This paper assesses the feasibility of using a spent fluid catalytic cracking catalyst (SFCC) as precursor for the production of geopolymers. The mechanical and structural characterization of alkali-activated SFCC binders formulated with different overall (activator + solid precursor) SiO2/Al2O3 and Na2O/SiO2 molar ratios are reported. Formation of an aluminosilicate ‘geopolymer’ gel is observed under all conditions of activation used, along with formation of zeolites. Increased SiO2/Al2O3 induces the formation of geopolymers with reduced mechanical strength, for all the Na2O/SiO2 ratios assessed, which is associated with excess silicate species supplied by the activator. This is least significant at increased alkalinity conditions (higher Na2O/SiO2 ratios), as larger extents of reaction of the spent catalyst are achieved. SiO2/Al2O3 and Na2O/SiO2 ratios of 2.4 and 0.25, respectively, promote the highest compressive strength (67 MPa). This study elucidates the great potential of using SFCC as precursor to produce sustainable ceramic-like materials via alkali-activation.
The catalyst used in fluid catalytic cracking (FCC) in the petrochemical industry is an aluminosilicate with a zeolitic structure, often on an alumina or silica-alumina support. The FCC process is conducted to obtain higher-octane gasoline through the breaking of the long chains of hydrocarbon molecules. When the FCC catalyst loses its catalytic properties (becoming ‘spent’), it is replaced, and the deactivated catalyst residue is discarded and treated as an inert waste (
In the search for ways to utilize this waste, the application of SFCC as an alternative supplementary cementitious material for the production of blended cements has been explored (
In the past decades great attention has been given to alternative cementitious materials known as ‘geopolymers’, which are binders produced through the chemical reaction between an aluminosilicate precursor and an alkaline activator. The main precursors used in geopolymers production are metakaolin (
The widespread interest in geopolymers for different industrial applications has motivated the assessment of low cost precursors such as SFCC in geopolymer production. Preliminary work activating SFCC catalysts to form geopolymers (
This study evaluates the feasibility of producing geopolymer materials based on a Colombian SFCC. The effect of formulation conditions, specifically overall Na2O/SiO2 and SiO2/Al2O3 molar ratios, in the microstructure and compressive strength of these materials is assessed. Detailed structural analysis of the activated SFCC catalyst is conducted using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), 29Si and 27Al MAS nuclear magnetic resonance spectroscopy, and scanning electron microscopy (SEM).
A spent fluid catalytic cracking catalyst (SFCC) from a Colombian petroleum company is used as the geopolymer precursor in this study. Before chemical activation, the SFCC was milled for 5 hours, using a ball mill. The resulting powder had specific gravity of 2630 kg/m3 and a mean diameter D (
Chemical composition of the SFCC. LOI is loss on ignition at 1000 °C
Component | mass % |
---|---|
SiO2 | 48.09 |
Al2O3 | 41.57 |
Fe2O3 | 0.91 |
CaO | 0.22 |
MgO | 0.13 |
K2O | 0.09 |
TiO2 | 0.85 |
LOI | 2.19 |
The X-ray diffractogram of the SFCC (
The alkali activator used was a commercial sodium silicate solution with 29.1 wt.% SiO2 and 10.2 wt.% Na2O and 60.0 wt.% H2O, and solid analytical grade NaOH pellets. This was dissolved in the mix water and allowed to cool until reaching room temperature prior to preparation of the specimens. All the activating solutions reach a pH higher than 14.
For the preparation of the geopolymer specimens, the alkaline activator was formulated to obtain overall (activator + solid precursor) SiO2/Al2O3 molar ratios of 2.0, 2.2, 2.4, 2.6, 2.8 and 3.0, and overall Na2O/SiO2 molar ratios of 0.20, 0.25 and 0.30. The amount of water in the alkali activator was adjusted to achieve a total H2O/Na2O ratio of 11 in all samples (
Chemical Composition of geopolymer systems (total H2O/Na2O molar ratio of 11)
Mix Composition (Total Molar Ratio) | Alkaline activator Solution | ||
---|---|---|---|
SiO2/Al2O3 | Na2O/SiO2 | Na2O/Al2O3 | Ms = SiO2/Na2O (molar ratio) |
2.0 | 0.40 | 0.08 | |
2.2 | 0.44 | 0.53 | |
2.4 | 0.20 | 0.48 | 0.90 |
2.6 | 0.52 | 1.22 | |
2.8 | 0.56 | 1.49 | |
3.0 | 0.60 | 1.72 | |
2.0 | 0.50 | 0.07 | |
2.2 | 0.55 | 0.42 | |
2.4 | 0.60 | 0.72 | |
2.6 | 0.25 | 0.65 | 0.97 |
2.8 | 0.70 | 1.19 | |
3.0 | 0.75 | 1.38 | |
2.0 | 0.60 | 0.06 | |
2.2 | 0.66 | 0.35 | |
2.4 | 0.72 | 0.60 | |
2.6 | 0.30 | 0.78 | 0.81 |
2.8 | 0.84 | 0.99 | |
3.0 | 0.90 | 1.15 |
The compressive strength was assessed for cylindrical paste samples of 30 mm (diameter) × 60 mm (height), using a universal testing instrument (Instron) at a displacement rate of 1 mm/min. Sample ends were flattened and made parallel using coarse sandpaper before testing. Each reported value corresponds to the average of 5 measurements. Powdered pastes were analyzed through:
X-ray diffraction (XRD), using a Bruker D8 Advance instrument with Cu Kα radiation and a nickel filter. The tests were conducted with a step size of 0.020°, for 2θ values between 3° and 60°.
Fourier transform infrared (FTIR) spectroscopy, with a PerkinElmer Spectrum 100 instrument. The KBr pellet technique was used to prepare the samples, which were scanned from 4000 to 400 cm−1.
Solid-state 29Si and 27Al magic angle spinning nuclear magnetic resonance (MAS NMR). The analysis of the SFCC was carried out with a Bruker 400 Ultrashield Avance II 400 spectrometer (9.4 T) using a MAS NMR probe for 5 mm rotors and a spinning speed of 5.0 kHz. 29Si MAS NMR spectra were acquired using a resonance frequency of 79.5 MHz, a pulse width of 5 μs and a relaxation delay of 5 s and 4000 scans. 27Al MAS NMR experiments were conducted at 104.23 MHz, with a pulse width of 5 μs, a relaxation delay of 0.25 s, and 2048 scans. MAS NMR spectra of the alkali activated specimens were obtained on a Varian Direct Drive VNMRS-600 spectrometer (14.1 T) using a MAS NMR probe for 4 mm o.d. zirconia rotors and a spinning speed of 10.0 kHz. 29Si MAS NMR spectra were acquired using a pulse width of 4 μs and a relaxation delay of 20 s, and more than 3600 scans. 27Al MAS NMR experiments were conducted at 156.3 MHz on the same instrument, with a pulse width of 0.5 μs, a relaxation delay of 2 s, and 1024 scans. All 29Si and 27Al chemical shifts are referenced to external samples of tetramethylsilane (TMS) and a 1.0 M aqueous solution of AlCl3·6H2O, respectively.
Scanning electron microscopy (SEM), conducted in a JEOL JSM-6490LV high vacuum microscope (3×10−6 torr) at an acceleration voltage of 20 keV, using carbon coated unpolished specimens.
Diffractogram of the unreacted spent FCC catalyst. An: andalusite, Am: analcime, F: faujasite, K: kyanite, M: mullite, Q: quartz, S: sillimanite.
The formulation of the specimens has a marked effect on strength. The highest mechanical strengths are obtained (
7-day compressive strengths of alkali-activated SFCC specimens.
These results are comparable with the strengths of metakaolin-based geopolymers with the same curing duration and similar formulation conditions, including the reduction in strength at high SiO2/Al2O3 ratios (
The X-ray diffractograms of the activated spent FCC pastes formulated with Na2O/SiO2 molar ratios of 0.20, 0.25 and 0.30 are shown in
In samples formulated with an Na2O/SiO2 ratio of 0.20 (
Diffractograms of activated SFCC pastes formulated with a Na2O/SiO2 ratio of (A) 0.20, (B) 0.25 and (C) 0.30. An: analcime, F: faujasite, U: ussingite, ZA: zeolite Na-A, Q: quartz, M: mullite, S: sillimanite and Ky: kyanite.
Upon increasing the Na2O/SiO2 molar ratio to 0.25 (
The spectrum of unreacted SFCC (
FTIR spectra of SFCC precursor, and activated pastes formulated with SiO2/Al2O3 (S/A) ratios as marked, at Na2O/SiO2 ratios of (A) 0.20, (B) 0.25, (C) 0.30.
All alkali-activated pastes display a main band between 1200 cm−1 and 800 cm−1 corresponding to the asymmetric stretching vibration mode of the Si-O-T linkage (where T may be Si or Al) (
The activation process also leads to a reduction in the intensities of the bands observed in the SFCC precursor at 460 cm−1, 526 cm−1 and 617 cm−1, associated with the dissolution of the zeolite phases, consistent with the XRD results (
Bands at 1630 cm−1 and 3450 cm−1 in the activated pastes are attributed to bending (H–O–H) and stretching vibrations (–OH) of water in the hydrated products, respectively (
In samples with an Na2O/SiO2 molar ratio of 0.20, a lower SiO2/Al2O3 ratio promotes shifting of the T-O-T band towards lower wavenumbers (1017 cm−1 for SiO2/Al2O3=2.4), suggesting the incorporation of higher contents of Al within the gel, compared with the samples activated with higher SiO2/Al2O3 ratio that show slight variations in the T-O-T band position (1022 cm−1 for SiO2/Al2O3=3.0). These results are consistent with the formation of a more Si-rich gel in the samples formulated with the highest SiO2/Al2O3 ratio. This is in agreement with the shifting of the band at 725 cm−1 towards higher wavenumbers at increased SiO2/Al2O3 ratios, as a consequence of the more Si-rich environment in the gel. The Si-O-T band is observed to be narrower and significantly more intense for the samples with SiO2/Al2O3 ratios of 2.4 and 2.6, because of the higher extent of dissolution of the SFCC, contributing more silicate and aluminate species for geopolymer gel formation. Similar trends are observed in specimens formulated with higher Na2O/SiO2 ratios (0.25 and 0.30).
At higher alkalinity conditions, associated with higher Na2O/SiO2 ratios, the alkali content favors the initial dissolution of the aluminosilicate precursor and its consequent reaction to form the geopolymer product. This leads to the shifting of the main T-O-T band towards lower wavenumbers as identified in the activated specimens when compared with the unreacted SFCC. Conversely, at low alkalinity conditions a higher proportion of unreacted precursor is identified, as the T-O-T band of the SFCC does not exhibit variations in the activated samples.
27Al MAS NMR spectrum of the unreacted SFCC.
The 29Si MAS spectrum of the unreacted SFCC (
29Si MAS NMR spectrum of the unreacted SFCC.
The 27Al MAS spectra of selected SFCC-based geopolymers (
27Al MAS NMR spectra of SFCC geopolymers formulated with a SiO2/Al2O3 ratio of 2.4, as a function of Na2O/SiO2 (N/S) ratio as marked.
A significant reduction of the Al(VI) band is identified in the activated SFCC, when compared with the unreacted SFCC, along with a significant increase in the intensity in the Al(IV) region of the spectra. The reduction in the intensity of the bands in the Al(VI) region is attributed to the dissolution of the dealuminated zeolite, leading to the formation of a highly crosslinked disordered aluminosilicate ‘geopolymer’ gel, as identified by FTIR.
Although the 27Al MAS NMR spectra of the activated SFCC specimens are dominated by the resonance associated with Al(IV), a small resonance centered between 2 and 3 ppm is also observed in the spectra. This resonance corresponds to small amounts of Al(VI) from the residual mullite phase (
The peak at –105 ppm in the 29Si MAS NMR spectrum of the unreacted SFCC (
29Si MAS NMR spectra of SFCC geopolymers formulated with a SiO2/Al2O3 ratio of 2.4, as function of the Na2O/SiO2 (N/S) ratio.
Low alkali content (Na2O/SiO2 of 0.20) promotes the broadening of the 29Si MAS NMR spectrum, along with a low intensity shoulder between –98 ppm and –120 ppm, corresponding to the Q4 sites of faujasite from the spent catalyst that are not completely consumed during the geopolymerisation process. Increasing the alkali content (higher Na2O/SiO2), the intensity of this shoulder is significantly reduced, confirming a higher extent of dissolution of faujasite from the SFCC at higher alkali content in the systems, as observed in the XRD results (
SEM images of geopolymers formulated with an Na2O/SiO2 molar ratio of 0.25, and SiO2/Al2O3 molar ratios of (A) 2.0 (B) 2.2 (C) 2.4 (D) 2.6, (E) 2.8 and (F) 3.0.
Geopolymers can be successfully produced from spent fluid catalytic cracking catalyst, which is available in high volumes in some parts of the world. Structural studies show that the alkali-activated SFCC pastes consist of an amorphous phase corresponding to an aluminosilicate ‘geopolymer’ type gel, along with different types of zeolites, depending on the contents of silicates and alkalis available in the systems. The mechanical strength of the activated SFCC is strongly influenced by the formulation conditions, where optimal overall SiO2/Al2O3 and Na2O/SiO2 ratios of 2.4 and 0.25, respectively, promoting a compressive strength of up to 67 MPa. Insufficient alkali content in the formulated geopolymers retards the initial dissolution of the zeolite phases present in the SFCC, and consequently a higher proportion of unreacted precursor is identified in the samples activated with an Na2O/SiO2 molar ratio of 0.20. These results elucidate that alkali-activation of SFCC is a viable method for managing and valorizing this industrial waste, with great potential for the production of ceramic-like materials with good mechanical performance.
This study was sponsored by the