catalysts in Petroleum Refining and Petrochemical Industries 1995, Proceedings of the 2nd International Conference on Catalysts in Petroleum refining and Petrochemical Industries

579 Pages·1996·15.13 MB·vxii, 1-604\579

by  M. Absi-Halabi, J. Beshara and A. Stanislaus (Eds.)

FOREWORD
The 2nd International Conference on Catalysts in Petroleum Refining and Petrochemical
Industries was held in Kuwait during the period April 22-26, 1995, under the auspises of
H.H. Sheikh Saad A1-Abdullah A1-Salem A1-Sabah, Kuwait's Crown Prince and Prime
Minister. The 1st conference was also held in Kuwait in 1989. The present conference was
scheduled to be held in 1993; however, it was postponed due to the events that
encompassed Kuwait and the Gulf region in 1990-1991.
The patronage of the conference, the organizing bodies, and the selective emphasis on the
role of catalysts in the petroleum and petrochemical industries reflect the keen interest of
the countries in the region in actively contributing to the development of these industries.
Petroleum-related industries are the main economic activities of most countries in the
region. The refining capacity in the Gulf Region exceeds 5 MM barrels/day and includes
some of the most sophisticated petroleum refining schemes in the world. The basic
petrochemical industry has been also growing steadily in the region since the early eighties.
The conference was attended by around 300 specialists in the catalysis field from both
academia and industry from over 30 countries. It provided a forum for the exchange of
ideas between scientists and engineers from the region with their counterparts from the
industrialized countries. A total of 62 scientific papers were presented. The papers were
carefully selected to include a blend of fundamental and applied research, and industrial
experience. Such a blend was thought to be essential for providing the participants from
both industry and academia with a chance to become familiar with the challenges facing
each group and the actions taken to meet them.
A number of keynote speakers, carefully selected from high ranking officials, policy
makers, and multinational company representatives, were also invited to address the
conference. The keynote presentations, which are published as a separate volume by the
Kuwait Institute for Scientific Research, provided the participants with an overview of the
directions the petroleum and petrochemical industries will take over the next decade.
The program of the conference included a field visit to one of Kuwait's most modem
refineries. A trip was also organized to one of Kuwait's oil fields. The partipants had a
chance to observe oil lakes and the extent of the damage incurred by the blowing up of
Kuwait's oil wells.
The success of the conference is perhaps difficult for the organizers to assess. However, the
quality of the papers in this volume provides some indication. Another indication is the
keen interest and encouragement expressed by numerous participants in attending the next
meeting, which will be held in Kuwait in 1998.
The Editors
vi
P R E F A C E
Catalysis plays an increasingly critical role in modern petroleum refining and basic
petrochemical industries. The market demands for and specifications of petroleum and
petrochemical products are continuously changing. They have impacted the industry
significantly over the past twenty years. Numerous new refining processes have been
developed and significant improvements were made on existing technologies. Catalysts
have been instrumental in enabling the industry to meet the continuous challenges posed by
the market. As we enter the 21st century, new challenges for catalysis science and
technology are anticipated in almost every field. Particularly, better utilization of
petroleum resources and demands for cleaner transportation fuels are major items on the
agenda. It is against this background that the 2nd International Conference on Catalysts in
Petroleum Refining and Petrochemical Industries was organized.
The papers from the conference were carefully selected from around 001 submissions. The
papers were refereed in terms of scientific and technical content and format in accordance
with internationally accepted standards. They were a mix of reviews providing an
overview of selected areas, original fundamental research results, and industrial
experiences.
The papers in the proceedings were grouped in the following sections for quick reference:
Plenary Papers
-
Hydroprocessing of Petroleum Residues dna Distillates
-
Fluid Catalytic Cracking
-
Oxidation Catalysis
-
Aromatization & Polymerization Catalysis
-
Catalyst Characterization dna Performance
-
The plenary papers were mostly reviews covering important topics related to the objectives
of the conference. The remaining sections cover various topics of major impact on modern
petroleum refining and petrochemical industries. A large number of papers dealt with
hydroprocessing of petroleum distillates and residues which reflects the concern over
meeting future sulfur-level specifications for diesel and fuel oils.
The task of editing this volume was facilitated by the efforts of the International Advisory
Committee and the Scientific Committee of the conference who reviewed all the papers.
The editorial board gratefully acknowledge this effort; the cooperation, time and effort of
all authors; and the management of the Kuwait Institute for Scientific Research for
allocating the required resources to prepare the manuscript of this volume.
The Editors
xi
ORGANIZING COMMITTEE
Jasem IA Besharah Chairman KISR
Khaled 1A Muhailan Rapporteur KFAS
Mamun Absi Halabi Coordinator KISR
Abbas ilA Khan Member KFAS
Anwar Abdullah Member GCC
Taher 1A Sahaf Member KU
Mohammad ilA Abbas Member KPC
Abdul-Karim Abbas Member KNPC
Bader IA Safran Member PIC
Faisal Mandani Member PAAET
Hassan Qabazard Member KISR
Mubarak IA Adwani Member KISR
IA Tayeb Wenada Member OAPEC
INTERNATIONAL ADVISORY COMMITTEE
Mamun Absi Halabi Chairman Kuwait
David L. Trimm Member Australia
Bernard Delmon Member Belgium
Burce .C Gates Member USA
Walter Kaminsky Member Germany
Yasuaki Okamoto Member Japan
Mario L. Occelli Member USA
Henrik Topsoe Member Denmark
SCIENTIFIC COMMITTEE
Taher 1A Sahaf Chairman KU
Anthony Stanislaus Rapporteur KISR
Abdullah .S 1A Nasser Member Mina Abdulla~NPC
Jaleel Shishtary Member Mina 1A Ahmadi/KNPC
Erdogan Alper Member KU
Mustafa .A .A Gholoum Member Shuaiba/KNPC
Faisal Mandani Member PAAET
Ezra Kam Member KISR
Xll .~
ACKNOWLEDGEMENTS
The Organizing Committee was deeply honored by the patronage of//. H. The Crown
Prince and Prime Minister Sheikh Saad A1-Abdullah A1-Salem AI-Sabah, which reflects
his keen interest in science and technology.
The Committee is also grateful for the financial support of the Kuwait Institute for Scientific
Research, the Kuwait Foundation for the Advancement of Science, the Kuwait National
Petroleum Company, the Kuwait Petroleum Corporation, Kuwait University, the Gulf
Cooperation Council, Public Authority for Applied Education and Training, the
Petrochemical Industries Company and the Organization of Arab Petroleum Exporting
Countries. The Committee would like also to express gratitude for the efforts of the Japan
Petroleum Institute in coordinating and supporting the participation of prominent Japanese
scientists in this event.
The Committee would like also to extend its deep appreciation for the effort and time put
forth by the distiguished keynote speakers, namely H.E. Mr. Hisham Al-Nazer, H.E. Mr.
Erwin Valera, H.E. Mr. Lulwanu Lukman, Mr. Abdullatif AI-Hamad, Mr. Charles DiBona,
Mr. John Yimoyines, Mr. .J Kent Murray, Mr. Mahmoud Yusef, Mr. Moayad Al-Qurtas,
Mr. Khalaf A1-Oteibeh, Mr. Khaled Buhamra, and Mr. Nader Sultan.
The Organizing Committee are also appreciative of the efforts of the members of the
International Advisory Committee and the Scientific Committee for their thorough work in
selecting and refereeing the submitted papers.
The Committee also acknowledges the help and guidance provided by Elsevier Science
Publishing Company and the advisory editors of this series ni preparing this proceedings.
We would like to thank our colleagues at the Kuwait Institute for Scientific Research, the
Kuwait Ministry of Oil, and the chairmen and cochairmen of the sessions, who provided
unlimited assistance at times when it was badly needed. Finally, we feel deeply indebted to
the participants who enriched the meeting with their serious discussions till the end.
DR. JASEM BESHARA
CHAIRMAN, ORGANIZING COMMITTEE
Catalysts in Petroleum Refining and Petrochemical Industries 1995
M. Absi-Halabi et .la (Editors)
(cid:14)9 1996 Elsevier Science B.V. llA rights reserved.
CONTROL OF CATALYST PERFORMANCE IN SELECTIVE OXIDATION OF
LIGHT HYDROCARBONS: CATALYST DESIGN AND OPERATIONAL
CONDITIONS
B. Delmon, P. Ruiz, S.R.G. Carraz~in, S. Korili, M.A. Vicente Rodriguez, Z. Sobalik
Catalyse et Chimie des Mat6riaux Divis6s, Universit6 Catholique de Louvain, Place Croix du
Sud 2/17 - 1348 Louvain-la-Neuve, Belgium
This paper is an attempt to summarize the situation with respect to the selective catalytic
oxidation of light alkanes using heterogeneous catalysts. Methane oxidation reactions and the
oxidation of butane to maleic anhydride will only be alluded to occasionally, because they have
been reviewed in detail in a large number of papers.
We shall first show that it is still far from clear which are the families of catalysts to be used
for the various reactions: mainly oxidative dehydrogenation or oxidation to oxygen-containing
molecules of ethane, propane or isobutane. Much research is still necessary for understanding
the mechanisms leading to high selectivity. In this context, we shall suggest that many concepts
inherited from the development in selective oxidation and ammoxidation of olefins are probably
of little use.
Conversely, much emphasis has to be laid on new data which opens promising
perspectives, namely (i) the occurrence of cooperation effects between two (or several) separate
phases and especially the role of spillover oxygen and the so-called "remote control" and (ii) the
occurrence of homogeneous non-catalysed reactions which occur at temperatures only slightly
higher than the catalytic ones and correspond to similar selectivities.
This suggests that research on selective catalytic oxidation, to be effective, should be
comprehensive: it should continue to involve a search for new active phases and efforts to
improve the already known catalysts. But research should also include investigations on the
role of spillover oxygen, the nature of this oxygen (more or less electrophilic), the donors that
can generate it, and the way this spillover oxygen reacts with the catalytic surface. Research
should also contemplate the problem of how homogeneous and heterogeneous reactions
proceed simultaneously or consecutively. In parallel with these research lines, chemical
engineering must develop new concepts and new reactors. Recent spectacular results in
methane coupling or oxidative dehydrogenations show that considerable progress can be made
if the problem of light alkane selective oxidation benefits from a multifacetted approach.
1. INTRODUCTION
Making valuable products from light hydrocarbons is presently one of the major challenges
for the petroleum and petrochemical industries. Among the various processes able to transform
light hydrocarbons to useful products, catalysis has a major role to play. Conceptually, the
cheapest and easiest route is through catalytic oxidation. The reason is that oxygen (pure or in
air) is cheap and possesses the high reactivity necessary to activate saturated hydrocarbons. For
that type of activation, heterogeneous and homogeneous catalysis are competing. Nevertheless,
the preference in principle goes to heterogeneous catalysis, especially if very large quantities
have to be transformed, as in the case of methane.
On the whole, a continuous progress towards a more selective oxidation of light saturated
hydrocarbons is observed, and recent announcements demonstrate that dramatic progress can
be made even in the very difficult case of methane activation, using either heterogeneous or
homogeneous catalysts.
The activation of light saturated hydrocarbons becomes increasingly more difficult as the
molecules become smaller, with methane reactions being the most difficult to control. On the
other hand, the occurrence of non-catalysed gas phase oxidation makes selectivity control very
complicted. This is a problem common to almost all oxidations, unless one of the products is
extremely stable (cid:12)9 examples are unsaturated nitriles (e.g. acrylonitrile in the ammoxidation of
propane) or maleic anhydride (in the oxidation of butane). There is a parallel trend in the
changes of reactivity with molecular weight in catalytic and non catalytic (gas phase) oxidation.
The challenge to catalysis to achieve selective reactions at lower temperature is thus equally
important for all light hydrocarbons.
The activation of very light hydrocarbons (propane, ethane and methane) in the presence of
oxygen has been achieved only at temperatures substantially or much higher than those used in
the reactions of other hydrocarbons. There is however little doubt that some mechanistic
similitudes exist and that the vast body of knowledge accumulated on the reaction of other
hydrocarbons (including unsaturated ones) with oxygen will be useful for improving the
efficiency of these difficult reactions. Nevertheless, the outstanding commercial success of the
oxidations and ammoxidations of light olefins and that of the oxidation of butane to maleic
anhydride has directed the fundamental research of the largest number of investigators to topics
which are probably not the most relevant to the new challenges set by the selective oxidation of
light alkanes. A much broader approach has certainly to be taken, compared to that used in
former investigations.
It is the aim of this contribution to highlight a few promising directions for research in the
area of selective reactions of light alkanes with oxygen (oxidation and oxidative
dehydrogenation). We shall emphasize three aspects:
(i) new concepts have been recently developed in a field which seemed to be well
established, namely the catalytic oxidation of olefins and butane, but where new powerful
methods of action have been discovered. We shall show that these new concepts are applicable
to the catalytic oxidation of the light saturated hydrocarbons, namely containing from one to
five carbon atoms. We shall present, in some cases for the first time, results which strongly
suggest that a cooperation between distinct phases in oxidation catalysts could play an
important role in the oxidation of light hydrocarbons, even perhaps in the coupling of methane.
(ii) we shall suggest, on the basis of new results from our and other laboratories, that the
intervention of non catalysed gas phase reactions must be accounted for and should be
investigated carefully.
(iii) we shall also show that catalyst discovery and development in the field of
heterogeneous oxidation of light hydrocarbons should be accompanied by innovative
developments on the chemical engineering side.
Before examining specifically these points, we shall "set the stage", namely attempt to give
an overview of the results published in literature on the selective reactions of light alkanes with
oxygen. The largest part of the contribution will consist in a critical overview of the parameters
traditionally believed to be crucial for activity and selectivity. We shall show that one
parameter, which probably has the largest importance, has been almost completely forgotten:
this is the ability for separate phases, inactive or poorly active, to enhance the activity of
potentially active and selective phases, via an oxygen spillover process. Results will be
presented which strongly suggest that the same sort of cooperation between phases can operate
in the reactions of light alkanes. At the end, we shall suggest that the existence of gas phase
oxidation reactions, the occurrence of the phase cooperation mentioned above and the other
particularities of light alkane oxidation are about to trigger new developments in chemical
engineering which will probably be as innovative and crucial for viable processes as the
development of fluidized bed reactors for oxidation or ammoxidation, and riser reactors (in the
case of butane oxidation) has been during the remarkable development of catalytic oxidation in
the last 25 years.
2. CATALYSTS ACTIVE IN THE SELECTIVE REACTION OF LIGHT
ALKANES WITH OXYGEN
The variety of catalysts which have been claimed to activate light alkanes is very large. The
only conspicuous exception concerns the reaction of butane to maleic anhydride; this is,
however, a special case considering the high stability of the product, namely maleic anhydride.
But this large diversity of formulations exists even in the ammoxidation of propane to
acrylonitrile, although the product is also particularly stable in this case. It cannot be therefore
concluded that given oxidation reactions take place only on a single family of catalysts.
In what follows, we present a series of tables concerning various reactions of light alkanes
with oxygen. We wish, however, to underline the fact that the data contained in the tables are
by no means comprehensive. We have selected them in view of our objectives, namely (i) to
underline the variety of formulations proposed for a single reaction, (ii) to extract from these
data a few conclusions and (iii) to speculate on the possible importance of some parameters.
We have avoided to overburden the tables with information on reaction conditions. These are
indeed very different, and correlating them with catalyst composition has little usefulness for
the moment (except perhaps for propane ammoxidation, where investigation is more
advanced). We do not present data concerning either methane or butane. In the case of methane
oxidation and oxidative coupling, innumerable articles (more than 1000) have been published,
together with many review papers. Concerning butane, the numerous articles and review
papers dealing with oxidation of maleic anhydride obscure the few scattered articles dealing
with oxidative dehydrogenation; dehydrogenation of butane has mainly been done in reactions
without oxygen.
In the tables, we omit the chemical symbol of oxygen and list only the elements combined
with oxygen in the catalysts, or oxygen when it is present in a phase indicated as such by the
authors (e.g., supports: MgO, SIO2), except if there is good ground to believe that well defined
metal oxide entities are crucial for catalytic activity (e.g., VO...).
In addition to the systems listed in Table I for the oxidative dehydrogenation of ethane,
other systems have been tested because they have proven to be active in other alkane
oxidations; this is particularly the case of many catalysts used in the oxidative coupling of
methane, VPO and magnesium phosphate catalysts (butane oxidation and propane
dehydrogenation, respectively) and MoVO catalysts. Various zeolites have also been tested.
This table, the largest to be presented here, perfectly illustrates the fact that no formulation
seems convincingly better than the others.
In the oxidative dehydrogenation of propane (Table II), the various magnesium vanadates
have been the object of many studies, but other systems seem to have comparable performances
(systems based on cerium, niobium, or vanadium, molybdates and noble metals on monoliths
used with very short contact time).
Because the direct dehydrogenation of isobutane to isobutene is now in operation
industrially, it is not surprising that relatively few publications deal with the corresponding
oxidative dehydrogenation to isobutene (about 20 in the past 6 years). On the whole, the
catalysts used are similar to those mentioned in the previous tables: phosphates, chromates,
molybdates. Active carbon has also been mentioned, but it is hard to imagine that the catalyst
could work a long time in the presence of oxygen. Table III gives two examples of the results
mentioned in literature.
Mention has been made of the selective oxidation (yield = 65%) of isobutene on UV
activated TiO2 50.
Table I.
Ethane oxidative dehydrogenation to ethylene
Catalyst Conversion Yield Selectivity Ref.
% % %
Ca-Ni 25 93.6 1
ceramic foam monoliths + Pt, Rh, Pd 80 70 2
Cd-La-A1 35 84 3
MgO based catalysts 45 73.7 4
Ce2(CO3)3 90 5
Mo-Si, Si-W or P-W/A1203 90 6
Cr-Zr-P 20-30 50-60 7
Li-Na-Mg 38 86 8
Li/MgO 75-79 70 9
Sr-Ce-Yb 49 10
Na-Mn 76.8 86.5 11
zeolites 86-90 12
La203-B aF2 59.1 84.7 31
heteropolyacid 76-98 14
Pt-cordierite 70 72 51
(electrocatalytic) 10.6 96.9 16
Mo-V-Nb-Sb 22-57 72-82 71
Mo-V-Nb-Sb-M 34 86 81
Na-K-Zr 85 86.4 91
Li-Ti+Mo, Sn or Sb 54.9 86.3 20
Li-Ti-Mn 46.9 74.6 21
V-P-U 22.7 22
Zn2TiO4+Bi 71.2 67.5 23
Co-P+promoter 68.1 80.5 24
Mo-Te 100 25
Mo-Bi-Ti-Mn-Si 100 26
Li/M~D+promoter 75 76 27
It seems that very few investigations concern the oxidation or oxidative dehydrogenation of
C5 alkanes. Oxidative dehydrogenation of isopentane to isoprene has been mentioned. Two
articles deal with MnO2, CoO/CaO3, NaOH/A1203, but in the presence of HI 51,52; this
obviously suggests the intervention of gas-phase reactions. The yields (Y) in isobutene were
relatively high (e.g., Y = 50-60% with a selectivity of 65 to 95%). Pentane can also produce
maleic anhydride and phthalic anhydride 53-57.
Considering in a general way the activation of light alkane by oxygen, the ammoxidation of
propane has certainly not to be forgotten. This process is already under industrial development.
If we try to get an overview of the recent work on the selective reactions of light alkanes
with oxygen, two remarks may be made:
(cid:12)9 Several lines have been followed, all inspired by former successful lines of research. It is
striking that the proposed catalysts are generally similar to those previously used in the selective
reaction of alkanes with oxygen: oxidative coupling of methane or oxidation of butane to maleic
anhydride. Many of them are also similar to catalysts used for the reactions of olefins with
oxygen (molybdates) or for dehydrogenation without oxygen (chromium containing catalysts).
Because of the success of vanadyl phosphate in butane oxidation, investigators tend to focus on
vanadium containing catalysts also in the case of other alkanes. Nevertheless, the data available
do not seem to exclude any other formulation.
(cid:12)9 On the other hand, the reaction of ethane, propane, isobutane, and pentanes with oxygen
described until now are poorly selective at high, and even at moderate conversions. One cannot
exclude the empirical discovery of completely new catalysts with outstanding performances.
However, a more systematic approach may also help find satisfactory catalysts. An in-depth
understanding of the principles involved ni catalytic selective oxidation is necessary to improve
activity, selectivity and resistance to ageing of catalysts. This is true as well for the catalysts to
be perhaps discovered as for those already cited.
Table II.
Propane oxidative dehydrogenation
Reactant Product tsylataC Conversion Yield Selectivity Ref.
% % %
Propane Propene Nb based catalysts 7 58 82
Propane Propene VMg, VMg+Ag, 01 84, 86.9 29
lacimehcortcelE
pumping of oxygen
)POE(
Propane Propene VMg and chloride of 23.1 03
Cu ,+ Li ,+ Ag ,+ Cd +2
Propane Propene noble metals (Pt,Pd) on 001 56 (total 13
enehtE ceramic foam shtilonom )snifelo
at short contact time, 5
ms
Propane Propene 4OoM59.0oC 91 60 32
Propane Propene =gM/V 33
1/2 32 46
2/2 32 95
3/2 32 49
Propane Propene VMgffiO2 52 60 43
Propane Propene 4OoMiN 20 12.5 62 53
34 14.8 34.5
Propane Propene 02sC/3FeC22OeC 41.3 33.5 81.1 36
Propane Propene FeV-supported Nd203 40.3 66.2 73
Propane Alkenes Vanadate stsylatac 50 50 83
enatuB
Hexane
Propane Propene 1A-dN-eF-V 40.3 66.2 26.7 93
Propane Propene gMV 01 56 40
Propane Propene CeO~CeF3 53.4 3 6.7 14
Propane Propene* 4OP3)3HN( + 21 *53 42
**elirtinolyrcA 3)3ON(ni + 36.7**
Vanadyl phosphate
Propane Propene NiMoOx (a=0.6-1.3; 29 1.81 34
x=number determined
by Ni or Mo valency)
Propane Propene 3021A supported 19 44
Pt/Cs/Sm
Propane Propene MgV206 %05( 17 45
OgM+sO2V calcined ta
610 ~
Propane Propene 2OiS/4OoMoC 4.1 77.9 46
Propane Propene 1A/4OV3aN/OHaN 20.9 16.6 79.8 74
The next sections will therefore indicate some of these fundamental aspects and suggest the
perspectives that some new f'mdings are opening.
Table HI.
Isobutane oxidative deh~,drosenation to isobutene.
Catalyst Selectivity (S) Ref.
Y203 + CeF3 high conversion 48
Ni2P207 S = 82 % 49
Zn2P207, Cr4(P207)3, M~2P207 S = 60-70 % 49
3. PARAMETERS TRADITIONALLY CONSIDERED IN SELECTIVE
OXIDATION
A very large amount of work has been devoted in the past to the oxidation of olefins
Callylic" oxidation to unsaturated aldehydes) and butane (to maleic anhydride). This has led to
the development of ideas and concepts which are quite naturally used in the new investigations
concerning light alkanes. It is necessary to examine these ideas and concepts and to evaluate in
a critical way their potential for discovering or improving catalysts in the new field that
oxidation of light alkanes constitutes. This will be done here shortly on the basis of classical
books or articles 53,58-62.
3.1. Doping
The idea is to add foreign ions as a solid solution in already active oxide structures. This is
logical. The oxidation of hydrocarbons involves oxygen from the catalyst lattice and
replenishment of the latter by molecular oxygen after the hydrocarbon molecule has been
dehydrogenated or oxidised. This is an oxido-reduction mechanism. Doping by elements of
other valencies can in principle change the oxido-reduction level of the surface. More precisely,
the really important parameters in the processes are the rates of (i) removal of oxygen by the
reaction with the hydrocarbon and (ii) reoxidation by 02. In principle, doping can alter these
rates, but very few measurements have been made along this line. Doping can also change
surface acidity, a parameter essential for the activation of alkanes.
Doping is certainly a good approach for modifying a catalyst. It should however be
underlined that it has seldom been verified that the doping elements were really incorporated in
the host oxide and did not spontaneously segregate out. There are indeed conspicuous instances
of such segregations. For example, it had been claimed that antimony in solid solution in tin
oxide SnO2 explained the high activity of Sb-Sn-O catalysts in oxidation. Actually, Sb has a
strong tendency to segregate out of SnO2 during the catalytic reaction 63-65. But in other
reactions, there seems to be indeed an effect of doping elements to alter the extent of oxidation-
reduction in the near surface layers (e.g., cobalt in V-P-O catalysts) 66.
It is therefore advisable to use the doping elements in quantities compatible with complete
solubility in the host oxide, and to check that they do not segregate during the catalytic reaction.
Cobalt, mentioned above as a useful dopant, could exert a catastrophic effect if segregated as
cobalt oxide, because of the high activity of the latter in complete oxidation.
3.2. Supports
It seems that supports have been considered with much circumspection in the early days of
allylic oxidation. Progressively silica began to be used, but it is considered as being generally
inert, and permitting only a better dispersion or a higher mechanical strength. However, real
supports are progressively appearing in the field of catalytic oxidation, as suggested by the
tables presented above. A conspicuous and well known example is TiO2 as a support for V205.
The advantage of using TiO2 (e.g., in o-xylene oxidation to phthalic anhydride) is probably not
to give isolated surface vanadium atoms, but rather to stabilise islets of a sub-oxide of
vanadium, V6013 over a broader range of oxido-reduction conditions 67-69. This