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Effect of catalyst modification on the conversion of methanol to light olefins over SAPO-34
~ A PA LE IY D CP AT L SS I A: GENERAL
ELSEVIER
Applied Catalysis A: General 138 (1996) 135-145
Effect of catalyst modification on the conversion of methanol to light olefins over SAPO-34 Miles J. van Niekerk * ,1, Jack C.Q. Fletcher, Cyril T. O'Connor Catalysis Research Unit, Departmentof Chemical Engineering, Universityof Cape Town, Private Bag, Rondebosch, 7700, South Africa
Received 31 July 1995; accepted 22 September 1995
Abstract The catalytic activity and selectivity of as-prepared and modified samples of SAPO-34 and Me-APSO-34 (Me = Co, Ni) for the conversion of methanol to olefins has been investigated. The catalytic performance for the conversion of methanol to light olefins of all the catalyst samples prepared was found to be closely related to the number of strong acid sites present. Mild steaming, encountered during deep-bed calcination, increased the lifetime of SAPO-34 due to the formation of stronger acid sites probably on the extemal surface of the crystallites. Selectivities to light olefins were typical of those previously reported and was essentially constant for all the catalysts investigated. The absence of Cs+ olefins is ascribed to the 'cage effect'. Dilution of the methanol with water as opposed to nitrogen increased the catalyst utilization value threefold and reduced the rate of coke formation during reaction. Treatments such as steaming, silanization and poisoning of strong sites by ammonia all reduced the number of strong acid sites and, thus, reduced catalytic performance. Keywords: SAPO-34; Methanol; Alkenes
I. I n t r o d u c t i o n T h e c a t a l y t i c c o n v e r s i o n o f m e t h a n o l to o l e f i n s is o f s i g n i f i c a n t i n d u s t r i a l i n t e r e s t e s p e c i a l l y w i t h r e s p e c t to the p r o d u c t i o n o f e t h e n e a n d p r o p e n e [1,2]. M a n y i n v e s t i g a t i o n s h a v e b e e n u n d e r t a k e n to s t u d y the e f f e c t o f v a r i o u s r e a c t i o n c o n d i t i o n s on the a c t i v i t y a n d s e l e c t i v i t y o f Z S M - 5 a n d m o d i f i e d Z S M - 5 f o r this
* Corresponding author. Tel. (+ 27-21) 6502516, fax. (+ 27-21) 6503775, e-mail [email protected]. t Present address: Department of Chemical Engineering, The Queen's University of Belfast, Belfast BT9 6ES, Northern Ireland. 0926-860X/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0926-860X(95)00240-5
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reaction [3-6]. The small pore silicoaluminophosphate molecular sieve, SAPO34, which is an isomorph of chabazite [7,8], has been shown to be a promising catalyst for this reaction [9]. This is due to the combination of a moderate acid strength and the steric restrictions imposed by the small pore structure of SAPO-34 [10]. This paper presents the results of a study of the effect of various modifications to the catalyst on the selectivity and lifetime of SAPO-34. The catalyst was also subjected to so-called 'deep bed' calcination. Samples of this catalyst were then modified by silanization, steaming and ammonia adsorption, the last-mentioned treatment being aimed at inertizing the strong acid sites. Samples were also prepared in which cobalt and nickel, respectively, were added during the synthesis procedure.
2. Experimental 2.1. Catalysts Three batches of SAPO-34 were synthesized. Table 1 lists the samples synthesized and modified. S1, $2 and $3 are all SAPO-34. The abbreviated names of each preparation are given in Table 1. Samples S 1 and $2 were made according to Example 34 and $3 according to Example 33, of US patent 4440 871 [11] Co-APSO-34 (Col) and the three Ni-APSO-34 samples (Nil, Ni2 and Ni3) were synthesized according to European Patent 0 161 488 [12] using the acetate salt of the relevant metal. The synthesis times of Nil, Ni2 and Ni3 were 96, 96 and 133 h, respectively, and the pH of the synthesis gels of these materials were 7.7, 11.3 and 7.6, respectively.
Table 1 Catalyst crystallinity, acidity and bulk composition o f as prepared (S 1 - $ 3 ) a n d modified S A P O - 3 4 catalysts Catalyst
Relative Crystallinity
HTD NH 3 (mmol/g)
P e a k temp. (°C)
AI20 3 (wt.-%)
SiO 2 (wt.-%)
P (wt.-%)
Sl $2 $3 $3 * $3 ' -SIL $3 * - S T M $3 * - A M M Co 1 Ni I Ni2 Ni3
0.59 0.87 1.00 1.86 0.68 0.71 1.86 1.34 0.33 0.00 1.32
O. 18 0.26 0.38 0.46 0.36 0.32 0.40 0.21 0.06 0.00 0.37
363 377 393 388 388 360 383 380 357 360
27.5 35.5 36.6 . . 34.9 40.1 33.0 31.1
0.4 0.5 0.4 . . 0.4 1.6 0.4 2.5
40.3 21.8 28.2 -
. .
Ni/Co (wt.-%)
-
. . 31.7 15.9 30.2 41.4
1.7 0.7 2.4 0.5
M.J. van Niekerk et al./Applied Catalysis A: General 138 (1996) 135-145
137
All catalyst samples were dried in air at 90°C for 12 h and then allowed to equilibrate at ambient conditions prior to calcination. Samples of S 1, $2, $3 as well as the Co-APSO-34 and Ni-APSO-34 samples were calcined in a 'shallow bed' configuration in a packed bed reactor (15 g catalyst, 16 mm diameter, 5 mm catalyst bed depth) in flowing dry air (standard GSHV = 100 h -~) at 550°C for 18 h. A sample of $3 was also 'deep-bed' calcined. This was done identically to 'shallow-bed' calcination except that the bed depth of the catalyst sample in the reactor was 180 mm. This sample is designated as $3 * A sample of $3 * was steamed by passing a wet nitrogen stream (50 m l / m i n , 12 kPa water vapor pressure) over a packed bed of catalyst (1.5 g catalyst, 16 mm diameter, 18 mm deep) for 120 min at 300°C. The sample was cooled in dry flowing nitrogen. This sample is referred to as $3 *-STM. In order to investigate the role of weak acid sites, ammonia (5% NH 3 in helium) was adsorbed onto a sample of $3 * at 100°C for 1 h. Weakly adsorbed ammonia was then desorbed by heating the catalyst from ambient temperature to 400°C (the catalytic reaction temperature) at 10°C/min. in flowing helium. The sample was then held at this temperature for 1 h. This sample is referred to as $3 *-AMM. Lastly silanization of a sample of $3 * was carried out by passing 0.5 g / h of tetraethoxysilane through a 1.25 g bed of SAPO-34 for 1.5 h at 350°C. The catalyst was then flushed with nitrogen at 350°C for 1 h. This catalyst, referred to as $3 *-SIL, was finally calcined in flowing dry air for 10 h at 500°C and cooled to room temperature in flowing dry nitrogen.
2.2. Catalyst characterization and reaction studies All catalyst samples were characterized using XRD, SEM and ammonia temperature programmed desorption (NH3-TPD). XRD spectra were obtained under identical experimental and sample conditions using Cu K cr radiation. The relative crystallinities of the samples were roughly approximated by summing the peak heights of the (100), ( - 2 1 0 ) and ( - 310) reflections and normalizing with respect to $3. NH3-TPD was carried out in the range 100 to 600°C (0.25 g sample, 10°C/min, 60 m l / m i n helium carrier flow) after calcination of the sample in air at 500°C for 12 h. The amount of ammonia which desorbed from the high temperature peak was reproducible to within 0.05 m m o l / g . Reactions were carried out in a fixed bed borosilicate glass reactor (id 16 mm). Methanol was fed from a double stage temperature controlled saturator using high purity nitrogen as a carder gas. When water was used as a diluent, the reactants were fed using a syringe pump. The reaction conditions used were: Temp. = 400°C, WHSV = 1.0 g methanol/g c a t . / h and methanol partial pressure = 21 kPa. Hydrocarbon reaction products were separated using a Supelco DH150 capillary column (FID) and CO and CO 2 were separated using a 25 m Carbosieve SII column (TCD). The conversion levels reported refer to the conversion of the combined methanol/dimethyl ether to hydrocarbons, since,
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methanol and dimethyl ether rapidly reach equilibrium. Product selectivities varied as the catalyst deactivated and the selectivities reported in this work are the averages of those observed when the methanol/dimethyl ether conversion was above 90%. Catalyst performance was evaluated in this work by measuring the catalyst utilization value (CUV) which was defined as the mass of methanol/dimethyl ether which is converted to hydrocarbons per gram of catalyst, before the conversion level dropped below half of the maximum conversion for that particular run. Catalyst lifetime is defined as the time that it takes for the methanol/dimethyl ether conversion level to drop below half of the maximum conversion for that particular run. Since, in almost all cases the maximum conversion was 100% this was usually equivalent to the time taken to reach 50% conversion. The total amount of coke deposited on the catalyst was determined from the mass of hydrocarbon combusted in air at 500°C (i.e., hard coke) after treating the coked sample at this temperature in flowing nitrogen for 30 min. Mass changes were recorded using a Stanton Redcroft STA 780 thermo-gravimetric balance.
3. Results
3.1. Catalyst characterization The relative crystallinity, amount of ammonia desorbing at high temperature in the TPD studies as well as the peak temperature and the elemental analysis of the samples is shown in Table 1. No elemental analyses were carried out on the modified samples. The compositions of the SAPO-34 and Me-APSO-34 materials were virtually as expected from the synthesis mixtures and are shown in Table 1. XRD spectra showed that S1 and $2 contained up to 15% of SAPO-5 impurity. The $3 and Col samples were highly crystalline and contained no SAPO-5 material. The Nil and Ni2 samples were almost completely amorphous. By increasing the synthesis time to 133 h a highly crystalline SAPO-34 product, Ni3, was formed which contained no SAPO-5 material. The $3 * material had a greater relative crystallinity than the $3 material. Generally, there was no difference in the relative crystallinities of the uncalcined and 'shallow bed' calcined material, indicating that 'shallow bed' calcination results in detemplation only and not crystallographic modification. Silanizing and steaming resulted in a significant reduction in the relative crystallinity of the sample. In the case of steaming, it is possible that this caused framework damage by dealumination. It is not clear why there should have been a loss of crystallinity upon silanization. Ammonia treatment, as expected, did not influence the relative crystallinity of the sample. Electron micrographs showed that both S 1 and $2 materials had very similar
M.I. van Niekerk et al./Applied Catalysis A: General 138 (1996) 135-145
139
Fig. 1. Electron micrograph of (a) SAPO-34 ($3), (b) SAPO-34 incorporating nickel in the synthesis (Ni3), (c) SAPO-34 incorporating cobalt in the synthesis (Col).
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M.J. uan Niekerk et al.// Applied Catalysis A: General 138 (1996) 135-145
0.006
$3" t
G'~ 0.005 I "~ 0.004 ' -lz
,
'
/I
0.003 ! Q.
Additional acidity ~
~\
0.002 LTDAmmonia
HTD Ammonia
0.001.
C100
150
2b0
25o 300 3So Temperature (degrees C)
4bo
.5o
sod
Fig. 2. NH3-TPD spectra of shallow bed calcined ($3), deep bed calcinced ($3 ° ) and silanized $3 * ($3 *-SIL) samples of SAPO-34.
morphologies consisting of 5-50 /zm agglomerates composed of small angular crystals in the shape of platelets. The $3 material consisted of 5-50 /xm agglomerates composed of small rectangular platelets (Fig. la). The Col crystals were cubic with sides of length approximately 1 /zm (Fig. lb) and the Ni3 material was almost identical in appearance to the $3 material (Fig. lc). The NH3-TPD spectra of the SAPO-34 and Me-APSO-34 materials exhibited two characteristic peaks, a low temperature desorption (LTD) peak between ! 95 and 244°C and a high temperature desorption (HTD) peak between 324 and 380°C. Only the HTD peak is indicative of strong acidity [13,14]. The amounts of strong acidity measured are shown in Table 1. $3 * possessed the greatest amount of strong acidity differing from the unmodified catalyst ($3) by the presence of an additional strong acidity at 460-480°C (Fig. 2). Ammonia treatment, silanizing and steaming the deep-bed calcined sample all caused a reduction in the amount of strong acidity.
3.2. Catalytic studies The results of a study of the performance of each catalyst for the conversion of methanol to olef'ms are presented in Table 2. Of the unmodified samples $3 had the greatest CUV and the C 2 / C 3 olefin ratios were slightly higher for the catalysts with the longer lifetimes. In spite of the high purity and crystallinity of Col, the lifetime of this catalyst was half that of $3, although there was little
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MJ. van Niekerk et al./Applied Catalysis A: General 138 (1996) 135-145
Table 2 Methanol-to-olef'm (MTO) activity and selectivity of unmodified and modified SAPO-34 catalysts Catalyst
S1 $2 $3 $3 * $3 * / H 2 0 a $3 *-SIL $3 *-STM $3 *-AMM Co 1 Ni I Ni2 Ni3
HTD NH 3
MTO
MTO
C 22-/C 32-
C22- _ C 42-
CH 4
% Coke
(mmol/g)
lifetime (h)
CUV ( g / g cat)
(Ratio)
selectivities (wt.-%)
(wt.-%)
content (wt.-%)
0.1 0.26 0.38 0.46 0.46 0.36 0.32 0.40 0.21 0.06 0.00 0.37
4.8 9.5 12.3 16.4 51.5 13.9 11.0 14.3 5.0 2.3 2.1 8.9
4.0 9.0 12.0 15.9 45.8 13.2 10.4 13.1 4.4 1.8 0.0 7.9
0.73 0.98 1.00 1.01 0.79 0.84 0.81 0.84 1.09 0.86 0.87 0.87
88.2 89.4 92.2 90.6 89.7 90.2 91.5 84.8 86.9 72.7 33.5 91.8
0.4 0.9 1.1 1.0 0.5 0.6 0.6 0.6 1.5 0.7 19.8 0.6
3.7 14.1 14.6 16.6 19.8 15.5 18.1 17.4 15.7 6.0 0.6 15.4
a Water was used to dilute the methanol feed (21 kPa) in this run only.
change in the C 2 / C 3 olefin ratio. The CUV of Nil was even less than that of Col and the C 2 / C 3 olefin ratio and the C 2 - C 4 olefin selectivity were significantly reduced. Ni2 had virtually no MTO activity. The relatively highly crystalline Ni3 was more active than any of the other SAPO-34 catalysts synthesized with cobalt or nickel but still had a lower CUV than unmodified $3.
18,
16,
14,
,_>, 12,
10, o~ 8.
> ---i o O
6.
I.--
4.
2,
Oq o
I 0.05
I 0.1
I 0.15
I 0.2
I 0.25
I 0.3
I 0.35
I 0.4
HTD acidity (retool NHa/g catalyst) Fig. 3. Relationship between HTD acidity and catalyst CUV.
I 0.45
0.5
142
MJ. van Niekerk et al./Applied Catalysis A: General 138 (1996) 135-145
As with Nil, the C 2 / C 3 olefin ratio and the C2-C 4 olefin selectivities of Ni3 were also less than those of the $3 or Col catalysts. Deep-bed calcined SAPO-34, viz. sample $3 *, had a consistently greater CUV than the shallow-bed calcined sample. When water was used as a diluent there was a three-fold increase in catalyst lifetime, the C2-C 4 olefin selectivity was essentially unchanged, and the methane selectivity and the C 2 / C 3 olefin ratio were reduced. Ammonia treatment, silanizing and steaming produced materials which exhibited reduced CUVs as expected from their reduced acidities. Generally, reduced amounts of strong acidity were accompanied by lower CUVs and lower C 2 / C 3 olefin ratios.
4. Discussion
It was generally observed in this study that the catalysts which had greater amounts of strong acidity, as determined by NH3-TPD, had greater CUVs and longer lifetimes. This is illustrated, for example, by the low CUV of Col in comparison to that of $2 and $3 even though the Col material has less impurities and is significantly more crystalline than the $2 and $3 materials. Samples containing significant amounts of SAPO-5 had reduced amounts of HTD acidity and, consistent with the above observations, had lower CUVs than the other catalysts. As shown in Fig. 3, it was the amount of acidity rather than crystallinity which had the most important effect on the performance of the catalyst. However, within a particular group of these samples (e.g., Ni 1, Ni2 and Ni3) the HTD acidity appeared to be related to catalyst crystallinity. Consistent with previous work [10], the light olefin selectivity of the various SAPO-34 and Me-APSO-34 catalysts investigated were very similar except for the Nil and Ni2 materials for which it was significantly reduced. These two catalysts were essentially inactive, but have been included in this study to demonstrate the relationship between amount of acidity and CUV at low acid site concentrations. The S1 material was found to have a lower C J C 3 olefin ratio compared to that of the other catalysts. This may be due to the presence of some SAPO-5 material (up to 15%) which would not have had the same shape selective properties as SAPO-34. The increased C 2 / C 3 olefin ratio of the Col catalyst compared to the $2 and $3 materials may be due to framework distortion resulting from cobalt incorporation into a tetrahedral position, as suggested by Bennett and Marcus [15]. An even more severe framework distortion might be expected if nickel were incorporated into a tetrahedral framework position since, unlike cobalt, four coordinate nickel is usually square-planar in structure. No significant framework distortions were, however, indicated by the XRD spectra of the Ni-APSO-34 materials suggesting that the nickel was not incorporated into the SAPO-34 framework of these samples. The decreased ethene selectivity
M J . van Niekerk et al. / Applied Catalysis A: General 138 (1996) 135-145
| 43
of these two catalysts is probably due to the ethene dimerization activity of occluded nickel oxide species. Inui et al. [16] have reported that ethene selectivities could be increased to as high as 90% ( C 2 / C 3 olefin ratio of approximately 17) by incorporating nickel into the SAPO-34 synthesis and they have ascribed this to a reduction in acid site strength. The NH3-TPD spectra of their Ni-APSO-34 catalysts [17], although showing a reduction in the amount of HTD acidity, showed no significant reduction in the HTD peak temperature, viz. average acid site strength, when compared to SAPO-34. It is unlikely that reduced acid site strength alone would account for increased ethene and light olefin selectivities. This may be illustrated by the low ethene selectivity of SAPO-11 which has significantly weaker acidity than SAPO-34 [18]. It was not possible in the present work to obtain the high ethene selectivities reported by Inui et al. and indeed the observation that up to one third of the nickel could be removed easily in one ion-exchange step confirmed that it was unlikely that nickel had been incorporated into the framework. When the dimensions of a molecule diffusing in a molecular sieve are similar to the internal dimensions of a cage, the so-called 'cage' effect can occur. For example when n-pentane diffuses into the chabazite cage, a SAPO-34 isomorph, its diffusivity is more than an order of magnitude less than that of n-decane which has a diffusion coefficient similar to ethane [19]. This phenomenon, together with the mild acid strength of SAPO-34, may be largely responsible for the virtual absence of Cs+ species in the product spectrum. The additional strong HTD acidity in the $3 * sample (Fig. 2) resulted in increased CUV and lifetime properties. When this sample was treated with ammonia ($3 *-AMM) these additional strong acid sites are poisoned and this is accompanied by a reduced CUV. These additional sites formed during the deep-bed calcination procedure may be similar to the 'super' acid sites which have been reported to form during the mild steaming which takes place at these conditions [20,21]. The TPD spectrum of the $3 *-SIL sample was very similar to that of the $3 * sample with the exception of the disappearance of the additional strong acidity associated with the latter (Fig. 2). Tetraethoxysilane molecules are too large to enter the pores easily and the deposition of a silica layer on the exterior of the crystallites will, thus, tend to eliminate external surface acid sites. This seems to indicate that the additional strong acidity may be formed mostly on the outside of the catalyst. This additional 'surface' acidity did not result in an increase in the C5+ selectivity. The external surface of the crystallite is readily accessible to dimethyl ether and water which may be expected to absorb preferentially on the surface acid sites, thus, displacing the less basic light olefins and thereby limiting their chain growth. The increased crystallinity of the $3 * sample may be a result of 'annealing' which can take place under mild steaming conditions and which allows aluminum, phosphorus or silicon to migrate to defect sites [22,23]. The direct steaming conditions used
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M.J. van Niekerk et al./Applied Catalysis A: General 138 (1996) 135-145
in this study are, by comparison, quite harsh and lead to a breakdown of the SAPO-34 framework which results in a reduction in crystallinity and a reduced catalyst performance. The use of water as opposed to nitrogen as a feed diluent results, inter alia, in reduced reaction product residence times. Water is a stronger base than the methanol feed or the olefin reaction products [24] and hence will tend to adsorb preferentially on the acid sites, thus, essentially reducing the residence times of coke precursors on the acid sites. This will reduce the rate of coke formation and enhance the CUV. Coke formation may be due to a cyclization reaction involving free radical coke precursors and may be inhibited by the presence of water. This is highlighted by the fact that although the CUV increased about threefold compared to the standard sample ($3 *) the wt.-% coke was only slightly higher.
5. Conclusions The performance of all the catalysts tested in this work, as illustrated by the CUV and lifetime properties, was shown to be directly related to the amount of strong acidity as determined by the area under the high temperature desorption peak in the NHa-TPD studies. The major products of reaction were C2-C 4 olefins. The almost complete absence of C5+ olefins is ascribed to the so-called 'cage effect'. Deep bed calcination of the SAPO-34 sample resulted in a increase in the relative crystallinity, amount of strong acidity and catalyst performance. This treatment led to the formation of additional stronger acid sites and is ascribed to in-situ mild steaming. Dilution of the methanol feed with water produced a three-fold increase in the catalyst lifetime. There was no evidence that nickel could be easily incorporated into the framework of SAPO-34 and neither the addition of cobalt nor nickel enhanced the performance of the catalyst. Silanization of a 'deep bed' calcined sample indicated that additional acidity arising from this calcination treatment formed on the external surface of the crystallites.
Acknowledgements The authors wish to thank the University of Cape Town, FRD, Sasol and AECI for funding this research.
References [1] N.Y. Chen and W.E. Garwood, Catal. gev. Sci. Eng., 28 (1986) 185. [2] A.A. Avidan, Stud. Surf. Sci. Catal., 36 (1988) 307.
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[3] [4] [5] [6] [7] [8] [9] [10] [l 1] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
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C.D. Chang, C.T.W. Chu and R.F. Socha, J. Catal., 86 (1984) 289. R.M. Dessau, J. Catal., 99 (1986) 111. K. Suzuki, Y. Kiyozumi, K. Matsuzaki and S. Shin, Appl. Catal., 35 (1987) 401. W.E. Kaeding and S.A. Butter, J. Catal., 61 (1980) 151. T. Inui, H. Matsuda, H. Okaniwa and A. Miyamoto, Appl. Catal., 58 (1990) 155. J. Liang, H. Li, S. Zhao, W. Guo, R. Wang and M. Ling, Appl. Catal., 64 (1990) 31. S.T. Wilson, B.M. Lok and E.M. Flanigen, US Patent 4310440, 1982. A.J. Marchi and G.F. Froment, Appl. Catal., 71 (1991) 139. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Carman and E.M. Flanigen, US Patent 4440871, 1985. B.M. Lok, B.K. Marcus and E.M. Flanigen, US Patent 0161 488, 1985. B.L. Meyers, T.H. Fleisch, G.J. Ray, J.T. Miller and J.B. Hall, J. Catal., I l0 (1988) 84. M. Sawa, M. Niwa and Y. Murakami, Appl. Catal., 53 (1989) 174. J.M. Bennet and B.K. Marcus, Stud. Surf. Sci. Catal., 37 (1988) 269. T. Inui, S. Phatanasri and H. Matsuda, J. Chem. Soc., Chem. Commun., (1990) 205. T. Inui, R.K. Grasselli and A.W. Sleight (Editors), Stud. Surf. Sci. Catal., 67 (1991) 233. J.S. Vaughan, PhD Thesis, University of Cape Town, 1991. N.Y. Chen, W.E. Garwood and F.G. Dwyer, in Shape Selective Catalysis in Industrial Applications, Marcel Dekker, New York, 1988, p. 56. R.A. Beyerlain, G.B. McVicker, L.N. Yacallo and J.J. Ziemiak, J. Phys. Chem., 92 (1988) 1967. R.M. Lago, W.O. Haag, R.J. Mokovsky, D.H. Olson, S.D. Hellring, K.D. Schmitt and G.T. Kerr, Proc. 7th Int. Zeolite Conf., (1986) 677. H.K. Beyer, I.M. Belanykaja, 1.V. Mishin and G. Borberly, Proc. Conf. Struct. React. Modified Zeolites, (1984) 133. M. Musa, V. Tarina, A.D. Stoica, E. Ivanov, D. Postinaru, E. Pop, G.R. Pop, R. Ganea, R. Birjega, G. Musca and E.A. Paukshtis, Zeolites, 7 (1987) 431. Y. Xu, C.P. Grey, J.M. Thomas and K. Cheetham, Catal. Lett., 4 (1990) 251.
ELSEVIER
Applied Catalysis A: General 138 (1996) 135-145
Effect of catalyst modification on the conversion of methanol to light olefins over SAPO-34 Miles J. van Niekerk * ,1, Jack C.Q. Fletcher, Cyril T. O'Connor Catalysis Research Unit, Departmentof Chemical Engineering, Universityof Cape Town, Private Bag, Rondebosch, 7700, South Africa
Received 31 July 1995; accepted 22 September 1995
Abstract The catalytic activity and selectivity of as-prepared and modified samples of SAPO-34 and Me-APSO-34 (Me = Co, Ni) for the conversion of methanol to olefins has been investigated. The catalytic performance for the conversion of methanol to light olefins of all the catalyst samples prepared was found to be closely related to the number of strong acid sites present. Mild steaming, encountered during deep-bed calcination, increased the lifetime of SAPO-34 due to the formation of stronger acid sites probably on the extemal surface of the crystallites. Selectivities to light olefins were typical of those previously reported and was essentially constant for all the catalysts investigated. The absence of Cs+ olefins is ascribed to the 'cage effect'. Dilution of the methanol with water as opposed to nitrogen increased the catalyst utilization value threefold and reduced the rate of coke formation during reaction. Treatments such as steaming, silanization and poisoning of strong sites by ammonia all reduced the number of strong acid sites and, thus, reduced catalytic performance. Keywords: SAPO-34; Methanol; Alkenes
I. I n t r o d u c t i o n T h e c a t a l y t i c c o n v e r s i o n o f m e t h a n o l to o l e f i n s is o f s i g n i f i c a n t i n d u s t r i a l i n t e r e s t e s p e c i a l l y w i t h r e s p e c t to the p r o d u c t i o n o f e t h e n e a n d p r o p e n e [1,2]. M a n y i n v e s t i g a t i o n s h a v e b e e n u n d e r t a k e n to s t u d y the e f f e c t o f v a r i o u s r e a c t i o n c o n d i t i o n s on the a c t i v i t y a n d s e l e c t i v i t y o f Z S M - 5 a n d m o d i f i e d Z S M - 5 f o r this
* Corresponding author. Tel. (+ 27-21) 6502516, fax. (+ 27-21) 6503775, e-mail [email protected]. t Present address: Department of Chemical Engineering, The Queen's University of Belfast, Belfast BT9 6ES, Northern Ireland. 0926-860X/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0926-860X(95)00240-5
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reaction [3-6]. The small pore silicoaluminophosphate molecular sieve, SAPO34, which is an isomorph of chabazite [7,8], has been shown to be a promising catalyst for this reaction [9]. This is due to the combination of a moderate acid strength and the steric restrictions imposed by the small pore structure of SAPO-34 [10]. This paper presents the results of a study of the effect of various modifications to the catalyst on the selectivity and lifetime of SAPO-34. The catalyst was also subjected to so-called 'deep bed' calcination. Samples of this catalyst were then modified by silanization, steaming and ammonia adsorption, the last-mentioned treatment being aimed at inertizing the strong acid sites. Samples were also prepared in which cobalt and nickel, respectively, were added during the synthesis procedure.
2. Experimental 2.1. Catalysts Three batches of SAPO-34 were synthesized. Table 1 lists the samples synthesized and modified. S1, $2 and $3 are all SAPO-34. The abbreviated names of each preparation are given in Table 1. Samples S 1 and $2 were made according to Example 34 and $3 according to Example 33, of US patent 4440 871 [11] Co-APSO-34 (Col) and the three Ni-APSO-34 samples (Nil, Ni2 and Ni3) were synthesized according to European Patent 0 161 488 [12] using the acetate salt of the relevant metal. The synthesis times of Nil, Ni2 and Ni3 were 96, 96 and 133 h, respectively, and the pH of the synthesis gels of these materials were 7.7, 11.3 and 7.6, respectively.
Table 1 Catalyst crystallinity, acidity and bulk composition o f as prepared (S 1 - $ 3 ) a n d modified S A P O - 3 4 catalysts Catalyst
Relative Crystallinity
HTD NH 3 (mmol/g)
P e a k temp. (°C)
AI20 3 (wt.-%)
SiO 2 (wt.-%)
P (wt.-%)
Sl $2 $3 $3 * $3 ' -SIL $3 * - S T M $3 * - A M M Co 1 Ni I Ni2 Ni3
0.59 0.87 1.00 1.86 0.68 0.71 1.86 1.34 0.33 0.00 1.32
O. 18 0.26 0.38 0.46 0.36 0.32 0.40 0.21 0.06 0.00 0.37
363 377 393 388 388 360 383 380 357 360
27.5 35.5 36.6 . . 34.9 40.1 33.0 31.1
0.4 0.5 0.4 . . 0.4 1.6 0.4 2.5
40.3 21.8 28.2 -
. .
Ni/Co (wt.-%)
-
. . 31.7 15.9 30.2 41.4
1.7 0.7 2.4 0.5
M.J. van Niekerk et al./Applied Catalysis A: General 138 (1996) 135-145
137
All catalyst samples were dried in air at 90°C for 12 h and then allowed to equilibrate at ambient conditions prior to calcination. Samples of S 1, $2, $3 as well as the Co-APSO-34 and Ni-APSO-34 samples were calcined in a 'shallow bed' configuration in a packed bed reactor (15 g catalyst, 16 mm diameter, 5 mm catalyst bed depth) in flowing dry air (standard GSHV = 100 h -~) at 550°C for 18 h. A sample of $3 was also 'deep-bed' calcined. This was done identically to 'shallow-bed' calcination except that the bed depth of the catalyst sample in the reactor was 180 mm. This sample is designated as $3 * A sample of $3 * was steamed by passing a wet nitrogen stream (50 m l / m i n , 12 kPa water vapor pressure) over a packed bed of catalyst (1.5 g catalyst, 16 mm diameter, 18 mm deep) for 120 min at 300°C. The sample was cooled in dry flowing nitrogen. This sample is referred to as $3 *-STM. In order to investigate the role of weak acid sites, ammonia (5% NH 3 in helium) was adsorbed onto a sample of $3 * at 100°C for 1 h. Weakly adsorbed ammonia was then desorbed by heating the catalyst from ambient temperature to 400°C (the catalytic reaction temperature) at 10°C/min. in flowing helium. The sample was then held at this temperature for 1 h. This sample is referred to as $3 *-AMM. Lastly silanization of a sample of $3 * was carried out by passing 0.5 g / h of tetraethoxysilane through a 1.25 g bed of SAPO-34 for 1.5 h at 350°C. The catalyst was then flushed with nitrogen at 350°C for 1 h. This catalyst, referred to as $3 *-SIL, was finally calcined in flowing dry air for 10 h at 500°C and cooled to room temperature in flowing dry nitrogen.
2.2. Catalyst characterization and reaction studies All catalyst samples were characterized using XRD, SEM and ammonia temperature programmed desorption (NH3-TPD). XRD spectra were obtained under identical experimental and sample conditions using Cu K cr radiation. The relative crystallinities of the samples were roughly approximated by summing the peak heights of the (100), ( - 2 1 0 ) and ( - 310) reflections and normalizing with respect to $3. NH3-TPD was carried out in the range 100 to 600°C (0.25 g sample, 10°C/min, 60 m l / m i n helium carrier flow) after calcination of the sample in air at 500°C for 12 h. The amount of ammonia which desorbed from the high temperature peak was reproducible to within 0.05 m m o l / g . Reactions were carried out in a fixed bed borosilicate glass reactor (id 16 mm). Methanol was fed from a double stage temperature controlled saturator using high purity nitrogen as a carder gas. When water was used as a diluent, the reactants were fed using a syringe pump. The reaction conditions used were: Temp. = 400°C, WHSV = 1.0 g methanol/g c a t . / h and methanol partial pressure = 21 kPa. Hydrocarbon reaction products were separated using a Supelco DH150 capillary column (FID) and CO and CO 2 were separated using a 25 m Carbosieve SII column (TCD). The conversion levels reported refer to the conversion of the combined methanol/dimethyl ether to hydrocarbons, since,
138
M.J. van Niekerk et aL / A p p l i e d Catalysis A: General 138 (1996) 135-145
methanol and dimethyl ether rapidly reach equilibrium. Product selectivities varied as the catalyst deactivated and the selectivities reported in this work are the averages of those observed when the methanol/dimethyl ether conversion was above 90%. Catalyst performance was evaluated in this work by measuring the catalyst utilization value (CUV) which was defined as the mass of methanol/dimethyl ether which is converted to hydrocarbons per gram of catalyst, before the conversion level dropped below half of the maximum conversion for that particular run. Catalyst lifetime is defined as the time that it takes for the methanol/dimethyl ether conversion level to drop below half of the maximum conversion for that particular run. Since, in almost all cases the maximum conversion was 100% this was usually equivalent to the time taken to reach 50% conversion. The total amount of coke deposited on the catalyst was determined from the mass of hydrocarbon combusted in air at 500°C (i.e., hard coke) after treating the coked sample at this temperature in flowing nitrogen for 30 min. Mass changes were recorded using a Stanton Redcroft STA 780 thermo-gravimetric balance.
3. Results
3.1. Catalyst characterization The relative crystallinity, amount of ammonia desorbing at high temperature in the TPD studies as well as the peak temperature and the elemental analysis of the samples is shown in Table 1. No elemental analyses were carried out on the modified samples. The compositions of the SAPO-34 and Me-APSO-34 materials were virtually as expected from the synthesis mixtures and are shown in Table 1. XRD spectra showed that S1 and $2 contained up to 15% of SAPO-5 impurity. The $3 and Col samples were highly crystalline and contained no SAPO-5 material. The Nil and Ni2 samples were almost completely amorphous. By increasing the synthesis time to 133 h a highly crystalline SAPO-34 product, Ni3, was formed which contained no SAPO-5 material. The $3 * material had a greater relative crystallinity than the $3 material. Generally, there was no difference in the relative crystallinities of the uncalcined and 'shallow bed' calcined material, indicating that 'shallow bed' calcination results in detemplation only and not crystallographic modification. Silanizing and steaming resulted in a significant reduction in the relative crystallinity of the sample. In the case of steaming, it is possible that this caused framework damage by dealumination. It is not clear why there should have been a loss of crystallinity upon silanization. Ammonia treatment, as expected, did not influence the relative crystallinity of the sample. Electron micrographs showed that both S 1 and $2 materials had very similar
M.I. van Niekerk et al./Applied Catalysis A: General 138 (1996) 135-145
139
Fig. 1. Electron micrograph of (a) SAPO-34 ($3), (b) SAPO-34 incorporating nickel in the synthesis (Ni3), (c) SAPO-34 incorporating cobalt in the synthesis (Col).
140
M.J. uan Niekerk et al.// Applied Catalysis A: General 138 (1996) 135-145
0.006
$3" t
G'~ 0.005 I "~ 0.004 ' -lz
,
'
/I
0.003 ! Q.
Additional acidity ~
~\
0.002 LTDAmmonia
HTD Ammonia
0.001.
C100
150
2b0
25o 300 3So Temperature (degrees C)
4bo
.5o
sod
Fig. 2. NH3-TPD spectra of shallow bed calcined ($3), deep bed calcinced ($3 ° ) and silanized $3 * ($3 *-SIL) samples of SAPO-34.
morphologies consisting of 5-50 /zm agglomerates composed of small angular crystals in the shape of platelets. The $3 material consisted of 5-50 /xm agglomerates composed of small rectangular platelets (Fig. la). The Col crystals were cubic with sides of length approximately 1 /zm (Fig. lb) and the Ni3 material was almost identical in appearance to the $3 material (Fig. lc). The NH3-TPD spectra of the SAPO-34 and Me-APSO-34 materials exhibited two characteristic peaks, a low temperature desorption (LTD) peak between ! 95 and 244°C and a high temperature desorption (HTD) peak between 324 and 380°C. Only the HTD peak is indicative of strong acidity [13,14]. The amounts of strong acidity measured are shown in Table 1. $3 * possessed the greatest amount of strong acidity differing from the unmodified catalyst ($3) by the presence of an additional strong acidity at 460-480°C (Fig. 2). Ammonia treatment, silanizing and steaming the deep-bed calcined sample all caused a reduction in the amount of strong acidity.
3.2. Catalytic studies The results of a study of the performance of each catalyst for the conversion of methanol to olef'ms are presented in Table 2. Of the unmodified samples $3 had the greatest CUV and the C 2 / C 3 olefin ratios were slightly higher for the catalysts with the longer lifetimes. In spite of the high purity and crystallinity of Col, the lifetime of this catalyst was half that of $3, although there was little
141
MJ. van Niekerk et al./Applied Catalysis A: General 138 (1996) 135-145
Table 2 Methanol-to-olef'm (MTO) activity and selectivity of unmodified and modified SAPO-34 catalysts Catalyst
S1 $2 $3 $3 * $3 * / H 2 0 a $3 *-SIL $3 *-STM $3 *-AMM Co 1 Ni I Ni2 Ni3
HTD NH 3
MTO
MTO
C 22-/C 32-
C22- _ C 42-
CH 4
% Coke
(mmol/g)
lifetime (h)
CUV ( g / g cat)
(Ratio)
selectivities (wt.-%)
(wt.-%)
content (wt.-%)
0.1 0.26 0.38 0.46 0.46 0.36 0.32 0.40 0.21 0.06 0.00 0.37
4.8 9.5 12.3 16.4 51.5 13.9 11.0 14.3 5.0 2.3 2.1 8.9
4.0 9.0 12.0 15.9 45.8 13.2 10.4 13.1 4.4 1.8 0.0 7.9
0.73 0.98 1.00 1.01 0.79 0.84 0.81 0.84 1.09 0.86 0.87 0.87
88.2 89.4 92.2 90.6 89.7 90.2 91.5 84.8 86.9 72.7 33.5 91.8
0.4 0.9 1.1 1.0 0.5 0.6 0.6 0.6 1.5 0.7 19.8 0.6
3.7 14.1 14.6 16.6 19.8 15.5 18.1 17.4 15.7 6.0 0.6 15.4
a Water was used to dilute the methanol feed (21 kPa) in this run only.
change in the C 2 / C 3 olefin ratio. The CUV of Nil was even less than that of Col and the C 2 / C 3 olefin ratio and the C 2 - C 4 olefin selectivity were significantly reduced. Ni2 had virtually no MTO activity. The relatively highly crystalline Ni3 was more active than any of the other SAPO-34 catalysts synthesized with cobalt or nickel but still had a lower CUV than unmodified $3.
18,
16,
14,
,_>, 12,
10, o~ 8.
> ---i o O
6.
I.--
4.
2,
Oq o
I 0.05
I 0.1
I 0.15
I 0.2
I 0.25
I 0.3
I 0.35
I 0.4
HTD acidity (retool NHa/g catalyst) Fig. 3. Relationship between HTD acidity and catalyst CUV.
I 0.45
0.5
142
MJ. van Niekerk et al./Applied Catalysis A: General 138 (1996) 135-145
As with Nil, the C 2 / C 3 olefin ratio and the C2-C 4 olefin selectivities of Ni3 were also less than those of the $3 or Col catalysts. Deep-bed calcined SAPO-34, viz. sample $3 *, had a consistently greater CUV than the shallow-bed calcined sample. When water was used as a diluent there was a three-fold increase in catalyst lifetime, the C2-C 4 olefin selectivity was essentially unchanged, and the methane selectivity and the C 2 / C 3 olefin ratio were reduced. Ammonia treatment, silanizing and steaming produced materials which exhibited reduced CUVs as expected from their reduced acidities. Generally, reduced amounts of strong acidity were accompanied by lower CUVs and lower C 2 / C 3 olefin ratios.
4. Discussion
It was generally observed in this study that the catalysts which had greater amounts of strong acidity, as determined by NH3-TPD, had greater CUVs and longer lifetimes. This is illustrated, for example, by the low CUV of Col in comparison to that of $2 and $3 even though the Col material has less impurities and is significantly more crystalline than the $2 and $3 materials. Samples containing significant amounts of SAPO-5 had reduced amounts of HTD acidity and, consistent with the above observations, had lower CUVs than the other catalysts. As shown in Fig. 3, it was the amount of acidity rather than crystallinity which had the most important effect on the performance of the catalyst. However, within a particular group of these samples (e.g., Ni 1, Ni2 and Ni3) the HTD acidity appeared to be related to catalyst crystallinity. Consistent with previous work [10], the light olefin selectivity of the various SAPO-34 and Me-APSO-34 catalysts investigated were very similar except for the Nil and Ni2 materials for which it was significantly reduced. These two catalysts were essentially inactive, but have been included in this study to demonstrate the relationship between amount of acidity and CUV at low acid site concentrations. The S1 material was found to have a lower C J C 3 olefin ratio compared to that of the other catalysts. This may be due to the presence of some SAPO-5 material (up to 15%) which would not have had the same shape selective properties as SAPO-34. The increased C 2 / C 3 olefin ratio of the Col catalyst compared to the $2 and $3 materials may be due to framework distortion resulting from cobalt incorporation into a tetrahedral position, as suggested by Bennett and Marcus [15]. An even more severe framework distortion might be expected if nickel were incorporated into a tetrahedral framework position since, unlike cobalt, four coordinate nickel is usually square-planar in structure. No significant framework distortions were, however, indicated by the XRD spectra of the Ni-APSO-34 materials suggesting that the nickel was not incorporated into the SAPO-34 framework of these samples. The decreased ethene selectivity
M J . van Niekerk et al. / Applied Catalysis A: General 138 (1996) 135-145
| 43
of these two catalysts is probably due to the ethene dimerization activity of occluded nickel oxide species. Inui et al. [16] have reported that ethene selectivities could be increased to as high as 90% ( C 2 / C 3 olefin ratio of approximately 17) by incorporating nickel into the SAPO-34 synthesis and they have ascribed this to a reduction in acid site strength. The NH3-TPD spectra of their Ni-APSO-34 catalysts [17], although showing a reduction in the amount of HTD acidity, showed no significant reduction in the HTD peak temperature, viz. average acid site strength, when compared to SAPO-34. It is unlikely that reduced acid site strength alone would account for increased ethene and light olefin selectivities. This may be illustrated by the low ethene selectivity of SAPO-11 which has significantly weaker acidity than SAPO-34 [18]. It was not possible in the present work to obtain the high ethene selectivities reported by Inui et al. and indeed the observation that up to one third of the nickel could be removed easily in one ion-exchange step confirmed that it was unlikely that nickel had been incorporated into the framework. When the dimensions of a molecule diffusing in a molecular sieve are similar to the internal dimensions of a cage, the so-called 'cage' effect can occur. For example when n-pentane diffuses into the chabazite cage, a SAPO-34 isomorph, its diffusivity is more than an order of magnitude less than that of n-decane which has a diffusion coefficient similar to ethane [19]. This phenomenon, together with the mild acid strength of SAPO-34, may be largely responsible for the virtual absence of Cs+ species in the product spectrum. The additional strong HTD acidity in the $3 * sample (Fig. 2) resulted in increased CUV and lifetime properties. When this sample was treated with ammonia ($3 *-AMM) these additional strong acid sites are poisoned and this is accompanied by a reduced CUV. These additional sites formed during the deep-bed calcination procedure may be similar to the 'super' acid sites which have been reported to form during the mild steaming which takes place at these conditions [20,21]. The TPD spectrum of the $3 *-SIL sample was very similar to that of the $3 * sample with the exception of the disappearance of the additional strong acidity associated with the latter (Fig. 2). Tetraethoxysilane molecules are too large to enter the pores easily and the deposition of a silica layer on the exterior of the crystallites will, thus, tend to eliminate external surface acid sites. This seems to indicate that the additional strong acidity may be formed mostly on the outside of the catalyst. This additional 'surface' acidity did not result in an increase in the C5+ selectivity. The external surface of the crystallite is readily accessible to dimethyl ether and water which may be expected to absorb preferentially on the surface acid sites, thus, displacing the less basic light olefins and thereby limiting their chain growth. The increased crystallinity of the $3 * sample may be a result of 'annealing' which can take place under mild steaming conditions and which allows aluminum, phosphorus or silicon to migrate to defect sites [22,23]. The direct steaming conditions used
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M.J. van Niekerk et al./Applied Catalysis A: General 138 (1996) 135-145
in this study are, by comparison, quite harsh and lead to a breakdown of the SAPO-34 framework which results in a reduction in crystallinity and a reduced catalyst performance. The use of water as opposed to nitrogen as a feed diluent results, inter alia, in reduced reaction product residence times. Water is a stronger base than the methanol feed or the olefin reaction products [24] and hence will tend to adsorb preferentially on the acid sites, thus, essentially reducing the residence times of coke precursors on the acid sites. This will reduce the rate of coke formation and enhance the CUV. Coke formation may be due to a cyclization reaction involving free radical coke precursors and may be inhibited by the presence of water. This is highlighted by the fact that although the CUV increased about threefold compared to the standard sample ($3 *) the wt.-% coke was only slightly higher.
5. Conclusions The performance of all the catalysts tested in this work, as illustrated by the CUV and lifetime properties, was shown to be directly related to the amount of strong acidity as determined by the area under the high temperature desorption peak in the NHa-TPD studies. The major products of reaction were C2-C 4 olefins. The almost complete absence of C5+ olefins is ascribed to the so-called 'cage effect'. Deep bed calcination of the SAPO-34 sample resulted in a increase in the relative crystallinity, amount of strong acidity and catalyst performance. This treatment led to the formation of additional stronger acid sites and is ascribed to in-situ mild steaming. Dilution of the methanol feed with water produced a three-fold increase in the catalyst lifetime. There was no evidence that nickel could be easily incorporated into the framework of SAPO-34 and neither the addition of cobalt nor nickel enhanced the performance of the catalyst. Silanization of a 'deep bed' calcined sample indicated that additional acidity arising from this calcination treatment formed on the external surface of the crystallites.
Acknowledgements The authors wish to thank the University of Cape Town, FRD, Sasol and AECI for funding this research.
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