Zirconia, tungstated - acidity

Keywords alkanes-isomerization iron-promoter n-pentane-isomerization platinum-promoter selectivity tungsten oxide zirconia-tungstated-acidity zirconia-tungstated-redox properties... 

The temperature-programmed desorption of low-molecular weight alcohols was studied over tungstated zirconia and acidic zeolites. Tungstated zirconia (WZ) acids have less tendency to deactivate due to carbon deposition and to form oligomeric products. Proton exchange is also less important in WZ. ... 

In order to enhance stability and acidity, zirconia aerogels were also doped with tungsten. Boyse et al. [46] reported that zirconia-tungstate aerogels prepared by one-step synthesis required a more elevated activation temperature in order to expel... 

Boyse, R.A. and Ko, E.I. Commercially available zirconia-tungstate as a benchmark catalytic material. Appl Catal A Gen. 1999,177, L131-L137. Santiesteban, J.G., Vartuli, J.C., Han, S., Bastian, R.D., and Chang, C.D. Influence of the preparative method on the activity of highly acidic WO c/Zr02 and the relative acid activity compared with zeolites. J. Catal. 1997, 168, 431 41. 

In addition to this, solid acid catalysts can also be used in the hydroisomerization cracking of heavy paraffins, or as co-catalysts in Fischer-Tropsch processes. In the first case, it could also be possible to transform inexpensive refinery cuts with a low octane number (heavy paraffins, n-Cg 20) to fuel-grade gasoline (C4-C7) using bifunctional metal/acid catalysts. In the last case, by combining zeolites with platinum-promoted tungstate modified zirconia, hybrid catalysts provide a promising way to obtain clean synthetic liquid fuels from coal or natural gas. 

Also for this reaction, namely esterification plus transesterification, mixed oxides, this time acidic in nature, appear to be the most promising alternative. Tungstated zirconia-alumina (WZA), sulfated zirconia-alumina and sulfated tin oxide were shown to be active in the transesterification of soybean oil with methanol at 200-300 °C and in the esterification of n-octanoic acid with methanol at 175-200 °C. Although the order of activities is different for the two reactions, WZA gives high conversions in both readions and it is stable under the reaction conditions [31]. Titania on zirconia, alumina on zirconia and zirconia on alumina also showed good performances [32, 6]. 

Furuta et al. tested a series of strong solid acids (alumina promoted sulfated zirconia, alumina promoted tungstated zirconia and sulfated tin oxide) for the transesterification of soybean oil with methanol at 200-300°C. Reaction yields over 90% were obtained for the alumina promoted tungstated zirconia at reaction times of 20 h using a flow reactor T = 250°C). The activity of the same catalyst was maintained for up to 100 h. 

Alcohol Dehydration Reactions as Chemical Precursors for Coke Formation and Acidity Probes in Tungstated Zirconia Catalysts... 

Tungstated zirconia (WZ) catalysts have been proposed as viable candidates for alkane isomerizations, especially those heavier than C4 [1]. In this work, alcohol dehydrations were used to rank the acidity of WZ with respect to that of HY zeolite, and to evaluate the ability of these catalysts to resist coking during a reaction that yields an olefin molecule as the primary product. The results from temperature-programmed reaction and infrared spectroscopy studies allowed us to gain some insight into the relative stability of WZ. 

F. 2002. 12-Tungstophosphoric acid/zirconia—a highly active stable solid acid—comparison with a tungstated zirconia catalyst. Chem. Commun. 1074-1075. 

Hence, our calculations emphasized a step-wise mechanism over most oxides H-abstraction leads to a surface hydroxyl, in conjunction with the alkyl radical formation, which rapidly rebounds to a nearby oxygen to form a surface alkoxy. This step-wise one-electron oxidation mechanism not only offers an energetically more favorable route than the one-step two-electron oxidation mechanism such as (5+2), but also provides a plausible solution to the puzzle as contrasting the EPR results [50] to the IR results [49] (cf. Fig. 6). We propose that EPR with higher time resolution (10 -10 s) detected the radical formation, whose stability was enhanced by the higher acidity of the tungstated zirconia catalyst which retarded the rebound process. On the contrary, IR with lower time resolution (10 -10 s) was unable to capture the active alkyl intermediates, showing the peaks related to the existence of both surface hydroxy and alkoxy after rebound. 

Boyse R A (1997) Crystallization behaviour of tungstate on zirconia and its relationship to acidic properties 1. Effect of preparation parameters. J Catal 171 191-207... 

In 2010, Zubir, et al. investigated the kinetic behavior of the heterogeneous catalyzed esterification of oleic acid with ethanol using tungstated zirconia as a catalyst in the temperature range of 303.15- 323.15K (Otera, 1993). 

MEL Chemicals Sulphated and Tungstated Zirconia — Solid, Strong Acid Catalysts. MELCat Doc. 2000. Machester England, 2000. 

Both titania (anatase more than rutile) and, even more, zirconia (tetragonal more than monoclinic), when sulfated or covered with tungsten oxide become very active for some hydrocarbon conversion reactions such as -butane skeletal isomerization [263]. For this reason, a discussion began on whether these materials have to be considered superacidic. Spectroscopic studies showed that the sulfate ions [264] as well as the tungstate ions [265,266] on ionic oxides in dry conditions, are tetracoordinated with one short S=0 and W=0 bond (mono-oxo structure) as shown in Scheme 9.3(11). Polymeric forms of tungstate species could also be present [267]. However, in the presence of water the situation changes very much. According to the Lewis acidity of wolframyl species, it is believed that it can react with water and be converted in a hydrated form, as shown in Scheme 9.3. Residual... 

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