Titania derived nanotubes and nanoparticles : catalyst supports in hydrogenation, oxidation and esterification reactions

摘要

Nanotubular titanates were synthesized by a simple methodology using a commercialudTiO2 (Degussa P25 containing anatase and rutile phases) and a base (KOH) solution.udPrior to the removal of KOH, the samples of TiO2 were aged for three different timeudintervals (0, 2 days, 61 days). The freshly prepared synthetic samples wereudcharacterized for their structural and morphological properties by BET, XRD,udRaman, TEM, HRTEM, EDX and SEM. Both TEM and SEM analysis revealed thatudageing time influenced the tubular structure and morphology of the new materials.udRaman and surface analysis data also showed that ageing time affected both theudstructural and surface properties of TiO2. The XRD results showed that theudcrystallinity of the TiO2 decreased with increasing ageing time. Energy dispersive Xrayudspectroscopy (EDX) showed that the tubes derived from TiO2 are comprised ofudpotassium, titanium and oxygen. Catalysts A, B and C were prepared by the additionudof 1 wt% Pd (wet impregnation) to the titanate formed after ageing of the TiO2 inudKOH for 0, 2 and 61 days, respectively. The catalysts were tested for the vapourudphase hydrogenation of phenol in a fixed-bed micro reactor within the temperatureudrange of 165 to 300oC under atmospheric pressure. Of the three catalysts, catalyst Budshowed the best activity (conversion 97%) and total selectivity to cyclohexanoneud(99%). In contrast, catalyst C, which showed a moderate activity favoured selectivityudto cyclohexanol. These results are attributed to differences in surface morphologiesudbetween the two catalysts B and C, associated with the surface area and a change inudthe surface acid-base properties. Catalyst B also showed a higher resistance towardsuddeactivation and maintained a higher total selectivity to cyclohexanone than didudcatalyst C.udA hydrothermal treatment of NaOH and TiO2 was employed to prepare twoudmaterials, TiO2-B and TiO2-C with relatively small crystallite size and large specificudsurface area. The hydrogenation of phenol was used to evaluate the activity of theududcatalysts Pd/TiO2-B and Pd/TiO2-C. The reaction proceeds in a single step andudinvolves the formation of a partially hydrogenated product, namely cyclohexanone.udThe larger surface area catalyst (Pd/TiO2-C, 89 m2/g) showed better activity andudselectivity to cyclohexanone than its counterpart (Pd/TiO2-B, 45 m2/g). The catalystudactivity showed significant dependency on the surface area whereas the selectivityudwas greatly influenced by surface basicity.udTitania derived nanotubes synthesized by treating P25 Degussa TiO2 with audconcentrated KOH solution and aged for 2 days was used as a catalyst support for theudhydrogenation of o-chloronitrobenzene (O-CNB) with Pd as the active phase. Theudvapour-phase hydrogenation of O-CNB was carried out in ethanol at 250 oC andudatmospheric pressure over a Pd/TiO2 derived nanotube catalyst (Pd/TiO2-M).udPd/TiO2-M gave complete conversion (100%) of O-CNB with a selectivity to orthochloroanilineud(O-CAN) of 86 %. The stability of the Pd/TiO2 catalyst was tested overud5 hours during which time the conversion slowly dropped to 80 % (selectivity 93 %)uddue to poisoning. TPR analysis revealed the existence of a strong palladium-supportudinteraction and this was found to be crucial to the overall activity of the catalyst.udIt has been found that gold supported on potassium titanate, KTiO2(OH) can, underudsome circumstances, exhibit a superior performance for the oxidation of carbonudmonoxide, relative to that obtained with titania as a support. It appears that theuddispersions of gold on the two types of support are sufficiently similar that otherudfactors are responsible for the improved activity noted. It may be that the higherudbasic character and detailed structural features of the titanate surface play a role.udThe effect of the addition of alkali metal ions on the anatase to rutile transformationudof titanium dioxide (P25 Degussa) was investigated using X-ray diffraction, Ramanudspectroscopy, and surface area measurements. Both Li and Cs ions accelerated theudanatase to rutile transformation whereas Na and K ions did not show any effect.udFurthermore, the effect was more pronounced after addition of the Li ions so that theududtransformation temperature dramatically decreased from ~800 oC for commercialudTiO2 to ~600 oC. The surface area of the TiO2 material decreased with sintering dueudto the increase in crystalline size. Moreover, the acceleration of the transformationudoccurred at lower temperatures and at higher Li content.udMesoporous nanocrystalline TiO2 (HSA TiO2) was prepared by hydrothermaludtreatment of TiO2 with NaOH. The material was very amorphous and underwent theudphase transformation from amorphous to anatase phase and subsequently fromudanatase to rutile phase with sintering. The anatase to rutile transformation wasuddelayed after doping and grain growth was inhibited. After sintering at 800 oC theudmaterial (HSA TiO2) still contained a significant amount of the anatase phase. Theudcomplete transformation only occurred at ~1000 oC.udThe esterification of benzoic acid and butyric acid with propanol over alkali metaludions supported on TiO2 was investigated. K/TiO2-D showed the highest conversionudfor both benzoic acid and butyric acid. The selectivity to propylbenzoate andudpropylbutyrate was influenced by the basic nature of the catalysts. Butyric acid wasudfound to be more reactive than benzoic acid. The difference in reactivity wasudexplained in terms of steric and inductive effects. The differences in boiling pointsudand pH values were also considered.