- Email: [email protected]
Silica deactivation of bead VOC catalysts
B ENVIRONMENTAL ELSEVIER
Applied Catalysis B: Environmental 15 (1998) 21-28
Silica deactivation
of bead VOC catalysts
C. Libanati”, D.A. Ulleniusb, C.J. PereiralTa,* aResearch Division, W! R. Grace and Co.-Corm., 7500 Grace Drive, Columbia MD, USA b Grace TEC Systems, 830 Prosper Road, De Pere WI, USA
Received 31 December 1996; received in revised form 23 April 1997; accepted 23 April 1997
Abstract Catalytic oxidation is a key technology for controlling the emissions of Volatile Organic Compounds (VOCs) from industrial plants. The present paper examines the deactivation by silica of bead VOC catalysts in a Aexographic printing application. Post mortem analyses of field-aged catalysts suggest that organosilicon compounds contained in the printing ink
diffuse into the catalyst and deposit as silica particles in the micropores. Laboratory activity evaluation of aged catalysts suggests that silica deposition is non-selective and that silica masks the noble metal active site. 0 1998 Elsevier Science B.V. Keywords:
Volatile organic compound control; Deactivation; Silica
1. Introduction
Environmental regulations require the control of Volatile Organic Compound (VOC) emissions from industrial and chemical plants. Control options include pollution prevention, minimization, and tailend control. A number of tail-end control techniques are available including thermal and catalytic oxidation, adsorption and biotreatment. Typically, the most cost-effective control option is selected for a particular application. Catalytic oxidation is a key technology for the complete oxidation of mixtures of VOCs. The oxidation reactor typically consists of beds containing monolith or pelleted catalysts. The reactor is designed *Correspondingauthor. Tel.: (301) 405-5575; e-mail: [email protected] ‘Present address: Department of Chemical Engineering, University of Maryland,
College Park, Maryland 20742
0926-860x/98/$19.00 c> 1998 Elsevier Science B.V. All rights resewed. PII SO926-3373(97)00033-7
to meet certain performance guarantees that allow the application to operate under environmental compliance. In order to properly size the catalyst bed, the deactivation behavior of the catalyst has to be understood. While the performance of fresh catalysts has received attention over the last several decades, there is a limited amount of open literature information on catalyst deactivation [I]. The deactivation of aluminasupported bead automobile catalysts by phosphorus has been studied in some detail [2]. The phosphorus was found to penetrate the catalyst in shell progressive manner and form a glassy phase on the surface of the alumina [3,4]. VOC catalyst deactivation of noble and base metal oxide catalysts by chlorinated hydrocarbons and sulfur has been reviewed recently [5]. The effect of silica on noble metal catalyst activity has been examined in several studies. The effect of adding hexamethyldisiloxane (HMDS) on the oxidation of hydrogen, methane and propylene over a
22
C. Libanati et al./Applied
Catalysis B: Environmental
Pt/alumina catalyst has been studied [6]. The oxidation rate of hydrogen at 600°C was negligibly affected by the presence of HMDS. In contrast, the rates of methane and propylene were significantly reduced. With the removal of HMDS from the inlet gas, the rate of propylene oxidation recovered nearly completely. In contrast, methane oxidation did not recover leading these investigators to propose that each of the reactions occurred over different active sites. Irreversible catalyst deactivation by HMDS vapors for the methane oxidation reaction has also been separately reported [7]. The study found that the deactivation effect was much smaller for butane oxidation. The authors propose that silicon atoms from the organosilicon compounds physically block active surface sites. The deactivation of noble metal ozone decomposition catalysts has been attributed to the presence of silica, phosphorus and sulfur in the aged catalyst
PI. The cyclohexane dehydrogenation reaction was studied over palladium black on which triethylsilane was decomposed at 250°C [9]. Using XPS characterization of the catalyst, the authors propose that silica
Fig. 1. Schematic
15 (1998) 21-28
coats the surface and diffuses into bulk palladium, thereby detrimentally impacting hydrogenation activity. Oxidation restores hydrogenation activity by converting surface silicon to silica which is either permeable to hydrogen or exists as islands on the palladium. Deactivation reports in the literature are largely based on laboratory studies. Laboratory aging studies, while useful, differ from field-deactivated catalysts in several important ways. Field catalysts may deactivate as a result of more than one type of deactivation mechanism. In VOC reactors, masking, poisoning, and sintering may all contribute to catalyst deactivation. The concentration of the poisoning species (and perhaps also of the primary reactants) can vary with time. In the printing industry, additional presses may come on line thereby changing the concentration of the poison and the composition of the exhaust. The poisoning species concentration can be below the analytical detection limits. Even poison concentrations in the ppb range can lead to a substantial build-up on the catalyst over its useful life of several years. Reactor operation and, as a result, catalyst
of VOC oxidation
reactor,
C. Libanati et &./Applied
Fig. 2. Photograph
Catalysis B: Environmental
of top of commercial
poisoning can be discontinuous. Units that emit VOCs may be shut down periodically. Catalyst manufacturers have a considerable incentive to understand deactivation mechanisms and to develop accelerated aging protocols that mimic field performance. This is accomplished by laboratory aging studies and by post mortem characterization and evaluation of field aged catalysts. The present paper discusses the results of field-aged bead catalysts in a flexographic printing application. The primary deactivation mechanism in the case of this catalyst is poisoning by silica. Aged bead catalysts are characterized and their activity for hexane oxidation is evaluated.
2. VOC catalyst reactor A schematic of the commercial catalyst reactor is shown in Fig. 1. The VOC inlet concentration
15 (1998) 21-28
reactor bed with VOC catalyst
23
canisters.
was approximately 1,500 ppm as Ci hydrocarbon. The solvent composition was 88 v% ethanol and 12 v% n-propyl acetate. The silica responsible for catalyst deactivation is believed to come from the organosilicon compounds contained in the printing ink. The design conditions are an inlet catalyst temperature of 343°C and a flow rate of 283 Nm3/min. Individual catalyst beads of approximately 3.1 mm diameter are packed in a bed 17.8 cm in depth. The catalyst beads, made of y-alumina, are impregnated with Pt and Pd in a surface shell of approximately 50-100 microns depth. The gas flows downward through the catalyst bed. Canisters 5.1 cm in diameter and 17.8 cm long were packed with catalyst beads and placed at several locations in the bed (Fig. 2). The canisters were installed in February 1989 and were recovered in the December 1993, that is, the catalyst was continuously aged for 4.5 years.
24
C. Libnnati et al. /Applied
Catalysis B: Environmental
3. Experimental The canisters containing aged catalyst were removed and separated into seven fractions, each fraction representing approximately a 2.5 cm bed depth. Beads within each fraction were mixed together for purposes of analytical characterization and testing. EDAX measurements of silicon on catalysts were performed on a Hitachi S570 instrument with a PGT detector. Electron microprobe scans for silicon were performed on beads in each fraction using a Cameca Camebax analyzer. Pore structure measurements were performed on an Micromeritics Autopore 9200 instrument. Ten cubic centimeters of catalyst from each section of the bed were loaded into a laboratory reactor 2.4 cm in diameter. Six hundred ppm of hexane and air was fed over the catalyst at a flow rate of 1.90 l/min (i.e. a space velocity of 11400 h-l). The temperature of the
Fig. 3. EDAX photographs
15 (1998) 21-28
reactor was increased and steady-state conversion of hexane at the reactor outlet was measured using a PID analyzer.
4. Results and discussion EDAX photographs of silicon profiles for fresh and aged beads are shown in Fig. 3. A map of the silica concentration for aged beads in the top 2.5 cm section of the bed (Fig. 3b) shows that silica has penetrated over 300 microns into the bead. This is confirmed by electron microprobe profiles of the silica in beads at various bed depths (Fig. 4. The thickness of the silicaladen zone in beads at the top of the bed is approximately 400 microns. The silica thickness in the beads for each inch of the bed is shown in Table 1. The silica concentration and the penetration of silica into the bead decreases from the top of the bed to the bottom.
of catalyst beads: (a) fresh, and (b) aged from the top 2.5 cm of the bed
C. Libanati et al. /Applied
Catalysis B: Environmental
25
15 (1998) 21-28
Fig. 3. (Continued)
Table 1 Si loading and thickness Bed section
1st 2.5 cm section 2nd section 3rd section 4th section 5th section 6th section 7th section
Table 2 Pore structure of fresh and aged catalyst
in beads by electron microprobe Si wt% by ICP
Si layer thickness,
5.9 5.2 4.7 2.9 2.38 0.5 0.56
400 311 258 221 236 106 67
pm
The pore structure of the fresh and aged catalyst from the top of the reactor is shown in Table 2. The surface area and micropore volume of the aged catalyst is lower than that of the fresh catalyst. This suggests that silica from the exhaust has been preferentially deposited within the micropores of the catalyst.
Vtotal.cm3/g V,,,, cm’lg V m,c, cm3/g d,,, A dmic, A Density, g/cm3 Surface area, m*/g
(Top 2.5 cm)
Fresh
Aged
1.17 0.45 0.72 5 500 120 0.66 245
1.Ol 0.41 0.59 6 100 140 0.77 178
Lightoff curves for hexane conversion for each section of the catalyst bed are shown in Fig. 5. The data from these intergral reactor experiments were analysed using standard reaction engineering methodologies, for example, see [lo]. The intrinsic oxidation activity of the catalyst increases as a function of bed depth. Arrhenius plots of this data are shown in Fig. 6. The rate constant decreases with increasing
26
C. Libanati et &./Applied
Catalysis B: Environmental
15 (1998) 21-28
4th inch
5th inch
6th inch
7th inch
Fig. 4. Electron microprobe
profiles of silica in aged beads at various bed depths.
silica loading. The activation energy for samples at various locations in the bed is fairly constant. The rate constant of beads as a function of the silica level is shown in Fig. 7. As can be seen, the residual activity over highly silica-deactivated beads is approximately 15% of fresh activity. The silica penetration depth of 3OWOO microns into the top layer of beads suggests that the precursor organosilicon compounds are in the gas phase rather
than in a particulate form. These precursor compounds diffuse into the bead catalyst and deposit as silica particles in the micropores. Electron microprobe results suggest that the silica profiles can be approximated by a shell progressive mechanism. The residual catalytic activity even in catalyst beads in the top layer that are loaded with approximately 10 w% of silica in the outer shell suggests that the deposition of silica is non-selective. In the top layer, the silica
27
C. Libanati et al. /Applied Catalysis B: Environmental 15 (1998) 21-28 100 A
80
1stInch
x 2nd Inch
.3rd 5 .-
Inch
60
f s
x 4th Inch
s 8
40 I 5th Inch
a 61h Inch
20
.7th
01 350
“‘I
,“I
“‘2 400
500
450
550
Inch
““’ 600
650
700
750
800
Temperature(F)
Fig. 5. Light-off
curves for the oxidation
of hexane in a laboratory
reactor.
a x Fresh 7 ?? 7th Inch 6
&6th Inch
35 > d 5 04
.5th
inch
x 4th Inch
.3rd
Inch
3 ?? 2nd Inch 2 .Isi
Inch
1 0.0014
0.0015
0.0016
0.0017
0.0018
0.0019
0.002
0.0021
l/T (l/K)
Fig. 6. Arrehenius
plots for hexane oxidation
penetrates far below the outer shell of active catalyst. This leads to the likely conclusion that silica masks the noble metal active site and is thus responsible for deactivation. The above data represents a realistic picture of poisoning by silica over the life of a commercial
data.
catalyst. The information has been incorporated into a simple reaction engineering model which is used to perform sensitivity calculations on the effect of precursor silica concentration on the deactivation rate and to design and size commercial reactors to meet specific lifetime guarantees.
28
C. Libanati et al./Applied
0
1
2
Catalysis B: Environmental
3
4 Si
5
15 (1998) 21-28
6
7
8
Level (%)
Fig. 7. Rate constant as a function of silica loading in beads.
5. Conclusions Data obtained for silica-aged VOC catalysts from a commercial reactor deployed in the printing industry has been reported. The silica has been found to penetrate the catalyst bead approximately via a shell progressive mechanism and to deposit in the micropores of the catalyst. Laboratory activity measurements using hexane indicate that the activation energy for the aged catalysts is the same as for fresh catalysts. The first-order reaction rate constant decreases as a function of silicon loading to about 15% of the fresh loading for the highly deactivated samples, indicating coverage of the active sites. Such data can be used to understand the deactivation mechanism in commercial reactors and, thereby, to design and size VOC catalyst reactors.
Acknowledgements The authors would like to thank Drs. M. Uberoi and R. Carman for helpful discussions. We would
also like to thank M. Gignac and M. Peters of Grace’s Analytical Department for the EDAX and EM data.
References [II J.J. Spivey, J.B. Butt, Catal. Today 11 (1992) 465-500. f21 L.L. Hegedus, K. Baron, J. Catal. 54 (1978) 115-119. [31 B. Angel&, K. Kirchner, Chem. Eng. Sci. 35 (1980) 20892091. [41 D.R. Liu, J.-S. Park, Appl. Catal. B 2 (1993) 49-70. [51 J.B. Butt, J.J. Spivey, SK. Agrawal, Catalyst deactivation, Stud. Surf. Sci. Catal. 88 (1994) 19-30. [61 S.J. Gentry, A. Jones, J. Appl. Chem. Biotech. 28 (1978) 727. [71 CF. Cullis, B.M. Willatt, J. Catal. 86 (1984) 187. PI R.M. Heck, R.J. Farauto, H.C. Lee, Catal. Today 13 (1992) 43. [91 G.V. Smith, J. Stochs, S. Degendra, T. Wiltowski, Catalysis of Organic Reactions, E. Pasco, Ed., Marcel1 Dekker, 137 (1991). [lOI J.W. Bee&man, C. Libanati, A reaction engineering analysis of a Pt/Pd on alumina catalyst for ethanol oxidation, The 5th World Congress of Chemical Engineering, 3, 627 (1996).
Applied Catalysis B: Environmental 15 (1998) 21-28
Silica deactivation
of bead VOC catalysts
C. Libanati”, D.A. Ulleniusb, C.J. PereiralTa,* aResearch Division, W! R. Grace and Co.-Corm., 7500 Grace Drive, Columbia MD, USA b Grace TEC Systems, 830 Prosper Road, De Pere WI, USA
Received 31 December 1996; received in revised form 23 April 1997; accepted 23 April 1997
Abstract Catalytic oxidation is a key technology for controlling the emissions of Volatile Organic Compounds (VOCs) from industrial plants. The present paper examines the deactivation by silica of bead VOC catalysts in a Aexographic printing application. Post mortem analyses of field-aged catalysts suggest that organosilicon compounds contained in the printing ink
diffuse into the catalyst and deposit as silica particles in the micropores. Laboratory activity evaluation of aged catalysts suggests that silica deposition is non-selective and that silica masks the noble metal active site. 0 1998 Elsevier Science B.V. Keywords:
Volatile organic compound control; Deactivation; Silica
1. Introduction
Environmental regulations require the control of Volatile Organic Compound (VOC) emissions from industrial and chemical plants. Control options include pollution prevention, minimization, and tailend control. A number of tail-end control techniques are available including thermal and catalytic oxidation, adsorption and biotreatment. Typically, the most cost-effective control option is selected for a particular application. Catalytic oxidation is a key technology for the complete oxidation of mixtures of VOCs. The oxidation reactor typically consists of beds containing monolith or pelleted catalysts. The reactor is designed *Correspondingauthor. Tel.: (301) 405-5575; e-mail: [email protected] ‘Present address: Department of Chemical Engineering, University of Maryland,
College Park, Maryland 20742
0926-860x/98/$19.00 c> 1998 Elsevier Science B.V. All rights resewed. PII SO926-3373(97)00033-7
to meet certain performance guarantees that allow the application to operate under environmental compliance. In order to properly size the catalyst bed, the deactivation behavior of the catalyst has to be understood. While the performance of fresh catalysts has received attention over the last several decades, there is a limited amount of open literature information on catalyst deactivation [I]. The deactivation of aluminasupported bead automobile catalysts by phosphorus has been studied in some detail [2]. The phosphorus was found to penetrate the catalyst in shell progressive manner and form a glassy phase on the surface of the alumina [3,4]. VOC catalyst deactivation of noble and base metal oxide catalysts by chlorinated hydrocarbons and sulfur has been reviewed recently [5]. The effect of silica on noble metal catalyst activity has been examined in several studies. The effect of adding hexamethyldisiloxane (HMDS) on the oxidation of hydrogen, methane and propylene over a
22
C. Libanati et al./Applied
Catalysis B: Environmental
Pt/alumina catalyst has been studied [6]. The oxidation rate of hydrogen at 600°C was negligibly affected by the presence of HMDS. In contrast, the rates of methane and propylene were significantly reduced. With the removal of HMDS from the inlet gas, the rate of propylene oxidation recovered nearly completely. In contrast, methane oxidation did not recover leading these investigators to propose that each of the reactions occurred over different active sites. Irreversible catalyst deactivation by HMDS vapors for the methane oxidation reaction has also been separately reported [7]. The study found that the deactivation effect was much smaller for butane oxidation. The authors propose that silicon atoms from the organosilicon compounds physically block active surface sites. The deactivation of noble metal ozone decomposition catalysts has been attributed to the presence of silica, phosphorus and sulfur in the aged catalyst
PI. The cyclohexane dehydrogenation reaction was studied over palladium black on which triethylsilane was decomposed at 250°C [9]. Using XPS characterization of the catalyst, the authors propose that silica
Fig. 1. Schematic
15 (1998) 21-28
coats the surface and diffuses into bulk palladium, thereby detrimentally impacting hydrogenation activity. Oxidation restores hydrogenation activity by converting surface silicon to silica which is either permeable to hydrogen or exists as islands on the palladium. Deactivation reports in the literature are largely based on laboratory studies. Laboratory aging studies, while useful, differ from field-deactivated catalysts in several important ways. Field catalysts may deactivate as a result of more than one type of deactivation mechanism. In VOC reactors, masking, poisoning, and sintering may all contribute to catalyst deactivation. The concentration of the poisoning species (and perhaps also of the primary reactants) can vary with time. In the printing industry, additional presses may come on line thereby changing the concentration of the poison and the composition of the exhaust. The poisoning species concentration can be below the analytical detection limits. Even poison concentrations in the ppb range can lead to a substantial build-up on the catalyst over its useful life of several years. Reactor operation and, as a result, catalyst
of VOC oxidation
reactor,
C. Libanati et &./Applied
Fig. 2. Photograph
Catalysis B: Environmental
of top of commercial
poisoning can be discontinuous. Units that emit VOCs may be shut down periodically. Catalyst manufacturers have a considerable incentive to understand deactivation mechanisms and to develop accelerated aging protocols that mimic field performance. This is accomplished by laboratory aging studies and by post mortem characterization and evaluation of field aged catalysts. The present paper discusses the results of field-aged bead catalysts in a flexographic printing application. The primary deactivation mechanism in the case of this catalyst is poisoning by silica. Aged bead catalysts are characterized and their activity for hexane oxidation is evaluated.
2. VOC catalyst reactor A schematic of the commercial catalyst reactor is shown in Fig. 1. The VOC inlet concentration
15 (1998) 21-28
reactor bed with VOC catalyst
23
canisters.
was approximately 1,500 ppm as Ci hydrocarbon. The solvent composition was 88 v% ethanol and 12 v% n-propyl acetate. The silica responsible for catalyst deactivation is believed to come from the organosilicon compounds contained in the printing ink. The design conditions are an inlet catalyst temperature of 343°C and a flow rate of 283 Nm3/min. Individual catalyst beads of approximately 3.1 mm diameter are packed in a bed 17.8 cm in depth. The catalyst beads, made of y-alumina, are impregnated with Pt and Pd in a surface shell of approximately 50-100 microns depth. The gas flows downward through the catalyst bed. Canisters 5.1 cm in diameter and 17.8 cm long were packed with catalyst beads and placed at several locations in the bed (Fig. 2). The canisters were installed in February 1989 and were recovered in the December 1993, that is, the catalyst was continuously aged for 4.5 years.
24
C. Libnnati et al. /Applied
Catalysis B: Environmental
3. Experimental The canisters containing aged catalyst were removed and separated into seven fractions, each fraction representing approximately a 2.5 cm bed depth. Beads within each fraction were mixed together for purposes of analytical characterization and testing. EDAX measurements of silicon on catalysts were performed on a Hitachi S570 instrument with a PGT detector. Electron microprobe scans for silicon were performed on beads in each fraction using a Cameca Camebax analyzer. Pore structure measurements were performed on an Micromeritics Autopore 9200 instrument. Ten cubic centimeters of catalyst from each section of the bed were loaded into a laboratory reactor 2.4 cm in diameter. Six hundred ppm of hexane and air was fed over the catalyst at a flow rate of 1.90 l/min (i.e. a space velocity of 11400 h-l). The temperature of the
Fig. 3. EDAX photographs
15 (1998) 21-28
reactor was increased and steady-state conversion of hexane at the reactor outlet was measured using a PID analyzer.
4. Results and discussion EDAX photographs of silicon profiles for fresh and aged beads are shown in Fig. 3. A map of the silica concentration for aged beads in the top 2.5 cm section of the bed (Fig. 3b) shows that silica has penetrated over 300 microns into the bead. This is confirmed by electron microprobe profiles of the silica in beads at various bed depths (Fig. 4. The thickness of the silicaladen zone in beads at the top of the bed is approximately 400 microns. The silica thickness in the beads for each inch of the bed is shown in Table 1. The silica concentration and the penetration of silica into the bead decreases from the top of the bed to the bottom.
of catalyst beads: (a) fresh, and (b) aged from the top 2.5 cm of the bed
C. Libanati et al. /Applied
Catalysis B: Environmental
25
15 (1998) 21-28
Fig. 3. (Continued)
Table 1 Si loading and thickness Bed section
1st 2.5 cm section 2nd section 3rd section 4th section 5th section 6th section 7th section
Table 2 Pore structure of fresh and aged catalyst
in beads by electron microprobe Si wt% by ICP
Si layer thickness,
5.9 5.2 4.7 2.9 2.38 0.5 0.56
400 311 258 221 236 106 67
pm
The pore structure of the fresh and aged catalyst from the top of the reactor is shown in Table 2. The surface area and micropore volume of the aged catalyst is lower than that of the fresh catalyst. This suggests that silica from the exhaust has been preferentially deposited within the micropores of the catalyst.
Vtotal.cm3/g V,,,, cm’lg V m,c, cm3/g d,,, A dmic, A Density, g/cm3 Surface area, m*/g
(Top 2.5 cm)
Fresh
Aged
1.17 0.45 0.72 5 500 120 0.66 245
1.Ol 0.41 0.59 6 100 140 0.77 178
Lightoff curves for hexane conversion for each section of the catalyst bed are shown in Fig. 5. The data from these intergral reactor experiments were analysed using standard reaction engineering methodologies, for example, see [lo]. The intrinsic oxidation activity of the catalyst increases as a function of bed depth. Arrhenius plots of this data are shown in Fig. 6. The rate constant decreases with increasing
26
C. Libanati et &./Applied
Catalysis B: Environmental
15 (1998) 21-28
4th inch
5th inch
6th inch
7th inch
Fig. 4. Electron microprobe
profiles of silica in aged beads at various bed depths.
silica loading. The activation energy for samples at various locations in the bed is fairly constant. The rate constant of beads as a function of the silica level is shown in Fig. 7. As can be seen, the residual activity over highly silica-deactivated beads is approximately 15% of fresh activity. The silica penetration depth of 3OWOO microns into the top layer of beads suggests that the precursor organosilicon compounds are in the gas phase rather
than in a particulate form. These precursor compounds diffuse into the bead catalyst and deposit as silica particles in the micropores. Electron microprobe results suggest that the silica profiles can be approximated by a shell progressive mechanism. The residual catalytic activity even in catalyst beads in the top layer that are loaded with approximately 10 w% of silica in the outer shell suggests that the deposition of silica is non-selective. In the top layer, the silica
27
C. Libanati et al. /Applied Catalysis B: Environmental 15 (1998) 21-28 100 A
80
1stInch
x 2nd Inch
.3rd 5 .-
Inch
60
f s
x 4th Inch
s 8
40 I 5th Inch
a 61h Inch
20
.7th
01 350
“‘I
,“I
“‘2 400
500
450
550
Inch
““’ 600
650
700
750
800
Temperature(F)
Fig. 5. Light-off
curves for the oxidation
of hexane in a laboratory
reactor.
a x Fresh 7 ?? 7th Inch 6
&6th Inch
35 > d 5 04
.5th
inch
x 4th Inch
.3rd
Inch
3 ?? 2nd Inch 2 .Isi
Inch
1 0.0014
0.0015
0.0016
0.0017
0.0018
0.0019
0.002
0.0021
l/T (l/K)
Fig. 6. Arrehenius
plots for hexane oxidation
penetrates far below the outer shell of active catalyst. This leads to the likely conclusion that silica masks the noble metal active site and is thus responsible for deactivation. The above data represents a realistic picture of poisoning by silica over the life of a commercial
data.
catalyst. The information has been incorporated into a simple reaction engineering model which is used to perform sensitivity calculations on the effect of precursor silica concentration on the deactivation rate and to design and size commercial reactors to meet specific lifetime guarantees.
28
C. Libanati et al./Applied
0
1
2
Catalysis B: Environmental
3
4 Si
5
15 (1998) 21-28
6
7
8
Level (%)
Fig. 7. Rate constant as a function of silica loading in beads.
5. Conclusions Data obtained for silica-aged VOC catalysts from a commercial reactor deployed in the printing industry has been reported. The silica has been found to penetrate the catalyst bead approximately via a shell progressive mechanism and to deposit in the micropores of the catalyst. Laboratory activity measurements using hexane indicate that the activation energy for the aged catalysts is the same as for fresh catalysts. The first-order reaction rate constant decreases as a function of silicon loading to about 15% of the fresh loading for the highly deactivated samples, indicating coverage of the active sites. Such data can be used to understand the deactivation mechanism in commercial reactors and, thereby, to design and size VOC catalyst reactors.
Acknowledgements The authors would like to thank Drs. M. Uberoi and R. Carman for helpful discussions. We would
also like to thank M. Gignac and M. Peters of Grace’s Analytical Department for the EDAX and EM data.
References [II J.J. Spivey, J.B. Butt, Catal. Today 11 (1992) 465-500. f21 L.L. Hegedus, K. Baron, J. Catal. 54 (1978) 115-119. [31 B. Angel&, K. Kirchner, Chem. Eng. Sci. 35 (1980) 20892091. [41 D.R. Liu, J.-S. Park, Appl. Catal. B 2 (1993) 49-70. [51 J.B. Butt, J.J. Spivey, SK. Agrawal, Catalyst deactivation, Stud. Surf. Sci. Catal. 88 (1994) 19-30. [61 S.J. Gentry, A. Jones, J. Appl. Chem. Biotech. 28 (1978) 727. [71 CF. Cullis, B.M. Willatt, J. Catal. 86 (1984) 187. PI R.M. Heck, R.J. Farauto, H.C. Lee, Catal. Today 13 (1992) 43. [91 G.V. Smith, J. Stochs, S. Degendra, T. Wiltowski, Catalysis of Organic Reactions, E. Pasco, Ed., Marcel1 Dekker, 137 (1991). [lOI J.W. Bee&man, C. Libanati, A reaction engineering analysis of a Pt/Pd on alumina catalyst for ethanol oxidation, The 5th World Congress of Chemical Engineering, 3, 627 (1996).