Phosphonate‐Modified UiO‐66 Brønsted Acid Catalyst and Its Use in Dehydra‐Decyclization of 2‐Methyltetrahydrofuran to Pentadienes

Phosphorus‐modified all‐silica zeolites exhibit activity and selectivity in certain Brønsted acid catalyzed reactions for biomass conversion. In an effort to achieve similar performance with catalysts having well‐defined sites, we report the incorporation of Brønsted acidity to metal–organic frameworks with the UiO‐66 topology, achieved by attaching phosphonic acid to the 1,4‐benzenedicarboxylate ligand and using it to form UiO‐66‐PO₃H₂ by post‐synthesis modification. Characterization reveals that UiO‐66‐PO₃H₂ retains stability similar to UiO‐66, and exhibits weak Brønsted acidity, as demonstrated by titrations, alcohol dehydration, and dehydra‐decyclization of 2‐methyltetrahydrofuran (2‐MTHF). For the later reaction, the reported catalyst exhibits site‐time yields and selectivity approaching that of phosphoric acid on all‐silica zeolites. Using solid‐state NMR and deprotonation energy calculations, the chemical environments of P and the corresponding acidities are determined.     

Insight into Three‐Coordinate Aluminum Species on Ethanol‐to‐Olefin Conversion over ZSM‐5 Zeolites

Commercial bioethanol can be readily converted into ethylene by a dehydration process using solid acids, such as Brønsted acidic H‐ZSM‐5 zeolites, and thus, it is an ideal candidate to replace petroleum and coal for the sustainable production of ethylene. Now, strong Lewis acidic extra‐framework three‐coordinate Al³⁺ species were introduced into H‐ZSM‐5 zeolites to improve their catalytic activity. Remarkably, Al³⁺ species working with Brønsted acid sites can accelerate ethanol dehydration at a much lower reaction temperature and shorten the unsteady‐state period within 1–2 h, compared to >9 h for those without Al³⁺ species, which can significantly enhance the ethanol dehydration efficiency and reduce the cost. The reaction mechanism, studied by solid‐state NMR, shows that strong Lewis acidic EFAl‐Al³⁺ species can collaborate with Brønsted acid sites and promote ethanol dehydration either directly or indirectly via an aromatics‐based cycle to produce ethylene.     

The porosity, acidity, and reactivity of dealuminated zeolite ZSM-5 at the single particle level: the influence of the zeolite architecture

A combination of atomic force microscopy (AFM), high‐resolution scanning electron microscopy (HR‐SEM), focused‐ion‐beam scanning electron microscopy (FIB‐SEM), X‐ray photoelectron spectroscopy (XPS), confocal fluorescence microscopy (CFM), and UV/Vis and synchrotron‐based IR microspectroscopy was used to investigate the dealumination processes of zeolite ZSM‐5 at the individual crystal level. It was shown that steaming has a significant impact on the porosity, acidity, and reactivity of the zeolite materials. The catalytic performance, tested by the styrene oligomerization and methanol‐to‐olefin reactions, led to the conclusion that mild steaming conditions resulted in greatly enhanced acidity and reactivity of dealuminated zeolite ZSM‐5. Interestingly, only residual surface mesoporosity was generated in the mildly steamed ZSM‐5 zeolite, leading to rapid crystal coloration and coking upon catalytic testing and indicating an enhanced deactivation of the zeolites. In contrast, harsh steaming conditions generated 5–50 nm mesopores, extensively improving the accessibility of the zeolites. However, severe dealumination decreased the strength of the Brønsted acid sites, causing a depletion of the overall acidity, which resulted in a major drop in catalytic activity.     

Synergic Effect of Active Sites in Zinc‐Modified ZSM‐5 Zeolites as Revealed by High‐Field Solid‐State NMR Spectroscopy

Understanding the nature of active sites in metal‐supported catalysts is of great importance towards establishing their structure–property relationships. The outstanding catalytic performance of metal‐supported catalysts is frequently ascribed to the synergic effect of different active sites, which is however not well spectroscopically characterized. Herein, we report the direct detection of surface Zn species and ¹H–⁶⁷Zn internuclear interaction between Zn²⁺ ions and Brønsted acid sites on Zn‐modified ZSM‐5 zeolites by high‐field solid‐state NMR spectroscopy. The observed promotion of C?H bond activation of methane is rationalized by the enhanced Brønsted acidity generated by synergic effects arising from the spatial proximity/interaction between Zn²⁺ ions and Brønsted acidic protons. The concentration of synergic active sites is determined by ¹H–⁶⁷Zn double‐resonance solid‐state NMR spectroscopy.     

Probing the Brønsted Acidity of the External Surface of Faujasite-Type Zeolites

We outline two methodologies to selectively characterize the Brønsted acidity of the external surface of FAU-type zeolites by IR and NMR spectroscopy of adsorbed basic probe molecules. The challenge and goal are to develop reliable and quantitative IR and NMR methodologies to investigate the accessibility of acidic sites in the large pore FAU-type zeolite Y and its mesoporous derivatives often referred to as ultra-stable Y (USY). The accessibility of their Brønsted acid sites to probe molecules (n-alkylamines, n-alkylpyridines, n-alkylphosphine- and phenylphosphine-oxides) of different molecular sizes is quantitatively monitored either by IR or ³¹P NMR spectroscopy. It is now possible, for the first time to quantitatively discriminate between the Brønsted acidity located in the microporosity and on the external surface of large pore zeolites. For instance, the number of external acid sites on a Y (LZY-64) zeolite represents 2 % of its total acid sites while that of a USY (CBV760) represents 4 % while the latter has a much lower framework Si/Al ratio.     

Framework Stability and Brønsted Acidity of Isomorphously Substituted Interlayer‐Expanded Zeolite COE‐4: A Density Functional Theory Study

COE‐4 zeolites possess a unique two‐dimensional ten‐ring pore structure with the Si(OH)₂ hydroxyl groups attached to the linker position between the ferrierite‐type layers, which has been demonstrated through the interlayer‐expansion approach in our previous work (H. Gies et al. Chem. Mater.­ 2012 , 24, 1536). Herein, density functional theory is used to study the framework stability and Brønsted acidity of the zeolite T‐COE‐4, in which the tetravalent Si is isomorphously substituted by a trivalent Fe, B, Ga, or Al heteroatom at the linker position. The influences of substitution energy and equilibrium geometry parameters on the stability of T‐COE‐4 are investigated in detail. The relative acid strength of the linker position is revealed by the proton affinity, charge analysis, and NH₃ adsorption. It is found that the range of the 〈T‐O‐Si〉 angles is widened to maintain the stability of isomorphously substituted T‐COE‐4 zeolites. The smaller the 〈O1‐T‐O2〉 bond angle is, the more difficult is to form the regular tetrahedral unit. Thus, the substitution energies at the linker positions increase in the following sequence: Al‐COE‐4 < Ga‐COE‐4 < Fe‐COE‐4 < B‐COE‐4. The adsorption of NH₃ as a probe molecule indicates that the acidity can affect the hydrogen‐bonding interaction between (N?H???O2) and (N???H?O2). The relative Brønsted‐acid strength of the interlayer‐expanded T‐COE‐4 zeolite decreases in the order of Al‐COE‐4 > Ga‐COE‐4 > Fe‐COE‐4 > B‐COE‐4. These findings may be helpful for the structural design and functional modification of interlayer‐expanded zeolites.     

Aluminum Redistribution during the Preparation of Hierarchical Zeolites by Desilication

A literature survey reveals a prominent reduction in the concentration of Brønsted acid sites in hierarchically organized zeolites with increasing mesoporous or external surface area independent of the framework type or synthesis route; this suggests a common fundamental explanation. To determine the cause, nature, and impact of the underlying changes in aluminum speciation, this study combines a multitechnique analysis that integrates basic characterization, a detailed synchrotron XRD and multiple‐quantum NMR spectroscopy assessment, and catalytic tests to correlate evolution of the properties with performance during successive steps in the preparation of hierarchical MFI‐type zeolites by desilication. The findings, subsequently generalized to FAU‐ and BEA‐type materials, identify the crucial impact of calcination on the protonic form, which is an integral step in the synthesis and regeneration of zeolite catalysts; on aluminum coordination; and the associated acidity trends.     

FT-IR study of hydrocarbon conversion on dealuminated HY zeolites in working conditions

The acidity of two dealuminated HY zeolites, obtained either by isomorphous substitution or by hydrothermal treatment at 550°C followed by acid leaching, was characterized by IR spectroscopy in dynamic or static modes. The probe molecules used were 2,6-dimethylpyridine, pyridine and deuterated acetonitrile. They showed that the steamed sample presented very strong acidic Brønsted sites, characterized by OH groups giving rise to a ν (OH) band at 3600 cm⁻¹, and Lewis acidity. The steamed sample was found active for n-hexane cracking at 400°C. Use of an IR cell working as a flow reactor and specific poisoning of hydroxyl groups by 2,6-dimethylpyridine evidenced the catalytic role played by the 3600 cm⁻¹ OH groups. On the other hand, cyclohexene conversion, a much less demanding reaction, occurred on both types of zeolite. All the accessible acidic hydroxyls appeared to be active and results were discussed according to the various reactions observed: cyclohexene isomerization, hydrogen transfer and cracking. The intensity of a band at 1586 cm⁻¹, assigned to carbonaceous deposits, was correlated to catalyst deactivation. The results showed that the 3600 cm⁻¹ OH groups were first consumed under cyclohexene flow, due to a rapid poisoning by coke. Adsorption of probe molecules on deactivated samples evidenced that the Lewis acid sites were almost unaffected whatever the reaction, suggesting that they do not play a significant role. This was confirmed by comparison of activity measurements on both types of zeolite towards cyclohexene conversion.     

Hierarchical ZSM‐5 Zeolites in Shape‐Selective Xylene Isomerization: Role of Mesoporosity and Acid Site Speciation

The isomerization of o‐xylene, a prototypical example of shape‐selective catalysis by zeolites, was investigated on hierarchical porous ZSM‐5. Extensive intracrystalline mesoporosity in ZSM‐5 was introduced by controlled silicon leaching with NaOH. In addition to the development of secondary porosity, the treatment also induced substantial aluminum redistribution, increasing the density of Lewis acid sites located at the external surface of the crystals. However, the strength of the remaining Brønsted sites was not changed. The mesoporous zeolite displayed a higher o‐xylene conversion than its parent, owing to the reduced diffusion limitations. However, the selectivity to p‐xylene decreased, and fast deactivation due to coking occurred. This is mainly due to the deleterious effect of acidity at the substantially increased external surface and near the pore mouths. A consecutive mild HCl washing of the hierarchical zeolite proved effective to increase the p‐xylene selectivity and reduce the deactivation rate. The HCl‐washed hierarchical ZSM‐5 displayed an approximately twofold increase in p‐xylene yield compared to the purely microporous zeolite. The reaction was followed by operando infrared spectroscopy to simultaneously monitor the catalytic performance and the buildup of carbonaceous deposits on the surface. Our results show that the interplay between activity, selectivity, and stability in modified zeolites can be optimized by relatively simple post‐synthesis treatments, such as base leaching (introduction of mesoporosity) and acid washing (surface acidity modification).     

Staining of fluid-catalytic-cracking catalysts: localising Brønsted acidity within a single catalyst particle

A time‐resolved in situ micro‐spectroscopic approach has been used to investigate the Brønsted acidic properties of fluid‐catalytic‐cracking (FCC) catalysts at the single particle level by applying the acid‐catalysed styrene oligomerisation probe reaction. The reactivity of individual FCC components (zeolite, clay, alumina and silica) was monitored by UV/Vis micro‐spectroscopy and showed that only clay and zeolites (Y and ZSM‐5) contain Brønsted acid sites that are strong enough to catalyse the conversion of 4‐fluorostyrene into carbocationic species. By applying the same approach to complete FCC catalyst particles, it has been found that the fingerprint of the zeolitic UV/Vis spectra is clearly recognisable. This almost exclusive zeolitic activity is confirmed by the fact that hardly any reactivity is observed for FCC particles that contain no zeolite. Confocal fluorescence microscopy images of FCC catalyst particles reveal inhomogeneously distributed micron‐sized zeolite domains with a highly fluorescent signal upon reaction. By examining laboratory deactivated FCC catalyst particles in a statistical approach, a clear trend of decreasing fluorescence intensity, and thus Brønsted acidity, of the zeolite domains is observed with increasing severity of the deactivation method. By comparing the average fluorescence intensities obtained with two styrenes that differ in reactivity, it has been found that the Brønsted acid site strength within FCC catalyst particles containing ZSM‐5 is more uniform than within those containing zeolite Y, as confirmed with temperature‐programmed desorption of ammonia.     

Comparison of the acidic properties of ZSM-5 zeolites isomorphously substituted by Ga,In, B and Fe

The acidic properties of isomorphously substituted MFI-type zeolites prepared and pretreated in the same way were studied to obtain an exact gradation of the intrinsic acidity caused by the incorporation of Ga, Fe, In and B instead of Al into the framework. The nature, strength and concentration of the acidic sites were studied by temperature-programmed desorption of ammonia (TPDA) and Fourier transform infrared spectroscopy. The assignment of the two peaks observed in the TPDA plots of all zeolite samples required the additional registration of the decomposition plot of the ammonium form in the cases of In-ZSM-5 and B-ZSM-5. A temperature program with two ramps and an intermediate isothermal hold was used to separate the low-temperature peak from the high-temperature peak. In this way, the Brönsted sites could be determined more clearly on samples containing a high portion of non-framework species that inhibit the egress of ammonia from the pores. The nature and the strength of the acidic sites were spectroscopically characterized by recording the stretching vibration of the bridged OH groups of Brönsted sites and the spectra of pyridine adsorbed on Brönsted and Lewis sites. The intrinsic acidity of the Brönsted sites on In-ZSM-5 could be clearly classified to be intermediate between the acid strength of B- and Fe-ZSM-5. A relation could be revealed between the frequencies of the OH-stretching vibration and the temperature of the peak maxima of ammonia desorption from the Brönsted sites of the isomorphously substituted ZSM-5 samples.     

Significant Influence of Zn on Activation of the C‐H Bonds of Small Alkanes by Brønsted Acid Sites of Zeolite

Herein, we analyze earlier obtained and new data about peculiarities of the H/D hydrogen exchange of small C₁–n‐C₄ alkanes on Zn‐modified high‐silica zeolites ZSM‐5 and BEA in comparison with the exchange for corresponding purely acidic forms of these zeolites. This allows us to identify an evident promoting effect of Zn on the activation of C? H bonds of alkanes by zeolite Brønsted sites. The effect of Zn is demonstrated by observing the regioselectivity of the H/D exchange for propane and n‐butane as well as by the increase in the rate and a decrease in the apparent activation energy of the exchange for all C₁–n‐C₄ alkanes upon modification of zeolites with Zn. The influence of Zn on alkane activation has been rationalized by dissociative adsorption of alkanes on Zn oxide species inside zeolite pores, which precedes the interaction of alkane with Brønsted acid sites.     

Catalytic activity of H-forms of zeolites in the isomerization of supercritical <Emphasis Type="Italic">n</Emphasis>-pentane and their physicochemical properties

The acidic properties of the H-forms of zeolites ZSM-5, Beta, Y, and mordenite are studied by diffuse reflectance IR spectroscopy using n-pentane as a probe molecule. The decreasing order of Brønsted acid site strengths is constructed. The isopentane selectivity in n-pentane isomerization under supercritical conditions (260°C, 130 atm) increases in the order H-ZSM-5 < H-Beta < H-mordenite(11) ≈ H-Y with decreasing strength of Brønsted sites. Catalytic data are analyzed jointly with the results of physicochemical studies of H-mordenite (temperature-programmed ammonia desorption, benzene adsorption, and IR spectroscopy). Under the supercritical conditions, the conversion of n-pentane on mordenite is determined by the total acidity of the zeolite and also by the accessibility of the acid sites inside the zeolite channels to the reactant.     

High Catalytic Performance of Mesoporous Dual Brønsted Acidic Ternary Poly (Ionic Liquids) for Friedel‐Crafts Alkylation

A newfangled cross‐linked dual Brønsted acidic ternary mesoporous poly (ionic liquids)(MPILs) with mesoporous structure was successfully synthesized with divinylbenzene as cross linker, 1‐vinyl‐3‐butyl imidazole bromide and sodium p‐styrene sulfonate as functional group through an ordinary post‐modification method and anion exchange process. A sponge‐like mesoporous tunnel structure was observed and the obtained P (BVS‐SO₃H)‐SO₃CF₃ sample appeared a relatively high thermal stability, a large surface area (up to 286.8 m²/g) and great pore volume (0.73 cm³/g). The abundant dual acidic group of sulfonic acid and trifluoromethanesulfonic acid of the composite in the polymer framework impart Brønsted acidity. For the sake of demonstrating our claims, the sample has been used as a novel solid acid catalyst for the reaction of alkylation of o‐xylene with styrene to 1‐diphenylethane (PXE). Under optimal reaction conditions (reaction under 120 °C for 3 hr, catalyst amount was 0.5 wt% of the reaction system, and the mass ratio of o‐xylene/styrene was 7.5:1, a 100% conversion of styrene and 93.7% PXE yield was acquired. After four times recycle, the yield remains 53.3%. Comparing with the commercial liquid acid catalyst, it processing a higher catalytic property and recyclability. Moreover, this fresh dual acidic heterogeneous catalyst owning a promising future applied in other acidic catalytic reactions and provide a new method to modify catalyst.     

Theoretical study of the adsorption of ethylene on alkali‐exchanged zeolites

The structures of alkali‐exchanged faujasite (X–FAU, X = Li⁺ or Na⁺ ion) and ZSM‐5 (Li–ZSM‐5) zeolites and their interactions with ethylene have been investigated by means of quantum cluster and embedded cluster approaches at the B3LYP/6‐31G(d, p) level of theory. Inclusion of the Madelung potential from the zeolite framework has a significant effect on the structure and interaction energies of the adsorption complexes and leads to differentiation of different types of zeolites (ZSM‐5 and FAU) that cannot be drawn from a typical quantum cluster model, H₃SiO(X)Al(OH)₂OSiH₃. The Li–ZSM‐5 zeolite is predicted to have a higher Lewis acidity and thus higher ethylene adsorption energy than the Li–FAU zeolites (16.4 vs. 14.4 kcal/mol), in good agreement with the known acidity trend of these two zeolites. On the other hand, the cluster models give virtually the same adsorption energies for both zeolite complexes (8.9 vs. 9.1 kcal/mol). For the larger cation‐exchanged Na–FAU complex, the adsorption energy (11.6 kcal/mol) is predicted to be lower than that of Li–FAU zeolites, which compares well with the experimental estimate of about 9.6 kcal/mol for ethylene adsorption on a less acidic Na–X zeolite. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 333–340, 2003     

Untersuchungen zur Acidität und katalytischen Spaltaktivität dekationisierter NaY-Zeolithe

Investigation on Acidity and Catalytic Activity of Deep-Bed Calcinated Zeolites NH₄ NaY NMR and infrared techniques are applied to decationated zeolites NaY to study Brönsted acidity. The results are compared with measurements of catalytic activity and crystallinity of this zeolites. The number of OH groups which are able to form a pyridinium ion (PyH⁺) increases with increasing exchange degree and with increasing temperature of the sample. The rate of pyridinium ion formation as an equivalent of Brönsted acidity and the catalytic activity increase similarly with increasing exchange degree up to such values where a loss of crystallinity occurs.     

The synthesis of metal–organic framework Al‐MIL‐53‐derived Brønsted acid catalyst and its application in the Mannich reaction

A metal–organic framework Al‐MIL‐53‐NH₂‐derived Brønsted acid catalyst (Al‐MIL‐53‐RSO₃H) has been synthesized employing a post‐synthetic modification strategy under mild conditions. The Al‐MIL‐53‐RSO₃H catalyst was successfully utilized in the nitro‐Mannich reaction taking advantage of its strong Brønsted acidity. Good to excellent yields of Mannich adducts were achieved for a variety of acylimine substrates in the presence of 0.1 mol% Al‐MIL‐53‐RSO₃H. Furthermore, the Al‐MIL‐53‐RSO₃H catalyst can be recycled five times without decreasing the yield and selectivity of Mannich adducts.     

Identity of two types of strong Brønsted acid sites in mazzite revealed by CO probe: IR study and periodic DFT modeling

The nature of catalytic Brønsted sites in mazzite is clarified by molecular modeling combined with spectroscopy. Density Functional Theory study for periodic models of high-silica mazzite evidence that most stable bridging hydroxyls, noticeably binding CO probe, fall into two categories: Brønsted sites located in larger channels, characterized by higher OH frequency of bare hydroxyl with very large redshift upon CO interaction, and lower-frequency sites located in smaller channels, showing lower redshift. This fully corresponds to two bands obtained for OH stretch in IR spectra. Very good agreement between theory and experiment found in this work not only confirms that Brønsted sites studied here belong to the strongest acid sites among known zeolites but also clarifies their identity in mazzite. Location of sites with exceptionally large red shift upon CO adsorption at 12-T wide channel very well conforms to both intuitive expectations and predictions for other zeolites from former studies.     

Novel Sol–Gel Synthesis of Acidic MgF2−x(OH)x Materials

Novel magnesium fluorides have been prepared by a new fluorolytic sol–gel synthesis for fluoride materials based on aqueous HF. By changing the amount of water at constant stoichiometric amount of HF, it is possible to tune the surface acidity of the resulting partly hydroxylated magnesium fluorides. These materials possess medium‐strength Lewis acid sites and, by increasing the amount of water, Brønsted acid sites as well. Magnesium hydroxyl groups normally have a basic nature and only with this new synthetic route is it possible to create Brønsted acidic magnesium hydroxyl groups. XRD, MAS NMR, TEM, thermal analysis, and elemental analysis have been applied to study the structure, composition, and thermal behaviour of the bulk materials. XPS measurements, FTIR with probe molecules, and the determination of N₂/Ar adsorption–desorption isotherms have been carried out to investigate the surface properties. Furthermore, activity data have indicated that the tuning of the acidic properties makes these materials versatile catalysts for different classes of reactions, such as the synthesis of (all‐rac)‐[α]‐tocopherol through the condensation of 2,3,6‐trimethylhydroquinone (TMHQ) with isophytol (IP).     

Elucidation of Adsorbate Structures and Interactions on Brønsted Acid Sites in H‐ZSM‐5 by Synchrotron X‐ray Powder Diffraction

Microporous H‐ZSM‐5 containing one Brønsted acid site per asymmetric unit is deliberately chosen to host pyridine, methanol, and ammonia as guest molecules. By using new‐generation in situ synchrotron X‐ray powder diffraction combined with Rietveld refinement, the slight but significant alteration in scattering parameters of framework atoms modified by the guest molecules enables the user to elucidate their adsorption geometries and interactions with the Brønsted acid sites in H‐ZSM‐5 in terms of atomic distances and angles within experimental errors. The conclusion, although demonstrated in the H‐ZSM‐5, is expected to be transferable to other zeolites. This approach provides a stepping stone towards the rational engineering of molecular interaction(s) with acid sites in zeolitic catalysis.