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Microwave effect in the dehydrogenation of tetralin and decalin with a fixed-bed reactor
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 2 4 2 e3 2 5 0
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Microwave effect in the dehydrogenation of tetralin and decalin with a fixed-bed reactor Yindee Suttisawat a, Hideki Sakai b, Masahiko Abe b, Pramoch Rangsunvigit c, Satoshi Horikoshi a,* a
Department of Materials and Life Sciences, Faculty of Science and Technology, Sophia University, 7-1 Kioicho, Chiyodaku, Tokyo 102-8554, Japan b Department of Pure and Applied Chemistry in Faculty of Science, Tokyo University of Science, Chiba 278-8510, Japan c The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok 10330, Thailand
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abstract
Article history:
This study investigates the effect of microwaves on the dehydrogenation of decalin and
Received 21 August 2011
tetralin. The reactions were studied in a fixed-bed reactor under microwave heating
Received in revised form
(MWH), and the results were compared with those under conventional heating (electrical
17 October 2011
heating) (CH). The result of this investigation showed that under the same operating
Accepted 25 October 2011
conditions, increase in the conversion of tetralin in the microwave-heated system was
Available online 9 December 2011
more than that in the conventional heated system. This demonstrated the microwave effect appeared in the dehydrogenation of tetralin. In addition, the activity and stability of
Keywords:
the catalyst also improved under MWH. The microwave effect resulted in increase in the
Microwave effect
conversion reaction of tetralin dehydrogenation by generation of a large temperature
Microwave heating
gradient from the catalyst surface to surrounding species and the subsequent mass
Mass diffusion
transfer in which both vectors have direction inverted from that of CH. Such a phenom-
Fixed-bed reaction
enon induced faster molecule desorption (product species) or enhancement of species
Decalin dehydrogenation
transport in the system. In addition, the contiguous strong adsorption of hydrocarbon can be reduced, leading to decrease in coke deposition. Conditionally, this benefit issue particularly resulted in increase in the reaction rate in which species transport or mass diffusion is the rate-limiting step. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Microwave energy has been widely used in fields that involve thermally activated processing, such as food processing and drying processing including wood, paper, ceramic, and recently, it has been frequently employed in chemical reactions [1]. The advantages of microwave heating (MWH) are multiple, for example, quick and volumetric heating can be achieved. In addition, it is possible to selectively heat the
desired region or a particular material because of difference in microwave absorption properties of different matter. The feature of selective heating leads to a reduction in energy consumption and in the time required for processing steps, such as the drying process. Microwaves can remarkably enhance organic synthesis reactions (higher yield and faster reaction compared with conventional reactions). This effect has provoked the interest of many researchers and has led them to apply the use of microwaves to other chemical
* Corresponding author. E-mail address: [email protected] (S. Horikoshi). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.10.111
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 2 4 2 e3 2 5 0
reactions, such as decomposition of toxic materials, regeneration of activated carbon saturated with volatile organic compounds, or even catalyst synthesis [2]. However, although it has been mentioned that MWH can have beneficial effects on chemical reactions over conventional methods, some publications have claimed that microwaves do not show any specific improvement in reaction efficiency when compared with a conventional system [3,4]. Microwaves have been widely discussed in terms of their thermal and nonthermal effects. The thermal effect on acceleration of a reaction has been explained in terms of very quick heating, hot spot, or superheating, while the effect is considered to be nonthermal when enhancement of a chemical reaction cannot be explained by temperature parameters, such as molecular polarization, molecule transport phenomena, and modification of thermodynamic parameters by microwave irradiation [5e7]. The nonthermal effects on enhancement of microwaveheated reactions are still not clear. It was suggested that poor sensitivity of temperature measurement and the resulting errors led to non-detection of the thermal effect [3,8,9]. Till now, the effect of microwaves and their mechanism of action are still under debate. In the present study, we investigated the dehydrogenation of tetralin and decalin under MWH in a fixed-bed reactor and demonstrated the microwave effect in reactions. The dehydrogenation/hydrogenation of organic chemical hydrides, such as decalin and methylcyclohexane, has been proposed as a potential process method for hydrogen storage. The hydrogen storage capacity of such organic chemical hydrides is 6e7 wt%, which satisfies the target value for hydrogen storage [10e12]. However, this reaction is constrained by deactivation of the catalyst due to coke deposition, leading to lower reaction efficiency. A number of supported metal catalysts have been used in this reaction such as Pt, PteRh, Pd, and Ni on activated carbon or PteSn on alumina [13,14]. In this study, we used activated carbon deposited with Pt and PteSn as catalysts in the dehydrogenation of tetralin and decalin. Pt and PteSn are good catalysts for this type of reaction, while activated carbon is an excellent material for microwave absorption [15]. The reactions were performed in the gas phase using a flow system. These experiments compared the dehydrogenation reaction conversions using MWH and conventional heating (electrical heating) (CH), two different heating techniques, while varying the parameters reaction temperature and feed flow rate (reaction space time).
2.
Experimental
2.1.
Materials and catalyst preparation
Pt or Pt and Sn deposited on activated carbon (Pt/AC and PteSn/AC, respectively) were used as catalysts in the dehydrogenation reactions of tetralin and decalin (24% cis-isomer and 76% trans-isomers). Pt/AC (Pt: 1 wt%) and PteSn/AC (Pt: 1 wt% and Sn: 0.6 wt%) were prepared by the incipient
Conversion ¼
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wetness impregnation method. An appropriate amount of each metal precursor (H2PtCl6$6H2O and SnCl4$5H2O, 98.5% purity; supplied by Wako Pure Chemical Industries, Ltd., Japan) was dissolved in deionized water and then introduced into the pores of the carbon support (charcoal activated carbon, 99% purity, granular size 0.72e1.4 mm, 0.59 cm3 g1, 1174 m2 g1). The wet sample was kept at room temperature for 1 h and then dried under an air atmosphere at 110 C for 2e3 h, followed by reduction under a flow of hydrogen at 320 C for 4 h.
2.2.
Dehydrogenation in fixed-bed reactor
The catalytic reactions were carried out under atmospheric pressure in a fixed-bed tubular reactor. The dehydrogenation reaction was performed under single-mode microwave dielectric heating (microwave magnetron capacity: 0e200 W, 2.45 GHz supplied by ARIOS company) and was compared with the reaction conducted under CH (120 W electrical heater). A quartz tube of 5 mm inside diameter was used as a reactor and loaded with 0.2 g of catalyst. A thermocouple, which does not absorb microwave irradiation, was used to measure the center temperature of the catalyst bed inside the reactor. Scheme 1 illustrates the experimental set up of the fixed-bed reaction using a single-mode microwave apparatus. In the procedure, a reactor tube was placed in the microwave cavity at the Emax position in electromagnetic field. Microwaves were produced by a generator with a 2.45 GHz magnetron, irradiating along the waveguide to the reactor tube, which was packed with the catalyst. A three-stub tuner and a plunger were used to control the microwave irradiation intensity and position of irradiation on the sample while a circulator absorbed the reflected microwaves from the cavity to minimize damage of the magnetron. Prior to the reaction, Ar was flowed over the catalyst bed for 15 min to reduce the air content inside the reactor. Microwaves with power of 60 W were then irradiated for 2e3 min to warm up the catalyst bed to approximately 100e120 C. The temperature was then increased for the reactions: 250e320 C for tetralin dehydrogenation and 280e340 C for decalin dehydrogenation. The reactant, decalin or tetralin, was fed into the single-pass continuous flow reactor packed with the granular catalyst by a syringe pump while the space time of reaction was varied from 4.13 to 10.33 gcatalyst h/moltetralin for tetralin dehydrogenation and from 9.74 to 42.86 gcatalyst h/moldecalin for decalin dehydrogenation. The temperature of the catalyst bed was controlled by tuning the microwave power in the range 100e120 W. The outlet liquid product was collected every 30e60 min and conversion of reactant (%) was analyzed by FID-GC (Shimadzu model 2014 connected with Zebron column model ZB-624).
2.3.
ðmoles of reactantðinletÞÞ ðmoles of reactantðoutletÞÞ moles of reactantðinletÞ
Definition of conversion and selectivity
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 2 4 2 e3 2 5 0
Scheme 1 e Experimental set up under microwave irradiation (side view).
Selectivity ¼
moles of product formed moles of reactant converted
Space time of reaction ðW=FÞ ¼
3.
amount of catalyst ðgÞ feed flow rate ðmole=hÞ
decalin dehydrogenation (Scheme 2), dehydrogenation of decalin into naphthalene must be performed at a reaction temperature much higher than that of tetralin into naphthalene because the reaction is highly endothermic and thermodynamically restricted at low temperatures (because of reverse and isomerization reactions) [14,16]. From individual studies of tetralin and decalin dehydrogenation, we can clearly observe the effect of microwaves on these reactions.
Experimental results and discussion 3.1.
The dehydrogenation reactions of tetralin and decalin under MWH were studied and compared with their dehydrogenation reactions under CH (electrical furnace). The results are separately reported in the following two sections for tetralin dehydrogenation using Pt-loaded granular activated carbon (Pt/AC) as the catalyst and decalin dehydrogenation using PteSn-loaded granular activated carbon (PteSn/AC) as the catalyst. Although tetralin is an intermediate in the reaction of
Scheme 2 e Dehydrogenation of decalin and tetralin.
Tetralin dehydrogenation
Fig. 1 shows the dehydrogenation of tetralin with Pt/AC catalyst as a function of reaction temperature and the comparison of MWH and CH methods. The reaction was performed at 250e320 C. The results show that to achieve equivalent tetralin conversions, the microwave-heated reaction required a reaction temperature ca. 20 C lower than the conventional reaction. In order to achieve 68% tetralin conversion, the reaction performed under MWH was conducted at a reaction temperature of 230 C, while the conventional reaction required a temperature of 250 C. The reaction under MWH reached 100% tetralin conversion at a reaction temperature of 290 C, while the reaction under CH approached 100% at a higher reaction temperature. However, no significant difference was found in the value of
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 2 4 2 e3 2 5 0
100
Tetralin conversion (%)
Tetralin conversion (%)
100
MWH
80
CH 60
40
20
MWH
80
CH 60 40 slope =
ΔX
20
0 240
260
280
300
Fig. 1 e Effect of temperature on tetralin dehydrogenation under microwave heating (MWH) and conventional heating (CH). Conditions: 1 wt% Pt/AC, space time of reaction [ 10.33 gcatalyst h/moltetralin.
naphthalene selectivity between the two systems (this data is not displayed here), where the naphthalene selectivity was greater than 98%. At a fixed temperature of 270 C, the feed flow rate was varied to investigate the effect of space time on tetralin conversion during the dehydrogenation reaction. The interesting results shown in Fig. 2 reveal that when the space time decreased from 10.33 to 4.13 gcat h/moltetralin, the conversion dramatically reduced from 86.5% to 40.3% in the case of the reaction under CH. In contrast, in the case of reaction under microwave irradiation, the conversion did not lower significantly; it slightly decreased from 97.1% to 89.2%. A remarkable decrease in conversion with increase in the space time suggests that the reaction is affected by species transport or
CH = 0.091
60
120
180
240
Time on stream (min)
320
Temperature (°C)
MW = 0.046
ΔT
0 0 220
ΔX ΔT
Fig. 3 e Test of catalyst and reaction stability of tetralin dehydrogenation under microwave heating (MWH) and conventional heating (CH). Conditions: 1 wt% Pt/AC, reaction temperature [ 270 C, space time [ 10.33 gcatalyst h/moltetralin.
a rate-limiting step of mass diffusion, particularly molecular desorption from active sites of the catalyst. It is well known that naphthalene, which is the product of the reaction, has two benzene rings, and consequently, has strong adsorption energy with the active sites of the catalyst. For this reason, species transport on active sites of the catalyst is restricted. Furthermore, the stability and activity of the catalyst were studied. The plot of tetralin conversion and time on stream for 4 h in Fig. 3 shows the catalyst and reaction stability under MWH and CH at a reaction temperature of 270 C with a space time of 10.33 gcat h/moltetralin. For the reaction under MWH, the results show that the conversion decreased from the original 98% to 89% and seemed to be stable after 3 h running time, while in the reaction under CH, the conversion continuously reduced from 86.5% to 66.8% even after an operating
MWH
80
PtOx
CH
PtOx
Intensity (a.u.)
Tetralin conversion (%)
100
60
40
c) spent Pt/AC-MW b) spent Pt/AC-CH a) fresh Pt/AC
20
0 0
3
6
9
12
15
W/F tetralin (g cat h/mol tetralin) Fig. 2 e Effect of space time on tetralin dehydrogenation under microwave heating (MWH) and conventional heating (CH). Conditions: 1 wt% Pt/AC, reaction temperature [ 270 C.
65
70
75
80
Binding energy (eV) Fig. 4 e XPS spectrum of Pt (4f electron) of Pt/AC catalyst: (a) fresh catalyst and (b) spent catalyst from conventional heated reaction and (c) spent catalyst from microwaveheated reaction.
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Scheme 3 e Model of heat transfer direction: (a) under microwave heating (MWH): Tcatalyst > Tsurrounding, heat transfers from the catalyst to the surrounding matter therefore mass also transfers in the same direction because of the coupling vector and (b) under conventional heating (CH): Tcatalyst < Tsurrounding, heat transfers from the surrounding matter to the catalyst.
time of 4 h. Rate of conversion decrease with time on stream in each reaction was calculated from the slope of the curve. The curve slope in the case of the conventional system is 0.091, while that in the microwave system is 0.046. This indicates that the rate of conversion in the conventional reaction decreases twice as fast as the rate of conversion in the microwave-heated reaction. Decrease in conversion caused by a loss of catalyst activity is a consequence of naphthalene deposition on the catalyst active sites, which then generates coke [17]. Thus, we assume that the rate of coke formation during the reaction under MWH is less than that during the reaction under CH. In the tetralin dehydrogenation reaction, it was revealed that the microwave-heated reaction showed better performance than the conventional heated reaction. The reaction under MWH resulted in a higher conversion at the same reaction temperature or space time and displayed better stability and activity of the catalyst than that of CH. The positive effect of MWH on tetralin dehydrogenation may result from the following: (i) Hot spots or localized superheating within the catalyst, during the microwave-heated reaction can be visually observed by arcing or sparking on the surface of the catalyst. As described by the Arrhenius reaction (k ¼ AeEa/RT), the reaction rate depends on two factors: a pre-exponential factor (A) and an exponential factor (eEa/RT). The presence of localized superheating (arcing and sparking) caused high temperatures at active sites of the catalyst, contributing to increase in the magnitude of the exponential part of the Arrhenius reaction and enhancement of the reaction rate [7,18e21]. However, the phenomenon of arcing or sparking on the catalyst surface was random and heterogeneous, and therefore, it was not easy to measure the exact temperature generated by sparking or arcing.
(ii) Nonthermal effects of microwave irradiation have been proposed, such as polarization of reactant molecules or modification of catalyst activity [7,22]. However, the nonthermal effects on enhancement of microwaveheated reactions are difficult to observe and discuss. In this study, the reactant is a nonpolar material; therefore, the microwave field should not have much effect on polarization. In addition, to clarify whether microwaves may affect some feature of the catalysts, we used X-ray photoelectron spectroscopy (XPS) to characterize the catalysts used in the reaction (fresh and spent catalysts) and to investigate the binding energy of metallic Pt (Pt4f level). The XPS spectra of a low energy band of Pt 4f7/2 and a high energy band of Pt 4f5/2 display discrete peaks at 69.7 eV and at 72.2 eV, respectively (Fig. 4). The peak positions indicating the binding energies of Pt in fresh and spent catalysts from microwave- and conventional heated reactions are identical. In the XPS spectra of the spent catalysts, however, the presence of small shoulder peaks can be observed at 71.4 and 74.9 eV, corresponding to the oxide of Pt (PtO or PtO2) [23]. Pt oxide would occur from an interaction of Pt with oxygen in the functional groups (carbonyl, carboxyl, hydroxyl and quinone) on the carbon surface [24,25]. (iii) The effect of microwave field on the improvement in molecular diffusion or transport of species in a reaction system has been mentioned [8,9]. Antonio and Deam have proved the hypothesis that if the transport of an active species is a rate-limiting step in a reaction, the temperature gradient under MWH enhances the diffusion of that species, and the overall reaction rate is changed under MWH compared with CH [9]. From the above results in this experiment, it is revealed that when the feed flow rate was increased or the space time lowered, the reaction conversion percentage dramatically decreased. This may imply that the tetralin dehydrogenation reaction is
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 2 4 2 e3 2 5 0
mass transfer in the same direction because of the coupling vector. This phenomenon possibly has two effects: (i) promoting desorption of product species (naphthalene) from the catalyst active sites, leading to facilitation of mass transfer in the system [8,26] and (ii) preventing a contiguous strong adsorption of hydrocarbon on active sites, causing decrease in coke deposition. Hence, the heating phenomenon from microwave inverts from conventional heating, species transport in the system under MWH was better than that under CH. Consequently, the overall reaction rate under MWH increased. This explains the MWH effect that enhances tetralin dehydrogenation compared to CH.
Decalin conversion (%)
100
80
60
MWH 40
CH
20
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0 0
60
120
180
240
300
Time on stream (min) Fig. 5 e Decalin dehydrogenation under microwave heating (MWH) and conventional heating (CH). Conditions: 1 wt% Pt/AC, reaction temperature [ 320 C, space time of reaction [ 42.86 gcatalyst h/moltetralin.
limited by the rate of mass diffusion or species transport, in particular, desorption of product species from active sites of the catalyst. This assumption was supported by the behavior of naphthalene adsorption. Besides hydrogen, naphthalene is a dominant product of the reaction. It is well known that naphthalene has strong adsorption energy because of the p-electrons of its aromatic rings. The strong adsorption of naphthalene on the catalyst active sites not only decelerated the reaction because of site blocking but also caused coke deposition. This led to deactivation of the catalyst as evidenced by decrease in dehydrogenation conversion as time on stream increased. An advantageous effect of microwaves on an increase of species transport in reaction correlated with temperature gradient and mass transfer which its direction inverted with that of conventional heating (Scheme 3). In case of tetralin dehydrogenation under MWH, the heat transfer generated from the Pt/AC catalyst to surrounding species also induced the driving force of
3.2.
Decalin dehydrogenation
Dehydrogenation of decalin was investigated under MWH and CH under the same experimental set up used for tetralin dehydrogenation. However, decalin dehydrogenation occurs preferentially at a higher reaction temperature than tetralin dehydrogenation; decalin dehydrogenation favors temperatures greater than 300 C [14]. Fig. 5 shows decalin conversion at a reaction temperature of 320 C plotted with time on stream in the dehydrogenation reaction using 1% Pt/AC as a catalyst under MWH and CH (electrical heater). The result shows that decalin conversion rapidly decreased from ca. 85% in the initial running period to 10e13% after an operating time of 180 min. The data is consistent between the conventional and microwave-heated reaction systems. Fig. 6 presents TEM images of the catalysts, fresh Pt/AC and spent Pt/AC. The large particle size of Pt on activated carbon was clearly observed, caused by metal sintering. The Pt particle size was increased from 5.34 to 10.63 nm after the dehydrogenation reaction. At a high operating temperature, Pt favored agglomeration and was present as larger particles, which induced decrease in the activity of the catalyst leading to lower conversion. It is well known that Sn is normally used as a catalyst promoter to improve the stability of a precious metal such as Pt by preventing the sintering and agglomeration of Pt, the so-called “geometric effect”. Moreover, addition of Sn to Pt also has an electronic effect because electrons from Sn possibly transfer
Fig. 6 e TEM images of 1% Pt/AC catalyst: (a) fresh catalyst and (b) spent catalyst.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 2 4 2 e3 2 5 0
Fig. 7 e TEM images of 1% PteSn/AC catalyst: (a) fresh catalyst and (b) spent catalyst from conventional heated reaction and (c) spent catalyst from microwave-heated reaction.
to Pt, which enables improvement in its catalytic activity [27,28]. Consequently, Sn was co-doped with Pt (1% PteSn/AC) on the activated carbon used as the catalyst in the decalin dehydrogenation. Fig. 7 shows TEM images of 1% PteSn/AC (fresh and spent catalyst from the decalin dehydrogenation reaction), which reveal small-sized and well-dispersed metal particles on the activated carbon. The average metal particle size observed from the TEM images was ca. 3.09 nm. This indicates the effect of Sn on the modification and stabilization of the Pt particles located on the catalyst. The dehydrogenation of decalin using 1% PteSn/AC as a catalyst was conducted at temperatures of 280e340 C. Fig. 8 shows the conversion of decalin and product selectivity of the reaction, comparing microwave irradiation with CH. The result shows that as the reaction temperature increases, conversion of decalin and selectivity of naphthalene increase, whereas selectivity of tetralin reduces. When comparing the reactions conducted in the microwave and conventional systems, at low reaction temperatures (280 C and 300 C), we observed that the decalin conversion under MWH was slightly higher than that under CH. At 280 C, the reaction conversion under microwave irradiation was 22.48% while that under
80
DL conv.
60 40 20 TL sel.
0 260
100
NL sel.
280
300
320
340
360
Temperature (°C) Fig. 8 e Effect of temperature on decalin dehydrogenation under microwave heating (MWH) and conventional heating (CH). Conditions: 1% PteSn/AC, space time of reaction [ 25.72 gcatalyst h/moldecalin. NL, naphthalene; DL, decalin; TL, tetralin.
Decalin conversion (%)
Decalin conversion and selectivity (%)
100
conventional reaction was 12.58%. Once the reactions were operated at a temperature higher than 300 C, decalin conversion was similar in the two systems. At 320 and 340 C, conversions were 63% and 78%, respectively. In addition, the product selectivities in the two systems were found to be equivalent. Naphthalene selectivity was greater than 80%, while selectivity of the tetralin product was lower than 20%. In summary, at different reaction temperatures, decalin conversions achieved using the conventional and microwave systems were almost identical. Fig. 9 shows decalin conversion in the dehydrogenation reaction with space times ranging from 9.74 to 42.86 gcatalyst h/ moldecalin at a reaction temperature of 320 C. The greater the space time, the higher the decalin conversion. At the higher space time of 42.86 gcatalyst h/moldecalin, decalin conversion was greater than 80%. When comparing decalin dehydrogenation in the conventional and microwave systems, the conversions were found to be relatively equivalent in each run at the corresponding space times. However, at space times lower than 25.72 gcatalyst h/moldecalin, it can be observed that conversions obtained by the microwave-heated reaction are slightly higher than those obtained by the conventional
80
60
MWH
40
CH
20
0 0
20
40
60
W/F decalin (g cat h/mol decalin) Fig. 9 e Effect of space time on decalin dehydrogenation under microwave heating (MWH) and conventional heating (CH). Conditions: 1 wt% PteSn/AC, reaction temperature [ 320 C.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 2 4 2 e3 2 5 0
Decalin conversion (%)
100 MWH 80 CH 60
40
20
0 30
90
150
210
270
330
Time on stream (min) Fig. 10 e Test of catalyst and reaction stability of decalin dehydrogenation under microwave heating (MWH) and conventional heating (CH). Conditions: 1 wt% PteSn/AC, reaction temperature [ 320 C, space time of reaction [ 42.86 gcatalyst h/moldecalin.
heated reaction. Fig. 10 illustrates the decalin conversion in the dehydrogenation reaction with time on stream in the two heating systems (microwave and conventional). At a running time of 90 min, the results show that the conversion obviously decreases from the original 80.8% to 77.7% in the microwave system and from 79.6% to 75.6% in the conventional system. After an operating time on stream of 210 min, the conversions were stabilized at ca. 74% in both systems. The above results indicate that in the case of decalin dehydrogenation, the reaction under MWH is identical to that observed under CH. This result is in contrast with that of tetralin dehydrogenation, where microwave-heated reaction shows improved performance over the conventional heated reaction. This leads to the question of why MWH has no specific effect on the decalin dehydrogenation reaction. As mentioned in the earlier tetralin dehydrogenation experiment, the microwave effect on dehydrogenation incorporated an acceleration of mass diffusion or species transport in the system because of the large temperature gradients from the carbon catalyst to the bulk nonpolar reactant. This feature contributes to the reaction, which is limited by mass diffusion or species transport. In other words, such an advantage of using microwaves should not yield much evidence of enhancement of the reaction, if this reaction system is not limited by mass diffusion or species transport in the system [9]. Wang et al. studied the kinetic model of decalin dehydrogenation over PteSn/g-Al2O3 at a reaction temperature of 325 C and mentioned that the surface reactions are the rate determining step of this reaction [12]. The dehydrogenation of decalin involves the isomerization reaction of cis- into trans-decalin and then dehydrogenation to tetralin and naphthalene, in addition to those reversible reactions. Furthermore, codoping of the Pt catalyst with Sn also assisted and modified the stability and activity of the Pt, reducing the problems of coke deposition and molecule diffusion. With explanation, we can observe the geometric effect of Sn in which sintering and
3249
agglomeration of Pt is prevented and smaller ensembles are obtained by dilution of Pt (Fig. 7). When the number of contiguous platinum atoms decreased, the multipoint adsorption of hydrocarbon molecules on the surface was hampered. Consequently, the deactivation rate of the catalyst by coke deposition was reduced. In addition, Sn modifies Pt electronically by donating electrons to the holes of the 5d band of the platinum atom. By this electronic effect, CeC bond hydrogenolysis does not occur because the hydrocarbon cannot be strongly adsorbed on the catalyst surface [27,28]. The sequence of the Sn effect leads to decreased coke deposition and improvement in molecule adsorption/desorption or transport of species on the active sites in the system. Therefore, by studying the conditions of decalin dehydrogenation with catalysis by PteSn-supported activated carbon, it can be inferred that the overall reaction rate is not dominated by a molecule diffusion step (molecule desorption/adsorption). This could explain the reason why the application of MWH in decalin dehydrogenation was not significantly advantageous over CH. However, in the experiments conducted at a low temperature and a high space time, in which the dominant reaction rate may be governed by mass diffusion, it can be noticed that the conversion achieved under MWH is slightly higher than that obtained under CH. In summary, note that the microwave effect, which enhanced the reaction efficiency over the conventional system, involved acceleration of molecule diffusion or transport of species (desorption/ adsorption) in the system. The microwave effect will, therefore, obviously be evident in reaction systems where the dominant reaction rate is governed by mass transport [29e33]. In addition, a noteworthy advantage of MWH on the dehydrogenation of organic hydrides (decalin and tetralin) was the low temperature of the outlet gas stream (H2 < 60 C) and matter of the surrounding system. A consequence of using microwaves as a selective heating tool is that only dielectrical materials can be heated up by a microwave. In this case, the catalyst bed was primarily heated, while the temperature of the surrounding substance remained quite low, leading to fast cooling of the outlet gas stream. This behavior should be useful for onboard hydrogen storage systems that require a low operating temperature while the PEM fuel cell system needs a feed of hydrogen, the temperature of which is not over 80 C [34,35]. This indicates that hydrogen can be produced from the dehydrogenation reaction in the microwave system at the temperature that favors the PEM fuel cell without requiring its transfer to a cooling unit.
4.
Conclusion
The microwave effects in the dehydrogenation reactions of decalin and tetralin were investigated using a heterogeneous fixed-bed reactor. The conversion of reactants was compared using different heating techniques, microwave and conventional (electrical heater) at the same operating conditions. Tetralin dehydrogenation under MWH exhibited improved conversion over the reaction under CH. Furthermore, the deactivation rate of the catalyst was lower under MWH than under CH. An advantageous effect of MWH was the large temperature gradient between the catalyst and surrounding
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species (direction of heat transfer inverted from that of CH). This led to acceleration of mass desorption and species transport in the system, reduction of coke deposition, and enhancement of tetralin dehydrogenation. Evidence of the microwave effect can be obviously observed in reactions that are controlled by mass transfer as the rate-limiting step.
Acknowledgement Financial support was provided to S.H. through a Grant-in-aid for young scientists (No. B-21750210) from the Japan Society for the Promotion of Science (JSPS), Ministry of Education, Culture, Sports, Science and Technology, Sport Science and Technology of Japan.
references
[1] Kingston HM, Haswell SJ. In: Microwave-enhanced chemistry. 1st ed. Washington, DC: American Chemical Society; 1997. [2] Loupy A. Microwave in organic synthesis. Weinheim, Germany: Wiley-VCH Verlag; 2006. [3] Raner KD, Strauss CR, Vyskoc F, Mokbel L. A comparison of reaction kinetics observed under microwave irradiation and conventional heating. J Org Chem 1993;58:950e3. [4] Strauss CR, Trainor RW. Developments in microwaveassisted organic chemistry. Aust J Chem 1995;48:1665e92. [5] Jacob J, Chia LHL, Boey FYC. Thermal and non-thermal interaction of microwave radiation with materials. J Mater Sci 1995;30:5321e7. ´ , Diez-Barra E. [6] Langa F, Cruz PDL, Hoz ADL, Dı´az-Ortiz A Microwave irradiation: more than just a method for accelerating reactions. Contem Org Synth; 1997:373e85. ´ , Moreno A. Microwaves in organic [7] Hoz A, Dı´az-Ortiz A synthesis: thermal and non-thermal microwave effect. Chem Soc Rev 2005;34:164e78. [8] Semenov VE, Rybakov K, II. What type transport phenomena can be induced by microwave field in solids and how these phenomena contribute to materials processing. in: Proceedings of the international symposium on novel materials processing by advanced electromagnetic energy sources, Osaka, Japan; March 19e22, 2004. p. 111e7. [9] Antonio C, Deam RT. Can “microwave effects” be explained by enhanced diffusion? Phys Chem Chem Phys 2007;9:2976e82. [10] Makowski P, Thomas A, Kuhn P, Goettmann F. Organic materials for hydrogen storage applications: from physisorption on organic solids to chemisorption in organic molecules. Energy Environ Sci 2009;2:480e90. [11] Sebastia´n D, Bordeje´ EG, Calvillo L, La´zaro MJ, Moliner R. Hydrogen storage by decalin dehydrogenation/naphthalene hydrogenation pair over platinum catalysts supported on activated carbon. Int J Hydrogen Energy 2008;33:1329e34. [12] Saito Y, Aramaki K, Hodoshima S, Saito M, Shoto A, Kuwano J, Otake K. Efficient hydrogen generation from organic chemical hydrides by using catalytic reactor on the basis of superheated liquid-film concept. Chem Eng Sci 2008; 63:4935e41. [13] Biniwale RB, Rayalu S, Devotta S, Ichikawa M. Chemical hydrides: a solution to high capacity hydrogen storage and supply. Int J Hydrogen Energy 2008;33:360e5. [14] Wang B, Goodman DW, Froment GF. Kinetic modeling of pure hydrogen production from decalin. J Catal 2008;253: 229e38.
[15] Mene´ndez JA, Arenillas A, Fidalgo B, Ferna´ndez Y, Zubizarreta L, Calvo EG, et al. Microwave heating process involving carbon materials. Fuel Proces Technol 2010;91:1e8. [16] Stull OR, Westrum Jr EF, Sinke GC. In: The chemical thermodynamics of organic compounds. 2nd ed. New York: John Wiley & Son; 1969. [17] Resini C, Lucarelli C, Jacquin MT, Liew KE, Gabellini I, Albonetti S, et al. Pt-Sn/g-Al2O3 and Pt-Sn-Na/ g-Al2O3 catalysts for hydrogen production by dehydrogenation of Jet A-1 fuel: Characterization and preliminary activity tests. Int J Hydrogen Energy; 2011. [18] Kla´n P, Litera´k J, Relich S. Molecular photochemical thermometer: investigation of microwave superheating effects by temperature dependent photochemical processes. J Photochem Photobiol A Chem 2002;143:49e57. [19] Lidsto¨m P, Tierney J, Wathey B, Westman J. Microwave assisted organic synthesis. Tetrahedron 2001;57:9225e83. [20] Djenni ZC, Hamada B, Chemat F. Atmospheric pressure microwave assisted heterogeneous catalytic reactions. Molecules 2007;12:1399e409. [21] Zhang X, Hayward DO, Mingos DMP. Apparent equilibrium shifts and hot-spot formation for catalytic reactions induced by microwave dielectric heating. Chem Commun 1999;11: 975e9283. [22] Perreux L, Loupy A. Nonthermal effects of microwaves in organic synthesis. In: Loupy A, editor. Microwave in organic synthesis. Weinheim, Germany: Wiley-VCH Verlag; 2006. p. 134e218. [23] Kim KS, Winograd N, Davis RE. Electron spectroscopy of platinum-oxygen surfaces and application to electrochemical studies. J Am Chem Soc 1971;93:6296e7. [24] Yue ZR, Jiang W, Wang L, Toghiani H, Gardner SD, Pittman CU. Adsorption of precious metal ions onto electrochemically oxidized carbon fibers. Carbon 1999;37:1607e18. [25] Adora S, Soldo-Olivier Y, Faure R, Durand R, Dartyge E, Baudelet F. Electrochemical preparation of platinum nanocrystallites on activated carbon studied by X-ray absorption spectroscopy. J Phys Chem B 2001;105:10489e95. [26] Dincov DD, Parrott KA, Pericleous KA. Heat and mass transfer in two-phase porous materials under intensive microwave heating. J Food Eng 2004;65:403e12. [27] Ertl G, Knozinger H, Weitkamp J. Handbook of heterogeneous catalysis 5. Weinheim, Germany: Wiley-VCH Verlag; 1997. [28] Srinivasan R, Davis BH. Paraffin dehydrocyclization. Part 9. Conversion of n-octane with Pt-Sn catalysts at atmospheric pressure. J Mol Catal 1994;88(3):343e58. [29] Gibson C, Mathews I, Samuel A. Microwave enhanced diffusion in polymeric materials. J Microwave Power Electromagn Energy 1998;23(1):17e28. [30] Leskovsek S, Smidovnik A, Koloini T. Kinetics of catalytic transfer hydrogenation of soybean oil in microwave and thermal field. J Org Chem 1994;59:7433e6. [31] Mallon FK, Ray WH. Enhancement of solid-state polymerization with microwave energy. J Appl Polym Sci 1998;69:1203e12. [32] Nakai Y, Tsujita Y, Yoshimizu H. Control of gas permeability for cellulose acetate membrane by microwave irradiation. Desalination 2002;145:375e7. [33] Booske JH, Cooper RF. How the coupling of microwave and RF energy in materials can affect solid state charge and mass transport and result in unique processing effects. In: WillertPorada M, editor. Advances in microwave & radio frequency processing. Berlin/Heidelberg: Springer; 2006. p. 461e71. [34] Satyapal S, Petrovic J, Read C, Thomas G, Ordaz G. The U.S. department of energy’s national storage project: progress towards meeting hydrogen-powered vehicle requirements. Catal Today 2007;120:246e56. [35] Crawley G. Proton exchange membrane (PEM) fuel cells. Fuel Cell Today 2006;March:1e12, www.fuelcelltoday.com.
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Microwave effect in the dehydrogenation of tetralin and decalin with a fixed-bed reactor Yindee Suttisawat a, Hideki Sakai b, Masahiko Abe b, Pramoch Rangsunvigit c, Satoshi Horikoshi a,* a
Department of Materials and Life Sciences, Faculty of Science and Technology, Sophia University, 7-1 Kioicho, Chiyodaku, Tokyo 102-8554, Japan b Department of Pure and Applied Chemistry in Faculty of Science, Tokyo University of Science, Chiba 278-8510, Japan c The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok 10330, Thailand
article info
abstract
Article history:
This study investigates the effect of microwaves on the dehydrogenation of decalin and
Received 21 August 2011
tetralin. The reactions were studied in a fixed-bed reactor under microwave heating
Received in revised form
(MWH), and the results were compared with those under conventional heating (electrical
17 October 2011
heating) (CH). The result of this investigation showed that under the same operating
Accepted 25 October 2011
conditions, increase in the conversion of tetralin in the microwave-heated system was
Available online 9 December 2011
more than that in the conventional heated system. This demonstrated the microwave effect appeared in the dehydrogenation of tetralin. In addition, the activity and stability of
Keywords:
the catalyst also improved under MWH. The microwave effect resulted in increase in the
Microwave effect
conversion reaction of tetralin dehydrogenation by generation of a large temperature
Microwave heating
gradient from the catalyst surface to surrounding species and the subsequent mass
Mass diffusion
transfer in which both vectors have direction inverted from that of CH. Such a phenom-
Fixed-bed reaction
enon induced faster molecule desorption (product species) or enhancement of species
Decalin dehydrogenation
transport in the system. In addition, the contiguous strong adsorption of hydrocarbon can be reduced, leading to decrease in coke deposition. Conditionally, this benefit issue particularly resulted in increase in the reaction rate in which species transport or mass diffusion is the rate-limiting step. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Microwave energy has been widely used in fields that involve thermally activated processing, such as food processing and drying processing including wood, paper, ceramic, and recently, it has been frequently employed in chemical reactions [1]. The advantages of microwave heating (MWH) are multiple, for example, quick and volumetric heating can be achieved. In addition, it is possible to selectively heat the
desired region or a particular material because of difference in microwave absorption properties of different matter. The feature of selective heating leads to a reduction in energy consumption and in the time required for processing steps, such as the drying process. Microwaves can remarkably enhance organic synthesis reactions (higher yield and faster reaction compared with conventional reactions). This effect has provoked the interest of many researchers and has led them to apply the use of microwaves to other chemical
* Corresponding author. E-mail address: [email protected] (S. Horikoshi). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.10.111
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 2 4 2 e3 2 5 0
reactions, such as decomposition of toxic materials, regeneration of activated carbon saturated with volatile organic compounds, or even catalyst synthesis [2]. However, although it has been mentioned that MWH can have beneficial effects on chemical reactions over conventional methods, some publications have claimed that microwaves do not show any specific improvement in reaction efficiency when compared with a conventional system [3,4]. Microwaves have been widely discussed in terms of their thermal and nonthermal effects. The thermal effect on acceleration of a reaction has been explained in terms of very quick heating, hot spot, or superheating, while the effect is considered to be nonthermal when enhancement of a chemical reaction cannot be explained by temperature parameters, such as molecular polarization, molecule transport phenomena, and modification of thermodynamic parameters by microwave irradiation [5e7]. The nonthermal effects on enhancement of microwaveheated reactions are still not clear. It was suggested that poor sensitivity of temperature measurement and the resulting errors led to non-detection of the thermal effect [3,8,9]. Till now, the effect of microwaves and their mechanism of action are still under debate. In the present study, we investigated the dehydrogenation of tetralin and decalin under MWH in a fixed-bed reactor and demonstrated the microwave effect in reactions. The dehydrogenation/hydrogenation of organic chemical hydrides, such as decalin and methylcyclohexane, has been proposed as a potential process method for hydrogen storage. The hydrogen storage capacity of such organic chemical hydrides is 6e7 wt%, which satisfies the target value for hydrogen storage [10e12]. However, this reaction is constrained by deactivation of the catalyst due to coke deposition, leading to lower reaction efficiency. A number of supported metal catalysts have been used in this reaction such as Pt, PteRh, Pd, and Ni on activated carbon or PteSn on alumina [13,14]. In this study, we used activated carbon deposited with Pt and PteSn as catalysts in the dehydrogenation of tetralin and decalin. Pt and PteSn are good catalysts for this type of reaction, while activated carbon is an excellent material for microwave absorption [15]. The reactions were performed in the gas phase using a flow system. These experiments compared the dehydrogenation reaction conversions using MWH and conventional heating (electrical heating) (CH), two different heating techniques, while varying the parameters reaction temperature and feed flow rate (reaction space time).
2.
Experimental
2.1.
Materials and catalyst preparation
Pt or Pt and Sn deposited on activated carbon (Pt/AC and PteSn/AC, respectively) were used as catalysts in the dehydrogenation reactions of tetralin and decalin (24% cis-isomer and 76% trans-isomers). Pt/AC (Pt: 1 wt%) and PteSn/AC (Pt: 1 wt% and Sn: 0.6 wt%) were prepared by the incipient
Conversion ¼
3243
wetness impregnation method. An appropriate amount of each metal precursor (H2PtCl6$6H2O and SnCl4$5H2O, 98.5% purity; supplied by Wako Pure Chemical Industries, Ltd., Japan) was dissolved in deionized water and then introduced into the pores of the carbon support (charcoal activated carbon, 99% purity, granular size 0.72e1.4 mm, 0.59 cm3 g1, 1174 m2 g1). The wet sample was kept at room temperature for 1 h and then dried under an air atmosphere at 110 C for 2e3 h, followed by reduction under a flow of hydrogen at 320 C for 4 h.
2.2.
Dehydrogenation in fixed-bed reactor
The catalytic reactions were carried out under atmospheric pressure in a fixed-bed tubular reactor. The dehydrogenation reaction was performed under single-mode microwave dielectric heating (microwave magnetron capacity: 0e200 W, 2.45 GHz supplied by ARIOS company) and was compared with the reaction conducted under CH (120 W electrical heater). A quartz tube of 5 mm inside diameter was used as a reactor and loaded with 0.2 g of catalyst. A thermocouple, which does not absorb microwave irradiation, was used to measure the center temperature of the catalyst bed inside the reactor. Scheme 1 illustrates the experimental set up of the fixed-bed reaction using a single-mode microwave apparatus. In the procedure, a reactor tube was placed in the microwave cavity at the Emax position in electromagnetic field. Microwaves were produced by a generator with a 2.45 GHz magnetron, irradiating along the waveguide to the reactor tube, which was packed with the catalyst. A three-stub tuner and a plunger were used to control the microwave irradiation intensity and position of irradiation on the sample while a circulator absorbed the reflected microwaves from the cavity to minimize damage of the magnetron. Prior to the reaction, Ar was flowed over the catalyst bed for 15 min to reduce the air content inside the reactor. Microwaves with power of 60 W were then irradiated for 2e3 min to warm up the catalyst bed to approximately 100e120 C. The temperature was then increased for the reactions: 250e320 C for tetralin dehydrogenation and 280e340 C for decalin dehydrogenation. The reactant, decalin or tetralin, was fed into the single-pass continuous flow reactor packed with the granular catalyst by a syringe pump while the space time of reaction was varied from 4.13 to 10.33 gcatalyst h/moltetralin for tetralin dehydrogenation and from 9.74 to 42.86 gcatalyst h/moldecalin for decalin dehydrogenation. The temperature of the catalyst bed was controlled by tuning the microwave power in the range 100e120 W. The outlet liquid product was collected every 30e60 min and conversion of reactant (%) was analyzed by FID-GC (Shimadzu model 2014 connected with Zebron column model ZB-624).
2.3.
ðmoles of reactantðinletÞÞ ðmoles of reactantðoutletÞÞ moles of reactantðinletÞ
Definition of conversion and selectivity
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 2 4 2 e3 2 5 0
Scheme 1 e Experimental set up under microwave irradiation (side view).
Selectivity ¼
moles of product formed moles of reactant converted
Space time of reaction ðW=FÞ ¼
3.
amount of catalyst ðgÞ feed flow rate ðmole=hÞ
decalin dehydrogenation (Scheme 2), dehydrogenation of decalin into naphthalene must be performed at a reaction temperature much higher than that of tetralin into naphthalene because the reaction is highly endothermic and thermodynamically restricted at low temperatures (because of reverse and isomerization reactions) [14,16]. From individual studies of tetralin and decalin dehydrogenation, we can clearly observe the effect of microwaves on these reactions.
Experimental results and discussion 3.1.
The dehydrogenation reactions of tetralin and decalin under MWH were studied and compared with their dehydrogenation reactions under CH (electrical furnace). The results are separately reported in the following two sections for tetralin dehydrogenation using Pt-loaded granular activated carbon (Pt/AC) as the catalyst and decalin dehydrogenation using PteSn-loaded granular activated carbon (PteSn/AC) as the catalyst. Although tetralin is an intermediate in the reaction of
Scheme 2 e Dehydrogenation of decalin and tetralin.
Tetralin dehydrogenation
Fig. 1 shows the dehydrogenation of tetralin with Pt/AC catalyst as a function of reaction temperature and the comparison of MWH and CH methods. The reaction was performed at 250e320 C. The results show that to achieve equivalent tetralin conversions, the microwave-heated reaction required a reaction temperature ca. 20 C lower than the conventional reaction. In order to achieve 68% tetralin conversion, the reaction performed under MWH was conducted at a reaction temperature of 230 C, while the conventional reaction required a temperature of 250 C. The reaction under MWH reached 100% tetralin conversion at a reaction temperature of 290 C, while the reaction under CH approached 100% at a higher reaction temperature. However, no significant difference was found in the value of
3245
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 2 4 2 e3 2 5 0
100
Tetralin conversion (%)
Tetralin conversion (%)
100
MWH
80
CH 60
40
20
MWH
80
CH 60 40 slope =
ΔX
20
0 240
260
280
300
Fig. 1 e Effect of temperature on tetralin dehydrogenation under microwave heating (MWH) and conventional heating (CH). Conditions: 1 wt% Pt/AC, space time of reaction [ 10.33 gcatalyst h/moltetralin.
naphthalene selectivity between the two systems (this data is not displayed here), where the naphthalene selectivity was greater than 98%. At a fixed temperature of 270 C, the feed flow rate was varied to investigate the effect of space time on tetralin conversion during the dehydrogenation reaction. The interesting results shown in Fig. 2 reveal that when the space time decreased from 10.33 to 4.13 gcat h/moltetralin, the conversion dramatically reduced from 86.5% to 40.3% in the case of the reaction under CH. In contrast, in the case of reaction under microwave irradiation, the conversion did not lower significantly; it slightly decreased from 97.1% to 89.2%. A remarkable decrease in conversion with increase in the space time suggests that the reaction is affected by species transport or
CH = 0.091
60
120
180
240
Time on stream (min)
320
Temperature (°C)
MW = 0.046
ΔT
0 0 220
ΔX ΔT
Fig. 3 e Test of catalyst and reaction stability of tetralin dehydrogenation under microwave heating (MWH) and conventional heating (CH). Conditions: 1 wt% Pt/AC, reaction temperature [ 270 C, space time [ 10.33 gcatalyst h/moltetralin.
a rate-limiting step of mass diffusion, particularly molecular desorption from active sites of the catalyst. It is well known that naphthalene, which is the product of the reaction, has two benzene rings, and consequently, has strong adsorption energy with the active sites of the catalyst. For this reason, species transport on active sites of the catalyst is restricted. Furthermore, the stability and activity of the catalyst were studied. The plot of tetralin conversion and time on stream for 4 h in Fig. 3 shows the catalyst and reaction stability under MWH and CH at a reaction temperature of 270 C with a space time of 10.33 gcat h/moltetralin. For the reaction under MWH, the results show that the conversion decreased from the original 98% to 89% and seemed to be stable after 3 h running time, while in the reaction under CH, the conversion continuously reduced from 86.5% to 66.8% even after an operating
MWH
80
PtOx
CH
PtOx
Intensity (a.u.)
Tetralin conversion (%)
100
60
40
c) spent Pt/AC-MW b) spent Pt/AC-CH a) fresh Pt/AC
20
0 0
3
6
9
12
15
W/F tetralin (g cat h/mol tetralin) Fig. 2 e Effect of space time on tetralin dehydrogenation under microwave heating (MWH) and conventional heating (CH). Conditions: 1 wt% Pt/AC, reaction temperature [ 270 C.
65
70
75
80
Binding energy (eV) Fig. 4 e XPS spectrum of Pt (4f electron) of Pt/AC catalyst: (a) fresh catalyst and (b) spent catalyst from conventional heated reaction and (c) spent catalyst from microwaveheated reaction.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 2 4 2 e3 2 5 0
Scheme 3 e Model of heat transfer direction: (a) under microwave heating (MWH): Tcatalyst > Tsurrounding, heat transfers from the catalyst to the surrounding matter therefore mass also transfers in the same direction because of the coupling vector and (b) under conventional heating (CH): Tcatalyst < Tsurrounding, heat transfers from the surrounding matter to the catalyst.
time of 4 h. Rate of conversion decrease with time on stream in each reaction was calculated from the slope of the curve. The curve slope in the case of the conventional system is 0.091, while that in the microwave system is 0.046. This indicates that the rate of conversion in the conventional reaction decreases twice as fast as the rate of conversion in the microwave-heated reaction. Decrease in conversion caused by a loss of catalyst activity is a consequence of naphthalene deposition on the catalyst active sites, which then generates coke [17]. Thus, we assume that the rate of coke formation during the reaction under MWH is less than that during the reaction under CH. In the tetralin dehydrogenation reaction, it was revealed that the microwave-heated reaction showed better performance than the conventional heated reaction. The reaction under MWH resulted in a higher conversion at the same reaction temperature or space time and displayed better stability and activity of the catalyst than that of CH. The positive effect of MWH on tetralin dehydrogenation may result from the following: (i) Hot spots or localized superheating within the catalyst, during the microwave-heated reaction can be visually observed by arcing or sparking on the surface of the catalyst. As described by the Arrhenius reaction (k ¼ AeEa/RT), the reaction rate depends on two factors: a pre-exponential factor (A) and an exponential factor (eEa/RT). The presence of localized superheating (arcing and sparking) caused high temperatures at active sites of the catalyst, contributing to increase in the magnitude of the exponential part of the Arrhenius reaction and enhancement of the reaction rate [7,18e21]. However, the phenomenon of arcing or sparking on the catalyst surface was random and heterogeneous, and therefore, it was not easy to measure the exact temperature generated by sparking or arcing.
(ii) Nonthermal effects of microwave irradiation have been proposed, such as polarization of reactant molecules or modification of catalyst activity [7,22]. However, the nonthermal effects on enhancement of microwaveheated reactions are difficult to observe and discuss. In this study, the reactant is a nonpolar material; therefore, the microwave field should not have much effect on polarization. In addition, to clarify whether microwaves may affect some feature of the catalysts, we used X-ray photoelectron spectroscopy (XPS) to characterize the catalysts used in the reaction (fresh and spent catalysts) and to investigate the binding energy of metallic Pt (Pt4f level). The XPS spectra of a low energy band of Pt 4f7/2 and a high energy band of Pt 4f5/2 display discrete peaks at 69.7 eV and at 72.2 eV, respectively (Fig. 4). The peak positions indicating the binding energies of Pt in fresh and spent catalysts from microwave- and conventional heated reactions are identical. In the XPS spectra of the spent catalysts, however, the presence of small shoulder peaks can be observed at 71.4 and 74.9 eV, corresponding to the oxide of Pt (PtO or PtO2) [23]. Pt oxide would occur from an interaction of Pt with oxygen in the functional groups (carbonyl, carboxyl, hydroxyl and quinone) on the carbon surface [24,25]. (iii) The effect of microwave field on the improvement in molecular diffusion or transport of species in a reaction system has been mentioned [8,9]. Antonio and Deam have proved the hypothesis that if the transport of an active species is a rate-limiting step in a reaction, the temperature gradient under MWH enhances the diffusion of that species, and the overall reaction rate is changed under MWH compared with CH [9]. From the above results in this experiment, it is revealed that when the feed flow rate was increased or the space time lowered, the reaction conversion percentage dramatically decreased. This may imply that the tetralin dehydrogenation reaction is
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 2 4 2 e3 2 5 0
mass transfer in the same direction because of the coupling vector. This phenomenon possibly has two effects: (i) promoting desorption of product species (naphthalene) from the catalyst active sites, leading to facilitation of mass transfer in the system [8,26] and (ii) preventing a contiguous strong adsorption of hydrocarbon on active sites, causing decrease in coke deposition. Hence, the heating phenomenon from microwave inverts from conventional heating, species transport in the system under MWH was better than that under CH. Consequently, the overall reaction rate under MWH increased. This explains the MWH effect that enhances tetralin dehydrogenation compared to CH.
Decalin conversion (%)
100
80
60
MWH 40
CH
20
3247
0 0
60
120
180
240
300
Time on stream (min) Fig. 5 e Decalin dehydrogenation under microwave heating (MWH) and conventional heating (CH). Conditions: 1 wt% Pt/AC, reaction temperature [ 320 C, space time of reaction [ 42.86 gcatalyst h/moltetralin.
limited by the rate of mass diffusion or species transport, in particular, desorption of product species from active sites of the catalyst. This assumption was supported by the behavior of naphthalene adsorption. Besides hydrogen, naphthalene is a dominant product of the reaction. It is well known that naphthalene has strong adsorption energy because of the p-electrons of its aromatic rings. The strong adsorption of naphthalene on the catalyst active sites not only decelerated the reaction because of site blocking but also caused coke deposition. This led to deactivation of the catalyst as evidenced by decrease in dehydrogenation conversion as time on stream increased. An advantageous effect of microwaves on an increase of species transport in reaction correlated with temperature gradient and mass transfer which its direction inverted with that of conventional heating (Scheme 3). In case of tetralin dehydrogenation under MWH, the heat transfer generated from the Pt/AC catalyst to surrounding species also induced the driving force of
3.2.
Decalin dehydrogenation
Dehydrogenation of decalin was investigated under MWH and CH under the same experimental set up used for tetralin dehydrogenation. However, decalin dehydrogenation occurs preferentially at a higher reaction temperature than tetralin dehydrogenation; decalin dehydrogenation favors temperatures greater than 300 C [14]. Fig. 5 shows decalin conversion at a reaction temperature of 320 C plotted with time on stream in the dehydrogenation reaction using 1% Pt/AC as a catalyst under MWH and CH (electrical heater). The result shows that decalin conversion rapidly decreased from ca. 85% in the initial running period to 10e13% after an operating time of 180 min. The data is consistent between the conventional and microwave-heated reaction systems. Fig. 6 presents TEM images of the catalysts, fresh Pt/AC and spent Pt/AC. The large particle size of Pt on activated carbon was clearly observed, caused by metal sintering. The Pt particle size was increased from 5.34 to 10.63 nm after the dehydrogenation reaction. At a high operating temperature, Pt favored agglomeration and was present as larger particles, which induced decrease in the activity of the catalyst leading to lower conversion. It is well known that Sn is normally used as a catalyst promoter to improve the stability of a precious metal such as Pt by preventing the sintering and agglomeration of Pt, the so-called “geometric effect”. Moreover, addition of Sn to Pt also has an electronic effect because electrons from Sn possibly transfer
Fig. 6 e TEM images of 1% Pt/AC catalyst: (a) fresh catalyst and (b) spent catalyst.
3248
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 2 4 2 e3 2 5 0
Fig. 7 e TEM images of 1% PteSn/AC catalyst: (a) fresh catalyst and (b) spent catalyst from conventional heated reaction and (c) spent catalyst from microwave-heated reaction.
to Pt, which enables improvement in its catalytic activity [27,28]. Consequently, Sn was co-doped with Pt (1% PteSn/AC) on the activated carbon used as the catalyst in the decalin dehydrogenation. Fig. 7 shows TEM images of 1% PteSn/AC (fresh and spent catalyst from the decalin dehydrogenation reaction), which reveal small-sized and well-dispersed metal particles on the activated carbon. The average metal particle size observed from the TEM images was ca. 3.09 nm. This indicates the effect of Sn on the modification and stabilization of the Pt particles located on the catalyst. The dehydrogenation of decalin using 1% PteSn/AC as a catalyst was conducted at temperatures of 280e340 C. Fig. 8 shows the conversion of decalin and product selectivity of the reaction, comparing microwave irradiation with CH. The result shows that as the reaction temperature increases, conversion of decalin and selectivity of naphthalene increase, whereas selectivity of tetralin reduces. When comparing the reactions conducted in the microwave and conventional systems, at low reaction temperatures (280 C and 300 C), we observed that the decalin conversion under MWH was slightly higher than that under CH. At 280 C, the reaction conversion under microwave irradiation was 22.48% while that under
80
DL conv.
60 40 20 TL sel.
0 260
100
NL sel.
280
300
320
340
360
Temperature (°C) Fig. 8 e Effect of temperature on decalin dehydrogenation under microwave heating (MWH) and conventional heating (CH). Conditions: 1% PteSn/AC, space time of reaction [ 25.72 gcatalyst h/moldecalin. NL, naphthalene; DL, decalin; TL, tetralin.
Decalin conversion (%)
Decalin conversion and selectivity (%)
100
conventional reaction was 12.58%. Once the reactions were operated at a temperature higher than 300 C, decalin conversion was similar in the two systems. At 320 and 340 C, conversions were 63% and 78%, respectively. In addition, the product selectivities in the two systems were found to be equivalent. Naphthalene selectivity was greater than 80%, while selectivity of the tetralin product was lower than 20%. In summary, at different reaction temperatures, decalin conversions achieved using the conventional and microwave systems were almost identical. Fig. 9 shows decalin conversion in the dehydrogenation reaction with space times ranging from 9.74 to 42.86 gcatalyst h/ moldecalin at a reaction temperature of 320 C. The greater the space time, the higher the decalin conversion. At the higher space time of 42.86 gcatalyst h/moldecalin, decalin conversion was greater than 80%. When comparing decalin dehydrogenation in the conventional and microwave systems, the conversions were found to be relatively equivalent in each run at the corresponding space times. However, at space times lower than 25.72 gcatalyst h/moldecalin, it can be observed that conversions obtained by the microwave-heated reaction are slightly higher than those obtained by the conventional
80
60
MWH
40
CH
20
0 0
20
40
60
W/F decalin (g cat h/mol decalin) Fig. 9 e Effect of space time on decalin dehydrogenation under microwave heating (MWH) and conventional heating (CH). Conditions: 1 wt% PteSn/AC, reaction temperature [ 320 C.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 2 4 2 e3 2 5 0
Decalin conversion (%)
100 MWH 80 CH 60
40
20
0 30
90
150
210
270
330
Time on stream (min) Fig. 10 e Test of catalyst and reaction stability of decalin dehydrogenation under microwave heating (MWH) and conventional heating (CH). Conditions: 1 wt% PteSn/AC, reaction temperature [ 320 C, space time of reaction [ 42.86 gcatalyst h/moldecalin.
heated reaction. Fig. 10 illustrates the decalin conversion in the dehydrogenation reaction with time on stream in the two heating systems (microwave and conventional). At a running time of 90 min, the results show that the conversion obviously decreases from the original 80.8% to 77.7% in the microwave system and from 79.6% to 75.6% in the conventional system. After an operating time on stream of 210 min, the conversions were stabilized at ca. 74% in both systems. The above results indicate that in the case of decalin dehydrogenation, the reaction under MWH is identical to that observed under CH. This result is in contrast with that of tetralin dehydrogenation, where microwave-heated reaction shows improved performance over the conventional heated reaction. This leads to the question of why MWH has no specific effect on the decalin dehydrogenation reaction. As mentioned in the earlier tetralin dehydrogenation experiment, the microwave effect on dehydrogenation incorporated an acceleration of mass diffusion or species transport in the system because of the large temperature gradients from the carbon catalyst to the bulk nonpolar reactant. This feature contributes to the reaction, which is limited by mass diffusion or species transport. In other words, such an advantage of using microwaves should not yield much evidence of enhancement of the reaction, if this reaction system is not limited by mass diffusion or species transport in the system [9]. Wang et al. studied the kinetic model of decalin dehydrogenation over PteSn/g-Al2O3 at a reaction temperature of 325 C and mentioned that the surface reactions are the rate determining step of this reaction [12]. The dehydrogenation of decalin involves the isomerization reaction of cis- into trans-decalin and then dehydrogenation to tetralin and naphthalene, in addition to those reversible reactions. Furthermore, codoping of the Pt catalyst with Sn also assisted and modified the stability and activity of the Pt, reducing the problems of coke deposition and molecule diffusion. With explanation, we can observe the geometric effect of Sn in which sintering and
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agglomeration of Pt is prevented and smaller ensembles are obtained by dilution of Pt (Fig. 7). When the number of contiguous platinum atoms decreased, the multipoint adsorption of hydrocarbon molecules on the surface was hampered. Consequently, the deactivation rate of the catalyst by coke deposition was reduced. In addition, Sn modifies Pt electronically by donating electrons to the holes of the 5d band of the platinum atom. By this electronic effect, CeC bond hydrogenolysis does not occur because the hydrocarbon cannot be strongly adsorbed on the catalyst surface [27,28]. The sequence of the Sn effect leads to decreased coke deposition and improvement in molecule adsorption/desorption or transport of species on the active sites in the system. Therefore, by studying the conditions of decalin dehydrogenation with catalysis by PteSn-supported activated carbon, it can be inferred that the overall reaction rate is not dominated by a molecule diffusion step (molecule desorption/adsorption). This could explain the reason why the application of MWH in decalin dehydrogenation was not significantly advantageous over CH. However, in the experiments conducted at a low temperature and a high space time, in which the dominant reaction rate may be governed by mass diffusion, it can be noticed that the conversion achieved under MWH is slightly higher than that obtained under CH. In summary, note that the microwave effect, which enhanced the reaction efficiency over the conventional system, involved acceleration of molecule diffusion or transport of species (desorption/ adsorption) in the system. The microwave effect will, therefore, obviously be evident in reaction systems where the dominant reaction rate is governed by mass transport [29e33]. In addition, a noteworthy advantage of MWH on the dehydrogenation of organic hydrides (decalin and tetralin) was the low temperature of the outlet gas stream (H2 < 60 C) and matter of the surrounding system. A consequence of using microwaves as a selective heating tool is that only dielectrical materials can be heated up by a microwave. In this case, the catalyst bed was primarily heated, while the temperature of the surrounding substance remained quite low, leading to fast cooling of the outlet gas stream. This behavior should be useful for onboard hydrogen storage systems that require a low operating temperature while the PEM fuel cell system needs a feed of hydrogen, the temperature of which is not over 80 C [34,35]. This indicates that hydrogen can be produced from the dehydrogenation reaction in the microwave system at the temperature that favors the PEM fuel cell without requiring its transfer to a cooling unit.
4.
Conclusion
The microwave effects in the dehydrogenation reactions of decalin and tetralin were investigated using a heterogeneous fixed-bed reactor. The conversion of reactants was compared using different heating techniques, microwave and conventional (electrical heater) at the same operating conditions. Tetralin dehydrogenation under MWH exhibited improved conversion over the reaction under CH. Furthermore, the deactivation rate of the catalyst was lower under MWH than under CH. An advantageous effect of MWH was the large temperature gradient between the catalyst and surrounding
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 2 4 2 e3 2 5 0
species (direction of heat transfer inverted from that of CH). This led to acceleration of mass desorption and species transport in the system, reduction of coke deposition, and enhancement of tetralin dehydrogenation. Evidence of the microwave effect can be obviously observed in reactions that are controlled by mass transfer as the rate-limiting step.
Acknowledgement Financial support was provided to S.H. through a Grant-in-aid for young scientists (No. B-21750210) from the Japan Society for the Promotion of Science (JSPS), Ministry of Education, Culture, Sports, Science and Technology, Sport Science and Technology of Japan.
references
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