3.2. One-Step Sol-Gel Method 2187 4. Structural Characterization 2188 4.1. XRD Studies 2188 4.2. FTIR Studies 2189 4.3. Raman Studies 2189 4.4. TG and DTG Analysis 2190 4.5. SEM and TEM 2190 5. Structure of Sulfated Zirconia 2191 6. Superacidity and Measurement 2192 6.1. Uncertainties about Superacidity 2192 6.2. Acidity Measurement 2193 7. Application of Sulfated Zirconia Catalysts 2193 7.1. Biginelli Reaction 2193 7.2. Knoevenagel Condensation 2194 7.3. Synthesis of Formamidine 2194 7.4. Acylation of Aromatic Compounds 2194 7.4.1. Acylation of Benzene 2194 7.4.2. Acylation of Substituted Benzenes 2194 7.4.3. Acylation of Naphthalenes and Anthracenes 2195

Organic syntheses and transformations catalyzed by sulfated zirconia.

Mesoporous silica-supported 12-tungstophosphoric acid catalysts for the liquid phase dehydration of d-xylose

A new efficient, mild and practical method for reduction of aromatic nitro compounds employing NaBH4/Raney nickel system is reported. The method is simple, inexpensive, easily scaled-up and applicable for large scale preparation of different substituted anilines as well as other heterocyclic aryl amines.

Rapid, efficient and selective reduction of aromatic nitro compounds with sodium borohydride and Raney nickel

A fluorescent probe capable of detecting H2S at submicromolar concentrations in cells

This critical review addresses heterogeneous catalysis in systems where multiple liquid phases coexist and where one of the phases is catalyst-philic. This technique provides built-in catalyst separation, and product recovery for organic reactions. Focus is placed on the components of the multiphasic systems with emphasis on the constituents of the catalyst-philic phases (PEGs, onium salts, ionic liquids) that incorporate the catalysts, as well as on the effects on catalytic efficiency. It collects a wide body of scattered information that is often labelled with different terms.

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Multiphasic heterogeneous catalysis mediated by catalyst-philic liquid phases

The selective reduction of nitroaromatics to the corresponding amines is an important transformation since many aromatic amines exhibit biological activities and find a multitude of industrial applications, being intermediates for the synthesis of dyes, pharmaceuticals and agrochemicals. A variety of nitroaromatics dissolved in organic solvents was reduced by using aqueous sodium sulfide, and tetrabutylammonium bromide (TBAB) as the phase transfer catalyst by choosing appropriate concentrations which resulted in three immiscible liquid phases. Compared to L-L PTC, the L-L-L PTC offers much higher rates of reaction, better selectivities and repeated use of catalyst. The selectivities for the desired products were 100%.

Liquid‐Liquid‐Liquid Phase Transfer Catalysis: A Novel and Green Concept for Selective Reduction of Substituted Nitroaromatics

Reactions in three immiscible liquid phases (L–L–L) are attractive, and one of the phases can be the locale of reaction which will have a dramatic effect on product distribution in complex reactions. Thus, converting a bi-liquid system into a tri-liquid phase is of considerable scientific and commercial interest. 4-Benzyloxy-vanillin is used as a perfume and also as a starting material for synthesis of thalifoline and ephedradine as alkaloids and in synthesis of flavonoid compounds. Etherification of vanillin with benzyl chloride under biphasic phase transfer catalysis leads to formation of 4-benzyloxy-vanillin, but selectively suffers due to side reactions. Waste minimization is a major theme of green chemistry. In the traditional liquid–liquid phase transfer catalysis, the catalyst is not recovered but disposed off causing load on environment. However, the transformation of two-liquid phases into three-liquid phases L–L–L, PTC leads to 100% conversion of the limiting reactant benzyl chloride with 100% selectivity to 4-benzyloxy-vanillin, using TBAB as a catalyst. The rates of reactions are very high under L–L–L PTC, and reaction can be completed within 1 h as against 8 h required in L–L PTC. The catalyst-rich middle phase is recycled many times, thereby leading to profitability. The current work deals with effect of different kinetics and process parameters leading to enhancement in rates and selectivities and greener aspects of phase transfer catalysis.

Novelties of reaction in the middle liquid phase in tri-liquid phase transfer catalysis: Kinetics of selective O-alkylation of vanillin with benzyl chloride

Reactions in three immiscible liquid phases are attractive, and one of the phases can be the locale of the reaction, which will have a dramatic effect on the product distribution in complex reactions. Thus, converting a bi-liquid (L-L) system into tri-liquid (L-L-L) phases is of considerable scientific and commercial interest. Such systems encounter mass-transfer resistances for transfer across two interfaces and the kinetic analysis becomes difficult. In the case of phase transfer catalysis (PTC), a majority of preparatory and industrial reactions are conducted in two phases. The L-L PTC reactions are conducted under milder conditions, using less-expensive solvents at much faster reaction rates and improved selectivities to desired products. However, the phase transfer catalyst is not recovered but treated as waste, because the quantities are very small and do not contribute much to the expensive product. L-L PTC can be elegantly modified to convert it to L-L-L PTC, to recover and reuse the catalyst and also to enhance selectivity, thereby improving profitability and environmental benefits. 3,4-Dimethyl aniline is a useful starting material for the production of riboflavin (vitamin B2) and also is used as a precursor for many drugs. It can be produced via the reduction of 4-nitro-o-xylene in toluene with aqueous sodium sulfide, using tetrabutylammonium bromide (TBAB) as a catalyst under L-L-L PTC at 80 °C. Comparison between tri- and bi-liquid phases was also conducted, and it was observed that, under L-L-L conditions, the rates of reaction of the formation of 3,4-dimethyl aniline had been dramatically enhanced. The kinetics and mechanism of complex L-L-L PTC processes have been explored in detail. The yield, based on the initial amount of reactant, was >95%, and the selectivity was 100%.

Intensification of rates and selectivity using tri-liquid versus Bi-liquid phase transfer catalysis : Insight into reduction of 4-nitro-o-xylene with sodium sulfide

Borohydride reductions, although common, have not been studied from kinetic and mechanistic view points due to complex chemistry and more so, there is no report on kinetics and mechanism of such reductions under phase transfer catalysis. The novelties of phase transfer catalysed chemoselective reduction of citronellal to citronellol using sodium borohydride were studied in detail in liquid–liquid phase transfer catalysis using tetrabutylammonium bromide (TBAB), tetrapropylammonium bromide (TPAB), tetraethyl ammonium bromide (TEAB) and tetrabutylammonium hydrogen sulfate (TBAHS). TBAB was the best catalyst at 30 °C. Citronellol is very extensively used in a number of perfume compositions. The effects of different parameters such as speed of agitation, phase volume ratio, catalyst concentration, sodium borohydride concentration, citronellal concentration and temperature have been studied. The reaction was found to be 100% selective towards the formation of the desired product. The kinetics and mechanism are presented to account for the effects of various parameters on conversion and rates. It was possible to establish both rate constant and distribution constant from the same set of data. The results are novel.

Novelties of kinetics of chemoselective reduction of citronellal to citronellol by sodium borohydride under liquid–liquid phase transfer catalysis

The alkylation of vanillin, containing three reactive groups, with benzyl chloride is challenging and can lead to both C- and O-alkylated products, particularly when liquid–liquid (L–L) phase transfer catalysis (PTC) is used. In the current studies, solid–liquid (S–L) PTC has been employed in the reaction of solid sodium salt of vanillin and benzyl chloride in toluene at 90 °C to make 4-benzyloxy vanillin, an ether, which is used as a perfume and also as a starting material for the synthesis of thalifoline, ephedradine as alkaloids and in synthesis of flavonoid compounds. The selectivity towards the desired product under S–L PTC is 100%. The rates of reaction are enhanced greatly by using trace quantities of water (the so-called omega phase (ω)) with 100% selectivity to the ether. Efficacy of various phase transfer agents such as TBAB (tetra- n -butyl ammonium bromide), TBAHS (tetra- n -butyl ammonium hydrogen sulfate), TPAB (tetra- n -propyl ammonium bromide), TEAB (tetraethyl ammonium bromide) and ETPB (ethyl triphenyl phosphonium bromide) was evaluated under otherwise similar conditions at 90 °C and explained. The order of activity was as follows: TBAB (maximum) > TBHS > TPAB > TEAB > ETPB (minimum). A theoretical model is developed to account for the enhancements in rates of reaction and selectivity to the ether. The kinetic constants have been evaluated and the apparent activation energy is determined as 9.64 kcal/mol. The results are novel.

Sharad V. Lande

Papers

Liquid‐Liquid‐Liquid Phase Transfer Catalysis: A Novel and Green Concept for Selective Reduction of Substituted Nitroaromatics

Novelties of reaction in the middle liquid phase in tri-liquid phase transfer catalysis: Kinetics of selective O-alkylation of vanillin with benzyl chloride

Intensification of rates and selectivity using tri-liquid versus Bi-liquid phase transfer catalysis : Insight into reduction of 4-nitro-o-xylene with sodium sulfide

Novelties of kinetics of chemoselective reduction of citronellal to citronellol by sodium borohydride under liquid–liquid phase transfer catalysis

Rate intensive and selective etherification of vanillin with benzyl chloride under solid-liquid phase transfer catalysis by aqueous omega phase