IR Spectroscopic Study of Alkane Interaction with the Brønsted Acid Sites of Hydrogen-Exchanged Zeolites

Diffuse-reflectance IR spectroscopy is used to study the interaction of C₃ and C₆ alkanes (propane, n-hexane, 3-methylpentane, and cyclohexane) with the Brønsted acid sites of hydrogen-exchanged mordenite, ferrierite, ZSM-5, and faujasite. It is found that a shift of the absorption band of the stretching vibrations of acidic Si(OH)Al groups toward lower frequencies (OH) due to the formation of a hydrogen bond with adsorbed alkanes increases in the following series: OH(propane) < OH(n-hexane) = OH(3-methylpentane). The accessibility of Si(OH)Al groups to alkane molecules is determined by the dimension of rings through which molecules enter zeolite channels and cavities. It follows from the measured OH values that the strength of Brønsted acid sites decreases in the following series: HZSM-5 > H-mordenite H-ferrierite HY. The difference between the three high-silica zeolites is not great. The results obtained are compared with the published IR data on Si(OH)Al groups of zeolites with adsorbed alkanes and other weak bases.     

Identifying the Role of Brønsted and Lewis Acid Sites in the Diels-Alder Cycloaddition of 2,5-DMF and Ethylene

The role of Lewis and Brønsted acid sites in the Diels-Alder cycloaddition (DAC) of ethylene to 2,5-dimethylfuran (2,5-DMF) to p-xylene was investigated. Amorphous silica catalysts containing Al³⁺ (ASA), Ga³⁺ (ASG), and In³⁺ (ASI) were prepared via homogeneous deposition-precipitation. Silica modified with Zr⁴⁺ (ASZ) was prepared by impregnation. Their acidic properties were characterized by various IR and NMR spectroscopic techniques. Measurements using pyridine as a probe molecule highlighted the presence of mostly Lewis acid sites (LAS) in all materials. Using CO as a probe, in contrast, demonstrated the existence of Brønsted acid sites (BAS) in ASA and ASG, which were nearly absent in ASI and ASZ. Differences in basic strength can explain the contrast in results observed between the two probe molecules. The highest p-xylene yield (~20 %) in the DAC reaction, could be achieved with ASA and ASG. The lack of BAS in ASI and ASZ resulted in inferior performance in the DAC, with p-xylene yields below 5 %. These results indicate the importance of BAS for the DAC reaction. Several other heterogeneous and homogeneous catalysts were explored for the DAC reaction to show the generality of our conclusion that BAS play a critical role in obtaining p-xylene from 2,5-DMF and ethylene.     

Brønsted Acid versus Phase-Transfer Catalysis in the Enantioselective Transannular Aminohalogenation of Enesultams

We have studied the enantioselective transannular aminohalogenation reaction of unsaturated medium-sized cyclic benzosulfonamides by using both chiral Brønsted acid and phase-transfer catalysis. Under optimized conditions, a variety of bicyclic adducts can be obtained with good yields and high enantioselectivities. The mechanism of the reaction was also studied by using computational tools; we observed that the reaction involves the participation of a conformer of the nine-membered cyclic substrate with planar chirality in which the stereochemical outcome is controlled by the relative reactivity of the two pseudorotational enantiomers when interacting with the chiral catalyst.     

Intramolecular cyclization of S-phenyl 3-arylpropynethioates by the action of Brønsted and Lewis acids

Russian Journal of Organic Chemistry -     

Steam-Induced Coarsening of Single-Unit-Cell MFI Zeolite Nanosheets and Its Effect on External Surface Brønsted Acid Catalysis

Commonly used methods to assess crystallinity, micro-/mesoporosity, Brønsted acid site density and distribution (in micro- vs. mesopores), and catalytic activity suggest nearly invariant structure and function for aluminosilicate zeolite MFI two-dimensional nanosheets before and after superheated steam treatment. Yet, pronounced reaction rate decrease for benzyl alcohol alkylation with mesitylene, a reaction that cannot take place in the zeolite micropores, is observed. Transmission electron microscopy images reveal pronounced changes in nanosheet thickness, aspect ratio and roughness indicating that nanosheet coarsening and the associated changes in the external (mesoporous) surface structure are responsible for the changes in the external surface catalytic activity. Superheated steam treatment of hierarchical zeolites can be used to alter nanosheet morphology and regulate external surface catalytic activity while preserving micro- and mesoporosity, and micropore reaction rates.     

Influence of the Br?nsted and Lewis Acid Sites in Platinum/Solid Acid Catalysts on the Hydrogenation of Benzene and Toluene

The hydrogenation of benzene and toluene was investigated over US-SSY, -Al₂O₃, and Ha1-MCM-41 supported platinum catalysts. The acidity of catalyst supports was measured by IR spectra of pyridine adsorption. On the Brönsted acid supported catalyst, both the hexacyclic saturation product and the pentacyclic one, the hydroisomerization product, were observed, and these two products were revealed to be primary reaction products. The two products were proposed to be formed competitively on Brönsted acid sites with carbonium ions as the reaction intermediate. By contrast, no hydroisomerization product was found on Lewis acid supported catalysts. It was inferred that the electron-deficient reaction intermediate formed on Lewis acids could not undertake any skeletal isomerization process to give a pentacyclic saturation product.     

A Supramolecular Model for the Co-Catalytic Role of Nitro Compounds in Brønsted Acid Catalyzed Reactions

Nitro compounds are known to change reaction rates and kinetic concentration dependence of Brønsted-acid-catalyzed reactions. Yet, no mechanistic model exists to account for these observations. In this work, an atomistic model for the catalytically active form for an alcohol dehydroazidation reaction is presented, which is generated by DFT calculations and consists of an H-bonded aggregate of two molecules of Brønsted acid and two molecules of nitro compound. The computed O−H stretching frequencies for the aggregate indicate they are stronger acids than the individual acid molecules and serve as predictors for experimental reaction rates. By applying the model to a chemically diverse set of potential promoters, it was predicted and verified experimentally that sulfate esters induce a similar co-catalytic effect. The important implication is that Brønsted-acid catalysis must be viewed from a supramolecular perspective that accounts for not only the pK_a of the acid and the bulk properties of a solvent, but also the weak interactions between all molecules in solution.     

Experimental and Computational Investigations of the Reactions between α,β-Unsaturated Lactones and 1,3-Dienes by Cooperative Lewis Acid/Brønsted Acid Catalysis

The reactions of α,β-unsaturated δ-lactones with activated dienes such as 1,3-dimethoxy-1-[(trimethylsilyl)oxy]-1,3-butadiene (Brassard's diene) are barely known in literature and show high potential for the synthesis of isocoumarin moieties. An in-depth investigation of this reaction proved a stepwise mechanism via the vinylogous Michael-products. Subsequent cyclisation and oxidation by LHMDS and DDQ, respectively, provided six mellein derivatives (30–84 %) and four angelicoin derivatives (40–78 %) over three steps. DFT-calculations provide insights into the reaction mechanism and support the theory of a stepwise reaction.     

Identification of Crucial Intermediates in the Formation of Humins from Cellulose-Derived Platform Chemicals Under Brønsted Acid Catalyzed Reaction Conditions

Humins are one of the undesirable products formed during the dehydration of sugars as well as the conversion of 5-hydroxymethylfurfural (HMF) to value-added products. Thus, reducing the formation of humins is an important strategy for improving the yield of the aforementioned reactions. Even after a plethora of studies, the mechanism of formation and the structure of humins are still elusive. In this regard, we have employed density functional theory-based mechanistic studies and microkinetic analysis to identify crucial intermediates formed from glucose, fructose, and HMF that can initiate the polymerization reactions resulting in humins under Brønsted acid-catalyzed reaction conditions. This study brings light into crucial elementary reaction steps that can be targeted for controlling humins formation. Moreover, this work provides a rationale for the experimentally observed aliphatic chains and HMF condensation products in the humins structure. Different possible polymerization routes that could contribute to the structure of humins are also suggested based on the results. Importantly, the findings of this work indicate that increasing the rate of isomerization of glucose to fructose and reducing the rate of reaction between HMF molecules could be an efficient strategy for reducing humins formation.     

Effect of addition of Lewis/Brönsted acids in the asymmetric aldol condensation catalyzed by trifluoroacetate salts of proline-based dipeptides

Proline-based dipeptides catalyze aldol condensations with good yield and stereoselectivity after addition of zinc or sodium acetate to the trifluoroacetate peptide. The chirality of the N-terminal proline in the catalyst determines the absolute configuration of the aldol product, but stereoselectivity depends on the configuration of both amino acids, and is higher for the enantiomeric pair R–S, S–R. Regarding the nature of the second component, optimal results in both yield and stereoselectivity are obtained when neutral unbranched primary amino acids are used.     

Brønsted Acid–Catalyzed Meyer–Schuster Rearrangement for the Synthesis of α,β-Unsaturated Carbonyl Compounds

An efficient and simple protocol for the Meyer–Schuster rearrangement of propargyl alcohols into α,β-unsaturated carbonyl compounds has been developed using catalytic amounts of p-toluenesulfonic acid in 1,2-dichloroethane.     

Reinterpretation of the Brønsted alpha for redox reactions based on the effect of substituents on hydride transfer reaction rates between NAD+ analogues

Rate and equilibrium constants have been determined spectrophotometrically for two sets of hydride transfer redox reactions between acridine and benzothiazoline derivatives and between pyridine and benzimidazoline derivatives that can be regarded as NAD+/NADH analogues. According to generally accepted ideas of the relation between equilibrium constants, K, and rate constants, k, these reactions would all have Br?nsted alpha values close to 0.5 since the equilibrium constants, K, for these reactions range from 10(-1) to 10(2). However, when the structural variation is in the hydride acceptor, the Br?nsted alpha is less than 0.5 (0.38 and 0.42, respectively), and when the structural variation is in the hydride donor, the Br?nsted alpha is greater than 0.5 (0.63 and 0.61, respectively) for the present systems. The Marcus theory of atom and group transfer can explain the difference of alpha values in terms of the tightness factor in the critical configuration. When the transition state is loose and symmetrical, the deviation of the Br?nsted alpha from 0.5 can be obtained by adding or subtracting a tightness factor that depends on the location of the substituents.     

Effect of Chemical Modification of the Tetrapyrrole Macrocycle Structure on the Spectral,Acid–Base,and Complexing Properties of tert-Butyl-Substituted Porphyrazines

Russian Journal of Organic Chemistry - The effect of chemical modification of the tetrapyrrole macrocycle structure on the spectral, acid–base, and complexing properties of...     

A Brønsted Acid Catalyzed Cascade Reaction for the Conversion of Indoles to α-(3-Indolyl) Ketones by Using 2-Benzyloxy Aldehydes

A Brønsted acid catalyzed, operationally simple, scalable route to several functionalized α-(3-indolyl) ketones has been developed and the long-standing regioisomeric issue has been eliminated by choosing appropriate carbonyls. A readily available and cheap bottle reagent was used as the catalyst. This protocol was also applicable to the synthesis of densely functionalized α-(3-pyrrolyl) ketones. A detailed mechanistic study confirmed the involvement of enolether as a reaction intermediate. Several postsynthetic modifications along with easy access to β-carboline, tryptamines, tryptophols, and spiro-indolenine proclaim the synthetic utility of this powerful building block. On the basis of this concept, functionalized carbazoles were constructed by a cascade annulation strategy.     

The Influence of Heteroatom M(M = B, Al, Ge, Ga, Fe)on the Adsorption Properties of M-ZSM-12 Zeolites

Heteroatom M-ZSM-12 zeolites(M=B, Al, Ge, Ga, Fe) are hydrothermally synthesized and it is proved that the heteroatom M is involved in the framework of synthesized molecular sieves by means of XRD, IR spectra. The results of adsorption and diffusion experiments indicate that the heteroatom M modifies the pore volume, specific surface area, and the channel of ZSM-12 molecular sieves.     

Brønsted Acidic Ionic Liquids: New,Efficient, and Green Promoter System for the Synthesis of 4(3H)-Quinazolinones

A simple, inexpensive, and efficient new synthesis of 4(3H)-quinazolinones from the reaction of 2-aminobenzamide with aroyl chlorides in the presence of two new halogen-free Brønsted acidic ionic liquids, 3-methyl-1-(4-sulfonic acid)butylimidazolium hydrogen sulfate [(CH₂)₄SO₃HMIM][HSO₄] and 1-(4-sulfonic acid) butylpyridinium hydrogen sulfate [(CH₂)₄SO₃HPY][HSO₄], green and reusable catalysts, with excellent product yields under solvent-free conditions is reported. The products could be separated simply from the catalyst, and the catalyst could be recycled without noticeably decreasing the catalytic activity.     

Effect of BaNH,CaNH,Mg3N2 on the activity of Co in NH3 decomposition catalysis

Development of active and non-noble metal-based catalyst for H2 production via NH3 decomposition is crucial for the implementation of NH3 as a H2 carrier.Co-based catalysts have received increasing attention because of its high intrinsic activity and moderate cost.In this work,we examined the effect of BaNH,CaNH and Mg3 N2 on the catalytic activity of Co in the NH3 decomposition reaction.The H2 formation rate ranks the order as Co-BaNH>Co-CaNH>Co-Mg3 N2≈Co/CNTs within a reaction temperature range of 300-550℃.It is worth pointing out that the H2 formation rate of Co-BaNH at 500℃reaches20 mmolH2 gcat-1 min-1,which is comparable to those of the active Ru/Al2 O3(ca.17 mmolH2 gcat-1 min1)and Ru/AC(21 mmolH2 gcat-1 min-1)catalysts under the similar reaction conditions.In-depth research shows that Co-BaNH exhibits an obviously higher intrinsic activity and much lower Ea(46.2 kJ mol-1)than other Co-based catalysts,suggesting that BaNH may play a different role from CaNH,Mg3 N2 and CNTs during the catalytic process.Combined results of XRD,Ar-TPD and XAS show that a[Co-N-Ba]-like intermediate species is likely formed at the interface of Co metal and BaNH,which may lead to a more energy-efficient reaction pathway than that of neat Co metal for NH3 decomposition.