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CHAPTER 1

INTRODUCTION

Catalysis is one of the most important and widely-spread concepts

of chemistry. Today, catalysis is the workhorse in the chemical industry with

approximately >90% of the products produced in catalytic processes. In

addition, life cannot exist without catalysis, as virtually all biochemical

processes that sustain life are reliant on enzymes, nature’s own catalysts. The

term catalysis was coined by Berzelius in 1836.

Catalysis is defined as the change in rate of a chemical reaction due

to the participation of a substance called catalyst, which is not consumed by

their action. However, the catalyst affects only the rate of the reaction; it

changes neither the thermodynamics of the reaction nor the equilibrium

composition. A catalyst can activate a definite molecule to lower its activation

energy of the corresponding transition state through the adsorption or the

coordination of the molecule on the catalyst (Chorkendorff and

Niemantsverdriet 2003).

More recently environmental concerns have driven the

development of specialized catalysts to prevent polluting substances from

being released into the environment. Since 1950s wide use of catalyst in the

petroleum and petrochemical industries was the most important factor

enabling man to tap into petroleum for energy as well as new raw materials

and new polymer materials resulting in a high level of living standard. After

late 1970 and through 1980, the technology of catalysis moved to better

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understanding of the catalytic function and to improve the catalytic selectivity

in order to save the energy and chemicals for the better economy and for a

cleaner process to meet environmental regulation. In the last 10 years,

catalysts find greater importance in the development of fuel cell technology to

convert fossil fuels into hydrogen, in utilization of toxic gases in advanced

industry, and the gasification of solid wastes to restore natural environment.

1.1 PETROLEUM REFINING

A petroleum refinery is a manufacturing operation where crude

petroleum (the raw material) is converted into usable finished products. In

other words, it is the manufacturing phase of the oil industry. The function of

the refinery is to convert crude oil into the finished products required by the

market in the most efficient and hence most profitable manner. Crude oil is

complex mixtures containing many different hydrocarbon compounds that

vary in appearance and composition from one oil field to another. Crude oil

range in consistency from water to tar-like solids, and in color from clear to

black. An average crude oil contains about 84% carbon, 14% hydrogen, 1-3%

sulfur, and less than 1% each of nitrogen, oxygen, metals, and salts.

Crude oil is generally classified as paraffinic, naphthenic, or

aromatic, based on the predominant proportion of similar hydrocarbon

molecules. Mixed-base crudes have varying amounts of each type of

hydrocarbon. Refinery crude base stocks usually consist of mixtures of two or

more different crude oil. Crude oil that contains appreciable quantities of

hydrogen sulfide or other reactive sulfur compounds is called sour. Those

with less sulfur are called sweet.

1.2 HYDROCARBON COMPOUNDS IN CRUDE OIL

The paraffinic series of hydrocarbon compounds found in crude oil

have the general formula CnH2n+2 and can be either straight chains (normal) or

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branched chains (isomers) of carbon atoms. The lighter, straight-chain

paraffin molecules are found in gases and paraffin waxes. Examples of

straight-chain molecules are methane, ethane, propane, and butane (gases

containing from one to four carbon atoms), and pentane and hexane (liquids

with five to six carbon atoms). The branched-chain (isomer) paraffins are

usually found in heavier fractions of crude oil and have higher octane

numbers than normal paraffins.

Aromatics are unsaturated ring-type (cyclic) compounds which

react readily because they have carbon atoms that are deficient in hydrogen.

All aromatics have at least one benzene ring as part of their molecular

structure. Naphthalenes are fused double-ring aromatic compounds. The most

complex aromatics, polynuclears (three or more fused aromatic rings) are

found in heavier fractions of crude oil.

Naphthenes are saturated hydrocarbon groupings with the general

formula CnH2n, arranged in the form of closed rings (cyclic) and found in all

fractions of crude oil except the very lightest fraction. Single-ring naphthenes

(monocycloparaffins) with five and six carbon atoms predominate, with two-

ring naphthenes (dicycloparaffins) found in the heavier ends of naphtha.

Alkenes are mono-olefins with the general formula CnH2n and

contain only one carbon-carbon double bond in the chain. The simplest alkene

is ethylene. Olefins are usually formed by thermal and catalytic cracking and

rarely occur naturally in unprocessed crude oil.

Dienes, also known as diolefins, have two carbon-carbon double

bonds. The alkynes, another class of unsaturated hydrocarbons, have a

carbon-carbon triple bond within the molecule. Both these series of

hydrocarbons have the general formula CnH2n-2. Diolefins such as

1,2-butadiene and 1,3-butadiene, and alkynes such as acetylene, occur in C5

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and lighter fractions from cracking. These compounds are more reactive than

paraffins or naphthenes and readily combine with other elements such as

hydrogen, chlorine, and bromine.

1.3 NONHYDROCARBON COMPOUNDS IN CRUDE OIL

1.3.1 Sulfur Compounds

Sulfur may be present in crude oil as hydrogen sulfide (H2S), as

compounds (e.g. mercaptans, sulfides, disulfides, thiophenes, etc.), or as

elemental sulfur. Each crude oil has different amounts and types of sulfur

compounds, but as a rule the proportion, stability, and complexity of the

compounds are greater in heavier crude-oil fractions. Hydrogen sulfide is a

primary contributor to corrosion in refinery processing units. Other corrosive

substances are elemental sulfur and mercaptans. Moreover, the corrosive

sulfur compounds have an obnoxious odor.

Pyrophoric iron sulfide results from the corrosive action of sulfur

compounds on the iron and steel used in refinery process equipment, piping,

and tanks. The combustion of petroleum products containing sulfur

compounds produces undesirable products such as sulfuric acid and sulfur

dioxide. Catalytic hydrotreating processes such as hydrodesulfurization

remove sulfur compounds from refinery product streams. Sweetening

processes either remove the obnoxious sulfur compounds or convert them to

odorless disulfides, as in the case of mercaptans.

1.3.2 Nitrogen and Oxygen Compounds

Nitrogen is found in lighter fractions of crude oil as basic

compounds, and more often in heavier fractions of crude oil as nonbasic

compounds. The decomposition of nitrogen compounds in catalytic cracking

and hydrocracking processes forms ammonia and cyanides that can cause

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corrosion. Oxygen compounds such as phenols, ketones, and carboxylic acids

occur in crude oil in varying amounts. The presence of such organic oxygen

containing molecule also affect the selectivity, stability and surface of the

sulfided hydrotreating catalysts (Furimsky 2000).

1.3.3 Trace Metals and Salts

Metals including nickel, iron, and vanadium are often found in

crude oil in small quantities and are removed during the refining process. The

burning of heavy fuel oil in refinery furnaces and boilers can leave deposits of

vanadium oxide and nickel oxide in furnace boxes, ducts, and tubes. It is also

desirable to remove trace amounts of arsenic, vanadium, and nickel prior to

processing as they can poison certain catalysts.

Crude oil often contains inorganic salts such as sodium chloride,

magnesium chloride, and calcium chloride in suspension or dissolved in

entrained water (brine). These salts must be removed or neutralized before

processing to prevent catalyst poisoning, equipment corrosion, and fouling.

Salt corrosion is caused by the hydrolysis of some metal chlorides to

hydrogen chloride (HCl) and the subsequent formation of hydrochloric acid

when crude is heated. Hydrogen chloride may also combine with ammonia to

form ammonium chloride (NH4Cl), which causes fouling and corrosion.

1.4 TYPES OF REACTION IN REFINERY

There are number of processes involved in petroleum refining as

shown in Figure 1.1. These include thermal, catalytic and hydroprocessing

upgrading processes. The hydroprocessing processes include three major

classes, namely, hydrotreating, hydrocracking, and hydrofinishing.

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Figure 1.1 Stream of petroleum processes

1.4.1 Catalytic Reforming

Reforming is an important process used to convert straight-chain

alkane and alkenes into branched and aromatic hydrocarbons (low-octane into

high-octane) and catalysed by bifunctional platinum/Al2O3. Both the metal

and support are involved in the reaction, making the system bifunctional

catalyst. This process involves reaction such as, dehydrogenation,

aromatization, hydrogenation, isomerization and cracking. The first three

reactions are catalysed by the metal and last two reactions are catalysed by

acidic sites on the alumina support. Most of the reactions (except

isomerization and hydrogenation) are strongly endothermic, and a net

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producer of hydrogen. Thermodynamics of this reaction therefore require high

temperature and low pressure. However, such conditions are favourable for

coke formation, which severely limits the life time of the catalysts. For this

reason, hydrogen is recycled at moderate pressures to limit the coke

formation. Hence, the actual reforming conditions (500 ºC, 5-20 bar, H2) are a

compromise between product quality and yield on one hand and reduce coke

formation on the other hand (Chorkendorff and Niemantsverdriet 2003).

Isomerisation is a reforming process in which the structure of the compound is

rearranged to give an isomer with desirable anti-knock properties.

Olefins obtained as by products during cracking can be

polymerized under acidic conditions using H2SO4 or H3PO4 to give high

molecular weight hydrocarbons which can be catalytically hydrogenated to

give branched alkanes.

1.4.2 Hydrocracking

Hydrocracking is a two-stage process combining catalytic cracking

and hydrogenation, wherein heavier feedstocks are cracked in the presence of

hydrogen to produce more desirable products. The process employs high

pressure, high temperature, a catalyst, and hydrogen.

Hydrocracking is used for feedstocks that are difficult to process by

either catalytic cracking or reforming, since these feedstocks are characterized

usually by a high polycyclic aromatic content and/or high concentration of the

two principal catalyst poisons, sulfur and nitrogen compounds. The

hydrocracking process largely depends on the nature of the feedstock and the

relative rates of the two competing reactions, hydrogenation and cracking.

Heavy aromatic feedstock is converted into lighter products very high

pressures (1,000-2,000 psi) and fairly high temperatures (750-1,500 °C), in

the presence of hydrogen and special catalysts. When the feedstock has a high

paraffinic content, the primary function of hydrogen is to prevent the formation of

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polycyclic aromatic compounds. Another important role of hydrogen in the

hydrocracking process is to reduce tar formation and to prevent build-up of

coke on the catalyst. Hydrogenation also serves to convert sulfur and nitrogen

compounds present in the feedstock to hydrogen sulfide and ammonia.

Hydrocracking produces relatively large amounts of isobutane for

alkylation feedstock. Hydrocracking also performs isomerization for pour-

point control and smoke-point control, both of which are important in high-

quality jet fuel. Noble metals such as Pt, Pd, etc., supported on alumina or

zeolites are used as hydrocracking catalyst.

1.4.3 Hydrotreating

The crude oil contains S, N, O, and metals (Ni, V) and makes high

demands on the catalyst performance. The presence of nitrogen compounds

leads to poor color, smell and subsequently NOx causes pollution of the

atmosphere. N-compounds also act as poison for hydrocracking and reforming

catalysts in the later stages of oil refining. Furthermore, the intensified

protection of the environment has led to the sharpening of the norms for the S,

N and metal content of the petroleum products (Stinner et al 2001).

The basis for hydrotreating was laid in 1910 by Bergius who

renewed coal into gaseous and liquid fractions applying non-catalytic

hydrogenation and cracking at high temperatures and pressures. Although the

yield of Bergius' process (hydrogenation of coal) was high, the products were

not suitable because they mainly contained hydrocarbons with high molecular

weights and in addition contained large amounts of oxygen, nitrogen and

sulphur compounds. An improvement was seen when catalysts were used for

this process. Hydrotreating catalysts originated in the 1920s when German

researchers developed unsupported metal sulfide catalysts for liquefying coal.

However, it was not until 1970s that the structures of these catalysts and the

mechanisms of their catalytic action began to be understood. It was

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established that under catalytic reaction conditions, most of the molybdenum

in industrial hydrotreating catalysts is present as small MoS2 particles in the

pores of -Al2O3. It was not until the 1980s that the location of the cobalt and

the nickel promoter ions in the hydrotreating catalysts was more or less

determined. The role of phosphate and fluorine additives is still under

investigation. Supports other than -Al2O3 like amorphous silica-alumina, are

also used in commercial units and their functions are topics of academic and

industrial research.

Hydrotreating is a generic name given to processes utilizing

hydrogen, which include Hydrodesulfurization (HDS), Hydrodenitrogenation

(HDN), Hydrodemetallization (HDM), hydrogenation and hydrofinishing.

Hydrotreating is the catalytic conversion of organic sulfur, nitrogen, oxygen,

and metal-containing molecules from crude oil at high hydrogen pressures

and includes the hydrogenation of unsaturated compounds and cracking of

petroleum feedstock to lower molecular hydrocarbons. As a consequence,

hydrotreating is the largest application of industrial catalyst on the basis of the

amount of material processed per year. Typically, hydrotreating is done prior

to processes such as catalytic reforming so that the catalyst is not

contaminated by untreated feedstock. Hydrotreating is also used prior to

catalytic cracking to reduce sulfur and improve product yields and to upgrade

middle-distillate petroleum fractions into finished kerosene, diesel fuel, and

heating fuel oil. In addition, hydrotreating converts olefins and aromatics to

saturated compounds.

HDS is a process employed to remove sulfur from petroleum and

other fossil fuel stocks through reaction with hydrogen in order to prevent

sulfur from poisoning the metal catalyst and also to remove the unpleasant

odour of lube oil.

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HDN is a process in which organonitrogen compounds are removed

from hydrocarbon feed stocks to produce processible, stable and

environmentally acceptable liquid fuels and lube base stocks. HDN is an

important reaction in petroleum refining, because it removes nitrogen, which

causes pollution when fuels are burnt and also poisons the catalyst when

petroleum distillates are processed. The central reaction in HDN is the

breaking of the C-N bond of nitrogen containing hydrocarbons, in which the

nitrogen atom is removed as ammonia and thereby the hydrocarbons are free

from nitrogen. Hydrogenation followed by hydrogenolysis is found to be a

very effective and advantageous method for removing nitrogen and sulfur

from petroleum. Since the remaining fragments of the N- and S- containing

compounds are left as pure hydrocarbons in the products.

Oxygen containing compounds are acidic and their presence,

especially in commercial petroleum products, is unwelcome. Most of the

oxygen exists as hydroxyl groups and carbonyl groups. Water formed as a

result of hydrodeoxygenation (HDO) is known to act as a poison for catalysts.

These compounds interact with sulfur and nitrogen thereby reducing the rate

of HDN and HDS.

1.5 SULFUR CONTAINING COMPOUNDS

Organo sulfur compounds are usually present in almost all fractions

of crude oil distillation. Higher boiling fractions contain relatively more sulfur

compounds with high molecular weight. Therefore, a wide spectrum of sulfur-

containing compounds should be considered from the viewpoint of their

reactivity in the hydrotreating processes. The common types of sulfur

compounds in liquid fuels are mercaptans, sulfides, disulfides, thiophene and

benzothiophene.

Figure 1.2 presents a qualitative relationship between the type and

size of sulfur containing organic molecules in various distillate fuel fractions

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and their relative reactivities (Song 2002). The reactivity ranking in

Figure 1.2 is based on well-known experimental observations and literature

information (Knudsen et al 1999, Whitehurst et al 1998 and Song and Ma

2003). HDS occurs directly through hydrogenolysis pathway for the sulfur

compounds without a conjugation structure between the lone pairs on S atom

and the -electrons on aromatic ring, disulfides, sulfides, thiols, and

tetrahydrothiophene.

Figure 1.2 Reactivity of organic sulfur compounds in

hydrodesulfurization (HDS)

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These sulfur compounds exhibit higher HDS reactivity than that of

thiophene by an order of magnitude because they have high electron density

on the sulfur atom and weak C–S bond. The reactivities of the

1 to 3 ring sulfur compounds decrease in the order thiophenes >

benzothiophenes > dibenzothiophenes (Frye and Mosby 1967, Kilanowski et

al 1978, Houalla et al 1980, Girgis and Gates 1991, Vasudevan and Fierro,

1996). In naphtha, thiophene is much less reactive than thiols, sulfides, and

disulfides that the latter can be considered to be virtually infinitely reactive in

practical high-conversion processes (Gates and Topsoe 1997). Similarly, in

gas oil, the reactivities of (alkyl-substituted) 4-methyldibenzothiophene and

4,6-dimethyldibenzothiophene (4,6-DMDBT) are much lower than other

sulfur-containing compounds (Kabe et al 1992, Ma et al 1994 and 1995).

Consequently, in deep HDS, the conversion of these key substituted

dibenzothiophenes largely determines the required reaction conditions.

Gates and Topsoe (1997) pointed out that, the 4-methyldibenzothiophene and

4,6-DMDBT are the most appropriate compounds for investigations of

candidate catalysts and reaction mechanisms.

1.6 NITROGEN CONTAINING COMPOUNDS

Nitrogen-containing compounds in petroleum and coal-derived

liquids are normally divided into two groups: heterocyclic and non

heterocyclic. Noncyclic nitrogen compounds such as aliphatic amines and

nitriles are present in oil in small amounts and are easy to denitrogenate.

Among non-heterocyclic nitrogen compounds, aniline derivatives are the

most important ones in HDN because they are always formed in the HDN

network of heterocyclic nitrogen compounds and they are more difficult to

denitrogenate than aliphatic ones. Heterocyclic nitrogen containing

compounds are present in larger amount and are very difficult to remove.

They can be divided into basic and nonbasic compounds. Nonbasic

compounds consist of five-membered heterocycles such as pyrrole, indole,

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carbazole, etc., The lone-pair electrons on the nitrogen atom of these

compounds are delocalized around the aromatic ring and are not available for

donation to acid sites on catalyst surfaces.

Table 1.1 Representative nitrogen-containing compounds in petroleum

crude, Shale oil and Cole-derived liquids

Compound Formula Structure

Nonheterocyclic compounds

AnilineC6H5NH2

Pentylamine C5H11NH2

Nonbasic heterocyclic compounds

PyrroleC4H5N

IndoleC8H7N

CarbazoleC12H9N

Basic heterocyclic compounds

PyridineC5H5N

QuinolineC9H7N

IndolineC8H9N

AcridineC13H9N

Benz(a)acridineC17H11N

Benz(c)acridineC17H11N

Dibenz(c,h)acridineC21H13N

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Basic compounds include six-membered heterocycles such as

pyridine, quinoline, acridine, etc. The lone-pair electrons on the nitrogen atom

of these compounds, on the contrary, are not tied up in the cloud of the

heterocyclic ring and are available for sharing with acid sites on catalyst

surfaces. The saturated five-membered heterocycles, like indoline are strongly

basic as aliphatic amines. Table 1.1 lists the various types of nitrogen-

containing compounds (without alkyl substitution).

1.7 HYDROTREATING CATALYSTS

Industrial hydrotreating catalyst contains molybdenum and cobalt

or nickel, supported on -Al2O3. MoO3/Al2O3 and Co/Ni/Al2O3 tested for the

removal of S, N and O atoms. It is found that MoO3/Al2O3 is highly active

than Co and Ni sulfide/Al2O3. Therefore, molybdenum sulfide is traditionally

considered to be the catalyst. As a consequence, cobalt and nickel are referred

to as the promoters of the Mo activity (Ho 1988 and Prins et al 1989). Cobalt

is used mainly as a promoter for sulfided Mo/Al2O3 in HDS, and nickel in

HDN. In addition to molybdenum and cobalt or nickel, hydrotreating catalysts

often contain additives such as phosphorus, boron, fluorine or chlorine, which

may influence the catalytic as well as the mechanical properties of the catalyst

(Lewandowski and Sarbak 2000, Jian et al 1995, Sun et al 2003 and Gioia and

Murena 1998). Typical hydrotreating catalysts need to be sulfided to activate

the reacting sites. Sulfiding is traditionally done during the start-up phase by

exposing the catalyst to the sulfur-containing liquid feed or to the gaseous

mixture of H2S and hydrogen at high temperature. The sulfiding procedure

has significant influence on the catalytic activity and stability.

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1.8 DIFFICULTIES OF HDN

HDN is typically more difficult than HDS. This can be partially

explained by the relative bond strength as shown in Table 1.2. The C N bond

energy (615 kJ/mol) is higher than the C N bond (308 kJ/mol), and the C N

bond is also stronger than the C S bond (259 kJ/mol). That is why the

aromatic nitrogen containing compounds, especially the aromatic nitrogen-

containing heterocycles must be saturated before they can be further

denitrogenated, unless some materials with metallic properties such as metal

carbides and nitrides are used as HDN catalysts. Heavier feedstocks such as

VGO, residue, light cycle oil from VGO and residue, and coal-derived liquids

usually contain much higher concentration of nitrogen containing compounds

than straight-run streams. They are more complex in structure and difficult to

denitrogenate. Nitrogen containing compounds often have a strong adsorption

capacity on the catalyst surface. Self-Inhibition by the high concentration of

these compounds as well as the secondary compounds leads to a much lower

HDN rate. More severe catalyst deactivation is caused by the high molecular

weight-aromatic compounds in the feedstock. During HDN, coke is also

formed, mainly by condensation of the basic nitrogen-containing compounds

on Lewis acid sites of the catalyst.

Table 1.2 Bond Energies between various atoms

BondEnergy,

kJ/molBond

Energy,

kJ/mol

436 391

413 308

348 615

614 891

839 259

358 577

799 347

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1.9 STRUCTURE OF HYDROTREATING CATALYSTS

1.9.1 Structure of the Oxidic Catalyst

Hydrotreating catalysts are usually prepared by a sequential pore

volume impregnation procedure or by co-impregnation (Prins et al 1989 and

Chianelli et al 1994). In the former method, -Al2O3 support is first

impregnated with an aqueous solution of ammonium heptamolybdate

(NH4)6Mo7O24, and then with an aqueous solution of nickel nitrate Ni(NO3)2

or cobalt nitrate Co(NO3)2. In the latter method, both the Mo and Ni

precursors are impregnated simultaneously. Massoth (1979) explained the

formation of monolayer structure of MoO3 over Al2O3 surface. The results

concluded that there is a strong interaction between molybdenum with the

hydroxyl groups on the Al2O3 surface has been assumed to result in a MoO3

monolayer structure. A similar conclusion was drawn in a combined 1H-NMR

and low temperature chemisorption study (Kraus and Prins 1996) and in

several EXAFS studies (Chiu et al 1984, Parham and Merrill 1984 and

Clausen et al 1986). Infrared emission spectroscopy could not detect bands

due to a MoO3 phase even at high loadings (15 wt.%) of molybdenum on

Mo/Al2O3 catalysts, indicating that the molybdenum is present in a highly

dispersed phase (Li et al 1991). Investigations on Ni and Co promoted

catalysts confirmed an interaction between molybdenum and nickel or cobalt

in the catalyst in the oxidic state. The infrared absorption bands of NO

adsorbed on CoMo/Al2O3 are shifted from those of NO on Co/Al2O3 (Topsoe

and Topsoe 1982), and Raman bands due to polymeric molybdenum oxide

species decrease in intensity with increasing cobalt loading in an oxidic

CoMo/Al2O3 catalyst (Gao and Xin 1993). The results suggested that nickel

or cobalt cations interact especially with the most highly polymerized

molybdenum oxide species. In this way the promoter cations stay at the

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surface and close to the molybdenum cations and are well positioned to form

the active Ni-Mo-S structure during sulfidation.

1.9.2 Structure of the Sulfidic Catalyst

The oxidic catalyst precursors are transformed into the actual

hydrotreating catalyst by sulfidation in a mixture of H2 and one or more

compounds containing sulfur. H2S, thiophene, CS2, dimethyl disulfide or the

oil fraction to be hydrotreated can be used for the sulfidation. The properties

of the sulfided catalyst depend to a great extent on the calcination and

sulfidation conditions. Calcination at high temperature induces a strong

interaction between molybdenum and cobalt or nickel cations and the Al2O3

support. Consequently, it is difficult to transform completely the oxidic

species into sulfides. Mossbauer spectroscopy of CoMo/Al2O3 catalysts

showed that, at increasingly high calcination temperatures, more Co2+ ions are

incorporated into the bulk of the alumina (Wivel et al 1984). The higher the

calcination temperature, the higher the sulfidation temperature needed to

bring these cations back to the surface to provide a high catalytic activity for

hydrotreating. At temperatures that are too high, however, the metal sulfide

particles sinter or do not form the catalytically active Co-Mo-S structure.

Optimum calcination and sulfidation temperatures are in the range of

673-773 K for Al2O3-supported catalysts (Prada Silvy et al 1989a). MoS2 has

a layer lattice, and the sulfur-sulfur interaction between successive MoS2

layers is weak (Van der Waals force). Crystals grow as platelets with

relatively large dimensions parallel to the basal sulfur planes and small

dimensions perpendicular to the basal planes. Investigations of model

catalysts consisting of MoS2 grown on -Al2O3 films on the surfaces of

MgAl2O4 supports have shown that MoS2 grows with its basal plane parallel

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to the (111) surface of -Al2O3 and perpendicular to the (100) -Al2O3 surface

(Sakashita and Yoneda 1999). This observation also suggested that the edges

of the MoS2 platelets are bonded to the (100) surface of -Al2O3 by Mo-O-Al

bonds (Figure 1.3).

(a) (111) - Al2O3 (b) (100) - Al2O3

Figure 1.3 Orientation of small MoS2 particles on (a) (111) and

(b) (100) -Al2O3 surface

Nickel may be present in three forms after sulfidation: as Ni3S2

crystallites on the support, as nickel atoms adsorbed on the edges of the MoS2

crystallites (the so-called Ni-Mo-S phase) and as nickel cations at octahedral

or tetrahedral sites in the -Al2O3 lattice (Figure 1.4). Analogously, cobalt can

be present as segregated Co9S8, as Co-Mo-S and as cobalt cations on the

support. Depending on the relative concentrations of nickel (or cobalt) and

molybdenum and on the pre-treatment conditions, a sulfided catalyst may

contain a relatively large amount of either Ni3S2 (or Co9S8) or the

Ni-Mo-S (or Co-Mo-S) phase.

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Figure 1.4 Three forms of nickel present in a sulfided NiMo/Al2O3

catalyst: as active sites on the MoS2 edges (the so-called

Ni-Mo-S phase), as segregated Ni3S2 and as Ni2+

ions in the

support lattice

Several in-situ characterization techniques such as Mossbauer,

infrared, and EXAFS confirmed the Ni-Mo-S (or Co-Mo-S) edge decoration

model. The infrared spectra of NO molecules adsorbed on sulfided

CoMo/Al2O3 catalysts indicated that as the cobalt content increased at fixed

molybdenum content, the number of NO molecules adsorbed on cobalt sites

increased and the number of NO molecules adsorbed on molybdenum sites

decreased (Topsoe and Topsoe 1983). Cobalt atoms at edge-decoration sites

cover molybdenum atoms and block adsorption of NO on these molybdenum

atoms. The observed behavior is therefore in accordance with the edge-

decoration location. EXAFS studies showed that a nickel atom in a sulfided

NiMo catalyst supported on -Al2O3 or on carbon is surrounded by four or

five sulfur atoms at a distance of 2.2 Å, by one or two molybdenum atoms at a

distance of 2.8 Å and by one nickel atom at a distance of 3.2 Å (Louwers and

Prins 1992). These data are consistent with a model in which the nickel atoms

are located at the MoS2 edges in the molybdenum plane in a square pyramidal

coordination. The nickel atoms are connected to the MoS2 by four sulfur

atoms and depending on the H2S partial pressure, a fifth sulfur atom may be

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present in the apical position in front of the nickel atom (Figure 1.5). Density

functional theory (DFT) calculations suggested a different edge-decoration

model (Byskov et al 1999). Instead of substituting molybdenum atoms at the

molybdenum edge, cobalt atoms were claimed to prefer to substitute

molybdenum atoms at the sulfur edge. Other DFT calculations

indicated that these particular edge positions are an artifact of the too small

MoS2 clusters used to model MoS2 in the calculations (Byskov et al 1999).

DFT calculations with larger MoS2 clusters showed that the most favorable

location of the promoter atoms is the substitutional position at the

molybdenum edge. The nickel and cobalt atoms extend, as it were, the MoS2

lattice by taking up molybdenum positions (Raybaud et al 2000). This

conclusion is in good agreement with the EXAFS results (Figure 1.5)

(Louwers and Prins 1992).

Figure 1.5 Structure of Ni-Mo-S phase as determined by EXAFS studies

The structure of the active phase has been a matter of great debate.

Voorhoeve and Stuive (1971) proposed the intercalation of co-promoter atoms

between alternating MoS2 layers (intercalation model), while Farragher and

Cossee (1973) suggested that the promoter ions are located in alternate layers

at the edges (pseudo–intercalation or decoration model). The remote control

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or contact synergy model of Delmon (1990), the physical contact between

separate Co9S8 and MoS2 crystallites offers an explanation for the

promotional effect, the first one causing spill–over hydrogen that enhancing

the activity of the MoS2 phase. Ratnasamy and Sivasanker (1980) suggested

the promoter ions to be located at the edges of the MoS2 layers. Topsoe and

Topsoe (1983) produced convincing experimental evidence for this on the

basis of IR studies of NO molecules adsorbed on these catalysts. Using

Mössbauer Emission Spectroscopy (MES), Topsoe et al (1981) and Wivel et

al (1984) assigned a specific Co signal to a so–called ‘Co–Mo–S’ interaction

phase different from Co9S8. Wivel et al (1981) observed a linear correlation

between the amount of co-promoter ions present in this ‘Co–Mo–S’ phase and

thiophene HDS activity. Later, the promoter atoms were found present at the

edges of MoS2 crystals was obtained by Analytical Electron Microscopy

(Sorensen et al 1985). The term ‘Co–Mo–S’ structures has been used

exclusively to describe the local structures at the edges of MoS2 slabs

involving Co, Mo, and S atoms. It was commonly proposed that the

catalytically active sites in hydrotreating catalyst are the molybdenum atoms

at the surfaces of the MoS2 crystallites, with at least one sulfur vacancy at a

site to allow the reacting molecule to bond chemically to the molybdenum

atom.

1.10 EFFECT OF MoO3 LOADING ON HYDROTREATING

PROCESS

It has been understood from the literature that an optimum level of

MoO3 loading is required for a particular support. The catalytic activity has

been found to increase as the MoO3 loading increased. Many reports are

available in this context. Muralidhar et al (1984 and 2000) studied the effect

of Mo loading on metal oxides supported hydrotreating catalyst by varying

the percentage of MoO3 from 2 to 16 wt.%. It is found that no XRD pattern

22

resembling that of MoO3 up to 14 wt. % and 8 wt.% of MoO3 on ZrO2-TiO2,

and MoS2/TiO2–Al2O3 (1:1) respectively, indicating that MoO3 is well

dispersed. In another study by Landau et al (2001), on AlMCM-41 supported

CoO-MoO3 only at 45 wt. % formation of MoO3 was observed when the

catalysts was synthesised by ultrasonic. However, on H-Al-MCM-41, up to

24 wt.% of Mo loading no crystallites due to MoO3 was observed

(Sardhar Basha et al 2006). Over SBA-15 and Al-SBA-15 supported NiMo

catalysts 8 wt.% MoO3 has been reported to be the optimum for better

dispersion. (Muthu Kumaran et al 2007). However, in another study it is

reported that until 17 wt.% of molybdenum, there is complete dispersion of

MoO3 without any crystalline species on SBA-15 support (Sundaramurthy et

al 2008b).

1.11 ROLE OF ADDITIVES IN HYDROTREATING

To increase activity and stability of the hydrotreating catalysts,

elements such as fluorine, phosphorous and boron are used as additives to the

support. Fluorinated hydroprocessing catalysts are used in refineries for

processing lubricant oil (Stanulonis et al 1982). With increasing demand for

hydrotreating, fluorine is widely studied as an additive in hydrotreating and

hydrocracking catalysts containing Mo and W supported on alumina, silica–

alumina, and zeolites. The enhancement of the activity by fluorine addition

has been attributed to high acidity, good dispersion, and high chemisorption

capacity for hydrogen. In most cases, fluorination of the catalyst is done by

impregnating the support with a fluoride salt, like NH4F (Ramirez et al 1990

and Lewandowski et al 1997). When fluorine is incorporated in CoMo-Al2O3

an increase in the rate of cyclohexane hydrogenation and decrease in the rate

of cyclohexene isomerization was reported (Boorman et al 1987).

Jiratova and Kraus (1986) reported that the addition of 3 wt.% F to

a raw CoMo-Al2O3 catalyst increased substantially both hydrogenation and

23

isomerization of cyclohexene. Ramirez et al (1990) showed that fluorine

incorporation enhanced appreciably and moderately the hydrogenation

activity of Mo and CoMo catalysts, respectively; this increase being only

roughly related to Mo dispersion in both types of catalysts.

The changes induced by fluorination on the surface distribution of

the oxidic tungsten species led to improvements in the sulfidation, significant

changes in the morphology of the WS2 structures, and increased activity for

both gas oil HDS and pyridine HDN. Moreover, it was found that the

promotion induced by fluoride was relatively higher for HDN than for HDS

(Benitez et al 1995).

The order of fluorine addition changes the dispersion of the nickel

and the tungsten species, incorporation of nickel with the tungsten edge sites,

and consequently the HDS activity of the catalysts. The order of fluorine

addition does not affect the catalyst surface area; neither does it affect the

amount of fluorine retained in the catalyst after calcination. The dispersion of

the metal species and incorporation of nickel with the tungsten edge sites also

increase in the order of W-Ni-F> W-F-Ni> F-W-Ni. Fluorine addition

enhances the thiophene HDS activity in the order W-Ni-F> W-F-Ni> F-W-Ni.

The HDS activity of NiW/A12O3 is enhanced more by the repartition of the

metal species than by partial solubilization of alumina in the fluorine-addition

step (Kwak and Moon 1999).

Guemez et al (1995) studied the effect of fluorine addition to

molybdena-alumina catalyst doubly promoted by Zn and Co for

hydrodenitrogenation of quinoline and found that HDN activity increased by

the addition of fluorine and attained maximum in the region 1.0-1.3 wt.% F.

The increased activity has been attributed to the increase in the number of

catalytically active metal sulfide sites. Marques et al (2011) studied the effect

of addition of Fluorine on acidity of alumina supported Ni-Mo catalyst.

24

Phosphorous, as phosphate, mainly alters the acid–base properties

of the support, which is directly correlated to the active phase dispersion

(Lewis et al 1992). Therefore phosphorus is frequently used to modify the

activity of hydrotreating catalyst. (Eijsbouts et al 1991, Lewis et al 1992). The

addition of phosphorus has been found to increase the formation of

polymolybdate species which are more active for hydrotreating reaction. It is

also observed that reducibility of these species is increased with phosphorous

loading. However, at high phosphorous loading the formation of crystalline

MoO3 species is reported (Maity et al 2008).

Addition of boron and phosphorous caused the formation of

extremely strong acid sites on the catalyst. Though the addition of boron and

phosphorous to NiMo/Al2O3 catalyst did not show any significant effect on

sulfur conversion, the HDN and HDS activities of the catalyst containing

1.7 wt.% boron and the one containing 2.7 wt.% phosphorous are comparable

to those of a commercial hydrotreating catalyst (Ferdous et al 2004).

It is reported in the literature that the addition of phosphorous to

NiMo/ -Al2O3-carbide catalyst has enhanced the dispersion of

Ni-Mo carbide phase, changed the nature of surface active sites and increased

HDN activity through accelerated C–N bond breaking, compared to the

NiMo/ -Al2O3 catalysts. HDN activity of phosphorous modified NiMo nitride

catalyst was enhanced by addition of phosphorous, with maximum activity at

1.6 wt.% of phosphorous loading. Also the high activity of phosphorous

modified catalyst has been attributed due to increased acidity of the catalyst

(Sundaramurthy et al 2007, 2008a).

25

1.12 SUPPORT EFFECT IN HYDROTREATING

Alumina has been widely used to as a support for industrial

hydrotreating catalyst. Alumina supported CoMo and NiMo hydrotreating

catalysts have been investigated extensively and the literature is exhaustive.

There has been considerable interest to study the use of other supports such as

silica, carbon, mixed metal oxides, zeolites and mesoporous materials. These

supports have been used either alone or mixed with alumina. The effect of

these materials as support for Ni-Mo and Co-Mo is discussed with reference

to hydrotreating process. Thus, it is appropriate to discuss in the following

section, the influence of supports on the surface area, dispersion of active

phases, the interaction of the active component with the support, and the

degree of sulfidation as these factors significantly affect the HDN activity and

selectivity.

1.12.1 Carbon

Carbon has been considered as a good support with regard to the

interaction of metals with support for hydrotreating catalyst. (Topsoe et al

1986, Vissers et al 1987, Boorman and Chong 1992). Carbon supported

Ni-Mo catalyst is less likely to be poisoned by basic nitrogen-containing

compounds during HDS (Hillerova et al 1991) and more resistant to coke

formation (Boorman and Chong 1992). The high activity of carbon-supported

tungsten oxide catalyst is attributed to the weak interaction of the active

metals with the support (Topsoe et al 1986) and complete sulfidation of

tungsten oxide (Louwers and Prins 1993).

Recently, mesoporous carbon substituted hydrotreating catalysts are

used as support for Ni-Mo catalyst. This catalyst has been reported to be

better than the commercial alumina supported catalysts. Hussain and Ihm

26

(2007) studied the mesoporous carbon supported Mo catalyst as a function of

molybdenum loading with respective pore structure of the carbon material and

found that CMK-3 supported Mo catalysts are superior to CMK-1 and Al2O3

supported catalysts due to its larger pore size and higher acidic functional

groups.

To improve the low density and low mechanical strength of

activated carbon, it was combined with alumina support. Liu et al (2011)

studied the alumina activated carbon (AAC) composite as the support for

NiMo HDS catalyst and found that AAC supported catalyst is better than

NiMo supported individually on Al2O3 and activated carbon in terms of

activity and stability . The outstanding activity of NiMo/AAC is attributed to

the mesoporous structure of the AAC support and good dispersion of Ni and

Mo species on the AAC.

1.12.2 Oxides as Support

Silica was often used as support because of its inert character and

its weak interaction with the sulfided phase which permits a better

understanding of the sulfided Mo phase on the support surface. NiMo/SiO2

and NiW/SiO2 catalysts were prepared by impregnation of the support with

dimethylformamide solution of molybdophosphoric and tungstophosphoric

acids and found to be more active for the HDN of a model mixture containing

pyridine (2000 ppm N) and thiophene (6000 ppm S) in cyclohexane than the

corresponding NiMo/Al2O3 and NiW/Al2O3 catalysts (Rives et al 2001). This

has been attributed to the weak interaction of the heteropoly acids with SiO2

than with Al2O3. This has also favored the formation of the more active type

II structure upon sulfidation of the former. However, there are also reports

stating that the activity is low when silica alone is used as support than when

SiO2 is used in combination with Al2O3 for hydrotreating (Massoth et al

27

1984). The increased activity in mixed oxide catalyst is due to the formation

of uniform monolayer with fine dispersion of active metal.

Oxides like titania and zirconia have been studied as supports for

hydrotreating catalyst. The high activity of titania supported catalyst is due to

the uniform distribution of hydroxyl groups on the surface which can provide

a homogenous surface for the adsorption of molybdate anions with the

tetrahedrally coordinated Ti4+. On the other hand, on Al2O3, A13+ ions are

octahedrally and tetrahedrally coordinated and hydroxyl groups are ordered in

parallel rows (Knozinger and Ratnasamy 1978). Such preferential

arrangement in rows requires an arrangement of the molybdate anions during

the impregnation process, which leads to molybdenum trioxide even at sub

monolayer loadings. These observations are in agreement with XPS data

recorded on the oxidic precursors by Nag (1987) and Caceres et al (1990).

The reducibility of molybdate species depends on the support. Molybdenum

on Mo/TiO2 is more easily reduced than on Mo/Al2O3; this result was

confirmed using high pressure and high temperature ESR technique. Kohno et

al (1986)

ZrO2 has been attracting attention of researchers as catalyst as and

as support due to its high thermal stability, extreme hardness, and high

specific mass of zirconia. It has both acidic and basic properties. Nag (1987)

reported that Mo oxidic species on ZrO2 are homogenously distributed and

chemically coordinated better than on Al2O3. According to Zaki et al (1986),

there cannot be the formation of monolayer of Mo on the ZrO2 and MoO3 can

only form aggregates. Large differences in CO hydrogenation, observed by

Mauchausse et al (1998) for Mo/ZrO2 and Mo/CeO2, compared to Mo/Al2O3

The variations in the density of the active sites depending on residual

Mo-O-Al bonds is responsible for the hydrogenation properties. This

property has been exploited for the hydrogenation of pyridine and

28

hydrodenitrogenation of piperidine (Portefaix et al 1989) on transition metals

sulfide hydrotreating catalyst. The high activity of Mo/ZrO2 has been

attributed to the increase in the number of active sites Mauge et al (1991).

However, pure titania and zirconia supports generally have low surface area

and porosity; their commercial applications are limited due to lack of thermal

stability and mechanical properties. With an aim to overcome these

disadvantages mixed oxides of these materials with -Al2O3 have been used as

supports to take advantage of favorable characteristics of both the systems.

The Hydrocracking catalyst is bifunctional, having both hydro-

dehydrogenation and acidic function. The hydro-dehydrogenation component

is usually a sulfide phase, while the acidic component is generally a zeolite or

an amorphous silica alumina (ASA). To obtain a good selectivity towards gas

oil and kerosene, the hydrocracking catalyst should be moderate and

significantly lower than that of zeolites. Nevertheless, too weak acidic

properties lead to a catalyst of low activity. Consequently, balanced acidity is

crucial to obtain high conversion and appropriate selectivity. The use of

oxides presenting moderate acidic properties as ASA is preferred to maximize

middle distillate products.

The influence of chelating agents on the hydrodesulfurization

(HDS) activity of -Al2O3 and SiO2-Al2O3 supported CoMo catalysts was

studied by Al-Dalama and Stanislaus (2006) and found that the addition of a

complexing agent (EDTA or nitriloacetic acid (NTA) to the impregnation

solution resulted in a remarkable improvement in the HDS activity of both

-Al2O3 and SiO2-Al2O3 supported CoMo catalyst.The HDS reaction of

dibenzothiophene was studied over Alumina-Silica-Alumina (ASA) supported

CoMo hydrotreating catalyst (Hensen et al 2007) and pointed out that the

surface acidity play a prime role. The high activity of ASA supported catalyst

29

is due to preferential adsorption of the dibenzothiophene molecule on the

active Co-Mo-S sites.

The effect of preparation methods of supports on hydroprocessing

of Maya heavy crude was studied over Al2O3-TiO2 binary oxide. The catalyst

supported on binary oxide was found to exhibit high activity and stability with

time-on-stream (Maity et al 2006).

Srinivas et al (1998) and Muralidhar et al (2003) have studied the

activity of MoO3 and WO3 catalysts supported on Al2O3-TiO2 oxide, pure

TiO2 and pure Al2O3 for HDS of thiophene. The catalyst supported on

Al2O3-TiO2 (1:1) showed 5 times higher activity than Al2O3-supported

catalysts and 2 times higher than TiO2 supported catalysts. From the low-

temperature oxygen chemisorption and temperature-programmed reduction

results, it was found that the presence of large number of easily reducible

MoO3 species on binary oxide compared with Al2O3-supported catalyst.

Pophal et al (1997) also found that alumina-titania supported catalysts were

highly efficient for hydrodesulfurization of 4,6-dimethyldibenzothiophene.

Maity et al (2003a and 2003b) have observed that catalyst supported on

Al2O3-TiO2 showed high activity compared with catalysts supported on Al2O3

and Al2O3-SiO2

The beneficial effect of Al2O3, zeolite mixed-Al2O3 and Al2O3-

TiO2, ZrO2-Al2O3 was discussed for the deep hydrodesulfurization of

4, 6-dimethyl dibenzothiophene (Bej et al 2004) and reported that removal of

sulfur atom was easier on mixed oxide supported catalyst than the others.

Titania-alumina of various compositions (atomic ratios: 1/9, 1/1

and 9/1) were prepared from the sulfates by co precipitation with aqueous

ammonia or urea. A maximum acidity was measured for TiO2-Al2O3 (1/9)

prepared with ammonia while no acid sites were found for TiO2-Al2O3 (1/9)

30

prepared with urea. Basic property appeared only for TiO2-Al2O3 (1/1)

prepared with ammonia when it was exposed to water vapor. The maximum

activities were observed on TiO2-Al2O3 (1/9 and 1/1) prepared with ammonia

for the isomerization of 1-butene and on TiO2-Al2O3 (1/9) prepared with

ammonia for the dehydration of 2-butanol, respectively (Rodenas et al 1981).

Zhang et al (2010) prepared Al2O3–ZrO2 composite supported

NiMo catalysts with various ZrO2 content and tested for hydrotreating. From

the results it is found that the composite supports prepared by the chemical

precipitation method existed as amorphous phase in the samples with

insufficient amount of ZrO2, and further incorporation of ZrO2 into supports

provided a better dispersion of NiMo species and easy reduction. The Lewis

acid sites of catalysts increased significantly by the introduction of ZrO2 into

alumina. The results showed that the ZrO2–Al2O3 supported NiMo catalysts

with suitable ZrO2 content exhibited much higher catalytic activity than Al2O3

supported catalyst.

1.12.3 Mesoporous MCM-41 as Support

Zeolites and related molecular sieves are largely being used for

various catalytic applications because of their unique structural and textural

properties. However, the performance of the zeolitic systems is limited by

diffusional limitations of bigger molecules for catalytic conversions within

the zeolitic channels. Hydrodenitrogenation being slower than HDS, may not

be affected by diffusional limitations, in which case variations in pore size

within a reasonable range may not affect the catalytic activity. However, with

high molecular weight nitrogen containing compounds, diffusional limitations

become promising with decreasing pore size (Katzer and Sivasubramanian

1979).

31

The major breakthrough came when Mobil reported the successful

synthesis of mesoporous M41S materials in 1992 (Beck et al 1992, Kresge

et al 1992). Since then, hexagonal mesoporous silica [HMS] (Tanev and

Pinnavaia 1995), mesoporous aluminophosphate (Zhao et al 1997), different

types of Mesoporous metallophosphates (Jones et al 2000) and other

mesoporous molecular sieves such as FSM-16 (Inagaki et al 1993), KIT

(Ryoo et al 1996) and so forth, were also synthesized successively. Among

the other members of the M41S family, the synthesis and catalytic

applications of MCM-41 type mesoporous materials have been investigated

extensively. This material exhibits hexagonal array of unidimensional

mesopores, the diameter of which can be tuned in the range between 20 and

100 Å with a narrow pore size distribution, through the proper choice of

surfactants as the template, auxiliary chemicals, and reaction conditions. The

unique properties of MCM-41 are exploited for the transformation of bulky

molecules. The large pore channels may reduce pore diffusion limitation and

allow the effective use of the active sites on the surface of the pore wall by the

reactants (Ying et al 1999).

After the discovery of MCM-41, this material has been used as a

carrier for mild hydrocracking (MHC). The MHC performance of a

NiMo/MCM-41 catalyst was compared with that of amorphous silica-alumina

and ultrastable low unit cell size Y zeolites (USY) having same Ni and Mo

content. From the point of the acidity of the carrier, USY zeolite showed the

highest amount of Bronsted acidity, most of the sites having medium-strong

acid strength. The acidity of MCM-41 was similar to that of amorphous silica-

alumina, both in number and acid strength distribution. Moreover, most of the

Bronsted acid sites in mesoporous MCM-41 and silica-alumina carriers are of

weak-medium strength, as it is required for producing diesel in MHC

operation. However, it should be taken into account that the acid

characteristics of the supports are modified when supporting the metals, and

32

in this particular case, the Mo strongly interacts with the acid sites, making

the strongest acid sites disappear from MCM-41 and amorphous silica-

alumina samples (Corma et al 1995). In case of MHC, selectivity to middle

distillates was equally important as the total conversion. The product

distribution showed that the NiMo/MCM-41 catalyst could produce the

lowest amount of gases and consequently the highest amount of gasoline.

Since, the acidities are very similar on the two catalysts; the differences in

selectivities were related to the regularity, size, and dimensionality of the

pores present in MCM-41(Corma et al 1996).

Further SiMCM-41 has been exchanged with proton and used as a

support for Ni-Mo catalyst and tested for HDS of dibenzothiophene (DBT).

Proton exchanged SiMCM-41 performs better than untreated

SiMCM-41.Most of the sodium cations contained in MCM-41 were removed

by proton exchange. This proton exchange has little effect on the structural

and acidic properties of SiMCM-41. It seems that using HNO3 for proton

exchange clears the pore channels of MCM-41 during the ion exchange and

thus the reconstruction of the mesostructure has contributed only to the

increase of the crystallinity (Li et al 2002). Wang et al (2001) reported that

the SiMCM-41 supported Co-Mo or Ni-W sulfides are more active for the

HDS of DBT, than conventional -Al2O3 supported catalyst. This is attributed

to the high surface area and mild acidity of support which lead to high

dispersion of the metal species on the surface. The HDS of vacuum gas oil

was studied over MCM-41 supported NiMo catalysts as a function of shape of

mesoporous material (tubular shaped MCM-41, non-tubular shaped MCM-41)

and compared with alumina and silica supported NiMo catalysts. The results

clearly showed that tubular shaped MCM-41 supported catalyst is better than

the others (Ling et al 2009).

33

The activity of AlMCM-41 supported Co-Mo catalyst was studied

for deep HDS of light cycle oil (LCO). The activity of this catalyst has been

reported to be much higher than commercial -Al2O3. (Kostova et al 2002).

AlMCM-41 supported CoO-MoO3 catalyst at normal metal loading level

(3 wt.% CoO-12 wt.% MoO3) showed less activity in converting DBT than

-Al2O3 supported catalyst. However, at high metal loading it was

substantially more active than -Al2O3 supported catalyst in DBT conversion

(Song and Madhusudan Reddy 1999). It is clearly evident from the results

MCM-41 can be a more suitable support for hydrotreating if proper amount of

CoO and MoO3 are loaded.

The effect of lanthanum addition to the mesoporous

Al-MCM-41 supported catalyst was studied by Song et al (2006), the results

showed higher HDS conversion in the lanthanum modified catalyst than

unmodified catalyst. The increased activity is due to the presence of more

acidic sites created by the addition of lanthanum.

The HDN activity of NiO-MoO3/H-AlMCM-41 catalysts as a

function of effect of molybdenum loading was reported by Sardhar Basha

et al (2006). The results showed that the impregnation of MoO3 up to

24 wt.% over H-AlMCMA-41 resulted in fine dispersion and high activity

towards HDN. It is also found that the protonation of the support remarkably

affected the activity of the catalysts. The same authors (2009) studied the

effect of order of impregnation of active and promoter metals over high

surface area Al-MCM-41 support and concluded that the catalyst prepared by

reverse order impregnation showed higher catalytic activity than prepared by

the other methods of impregnation. The effect of addition of TiO2 into

MoP/MCM-41 catalyst was studied towards the HDN of quinoline and

decahydroquinoline and found that the catalyst containing 5 wt.% of TiO2

showed maximum hydrodenitrogenation activity (Duan et al 2010).

34

1.12.4 Mesoporous SBA-15 as Support

Though MCM-41 type material has high surface area, because of its

small wall thickness the hydrothermal stability is much less. Due to this

reason, there is serious limitation to its practical application (Kooyman et al

2003). The discovery of SBA-15 has opened new possibilities for the

preparation of efficient catalysts for hydrotreating. SBA-15 presents several

advantages such as high surface area (800 m2g-1) and a hexagonal

arrangement of mesopores with sizes from 4 to 30 nm. Pore wall thicknesses

of around 3–6 nm in SBA-15 is responsible for high thermal and

hydrothermal stability (Zhao et al 1998a and 1998b). In recent years different

research groups have employed SBA-15 as a promising support for

hydrotreating catalysts. Vradman et al (2003) reported high activities for HDS

and hydrogenation using Ni-W-S/SBA-15 as catalyst compare to commercial

-Al2O3 supported catalyst.

Ni and Fe supported on SBA-15 catalyst showed high

hydrocracking activity because of the good hydrogenation properties of nickel

Byambajav and Ohtsuka (2003). AlSBA-16 has been reported to be an

excellent support for NiMo catalysts in the conversion of the refractory sulfur

compound (Klimova et al 2004). It is inferred from the detailed study of

method of incorporation of aluminium in to SBA-16 that, the post-synthetic

alumination of SBA-16 by reaction with AlCl3 or Al (i-PrO)3 has number of

advantages than with sodium aluminate or SBA-16 synthesized by direct

method. It is concluded that grafting with AlCl3 or Al(i-PrO)3 improves the

activity of SBA-15; this can be attributed to good dispersion of Ni and Mo

active phases and to the bifunctional character of these catalyst namely, to the

participation of both types of sites, coordinatively unsaturated sites of NiMoS

active phase and Bronsted acids sites of the support. Carlos Amezcua et al

(2005) studied the addition of TiO2 to NiMo supported SBA-16 catalyst by

35

different methods (post-synthetic, chemical grafting and incipient wetness

impregnation) and found that, the best dispersion of TiO2 in the SBA-16 pore

channels was obtained by grafting methods.

The activity of SBA-15 supported Mo, CoMo and NiMo for

thiophene hydrodesulfurization and cyclohexene hydrogenation reaction was

compared with that of alumina Murali Dhar et al (2005). Further (Shelu et al

2008) the effect of addition of ZrO2 on SBA-15 was studied for hydrotreating.

A significant increase in the activity was reported up to 25 wt.%. A

comparative study indicated that ZrO2-SBA-15 and TiO2-SBA-15 supported

CoMo are better than pure SBA-15 supported catalyst for

hydrodesulfurization.

A systematic study of effect of variation of molybdenum and

promoter content and catalytic functionalities over Al-SBA-15 support was

made and reported that molybdenum is dispersed well up to 8 wt.% and

maximum activity at 8 wt.% (Muthu Kumaran et al 2007). The effect of

MoO3 loading and the addition of boron and aluminium over SBA-15 and its

hydrotreating activity towards the gas oil were studied by Sundaramurthy et al

(2008b). 17 wt.% of molybdenum loaded catalyst was found to have fine

dispersion and also the incorporation of Boron and Aluminium increases the

activity. The activity of all the catalyst found to be higher than that of

commercial Al2O3 supported catalysts.

A detailed systematic study was made on Mo, CoMo and NiMo

supported on Al-SBA-15 with various Si/Al ratios for HDS (Klimova et al

2008). The activity of Mo and NiMo/SBA-15 catalysts found to increase with

Al incorporation into the support.

The effect of Si/Al ratio and activity of Al-SBA-15 supported

hydrotreating catalysts was studied by Muthu Kumaran et al (2006) and found

36

that the molybdenum dispersion and anion vacancies and catalytic activities

are significantly influenced by the aluminium content. The result concluded

that the anion vacancy on molybdenum sulfide phase as well as its dispersion

decreases with increasing Si/Al ratio.

There are numerous studies reported the use of various supports

with and without modifications for hydrotreating catalyst. SBA-15 support is

found to be a suitable support towards hydrotreating process with a main

focus on the hydrodesulfurization activity of the model compound at high

pressure condition. However, studies on hydrodenitrogenation using SBA-15

supported NiMo catalyst is very much limited and not explored fully. Hence,

the studies on NiMo supported on Si-SBA-15 and modified SBA-15 is

focused in depth with relevance to hydrodenitrogenation of some nitrogen

containing compounds as model compounds.

1.13 SCOPE OF THE PRESENT STUDY

The work described in this thesis is principally aimed at studying

SBA-15 supported NiO-MoO3 catalysts with respect to their physical and

chemical properties and their activity towards HDN of methylcyclohexylamine

(MCHA). The following are the main objectives:

Synthesis of mesoporous Si-SBA-15 and Al-SBA-15 with

varying Si/Al ratio values (Si/Al = 10, 20, 30 and 40).

Synthesis of mesoporous NiO-MoO3 catalysts supported on

Al-SBA-15 by wet impregnation of catalytic components such

as NiO and MoO3.

Synthesis of series of catalysts with various weight % of

MoO3 (8, 12, 18 and 24 wt.%) at constant weight % of

NiO (3 wt.%) on Al-SBA-15(10) support.

37

Synthesis of catalysts by changing the order of

impregnation of the active components NiO and MoO3

(normal, reverse and co-impregnation).

Synthesis of series of NiO-MoO3 catalysts supported on

Al-SBA-15 with varying Si/Al ratio values (Si/Al = 10, 20, 30

and 40).

Synthesis of AlMCM-41 and - Al2O3 supports.

Synthesis of mesoporous AlMCM-41 and -Al2O3 supported

NiO-MoO3 by wet impregnation of catalytic components such

as NiO (3 wt.%) and MoO3 catalysts (12 wt.%).

Synthesis of fluorine and phosphorus modified SBA-15

supports.

Synthesis of mesoporous P-SBA-15 and F-SBA-15 supported

NiO-MoO3 by wet impregnation of catalytic components such

as NiO (3 wt.%) and MoO3 (12 wt.%) catalysts.

Characterization of the synthesized NiO-MoO3/SBA-15

catalysts using XRD, Nitrogen adsorption-desorption

measurements, DRS, TGA, ICP-OES, 27Al MAS NMR,31P MAS NMR, SEM, TEM, FT-IR, TPD, TPR and RAMAN

techniques.

Evaluation of effect of sulfiding agents on the

hydrodenitrogenation of methylcyclohexylamine.

Evaluation of the catalytic activity of all the catalysts for

hydrodenitrogenation of methylcyclohexylamine.

Correlation of catalytic activity with their physico-chemical

properties.