Influence of Mixed Supports on the Steam Catalytic Reforming of

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Influence of Mixed Supports on the Steam Catalytic Reforming of Wood Vinegar Xiwei Xu,*,†,‡,§ Enchen Jiang,*,† Yan Sun,† and Zhiyu Li† †

College of Materials and Energy, South China Agricultural University, Guangzhou 510640, China Key Laboratory of Biomass Energy and Material (KLBEM), Jiangsu Province, Nanjing 210007, China § State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, China ‡

ABSTRACT: Obtaining hydrogen from renewable energy is a novel method. Instead of single support catalysts, mixed zeolite supports were used in this study to obtain hydrogen by steam catalytic reforming of wood vinegar. This technique improves the activity and extends the life of catalysts. The results show that, compared with a single support catalyst, mixed supports are beneficial for improving H2 concentration and anticarbon ability. H2 content and yield increased from 54.77% to 60.32% and 18.91 mg/g sample to 22.47 mg/g sample, respectively, when γ-Al2O3 mixed MCM-41 supports were used instead of single γAl2O3. Moreover, the amount of carbon deposition significantly decreased from 9.79% to 5.85%. The admixture of support γAl2O3 with MCM-41 optimized the diversity of acidic intensity, structure, pore characteristics, and heat and mass transfer ability of mixed catalysts, and improved the reactivity and resistance to carbon deposition of catalysts. presence of Ni. Moreover, Deshmane et al.13 found out that the addition of Sn can effectively improve the catalyst’s competence to prevent carbon deposition. Zeolite is gaining popularity in the catalytic industry because of this material’s excellent pore structure and surface area. Uddin14 reported the approach of methane decomposition using zeolite Ni/Y catalyst in a fixed bed reactor to produce H2. The Ni/ZSM-5 catalyst is effective for the selectivity of H2 products.15 However, carbon deposition is a serious problem hindering the regeneration and life of catalysts. Many researchers have attempted to discover new catalysts that can reduce carbon depositions and solve the problem. Azad et al.16 found that carbon depositions on Ni/ZrO2 catalysts were significantly higher than that on Ni/γ-Al2O3, suggesting that carbon deposition is strongly dependent on the type of support. For example, Williams prepared several nickel-based catalysts with different supports (i.e., Ni/γ-Al2O3, Ni/MgO, Ni/CeO2, Ni/ZSM-5, Ni−Al, Ni−Mg−Al, and Ni/CeO2/γ-Al2O3) to produce hydrogen, and investigated the catalysts’ ability to prevent coke formation.16 Given its special pore structure, MCM-41 is beneficial for the steam catalytic reforming of macromolecular organic compounds (i.e., phenol derivate). Due to the larger pore size, the macromolecular organic compounds will catalytically crack into the intermediate products, which are small molecular organic compounds. And the intermediate can easily go through micropore zeolites and undergo further reactions. MCM-41 plays an important role in preventing macromolecular organic compounds from forming carbon on the surface or in the pores of micropore zeolites during the reaction. Due to the pore structure, the acidic intensity and the BET of supports significantly influence the activity and carbon

1. INTRODUCTION The huge demand for fuels and chemicals is constantly growing as the world’s economy continues to develop.1,2 Obtained from fossil sources, synthesis gas is a building block in the chemical industry and in fuel synthesis.3,4 Obtaining synthesis gas from renewable energy is a promising method of reducing fossil and environmental crises. Biomass from agriculture and forest residues is one of the most abundant renewable resources.5,6 Wood vinegar feedstock was the light liquid in the upper of the crude bio-oil from the pyrolysis of biomass. It appeared as a dark brown liquid with a strong smoky smell. The main components of wood vinegar were acetic acid and phenols. This substance cannot be used widely because of its high water content, low heat value, and complex compounds. However, the main components of wood vinegar, namely, acids, ketones, alcohol, and phenols, can be converted into synthesis gas by steam catalytic reforming. The steam catalytic reforming of wood vinegar is a new fuel processing technology that is environmentally friendly and promising for hydrogen generation purposes. Many researchers have proposed methods for obtaining hydrogen from renewable energy. Soares obtained H2 production by glycerol APR over Ptx-Fey/γ-Al2O3 catalysts at low temperature.7 Elia Gianotti found that partial dehydrogenation of fuels is a promising method to obtain high-purity hydrogen, which can feed a fuel-cell-based power unit.8 Numerous researchers have also explored hydrogen production via steam reforming of ethanol9,10 or acetic acid.11 During the process, catalysts play an important role. Many researchers have found that small organic molecules in bio-oil, such as acids, aldehydes, and ketones, can be effectively converted into hydrogen by Ni-base catalysts. For example, Wang et al.12 proved that the reactivity and selectivity of Nibased and Ru-based catalysts in obtaining hydrogen from acetic acid catalytic reforming are higher when catalysts with different metal active materials are chosen. The catalytic activity of catalysts remains high even at low temperatures with the © XXXX American Chemical Society

Received: November 14, 2016 Revised: December 30, 2016 Published: January 13, 2017 A

DOI: 10.1021/acs.energyfuels.6b03000 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

3. RESULTS AND DISCUSSION 3.1. Characterization of Fresh Catalysts. 3.1.1. N2Physisorption Analysis. The BET surface area, the pore

resistance of catalysts. Therefore, we chose different styles of supports, such as micropore, mesopore zeolites, and γ-Al2O3, and we analyzed the effect of single and mixed supports on the activity and carbon resistance of catalysts. In conclusion, we choose Sn or Ni as the active metal and used catalysts that mixed micropore and mesopore zeolite supports, aiming to improve the catalytic activity, reduce the amount of carbon deposition, and extend the life of the catalysts.

Table 1. N2-Physisorption Data of Catalysts with Different Supports

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The supports ZSM-5, HBeta, γ-Al2O3, and MCM-41 were purchased from the catalyst plant of the University of Nankai. The mixed support γ-Al2O3 + MCM-41 was made of the same masses of γ-Al2O3 and MCM-41 by mechanical mixing. The method for preparing ZSM-5 + MCM-41 and HBeta + MCM-41 is the same as that for preparing γ-Al2O3 + MCM-41. SnCl2 and nickel nitrate nonahydrate (Sigma) were dissolved in hydrochloric acid aqueous solution. The molar ratio of Ni and Sn was 3:1. The contents of Ni and Sn are 10% and 3.3%. The single support ZSM-5, HBeta, and γ-Al2O3 and mixed support ZSM-5 + MCM-41, HBeta + MCM41, and γ-Al2O3 + MCM-41 (d < 0.45 mm) were impregnated in the solution. The mixture was subjected to ultrasonic shaking at room temperature for 30 min and maintained at room temperature for 2 h. The mixture was dried at 100 °C for 24 h and then calcined at 500 °C for 3h in air. 2.2. Characterization. The amount of carbon deposition on the spent catalysts was tested with the STA449CJupiter thermogravimetric (TG-DSC) analyzer. The heating rate was 10 °C/min The scanning electron microscopy (SEM) pattern and the Fourier transmission infrared (FT-IR) spectra of the surface morphology of the fresh catalysts and the carbon deposited on the spent catalysts were analyzed in the same way as in our previous research. 2.3. Catalytic Runs. The experimental setup is the same as that in our previous research.17 Carrier gas N2 (100 mL/min) was introduced with a mass flow controller. Bio-oil was injected with a peristaltic pump. The fixed bed was made of a 30 mm i.d. quartz tube, and 10 g of catalyst was placed in the middle of the tube every time. And the feed rate is 16 g/h. The reaction temperature is 600 °C. Before the reaction, reduction of catalysts was taken at 750 °C for 1 h with 10%/ 90% H2/N2. The part of the quartz tube which got out of the oven was kept at 250 °C to avoid condensation of bio-oil. The wood vinegar sample was extracted with CCl4, and then the solvent was removed by vacuum distillation, and the sample was analyzed by GC-MS. The condition 19091IV-136INNOWAX capillary column was chosen. The initial temperature was set at 45 °C and held for 10 min, and the temperature was increased to 120 °C at the heating rate of 10 °C/min. Finally, the temperature was increased to 250 °C at the heating rate of 5 °C/min. The temperature at the entrance was 250 °C, and the ratio of diversion was 10:1 under the nitrogen flow rate of 1 cm3/min. The condition of MS:EI was chosen as the ion source. And the gas products were analyzed by GC using a gas chromatograph (Agilent 19091N-133) equipped with a HP-plot-Q column (30 m × 250 μm × 0.25 μm) and a TCD detector.

WCH 4 =

VCH 4 × 100% VH 2 + VCH 4 + VCO2 + VCO

(1)

WCO2 =

VCO2 × 100% VH 2 + VCH 4 + VCO2 + VCO

(2)

Ni/Sn/HBeta Ni/Sn/HZSM-5 Ni/Sn/γ-Al2O3 Ni/Sn/MCM-41 Ni/Sn/HBeta+MCM-41 Ni/Sn/HZSM-5+MCM-41 Ni/Sn/γ-Al2O3+MCM-41

45 64 54 31 283 246 103

Total pore volume 3 BET[cm /g] 0.13 0.17 0.34 0.20 0.30 0.26 0.28

(P/P0 (P/P0 (P/P0 (P/P0 (P/P0 (P/P0 (P/P0

= = = = = = =

0.99) 0.99) 0.99) 0.99) 0.99) 0.99) 0.99)

Pore size [nm] 4 3 10 12 − − −

Figure 1. N2-physisorption measurement of single support and mixed support catalysts.

volumes, and the microporosity of the single and mixed support catalysts were determined from their N2 adsorption isotherms. The results are summarized in Table 1. The Ni/Sn/ZSM-5, Ni/ Sn/γ-Al2O3, Ni/Sn/HBeta, and Ni/Sn/MCM-41 catalysts showed relatively low surface areas of 64, 54, 45, and 31 m2/ g, respectively, thereby indicating that these catalyst nearly had no pores accessible for N2 molecules, and the channels were fully filled. The BET surface area of mixed supports fluctuated between 103 and 283 m2/g, which was much higher than those of single support catalysts. Besides, the pore size for mixed supports catalysts was much larger than the single supports catalysts. It was possible that the new materials with different

2.4. Definition. WCH4 is calculated using eq 2, where VCH4, VCO2, and VH2 are the volumes of CH4, CO2, and H2 in the gas sample.

H2 yield =

Catalysts

BET Surface area[m2/g]

WH2∗ V ∗ 2 × 100% (mg/g sample) mass of feedstock ∗22.4 (3)

V: the volume of products. B


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Energy & Fuels

Table 3. Composition and physical properties of wood vinegar from pine nut shells Water content (wt %)

Viscosity at 20 °C (cSt)

pH

74

1.08

2.9

Table 4. Composition Range of Wood Vinegar

Figure 2. NH3-TPD pattern of the Ni/Sn/support catalysts.

structure were formed between the two different supports in the mixed catalysts. For all single support catalysts, the shape of their isotherms is that of a Type-V isotherm in Figure 1, thereby suggesting microporosity development. Not only micropores but also mesopores were found at relatively high pressure (Figure 1). Nitrogen adsorption/desorption isotherms of mixed support catalysts can be considered as type V. It was accepted that type IV is related to the mesoporous materials. The isotherms are associated with an H4 type hysteresis loop for Ni/Sn/ZSM-5, Ni/Sn/HBeta and Ni/Sn/ZSM-5+MCM-41, Ni/Sn/HBeta +MCM-41 and with an type-H3 for Ni/Sn/γ-Al2O3 and Ni/ Sn/γ-Al2O3+MCM-41, which indicates that there are aggregated or agglomerated particles with slit-shaped pores in the materials, and the H4 type hysteresis loop induced that the pores in the catalysts formed via materials with layered structure. 3.1.2. Analysis of NH3-TPD. The NH3-TPD patterns of single support and mixed support catalysts are presented in Figure 2. The results show that there are weak acidic sites for all catalysts. There are only very weak acidic sites for Ni/Sn/ MCM-41 catalysts. Because MCM-41 was made of Si, in the case of Ni/Sn/ZSM-5, Ni/Sn/γ-Al2O3, and Ni/Sn/HBeta, NH3 desorption peaks appeared among 200−400 °C, which are assigned to moderate acidic sites. The strong acidic sites for mixed support catalysts Ni/Sn/ HBeta+MCM-41, Ni/Sn/ZSM-5+MCM-41, and Ni/Sn/γAl2O3+MCM-41 centered at Tmax of 520, 535, and 602 °C, respectively, are much stronger than single support catalysts. Moreover, the amount of strong acidic sites for all mixed support catalysts are significantly higher than those for single support catalysts. This is possibly the reason why the activity and carbon resistance of mixed support catalysts are higher than those of single support catalysts. It is also indicated that there are new phases formed between the different supports for mixed support catalysts. It is possible that new materials form during the process of preparation of the catalyst. That means there is a part of the different single support resolving in the

Molecular formula

No.

RT

Name

1 2 3

6.716 7.556 7.728

4 5 6 7 8 9

7.802 8.848 9.029 9.095 9.679 9.852

10 11 12 13 14

10.016 11.317 12.814 12.880 13.539

15 16 17 18 19 20 21 22 23 24 25 26 27

13.966 15.349 16.139 16.213 16.625 17.596 17.744 17.851 18.048 19.118 19.299 19.678 22.385

28 29

23.307 27.685

2-Propanone, 1-hydroxy2-Cyclopenten-1-one 2-Cyclopenten-1-one, 2methyl1-Hydroxy-2-butanone Acetic acid 1,2-Ethanediol, diacetate Furfural 2-Furanmethanol, tetrahydro2-Cyclopenten-1-one, 3methylPropanoic acid Butyrolactone 1,2-Cyclopentanedione 1,2-Cyclopentanedione 1,2-Cyclopentanedione, 3methylPhenol, 2-methoxyPhenol, 2-methoxy-4-methylPhenol, 2-methylPhenol Phenol, 4-ethyl-2-methoxyPhenol, 4-methylPhenol, 4-methylButanal, 3-methylPhenol, 2-methoxy-4-propylEugenol Phenol, 3,4-dimethyl2-Methoxy-4-vinylphenol Phenol, 2-methoxy-4-(1propenyl) Anthracene, 2-methylBenzeneacetic acid, 4-hydroxy3-

Area%

C3H6O2 C5H6O C6H8O

11.04 1.49 0.49

C4H8O2 C2H4O2 C6H10O4 C5H4O2 C5H10O2 C6H8O

1.55 20.72 1.40 3.41 1.25 1.06

C3H6O2 C4H6O2 C4H4O2 C5H6O2 C6H8O2

2.17 0.84 1.03 1.48 2.84

C7H8O2 C8H10O2 C7H8O C6H6O C9H12O2 C7H8O C7H8O C5H10O C10H14O2 C10H12O2 C8H10O C9H10O2 C10H12O2

12.54 9.09 1.06 2.33 6.43 1.41 1.09 2.51 1.53 0.96 1.47 1.09 3.15

C15H12 C9H10O4

1.90 2.69

solution of nickel nitrate and iron nitrate during the wet impregnation that forms a new structural material. 3.2. Composition and Properties of Pine Nut Shell and Wood Vinegar from the Pine Nut Shell. Table 2 shows the physical and chemical properties of pine nut shells. The content of volatile matter is about 72.10%, which is the resource of wood vinegar. And the sulfur content is lower, which indicates pine nut shell is an environmentally renewable energy. Wood vinegar is obtained from the pyrolysis of pine nut in a fixed bed. The feed was heated from room temperature to 500 °C at the rate of 10 °C/min and maintained for 1 h. Wood vinegar was tan in color and had a strong smoky smell. Table 3

Table 2. Physical and Chemical Properties of Pine Nut Shells Proximate analysis (as received, %wt)

Ultimate analysis (dry basis, %wt)

Heating value (MJ/kg)

Moisture

Volatile matter

Fixed Carbon

Ash

C

H

N

S

HHV

LHV

8.54

72.10

18.88

0.48

47.59

6.28

0.56

1.16

19.24

17.40

C


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Energy & Fuels Table 5. Effect of Temperature on the Steam Reforming of Wood Vinegar Gas composition (vol %)

Gas yield (mg/g sample)

Wgas/% Reforming Temp (°C)

V(mL/min)

H2

CO

CH4

CO2

C2H4

C2H6

H2

CO

CH4

CO2

C2H4

C2H6

500 600 700 800

21.46 47.05 87.50 114.29

19.75 33.52 33.26 42.24

26.76 16.53 10.51 8.45

22.94 22.69 25.91 20.82

24.59 22.50 27.03 26.27

5.76 4.53 3.20 2.22

0.20 0.23 0.09 −

1.51 5.63 10.39 17.24

28.71 38.89 45.98 48.29

14.07 30.50 64.78 67.99

41.46 83.18 185.83 235.90

6.18 10.66 14.00 12.69

0.23 0.58 0.42 −

Figure 3. Catalytic performance of single-support catalysts on wood vinegar steam catalytic reforming.

Table 6. Composition and Yield of Gases after Wood Vinegar Catalytic Reforming Gas composition (vol %)


Catalysts

V (mL/min)

H2

CO

CH4

CO2

C2H4

H2

CO

CH4

CO2

C2H4

Ni/Sn/HBeta Ni/Sn/HZSM-5 Ni/Sn/γ-Al2O3

114.75 90.78 96.68

52.65 52.28 54.77

15.88 21.73 12.91

10.29 9.52 10.21

17.69 13.90 20.76

3.34 2.61 1.35

21.58 16.95 18.91

91.11 98.63 62.41

33.74 24.69 28.20

173.99 108.16 172.04

19.16 11.85 6.53

shows the physical properties of wood vinegar. The water content is 74 wt %, and the viscosity at 20 °C is 1.08 cSt. Table 4 shows the composition range of wood vinegar. More than 30 organic materials were detected with GC-MS; these materials include acids, ketones, esters, and phenols. The content of acids and phenols is higher than that of the others.

The content of acetic acid and guaiacol is 20.72% and 12.54%, respectively. 3.3. Steam Catalytic Reforming of Wood Vinegar. 3.3.1. Effect of Temperature on the Steam Reforming of Wood Vinegar. Temperature plays an important role in the steam catalytic reforming of wood vinegar. The steam catalytic D


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Figure 4. Catalytic performance of different mixed support catalysts for wood vinegar steam reforming.

Table 7. Composition and Yield of Gases after Wood Vinegar Catalytic Reforming with Mixed Support Catalysts Gas composition (vol %)


Catalysts

V (mL/min)

H2

CO

CH4

CO2

C2H4

H2

CO

CH4

CO2

C2H4

Ni/Sn/HBeta + MCM-41 Ni/Sn/HZSM-5 + MCM-41 Ni/Sn/γ-Al2O3 + MCM-41

79.97 53.55 104.29

52.55 59.36 60.32

18.40 16.90 8.83

12.31 6.66 8.71

15.58 15.81 21.55

1.14 1.20 0.58

15.01 11.35 22.47

73.57 45.25 46.04

28.13 10.19 25.95

106.79 72.57 192.64

4.56 6.53 3.02

Table 8. Distribution of Products after Catalytic Reforming of Wood Vinegar catalysts Ni/Sn/HBeta Ni/Sn/HBeta + MCM-41 Ni/Sn/HZSM-5 Ni/Sn/HZSM-5 + MCM-41 Ni/Sn/γ-Al2O3 Ni/Sn/γ-Al2O3 + MCM-41

mass of reactant (g)

Liquid products (%)

Gas products (%)

Mass of carbon left (%)

28.9 29.6

47.09 53.61

49.45 44.36

3.46 2.03

30.4 29.5

52.53 57.46

44.18 39.83

3.29 2.71

31.2 30.6

52.08 49.97

44.71 48.07

3.21 1.96

Table 9. Conversion of Organic Material in the Wood Vinegar Catalytic Reforming

reforming of wood vinegar was investigated at 500 °C, 600 °C, 700 °C, and 800 °C. SiO2 was chosen as the catalyst alternative. The feed rate was 0.3 g/min, and the flow rate of carrier gas was 100 mL/min. Table 5 shows the influence of temperature on gas products. The gas flow rate and the gas yield were very low at 500 °C. The flow rate of the gas products increased from 21.46 mL/min

Catalysts

The organic material content of reactant (%)

The organic material content of left material (%)

Conversion (%)

Ni/Sn/HBeta Ni/Sn/HBeta + MCM-41 Ni/Sn/HZSM-5 Ni/Sn/HZSM-5 + MCM-41 Ni/Sn/γ-Al2O3 Ni/Sn/γ-Al2O3 + MCM-41

18 18 18 18 18 18

1.5 3.2 2.9 4.1 2.4 1.9

91.67 82.22 83.89 77.22 86.67 89.44

to 114.29 mL/min. The H2 yield increased from 1.51 mg/g to 17.29 mg/g sample. Wood vinegar can take the following reaction: wood vinegar → CO, H2, CO2, H2O, and CH4 + hydrocarbons. The reaction is endothermic. Therefore, high temperature is beneficial for gas yield. The steam catalytic reforming of gases, CO + H2O = CO2 + H2, is enhanced at high temperature. E


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Energy & Fuels Table 10. Conversion of Organic Material in the Wood Vinegar Conversion (%) catalysts

2-Propanone, 1-hydroxy-

Acetic acid

furfural

guaiacol

2-methoxy-4-methyl-Phenol

4-ethyl-2-methoxy-Phenol

Ni/Sn/HBeta Ni/Sn/HBeta + MCM-41 Ni/Sn/HZSM-5 Ni/Sn/HZSM-5 + MCM-41 Ni/Sn/γ-Al2O3 Ni/Sn/γ-Al2O3 + MCM-41

100% 100% 100% 100% 100% 100%

100% 100% 100% 100% 100% 100%

100% 100% 99% 97% 100% 98%

100% 100% 96% 95% 96% 100%

92% 94% 96% 95% 98% 96%

91% 90% 97% 92% 94% 97%

obvious that the acidic intensity and amount of HBeta and ZSM-5 are higher than γ-Al2O3 (in Figure 2). For Ni/Sn/ZSM5 and Ni/Sn/HBeta catalyst, it is generally accepted that the shape selectivity also plays an important role in the distribution of gas products. Moreover, it was obvious that the CO concentration competed with the H2 concentration for all catalysts. When the CO concentration increased, the H2 concentration decreased with time. It was possible that the organic compounds in wood vinegar trended to produce CO instead of H2 with time. For example, acetic acid took reaction to produce ketone (ketonization of acetic acid 2CH3COOH → (CH3)2CO + H2O + CO) instead of H2 (steam reforming of acetic acid: CH3COOH + 2H2O → 2CO2 + 4H2). Moreover, the reverse water gas shift reaction (H2 + CO2 = CO + H2O) became the main reaction with time instead of the water gas shift reaction (CO + H2O = CO2 + H2). Similarly, the CH4 concentration competed with the CO2 concentration. And the change of the main reaction was decided by the content of active materials and the resistance to carbon deposition of catalysts. Table 6 shows the content of gases after steam catalytic reforming. The hydrogen content in gas products by Ni/Sn/ HBeta catalyst is not the highest compared to other catalysts, but the gas flow rate is the highest, which reaches 114 mL/min; therefore, the yield of gases is higher with Ni/Sn/HBeta catalyst in 2 h. The hydrogen yield reaches up to 21.58 mg/g sample. In our previous research,18 the H2 yield is 7.10 mg/g sample and 15.29 mg/g sample with γ-A12O3 and Ni/γ-A12O3, respectively. In the study, the activity of Ni/γ-A12O3 has been enhanced when adding Sn. It is possible that Sn is usually used as an active material for organic catalytic reforming. Ferrari et al.19 reported that the addition of Sn into Ni/γ-A12O3 can improve the adsorption capacity of CO2 and CH4 because Sn can form a Sn−CO bond and a Sn−H bond with CO and CH4, respectively. The strong adsorption capacity from oxygen atom valence bonds can prevent carbon deposition. Moreover, the H bond on the surface of this catalyst can effectively prevent Ni oxidation. It was also possible that the surface acidity of catalysts can be changed by adding active materials, such as Ni and Sn. Active substances, such as Ni and Sn, cover acidic sites on the surface of the molecular sieve (i.e., Bronsted acid and Lewis acid), resulting in a decline of the total acidic amount of the catalyst surface. On the other hand, the synergy of unsaturated Ni cation on the catalyst surface would be formed as a new Lewis acid center to compensate for the previously covered Lewis acidic sites.20,21 Therefore, the decline of acidic sites indicated that the amount of acidic site was covered by Ni. 3.3.3. Effect of Mixed Support Catalysts on the Catalytic Reforming of Wood Vinegar. Ni/Sn/HBeta + MCM-41, Ni/

Figure 5. Catalytic performance of Ni/Sn/γ-Al2O3 + MCM-41 catalyst.

H2 increases with temperature. However, the content of CO, C2H4, and C2H6 demonstrates a downward tendency, as shown in Table 5. The content of CH4 and CO2 reached the peak at 700 °C. The yield of gases was significantly low without the catalyst, except CO2. Thus, most of the organic materials in wood vinegar were converted into CO2 without the catalyst. 3.3.2. Characteristics of Wood Vinegar Steam Catalytic Reforming by Single Carrier Ni/Sn Catalyst. The experimental conditions were as follows. The reaction temperature was 600 °C. The wood vinegar injection volume was 0.25 g/min. The flow rate of the carrier gas N2 was 100 mL/min. The catalyst volume was 10 g. The used catalysts were Ni/Sn/HBeta, Ni/ Sn/HZSM-5, and Ni/Sn/γ-Al2O3. The change in gas content with a single carrier catalyst is shown in Figure 3. When Ni/Sn/HBeta catalyst was used, the content of hydrogen remained at around 65% during the initial hour. Hydrogen content dropped significantly from 65% to 30% with time. CO content increased from 5% to 35%, and CH4 content increased from 5% to 16%. When Ni/Sn/HZSM5 catalyst was used, hydrogen content gradually fell to 40% from 65%. However, the content of CO, CH4, and C2 slightly rose. CO content increased from about 10% to 25%. The catalytic activity of Ni/Sn/γ-Al2O3 was more durable than that of other catalysts. The change in content of hydrogen was not obvious. It decreased from 65% to 40% for 2 h. CO content dropped slightly at first and then subsequently rose; by contrast, CO2 followed an opposite change trend. CH4 and C2 contents maintained a rising trend. The effect of supports on the catalytic reforming reaction of wood vinegar is obvious. The results show that single support Ni/Sn/HBeta catalyst is more active than other single support catalysts from Figure 3 and Table 6. It is possible that acidic intensity and amount of support play an important role. It is F


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Figure 6. SEM patterns of fresh and spent catalysts.

rate increased from 96 mL/min with Ni/Sn/γ-Al2O3 to 104 mL/min with Ni/Sn/γ-Al2O3 + MCM-41. The gas yield or gas content by Ni/Sn/HBeta + MCM-41 and Ni/Sn/HZSM-5 + MCM-41 catalyst are lower than those by single support catalysts. Mixed zeolites support catalysts are not beneficial for the catalytic reforming of wood vinegar for producing gas. However, the gas yield by Ni/Sn/γ-Al2O3 + MCM-41 catalyst is higher than both Ni/Sn/MCM-41 and Ni/ Sn/γ-Al2O3. The acidic amount and surface and pore structure play an important role. Not only the acidic amount and intensity of Ni/Sn/γ-Al2O3 + MCM-41 are higher than those of single support Ni/Sn/γ-Al2O3 catalyst, but also the surface area and pore size are bigger. Therefore, the gas yield is low and the carbon deposition is high when only γ-Al2O3 is used. The acidic amount, acidic distribution, BET surface area, and pore structure were all changed when mixed MCM-41 and γ-Al2O3 were used. Thus, the gas yield is improved, the carbon deposition is decreased, and the life of catalysts is extended. Besides, Huang22 investigated the thermal conductivity of MCM-41. And the results show that it is obviously anisotropic, that its largest value is along the length of the pores, and that it has quasi-one-dimensional characteristic. Moreover, MCM-41

Sn/HZSM-5 + MCM-41, and Ni/Sn/γ-Al2O3 + MCM-41 were chosen as catalysts, and the ratio is 1:1 based on mass. The reaction temperature was 600 °C, and the feed rate was 0.25 g/ min. The usage of catalysts was 10 g. The variety of gas contents is shown in Figure 4. The H2 content decreased from 60% to 53% after 2 h with Ni/Sn/ HBeta + MCM-41. The reduction in H2 content by the mixed support catalyst was lower than that with the single support catalyst. Thus, the catalyst life was extended by adding mesoporous support MCM-41 in the catalysts. The activity of Ni/Sn/γ-Al2O3 + MCM-41 was the best among the three mixed support catalysts. The H2 yield of 22.47 mg/g of sample was about 4 mg/g sample more than that with the single support. The content and yield of H2 were 65% and only decreased 4% after 2 h. The content of CO, CH4, CO2, and C2 remained unchanged. Tables 6 and 7 show that the gas flow rate is less when mixed support catalysts Ni/Sn/HBeta + MCM-41 and Ni/Sn/HZSM5 + MCM-41 are used than with single support catalyst. The gas flow rate decreased from 114.75 mL/min and 90.78 mL/ min to 79.97 mL/min and 53.55 mL/min, respectively. The H2 yield decreased significantly. On the other hand, the gas flow G


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Energy & Fuels

Figure 8. TPO pattern of spent catalysts.

Therefore, the mixture of MCM-41 and γ-Al2O3 enhanced the mass and heat transfer and improved the reactivity of catalysts. 3.3.4. Mass Balance and Mechanism during the Catalytic Steam Reforming of Wood Vinegar. 3.3.4.1. Distribution of Products. Wood vinegar contains some mixed organic materials with large viscosities. The feed rate is slightly different every time because of the large-viscosity material, although it has been stirred at low temperature before the reaction. Therefore, the total mass of reactant is not the same for every reaction. Some brownish black material was left above the bed layer of the catalyst inside the reactor after the reaction. Carbon deposition was left inside the reactor from the catalytic cracking or the condensation of the macro-organic material in the wood vinegar. Moreover, a circle of brown residues were deposited at the outlet of the quartz tube, which may be the organic material that was brought out by gas and cooled down at the outlet. The reactor before and after the reaction was weighed, and the mass of the remaining carbon deposition was obtained. Table 8 shows the product distribution from steam catalytic reforming with different support catalysts. The liquid products are highest in all samples, which accounts for nearly half of the whole products. The amount of remaining carbon deposition is lower than 5%. The gas products are slightly lower than the liquid products. The percentage of gas products is Ni/Sn/ HBeta > Ni/Sn/γ-Al2O3 + MCM-41 > Ni/Sn/γ-Al2O3 ≈ Ni/ Sn/HBeta + MCM-41 ≈ Ni/Sn/HZSM-5 > Ni/Sn/HZSM-5 + MCM-41. The mass of carbon left significantly decreased with mixed support catalysts, which is beneficial from the optimization of acidic amount, acidic distribution, BET surface

Figure 7. FT-IR patterns of carbon deposit on spent catalysts.

is beneficial for the steam catalytic reforming of macromolecular organic compounds (i.e., phenol derivate). Due to the larger pore size, the macromolecular organic compounds will catalytic crack into the intermediate products which are small molecular organic compounds. And the intermediate can easily go through the pore of γ-Al2O3 and take further reactions. H


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Energy & Fuels Table 11. Mass Change of Spent Catalysts Heated at High Temperature under Air Peak temperature/°C (Mass change) Catalyst Ni/Sn/HBeta Ni/Sn/HBeta + MCM-41 Ni/Sn/HZSM-5 Ni/Sn/HZSM-5 + MCM-41 Ni/Sn/γ-Al2O3 Ni/Sn/γ-Al2O3 + MCM-41(2h)

Peak 1 85 128 82 91 85 113

Peak 2

(−2.88%) (−1.58%) (−1.76%) (−1.98%) (−1.56%) (−2.04%)

598 605 583 562 479 549

area, and pore structure of catalysts and enhancement of heat and mass transfer of catalysts. 3.3.4.2. Conversion of Organic Material with Different Support Catalysts. The content of organic material in the crude wood vinegar is 18%. The organic material content of wood vinegar after steam catalytic reforming is shown in Table 9. The conversion of organic material is highest at 91.67% with Ni/Sn/HBeta catalyst. The second is Ni/Sn/γ-Al2O3 + MCM41 mixed support catalyst, which is 89.44%. Ni/Sn/HZSM-5 + MCM-41 is the least at 77.22%. Table 9 shows that the support plays a significant role in the conversion of organic materials. The conversion is lower with mixed supports than a single support catalyst, except for Ni/Sn/γ-Al2O3 + MCM-41. Table 10 shows the conversion of some organic materials, which are the main composition of wood vinegar. More than 30 organic materials are found in crude wood vinegar. The type of organic material decreases to less than 10 after catalytic reforming. Most of the organic materials have been transferred into gases, H2O, and some small-molecule organic materials. The water content is 82% in the crude wood vinegar. The water content is 98% when Ni/Sn/HBeta is used as the catalyst. The main ingredients of wood vinegar are acetic acid, 2methoxyphenol, 2-methoxy-4-methylphenol, furfural, and 4ethyl-2-methoxyphenol. From Table 10 we can know the conversion of organic material in the wood vinegar. The content of acetic acid is 20.72% in the crude wood vinegar. Acetic acid almost cannot be detected after the reaction. The content of ketone increases after the reaction. Acetic acid takes the following reaction to produce ketone. 2CH3COOH → (CH3)2 CO + H 2O + CO

(5)

Degradation of acetic acid: (6)

CH3COOH → CO + H 2

(7)

Peak 4

0.31% 0.41% 1.59% 2.03% 1.96% 613 (−0.29%)

1.14%

3.3.5. Life of Ni/Sn/γ-Al2O3 + MCM-41 Mixed Support Catalyst. Figure 5 showed that the content of H2 and CO2 decreased significantly, and the content of CH4 and CO slightly increased at the first 0.5 h. The content of gases remained nearly the same during the next 4 h. However, the content of H2 and CO2 decreased slowly during the fifth hour. The catalyst activity decreased because of the carbon deposition or the sintering or loss of active material in catalysts. 3.4. Carbon Deposition. 3.4.1. SEM Pattern of Catalyst. Carbon deposition will cause the poisoning of the catalyst or block the pore of the catalyst. The SEM patterns of fresh and spent catalysts are observed to investigate carbon deposition. The results are shown in Figure 6. The fresh catalyst was compared with the used catalyst. Some small pores were found in the surface of the fresh catalyst Ni/Sn/HBeta + MCM-41. However, the used catalyst was filled with layers of carbon, and the pores disappeared. Some small particles adhered on the surface for the fresh mixed catalysts of Ni/Sn/ZSM-5 + MCM41. The metal particle was absorbed by the surface during catalyst preparation. After the reaction, the diameters of the particles were enlarged, and clusters of particles became stacked. Small pores were found on the surface of the fresh Ni/Sn/γ-Al2O3 + MCM-41 catalyst. The catalysts were covered with porous and loose materials after the reaction. Thus, the life of Ni/Sn/γ-Al2O3 + MCM-41 was longer than that of other catalysts. The reactants can interact with the active center by going through the porous and loose materials. However, the reactant for the other catalysts was prohibited to touch the active part by carbon deposition or organic polymer on the surface of used catalysts. No fiber coke was found on the spent catalyst, which is the key factor for the deactivation of the catalyst.

(4)

CH3COOH → CO2 + CH4

Peak 3

Moreover, the alcohols and aldehydes also cannot be detected after reaction. The small molecule organic materials are possibly transformed into COx and H2 by catalytic cracking and steam reforming according to the following equations.

Some researchers have found that the carboxyl groups are converted into ketone groups as a function of solid acid catalysts, and ketone is then converted into gases. Bossola23 discovered that hydrogen obtained from the steam catalytic reforming of acetic acid was a possible method for acetic acid conversion. Steam reforming of acetic acid: CH3COOH + 2H 2O → 2CO2 + 4H 2

(−8.21%) (−9.95%) (−6.20%) (−6.17%) (−9.79%) (−5.56%)

The conversion of phenols is higher than 90%. Most of them have been converted into gases. A part of them has been converted into phenol, benzene, and CH4 by dimethoxy or dihydroxylation according to the following equations. I


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Energy & Fuels 3.4.2. FT-IR Pattern of Spent Catalysts. The FT-IR pattern of spent catalysts is shown in Figure 7. The peaks on the FT-IR are attributed to different chemical groups in the spent catalysts. The peaks at 3400 and 1600 cm−1 can be attributed to the bending of the adsorbed water molecules and OH groups.24−26 The peak around 1620 cm−1 corresponds to the stretching vibration CC. The peaks at 956, 2923, 2852, and 2723 cm−1 can be attributed to the stretching vibrations of − CH2 and − CH3. The peaks at 1461 and 1377 cm−1 are the bending vibrations of CH2 and − CH3. The band around 1100 cm−1 may be attributed to the stretching vibration of C−N or the bending of the C−O bond.27 The infrared absorption characteristic peak for γ-Al2O3 is at 570 cm−1. The adsorption peaks for Ni are at 587 and 780 cm−1. The peaks at 540 and 460 cm−1 are attributed to Sn. The types of organic materials on the different supports are similar, but their amounts are not the same. 3.4.3. TPO Analysis of Carbon Deposition. Figure 8 and Table 11 show the carbon deposition. Figure 8 shows the mass loss of the spent catalyst. Peaks 1 and 2 in Table 11 show the amount of carbon deposition at different temperature zones. The mass loss from Peak 1 corresponds to H2O and smallmolecule organic material, such as alcohol, aldehyde, and acid, which will be oxidized and transferred into CO2 and H2O around 30−280 °C. Mass loss is the highest for the Ni/Sn/ HBeta catalysts. The mass loss for the single support catalyst for Peak 1 is lower than for mixed support, except for Ni/Sn/ HBeta. The structure of catalysts plays an important role in the carbon deposition. The mass loss for Peak 2 is carbon deposition. The amount of carbon deposition for single support catalysts is higher than the mixed catalysts, especially for Ni/ Sn/γ-Al2O3 + MCM-41 catalysts. The mass loss for Ni/Sn/γAl2O3 is 9.79%. However, it is only 5.85% when Ni/Sn/γ-Al2O3 + MCM-41 catalyst is used. The mass loss for the single Ni/Sn/ HBeta catalyst is lower than the mixed support catalysts. Moreover, the amount of carbon deposition for Ni/Sn/HZSM5 is nearly the same with mixed support catalysts. Thus, mesopore zeolite is beneficial for the carbon tolerant of solid acid catalysts but not for micropore zeolites. The reason is that both the weak and strong acidic sites, intensity, and amount of acidic sites of Ni/Sn/γ-Al2O3 + MCM-41 are lower than Ni/ Sn/HBeta + MCM-41 and Ni/Sn/ZSM-5 + MCM-41. And it is generally accepted that the acidic intensity and amount are a key factor for carbon deposition. Another reason is that the structure of catalysts, the pore, and the surface area of zeolites are different with γ-Al2O3. Therefore, some organic reactants can form carbon deposition on γ-Al2O3 but not on zeolites. Thus, the carbon deposition for single micropore zeolites is lower than that for γ-Al2O3. The structure of mixed catalysts Ni/Sn/γ-Al2O3 + MCM-41 was significantly changed and kept the advantage of both acidic intensity of γAl2O3 and good structure of zeolites. However, the advantage for both zeolites mixed support is not obvious. The mixed supports of solid catalyst and mesopore zeolites are a promising method to decrease carbon deposition. Moreover, Peak 3 shows the increased mass of spent catalyst at higher temperature. The remaining Fe and Ni, which come from the reduction of catalyst, were oxidized again at high temperature. The increased amount for single zeolite catalyst is less than the mixed catalyst except for γ-Al2O3. However, the amount is opposite for γ-Al2O3. It is induced that mixed support can reduce the consumption rate of active materials.

4. CONCLUSION (1) The single support catalyst is beneficial for the catalytic reforming of wood vinegar to produce rich H2. The yield is 21.58 mg/g sample, and the H2 concentration is 52.65% by Ni/Sn/HBeta. However, the H2 yield decreased with time. The catalytic activity of the catalyst is Ni/Sn/HBeta > Ni/Sn/γ-Al2O3 > Ni/Sn/HZSM-5 because of the differences in the acidic intensity and structure of the supports. (2) The gas concentration slightly decreased after 2 h when the mixed support catalyst was used. The H2 yield for the two zeolite mixed support decreased. However, the average concentration and yield increased to 60.32% and 22.47 mg/g sample, respectively, for Ni/Sn/γ-Al2O3 + MCM-41. The mixed support (i.e., mix of solid acid and zeolite catalysts) can extend the life of catalysts and improve catalytic efficiency. (3) The conversion of organic material in wood vinegar is about 77%−91% after catalytic reforming. The conversion of rich organic material is above 90%. The main reaction is steam catalytic reforming, hydrodeoxygenation, and steam reforming of gases, such as methane steam reforming. The conversion of organic material by mixed support catalysts is slightly higher than single support. (4) The carbon deposition significantly decreased from 9.79% to 5.85% when mixed support catalyst Ni/Sn/γAl2O3 + MCM-41 was used instead of single support catalyst Ni/Sn/γ-Al2O3.

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AUTHOR INFORMATION

Corresponding Authors

*(Xiwei Xu) E-mail: [email protected]. *(Enchen Jiang) E-mail: [email protected]. ORCID

Enchen Jiang: 0000-0002-8431-5717 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Science and Technology Planning Project of Guangdong Province, China (Grant No. 2016A020210073), the National Science Foundation of China (Grant No. 51576071), Key Laboratory of Biomass Energy and Material (KLBEM) (Grant No. JSBEM201706), and the State Key Laboratory of Pulp and Paper Engineering (Grant No. 20140536).

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Influence of Mixed Supports on the Steam Catalytic Reforming of

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