Studies of oxidative dehydrogenation of ethanol over manganese oxide octahedral molecular sieve catalysts

ELSEVIER

Microporousand MesoporousMaterials2I ( 1998)315-324

Studies of oxidative dehydrogenation of ethanol over manganese oxide octahedral molecular sieve catalysts H. Zhou a, J.Y. Wang a, X. Chen a, Chi-Lin O’Young b, Steven L. Suib a*cgd,* a U-60, Department of’Chemistry. University qj Connecticut. Stows. CT 062694060. USA b Texaco ResearchCenter, Texaco Inc. PO Box 509, Beucon, NY 12508, USA ’ Department of Chemical Engineering, University of Connecticut, Storrs, CT 06269, USA d institute of Materials Science, University of Connecticut, Storrs. CT 06269, USA Received 25 August 1997:received in revised form 16 January 1998;accepted 16 January 1998

Abstract

A seriesof manganeseoxide octahedral molecular sieve (OMS) materials were prepared and characterized by X-ray diffraction and surface area measurements (Brunauer-Emmett-Teller). Some first row transition metals (Ni’ ‘, Cd’, Fe3+, Co’+, M g” ) were doped into the framework of these octahedral molecular sieve materials to give [Ml--0MS materials. The catalytic activities of these materials were studied in the oxidative dehydrogenation of ethanol in a flow reactor at different temperatures and at various ratios of ethanol to OZ. The influence of space velocity on the reaction was also tested. The experimental results showed that both the conversions and selectivities of this reaction over each individual OMS material are highly dependent on the nature of OMS materials and the transition metal dopants in their framework structures. The [Co]-OMS-1 materials show the highest conversion and selectivity to acetaldehyde of all materials. Basic sites on both OMS-1 and OMS-2 materials may be responsible for catalytically active sites in this reaction. 0 1998 Elsevier Science B.V. All rights reserved. Keywords:

Oxidative

dehydrogenation;

Manganese oxide; Molecular sieve

1. Introduction Manganese oxide octahedral molecular sieve (OMS) materials [ 1,2] are a class of synthetic manganese oxide materials with tunnel structures similar to the naturally occurring minerals todorokite and hollandite. The basic structural unit of both todorokite and hollandite is the MnO, octahedron arranged in tunnel-like microstructures. * Corresponding author. 1387-181l/98/$19.00 0 1998Elsevier ScienceB.V. All rights reserved PII: Sl387-1811(98)00034-1

The oxidation states of manganesein these materials are Mn4+, Mn3+ and Mn2+ [3-61. The presenceof one-dimensional tunnels in these materials allows their utilization in a manner similar to molecular sieves,like the microporous crystalline inorganic solids such as zeolites and clays [ 7.81. Molecular sieves are able to differentiate molecules on the basis of size and shape. Furthermore, these unique properties are considered to be important in both heterogeneouscatalysis and gas sorption.

The structure of synthetic todorokite-like molecular sieves (OMS-I ) consists of chains of MnO, octahedra with edge sharing to form one-dimensional 3 x 3 tunnel structures. as shown in Fig. 1 [I-- 3,9, lo]. The average oxidation state of manganese in OMS-I is +3.5 [ 1.111. The pore size of these materials is about 6.9 A [ l,l2-141. OMS-1 is usually thermally stable above 500’ C in air [ 11. Some work shows that OMS-I materials evolve oxygen at high temperatures [ 151. This oxygen evolution and high thermal stability may make OMS-1 materials useful as oxidation catalysts [l,I5]. Hollandite-like materials (OMS-2) consist of

OMS-1

Ms&~M~,$&,Mr&,O,, I 4.474.55 H 20 I . Fig. I. Structure ofsynthetic

todorokire-like

catalysts (OMS-I ).

OMS-2

Fig. 2. Structure of synthetic hollandite-like

catalysts (OMS-2 ).

the same structural units of MnO, octahedra joined at the edges, but form 2 x 2 one-dimensional tunnel structures, as shown in Fig. 2 [ 1214]. The average oxidation state of manganese in OMS-2 is +3.9. and the pore size of OMS-2 is about 4.6 A [ 1l-141. It is thermally stable above 600 C in air [Ill. Some first row transition metal cations and divalent cations can be doped into the framework of both OMS-1 and OMS-2 materials tr.) give doped OMS-1 and OMS-2 materials which are symbolized as [Ml-OMS-I and [Ml--OMS-2 [2]. Here, M in brackets stands for transition metals other than manganese doped in the framework PI. In the present work, the catalytic activities of both [ Ml-OMS-1 and [ Ml---OMS-2 materials were investigated in the oxidative dehydrogenation of ethanol to acetaldehydc in a flow system at different temperatures and with different ralios of ethanol to OZ. Currently, one of the commercial techniques for production of acetaldehyde is catalytic oxidation of ethanol. which is carried out by passing alcohol vapors and preheated air over a silver oxide catalyst at 4XO’C at a conversion of 74% to 82% and a selectivity to acetaldehyde generally no more than 80% [ 161. There are two major disadvantages of this catalyst system: high cost of the catalysts and high reaction temperature. Other catalysts have. therefore, been intensively studied for oxidative dehydrogenation of ethanol to produce acetaldehyde. Mainly transition metal oxides or sy-stems with mixtures of diRerent transition metal oxides have been studied. The results have shown that temperatures can be lowered to yield comp’ztitive conversions and selectivities to acetaldehydc [ 172 11. The [M ]-OMS can be considered as a mixture of different transition metal oxides and may be useful as oxidation catalysts [ 1,151. The experimental results of this research showed that the conversions and selectivities of this reaction are highly dependent on the structure of the OMS materials and the transition metal dapants in their frameworks. The influence of space velocity and different ratios of oxygen content to ethanol in the oxidative dehydrogenation reaction were also studied.

3111

H. Zhou et ~1. ’ Microporous und Mesoporous Marrrials 21 ( 1998) 31.5 324

2. Experimental section 2.1. Reagents

All reagents were of analytical grade, unless otherwise noted. Distilled deionized water (DDW ) was used to prepare materials. Hydrogen peroxide (30% aqueous solution with stabilizer) was obtained from J.T. Baker, Inc. The water used to dilute the aqueous hydrogen peroxide was DDW. The total concentration of H202 was determined by titration with a 0.1 N solution of KMnO,. 2.2. Cutulysts

The catalysts were [Ml-OMS-land [Ml-OMS-2-type materials. [M] stands for other metals besides manganese doped into the framework of OMS materials. The [Ml-.OMS-I materials were synthesized hydrothermally in autoclaves [ 151, whereas the [ Ml--OMS-2 sampleswere synthesized by reflux and precipitation methods [22]. 2.3. Preptrration qf’[MJ-OMS-I

materiuls

An aqueous solution of 8 ml of 0.10 M MCI, (M = Mg, Co, Ni. Cu, and Fe) was added to 60 ml of 0.050 M MnCl, solution. [ 151A 75 ml solution of 6 M NaOH was then added to obtain an M-doped Mn(OH), suspension. A 60 ml solution of 0.20M NaMnO, was added dropwise to the suspension. The resulting precipitate was aged at room temperature for 1 week before it was filtered, and washed with DDW until no chloride ions were detected in the supernatant. The washed product (Na-buserite) was exchanged with 500 ml of 0.5 M MgCI, solution for 3-8 h to give ML+-doped Mg-buserite. The M-doped Mg-buserite was subsequently filtered and washed with DDW before it was autoclaved at 160°C for 30 h. The autoclaved product was finally washed and freeze-dried. Normally, the yield of the final product was about 3 g. 2.4. Preparation oj‘(MJ-OMS-2

muterials

KMnO, ( 13.3 g) dissolved in 225 ml DDW was added to a solution of 19.8 g MnSO,. H,O in

67.5 ml DDW and 6.8 ml of concentrated nitric acid [22]. The resulting black precipitate was stirred and refluxed at 100°C for 24 h. The initial pH of the solution was 1.7. The precipitate was washed seven times with lOOm1 of DDW. The final product was filtered and dried at 120°C overnight. Typical yield was about 17 g. [Ml-OMS-2 materials doped with different first row transition metals were prepared by adding aqueous solutions of dopants prior to refluxin,g. Other synthetic steps are the same as the undoped materials. [Nil-OMS-2( 1) and [Nil-OMS-2(2) were prepared by adding different amounts ol aqueous solutions of Ni( NO&. hH,O to the realctant solution. 2.5. Su$~e area measurements

The surface areas of both [Ml-OMS-1 artd [ Ml-OMS-2 materials were determined by Brunauer--Emmett-Teller (BET ) measurements using nitrogen gas and a multi-point method. The surface areas of [Ml-OMS-1 materials range from 140 to 180m2g-‘, and 100 t.o 120m2 g-’ li.,r [Ml-OMS-2 materials. 2.6. X-ru.v powder difktion

i XRD) *studies

Both [Ml-OMS-1 and [Ml-OMS-2 materials were characterized with XRD methods. Data were collected with a Scintag 2000 PDS with Cu Ka X-radiation, a beam voltage of 45 kV, and 40 mA beam current. 2.7. Fourier transform @ared measurrmen ts

(FTIR)

A sample wafer made of 19.00mg [Ml-OMS sample mixed with 1.OOmg pure KBr was thermally dried overnight at 110°C in an oven. A background spectrum was recorded from a pure KBr wafer at room temperature with a Gala;lcy Model 4020 FTIR spectrometer. A sample wafer was located at the center of the sample chamber, where an IR light beam passedthrough the sample wafer. Then IR spectra in the transmittance mode were taken from 100 cm--’ to 600cm-‘. The

318

H. Zllou et al. ! Microporous and Mesoporous Materials 21 (1998) 315.324

sample chamber was purged by N, during the measurements. 2.8. Apparatus andprocedures

The apparatus used for catalytic studies is shown in Fig. 3. The catalyst (20 mg pretreated with He gas at 250°C for 1 h) is located at the internal middle point of a Pyrex tube which has a 0.25 in diameter and is placed inside a furnace. Both ends of the catalyst bed were capped by glass wool. The ethanol was carried into the reactor with a gas mixture of He and 0, from a bubbler placed in an ice bath. The ethanol carried out was then passed through the catalyst bed where the oxidative dehydrogenation of ethanol occurred. The temperature of the catalyst bed was set at a certain point. controlled, and monitored with a temperature controller and digital thermometer. Gas chromatography mass spectrometry (GC-MS) methods were used for the identification of reaction products. In situ GC analysis was done for the quantitative analysis of reaction products. The quantitative analysis of each individual product and unreacted ethanol was determined according to their thermal conductivity response factors. Therefore. both conversions and selectivit-

-8 7-l i Ii L 5

67

3

ies for each individual product were readily calculated. The GC was a Hewlett-Packard 5880 A with a thermal conductivity detector (TCD), and H:e was used as carrier gas. Two columns in series were used for product separation. One was a Poropak-Q column, and the other was a Poropak T column. The oven temperature was 16O”C,and the temperature profile was isothermal. The GC MS was a Hewlett-Packard model 5890 series II chromatograph with a Hewlett-Packard model 5971 series mass-selective detector. The column used was HP. 1 (methyl silicone gum) Instrument Test with 5 m x 0.53 m x 2.65 pm film thickness. The temperature profile included three levels which are: level 1 at 5”C, which remained at this temperature for 1 min. then went to 20°C with an increasing rate of 5°C min-l; level 2 started at 20°C then went to 150°C with an increasing rate of 35°C min-‘; level 3 started at 150°C then went to 240°C with an increasing rate of 30°C min- ’ and stayed at this temperature for 13 min. The solvent delay time was 1 min. The range of ion mass detected was from 14 to 150 m/r. The catalysts investigated are: [ Ml-OMS- 1, including [Al]-OMS-1, [Co]-OMS-1, [Cu]OMS-I, [Fe]-OMS-1, [Mg]-OMS-1, [Nil-OMS-1 and [Zn]--OMS-1; [Ml-OMS-2, including [Al]OMS-2, [Co]-OMS-2, [Cu]--OMS-2, [Fe]-OMS-2, [Mg]-OMS-2, [Nil-OMS-2 and [Zn]-OMS-2; here M represents the same cations, which alre Al, Co, Cu, Fe. Mg, Ni or Zn ions doped in the framework of the manganese oxide OMS materials. The particle sizes of both [Ml-OMS-I and [Ml--OMS-2 catalysts were selected within the range of 300 to 500 pm.

9

1 .1

1. gas mixture

2. bubbler

3. ethanol

5. flow reactor

6. glasswool 7. catalyst

3. Results 3.1. Catalytic results

4. furnace 8. transfer line

9. GC Fig. 3. Reaction apparatus diagram of oxidative dehydrogenation of ethanol over [Ml- OMS catalysts.

The reaction products analyzed with G(:-MS and GC methods were CO,, H,O, acetaldehyde (CH,CHO), and ethanol which was unreacted and remained in the system after reaction. The equations for three possible chemical pathways are

:\I9

H. Zhou et al. i Microporous and Mesoporous Muterials 21 ( 1998) 315 -324

listed as follows: [MJ-OMS

CH,CH,OH + 0.502 ------+CH,CHO + Hz0

( 1)

A W-C’MS

2C02 + 2H,O

CH,CHO + 2.502-

(2)

A IMWMS

CHJH,OH

-t 30, -----+2CO, + 3H,O A

(3)

The conversion and selectivity are defined as follows: ( 1) conversion equals the number of moles of reactant consumed divided by the number of moles of the total reactant passed into the reactor; (2) selectivity of a desired product equals the number of moles of the reactant converted to the desired products divided by number of moles of the total consumed reactant. Table 1 shows the conversions and selectivities to acetaldehyde and carbon dioxide for the oxidative dehydrogenation of ethanol over each individual [Ml---OMS-1 catalyst with 1% 0, in He at 300°C. The results of Table 1 show that [Co]--OMS-1 gives 82.8% conversion, which is the highest among all the [M ]-OMS-I catalysts tested here. The [Fe]--OMS-1 catalyst gives 92.8% selectivity to acetaldehyde in the reaction, which is the highest one among the [Ml--OMS-I catalysts tested here. The [Cu]-OMS-1 catalyst gives 23.7% selectivity for CO1 in this reaction, which is the highest selectivity to COz for all [Ml-OMS-1 catalysts tested here. Table 1

Conversions and selectivities of oxidative dehydrogenation of ethanol over [Ml-OMS-I catalysts with 1% 0, in He at 3OO’C Catalyst

C” (8)

5’1b 6)

s, c (%)

[Co]-OMS-1 [Mg]m-OMS-I [Cu] -OMS-I [Znj-OMS-1 [Nil -OMS-1 [Fe]-OMS-1

82.8 72.1 66.4 64.2 58.0 28.5

81.1 76.7 77.7 84.8 X6.6 92.8

18.9 23.7 22.3 15.2 1x.4 7.2

’ Conversion. b Selectivity to acetaldehyde. ’ Selectivity to carbon dioxide.

Table 2 shows the conversions, selectivities to acetaldehyde and carbon dioxide for oxidative dehydrogenation of ethanol over each individual [Ml-OMS-2 catalyst with 1% 0, in He at 300°C. The results show that [Fe]--OMS-2 gives 71.5% conversion, which is the highest one among all the [Ml-OMS-2 catalysts studied here. The [Fe]-OMS-2 catalyst also gives 78.9% selectivity to acetaldehyde of the reaction, which is the highest one among all the [Ml-OMS-2 catalysts tested here. The [Cu]-OMS-2 catalyst gives 40.1% selectivity of CO1 in the reaction, which is the highest selectivity to COz for all of the [ Ml-OMS-2 materials tested here. Table 3 shows a comparison of conversions over both [ Mg]-OMS-1 materials and [Ml-OMS-2 materials with 1% 0, in He at 300°C. The results suggest that all the conversions over [Ml--OM:S-2 Table 2 Conversions and selectivities of oxidative dehydrogenation of ethanol over [ Ml-OMS-2 catalysts with 1% O2 in He at 300°C Catalyst

c” @)

s, b (%)

[Fe]--OMS-2 [Co]--OMS-2 [Cu]--OMS-2 [Zn] OMS-2 KmOMS-2 [Nil- OMS-2

71.5 63.0 58.5 57.0 55.8 48.4

78.9 72.0. 59.9 66.5 64.6 78.0

s, (: (%) -21 I 28 0 40.I 33 5 35.4 22.0

a Conversion. b Selectivity to acetaldehyde. ’ Selectivity to carbon dioxide. Table 3 Comparison of conversions and selectivities of oxidative dehydrogenation of ethanol over [Mg]-OMS-1 and [Ml--OMS-2 Catalysts with I% 0, in He at 3OO’C Catalyst

c” (%Jg)

s, b (%)

s; r (6)

[Co] -OMS-2 [Vu)- OMS-2 [Fe] -OMS-2 [Mg]-OMS-I [Nil -OMS-2 [Zn]-OMS-2 K--OMS-2

63.0 58.5 71.5 72.1 48.4 57.0 55.8

72.0. 59.9 78.9 76.7 78.0 66.5 64.6

28.0 40. I 21.1 23.7 22.0 33.5 35.4 .-

’ Conversion. ’ Selectivity to acetaldehyde. ’ Selectivity to carbon dioxide.

materials are lower than those with [Mg]-OMS-1 catalysts. 3.2. The efltict of reuction tmpmmrc Table 4 shows a comparison of conversions and selectivities in the oxidative dehydrogenation of ethanol over [Co]-OMS-1 catalyst with 1% O2 in He gas at 250-C and 3OO’C. When lowering the temperature of the reaction from 300 to 250 C, but keeping other experimental parameters identical, both conversions and selectivities to CO, were decreased from 82.8 to 78.3% and from 18.9 to 14.1% respectively. However, the selectivity of acetaldehyde increased from 81.1% to 85.9% on lowfering the temperature of the reaction from 300 to 25O’C. 3.3. The t$fkct qf’chunge in o.y’grn con tent Table 5 shows a comparison of the conversions and selectivities of oxidative dehydrogenation of ethanol over [Co]-OMS-1 catalysts with gas mixTable 4 Conversions and selectlvities of oxidativc dehydrogenation of ethanol over [Co] -OMS-I catalyst with I % 0: in He at 250 C and 300 C Cataly5t

(” (%I

s, b (%)

[Co] OMS-I [Co- OMS-I

81.8 78.3

81.1 85.9

Table 5 Comparison of conversions and selectivities of the reaction over [Co] OMS-I catalyst with different concentrations of O2 at 300 c

(%)

0: (5%)

P

[Co] -OMS-I [Co~OMS-I [Co] OMS-I [Co] OMS-I

I 1 5 ?I

82.8 85.2 89.4 100

’ Conversion. h Selectivity to acetaldehyde. ’ Selectivity to carbon dioxide.

The SV is defined as the number of moles of reactant passed over a unit weight of the catalyst within a unit of time. Here. the reactant is ethanol, the catalyst is [Co]--OMS-1: and units of weight and time are gram and hour respectively. Table 6 shows a comparison of the conversions and selectivities of oxidative dehydrogenation of ethanol ov-er [Co]--OMS-I catalysts with 1% 0, in He. but with different SVs at 300°C. The SV here is defined as the number of moles of reactant passing through 1 g of the catalyst in a certain unit of time. The results show that both the conversion and se.lectivity to CO, were decreased with increasing SV of ethanol, but the selectivity to acetaldehydz was increased.

Fig. 4 shows activity versus time-on-stream of [Co]--OMS-1 catalysts in the oxidative dehydrogenation of ethanol with 1%’ 0, in He gas at 300°C. The catalytic activity did not decrease significantly over a 3 h period.

’ Conversion. ’ Selectivity to acetaldehyde. ’ Selectivity to carbon dioxide.

Catalyst

tures of oxygen and He in which the oxygen content was varied from 1 to 21% at 300°C. The results show that both the conversion and the selectivity to CO, were increased with increasing concentration of O,, but the selectivity of acetaldehyde was decreased. When air was used as an oxidant, both the conversion and selectivity to CO, became 100%.

s, ” (%)

s, c (%I

81.1 7x.5 71.4 0

IX.!, 21.5 ‘7.6 I00

4. Discussion 4.1. CrrtuJl~tic resulr.5 Various cations doped in the framework of [M] -OMS-I might result in different physicochemical properties of [Ml-OMS-1 materials, which may be due to the nature of these various cations [2]. Thermal and desorptive properties [l-i] are also known to vary when these various cations are doped into both OMS-1 and OMS-2 materials [I 151. Therefore. it is possible that [Ml-OMS-I

H. Zhou et ul. Table 6 Comparison

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Mimqmraus and Mesoporous Muturials 21 (199X) 31.5 324

of conversions and selectivities of the reaction over [Co] OMS- I catalyst with I % OL in He and different SVs at 300’ C

Catalyst

SV” (molg

[Co]-OMS-I [Co] -OMS-I [Co1 --OMS- I

3.6x IO ~z 6.1 X IO 1 1.1x10 ’

‘h

‘)

c (%7c)

s, c (%)

sz d (9%)

89.7 82.8 76.6

78.4 XI.1 86.8

21.6 18.9 13.2

L SV of ethanol (moles of ethanoI,‘gram of catalyst per hour). h Conversion. c Selectivity to acetaldehyde. d Selectivity to carbon dioxide.

0

01 0

Selectivity

of Carbon

Dioxide

100 Time

200

300

(min)

Fig. 4. Activity versus time-on-stream O2 in He at 300 C.

for (Co] OMS-I with 1%

materials with different types of transition metal dopant would be expected to give different catalytic activities in the oxidative dehydrogenation of ethanol. The results shown in Table 1 are consistent with the above hypothesis. Table 1 shows that, under similar experimental conditions, each individual [M ]--OMS-1 material gives different conversions as well as different selectivities to both carbon dioxide and acetaldehyde. Focusing on the conversion data, it is shown that [Mg] OMS- 1 catalyst is very active and only [Co]-OMS-1 catalyst surpasses this activity. The conversions of all other catalysts are less than that of [Mg]---OMS-I material, especially the conversion of [Fe]-OMS-1 catalyst which was only 28.5%, the lowest one among all these catalysts. We have observed that the catalysts containing transition metal framework dopants are active and selective for dehydrogenation of ethanol to acetal-

dehyde [22--271. This is consistent with the effect of transition metal cations for the catalytic activity of [Ml-OMS-1 in total oxidation of CO [ 11. Table 1 shows that the selectivity of acetaldehyde in the reaction with [Mg]-OMS-1 catalyst is considerably less than the selectivity to acetaldehyde obtained when doped [Ml-OMS-1 materials were used. Among all the catalysts tested, [Fe]---OMS-I catalyst gives the highest selectivity to acetaldehyde in the oxidative dehydrogenation of ethanol. The data of Table 1 show that a systematic trend is observed for these [M ]--OMS- I materials doped with transition metal cations in their frameworks with respect to the conversion and selectivity. There is an inversely proportional trend between the conversion and selectivity to acetaldehyde. Materials with high conversions give lower selecrtivites to acetaldehyde. For example, among these materials, [Co]-OMS-1 gives the highest conversion and the lowest selectivity t.o acetaldehydc, whereas [ Fe]--OMS-1 gives the lowest conversion, but the highest selectivity to acetaldehyde. Th.e most active catalysts promote total oxidation of ethanol to CO,, diminishing the yield of CH,CHO. In addition, for these most active catttlysts, CH,CHO can be further oxidized to CO;,. These observations may explain the inversely proportional trend of high activity and low selectivity to CH,CHO. Another interesting observation from the data of Table 1 is that both conversion and selectivity to acetaldehyde with most transition metal cations, except Co, only increased the selectivity to acetaldehyde, but decreased the overall conversion in relation to undoped [ Mg]-OMS-1 materials. Only [Co]--OMS-1 catalysts increased both the conver-

322

H. Zhou rt al. i Microporous and Mesoporous Materials 21 (I998) 315-324

sion and selectivity to acetaldehyde: however, the reason why Co” dopants in the framework affect the catalytic activity and selectivity of [Co]-OMS-1 materials is still not clear. Acetic acid is a potential product resulting from further oxidization of acetaldehyde which was not detected in our experiments. This may suggestthat [ Ml-OMS-1 materials are mild catalysts for oxidation reactions and in the selective oxidative dehydrogenation of ethanol. Transition metal cations doped into frameworks of OMS-2 materials are also expected to show differences in both catalytic activity and selectivity in much the same way as with the [Ml---OMS-I materials discussedabove. Table 2 shows that conversions of all doped [M] OMS-2 materials, except [ Nil-OMS-2, were significantly increased to different extents in relation to the undoped K--OMS-2 material. The conversion of [Fe] -OMS-2 is the highest one, whereas the conversion of [Nil-OMS-2 is even lower than the conversion of undoped OMS-2 material ( K-OMS-2). There is no obvious relationship between conversion and selectivity to acetaldehyde for the [Ml-OMS-2 materials as was the casefor [M ]-OMS- 1 materials discussed above. The conversions and selectivities to acetaldehyde for [ Fe]--OMS-2, [Co]-OMS-2, and [Zn]-OMS-2 catalysts were increased significantly with respect to undoped K--OMS-2 material. The conversions and selectivities to acetaldehyde for [Cu]-OMS-2 catalysts were decreasedwith respect to K-OMS-2 material. However, the conversion of [ Nil-OMS-2 was decreased,but the selectivity to acetaldehyde was increased significantly again with respect to K-OMS-2. These data suggestthat transition metal cations doped into the frameworks of OMS-2 materials play a more critical role in catalytic activity and selectivity than in the OMS-1 materials. The exact role of the transition metal dopants is much more complicated for the OMS-2 systems than the OMS-1 systems. We have critically investigated the optical, thermal, redox, structural and other properties of transition-metal-doped OMS-2 systems in other reactions [ 1)151. The Fe and Co systems show variable oxidation states; the Zn system is fixed

at Zn”. The thermal stabilities of Fe2 ’ and Zn’+ dopants are much less than Co” and Cu* ‘. The surface areas of all materials are about the same. There are no obvious trends between the physicochemical properties and these catalytic data. The major differences between [ Ml-OMS-1 and [ Ml-OMS-2 materials can be examined from three aspects. The first is the tunnel opening sizle. The tunnel opening size of [Ml-OMS-1 is around 6.9 A, whereas the tunnel opening size of [Ml-OMS-2 materials is only around 4.6 A. The kinetic diameter of CH&HO is about 4.5 A. which is approximately equal to the pore size of OMS-2 and leads to better shape selectivity than with OMS-1 [28]. A second factor is the average oxidation state of Mn. The average oxidation state of Mn in [ Ml-OMS-1 materials is about + 3 5 and for [Ml---OMS-2 it is about +3.8. A third factor is surface area. The surface areas of [Ml-CIMS-I and [ Ml-OMS-2 are in the ranges of 140 to 180rn’g-’ and 100 to 130 m2 g-’ respectively. Table 3 shows that, compared with conversion over [ME]-OMS-1. all the conversions from each transition-metal-doped [MI--OMS-2. as well as undoped OMS-2 material (K--OMS-2), are lower than the conversion of [Mg]-OMS-1. These results can probably be explained by the fact:, that [M]~-OMS-1 materials have relatively larger surface areas than [Ml-OMS-2 materials and that the opening size of [Ml-OMS-1 materials is large enough to allow ethanol to get into the pores of [Ml-MOMS- 1 materials, which might provide shape-selectiveeffects [ 11,29,28]. Since it has been found that basic sites of molecular sieves play a very important role in the dehydrogenation of alcohols [ 22,301, this suggests that both OMS-1 and OMS-2 materials also have basic sites. However, acid sites have already been found in thesematerials [ 1,2]. Similar observations of both acid and base properties have also been discovered in zeolite molecular sieves [26,30-341 and in the natural manganesenodule counterparts of OMS materials [35.36]. 4.2. Thr eflkct of rruction temperuture

Table 4 shows that temperature is also an effective factor that significantly influences the conver-

H. Zhou et al. 1 Microporous and Mesoporous Materials 21 (1998) 315-324

sions and selectivities to acetaldehyde and carbon dioxide for oxidative dehydrogenation of ethanol. For [Co]-OMS-1 material, on lowering the reaction temperature (while keeping other reaction parameters unchanged), the conversion and selectivity to carbon dioxide were decreased,while the selectivity to acetaldehyde was increased. As mentioned previously, the products of this reaction were carbon dioxide, water, and acetaldehyde. Acetaldehyde was a product of oxidative dehydrogenation of ethanol. Carbon dioxide was either from total oxidation of ethanol or acetaldehyde. Water can also be produced from either the oxidative dehydrogenation of ethanol or the total oxidation of both ethanol and acetaldehyde. It is also understandable that, in the presence of [Co]-OMS-1, at the lower reaction temperature there will be smaller amounts of both ethanol and acetaldehyde oxidized to produce carbon dioxide and water. Consequently, the apparent overall reaction conversion and selectivity to carbon dioxide were decreasedand the selectivity to acetaldehyde was increased. Similar phenomena are found for the same reactions over zeolites 1311. 4.3. The efect of change in 0, content

The data of Table 5 suggest that the concentration of 0, also influences the conversion and selectivity to acetaldehyde as well as carbon dioxide. Since the total oxidation of both ethanol and acetaldehyde are more favorable under oxygenrich conditions, the conversion may be increased with a decreasein selectivity to acetaldehyde. 4.4. The efects of SV

The influence of SV on conversions and selectivities can be explained by considering that the higher SVs result in a shorter residence time of ethanol in the bed of OMS materials where both oxidative dehydrogenation of ethanol and further oxidation (complete oxidation) occur. As mentioned previously, the carbon dioxide could be due to total oxidation from either ethanol or acetaldehyde, which were both catalyzed by OMS-1. The shorter residence time would cause a decrease in conversion and the higher selectivity to acetaidehyde.

323

Shorter residencetimes may inhibit total oxidation of both C,H,OH and CH,CHO. 4.5. Catalytic activity and deactivation of catalysts

Data of Fig. 4 show a rapid approach to steady state of the conversion and selectivity to acetaldehyde. No severecatalytic deactivation during a 3 h period of reaction time was observed. These data suggest that turnover frequencies (TOFs) of these two catalysts are quite low in this reaction. Such low TOFs have been observed for all the [Ml-OMS-1 and [Ml-OMS-2 catalysts tested in oxidative dehydrogenation of ethanol.

5. Conclusions From the results discussed above, the following observations have been made. ( 1) Both OMS- 1 and OMS-2 materials are active towards oxidative dehydrogenation of ethanol and show high selectivities to acetaldehyde. (2) Incorporation of transition metal dopants in the frameworks of both [Ml-OMS-1 and [Ml-OMS-2 leads to a variation in the catalytic activities of these materials. The framework transition metal dopants of [Ml--OMS-1 enhance the selectivity to acetaldehyde. Among all the [Ml-OMS-1 materials tested, the [Co]-OMS-1 material demonstrated both the highest overall conversion and selectivity to acetaldehyde. The transition metal cations doped in the framework. of [Ml--OMS-2 materials may behave differently from similar cations in [Ml---OMS-1 materials Among the [Ml-OMS-2 materials tested, the: [ Fe]-OMS-2 material shows both the highest conversion and selectivity to acetaidehyde. (3) The catalytic activities of both [ Ml-OMS-I and [Ml-OMS-2 materials may be due to basic: sites in these materials.

Acknowledgement We thank the US Department of Energy, Office; of Basic Energy Sciences, Division of Chemical

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H. Zhou et al. / Microporous and Mesoporow

Sciences and Texaco. Inc. for support of this research. References

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