Chapter 5 Pore shaping to enhance the quality of adsorbents
5.4 Results and discussion
CHAPTER 5 PORE SHAPING TO ENHANCE THE QUALITY OF ADSORBENTS
79 then flown into the quartz tube at 100 sccm rate from one end through the stopper. Target temperature, rising rate and holding time has been set to the temperature controller. After reaching the target temperature, methane gas has been flown at same rate. Ar and methane was flowing at the same time from the same end of quartz tube. Exhaust gas is discharged by a tube through the other end of quartz tube. A cooling bath is used to reduce the temperature before releasing the gas to environment. Unlike CH4, benzene is liquid at room temperature. When pyrolyzed with benzene, an HPLC column is used to precisely control the flow of liquid benzene. The liquid benzene is introduced into the quartz tube from the same end where Ar is flowing. A tape heater is wrapped around that end of quartz tube to evaporate the benzene. Argon acts as carrier gas and push the vapor benzene into the pores.
When the pyrolization is finished, both the Argon and pyrolization material flow is stopped. The sample is kept inside the quartz tube until it reaches the room temperature by natural cooling. The complete setup and procedure is shown in Fig. 5.5.
Fig. 5.5. Schematic diagram of the experimental setup for pyrolysis.
CHAPTER 5 PORE SHAPING TO ENHANCE THE QUALITY OF ADSORBENTS
80 shows the plot for benzene pyrolyzed samples. The PSD curves are drawn by minimizing the roughness of distribution so that all the peaks can be seen clearly. NLDFT model for N2 at 77 K on carbon slit pores is considered to evaluate the PSD. The regularization value is considered 0.0001 for this model.
Methane is in gaseous phase at room temperature. The flow rate of carrier (Argon) gas and methane is considered constant in this experiment. The time of pyrolization is also considered fixed and only the temperature varied. Mass of the yield increased with pyrolization temperature. Surface area, pore volume and PSD peaks decrease with the pyrolization temperature. At 1000 °C, the PSD peak of 0.8 nm is completely vanished (Fig.
5.7). This indicates that the most of the unusable pores are removed. However, the usable pores also decreased at this temperature.
Benzene is in liquid phase at room temperature. In order to achieve similar flow rate of methane pyrolization experiment, the liquid benzene flow rate have to be very low.
Hence, an HPLC pump is used to precisely control the flow rate. Vapor benzene flow rate can be calculated from the liquid flow rate by the following equation (5.1).
22.4
liq liq vap
m m
M ρ ×
= ×
(5.1)
Here, the unit of mass flow rate, molecular weight and density is mL/min, g/mol and g/L respectively.
Pyrolysis at 900 °C for 30 minutes at 50 mL/min gas benzene flow rate remove all the pores from Maxsorb III and the surface area becomes zero. N2 adsorption do not occur (Fig. 5.8) and there is no peak in the PSD plot (Fig. 5.9). When the temperature is decreased to 800 °C keeping the pyrolization time and flow rate constant, similar incident occurs.
This indicates that pyrolization time and flow rate should be decreased too.
CHAPTER 5 PORE SHAPING TO ENHANCE THE QUALITY OF ADSORBENTS
81 Fig. 5.6. Ad/desorption isotherm of N2 onto parent and CH4 pyrolyzed Maxsorb III.
Fig. 5.7. Comparison of PSD among parent and methane pyrolyzed Maxsorb III.
0 200 400 600 800 1000
0.00 0.20 0.40 0.60 0.80 1.00
Vads/des[cm3g–1]
Relative pressure (P/P0)
Parent Maxsorb III – ads. Parent Maxsorb III – des.
Tₚ = 800 °C, t = 30 min – ads. Tₚ = 800 °C, t = 30 min – des.
Tₚ = 850 °C, t = 30 min – ads. Tₚ = 850 °C, t = 30 min – des.
Tₚ = 900 °C, t = 30 min – ads. Tₚ = 900 °C, t = 30 min – des.
Tₚ = 1000 °C, t = 30 min – ads. Tₚ = 1000 °C, t = 30 min – des.
0.00 0.50 1.00 1.50 2.00 2.50 3.00
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Pore volume (cm3g–1nm–1)
Pore width (nm)
Parent Maxsorb III
Pyrolization temperature 800 °C, 30 min Pyrolization temperature 850 °C, 30 min Pyrolization temperature 900 °C, 30 min Pyrolization temperature 1000 °C, 30 min
dV dW
CHAPTER 5 PORE SHAPING TO ENHANCE THE QUALITY OF ADSORBENTS Table 5.6. Maxsorb III pyrolization with methane (CH4).
Initial mass before degassing
[g]
Initial mass after degassing
[g]
Pyr.
temp.
[°C]
Gas flow rate [mL/min]
Pyr.
time [min]
Final mass after pyrolysis
[mg]
Mass increase
[%]
BET Surface area [m2/g]
Total pore volume (cm3/g)
Pore size distribution
Ar CH4 Peak (W, dV/dW)
[nm, cm3 g–1 nm–1]
Range [nm]
Parent Maxsorb III 3140 2.01 (1.67, 1.6) 0.1 – 4.4
0.2223 0.2004 800 50 50 30 0.2027 1.15 2762.07 ± 19.15 1.37 (0.77, 0.85), (1.08, 2.30), (1.30, 1.72), (1.42, 0.67), (1.63, 0.82), (1.94, 1.13), (2.30, 0.85), (2.53, 0.64)
0.68 – 3.60
0.2000 0.1885 850 50 50 30 0.2355 24.27 1786.63±25.31 0.90 (0.80, 0.57), (1.08, 1.23), (1.30, 1.10), (1.41,0.43), (1.63, 0.56), (1.94, 0.73), (2.30, 0.53), (2.53, 0.39)
0.76 – 3.57
0.2044 0.1879 900 50 50 30 0.2511 33.63 1916.21 ± 18.67 0.94 (0.80, 0.78), (1.08, 1.38), (1.30, 1.18), (1.41, 0.44), (1.63, 0.57), (1.94, 0.73), (2.30, 0.49), (2.53, 0.34)
0.68 – 3.52
0.2116 0.1897 1000 50 50 30 0.3318 74.91 760.68 ± 17.33 0.39 (1.17, 0.66), (1.30, 0.42), (1.41, 0.22), (1.63, 0.25), (1.94, 0.28), (2.30, 0.15), (2.52, 0.10)
1.09 – 3.50
CHAPTER 5 PORE SHAPING TO ENHANCE THE QUALITY OF ADSORBENTS Table 5.7. Maxsorb III pyrolization with benzene (C6H6).
Initial mass before degassing
[g]
Initial mass after
degassing [g]
Pyr.
temp.
[°C]
Liquid C6H6
flow rate [mL/min]
Gas flow rate [mL/min] Pyr.
time
[min]
Final mass after
pyrolysis [mg]
Mass increase
[%]
BET Surface area
[m2/g]
Total pore volume [cm3/g]
Pore size distribution
Ar C6H6 Peak (W, dV/dW)
[nm, cm3 g–1 nm–1] Range [nm]
Parent Maxsorb III 3140 2.01 (1.67, 1.6) 0.1 – 4.4
0.2107 0.1972 900 0.20 50 50 30 0.4341 120.13 0.1104 ± 0.0387 0.0012 (19.2, 0.0016), (21.0, 0.00014), (25.0, 0.00009), (29.9, 0.00007)
17.5 – 33.9
0.2018 0.1998 800 0.20 50 50 30 0.4721 136.29 0.2739 ± 0.0423 0.0022 (9.91, 0.00032), (12.9,
0.0016) 9.49 – 21.9
0.2127 0.2047 800 0.10 50 25 10 0.3572 74.50 975.75 ± 17.51 0.5005 (1.09, 0.70), (1.3, 0.77), (1.41, 0.31), (1.63, 0.34), (1.94, 0.40), (2.3, 0.26)
1.05 – 3.76
0.2073 0.1692 800 0.02 50 05 05 0.2221 31.26 2587.47 ± 26.02 1.2811 (0.77, 0.78), (1.08, 2.01), (1.3, 1.64), (1.42, 0.63), (1.63, 0.78), (1.94, 1.06), (2.30, 0.77)
0.67 – 3.76
0.2043 0.1951 800 0.02 50 05 10 0.2450 25.58 2165.79 ± 22.47 1.0770 (0.77, 0.72), (1.08, 1.49), (1.30, 1.42), (1.41, 0.53), (1.63, 0.67), (1.94, 0.89), (2.30, 0.63)
0.69 – 3.76
CHAPTER 5 PORE SHAPING TO ENHANCE THE QUALITY OF ADSORBENTS
84 Fig. 5.8. Ad/desorption isotherm of N2 onto parent and C6H6 pyrolyzed Maxsorb III.
Fig. 5.9. Comparison of PSD among parent and benzene pyrolyzed Maxsorb III.
0 200 400 600 800 1000
0.00 0.20 0.40 0.60 0.80 1.00
Vads/des[cm3g–1]
Relative pressure (P/P0)
Parent Maxsorb III – ads.
Parent Maxsorb III – des.
Tₚ = 900 °C, t = 30 min, ṁᵥₐₚ = 50 mL/min – ads.
Tₚ = 900 °C, t = 30 min, ṁᵥₐₚ = 50 mL/min – des.
Tₚ = 800 °C, t = 30 min, ṁᵥₐₚ = 50 mL/min – ads.
Tₚ = 800 °C, t = 30 min, ṁᵥₐₚ = 50 mL/min – des.
Tₚ = 800 °C, t = 10 min, ṁᵥₐₚ = 25 mL/min – ads.
Tₚ = 800 °C, t = 10 min, ṁᵥₐₚ = 25 mL/min – des.
Tₚ = 800 °C, t = 5 min, ṁᵥₐₚ = 5 mL/min – ads.
Tₚ = 800 °C, t = 5 min, ṁᵥₐₚ = 5 mL/min – des.
Tₚ = 800 °C, t = 10 min, ṁᵥₐₚ = 5 mL/min – ads.
Tₚ = 800 °C, t = 10 min, ṁᵥₐₚ = 5 mL/min – des.
0.00 0.50 1.00 1.50 2.00 2.50 3.00
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Pore volume (cm3g–1nm–1)
Pore width (nm)
Parent Maxsorb III
Tₚ = 900 °C, t = 30 min, ṁᵥₐₚ = 50 mL/min Tₚ = 800 °C, t = 30 min, ṁᵥₐₚ = 50 mL/min Tₚ = 800 °C, t = 10 min, ṁᵥₐₚ = 25 mL/min Tₚ = 800 °C, t = 5 min, ṁᵥₐₚ = 5 mL/min Tₚ = 800 °C, t = 10 min, ṁᵥₐₚ = 5 mL/min
dV dW
CHAPTER 5 PORE SHAPING TO ENHANCE THE QUALITY OF ADSORBENTS
85 At this point, the pyrolysis temperature is set to 800 °C, the flow rate is decreased to half (0.25 mL/min) and the pyrolysis time is one third (10 min). For this conditions, all the pores were not blocked. However, the yield amount is still high and surface area is about one third of parent Maxsorb III. Smaller micropores around 0.8 nm were completely blocked and a lot of usable pores are decreased too.
At same pyrolysis temperature, the flow rate (5 mL/min) and pyrolization temperature is further decreased. At this conditions, surface area and pore volume change is not so significant. Smaller micropores still remains and the improvement is not so significant. Hence, the pyrolization time is increased to 10 min keeping all the other conditions unchanged. Some of the regular micropores and mesopores were removed at this condition.