Catalysis: Volume 16by James J Spivey
There is an increasing challenge for chemical industry and research institutions to find cost-efficient and environmentally sound methods of converting natural resources into fuels chemicals and energy. Catalysts are essential to these processes and the Catalysis Specialist Periodical Report series serves to highlight major developments in this area. This series
There is an increasing challenge for chemical industry and research institutions to find cost-efficient and environmentally sound methods of converting natural resources into fuels chemicals and energy. Catalysts are essential to these processes and the Catalysis Specialist Periodical Report series serves to highlight major developments in this area. This series provides systematic and detailed reviews of topics of interest to scientists and engineers in the catalysis field. The coverage includes all major areas of heterogeneous and homogeneous catalysis and also specific applications of catalysis such as NOx control kinetics and experimental techniques such as microcalorimetry. Each chapter is compiled by recognised experts within their specialist fields and provides a summary of the current literature. This series will be of interest to all those in academia and industry who need an up-to-date critical analysis and summary of catalysis research and applications. Catalysis will be of interest to anyone working in academia and industry that needs an up-to-date critical analysis and summary of catalysis research and applications. Specialist Periodical Reports provide systematic and detailed review coverage in major areas of chemical research. Compiled by teams of leading experts in their specialist fields, this series is designed to help the chemistry community keep current with the latest developments in their field. Each volume in the series is published either annually or biennially and is a superb reference point for researchers. www.rsc.org/spr
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Catalysis Volume 16
A Review of Recent Literature
By James J. Spivey
The Royal Society of ChemistryCopyright © 2002 The Royal Society of Chemistry
All rights reserved.
BY MEHRI SANATI, BJORN HARRYSSON, MOSTAFA FAGHIHI, BORJE GEVERT, AND SVEN JARAS
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 γ-Al2O3 are 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 material.
The typical feedstock for laboratory tests is usually either a mixture of model mono-compounds and/or a mixture of different aromatic hydro-carbons. 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 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, respectively.
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), hydrodenitro-genation (HDN) and hydrodeoxygenation (HD0). 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 hydro-conversion, which enables a change in the molecular weight and structure of organic molecules. Examples are hydrogenation (HYD) and hydrodearo-matization (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. 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.
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 hydrode-sulfurization 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.
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 pressure.
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.
Thus, the use of these catalysts depends strongly on severe pre-treatment conditions, unless the sulfur tolerance can be greatly improved for the noble 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 Navalikhina.32 Special attention to the preparation methods was discussed in more detail by P. Grange and X. Vanhaeren. A comprehensive review of the hydrodeoxygenation, with particular focus on upgrading of bio-oils, was published by Furimsky. Catalyst deactivation during hydroprocessing, including the adverse effects of the 0-compounds, was reviewed by Furimsky and Massoth.
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 hydrodearomatization.
A comprehensive review of the reactions during hydroprocessing has been published by TopsØe 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 γ-Al,2O3 supported hydro-genation catalysts, using the conventional CoMo, NiMo, NiW, and platinium group metals (including ruthenium, rhodium, palladium and platinum). 0n 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 mono- aromatic rings such as benzene.1 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.
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.
2.1 Reactivity in Hydrogenation Reactions. - The recent reactivity studies have been reviewed by Moreau and Geneste, Girgis and Gates and Stanislaus and Cooper. 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:
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 - 105
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 rr- 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 thermo-dynamic 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/Al203 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. The total aromaticity is generally given by resonance energy (RE);105 resonance energy increases with the number of aromatic rings, independently of the presence or the absence of heteroatoms in the rings.
The magnitude of resonance energy per ring was less for naphthalene compared with benzene, and consequently the hydrogenation rate was low for benzene. The 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 phenan-threne and anthracene, the behaviour is shown in Table 2.
The increase in resonance energy with the angularity of the system, e.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 π 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 π-bonds involving an electron transfer from the aromatic ring to the unoccupied d-metal orbitals. Since the π-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 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/Al2O3 and Ni-Mo-S/Al2O3, the reactivity pattern of aromatic and alkylated aromatic compounds is the opposite. In other words, addition of methyl groups to aromatic compounds gives rise to an enhancement of the reactivity of these molecules for hydro-genation; the corresponding hydrogenation rate for the same homologous series (as stated above) will decrease in the order benzene < toluene < p-xylene < m-xylene < o-xylene over moderate reaction conditions.
Both the number and position of the substituted groups in benzene ring affected the hydrogenation activity. The 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. The lowest reactivity of ortho-isomers has been attributed to steric effects.
Steric hindrance of neighbouring methyl groups in ortho-positions had a more significant effect on the formation of π-bonded complexes.
Pondi and Vannice69 observed that the relative rate of toulene and benzene hydrogenation with an equimolar gas mixture for the Pd supported catalysts in Al2O3 and SiO2-Al2O3 was less than unity. A value less than unity indicated that toluene was less reactive than benzene on Pd catalysts. 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.
For the three catalytic systems, Pt/SiO2-Al2O3, Pd/SiO2-Al2O3, and PtPd/ SiO2-Al2O3, 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. This reactivity sequence was related to a decrease in the resonance energy per aromatic ring as well as to differences in the π-electron cloud density in the aromatic ring because of the inductive effect of the methyl group in toluene. 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. 0f 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.
Excerpted from Catalysis Volume 16 by James J. Spivey. Copyright © 2002 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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Meet the Author
Professor Spivey is the McLaurin Shivers Professor of Chemical Engineering at Louisiana State University and Director of the DOE Energy Frontier Research Center at LSU. Professor Spivey's research interests include the application of the principles of heterogeneous catalysis to catalytic combustion, control of sulfur and nitrogen oxides from combustion processes, acid/base catalysis (e.g., for condensation reactions), hydrocarbon synthesis, and the study of catalyst deactivation.
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