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On the mechanism of the selective oxidation of butane and 1-butene on vanadyl phosphates
27
Catalysis Today, l(l987) 27-36 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
ON THE MECHANISM
OF THE SELECTIVE
OXIDATIONOF BUTANE AND I-BUTENE ON VANADYL
PHOSPHATES
SSZAKACS, 'Central
H.WOLF2,
G.MINK',
Res.Inst.Chem.,
Bureau of Techn.
I.BERT6T13
Hung.Acad.Sci.,
Development
, N.WDSTNECK2, H-1525
H-1108 Budapest,
2Centr.Inst.0rg.Chem.,
Acad.Sci.GDR,
3Res.Lab.Inorg.Chem.,
Hung.Acad.Sci.,
B.LOCKE'
Budapest;
and H.SEEBOTH2
present
address:
Olto u. 16, 1X.36.
Berlin/Adlershof,
Rudower
H-1112 Budapest,
Chausse
5.
Budaiirsi ut 45.
ABSTRACT The oxidation of 1-butene and n-butane to maleic anhydride was studied on Wand($-VOPO and on (VO)$P$O7,, these serving as oxidizing agents or catalysts. As shown by tfle oxygen bal n es in catalytic tests, partial reduction of the vanadium (V) and oxidation of the vanadium (IV) catalysts occur initially. On the basis of kinetic studies and suppplementary DSC, TG-MS and XPS information, a mobile 0- radical is considered to be the active species in both the stoichiometric and the catalytic oxidation, characterized by closely similar initial conversions.
INTRODUCTION Vanadium-phosporus ducing maleic
oxide catalysts
anhydride
the main features
of the reaction
cation of the key intermediates
mechanism
as butadiene
2-4). There are still controversies, oxygen
have opened a possible
(MA) from C4-feedstocks
(ref.1).
new route for pro-
In case of n-butenes
have been revealed
, crotonaldehyde
however,
concerning
by the identifi-
and furan
the mechanism
(refs. of the
transfer.
Proposed
mechanisms,
oxygen with gaseous both components
based on kinetic data, involve:
hydrocarbon
in the adsorbed
in a redox cycle Recent results
by Hodnett
in catalytic
hydrocarbons
and Delmon
(ref.101
also support
of butane and MA is concerned.
work was to give additional
by a comparative
study of the selective
in an inert atmosphere
0920-5861/87/$03.50
or the reaction
with
of lattice oxygen
a redox mechanism; the yields were
tests and in the absence of gass-phase
The aim of the present lattice oxygen
state (ref.6);
of adsorbed
mechanism
(refs.7-9).
as far as initial conversions comparable
the reaction
(ref.5); a Langmuir-Hinshelwood
information oxidation
and also in the catalytic
0 1987 Elsevier Science Publishers B.V.
02. on the role of of the title
tests.
EXPERIMENTAL Reactor The reactions were carried out in a fluidized bed microreactor able to operate either in a pulse reactor mode or as a stationary flow reactor. The fused silica reactor of about 1 cm3 in volume was vibrated by an external vibrator. Purified He with less than 5 ppm O2 was used as carrier gas. Flow rates of 0 .5 cm3*s'I were kept constant in both operation modes. The pulsing valve with a pulse volume of 13 cm3 was thermostated at 400 K. The composition of the injected mixture, as well as that of the stationary flow, was either 0.3 volume % of C4-hydrocarbon in He, or, in catalytic tests, 0.3 % hydrocarbon + 1 % O2 in He. When injecting MA, the conditions, as indicated later, were different. Analysis The feed and all the components of the outlet were analysed by GC equipped with four columns (Table 11. After a group separation the reaction products were analysed according to Table 1. TABLE 1 GC analysis conditions Column I 2
Group 02, CH4, CO COP and C4-
Length and I.D. 1
mx4mm
10.5 m x 4 mm
T(K) 293
Support Stat.phase Linde 13X
-
Detector TCD
293
CHRO~SORB W/AW
30% pimelic ac.dinitrile
TCD
-hydrocarbons 3
Aldehydes, ketones,furan
5
mx4mm
400
_ " _
30% Carbowax -3000
TCD
4
Acids and MA
2
mx4mm
400
_ " _
30% FFAP + 3% H3P04
FID
The total analysis required about 20 min; i.e. this was the approximate time interval between two subsequent pulses or samplings. Precise carbon and oxygen balances in percentages have been determined as the sum of the given kind of atoms in the outlet related to that in the inlet. The conversion is defined as the percentage fraction of the feed compound transformed to other compounds, excluding isomers. Hereafter we also calculate selectivity (Si) values as s
i
=
the amount of carbon present in product "i" the amount of carbon in the reacted fraction of the feed ' loo%
Catalysts Stoichiometricor-VDFD4,P-VOPD4 and fV0)2P207 catalysts were prepared by the method propo_sTdby Ladwig (ref.11). The specific surface areas were 4.7, 8.8 and 6.3 rn2g
, respectively. For the experiments. a grain size of 0.2 - 0.4 mm
was used. The thermal behaviour of the catalysts under He or Ar flow was studied in SETARAM DSC-Ill microcalorimet~r coupled to a single column GC and in a Perkin Elmer thermal surface
balance
electronic
connected
with a Balzers @IS-511
state of the catalysis
mass spectrometer.The
at 300 and 770 K was studied by ESCA,
using a KRATOS XSA~OO jnstrument. RESULTS AND DISCUSSION Oxidation by Lattice Oxygen The mobility of lattice oxygen in catalytic oxidation is of fundamental importance.
Therefore
the interaction
MA with the catalysts
of the title hydrocarbons
in the absence of gaseous
Oxidation of I-butene by&-
and of the product
O_, has first been studied.
and P-VOPO4
Results obtained at 720 K by pulse method are collected ‘inTable 2. fn these experiments, besides the main products
i.e. CO, COP, MA, furan and butadiene,
minor products (MP). i.e. ~H3DOH, CH3CH0, C2H~COOH. ~~H3COOH
and C3H5CH0,
have
also been observed. The amount of hydrocarbon injected in one pulse was co~ensurate with the number of surface vanadyl type oxygens, assumed as reactive species in oxidation.
In this context it seems to be strange that the conversion unchanged
with
increasing
is practically
pulse number, though one pulse of butene consumes ap-
proximately the total oxidation capacity of one surface layer. We therefore consider that during the 20 min required for analysis between subsequent pulses, a closely complete equilibration of bulk and surface oxygens occurs, resulting always in a closely complete surface coverage by vanadium-(V) species. (As estimated, only about 1 % of the bulk oxidation capacity of the solid was consu~d by one pulse.1 Nevertheless, there are systematic trends with increasing pulse number: (5) the selectivity of MA formation decreases, and (ii) the carbon balance, which initially varied at around 100 I, tends to decrease as a result of the deposition of carbonoceaus species, Both of these phenomena are more pronounced in case of the=-phase. As Table 2 also
shows@ parallel to the oxidation by lattice
oxygen, a catalytic process - the is~risation
of I-butene - also occurs and
the isomerisation activity of the catalysts moderately increases with the pulse number.
30 TABLE 2 Oxidation 'IMP
of 1-butene by+
- the summarized
Phase No
*3
and B-VOPO4
selectivity
inthe
pulse reactor at 720 K.m = 0.10 g,
of minor products;
No - number of pulses.
Conv. (%)
CO2
Butene Selectivity (X) CO MA furan b.diene tMP l-b.
isomers(%) Cis Trans
:
85 84
23 17
23 21 17
4
;k!
21
15
1 2
79
11 IO
23 20
::
14 12
20
81 ::
1;
z:
G ;
The reaction
53 31 52
0.3 0.4
2.7 2,;
14
0.8
64 69
0.9 1.3
62 68 :83
2.6 2.0 0.6
65 60 76
16 20 11
::
103 96
9:2
0.2
59
19
:;
::
3.1
4.1 5.6
75 69
13 15
1:
1:;
1.3 1.1
;:"5
5.3 6.2
68
15
1:
102 99
1::
4:3 33.;
4.9 3.7
65 66
16 15
3;
;:
of I-butene with
were formed. The main oxidation
been recently
(ref.13)
only trace amounts
showed an appreciable - vanadium
high catalytic
It was assumed
of product
deficit,
as it has also
system.
(IV)
activity
On the
towards
butene
MA with the solids
that the carbon deficiency
primarily
by the strong adsorption
teraction
of MA with VOP04 and (VO)2P207
shown in Tables
of MA or its decomposition has therefore
2 and 3 is caused products.
The in-
also been studied.
As the carbon balance A-VOP04.
shows, no deposition The initial
case of the first pulse,
In the subsequent creasing
pulses,
is caused
conversion
of MA occurs
by the oxidation
however,
initially
on pure,
of about 25 %, observed
with decreasing
in the
of MA with surface oxygens. conversion,
surface oxygen deficiency,
a moderate
of I-butene with
in pulse reactor.
deposition
i.e. with
Reaction
Conv. (%)
CO2
24.4 12.4
4.8 9.7
(VO),P207
Selectivity (%) CO b.diene
3.6 -
43 42
MA
::1
T=720 K; m=O.lO
Isomers (%) l-b. Cis Trans
;':
26 29
::
in-
of carbon also occurs.
TABLE 3
:
The
are given in Table 4.
stoichiometric
No
of MA
as was found by Morselli
(Table 3).
The interaction
results
(VO)2P207.
in the butane
other hand, this solid shows extremely isomerisation
onto
product was butadiene,
The carbon balance observed
(%)
(VO),P,O,
When pulses of butene were injected
et al.tref.12).
Carbon balance
Carbon balance
88 94
g
(%)
31 TABLE 4 The transformation of MA on P-VOP04 (m=0.40 g) and on (VO)2P207 (m=0.40 g) at 750 K. Pulse volume = 1 cm3, injected amount of MA: Z*lO-* mg per pulse. Solid charge
No
Selectivity (X)
Conversion (%I
co
co7
C-balance (92
25.3
82
18.3
102 :: 95
P-VOP04
: :
24.6 17.3 13.3
1; 63
95 f -
(VO)*P*O7
2 1 3
91 :;
40 41 26
10.8 11.0 trace
65: 43
On the contrary, almost complete conversion of MA occurs on (VO12P207, which, according to the carbon balance, largely refers to the strong adsorption of MA or its decomposition products. This process seems to be vigorous on vanadium of lower valence state. Reaction of n-butane withoc- and B-VOP04 Because of its lower reactivity, butane was brought into contact with the catalysts at higher temperatures, generally at 820 K. The initial stage of these reactions was studied by pulse technique;,After several pulses the reactor was turned to the flow reactor mode for studying the so-called quasi-steady-state. The reaction products were MA, CO2 and CO, with trace amounts of aldehydes, ketones and acids. Butadiene and furan became detectable only after the 4th pul-se.
loo % 80
1
0-p 0
C -balance x~,x_x-x-c
1
3-x
3N04s I5 time,mln s 25
Fig.1. Reaction of n-butane witho(-VOP04 in pulse and in flow reactor mode, in the absence of gas phase 02, T = 820 K, m = 0.4 g
32 Results obtained and-VOPO4 are shown in Fig.1. One can see that the initial selectivity (SW) is not too high, though it increases moderately with the pulse number. blhenthe system is put into the flow reactor mode, a significant increase of Sm
accompanied with a sudden decrease of the conversion occurs. Re-
sembling a quasi-steady-state,both values then
become
more or less stabiliz-
ed. At this stage, according to the C-balance, no further deposition of carbon occurs. We consider that the observed phen~ena are related to the actual surface oxygen concentration: During pulse reactor operation, as discussed before, an almost complete reconstruction of the surface occurs between subsequent pulses. Thus, for several pulses, a practically intact vanadium-(V) surface is exposed to the injected hydrocarbon. (After the five pulses shown in Fig.1, less than 2 X of the bulk oxidation capacity has been consumed.) On the contrary, during 25 min. of flow reactor operation, about 15% of the bulk oxidation capacity has been utilized, and the observed quasi-steady state very likely refers to an even more reduced surface. As the results show, surface oxygen deficiency favours the fo~ation of MA. Results obtained on B-VOPO4 were qualitatively identical with those shown in Fig.1; however, the initial deposition of carbon was less p~nounced. In both cases, besides CO, CO2 and MA as main products, butadiene and furan have also been identified on reduced surfaces. Separate tests were devoted to study the reaction of these MA intermediates,giving the following products: Butadiene t VDPO4 : furan, MA, CO, CO2 + VOPO4 : MA, co, co*
Furan
The catalytic oxidation of 1-butene and n-butane In the catalytic testsboth the pulse and flow reactor techniques were used. Results obtained on&
and Is-YOPO4catalysts are presented in Figs. 2 and 3.
One of the most characteristic features of these reactions is, that in the initial stage, in spite of the presence of gas phase 02, the catalysts serve as oxygen sources, as demonstrated by the oxygen balance. Even a rough comparison of
O-balance curves in Figs. 2A and 2B clearly shows that at lower temperature
(720 K) much more lattice oxygen participates in the reaction than at higher temperature, notwithstandingthe large charge in the latter case. At 820 K the quasi-steady
state,
characterized
by 0-ba'lances
of about
100%
has been readily
achieved after several pulses of I-butene. In contrast with the initial reduction of vanadi~-(V~ catalysts, the fVO),P,O, was partially oxidized in the early stage of the reactions, as deduced from the observed O-deficits. As an example of the phenomenon, Fig.4. shows the results obtained in the catalytic oxidation of n-butane.
13-t q
A
\o-hklnce
140
%.
1201 100
0-0-o
'\ .C-balance ‘b-o x----_xx--x-x-----x~
x-
i
80
+,&he
6
conv
40 20 0i
0
12
3 4 NO
5
6
0
10 40 time, min
NO
Fig.2. Oxidation of 1-butene with 02 onkVOPO4. B: T = 820 K (m = 0.40 g).
%
1~ 140120IOO-
-o
O-balance -o-o,,
%
60-
.&G;;&:-o
40-
0-
0
“0 \
140
o.
120
0
O-balance o-0 '.
-“o_
x__.-x-x-----+--x +-~.&E-!!._
20-
J
B
160
C-balance
8&
0
A: T = 720 K (m = 0.10 g);
A 0
I I 012345
01s OJ!J SC02 A- AY--I 1 2 3 NO
Fig.3. Catalytic
--\
;t
Sb.diene
410
oxidation
B: n-butane
0
30 time,min
on p-VOPO,.
0
1
2 NO
3
20 40 time,min
A: 1-butene + 02 at 720 K (m B 0.10 9);
+ 02 at 820 K (m = 0.40 9).
01
2
3 4 NO
5
6
10 40 time, min
Fig.4. The butane f 02 reaction on (\tO),P207 T = 820 K, m = 0.40 g, The thermal behaviour of the catalysts and the assumed route for oxygen transfer Upon programmed heating up to 970 K in He or Ar flow, no evolution of O2 gas Above this temperature, our VOPO4 samples started to release 0 in 2 two steps, giving peak maxima at 1025 and 1095 K, as observed by DSC - GC, and
was observed.
at 1035 and ?lOOK, as detected by TG-MS. On (VO)2P207, only one 02 peak at about 1100 K was observed. XPS spectra on pelleted catalysts were run on a KRATOS XSAM 800 instrument, using Mg K~radiation. The first spectra were obtained for each sample at room temperature. Then the sample was heated up to 770 K (measured in the sample holder) inside the instrument in UHV, and the second measurement was made at this temperature. The third,series of spectra were run after the catalyst was cooled to room temperature. All the spectra were referred to the 4 f 7/2 line of gold, evaporated previously onto the samples. Upon heating the catalysts to 770 K there was no change in the shape or in the position of the P 2p doublet. The heat-treatmentresulted in a moderate reduction of vanadium in the&- and@-vanadyl phosphates as deduced from the broadening of the Y 2p 3/2 lines towards lower binding energies in the spectra taken at 770 K and also at 300 K after the heat-treatment. On the contrary, characteristicand reversible changes were observed at high temperature in the 0 Is as well as in the valence band regions for a?1 samples. As an example, spectra obtained on VOPO4 at 770 K and at 300 K after the heattreatment are shown in Fig.5. On all samples, irrespective of being heated or not, at room temperature the 0 IS peak consisted of two components. The smaller one with an intensity of 7-12 % was shifted by 1.2 - 1.8 eV towards the higher B.E. side of the main 0 fs peak. At 770 K, however, as shown in Fig.5 reversible formation of a new
0 BE. eV
30
Fig. 5. The 0 1s and the valence treatment for/S-VOP04. oxygen
species was observed,
ly stable peak. The relative on VOP04 and on (VO)3P207 ture resulted valence
regions
at 770 K and at 300 K after the heat
which overlapped intensity
catalysts.
also in the reversible
appearance
of a new, intense
electrons
7.lt0.8
(B.E. = 2.2 2 0.3 eV), in agreement
forms upon heat-treatment,
observations,
presumably
A comparison
to butene oxidation
that under similar in inert atmosphere
condition
with other data (ref.15).
we suggest that a new oxygen
species
type of process:
v4++ OS
of the data given in Table
and 2B, all referring
to
of DOS of the V 3d valence
by the following
v5++ 02- -
for C Zp,
and 9.621 eV, respectively,
this new peak refers to the increase
On the basis of the above
in tempera-
line in the
region at about 2 eV B.E. As in this region the B.E. values
all probability
served
form was 8-12 %, both
At the same time, the increase
0 2p and P 3p lines (ref.14) are 6.4+1.9,
strates
with the above small but relative-
of this reversible
1 with the results on VOP04 catalysts,
shown in Figs.2A clearly
demon-
the initial rates and selectivities
are close to those found in the catalytic
ob-
tests.
36 When comparing Fig. 1. with Fig. 38, it is also seen that the initial rates and selectivities
are similar also for butane,
both in the absence and the presence
of 02, with good agreement with the recent results by Hodnett and Delmon (ref.10). We therefore consider that both in stoichiometricand catalytic oxidation, the same oxygen
species
(the proposed 0: ion-radical)
is the reactive
one,
which is also responsible for the enhanced migration of bulk oxygen. Due to surface oxygen deficiency under steady-state oxidation, the surface concentration of this oxygen ion-radical can
not be too high and therefore the selective
oxidation to MA is still allowed. The transformation
of butane and butene
to MA presumably involves the abstrac-
tion of hydrogens by 02, and also the addition reactions of these 0' species. The catalytic cycle also,of course, involves the reoxidation of the solid. Under steady-state condition, this latter process seems to be the rate-controlling one, resulting
in an oxygen-deficientsurface.
The oxidation of butane to MA is considered to occur via butene. However, further
as the first step is the rate-controlling
intermediates
of these assumed n-butane
are not easily detectable.
intermediates
was observed.
with VOPO4 type catalysts
traces of butadiene after a moderate
On the contrary,
in the reaction
of
in the absence of gas phase 02, no butene but
and furan have been detected
reduction
one, butene and the
In our catalytic tests, neither
after several pulses,
i.e.
of the solids.
ACKNOWLEDGEMENT We gratefully
acknowledge
the help of Dr G.Ladwig and Dr B.Kubias in the prep-
aration of the catalysts. REFERENCES B.K. Hodnett. Catal.Rev. Sci.Eng., 27 (1985) 373-424. M. Ai, P. Boutry and R. Montarnal, Bull.Soc.Chim.Fr., (1970) 2775-2782. 3 M. Ai, P. Boutry. R. Montarnal and G. Thomas, Bull.Soc.Chim.Fr.. (1970) 2783-2789, 43 (1979) 3490-3495. 4 M. Ai, Bull.Chem.Soc.Jpn., P. Sunder-land, Ind.Eng.Cham., Prod.Res.Dev., 15 (1976) 99-99. 65 F. Cavani, G. Centi, I. Manenti, A. Riva and F. Trifiro, Ind.Eng.Chem.Prod. Res.Dev.. 22 (1983) 565-570. R.L. Varma and D.N. Saraf. J.Catal., 55 (1978) 361-372. I: R.L. Varma and D.N. Saraf, J.Catal., 55 (1978) 373-382. 9 E. Bordes and P. Courtine, J.Catal., 57 (1979) 236-252. B.K. Hodnett and B. Delmon, Appl.Catal., 15 (1985) 141-150. 338 (1965 266-270. :': G. Ladwig, Z.Anorg.Allg.Chem., L. Morselli, F. Trifiro and L. Urban, J.Catal., 75 (1982) 112-121. 1; F. Cavani, G. Centi and F. Trifiro, Appl.Catal., 15 (1985) 151-160. 14 Handbook of Chem. and Phys., 60th Edition, CRC Press, Inc., Boca Raton. Florida, 1979-80. p. E-192. 42 (1981) 15 N. Betham. A.F. Orchard and G. Thorton, J.Phys.Chem.Solids, 1051-1055.
:
Catalysis Today, l(l987) 27-36 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
ON THE MECHANISM
OF THE SELECTIVE
OXIDATIONOF BUTANE AND I-BUTENE ON VANADYL
PHOSPHATES
SSZAKACS, 'Central
H.WOLF2,
G.MINK',
Res.Inst.Chem.,
Bureau of Techn.
I.BERT6T13
Hung.Acad.Sci.,
Development
, N.WDSTNECK2, H-1525
H-1108 Budapest,
2Centr.Inst.0rg.Chem.,
Acad.Sci.GDR,
3Res.Lab.Inorg.Chem.,
Hung.Acad.Sci.,
B.LOCKE'
Budapest;
and H.SEEBOTH2
present
address:
Olto u. 16, 1X.36.
Berlin/Adlershof,
Rudower
H-1112 Budapest,
Chausse
5.
Budaiirsi ut 45.
ABSTRACT The oxidation of 1-butene and n-butane to maleic anhydride was studied on Wand($-VOPO and on (VO)$P$O7,, these serving as oxidizing agents or catalysts. As shown by tfle oxygen bal n es in catalytic tests, partial reduction of the vanadium (V) and oxidation of the vanadium (IV) catalysts occur initially. On the basis of kinetic studies and suppplementary DSC, TG-MS and XPS information, a mobile 0- radical is considered to be the active species in both the stoichiometric and the catalytic oxidation, characterized by closely similar initial conversions.
INTRODUCTION Vanadium-phosporus ducing maleic
oxide catalysts
anhydride
the main features
of the reaction
cation of the key intermediates
mechanism
as butadiene
2-4). There are still controversies, oxygen
have opened a possible
(MA) from C4-feedstocks
(ref.1).
new route for pro-
In case of n-butenes
have been revealed
, crotonaldehyde
however,
concerning
by the identifi-
and furan
the mechanism
(refs. of the
transfer.
Proposed
mechanisms,
oxygen with gaseous both components
based on kinetic data, involve:
hydrocarbon
in the adsorbed
in a redox cycle Recent results
by Hodnett
in catalytic
hydrocarbons
and Delmon
(ref.101
also support
of butane and MA is concerned.
work was to give additional
by a comparative
study of the selective
in an inert atmosphere
0920-5861/87/$03.50
or the reaction
with
of lattice oxygen
a redox mechanism; the yields were
tests and in the absence of gass-phase
The aim of the present lattice oxygen
state (ref.6);
of adsorbed
mechanism
(refs.7-9).
as far as initial conversions comparable
the reaction
(ref.5); a Langmuir-Hinshelwood
information oxidation
and also in the catalytic
0 1987 Elsevier Science Publishers B.V.
02. on the role of of the title
tests.
EXPERIMENTAL Reactor The reactions were carried out in a fluidized bed microreactor able to operate either in a pulse reactor mode or as a stationary flow reactor. The fused silica reactor of about 1 cm3 in volume was vibrated by an external vibrator. Purified He with less than 5 ppm O2 was used as carrier gas. Flow rates of 0 .5 cm3*s'I were kept constant in both operation modes. The pulsing valve with a pulse volume of 13 cm3 was thermostated at 400 K. The composition of the injected mixture, as well as that of the stationary flow, was either 0.3 volume % of C4-hydrocarbon in He, or, in catalytic tests, 0.3 % hydrocarbon + 1 % O2 in He. When injecting MA, the conditions, as indicated later, were different. Analysis The feed and all the components of the outlet were analysed by GC equipped with four columns (Table 11. After a group separation the reaction products were analysed according to Table 1. TABLE 1 GC analysis conditions Column I 2
Group 02, CH4, CO COP and C4-
Length and I.D. 1
mx4mm
10.5 m x 4 mm
T(K) 293
Support Stat.phase Linde 13X
-
Detector TCD
293
CHRO~SORB W/AW
30% pimelic ac.dinitrile
TCD
-hydrocarbons 3
Aldehydes, ketones,furan
5
mx4mm
400
_ " _
30% Carbowax -3000
TCD
4
Acids and MA
2
mx4mm
400
_ " _
30% FFAP + 3% H3P04
FID
The total analysis required about 20 min; i.e. this was the approximate time interval between two subsequent pulses or samplings. Precise carbon and oxygen balances in percentages have been determined as the sum of the given kind of atoms in the outlet related to that in the inlet. The conversion is defined as the percentage fraction of the feed compound transformed to other compounds, excluding isomers. Hereafter we also calculate selectivity (Si) values as s
i
=
the amount of carbon present in product "i" the amount of carbon in the reacted fraction of the feed ' loo%
Catalysts Stoichiometricor-VDFD4,P-VOPD4 and fV0)2P207 catalysts were prepared by the method propo_sTdby Ladwig (ref.11). The specific surface areas were 4.7, 8.8 and 6.3 rn2g
, respectively. For the experiments. a grain size of 0.2 - 0.4 mm
was used. The thermal behaviour of the catalysts under He or Ar flow was studied in SETARAM DSC-Ill microcalorimet~r coupled to a single column GC and in a Perkin Elmer thermal surface
balance
electronic
connected
with a Balzers @IS-511
state of the catalysis
mass spectrometer.The
at 300 and 770 K was studied by ESCA,
using a KRATOS XSA~OO jnstrument. RESULTS AND DISCUSSION Oxidation by Lattice Oxygen The mobility of lattice oxygen in catalytic oxidation is of fundamental importance.
Therefore
the interaction
MA with the catalysts
of the title hydrocarbons
in the absence of gaseous
Oxidation of I-butene by&-
and of the product
O_, has first been studied.
and P-VOPO4
Results obtained at 720 K by pulse method are collected ‘inTable 2. fn these experiments, besides the main products
i.e. CO, COP, MA, furan and butadiene,
minor products (MP). i.e. ~H3DOH, CH3CH0, C2H~COOH. ~~H3COOH
and C3H5CH0,
have
also been observed. The amount of hydrocarbon injected in one pulse was co~ensurate with the number of surface vanadyl type oxygens, assumed as reactive species in oxidation.
In this context it seems to be strange that the conversion unchanged
with
increasing
is practically
pulse number, though one pulse of butene consumes ap-
proximately the total oxidation capacity of one surface layer. We therefore consider that during the 20 min required for analysis between subsequent pulses, a closely complete equilibration of bulk and surface oxygens occurs, resulting always in a closely complete surface coverage by vanadium-(V) species. (As estimated, only about 1 % of the bulk oxidation capacity of the solid was consu~d by one pulse.1 Nevertheless, there are systematic trends with increasing pulse number: (5) the selectivity of MA formation decreases, and (ii) the carbon balance, which initially varied at around 100 I, tends to decrease as a result of the deposition of carbonoceaus species, Both of these phenomena are more pronounced in case of the=-phase. As Table 2 also
shows@ parallel to the oxidation by lattice
oxygen, a catalytic process - the is~risation
of I-butene - also occurs and
the isomerisation activity of the catalysts moderately increases with the pulse number.
30 TABLE 2 Oxidation 'IMP
of 1-butene by+
- the summarized
Phase No
*3
and B-VOPO4
selectivity
inthe
pulse reactor at 720 K.m = 0.10 g,
of minor products;
No - number of pulses.
Conv. (%)
CO2
Butene Selectivity (X) CO MA furan b.diene tMP l-b.
isomers(%) Cis Trans
:
85 84
23 17
23 21 17
4
;k!
21
15
1 2
79
11 IO
23 20
::
14 12
20
81 ::
1;
z:
G ;
The reaction
53 31 52
0.3 0.4
2.7 2,;
14
0.8
64 69
0.9 1.3
62 68 :83
2.6 2.0 0.6
65 60 76
16 20 11
::
103 96
9:2
0.2
59
19
:;
::
3.1
4.1 5.6
75 69
13 15
1:
1:;
1.3 1.1
;:"5
5.3 6.2
68
15
1:
102 99
1::
4:3 33.;
4.9 3.7
65 66
16 15
3;
;:
of I-butene with
were formed. The main oxidation
been recently
(ref.13)
only trace amounts
showed an appreciable - vanadium
high catalytic
It was assumed
of product
deficit,
as it has also
system.
(IV)
activity
On the
towards
butene
MA with the solids
that the carbon deficiency
primarily
by the strong adsorption
teraction
of MA with VOP04 and (VO)2P207
shown in Tables
of MA or its decomposition has therefore
2 and 3 is caused products.
The in-
also been studied.
As the carbon balance A-VOP04.
shows, no deposition The initial
case of the first pulse,
In the subsequent creasing
pulses,
is caused
conversion
of MA occurs
by the oxidation
however,
initially
on pure,
of about 25 %, observed
with decreasing
in the
of MA with surface oxygens. conversion,
surface oxygen deficiency,
a moderate
of I-butene with
in pulse reactor.
deposition
i.e. with
Reaction
Conv. (%)
CO2
24.4 12.4
4.8 9.7
(VO),P207
Selectivity (%) CO b.diene
3.6 -
43 42
MA
::1
T=720 K; m=O.lO
Isomers (%) l-b. Cis Trans
;':
26 29
::
in-
of carbon also occurs.
TABLE 3
:
The
are given in Table 4.
stoichiometric
No
of MA
as was found by Morselli
(Table 3).
The interaction
results
(VO)2P207.
in the butane
other hand, this solid shows extremely isomerisation
onto
product was butadiene,
The carbon balance observed
(%)
(VO),P,O,
When pulses of butene were injected
et al.tref.12).
Carbon balance
Carbon balance
88 94
g
(%)
31 TABLE 4 The transformation of MA on P-VOP04 (m=0.40 g) and on (VO)2P207 (m=0.40 g) at 750 K. Pulse volume = 1 cm3, injected amount of MA: Z*lO-* mg per pulse. Solid charge
No
Selectivity (X)
Conversion (%I
co
co7
C-balance (92
25.3
82
18.3
102 :: 95
P-VOP04
: :
24.6 17.3 13.3
1; 63
95 f -
(VO)*P*O7
2 1 3
91 :;
40 41 26
10.8 11.0 trace
65: 43
On the contrary, almost complete conversion of MA occurs on (VO12P207, which, according to the carbon balance, largely refers to the strong adsorption of MA or its decomposition products. This process seems to be vigorous on vanadium of lower valence state. Reaction of n-butane withoc- and B-VOP04 Because of its lower reactivity, butane was brought into contact with the catalysts at higher temperatures, generally at 820 K. The initial stage of these reactions was studied by pulse technique;,After several pulses the reactor was turned to the flow reactor mode for studying the so-called quasi-steady-state. The reaction products were MA, CO2 and CO, with trace amounts of aldehydes, ketones and acids. Butadiene and furan became detectable only after the 4th pul-se.
loo % 80
1
0-p 0
C -balance x~,x_x-x-c
1
3-x
3N04s I5 time,mln s 25
Fig.1. Reaction of n-butane witho(-VOP04 in pulse and in flow reactor mode, in the absence of gas phase 02, T = 820 K, m = 0.4 g
32 Results obtained and-VOPO4 are shown in Fig.1. One can see that the initial selectivity (SW) is not too high, though it increases moderately with the pulse number. blhenthe system is put into the flow reactor mode, a significant increase of Sm
accompanied with a sudden decrease of the conversion occurs. Re-
sembling a quasi-steady-state,both values then
become
more or less stabiliz-
ed. At this stage, according to the C-balance, no further deposition of carbon occurs. We consider that the observed phen~ena are related to the actual surface oxygen concentration: During pulse reactor operation, as discussed before, an almost complete reconstruction of the surface occurs between subsequent pulses. Thus, for several pulses, a practically intact vanadium-(V) surface is exposed to the injected hydrocarbon. (After the five pulses shown in Fig.1, less than 2 X of the bulk oxidation capacity has been consumed.) On the contrary, during 25 min. of flow reactor operation, about 15% of the bulk oxidation capacity has been utilized, and the observed quasi-steady state very likely refers to an even more reduced surface. As the results show, surface oxygen deficiency favours the fo~ation of MA. Results obtained on B-VOPO4 were qualitatively identical with those shown in Fig.1; however, the initial deposition of carbon was less p~nounced. In both cases, besides CO, CO2 and MA as main products, butadiene and furan have also been identified on reduced surfaces. Separate tests were devoted to study the reaction of these MA intermediates,giving the following products: Butadiene t VDPO4 : furan, MA, CO, CO2 + VOPO4 : MA, co, co*
Furan
The catalytic oxidation of 1-butene and n-butane In the catalytic testsboth the pulse and flow reactor techniques were used. Results obtained on&
and Is-YOPO4catalysts are presented in Figs. 2 and 3.
One of the most characteristic features of these reactions is, that in the initial stage, in spite of the presence of gas phase 02, the catalysts serve as oxygen sources, as demonstrated by the oxygen balance. Even a rough comparison of
O-balance curves in Figs. 2A and 2B clearly shows that at lower temperature
(720 K) much more lattice oxygen participates in the reaction than at higher temperature, notwithstandingthe large charge in the latter case. At 820 K the quasi-steady
state,
characterized
by 0-ba'lances
of about
100%
has been readily
achieved after several pulses of I-butene. In contrast with the initial reduction of vanadi~-(V~ catalysts, the fVO),P,O, was partially oxidized in the early stage of the reactions, as deduced from the observed O-deficits. As an example of the phenomenon, Fig.4. shows the results obtained in the catalytic oxidation of n-butane.
13-t q
A
\o-hklnce
140
%.
1201 100
0-0-o
'\ .C-balance ‘b-o x----_xx--x-x-----x~
x-
i
80
+,&he
6
conv
40 20 0i
0
12
3 4 NO
5
6
0
10 40 time, min
NO
Fig.2. Oxidation of 1-butene with 02 onkVOPO4. B: T = 820 K (m = 0.40 g).
%
1~ 140120IOO-
-o
O-balance -o-o,,
%
60-
.&G;;&:-o
40-
0-
0
“0 \
140
o.
120
0
O-balance o-0 '.
-“o_
x__.-x-x-----+--x +-~.&E-!!._
20-
J
B
160
C-balance
8&
0
A: T = 720 K (m = 0.10 g);
A 0
I I 012345
01s OJ!J SC02 A- AY--I 1 2 3 NO
Fig.3. Catalytic
--\
;t
Sb.diene
410
oxidation
B: n-butane
0
30 time,min
on p-VOPO,.
0
1
2 NO
3
20 40 time,min
A: 1-butene + 02 at 720 K (m B 0.10 9);
+ 02 at 820 K (m = 0.40 9).
01
2
3 4 NO
5
6
10 40 time, min
Fig.4. The butane f 02 reaction on (\tO),P207 T = 820 K, m = 0.40 g, The thermal behaviour of the catalysts and the assumed route for oxygen transfer Upon programmed heating up to 970 K in He or Ar flow, no evolution of O2 gas Above this temperature, our VOPO4 samples started to release 0 in 2 two steps, giving peak maxima at 1025 and 1095 K, as observed by DSC - GC, and
was observed.
at 1035 and ?lOOK, as detected by TG-MS. On (VO)2P207, only one 02 peak at about 1100 K was observed. XPS spectra on pelleted catalysts were run on a KRATOS XSAM 800 instrument, using Mg K~radiation. The first spectra were obtained for each sample at room temperature. Then the sample was heated up to 770 K (measured in the sample holder) inside the instrument in UHV, and the second measurement was made at this temperature. The third,series of spectra were run after the catalyst was cooled to room temperature. All the spectra were referred to the 4 f 7/2 line of gold, evaporated previously onto the samples. Upon heating the catalysts to 770 K there was no change in the shape or in the position of the P 2p doublet. The heat-treatmentresulted in a moderate reduction of vanadium in the&- and@-vanadyl phosphates as deduced from the broadening of the Y 2p 3/2 lines towards lower binding energies in the spectra taken at 770 K and also at 300 K after the heat-treatment. On the contrary, characteristicand reversible changes were observed at high temperature in the 0 Is as well as in the valence band regions for a?1 samples. As an example, spectra obtained on VOPO4 at 770 K and at 300 K after the heattreatment are shown in Fig.5. On all samples, irrespective of being heated or not, at room temperature the 0 IS peak consisted of two components. The smaller one with an intensity of 7-12 % was shifted by 1.2 - 1.8 eV towards the higher B.E. side of the main 0 fs peak. At 770 K, however, as shown in Fig.5 reversible formation of a new
0 BE. eV
30
Fig. 5. The 0 1s and the valence treatment for/S-VOP04. oxygen
species was observed,
ly stable peak. The relative on VOP04 and on (VO)3P207 ture resulted valence
regions
at 770 K and at 300 K after the heat
which overlapped intensity
catalysts.
also in the reversible
appearance
of a new, intense
electrons
7.lt0.8
(B.E. = 2.2 2 0.3 eV), in agreement
forms upon heat-treatment,
observations,
presumably
A comparison
to butene oxidation
that under similar in inert atmosphere
condition
with other data (ref.15).
we suggest that a new oxygen
species
type of process:
v4++ OS
of the data given in Table
and 2B, all referring
to
of DOS of the V 3d valence
by the following
v5++ 02- -
for C Zp,
and 9.621 eV, respectively,
this new peak refers to the increase
On the basis of the above
in tempera-
line in the
region at about 2 eV B.E. As in this region the B.E. values
all probability
served
form was 8-12 %, both
At the same time, the increase
0 2p and P 3p lines (ref.14) are 6.4+1.9,
strates
with the above small but relative-
of this reversible
1 with the results on VOP04 catalysts,
shown in Figs.2A clearly
demon-
the initial rates and selectivities
are close to those found in the catalytic
ob-
tests.
36 When comparing Fig. 1. with Fig. 38, it is also seen that the initial rates and selectivities
are similar also for butane,
both in the absence and the presence
of 02, with good agreement with the recent results by Hodnett and Delmon (ref.10). We therefore consider that both in stoichiometricand catalytic oxidation, the same oxygen
species
(the proposed 0: ion-radical)
is the reactive
one,
which is also responsible for the enhanced migration of bulk oxygen. Due to surface oxygen deficiency under steady-state oxidation, the surface concentration of this oxygen ion-radical can
not be too high and therefore the selective
oxidation to MA is still allowed. The transformation
of butane and butene
to MA presumably involves the abstrac-
tion of hydrogens by 02, and also the addition reactions of these 0' species. The catalytic cycle also,of course, involves the reoxidation of the solid. Under steady-state condition, this latter process seems to be the rate-controlling one, resulting
in an oxygen-deficientsurface.
The oxidation of butane to MA is considered to occur via butene. However, further
as the first step is the rate-controlling
intermediates
of these assumed n-butane
are not easily detectable.
intermediates
was observed.
with VOPO4 type catalysts
traces of butadiene after a moderate
On the contrary,
in the reaction
of
in the absence of gas phase 02, no butene but
and furan have been detected
reduction
one, butene and the
In our catalytic tests, neither
after several pulses,
i.e.
of the solids.
ACKNOWLEDGEMENT We gratefully
acknowledge
the help of Dr G.Ladwig and Dr B.Kubias in the prep-
aration of the catalysts. REFERENCES B.K. Hodnett. Catal.Rev. Sci.Eng., 27 (1985) 373-424. M. Ai, P. Boutry and R. Montarnal, Bull.Soc.Chim.Fr., (1970) 2775-2782. 3 M. Ai, P. Boutry. R. Montarnal and G. Thomas, Bull.Soc.Chim.Fr.. (1970) 2783-2789, 43 (1979) 3490-3495. 4 M. Ai, Bull.Chem.Soc.Jpn., P. Sunder-land, Ind.Eng.Cham., Prod.Res.Dev., 15 (1976) 99-99. 65 F. Cavani, G. Centi, I. Manenti, A. Riva and F. Trifiro, Ind.Eng.Chem.Prod. Res.Dev.. 22 (1983) 565-570. R.L. Varma and D.N. Saraf. J.Catal., 55 (1978) 361-372. I: R.L. Varma and D.N. Saraf, J.Catal., 55 (1978) 373-382. 9 E. Bordes and P. Courtine, J.Catal., 57 (1979) 236-252. B.K. Hodnett and B. Delmon, Appl.Catal., 15 (1985) 141-150. 338 (1965 266-270. :': G. Ladwig, Z.Anorg.Allg.Chem., L. Morselli, F. Trifiro and L. Urban, J.Catal., 75 (1982) 112-121. 1; F. Cavani, G. Centi and F. Trifiro, Appl.Catal., 15 (1985) 151-160. 14 Handbook of Chem. and Phys., 60th Edition, CRC Press, Inc., Boca Raton. Florida, 1979-80. p. E-192. 42 (1981) 15 N. Betham. A.F. Orchard and G. Thorton, J.Phys.Chem.Solids, 1051-1055.
: