G. Murali Dhar

Mixed oxide supported hydrodesulfurization catalysts—a review

Abstract Support effects form important aspect of hydrodesulfurization (HDS) studies and mixed oxide supports received maximum attention in the last two decades. This review will focus attention on studies on mixed oxide supported Mo and W catalysts. For convenience of discussion, these are divided into Al 2 O 3 containing mixed oxide supports, TiO 2 containing mixed oxide supports, ZrO 2 containing mixed oxide supports and other mixed oxide supports containing all the rest. TiO 2 containing mixed oxides received maximum attention, especially TiO 2 –Al 2 O 3 supported catalysts. A brief discussion about their prospects for application to ultradeep desulfurization is also included. An overview of the available literature with emphasis on research carried out in our laboratory form the contents of this publication.

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.

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 .

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.

Pore size analysis of ZSM-5 catalysts used in n-heptane aromatization reaction: an evidence for molecular traffic control (MTC) mechanism

Gas adsorption studies were carried out for pore analysis of ZSM-5 catalysts used in n-heptane aromatization, to understand the deactivation behavior of zeolite channels. Results suggested that the alumina binder protected the zeolite catalyst by accommodating most of the coke. Analysis of the zeolitic part of the pores indicated that the straight channels are mainly blocked at pore opening and provided an evidence for molecular traffic control mechanism (MTC).

Synthesis, Characterization, and Evaluation of Mesoporous MCM-41-supported Molybdenum Hydrotreating Catalysts

Abstract Mesoporous MCM-41 material with high surface area and narrow pore size distribution was synthesized and used as a support for Mo, CoMo, and NiMo catalysts. The molybdenum loading was varied from 2–14 wt% on MCM-41. On 10 wt% Mo/MCM-41, the promoter Co or Ni concentration was varied from 1–5 wt%. All the catalyst samples were characterized by surface area, low temperature oxygen chemisorption, x-ray diffraction (XRD), and temperature programmed reduction methods. Characterization results show that Mo is well dispersed on MCM-41 up to 10 wt%. The catalytic activities were evaluated for thiophene hydrodesulphurization (HDS), cyclohexene hydrogenation (HYD), and furan hydrodeoxygenation (HDO). All three catalytic functionalities vary in a similar manner to that of oxygen chemisorption as a function of Mo loading, indicating that there is a correlation between oxygen uptake and catalytic sites. The activities of these catalysts were compared with γ-Al2O3- and amorphous SiO2-supported catalysts. It was found that MCM-41-supported Mo catalysts displayed superior activities.