Propene Oligomerization and Xylene and Methyl-Pentene Isomerization over SAPO-11 and MeAPSO-11

J. Weitkamp, H.G. Karge, H. Pfeifcr and W. Holderich (Eds.) Zeolites and Related Microporous Materials: Stare of [he Art 1994 Studies in Surface Science and Calalysis, Vol. 84 0 1994 Elsevier Science B.V. All rights reserved.

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Propene Oligomerization and Xylene and Methyl-Pentene Isomerhation over SAPO-11 and MeAPSO-11 J. S.Vaughan, C.T.OConnor and J.C.Q.Fletcher. Catalysis Research Unit, Department of Chemical Engineering, University of Cape Town, Rondebosch, 7700, South Africa

The molecular sieves SAPO-I1 and MeAPSO-11, where Me = Co, Fe, Mn and Ni, were synthesized and the acidity of these catabsts and modijed forms thereof tested using the isomerization of m-xylene and 2-methyl-2-pentene (2M2P) as probe reactions. The activity of the catalystsfor high pressure propene oligomerization was also evaluated. The modjcations included steaming, acid washing, metal impregnation,silanization,pelletization and extrusion. In the conversion of m-xylene the catalysts all showed the high pardortho ratio expectedfor IOMR structures. In the case of 2M2P isomerization the ratio of methyl shijt products (c/t3M2P) to dauble bondsh@ (c/t-4A42P) indicated the presence of strong acid sites. In both these reactions di&vsional resistances due either to the presence of non-framework intraparticular matter or to amorphous acidic material on the external surface of the crystallites masked the activity of the catalysts. This was indicated by the unusually high extent of disproportionation of the m-xylene and the poor correlations between the activity and the ammonia-TPD results. SAPO-I I madified by mild steaming yielded a remarkably high catalyst utilization value of over I000 g-liquid/gcatalystfor propene oligomerization. More than 50% of the reaction products were dimers of propene. 1. Introduction Silicoaluminophosphate molecular sieves and those into which metals have been incorporated, viz. SAPO-n and MeAPSO-n have been shown to possess mild acidity and hence are particularly interesting for a wide range of hydrocarbon reactions [l-61. At Si and metal contents of less than 2 wt%, there exists a good correlation between the number of acid sites generated and the framework composition [7,8]. In the present paper a series of SAPO-11 and MeAPSO- 11 samples have been synthesized and their acidic and shape selective properties determined using ammonia TPD and probe reactions. Acidity was probed by comparing the methyl and double bond shift reactions of 2-methyl-2-pentene (2M2P) and the shape selectivity using the isomerization of mxylene. The activity of these catalysts and modified forms thereof for propene oligomerization was determined using the catalyst utilization value (gliquid/gcatalyst) as the criterion of performance. In particular steamed SAPO-11 was studied since this treatment has been shown to enhance the activity of ZSM-5 for this reaction [ 111. 2. Experimental

2.1 Catalyst synthesis, modification and characterization SAPO-11 and MeAPSO-11 (Me = Co, Fe and Mn) molecular sieves were synthesized according to the relevant patents [9,10]. NiAPSO-11 was synthesized by substitution of Ni for Mn in the MnAPSO-11 synthesis recipe. The -150 pm size fraction was used in all catalytic tests. Post-

1710

synthesis modifications were carried out on pre-calcined samples. Calcination was carried out using 10 g of as-synthesized catalyst at 500°C in flowing nitrogen (cu. 4"C/min) and kept at this temperature for 12 hours before air was passed over the catalyst for a firther 24 hours. The following post-synthesis modifications of calcined SAPO-11 were carried out : (i) Ni and Co impregnation by the incipient wetness technique (cu. 4 wt% metal loading). (ii) Acid washing (5 g of the calcined catalyst was stirred in 100 me of 2M HNO, at 80°C for 2 hours, abbreviated hereafter as Acid-SAPO-l l). (i) Mild and severe steaming (Mdd - 60 Torr H,O 100 mi?/min 500°C 2 hours, abbreviated hereafter as MS-SAPO-I 1; Severe - 100 Torr H,O 100 mQ/min500°C for 12 hours, abbreviated hereafter as SS-SAPO-11). (iv) Silanization with tetraethoxysilane (2 g catalyst, reactor temperature=35O0C, WHSV=l kl, time=1.5 h, abbreviated as Silan-SAPO-11). Extrudates of SAPO-11 were made using a 75205 mixture of catalyst:binder:celluloseand were extruded under 5 tonnes pressure (Ext-SAF'O-11). Pellets of SAPO-11 were pressed under 10 tonnes pressure and then crushed to i 2 mm size fraction (Pell-SAPO-11). Aluminium, phosphorous and silicon contents were determined using ICP and the metal contents determined using atomic absorption spectroscopy (AA). The as-synthesized samples were also characterised by XRD, SEM, BET surface area and hexane sorption capacity. Relative crystallinities were determined by summation of the 5 most intense reflections in the 28 range of 15-25" and normalizing with respect to SAPO-11. Ammonia TPD experiments were performed in a quartz sample cell containing cu. 0.25 g of catalyst. The catalyst was calcined in flowing air for 240 minutes at 500°C before being cooled to 100°C in flowing helium. Ammonia was then adsorbed from a 4% NH, in helium mixture for 60 minutes and the physisorbed ammonia was removed by purging the system with helium for a fbrther 24 hours. For the NH,-TPD spectra, recorded by measuring the ammonia desorbed using a TCD, the temperature was ramped at 10"C/min from 100°C to 600°C and maintaining the final temperature for 30 minutes. NH, mass balances, checked by titration of the exiting stream, were always better than 95%. FT-IR spectra of the catalysts (1% in Kl3r) were recorded using a Nicolet 5ZDX FT-IR spectrometer at a resolution of 4 cm-'. 2.2 Reaction Procedures M-xylene and 2-methyl-2-pentene were obtained from Aldrich (AR 99%). The oligomerization feed mixture (86 wt% propene, 14 wt% propane) was obtained from Sasol. For all of the reactions the catalysts were first calcined in-situ in flowing air at 500°C for at least 12 hours. Xylene isomerization was carried out in a 16 mm ID quartz reactor packed with ca. 0.5 g of catalyst and the m-xylene was fed to the reactor via a double stage saturator (partial pressure of 1 kPa in nitrogen). The reaction conditions were 450"C, WHSV = 1 k1and 115 kPa. 2-Methyl-2pentene isomerization was carried out in a 24 mm ID borosilicate glass reactor packed with ca. 0.25 g of catalyst and the 2-methyl-2-pentene was fed to the reactor via a double stage saturator (partial pressure of 18 kPa in nitrogen). The reaction conditions were 200"C, WHSV = 11 h 1 and 115 kPa. The propene oligomerization apparatus has been described elsewhere [ 111. In the start-up procedure the reactor was cooled to 100°C under nitrogen (5 MPa) before the propene-propane feed mixture was passed over the catalyst bed. Once the feed contacted with the catalyst bed the reactor was kept at 100°C for 5 minutes before being heated at 6"C/min to

171 1

220°C. Depending on the activity of the catalyst the reactor temperature was then set so that the required catalyst bed temperature was achieved. Propene oligomerization was carried out at 250"C, WHSV = 12*2 h 1 and 5.0 MPa. Mass balances for all the reaction studies were greater than 95%. 2.3 Analysis The reaction products were analyzed by gas chromatography. A 30 m Supelcowax 10 capillary column was used to separate the xylene isomerkition reaction products. The 2-methyl-2-pentene and propene oligomerization products were analyzed using a 30 m, 0.352 mm I.D. hsed silica megabore column coated with a 1.5 p m film of DB- 1 (1 00 % methylpolysiloxane). Oligomer groupings were assigned on the basis of GC-MS analysis of the liquid samples. Samples of the deactivated catalysts (cu. 15 mg) were heated in a flowing nitrogen stream to 500°C in the thermogravimetricbalance and held at that temperature for 60 minutes before air was passed over the coked catalyst for a firther 120 minutes. This was done in order to determine the relative amounts of '%oft''and "hard" coke respectively. 3. Results and Discussion

3.1 Synthesis and characterization The elemental analysis of each catalyst is shown in Table 1 , For all the sieves shown the (Al+Me):(Si+P) ratios were greater than 1 indicating that there was an excess of either Al or the metal present. This has been previously observed [9,10] and indicates that there must be non-framework oxides of these elements present which XRD experiments did not observe. The amount of silicon present in the sieves was lower than the values given in the patents [9,10] and, being less than 2 wt%, was assumed to have substituted for P only [3]. Metal contents for the MeAPSO-11 sieves were all less than 5.5 wt% and the respective amounts of Fe, Co and Mn corresponded well to those given in the' patent [lo]. Acid washing of aluminium.

I

1

h

c ._ .c

0)

-0

t

au

10

15

20 25 2 Theta (degrees)

30

35

Figure 1. XRD spectra of SAPO-11 and CoAPSO-11.

SAPO-11 in nitric acid at 80°C removed 8% of the

The XRD spectra for SAPO-11 and CoApSO-11 are shown in Figure 1 as examples. All the patterns indicated that the AEL structure type was the only crystalline phase present and no broad

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baseline indicative of amorphous material was seen. Table 1 also shows that addition of metals to the synthesis gel decreased the crystallinity. FT-IR spectra of the catalysts all showed features typical of SAPO-11[12]. N,-BET surface areas and n-hexane sorption capcity correlated well Table 1. Chemical analysis (molar ratios), surface area, n-hexane adsorption,YOrelative crystallinity data T-atoms composition (SixALp&4e,)0 Catalyst

X

Y

m

z

$+w)l

%re1 cryst.

LOI'

100

10.0

72 61 60 70 99 99

11.0 14.2

(X+Z)

SAPO-I 1

NiAPSO-I 1

FeAPSO-11 COAPSO-I 1 MnApso-11

Acid-SAPO-112 MS-SAPO- 1I Ni(imp)-SAPO- 1 1 Co(imp)-SAPO-11 Silan-SAPO-11

0.006 0.015 0.005 0.016 0.01

0.552 0.442 0.510 0.434 0.532 0.388 0.529 0.407 0.495 0.450

0.00

0.04 0.07 0.05 0.04

1.23 1.23 1.54 1.36 1.17

-

16.0

11.0

100

-

100 98

-

Surface Area (m'k) 236 200 120 128 193 168 70 89

n-hexane sorption 4.5 3.8 2.8 2.8 3.0 3.0 3.5 1.9 2.1

-

1. LO1 - wt% loss on ignition at 600°C in air 2. 8% of Al removed by acid washing 3. Steamed SAPO-11 has the same chemical composition as SAPO-I 1

-

with each other. Incorporation of metals into the synthesis gel decreased the surface area and the extent of hexane sorption. The low % crystallinities of the F MeAPSOs which may be due to structural defects are consistent .with the low hexane sorption E z: capacity. The excess oxides of Al and Me also possibly caused .-C diffusional restrictions in the pores. Although acid washing and ! E steaming had no effect on the % crystallinity these treatments 200 300 400 500 600 reduced the hexane sorption capacity significantly. Acid Temperature ( c) washing would have been Figure 2. Ammonia TPD spectra of SAPO-11, NIAPSO-11 and cO~~S0-11. expected to clean out the pores but may have increased diffusional resistances by causing the introduction of extra-framework oxides of aluminium. Electron micrographs showed the SAPO-11samples to consist of spherical agglomerates of about 5 microns diameter. The MeAPSOs showed clear evidence of separate amorphous-like material

-

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amongst the intertwined regular crystallites. The NH,-TF'D data (Table 2, Figure 2) showed that the catalysts all possessed mild acidity, with most of the ammonia being desorbed between 250-3 10°C (low temperature desorption-LTD). There were no distinct high temperature desorption (HTD) peaks typical of strong acid sites. The assignment of HTD acid sites above 450°C was arbitrary. As has been observed previously [2] when Co and Mn were added to the SAPO-11 synthesis gel the number of acid sites increased compared to the SAPO-11 sample. This indicated that Coz+and Mn2+may have substituted for On the other hand Ni and Fe addition to the synthesis gel had no effect on the number of acid sites compared to SAF'O-11 although the LTD peak shifted to slightly lower temperatures which indicated slight weaker acid strength. Steaming and acid washing SAPO-11 reduced the number of acid sites. Since in general reduced hexane sorption capacity did not indicate reduced acidity, this loss of acidity by SAPO-11 after steaming or acid washing could not be ascribed to diffusional problems for the ammonia. It must therefore be due to loss of Si or SAPO type centres from the structure [13]. Silanized SAPO-11 also had fewer acid sites than SAPO-11. Ni and Co impregnation of SAPO-11 had no effect on the number or strength of acid sites. Table 2. NH,-TPD data. Catalyst

LTD

HTD ("C)

LTD (mm0Ug)

HTD (m0Vg)

Total (m0Ug)

SAPO-1 1 NiAPSO- 1 1 FeAPSO-I 1 COAPSO-I 1 MnAPso-I1

312 256 282 310 293 310 330 300 317 310

48 1

0.24 0.23 0.25 0.41 0.32 0.20 0.20 0.22 0.26 0.12

0.19 0.22 0.19 0.22 0.22 0.09 0.1 1 0.12 0.16 0.10

0.43 0.45 0.44 0.63 0.54 0.29 0.31 0.34 0.42 0.22

MS-SAPO-I1 ss-SAPO-1 I Acid-SAPO- 1 1 Ni(imp)SAPO- 1 1 Silan-SAPO-1 1

("C)

405 410,590 420,590 436 480 480

3.2 Xylene isomerization The conversion of m-xylene has been used extensively to probe the shape selective and acidic properties of medium pore molecular sieves [ 14,151. The conversion and selectivity data for m-xylene isomerization over the catalysts used in this study are shown in Table 3. All the catalysts studied showed a gradual deactivationwith time on stream. As expected for the 10 MR structure the p d o r t h o ratios were all greater than the equilibrium ratio (at 45OOC) of 0.95 and are similar to those reported previously [2]. The extent of disproportionation was however unusually high especially during the initial period. This may be due to large amounts of external acidity arising from the non-framework material referred to earlier. After some time on stream the ratio of disproportionation to isomerization activity decreased with a concomitant increase in the plo ratio indicating possibly that these external non-shape selective sites had deactivated and that the intraparticular isomerization reaction was favoured. Non-framework material present in the case of the MeAPSOs may also be located within pores and be responsible for enhancing the shape

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selective isomerization by reducing the effective pore diameter. 3.3 2-methyl-2-pentene isomerization

The 2M2P isomerization has been proposed to probe the relative strength of weak to medium acidity catalysts [16]. The conversion and selectivity data for these tests are shown in Table 4. All catalysts deactivated to a steady state conversion level and the oligomerization products were always below 2 wt%. The ratio of 2M2P to 2MlP was cu. 3.3 which indicated that equilibrium conversion was achieved in all cases. Although this reaction should discriminate on the basis of acid strength there was no correlation between the ratio of methyl shift:double bond shift and the ammonia TPD results. This may be due to diffusional resistances being the controlling factor in the reaction. The presence of such resistances has already been highlighted in the discussion of the surface area and hexane sorption results. SAPO-11 showed a yield of about 17% methyl shift products which is twice to three times greater than in the case of the MeAPSO-11 samples. Assuming that the methyl shift reaction is transition state shape selectively controlled, the high R1 value for SAPO-11 may be due to the openness of its pores as indicated by the hexane sorption capacity. It is clear then that the presence of diffusional constraints in the sample studied inhibited the validity of the use of this probe reaction as an indicator of acid strength. Table 3. m-Xylene Isomerhation. Initial (t=5 minutes) Dispflsom Conv. (YO) pardortho SAPO- 1 1 NiAPSO- 1 1 FeAPSO-I 1 COAPSO- 1 1 W S O - I1 Acid-SAPO-11

8.0 6.0 12.8 9.8 10.3 6.7

0.9 0.85

2.5 3.2

1.4

1.55 2.3 1.4

m

1.4

Steady state (t=80 minutes) Conv. (%) pardortho DispDsom 3.7 3.9 4.7 3.8 4.0 3.1

1.74 1.28 1.46 1.78 1.74 1.2

1.3 0.9 3.1

1 .o

1 .o 1.4

Table 4.2-methyl-2-pentene isomerization. Conv. SAPO-I 1 "SO-I 1 FeAPSO-11 COAPSO- 1 1 W S O - I1

55.7 42.8 34.4 33.8 42.6

Initial (t=5 minutes) R1' (2M2P:2MIP) 1.so 0.83 0.15

0.11 0.83

3.3 3.3 3.3 3.3 3.3

Steady state (t=80 minutes) Conv. R1 (2M2P:2M1P) 40.8 37.7 30.9 28.3 38.0

0.78 0.20 0.13 0.10 0.33

3.3 3.3 3.3 3.3 3.3

* - Ratio of methyl shift (ch-3MZP) to double bond shift (ch-4M2P) products 3.4 Propene oligomerization The initial conversions and catalyst utilization values (gliquid product/gcatalyst) are shown in Table 5. Although the conversions vary considerably the objective of this work was not to make mechanistic interpretations but rather to compare the performance of the catalysts with respect to their ability to convert propene into liquid oligomers and the catalyst lifetime. The CUV is indicative ofthese properties. The incorporationof metals into the SAPO-11 synthesis gel resulted

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in decreased oligomerization performance levels (CUV) compared to SAPO- 11, except for W S O - 1 1 which performed as well as SAPO-11. CoAPSO-11, NiAPSO-11 and FeAPSO-11 deactivated rapidly with time on stream. Extrudates and pellets of SAPO-11 performed as well, if not better in the latter case, than the powder form of SAPO-11. Steaming increased the CUV of SAPO-11 dramatically and in the case of severe steaming the fraction of dimer increased significantly. Silanizing and acid washing the catalyst reduced the CUV of SAPO-11. The metal impregnated sieves were completely inactive. In both these cases reduced activity was probably due to diffusional resistances as indicated by the correlation with hexane sorption capacity. The absence of a correlation between TPD data and propene oligomerization for the metal impregnated samples may be due to differences in the counterdifisional resistance of the samples for ammonia and products of oligomerization respectively. Deactivated SAPO- 11 was hlly regenerated by recalcination in air at 500°C to similar activity levels. Notwithstanding the conversion levels in all instances the dimer fraction was the major reaction product. A linear relationship was evident between the C,,+ yield and conversion, which indicated a consecutive reaction mechanism. H-NMR spectra showed that steaming of SAPO-11 produced a more linear product. No aromatics were observed for any case. The amount of coke on the deactivated catalyst after oligomerization, as determined by TG-DTA, corresponded approximately in almost all cases to the amount needed for pore filling ( i e . same as LOI). The amount of "hard coke" was always very low (<0.1 wt%). FT-IR which indicated mainly the presence of aliphatic coke [ 171 showed that deactivation was probably due to pore blockage as a result of the inability of the higher oligomers to diffise out the pores. Table 5. Pmpene ollgomerlzatlon activity and selectivity data for SAPO-11, modified SAPO-11 and MeAPSO11.

Max conv.

SAPO-I 1 Ext-SAPO- 1 1 Pell-SAPO-11 MS-SAPO-1 1 ss-SAPO-I 1 Silan-SAPO- 1 1 Acid-SAPO- 1 1 NiAPSO-11 FeAPSO-11 COAPSO- 1 1 W S O - I1

(%)

CVV' (gliquidgcat)

78 84 95 93 89 33 55 60 60 65 59

702 416 1368 1674 1762 12 35 16 43 47 680

wt% oligomer fraction

Dimer

Trimer

C,*+

56.3 57.5 65.0 53.7 70.3 69.8 47.9 56.2 60.4 73.3 54.9

22.3 28.9 27 .O 27.7 19.5 23.0 24.4 30.4 26.2 20.8 20.9

21.2 13.6 8.0 18.6 10.1 7.0 27.7 13.4 13.4 5.8 24.0

* CUV - defined as the yield of liquid product collected from maximum conversion to 112 life conversion. 5. Conclusions

The acidity of the catalysts prepared in this study as determined by ammonia-TPD did not correlate with that indicated by the use of the stated probe reactions. Although the expected

1716 pardortho ratios and extents of methyl shift reaction were obtained for the reactions of rn-xylene and 2M2P respectively the presence of diffusional resistances, especially in the case of the MeAPSOs, led to higher than expected disproportionation and lower than expected methyl shift reactions. Surface area and hexane sorption capacity measurements also showed than there was significant pore mouth blockage or intraparticular difisional resistances possibly due to the presence of residual non-framework material. This had a strong influence on the catalytic activity of the samples. Steaming of SAPO-11 resulted in a remarkably higher catalyst utilization value for propene oligomerization. The major products were the dimers of propene. No aromatics were formed and the oligomer product was predominantly linear.

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