Phenol alkylation with isobutene — influence of heterogeneous Lewis and/or Brønsted acid sites

Acidic solid catalysts with different types of acidity were used to study the liquid-phase alkylation of phenol with isobutene. A phosphonium ionic liquid immobilized on silica type carrier exhibiting pure Lewis acidity, Amberlyst 15 with pure Brønsted acidity as well as WO₃/ZrO₂ with both types of acid sites were used for this study. The active sites are postulated based on pyridine-FT-IR and NH₃-TPD studies, BET analyses, MAS NMR and XRD measurements. The different properties of the chosen catalysts are mirrored in the product distribution of the reaction mixture. It was found that WO₃/ZrO₂ is a very active and selective catalyst for the production of 2,4-di-tert-butylphenol under mild reaction conditions.     

Synthesis and application of a novel strong and stable supported ionic liquid catalyst with both Lewis and Brønsted acid sites

Poly(4-vinylpyridine)-supported ionic liquid with both Lewis and Brønsted acid sites was easily prepared from its starting materials and used as a novel and highly efficient heterogeneous catalytic system for the synthesis of biscoumarins by two-component one-pot domino Knoevenagel-type condensation/Michael reaction between various aliphatic and aromatic aldehydes with 4-hydroxycoumarin. The Lewis and Brønsted acidic sites loading in [P₄VPy-BuSO₃H]Cl-X(AlCl₃) were found to be 2.15 and 0.9 mmol per gram of catalyst, respectively. The effect of the simultaneous presence of Lewis and Brønsted acid sites was evaluated. The catalyst was characterized by Fourier transform infrared spectroscopy (FT-IR), thermal gravimetric analysis (TGA), scanning electron microscopy (SEM), elemental analysis, and atomic absorption technique. The catalyst is stable (as a bench top catalyst) and reusable.     

Lewis and Brønsted acid catalysis with AlMCM-41 and AlMMS: dependence on exchange cation

Mesoporous solid acid catalysts based on AlMCM-41 and AlMMS have been prepared. The two catalysts exhibit similar unidimensional pore structures with hexagonal symmetries. AlMMS shows less long-range order than AlMCM-41 but is considerably easier to synthesise. The catalytic activities have been measured and compared in the Lewis-acid-catalysed alkylation of toluene with benzyl chloride, and the Brønsted-acid-catalysed alkylation of toluene with benzyl alcohol. Activities have been measured for catalysts ion-exchanged with H⁺, Fe³⁺, Al³⁺ and Na⁺, and following thermal activation at temperatures of 150–350°C. They have also been compared with K10, a mesoporous acid-treated clay catalyst. Results show that the acid-treated clay is the most active of the three catalysts in both reactions. For all catalysts, the Fe³⁺ forms exhibit the highest Lewis acid catalytic activities, and the Al³⁺ and H⁺ forms show higher Brønsted acid activities. Infrared spectra of adsorbed pyridine show relative concentrations of Lewis and Brønsted acid sites consistent with this. Differences in the dependence of catalytic activities on thermal activation temperature are interpreted in terms of the hydration properties of the catalysts.     

The roles of BrØnsted and Lewis surface acid sites in acid-treated montmorillonite supported ZnCl2 alkylation catalysts

The catalyst known as clayzic (ZnCl₂ supported on acid-treated montmorillonite) exhibits both BrØnsted and Lewis surface acidities. The relative catalytic activities of these two types of acid site have been determined in a range of reactions, by comparing the activities of clayzic with those of related catalysts which exhibit only one type of surface acidity. Acid-treated montmorillonite has been used as the BrØnsted acid catalyst in this comparison, and ZnCl₂ supported on a mesoporous silica has been used as the Lewis acid catalyst. The results confirm that the relative activities of the two types of acid site depend on both the nature of the reaction and the polarity of the reaction medium. They will be of value in optimising the choice of support and the ZnCl₂ loading in clayzic-type catalysts for specific reactions.     

Density-Functional Theoretical Study on the Role of Lewis and Brønsted Acid Sites on CeO2(110) Surfaces for Nitrile Hydration

Hydration of cyanopyridine on CeO₂(110) surfaces was studied using periodic DFT+U calculations. One of two adsorption modes of 2-cyanopyridine occurs with two-point interaction which causes substrate specificity. A catalytic cycle for the hydration of 2-cyanopyridine was proposed. Cooperativity of Lewis and Brønsted acid sites was found to stabilize the intermediates for the hydration.     

Synthesis of ionones on solid Brønsted acid catalysts: Effect of acid site strength on ionone isomer selectivity

The effect of Brønsted acid site strength on the liquid-phase conversion of pseudoionone to ionone isomers (α-, β- and γ-ionone) was studied on resin Amberlyst 35W, silica-supported heteropolyacid (HPAS) and silica-supported triflic acid (TFAS). Catalyst acidity was probed by temperature-programmed desorption of NH₃ coupled with infrared spectra of adsorbed pyridine. The initial pseudoionone conversion rate followed the order: TFAS > Amberlyst 35W ≈ HPAS. Synthesis of the three ionone isomers occurred via a common cyclic carbocation intermediate formed from the activation of the pseudoionone molecule on Brønsted acid sites. Initial ionone mixtures containing a α:β:γ isomer distribution of about 40:20:40 were formed, irrespective of the acid site strength. But the ionone mixture composition changed with the progress of the reaction because γ-ionone was consecutively converted to α-ionone on HPAS and Amberlyst 35W, whereas the stronger acid sites of TFAS converted γ-ionone to β-ionone.     

Inulin conversion to hydroxymethylfurfural by Brønsted acid in ionic liquid and its physicochemical characterization

A simple conversion process of inulin polymer into hydroxymethylfurfural (HMF) was developed using Brønsted acid catalyst (HCl) in the presence of an ionic liquid, 1-octyl-3-methylimidazolium chloride ([OMIM]Cl). In addition, the physicochemical properties of inulin particle and its depolymerixation products were scrutinized. FESEM and XRD diffraction frequency showed that inulin particles are clustered in a granulated formation and their molecular structure is highly amorphous. FT-IR analysis identified five characteristic frequency regions: Region 1; 700–900, Region 2; 900–1,200, Region 3; 1,200–1,350; Region 4; 1,350–1,500, and Region 5; 1,530–1,800 cm^?1. HPLC analysis confirmed that the major composition of inulin consists of fructose and glucose. The synthesis of HMF was significantly affected by the Brønsted catalyst and its concentration. Its highest yield (63.1±5.1 dwt%) was achieved at 0.3M HCl in the presence of [OMIM]Cl. No presence of the Brønsted catalyst exhibited negligible HMF yield (2.3±1.1 dwt%). Our results demonstrate that the Brønsted catalyst plays a pivotal role in the catalytic process of HMF synthesis from inulin polymer.     

Discrepancy between TPD- and FTIR-based measurements of Brønsted and Lewis acidity for sulfated zirconia

Temperature-programmed desorptions (TPD) of isopropylamine (IPA), NH₃, and pyridine were compared with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of pyridine to determine the effect of H₂O on the Brønsted and Lewis acidities of two sulfated zirconia (SZ) catalysts. Although the traditional interpretation of pyridine infrared spectra showed an apparent increase in Brønsted acidity upon treating SZ with H₂O, TPD spectra showed that H₂O displaced IPA from approximately one-fifth of the Lewis sites with no corresponding increase in Bronsted acidity. Water treatment prior to TPD displaced similar amounts of both NH₃ and pyridine. The primary effect of H₂O is displacement of weakly adsorbed basic probe molecules from Lewis sites, rather than the conversion of Lewis sites to Brønsted sites. Finally, different types of analyses (e.g. infrared or TPD) of catalyst acidity yield dramatically different conclusions regarding Brønsted and Lewis acidity.     

Synthesis of a Novel Strong Brønsted Acidic Ionic Liquid and its Catalytic Activities for the Oxathioacetalization

The novel efficient procedure has been developed for the oxathioacetalization of carbonyl compounds and 2-mercaptoethanol at room temperature under solvent-free condition. The results showed that the novel multi-SO₃H functionalized ionic liquid was very efficient for the reactions with the good to excellent yields in short time. Operational simplicity, without need of any solvent, room temperature, low cost of the catalyst used, high yields, applicability to large-scale reactions, reusability and chemoselectivity are the key features of this methodology.     

Stability of Fe/ZSM‐5 de‐NOx catalyst: effects of iron loading and remaining Brønsted acid sites

The stability of Fe/ZSM‐5 de‐NOx catalyst has been investigated. The samples are prepared by sublimation of iron chloride. Substantial amount of protons are found to remain in the fresh catalyst after washing and calcination. After 10 h exposure to wet exhaust gas at 600^°C, the catalyst is severely deactivated. The presence of steam induces dealumination of the ZSM‐5 matrix, because the protons provide the point of attack by water. In addition, highly reactive distorted tetrahedral iron species and tetrahedral species change to less reactive octahedral iron ions or iron agglomerates upon aging treatment. With the second sublimation of iron chloride, the iron loading is increased and thus the concentration of remaining protons is reduced. Also, the catalyst turns out to preserve more reactive iron ions after aging treatment. In conclusion, the second sublimation is believed to bring about a remarkable improvement in the stability of the Fe/ZSM‐5 catalyst although its de‐NOx activity is slightly decreased. This revised version was published online in July 2006 with corrections to the Cover Date.     

Reactivity of Brönsted acid ionic liquids as dual solvent and catalyst for Fischer esterifications

Several water-stable ionic liquids with different acidity and affinity were synthesized and applied as both solvents and acid catalysts for Fischer esterification of ethanol reacting with four aliphatic carboxylic acids (acetic acid, n-hexanoic acid, lauric acid, and stearic acid). Among the studied ionic liquids, [(n-bu-SO₃H) MIM][HSO₄] (3-butyl-1-(butyl-4-sulfonyl) imidazolium sulfate) and [(n-bu-SO₃H) MIM][p-TSO] (3-butyl-1-(butyl-4-sulfonyl) imidazolium toluenesulfonate) show higher reactivity for the production of ethyl esters. The catalytic activities of these ionic liquids are strongly dependent on the acidity of their anions and cations, as well as their hydrophilicity and affinity with the reactants. Water refluxing through the condenser may be another important reason for obtaining high conversion of esterification, indicating a water-sequester process is still needed in order to obtain a higher yield of ester in the ionic liquid catalyzed esterification system. Kinetics studies show the conversions of the acids increase with reaction temperature and time, and reach equilibrium within about two hours. The apparent activation energies are 39.1±2.0, 49.7±2.5, 51.4±2.5 and 59.3±3.0 kJ·mol⁻¹ for the formation of ethyl acetate, ethyl n-hexanoate, ethyl laurate and ethyl stearate, respectively.     

Rapid generation of molecular complexity in the Lewis or Brønsted acid-mediated reactions of methylenecyclopropanes

Although they are highly strained, methylenecyclopropanes (MCPs) are readily accessible molecules that have served as useful building blocks in organic synthesis. MCPs can undergo a variety of ring-opening reactions because the release of cyclopropyl ring strain (40 kcal/mol) can provide a thermodynamic driving force for reactions and the π-character of the bonds within the cyclopropane can afford the kinetic opportunity to initiate the ring-opening. Since the 1970s, the chemistry of MCPs has been widely explored in the presence of transition metal catalysts, but less attention had been paid to the Lewis or Br?nsted acid mediated chemistry of MCPs. During the past decade, significant developments have also been made in the Lewis or Br?nsted acid mediated reactions of MCPs. This Account describes chemistry developed in our laboratory and by other researchers. Lewis and Br?nsted acids can be used as catalysts or reagents in the reactions of MCPs with a variety of substrates, and substituents on the terminal methylene or on the cyclopropyl ring of MCPs significantly affect the reaction pathways. During the past decade, we and other researchers have found interesting transformations based on this chemistry. These new reactions include the ring expansion of MCPs, cycloaddition reactions of MCPs with aldehydes and imines, cycloaddition reactions of MCPs with nitriles in the presence of strong Br?nsted acid, radical reactions of MCPs with 1,3-dicarbonyl compounds, intramolecular Friedel-Crafts reactions of MCPs with arenes, acylation reactions of MCPs, and the reaction of MCPs with 1,1,3-triarylprop-2-yn-1-ols or their methyl ethers. These Lewis or Br?nsted acid mediated reactions of MCPs can produce a variety of new compounds such as cyclobutanones, indenes, tetrahydrofurans, and tetrahydroquinolines. Finally, we have also carried out computational studies to explain the mechanism of the Br?nsted acid mediated reactions of MCPs with acetonitrile.     

Effect of cross-linking degree on proton conductivity of a Schiff-Base network impregnated with Brønsted acids

The cross-linking degree (crystallinity) and proton sources of conductors strongly impacts proton-conducting properties, but the attention is still limited, especially for covalent-organic frameworks. Here, we prepared a couple of cross-linking degrees of Schiff-Based Network by solvothermal (high surface specific area, SNW-S) and microwave-assisted (low cross-linking structure, SNW-M) strategies, respectively; subsequently, Brønsted acids (BAs)-entrapped Schiff-Based Network (SNW) composites as fillers are blended into polyvinyl pyrrolidone/poly(vinylidene fluoride) (PP) blends to form hybrid membranes. Varied crystalline degrees and microstructures of the SNWs are monitored by XRD, FTIR, and nitrogen adsorption, coupled with relevant conduction measurements like water uptake and proton conductivity (PC) for the composites and hybrid membranes. Structurally, under the aid of “acid–base pairs” between amine groups in Schiff-Based Network and Brønsted acids as proton carriers, the resulting H₂SO₄-incorporated SNW-M with poor cross-linking displays the high proton conductivity (4.96 × 10⁻³ S/cm) with an activation energy of 0.13 eV, whereas H₂SO₄ and H₃PO₄-incorporated SNW-S appear the relatively lower conductivities, imputable to the differences in the cross-linking degree of Schiff-Based Network and interactions between proton carriers and active sites in Schiff-Based Network. Additionally, the hybrid membranes also show σ values in the identical level of the composites, reaching to 8.02 × 10⁻³ S/cm at 323 K.     

Hydroisomerization of n-hexadecane over Brønsted acid site tailored Pt/ZSM-12

Hydroisomerization of n-hexadecane is performed over ZSM-12 framework having tailored Brønsted acidity to investigate the effect in terms of product selectivity and yield. For this purpose, pure phase of ZSM-12 (bulk molar ratio Si/Al ~ 60) has been synthesized using TEABr as a structure directing agent. The framework Brønsted acidity is tailored with group II elements (M) viz. Ca, Ba and Mg, by means of ion-exchange method. The samples so prepared have been characterized for phase purity, textural parameters, morphology by employing powder X-ray diffraction, nitrogen adsorption–desorption isotherm measurement at 77 K, and scanning electron microscopy technique, respectively. Similarly, % metal exchange is estimated using inductively coupled plasma technique. The quantification of Brønsted acidity for H⁺–M⁺⁺–ZSM-12 samples has been estimated by means of ammonia temperature programmed desorption (NH₃-TPD) and Fourier transform infrared spectroscopy of ammonia (NH₃-FTIR). The well characterized H⁺–M⁺⁺–ZSM-12 samples were loaded with Platinum (Pt, 0.5 wt%) and subjected to hydroisomerization of n-hexadecane using an up-flow fixed bed reactor to verify the effect of process parameters like temperature and WHSV. Pt/H⁺–Ba²⁺–ZSM-12 with tailored Brønsted acidity in the range of about 25 % demonstrated the optimum performance among all the catalysts with an increased isomer selectivity and yield (89.2 and 80.3 %, respectively) by about 4 wt% at a conversion level of about 90 % compared to Pt/H⁺–ZSM-12 framework at 568 K. Such enhancement in isomer selectivity and yield is found to be significant from commercial application point of view. Based on the obtained trend, the potential benefits of implementation of Pt/H⁺–Ba²⁺–ZSM-12 (bulk molar ratio Si/Al ~ 60) framework for cold flow property improvement of ‘bio-ATF’ have been envisaged.     

Synthesis of a silica-immobilized Brönsted acidic ionic liquid catalyst and hydrolysis of cellulose in water under mild conditions

A silica immobilized imidazolium-type acidic ionic liquid catalyst was shown to be a better catalyst than n-propylsulfonic acid silica (PrSO₃H-SiO₂) and sulfonic acid silica (SO₃H-SiO₂) for the hydrolysis of untreated Sigmacell Cellulose (DP ~ 450) in water. For example, new catalyst produced the highest TRS yield of 48.1% after 3 h at 190 °C, whereas cellulose samples heated with PrSO₃H-SiO₂ and SO₃H-SiO₂ catalysts produced only 19.9% and 13.2% TRS yields, respectively, under identical conditions. The new catalyst could be recycled up to four cycles with a small loss in catalytic activity.     

Brønsted acidity of silica silanol groups induced by adsorption of acids

Surface silanol groups of silica, which never revealed any Brønsted acidity, are shown to donate protons to adsorbed basic molecules, such as ammonia, pyridine or 2,6-dimethylpyridine, after addition of acids such as SO₂ or NO₂. The latter, when coadsorbed with bases, interact with the oxygen atoms of silanols leading to OH acidity increase and to protonation. Bases, in turn, enhance chemisorption of SO₂ or NO₂, and strongly held coadsorption products are formed as a result. The proposed mechanism of induced Brønsted acidity could account for the promoting effect of acidic gases in reactions catalysed by metal oxides.     

Palladium-catalyzed hydroesterification of olefins with isosorbide in standard and Brønsted acidic ionic liquids

The palladium catalyzed hydroesterification of 1-octene with isosorbide was studied in standard and Brønsted acidic ionic liquids as reaction media. High conversions and selectivities towards the targeted isosorbide diesters have been achieved by using a catalytic system composed of Pd(OAc)₂ and trisulfonated triphenylphosphine dissolved in 1-(4-sulfonic acid)-butyl-3-methylimidazolium tosylate. The catalytic phase can be reused several times without any significant decrease in activity and selectivity.