Catalytic Hydrodearomatization

1 Catalytic Hydrodearomatization BY MEHRI SANATI, BJORN HARRYSSON, MOSTAFA FAGHIHI, BORJE GEVERT, AND SVEN JARAS

1

Introduction

Hydroprocessing of various feeds for the production of fuels is extensively practised in the petroleum industry, and to some extent in coal liquefaction and in the upgrading of synthetic fuels and lubricant oils. Another promising area where hydroprocessing can be applied is the development of renewable non-fossil fuels (pyrolitic bio-oil) for the elimination of the oxygen-containing molecules and the improvement of the H/C ratio. Hydroprocessing reactions occur on the active sites of the catalysts. Also, a suitable pore size distribution of the catalysts is required to ensure the access of reactant molecules to the active sites. The catalysts used in hydroprocessing consist of a molybdenum catalyst that is supported on a high surface area carrier in the 100-300 m2/g range, most commonly alumina, and is promoted by either cobalt or nickel. The concentration by weight of the metal is usually 1-4% for Co and Ni, and 8-16% for Mo.' The catalysts are active in the sulfided state, being either presulfided or sulfided on stream with a sulfur containing feed. Monometallic (Pt or Pd) and bimetallic (Pt-Pd) catalysts of noble metal supported on y-A1203are known to be highly active in the hydrogenation of aromatics under mild conditions. However, noble metal catalysts are easily poisoned by a small amount of sulfur; severe pre-treatment of the feedstock is needed to reduce sulfur to a few ppm. Recent studies have dealt with how to improve the activity of these catalysts and their sulfur tolerance, e.g. by adding a second transition metal or using different support The typical feedstock for laboratory tests is usually either a mixture of model mono-compounds and/or a mixture of different aromatic hydrocarbons. 12-13 In industrial feeds, however, several types of aromatics are present, whose hydrogenation activities differ considerably. The composition and concentration of various nitrogen and sulfur compounds also significantly influence the activity. The process is normally carried out in a trickle-bed reactor at an elevated temperature and hydrogen pressure. In the case of severe deactivation, an ebullating bed reactor might be used but this type of reactor is not suitable due to back-mixing when a high conversion is needed. The specific characteristic of a trickle bed reactor is that a part of the catalytic surface is covered by a liquid Catalysis, Volume 16 0The Royal Society of Chemistry, 2002 1

2

Catalysis

and the other part by a gas. In the common set up, the liquid phase flows downwards through the reactor concurrently with a gas phase that partly consists of vaporized compounds. The temperature and pressure ranges for the hydrogenation of aromatic hydrocarbons in a liquid phase batch reactor were reported to be 450-700 K and 3.5-17 MPa, re~pectively.'~-~~~*~-'~ Hydroprocessing catalysts are quite versatile, exhibiting activity for a number of important reactions. Those of major interest in hydroprocessing that might be referred to as hydrorefining correspond to removal of heteroatoms; hydrodesulfurization (HDS), hydrometallization (HDM), hydrodenitrogenation (HDN) and hydrodeoxygenation (HDO). These reactions involve hydrogenolysis of C-heteroatom bonds. The removal of sulfur and nitrogen is necessary to meet environmental limits. Sulfur may also cause problems with catalyst poisoning and corrosion. HDN is needed to avoid catalyst poisoning of acid sites and improving stability in lube oils. An important reaction in petrochemical industry and refineries is hydroconversion, which enables a change in the molecular weight and structure of organic molecules. Examples are hydrogenation (HYD) and hydrodearomatization (HDA). When oil is hydrotreated, the reduction of aromatic compounds competes with the removal of sulfur and nitrogen. The purpose of hydrotreating (in the latter sense) is to improve the stability and quality of the product. The reduction of aromatic compounds, especially polyaromatics, gives a higher stability to the product, as well as affecting the solubility and colour of the product. Aromatics in fuels not only lower the quality and produce undesired exhaust emissions, they also have potential hazardous and carcinogenic effects.26 Thus polyaromatic compounds are removed to meet health and environmental regulations. The growing understanding of health hazard associated with these emissions is leading to limitation in the use of aromatics in both Europe and the United States.27 The process to make cleaner fuels that are more environmentally friendly is often accompanied by desulfurization and hydrodearomatization. Decreasing the aromatic content increases the cetane number in diesel fuel. Two approaches, a single-stage process and a two-stage process, have been proposed for distillate fuels (particularly diesel fuels) to meet these strict standards for diesel fuels. The single-stage process combines severe hydrodesulfurization and hydrogenation using a single conventional sulfided CoMo, NiMo or NiW catalyst. In order to reach the necessary aromatic saturation the H2 pressure needs to be substantially higher than the H2 pressure at which current hydrodesulfurization units operate.28 The two-stage system uses a conventional hydrotreating catalyst in the first reactor and a noble metal catalyst in the second; this yields a low aromatic diesel stream at moderate hydrogen p r e s s ~ r e . ~ ~ ~ ~ ~ This latter system is highly active for the reduction of aromatics but is very susceptible to sulfur poisoning; the sulfur concentration at the inlet of the second reactor must be reduced to a few parts per million.31 Thus, the use of these catalysts depends strongly on severe pre-treatment conditions, unless the sulfur tolerance can be greatly improved for the noble

1: Catalytic Hydrodearomatization

3

metal catalyst. A number of recent studies have attempted to address this problem by developing catalysts with a high resistance to the sulfur poisoning and at the same time retaining a high hydrogenation activity. In spite of the large number of articles published in recent years, the subject has been widely reviewed. The catalytic aspects of the hydrogenation were discussed by Krylov and N a ~ a l i k h i n a Special .~~ attention to the preparation methods was discussed in more detail by P. Grange and X. Vanhaeren.27 A comprehensive review of the hydrodeoxygenation, with particular focus on upgrading of bio-oils, was published by F u r i m ~ k y .Catalyst ~~ deactivation during hydroprocessing, including the adverse effects of the 0-compounds, was reviewed by Furimsky and M a s ~ o t h . ~ ~ In this review, the primary focus is on the most recently reported work in the literature for both basic and industrial aspects of hydrodearomatization reactions. It is an extension and update of recent studies dealing with the aromatic reduction in different petrochemical feedstocks. These reviews, which have recently appeared in literature, provide comprehensive information regarding hydrodear~matization.~~. 35-37 A comprehensive review of the reactions during hydroprocessing has been published by Topsoe et al.

2

Hydrogenation of Mono-, Di-, Tri-, Multiring and Mixtures of Aromatic Compounds

In recent years an increasing awareness of the use of aromatics contained in different feedstocks, especially distillate fuel (in particular diesel and gas oils), with respect to the adverse effects of undesired emissions and potential health risks, has received considerable attention. In addition, a high aromatic content is associated with poor fuel quality, giving low cetane number in diesel fuel and a high smoke point in jet fuel. To date, a number of the model compounds that are representative of components in industrial feeds, have been extensively studied on several catalysts. These include both unsupported and y-A1203 supported hydrogenation catalysts, using the conventional CoMo, NiMo, NiW, and platinium group metals (including ruthenium, rhodium, palladium and platinum). On all catalysts, the rate of hydrogenation generally increases with the number of aromatic rings present, i.e. a low rate of hydrogenation is observed for monoaromatic rings such as benzene.' The greater reactivity for hydrogenation with higher fused ring systems, such as naphthalene and anthracene, is due to the fact that the resonance energy of the second ring of these multiple compounds is less than for benzene.35 Table 1 of this review shows the recent related publications on hydrodearomatization and the catalytic systems, reaction conditions and product selectivities for these studies. The choice of model compounds were often the mono-aromatics compounds or a mixture of the aromatics in order to simulate a composition similar to the industrial feedstock in refinery.

T = 195-250 "C,P = 5 MPa, WHSV Toluene and 4.5 h-l, aromatic reactivity follows: naphthalene mixture (20% Mono and 2% naphthalene >> toluene > tetralin. This order was related to a decrease in Di) and the amount of the resonance energy per atomic ring, sulfur was varied from difference in the .n-electron cloud 0.065 to 0.070 (DBT). density in the aromatic ring, as well as inductive effect of the methyl group

2000 PtPd/Si02-A1203, Pt/Si02-A1203 and Pd/Si02-A1203

Oil fraction containing 32-40 vol YOaromatics (20.5 Mono, 17.3 Di, 2.2 Tri) and 172-474 ppm sulfur

T = 300 "C,P = 4.9 MPa and LHSV 1.5 h-l, the long-term stability test under industrial operating conditions demonstrated the excellent stability of the catalyst

PtPd/Si02-A1203catalyst shows the highest activity for the hydrogenation of mono- and di- in the presence of S, Pt/ Si02-A1203has the lowest activity. The excellent activity of PtPd catalyst was attributed to the electron - deficient platinum species (site isolation) of isolated Pt cluster on the Pd surface

Enhancement of catalytic activity, which depended on the Pd/(Pt+Pd) weight ratio and reached a maximum of aromatic hydrogenation of about 0.7 Pd/(Pt+Pd) weight ratio. Hydrated mono, di, tri was about 30,4 and 3 vol YO,respectively

4

3

2

Radioacti~e~~S-labeled DBT was used to measure of catalytic HDY activity of phenanthrene, which was monitored by a change in unreacted [3sS]DBTto form [3sS]H2S

Phenanthrene + 1 wt YOdibenzothiophene (DBT)

RefW

38

Products I remarks

Recycle solvent in coal The tetra-aromatic, for example tetrahydronaphthalene, formed much liquefaction, more than decahydronaphthalenes naphthalenes: 15.8, phenanthrenes 3.25 anthracene 2.3, pyrene 3.9 (wt %)

Substrate

2000 Pt-Pd/Si02-Al2O3

2000 Ni-MolAlZO3, support T = 290 to 330 "C,150 t/d pilot plant, y-alumina cylindrical solvent dimethyl disulfide continuously grain added at a conc. 0.5% wt. The hydrogenation rate increases in the order naphthalenes > phenanthrenes > anthracenes > pyrenes 2000 y-alumina-supported T = 260 and 280 "C,P = 5 MPa, Pt-Pd HYD activities over the bimetallic catalyst (Pt-Pd) were higher than that of the mono-metalliccatalysts together

Reaction conditions I remarks

Hydrogenation of polyarornatics

Year Catalyst

Table 1

P

Cyclohexane and methylcyclopentane isomer, a total selectivity to CH+MCP above go%, the concentration of MCP increased withreaction temperature for both catalysts

T = 200-300 "C, P = 3.0 MPa, Pt/Beta has a higher sulfur resistance than Pt/ WOx-ZrO2. The lower sulfur resistance of P W Z r could be explained by a strong interaction of part of the Pt with surface W6+

7

EXAFS (X-ray absorptionfine structure) for characterisation of the catalysts, structure, interaction between Pt and Pd in the Pt-Pd/Si02-Al,03 catalyst. The amount of Pt (0.5 wtY0)-Pd/SiO~-A1~0~ was sufficient for obtaining optimum catalytic performance. Regarding the activities of aromatic hydrogenation, it was assumed that the Pd sites dispersed on Pt particle were responsible for the high hydrogenation activity

Fixed bed reactor, 200-300 "C, total pressure 4.9 MPa, LHSV 1.5 h- and hydrogen to feed oil ratio 500 NYl. EXAFS characterization of the catalysts

1999 PtPd/Si02-A1203, Pt/Si02-A1203 and Pd/Si02-A1203

LCO (middle distillate refinery product) / SRLGO feed obtained from refinery

6

Dependency of solvent effect on the hydrogenation activity. Toluene conversion in MeOH, EtOH, 1-PrOH (alcohol as a solvent), Ru/A1203 was 95, 100 and 100, respectively. Toluene conversion in MeOH, EtOH, 1-PrOH (alcohol as a solvent), Pt/A1203 was nil

5

39

T = 120 "C, P = 6 MPa, ratio substrate : Mono-aromatic solvent = 3 g : (6-0.75 g), reaction time compounds, toluene, phenol, etc. was 2 h

Benzene, n-heptane / benzene mixture (25 wt YObenzene), in the presence (200 ppm) and absence of sulfur

The dependency of the catalytic activity on thesupport has been considered

Alumina support provides a higher Black oil catalytic activity than the hydrotreatment hydrogenation of aromatics, under reaction conditions as used in industry

1999 Ru/A1203and Pt/ A1203

2000 Mo-Ni supported on y-A1203,or hydrated titanium dioxide (HTD), or palygorskitemontmorillonite clay (PMC) 2000 Pt/WO,-ZrO2, (12.7 wt YOW) and Pt/Beta (Si/Al= 12)

s3 z8'

b

ul

6' 3

i;.

se

3

B3

=L

L

* *

40

The role of hydrogen partial pressure in the hydrogenation of benzene, toluene and three isomeric forms of xylene over Ni/Si02; TOF decreases in the order benzene>toluene>p-xylene>m-xylene> o-xylene, representing the order of increasing stability and decreasing reactivity, of the adsorbed x complex

Fixed bed glass reactor (i.d. = 15 mm), Benzene, toluene, o-xylene, rn-xylene atmospheric pressure, temperature range 120 "C 5 T 5 250 "C, space and p-xylene velocity 2 x lo3 h-l, hydrogen to pressure was varied from 1.9. 9.6. MPa at constant aromatic pressure 0.004 MPa using dry nitrogen as the diluent; when aromatic pressure was varied 0.001-0.006 MPa, the hydrogen pressure was constant, 0.094 MPa and again nitrogen as the make-up gas

1999 Ni/SiO;?

9

Improve selectivity of benzene hydrogenation to cyclehexene. Ru-B/Si02 (amorphous) exhibited better selectivitytowards cyclohexene than the corresponding RdSiO2

Selective hydrogenation of benzene

Autoclave with a magnetic stirrer and 1000 rpm stirring, T = 150 "C, P = 4 MPa

1999 Ru-B/Si02 amorphous and Ru / Si02

Ref (s) 8

Benzene, xylene, mesitylene

T = 220 "C, P = 6 MPa, WHSV 2 h-l, H2 / hydrocarbon volume ratio 800, catalyst with particle size 40-60 mesh, the feed was a mixture containing 20 wt% hydrocarbon with 80 wt% n-hexane

1999 Pt (or Pd)/ y-A1203 Pt (or Pd)/ PLC, PLC is pillared clay which are known as crosslinked smectites, a new class of molecular sieve-like materials with a large pore size

Products / remarks Both catalysts, Pt (or Pd)/y-A1203and Pt (or Pd)/PLC showed the same saturation activity for benzene, although the selectivityfor cyclohexane Pd/Al-PLC is lower than Pt/Al-PLC. Generally, Pt (or Pd)/ PLC show much higher catalytic activity for the hydrogenation of large aromatic molecules, large pore structure and weak acidity

Substrate

Reaction conditions / remarks

(contd.)

Year Catalyst

Table 1

o\

-

Benzene, toluene

Phenanthrene

T = 200-400 “C,P = 5 MPa, a dual noble metal catalyst Pt(2)-Pd( 10) showed similar hydrosulfurization results and better hydrogenaton compared with a Co-Mo catalyst

1999 Alumina-supported Pt and Pd, Pt (or Pd)/ A1203,presulfided (mixture of 5% H2S in H2) Co-Mo catalyst

1998 Modified catalysts Micro-catalytic stainless steel reactor, L = 10 cm, d = 6 mm, T = 50-250 “C, containing 0.35% Pt-Al203, introducing flow rate H2 20 cm3/ min a second metal Ir, Rh, Re and U and fluorination and chlorination with different halogen contents of 1.3 and 6 wtYo

Distillatefuels

Low temperature catalytic hydrogenation processes over noble metal catalyst without addition of dibenzothiophene

1999 Two stage, new design catalyst concept in the hypothetical stage, First Ni-Mo or Co-Mo. Second noble-metal, which will be supported as bimodal on zeolite with two pore openings, < 5 A and in laqge pore opening <6A

Benzene to cyclohexane, toluene to methylcyclohexane, respectively.Toluene possesses higher activity than benzene on the monometallic catalysts, the benzene hydrogenation activity of the bimetallic catalysts follows the order PtRh > PtIr > PtRe > Pt > PtU, the corresponding order for toluene hydrogenation follows PtRh > PtRe > PtIr > Pt > PtU

Dihydrophenanthrene, tetrahydrophenanthrene, octahydrophenanthrene perhydrophenanthrene Pt-Pd catalyst showed better performance in hydrogenation of phenanthrene in presence of 1 wt% DBT than the conventional Mo-based catalyst

Multiple catalyst bed to achieve deep hydrosulfurization and hydrogenation, first stage ‘HDS’ Ni-Mo or Co-Mo catalyst, second stage ‘HDY’ over noble-metal catalyst supported on dealuminated mordenite

43

Tetralin with benzothiophene as a model compounds for sulfur poisoning (0- 1000 ppm sulfur content)

1998 Pt/yA1203

Continuous down flow fixed-bed reaction system, WHSV ranging from 2.0 to 12 (g of feed/h:g of catalyst), investigation of the relation between sulfur-poisoningand catalytic properties of Ptly-A1203catalysts for aromatic hydrogenation. T = 270 "C,total pressure 3.2 MPa

n-Propylbenzene alone; n-propylbenzene and hydrogen sulfide; n-propylbenzene, hydrogen sulfide and ammonia; n-propylbenzene, hydrogen sulfide, ammonia and water

1998 Conventional NiMoI Batch autoclave reactor, pressure 6.9 yAlzO3,3.2 wt% NiO MPa, T = 330,350 and 375 "C and 15.4 wt% Moo3, presulfiding with a 1: 9 H2S-H2 mixture at 325 "C, 8 h

Industrial conditions, a-A1203 Benzene supported nickel showed the best activity, with 40% nickel concentration and optimum metal area of 10.8 m2/g

1998 Ni on different supports

Substrate

Reaction conditions I remarks

(contd,)

Year Catalyst

Table 1

44

45

46

The reaction is moderately reduced by sulfide and severely inhibited by ammonia, addition of water did not further affect the aromatic hydrogenation

The material balance is about 98% for the catalytic performance tests. CO chemisorption and extended X-ray absorption h e structure (EXAFS) spectroscopy revealed that the decline of the reaction rate is caused by the formation of PtS and the reduction of PtS was achieved by hydrogen reactivation

RefW

Hydrogenated cyclic compounds

Products I remarks

00

1. Dimethylcyclohexane, ethylbenzene, decalins, tetralin 2. Perhydroanthracene 3. Benzene, cyclohexane, hexylbenzene, hexylcyclohexane, cyclohexylbenzene, bicyclohexyl, dodecahydro-, octahydro-, hexahydro-, tetrahydrocarbazole 4. Benzene, cyclohexane, hexylbenzene, hexylcyclohexane, cyclohexylbenzene, bicyclohexyl, biphenyl, tetrahydro-, hexahydrodibenzothiophene 5 . Different hydrocarbons etc. The results showed that the higher sulfur tolerance of the bimetallic Pd-Pt was achieved when Pd-Pt was supported on the highest acidity zeolite, the sulfur tolerance decreasing when the acidity of support decreases

Naphthalene Anthracene Carbazole Dibenzothiophene Coal-derived oil

1. 2. 3. 4. 5.

For the proupose of the hydrogenation of aromatics

Characterization of the catalysts by different technique, i. e. EXAFS measurement

1998 Pt and Pd, zeolitesupported catalysts, (Pd : Pt mole ratio of 4 : 1). The sulfidation of the catalysts was done in a 1000 ppm H2S/N2stream

Tetralin, trans- and cis- decalins. All samples demonstrate high hydrogenation activity in the first step when tetralin is formed. In the second step, trans- and cis- decalins are formed, the tungsten catalysts are two times more active than the molybdenum catalysts. Mo for mild hydrogenation, while tungsten is superior for deep hydrogenation

Autoclave, P = 10 MPa HZ, T = 430 "C The addition of phosphoric acid to the catalyst was an attempt to change the chemical properties of the catalyst, in order to increase activity of heteroatom removal

Naphthalene

1998 Phosphoric acidpromoted Mn203-Ni0

1998 (P)NiMo/Ti-HMS T = 325 "C, total pressure 4.4 MPa, and (P)NiW/Ti-HMS, 7 h, naphthalene dissolved in nphosphorus hexadecane concentation was 0.2 wt% P205,NiMo/ Ti-HMS, NiW/ Ti-HMS, sulfidation mixture (H2 15 vol YO H2S)

10

(contd.)

50

51

1-Methylnaphthalene, Examination of the hydrogenation of 1-methylnaphthalene was used to coal-derived middle quantify the conversion rate of the distillate hydrogenation of the aromatics contents in the coal-derived middle distillate At low Ru content (Ru 0.2) cyclohexane is the hydrogenation product. As the Ru content increases cyclohexane formation decreases and hydrogenolysis product (methane) will be formed

Ref($1 49

Products I remarks Cumene hydrogenation yielded isopropylcyclohexane is the only product, Hydrogenation of cumene, in the absence of sulfur. 5 wt% cumene and 95 wt% tetradecane Mo2C possesses higher activity than noble metal catalyst, the thermodynamic equilibrium conversion was 99.99%, indicating neither Mo2C nor Ptly-Al203 achieve equilibrium conversion. MoS2 is not an active catalyst for hydrogenation

Substrate

First-order reaction with respect to aromatic and partial pressure of hydrogen. P=4-12MPa, T=400"C

Benzene Catalytic test were performed in a continuous flow fixed bed microreactor, atmospheric (0.10 MPa) pressure, T = 250 "C, H2 flow = 60 cm3/ min, weight of catalyst = 20-40 mg, the conversion level was kept below 10%

1998 Rdalumina catalysts (0.21-5.1 1 wt% Ru)

Simultaneous hydrogenation, 5 wt% cumene and 95 hydrosulfurization and wt% tetradecane, in hydrooxygenation were observed with the absence of sulfur minimal deactivation of Mo2C up to 30 ppm for sulfur, 2000 ppm oxygen and 5 wt % cumene, Ptly-A1203 deactivated immediately upon addition of sulfur. P=5.1 MPa, T=250"C, 5.3 pmol s-l (5 cm3h-l)

Reaction conditions I remarks

1998 Ni-Mo/alumina

Year Catalyst

Table 1

0

w

1997 Pt/Y-zeolite

Reaction conditions were close to realistic industrial conditions, temperature dependency with a max. in the reaction rate at 325 "C, the pressure dependency of the rate with respect to the ratio of the toluene/H2 was sensitive to the level of the sulfur in feed Toluene

Methylcyclohexenes and methylcyclohexane

Higher acidity of Pt/MCM-41 has been favoured over the higher activity of Pt/A1203,for aromatics hydrogenation

Naphthalene in n-hexadecane was used to simulate the aromatic in diesel fuels

1997 Pt/MCM-41, Pt/A1203 T = 180"C, P = 4.2 MPa, concurrent down-flow trickle bed reactor, the metal dispersion on MCM-4lwas higher than that on A1203

Main components of the hydrogenation product, Naphthalene to decalins and tetralin; acenaphthene to tetrahydroacenaphthene; phenanthrene to dihyrophenanthrene, tetrahydrophenanthrene, octahydrophenanthrene; anthracene to dihydroanthracene, terahydroanthracene and octahydroanthracene; fluoranthene to tetrahydrofluoranthene; and pyrene to dihydropyrene

Toluene, solvent was Methylcyclohexenesand either in mixture of methylcyclohexane water (33.4 mol%) and methanol (66.6 mol%) or methanol

A light fraction of Trickle bed reactor P = 9.8 MPa, T = 350 "C, hydrogen anthracene oil dm3 s - l , the main flow 4.0 x dissolved in toluene purpose of the experiment was to obtain information on the effectiveness factor for both the wetted and dry zones of the catalyst

1997 Ruthenium supported T = 303 "C, atmospheric pressure (0.10 MPa), a semi-batch magnetically on silica and silicastirred reactor with a coolingjacket active charcoal contained 5 wt YOof the metal by sol-gel method

1998 Ni-Moly-AlzO3, catalyst was presulfided by a mixture of 10% of H2S in H2

53

52

11

13

Naphthalene in the Decahydronaphthalene absence and present of Tetrahydronaphthalene sulfur (Benzothiophene), n-tridecane was used as a solvent

Tetralin, in presence of H2S, or dimethyl disulfide in solution in n-heptane

T = 200 "C, in a microautoclave reactor, mordenite-supported Pt and Pd catalysts are more active than Y zeolite supported Pt and Pd, respectively. Y zeolite supported catalysts afford higher yield of cisdecalin. Mordenite-supported catalysts give higher yield of trans-decalin.The addition of sulfur decreases the activity of all catalysts tested

T = 300 "C, the activity for RuKYd was higher than for NiMoIalumina. The activity of the RuKYd was dependent on the sulfidation method, the catalyst sulfided by dimethyl disulfide was less active than when sulfided by H2S/H2mixture

Cyclohexene, 1-hexene Cyclohexane, hexane Competitive hydrogenation of cyclohexene and \-hexene pore, with a constriction of 5 A yielded higher rate of hydrogenation for 1-hexene. Other catFlysts supported on a constriction (7 A) and activated coal display comparable rates for both reactants. Gas-phase fixed-bed reactor, T = 100 "C, WHSV = 25-50 h-'

1997 Pt and Pd supported on mordenite (HM38), a Y zeolite (HY), A1203 and Ti02

1997 Sulfided Ru on dealuminated Y zeolite (RuKYd), NiMo/alumina

1997 Pt supported on carbon fibers; small constriction (5 A), large constriction (7 A)

ProductsI remarks

Substrate

Reactionconditions I remarks

(contd.)

Year Catalyst

Table 1

56

55

54

Ref(s)

N

c

To develop high sulfur tolerant Pt catalysts for hydrogenation of aromatics, the choice of catalyst support and/or the finding of the second metal (for the formation of bimetallic interactions, so the sulfur adsorption decreases) is necessary Tetralin, cis- and trans-decalin, both MCM-41 and USY supports showed the greatest sulfur resistance. Ptf (A1)MCM41 presented the highest activity for hydrogenation of aromatics in LCO feedstock, especially at low temperature (300 "C)

Normal sulfur poisoning, tetralin with 1000 ppm sulfur, T = 270 "C, P = 3.2 MPa. With more severe conditions, 19 atm and 2000 pprn sulfur, WHSV 4.8 (g of feed/h:g of catalyst). Continuous fixed bed reactor

Batch reactor, (reaction conditions for 1. Hydrotreatment of naphthalene + 200 naphthalene hydrogenation) T = 225-275 "C, total P = 5.0 MPa, PPm (DBT) dibenzothiophen solvent was n-decane; LCO hydrogenation, T = 300-350 "C, 2. light cycle oil (LCO) (400 ppm total P = 5.0 MPa, WHSV = 4 hsulfur, 70 wt 'YO aromatic)

1997 Wy-A1203

1997 Pt/ (two MCM-41 supports, differing in their chemical composition) Wamorphous mesoporous silicaalumina (MSA Si/Al= 100) Pt supported on a commercially amorphous silicaalumina (ASA) WUSY zeolite Ptfy-AlzO3 PtfSiOZ The nominal Pt = 0.5- 1 wt%

w

c ,

61

Naphthalene dissolved cis-Decalin, trans-decalin The selectivity of cis-decalin was studied in n-hexadecane for HYD purpose

1997 Pt/A1203,Pt/MCM-41 Concurrent down flow trickle bed reactor, T = 180 "C, P = 4.2 MPa, LHSV = 2.8 h- l , H2/liquidfeed 800 mVml, the hydrogenation activity of PdMCM-41 is higher than that of PdA1203, but cis-decalin selectivity was lower than with Pt/A1203

-

60

-

59

Tetralin in presence of From HDY of tetralin the major product The hydrogenation activity was very large amounts of H2S was cis- and trans-decalin high in the presence of H2S and (1.85%) roughly 300 times the activity (expressed per metal atom) of an industrial NiMo/ A1203 catalyst. The active phase, which consists of cluster of ca. 50 Ru-S, is located in the zeolite framework

From HDY of toluene the major product was methylcyclohexanewith a 50% selectivity, the others were ethylcyclopentane,methylcyclopentane and dimethylcyclopent anes; From HDY of tetralin the major product was cis- and trans-decalin with a 90% selectivity, small quantities of methylindanes and methylcyclopentanes were observed

Tetralin and toluene in the presence of 1.9% H2S

1997 Ruthenium sulfide supported in a Y zeolite, HYd (commercial dealuminated) and KYd (prepared from commercial dealuminated HYd zeolite) and subsequent sulfidation

-

Microreactor system, for toluene HYD, T = 280-390 "C and P 4.5 MPa; for tetralin HDY, T=250-300 "C, P 4.5 Mpa. For tetralin HDY, the specific activities were found to decrease in the order: RuHYd > RuHY > RuKHYd >> NiMo/ A1203> RuKY For toluene HYD, the specific activities were decreased in the order: RuHYd > RuKHYd > RuHY >> RuKY NiMo/A1203

Ref(s)

1997 Ru dispersed in a series of zeolites with various acidic properties; HY, KY (prepared from Nay), KHYd (prepared from commercial dealuminated HYd zeolite) and subsequent sulfidation

ProductsI remarks

Substrate

Reaction conditions / remarks

(contd.)

Year Catalyst

Table 1

P

b-

Batch reactor, T = 310-350 "C, P = 5.0-8.0 Mpa. This study focused on the difference in kinetics, between powdered and pellet catalysts in liquidphase HDY reactions of the aromatic ring. The inhibitory effect of 1-methylnaphthalene was clearly observed for the powdered catalyst

1997 Ni/SiOz

Gas-phase hydrogenation, T = 120-250 "C, the hydrogen pressure was constant, 0.095 MPa; where the pressure of each aromatic was varied in the range 0.001-0.006 MPa, the TOF of the three xylenes increased in the order o-xylene < rn-xylene < p-xylene, over the entire temperature range studied

1997 Ruthenium supported Autoclave, initial P = 10 MPa (H2), T = 430 "C on mixed oxides (Mn203-Zn0, Mn203-Ni0, Mn203-La203) with different molar compositions. The supports were loaded with 0.1,0.2, 0.5 and 1.0 wt ruthenium

1997 Powdered, cylindrical and trilobed commercial Co-Moly-Al203

I. Etthylcyclohexane, decalin, tetralin, others I I. Per-, octa-,tetr ahydr o-anthracene, others 111. Different aromates

Stereoisomeric product mixtures of the saturated dimethylcyclohexane

0-,m-and p-xylene

Methyltetralins and methyldecalins

I. Naphthalene 11. Anthracene 111. Coal-derived oil

1-Methylnaphthalene diluted in a C14-C16 normal paraffin mixture

64

63

62

c #

5' 3

g

'i$

2 8

k

k

is

' 5 i2

The analysis of product samples from long-term stability tests (25 days), collected periodically, found that aromatic conversion ranges from 3- 10 wt%, depending on precursors and pretreatment conditions

Commercial diesel

Synthetic crude The aromatics conversion was 29% for Pt distillates from on pillared interlayered clay (PILCj Canadian oil sands of alumina and 44% for Pt/Y-zeolite varying sulfur content, high > 100 ppm and low < 10 ppm, monoaromatics content 31 mass%; diaromatics 8 mass% and triaromatics 0.8 mass%

The bimetallic catalysts possess a relatively high selectivity for aromatic reduction and other hydrotreating processes, T = 340 "C,P 2 4 MPa

T = 310 "C, P = 7.0 MPa, for kinetic investigation the temperature was 320, 340 and 360 "C. The enhanced activity of Pt/Y-zeolite-alumina catalyst is attributed to unique structure of the support, producing well-dispersed Pt metal clusters

1996 Pt-PdlrAl203

1996 Pt on pillared interlayered clay (P1LC)-alumina and Y-zeolite-alumina

-

- -

cis- and trans-dimethylcyclohexane;the distribution of stereoisomerswas dependent on the reactant pressure as well as on temperature. The stereoisomers were governed by the adsorptioddesorption kinetics of an intermediate cyclic olefin; the configuration of the final product depended on the orientation of the olefin double bond upon adsorption

0-, p-xylene

Gas-phase hydrogenation, from the kinetic investigation suggested a noncompetitive adsorption of hydrogen and aromatic compounds on the catalyst surface. Thermodynamics suggested the stepwise hydrogen addition to the aromatic molecule

1997 NilAl2O3

~

Products I remarks

~~~

Substrate

~

Reaction conditions I remarks

(contd)

Year Catalyst

Table 1

67

66

65

Ref(s)

m

c

69

70

71

The relative rate of toluene and benzene HDY with an equimolar gas mixture showed that toluene was less reactive than benzene on Pd catalysts, whereas the value for toluene HDY indicates that is favoured on Pt catalysts The catalysts showed activity and selectivity to HDY reaction of aromatics

Naphthalene, decalin and several products with the same formula C10H18

The reaction parameters such as metal Methyl benzoate loading, temperature and solvents effect have been studied

Tetralin Flow reactor, T = 300 "C, P = 7 MPa (total). DMDS and H2S were used to study the effects of the sulfidation method and its dependencies on hydrogenation activity. It was found that the H2S method was the most suitable

1996 Sulfied ruthenium on KY zeolite

1996 Pd/Si02-A1203 (0.22-1.64 wt0/0Pd), Pdq-Al203 (1.67 wt YOPd), Ptlq-Al203 (0.78 wt% Pt) and commercial P d C (3.61wtYo Pd)

1996 Pt supported on inorganic polymer, silicapolysulfoalumoxane, with different metal loading

68

The main reaction product was the completely hydrogenated cycloalkane

Di- and tri-substituted T = 95-125 "C, P = 2-4 MPa. The aromatics, such as oactivity of different substituent positions decreased in the order para > m-and p-xylenes,pcymene and meta > ortho, the trisubstituted mesitylene benzene (mesitylene) had a lower reaction rate than the disubstituted compounds (xylenes) Benzene and toluene T = 80 "C, P = 0.007 MPa (total aromatic partial pressure) and 0.095 MPa (hydrogen partial pressure)

1996 Commercial preactivated catalyst, nickel-alumina

(contd.)

Reaction conditions / remarks Substrate

1996 Platina on mesoporous aluminosilicate (Al-MCM-4 1) support with varying WA1 ratios using different aluminum sources (aluminum isopropoxide, pseudo boehmite and aluminum sulfate)

A 30 ml stainless-steel tubing bomb I Naphthalene batch reactor was used. The reactor I1 Phenanthrene was heated in a fluidized sand batch under vertical shaking (240 cycles/ min), P=7-10.5 MPa H2, T = 200-300 "C. The catalyst was active for hydrogenation of large aromatics, the activity was significantly different depending on the synthesis conditions, especially with respect to the source of aluminium

1996 Ammonium T = 350,400 and 450 "C, P = 7.2 MPa, 1. Naphthalene tetrathiomolybdate as hydrogedreactant molar ratio (5: l), 2. Phenanthrene a catalyst precursor microautoclave reactor, the rate of the 3. Pyrene reaction is most important for bicyclic compounds, as ring size increases, kinetic play a less important role and thermodynamics become the driving force for the outcome of the reaction

Year Catalyst

Table 1

I Decalin tetralin I1 Tetra-, di-, octa-, tetradecahydrophenanthrene

1. Tetralin the only hydrogenation product 2. Primary product is dihydrophenanthrene and tetrahydrophenanthrene. Octahyrophenanthrene is a hydrogenated product of the primary products 3. The major product at all temperatures for pyrene hydrogenation is dihydropyene. Secondary hydrogenation products are tetrahydropyrene and hexahydropyrene

Products/ remarks

w

73

72

Ref

Benzene, toluene

T=30°C P=O.lOMPa

T = 350°C P = 6 MPa. The hydrogenation was performed over a large range of H2S partial pressures. The Nualumina is nearly inactive

1996 Silica-supported carboxymethylcellulose platinum complex (Si02-CMCPt> 1996 Zeolite-supported noble metal catalysts

1996 Mo, Ni, Ni-Mo and Ni-Mo-P/alumina

Toluene in the presence of H2S

Low temperature hydrogenation of aromatics in the absence and presence of DBT

Tetralin, cis- and trans-decalins and others. The equilibrium conversion decreases with increasing temperature. Tetralin becomes the dominant product at higher temperatures

Naphthalene, the solvent used was tridecane. The hydrogenation reaction was started when bezothiophene was added to the solvent

Tubing bombs, T = 200 and 280 "C, P = 6.99 MPa H2. The Pd/TiOZ showed higher sulfur resistance in comparison with the other catalysts

1996 Pf/A1203,PdA1203, Pd/Ti02 and NiMo/ A1203(commercial)

Methylcyclohexane

Cyclohexane and methylcyclohexane

Tetralin cis- trans-decalins

Naphthalene in hexadecane (as a solvent) was used. In the reaction with sulfided catalysts, an additional 1.5 wt.% Dimethyl disulfide was added as a sulfur source

Tubing bomb microreactor, T = 310 "C, P = 6.90 MPa cold hydrogen pressure. The Pt-promoted catalysts were more active than the original catalysts in the oxide forms, whereas the activities of the sulfide catalysts both in promoted and nonpromoted were similar

1996 Pt, Ru and Ir promotion on three commercial A1203supported catalysts (NiMo, CoMo and NiW)

78

77

76

Reaction conditions I remarks

The hydrogenation has been carried out under pressure of different coke oven gases = 55 vol.% H2 instead of pure hydrogen, influence of reaction time and temperature were studied T = 300-450 "C, P = 11-25 MPa, 16 h

Mathematical calculation of hydrogenation

Kinetic investigation of gas-phase hydrogenation, T = 130-220 "C, differential fixed bed reactor, hydrogen pressure was varied over 0.04-0.09 MPa and xylene pressure over 0.01-0.035 MPa, helium was a makeup to adjust the flow to 10 mmollmin

Gas-phase hydrogenation, T = 145-220 "C,m-xylene partial pressure was varied over 0.01-0.035 MPa and hydrogen partial pressure was in the range 0.4-0.9 bar

1996 Commercial nickelmolybdenum (3 wt% NiO, 15 wt?h Moo3 on alumina) and Pd (5 wt% on alumina)

1996

1996 Ni/A1203

1996 17 wt .Yo Ni/A1203

(contd.)

Year Catalyst

Table 1

Dihydrophenanthrene, tetra hy drophenanthrene, octahydrophenanthrene; the maximum yield was at 370 "C

ProductsI remarks

m-Xylene

0-, p-Xylene

The distribution of stereoisomers depended only on temperature, mixtures of stereoisomeric products of the saturated dimeth y lcyclohexane. Rate maximum for saturated product was at about 405 K

Main product, cis- and transdimethylcyclohexaneswas formed in non-equilibrium ratios; the kinetic results showed that the reaction proceeds through consecutive hydrogen addition steps on the catalyst surface and cyclic olefin was intermediate product

Benzene, naphthalene Cyclohexane, decalin and pyrene

Phenanthrene

Substrate

82

81

80

79

Ref0)

0

h,

84

Cyclohexane, the data obtained from benzene hydrogenation were found to be in very good agreement with the percentages of metal exposed Completely hydrogenated cycloalkane and trace amounts of cycloalkenes, hydrogenation rate decreased with increasing length of the substituent in the benzene ring

Benzene The anionic exchange on a low and high specific surface area ceria, exhibits the same amount of exposed metallic Rh atoms, hence the high surface area ceria contain 50% more Rh

Benzene, toluene, T =95-125 "C, P = 2-4 MPa. A ethylbenzene, cumene model on competitive adsorption of hydrogen and the aromatic compound fitted the experimental data. The hydrogenation rate was based on a sequential addition mechanism of adsorbed hydrogen to the aromatic nucleus

1996 0.15-0.33 wt % Rhl CeOz catalysts

1996 Commercial preactivated catalyst of nickel-alumina (Ni 16.6 wt%)

85

83

Tetralin and two isomers of decalin, tetralin is identified as a primary product, decalins are identified as non-primary products

Naphthalene Batch reactor at 350 "C and 17 MPa, kinetic investigation. Vanadium sulfide deposits led to decrease hydrogenation rate in the naphthalene network; the results showed a sequential hydrogenation of naphthalene to form tetralin which was hydrogenated to give decalin

1996 Ni-Mo/yAlzO3 enriched with various amounts of nickel and vanadium by contact with solutions of the respective meta1 naphthenates. CS2 was added to the mixture so that the catalysts was presulfided

Naphthalene dissolved The catalytic activity of Pt-aluminium borate was higher than that of Ptlyin n-hexadecane A1203,but its cis, trans decalin selectivity is lower than that of Ptly-Al203 catalyst owing to the higher acidity; too much boron (A1:B = 8) degrades the hydrogenation activity

P=5.17 MPa, LHSV=2.8 h-'

Continuous fixed-bed reactor, T = 270 "C, P = 1.2-3.3 MPa, kinetic investigation of sulfur on the deactivation of catalysts

1996 Pt-aluminium borate

1995 Pt/y-A1203

Tetraline

1. Tetralin

A Langmuir-Hinshelwoodreaction model, which was based on a chemisorption scheme with irreversible surface reaction control for tetralin and reversible surface reaction for sulfur poisoning was proposed to describe the deactivation model of the catalysts

5. Dodecahydrochrysene 6. Dodecahydro-l,2-benzanthracene

4. Octahydrophenanthrene

3. 1,2,3,4,5,6,7,8,-octahydroanthracene

2. 9,lO dihydroanthracene

1. Naphthalene + hexane 2. Anthracene with lithium diisopropylamide 3. Anthracene with potassium bistrimethylsilylamide 4. Phenanthrene with the lithium diisopropylamide 5. Chrysene 6. 1,Zbenzanthracene

88

87

86

Ref (s)

1996 Lithium Autoclave, T = 250 "C, 4- 18 h, P = 7 MPa diisopropylamide Potassium bistrimethylsilylamide

Year Catalyst

Reaction conditions I remarks

~

Products I remarks

(contd.) Substrate

Table 1

t 4

h)

1995 Rh and Ni organometallic complexes anchored on USY zeolites

Mild reaction conditions, 80 "C and 0.6 MPa of HzO, the strong cooperative effect between the zeolite surface and the transition metal surface was thought to be responsible for the enhancement of hydrogenation reaction

1995 Ptlalumina-aluminum P = 6.8 MPa, T = 350 "C,trickle bed reactor, LHSV = 2.8 hphosphate

Tetralin, decalin, Walumina-aluminium phosphate had a better hydrogenation activity and lower cis-decalin selectivity than Wy-A1203, due to the higher acidity of the support

Trickle bed reactor, P = 5.17 MPa, T =240 "C

1995 Pt/A1203-aluminium phosphate

Benzene, toluene, a-methylstyrene

Total conversion of benzene, toluene and a-methylstyrene using zeolite containing the Rh complex was achieved after reaction times of 6, 18 and 24 h respectively. The Ni complex was less active than the corresponding Rh complex. The positive influence of the zeolite was attributed to an increase in the concentration of reactants inside the pores

Naphthalene dissolved The result showed that the catalyst had a in n-hexadecane better hydrogenation activity and lower cis-decalin selectivity than Pt/y-A1203 catalyst due to the higher acidity of the support

A solution of naphthalene in nhexadecane was used to simulate the aromatic in diesel fuels

Pt-Pd combination catalyst showed the highest sulfur resistance. The interpretation of the spectroscopic investigation was that the role of Pd in enhancing sulfur resistance was due to decreasing the electron density of Pt and thereby inhibiting the adsorption of H2S

Hydrogenation feed contain 1000 ppm Tetralin + 1000 ppm sulfur and sulfur, H2/hydrocarbon = 2.7, straight run distillate T =280 "C,P = 2.62 MPa diesel

1995 Pt/y-A1203,modified by adding a second metal, Co, Mo, Ni, Re, Ag and Pd

90

16

15

89

w h,

T = 200-260 "C, P = 1.7-8.7 MPa and LHSV 1.5-8.0 liquid hourly space velocity, trickle bed reactor

1995 Naphthalene in inert solvent n-hexadecane

1. o-xylene 2. Naphthalene 3. Phenanthrene 4. Pyrene 5. Anthracene 6. Chrysene

Batch autoclave, T = 350 "C,P = 6.9 MPa H2, presulfiding at 400 "Cfor 135 min. in 10% H2S in H2. Cyclohexane was used as a solvent. Focus on the relationship between molecular structure and hydrogenation reactivity in heavy oil processing. Equilibrium ratios were much larger than unity for benzenic, larger than unity for the naphthenic and smaller than unity for the PHE

1995 Presulfided CoMoI A1203

PtlA1203

Substrate

Reaction conditions I remarks

(contd.)

Year Catalyst

Table 1

92

91

Ref0)

Tetralin and cis- and trans-decalin. The reaction was sequential, i. e. naphthalene hydrogenated to tetralin, followed by sequence hydrogenation to cis- and transdecalin. The apparent activation energies for hydrogenation of tetralin to cis- and trans-decalin were found to be 9.88 and 7.25 kcallmol, respectively

m1. cis-trans-l,2-dimethylcyclohexane, xylene, p-xylene, cis- and trans- 1,3dimethyl- and -1,4dimethylcyclohexanes 2. Tetralin and cis-trans-decalins 3. Di-, tetra-, octa- and perhydrophenanthrene 4. Di-, tetra-, hexa-, deca- and perhydropyrene 5 . Di-, tetra-, octa- and perhydroanthracene 6. Di-, tetra-, hexa-, octa-, dodeca- and perhydrochrysene

Products I remarks

Formation of surface sulfide on the catalyst

The synergistic effects between Ni(Co) Benzene and Mo and between metal loading and acidity of zeolite was investigated. The synergistic effect occurs at a Ni/ (Ni+Mo) ratio of about 0.4-05 and was related to the formation of Ni-Mo-S phases within the structure of zeolite

1995 Molybdenum nitride

1995 Ni-Mo sulfide catalysts supported on zeolites (HY), conventional Ni-Mo/ A1203

Vacuum gas oil

Stirred autoclave having a Parr 4561 Selective assembly system, T = 150-250 "C, P up hydrogenation of to about 6.90 MPa. The hydrogenation benzene in gasoline was carried out in a biphasic system of water and gasoline. The benzene was selectively solubilized in the water for further hydrogenation in the presence of the water-soluble catalyst mixture

1995 Catalyst comprising a mixture of catalyticallyactive mixture of water-soluble, organometallic compounds. Catalyst (l), M[L],[X], wherein M is a metal selected from Cr, Fe, Co, Ni, Mo, Ru, Rh, Pd, Ta, W, Re, Os, Ir, Pt, La and Ce; L is an aromatic hydrocarbon; X is halogen; x and y are integers from 1 to 10. Catalyst (2), tris(tripheny1phosphine)rhodium(1)halide or tris(tripheny1phosphine)ruthenium(1)halide

Cyclohexane, Ni-Mo sulfide in the zeolite is highly dispersed and responsible for the high hydrogenation activity, which was comparable to the conventional Ni-Mo/A1203

The combination of both catalysts was found to produce conversion in excess of 40% which was much more than either of the catalysts separately

95

94

93

r,

3

6'

5

G-

$5

B

Q.

?

k

%

s

2

..

Naphthalene 1995 Mordenite-supported The isomers of trans- and cis-decalin Pt catalysts takes place on acid sites 1995 17 wt.% Ni/A1203 Differential microreactor, atmospheric Ethylbenzene pressure, T = 130- 190 "C, ethylbenzene partial pressure was varied from 0.01 MPa to 0.035 MPa, hydrogen partial pressure was kept in the range of 0.04-0.09 MPa 1995 Molybdenum Propylbenzene Total P = 5 MPa (hydrogen at 4.14 oxynitride MPa, cyclohexane at 0.85 bar and propylbenzene at 0.06 MPa), T=267-397 "C

1995 NiMo on Y zeolite

The carbon atoms on the surface and Benzene in the bulk of Pd particles of the catalyst showed a capacity for chemisorption and hydride formation o-Xylene T = 380 "C , P = 6 MPa, various amount of o-xylene (5, 10 and 20 mol% was added to n-heptane)

1995 PdIC

Substrate

Reaction conditions I remarks

(contd.)

Year Catalyst

Table 1 Ref(s)

Kinetic investigation of ethylbenzene hydrogenation. Rate maximum was obseved at 160- 175"C, depending on the concentration ratio of the reactant, main hydrogenation product was ethylcyclohexane Propylcyclohexane,characterization through, chemical analysis. TPD of propylbenzene and preadsorbed benzene at room temperature and successive catalytic runs indicated that, during catalytic runs, the surface of the catalyst will be chemically modified; an oxycarbonitride is formed without decrease of the specific surface area

101

100

96 The main product was cyclohexane, the turnover frequency in benzene hydrogenation was a structure-insensitive reaction 97 The rate of formation of m- and p isomers allow an estimation of the acid activity of a bifunctional catalyst. The hydrogenation activity was defined as the rate of formation of all Cg naphthenic compounds, which is proportional to the content of Ni and Mo of the catalyst and independent of the zeolite content Tetralin, trans- and cis-decalin 98,99

ProductsI remarks

a

h,

I : Catalytic Hydrodearomatization

27

2.1 Reactivity in Hydrogenation Reactions. - The recent reactivity studies have been reviewed by Moreau and G e n e ~ t e ,Girgis ~ ~ and Gates36 and Stanislaus and Cooper.37 In these reviews, the reactivity of aromatic compounds was defined as the overall conversion of aromatic compounds to fully and/or partially hydrogenated products. The hydrogenation reactivity of aromatic hydrocarbon was affected by the following factors: 0

0

0

0

0

0 0

0

0

aromaticity; the aromatic character of a molecule is a measure of its degree of unsaturation and on its thermodynamic stability the total aromaticity; generally given by total resonance energy which is defined as the value obtained by subtracting the actual energy of the molecule from that of the most stable contributing structure102-10s partial resonance energy; as the number of fused aromatic rings was increased, the resonance stabilisation energy per aromatic ring was decreased hydrogenation reactivity related to geometric modification of model compounds the contribution of the structural and geometrical effects of organic molecules to the reactivity; this is taken into account where interaction between the molecule and the catalyst surface was an important parameter differences in the n- electron cloud density in alkylated aromatics electronic effects; when the model compounds was substituted by alkyl or aryl groups, the slight differences observed in reactivity was accounted for in terms of electronic effect the presence of bulkier substituents; a significant effect on reactivity was assumed to be due to the steric effects. These effects have been most important when fused multiring aromatics were hydrogenated relationship to the reaction rate constant

Unlike olefin hydrogenation, high hydrogen pressures are required to effect ring saturation in aromatics hydrogenation. This is partly due to the low reactivity of the aromatic structure as a result of resonance stabilization of the conjugated system and partly due to limitations determined by the thermodynamic equilibrium at the pressures and temperatures employed. Therefore, most studies on the reactivity of aromatics have been conducted at pressures and temperatures that favour low equilibrium concentrations of aromatics. Relative hydrogenation reactivates of one ring in the multi-aromatic model compounds over a sulfided NiMo/A1203 were correlated to the rate constant by Moreau et al.,35 the correlation showed the following order: benzene (1) < phenanthrene (4) < naphthalene (20) < anthracene (40) where the numbers are relative rate constants. The reactivity of the aromatic compounds was correlated to the aromaticity of the rings.35-106 The total aromaticity is generally given by resonance energy (RE);lo5 resonance energy increases with the number of aromatic rings, independently of the presence or the absence of heteroatoms in the rings.35

28

Catalysis

Table 2

Hydrogenation of mono- di- and tri-aromatic hydrocarbons, positions for addition reaction in di- and tri-aromatic

Hydrogenat ion reaction

Total resonance energy (kcal mol-')

Resonance energy1 ring (kcal mol-')

36-40

40

59-75

28

1

9

+H2=Q

71-105

10

10

The magnitude of resonance energy per ring was less for naphthalene compared with benzene, and consequently the hydrogenation rate was low for benzene.lo6The low aromatic character of one of the rings in the naphthalene molecule is experimentally shown by its ability to undergo addition reactions across 1,2-positions, and the corresponding positions are 9,10 for phenanthrene and anthracene,lo7the behaviour is shown in Table 2. The increase in resonance energy with the angularity of the system,105e.g. anthracene and phenanthrene which are isomeric hydrocarbons (containing the same total number of aromatic rings) resulted in lower reactivity for phenanthrene. It is commonly argued that the presence of methyl substituents on the benzene ring stabilizes the adsorbed n complex with the resultant introduction of a higher energy barrier to aromatic ring hydrogenation. The hydrogenation rate in the presence of the noble metal catalysts decreased in order benzene > toluene > p-xylene > m-xylene > o-xylene under moderate reaction conditions, confirming this hypothesis. The bonding of a molecule on the surface of solid catalysts depends on the local electron density of states on the adsorbing metal atoms. Aromatic compounds are bonded to the solid surface of the catalysts via n-bonds involving an electron transfer from the aromatic ring to the unoccupied d-metal orbitals.lo8 Since the n-electron cloud density in toluene is higher than that of benzene, it would be expected that toluene is more strongly adsorbed on the metal surface. Consequently, a strong interaction between aromatic

1: Catalytic Hydrodearomatization

29

compounds and the metal atoms will lead to a reduction in the hydrogenation rate of the former. An increase in electron density cloud with the addition of another methyl groups can account for the lower reactivity of xylene. For metal sulfide catalysts, such as Ni-W-S/A1203 and Ni--Mo-S/A1203, the reactivity pattern of aromatic and alkylated aromatic compounds is the opposite.37In other words, addition of methyl groups to aromatic compounds gives rise to an enhancement of the reactivity of these molecules for hydrogenation; the corresponding hydrogenation rate for the same homologous series (as stated above) will decrease in the order benzene < toluene < p-xylene < rn-xylene < o-xylene over moderate reaction conditions. Both the number and position of the substituted groups in benzene ring affected the hydrogenation activity.68The observed reaction rate of a trisubstituted benzene (mesitylene) over a commercial pre-activated catalyst particles of nickel-alumina was lower than the reaction rate of the disubstituted (xylene) benzene? The activity of the different substituent positions decreased in the order para > meta > ortho.68The lowest reactivity of ortho-isomers has been attributed to steric effects.37 Steric hindrance of neighbouring methyl groups in ortho-positions had a more significant effect on the formation of n-bonded complexes. Pondi and V a n n i ~ observed e~~ that the relative rate of toulene and benzene hydrogenation with an equimolar gas mixture for the Pd supported catalysts in A1203and Si02-A1203 was less than unity. A value less than unity indicated that toluene was less reactive than benzene on Pd catalyst^.^^ In the same experiment a threefold higher value for relative rate was reported for Pt catalysts; the authors interpretation was that toluene hydrogenation was favoured on Pt.69 For the three catalytic systems, Pt/Si02-A1203, Pd/Si02--A1203, and PtPdl Si02-A1203, under moderate operating conditions and in the presence of 113 ppm sulfur, the order of aromatic reactivity was as the following sequence: naphthalene >> toluene > tetralin.4 This reactivity sequence was related to a decrease in the resonance energy per aromatic ring as well as to differences in the n-electron cloud density in the aromatic ring because of the inductive effect of the methyl group in t01uene.~The bimetallic catalyst exhibited higher hydrogenation activity for those three aromatic compounds. This enhancement in the aromatic hydrogenation activity in presence of sulfur was attributed to the synergetic effect between Pt and Pd.4 Of course, there are a variety of interpretations concerning the noble bimetallic effect on catalytic activity, but obviously the greater reactivity is a result of the presence of an addition effect, as opposed to a synergetic effect. In agreement with the literature data, in addition to the influence of resonance energy and stereochemistry, additional factors such as choice of the catalyst metal and reaction conditions have a significant effect on the reactivity of the aromatic molecules. Hydrogenation activity on a commercial Ni-alumina catalyst decreased with increasing length of the substituent in the benzene ring, thus benzene appeared to be the most reactive.85

Catalysis

30 100

iE

80

W

60 I ,

0

40

"

20 0 150

200

250

300

350

Pt (3)-Pd (10)

400

450

Temperature ('C) Figure 1

Conversion of PHE hydrogenation as a function of temperature on several catalysts (fromref 42)

The rate of hydrogenation of derivatives of benzene varies with the nature, number and position of the substituents. The ratio KTIB (toluene to benzene) of the adsorption coefficients is very large on ruthenium, about 9-10 on rhodium and about 1 on palladium. This behaviour was interpreted as an effect of a considerable electron transfer from methyl groups of the aromatics to the metal. Large KTIBratios are associated with the electron-deficient character of the metal.log Pyrene, fluoranthene, anthracene and fluorene were hydrogenated over a Pt, Pd, Rh and Ru catalyst in a study by Sakanishi et al. 110Also here the results showed a change in reactivity order depending on the catalyst used. However, the difference in reactivity can not be explained only by the structural properties of aromatic compounds. The reactivity will be influenced by the properties of the catalysts. The appearance of differences in reactivity depended on varying the type of catalyst and diversities recorded from the different reported works confirmed that more research is needed with a special focus on the reactivity aspects.

2.2 Thermodynamics of Hydrogenation. - One important property of the hydrodearomatization reaction is that the maximum equilibrium conversion occurs at temperatures close to or even below the temperatures needed for the hydrodearomatization rate to be of industrial interest and this is clearly seen in Figure 1. To the right of the maximum of the curves the equilibrium rate is limiting the hydrogenation process and to the left, the hydrogenation rate is limited by the temperature.2 This is why catalysts from the platinum group with a high activity are preferred rather than less active sulfided catalysts. Keane and Patterson40 included equilibrium curves for benzene, toluene and o-xylene at 0.1 MPa.

1 :Catalytic Hydrodearornat iza tion

31

Djega-Mariadassou et al. lol present equilibrium data for ethylbenzene at 5.0 MPa total pressure and a hydrogen pressure of 4.1 MPa.

2.3 Reaction Pathways and Kinetics of Hydrogenation. - The reaction network for hydro-dearomatization has been studied by several groups and will be presented below. Reaction order, activation energies and thermodynamic values will be presented. 2.3.1 Reaction Network. The hydrogenation of a one-ring system can be influenced by the presence of other functional groups. The reaction in this case will start with either hydrogenation of the functional group or hydrogenation of the aromatic ring. Takagi et aZ.6 have studied the effect of solvent for the hydrogenation of benzyl alcohol. This makes it possible to control the selectivity in the reaction. When hydrogenating monoaromatics the major product will be the hydrogenated ring, but also some cyclic mono alkenes are also formed.'" It is this alkene that according to Salmi et aZ.65981385 undergoes 'roll-over' in the adsorbed state and two different cis and trans isomers are formed. In the same way naphthalene is hydrogenated first to tetralin and finally to decalin in both the cis and trans f0rms.83992Huang et al.92present a figure, originating from Weitcamp, l2 claiming that several other intermediates exist. They present indications of the existence of octalin, as this compound is a necessary intermediate for the formation of the isomerized products. Qian et al.42 presented phenanthrene (an aromatic with three rings) dearomatization using both PdPt catalysts and sulfided CoMo catalysts. Total pressure was kept at 5 MPa with decalin as solvent and 1% aromatic, at various temperatures. Hydrogenation of phenanthrene proceeds through a network of reversible reactions. The middle ring was the most likely to be hydrogenated first. 2.3.2 Reaction Order. Keana hydrogenated xylenes over a nickel catalyst on silica. The turnover frequency (TOF) followed the following general expression.

TOF = kPxmPnH2 where the reaction order for the xylenes varied from 0.1 at 393 to about 0.44 at 530°C. The value of n for the hydrogen pressure increased with temperature from 0.7 to 2.3. 2.3.3 Activation Energies. Huang and Kang92 and Corma et aZ.58 provide a table of older results on apparent activation energies for hydrogenation of aromatic hydrocarbons, mainly benzene. Discussion of activation energies is given by Keane and P a t t e r s s ~ n . ~ ~ * ~ The true activation energy can be calculated from the observed apparent activation energy by subtracting the sum of adsorption energies of the aromatic and the hydrogen. The adsorption energies provide the major contribution to the true activation energy. Adsorption energies for an aromatic are in the range 100-140 kJ mol-'.

32

Catalysis

2.4 Catalysts and Nature of Catalytic Sites. - Because of more stringent environmental regulations, greater attention has been focused on the reduction of aromatics in distillate fractions. Also important is the use of deep catalytic hydrogenation of the aromatic compounds to produce petrochemicals, e.g. conversion of benzene into cyclohexane in the synthesis of caprolactam. The choice of catalyst type varies according to the application, nature of the feedstocks and desired activity/selectivity of the reactions. Conventional hydrotreating catalysts containing sulfided mixed oxides (NiMo, NiW, CoMo) can only accomplish moderate levels of aromatic saturation under typical hydrotreating conditions. Because of the thermodynamic limitations in hydrodearomatization, a deep level of aromatic saturation has not been achieved by increasing operation severity (high temperature and high Hz pressure). Among the conventional catalysts, NiMo and NiW are preferred for aromatic saturation. NiW catalysts have the highest activity for aromatic hydrogenation at low hydrogen sulfide partial pressures,37but their use has been limited due to the higher cost. Models of the active phases and structural aspects of these mixed oxides (NiMo, NiW, CoMo) have been recently reviewed by Topsoe and co-workers. Transition metals, as well as platinium-group metals, have been used as the catalyst for this type of hydrogenation. Platinum-group metals have the advantage of high hydrogenation activity at low temperature. The catalyst activity for the hydrogenation of benzene was reported to be in order Rh > Ru >> Pt >> Pd >> Ni > C0.113 Three categories of catalysts have been studied for hydrodearomatization in literature. The first category is a mono-metallic noble-metal catalyst. Obviously, these catalysts will be used in the hydrogenation processes in the absence of sulfur in the feedstocks. The second category of catalysts consists of two noble metals and is known as a bimetallic noble-metal catalyst. Several combinations of Pt, Pd, Rh and Re have been proposed. The third is a combination of the conventional hydrotreating catalysts promoted by a noble metal; these catalysts consist of three metals. An attribute of the noble-metal based catalysts is that they are active at lower temperatures; thereby they are the preferred catalysts for deep aromatic saturation. The main drawback of noble-metal hydrogenation catalysts is that they are poisoned by small amounts of sulfur and nitrogen organic compounds present in the feed. However, because in the industrial feeds several heteroaromatic compounds are present, considerable attention has been paid recently to developing catalysts with the high hydrogenation activity while maintaining strong resistance to poisoning by the small amounts of sulfur- and nitrogencontaining compounds in the feed stream. Several attempts have been made to minimize the poisoning effects of heteroatoms. The resistance to poisoning by heteroatoms is improved by adding a second noble metal to Pt, changing the particle size, or changing the acid-base properties of the carrier. With respect to the latter, it is reported that the sulfur tolerance of the noble metal catalysts can be greatly enhanced by using acidic supports such as zeolites, whereas less

I : Catalytic Hy drodearomat iza tion

33

acid supports such as Si02-Al203 can generate moderate sulfur resistance.28.3 1,37,41,114- 115 The studies performed by Vannice et al.69J16-117found that the activity of the Pt or Pd catalyst for benzene and toluene hydrogenation was much higher when loaded on Si02-Al203 compared with an A1203-supported catalyse. The enhancement of the activity was explained by the more acidic nature of the Si02-Al2O3 as a support. The exceptional performance of the PtPd catalyst in aromatic hydrogenation was interpreted in terms of the electron-deficient platinum species (isolated Pt cluster on Pd surface) in the resultant PtPd particle^.^ It has been reported in literature118y119 that the high sulfur tolerance and higher activity of zeolite-supported Pt or Pd catalysts arises from the formation of an electron-deficient cluster of metal particles, Pt6+ or Pds+ upon the hydrogenation of the aromatic compounds. The close contact between the strong acidic support and the small cluster of Pt or Pd atoms makes it possible for the electron to be withdrawn from the noble metal thereby creating an electron-deficient metal particle. 120~121In the case of zeolite-supported bimetallic PtPd catalysts, it seems that the molar ratio of the PtRd is seldom greater than unity. Also the role of the acidic support is to modify the active site, which is the noble metal. The catalytic cracking reaction of the acidic support, under hydrogenation conditions, is undesired as this lowers the selectivity towards the desired hydrodearomatization reaction. The hypotheses that the deactivation effect is related to acidic support has been proven by using a less acidic carrier. In these studies, the Si02-Al203 carrier was replaced by the more acidic support such as zeolites in order to minimise the effect of the cracking activity, which accelerate the catalytic deactivation by coke and results in enhanced yields of naphtha and g a ~ . ~ . ~ The sulfur resistance and saturation activity of the aromatic catalysts was improved by using MCM-41 materials as a support for noble metal catalyst^.^^ The higher saturation activity of naphthalene on Pt/MCM-41, in comparison with the Pt/A1203,was attributed to the medium-strong acidity of the MCM41 support and its higher metal d i s p e r s i ~ n . ~ ~ Different s t u d i e have ~ ~ ~been ~ ~devoted ~ to the nature of the active phase on the supported mono-noble metals such as ruthenium or rhodium in hydrodearomatization. The ruthenium or rhodium sulfide phase was identified as the active phase for aromatic hydrogenation. Aromatics hydrogenation in the presence of H2S shows that a weak Pt (or Pd)-S sulfide bond is formed.2 It was further found that the synergetic effect observed on the bimetallic PtPd was not as significant as those achieved with typical CoMo catalysts.2 The observed activity in Figure 1 indicates clearly the absence of synergetic effect between the bimetallic Pt and Pd. However, the activity results revealed the presence of the additional effect from the bimetallic combination of PtPd. The transition metal catalysts used in hydrodearomatization, the role of

34

Catalysis

preparation, the nature of the metal precursor and support and the ultimate dispersion of the active metal phase and their bearing on the catalytic data were carefully reviewed by Krylov et aZ.32 3

Industrial Aspects

The aim of this section is to illustrate the importance of aromatic saturation in the refining industry. There were a number of commercial processes having aromatic saturation as their major objective before the growing interest in removal of aromatics in diesel began in 1991. For example, aromatic saturation is used in the production of kerosene and jet fuel for smoke point i m p r ~ v e r n e n t . ' ~ ~A- ' ~ high ~ smoke point due to a low aromatic content improves the quality of a flame, producing a greater flame without smoke and d e p 0 ~ i t s .A l ~high ~ smoke point reduces the effects of the radiation from the jet flame on exposed mechanical parts in a jet engine. The removal of aromatics is essential in the production of lubricants and solvents to both control the technical performance of the product and the impact on health and environment. A low aromatic content improves the viscosity index (VI) of engine oil. The lower the VI, the lower the variation of the viscosity with temperature.126 In 1993 the European Community decided to label lubricants based on the DMDS extractable content of the lubricant according to the Institute of Petroleum method IP 346. The polyaromatics in the lubricant are extracted in to a DMDS phase and the 3% limit for the DMDS extractable content is based on a correlation with skin cancer in skin painting tests on mice. 127 In the production of food/medical grade oils, a complete reduction of aromatics according to the demands in the FDA, DAB or BS pharmacopoeia is needed. The production of medical white oil represents the most severe process condition used in hydrorefining. The first extraction stage and the first hydrotreating stage remove the major part of the nitrogen, sulfur and the aromatics. The intermediate product will meet the demands on the less refined product technical white oil. A low space velocity, high pressure and noble metal catalyst is needed in the second hydrotreating stage to overcome the thermodynamic limitations for the reduction of the remaining aromatics in the medical white oil. The production routes for lubricants and food/medical grade oil by extraction and acid/clay treatment have gradually been replaced by catalytic hydroprocessing routes. 128-130 It is an established practice in fuel refining to pretreat FCC feed stocks. One of the main objectives in the hydrotreating of FCC-feed stock is to reduce sulfur to minimize sulfur oxide release during regeneration of the catalyst and to meet the sulfur specification of the products. A more severe hydrotreating of the FCC-feed and a change of the catalyst from a CoMo catalyst to a catalyst containing NiMoP, a higher pressure, and a lower space velocity will increase the reduction of the nitrogen and aromatic content in the FCC-feed. The hydrogenation of aromatic and nitrogen components in FCC feed stocks

I :Catalytic Hydrodearomat iza t ion

Table 3

35

Swedish and Californian diesel specijications

Class

Class I

Sweden Class 11

Class 111

California >50 000 bpsd

Density, kgm-3 Sulfur, ppm Nitrogen, ppm Aromatics, vol% Cetane index Cetane number IBP, "C FBP, "C

800-820 10

800-820 50

800-830 500

5 50

10 47

25 47

180 300

180 310

180 330

830-860 500 10 10 48 170-215

305-350

decreases the coke deposited on the catalyst and improves the selectivity to more valuable products.1279131-132 Aromatic saturation is also desired in residue desulfurization for transportation fuels and in hydrocracking to increase the hydrogedcarbon ratio. Hydrogenation of aromatics is desired in the hydrotreating of straight run gas oil to improve the cetane number in diesel, and is even more necessary because the use of other low quality sources of various diesel feedstocks as straight run gas oil, visbreaker gas oil, coker gas oil and light cycle oil is increasing.133 The Swedish (1991) and the California (1993) authorities were the first to introduce legislation limiting the aromatic content in diesel. Table 3 shows the Swedish and Californian diesel specifications. The motivation for the legislation is that the reduction of the aromatic content and sulfur content results in reduced particulate and NO, emissions from the diesel engine. The specifications for fuel oil and diesel will become even more stringent for Europe and USA. 134-135 Asian countries are already moving towards similar tighter specifications.136 Changes in fuel specifications have been the dominant force for the development and introduction of new catalysts and processes for hydrotreating. Several new processing schemes have been introduced since 1991. Stricter specifications will continue to require improvements in performance. As highlighted above, thermodynamic considerations will limit the use of higher reaction temperatures in deep hydrogenation of aromatics. In current commercial processes for deep hydrogenation of aromatics, a two-stage process is used. In the first stage the sulfur and nitrogen contents are reduced by using a NiMo catalyst on alumina. Some aromatic saturation does also occur during this first step, but the main objective in the first step is to reduce sulfur and nitrogen. This allows for the use of a noble-metal catalyst at low reaction temperatures in the second stage after an intermediate gas separation and stripping in between the two stages.137Examples of licensors of this twostage technology are Shell, Topsoe, Lummus-Criterion and IFP. An option with the Synsat process from Criterion-Lummus is to remove the H2S and NH3 containing by-product gases within a single reactor. The Synsat 1339136~138-140

Catalysis

36 Fresh Feed

~

Recycle gas Make-upH2

Recvcle

Gas to VapodLiquid Separation and Product to stripping

Reactor 1ene;th

Figure 2

(a) SynSat reactor; (b) temperature and hydrogen partial pressure proJles

reactor, shown in Figure 2, also includes an optional counter-current gas flow in the bottom catalyst bed to further reduce the H2S content, increase the hydrogen partial pressure and maintain the temperature profile. 38 The catalysts commonly used in the second stage for aromatic saturation is a sulfur-resistant Pd-Pt, noble-metal catalyst on an acidic zeolite. Much attention has been focused on ways to increase the sulfur tolerance of noble metal catalysts. l4 A comparative in 1992 by Cooper et al. shows that a two-stage operation with an inter-stage stripper and sulfur-tolerant noble metal catalyst is the optimum for deep aromatic saturation. It has to be mentioned that diesel meeting the tightening specifications can also be obtained by hydrocracking or mild hydrocracking where the catalyst also contains an acidic function as well as the hydrogenating-dehydrogenating function.143The acidic function introduces a shift in boiling range and a ring opening. The ring opening results in a further improvement of the cetane number after a prior saturation of the aromatic ring. Hydrocracking is outside 3

339

1: Catalytic Hydrodearomatization

37

the scope of the present paper but there are some processes for deep aromatic saturation including an option for ring opening, boiling range shift or/and a cold flow improvement, e.g. the Akzo-Fina CFI and the SynShift processes.

4

Summary and Conclusion

In this paper we have reviewed the literature regarding the state of aromatic hydrogenation processes including the kinetic, thermodynamic, catalytic chemistry and industrial aspects. It is possible to produce a diesel fuel with a low aromatic content, high cetane number, uncoloured and with a low sulfur content. The low sulfur content is a necessity since the effective dearomatization catalysts are sensitive to sulfur. In the recent years several solutions to the severe sulfur deactivation of the dearomatization catalyst have been presented. However, the sulfur tolerance has to be further increased. The interesting temperatures are around 340 "C. At higher temperatures, the equilibrium limits the yield. At 370 "C, uncatalytic hydrogen donor reactions are possible and aromatics are formed from hydrogenated products. Higher temperatures can be used, depending on the hydrogen partial pressure. The higher the pressure the higher the temperature that can be used. If a more active catalyst is found, a lower temperature could be used, making it possible to use lower hydrogen partial pressure. This would lead to large savings in the production of low aromatic compounds. In the near future, it would be of interest to develop more basic knowledge on the industrial process. This will reduce the cost for dearomatization. There is a need for experiments with more industrial relevant model compounds and to find equilibrium data for model compounds under industrial conditions. There is also a need for experiments with industrial feeds, e.g. feedstocks to FCC and light cycle oils from FCC that can be used as diesel fuel. This could achieve a valuable increase of the cetane number, especially in the USA. Other interesting oils are base oils for lube stock. The sulfur resistance by using a zeolite carrier has been demonstrated. However, it is of great interest to increase this resistance or to develop completely new concepts since most feedstocks do contain significant levels of sulfur. Deep desufurization before dearomatization is costly. The need for more knowledge of selective hydrogenation 1 benzene --+ cyclohexene (not cyclohexane). 2 competitive reduction of benzene in gasoline by hydrogenation of a light fraction from catalytic reformer to reduce benzene in gasoline. 3 hydrogenation of specific aromatic compounds in competition with other aromatics in the feed. 4 developing catalysts with a high hydrogenation activity while at the same time maintaining strong resistance to poisoning of sulfur in the form of hydrogen sulfide and benzothiophenes.

Catalysis

38

5 !studying the influence of possible poisons like hydrogen sulfide, ammonia,

and water on the kinetic of hydrodehydroaromatization on one ring and two ring and multiple aromatic model compounds (the major portion of the aromatic content in diesel feedstocks) in the production of low aromatic diesel fuels. 6 more research to develop both sulfur- and nitrogen-resistant catalysts, because in industrial feeds, several heteroaromatics are present. 7 studying new methods for hydrogenation of aromatics like under superzritical conditions. I

5 1 2 3 4 5 6 7 8 9 10 11 12 13

14 15 16 17 18

19 20 21 22 23 24

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42

Catalysis

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