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New developments and concepts in enhancing activities of heterogeneous metathesis catalysts
Journal
of Molecular
Catalysis,
NEW DEVELOPMENTS OF HETEROGENEOUS ROBERT Research
28 (1985)
117 - 131
117
AND CONCEPTS IN ENHANCING ACTIVITIES METATHESIS CATALYSTS
L. BANKS and SIMON G. KUKES and Development,
Phillips Petroleum
Company,
Bartlesville,
OK 74004
(U.S.A.)
Summary Our continuing investigations of heterogeneous metathesis catalysts have led to three findings: (1) Studies of MgO/W03* SiOz combinations suggest that MgO plays an unexpected beneficial role in olefin metathesis catalysis: that of generating gas-phase ‘excited species’ (e.g. ally1 or allyl-0x0 radicals), which are initiators or precursors of metathesis sites, producing dramatic increases in metathesis activities. The proposed new concept is indirectly supported by findings that the activities of Moo3 metathesis catalysts are not likewise enhanced by MgO; more significantly, enhancement of WOJ catalysts does not occur when Moos, a radical scavenger, is present. (2) The catalytic activities of conventional heterogeneous metathesis catalysts are increased significantly by admixing minor amounts of elemental S, Si, Mg, Ba, Sn, Zn, Sb or W with the metathesis catalysts, and treating the admixtures at elevated temperature under an inert atmosphere. The enhanced activity is attributed to a partial reduction of the catalysts by the added reducing metals or elements. (3) An unusual and interesting catalytic behavior is exhibited by rhenium oxide supported on Th3(P0& : the metathesis activity increased by more than 50% when small amounts of oxygen were added to the olefin feed. Other heterogeneous metathesis catalysts, including Re207* AlsO and Re,O,- A1P04, do not show this behavior when oxygen is added; in most cases, traces of oxygen produce a decline in metathesis activity. This unusual behavior has fundamental implications: key roles for oxygen ligands have been claimed for both homogeneous and heterogeneous metathesis catalysts.
Introduction Among the most extensively studied heterogeneous metathesis catalysts are the oxides of tungsten, molybdenum and rhenium supported on high surface area inorganic substrates [ 11.Silica and alumina are the most widely used supports; however, activity with other inorganic substrates, such as metallo-phosphates, has been demonstrated [ 2, 31. Numerous studies to 0304-5102/85/$3.30
0 Elsevier Sequoia/Printed
in The Netherlands
118
elucidate the mechanistic aspects and nature of the active sites of these catalysts and to improve, or extend, their effectiveness for olefin metathesis have been reported [ 11. Various modifications and treatments to increase activity or selectivity have been found: for example, neutralizing acidic-type sites increases the selectivity towards primary metathesis products; addition of nonacidic selective isomerization catalysts shifts the carbon-carbon double bonds of reactants or products (essential for certain applications) [ 41; pre-reduction with reducing gases [5] or metallic sodium [ 61 increases the activity or selectivity of some catalysts; an addition of small amounts of chelating diolefins [7] or ammonia [8] to olefin feed increases the extent of metathesis reactions over tungsten oxide-silica. Key roles for oxygen ligands in metathesis catalysts have been proposed [ 91. In our continuing investigation ofheterogeneous catalysts for metathesistype reactions, three interesting and potentially important findings or observations have emerged: (1) Dramatic activity increases of tungsten oxide-silica in the presence of a magnesia catalyst suggest that catalytic sites on tungsten catalysts are being produced by gas-phase ‘excited species’ generated on the surface of the magnesia. (2) Minor amounts of solid reducing metals or elements are effective agents for increasing the initial activities of conventional tungsten and molybdenum oxide catalysts. (3) Significant increases in the metathesis activities of rhenium oxidethorium phosphate occur when traces of oxygen are added to the olefin feed.
Activity
enhancement
by magnesium
oxide
The incorporation of highly selective double-bond isomerization catalysts with heterogeneous metathesis catalysts is an established technique for obtaining high conversions in certain metathesis applications [ 3, 4, lo]. The assigned role of the isomerization catalyst is to shift the double bonds of symmetrical olefins, permitting ‘productive metathesis’ to occur (e.g., the synthesis of linear olefins), or to maintain an equilibrium concentration among the isomers, thus replenishing the key reactant being depleted in the metathesis step (e.g., ethylene cleavage of /3-diisobutylene in the Phillips commercial neohexene process). Magnesium oxide, a very selective isomerization catalyst, is effective for these applications; it is compatible with tungsten oxide-silica and other heterogeneous metathesis catalysts under both reaction and activation conditions [6]. A second use of magnesia in olefin metathesis catalysts is in the pretreatment of feed streams’to olefin metathesis units: at ambient temperature it is extremely effective for removing from olefins very low concentrations of contaminants (e.g., peroxides and hydroperoxide compounds) [ 61. Commercially, it is used in conjunction with other known adsorptive materials for the purification of olefin feed
119 TABLE 1 Metathesis activities of tungsten oxide-silica Catalyst systema
WOa*Si02 (1.5 g)
Reaction temp. (“C)
combined catalyst systems
Propylene conversion ( %)b
Secondary reaction products (%)c l-Butene
cs plus
400 425 400 250 350 400
4.2 13.2 0.3 30.3 44.3 46.7
trace 0.9
trace trace
0.7 2.4 7.6
0.2 1.6 5.3
MgO (2.6 g)/WOs’SiOz (1.5 gY
400
44.6
1.1
0.7
CaO (3.4 g)/WOa*SiOz (i.5 g)”
400
44.3
1.0
0.6
MgO (3.6 g) MgO (2.6 g)/WOs*SiOz (1.5 g)*
=Activation at 600 “C with air followed by treatment for 15 min with CO. bActivity test at 0.1 MPa and propylene rate of 0.15 1 min-‘. CPercentage converted propylene. *Two catalysts mixed together (Fig. 1A). eCatalysts in separate layers (Fig. 1B; WOs’SiO; on bottom).
streams. Evidence of an additional, and more intriguing, role of magnesium oxide has emerged: results of continued investigations of tungsten oxide and magnesium oxide catalyst combinations suggest that gas-phase ‘excited species’ are generated on the surface of magnesia, and these species initiate, or are precursors of, catalytic sites resulting in metathesis activity increases by as much as lOO-fold. Table 1 provides a comparison of the activities of combined magnesium oxide/tungsten oxide catalyst systems for the metathesis of olefins. At 400 “C!, with tungsten oxide-silica alone, propylene conversion was 4%. Magnesia, at these conditions, had a low activity for olefin metathesis (0.3%). However, a mixture of tungsten oxide-silica and magnesia had very high metathesis activity; equilibrium conversions of propylene were achieved even at 350 “C (50 “C lower than the temperature of the control tests). The yields of secondary products are a measure of the double-bond isomerization activity of the catalyst systems. Although isomerization occurred with the mixed catalyst system (Table l), the data indicate that isomerization was not a major contributing factor to the high propylene conversion. This conclusion was confirmed by placing magnesium oxide in a layer or zone immediately above the tungsten oxide-silica (Fig. 1B); very high metathesis activity was obtained (Table 1) and the yield of secondary products was less than 15% of that obtained with the two catalysts mixed, demonstrating that double-bond isomerization activity was not a major factor contributing to the dramatic activity enhancement. When tungsten oxide-silica was placed ahead of magnesia, propylene conversions (data not shown) were
OLEFIN
PURIFICATION SECTION I25”C)
A
B
Fig. 1. Experimental systems: catalyst bed c:onfigurations.
MgO+ WOySiO2
/ / ,
WOg*SiO2
L:
1 46.7%
T
44.6%
Fig. 2. Comparison of metathesis activities of magnesia and tungsten oxide-silica catalyst combinations.
the same as obtained in similar tests on tungsten oxide-silica alone. A summary of these results appears in Fig. 2. Combined catalyst systems in which calcium oxide, alumina, or basetreated alumina were used in place of magnesia were also found to be very active for olefin metathesis (data for the CaO/W0s*Si02 system shown in Table 1). To determine the temperature range over which the magnesium oxide is most effective for enhancing the activity, tests were made with magnesium oxide placed in a separate zone 12 cm above the tungsten oxide-silica catalyst (Fig. ID). This permitted temperature variation of the magnesium oxide while maintaining the temperature of metathesis constant (e.g. 325 “C). The results (Table 2) indicate that the temperature range of 300 - 400 “C is the most effective for ‘enhancing’ or ‘promoting’ the metathesis activity of tungsten oxide-silica; with magnesia at 300 “C, the metathesis activity was almost two orders of magnitude greater than that obtained with magnesia at 200 “C. Investigation of the effects of magnesium oxide on molybdena-containing metathesis catalysts uncovered an unexpected phenomenon. In contrast to tungsten oxide catalysts, activities of molybdena catalysts were not enhanced by magnesia and, what is possibly of more significance, molybdena prevented magnesia from enhancing the activity of tungsten oxide catalysts. These results are shown in Table 3: without magnesium oxide, the
121 TABLE
2
Effect of magnesium oxide temperature tungsten oxide catalyst systema Magnesium
oxide
temperature
Propylene
on
metathesis
conversion
activity
of magnesium
(“C)
(%)b
Relative metathesis activityC
200 250 300 350 400 450 500
0.7 a.4 31.6 29.6 27.4 23.1 14.8
1 13 90 18 66 50 27
oxide/
aTwo-zone reactor (Fig. 1D) with magnesium oxide (2.0 g) approximately 12 cm above tungsten oxide-silica (0.75 g). Both catalysts activated at 600 “C with air followed by treatment for 15 min with CO. bTemperature of tungsten oxide-silica maintained constant at 325 “C. Other test conditions: 0.1 MPa, propylene rate of 0.15 1 min-‘. CBased on activity of tungsten oxide-silica when magnesium oxide temperature was 200 “C. TABLE Effect
3 of magnesium
oxide
on catalyst
Metathesis catalyst systema
Control:
WOs*SiOs (1.5 g)
systems
containing
molybdenum
oxide
Propylene conversion (%) b
Relative metathesis activityC
Without MgO With MgOd
Without MgO
With MgOd > 10
25.6
43.8f
1.0
MoOa*SiOs (1.5 g)
24.6
25.6
0.9
MoOs*WOs*Si02
(1.5 g)
28.4
29.0
1.2
1.3
MoOs*Si02(0.66
g)/WOs*SiOz(O.66
g)e
14.6
14.0
0.5
0.5
g)e
21.9
44.5
1.1
WOs’Si02(0.66
aActivated
g)/MoOs’Si02(0.66
at 600 “C with air followed
by treatment
1.0
> 10
for 15 min with CO.
bTest at 425 “C, 0.1 MPa and propylene rate of 0.15 1 min-‘. CBased on activity of WOs*SiOz obtained in this series of experiments. d3.0 g MgO on top of metathesis catalysts (Figs. 1B and 1C). eMetathesis catalysts in separate layers (Figs. 1B and 1C). fConversion 39.0% at 375 “C.
metathesis activities of tungsten oxide-silica, molybdenum oxide-silica and molybdenum oxide-tungsten oxide-silica were, essentially the same; with magnesia in a zone ahead of these catalysts, no significant changes in the activities of the molybdena-silica or of the molybdena-tungsten oxide-silica occurred, while the activity of the tungsten oxide-silica catalyst increased by a factor greater than 10. Additional evidence of the detrimental effect of molybdena on the enhancement of activity was provided by inserting a layer of the molybdena-
122
MoO3.Si02
WO3.SiO2
f
25.6% * 43.6%
*PROPYLENE
$
24.6% * 25.6%
CONVERSION
Fig. 3. Comparison metathesis catalysts
MOO3’WO3’ SiOl
MoOg’Si02
21.9%* 44.5%
26.4%* 29.0% WITHOUT
WO3.SiO2
MgO PRESENT.
of metathesis activities of molybdenum with and without magnesia present.
oxide/tungsten
oxide-silica
silica catalyst between magnesia and tungsten oxide-silica (Fig. 1C). Propylene conversions with magnesia and without magnesia were the same (Table 3). The positions of the two metathesis catalysts were then reversed, with tungsten oxide-silica ahead of the molybdena catalysts. The expected enhancement in activity by magnesia was obtained. A summary of these tests is presented in Fig. 3. Various other metal oxides supported on silica have been screened to determine their effect on the activities of both tungsten oxide-silica and magnesia/tungsten oxide-silica catalyst systems (Table 4). In the series without magnesia, in which the supported metal oxides were placed above the tungsten oxide-silica catalyst (Fig. lB), metathesis conversions at least double the control were obtained with the oxides of Ba, Zr, Cr, Ag and Th. With magnesia added to the system (Fig. 1C: i.e., magnesia/metal oxide-silica/ tungsten oxide-silica), oxides of Zn, Zr, K and Cr gave higher propylene conversions, oxides of Th, Sr, Cs, Ba, Pd and Sb had no significant effect, and oxides of Mn, V, Co, Cu, Ag, Re, Bi, Sn, Ni, Fe and Pb reduced propylene conversion. These screening tests were limited to one set of conditions: the behavior of these metal oxides over a range of compositions, pretreatment conditions, or reaction conditions was not investigated. Several explanations for the dramatic increase in metathesis activity occurring when magnesia is combined with tungsten oxide-silica have been proposed. One of the earliest, and one of the simplest suggested reasons was that magnesia, acting as a ‘guard-bed’, removes or destroys traces of contaminants from the feed that are detrimental to metathesis catalysts (e.g., by destroying or hindering the formation of metallocarbene catalytic sites). This explanation stems from earlier feed purification studies of Rycheck and Pennella [ 111. They showed that, with a guard-chamber, placed directly ahead of the reactor, containing calcium oxide at 100 - 300 “C, zirconium dioxide (100 - 300 “C), titanium dioxide (100 - 250 “C), or zinc oxide (100 - 200 “C), the conversion of propylene over tungsten oxide-silica was substantially higher. However, as is evident from the data in Table 3, the failure of magnesia
123 TABLE
4
Effects of various metal oxides on metathesis magnesia/tungsten oxide-silica catalyst systems Supported oxidesa
metal
Propylene
conversion
(Without MgO) b MO;SiOa/WOs.Si02 none (control) unpromoted SiOz ZnO*SiOa Zr02’Si02 K20.Si02 CrOs’Si02 ThOa’SiOz SrO*SiOz CsaO.SiOa BaO*SiOa PdO*SiOa SbsOa*SiOa PbOa’SiOa Fe20s*Si02 NiO*SiOz SnOa’SiOa BiaOs*SiOa Rea07*SiOa Ag,O-SiOa CuO*SiOz CoO*SiOz VaO,*SiOz MnO.SiOa
4.4 3.1
(0.2) (0.3) (1.9)
12.0 6.1 11.0 8.9 6.3 (9.2) 12.3 5.8 (9.2) 4.6 1.6 5.1 (9.3) (6.8) 4.1 9.6
1.6
activity
of
tungsten
oxide-silica
and
(%) (With MgO)C MgO/MO,*SiOa/WOs*SiOa 31.1 31.2 43.1 42.6 42.1 42.0 38.6 38.4 36.0 35.1 33.1 32.0 24.0 11.2 14.8 13.4 11.6 11.1 9.1 6.6 6.5 6.1 2.3
a5% metal, as metal oxide, supported on silica. bFigure 1B: 0.15 g MO,.Si02 on top of 0.15 g WOa’SiOa. Tests at 0.1 MPa, 0.15 1 min-’ propylene at 400 “C, except data in () obtained at 315 “C. Catalyst systems activated and treated with CO (15 min) at 600 “C. CFigure 1C: 2.0 g Mg0/0.15 g MO;Si0,/0.15 g WOs*SiOa. Tests at 315 “C. Catalyst systems activated as noted in footnote b.
to benefit the metathesis activity when molybdena is present makes the ‘guard-bed theory’ difficult to rationalize. Another suggested explanation was that metathesis was occurring over magnesium oxide in addition .to occurring over tungsten oxide-silica. This suggestion also stemmed from previous studies; we have demonstrated that, under certain conditions, magnesium oxide is a metathesis catalyst [12]. Also, Iwamato and Lunsford [13] have implied that metathesis can occur over MgO; at 300 - 450 “C, hydrocarbons having carbon numbers smaller and greater than the reactant (propylene) were formed. However, results of control tests made in connection with the current investigation (i.e., magnesium oxide both alone and downstream from tungsten oxide-silica), along with results of the tests with molybdena-containing catalysts and the sup-
124
ported metal oxides, indicate that, under the conditions employed, metathesis was not occurring to any appreciable extent on the surface of magnesium oxide. A feasible alternative explanation of the activity enhancement phenomenon, supported by indirect experimental evidence, proposed that gas-phase ‘excited species’ are generated on the surface of magnesium oxide, and that these species are initiators or precursors of catalytic metathesis on the tungsten catalysts. Tungsten oxide-silica catalysts are noted for very low site densities [ 141; thus a gas-phase concentration of only a few ppm of postulated species would be sufficient to increase significantly the relative number of metathesis sites, producing dramatic increases in catalytic activity. Several possibilities for the nature of the postulated species exist. These include allyl, oxygen ions and oxygen-containing radicals; all are known to form on the surface of magnesium oxide [13, 151. Results of a preliminary experiment using an EPR-matrix isolation technique [16] indicate that low concentrations of gas-phase n-ally1 radicals are formed when propylene contacts magnesia at 350 “C or 400 “C*; at 400 “C, gas-phase radical formation was about one-tenth that formed over bismuth oxide [16]. The likelihood that oxygen ions or oxygen-containing radicals produced by magnesia are involved in the creation of metathesis sites must be considered; key roles for oxygen ligands have been claimed for both homogeneous and heterogeneous metathesis catalysts (e.g., the spectator oxy group concept advanced by Rappe and Goddard [9]). The most convincing evidence supporting an ‘excited species’ explanation is provided by the ineffectiveness of magnesia in enhancing the metathesis activity when molybdena is present; molybdena is a known radical trap, scavenging radicals produced elsewhere in the system [ 161. Preliminary studies indicate that gas-phase ‘excited species’ can also be generated over calcium oxide, alumina and certain supported metal oxides (Table 4); various other metal oxides behave like molybdena, destroying the excited species. Based on current evidence, the ‘excited species’ concept seems to be the most plausible explanation for the role of magnesia in the promotion of metathesis reactions. However, it is conceivable that feed contaminants are converted to inert species by magnesia and reconverted to ‘catalyst poisons’ by molybdena. In summary, the dramatic enhancement of activity of tungsten oxidesilica catalysts by magnesia and the failure to generate such enhancement when molybdena, a radical scavenger, is present suggest a new concept in heterogeneous metathesis catalysis. The ‘excited species’ concept provides an additional opportunity for elucidating the nature of surface sites responsible for metathesis.
*EPR
data courtesy
of J.H.
Lunsford.
125
Activity enhancement
by solid reducing metals and elements
The initial activities of most heterogeneous metathesis catalysts can be increased by treating the catalyst with reducing gases (carbon monoxide, hydrogen, hydrocarbons) at elevated temperatures prior to metathesis [ 51. Treatment with metallic sodium is also effective; a significant increase in the metathesis activity of molybdena-alumina was obtained by mixing the catalyst with metallic sodium and heating to a temperature above the melting point of sodium [ 6 1. In recent studies in this area, we have found that other solid reducing metals and elements are also effective ‘activators’ for heterogeneous metathesis catalysts [17, 181. The data in Table 5 show that, in addition to sodium, the following are also effective in increasing the initial activity of conventional tungsten oxidesilica metathesis catalysts: metallic lithium, barium, magnesium, tin, aluminum, silicon, antimony, tungsten, manganese and zinc. These data were obtained by adding elemental metal powders (e.g., 2 - 10%) to the top of the metathesis catalyst bed and treating the metal/catalyst combinations with a flowing inert gas atmosphere for 30 - 60 min at 450 - 720 “C. During the
TABLE 5 Enhancement treatment
in metathesis activity of tungsten oxide-silica
‘Activator’ (elemental metal)
lithium sodium barium magnesium tin magnesium/tin aluminum silicon antimony tungsten manganese zinc
Amount (wt.%)
2 2 5 8 5 10 8 512.5 3 4 2 6 8 27 10
Nitrogen treatment (C) 450 450 720 720 680 670 660 550 680 600 600 600 600 650 625
by reducing metals-nitrogen
Propylene conversion (%)a Control test 34.8 2.1 8.7 2.3 2.3 < Id 17.9 2.3 2.3 17.2 17.9 16.2 16.2 13.5 13.9
With activator 44.6 49.0 34.4 36.9 17.9 17.7d 41.3 17.9 15.5 48.5 32.0 40.9 39.5 21.1 49.0
After calcinationb
9.0
1.3
47.9 25.2 19.3 7.6 4.9
Activity enhancementC
>2 > 100 8 40 11 > 20 7 11 9 > 10 2.4 8 6 2 > 10
aAverage conversion for first-hour operation; tests at 400 - 500 “C, 1 atm pressure, 0.2 1 min-’ (g catalyst)-‘. bMetal/W03*SiOz regenerated in air at 550 - 600 “C. CRatio of activities with and without metal activator. dActivity test at 200 “C.
0.05 -
126
which were essential to achieve enhanced high-temperature treatments, activities, the color of the supported tungsten oxide catalyst changed from yellow to blue or blue-grey, indicating a partial reduction of tungsten oxide. Exposure of the reducing metal/metathesis catalyst to oxygen at high temperatures, as encountered in catalyst regeneration, eliminated the benefit of the metal. However, for the silicon/WOs* SiOz system, activity enhancement did not begin to deteriorate until after several regeneration-reaction cycles; this behavior is attributed to the slow rate of oxidation of silicon. As is evident from the range of conversions for the control tests reported in Table 5, these studies were made at several different test conditions; thus, the effectiveness of the different ‘activators’ cannot be directly compared. Additional data showing the effects of metallic zinc on the activity of tungsten oxide-silica are presented in Fig. 4. Equilibrium conversions of propylene were obtained when 10% and 40% zinc were added to the top of the bed and the mixtures were treated in flowing nitrogen for 45 min at 625 “C. 50
x
QlO%Z”
x
40%
zn
40
30 PROPYLENE CONVERSION, %
20
10
0 30
60 TIME
Fig. 4. Enhancement
in metathesis
90
120
ON STREAM
activity
150
180
(min)
of WO3*SiO2 by metallic
zinc.
With lower amounts of zinc, the level of activity enhancement decreased during the first hour of operation. ESR spectra of zinc/tungsten oxidesilica, obtained in situ at 25 “C, were similar to those obtained with tungsten oxidesilica following carbon monoxide reduction. As already noted, the benefit of metallic zinc was lost when the metal/catalyst combination was regenerated in air at 600 “C. The activity of molybdenum oxide-silica metathesis catalysts is also enhanced by solid reducing metals. As shown by the data in Table 6, adding lithium, sodium, barium, magnesium, or magnesium/tin to MOO,. SiO? and treating in flowing nitrogen for 30 min at 450 - 730 “C increased the initial activity of the metathesis catalyst by a factor of two to three. Silicon was not effective for improving the activity of molybdena-silica systems.
127 TABLE
6
Enhancement in metathesis nitrogen treatment ‘Activator’ (elemental
metal)
none (control) lithium sodium barium magnesium magnesium/tin
activity
Amount (wt.%)
8 2 5 3 913
of molybdenum
oxide-silica
Nitrogen treatment
Propylene conversion
(“CY
(%)b
(550) 450 450 730 660 550
9.8 17.7 15.4 16.0 20.9 22.9
by reducing
Relative activity
1.0 2.2 1.8 2.0 2.8 3.1
aTreatment for 30 min with nitrogen after metals added to top of catalyst bAverage conversion of propylene for first-hour operation. Tests at 350 pressure and propylene flow of 0.1 1 min-’ (g catalyst)-‘.
TABLE
metals-
bed. - 360
C,
1 atm
7
Enhancement Catalyst
in activities
of metathesis
system
catalysts Propylene
(A)
WOs*SiOz WOs l SiOz
(control) plus 3% sulfur
(B)
MoOs*SiOs MoOa*SiOz
(C)
WSz*SiOz (I) (I) after calcination (II) plus 7% sulfur
(control) plus 3% sulfur (II)
by elemental conversion
sulfur
treatmenta
(%)b
Relative activity
8.7 34.3
1 4.2
9.8 20.7
1 2.8
4.4 17.6 34.0
0.2 1 3.0
Vreatment in flowing nitrogen for 30 min at 600 - 620 “C!. bAverage conversion of propylene for first-hour operation at 500 “C, 1 atm pressure 0.1 1 min-’ (g catalyst)-‘.
and
Elemental sulfur was also found to be an effective agent for improving the metathesis activities of supported tungsten oxide and molybdenum oxide catalysts. Placing 3% elemental sulfur on top of tungsten oxide-silica, and heating the mixture in flowing nitrogen for 30 min at 620 ‘C, increased the initial (first hour) activity of the metathesis catalyst by a factor of four (Table 7). The sulfur/nitrogen-treated catalyst was blue-grey, an indication that at least part of the tungsten oxide had been reduced to sub-oxides. Treatment of molybdenum oxide-silica with 3% sulfur increased the initial activity by a factor of three (Table 7(B)). In a control experiment, Table 7(C), tungsten oxide-silica was treated with hydrogen sulfide at 370 “C; the low activity of the sulfided catalyst indicated the active species resulting from the addition of elemental sulfur to the catalyst was not tungsten sulfide. This was further
128
confirmed by calcining the sulfided catalyst at 550 “C (converting WS? to WOs); the relative activity increased from 0.2 to 1, and then to 3 following addition of 7% elemental sulfur. The enhanced metathesis activities obtained in this group of experiments is attributed, at least in part, to partial reduction of the supported tungsten and molybdenum oxides by the added reducing metals or elements.
Activity enhancement of rhenium oxide-thorium
phosphate by oxygen
Early catalyst studies in our laboratory demonstrated that metallophosphates were effective supports or substrates for heterogeneous metathesis catalysts [2]. Recently we re-investigated this class of supports for unique catalytic behavior and found that the rhenium oxide-thorium phosphate system possesses an unusual and interesting catalytic property: its activity for metathesis reactions is enhanced by small amounts of oxygen in the olefin feed. As shown in Fig. 5, propylene conversion jumped more than 50% following the addition of 0.4% oxygen (i.e. 1.9% air) to the propylene feed. When the addition of oxygen was stopped, the conversion dropped rapidly to a level slightly lower than the original conversion. Injections of nitrogen containing 30 - 40 ppm oxygen into the feed caused slight increases in the propylene conversion (Fig. 6). Tests at lOO”C, 0.1 MPa and 12 WHSV
20 PROPYLENE CONVERSION % 10 0 NONE (CONTROL) n 0.4% O2 11.9% Air)
0
Fig. 5. Effects WHSV.)
0
I
I
I
I
I
I
I
I
I
40
00
120
160
200
240
260
320
360
Time On Stream
(mid
of oxygen
on Re207*Th3(PO&
catalyst.
(Tests
at 100 ‘C, 0.1 MPa and 12
The activities of other catalyst systems, including rhenium oxidealumina (Fig. 7) and rhenium oxide-aluminum phosphate, did not increase when oxygen was added to the propylene; in most cases, as shown in Table 8, traces of oxygen were detrimental to metathesis activity. Under the conditions employed in these studies (Le. 100 “C), the rate of decline in activity of
129 Tests at lOO”C, 0.1 MPa and 12 WHSV 304
PROPYLENE CONVERSION % PROPYLENE PLUS 0 NONE (CONTROL1 n 1.2% AIR A 1.8% N2i”30 v 1.6% N2k40
-0
40
00
120
PPm 021 PPm 021 160
200
Time On Stream
240
200
320
360
(mid
Fig. 6. Effects of oxygen and nitrogen on Rez07 -Th3(PO& MPa and 12 WHSV.)
catalyst. (Tests at 100 “C, 0.1
Tests at IQCI??., 0.1 MPa and 12 WHSV
PROPYLENE CONVERSION, %
0
40
60
120
160
Time on Stream
200
240
200
3 D
(min)
Fig. 7. Effects of oxygen on Rez07*A1203 catalyst. (Tests at 100 “C, 0.1 MPa and 12 WHSV.)
conventional rhenium oxide-alumina increased slightly following the addition of oxygen (Fig. 7). Nakamura and Echigoya [ 191 obtained enhanced activity of low-loading (Re/Al = 1:199) rhenium oxide-alumina catalysts by the adsorption of oxygen onto the prereduced catalyst or by injecting oxygen onto the catalyst during the course of the reaction. The adsorption of oxygen yielded a paramagnetic species on the surface, which was assigned as 02- bound to A13+located in the vicinity of the active site Re@” l)+. The unusual behavior of rhenium oxide-thorium phosphate catalysts has fundamental implications; key roles for oxygen ligands have been claimed
130 TABLE
8
Related
metathesis
Catalyst
system
catalysts:
effect
of adding oxygen
Propylene
feeds
(%)a
(no 02)
- 0.3% 02 in feed
7.0 0.2 3.3 0.7
6.9
Control
Re,07.A1P04 Rez07*Th02 Rez07*Si02 Mo03*ThJ(P0&,
conversion
to olefin
=Tests at 100 “C, 0.1 MPa and 12 WHSV.
for both homogeneous and heterogeneous metathesis catalysts. Based on theoretical energy calculations, Rappe and Goddard [9] concluded that ‘spectator’ metal-oxygen bonds are essential for metathesis. Experimental The activities for the metathesis of propylene of various catalysts and combinations of catalysts were obtained at atmospheric pressure (0.1 MPa) in a conventional continuous-flow system with a fixed catalyst bed in a tubular reactor. As depicted in Fig. 1, four types of catalyst configurations were used: (A) conventional (one catalyst or two catalysts mixed); (B) layers or separate zones of two catalysts; (C) layers or separate zones of three different catalysts; and (D) two zones separated by 12 cm. For A, B and C, the tubular quartz reactors were 1.2 cm X 20 cm; for D the reactor was 1.6 cm X 30 cm. A tubular metal reactor was used in several of the experiments reported in Tables 5 and 6. The reactors were positioned in a temperaturecontrolled electric furnace; temperatures were measured in the center of the catalyst bed. The reaction conditions for the metathesis tests, along with the amounts and types of catalyst loading, and the catalyst activation and treatment conditions, are shown in the tables and figures. The catalysts were usually initially activated at 550 “C or 600 “C with dry air (0.2 1 min-‘) for 1 h. The olefin feed (Phillips polymerization grade propylene) passed at ambient temperature through purification columns containing Alcoa’s H151 alumina, 13X molecular sieves and magnesium oxide (0.2 1 of each), through a rotameter and by downward flow through the reactor. The reactor product stream was analyzed on-line automatically, usually every 20 min, using gasliquid chromatography. The expected classical metathesis product distribution was obtained in all cases. The silica-based metathesis catalysts and the silica-supported metal oxides were prepared by impregnating Davison Grade 59 silica gel (20 - 40 mesh size) via the conventional incipient wetness technique with aqueous
131
solutions of appropriate inorganic compounds, followed by drying at 100 “C and calcining in dry air at 500 “C. The silica gel had a surface area of 340 m* gg’ and a pore volume of 1.15 ml g-l. The tungsten oxide catalysts contained 6.0% and 9.2% WOs respectively, and the molybdena catalysts 5.8% and 8.0% MOO, respectively. The dual-promoted catalyst contained 2.9% Moos and 4.6% WOs (the total metal atom concentrations were the same for the three metathesis catalysts compositions used in Table 3). The concentration of metals in the supported metal oxides (Table 4) was 5%. Magnesium oxide, a commercial preparation, had a surface area of 150 m* g-’ and a pore volume of 0.55 ml g-l; it was sized to 20 . 40 mesh before use. Thorium phosphate supports were prepared by combining aqueous solutions of dibasic ammonium phosphate and thorium nitrate, drying at 100 - 200 “C overnight and calcining at 550 “C! for 2 h in flowing air. Supports with surface areas ranging from 17 - 120 m* g-’ were obtained. Rhenium oxide (14%) was added via the conventional incipient wetness technique. The propylene conversion data shown in the tables are average values calculated from product analyses obtained during the time indicated. The relative catalyst activities shown in Tables 2 and 3 are based on the LangmuirHinshelwood model for the reversible metathesis reaction: 2 propylene + ethylene + 2-butene. An average experimental value of 0.42 was used for the fraction of propylene converted at equilibrium.
References 1 R. L. Banks, in C. Kemball and D. A. Dowden (eds.), Specialist Periodical Reports Catalysis, Vol. IV, The Chemical Society, London, 1981, pp. 100 - 129. 2 L. F. Hecklesberg, R. L. Banks and G. C. Bailey, Ind. Eng. Chem., Prod. Res. Dev., 8
(1969) 259. 3 R. L. Banks,
Catalysis; Topics in Current Chemistry, Series No. 5, Springer-Verlag, New York, 1972, p. 39. 4 R. L. Banks, D. S. Banasiak, P. S. Hudson and J. R. Norell, J. Mol. Cotal., 15 (1982) 21. 5 R. L. Banks, Am. Chem. Sot., Div. Pet. Chem. Prepr., 17(3) (1972) A21 - 26. 6 U.S. Pats. 3 865 751 (1975), 3 996 166 (1976) and 4 071 471 (1978) to R. L. Banks and J. R. Kenton. 7 F. Pennella and R. L. Banks, J. Catal., 31 (1973) 304. 8 J. Fathikalajahi and G. B. Wills, J. Mol. Catal., 8 (1980) 127. 9 A. K. Rappe and W. A. Goddard, III, J. Am. Chem. Sot., 104 (1982) 448. 10 R. L. Banks,J. Mol. Catal., 8 (1980) 269. 11 U.S. Pat. 4 188501 (1980) to M. R. Rycheck and F. Pennella. 12 U.S. Pat. 3 546 313 (1970) to R. L. Banks. 13 M. Iwamato and J. H. Lunsford, J. Phys. Chem., 84 (1980) 3079. 14 A. J. Moffat, A. Clark and M. M. Johnson, J. Catal., 22 (1971) 379:’ 15 Y. Takita, M. Iwamoto and J. H. Lunsford, J. Phys. Chem., 84 (1980) 1710. 16 W. Martir and J. H. Lunsford, J. Am. Chem. Sot., 103 (1981) 3728. 17 U.S. Pat. 4 368 471 (1983) to S. G. Kukes. 18 U.S. Pat. 4 465 890 (1984) to S. G. Kukes and R. L. Banks. 19 R. Nakamura and E. Echigoya, J. Mol. Catal., 15 (1982) 147.
of Molecular
Catalysis,
NEW DEVELOPMENTS OF HETEROGENEOUS ROBERT Research
28 (1985)
117 - 131
117
AND CONCEPTS IN ENHANCING ACTIVITIES METATHESIS CATALYSTS
L. BANKS and SIMON G. KUKES and Development,
Phillips Petroleum
Company,
Bartlesville,
OK 74004
(U.S.A.)
Summary Our continuing investigations of heterogeneous metathesis catalysts have led to three findings: (1) Studies of MgO/W03* SiOz combinations suggest that MgO plays an unexpected beneficial role in olefin metathesis catalysis: that of generating gas-phase ‘excited species’ (e.g. ally1 or allyl-0x0 radicals), which are initiators or precursors of metathesis sites, producing dramatic increases in metathesis activities. The proposed new concept is indirectly supported by findings that the activities of Moo3 metathesis catalysts are not likewise enhanced by MgO; more significantly, enhancement of WOJ catalysts does not occur when Moos, a radical scavenger, is present. (2) The catalytic activities of conventional heterogeneous metathesis catalysts are increased significantly by admixing minor amounts of elemental S, Si, Mg, Ba, Sn, Zn, Sb or W with the metathesis catalysts, and treating the admixtures at elevated temperature under an inert atmosphere. The enhanced activity is attributed to a partial reduction of the catalysts by the added reducing metals or elements. (3) An unusual and interesting catalytic behavior is exhibited by rhenium oxide supported on Th3(P0& : the metathesis activity increased by more than 50% when small amounts of oxygen were added to the olefin feed. Other heterogeneous metathesis catalysts, including Re207* AlsO and Re,O,- A1P04, do not show this behavior when oxygen is added; in most cases, traces of oxygen produce a decline in metathesis activity. This unusual behavior has fundamental implications: key roles for oxygen ligands have been claimed for both homogeneous and heterogeneous metathesis catalysts.
Introduction Among the most extensively studied heterogeneous metathesis catalysts are the oxides of tungsten, molybdenum and rhenium supported on high surface area inorganic substrates [ 11.Silica and alumina are the most widely used supports; however, activity with other inorganic substrates, such as metallo-phosphates, has been demonstrated [ 2, 31. Numerous studies to 0304-5102/85/$3.30
0 Elsevier Sequoia/Printed
in The Netherlands
118
elucidate the mechanistic aspects and nature of the active sites of these catalysts and to improve, or extend, their effectiveness for olefin metathesis have been reported [ 11. Various modifications and treatments to increase activity or selectivity have been found: for example, neutralizing acidic-type sites increases the selectivity towards primary metathesis products; addition of nonacidic selective isomerization catalysts shifts the carbon-carbon double bonds of reactants or products (essential for certain applications) [ 41; pre-reduction with reducing gases [5] or metallic sodium [ 61 increases the activity or selectivity of some catalysts; an addition of small amounts of chelating diolefins [7] or ammonia [8] to olefin feed increases the extent of metathesis reactions over tungsten oxide-silica. Key roles for oxygen ligands in metathesis catalysts have been proposed [ 91. In our continuing investigation ofheterogeneous catalysts for metathesistype reactions, three interesting and potentially important findings or observations have emerged: (1) Dramatic activity increases of tungsten oxide-silica in the presence of a magnesia catalyst suggest that catalytic sites on tungsten catalysts are being produced by gas-phase ‘excited species’ generated on the surface of the magnesia. (2) Minor amounts of solid reducing metals or elements are effective agents for increasing the initial activities of conventional tungsten and molybdenum oxide catalysts. (3) Significant increases in the metathesis activities of rhenium oxidethorium phosphate occur when traces of oxygen are added to the olefin feed.
Activity
enhancement
by magnesium
oxide
The incorporation of highly selective double-bond isomerization catalysts with heterogeneous metathesis catalysts is an established technique for obtaining high conversions in certain metathesis applications [ 3, 4, lo]. The assigned role of the isomerization catalyst is to shift the double bonds of symmetrical olefins, permitting ‘productive metathesis’ to occur (e.g., the synthesis of linear olefins), or to maintain an equilibrium concentration among the isomers, thus replenishing the key reactant being depleted in the metathesis step (e.g., ethylene cleavage of /3-diisobutylene in the Phillips commercial neohexene process). Magnesium oxide, a very selective isomerization catalyst, is effective for these applications; it is compatible with tungsten oxide-silica and other heterogeneous metathesis catalysts under both reaction and activation conditions [6]. A second use of magnesia in olefin metathesis catalysts is in the pretreatment of feed streams’to olefin metathesis units: at ambient temperature it is extremely effective for removing from olefins very low concentrations of contaminants (e.g., peroxides and hydroperoxide compounds) [ 61. Commercially, it is used in conjunction with other known adsorptive materials for the purification of olefin feed
119 TABLE 1 Metathesis activities of tungsten oxide-silica Catalyst systema
WOa*Si02 (1.5 g)
Reaction temp. (“C)
combined catalyst systems
Propylene conversion ( %)b
Secondary reaction products (%)c l-Butene
cs plus
400 425 400 250 350 400
4.2 13.2 0.3 30.3 44.3 46.7
trace 0.9
trace trace
0.7 2.4 7.6
0.2 1.6 5.3
MgO (2.6 g)/WOs’SiOz (1.5 gY
400
44.6
1.1
0.7
CaO (3.4 g)/WOa*SiOz (i.5 g)”
400
44.3
1.0
0.6
MgO (3.6 g) MgO (2.6 g)/WOs*SiOz (1.5 g)*
=Activation at 600 “C with air followed by treatment for 15 min with CO. bActivity test at 0.1 MPa and propylene rate of 0.15 1 min-‘. CPercentage converted propylene. *Two catalysts mixed together (Fig. 1A). eCatalysts in separate layers (Fig. 1B; WOs’SiO; on bottom).
streams. Evidence of an additional, and more intriguing, role of magnesium oxide has emerged: results of continued investigations of tungsten oxide and magnesium oxide catalyst combinations suggest that gas-phase ‘excited species’ are generated on the surface of magnesia, and these species initiate, or are precursors of, catalytic sites resulting in metathesis activity increases by as much as lOO-fold. Table 1 provides a comparison of the activities of combined magnesium oxide/tungsten oxide catalyst systems for the metathesis of olefins. At 400 “C!, with tungsten oxide-silica alone, propylene conversion was 4%. Magnesia, at these conditions, had a low activity for olefin metathesis (0.3%). However, a mixture of tungsten oxide-silica and magnesia had very high metathesis activity; equilibrium conversions of propylene were achieved even at 350 “C (50 “C lower than the temperature of the control tests). The yields of secondary products are a measure of the double-bond isomerization activity of the catalyst systems. Although isomerization occurred with the mixed catalyst system (Table l), the data indicate that isomerization was not a major contributing factor to the high propylene conversion. This conclusion was confirmed by placing magnesium oxide in a layer or zone immediately above the tungsten oxide-silica (Fig. 1B); very high metathesis activity was obtained (Table 1) and the yield of secondary products was less than 15% of that obtained with the two catalysts mixed, demonstrating that double-bond isomerization activity was not a major factor contributing to the dramatic activity enhancement. When tungsten oxide-silica was placed ahead of magnesia, propylene conversions (data not shown) were
OLEFIN
PURIFICATION SECTION I25”C)
A
B
Fig. 1. Experimental systems: catalyst bed c:onfigurations.
MgO+ WOySiO2
/ / ,
WOg*SiO2
L:
1 46.7%
T
44.6%
Fig. 2. Comparison of metathesis activities of magnesia and tungsten oxide-silica catalyst combinations.
the same as obtained in similar tests on tungsten oxide-silica alone. A summary of these results appears in Fig. 2. Combined catalyst systems in which calcium oxide, alumina, or basetreated alumina were used in place of magnesia were also found to be very active for olefin metathesis (data for the CaO/W0s*Si02 system shown in Table 1). To determine the temperature range over which the magnesium oxide is most effective for enhancing the activity, tests were made with magnesium oxide placed in a separate zone 12 cm above the tungsten oxide-silica catalyst (Fig. ID). This permitted temperature variation of the magnesium oxide while maintaining the temperature of metathesis constant (e.g. 325 “C). The results (Table 2) indicate that the temperature range of 300 - 400 “C is the most effective for ‘enhancing’ or ‘promoting’ the metathesis activity of tungsten oxide-silica; with magnesia at 300 “C, the metathesis activity was almost two orders of magnitude greater than that obtained with magnesia at 200 “C. Investigation of the effects of magnesium oxide on molybdena-containing metathesis catalysts uncovered an unexpected phenomenon. In contrast to tungsten oxide catalysts, activities of molybdena catalysts were not enhanced by magnesia and, what is possibly of more significance, molybdena prevented magnesia from enhancing the activity of tungsten oxide catalysts. These results are shown in Table 3: without magnesium oxide, the
121 TABLE
2
Effect of magnesium oxide temperature tungsten oxide catalyst systema Magnesium
oxide
temperature
Propylene
on
metathesis
conversion
activity
of magnesium
(“C)
(%)b
Relative metathesis activityC
200 250 300 350 400 450 500
0.7 a.4 31.6 29.6 27.4 23.1 14.8
1 13 90 18 66 50 27
oxide/
aTwo-zone reactor (Fig. 1D) with magnesium oxide (2.0 g) approximately 12 cm above tungsten oxide-silica (0.75 g). Both catalysts activated at 600 “C with air followed by treatment for 15 min with CO. bTemperature of tungsten oxide-silica maintained constant at 325 “C. Other test conditions: 0.1 MPa, propylene rate of 0.15 1 min-‘. CBased on activity of tungsten oxide-silica when magnesium oxide temperature was 200 “C. TABLE Effect
3 of magnesium
oxide
on catalyst
Metathesis catalyst systema
Control:
WOs*SiOs (1.5 g)
systems
containing
molybdenum
oxide
Propylene conversion (%) b
Relative metathesis activityC
Without MgO With MgOd
Without MgO
With MgOd > 10
25.6
43.8f
1.0
MoOa*SiOs (1.5 g)
24.6
25.6
0.9
MoOs*WOs*Si02
(1.5 g)
28.4
29.0
1.2
1.3
MoOs*Si02(0.66
g)/WOs*SiOz(O.66
g)e
14.6
14.0
0.5
0.5
g)e
21.9
44.5
1.1
WOs’Si02(0.66
aActivated
g)/MoOs’Si02(0.66
at 600 “C with air followed
by treatment
1.0
> 10
for 15 min with CO.
bTest at 425 “C, 0.1 MPa and propylene rate of 0.15 1 min-‘. CBased on activity of WOs*SiOz obtained in this series of experiments. d3.0 g MgO on top of metathesis catalysts (Figs. 1B and 1C). eMetathesis catalysts in separate layers (Figs. 1B and 1C). fConversion 39.0% at 375 “C.
metathesis activities of tungsten oxide-silica, molybdenum oxide-silica and molybdenum oxide-tungsten oxide-silica were, essentially the same; with magnesia in a zone ahead of these catalysts, no significant changes in the activities of the molybdena-silica or of the molybdena-tungsten oxide-silica occurred, while the activity of the tungsten oxide-silica catalyst increased by a factor greater than 10. Additional evidence of the detrimental effect of molybdena on the enhancement of activity was provided by inserting a layer of the molybdena-
122
MoO3.Si02
WO3.SiO2
f
25.6% * 43.6%
*PROPYLENE
$
24.6% * 25.6%
CONVERSION
Fig. 3. Comparison metathesis catalysts
MOO3’WO3’ SiOl
MoOg’Si02
21.9%* 44.5%
26.4%* 29.0% WITHOUT
WO3.SiO2
MgO PRESENT.
of metathesis activities of molybdenum with and without magnesia present.
oxide/tungsten
oxide-silica
silica catalyst between magnesia and tungsten oxide-silica (Fig. 1C). Propylene conversions with magnesia and without magnesia were the same (Table 3). The positions of the two metathesis catalysts were then reversed, with tungsten oxide-silica ahead of the molybdena catalysts. The expected enhancement in activity by magnesia was obtained. A summary of these tests is presented in Fig. 3. Various other metal oxides supported on silica have been screened to determine their effect on the activities of both tungsten oxide-silica and magnesia/tungsten oxide-silica catalyst systems (Table 4). In the series without magnesia, in which the supported metal oxides were placed above the tungsten oxide-silica catalyst (Fig. lB), metathesis conversions at least double the control were obtained with the oxides of Ba, Zr, Cr, Ag and Th. With magnesia added to the system (Fig. 1C: i.e., magnesia/metal oxide-silica/ tungsten oxide-silica), oxides of Zn, Zr, K and Cr gave higher propylene conversions, oxides of Th, Sr, Cs, Ba, Pd and Sb had no significant effect, and oxides of Mn, V, Co, Cu, Ag, Re, Bi, Sn, Ni, Fe and Pb reduced propylene conversion. These screening tests were limited to one set of conditions: the behavior of these metal oxides over a range of compositions, pretreatment conditions, or reaction conditions was not investigated. Several explanations for the dramatic increase in metathesis activity occurring when magnesia is combined with tungsten oxide-silica have been proposed. One of the earliest, and one of the simplest suggested reasons was that magnesia, acting as a ‘guard-bed’, removes or destroys traces of contaminants from the feed that are detrimental to metathesis catalysts (e.g., by destroying or hindering the formation of metallocarbene catalytic sites). This explanation stems from earlier feed purification studies of Rycheck and Pennella [ 111. They showed that, with a guard-chamber, placed directly ahead of the reactor, containing calcium oxide at 100 - 300 “C, zirconium dioxide (100 - 300 “C), titanium dioxide (100 - 250 “C), or zinc oxide (100 - 200 “C), the conversion of propylene over tungsten oxide-silica was substantially higher. However, as is evident from the data in Table 3, the failure of magnesia
123 TABLE
4
Effects of various metal oxides on metathesis magnesia/tungsten oxide-silica catalyst systems Supported oxidesa
metal
Propylene
conversion
(Without MgO) b MO;SiOa/WOs.Si02 none (control) unpromoted SiOz ZnO*SiOa Zr02’Si02 K20.Si02 CrOs’Si02 ThOa’SiOz SrO*SiOz CsaO.SiOa BaO*SiOa PdO*SiOa SbsOa*SiOa PbOa’SiOa Fe20s*Si02 NiO*SiOz SnOa’SiOa BiaOs*SiOa Rea07*SiOa Ag,O-SiOa CuO*SiOz CoO*SiOz VaO,*SiOz MnO.SiOa
4.4 3.1
(0.2) (0.3) (1.9)
12.0 6.1 11.0 8.9 6.3 (9.2) 12.3 5.8 (9.2) 4.6 1.6 5.1 (9.3) (6.8) 4.1 9.6
1.6
activity
of
tungsten
oxide-silica
and
(%) (With MgO)C MgO/MO,*SiOa/WOs*SiOa 31.1 31.2 43.1 42.6 42.1 42.0 38.6 38.4 36.0 35.1 33.1 32.0 24.0 11.2 14.8 13.4 11.6 11.1 9.1 6.6 6.5 6.1 2.3
a5% metal, as metal oxide, supported on silica. bFigure 1B: 0.15 g MO,.Si02 on top of 0.15 g WOa’SiOa. Tests at 0.1 MPa, 0.15 1 min-’ propylene at 400 “C, except data in () obtained at 315 “C. Catalyst systems activated and treated with CO (15 min) at 600 “C. CFigure 1C: 2.0 g Mg0/0.15 g MO;Si0,/0.15 g WOs*SiOa. Tests at 315 “C. Catalyst systems activated as noted in footnote b.
to benefit the metathesis activity when molybdena is present makes the ‘guard-bed theory’ difficult to rationalize. Another suggested explanation was that metathesis was occurring over magnesium oxide in addition .to occurring over tungsten oxide-silica. This suggestion also stemmed from previous studies; we have demonstrated that, under certain conditions, magnesium oxide is a metathesis catalyst [12]. Also, Iwamato and Lunsford [13] have implied that metathesis can occur over MgO; at 300 - 450 “C, hydrocarbons having carbon numbers smaller and greater than the reactant (propylene) were formed. However, results of control tests made in connection with the current investigation (i.e., magnesium oxide both alone and downstream from tungsten oxide-silica), along with results of the tests with molybdena-containing catalysts and the sup-
124
ported metal oxides, indicate that, under the conditions employed, metathesis was not occurring to any appreciable extent on the surface of magnesium oxide. A feasible alternative explanation of the activity enhancement phenomenon, supported by indirect experimental evidence, proposed that gas-phase ‘excited species’ are generated on the surface of magnesium oxide, and that these species are initiators or precursors of catalytic metathesis on the tungsten catalysts. Tungsten oxide-silica catalysts are noted for very low site densities [ 141; thus a gas-phase concentration of only a few ppm of postulated species would be sufficient to increase significantly the relative number of metathesis sites, producing dramatic increases in catalytic activity. Several possibilities for the nature of the postulated species exist. These include allyl, oxygen ions and oxygen-containing radicals; all are known to form on the surface of magnesium oxide [13, 151. Results of a preliminary experiment using an EPR-matrix isolation technique [16] indicate that low concentrations of gas-phase n-ally1 radicals are formed when propylene contacts magnesia at 350 “C or 400 “C*; at 400 “C, gas-phase radical formation was about one-tenth that formed over bismuth oxide [16]. The likelihood that oxygen ions or oxygen-containing radicals produced by magnesia are involved in the creation of metathesis sites must be considered; key roles for oxygen ligands have been claimed for both homogeneous and heterogeneous metathesis catalysts (e.g., the spectator oxy group concept advanced by Rappe and Goddard [9]). The most convincing evidence supporting an ‘excited species’ explanation is provided by the ineffectiveness of magnesia in enhancing the metathesis activity when molybdena is present; molybdena is a known radical trap, scavenging radicals produced elsewhere in the system [ 161. Preliminary studies indicate that gas-phase ‘excited species’ can also be generated over calcium oxide, alumina and certain supported metal oxides (Table 4); various other metal oxides behave like molybdena, destroying the excited species. Based on current evidence, the ‘excited species’ concept seems to be the most plausible explanation for the role of magnesia in the promotion of metathesis reactions. However, it is conceivable that feed contaminants are converted to inert species by magnesia and reconverted to ‘catalyst poisons’ by molybdena. In summary, the dramatic enhancement of activity of tungsten oxidesilica catalysts by magnesia and the failure to generate such enhancement when molybdena, a radical scavenger, is present suggest a new concept in heterogeneous metathesis catalysis. The ‘excited species’ concept provides an additional opportunity for elucidating the nature of surface sites responsible for metathesis.
*EPR
data courtesy
of J.H.
Lunsford.
125
Activity enhancement
by solid reducing metals and elements
The initial activities of most heterogeneous metathesis catalysts can be increased by treating the catalyst with reducing gases (carbon monoxide, hydrogen, hydrocarbons) at elevated temperatures prior to metathesis [ 51. Treatment with metallic sodium is also effective; a significant increase in the metathesis activity of molybdena-alumina was obtained by mixing the catalyst with metallic sodium and heating to a temperature above the melting point of sodium [ 6 1. In recent studies in this area, we have found that other solid reducing metals and elements are also effective ‘activators’ for heterogeneous metathesis catalysts [17, 181. The data in Table 5 show that, in addition to sodium, the following are also effective in increasing the initial activity of conventional tungsten oxidesilica metathesis catalysts: metallic lithium, barium, magnesium, tin, aluminum, silicon, antimony, tungsten, manganese and zinc. These data were obtained by adding elemental metal powders (e.g., 2 - 10%) to the top of the metathesis catalyst bed and treating the metal/catalyst combinations with a flowing inert gas atmosphere for 30 - 60 min at 450 - 720 “C. During the
TABLE 5 Enhancement treatment
in metathesis activity of tungsten oxide-silica
‘Activator’ (elemental metal)
lithium sodium barium magnesium tin magnesium/tin aluminum silicon antimony tungsten manganese zinc
Amount (wt.%)
2 2 5 8 5 10 8 512.5 3 4 2 6 8 27 10
Nitrogen treatment (C) 450 450 720 720 680 670 660 550 680 600 600 600 600 650 625
by reducing metals-nitrogen
Propylene conversion (%)a Control test 34.8 2.1 8.7 2.3 2.3 < Id 17.9 2.3 2.3 17.2 17.9 16.2 16.2 13.5 13.9
With activator 44.6 49.0 34.4 36.9 17.9 17.7d 41.3 17.9 15.5 48.5 32.0 40.9 39.5 21.1 49.0
After calcinationb
9.0
1.3
47.9 25.2 19.3 7.6 4.9
Activity enhancementC
>2 > 100 8 40 11 > 20 7 11 9 > 10 2.4 8 6 2 > 10
aAverage conversion for first-hour operation; tests at 400 - 500 “C, 1 atm pressure, 0.2 1 min-’ (g catalyst)-‘. bMetal/W03*SiOz regenerated in air at 550 - 600 “C. CRatio of activities with and without metal activator. dActivity test at 200 “C.
0.05 -
126
which were essential to achieve enhanced high-temperature treatments, activities, the color of the supported tungsten oxide catalyst changed from yellow to blue or blue-grey, indicating a partial reduction of tungsten oxide. Exposure of the reducing metal/metathesis catalyst to oxygen at high temperatures, as encountered in catalyst regeneration, eliminated the benefit of the metal. However, for the silicon/WOs* SiOz system, activity enhancement did not begin to deteriorate until after several regeneration-reaction cycles; this behavior is attributed to the slow rate of oxidation of silicon. As is evident from the range of conversions for the control tests reported in Table 5, these studies were made at several different test conditions; thus, the effectiveness of the different ‘activators’ cannot be directly compared. Additional data showing the effects of metallic zinc on the activity of tungsten oxide-silica are presented in Fig. 4. Equilibrium conversions of propylene were obtained when 10% and 40% zinc were added to the top of the bed and the mixtures were treated in flowing nitrogen for 45 min at 625 “C. 50
x
QlO%Z”
x
40%
zn
40
30 PROPYLENE CONVERSION, %
20
10
0 30
60 TIME
Fig. 4. Enhancement
in metathesis
90
120
ON STREAM
activity
150
180
(min)
of WO3*SiO2 by metallic
zinc.
With lower amounts of zinc, the level of activity enhancement decreased during the first hour of operation. ESR spectra of zinc/tungsten oxidesilica, obtained in situ at 25 “C, were similar to those obtained with tungsten oxidesilica following carbon monoxide reduction. As already noted, the benefit of metallic zinc was lost when the metal/catalyst combination was regenerated in air at 600 “C. The activity of molybdenum oxide-silica metathesis catalysts is also enhanced by solid reducing metals. As shown by the data in Table 6, adding lithium, sodium, barium, magnesium, or magnesium/tin to MOO,. SiO? and treating in flowing nitrogen for 30 min at 450 - 730 “C increased the initial activity of the metathesis catalyst by a factor of two to three. Silicon was not effective for improving the activity of molybdena-silica systems.
127 TABLE
6
Enhancement in metathesis nitrogen treatment ‘Activator’ (elemental
metal)
none (control) lithium sodium barium magnesium magnesium/tin
activity
Amount (wt.%)
8 2 5 3 913
of molybdenum
oxide-silica
Nitrogen treatment
Propylene conversion
(“CY
(%)b
(550) 450 450 730 660 550
9.8 17.7 15.4 16.0 20.9 22.9
by reducing
Relative activity
1.0 2.2 1.8 2.0 2.8 3.1
aTreatment for 30 min with nitrogen after metals added to top of catalyst bAverage conversion of propylene for first-hour operation. Tests at 350 pressure and propylene flow of 0.1 1 min-’ (g catalyst)-‘.
TABLE
metals-
bed. - 360
C,
1 atm
7
Enhancement Catalyst
in activities
of metathesis
system
catalysts Propylene
(A)
WOs*SiOz WOs l SiOz
(control) plus 3% sulfur
(B)
MoOs*SiOs MoOa*SiOz
(C)
WSz*SiOz (I) (I) after calcination (II) plus 7% sulfur
(control) plus 3% sulfur (II)
by elemental conversion
sulfur
treatmenta
(%)b
Relative activity
8.7 34.3
1 4.2
9.8 20.7
1 2.8
4.4 17.6 34.0
0.2 1 3.0
Vreatment in flowing nitrogen for 30 min at 600 - 620 “C!. bAverage conversion of propylene for first-hour operation at 500 “C, 1 atm pressure 0.1 1 min-’ (g catalyst)-‘.
and
Elemental sulfur was also found to be an effective agent for improving the metathesis activities of supported tungsten oxide and molybdenum oxide catalysts. Placing 3% elemental sulfur on top of tungsten oxide-silica, and heating the mixture in flowing nitrogen for 30 min at 620 ‘C, increased the initial (first hour) activity of the metathesis catalyst by a factor of four (Table 7). The sulfur/nitrogen-treated catalyst was blue-grey, an indication that at least part of the tungsten oxide had been reduced to sub-oxides. Treatment of molybdenum oxide-silica with 3% sulfur increased the initial activity by a factor of three (Table 7(B)). In a control experiment, Table 7(C), tungsten oxide-silica was treated with hydrogen sulfide at 370 “C; the low activity of the sulfided catalyst indicated the active species resulting from the addition of elemental sulfur to the catalyst was not tungsten sulfide. This was further
128
confirmed by calcining the sulfided catalyst at 550 “C (converting WS? to WOs); the relative activity increased from 0.2 to 1, and then to 3 following addition of 7% elemental sulfur. The enhanced metathesis activities obtained in this group of experiments is attributed, at least in part, to partial reduction of the supported tungsten and molybdenum oxides by the added reducing metals or elements.
Activity enhancement of rhenium oxide-thorium
phosphate by oxygen
Early catalyst studies in our laboratory demonstrated that metallophosphates were effective supports or substrates for heterogeneous metathesis catalysts [2]. Recently we re-investigated this class of supports for unique catalytic behavior and found that the rhenium oxide-thorium phosphate system possesses an unusual and interesting catalytic property: its activity for metathesis reactions is enhanced by small amounts of oxygen in the olefin feed. As shown in Fig. 5, propylene conversion jumped more than 50% following the addition of 0.4% oxygen (i.e. 1.9% air) to the propylene feed. When the addition of oxygen was stopped, the conversion dropped rapidly to a level slightly lower than the original conversion. Injections of nitrogen containing 30 - 40 ppm oxygen into the feed caused slight increases in the propylene conversion (Fig. 6). Tests at lOO”C, 0.1 MPa and 12 WHSV
20 PROPYLENE CONVERSION % 10 0 NONE (CONTROL) n 0.4% O2 11.9% Air)
0
Fig. 5. Effects WHSV.)
0
I
I
I
I
I
I
I
I
I
40
00
120
160
200
240
260
320
360
Time On Stream
(mid
of oxygen
on Re207*Th3(PO&
catalyst.
(Tests
at 100 ‘C, 0.1 MPa and 12
The activities of other catalyst systems, including rhenium oxidealumina (Fig. 7) and rhenium oxide-aluminum phosphate, did not increase when oxygen was added to the propylene; in most cases, as shown in Table 8, traces of oxygen were detrimental to metathesis activity. Under the conditions employed in these studies (Le. 100 “C), the rate of decline in activity of
129 Tests at lOO”C, 0.1 MPa and 12 WHSV 304
PROPYLENE CONVERSION % PROPYLENE PLUS 0 NONE (CONTROL1 n 1.2% AIR A 1.8% N2i”30 v 1.6% N2k40
-0
40
00
120
PPm 021 PPm 021 160
200
Time On Stream
240
200
320
360
(mid
Fig. 6. Effects of oxygen and nitrogen on Rez07 -Th3(PO& MPa and 12 WHSV.)
catalyst. (Tests at 100 “C, 0.1
Tests at IQCI??., 0.1 MPa and 12 WHSV
PROPYLENE CONVERSION, %
0
40
60
120
160
Time on Stream
200
240
200
3 D
(min)
Fig. 7. Effects of oxygen on Rez07*A1203 catalyst. (Tests at 100 “C, 0.1 MPa and 12 WHSV.)
conventional rhenium oxide-alumina increased slightly following the addition of oxygen (Fig. 7). Nakamura and Echigoya [ 191 obtained enhanced activity of low-loading (Re/Al = 1:199) rhenium oxide-alumina catalysts by the adsorption of oxygen onto the prereduced catalyst or by injecting oxygen onto the catalyst during the course of the reaction. The adsorption of oxygen yielded a paramagnetic species on the surface, which was assigned as 02- bound to A13+located in the vicinity of the active site Re@” l)+. The unusual behavior of rhenium oxide-thorium phosphate catalysts has fundamental implications; key roles for oxygen ligands have been claimed
130 TABLE
8
Related
metathesis
Catalyst
system
catalysts:
effect
of adding oxygen
Propylene
feeds
(%)a
(no 02)
- 0.3% 02 in feed
7.0 0.2 3.3 0.7
6.9
Control
Re,07.A1P04 Rez07*Th02 Rez07*Si02 Mo03*ThJ(P0&,
conversion
to olefin
=Tests at 100 “C, 0.1 MPa and 12 WHSV.
for both homogeneous and heterogeneous metathesis catalysts. Based on theoretical energy calculations, Rappe and Goddard [9] concluded that ‘spectator’ metal-oxygen bonds are essential for metathesis. Experimental The activities for the metathesis of propylene of various catalysts and combinations of catalysts were obtained at atmospheric pressure (0.1 MPa) in a conventional continuous-flow system with a fixed catalyst bed in a tubular reactor. As depicted in Fig. 1, four types of catalyst configurations were used: (A) conventional (one catalyst or two catalysts mixed); (B) layers or separate zones of two catalysts; (C) layers or separate zones of three different catalysts; and (D) two zones separated by 12 cm. For A, B and C, the tubular quartz reactors were 1.2 cm X 20 cm; for D the reactor was 1.6 cm X 30 cm. A tubular metal reactor was used in several of the experiments reported in Tables 5 and 6. The reactors were positioned in a temperaturecontrolled electric furnace; temperatures were measured in the center of the catalyst bed. The reaction conditions for the metathesis tests, along with the amounts and types of catalyst loading, and the catalyst activation and treatment conditions, are shown in the tables and figures. The catalysts were usually initially activated at 550 “C or 600 “C with dry air (0.2 1 min-‘) for 1 h. The olefin feed (Phillips polymerization grade propylene) passed at ambient temperature through purification columns containing Alcoa’s H151 alumina, 13X molecular sieves and magnesium oxide (0.2 1 of each), through a rotameter and by downward flow through the reactor. The reactor product stream was analyzed on-line automatically, usually every 20 min, using gasliquid chromatography. The expected classical metathesis product distribution was obtained in all cases. The silica-based metathesis catalysts and the silica-supported metal oxides were prepared by impregnating Davison Grade 59 silica gel (20 - 40 mesh size) via the conventional incipient wetness technique with aqueous
131
solutions of appropriate inorganic compounds, followed by drying at 100 “C and calcining in dry air at 500 “C. The silica gel had a surface area of 340 m* gg’ and a pore volume of 1.15 ml g-l. The tungsten oxide catalysts contained 6.0% and 9.2% WOs respectively, and the molybdena catalysts 5.8% and 8.0% MOO, respectively. The dual-promoted catalyst contained 2.9% Moos and 4.6% WOs (the total metal atom concentrations were the same for the three metathesis catalysts compositions used in Table 3). The concentration of metals in the supported metal oxides (Table 4) was 5%. Magnesium oxide, a commercial preparation, had a surface area of 150 m* g-’ and a pore volume of 0.55 ml g-l; it was sized to 20 . 40 mesh before use. Thorium phosphate supports were prepared by combining aqueous solutions of dibasic ammonium phosphate and thorium nitrate, drying at 100 - 200 “C overnight and calcining at 550 “C! for 2 h in flowing air. Supports with surface areas ranging from 17 - 120 m* g-’ were obtained. Rhenium oxide (14%) was added via the conventional incipient wetness technique. The propylene conversion data shown in the tables are average values calculated from product analyses obtained during the time indicated. The relative catalyst activities shown in Tables 2 and 3 are based on the LangmuirHinshelwood model for the reversible metathesis reaction: 2 propylene + ethylene + 2-butene. An average experimental value of 0.42 was used for the fraction of propylene converted at equilibrium.
References 1 R. L. Banks, in C. Kemball and D. A. Dowden (eds.), Specialist Periodical Reports Catalysis, Vol. IV, The Chemical Society, London, 1981, pp. 100 - 129. 2 L. F. Hecklesberg, R. L. Banks and G. C. Bailey, Ind. Eng. Chem., Prod. Res. Dev., 8
(1969) 259. 3 R. L. Banks,
Catalysis; Topics in Current Chemistry, Series No. 5, Springer-Verlag, New York, 1972, p. 39. 4 R. L. Banks, D. S. Banasiak, P. S. Hudson and J. R. Norell, J. Mol. Cotal., 15 (1982) 21. 5 R. L. Banks, Am. Chem. Sot., Div. Pet. Chem. Prepr., 17(3) (1972) A21 - 26. 6 U.S. Pats. 3 865 751 (1975), 3 996 166 (1976) and 4 071 471 (1978) to R. L. Banks and J. R. Kenton. 7 F. Pennella and R. L. Banks, J. Catal., 31 (1973) 304. 8 J. Fathikalajahi and G. B. Wills, J. Mol. Catal., 8 (1980) 127. 9 A. K. Rappe and W. A. Goddard, III, J. Am. Chem. Sot., 104 (1982) 448. 10 R. L. Banks,J. Mol. Catal., 8 (1980) 269. 11 U.S. Pat. 4 188501 (1980) to M. R. Rycheck and F. Pennella. 12 U.S. Pat. 3 546 313 (1970) to R. L. Banks. 13 M. Iwamato and J. H. Lunsford, J. Phys. Chem., 84 (1980) 3079. 14 A. J. Moffat, A. Clark and M. M. Johnson, J. Catal., 22 (1971) 379:’ 15 Y. Takita, M. Iwamoto and J. H. Lunsford, J. Phys. Chem., 84 (1980) 1710. 16 W. Martir and J. H. Lunsford, J. Am. Chem. Sot., 103 (1981) 3728. 17 U.S. Pat. 4 368 471 (1983) to S. G. Kukes. 18 U.S. Pat. 4 465 890 (1984) to S. G. Kukes and R. L. Banks. 19 R. Nakamura and E. Echigoya, J. Mol. Catal., 15 (1982) 147.