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Conversion of C2–C3 alkanes on heteropoly oxometalates
~
A PT PA LE IY DSS CA L I A: GENERAL
ELSEVIER
Applied Catalysis A: General 146 (1996) 65-86
Conversion of C2-C 5 alkanes on heteropoly oxometalates J.B. Moffat Department of Chemistry and the Guelph-Waterloo Centre for Graduate Work in Chemistry, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1
Abstract
Studies of the conversion of ethane and isobutane on heteropoly oxometalates are reviewed. The mechanisms of the processes are shown to be related to the elemental composition of the anion of the catalyst and the acidic strength of the solid acids. With unsupported heteropoly oxometalates the charge balancing cations play a prominent role particularly in leading to microporous structures. Although participation of the gas phase undoubtedly occurs in the aforementioned processes, the catalyst plays an important role in the C - H bond scission. Both the conversion and selectivities in these processes are related to the strength of the terminal oxygen-anionic metal bond which itself is dependent upon the nature of the peripheral metal in the anion of the catalyst. The reductant employed in the oxidation processes appears to act primarily as a regenerator of the active oxygen species in the catalyst. Keywords: Heteropoly oxometalates; Ethane; Isobutane; Metal-oxygen cluster compounds
1. Introduction
Research in catalysis is frequently driven by market and, most recently, environmental requirements. In many instances, while resource materials may be sold and employed as such, the catalytic conversion of these may result in the formation of products which are more immediately usable and the sale of which is more profitable even after subtraction of the production costs. There is also considerable impetus to find new environmentally-friendly catalysts to replace those which, for a number of reasons, may be currently less acceptable. Thus, for example, the conversion of alkanes to a variety of chemical products has attracted considerable attention in recent years [1-39]. The present contribution focuses on the conversion of alkanes with two or more carbon atoms with 0926-860X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. Pll S0926- 860X(96)00169-X
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J.B. Moffat /Applied Catalysis A: General 146 (1996) 65-86
metal-oxygen cluster compounds (also known as heteropoly oxometalates) as heterogeneous catalysts for such purposes.
2. Metal-oxygen cluster compounds (heteropoly oxometalates) Metal-oxygen cluster compounds (MOCC) are ionic solids with discrete cations and anions. While a wide variety of compositions and structures has been identified [40] only those of Keggin structure will be considered here. Although a number of isomers of the Keggin structure is known, since the oL form is generally more stable, discussion will be restricted to this isomer. The Keggin anion has overall T0 symmetry with a central atom such as phosphorus, silicon or one of a large number of elements bonded to four oxygen atoms arranged tetrahedrally around the central atom (Fig. 1). Twelve octahedra with a peripheral metal atom, such as tungsten, at each of their approximate centres and oxygen atoms at their vertices, surround the central tetrahedron and are arranged in four groups of three edge-shared octahedra, M30~3. Three types of oxygen atoms are present in the anion: those which join the central atom to a peripheral metal atom, those which connect two of the latter and those, of which there are twelve, which are bonded to the peripheral metal atom only and protrude from the anion. A variety of cations is possible, ranging from single atom to multiatom-multielement species. With the proton, heteropoly acids are formed. Although considerable powder X-ray diffraction data exist, relatively little single crystal
Fig. 1. The Keggin structure for the ~-[XMI2O40 ]n- anion. Bigger circles, central and peripheral atoms; smaller circles, oxygen atoms.
J.B. Moffat / Applied Catalysis A: General 146 (1996) 65-86
67
information is available. X-ray and neutron diffraction studies of the six-hydrate of 12-tungstophosphoric acid (H3PW1204o. 6H20) have shown that this solid acid has a cubic (Pn3m) structure with each proton hydrogen-bonded to two water molecules in an approximately coplanar configuration and the water molecules hydrogen-bonded to the terminal oxygen atoms of the anions (Fig. 2) [41 ]. Single-crystal data is also available for 12-molybdophosphoric acid [42-45]. Although the heteropoly acids appear to have open structures, the BET N 2 surface areas are low (Table 1). However, photoacoustic (PAS) FTIR measurements have shown that polar molecules such as ammonia are capable of penetrating into the bulk structure of these solids [46]. Thus, on exposure of 12-tungstophosphoric acid (HPW) to gaseous ammonia, molecules of the latter penetrate into the solid to interact with the protons, forming NH~- ions, as evidenced from the IR band at 1429 cm-1, until three ammonia molecules per anion have entered the structure (Fig. 3). The phenomenon observed with ammonia is also found with other polar molecules such as pyridine and methanol [47,48].
Fig. 2. Crystallographic arrangement of protons, anions, and water molecules in hydrated 12-tungstophosphoric acid [41].
J.B. Moffat / Applied Catalysis A: General 146 (1996) 65-86
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Table 1 Surface areas a o f heteropoly oxometalates Cation
H Na Ag K NH 4 Rb Cs MeNH 3 Me 4 N BETN 278K,m
Anion S i W 1204~14
P M o j 2 0403
PWl 204o 3
AsW~ 2 0403
3.2 1.3 3.3 116.9 116.3 153.4 3.0 -
8 3.5 39.9 193.4 145.5 1.3 -
8 3.7 3.0 90.0 128.2 162.9 3.0 4.5
6.5 46.0 82.1 101.4 65.4 2.1 -
2 g-l.
The heteropoly acids contain Br/Jnsted acid sites but no evidence of Lewis acid sites [46,47]. Titrations with Hammett indicators have shown the presence of very strong BriSnsted acid sites. Microcalorimetric measurements of the
4oo
'
32bo
'
a,~o
'
~6bo
'
8 b o c M -~
Fig. 3. P A S spectra o f (a) 12-tungstophosphoric acid pre-evacuated at 473 K, ( b ) - ( e ) after stepwise dosing with a m m o n i a at 423 K. (The a m m o n i u m salt (f) is included for comparison).
J.B. Moffat / Applied Catalysis A: General 146 (1996) 65-86
69
250
2OO
a-,
-~150
2
"-D
'-~100
C~ 5O
I
0(~
1
I
2 ' MOLECULES
I
3 NH a / KU
'
t
4
'
5
Fig. 4. Differential heats of ammonia sorption on HPW at 323 K. (1) Activation: 423 K / 2 h/UHV; (2) activation: 523 K / 1 h / U H V .
differential heat of adsorption of ammonia [49] produce values of 150-200 kJ mol -~ in the same range ( > 120 kJ mol -~) as suggested to be indicative of superacidity (Fig. 4) [50,51]. The thermal stability of the metal-oxygen cluster compounds is found to be dependent on both the composition of the anion and the nature of the cation (Table 2) [52]. Photoacoustic FTIR spectra of HPW after heating at various temperatures show that the set of bands below 1200 cm -1 attributed to the Keggin structure, although somewhat diminished in intensity, nevertheless re-
Table 2 DTAs of alkylammonium 12-heteropoly salts R
RPW salts Pr4N Me 4 N Me3NH MezNH 2 MeNH 3 NH 4 RPMo salts Pr4N Me4N Me3N H Me 2NH 2 MeNH 3 NH 4
DTA peaks (K) a Exo
Endo
765m, br 808m, br 763m, br; 788m, br 738m, br; 790m, br 728w, br; 788m, br Stable to at least 823 K
51 lw, br; 698w, br b 485w; 549w b b
51 lw b
496w; 504w; 539w; 556w; 663s 523W, br; 661m; 691s 583W; 595W; 668m 606m; 663m, br 631m b
b
578w; 596m 598m; 615m, br 773s
a w, Weak; m, medium; s, strong; br, broad. b No peak observed below 873 K.
J.B. Moffat /Applied Catalysis A: General 146 (1996) 65-86
70
H3PW12040-n H20
.~
c)
t/)
~ Ii~, ~L-
*
Z
LII),I). C. . . .
350 ° ^
a) 25*
] ~ 0J_. '
,
2__J_ ~ld0.
/\
I
1$d11. J
, . 11~1~ '
M-].
Fig. 5. PAS FTIR spectra of 12-tungstophosphoric acid showing characteristic bands in 1100-800 cm I region and effect of heating in vacuo.
main at 450°C (Fig. 5) [46]. In particular, the bands at 1080 and 980 cm -1 associated with the triply degenerate asymmetric stretching vibration of the central P O 4 tetrahedron and a stretching vibration of the terminal oxygen atoms of the anion, respectively, are evident at 450°C. Temperature-programmed desorption experiments with the heteropoly acids HPW, 12-molybdophosphoric acid (HPMo) and 12-tungstosilicic acid (HSiW) show that the water molecules hydrogen-bonded to the protons and to the anions are desorbed from the solid acids at temperatures near 200°C (Fig. 6) [53]. Additionally, at temperatures between 400 and 600°C, dependent upon the elemental composition, water resulting from the extraction of anionic oxygen atoms by protons, is associatively desorbed. As a result of the low surface areas of the acids, the rates of reactions employing these as heterogeneous catalysts with nonpolar molecules such as methane are relatively low [54]. Consequently, it is advantageous to enhance the contact between reactants and catalyst by supporting the solid acid on a high surface area material. In studies of the partial oxidation of methane on various of the MOCC evidence was found for the enhancement of the thermal stability of the MOCC, apparently resulting from the strong interaction between the latter and the silica support [55].
J.B. Moffat / Applied Catalysis A." General 146 (1996) 65-86
71
d uJ (n z 0 (1) uJ no O~ 0 UJ I14J
I 773
I
I 573 TEMP/aK
I
I 373
Fig. 6. Temperature-programmed desorption profiles for 12-tungstophosphoric acid, 12-tungstosilicic acid, and 12-molybdophosphoric acid after pretreatment at 25°C for 16 h.
Further information on the supported heteropoly acids was obtained from the first application of 31p NMR to such systems which was published in 1990 [56,57]. In that work 3]p NMR spectra of 23 wt.-% H P M o / S i O 2 samples which had been heated at a number of temperatures for various periods of time showed one relatively broad peak whose positions and full widths at half maximum (FWHM) are recorded in Table 3. The samples heated at temperatures of 550°C or less showed peaks at positions consistent with the 3]p NMR spectra of heteropolymolybdates [40]. Since after use in a methane oxidation test reaction
Table 3 Effect of pretreatment conditions on the 31p NMR parameters of silica supported 12-molybdophosphoric acid catalyst a Pretreatment
NMR 3z p peak position (ppm)
FWHM (Hz)
Bulk HPMo 200°C, 2 h 350°C, 16 h 450°C, 16 h 550°C, 16 h (570°C, 4 h) b 600°C, 16h 730°C, 16 h
- 8 - 8 - 8 -8 - 8 -- 8 -5 -5
1.5 1.0 1.2 1.1 1.2 1.1 1.8 9.4
a 23 wt.-% H P M o / S i O 2 calcined at the temperatures and for the periods of time shown. b Methane oxidation at conditions shown.
J.B. Moffat / Applied Catalysis A: General 146 (1996) 65-86
72
Table 4 Effect of loading of H P M o / S i O 2 a on 31p NMR parameters Loading (wt.-%)
NMR 31p peak position (ppm)
FWHM (Hz)
Bulk HPMo
- 8
1.5
1.2 8.3 16. 31 39
-8 -8 -8 -8 -8
1.5 1.2
1.0 1.1 4.1
a Pretreated at 350°C for 2 h.
at 570°C for 4 h the position and FWHM of the 3 1 p NMR line are unchanged from those observed for HPMo at lower temperatures, the structure of the heteropoly anion is retained under catalytic operating conditions. For temperatures of 600 ° and higher it is clear, however, that the anion is no longer in existence. 31p NMR spectra of H P M o / S i O 2 samples of various loadings which had previously been calcined at 350 ° for 2 h again showed peaks characteristic of bulk HPMo up to and including a loading of 31 wt.-% HPMo (Table 4). Information on the dispersion of the supported species on the silica surface has been obtained from X-ray photoelectron spectroscopy [56] (Fig. 7). The
1.0 J
Q.
co
I
::~ (,ej 0.8-
s
i__
+
0
0 .< n-
/J t
0.6-
0
~(/.30-4" 7
ILl I-" ~-~ 0.2O3 Q_
X 0.0
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0 14
0.16
0 18
0.20
0.22
0.24
HPMo LOADING (KU.nm-2) Fig. 7. Evolution of the XPS Mo3d/Sizp intensity ratio versus the !2-mo]ybdophosphoric acid loading for the series of catalysts calcined at 350°C (C)), 2 h and after catalytic test at 570°C, 3 h ( + ) , as calculated for a monolayer dispersion (---).
J.B. Moffat / Applied Catalysis A: General 146 (1996) 65-86
73
MO3d/Si2p intensity ratio increases linearly with the loading up to approximately 0.04 KU nm -2 or approximately l0 wt.-% HPMo, with a slope similar to that of the calculated line for a monolayer dispersion. For this loading range single anions or perhaps small aggregates of these are dispersed on the surface. For higher loadings a change in slope signals the appearance of larger aggregates up to a plateau at 0.10 K U / n m 2 or approximately 23 wt.-% HPMo, at which loading large aggregates are formed. After use in a methane oxidation reaction the curve for the intensity ratio has a similar shape but is shifted to lower values, indicative of the small loss of molybdenum during the reaction. Subsequently, 31p spin-lattice relaxation measurements on H P M o / S i O 2 have shown that the apparent monolayer is reached at a loading of approximately 35 wt.-% HPMo [57]. The results of the aforementioned studies show that HPMo can be deposited uniformly on the surface of silica in a highly dispersed form up to a loading of approximately 10 wt.-% [56]. For loadings between this and 25%, aggregates begin to form, while at higher loadings particles of the acid are present.
3. Oxidative dehydrogenation of ethane The first report on the conversion of ethane on MOCC appeared in 1994 [34]. Ethane is converted to both ethylene and acetaldehyde, on MOCC supported on silica, at reaction temperatures between 450 and 570°C. With N20 as the oxidant the conversion of ethane is strongly dependent on the elemental composition of the SiO2-supported heteropoly acids, with those containing molybdenum producing the highest conversion. The unsupported HPMo yields a conversion a factor of ten smaller than that obtained with the SiO2-supported acid. With oxygen as an oxidant the conversion on H P M o / S i O 2 is higher than that with N20 but no acetaldehyde is detected with the former oxidant. As previously observed with methane, the loading plays a significant role, particularly with the conversions (Fig. 8). The conversion of ethane increases with loading, reaching a maximum for 20-25 wt.-% HPMo on silica, and decreases. Concomitantly, the selectivities to C2H4, CO and CO 2 increase, while that to acetaldehyde decreases. With decreasing contact time, the conversion of C2H6, with N20 as oxidant, decreased, the selectivities to acetaldehyde and ethylene increased and that to CO x decreased, implying that acetaldehyde and ethylene are primary products (not shown). At conversions of approximately 3% the selectivities to acetaldehyde and ethylene sum to greater than 70%. With 20% H P M o / S i O 2 and N20 as oxidant, an increase in reaction temperature produces increases in conversion and selectivities to CO x at the expense of acetaldehyde. With oxygen as an oxidant the production of CO is favoured at all reaction temperatures.
J.B. Moffat /Applied Catalysis A: General 146 (1996) 65-86
74 60
o .-n a.~ 4 0
2
-.lm
4-,
0
m ¢~ 20
1
0
b 10
0
, 20
, 3O
0 4O
LoadinglwtZl Fig. 8. Effect of loading of HPMo on conversion of ethane and selectivity over supported catalyst; TR = 540°C, W = 0.5 g, F = 25 ml/min, C2H 6 / N 2 0 = 4 / 1 , ( O ) conversion of C2H 6, (zx) selectivity of C 2H4, (open hexagon) CH 3CHO, ('7) CO,, ( ~ ) CO, (cLosed hexagon) yield of CH~CHO.
In the absence of an oxidant acetaldehyde is formed on the catalyst from ethane during the initial stages of the reaction but is no longer found in the product stream after a number of hours on stream while, concomitantly, the selectivity to ethylene increases (Fig. 9). This suggests that the formation of acetaldehyde requires the participation of the catalyst while ethylene does not. Further, when ethane and nitrous oxide react in the absence of the catalyst no II
100
II
9retreatment ¢3 o b~
--t mo o -,=
.~ 50 ¢J
1
c)
pretreatme6t~]
¢/)
0
1
2
.with 0 2
i /
4
6
5
o
=z
8
9
Time on Stream(hr) Fig. 9. Effect of time on stream on ethane conversion and selectivity over 20 wt.-% HPMo/SiO2: TR = 510°C, W = 1.00 g, F = 5 ml/min, C2H 6 only and no oxidants, ( 0 ) conversion of C2H 6, ( ~ ) selectivity of CO, ( V ) C02, (~.) C2H4, (hexagon) CH3CHO.
J.B. Moffat / Applied Catalysis A: General 146 (1996) 65-86
75
60 50
4O •~
2
30
q}
i
9,,
o
2o
10
450
t soo
Pretreatment
i
5so
i 800
i 850
0
Temperature[*CI
Fig. 10. Effect of pretreatment temperature on ethane conversion and selectivity over 20 wt.-% HPMo/SiO2: TR = 540°C, W = 0.5 g, F = 25 ml/min, C2H 6 / N 2 0 = 4 / 1 , ( 0 ) conversion of C~H 6, (O) selectivity of CO, (~7) CO2, (/x) C2H4 ' (hexagon) CH3CHO.
acetaldehyde is observed. Since the initial selectivity to acetaldehyde in the absence of an oxidant is similar to that observed in the presence of an oxidant and since no acetaldehyde is produced in the absence of a catalyst, it can be concluded that there is little or no participation of a homogeneous process in the generation of this product. Thus, the oxidant is apparently acting as a regenerator of the active sites on the catalyst for the production of acetaldehyde. Since nitrous oxide restores the ability of the catalyst to produce acetaldehyde, while oxygen does not, the former is more effective than the latter in replacing the lost oxidation sites of the catalyst. The dependence of the conversion of ethane and the selectivities to the various products on the pretreatment temperature provide further evidence for the enhancement of the thermal stability of 12-molybdophosphoric acid when supported on silica (Fig. 10). Up to a pretreatment temperature of 500°C the conversion of ethane and the selectivities to the various products remains unchanged. The conversion and selectivity to acetaldehyde begin to decrease between 500 and 550°C while the selectivities to the carbon oxides begin to increase. The selectivity to ethylene remains relatively unchanged for pretreatment temperatures of 450-650°C. These observations are similar to those found with methane, as noted above. Analogously to the results found with methane it is expected that the oxidation of ethane proceeds through a radical mechanism [17] involving either the bridged or the terminal oxygen of the anion. C2H 6 + O-----) C2H 5 + O H -
76
J.B. Moffat / Applied Catalysis A: General 146 (1996) 65-86 0.0
<
,1u =E -1.0
10 e,. w \
5
\\ \ \
a z
o m
o
PW a
SiW b
PMo c
Fig. 11. Magnitude of negative charges on bridging (b) and outer (o) oxygen atoms; binding energies between terminal oxygen atoms and peripheral metal atoms in the anions shown in abbreviated notation.
The ethyl radicals may enter the gas phase where they suffer deep oxidation. Alternatively, the ethyl radicals may remain in contact with the anion where ethylation occurs. Extended Hiickel calculations have shown that the terminal oxygen atoms of anions containing molybdenum in the peripheral metal positions are more labile than those where tungsten occupies the latter positions (Fig. 11) [58]. This may explain the markedly different results for the conversion of ethane obtained with the supported acids containing these two metals. Nitrous oxide may decompose in the gas phase to form O - [7,25] or may interact directly with the Keggin anion to regenerate the depleted oxygen in a manner similar to that proposed by Erdohelyi et al. [25] for silica-supported alkali molybdate catalysts. N20 + M o S + ~ N 2 -q- M o 6 + _ O where the Mo occupy the peripheral metal positions in the Keggin anion. Supplementary experiments have shown that both ethylene and ethanol are converted by N 2 0 in the presence of HPMo/SiO 2 to, among other products, acetaldehyde. Since earlier work has shown that both methanol and formaldehyde are formed in the oxidation of methane on HPMo/SiO 2 it appears reasonable to suggest that ethanol may be a primary product in the oxidative conversion of ethane, albeit one whose lifetime under the conditions of the
J.B. Moffat / Applied Catalysis A: General 146 (1996) 65-86
77
reaction is relatively short. Ethanol could form from the protons in HPMo and the ethoxy group resulting from the ethylation of the Keggin anion or from the reaction of the ethoxy group with water. A flow diagram consistent with the aforementioned observations shows acetaldehyde being derived from ethanol and ethylene with the latter forming from ethanol: C~H4
C2I~
-~
C2HsOH
--o
CHsCHO
,,,
Z CO, COz
The ratios of ethylene/acetaldehyde and C O / C O 2 from ethanol are 1.8 and 2.9, respectively, while under similar reaction conditions those from ethane are 1.6 and 2.5, respectively. Although not conclusive, these observations provide some support for the contention that the conversion of ethane proceeds through ethanol. A somewhat similar mechanism has been suggested for the conversion of ethane on B203-AI203 [13] and B203-P205 [30] although, in contrast with the present results, oxidation of ethylene on these catalysts did not produce acetaldehyde, formaldehyde or carbon monoxide. Earlier work on the partial oxidation of methane on the metal-oxygen cluster compounds showed that the introduction of small quantities of tetrachloromethane (TCM) produced enhanced conversions and selectivities to C 2 hydrocarbons, particularly ethylene [59-61]. Similar studies with ethane demonstrated that the conversion of ethane and the selectivity to CO 2 increased on addition of TCM while the selectivities to ethylene and CO decreased [62]. Small quantities of ethyl chloride were also produced. Separate experiments with ethylene and nitrous oxide showed that the introduction of 0.2 mol-% of TCM into the feedstream increased the selectivity to acetaldehyde by a factor of four, from 13.3 to 54.9%, at a conversion of 4.5 __ 0.3%. Since earlier experiments on the conversion of methane in the presence of TCM showed that strong interaction of the latter with the surface of the catalyst was occurring [59-61], the chloromethane is evidently altering the heterogeneous process although the possibility of some influence on the purely gas phase reactions cannot be excluded. Most recently, the conversion of ethane has been examined on salts of molybdophosphoric acid with both potassium and ammonium cations [63]. The Keggin anion was modified by substitution of one or two antimony atoms for molybdenum producing a thermal stabilization of the structure by 50-100 degrees. However, the addition of antimony led to a decrease in the catalytic activity as compared with the antimony-free compound. In all cases, the
78
J.B. Moffat / Applied Catalysis A: General 146 (1996) 65-86
conversions of ethane ranged from 1 to 7% while the selectivities to ethylene were 34-56%. No evidence for the formation of acetaldehyde was reported. With samples containing potassium, iron and antimony the selectivities to ethylene reached as high as 86% at a conversion of 2.6%.
4. Conversion of C 4 alkanes
Studies of the conversion of butane and isobutane on metal-oxygen cluster compounds are more numerous than those with other alkanes. This is in contrast with that which is found in general. Kung [2] has noted that with heterogeneous catalysts of all types there are fewer studies of the oxidative dehydrogenation of butane than ethane or propane. The interest in C 4 hydrocarbons is twofold. The skeletal isomerization of butane to form isobutane is of industrial importance since the latter hydrocarbon can serve as a precursor to methyl tert-butyl ether which has been employed as an octane enhancer in some countries to compensate for the removal of tetraethyl lead from gasoline [64]. In addition, any processes which lead to the conversion of linear hydrocarbons to branched species are of potential importance in producing environmentally friendly substitutes for volatile organic compounds [65]. The cesium salt of HPW, Cs2.5H0.5PW12040 , has been shown to catalyze the skeletal isomerization of n-butane to isobutane at 300°C [66]. At this temperature, the selectivity to the latter compound is approximately 83% with the next highest selectivity, 8.5%, for propane and propene. Conversions of n-butane up to 20% were obtained. Similar studies with the aforementioned catalyst but where Pt(NH3)4C12 had been added in the preparative process yielded selectivities to isobutane as high as 95% at a conversion of 34%, in the presence of hydrogen at 300°C [67]. More detailed accounts of this work have also recently appeared [68] in which the relative amounts of protons and cesium have been varied in the aforementioned salt of HPW and the maximum in activity was confirmed for the salt with 2.5 Cs and 0.5 H. Most recently, a number of the cesium salts of HPW have been synthesized and characterized by a variety of techniques [69]. These authors conclude that for the n-butane isomerization reaction the highest conversion is achieved with cesium contents of 2 cations per anion.
5. Conversion of isobutane
The conversion of isobutane to methyl methacrylate (MMA) has recently been investigated with metal-oxygen cluster compounds [70]. Although a
J.B. Moffat / Applied Catalysis A: General 146 (1996) 65-86
79
number of technologies are available for the production of M M A and some are in commercial operation in various countries, that based on acetone cyanohydrin remains dominant [71]. In view of tightening environmental regulations and changes in production costs alternative processes may become more attractive. The oxidation of isobutane to methacrylic acid (MAA) has been studied on dicationic salts of 12-molybdophosphoric acid with the ammonium and potassium cations in various relative concentrations and in some preparations with iron as a third cation [70]. At 350°C the conversion of isobutane increased from 4 to 6% as the number of iron cations increased from 0 to 1.5 per anion, while the yield of methacrylic acid increased from 1 to 3%. However, the selectivity to MAA is adversely affected by the introduction of iron. Methacrolein, isobutyric acid, acetic acid and carbon oxides were produced with yields similar to those of methacrylic acid. These authors note that iron could alter the redox properties of molybdenum in the Keggin anion as well as influencing the acidity of the catalyst. The conversion of isobutane to MAA and methacrolein has also been investigated on a Cs salt of 12-molybdophosphoric acid [72] with nickel, manganese or iron as a second cation. With CSxH3_xPMOl2040 at 340°C the conversion increased with x to a maximum of 17% at a value of x equal to 2.85. However, at this composition the selectivities to MAA and methacrolein (MAL) were only 5 and 10%, respectively. The maximum values of the selectivities of MAA and MAL were 34 and 18, respectively, observed at values of x of 2 and 0, respectively. The selectivity to carbon oxides was a maximum at 81% for x equal to 2.85. The introduction of transition metal ions together with 2.5 Cs + and the appropriate quantities of protons to provide charge neutralization produced conversions up to 24% (with Ni2÷), selectivities to MAA of 35% (with Fe 3+) and to MAL of 15% (with Co2+).
6. Discussion The activation and conversion of alkanes containing two or more carbon atoms on metal-oxygen cluster compounds involves a number of factors relating to both compositional and morphological properties of these solids. As noted in the preceding discussion, the acidity of the MOCC is an important variable in many processes. It is well known that the assessment of the acidity and the distribution of the acidic strength of surface sites of solids is a complex problem and the correlation of the results obtained from various techniques has not yet been accomplished [73]. However, the least ambiguous of the methods currently available may be that which evaluates the performance of the catalyst in one or more processes in which a variety of catalysts has been previously tested.
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Investigations in the early part of the last decade on the conversion of methanol to hydrocarbons containing two or more carbon atoms (the so-called methanol to gasoline (MTG) process) showed that HPW was vastly superior to HPMo as a catalyst for this process with the latter catalyst producing considerable quantities of carbon oxides [74]. Since this process is known to require relatively strong acidic sites the aforementioned results provide experimental conformation of the predictions of the results of extended Hfickel calculations mentioned earlier in this report [58]. These calculations showed that the charges on the terminal oxygen atoms of those Keggin anions containing molybdenum in the peripheral metal positions are higher than on those containing tungsten and thus that the mobility of the proton and hence the acidic strength in the MOCC containing the latter is higher than in those containing the former element. Surprisingly, tests of the same MTG reaction with the ammonium salt of HPW produced selectivities superior to those obtained with the parent acid [75,76]. Analyses of the ammonium salt (NH4PW) by various methods including PAS FTIR showed that free protons, in addition to those bound in NH~were present and consequently, although the salt was synthesized from stoichiometric quantities of the preparative reagents the resulting solid was nonstoichiometric [46]. Thus the improved selectivities in the MTG process obtained with NH4PW could be attributed to the residual protons with acid strength distributions perturbed by the presence of the ammonium cations. It should be noted that the surface area of NH4PW was significantly higher ( > 100 m 2 g - l ) [75,76] than that of HPW ( < 10 m 2 g - l ) [73] and consequently adds an additional factor to the functionalities of the catalyst. This will be explored further later in this discussion. Similar observations have been made with NH4PW employed catalytically in the alkylation of toluene to form xylenes [77]. However, in this example, since the selectivity to p-xylene exceeded that expected at equilibrium it is apparent that, in addition to the acidic properties of NH4PW, its pore structure played a significant role in the shape selective process. It is important to note that, although it may be contended that additional protons can be produced from the ammonium ions of NH4PW resulting from the loss of ammonia at higher temperatures, both the aforementioned MTG and toluene alkylation processes took place at temperatures lower than those shown from TPD to be required for ammonia desorption. The aforementioned recent results obtained by various workers [66-69] with nonstoichiometric cesium compounds and their conclusions concerning the variations of acidity with the relative concentrations of Cs + and H ÷ are evidently consistent with the earlier results obtained with nonstoichiometric NHaPW [75-77]. As noted above, the early work with NH4PW showed that its surface area was relatively high. Subsequent studies in the late 80's showed that the high surface area of NH4PW resulted largely from the presence of micropores
J.B. Moffat / Applied Catalysis A: General 146 (1996) 65-86
81
[77,78]. Further, it was demonstrated that high surface area, microporous MOCC could be prepared from 12-tungstophosphoric, 12-molybdophosphoric, 12tungstosilicic, 12-molybdosilicic and 12-tungstoarsenic acids with a number of the monovalent cations of the 1A group [78-86]. Much of these results came from the suitable analysis of nitrogen adsorption-desorption isotherms, an example of which is shown in Fig. 12 [79,80]. These isotherms exhibit a sharp increase in the quantity adsorbed at low partial pressures which is indicative of the presence of micropores. It is also of interest to note that the cesium salt of HPW displays a hysteresis loop showing the presence of mesopores. The more recent work with CsPW compounds is in semiquantitative agreement with these earlier results. For purposes of illustration, Tables 1 and 5 show the surface areas and mean pore radii for the various salts of the heteropoly acids. It is of interest to note that the sodium salts show no evidence of microporosity and possess surface areas similar to those of the parent acids. Confirmation of the existence of the
I
90
PW
salts
60 •
y 30
i
0.5
1.0
PIPo Fig. 12. Nitrogen adsorption-desorption isotherms at 78 K for high surface area cesium, ammonium, and potassium salts of 12-tungstophosphoric acid.
J.B. Moffat /Applied Catalysis A: General 146 (1996) 65-86
82
Table 5 Mean micropore radii (A) of high surface area MOCC Cation
Anion
K+ NH~Rb + Cs +
PW
PMo
SiW
AsW
8.8 10.3 13.9
9.3 13.0
9.5 10.3 10.5
11.2 11.3 9.4 9.9
14.3
microporous structure in various of the MOCC has recently been provided by 129Xe NMR [87,88]. In view of the recent work on dicationic MOCC discussed earlier in this report the influence of the second cation on the morphological properties of these materials is of interest. Ion exchange studies of the MOCC have shown that exchange of the cations from the high surface area, microporous MOCC is possible although complete substitution is not possible [89]. Further, the extent of the exchange is dependent on the relative sizes of the entering and exiting cations. However, most significantly such exchange can be performed with retention of structure, both crystallographic and morphological, although not surprisingly, shifts reflecting differences in sizes of the cations are observed (Figs. 13 and 14) [90]. In the studies of the oxidation of isobutane to methacrylic acid on the N H 4 / K salts of HPMo as noted earlier in this report it is expected that high surface areas and microporous structures should be retained in the presence of the cations. As seen from Fig. 13 the ion exchange of NH~- by K + alters the
80
60 G) m
40
J
1
I
25
50
75
100
Fig. 13. BET surface area as a function of the cation composition for the ion exchanged K ÷ / N H ~ - / P M o ~ 2 0 ~ 3 system.
83
J.B. Moffat / Applied Catalysis A: General 146 (1996) 65-86 11,70
,
,
,
11,65
o. ~
11.60
11.55 0
I 25
i 50
I 75
100
P e r c e n t K+
Fig. 14. Lattice parameter as a function of cation composition for the NH~-/K+/PMol20~03 ion exchange system.
surface area from approximately 75 to 44 m 2 g-~. Although the authors of the aforementioned paper on the oxidation of isobutane do not discuss changes in the morphological properties of their catalysts with cationic composition they do note that the surface areas of the fresh catalysts were approximately 140 mZ/g [701. In the work discussed in the present report, metal-oxygen cluster compounds have frequently been modified by the addition of multivalent cations. However, the structural location of these is uncertain. Recent single crystal X-ray diffraction studies have shown that it is difficult, if not impossible, to synthesize MOCC with Keggin structure from multivalent cations [45]. Finally, some comments concerning the mechanism through which the MOCC activate alkanes is of relevance. In earlier work on the activation and partial oxidation of methane, MOCC containing molybdenum were found to be superior to those of tungsten [54]. In studies with HPMo supported o n SiO 2, as the protons are replaced by cesium the tumover rates for methane decrease to the value expected for silica itself while the selectivity to formaldehyde becomes vanishingly small. Concomitantly the selectivities to CO and CO 2 decrease and increase, respectively, approaching those values found for the support. Similar results were obtained on exchange of the protons by various monovalent cations [91]. These results, together with those from other experiments, suggested that the proton played a role in the oxidative capabilities of the catalyst. This participation is probably indirect with the proton extracting oxygen atoms from the Keggin anion to produce oxygen vacancies which are required for the activation of the oxygen source and therefore the methane.
J.B. Moffat / Applied Catalysis A: General 146 (1996) 65-86
84
Many years ago, Bohme and Fehsenfeld [92] showed that the O - ion is capable of stripping a hydrogen atom from an alkane: 0
+ RH --->O H - + R
Since that time many workers have studied the conversion of methane and have concluded that hydrogen atoms are extracted from the alkane by O followed by the reaction of alkyl radicals with surface 0 2- ions. A similar mechanism may be extant in the oxidation of ethane and higher hydrocarbons on metal-oxygen cluster compounds with the oxygen vacancies being replaced by O - from the oxidant. The alkyl radicals which are formed from the C - H scission can then interact with anionic oxygen atoms possessing a higher electron density. Since these will be relatively labile, where the anions contain molybdenum, the alkyl radicals may extract such atoms from the anion to form a peroxy radical. Acid-catalyzed processes, such as the conversion of methanol, in which the anions are methylated, thus show an interesting correspondence with redox processes. However, in the former reaction an alkyl ion and relatively tightly bound anionic oxygen atoms are involved while in the latter alkyl radicals and labile anionic oxygen atoms are central to the mechanism. It is probable that the first step in the conversion of an alkane on heteropoly oxometalates involves the formation of alkyl radicals. Since the conversion decreases with the temperature at which the catalyst is pretreated the C - H bond dissociation is apparently a surface-initiated process. The alkyl radicals can undergo further reactions on the surface or desorb into the gas phase where reaction with the oxidant to form an alkene occurs. The desorption process and the subsequent formation of the alkene is expected to be favoured by increasing temperatures. The alkyl radicals which are adsorbed on the surface of the catalyst can produce the corresponding alkenes or can interact with the oxygen species to produce alkoxides. This process has its analogue in the alkanol conversion process in which, for example, the heteropoly anion becomes methylated by methyl cations attaching themselves to the terminal oxygen atoms of the anion [93]. These oxygen atoms can be extracted by the alkyl groups to form an alkoxide and ultimately the corresponding aldehyde. Thus the selectivity to the latter is expected be dependent on the lability of the anionic terminal oxygen atoms which has been predicted to be higher in molybdenum-containing heteropoly anions than in those containing tungsten [58].
Acknowledgements The financial support of the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.
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85
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A PT PA LE IY DSS CA L I A: GENERAL
ELSEVIER
Applied Catalysis A: General 146 (1996) 65-86
Conversion of C2-C 5 alkanes on heteropoly oxometalates J.B. Moffat Department of Chemistry and the Guelph-Waterloo Centre for Graduate Work in Chemistry, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1
Abstract
Studies of the conversion of ethane and isobutane on heteropoly oxometalates are reviewed. The mechanisms of the processes are shown to be related to the elemental composition of the anion of the catalyst and the acidic strength of the solid acids. With unsupported heteropoly oxometalates the charge balancing cations play a prominent role particularly in leading to microporous structures. Although participation of the gas phase undoubtedly occurs in the aforementioned processes, the catalyst plays an important role in the C - H bond scission. Both the conversion and selectivities in these processes are related to the strength of the terminal oxygen-anionic metal bond which itself is dependent upon the nature of the peripheral metal in the anion of the catalyst. The reductant employed in the oxidation processes appears to act primarily as a regenerator of the active oxygen species in the catalyst. Keywords: Heteropoly oxometalates; Ethane; Isobutane; Metal-oxygen cluster compounds
1. Introduction
Research in catalysis is frequently driven by market and, most recently, environmental requirements. In many instances, while resource materials may be sold and employed as such, the catalytic conversion of these may result in the formation of products which are more immediately usable and the sale of which is more profitable even after subtraction of the production costs. There is also considerable impetus to find new environmentally-friendly catalysts to replace those which, for a number of reasons, may be currently less acceptable. Thus, for example, the conversion of alkanes to a variety of chemical products has attracted considerable attention in recent years [1-39]. The present contribution focuses on the conversion of alkanes with two or more carbon atoms with 0926-860X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. Pll S0926- 860X(96)00169-X
66
J.B. Moffat /Applied Catalysis A: General 146 (1996) 65-86
metal-oxygen cluster compounds (also known as heteropoly oxometalates) as heterogeneous catalysts for such purposes.
2. Metal-oxygen cluster compounds (heteropoly oxometalates) Metal-oxygen cluster compounds (MOCC) are ionic solids with discrete cations and anions. While a wide variety of compositions and structures has been identified [40] only those of Keggin structure will be considered here. Although a number of isomers of the Keggin structure is known, since the oL form is generally more stable, discussion will be restricted to this isomer. The Keggin anion has overall T0 symmetry with a central atom such as phosphorus, silicon or one of a large number of elements bonded to four oxygen atoms arranged tetrahedrally around the central atom (Fig. 1). Twelve octahedra with a peripheral metal atom, such as tungsten, at each of their approximate centres and oxygen atoms at their vertices, surround the central tetrahedron and are arranged in four groups of three edge-shared octahedra, M30~3. Three types of oxygen atoms are present in the anion: those which join the central atom to a peripheral metal atom, those which connect two of the latter and those, of which there are twelve, which are bonded to the peripheral metal atom only and protrude from the anion. A variety of cations is possible, ranging from single atom to multiatom-multielement species. With the proton, heteropoly acids are formed. Although considerable powder X-ray diffraction data exist, relatively little single crystal
Fig. 1. The Keggin structure for the ~-[XMI2O40 ]n- anion. Bigger circles, central and peripheral atoms; smaller circles, oxygen atoms.
J.B. Moffat / Applied Catalysis A: General 146 (1996) 65-86
67
information is available. X-ray and neutron diffraction studies of the six-hydrate of 12-tungstophosphoric acid (H3PW1204o. 6H20) have shown that this solid acid has a cubic (Pn3m) structure with each proton hydrogen-bonded to two water molecules in an approximately coplanar configuration and the water molecules hydrogen-bonded to the terminal oxygen atoms of the anions (Fig. 2) [41 ]. Single-crystal data is also available for 12-molybdophosphoric acid [42-45]. Although the heteropoly acids appear to have open structures, the BET N 2 surface areas are low (Table 1). However, photoacoustic (PAS) FTIR measurements have shown that polar molecules such as ammonia are capable of penetrating into the bulk structure of these solids [46]. Thus, on exposure of 12-tungstophosphoric acid (HPW) to gaseous ammonia, molecules of the latter penetrate into the solid to interact with the protons, forming NH~- ions, as evidenced from the IR band at 1429 cm-1, until three ammonia molecules per anion have entered the structure (Fig. 3). The phenomenon observed with ammonia is also found with other polar molecules such as pyridine and methanol [47,48].
Fig. 2. Crystallographic arrangement of protons, anions, and water molecules in hydrated 12-tungstophosphoric acid [41].
J.B. Moffat / Applied Catalysis A: General 146 (1996) 65-86
68
Table 1 Surface areas a o f heteropoly oxometalates Cation
H Na Ag K NH 4 Rb Cs MeNH 3 Me 4 N BETN 278K,m
Anion S i W 1204~14
P M o j 2 0403
PWl 204o 3
AsW~ 2 0403
3.2 1.3 3.3 116.9 116.3 153.4 3.0 -
8 3.5 39.9 193.4 145.5 1.3 -
8 3.7 3.0 90.0 128.2 162.9 3.0 4.5
6.5 46.0 82.1 101.4 65.4 2.1 -
2 g-l.
The heteropoly acids contain Br/Jnsted acid sites but no evidence of Lewis acid sites [46,47]. Titrations with Hammett indicators have shown the presence of very strong BriSnsted acid sites. Microcalorimetric measurements of the
4oo
'
32bo
'
a,~o
'
~6bo
'
8 b o c M -~
Fig. 3. P A S spectra o f (a) 12-tungstophosphoric acid pre-evacuated at 473 K, ( b ) - ( e ) after stepwise dosing with a m m o n i a at 423 K. (The a m m o n i u m salt (f) is included for comparison).
J.B. Moffat / Applied Catalysis A: General 146 (1996) 65-86
69
250
2OO
a-,
-~150
2
"-D
'-~100
C~ 5O
I
0(~
1
I
2 ' MOLECULES
I
3 NH a / KU
'
t
4
'
5
Fig. 4. Differential heats of ammonia sorption on HPW at 323 K. (1) Activation: 423 K / 2 h/UHV; (2) activation: 523 K / 1 h / U H V .
differential heat of adsorption of ammonia [49] produce values of 150-200 kJ mol -~ in the same range ( > 120 kJ mol -~) as suggested to be indicative of superacidity (Fig. 4) [50,51]. The thermal stability of the metal-oxygen cluster compounds is found to be dependent on both the composition of the anion and the nature of the cation (Table 2) [52]. Photoacoustic FTIR spectra of HPW after heating at various temperatures show that the set of bands below 1200 cm -1 attributed to the Keggin structure, although somewhat diminished in intensity, nevertheless re-
Table 2 DTAs of alkylammonium 12-heteropoly salts R
RPW salts Pr4N Me 4 N Me3NH MezNH 2 MeNH 3 NH 4 RPMo salts Pr4N Me4N Me3N H Me 2NH 2 MeNH 3 NH 4
DTA peaks (K) a Exo
Endo
765m, br 808m, br 763m, br; 788m, br 738m, br; 790m, br 728w, br; 788m, br Stable to at least 823 K
51 lw, br; 698w, br b 485w; 549w b b
51 lw b
496w; 504w; 539w; 556w; 663s 523W, br; 661m; 691s 583W; 595W; 668m 606m; 663m, br 631m b
b
578w; 596m 598m; 615m, br 773s
a w, Weak; m, medium; s, strong; br, broad. b No peak observed below 873 K.
J.B. Moffat /Applied Catalysis A: General 146 (1996) 65-86
70
H3PW12040-n H20
.~
c)
t/)
~ Ii~, ~L-
*
Z
LII),I). C. . . .
350 ° ^
a) 25*
] ~ 0J_. '
,
2__J_ ~ld0.
/\
I
1$d11. J
, . 11~1~ '
M-].
Fig. 5. PAS FTIR spectra of 12-tungstophosphoric acid showing characteristic bands in 1100-800 cm I region and effect of heating in vacuo.
main at 450°C (Fig. 5) [46]. In particular, the bands at 1080 and 980 cm -1 associated with the triply degenerate asymmetric stretching vibration of the central P O 4 tetrahedron and a stretching vibration of the terminal oxygen atoms of the anion, respectively, are evident at 450°C. Temperature-programmed desorption experiments with the heteropoly acids HPW, 12-molybdophosphoric acid (HPMo) and 12-tungstosilicic acid (HSiW) show that the water molecules hydrogen-bonded to the protons and to the anions are desorbed from the solid acids at temperatures near 200°C (Fig. 6) [53]. Additionally, at temperatures between 400 and 600°C, dependent upon the elemental composition, water resulting from the extraction of anionic oxygen atoms by protons, is associatively desorbed. As a result of the low surface areas of the acids, the rates of reactions employing these as heterogeneous catalysts with nonpolar molecules such as methane are relatively low [54]. Consequently, it is advantageous to enhance the contact between reactants and catalyst by supporting the solid acid on a high surface area material. In studies of the partial oxidation of methane on various of the MOCC evidence was found for the enhancement of the thermal stability of the MOCC, apparently resulting from the strong interaction between the latter and the silica support [55].
J.B. Moffat / Applied Catalysis A." General 146 (1996) 65-86
71
d uJ (n z 0 (1) uJ no O~ 0 UJ I14J
I 773
I
I 573 TEMP/aK
I
I 373
Fig. 6. Temperature-programmed desorption profiles for 12-tungstophosphoric acid, 12-tungstosilicic acid, and 12-molybdophosphoric acid after pretreatment at 25°C for 16 h.
Further information on the supported heteropoly acids was obtained from the first application of 31p NMR to such systems which was published in 1990 [56,57]. In that work 3]p NMR spectra of 23 wt.-% H P M o / S i O 2 samples which had been heated at a number of temperatures for various periods of time showed one relatively broad peak whose positions and full widths at half maximum (FWHM) are recorded in Table 3. The samples heated at temperatures of 550°C or less showed peaks at positions consistent with the 3]p NMR spectra of heteropolymolybdates [40]. Since after use in a methane oxidation test reaction
Table 3 Effect of pretreatment conditions on the 31p NMR parameters of silica supported 12-molybdophosphoric acid catalyst a Pretreatment
NMR 3z p peak position (ppm)
FWHM (Hz)
Bulk HPMo 200°C, 2 h 350°C, 16 h 450°C, 16 h 550°C, 16 h (570°C, 4 h) b 600°C, 16h 730°C, 16 h
- 8 - 8 - 8 -8 - 8 -- 8 -5 -5
1.5 1.0 1.2 1.1 1.2 1.1 1.8 9.4
a 23 wt.-% H P M o / S i O 2 calcined at the temperatures and for the periods of time shown. b Methane oxidation at conditions shown.
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Table 4 Effect of loading of H P M o / S i O 2 a on 31p NMR parameters Loading (wt.-%)
NMR 31p peak position (ppm)
FWHM (Hz)
Bulk HPMo
- 8
1.5
1.2 8.3 16. 31 39
-8 -8 -8 -8 -8
1.5 1.2
1.0 1.1 4.1
a Pretreated at 350°C for 2 h.
at 570°C for 4 h the position and FWHM of the 3 1 p NMR line are unchanged from those observed for HPMo at lower temperatures, the structure of the heteropoly anion is retained under catalytic operating conditions. For temperatures of 600 ° and higher it is clear, however, that the anion is no longer in existence. 31p NMR spectra of H P M o / S i O 2 samples of various loadings which had previously been calcined at 350 ° for 2 h again showed peaks characteristic of bulk HPMo up to and including a loading of 31 wt.-% HPMo (Table 4). Information on the dispersion of the supported species on the silica surface has been obtained from X-ray photoelectron spectroscopy [56] (Fig. 7). The
1.0 J
Q.
co
I
::~ (,ej 0.8-
s
i__
+
0
0 .< n-
/J t
0.6-
0
~(/.30-4" 7
ILl I-" ~-~ 0.2O3 Q_
X 0.0
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0 14
0.16
0 18
0.20
0.22
0.24
HPMo LOADING (KU.nm-2) Fig. 7. Evolution of the XPS Mo3d/Sizp intensity ratio versus the !2-mo]ybdophosphoric acid loading for the series of catalysts calcined at 350°C (C)), 2 h and after catalytic test at 570°C, 3 h ( + ) , as calculated for a monolayer dispersion (---).
J.B. Moffat / Applied Catalysis A: General 146 (1996) 65-86
73
MO3d/Si2p intensity ratio increases linearly with the loading up to approximately 0.04 KU nm -2 or approximately l0 wt.-% HPMo, with a slope similar to that of the calculated line for a monolayer dispersion. For this loading range single anions or perhaps small aggregates of these are dispersed on the surface. For higher loadings a change in slope signals the appearance of larger aggregates up to a plateau at 0.10 K U / n m 2 or approximately 23 wt.-% HPMo, at which loading large aggregates are formed. After use in a methane oxidation reaction the curve for the intensity ratio has a similar shape but is shifted to lower values, indicative of the small loss of molybdenum during the reaction. Subsequently, 31p spin-lattice relaxation measurements on H P M o / S i O 2 have shown that the apparent monolayer is reached at a loading of approximately 35 wt.-% HPMo [57]. The results of the aforementioned studies show that HPMo can be deposited uniformly on the surface of silica in a highly dispersed form up to a loading of approximately 10 wt.-% [56]. For loadings between this and 25%, aggregates begin to form, while at higher loadings particles of the acid are present.
3. Oxidative dehydrogenation of ethane The first report on the conversion of ethane on MOCC appeared in 1994 [34]. Ethane is converted to both ethylene and acetaldehyde, on MOCC supported on silica, at reaction temperatures between 450 and 570°C. With N20 as the oxidant the conversion of ethane is strongly dependent on the elemental composition of the SiO2-supported heteropoly acids, with those containing molybdenum producing the highest conversion. The unsupported HPMo yields a conversion a factor of ten smaller than that obtained with the SiO2-supported acid. With oxygen as an oxidant the conversion on H P M o / S i O 2 is higher than that with N20 but no acetaldehyde is detected with the former oxidant. As previously observed with methane, the loading plays a significant role, particularly with the conversions (Fig. 8). The conversion of ethane increases with loading, reaching a maximum for 20-25 wt.-% HPMo on silica, and decreases. Concomitantly, the selectivities to C2H4, CO and CO 2 increase, while that to acetaldehyde decreases. With decreasing contact time, the conversion of C2H6, with N20 as oxidant, decreased, the selectivities to acetaldehyde and ethylene increased and that to CO x decreased, implying that acetaldehyde and ethylene are primary products (not shown). At conversions of approximately 3% the selectivities to acetaldehyde and ethylene sum to greater than 70%. With 20% H P M o / S i O 2 and N20 as oxidant, an increase in reaction temperature produces increases in conversion and selectivities to CO x at the expense of acetaldehyde. With oxygen as an oxidant the production of CO is favoured at all reaction temperatures.
J.B. Moffat /Applied Catalysis A: General 146 (1996) 65-86
74 60
o .-n a.~ 4 0
2
-.lm
4-,
0
m ¢~ 20
1
0
b 10
0
, 20
, 3O
0 4O
LoadinglwtZl Fig. 8. Effect of loading of HPMo on conversion of ethane and selectivity over supported catalyst; TR = 540°C, W = 0.5 g, F = 25 ml/min, C2H 6 / N 2 0 = 4 / 1 , ( O ) conversion of C2H 6, (zx) selectivity of C 2H4, (open hexagon) CH 3CHO, ('7) CO,, ( ~ ) CO, (cLosed hexagon) yield of CH~CHO.
In the absence of an oxidant acetaldehyde is formed on the catalyst from ethane during the initial stages of the reaction but is no longer found in the product stream after a number of hours on stream while, concomitantly, the selectivity to ethylene increases (Fig. 9). This suggests that the formation of acetaldehyde requires the participation of the catalyst while ethylene does not. Further, when ethane and nitrous oxide react in the absence of the catalyst no II
100
II
9retreatment ¢3 o b~
--t mo o -,=
.~ 50 ¢J
1
c)
pretreatme6t~]
¢/)
0
1
2
.with 0 2
i /
4
6
5
o
=z
8
9
Time on Stream(hr) Fig. 9. Effect of time on stream on ethane conversion and selectivity over 20 wt.-% HPMo/SiO2: TR = 510°C, W = 1.00 g, F = 5 ml/min, C2H 6 only and no oxidants, ( 0 ) conversion of C2H 6, ( ~ ) selectivity of CO, ( V ) C02, (~.) C2H4, (hexagon) CH3CHO.
J.B. Moffat / Applied Catalysis A: General 146 (1996) 65-86
75
60 50
4O •~
2
30
q}
i
9,,
o
2o
10
450
t soo
Pretreatment
i
5so
i 800
i 850
0
Temperature[*CI
Fig. 10. Effect of pretreatment temperature on ethane conversion and selectivity over 20 wt.-% HPMo/SiO2: TR = 540°C, W = 0.5 g, F = 25 ml/min, C2H 6 / N 2 0 = 4 / 1 , ( 0 ) conversion of C~H 6, (O) selectivity of CO, (~7) CO2, (/x) C2H4 ' (hexagon) CH3CHO.
acetaldehyde is observed. Since the initial selectivity to acetaldehyde in the absence of an oxidant is similar to that observed in the presence of an oxidant and since no acetaldehyde is produced in the absence of a catalyst, it can be concluded that there is little or no participation of a homogeneous process in the generation of this product. Thus, the oxidant is apparently acting as a regenerator of the active sites on the catalyst for the production of acetaldehyde. Since nitrous oxide restores the ability of the catalyst to produce acetaldehyde, while oxygen does not, the former is more effective than the latter in replacing the lost oxidation sites of the catalyst. The dependence of the conversion of ethane and the selectivities to the various products on the pretreatment temperature provide further evidence for the enhancement of the thermal stability of 12-molybdophosphoric acid when supported on silica (Fig. 10). Up to a pretreatment temperature of 500°C the conversion of ethane and the selectivities to the various products remains unchanged. The conversion and selectivity to acetaldehyde begin to decrease between 500 and 550°C while the selectivities to the carbon oxides begin to increase. The selectivity to ethylene remains relatively unchanged for pretreatment temperatures of 450-650°C. These observations are similar to those found with methane, as noted above. Analogously to the results found with methane it is expected that the oxidation of ethane proceeds through a radical mechanism [17] involving either the bridged or the terminal oxygen of the anion. C2H 6 + O-----) C2H 5 + O H -
76
J.B. Moffat / Applied Catalysis A: General 146 (1996) 65-86 0.0
<
,1u =E -1.0
10 e,. w \
5
\\ \ \
a z
o m
o
PW a
SiW b
PMo c
Fig. 11. Magnitude of negative charges on bridging (b) and outer (o) oxygen atoms; binding energies between terminal oxygen atoms and peripheral metal atoms in the anions shown in abbreviated notation.
The ethyl radicals may enter the gas phase where they suffer deep oxidation. Alternatively, the ethyl radicals may remain in contact with the anion where ethylation occurs. Extended Hiickel calculations have shown that the terminal oxygen atoms of anions containing molybdenum in the peripheral metal positions are more labile than those where tungsten occupies the latter positions (Fig. 11) [58]. This may explain the markedly different results for the conversion of ethane obtained with the supported acids containing these two metals. Nitrous oxide may decompose in the gas phase to form O - [7,25] or may interact directly with the Keggin anion to regenerate the depleted oxygen in a manner similar to that proposed by Erdohelyi et al. [25] for silica-supported alkali molybdate catalysts. N20 + M o S + ~ N 2 -q- M o 6 + _ O where the Mo occupy the peripheral metal positions in the Keggin anion. Supplementary experiments have shown that both ethylene and ethanol are converted by N 2 0 in the presence of HPMo/SiO 2 to, among other products, acetaldehyde. Since earlier work has shown that both methanol and formaldehyde are formed in the oxidation of methane on HPMo/SiO 2 it appears reasonable to suggest that ethanol may be a primary product in the oxidative conversion of ethane, albeit one whose lifetime under the conditions of the
J.B. Moffat / Applied Catalysis A: General 146 (1996) 65-86
77
reaction is relatively short. Ethanol could form from the protons in HPMo and the ethoxy group resulting from the ethylation of the Keggin anion or from the reaction of the ethoxy group with water. A flow diagram consistent with the aforementioned observations shows acetaldehyde being derived from ethanol and ethylene with the latter forming from ethanol: C~H4
C2I~
-~
C2HsOH
--o
CHsCHO
,,,
Z CO, COz
The ratios of ethylene/acetaldehyde and C O / C O 2 from ethanol are 1.8 and 2.9, respectively, while under similar reaction conditions those from ethane are 1.6 and 2.5, respectively. Although not conclusive, these observations provide some support for the contention that the conversion of ethane proceeds through ethanol. A somewhat similar mechanism has been suggested for the conversion of ethane on B203-AI203 [13] and B203-P205 [30] although, in contrast with the present results, oxidation of ethylene on these catalysts did not produce acetaldehyde, formaldehyde or carbon monoxide. Earlier work on the partial oxidation of methane on the metal-oxygen cluster compounds showed that the introduction of small quantities of tetrachloromethane (TCM) produced enhanced conversions and selectivities to C 2 hydrocarbons, particularly ethylene [59-61]. Similar studies with ethane demonstrated that the conversion of ethane and the selectivity to CO 2 increased on addition of TCM while the selectivities to ethylene and CO decreased [62]. Small quantities of ethyl chloride were also produced. Separate experiments with ethylene and nitrous oxide showed that the introduction of 0.2 mol-% of TCM into the feedstream increased the selectivity to acetaldehyde by a factor of four, from 13.3 to 54.9%, at a conversion of 4.5 __ 0.3%. Since earlier experiments on the conversion of methane in the presence of TCM showed that strong interaction of the latter with the surface of the catalyst was occurring [59-61], the chloromethane is evidently altering the heterogeneous process although the possibility of some influence on the purely gas phase reactions cannot be excluded. Most recently, the conversion of ethane has been examined on salts of molybdophosphoric acid with both potassium and ammonium cations [63]. The Keggin anion was modified by substitution of one or two antimony atoms for molybdenum producing a thermal stabilization of the structure by 50-100 degrees. However, the addition of antimony led to a decrease in the catalytic activity as compared with the antimony-free compound. In all cases, the
78
J.B. Moffat / Applied Catalysis A: General 146 (1996) 65-86
conversions of ethane ranged from 1 to 7% while the selectivities to ethylene were 34-56%. No evidence for the formation of acetaldehyde was reported. With samples containing potassium, iron and antimony the selectivities to ethylene reached as high as 86% at a conversion of 2.6%.
4. Conversion of C 4 alkanes
Studies of the conversion of butane and isobutane on metal-oxygen cluster compounds are more numerous than those with other alkanes. This is in contrast with that which is found in general. Kung [2] has noted that with heterogeneous catalysts of all types there are fewer studies of the oxidative dehydrogenation of butane than ethane or propane. The interest in C 4 hydrocarbons is twofold. The skeletal isomerization of butane to form isobutane is of industrial importance since the latter hydrocarbon can serve as a precursor to methyl tert-butyl ether which has been employed as an octane enhancer in some countries to compensate for the removal of tetraethyl lead from gasoline [64]. In addition, any processes which lead to the conversion of linear hydrocarbons to branched species are of potential importance in producing environmentally friendly substitutes for volatile organic compounds [65]. The cesium salt of HPW, Cs2.5H0.5PW12040 , has been shown to catalyze the skeletal isomerization of n-butane to isobutane at 300°C [66]. At this temperature, the selectivity to the latter compound is approximately 83% with the next highest selectivity, 8.5%, for propane and propene. Conversions of n-butane up to 20% were obtained. Similar studies with the aforementioned catalyst but where Pt(NH3)4C12 had been added in the preparative process yielded selectivities to isobutane as high as 95% at a conversion of 34%, in the presence of hydrogen at 300°C [67]. More detailed accounts of this work have also recently appeared [68] in which the relative amounts of protons and cesium have been varied in the aforementioned salt of HPW and the maximum in activity was confirmed for the salt with 2.5 Cs and 0.5 H. Most recently, a number of the cesium salts of HPW have been synthesized and characterized by a variety of techniques [69]. These authors conclude that for the n-butane isomerization reaction the highest conversion is achieved with cesium contents of 2 cations per anion.
5. Conversion of isobutane
The conversion of isobutane to methyl methacrylate (MMA) has recently been investigated with metal-oxygen cluster compounds [70]. Although a
J.B. Moffat / Applied Catalysis A: General 146 (1996) 65-86
79
number of technologies are available for the production of M M A and some are in commercial operation in various countries, that based on acetone cyanohydrin remains dominant [71]. In view of tightening environmental regulations and changes in production costs alternative processes may become more attractive. The oxidation of isobutane to methacrylic acid (MAA) has been studied on dicationic salts of 12-molybdophosphoric acid with the ammonium and potassium cations in various relative concentrations and in some preparations with iron as a third cation [70]. At 350°C the conversion of isobutane increased from 4 to 6% as the number of iron cations increased from 0 to 1.5 per anion, while the yield of methacrylic acid increased from 1 to 3%. However, the selectivity to MAA is adversely affected by the introduction of iron. Methacrolein, isobutyric acid, acetic acid and carbon oxides were produced with yields similar to those of methacrylic acid. These authors note that iron could alter the redox properties of molybdenum in the Keggin anion as well as influencing the acidity of the catalyst. The conversion of isobutane to MAA and methacrolein has also been investigated on a Cs salt of 12-molybdophosphoric acid [72] with nickel, manganese or iron as a second cation. With CSxH3_xPMOl2040 at 340°C the conversion increased with x to a maximum of 17% at a value of x equal to 2.85. However, at this composition the selectivities to MAA and methacrolein (MAL) were only 5 and 10%, respectively. The maximum values of the selectivities of MAA and MAL were 34 and 18, respectively, observed at values of x of 2 and 0, respectively. The selectivity to carbon oxides was a maximum at 81% for x equal to 2.85. The introduction of transition metal ions together with 2.5 Cs + and the appropriate quantities of protons to provide charge neutralization produced conversions up to 24% (with Ni2÷), selectivities to MAA of 35% (with Fe 3+) and to MAL of 15% (with Co2+).
6. Discussion The activation and conversion of alkanes containing two or more carbon atoms on metal-oxygen cluster compounds involves a number of factors relating to both compositional and morphological properties of these solids. As noted in the preceding discussion, the acidity of the MOCC is an important variable in many processes. It is well known that the assessment of the acidity and the distribution of the acidic strength of surface sites of solids is a complex problem and the correlation of the results obtained from various techniques has not yet been accomplished [73]. However, the least ambiguous of the methods currently available may be that which evaluates the performance of the catalyst in one or more processes in which a variety of catalysts has been previously tested.
80
J.B. Moffat / Applied Catalysis A: General 146 (1996) 65-86
Investigations in the early part of the last decade on the conversion of methanol to hydrocarbons containing two or more carbon atoms (the so-called methanol to gasoline (MTG) process) showed that HPW was vastly superior to HPMo as a catalyst for this process with the latter catalyst producing considerable quantities of carbon oxides [74]. Since this process is known to require relatively strong acidic sites the aforementioned results provide experimental conformation of the predictions of the results of extended Hfickel calculations mentioned earlier in this report [58]. These calculations showed that the charges on the terminal oxygen atoms of those Keggin anions containing molybdenum in the peripheral metal positions are higher than on those containing tungsten and thus that the mobility of the proton and hence the acidic strength in the MOCC containing the latter is higher than in those containing the former element. Surprisingly, tests of the same MTG reaction with the ammonium salt of HPW produced selectivities superior to those obtained with the parent acid [75,76]. Analyses of the ammonium salt (NH4PW) by various methods including PAS FTIR showed that free protons, in addition to those bound in NH~were present and consequently, although the salt was synthesized from stoichiometric quantities of the preparative reagents the resulting solid was nonstoichiometric [46]. Thus the improved selectivities in the MTG process obtained with NH4PW could be attributed to the residual protons with acid strength distributions perturbed by the presence of the ammonium cations. It should be noted that the surface area of NH4PW was significantly higher ( > 100 m 2 g - l ) [75,76] than that of HPW ( < 10 m 2 g - l ) [73] and consequently adds an additional factor to the functionalities of the catalyst. This will be explored further later in this discussion. Similar observations have been made with NH4PW employed catalytically in the alkylation of toluene to form xylenes [77]. However, in this example, since the selectivity to p-xylene exceeded that expected at equilibrium it is apparent that, in addition to the acidic properties of NH4PW, its pore structure played a significant role in the shape selective process. It is important to note that, although it may be contended that additional protons can be produced from the ammonium ions of NH4PW resulting from the loss of ammonia at higher temperatures, both the aforementioned MTG and toluene alkylation processes took place at temperatures lower than those shown from TPD to be required for ammonia desorption. The aforementioned recent results obtained by various workers [66-69] with nonstoichiometric cesium compounds and their conclusions concerning the variations of acidity with the relative concentrations of Cs + and H ÷ are evidently consistent with the earlier results obtained with nonstoichiometric NHaPW [75-77]. As noted above, the early work with NH4PW showed that its surface area was relatively high. Subsequent studies in the late 80's showed that the high surface area of NH4PW resulted largely from the presence of micropores
J.B. Moffat / Applied Catalysis A: General 146 (1996) 65-86
81
[77,78]. Further, it was demonstrated that high surface area, microporous MOCC could be prepared from 12-tungstophosphoric, 12-molybdophosphoric, 12tungstosilicic, 12-molybdosilicic and 12-tungstoarsenic acids with a number of the monovalent cations of the 1A group [78-86]. Much of these results came from the suitable analysis of nitrogen adsorption-desorption isotherms, an example of which is shown in Fig. 12 [79,80]. These isotherms exhibit a sharp increase in the quantity adsorbed at low partial pressures which is indicative of the presence of micropores. It is also of interest to note that the cesium salt of HPW displays a hysteresis loop showing the presence of mesopores. The more recent work with CsPW compounds is in semiquantitative agreement with these earlier results. For purposes of illustration, Tables 1 and 5 show the surface areas and mean pore radii for the various salts of the heteropoly acids. It is of interest to note that the sodium salts show no evidence of microporosity and possess surface areas similar to those of the parent acids. Confirmation of the existence of the
I
90
PW
salts
60 •
y 30
i
0.5
1.0
PIPo Fig. 12. Nitrogen adsorption-desorption isotherms at 78 K for high surface area cesium, ammonium, and potassium salts of 12-tungstophosphoric acid.
J.B. Moffat /Applied Catalysis A: General 146 (1996) 65-86
82
Table 5 Mean micropore radii (A) of high surface area MOCC Cation
Anion
K+ NH~Rb + Cs +
PW
PMo
SiW
AsW
8.8 10.3 13.9
9.3 13.0
9.5 10.3 10.5
11.2 11.3 9.4 9.9
14.3
microporous structure in various of the MOCC has recently been provided by 129Xe NMR [87,88]. In view of the recent work on dicationic MOCC discussed earlier in this report the influence of the second cation on the morphological properties of these materials is of interest. Ion exchange studies of the MOCC have shown that exchange of the cations from the high surface area, microporous MOCC is possible although complete substitution is not possible [89]. Further, the extent of the exchange is dependent on the relative sizes of the entering and exiting cations. However, most significantly such exchange can be performed with retention of structure, both crystallographic and morphological, although not surprisingly, shifts reflecting differences in sizes of the cations are observed (Figs. 13 and 14) [90]. In the studies of the oxidation of isobutane to methacrylic acid on the N H 4 / K salts of HPMo as noted earlier in this report it is expected that high surface areas and microporous structures should be retained in the presence of the cations. As seen from Fig. 13 the ion exchange of NH~- by K + alters the
80
60 G) m
40
J
1
I
25
50
75
100
Fig. 13. BET surface area as a function of the cation composition for the ion exchanged K ÷ / N H ~ - / P M o ~ 2 0 ~ 3 system.
83
J.B. Moffat / Applied Catalysis A: General 146 (1996) 65-86 11,70
,
,
,
11,65
o. ~
11.60
11.55 0
I 25
i 50
I 75
100
P e r c e n t K+
Fig. 14. Lattice parameter as a function of cation composition for the NH~-/K+/PMol20~03 ion exchange system.
surface area from approximately 75 to 44 m 2 g-~. Although the authors of the aforementioned paper on the oxidation of isobutane do not discuss changes in the morphological properties of their catalysts with cationic composition they do note that the surface areas of the fresh catalysts were approximately 140 mZ/g [701. In the work discussed in the present report, metal-oxygen cluster compounds have frequently been modified by the addition of multivalent cations. However, the structural location of these is uncertain. Recent single crystal X-ray diffraction studies have shown that it is difficult, if not impossible, to synthesize MOCC with Keggin structure from multivalent cations [45]. Finally, some comments concerning the mechanism through which the MOCC activate alkanes is of relevance. In earlier work on the activation and partial oxidation of methane, MOCC containing molybdenum were found to be superior to those of tungsten [54]. In studies with HPMo supported o n SiO 2, as the protons are replaced by cesium the tumover rates for methane decrease to the value expected for silica itself while the selectivity to formaldehyde becomes vanishingly small. Concomitantly the selectivities to CO and CO 2 decrease and increase, respectively, approaching those values found for the support. Similar results were obtained on exchange of the protons by various monovalent cations [91]. These results, together with those from other experiments, suggested that the proton played a role in the oxidative capabilities of the catalyst. This participation is probably indirect with the proton extracting oxygen atoms from the Keggin anion to produce oxygen vacancies which are required for the activation of the oxygen source and therefore the methane.
J.B. Moffat / Applied Catalysis A: General 146 (1996) 65-86
84
Many years ago, Bohme and Fehsenfeld [92] showed that the O - ion is capable of stripping a hydrogen atom from an alkane: 0
+ RH --->O H - + R
Since that time many workers have studied the conversion of methane and have concluded that hydrogen atoms are extracted from the alkane by O followed by the reaction of alkyl radicals with surface 0 2- ions. A similar mechanism may be extant in the oxidation of ethane and higher hydrocarbons on metal-oxygen cluster compounds with the oxygen vacancies being replaced by O - from the oxidant. The alkyl radicals which are formed from the C - H scission can then interact with anionic oxygen atoms possessing a higher electron density. Since these will be relatively labile, where the anions contain molybdenum, the alkyl radicals may extract such atoms from the anion to form a peroxy radical. Acid-catalyzed processes, such as the conversion of methanol, in which the anions are methylated, thus show an interesting correspondence with redox processes. However, in the former reaction an alkyl ion and relatively tightly bound anionic oxygen atoms are involved while in the latter alkyl radicals and labile anionic oxygen atoms are central to the mechanism. It is probable that the first step in the conversion of an alkane on heteropoly oxometalates involves the formation of alkyl radicals. Since the conversion decreases with the temperature at which the catalyst is pretreated the C - H bond dissociation is apparently a surface-initiated process. The alkyl radicals can undergo further reactions on the surface or desorb into the gas phase where reaction with the oxidant to form an alkene occurs. The desorption process and the subsequent formation of the alkene is expected to be favoured by increasing temperatures. The alkyl radicals which are adsorbed on the surface of the catalyst can produce the corresponding alkenes or can interact with the oxygen species to produce alkoxides. This process has its analogue in the alkanol conversion process in which, for example, the heteropoly anion becomes methylated by methyl cations attaching themselves to the terminal oxygen atoms of the anion [93]. These oxygen atoms can be extracted by the alkyl groups to form an alkoxide and ultimately the corresponding aldehyde. Thus the selectivity to the latter is expected be dependent on the lability of the anionic terminal oxygen atoms which has been predicted to be higher in molybdenum-containing heteropoly anions than in those containing tungsten [58].
Acknowledgements The financial support of the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.
J.B. Moffat/ Applied CatalysisA: General 146 (1996) 65-86
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