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A study of the deactivation and regeneration behaviour and related catalytic properties of modified zeolite catalysts
B. Dclrnon and G.F. Frorncnt (Eds.) Caialyst Deactivation 1994 Studies in Surface Science and Catalysis, Vol. 88 0 1994 Elsevicr Science B.V. All rights reserved.
215
A study of the deactivation and regeneration behaviour and related catalytic properties of modified zeolite catalysts Lingao Zhang", Songying Chen, and Shaoyi Peng State Key Laboratory of Coal Conversion and Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, China The coking and oxidative regeneration behaviour of HY-type zeolite catalysts containing additive elements such as Pt, Cu, or Zn were investigated. It was found that Pt or Cu element causes different variations to the cracking activity in n-heptane conversion as compared with parent HY, and that accelerated oxidation of coke was observed in temperature-programmed oxidation (TPO) of coked PtHY and CuHY catalysts. XPS and other techniques provide information concerning the distribution and variations of metallic species after sample calcination, reduction, deactivation and regeneration processes, and it is postulated that electron transfer between these metallic species and coke, or hydrogen, or oxygen molecules is responsible for the experimentally observed behaviour.
1. INTRODUCTION
Coke formation on the surface of catalysts is hardly avoidable in most processes of catalysed hydrocarbon conversion, as it occurs along with the reactant-to-product conversion and is an integral role of the active sites [l].It has been demonstrated that intra-crystalline coking in zeolites is a shape-selective process controlled by the pore size and its configuration, thus large-pore zeolites such as Y deactivates much more rapidly than medium-pore zeolite, e.g. ZSMS [2]. The acidity, porosity and other properties of zeolite catalysts can, however, also be altered by postsynthesis modification methods, resulting in further decrease of the coking rate of the catalysts, e.g. rare earth elements in REY catalysts contribute both to the variation of acidic function and therefore a lowered coking deactivation, and further to the hydrothermal stability of FCC catalysts during oxidative regeneration [3].This work is aimed a t investigating the coking deactivation and oxidative regeneration of some zeolite catalysts modified by impregnating, ion-exchanging, or physical mixing with elements of Pt, Cu, Zn, or Ga. The effects of these additive elements are evaluated by a combination of several experimental techniques.
* present address: Department of Chemistry, University of Cambridge, Cambridge CB2 ZEW, UK
216 2. EXPERIMENTAL 2.1 Reaction testing and regeneration of deactivated catalysts
The additive elements were introduced by ion-exchanging HY or HZSM-5 zeolites with an aqueous solution of CuC12 or ZnCl2 or GaCh, or by impregnating HY with an aqueous solution of H2PtCl6, followed by filtration, washing, drying and calcination. Reaction was carried out by introducing a continuous flow of n-heptane vapour in either N2 or H2 stream into a catalyst bed at 500 "C with an on-line GC for product analysis, from which conversion and selectivity were calculated. For the deactivated catalysts (with 11-20 wt.'X coke determined by thermogravimetry), regeneration was attempted by using temperature-programmed oxidation (TPO) at a heating rate of 10 K/min in a stream of 5-21 vol.%) 0 2 in He (or Ar) with GC determination of the effluent gaseous composition. 2.2 Characterization of metallic and acidic function
Temperature programmed reduction(TPR), (re-)oxidation(TPO), or desorption of NH3(TPD) was used to characterize the metallic function, or the acidic properties of various samples (calcined, reduced, coked, and regenerated), with sample surface areas and pore volumes measured on a Micromeritics Digisorb 2500 using the 3parameter BET equation. X-ray photoelectron spectroscopy (XPS) experiments were performed in a Perkin-Elmer PHI-5300 instrument with Mg Ka (hu=1253.6 eV) radiation, and the surface acidic characters were examined after the adsorption of pyridine into various samples following similar procedures of Kaliaguine et aZ. [4], where the surface atomic compositions were calculated in the usual procedures. Binding energies (B.E.) were calibrated with residual carbon (Cls) B.E. at 284.6 eV, and surface acidity variations were also compared with pyridine chemisorption studies by infrared spectroscopy (IR) and thermogravimetry(TG).
3. RESULTS AND DISCUSSIONS 3.1. The variations in catalytic activity of n-heptane cracking It was observed that Pt prolonged the catalytic activity during the initial hours of time-on-stream, but both Cu and Zn caused quicker deactivation compared with HY This could be rationalized since Pt with its remarkable hydrogenating ability is believed to assume in situ hydrogenolysis of coke precursors [5], while both Cu and Zn in their partially reduced states present some basicity which could inhibit the initial acidic cracking activity for heptane. The aromatizing ability of Zn in HY would also produce a negative effect so either Cu or Zn element could not perform comparable cracking to the coke precursors as in the case of Pt. Thus, deactivation was remarked with both CuHY and ZnHY (Figure 1). The shape selective limitation and the lower acidity density of medium-pore ZSM-5 zeolite are considered to be the limiting factors in the comparatively lower coking rate of ZSMS catalysts containing Pt, or Cu, or Ga elements under similar reaction conditions. 3.2. The additive effect in TPO regeneration of coked catalysts TPO regeneration of coked catalysts indicates that different minima occurred in the oxygen concentration downstream from the reactor (Figure 2). Those at lower
217
temperatures could correspond to the oxidation of coke at the metallic sites [6], while these at higher temperatures may be attributed to the oxidation of coke at the zeolitic support sites [7-101. For coked-PtHY the oxidation would yield mostly C02 at higher temperatures, as Pt exhibited an evident catalytic effect which was accompanied by an instantaneous rise (>501'C) in local temperature, resulting in severe sintering of PtHY sample. Therefore TPO-regenerated PtHY demonstrated a much poor recovery of the original activity. The recovery of cracking activity was neither significant with regenerated CuHY nor with ZnHY, and N2-BET data indicated cn. 47% reduction in surface area and porosity after regeneration. The coked-ZnHY exhibited a minimum only at higher temperature due to the inability of Zn to activate dioxygen at lower temperatures [7]. It may be speculated that catalyzed oxidation of coke molecules in Pt- or Cu-containing HY catalysts could involve the spillover of oxygen species from the metallic sites of either Pt or Cu where coke could be oxidized at some lower temperatures onto the zeolitic sites, where the coke molecules are less reactive toward gaseous dioxygen.
1.5PtHY 0.1PLN Y
HY ZnHY Cul IY
0
5
10
15
20
25
30
TIME-ON-STREAM (t), HR Figure 1.Conversion of n-heptane (%) vs. time-on-stream (t), T=500 O C a: 1.5(~t.~/~)PtHY; b: O.l(wt.'%)PtHY;c: HY; d: ZnHY; e: CuHY; a': TPO-regenerated 1.5PtHY; d': TPO-regenerated ZnHY; 3.3. The state of metallic species characterised by other techniques XPS results provide some information on surface atomic distributions in various samples. It is realized that all these three elements exist in different oxidation states (Table 1). With CuHY or ZnHY, surface metallic dispersion (M/Si values) increased after coking in either N2 or H2 stream; but for coked-PtHY sample this variation was insignificant. However, the surface carbon (atom '%) was enriched for the samples deactivated in H2 atmosphere as compared with those in N2.
218
Table 1 XPS data of surface chemical compositions and elemental dispersion factors ~
Sample
Si/A1
ZnHY(c) ZnHY(d)N ZnHY(r) ZnHYfd)H CuHY(c) CuHY(d)N CuHY(r) CuHY(d)H O.lPtHY(c) O.lPtHY(d)N O.lPtHY(r) O.lPtHY(d)H 0.5PtHY(r) 0.5PtHY(d)H l.OPtHY(r) l.OPtHY(d)H
4.48 3.92 5.31 3.63 4.67 4.93 4.53 5.23 4.33 4.23 4.58 4.86 3.85 4.86 3.15 3.92
6 C, atm%
8.1 10.6 0.64 6.3 6.6 15.2 4.1 7.2
M/Si*102 0.74 1.4 0.77 1.5 2.8 6.3 2.6 6.9 1.2 1.0 1.2 1.7 2.1 1.7 1.7 1.4
Binding Energy(B.E. ): eV 1022.9 1022.7 1023.4 1021.6 1023.2 932.0 933.6 930.6 932.0 933.1 313.9
1024.4 1024.9 1024.6 933.9 935.9 933.8 935.2 316.0
315.8 313.6 314.8 316.5 314.4 316.8 315.5 313.3 314.9 314.4
Si/A1: values in the measured surface layer; the bulk Si/Al is 7.16; 6 C: surface carbon on an atomic basis calculated by subtracting the amount of carbon element after and before coking; M/Si: surface metallic dispersion factor, where M=Zn, Cu, or Pt in the particular catalyst, binding energies (B.E.) are shown for Zn 2p3/2, Cu2p3/2, or Pt3d5/2, respectively; c: calcined; r: reduced; d: deactivated; N or H: coked in N2 or H2 stream. Table 2 The distributions of Cu species as revealed by Auger electron parameters (unit; eV) Cu(LMM) CuHY(c) CuHY(r) CuHY(d)H CuHY(d)N
335.9, 339.9 335.9, 339.9 337.4 336.6'339.4
Ek 913.7,917.7 913.7,917.7 916.2 914.2,917.0
CI
1847.6, 1849.7 1847.5,1849.7 1849.3 1850.1,1850.6
Cu(LMM): Auger electron parameters; Ek:kinetic energy of relevant electrons; a: correlation factors, indicative of Cu species of different oxidation states.
219 1
# 200 300 400 500 600 TEMPERATURE, "C Figure 2. TPO of coked catalysts (heating rate=lO K/min) a: 1.5(wt.'Xl)PtHY;b: O.l(wt.'%)PtHY;c: HY; d: ZnHY; e: CuHY; f Pt/Si02+A1203 Figure 3 shows a comparison of Pt species with varied oxidation states in the O.1PtHY (Pt: 0.lwt. '% ) sample after calcination, reduction, and coking. It can be seen that after reduction or deactivation some of the platinic species were reduced to platinous states with Pt 3d5/2 electron binding energies(B.E.) of 313.6 eV for H2 reduced samples, whereas coking in H2+n-heptane or Nz+n-heptane also resulted in the appearance of platinous species at B.E. 314.8 eV and 313.9eV, respectively. The distribution of cupric/cuprous species in the CuHY sample is tabulated in Table 2, and it can be seen that coking caused partial reduction of Cu2+to Cu+/CuO species. The appearance of electron-deficient Cu species at B.E. of 935.2eV and 935.9eV for the coked CuHY(d)H and CuHY(d)N samples may suggest that there are electron donating contribution from these Cu species to the condensed aromatic-ring structures associated with the coke molecules, as similar increase in values of binding energies was observed in CuF2 or CuC12 compounds [ll],where it is believed that electron transfer from Cu2+ to either the F- or C1- ions is responsible for the increased B.E. values of Cu species.
220
dE/N(E)
a.u.
333
328 323
318
313 308
Binding Energy (B.E.), eV Figure 3. XPS (Pt 3d5/2) spectra of 0.1 PtHY catalysts after different treatment a: coked in N?.+n-heptane at 450"C for 2 hours; b: after calcination; c: after reduction in H2; d: coked in H2m-heptane at 450 "C for 2 hours The reductive/oxidative properties of transitional metal elements in these zeolite catalysts were also examined by TPR and TPO, and it is shown that metallic species in certain cation locations may migrate under calcination, reduction, and reaction conditions [7].The different treatment, e.g. coking or even the oxidative regeneration, will produce metallic species of varied oxidation states with different distributions in the molecular sieve structures as exemplified by the above XI'S data. The redox properties of these metallic cations exhibit the influence of hydrogen and/or coke molecules, and it is further postulated that the electron transfer with oxygen species are considered responsible for their catalyzed performance in the TPO regeneration processes, as shown in Figure 2. 3.4. The variations of surface and bulk acidic characters after coking
The variations of acidic properties in the surface layers and in the bulk solid catalysts after calcination, reduction, or coking were examined by pyridine Nls XI'S [4,aand by the pyridine infrared adsorption techniques, respectively. This provides a means to compare the changes in the characteristic Br'dnsted and Lewis acidity functions after those treatment conditions. First of all, TPD of ammonia revealed that both coked and regenerated samples exhibited much decreased acidity as compared with either calcined or reduced samples before the reaction of n-heptane conversion in either N2 or H2 stream [7]. The adsorption of pyridine may cause further perturbation to the Pt4+ or Pt 2+ species in the zeolite as indicated by the increase in binding energies of I't,?d5/2 electrons, as shown in Table 3 and Figure 4.
22 1
Table 3 Binding energy (B.E.: eV) of zeolite constituent elements in PtHY catalysts as measured by XPS after pyridine adsorption Sample
A12p
Si2p
HY HY(d)H O.lPtHY(c) O.lPtHY(r) 0.5PtHY(c) 0.5PtHY(r) 0.5PtHY(d)H l.OPtHY(c) 1.OPtHY(r) l.OPtHY(d)H 1.5PtHY(c) 1.5PtHY(r) 1.5PtHY(d)H
74.4 74.4 74.3 74.4 74.3 74.3 74.5 74.3 74.5 74.5 74.3 74.4 74.1
102.6 102.8 102.5 102.6 102.5 102.6 102.7 102.6 102.6 102.8 102.6 102.6 102.8
Pt3d5/z
Pt4f7/2
314.9, 314.4 313.2,315.2 312.2, 314.8 313.4, 313.6, 314.2,
317.7 316.8 315.7 315.8 317.3 316.7
72.5 74.1 72.1 71.4 71.4 71.6 71.5 71.7 71.9 71.6 71.4
Sample designation is the same as in Table 3
I
1
314.9
COUIl tS
dE/N( E) a.u.
333
328 323
318
313
308
Binding Energy (B.E.), eV Figure 4. XPS spectra (Pt3d5/2) of pyridine adsorbed 0.5PtHY samples after different pretreatment a: after calcination; b: after Hz-reduction; c: after coking in Nz.
222
It was found by Nls XPS studies of pyridine-adsorbed samples that after deactivation the surface acidic function changes in a different manner with the bulk acidity measured by infrared characteristic absorption bands of pyridine adsorbed samples [7], which would suggest different distributions of the acidic properties in the sample catalysts. The effects of additive elements on the overall acidic features of modified zeolite catalysts are dependent on sample pretreatment and/or reaction condition, which will contribute differently to the induced acidity on the surface and in bulk bifunctional properties, as examined by the reaction of n-heptane shown in Figure 1. 4. CONCLUSIONS
It can be concluded that additives such as Cu, Zn, or Pt element in HY-type molecular sieve catalysts play an important role in affecting both the deactivation by coking during hydrocarbon reactions and subsequent oxidative regeneration. The properties of metallic species can be better understood with a combination of several experimental techniques to characterize the varied oxidation states and the migration of metallic species in the zeolite crystallite after different treatment. Evidence suggests that the redox properties of metal cations or the electron transfer with hydrogen or oxygen species are responsible for the catalytic function in the deactivation and regeneration processes. This mechanism can also account for the interaction of metallic species with the deposited coke molecules, or with the oxygen molecules used to regenerate the coked catalysts.
Acknowledgements Financial support from National Natural Science Foundation of China and Beijing Zongguancun Associated Centre of Analysis and Measurement is gratefully acknowledged. Experimental assistance of Professor Daming Feng with XPS measurements is much appreciated. REFERENCES
1. G.A. Sormajai, Catalyst Design-Progress and Perspectives, (L.L. Hegedus Ed), John Wiley, New York, 1987. 2. S. Bhatia, J. Beltramini, and D.D. Do, Catal. Rev.- Sci. Eng., 31 (1989) 431. 3. W.O. Haag and N.Y. Chen, Catalyst Design -Progress and Perspectives, (L.L. Hegedus Ed), John Wiley, New York, 1987. 4. R. Borade, A. Adnot, and S. Kaliaguine, J. Chem. SOC.,Faraday Trans., 86 (1990) 3949. 5. P. Gallezot, Catal. Rev. - Sci. Eng., 20, (1979) 121. 6. J. Barbier, Catalyst Deactivation 1987, B. Delmon et al. (Eds.), p. 1 Elsevier, Amsterdam, 1987. 7. L.G. Zhang, Ph.D. Thesis, Inst. of Coal Chem., Chinese Acad. of Sci., 1993. 8. J. Novakova and Z. Dolejsek, Zeolites, 10 (1990) 189. 9. P. Magnoux and Guisnet, Appl. Catal., 38 (1988) 341. 10. A. Schraut, G. Emig, and H.-G. Sockel, Appl. Catal., 29 (1986)311. 11.S. W. Gaarenstroom and N. Winograd, J. Chem. Phys., 67 (1977) 3500.
215
A study of the deactivation and regeneration behaviour and related catalytic properties of modified zeolite catalysts Lingao Zhang", Songying Chen, and Shaoyi Peng State Key Laboratory of Coal Conversion and Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, China The coking and oxidative regeneration behaviour of HY-type zeolite catalysts containing additive elements such as Pt, Cu, or Zn were investigated. It was found that Pt or Cu element causes different variations to the cracking activity in n-heptane conversion as compared with parent HY, and that accelerated oxidation of coke was observed in temperature-programmed oxidation (TPO) of coked PtHY and CuHY catalysts. XPS and other techniques provide information concerning the distribution and variations of metallic species after sample calcination, reduction, deactivation and regeneration processes, and it is postulated that electron transfer between these metallic species and coke, or hydrogen, or oxygen molecules is responsible for the experimentally observed behaviour.
1. INTRODUCTION
Coke formation on the surface of catalysts is hardly avoidable in most processes of catalysed hydrocarbon conversion, as it occurs along with the reactant-to-product conversion and is an integral role of the active sites [l].It has been demonstrated that intra-crystalline coking in zeolites is a shape-selective process controlled by the pore size and its configuration, thus large-pore zeolites such as Y deactivates much more rapidly than medium-pore zeolite, e.g. ZSMS [2]. The acidity, porosity and other properties of zeolite catalysts can, however, also be altered by postsynthesis modification methods, resulting in further decrease of the coking rate of the catalysts, e.g. rare earth elements in REY catalysts contribute both to the variation of acidic function and therefore a lowered coking deactivation, and further to the hydrothermal stability of FCC catalysts during oxidative regeneration [3].This work is aimed a t investigating the coking deactivation and oxidative regeneration of some zeolite catalysts modified by impregnating, ion-exchanging, or physical mixing with elements of Pt, Cu, Zn, or Ga. The effects of these additive elements are evaluated by a combination of several experimental techniques.
* present address: Department of Chemistry, University of Cambridge, Cambridge CB2 ZEW, UK
216 2. EXPERIMENTAL 2.1 Reaction testing and regeneration of deactivated catalysts
The additive elements were introduced by ion-exchanging HY or HZSM-5 zeolites with an aqueous solution of CuC12 or ZnCl2 or GaCh, or by impregnating HY with an aqueous solution of H2PtCl6, followed by filtration, washing, drying and calcination. Reaction was carried out by introducing a continuous flow of n-heptane vapour in either N2 or H2 stream into a catalyst bed at 500 "C with an on-line GC for product analysis, from which conversion and selectivity were calculated. For the deactivated catalysts (with 11-20 wt.'X coke determined by thermogravimetry), regeneration was attempted by using temperature-programmed oxidation (TPO) at a heating rate of 10 K/min in a stream of 5-21 vol.%) 0 2 in He (or Ar) with GC determination of the effluent gaseous composition. 2.2 Characterization of metallic and acidic function
Temperature programmed reduction(TPR), (re-)oxidation(TPO), or desorption of NH3(TPD) was used to characterize the metallic function, or the acidic properties of various samples (calcined, reduced, coked, and regenerated), with sample surface areas and pore volumes measured on a Micromeritics Digisorb 2500 using the 3parameter BET equation. X-ray photoelectron spectroscopy (XPS) experiments were performed in a Perkin-Elmer PHI-5300 instrument with Mg Ka (hu=1253.6 eV) radiation, and the surface acidic characters were examined after the adsorption of pyridine into various samples following similar procedures of Kaliaguine et aZ. [4], where the surface atomic compositions were calculated in the usual procedures. Binding energies (B.E.) were calibrated with residual carbon (Cls) B.E. at 284.6 eV, and surface acidity variations were also compared with pyridine chemisorption studies by infrared spectroscopy (IR) and thermogravimetry(TG).
3. RESULTS AND DISCUSSIONS 3.1. The variations in catalytic activity of n-heptane cracking It was observed that Pt prolonged the catalytic activity during the initial hours of time-on-stream, but both Cu and Zn caused quicker deactivation compared with HY This could be rationalized since Pt with its remarkable hydrogenating ability is believed to assume in situ hydrogenolysis of coke precursors [5], while both Cu and Zn in their partially reduced states present some basicity which could inhibit the initial acidic cracking activity for heptane. The aromatizing ability of Zn in HY would also produce a negative effect so either Cu or Zn element could not perform comparable cracking to the coke precursors as in the case of Pt. Thus, deactivation was remarked with both CuHY and ZnHY (Figure 1). The shape selective limitation and the lower acidity density of medium-pore ZSM-5 zeolite are considered to be the limiting factors in the comparatively lower coking rate of ZSMS catalysts containing Pt, or Cu, or Ga elements under similar reaction conditions. 3.2. The additive effect in TPO regeneration of coked catalysts TPO regeneration of coked catalysts indicates that different minima occurred in the oxygen concentration downstream from the reactor (Figure 2). Those at lower
217
temperatures could correspond to the oxidation of coke at the metallic sites [6], while these at higher temperatures may be attributed to the oxidation of coke at the zeolitic support sites [7-101. For coked-PtHY the oxidation would yield mostly C02 at higher temperatures, as Pt exhibited an evident catalytic effect which was accompanied by an instantaneous rise (>501'C) in local temperature, resulting in severe sintering of PtHY sample. Therefore TPO-regenerated PtHY demonstrated a much poor recovery of the original activity. The recovery of cracking activity was neither significant with regenerated CuHY nor with ZnHY, and N2-BET data indicated cn. 47% reduction in surface area and porosity after regeneration. The coked-ZnHY exhibited a minimum only at higher temperature due to the inability of Zn to activate dioxygen at lower temperatures [7]. It may be speculated that catalyzed oxidation of coke molecules in Pt- or Cu-containing HY catalysts could involve the spillover of oxygen species from the metallic sites of either Pt or Cu where coke could be oxidized at some lower temperatures onto the zeolitic sites, where the coke molecules are less reactive toward gaseous dioxygen.
1.5PtHY 0.1PLN Y
HY ZnHY Cul IY
0
5
10
15
20
25
30
TIME-ON-STREAM (t), HR Figure 1.Conversion of n-heptane (%) vs. time-on-stream (t), T=500 O C a: 1.5(~t.~/~)PtHY; b: O.l(wt.'%)PtHY;c: HY; d: ZnHY; e: CuHY; a': TPO-regenerated 1.5PtHY; d': TPO-regenerated ZnHY; 3.3. The state of metallic species characterised by other techniques XPS results provide some information on surface atomic distributions in various samples. It is realized that all these three elements exist in different oxidation states (Table 1). With CuHY or ZnHY, surface metallic dispersion (M/Si values) increased after coking in either N2 or H2 stream; but for coked-PtHY sample this variation was insignificant. However, the surface carbon (atom '%) was enriched for the samples deactivated in H2 atmosphere as compared with those in N2.
218
Table 1 XPS data of surface chemical compositions and elemental dispersion factors ~
Sample
Si/A1
ZnHY(c) ZnHY(d)N ZnHY(r) ZnHYfd)H CuHY(c) CuHY(d)N CuHY(r) CuHY(d)H O.lPtHY(c) O.lPtHY(d)N O.lPtHY(r) O.lPtHY(d)H 0.5PtHY(r) 0.5PtHY(d)H l.OPtHY(r) l.OPtHY(d)H
4.48 3.92 5.31 3.63 4.67 4.93 4.53 5.23 4.33 4.23 4.58 4.86 3.85 4.86 3.15 3.92
6 C, atm%
8.1 10.6 0.64 6.3 6.6 15.2 4.1 7.2
M/Si*102 0.74 1.4 0.77 1.5 2.8 6.3 2.6 6.9 1.2 1.0 1.2 1.7 2.1 1.7 1.7 1.4
Binding Energy(B.E. ): eV 1022.9 1022.7 1023.4 1021.6 1023.2 932.0 933.6 930.6 932.0 933.1 313.9
1024.4 1024.9 1024.6 933.9 935.9 933.8 935.2 316.0
315.8 313.6 314.8 316.5 314.4 316.8 315.5 313.3 314.9 314.4
Si/A1: values in the measured surface layer; the bulk Si/Al is 7.16; 6 C: surface carbon on an atomic basis calculated by subtracting the amount of carbon element after and before coking; M/Si: surface metallic dispersion factor, where M=Zn, Cu, or Pt in the particular catalyst, binding energies (B.E.) are shown for Zn 2p3/2, Cu2p3/2, or Pt3d5/2, respectively; c: calcined; r: reduced; d: deactivated; N or H: coked in N2 or H2 stream. Table 2 The distributions of Cu species as revealed by Auger electron parameters (unit; eV) Cu(LMM) CuHY(c) CuHY(r) CuHY(d)H CuHY(d)N
335.9, 339.9 335.9, 339.9 337.4 336.6'339.4
Ek 913.7,917.7 913.7,917.7 916.2 914.2,917.0
CI
1847.6, 1849.7 1847.5,1849.7 1849.3 1850.1,1850.6
Cu(LMM): Auger electron parameters; Ek:kinetic energy of relevant electrons; a: correlation factors, indicative of Cu species of different oxidation states.
219 1
# 200 300 400 500 600 TEMPERATURE, "C Figure 2. TPO of coked catalysts (heating rate=lO K/min) a: 1.5(wt.'Xl)PtHY;b: O.l(wt.'%)PtHY;c: HY; d: ZnHY; e: CuHY; f Pt/Si02+A1203 Figure 3 shows a comparison of Pt species with varied oxidation states in the O.1PtHY (Pt: 0.lwt. '% ) sample after calcination, reduction, and coking. It can be seen that after reduction or deactivation some of the platinic species were reduced to platinous states with Pt 3d5/2 electron binding energies(B.E.) of 313.6 eV for H2 reduced samples, whereas coking in H2+n-heptane or Nz+n-heptane also resulted in the appearance of platinous species at B.E. 314.8 eV and 313.9eV, respectively. The distribution of cupric/cuprous species in the CuHY sample is tabulated in Table 2, and it can be seen that coking caused partial reduction of Cu2+to Cu+/CuO species. The appearance of electron-deficient Cu species at B.E. of 935.2eV and 935.9eV for the coked CuHY(d)H and CuHY(d)N samples may suggest that there are electron donating contribution from these Cu species to the condensed aromatic-ring structures associated with the coke molecules, as similar increase in values of binding energies was observed in CuF2 or CuC12 compounds [ll],where it is believed that electron transfer from Cu2+ to either the F- or C1- ions is responsible for the increased B.E. values of Cu species.
220
dE/N(E)
a.u.
333
328 323
318
313 308
Binding Energy (B.E.), eV Figure 3. XPS (Pt 3d5/2) spectra of 0.1 PtHY catalysts after different treatment a: coked in N?.+n-heptane at 450"C for 2 hours; b: after calcination; c: after reduction in H2; d: coked in H2m-heptane at 450 "C for 2 hours The reductive/oxidative properties of transitional metal elements in these zeolite catalysts were also examined by TPR and TPO, and it is shown that metallic species in certain cation locations may migrate under calcination, reduction, and reaction conditions [7].The different treatment, e.g. coking or even the oxidative regeneration, will produce metallic species of varied oxidation states with different distributions in the molecular sieve structures as exemplified by the above XI'S data. The redox properties of these metallic cations exhibit the influence of hydrogen and/or coke molecules, and it is further postulated that the electron transfer with oxygen species are considered responsible for their catalyzed performance in the TPO regeneration processes, as shown in Figure 2. 3.4. The variations of surface and bulk acidic characters after coking
The variations of acidic properties in the surface layers and in the bulk solid catalysts after calcination, reduction, or coking were examined by pyridine Nls XI'S [4,aand by the pyridine infrared adsorption techniques, respectively. This provides a means to compare the changes in the characteristic Br'dnsted and Lewis acidity functions after those treatment conditions. First of all, TPD of ammonia revealed that both coked and regenerated samples exhibited much decreased acidity as compared with either calcined or reduced samples before the reaction of n-heptane conversion in either N2 or H2 stream [7]. The adsorption of pyridine may cause further perturbation to the Pt4+ or Pt 2+ species in the zeolite as indicated by the increase in binding energies of I't,?d5/2 electrons, as shown in Table 3 and Figure 4.
22 1
Table 3 Binding energy (B.E.: eV) of zeolite constituent elements in PtHY catalysts as measured by XPS after pyridine adsorption Sample
A12p
Si2p
HY HY(d)H O.lPtHY(c) O.lPtHY(r) 0.5PtHY(c) 0.5PtHY(r) 0.5PtHY(d)H l.OPtHY(c) 1.OPtHY(r) l.OPtHY(d)H 1.5PtHY(c) 1.5PtHY(r) 1.5PtHY(d)H
74.4 74.4 74.3 74.4 74.3 74.3 74.5 74.3 74.5 74.5 74.3 74.4 74.1
102.6 102.8 102.5 102.6 102.5 102.6 102.7 102.6 102.6 102.8 102.6 102.6 102.8
Pt3d5/z
Pt4f7/2
314.9, 314.4 313.2,315.2 312.2, 314.8 313.4, 313.6, 314.2,
317.7 316.8 315.7 315.8 317.3 316.7
72.5 74.1 72.1 71.4 71.4 71.6 71.5 71.7 71.9 71.6 71.4
Sample designation is the same as in Table 3
I
1
314.9
COUIl tS
dE/N( E) a.u.
333
328 323
318
313
308
Binding Energy (B.E.), eV Figure 4. XPS spectra (Pt3d5/2) of pyridine adsorbed 0.5PtHY samples after different pretreatment a: after calcination; b: after Hz-reduction; c: after coking in Nz.
222
It was found by Nls XPS studies of pyridine-adsorbed samples that after deactivation the surface acidic function changes in a different manner with the bulk acidity measured by infrared characteristic absorption bands of pyridine adsorbed samples [7], which would suggest different distributions of the acidic properties in the sample catalysts. The effects of additive elements on the overall acidic features of modified zeolite catalysts are dependent on sample pretreatment and/or reaction condition, which will contribute differently to the induced acidity on the surface and in bulk bifunctional properties, as examined by the reaction of n-heptane shown in Figure 1. 4. CONCLUSIONS
It can be concluded that additives such as Cu, Zn, or Pt element in HY-type molecular sieve catalysts play an important role in affecting both the deactivation by coking during hydrocarbon reactions and subsequent oxidative regeneration. The properties of metallic species can be better understood with a combination of several experimental techniques to characterize the varied oxidation states and the migration of metallic species in the zeolite crystallite after different treatment. Evidence suggests that the redox properties of metal cations or the electron transfer with hydrogen or oxygen species are responsible for the catalytic function in the deactivation and regeneration processes. This mechanism can also account for the interaction of metallic species with the deposited coke molecules, or with the oxygen molecules used to regenerate the coked catalysts.
Acknowledgements Financial support from National Natural Science Foundation of China and Beijing Zongguancun Associated Centre of Analysis and Measurement is gratefully acknowledged. Experimental assistance of Professor Daming Feng with XPS measurements is much appreciated. REFERENCES
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