Solid Lewis Acid Research Articles

Fly ash-supported cerium triflate as an active recyclable solid acid catalyst for Friedel–Crafts acylation reaction

An efficient solid Lewis acid, has been synthesized by loading cerium triflate (7 wt%) on the acid activated fly ash with high silica content (81%). The physico-chemical properties of synthesized fly ash-supported cerium triflate catalyst (CFT) were monitored by XRD, FT-IR spectroscopy, FT-IR spectroscopy of the ammonia adsorbed catalyst, SEM-EDAX, TEM, Flame Atomic Absorption Spectrophotometer and TG-DTA study. The increased concentration of silica surface hydroxyl groups on activated fly ash have a major influence on the loading of cerium triflate. The catalytic activity of the catalyst CFT was tested in the acylation of veratrole using acetic anhydride as the acylating agent. The proposed model structure of CFT shows that the triflate species withdraws the electron density from the surface cerium making it electron deficit and generate Lewis acidity on the surface of fly ash as confirmed by NH 3 adsorbed FT-IR spectrum. The activity data indicate that this heterogeneous catalyst is very active, corresponding to high conversion (88%) of veratrole to 3,4-dimethoxyacetophenone. The catalyst could be easily recovered and reused giving similar conversion up to three reaction cycles indicating its stability under experimental conditions. Thus fly ash-supported cerium triflate is a novel and efficient catalyst and is a promising way of bulk utilization of waste fly ash by developing cost effective catalyst system for industrially important Friedel–Crafts acylation reactions.

Protonated Titanate Nanotubes as Solid Acid Catalyst

Protonated titanate nanotubes are demonstrated to function as a highly active solid Lewis acid catalyst even near room temperature. The high catalytic activity for the reaction can be attributed to the unique nanotube structure, which contains both Brønsted and Lewis acid sites.

Water Resistant, Catalytically Active Nb and Ta Isolated Lewis Acid Sites, Homogeneously Distributed by Direct Synthesis in a Beta Zeolite

Nb and Ta solid Lewis acids able to work in the presence of water have been prepared by incorporating these elements in the framework of zeolite Beta by a direct synthesis procedure. Characterization of the samples by extended X-ray absorption fine structure and X-ray absorption near edge structure indicate that the calcined samples contain tetracoordinated Nb5+ (or Ta5+) with three siloxane bridges and one Nb═O (or Ta═O) bonds. The presence of Lewis acidity has been characterized by IR of adsorbed pyridine and cyclohexanone. The Nb- and Ta-Beta samples are catalytically active for the Meerwein−Ponndorf−Verley reduction of cyclohexanone with 2-butanol and the etherification of 2-butanol with p-methoxybenzalcohol. Both reactions can be successfully combined in a domino reaction to obtain the ether starting with p-methoxybenzaldehyde. The catalytic activity of Nb- and Ta-Beta has been compared with that of the homogeneous counterparts (NbCl5 and TaCl5) reported in the literature for the intramolecular carbo...

A Highly Efficient and Environmentally Friendly CMC‐Supported Lanthanide Catalyst for One‐pot Synthesis of Substituted Imidazoles

AbstractA CMC‐supported lanthanide catalyst was prepared from CMC and isopropoxy lanthanide. This solid Lewis acid catalyst was used in synthesis of substituted imidazoles under mild conditions, showing catalytic activity comparable to the expensive lanthanide Lewis acid Ln(OTf)3. Furthermore, the catalyst could be readily recycled and reused for five times without any appreciable loss in catalytic activity.

Supported high surface AlF3: a very strong solid Lewis acid for catalytic applications

A series of HS-AlF3/Al2O3 with different HS-AlF3 loading was prepared by impregnation method using aluminium isopropylate in isopropyl alcohol as a solvent and non-aqueous HF solution. Post-fluorination of the supported AlF3 precursor was carried out under moderate conditions with CHClF2 or HF. The catalytic activity of the supported HS-AlF3 was assessed by study of the CHClF2 dismutation and CBrF2CBrFCF3isomerization reactions. All the supported HS-AlF3 samples were found to be effective for CHClF2 dismutation, whereas HS-AlF3/γ-Al2O3a exhibited superior activity, which was about 10–30 times higher than that of HS-AlF3/α-Al2O3, in terms of isomerization activity.

A General Method for the Preparation of Ethers Using Water‐Resistant Solid Lewis Acids.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

A General Method for the Preparation of Ethers Using Water‐Resistant Solid Lewis Acids

A General Method for the Preparation of Ethers Using Water‐Resistant Solid Lewis Acids

A General Method for the Preparation of Ethers Using Water‐Resistant Solid Lewis Acids

This most ethereal of cascades: Water-resistant single isolated Lewis acids within the framework of molecular sieves act as excellent general catalysts for the synthesis of ethers (see scheme). On this basis, an environmentally friendly process has been developed for the preparation of fine chemicals that involves a one-pot Meerwein–Ponndorf–Verley reduction/etherification cascade.

Water-resistant Lewis-acid sites: carbonyl-ene reactions catalyzed by tin-containing, hydrophobic molecular sieves

Two solid Lewis acids, namely Sn-Beta and Sn-MCM-41, have been applied to the Carbonyl- Ene reaction of β-pinene and α-methylstyrene with paraformaldehyde. For the more bulky molecule β-pinene the mesoporous material Sn-MCM-41 has been found to be the better catalyst whereas for the smaller α-methylstyrene Sn-Beta has shown a higher activity. Best selectivities have been observed in nitrile solvents, such as acetonitrile and butyronitrile, whereas reactions in toluene or dioxane have been less selective. The catalysts are water resistant what has been evidenced by addition of up to 200 wt% (with respect to the catalyst) of water without loss of activity or selectivity. Sn-MCM-41 has been employed in several cycles with intermediate regeneration, always recovering the high activity.

Continuous reactions in supercritical fluids; a cleaner, more selective synthesis of thymol in supercritical CO2

Continuous fixed-bed catalytic Friedel–Crafts alkylation of m-cresol with different alkylating agents, isopropanol (IPA) and propylene, has been carried out using supercritical CO2, scCO2, as an alternative and more environmentally friendly reaction medium, for the synthesis of the fine chemical thymol. Both a solid Lewis acid catalyst (γ-Al2O3) and a solid Bronsted acid catalyst (Nafion® SAC-13) have been investigated over a range of reaction conditions to optimise yield and selectivity for thymol. The reaction product distribution was found to be related to the type of catalyst employed. This is likely to have been due to the different reaction pathways through which the reaction occurred, a direct Friedel–Crafts alkylation in the case of Bronsted type acids and a Fries rearrangement when employing the Lewis catalyst. The new technique of 2DCOR-GC analysis was employed to establish the order of formation of the different species generated in the reaction over the two catalysts in scCO2.

Novel heterogeneously catalysed selective, solventless monotetrahydropyranylation of different symmetrical 1,n-diols and various alcohols using ZrO 2-pillared clay

ZrO 2-pillared clay (Zr-PILC) a mild recyclable solid Lewis acid catalyst prepared by a short and simple novel method by us, has been used for the selective, monotetrahydropyranylation under solvent-free condition both by heating and microwave irradiation of 1,n-symmetrical diols and simple alcohols with good selectivity and conversion in high yields for the first time. HO−( CH 2) n− OH+ DHP → MW Zr-PILC HO−( CH 2) n− OTHP A comparative study with previously reported methods and other acid catalysts has been done to demonstrate the efficacy of the present developed methodology.

Involvement of solid acid on Al- and Ga-doped porous silica in the Diels–Alder reaction

The Diels–Alder reaction of 2,3-dimethylbutadiene with p-benzoquinone using Al- and Ga-doped porous silica, i.e., MCM-41, zeolite, and amorphous silica, was studied focusing on individual effects of types, strength, and amount of acid sites and pore size of solid acid catalysts. MCM-41 and amorphous silica-based catalysts exhibited high activity for the Diels–Alder reaction, while zeolites only showed low activity. Comparison with IR spectra of adsorbed pyridine clearly indicates that solid Lewis acid sites are effective for the reaction. Turnover numbers of each catalyst were compared based on the number of total acid sites measured by NH 3-temperature programmed desorption (NH 3-TPD). Al-MCM-41 and Ga-MCM-41 showed higher turnover numbers than that of AlCl 3, which is a typical homogeneous Lewis acid catalyst. The catalysts having stronger acid sites basically showed higher turnover numbers in the Diels–Alder reaction. The effect of pore size was examined by comparison of Ga-MCM-41 and H-type mordenite (H-MOR) evacuated at 973 K. Through dehydration of H-MOR by evacuation at 973 K, Brønsted acid sites of H-MOR were converted to Lewis acid sites of which acid strength is comparable to Ga-MCM-41. Based on the number of Lewis acid sites, Ga-MCM-41 and H-MOR after evacuation at 973 K showed the same turnover number, although they have different pore sizes. These results clearly demonstrate that the catalytic activity depends on strength and amount of Lewis acid sites, while there is not much effect of pore size on the catalytic activities of the Diels–Alder reaction of 2,3-dimethylbutadiene with p-benzoquinone.

Simulation and Design of a Non-adiabatic Multiphase Microreactor

We have identified a novel, multiphase microreactor configuration for the direct fluorination of toluene. With this reactor the fluorine and toluene are separated by a macroporous membrane impregnated with a solid Lewis acid catalyst, which influences the fluorine directly when being absorbed into the liquid phase. Reaction occurs in the membrane adjacent to the gas-phase. According to the literature fluorine is turned into an electrophile in the presence of acidic catalysts improving the selectivity towards monofluorinated products. The direct fluorination of toluene is a very fast and exothermic reaction and therefore heat management is of great importance to avoid thermal runaway. Also, the selectivity of the process deteriorates at elevated temperatures due to increasing radical reactions.To address this issue a detailed model of the heat and mass transfer has been developed. The catalytic membrane is modelled as an idealised system and the porosity, tortuosity and catalytic activity are parameters in a sensitivity analysis to obtain bench-mark values for the reactor performance and the membrane design. The software package FEMLAB has been employed to solve the partial differential equations simultaneously by means of the finite element technique.The performance of the membrane reactor configuration is compared to a falling film microreactor. It is shown that the membrane reactor has great potential to improve the selectivity of the direct fluorination reaction towards mono-fluorinated products by means of a thin solid acid catalyst layer. The influence of the reactor size and the aspect ratio on the temperature in the reaction plane and hot spot formation is studied. With the falling film reactor rising channel size results in increasingly prominent hot spot formation and the selectivity towards monofluorinated products deteriorates. The membrane supports the heat removal because the reaction plane is located inside the solid grid. In this manner the membrane catalyst increases the selectivity of the process and also shifts the optimal operation temperature to higher values towards room temperature. Higher production rate per channel can be achieved requiring less cooling.

Structural insights into aluminum chlorofluoride (ACF).

The structure of the very strong solid Lewis acid aluminum chlorofluoride (ACF, AlCl(x)F(3-x), x = 0.05-0.3) was studied by IR, ESR, Cl K XANES, (19)F MAS NMR, and (27)Al SATRAS NMR spectroscopic methods and compared with amorphous aluminum fluoride conventionally prepared by dehydration of alpha-AlF(3) x 3H(2)O. The thermal behavior of both compounds was investigated by DTA and XRD. In comparison to ACF, amorphous AlF(3) prepared in a conventional way is not catalytically active for the isomerization reaction of 1,2-dibromohexafluoropropane, which requires a very strong Lewis acid. Both compounds are mainly built up of corner-sharing AlF(6) octahedra forming a random network. The degree of disorder in ACF is higher than in amorphous AlF(3). Terminal fluorine atoms were detected in ACF by (19)F NMR. The chlorine in ACF does not exist as a separate, crystalline AlCl(3) phase. Additionally, chlorine-containing radicals, remaining from the synthesis, are trapped in cavities of ACF. These radicals are stable at room temperature but do not take part in the catalytic reaction.

Water-resistant solid Lewis acid catalysts: Meerwein–Ponndorf–Verley and Oppenauer reactions catalyzed by tin-beta zeolite

The catalytic activity of Sn-beta zeolite in the Meerwein–Pondorf–Verley (MPV) reduction of carbonyl compounds with secondary alcohols as reductants and Oppenauer oxidation of alcohols were performed with quantitative yields to the corresponding product. This catalyst exhibits a behavior not observed with Ti- and Al-beta zeolites, with excellent activity and selectivity even after four catalytic recycles, and good stereoselectivities to the thermodynamic less favorable cis-alcohol isomer when alkyl-cylohexanones were used as substrates. A prochiral ketone has been reduced within an enantiomeric excess close to 50% when using a chiral alcohol as the reducing reactant. Other transition metals incorporated in the beta zeolite framework (V or B) led to poor catalytic activities for this reaction. IR studies using cyclohexanone as probe molecule over beta zeolites demonstrated that the more specific Lewis acid sites in the framework of Sn-beta are responsibles for its better catalytic activity with respect to Ti- or Al-beta. The influence of the structure of the alcohol and ketone shows that reaction transition states that by size can be fitted within the pore of beta may have problems forming on the Lewis acid sites due to the shielding effect of the neighbor framework oxygen atoms. Finally, Sn-beta containing organic groups attached to the structure is able to perform the MPV in the presence of water, offering a real alternative to conventional aluminum iso-propoxide catalysts.

Catalysts from MCM-41: framework modification, pore size engineering, and organic–inorganic hybrid materials

The mesoporous material, MCM-41, is characterized by an extremely high surface area and a one-dimensional system of cylindrical pores with a narrow size distribution in the mesopore range. Adjustment of the pore size over the range 2.0–4.0 nm can be easily achieved by selection of a templating agent. However, the material has little intrinsic catalytic activity. In order to take advantage of the unique opportunities inherent in the defined porosity, catalytic activity has to be introduced. This can be done by inorganic ions, or by affixing organic/metal-organic active groups on the material. We describe catalysts for high temperature gas-phase reactions where functionalization has been achieved by the introduction of framework alumina and platinum. For liquid-phase reactions, we developed solid bases as well as solid Lewis acids based upon modified MCM-41. Through pore size engineering, i.e. matching the pore size to the size of the reacting molecules, the selectivity of the catalyst can be increased, as shown for the preparation of linear alkyl benzenes.

Anhydrous zirconium(IV) sulfate and tin(IV) sulfate: solid Lewis acid catalysts in liquid-phase hydro-acyloxy-addition reactions?

Anhydrous zirconium(IV) and tin(IV) sulfate [Zr(SO4)2 and Sn(SO4)2] were prepared from their corresponding metal tetrachlorides and fuming H2SO4/10% SO3. Properties of interest for their potential use as heterogeneous Lewis acid catalysts, such as their susceptibility to undergo hydrolysis, are studied. In the presence of water the metal sulfates are converted into their metal oxides (ZrO2 ·2H2O and SnO2·2H2O) and the Brønsted acid H2SO4, which can also be catalytically active. This is illustrated by the solvent-free liquid-phase hydro-acyloxy-addition of acetic acid (2) to camphene (1) (molar ratio 1: 2 = 1:1) giving the fragrance isobornyl acetate (3). Both anhydrous Zr(SO4)2 and Sn(SO4)2 are inactive as heterogeneous solid Lewis acid catalysts; no conversion of 1 into 3 is found. Upon addition of water, however, 1 is efficiently converted into 3. Moreover, the conversion of 1 into 3 continues even after removal of the solid Lewis acid catalyst by filtration. These results show that after addition of water the reaction becomes homogeneously catalysed by dissolved H2SO4.

Ligand control of the catalytic activities of Al 3+-immobilized solid Lewis acids

Treatment of anthracenebisresorcinol 1 (a tetraol) with Al(CH 3) 2Cl or Al(O iPr) 3 affords an amorphous solid formulated as 1 4−·2[AlCl] 2+ or 1 4−·2[AlO iPr] 2+ ( 1 4− is the deprotonated tetraanionic species of host 1). These solids (AlCl and AlO iPr hosts) as recoverable and reusable Lewis acids catalyze the Diels–Alder reaction of acrolein with 1,3-cyclohexadiene, where the former (AlCl) at 3 mol% exhibits a much higher catalytic activity with a halflife of the reaction of τ=1 min than the latter (AlO iPr) with τ=80 min. The efficient solid-state catalysis by the AlCl host with a turnover rate of 30/min is based on the combination of space vacancy (microporosity) and coordination vacancy (coordinative unsaturation) as revealed by 27Al MAS NMR spectroscopy.

Heterogeneous catalysis for the acetylation of benzo crown ethers

Heterogeneous catalytic acetylation of benzo-15-crown-5 was investigated using solid Lewis acids consisting of sulfated zirconia calcined at 625°C or K10 exchanged by Cu, Zn, Fe or Sn ions and treated at 250°C. The nature of the transition metal ions introduced in the mesoporous clay plays an important role in the activity of the catalysts. Sn-exchanged K10 shows the best catalytic properties and could suit for practical preparative purposes. 4′-Acetyl-benzo-15-crown-5 was produced with 90% yield under optimal conditions.

ChemInform Abstract: Novel Oxidative Coupling of 2‐Naphthols to 1,1′‐Bi‐2‐naphthols Catalyzed by Solid Lewis Acids Using Atmospheric Oxygen as Oxidant.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.