Isopropanol chemisorption

Isopropanol chemisorption and temperature programmed surface reaction (TPSR) towards propylene allowed the determination of the number of active acid sites and the activation energy in order to compare the acid properties of HPAs with various catalytic materials. 

The present investigation presents a detailed examination of the conditions (solvents, substrate-resin ratio, time of the exchange) to optimize the synthesis of fosfotungstic and fosfomolybdic Wells-Dawson acids through the ion exchange method. Additionally, isopropanol chemisorption and temperature programmed surface reaction was applied to obtain information of the nature and number of the active acid sites. 

Previous investigations demonstrated for the first time in the literature that in situ isopropanol chemisorption and quantitative temperature programmed surface reaction are suitable to be used to determine the nature, number, and acid strength of the surface/bulk active sites of tungsten oxide-based catalysts and particularly of the heteropoly compounds [15]. 

Table 5.1 presents the atoms coordination, specific surface area, external and microporous area, total pore volume, micropore volume, and diameter (in the mesoporous range) of the WeUs-Dawson and Keggin HPAs, along with monolayer supported and mesostructured tungsten-based materials and bulk WO3. Table 5.2 compares the temperatures of isopropanol chemisorption, number of surface active acid sites (Ns), temperatures of propylene desorption, and activation energy of isopropoxy surface reaction towards propylene. 

Temperature of isopropanol chemisorption, maximum number of active sites for isopropanol chemisorption, and activation energy of surface reaction of tungsten- and molybdenum-based bulk and supported catalysts... 

This observation evidences that the loss of water leads to the shortening of the distance between the Wells-Dawson units and the decrease of the available active sites for isopropanol chemisorption. 

Parfitt and his co-workers pointed out (Day, Parfitt and Peacock, 1971) that there is an important difference between ethanol and isopropanol in their interactions with rutile. Whereas ethanol can displace water and undergo dissociative chemisorption to form the surface ethoxide, isopropanol is more readily adsorbed in the molecular form. This is consistent with the hydrophobic nature of ethanol-treated Ti02 and the autophobic nature of the ethanol monolayer. The latter effect is manifested in the form of a Type I isotherm, which is remarkably similar to that given by ethanol on alumina (see Figure 10.15). 

Other methods—indirect, but not utilizing chemisorption— have been used to determine site densities. In 1958 the magnetic moment of surface atoms was taken to be a measure of the number of catalytically active atoms ( ). The number of surface free valencies was used for the same purpose in 1964 ( ). In 1965 Mellor and coworkers ( ) oxidized KI over silica-alumina by chemical analysis the number of surface atoms capable of oxidation was determined and taken to be the site density. The number of hydrogen vibrations on the ZnO surface which catalyzed the dehydrogenation of isopropanol was used to calculate the site density (91). 

The chemisorption of isopropanol at 313 K leads to the coverage of the heteropolyacids with a stable monolayer of adsorbed isopropoxy species and avoids further surface reaction. These adsorbed intermediate-reactive alkoxy species further react and desorb as propylene (or other product depending on the nature of the site) upon controlled heating during the TPSR experiment. Therefore, the quantification of the desorbed product is proportional to the total number of acid sites active on isopropanol dehydration towards propylene. 

Isopropanol adsorption was performed at several temperatures in order to determine the more suitable conditions for the alcohol chemisorption on the active sites avoiding further reaction. The observation that no molecular isopropanol is detected in the TPSR spectra indicates that no physisorption (or weak chemisorption) of the alcohol is produced even at 313 K over heteropolyacids (spectra not shown). The maximum amount of surface isopropoxy species over tungsten oxide WO3 and monolayer supported tungsten oxide species was obtained through isopropanol adsorption at 383 and 343 K, respectively. 

Photoacoustic Fourier transform infrared (FTIR) spectroscopy studies performed by Moffat and coworkers [33] on Ci to C4 alcohols adsorbed over phospho-tungstic Keggin acidH3PWi2O40, demonstrated the formation of alkoxyl intermediates. The authors observed the chemisorption of 3-4 isopropanol molecules per Keggin anion after 5 to 10 h of exposure to the alcohol at 25°C. The alcohol initially adsorbs as a protonated species /-C3H7OHJ and forms isopropoxyl species upon evacuation at about 50°C. 

The previous sections discussed the applications of methanol chemisorption as a tool to describe the surfaces of oxide catalysts. Other molecules, such as isopropanol and formic acid, have also been used as chemical probes to measure the number of surface active sites of metal oxide catalysts but to a lesser extent. The ability of isopropanol to distinguish between surface redox and acid sites and the observation that it adsorbs as a stable monolayer of surface isopropoxy species over oxide materials, also makes this molecule a suitable surface chemical probe. 

Fein et al. [65] also investigated the TOFs of bulk metal oxides toward formic acid oxidation through the dissociative chemisorption of the HCOOH to surface formate species HCOO-M. The authors obtained similar structure-activity relationships as observed for methanol and isopropanol. 

Maximum number of active sites for adsorption/reaction of isopropanol calculated from the addition of die amount of propylene produced during chemisorption and TPSR experiments. 

Banares, M.A. and Wachs, I.E. Molecular structures of suported metal oxide catalysts under different environments. J. Raman Spectrosc. 2002, 33, 359-380. Gambaro, LA. and Briand, L.E. In-situ quantification of the active acid sites of H6P2Wis062J H20 heteropoly-acid through chemisorption and temperature programmed surface reaction of isopropanol. Appl Catal 2004, 264, 151-159. Badlani, M. Methanol A Smart Chemical Probe Molecule. Master thesis, Lehigh University, Bethlehem, PA, 2000. 

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