The review covers the recent developments in the field of novel micro-mesoporous materials obtained by zeolite recrystallization. The materials are classified into three distinctly different groups ...depending on the degree of recrystallization: (i) coated mesoporous zeolites (RZEO-1); (ii) micro-mesoporous nanocomposites (RZEO-2); and (iii) mesoporous materials with zeolitic fragments in the walls (RZEO-3). The first part of the review is focused on the analysis of the synthetic strategies leading to different types of recrystallized materials. In the second part, a comprehensive view on their structure, texture and porosity in connection with acidic and diffusion properties is given. The last part is devoted to the catalytic applications of recrystallized materials. The advantages and disadvantages with respect to pure micro- and mesoporous molecular sieves and other hierarchical zeolites are critically analyzed and the future opportunities and perspectives are discussed.
In situ MAS NMR studies on the monitoring of hydrothermal synthesis of zeolites are reviewed. The first part of the review contains information on the experimental techniques used for the in situ NMR ...studies in static and MAS conditions. In the second part, the main capabilities of the in situ 1H, 11B, 13C, 14N, 19F, 23Na, 27Al, 29Si and 31P MAS NMR for the elucidation of the mechanism of hydrothermal synthesis of zeolites are examined and the data on NMR lines identification are summarized. In the last part the main application areas of the techniques are considered and illustrated with examples taken from the mechanistic studies of zeolites A, X, MFI and BEA synthesis. A cross‐reference index between the materials studied, the experimental approaches used, the mechanistic information obtained, and the corresponding literature sources is established.
The potential of in situ multinuclear MAS NMR spectroscopy for monitoring hydrothermal synthesis of zeolites is demonstrated in the Review. The technique enables the detection of intermediate species stable only under hydrothermal conditions, provides information on the kinetics of synthesis and gives insight into the mechanism of gel formation, gel aging, crystal nucleation and growth.
1,3‐Butadiene was synthesized from ethanol using zirconium‐containing zeolite beta (ZrBEA) catalysts doped with 1 wt % silver. The Zr was planted using post‐synthesis modification by dealumination of ...the parent zeolite followed by treatment with ZrOCl2 in a DMSO solution. FTIR and NMR spectroscopy were used to investigate the planting process by preparing materials with different Si/Al ratios and crystal sizes. The results showed preferential grafting of Zr to the terminal silanols present on the external surface of the zeolite crystals instead of incorporation of Zr into silanol nests. The grafting yielded highly accessible Zr(OSi)3OH open sites with high Lewis acidity, as confirmed by FTIR spectroscopy of adsorbed CO. These sites are shown to be extremely active for the conversion of ethanol to butadiene. Ag/ZrBEA catalysts prepared using the post‐synthesis method showed significant advantages compared with Ag/ZrBEA catalysts synthesized using a conventional hydrothermal procedure. The best catalyst performance in terms of butadiene formation rate (3 μmol g−1 s−1) was observed over Ag/Zr(3.5)BEA(75) (containing 3.5 wt % Zr), which had the smallest crystal size and the highest content of Zr open sites of the prepared catalysts.
The Ag goes to Zeolites! Ag/ZrBEA (zirconium‐containing zeolite beta) catalysts prepared using a post‐synthesis method show significant advantages compared with Ag/ZrBEA catalysts synthesized using a conventional hydrothermal procedure. Parent AlBEA materials with different Si/Al ratios and crystal sizes are used to determine the best catalyst for the conversion of ethanol to butadiene.
The synthesis of buta‐1,3‐diene from ethanol has been studied over metal‐containing (M=Ag, Cu, Ni) oxide catalysts (MOx=MgO, ZrO2, Nb2O5, TiO2, Al2O3) supported on silica. Kinetic study of a wide ...range of ethanol conversions (2–90 %) allowed the main reaction pathways leading to butadiene and byproducts to be determined. The key reaction steps of butadiene synthesis were found to involve ethanol dehydrogenation, acetaldehyde condensation, and the reduction of crotonaldehyde with ethanol into crotyl alcohol. Catalyst design included the selection of active components for each key reaction step and merging of these components into multifunctional catalysts and adjusting the catalyst functions to achieve the highest selectivity. The best catalytic performance was achieved over the Ag/ZrO2/SiO2 catalyst, which showed the highest selectivity towards butadiene (74 mol %).
Losing H2: Metal‐promoted oxides supported on silica are efficient catalysts for butadiene synthesis from ethanol. The metal promoters allow ethanol dehydrogenation, the metal oxide components are efficient in acetaldehyde condensation and reduction of crotonaldehyde with ethanol, whereas the silica support allows the dehydration steps. The optimized catalyst provides 74 mol % selectivity to butadiene at 88 % ethanol conversion at 593 K.
The Lewis and Brønsted acid properties of SnBEA zeolites as well as SnO2 supported on silica BEA have been examined by means of IR spectroscopy of adsorbed pyridine, 2,6-ditertbutylpyridine, and ...deuterated acetonitrile. Three types of surface sites have been detected and assigned to (i) framework Sn centers possessing Lewis acid properties; (ii) weak Brønsted acid sites associated with framework tin atoms; and (iii) nonframework SnO2 particles which possess Lewis acidity. The total amount of Lewis sites can be determined using pyridine as a probe molecule, while the type of Lewis sites can be distinguished by FTIR of adsorbed acetonitrile. The band at 2316 cm–1 is attributed to strong framework sites, whereas the band at 2287 cm–1 is attributed to nonframework sites. Brønsted acid sites can be characterized using 2,6-ditertbutylpyridine (bands at 3363, 1613, and 1530 cm–1) and deuterated acetonitrile (band at 2308 cm–1). The relative amount of Lewis and Brønsted acid sites on SnBEA can be varied by treatment in hydrogen at different temperatures.
Fourier transform infrared spectroscopy and density functional theory calculations have been used to elucidate the nature of active sites of ZrBEA zeolite responsible for the catalytic synthesis of ...butadiene. We show that the content of open Zr(IV) Lewis acid sites, represented by isolated Zr atoms in tetrahedral positions of the zeolite crystalline structure connected to three −O–Si linkages and one OH group, correlates with the catalytic activity in the process of conversion of ethanol into butadiene. The higher catalytic activity of the open sites is attributed to their higher acid strength and steric accessibility. The study suggests that the control of such open sites plays a crucial role for the further design of the optimal multifunctional zeolite-based catalysts.
We report a mild Lewis acid induced isomerization of donor–acceptor cyclopropanes, containing an alkenyl moiety and diverse electron-withdrawing group(s) at the adjacent positions, into substituted ...cyclopentenes. We have found that 1,1,2-trisubstituted cyclopent-3-enes were exclusively obtained in yield of 51–99% when cyclopropanes with a 2-substituted alkenyl group as a donor underwent isomerization. For cyclopropanes bearing a trisubstituted alkenyl group either the corresponding cyclopent-3-enes or isomeric cyclopent-2-enes having two acceptor groups at the C(1) atom were formed, with the reaction selectivity being determined by the applied Lewis acid. We have shown that the reactivity of the donor–acceptor cyclopropane increases with the increase of the electron-donating character of (hetero)aromatic group attached to the alkenyl moiety. The synthetic utility of the developed methodology was also demonstrated through the synthesis of polysubstituted cyclopentane and piperidine derivatives.
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•MPVO reaction of crotonaldehyde and ethanol has been studied over Zr-containing catalysts.•The best catalyst performance in terms of activity and selectivity is observed over ...ZrBEA.•The results point to the key role of Zr4+ Lewis acid sites.•Isolated Zr atoms in zeolite crystalline structure are found to be the most efficient.
A series of Zr-containing catalysts including bulk ZrO2, ZrO2 supported on silica, titania and ceria, Zr-BEA zeolite and Zr-MCM-41 mesoporous material was prepared and characterized by X-ray diffraction, nitrogen adsorption-desorption, X-ray photoelectron spectroscopy, 29Si MAS NMR, TPD NH3, and FTIR of adsorbed CO. The catalytic materials were tested in the MPVO reaction of crotonaldehyde and ethanol in the temperature range of 473–573K. The results showed that the activity of Zr-based catalysts in the MPVO reduction of crotonaldehyde correlates with the amount of Zr4+ Lewis acid sites in the catalysts. Tetrahedral Zr atom sites isolated within the crystalline structure of zeolite BEA were found to be the most efficient. The best catalyst performance in terms of selectivity was observed over ZrBEA: 72% selectivity to crotyl alcohol was achieved at crotonaldehyde conversion of 15%.
MBEA zeolites are known to catalyze carbon–carbon coupling reactions such as acetaldehyde condensation, which is an important step in a range of industrially relevant processes, e.g., a sustainable ...butadiene synthesis. The widely accepted mechanism of the reaction includes a separate enolization step via an α-proton transfer to zeolite. By combining the H–D exchange activity studies, FTIR spectroscopy, and DFT calculations, we show that such a mechanism is indeed feasible for the SnBEA zeolite. For the ZrBEA and TiBEA zeolites, on the other hand, experimental evidence suggests that a separate enolization step is unlikely. We propose the possibility of an alternative concerted single-step mechanism that involves coadsorption of two aldehyde molecules at the open M(IV) Lewis acid site and a subsequent proton transfer between the adsorbates in a collective transition state stabilized by the M–OH group of the open site. The study suggests that the nature of a zeolite dopant can thus be used to control the activity of the catalyst by modifying the reaction mechanism.
A silica‐supported Ag catalyst has been shown to be an efficient heterogeneous catalyst for the oxidant‐free dehydrogenation of ethanol into acetaldehyde. The reaction mechanism has been investigated ...by in situ FTIR spectroscopy. The kinetic isotope effects for proton and hydride ion have been studied by using CH3CD2OH and CH3CH2OD as labeled reactants. The results indicate that OH bond activation and the formation of a hydrogen‐bonded complex take place on the silica support and that the Ag particles are necessary for the activation of the CH bond. The kinetic isotope effect (kH/kD) is 1.9 for CH3CD2OH and 1.8 for CH3CH2OD. The concerted mechanism of CH cleavage on the Ag sites and proton ion on the silica sites is proposed to account for the results of the spectroscopic and kinetic experiments.
Every mechanism should have a silver lining: Ethanol is selectively converted into acetaldehyde over Ag supported on silica. The Ag particles and SiOH groups of silica act in a complementary manner in a concerted mechanism, which is proposed based on in situ FTIR spectroscopic studies and kinetic experiments with specifically deuterated ethanol. The proposed mechanism shows the way for the design of efficient supported Ag catalysts.
