Amorphous titania

Although the mesoporous materials, such as Ti-MCM-41, have lower intrinsic epoxidation selectivity than TS-1 and Ti-beta, they must nevertheless be used as catalysts for reactions of large molecules typical in the fine chemicals industry. It is, therefore, interesting to elucidate how these ordered mesoporous materials compare with the earlier generation of amorphous titania-silica catalysts. Guidotti et al (189) recently compared Ti-MCM-41 with a series of amorphous titania-silica catalysts for the epoxidation of six terpene molecules of interest in the perfumery industry (Scheme 6). Anhydrous TBHP was used as the oxidant because the catalytic materials are unstable in water. The physical characteristics of these catalysts are compared in Table XII. 

Comparative catalytic activities (turnover numbers and selectivity (in parentheses)) of ordered Ti-MCM-41 (A) and amorphous titania-silica (B-E) catalysts in the epoxidation of unsaturated cyclic terpenes (1-6) using anhydrous TBHP... 

Crystallinity. It seems that the degree of crystallinity of Ti02 is an important factor in obtaining active Ti02 (46). Indeed, amorphous titania is poorly active. An optimum calcination temperature of amorphous samples presumably corresponds to a compromise between an enhanced crystallinity, together with a decreased density of lattice defects, and limited decreases in surface area and coverage by OH groups. 

Yanagisawa K., Ovenstone J. (1999) Crystallization of Anatase from Amorphous Titania Using the Hydrothermal Technique Effects of Starting Material and Temperature, J. Phys. Chem B. 103(37), 7781-7787. 

Chen F, Deng Z, Li X, Zhang J, Zhao J. Visible hght detoxification by 2,9,16,23-tetracar-boxyl phthalocyanine copper modified amorphous titania. Chem Phys Lett 2005 415 85-8. 

Given their modest acidity and low temperature, LPD methods are ideally suited to polymer substrates. We applied Method 1 (pH =3.8 room temperature) to unactivated PMR-15. RBS analysis showed that the deposited titania was 90 nm thick after 24 h and a 450 nm film was deposited in 48 h. Variability in the onset time for film formation may account for the seemingly slower initial growth. Method 1 titania growth on clean silicon wafers for these same time intervals gave 250 and 450 nm respectively. All samples were amorphous, as had been reported for this method on variously treated silicon wafers.12 An adherent, amorphous, titania film (420 nm thick in 48 h) also formed on a PMR-15 surface that had been sulfonated by exposure to SO3 gas. 

Data for several titania systems are given In Table 1 for the photodegradation of dichloromethane. First of all, data for unsupported titania should be compared. These data suggest that the rutile form of titania is more active than the anatase form which In turn Is more active than the amorphous titania. In general this is also true for the supported analogs. Reasons why an ordered high temperature form Is more active than a low temperature or amorphous form may be due to the propensity of the former to form defect adsorption sites (See below). 

We have studied the phase transformation from nanometer-sized amorphous titania (Ti02) to nanocrystalline anatase at 300 - 400° C (unpublished). Amorphous titania samples were prepared by fast hydrolysis of titanium ethoxide in water at 0° C (Zhang et al. 2001). The extent of transformation was monitored using XRD determination of the phase mixture as a function of time. We also found that the transformation kinetics do not follow the widely employed JMAK equation. 

Yamanaka T, Fukumori Y (1995) Molecular aspects of the electron transfer system which participates in the oxidation of iron by Thiobacillus ferrooxidans. FEMS Microbiol Rev 17 401-413 Yanagisawa K, Ovenstone J (1999) Crystallization of anatase from amorphous titania using the hydrothermal technique Effects of starting material and temperature. J Phys Chem B 103 7781-87 Yang J, Mei S, Ferreira IMF (2000) Hydrothermal synthesis of nanosized titania powders lirfluence of peptization and peptizing agents on the crystalline phases and phase transition. J Am Ceram Soc 83 1361-1368... 

Epoxidation of cycloalkenones over amorphous titania-silica aerogels... 

Hutter and co-workers made mesoporous amorphous titania-silica aerogels by the sol-gel method, including supercritical carbon dioxide extraction in the workup.267 These were used with cumenehydroperoxide to epoxidize 1-hexene in 95% selectivity and cyclohexene in 100% selectivity. The authors state that their catalysts are superior to titanium-/ and titanium MCM 41 for the oxidation of bulky compounds such as limonene (4.55). 

H, Z, Zhang and J, F, Banfield, Kinetics of crystallization and crystal growth of nanocrystalline anatase in nanometer-sized amorphous titania, Chem. Mater, 14 4145 154, 2002... 

Depending on the voliune filHng factor of the matrix material, substantial shrinkage of the porous network accompanied by crack formation may occur for low filHng fractions during template removal. Furthermore, annealing the repHca material at elevated temperatures may lead to a transformation from one modification or crystal phase to another that is thermodynamically more stable at these temperatures, as shown for the thermal conversion of amorphous titania or its anatase phase to the rutile structure [79]. 

The surface hydroxyl groups compete with the embedded chemical sensitizer for photoholes yielding hydroxyl species which tend to recombine forming oxygen bridges. As a result, amorphous titania exhibits an effective photopolymerization on the exposed surfaces and looses a solubility, while on the non-exposed surfaces it can be readily removed by washing in the water-isopropanol mixture. The dense photopolymerized titania on the exposed surfaces forms a confined medium for Pd nanophase formation. 

TiOz Polymorph The Interest of Anatase and Amorphous Titania... 

Men ndez, R., Alvarez, F, Betas, C., Nacimiento, E, Alcantara, R, Tirado, J. L., and Ortiz, G. F. [2014]. Self-organized amorphous titania nanotubes with deposited graphene film like a new heterostructured electrode for lithium ion batteries, J. Power Sources, 248, pp. 886-893. 

The microemulsion system water/Triton X-lOO/n-hexanol/cyclohexane has been used recently to cause reaction between TiOCl2 and NH4OH, and obtain amorphous titania nanoparticles [267]. The particles crystallized to anatase at 460°C and to rutile at 850°C. The crystallite size was 10-36 nm (500°-900°C), but the particles agglomerated easily during heating. 

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