Synthesis, characterization and catalytic testing of SAPO-18, MgAPO-18, and ZnAPO-18 in the MTO reaction

~

APPLIED CATALYSS I AG : ENERAL

Applied Catalysis A: General 142 (1996) L 197-L207

ELSEVIER

Letter

Synthesis, characterization and catalytic testing of SAPO-18, MgAPO-18, and ZnAPO-18 in the MTO reaction R u n e W e n d e l b o *, D u n c a n A k p o r i a y e , A n n e A n d e r s e n , I v a r M. Dahl, H e l l e B. M o s t a d SINTEF, P.O. Box 124 Blindern, Oslo, Norway

Received 13 November 1995; revised 9 March 1996; accepted 18 March 1996

Abstract SAPO-18, MgAPO-18 and ZnAPO-18 have been successfully synthesised using TEAOH as organic additive to the synthesis gel. SAPO-18 has catalytic properties comparable to SAPO-34, whereas the MeAPO-18s had much higher selectivities to C ~ - C 4 and shorter catalytic lifetime than SAPO-18. Keywords: MeAPO; SAPO; AEI; MTO

1. Introduction

SAPO-18 and its MeAPO analogs are microporous, crystalline alumino phosphate based compounds isostructural with A1PO4-18 [1,2], and are structurally closely related to chabazite/SAPO-34, see Fig. 1. The IZC designations for the two structures are 'AEI' and 'CHA', respectively. Both structures can be seen as being built entirely out of double six-rings (D6Rs). They have the same channel size, but the cavities have different geometries. The SAPOs and MeAPOs possess negatively charged crystal lattices with associated charge compensating cations in the pores, whereas the pure aluminum phosphate analogues have uncharged lattices [3]. SAPO-34 has proven to be a good catalyst for the methanol to olefin reaction (MTO) [4], and since SAPO-18 * Corresponding author. 0926-860X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S 0 9 2 6 - 8 6 0 X ( 9 6 ) 0 0 1 1 8 - 4

L198

R. Wendelbo et al. /Applied Catalysis A: General 142 (1996) L197 L207

CHA

AEI

Fig. 1. The structures AEI viewed normal to the (100)-plane (a) and CHA viewed normal to the (~0)-plane (b).

has a similar pore geometry [2], it should accordingly be expected to be a good MTO catalyst too. That this is the case has recently been demonstrated by Chen et al. [5-7]. The same applies to MeAPOs such as the Mg- and Zn-containing varieties. A mechanistic model for the MTO reaction in SAPO-34 indicates that MeOH reacts with relatively large carbocations inside the large cavity, and that the products are formed by the cracking of these carbocations [8], selectively letting out molecules small enough to penetrate the 8R pore openings. The A1PO4-based molecular sieves are crystallized in gels containing organic additives, very often amines or alkyl ammonium ions, and the products formed will often depend on the kind of organic additive [3]. The same alkyl-ammonium cation can act as a charge compensating species in a SAPO or MeAPO synthesis, and as a neutral species in an A1POa synthesis. In the latter it must form an ion pair with O H - or another anion. This is why it is often found that uncharged and charged structure analogs cannot be made with the same organic additive [9]. Following the original Union Carbide recipe [10], SAPO-34 is readily formed when Si is added to the synthesis gel, whereas, in a similar formulation without Si, A1PO4-18 is formed [1]. It took some 4 years before A1PO4-34 was successfully synthesized also using TEAOH as organic additive, but with AI(HzPO4) 3 as the source of A1, large excess of TEAOH and phosphoric acid and seeding with ALPO4-5 [11], and later by using polyphosphoric acid and wet milling of the gel [12]. SAPO-34 and A1PO4-18 have also been reported as being synthesized with the additive cyclohexylamine instead of TEAOH [13]. Recently the syntheses of SAPO-18 [5-7] and CoAPO-18, MgAPO-18 and ZnAPO-18 [14,15] were reported, using the organic additive N, N-diisopropylethylamine, and the authors claimed that SAPO-18 could not be

R. Wendelbo et al. /Applied Catalysis A: General 142 (1996) L197-L207

L 199

synthesized using TEAOH [6]. In the present communication we demonstrate that a well crystalline, and fairly pure SAPO-18 can indeed be crystallized using TEAOH as organic additive, and also MeAPO-18s exemplified by Mg- and ZnAPO-18. Results from catalytic testing of the materials in the MTO reaction are also presented.

2. Experimental 2.1. Synthesis SAPO-18 was prepared according to example 2 in [16]; 81.6 g of Al-isopropoxide (Jansen) was mixed with 108 g of distilled water in a 1-1 polypropylene bottle, and the bottle was shaken for 1 min. 45 g of 85% phosphoric acid was added and the bottle was again shaken for 1 min and then cooled under running tap water. Thereafter 0.6 g of 37% HC1 was added followed by shaking, and then 3.0 g 30% silica sol (DuPont Ludox LS-30) was added, and the bottle was again shaken. The addition of HC1 has previously been found to allow better control of Si substitution in SAPO-34 [17], and has for the same purpose been used in this case. After 15 min, the gel was filtered for 10 min (water suction). During the filtration step the weight was reduced by 100 g. One third of the filtercake was transferred to a 250 ml plastic bottle and 49 g of 40% TEAOH (Aldrich) was added, with subsequent shaking. The overall gel composition was: A1203:0.98 P205:0.015 HC1:0.075 SiO2:TEA20:41 H20:2.8 i-C3HvOH assuming that water, HC1 and isopropanol were lost in equal proportions and that no other components were lost during the filtration step. The gel was transferred to a Teflon lined stainless steel autoclave and was aged at room temperature for 12 h, and then crystallized for 120 h at 215°C while agitated. MgAPO-18 and ZnAPO-18 were synthesized like SAPO-18, using Mg and Zn nitrates in place of the colloidal silica, in equivalent amounts on a molar basis; the Mg and Zn nitrates were dissolved in the phosphoric acid 20 min prior to mixing of the other reagents. A1PO4-18 was synthesized by adding a solution of 36.3 g of distilled water and 15.0 g of 85% phosphoric acid to 27.2 g of Al-isopropoxide in a 250 ml polypropylene bottle, thereafter the bottle was shaken for 1 min. 49 g of 40% TEAOH (Aldrich) was then added, with subsequent shaking to produce a gel with composition: A1203:P2Os:TEA20:60 H 2 0 : 6 i-C3HvOH. The gel was transferred to a Teflon lined stainless steel autoclave and was aged at room temperature for 6 h, and then crystallized for 69 h at 215°C while agitated. All solid products were recovered by centrifugation and washed once in distilled water, dried over night at 100°C, and finally calcined for 4 h at 550°C in flowing dry air. The calcined samples were stored under N e.

L200

R. Wendelbo et al./Applied Catalysis A: General 142 (1996) L197-L207

2.2. Characterization X-ray diffraction was performed on a Siemens diffractometer type D 5000 with Ni-filtered Cu K a-radiation and run in a stepscan mode with steps of 0.02 degrees and 1 s collection time per step. Ammonia-TPD was performed on an Altamira AMI-1 system. Ammonia was adsorbed at 100°C followed by flushing with He for 30 min at 100°C before heating at a rate of 20°C/min in a flow of 30 ml H e / m i n . The same procedure was repeated, but with adsorption and flushing at 300°C. The samples were analysed by SEM with a JEOL JSM-840 instrument. Crystal sizes were determined from SEM micrographs as a range from the smallest to the largest dimension observed. Si, Mg, and Zn content was determined by XRF. Micropore volumes were measured with N2, using a Carlo Erba Sorbtomatic 1800 with full adsorption isotherms.

2.3. Catalytic testing The calcined catalyst was pressed at 60 MPa for 1 min. The tablets were ground and sieved and 1.0 g of the fraction 3 5 - 7 0 mesh was filled in a stainless steel fixed bed reactor. Methanol diluted with 60% N 2 w a s fed at 420°C with W H S V = 1. The product stream was analyzed every 20 min by GC. The lifetime was taken as the time of breakthrough of dimethyl ether (tDME) defined as the time span with more than 99% conversion of MeOH and DME.

3. Results SAPO- 18, MgAPO- 18 and ZnAPO- 18 crystallize in water-TEAOH medium within a narrow window of gel composition, temperature and time, as defined by the recipe given above. At prolonged crystallization times ( > 150 h) dense crystalline phases tend to appear in the product, replacing the AEI phase. At lower water contents AFI appears as a contaminant, and at higher water contents CHA appears. The XRD traces of calcined SAPO- 18, MgAPO- 18, ZnAPO- 18, and A1PO4-18 compared to the simulated diffraction pattern of calcined AEI are presented in Fig. 2. The ZnAPO-18 and the A1PO4-18 contain minor amounts of AFI phase, whereas the other samples are essentially pure. The ammonia TPD profiles for the four samples are presented in Fig. 3a. The results show that the SAPO-18 exhibits one low temperature peak and one high temperature peak. The MeAPO-18s, and to a lesser extent the A1PO4 form have shoulders towards the high temperature side, but no discrete high temperature peaks. Integration of the SAPO-18 high temperature peak above the local minimum at 285°C yields 0.25 m m o l / g , coinciding fairly well with the 0.28

R. Wendelbo et al. / Applied Catalysis A: General 142 (1996) L I 97-L207

L201

2000

I i

'~

looo

ALP04-18

±

4

r

6

8

10

;

,

12

14

"

16

18

20

22

24

,

,-

26

28

[

30

32

34

36

38

Degrees 2 theta Fig. 2. XRD profiles of calcined SAPO-18, MgAPO-18, ZnAPO-18, and A1PO4-18 compared to simulated AE1. Peaks marked with * are characteristic of the AFI structure.

mmol of S i / g obtained from analysis. This indicates that each Si in the SAPO-18 corresponds to a BrCnsted acidic site. The TPD profiles of the MeAPOs have also been integrated from 285°C and up, yielding much lower values than the SAPO, but the MeAPOs also have much less subtituents (Mg and Zn) incorporated in the lattice than Si in the SAPO (Table 1). Finally, the A1PO4 yields an integral ( > 285°C) of 0.02 m m o l / g , although it has no measurable amount of substituents in the lattice. In order to verify that the high temperature shoulders in the TPD traces of the MeAPOs and the A1PO4 do represent strongly adsorbed ammonia, a new set of TPD experiments were performed where ammonia was adsorbed at 300°C, and TPD was run from 300 to 600°C. The results are shown in Fig. 3b, and the integrated areas recalculated to m m o l / g are given in Table 1. In Fig. 4 SEM micrographs of the different samples are presented, clearly showing the platelet-like shape of the crystallites in all the materials, typical of the AEI materials. The SAPO-, MgAPO-, and ZnAPO-18s all have a crystal size distribution in the same range, 0.1-2.0 lxm. The crystal size range (Table 1) indicates the minimum and maximum diameters observed with SEM. It should be noted that details smaller than 0.1 Ixm could not be distinguished on the micrographs. N 2 adsorption data (Table 1) confirm the high crystallinity of the SAPO-18. Micropore volumes have not been measured on the MgAPO-18 and ZnAPO-18, but the XRD intensities indicate the same degree of crystallinity as for the SAPO-18, keeping in mind that the ZnAPO-18 contains some 10% impurities. In Table 2 are compiled data on catalyst lifetime (toME), C ~ - C 4 selectivities

L202

R. Wendelbo et al. / Applied Catalysis A: General 142 (1996) L197-L207

3000

(a)

2500

2000

15~

1000

O r

100

-

- - - I

150

. . . .

200

250

300

350

400

I

I

450

500

550

600

Temperature, degrees C

3so

-

I

\

250 2O0

MgAPO-18

15o

ZnAPO-18

5o +

300

350

400

450

500

550

600

650

Temperature, degrees C Fig. 3. (a) NH 3 -TPD profiles of SAPO-I8, MgAPO-I8, ZnAPO-18, and AIPO4-18, (b) NH3-TPD profiles of SAPO-18 after adsorption and purging at the temperatures indicated.

at tDME, tDME being defined as the moment that dimethyl ether in the product stream exceeds 1% under the given testing conditions. At /DME the selectivities of MgAPO-18 and ZnAPO-18 are very significantly shifted towards higher olefins as compared to SAPO-18, whereas the MeAPOs have C 3 / C 2 ratios twice that of the SAPO. The product distribution during the reaction time over the SAPO-18 is presented in Fig. 5. Clearly, the time

R. Wendelbo et al. /Applied Catalysis A: General 142 (1996) L197-L207

L203

Table l Catalyst characterization data: micropore volumes, acidity and crystal size of SAPO- 18, MgAPO- 18, ZnAPO- 18 and AIPO 4-18 Material

Micropore volume a ( m l / g )

Si, Mg, Zn (mmol/g)

Bronsteds strong acid sites b (mmol NH 3 / g )

Crystal size range (p.m)

SAPO- 18 MgAPO-18 ZnAPO- 18 A1PO4-18

0.25 n.m. n.m. 0.28

0.28 0.14 0.10 0.00

0.25/0.22 0.10/0.08 0.08/0.06 0.03/0.02

0.1-2.0 0.1-2.0 0.1-2.0 0.2-8.0

Micropore volumes measured with N 2 adsorption. b Acidity measured as NH 3 desorbed in the high temperature peak ( > 285°C) in Fig. 3a, and ( > 300°C) in Fig. 3b. n.m. = not measured.

tO]

Fig. 4. SEM images of (a) SAPO- 18, (b) MgAPO- 18, (c) ZnAPO- 18 and (d) A1PO4-18. Table 2 Catalytic test data for SAPO-18, MgAPO-18, ZnAPO-18 and AIPO4-18: lifetime and C 2 -C4-selectivity Material SAPO- 18 MgAPO- 18 ZnAPO- 18 A1PO4-18

Lifetime a (tDME) ' rain tos

C2_sel ' wt.-% at

345

47 27 32

190

65 0

a Lifetime taken as tom E.

tDM E

C3_sel ' wt.-% at 40 47 45 -

tDM E

C4_sel" wt.-% at tDME 11 22 19 -

R. Wendelbo et al. / Applied Catalysis A: General 142 (1996) LI 97-L207

L204 50

T

ethene

J 45 - -

40

e}

--

aS+ ~

propene

"6 i ..~ aoL

r l

,o "

o~ 2s~

Z= 2o .$ 15

L

butenes

lO ~

propane 0

0

50

100

150

200

250

300

350

time (minutes) Fig. 5. Product distribution expressed as wt.-% carbon for the conversion of methanol over SAPO-18 at 420°C and W H S V = 1.

averaged selectivities to lower olefins are higher on SAPO-18 than on the MeAPOs.

4. Discussion Pure and well crystalline SAPO-18 and MgAPO-18 can be synthesized using TEAOH as an organic additive, using recipes that in many aspects are similar to those used for making SAPO-34 and MgAPO-34. Considering the similarity between the AEI and CHA structures, this is perhaps not unexpected. Lower Si and water contents than hitherto used for making SAPO-34 seem to be among the key variables for obtaining the AEI-structure. Correspondingly, MgAPO-18 and ZnAPO-18 are made with less Mg and Zn, and less water than used for making MgAPO-34 and ZnAPO-34. The gels are prepared with the same initial water content as SAPO-34 and MgAPO-34, but the water content is subsequently reduced to a level typical of the A1PO4-18 synthesis gel [1]. Since SAPO-18 cannot be made by adding Si to the A1PO4-18 recipe, and A1PO4-18 cannot be made by excluding Si from a SAPO-34 recipe, this implies an intricate nucleation and crystal growth behaviour where some initial nucleation or precursor formation requires a higher water content than the subsequent crystal growth of SAPO-18. From the XRD traces in Fig. 2, it can be seen that the A1PO4-18 and the ZnAPO-18 samples contain traces of the AFI structure, but with further optimization of the synthesis we believe these phases can also be made pure. In SAPO-18 the level of Si incorporated corresponds to the

R. Wendelbo et al./ Applied Catalysis A: General 142 (1996) L197-L207

L205

amount of Si in the synthesis gel, whereas the MgAPO-18 and the ZnAPO-18 have much lower contents of Mg and Zn than intended (Table 1). These results probably reflect the loss of Mg and Zn during the filtration step, but this has not been checked. Chen et al. [18] also obtained less substitution (about half) with Mg and Zn than with Si. Using TEAOH as a template, it was not possible to make SAPO-18 with (Si + A1 + P ) / S i lower than about 30, close to the results of Chen et al. [18] who obtained ratios down to 25 using diisopropylamine as a template. It appears that the number of strong BrCnsted acid sites in our SAPO-18 sample approximately equals the number of Si in the structure, indicating that most, or all of the Si in this material occurs as isolated Si atoms in the lattice. There is also a clear relationship between Mg and Zn content and the number of BrCnsted strong acid sites in the MgAPO-18 and the ZnAPO-18. For the MgAPO-18 and the ZnAPO-18 the number of acid sites are small compared to the contents of Mg and Zn (Table 1), particularly if the A1PO4-18 peak is considered as background level. It is very likely that some of the Mg 2+ and Zn 2+ are not in lattice positions, but occur as exchangeable cations, reducing the number of BrOnsted acid sites. The TPD results (Fig. 3b) further imply the interpretation that the SAPO-18 and ZnAPO-18 have weaker acid strength than the MgAPO-18, the high T peaks emerging at lower temperatures, but here other effects, e.g. readsorption may contribute. Since readsorption increases with the density of acid sites it can be concluded that the MgAPO-18 has stronger acid sites than the SAPO-18. Flanigen et al. [19] measured higher butane cracking rates on MgAPO-34 than on ZnAPO-34 and SAPO-34, supporting the evidence that Mg as substituent in A1PO4 lattices of the closely related AEI and CHA type structures induces higher acid strength than Zn or Si. The fact that also the A1PO4-18 appears to have some, although very few strong acid sites (Fig. 3b), is probably an artifact since this sample has no measurable lattice substituents. Its TPD profile might accordingly be used as background for the other profiles, leading to a reduction in the peak areas in Table 2, but we do not have sufficient information to assess this question at the present. The SAPO-, MgAPO-, and ZnAPO-18 form thin plates of the same dimensions (Fig. 4 and Table 1) which is fortunate for the study and comparison of catalytic activity, because diffusion induced effects should be equal. It appears that growth in the crystallographic z-direction in the AEI materials is hampered, presumably related to the shifts in D6R orientation in this direction (Fig. 1). The square planar shape of the crystals reflects the equivalence of the internal structure in the crystallographic x and y directions [2]. The lower ethene selectivity and higher propene selectivity (at tDME) of the Mg- and ZnAPO-18 catalysts as compared to SAPO-18 (Table 2) is analogous to the results of Chen et al. [15], and as obtained with materials with a CHA structure [20]. It thus seems that the nature of the substituting element is more

L206

R. Wendelbo et a l . / Applied Catalysis A: General 142 (1996) LI97-L207

important than the difference in pore geometry between the AEI and the CHA structures. Concerning total conversion, the catalytic lifetimes of the different AEI materials correlate fairly well with the number of strong acid sites. This observation is, however, insufficient to conclude anything about the deactivation mechanism. In general, other factors in addition to the nature of the substituent and differences in supercage geometry may play important roles when comparing catalysts. Such factors are crystal size distribution, density of acid sites, overall crystallinity and defect density. In the present set of catalysts we have minimized these factors, although we have little control of the defect site density. The influence of these factors are presently under study in our laboratory, and will be the subject of a future publication.

5. Conclusion Pure and well crystalline SAPO-18 and MgAPO-18, and a fairly pure ZnAPO-18 have been synthesized. All three materials are active MTO catalysts in contrast to A1POa- 18, but MgAPO-18 and ZnAPO-18 are definitely inferior to SAPO-18 regarding catalytic lifetime and ethene selectivity.

Acknowledgements We are indebted to Norsk Hydro AS for supporting the present work, and to Steinar Kvisle and Terje Fuglerud for fruitful discussions.

References [l] S.T. Wilson, B.M. Lok and E.M. Flanigen, US 4,310,440 (1982). [2] A. Simmen, L.B. McCusker, Ch. Baerlocher and W.M. Meier, Zeolites, 11 (1991) 654-661. [3] B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan and E.M. Flanigen, J. Am. Chem. Soc., 106 (1984) 6092-6093. [4] S.W. Kaiser, EPA 0,105,512 (1983). [5] J. Chen, P.A. Wright, S. Natarajan and J.M. Thomas, Stud. Surf. Sci. Catal., 84 (1994) 1731-1738. [6] J. Chen, J.M. Thomas, P.A. Wright and R.P. Townsend, Cat. Lett., 28 (1994) 241-248. [7] J. Chen, P.A. Wright, J.M. Thomas, S. Natarajan, L. Marchese, S.M. Bradley, G. Sankar, C.R.A. Catlow, P.L. Gai-Boyes, R.P. Townsend and C.M. Lok, J. Phys. Chem., 98 (1994) 10216-10224. [8] I.M. Dahl and S. Kolboe, J. Catal., (1994) 458-464. [9] E.M. Flanigen, R.L. Patton and S.T. Wilson, Stud. Surf. Sci. Catal., 37 (1987) 13-27. [10] B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan and E.M. Flanigen, US 4,440,871 (1984). [11] D.A. Lesch, R.L. Patton and N.A. Woodward, EPA 0,293,939 (1988). [12] E. Jahn and H. Gies, Extended abstract, 9th. Int. Zeol. Conf. Montreal, RP241, 1992.

R. Wendelbo et al. / Applied Catalysis A: General 142 (1996) L I 97-L207

L207

[13] J. J~inchen, I. Girnus, U. Lohse, M.P.J. Peeters, J.W. de Haan, L.J.M. van de Ven and J.H.C. van Hooff, Ext. abstract, German workshop on zeolite chemistry, Leipzig, 1993. [14] L. Marchese, J. Chen, J.M. Thomas, S. Colluccia, A. Zecchina, J. Phys. Chem., 98(50) (1994) 13350-13356. [15] J. Chen and J.M. Thomas, J. Chem. Soc. Chem. Commun., (1994) 603-604. [16] R. Wendelbo, Int. appl. PCT/NO94/00130. [17] R. Wendelbo, H.M. Oren and S. Kvisle, No. 174341 (1994). [18] J. Chen, J.M. Thomas and G. Sankar, J. Chem. Soc. Faraday Trans., 90(22) (1994) 3455-3459. [19] E.M. Flanigen, B.M. Lok, R. Lyle Patton and S.T. Wilson, in Proceedings of the 7th IZC, Tokyo, 1986, pp. 103-112. [20] S.W. Kaiser, No. appl. 872505 (1987).