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Use of zeta potential measurements in catalyst preparation
10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.
Use of zeta potential measurements in catalyst preparation Stuart Soled,a William Wachter,b Hyung Woa a
ExxonMobil Research and Engineering Company, Corporate Strategic Research, Annandale, NJ 08801, USA b ExxonMobil Research and Engineering Company, ExxonMobil Process Research, Annandale, NJ 08801, USA
Abstract We illustrate two applications where zeta potential measurements have provided useful information for catalyst preparations. In the first case, we decribe how maximizing the electrostatic attraction between the active complex in an impregnating solution and the support leads to smaller metal particles upon reduction. This approach allowed synthesis of small platinum crystallites on a yttria-modifed amorphous silica-alumina support. The yttrium oxide not only titrates some acid sites but it provides more positively charged surface regions (at a given solution pH) on the support that better disperse the anionic chloroplatinate anion and the subsequently formed Pt crystallites. In the second application, we studied the attrition resistance of FCC (fluid cat cracking) catalysts, ~70 micron spray dried particles formed from micron-sized USY (ultrastable Y) zeolite crystals and submicron sol particles. The attrition is minimized when the larger zeolite particles are uncharged while the submicron-sized sol particles are highly charged. The results suggest that the stable colloid formed from nanocrystalline haloing provides an optimized dried and calcined agglomerate. Keywords: zeta potential, fluid cat cracking
1. Introduction Surface charging of small oxide particles can provide a useful tool to enhance catalyst syntheses. In this paper we describe two applications where surface charges are used advantageously- in the first, matching complementary charges on the support surface and metal impregnate complex are used to optimize Pt metal dispersion. This approach was described many years ago by Brunelle [1], and more recently expanded and refined by Regalbuto [2]. To determine the zero point of charge of a support, they measure the buffering action of the surface as it is exposed to solutions with different amounts of acid or base. Zeta potential measurements use a different approach to measure (near) surface charge but the concepts of optimizing support and impregnate electrostatic interactions remain the same. In the bifunctional catalyst briefly described here, a surface yttrium oxide partial monolayer is added to an amorphous silica-alumina support to temper its acidic properties and Pt is added to this modified support to allow bifunctional catalysis. In the second application, we discuss the use of surface charging concepts and zeta potential measurements to optimize the attrition resistance of fluid cat cracking (FCC) catalyst composite particles. The FCC particles consist of micron size zeolite particles held together by submicron sol particles to form 50-70μ composites. In this study, we start with a suite of USY zeolites of variable bulk Si-Al ratios and first determine their relative surface compositions using isoelectric points (IEP). We established an excellent
102
S. Soled et al.
correlation between the measured isoelectric points and the XPS-determined relative surface concentrations of Si and Al. We then show how appropriately matching the IEP of the zeolites with the properties of the sol particles can create an attrition-resistant composite. Small particle metal oxides lower their free energies by terminating their surfaces with the lowest possible charge, which normally means with oxygen. Such an atomic arrangement would create a stoichiometry richer in oxygen than the (crystallographically imposed) bulk stoichiometry and would generate a net negative charge on small oxide particles- this of course, does not occur. Instead, the surfaces normally terminate in hydroxyl groups rather than oxide anions. The hydroxyls effectively lower the surface anion charge from –2 to –1 and allow the particles to maintain a neutral total charge. Some of the hydroxyls may, on calcination, condense with release of water. This condensation creates bridged oxygen anions or coordinatively unsaturated metal cations but it maintains the neutral charge of the metal oxide (or more correctly, metal oxyhydroxide) particles. When small oxide (i.e. oxyhydroxide) particles contact liquid water they no longer have to remain electrically neutral; surface charges can develop because ions in solution are available to neutralize these charges by forming a classical double layer surrounding the particle. Consequently, surface hydroxyls on small oxide particles in aqueous suspension will ionize and their surfaces develop a net positive or negative charge, as a function of the pH (the protonation or deprotonation driving force) in the contacting solution. Zeta potential refers to the charge at the interface of the shear plane separating the tightly held compact layer and the more loosely held diffuse layer of the classical double layer. The isoelectric point represents the pH at which the zeta potential equals zero, and it reveals information about the near surface cation composition of the oxide support. At a specific pH for each solid, small but equal amounts of M(OH2)+ and MO– co-exist, with M(OH) being the most abundant surface species; this represents the isoelectric point. Consequently, the zeta potential is positive at pH values below the isoelectric point, where M(OH2)+ become the majority surface species as protons are donated from the hydronium ions, and negative at pH values above the isoelectric point. There is a difference between the zero point of charge and the IEP, but for most cases considered here (i.e. weak specific anion adsorptions), they are closely related. The IEP gives information about the chemical nature of the metal oxide support because oxides containing cations with high charge to radius ratio (for ex. Si+4) have low IEPs and oxides with lower charge-density cations have higher IEPs. This occurs because at high charge densities (e.g. for Si+4) a large driving force (low pH) is required to protonate the OH group as it is close to the small and already highly charged Si+4 cation. In other words, a protonated OH2+ on silica (if it were even to exist) would donate its proton to a solution at low pH. This sounds strange since silica classically acts as a non-acidic inert support in gaseous environments typical of most catalytic reactions, but in aqueous suspension it is very acidic. In contrast, Al+3 cations, because they have a lower charge and are larger than Si+4, are more easily protonated and therefore have a higher isoelectric point (~9). Ti+4 like Si+4 is tetravalent, but it has a larger cationic radius and thereby is more easily protonated: its isoelectric point is between 6 and 7 [3]. For mixed oxide supports the average surface population of the cations at the measured IEP reflects a molar average of the individual metal oxide IEP’s. Historically, electrophoresis measurements were used to measure isoelectric points, but fortunately, during the last couple of decades, simple and inexpensive laboratory instrumentation has become available to measure zeta potential even in relatively high
Use of zeta potential measurements in catalyst preparation
103
concentration suspensions. These instruments measure acoustic signals created when the double layer around small particles distorts when these particles are placed in a megahertz ac electric field.
2. Experimental Table 1 lists the suite of ultrastable Y zeolites zeolites investigated in this study. Five grams of each zeolite was added to 200 cc of water and dispersed for 5 minutes using an ultrasonic dispersion probe and then measured with a Matec 8050 electrokinetic instrument. Zeta potentials were monitored during titrations using 1N HCl or 1N NaOH; titration with HCl if the initial zeta potential was negative and with NaOH if the initial zeta potential was positive. The crystallinity of the samples was determined by x-ray diffraction following ASTM Procedure D-3906-91. XPS measurements were performed on a Leybold-Heraeus ultra high vacuum system equipped with an Al Kα x-ray source (hυ=1486.6 eV) and a hemispherical energy analyzer. Photoemission spectra were obtained normal to the analyzed surface of pressed wafer samples with the electron analyzer operating at 50 eV pass energy. Surface areas were determined by a multipoint BET measurement after outgassing at 300C. The Davison Attrition Index (DI) uses 7.0 cc of sample catalyst which is screened to remove particles in the 0 to 20 micron range. The remaining particles are then contacted in a hardened steel jet cup having a precision bored orifice through which an air jet of humidified (60%) air is passed at 21 liter/minute for 1 hour. The DI is defined as the percent of 0-20 micron fines generated during the test relative to the amount of >20 micron material initially present, i.e., the DI = 100 x (wt% of 0 – 20 micron material formed during test)/ (wt% of original 20 microns or greater material before the test). The lower the DI, the more attrition resistant is the catalyst. The zeolites were obtained from an external source; consequently, the details regarding their modification are not known. Table 1. Properties of USY zeolites. Sample Label
% wt. Na
Bulk Si/Al
XPS Si(2p)/Al(2p)
BET Surface Area (m2/g)
% Crystallinity
A
0.86
4.62
8.82
543
93
B
1.5
3.54
4.6
590
98
C
0.43
2.99
3.76
569
79
D
0.15
2.66
1.73
560
67
E
0.03
6.55
3.56
642
88
F
0.14
2.74
1.58
543
66
G
1.5
2.38
1.65
490
67
H
0.11
2.57
0.93
498
83
I
0.15
2.69
0.86
593
91
J
0.65
2.71
0.86
667
98
K
2.8
2.61
1.45
605
99
104
S. Soled et al.
For the supported SiO2-Al2O3 catalysts, an amorphous silica alumina with a bulk concentration of 55%SiO2 and 45%Al2O3 was used. Aqueous yttrium nitrate was impregnated via incipient wetness impregnation onto the supports, and the samples were then dried and calcined at 450C. 0.3% wt. Pt was impregnated using a hexachlorplatinate precursor.
3. Results and discussion
7.4
60%
7
48%
6.6
36%
6.2
24%
5.8
12%
5.4 0
2
4
6
8
10
12
14
16
% Pt dispersion (H/M)
isoelectric point
Professor Brunelle in 1978 authored a masterful review describing a strategy for optimizing supported metal catalyst preparations by maximizing the electrostatic attraction between the precursor and support [1]. Others have followed and expanded on this protocol over the years, with many of the newer studies described by Regalbuto [2]. Our work is also based on enhancing electrostatic interactions but uses zeta potential measurements to chose the optimum preparation method for synthesizing Pt clusters on modified silica-alumina supports. In this study we were interested in tempering the acidity of amorphous silica-alumina by titrating with partial monolayers of yttrium oxide so that the residual acid sites would have strengths similar to chlorided or fluorided alumina [4]. The application involved bifunctional catalysis so optimizing platinum dispersion on the modified silica-alumina supports was important. Yttrium oxide is mildly basic and disperses readily onto the silica-alumina surface [5]. Since the isoelectic point of an oxide is related to the charge to radius ratio of its surface cations, the large trivalent yttrium cations have high isoelectric points- above 11 [3]. We determined that as the yttria surface population increases, the isoelectric point of the modified silica-alumina increases as does the hydrogen chemisorption uptakes (dual isotherm measurements at 40oC) of the reduced hexachloroplatinate anion (see Figure 1). The IEP represents an average surface state, so that below monolayer coverage we measure a contribution from regions of silica-alumina and regions of yttrium oxide. At a given pH, the regions of the support with a higher positive charge more strongly attract the dicholorplatinate anion and produce more dispersed Pt. It is reasonable to assume (although not proven here) that the Pt is preferentially located on the yttrium oxide.
0%
18
% Y2O3 in Y2O3/SiO2-Al2O3 Figure 1. Isoelectric Point and Strong Hydrogen Chemisorption of .3%Pt/Yttria Modified-Silica Alumina.
Use of zeta potential measurements in catalyst preparation
105
The second problem we addressed concerns a common issue in catalyst synthesis, namely how to insure that larger particles formed from assembling smaller components maintain physical integrity. In the case of FCC catalysts, 50-70μ spray dried agglomerates were created from micron sized zeolites held together with submicron sol particles. The question we addressed is how to minimize the attrition of the composite. The suite of zeolites chosen consisted of ultrastable Y zeolites of 1-3 micron size with the properties shown in Table 1. The IEP of each of the USY samples was measured by titration with either HCl or NaOH. The fractional surface aluminum concentration for the suite of USY samples was also measured using XPS, and Figure 2 compares the two measurements.
isoelectric point
10 8 6 4 2 0
0
0.1
0.2
0.3
0.4
0.5
0.6
XPS surface Al/(Si + Al) Figure 2. Isoelectric point vs. XPS fractional aluminum content for suite of USY zeolites.
Although we expect XPS to sample to a penetration depth of ~40Å, the fractional aluminum content correlates well with the measured isoelectric points. The variation of isoelectric points from 2 to 9 suggests that for some of the USY samples the surface is silica-rich whereas other samples have predominantly alumina-rich surfaces. Because these samples were obtained from an outside source, the exact methodology used to surface enrich the USY samples is not know, although it would interesting to understand how to do this. Note that although the XPS ratios do correlate with the isoeletric points, the XPS ratios do not extend from aluminum contents of 0 to 1, suggesting that the subsurface layers are not as enriched in Si or Al as the surface. We did have concerns about the stability of the zeolites in acidic media, so we checked the time dependence of our measurements. When we changed the “soak” time in the acid from 15 to 150 seconds between measuring each data point, the zeta potential measurements did not change substantively. This probably results from the low concentration of acid present during titration (~5ml of 1 N HCl in 200 cc water) and the low reactivity at room temperature. Two different sols were used to bind the USY particles together. One was a SuperD silica sol prepared by reacting a sodium silicate solution with an aluminum sulfate/sulfuric acid solution under high shear to a pH 3.0 and the other was an aluminum chlorohydrol sol stable at pH 4.3. In Figure 3 we show schematically what we are attempting to achieve in the spray drying process, with the relative sizes of the USY zeolites and sol particles depicted.
106
S. Soled et al.
composite catalyst
+
(70μ)
submicron sol particles
zeolite, clay or alumina binder 1μ
Figure 3. Binding of components of an FCC catalyst.
The attrition of the composite FCC particles was measured using the Davison Attrition Test. Higher values of the index indicate weaker particles that attrit easier. A series of catalysts were prepared by spray drying the USY zeolites with one of the two sols and the attrition of the composite was measured. The results are shown in Figure 4.
Davison Attrition Index (increasing weakness) ==>
60 50 40 30 20 10 0
0
1
2
3
4
5
⏐sol pH - zeolite isoelectric point⏐
6
Figure 4. Correlation of FCC Agglomerate Strength and the difference between the absolute valuer of the sol pH and the zeolite IEP.
It is intriguing that the composite is strongest when the larger zeolite crystals have their IEP near to or at the pH of the sol. In the pH stability range of the small sol particles (2-4 for Super-D and 4-5 for alumina chlorohydrol) the sol nanoparticles are electrotatically stabilized with a high surface charge. The results in Figure 4 suggest that at the pH where the sol particles are highly charged, the zeolite should have no surface charge. This result may seem counterintuitive as one might think that the sol particles and the zeolites should be oppositely charged. However, this is not what this data suggests, so another phenomena is operating to create the stable agglomerate. A publication by Jennifer Lewis may help explain this phenomena [6]. She introduced the concept of nanoparticle haloing as a self-organizing process that imparts stability to naturally attractive particles by decorating their superficial areas with highly charged nanoparticles present at critical concentrations. She observed and calculated this effect for the case of micron sized silica particles decorated with a zirconia sol and found that the most stable arrangement occurred when the larger silica particles were
Use of zeta potential measurements in catalyst preparation
107
uncharged and the smaller zirconia nanoparticles had a high charge. Figure 5 represents this phenomena [7].
Figure 5. Representation of Nanoparticle Haloing (from 7).
We are hypothesizing that the FCC particles we have studied behave in an analogous way, with the zeolite crystallites replacing the silica spheres and the zirconia sol being replaced by either Super-D or alumina sol. This stable collodial suspension so formed by nanoparticle haloing has an optimized mixing of the sol and zeolite crystals. On drying and calcination, a porous but strong network forms to hold the agglomerates together. This approach has allowed formation of strong catalysts, even when using the Al chlorohydrol sol. The latter sol was not known to provide stable agglomerates.
4. Conclusion Zeta potential measurements have been used to successfully develop optimized metal dispersion on support oxides. They have also provided a useful tool for the design of attrition resistant FCC catalyst particles.
Acknowledgments The authors wish to thank Joe Baumgartner for help with some of these measurements.
References 1. 2. 3. 4. 5. 6. 7.
J. P. Brunelle, 1978, Pure Appl. Chem., 50, 1211. J. R. Regalbuto, 2009, in “Synthesis of Solid Catalysts”, (ed. K.P. de Jong) Wiley-VCH, p. 33. G. A. Parks and P.L. De Brnyn, 1962, J. Phys. Chem., 66, 967. S. Soled, G. B. McVicker, W.E. Gates and S. Miseo, 1995, US Pat. 5,457,253. S. L. Soled, G. McVicker, S. Miseo, W. Gates, and J. Baumgartner, 1996, Stud. Surf. Sci. Catal., 101A, 563-572 J.A. Lewis, 2001, Langmuir, 17, 8414. M. Jacoby, Jan. 7, 2002, Chemical and Engineering News, p. 11.
Use of zeta potential measurements in catalyst preparation Stuart Soled,a William Wachter,b Hyung Woa a
ExxonMobil Research and Engineering Company, Corporate Strategic Research, Annandale, NJ 08801, USA b ExxonMobil Research and Engineering Company, ExxonMobil Process Research, Annandale, NJ 08801, USA
Abstract We illustrate two applications where zeta potential measurements have provided useful information for catalyst preparations. In the first case, we decribe how maximizing the electrostatic attraction between the active complex in an impregnating solution and the support leads to smaller metal particles upon reduction. This approach allowed synthesis of small platinum crystallites on a yttria-modifed amorphous silica-alumina support. The yttrium oxide not only titrates some acid sites but it provides more positively charged surface regions (at a given solution pH) on the support that better disperse the anionic chloroplatinate anion and the subsequently formed Pt crystallites. In the second application, we studied the attrition resistance of FCC (fluid cat cracking) catalysts, ~70 micron spray dried particles formed from micron-sized USY (ultrastable Y) zeolite crystals and submicron sol particles. The attrition is minimized when the larger zeolite particles are uncharged while the submicron-sized sol particles are highly charged. The results suggest that the stable colloid formed from nanocrystalline haloing provides an optimized dried and calcined agglomerate. Keywords: zeta potential, fluid cat cracking
1. Introduction Surface charging of small oxide particles can provide a useful tool to enhance catalyst syntheses. In this paper we describe two applications where surface charges are used advantageously- in the first, matching complementary charges on the support surface and metal impregnate complex are used to optimize Pt metal dispersion. This approach was described many years ago by Brunelle [1], and more recently expanded and refined by Regalbuto [2]. To determine the zero point of charge of a support, they measure the buffering action of the surface as it is exposed to solutions with different amounts of acid or base. Zeta potential measurements use a different approach to measure (near) surface charge but the concepts of optimizing support and impregnate electrostatic interactions remain the same. In the bifunctional catalyst briefly described here, a surface yttrium oxide partial monolayer is added to an amorphous silica-alumina support to temper its acidic properties and Pt is added to this modified support to allow bifunctional catalysis. In the second application, we discuss the use of surface charging concepts and zeta potential measurements to optimize the attrition resistance of fluid cat cracking (FCC) catalyst composite particles. The FCC particles consist of micron size zeolite particles held together by submicron sol particles to form 50-70μ composites. In this study, we start with a suite of USY zeolites of variable bulk Si-Al ratios and first determine their relative surface compositions using isoelectric points (IEP). We established an excellent
102
S. Soled et al.
correlation between the measured isoelectric points and the XPS-determined relative surface concentrations of Si and Al. We then show how appropriately matching the IEP of the zeolites with the properties of the sol particles can create an attrition-resistant composite. Small particle metal oxides lower their free energies by terminating their surfaces with the lowest possible charge, which normally means with oxygen. Such an atomic arrangement would create a stoichiometry richer in oxygen than the (crystallographically imposed) bulk stoichiometry and would generate a net negative charge on small oxide particles- this of course, does not occur. Instead, the surfaces normally terminate in hydroxyl groups rather than oxide anions. The hydroxyls effectively lower the surface anion charge from –2 to –1 and allow the particles to maintain a neutral total charge. Some of the hydroxyls may, on calcination, condense with release of water. This condensation creates bridged oxygen anions or coordinatively unsaturated metal cations but it maintains the neutral charge of the metal oxide (or more correctly, metal oxyhydroxide) particles. When small oxide (i.e. oxyhydroxide) particles contact liquid water they no longer have to remain electrically neutral; surface charges can develop because ions in solution are available to neutralize these charges by forming a classical double layer surrounding the particle. Consequently, surface hydroxyls on small oxide particles in aqueous suspension will ionize and their surfaces develop a net positive or negative charge, as a function of the pH (the protonation or deprotonation driving force) in the contacting solution. Zeta potential refers to the charge at the interface of the shear plane separating the tightly held compact layer and the more loosely held diffuse layer of the classical double layer. The isoelectric point represents the pH at which the zeta potential equals zero, and it reveals information about the near surface cation composition of the oxide support. At a specific pH for each solid, small but equal amounts of M(OH2)+ and MO– co-exist, with M(OH) being the most abundant surface species; this represents the isoelectric point. Consequently, the zeta potential is positive at pH values below the isoelectric point, where M(OH2)+ become the majority surface species as protons are donated from the hydronium ions, and negative at pH values above the isoelectric point. There is a difference between the zero point of charge and the IEP, but for most cases considered here (i.e. weak specific anion adsorptions), they are closely related. The IEP gives information about the chemical nature of the metal oxide support because oxides containing cations with high charge to radius ratio (for ex. Si+4) have low IEPs and oxides with lower charge-density cations have higher IEPs. This occurs because at high charge densities (e.g. for Si+4) a large driving force (low pH) is required to protonate the OH group as it is close to the small and already highly charged Si+4 cation. In other words, a protonated OH2+ on silica (if it were even to exist) would donate its proton to a solution at low pH. This sounds strange since silica classically acts as a non-acidic inert support in gaseous environments typical of most catalytic reactions, but in aqueous suspension it is very acidic. In contrast, Al+3 cations, because they have a lower charge and are larger than Si+4, are more easily protonated and therefore have a higher isoelectric point (~9). Ti+4 like Si+4 is tetravalent, but it has a larger cationic radius and thereby is more easily protonated: its isoelectric point is between 6 and 7 [3]. For mixed oxide supports the average surface population of the cations at the measured IEP reflects a molar average of the individual metal oxide IEP’s. Historically, electrophoresis measurements were used to measure isoelectric points, but fortunately, during the last couple of decades, simple and inexpensive laboratory instrumentation has become available to measure zeta potential even in relatively high
Use of zeta potential measurements in catalyst preparation
103
concentration suspensions. These instruments measure acoustic signals created when the double layer around small particles distorts when these particles are placed in a megahertz ac electric field.
2. Experimental Table 1 lists the suite of ultrastable Y zeolites zeolites investigated in this study. Five grams of each zeolite was added to 200 cc of water and dispersed for 5 minutes using an ultrasonic dispersion probe and then measured with a Matec 8050 electrokinetic instrument. Zeta potentials were monitored during titrations using 1N HCl or 1N NaOH; titration with HCl if the initial zeta potential was negative and with NaOH if the initial zeta potential was positive. The crystallinity of the samples was determined by x-ray diffraction following ASTM Procedure D-3906-91. XPS measurements were performed on a Leybold-Heraeus ultra high vacuum system equipped with an Al Kα x-ray source (hυ=1486.6 eV) and a hemispherical energy analyzer. Photoemission spectra were obtained normal to the analyzed surface of pressed wafer samples with the electron analyzer operating at 50 eV pass energy. Surface areas were determined by a multipoint BET measurement after outgassing at 300C. The Davison Attrition Index (DI) uses 7.0 cc of sample catalyst which is screened to remove particles in the 0 to 20 micron range. The remaining particles are then contacted in a hardened steel jet cup having a precision bored orifice through which an air jet of humidified (60%) air is passed at 21 liter/minute for 1 hour. The DI is defined as the percent of 0-20 micron fines generated during the test relative to the amount of >20 micron material initially present, i.e., the DI = 100 x (wt% of 0 – 20 micron material formed during test)/ (wt% of original 20 microns or greater material before the test). The lower the DI, the more attrition resistant is the catalyst. The zeolites were obtained from an external source; consequently, the details regarding their modification are not known. Table 1. Properties of USY zeolites. Sample Label
% wt. Na
Bulk Si/Al
XPS Si(2p)/Al(2p)
BET Surface Area (m2/g)
% Crystallinity
A
0.86
4.62
8.82
543
93
B
1.5
3.54
4.6
590
98
C
0.43
2.99
3.76
569
79
D
0.15
2.66
1.73
560
67
E
0.03
6.55
3.56
642
88
F
0.14
2.74
1.58
543
66
G
1.5
2.38
1.65
490
67
H
0.11
2.57
0.93
498
83
I
0.15
2.69
0.86
593
91
J
0.65
2.71
0.86
667
98
K
2.8
2.61
1.45
605
99
104
S. Soled et al.
For the supported SiO2-Al2O3 catalysts, an amorphous silica alumina with a bulk concentration of 55%SiO2 and 45%Al2O3 was used. Aqueous yttrium nitrate was impregnated via incipient wetness impregnation onto the supports, and the samples were then dried and calcined at 450C. 0.3% wt. Pt was impregnated using a hexachlorplatinate precursor.
3. Results and discussion
7.4
60%
7
48%
6.6
36%
6.2
24%
5.8
12%
5.4 0
2
4
6
8
10
12
14
16
% Pt dispersion (H/M)
isoelectric point
Professor Brunelle in 1978 authored a masterful review describing a strategy for optimizing supported metal catalyst preparations by maximizing the electrostatic attraction between the precursor and support [1]. Others have followed and expanded on this protocol over the years, with many of the newer studies described by Regalbuto [2]. Our work is also based on enhancing electrostatic interactions but uses zeta potential measurements to chose the optimum preparation method for synthesizing Pt clusters on modified silica-alumina supports. In this study we were interested in tempering the acidity of amorphous silica-alumina by titrating with partial monolayers of yttrium oxide so that the residual acid sites would have strengths similar to chlorided or fluorided alumina [4]. The application involved bifunctional catalysis so optimizing platinum dispersion on the modified silica-alumina supports was important. Yttrium oxide is mildly basic and disperses readily onto the silica-alumina surface [5]. Since the isoelectic point of an oxide is related to the charge to radius ratio of its surface cations, the large trivalent yttrium cations have high isoelectric points- above 11 [3]. We determined that as the yttria surface population increases, the isoelectric point of the modified silica-alumina increases as does the hydrogen chemisorption uptakes (dual isotherm measurements at 40oC) of the reduced hexachloroplatinate anion (see Figure 1). The IEP represents an average surface state, so that below monolayer coverage we measure a contribution from regions of silica-alumina and regions of yttrium oxide. At a given pH, the regions of the support with a higher positive charge more strongly attract the dicholorplatinate anion and produce more dispersed Pt. It is reasonable to assume (although not proven here) that the Pt is preferentially located on the yttrium oxide.
0%
18
% Y2O3 in Y2O3/SiO2-Al2O3 Figure 1. Isoelectric Point and Strong Hydrogen Chemisorption of .3%Pt/Yttria Modified-Silica Alumina.
Use of zeta potential measurements in catalyst preparation
105
The second problem we addressed concerns a common issue in catalyst synthesis, namely how to insure that larger particles formed from assembling smaller components maintain physical integrity. In the case of FCC catalysts, 50-70μ spray dried agglomerates were created from micron sized zeolites held together with submicron sol particles. The question we addressed is how to minimize the attrition of the composite. The suite of zeolites chosen consisted of ultrastable Y zeolites of 1-3 micron size with the properties shown in Table 1. The IEP of each of the USY samples was measured by titration with either HCl or NaOH. The fractional surface aluminum concentration for the suite of USY samples was also measured using XPS, and Figure 2 compares the two measurements.
isoelectric point
10 8 6 4 2 0
0
0.1
0.2
0.3
0.4
0.5
0.6
XPS surface Al/(Si + Al) Figure 2. Isoelectric point vs. XPS fractional aluminum content for suite of USY zeolites.
Although we expect XPS to sample to a penetration depth of ~40Å, the fractional aluminum content correlates well with the measured isoelectric points. The variation of isoelectric points from 2 to 9 suggests that for some of the USY samples the surface is silica-rich whereas other samples have predominantly alumina-rich surfaces. Because these samples were obtained from an outside source, the exact methodology used to surface enrich the USY samples is not know, although it would interesting to understand how to do this. Note that although the XPS ratios do correlate with the isoeletric points, the XPS ratios do not extend from aluminum contents of 0 to 1, suggesting that the subsurface layers are not as enriched in Si or Al as the surface. We did have concerns about the stability of the zeolites in acidic media, so we checked the time dependence of our measurements. When we changed the “soak” time in the acid from 15 to 150 seconds between measuring each data point, the zeta potential measurements did not change substantively. This probably results from the low concentration of acid present during titration (~5ml of 1 N HCl in 200 cc water) and the low reactivity at room temperature. Two different sols were used to bind the USY particles together. One was a SuperD silica sol prepared by reacting a sodium silicate solution with an aluminum sulfate/sulfuric acid solution under high shear to a pH 3.0 and the other was an aluminum chlorohydrol sol stable at pH 4.3. In Figure 3 we show schematically what we are attempting to achieve in the spray drying process, with the relative sizes of the USY zeolites and sol particles depicted.
106
S. Soled et al.
composite catalyst
+
(70μ)
submicron sol particles
zeolite, clay or alumina binder 1μ
Figure 3. Binding of components of an FCC catalyst.
The attrition of the composite FCC particles was measured using the Davison Attrition Test. Higher values of the index indicate weaker particles that attrit easier. A series of catalysts were prepared by spray drying the USY zeolites with one of the two sols and the attrition of the composite was measured. The results are shown in Figure 4.
Davison Attrition Index (increasing weakness) ==>
60 50 40 30 20 10 0
0
1
2
3
4
5
⏐sol pH - zeolite isoelectric point⏐
6
Figure 4. Correlation of FCC Agglomerate Strength and the difference between the absolute valuer of the sol pH and the zeolite IEP.
It is intriguing that the composite is strongest when the larger zeolite crystals have their IEP near to or at the pH of the sol. In the pH stability range of the small sol particles (2-4 for Super-D and 4-5 for alumina chlorohydrol) the sol nanoparticles are electrotatically stabilized with a high surface charge. The results in Figure 4 suggest that at the pH where the sol particles are highly charged, the zeolite should have no surface charge. This result may seem counterintuitive as one might think that the sol particles and the zeolites should be oppositely charged. However, this is not what this data suggests, so another phenomena is operating to create the stable agglomerate. A publication by Jennifer Lewis may help explain this phenomena [6]. She introduced the concept of nanoparticle haloing as a self-organizing process that imparts stability to naturally attractive particles by decorating their superficial areas with highly charged nanoparticles present at critical concentrations. She observed and calculated this effect for the case of micron sized silica particles decorated with a zirconia sol and found that the most stable arrangement occurred when the larger silica particles were
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uncharged and the smaller zirconia nanoparticles had a high charge. Figure 5 represents this phenomena [7].
Figure 5. Representation of Nanoparticle Haloing (from 7).
We are hypothesizing that the FCC particles we have studied behave in an analogous way, with the zeolite crystallites replacing the silica spheres and the zirconia sol being replaced by either Super-D or alumina sol. This stable collodial suspension so formed by nanoparticle haloing has an optimized mixing of the sol and zeolite crystals. On drying and calcination, a porous but strong network forms to hold the agglomerates together. This approach has allowed formation of strong catalysts, even when using the Al chlorohydrol sol. The latter sol was not known to provide stable agglomerates.
4. Conclusion Zeta potential measurements have been used to successfully develop optimized metal dispersion on support oxides. They have also provided a useful tool for the design of attrition resistant FCC catalyst particles.
Acknowledgments The authors wish to thank Joe Baumgartner for help with some of these measurements.
References 1. 2. 3. 4. 5. 6. 7.
J. P. Brunelle, 1978, Pure Appl. Chem., 50, 1211. J. R. Regalbuto, 2009, in “Synthesis of Solid Catalysts”, (ed. K.P. de Jong) Wiley-VCH, p. 33. G. A. Parks and P.L. De Brnyn, 1962, J. Phys. Chem., 66, 967. S. Soled, G. B. McVicker, W.E. Gates and S. Miseo, 1995, US Pat. 5,457,253. S. L. Soled, G. McVicker, S. Miseo, W. Gates, and J. Baumgartner, 1996, Stud. Surf. Sci. Catal., 101A, 563-572 J.A. Lewis, 2001, Langmuir, 17, 8414. M. Jacoby, Jan. 7, 2002, Chemical and Engineering News, p. 11.