Journal Pre-proof The behavior of the surface Si/(Si+Al) ratio in the FCC catalyst deactivation ´ ´ Alvaro A. Amaya, Carlos A. Gonzalez, Fernando Mart´ınez O
PII:
S0368-2048(19)30257-9
DOI:
https://doi.org/10.1016/j.elspec.2019.146932
Reference:
ELSPEC 146932
To appear in:
Journal of Electron Spectroscopy and Related Phenomena
Received Date:
5 September 2019
Revised Date:
17 December 2019
Accepted Date:
22 December 2019
´ Gonzalez ´ Please cite this article as: Amaya AA, CA, Mart´ınez O F, The behavior of the surface Si/(Si+Al) ratio in the FCC catalyst deactivation, Journal of Electron Spectroscopy and Related Phenomena (2019), doi: https://doi.org/10.1016/j.elspec.2019.146932
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The behavior of the surface Si/(Si+Al) ratio in the FCC catalyst deactivation
Álvaro A. Amaya a, Carlos A. González b, Fernando Martínez O. a*
a
Centro de Investigaciones en Catálisis - CICAT, Universidad Industrial de Santander - UIS,
Escuela de Química, km 2 vía el Refugio, Piedecuesta, Santander, Colombia.
Ecopetrol S.A., Instituto Colombiano del Petróleo, km 7 vía Piedecuesta, Santander, Colombia.
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b
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*
[email protected]
Highlights
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Surface Si/(Si+Al) ratio depends on the deactivation of the catalyst. Surface Si/(Si+Al) ratio increases when the FCC catalyst is deactivated. Excess Al2O3 is probably removed from the surface of the catalyst.
Abstract
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Herein, the surface Si/(Si+Al) ratio of a series of FCC catalysts was studied by X-ray Photoelectron Spectroscopy (XPS). Firstly, some of the interferences of the XPS signals from Si, and Al elements were studied and fixed by using the linear multiple regression method (LMR). Subsequently, the Si/(Si+Al) ratio from a set of FCC catalysts was analyzed and this information was correlated with the surface area analysis (BET method). This work showed that the surface Si/(Si+Al) ratio of low deactivated FCC catalyst was lower than its bulk Si/(Si+Al) ratio, suggesting that there is a high
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content of Al2O3 on the surface of the catalyst when it is slightly deactivated. The surface Si/(Si+Al) ratio of the FCC catalyst increased as the surface area decreased (total and microporous) due to the deactivation of the catalyst; it means that the surface Si/(Si+Al) ratio is strongly affected by the deactivation of the catalyst. The surface Si/(Si+Al) ratio of the equilibrium FCC catalyst samples (Ecat) were very similar to their bulk Si/(Si+Al) ratio. This fact suggests that the surface Si/(Si+Al) ratio of an FCC catalyst progressively increases up to reach the bulk Si/(Si+Al) ratio, therefore the surface Si/(Si+Al) ratio is related to the FCC catalyst deactivation phenomenon.
Keywords: FCC catalyst; X-ray Photoelectron Spectroscopy; surface Si/(Si+Al) ratio.
1. Introduction The FCC process is used for gasoline production in many refineries around the world. The FCC catalyst is principally composed of zeolite (USY) and matrix components: alumina (Al2O3), clay and a binder (SiO2) [1]. The catalyst employed in the FCC process is seriously damaged by extreme
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conditions of temperature, moisture and some contaminants such as Ni and V [2-4]. When an FCC catalyst is deactivated, the microporous area (related to zeolite) decreases due to dealumination
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processes, and it leads to a low gasoline yield and high coke and H2 production.
FCC catalyst is deactivated in two ways: a reversible route caused mainly by coke formation [5,6],
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and a non-reversible route due to sintering (loss of surface area), dealumination of zeolite and metal deposition (mainly Ni and V) [7-10]. Although the surface area (microporous and total) is the most
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common parameter used when determining the degree of deactivation of an FCC catalyst, other properties are affected as well and they can provide information about the state of the catalyst.
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Knowing the surface properties of the FCC catalyst is very important to explain its deactivation because the first cracking reactions take place on the surface of the catalyst [11]. However, there are few studies about the surface characterization by X-ray Photoelectron Spectroscopy (XPS)
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regarding the FCC catalysts. In this work, a set of FCC catalysts (with different deactivation degree) were studied by XPS, with the purpose of analyzing their surface Si/(Si+Al) ratio. Subsequently, this parameter was correlated with the surface area analysis (BET method) to find a relationship between the deactivation degree of the FCC catalyst and its surface Si/(Si+Al) ratio.
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2. Experimental
2.1 Samples and raw materials The FCC catalyst samples used in this work were: equilibrium catalysts (FCC-1 to FCC-3) and artificially deactivated catalysts obtained by Cyclic Metal Impregnation (CMI) and Cyclic Propylene Steaming (CPS) methods; they are named FCC-4 to FCC-16. More information about the
FCC-4 to FCC-16 samples is shown in Table 1. All FCC catalysts used in this work were obtained by using the same fresh FCC catalyst precursor. The FCC catalyst samples were calcined at 600 °C for 3 h in a static furnace before XPS analysis. The total content of lanthanum (expressed as % RE2O3) in the FCC catalysts was about 2.0 – 5.0% w/w. Total Ni and V concentration in the samples was about 4000 and 6000 ppm, respectively. Al2O3 and SiO2 are commercially reagents and they were used as reference samples. ASA sample (amorphous alumino-silicate) was synthesized employing the sol-gel method by using a synthesis methodology similar to the reported by La Parola et al. [12]. Ni/SiO2(5) sample was synthesized by
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incipient wetness impregnation method employing a solution of Ni(NO3)2.6H2O and SiO2; Ni
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concentration was adjusted to 5.0% w/w.
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2.2 XPS analysis
XPS analysis was performed on a SPECS® XPS/ISS/UPS Surface Characterization Platform. The samples were analyzed by using a monochromatic Al X-ray source operated at 200 W/12 kV. The
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pass energy of the hemispherical analyzer was adjusted to 100 eV for the high-resolution spectra to improve the detection of elements present in low concentration; the pass energy used did not have a
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strong effect on the FWHM values of the signals studied (see SI 1). Step-size was adjusted to 0.05 eV, Dwell time was set at 0.1 s and high-resolution spectra were recorded with 30 scans (FWHM of Ag 3d5/2 = 1.54 eV). Surface charge compensation was controlled with a Flood Gun. The scale of
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binding energy was calibrated adjusting the C-H component of the adventitious carbon to 284.8 eV. The Relative Sensitivity Factors (RSF) employed were: C 1s (1.0), O 1s (2.77), Si 2p (0.85), Si 2s (0.98), Al 2p (0.57), Al 2s (0.78), La 3d5/2 (24.45) and Ni 2p (19.06). The XPS spectra were analyzed by using CasaXPS software. All signals were treated by using Shirley background.
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3. Results and discussion
3.1 Determination of analysis conditions and data interpretation
3.1.1 Silicon Si 2p and Si 2s signals are commonly used to study the XPS spectrum from a sample containing Si element. As shown in Figure 1, the Si 2p signal observed in a common aluminosilicate is located
very near to the maximum point of the Al 2p satellite signal (named as Al 2p*), therefore, an imminent interference is shown. Additionally, La 4d signal overlaps with the Si 2p signal. Table 2 shows that the discrepancy between the area of Si 2p and Si 2s signals can be greater than 60% in FCC catalyst samples. Linear multiple regression method with normalized values was used to relate the corrected area of Si 2s, Al 2p and La 3d with the Si 2p signal observed (named as Si 2p**). The result obtained is summarized in Equation 1 and Figure 2.a (Ac represents the corrected area of the respective signal in brackets). Eq. 1
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Ac(Si 2p∗∗ ) = 0.992 × Ac(Si 2s) + 0.068 × Ac(Al 2s) + 0.175 × Ac(La 3d)
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This mathematical expression shows that the interference caused by La 4d signal is high because of its high normalized coefficient. Additionally, Equation 1 also evidences the interference caused by Al 2p* signal to the Si 2p signal. Those interferences indicate that the Si 2p signal observed in an
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FCC catalyst cannot be used to study the Si content in these materials. On the other hand, even though the Si 2s signal is near to the Al 2s satellite signal (Al 2s*), it is observed that the maximum
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point of the Al 2s* signal is displaced from the Si 2s signal. Therefore, the Al 2s* signal does not significantly overlap with the Si 2s signal. In this work, Si 2s signal was selected to study the Si
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3.1.2 Aluminum
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content in the FCC catalysts samples.
When aluminum is analyzed in FCC catalysts by using XPS, both Al 2s and Al 2p signals have interference problems. Al 2s signal overlaps with the Si 2p* and Ni 3s signals, while the Al 2p signal overlaps with the Ni 3p and V 3s signals. The interference effect due to the Ni 3s, Ni 3p and V 3s signals is significantly important when Ni and V concentrations are very high and it should be taken into account for the XPS analysis. A good solution to quantify the aluminum content in the
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FCC samples with high nickel and vanadium concentration was achieved by calculating the corrected area of the Al 2p signal employing the Al 2s, Si 2s and Ni 2p signals. The observed Al 2s signal (called as Al 2s**) could be simulated according to the Equation 2 by using FCC catalysts with negligible surface Ni and V concentration. The coefficients of proportionality were calculated by using the multiple linear regression method.
Eq. 2
Ac(Al 2s ∗∗ ) = 1.02 × Ac(Al 2p) + 0.085 × Ac(Si 2s)
According to this equation, the contribution of Si 2p* signal to the Al 2s** signal is 0.085 times the corrected area of the Si 2s signal. On the other hand, the Ni 2p signal was used to measure the area of the Ni 3s signal, which is overlapped with the Al 2s signal. Equation 3 shows the relation between Ac(Ni 2p) and Ac(Ni 3s) obtained from the NiSiO3(5) sample; this procedure is indicated in the supporting information SI 2. Eq. 3
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Ac(Ni 3s) = 0.58 × Ac(Ni 2p)
corrected area of the Al 2s** signal with the signals composing it.
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Finally, Equation 4 was obtained by including Equation 3 in Equation 2. This expression relates the
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Ac(Al 2s∗∗ ) = 1.02 × Ac(Al 2p) + 0.085 × Ac(Si 2s) + 0.58 × Ac(Ni 2p)
Eq. 4
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Equation 4 was rearranged to obtain an expression that relates the corrected area of the of Al 2p signal with the corrected area of the Al 2s, Si 2s and Ni 2p signals. Ac(Al 2s ∗∗ ) − 0.085 × Ac(Si 2s) − 0.58 × Ac(Ni 2p) 1.02
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Ac(Al 2p) =
Eq. 5
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Equation 5 allows us to know the Al content easily since the aluminum content is quantified using the Al 2p signal (calculated using the Al 2s **, Si 2s and Ni 2p signals) in the FCC samples with high or low V and Ni concentration. Additionally, Al 2p signal can be directly used in FCC catalyst samples where surface Ni and V is not observed.
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3.2 Study of the surface Si/(Si+Al) ratio in FCC catalysts with different deactivation degree The surface content of Si and Al (without adventitious carbon contribution) and the surface Si/(Si+Al) ratio were studied for each FCC catalyst (FCC-1 to FCC-16) by using the analytical conditions mentioned above. A correlation between these parameters is indicated in Table 3 and Figure 3a-c. As shown, a direct relationship between the total surface area and the surface Si/(Si+Al) ratio was observed (Figure 3c); this trend is also perceived between the microporous surface area (zeolite) and the surface Si/(Si+Al) ratio (see Figure 3a).
The deactivation of the FCC catalyst is evidenced by the loss of surface area, the lower the surface area, the higher the deactivation or damage of the FCC catalyst is [14]. Therefore, Figure 3a suggests that the surface Si/(Si+Al) ratio increases as the FCC catalyst deactivation (determined by the surface area) increases. The surface Si/(Si+Al) ratio of an FCC catalyst with low degree deactivation (high surface area) was about 0.24, while a high deactivated FCC catalyst (FCC-1) showed a surface Si/(Si+Al) ratio of 0.46. The bulk Si/(Si+Al) ratio of the FCC catalysts studied was 0.45. A surface Si/(Si+Al) ratio significantly lower than the Si/(Si+Al) ratio observed in bulk, suggests
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that a high aluminum-content component is present on the surface; according to the general FCC formulation, this component must be Al2O3 because the other components have Si/(Si+Al) ratio
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equal or higher than 0.5.
When the deactivation degree increases (low surface area), surface Si/(Si+Al) ratio increases until
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reaching similar values to the observed in the bulk of the catalyst. Two possible ways are expected to increase the surface Si/(Si+Al) ratio in the catalyst: loss of surface Al (or a high-content
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aluminum component) or Si migration from the bulk to the surface. According to Figure 4d, when FCC is being deactivated, the change of Al concentration is more significant than the change of Si concentration. It could suggest that the surface Si/(Si+Al) ratio is increased by losing of Al atoms
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from the surface and not for Si migration to the surface.
According to the results observed in this work, FCC catalyst must have a high content of Al2O3 on its surface when it is not significantly deactivated. This extra-content of Al2O3 on the surface of the
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catalyst is removed as the FCC catalyst is deactivated, allowing that surface and bulk Si/(Si+Al) ratios tend to be similar. This phenomenon is also observed in FCC catalysts with a high content of Si on the surface; in this situation, the surface Si/(Si+Al) ratio decreases until reaching a value close to that observed in bulk [7,15,16]. XPS has not been widely used to study the surface chemical composition of FCC catalysts, and a relationship between the Si/(Si+Al) ratio and the deactivation
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degree of the catalyst has not been reported yet in the literature. Other non-surface techniques, such as SEM-EDS, are widely used for this purpose [17,18]. However, this tool cannot provide accurate information about the firsts 10 nanometers of the catalyst, then, a possible phenomenon of surface dealumination would go unnoticed if non-surface techniques are used to characterize these materials.
4. Conclusion
Si content in an FCC catalyst could be studied employing the Si 2s signal whereas Al content cannot be studied using a single signal. In the last case, a mathematic model was required to know the area of Al 2p signal without interferences in the FCC samples with a high or low concentration of V and Ni. The surface Si/(Si+Al) ratio of a low deactivated FCC catalyst (high surface area) is lower than the bulk Si/(Si+Al) ratio. It shows that the surface of an FCC catalyst is rich in Al2O3. When an FCC catalyst is deactivated, the surface Si/(Si+Al) ratio tends to be equal to the bulk Si/(Si+Al) ratio, which indicates that an extra content of Al2O3 is progressively removed according to the
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deactivation degree of the FCC catalyst. The higher values of the surface Si/(Si+Al) ratio were observed in the Ecat samples. Although some FCC catalysts deactivated by CMI and CPS had
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values of the surface area similar to the ones found in the Ecat samples, their surface Si/(Si+Al)
ratios were lower than the measured in the Ecat catalysts. This last fact shows the difficulty of the current deactivation methods (CMI or CPS) to reach the state of the deactivation of an equilibrium
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catalyst.
5. Acknowledgements
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This work was financially supported by Empresa Colombiana de Petróleos - Ecopetrol, Instituto Colombiano del Petróleo -ICP and Universidad Industrial de Santander -UIS (cooperation
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project No. 5219040). Thanks to Surf-Lab Laboratory for XPS data acquisition.
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FIGURES
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Figure 1. XPS spectra of a) FCC-1, b) Al2O3 and c) SiO2 samples.
0.80 0.60 0.40
0.20 0.00 0.00
0.20
0.40
0.60
0.80
1.00
100,000
y = 0.9947x + 282.81 R² = 0.9984
80,000 60,000
40,000 20,000 0
0
Experimental Ac (Si 2p**) / normalized
25,000
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b)
y = 1.0011x - 0.0007 R² = 0.9993
1.00
Calculated Ac (Al 2s**) / a.u.
Calculated Ac (Si 2p**) / normalized
a)
50,000
75,000 100,000
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Experimental Ac( Al 2s**) / a.u.
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Figure 2. Experimental and calculated areas of a) Si 2p** and a) Al 2s** signals.
Figure 3. Relationship between the surface Si/(Si+Al) ratio and the surface area analysis (a-c). d) Si and Al concentration as a function of the surface Si/(Si+Al) ratio.
TABLES
Table 1. Deactivation conditions used in the samples FCC-4 to FCC-16. % Steam
9
65
Temperature (°C) 705
CPS
20
50
780
FCC-6
CPS
30
20
760
FCC-7
CPS
30
65
760
FCC-8
CPS
100
50
780
FCC-9
CMI
20
50
780
FCC-10
CMI
60
20
780
FCC-11
CMI
60
50
760
FCC-12
CMI
60
50
FCC-13
CMI
60
50
FCC-14
CMI
60
FCC-15
CMI
100
FCC-16
CMI
180
780
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800
65
780
50
780
50
780
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FCC-5
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Cycles
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FCC-4
Deactivation method CPS
Sample
Table 2. Corrected area of the Al 2p, Al 2s, Si 2s, Si 2p and La 3d signals found in the FCC catalysts and synthesized samples. Corrected Area Sample
𝐒𝐢 a
b
𝐒𝐢+𝐀𝐥
𝐀𝐥 𝟐𝐬 𝐒𝐢 𝟐𝐩 𝐀𝐥 𝟐𝐩 𝐒𝐢 𝟐𝐬
Al 2s
Si 2s
Si 2p
La 3d
FCC-1 FCC-2 FCC-3
17,128
18,354
13,663
19,138
642
0.44
1.07
1.40
37,059 32,176
40,836 35,098
30,072 24,947
37,543 36,787
929 1590
0.45 0.44
1.10 1.09
1.25 1.47
FCC-4 FCC-6 FCC-7 FCC-8
26,531 54,589 53,563 20,645
27,759 56,577 55,003 21,352
8477 17,542 19,402 10,735
13,843 28,822 32,809 17,445
798 1660 2206 1056
0.24 0.24 0.27 0.34
1.05 1.04 1.03 1.03
FCC-9
52,496 91,701
55,077 97,140
16,077 -
26,081 -
1541 -
0.23 0.00
1.05 1.06
1.62 -
64,005
N.A.
69,866 45,967
76,339 53,845
-
1.00 0.42
-
1.09 1.17
1.63 1.64 1.69 1.63
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Al2O3 SiO2 AAS
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Al 2p
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La 3d signal is obtained according to Amaya et al. [13] Calculated using the signals Al 2p and Si 2s without any modification.
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a
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FCC-5 sample is not used for the determination of the coefficients mentioned on equations 1 and 2. N.A.= Not analyzed AAS= Amorphous Alumino-Silicate
Table 3. Atomic concentration of Si and Al and surface area analysis of the FCC catalysts samples
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% Atomic concentration
Sample
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FCC-1 FCC-2 FCC-3 FCC-4 FCC-5 FCC-6 FCC-7 FCC-8 FCC-9 FCC-10 FCC-11 FCC-12 FCC-13 FCC-14
Si
Al
15.93 14.56 15.58 9.24 10.87 9.17 9.72 12.26 8.27 9.21 7.95 11.21 10.46 11.95
18.84 19.23 19.85 28.66 31.75 28.02 26.09 23.00 26.87 26.59 25.33 22.76 25.00 22.58
Surface area (m2/g) 𝐒𝐢 𝐒𝐢 + 𝐀𝐥 Mesoporous Microporous Total 0.46 0.44 0.43 0.24 0.26 0.25 0.27 0.35 0.24 0.26 0.24 0.33 0.29 0.35
51 58 62 56 49 52 48 56 62 60 67 42 45 58
105 83 111 203 155 183 174 112 209 180 192 143 123 115
156 141 173 259 204 235 222 168 271 240 259 185 168 173
13.61 13.03
20.27 18.67
0.40 0.41
33 33
95 112
129 145
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FCC-15 FCC-16