- Email: [email protected]
Isomerization of 1-butene over SAPO-11 catalysts synthesized by varying synthesis time and silica sources
Applied Catalysis A: General 259 (2004) 227–234
Isomerization of 1-butene over SAPO-11 catalysts synthesized by varying synthesis time and silica sources Ville Nieminen a , Narendra Kumar a , Teemu Heikkilä b , Ensio Laine b , Jose Villegas a , Tapio Salmi a , Dmitry Yu. Murzin a,∗ a
Process Chemistry Centre, Laboratory of Industrial Chemistry, Åbo Akademi University, FIN-20500 Åbo/Turku, Finland b Department of Physics, Turku University, FIN-20014 Åbo/Turku, Finland Received in revised form 25 July 2003; accepted 19 September 2003
Abstract A series of SAPO-11 catalyst samples were synthesized by varying source of silica and synthesis time. Products were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), nitrogen adsorption and FTIR spectroscopy of pyridine adsorption as well as evaluated towards 1-butene skeletal isomerization. By using waterglass as a source of silica, tridymite phase instead of SAPO-11 was obtained. The combination of colloidal silica and aluminum oxide as sources of silica and alumina resulted in SAPO-11 phase but small amounts of other phases, such as tridymite, were obtained. The number of the acid sites was observed to increase if synthesis time was prolonged indicating more silicon incorporation into the framework. Acidity and phase distribution had an influence on the conversion of 1-butene and selectivity to isobutene. The selectivity to isobutene was higher with catalysts, which contained more SAPO-11 phase. © 2003 Elsevier B.V. All rights reserved. Keywords: Isomerization; 1-Butene; SAPO-11 catalysts
1. Introduction Skeletal isomerization of linear butenes to isobutene is an interesting reaction from industrial point of view, since the product is used in the production of octane enhancing fuel additives. Ten-membered-ring (10-MR) zeolites and microporous molecular sieves, such as ZSM-22 and SAPO-11, are good catalysts for selective skeletal isomerization of n-butenes [1]. This is due to sufficient Brønsted acid function and shape selective pore structure. The yield of isobutene in ferrierite has been observed to be proportional to the concentration of the Brønsted acid sites [2]. However, the high selectivity towards isobutene is due to pore structure rather than the acidity [3,4]. The incorporation of silicon into the framework of a hypothetical aluminophosphate molecular sieve structure
∗ Corresponding author. Tel.: +358-2-215-4985; fax: +358-2-215-4479. E-mail address: [email protected] (D.Yu. Murzin).
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2003.09.038
(AlPO4 ) results in the formation of silicoaluminophosphates, SAPOs, which have Brønsted acidity and ion-exchange capacity. SAPO-11 is a silicoaluminophosphate type microporous molecular sieve with AEL structure, consisting of non-intersecting elliptical 10-MR pores of 0.39 nm × 0.64 nm [5]. The acidic properties of SAPO depend on the content, location and distribution of Si [6]. Hence, synthesis procedure and reagents used can influence the physico-chemical and therefore the catalytic properties of SAPO materials. Studies of the influence of synthesis parameters on the physico-chemical properties of SAPO materials have been reported [7–15]. However, the effect of the synthesis parameters on the catalytic behavior in the skeletal isomerization of linear butenes over SAPO-11 has not been studied to large extent [12,13,16]. The increased selectivity and short synthesis times of the catalyst as well as low reagent prices are attractive for the industry. In this work, we have investigated how silica source and synthesis time affect the physico-chemical and catalytic properties of the SAPO-11 catalyst in the skeletal isomerization of 1-butene.
228
V. Nieminen et al. / Applied Catalysis A: General 259 (2004) 227–234
2. Experimental 2.1. Synthesis The SAPO-11 catalysts were synthesized as mentioned in Ref. [17] with some modifications. The reagents used in the preparation were ortho-phosphoric acid (Merck), aluminum isopropoxide (Aldrich), di-n-propylamine (Aldrich), Ludox AS30 (Aldrich) and distilled water. A gel solution was prepared and introduced in an autoclave and synthesis was carried out at 673 K in an oven under static condition. Sodium silicate (i.e. waterglass, Na2 O 7.5–8.5%, SiO2 25.5–28.5%, pH 11.5, Merck), Ludox AS30 (30 wt.-% suspension in water, pH 8.9, surface area 220 m2 g−1 , Aldrich) and Ludox AS40 (40 wt.% suspension in water, pH 9.1, surface area 135 m2 g−1 , Aldrich) were used in the synthesis to investigate the influence of silica sources on the catalytic properties of SAPO-11. The influence of synthesis time was studied by carrying out preparation of SAPO-11 using Ludox AS30 as a source of silica at 24, 48 and 72 h. After the preparation, crystalline material was filtered and washed thoroughly with distilled water. The samples were dried at 373 K and the calcination was carried out at 873 K. The samples prepared with the silica sources Ludox AS30, Ludox AS40 and waterglass were denoted as SAPO-11-L30, SAPO-11-L40 and WG, respectively. The samples prepared in 24, 48 and 72 h were denoted as SAPO-11-24 h, SAPO-11-48 h and SAPO-11-72 h, respectively. Note that SAPO-11-L30 and SAPO-11-48 h is the same catalyst but the notation differs depending on to which synthesis procedure it is compared. 2.2. Characterization The characterization of the SAPO-11 catalysts was carried out by using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), nitrogen adsorption and pyridine adsorption with FTIR spectrometer. X-ray powder diffraction measurements were done on a Philips PW1820-based diffractometer using a Cu X-ray tube. All samples were measured through an angular range of 5–40◦ (2θ) using 0.02◦ steps and 1 s measuring time for each step. The measured diffractograms were analyzed using X’Pert HighScore software (Philips, 2001) and the Powder Diffraction File (PDF)-database (PDF-2, sets 1–46, 1996, ICDD). The PDF database was used to identify the sample composition. The sums of background intensities were compared to get an estimate of relative crystallinity. The morphology was investigated by the scanning electron microscope (Cambridge Leica 360). The surface areas of the microporous materials were determined by nitrogen adsorption using a Sorptometer 1900 (Carlo Erba Instruments). The samples were outgassed at 573 K for 3 h before measuring the surface area. The FTIR spectrometry of adsorbed pyridine was used to investigate the acidic properties of the synthesized catalysts. The FTIR spectrometer (ATI Matson infinity spectrometer) was equipped with an in situ cell comprising ZnSe windows.
The samples were pressed into self-supporting discs (weight 24–27 mg and radius 0.65 cm), activated in vacuo at 723 K for 1 h prior to pyridine adsorption at 373 K. The physisorbed pyridine was desorbed in vacuo at 523 (for 1 h), 623 (for 40 min) and 723 K (for 0.5 h) after which each evacuation the FTIR spectra were recorded at 373 K. The varying outgassing temperatures were used in order to define the strong (sites retaining pyridine at 723 K), moderate and strong (sites retaining pyridine at 623 K) as well as weak, moderate and strong (sites retaining pyridine at 523 K) acidity regions. The Brønsted and Lewis acid sites can be distinguished by the bands of chemisorbed pyridinium ion at 1545 cm−1 and coordinatively bonded pyridine at 1455 cm−1 , respectively [18,19]. The band at 1490 cm−1 is associated with pyridine adsorbed on both Brønsted and Lewis acid sites. 2.3. Catalyst evaluation Skeletal isomerization of 1-butene to isobutene was investigated over synthesized SAPO-11 catalysts in a fixed-bed reactor at near atmospheric pressure. The product analyses were carried out on-line using a gas chromatograph (Varian 3700) equipped with a flame-ionization detector (FID) and capillary column (50 m × 0.32 mm i.d. fused silica PLOT Al2 O3 –KCl). The reactant was diluted with nitrogen to obtain a partial pressure of 0.5 atm. The conversion of 1-butene, selectivity to isobutene and yield of isobutene were investigated as a function of time-on-stream (TOS) at the weight hourly space velocity (WHSV) of 5 h−1 and temperature 673 K. Since the double bond isomerization of 1-butene is much faster than the skeletal isomerization, the three n-butene isomers 1-butene, cis-2-butene and trans-2-butene were considered as reactants in the calculations. Thus, the conversion of 1-butene, the selectivity to isobutene and the yield of isobutene are defined as follows (Qm is quantity in moles): conversion (mole%) (Qm )1-butene,in − (Qm )n-butenes,out = × 100 (Qm )1-butene,in selectivity to isobutene (mole%) (Qm )isobutene,out × 100 = (Qm )1-butene,in − (Qm )n-butenes,out yield of isobutene (mole%) =
(Qm )isobutene,out × 100 (Qm )1-butene,in
3. Results 3.1. Characterization 3.1.1. XRD The XRD patterns of the synthesized catalysts are presented in Figs. 1 and 2. All catalysts synthesized with Ludox AS30 and Ludox AS40 as sources of silica produced SAPO-11 as a major phase. Samples had small extra
V. Nieminen et al. / Applied Catalysis A: General 259 (2004) 227–234
SAPO-11-72h
4000
WG
8000
6000
Counts
3000
Counts
229
2000
4000
2000
1000
0
0 5
10
15
20
25
30
35
5
40
10
15
20
35
40
25
30
35
40
3000
Counts
3000
Counts
30
SAPO-11-L40
4000
SAPO-11-48h
4000
25
2θ
2θ
2000
2000
ATO 1000
1000
0
0
5
5
10
15
20
25
30
35
40
2θ
15
20
2θ
Fig. 2. XRD diffractrograms for the samples synthesized with Ludox AS40 (L40) and waterglass (WG) as a sources of silica.
SAPO-11-24h
4000
10
well. When SAPO-11-L40 was taken as a reference, the proportional crystallinities of SAPO-11-24 h, SAPO-11-48 h and SAPO-11-72 h were ∼100, 90 and 92%, respectively. The amounts of minor crystalline phases were rather small in all samples (up to 3%) being highest in SAPO-11-48 h. The synthesis by using waterglass as a source of silica, however, did not end up to SAPO-11 phase. By using PDF database, the material can be identified to be tridymite, most likely to tridymite-2H, syn.
Counts
3000
2000
1000
0 5
10
15
20
25
30
35
40
2θ
Fig. 1. XRD diffractrograms for the samples synthesized with 24, 48 and 72 h.
peaks for example at 2θ values of 13.3 and 20.3◦ which are most likely caused by small amounts of tridymite, AlPO4 -11 and/or AlPO4 -tridymite [20]. SAPO-11-48 h had an additional extra peak at 8.5◦ , indicating the presence of SAPO-31 phase. The XRD pattern of SAPO-11-72 h was nearly identical to that of SAPO-11-48 h, although the amount of minor phases was smaller and no peak at 8.5◦ was observed. The diffractograms of SAPO-11-24 h and SAPO-11-L40 were practically identical with each other as
3.1.2. Scanning electron microscope The scanning electron micrographs of SAPO-11 catalysts synthesized by using Ludox AS30 and Ludox AS40 as sources of silica are presented in Fig. 3. The shapes of the crystals were spherical being similar to those found for SAPO-11 type of microporous materials [7]. The source of silica or synthesis time did not have a strong effect on the morphology of the crystals with the only exception of a sample synthesized from waterglass. Crystal size of SAPO-11-48 h was slightly larger than that of others. 3.1.3. Surface area The measured surface areas are given in Table 1. The changes in the surface areas dependent on the synthesis
230
V. Nieminen et al. / Applied Catalysis A: General 259 (2004) 227–234
Fig. 3. SEM images of SAPO-11-24 h (upper left) SAPO-11-48 h (upper right) SAPO-11-72 h (lower left) and SAPO-11-L40 (lower right).
methods are rather minor being from 229 to 257 m2 g−1 . The samples with higher degree of crystallinity showed lower surface area. Analogous observation has been reported before for SAPO-11 catalysts [7].
tain either of the acid sites, as was expected from the XRD result. The amount of Brønsted acid sites is somewhat lower on SAPO-11-24 h compared to others, which had approximately the same amount of Brønsted acid sites. A tendency over these samples is that the less crystalline materials contained slightly more Lewis acid sites. It is notable that the number of strong acid sites over all catalysts was rather low.
3.1.4. Acidity measurements In order to discriminate the weak, moderate and strong acid sites, desorption of pyridine was carried out at 523, 623 and 723 K, respectively. An example of the absorption bands is given in Fig. 4 and integrated areas of the relevant absorption bands are reported in Table 1. All synthesized materials with SAPO-11 phase contained Brønsted and Lewis acid sites. The catalyst synthesized with waterglass did not con-
3.2. Catalyst evaluation The synthesized SAPO-11 catalysts were tested towards 1-butene skeletal isomerization at the WHSV of 5 h−1 and temperature 473 K. The results are presented in Figs. 5
Table 1 Specific surface area, proportional crystallinity and the amount (cm−1 g−1 ) of Lewis acid sites (LAS) and Brønsted acid sites (BAS) after pyridine desorption at 523, 623 and 723 K Catalyst sample
Surface area (m2 g−1 )
Proportional crystallinity (%)
SAPO-11-24 h SAPO-11-48 h/L30 SAPO-11-72 h SAPO-11-L40
232 240 257 229
∼100 90 92 100
a
523 K
723 Ka
623 K
BAS
LAS
BAS
LAS
BAS
LAS
30 41 36 37
8 12 10 9
18 28 23 25
4 7 7 5
6 7 11 12
2 5 6 3
Rough approximates due to the weak absorption bands and a consequent low signal-to-noise ratio.
V. Nieminen et al. / Applied Catalysis A: General 259 (2004) 227–234
231
25
0.4
SAPO-11-48h SAPO-11-24h SAPO-11-72h
Lewis
Brønsted
723 K
20
Yield, mol-%
Absorbance
0.3
623 K 0.2
0.1
15
10
5
523 K 0.0 1700
0
1650
1600
1550
1500
Wavenumber / cm
1450
0
1400
50
100
150
200
250
300
Time-On-Stream / min
-1
80
Fig. 4. FTIR spectra of adsorbed pyridine on SAPO-11-48 h after evacuation at 523, 623 and 723 K.
70
50 40 30 20
SAPO-11-48h SAPO-11-24h SAPO-11-72h
10 0 0
50
100
150
200
250
300
Time-On-Stream / min 50
SAPO-11-48h SAPO-11-24h SAPO-11-72h
40
Conversion, mol-%
and 6. The main products were propene, isobutane, n-butane, n-butene, isobutene, pentenes and octenes. Small fractions of hexenes and heptenes as well as traces of C1–C3 were also obtained. In general, the selectivity to isobutene over synthesized catalysts was rather low, although the conditions were not optimal to reach high yields of isobutene. This is probably due to the presence of other phases than SAPO-11. The minor crystalline phases can also contribute to the catalytic activity. For instance, AlPO4 have been reported to be active in transformations of 1-butene [12]. The phases other than SAPO-11 decrease the selectivity to isobutene of the synthesized SAPO-11 catalysts. The SAPO-11-48 h and SAPO-11-72 h catalysts exhibited higher conversion of 1-butene and lower selectivity to isobutene than SAPO-11-24 h. The yield of isobutene, however, was approximately at the same level over these three catalysts synthesized with varying synthesis times. The effect of the silicon source on the catalytic performance was more pronounced. The conversion of 1-butene and the yield of isobutene over SAPO-11-L40 were lower than those over SAPO-11-L30 were. The catalyst synthesized with waterglass as a source of silica did not exhibit any catalytic activity at all.
Selectivity, mol-%
60
30
20
10
0 0
50
100
150
200
250
300
Time-On-Stream / min
4. Discussion 4.1. Synthesis and characterization Synthesis time had a minor influence on the silicon incorporation into the framework, which is responsible for the acidity of SAPO materials. However, the acidity of SAPO materials is not directly proportional to silicon content as the aluminum content is in zeolites, but also related to the silicon substitution mechanism and distribution of Si. Mechanistically, three different isomorphic silicon substitution mechanisms have been proposed for
Fig. 5. The conversion of 1-butene, selectivity to isobutene and yield of isobutene of SAPO-11-24 h, SAPO-11-48 h and SAPO-11-72 h as a function of time-on-stream at the WHSV = 5 h−1 and temperature 673 K.
silicon substitution into the hypothetical AlPO4 framework: replacement of one aluminum by one silicon (SM1), replacement of one phosphorous by one silicon (SM2) or replacement of aluminum–phosphorous pairs by two silicon atoms (SM3) [21]. SM2 mechanism produces more Brønsted acid sites than SM3, since in the latter one the tetrahedral network remains neutral when both P and Al are simultaneously replaced by Si. The combination of SM2
232
V. Nieminen et al. / Applied Catalysis A: General 259 (2004) 227–234
Fig. 6. The conversion of 1-butene, selectivity to isobutene and yield of isobutene of SAPO-11-L30 and SAPO-11-L40 as a function of time-on-stream at the WHSV = 5 h−1 and temperature 673 K.
and SM3 mechanisms of Si incorporation has been proposed to be prevailing in the SAPO-11 synthesis [7]. The SAPO-11-48 h and SAPO-11-72 h catalysts contained more Lewis and Brønsted acid sites than SAPO-11-24 h. Due to the increase in the number of the acid sites with prolonged synthesis times, more silicon are supposed to be incorporated into the SAPO-11 framework in SAPO-11-72 h and
SAPO-11-48 h than in SAPO-11-24 h, if the mechanism remains the same during the whole synthesis. In fact, it has been observed for SAPO-5 that prolonged synthesis time increases silicon content and therefore catalysts acidity [9]. When di-n-propylamine is used as the organic template, both SAPO-11 and SAPO-31 can be crystallized [22]. Under the crystallization process of SAPO-11, with prolongation of crystallization time the alteration of crystallization type between SAPO-11 and SAPO-31 has been reported. This was supposed to result from the material exchange between the solution phase and the solid phase [23]. In addition, contradictory studies concerning phase transitions between SAPO-31 and SAPO-11 phases as a function of synthesis time have been reported [12,13]. In this study, an indication of such a phase transition to SAPO-31 was observed only in the XRD pattern of SAPO-11-48 h. Especially, no phase oscillation [12] as a function of time was observed. Synthesis time had a minor influence on the crystallinity, which was higher with shorter synthesis time of 24 h than with 48 or 72 h. This is different from the results reported before showing crystallinity at nearly 100% level already after 8 h without further decrease thereafter [12]. Silica sources used in the synthesis of SAPO-11 have a strong influence on the resulting catalyst. When waterglass was used as a source of silica, SAPO-11 phase was not obtained at all (Fig. 1). The resulting phase, tridymite-2H, syn, indicated that this silica source did not react with phosphorous acid. The reason for such a behavior of sodium silicate could be the presence of large amount of sodium species in it, which might hinder the nucleation process in SAPO-11 cystallization. Small amounts of alkali cations in the reaction mixture have been reported to prevent the nucleation of SAPO-5 [24]. An explanation was offered in terms of electrostatic interactions in the liquid phase between the alkali ions and the building units of SAPO-5: the alkali cations tend to precipitate an amorphous phase, which causes a significant decrease in crystallinity of the products. Opposite to waterglass, colloidal silica as a source of silica led to a synthesis of SAPO-11 phase. Regardless of the synthesis time, SAPO-11 was always contaminated with tridymite and AlPO4 phases. This is reasonable, since dense phases, such as AlPO4 -tridymite are known to be formed first during the synthesis of SAPO-11 [7]. Furthermore, tridymite is known to be a by-product in the synthesis of SAPO-11, if less reactive alumina source is used [7]. The use of aluminum isopropoxide has been reported to provide reduced crystallinity in case of the synthesis of SAPO-34 [25]. This was assumed to be due to some interference of the organic residue from the Al-source on the crystallization process. The colloidal silica sources instead, i.e. Ludox AS30 and Ludox AS40, are polymeric silica materials, which need to be broken into small components in order to be incorporated into the hypothetical AlPO4 framework. They differ in surface area and concentration. In the synthesis of SAPO-11, this difference did not have a major effect on the number of the acid sites, but evidently on the crystallinity. Obtained crystallinity was
V. Nieminen et al. / Applied Catalysis A: General 259 (2004) 227–234
higher in the synthesis with Ludox AS40 with higher concentration but lower surface area of the suspension. However, the combination of the used silica and alumina sources is not optimal to obtain phase pure SAPO-11. 4.2. 1-Butene isomerization In the catalyst synthesized with waterglass, no acid sites were present and obviously, no 1-butene conversion was observed either. However, the acidity is not directly related to the catalytic activity in case of SAPO-11-L30 and SAPO-11-L40: the number of Brønsted acid sites is at the same level over both catalysts, but conversion of 1-butene was substantially higher over the former one. The divergence in the catalytic behavior is probably related to differences in crystallinity and further, distribution of the impure phases as well as differences in location and distribution of the acid sites. The effect of synthesis time on the yield of isobutene was rather minor; although SAPO-11-24 h exhibited lower conversion of 1-butene, the yield of isobutene was at the same level as over SAPO-11-48 h and SAPO-11-72 h, which is due to the higher selectivity to isobutene over SAPO-11-24 h. Lower conversion of SAPO-11-24 h can be related to lower number of the Brønsted acid sites. While comparing the selectivities, the SAPO-11 catalysts showing lower 1-butene conversion exhibit clearly higher selectivity to isobutene. However, proper comparison of selectivity should be made at the same conversion level. Therefore, in Fig. 7 the selectivities are plotted as a function of conversion, presuming that the effect of coke formation is the same being independent on the conversion of 1-butene. As can be seen, the catalysts with similar proportional crystallinities exhibit similar selectivity to isobutene at the same conversion levels and further, the selectivity is higher with higher crystallinity. High selectivity to isobutene is associated with a particular pore structure [4]. Almost 100% selectivity to isobutene has been reported at conversion
Selectivity to isobutene, mol-%
90
SAPO-11-L40 SAPO-11-24h SAPO-11-48h SAPO-11-72h
80
70
60
50
233
level corresponding to thermodynamic equilibrium concentration over catalysts having AlPO4 11-framework structure [26,27]. This was explained by the selective monomolecular mechanism, which takes place in the monodimensional elliptical pores of AlPO4 11-structure hindering dimerization reactions and consequent bimolecular, less selective reaction mechanism. It is reasonable to suppose that the selectivity to isobutene over evaluated catalysts is affected by other, non-porous XRD invisible phases present in the catalysts, on which the non-selective n-butene transformation takes place. Furthermore, the lower selectivity to isobutene over SAPO-11-48 h and SAPO-11-72 h can be understood in terms of smaller amount of porous structure due to lower crystallinity. Hence, bimolecular reaction pathway is not as restricted over above-mentioned catalysts as in the case of SAPO-11-L40 and SAPO-11-24 h with higher crystallinity containing larger amount of porous solid catalyst. In addition, Lewis acidity, especially in combination with Brønsted acid sites, enhances di-, oligomerization/cracking reactions leading to higher yield of by-products [2]. This could be one contributing factor for higher conversion of 1-butene over SAPO-11-48 h and SAPO-11-72 h compared to SAPO-11-24 h and SAPO-11-L40, since the former contained slightly more Lewis acid sites.
5. Conclusions SAPO-11 catalysts synthesized with varying silica sources and synthesis times have been characterized by XRD, SEM, nitrogen adsorption and FTIR spectroscopy of pyridine adsorption as well as evaluated towards 1-butene skeletal isomerization. The choice of silica source in the synthesis of SAPO-11 catalyst has a pronounced effect on the conversion of 1-butene. Waterglass as a source of silica does not result in either acid sites of a SAPO-11 phase at all and therefore the solid material does not exhibit catalytic activity. The combination of aluminum isopropoxide and colloidal silica as sources of alumina and silica does produce a pure SAPO-11 phase but contamination with other phases, such as tridymite, was unavoidable. The use of two different kinds of colloidal silica as Si sources resulted in notable differences in crystallinity and catalytic activity. Prolonging the synthesis time increased the acid site concentration, which indicates that more silicon is incorporated into the hypothetical AlPO4 framework with increasing synthesis time. The conversion of 1-butene depended on the concentration of the acid sites and the presence of phases others than SAPO-11. Higher selectivity to isobutene was obtained over more crystalline materials.
40 20
30
40
50
Acknowledgements
Conversion, mol-%
Fig. 7. Selectivity to isobutene plotted as a function of 1-butene conversion of SAPO-11 samples.
This work is part of the activities at the Åbo Akademi Process Chemistry Centre within the Finnish Centre of
234
V. Nieminen et al. / Applied Catalysis A: General 259 (2004) 227–234
Excellence Programme (2000–2005) by the Academy of Finland. Financial support from Fortum foundation and Graduate School of Material Research is gratefully acknowledged. References [1] P. Mérideau, C. Naccache, Adv. Catal. 44 (1999) 505. [2] B. Wichterlova, N. Zilkova, E. Uvarova, J. Cejka, P. Sarv, C. Paganini, J.A. Lercher, Appl. Catal. A: Gen. 182 (1999) 297. [3] C.M. López, F.J. Machado, J. Goldwasser, B. Méndez, K. Rodr´ıguez, M.M. Ram´ırez-Agudelo, Zeolites 19 (1997) 133. [4] H.H. Mooiwer, K.P. De Jong, B. Kraushaar-Czarnetzki, W.H.J. Stork, B.C.H. Krutzen, Stud. Surf. Sci. Catal. 84 (1994) 2327. [5] J.A. Martens, P.J. Grobet, P.A. Jacobs, J. Catal. 126 (1990) 299. [6] J.M. Campelo, F. Lafont, J.M. Marinas, Appl. Catal. A: Gen. 152 (1997) 53. [7] M. Alfonzo, J. Goldwasser, C.M. Lopez, F.J. Machado, M. Matjushin, B. Méndez, M.M. Ramirez de Agudelo, J. Mol. Catal. A: Chem. 98 (1995) 35. [8] C.M. López, K. Rodr´ıguez, B. Méndez, A. Montes, F.J. Machado, Appl. Catal. A: Gen. 197 (2000) 131. [9] T. Xiao, L. An, H. Wang, Appl. Catal. A: Gen. 130 (1995) 187. [10] P. Mérideau, V.A. Tuan, V.T. Nghiem, S.Y. Lai, L.N. Hung, C. Naccache, J. Catal. 169 (1997) 55. [11] A. Bhattacharya, J. Das, S. Mitra, S.K. Roy, J. Chem. Tech. Biotechnol. 54 (1992) 399.
[12] C.M. López, F.J. Machado, B. Méndez, M. Pinto, V. Sazo, J. Goldwasser, M.M. Ram´ırez, Top. Catal. 10 (2000) 65. [13] C.M. López, F.J. Machado, J. Goldwasser, B. Méndez, K. Rodr´ıguez, Zeolites 19 (1997) 133. [14] X.S. Zhao, G.Q. Lu, A.K. Whittaker, J. Drennan, H. Xu, Microporous Mesoporous Mater. 55 (2002) 51. [15] D. Demuth, G.D. Stucky, K.K. Unger, F. Schüth, Microporous Mater. 3 (1995) 473. [16] H. Tian, L. Shin, J. Yuan, M.E. Bekheet, C. Li, React. Kinet. Catal. Lett. 64 (1998) 119. [17] B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan, E.M. Flanigen, US Patent 4440871 (1984). [18] C.A. Emeis, J. Catal. 141 (1993) 347. [19] G. Busca, Catal. Today 41 (1998) 191. [20] M.M. Treacy, J.B. Higgins, Collection of Simulated XRD Powder Patterns for Zeolites, fourth revised ed., Elsevier, Amsterdam, 2001. [21] B.M. Lock, C.A. Messina, R.L. Patton, R.T. Garjek, R.T. Cannon, E.M. Flaningen, J. Am. Chem. Soc. 106 (1984) 6092. [22] A.K. Sinha, S. Sainkar, S. Sivasanker, Microporous Mesoporous Mater. 31 (1999) 321. [23] Z.-M. Wang, Z.-F. Yan, in: Proceedings of the 22nd ACS National Meeting, Chicago, IL, USA, 26–30 August 2001 (Abstract of Papers). [24] N. Dumont, Z. Gabelica, E.G. Derouane, F. Di Rentzo, Microporous Mater. 3 (1994) 71. [25] M. Popova, Ch. Minchev, V. Kanazirev, Appl. Catal. A: Gen. 169 (1998) 227. [26] J. Cejka, B. Wichterlova, P. Sarv, Appl. Catal. A 179 (1999) 217. [27] J. Houzvicka, V. Ponec, Catal. Rev. Sci. Eng. 39 (1997) 319.
Isomerization of 1-butene over SAPO-11 catalysts synthesized by varying synthesis time and silica sources Ville Nieminen a , Narendra Kumar a , Teemu Heikkilä b , Ensio Laine b , Jose Villegas a , Tapio Salmi a , Dmitry Yu. Murzin a,∗ a
Process Chemistry Centre, Laboratory of Industrial Chemistry, Åbo Akademi University, FIN-20500 Åbo/Turku, Finland b Department of Physics, Turku University, FIN-20014 Åbo/Turku, Finland Received in revised form 25 July 2003; accepted 19 September 2003
Abstract A series of SAPO-11 catalyst samples were synthesized by varying source of silica and synthesis time. Products were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), nitrogen adsorption and FTIR spectroscopy of pyridine adsorption as well as evaluated towards 1-butene skeletal isomerization. By using waterglass as a source of silica, tridymite phase instead of SAPO-11 was obtained. The combination of colloidal silica and aluminum oxide as sources of silica and alumina resulted in SAPO-11 phase but small amounts of other phases, such as tridymite, were obtained. The number of the acid sites was observed to increase if synthesis time was prolonged indicating more silicon incorporation into the framework. Acidity and phase distribution had an influence on the conversion of 1-butene and selectivity to isobutene. The selectivity to isobutene was higher with catalysts, which contained more SAPO-11 phase. © 2003 Elsevier B.V. All rights reserved. Keywords: Isomerization; 1-Butene; SAPO-11 catalysts
1. Introduction Skeletal isomerization of linear butenes to isobutene is an interesting reaction from industrial point of view, since the product is used in the production of octane enhancing fuel additives. Ten-membered-ring (10-MR) zeolites and microporous molecular sieves, such as ZSM-22 and SAPO-11, are good catalysts for selective skeletal isomerization of n-butenes [1]. This is due to sufficient Brønsted acid function and shape selective pore structure. The yield of isobutene in ferrierite has been observed to be proportional to the concentration of the Brønsted acid sites [2]. However, the high selectivity towards isobutene is due to pore structure rather than the acidity [3,4]. The incorporation of silicon into the framework of a hypothetical aluminophosphate molecular sieve structure
∗ Corresponding author. Tel.: +358-2-215-4985; fax: +358-2-215-4479. E-mail address: [email protected] (D.Yu. Murzin).
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2003.09.038
(AlPO4 ) results in the formation of silicoaluminophosphates, SAPOs, which have Brønsted acidity and ion-exchange capacity. SAPO-11 is a silicoaluminophosphate type microporous molecular sieve with AEL structure, consisting of non-intersecting elliptical 10-MR pores of 0.39 nm × 0.64 nm [5]. The acidic properties of SAPO depend on the content, location and distribution of Si [6]. Hence, synthesis procedure and reagents used can influence the physico-chemical and therefore the catalytic properties of SAPO materials. Studies of the influence of synthesis parameters on the physico-chemical properties of SAPO materials have been reported [7–15]. However, the effect of the synthesis parameters on the catalytic behavior in the skeletal isomerization of linear butenes over SAPO-11 has not been studied to large extent [12,13,16]. The increased selectivity and short synthesis times of the catalyst as well as low reagent prices are attractive for the industry. In this work, we have investigated how silica source and synthesis time affect the physico-chemical and catalytic properties of the SAPO-11 catalyst in the skeletal isomerization of 1-butene.
228
V. Nieminen et al. / Applied Catalysis A: General 259 (2004) 227–234
2. Experimental 2.1. Synthesis The SAPO-11 catalysts were synthesized as mentioned in Ref. [17] with some modifications. The reagents used in the preparation were ortho-phosphoric acid (Merck), aluminum isopropoxide (Aldrich), di-n-propylamine (Aldrich), Ludox AS30 (Aldrich) and distilled water. A gel solution was prepared and introduced in an autoclave and synthesis was carried out at 673 K in an oven under static condition. Sodium silicate (i.e. waterglass, Na2 O 7.5–8.5%, SiO2 25.5–28.5%, pH 11.5, Merck), Ludox AS30 (30 wt.-% suspension in water, pH 8.9, surface area 220 m2 g−1 , Aldrich) and Ludox AS40 (40 wt.% suspension in water, pH 9.1, surface area 135 m2 g−1 , Aldrich) were used in the synthesis to investigate the influence of silica sources on the catalytic properties of SAPO-11. The influence of synthesis time was studied by carrying out preparation of SAPO-11 using Ludox AS30 as a source of silica at 24, 48 and 72 h. After the preparation, crystalline material was filtered and washed thoroughly with distilled water. The samples were dried at 373 K and the calcination was carried out at 873 K. The samples prepared with the silica sources Ludox AS30, Ludox AS40 and waterglass were denoted as SAPO-11-L30, SAPO-11-L40 and WG, respectively. The samples prepared in 24, 48 and 72 h were denoted as SAPO-11-24 h, SAPO-11-48 h and SAPO-11-72 h, respectively. Note that SAPO-11-L30 and SAPO-11-48 h is the same catalyst but the notation differs depending on to which synthesis procedure it is compared. 2.2. Characterization The characterization of the SAPO-11 catalysts was carried out by using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), nitrogen adsorption and pyridine adsorption with FTIR spectrometer. X-ray powder diffraction measurements were done on a Philips PW1820-based diffractometer using a Cu X-ray tube. All samples were measured through an angular range of 5–40◦ (2θ) using 0.02◦ steps and 1 s measuring time for each step. The measured diffractograms were analyzed using X’Pert HighScore software (Philips, 2001) and the Powder Diffraction File (PDF)-database (PDF-2, sets 1–46, 1996, ICDD). The PDF database was used to identify the sample composition. The sums of background intensities were compared to get an estimate of relative crystallinity. The morphology was investigated by the scanning electron microscope (Cambridge Leica 360). The surface areas of the microporous materials were determined by nitrogen adsorption using a Sorptometer 1900 (Carlo Erba Instruments). The samples were outgassed at 573 K for 3 h before measuring the surface area. The FTIR spectrometry of adsorbed pyridine was used to investigate the acidic properties of the synthesized catalysts. The FTIR spectrometer (ATI Matson infinity spectrometer) was equipped with an in situ cell comprising ZnSe windows.
The samples were pressed into self-supporting discs (weight 24–27 mg and radius 0.65 cm), activated in vacuo at 723 K for 1 h prior to pyridine adsorption at 373 K. The physisorbed pyridine was desorbed in vacuo at 523 (for 1 h), 623 (for 40 min) and 723 K (for 0.5 h) after which each evacuation the FTIR spectra were recorded at 373 K. The varying outgassing temperatures were used in order to define the strong (sites retaining pyridine at 723 K), moderate and strong (sites retaining pyridine at 623 K) as well as weak, moderate and strong (sites retaining pyridine at 523 K) acidity regions. The Brønsted and Lewis acid sites can be distinguished by the bands of chemisorbed pyridinium ion at 1545 cm−1 and coordinatively bonded pyridine at 1455 cm−1 , respectively [18,19]. The band at 1490 cm−1 is associated with pyridine adsorbed on both Brønsted and Lewis acid sites. 2.3. Catalyst evaluation Skeletal isomerization of 1-butene to isobutene was investigated over synthesized SAPO-11 catalysts in a fixed-bed reactor at near atmospheric pressure. The product analyses were carried out on-line using a gas chromatograph (Varian 3700) equipped with a flame-ionization detector (FID) and capillary column (50 m × 0.32 mm i.d. fused silica PLOT Al2 O3 –KCl). The reactant was diluted with nitrogen to obtain a partial pressure of 0.5 atm. The conversion of 1-butene, selectivity to isobutene and yield of isobutene were investigated as a function of time-on-stream (TOS) at the weight hourly space velocity (WHSV) of 5 h−1 and temperature 673 K. Since the double bond isomerization of 1-butene is much faster than the skeletal isomerization, the three n-butene isomers 1-butene, cis-2-butene and trans-2-butene were considered as reactants in the calculations. Thus, the conversion of 1-butene, the selectivity to isobutene and the yield of isobutene are defined as follows (Qm is quantity in moles): conversion (mole%) (Qm )1-butene,in − (Qm )n-butenes,out = × 100 (Qm )1-butene,in selectivity to isobutene (mole%) (Qm )isobutene,out × 100 = (Qm )1-butene,in − (Qm )n-butenes,out yield of isobutene (mole%) =
(Qm )isobutene,out × 100 (Qm )1-butene,in
3. Results 3.1. Characterization 3.1.1. XRD The XRD patterns of the synthesized catalysts are presented in Figs. 1 and 2. All catalysts synthesized with Ludox AS30 and Ludox AS40 as sources of silica produced SAPO-11 as a major phase. Samples had small extra
V. Nieminen et al. / Applied Catalysis A: General 259 (2004) 227–234
SAPO-11-72h
4000
WG
8000
6000
Counts
3000
Counts
229
2000
4000
2000
1000
0
0 5
10
15
20
25
30
35
5
40
10
15
20
35
40
25
30
35
40
3000
Counts
3000
Counts
30
SAPO-11-L40
4000
SAPO-11-48h
4000
25
2θ
2θ
2000
2000
ATO 1000
1000
0
0
5
5
10
15
20
25
30
35
40
2θ
15
20
2θ
Fig. 2. XRD diffractrograms for the samples synthesized with Ludox AS40 (L40) and waterglass (WG) as a sources of silica.
SAPO-11-24h
4000
10
well. When SAPO-11-L40 was taken as a reference, the proportional crystallinities of SAPO-11-24 h, SAPO-11-48 h and SAPO-11-72 h were ∼100, 90 and 92%, respectively. The amounts of minor crystalline phases were rather small in all samples (up to 3%) being highest in SAPO-11-48 h. The synthesis by using waterglass as a source of silica, however, did not end up to SAPO-11 phase. By using PDF database, the material can be identified to be tridymite, most likely to tridymite-2H, syn.
Counts
3000
2000
1000
0 5
10
15
20
25
30
35
40
2θ
Fig. 1. XRD diffractrograms for the samples synthesized with 24, 48 and 72 h.
peaks for example at 2θ values of 13.3 and 20.3◦ which are most likely caused by small amounts of tridymite, AlPO4 -11 and/or AlPO4 -tridymite [20]. SAPO-11-48 h had an additional extra peak at 8.5◦ , indicating the presence of SAPO-31 phase. The XRD pattern of SAPO-11-72 h was nearly identical to that of SAPO-11-48 h, although the amount of minor phases was smaller and no peak at 8.5◦ was observed. The diffractograms of SAPO-11-24 h and SAPO-11-L40 were practically identical with each other as
3.1.2. Scanning electron microscope The scanning electron micrographs of SAPO-11 catalysts synthesized by using Ludox AS30 and Ludox AS40 as sources of silica are presented in Fig. 3. The shapes of the crystals were spherical being similar to those found for SAPO-11 type of microporous materials [7]. The source of silica or synthesis time did not have a strong effect on the morphology of the crystals with the only exception of a sample synthesized from waterglass. Crystal size of SAPO-11-48 h was slightly larger than that of others. 3.1.3. Surface area The measured surface areas are given in Table 1. The changes in the surface areas dependent on the synthesis
230
V. Nieminen et al. / Applied Catalysis A: General 259 (2004) 227–234
Fig. 3. SEM images of SAPO-11-24 h (upper left) SAPO-11-48 h (upper right) SAPO-11-72 h (lower left) and SAPO-11-L40 (lower right).
methods are rather minor being from 229 to 257 m2 g−1 . The samples with higher degree of crystallinity showed lower surface area. Analogous observation has been reported before for SAPO-11 catalysts [7].
tain either of the acid sites, as was expected from the XRD result. The amount of Brønsted acid sites is somewhat lower on SAPO-11-24 h compared to others, which had approximately the same amount of Brønsted acid sites. A tendency over these samples is that the less crystalline materials contained slightly more Lewis acid sites. It is notable that the number of strong acid sites over all catalysts was rather low.
3.1.4. Acidity measurements In order to discriminate the weak, moderate and strong acid sites, desorption of pyridine was carried out at 523, 623 and 723 K, respectively. An example of the absorption bands is given in Fig. 4 and integrated areas of the relevant absorption bands are reported in Table 1. All synthesized materials with SAPO-11 phase contained Brønsted and Lewis acid sites. The catalyst synthesized with waterglass did not con-
3.2. Catalyst evaluation The synthesized SAPO-11 catalysts were tested towards 1-butene skeletal isomerization at the WHSV of 5 h−1 and temperature 473 K. The results are presented in Figs. 5
Table 1 Specific surface area, proportional crystallinity and the amount (cm−1 g−1 ) of Lewis acid sites (LAS) and Brønsted acid sites (BAS) after pyridine desorption at 523, 623 and 723 K Catalyst sample
Surface area (m2 g−1 )
Proportional crystallinity (%)
SAPO-11-24 h SAPO-11-48 h/L30 SAPO-11-72 h SAPO-11-L40
232 240 257 229
∼100 90 92 100
a
523 K
723 Ka
623 K
BAS
LAS
BAS
LAS
BAS
LAS
30 41 36 37
8 12 10 9
18 28 23 25
4 7 7 5
6 7 11 12
2 5 6 3
Rough approximates due to the weak absorption bands and a consequent low signal-to-noise ratio.
V. Nieminen et al. / Applied Catalysis A: General 259 (2004) 227–234
231
25
0.4
SAPO-11-48h SAPO-11-24h SAPO-11-72h
Lewis
Brønsted
723 K
20
Yield, mol-%
Absorbance
0.3
623 K 0.2
0.1
15
10
5
523 K 0.0 1700
0
1650
1600
1550
1500
Wavenumber / cm
1450
0
1400
50
100
150
200
250
300
Time-On-Stream / min
-1
80
Fig. 4. FTIR spectra of adsorbed pyridine on SAPO-11-48 h after evacuation at 523, 623 and 723 K.
70
50 40 30 20
SAPO-11-48h SAPO-11-24h SAPO-11-72h
10 0 0
50
100
150
200
250
300
Time-On-Stream / min 50
SAPO-11-48h SAPO-11-24h SAPO-11-72h
40
Conversion, mol-%
and 6. The main products were propene, isobutane, n-butane, n-butene, isobutene, pentenes and octenes. Small fractions of hexenes and heptenes as well as traces of C1–C3 were also obtained. In general, the selectivity to isobutene over synthesized catalysts was rather low, although the conditions were not optimal to reach high yields of isobutene. This is probably due to the presence of other phases than SAPO-11. The minor crystalline phases can also contribute to the catalytic activity. For instance, AlPO4 have been reported to be active in transformations of 1-butene [12]. The phases other than SAPO-11 decrease the selectivity to isobutene of the synthesized SAPO-11 catalysts. The SAPO-11-48 h and SAPO-11-72 h catalysts exhibited higher conversion of 1-butene and lower selectivity to isobutene than SAPO-11-24 h. The yield of isobutene, however, was approximately at the same level over these three catalysts synthesized with varying synthesis times. The effect of the silicon source on the catalytic performance was more pronounced. The conversion of 1-butene and the yield of isobutene over SAPO-11-L40 were lower than those over SAPO-11-L30 were. The catalyst synthesized with waterglass as a source of silica did not exhibit any catalytic activity at all.
Selectivity, mol-%
60
30
20
10
0 0
50
100
150
200
250
300
Time-On-Stream / min
4. Discussion 4.1. Synthesis and characterization Synthesis time had a minor influence on the silicon incorporation into the framework, which is responsible for the acidity of SAPO materials. However, the acidity of SAPO materials is not directly proportional to silicon content as the aluminum content is in zeolites, but also related to the silicon substitution mechanism and distribution of Si. Mechanistically, three different isomorphic silicon substitution mechanisms have been proposed for
Fig. 5. The conversion of 1-butene, selectivity to isobutene and yield of isobutene of SAPO-11-24 h, SAPO-11-48 h and SAPO-11-72 h as a function of time-on-stream at the WHSV = 5 h−1 and temperature 673 K.
silicon substitution into the hypothetical AlPO4 framework: replacement of one aluminum by one silicon (SM1), replacement of one phosphorous by one silicon (SM2) or replacement of aluminum–phosphorous pairs by two silicon atoms (SM3) [21]. SM2 mechanism produces more Brønsted acid sites than SM3, since in the latter one the tetrahedral network remains neutral when both P and Al are simultaneously replaced by Si. The combination of SM2
232
V. Nieminen et al. / Applied Catalysis A: General 259 (2004) 227–234
Fig. 6. The conversion of 1-butene, selectivity to isobutene and yield of isobutene of SAPO-11-L30 and SAPO-11-L40 as a function of time-on-stream at the WHSV = 5 h−1 and temperature 673 K.
and SM3 mechanisms of Si incorporation has been proposed to be prevailing in the SAPO-11 synthesis [7]. The SAPO-11-48 h and SAPO-11-72 h catalysts contained more Lewis and Brønsted acid sites than SAPO-11-24 h. Due to the increase in the number of the acid sites with prolonged synthesis times, more silicon are supposed to be incorporated into the SAPO-11 framework in SAPO-11-72 h and
SAPO-11-48 h than in SAPO-11-24 h, if the mechanism remains the same during the whole synthesis. In fact, it has been observed for SAPO-5 that prolonged synthesis time increases silicon content and therefore catalysts acidity [9]. When di-n-propylamine is used as the organic template, both SAPO-11 and SAPO-31 can be crystallized [22]. Under the crystallization process of SAPO-11, with prolongation of crystallization time the alteration of crystallization type between SAPO-11 and SAPO-31 has been reported. This was supposed to result from the material exchange between the solution phase and the solid phase [23]. In addition, contradictory studies concerning phase transitions between SAPO-31 and SAPO-11 phases as a function of synthesis time have been reported [12,13]. In this study, an indication of such a phase transition to SAPO-31 was observed only in the XRD pattern of SAPO-11-48 h. Especially, no phase oscillation [12] as a function of time was observed. Synthesis time had a minor influence on the crystallinity, which was higher with shorter synthesis time of 24 h than with 48 or 72 h. This is different from the results reported before showing crystallinity at nearly 100% level already after 8 h without further decrease thereafter [12]. Silica sources used in the synthesis of SAPO-11 have a strong influence on the resulting catalyst. When waterglass was used as a source of silica, SAPO-11 phase was not obtained at all (Fig. 1). The resulting phase, tridymite-2H, syn, indicated that this silica source did not react with phosphorous acid. The reason for such a behavior of sodium silicate could be the presence of large amount of sodium species in it, which might hinder the nucleation process in SAPO-11 cystallization. Small amounts of alkali cations in the reaction mixture have been reported to prevent the nucleation of SAPO-5 [24]. An explanation was offered in terms of electrostatic interactions in the liquid phase between the alkali ions and the building units of SAPO-5: the alkali cations tend to precipitate an amorphous phase, which causes a significant decrease in crystallinity of the products. Opposite to waterglass, colloidal silica as a source of silica led to a synthesis of SAPO-11 phase. Regardless of the synthesis time, SAPO-11 was always contaminated with tridymite and AlPO4 phases. This is reasonable, since dense phases, such as AlPO4 -tridymite are known to be formed first during the synthesis of SAPO-11 [7]. Furthermore, tridymite is known to be a by-product in the synthesis of SAPO-11, if less reactive alumina source is used [7]. The use of aluminum isopropoxide has been reported to provide reduced crystallinity in case of the synthesis of SAPO-34 [25]. This was assumed to be due to some interference of the organic residue from the Al-source on the crystallization process. The colloidal silica sources instead, i.e. Ludox AS30 and Ludox AS40, are polymeric silica materials, which need to be broken into small components in order to be incorporated into the hypothetical AlPO4 framework. They differ in surface area and concentration. In the synthesis of SAPO-11, this difference did not have a major effect on the number of the acid sites, but evidently on the crystallinity. Obtained crystallinity was
V. Nieminen et al. / Applied Catalysis A: General 259 (2004) 227–234
higher in the synthesis with Ludox AS40 with higher concentration but lower surface area of the suspension. However, the combination of the used silica and alumina sources is not optimal to obtain phase pure SAPO-11. 4.2. 1-Butene isomerization In the catalyst synthesized with waterglass, no acid sites were present and obviously, no 1-butene conversion was observed either. However, the acidity is not directly related to the catalytic activity in case of SAPO-11-L30 and SAPO-11-L40: the number of Brønsted acid sites is at the same level over both catalysts, but conversion of 1-butene was substantially higher over the former one. The divergence in the catalytic behavior is probably related to differences in crystallinity and further, distribution of the impure phases as well as differences in location and distribution of the acid sites. The effect of synthesis time on the yield of isobutene was rather minor; although SAPO-11-24 h exhibited lower conversion of 1-butene, the yield of isobutene was at the same level as over SAPO-11-48 h and SAPO-11-72 h, which is due to the higher selectivity to isobutene over SAPO-11-24 h. Lower conversion of SAPO-11-24 h can be related to lower number of the Brønsted acid sites. While comparing the selectivities, the SAPO-11 catalysts showing lower 1-butene conversion exhibit clearly higher selectivity to isobutene. However, proper comparison of selectivity should be made at the same conversion level. Therefore, in Fig. 7 the selectivities are plotted as a function of conversion, presuming that the effect of coke formation is the same being independent on the conversion of 1-butene. As can be seen, the catalysts with similar proportional crystallinities exhibit similar selectivity to isobutene at the same conversion levels and further, the selectivity is higher with higher crystallinity. High selectivity to isobutene is associated with a particular pore structure [4]. Almost 100% selectivity to isobutene has been reported at conversion
Selectivity to isobutene, mol-%
90
SAPO-11-L40 SAPO-11-24h SAPO-11-48h SAPO-11-72h
80
70
60
50
233
level corresponding to thermodynamic equilibrium concentration over catalysts having AlPO4 11-framework structure [26,27]. This was explained by the selective monomolecular mechanism, which takes place in the monodimensional elliptical pores of AlPO4 11-structure hindering dimerization reactions and consequent bimolecular, less selective reaction mechanism. It is reasonable to suppose that the selectivity to isobutene over evaluated catalysts is affected by other, non-porous XRD invisible phases present in the catalysts, on which the non-selective n-butene transformation takes place. Furthermore, the lower selectivity to isobutene over SAPO-11-48 h and SAPO-11-72 h can be understood in terms of smaller amount of porous structure due to lower crystallinity. Hence, bimolecular reaction pathway is not as restricted over above-mentioned catalysts as in the case of SAPO-11-L40 and SAPO-11-24 h with higher crystallinity containing larger amount of porous solid catalyst. In addition, Lewis acidity, especially in combination with Brønsted acid sites, enhances di-, oligomerization/cracking reactions leading to higher yield of by-products [2]. This could be one contributing factor for higher conversion of 1-butene over SAPO-11-48 h and SAPO-11-72 h compared to SAPO-11-24 h and SAPO-11-L40, since the former contained slightly more Lewis acid sites.
5. Conclusions SAPO-11 catalysts synthesized with varying silica sources and synthesis times have been characterized by XRD, SEM, nitrogen adsorption and FTIR spectroscopy of pyridine adsorption as well as evaluated towards 1-butene skeletal isomerization. The choice of silica source in the synthesis of SAPO-11 catalyst has a pronounced effect on the conversion of 1-butene. Waterglass as a source of silica does not result in either acid sites of a SAPO-11 phase at all and therefore the solid material does not exhibit catalytic activity. The combination of aluminum isopropoxide and colloidal silica as sources of alumina and silica does produce a pure SAPO-11 phase but contamination with other phases, such as tridymite, was unavoidable. The use of two different kinds of colloidal silica as Si sources resulted in notable differences in crystallinity and catalytic activity. Prolonging the synthesis time increased the acid site concentration, which indicates that more silicon is incorporated into the hypothetical AlPO4 framework with increasing synthesis time. The conversion of 1-butene depended on the concentration of the acid sites and the presence of phases others than SAPO-11. Higher selectivity to isobutene was obtained over more crystalline materials.
40 20
30
40
50
Acknowledgements
Conversion, mol-%
Fig. 7. Selectivity to isobutene plotted as a function of 1-butene conversion of SAPO-11 samples.
This work is part of the activities at the Åbo Akademi Process Chemistry Centre within the Finnish Centre of
234
V. Nieminen et al. / Applied Catalysis A: General 259 (2004) 227–234
Excellence Programme (2000–2005) by the Academy of Finland. Financial support from Fortum foundation and Graduate School of Material Research is gratefully acknowledged. References [1] P. Mérideau, C. Naccache, Adv. Catal. 44 (1999) 505. [2] B. Wichterlova, N. Zilkova, E. Uvarova, J. Cejka, P. Sarv, C. Paganini, J.A. Lercher, Appl. Catal. A: Gen. 182 (1999) 297. [3] C.M. López, F.J. Machado, J. Goldwasser, B. Méndez, K. Rodr´ıguez, M.M. Ram´ırez-Agudelo, Zeolites 19 (1997) 133. [4] H.H. Mooiwer, K.P. De Jong, B. Kraushaar-Czarnetzki, W.H.J. Stork, B.C.H. Krutzen, Stud. Surf. Sci. Catal. 84 (1994) 2327. [5] J.A. Martens, P.J. Grobet, P.A. Jacobs, J. Catal. 126 (1990) 299. [6] J.M. Campelo, F. Lafont, J.M. Marinas, Appl. Catal. A: Gen. 152 (1997) 53. [7] M. Alfonzo, J. Goldwasser, C.M. Lopez, F.J. Machado, M. Matjushin, B. Méndez, M.M. Ramirez de Agudelo, J. Mol. Catal. A: Chem. 98 (1995) 35. [8] C.M. López, K. Rodr´ıguez, B. Méndez, A. Montes, F.J. Machado, Appl. Catal. A: Gen. 197 (2000) 131. [9] T. Xiao, L. An, H. Wang, Appl. Catal. A: Gen. 130 (1995) 187. [10] P. Mérideau, V.A. Tuan, V.T. Nghiem, S.Y. Lai, L.N. Hung, C. Naccache, J. Catal. 169 (1997) 55. [11] A. Bhattacharya, J. Das, S. Mitra, S.K. Roy, J. Chem. Tech. Biotechnol. 54 (1992) 399.
[12] C.M. López, F.J. Machado, B. Méndez, M. Pinto, V. Sazo, J. Goldwasser, M.M. Ram´ırez, Top. Catal. 10 (2000) 65. [13] C.M. López, F.J. Machado, J. Goldwasser, B. Méndez, K. Rodr´ıguez, Zeolites 19 (1997) 133. [14] X.S. Zhao, G.Q. Lu, A.K. Whittaker, J. Drennan, H. Xu, Microporous Mesoporous Mater. 55 (2002) 51. [15] D. Demuth, G.D. Stucky, K.K. Unger, F. Schüth, Microporous Mater. 3 (1995) 473. [16] H. Tian, L. Shin, J. Yuan, M.E. Bekheet, C. Li, React. Kinet. Catal. Lett. 64 (1998) 119. [17] B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan, E.M. Flanigen, US Patent 4440871 (1984). [18] C.A. Emeis, J. Catal. 141 (1993) 347. [19] G. Busca, Catal. Today 41 (1998) 191. [20] M.M. Treacy, J.B. Higgins, Collection of Simulated XRD Powder Patterns for Zeolites, fourth revised ed., Elsevier, Amsterdam, 2001. [21] B.M. Lock, C.A. Messina, R.L. Patton, R.T. Garjek, R.T. Cannon, E.M. Flaningen, J. Am. Chem. Soc. 106 (1984) 6092. [22] A.K. Sinha, S. Sainkar, S. Sivasanker, Microporous Mesoporous Mater. 31 (1999) 321. [23] Z.-M. Wang, Z.-F. Yan, in: Proceedings of the 22nd ACS National Meeting, Chicago, IL, USA, 26–30 August 2001 (Abstract of Papers). [24] N. Dumont, Z. Gabelica, E.G. Derouane, F. Di Rentzo, Microporous Mater. 3 (1994) 71. [25] M. Popova, Ch. Minchev, V. Kanazirev, Appl. Catal. A: Gen. 169 (1998) 227. [26] J. Cejka, B. Wichterlova, P. Sarv, Appl. Catal. A 179 (1999) 217. [27] J. Houzvicka, V. Ponec, Catal. Rev. Sci. Eng. 39 (1997) 319.