Palygorskite

Fuel 107 (2013) 699–705

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CO2 reforming of toluene as model compound of biomass tar on Ni/Palygorskite Tianhu Chen a,⇑, Haibo Liu a,b, Peichao Shi a, Dong Chen a, Lei Song a, Hongping He c, Ray L. Frost b,⇑ a

School of Resources and Environmental Engineering, Hefei University of Technology, Hefei, China School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, Australia c Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China b

h i g h l i g h t s " We have made on palygorskite-supported nickel catalysts. " We tested catalytic CO2 reforming of biomass tar. " The influence of reaction temperature and concentration of CO2 on H2 yield has been studied. " As reaction temperature and concentration of CO2 increased, the amount of carbon deposit decreased.

a r t i c l e

i n f o

Article history: Received 10 September 2012 Received in revised form 4 December 2012 Accepted 10 December 2012 Available online 23 December 2012 Keywords: Toluene Biomass tar CO2 reforming Palygorskite Catalytic cracking

a b s t r a c t Catalytic CO2 reforming of biomass tar on palygorskite-supported nickel catalysts using toluene as a model compound of biomass tar was investigated. The experiments were performed in a bench scale installation a fixed bed reactor. All experiments were carried out at 650, 750, 800 °C and atmospheric pressure. The effect of Ni loading, reaction temperature and concentration of CO2 on H2 yield and carbon deposit was investigated. Ni/Palygorskite (Ni/PG) catalysts with Ni/PG ratios of 0%, 2%, 5% and 8% were tested, the last two show the best performance. H2 yield and carbon deposit diminished with the increase of reaction temperature, Ni loading, and CO2 concentration. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction With the depletion of fossil fuel sources as well as global warming issues, the utilization of biomass is receiving increased attention as a potential source of renewable energy. Among the biomass utilization technologies, gasification has attracted increased interest by producing a gas rich in H2 and CO [1,2]. Biomass gasification is a promising technology for fuels for power generation and chemical production. One of the main disadvantages for commercialising biomass gasification is the product gas quality. Among the impurities present in product gas, tar represents a serious impediment that has received significant attention in literature [3–5]. In addition, tar can deposit on surfaces of filters, heat exchangers and engines, reducing component performance ⇑ Corresponding authors. Tel.: +86 551 2903990 (T. Chen), tel.: +61 7 3138 2407; fax: +61 7 3138 1804 (R.L. Frost). E-mail addresses: [email protected], [email protected] (T. Chen), [email protected] (R.L. Frost). 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.12.036

and increasing maintenance requirements. It must also be remembered that tar can polymerize to form more complex structures and aerosols. Tar removal methods can be divided into two types: primary and secondary [3]. Primary methods include the measures taken in the gasification step to prevent tar form being in the fluidized bed furnaces or to convert it into small-molecule gases. Secondary methods include chemical or physical treatments downstream of the fluidized bed furnaces. Among these measures, the catalytic reforming process is very attractive technique for tar removal. The catalysts used in the catalytic reforming process include metallic catalysts, mainly Ni-based catalysts [5,6–10], alkali metallic catalysts, dolomites, olivine [11] or a combination of metals on mineral substrate [12]. Tar is a complex mixture of organic compounds which formed in the thermochemical processing of biomass. Therefore, many researchers used model compound in the tar remove studies, example of phenol [13,14], naphthalene [15,16] and toluene [5,9,10,17]. In the present research work, toluene was chosen as a model compound of biomass tar because it is found in significant

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quantities, especially at low reaction temperature, being clearly surpassed by benzene [18]. Various kinds of catalysts and operationing conditions were described in studies about tar catalytic cracking using toluene as model compound. Simell et al. [19] studied tar-decomposing activities of dolomites and limestone catalysts using toluene as a model compound. Their tests were carried out in a fixed-bed reactor at 900 °C under 2 MPa using an N2–H2O–CO2 gas mixture. Wang and Gorte [20] analyzed 1 wt% Pd/alumina and 1 wt% Pt/ceria catalysts in a fixed bed reactor. The S/C ratio was from 1:1 to 3:1 and reaction temperature was from 853 to 1043 °C. Juutilainen et al. [21] studied several catalysts (nickel based-catalyst, zirconia, dolomite, alumina, alumina-doped zirconia) at different temperatures from 550 to 900 °C. In their tests, they used a synthetic gasification gas mixture in the presence of oxygen from 1% to 20%. Simell et al. [22] tested several catalysts included alumina, dolomite, SiC and nickel-based catalysts. These researchers studied toluene decomposition in different gas atmospheres. The tests were carried out in a fixed-bed tube reactor at 900 °C under 2 and 5 MPa pressure. Swierczynski et al. [9] studied olivine and Ni/olivine catalysts in the fixed bed using toluene as a model compound with reaction temperatures from 560 to 850 °C. The ratios of steam/toluene varied from 7.5 to 24. Kuhn et al. [11] evaluated four olivine catalysts by reforming model compounds (naphthalene, toluene and methane). Treatments for olivine catalysts prior to reaction and the formation of free Fe phase played an important role in catalyst activity. Zhang et al. [12] employed Ni/olivine catalysts doped with CeO2 in toluene steamreforming experiment. The ratio of S/C was 5 at temperatures from 700 to 830 °C. Nordgreen et al. [23] analyzed metallic iron catalysts on tar decomposition at temperatures from 700 to 900 °C. Tar decomposition on nature olivine and Ni/olivine in dry-reforming (methane conversion) and steam-reforming (methane conversion) was studied at temperatures from 600 to 800 °C [24]. As mention above, Ni-based catalysts have been widely used in biomass gasification or biomass gas purification. In order to enhance the nickel catalysts performance, palygorskite (PG) was employed as a catalyst support. PG is a silicate clay mineral which has a special structure and physico-chemical properties. PG has fibrous morphology with 30–40 nm in diameter and several micrometers in length. PG has large surface area, is mesoporous and thus exhibits excellent surface activity and adsorption properties [25–27]. PG is an excellent natural nanometer mineral material which can be used as catalyst carrier [28–30]. Research using PG as a catalyst or catalyst carrier in the reforming of biomass tar has not been reported in recent years. As a greenhouse gas, carbon dioxide had aroused widespread attention because it can lead to temperature rise of the global atmosphere. Carbon dioxide can be used to make syngas production which can react with methane. Lemonidou and Vasalos [31] investigated methane reforming by carbon dioxide over 5 wt% Ni/CaO–Al2O3 catalyst. Catalytic activity was investigated at temperatures between 630 and 850 °C with an equimolar CH4 and CO2 mixture. Tsipouriari and Verykios [32] studied the kinetic behavior of the Ni/La2O3 catalyst in the reforming reaction of methane with carbon dioxide as a function of temperature and partial pressure of CH4 and CO2. Tsipouriari and Verykios [33] studied mechanistic aspects of carbon and oxygen reaction pathways to form CO over Ni/La2O3 and Ni/Al2O3 using isotopically labeled molecules. It was found that the quantity of reversibly adsorbed CH4 and the active carbon-containing intermediate species in the carbon pathway to form CO originating from CH4 is higher than the respective quantities derived from the CO2 molecule. In fact, carbon dioxide was included in biomass gasification gas. In this context, it has been considered of interest to study the performance of Ni/PG in a fixed bed reactor at temperatures between 650 and 800 °C with CO2. In this work, the influence of Ni

loading on H2 yields is analyzed. The influence of CO2 concentration (C(CO2)/C(C7H8)) on H2 yields is analyzed with C(CO2)/C(C7H8) ratios varied from 0.9 to 4.54. Especially, CO2-TPD and TPO are applied to investigate the effect of the introduction of Ni/NiO on sorption of CO2, to investigate the role of CO2 in the catalytic reforming reaction and to evaluate the catalytic performance of Ni/PG.

2. Experimental 2.1. Experimental system The experimental system is a bench-scale installation using a fixed reactor. A schematic of installation is shown in Fig. 1. The reactor is made of quartz reactor and the inner diameter of reactor is 25 mm. The evaporating pot is also made of stainless steel with cavity volume of 84.78 cm3. Toluene is introduced into the evaporating pot using a peristaltic pump and then nitrogen is introduced into the evaporating pot and carries toluene gas into the quartz reactor. Toluene is delivered by peristaltic pump with a flow rate of 5.92 mg/min. The gases needed in this installation (hydrogen for reducing the catalyst, argon for chromatograph and nitrogen) are metered by a mass flow controller. The H2 concentration in the exit gas is collected and determined by gas chromatography (GC-7890T). One column (C-2000) is used with TCD detector and argon is used as carrier gas. The time required for product gas analysis is 10 min. The experimental system worked at atmospheric pressure and reaction time was 20 min. All the experiments were carried out at temperatures between 650 and 800 °C. The total gas flow rate is 250 cm3/min using nitrogen as carrier gas. The nitrogen gas flow rate is from 200 to 250 cm3/ min and CO2 flow rate is from 0 to 50 cm3/min with ratios of C(CO2)/C(C7H8) from 0 to 4.54. The inlet toluene-feeding rate is constant. The catalytic reaction bed is composed of silica wool as filter supporting bed and catalyst (2 g) with a size of 0.85–2 mm. 2.2. Catalysts preparation The catalysts were prepared in our laboratory by incipient impregnation. Three catalysts were prepared and Ni loading is 0, 2, 5 and 8 wt%. A defined concentration of solution of nickel nitrate and PG were mixed and then were strongly stirred. The mixed samples were dried in vacuum conditions at 105 °C, smashed and sieved (0.85–2 mm). The particles were calcined in nitrogen atmosphere at 500 °C for 3 h and reduced by hydrogen at 500 °C for 2 h. The flow rate of hydrogen was 100 mL/min. The catalysts were labeled as NiX/PG, X represents Ni loading. The calcined catalysts were characterized by X-ray diffraction (XRD). The morphology of reduced catalysts is characterized by transmission electron microscope (TEM). The selected area electron diffraction and energy spectrum (EDS) were also employed to characterize catalysts.

Fig. 1. Schematic of the experimental system.

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2.3. Temperature programmed oxidation (TPO) and temperature programmed desorption (CO2-TPD) Temperature programmed oxidation (TPO) is a procedure that allows exposure of a sample to a gaseous flow of oxygen diluted in an inert gas during an increase in temperature. CO2, CO and H2O formed and O2 consumed are quantified versus temperature by a mass spectrometer (HIDEN, QIC-20). TPO analysis was performed with 20 mg of catalyst placed in a U-shaped quartz tube. The oxidation gaseous mixture (O2 = 5 mL/min and Ar = 45 mL/ min) was passed through the reactor heated from 50 to 800 °C (10 °C/min). The surface adsorption property of CO2 of the calcined catalysts was measured by temperature-programmed desorption of carbon dioxide (CO2-TPD). Experiments were carried out in a flow-through quartz reactor using a mass spectrometer as the detector for the down-stream gas flow. Argon was the carrier gas at a flow rate of 80 ml/min. Before starting the TPD experiment, the sample (250 mg) was pre-treated with argon at 650 °C for 1 h and cooled to room temperature. Then CO2 (15 ml/min) was introduced into quartz reactor for 1.5 h. Following this adsorption, the sample was flushed with argon at room temperature for 2.5 h to remove any physisorbed CO2. After the argon was flushed, the sample was heated to 900 °C at a heating rate of 10 °C/min in an argon flow. The desorbed CO2 was measured by the mass spectrometer with the signal taken to be m/e = 44.

3. Results and discussion 3.1. Catalyst characterization The XRD spectra of the reduced catalysts with different Ni loading are shown in Fig. 2. These peaks at 2h = 8.3°, 13.7°, 19.7°, 24° and 20.8°, 26.5° are observed and attributed to palygorskite and quartz, respectively. When Nix/PG is reduced at 500 °C, a new phase was found at 2h = 37.2°, 43.2°, 62.6° and ascribed to nickel oxide, especially on Ni8/PG. Previous report [34] has showed that NiO had a reduction peak at around 470 °C according to the result of H2-TPR. In addition, the peak had a dramatic decrease after the NiO was supported on PG and a new peak was observed at higher temperature over 500 °C. The latter peak was ascribed to the interaction between PG and NiO resulting to the increase of reduction temperature. Therefore, Ni and NiO coexist on the surface of PG under the annealing temperature of 500 °C. The reason that Ni phase is unable to find in the XRD patterns should be assigned to the low amount or/and the very small grain size of crystal. Meanwhile, reflection intensity of palygorskite decreased obviously, which

Q

Relative intensity/C.p.s.

2000

3.2. Effect of reaction temperature and C(CO2)/C(C7H8) ratio on H2 yield Fig. 5 shows the effect of reaction temperature on CO2 reforming of toluene on Nix/PG (x = 0, 2, 5, 8). The experiments are carried out at different temperatures varying from 650 to 800 °C and different C(CO2)/C(C7H8) ratio ranged from 0 to 4.54. The H2 yield experiences a little decrease with increasing reaction temperature regardless of the Ni loading. Furthermore, H2 yield increases a little and then decreases with the increase of C(CO2)/C(C7H8) ratio. Undoubtedly, the H2 yield process changed with the ration of C(CO2)/C(C7H8). The main reactions are as follows: Catalytic cracking reaction

P---Palygorskite Q---Quartz N---Nickel oxide

P

N

N

P Q P

P

contributes to the change of palygorskite structure under high temperature. The TEM, selected area electron diffraction and EDS of Ni5/PG are shown in Fig. 3. Some crystal grains, which are identified as nickel oxide according to the XRD pattern, are found and highly dispersive on palygorskite crystals. In addition, Ni is observed on the Ni5/PG except Mg, O, Si, etc., which in part demonstrates the existence of NiO on palygorskite crystals. The CO2-TPD of Ni5/PG and PG is shown in Fig. 4. Three desorption peaks are observed for Ni5/PG and PG. The three desorption temperatures are about 100, 475 and 700 °C, respectively. Especially, the first peak and the third peak become large after Ni loading on PG. To the best of our knowledge, the surface adsorption water, crystal water and structural water of PG would be removed gradually with increasing temperature. Therefore, the change of desorption temperature of CO2 is ascribed to desorption of different water on/in PG. The first adsorption of CO2 to PG should be contributed to physical adsorption, so have a low desorption temperature. The second and the third adsorption should be assigned to chemisorptions between CO2 and PG which gives birth to a strong bonding force. However, the addition of Ni considerably improves the adsorption capacity of Ni5/PG, especially the first and the third adsorption site. The increase of adsorption capacity of Ni5/PG should be contributed to the interaction of NiO or/and Ni and CO2 related to the bonding between Ni species and O species except the interaction between CO2 and PG due to the introduction of NiO and Ni. However, as well known [35], the introduction of CO2 is able to reduce carbon deposition based on the reaction of NiC + CO2 = Ni + 2CO, which indicates the interaction between Ni and O species is not very stronger. Otherwise, desorption of CO does not occur at relative low temperature and the objective of reducing carbon deposition does not come true. Therefore, the increase of adsorption capacity of CO2 is absolute coupling between PG, Ni/NiO and CO2. In a word, the addition of Ni makes PG more feasible to adsorption more CO2 to react with carbon deposit, especially the improvement of adsorption capacity of Ni5/PG at high adsorption temperature.

N PG Ni2/PG Ni5/PG Ni8/PG

10

20

30

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50

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70

2 theta/degree Fig. 2. XRD patterns of NiX/PG (x = 0, 2, 5, 8).

Fig. 3. TEM images (A) and EDS (B) of Ni5/PG.

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MS intensity of (m/e=44) CO2

702

more H2 is consumed by the water–gas shift reaction than H2 produced by catalytic reaction of the toluene, which results to the decrease of H2 yield at high temperature. As we know, the dry reforming reaction precede water–gas swift, which lead to the increase of H2 yield at low C(CO2)/C(C7H8) ratio. However, water–gas shift need to consume CO2. Therefore, high C(CO2)/C(C7H8) ratio benefits the reaction and result to the decrease of H2 yield with the increase of C(CO2)/C(C7H8) ratio. Consequently, H2 yield experiences an increase and then decrease with increasing C(CO2)/C(C7H8) ratio. Moreover, different reaction temperature has a different effect on the H2 yield under the varying C(CO2)/C(C7H8) ratio. When temperature comes to 650 °C, the inflect point of H2 yield occurs as the C(CO2)/C(C7H8) ratio is 0.91 as seen Fig. 6. However, when temperature increases to 800 °C, the reflect point occurs as the C(CO2)/ C(C7H8) ratio is 1.81. This phenomenon is ascribed to the joint effect of the temperature and CO2 amount on the several reactions mentioned above.

Ni5/PG

PG

100

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600

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800

Temperature/ oC Fig. 4. CO2-TPD Ni5/PG and pure PG.

100

3.3. Effect of Ni loading on H2 yield

650oC(PG) 650oC

Ni2/PG 80

The effect of Ni loading on H2 yield at different temperature and C(CO2)/C(C7H8) ratio is shown in Fig. 6. The results show a significant increase in H2 yield after the addition of Ni, which indicates the activity of Ni to catalytic CO2 reforming of toluene or/and to catalytic cracking of toluene. On the one hand, at low reaction temperature (650 °C), the effect of Ni loading is more evident than that at high temperature (800 °C). H2 yield over Ni8/PG decreases more obvious than that over Ni2/PG and Ni5/PG. The other hand, as mentioned above, high temperature (800 °C) does not support the increase of H2 yield, which ascribed to the water–gas reaction. The relative low Ni loading (2% or 5%) benefit the increase of H2 yield, which should be attributed to the optimum dispersive of active composition on PG. It is proposed that high loading (8%) of Ni decreases the dispersion and high temperature also makes the aggregation of active composition. Therefore, H2 yield decreases at high temperature (800 °C) and high loading (8%) of Ni.

750oC

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800oC

40 20 1000

Ni5/PG

H2 yield/mL

80 60 40 20 1000

Ni8/PG

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650 C

PG Ni2/PG Ni5/PG

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H2 yield/mL

0

Ni8/PG

C(CO2)/C(C7H8) Fig. 5. Effect of reaction temperature and C(CO2)/C(C7H8) ratio on H2 yield.

C7 H8 ! C þ H2 þ CH4 þ C2 H4 þ Cn Hm þ   

b

80

PG o

800 C

Ni2/PG

ð1Þ

Dry reforming

C7 H8 þ 7CO2 ! 14CO þ 4H2

1000

Ni5/PG Ni8/PG

60

ð2Þ 40

Water–gas shift

H2 þ CO2 ! CO þ H2 O

ð3Þ

The water–gas shift reaction is significantly affected by reaction temperature. The water–gas shift reaction consumes H2, CO2and produces CO, thus significantly affects the H2 yield. When the reaction temperature increased, the catalytic cracking reaction of toluene and water–gas shift reaction are also strengthened. But

20 0

0

1

2

3

4

C(CO2)C(C7H8) Fig. 6. Effect of Ni loading on H2 yield (a: 650 °C; b: 800 °C).

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3.4. Effect of reaction temperature, C(CO2)/C(C7H8) ratio, and Ni loading on carbon deposit

650 °C. The results show that catalytic CO2 reforming of toluene over Nix/PG (x = 2, 5, 8) is effective.

Fig. 7 presents the effect of reaction temperature on carbon deposit over Ni5/PG and the effect of Ni loading on carbon deposit at reaction temperature of 800 °C. It can be observed that carbon deposit decreases by 0.8–1.8% dramatically after the addition of CO2 regardless of the reaction temperature, Ni loading, and C(CO2)/ C(C7H8) ratio. Fig. 7a shows when reaction temperature increases from 650 to 800 °C, carbon deposit decreases by 0.7–1.3% regardless of the C(CO2)/C(C7H8) ratio. Meanwhile, carbon deposit decreases evidently with the increase of reaction temperature at any C(CO2)/C(C7H8) ratio. Besides, the addition of CO2 reduces the carbon deposit dramatically at any reaction temperatures, however, no obvious changes of carbon deposit are observed when the ratio is over 0.9. The reaction formula between CO2 and carbon deposit is as follow. It is speculated that C(CO2)/C(C7H8) ratio low 0.9 is enough to react with carbon deposit. So, no obvious changes of carbon deposit are observed when C(CO2)/C(C7H8) ratio is over 0.9.

3.5. TPO of reacted catalysts

C þ CO2 ¼ 2CO

ð4Þ

Fig. 7b displays that carbon deposit decreases by 0.8% or so over Nix/PG (x = 2, 5, 8) after the addition of CO2. The carbon deposit over Ni5/PG and Ni8/PG is lower than that over Ni2/PG when C(CO2)/C(C7H8) ratio is lower than 0.9, which should be ascribed to the more reactivity sites after high loading of Ni supporting on PG. By the comparison of Fig. 7a and b, it is found that high temperature benefits the decrease of carbon deposit, which is assigned to the endothermic reaction (4). Besides, it is concluded that high temperature (800 °C), high loading of Ni (5% or 8%), and the addition of CO2 play an important role in decreasing carbon deposit. The carbon deposit is lower than 0.06%, when Ni loading is over 2% and reaction temperature is 800 °C, proves that the addition of CO2 can inhibit the formation of carbon deposit and produce more CO (reaction (4)). The carbon deposit is lower than 0.36% when Ni loading comes to 5% and reaction temperature is only

TPO profiles of reacted catalysts as a function of different C(CO2)/ C(C7H8) ratio is presented in Fig. 8a. Three CO2 peaks are observed from the TPO and assigned to carbonate, inactive carbon, and metal carbon composite according the report [36], respectively. After the introduction of CO2, the intensity of CO2 experiences a dramatic decrease, especially the inactive carbon (the second CO2 peak). This discloses that the introduction of CO2 strongly enhances the resistance of catalysts to carbon deposit. However, when C(CO2)/C(C7H8) ratio is over 0.9, no obvious changes of CO2 signal are observed, which denotes this ratio of C(CO2)/C(C7H8) is enough to resist carbon deposit agrees well with the carbon deposit detected by TC. Besides, high C(CO2)/C(C7H8) ratio makes against to H2 yield (Fig. 5). Therefore, high C(CO2)/C(C7H8) ratio is not necessary for catalytic CO2 reforming of toluene. As shown in Fig. 8b, CO2 intensity (especially the inactive carbon and carbonate peak) has an obvious decrease with increasing reaction temperature, especially as the temperature increasing to 800 °C. As mentioned above, high temperature supports the reaction (4), so high temperature possesses high resistance to carbon deposit. However, high temperature goes against to H2 yield (Fig. 5). Therefore, high reaction temperature is not necessary for catalytic CO2 reforming of toluene. Fig. 9 displays the TPO profiles of reacted catalyst as a function of different Ni loading. As shown in Fig. 9a, CO2 intensity has a considerable increase with the increasing Ni loading, which suggests that high Ni loading favor the formation of carbon deposit without the introduction of CO2. As we know, the more the reactivity sites, the better the catalytic cracking of toluene. Consequently, high Ni loading favors the catalytic cracking of toluene without CO2 resulting to high carbon deposit and high H2 yield (Fig. 6a). When CO2 is

650oC Ni5/PG

2.5 Ni5/PG

a

o

650 C

2.0

a

Cc /Ct =0

5E-10

Cc /Ct =0.91

o

750 C

Cc /Ct =1.81

o

1.5

800 C

Cc /Ct =2.72

0.5 0.0

b

o

800 C

Ni2/PG

1.0

Cc /Ct =4.54

MS intensity of CO2 (a.u)

Carbon deposite/%

1.0

C c /Ct=4.54 Ni5/PG

b

650oC

Ni5/PG

750oC

Ni8/PG

800oC

0.5

0.0

0

1

2

3

4

C(CO2)/C(C7H8) Fig. 7. Effect of reaction temperature and Ni loading on carbon deposit (a: Ni5/PG; b: 800 °C).

100

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Temperature/oC Fig. 8. TPO profiles of reacted Ni5/PG with different Cc/Ct (a) and different reaction temperature (b).

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a

o

650 C 4E-10

MS intensity of CO2/a.u.

Ni2/PG Ni5/PG Ni8/PG

o

b

800 C

Fig. 10. Flow chart of toluene dehydrogenation.

Ni2/PG Ni5/PG Ni8/PG

of toluene, which decreases carbon deposit, and react with H2 resulting to the decrease of H2 yield. The decrease of carbon deposit and H2 yield is in positive relative to the increase of reaction temperature because high temperature favors the two reactions. 4. Conclusions

100

200

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500

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800

o

Temperature/ C Fig. 9. TPO profiles of reacted different catalysts with different reaction temperature (a: 650 °C; b: 800 °C).

introduced (C(CO2)/C(C7H8) ratio 4.54), CO2 peak intensity decreases significantly over Ni5/PG and Ni8/PG (Fig. 9b) compared to without CO2 (Fig. 9a). Meanwhile, no obvious change is found over Ni2/PG between with and without CO2. The three catalysts almost have the same CO2 peak area, namely the same carbon deposit. Evidently, Ni2/PG has an inferior catalytic reactivity giving birth to relative low carbon deposit and H2 yield. Therefore, CO2 really has good capacity in removing carbon deposit and Ni5/PG and Ni8/PG show better catalytic reactivity.

3.6. Decomposition mechanism The products of catalytic CO2 reforming of toluene over contain H2, H2O, CO, CxHy with two to five carbons, benzene, and benzene series (not shown here). According to the catalytic cracking reaction of toluene (1), C, H2, and CxHy are the main products. However, the introduction of CO2 gives birth to CO which results from the catalytic CO2 reforming of toluene and the reaction between CO2 and C. The later reaction meanwhile greatly makes against the formation of carbon deposit. Therefore, CO2 plays two important roles in catalytic CO2 reforming of toluene. The addition of Ni considerably improves the catalytic reforming reaction compared to the pure PG by increasing the adsorption of CO2. Wang [37] reported that five paths of dehydrogenation reaction of toluene are speculated (Fig. 10). Dehydrogenation reaction easily occurred according to path 1 and path 2, but dehydrogenation according to path 3, path 4 and path 5 needs to absorb more energy. Therefore, according to thermodynamics, toluene dehydrogenation reaction-sequence was: path 1 < path 2 < path 3  path 4  path 5. Based on the results, the reaction process was suggested as follows. Firstly, the toluene is adsorbed onto reactive sites where metal-catalyzed dehydrogenation occurs. Carbon dioxide is also adsorbed onto the catalyst support, generating CO and O. At an appropriate temperature, the O migrates to the reactive sites and leads to the oxidation of toluene mainly producing CO and H2. Meanwhile, CO2 also react with C resulting from catalytic cracking

Palygorskite-supported Ni with high dispersion is prepared. The addition of Ni greatly increases the adsorption of CO2 on PG. The Ni/PG catalysts play a significant role in catalytic CO2 reforming of toluene result to the increase of the H2 yield. Ni loading affects the H2 yield and the order is as follows: PG < Ni2/PG < Ni5/ PG  Ni8/PG. In addition, when the C(CO2)/C(C7H8) ratios increases from 0 to 4.5 or the reaction temperature increases from 650 to 800 °C, the H2 yield has a slight decrease, however, carbon deposit also experiences a decrease. The introduction of CO2 promotes the generation of CO and inhibits the formation of carbon deposit. Acknowledgements The authors express their gratitude to the national Natural Science Foundation of China (No. 50774027), Nationl High-Tech Research and Development Program (863) project (No. 2007AA06Z118) for providing financial support for the work. References [1] Christopher B, Field J, Elliott C, David BL. Biomass energy: the scale of the potential resource. Trends Ecol Evol 2008;23(2):65–72. [2] Li CS, Suzuki K. Tar property, analysis, reforming mechanism and model for biomass gasification – an overview. Renew Sust Energ Rev 2009;13:594–604. [3] Devi L, Ptasinski KJ, Janssen FJJG. A review of the primary measures for tar elimination in biomass gasification processes. Biomass Bioenergy 2003;24:125–40. [4] Zhang RQ, Brown RC, Suby A, Cummer K. Catalytic destruction of tar in biomass derived producer gas. Energy Convers Manage 2004;45:995–1014. [5] Bona S, Guillen P, Alcalde JG, Garcia L, Bilbao R. Toluene steam reforming using coprecipitated Ni/Al catalysts modified with lanthanum or cobalt. Chem Eng J 2008;137:587–97. [6] Li CS, Hirabayashi D, Suzuki K. Development of new nickel based catalyst for biomass tar steam reforming producing H2-rich syngas. Fuel Process Technol 2009;90:790–6. 2 [7] Li CS, Hirabayashi D, Suzuki K. A crucial role of O on mayenite 2 and O2 structure for biomass tar steam reforming over Ni/Ca12Al14O33. Appl Catal B 2009;88:351–60. [8] Kuhn JN, Zhao ZK, Naber AS, Felix LG, Slimane RB, Choi CW, et al. Ni-olivine catalysts prepared by thermal impregnation: structure, steam reforming activity and stability. Appl Catal A 2008;341:43–9. [9] Swierczynski D, Courson C, Kiennemann A. Study of steam reforming of toluene used as model compound of tar produced by biomass gasification. Chem Eng Process 2008;47:508–13. [10] Swierczynski D, Libs S, Courson C, Kiennemann A. Steam reforming of tar from a biomass gasification process over Ni/olivine catalyst using toluene as a model compound. Appl Catal B 2007;74:211–22. [11] Kuhn JN, Zhao ZK, Felix LG, Slimane RB, Choi CW, Ozkan US. Olivine catalysts for methane- and tar-steam reforming. Appl Catal B 2008;81:14–26.

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