- Email: [email protected]
DBH (deboronated borosilicate molecular sieve)-catalyzed oxidation reactions
/ ELSEVIER
APPLIED CATALYSS I AG : ENERAL
Applied Catalysis A: General 143 (1996) 29-51
The CVD F e / M o / D B H ( deboronated borosilicate molecular sieve)-catalyzed oxidation reactions Jin S. Y o o
*
Amoco Research Center, P.O. Box 3011, Naperville, IL 60566, USA
Abstract
The CVD F e / M o / D B H (deboronated borosilicate molecular sieve) catalyst exhibits a paraselective property for the gas phase O 2 oxidations of a variety of alkylaromatic compounds. Novel reactions studied with the catalyst include selective synthesis of aldehydes, activation and utilization of CO z in the oxidation reactions, syntheses of oxygenates from alkenylaromatics, oxidative dehydrogenation reactions of alkylaromatics and alkanes. Terephthaldehyde, benzaldehyde and their derivatives are prepared from methylbenzenes, alkylaromatics, alkenylaromatics, and para-substituted toluene derivatives. Alkenylaromatics and olefins are formed from alkylaromatics and alkanes, respectively, as an initial dehydrogenated product. These reactions are described using the representative substrate(s) as an example, and the results are interpreted based on the para-selectivity based on the effects of various para-oriented substituents such as alkyl, alkenyl, hydroxy, methoxy and halide group. These reactions share a reaction mechanism involving the rate-determining hydrogen abstraction initiated by a common active species, ferric molybdate. Keywords: Para-selective oxidation; CVD Fe/Mo/DBH; Methylaromatics; Alkenylaromatics; Alkylaromatics; Oxidative dehydrogenation; CO 2 activation
1. I n t r o d u c t i o n
The chemical vapor deposition technology has been utilized for depositing highly and uniformly dispersed fine particles of catalytic materials such as metal and metal oxides on a supporting matrix. The ultra thin layer of silica was
* Correspondence address: 2315 Mast Court, Flossmore, IL 60422, USA, tel./fax. (+ 1-708) 7984998, e-mail jsyoo @amoco.corn 0926-860X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. Pll S0926- 860X(96)00069-5
30
ZS. Yoo /Applied Catalysis A: General 143 (1996) 29-51
prepared by the CVD of Si(OMe) 4 on "),-alumina, titania and zirconia [1-3]. The silica monolayer deposited on these metal oxides was employed in the double bond isomerization of olefins [2] and the Beckmann rearrangement of cyclohexanone oxime [4]. The CVD technique has also been used for controlling the pore opening size of zeolites such as mordenite [5,6] and HZSM-5 [7,8]. The Pt-loaded H-mordenite has been modified by the CVD of silica to improve the shape selectivity for hydrocracking of paraffins. Other applications were directed to catalytic cracking, isomerization, alkylation, disproportionation and oxidation. The silica modified HZSM-5 has been noted for alkylation of toluene with methanol and toluene disproportionation to make p-xylene. The ultra-thin germanium oxide layer in a monoatomic order thickness on zeolite was also prepared and studied by others [9-11]. In our laboratory, efforts to prepare novel catalysts by depositing the catalytically active species using the surface silanols as an anchoring site are continuing. We have succeeded in making the CVD Fe/Mo/DBH(deboronated borosilicate molecular sieve) by the CVD technique. The CVD F e / M o / D B H catalysts prepared in small batches in laboratory scale, that have slightly varied metal compositions, are referred to as the catalyst hereafter. The active catalytic species, ferric molybdate, is uniformly dispersed as fine particles (2-40 nm) on the surface of the catalyst, and it catalyzed various oxidation reactions through hydrogen abstraction as an initial reaction. The encouraging results that terephthaldehyde was formed in high yield from the gas phase O 2 oxidation of p-xylene prompted us to look into other types of oxidation reactions. In an early attempt to selectively synthesize useful and reactive bifunctional aromatic aldehydes such as terephthaldehyde and its derivatives, the gas phase O 2 oxidation was undertaken with various hydrocarbon substrates over the same catalyst. Some findings have previously been reported. These include the para-selective oxidation of p-xylene to terephthaldehyde and p-tolualdehyde [12,13], the catalytic activity and selectivity in para-selective O 2 oxidation reactions in the presence of CO 2 [14], the preferential oxidation of p-xylene from xylene isomer mixture containing ethylbenzene utilizing an unusual reactivity gap between p-xylene and other isomers [15], the identification that DBH as the most promising supporting matrix for the catalyst preparation among various zeolites [16], the oxidation reactions of polymethylated benzenes in order to expand the para-selectivity of the catalyst [17], the detailed CVD catalyst preparation techniques [18], the characterization of the catalyst [19] and the one-step hydroxylation of benzene to phenol [20]. In this paper, various oxidation reactions catalyzed over CVD F e / M o / D B H are reported using representative substrates. Based on the para-selectivity of the catalyst, a reaction mechanism is postulated that the initial slow step of hydrogen abstraction(s) is promoted by the common active species, ferric molybdate, which provides the "pair" and "single" sites.
J.S. Yoo/Applied Catalysis A: General 143 (1996) 29-51
31
2. Experimental The method of preparing the CVD F e / M o / D B H catalyst by the chemical vapor deposition technique, evaluation and characterization of the catalyst, and analyses of products with an on-line GC were described in detail elsewhere [12-14]. All runs discussed hereafter followed the procedures described in the above papers. When the reactor reached the desired reaction temperature under the flow of purge gas, the purge gas flow was stopped and the feed gas was passed through the catalyst bed in the reaction under the following conditions: Catalyst load:
0.5-10 g
Molar ratio of O J s u b s t r a t e :
10-40
Substrate concentration in feed:
0.1-0.74 mol%
WHSV:
0.1-1.1 h i
Contact time:
0.1-0.5 s
Reactor temperature:
150-500°C
Gas flow rate:
100-4000 sccm
3. Results and discussions 3.1. Methylaromatics A series of methylaromatics such as toluene, xylenes, pseudocumene, and durene were subjected to gas phase O 2 oxidation over the catalyst under similar reaction conditions.
3.1.1. Toluene vs p-xylene The representative results from the gas phase 0 2 oxidations of toluene and p-xylene over the catalyst are compared in Table 1. At 350°C, only 13.4% of toluene was converted to benzaldehyde in 63.7 mol% selectivity whereas 73.7% of p-xylene was oxidized to form two aldehyde products, terephthaldehyde in 49.2 mol% and p-tolualdehyde in 25.2 mol%. This shows the conversion of
J.S. Yoo /Applied Catalysis A: General 143 (1996) 29-51
32
Table 1 Oxidations of toluene and p-xylene over CVD F e / M o / D B H Substrate Temp. (°C) Conversion (%) Product selectivity (%) Benzaldehyde Terephthaldehyde p-Tolualdehyde Maleic anhydride By-products CO CO 2
Toluene
p-Xylene
350
400
325
350
13.4
32.6
46.0
73.7
63.7
60.5 48.6 29.2
49.2 25.2
1.5 4.3 10.6
1.8 5.8 16.1
0.0
10.5
0.0 36.3
4.0 24.8
p-xylene was much higher than that of toluene and also the aldehyde formation was much greater with p-xylene than with toluene. The combined yield of p-tolualdehyde (monoaldehyde) and terephthaldehyde (dialdehyde) from pxylene was 54.8% compared to the yield of benzaldehyde from toluene which was merely 8.5%. Toluene reflected the greater substrate burning as shown by the high CO x selectivity (36.3%) 3.1.2.
p-Xylene, o-xylene, and m-xylene
The xylene isomers were individually oxidized under the standard conditions in the gas phase, and the representative results are summarized in Table 2. The conversion of p-xylene was very high, 74%, while o-xylene and m-xylene exhibited a very limited conversion. 3.1.3. Pseudocumene vs durene
The results of pseudocumene and durene are compiled in Table 3. As shown below, three monoaldehyde products were expected from pseudocumene de-
Table 2 Gas phase 02 oxidations of xylene isomers Substrate
p-xylene
o-xylene
m-xylene
Conversion (mol%) Product selectir, ity (mol%) Monoaldehyde Dialdehyde By-products
74
6
3
CO + CO 2
25
33
49
trace
2 22
0 66
0 2 / x y l e n e : 4 3 / 1 , contact time: 0.15 s, WHSV: 0.32 h - t , 350°C.
67 0 0 33
33
J.S. Yoo /Applied Catalysis A: General 143 (1996) 29-51 Table 3 Gas phase O 2 oxidations of pseudocumene and durene Substrate
Pseudocumene
Durene
Temp. (°C) Conversion (%)
450 2.75
450 74.8 a
Product selectivity (mol%) 2,5 -dimethylbenzaldehyde 2,4-Dimethylbenzaldehyde 3,4-Dimethylbenzaldehyde 2-Methylterephthaldehyde COy 2,5 -Dimethylterephthaldehyde 2,3-Dimethylphthalic anhydride 2,3 -Dimethylphthaldehyde 3 -Methyl-4-formylphthaldehyde 2,4,5-Triformyltoluene 2,3,5,6-Tetraformylbenzene 2,4,5 -Trimethylbenzaldehyde Maleic anhydride
9.7 8.2 11.7 47.9 19.5 47.5 8.9 9.9 6.9 4.9 trace 10.9 t0.9
Pseudocumene feed: premixed gas, 4.0 vol% 02 and 4.0 vol% N2 in He 02/pseudocumene: 2 8 / 1 , contact time: 0.21 s, WHSV: 0.34 h -j . Durene feed: durene solution in benzene (molar ratio of durene/benzene = 2.2) (pump rate = 0.8 g / h ) , feed gas: 6.0% 02 and 6.0% N 2 in He. a Area % excluding CO x.
pending on which methyl substituent was oxidized. Indeed all three monoaldehydes were obtained with comparable selectivities; 2,5-dimethylbenzaldehyde in 9.7%, 2,4-dimethylbenzaldehyde in 8.2%, and 3,4-dimethylbenzaldehyde in 11.7%. Of the two possible dialdehyde products, only one was obtained. That was 2-methylterephthaldehyde which was the result of the oxidation of two para-oriented methyl groups and its selectivity was very high, 47.9%.
CH~
CH3
CH3
CH,
H3C-~-CH , ~ OHC-~)-CHO + OHC-~-CH,
* H3C-~-CHO
main product
o.@o .
x,c-
-c.,
(&,lo\ •
.,C
o/O
.
co..,,o
J.S. Yoo/Applied Catalysis A: General 143 (1996) 29-5l
34
Compared with pseudocumene, durene was much more reactive as seen by its high conversion: 27.5% of pseudocumene vs 74.8% of durene. From durene, as shown below, one can expect one monoaldehyde from the oxidation of one methyl group, two dialdehydes from the oxidation of two methyl groups, one trialdehyde from the oxidation of three methyl groups, and one tetraldehyde from the oxidation of four methyl groups. One dialdehyde, 2,5-dimethylterephthaldehyde, had a very high selectivity. (47.5%). Only a trace quantity of the tetraldehyde was produced. Other aldehyde were produced in moderate selectivities.
~C. [~.C]Bj u~c-'-,~,,.'-cu~
02
H3C.[~..CHO
+ H3C~ C H O
oac - ~ , ~ j - cu~ =ain
u,c ~ - C H O
product
s,c- ~ . . . c a o H,c'~'CH,
+
+
u,c-[~-cao OHC'~'CHO
+
trace
COx
+
1~0
amount
The catalyst has been known to oxidize the methyl substituent to formyl in many methylaromatics [12,13,19]. Based on the studies of the various methylaromatics given above, it can be concluded that the reactivity of the substrate and the product selectivities depend on the number of methyl substituents and their relative positions. Presence of a second methyl substituent increased the reactivity as shown by the increased conversion of p-xylene compared with that of toluene. Especially the presence of the second methyl group which was in para position to the first methyl group (these two methyl groups are called hereafter para-oriented methyl groups) remarkably enhanced the performance of the catalyst as evidenced in the reaction of p-xylene, pseudocumene and durene. The great differences observed in the p-xylene oxidation and the toluene oxidation in terms of its conversion and selectivities of aldehyde products indicate that the reaction path of the formation of terephthaldehyde and ptolualdehyde from p-xylene is not the same as that of the formation of
J.S. Yoo / Applied Catalysis A: General 143 (1996) 29-51
35
benzaldehyde from toluene. It is believed that the former involves two different active sites: the interaction of two para-oriented methyl substituents of the substrate with the " p a i r " site to form terephthaldehyde, and of one methyl group with the "single" site of the catalyst to produce p-tolualdehyde. On the other hand, the latter, the reaction of toluene, involves the interaction of the methyl substituent with the "single" site of the catalyst. The reactivity of xylene isomers was in the order of p-xylene >> o-xylene > m-xylene. The para selectivity of the catalyst was clearly proven here. Utilizing this big reactivity gap between p-xylene and other isomers, p-xylene was preferentially oxidized from the xylene mixtures [15]. Once p-xylene is preferentially oxidized in the xylene mixture, the resulting aldehydes are separated from the unreacted o- and m-xylene by a simple separation method such as distillation. The unreacted o- and m-xylene can be recycled to the isomerization unit. This implicates that the costly process of conventional p-xylene separation can be eliminated, and that a xylene mixture can be used in place of the separated pure p-xylene for the synthesis of aldehydes and other oxygenates. Even though durene molecule was bulkier than pseudocumene molecule, durene was much more reactive than pseudocumene. Durene has two pairs of para-oriented methyl substituents while pseudocumene has only one such pair and the third methyl group is in ortho- and meta-position to the other two methyl groups. The chance of para-selective oxidation would be doubled by the presence of two pairs of para-oriented methyl groups in durene. The major oxidation product of pseudocumene was 2-methylterephthaldehyde. This is in agreement with the fact that the para-oriented methyl groups in pseudocumene were readily oxidized as in the case of p-xylene. 2,5-Dimethylterephthaldehyde was the main product from durene oxidation and formed as a result of the oxidation of the para-oriented methyl substituents. Since only a trace amount of the tetraldehyde, 2,3,5,6-tetraformylbenzene, was observed, it can be said that the second set of para-oriented methyl groups enhanced the reactivity of the first oxidation step but did not participate in the second oxidation step.
3.2. Oxidative dehydrogenation of alkylaromatics When an alkyl group in alkylaromatics becomes ethyl or higher, the oxidative dehydrogenation reaction takes place at the initial step, and the vinyl group is formed over the catalyst under the standard gas phase 0 2 oxidation conditions [24]. Alkylaromatics such as ethylbenzene, p-diethylbenzene, p-ethyltoluene, p-t-butylethylbenzene and cumene were converted to styrene, p-divinylbenzene and p-ethylstyrene, p-vinyltoluene, p-t-butylstyrene, and a-methylstyrene, respectively, at the initial step. The oxidative dehydrogenation reaction was discussed using ethylbenzene, p-ethyltoluene p-diethylbenzene, and low alkanes as representative substrates.
36
J.S. Yoo/Applied Catalysis A: General 143 (1996) 29-51
3.2.1. Ethylbenzene and styrene The results of ethylbenzene oxidation are shown as a function of temperature in Table 4. The conversion increased with the temperature. At low temperatures styrene was the main product but as the temperature increased, less styrene was detected and more benzaldehyde was produced. At high temperatures maleic anhydride became predominant. Based on these observations, it was suspected that the catalyst oxidized the ethyl substituent in ethylbenzene to vinyl group via oxidative dehydrogenation reaction. The initial product, styrene, was then immediately oxidized further to produce benzaldehyde, benzoic acid and maleic anhydride along with carbon oxides as shown below.
~
-CH-CH~
initialp r o d u c t
~
-CHO
~-COOH
0 HC -
II
C
+
CO,
÷
H20
HC 0
In order to confirm that the oxidation products obtained in the oxidation of ethylbenzene were derived from further oxidation of the styrene produced in the initial stage, pure styrene was subjected to the identical oxidation conditions used in the oxidation of ethylbenzene above. The results of styrene oxidation reactions are summarized in Table 5. As expected, benzaldehyde and benzoic acid were the major products from styrene.
3.2.2. p-Ethyltoluene The results of the oxidation of p-ethyltoluene are summarized in Table 4. The reaction paths are illustrated in the scheme below for the reaction at 375°C. Presence of p-methylstyrene which was formed through the oxidative dehydrogenation reaction at the ethyl substituent site in p-ethyltoluene, was observed. Subsequent oxidation of the vinyl group in the resulting p-methylstyrene was shown by the substantial amounts of p-tolualdehyde and p-toluic acid in the product mixture. It should be noted that the oxidative dehydrogenation reaction
37
J.S. Yoo/Applied Catalysis A: General 143 (1996) 29-51
proceeded faster than the oxidation of two methyl groups in p-xylene to aldehydes. A small amount (5.6%-8.4%) of p-methylacetophenone was also produced as expected from the alternate oxidation of p-methylstyrene.
3.2.3. p-Diethylbenzene Since considerable exothermicity was observed with p-diethylbenzene, the reactor temperature was lowered and the flow rate of the feed gas was increased in this study. This observation led us to conclude that p-diethylbenzene was far more reactive than p-ethyltoluene or p-xylene in the gas phase 0 2 oxidation over the catalyst. The results are summarized in Table 4. Two dehydrogenation products, p-ethylstyrene and p-divinylbenzene, were identified.
u,c-
©
-CS, CH3
--, O=
39.91
S,C-
©
-Ca-C~,
7.31
-~
~,
H,C-
©
-COCll~
6.9t
,1.
H3G-Q-GHO 17.2!1;
BsC-Q-COOH 23.0t
H3C-O-GH2CHO trace
OHC-O-CHO 0.6t
As shown in the scheme below, p-ethylstyrene and p-divinylbenzene were initially formed at two different active sites, e.g., "single" and " p a i r " site, respectively, from the p-diethylbenzene substrate, and these products were subsequently further oxidized via two different reaction paths, p-Ethylstyrene was converted to p-ethylbenzaldehyde and then to p-ethylbenzoic acid while much more reactive p-divinylbenzene was quickly oxidized to p-vinylbenzaldehyde and then to terephthaldehyde. In the study of the oxidation of various alkylaromatics where the alkyl substituent is ethyl or higher, the results showed that the catalyst oxidized the ethyl group to a vinyl group via oxidative dehydrogenation reaction, and if methyl substituent was present at the same time the catalyst preferred the ethyl over the methyl for oxidation and left the methyl group unchanged. Conversion of ethylbenzene to styrene, p-ethyltoluene to p-methylstyrene,
38
J.S. Yoo /Applied Catalysis A: General 143 (1996) 29-51
and p-diethylbenzene to 4-ethylstyrene and p-divinylbenzene are such examples. This is in contrast to the activity shown by the same catalyst on p-xylene.
~o~
~c-ce-< ~ -c.~ ;02
u2c-clt-~
-cu-cu2
~ 02
OHC-~-C~Hs
H2C-CH-~-GHO
HOOt~-
OH¢-~-CHO
-C:;~ls
p-Xylene was converted to terephthaldehyde indicating that the catalyst oxidized the methyl substituent to a formyl group. However, the methyl group remained the same in p-ethyltoluene since p-ethylbenzaldehyde was not observed. One can conclude that the ethyl group in p-ethyltoluene rendered the methyl group unreactive toward oxidation, while the ethyl group in para-position was being oxidized. Similar study done on p-(t-butyl)ethylbenzene [24] showed the dehydrogenation occurring at the ethyl substituent. These findings point to the conclusion that the a,[3-hydrogen elimination reaction at an ethyl site must be easier and faster than the oxidation of methyl to a formyl group by this catalyst. The initially formed vinyl group was very reactive and oxidized immediately to aldehyde or ketone which then changed to carboxylic acid. This was why benzaldehyde and benzoic acid were observed from ethylbenzene via styrene. In the same manner p-methylstyrene produced from p-ethyltoluene underwent oxidation and formed p-tolualdehyde and p-toluic acid. Oxidation at the a-carbon of p-methylstyrene produced p-methylacetophenone, p-Ethylstyrene (initially produced from p-diethylbenzene) formed p-ethylbenzaldehyde and p-ethylbenzoic acid. p-Divinylbenzene (produced again from p-diethylbenzene)
J,S. Yoo / Applied Catalysis A: General 143 (1996) 29-51
39
Table 4 Gas phase 02 oxidation of ethylbenzene (EB), p-ethyltoluene (ET) and p-diethylbenzene (DEB) Substrate
EB
Temp.°C Conversion (%)
325 5.2
375 22.6
25.3 37.9 11.6
7.1 28.1
ET
DEB
375 39.0
325 27.1 a
Productivi~ selectit, ity (%) Styrene Benzaldehyde Benzoic acid Maleic anhydride CO CO 2 p-Methylstyrene p-Tolualdehyde p-Toluic acid p-Methylacetophenone Terephthaldehyde
24.7 12.1 17.8
10.7 24.4
14.8 24.9 7.3 17.2 23.0 6.9 0.6 10.0 c 1.6 17.3 5.5 26.6 7.7 26.3 4.1
o-Diethylbenzene b p-Divinylbenzene Terephthaldehyde p-Vinylbenzaldehyde p-Ethylstyrene p-Ethylbenzaldehyde p-Ethylbenzoic acid Unidentified products
Reaction conditions for ethylbenzene: 325 and 375°C, WHSV: 0.57 h l, feed: 6% 0 2 and 6% N 2 in He, flow rate: 400 sccm, 0 2 / E T : 7.51/1 in a micro-reactor loaded with 0.5 g catalyst. Reaction conditions for diethylbenzene: feed: 6% O 2 and 6% N 2 in He, flow rate: 800 sccm, 325°C in a micro-reactor loaded with 0.5 g catalyst. Area %. b May be the isomerization product of p-diethylbenzene. c Excluding CO 2.
formed p-vinylbenzaldehyde and terephthaldehyde. The path through monovinyl intermediate, p-ethylstyrene, gave the higher yield as shown by the combined area percentage of 15% compared with the combined area percentage of 8.5%. Table 5 Oxidation of styrene Temp, (°C)
275
300
325
Feed type
I
II
I
II
I
Conversion (%)
14.4
12.3
34.5
265. l
73.3
22.2 3.4 9.2 53.4 6.3 5.5
24.0 4.2 9.7 56.6 5.5
19.9 1.9 6.5 57.3 6.6 5.6
21.7 2.5 7.7 60.5 5.7
18.9 0.9 3.8 58.8 7.9 5.9
1I
Product selectiuity (%) Benzaldehyde Phenylacetaldehyde Acetophenone Benzoic acid CO CO 2
21.2 1.5 5.2 62.5 6.2
Feed I: 0.17% styrene, 5.0% 02, 18.3% N 2 in He; Feed II: 0.17% styrene, 5.0% 02, 18.4% N 2 in CO 2.
40
J.S. Yoo /Applied Catalysis A: General 143 (1996) 29-51
through divinyl intermediate, p-divinylbenzene. It suggests that these two paths are competing in the oxidative dehydrogenation of p-diethylbenzene. This argument would be expanded further in the CO 2 oxidation section later. Since benzaldehyde and benzoic acid obtained from the oxidation of ethylbenzene were again the major products from styrene oxidation, it is safe to assume that ethylbenzene, p-ethyltoluene and p-diethylbenzene produced the corresponding vinylaromatics initially even though a large amount of vinyl derivative was not found in the final product mixture. The para-selectivity of the catalyst favorably influenced the oxidative reactions of these three substrates, and the reactivity of p-diethylbenzene was greater than that of p-xylene. Based on the reaction mechanism postulated on the p-xylene oxidation [13], it was theorized that the oxidative dehydrogenation of the ethyl group in the p-diethylbenzene substrate must occur in two ways, using the "single" site or the " p a i r " sites of the catalyst [13]. If just one ethyl group in the p-diethylbenzene substrate interacted with the "single" site of the catalyst in the stand-on manner, p-ethylstyrene was produced as a result of single o~,[3-hydrogen elimination reaction. Once p-ethylstyrene was formed, the second ethyl group became much more difficult to be oxidized, and rather the vinyl group in p-ethylstyrene was oxidized further to p-ethylbenzaldehyde, which eventually converted to p-ethylbenzoic acid. On the other hand, if two ethyl groups in p-diethylbenzene interacted in a concerted manner on the " p a i r " sites of the catalyst in the lying-on fashion, p-divinylbenzene was formed as a result of the concerted double oL,[3-hydrogen elimination reactions at the two ethyl groups. p-Divinylbenzene was extremely reactive and changed to terephthaldehyde and p-vinylbenzaldehyde upon the subsequent oxidation.
3.2.4. Alkanes Besides alkylaromatic substrates, this work was extended to low alkanes such as n-butane, propane and ethane under the standard gas phase O 2 oxidation conditions [24]. n-Butane was subjected to the gas phase O 2 oxidation in a quartz reactor loaded with 10 g of the same F e / M o / D B H catalyst. A premixed feed containing 0.5% n-butane, 28% 0 2 and 3% N 2 in CO 2 was oxidized, and the results are summarized along with the reaction conditions in Table 6. In the presence of CO 2 in O 2, n-butane was converted to maleic anhydride, and an anticipated dehydrogenated intermediates, n-butene a n d / o r butadiene, were not detected in the reaction effluent. The premixed gases consisting of 2% propane (or ethane), 4% 0 2 and 4% N 2 in He were subjected to the oxidation reaction in the reactor loaded with 5.0 g of CVD Z r / M o / D B H catalyst. The CVD Z r / M o / D B H catalyst was prepared from ZrCI4, MoOeCI 2 and borosilicate molecular sieve by the chemical vapor deposition method. Metal composition of the resulting catalyst was 10.45% Mo and 4.1% Zr, and the atomic ratio of M o / Z r was 2.4/1. These results are also combined in Table 5. Propane and ethane were dehydrogenated to propylene and
J.S. Yoo / Applied Catalysis A: General 143 (1996) 29-51
41
Table 6 Oxidation of n-butane over CVD F e / M o / D B H , propane and ethane over CVD Z r / M o / D B H , 10.45% Mo, 4.1% Zr and M o / Z r = 2.4/1 Alkane
n-butane
Oxidant
02/CO 2
Temp. (°C) Conv. (%)
400 14.5
Propane
Ethane
02 425 34.0
02
500 3.9
550 10.3
550 2.0
600 3.8
0.2 85.2
0.6 70.3
88.5
86.3
10.4 3.7
21.0 4.7
11.0
13.7
Product selectit,ity (%) Ethylene Propylene Maleic anhydride CO CO 2
8.9 91.4 -
3.7 96.3 -
For n-butane, premixed feed: 0.5% n-butane, 28% 02 and 3% N 2 in C02, flow rate: 800 sccm, WHSV: 0.114 h - j , contact time: 2.10 s, 02/n-butane: 56/1. For propane and ethane, premixed feed: 2.0% propane or ethane, WHSV: 1.135 h -~ , contact time: 0.84 s, 0 2 / p r o p a n e or ethane: 2/1.
ethylene, respectively, under identical oxidation conditions. It is not surprising to observe that conversion of ethane is much smaller than those of propane and n-butane, and that the initial products, n-butene and butadiene, are completely oxidized to form maleic anhydride under the conditions studied. Similar results were also obtained from these three substrates over the (CVD F e / M o / D B H ) catalyst. In summary, n-butane was converted to maleic anhydride, while propane and ethane were oxidatively dehydrogenated to propylene and ethylene, respectively, by the gas phase O 2 oxidation. 3.3. Para-substituted toluene derivatiues Para-substituted toluene derivatives, CH3-C6H4-X, where X stands for OH, OCH 3, C1, CHO, C2H 5, and CH=CH2, were selected to study the effects of the substituent on the oxidation of methyl groups in these toluene derivatives. 3.3.1. p-Cresol The 0 2 oxidation of p-cresol over the catalyst failed to produce p-hydroxybenzaldehyde, the expected product, because this initial product was so reactive that the tarry material resulted instead even at low temperatures such as 200°C. To circumvent this problem, the catalyst surface was modified by coating with Si(OMe) 4 by the CVD method [15]. The reaction over the new modified catalyst produced p-hydroxybenzaldehyde, phenol, and carbon oxides in selectivities of 14.1, 5.5, and 80.5%, respectively, at 9.1% p-cresol conversion at 250°C. Extensive burning was observed probably due to the highly reactive nature of p-cresol and silica-coated catalyst itself.
42
J.S. Yoo /Applied Catalysis A: General 143 (1996) 29-51
HO_~_CHs
O~
HO-~-CHO
+
CH30-~-CH 3
~
CH30-~-CHO
+
~ - O H
cox
+
CO~
+
~o
3.3.2. p-Methylanisole To avoid extensive burning and other drawbacks experienced with p-cresol, p-methylanisole was chosen in which the hydroxy group in p-cresol was replaced by the inactive rnethoxy group. Approximately 26 area% of the starting substrate was converted to a sole product, p-anisaldehyde. Selectivities to p-anisaldehyde and to CO x were 81 and 15%, respectively, under the 0 2 oxidation conditions. main path
~ H,c-
-clio
-.
0 He - C~ II HC- C~ II 0 IJ
a,c-
-cooH
-. ~
0
-*
eo~
+
I~O
OHC-~-CHO (minor product) 3.3.3. p-Tolualdehyde As shown in Table 7, approximately 46 mol% of p-tolualdehyde was converted to yield p-toluic acid, maleic anhydride, CO 2 and terephthaldehyde in selectivities of 43%, 19%, 32% and 3%, respectively.
Table 7 Gas phase Oz oxidation of p-tolualdehyde Substrate
p-Tolualdehyde
p-Xylene
Temp. (°C) WHSV ( h - ) Conversion (%)
350 0.38 46
350 0.32 78
Product selectivity (%) Terephthaldehyde p-Tolualdehyde Maleic anhydride p-Toluic acid Toluene CO 2
3 19 43 3 32
44 18 14 4 2 18
The pump rate of p-xylene: 0.15-0.19 g/h, contact time: 0.15 s, WHSV: 0.32-0.38 h -I .
J.S. Yoo/Applied Catalysis A: General 143 (1996) 29-51
43
3.3.4. p- and m-Chlorotoluene The oxidation of p- and m-chlorotoluene are summarized in Table 8. As was expected, the p-chloro-substituted substrate, p-chlorotoluene, was much more reactive than the m-chloro-substituted counterpart. In a typical run, 28% of p-chlorotoluene was converted to form mainly p-chlorobenzaldehyde in 61.6% selectivity at 325°C, while only 10.8% of m-chlorotoluene was converted at 350°C to m-chlorobenzaldehyde and p-chlorobenzaldehyde in selectivities of 26.1% and 23.6%, respectively. These results indicated that the catalyst exhibited catalytic activity for both p- and m-chlorotoluene. It was observed that at elevated temperatures, 400-450°C, the oxidation of m-chlorotoluene yielded more p-chlorobenzaldehyde than m-chlorobenzaldehyde and chlorobenzene. Isomerization must occur during or after the oxidation reaction and become more important at higher temperatures. The para-oriented substituents (X) played a key role in defining the reactivity of substrate and the product selectivity in para-substituted toluenes, H3CC6H4-X. When the para-oriented substituents were OH, OCH 3 and C1, these substituents remained unchanged and the methyl group was selectively oxidized to aldehyde. For example, p-cresol, p-methylanisole and p-chlorotoluene were converted to p-hydroxybenzaldehyde, p-anisaldehyde and p-chlorobenzaldehyde, respectively. On the other hand, formyl ( - C H O ) , ethyl ( -CzH 5) and vinyl ( - C H = C H 2) substituents themselves were oxidized by the catalyst and rendered the para-methyl group in the substrates unreactive, p-Tolualdehyde was converted to p-toluic acid, and p-ethyltoluene to p-methylstyrene which was further oxidized to produce mainly p-tolualdehyde and p-toluic acid.
Table 8 Gas phase 02 oxidation of p- and m-chlorotoluene. Catalyst B: CVD F e / M o / D B H Substrate
p-chlorotoluene
Temp. (°C) Conversion (%)
350 33.7
375 54.4
400 76.5
m-chlorotoluene 350 8.2
400 15.8
450 33.2
59.4 0.3 1.4 12.2 6.8 19.2
53.4 0.2 2.4 15.6 7.8 20.0
42.3 0.1 4.1 16.1 11.1 25.9
24.0 35.2 18.4 8.3
26.9 17.8 9.7 9.1
38.1 7.8 5.3 5.3 14.6
25.3
27.9
Product selectiz~i~., % pCBA mCBA CB MA CO CO 2
11.4
14.2
pCBA: p-chlorobenzaldehyde, mCBA: m-chlorobenzaldeyde, CB: chlorobenzene, MA: maleic anhydride.
44
J.S. Yoo /Applied Catalysis A: General 143 (1996) 29-51
H3C-~-Cl
02
OHC-~-C1
+
co,
+
~o
OHC-G-C1 + OHC-~ ~z
\CI
+
co,
+
u=o
Chemically very reactive substituent groups such as formyl and vinyl are attacked by the catalyst more favorably than the methyl substituent. Conversely, the chemically stable methoxy substituent would not compete with methyl for the oxidation by the catalyst. In the case of an ethyl substituent, as discussed in the above section, the presence of e~ and [3 hydrogens facilitates the dehydrogenation of the ethyl group.
3.4. Oxidation in the presence of CO z In the course of study to find ways to suppress substrate burning in the gas phase 0 2 oxidations of p-xylene over the catalyst, carbon dioxide was added to the O 2. Unexpectedly, the p-xylene conversion was dramatically enhanced by the presence of CO 2 in the system. The same phenomenon was observed with various alkylaromatics. Efforts to elucidate the role played by CO 2 in the oxidation of a variety of hydrocarbons are still being made. Our current understanding on the role of CO 2 is presented here using p-xylene, styrene and p-ethyltoluene as representative substrates.
3.4.1. p-Xylene p-Xylene was subjected to the gas phase 0 2 oxidation in three different types of feeds by varying reaction temperatures from 250 to 350°C. The results are in Table 9. In the presence of CO z, even at 250°C, the conversion increased and more aldehydes, both p-tolualdehyde and terephthaldehyde, were obtained. This indicated CO 2 was activated on the catalyst even under this mild reaction conditions. 3.4.2. Styrene Contrary to p-xylene oxidation, the presence of C O 2 in O2-containing feed gas adversely affected the oxidation of styrene as seen in Table 5. The
J.S. Yoo /Applied Catalysis A." General 143 (1996) 29-51
45
Table 9 Oxidation of p-xylene Temp. (°)C
250
Feed
1
lI
III
300 I
II
IlI
350 I
I1
III
375 I
I1
Cony. (%)
8.6
23.4
0.6
17.6
33.3
5,1
41.2
65.5
4.1
60.7
84.1
32.4 3.3 0.0 0.0 20.0 23.8 0.0 0.0
54.5 4.7 0,0 0.0 21,4 17.8 0.0 -
44.9 0.0 0.0 0.0 29.2 25.9 0.0 -
57.9 16.4 1.3 0.0 6.2 7.5 0.0 10.7
57.2 27.5 1.5 0.0 6.9 6.8 0.0 --
69.4 13.8 0.0 0.0 8.1 8.7 0.0 -
50.2 23.5 2.4 2.4 3.5 1.7 0.6 15.6
40.9 33.6 2.7 6.0 4.8 0.6 3.4 -
60.5 28.4 1.2 0.0 5.4 4.5 0.0 -
40.6 32.6 2.7 5.8 3.1 0.4 4.7 20.2
40.6 30.2 3.1 13.7 4.8 0.0 7.5 -
Product selectit, iO, (%) TOAL TPAL BZAL MA TOL TMBPM CO CO 2
Catalyst 5:6.8 wt% Mo, 1.88 wt% Fe, 540 ppm B, M o / F e = 2.1, 0.508 g (1.4 ml) in a quartz micro-reactor. Oxidation conditions: WHSV: 0.22 h - I , Contact time: 0.21 s, Gas flow rate: 400 sccm. Feed gas blends: 1: 0.1% p-xylene, 1.0% 02, 1.0% N 2 in He. II: 0.1% p-xylene, 1.0% 02, 1.0% N 2 in CO 2. III: 0.1% p-xylene, 1.0%N 2 i n C O 2. TOAL: p-tolualdehyde, TPAL: terephthaldehyde, BZAL: benzaldehyde, MA: maleic anhydride, TOL: tolualdehyde, TMBPM: trimethylbiphenylmethane.
conversion decreased when C O 2 product distribution was observed.
was
present and no appreciable change in
3.4.3. p-Ethyltoluene The promoting effect of
C O 2 o n the conversion of the p-ethyltoluene substrate was found to be very remarkable as illustrated in Table 10. The p-ethyltoluene conversion increased from 39.0% to 60.9%, and selectivity to p-tolualdehyde and p-toluic acid increased also from 17.2% to 20.0%, and
Table 10 Oxidation of p-ethyltoluene over the catalyst (10 g) Temp. (°C)
350
375
400
Feed type
I
II
l
II
1
II
Conversion (%)
4.7
11.9
39.0
60.0
70.0
81.1
14.7 14.8 5.6 23.0 0.3 12.9 24.6
19.4 22.5 8.4 29.0 0.8 14.4
7.3 17.2 6.9 23.0 0.6 14.8 24.9
6.4 20.0 6.7 34.8 1.2 22.0
2.7 16.7 5.6 15.8 1.3 19.1 29.8
2.8 22.6 6.7 21.3 2.4 28.9
Product selectil~iO, (%) p-Methylstyrene p-Tolualdehyde p-Me-acetophenone p-Toluic acid Terephthaldehyde CO CO 2
Feed I: 0.16% p-ET(3.5 g / h ) , 5.5% O 2, 20% N 2 in He. II: 0.16% p-ET(3.5 g / h ) , 5.6% 0 2, 2(1% N 2 in CO_,. WHSV: 0.36-0.37 h - l , contact time: 0.21 s.
46
J.S. Yoo/Applied Catalysis A: General 143 (1996) 29-51
23.0% to 34.8%, respectively, at 375°C in the presence of CO 2. In spite of this increase in conversion and in aldehyde production, the amount of p-methylstyrene remained the same in both streams. This may indicate that p-methylstyrene was immediately consumed to form p-tolualdehyde and then to p-toluic acid. Interestingly, selectivity to p-methylacetophenone stayed low and did not change in both streams and at all temperatures. The presence of CO 2 did not make any difference in the formation of p-methylacetophenone. This probably indicates that a different reaction path which does not involve the formation of the vinyl group leads to the formation of this product. Aldehyde yield was significantly increased by the presence of CO 2 in the 0 2 feed gas in the oxidation of p-xylene and of p-ethyltoluene. However, in the case of styrene oxidation addition of carbon dioxide decreased substrate conversion. It should be pointed out that this finding in the gas phase 0 2 oxidation over the catalyst contradicts results observed in the liquid phase 02 oxidation of styrene with Fe and Rh-complex catalysts [23]. The presence of CO 2 favorably promoted the single O-transfer reaction in the liquid phase oxidation of styrene. The single O-transfer products such as styrene epoxide and phenylacetaldehyde were remarkably enhanced in the presence of CO 2. Considering the fact that the p-xylene reaction and the p-ethyltoluene reaction involve oL,[3-hydrogen abstraction as the initial step which is followed by the oxidation of vinyl to formyl group, whereas the styrene reaction involves only the oxidation of vinyl to formyl group, it can be argued that CO 2 is directly tied to the hydrogen abstraction step to form the vinyl group, but it does not participate in the conversion of the resulting vinyl group to oxygenates in the subsequent oxidation step. Since the hydrogen abstraction is the slow rate-determining step, the beneficial effect of CO 2 on the overall reaction becomes predominant. In short, the slow rate-determining step of hydrogen abstraction may be dramatically accelerated by the presence of CO 2 in the O 2 oxidation. It has been speculated that the peroxocarbonate species, CO 4 , may be responsible for this promoting effect, as this species has already been proposed with some catalysts [22,23]. The peroxocarbonate species may be generated on the molybdenum or iron center in ferric molybdate [19].
O._____C//0 0 0
[
0
\ /~o~ /e~
0
o ~ Mo~ 0
Si
IIIIIIIDBHIIIIIIIIIIII
J.S. Yoo / Applied Catalysis A: General 143 (1996) 29-51
Mo - Mo distance 6.77 7.08
Fe - Fe distance 5.43 A
7.23 A
47
H - H distance (between two CHa groups)
6.37- 7.23 A
Fig. 1. Molecular model of ferric molybdate local cluster showing octahedrally coordinated Fe 3+ and tetrahedrally coordinated Mo 6+ ions. The superposition of p-xylene indicates close size correspondence of M o - M o paired sites and size of p-xylene molecule.
3.5, Para-selectivity The para-selective property of the catalyst has been demonstrated in the gas phase 0 2 oxidations using various alkylaromatics as a substrate. The current thought on the nature of para-selectivity exhibited by the catalyst is discussed using toluene, p-xylene and its isomers as example. Ferric molybdate, Fez(MoO4)3, has been identified to be an active species responsible for the para-selective oxidation reactions [12,19]. Although a unit cell of this molecule contains eight inequivalent iron sites and 12 inequivalent Mo sites, the XRD database [21] indicates that all of the iron sites are octahedral with an average F e - O distance of 1.99 A, and all of the Mo sites are tetrahedral with an average M o - O distance of 1.77 ,~. A chemical model built based on the crystalline structure of Fe2(MoO4)3o is illustrated in Fig. 1. The average F e - O bond distance in the model is 1.98 A and the average M o - O distance is 1.76 A. It shows the distance between two Mo centers, the possible " p a i r " site of the catalyst, in the range of 6.77-7.23 ,~ and the distance between H of one meth~¢l group and H of the other methyl group in p-xylene in the range of 6.37-7.23 A. Thus, when the p-xylene molecule aligns with the " p a i r " sites of the catalyst, the two Mo centers sit right on top of the two hydrogen atoms of two different methyl groups in a p-xylene molecule and, at the same time, the center iron atom in - M o - O - F e - O - M o interacts with w-electrons of the benzene ring in p-xylene. The good geometrical fit between the active " p a i r " sites of the catalyst and the molecular dimension of the substrate may allow the abstraction of two hydrogen atoms, one from each methyl group, to occur in a concerted manner to form the diradical, "CH2-C6H4-CH~, which is more stable than the single o
o
48
J.S. Yoo /Applied Catalysis A: General 143 (1996) 29-51
radical counterpart, and eventually to lead to the formation of terephthaldehyde. On the other hand, if just one methyl group in p-xylene or in toluene stands up on the Mo center of the catalyst through the "single" site, the mono-radical is formed via the abstraction of one hydrogen atom from the methyl group, "CHz-C6H4-CH 3 from p-xylene and "CH2C6H5 from toluene. The monoradical leads to p-tolualdehyde or benzaldehyde. Since the diradical, "CH2-C6H 4CH 2, is much more stable than the mono-radical, "CHz-C6Hs, it is not surprising to observe that the diradical formation from p-xylene is much more favored over the mono-radical formation from either p-xylene or toluene. These two different types of active sites may function competitively during the gas phase 0 2 oxidation. Thus, terephthaldehyde and p-tolualdehyde are formed in the p-xylene oxidation and the molar ratio of these two aldehydes in the product can be controlled to a large extent by the catalyst surface modification as well as the oxidation variable optimization. In essence, the geometrical alignment of the p-xylene molecule with the " p a i r " site of the ferric molybdate, gives rise to the para-selective property of the catalyst. The formation of terephthaldehyde and p-tolualdehyde from the oxidation of p-xylene on bulk ferric molybdate under more severe conditions [12] gives credence to this explanation. The " p a i r " sites of the catalyst cannot align with the two methyl hydrogen atoms in o-xylene and in m-xylene. Furthermore, presence of the second methyl group in o-xylene hinders the interaction of the substrate with the catalyst. In the case of m-xylene, the second methyl group does not favor substrate-catalyst interaction at the other methyl substituent because the methyl substituent is known to be ortho- or para-directing. pair site
,o_©_o, . .,o_©_o,.
o.o_©_o.o
single site
single site
o.0_©
It is, therefore, not difficult to understand the low reactivity of o-xylene and no reactivity of m-xylene with the catalyst. Increased size of substrate molecules
J.S. Yoo / Applied Catalysis A: General 143 (1996) 29-51
49
did not affect the para-selectivity of the catalyst as seen in the reaction of pseudocumene and of durene. In fact, the larger durene underwent oxidation better than the less bulky pseudocumene. When the pore size of the fresh borosilicate molecular sieve is compared with the molecular volume of these substrates, it is difficult to rationalize that the para-selectivity of the catalyst stems from the shape of the pore, especially the internal channel structure of the DBH matrix. The same concept can also be applied to the oxidative dehydrogenation of alkylaromatics such as ethylbenzene, p-ethyltoluene and p-diethylbenzene. For p-diethylbenzene, it is thought that an alignment of the a-hydrogen atoms from two ethyl groups with the "pair" sites triggers the double a,[3-hydrogenation elimination to form divinylbenzene, and that one ethyl group interacting with the "single" site induces single oL,[3-hydrogen elimination to produce ethylstyrene. The oxidative dehydrogenation reactivities of these three substrates are defined by the para-selective nature of the catalyst, and their reactivities are in the order of p-diethylbenzene > ethyltoluene > ethylbenzene.
4. Conclusions
The CVD F e / M o / D B H catalyst exhibited a unique para-selective property for the gas phase 0 2 oxidations of various hydrocarbon substrates. Novel reactions catalyzed by the catalyst include the selective synthesis of terephthaldehyde, p-tolualdehyde, benzaldehyde and their derivatives, the activation and utilization of CO 2 as an oxidant for improving the catalytic activity and stability, the syntheses of oxygenates from alkenylaromatics, the oxidative dehydrogenation reactions of alkylaromatics to the corresponding alkenylaromatics. Terephthaldehyde, benzaldehyde and their derivatives were prepared by the para-selective oxidation of methylaromatics, alkylaromatics, alkenylaromatics and para-substituted toluene derivatives. The results were interpreted on the basis of para-selectivity of the catalyst having the "pair" and "single" sites on the surface of the ferric molybdate species by emphasizing the predominant effects of various para-oriented substituents. The reactivities of the hydrocarbon substrates are also closely tied to the geometrical as well as electronic structure of para-substituents. Thus, the feasibility of separating p-xylene from its isomers via the preferential oxidation using the big reactivity gap was demonstrated. The yields of the aldehyde products were also dramatically enhanced by running the oxidations of these substrates except alkenylaromatics in the presence of CO 2 in the O 2 containing feed. The role of CO z played for these reactions is to promote/accelerate the hydrogen abstraction reaction step, probably the rate-determining reaction, via the peroxocarbonate species, CO 4 . It appears that carbon dioxide does not
50
J.S. Yoo/Applied Catalysis A: General 143 (1996) 29-51
participate in the secondary oxidation of the vinyl group resulted from the initial dehydrogenation step. Under identical conditions, oxidative dehydrogenation occurred to form unsaturated hydrocarbon as an initial product when the alkyl group in the alkylaromatics became ethyl or higher. The reactivities of the alkylaromatics over the catalyst for the gas phase 0 2 oxidation are in the order of ethylbenzene < p-ethyltoluene < p-diethylbenzene. The para-oriented ethyl groups in p-diethylbenzene undergoes oxidative dehydrogenation much faster than the para-methyl group in p-xylene. Under similar gas phase 0 2 oxidation conditions, n-butane was oxidized to maleic anhydride, while propane and ethane were oxidatively dehydrogenated to propylene and ethylene, respectively. Finally, all the reactions studied share the reaction mechanism involving the rate-determining hydrogen abstraction(s) initiated by a common active phase, ferric molybdate, which provides the "pair" and "single" sites for the reactions.
Acknowledgements The author is indebted to P.S. Lin, S.D. Elfline, and M.S. Kleefisch for providing experimental data, and also greatly appreciates C. Choi-Feng and G.W. Zajac for the catalyst characterization.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
M. Niwa, N. Katada and Y, Murakami, J. Phys. Chem., 94 (1990) 6441. N. Katada, T. Toyama and M. Niwa, J. Phys. Chem., 98 (1994) 7647. M. Niwa, N. Katada and Y. Murakami, J. Catal., 134 (1992) 340. T. Jin and J.M. White, Surf. Interface Anal., 11 (1988) 157. W.W. Kaeding and L.B. Young and C.C. Chu, J. Catal., 89 (1984) 267. M. Niwa, Y. Kawashima and Y. Murakami, J. Chem. Soc. Faraday Trans. I, 81 (1985) 2757. E. Moretti, M. Padovan, M. Solari, C Marano and R. Covini, German Pat., 3,301,798 (1983). M. Niwa, M. Kato, T. Hattori and Y. Murakami, J. Phys. Chem., 90 (1986) 6233. T. Hibino, M. Niwa, Y. Murakami, S. Kohai and T. Hanaichi, J. Phys. Chem., 93 (1989) 7847. T. Hibino, M. Niwa, Y. Murakami and M. Sano, J. Chem. Soc. Faraday Trans. I, 85 (1989) 2327. N. Katada and M. Niwa, Catal. Lett., 32 (1995) 131. J.S. Yoo, J.A. Donohue and C. Choi-Feng, Symposium on Advanced Techniques in Catalyst Preparation, 209th ACS National Meeting, April 1-7, 1995, Anaheim, CA, preprint 110. J.S. Yoo, J.A. Donohue, M.S. Kleefisch, P.S. Lin and S.D. Elfline, Appl. Catal., 105 (1993) 83. J.S. Yoo, P.S. Lin and S. D. Elfline, Appl. Catal. A, 106 (1993) 259. J.S. Yoo, J.A. Donohue and M.S. Kleefisch, Appl. Catal. A, 110 (1994) 75. J.S. Yoo, C. Choi-Feng and J.A. Donohue, Appl Catal. A, 118 (1994) 87. J.S. Yoo, P.S. Lin and S.D. Elfline, Appl. Catal. A, 124 (1995) 139. J.S. Yoo, J.A. Donohue and C. Choi-Feng, Symposium on Advanced Techniques in Catalyst Preparation, 209th ACS National Meeting, Anaheim, CA, April 2-7, 1995, preprint pp. 103-113. G.W. Zajac, C. Choi-Feng, J. Faber, J.Z. Shyu, R. Patel, H. Hochst and J.S. Yoo, J. Catal., 151 (1995) 338.
J.S. Yoo /Applied Catalysis A: General 143 (1996) 29-51 [20] [21] [22] [23] [24]
J.S. Yoo, S.S. Grimmer, A.R. Sohail and C. Choi-Feng, Catal. Lett., 29 (1994) 299. V. Massarotti, G. Flor and A. Marinit, J. Appl. Cryst., 14 (1981) 4. J.L. Dubois, M. Minoun and C.J. Caeron, Catal. Lett., 6 (1990) 967. M. Aresta, C. lZragale, E. Quaranta and I. Tommasi, J. Chem. Soc. Chem. Commun., (1992) 315. J.S. Yoo, Appl. Catal. A, in press.
51
APPLIED CATALYSS I AG : ENERAL
Applied Catalysis A: General 143 (1996) 29-51
The CVD F e / M o / D B H ( deboronated borosilicate molecular sieve)-catalyzed oxidation reactions Jin S. Y o o
*
Amoco Research Center, P.O. Box 3011, Naperville, IL 60566, USA
Abstract
The CVD F e / M o / D B H (deboronated borosilicate molecular sieve) catalyst exhibits a paraselective property for the gas phase O 2 oxidations of a variety of alkylaromatic compounds. Novel reactions studied with the catalyst include selective synthesis of aldehydes, activation and utilization of CO z in the oxidation reactions, syntheses of oxygenates from alkenylaromatics, oxidative dehydrogenation reactions of alkylaromatics and alkanes. Terephthaldehyde, benzaldehyde and their derivatives are prepared from methylbenzenes, alkylaromatics, alkenylaromatics, and para-substituted toluene derivatives. Alkenylaromatics and olefins are formed from alkylaromatics and alkanes, respectively, as an initial dehydrogenated product. These reactions are described using the representative substrate(s) as an example, and the results are interpreted based on the para-selectivity based on the effects of various para-oriented substituents such as alkyl, alkenyl, hydroxy, methoxy and halide group. These reactions share a reaction mechanism involving the rate-determining hydrogen abstraction initiated by a common active species, ferric molybdate. Keywords: Para-selective oxidation; CVD Fe/Mo/DBH; Methylaromatics; Alkenylaromatics; Alkylaromatics; Oxidative dehydrogenation; CO 2 activation
1. I n t r o d u c t i o n
The chemical vapor deposition technology has been utilized for depositing highly and uniformly dispersed fine particles of catalytic materials such as metal and metal oxides on a supporting matrix. The ultra thin layer of silica was
* Correspondence address: 2315 Mast Court, Flossmore, IL 60422, USA, tel./fax. (+ 1-708) 7984998, e-mail jsyoo @amoco.corn 0926-860X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. Pll S0926- 860X(96)00069-5
30
ZS. Yoo /Applied Catalysis A: General 143 (1996) 29-51
prepared by the CVD of Si(OMe) 4 on "),-alumina, titania and zirconia [1-3]. The silica monolayer deposited on these metal oxides was employed in the double bond isomerization of olefins [2] and the Beckmann rearrangement of cyclohexanone oxime [4]. The CVD technique has also been used for controlling the pore opening size of zeolites such as mordenite [5,6] and HZSM-5 [7,8]. The Pt-loaded H-mordenite has been modified by the CVD of silica to improve the shape selectivity for hydrocracking of paraffins. Other applications were directed to catalytic cracking, isomerization, alkylation, disproportionation and oxidation. The silica modified HZSM-5 has been noted for alkylation of toluene with methanol and toluene disproportionation to make p-xylene. The ultra-thin germanium oxide layer in a monoatomic order thickness on zeolite was also prepared and studied by others [9-11]. In our laboratory, efforts to prepare novel catalysts by depositing the catalytically active species using the surface silanols as an anchoring site are continuing. We have succeeded in making the CVD Fe/Mo/DBH(deboronated borosilicate molecular sieve) by the CVD technique. The CVD F e / M o / D B H catalysts prepared in small batches in laboratory scale, that have slightly varied metal compositions, are referred to as the catalyst hereafter. The active catalytic species, ferric molybdate, is uniformly dispersed as fine particles (2-40 nm) on the surface of the catalyst, and it catalyzed various oxidation reactions through hydrogen abstraction as an initial reaction. The encouraging results that terephthaldehyde was formed in high yield from the gas phase O 2 oxidation of p-xylene prompted us to look into other types of oxidation reactions. In an early attempt to selectively synthesize useful and reactive bifunctional aromatic aldehydes such as terephthaldehyde and its derivatives, the gas phase O 2 oxidation was undertaken with various hydrocarbon substrates over the same catalyst. Some findings have previously been reported. These include the para-selective oxidation of p-xylene to terephthaldehyde and p-tolualdehyde [12,13], the catalytic activity and selectivity in para-selective O 2 oxidation reactions in the presence of CO 2 [14], the preferential oxidation of p-xylene from xylene isomer mixture containing ethylbenzene utilizing an unusual reactivity gap between p-xylene and other isomers [15], the identification that DBH as the most promising supporting matrix for the catalyst preparation among various zeolites [16], the oxidation reactions of polymethylated benzenes in order to expand the para-selectivity of the catalyst [17], the detailed CVD catalyst preparation techniques [18], the characterization of the catalyst [19] and the one-step hydroxylation of benzene to phenol [20]. In this paper, various oxidation reactions catalyzed over CVD F e / M o / D B H are reported using representative substrates. Based on the para-selectivity of the catalyst, a reaction mechanism is postulated that the initial slow step of hydrogen abstraction(s) is promoted by the common active species, ferric molybdate, which provides the "pair" and "single" sites.
J.S. Yoo/Applied Catalysis A: General 143 (1996) 29-51
31
2. Experimental The method of preparing the CVD F e / M o / D B H catalyst by the chemical vapor deposition technique, evaluation and characterization of the catalyst, and analyses of products with an on-line GC were described in detail elsewhere [12-14]. All runs discussed hereafter followed the procedures described in the above papers. When the reactor reached the desired reaction temperature under the flow of purge gas, the purge gas flow was stopped and the feed gas was passed through the catalyst bed in the reaction under the following conditions: Catalyst load:
0.5-10 g
Molar ratio of O J s u b s t r a t e :
10-40
Substrate concentration in feed:
0.1-0.74 mol%
WHSV:
0.1-1.1 h i
Contact time:
0.1-0.5 s
Reactor temperature:
150-500°C
Gas flow rate:
100-4000 sccm
3. Results and discussions 3.1. Methylaromatics A series of methylaromatics such as toluene, xylenes, pseudocumene, and durene were subjected to gas phase O 2 oxidation over the catalyst under similar reaction conditions.
3.1.1. Toluene vs p-xylene The representative results from the gas phase 0 2 oxidations of toluene and p-xylene over the catalyst are compared in Table 1. At 350°C, only 13.4% of toluene was converted to benzaldehyde in 63.7 mol% selectivity whereas 73.7% of p-xylene was oxidized to form two aldehyde products, terephthaldehyde in 49.2 mol% and p-tolualdehyde in 25.2 mol%. This shows the conversion of
J.S. Yoo /Applied Catalysis A: General 143 (1996) 29-51
32
Table 1 Oxidations of toluene and p-xylene over CVD F e / M o / D B H Substrate Temp. (°C) Conversion (%) Product selectivity (%) Benzaldehyde Terephthaldehyde p-Tolualdehyde Maleic anhydride By-products CO CO 2
Toluene
p-Xylene
350
400
325
350
13.4
32.6
46.0
73.7
63.7
60.5 48.6 29.2
49.2 25.2
1.5 4.3 10.6
1.8 5.8 16.1
0.0
10.5
0.0 36.3
4.0 24.8
p-xylene was much higher than that of toluene and also the aldehyde formation was much greater with p-xylene than with toluene. The combined yield of p-tolualdehyde (monoaldehyde) and terephthaldehyde (dialdehyde) from pxylene was 54.8% compared to the yield of benzaldehyde from toluene which was merely 8.5%. Toluene reflected the greater substrate burning as shown by the high CO x selectivity (36.3%) 3.1.2.
p-Xylene, o-xylene, and m-xylene
The xylene isomers were individually oxidized under the standard conditions in the gas phase, and the representative results are summarized in Table 2. The conversion of p-xylene was very high, 74%, while o-xylene and m-xylene exhibited a very limited conversion. 3.1.3. Pseudocumene vs durene
The results of pseudocumene and durene are compiled in Table 3. As shown below, three monoaldehyde products were expected from pseudocumene de-
Table 2 Gas phase 02 oxidations of xylene isomers Substrate
p-xylene
o-xylene
m-xylene
Conversion (mol%) Product selectir, ity (mol%) Monoaldehyde Dialdehyde By-products
74
6
3
CO + CO 2
25
33
49
trace
2 22
0 66
0 2 / x y l e n e : 4 3 / 1 , contact time: 0.15 s, WHSV: 0.32 h - t , 350°C.
67 0 0 33
33
J.S. Yoo /Applied Catalysis A: General 143 (1996) 29-51 Table 3 Gas phase O 2 oxidations of pseudocumene and durene Substrate
Pseudocumene
Durene
Temp. (°C) Conversion (%)
450 2.75
450 74.8 a
Product selectivity (mol%) 2,5 -dimethylbenzaldehyde 2,4-Dimethylbenzaldehyde 3,4-Dimethylbenzaldehyde 2-Methylterephthaldehyde COy 2,5 -Dimethylterephthaldehyde 2,3-Dimethylphthalic anhydride 2,3 -Dimethylphthaldehyde 3 -Methyl-4-formylphthaldehyde 2,4,5-Triformyltoluene 2,3,5,6-Tetraformylbenzene 2,4,5 -Trimethylbenzaldehyde Maleic anhydride
9.7 8.2 11.7 47.9 19.5 47.5 8.9 9.9 6.9 4.9 trace 10.9 t0.9
Pseudocumene feed: premixed gas, 4.0 vol% 02 and 4.0 vol% N2 in He 02/pseudocumene: 2 8 / 1 , contact time: 0.21 s, WHSV: 0.34 h -j . Durene feed: durene solution in benzene (molar ratio of durene/benzene = 2.2) (pump rate = 0.8 g / h ) , feed gas: 6.0% 02 and 6.0% N 2 in He. a Area % excluding CO x.
pending on which methyl substituent was oxidized. Indeed all three monoaldehydes were obtained with comparable selectivities; 2,5-dimethylbenzaldehyde in 9.7%, 2,4-dimethylbenzaldehyde in 8.2%, and 3,4-dimethylbenzaldehyde in 11.7%. Of the two possible dialdehyde products, only one was obtained. That was 2-methylterephthaldehyde which was the result of the oxidation of two para-oriented methyl groups and its selectivity was very high, 47.9%.
CH~
CH3
CH3
CH,
H3C-~-CH , ~ OHC-~)-CHO + OHC-~-CH,
* H3C-~-CHO
main product
o.@o .
x,c-
-c.,
(&,lo\ •
.,C
o/O
.
co..,,o
J.S. Yoo/Applied Catalysis A: General 143 (1996) 29-5l
34
Compared with pseudocumene, durene was much more reactive as seen by its high conversion: 27.5% of pseudocumene vs 74.8% of durene. From durene, as shown below, one can expect one monoaldehyde from the oxidation of one methyl group, two dialdehydes from the oxidation of two methyl groups, one trialdehyde from the oxidation of three methyl groups, and one tetraldehyde from the oxidation of four methyl groups. One dialdehyde, 2,5-dimethylterephthaldehyde, had a very high selectivity. (47.5%). Only a trace quantity of the tetraldehyde was produced. Other aldehyde were produced in moderate selectivities.
~C. [~.C]Bj u~c-'-,~,,.'-cu~
02
H3C.[~..CHO
+ H3C~ C H O
oac - ~ , ~ j - cu~ =ain
u,c ~ - C H O
product
s,c- ~ . . . c a o H,c'~'CH,
+
+
u,c-[~-cao OHC'~'CHO
+
trace
COx
+
1~0
amount
The catalyst has been known to oxidize the methyl substituent to formyl in many methylaromatics [12,13,19]. Based on the studies of the various methylaromatics given above, it can be concluded that the reactivity of the substrate and the product selectivities depend on the number of methyl substituents and their relative positions. Presence of a second methyl substituent increased the reactivity as shown by the increased conversion of p-xylene compared with that of toluene. Especially the presence of the second methyl group which was in para position to the first methyl group (these two methyl groups are called hereafter para-oriented methyl groups) remarkably enhanced the performance of the catalyst as evidenced in the reaction of p-xylene, pseudocumene and durene. The great differences observed in the p-xylene oxidation and the toluene oxidation in terms of its conversion and selectivities of aldehyde products indicate that the reaction path of the formation of terephthaldehyde and ptolualdehyde from p-xylene is not the same as that of the formation of
J.S. Yoo / Applied Catalysis A: General 143 (1996) 29-51
35
benzaldehyde from toluene. It is believed that the former involves two different active sites: the interaction of two para-oriented methyl substituents of the substrate with the " p a i r " site to form terephthaldehyde, and of one methyl group with the "single" site of the catalyst to produce p-tolualdehyde. On the other hand, the latter, the reaction of toluene, involves the interaction of the methyl substituent with the "single" site of the catalyst. The reactivity of xylene isomers was in the order of p-xylene >> o-xylene > m-xylene. The para selectivity of the catalyst was clearly proven here. Utilizing this big reactivity gap between p-xylene and other isomers, p-xylene was preferentially oxidized from the xylene mixtures [15]. Once p-xylene is preferentially oxidized in the xylene mixture, the resulting aldehydes are separated from the unreacted o- and m-xylene by a simple separation method such as distillation. The unreacted o- and m-xylene can be recycled to the isomerization unit. This implicates that the costly process of conventional p-xylene separation can be eliminated, and that a xylene mixture can be used in place of the separated pure p-xylene for the synthesis of aldehydes and other oxygenates. Even though durene molecule was bulkier than pseudocumene molecule, durene was much more reactive than pseudocumene. Durene has two pairs of para-oriented methyl substituents while pseudocumene has only one such pair and the third methyl group is in ortho- and meta-position to the other two methyl groups. The chance of para-selective oxidation would be doubled by the presence of two pairs of para-oriented methyl groups in durene. The major oxidation product of pseudocumene was 2-methylterephthaldehyde. This is in agreement with the fact that the para-oriented methyl groups in pseudocumene were readily oxidized as in the case of p-xylene. 2,5-Dimethylterephthaldehyde was the main product from durene oxidation and formed as a result of the oxidation of the para-oriented methyl substituents. Since only a trace amount of the tetraldehyde, 2,3,5,6-tetraformylbenzene, was observed, it can be said that the second set of para-oriented methyl groups enhanced the reactivity of the first oxidation step but did not participate in the second oxidation step.
3.2. Oxidative dehydrogenation of alkylaromatics When an alkyl group in alkylaromatics becomes ethyl or higher, the oxidative dehydrogenation reaction takes place at the initial step, and the vinyl group is formed over the catalyst under the standard gas phase 0 2 oxidation conditions [24]. Alkylaromatics such as ethylbenzene, p-diethylbenzene, p-ethyltoluene, p-t-butylethylbenzene and cumene were converted to styrene, p-divinylbenzene and p-ethylstyrene, p-vinyltoluene, p-t-butylstyrene, and a-methylstyrene, respectively, at the initial step. The oxidative dehydrogenation reaction was discussed using ethylbenzene, p-ethyltoluene p-diethylbenzene, and low alkanes as representative substrates.
36
J.S. Yoo/Applied Catalysis A: General 143 (1996) 29-51
3.2.1. Ethylbenzene and styrene The results of ethylbenzene oxidation are shown as a function of temperature in Table 4. The conversion increased with the temperature. At low temperatures styrene was the main product but as the temperature increased, less styrene was detected and more benzaldehyde was produced. At high temperatures maleic anhydride became predominant. Based on these observations, it was suspected that the catalyst oxidized the ethyl substituent in ethylbenzene to vinyl group via oxidative dehydrogenation reaction. The initial product, styrene, was then immediately oxidized further to produce benzaldehyde, benzoic acid and maleic anhydride along with carbon oxides as shown below.
~
-CH-CH~
initialp r o d u c t
~
-CHO
~-COOH
0 HC -
II
C
+
CO,
÷
H20
HC 0
In order to confirm that the oxidation products obtained in the oxidation of ethylbenzene were derived from further oxidation of the styrene produced in the initial stage, pure styrene was subjected to the identical oxidation conditions used in the oxidation of ethylbenzene above. The results of styrene oxidation reactions are summarized in Table 5. As expected, benzaldehyde and benzoic acid were the major products from styrene.
3.2.2. p-Ethyltoluene The results of the oxidation of p-ethyltoluene are summarized in Table 4. The reaction paths are illustrated in the scheme below for the reaction at 375°C. Presence of p-methylstyrene which was formed through the oxidative dehydrogenation reaction at the ethyl substituent site in p-ethyltoluene, was observed. Subsequent oxidation of the vinyl group in the resulting p-methylstyrene was shown by the substantial amounts of p-tolualdehyde and p-toluic acid in the product mixture. It should be noted that the oxidative dehydrogenation reaction
37
J.S. Yoo/Applied Catalysis A: General 143 (1996) 29-51
proceeded faster than the oxidation of two methyl groups in p-xylene to aldehydes. A small amount (5.6%-8.4%) of p-methylacetophenone was also produced as expected from the alternate oxidation of p-methylstyrene.
3.2.3. p-Diethylbenzene Since considerable exothermicity was observed with p-diethylbenzene, the reactor temperature was lowered and the flow rate of the feed gas was increased in this study. This observation led us to conclude that p-diethylbenzene was far more reactive than p-ethyltoluene or p-xylene in the gas phase 0 2 oxidation over the catalyst. The results are summarized in Table 4. Two dehydrogenation products, p-ethylstyrene and p-divinylbenzene, were identified.
u,c-
©
-CS, CH3
--, O=
39.91
S,C-
©
-Ca-C~,
7.31
-~
~,
H,C-
©
-COCll~
6.9t
,1.
H3G-Q-GHO 17.2!1;
BsC-Q-COOH 23.0t
H3C-O-GH2CHO trace
OHC-O-CHO 0.6t
As shown in the scheme below, p-ethylstyrene and p-divinylbenzene were initially formed at two different active sites, e.g., "single" and " p a i r " site, respectively, from the p-diethylbenzene substrate, and these products were subsequently further oxidized via two different reaction paths, p-Ethylstyrene was converted to p-ethylbenzaldehyde and then to p-ethylbenzoic acid while much more reactive p-divinylbenzene was quickly oxidized to p-vinylbenzaldehyde and then to terephthaldehyde. In the study of the oxidation of various alkylaromatics where the alkyl substituent is ethyl or higher, the results showed that the catalyst oxidized the ethyl group to a vinyl group via oxidative dehydrogenation reaction, and if methyl substituent was present at the same time the catalyst preferred the ethyl over the methyl for oxidation and left the methyl group unchanged. Conversion of ethylbenzene to styrene, p-ethyltoluene to p-methylstyrene,
38
J.S. Yoo /Applied Catalysis A: General 143 (1996) 29-51
and p-diethylbenzene to 4-ethylstyrene and p-divinylbenzene are such examples. This is in contrast to the activity shown by the same catalyst on p-xylene.
~o~
~c-ce-< ~ -c.~ ;02
u2c-clt-~
-cu-cu2
~ 02
OHC-~-C~Hs
H2C-CH-~-GHO
HOOt~-
OH¢-~-CHO
-C:;~ls
p-Xylene was converted to terephthaldehyde indicating that the catalyst oxidized the methyl substituent to a formyl group. However, the methyl group remained the same in p-ethyltoluene since p-ethylbenzaldehyde was not observed. One can conclude that the ethyl group in p-ethyltoluene rendered the methyl group unreactive toward oxidation, while the ethyl group in para-position was being oxidized. Similar study done on p-(t-butyl)ethylbenzene [24] showed the dehydrogenation occurring at the ethyl substituent. These findings point to the conclusion that the a,[3-hydrogen elimination reaction at an ethyl site must be easier and faster than the oxidation of methyl to a formyl group by this catalyst. The initially formed vinyl group was very reactive and oxidized immediately to aldehyde or ketone which then changed to carboxylic acid. This was why benzaldehyde and benzoic acid were observed from ethylbenzene via styrene. In the same manner p-methylstyrene produced from p-ethyltoluene underwent oxidation and formed p-tolualdehyde and p-toluic acid. Oxidation at the a-carbon of p-methylstyrene produced p-methylacetophenone, p-Ethylstyrene (initially produced from p-diethylbenzene) formed p-ethylbenzaldehyde and p-ethylbenzoic acid. p-Divinylbenzene (produced again from p-diethylbenzene)
J,S. Yoo / Applied Catalysis A: General 143 (1996) 29-51
39
Table 4 Gas phase 02 oxidation of ethylbenzene (EB), p-ethyltoluene (ET) and p-diethylbenzene (DEB) Substrate
EB
Temp.°C Conversion (%)
325 5.2
375 22.6
25.3 37.9 11.6
7.1 28.1
ET
DEB
375 39.0
325 27.1 a
Productivi~ selectit, ity (%) Styrene Benzaldehyde Benzoic acid Maleic anhydride CO CO 2 p-Methylstyrene p-Tolualdehyde p-Toluic acid p-Methylacetophenone Terephthaldehyde
24.7 12.1 17.8
10.7 24.4
14.8 24.9 7.3 17.2 23.0 6.9 0.6 10.0 c 1.6 17.3 5.5 26.6 7.7 26.3 4.1
o-Diethylbenzene b p-Divinylbenzene Terephthaldehyde p-Vinylbenzaldehyde p-Ethylstyrene p-Ethylbenzaldehyde p-Ethylbenzoic acid Unidentified products
Reaction conditions for ethylbenzene: 325 and 375°C, WHSV: 0.57 h l, feed: 6% 0 2 and 6% N 2 in He, flow rate: 400 sccm, 0 2 / E T : 7.51/1 in a micro-reactor loaded with 0.5 g catalyst. Reaction conditions for diethylbenzene: feed: 6% O 2 and 6% N 2 in He, flow rate: 800 sccm, 325°C in a micro-reactor loaded with 0.5 g catalyst. Area %. b May be the isomerization product of p-diethylbenzene. c Excluding CO 2.
formed p-vinylbenzaldehyde and terephthaldehyde. The path through monovinyl intermediate, p-ethylstyrene, gave the higher yield as shown by the combined area percentage of 15% compared with the combined area percentage of 8.5%. Table 5 Oxidation of styrene Temp, (°C)
275
300
325
Feed type
I
II
I
II
I
Conversion (%)
14.4
12.3
34.5
265. l
73.3
22.2 3.4 9.2 53.4 6.3 5.5
24.0 4.2 9.7 56.6 5.5
19.9 1.9 6.5 57.3 6.6 5.6
21.7 2.5 7.7 60.5 5.7
18.9 0.9 3.8 58.8 7.9 5.9
1I
Product selectiuity (%) Benzaldehyde Phenylacetaldehyde Acetophenone Benzoic acid CO CO 2
21.2 1.5 5.2 62.5 6.2
Feed I: 0.17% styrene, 5.0% 02, 18.3% N 2 in He; Feed II: 0.17% styrene, 5.0% 02, 18.4% N 2 in CO 2.
40
J.S. Yoo /Applied Catalysis A: General 143 (1996) 29-51
through divinyl intermediate, p-divinylbenzene. It suggests that these two paths are competing in the oxidative dehydrogenation of p-diethylbenzene. This argument would be expanded further in the CO 2 oxidation section later. Since benzaldehyde and benzoic acid obtained from the oxidation of ethylbenzene were again the major products from styrene oxidation, it is safe to assume that ethylbenzene, p-ethyltoluene and p-diethylbenzene produced the corresponding vinylaromatics initially even though a large amount of vinyl derivative was not found in the final product mixture. The para-selectivity of the catalyst favorably influenced the oxidative reactions of these three substrates, and the reactivity of p-diethylbenzene was greater than that of p-xylene. Based on the reaction mechanism postulated on the p-xylene oxidation [13], it was theorized that the oxidative dehydrogenation of the ethyl group in the p-diethylbenzene substrate must occur in two ways, using the "single" site or the " p a i r " sites of the catalyst [13]. If just one ethyl group in the p-diethylbenzene substrate interacted with the "single" site of the catalyst in the stand-on manner, p-ethylstyrene was produced as a result of single o~,[3-hydrogen elimination reaction. Once p-ethylstyrene was formed, the second ethyl group became much more difficult to be oxidized, and rather the vinyl group in p-ethylstyrene was oxidized further to p-ethylbenzaldehyde, which eventually converted to p-ethylbenzoic acid. On the other hand, if two ethyl groups in p-diethylbenzene interacted in a concerted manner on the " p a i r " sites of the catalyst in the lying-on fashion, p-divinylbenzene was formed as a result of the concerted double oL,[3-hydrogen elimination reactions at the two ethyl groups. p-Divinylbenzene was extremely reactive and changed to terephthaldehyde and p-vinylbenzaldehyde upon the subsequent oxidation.
3.2.4. Alkanes Besides alkylaromatic substrates, this work was extended to low alkanes such as n-butane, propane and ethane under the standard gas phase O 2 oxidation conditions [24]. n-Butane was subjected to the gas phase O 2 oxidation in a quartz reactor loaded with 10 g of the same F e / M o / D B H catalyst. A premixed feed containing 0.5% n-butane, 28% 0 2 and 3% N 2 in CO 2 was oxidized, and the results are summarized along with the reaction conditions in Table 6. In the presence of CO 2 in O 2, n-butane was converted to maleic anhydride, and an anticipated dehydrogenated intermediates, n-butene a n d / o r butadiene, were not detected in the reaction effluent. The premixed gases consisting of 2% propane (or ethane), 4% 0 2 and 4% N 2 in He were subjected to the oxidation reaction in the reactor loaded with 5.0 g of CVD Z r / M o / D B H catalyst. The CVD Z r / M o / D B H catalyst was prepared from ZrCI4, MoOeCI 2 and borosilicate molecular sieve by the chemical vapor deposition method. Metal composition of the resulting catalyst was 10.45% Mo and 4.1% Zr, and the atomic ratio of M o / Z r was 2.4/1. These results are also combined in Table 5. Propane and ethane were dehydrogenated to propylene and
J.S. Yoo / Applied Catalysis A: General 143 (1996) 29-51
41
Table 6 Oxidation of n-butane over CVD F e / M o / D B H , propane and ethane over CVD Z r / M o / D B H , 10.45% Mo, 4.1% Zr and M o / Z r = 2.4/1 Alkane
n-butane
Oxidant
02/CO 2
Temp. (°C) Conv. (%)
400 14.5
Propane
Ethane
02 425 34.0
02
500 3.9
550 10.3
550 2.0
600 3.8
0.2 85.2
0.6 70.3
88.5
86.3
10.4 3.7
21.0 4.7
11.0
13.7
Product selectit,ity (%) Ethylene Propylene Maleic anhydride CO CO 2
8.9 91.4 -
3.7 96.3 -
For n-butane, premixed feed: 0.5% n-butane, 28% 02 and 3% N 2 in C02, flow rate: 800 sccm, WHSV: 0.114 h - j , contact time: 2.10 s, 02/n-butane: 56/1. For propane and ethane, premixed feed: 2.0% propane or ethane, WHSV: 1.135 h -~ , contact time: 0.84 s, 0 2 / p r o p a n e or ethane: 2/1.
ethylene, respectively, under identical oxidation conditions. It is not surprising to observe that conversion of ethane is much smaller than those of propane and n-butane, and that the initial products, n-butene and butadiene, are completely oxidized to form maleic anhydride under the conditions studied. Similar results were also obtained from these three substrates over the (CVD F e / M o / D B H ) catalyst. In summary, n-butane was converted to maleic anhydride, while propane and ethane were oxidatively dehydrogenated to propylene and ethylene, respectively, by the gas phase O 2 oxidation. 3.3. Para-substituted toluene derivatiues Para-substituted toluene derivatives, CH3-C6H4-X, where X stands for OH, OCH 3, C1, CHO, C2H 5, and CH=CH2, were selected to study the effects of the substituent on the oxidation of methyl groups in these toluene derivatives. 3.3.1. p-Cresol The 0 2 oxidation of p-cresol over the catalyst failed to produce p-hydroxybenzaldehyde, the expected product, because this initial product was so reactive that the tarry material resulted instead even at low temperatures such as 200°C. To circumvent this problem, the catalyst surface was modified by coating with Si(OMe) 4 by the CVD method [15]. The reaction over the new modified catalyst produced p-hydroxybenzaldehyde, phenol, and carbon oxides in selectivities of 14.1, 5.5, and 80.5%, respectively, at 9.1% p-cresol conversion at 250°C. Extensive burning was observed probably due to the highly reactive nature of p-cresol and silica-coated catalyst itself.
42
J.S. Yoo /Applied Catalysis A: General 143 (1996) 29-51
HO_~_CHs
O~
HO-~-CHO
+
CH30-~-CH 3
~
CH30-~-CHO
+
~ - O H
cox
+
CO~
+
~o
3.3.2. p-Methylanisole To avoid extensive burning and other drawbacks experienced with p-cresol, p-methylanisole was chosen in which the hydroxy group in p-cresol was replaced by the inactive rnethoxy group. Approximately 26 area% of the starting substrate was converted to a sole product, p-anisaldehyde. Selectivities to p-anisaldehyde and to CO x were 81 and 15%, respectively, under the 0 2 oxidation conditions. main path
~ H,c-
-clio
-.
0 He - C~ II HC- C~ II 0 IJ
a,c-
-cooH
-. ~
0
-*
eo~
+
I~O
OHC-~-CHO (minor product) 3.3.3. p-Tolualdehyde As shown in Table 7, approximately 46 mol% of p-tolualdehyde was converted to yield p-toluic acid, maleic anhydride, CO 2 and terephthaldehyde in selectivities of 43%, 19%, 32% and 3%, respectively.
Table 7 Gas phase Oz oxidation of p-tolualdehyde Substrate
p-Tolualdehyde
p-Xylene
Temp. (°C) WHSV ( h - ) Conversion (%)
350 0.38 46
350 0.32 78
Product selectivity (%) Terephthaldehyde p-Tolualdehyde Maleic anhydride p-Toluic acid Toluene CO 2
3 19 43 3 32
44 18 14 4 2 18
The pump rate of p-xylene: 0.15-0.19 g/h, contact time: 0.15 s, WHSV: 0.32-0.38 h -I .
J.S. Yoo/Applied Catalysis A: General 143 (1996) 29-51
43
3.3.4. p- and m-Chlorotoluene The oxidation of p- and m-chlorotoluene are summarized in Table 8. As was expected, the p-chloro-substituted substrate, p-chlorotoluene, was much more reactive than the m-chloro-substituted counterpart. In a typical run, 28% of p-chlorotoluene was converted to form mainly p-chlorobenzaldehyde in 61.6% selectivity at 325°C, while only 10.8% of m-chlorotoluene was converted at 350°C to m-chlorobenzaldehyde and p-chlorobenzaldehyde in selectivities of 26.1% and 23.6%, respectively. These results indicated that the catalyst exhibited catalytic activity for both p- and m-chlorotoluene. It was observed that at elevated temperatures, 400-450°C, the oxidation of m-chlorotoluene yielded more p-chlorobenzaldehyde than m-chlorobenzaldehyde and chlorobenzene. Isomerization must occur during or after the oxidation reaction and become more important at higher temperatures. The para-oriented substituents (X) played a key role in defining the reactivity of substrate and the product selectivity in para-substituted toluenes, H3CC6H4-X. When the para-oriented substituents were OH, OCH 3 and C1, these substituents remained unchanged and the methyl group was selectively oxidized to aldehyde. For example, p-cresol, p-methylanisole and p-chlorotoluene were converted to p-hydroxybenzaldehyde, p-anisaldehyde and p-chlorobenzaldehyde, respectively. On the other hand, formyl ( - C H O ) , ethyl ( -CzH 5) and vinyl ( - C H = C H 2) substituents themselves were oxidized by the catalyst and rendered the para-methyl group in the substrates unreactive, p-Tolualdehyde was converted to p-toluic acid, and p-ethyltoluene to p-methylstyrene which was further oxidized to produce mainly p-tolualdehyde and p-toluic acid.
Table 8 Gas phase 02 oxidation of p- and m-chlorotoluene. Catalyst B: CVD F e / M o / D B H Substrate
p-chlorotoluene
Temp. (°C) Conversion (%)
350 33.7
375 54.4
400 76.5
m-chlorotoluene 350 8.2
400 15.8
450 33.2
59.4 0.3 1.4 12.2 6.8 19.2
53.4 0.2 2.4 15.6 7.8 20.0
42.3 0.1 4.1 16.1 11.1 25.9
24.0 35.2 18.4 8.3
26.9 17.8 9.7 9.1
38.1 7.8 5.3 5.3 14.6
25.3
27.9
Product selectiz~i~., % pCBA mCBA CB MA CO CO 2
11.4
14.2
pCBA: p-chlorobenzaldehyde, mCBA: m-chlorobenzaldeyde, CB: chlorobenzene, MA: maleic anhydride.
44
J.S. Yoo /Applied Catalysis A: General 143 (1996) 29-51
H3C-~-Cl
02
OHC-~-C1
+
co,
+
~o
OHC-G-C1 + OHC-~ ~z
\CI
+
co,
+
u=o
Chemically very reactive substituent groups such as formyl and vinyl are attacked by the catalyst more favorably than the methyl substituent. Conversely, the chemically stable methoxy substituent would not compete with methyl for the oxidation by the catalyst. In the case of an ethyl substituent, as discussed in the above section, the presence of e~ and [3 hydrogens facilitates the dehydrogenation of the ethyl group.
3.4. Oxidation in the presence of CO z In the course of study to find ways to suppress substrate burning in the gas phase 0 2 oxidations of p-xylene over the catalyst, carbon dioxide was added to the O 2. Unexpectedly, the p-xylene conversion was dramatically enhanced by the presence of CO 2 in the system. The same phenomenon was observed with various alkylaromatics. Efforts to elucidate the role played by CO 2 in the oxidation of a variety of hydrocarbons are still being made. Our current understanding on the role of CO 2 is presented here using p-xylene, styrene and p-ethyltoluene as representative substrates.
3.4.1. p-Xylene p-Xylene was subjected to the gas phase 0 2 oxidation in three different types of feeds by varying reaction temperatures from 250 to 350°C. The results are in Table 9. In the presence of CO z, even at 250°C, the conversion increased and more aldehydes, both p-tolualdehyde and terephthaldehyde, were obtained. This indicated CO 2 was activated on the catalyst even under this mild reaction conditions. 3.4.2. Styrene Contrary to p-xylene oxidation, the presence of C O 2 in O2-containing feed gas adversely affected the oxidation of styrene as seen in Table 5. The
J.S. Yoo /Applied Catalysis A." General 143 (1996) 29-51
45
Table 9 Oxidation of p-xylene Temp. (°)C
250
Feed
1
lI
III
300 I
II
IlI
350 I
I1
III
375 I
I1
Cony. (%)
8.6
23.4
0.6
17.6
33.3
5,1
41.2
65.5
4.1
60.7
84.1
32.4 3.3 0.0 0.0 20.0 23.8 0.0 0.0
54.5 4.7 0,0 0.0 21,4 17.8 0.0 -
44.9 0.0 0.0 0.0 29.2 25.9 0.0 -
57.9 16.4 1.3 0.0 6.2 7.5 0.0 10.7
57.2 27.5 1.5 0.0 6.9 6.8 0.0 --
69.4 13.8 0.0 0.0 8.1 8.7 0.0 -
50.2 23.5 2.4 2.4 3.5 1.7 0.6 15.6
40.9 33.6 2.7 6.0 4.8 0.6 3.4 -
60.5 28.4 1.2 0.0 5.4 4.5 0.0 -
40.6 32.6 2.7 5.8 3.1 0.4 4.7 20.2
40.6 30.2 3.1 13.7 4.8 0.0 7.5 -
Product selectit, iO, (%) TOAL TPAL BZAL MA TOL TMBPM CO CO 2
Catalyst 5:6.8 wt% Mo, 1.88 wt% Fe, 540 ppm B, M o / F e = 2.1, 0.508 g (1.4 ml) in a quartz micro-reactor. Oxidation conditions: WHSV: 0.22 h - I , Contact time: 0.21 s, Gas flow rate: 400 sccm. Feed gas blends: 1: 0.1% p-xylene, 1.0% 02, 1.0% N 2 in He. II: 0.1% p-xylene, 1.0% 02, 1.0% N 2 in CO 2. III: 0.1% p-xylene, 1.0%N 2 i n C O 2. TOAL: p-tolualdehyde, TPAL: terephthaldehyde, BZAL: benzaldehyde, MA: maleic anhydride, TOL: tolualdehyde, TMBPM: trimethylbiphenylmethane.
conversion decreased when C O 2 product distribution was observed.
was
present and no appreciable change in
3.4.3. p-Ethyltoluene The promoting effect of
C O 2 o n the conversion of the p-ethyltoluene substrate was found to be very remarkable as illustrated in Table 10. The p-ethyltoluene conversion increased from 39.0% to 60.9%, and selectivity to p-tolualdehyde and p-toluic acid increased also from 17.2% to 20.0%, and
Table 10 Oxidation of p-ethyltoluene over the catalyst (10 g) Temp. (°C)
350
375
400
Feed type
I
II
l
II
1
II
Conversion (%)
4.7
11.9
39.0
60.0
70.0
81.1
14.7 14.8 5.6 23.0 0.3 12.9 24.6
19.4 22.5 8.4 29.0 0.8 14.4
7.3 17.2 6.9 23.0 0.6 14.8 24.9
6.4 20.0 6.7 34.8 1.2 22.0
2.7 16.7 5.6 15.8 1.3 19.1 29.8
2.8 22.6 6.7 21.3 2.4 28.9
Product selectil~iO, (%) p-Methylstyrene p-Tolualdehyde p-Me-acetophenone p-Toluic acid Terephthaldehyde CO CO 2
Feed I: 0.16% p-ET(3.5 g / h ) , 5.5% O 2, 20% N 2 in He. II: 0.16% p-ET(3.5 g / h ) , 5.6% 0 2, 2(1% N 2 in CO_,. WHSV: 0.36-0.37 h - l , contact time: 0.21 s.
46
J.S. Yoo/Applied Catalysis A: General 143 (1996) 29-51
23.0% to 34.8%, respectively, at 375°C in the presence of CO 2. In spite of this increase in conversion and in aldehyde production, the amount of p-methylstyrene remained the same in both streams. This may indicate that p-methylstyrene was immediately consumed to form p-tolualdehyde and then to p-toluic acid. Interestingly, selectivity to p-methylacetophenone stayed low and did not change in both streams and at all temperatures. The presence of CO 2 did not make any difference in the formation of p-methylacetophenone. This probably indicates that a different reaction path which does not involve the formation of the vinyl group leads to the formation of this product. Aldehyde yield was significantly increased by the presence of CO 2 in the 0 2 feed gas in the oxidation of p-xylene and of p-ethyltoluene. However, in the case of styrene oxidation addition of carbon dioxide decreased substrate conversion. It should be pointed out that this finding in the gas phase 0 2 oxidation over the catalyst contradicts results observed in the liquid phase 02 oxidation of styrene with Fe and Rh-complex catalysts [23]. The presence of CO 2 favorably promoted the single O-transfer reaction in the liquid phase oxidation of styrene. The single O-transfer products such as styrene epoxide and phenylacetaldehyde were remarkably enhanced in the presence of CO 2. Considering the fact that the p-xylene reaction and the p-ethyltoluene reaction involve oL,[3-hydrogen abstraction as the initial step which is followed by the oxidation of vinyl to formyl group, whereas the styrene reaction involves only the oxidation of vinyl to formyl group, it can be argued that CO 2 is directly tied to the hydrogen abstraction step to form the vinyl group, but it does not participate in the conversion of the resulting vinyl group to oxygenates in the subsequent oxidation step. Since the hydrogen abstraction is the slow rate-determining step, the beneficial effect of CO 2 on the overall reaction becomes predominant. In short, the slow rate-determining step of hydrogen abstraction may be dramatically accelerated by the presence of CO 2 in the O 2 oxidation. It has been speculated that the peroxocarbonate species, CO 4 , may be responsible for this promoting effect, as this species has already been proposed with some catalysts [22,23]. The peroxocarbonate species may be generated on the molybdenum or iron center in ferric molybdate [19].
O._____C//0 0 0
[
0
\ /~o~ /e~
0
o ~ Mo~ 0
Si
IIIIIIIDBHIIIIIIIIIIII
J.S. Yoo / Applied Catalysis A: General 143 (1996) 29-51
Mo - Mo distance 6.77 7.08
Fe - Fe distance 5.43 A
7.23 A
47
H - H distance (between two CHa groups)
6.37- 7.23 A
Fig. 1. Molecular model of ferric molybdate local cluster showing octahedrally coordinated Fe 3+ and tetrahedrally coordinated Mo 6+ ions. The superposition of p-xylene indicates close size correspondence of M o - M o paired sites and size of p-xylene molecule.
3.5, Para-selectivity The para-selective property of the catalyst has been demonstrated in the gas phase 0 2 oxidations using various alkylaromatics as a substrate. The current thought on the nature of para-selectivity exhibited by the catalyst is discussed using toluene, p-xylene and its isomers as example. Ferric molybdate, Fez(MoO4)3, has been identified to be an active species responsible for the para-selective oxidation reactions [12,19]. Although a unit cell of this molecule contains eight inequivalent iron sites and 12 inequivalent Mo sites, the XRD database [21] indicates that all of the iron sites are octahedral with an average F e - O distance of 1.99 A, and all of the Mo sites are tetrahedral with an average M o - O distance of 1.77 ,~. A chemical model built based on the crystalline structure of Fe2(MoO4)3o is illustrated in Fig. 1. The average F e - O bond distance in the model is 1.98 A and the average M o - O distance is 1.76 A. It shows the distance between two Mo centers, the possible " p a i r " site of the catalyst, in the range of 6.77-7.23 ,~ and the distance between H of one meth~¢l group and H of the other methyl group in p-xylene in the range of 6.37-7.23 A. Thus, when the p-xylene molecule aligns with the " p a i r " sites of the catalyst, the two Mo centers sit right on top of the two hydrogen atoms of two different methyl groups in a p-xylene molecule and, at the same time, the center iron atom in - M o - O - F e - O - M o interacts with w-electrons of the benzene ring in p-xylene. The good geometrical fit between the active " p a i r " sites of the catalyst and the molecular dimension of the substrate may allow the abstraction of two hydrogen atoms, one from each methyl group, to occur in a concerted manner to form the diradical, "CH2-C6H4-CH~, which is more stable than the single o
o
48
J.S. Yoo /Applied Catalysis A: General 143 (1996) 29-51
radical counterpart, and eventually to lead to the formation of terephthaldehyde. On the other hand, if just one methyl group in p-xylene or in toluene stands up on the Mo center of the catalyst through the "single" site, the mono-radical is formed via the abstraction of one hydrogen atom from the methyl group, "CHz-C6H4-CH 3 from p-xylene and "CH2C6H5 from toluene. The monoradical leads to p-tolualdehyde or benzaldehyde. Since the diradical, "CH2-C6H 4CH 2, is much more stable than the mono-radical, "CHz-C6Hs, it is not surprising to observe that the diradical formation from p-xylene is much more favored over the mono-radical formation from either p-xylene or toluene. These two different types of active sites may function competitively during the gas phase 0 2 oxidation. Thus, terephthaldehyde and p-tolualdehyde are formed in the p-xylene oxidation and the molar ratio of these two aldehydes in the product can be controlled to a large extent by the catalyst surface modification as well as the oxidation variable optimization. In essence, the geometrical alignment of the p-xylene molecule with the " p a i r " site of the ferric molybdate, gives rise to the para-selective property of the catalyst. The formation of terephthaldehyde and p-tolualdehyde from the oxidation of p-xylene on bulk ferric molybdate under more severe conditions [12] gives credence to this explanation. The " p a i r " sites of the catalyst cannot align with the two methyl hydrogen atoms in o-xylene and in m-xylene. Furthermore, presence of the second methyl group in o-xylene hinders the interaction of the substrate with the catalyst. In the case of m-xylene, the second methyl group does not favor substrate-catalyst interaction at the other methyl substituent because the methyl substituent is known to be ortho- or para-directing. pair site
,o_©_o, . .,o_©_o,.
o.o_©_o.o
single site
single site
o.0_©
It is, therefore, not difficult to understand the low reactivity of o-xylene and no reactivity of m-xylene with the catalyst. Increased size of substrate molecules
J.S. Yoo / Applied Catalysis A: General 143 (1996) 29-51
49
did not affect the para-selectivity of the catalyst as seen in the reaction of pseudocumene and of durene. In fact, the larger durene underwent oxidation better than the less bulky pseudocumene. When the pore size of the fresh borosilicate molecular sieve is compared with the molecular volume of these substrates, it is difficult to rationalize that the para-selectivity of the catalyst stems from the shape of the pore, especially the internal channel structure of the DBH matrix. The same concept can also be applied to the oxidative dehydrogenation of alkylaromatics such as ethylbenzene, p-ethyltoluene and p-diethylbenzene. For p-diethylbenzene, it is thought that an alignment of the a-hydrogen atoms from two ethyl groups with the "pair" sites triggers the double a,[3-hydrogenation elimination to form divinylbenzene, and that one ethyl group interacting with the "single" site induces single oL,[3-hydrogen elimination to produce ethylstyrene. The oxidative dehydrogenation reactivities of these three substrates are defined by the para-selective nature of the catalyst, and their reactivities are in the order of p-diethylbenzene > ethyltoluene > ethylbenzene.
4. Conclusions
The CVD F e / M o / D B H catalyst exhibited a unique para-selective property for the gas phase 0 2 oxidations of various hydrocarbon substrates. Novel reactions catalyzed by the catalyst include the selective synthesis of terephthaldehyde, p-tolualdehyde, benzaldehyde and their derivatives, the activation and utilization of CO 2 as an oxidant for improving the catalytic activity and stability, the syntheses of oxygenates from alkenylaromatics, the oxidative dehydrogenation reactions of alkylaromatics to the corresponding alkenylaromatics. Terephthaldehyde, benzaldehyde and their derivatives were prepared by the para-selective oxidation of methylaromatics, alkylaromatics, alkenylaromatics and para-substituted toluene derivatives. The results were interpreted on the basis of para-selectivity of the catalyst having the "pair" and "single" sites on the surface of the ferric molybdate species by emphasizing the predominant effects of various para-oriented substituents. The reactivities of the hydrocarbon substrates are also closely tied to the geometrical as well as electronic structure of para-substituents. Thus, the feasibility of separating p-xylene from its isomers via the preferential oxidation using the big reactivity gap was demonstrated. The yields of the aldehyde products were also dramatically enhanced by running the oxidations of these substrates except alkenylaromatics in the presence of CO 2 in the O 2 containing feed. The role of CO z played for these reactions is to promote/accelerate the hydrogen abstraction reaction step, probably the rate-determining reaction, via the peroxocarbonate species, CO 4 . It appears that carbon dioxide does not
50
J.S. Yoo/Applied Catalysis A: General 143 (1996) 29-51
participate in the secondary oxidation of the vinyl group resulted from the initial dehydrogenation step. Under identical conditions, oxidative dehydrogenation occurred to form unsaturated hydrocarbon as an initial product when the alkyl group in the alkylaromatics became ethyl or higher. The reactivities of the alkylaromatics over the catalyst for the gas phase 0 2 oxidation are in the order of ethylbenzene < p-ethyltoluene < p-diethylbenzene. The para-oriented ethyl groups in p-diethylbenzene undergoes oxidative dehydrogenation much faster than the para-methyl group in p-xylene. Under similar gas phase 0 2 oxidation conditions, n-butane was oxidized to maleic anhydride, while propane and ethane were oxidatively dehydrogenated to propylene and ethylene, respectively. Finally, all the reactions studied share the reaction mechanism involving the rate-determining hydrogen abstraction(s) initiated by a common active phase, ferric molybdate, which provides the "pair" and "single" sites for the reactions.
Acknowledgements The author is indebted to P.S. Lin, S.D. Elfline, and M.S. Kleefisch for providing experimental data, and also greatly appreciates C. Choi-Feng and G.W. Zajac for the catalyst characterization.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
M. Niwa, N. Katada and Y, Murakami, J. Phys. Chem., 94 (1990) 6441. N. Katada, T. Toyama and M. Niwa, J. Phys. Chem., 98 (1994) 7647. M. Niwa, N. Katada and Y. Murakami, J. Catal., 134 (1992) 340. T. Jin and J.M. White, Surf. Interface Anal., 11 (1988) 157. W.W. Kaeding and L.B. Young and C.C. Chu, J. Catal., 89 (1984) 267. M. Niwa, Y. Kawashima and Y. Murakami, J. Chem. Soc. Faraday Trans. I, 81 (1985) 2757. E. Moretti, M. Padovan, M. Solari, C Marano and R. Covini, German Pat., 3,301,798 (1983). M. Niwa, M. Kato, T. Hattori and Y. Murakami, J. Phys. Chem., 90 (1986) 6233. T. Hibino, M. Niwa, Y. Murakami, S. Kohai and T. Hanaichi, J. Phys. Chem., 93 (1989) 7847. T. Hibino, M. Niwa, Y. Murakami and M. Sano, J. Chem. Soc. Faraday Trans. I, 85 (1989) 2327. N. Katada and M. Niwa, Catal. Lett., 32 (1995) 131. J.S. Yoo, J.A. Donohue and C. Choi-Feng, Symposium on Advanced Techniques in Catalyst Preparation, 209th ACS National Meeting, April 1-7, 1995, Anaheim, CA, preprint 110. J.S. Yoo, J.A. Donohue, M.S. Kleefisch, P.S. Lin and S.D. Elfline, Appl. Catal., 105 (1993) 83. J.S. Yoo, P.S. Lin and S. D. Elfline, Appl. Catal. A, 106 (1993) 259. J.S. Yoo, J.A. Donohue and M.S. Kleefisch, Appl. Catal. A, 110 (1994) 75. J.S. Yoo, C. Choi-Feng and J.A. Donohue, Appl Catal. A, 118 (1994) 87. J.S. Yoo, P.S. Lin and S.D. Elfline, Appl. Catal. A, 124 (1995) 139. J.S. Yoo, J.A. Donohue and C. Choi-Feng, Symposium on Advanced Techniques in Catalyst Preparation, 209th ACS National Meeting, Anaheim, CA, April 2-7, 1995, preprint pp. 103-113. G.W. Zajac, C. Choi-Feng, J. Faber, J.Z. Shyu, R. Patel, H. Hochst and J.S. Yoo, J. Catal., 151 (1995) 338.
J.S. Yoo /Applied Catalysis A: General 143 (1996) 29-51 [20] [21] [22] [23] [24]
J.S. Yoo, S.S. Grimmer, A.R. Sohail and C. Choi-Feng, Catal. Lett., 29 (1994) 299. V. Massarotti, G. Flor and A. Marinit, J. Appl. Cryst., 14 (1981) 4. J.L. Dubois, M. Minoun and C.J. Caeron, Catal. Lett., 6 (1990) 967. M. Aresta, C. lZragale, E. Quaranta and I. Tommasi, J. Chem. Soc. Chem. Commun., (1992) 315. J.S. Yoo, Appl. Catal. A, in press.
51