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Methanol conversion to hydrocarbons using modified clinoptilolite catalysts
Applied Catalysis, 43 (1988) 133-140 Elsevier Science Publishers B.V., Amsterdam -
133 Printed in The Netherlands
Methanol Conversion to Hydrocarbons Using Modified Clinoptilolite Catalysts Investigation of Catalyst Lifetime and Reactivation G.J. HUTCHINGS**,T.
THEMISTOCLEOUS
and R.G. COPPERTHWAITE
Catalysis Research Programme, Department of Chemistry, University of the Witwatersrand, PO Wits, 2050, Johannesburg (South Africa) (Received 23 February 1988, accepted 13 May 1988)
ABSTRACT A study of the deactivation and reactivation of modified clinoptilolite catalysts for methanol conversion to hydrocarbons is reported. Clinoptilolite catalysts, modified by either ammonium ion exchange or hydrochloric acid treatment, exhibit a short useful catalyst lifetime for this reaction (ca. 2-3 h) due to a high rate of coke deposition (3-5*10-3 g carbon/g catalyst/h). A comparative study of reactivation using oxygen, nitrous oxide and ozone/oxygen as oxidants indicated that nitrous oxide reactivation gives improved catalytic performance when compared to the activity and lifetime of the fresh catalyst. Both oxygen and ozone/oxygen were found to be ineffective for the reactivation of clinoptilolite. Initial studies of in situ on-line reactivation are also described.
INTRODUCTION
The production of synthetic fuels from methanol using zeolite catalysts remains an intensely researched field and the process has recently been commercialised in New Zealand [ 11. By far the greater majority of studies have concentrated on the use of the zeolite catalyst H-ZSM-5 [Z-4], which by virtue of a combination of pore size and Bronsted acidity converts methanol into gasoline range chemicals at high conversion [ 3, 51. Alternative catalyst systems have also been investigated, e.g. the non-zeolite catalyst W03/A1203 [68] is active but demonstrates high methane selectivity. Recently [9, lo] natural clinoptilolite modified by treatment with mineral acids or ammonium salts was shown to be an active catalyst for methanol conversion. Modified clinoptilolite was shown to give a product distribution which predominates in propene and butenes, with low methane, and is therefore of interest for the *Current address: Leverhulme Centre for Innovative Catalysis, Department of IPI Chemistry, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, U.K.
0166-9834/88/$03.50
0 1988 Elsevier Science Publishers B.V.
134
production of precursors for the Mobil olefins-to-gasoline and distillate (MOGD) process. Due mainly to the small pore structure of clinoptilolite this zeolite demonstrates a much shorter active lifetime than H-ZSM-5 and hence it is important that this zeolite can be successfully reactivated and re-used. In this paper we present results for catalyst deactivation and a comparative study of reactivation with oxygen, nitrous oxide and ozone/oxygen as oxidants. In addition our initial attempts to reactivate this catalyst with on-line procedures are also described. EXPERIMENTAL
The natural clinoptilolite (NC) was from Zululand, South Africa and contains 75-80 mass-% clinoptilolite with minor amounts of quartz, cristobolite and potassium feldspar. The natural clinoptilolite was modified by the following two procedures. Sample NC-A was prepared by treating NC (100 g) with aqueous ammonium sulphate (1 M, 1 1) at 25’ C for 40 min. The sample was collected by filtration and washed with distilled water. This procedure was repeated a total of four times. Sample NC-B was prepared by treating NC (100 g) with aqueous hydrochloric acid (1 M, 11) at 74’ C for 8 h. The sample was collected by filtration and washed with distilled water. This procedure was repeated a total of three times. The treated zeolites NC-A and NC-B were dried (120’ C, 3 h) and calcined (500’ C, 3 h) prior to use. Modified samples NC-A and NC-B were analysed using X-ray diffraction and it was determined that the treatments had not significantly affected the structure of the zeolite component. The samples were also characterized by Xray fluorescence and the surface areas were determined by nitrogen adsorption according to the BET method and the data are shown in Table 1. Ammonium ion exchange causes a substantial decrease in surface area without significantly effecting the silicon:aluminium mole ratio. Treatment with 1 M hydrochloric acid causes a significant increase in both silicon:aluminium ratio and surface area as has been noted previously for clinoptilolite [ 91. The modified clinoptilolites were used as catalysts for methanol conversion as previously described [ 41. Deactivated samples were reactivated off-line with oxygen, nitrous oxide or ozone/oxygen as oxidants using methods cited before TABLE 1 Analysis of modified clinoptilolite Sample
Si/Al mole ratio
Fe (W)
Ca (So)
Na (%)
K (%)
Surface area (m2 g-‘1
NC
5.0
NC-A NC-B
4.8 8.2
0.83 0.79 0.66
0.98 0.57 0.32
0.88 0.04 0.04
3.03 1.54 1.66
21 11 97
135 [ 11,121. On-line reactivation was attempted by replacing the nitrogen carrier gas, while the catalyst was being used for methanol conversion, with an equivalent volumetric flow-rate of nitrogen-oxygen (0.4 mol-% oxygen in nitrogen).
RESULTS AND DISCUSSION
Catalyst deactivation The effect of the modification procedures on the catalytic activity of clinoptilolite were investigated at 400 o C and typical results are given in Table 2. In general C&C, alkene selectivities were high (60% by mass) and only the hydrochloric acid modified clinoptilolite NC-B demonstrated significant C, selectivities. After use catalyst samples were found to contain ca. l-2% by mass carbon. The lowest rate of carbon deposition was noted for the hydrochloric acid treated sample NC-B (3*10e3 g carbon/g catalyst/h) and consequently this catalyst exhibited a longer effective catalyst lifetime (ca. 3 h) compared to sample NC-A (ca. 2 h). Selectivity to hydrocarbon products varies with reaction time, and typical data are shown in Fig. 1 for catalyst NC-A. The major selectivity changes occur within the initial 1 h of reaction and generally selectivity to C3 and C, hydrocarbons decreased while the C, selectivity increased. Based on these studies it is clear that modified clinoptilolites are rapTABLE 2
Catalyticperformance of treated clinoptilolite catalyst, 400°C Catalyst WHSV (h-l) Time on line (min ) Conversion (% ) Selectivity (% by mass) CH, W-L W-&i CJ-& C,Hs Cd C, :;
TotalC&C, wg/h**
NC
NC-A 0.083
0.097 60 64.5
240 6.5
5.8 15.8 1.8 44.4 0 19.1 10.0 2.8 0.1
14.4 13.1 2.3 4.1 0 14.7 11.6 2.8 0
7.5 28.3 4.0 35.6* -
71.2
84.6
81.1 0.48
NC-B
60 99.1
0.097
250 23.1
-
16.7 6.4 1.2 0.3
65 96.8
265 47.1
15.0 5.1 2.1 0
4.3 15.7 1.1 27.8 4.0 13.0 23.5 9.2 1.3
6.8 12.3 1.0 26.1 2.7 20.5 22.7 7.2 0.6
82.9
61.6
9.8 28.6 4.4 34.9*
0.50
*Total C!, fraction. **Carbon deposition averaged over run time (g carbon/100 g catalyst/h).
62.6 0.30
I OG
50
IGC
25G
200
250
TIME ON LZNE
300
350
400
450
!mln)
Fig. 1. Effect of time-on-line on selectivity, catalyst NC-A, 4OO”C, methanol WHSV =0.083 h-l. (A) Cl, (0) Cz, (0) ‘% (+) CL (0) C,.
idly deactivated by coke deposition which both limits the effective lifetime to 2-3 h and affects the observed selectivity to hydrocarbon products. Reactivation with oxygen, nitrous oxide and ozone/oxygen Modified clinoptilolite NC-B was deactivated by reaction with methanol (400’ C, gas hourly space velocity (GHSV) = 0.1 h-l) for 7 h. The deactivated NC-B was then reactivated in situ in the reactor using either an oxygen treatment (500°C 3 h, oxygen GHSV=3600 h-l) to give sample NC-Bl, or using an nitrous oxide treatment (500°C 4 h, nitrous oxide GHSV= 3600 h-l) to give sample NC-B2. Analysis of the catalyst following reactivation demonstrated that both treatments essentially removed most of the carbonaceous residues, and analysis of the reactor effluent gases during reactivation confirmed the presence of carbon dioxide which decreased with increasing reactivation time. Following reactivation both samples were tested for methanol conversion under identical conditions and the results are shown in Table 3. Nitrous oxide reactivation is found to be far more effective than oxygen alone which is evidenced by a comparison of the effective catalyst lifetime for methanol conversions (Fig. 2). Oxygen reactivation does not restore the catalyst performance to that of fresh NC-B. However, nitrous oxide reactivation gives improved catalyst performance, in terms of conversion and lifetime, when compared to fresh NC-B. Neither oxygen nor nitrous oxide treatments significantly affect the product selectivity observed for methanol conversion.
137 TABLE 3 Catalytic performance of the fresh and reactivated NC-B catalyst NC-B3
NC-B2
NC-B1
Catalyst
NC-B
TOL (min)
120
240
360
120
240
360
120
240
360
60
120
250
Conversion (%) 96.5 Selectivity (% by mass) CH, 9.5 W-L 21.0 C,H, 2.5 C& 27.4 W-b 5.2 c3 total 32.5 C4 20.5 C, 9.9 C, 2.7 C 7+ 1.2
95.1
63.7
96.3
83.9
35.4
99.4
98.4
66.9
98.9
40.1
4.9
10.1 20.0 2.1 24.3 4.4 28.7 22.0 13.0 3.4 0.6
10.8 19.8 1.7 28.0 3.6 31.6 19.4 10.0 5.5 1.1
7.0 19.1 1.6 29.5 4.7 34.2 21.6 13.9 2.2 0.4
8.6 18.5 1.7 30.2 4.0 34.2 19.7 13.3 3.2 0.6
15.3 21.4 2.1 27.6 2.4 30.0 15.5 9.7 4.6 1.3
8.8 21.4 2.2 25.1 5.2 30.3 22.8 12.6 1.3 0.6
6.7 17.0 1.2 27.5 3.9 31.4 24.7 14.2 4.0 0.7
10.0 21.0 2.0 29.4 3.9 33.3 18.3 11.2 3.1 0.9
4.0 15.6 1.0 35.5 5.1 40.6 14.2 6.1 2.1 16.5
10.8 21.7 2.0 28.9 0.4 29.3 18.1 10.5 4.7 3.0
26.9 28.1 1.7 21.8 1.2 23.0 6.6 5.0 6.0 2.7
100
0
0
60
120
180
240
TIME ON LINE
300
360
420
480
bin)
Fig. 2. Comparison of regeneration of modified clinoptilolite with oxygen, nitrous oxide and ozone. (0 ) Fresh NC-B, (0) NC-B1 following reactivation with oxygen, (0) NC-B2 following reactivation with nitrous oxide, (A ) NC-B3 following reactivation with ozone/oxygen mixture.
Previous studies with modified clinoptilolite [9] have indicated that catalysts can be successfully reactivated by calcination in air. However, no experimental details were given in that study of how the calcinations were affected. The results of the present study demonstrate that off-line oxygen reactivation in situ in the reactor may require extensive reactivation periods ( > 3 h). Tra-
138
ditionally [ 111 air is utilized as the oxidizing medium for industrially used zeolites but this study indicates that air will not be a suitable oxidant for clinoptilolite based catalysts. However the catalysts can be successfully reactivated with a short ( < 3 h) nitrous oxide treatment which can give enhanced catalyst performance. Further studies are required to investigate the effect of low nitrous oxide concentrations since the current study utilized pure nitrous oxide. Deactivated clinoptilolite NC-B was reactivated in situ in the reactor using an ozone/oxygen mixture (2OO”C, 4 h, 3 mol-% ozone, total GHSV= 16000 h-l ) to give sample NC-B3. As has been previously observed for ozone regeneration only partial coke removal was affected, Following reactivation the sample NC-B3 was utilized as a catalyst for methanol conversion and the results are shown in Table 3 and Fig. 2. It is apparent that ozone reactivation at 200’ C does restore total initial catalyst activity, but that the effective lifetime for this material is very short (ca. 1 h) compared to either oxygen or nitrous oxide reactivation at higher temperature. On-line reactivation studies Catalyst deactivation for zeolites during methanol conversion is largely considered to be due to the deposition of carbonaceous deposits within the pore structure of the zeolite. The nature of the pore structure has been shown [ 131 to be a major factor controlling the rate of catalyst deactivation and hence small pore zeolites, e.g. clinoptilolite, deactivate more rapidly than zeolites with intermediate pore sizes, e.g. H-ZSM-5. Whilst catalyst activity is normally restored by oxidation of these carbonaceous deposits this requires the catalyst to be taken off line which entails loss of production time. We have studied the possibility of in situ reactivation whilst the catalyst NC-A was operated for methanol conversion by adding a trace concentration of oxygen (0.4 mol-% ) to the nitrogen carrier gas. The results shown in Table 4 demonstrate that addition of oxygen in this way does not appear to decrease the deactivation rate. On the contrary, addition of trace levels of oxygen significantly decreases the active catalyst lifetime when the catalyst is operated at 400°C (Fig. 3). At this temperature no effect on product selectivity was observed. The reaction temperature of 400°C is considered to be optimal for this modified clinoptilolite and hence it can be concluded that this method of on-line reactivation is not feasible with this zeolite. At 500’ C addition of oxygen does not significantly affect conversion but it does decrease the methane selectivity and increase the selectivity to C, and C5 hydrocarbons. At this temperature it is possible that the oxygen may be reacting with a surface intermediate which is a precursor to methane but not a precursor to carbon-carbon bond formation. These results therefore demonstrate that on-line reactivation with oxygen is not a feasible process.
139 TABLE 4 Effect of gas phase oxygen addition on catalyst performance None
Additive T (“C) WHSV (‘C) Time on line (min) Conversion (% )
None
Oxygen*
400 500 0.083 0.101 10 90 180 15 90 180 15 98.5 96.7 51.6 98.8 95.0 26.7 100
Selectivity (% by mass) CH, 5.5 7.0 C,H, 24.4 29.1 C~HG 2.7 3.6 C3 42.2 35.7 C, 14.4 16.8 C5 9.8 6.4 CS 0.8 1.3 c‘7+ 0.2 0.2
9.6 12.1 19.4 30.1 45.9 42.9 4.1 6.2 6.4 34.1 25.3 20.5 15.1 9.6 9.5 4.9 0.7 1.0 1.3 0 0.1 0.1 0.1 0
29.0 39.4 7.0 16.2 7.1 0.9 0.2 0
Oxygen*
400 500 0.062 0.074 90 180 30 90 180 96.4 38.9 92.3 90.3 28.4
6.2 8.1 24.5 34.1 2.6 3.6 44.4 40.0 17.2 6.8 4.5 6.8 0.3 0.3 0.3 0.2
10.0 30.8 3.9 33.9 15.1 5.9 0.4 0
10.3 35.6 4.1 22.8 19.4 7.8 0 0
11.8 44.4 4.9 24.2 11.2 3.5 0 0
20.1 37.6 5.6 18.2 16.4 2.1 0 0
*0.4% oxygen/99.6%, nitrogen carrier gas.
0
0
50
IOG TIME 0% LINE
150
200
250
!mln)
Fig. 3. Effect of oxygen on methanol conversion for catalyst NC-A at 400°C. (0 ) Methanol WHSV=0.083 h-l, no oxygen added, (A ) methanol WHSV=0.062 h-‘, 0.4 mol-% oxygen in carrier gas.
The deactivation observed with oxygen addition requires further comment. In our previous studies we have shown that H-ZSM-5 is also rapidly deactivated on addition of low oxygen concentrations during methanol conversion
140 [ 14,151. With H-ZSM-5 this effect has been attributed to a facile oxidation of a crucial surface intermediate to form formaldehyde which in the presence of acid sites polymerizes to generate coke. The results of the current study could indicate that methanol conversion to hydrocarbon with clinoptilolite and HZSM-5 operate via a common mechanistic pathway, however additional studies are required to fully elucidate the mechanism of methanol conversion with clinoptilolite. The result of this study may have application to the general operation of zeolite catalysts. Zeolites used in hydrogen transformation or formation reactions are, as stated previously, reactivated with oxygen/nitrogen mixtures. Following reactivation the oxygen is purged from the reactor prior to re-use of the catalyst. It is clear that oxygen removal has to be carried out effectively since residual oxygen could lead to some catalyst deactivation. In conclusion the results of this study demonstrate that modified clinoptilolite catalysts can be successfully reactivated using a facile nitrous oxide treatment. However reactivation with oxygen or ozone/oxygen mixture is not found to be effective. ACKNOWLEDGEMENTS
We thank Mr. Kim Pratley for useful discussions and for supplying samples of clinoptilolite and we thank the University of the Witwatersrand for financial support. REFERENCES 1 2 3 4 5 6 7
8 9 10 11 12 13 14 15
C.J. Maiden, in D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak (Eds. ), Methane Conversion, Studies in Surface Science and Catalysis, Vol. 36, Elsevier, Amsterdam, 1988, p. 1. C.D. Chang, Catal. Rev. Sci. Eng., 25 (1983) 1. G.J. Hutchings, Chem. Brit., 23 (1987) 762. G.J. Hutchings, F.M. Gottschalk, M.V.M. Hall and R. Hunter, J. Chem. Sot. Faraday Trans. 1,83 (1987) 571. S.M. Csicsery, Chem. Brit., 21 (1985) 473. G.A. Olah, U.S. Patent 4373109 (1983). G.A. Olah, H. Doggweiler, J.D. Felberg, S. Frohlich, M.J. Grdina, R. Karpeles, T. Kevmi, S. Laba, W.H. Ip, K. Lammertsma, G. Salem and D.C. Tabar, J. Am. Chem. Sot., 106 (1984) 2143. W.M. Ip, Ph.D. Thesis, University of Southern California, Los Angeles, 1982. H. Sako, M. Nitta and K. Aomura, Appl. Catal., 16 (1985) 249. H. Sakoh, M. Nitta and K. Aomura, Kogakubu Kenkyu Hokoka (Hokkaido Daigaku), 128 (1985) 69. R.G. Copperthwaite, G.J. Hutchings, P. Johnston and SW. Orchard, J. Chem. Sot. Faraday Trans. 1,82 (1986) 1007. R.G. Copperthwaite, G. Foulds, T. Themistocleousand G.J. Hutchings, J. Chem. Sot., Chem. Commun., (1987) 748. P. Dejaifve, A. Auroux, P.C. Gravelle, J.C. Vedrine, Z. Gabelica and E.G. Derouane, J. Catal., 70 (1981) 123. R. Hunter, G.J. Hutchings and W. Pick], J. Chem. Sot., Chem. Commun., (1987) 1369. G.J. Hutchings, L. Jansen van Rensburg, W. Pick1 and R. Hunter, J. Chem. Sot., Faraday Trans. 1,84 (188) 1311.
133 Printed in The Netherlands
Methanol Conversion to Hydrocarbons Using Modified Clinoptilolite Catalysts Investigation of Catalyst Lifetime and Reactivation G.J. HUTCHINGS**,T.
THEMISTOCLEOUS
and R.G. COPPERTHWAITE
Catalysis Research Programme, Department of Chemistry, University of the Witwatersrand, PO Wits, 2050, Johannesburg (South Africa) (Received 23 February 1988, accepted 13 May 1988)
ABSTRACT A study of the deactivation and reactivation of modified clinoptilolite catalysts for methanol conversion to hydrocarbons is reported. Clinoptilolite catalysts, modified by either ammonium ion exchange or hydrochloric acid treatment, exhibit a short useful catalyst lifetime for this reaction (ca. 2-3 h) due to a high rate of coke deposition (3-5*10-3 g carbon/g catalyst/h). A comparative study of reactivation using oxygen, nitrous oxide and ozone/oxygen as oxidants indicated that nitrous oxide reactivation gives improved catalytic performance when compared to the activity and lifetime of the fresh catalyst. Both oxygen and ozone/oxygen were found to be ineffective for the reactivation of clinoptilolite. Initial studies of in situ on-line reactivation are also described.
INTRODUCTION
The production of synthetic fuels from methanol using zeolite catalysts remains an intensely researched field and the process has recently been commercialised in New Zealand [ 11. By far the greater majority of studies have concentrated on the use of the zeolite catalyst H-ZSM-5 [Z-4], which by virtue of a combination of pore size and Bronsted acidity converts methanol into gasoline range chemicals at high conversion [ 3, 51. Alternative catalyst systems have also been investigated, e.g. the non-zeolite catalyst W03/A1203 [68] is active but demonstrates high methane selectivity. Recently [9, lo] natural clinoptilolite modified by treatment with mineral acids or ammonium salts was shown to be an active catalyst for methanol conversion. Modified clinoptilolite was shown to give a product distribution which predominates in propene and butenes, with low methane, and is therefore of interest for the *Current address: Leverhulme Centre for Innovative Catalysis, Department of IPI Chemistry, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, U.K.
0166-9834/88/$03.50
0 1988 Elsevier Science Publishers B.V.
134
production of precursors for the Mobil olefins-to-gasoline and distillate (MOGD) process. Due mainly to the small pore structure of clinoptilolite this zeolite demonstrates a much shorter active lifetime than H-ZSM-5 and hence it is important that this zeolite can be successfully reactivated and re-used. In this paper we present results for catalyst deactivation and a comparative study of reactivation with oxygen, nitrous oxide and ozone/oxygen as oxidants. In addition our initial attempts to reactivate this catalyst with on-line procedures are also described. EXPERIMENTAL
The natural clinoptilolite (NC) was from Zululand, South Africa and contains 75-80 mass-% clinoptilolite with minor amounts of quartz, cristobolite and potassium feldspar. The natural clinoptilolite was modified by the following two procedures. Sample NC-A was prepared by treating NC (100 g) with aqueous ammonium sulphate (1 M, 1 1) at 25’ C for 40 min. The sample was collected by filtration and washed with distilled water. This procedure was repeated a total of four times. Sample NC-B was prepared by treating NC (100 g) with aqueous hydrochloric acid (1 M, 11) at 74’ C for 8 h. The sample was collected by filtration and washed with distilled water. This procedure was repeated a total of three times. The treated zeolites NC-A and NC-B were dried (120’ C, 3 h) and calcined (500’ C, 3 h) prior to use. Modified samples NC-A and NC-B were analysed using X-ray diffraction and it was determined that the treatments had not significantly affected the structure of the zeolite component. The samples were also characterized by Xray fluorescence and the surface areas were determined by nitrogen adsorption according to the BET method and the data are shown in Table 1. Ammonium ion exchange causes a substantial decrease in surface area without significantly effecting the silicon:aluminium mole ratio. Treatment with 1 M hydrochloric acid causes a significant increase in both silicon:aluminium ratio and surface area as has been noted previously for clinoptilolite [ 91. The modified clinoptilolites were used as catalysts for methanol conversion as previously described [ 41. Deactivated samples were reactivated off-line with oxygen, nitrous oxide or ozone/oxygen as oxidants using methods cited before TABLE 1 Analysis of modified clinoptilolite Sample
Si/Al mole ratio
Fe (W)
Ca (So)
Na (%)
K (%)
Surface area (m2 g-‘1
NC
5.0
NC-A NC-B
4.8 8.2
0.83 0.79 0.66
0.98 0.57 0.32
0.88 0.04 0.04
3.03 1.54 1.66
21 11 97
135 [ 11,121. On-line reactivation was attempted by replacing the nitrogen carrier gas, while the catalyst was being used for methanol conversion, with an equivalent volumetric flow-rate of nitrogen-oxygen (0.4 mol-% oxygen in nitrogen).
RESULTS AND DISCUSSION
Catalyst deactivation The effect of the modification procedures on the catalytic activity of clinoptilolite were investigated at 400 o C and typical results are given in Table 2. In general C&C, alkene selectivities were high (60% by mass) and only the hydrochloric acid modified clinoptilolite NC-B demonstrated significant C, selectivities. After use catalyst samples were found to contain ca. l-2% by mass carbon. The lowest rate of carbon deposition was noted for the hydrochloric acid treated sample NC-B (3*10e3 g carbon/g catalyst/h) and consequently this catalyst exhibited a longer effective catalyst lifetime (ca. 3 h) compared to sample NC-A (ca. 2 h). Selectivity to hydrocarbon products varies with reaction time, and typical data are shown in Fig. 1 for catalyst NC-A. The major selectivity changes occur within the initial 1 h of reaction and generally selectivity to C3 and C, hydrocarbons decreased while the C, selectivity increased. Based on these studies it is clear that modified clinoptilolites are rapTABLE 2
Catalyticperformance of treated clinoptilolite catalyst, 400°C Catalyst WHSV (h-l) Time on line (min ) Conversion (% ) Selectivity (% by mass) CH, W-L W-&i CJ-& C,Hs Cd C, :;
TotalC&C, wg/h**
NC
NC-A 0.083
0.097 60 64.5
240 6.5
5.8 15.8 1.8 44.4 0 19.1 10.0 2.8 0.1
14.4 13.1 2.3 4.1 0 14.7 11.6 2.8 0
7.5 28.3 4.0 35.6* -
71.2
84.6
81.1 0.48
NC-B
60 99.1
0.097
250 23.1
-
16.7 6.4 1.2 0.3
65 96.8
265 47.1
15.0 5.1 2.1 0
4.3 15.7 1.1 27.8 4.0 13.0 23.5 9.2 1.3
6.8 12.3 1.0 26.1 2.7 20.5 22.7 7.2 0.6
82.9
61.6
9.8 28.6 4.4 34.9*
0.50
*Total C!, fraction. **Carbon deposition averaged over run time (g carbon/100 g catalyst/h).
62.6 0.30
I OG
50
IGC
25G
200
250
TIME ON LZNE
300
350
400
450
!mln)
Fig. 1. Effect of time-on-line on selectivity, catalyst NC-A, 4OO”C, methanol WHSV =0.083 h-l. (A) Cl, (0) Cz, (0) ‘% (+) CL (0) C,.
idly deactivated by coke deposition which both limits the effective lifetime to 2-3 h and affects the observed selectivity to hydrocarbon products. Reactivation with oxygen, nitrous oxide and ozone/oxygen Modified clinoptilolite NC-B was deactivated by reaction with methanol (400’ C, gas hourly space velocity (GHSV) = 0.1 h-l) for 7 h. The deactivated NC-B was then reactivated in situ in the reactor using either an oxygen treatment (500°C 3 h, oxygen GHSV=3600 h-l) to give sample NC-Bl, or using an nitrous oxide treatment (500°C 4 h, nitrous oxide GHSV= 3600 h-l) to give sample NC-B2. Analysis of the catalyst following reactivation demonstrated that both treatments essentially removed most of the carbonaceous residues, and analysis of the reactor effluent gases during reactivation confirmed the presence of carbon dioxide which decreased with increasing reactivation time. Following reactivation both samples were tested for methanol conversion under identical conditions and the results are shown in Table 3. Nitrous oxide reactivation is found to be far more effective than oxygen alone which is evidenced by a comparison of the effective catalyst lifetime for methanol conversions (Fig. 2). Oxygen reactivation does not restore the catalyst performance to that of fresh NC-B. However, nitrous oxide reactivation gives improved catalyst performance, in terms of conversion and lifetime, when compared to fresh NC-B. Neither oxygen nor nitrous oxide treatments significantly affect the product selectivity observed for methanol conversion.
137 TABLE 3 Catalytic performance of the fresh and reactivated NC-B catalyst NC-B3
NC-B2
NC-B1
Catalyst
NC-B
TOL (min)
120
240
360
120
240
360
120
240
360
60
120
250
Conversion (%) 96.5 Selectivity (% by mass) CH, 9.5 W-L 21.0 C,H, 2.5 C& 27.4 W-b 5.2 c3 total 32.5 C4 20.5 C, 9.9 C, 2.7 C 7+ 1.2
95.1
63.7
96.3
83.9
35.4
99.4
98.4
66.9
98.9
40.1
4.9
10.1 20.0 2.1 24.3 4.4 28.7 22.0 13.0 3.4 0.6
10.8 19.8 1.7 28.0 3.6 31.6 19.4 10.0 5.5 1.1
7.0 19.1 1.6 29.5 4.7 34.2 21.6 13.9 2.2 0.4
8.6 18.5 1.7 30.2 4.0 34.2 19.7 13.3 3.2 0.6
15.3 21.4 2.1 27.6 2.4 30.0 15.5 9.7 4.6 1.3
8.8 21.4 2.2 25.1 5.2 30.3 22.8 12.6 1.3 0.6
6.7 17.0 1.2 27.5 3.9 31.4 24.7 14.2 4.0 0.7
10.0 21.0 2.0 29.4 3.9 33.3 18.3 11.2 3.1 0.9
4.0 15.6 1.0 35.5 5.1 40.6 14.2 6.1 2.1 16.5
10.8 21.7 2.0 28.9 0.4 29.3 18.1 10.5 4.7 3.0
26.9 28.1 1.7 21.8 1.2 23.0 6.6 5.0 6.0 2.7
100
0
0
60
120
180
240
TIME ON LINE
300
360
420
480
bin)
Fig. 2. Comparison of regeneration of modified clinoptilolite with oxygen, nitrous oxide and ozone. (0 ) Fresh NC-B, (0) NC-B1 following reactivation with oxygen, (0) NC-B2 following reactivation with nitrous oxide, (A ) NC-B3 following reactivation with ozone/oxygen mixture.
Previous studies with modified clinoptilolite [9] have indicated that catalysts can be successfully reactivated by calcination in air. However, no experimental details were given in that study of how the calcinations were affected. The results of the present study demonstrate that off-line oxygen reactivation in situ in the reactor may require extensive reactivation periods ( > 3 h). Tra-
138
ditionally [ 111 air is utilized as the oxidizing medium for industrially used zeolites but this study indicates that air will not be a suitable oxidant for clinoptilolite based catalysts. However the catalysts can be successfully reactivated with a short ( < 3 h) nitrous oxide treatment which can give enhanced catalyst performance. Further studies are required to investigate the effect of low nitrous oxide concentrations since the current study utilized pure nitrous oxide. Deactivated clinoptilolite NC-B was reactivated in situ in the reactor using an ozone/oxygen mixture (2OO”C, 4 h, 3 mol-% ozone, total GHSV= 16000 h-l ) to give sample NC-B3. As has been previously observed for ozone regeneration only partial coke removal was affected, Following reactivation the sample NC-B3 was utilized as a catalyst for methanol conversion and the results are shown in Table 3 and Fig. 2. It is apparent that ozone reactivation at 200’ C does restore total initial catalyst activity, but that the effective lifetime for this material is very short (ca. 1 h) compared to either oxygen or nitrous oxide reactivation at higher temperature. On-line reactivation studies Catalyst deactivation for zeolites during methanol conversion is largely considered to be due to the deposition of carbonaceous deposits within the pore structure of the zeolite. The nature of the pore structure has been shown [ 131 to be a major factor controlling the rate of catalyst deactivation and hence small pore zeolites, e.g. clinoptilolite, deactivate more rapidly than zeolites with intermediate pore sizes, e.g. H-ZSM-5. Whilst catalyst activity is normally restored by oxidation of these carbonaceous deposits this requires the catalyst to be taken off line which entails loss of production time. We have studied the possibility of in situ reactivation whilst the catalyst NC-A was operated for methanol conversion by adding a trace concentration of oxygen (0.4 mol-% ) to the nitrogen carrier gas. The results shown in Table 4 demonstrate that addition of oxygen in this way does not appear to decrease the deactivation rate. On the contrary, addition of trace levels of oxygen significantly decreases the active catalyst lifetime when the catalyst is operated at 400°C (Fig. 3). At this temperature no effect on product selectivity was observed. The reaction temperature of 400°C is considered to be optimal for this modified clinoptilolite and hence it can be concluded that this method of on-line reactivation is not feasible with this zeolite. At 500’ C addition of oxygen does not significantly affect conversion but it does decrease the methane selectivity and increase the selectivity to C, and C5 hydrocarbons. At this temperature it is possible that the oxygen may be reacting with a surface intermediate which is a precursor to methane but not a precursor to carbon-carbon bond formation. These results therefore demonstrate that on-line reactivation with oxygen is not a feasible process.
139 TABLE 4 Effect of gas phase oxygen addition on catalyst performance None
Additive T (“C) WHSV (‘C) Time on line (min) Conversion (% )
None
Oxygen*
400 500 0.083 0.101 10 90 180 15 90 180 15 98.5 96.7 51.6 98.8 95.0 26.7 100
Selectivity (% by mass) CH, 5.5 7.0 C,H, 24.4 29.1 C~HG 2.7 3.6 C3 42.2 35.7 C, 14.4 16.8 C5 9.8 6.4 CS 0.8 1.3 c‘7+ 0.2 0.2
9.6 12.1 19.4 30.1 45.9 42.9 4.1 6.2 6.4 34.1 25.3 20.5 15.1 9.6 9.5 4.9 0.7 1.0 1.3 0 0.1 0.1 0.1 0
29.0 39.4 7.0 16.2 7.1 0.9 0.2 0
Oxygen*
400 500 0.062 0.074 90 180 30 90 180 96.4 38.9 92.3 90.3 28.4
6.2 8.1 24.5 34.1 2.6 3.6 44.4 40.0 17.2 6.8 4.5 6.8 0.3 0.3 0.3 0.2
10.0 30.8 3.9 33.9 15.1 5.9 0.4 0
10.3 35.6 4.1 22.8 19.4 7.8 0 0
11.8 44.4 4.9 24.2 11.2 3.5 0 0
20.1 37.6 5.6 18.2 16.4 2.1 0 0
*0.4% oxygen/99.6%, nitrogen carrier gas.
0
0
50
IOG TIME 0% LINE
150
200
250
!mln)
Fig. 3. Effect of oxygen on methanol conversion for catalyst NC-A at 400°C. (0 ) Methanol WHSV=0.083 h-l, no oxygen added, (A ) methanol WHSV=0.062 h-‘, 0.4 mol-% oxygen in carrier gas.
The deactivation observed with oxygen addition requires further comment. In our previous studies we have shown that H-ZSM-5 is also rapidly deactivated on addition of low oxygen concentrations during methanol conversion
140 [ 14,151. With H-ZSM-5 this effect has been attributed to a facile oxidation of a crucial surface intermediate to form formaldehyde which in the presence of acid sites polymerizes to generate coke. The results of the current study could indicate that methanol conversion to hydrocarbon with clinoptilolite and HZSM-5 operate via a common mechanistic pathway, however additional studies are required to fully elucidate the mechanism of methanol conversion with clinoptilolite. The result of this study may have application to the general operation of zeolite catalysts. Zeolites used in hydrogen transformation or formation reactions are, as stated previously, reactivated with oxygen/nitrogen mixtures. Following reactivation the oxygen is purged from the reactor prior to re-use of the catalyst. It is clear that oxygen removal has to be carried out effectively since residual oxygen could lead to some catalyst deactivation. In conclusion the results of this study demonstrate that modified clinoptilolite catalysts can be successfully reactivated using a facile nitrous oxide treatment. However reactivation with oxygen or ozone/oxygen mixture is not found to be effective. ACKNOWLEDGEMENTS
We thank Mr. Kim Pratley for useful discussions and for supplying samples of clinoptilolite and we thank the University of the Witwatersrand for financial support. REFERENCES 1 2 3 4 5 6 7
8 9 10 11 12 13 14 15
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