography reveal the final material has a cubic close packed symmetry (Fm3̅m) with isolated spherical mesopores interconnected through micropores. Heating of the synthesis medium from room temperature to 70 °C results in a mesopore size increase from 7.0 to 11.2 nm. Stepwise addition of the silicate source allows isolation of a sequence of intermediates that upon characterization with small-angle X-ray scattering uncovers the formation process via formation and aggregation of individual silica-covered Pluronic micelles.
The metal−organic frameworks MIL-47 (VIVO{O2C-C6H4-CO2}) and MIL-53(Al) (AlIII(OH)·{O2C-C6H4-CO2}) are capable of separating ethylbenzene and styrene. Both materials adsorb up to 20−24 wt % of both ...compounds. Despite the fact that they have identical building schemes, the reason for preferential adsorption of styrene compared to ethylbenzene is very different for the two frameworks. For MIL-47, diffraction experiments reveal that styrene is packed inside the pores in a unique, pairwise fashion, resulting in separation factors as high as 4 in favor of styrene. These separation factors are independent of the total amount of adsorbate offered. This is due to co-adsorption of ethylbenzene in the space left available between the packed styrene pairs. The separation is of a non-enthalpic nature. On MIL-53, the origin of the preferential adsorption of styrene is related to differences in enthalpy of adsorption, which are based on different degrees of framework relaxation. The proposed adsorption mechanisms are in line with the influence of temperature on the separation factors derived from pulse chromatography: separation factors are independent of temperature for MIL-47 but vary with temperature for MIL-53. Finally, MIL-53 is also capable of removing typical impurities like o-xylene or toluene from styrene−ethylbenzene mixtures.
Gallium oxide nanorods with unprecedented small dimensions (20–80 nm length and 3–5 nm width) were prepared using a novel, template‐free synthesis method. This nanomaterial is an excellent ...heterogeneous catalyst for the sustainable epoxidation of alkenes with H2O2, rivaling the industrial benchmark microporous titanosilicate TS‐1 with linear alkenes and being much superior with bulkier substrates. A thorough characterization study elucidated the correlation between the physicochemical properties of the gallium oxide nanorods and their catalytic performance, and underlined the importance of the nanorod morphology for generating a material with high specific surface area and a high number of accessible acid sites.
Selective epoxidation: Gallium oxide nanorods with unprecedented small dimensions (20–80 nm length and 3–5 nm width) were prepared using a novel, template‐free synthesis method. The nanorods are much superior to conventional gallium oxide both in terms of specific surface area and number of acid sites and display excellent performance as epoxidation catalyst with H2O2 as the oxidant (see picture).
Ammonia treatment of USY zeolite has led to a new hierarchical material. The local and global structural changes during the transformation have been monitored by XRD and by 1H, 29Si, and 27Al ...solid-state NMR. A wealth of 1D and 2D NMR protocols were applied, including 1H DQ–SQ (double-quantum–single-quantum), 27Al MQMAS (multiple-quantum magic angle spinning), 29Si MAS (magic angle spinning) and CPMAS (cross-polarization magic angle spinning), and 1H–29Si HETCOR (heteronuclear correlation). The effects of aqueous ammonia treatment, different thermal post-treatments, and rehydration were studied. An increasing loss of crystallinity was observed upon increasing duration of ammonia treatment. Under the experimental conditions, a few percent of silica was lost into solution and no loss of aluminum was observed. However, increasing numbers of silanol groups were detected. The progressive transformation induces formation of mesopores, reduction of the fraction of the sample exhibiting Bragg crystallinity, and apparition of a dense, amorphous aluminosilicate phase. The latter contains ammonium ions and strongly bound water, both of which are resistant to thermal decomposition up to 350 °C. After about 24 h of treatment, the zeolite fraction is completely transformed into the amorphous phase. At intermediate stages, a complex hierarchical material is obtained with mesopores and zeolitic micropores next to a dense amorphous aluminosilicate, containing ammonium ions, highly structured water, and silanol nests.
The molecular steps involved in the self‐assembly of Cu3(BTC)2 (BTC=1,3,5‐benzenetricarboxylic acid) metal–organic frameworks that enclose Keggin‐type H3PW12O40 heteropolyacid molecules were ...unraveled by using solution 17O, 31P, and 183W NMR spectroscopy, small‐angle X‐ray scattering, near‐IR spectroscopy, and dynamic light scattering. In aqueous solution, complexation of Cu2+ ions with Keggin‐type heteropolyacids was observed. Cu2+ ions are arranged around the Keggin structure so that linking through benzenetricarboxylate groups results in the formation of the Cu3(BTC)2 MOF structure HKUST‐1. This is a unique instance in which a templating mechanism that relies on specific molecular‐level matching and leads to explicit nanoscale building units can be observed in situ during formation of the synthetic nanoporous material.
The template is the key: A strong noncovalent interaction between Cu2+ ions and Keggin‐type heteropolyacids leads to spatial organization that can be exploited for direct templating of a metal–organic framework (see figure; BTC=1,3,5‐benzenetricarboxylic acid).
Catalyst Design by NH4OH Treatment of USY Zeolite Van Aelst, Joost; Verboekend, Danny; Philippaerts, An ...
Advanced functional materials,
December 9, 2015, Letnik:
25, Številka:
46
Journal Article
Recenzirano
Odprti dostop
Hierarchical zeolites are a class of superior catalysts which couples the intrinsic zeolitic properties to enhanced accessibility and intracrystalline mass transport to and from the active sites. The ...design of hierarchical USY (Ultra‐Stable Y) catalysts is achieved using a sustainable postsynthetic room temperature treatment with mildly alkaline NH4OH (0.02 m) solutions. Starting from a commercial dealuminated USY zeolite (Si/Al = 47), a hierarchical material is obtained by selective and tuneable creation of interconnected and accessible small mesopores (2–6 nm). In addition, the treatment immediately yields the NH4+ form without the need for additional ion exchange. After NH4OH modification, the crystal morphology is retained, whereas the microporosity and relative crystallinity are decreased. The gradual formation of dense amorphous phases throughout the crystal without significant framework atom leaching rationalizes the very high material yields (>90%). The superior catalytic performance of the developed hierarchical zeolites is demonstrated in the acid‐catalyzed isomerization of α‐pinene and the metal‐catalyzed conjugation of safflower oil. Significant improvements in activity and selectivity are attained, as well as a lowered susceptibility to deactivation. The catalytic performance is intimately related to the introduced mesopores, hence enhanced mass transport capacity, and the retained intrinsic zeolitic properties.
Postsynthetic NH4OH treatment is performed on USY (ultra‐stable Y) zeolite to sustainably develop superior catalysts. Extensive characterization shows the selective creation of small mesopores by partial zeolite densification without significant material leaching. The hierarchical zeolite outperforms the conventional USY for acid‐catalyzed isomerization of α‐pinene and metal‐catalyzed conjugation of safflower oil, which is attributed to its enhanced intracrystalline mass transport capacity.
The formation of silicate nanoaggregates (NAs) at the very early stages of precursor sols and zeolite beta crystallization from silicate nanoparticles (NPs) are investigated in detail using a ...combination of different analysis methods, including liquid‐state 29Si, 27Al, 14N, and 1H NMR spectroscopy, mass spectrometry (MS), small‐angle X‐ray scattering (SAXS), X‐ray diffraction (XRD), and transmission electron microscopy at cryogenic temperatures (cryo‐TEM). Prior to hydrothermal treatment, silicate NAs are observed if the Si/OH ratio in the reaction mixture is greater than 1. Condensation of oligomers within the NAs then generates NPs. Aluminum doped into the synthesis mixtures is located exclusively in the NPs, and is found exclusively in a state that is fourfold connected to silicate, favoring their condensation and aggregation. These results are in agreement with general trends observed for other systems. Silicate NAs are essential intermediates for zeolite formation and are generated by the aggregation of hydrated oligomers, aluminate, and templating cations. Subsequent further intra‐nanoaggregate silicate condensation results in the formation of NPs. 1H and 14N liquid NMR as well as diffusion ordered spectroscopy (DOSY) experiments provide evidence for weakly restricted rotational and translational mobility of the organic template within NAs as a consequence of specific silicate–template interactions. NAs thus appear as key species in clear sols, and their presence in the precursor sol favors silicate condensation and further crystallization, promoted either by increasing the Si/OH ratio or by heating.
Mechanism analysis: The formation of zeolite beta (see figure) is monitored by liquid‐state NMR, ESI‐MS, SAXS, XRD, cryo‐TEM, and HRSEM. The complementary methods allow the monitoring of all processes occurring during silicate speciation and particle formation up to the final stages of zeolite beta crystallization. The analyses reveal the zeolite formation mechanism and allow the identification of crucial species as well as spectators.
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