Catalysts For Acid-catalyzed Reactions Research Articles

Sulfonated organosilica-aluminum phosphates as useful catalysts for acid-catalyzed reactions: Insights into the effect of synthesis parameters on the final catalyst

The synthesis, by a sol-gel method, of organosilica-aluminum phosphates, (X)-AlPO(Y)-(Z), using three different silica precursors (X) 2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane (C), 3-(mercaptopropyl)trimethoxysilane (MPTMS) or Bis[3-(triethoxysilyl)propyl]tetrasulfide (B), with varying Al/P molar ratio (Y = 3–10) and pH of the final gel (Z = 3–9), maintaining an Al/Si= 3 molar ratio, has been carried out. Materials were characterized by XRF, SEM-EDX, XRD, 1H–29Si CP, 27Al and 31P MAS NMR and nitrogen adsorption. The acidity of the solids was evaluated by TGA, XRF and XPS and their catalytic behavior in the etherification of glycerol with tert-butyl alcohol, under microwave irradiation. The solids with an Al/P molar ratio between 3 and 6, synthesized at a pH value over 3 with precursor C, exhibiting the highest values of acidity and predominantly a mesoporous character, showed the best catalytic behavior.

A solid iron salt catalyst for selective conversion of biomass-derived C5 sugars to furfural

Salt is widely applied as catalyst but it is difficult to separate the soluble salt from the reaction medium. To solve this issue, in this study, a solid salt catalyst (FeClx-D008) was prepared by using a solid acidic resin (D008) and FeCl3 via a partial ion exchange method as the demonstration of proof of concept. The introduction of FeCl3 onto D008 replaced some Brønsted acid sites with newly generated Lewis acid sites. The synergistic effect of the Brønsted and Lewis acidic sites rendered the catalyst with excellent catalytic selectivity for the production of furfural from C5 sugars (highest yield: 96.3%). The kinetic study showed that the introduction of FeClx onto D008 enhanced the activation energy (AE) from xylose to polymer while lowered the AE from xylose to furfural. The FeClx species could strongly coordinate with the -SO3− group in D008 with the high resistivity to leaching. The successful immobilization of FeClx species on D008 solved the issue for separation of soluble salts from the reaction medium, which is also an attempt for application of resin-based solid salt as the catalyst in acid-catalyzed reactions.

Sulfated tin oxide with macro- and mesopores controlled using an integrated sol-gel and surfactant template route

Sulfated tin oxide was synthesized by a sol-gel process and pores were formed in the ceramic powders using Pluronic F-127 as a soft template. Sulfate ions linked to the tin oxide promoted the formation of acid sites on the particle surfaces. After calcination, the sulfate ions remained on the oxide particles as uni- and bidentate complexes, so changes in the tin atom coordination numbers at the second shells could be observed by X-ray absorption spectroscopy. The tin oxides showed a cassiterite nanocrystalline structure consisting of aggregates of spherical particles, with crystallite size of around 8 nm, radius of gyration ∼3.7 nm, and mean particle size between 7 and 10 nm, according to X-ray diffraction and small angle X-ray scattering measurements, together with the use of transmission electron microscopy images. The use of Pluronic F-127 as a template for the pores resulted in the formation of meso- and macropores sized around 7.5 nm and 60 μm, respectively, surface area of 128 m2 g−1 and porosities of 71%, after calcination at 500 ºC. The porous sulfated tin oxide was used to catalyze the isopropanol dehydration reaction, with production of propene. The catalytic activity and the temperature-programmed desorption of ammonia confirmed the presence of acid sites on the surface, suggesting that this material could be employed as a heterogeneous catalyst in acid-catalyzed reactions.

Exfoliated HNb3O8 nanosheets of enhanced acidity prepared by efficient contact of K2CO3 with Nb2O5

Although exfoliated HNb3O8 nanosheet (e-HNb3O8) is known to be a powerful catalyst in acid-catalyzed reactions, the preparation technique has limitations in its high-yield production. We herein report the enhanced exfoliation efficiency and recovery of e-HNb3O8 by contacting Nb2O5 solid with a diluted K2CO3 solution before ball milling (mechanochemical activation). The so-obtained e-HNb3O8 showed a larger specific surface area, a higher density of total acid sites and an improved catalytic performance in the dehydration of 2-heptanol and formic acid, compared to the corresponding sample prepared by the conventional solid-state mixing. The ball-milled K2CO3–Nb2O5 mixtures, layered KNb3O8, ion-exchanged HNb3O8 and exfoliated HNb3O8 samples were characterized by PXRD, TEM-EDS, TGA-MS and SEM-EDS. The characterization results revealed more intercalated K+ ions in KNb3O8 particles as well as more homogeneous K2CO3–Nb2O5 mixture when the employed preparation technique was used. This is achieved by more infiltration of K2CO3 into Nb2O5 lattice before ball milling, thereby resulting in the above positive findings. Therefore, our approach has great potential to be extended to other niobate-containing nanosheet-type materials.

A “one-step” sulfonic acid PMO as a recyclable acid catalyst

The synthesis and catalytic activity of a new sulfonic acid-functionalized periodic mesoporous organosilica (PMO) obtained by an in situ oxidation process have been reported. This material was prepared by co-condensation of a homemade thiol-functionalized bis-silane precursor (EtO)3Si-R-Si(OEt)3 (R=CH2CHSH) with 1,2-bis(triethoxysilyl)ethane (BTEE) in the presence of a nonionic triblock co-polymer. Thiol groups were oxidized either in situ during the formation of the mesostructure, or by a post-synthesis oxidation of the solid material. The in situ generated sulfonated PMO material had a well-ordered hexagonal mesoporous structure and a narrow pore size distribution. The sulfonic acid concentration of these materials was in the range of 0.85–1.48mmol H+ g−1. The catalytic performance of these sulfonic acid-functionalized materials was evaluated and compared with the commercial resin Amberlyst-15 in the esterification of acetic acid with benzyl alcohol. The in situ sulfonated PMO has shown to be an efficient, robust, and recyclable catalyst in acid-catalyzed reactions throughout consecutive catalytic cycles.

Synthesis of a nano-crystalline solid acid catalyst from fly ash and its catalytic performance

The synthesis of nano-crystalline activated fly ash catalyst (AFAC) with crystallite size of 12 nm was carried out by chemical and thermal treatment of fly ash, a waste material generated from coal-burning power plants. Fly ash was chemically activated using sulfuric acid followed by thermal activation at 600 °C. The variation of surface and Physico-chemical properties of the fly ash by activation methods resulted in improved acidity and therefore, catalytic activity for acid catalyzed reactions. The AFAC was characterized by X-ray diffraction, FT-IR spectroscopy, N 2-adsorption–desorption isotherm, scanning electron microscopy, flame atomic absorption spectrophotometry and sulfur content by CHNS/O elemental analysis. It showed amorphous nature due to high silica content (81%) and possessed high BET surface area (120 m 2/g). The catalyst was found to be highly active solid acid catalyst for liquid phase esterification of salicylic acid with acetic anhydride and methanol giving acetylsalicylic acid and methyl salicylate respectively. A maximum yield of 97% with high purity of acetylsalicylic acid (aspirin) and a very high conversion 87% of salicylic acid to methyl salicylate (oil of wintergreen) was obtained with AFAC. The surface acidity and therefore, catalytic activity in AFAC was originated by increased silica content, hydroxyl content and higher surface area as compared to fly ash. The study shows that coal generated fly ash can be converted into potential solid acid catalyst for acid catalyzed reactions. Furthermore, this catalyst may replace conventional environmentally hazardous homogeneous liquid acids making an ecofriendly; solvent free, atom efficient, solid acid based catalytic process. The application of fly ash to synthesize a solid acid catalyst finds a noble way to utilize this abundant waste material.

Preparation and catalytic testing of mesoporous sulfated zirconium dioxide with partially tetragonal wall structure

Methods were explored to synthesize sulfated mesoporous zirconia with crystalline pore walls of tetragonal crystal structure. The material has been characterized by small and large angle X-ray diffraction, nitrogen physisorption, transmission electron microscopy (TEM) and catalytic tests using n-butane isomerization to iso-butane and alkylation of 1-naphthol with 4- tert-butylstyrene as probe reactions. It has been found that sulfate deposition is crucial for the transformation of a mesoporous precursor with amorphous pore walls into a material with crystalline pore walls maintaining the mesoporous morphology with narrow pore size distributions. TEM shows no ordered stacking of the pores. As a catalyst for acid catalyzed reactions of large molecules, mesoporous sulfated zirconia is superior to microporous sulfated zirconia.