SUPPORT EFFECT ON CATALYTIC FUNCTIONALITIES OF HYDROTREATING CATALYSTS

Abstract

The removal of sulfur, nitrogen, oxygen and metals from oil by reductive treatments in socalled hydrotreating process has been of paramount importance ever since oil began to be used as an energy source. Oil and oil products have to be purified because most of the catalysts which are used further processing of oil products cannot tolerate sulfur and metals. A further reason for clean up is to diminish air polluting emissions due to oxides of sulfur and nitrogen which contribute to acid rain. Early in this century it was discovered that sulfur containing compounds poisoned many metallic catalysts (Pt-catalysts) but the transition metals such as Mo and W, sulfides retained the ability to hydrogenate aromatic compounds in presence of sulfur. Although the origin of hydrotreating catalysts goes back to the 1920s when German researchers developed catalysts to liquefy coal and later on it is applied to petroleum and tar upgrading as well. The alumina supported catalysts first appeared just before World War II and emerged as the modern petroleum processing catalysts used today in due course, in virtually every refinery in the world. The hydrotreating catalysts fall into a special category due to their exceptional resistance to poisons. The major reactions catalyzed by these materials are hydrogenation of olefins, ketones and aromatics (HYD), hydrodesulfurisation (HDS), hydrodenitrogenation (HDN), hydrodemetallation (HDM) and hydrodeoxygenation (HDO). Molybdenum and tungsten sulfides are main active components in hydrotreating catalysts and these are often promoted by Co and Ni. Less often promoter like Fe and Pb are also used. Different compositions are used for different applications, Co-Mo catalysts are favoured for hydrodesulfurization while Ni-Mo is preferred when hydrogenation is important. In the laboratory scale many 1st, 2nd and 3rd row transition metal sulfides are also reported to have higher activity than well known Co-Mo and Ni-W catalysts systems. The Mo and Wsystems are generally supported on y-Al203. Support plays an important role in controlling the properties of the hydrotreating catalysts. However, supports other than y-alumina are not studied in detail until recently. In the last decade there has been considerable interest on support effects due to the realization that these data are very useful in preparing improved catalysts. With this view in mind four supports, Zr02, Ti02, Ti02-Zr02 and sepiolite have been studied in order to understand the role of support and origin of catalytic activity. This thesis deals with support effects where Zr02, Ti02, Ti02-Zr02 and sepiolite were used as a support for Mo active component. Some of the supports are commercial in nature while others are prepared in laboratory under controlled conditions. Both promoted and unpromoted catalysts were prepared and the supports and finished catalysts in oxides state were examined by XRD, IR, ESCA, surface area, pore volume etc. physico-chemical characterization techniques. The oxygen chemisorption on sulfided catalysts was also evaluated. The activities were studied for hydrodesulphurization (HDS), hydrogenation (HYD), hydrodeoxygenation (HDO) reactions and an attempt is made to relate dispersion of the active component with catalytic activities. The studies carried out on four supports is presented in this thesis. For convenience of presentation the thesis is divided into six chapters. The first chapter presents about the importance of hydrotreating catalysts as well as support effects on hydrotreating reactions. The various structural models proposed also have been described in this chapter. This chapter also gives a brief account about the significance of application of probe molecules like 02, NO, and CO in advancement of knowledge about active sites on hydrodesulfurization catalysts. Ill Special emphasis is given to literature on support effects with specific reference to Zr02, Ti02 and Ti02-Zr02. Chapter 2 deals with the experimental procedure and the techniques employed in this investigation. The details of the support and catalyst preparation have been described. Schematic drawings of the volumetric adsorption system catalytic flow micro reactor, temperature programmed reduction system are also included. The experimental details of catalytic activity determinations, GC analysis of product distribution, surface area measurements, Low Temperature Oxygen Chemisorption (LTOC), determination of pore volume and pore size distribution by means of mercury penetration porosimeter, and the details of various catalyst characterization techniques like SEM, XRD, ESCA and FTIR are used. Chapter 3 deals with the series of Zr02-supported molybdenum sulfide catalysts. They were characterized by low temperature oxygen chemisorption in sulfided state, X-ray diffraction, Infrared and surface area measurements. The catalytic activities of these catalysts were evaluated by using hydrodesulfurization (HDS) of thiophene, hydrogenation (HYD) of cyclohexene and hydrodeoxygenation (HDO) of tetrahydrofuran as model reactions. The activities of these catalysts are found to correlate with their oxygen up take values. It was noted from oxygen chemisorption and > activity data that the monolayer formation of Mo occurs at 6 wt% Mo on these catalysts. The Co or Ni promotional effect also has been studied. These results are interpreted based ona surface structural model, showing a highly dispersed monolayer of amorphous Mo-sulfide phase at lower Mo-loadings and a crystalline phase at higher loadings. Chapter 4 includes studies on Ti02 supported molybdenum sulfide catalysts. Prepared catalysts have been characterized by surface area measurement, X-ray diffraction, TPR, FTIR and IV LTOC. The HDS, HYD and HDO activities are also studied. The catalytic activities and oxygen chemisorption increase linearly upto 8Wt. %Mo loading and then decreases at higher loadings. X-ray diffraction results indicated presence of Mo03 crystallite at loading about 8wt %. These results are in agreement with the oxygen chemisorption and activity variation with loading. Pure support showed considerable activity for all the reactions studied and also significant oxygen uptake. Corrections were made for the support contributions. Oxygen chemisorption was found to correlate with activities HDS, HYD and HDO. Generally available zirconia and titania are oflow surface area and these surface areas decrease at higher temperatures. And also, at reaction condition zirconia and titania supported catalysts are not structurally stable, i.e., there is possibility to phase transformations. An approach to avoid these disadvantages is to employ mixed oxides as supports for hydrotreating catalysts. These supports are thermally stable and also of high surface area. The catalytic properties and catalytic activities of mixed oxide supported catalysts have been discussed in Chapter 5. Aseries of catalysts were prepared by homogeneous precipitation method using urea hydrolysis. Various catalysts with different Ti/Zr molar ratios as well as end members were prepared by this method. This method is found to give high surface area supports. The prepared catalysts are characterized by XRD, BET surface area, TPR, FTIR, ESCA and oxygen chemisorption. On all these catalysts HDS, HYD and HDO model reactions were carried out. There is also a good correlation between oxygen chemisorption and HDS or HYD activity. The maximum activity is found at 12 wt %Mo-loading. From Temperature Programme Reduction (TPR) results, it was found that there were two distinguishable molybdenum phases which probably may belong to Zr02 and Ti02. Mixed oxide supported catalysts are about 7 times more active than commercial y-Al203 supported catalysts. Chapter 6 deals with sepiolite clay supported molybdenum catalysts. Due to porous nature, low cost and metal-tolerance, sepiolite supported catalysts have an advantage in hydrotreating processes. The catalysts with varying Mo wt %were prepared by impregnation method. Adetailed characterization has been carried out by BET surface area, pore volume, pore size distribution. XRD, SEM, TPR, FTIR and oxygen chemisorption. The HDS and HYD activities were evaluated for these catalysts in micro catalytic reactor. Results indicate that the activity and oxygen chemisorption gradually increase upto monolayer coverage (6 wt % of Mo) and again both decrease with further increase in loading. These results are interpreted in terms of the models proposed in literature. The promotional effects of Co and Ni is also discussed in this chapter. The objective of thesis is to elucidate the effect of support on catalytic functionalities of hydrotreating catalyst and generate structure-activity relationships using low temperature oxygen chemisorption data. All together a comparative analysis of various support effects on physicochemical properties and characteristic activities are discussed with reference to y-A1203 support (Data related to y-Al203 supported catalyst has been taken from ref 171). A general observation indicates that all the supported catalysts studied showed higher HDS and lower HYD activities compared to standard y-Al203 supported catalyst. These discussions will give an overview in understanding the various factors influencing the hydrotreating functionalities.