Incorporation of Co(II) in dealuminated BEA zeolite at lattice tetrahedral sites evidenced by XRD, FTIR, diffuse reflectance UV-Vis, EPR, and TPR

The Synthesis of Different Series of Cobalt BEA Zeolite Catalysts by Post-Synthesis Methods and Their Characterization

Three series of zeolite catalysts Co all-silica and Co Al-containing zeolites beta were prepared for use in the selective oxidative dehydrogenation of propane to propylene. Two series of zeolite catalysts Co all-silica were prepared by a two-step postsynthesis method at pH = 2.5 and pH = 3.0–9.0, respectively, which allows the incorporation of cobalt into SiBEA zeolite in the form of isolated framework pseudo-tetrahedral Co(II) species. The incorporation of Co ions into vacant T-atom sites and their reaction with silanol groups were demonstrated by NMR and FTIR methods. The generation of Lewis acid sites without the formation of Brønsted sites was proved by FTIR using pyridine and CO as probe molecules. The state of cobalt in three series of prepared and calcined zeolite catalysts was characterized by DR UV-vis. This technique allowed to show that for low Co content (<2 wt.%) cobalt is present in the form of framework pseudo-tetrahedral Co(II) species. For higher Co content (>2 wt.%), both framework pseudo-tetrahedral and extra-framework octahedral Co(II) species are present. The Co Al-containing zeolite beta series prepared on non-dealuminated support shows the presence of extra-framework octahedral Co(II) only.

Atomically Dispersed Co2+ Sites Incorporated into a Silicalite-1 Zeolite Framework as a High-Performance and Coking-Resistant Catalyst for Propane Nonoxidative Dehydrogenation to Propylene

Propane nonoxidative dehydrogenation (PDH) is a promising route to produce propylene with the development of shale gas exploration technology. Co-based catalysts with low cost and low toxicity could activate C-H effectively, but they suffer from deactivation with coke formation. In this work, a catalyst formed by incorporating highly dispersed Co sites into a Silicalite-1 zeolite framework (Co-Silicalite-1) is synthesized by a hydrothermal protocol in the presence of ammonia, which exhibits superior propane dehydrogenation catalytic performance with 0.0946 mmol C₃H₆·s^-1·g_Co^-1 and propylene selectivity higher than 98.5%. It also shows outstanding catalytic stability and coking resistance in a 3560 min time-on-stream. Combined characterization results demonstrate that the tetrahedrally coordinated Co²⁺ site serves as the PDH catalytic active site, which is stabilized by Si-O units of the zeolite framework. Incorporation of Co sites into the zeolite framework could avoid the reduction of Co species to metallic Co. Moreover, the catalytic performance is improved by the enhanced propane adsorption and propylene desorption.

Towards a better understanding of Lewis acidic aluminium in zeolites

Zeolites are a class of materials that are of great relevance for industrial catalysis. Several fundamental questions relating to the structure and role of the Lewis acid sites in these materials remain unanswered. Proposals for the origin of such species can broadly be classified into three categories, which have distinct structures: extra-framework, framework-associated and framework aluminium. In this Perspective, we review each of these proposals and proceed to analyse their suitability to understand experimental results. Contrary to traditional belief, the number of Lewis acid sites does not always correlate to extra-framework aluminium content. As a result, we highlight that the terms 'extra-framework' and 'framework-associated' aluminium should be used with caution. We propose how the usage of different characterization techniques can enable the closure of knowledge gaps concerning the strength, multiplicity, localization and structure of catalytically active Lewis acid sites in zeolites.

Microporous Frameworks as Promising Platforms for Antibacterial Strategies Against Oral Diseases

Nowadays, the heavy burden of oral diseases such as dental caries, periodontitis, endodontic infections, etc., and their consequences on the patients' quality of life indicate a strong need for developing effective therapies. Bacterial infections played an important role in the field of oral diseases, in-depth insight of such oral diseases have given rise to the demand for antibacterial therapeutic strategies. Recently, microporous frameworks have attracted tremendous interest in antibacterial application due to their well-defined porous structures for drug delivery. In addition, intensive efforts have been made to enhance the antibacterial performance of microporous frameworks, such as ion doping, photosensitizer incorporation as building blocks, and surface modifications. This review article aims on the major recent developments of microporous frameworks for antibacterial applications against oral diseases. The first part of this paper puts concentration on the cutting-edge researches on the versatile antibacterial strategies of microporous materials via drug delivery, inherent activity, and structural modification. The second part discusses the antibacterial applications of microporous frameworks against oral diseases. The applications of microporous frameworks not only have promising therapeutic potential to inhibit bacterial plaque-initiated oral infectious diseases, but also have a wide applicability to other biomedical applications.

The Impact of Reduction Temperature and Nanoparticles Size on the Catalytic Activity of Cobalt-Containing BEA Zeolite in Fischer–Tropsch Synthesis

A goal of this work was to investigate the influence of the preparation procedure and activation conditions (reduction temperature and reducing medium: pure hydrogen (100% H2) or hydrogen-argon mixture (5% H2-95% Ar)) on the activity of Co-containing BEA zeolites in Fischer–Tropsch synthesis. Therefore, a series of CoBEA zeolites were obtained by a conventional wet impregnation (Co5.0AlBEA) and a two-step postsynthesis preparation procedure involving dealumination and impregnation steps (Co5.0SiBEA). Both types of zeolites were calcined in air at 500 °C for 3 h and then reduced at 500, 800 and 900 °C for 1 h in 100 % H2 and in 5% H2–95% Ar mixture flow. The obtained Red-C-Co5.0AlBEA and Red-C-Co5.0SiBEA catalysts with various physicochemical properties were tested in Fischer–Tropsch reaction. Among the studied catalysts, Red-C-Co5.0SiBEA reduced at 500 °C in pure hydrogen was the most active, presenting selectivity to liquid products of 91% containing mainly C7–C16 n-alkanes and isoalkanes as well as small amount of olefins, with CO conversion of about 11%. The Red-C-Co5.0AlBEA catalysts were not active in the Fischer–Tropsch synthesis. It showed that removal of aluminum from the BEA zeolite in the first step of postsynthesis preparation procedure played a key role in the preparation of efficient catalysts for Fischer–Tropsch synthesis. An increase of the reduction temperature from 500 to 800 and 900 °C resulted in two times lower CO conversion and a drop of the selectivity towards liquid products (up to 62%–88%). The identified main liquid products were n-alkanes and isoalkanes. The higher activity of Red-C-Co5.0SiBEA catalysts can be assigned to good dispersion of cobalt nanoparticles and thus a smaller cobalt nanoparticles size than in the case of Red-C-Co5.0AlBEA catalyst.

The Catalytic Performance of Ni-Co/Beta Zeolite Catalysts in Fischer-Tropsch Synthesis

The influence of nickel introduction on the catalytic performance of cobalt micro- and mesoporous Beta zeolite catalysts in Fischer–Tropsch Synthesis was studied. Catalysts containing 3 wt% of nickel and 10 wt% of cobalt were prepared by co-impregnation and sequential impregnation and comprehensively characterized by XRD, XPS, NH3-TPD, TPR-H2 and TEM EDX techniques. Neither the dealumination of Beta zeolite nor the incorporation of Co and Ni affected its structure, as shown by XRD and BET investigations. The presence of nickel results in the decrease in the temperature of the cobalt oxide reduction, evidenced by TPR-H2 and the increase of CO conversion. Among all the tested catalysts, the best catalytic properties in FTS showed that based on microporous dealuminated zeolites with a very high CO conversion, near 100%, and selectivity to liquid products of about 75%. In case of dealuminated samples, the presence of Ni decreased the selectivity to liquid products. All catalysts under study showed high resistance to deactivation during the whole time of synthesis (24 h). The very high stability of nickel-cobalt based Beta catalysts was probably due to the hydrogen spillover from metallic nickel particles to cobalt oxides, which decreased re-oxidation of the active phase, sintering and the creation of the carbon on the catalyst surface. Moreover, the presence of Ni on the surface of cobalt-based Beta catalysts could obstruct the formation of graphitic carbon and, in consequence, delay the deactivation of the catalyst.

Cobalt-containing zeolitic imidazole frameworks for C–H activation using visible-light redox photocatalysis

Rationalising the photocatalytic activity of different cobalt ZIFs, provides an improved understanding of photocatalytic C–H activation.

Dealuminated Beta Zeolite Modified by Alkaline Earth Metals

Alkaline Earth metals (Mg, Sr, and Ca) were incorporated into the dealuminated mesoporous beta zeolite (DeAlBeta) by the two-step postsynthesis method. Physicochemical properties of both unmodified and alkaline Earth metal-modified DeAlBeta zeolite were characterized by XRD, DR UV-vis, FTIR, TPD of NH3 and CO2, NMR, and XPS. The dealumination of beta zeolite led to decrease of its acidity and basicity. The incorporation of alkaline Earth metals into the framework of dealuminated beta zeolite did not affect its structure. The modification of DeAlBeta with a small amount of alkaline Earth metals increases the number of acidic centers, which may be related to the formation of framework Mg (Ca or Sr) (II) Lewis acidic sites.

Influence of Levulinic Acid Hydrogenation on Aluminum Coordination in Zeolite-Supported Ruthenium Catalysts: A 27 Al 3QMAS Nuclear Magnetic Resonance Study

The influence of a highly oxygenated, polar protic reaction medium, that is, levulinic acid in 2-ethylhexanoic acid, on the dealumination of two zeolite-supported ruthenium catalysts, namely Ru/H-β and Ru/H-ZSM-5, has been investigated by 27 Al triple-quantum magic-angle spinning nuclear magnetic resonance spectroscopy (3QMAS NMR). Upon use of these catalysts in the hydrogenation of levulinic acid, the heterogeneity in aluminum speciation is found to increase for both Ru/H-ZSM-5 and Ru/H-β. For Ru/H-ZSM-5, the symmetric, tetrahedral framework aluminum species (FAL) were found to be mainly converted into distorted tetrahedral FAL species, with limited loss of aluminum to the solution by leaching. A severe loss of both FAL and extra-framework aluminum (EFAL) species into the liquid phase was observed for Ru/H-β instead. The large decrease in tetrahedral FAL species, in particular, results in a significant decrease in strong acid sites, as corroborated by Fourier transform infrared spectroscopy (FT-IR). This decrease in acidity, evidence of the inferior stability of the strongly acidic sites in Ru/H-β relative to Ru/H-ZSM-5 under the applied conditions, is considered as the main reason for differences seen in catalyst performance.

Probing zeolites by vibrational spectroscopies

This review addresses the most relevant aspects of vibrational spectroscopies (IR, Raman and INS) applied to zeolites and zeotype materials. Surface Brønsted and Lewis acidity and surface basicity are treated in detail. The role of probe molecules and the relevance of tuning both the proton affinity and the steric hindrance of the probe to fully understand and map the complex site population present inside microporous materials are critically discussed. A detailed description of the methods needed to precisely determine the IR absorption coefficients is given, making IR a quantitative technique. The thermodynamic parameters of the adsorption process that can be extracted from a variable-temperature IR study are described. Finally, cutting-edge space- and time-resolved experiments are reviewed. All aspects are discussed by reporting relevant examples. When available, the theoretical literature related to the reviewed experimental results is reported to support the interpretation of the vibrational spectra on an atomic level.

Remarkable Effect of the Preparation Technique on the State of Cobalt Ions in BEA Zeolites Evidenced by FTIR Spectroscopy of Adsorbed CO and NO, TPR and XRD

The state of cobalt in two BEA zeolites was studied by XRD, TPR, and FTIR spectroscopy using CO and NO as probe molecules. One of the samples, CoAlBEA (0.4 wt % of Co), was prepared by conventional ion exchange and the other, CoSiBEA (0.7 wt % Co), by a two-step postsynthesis method involving dealuminated SiBEA zeolite. The introduction of Co into SiBEA leads to an increase of unit cell parameters of the BEA structure and to the consumption of silanol groups in vacant T-sites of the dealuminated zeolite. In contrast, no structural changes are observed after incorporation of cobalt into AlBEA by ion-exchange. The reduction temperature of cobalt in CoSiBEA zeolite (1130 K), is much higher than for CoAlBEA and indicates a strong interaction of cobalt ions with SiBEA. Low-temperature CO adsorption on CoAlBEA results in (i) H-bonded CO, (ii) Co(3+)-CO adducts (2,208 cm(-1)) and (iii) a small amount of Co(2+)-CO complexes (2,188 cm(-1)). In agreement with these results, NO adsorption leads to the appearance of (i) NO(+) (2,133 cm(-1), formed with the participation of the zeolite acidic hydroxyls), (ii) Co(3+)-NO (1932 cm(-1)), and (iii) a small amount of Co(2+)(NO)(2) dinitrosyls (nu(s) = 1,898 and nu(as) = 1,814 cm(-1)). Low-temperature CO adsorption on CoSiBEA leads to formation of two kinds of Co(2+)-CO adducts (2,185 and 2,178 cm(-1)). No Co(3+) cations are detected. In line with these results, adsorption of NO reveals the existence of two kinds of Co(2+)(NO)(2) dinitrosyls (nu(s) = 1,888 and nu(as) = 1,808 cm(-1) and nu(s) = 1,878 and nu(as) = 1,799 cm(-1), respectively).