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Ind. Eng. Chem. Res. 1998, 37, 2378-2382
Development of Borosilicate Multifunctional Zeolite Catalyst for Skeletal Isomerization of Normal Butenes Armond Boghoz† and Mohammad Soltanieh*,† Chemical Engineering Department, Sharif University of Technology, P.O. Box 11365/8639, Tehran, Iran
Rostam Mondegarian‡ and Mohsen Karbalaee‡ Research Institute of Petroleum Industry, NIOC, P.O. Box 18745/4391, Tehran, Iran
In this work borosilicate zeolite catalysts for skeletal isomerization of normal alkenes, in particular n-butenes, have been synthesized and characterized. The HZSM-5 catalyst type was confirmed through X-ray diffraction and scanning electron microscopy tests. A BET surface area of approximately 350 m2/g, pore size of at least 5 Å, B/Si ratio of less than 1 wt %, and surface acidity of approimately 350 µmol of NH3/g were determined. The performance of this stable, active, and multifunctional catalyst was measured for skeletal isomerization of n-butenes to isobutene in an experimental reactor. It was observed that the selectivity of the isobutene reaction increases with a decrease in the acidity of zeolite, increase in temperature up to 600 °C, and a decrease in contact time. On the other hand, catalyst activity decreases at temperatures above 550 °C and with an increase in contact time due to the formation of side products and carbonaceous residues on the catalyst surface. Catalyst performance for skeletal isomerization of n-butenes was found to be satisfactory, namely an average yield of around 25 mol %, conversion of more than 50 mol %, and selectivity to isobutene of 45 mol % were observed. In addition, in this research a catalyst for skeletal isomerization of normal butenes was modified for sketetal dehydroisomerization of normal butane. This concept will be quantified in future publications. Industrial importance of this reaction is in the production of methyl tert-butyl ether from isobutene and methanol. Introduction In the production of MTBE (methyl tert-butyl ether) which is an oxygenate for production of unleaded gasoline, isobutene is used as a main reactant along with methanol (Seddon, 1992). The present sources of butenes including isobutene are mainly from byproducts of thermal and catalytic crackers (Seddon, 1992; O’Young et al., 1993). Other possible sources are production by isomerization of n-butenes taken from thermal or catalytic crackers (Figure 1) and dehydrogenation of isobutane taken from field butanes or produced by isomerization of n-butane (Seddon, 1992; O’Young et al., 1993; Sikkenga et al., 1984). However, these sources are limited and do not supply the existing demand. Therefore, there is a continuing interest in the development of new skeletal isomerization catalyst and processes to give optimum results of various industrial requirements (O’Young et al., 1993; Sikkenga et al., 1984, Sikkenga, 1985a,b; Petrochemical Processor, 1995). Processes for skeletal isomerization of n-olefins (e.g., to produce isobutene) are relatively nonselective, inefficient, and short-lived because of the unsaturated nature of these compounds (O’Young et al., 1993). The isobutene yield on the best contemporary catalyst is only about 40% as a result of thermodynamical limitation at high reaction temperature (Houzvicka et al., 1997a,b). Zeolite materials, especially in their hydrogen forms are known to behave quite effectively in catalyzing olefins * To whom correspondence is addressed. † Tel.: 600 5819. Fax: 601 2983. ‡ Tel.: 591021-41. Fax: 615 3397.
Figure 1. Integrated isomerization and MTBE production.
(Houzvicka et al., 1997a,b; O’Young et al., 1993; Sikkenga et al., 1984; Bellussi et al., 1992). The most suitable materials for skeletal isomerization of n-butenes are the zeolite-type catalysts in the group of 10-membered ring molecular sieves include ferrierite, ZSM-22, SAPO-11, and ZSM-5. All of these catalysts are very stable, but the resulting yield and selectivity of the ZSM-5 are lower (Houzvicka et al., 1997b). Strong acidity and its high density of active sites are responsible for side products formation (Houzvicka et al., 1997a). The ZSM-5 catalysts having channels formed by 10-membered rings are very stable, but not selective (Houzvicka et al., 1997b; O’Young et al., 1993). Olefin isomerization processes can be directed toward skeletal isomerization by using HZSM-5 type borosilicate zeolites with sufficient acidity (Houzvicka et al., 1997a; O’Young et al., 1993; Sikkenga et al., 1984). Therefore, in the present work authors tried to investigate whether a substitution of aluminum (Al) in the zeolite skeleton of the ZSM-5
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Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 2379
were preferrably used for catalyst performance evaluation experiments were as follows (Boghoz, 1997):
Table 1. Molar Ratios of Components in Reaction Mixture (Boghoz, 1997) samples X Y Z
1
2
3
4
8.65 31.16 1648.4
8.65 31.24 1652.1
4.33 15.62 826.1
2.17 7.81 413.1
by the boron (B) can improve the selectivity without a decrease in stability. Furthermore, the objective of this research is the development of an improved process for the skeletal isomerization of normal alkenes, in particular n-butenes. In this work the results of synthesis and characterization of a special type borosilicate zeolite catalyst for conversion of n-butenes to isobutene is presented. These catalysts can be used in an integrated process for skeletal isomerization of linear butenes to produce isobutene (isobutylene), which is then used in the production of MTBE (Figure 1). For conversion of n-butene to isomers at equilibrium we have
[1-butene T trans-2-butene T cis-2-butene] f isobutene (1) Gibbs free energy of formation for these equilibrium conversions to isubutene at 550 °C are -7.5, -4.0, and -6.55 kJ/gmol, respectively (Yaws, 1992). These reactions are favorable and thermodynamically feasible. Equilibrium conversion for each reaction at 550 °C and 1 atm is about 50%. Experimental Section Preparation and Characterization of the Catalyst. HZSM-5-type borosilicate zeolites (Table 1, samples 1-4) were synthesized by mixing a water solution of sodium metaborate (NaBO2‚4H2O) and tetrapropylammonium (TPA) as an organic template with solid powder of silicon oxide as a source of silicon (Kolts, 1981). pH of this milky slurry was kept within the range of about 10.8-11.0 by adding solid sodium hydroxide (NaOH). The composition of the resulting medium corresponded to molar ratios X(TPA)2O, 1.0(B2O3), Y(SiO2), Z(H2O) in which values of X, Y, and Z for each sample are given in Table 1 (Boghoz, 1997). Crystallization of each sample was carried out in Teflonlined autoclaves at 165 °C for 5 days. The crystalline product was recovered by suitable filteration with washing and then was mildly dried at 120 °C for 3 days. The organic template and water of hydration were then removed by calcining the crystalline product for 8 h at 580 °C in a stream of air. The rate of temperature rise was 4 °C/min. Borosilicate zeolites, which were prepared by the above-mentioned procedure, and were all in sodium form (Table 1, samples 1-4), were characterized by XRD and SEM tests and the HZSM-5 type was confirmed. Typical XRD and SEM test results are represented in Figures 2 and 3. BET surface area in the range of approximately 270-350 m2/g, pore size of at least 5 Å and a B/Si ratio less than 1 wt % were measured (Boghoz, 1997). Catalyst Testing System. Catalyst performance evaluation experiments were carried out in a oncethrough microreactor setup, in which a stream of 1-butene (99.9%) in the vapor phase was contacted with a catalytic material containing borosilicate catalyst at a suitable reaction temperature, pressure, and space velocity (Figure 4). Reactor operating conditions which
temperature pressure WHSV H2/HC
350-600 °C 1-2 bar (absolute) 1.0-10.0 h-1 0.5-1.5 mol/mol
Major equipments used in microreactor system are Pyrex glass type 1 cm (i.d.) × 50 cm (length) equipped with glass filter (effective catalyst bed length is about 10 cm), an electrical labratory oven (1000 °C maximum) with digital temperature control system, two mass flowmeters with digital control panel, and a gas chromatograph model HP5890 calibrated for analyzing refinery gases including butenes. Flow-controlled streams of 1-butene and H2 gases were mixed uniformly through the mixing chamber (3-cm diameter tube). Mixed feed gas flows through two three-way valves to the reactor inlets. By suitable arrangement of valves openings, feed gas can be directed into the sampling system, vent system, or reactor inlet. The reactor which is made from Pyrex glass consists of a preheating zone. The reactor is placed in a vertical position (downflow) in an electrical heater (labaratory oven), in which the reactor is kept isothermal. The heating system can be controlled by a digital temperature control system. For data evaluation, yield of any compound named Xi (some authors call it relative concentration) in an effluent was determined in mol % by an integrator from a GC signal (integrated peak area). So conversion is defined by
conversion (mol %) ) [ Xi (i * linear butenes)/
∑
∑Xi] × 100%
(2)
The sum in the denominator comprises all compounds detected; that in the numerator are the overall products except linear butenes. Selectivity is defined by
selectivity (mol %) ) [Xisobutene/ Xi (i * linear butenes)] × 100% (3)
∑
Isobutene formation is related to the formation of all products by selectivity. In these equations, the linear butenes are excluded from the indicated sums. This is because linear butenes are in equlibrium, and thus, all these three butenes can be considered as feed. Experimental results will be described in the following section. Results and Discussion Through the experiments, the effects of catalyst acidity, reaction temperature (catalyst-bed temperature), residence time (weight hourly space velocity), and zeolite structure were studied, as described below. Catalyst Acidity. Sodium form borosilicate zeolites direct 1-butene isomerization toward cis and trans isomerization. These zeolities may be converted to the hydrogen form through cation-exchange with ammonium ions in order to remove sodium and then be calcined for discarding ammonia. Acidity of these zeolites were evaluated by the ammonia chemisorption (ASTM 4824) method. Measured acidity for samples 1-4 are presented in Table 2. Test results for sodium and hydrogen forms of sample 2 made in this work are summarized in Table 3. This hydrogen form boro-
2380 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998
Figure 2. Typical zeolite XRD test results (Boghoz, 1997).
Figure 3. Typical zeolite SEM test results (Boghoz, 1997).
silicate zeolite was found to demonstrate sufficient acidic strength for the skeletal isomerization process of n-butenes. Reaction Temperature. The dependence of isobutene yield on reaction temperature is given in Figure 5. It can be seen that isomerization and side reactions, in particular dimerization, strongly depend on temperature. However, dimerization is a reaction thermodynamically favored at low temperature (Houzvika and Ponec, 1997). Not only the overall yield but also the selectivity strongly increases with increasing temperature. This effect was pronounced by the HZSM-5-type catalyst, because it allows unhindered dimerization and hydrogen transfer reactions at the crossing of channels (Houzvika and Ponec, 1997). The main side reactions were dimerization and oligomerization. Either decrease in yield of C32- and C5+ or increase of isobutene selectivity are indications of suppressing these side reactions by increasing temperature. However, it was stable at temperatures up to 550 °C. Above 550 °C, faster deactivation of the catalyst was observed due to formation of carbonaceous residues (coke). Contact Time. The effect of apparent contact time with catalyst active sites on isobutene selectivity was
studied by changing the reactant feed rate. The results are given in Figure 6 in terms of weight hourly space velocity (WHSV). With an increase in WHSV from 3 to 10 h-1 (decreasing contact time), selectivity of isobutene increases. Conversion of linear C4 olefins decreased with increase in WHSV. Contact time of olefin molecules with acidic sites at high temperature varies with space velocity. A reactive molecule of isobutene formed in a pore must undergo a random collision path inside the pore before it can go out of the catalyst pore to the gas phase; this causes the formation of various byproducts (Houzvicka et al., 1997b). The main byproducts (propene and pentenes) are formed by a sequence of reactions: butene dimerization > isomerization > cracking. Therefore, decreasing contact time has the same effect as dilution of active sites and leads to suppression of consecutive reactions. Furthermore, contacting time of n-butene molecules with acidic sites at high temperatures increases by a decrease in WHSV. This causes faster deactivation of the catalyst due to cracking of n-butene molecules and the formation of carbonaceous residues (coke). Zeolite Structure. The effect of the zeolite crystalline structure was studied by preparing HZSM-5-type borosilicate zeoilites with different B/Si molar ratios. Zeolite composition and crystalline structure are highly dependent on the B/Si ratio in the zeolite. Through controlling the quantity of boron participating in the reaction mixtures when preparing the zeolites, it is possible to vary the B/Si molar ratio in the final product. The effect of zeolite structure on the skeletal isomerization process was investigated by preparing three zeolite catalysts with different B/Si molar ratios and evaluating their catalytic perfomance through experiments. In reaction mixtures used for the preparation of samples 2-4, the B/Si molar ratios utilized were changed as shown in Table 1. These zeolites were characterized with the SEM method and the HZSM-5 structure was confirmed. It was also observed that by the increasing B/Si molar ratio, zeolites with uniformly structured pores and regular geometry were formed. This is due to increasing the influence of the BO4 tetrahedra in the crystalline structure. The results of tests are presented in Table 4. It was found that by
Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 2381
Figure 4. Catalyst testing system flow diagram (Boghoz, 1997). Table 2. Measured Acidity for Samples (Boghoz, 1997) samples
acidity (µmol of NH3/g)
1 2 3 4
403 372 315 230
Table 3. Test Results for Sodium and Hydrogen Forms of Sample 2 Catalysta samples
conversion (%)
isobutene (%)
n-butenes (%)
C32- plus C5+
no. 2 (H-form) no. 2 (Na-form)
64.8 9.4
18.4
35.2 90.6
39.3 2.7
a Tests were made using 2.4 g of sample no. 2 at a temperature of 550 °C, a total pressure of 1 bar, weight hourly space velocity of 6 h-1, and hydrogen-to-n-butene ratio of 0.5. Time-on-stream for GC samples is 2.5 h (Boghoz, 1997).
increasing the B/Si ratio in the zeolites, conversion of n-butenes was decreased and selectivity to isobutene formation increased. Selectivites to byproducts formation (namely C32- + C5+) also decreased. Therefore, it can be concluded that by increasing the B/Si molar ratio, the acidic properties of zeolite decrease. Furthermore, by increasing boron substitution into the skeleton of HZSM-5 catalysts, the density of active sites decreases, leading to a suppression of consecutive reactions. Catalyst Stability. The group of 10-membered ring molecular sieves including ZSM-5 are very stable (Houzvicka et al., 1997a,b). Isobutene formation over the 10membered ring borosilicate sieves as a function of timeon-stream are given in Figure 7. These borosilicate zeolites show an excellent stability. Conclusions On the basis of the results from catalytic reactions, we conclude that the sodium-form borosilicate does not have activity for conversion of n-butenes to isobutene by skeletal isomerization. However, hydrogen-form ZSM-5-type borosilicate zeolites with suitable B/Si ratios do have enough activity for skeletal isomerization of
Figure 5. Effects of reaction temperature. Tests were made using 2.4 g of sample no. 1 with total pressure of 1 bar, weight hourly space velocity of 6 h-1, and a hydrogen-to-n-butene ratio of 0.5. Time-on-stream for GC samples is 2.5 h (Boghoz, 1997).
Figure 6. Effects of contact time. Tests were made using 2.4 g of sample no. 3 with a temperature of 550 °C, a total pressure of 1 bar, and hydrogen-to-n-butene ratio of 0.5. Time-on-stream for GC samples is 2.5 h (Boghoz, 1997).
n-butenes. The main side reactions such as dimerization and oligomerization are suppressed by increasing the reaction temperature up to 600 °C. Due to the
2382 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 Table 4. Test Results for Borosilicate Catalysts with Different B/Si Ratiosa samples conversion (%) selectivity (%) iC42C32- + C5+
2
3
4
64.8
52.4
51.2
28.4 60.7
36.2 59.1
38.3 56.6
a
Tests were made using 2.4 g of samples 2-4 at a temperature of 550 °C, a total pressure of 1 bar, weight hourly space velocity of 6 h-1, and hydrogen-to-n-butene ratio of 0.5. Time-on-stream for GC samples is 2.5 h (Boghoz, 1997).
Figure 7. Effects of time-on-stream. Tests were made using 2.4 g of sample no. 1 with a total pressure of 1 bar, weight hourly space velocity of 6 h-1 and hydrogen-to-n-butene ratio of 0.5 (Boghoz, 1997).
formation of carbonaceous residues, a trend of decreasing catalyst activity with reaction temperatures higher than 550 °C is observed. A decrease in contact time has the same effect as dilution of active sites and will lead to suppression of consecutive reactions. Also, a trend of decreasing catalyst activity with increasing contact time due to the formation of carbonaceous residues is observed. The acidic properties of the zeolite are decreased by increasing the B/Si ratio in the skeleton of the HZSM-5-type borosilicate catalyst, leading to suppression of side products formation. Therefore, selectivity to isobutene formation is increased by increasing the B/Si ratio. A combination of acidity and
geometrical constraint yields a catalytic site which tends to be active for skeletal isomerization of n-butenes. Acknowledgment Support of this work was provided by the NIOC, Research Institute of Petroleum Industry (RIPI). Literature Cited Bellussi, G.; Giusti, A.; Zanibelli, L. Dehydroisomerization catalyst and its use in the preparation of isobutene from n-butane. U.K. Patent 2,246,524, 1992. Boghoz, A. Catalyst Synthesis and Performance Analysis for Skeletal Dehydroisomerization of Normal Butane. Ph.D. Dissertation, Sharif University of Technology, Tehran, Iran, 1997. Boghoz, A.; Soltanieh, M.; Mondegarian, R.; Karbalaee, M. An Skeletal Isomerization of Normal Alkenes. Paper Presented at 12th International Congress of Chemical and Process Engineering, CHISA 96, Prague, Czech Republic, 1996. Houzvicka, J.; Ponec, V. Skeletal Isomerization of Butene: On the Role of Bimolecular Mechanism. Ind. Eng. Chem. Res. 1997, 36, 1424. Houzvicka, J.; Klik, R.; Kubelkova, L.; Ponec, V. The Role of the Density of Active Sites in Skeletal Isomerization of n-Butene. Appl. Catal. A: General 1997a, 150, 101. Houzvicka, J.; Hansildaar, S.; Ponec, V. The Shape Selectivity in the Skeletal Isomerization of n-Butene to Isobutene. J. Catal. 1997b, 167, 273. Kolts, M. R. Crystalline borosilicate and process of preparation. U.S. Patent 4,269,813, 1981. O’Young, C.; Browne, J. E.; Matteo, J. F.; Sawicki, R. A.; Hazen, J. Bimetallic catalysts for dehydroisomerization of n-butane to isobutene. U.S. Patent 5,198,597, 1993. Petrochemical Processor “ 95 ”. Hydrocarbon Process. 1995, 74 (3), 125. Seddon, D. Reformulated Gasoline, Opportunities for New Technology. Catal. Today 1992, 15, 1. Sikkenga, D. L. Process to convert linear alkenes selectively using high concentrations of AMS-1B crystalline borosilicate catalyst. U.S. Patent 4,503,282, 1985a. Sikkenga, D. L. Nonaqueous exchange of acidic sites on molecular sieves. U.S. Patent 4,550,091, 1985b. Sikkenga, D. L.; Nevitt, T. D.; Jerome, N. F. Process to convert linear alkenes. U.S. Patent 4,433,190, 1984. Yaws, C. L. Thermodynamic and Physical Property Data; Gulf Publ. Comp.: Houston, TX, 1992.
Received for review August 4, 1997 Revised manuscript received January 6, 1998 Accepted February 24, 1998 IE970539E
