Research on carbonization of agricultural by-products of coffee bean hulls and corn cobs by hydrothermal carbonization method, application as adsorbent and catalyst materials

The surface structure of the original raw coffee bean hull/corn cob material and Hydrochar is shown in Figure 3.23.

(a) Coffee bean hulls

(b) Corn cob

(d) Corn Cob Hydrochar

Figure 3.23. SEM images of raw materials before activation (a) coffee bean hull, (b) corn cob,

The results in Figure 3.23 a, b show that the surface structure of the original coffee bean hull/corn cob material samples all have a rough, dense structure with very few pores. The results in Figure 3.23 c, d show that the coffee bean hull/corn cob Hydrochar samples all have a non-uniform, dense surface structure with very few pores.

Furthermore, the results of SEM image analysis in Figure 3.24 and TEM image in Figure

3.25 shows an insight into the structure and surface morphology of biochar samples activated by different methods which have undergone great changes compared to the original base materials.

(a) CH hydrogen

(b) Hydrogen CC

(d) CC magnet

(e) CH impreg

(f) CC impreg

(g) CH active

(h) CC activ

(i) CH biochar

(k) CC biochar

Figure 3.24. SEM images of activated biochar samples: (a) CH hydrogen , (b) CC hydrogen ,

(i) CH biochar , (k) CC biochar.

The SEM image results in Figure 3.24 show that magnetic Hydrochar has a dense, heterogeneous surface structure and few pores, possibly due to the formation of Fe 3 O 4 molecules penetrating into the pores. The coal samples from Hydrochar (CH hydrogen , CC hydrogen , CH impreg , CC impreg , CH activ , CC activ ) and activated Biochar (CH biochar , CC biochar ) show that the surface structure and morphology are both porous, layered, with irregular shape distribution and heterogeneous pore size. The high porosity and large surface area of the coal samples may be related to the release of organic compounds formed during the activation of Hydrochar or raw materials with KOH solution. In addition, KOH is a strong alkali that can combine with carbon to form hydrogen, CO, CO 2 gases , thereby forming pores and facilitating the formation of oxygen-containing functional groups on the material surface according to the reactions [143, 144]:

6 KOH  2 K + 2 K 2 CO 3 + 3 H 2 K 2 CO 3 + 2C  2K + 3CO K 2 CO 3  K 2 O + CO 2

K 2 O + 2C  2K + CO

The SEM image results in Figure 3.24 i, k show that the activated Biochar sample (CH biochar , CC biochar ) has a better porous surface structure than the coal samples from Hydrochar (CH hydro , CC hydro , CH impreg , CC impreg , CH activ , CC activ ), possibly because Biochar formed during the pyrolysis process in an inert gas environment at high temperature (500 o C) has a larger specific surface area than Hydrochar and raw materials. In addition, in a hydrothermal environment, the above reactions do not occur as vigorously as in impregnation conditions, resulting in the surface area of the coal samples from Hydrochar being smaller than that of activated Biochar.

Furthermore, the magnetic Hydrochar sample also analyzed the TEM image in Figure 3.25 to add data to evaluate the magnetization.

(a) CH magnet

(b) CC magnet

Figure 3.25. TEM images of magnetic Hydrochar: (a) coffee bean hull, (b) corn cob.

The TEM image results in Figure 3.25 show that black iron nanoparticles (Fe 3 O 4 ) were distributed on the surface of CH magnet , CC magnet , based on the reactions:

FeCl 3 + NaOH  Fe(OH) 3 + NaCl Fe(OH) 3  Fe 2 O 3 + H 2 O

Fe 2 O 3 + C  Fe 3 O 4 / Fe + CO / CO 2

The surface morphology of CH magnet and CC magnet did not have a definite shape but were distributed in arrays. The results were completely consistent with other published studies [112]. To further clarify the morphology of activated biochar samples, the BET specific surface area analysis method and the adsorption and desorption capacity of N 2 as well as the pore distribution were studied.

3.3.2. Specific surface area (BET) of activated biochars

The results in Table 3.5 show the specific surface areas of biochar samples activated by different methods.

Table 3.5. Specific surface area of different types of activated carbon.

STT

Material	Symbol	BET (m2 / g)	Hole size (A o )	Volume hole (cc/g)
1	Hydrothermally activated hydrochar	CH hydrogen	703.9	15.40	0.482
2	Hydrothermally activated hydrochar	CC hydrogen	741.1	11.20	0.427
3	Magnetic Hydrochar	CH magnet	157.9	12.80	0.239
4	Magnetic Hydrochar	CC magnet	165.7	17.00	0.232
5	Impregnated activated hydrochar	CH impreg	743.8	12.20	0.448
6	Impregnated activated hydrochar	CC impreg	861.7	15.60	0.552
7	Activated Hydrochar	CH activ	950.4	16.40	0.610
8	Activated Hydrochar	CC activ	965.9	15.40	0.547
9	Activated Biochar	CH biochar	1344.8	13.60	0.800
10	Activated Biochar	CC biochar	1707.3	12.40	1,022

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The specific surface areas of the charcoals are arranged in decreasing order as follows: Activated Biochar > Activated Hydrochar > Impregnated Activated Hydrochar > Hydrothermally Activated Hydrochar > Magnetic Hydrochar, consistent with the above SEM morphological observations. The results show that all the charcoal samples derived from corn cobs have larger surface areas than those derived from coffee bean hulls, possibly due to

The composition of coffee bean hulls contains many lignin molecules, so it is more difficult to decompose than corn cobs. The pyrolysis method gives materials with a larger specific surface area than the hydrothermal carbonization method, CC biochar reaches 1707.3 m 2 /g and CH biochar reaches

1344.8 m 2 /g. This result is consistent with the results of surface structure analysis by image.

SEM above. In addition, comparing the coal samples obtained through the HTC method, activated Hydrochar has the largest specific surface area as follows: CC activ reaches 965.9 m 2 /g and CH activ reaches 950.4 m 2 /g. Comparing hydrothermally activated Hydrochar (CH hydro , CC hydro ) and impregated activated Hydrochar (CH impreg , CC impreg ), CH impreg ,

CC impreg has a larger specific surface area, the difference is  BET = 39.9 m 2 /g and  BET = 120.6 m 2 /g, respectively). The difference in specific surface area above may be due to the fact that the impregnation activation method uses a very large amount of KOH (KOH/Hydrochar ratio is 1:1) compared to the hydrothermal method (KOH/biomass ratio is 1:6.67, corresponding to KOH/Hydrochar ratio is 1:3.98). In addition, in a hydrothermal environment, the above reactions do not occur as vigorously as in impregnation conditions. However, the obtained hydrothermally activated Hydrochar still has a much increased specific surface area, about 24 times compared to the original Hydrochar. This result is completely consistent with other published studies [79, 84, 85, 145-147]. This suggests that the adsorption capacity and catalytic activity will be positive.

The specific surface area of magnetic Hydrochar samples CH magnet and CC magnet after magnetization is generally larger than that of Hydrochar, but lower than that of other coal samples (CH magnet reaches 157.9 m 2 /g, CC magnet reaches 165.7 m 2 /g). This may be due to the formation of Fe 3 O 4 molecules that have penetrated into the pores or occupied the empty surface area, leading to a decrease in the specific surface area of CC magnet and CH magnet . They believe that magnetic Hydrochar will reduce the adsorption capacity and catalytic activity. The research results are similar to other published studies [112].

3.3.3. Nitrogen adsorption and desorption curves of activated biochar samples

The results of nitrogen adsorption and desorption isotherms of activated biochar samples are shown in (Figure 3.26) and the pore distribution (Figure 3.27).

Adsorption capacity (cm 3 /gSTP)

700

600

Adsorption Desorption

(a)

CH biochar

700

Adsorption capacity (cm 3 /gSTP)

600

Adsorption

Desorption

(b)

CC biochar

500

400

300

200

100

CH activ CH impreg

CH hydrogen CH magnet

Hydrochar CH 0.0 0.2 0.4 0.6 0.8 1.0 1.2

Relative pressure (P/P o )

500

400

300

200

100

CC impreg CC activ

CC hydrogen CC magnet

Hydrochar CC 0.0 0.2 0.4 0.6 0.8 1.0 1.2

Relative pressure (P/P o )

Figure 3.26. Adsorption and desorption curves of different activated biochar samples derived from: (a) coffee bean hulls, (b) corn cobs.

0.10

Pore volume (cc/A o /g)

0.08

0.06

0.04

0.02

0.00

(a)

Hydrochar

CH hydrogen CH magnet CH impreg CH activ

CH biochar

0 10 20 30 40 50 60

Pore size (A o )

0.10

Pore volume (cc/A o /g)

0.08

0.06

0.04

0.02

0.00

(b)

Hydrochar

CC hydrogen CC magnet CC impreg

CC activ CC biochar

0 10 20 30 40 50 60

Pore size (A o )

Figure 3.27. Pore size distribution curves of different activated biochar samples derived from: (a) coffee bean hulls, (b) corn cobs.

The results in Figure 3.26 show that the nitrogen adsorption and desorption isotherms of the coal samples have a hybrid shape between type I and type IV isotherms according to the IUPAC (International Union of Pure and Applied Chemistry Nomenclature) classification definition. Type I isotherms usually represent the characteristics of micro-sized materials. Type IV isotherms indicate the characteristics of meso-sized materials. In addition, the nitrogen adsorption and desorption isotherms show that the adsorption capacity of Hydrochar increases significantly after activation. The results in Figure 3.27 show that the pore size distribution of the coal samples ranges from 0.2 to 59.8 A˚, the pore size is mainly concentrated at about 15.4 A˚, indicating the existence of mainly micro-pores in the structure of the material samples. This predicts the adsorption capacity of the initial raw materials and

Hydrochar will increase significantly after activation, which can be applied to treat pollutants in water or air environment. Moreover, the results of nitrogen adsorption and desorption isotherms as well as the pore size distribution of activated biochar samples are consistent with the above SEM and BET analysis results. In addition, to evaluate the overall properties of the material, the properties of crystal structure, chemical bonding characteristics of surface functional groups, as well as the content of surface functional groups were also analyzed based on XRD, FTIR and Boehm titration methods.

3.3.4. XRD patterns of activated biochar samples

21.6

30.5

35.6

43.3 57.1 62.2

53.5

Carbon

FeO, Fe 3 O 4

CH magnet

CC magnet

CH hydrogen

CC hydrogen CH magnet

CC magnet

CH impreg CC impreg

CH activ

CC activ

CH biochar CC biochar

Intensity (cps)

10 20 30 40 50 60 70

Angle 2θ

10 20 30 40 50 60 70 80

Angle 2θ

Figure 3.28. XRD patterns of biochar samples activated by different methods.

The XRD results in Figure 3.28 show that the structure of all activated biochar samples has a 2θ diffraction spectrum in the range of 20 - 24°, which is the characteristic reflection spectrum of amorphous carbon. Therefore, all coal samples have an amorphous structure. This result is completely consistent with other published studies [148]. However, magnetic Hydrochar (CH magnet , CC magnet ) also has diffraction peaks at 2θ in the range of 30.5; 35.6; 43.3; 53.5; 57.1 and 62.2°, showing the presence of FeO, Fe 3 O 4 on magnetic activated carbon. The results are similar to those published

Zhou et al.'s peak positions and relative high intensities corresponded to the characteristic peaks of Fe 3 O 4 [149]. This showed that the CH magnet and CC magnet samples were successfully magnetized, consistent with the findings of the SEM and TEM analysis results above. The results were similar to those of Ma et al.'s study [112] for MCA magnetic activated carbon samples derived from corn cobs.

3.3.5. Fourier transform infrared spectra of activated biochar samples

Fourier transform infrared spectroscopy (FTIR) was used to analyze the characteristic chemical bond vibrations between atoms of Hydrochar samples and activated biochar samples in Figure 3.29 in the vibrational region from 400 - 4000 cm -1 .

CH magnet

592

Fe 3 O 4

CC magnet

592

-OH

-CH n

C=O -OH COC

Hydrochar

C= CCO

Hydrochar CC

CH hydrogen

CC hydrogen CH magnet CC magnet CH impreg

CC impreg CH activ CC activ CH biochar

CC biochar

FeO

3 4

Transmittance (% )

Transmittance (%)

1400 1200 1000 800 600 400

Wavelength ( cm - 1 )

4000 3500 3000 2500 2000 1500 1000 500

Wavelength (cm -1 )

Figure 3.29. FTIR spectra of Hydrochar and different types of activated carbons derived from coffee bean hulls and corn cobs.

The FTIR spectra results in Figure 3.29 show that all Hydrochar and activated biochar samples obtained after activation have oscillations in the wide range of 3354 - 3427 cm -1 , corresponding to the oscillations of (OH) bonds of alcohol, phenol and carboxylic acid in lignin and cellulose, demonstrating the presence of "free" hydroxyl groups on the surface of activated biochar [150, 151]. The peaks oscillating in the range of 2924 - 2926 cm -1 and around 1585 - 1620 cm -1 indicate the presence of

Research on carbonization of agricultural by-products of coffee bean hulls and corn cobs by hydrothermal carbonization method, application as adsorbent and catalyst materials - 14

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