Table Of ContentStudies in Surface Science and Catalysis
Advisow Editors: Delrnon and J.T. Yates
6.
Vol. 68
STRUCTURE-ACTIVITY
AND SELECTIVITY
RELATIONSHIPS IN
HETEROGENEOUS
CATA LY S IS
Proceedings of the ACS Symposium on Structure-Activity Relationships
in Heterogeneous Catalysis, Boston, MA, April 22-2 7, 1990
Editors
R. K. Grasselli
Mobil Central Research Laboratory, P. 0. Box 1025, Princeton, N. 08543, ISA
and
A.W. Sleight
Department of Chemistry, Oregon State University, Corvallis, OR 9733 I, USA
ELSEVlE R Amsterdam - Oxford - New York - Tokyo 1991
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of
Library Congress Cataloging-in-Publication Data
ACS Symposium on Structure-Activity Relationshlps in Heterogeneous
Catalysis (1990 Boston. Mass.)
in
Structure-activity and selectivity relationships heterogeneous
catalysis proceedings of the ACS Symposium on Structure-Activity
Relationships in Heterogeneous Catalysis. Boston. MA, April 22-27.
1990 / editors.- -R .K. Grasselli and A.W. Sleight.
p. cm. (Studies in surface science and catalysis ; vol.
67)
Includes bibllographlcal references and index.
ISBN 0-444-88942-6 (U.S.)
1. Catalysts--Structure-activlty relatlonshlps--Congresses.
2. Heterogeneous catalysis--Congresses. I. Grasselli. Robert K..
1930- . 11. Sleight. A W. 111. Title. IV. Serfas Studias in
surface science and catalysis ; 67.
OD505.A27 1990
541.3’95--dc20 91- 16425
CIP
ISBN 0-444-88942-6
0 Elsevier Science Publishers B.V.. 1991
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IX
PREFACE
Structure plays an important role in heterogeneous catalysis. It provides a framework for
the arrangement and strategic placement of key catalytic elements, hosting them in a
prescribed manner so that their respective electronic properties can exhibit their desired
catalytic functions and mutual interactions. Under reaction conditions these framework
structures and their key catalytic guests undergo dynamic processes becoming active
participants of the overall catalytic process. They are not mere static geometric forms. The
framework provides the necessary crystal structure stabilization and hence acts as a
template. Non-stoichiometry and vacancy rearrangements of the solids are important
factors contributing to these dynamic processes of catalytic reactions. The dynamics of
catalytic structures are particularly vivid in selective oxidation catalysis where the lattice of
a given catalytic solid partakes as a whole, not only its surface, in the redox processes of the
reaction. The catalyst becomes actually a participating reagent. By proper choice of key
catalytic elements and their host structures preferred catalytic pathways can be selected
over less desired ones. However, not only in selective redox catalysis does structure play
an important role, its importance is also well documented, among others, in shape selective
zeolite catalysis, enantioselective hydrogenation and hydrodesulfurization.
The contributions presented in this book address the dynamic character of the solid state
under catalytic reaction conditions. By relating structure to activity and selectivity in
heterogeneous catalysis our understanding of such correlations has been significantly
enhanced through the use of sophisticated spectroscopic means, surface science and
modeling. Nonetheless, the ultimate test of the correlations remains the actual catalytic
reaction.
The individual contributors who made this update of structure activity and selectivity
correlations in heterogeneous catalysis possible are herewith sincerely thanked.
R. K. Grasselli A. W. Sleight
Mobil Central Research Laboratory Department of Chemistry
P. 0. Box 1025 Oregon State University
Princeton, NJ 08543 Corvallis, OR 97331
Princeton: February 28,1991
x
ACKNOWLEDGMENT
The editors gratefully acknowledge the financial support of the corporations:
following
Alcoa DuPont
Allied Signal Exxon
Amoco W. R. Grace
Arc0 Mobil
Ashland Monsanto
8.P . America Petroleum Research Fund
R.K. Grasselii and A.W. Sleight (Editors),S tructure-Actiuity and SeGctiurty Relationships in 1
Heterogeneous Catalysis
0 1991 Elsevier Science Publishers B.V., Amsterdam
Redox Dynamics and StructurelActivity Relationships
in Vanadium-Oxide on Ti02 Catalyst
G. CENTI, M. LOPEZ GRANADOSa, D. PINELLI and F. TRIFIRO'
Dept. of Industrial Chemistry and Materials, V.le Risorgimento 4,40136 Bologna (frafy)
ABSTRACT
The in-situ evolution of crystallites of V2O5 on the Ti02 surface during interaction with the o-xylene/air
reagent mixture involves together with their sprcadin on the titania surface partial reduction and the
formation of an amorphous phase characterized by a V:'V '' ratio of 2:l and an band centered at 995
IR
cm-I. The phase does not form directly by reduction, but involves a preliminary reduction to a phase with a
lower mean oxidation state. This reduced vanadium-oxide phase is then partially reoxidized to the active
phase with a decrease in the formation of the intermediate phthalide and an increase in the selectivity to
phthalic anhydride. A similar evolution in the catalytic behavior and in the mean oxidation state is observed
in unsupported V205 which aftcr about 200 hours, transforms into V307, This phase has a VV:V1' ratio
similar to that present in the active phase of vanadium-oxide supported on Ti02, but does not show its
characteristic IR band centered at 995 cm-'. Using a prereduced unsupported vanadium-oxide, it is possible
to decrease the activation time considerably and to obtain a final catalyst, after in-situ treatment, whose
catalytic behavior is very comparable to that of V/Ti/O, but again characterized by the presence of a phase
(v307) different from that present in the V/Ti/O system. It is suggested that the catalytic behavior in
o-xylene conversion to phthalic anhydride is not rclatcd to the presence of an unique special surface structure
ofvanadium-oxide on the Ti02 surface, but rather to a suitablc V':V'' ratio and surface distribution. Thcse
features can also be realized in unsupported vanadium-oxide using a suitable preparation and activation
procedure.
INTRODUCTION
Vanadium-oxide supported on an oxide matrix, in particular on TiO2, is widely used for catalytic
partial oxidation of hydrocarbons. Over the years a number of observations have been made
suggesting the advantages to be gained by supporting the catalytically active vanadium-oxide on the
surface of another oxide [l and references therein]. However, a fundamental question arises from
this concept: Are these variations to be attributed to a change in the catalytic behavior related to an
increase in the available surface area, or is there a change in the local structure of the vanadium
oxide species stabilized by interaction with the support ?
a On leave from the Institute of Catalysis y Petroleoquimica, Madrid (Spain)
2
It has now become clear that under suitable preparation conditions vanadium oxide may be
supported Ti02 in a well-dispersed form with the formation of a monolayer of the active oxide
on
the support, and that the support appears to play a crucial role in facilitating the formation of the
on
active structures due to geometrical or chemical factors [ 1-22 and references therein]. Most authors
have focused their attention this monolayer concept and characterization of the monolayer, a
on on
variety of contradictory possible surface configurations have suggested, such (i) mono-0x0,
been as
hydroxyoxo, di-oxo vanadate, (ii) tetrahedrally or octahedrally coordinated vanadium species, (iii)
isolated, polymeric, bidimensional or tridimensional species, and (iv) clusters or coherent lamellae,
of amorphous or of paracrystalline vanadium oxide. Considerable confusion thus exists regarding
the nature of the local surface configuration of vanadium oxide Ti02, also because sometimes
on
the multiple molecular states that can be present simultaneously in the supported metal oxide are
not sufficiently taken into account. Also problematic is the analysis of the relationship between the
possible surface configurations with the activity/selectivity in o-xylene oxidation. Relatively
fragmentary information exists on this fundamental aspect, and generally, very few papers analyze
the evolution of the catalyst in the reaction medium, which is a fundamental aspect to assess the real
nature of the active phase and the role of Ti& in its stabilization.
No unequivocal answer, for example, can be found in the literature on the basic problem of the
difference between the catalytic behavior of supported and unsupported vanadium oxide. the
Does
presence of the support improve only the activity or also the selectivity? This, in turn, determines
the importance of the various interpretations of the molecular structure of vanadium-oxide Ti@,
on
on the catalytic behavior.
In this work the process of transformation of the catalyst in the reacting medium is studied in
order to correlate the type of transformations with the catalytic behavior and the nature of the active
phase. The time-on-stream evolution of structural and catalytic properties of vanadium-oxide
v IV
supported or unsupported on Ti02 were studied in a flow reactor, by characterizing the :V
V
ratio through chemical analysis, and the surface structure by means of various physicochemical
techniques.
The catalysts were prepared by solid state reaction. This method, despite its inherent simplicity,
leads to a system whose catalytic performance compares well to that obtained by other methods
such as wet impregnation and grafting techniques [16-191. However, as compared to other
preparation methods, preparation by solid state reaction has many advantages: (i) possible
interference by other reactants is eliminated, (ii) the starting situation is clearly defined for a better
correlation of the evolution of the catalytic system to its catalytic performance, and (iii) a more clear
distinction of the solid state reactions occumng between the V2O5 and Ti02 during calcination and
during in-situ heating treatments in the presence of the o-xylenelair reacting mixture is possible.
EXPERIMENTAL
Catalyst Preparation The catalysts were prepared by solid state reaction of Ti02 and V2O5
(Carlo Erba reagent grade). Generally, 7.7 wt.% V2O5 was used, an amount typical for industrial
preparations. Anatase and rutile prepared by Tic14 hydrolysis were used in order to obtain highly
pure Ti& supports and to exclude interference from doping. After mixing and gently grinding (1
min, in order to have good mixing but avoiding mechanico-chemical alterations of the samples), the
powder was calcined in an oven at 500°C for 16h or longer in a static air atmosphere. The samples
were then treated in-situ in long-run (about 500 hours) catalytic tests with a reagent mixture of 1.5%
v/v 0-x lene in air (reaction temperature around 320'C).
Yv v
A V -V mixed valence sample of unsupported vanadium-oxide was prepared by dropping a
IV
solution of V -oxalate into an ammonia solution (pH around 9), filtration, washing and drying at
80 'C. and calcination at 280 'C.
3
Catalytic Tests The catalysts were tested in a conventional laboratory apparatus with a tubular
fixed bed reactor working at atmospheric pressure and on-line gas chromatographic analyses of
reagent and product compositions. The standard reactant composition was 1.5 % o-xylene, 20.5%
02 and 78% N2. The catalyst (0.52 g) was loaded as grains (0.250-0.420 mm). A thermocouple,
placed in the middle of the catalyst bed, was used to verify that the axial temperature profile was
within 3-5 'C.
Characteerizution The characterization by chemical analysis of the vanadium-oxide species
present in VTiO samples and of the mean valence state of vanadium was performed as follows. The
samples (about 0.5 g) were moistened at room temperature (r.t.) with 50 ml of a dilute (4 M) H2SO4
solution or with an ammonia solution (4 M) for fifteen minutes under stirring and then filtered. The
amount of vanadium was determined separately in the filtered solution and in the residual sample
dissolved in boiling concentrated H2SO4 (16 M). The total amounts and the valence state of
vanadium were determined by a titrimetric method. In particular, a part of each fraction was titrated
with 0.1 KMnO4 to determine the amount of reduced vanadium species and then with Fe2' to
determine total amount of vanadium; another part was titrated with Fe2+i n order to determine the
amount of V(V). From the balance it is possible to quantify the total and relative amounts of V(1V)
and V(V) and of any V(I1I) present. The vanadium species extracted by the r.t. dilute sulphuric acid
or ammonia solution will be, hereinafter, called soluble or weakly-interacting species, whereas the
remaining species determined after dissolving the residue will be called insoluble or strongly
interacting species. Other reactivity and spectroscopic analyses were carried out as previously
reported [ 16-20,23,24].
RESULTS AND DISCUSSION
Formation of an Interacting Vw Layer. The solid state reaction of V2O5 and Ti02 in air at
temperatures in the 400-5OO'C range, leads, in the absence of any reducing agent, to the formation
of relevant amounts of V". Chemical analysis shows, in fact, the formation of V" species that
cannot be dissolved in an acid or basic aqueous medium, in contrast to other supported V'"- and
V
V -oxide species. Shown in Table I is the amount of VIV formed during the calcination of V2O5
and Ti02 that is approximately equal (for catalysts with a surface area of about 10 m2/g) to the
reference monolayer estimated on the basis of a geometrical coverage of the titania surface. The
2
reference monolayer is about 0.1% w/w of V2O5 per m of Ti02. For the sake of comparison, the
amounts of the various species of vanadium determined by chemical analysis are all expressed in
Table I as % by weight of equivalent moles of VO2.5. The formation of these V" surface sites
probably occurs by a specific reaction between hydroxyl groups of Ti02 surface and Vv sites and
the formation of this species is the driving force for the spontaneous reduction of Vv in oxidizing
conditions and for its surface migration. The mechanism of formation probably involves the
preliminary formation of Ti3+ sites by dehydroxylation with consecutive electron transfer to V sites
and formation of stable Ti-0-V bonds.
In contrast with what happens with rutile samples, the addition of a reducing agent during the
heat treatment does not further increase the amount of insoluble V" in anatase samples (Table I).
Similar results are obtained if the heat treatment is erformed in-situ during the catalytic tests
(Table I). In rutile samples, the amount of insoluble VR increases further up to a limiting value of
around 3.8% w/w after long-term catalytic tests. The redox and chemical (solubility) properties of
the V'" sites are altered considerably by the interaction with the Ti02 surface, in comparison with
those of the VrV- oxide. XRD and ESR data clearly exclude the formation of a solid solution in
anatase samples, in contrast to that observed for rutile samples. In particular, ESR characterization
[23] shows the presence in anatase samples of several iso1at:d surface and unsaturated vanadyl ions
in slightly different distorted octahedral environments. Some of these species may interconvert with
4
Table I Chemical analysis data (k 0.15%) of the distribution of vanadium oxide species in
samples prepared by solid state reaction of v205 (7.7% w/w) and Ti02.
I I
Nature of Ti02 Anatase (9.8 m2fg) Ti02 Rutile (8.9 m2/g)
treatment
% wlw of equivalent v205 % wfw of equivalent v205
V(1V) V(V) V(1V) V(V) V(1V) V(V) V(IV) V(V)
insoluble soluble insoluble soluble
1.7 1.7
0.9 - - 6.8 0.8 6.9
0.9 - 6.8 1.8 - 5.9 -
0.9 - 0.3 6.5 1.8 - 0.3
5.6
0.9 - 3.1 3.1 1.6 1.2 4.9
0.9 - 2.0 4.8 3.1 - 0.9 3.1
(a) mixing; (b) calcination at 500'C for 24 hours; (c) calcination in the presence of a reducing agent (10% v/v
of NH3); (d) sample c after subsequent calcination at 400'C for 3 hours; (e) sample a after heating (320'0 in
a flow of o-xylenelair for 24 hours or (f) for 1440 hours.
each other by the addition of suitable probe molecules, indicating the presence of Lewis acidity. In
rutile samples, in contrast, no isolated surface vanadyl species could be detected, even for amounts
of insoluble V" species much below those necessary for monolayer coverage. The spectrum
ESR
is always characterized by a broad unstructured signal with a g value of about 1.98 that is
IV
characteristic of near-lying V paramagnetic centres. This broad signal is overlapped by another
v4'
signal showing hyperfine structure which can be attributed to isolated non-vanadylic sites in
substitutional positions in the rutile structure.
A further difference characterizes the insoluble VIV sites in anatase and rutile samples (Table
II).
Whereas in anatase samples all the V" sites could be reduced and are accessible to gaseous
reactants such as Hz, only a fraction (around 20-30%) of the insoluble sites in mile samples
V"
are accessible to gaseous reactants, indicating that only a reduced fraction of these sites is localized
at the surface or in subsurface layers. A further difference between insoluble V" sites in anatase
and rutile samples is shown in Table 11. V" sites can be more easily reduced to V"' in anatase
samples as compared to rutile samples, but cannot be oxidized to VV as occurs in rutile samples.
ESR and reactivity data thus indicate a homogeneous distribution of these V" sites on the
surface of anatase, with a mean estimated distance of 4.5 8, from V centers and the presence of
islands of V2O4 on the surface of the rutile samples.
The presence of these V" surface sites also modifies the reactivity in o-xylene oxidation of
titania which is enhanced considerably as compared to pure Ti@, even though the activation of
o-xylene is relatively not selective to phthalic anhydride (around 30%). The catalytic behavior is
stable in the case of V"- modified Ti02 anatase, whereas the selectivity to phthalic anhydride
improves with time on-stream in the rutile sample as a consequence of the possibility of oxidation
of insoluble VIV to Vv in these catalysts (see Table 11).
Nature of the Active Layer. On the V'"-modified surface of Ti02 after calcination only V2O5
crystallites are present, but this phase transforms to a partially reduced amorphous phase during the
consecutive in-situ treatment in a flow of o-xylene/air. Stable catalytic behavior may be reached in
about 500 hours of time on stream. The characterization of the nature of the upper layer on
VIv-modified Ti02 surfaces may be realized after its extraction with a dilute sulphuric acid
solution. The analysis of this upper layer indicated (i) a mean valence state of vanadium of 4.71 that
corresponds to a V v :V IV ratio of 2:1 and (ii) the presence of a characteristic band centred at 995
IR
5
Table I1 Rates of reduction (2% H2 in helium) and of consecutive oxidation (20% 02 in
nitrogen) at 400'C of the insoluble V" species in anatase and rutile samples after long-term
catalytic tests.
sample amount of Rate of Reduction Fraction Reduced Rate of Oxid. Fraction Oxidized
insoluble V'" moles 0 removedl of insoluble V" mmoles 0 inserted/ of insoluble VrV
mmoles V204 % mmoles V204 %
unutuse 0.9 10.97 100 0 0
rutile 3.1 1.13 24 0.12 23
1 V
cm- due to the symmetrical stretching mode of V =O. The removal of this phase decreases the
activity in o-xylene oxidation but especially the selectivity, which drops from about 75% to 30%.
The phase is amorphous and no evidence was found of the presence of residual V2O5
XRD
crystallites characterized by a defined sharp band at 1020 cm-' of V=O stretching mode. The shift
to lower frequencies of vV=O in comparison to crystalline V2O5 may be attributed to the electronic
effect of neighborin8 V " sites or to the presence of coordinatively adsorbed water that causes a
weakening of the V =O double bond. In general, FT-IR spectra of the catalyst after different times
show that a good correlation exists between the frequency of the V=O stretching band in the
samples and the oxidation state of the vanadium oxide deposed on the surface of the TiO2. The
band changes position in the spectrum and decreases in frequency from 1020 cm-*, corresponding
to pure crystalline V2O5, to lower values proportional to the degree of reduction. In agreement, the
consecutive oxidation of a V-Ti02 sample after long-run catalytic tests indicates a shift to higher
frequencies and the appearence of a further band at about 1010 cm-'. A corresponding increase in
the mean oxidation state of vanadium from 4.72 to 4.91 is observed. This suggests that the
modification of initial VzO5 particles in the reaction mt dium is not completely reversible by
consecutive reoxidation.
Similar results are found in rutile samples, but both the time necessary to reach a certain mean
valence state of vanadium as well as the stability of the reduced catalyst against consecutive
oxidation are indicative of the formation of less stable, partially reduced, vanadium-oxide species
on the mile surface in comparison to the anatase surface.
Wide line solid state 'lV-NMR characterization of the local coordination of V5+ sites in the
active phase [24] indicates a significant shift of the asymmetric resonance peak due to axial
shielding and a general broadening of the peak as compared to the reference signal for V2O5. The
change is analogous to that observed in hydrated V2O5 and could be interpreted as a change from
the nearly five-fold coordination of vanadium in the initial crystalline VzO5 to a nearly octahedral
coordination in the active phase after long-term catalyac tests. No great differences in the
"V-NMR spectra are observed in the anatase and rutile samples after long-term catalytic tests.
XPS characterization of the depth profile of vanadium in anatase samples [24] shows a
considerable change in the V/Ti atomic ratio (from about 0.47 to about 0.27) after removal of the
first nm of thickness from the catalyst using Ne+ ions for the: sputtering. The V/Ti atomic ratio then
decreases at a slower rate to nearly zero with the removal of a further 12-14 nm. This indicates the
presence of two Vv phases on the surface of Ti02 anatase, the first corresponding to the monolayer
and the second present in amorphous aggregates with estimated thicknesses of about 10-15 nm. In
rutile samples, on the contrary, the presence of the first monolayer species is not observed, but
rather only the second species.
Dynamics ofin-Situ Evolution. Fig. 1 shows that the V20g-TiQ catalyst in a pilot plant reactor
undergoes a first deep reduction and then partial reoxidation with the formation of the final active
6
5
4.75
4.5
425
4
(cid:145)L
3.75
3.75
V204 V6013 V409 V307 V2&
Crystalline phases of vanadium-oxide
Fig. Effect of time on-stream in 0-xylene oxidation on the mean valence state of vanadium in the
1
soluble part of V-oxide on and mean valence state of vanadium in some crystalline phases of
Ti@.
vanadium-oxide.
catalyst whose characteristics were discussed above. The V2O5 is first reduced to a phase with a
valence state similar to that of v6013, however XRD analysis shows the presence of only an
amorphous vanadium-oxide phase. It is thus not possible to make a definite attribution. After this
stage, the catalyst starts to be progressively reoxidized and reaches a final stable mean valence state
in the soluble part of vanadium similar to that present in the V307 phase. Also in this case, the
vanadium-oxide phase is XRD amorphous. It should be noted that the crystalline V307 is prepared
by solid state reaction of v6013 with V2O5 [25] and it is reasonable to hypothesize that a similar
mechanism occurrs in the transformation of the V205-Ti02 mixture to the final active catalyst. The
correlation of this effect with the catalytic behavior is complex because different catalytic
two
effects take place at the same time: 1) The spreading of vanadium on the Ti02 surface, and 2) the
reduction and consecutive partial oxidation of the V-oxide upper layer and the consequent change
in the nature of the supported phases.
In order to obtain a better understanding of the dynamics of these redox transformations as well
as the role of titanium oxide in determining the final state of the catalyst, we carried out an
analogous experiment where the evolution of an unsupported pure commercial (Fig. 2) was
V2O5
followed. In this case, the concurrent process of spreading of vanadium-oxide on the surface of the
titanium oxide is not present.
The commercial V2O5 also undergoes a similar reduction-reoxidation process of the supported
V2O5 (Fig. 2), but the parallel change in the catalytic behavior may be more clearly correlated to
the dynamics of phase transformation. The selectivity in phthalic anhydride is very high at the
beginning, but decreases reaching a minimum value after about 40 h, and then increases by further
in- siru treatment up to about 200 hours. A parallel evolution is observed in the activity, whereas the
selectivities to COX and phthalide pass through a maximum. The characterization of the
FT-IR
catalyst after 200 hours of time on stream indicates the presence of a complex spectrum, whose
characteristics are very similar to those of V307 plus some residual V2O5 particles (Fig. 3). This
compound has a VV:VIv ratio (2:l) similar to that present in the the active phase of the V/ri/O
catalyst after 200 hours in a stream of o-xylene/air (see Fig. 2). It should be observed that the IR
spectrum of V2O5 after in-siru conditioning (Fig. 3) is very different from that shown by V2O5
