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Formaldehyde production by catalytic dehydrogenation of methanol in inorganic membrane reactors
applied catalysis A ELSEVIER
Applied Catalysis A: General, 109 ( 1994) 63-76
Formaldehyde production by catalytic dehydrogenation of methanol in inorganic membrane reactors Jingfa Deng”, Jingtao Wu Department (Received
of Chemistry,Fudan 15 December 1992,
University, Shanghai, People’s Republic of China
revised manuscript received 22 October 1993)
Abstract Palladium/ceramic membranes were prepared by electroless plating and porous alumina membranes were prepared by the sol-gel process, respectively. Characterization of the membranes showed
that they had a high permeability and selectivity to hydrogen. The catalytic components copper and phosphorus were deposited onto the porous alumina membrane by the sol-gel process and thus a catalytic inorganic membrane was prepared. The catalytic dehydrogenation of methanol to formaldehyde was carried out in membrane reactors incorporating the palladium/ceramic membrane, the
porous alumina membrane and the catalytic porous membrane, respectively. The yields of formaldehyde increased significantly in these membrane reactors compared with that in the conventional reactor. At moderate reaction conditions, the yield increased by 15% with the palladium/ceramic membrane, 8% with the c:atalytic porous membrane and 5% with the porous alumina membrane. The properties and characteristics of the three membranes were compared and discussed. Key words: ahrmina membrane; composite membrane; dehydrogenation catalyst; membrane reactor; methanol; palladium membrane
1. Introduction
Membrane reactors have generated great interest in recent years because they have many favorable characteristics compared to conventional reactors [ 11, one of which is that they can shift the thermodynamic equilibrium of a reversible reaction system toward the product side. For membrane reactors used in the catalytic reactions, inorganic membranes are usually the best choice due to their high thermal stability [ 2,3]. Kikuchi et al. developed a new *Corresponding author. Fax. ( + 86-2
1)5490535
0926-860X/94/$07.00 0 1’994Elsevier Science B.V. All rights reserved SSDl0926-860X(93)EO218-2
64
J. Deng, J. Wu /Applied
Catalysis A 109 (1994) 63-76
type of inorganic membrane [ 4,5], the palladium/ceramic membrane, which had a much higher hydrogen permeability than that of an ordinary palladium membrane, yet the high selectivity to hydrogen remained unchanged. Yoldas successfully prepared crack-free and pinhole-free porous alumina membranes by the sol-gel process [ 61. Both of these membranes show promise for use in catalytic reactions. Until now, most of the research work on membrane catalysis involving the combinations of reactions and separation has concentrated on dehydrogenation reactions [ 7,8]. In the present work, the dehydrogenation of methanol to formaldehyde was chosen for investigation. Song and Hwang have studied the oxidative dehydrogenation of methanol to formaldehyde in a porous vycor glass membrane reactor [ 9 1. The product of this procedure is an aqueous formaldehyde solution which has a concentration of not more than 40%. It is difficult to get formaldehyde in the high concentration which is sometimes required. Direct catalytic dehydrogenation of methanol results in simpler products with a high concentration of formaldehyde [ I I]. However, the conversion of methanol is low due to thermodynamic equilibrium limitations. Zaspalis et al. reported the dehydrogenation of methanol over yA&O3 membrane reactors. The conversion of methanol was significantly improved, but the selectivity of formaldehyde was low [ lo]. In the present work, the reaction was carried out in membrane reactors using a Cu-P/Si02 catalyst. The conversion of methanol was found to be increased greatly with a very high selectivity of formaldehyde and thus the reaction may have potential for industrial manufacture. While the combination of reaction and separation is one of the breakthroughs of membrane reactors, the combination of the membrane and catalyst is also significant. Not only can it simplify the process operation, but it can also further enhance the conversion of the reactants. The most direct way to deposit copper and phosphorus onto a membrane is by impregnation. However, an impregnated Cu-P/A&O, membrane catalyst showed lower selectivity of formaldehyde than that of the Cu-P/SiO, catalyst, probably due to the different interaction between copper-alumina and copper-silica. In order to improve formaldehyde selectivity, a new type of Cu-P/Si02 membrane catalyst was prepared by the sol-gel process. Through SiOz can form a porous membrane itself, the stability of this membrane is much lower than that of a A&O3 membrane. Therefore, the Cu-P/Si02 catalyst was deposited onto the porous alumina membrane. This membrane catalyst prepared by the sol-gel process showed satisfactory activity in the dehydrogenation of methanol to formaldehyde. The solgel process proved to be a successful method for preparing a membrane catalyst. In the present paper, we report the preparation of three different kinds of inorganic membranes: a palladium/ceramic membrane, a porous alumina membrane and a catalytic inorganic membrane. The effects of these membrane reactors in promoting the dehydrogenation of methanol to formaldehyde are investigated. The properties of these membrane reactors are compared.
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2. Experimental 2.1. Membrane preparation 2.1.1. Palladium/ceramic membrane The palladium/ceramic membrane was prepared according to Kikuchi and coworkers [4] by electroless plating on the outer surface of a porous ceramic tube (O.D. 12 mm, I.D. 10 mm, average porous size 0.5 pm). The thickness of the palladium layer was about 20 pm. 2.1.2. Porous alumina membrane The porous alumina membrane was prepared by the sol-gel process according to Yoldas [ 61. Boehmite ( y-AlOOH) sols were obtained by hydrolysis of aluminum isopropoxide. A porous ceramic tube was then dipped into the sol and after a certain contact time, the tube was dried at room temperature and finally heated and calcined at 823 K for 24 h. Repetition of this procedure was required. 2.1.3. Catalytic inorganic membrane The catalytic inorganic membrane was prepared also by the sol-gel process. The catalytic components copper and phosphorus were added to a silica sol prepared by the hydrolysis of tetraethyl silicate, which was then deposited onto the porous alumina membrane. The catalytic membrane was reduced in a hydrogen atmosphere at 673 K for one hour before reaction. For comparison, a conventional Cu-P/SiOZ catalyst was prepared by an ion-exchange method [lo]. This catalyst was also used in the palladium/ceramic membrane and the porous alumina membrane reactors. 2.2. Membrane reactor The schematic diagram of the membrane reactor is shown in Fig. 1. For the palladium/ ceramic membrane and porous alumina membrane reactors which both have a separate catalyst bed, methanol is carried by argon through the outer path, where it encounters the catalyst bed placed in the middle of the outer tube. The hydrogen produced from the reaction permeates through the membrane into the inner tube and is carried away by a sweeping gas. For the catalytic inorganic membrane reactor, methanol is carried through the inner path. A short glass rod with a diameter a little less than the inner diameter of the membrane is fixed at the middle of the inner tube to enable the continuous contact of methanol with the catalytic components deposited on the membrane. 2.3. Characterization Scanning electron microscopy (SEM) was carried out using a Hitachi S-520 instrument. Thermal analysis was performed using a DuPont 1090 apparatus. Pore size distributions were measured using a Micromeritics Digisorb-2600 adsorption apparatus employing nitrogen adsorption at 77 K.
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CaraIysis A IO9 (1994) 63-76
Inlet (Inner path)
Inlet (Outer
Quartz
Glass
tube
stick
membrane
:. -I :
path)
Catalyst
-
Outlet (Outer
path)
Outlet (Inner
path)
Fig. 1. Schematic diagram of the membrane reactor.
A gas permeation test was carried out in a membrane reactor without a catalyst bed. The pure sample gas was introduced into the inner tube and the permeated gas was swept out by the sweeping gas (argon or nitrogen) in the outer tube. There was adifference in pressure (0.2 atm) over the inner and outer tube and the sweeping flow-rate was 400 ml/mm. The total gas volume was determined by a wet gas meter. The compositions of the mixing gas was determined by a gas chromatograph. Thus the amount of permeated gas could be obtained.
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2.4. Products analysis
The products were analyzed by an on-line gas chromatograph with a thermal conductivity detector. Two different columns were employed. A Porapak N column was used to separate HCHO, CH,OH, CH,OCH, and HCOOCH,. A TDX-01 column (provided by Tian-Jing No. 2 Chemical Reagent Company) was used to separate CO, C02, H2 and CH,. The alternation of the two Scolumns was performed by two valves connected into the system. A dry ice-acetone cold well was employed in front of the TDX-01 column to remove CH,OH, CH,OCH,, HCOOCH3 and HCHO. The carrier gas was argon. The temperature of the columns in use was 120°C. The quantitative method was as follows: In the TDX-01 column, H,, CH4, CO and CO:! were separated perfectly and the compositions were calculated by the normalization method (all other species were condensed and removed by the cold well before entering the column). With the compositions of these four species, a combined responsive factor was obtained for the first peak of the Porapak N column which is a mixture of the four species (the Porapak N column could not separate the above four species). Thus the compositions of all the species could be calculated. For the palladium/ceramic membrane reactor, the products were collected only at the outer tube, for only hydrogen could permeate through the membrane. For the porous membrane reactor, all the products could permeate through the membrane with different permeabilities. Thus products from both the outer and inner tube were analyzed and then collected by two cold wells of liquid nitrogenethanol separately. The weight of the condensed products were measured. The conversion of methanol is calculated as follows: C%=
WiCi%+rv,
C,%
Wi+W~
Ci, C, are the methanol conversions in the inner and outer tube respectively, Wi, W, are the weights of the condensed products in the inner and outer tube, respectively. The experimental re,sults show that the main by-products were CO, CH30CH3 and CH4. Only a small amount of CO, and HCOOCH, was found.
3. Results and discussion
3.1. Membranes
3.1.1. Palladium/ceramic membrane The palladium/ceramic membrane was characterized by a gas permeation test. The results were in good agreement with those of Kikuchi and coworkers [4]. No nitrogen was detected in the permeation side when a mixture of hydrogen and nitrogen (mole ratio 1: 1) passed through the membrane. The activation energy for the hydrogen flow through the palladium membrane was found to be 9.2 kJ/mol.
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Catalysis A 109 (1994) 63-76
3.1.2. Porous alumina membrane In order to obtain a crack-free and pinhole-free porous membrane, different conditions in the preparation stage were studied, as the properties of the membrane were very sensitive to these conditions.. The SEM photograph (Fig. 2) shows that a satisfactory membrane was obtained when the concentration of the sol was 0.5 mol Al/mol, the dipping time was 120 s, the gelation time was 48 h and the heating rate was 50 K/h. We also investigate the effect of the different amounts of acid added to peptize the boehmite sol on the membrane pore size and pore volume (Table 1) . The pore size distributions are also given in Fig. 3. The results show that the pore volume and the pore size decrease with increasing acid concentration, which is in accordance with the results of Yoldas [ 61. Sol C has a pore size near 4 nm with a sharp pore size distribution. In the present work, sol C was chosen for preparation of the porous alumina membrane. DSC analysis exhibited three endothermal peaks in the transformation from boehmite gel
Fig. 2. Electron micrograph right side: porous ceramic.
of the cross-section
of a porous alumina membrane.
Left side: alumina membrane;
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Catalysis A 109 (1994) 63-76
Table 1 Effect of different amounts of acid added to the pore volume and pore size of the membrane Al Sol
A B C
HCl/boehmite (mol/mol)
sol
0.05 0.07 0.10
Viscosity (mPa-s)
BET
VPme (cm’/g)
Diameter
(m*/g)
1.14 1.19 1.29
265.4 250.7 274.8
0.387 0.343 0.309
4.8 4.6 3.7
(nm)
A Sol B
Fig. 3. Pore size distribution
curves for membranes
Temperature(
obtained from sol A, B, C.
‘C)
Fig. 4. DSC profile of boehmite gel
to the alumina membrane (Fig. 4)) which is in accordance with Klein et al. [ 121. The first peak (373 K) is assigned to water evaporation. The second peak (573 K) is related to the transformation of boehmite sol to the alumina membrane. The third peak (723 K) can be assigned to the transformation of different alumina phases.
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Thermogravimetric analysis showed there was a continuous loss of weight from room temperature to 823 K (Fig. 5), which was chosen to be the calcinating temperature. A permeation test was carried out to examine the porous membrane (Fig. 6). The experimental results are in good agreement with the theoretical value (Knudsen diffusion). 3.2. Performances
ofthe membrane reactors
The dehydrogenation of methanol was carried out in the palladium/ceramic membrane reactor, the porous alumina membrane reactor and the catalytic membrane reactor, respectively, Fig. 7 shows the conversion of methanol and the selectivity of formaldehyde in the palladium/ceramic membrane reactor as a function of reaction temperature. At a given W(g) is the weight of the catalyst, F (mol/ space time [ W/F=:2430 gcat*min*mol-‘, min) is the feed rate] the conversion of methanol in the conventional reactor is very close to the thermodynamic equilibrium value. The methanol conversion in the membrane reactor, however, significantly exceeds the equilibrium value. The selectivity of formaldehyde is only slightly lower than that in the conventional reactor. The feed rate has an important effect on the efficiency of the membrane reactor as is illustrated in Fig. 8 for the case of the palladium/ceramic membrane reactor. At high values of W/F (small feed rates), the reaction process is mainly controlled by the thermodynamics, and the improveme,nt of methanol conversion is very obvious in the membrane reactor. As W/F decreases, the thermodynamic factor becomes less dominant, and the dynamic factor becomes more important, thus the improvement in the membrane reactor becomes less obvious.
loo
90
80
70
60
Temperature(
OC)
Fig. 5. TGA profile of boehmite gel.
J. Deng, J. Wu /Applied
600
Fig. 6. Gas permeation
6.50
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Catalysis A 109 (1994) 63-76
700
750
Tempsature
(K)
test of a porous alumina membrane.
(H) HZ; (0)
BOO
Ar; (A) C&OH; (V) HCHO.
100
90
80
8
7o
.$ 60 il P g 50 v "0 40 ii s E3 30
20
10 6””
650
700
Temperature
750
800
(IQ
Fig. 7. Methanol conversion and formaldehyde selectivity vs. reaction temperature in a palladium/ceramic membrane reactor. W/F=2430 gcat*min.mol-‘; sweeping flow rate=380 ml/min. (- - -) Conversion of thermodynamic equilibrium; (0, m) conversion and selectivity in a conventional reactor; (A, V) conversion and selectivity in a palladium/ceramic membrane reactor.
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Catalysis A 109 (1994) 63-76
10
0
2000
1000 W/P
3000
4000
(gcat.min/mol)
Fig. 8. Methanol conversion and formaldehyde yield vs. W/F in a palladium/ceramic membrane reactor. Reaction temperature 673 K; sweeping flow rate= 380 mUmin. (0, 0) Conversion and yield in a conventional reactor; (A, A) conversion and yield in a palladium/ceramic reactor.
The effect of the membrane reactor depends on the permeability of hydrogen, which is related to the flow rate of the sweeping gas. Fig. 9 illustrates the relationship between methanol conversion and the flow rate of sweeping argon in the palladium/ceramic membrane reactor. The conversion increases sharply with the argon flow rate at first and then, at a high flow rate the curve gradually flattens where the main factor determining the reaction process is the reaction rate. For the porous alumina membrane reactor and the catalytic membrane reactor, the feed rate and the sweeping flow rate have similar effects on methanol conversion as are shown in Figs. 10 and 11. Finally, we compared the yield of formaldehyde in the palladium/ceramic membrane reactor, the porous alumina membrane reactor, the catalytic inorganic membrane reactor and the conventional reactor as shown in Fig. 12. The conversions of methanol in all three membrane reactors are higher than that in the conventional reactor. The palladium/ceramic membrane reactor has the most significant increase of conversion due to its extraordinarily high selectivity for hydrogen permeation. For the porous alumina membrane reactor and the inorganic catalytic membrane reactor, methanol conversion has the higher value in the latter. This can be explained by the different steps of the reaction in the two membrane reactors. In the porous alumina membrane reactor with a separate catalyst bed, the reaction includes the following steps: (1) the reactant enters the grains of the catalyst through diffusion; (2) reaction in the catalyst grains; (3) the product species exit out of the grains through diffusion and mix with species flow outside the grains (composed of reactant species and product species); (4) one of the product species (Hz) is separated from the mixture by the membrane. For catalytic reactions, the conversion in the pores of the catalyst
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Catalysis A 109 (1994) 63-76
1800 Flow rate of Ar/ cm’min~’ Fig. 9. Methanol conversion vs. flow rate of sweeping argon in a palladium/ceramic membrane reactor. Reaction (- - - - -) Thermodynamic equilibrium conversion. temperature 673 K, W/F=2430 gcat.min*mol-‘.
60
20
10 500
loo0
1500 W/F @at
2000
2500
3000
mirdmol)
Fig. 10. Methanol conversion as a function of W/F and a sweeping flow rate in a porous alumina membrane reactor. Reaction temperature 673 K. (0) Conventional reactor; (v) sweeping flow rate 120 ml/mm; (m) sweeping flow rate 60 mllmin; (A) sweeping flow rate 30 ml/mitt.
.I. Deng, J. Wu /Applied
Catalysis A 109 (1994) 63-76
60
45 c ;;40 x s 335 @ 030 14 % ~25 .B I *20 B u 15 10
L
500
1000
2000
1500
2500
3000
W/F (gcat.min/mol) Fig. 11. Methanol conversion as a function of W/F and a sweeping flow rate in a catalytic membrane reactor. Reaction temperature 673 K. (0) Conventional reactor; (V) sweeping flow rate 120 ml/mm; (H) sweeping flow rate 60 ml/mm; (A) sweeping flow rate 30 ml/min.
60 55 50 g 45 Q) x 40 f 4 35 ,E g 30 % v 3 j?
25 20 15 10 600
650
700
Temperature
750
800
(K)
Fig. 12. Formaldehyde yields in various reactors. W/F= 1600 gcat-min.mol-‘. (V) Conventional reactor; (0) palladium/ceramic reactor; (A) porous alumina membrane reactor; ( n ) catalytic membrane reactor.
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grains is usually much higher than that at the external surface due to the active surface area of the grains. In the above steps, separation takes place after the species coming out of the grains have mixed with the species flow outside the grains, thus the efficiency of the separation is lowered. In the catalytic membrane reactor, the reaction includes the following steps: ( 1) the reactant moves into the catalytic membrane; (2) reaction in the catalytic membrane; (3) the product species permeate to the other side of the membrane. Thus, the high conversion species in the catalytic membrane undertake separation before mixing with the low conversion species flow. In this way, the separation efficiency is enlarged and so is the conversion of the reactant. 3.2. Comparison of the three membrane reactors As it has been illustrated, the palladium/ceramic membrane reactor has the greatest effect in increasing methanol conversion among the three membrane reactors studied. The product of the reaction can be collected entirely at one outlet of the membrane reactor, while for the other two porous membrane reactors, the product should be collected at both outlets of the reactor. However, the durability of the palladium/ceramic membrane is less than those of the other two porous membranes. The palladium/ceramic membrane cannot be used for reactions at temperatures below 573 K, when the (Y,p transformation of PdH will take place and results in cracks on the membrane [4]. This causes trouble and inconvenience for operation. Of course, the price of the palladium/ceramic membrane is higher than those of the other two. The alumina membrane reactor and the catalytic membrane reactor, although producing only minor improvements in methanol conversion, are much more durable and cheaper than the palladium/ceramic membrane, and therefore have greater industrial potential. Of the two membrane reactors, the catalytic membrane reactor has a higher methanol conversion with a simpler operating process. However, the absolute quantity of the product is small on account of the limited amount of catalytic material that is deposited on the membrane. Another problem of the catalytic membrane reactor is that once the catalyst is deactivated, the whole catalytic membrane becomes useless and should be replaced. Future work will address the:se problems.
4. Conclusions ( 1) Two types of inorganic membrane: the palladium/ceramic membrane and the porous alumina membrane were prepared by electroless plating and the sol-gel process, respectively. Experimental results show that they are both effective for use in catalytic dehydrogenation reactions. (2) Copper and phosphorus catalytic components were deposited onto the porous alumina membrane by the sol-gel process, thereby producing an active and selective catalytic membrane. (3) The dehydrogenation of methanol was studied in membrane reactors incorporating these membranes as well as in a conventional fixed bed reactor for comparison. The palladium/ceramic membrane reactor produced the highest increase of methanol conversion, while the catalytic membrane reactor ranked the second, with the porous alumina membrane
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Catalysis A IO9 (1994) 63-76
reactor achieving least improvement. At moderate reaction conditions (reaction temperature 673 K, W/F around 1000 gcat. min * mol- ‘) , the yield of formaldehyde increased by 15% in the palladium/ceramic membrane reactor, 8% in the catalytic porous membrane reactor and 5% in the porous alumina membrane reactor above that achieved in a conventional single pass fixed bed reactor employing the same catalyst. (4) The properties and characteristics of the three membranes were compared. It is concluded that inorganic porous membranes (catalytic or non-catalytic) have a greater potential for industrial use than the palladium/ceramic membrane.
References
8 9 10 11 12
H.P. Hsieh, Catal. Rev.-Sci. Eng., 33 (l/2) (1991) 1. V.M. Gryaznov, Platinum Metals Rev., 30 (2) ( 1986) 68. 0. Shinji, M. Misono and Y. Yoneda, Bull. Chem. Sot. Jpn., 55 (1982) 2760. S. Uemiya, Y. Kude, K. Sugino, N. Sato, T. Matsuda and E. Kikuchi, Chem. L&t., (1988) 1687. E. Kikuchi, S. Uemiya, N. Satao and H. moue, Chem. Lett., (1989) 489. B.E. Yoldas, J. Mater. Sci., 10 (1975) 1856. S. Uemiya, N. Sato, H. Ando, T. Matsuda and E. Kikuchi, Appl. Catal. A, 67 (1991) 223. A.M. Champagnie, T.T. Tsotsis, R.G. Minet and E. Wagner, J. Catal., 134 (1992) 713. J.Y. Song and S-T. Hwang, J. Membr. Sci., 57 (1991) 95. U.T. Zaspalis, W. van Praag, K. Keizer, J.G. van Ommen, J.R.H. Ross and A.J. Burggraaf, App. Catal., 74 (1991) 205. T. Yamamoto, A. Sbinwda, T. Oknhara and M. Misono, Chem. Lett., ( 1988) 273. L.C. Klein, C. Yu, R. Woodman and R. Pavlik, Catal. Today, 14 (1992) 165.
Applied Catalysis A: General, 109 ( 1994) 63-76
Formaldehyde production by catalytic dehydrogenation of methanol in inorganic membrane reactors Jingfa Deng”, Jingtao Wu Department (Received
of Chemistry,Fudan 15 December 1992,
University, Shanghai, People’s Republic of China
revised manuscript received 22 October 1993)
Abstract Palladium/ceramic membranes were prepared by electroless plating and porous alumina membranes were prepared by the sol-gel process, respectively. Characterization of the membranes showed
that they had a high permeability and selectivity to hydrogen. The catalytic components copper and phosphorus were deposited onto the porous alumina membrane by the sol-gel process and thus a catalytic inorganic membrane was prepared. The catalytic dehydrogenation of methanol to formaldehyde was carried out in membrane reactors incorporating the palladium/ceramic membrane, the
porous alumina membrane and the catalytic porous membrane, respectively. The yields of formaldehyde increased significantly in these membrane reactors compared with that in the conventional reactor. At moderate reaction conditions, the yield increased by 15% with the palladium/ceramic membrane, 8% with the c:atalytic porous membrane and 5% with the porous alumina membrane. The properties and characteristics of the three membranes were compared and discussed. Key words: ahrmina membrane; composite membrane; dehydrogenation catalyst; membrane reactor; methanol; palladium membrane
1. Introduction
Membrane reactors have generated great interest in recent years because they have many favorable characteristics compared to conventional reactors [ 11, one of which is that they can shift the thermodynamic equilibrium of a reversible reaction system toward the product side. For membrane reactors used in the catalytic reactions, inorganic membranes are usually the best choice due to their high thermal stability [ 2,3]. Kikuchi et al. developed a new *Corresponding author. Fax. ( + 86-2
1)5490535
0926-860X/94/$07.00 0 1’994Elsevier Science B.V. All rights reserved SSDl0926-860X(93)EO218-2
64
J. Deng, J. Wu /Applied
Catalysis A 109 (1994) 63-76
type of inorganic membrane [ 4,5], the palladium/ceramic membrane, which had a much higher hydrogen permeability than that of an ordinary palladium membrane, yet the high selectivity to hydrogen remained unchanged. Yoldas successfully prepared crack-free and pinhole-free porous alumina membranes by the sol-gel process [ 61. Both of these membranes show promise for use in catalytic reactions. Until now, most of the research work on membrane catalysis involving the combinations of reactions and separation has concentrated on dehydrogenation reactions [ 7,8]. In the present work, the dehydrogenation of methanol to formaldehyde was chosen for investigation. Song and Hwang have studied the oxidative dehydrogenation of methanol to formaldehyde in a porous vycor glass membrane reactor [ 9 1. The product of this procedure is an aqueous formaldehyde solution which has a concentration of not more than 40%. It is difficult to get formaldehyde in the high concentration which is sometimes required. Direct catalytic dehydrogenation of methanol results in simpler products with a high concentration of formaldehyde [ I I]. However, the conversion of methanol is low due to thermodynamic equilibrium limitations. Zaspalis et al. reported the dehydrogenation of methanol over yA&O3 membrane reactors. The conversion of methanol was significantly improved, but the selectivity of formaldehyde was low [ lo]. In the present work, the reaction was carried out in membrane reactors using a Cu-P/Si02 catalyst. The conversion of methanol was found to be increased greatly with a very high selectivity of formaldehyde and thus the reaction may have potential for industrial manufacture. While the combination of reaction and separation is one of the breakthroughs of membrane reactors, the combination of the membrane and catalyst is also significant. Not only can it simplify the process operation, but it can also further enhance the conversion of the reactants. The most direct way to deposit copper and phosphorus onto a membrane is by impregnation. However, an impregnated Cu-P/A&O, membrane catalyst showed lower selectivity of formaldehyde than that of the Cu-P/SiO, catalyst, probably due to the different interaction between copper-alumina and copper-silica. In order to improve formaldehyde selectivity, a new type of Cu-P/Si02 membrane catalyst was prepared by the sol-gel process. Through SiOz can form a porous membrane itself, the stability of this membrane is much lower than that of a A&O3 membrane. Therefore, the Cu-P/Si02 catalyst was deposited onto the porous alumina membrane. This membrane catalyst prepared by the sol-gel process showed satisfactory activity in the dehydrogenation of methanol to formaldehyde. The solgel process proved to be a successful method for preparing a membrane catalyst. In the present paper, we report the preparation of three different kinds of inorganic membranes: a palladium/ceramic membrane, a porous alumina membrane and a catalytic inorganic membrane. The effects of these membrane reactors in promoting the dehydrogenation of methanol to formaldehyde are investigated. The properties of these membrane reactors are compared.
J. Deng, J. Wu /Applied
Catalysis A 109 (1994) 63-76
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2. Experimental 2.1. Membrane preparation 2.1.1. Palladium/ceramic membrane The palladium/ceramic membrane was prepared according to Kikuchi and coworkers [4] by electroless plating on the outer surface of a porous ceramic tube (O.D. 12 mm, I.D. 10 mm, average porous size 0.5 pm). The thickness of the palladium layer was about 20 pm. 2.1.2. Porous alumina membrane The porous alumina membrane was prepared by the sol-gel process according to Yoldas [ 61. Boehmite ( y-AlOOH) sols were obtained by hydrolysis of aluminum isopropoxide. A porous ceramic tube was then dipped into the sol and after a certain contact time, the tube was dried at room temperature and finally heated and calcined at 823 K for 24 h. Repetition of this procedure was required. 2.1.3. Catalytic inorganic membrane The catalytic inorganic membrane was prepared also by the sol-gel process. The catalytic components copper and phosphorus were added to a silica sol prepared by the hydrolysis of tetraethyl silicate, which was then deposited onto the porous alumina membrane. The catalytic membrane was reduced in a hydrogen atmosphere at 673 K for one hour before reaction. For comparison, a conventional Cu-P/SiOZ catalyst was prepared by an ion-exchange method [lo]. This catalyst was also used in the palladium/ceramic membrane and the porous alumina membrane reactors. 2.2. Membrane reactor The schematic diagram of the membrane reactor is shown in Fig. 1. For the palladium/ ceramic membrane and porous alumina membrane reactors which both have a separate catalyst bed, methanol is carried by argon through the outer path, where it encounters the catalyst bed placed in the middle of the outer tube. The hydrogen produced from the reaction permeates through the membrane into the inner tube and is carried away by a sweeping gas. For the catalytic inorganic membrane reactor, methanol is carried through the inner path. A short glass rod with a diameter a little less than the inner diameter of the membrane is fixed at the middle of the inner tube to enable the continuous contact of methanol with the catalytic components deposited on the membrane. 2.3. Characterization Scanning electron microscopy (SEM) was carried out using a Hitachi S-520 instrument. Thermal analysis was performed using a DuPont 1090 apparatus. Pore size distributions were measured using a Micromeritics Digisorb-2600 adsorption apparatus employing nitrogen adsorption at 77 K.
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CaraIysis A IO9 (1994) 63-76
Inlet (Inner path)
Inlet (Outer
Quartz
Glass
tube
stick
membrane
:. -I :
path)
Catalyst
-
Outlet (Outer
path)
Outlet (Inner
path)
Fig. 1. Schematic diagram of the membrane reactor.
A gas permeation test was carried out in a membrane reactor without a catalyst bed. The pure sample gas was introduced into the inner tube and the permeated gas was swept out by the sweeping gas (argon or nitrogen) in the outer tube. There was adifference in pressure (0.2 atm) over the inner and outer tube and the sweeping flow-rate was 400 ml/mm. The total gas volume was determined by a wet gas meter. The compositions of the mixing gas was determined by a gas chromatograph. Thus the amount of permeated gas could be obtained.
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61
2.4. Products analysis
The products were analyzed by an on-line gas chromatograph with a thermal conductivity detector. Two different columns were employed. A Porapak N column was used to separate HCHO, CH,OH, CH,OCH, and HCOOCH,. A TDX-01 column (provided by Tian-Jing No. 2 Chemical Reagent Company) was used to separate CO, C02, H2 and CH,. The alternation of the two Scolumns was performed by two valves connected into the system. A dry ice-acetone cold well was employed in front of the TDX-01 column to remove CH,OH, CH,OCH,, HCOOCH3 and HCHO. The carrier gas was argon. The temperature of the columns in use was 120°C. The quantitative method was as follows: In the TDX-01 column, H,, CH4, CO and CO:! were separated perfectly and the compositions were calculated by the normalization method (all other species were condensed and removed by the cold well before entering the column). With the compositions of these four species, a combined responsive factor was obtained for the first peak of the Porapak N column which is a mixture of the four species (the Porapak N column could not separate the above four species). Thus the compositions of all the species could be calculated. For the palladium/ceramic membrane reactor, the products were collected only at the outer tube, for only hydrogen could permeate through the membrane. For the porous membrane reactor, all the products could permeate through the membrane with different permeabilities. Thus products from both the outer and inner tube were analyzed and then collected by two cold wells of liquid nitrogenethanol separately. The weight of the condensed products were measured. The conversion of methanol is calculated as follows: C%=
WiCi%+rv,
C,%
Wi+W~
Ci, C, are the methanol conversions in the inner and outer tube respectively, Wi, W, are the weights of the condensed products in the inner and outer tube, respectively. The experimental re,sults show that the main by-products were CO, CH30CH3 and CH4. Only a small amount of CO, and HCOOCH, was found.
3. Results and discussion
3.1. Membranes
3.1.1. Palladium/ceramic membrane The palladium/ceramic membrane was characterized by a gas permeation test. The results were in good agreement with those of Kikuchi and coworkers [4]. No nitrogen was detected in the permeation side when a mixture of hydrogen and nitrogen (mole ratio 1: 1) passed through the membrane. The activation energy for the hydrogen flow through the palladium membrane was found to be 9.2 kJ/mol.
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Catalysis A 109 (1994) 63-76
3.1.2. Porous alumina membrane In order to obtain a crack-free and pinhole-free porous membrane, different conditions in the preparation stage were studied, as the properties of the membrane were very sensitive to these conditions.. The SEM photograph (Fig. 2) shows that a satisfactory membrane was obtained when the concentration of the sol was 0.5 mol Al/mol, the dipping time was 120 s, the gelation time was 48 h and the heating rate was 50 K/h. We also investigate the effect of the different amounts of acid added to peptize the boehmite sol on the membrane pore size and pore volume (Table 1) . The pore size distributions are also given in Fig. 3. The results show that the pore volume and the pore size decrease with increasing acid concentration, which is in accordance with the results of Yoldas [ 61. Sol C has a pore size near 4 nm with a sharp pore size distribution. In the present work, sol C was chosen for preparation of the porous alumina membrane. DSC analysis exhibited three endothermal peaks in the transformation from boehmite gel
Fig. 2. Electron micrograph right side: porous ceramic.
of the cross-section
of a porous alumina membrane.
Left side: alumina membrane;
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Table 1 Effect of different amounts of acid added to the pore volume and pore size of the membrane Al Sol
A B C
HCl/boehmite (mol/mol)
sol
0.05 0.07 0.10
Viscosity (mPa-s)
BET
VPme (cm’/g)
Diameter
(m*/g)
1.14 1.19 1.29
265.4 250.7 274.8
0.387 0.343 0.309
4.8 4.6 3.7
(nm)
A Sol B
Fig. 3. Pore size distribution
curves for membranes
Temperature(
obtained from sol A, B, C.
‘C)
Fig. 4. DSC profile of boehmite gel
to the alumina membrane (Fig. 4)) which is in accordance with Klein et al. [ 121. The first peak (373 K) is assigned to water evaporation. The second peak (573 K) is related to the transformation of boehmite sol to the alumina membrane. The third peak (723 K) can be assigned to the transformation of different alumina phases.
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Thermogravimetric analysis showed there was a continuous loss of weight from room temperature to 823 K (Fig. 5), which was chosen to be the calcinating temperature. A permeation test was carried out to examine the porous membrane (Fig. 6). The experimental results are in good agreement with the theoretical value (Knudsen diffusion). 3.2. Performances
ofthe membrane reactors
The dehydrogenation of methanol was carried out in the palladium/ceramic membrane reactor, the porous alumina membrane reactor and the catalytic membrane reactor, respectively, Fig. 7 shows the conversion of methanol and the selectivity of formaldehyde in the palladium/ceramic membrane reactor as a function of reaction temperature. At a given W(g) is the weight of the catalyst, F (mol/ space time [ W/F=:2430 gcat*min*mol-‘, min) is the feed rate] the conversion of methanol in the conventional reactor is very close to the thermodynamic equilibrium value. The methanol conversion in the membrane reactor, however, significantly exceeds the equilibrium value. The selectivity of formaldehyde is only slightly lower than that in the conventional reactor. The feed rate has an important effect on the efficiency of the membrane reactor as is illustrated in Fig. 8 for the case of the palladium/ceramic membrane reactor. At high values of W/F (small feed rates), the reaction process is mainly controlled by the thermodynamics, and the improveme,nt of methanol conversion is very obvious in the membrane reactor. As W/F decreases, the thermodynamic factor becomes less dominant, and the dynamic factor becomes more important, thus the improvement in the membrane reactor becomes less obvious.
loo
90
80
70
60
Temperature(
OC)
Fig. 5. TGA profile of boehmite gel.
J. Deng, J. Wu /Applied
600
Fig. 6. Gas permeation
6.50
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Catalysis A 109 (1994) 63-76
700
750
Tempsature
(K)
test of a porous alumina membrane.
(H) HZ; (0)
BOO
Ar; (A) C&OH; (V) HCHO.
100
90
80
8
7o
.$ 60 il P g 50 v "0 40 ii s E3 30
20
10 6””
650
700
Temperature
750
800
(IQ
Fig. 7. Methanol conversion and formaldehyde selectivity vs. reaction temperature in a palladium/ceramic membrane reactor. W/F=2430 gcat*min.mol-‘; sweeping flow rate=380 ml/min. (- - -) Conversion of thermodynamic equilibrium; (0, m) conversion and selectivity in a conventional reactor; (A, V) conversion and selectivity in a palladium/ceramic membrane reactor.
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Catalysis A 109 (1994) 63-76
10
0
2000
1000 W/P
3000
4000
(gcat.min/mol)
Fig. 8. Methanol conversion and formaldehyde yield vs. W/F in a palladium/ceramic membrane reactor. Reaction temperature 673 K; sweeping flow rate= 380 mUmin. (0, 0) Conversion and yield in a conventional reactor; (A, A) conversion and yield in a palladium/ceramic reactor.
The effect of the membrane reactor depends on the permeability of hydrogen, which is related to the flow rate of the sweeping gas. Fig. 9 illustrates the relationship between methanol conversion and the flow rate of sweeping argon in the palladium/ceramic membrane reactor. The conversion increases sharply with the argon flow rate at first and then, at a high flow rate the curve gradually flattens where the main factor determining the reaction process is the reaction rate. For the porous alumina membrane reactor and the catalytic membrane reactor, the feed rate and the sweeping flow rate have similar effects on methanol conversion as are shown in Figs. 10 and 11. Finally, we compared the yield of formaldehyde in the palladium/ceramic membrane reactor, the porous alumina membrane reactor, the catalytic inorganic membrane reactor and the conventional reactor as shown in Fig. 12. The conversions of methanol in all three membrane reactors are higher than that in the conventional reactor. The palladium/ceramic membrane reactor has the most significant increase of conversion due to its extraordinarily high selectivity for hydrogen permeation. For the porous alumina membrane reactor and the inorganic catalytic membrane reactor, methanol conversion has the higher value in the latter. This can be explained by the different steps of the reaction in the two membrane reactors. In the porous alumina membrane reactor with a separate catalyst bed, the reaction includes the following steps: (1) the reactant enters the grains of the catalyst through diffusion; (2) reaction in the catalyst grains; (3) the product species exit out of the grains through diffusion and mix with species flow outside the grains (composed of reactant species and product species); (4) one of the product species (Hz) is separated from the mixture by the membrane. For catalytic reactions, the conversion in the pores of the catalyst
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Catalysis A 109 (1994) 63-76
1800 Flow rate of Ar/ cm’min~’ Fig. 9. Methanol conversion vs. flow rate of sweeping argon in a palladium/ceramic membrane reactor. Reaction (- - - - -) Thermodynamic equilibrium conversion. temperature 673 K, W/F=2430 gcat.min*mol-‘.
60
20
10 500
loo0
1500 W/F @at
2000
2500
3000
mirdmol)
Fig. 10. Methanol conversion as a function of W/F and a sweeping flow rate in a porous alumina membrane reactor. Reaction temperature 673 K. (0) Conventional reactor; (v) sweeping flow rate 120 ml/mm; (m) sweeping flow rate 60 mllmin; (A) sweeping flow rate 30 ml/mitt.
.I. Deng, J. Wu /Applied
Catalysis A 109 (1994) 63-76
60
45 c ;;40 x s 335 @ 030 14 % ~25 .B I *20 B u 15 10
L
500
1000
2000
1500
2500
3000
W/F (gcat.min/mol) Fig. 11. Methanol conversion as a function of W/F and a sweeping flow rate in a catalytic membrane reactor. Reaction temperature 673 K. (0) Conventional reactor; (V) sweeping flow rate 120 ml/mm; (H) sweeping flow rate 60 ml/mm; (A) sweeping flow rate 30 ml/min.
60 55 50 g 45 Q) x 40 f 4 35 ,E g 30 % v 3 j?
25 20 15 10 600
650
700
Temperature
750
800
(K)
Fig. 12. Formaldehyde yields in various reactors. W/F= 1600 gcat-min.mol-‘. (V) Conventional reactor; (0) palladium/ceramic reactor; (A) porous alumina membrane reactor; ( n ) catalytic membrane reactor.
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Catalysis A 109 (I 994) 63-76
75
grains is usually much higher than that at the external surface due to the active surface area of the grains. In the above steps, separation takes place after the species coming out of the grains have mixed with the species flow outside the grains, thus the efficiency of the separation is lowered. In the catalytic membrane reactor, the reaction includes the following steps: ( 1) the reactant moves into the catalytic membrane; (2) reaction in the catalytic membrane; (3) the product species permeate to the other side of the membrane. Thus, the high conversion species in the catalytic membrane undertake separation before mixing with the low conversion species flow. In this way, the separation efficiency is enlarged and so is the conversion of the reactant. 3.2. Comparison of the three membrane reactors As it has been illustrated, the palladium/ceramic membrane reactor has the greatest effect in increasing methanol conversion among the three membrane reactors studied. The product of the reaction can be collected entirely at one outlet of the membrane reactor, while for the other two porous membrane reactors, the product should be collected at both outlets of the reactor. However, the durability of the palladium/ceramic membrane is less than those of the other two porous membranes. The palladium/ceramic membrane cannot be used for reactions at temperatures below 573 K, when the (Y,p transformation of PdH will take place and results in cracks on the membrane [4]. This causes trouble and inconvenience for operation. Of course, the price of the palladium/ceramic membrane is higher than those of the other two. The alumina membrane reactor and the catalytic membrane reactor, although producing only minor improvements in methanol conversion, are much more durable and cheaper than the palladium/ceramic membrane, and therefore have greater industrial potential. Of the two membrane reactors, the catalytic membrane reactor has a higher methanol conversion with a simpler operating process. However, the absolute quantity of the product is small on account of the limited amount of catalytic material that is deposited on the membrane. Another problem of the catalytic membrane reactor is that once the catalyst is deactivated, the whole catalytic membrane becomes useless and should be replaced. Future work will address the:se problems.
4. Conclusions ( 1) Two types of inorganic membrane: the palladium/ceramic membrane and the porous alumina membrane were prepared by electroless plating and the sol-gel process, respectively. Experimental results show that they are both effective for use in catalytic dehydrogenation reactions. (2) Copper and phosphorus catalytic components were deposited onto the porous alumina membrane by the sol-gel process, thereby producing an active and selective catalytic membrane. (3) The dehydrogenation of methanol was studied in membrane reactors incorporating these membranes as well as in a conventional fixed bed reactor for comparison. The palladium/ceramic membrane reactor produced the highest increase of methanol conversion, while the catalytic membrane reactor ranked the second, with the porous alumina membrane
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Catalysis A IO9 (1994) 63-76
reactor achieving least improvement. At moderate reaction conditions (reaction temperature 673 K, W/F around 1000 gcat. min * mol- ‘) , the yield of formaldehyde increased by 15% in the palladium/ceramic membrane reactor, 8% in the catalytic porous membrane reactor and 5% in the porous alumina membrane reactor above that achieved in a conventional single pass fixed bed reactor employing the same catalyst. (4) The properties and characteristics of the three membranes were compared. It is concluded that inorganic porous membranes (catalytic or non-catalytic) have a greater potential for industrial use than the palladium/ceramic membrane.
References
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H.P. Hsieh, Catal. Rev.-Sci. Eng., 33 (l/2) (1991) 1. V.M. Gryaznov, Platinum Metals Rev., 30 (2) ( 1986) 68. 0. Shinji, M. Misono and Y. Yoneda, Bull. Chem. Sot. Jpn., 55 (1982) 2760. S. Uemiya, Y. Kude, K. Sugino, N. Sato, T. Matsuda and E. Kikuchi, Chem. L&t., (1988) 1687. E. Kikuchi, S. Uemiya, N. Satao and H. moue, Chem. Lett., (1989) 489. B.E. Yoldas, J. Mater. Sci., 10 (1975) 1856. S. Uemiya, N. Sato, H. Ando, T. Matsuda and E. Kikuchi, Appl. Catal. A, 67 (1991) 223. A.M. Champagnie, T.T. Tsotsis, R.G. Minet and E. Wagner, J. Catal., 134 (1992) 713. J.Y. Song and S-T. Hwang, J. Membr. Sci., 57 (1991) 95. U.T. Zaspalis, W. van Praag, K. Keizer, J.G. van Ommen, J.R.H. Ross and A.J. Burggraaf, App. Catal., 74 (1991) 205. T. Yamamoto, A. Sbinwda, T. Oknhara and M. Misono, Chem. Lett., ( 1988) 273. L.C. Klein, C. Yu, R. Woodman and R. Pavlik, Catal. Today, 14 (1992) 165.