One step microwave-assisted synthesis of nanocrystalline WO_x–ZrO₂ acid catalysts

Abstract

Nanocrystalline ZrO2 and WOx-ZrO2 catalysts with a good control over the zirconia phase and the nature of the W species were synthesized in a microwave-assisted aqueous route. The zirconia polymorphs monoclinic ZrO2 (m-ZrO2) and tetragonal ZrO2 (t-ZrO2) were isolated through an accurate control of the synthetic conditions, in acidic and basic media, respectively.The evolution of the zirconia and W species under annealing of the WOx-ZrO2 nanocomposites at 500 o C and 800 o C was studied, and the resulting materials were tested as catalysts for the reactions of aldoxime dehydration and hydration of alkynes for the production of nitriles and carbonyls, respectively. In the dehydration reaction the most active species are W(VI) in tetrahedral coordination, independently of the ZrO2 polymorph, while in the hydration reaction the zirconia phase plays a key role, as the tungsten doped t-ZrO2 appears the most active catalyst.

Structure–reactivity relationship in isolated Zr sites present in Zr-zeolite and ZrO2 for the Meerwein–Ponndorf–Verley reaction

“…Different zirconia based materials were prepared following previously reported procedures. 29,30 Two zirconia samples with pure crystalline phase were synthesized hydrothermally (m-ZrO2-H sample) and solvothermally (t-ZrO2-S sample). Urea was used in both synthesis as homogeneous precipitating agent, and the only difference was the solvent used, water and methanol for obtaining pure monoclinic m-ZrO2-H and pure tetragonal t-ZrO2-S polymorphs, respectively ( Figure 1).…”

“…Urea was used in both synthesis as homogeneous precipitating agent, and the only difference was the solvent used, water and methanol for obtaining pure monoclinic m-ZrO2-H and pure tetragonal t-ZrO2-S polymorphs, respectively ( Figure 1). Following a microwave-assisted route 30 we were able to obtain pure monoclinic zirconia (m-ZrO2-A sample). By introducing urea in the reaction media, a sample containing 89% tetragonal and 11% monoclinic zirconia was also synthesized (t-ZrO2-U sample).…”

“…Finally the materials were annealed at 400 o C in a muffle furnace in air atmosphere, with a temperature ramp rate of 5ºC/min and a dwell time of 2 h. Two more materials were obtained through microwave-assisted synthesis. 30 47 mmol of ZrOCl2·8H2O were dissolved in 80 mL of mili-Q water and this solution was transferred into two Teflon cups of 100 mL each and introduced in a microwave oven Synthos 3000 from Anton Paar. After sealing it, the temperature was increased at a heating rate of 20 o C/min until 220 o C, with a dwell time of 10 minutes.…”

Stabilization of ZrO2 Powders via ALD of CeO2 and ZrO2

“…This can be important in some catalytic applications. For example, the acid-base properties of zirconia are reported to change with the different polymorphs [25]. The ability to stabilize a particular phase without significantly changing the surface area and without the addition of dopants could be very useful.…”

Modulating Properties of Pure ZrO2 for Structure–activity Relationships in Acid‐base Catalysis: Contribution of the Alginate Preparation Route

“…The precipitation routes have been largely developed to offer a large spectrum of properties, in terms of mesoporosity, specific surface area, crystalline phases present and crystallite size, surface hydroxyl level, and distribution of acid‐base adsorption sites. These properties have been tuned by varying the pH, the temperature, the pressure, the heating mode, the additives, the duration of precipitation and/or maturation, and the drying and calcination conditions. In a detailed study by Bell et al, the most interesting samples (large specific surface areas, high proportion of metastable phase) were obtained after 10 days of precipitate digestion at 100 °C.…”