Adsorption titanium silicalite

The effect of zeolite porosity on the reaction rate was also well demonstrated in liquid-phase oxidation over titanium-containing molecular sieves. Indeed, the remarkable activity in many oxidations with aqueous H2O2 of titanium silicalite (TS-1) discovered by Enichem is claimed to be due to isolation of Ti(IV) active sites in the hydrophobic micropores of silicalite.[42,47,68 69] The hydrophobicity of this molecular sieve allows for the simultaneous adsorption within the micropores of both the hydrophobic substrate and the hydrophilic oxidant. The positive role of hydrophobicity in these oxidations, first demonstrated with titanium microporous glasses,[70] has been confirmed later with a series of titanium silicalites differing by their titanium content or their synthesis procedure.[71] The hydrophobicity index determined by the competitive adsorption of water and n-octane was shown to decrease linearly with the titanium content of the molecular sieve, hence with the content in polar Si-O-Ti bridges in the framework for Si/Al > 40.[71] This index can be correlated with the activity of the TS-1 samples in phenol hydroxylation with aqueous H2C>2.[71] The specific activity of Ti sites of Ti/Al-MOR[72] and BEA[73] molecular sieves in arene hydroxylation and olefin epoxidation, respectively, was also found to increase significantly with the Si/Al ratio and hence with the hydrophobicity of the framework. 

More promising from an industrial perspective, however, is the separation of the oxidation zone from the aqueous one effected by the catalytic material itself, through the selective adsorption of the reagents. The introduction of Titanium Silicalite-1 (TS-1), in which the hydrophobic properties of the pores protect the active sites from the inhibition of the external aqueous medium, was a demonstration of the concept. The catalyst, the substrate and the aqueous soluhon of hydrogen peroxide can, in this case, be mixed together, with a great simplification of the process and also a reduction of the hazards. Three commercial processes. 

Figure 1. Variations with coverage of the differential heats of ammonia adsorption on titanium-silicalites with different titanium contents (in pmol/g).

UV-vis spectrum of the calcinated material showed an adsorption band at about 205 nm, characteristic of titanium silicalites. The absence of signal beyond 275 nm indicated that the material was free from extraframework oxide species. S.E.M. pictures revealed that the sample was in the form of very small uniform crystals of about 0.3 pm in size. 

ACIDITY STUDIES ON TITANIUM SILICALITES-1 (TS-1) BY AMMONIA ADSORPTION USING MICROCALORIMETRY... 

Similarly, the acidity of titanium-silicalites with different titanium contents was characterized by adsorption calorimetry at 353 K of various probe molecules by Muscas et al. [247]. These molecular sieves had a molar compo-... 

The adsorption properties of titanium silicalites-1 synthesized via two different routes, viz. in the presence or the absence of sodium in the precursor gel, have been compared by Auroux et al. [248]. Adsorption calorimetric measurements of a basic probe (NH3) and an acidic probe (SO2) showed that these solids were very acidic compared to a silicalite-1 sample. The presence of Na in the different samples decreased the number and the strength of the acid sites. The modification strongly depended on the synthesis procedure [248]. 

In consistence with the direct observation of the differential heats and isotherms, it can be noticed in Table 1 that the initial heat of ammonia adsorption is strongly affected by the presence of a small amount of titanium. Qjnj increases from 77 kJ/mol for pure silicalite up to about 200 kJ/mol for only 0.26 wt% Ti. Then for increasing titanium content, has a slight tendency to decrease. 

As a difference with the silicalite, TS-1 shows a characteristic IR band at 960 cm l. Adsorption of water shifts upwards the 960 cm peak to 970-975 cm l[15]. By increasing the Ti molar fraction in TS-1 from 0 to 0.025 a linear increase of the above mentioned IR band intensity with x was observed [15]. This means that the 960 cm l IR band is in someway related to the presence of lattice titanium. 

The ODH of propane over titanium and vanadium containing zeolites and nonzeolitic catalysts revealed that Ti-silicalite was the most active. The addition of water caused an increase in selectivity, probably due to a competitive adsorption on the active sites. The reaction is proposed to occur on the outer surface of the Ti-silicalite crystallites on Lewis acid sites, and a sulfation of the catalyst, which increases the acidity of these sites, results in a further increase of the catalytic activity. The maximum conversion obtained was 17% with a propene selectivity of up to 74% [65]. Comparison of propane oxidation and ammoxidation over Co-zeolites shows an increase in conversion and propene selectivity during ammoxidation. For a conversion of 14%, 40% propene selectivity was obtained with ammonia, whereas, at 10% conversion the propene selectivity was only 12% with oxygen. The increase in activity and selectivity can be due to the formation of basic sites via ammonia adsorption [38]. 

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