- Email: [email protected]
Medium and large pore zeolites in n-hexene skeletal isomerization
Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) 9 2004 Elsevier B.V. All fights reserved.
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M E D I U M AND L A R G E PORE Z E O L I T E S IN N-HEXENE SKELETAL ISOMERIZATION Tiitta, M., Harlin, E., Makkonen, J., Root, A., Sandelin, F. and Osterholm, H. Fortum Oil and Gas Oy, PO Box 310, FIN-06101 Porvoo, Finland. Tel: +358-1045 27568. Fax: +358-1045 27072. E-mail: [email protected]
ABSTRACT The activity of medium and large pore zeolites, Y, beta, ZSM-22 and ferrierite was compared in n-hexene skeletal isomerization. Zeolite structure determined the activity, selectivity and stability of the catalyst. Ferrierite performed best in the n-hexene isomerization, while Y-zeolites exhibited the lowest activity. Surprisingly, the amount of acid sites played a minor role in the reaction. However, the zeolites with the highest acid site strength were the most active, also within the zeolite type. The selectivity to isohexene with beta- and Y-zeolite increased with decreasing conversion. More dimerized products were formed on beta-and Y-zeolites than on ferrierite and ZSM-22. The carbon contents of used catalysts increased with increasing micropore area of the zeolite. Keywords: skeletal isomerization, n-hexene, beta, Y-zeolite, ZSM-22, ferrierite, acidity INTRODUCTION Skeletal isomerization of light olefins has been a topic of numerous publications over the last decade [1 - 5]. Most of the studies have been focused on the isomerization of butenes. Tertiary butenes are the sources in the production of methyl tertiary butyl ethers (MTBE) and isooctenes [6,7]. They are also important intermediates in the chemical industry [8]. Several different zeolite catalysts have been found to be active and selective in the skeletal isomerization of butenes [9-13]. The catalysts showing the best performance in n-butene skeletal isomerization have been AEL, FER, MTT and TON structure type zeolites [12]. For a comprehensive review of the skeletal isomerization catalysts, see the papers of de Jong et al. [12] and Houzvicka et al. [13]. Neste has developed the TAME (tertiary amyl methyl ether) process and that first unit has been in operation in Porvoo, Finland, since 1995 [14]. Tertiary C5-C7 olefins and methanol are the reactants in the TAME unit [15]. Several publications have also been published reporting the results of isomerization of pentene and hexene. The skeletal isomerization of n-hexene on alumina catalysts has been the topic of the publications in 1980s. The skeletal isomerization of n-hexene on alumina required, however, reaction temperatures above 300~ [16]. On zeolites, the skeletal isomerization of n-hexene takes place at lower temperatures [ 17-20]. The skeletal isomerization of n-hexene on ZSM-5 zeolites has been throughly studied [18-20]. These investigations have clarified the shape selective properties of ZSM-5 zeolite and reaction routes to different components in the reaction products including coke. Abbot et al [18,19] compared the performance of ZSM-5 and Y-zeolite. Firstly, a double bond shift reaction on 1-hexene occurs for both catalysts. The cis-2-hexene/trans-2-hexene ratio is nearer the equilibrium on ZSM-5 than on HY at all 1-hexene conversion levels. Secondly, the n-hexenes are skeletal isomerized. The relative rate of skeletal isomerization was significantly higher on ZSM-5 than on Y-zeolite. The product distribution was also different. The amount of cracking andpolymerization products was smaller on ZSM-5 than on Y-zeolite. With Y-zeolite there were also aromatic molecules in the product. Butt et al. [20] have compared Pd/SAPO-11, SAPO-11, ZSM-5 and mordenite in 1-hexene isomerization. They found that ZSM-5 zeolite was the most active. Mainly cracked and polymerized products were formed. Mordenite catalysed only the double bond shift reaction at temperatures below 300~ A minor amount of skeletal isomerized products could be observed at a reaction temperature of 450~ The yield to skeletal isomerized product was the highest with SAPO-11 catalyst. The addition of palladium into SAPO-11 decreased the amount of skeletal isomerized product.
2324 Wu et al. [21] have investigated the differences between SAPO-11, ZSM-11, ZSM-12, ferrierite and Y-zeolite in 1-hexene skeletal isomerization. The ferrierite yielded the highest amount of skeletal isomerized products. 69 wt-% of the product was isohexenes at a reaction temperature of 270~ at a pressure of 0.2 MPa. Mostly cracked and dimerized products were formed on ZSM-! 1. Y-zeolite catalysed mainly the double bond shift reaction. The amount of skeletal isomerized products was only 5 wt-%. They concluded that besides the acid strength and acid site density, the pore size of zeolite is important in the skeletal isomerization of 1-hexene. In our earlier work, ferrierites with different properties were compared with the commercial Cs-CT-olefin feed. It was found that the ferrierite catalysts having a small particle size, low total aluminium content but a high percentage of aluminium in framework positions show the highest activity and selectivity for the reaction and stable catalytic performance with time-on-stream [22]. Because only a few investigations of the skeletal isomerization of n-hexenes have appeared, we initiated a comparative study of the ferrierite, ZSM-22, beta- and Y-zeolite catalysts with different acidic properties. The target was to study the relationship between zeolite type, acidity and activity in the skeletal isomerization of n-hexene. EXPERIMENTAL
SECTION
Zeolites Ferrierite was purchased from Zeolyst International Corp. Four Y-zeolites and four beta zeolites with various aluminium contents were from TOSOH Corp. ZSM-22 was prepared at Abo Akademi University [3]. All the zeolites were transformed to the proton form via ammonium exchange and calcination except one Y-zeolite sample (Y 1).
Characterization of zeolites Si and AI were measured by wavelength dispersive X-ray fluorescence spectrometry (XRF, Bruker AXS $4). The instrument is sequential with Rh end-window X-ray tube. The samples were dried for 2h at 110~ and equilibrated over night in 29% relative humidity over saturated CaCI2xH20 water solution. 500 mg of sample was weighed into a platinum crucible, mixed with lithium tetraborate and 100 mg NaI to a total weight of 10 g. The powder mixture was heated at 1350~ for 8 min and the melt was poured into a mold, where it was allowed to cool under controlled conditions. The resulting glass bead is a solid solution suitable for Si and A1 determination in the XRF instrument towards a calibration line. The calibration line is made with samples of known composition and of similar matrix. Na was measured by atomic absorption spectrometry (AAS, Perkin Elmer 4100). Dissolution was made in acidic medium and was further diluted with water. The X-ray diffraction patterns were collected between 2 and 75 ~ 2 0 in reflection mode with a Siemens DS00 instrument (XRD), equipped with a Cu-anode and a curved graphite monochromator in the reflected beam. The samples were dried in the sample holder at 110~ for at least 2h. The samples were covered by a half-cylindrical Mylar window, to avoid absorption of moisture and prevent eventual formation of crystalline hydrates. Crystallinity was measured between 5 and 40~ after removal of K~2 and instrument background. The instrument background was measured under the same conditions without a sample holder. The degree of crystallinity was measured from the area under the peaks over the total area. Surface area and porosity were measured by N2-adsorption/desorption at liquid nitrogen temperature, according to ASTM D4641. The instrument used was ASAP 2400 from Micromeritics. Data were evaluated by DataMaster vs. 2 software. The surface area was calculated by the BET equation (Brunauer-Emmett and Teller), mesoporous area and volume by BJH equation (Barrett-Joyner-Halenda) from adsorption isotherm. The microprorous area and microporosity were evaluated by the t-Plot method. 300 mg of sample was weighed into sample bottles, filler rods and isothermal jackets were used. The pretreatment was carried out under vacuum at 300~ for at least 6 h, the sample was weighed and analysed. The morphology of the as-received zeolites was evaluated with a Jeol JSM 840A Scanning Electron Microscopy (SEM). The powder samples were placed on carbon tape as a thin layer. The powder layer was first coated by carbon and thereafter by gold, to obtain a good conducting surface and thus SEM pictures with good resolution. Total acidity was obtained by temperature programmed desorption (NH3-TPD). The instrument is AMI100 by Altamira, equipped with a thermal conductivity detector (TCD). The samples were dried for at least 2h at 110~ equilibrated in 29% humidity over night. 40 mg sample was mounted in the sample tube,
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pretreated in helium flow at 80~ for 20 min and ramped to 500~ 20 ~ where it was held isothermally for lh. Ammonia (10% in helium) was flushed through the sample for lh at 100~ followed by helium flush at the same temperature for lh. The TPD temperature profile was: 30 min at 100~ ramp 20~ to 500~ hold 30 min. After the TPD measurement NH3 was adsorbed at 150~ followed by TPD measurement with the above mention temperature profile. The procedures were repeated at 200, 250 and 300~ The quantitation was made by 941al pulses of 10%NH3 in helium. The TPD curves and pulse calibration curves were quantified by the PeakFit program, which allows resolution of the TPD curves into a set of Gaussian curves. The desorption temperature maxima were also recorded. The number of protons in different OH groups was measured by ~H MAS NMR. The instrument used was a Chemagnetics CMX-Infinity 400. The samples were dried at 450~ for 3 h. Peaks at 4.2 and 5.3 ppm were interpreted as acidic OH. SiOH and A1OH at 1.8 and 2.8 ppm, respectively, are not acidic. The acidic OH groups are Br6nsted acid sites. For determination of the number of Lewis acid sites, NH3 is adsorbed at various temperatures. The adsorption was made in the NH3-TPD instrument under similar conditions as in the NH3-TPD analysis, the treatment times were extended, from 1 to 3h, due to a 10-fold increase in sample amount. From the peak positions and line shapes NH3 and NH4 + could be identified. NH3 can be physisorbed or bound to very weak acid sites. Physisorbed NH3 was usually not present in samples, with adsorption temperatures >200~ For more information of the acidity determination methods, see our recent paper [23,24]. The carbon contents of the used catalysts were determined with a CHN-2000 analyser, from the LECO Corporation. 50 mg of the catalyst was weighed into a crucible and burnt in oxygen at 900~ Carbon was analysed as CO2 by an IR-detector. The measuring range was 0.5 - 100%.
Activity measurements The zeolites were tested in a microreactor system (fixed bed plug flow reactor). The feed was a mixture of n-hexene (Aldrich, purity 98%) and n-hexane (LabScan, purity 95%). The feed and products were analysed with HP-5890 gas chromatograph (GC) equipped with a capillary column DB-1 and a flame ionisation detector (FID). A few products were analysed by mass spectrometry (VG 7070E) to confirm the identification of the GC peaks. 250 mg of each catalyst was placed in the reactor. The pretreatment of the catalyst in the reactor was at 400~ for 2h in nitrogen flow. After the pretreatment, the temperature was decreased into the reaction temperature 225~ and feed was introduced into the reactor. The reaction was performed at 225~ in atmospheric pressure. The weight per hour space velocity was 20 g feed/g catalyst in hour. The duration of each test was at least 48h. Four to 14 samples were taken during the test run. The conversion, selectivity and yield in the skeletal isomerization were calculated with the following formula: Conversion (%) =
100*(n-hexene in (wt-%) - n-hexene out (wt-%) (n-hexene in (wt-%))
(1)
Selectivity (%) =
100*amount of product (wt-%) amount of all products (wt-%)
(2)
Yield ( % ) =
Conversion (%) x Selectivity (%) 100
(3)
After the test run the catalyst was purged with nitrogen at reaction temperature for one hour. The reactor was cooled to room temperature in nitrogen flow. The different activities and rates of deactivation were estimated for the tested catalysts by a simplified model containing a power law equation describing the main skeletal isomerization reaction only. The stoichiometric constants being one mole n-hexene reacting to one mole i-hexene. The deactivation was assumed to follow a hyperbolic function. The equations were selected to compare the catalysts and are not to be taken as an attempt to describe any mechanism. The rate equation is then:
2326
1 r = k 1Cn_otel 9~ l+kdt
(4)
Where kl and kd are the estimated rate- and deactivation constants. Cn-olef is the n-hexene concentration (mol/m 3) and t is the time (h). The reactor was modelled as a packed bed plug flow reactor [25]. The differential equations were solved numerically by a in house Fortran based program package. The differential equations were solved by a backward difference method for stiff systems, based on the LSODE software [26]. The objective function was the sum of squares of the difference between the estimated and measured hexene concentrations. The objective function was minimised by Simplex method followed by a Leveberg-Marquart type of algorithm close to the solution [27]. RESULTS AND DISCUSSION The structural properties of zeolites involved in this study are summarised in Table 1. ZSM-22 has the lowest surface area due to the one-dimensional pore system and medium pore size. Y-zeolites have the highest surface areas having a three-dimensional pore system and largest pores within this set of zeolites. Ferrierite is a medium pore zeolite having two-dimensional pore system with BET surface area of 330m2/g. Beta zeolites belong to the large pore zeolites having a two-dimensional pore system. BET surface areas of our beta samples varied between 5 5 0 - 585m2/g. The sample Y1 had lower surface area than other Y-zeolite because of its high sodium content (lmmol/g). The sodium contents of other zeolites were under 0.0 l mmol/g. The mesopore area of Y-zeolites increased with decreasing aluminium content and micropore areas had a maximum with aluminium content of 6.6wt-%. This phenomenon originated to the used dealumination method. Ferrierite, ZSM-22 and Y-zeolites had crystallinities between 70 - 80%. Beta zeolites had lower crystallinities due to their polymorphism nature. The polymorphism also caused the small crystallite size. The crystallite size of ferrierite was 36nm and the crystallite size of ZSM-22 was 32nm. The crystallite sizes of Y-zeolites were between 34 - 51 nm. The particle sizes of all these zeolites were small (< 2 lam) according to SEM-pictures. The particles of ferrierite were like plates and the particles of ZSM-22 were like needles. The particles of beta and Y-zeolites were spherical. Table 1. Structural properties of zeolites. Zeolite type
AI, wt-% (XRF)
BET, m2/g
Ferrierite ZSM-22 Beta 1 Beta 2 Beta 3 Beta 4 Y 1 Y2 Y3 Y4
1.4 0.9 2.3 1.0 0.8 0.2 10.7 9.9 6.6 0.9
330 210 555 585 575 550 615 660 705 65O
Micropore area t-plot, m2/g 300 180 520 555 535 510 575 630 655 555
Mesopore area BJH, mZ/g
Cryst., %
Crystallite size, nm
43 30 85 85 95 95 55 50 80 120
76 70 54 52 51 49 73 74 70 70
36 32 24 25 22 23 34 51 44 44
The acidic properties of the zeolites are summarized in Table 2. All the studied zeolites have Br0nsted acid sites. The ferrierite had more acid sites than ZSM-22. The strength of acid sites for ferrierite and ZSM-22 were similar. The amount of Br6nsted acid sites did not correlate with the aluminium contents. There were a large amount of aluminium in extraframework position in some Y- and beta zeolites [24]. The amount of Br6nsted acid sites of beta zeolites was between 100-380~tmol/g, and the total acidity between 100-4801amol/g. In sample Betal there was a large amount of Lewis acid sites. The amount of Br6nsted acid sites in Y-zeolites was between 10-1200~tmol/g. The amount of Lewis acid sites was also high in the samples YI and Y2. This was due to the large amount of non-framework A1 in these samples (from 27A1MAS NMR, not shown). The acid sites strength of beta and Y- zeolites was less than that of ferrierite and ZSM-22. The
2327
sample Y I had the lowest acid strength. The sodium content of this sample was high, and the result indicated that sodium located in the strongest acid sites. Table 2. Acidic properties of zeolites. Br6nsted acid sites, pmol/g * Ferrierite 480 ZSM-22 200 Beta 1 380 Beta 2 310 Beta 3 150 Beta 4 100 Y 1 230 Y2 1200 Y3 680 Y4 10 * measured with ~H MAS NMR n.m. Zeolite type
Lewis ~tmol/g*
sites,
n,m,
10 100 5 0 0 230 310 n.m. n.m. = not measured
NH3-TPD, lamol/g 510 360 560 345 245 95 430 1240 985 111
NH3-TPD, max Td, ~ 450 450 375 385 370 360 330 360 380 360
Fig. 1 shows the performance of different zeolites in the skeletal isomerization of n-hexene. ZSM-22 and ferrierite had the highest activity. Beta zeolites were more active than Y-zeolites. The yield at the same conversion level was higher when ferrierite was used instead of ZSM-22. The yield at same conversion level was higher for ZSM-22 than for beta- and Y-zeolites. In skeletal isomerization of n-hexene, as in the case of n-butene, the structural type of the zeolite seems also to be the predominant property [14].
I-]
!
,t
4O e-
A Beta zeolites
x ,,c O 30
A Y zeolites [] ZSM-22 zeolites
O
9 Ferrierite "~ 20 >.
i 0
10
20
30
40
50
60
Conversion, %
Figure 1. Yield of isohexene versus conversion on n-hexene. Table 3 summarises the results of the skeletal isomerization tests and estimations of rate constants for skeletal isomerization and deactivation. Because the rate equation was very simplified and the purpose was to compare different catalysts the rate constant values were normalized. The deactivation constants of ZSM-22 are probably too high because of correlation of the parameters for that catalyst. The rate constant of n-hexene skeletal isomerization was highest for ZSM-22 and then for ferrierite. The differences in the rate constants between beta zeolites were minor irrespectively their large differences in the acidities. Y-zeolites had the lowest rate constants. The catalysts having the strongest acid sites had the highest rate within the series of beta zeolites (Fig. 2). The rate constants slightly increased with increasing amount of Br~nsted acid sites and total acid sites within the zeolite type.
2328 1,0E+03
1,0E+02
9 Ferrierite [] ZSM-22 [ 9A ~eta
[
1,OE+O1
A 1,0E+00 300
,
,
~
.
320
340
360
380
.
.
.
400
420
440
460
NH3-TPD max Td, *C
Figure 2. Rate constant versus the maximum temperature of NH3-TPD measurement. Table 3. Performance of different zeolites in n-hexene skeletal isomerization. Test
Zeolite
k~, norm.
kd, 1/s
1 2 3 4 5 6 7 8 9 10 11 12
Ferrierite Ferrierite ZSM-22 ZSM-22 Betal Beta2 Beta3 Beta4 YI Y2 Y3 Y4
27 26 150 110 6.9 8.4 6.6 6.2 2.0 1.3 2.3 1.0
0.017 0.018 1.2 1.1 0.10 0.08 0.12 0.11 3.0 0.11 0.28 0.12
Yield after 2 hour, % 49 48 52 46 10 11 9 11 1 3 3 2
Yield after 48 hour, % 30 27 3 2 2 4 2 3 0 1 1 1
Carbon content, wt-% 5.6 5.5 nm 3.9 13.2 13.9 11.9 10.1 15.1 16.3 17.3 7.7
All the zeolite catalysts deactivated during the test run. The lowest deactivation rate constant was with ferrierite and the highest deactivation rate constant was with ZSM-22. The low deactivation rate of ferrierite compared to ZSM-22 has different reasons based on their structural differences. The beta zeolites deactivated with similar rate though they had large differences in the amount of acid sites. Y-zeolite with the highest sodium content and the lowest strength of acid sites deactivated fastest. There was no correlation between the carbon content of used catalyst and the deactivation rate. The carbon contents of used catalysts after the test runs varied somewhat. ZSM-22 and ferrierite had the lowest carbon contents. The carbon contents of beta zeolites were between 10-14wt-%, and Y-zeolites between 8-17 wt-%. The carbon contents of used catalysts increased with increasing micropore areas as seen in Fig. 3. The carbon contents did not correlate with the activity of the catalyst. This gives an indication that the low activity of beta and Y-zeolites could be due to the low rate of desorption of the products followed by the fast coke formation. The catalyst Y4 had very low BrOnsted acidity and because of that could differ from the other catalysts.
2329
18
16
m m
m .~
12
m
~ 10 o e-
m
8
Catalyst Y4
C
o
6
c
o
mm
4
L
r
2
200
300
400
500
600
700
M i c r o p o r e area, mZlg
Figure 3. Carbon contents of used catalysts versus micropore area. The selectivities of all the catalysts were between 70-100%. There were, however, differences in the product distributions between the catalysts. More C8+-products (dimers and trimers) were formed on betaand Y-zeolites than on ferrierite and ZSM-22 zeolites. The large pore size and high amount of acid sites of beta- and Y-zeolite could explain the result. The product after two hour test (first sample) had always the most C8+-products compared with the other samples in the same test (Fig. 4).
A 1. sample 3,5
~
3
9 Heavies (C8+) (Beta zeolites)
c
~ 2,5
i
U
§
=
13Heavies (C8+) (Ferrierite)
2
1.sample
~" ~ "-
A Heavies (C8+) (Y zeolites)
! 9 Heavies (C8+) (ZSM-22 zeolites)
/k
"I
1
--=
A
1.sample
0,5
0
10
20
30
40
50
60
70
Conversion, %
Figure 4. Increase of the amount of C8+- components in the product versus conversion of n-hexene. CONCLUSIONS Most important for the high activity and selectivity in n-hexene isomerization is the structure type of the zeolite. Medium pore zeolites were more active and selective than large pore zeolites. No general relationship between the amount of total acid sites or Br0nsted acid sites to activity or selectivity in n-hexene skeletal isomerization was observed but the strength of acidity was found to be important. The catalysts with the highest acid strength had the highest activity and selectivity. Ferrierite had the highest activity and selectivity to isohexene. ZSM-22 zeolite had the highest initial activity but it deactivated fast under the conditions used. Beta zeolites were more active than Y-zeolites. The selectivity towards isoolefins with beta- and Y-zeolites increased with decreasing conversion.
2330 The carbon contents of used catalysts depended on the micropore area of the zeolites, and they were independent on the activity of the catalyst. ACKNOWLEDGEMENTS We wish to thank Dr. N. Kumar (Abo Akademi University) for preparation of ZSM-22 zeolite. R. Heikkil~i, H. Leuku, J. Tuovila, and R. Vuorenmaa (Fortum Oil and Gas) are thanked for carrying out different experiments and measurements. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.
Houzvicka, J., Hansildaar, S., Ponec., J. Catal. 167 (1997) 273-278. Guisnet, N., Andy, P., Gnep. N.S., Benazzi, E., Travers, C., J. Catal. 158 (1996) 551-560. Byggningsbacka, R., Kumar, N., Lindfors, L.-E., J. Catal. 178 (1998) 611-616. Se, G., Jeong, H. S., Hong, S. B., Uh, Y. S., Catal. Lett. 36 (1996) 249-253. Simon, M., Suib, S.L., O'Young, C.-L., J. Catal. 147 (1994) 484-493. de Young, K.P., Bosch, W., Morgan, T.D.B., Stud. Surf. Sci. Catal. 94 (1995) 15-32. Hunszinger, P., J~irvelin, H., Purola, V.-M., Nurminen, M., Birkhoff, R., Proceed. NPRA Annual Meeting, 2003. Dooley, K.M., Williams, J.A., Gates, B.C., Albright, R.L., J. Catal. 74 (1982) 361-370. Kumar, N., Nieminen, V., Lindfors, L.-E., Salmi, T., Murzin, D. Yu., Laine, E., Heikkil~i, T., Cat. Lett. 78 (2002) 105-110. Tian, H., Li., C., Stud. Surf. Sci. Catal. 105 (1997) 1397-1404. Moiweer, H.H., de Jong, K.P., Kraushaar-Czarnetski, B., Stork, W.H.J., Krutzen, B.C.H., Stud. Surf. Sci. Catal. 84 (1994) 2327-2344. van Donk, S., Bitter, J.,de Jong, K., Appl. Catal. A: General 212 (2001) 97-104. Houzvicka, J., Ponec, V., Catal. Rev.-Sci. Eng. 39 (1997) 319-344. Jakkula, J., J~irvelin, H., Kivi, J., World Petr. Conf., Book of Abstract, Stanvanger, 1994. Ignatius, J., J~irvelin, H., Lindqvist, P., Hydrocarbon Proc. 1995, 51-53. Baumgarten, E., Hollenberg, D., Htiffkers, H., J. Catal. 80 (1983) 419-426. Feng, X., Lee, J.S., Lee, J.W., Lee. J.Y., Wei, D., Hailer, G.L. Chem. Eng. J. 64 (1996) 255-161. Abbot, J., Wojiechowski, B. W., Can. J. Chem. Eng. 63 (1985) 451 - 461. Abbot, J., Corma, A., Wojciechowski, B. W., J. Catal. 92 (1985) 398-408. Choung, S.-J., Butt, J. B., Korean J. Chem. Eng. 7 (1993) 175 - 181. Wu, Z., Wang, Q., Xu, L., Xie, S., Stud. Surf. Sci. Catal. 142 (2002) 747-754. Nicolaides, C.P., Makkonen, J., Tiitta, M., Stud. Surf. Sci. Catal. 135 (2001)4256-4260. Root, A., Tiitta, M., Osterholm, H., Book of Abstract, EuropaCat, Rimini, Italia, 1999. Root, A., Tiitta, M., t)sterholm, H., Microporous Mesoporous Mat., 2003, submitted. H. Scott Fogler, Elements of Chemical Reaction Engineering, 3d Ed, Prentice-Hall, New Jersey, (1999) 14-15. A.C. Hindmarsh, A Systematized Collection of ODE-Solvers in Scientific Computing, R. Steple et al. Eds, IMACS North-Holland, Amsterdam (1983), 55-64. D.W. Marquart, SIAM J., (1963), 431-441.
2323
M E D I U M AND L A R G E PORE Z E O L I T E S IN N-HEXENE SKELETAL ISOMERIZATION Tiitta, M., Harlin, E., Makkonen, J., Root, A., Sandelin, F. and Osterholm, H. Fortum Oil and Gas Oy, PO Box 310, FIN-06101 Porvoo, Finland. Tel: +358-1045 27568. Fax: +358-1045 27072. E-mail: [email protected]
ABSTRACT The activity of medium and large pore zeolites, Y, beta, ZSM-22 and ferrierite was compared in n-hexene skeletal isomerization. Zeolite structure determined the activity, selectivity and stability of the catalyst. Ferrierite performed best in the n-hexene isomerization, while Y-zeolites exhibited the lowest activity. Surprisingly, the amount of acid sites played a minor role in the reaction. However, the zeolites with the highest acid site strength were the most active, also within the zeolite type. The selectivity to isohexene with beta- and Y-zeolite increased with decreasing conversion. More dimerized products were formed on beta-and Y-zeolites than on ferrierite and ZSM-22. The carbon contents of used catalysts increased with increasing micropore area of the zeolite. Keywords: skeletal isomerization, n-hexene, beta, Y-zeolite, ZSM-22, ferrierite, acidity INTRODUCTION Skeletal isomerization of light olefins has been a topic of numerous publications over the last decade [1 - 5]. Most of the studies have been focused on the isomerization of butenes. Tertiary butenes are the sources in the production of methyl tertiary butyl ethers (MTBE) and isooctenes [6,7]. They are also important intermediates in the chemical industry [8]. Several different zeolite catalysts have been found to be active and selective in the skeletal isomerization of butenes [9-13]. The catalysts showing the best performance in n-butene skeletal isomerization have been AEL, FER, MTT and TON structure type zeolites [12]. For a comprehensive review of the skeletal isomerization catalysts, see the papers of de Jong et al. [12] and Houzvicka et al. [13]. Neste has developed the TAME (tertiary amyl methyl ether) process and that first unit has been in operation in Porvoo, Finland, since 1995 [14]. Tertiary C5-C7 olefins and methanol are the reactants in the TAME unit [15]. Several publications have also been published reporting the results of isomerization of pentene and hexene. The skeletal isomerization of n-hexene on alumina catalysts has been the topic of the publications in 1980s. The skeletal isomerization of n-hexene on alumina required, however, reaction temperatures above 300~ [16]. On zeolites, the skeletal isomerization of n-hexene takes place at lower temperatures [ 17-20]. The skeletal isomerization of n-hexene on ZSM-5 zeolites has been throughly studied [18-20]. These investigations have clarified the shape selective properties of ZSM-5 zeolite and reaction routes to different components in the reaction products including coke. Abbot et al [18,19] compared the performance of ZSM-5 and Y-zeolite. Firstly, a double bond shift reaction on 1-hexene occurs for both catalysts. The cis-2-hexene/trans-2-hexene ratio is nearer the equilibrium on ZSM-5 than on HY at all 1-hexene conversion levels. Secondly, the n-hexenes are skeletal isomerized. The relative rate of skeletal isomerization was significantly higher on ZSM-5 than on Y-zeolite. The product distribution was also different. The amount of cracking andpolymerization products was smaller on ZSM-5 than on Y-zeolite. With Y-zeolite there were also aromatic molecules in the product. Butt et al. [20] have compared Pd/SAPO-11, SAPO-11, ZSM-5 and mordenite in 1-hexene isomerization. They found that ZSM-5 zeolite was the most active. Mainly cracked and polymerized products were formed. Mordenite catalysed only the double bond shift reaction at temperatures below 300~ A minor amount of skeletal isomerized products could be observed at a reaction temperature of 450~ The yield to skeletal isomerized product was the highest with SAPO-11 catalyst. The addition of palladium into SAPO-11 decreased the amount of skeletal isomerized product.
2324 Wu et al. [21] have investigated the differences between SAPO-11, ZSM-11, ZSM-12, ferrierite and Y-zeolite in 1-hexene skeletal isomerization. The ferrierite yielded the highest amount of skeletal isomerized products. 69 wt-% of the product was isohexenes at a reaction temperature of 270~ at a pressure of 0.2 MPa. Mostly cracked and dimerized products were formed on ZSM-! 1. Y-zeolite catalysed mainly the double bond shift reaction. The amount of skeletal isomerized products was only 5 wt-%. They concluded that besides the acid strength and acid site density, the pore size of zeolite is important in the skeletal isomerization of 1-hexene. In our earlier work, ferrierites with different properties were compared with the commercial Cs-CT-olefin feed. It was found that the ferrierite catalysts having a small particle size, low total aluminium content but a high percentage of aluminium in framework positions show the highest activity and selectivity for the reaction and stable catalytic performance with time-on-stream [22]. Because only a few investigations of the skeletal isomerization of n-hexenes have appeared, we initiated a comparative study of the ferrierite, ZSM-22, beta- and Y-zeolite catalysts with different acidic properties. The target was to study the relationship between zeolite type, acidity and activity in the skeletal isomerization of n-hexene. EXPERIMENTAL
SECTION
Zeolites Ferrierite was purchased from Zeolyst International Corp. Four Y-zeolites and four beta zeolites with various aluminium contents were from TOSOH Corp. ZSM-22 was prepared at Abo Akademi University [3]. All the zeolites were transformed to the proton form via ammonium exchange and calcination except one Y-zeolite sample (Y 1).
Characterization of zeolites Si and AI were measured by wavelength dispersive X-ray fluorescence spectrometry (XRF, Bruker AXS $4). The instrument is sequential with Rh end-window X-ray tube. The samples were dried for 2h at 110~ and equilibrated over night in 29% relative humidity over saturated CaCI2xH20 water solution. 500 mg of sample was weighed into a platinum crucible, mixed with lithium tetraborate and 100 mg NaI to a total weight of 10 g. The powder mixture was heated at 1350~ for 8 min and the melt was poured into a mold, where it was allowed to cool under controlled conditions. The resulting glass bead is a solid solution suitable for Si and A1 determination in the XRF instrument towards a calibration line. The calibration line is made with samples of known composition and of similar matrix. Na was measured by atomic absorption spectrometry (AAS, Perkin Elmer 4100). Dissolution was made in acidic medium and was further diluted with water. The X-ray diffraction patterns were collected between 2 and 75 ~ 2 0 in reflection mode with a Siemens DS00 instrument (XRD), equipped with a Cu-anode and a curved graphite monochromator in the reflected beam. The samples were dried in the sample holder at 110~ for at least 2h. The samples were covered by a half-cylindrical Mylar window, to avoid absorption of moisture and prevent eventual formation of crystalline hydrates. Crystallinity was measured between 5 and 40~ after removal of K~2 and instrument background. The instrument background was measured under the same conditions without a sample holder. The degree of crystallinity was measured from the area under the peaks over the total area. Surface area and porosity were measured by N2-adsorption/desorption at liquid nitrogen temperature, according to ASTM D4641. The instrument used was ASAP 2400 from Micromeritics. Data were evaluated by DataMaster vs. 2 software. The surface area was calculated by the BET equation (Brunauer-Emmett and Teller), mesoporous area and volume by BJH equation (Barrett-Joyner-Halenda) from adsorption isotherm. The microprorous area and microporosity were evaluated by the t-Plot method. 300 mg of sample was weighed into sample bottles, filler rods and isothermal jackets were used. The pretreatment was carried out under vacuum at 300~ for at least 6 h, the sample was weighed and analysed. The morphology of the as-received zeolites was evaluated with a Jeol JSM 840A Scanning Electron Microscopy (SEM). The powder samples were placed on carbon tape as a thin layer. The powder layer was first coated by carbon and thereafter by gold, to obtain a good conducting surface and thus SEM pictures with good resolution. Total acidity was obtained by temperature programmed desorption (NH3-TPD). The instrument is AMI100 by Altamira, equipped with a thermal conductivity detector (TCD). The samples were dried for at least 2h at 110~ equilibrated in 29% humidity over night. 40 mg sample was mounted in the sample tube,
2325
pretreated in helium flow at 80~ for 20 min and ramped to 500~ 20 ~ where it was held isothermally for lh. Ammonia (10% in helium) was flushed through the sample for lh at 100~ followed by helium flush at the same temperature for lh. The TPD temperature profile was: 30 min at 100~ ramp 20~ to 500~ hold 30 min. After the TPD measurement NH3 was adsorbed at 150~ followed by TPD measurement with the above mention temperature profile. The procedures were repeated at 200, 250 and 300~ The quantitation was made by 941al pulses of 10%NH3 in helium. The TPD curves and pulse calibration curves were quantified by the PeakFit program, which allows resolution of the TPD curves into a set of Gaussian curves. The desorption temperature maxima were also recorded. The number of protons in different OH groups was measured by ~H MAS NMR. The instrument used was a Chemagnetics CMX-Infinity 400. The samples were dried at 450~ for 3 h. Peaks at 4.2 and 5.3 ppm were interpreted as acidic OH. SiOH and A1OH at 1.8 and 2.8 ppm, respectively, are not acidic. The acidic OH groups are Br6nsted acid sites. For determination of the number of Lewis acid sites, NH3 is adsorbed at various temperatures. The adsorption was made in the NH3-TPD instrument under similar conditions as in the NH3-TPD analysis, the treatment times were extended, from 1 to 3h, due to a 10-fold increase in sample amount. From the peak positions and line shapes NH3 and NH4 + could be identified. NH3 can be physisorbed or bound to very weak acid sites. Physisorbed NH3 was usually not present in samples, with adsorption temperatures >200~ For more information of the acidity determination methods, see our recent paper [23,24]. The carbon contents of the used catalysts were determined with a CHN-2000 analyser, from the LECO Corporation. 50 mg of the catalyst was weighed into a crucible and burnt in oxygen at 900~ Carbon was analysed as CO2 by an IR-detector. The measuring range was 0.5 - 100%.
Activity measurements The zeolites were tested in a microreactor system (fixed bed plug flow reactor). The feed was a mixture of n-hexene (Aldrich, purity 98%) and n-hexane (LabScan, purity 95%). The feed and products were analysed with HP-5890 gas chromatograph (GC) equipped with a capillary column DB-1 and a flame ionisation detector (FID). A few products were analysed by mass spectrometry (VG 7070E) to confirm the identification of the GC peaks. 250 mg of each catalyst was placed in the reactor. The pretreatment of the catalyst in the reactor was at 400~ for 2h in nitrogen flow. After the pretreatment, the temperature was decreased into the reaction temperature 225~ and feed was introduced into the reactor. The reaction was performed at 225~ in atmospheric pressure. The weight per hour space velocity was 20 g feed/g catalyst in hour. The duration of each test was at least 48h. Four to 14 samples were taken during the test run. The conversion, selectivity and yield in the skeletal isomerization were calculated with the following formula: Conversion (%) =
100*(n-hexene in (wt-%) - n-hexene out (wt-%) (n-hexene in (wt-%))
(1)
Selectivity (%) =
100*amount of product (wt-%) amount of all products (wt-%)
(2)
Yield ( % ) =
Conversion (%) x Selectivity (%) 100
(3)
After the test run the catalyst was purged with nitrogen at reaction temperature for one hour. The reactor was cooled to room temperature in nitrogen flow. The different activities and rates of deactivation were estimated for the tested catalysts by a simplified model containing a power law equation describing the main skeletal isomerization reaction only. The stoichiometric constants being one mole n-hexene reacting to one mole i-hexene. The deactivation was assumed to follow a hyperbolic function. The equations were selected to compare the catalysts and are not to be taken as an attempt to describe any mechanism. The rate equation is then:
2326
1 r = k 1Cn_otel 9~ l+kdt
(4)
Where kl and kd are the estimated rate- and deactivation constants. Cn-olef is the n-hexene concentration (mol/m 3) and t is the time (h). The reactor was modelled as a packed bed plug flow reactor [25]. The differential equations were solved numerically by a in house Fortran based program package. The differential equations were solved by a backward difference method for stiff systems, based on the LSODE software [26]. The objective function was the sum of squares of the difference between the estimated and measured hexene concentrations. The objective function was minimised by Simplex method followed by a Leveberg-Marquart type of algorithm close to the solution [27]. RESULTS AND DISCUSSION The structural properties of zeolites involved in this study are summarised in Table 1. ZSM-22 has the lowest surface area due to the one-dimensional pore system and medium pore size. Y-zeolites have the highest surface areas having a three-dimensional pore system and largest pores within this set of zeolites. Ferrierite is a medium pore zeolite having two-dimensional pore system with BET surface area of 330m2/g. Beta zeolites belong to the large pore zeolites having a two-dimensional pore system. BET surface areas of our beta samples varied between 5 5 0 - 585m2/g. The sample Y1 had lower surface area than other Y-zeolite because of its high sodium content (lmmol/g). The sodium contents of other zeolites were under 0.0 l mmol/g. The mesopore area of Y-zeolites increased with decreasing aluminium content and micropore areas had a maximum with aluminium content of 6.6wt-%. This phenomenon originated to the used dealumination method. Ferrierite, ZSM-22 and Y-zeolites had crystallinities between 70 - 80%. Beta zeolites had lower crystallinities due to their polymorphism nature. The polymorphism also caused the small crystallite size. The crystallite size of ferrierite was 36nm and the crystallite size of ZSM-22 was 32nm. The crystallite sizes of Y-zeolites were between 34 - 51 nm. The particle sizes of all these zeolites were small (< 2 lam) according to SEM-pictures. The particles of ferrierite were like plates and the particles of ZSM-22 were like needles. The particles of beta and Y-zeolites were spherical. Table 1. Structural properties of zeolites. Zeolite type
AI, wt-% (XRF)
BET, m2/g
Ferrierite ZSM-22 Beta 1 Beta 2 Beta 3 Beta 4 Y 1 Y2 Y3 Y4
1.4 0.9 2.3 1.0 0.8 0.2 10.7 9.9 6.6 0.9
330 210 555 585 575 550 615 660 705 65O
Micropore area t-plot, m2/g 300 180 520 555 535 510 575 630 655 555
Mesopore area BJH, mZ/g
Cryst., %
Crystallite size, nm
43 30 85 85 95 95 55 50 80 120
76 70 54 52 51 49 73 74 70 70
36 32 24 25 22 23 34 51 44 44
The acidic properties of the zeolites are summarized in Table 2. All the studied zeolites have Br0nsted acid sites. The ferrierite had more acid sites than ZSM-22. The strength of acid sites for ferrierite and ZSM-22 were similar. The amount of Br6nsted acid sites did not correlate with the aluminium contents. There were a large amount of aluminium in extraframework position in some Y- and beta zeolites [24]. The amount of Br6nsted acid sites of beta zeolites was between 100-380~tmol/g, and the total acidity between 100-4801amol/g. In sample Betal there was a large amount of Lewis acid sites. The amount of Br6nsted acid sites in Y-zeolites was between 10-1200~tmol/g. The amount of Lewis acid sites was also high in the samples YI and Y2. This was due to the large amount of non-framework A1 in these samples (from 27A1MAS NMR, not shown). The acid sites strength of beta and Y- zeolites was less than that of ferrierite and ZSM-22. The
2327
sample Y I had the lowest acid strength. The sodium content of this sample was high, and the result indicated that sodium located in the strongest acid sites. Table 2. Acidic properties of zeolites. Br6nsted acid sites, pmol/g * Ferrierite 480 ZSM-22 200 Beta 1 380 Beta 2 310 Beta 3 150 Beta 4 100 Y 1 230 Y2 1200 Y3 680 Y4 10 * measured with ~H MAS NMR n.m. Zeolite type
Lewis ~tmol/g*
sites,
n,m,
10 100 5 0 0 230 310 n.m. n.m. = not measured
NH3-TPD, lamol/g 510 360 560 345 245 95 430 1240 985 111
NH3-TPD, max Td, ~ 450 450 375 385 370 360 330 360 380 360
Fig. 1 shows the performance of different zeolites in the skeletal isomerization of n-hexene. ZSM-22 and ferrierite had the highest activity. Beta zeolites were more active than Y-zeolites. The yield at the same conversion level was higher when ferrierite was used instead of ZSM-22. The yield at same conversion level was higher for ZSM-22 than for beta- and Y-zeolites. In skeletal isomerization of n-hexene, as in the case of n-butene, the structural type of the zeolite seems also to be the predominant property [14].
I-]
!
,t
4O e-
A Beta zeolites
x ,,c O 30
A Y zeolites [] ZSM-22 zeolites
O
9 Ferrierite "~ 20 >.
i 0
10
20
30
40
50
60
Conversion, %
Figure 1. Yield of isohexene versus conversion on n-hexene. Table 3 summarises the results of the skeletal isomerization tests and estimations of rate constants for skeletal isomerization and deactivation. Because the rate equation was very simplified and the purpose was to compare different catalysts the rate constant values were normalized. The deactivation constants of ZSM-22 are probably too high because of correlation of the parameters for that catalyst. The rate constant of n-hexene skeletal isomerization was highest for ZSM-22 and then for ferrierite. The differences in the rate constants between beta zeolites were minor irrespectively their large differences in the acidities. Y-zeolites had the lowest rate constants. The catalysts having the strongest acid sites had the highest rate within the series of beta zeolites (Fig. 2). The rate constants slightly increased with increasing amount of Br~nsted acid sites and total acid sites within the zeolite type.
2328 1,0E+03
1,0E+02
9 Ferrierite [] ZSM-22 [ 9A ~eta
[
1,OE+O1
A 1,0E+00 300
,
,
~
.
320
340
360
380
.
.
.
400
420
440
460
NH3-TPD max Td, *C
Figure 2. Rate constant versus the maximum temperature of NH3-TPD measurement. Table 3. Performance of different zeolites in n-hexene skeletal isomerization. Test
Zeolite
k~, norm.
kd, 1/s
1 2 3 4 5 6 7 8 9 10 11 12
Ferrierite Ferrierite ZSM-22 ZSM-22 Betal Beta2 Beta3 Beta4 YI Y2 Y3 Y4
27 26 150 110 6.9 8.4 6.6 6.2 2.0 1.3 2.3 1.0
0.017 0.018 1.2 1.1 0.10 0.08 0.12 0.11 3.0 0.11 0.28 0.12
Yield after 2 hour, % 49 48 52 46 10 11 9 11 1 3 3 2
Yield after 48 hour, % 30 27 3 2 2 4 2 3 0 1 1 1
Carbon content, wt-% 5.6 5.5 nm 3.9 13.2 13.9 11.9 10.1 15.1 16.3 17.3 7.7
All the zeolite catalysts deactivated during the test run. The lowest deactivation rate constant was with ferrierite and the highest deactivation rate constant was with ZSM-22. The low deactivation rate of ferrierite compared to ZSM-22 has different reasons based on their structural differences. The beta zeolites deactivated with similar rate though they had large differences in the amount of acid sites. Y-zeolite with the highest sodium content and the lowest strength of acid sites deactivated fastest. There was no correlation between the carbon content of used catalyst and the deactivation rate. The carbon contents of used catalysts after the test runs varied somewhat. ZSM-22 and ferrierite had the lowest carbon contents. The carbon contents of beta zeolites were between 10-14wt-%, and Y-zeolites between 8-17 wt-%. The carbon contents of used catalysts increased with increasing micropore areas as seen in Fig. 3. The carbon contents did not correlate with the activity of the catalyst. This gives an indication that the low activity of beta and Y-zeolites could be due to the low rate of desorption of the products followed by the fast coke formation. The catalyst Y4 had very low BrOnsted acidity and because of that could differ from the other catalysts.
2329
18
16
m m
m .~
12
m
~ 10 o e-
m
8
Catalyst Y4
C
o
6
c
o
mm
4
L
r
2
200
300
400
500
600
700
M i c r o p o r e area, mZlg
Figure 3. Carbon contents of used catalysts versus micropore area. The selectivities of all the catalysts were between 70-100%. There were, however, differences in the product distributions between the catalysts. More C8+-products (dimers and trimers) were formed on betaand Y-zeolites than on ferrierite and ZSM-22 zeolites. The large pore size and high amount of acid sites of beta- and Y-zeolite could explain the result. The product after two hour test (first sample) had always the most C8+-products compared with the other samples in the same test (Fig. 4).
A 1. sample 3,5
~
3
9 Heavies (C8+) (Beta zeolites)
c
~ 2,5
i
U
§
=
13Heavies (C8+) (Ferrierite)
2
1.sample
~" ~ "-
A Heavies (C8+) (Y zeolites)
! 9 Heavies (C8+) (ZSM-22 zeolites)
/k
"I
1
--=
A
1.sample
0,5
0
10
20
30
40
50
60
70
Conversion, %
Figure 4. Increase of the amount of C8+- components in the product versus conversion of n-hexene. CONCLUSIONS Most important for the high activity and selectivity in n-hexene isomerization is the structure type of the zeolite. Medium pore zeolites were more active and selective than large pore zeolites. No general relationship between the amount of total acid sites or Br0nsted acid sites to activity or selectivity in n-hexene skeletal isomerization was observed but the strength of acidity was found to be important. The catalysts with the highest acid strength had the highest activity and selectivity. Ferrierite had the highest activity and selectivity to isohexene. ZSM-22 zeolite had the highest initial activity but it deactivated fast under the conditions used. Beta zeolites were more active than Y-zeolites. The selectivity towards isoolefins with beta- and Y-zeolites increased with decreasing conversion.
2330 The carbon contents of used catalysts depended on the micropore area of the zeolites, and they were independent on the activity of the catalyst. ACKNOWLEDGEMENTS We wish to thank Dr. N. Kumar (Abo Akademi University) for preparation of ZSM-22 zeolite. R. Heikkil~i, H. Leuku, J. Tuovila, and R. Vuorenmaa (Fortum Oil and Gas) are thanked for carrying out different experiments and measurements. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.
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