Thiophene-Based Covalent Triazine Frameworks as Visible-Light-Driven Heterogeneous Photocatalysts for the Oxidative Coupling of Amines

This study reports on a metal-free Covalent Triazine Framework (CTF) incorporating bithiophene structural units (TP-CTF) with a semicrystalline structure as an efficient heterogeneous photocatalyst under visible light irradiation. The physico-chemical properties and composition of this material was confirmed via different characterization solid-state techniques, such as XRD, TGA, CO2 adsorption and FT-IR, NMR and UV-Vis spectroscopies. The compound was synthesized through a solvothermal process and was explored as a heterogeneous photocatalyst for the oxidative coupling of amines to imines under visible light irradiation. TP-CTF demonstrated outstanding photocatalytic activity, with high conversion rates and selectivity. Importantly, the material exhibited exceptional stability and recyclability, making it a strong candidate for sustainable and efficient imine synthesis. The low bandgap of TP-CTF enabled the efficient absorption of visible light, which is a notable advantage for visible-light-driven photocatalysis.

Synthesis, Characterization, and NH3-SCR Catalytic Performance of Fe-Modified MCM-36 Intercalated with Various Pillars

Two series of MCM-36 zeolites intercalated with various pillars and modified with iron were synthesized, analyzed with respect to their physicochemical properties, and tested as catalysts for the NH₃-SCR process. It was found that the characteristic MWW morphology of MCM-36 can be obtained successfully using silica, alumina, and iron oxide as pillars. Additionally, one-pot synthesis of the material with iron resulted in the incorporation of monomeric Fe³⁺ species into the framework positions. The results of catalytic tests revealed that the one-pot synthesized sample intercalated with silica and alumina was the most efficient catalyst of NO reduction, exhibiting ca. 100% activity at 250 °C. The outstanding performance of the material was attributed to the abundance of Lewis acid sites and the beneficial influence of alumina on the distribution of iron species in the zeolite. In contrast, the active centers originating from the Fe₂O₃ pillars improved the NO conversion in the high-temperature range. Nevertheless, the aggregated particles of the metal oxide limited the access of the reacting molecules to the inner structure of the catalyst, which affected the overall activity and promoted the formation of N₂O above 300 °C.

Hybrid organic-inorganic nanoparticles with associated functionality for catalytic transformation of biomass substrates

We present the one-pot synthesis of functionalized organosilica nanoparticles to generate multi-functional hybrid catalysts. Octadecyl, alkyl-thiol and alkyl-amino moieties were used separately and in different combinations, to generate different hybrid spherical nanoparticles with tunable acidic, basic and amphiphilic properties, covalently incorporating up to three organic functional elements onto the surface of the nanoparticles. Several parameters were optimised such as the concentration of the base employed during the hydrolysis and condensation synthesis process that showed a strong influence on the particle size. The physico-chemical properties of the hybrid materials were fully characterized by XRD, elemental and thermogravimetric analysis, electron microscopy, nitrogen adsorption isotherms and ¹³C and ²⁹Si NMR spectroscopy. Finally, the potential uses of the prepared materials as amphiphilic catalysts, with acidic or basic properties for the conversion of biomass molecules into platform chemicals were evaluated.

Hydrotalcite-Modified Clinoptilolite as the Catalyst for Selective Catalytic Reduction of NO with Ammonia (NH3-SCR)

A series of clinoptilolite-supported catalysts, modified with hydrotalcite-like phase (HT) by co-precipitation, were prepared and tested in NH₃-SCR reactions. It was found that deposition of HT on clinoptilolite increased conversion of NO within 250-450 °C, and that the positive impact on the catalytic activity was independent of HT loading. The promoting effect of clinoptilolite was attributed to Brönsted acid sites present in the zeolite, which facilitated adsorption and accumulation of ammonia during the catalytic process. Concentration of N₂O in the post-reaction gas mixture reached its maximum at 300 °C and the by-product was most likely formed as a consequence of NH₄NO₃ decomposition or side reaction of NH₃ oxidation in the high-temperature region. The gradual elimination of nitrous oxide, noticed as the material with the highest concentration of hydrotalcite phase, was attributed to the abundance of oligomeric iron species and the superior textural parameters of the material. UV-Vis experiments performed on the calcined samples indicated that Fe sites of higher nuclearity were generated by thermal decomposition of the hydrotalcite phase during the catalytic reaction. Therefore, calcination of the materials prior to the catalytic tests was not required to obtain satisfactory overall catalytic performance in NO reductions.

Interrogating the Behaviour of a Styryl Dye Interacting with a Mesoscopic 2D-MOF and Its Luminescent Vapochromic Sensing

In this contribution, we report on the solid-state-photodynamical properties and further applications of a low dimensional composite material composed by the luminescent trans-4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) dye interacting with a two-dimensional-metal organic framework (2D-MOF), Al-ITQ-HB. Three different samples with increasing concentration of DCM are synthesized and characterized. The broad UV-visible absorption spectra of the DCM/Al-ITQ-HB composites reflect the presence of different species of DCM molecules (monomers and aggregates). In contrast, the emission spectra are narrower and exhibit a bathochromic shift upon increasing the DCM concentration, in agreeance with the formation of adsorbed aggregates. Time-resolved picosecond (ps)-experiments reveal multi-exponential behaviors of the excited composites, further confirming the heterogeneous nature of the samples. Remarkably, DCM/Al-ITQ-HB fluorescence is sensitive to vapors of electron donor aromatic amine compounds like aniline, methylaniline, and benzylamine due to a H-bonding-induced electron transfer (ET) process from the analyte to the surface-adsorbed DCM. These findings bring new insights on the photobehavior of a well-known dye when interacting with a 2D-MOF and its possible application in sensing aniline derivatives.

Titanium-silicon ferrierites and their delaminated forms modified with copper as effective catalysts for low-temperature NH3-SCR

Titanium-silicon ferrierites with different Si/Ti ratios and their delaminated forms were modified with copper by ion-exchange. The obtained samples were characterized with respect to their chemical composition (ICP-OES), structure (XRD), texture (N2 sorption), morphology (SEM), form and aggregation of titanium and copper species (UV-vis-DRS), reducibility of deposited copper species (H2-TPR) and surface acidity (NH3-TPD). The porous structure of the zeolitic samples strongly influenced the form and aggregation of deposited copper species. In the case of the three dimensional microporous structure of ferrierites (Ti-FER), copper was deposited mainly in the form of aggregated copper oxide species, in contrast to the open micro- and mesoporous structure of delaminated ferrierites (Ti-ITQ-6), where mainly copper in the form of monomeric cations was identified. It was shown that monomeric copper cations are more catalytically active in NO to NO2 oxidation than aggregated copper oxide species and, therefore, for the low-temperature conversion of nitrogen oxides the fast SCR reaction pathway is more effective for delaminated ferrierites modified with copper (Cu-Ti-ITQ-6) than for microporous three dimensional ferrierite catalysts (Cu-Ti-FER).

Ferrierite and Its Delaminated Forms Modified with Copper as Effective Catalysts for NH3-SCO Process

Ferrierites and their delaminated forms (ITQ-6), containing aluminum or titanium in the zeolite framework, were synthetized and modified with copper by an ion-exchange method. The obtained samples were characterized with respect to their chemical composition (ICP-OES), structure (XRD, UV-Vis DRS), textural parameters (N₂-sorption), surface acidity (NH₃-TPD), form and reducibility of deposited copper species (UV-Vis DRS and H₂-TPR). Ferrierites and delaminated ITQ-6 zeolites modified with copper were studied as catalysts for the selective catalytic oxidation of ammonia to dinitrogen (NH₃-SCO). It was shown that aggregated copper oxide species, which were preferentially formed on Ti-zeolites, were catalytically active in direct low-temperature ammonia oxidation to NO, while copper introduced into Al-zeolites was present mainly in the form of monomeric copper cations catalytically active in selective reduction of NO by ammonia to dinitrogen. It was postulated that ammonia oxidation in the presence of the studied catalysts proceeds according to the internal-selective catalytic reduction mechanism (i-SCR) and therefore the suitable ratio between aggregated copper oxide species and monomeric copper cations is necessary to obtain active and selective catalysts for the NH₃-SCO process. Cu/Al-ITQ-6 presented the best catalytic properties possibly due to the most optimal ratio of these copper species.

A Lamellar MWW Zeolite With Silicon and Niobium Oxide Pillars: A Catalyst for the Oxidation of Volatile Organic Compounds

In this work, an MWW-type zeolite with pillars containing silicon and niobium oxide was synthesized to obtain a hierarchical zeolite. The effect of niobium insertion in the pillaring process was determined by combining a controllable acidity and accessibility in the final material. All pillared materials had niobium occupying framework positions in pillars and extra-framework positions. The pillared material, Pil-Nb-4.5 with 4.5 wt % niobium, did not compromise the mesoporosity formed by pillaring, while the increase of niobium in the structure gradually decreased the mesoporosity and ordering of lamellar stacking. The morphology of the pillared zeolites and the niobium content were found to directly affect the catalytic activity. Specifically, we report on the activity of the MWW-type zeolites with niobium catalyzing the gas-phase oxidation of volatile organic compounds (VOCs), which is an important reaction for clean environmental. All produced MWW-type zeolites with niobium were catalytically active, even at low temperatures and low niobium loading, and provided excellent conversion efficiencies.

MCM-22, MCM-36, and ITQ-2 Zeolites with Different Si/Al Molar Ratios as Effective Catalysts of Methanol and Ethanol Dehydration

MCM-22, MCM-36, and ITQ-2 zeolites with the intended Si/Al molar ratios of 15, 25, and 50 were synthetized and tested as catalysts for dehydration of methanol to dimethyl ether and dehydration of ethanol to diethyl ether and ethylene. The surface concentration of acid sites was regulated by the synthesis of zeolite precursors with different aluminum content in the zeolite framework, while the influence of porous structure on the overall efficiency of alcohol conversion was analyzed by application of zeolitic materials with different types of porosity—microporous MCM-22 as well as microporous-mesoporous MCM-36 and ITQ-2. The zeolitic samples were characterized with respect to their: chemical composition (ICP-OES), structure (XRD, FT-IR), texture (N₂ sorption), and surface acidity (NH₃-TPD). Comparison of the catalytic activity of the studied zeolitic catalysts with other reported catalytic systems, including zeolites with the similar Si/Al ratio as well as γ-Al₂O₃ (one of the commercial catalysts for methanol dehydration), shows a great potential of MCM-22, MCM-36, and ITQ-2 in the reactions of alcohols dehydration.

MOFs based on 1D structural sub-domains with Brønsted acid and redox active sites as effective bi-functional catalysts

Low-dimensional MOF-type catalysts containing Brønsted acid and redox active sites, based on assembled 1D organic–inorganic nanoribbons, for one-pot two-step reactions.

Synthesis of 2D and 3D MOFs with tuneable Lewis acidity from preformed 1D hybrid sub-domains

Novel MOF-type materials with different morphologies based on assembled 1D organic-inorganic sub-domains were prepared using specific monodentate benzylcarboxylate spacers with functional substituents in the para-position as structure modulating agents. The combination of electron-withdrawing or electron-donating functions in the organic spacers with suitable solvothermal synthesis conditions allowed modulating the structuration level (2D or 3D), vacancies, physico-chemical properties and Lewis acidity strength of the metal-organic structures. Furthermore, bimetallic (Al/Fe) MOF-type materials were synthesized by a one-pot direct process without modification of the structural framework. The activity of these hybrid materials as Lewis acid catalysts was evaluated to prepare cyanohydrins as precursors for the synthesis of biologically active compounds, and for aerobic oxidation of thiols to disulfides. The catalytic results showed that the derived MOFs exhibited modulatable Lewis acid capacities which are a function of the morphology, functionality of monodentate substituents present in the networks and a cooperative effect between metallic nodes of different nature.

Acid properties of organosiliceous hybrid materials based on pendant (fluoro)aryl-sulfonic groups through a spectroscopic study with probe molecules

Hybrid heterogeneous catalysts containing (fluoro)aryl-sulfonic groups were characterized through combined spectroscopic techniques and adsorption of probe molecules, with their reactivity being modulated for acetal formation.

Chiral hybrid materials based on pyrrolidine building units to perform asymmetric Michael additions with high stereocontrol

A new chiral mesoporous hybrid material was synthesized based on pyrrolidine units included in a siliceous framework, HybPyr, and integrated into the organic-inorganic structure, from a specific bis-silylated precursor. A fluoride sol-gel methodology under soft synthesis conditions and in the absence of sophisticated structural directing agents allowed the generation of a mesoporous architecture with a homogeneous distribution of active chiral moieties along the network. The hybrid material was studied by means of different characterization techniques (TGA, NMR and IR spectroscopy, chemical and elemental analyses, TEM, and textural measurements), verifying the stability and integrity of the asymmetric active sites in the solid. The hybrid material, HybPyr, is an excellent asymmetric heterogeneous and recyclable catalyst for enantioselective Michael addition of linear aldehydes to β-nitrostyrene derivatives with high stereocontrol of the reaction products.

Unraveling Competitive Electron and Energy-Transfer Events at the Interfaces of a 2D MOF and Nile Red Composites: Effect of the Length and Structure of the Linker

The distribution and interactions of organic molecules adsorbed on the surface of materials play important roles in many catalytic and photonic processes. Here, we show that the length and chemical structure of the linker in new Al-ITQ metal-organic frameworks (MOFs) are fundamental for the dynamics of the dye Nile Red (NR) adsorbed on its surface. For the studied composites using Al-ITQ-4-ethylbenzoic acid (EB), Al-ITQ-4-aminobenzoic acid (AB), and Al-ITQ-EB exposed to the aniline (AN) or N, N-dimethylaniline (DMA) atmospheres, we observed a very fast (∼1.2 ps) intramolecular charge-transfer reaction in adsorbed NR molecules. For NR@Al-ITQ-EB, where the linker has a shorter aliphatic chain (two carbons), the dye molecules present a homoenergy-transfer (ET) process, which is faster (∼90 ps) than in the previously reported NR@Al-ITQ-4-heptylbenzoic acid composite with longer aliphatic chain (seven carbons, ∼220 ps). The more polar environment created by the Al-oxide nodes in Al-ITQ-EB surface around the NR populations strongly favors the ET event. When the linker structure contains phenyl amine moieties, the resulting NR@Al-ITQ-AB composites show different and rich photodynamics, in which a fast electron transfer reaction from the MOF aniline moiety to the adsorbed NR occurs in ∼17 ps, inhibiting the ET process between the dye molecules near the MOF surface. This process also was confirmed in Al-ITQ-EB MOF exposed to AN and DMA gas atmospheres, as well as NR in pure aniline. The obtained results demonstrate how modifications in the length and structure of the organic linker in this MOF change the interface interactions and outcome of the photoinduced processes in the composites. Our findings on dye-MOF interface photobehavior are relevant to the design of new materials in which the interface plays a key role in their performance in the fields of catalysis and photonics.

Dandelion-Like Microspherical MCM-22 Zeolite Using BP 2000 as a Hard Template

The chemistry between layered MWW zeolite and carbon black pearls (BP 2000) as an inexpensive hard template was investigated to develop a rational one-pot synthesis of MCM-22 microspheres. The characterization results showed that the insertion of BP 2000 in the gel synthesis did not substantially compromise the crystallinity and microporosity, and the microscopic analyses showed that BP 2000 played a key role in controlling the final morphology of the MCM-22 zeolite, creating beautiful dandelion-like microspherical particles. The morphology obtained is due to the tortuous shape of the hard template, the particular MWW particle crystals, the interaction with the external surface of the MWW zeolitic precursor, and the synthesis conditions. The stacking of MWW crystals with edge-to-face orientations generates meso-/macrovoids, allowing access to the interiors of the microspheres. The microspheres were homogeneous with sizes ranging from 6 to 8 μm with an increase of the external surface and a macroporous size distribution centered at 200 nm, which is two times that of the traditional MCM-22 zeolite.

How Does the Surface of Al-ITQ-HB 2D-MOF Condition the Intermolecular Interactions of an Adsorbed Organic Molecule?

In this work, we unravel how the two-dimensional Al-ITQ-4-heptylbenzoic acid (HB) metal-organic framework (MOF) changes the interactions of Nile red (NR) adsorbed on its surface. Time-resolved emission experiments indicate the occurrence of energy transfer between adsorbed NR molecules, in abnormally long time constant of 2-2.5 ns, which gets shorter (∼0.25 ns) when the concentration of the surface-adsorbed NR increases. We identify the emission from local excited state of aggregates and charge transfer and energy transfer between adsorbed molecules. Femtosecond emission studies reveal an ultrafast process (∼425 fs) in the NR@Al-ITQ-HB composites, assigned to an intramolecular charge transfer in NR molecules. A comparison of the observed photobehavior with that of NR/SiO₂ and NR/Al₂O₃ composites suggests that the occurrence of energy transfer in the NR@MOF complexes is a result of specific and nonspecific interactions, reflecting the different surface properties of Al-ITQ-HB that are of relevance to the reported high catalytic activity. Our results provide new knowledge for further researches on other composites with the aim to improve understanding of photocatalytic and photonic processes within MOFs.

Exploring the Photodynamics of a New 2D-MOF Composite: Nile Red@Al-ITQ-HB

In this work, we unravel the photodynamics of Nile Red (NR) interacting with Al-ITQ-HB nanostructure, a new layer-type metal-organic framework (MOF) with potential catalytic and photonic applications. Steady-state spectroscopy reveals the presence of NR monomers and aggregates when interacting with the MOF structure. Time-resolved experiments provide emission lifetimes of the interacting monomers, H- and J-type aggregates. We observed contributions from two monomer populations having different environments. One monomer species emits from the local-excited state and another from a photoproduced charge-separated state resulting from an ultrafast intramolecular charge transfer (ICT). Femtosecond fluorescence experiments reveal that the ICT process occurs in ∼1 ps. Fluorescence microscopy on single crystals and agglomerates of the composites shows a homogenous distribution of the dye lifetimes within the material. This study shows that the photobehavior of NR in Al-ITQ-HB MOF is dictated by its location within the material. The reported findings using a well-known polarity probe and a new two-dimensional MOF provide information on the microenvironment of this material, which may help for designing smart MOFs with potential applications in photonics and nanocatalysis.

Organic-Inorganic Hybrid Materials: Multi-Functional Solids for Multi-Step Reaction Processes

The design of new hybrid materials with tailored properties at the nano-, meso-, and macro-scale, with the use of structural functional nanobuilding units, is carried out to obtain specific multi-functional materials. Organization into controlled 1D, 2D, and 3D architectures with selected functionalities is key for developing advanced catalysts, but this is hardly accomplished using conventional synthesis procedures. The use of pre-formed nanostructures, derived either from known materials or made with specific innovative synthetic methodologies, has enormous potential in the generation of multi-site catalytic materials for one-pot processes. The present concept article introduces a new archetype wherein self-assembled nanostructured builder units are the base for the design of multifunctional catalysts, which combine catalytic efficiency with fast reactant and product diffusion. The article addresses a new generation of versatile hybrid organic-inorganic multi-site catalytic materials for their use in the production of (chiral) high-added-value products within the scope of chemicals and fine chemicals production. The use of those multi-reactive solids for more nanotechnological applications, such as sensors, due to the inclusion of electron donor-acceptor structural arrays is also considered, together with the adsorption-desorption capacities due to the combination of hydrophobic and hydrophilic sub-domains. The innovative structured hybrid materials for multipurpose processes here considered, can allow the development of multi-stage one-pot reactions with industrial applications, using the materials as one nanoreactor systems, favoring more sustainable production pathways with economic, environmental and energetic advantages.

Growth-modulating agents for the synthesis of Al-MOF-type materials based on assembled 1D structural subdomains

Specific long-alkyl aromatic mono-carboxylate linkers, combined with metallic nodes, favors the formation of 1D metalorganic nanoribbon-type structural sub-units which are assembled to generate novel organized Al-MOF-type solids.

Corrigendum: Generation of subnanometric platinum with high stability during transformation of a 2D zeolite into 3D

This corrects the article DOI: 10.1038/nmat4757.

Generation of subnanometric platinum with high stability during transformation of a 2D zeolite into 3D

Single metal atoms and metal clusters have attracted much attention thanks to their advantageous capabilities as heterogeneous catalysts. However, the generation of stable single atoms and clusters on a solid support is still challenging. Herein, we report a new strategy for the generation of single Pt atoms and Pt clusters with exceptionally high thermal stability, formed within purely siliceous MCM-22 during the growth of a two-dimensional zeolite into three dimensions. These subnanometric Pt species are stabilized by MCM-22, even after treatment in air up to 540 °C. Furthermore, these stable Pt species confined within internal framework cavities show size-selective catalysis for the hydrogenation of alkenes. High-temperature oxidation-reduction treatments result in the growth of encapsulated Pt species to small nanoparticles in the approximate size range of 1 to 2 nm. The stability and catalytic activity of encapsulated Pt species is also reflected in the dehydrogenation of propane to propylene.

Single-Layered Hybrid Materials Based on 1D Associated Metalorganic Nanoribbons for Controlled Release of Pheromones

A new family of stable layered organic-inorganic materials has been prepared, in one-step solvothermal process. They are based on an ordered nickel cluster-type nanoribbons separated from each other by specific alkyl (heptyl- or dodecyl-) arylic mono-carboxylate moieties acting as molecular spacers, perpendicular to the 1D inorganic chains. These organic spacers contain hydrocarbon tails with different length which control the separation level between inorganic 1D sub-units, inhibiting the 3D growth of conventional DUT-8-type metal-organic frameworks (MOFs). The lamellar nature of the materials formed was studied and confirmed by different characterization techniques, showing the structural location of individual organic and inorganic building units. They have been successfully used as a long-lasting biodegradable and water-proof materials for controlled release of chemicals, such as pheromones for sustainable treatment of insect plagues.

Hierarchically structured ZSM-5 obtained by optimized mesotemplate-free method as active catalyst for methanol to DME conversion

In the presented studies, a new method for the synthesis of hierarchical porous materials with ZSM-5 zeolite properties was applied.

Influencing the activity and selectivity of alkylaromatic catalytic transformations by varying the degree of delamination in MWW zeolites

Increasing the degree of delamination of MWW layered precursors has a positive effect regarding selectivity for the alkylation product, as alkylation is favored vs. disproportionation when increasing the ratio of external to internal surface area.

Synthesis and characterization of new ruthenium N-heterocyclic carbene Hoveyda II-type complexes. Study of reactivity in ring closing metathesis reactions

This manuscript describes the synthesis and structural study of new second generation Hoveyda-Grubbs catalysts: 1,3-dimesityl-acenaphthylenyl-4,5-imidazolin-2-ylidene (BIAN-NHC) ruthenium isopropoxybenzylidene dichloride and 1,3-bis(2,6-dimethylphenyl)-2,3-dihydro-1H-imidazole Cl(2)Ru(=CH-o-O-i-PrC(6)H(4)) . The electrochemical and catalytic behavior of these new complexes was compared with the conventional NHC carbene Hoveyda II IMes-type complexes and for ring closing metathesis reactions.

Layered Materials with Catalytic Applications: Pillared and Delaminated Zeolites from MWW Precursors

Delaminated and pillared zeolites are an innovative family of molecular sieves which introduced a different concept inside the synthesis of active catalysts or inorganic supports. These types of materials exhibit an elevated accessibility due to their open structure, characterized by the high external surface area without imposed restrictions controlled by the pore sizes. These open zeolites are conformed by crystalline ordered (pillared zeolites) or disordered (delaminated zeolites) individual layers, exhibiting textural properties which are favorable to carry out catalytic processes in which it is necessary to employ catalysts with completely accessible active sites. The elevated external surface area of these zeolites is profitable to generate more specific organic-inorganic materials, acting in this case as stable inorganic matrixes. The preparation of this open type-zeolites family is based on the modification of, previously synthesized, zeolitic precursors which are preexpanded to obtain the final delaminated or pillared zeolites which exhibit very different physicochemical properties compared with the starting precursors. Along this paper, the most relevant MWW-type high accessible zeolitic materials will be considered. Their nature, characteristics, and reactivity will be shown in the function of the employed synthesis method for their preparation and the postsynthesis treatments carried out, tuning their properties.

Hybrid organic–inorganic structured materials as single-site heterogeneous catalysts

Catalyst selectivity is associated with well-defined homogeneous active sites. Transition metal complexes and organocatalysts are highly active and selective in the homogeneous phase, and their heterogenization by incorporating them into inorganic solid materials allows combining their excellent catalytic activity with improved separation, recovering and recycling properties. In this article, we present the structural characteristics and catalytic properties of hybrid organic–inorganic materials in which the molecular catalysts are part of the inorganic structure, emphasizing the possibilities of periodic mesoporous hybrid materials and coordination polymers as single-site solid catalysts.