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Ind. Eng. Chem. Res. 2007, 46, 4486-4496
Utilizing Colloidal Silica and Aluminum-Doped Colloidal Silica as a Binder in FCC Catalysts: Effects on Porosity, Acidity, and Microactivity Brian T. Holland,*,† Velu Subramani,‡ and Santosh K. Gangwal‡ Nalco Company, 1601 West Diehl Road, NaperVille, Illinois 60563, and RTI International, Research Triangle Park, North Carolina 27709
This paper reports on the synthesis and characterization of fluid catalytic cracking (FCC) catalysts using two different colloidal silica materials as binders, where one was doped with 2 wt % Al2O3 and the other was without any dopant. The physicochemical properties such as attrition resistance, particle size distribution, surface areas, porosity, morphology, chemical environment, and acidity were investigated employing various analytical and spectroscopic techniques such as N2 sorption, scanning electron microscopy, transmission electron microscopy, 29Si and 27Al MAS NMR, and temperature programmed desorption of ammonia (NH3-TPD). It was observed that the FCC catalysts synthesized using colloidal silica have larger particle sizes than that synthesized using sodium silicate (baseline catalyst) as a binder and lower than that of a commercial FCC catalyst. They also exhibit relatively lower attrition resistance than the commercial and the baseline FCC catalysts; however, they exhibit higher BET and matrix surface area than the commercial and the baseline FCC catalysts. Use of aluminum doped colloidal silica provided increased acidity of the matrix at low levels compared to the commercial FCC catalyst, which contained twice the amount of aluminum. This led to increased activity and selectivity in microactivity testing (MAT), especially for gasoline yield (53.28 wt %), compared to colloidal silica as a binder (49.66 wt %). In addition, the FCC catalysts made with aluminum doped colloidal silica offered similar dry gas amounts (1.34 wt %) and slightly higher coke values (2.98 wt %) compared to that made with Al-free colloidal silica, which produced 1.42 and 2.59 wt % dry gas and coke, respectively. Both these values are significantly lower than the commercial FCC catalyst, possibly as a result of the unique porosity observed with the FCC catalysts formulated with colloidal particles. 1. Introduction The fluid catalytic cracking (FCC) process changed significantly when zeolites were introduced in the 1960s1 to replace silica-alumina cracking catalysts. The zeolite cracking catalysts provided major benefits over the amorphous silica-alumina catalysts in terms of gasoline yield. At a constant coke yield, the zeolite catalyst increased the oil conversion and gasoline yield compared to amorphous silica-aluminas, while reducing the dry gas and heavy cycle oil (HCO) yield.2 However, due to lower olefinicity of products generated by the zeolite catalyst, the gasoline research octane number (RON) was reduced. The modern FCC catalyst contains a zeolite and a matrix, which typically consists of kaolin clay and a binder such as silica, alumina, or silica-alumina. Although zeolites revolutionized the FCC industry in the 1960s and newer zeolites are constantly being developed in both academia and corporate research and development, the source of binder in the matrix has remained the same, such as sodium silicate, sodium aluminate, aluminum chlorohydrate, and boehmite. One major process step involved with using sodium silicate as a SiO2 source is that the FCC catalyst needs to be washed and/or ionexchanged to remove sodium since the presence of sodium is detrimental to the development of acidic sites in FCC catalysts. Colloidal SiO2 without Na cation could be considered as a potential alternative SiO2 source as this can significantly reduce the processing time and cost by eliminating the washing and ion-exchange steps. Colloidal silica with even greater added benefits, such as the one having Al3+ dopant that can improve * To whom correspondence should be addressed. Fax: (630) 3052565. E-mail:
[email protected]. † Nalco Company. ‡ RTI International.
the catalyst acidity and gasoline yield, could overcome the prejudice of its high cost compared to traditional binders. The matrix typically provides the physical characteristics of the catalysts, for instance mesoporosity and macroporosity, attrition resistance, and stability toward heat and steam.3 In addition to physical characteristics, the matrix can also provide catalytic properties such as acidity for cracking heavier oil fractions (bottoms). These heavier oil fractions are too large to be cracked efficiently by the zeolite present in the FCC catalyst due to pore size restriction. The larger pores provided by the matrix and the increased acidity increase the bottoms cracking activity. It has also been shown that an active matrix component, such as boehmite alumina, can act as a metal trap to take out unwanted metals.4 Although the function of the active matrix components is to influence catalyst performance, the influence is typically less than what is provided by the zeolite. In contrast to the active matrix components, pure amorphous silica binder, e.g., colloidal silica, increases attrition resistance but offers very little catalytic activity due to the low acidity associated with it.5 However, the low coke and gas yield associated with a pure amorphous silica binder makes it attractive for running the FCC unit under harsher conditions, thus improving the gasoline octane and bottoms cracking.6 The active matrix components typically include alumina and/ or silica-alumina gels. For alumina, pseudoboehmite is most commonly used in FCC formulations. The pseudoboehmite has high surface area (200-300 m2/g) and has considerable acidity when activated. Al-Khattaf showed that FCC catalysts prepared with USY zeolite, kaolin clay, and silica sol with part of the kaolin replaced with (0, 5, 10, and 20 wt %) boehmite gave catalysts with higher surface areas and acidities as the boehmite content increased.5 As a result of the higher surface areas and
10.1021/ie0702734 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/27/2007
Ind. Eng. Chem. Res., Vol. 46, No. 13, 2007 4487 Table 1. Physical Properties of Colloidal Silica Employed in the Present Study colloidal silica colloidal silica-I colloidal silica-II
stabilizing ion +
NH4 NH4+
solid content (wt %)
Al2O3 content (wt %)
pH
particle sizea (nm)
32.5 20.2
0 2
9.1 9.2
9.1 7.0
a Particle sizes were determined by the equation: surface area ) 6/FD, where F ) density of SiO ) 2.2 g/cm3 and D ) particle diameter. The surface area 2 was measured by five-point BET method.
Scheme 1. FCC Catalysts Synthesis Recipe Using Colloidal Silica as a Binder
Table 2. List of Samples Synthesized and Used for Characterization sample
description
FCC-1 FCC-2 FCC-3 FCC-4
fresh FCC synthesized using colloidal silica-I fresh FCC synthesized using colloidal silica-II commercial FCC fresh FCC synthesized using sodium silicate as a binder steamed FCC-1 steamed FCC-2 steamed FCC-3 steamed FCC-4
FCC-1S FCC-2S FCC-3S FCC-4S
acidities with increasing alumina content, the microactivity testing (MAT) of hydrotreated vacuum gas oil (VGO) showed an increasing gas and coke selectivity with concomitant decrease in gasoline and light cycle oil (LCO) yield. Also, gas olefinicity was found to increase with increasing alumina content. Otterstedt et al. found similar results over an active matrix (25 wt % boehmite) for an FCC catalyst, except that LCO values increased.7 The acid strength of alumina is less than that of silicaalumina gels. The silica and alumina species can be added as separate entities, for example, silica sol and aluminum salts, which allows for some exchange of silicon atoms with those of aluminum. However, a more efficient method of alumina-silica gel synthesis is to add sodium silicate to an aluminum salt under controlled conditions. The materials generated from this type of process can offer controlled properties such as porosity and acidity depending on SiO2/Al2O3 ratio, pH of the solution, temperature and time of the reaction, aging temperature and time, and counterions present during the reaction.8-11 One of the more important aspects of a silica-alumina gel in an FCC catalyst is the acidity associated with it. Occelli et al.12 showed that a sodium silicate/aluminum salt (silica-alumina gel) had significant tetrahedral aluminum even after steam treatment. Tetrahedral aluminum is typically associated with aluminum atoms that are coupled with a proton (Brønsted acid site) providing the amorphous silica-alumina its acidity. For silicaalumina gels it has been shown that increasing alumina content up to ca. 30 wt % provides the most Brønsted acidity. Often the amount of alumina added and the surface area of the matrix are used to describe how the matrix will affect the performance of an FCC catalyst. Although good correlation has been developed for the alumina and surface area effect on an FCC catalyst performance,5,7 little effort has been put forth on describing the effects of porosity and the type of acidity that is present.6 Recently, Kortunov et al. followed the diffusion of n-octane through the pores of an FCC catalyst and described the limiting factor as transport through the mesopores and macropores, not the diffusivity within the zeolite.13 These results indicate that the matrix porosity and the strength and amount of accessible acid sites play a larger role than once realized. Of late, Nalco has developed an aluminum doped colloidal silica (colloidal aluminosilicate) material that exhibits some very
unique properties.14 These properties include neutral pH stability that is typically not possible with pure siliceous colloids.15 Besides pH neutral stability, these new Nalco aluminosilicate particles offer enhanced acidity. The synthesis involves the controlled addition of an aluminum/silicic acid solution to a catalyst, such as NaOH, KOH, or NH4OH. The intimate mixture of aluminum and silicic acid provides a highly homogeneous aluminosilicate particle. A significant advantage of using either colloidal silica or colloidal aluminosilicate particles is that if it is grown with the right catalyst, e.g., NH4OH, there is no need for further washing or ion exchange. The objective of the present study was therefore to synthesize FCC catalysts using colloidal silica and novel aluminum doped colloidal silica as binders and to investigate the effect of these binders on the physicochemical properties, especially pore structure, acidity, and microactivity, of the resulting FCC catalysts. 2. Experimental Section 2.1. Catalyst Synthesis. The USY zeolite and kaolin clay were supplied by a commercial vendor. Two different colloidal silica samples, whose properties are listed in Table 1, were used in this study. While the colloidal silica-I was a commercial product from Nalco (Product No. 1130C), the colloidal silicaII was an experimental aluminosilicate colloidal material containing a theoretical value of 2 wt % Al2O3. The particle formation in these colloidal silica samples was described elsewhere.14 Both colloidal silica-I and colloidal silica-II were deionized with cation-exchange resin Dowex 650C H+ and stabilized with NH4OH at a pH of ca. 9.0. These samples contained only about 0.2 wt % Na2O based on the elemental analysis using inductively coupled plasma (ICP) analyses performed at Galbraith Laboratory, Knoxville, TN. FCC catalysts containing 35 wt % USY zeolite (with an average particle size of ∼2.8 µm), 40-45 wt % kaolin clay (86% 50.0 nm). Table 4 summarizes the N2 adsorption and the mercury porosimetry data. The micropores are typically generated from the zeolite(s) in the FCC catalyst while the matrix contributes to the mesoporosity of the sample. USY and REY are the typical
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Figure 7. TEM microgram of FCC-2S. (a) Low-magnification of cross section; (b) higher magnification showing colloidal material around larger particle (zeolite or kaolin); (c) higher magnification showing colloidal pore structure; (d) higher magnification showing colloidal pore structure and close-packing.
zeolites used, and they have a micropore size of ca. 0.7 nm. The data shown in Table 4 indicate that the BET surface areas (sum of zeolite surface area and matrix surface area) of fresh FCC catalysts synthesized using colloidal silica and aluminumdoped colloidal silica (FCC-1 and FCC-2, respectively) and commercial FCC (FCC-3) sample. On the other hand, the micropore surface areas (zeolite surface areas) of the colloidal synthesized samples are lower than that of the commercial FCC catalyst and the external surface areas (matrix surface areas) are higher than that of the commercial FCC, suggesting that the type of zeolite and matrix used in the commercial catalyst and in the colloidal FCC catalysts are different. It is also interesting to note that the matrix surface areas of the fresh FCC catalysts synthesized using colloidal particles are higher than those of the commercial FCC (FCC-3) and the baseline catalyst (FCC-4) synthesized using sodium silicate as the silica source. However, steaming did not appreciably affect the matrix surface area of the commercial FCC and the baseline FCC, while it significantly decreased the matrix surface area of the colloidal-based FCC catalysts (Table 4). Such a drop in external surface area upon steaming suggests a considerable change in the physical properties of colloidal binders. Often, steaming can cause rearrangement of the matrix, which ultimately lowers the surface area as depicted in Figure 3.6 Similar to matrix surface area, the total pore volume also does not change upon steaming the commercial FCC catalyst. However, a significant change in total pore volume of the FCC catalysts synthesized using colloidal particles upon steaming is once again an indication of change in the physical properties of the colloidal binder system, as shown in Figure 3. A significant loss in BET surface area after steaming for FCC1S, FCC-2S, and FCC-3S catalysts is due to the loss of zeolite surface area. These changes could be attributed to either plugging of zeolite pores or a degradation of the zeolite. It should be pointed out that the commercial FCC catalyst (FCC3) appears to have substantially more zeolite in the mixture compared to the FCC catalysts synthesized using colloidal particles, as indicated by its higher zeolite surface area before and after steaming. Similar to zeolite surface area, the micropore volumes also decrease substantially after steaming for FCC-1S, FCC-2S, and
Figure 8. 29Si MAS NMR spectra of fresh FCC catalysts. (a) FCC-3; (b) FCC-1; (c) FCC-2.
Figure 9. 29Si MAS NMR spectra of steamed FCC catalysts. (a) FCC-3S; (b) FCC-1S; (c) FCC-2S.
FCC-3S catalysts. This change in micropore volume could also be attributed to micropore plugging or zeolite degradation. The observed difference in the micropore volume between the commercial FCC (FCC-3) and those synthesized in the present study further supports that the commercial FCC catalyst has more zeolite in the mixture. 3.2.2. Mercury Porosimetry. When pore sizes exceed about 100 nm, mercury porosimetry is the preferred method to measure them. The porosity data obtained from mercury intrusion into the fresh and steamed FCC catalysts are included in Table 4. As can be seen, the total pore area is substantially higher for the colloidal-based fresh FCC catalysts than for the fresh commercial and the baseline FCC catalysts. However, the total pore area of these synthesized samples dropped significantly compared to that of the commercial sample upon steaming. These results are consistent with those determined from N2 adsorption employing the t-plot method (Table 4) and suggest a considerable rearrangement of the colloidal particles.
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Ind. Eng. Chem. Res., Vol. 46, No. 13, 2007 Scheme 3. Schematic Representation of the Binding Action of Colloidal Particles on Zeolite and Clay during the Synthesis of FCC Catalysts
Figure 10. 27Al MAS NMR spectra of steamed FCC catalysts. FCC-3S (s); FCC-1S, (---); FCC-2S (- -).
Scheme 2. Porosity Development in Colloidal Particlesa
a Side view depicts colloidal particles close-packing giving a small mesopore, which is measured by Hg intrusion. Top view shows the larger mesopore (cavitiy), which is measured by N2 adsorption.
The total intrusion volumes from mercury porosimetry are also of considerable interest. Similar to that observed in the total pore volume of the FCC catalysts, as determined by N2 adsorption, the total intrusion pore volume determined by mercury porosimetry also exhibits similar trends for all the FCC catalysts of the present study. All the steamed samples appear to be very similar based on the data reported in Table 4. However, this is far from the truth. The following analysis of N2 and Hg pore size distribution data will show that the commercial FCC catalyst is significantly different in the mesopore and small macropore size regime from the FCC catalysts synthesized with colloidal particles. There will be an overlap of the pore size distribution plots for nitrogen adsorption and mercury intrusion, ca. 30-100 nm. However, the pore sizes determined from the two techniques are considerably different. The reason for this is depicted in Scheme 2, which shows colloidal spheres that are close-packed and the pore structure that is generated. The side view of the colloidal spheres in Scheme 2 depicts a pore structure going into a larger (interstitial) void (top view). Mercury intrusion measures the pores going into the larger void (side view), while nitrogen adsorption measures the large void shown in the top view. As a result, the pores determined by mercury intrusion are less than those determined by nitrogen adsorption. These types of pores are sometimes referred to as “ink-bottle” pores, and in this case, both ends are open (narrow openings leading to a larger cavity). The pore size distributions of the fresh and steamed commercial FCC catalysts and the colloidal-based FCC catalysts are presented in Figures 4-6. Both the nitrogen adsorption portion of the curve (left side) and the mercury intrusion (right
side) are presented as DV/(D log(d)) (cm3/g), which gives volume per given pore size range (pore size distribution). The dashed lines are for fresh catalysts, and the solid lines are for steamed catalysts. From these figures, it is apparent that the macropores centered at ca. 550-700 nm are unaffected by steaming; however, this is definitely not the case for the mesopores. The fresh commercial FCC catalyst (dashed line, Figure 4) has a broad peak centered ca. 8 nm from nitrogen adsorption and a sharper peak from mercury intrusion at ca. 6 nm. After steaming (solid lines) doublets appear for nitrogen adsorption, which shift to larger pore sizes and are extremely broad. Likewise, for the mercury intrusion plot after steaming, a doublet appears with the lower pore size centered ca. 7 nm and the larger pore ca. 11 nm. The pores determined by mercury intrusion are also broad, but not to the degree of the pores determined by nitrogen adsorption. Although the pore sizes determined by mercury intrusion are well within the mesopore range (50 nm). The use of colloidal silica as a binder in FCC catalysts (FCC1) changes the pore structure considerably compared to the commercial FCC catalyst (Figure 5). The macropore centered at ca. 650 nm does not change significantly after steaming, but is at a higher value compared to the commercial FCC catalyst (∼550 nm). The mesopores and small macropores change drastically after steaming as with the commercial FCC catalyst, but the FCC catalysts generated with colloidal silica have a radically different pore structure. Whereas the large portion of the commercial FCC catalyst pore structure comes from the large macropore (∼550 nm) as shown in Figure 4, there appears to be a more balanced pore size distribution between the mesopores, small macropores, and the large macropore (∼650 nm) for the FCC catalyst made with colloidal silica. The fresh colloidal silica FCC catalyst has a large mesopore peak, as measured by nitrogen adsorption, at ca. 10 nm. This peak is broad with considerable pore volume. The mercury intrusion peak for the fresh catalyst is shifted to a lower pore size of ca. 8.5 nm and is narrower compared to the peak obtained by nitrogen adsorption. Both peaks shift to higher values after steaming, ∼12 nm for nitrogen adsorption and ∼10 nm for mercury intrusion. The pore volume associated with these peaks, in terms of DV/(D log(d)), appears to change only slightly. Unlike the commercial FCC catalyst, the considerable loss in surface area and little change in pore volume indicate a transformation of smaller pores into larger pores as depicted in Figure 3. In this case it is the mesopores transforming into macropores. The pore size distribution plots in Figure 5 clearly show the change from mesopores to the smaller macropores. In addition,
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the data in Table 4 show a decrease in external surface area by 28 m2/g and pore volume by 0.08 cm3/g after steaming. A portion of the pore volume is lost from the zeolite, ∼0.02 cm3/ g; however, the majority is due to the loss in mesoporosity. The loss of both the surface area and pore volume in the mesopore region indicates a decrease in the amount of pores in that size regime and/or a shift to larger pores. As shown in Figure 3, the colloidal particles rearrange into a lower surface area and larger pore size material after steaming. The larger pores that have shifted into the macropore region, once again, are not easily detected by nitrogen adsorption; however, mercury intrusion values in Table 4 indicate a considerable loss in total pore area and a slight change in total intrusion volume after steaming. It can be seen from Figure 6 that the macropore peak for the FCC-2 containing 2 wt % Al2O3 in the colloidal silica is at a higher value (∼700 nm) versus the other two FCC catalysts. The mesopore pore structure is similar to that of the FCC-1 made with Al-free colloidal silica (Figure 5), with some notable differences. The fresh FCC-2 catalyst has a nitrogen adsorption peak centered at ca. 8 nm that is broad and has considerable pore volume, but unlike the FCC-1 catalyst, the mesopores determined by mercury intrusion extend below 10 nm and reach the measurement limit of the instrument. The pore volume of this regime appears to be quite significant, although no distinct peak is observed. After steaming the peaks shift to higher values, ∼12 nm for nitrogen adsorption and ∼10 nm for mercury intrusion, which are more in line with the steamed colloidal silica FCC catalyst. The pore volume associated with these peaks before and after steaming, in terms of DV/(D log(d)), appears to change to a higher degree than in the colloidal silica FCC catalyst; however, the data in Table 4 indicate a minimal change. As with the FCC catalyst made with colloidal silica, a small portion of the pore volume (∼0.02 cm3/g) is lost from the zeolite; however, once again the majority loss in pore volume is due to the loss in mesoporosity. To conclude, the use of colloidal materials as binders offers a unique pore structure to FCC catalysts. Considering the t-method external surface area, mercury total pore area, and total intrusion volume, there appears to be very little difference between the commercial FCC catalyst and the two FCC catalysts synthesized using colloidal particles after steaming. However, the pore size distribution plots show a much more defined pore structure for the later samples in the mesopore and small macropore range. On the basis of the results of Kortunov et al.,13 the ability of the reactants to diffuse through the matrix is the limiting factor in catalytic cracking, not the cracking on the zeolite. As a result, the location of the binder and the pore structure it creates in an FCC catalyst may be important and this aspect has been further investigated by TEM measurements. 3.3. Transmission Electron Microscopy. In order to determine the location of the colloidal binder, TEM micrographs were taken of a cross section of the steamed FCC-2S synthesized using colloidal silica containing 2 wt % Al2O3. Figure 7a is an edge of the cross-sectioned FCC catalyst particle at low magnification showing the open macropore structure between the larger particles of zeolite Y and/or the kaolin clay. Under higher magnification, the edges of the larger particles appear to be coated by the colloidal binder (Figure 7b,c). As the magnification increases, it becomes easier to see the colloidal binder located on the outside of the larger particles shown in Figure 7a. At the highest magnification (Figure 7d), the original spherical nature of the particles becomes difficult to distinguish; however, there appears to be a pore structure within the mass.
Figure 11. NH3-TPD profiles of fresh (top panel) and steamed (bottom panel) FCC catalysts.
Based on the TEM results, and the nitrogen adsorption and mercury intrusion results, it appears that the macropore structure (550-700 nm) that is unaffected by steaming is a direct result of the zeolite and clay components of the FCC catalyst. All three FCC catalysts discussed in this paper exhibit this nonchanging larger macropore peak. The same cannot be said for the mesopore and smaller macropore structures in the FCC catalysts, which exhibit pronounced changes after steaming for all three FCC catalysts. Not knowing what the commercial FCC catalyst is bound with, Scheme 3 is used to describe the location of only the colloidal particles in reference to the zeolite and clay particles for the FCC catalysts synthesized with colloidal silica and 2 wt % Al2O3 doped colloidal silica. The colloidal particles appear to coat the larger zeolite and clay particles, which in turn create a path to which the reactants must diffuse through to reach the zeolite particles. Controlling the porosity of this path to the active component may be of considerable importance in terms of activity, not just in FCC catalysts, but in any catalyst employing a larger active component (zeolites). Also, from the data in Table 4, the surface area can be misleading into what the porosity or pore size distribution actually is and may not be the best method of determining a reactive versus nonreactive FCC catalyst matrix.
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Table 6.
27Al
MAS NMR Data of Steamed FCC Catalysts and Colloidal Silica Samplesa AlO4
a
AlO5
AlO6
sample
CS (ppm)
concn (%)
CS (ppm)
concn (%)
CS (ppm)
concn (%)
FCC-1S FCC-2S FCC-3S colloidal silica-I colloidal silica-II
60 56 60 NA 52
18 26 25 NA 62
30 30 35 NA NA
50 38 15 NA NA
-3.7 -1.0 5.7 NA 4.9
32 36 60 NA 38
CS ) chemical shift; AlO4 ) tetrahedral aluminum; AlO5 ) pentacoordinate aluminum; AlO6 ) octahedral aluminum.
3.4. Acidity Effect of Aluminum. 3.4.1. 29Si and 27Al MAS NMR. The porosity of the colloidal based FCC catalysts is of considerable interest, but just as much interest lies in the effect aluminum may have on activity as part of the colloidal silica matrix. In order to determine the location and chemical environment of aluminum in the FCC catalysts, both 29Si and 27Al MAS NMR spectra have been recorded on fresh as well as steamed samples. For the sake of comparison, the spectra of both colloidal silica samples, namely colloidal silica-I and colloidal silica-II, have also been obtained. Figures 8 and 9 show the 29Si MAS NMR spectra of fresh and steamed FCC catalysts, respectively, while the chemical shift values of fresh and steamed catalysts together with those of the colloidal silica samples are gathered in Table 5. The chemical shift data of colloidal silica samples show essentially no difference between Al-free and Al-containing colloidal samples, indicating that the aluminum is not affecting the location and coordination of silica framework. This could be due to low amounts of Al2O3 (1.95 wt % based on solids as determined by ICP) in the sample. It is interesting to note from Figure 8 that the 29Si MAS NMR spectra and the chemical shift values (Table 5) of fresh FCC catalysts synthesized (FCC-1 and FCC-2) are almost identical with that of the commercial sample (FCC-3) except that the peak around -91 ppm, which is relatively strong in FCC-3, is weak. The observed chemical shift values are also similar for all three fresh FCC catalysts and are typical for FCC catalysts.12 The fresh commercial FCC catalyst (FCC-3) has chemical shift values corresponding to zeolite, silica, and clay matrix, with the largest contribution from the zeolite. The chemical shift values for the fresh FCC-2 catalyst are also similar, except for a peak at -88.3 ppm due to the matrix Q4(2Si,2Al) compared to the peak at -92.7 ppm due to the matrix kaolin clay present in the FCC catalyst. Upon steaming, the peaks associated with the zeolite become a single sharp peak centering at -107.4 ppm (Figure 9) with shoulders that could be associated with the matrix or the zeolite. The steamed colloidal silica based FCC catalyst (FCC-1) exhibits a narrow peak associated with the zeolite at -107.3 ppm with matrix peaks which are associated with the colloidal silica at -109.8 ppm for Q4 and -97.1 ppm for Q3 (Table 5). The FCC-2 exhibits the narrow peak associated with the zeolite at -107.3 ppm with matrix peaks at -110.7 ppm for Q4(4Si), -100.3 ppm for Q3(3Si) or Q4(3Si,1Al), and -93.1 ppm for Q3(2Si,1Al) or kaolin clay. The slight shift in peak positions between the neat colloidal silica and the colloidal silica based FCC catalyst might be due to the overlapping of different silica components from zeolite, kaolin clay, and colloidal silica present in the FCC catalyst. The 27Al MAS NMR spectra of steamed FCC catalysts are shown in Figure 10, while their chemical shift values together with that of colloidal silica samples are gathered in Table 6. Colloidal silica-I showed no appreciable signal, while colloidal silica-II doped with 2 wt % Al2O3 exhibited a significant peak at 52 ppm, which in aluminosilicate materials is typically
associated with framework aluminum (in a silica matrix). In addition to the tetrahedral Al, a large peak around 0 ppm for octahedral Al has also been observed in colloidal silica-II, which suggests that the deionization step involved in making the material and stabilizing with ammonia generated extra framework aluminum. The 27Al MAS NMR spectra of the steamed FCC catalysts shown in Figure 10 are of sizable interest. The spectrum for the steamed commercial FCC sample (FCC-3S) exhibits an intense peak around 0 ppm together with weak peaks around 45 and 60 ppm, indicating the existence of Al in three different coordination environments, namely, octahedral, pentacoordinate, and tetrahedral, respectively,18 with the majority (about 60%) of aluminum in the octahedral coordination and 25% as tetrahedral aluminum and 15% as pentacoordinate aluminum (Figure 10 and Table 6). On the other hand, the steamed colloidal silica based FCC catalyst (FCC-1S) has a significantly different ratio in that the majority of aluminum is pentacoordinate (50%) with 18% as tetrahedral aluminum and 32% as octahedral aluminum. Since the colloidal silica contains negligible aluminum, the aluminum species are from the zeolite and the kaolin clay. The steamed sample FCC-2S made with 2 wt % Al2O3 doped colloidal silica changes significantly compared to the Al-free analogue (FCC-1S). Pentacoordinate aluminum is still the major aluminum species, but its contribution is reduced to 38% with the octahedral aluminum species at 36%. The most interesting difference between the samples FCC-1S and FCC-2S is the tetrahedral aluminum, which has increased to 26% in FCC-2S. Also, the tetrahedral peak has shifted from 60 ppm for FCC-1S to 56 ppm for FCC-2S. The chemical shift of the tetrahedral aluminum indicates that there is still a significant portion of the tetrahedral aluminum coming from the doped Al2O3 in the colloidal silica-II containing 2 wt % Al2O3 that showed a tetrahedral Al peak at 52 ppm (Table 6). 3.4.2. NH3-TPD. NH3-TPD results are presented in Figure 11 and Table 7. It can be seen that the commercial FCC catalyst (FCC-3) exhibits two intense peaks, one centering at 130 °C and the other at 480 °C. The low-temperature peak is generally assigned to the weak acid sites, while the peak at high temperature is attributed to the strong acidic sites of zeolites in FCC catalysts. The FCC-1 and FCC-2 catalysts synthesized using colloidal silica-I and colloidal silica-II exhibit an intense peak only in the low-temperature region, around 110 °C. The high-temperature intense peak observed around 480 °C is completely absent in these samples; instead, weak shoulders around 300 and 630 °C are noticeable. The acid site densities calculated from integration of peak area (Table 7) reveals that the commercial FCC (FCC-3) has significantly stronger acidity than the catalysts made with the colloidal materials (FCC-1 and FCC-2). The stronger acidity of FCC-3 could be due to the type of zeolite(s) and composition of the matrix, which are unknown. However, the overall Al2O3 content of the commercial catalyst shown in Table 7 is significantly higher (SiO2/Al2O3 mole ratio (SAR) ) 1.7) than
Ind. Eng. Chem. Res., Vol. 46, No. 13, 2007 4495 Table 7. Acid Densities and SiO2/Al2O3 Molar Ratios of Fresh and Steamed FCC Catalystsa acid site density of steamed sample
acid site density of fresh sample
a
SARb
sample
weak
strong
total
total
fresh sample
steamed sample
FCC-1 FCC-2 FCC-3
0.0251 0.0270 0.0241
0.0058 0.0058 0.0235
0.0309 0.0328 0.0476
0.0076 0.0085 0.0093
4.2 4.2 1.7
4.8 4.6 2.1
Acid site densities are expressed in mmol/g sample. b SAR ) silica to alumina molar ratio (SiO2/Al2O3).
Table 8. Microactivity Test (MAT) Data of Steamed FCC Catalysts cracking product (wt %)
FCC-1S
FCC-2S
FCC-3S
dry gas C3 nC4 iC4 iC4) C3) nC4) total LPG gasoline LCO HCO coke conversion
1.42 0.76 0.67 3.80 0.90 3.15 2.84 12.11 49.66 20.72 13.50 2.59 65.78
1.34 0.74 0.69 3.92 0.64 2.99 2.59 11.57 53.28 19.84 11.00 2.98 69.17
1.81 1.06 0.95 4.71 0.55 3.19 2.61 13.08 53.27 19.45 8.25 4.15 72.30
the synthesized samples (SAR ) 4.2). Hence, the high acidity of the commercial sample could be attributed to the high Al content. Steaming dramatically decreased the intensities of both hightemperature and low-temperature peaks on all the FCC catalysts, indicating that steaming drastically decreases the acidity of FCC catalysts. For the commercial FCC catalyst, the high temperature desorption peak at 480 °C is lost or appears as a weak shoulder at a relatively lower temperature, around 430 °C, indicating that steaming removes most of the strong acidic sites in this sample. As observed in the fresh samples, the overall acidity of commercial sample is higher than those of the synthesized samples (Table 7). Among the synthesized samples, the FCC catalyst synthesized using 2 wt % Al2O3 doped colloidal silica (FCC-2S) possesses relatively higher acidity than the Al-free analogue (FCC-1S). The SiO2/Al2O3 mole ratio (SAR) and the NH3-TPD results after steaming indicate that the commercial FCC catalyst has a lot less acidity relative to its high Al2O3 content than the FCC catalysts synthesized from colloidal silica. In addition, although doping the colloidal silica with 2 wt % Al2O3 does not result in any noticeable increase in alumina (SAR) 4.2 for both samples), the higher acidity of FCC-2S synthesized using colloidal silica doped with 2 wt % Al2O3 compared to FCC-1S is noteworthy. 3.5. Microactivity Test (MAT). In order to understand the effect that 2 wt % Al2O3 doped colloidal silica has on activity, microactivity testing (MAT) was conducted on all three steamed FCC catalysts. The data are summarized in Table 8. It becomes apparent when looking at the data in the table that the gasoline yield on FCC-2S (53.28 wt %) is substantially higher than that on FCC-1S (49.66 wt %). The increase in gasoline yield is notable as this is the major significant difference observed in the FCC catalyst synthesized using Al2O3 doped colloidal silica. Also, the amount of gasoline generated by this sample is essentially equal to that of the commercial FCC catalyst (53.27 wt %). The conversion defined as (100 - (LCO wt % + HCO wt %)) of the FCC-2S catalyst is also significantly higher (69.17 wt %) than that obtained over FCC-1S without doped Al2O3 (65.78 wt %), with the majority of the conversion coming in the form of gasoline. Although the conversion is higher with the commercial FCC catalyst (72.30 wt %), it is apparent that
much of this increase is due to dry gas, coke, and (C3, nC4, and iC4) products compared to the FCC catalysts synthesized in the present study. The FCC catalyst made with the 2 wt % Al2O3 doped colloidal silica (FCC-2S) offers other interesting results that are not typical when aluminum is added to an FCC catalyst matrix. In general, the dry gas, light olefin selectivity, and coke products increase with aluminum addition to the matrix;5,6 however, the dry gas actually decreases for the FCC-2S (1.34 wt %) compared to the commercial catalyst, which gave a higher dry gas value of 1.81 wt %. As with the dry gas, the light olefin selectivity ((C3), iC4), and nC4))/VGO conversion) decreases with the addition of 2 wt % Al2O3 into the colloidal silica (9.0%) versus Al-free colloidal silica (10.5%). The coke, on the other hand, does increase slightly for the 2 wt % Al2O3 doped colloidal silica (2.98 wt %) versus the colloidal silica (2.59 wt %). Another interesting result is the slight effect the 2 wt % Al2O3 doped colloidal silica in the FCC catalyst has on the paraffins (C3, nC4, and iC4) compared to the FCC catalyst made with Al-free colloidal silica. The values of these products typically increase with increasing aluminum content in the matrix of an FCC catalyst.5 With only 2 wt % Al2O3 in the colloidal silica it may be possible that these very interesting results may also be due to the unique porosity of the colloidal based FCC particles. If the reactants have a less encumbered pathway through the matrix, the overcracking of desired products may be less likely to occur. Further work is being conducted to show the significance of porosity on FCC catalysts. 4. Conclusions The use of colloidal silica or 2 wt % Al2O3 doped colloidal silica as a binder in FCC catalysts has been shown to be quite effective in terms of attrition resistance and porosity. The somewhat lower attrition of the synthesized FCC samples compared to a commercial sample could be due to reasons other than the use of colloidal silica as a binder. These reasons potentially include conditions and scale of spray drying, and postsynthesis modifications. The porosity generated by the colloidal silica binders is of extreme interest due to the need of the reactants to pass through the colloidal binder system before reaching the active component (zeolite). In addition, the porosity due to the colloidal binder appears to be quite narrow compared to that in the commercial FCC catalyst, which may be important in preventing areas of increased coking due to plugging of smaller pores. The doping of Al2O3 to the colloidal silica also appears to be a more effective method of imparting acidity than through separate sources of alumina or even conventional precipitated silica-alumina as described in the literature. The gain in acidity with the Al2O3 doped colloidal silica appears to have a direct effect on gasoline yield. Literature Cited (1) Plank, C. J.; Rosinski, E. J. Metal-acid zeolite catalysts: a breakthrough in catalytic cracking technology. In Chem. Eng. Prog., Symp. Ser. 1967, 63 (No. 73), 26-30.
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(2) Blazek, J. J. Zeolitic catalyst proved on wax stock. Oil Gas J. 1971, 69 (45), 66-68. (3) Oblad, A. G. Molecular Sieve Cracking Catalysts. Oil Gas J. 1972, 70 (13), 84-88. (4) Scherzer, J. Correlation between catalyst formulation and catalytic properties. In Fluid catalytic cracking: science and technology studies in surface science and catalysis; Magee, J. S., Mitchell, M. M., Jr., Eds.; Elsevier Science: New York, 1993; Vol. 76, 145-182. (5) Al-Khattal, S. The influence of alumina on the performance of FCC catalysts during hydrotreated VGO catalytic cracking. Energy Fuels 2003, 17, 62-68. (6) Scherzer, J. Octane-enhancing, zeolitic FCC catalysts: scientific and technical aspects. Catal. ReV.sSci. Eng. 1989, 31 (3), 215-354. (7) Otterstedt, J.-E.; Zhu, Y.-M.; Sterte, J. Catalytic cracking of heavy oil over catalysts containing different types of zeolite Y in active and inactive matrices. Appl. Catal. 1988, 38, 143-155. (8) Plank, C. J.; Drake, L. C. Differences between silica and silicaalumina gels. I. Factors affecting the porous structure of these gels. J. Colloid Sci. 1947, 2, 399-412. (9) Oblad, A. G.; Milliken, T. H.; Mills, G. A. Chemical characteristics and structure of cracking catalysts. AdV. Catal. 1951, 3, 199-247. (10) Manton, M. R. S.; Davidtz, J. C. Controlled pore sizes and active site spacing determining selectivity in amorphous silica-alumina catalysts. J. Catal. 1979, 60, 156-166. (11) Courty, P.; Marcilly, C. In Preparations of Catalysts, III; Elsevier: Amsterdam, 1983; p 485. (12) Occelli, M. L.; Kalwei, M.; Wolker, A.; Eckert, H.; Auroux, A.; Gould, S. A. C. The use of nuclear magnetic resonance, microcalorimetry
and atomic force microscopy to study the aging and regeneration of fluid cracking catalysts. J. Catal. 2000, 196, 134-148. (13) Kortunov, P.; Vasenkov, S.; Karger, J.; Fe Elia, M.; Perez, M.; Stocker, M.; Papadopoulos, G. K.; Theodorou, D.; Drescher, B.; McElhiney, G.; Bernauer, B.; Krystl, V.; Kocirik, M.; Zikanova, A.; Jirglova, H.; Berger, C.; Glaser, R.; Weitkamp, J.; Hansen, E. W. Diffusion in fluid catalytic cracking catalysts on various displacement scales and its role in catalytic performance. Chem. Mater. 2005, 17, 2466-2474. (14) Holland, B. T.; Batllo, F.; MacDonald, D.; Ortiz, C. Y. Colloidal compositions and methods for preparing the same. U.S. Patent 2005234136 A1, 2005. (15) Alexander, G. B. Aluminosilicate aquasols and their preparation. U.S. Patent 2,974,108, 1961. (16) Brinker, J. C.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: San Diego, 1990; pp 518-521. (17) Holland, B. T. Unpublished results, Nalco Company: Naperville, IL, 2005. (18) Gilson, J. P.; Edwards, G. C.; Peters, A. W.; Rajagopalan, K.; Wormsbecher, R. F.; Roberie, T. G.; Shatlock, M. P. Penta-co-coordinated aluminum in zeolites and aluminosilicates. J. Chem. Soc., Chem. Commun. 1987, 91-92.
ReceiVed for reView February 21, 2007 ReVised manuscript receiVed April 20, 2007 Accepted April 24, 2007 IE0702734
