the chemistry of selective ring-opening catalysts.pdf

Review

The chemistry of selective ring-opening catalysts

Hongbin Du *, Craig Fairbridge, Hong Yang, Zbigniew Ring

National Center for Upgrading Technology, 1 Oil Patch Drive, Devon, Alta., Canada T9G 1A8

Received 6 June 2005; accepted 15 June 2005

Available online 30 August 2005

Abstract

Bitumen-derived crude and heavy oils require severe processing and produce middle distillate product with poor ignition quality. This

becomes a concern to refiners as tighter specifications on transportation fuel are promulgated. One process to address this issue is selective ring

opening of cycloparaffins to reduce the number of ring structures, while retaining the carbon number of a productmolecule. The process involves

bifunctional catalysts, both metal and acid sites, working in high-pressure, high-temperature reactor systems in the presence of hydrogen. The

acidic sites catalyze dehydrogenation, cracking, isomerization and dealkylation, while the metal sites promote hydrogenation, hydrogenolysis

and isomerization. Various compounds containing single, double and multiple rings have been used to model the ring-opening reactions and a

number of mechanisms have been proposed. The five-membered ring readily undergoes ring-opening reaction on either acid or metal catalysts

with the selectivity and activity dependent on the nature of the supported metal catalyst. The ring opening of six-membered ring compounds is

secondary, requiring an acidic function to isomerize a six-membered ring cycloparaffin to a five-membered ring. A balanced metallic-acidic

function catalyst is necessary to achieve optimal performance. A system dominated by acid function results in excess cracking, while a catalytic

systemwith high concentration of metals leads to mainly hydrogenation. Commercial hydrocracking catalysts using transitionmetal sulfides on

acidic supports usually require severe operating conditions due to their low activities of the metal sulfide compared to metal sites, leading to

extensive cracking of cycloparaffin side chains. Noble metals supported on acidic oxides are the most active catalysts for selective ring opening,

but these catalysts are very sensitive to poisoning by sulfur compounds in petroleum feedstocks. An understanding of the chemistry of selective

ring-opening catalysts, combined with theoretical studies of structure–activity relationships and high throughput experimentation methods,

provides opportunities in searching for new generations of selective ring-opening catalysts with high-performance and sulfur resistance.

Crown Copyright # 2005 Published by Elsevier B.V. All rights reserved.

Keywords: Ring opening; Catalysts; Model compound; Mechanism; Bifunctional catalysts

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2. Ring-opening catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1. Cracking on solid acid catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2. Hydrocracking on bifunctional catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.3. Hydrogen spillover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3. Selective ring opening of single-ring compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.1. Activity and selectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.2. Proposed mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.3. Electronic effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.4. Geometric effect. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.5. Support effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

www.elsevier.com/locate/apcata

Applied Catalysis A: General 294 (2005) 1–21

* Corresponding author. Present address: State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, China.

Fax: +86 25 8331 4502.

E-mail address: [email protected] (H. Du).

0926-860X/$ – see front matter. Crown Copyright # 2005 Published by Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2005.06.033

H. Du et al. / Applied Catalysis A: General 294 (2005) 1–212

4. Selective ring opening of multiple-ring compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4.1. Indan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4.2. Decalin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4.3. Tetralin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.4. Naphthalene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.5. Phenanthrene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.6. Fluorene and fluoranthene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5. Sulfur tolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

6. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

1. Introduction

Health concerns and local environmental awareness have

prompted worldwide regulatory actions on transportation

systems to limit emissions of criteria air contaminants

(hydrocarbons, carbon monoxide, nitrogen oxides and

particulate matter). In addition, concerns for global climate

change have highlighted the need to improve transportation

system efficiency to decrease emissions of carbon dioxide.

These regulations and concerns call for new technologies to

produce high quality liquid fuels and internal combustion

engines with maximum combustion effectiveness and

minimum emissions. The regulations primarily involve

reductions of sulfur levels, aromatics (particularly benzene),

olefins, volatility and the use of oxygenates in gasoline. The

sulfur compounds in fuels contribute to the emission of

sulfates with particulate matter, and have a deleterious effect

on the ability of future automobile engines to meet the more

stringent emissions standards. Aromatics in middle-dis-

tillate fuels produce particulates in the exhaust gases and, in

addition, have poor ignition properties, i.e. low cetane

number in diesel fuel and high smoke point in jet fuel.

Across the world, gasoline and diesel specifications are

becoming stricter, especially with regard to sulfur. In the US,

the Reformulated Gasoline Program Phase II, which took

effect on 1 January 2002, mandated a reduction in volatile

organic compounds (VOC) by 27%, nitrogen oxides (NOx)

by 7%, and toxic pollutants (aromatics, especially benzene)

by 22%. In 2004, the Environment Protection Agency

started to implement the Tier 2 Gasoline Sulfur Control

Program for cleaner gasoline, which limits the maximum

sulfur level to 300 ppm, and by January 2006 to 80 ppm. The

current maximum concentration of sulfur in on-road diesel is

limited to 500 ppm and 2000–5000 ppm for off-road diesel.

By June 2006, the new EPA regulations will limit sulfur

content to 15 ppm. Additional specifications for minimum

cetane index at 40, and maximum aromatics content at

35 vol% will also be imposed.

Canada has also launched programs for a staged sulfur

reductions in gasoline that introduced the specification of

150 ppm July 1, 2002, then to 30 ppm by January 1, 2005. A

maximum of 500 ppm sulfur in on-road diesel fuel was

mandated on January 1, 1998, with 15 ppm slated for June

2006.

In Europe, the European Commission mandated a

reduction in diesel sulfur levels from 500 to 350 wt ppm in

2000, and a further reduction to 50 wt ppm in 2005. The

regulations require higher average cetane number (currently

51 and 53 by 2005) than in the US and an even higher level of

55 is under consideration and will put additional pressure on

refiners. For gasoline, the regulations require reduction of

sulfur content from 150 to 50 ppm, and aromatics from 45 to

35 vol% by 2005. Future regulations will likely tighten the

limitation for the aromatic and toxics content and increase

fuel efficiency to reduce the resultant vehicle CO2 emissions.

This will probably promote light-duty, diesel-fuelled vehicle

production and the use of ultra-low sulfur diesel (ULSD).

In the last few years, refiners in the US and Canada have

invested in major refining expansion and revamping

construction projects because of the ultra-low sulfur fuel

requirements. At the same time, they are also facing

challenges of meeting other requirements including the

replacement of methyl tertiary butyl ether (MTBE) and

reduction of aromatics [1–3]. In addition, refiners have to cope

with increasingly heavier and poorer quality feedstocks.

According to the US Energy Information Administration, the

quality of crudes processed in the US declined between 1997

and 2001 at the approximate rates of �0.128 API/year and+0.057 wt% sulfur/year. This trend is expected to continue in

the next 5–10 years, forcing US refineries to import heavier

crudes due to the worldwide increase in crude demand,

decreasedproductionof light crudes frommature regions such

as the US and North Sea, and geopolitical factors. Increased

heavy oil supplies arose from Canada and Venezuela.

With 174 billion barrels of established bitumen reserves

(recoverable in situ and mineable), the Canadian oil sands

resource has been increasingly recognized as a strategic

source of North America energy supply. Alberta bitumen

production reached approximately 1 million b/d (barrels per

day) in 2003, accounting for 53% of Alberta’s total crude oil

and equivalent production. According to the Alberta Energy

Utilities Board (AEUB), total bitumen production will

increase to 3.5 million b/d by 2017, representing 2 million b/

d of synthetic crude oil (SCO) and 1.5 million b/d of

unprocessed crude bitumen. Compared to conventional

crude oil, SCO has the valued quality attributes of low sulfur

content and zero residues. However, SCO also has some

major disadvantages, largely related to its high aromaticity.

H. Du et al. / Applied Catalysis A: General 294 (2005) 1–21 3

Fig. 1. Cetane numbers of various compounds [76].

The heavy gas oil (HGO) bottom cut from SCO (about

38 vol%) contains more than 90% of cyclic hydrocarbons

which causes processing problems, particularly in fluid

catalytic cracking (FCC) units, which constitutes approxi-

mately 80% of the US conversion capacity. This has led to its

limited acceptance in a conventional FCC refinery intake. In

addition to the poor HGO quality, SCO also yields poor

quality middle distillate (potentially jet and diesel fuel

blending components). These factors have resulted in a price

differential between SCO and other light crudes. Similar

problems exist with other highly aromatic streams, such as

those from cokers and FCC units. These potential motor fuel

components have low cetane numbers and poor ignition

qualities. This will become a particular difficulty for refiners

if higher cetane specifications are introduced worldwide in

the near future. In order to accept larger volumes of bitumen-

derived crudes, both the producers and refiners will have to

address the quality issues and refining challenges. One way

to do this is to develop new catalytic processes that will

improve the quality of these streams. There has been a

growing demand for refining catalysts driven primarily by

refiners’ needs to meet the already legislated regulatory

requirements without capital infusion. It will require new

generation of high-performance hydroprocessing catalysts

to make higher quality fuels of the future.

Hydrogenation and hydrocracking are two commercially

proven refining technologies addressing the high aromaticity

issues [4]. The conventional hydrotreating catalysts help

saturate aromatic rings to naphthenes (cycloparaffins or

cycloalkanes) [5,6]. However, the increase of cetane number

via hydrogenation of aromatics is insufficient to significantly

increase thequality of themiddle distillates because thecetane

numbers of individual cycloalkanes, such as decalin are rather

low (Fig. 1). Early studies by Wilson et al. demonstrated that

the hydrogenation or a combination of hydrogenation and

aromatics extraction of bitumen-derived middle distillates

could only produce a distillate with 40 cetane number [7–9].

However, approximately97%of thearomatic carbonhad tobe

converted by severe hydroprocessing to achieve this.

Hydrocracking is, on the other hand, a process designed to

produce lighter and higher quality naphtha and middle

distillate fuel blending stocks by saturating aromatic rings and

reducing the number of ring structures via carbon–carbon

Fig. 2. b-Scission of linear and

bond scission in the presence of hydrogen [10,11]. However,

because of its extensive overcracking there is limited net

gain in production of desirable paraffins via this route, and

the improved distillate fuel quality results primarily from a

combination of hydrogenation of aromatics and a concentra-

tion of paraffins in a reduced volume of distillate product. The

ideal process would be a selective ring-opening catalyst that

can reduce the number of ring structures while retaining the

carbon number of a product molecule.

2. Ring-opening catalysis

2.1. Cracking on solid acid catalysts

Selective ring opening of naphthenes with minimum

cleavage of side chains is very complex and represents a

challenge to catalyst researchers. It is known that ring

opening can be catalyzed by the acid sites, e.g. the Bronsted

sites, via carbenium intermediates [12]. The reaction is

initiated by protolytic cracking, accompanied by protolytic

dehydrogenation, hydride transfer, skeletal isomerization,

b-scission and alkylation, as in the cases of cracking of

aliphatics over acid catalysts. The latter mechanisms have

been extensively studied [13]. The cracking of endocyclic

C–C bonds in cyclic hydrocarbons, however, is much slower

than those of aliphatics, presumably because of a higher

tendency of the alkenyl cation, formed by b-scission of a

cycloalkyl cation, to recyclize, or because of a lower b-

scission rate in cylcoalkylcarbon ions caused by an

unfavorable orientation of the p-orbital at the positively

charged carbon atom and the b-bond to be broken (Fig. 2)

cyclic hydrocarbons [14].


[14]. As a result, the overall reaction is predominated by

isomerization and subsequent hydrocracking (b-scission) of

side chains of cyclic hydrocarbons, particularly of those

having substituents with more than five carbon atoms,

leading to significant dealkylation of pendant substituents on

the ring. The yields of desired high cetane ring-opened

products are usually low due to a high extent of consecutive

cracking reactions and a fast catalyst deactivation.

2.2. Hydrocracking on bifunctional catalysts

Starting in the 1950s, bifunctional catalysts consisting of

highly dispersed metal particles for hydrogenation or

dehydrogenation and an acidic support for cracking or

isomerization were introduced in refining processes, such as

catalytic reforming of naphtha, hydrotreating, hydrocrack-

ing and hydroisomerization [2]. These processes improved

fuel quality and at the same time avoided catalyst decay and

the formation of coke deposits. Early studies by Mills et al.

[15] and by Weisz and Swegler [16] described the reactions

over the bifunctional catalysts as proceeding over two

distinct catalytic sites: the reactants are first converted into

olefins on the metal site via hydrogenation/dehydrogenation

reactions (Eq. (1)); then the formed olefins are protonated at

the acid sites, leading to the formation of carbenium ions,

which subsequently undergo skeletal isomerization, crack-

ing or alkylation (Eq. (2)). The products are finally desorbed

from the acid sites as olefins, which are hydrogenated on the

metal sites in the presence of hydrogen (Eq. (3)). It was later

recognized that the reactions could also occur on one

reaction site, owing to activated hydrogen species, i.e.

spillover hydrogen [17]. The metal co-catalyst provides

spillover hydrogen, which migrates to the acid sites and

saturates the carbenium intermediates. In other words, the

acidic support cannot only initiate the formation of

carbenium ions pyrolytically or by the addition of protons

to olefins, but also hydrogenate carbenium ions with hydride

ions and promote their desorption as the saturated products.

In addition, the ring opening of naphthenes can also proceed

on certain metal sites via direct hydrogenolysis of an

endocyclic C–C bond [12], i.e. cleavage of a C–C bond with

the addition of hydrogen. Overall, the activity and selectivity

of ring opening on bifunctional catalysts are strongly

dependent on the properties of the metal and support as well

as on reaction conditions. These parameters include the type

of metal, metal particle size, acidity of the support, pore size

of the support, the interface length and strength balance

between the metal and acid sites, working conditions, such

as temperature, hydrogen pressure, etc. This has provided

opportunities to devise catalysts for highly selective ring

opening of naphthenic compounds in fuel without significant

dealkylation of any pendant substituents on the ring.

(1)

(2)

(3)

2.3. Hydrogen spillover

The classical model for bifunctional catalysis proposed

byMills et al. [15] and byWeisz and Swegler [16] envisaged

the reaction in three steps (Eqs. (1)–(3)), involving a gas

phase diffusion of olefinic intermediates between the two

catalytic sites. The model successfully explained a number

of experimental observations, but failed to account for the

role of hydrogen and the synergy between the two catalyst

components in controlling the selectivity and activity [17].

The extension of the classical model by incorporating the

hydrogen spillover concept allows a better interpretation of

experimental results, including those that did not fit into the

old theorem. The new model involves the formation of

mobile hydrogen surface species, i.e. spilt-over hydrogen

that allows the hydroconversion of the hydrocarbon at a

certain distance from the metal so that all reaction steps can

occur on one reaction site.

Spillover is now a well-known phenomenon in hetero-

geneous catalysis, involving the transport of active species

sorbed or formed on one surface onto another that does not

sorb or form the active species under the same conditions.

Several small molecules have been known to exhibit

spillover effects upon interactions with noble metals,

including hydrogen, oxygen, nitrogen, carbon monoxide

and organic species [18]. Hydrogen spillover plays an

important role in petroleum processes, e.g. in hydrotreating,

hydrocracking, hydrogenation and hydroisomerization.

A number of excellent reviews on this subject have been

published [17–21].

The nature of the spilt-over hydrogen species has been

discussed in the literature. Depending on the system studied,

different species have been claimed, including H atoms, H

ions, ion pairs, H3 species and bound species. Roland et al.

[18] proposed a model that describes the spilt-over species

as electron donors adsorbed on the surface, which

corresponds to H atoms (occupied, weakly chemisorbed)

or H+ ions (empty, strongly chemisorbed). Their ratio is

determined by the chemical properties (e.g. the presence of

Lewis and Bronsted acid sites) and the electronic properties

(e.g. electron density) of the solid. In bifunctional catalysts,

spilt-over hydrogen can donate an electron to the support to

form a proton on a Bronsted site, and a hydride ion may form

on a Lewis site in the process of charge balancing [13]. An

olefin formed on the metal by dehydrogenation can react


with the proton on a Bronsted site to form a carbenium ion,

which may then combine with the migrated hydride ion from

a Lewis site to produce a saturated compound. Alternatively,

the carbenium ion formed by addition of a proton to an olefin

on a Bronsted site can migrate to a Lewis site where it reacts

with a hydride ion.

The creation of catalytic active sites by spilt-over

hydrogen has been recognized and used in supported metal

catalysts for hydrogenation (or dehydrogenation), hydro-

cracking and hydroisomerization. For instance, spillover of

hydrogen from a metal under certain conditions can make

the inert silica, or alumina, or carbon support active for

hydrogenation of olefin or benzene [22–27]. A number of

studies showed that mechanical mixtures of supported metal

catalyst and a support exhibited higher hydrogenation

activities than the supported metal catalyst alone. The

increased activities have been attributed to hydrogen

spillover. Without activation by hydrogen spillover, neither

silica nor alumina will adsorb hydrogen and both are inert

for hydrogenation catalysis.

Several studies have implied that hydrogen spillover can

give rise to Bronsted acid sites on oxide supports, including

zeolites. Fujimoto [26] found that mechanically mixed

catalyst Pt/SiO2 + H-ZSM-5 showed both high selectivity

and high conversion for benzene hydrogenation, equal to

those obtained over a Pt/ZSM-5 catalyst, while Pt/SiO2 or

H-ZSM-5 alone were not effective catalysts. He attributed

the activity of the mechanical mixture to the generation of

Bronsted acid sites by hydrogen spillover. Ohgoshi et al.

[27] demonstrated that the hybrid catalysts Pt/KA + NaYor

H-ZSM-5 showed high activity for isobutene hydrogena-

tion, while the Pt/KA catalyst and zeolite Y or H-ZSM-5

alone failed to hydrogenate isobutene because isobutene is

too big to fit into the pores of KA zeolite (3 A), and H2

cannot be activated on NaY or H-ZSM-5 zeolite. They

concluded that H2 was activated on Pt/KA. The resulting H

atoms spilt-over onto the NaYor H-ZSM-5 zeolite, and then

hydrogenated isobutene on the zeolite. Further study using a

H-ZSM-5 zeolite that contained almost no SiOH groups

mixed with Pt/KA yielded zero conversion of isobutene,

indicating the OH groups play an important role for

migration of spilt-over hydrogen. In the bifunctional CoMo/

zeolite hydrocracking catalysts, Co-Mo adsorbs H2 and

Fig. 3. Non-selective and selective hydr

supplies spilt-over hydrogen to acidic sites, where it forms

acidic hydroxyl groups and promotes acid-catalyzed

reactions [21].

3. Selective ring opening of single-ring compounds

3.1. Activity and selectivity

When adsorbed on metal surface at certain conditions,

hydrocarbon molecules undergo dehydrogenation/hydroge-

nation, skeletal isomerization and hydrogenolysis, including

ring opening/cyclization, and ring contraction/enlargement.

Certain noble metals, such as Pt, Pd, Ir, Ru and Rh have been

found to be selectively active for the ring opening for cyclic

hydrocarbons to the corresponding paraffins with the same

carbon number. The activity and selectivity depend mainly

on the metal catalysts, such as the type of metal, particle

size, crystal morphology, etc. The ring opening of cyclic

hydrocarbons on noble metals is highly sensitive to the

catalyst structure, particularly alkylcyclopentanes and

alkylcyclobutanes, which have been used as molecular

probes to characterize various types of catalysts. Several

excellent reviews have been published on this subject

[14,28–33].

The ring opening of methylcyclopentane (MCP) on

supported metal catalysts has been extensively studied

[30,34–37]. The reaction produces n-hexane (nHx), 2-

methylpentane (2MP) and 3-methylpentane (3MP). The

product distribution is dependent on the properties of the

metal. Over supported Pt catalysts with small particle sizes,

e.g. low loading and highly dispersed Pt, the ring opening of

MCP is non-selective (the rupture of endocyclic C–C bonds

is statistical, producing 2MP, 3MP and nHx in a ratio of

2MP:3MP:nHx = 40:20:40 related to the number of bond

types in the molecule) (Fig. 3). On the other hand, the ring

opening of MCP on large metal particles and metal surfaces

with low Miller index produces selectively 2MP and 3MP,

but no nHx. In some cases, the ring opening does not follow

the ‘selective’ or ‘non-selective’ mechanism, instead

producing unusually high amount of nHx or 3MP relative

to its statistical ratio. These ‘partially selective’ mechanisms

compete with the non-selective and selective mechanism,

ogenolysis of methylcyclopentane.


occurring on high loaded 10% Pt/Al2O3 at high-temperature

[30], or Pt/zeolite [38–40].

The ring opening of MCP on Ir [12] and Rh [41], on the

other hand, is less sensitive to the particle size of the metals

in comparison with Pt catalysts. However, these metals

exhibit higher activity and selectivity to open the C5 ring in

bisecondary positions [36,42]. Other metal catalysts, such as

Co, Ni, Ru and Os, show extensive hydrogenolysis, yielding

a significant amount of fragments [41,42].

Generally, the ring-opening activities of alkylcyclopen-

tanes on metal catalysts decrease with increased number of

ring substitutents [34]. This is especially the case for Ir, and

to lesser extent Ru, Rh and Ni [36], all of which show a

preference for cleaving unsubstituted ring C–C bonds. A

significant decrease in activity was observed for cleaving

substituted ring C–C bonds over these metals. In compar-

ison, Pt is better able to break substituted ring C–C bonds

than Ir, Ru, Rh and Ni. However, the ring-opening rate over

Pt catalysts is sensitive to the cis-/trans-isomer ratio,

decreasing with increasing trans-isomer concentration. In

other words, Pt requires a cis-substitution of isomers when

breaking a tertiary–tertiary C–C ring bond.

The reaction conditions have also been found to affect the

product distribution. Changes in selectivity and activity of

MCP ring opening on supported Pt metal catalysts with

hydrogen pressure have been reported [34,43]. Higher

hydrogen pressures favor ring opening further from the

substituents. The increase in hydrogen partial pressure leads

to increased ring-opening activity on supported Pt catalysts at

relatively low temperatures (<300 8C). This reaches a

maximum and thereafter shows a steady decline, or a very

broad plateau with further increase in hydrogen partial

pressure. The hydrogen pressure at which the maximum

activity is obtained is a function of the support. However,

these volcano-type plots of ring-opening activity versus

hydrogen pressure are independent of the support and the

reduction temperature. The latter determines the particle size,

morphology and structure of themetal particle on the support,

and influences the activity. These resultswere attributed to the

competitive adsorption of hydrogen and hydrocarbon on the

metal surface. Increase in hydrogen pressure diminishes

dehydrogenation activity, leading to an increase in the overall

selectivity towards the ring-opening reaction.

Hydrogen pressure dependence on activity and selectivity

of ring opening has also been observed over other transition

metal catalysts [41,42,44–47]. The relationships between

the product formation rate and hydrogen pressure varies with

catalytic systems, reflecting differences in reaction mechan-

ism. Over Rh catalysts [45,46], the ring opening of 1,2-

dimethylcyclopropane goes through a maximum, and then a

minimum and then a continuous increase along with the

increase in hydrogen pressure. On Pd, Ni and Cu catalysts

[45], the formation rates of one product increases and levels

off with increasing hydrogen pressure, while for the other

product the curve passes through a maximum. These

observations were attributed to the changes of formation of

adsorbed intermediates at different hydrogen pressures. A

dissociative adsorption of hydrocarbon via the rupture of the

C–H bond(s) was assumed at low hydrogen pressure. An

increase in hydrogen pressure inhibits this C–H dissociation,

slowing down the reaction rate. In the case of Rh catalyst,

new adsorbed species were presumably formed via scission

of the ring C–C bond at high hydrogen pressure. These

species were desorbed to produce ring-opening compounds.

Compared to the five-membered ring cyclopentanes, the

ring opening of the four-, and six-membered ring

compounds are less extensively studied. Cyclobutanes are

more reactive towards ring opening than cyclopentanes due

to higher ring strain. The reactions can take place at low

temperature (<100 8C), exhibiting relatively low selectivity,

i.e. statistical product distribution [44,46]. On the other

hand, the ring opening of cyclohexanes over metal catalysts

[36,48,49] is much slower than five-membered rings, even

over the most active metal Ir [36]. Instead, cracking and

aromatization dominate the total reaction. For a significant

conversion of six-membered rings into alkanes without loss

of molecular weight, additional acid function is required to

convert the six-membered rings to the five-membered rings

via skeletal isomerization. The latter then readily undergo

selective ring opening on acid or metal sites.

3.2. Proposed mechanism

There have been several mechanisms proposed to account

for ring opening on metal surfaces, two of which are well

recognized: the multiplet mechanism and the dicarbene

mechanism [30,33]. The two mechanisms differ mainly in

how the reaction intermediate forms on the metal surface.

In the multiplet mechanism, cyclic hydrocarbons

physically adsorb either edgewise on two metal atoms, or

flat-lying on the metal surface. The former is called the

‘doublet’ mechanism, and the latter called the ‘sextet-

doublet’ mechanism. Both mechanisms compete with each

other in the reaction. The doublet mechanism is thought to

occur on small metal particles, and is used to explain the

selective hydrogenolysis of bisecondary C–C bonds.

According to this mechanism, cyclic hydrocarbon is

adsorbed perpendicular to the metal surface via a

bisecondary C–C bond, which then reacts with chemisorbed

hydrogen in a push–pull manner to produce ring-opening

products. Due to steric hindrance, the edge-wise adsorption

of the tertiary–secondary or tertiary–tertiary C–C bonds is

limited. In the ‘sextet-doublet’ mechanism, on the other

hand, the cyclic molecule is physically adsorbed flat-lying

on the metal surface, with the carbon atoms of the ring

located over the interstices of the metal plane, e.g. Pt(1 1 1).

For the five-membered ring cyclopentanes, one C–C bond

has to be stretched (Fig. 4), and this bond is readily attacked

by neighboring, adsorbed hydrogen, leading to hydrogeno-

lysis of the ring. The tertiary–secondary C–C bonds in

alkylcyclopentane could be ruptured via the five-atom

adsorbed mode. For cyclobutanes, all four C–C bonds are


Fig. 4. Hydrogenolysis of methylcyclopentane via the multiplet mechanism [30].

stretched, resulting in higher reactivity but lower selectivity.

In contrast, there is no stretching when cyclohexanes and

paraffin adsorb on the metal surface, as all the carbon atoms

can fit the interstices of the Pt(1 1 1) plane. As a result,

cyclohexanes and paraffin are usually not hydrogenolysized,

but preferably dehydrogenated or isomerized.

Alternatively, the dicarbene mechanism involves the

chemisorption of the cyclic hydrocarbon molecules on the

metal surface after the rupture of several C–H bonds, forming

carbon–metal bonds or p-adsorbed olefins (Fig. 5). The non-

selective ring opening of MCP on metal catalysts can be

accounted for by thep-adsorbed olefinmechanism. Similar to

that in the sextet-doublet mechanism, MCP is adsorbed

parallel to the metal surface, but in a quasi-planar manner

involving only one metal atom. The selective ring opening of

MCP, on the other hand, involves 1,2-dicarbene complexes

that bond to several metal atoms and stand perpendicular to

the metal surface. Owing to steric hindrance, the hydro-

genolysis of the tertiary–secondaryC–Cbonds is retarded.On

some occasions, however, an exocyclic alkyl substituent

participates in the formation of a metallocyclobutane

intermediate, resulting in the selective breaking of tertiary–

tertiary and tertiary–secondary C–C bonds (Fig. 5b).

Fig. 5. Hydrogenolysis of methylcyclopenta

3.3. Electronic effect

As various mechanisms were proposed to account for

experimental facts such as selectivity, structural effect and

kinetic data, the nature of the catalyst–reactant interactions

remains unknown. In ring opening (cyclization), and

isomerization catalysis, metallocyclobutanes, metallocar-

benes and metallocarbynes were postulated as the possible

reaction intermediates, depending on the nature of the sites.

To generalize and simplify the reaction mechanisms, Garin

and Maire [29] proposed an agostic precursor as the first

species adsorbed on the metal surface, which consists of an

M H–C with a hydrogen atom bonded simultaneously to

both a carbon atom of a reactant and a transition metal atom

(Fig. 6). This species initiates the formation of a s-alkyl or

carbene species that further reacts to give rise to products of

ring opening (cyclization), hydrogenation (dehydrogena-

tion) and/or isomerization. It was proposed that an

electronic (or ligand) effect governs the relative contribu-

tion of s-alkyl or carbene precursor, while a geometric (or

ensemble size) effect determines the pathway to the bond

shift (isomerization), the cyclic, and the hydrogenolysis

reactions.

ne via the dicarbene mechanism [30].


Fig. 6. Proposed reaction mechanism of n-C5H12 on a metal catalyst,

involving an agostic precursor species [29].

Fig. 7. Simplified reaction mechanism for the catalytic reforming of

hydrocarbons on transition metal surfaces [54].

In hydrocracking/isomerization of 2-methyl- and 3-

methylpentane on several group VIII metal catalysts [29],

studies have shown that Co and Ni tend to cleave multiple

bonds, leading to extensive cracking, while Pd, Pt and Ir

selectively rupture primary–secondary/tertiary, secondary–

secondary/tertiary (demethylation) and secondary–second-

ary C–C bonds, respectively. The differences in the

selectivity and activity of metals in hydrogenolysis and

isomerization may be related to their electronic structure

(density of states), as proposed by Saillard and Hoffmann

[50]. When a H2 or a hydrocarbon molecule (e.g. CH4)

approaches a metal surface, the electron transfer from d

orbitals of the metal M to a C–H s* antibond (M! s*)

dominates the early stages of the reaction, which weakens

the C–H s bond and forms the M–H bond. This is different

from transition metal complexes in which s!M electron

transfer leads the reaction because of the higher energy of

the occupied metal orbitals on the surface. For transition

metals at the left side of the periodic table, the surface is

positive relative to the bulk, while for metals at the right side

of the periodic table, the metal surface is negative. This is in

accordance with the fact that the heats of chemisorption of

hydrogen on metal surfaces decrease from left to right across

a periodic row [50,51]. These metals are more likely to form

metallocarbynes or carbenes, leading to multiple C–C

rupture. Within the same series of transition metals, e.g.

group VIII metal, the charge transfer from the bulk to the

surface decreases as d level occupancy for a metal surface

increases in traversing a periodic row [50,52]. Consistently,

the heats of chemisorption for hydrogen atoms on the metal

surface decrease in going from 3d to 4d to 5d (i.e. decrease

with increased atomic weight).

Based on the product distribution, hydrogenolysis

reaction on metal catalysts can be classified into two groups

[29]: the C2-unit mode, involving rupture of C–C bonds

between primary and secondary carbon atoms; and the iso-

unit mode, involving rupture of a C–C bond with a tertiary

carbon atom. In the C2-unit mode, metallocarbene species

may be formed upon removing at least two hydrogen atoms,

which lead to hydrogenolysis over isomerization. On the

other hand, in the iso-unit mode where only one H atom is

available on the tertiary carbon, s-alkyl species are formed,

leading to higher isomerization than hydrogenolysis. The

relative contribution of the two modes is determined by the

electronic effect, i.e. the nature of the metal catalyst.

The C2-unit mode is favored on monometallic Ru, Ir and

bimetallic Pt-Ru, Pt-Co, Pt-Ir and Pt-Mo catalysts. The C2-

unit mode-dominated reactions, e.g. catalyzed by Ir catalysts,

were thought to proceed via a dicarbene (or doublet)

mechanism. The reactant bonds perpendicularly to the metal

surface. The reactions are usually not sensitive to the particle

size of metals but to the number of substituents because of

steric hindrance. On the other hand, the iso-unit mode is

dominant onNi, Pt, Pd, Pt-Ni, Pt-WandPt-Pd catalysts. In this

case, the reactant is adsorbed parallel to themetal surface, and

the reaction proceeds via a multiplet mechanism. The particle

size of metal influences the selectivity of the reactions.

It is now widely accepted that the reactions of

hydrocarbon on metal are initiated by dissociative adsorp-

tion of the alkanes on the metal surface and followed by

subsequent hydride elimination from adsorbed alkyl

intermediates. The former is the rate-determining step in

the reaction, while the latter is thought to determine the

selectivity. Zaera [53–55] proposed that b-hydride elimina-

tion from the surface intermediate accounts for the

production of olefins, g-hydride elimination is responsible

for isomerization and cyclization, and a-hydride elimination

leads to hydrogenolysis products (Fig. 7). The relative rates

for a-, b- and g-dehydrogenations determine selectivity of

metal catalysts in hydrocarbon reforming. In general, the

reactivity for a-, b- and g-dehydrogenations increases with

early transition metals and with decreasing hydrocarbon


chain lengths. The b-hydride elimination is the most

favorable. However, the relative rates for a-, b- and g-

dehydrogenations within the metal also change across the

periodic table, which results in the different catalytic

performance of different metals in hydrocarbon reforming.

For instance, Pt(1 1 1) displays comparable rates for a- and

g-dehydrogenations, while Ni(1 0 0) only shows a-dehy-

drogenation. These results explain the unique ability of Pt

for catalyzing hydrocarbon reforming as opposed to Ni,

which facilitates extensive cracking.

3.4. Geometric effect

The geometric effect on the catalytic properties is

exemplified by the particle size effects, which have been

extensively studied [28,30,31]. In hydroconversion of

hydrocarbons, large metal particle size promotes selective

hydrogenolysis, and small particle size favors non-selective

isomerization and hydrogenolysis. For instance, ring open-

ing on supported Pt metal catalysts exhibit pronounced

particle size effects on the selectivity. As illustrated by the

hydrogenolysis of MCP [56,57], catalysts with large Pt

particle size lead to selective ring opening, favoring 2MP,

and 3MP with the formation of nHx suppressed. Small Pt

particle size produces non-selectively nHx, 2MP and 3MP in

a statistical ratio. Other metals, e.g. Rh and Ir, favor the

selective hydrogenolysis with little effect of particle size.

The particle size has also affected the turnover rate

(activity) in the hydrogenolysis of cyclic hydrocarbons

[30,31]. On Pt/Al2O3, the turnover rate of the hydrogeno-

lysis of cyclopentane was shown to decrease with decreased

particle size. Similar results were observed on Rh/Al2O3. On

Ir/Al2O3, however, the hydrogenolysis of cyclopentane is

insensitive to the particle size. Pd/Al2O3, on the other hand,

presented a slight increase in the hydrogenolysis of

cyclopentane with decreasing particle size.

The origin of particle size effects is not clear. Some

possible explanations include electronic effects, morphol-

ogy, and support effects [28,30,31]. As the particle size is

reduced, the electronic bands of the metal particle become

distinct, and the electron energies increase. For most metal

particles, the electron energies begin to increase as the

particle size is reduced below 5 nm. For some metals, the

energies level off below a certain size, e.g. Pd at about 1 nm,

while for others, e.g. Pt, the increase in energies continue

with the decrease in particle size. As a result, the electronic

configurations of the surface atoms are different from those

of large particles, leading to changes in the catalytic

properties. On the other hand, the decrease in particle size

likely changes the morphology of the metal particle, which

could influence the catalytic properties. For instance, a large

fcc metal particle crystallizes in an octahedron geometry (8

faces), while most small fcc metal particles (d = 10 nm)

adopt cubooctahedral geometry (14 faces) as demonstrated

by quantum-mechanical calculations and experimental

studies. For even smaller particles a non-fcc arranged

icosahedron (20 faces) might become more stable [31]. As

the particle size decreases, the number of atoms of low

coordination at the edges or corners of the crystallites

increases, while the fraction of face atoms diminishes.

Therefore, small particles would favor catalysis by atoms of

low coordination, e.g. the non-selective, statistical hydro-

genolysis, giving higher rates. Large particles would favor

catalysis by face atoms of high coordination numbers, e.g.

selective hydrogenolysis. The effect of surface atom

coordination on the catalytic properties of the metal can

also account for the changes in the selectivity of the ring

opening of MCP over different metal surfaces. In controlled

tests using metal films with oriented faces [58,59] or

supported metal catalysts where the specific facets of the

metal particles were selectively prepared [41,60], studies

showed that the hydrogenolysis activity and product

selectivity varied from facet to facet of metal crystal. High

activities and non-selective hydrogenolysis were observed

on small polycrystalline particles with disordered surfaces,

or on flat, stepped, and kinked noble-metal single-crystal

surfaces of high Miller index containing high concentrations

of low-coordination sites. This particle size phenomenon

observed in heterogeneous catalysis is the basis of the

current interest in nanotechnology related to chemistry and

materials.

3.5. Support effect

Besides the above-mentioned electronic and geometric

effects, catalyst supports also play an important role in

heterogeneous catalysis. As demonstrated in many catalytic

processes, the supports are not inert in selective ring-

opening catalysis as previously thought, but alter the

catalytic properties of the metal particles through electronic

interactions, or by space confinements. The support effect

has attracted much attention and stimulated extensive

research worldwide. Excellent reviews have been published

on this subject [32,33,61,62].

A ‘partly’ selective mechanism for ring opening of MCP

was observed on noble metals supported on acidic oxides,

which produced a higher proportion of nHx than the

theoretical ratio, according to a non-selective mechanism.

The formation of nHx appeared to increase with increasing

support acidity [33,63,64]. Kramer and Zuegg [57] used the

adlineation model, coined by Schwab and Pietsch [65], to

account for the partial selectivity for nHx and 2MP. The

model assumed that ring opening occurs at the phase

boundary of metal and support. The support, i.e. an acidic

site, interacts with the tertiary C–H of MCP because this H

atom is most susceptible to attack by an acidic or cationic

site, while the neighboring carbon is attached to the metal

site. Ring opening of MCP adjacent to the methyl group

occurs, leading to the formation of additional nHx (Fig. 8).

This partly selective mechanism competes with a non-

selective mechanism occurring exclusively on the metal

surface. Alternatively, the partly selective ring opening of


Fig. 8. Partly selective ring opening of methylcyclopentane.

MCP may be explained by electronically asymmetric pairs

of metal atoms resulting from strong metal–support

interactions. The cationic sites of the support in bifunctional

catalysts exert an electric field on adjacent metal sites,

leading to the formation of partially charged metal atoms

that have been confirmed by various methods [33,66]. The

resulting Md�–Md+ exhibit functionalities similar to those of

the acid site–metal pair to selectively produce nHx and 2MP

but not 3MP (Fig. 8) [63].

Another ‘partly’ selective mechanism for ring opening of

MCP has been observed over zeolite-supported metal

catalysts, for example, on SAPO-11 [67], LTL [38,39,43]

and FAU [43,68]. Over these catalysts, significantly higher

selectivities to 3MP were observed compared to other

supported metal catalysts of similar particle size. These

enhanced selectivities were attributed to the constraint of the

one-dimension pores of zeolites that result in a preferred

orientation of the incoming MCPmolecule with its long axis

parallel to the direction of the pores. When the MCP

molecule reaches a metal particle inside a zeolite pore, ring

opening preferentially takes place through one of its ends,

resulting in a 3MP/2MP product ratio higher than the

statistical value of 0.5 (Fig. 8). Similarly, a higher 3MP/nHx

ratio than the statistical value of 0.5 can be explained by the

steric hindrance of the methyl group of MCP in zeolite pores

that restrict the rotation of the molecule when it approaches

the metal particle with its methyl end.

4. Selective ring opening of multiple-ring

compounds

Compared to the number of studies using mono C4, C5

and C6 naphthenic rings, there are fewer studies on ring

opening of more complex molecules containing multiple

rings. Recently, model compounds containing two fused

rings, such as indan, decalin, tetralin and naphthlene have

been used to model the upgrading of heavy oil fractions.

Unlike ring opening of alkylcyclopentanes, which can

readily occur at low temperature by hydrogenolysis on noble

metal catalysts, ring opening of fused C6 naphthenic rings

requires the addition of an acidic function to promote the

isomerization of the C6 rings into the more reactive C5 rings

for achieving reasonable ring-opening activity. The ring-

opening reactions are complex, usually accompanied by

hydrogenation, isomerization and cracking.

4.1. Indan

The indan molecule consists of a C5 ring fused to a

benzene ring, and represents a probable reaction intermediate

of reactions of heavy oil fractions on ring-opening catalysts.

The ring opening of indan was recently evaluated using noble

metal catalysts supported on boehmite by Nylen et al. [69].

They found that the hydrogen pressure plays a key role in the

reaction. At atmospheric pressure the ring opening of the C5

ring was dominant, leading to 2-ethyltoluene (60–70 mol%)

and minor amounts of n-propylbenzene (7–10%) (Eq. (4)).

The Ir and Pt-Ir catalysts showed superior catalytic activities

to the Pt catalyst. Pt was shown to selectively open the C5

naphthenic ring into 2-ethyltoluene with a selectivity

approximately seven times higher than for n-propylbenzene,

while Ir displays relatively high cracking activity. At high-

pressure (40 bar) and low-to-moderate temperature, hydro-

genation of indan into hexahydroindan was favored.

Increasing temperature resulted in increasing both the ring

opening and subsequent undesired cracking products.

(4)

4.2. Decalin

The six-member hydrocarbon rings are more stable than

five-member rings, with the ring strain energies of about

1 kcal/mol and 6–7 kcal/mol, respectively. McVicker et al.

[36] showed that the ease of ring opening of two-ring

naphthenes is directly related to the relative number of

saturated five-member rings in the molecule. Therefore, the

ring opening of decalin (4.4% conversion on 0.9% Ir/Al2O3)

with two saturated six-member rings is much slower than

perhydroindan (68%) with one six- and one five-member

ring, and bicyclo[3,3,0]octane of two five-member rings. An

acidic function in the catalyst has to be used to achieve a

significant amount of ring-opening products by promoting

the isomerization of six-member ring structures into the five-

member ones.

On acidic catalysts, the direct ring opening of decalin was

thought to proceed via cleavage of the corresponding

carbonium ion, which is initiated by the attack of a Bronsted

acid site on a C–C bond of decalin [70]. The alkylnaphthene

carbenium ion can further undergo b-scission and/or

isomerization to form lighter naphthenes or gaseous

molecules. Meanwhile, bimolecular reactions may occur,


Fig. 9. Proposed mechanism for direct ring opening over acid catalyst [70]. PC, pyrolytic cracking; ISO, isomerization; TA, transalkylation; b, b-scission; HT,

hydride transfer.

such as hydrogen transfer, transalkylation or disproportio-

nation to produce various naphthenic species, as shown in

Fig. 9. Alternatively, ring opening of decalin can take place

as a result of isomerization (ring contraction), which

produces five-member ring hexahydroindan. In other words,

the ring-opening products of decalin originated from the

isomerization products. Indeed, stereo- and skeletal iso-

merizations of decalin were found to dominate the reaction

of decalin on proton-form zeolites including medium-pore

ZSM-5, MCM-22, large-pore HY, H-Beta, H-Mordenite,

UTD-1 and extra-large-pore H-MCM-41 [71,72]. The

decalin isomers were then consumed by consecutive

reactions of ring opening and cracking, yielding more than

200 products. Stereoisomerization of decalin was thought to

occur on a Bronsted site via a pentacoordinated carbocation

(Fig. 10b), or carbenium ion formed by protonic dehy-

drogenation, or proton exchange, or on Lewis acid sites by

hydride abstraction (Fig. 10c). The cis-decalin is much more

Fig. 10. Isomerization of cis-decalin to trans-decalin [71].

reactive than trans-decalin, and converts much more

selectively to ring-opening products than trans-decalin.

The latter mainly converts to cracking products. The skeletal

isomerization was initiated by ring contraction of decalin

carbocations or carbenium ions to methylbicyclo[4,3,0]no-

nanes, followed by their isomerization or further ring

contraction (Fig. 11). The isomers can undergo further

isomerization to yield a complicated mixture of isomers.

Among those isomers, methylbicyclo[4,3,0]nonanes,

dimethylbicyclo[3,2,1]octanes, dimethylbicyclo[3,3,0]oc-

tanes and methylbicyclo[3,3,1]nonanes are the most

abundant. The pore size of zeolites influences the isomer

product distribution, while the acidity and temperature do

not affect the selectivity of isomerization. Decalin hardly

penetrates the channels of the medium-pore zeolites (ZSM-

5, MCM-22); these zeolites are much less active than large-

pore zeolites as the reaction takes place only on external

surfaces [70]. In addition, the pore topology of zeolite has a

strong influence on diffusion and, consequently, on activity

and selectivity. As a result, dimethylbicyclooctanes were

Fig. 11. Proposed isomerization reaction network for decalin [71].


Fig. 12. Proposed mechanism for isomerization and ring opening of decalin over acidic catalyst [71]. PC, pyrolytic cracking; ISO, isomerization; DS,

desorption; b, b-scission; HT, hydride transfer.

found to be more abundant over H-Y than over H-Beta

zeolites, while more methylbicyclononanes are formed over

Beta zeolites [71]. The isomers are then converted into ring-

opening and cracking products. The ring strain and the

number of alkyl substituents in the isomers affect the ring

opening. Strain in the hydrocarbon ring results in ring

opening following the order C3 > C4� C5 � C7� C6.

The isomers with more alky substitutes are more prone to

ring opening by acid because these compounds consist of

more tertiary carbon atoms that more easily form carbenium

ions than the secondary carbon. As a result, the main ring-

opening products on large-pore zeolites HY, H-Beta, and H-

Mordenite [71] are propyl-ring-opened products (ROP)

(from methylbicyclo[4,3,0]nonanes), followed by ethyl-

ROP (from dimethyl bicycle[3,2,1]octane with 1 or 2 methyl

groups on the five member ring), methyl-ROP (from

dimethyl bicycle[3,2,1]octane with 2 methyl groups on

the six member ring), ethylpropyl-ROP and butyl-ROP. The

different isomer product distributions on different zeolites

are reflected in the ring-opening product (ROP) distribu-

tions, i.e. relatively more ethyl-ROPs and less propyl-ROPs

are observed over H-Y zeolites in comparison with H-Beta.

Fig. 13. Proposed mechanism for isomerization and ring opening of decalin over

desorption; b, b-scission; HT, hydride transfer; HYG, de-/hydrogenation; RO, ri

Amechanism for decalin isomerization and ring opening has

been proposed (Fig. 12), involving pyrolytic dehydrogena-

tion, pyrolytic cracking, hydride transfer, b-scission and

olefin desorption and adsorption.

Introduction of noble metal onto acidic zeolite catalysts

reduces the strength of Bronsted acid sites, and significantly

enhances isomerization and ring opening of decalin [72,73],

while the cracking is reduced. Stereosiomerization of cis-

decalin to trans-decalin proceeds on a metal site via the

dehydrogenation–hydrogenation mechanism (Fig. 10a), in

addition to the stereoisomerization facilitated solely by acid

sites (Fig. 10b and c). The skeletal isomerization occurs on

Bronsted acid sites as shown in Fig. 13, which is an essential

step for the ring opening of decalin. In the absence ofBronsted

acid sites, neither isomerization nor ring-opening reactions

occur. The main isomerization products are methylbicy-

clo[4,3,0]nonanes and dimethylbicyclo[3,2,1]octanes, simi-

lar to those on proton-form zeolites [70,71]. However, the

distributions of ring-opening products over Pt-modified

zeolites are different from those obtained on H-zeolites.

The dominant ring-opening products are methyl-cyclohex-

anes, followed by ethyl-cyclohexanes. Ring opening yields of

bifunctional catalyst [73]. PC, pyrolytic cracking; ISO, isomerization; DS,

ng opening.


as high as 30 mol%were obtained; in comparison just 8 mol%

of ring-opening products were achieved on H-zeolites. The

differences were attributed to the addition of Pt. In addition to

ring opening initiated byBronsted acid sites as observedonH-

zeolites, Pt particles participated in the ring-opening reaction:

(1) to form olefin by dehydrogenation of decalin isomers or

ring-opening products, which were then protonated over

Bronsted acid sites and underwent isomerization and ring

opening; (2) to directly open rings of isomers and (3) to

provide spillover hydrogen to suppress the secondary

reactions (e.g. cracking) and to prevent the catalyst

deactivation.

4.3. Tetralin

Ring opening of tetralin via cracking on acidic supports

produced mainly C1 to C4 fractions, benzene, naphthalene

and C10 olefin/naphthene aromatics (mainly methylindanes

and naphthalene) [70,72]. Their relative selectivity varied

from one zeolite to another depending on pore topology and

size. Extensive cracking dominated on zeolites with medium

pore sizes (ZSM-5, MCM-22, ITQ-2) to form C3 and C4

fractions. Large pore zeolites (Beta, Y) were able to open the

naphthenic ring and minimize the dealkylation of the formed

alkylaromatics. Extra-large pore zeolites (UTD-1) and

mesoporous MCM-41 favored ring opening. A mechanism

was proposed to account for the reaction, involving the

attack of a Bronsted acid site either on the aromatic ring or

on the naphthenic ring (Fig. 14). The resulting carbenium

and carbonium ions underwent isomerization (ring contrac-

tion), b-scission and protolytic cracking to form ring-

opening products and isomerization products such as

Fig. 14. Proposed reaction networks for catalytic cracking of tetralin

methylindanes and methylindenes. Subsequently, dealkyla-

tion occurred to produce cracking products, while dis-

proportionation between carbenium and olefins led to heavy

products and coke. Fast hydride transfer between tetralin and

the adsorbed carbenium ions was attributed to the formation

of naphthalene, which was found to be one of the primary

products.

On bifunctional catalysts, ring opening of tetralin was

believed to occur exclusively on Bronsted acid sites, while

metal sites hydrogenated tetralin to decalin and promoted its

isomerization (ring contraction) which is a crucial step in the

ring opening of decalin as described in the previous section.

Hydrogenation of tetralin to decalin was found to dominate

on supported noble metal catalysts, especially at low

temperature [72,74–79]. The hydrogenation of tetralin was

reported to take place not only on metal centers but also on

acid sites by hydrogen spillover from the metal surface. The

formed decalin underwent skeletal isomerization, and then

subsequent ring-opening and cracking reactions. The rate of

isomerization, and hence the ring-opening rate increased

with the decrease in the distance between the metal and acid

sites and with the increase in metal loading. The ring-

opening product yields and product distribution were related

to the topology of zeolite structure, i.e. diffusional

limitations. The acidity, on the other hand, shifted the

optimal (maximum yield) reaction temperature for ring

opening. Mild acidity favored ring opening.

Supported noble metal catalysts are very sensitive to

sulfur compounds. Efforts have been made to improve the

sulfur resistance of noble metal-based catalysts for

hydrogenation. For instance, bimetallic catalysts, e.g. Pd-

Pt, have demonstrated moderate sulfur tolerance in

[70]. Iso, isomerization; PC, pyrolytic cracking; b, b scission.


hydrogenation [74,77,79]. However, the influence of sulfur

on the activity for ring opening of naphthenes is still unclear.

Rodriguez-Castellon et al. [74] found that the ring opening

and cracking of tetralin on Pt-Pd catalysts supported on

zirconium doped mesoporous silica were suppressed in the

presence of sulfur compounds, but not as much as

hydrogenation.

Ring opening of tetralin on transition metal catalysts, e.g.

NiMo and NiW supported on acidic materials, such as

silica–alumina or zeolites has been investigated [80–86].

These catalysts are able to catalyze the ring-opening reaction

of naphthalenes in the presence of sulfur compounds. Such

catalysts are usually used at severe operating conditions,

such as high hydrogen pressure and high-temperature, due to

their relatively low activities. Unlike noble metal catalysts,

the loading of NiW onto zeolites increases the amounts of

Bronsted acid sites [80,85]. However, the additional acid

sites themselves do not catalyze hydrocracking of tetralin.

The increased conversion of tetralin over pure zeolites can

be attributed to the bifunctionality. Sato et al. [80–82]

reported hydrocracking of tetralin over bifunctional NiW

sulfide catalysts supported on zeolites USY, HY and

mordenite (MOR) under typical hydrocracking conditions

at moderate temperature (623 K) and high hydrogen

pressure (6.1 MPa) with a low H2/feed ratio. They found

that the ring opening of tetralin required relatively strong

acid sites, and was related to the hydrogen transferability of

supports: NiW/USY > NiW/HY > NiW/MOR > NiW/

Al2O3. The NiW/Al2O3 catalyst functions mainly as a

hydrogenation catalyst, yielding decalin with small propor-

tions of heavy compounds, while zeolites and NiW/zeolite

catalysts produced a complex mixture of more than 300

Fig. 15. Proposed bimolecular mechanism for catalytic

compounds, consisting of products of cracking (alkane

gases, benzene), hydrogenation (decalin), isomerization

(decalin isomers), ring opening (alkylbenzene, monocyclo-

paraffins), dehydrogenation (naphthalene) and alkylation

(alkyltetralin, tricyclic compounds). NiW was found to

hydrogenate aromatic or olefinic compounds that were

produced in the reactions, and to supply spillover hydrogen

to the acid sites. Due to an insufficient supply of hydrogen

with NiW sulfide, hydrocracking of tetralin proceeded

mainly via a bimolecular pathway (Fig. 15), in addition to a

unimolecular mechanism, at the initial reaction stage,

leading to the formation of heavy compounds. In the long

run, the unimolecular process was dominant, and the heavy

compounds were gradually converted to light products. A

careful balance between the metal active sites of hydro-

genation and acid sites of cracking was necessary for

optimal performance for selective ring opening. Under

conditions with high hydrogen/tetralin ratios, on the other

hand, hydrocracking of tetralin proceeds via a monomole-

cular mechanism over transition metal or metal sulfide

catalysts [83–85], producing cracking compounds (volatiles,

benzene, toluene, xylene), ring opening (n-propylbenzene,

n-butylbenzene), isomerization (indan), hydrogenation

(decalin) and dehydrogenation (naphthalene) compounds.

No products heavier than decalins were observed [83–85].

4.4. Naphthalene

Hydrogenation of naphthalene to tetralin is dominant in

the hydroconversion of naphthalene over bifunctional

catalysts at low temperatures [40,87–95]. The hydrogena-

tion rate of naphthalene to tetralin is an order of magnitude

cracking of tetralin [81]. A, acid, DS, desorption.


Fig. 16. Hydrogenation networks for naphthalene over bifunctional cata-

lysts [88]. M, metal sites; A, acid sites; Hsp, spilt-over hydrogen.

higher than that of tetralin to decalin, due to the strong

adsorption of naphthalene on the metal surface [87].

Because of a thermodynamic limitation, the hydrogenation

conversions decreased with the increase in reaction

temperature, while the isomerization and ring-opening

reaction increased [91]. In a bifunctional catalyst system,

the hydrogenation takes place via conventional metal-

catalyzed hydrogenation, or acid site induced hydrogena-

tion, involving migration of spillover hydrogen from metal

sites (Fig. 16). The latter was thought to account for the

increased hydrogenation activity of metal catalysts on acidic

supports [88,94].

Ring opening of naphthalene is a secondary reaction,

converting saturated and/or isomerized products on metal or

Bronsted sites. Albertazzi et al. [90] showed that Pd(4)-Pt(1)

supported on Mg/Al mixed oxide, which is a basic support

with no Bronsted acid sites, produced an appreciable amount

of ring opening (18 mol% yield), mainly alkycyclohexanes,

at moderate conditions around 300 8C. In the presence of

acid sites, the yield of ring opening increased up to 30 mol%

[91].

The ring opening increased with the increase in reaction

temperature. However, consequent cracking reactions of

ring-opening products are favored at high temperatures

(>375 8C), leading to lighter products. The optimal

Fig. 17. Hydroconversion networks for 1-methylnaphthalene over bifunctional ca

transfer; HYG, de-/hydrogenation.

operating temperature exists, e.g. at ca. 350 8C for ring

opening of 1-methylnaphthalene over Pt/USY [40].

The ring-opening product yields are also dependent on

the Bronsted acidity. Arribas and Martinez [40] studied the

influence of zeolite acidity for the coupled hydrogenation

and ring opening of 1-methylnaphthalene on Pt/USY

catalysts with varied aluminum content (i.e. acidity). They

found that selective ring opening was favored over zeolites

with a low density of Bronsted acid sites. Strong and high

density of Bronsted acid sites lead to extensive cracking and

dealkylation. They also found that the selective ring opening

of 1-methylnaphthalene increased with decreasing space

velocity and increasing total pressure with both cases

exhibiting a maximum for ring-opening conversion. The

unimolecular mechanism was proposed for the hydrocrack-

ing of 1-methylnaphthalene (Fig. 17).

4.5. Phenanthrene

Few studies on selective ring opening of three-ring cyclic

compounds have been reported. Several early studies were

carried out to elucidate the reaction networks for hydro-

conversion of three- or four-ring compounds on bifunctional

catalysts under hydrocracking conditions. Nonetheless,

these studies shed insights into understanding of the ring

opening of these model compounds. The studies are

summarized as follows.

Phenanthrenewas considered a suitable model compound

for hydrogenation and hydrocracking of liquefied coal over

bifunctional catalysts. Lemberton and Guisnet [96] carried

out a model test of hydroconversion of phenanthrene over

NiMo sulfide supported on alumina at 430 8C, 10 MPa of H2

in a stainless steel autoclave. They found that phenanthrene

hydroconversion proceeded through multi-step hydrogena-

tion, isomerization and cracking reactions. Phenanthrene

initially was hydrogenated into dihydrophenanthrene, which

in turn was hydrogenated into tetrahydro- and octahydro-

phenanthrenes. But further hydrogenation into perhydro-

talysts [40]. ISO, isomerization; DS, desorption; b, b-scission; HT, hydride


Fig. 18. Proposed mechanism for phenanthrene hydroconversion [96]. ISO, isomerization; DS, desorption; b, b-scission; HT, hydride transfer.

phenanthrene was not observed. On NiMo/Al2O3 with only

weak acidity, ring opening took place through the opening of

the saturated central ring of phenanthrene, yielding mainly

2-ethyl biphenyl, or the opening of a saturated terminal ring,

yielding mainly butyl naphthalene, methylpropyl naphtha-

lene and probably diethyl naphthalene. The former was

proposed to occur on an acid site, initiated by an acid attack

to a benzene ring (Fig. 18), via cleavage of aromatic C–

secondary C bond rather than cleavage of the 9–10 C–C

bond because of the formation of stable conjugated

carbocation (2-ethyl biphenyl). The ring opening of the

saturated terminal ring, on the other hand, was thought to

proceed via a bifunctional mechanism, involving isomer-

ization of the C6 ring into the C5 ring, followed by ring

opening of the isomers on acid sites. Cleavage of aromatic

C–secondary C bond was highly disfavored because of the

involvement of an unstable primary carbocation.

Similar results were obtained by Korre et al. [97] on a

presulfided NiW/USY catalyst at 350 8C, 68.1 atm of H2 in a

batch autoclave. They presented detailed kinetic studies on

the hydrocracking of phenanthrene. A reaction network

consisting of hydrogenation, isomerization, ring opening

and dealkylation was established (Fig. 19), which was then

used to obtain various reaction rates, equilibrium and

adsorption parameters. Di- and tetrahydrophenanthrenes

were found as the primary products. The latter was

hydrogenated further to octahydrophenanthrene, but with

no perhydrophenanthrene observed. Terminal ring saturated

tetra- and octahydrophenanthrenes then underwent isomer-

ization and subsequent ring opening, while the ring-opening

products (dimethyl- or ethylbiphenyls) of central ring

saturated dihydrophenanthrene were not observed. The

ring-opening conversion of the latter was thought to proceed

via reversible dehydrogenation/hydrogention to the terminal

ring saturated compounds. Kinetic studies showed that

hydrogenations of terminal aromatic rings were favored over

those of the central ring, and isomerization of sym-

octahydrophenanthrene was favored over those of asym-

octahydrophenanthrene. Isomerizations and ring opening

preferably occurred at position a to an aromatic ring or a

tertiary carbon because of the ability of the parent structure

with tertiary carbons or aromatic rings to stabilize a

carbocation intermediate. Therefore, isomerizations proceed

in the following orders: tetrahydrophenanthrene > sym-

octahydrophenanthrene > asym-octahydrophenanthrene >tetralin. Similar trends have been observed for ring opening

and dealkylation.

4.6. Fluorene and fluoranthene

The hydrogenation of fluorene initially occurred over

bifunctional catalysts [98]. The primary product hexahydro-

fluorene was then converted by isomerization, ring opening

and dealkylation. However, isomerization of hexahydrofluor-

ene took place in parallel with ring opening, and the ring

opening of hexahydrofluorene and its isomers occurred

through the central five-member ring (Fig. 20). In the presence

of a strong acid (e.g. on NiMo/zeolite Y), ring opening of

hexahydrofluorenes occurred preferably in parallel with

isomerization and dealkylation, yielding mainly cyclohexyl-

methylenebenzene and cyclohexyltoluene.

The hydroconversion of fluoranthene follows a similar

mechanism. Initial hydrogenation of fluoranthene on NiMo/

zeolite Y yields tetrahydrofluoranthene as the major primary

product, which is then converted by cracking and

isomerization to produce mainly 1-phenyltetralin, 2-phe-

nyltetralin, 2-phenlymethylindan, tetralin and benzene. Ring

opening of the five-member ring of tetrahydrofluoranthene

immediately followed the hydrogenation of fluoranthene

due to its high ring strain. The formed 1-phenyltetralin can

isomerize to the thermodynamically more stable isomer 2-

phenyltetralin. 1-Phenyltetralin can recombine with a proton


Fig. 19. Phenanthrene hydroconversion networks over a presulfided NiW/

USY catalyst [97]. Relative pseudo-first order reaction constants are shown.

to form a carbenium ion, which rearranges to a more stable

benzylic carbocation by 1,5- and/or 1,3-hydride transfer

from the saturated ring. The latter is subject to a

hydrogenolytic cleavage of the C–C bond between two

aromatic rings to give tetralin and benzene. 2-Phenyltetralin,

on the other hand, does not form benzylic carbocation

because of unfavorable hydride transfer in this conforma-

tion. As a result, isomerization of 2-phenyltetralin took

place, yielding 2-phenylindan. The reaction networks for

hydrogenation and ring opening of fluoranthene are

summarized in Fig. 21.

5. Sulfur tolerance

The discussion thus far has considered ring-opening

reactions in model compounds where noble metal catalysts

are the most active catalysts for selective ring opening of

naphthenes. However, these catalysts are very sensitive to

sulfur compounds in real hydrocarbon feedstocks, which

normally need a severe hydrotreating pretreatment to reduce

the sulfur concentration to below a few ppm. The application

of these catalysts would thus be limited by such severe

pretreatment conditions, unless the sulfur tolerance pro-

blems could be solved. It is believed that the sulfur tolerance

of a noble metal catalyst is related to the electron density of

metal clusters. The sulfur poisoning is mainly due to the fact

that adsorption of H2S decreases the metal–support

interaction, which promotes platinum migration and leads

to a growth of platinum particle size [100]. Therefore, the

sulfur tolerance can be increased by decreasing Pt electron

density and/or enhancing metal–support interactions. By

choosing tetralin hydrogenation as a model reaction for

aromatics reduction, Chiou et al. [101] investigated the

effect of sulfur on the deactivation of g-Al2O3 supported Pt

catalysts. They found that the reversible reaction scheme is

unable to explain the decreased reactivation rate with the

severity of sulfur poisoning. TPR, fast FT-IR and electron

probe microanalysis (EPMA) suggested that the interactions

between CO and Pt, and H2 and Pt were weakened due to the

increase in sulfur poisoning. A sintering of platinum

particles occurred, promoted by the H2S adsorption at the

Pt–alumina interface, which may have decreased the Pt–

alumina interaction.

It is possible to improve sulfur resistance through the

addition of a second metal component to Pt or Pd catalyst,

i.e. via bimetallic interaction. There are well-documented

examples that alloying of metals can increase the activity,

selectivity and stability in catalytic reactions [102]. The

formation of the bimetallic phase could inhibit the Pt or Pd

migration. In addition, electron transfer from Pt or Pd to the

second metal would increase the sulfur tolerance of the

catalyst. Fujikawa et al. [103] performed screening tests of

various SiO2-Al2O3 noble metal catalysts (Pt, Pd, Re, Sn, Ir,

Ni, Mo and Ge) with hydrotreated light cycle oil (LCO)/

straight-run light gas oil (SRLGO) feedstocks containing

32–34 vol% aromatics and 172–474 ppm sulfur at a

temperature of 573 K, H2 pressure of 4.9 MPa, and LHSV

of 1.5 h�1. They found that the Pt-Pd/SiO2-Al2O3 catalyst

was the most highly active catalyst for aromatic hydro-

genation under the applied conditions. The co-existence of

Pt with Pd on SiO2-Al2O3 remarkably enhanced the catalytic

activity, which depended on the Pd/(Pt + Pd) weight ratio

and reached a maximum at about 0.7 Pd/(Pt + Pd) weight

ratio. TEM studies indicated that all active metals form Pt-

Pd bimetallic agglomerates on the catalyst. Long-term

stability tests demonstrated the excellent stability of the Pd-

Pt/SiO2-Al2O3 catalyst. Similar results have been reported

by Lin et al. [100]. The supported Pt catalysts on g-Al2O3

were modified by adding a secondmetal, such as Co,Mo, Ni,

Re, Ag and Pd. The results showed that Pd-Pt catalyst had

the highest sulfur resistance. Long-term stability tests over

Pt and Pd-Pt catalysts indicated the catalyst exhibited much


Fig. 20. Postulated mechanism for fluorene hydroconversion [98]. ISO, isomerization; DS, desorption; b, b-scission.

Fig. 21. Postulated mechanism for fluoranthene hydroconversion [99].

better catalytic performance for hydrogenation of diesel

feedstocks containing 28.4% aromatics, 369 ppm sulfur,

44.8 ppm nitrogen under the following commercial diesel

hydrotreating conditions: WHSV 3.0 h�1, H2 to oil mole

ratio 2.5, temperature 340 8C, pressure 580 psig. Pd-Pt

bimetallic interaction played a crucial role in improving

sulfur resistance. Fast FT-IR studies of CO adsorption

suggested that electrons are transferred from Pt to Pd by

bimetallic interaction, inhibiting the adsorption of H2S. In

addition, it was inferred that the Pd inhibited agglomeration

of Pt particles during hydrogen regeneration.

Another approach to enhance the sulfur resistance of

noble metal catalysts is to fine tune the metal–support

interaction. It has been reported that the sulfur resistance of

Pt catalyst can be increased by using acidic support, such as

SiO2-Al2O3 or zeolites. The interaction between the strong

acid site and the small cluster of noble metal results in the

electrons being withdrawn from the noble metal, thus

creating an electron-deficient metal particle [89,104–107].

This decreases the strength of the bonding interaction

between sulfur and metal and increases the sulfur tolerance

of the catalyst. Song and Schmitz [89] studied the

hydrogenation of naphthalene in n-tridecane at 200 8C in

the absence and presence of benzothiophene over mordenite

and zeolite Y-supported Pt and Pd, compared with Al2O3-

and TiO2-supported Pt and Pd catalysts. Both zeolite-

supported catalysts are substantially more active than the

Al2O3- and TiO2-supported catalysts. The presence of

benzothiophene decreases the activity of all the catalysts.

However, there are significant improvements in sulfur

tolerance of the noble metals when supported on zeolites.

Mordenite-supported Pd catalyst has shown the best activity

and the highest sulfur resistance [89].

6. Concluding remarks

Currently, there is increasingly interest in selective ring-

opening catalysis that can produce middle distillate with

high cetane number with bitumen-derived crude and heavy


oils. Ring opening of cycloparaffins can be catalyzed via

carbenium intermediates by protolytic cracking on the

Bronsted sites. The cracking of endocyclic C–C bonds in

cyclic hydrocarbons is much slower than those of aliphatics.

As a result, the overall reaction is predominated by

isomerization and subsequent hydrocracking (b-scission)

of side chains of cyclic hydrocarbons, particularly of those

having substituents with more than five carbon atoms,

leading to significant dealkylation of pendant substituents on

the ring. The yields of desired high cetane ring-opened

products are usually low due to a high extent of consecutive

cracking reactions and a fast catalyst deactivation.

On the other hand, the ring opening of cycloparaffins can

proceed on certain metal catalysts via direct hydrogenolysis

of an endocyclic C–C bond, i.e. cleavage of a C–C bond with

the addition of hydrogen. The reaction is accompanied by

dehydrogenation/hydrogenation, skeletal and stereo isomer-

ization. The activity and selectivity depend mainly on the

type of metal, particle size, crystal morphology, etc. Certain

noble metals, such as Pt, Pd, Ir, Ru and Rh have been found

to be selectively active for the ring opening for cyclic

hydrocarbons to the corresponding paraffins with the same

carbon number. However, these metal catalysts lack activity

for ring opening of the six-member hydrocarbon rings.

The presence of the acid function is necessary for

selective ring opening of six-membered ring naphthenes in

feedstocks. The acid function promotes the formation of

intermediate carbenium ions, which isomerize into five-

membered rings that readily undergo subsequent ring

opening on acid or metal sites. The metal function supplies

spilt-over hydrogen to the acid sites to saturate the

intermediate carbenium ions and prevent the formation of

coke. The acid number, strength and distance between the

acid site and the metal site strongly influence the catalytic

performance. Insufficient supply of spilt-over hydrogen

leads to mainly acidic cracking as in the monofunctional

catalyst, and favors coke formation, whereas in a catalytic

system with spilt-over hydrogen in a sufficiently high

concentration, hydrogenation dominates, and admixing of a

metal-free component can further increase the conversion.

The optimal catalytic system requires a well-balanced

metallic-acidic function. In selective ring-opening catalysis,

the preferred acid function isomerizes six-membered ring

naphthenes to five-membered ring naphthenes with the

minimum number of ring substituents [36]. Such non-

branching ring contraction allows maximal ring-opening

rates and product selectivities and minimal undesired

hydrocracking and secondary acyclic paraffin isomerization

reactions.

The commercial process for selective ring opening

involves bifunctional catalysts, both metal and acid sites,

working together in high-pressure, high-temperature reactor

systems in the presence of hydrogen. The acidic sites

catalyze dehydrogenation, cracking, isomerization and

dealkylation, while the metal sites promote hydrogenation,

hydrogenolysis and isomerization. The widely available

catalysts using transition metal sulfides on acidic supports

usually require severe operating conditions due to their low

activities of the metal sulfide compared to the metal sites,

leading to extensive cracking of cycloparaffin side chains.

Noble metals supported on acidic oxides are highly active

catalysts for selective ring opening, but these catalysts are

very sensitive to poisoning by sulfur compounds in

petroleum feedstocks. There is a need to develop new

generations of selective ring-opening catalysts with high-

performance and sulfur resistance that could meet the future

increasing cetane specification.

The search for such catalysts through the conventional

trial-and-error method would be tedious. During the last

decade, rapid development of modern computing techniques

and computational chemistry have enabled one to run

theoretical calculations at a relatively low cost, and at the

same time allowed for the accurate descriptions of the

molecular structures, spectroscopy and predictions of

chemical reactivity. The application of computational

chemistry in heterogeneous catalysis has grown rapidly in

recent years, which has helped to better understand the

reaction mechanism, to explain the experimental data, and to

devise new catalysts. For instance, quantum theoretical

calculations of electron densities of alloy catalysts, aided by

modern characterization techniques, such as X-ray photo-

spectroscopy, scanning tunnel microscopy, XANES, TPD,

etc., have provided insights into understanding the origin of

alloying effects of the metal catalysts. These studies have

yielded qualitative relationships of the chemisorption bond

strength of the reactants on the metal catalysts and catalytic

activity [28,53,102,108]. Recently, Jacobsen et al. [109]

found that the catalytic activity for ammonia synthesis is a

function of nitrogen adsorption energy on the surfaces of the

various metals, which exhibits a typical volcano curve. The

ideal catalyst should possess an optimal binding energy; the

metals that bind nitrogen either too weakly or too strongly

are poor catalysts. Guided by the theoretical studies, the

groups found theoretically, and confirmed experimentally,

that the alloys Co–Mo, Fe–Ru, Fe–Co and Ni–Mo were

more active than pure Co, Mo, Ru or Fe. Similar strategies

have been applied to search for hydrodesulfurization

catalysts from sulfur binding energy–activity correlations

[110–117], and hydrogenation catalysts from hydrogen

dissociation energy–activity correlations [118]. It is

expected that such strategies will provide opportunities in

searching for new metal catalysts with high-performance

and sulfur resistance for selective ring-opening catalysis.

In comparison, hydrogenolysis of C–C bonds in a

hydrocarbon is much more complex, and is usually

accompanied by dehydrogenation, isomerization, cycliza-

tion, etc. Nonetheless, Zaera [53–55] discovered a relation-

ship between the selectivity of metal catalysts in

hydrocarbon reforming and the ability of metal to catalyze

a-, b- and g-dehydrogenations from chemisorbed surface

alkyl intermediates. The latter varies across the periodic

table, and can be correlated to temperature-programmed


desorption of alkyl halides on the metal surface. The

temperature-programmed desorption data, which corre-

spond to the temperature maxima for the desorption of

methane, ethylene or iso-butane via a-, b- or g-hydride

eliminations, respectively, indicate the degree of difficulty of

the dehydrogenation reactions. Metals with the ability to

promote a-hydride elimination facilitate hydrogenolysis in

hydrocarbon reforming. Such data, though scarce and

scattered in the literature, have yielded some useful insights

into the hydrocarbon reforming catalysis. Research into this

direction and establishment of quantitative structure–

activity relationships could provide opportunities to discover

new ring-opening catalysts that could meet future fuel

specifications.

High throughput experimentation methods, invented in

the pharmaceutical industry for fast discovery of new drugs

has attracted a lot of attention in the catalysis research field

[119]. Many research institutes and companies have adopted

this technology when searching for new catalysts or for

improving existing catalyst performance. This method

allows the synthesis and catalyst testing of tens and even

hundreds of materials at one time, greatly speeding up the

catalyst development process. Such techniques will provide

new opportunities in searching for bi- or multi-metallic

metal catalyst with high-performance and sulfur resistance

for selective ring-opening catalysis, and/or improving

hydrocracking catalysts with well-balanced acid and metal

functions. With the guidance of today’s advanced computa-

tional methods, the search will be even more markedly

shortened by zeroing in a limited number of promising

candidates.

Acknowledgements

Partial funding for the National Centre for Upgrading

Technology (NCUT) has been provided by the Canadian

Program for Energy Research and Development (PERD),

the Alberta Research Council (ARC) and the Alberta Energy

Research Institute (AERI). The authors thank Mr. Norman

Sacuta for proofreading the manuscript.

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the chemistry of selective ring-opening catalysts.pdf

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selective ring opening

singlering compounds

membered ring cycloparaffin

number of ring structures

metal sites

active catalysts

acid sites

metal catalystswith