T.S.R. Prasada Rao

Characterization and evaluation of ZrO2 supported hydrotreating catalysts

A series of zirconia supported molybdenum catalysts were prepared and characterized by BETSA, XRD, TPR, FTIR, XPS and oxygen chemisorption. Thiophene, cyclohexene and tetrahydrofuran were taken as model compounds for evaluating their hydrodesulfurization (HDS), hydrogenation (HYD) and hydrodeoxygenation (HDO) activities, respectively. The XRD results indicate that Mo is present as a monolayer up to 6 wt.% loading, whereas MoO3 crystalline growth is observed beyond this loading. O2-uptake and catalytic activities also increase up to 6 wt.% Mo-loading but above this loading both start decreasing. There are good correlations between O2 uptake and all catalytic activities. TPR and FTIR results indicate that at lower loading, MoO3 is present as tetrahedra form and at moderate loading, both tetrahedral and octahedral forms are found. XPS results reveal that the electron transfer is taking place from support to molybdenum.

Studies on physico-chemical characterization and catalysis on high surface area titania supported molybdenum hydrotreating catalysts

A series of titania supported molybdenum catalysts were prepared by incipient wetness impregnation method and characterized by BET surface area, XRD, TPR, FTIR, ESCA, and low temperature oxygen chemisorption. Thiophene, cyclohexene and tetrahydrofuran were taken as model compounds for evaluating catalytic activities for hydrodesulfurization (HDS), hydrogenation (HYD), and hydrodeoxygenation (HDO) reactions, respectively. XRD results indicate that molybdenum oxide species are dispersed as a monolayer on the support up to 8 wt.% Mo and the formation of crystalline MoO 3 is observed above this loading. FTIR and TPR results showed that molybdenum oxide species were present predominantly in tetrahedral form at lower loading and polymeric octahedral forms are dominant at higher loading. Both oxygen chemisorption and rates of reaction were found to increase with increasing Mo loading up to 8 wt.% and then decrease with further increase in loading. HDS and HYD activities are more or less same but HDO activity is two times higher than HDS and HYD activities. The results are also interpreted with the help of other parameters, like dispersion, equivalent molybdenum surface area, surface coverage, crystalline size, quasi-turnover frequencies and intrinsic activities. ESCA results suggest that electron transfer is taking place from support to metal.

TiO2–ZrO2 mixed oxide as a support for hydrotreating catalyst

Pure TiO2, ZrO2 and TiO2–ZrO2 mixed oxides are prepared by urea hydrolysis. Hydrotreating catalysts containing 12 wt% molybdenum are prepared using these oxides and characterized by BET surface area, pore volume, XRD and oxygen chemisorption. It is observed that oxides produced by the method of urea hydrolysis have higher surface area as compared to those available commercially. With increasing zirconia content in the mixed oxide, the surface area increases and a maximum value is obtained for a mixed oxide having Ti and Zr molar ratio of 65/35. XRD results indicate that mixed oxides are poorly crystalline in nature. Thiophene hydrodesulfurization, cyclohexene hydrogenation and tetrahydrofuran hydrodeoxygenation are taken as model reactions for evaluating catalytic activities. It is found that both O2 uptake and catalytic activities increase with increasing zirconia content in mixed oxide and reach maximum values for the 12 wt% Mo/TiO2–ZrO2 (65/35) catalyst. With further increases in zirconia content, O2 uptake and catalytic activities decrease and the lowest values are observed for the pure ZrO2 supported catalyst.

Reaction pathways for the aromatization of paraffins in the presence of H-ZSM-5 and Zn/H-ZSM-5

The effect of the dehydrogenating component of H-ZSM-5 and Zn/H-ZSM-5 catalysts on n-heptane aromatization reaction was studied. An increase in the yield of aromatics observed over Zn/H-ZSM-5 is explained by the enhanced production of olefins, facilitating the aromatization reaction by the effective dehydrogenating action of zinc on paraffins. A significant change in the selectivities of toluene and C9+ aromatics was observed after addition of zinc. The increase in the yield of toluene observed over Zn/H-ZSM-5 from both n-heptane and Bombay High (BH) light naphtha indicates the possible occurrence of direct dehydrocyclization of paraffins in the presence of zinc. Zinc also facilitates the effective conversion of C6–C8 oligomers to the corresponding aromatics, an in that way suppresses the further oligomerization of C6–C8 oligomers on one hand, and cracking of C6–C8 oligomers on the other; which is reflected in the decreased yields of fuel gas and C9+ aromatics in the product. The product pattern of the BH light naphtha aromatization suggests that the presence of zinc also suppresses the acid catalyzed alkyl transfer reactions which generally take place over H-ZSM-5. Reaction pathways have been established to explain the selectivity changes that occurred in presence of zinc.

Characterization and hydrodesulfurization catalysis on WS2 supported on mesoporous Al-HMS material

Abstract Hexagonal mesoporous silica (HMS) and Al–HMS materials with high surface area and mesoporous structure were synthesized following the neutral templating path way. These materials were used as a support for the first time in preparing W/HMS, and W/Al–HMS catalysts and their Co, and Ni promoted analogues. On Al–HMS support (Si/Al=35) the tungsten loading was varied from 10 to 25 wt.%. On a catalyst containing 21 wt.% W the promoter concentration was varied from 1 to 5 wt.%. Oxygen uptakes and crystallite sizes of WS 2 derived from it indicated that WS 2 is well dispersed at all the loadings studied. The hydrodesulfurization and hydrogenation activities varied in a similar manner to that of oxygen uptakes as a function of W loading indicating that there exists a correlation between oxygen chemisorption and catalytic sites. The HMS and Al–HMS mesoporous material supported WS 2 catalysts showed outstanding activities in comparison with WS 2 supported on γ-Al 2 O 3 .

The determination of accessible Pt metal fraction in Pt–Sn/Al2O3 reforming catalyst

Abstract An attempt has been made to provide a simple method for the determination of accessible Pt metal fraction in Pt–Sn/Al 2 O 3 catalyst. The complications in Pt–Sn/Al 2 O 3 system arise mainly due to uncertainties involved in the oxidation states of tin: Sn 0 , Sn 2+ and Sn 4+ in the reduced catalyst. H 2 , O 2 chemisorption/titrations originally applied to Pt/Al 2 O 3 were extended to the Pt–Sn/Al 2 O 3 system. Two oxygen titration cycles OT1 and OT2 at room temperature were carried out using a dynamic pulse flow technique. Only part of the total Sn in the catalyst is reduced to Sn 0 (15%), which contributes to oxygen uptake. While OT1 is a linear function of the Sn content in the catalyst, the second oxygen titration OT2 gives the accessible Pt metal fraction only. It has been concluded that oxygen chemisorption/titrations at temperatures higher than ambient under dynamic conditions are not reliable for the determination of metal dispersions.

Oxidation of Sulphides by Molecular Oxygen - Aldehyde System in the Absence of Metal Catalyst

Abstract Molecular Oxygen/2-methylpropanal system provides a simple, efficient, and environmentally acceptable synthetic tool to convert sulphides to sulphones in near quantitative yields and to sulphoxides in high selectivity and good yields under ambient conditions. +Presented at the Int. Conf. on Org. Syn. (ICOS-10) at IISc, Bangalore, (India), December 11-16, 1994

Catalytic functionalities of TiO2 based SiO2, Al2O3, ZrO2 mixed oxide hydroprocessing catalysts

Thiophene HDS and cyclohexene HYD activities on three mixed oxides supported Mo and promoted by Co or Ni are presented. A comparative study indicated that Ti containing mixed oxide supports show outstanding activity for HDS. The trends of variation of activities indicated that the two functionalities originate from different set of catalytic sites on molybdenum sulfide phase.

Studies on sepiolite supported hydrotreating catalysts

Abstract Sepiolite is a hydrous magnesium silicate, receiving considerable attention in catalysis as support as well as catalyst. In this investigation a sepiolite was used as support for MoO 3 and the sulfided form used for hydrodesulfurization and hydrogenation reaction using thiophene and cyclohexene as model compounds respectively. The catalysts were characterized by surface area measurement, X-ray diffraction, IR and temperature programmed reduction methods. These results indicated that molybdenum is present as a monolayer upto 6 wt.% Mo loading and beyond this loading MoO 3 crystallite growth is observed. The low temperature oxygen chemisorption results indicated that there is no crystallite growth upto the monolayer region and beyond this there is rapid growth of MoS 2 crystallites. It was proposed that molybdenum interacts with basic hydroxyl groups attached to support. Linear correlations passing through origin are obtained between catalytic activities for thiophene hydrodesulfurization or cyclohexene hydrogenation and oxygen uptakes indicating that the anion vacancies created on reductive sulfiding are the seat of catalytic activity. Since correlations are obtained in the case of both HDS and HYD it is clear that oxygen chemisorption is not specific to any one of the functionalities but measures anion vacancies which are related to the state of dispersion of molybdenum.

Effect of sintering on the catalytic functionalities of MoS2/Al2O3 catalysts

Effect of sintering on physico-chemical and catalytic properties of Mo, Co-Mo, Ni-Mosupported on γ-Al2O3 is reported. Such effects on hydrodesulfurization (HDS), hydrogenation (HYD) and hydrodeoxygenation (HDO) are investigated as a function of sintering temperature. The results indicated that HDS and HYD have different optimum calcination temperatures and these functionalities originate from different sites. The results are discussed in the light of molybdenum sulfide dispersion, promotional effects and phase transformations of active component, promoters and support.