Acidic and basic properties of titania–alumina mixed oxides; active sites for propan-2-ol dehydration

Abstract

Ti0,-AI,O, mixed oxides have been prepared by cohydrolysis of Ti4' and A13+ isopropoxides, Textural and acid-base properties were investigated using nitrogen physisorption, adsorption of probe molecules followed by FTIR spectroscopy, and test-reactions. Catalysts were found to exhibit high surface areas and pore volumes. Results obtained from adsorbed CO, probe molecules and COS hydrolysis activity agreed well and showed that the hydroxy basicity of mixed oxides was lower than that of t h e two oxide components. Pyridine and 2,6-dimethylpyridine were used to titrate Lewis and Brsnsted acidic sites, respectively. A linear inerease of t h e number of strong Lewis acid sites with alumina content was observed, whereas t h e number of Br0nsted acid sites reached a maximum for 30% Ti0,-70% AI,O, compared with that expected from t h e composition and t h e activity of pure oxides. Comparison between probe-molecule adsorption and test reaction, points out that both types of site are involved in propan-2-01 dehydration. A transformation of Lewis acid sites into Bransted sites is proposed when propan-2-01 is chemisorbed on Lewis acid sites.Mixed oxides are increasingly used in catalysis because they can present properties different from those of the component single-metal oxides. A typical case is that of silica-alumina. In a recent paper, we examined the acidic and basic properties of Zr0,-Al,O, and Zr0,-TiO, mixed oxides using adsorption of specific probe molecules followed by FTIR, and activity towards two test-reactions : propan-2-01 dehydration and carbon oxysulfide hydrolysis.2 This work concerns Ti0,-A1203 mixed oxides. Our aim was not only to determine their acidic and basic properties but also to correlate probe adsorption and catalytic activities in order to specify the nature of the active sites. For this we chose test reactions with well characterized basicity and acidity. The mechanism of COS hydrolysis is now well e~tablished.~ In particular, the active sites are basic hydroxy groups, also involved in CO, adsorption in the form of hydrogen carbonate species, explaining the good correlation between catalyst activity and the 6(OH) band intensity of HOCO, -~p e c i e s .~ Conversely, propan-2-01 dehydration is largely used as an acidity test, but the nature of its active sites is more complex. Indeed, catalytic activity of Zr0,-TiO, mixed oxides was found to follow Bransted acid sites detected by 2,6-dimethylpyridine (lutidine), whereas for Zr0,-Al,O, activity seemed to be determined by the strong A13 + Lewis acidity, evidenced by pyridine.* Owing to the presence of strong Lewis acidity on pure Al,O, and some Brarnsted acidity on pure TiO,, Al,O,-TiO, mixed oxides are expected to present both types of active site and are therefore interesting model catalysts.It has recently been reported that use in hydrotreatment reactions of TiO,-Al,O, as supports for molybdenum sulfided catalysts, increases the specific rate for hydrodesulfurization of t h i~p h e n e .~ Moreover, McVicker and Ziemiak showed that these mixed ...

High-Temperature NO x Storage and Sulfur-Resistance of the Lithium-Based Lean-Burn NO x Trap Catalyst Pt/Li/TiO2–Al2O3

“…Similar results were also reported by other researchers [5][6][7][8]. 10 Lithium-Based Lean-Burn NO x Trap Catalyst 237…”

“…Matsumoto et al [9] found that the decomposition temperature of sulfates on TiO 2 support is lower than that on Al 2 O 3 support under reducing conditions. Lahousse et al [10] studied the acidic and basic properties of TiO 2 -Al 2 O 3 mixed oxides and its activity for propanediol dehydration. Their results show that TiO 2 -Al 2 O 3 mixed oxides exhibit better thermal stability and larger acidity than the two single oxides.…”

Upgrading of furfural to biofuel precursors via aldol condensation with acetone over magnesium hydroxide fluorides MgF2−x(OH)x

“…Such correlation was already reported between the activity for carbon oxysulfide hydrolysis of various mixed oxides (TiO 2 -Al 2 O 3 , TiO 2 -ZrO 2 and ZrO 2 -Al 2 O 3 ) and the intensity of the band at 1225 cm -1 . 48,49 As observed in Fig. 4 attributed to the presence of carbonates species produced by reaction with CO 2 .…”

“…For all other samples, several bands appeared between 1100 -1900 cm -1 , showing that some basic sites remained available to adsorb CO 2 and to form carbonate and bicarbonate species. [48][49][50] The decrease of the fluorine content is linked to an increase of several bands, including notably those located at 1358 and 1460 cm -1…”

Gas phase synthesis of isopropyl chloride from isopropanol and HCl over alumina and flexible 3-D carbon foam supported catalysts

“…The scheme describes four simultaneous processes of isopropyl chloride, propylene, n-propyl chloride and diisopropyl ether formation from isopropanol. In summary, the entire process can be presented as a combination of parallel reaction carried out on γ-alumina surface according to E1 and Snull1 mechanisms [52][53][54][55][56][57][58][59][60][61][62]. First, step 1 depicts the protonation stage -interaction of isopropanol with γ-alumina acid sites via OH-group [55,61] (which can also result in partial transformation of Lewis acid sites into Brønsted sites [62]), followed by dehydration step by E1 mechanism towards secondary propyl carbocation I (step 2).…”