Ind. Eng. Chem. Res. 2001, 40, 1065-1075
1065
Spray-Dried Iron Fischer-Tropsch Catalysts. 1. Effect of Structure on the Attrition Resistance of the Catalysts in the Calcined State Rong Zhao,† James G. Goodwin, Jr.,*,‡ K. Jothimurugesan,§ Santosh K. Gangwal,| and James J. Spivey⊥ Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, Department of Chemical Engineering, Clemson University, Clemson, South Carolina 29634, Department of Chemical Engineering, Hampton University, Hampton, Virginia 23668, Research Triangle Institute, P.O. Box 12194, Research Triangle Park, North Carolina 27709-2194, and Department of Chemical Engineering, North Carolina State University, Raleigh, North Carolina 27695
The use of Fe Fischer-Tropsch (F-T) catalysts in slurry bubble column reactors (SBCRs) has been problematic in the past because of their poor attrition resistance. Recently, we have reported the preparation of spray-dried Fe F-T catalysts having attrition resistance suitable for SBCR use, but the reason for this improvement was not clear. This paper focuses on research done to better understand the reason for the high attrition resistance of some of the Fe catalysts prepared. Understanding the relationship between the catalyst attrition resistance and composition/ structure is important for the preparation of attrition-resistant Fe catalysts. In the present study, two series of spray-dried Fe F-T catalysts having the composition Fe/Cu/K/SiO2 but with different amounts of precipitated and/or binder SiO2 were investigated. All of the catalysts studied were evaluated in their calcined form. This was done to minimize any possible attrition due to Fe phase change (such as can occur during activation and F-T synthesis) in order to address the effect of the other catalyst properties. A companion paper addresses attrition due to phase change after carburization. It was found that particle density, principally among other particle properties of the catalysts, correlated with the intrinsic catalyst attrition resistance. Changes in fluidization in the jet cup attrition test with changes in particle density only had minimal effects on the results. Particle density differences reflected differences in the catalyst inner structure. Differences in SiO2 type and concentration resulted in different structures for the SiO2 network and therefore affected the catalyst structure. Introduction Fischer-Tropsch synthesis (FTS) is the major route for converting syngas (CO + H2) made from coal or natural gas into a wide variety of hydrocarbons.3,4 Iron catalysts are the preferred catalysts for FTS based on coal and have been a continuing research focus.5-10 Such catalysts are relatively inexpensive, possess reasonable activity for FTS, and have excellent water gas shift (WGS) activity compared to cobalt catalysts. Such WGS capability enables iron Fischer-Tropsch (F-T) catalysts to process low H2/CO ratio syngas without an external shift reaction step. In the development of FTS over the past 20 years, the application of slurry bubble column reactors (SBCRs) has drawn much attention. This is due to their excellent heat removal capability during reaction. However, commercial use of SBCRs is just now being applied. One of the major drawbacks in the industrial application of SBCRs is catalyst attrition, especially when iron catalysts are used.11,12 The attrition of catalysts in SBCRs causes plugging problems of down* To whom all correspondence should be addressed. Email:
[email protected]. Tel: (864) 656-3055. Fax: (864) 656-0784. † University of Pittsburgh. ‡ Clemson University. § Hampton University. | Research Triangle Institute. ⊥ North Carolina State University.
stream filters as well as lower product quality. The use of supported iron catalysts can improve catalyst attrition resistance, but at the expense of lower specific catalyst activity.3,4,13,14 To improve the physical strength of the catalysts without sacrificing activity, spray drying has been recently used in the preparation of iron F-T catalysts.1,2,12,15,16 Dupont had used spray drying in combination with catalyst composition to successfully improve the attrition resistance of fluidized-bed vanadyl phosphate and multicomponent molybdate (MCM) acrylonitrile catalysts.17-19 It was suggested that the physical strength improvement of these catalysts was possibly related to the formation of the SiO2 incorporated into an “egg-shell” structure.19 In a recent investigation of the preparation procedures of spray-dried Fe catalysts,15 it was also suggested that the SiO2 concentration might be higher in the surface region of the resulting catalyst particles. However, these conclusions cannot fully explain the fact that the attrition resistance of some of the spray-dried Fe catalysts studied,1,2,16 prepared under very similar conditions, varied over a wide range but did not correlate linearly with the SiO2 concentration. If the egg-shell structure of SiO2 were responsible for the improvement of the catalyst physical strength, the thickness of such a shell would have increased with the SiO2 concentration and should have resulted in better catalyst attrition resistance. This inspired our interest to explore in greater detail the catalyst properties
10.1021/ie000644f CCC: $20.00 © 2001 American Chemical Society Published on Web 01/16/2001
1066
Ind. Eng. Chem. Res., Vol. 40, No. 4, 2001
affecting the physical strength of spray-dried Fe catalysts. In the present research, the characteristics of two series of spray-dried Fe catalysts were studied. The goal was to investigate the relationship between the catalyst attrition resistance and the structure and particle properties of the catalysts in the calcined state as affected by the refractory SiO2 present. An understanding of this relationship would help us to separate the effects on catalyst attrition of composition and spray drying from those of the Fe phase change during activation and reaction. This understanding should also apply, in part at least, where other metals are used besides Fe. The effects of the types and concentrations of binder material, i.e., refractory SiO2 (precipitated or binder SiO2), as well as the morphology and porosity of the catalyst particles on attrition are addressed. The effect of the Fe phase change during carburization on catalyst attrition is the subject of the second paper in this series. Experimental Section Catalyst Preparation. Two series of iron catalysts were prepared for this study. One series of catalysts was prepared without precipitated SiO2 but with different weight percentages of binder SiO2. The other series of catalysts was prepared with different levels of precipitated SiO2 and with roughly 10 wt % of binder SiO2. For both series, catalysts having compositions of 100Fe/ 5Cu/4.2K/xSiO2 were prepared. First, precipitation was induced by the addition of ammonium hydroxide to an aqueous solution containing Fe(NO3)3‚9H2O, Cu(NO3)2‚ 2.5H2O, and Si(OC2H5)4 (if added to give precipitated SiO2) in the desired ratio. Potassium promoter was added as aqueous KHCO3 to this precipitate, which was then slurried with the binder SiO2 precursor. The final step was to spray dry each catalyst at 250 °C in a Niro spray drier, which is directly scalable for the production of commercial quantities. After spray drying, each catalyst was then calcined at 300 °C for 5 h in a muffle furnace. The detailed preparation conditions and procedures can be found in refs 2 and 16. As stated above, two different types of SiO2 (precipitated SiO2 and binder SiO2) were used in the preparation of the catalysts studied. The term SiO2 in this paper refers to either or both of them. The following nomenclature is used: the letter P represents precipitated SiO2, while the letter B stands for binder SiO2. The numbers in the parentheses represent the concentration of each type of SiO2 (as the weight percentage of the total catalyst weight). For the series of catalysts without precipitated SiO2, P(0) is used. For example, a catalyst designated as Fe/P(0)/B(11) refers to an iron catalyst prepared without precipitated SiO2 but with 11 wt % of binder SiO2. Because the concentrations of Cu and K in the initial slurry during preparation were not varied, they are not addressed in the nomenclature used. In addition, the total concentration of SiO2 is expressed relative to the total weight of the catalyst and was used in the comparisons of catalyst attrition resistance and other catalyst properties. After preparation and calcination, the catalysts were tested in a fixed-bed reactor and were found to have good F-T reaction activity and selectivity comparable to those of commercial Fe F-T catalysts.1,2,16 Catalyst Characterization. (a) Attrition Resistance. The calcined catalysts were sieved between
standard sieves of 38 and 90 µm before attrition testing. The sieving was applied until particles no longer passed through. The attrition resistances of the calcined catalysts were evaluated using the jet cup test, a proposed ASTM method20 which has been demonstrated to measure attrition that is characteristic of an SBCR21 and similar to that produced by the ASTM air-jet test.22 In the jet cup tests, 5 g of each sample was used with an air jet having a flow rate of 15 L/min (with a relative humidity of 60 ( 5%) at room temperature for 1 h. The detailed system configuration and test procedure can be found elsewhere.21 The fines were collected by a thimble filter at the outlet of the jet cup chamber. The weight of the fines collected was divided by the weight of the total sample recovered to calculate the weight percentage of fines lost, one of the attrition indices used in this paper. In addition, an ultrasonic attrition test21 was used to also determine the attrition resistance of some of the catalysts. The testing procedure and system set up have been described in our earlier paper.21 In this study, a power input of 150 W was used and 0.5 vol % of the sample was loaded for each measurement. (b) Particle Size Distribution (PSD). PSDs of the samples both as prepared (in the calcined state) and after jet cup testing were measured using a Leeds & Northrup Microtrac 7990-11 laser particle size analyzer. Each sample was put into 50 mL of deionized water and dispersed using an ultrasonic bath. Such an ultrasonic bath was used to better disperse the catalyst sample in the distilled water and produce more reproducible data. The ultrasonic bath was used for a minimal period of time (approximately a few seconds), and no significant breakage of catalyst particles was observed after using the ultrasonic bath by comparing the Microtrac results of the same sample without and with use of ultrasound. The results of several measurements of the same sample were averaged in order to minimize the error. The detailed sampling and measuring procedures have been described elsewhere.21 In this study, change in the volume moment, a type of average particle size commonly used to represent a particular PSD, has been selected as a useful indicator of the attrition process in addition to the percent fines generated. The calculation of the volume moment is described in ref 21. The fines and the particles remaining in the jet cup system chamber were measured separately for their PSDs. Both distributions were combined according to the weight percentage of each portion to calculate the average particle size (volume moment). (c) Particle Density. The particle and skeletal densities of both series of catalysts were determined using low-pressure mercury displacement and N2 physisorption pore-volume measurements, where particle density is the particle mass divided by its volume and skeletal density is the particle mass divided by its volume excluding all of the open pores.23 (d) Brunauer-Emmett-Teller (BET) Surface Area and Pore Size Distribution. The BET surface areas and pore size distributions (micropore and mesopore) of the catalysts were determined by N2 physisorption using a Micromeritics ASAP 2010 automated system. These parameters were determined both for catalyst samples as prepared (in the calcined state) and for ones after attrition testing. Each sample was de-
Ind. Eng. Chem. Res., Vol. 40, No. 4, 2001 1067 Table 1. Jet Cup Attrition Resistance Test Results for the Spray-Dried Iron Catalysts
catalyst
precipitated silica (pbw)
binder silica (wt %)
original volume moment (µm)a,b
volume moment after jet cup test (µm)a,b
net change in volume moment (%)
jet cup fines (wt %)c,d
Fe/P(0)/B(4) Fe/P(0)/B(7) Fe/P(0)/B(11) Fe/P(0)/B(14) Fe/P(0)/B(17) Fe/P(4)/B(10) Fe/P(7)/B(10) Fe/P(10)/B(10) Fe/P(12)/B(10) Co/Zr/SiO2e Co/Ru/Al2O3f
0 0 0 0 0 3.76 6.97 9.73 12.1 N/A N/A
3.85 7.41 10.7 13.8 16.7 10.3 9.97 9.67 9.41 N/A N/A
78.0 86.7 88.8 69.9 63.5 103.0 83.0 90.2 85.9 79.9 75.1
39.2 45.4 67.8 47.6 23.7 37.3 34.1 27.7 30.1 45.16 63.7
49.7 47.6 23.6 31.9 62.7 63.8 58.9 69.3 65.0 43.5 15.2
26.6 21.8 8.5 18.2 51.6 26.6 33.9 39.6 41.3 31.1 5.7
a Volume moment is a volume mean diameter of the particles (see ref 21). b Average of three PSD measurements; error ) (5% of the value measured. c Fines wt % ) (weight of fines collected in the thimble filter/weight of the total catalyst recovered) × 100%. d Error ) (10% of the value measured. e Co/Zr/SiO2 was a Davison 952 silica-supported 20 wt % cobalt catalyst, promoted with 8.5 wt % ZrO2. f Co/Ru/SiO was a Vista B supported 20 wt % cobalt catalyst, promoted with 0.5 wt % Ru. 2
gassed in the Micromeritics system at 100 °C for 1 h and then at 300 °C for 2 h prior to each measurement. (e) Reducibility. The reducibilities of the calcined iron catalysts as prepared were measured by temperature-programmed reduction (TPR) using an Altamira AMI-1 system. The TPR measurements were carried out using 5% H2 in Ar with a flow rate of 30 cm3/min and a temperature ramp from 30 °C up to 900 °C at 5 °C/min. (f) Acid Leaching. To study the SiO2 network incorporated in the catalysts, acid leaching was performed by treating the catalysts using a 30% HCl solution (pH ) 1) for 48 h. After the iron dissolved, the residue of each sample was washed several times using deionized water. After filtration, the residue was dried under vacuum at room temperature in order to avoid any possible agglomeration caused by heating. (g) Particle Morphology. Particle morphology was obtained using a Philips XL30 FEG scanning electron microscope (SEM) for each catalyst (as prepared and after attrition testing) and for the SiO2 structure remaining after acid leaching. The samples were coated with palladium before the measurements to avoid charging problems. Both fines and particles remaining in the chamber were analyzed using SEM. The inner morphology of the catalyst particles was investigated at high magnification using a Hitachi H-7100 transmission electron microscope (TEM) and microtomed catalyst samples. The TEM was operated at 50 kV. (h) Phase and Crystallinity. X-ray powder diffraction (XRD) patterns were obtained with a Philips X’pert X-ray diffractometer using Cu KR radiation. A 0.5° beam slit and a parallel-beam detection system with a graphite monochromator was used. XRD was done for each catalyst sample as prepared (calcined state) and for some samples after acid leaching using a continuous scan mode at a scan rate of 0.02° (2θ)/step 0.8 s exposure time/step from 10° to 80°. A Fe2O3 (hematite) sample (99.98% purity) purchased from Aldrich Chemicals was used as a reference. (i) Elemental Analysis. The powder samples prepared for SEM study were used for particle surface composition analysis using energy-dispersive X-ray spectroscopy (EDXS). Cross sections of catalyst particles were prepared by mixing the particles with a lowviscosity polymer gel and, after consolidation of the polymer gel, cutting using a microtome. The microtomed samples were mounted on an aluminum stub, and the resulting flat surface was used for EDXS.
Figure 1. Weight percentage of fines lost in the jet cup test vs total SiO2 concentration in the catalysts without precipitated SiO2.
Results Catalyst Attrition. The attrition results for all of the iron catalysts studied are summarized in Table 1. Two parameters, weight percentage of fines lost and net change in the volume moment, are used in this paper as indications of catalyst attrition. Although the weight percentage of fines lost has been suggested to be affected by many fluidization properties,21 it is still reported and used as the major attrition index in this paper because any production of fines, which can cause filter plugging problems and product contamination, is critical in SBCR operation. A plot of the weight percentage of fines lost versus the total concentration of SiO2 (relative to the total catalyst weight) in the catalysts is shown in Figure 1 for the series of catalysts with only binder SiO2 and in Figure 2 for the series of catalysts with both types of SiO2. For the series without precipitated SiO2, an intermediate concentration of binder SiO2 resulted in optimum attrition resistance (Table 1 and Figure 1). The catalyst with 11 wt % of binder SiO2 but no precipitated SiO2, Fe/P(0)/B(11), was the most attrition-resistant one among all of the catalysts tested. This has been shown to be completely reproducible.1,2 With the addition of precipitated SiO2, the catalyst attrition (as indicated by the change in the average particle size or volume moment) was greater than that with binder SiO2 alone but did not vary by much for the different concentrations of precipitated SiO2. However, for the same series of
1068
Ind. Eng. Chem. Res., Vol. 40, No. 4, 2001 Table 2. Comparison of Results from Ultrasonic and Jet Cup Attrition Tests
catalyst Fe/P(0)/B(9) Fe/P(7)/B(10) Fe/P(10)/B(10)
ultrasonic test jet cup test ultrasonic test jet cup test ultrasonic test jet cup test
net change in volume moment (%)a
fines lost (wt %)b
29.4 24.0 56.5 58.9 47.5 69.3
13.5 4.8 33.6 33.9 28.0 39.6
a Error ) (5% of the value measured. b Error ) (10% of the value measured.
Figure 2. Weight percentage of fines lost in the jet cup test vs total SiO2 concentration in the catalysts.
Figure 4. XRD results for the calcined iron catalysts as prepared and for Fe2O3.
Figure 3. Accumulated fines lost vs time on stream of jet cup attrition testing.
catalysts, the amount of fines generated during attrition testing increased significantly with an increase in the concentration of the precipitated SiO2 (Table 1 and Figure 2). The results for two supported cobalt catalysts are also listed in Table 1 as benchmarks. One of the benchmark catalysts consisted of 20 wt % of cobalt and 8.5 wt % of ZrO2 supported on spray-dried Davison 952 silica, and the other one consisted of 20 wt % of cobalt and 0.5 wt % of Ru supported on spray-dried Vista B alumina. Both of the cobalt catalysts were prepared using incipient wetness and have been tested under typical F-T reaction conditions using a laboratory SBCR.21 In laboratory SBCR runs of 240 h, they were both found to be adequate in terms of attrition resistance (since attrition typically is greater during the initial days of a run). The attrition rates during jet cup attrition of these benchmark catalysts and selected spray-dried Fe catalysts are shown in Figure 3. The results indicate that some of the spray-dried iron catalysts (in their calcined state) which contain only binder SiO2 have a strong potential for SBCR use, especially because they are also attrition-resistant after carburization (the subject of the second paper in this series). The catalyst attrition properties were also determined using an ultrasonic test.21 We were concerned that differences in the particle density of the catalysts would affect their fluidization and might impact the resulting
attrition measurements using the jet cup test. Because fluidization is not an issue when using ultrasound to induce attrition, it was felt that the ultrasonic test would help to indicate if the jet cup results were biased in any way. Comparisons of the ultrasonic test and jet cup results are summarized in Table 2. The fines “lost” or more accurately “produced” during the ultrasonic test were based on all of the particles of less than 22 µm, similar to the sizes of particles recovered from the top of the jet cup. The ultrasonic test results showed a trend for these calcined catalysts similar to the jet cup results. The differences seen between tests are due to differences in the means causing attrition in the different tests. Catalyst Chemical Properties. The XRD results (Figure 4) verify that all of the calcined Fe catalysts were in an essentially similar crystalline state and consisted primarily of hematite, Fe2O3. Other components, such as even SiO2 (binder SiO2 and/or precipitated SiO2), were not detectable by XRD for any of the catalysts. The reducibilities of the catalysts were determined using H2 TPR, and the results indicate that H2 consumption during reduction decreased with an increase in the total concentration of SiO2 (Table 3). This change is due mainly to a decrease in the total concentration of iron oxide in the catalysts because the calculated reducibilities of the iron catalysts (on an Fe basis) are approximately the same for all of the catalysts. The TPR curve for each catalyst was similar to others in the literature.24 The addition of precipitated SiO2 did not appear to affect the TPR peak positions or Fe reducibility. However, because there was such a large amount of Fe present, it is likely that any differences in Fe-SiO2 interactions which may have been important in improving the attrition resistance could have
Ind. Eng. Chem. Res., Vol. 40, No. 4, 2001 1069 Table 3. TPR Results for the Calcined Spray-Dried Iron Catalysts
catalyst
H2 consumeda (mmol of H2/ g of catalyst)
Fe reducibility (%)
Fe/P(0)/B(4) Fe/P(0)/B(7) Fe/P(0)/B(11) Fe/P(0)/B(14) Fe/P(0)/B(17) Fe/P(4)/B(10) Fe/P(7)/B(10) Fe/P(10)/B(10) Fe/P(12)/B(10)
39.0 38.8 34.5 34.9 34.1 34.6 32.8 33.1 31.6
74 76 70 74 74 74 73 77 76
a
Error ) (3% of the value measured.
been masked by the reduction of the bulk Fe oxide during TPR. The XRD (Figure 4) and TPR (Table 3) results indicate that these catalysts were not very different chemically. Catalyst Particle Properties. Catalyst PSDs were determined using Microtrac analysis,21 and the average particle size is reported as the volume moment21 (Table 1). Although the catalyst samples were sieved (38-90 µm) before jet cup testing, it can be seen from Table 1 that the volume moment values of different catalysts before attrition testing (but after sieving) still varied significantly, indicating some differences in the particle morphology and PSD of the various catalysts. These PSD differences probably resulted during spray drying because of the compositional differences. The BET surface areas and average pore sizes (microand mesopores) were measured using N2 physisorption for the catalysts both as prepared in the calcined state and after attrition measurement (Table 4). The BET surface area for each series of catalysts generally increased with an increase in the total concentration of SiO2 (Figure 5). The catalysts having precipitated SiO2 had relatively higher surface areas; however, this appears to have been mainly due to the fact that those catalysts also had higher total SiO2 concentrations. The pore volume (micro- and mesopore) also slightly increased with an increase in the concentration of SiO2. The series of catalysts with precipitated SiO2 appears to have had higher pore volumes because of higher total SiO2 concentrations. The average pore radius (calculated using 2 × total pore volume/surface area) varied slightly for all of the catalysts. In general, there was a decrease in the average pore size for both series of catalysts as the SiO2 concentration increased, related obviously to the large increase in the BET surface area. For all of the catalysts, the micro- and mesopore volumes remained essentially unchanged after the jet cup test. The
Figure 5. BET surface area vs total SiO2 concentration in the catalysts.
BET surface areas appeared to decrease slightly for some of the catalysts after attrition testing, and the average pore radius increased slightly for those catalysts, accordingly, but there was no obvious trend with composition. Neither the micro- and mesopore volumes nor the BET surface area of these catalysts was found to be related to catalyst attrition. Consequently, there was no clear trend for the dependence of attrition on the micro- and mesopore size for either catalyst series. Although not apparently affecting the attrition properties of these catalysts, these parameters can be very important for the catalytic properties of these catalysts. The macropore volume was measured using mercury intrusion, and the results are summarized in Table 5. It appears that the series of catalysts with precipitated SiO2 had relatively higher macropore volumes. The total pore volume of this series of catalysts, as determined by both N2 physisorption and mercury intrusion, gradually increased with an increase in the SiO2 concentration. For the series of catalyst with only binder SiO2, the macropore volume went through a minimum value with an increase in the concentration of binder SiO2. The average particle densities were determined using low-pressure mercury displacement and are listed in Table 5. On the basis of the particle densities and pore volumes measured using N2 physisorption and mercury intrusion, the average catalyst skeletal densities were calculated and are also listed in Table 5. The skeletal density is defined as the density of a particle excluding all of the open pores, whereas the particle density is defined as the density of a particle including its pores. It can be seen that the particle density varied with the SiO2 concentration in a relatively narrow range. For the
Table 4. BET Surface Areas and Average Pore Sizes of the Iron Catalysts
catalyst
total SiO2 concn (wt %)
Fe/P(0)/B(4) Fe/P(0)/B(7) Fe/P(0)/B(11) Fe/P(0)/B(14) Fe/P(0)/B(17) Fe/P(4)/B(10) Fe/P(7)/B(10) Fe/P(10)/B(10) Fe/P(12)/B(10)
3.85 7.41 10.7 13.8 16.7 14.1 16.9 19.4 21.6
a
BET surface areaa (m2/g) fresh attrited 101 125 146 177 158 179 191 217 245
94.2 108 137 173 168 180 177 189 244
micro- and mesopore volumeb (cm3/g) fresh attrited
average pore radius (Å) fresh attritted
0.29 0.28 0.28 0.37 0.33 0.34 0.37 0.36 0.39
43.6 35.3 32.0 33.8 37.3 35.2 36.9 30.8 30.2
Error ) (5% of the value measured. b Error ) (12% of the value measured.
0.28 0.26 0.29 0.34 0.34 0.34 0.35 0.33 0.40
44.7 36.1 33.9 33.3 37.7 35.8 37.3 33.4 32.6
1070
Ind. Eng. Chem. Res., Vol. 40, No. 4, 2001
Table 5. Porosity of the Spray-Dried Iron Catalysts Measured by N2 Adsorption (Micro- and Mesopore) and Mercury Porosimetry (Macropore)
catalyst
total SiO2 concn (wt %)
pore volume (micro- and mesopore) (cm3/g)a
pore volume (macropore) (cm3/g)b
total pore volume (cm3/g)
particle density (g/cm3)c
skeletal density (g/cm3)d
Fe/P(0)/B(4) Fe/P(0)/B(7) Fe/P(0)/B(11) Fe/P(0)/B(14) Fe/P(0)/B(17) Fe/P(4)/B(10) Fe/P(7)/B(10) Fe/P(10)/B(10) Fe/P(12)/B(10)
3.85 7.41 10.7 13.8 16.7 14.1 16.9 19.4 21.6
0.29 0.28 0.28 0.37 0.33 0.34 0.37 0.36 0.39
0.22 0.17 0.13 0.19 0.27 0.22 0.29 0.32 0.31
0.51 0.45 0.41 0.56 0.60 0.56 0.66 0.68 0.70
1.03 1.11 1.33 1.09 0.90 0.95 0.89 0.96 0.92
2.17 2.21 2.91 2.77 1.93 2.02 2.16 2.75 2.55
a Measured using N physisorption; error ) (5% of the value measured. b Measured using mercury porosimetry; error ) (10% of the 2 value measured. c Determined using low-pressure mercury displacement; error ) (5% of the value measured. d Calculated using the particle density and total pore volume.
Figure 6. Morphology of the series of iron catalysts without precipitated silica (left, catalysts as prepared; right, catalysts after jet cup attrition testing).
series of catalysts without precipitated SiO2, the average particle density reached a maximum value for an optimum concentration of binder SiO2, due partly to changes in the macropore volume. Catalyst Morphology. Figures 6 and 7 illustrate the morphologies of the calcined catalysts without and with precipitated SiO2, respectively. In these figures, SEM micrographs of both the catalyst particles as prepared in the calcined state (on the left) and the particles remaining in the jet cup chamber after attrition testing (on the right) are shown. It is apparent that some catalysts as prepared had particles that were less
Figure 7. Morphology of the series of iron catalysts with precipitated silica (left, catalysts as prepared; right, catalysts after jet cup attrition testing).
agglomerated. There was relatively less agglomeration observed for the series of catalysts with precipitated SiO2 (Figure 7) except for the catalyst [Fe/P(4)/B(10)] with the smallest amount of precipitated SiO2. However, this was not found to relate significantly to the attrition resistance. The presence of agglomerates of primary particles after preparation means that measurements have to be made to determine if changes in PSD are due to primary particle breakage or simply breakup of the agglomerates. For the catalysts with precipitated SiO2, some particles observed appear to have had a structure with interior holes. In Figure 7, the holes appear as dark
Ind. Eng. Chem. Res., Vol. 40, No. 4, 2001 1071
Figure 8. Morphology of the iron catalysts at higher magnification: (A) catalyst particles with holes [Fe/P(10)B(10)]; (B) cross section of a particle with an inner hole [Fe/P(10)B(10)].
spots on the catalyst particles. SEM micrographs taken at higher magnification indicate that these dark spots were actually openings to larger interior holes in the particles (Figure 8A). Such inner holes can also be observed in the SEM micrograph of the cross section of a catalyst particle (Figure 8B). Particles with interior holes were seldom observed for the series of catalysts without precipitated SiO2, even for those having higher concentrations of binder SiO2. The decreases in particle size seen in the SEM micrographs following attrition testing are similar to the results from PSD measurement. Because it appears that the agglomerates formed during catalyst preparation were broken apart during attrition testing, a significant portion of the changes in the average particle size (volume moment) for Fe/P(0)/B(4), Fe/P(0)/B(7), and Fe/P(4)/B(10) following attrition testing was probably due simply to a breaking apart of the particle agglomerates. Fe/P(0)/B(11) did not appear to change as much in the average particle size as the other catalysts. For this catalyst, some agglomerates still remained even after attrition testing. The typical inner morphology of the catalysts as determined using TEM is shown in Figure 9 for Fe/P(0)/ B(11). It was observed that the catalyst particles consisted of nearly spherical primary SiO2 and Fe2O3 particles on a nanometer scale. This was the case for all of the catalysts in this study. Because of the overlapping of these primary particles, it was difficult to perform further analysis of the inner structure of the catalysts. SiO2 Network. The distribution of SiO2 incorporated in the catalyst particles was examined for cross sections of the catalyst particles using EDXS. As illustrated by Figure 10, Si and Fe were evenly distributed across the catalyst particles. This measurement was carried out for all of the catalysts of both series, and the same result was found for all. To study the SiO2 network, which obviously plays an important role in the attrition resistance of the catalysts, acid leaching was applied to both series of catalysts. Because the strong signal of iron oxide during XRD measurements made the analysis of the structure of the SiO2 network in the original catalysts very
Figure 9. TEM of a microtomed Fe/P(0)/B(11) particle.
difficult, such a drastic sample treatment was necessary. The XRD patterns of some of the SiO2 structures remaining after acid leaching are shown in Figure 11. The single broad peak in these XRD patterns indicates that these structures were relatively X-ray amorphous. EDXS results following acid leaching of all of the catalysts show that these amorphous structures consisted of only Si and O (Figure 12 shows a typical example). The iron oxide(s) was, thus, essentially completely removed during acid leaching. Typical morphologies of the SiO2 structures remaining after acid leaching are shown in Figures13 and 14 for the series of catalysts without or with precipitated SiO2, respectively. Because some of the refractory SiO2 agglomerated severely during drying of the residue, SEM micrographs at higher magnification are also shown in Figures 13 and 14. It is apparent at higher magnification that the large particles observed after acid leaching were actually agglomerates of smaller spherical particles. In the absence of precipitated SiO2, the SiO2 structure made up only of binder SiO2 (Figure 13) agglomerated severely during drying after acid leaching, except for SiO2 in catalyst Fe/P(0)/B(11), which agglomerated the least among this series of catalysts. To the contrary, in the presence of precipitated SiO2, the SiO2 network after acid leaching and drying formed individual spherical particles generally with less agglomeration. In addition, some of these SiO2 structures made of both types of SiO2 appear to have had inner holes, which is obvious at higher magnification (Figure 14). These hole structures are considered to be related to those observed in the corresponding catalysts before acid leaching. Discussion Catalyst Attrition Resistance. As mentioned earlier in this paper, two different attrition parameters were used in this study, i.e., net change in the volume moment and weight percentage of fines lost (Table 1). These two parameters differ mathematically and have different physical meanings. Weight percentage of fines lost is a direct representation of the amount of fines (ca.
