Dehydration of Fructose to 5-Hydroxymethylfurfural: Effects of Acidity and Porosity of Different Catalysts in the Conversion, Selectivity, and Yield

There is a demand for renewable resources, such as biomass, to produce compounds considered as platform molecules. This study deals with dehydration of fructose for the formation of 5-hydroxymethylfurfural (HMF), a feedstock molecule. Different catalysts (aluminosilicates, niobic acid, 12-tungstophosphoric acid—HPW, and supported HPW/Niobia) were studied for this reaction in an aqueous medium. The catalysts were characterized by XRD, FT-IR, N2 sorption at −196 °C and pyridine adsorption. It was evident that the nature of the sites (Brønsted and Lewis), strength, quantity and accessibility to the acidic sites are critical to the conversion and yield results. A synergic effect of acidity and mesoporous area are key factors affecting the activity and selectivity of the solid acids. Niobic acid (Nb2O5·nH2O) revealed the best efficiency (highest TON, yield, selectivity and conversion). It was determined that the optimum acidity strength of catalysts should be between 80 to 100 kJ mol−1, with about 0.20 to 0.30 mmol g−1 of acid sites, density about 1 site nm−2 and mesoporous area about 100 m2 g−1. These values fit well within the general order of the observed selectivity (i.e., Nb2O5 > HZSM-5 > 20%HPW/Nb2O5 > SiO2-Al2O3 > HY > HBEA).

Physicochemical Effects of Niobic Acid Addition Into Dental Adhesives

The incorporation of metallic oxides in dental adhesives has been a strategy to confer improved radiopacity and physicochemical properties for polymers. Tailoring the structure of these fillers could contribute to their application in therapeutic strategies for dental restorations. The aim of this study was to evaluate the incorporation of niobic acid into experimental dental adhesives, and compare these adhesives to niobium pentoxide containing adhesives. A control group without Nb2O5·n H2O or Nb2O5 was also used for comparison. Niobium-based particles have been used as a feasible approach, mainly because of their bioactivity. In this study, hydrated niobium pentoxide, also called niobic acid (Nb2O5·n H2O), was incorporated into an experimental dental adhesive as a potential catalyst for monomer conversion. A base resin for dental adhesive was formulated with methacrylate monomers and photoinitiators. Two types of oxides were tested as filler for this adhesive: Nb2O5·n H2O or niobium pentoxide (Nb2O5). Both fillers were added separately into the experimental adhesive at 0, 2.5, 5, and 10 wt.%. One group without Nb2O5·n H2O or Nb2O5 (0 wt.% of filler addition) was used as a control group. The formulated materials were analyzed for radiopacity according to the ISO 4049 and used FTIR analysis to assess the degree of conversion (DC) and the maximum polymerization rate (RPmax). Mechanical properties were analyzed by ultimate tensile strength (UTS) in a testing machine. Softening in solvent was conducted by measuring Knoop microhardness before and after immersion of samples in ethanol. Normality of data was assessed with Shapiro-Wilk, and comparisons between factors were conducted with two-way ANOVA and Tukey at 5% of significance. Both fillers, Nb2O5 or Nb2O5·n H2O, increased the radiopacity of dental adhesives in comparison to the unfilled adhesive (p < 0.05). There were no differences among groups for the ultimate tensile strength (p > 0.05), and all groups containing Nb2O5 or Nb2O5·n H2O improved the resistance against softening in solvent (p < 0.05). The groups with 5 and 10 wt% addition of Nb2O5 showed decreased DC compared to the control group (p < 0.05), while the addition of Nb2O5·nH2O up to 10 wt% did not alter the DC (p > 0.05). The polymerization rate did not change among groups (p > 0.05). In conclusion, Nb2O5·n H2O is a promising filler to be incorporated into dental adhesives providing proper mechanical properties, improved resistance against solvents, and increased radiopacity, without changing the DC.

Impact of Thermal Treatment of Nb2O5 on Its Performance in Glucose Dehydration to 5-Hydroxymethylfurfural in Water

The cascade dehydration of glucose to 5-hydroxymethylfurfural (HMF) was carried out in water over a series of Nb2O5 catalysts, which were derived from the thermal treatment of niobic acid at 300 and 550 °C, under air or inert atmosphere. Amorphous niobic acid showed high surface area (366 m2/g) and large acidity (2.35 mmol/g). With increasing the temperature of the thermal treatment up to 550 °C, the amorphous Nb2O5 was gradually transformed into a pseudohexagonal phase, resulting in a decrease in surface area (27–39 m2/g) and total acidity (0.05–0.19 mmol/g). The catalysts’ performance in cascade dehydration of glucose realized in pure water was strongly influenced by the total acidity of these materials. A remarkable yield of 37% HMF in one-pot reaction in water was achieved using mesoporous amorphous niobium oxide prepared by thermal treatment of niobic acid at 300 °C in air. The best-performing catalyst displayed a total acidity lower than niobic acid (1.69 mmol/g) which afforded a correct balance between a high glucose conversion and limited further conversion of the target product to numerous polymers and humins. On the other hand, the treatment of niobic acid at 550 °C, independently of the atmosphere used during the sample preparation (i.e., air or N2), resulted in Nb2O5 catalysts with a high ratio of Lewis to Brønsted acid sites and poor total acidity. These materials excelled at catalyzing the isomerization step in the tandem process.

Arylation of indoles using cyclohexanones dually-catalyzed by niobic acid and palladium-on-carbons

3-Arylindoles could be effectively constructed from indoles and cyclohexanones in the presence of the dual catalysts of N2O5/C and Pd/C.