Characterization of acidity in [B], [Al], and [Ga] isomorphously substituted ZSM‐5: Embedded DFT/UFF approach

The structure and electronic properties of the Brønsted acid site in B, Al or Ga isomorphously substituted ZSM‐5 zeolites were studied by using quantum cluster and embedded ONIOM approaches. In the former approach, zeolites are modeled by 5T and 12T quantum clusters, where T represents a Si or Al atom. In the latter model, called “Embedded ONIOM”, the long‐range interactions of the zeolite lattice beyond the 12T quantum cluster is included via optimized point charges added to the ONIOM(B3LYP/6‐31G(d,p):UFF). Inclusion of the extended zeolitic framework covering the nanocavity has an effect on the structure and adsorption properties. We found that the OH distances and v OH of the acidic proton in zeolite obtained from both models can predict the trend of acid strength as: B‐ZSM‐5 < Ga‐ZSM‐5 < Al‐ZSM‐5, which is in very good agreement with the experimental sequence. Furthermore, the PA data calculated from E‐ONIOM is also consistent with the experimental trend: B‐ZSM‐5 < Ga‐ZSM‐5 < Al‐ZSM‐5. It has, therefore, been demonstrated that our embedded ONIOM model provides accurate performance and can be one of the useful and affordable methods for future mechanistic studies involving petrochemical reactions. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

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.     

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.     

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.     

Vapor‐Phase Reaction of Citronellal over Mesoporous Molecular Sieves MCM‐41 and Zeolites

The vapor‐phase reaction of citronellal (CTN) at 220 °C and atmospheric pressure has been studied using mesoporous molecular sieves and zeolites in a fixed‐bed reactor. The primary products included isopulegol (IPG), menthone, and pulegol with subsequent reactions to form cyclic hydrocarbons. The CTN conversion and the product selectivity depend on the acidity and the textural property of catalysts. Lewis and/or Brönsted acid sites are essential for catalyzing this reaction. An increase of SiO₂/Al₂O₃ mol ratio diminishes the acid amount of all catalysts and enhances both the surface area and the structural order of MCM‐41. The catalytic activity follows the order of MCM‐41 > HZSM‐5 > Hβ > USY, in accordance with the relative total acid amount except that of MCM‐41. Despite its low acidity, Si‐MCM‐41 exhibits the best catalytic performance due to its uniform mesopores, large surface area and good stability; the CTN conversion and the IPG yield attain 91.9% and 58.6%, respectively, after at least 25 h time‐on‐stream.     

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.     

Impacts of Composition and Post‐Treatment on the Brønsted Acidity of Steam‐Treated Faujasite: Insights from FTIR Spectroscopy

A detailed FTIR study of the effects of steaming and acid leaching on protonated Y faujasite (FAU) and EMT zeolites is provided and the results are thoroughly analysed. In particular, emphasis is placed on the Brønsted acidic evolution and acidic strength measurements for a large series of as‐modified zeolites using CO as a sensitive probe to distinguish various protonic sites. While an increase of acidity for framework OH groups is observed during the strengthening of dealumination for both FAU and EMT series, the steaming process also generates a large variety of additional Brønsted acidic groups. Regarding acidic strength, these heterogeneous OH groups are sensitive to post‐treatments and their existence strongly depends on the initial composition of the zeolites. The presence of residual Na⁺ cations in the starting materials induces dramatic Brønsted acidic changes after steaming. As a result, steamed zeolites that initially contain traces of sodium possess unusual acidic Brønsted groups with low acidity. This result contradicts the trend generally observed with framework OH groups, for which steaming results in an increase of Brønsted acidic strength. The study reveals that the situation is indeed more complex, as some compositions and post‐treatments strongly influence the Brønsted acidity of as‐steamed zeolites both in their nature and their corresponding acidic strength. By linking these IR‐compiled features to the as‐exposed modifications, a large acidity scale better suited to characterizing catalysts having Brønsted acidity expanding from lowest to highest strength is proposed.     

Thiiranation of 2′‐adamantylidene‐9‐benzonorbornenylidene using 4,4′‐oligothiodimorpholine and brønsted acid

On leaving 4,4′‐dithiodimorpholine 6 powder undisturbed at room temperature over 10 years, it led to the formation of 4,4′‐tetrathiodimorpholine 7 . Reactions of 2′‐adamantylidene‐9‐benzonorbornenyidene 1 with 6, 7 , and 4,4′‐thiodimorpholine 8 and a Brønsted acid in CH₂Cl₂ at room temperature proceeded to afford the corresponding thiiranes, 2 and 3 . The order of reactivity of 4,4′‐oligothiodimorpholines combined with a Brønsted acid is 7 > 6 > 8 . The thiirane 3 was transformed to 1 and 2 under the reaction conditions. © 2009 Wiley Periodicals, Inc. Heteroatom Chem 20:12–18, 2009; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20505     

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.     

Deactivation of Cu‐Exchanged Automotive‐Emission NH3‐SCR Catalysts Elucidated with Nanoscale Resolution Using Scanning Transmission X‐ray Microscopy

To gain insight into the underlying mechanisms of catalyst durability for the selective catalytic reduction (SCR) of NO_x with an ammonia reductant, we employed scanning transmission X‐ray microscopy (STXM) to study Cu‐exchanged zeolites with the CHA and MFI framework structures before and after simulated 135 000‐mile aging. X‐ray absorption near‐edge structure (XANES) measurements were performed at the Al K‐ and Cu L‐edges. The local environment of framework Al, the oxidation state of Cu, and geometric changes were analyzed, showing a multi‐factor‐induced catalytic deactivation. In Cu‐exchanged MFI, a transformation of Cu^II to Cu^I and Cu_xO_y was observed. We also found a spatial correlation between extra‐framework Al and deactivated Cu species near the surface of the zeolite as well as a weak positive correlation between the amount of Cu^I and tri‐coordinated Al. By inspecting both Al and Cu in fresh and aged Cu‐exchanged zeolites, we conclude that the importance of the preservation of isolated Cu^II sites trumps that of Brønsted acid sites for NH₃‐SCR activity.     

Inverse sandwich complexes based on low‐valent group 13 elements and cyclobutadiene: A theoretical investigation on E‐C4H4‐E (E = Al,Ga, In,Tl)

The chemistry of the low‐valent Group 13 elements (E = B, Al, Ga, In, Tl) has formed the recent hot topic. Recently, a series of low‐valent Group 13‐based compounds have been synthesized, i.e., [E‐Cp*‐E]⁺ (E = Al, Ga, In, Tl) cations, which have been termed as the interesting “inverse sandwich” complexes. To enrich the family of inverse sandwiches, we report our theoretical design of a new type of inverse sandwiches E‐C₄H₄‐E (E = Al, Ga, In, Tl) for stabilizing the low‐valent Group 13 elements. The calculated dissociation energies indicate that unlike [E‐Cp‐E]⁺ that dissociates via loss of the charged atom E⁺, E‐C₄H₄‐E dissociates via loss of the neutral atom E with the bond strengths of Al > Ga > In > Tl. Moreover, E‐C₄H₄‐E are more stable in dissociation than [E‐Cp‐E]⁺ cations. By comparing with other various isomers, we found that the inverted E‐C₄H₄‐E should be kinetically quite stable with the least conversion barriers of 33.5, 33.5, 35.2, and 36.9 kcal/mol for E = Al, Ga, In, and Tl, respectively. Furthermore, replacement of cyclobutadiene‐H atoms by the highly electron‐positive groups such as SiH₃ and Si(CH₃)₃ could significantly stabilize the inverted form in thermodynamics. Possible synthetic routes are proposed for E‐C₄H₄‐E. With no need of counterions, the newly designed neutral complexes E‐C₄H₄‐E welcome future synthesis. © 2012 Wiley Periodicals, Inc.     

B、Al、Ga同晶取代丝光沸石的从头计算

用从头计算Hartree Fock方法研究了B、Al、Ga等同晶取代进入丝光沸石骨架后可能存在的位置,确定了与电荷平衡质子结合的氧位置,考察了B、Ga等杂原子进入骨架对丝光沸石Brnsted酸性的影响.能量分析表明B、Al、Ga在丝光沸石骨架中最容易进入T3和T4位;当Al、Ga在T4位时,质子与O10结合为能量最低即最稳定结构,而当B在T4位时,质子与O2或O10结合比较稳定.质子亲合势分析表明与硅铝丝光沸石相比, B和Ga进入骨架导致丝光沸石分子筛的Brnsted酸性有所减弱,其酸性依次为：B ZSM 5Ga ZSM 5< Al ZSM 5.     

Thermodynamics of Carbon Dioxide Adsorption on the Protonic Zeolite H‐ZSM‐5

Adsorption of carbon dioxide on H‐ZSM‐5 zeolite (Si:Al=11.5:1) was studied by means of variable‐temperature FT‐IR spectroscopy, in the temperature range of 310–365 K. The adsorbed CO₂ molecules interact with the zeolite Brønsted‐acid OH groups bringing about a characteristic red‐shift of the O? H stretching band from 3610 cm^?1 to 3480 cm^?1. Simultaneously, the ν₃ mode of adsorbed CO₂ is observed at 2345 cm^?1. From the variation of integrated intensity of the IR absorption bands at both 3610 and 2345 cm^?1, upon changing temperature (and CO₂ equilibrium pressure), the standard adsorption enthalpy of CO₂ on H‐ZSM‐5 is ΔH⁰=?31.2(±1) kJ mol^?1 and the corresponding entropy change is ΔS⁰=?140(±10) J mol^?1 K^?1. These results are discussed in the context of available data for carbon dioxide adsorption on other protonic, and also alkali‐metal exchanged, zeolites.     

Effects of Spatial Constraints and Brønsted Acid Site Locations on para‐Terphenyl Ionization and Charge Transfer in Zeolites

The locations of Brønsted acid sites (BAS) in the channels of medium‐pore zeolites have a significant effect on the spontaneous ionization of para‐terphenyl (PP₃) insofar as spatial constraints determine the stability of transition states and charge‐transfer complexes relevant to charge separation. The ionization rates and ionization yield values demonstrate that a strong synergy exists between the H⁺ polarization energy and spatial constraints imposed by the channel topology. Spectroscopic and modeling results show that PP₃ incorporation, charge separation, charge transfer and charge recombination differ dramatically among zeolites with respect to channel structure (H‐FER, H‐MFI, H‐MOR) and BAS density in the channel. Compartmentalization of ejected electrons away from the initial site of ionization decreases dramatically the propensity for charge recombination. The main mode of PP₃^.+ decay is hole transfer to form AlO₄H^.+ ??? PP₃ charge‐transfer complexes characterized by intense absorption in the visible range. According to the nonadiabatic electron‐transfer theory, the small reorganization energy in constrained channels explains the slow hole‐transfer rate.     

Effect of Brønsted/Lewis Acid Ratio on Conversion of Sugars to 5‐Hydroxymethylfurfural over Mesoporous Nb and Nb‐W Oxides

A series of mesoporous Nb and Nb‐W oxides were employed as highly active solid acid catalysts for the conversion of glucose to 5‐hydroxymethylfurfural (HMF ). The results of solid state ³¹P MAS NMR spectroscopy with adsorbed trimethylphosphine as probe molecule show that the addition of W in niobium oxide increases the number of Brønsted acid sites and decreases the number of Lewis acid sites. The catalytic performance for Nb‐W oxides varied with the ratio of Brønsted to Lewis acid sites and high glucose conversion was observed over Nb₅W₅ and Nb₇W₃ oxides with high ratios of Brønsted to Lewis acid sites. All Nb‐W oxides show a relatively high selectivity of HMF , whereas no HMF forms over sulfuric acid due to its pure Brønsted acidity. The results indicate fast isomerization of glucose to fructose over Lewis acid sites followed by dehydration of fructose to HMF over Brønsted acid sites. Moreover, comparing to the reaction occurred in aqueous media, the 2‐butanol/H₂O system enhances the HMF selectivity and stabilizes the activity of the catalysts which gives the highest HMF selectivity of 52% over Nb₇W₃ oxide. The 2‐butanol/H₂O catalytic system can also be employed in conversion of sucrose, achieving HMF selectivity of 46% over Nb₅W₅ oxide.     

Protonation of an Oxo‐Bridged Diiron Unit Gives Two Different Iron Centers: Synthesis and Structure of a New Class of Diiron(III)‐μ‐hydroxo Bisporphyrins and the Control of Spin States by Using Counterions

Reported herein is a hitherto unknown family of diiron(III)‐μ‐hydroxo bisporphyrins in which two different spin states of Fe are stabilized in a single molecular framework, although both cores have identical molecular structures. Protonation of the oxo‐bridged dimer ( 2 ) by using strong Brønsted acids, such as HI, HBF₄, and HClO₄, produce red μ‐hydroxo complexes with I₃^? ( 3 ), BF₄^? ( 4 ), and ClO₄^? ( 5 ) counterions, respectively. The X‐ray structure of the molecule reveals that the Fe? O bond length increases on going from the μ‐oxo to the hydroxo complex, whereas the Fe‐O(H)‐Fe unit becomes more bent, which results in the smallest known Fe‐O(H)‐Fe angles of 142.5(2) and 141.2(1)° for 3 and 5 , respectively. In contrast, the Fe‐O(H)‐Fe angle remains unaltered in 4 from the corresponding μ‐oxo complex. The close approach of two rings in a molecule results in unequal core deformations in 3 and 4 , whereas the cores are deformed almost equally but to a lesser extent in 5 . Although 3 was found to have nearly high‐spin and admixed intermediate Fe spin states in cores I and II, respectively, two admixed intermediate spin states were observed in 4 . Even though the cores have identical chemical structures, crucial bond parameters, such as the Fe? N_p, Fe? O, and Fe???Ct_p bond lengths and the ring deformations, are all different between the two Fe^III centers in 3 and 4 , which leads to an eventual stabilization of two different spin states of Fe in each molecule. In contrast, the two Fe centers in 5 are equivalent and assigned to high and intermediate spin states in the solid and solution states, respectively. The spin states are thus found to be dependent on the counterions and can also be reversibly interconverted. Upon protonation, the strong antiferromagnetic coupling in the μ‐oxo dimer (J, ?126.6 cm^?1) is attenuated to almost zero in the μ‐hydroxo complex with the I₃^? counterion, whereas the values of J are ?36 and ?42 cm^?1, respectively, for complexes with BF₄^? and ClO₄^? counterions.     

Synthesis of ϵ‐Caprolactam from Cyclohexanone Oxime Using Zeolites Hβ, HZSM‐5, and Alumina Pillared Montmorillonite

The Beckmann rearrangement of cyclohexanone oxime (CHO) to ?‐caprolactam (?‐C) was studied in a plug flow reactor at 300–400°C under atmospheric pressure by using Hβ, ZSM‐5, and alumina pillared montmorillonite. With Hβ(X) Y zeolites, raising the SiO₂/Al₂O₃ molar ratio (X) results in the enhancement of catalyst acid strength with concomitant decrease of the total acid amount. In creasing the calcination temperature (Y) causes remarkable diminution of catalyst surface area, acid strength, and acid amount. A similar trend was found for AlPMY catalysts. In there action of CHO, the initial catalytic activity correlates well with the total acid amount of various catalysts except for Hβ(10) Y (Y > 600°C). The reaction proceeds on both Brönsted and Lewis acid sites and the catalyst deactivation most likely occurs at the strong Brönsted acid sites. The effect of solvents in the feed on the catalytic results was also investigated; it was found that polar solvents such as ethanol or n‐butanol give high ?‐C yield and longer catalyst life time. In the reaction of CHO/C₂H₅OH over Hβ(10)800 at 400°C and W/F 74.6 gh/mol, the CHO conversion and ?‐C yield remain 100% and 92%, respectively, for at least 20 h time‐on‐stream. The reaction paths and the mechanism for ?‐C formation are proposed.     

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.     

Brønsted Acidity and Aluminum Substitution Characteristics of Si(nAi) Sites in Zeolites: A Theoretical MNDO Calculation

The Brønsted acidity of the various Si(nAl) sites present in zeolites is evaluated from proton binding energy and LUMO energy calculated by the semiempirical MNDO quantum chemical method. The two calculated energy values both exhibit a linear correlation with the existing ²⁹Si NMR chemical shift and the IR hydroxyl stretching frequency data. The inter-convertibility between different Si(nAl) sites during an alumination or dealumination process is also evaluated based on the calculated substitution energy. The results indicate that alumination processes are less favorable to occur in zeolites than dealumination processes and the latter is more likely to occur for Si(nAl) clusters that contain the maximum number of aluminum nearest neighbors.     

Post‐Synthesis Stabilization of Germanosilicate Zeolites ITH,IWW, and UTL by Substitution of Ge for Al

Germanosilicate zeolites often suffer from low hydrothermal stability due to the high content of Ge. Herein, we investigated the post‐synthesis introduction of Al accompanied by stabilization of selected germanosilicates by degermanation/alumination treatments. The influence of chemical composition and topology of parent germanosilicate zeolites ( ITH , IWW , and UTL ) on the post‐synthesis incorporation of Al was studied. Alumination of ITH (Si/Ge=2–13) and IWW (Si/Ge=3–7) zeolites resulted in the partial substitution of Ge for Al (up to 80 %), which was enhanced with a decrease of Ge content in the parent zeolite. In contrast, in extra‐large pore zeolite UTL (Si/Ge=4–6) the hydrolysis of the interlayer Ge?O bonds dominated over substitution. The stabilization of zeolite UTL was achieved using a novel two‐step degermanation/alumination procedure by the partial post‐synthesis substitution of Ge for Si followed by alumination. This new method of stabilization and incorporation of strong acid sites may extend the utilization of germanosilicate zeolites, which has been until now been limited.