Synthesis of highly siliceous ZSM-5 using diaminoalkanes as templates

Applied Catalysis, 73 ( 1991 ) 27 37

27

Elsevier Science Publishers B.V., Amsterdam

Synthesis of highly siliceous ZSM-5 using diaminoalkanes as templates M.G. Howden* and J.J.C. Botha Catalysis Programme, Division of Energy Technology, CSIR, P.O. Box 395, Pretoria 0001 (South Africa), tel. (+27 12)8414462, fax. (+27-I2)8412135 (Received 13 July 1990, revised manuscript received 7 February 1991 )

Abstract A series of c~,o)-diaminoalkanes, in which the alkane chain contained from 2 to 8 carbon atoms, was investigated regarding the suitability of these organic directing compounds in the synthesis of zeolite ZSM-5 having a silica-to-alumina mole ratio of around 250: 1. For ZSM-5 with this composition, 1,6diaminohexane was found to be the most satisfactory compound. Unfortunately, the product obtained is prone to include some unwanted zeolite ZSM-48, and synthesis conditions to minimise the fi)rmation of this material were identified. Nevertheless, from the conversion of methanol, the resulting ZSM-5 catalyst was still fbund to be capable of producing good light alkene selectivities of around 65%.

Keywords: catalyst preparation (templates), diaminoalkanes, synthesis, zeolites, ZSM-5.

INTRODUCTION

In the synthesis of zeolite ZSM-5, the best results are obtained when the composition of the product has a silica-to-alumina mole ratio of around 90:1. Products with a higher ratio are readily prepared if tetrapropylammonium cations are present as the organic directing agent. However, as the latter material is both expensive and scarce, other chemicals should preferably be considered for use in practice. The use of c~¢o-diaminoalkanes has proved successful if the carbon chain of the molecule contains approximately six carbon atoms [1-3]. As would be expected, in the initial work carried out with these templates the silica-toalumina mole ratio of the reaction mixture was generally in the region of 90: 1 [1-4]. Some restrictions on the composition were indicated from the work of van der Gaag et al. [5] who showed that the diaminohexane was not suitable for the synthesis ZSM-5 when silica-to-alumina mole ratios in excess of 200:1 were required. In the extreme, Franklin and Lowe [6,7] found that in the alumina-free synthesis, ZSM-48 was often preferentially crystallised, except when low reaction temperatures were used.

28

Previous work [8,9 ] has shown that significantly higher alkene selectivities from the dehydration of methanol are obtainable, if the ZSM-5 catalyst has a silica-to-alumina mole ratio in the region of 250:1. There are two techniques for preparing ZSM-5 with this sort of composition: either an average product is dealuminated, or the zeolite is synthesised directly with the correct amount of aluminium. Although tetrapropylammonium cations could be used for the latter method, we have recently been successful with a,o)-diaminoalkanes. Aspects of this approach, as well as the attainable alkene selectivities, are outlined in this paper. Naturally, there will always be certain problems regarding the purity of the product when synthesising highly siliceous ZSM-5 with the aid of a,o)-diaminoalkanes as the templating ingredient. With a view to reducing the impurities, we studied the effect of various parameters used during synthesis. The parameters investigated were: length of the carbon chain present in the diaminoalkane, the silica-to-alumina ratio, and the sodium content of the reaction mixture. Jacobs and Martens [10] expressed the view that in the absence of tetrapropylammonium cations, the synthesis of ZSM-5 required the presence of seeds for crystallisation. The most convenient method for satisfying this requirement would be to include a small amount of tetrapropylammonium cations in the reaction mixture. It has been shown [ 11 ] that the minimum amount should be sufficient to fill 10% of the intersections of the ZSM-5 crystals. EXPERIMENTAL

All the samples were prepared from mixtures having the following basic formula: xNa20 5DAA 0.084Pr4NBr yA1203 20SIO2 1000H20, where DAA is a,o)diaminoalkane. The recipe supplies sufficient tetrapropylammonium cations to fill 10% of the ZSM-5 intersections, together with 24 DAA molecules per unit cell. The latter value is about three times the amount that can possibly be incorporated into the zeolite, and can be considered a moderate excess. The silica used was a precipitated variety, Neosyl ET, obtained from Crosfield Chemicals. The other chemicals in the formula were obtained from sodium hydroxide, aluminium sulphate and the appropriate organic compounds, all of which were of analytical grade. The actual concentrations used for the different samples are given in Table 1. The pH of the mixtures was about 12.4, and the crystallisations were carried out in an autoclave in which the mixtures were continuously stirred at 25 r/min. The reaction temperature was 170 ° C and the duration of the crystallisations was 36 h. In order to identify the types of zeolite present, X-ray diffractometry (XRD) and scanning electron microscopy (SEM) were carried out on the samples as synthesised. Magic angle spinning nuclear magnetic resonance (NMR) spectra for 27A1were carried out after calcination at 630 ° C. Sodium was determined

29 TABLE 1

Concentrations of chemicals used in the different preparation mixtures. Preparation

x

Parent alkane of DAA

y

SiO2 : AI~O:~

mole ratio

1 2 3 4 5 6

2.10 2.10 2.10 2.10 2.10 2.10

Ethane Propane Butane Pentane Hexane Octane

0.080 0.080 0.080 0.080 0.080 0.080

250:1 250 : 1 250:1 250:1 250:1 250:1

7 3* 8 9 5* 10

2.10 2.10 2.10 2.10 2.10 2.10

Butane Butane Butane Hexane Hexane Hexane

0.133 0.080 0.047 0.133 0.080 0.047

150:1 250:1 425:1 150:1 250 : 1 425:1

11 12 5* 13

1.00 1.50 2.10 2.90

Hexane Hexane Hexane Hexane

0.080 0.080 0.080 0.080

250 : 1 250 : 1 250:1 250 : 1

*Details of certain preparations, marked with asterisks, are repeated below purely for convenience

in showing the variations investigated.

by chemical analysis of the samples after calcination and again after cation exchange, involving four exchanges at ambient temperature with a 1 M ammonium salt solution in the ratio of 10 g material per 100 g solution, followed by deammoniation at 550 ° C. For activity determinations, catalysts were prepared by using the following steps: calcining the synthesised zeolites at 630 ° C, cation exchanging as mentioned above, mixing with half its mass of pseudo-boehmite binder and extruding, and finally calcining at 550 ° C. The reaction studied was the conversion of methanol to light alkenes. In this test the methanol was diluted to 24% with nitrogen and passed, at a mass hourly space velocity ( M H S V ) of 0.66 in relation to the zeolite portion alone, over the catalyst at 450 ° C. RESULTS AND DISCUSSION

Zeolites formed With minor exceptions which will be discussed in the text, it was found from X R D that all the preparations consisted essentially of zeolite ZSM-5. A section of the diffractograms, scanned between 10 ° and 14 ° 0, of preparations 1 to 10

30

G "0

"°9"°

Po t

o~ G~

4~

/

@

®

.o ¢o .~

G

oo

Fig. 1. X - r a y diffractograms o f p r e p a r a t i o n s 1 to 10, s c a n n e d b e t w e e n 10 ° a n d 14 ° 0.

is reproduced in Fig. 1. The diffractogram of preparation 1 shows that a significant amount of amorphous material was found when diaminoethane was used as the principal template. By observing the changes in the diffractograms moving across from preparations 1 to 6, it can be seen that as the size of the alkane group was increased, the amount of amorphous material diminished and the crystallinity of the ZSM-5 increased. These data are substantiated by the scanning electron micrographs of the six preparations, shown in Fig. 2. The magic angle spinning NMR spectra for 27A1 are shown in Fig. 3. The aluminium in all the preparations gives a single peak around 56 ppm, which is typical for the tetrahedral co-ordination in zeolite ZSM-5. This must be the only significant location for the aluminium atoms as no other peaks are evident. As in the case of the X-ray diffractometry, the intensity of this peak from preparations 1 to 6 grew as the size of the alkane group increased. As the same concentration of aluminium was used in each of these preparations, this increase in peak intensity must be related to the growth in crystalline material. Apart from the peaks identified with ZSM-5, the X-ray diffractogram of sample i gave a solitary broad peak at 0.344 nm. This peak was attributed to the presence of analcime. As the length of the alkane group of the diaminoalkane was increased, the presence of analcime decreased; with diaminopentane it was very small, and with diaminohexane and diaminooctane it was not detected. Fig. 1 also includes sections of the diffractograms of preparations with vat-

31

Prep. 2 5,urn

Prep. 4 5jum

Prep. 5 5~m Fig. 2. Scanning electron micrographs of preparations 1 to 6.

Prep. 6 5jura

32

50

6 (ppm)

0

50

0

6(ppm)

Fig. 3. Magic angle spinning NMR spectra of 27A1at 78.2 MHz, of preparations 1 to 10. Samples were scanned between 3=80 and - 5 ppm.

ious silica-to-alumina ratios; samples 7, 3 and 8 when using diaminobutane, and samples 9, 5 and 10 when using diaminohexane as the main template. The crystallinity, as estimated from the XRD intensities of the preparations, decreased as the silica-to-alumina ratio was raised, with 1,4-diaminobutane tending to yield more amorphous material in highly siliceous ZSM-5 than its hexane counterpart. Therefore, depending on the diaminoalkane used, there is a limitation on the silica-to-alumina ratio in ZSM-5 when using these compounds as templates. Nevertheless, in the techniques used in this work 1,6 diaminohexane can be used to synthesise a satisfactorily crystalline ZSM-5 with a silica-to-alumina mole ratio in the region of 250: 1. This value is slightly higher than that observed by van der Gaag et al. [5]. The N M R data of preparations 7, 3 and 8, and 9, 5 and 10 also show a decrease in peak intensity with increasing silica-to-alumina ratio. However, this change in intensity is influenced mostly by the aluminium content of the preparation. Nevertheless, all the preparations do show again the presence of only the ZSM-5 tetrahedrally co-ordinated aluminium. For sample 3 three XRD peaks at 0.376, 0.372 and 0.365 nm, which are associated with ZSM-5, are clearly marked. As the length of the carbon chain in the diamino compound was increased, the height of the latter peak was reduced; with diaminooctane it was not detected. This peak is the (133) reflection and its extinction provides evidence of the presence of tetragonal ZSM11 instead of orthorhombic ZSM-5 [12,13]. This confirms that 1,8-diaminooctane is an ideal template for producing ZSM-11 [2,7,14]. Furthermore, the reduced diffraction intensity of the (133) peak means that the zeolite crystal-

33

Prep. II 5jum Fig. 4. Scanning electron micrograph of preparation 11.

lised with the aid of the diaminopentane and diaminohexane compounds produced ZSM-5 with significant ZSM-11 inter-growths. The diffractograms of samples 5, 9 and 10 also produced peaks at 0.418, 0.388 and 0.358 nm. These peaks are caused by the presence of zeolite ZSM-48 [ 15 ]. As the aluminium content of the preparation was reduced, the proportion of ZSM-48 increased. This trend is in agreement with the results of Franklin and Lowe [6,7] who found that at this reaction temperature, when no aluminium is used in the preparation mixture, ZSM-48 crystallises instead of ZSM-5. This means that besides crystallinity as discussed above, the obvious purity requirements of the zeolite ZSM-5 product also places a limit on extending the silicato-alumina ratio. From the organic directing compounds tested above, 1,6-diaminohexane was adequately suitable when preparing ZSM-5 with a silica-to-alumina mole ratio of around 250: 1. Naturally, another objective would be to synthesise the purest possible ZSM-5, which means that when using diaminohexane the minimum amount of ZSM-48 should be formed. Therefore, centred around preparation 5, the effects of variation in the sodium content of the preparation mixture were also studied in this work. With the low amount of sodium that was used in preparation 11, it was found from XRD intensities that the product probably contained more ZSM-48 than ZSM-5. This is confirmed by the SEM in Fig. 4 which clearly shows a high proportion of the needle structure of ZSM-48 [ 16 ]. Increasing the sodium oxide concentration in the reaction by shifting from sample 11 to 13 did reduce the amount of ZSM-48 formed, but some of this mineral was still present in sample 13, in which the highest concentration (x=2.90) of Na~O was used. The sodium concentration in the reaction mixture has a strong influence on the formation of ZSM-48, and to keep formation of the latter to a minimum it

34 is obviously necessary to avoid a low sodium content. However, controlling this parameter alone is unlikely to result in the production of pure ZSM-5.

Sodium content

The sodium content of ZSM-5 after synthesis gives a good indication of the purity of the product; excessive amounts of sodium generally indicate the presence of non-zeolitic material. The results on samples 1 to 6 are given in Table 2, and show that as the length of the alkane group was increased the sodium content decreased. This indicates that the amount of amorphous material diminished while the crystallinity of the zeolite increased. When diaminohexane and diaminooctane were used, the materials had sodium contents expected of fully crystalline ZSM-5 or ZSM-11. Theoretically all the sodium in ZSM-5 ought to be cation-exchangeable. Therefore, the amount of sodium remaining after cation exchange also indicates the purity of the product; the lower this residual sodium, the purer the zeolite. As can be seen in Table 2, the preparation in which 1,6-diaminohexane was used produces the best crystalline ZSM-5. Although sample 8, which used diaminooctane, had an initially acceptable sodium content, it was not possible to remove enough of it through cation exchange. We are unable to explain this phenomenon. Valyocsik and Rollmann [2] found that their preparations contained very low concentrations of sodium. This was attributed to diaminoalkanes completely filling the channels of the products, thereby making inclusion of sodium cations impossible. The preparations detailed in this work did not have their channels completely filled with these templates [ 17], and were therefore capable of accommodating a higher concentration of sodium cations. TABLE2 The Na20 contentsof preparations 1 to 6 Preparation

1 2 3 4 5 6

Parent alkane of DAA Ethane Propane Butane Pentane Hexane Octane

Mass percentageNa20 in preparation As syn.

After exch.

2.13 2.80 2.26 0.75 0.51 0.45

1.03 0.57 0.33 0.25 0.02 0.19

35

Catalytic activity When using the reaction conditions mentioned in the experimental section, it is expected that ZSM-5 with a silica-to-alumina mole ratio of 250: 1 should give a light alkene (C~ to C~ ) selectivity of approximately 65 %, of which about 15% will be ethene [9]. The actual results obtained on the first thirteen preparations are shown in Table 3. The low conversion level of sample 1, and to a lesser extent that of sample 2, is attributed to the lack of active sites in preparations that contain a high amount of amorphous material. Secondly, at this reaction temperature, the ethene is derived mostly from the cracking of propene and butene [ 18,19 ]. The low ethene selectivities for these two samples imply low cracking activity, which we believe must also be due to the absence of highly active sites caused by the presence of amorphous material. Thus, neither the diaminoethane nor the diaminopropane are good templates for preparing highly siliceous ZSM-5. The preparations, having a silica-to-alumina mole ratio of 250: 1, made with the remaining four organic compounds (diaminobutane, diaminopentane, diaTABLE3

The conversion of methanol to light alkenes over the extruded samples The methanol feed was diluted to 24% in nitrogen, reaction temperature was 450 ~C and the MHSV was 0.66 in relation to the zeolite portion alone Preparation

Conversion (mass-%)

Light alkene selectivity C2-C.~,

C~

C:T C~

(mass-%)

(mass- % )

(mass- % )

1 2 3 4 5 6

74.1 98.0 99.9 99.3 99.9 99.9

63.5 62.7 69.3 58.1 64.6 68.9

6.7 6.9 13.6 12.8 14.9 15.3

56.8 55.8 55.7 45.3 49.7 53.6

7 3 8 9 5 l0

100 99.9 77.4 100 99.9 100

56.4 69.3 48.0 57.5 64.6 68.8

17.4 13.6 2.3 19.7 14.9 13.9

39.0 55.7 45.7 37.8 49.7 54.9

11 12 5 13

100 100 99.9 99.9

66.0 69.0 64.6 71.3

17.8 18.0 14.9 15.2

48.2 51.0 49.7 56.1

~b

minohexane and diaminooctane) all yielded catalysts which produced an alkene conversion and selectivity expected of ZSM-5 with this particular composition [9]. However, the choice of one of these three materials would rest on its respective residual sodium content mentioned above: the sodium content usually has a direct relationship to the rate at which coke is deposited. This seems to indicate 1,6-diaminohexane as being the most suitable template. As mentioned earlier, proceeding from preparation 3 through to 6, the amount of ZSM-5 decreases at the expense of ZSM-11 until in the last sample only the latter is present. The results given in Table 3 show that there is little difference in the alkene selectivity between these preparations. It may therefore be concluded that the alkene selectivity of ZSM-11 is similar to that of ZSM-5. 1,5-Diaminopentane is both extremely scarce and expensive and was included in the investigation primarily to complete the series of organic compounds. This practical implication provides another reason for rejecting this material for the synthesis of ZSM-5. As explained above with preparations 1 and 2, the decrease in activity and ethene selectivity found with sample 8 is attributed to insufficient crystallisation of the ZSM-5. This catalyst was made with the aid of 1,4-diaminobutane, while that synthesised with 1,6-diaminohexane, and having the same silica-toalumina mole ratio of 425:1 (sample 10), produced full conversion. This additional information confirms that the latter compound is more suited as a template when synthesising zeolite ZSM-5 with a high silica-to-alumina ratio. It can be seen that as the silica-to-alumina ratio is increased (preparations 9, 5 and 10) the normal decrease in ethene and simultaneous increase in C3 to C~ alkene selectivity takes place [9]. There is a similar change in alkene selectivity with the set of samples 11, 12, 5 and 13. This means that in these samples the silica-to-alumina ratio in the ZSM-5 in increasing. The change in composition is attributed to the simultaneous formation of zeolite ZSM-48, which crystallises in a highly siliceous form that is free of aluminium [16], thereby causing the ZSM-5 to be richer in aluminium. The higher the concentration of ZSM-48, the more aluminium is present in ZSM-5 and, naturally, the lower is the silica-to-alumina ratio. In this work preparation 11 would have the lowest ratio and sample 13 the highest. The main intention of this investigation was to produce a highly siliceous ZSM-5 to enhance alkene selectivity, and thus the presence of ZSM-48 is counter-productive to this aim and its concentration should be kept to a minimum. CONCLUSIONS In the synthesis of highly siliceous ZSM-5, 1,6-diaminohexane has proved to be an excellent template. When using this compound in the synthesis process, the limit to which the silica-to-alumina mole ratio can be raised is around 250: 1. Unfortunately, the synthesis is inclined to include some ZSM-48, which

37 r e d u c e s t h e s i l i c a - t o - a l u m i n a r a t i o of t h e Z S M - 5 . I n o r d e r t o m i n i m i s e t h e a m o u n t of Z S M - 4 8 , low c o n c e n t r a t i o n s of s o d i u m i n t h e r e a c t i o n m i x t u r e s h o u l d p r e f e r a b l y be avoided.

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