Ind. Eng. Chem. Res. 2010, 49, 6815–6823
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Catalytic Cracking of 2,2,4-Trimethylpentane on FAU, MFI, and Bimodal Porous Materials: Influence of Acid Properties and Pore Topology Rhona Van Borm,† Alexander Aerts,‡ Marie-Franc¸oise Reyniers,*,† Johan A. Martens,‡ and Guy B. Marin† Laboratorium Voor Chemische Technologie, Ghent UniVersity, Krijgslaan 281 (S5), B-9000 Gent, Belgium, and Centrum Voor OpperVlaktechemie en Katalyse, Katholieke UniVersiteit LeuVen, Kasteelpark Arenberg 23, B-3001 HeVerlee, Belgium
Cracking experiments using 2,2,4-trimethylpentane as a model component have been performed on five FAU and three MFI zeolites. In addition to these eight commercially available catalysts, two newly developed zeotype materials with bimodal pore structure, BIPOMs, have been investigated. Both BIPOMs possess an MFI ultramicropore (99.99 wt %) and liquid nitrogen used for the cooling of the GC were supplied by Air Liquide, Belgium. A number of commercially available zeolites was tested. Three USY zeolites were delivered by Zeolyst International. CBV500 is obtained by steaming of a parent Y zeolite with Si/Al ratio 2.6, CBV720 and CBV760 are the result of a second steam treatment at higher temperature followed by acid leaching. Another USY (LZ-Y20) and one Y (Y62) zeolite are products from the Linde Division of Union Carbide Corporation (cur-
Table 1. Properties of the Zeolites Tested in This Study zeolite
type
LZ-Y20 Y62 CBV500 CBV720 CBV760 CBV3020E CBV5524G CBV8014 BIPOM1 BIPOM3
H-USY NH4-Y NH4-USY H-USY H-USY H-ZSM-5 NH4-ZSM-5 NH4-ZSM-5 BIPOM BIPOM
a
Si/Albulk Si/Alframe 2.6 2.6 2.6 15 30 15 25 40 50 50
30.0 2.6 3.9 16.0 100 18.4 25.2 40.0 47.0 50
Vmicro Ct (mol NH3 SBET kgcat-1) (m2 g-1) (cm3 g-1) 0.99 3.49 1.50 0.60 0.23 0.54 0.35 0.35 0.13 0.09
481 739 735 673 526 400 425 425 554 467
0.19 0.34 0.27 0.27 0.25 0.16 0.18 0.17 0.64 0.28a
Pores with diameters up to 10 nm.
rently a part of Dow Chemical Company). The Si/Al ratio of these FAU zeolites varies between 2.6 and 30. A detailed characterization of these FAU zeolites is given elsewhere22-28 (see Table 1). Three MFI zeolites with Si/Al ratio 15-40 were also acquired from Zeolyst International: CBV3020E, CBV5524G, and CBV8014. The most important features of these commercially available zeolites are summarized in Table 1. Besides these, two newly developed BIPOM materials were prepared starting from a “clear solution” containing MFI precursor nanoparticles. Clear solution was prepared by dissolving an amount of aluminum metal powder (Acros, > 99%) in 40 wt % aqueous tetrapropylammonium hydroxide (TPAOH, Alfa). Dissolution was carried out at room temperature in a polypropylene bottle under gentle stirring for 24 h. The TPA aluminate containing TPAOH solution was then combined with tetraethyl orthosilicate (TEOS, Acros, 98%), in a polypropylene bottle under continuous stirring. After hydrolysis of TEOS, recognized by the homogenization of the two liquids, deionized water was added and stirring continued for 24 h. The molar Al:TEOS:TPAOH: H2O ratio of the clear solution was 0.5:25: 9:400. From this clear solution BIPOM1 was synthesized by room temperature precipitation of the MFI precursor nanoparticles with ethanolic cetyltrimethylammonium bromide and calcination of the precipitate. Details of the synthesis procedure are described elsewhere.29,30 The particle size of BIPOM1 was 1-6 µm. The material did not exhibit long-range order and did not present wide angle XRD. To obtain BIPOM3, 20 mol % of TEOS used in clear solution synthesis was replaced with phenyltrimethoxysilane (PhTMS, Acros). After hydrolysis of the TEOS/PhTMS mixture in aqueous TPAOH, a clear solution was obtained with molar Al:PhTMS:TEOS:TPAOH:H2O ratio of 0.5:5:20:9:400. The clear solution was transferred to a 100 mL stainless steel autoclave and heated in an air oven at 393 K for 3 days without stirring. The autoclave was cooled to room temperature using cold water. The reaction mixture was centrifuged at 12 000 rpm for 30 min, and then, the crystals were separated from the mother liquor and redispersed in deionized water. This centrifugation/washing procedure was repeated 3 times. Finally, the crystals were transferred to porcelain crucibles and dried at 333 K in an air oven for 24 h. Calcination was carried out with air in a muffle furnace at 823 K for 5 h using a heating rate of +1 K min-1. The BIPOM3 material had an XRD pattern typical of MFI type zeolite. The particle size was around 1 µm. The most important features of the BIPOM zeolites are also summarized in Table 1. Catalytic Experiments. Cracking has been studied in a recycle electro balance reactor.31 In such reactor the weight of the catalyst basket is measured continuously and registered by means of a Sartorius type 4436 MP8-1 electro balance. Changes occur because of adsorption and coke formation. The use of external recirculation allows to work at gradientless conditions
Ind. Eng. Chem. Res., Vol. 49, No. 15, 2010 Table 2. Range of Cracking Conditions zeolite
T (K)
LZ-Y20 Y62 CBV500 CBV720 CBV760 CBV3020E CBV5524G CBV8014 BIPOM1 BIPOM3
698-748 723-763 723-748 723-748 723-748 723-748 748 748 748 748
a
0 0 a piC8 (103 Pa) W/FiC8 (kgcat s mol-1) X (mol %)
6-15 3-9 3-8 3-8 3-8 3-8 7 7 7 7
8-90 17-160 10-144 14-132 13-215 30-122 13-63 46-72 40-148 68-164
17-60 1-14 40-70 21-52 4-33 1-14 2-9 2-4 1-2 1-2
The catalyst mass W refers to the fresh zeolite before pretreatment.
and at high conversion. Online gas chromatographic analysis of the reactor effluent allows to determine activity and selectivity. The zeolite powder is first pressurized (12 bar, 15 min) using a hydraulic press and then pelletized. The fraction of pellets with a resulting diameter between 500 and 710 µm is retained. The desired amount of catalyst pellets (W ) 75-350 mg) is loaded into the catalyst basket which is then attached to the electro balance. Next, the reactor is closed. Before the start of an experiment all O2 is dispelled from the reactor by flushing for 12 h with a sufficiently high N2 flow. Prior to its use in catalytic experiments the zeolite pellets are pretreated by heating them at a rate of +5 K min-1 and keeping them at 773 K for 1 h under a flow of 45 NmL s-1 N2. This pretreatment ensures that all water and other contaminants adsorbed during loading of the reactor are removed and that zeolites supplied in ammonium-form are converted into hydrogen-form. Next, the temperature of the reactor is set to the desired value (T ) 698-763 K). The reaction temperature was chosen slightly below industrial operating temperatures (755-838 K) to avoid the interference of thermal cracking, which occurs via a radical mechanism, with the mechanism of catalytic cracking. Throughout the experiment, the reaction temperature is maintained constant. An overview of the experimental conditions is given in Table 2. 0 The flow rate of 2,2,4-trimethylpentane (FiC8 ) 1.3-6.3 -1 mol s ) out of a glass reservoir is controlled by adjusting the He pressure above the liquid hydrocarbon. Then, the liquid feed is evaporated and diluted with N2 to achieve the desired 0 hydrocarbon partial pressure (piC8 ) 3-15 kPa). Once a stable flow is obtained and the reactor temperature is equilibrated after the pretreatment, the reactant mixture can be fed. Perfect mixing inside the reactor is created by a ventilator driven by a DC motor (Dunkermotoren, type BG 43X50). Rotational speeds between 7000 and 9000 rpm were applied. After exiting the reactor, the effluent is mixed with the internal standard, dimethyl ether. An online sampling system allows storage of up to five gaseous samples. The samples can be injected simultaneously in two GCs. A system of three valves determines if a sample is stored or injected, enables storage of up to five gaseous samples, and allows the choice between both GCs. A Chrompack CP-9003 gas chromatograph with flame ionization detector is used to detect the individual hydrocarbons in the effluent. The components are separated on a capillary CP-Sil PONA CB for ASTM D 5134-90 fused silica column (50 m length, 0.21 mm internal diameter, 0.5 µm film thickness). A temperature program is applied for a good separation (1 min at 223 K, +6 K min-1, 3 min at 473 K). It allows to detect all hydrocarbon types with maximum 10 carbon atoms, alkanes, alkenes, cyclics, as well as aromatics. A Chrompack CP-9000 gas chromatograph with thermal conductivity detector is used to quantify the amount of N2 in the effluent. The components are separated on a packed Alltech SS Porapak N column (3 m
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length, 3.18 mm internal diameter). A temperature program is applied for a good separation (2 min at 373 K, +10 K min-1, 8 min at 423 K). After the elution of dimethyl ether the flow direction through the column is reversed to cause the heavier hydrocarbons to be removed from the column. The molar flow rate of any species i is calculated from the GC peak area, Ai, the calibration factor, CFi, the molecular mass of both species i, MWi, and internal standard, MWDME, and the molar flow rate of the latter, FDME, via Fi ) FDME
MWDMEAiCFi MWiADMECFDME
(1)
The conversion of 2,2,4-trimethylpentane is expressed as the number of moles of reactant converted per mole of reactant fed XiC8 ) 100
0 FiC8 - FiC8 0 FiC8
(2)
with F0iC8 and FiC8 the feed, respectively outlet, molar flow rates of 2,2,4-trimethylpentane. The selectivity toward a product i is obtained as the number of moles of product i formed per mole of 2,2,4-trimethylpentane converted Si ) 100
Fi 0 FiC8
- FiC8
(3)
Note that as selectivity is expressed on a molar basis, the total sum of product selectivities can be higher than 100%. Generally, activities of catalysts are compared on the basis of the mass of the catalyst, W, that is, using the space time 0 W/FiC8 . However, a more fair comparison would be based on the number of active sites available, NH+, that is, using the site time NH+/F0iC8. The number of acid sites used in this expression, is obtained by multiplying the catalyst mass with the acid site concentration, Ct, given in Table 1 for the investigated catalysts NH+ ) WCt
(4)
For all experimental conditions the absence of external mass and heat transfer limitations was verified using correlations from literature32 and by performing test experiments with different pellet sizes of FAU (LZ-Y20) and MFI (CBV3020E). No effect of the pellet size on catalyst activity was observed. Note that both methods only allow to study the absence of mass and heat transfer limitations at pellet level. The data reported in this paper refer to the fresh catalyst and were obtained by extrapolating the values of conversion and product selectivities, observed as a function of time on stream, to time on stream zero in order to exclude the effect of coke formation on the observed catalyst behavior, which falls beyond the scope of this work. As coke formation is inhibited on MFI zeolites33 and plays therefore only a minor role, the deviation from the trendlines because of extrapolation of both conversion and selectivity values toward time on stream zero is much less pronounced for MFI than for FAU, which is highly susceptible to coke formation during catalytic cracking. Results and Discussion Characterization. In this study, five commercially available FAU zeolites differing in acid properties (see Table 1), such as concentration of acid sites, bulk Si/Al ratio, framework Si/Al ratio, and presence of EFAl, have been tested in catalytic
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Figure 1. Elementary steps involved in the catalytic cracking of alkanes on zeolites.
cracking of 2,2,4-trimethylpentane. The Y zeolite (Y62) contains the highest concentration of acid sites (3.49 mol kgcat-1) as determined by NH3-TPD. The decreasing acid site concentration of CBV500 (1.51 mol kgcat-1), CBV720 (0.62 mol kgcat-1), and CBV760 (0.24 mol kgcat-1) clearly indicates that upon (more stringent) dealumination (more) Al is removed from the zeolite framework. The total concentration of active sites, Ct, determined via NH3-TPD does not correlate very well with the amount of framework Al. This is explained assuming that NH3 reacts with all the acidic sites present in the zeolite, i.e. with both Bro¨nsted (framework Al) and Lewis (EFAl) acid sites. The total micropore volume of BIPOM1 was 0.64 cm3 g-1, according to nitrogen physisorption analysis. This material has a bimodal pore network, distributed over super- and ultramicropores.19 Ultramicropores are MFI type.19 The supermicropores have larger dimensions, estimated to be 1.3-2.1 nm.29 The unusually high micropore volume of BIPOM1 is because of the large contribution of supermicropores. BIPOM3 has a total pore volume of 0.28 cm3 g-1. This material contains 0.18 cm3 g-1 micropores and 0.10 cm3 g-1 mesopores with diameter up to 10 nm. A colloidal MFI obtained via the clear solution method without PhTMS substitution has a total micropore volume of 0.13 cm3 g-1.29 Thus, compared to reference colloidal MFI, BIPOM3 presents enhanced microporosity of 0.05 cm3 g-1, templated by the organosilane compound PhTMS. Reaction Mechanism. Catalytic cracking of alkanes on zeolites is a well-known autocatalytic process. In literature, the debate on the exact nature of the intermediate surface species involved in hydrocarbon conversion processes on zeolites, carbenium ions or alkoxide species, is still ongoing.34-39 In this work, a reaction mechanism based on carbenium ion chemistry is assumed40 (see Figure 1). Two steps have been identified to initiate the catalytic cycle. In the monomolecular mechanism (Figure 1, step a), also referred to as protolytic scission, the C-C or C-H bond of a gas phase alkane is protonated leading to a smaller alkane or hydrogen in the gas phase and the complementary adsorbed carbenium ion via a nonclassical pentacoordinated carbonium ion.35,41-43 Once these initial carbenium ions on the zeolite surface are formed, gas phase alkanes can undergo hydride transfer reactions with these carbenium ions (step e), which corresponds to the bimolecular mechanism.35,42 If alkenes are present in the feed, these can be protonated by the zeolite leading to the formation of carbenium ions on the surface (step b). The carbenium ion intermediates can then undergo β-scission reactions, leading to a smaller carbenium ion and a gas phase alkene (step c) or can, in the reverse reaction, be alkylated with a gas phase alkene. The latter can lead to molecules larger than those present in the feed and is
Figure 2. Fresh catalyst 2,2,4-trimethylpentane conversion on five FAU zeolites as a function of site time at 748 K and 7 kPa partial pressure. Experimental: [, Y62; 9, LZ-Y20; 2, CBV500; b, CBV720; 1, CBV760. The solid lines represent trendlines obtained by fitting the data series to a logarithmic curve.
sometimes referred to as oligomeric cracking.42 Carbenium ions can also rearrange on the surface leading to skeletal isomers (step d). The catalytic cycle terminates when the carbenium ion desorbs from the surface, either via hydride transfer, leading to a gas phase alkane (step e), either via deprotonation, leading to a gas phase alkene and a restored active site (step b). Monomolecular cracking prevails at low alkane conversion, at low alkane partial pressure, at low alkene concentration, at high temperatures, and on small and medium pore zeolites having a low concentration of Bro¨nsted acid sites.2,35,44 At high alkane conversion, at high reactant partial pressure, at low temperatures, and on large pore zeolites with high site density, the bimolecular (and oligomeric) cracking mechanism dominates.2,35 Activity. Since the zeolites studied in this work contain different numbers of acid sites, comparing their catalytic activity on a mass basis could lead to erroneous interpretations. It is more appropriate to compare the catalytic activity per acid site basis. Therefore, Figure 2 presents the conversion of 2,2,4trimethylpentane on each of the fresh faujasite catalysts as a 0 function of site time, NH+/FiC8 . Conversions between 1 and 70 mol % have been obtained (see Table 2). Y62 (Si/Albulk ) 2.6), containing no EFAl and having the highest concentration of acid sites (3.49 mol kgcat-1) possesses by far the lowest cracking activity. This indicates that the presence of EFAl in faujasites greatly enhances the catalytic activity in alkane conversion. Although Y62, LZ-Y20, and CBV500 have an equal Si/Albulk ratio of 2.6, their activities in 2,2,4-trimethylpentane cracking differ markedly. This implies that the catalytic activity is not only controlled by the Si/Albulk ratio. Within the faujasite CBV series, CBV500 (Si/Albulk ) 2.6) shows the highest activity per acid site, followed by CBV720 (Si/Albulk ) 15) and CBV760 (Si/Albulk ) 30), which has the lowest activity. Similar catalytic behavior of the individual sites has been observed in hydroisomerization of decane and heptanes22 and in m-xylene transformation.24 The higher activity of LZ-Y20 compared to Y62 was also found in isobutene cracking.45 LZ-Y20 (Si/Albulk ) 2.6) and CBV720 (Si/Albulk ) 15) have a comparable activity. Ordering the five faujasites according to increasing site time yield leads to Y62 , CBV760 < LZ-Y20 ≈ CBV720 < CBV500. At 748 K, 7 kPa partial pressure and site time 67 kgcat s mol-1 the site time yields in 2,2,4-trimethylpentane cracking on faujasites range from 2.2 10-4 s-1 to 8.9 10-3 s-1. In Figure 3, conversion of 2,2,4-trimethylpentane on MFI zeolites is presented as a function of site time. Conversions ranging from 1 to 14 mol % have been achieved (see Table 2). Within the MFI series activity of the individual sites increases with decreasing Si/Albulk ratio: CBV8014 (Si/Albulk ) 40) < CBV5524G (Si/Albulk ) 25) < CBV3020E (Si/Albulk ) 15). At
Ind. Eng. Chem. Res., Vol. 49, No. 15, 2010
Figure 3. Fresh catalyst 2,2,4-trimethylpentane conversion achieved on three MFI zeolites as a function of site time at 748 K and 7 kPa partial pressure. Experimental: ], CBV3020E; 0, CBV5524G; 4, CBV8014. The solid lines represent trendlines obtained by fitting the data series to a linear curve.
Figure 4. Fresh catalyst 2,2,4-trimethylpentane conversion achieved on two BIPOMs and one MFI zeolite as a function of site time at 748 K and 7 kPa partial pressure. Experimental: ×, BIPOM1; +, BIPOM3; 4, CBV8014. The solid lines represent trendlines obtained by fitting the data series to a linear curve.
748 K, 7 kPa partial pressure and site time 65 kgcat s mol-1 the site time yields in 2,2,4-trimethylpentane cracking on MFI zeolites range from 5.7 10-4 s-1 to 2.2 10-3 s-1. When comparing the site time yields of the FAU and the MFI zeolites (Figure 2 and Figure 3) it can be seen that 2,2,4-trimethylpentane conversion is noticeably higher on the faujasites. The catalyst activity per acid site is a factor 3-4 lower on MFI than on FAU. This can be caused by shape selectivity effects. Two types of shape selectivity have been reported to occur during fluid catalytic cracking in MFI zeolites:33,46 reactant shape selectivity, where access of certain reactants is hindered or even entirely blocked, and transition state shape selectivity, where the formation of certain transition states is slowed down or even prevented. An example of the former type of shape selectivity in MFI is the slower diffusion of mono- and dibranched alkanes compared to their linear isomers.47,48 Examples of the latter type are the restriction of hydride transfer reactions and the prevention of coke formation.33,44,46 However, discriminating between different types of shape selectivity based on experimental results is not always straightforward and some types may occur simultaneously. Exclusion of 2,2,4-trimethylpentane from the MFI pores seems unlikely as the effective catalytic pore size at 573 K lies between 0.662 and 0.727 nm and increases to 0.764 nm at 643 K,49 while the kinetic diameter of the reactant amounts to 0.62 nm only.50 Moreover, 2,2-dimethylbutane also possessing a quaternary carbon atom has been reported to diffuse into MFI (Si/Albulk ) 100) at 423 K.51 The selectivity patterns may give more insight into the governing type(s) of shape selectivity in MFI. Figure 4 depicts the 2,2,4-trimethylpentane conversion on the two BIPOM catalysts (Si/Albulk ) 50) as a function of site time. The conversion obtained on the commercial MFI CBV8014 with a Si/Albulk ratio of 40 is added for comparison. The catalytic activity in 2,2,4-trimethylpentane cracking of BIPOM1 and BIPOM3 is almost equal. The maximum conversion obtained
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Figure 5. Product distribution per carbon number of 2,2,4-trimethylpentane cracking on fresh FAU (CBV760) at 29 mol % conversion, 748 K, 7 kPa partial pressure and space time 109 kg s mol-1, grouped into (a) alkanes and alkenes, and (b) linear and branched acyclic hydrocarbons.
on the BIPOMs under the conditions tested amounts to 2 mol % (see Table 2). This is not very high, but it is only a little lower than what is obtained on a commercial MFI zeolite with similar amount of Al. The BIPOM1 material does not present the long-range order of a zeolite material. The small size of the zeolitic nanoslabs this material is composed of may explain why acidity characteristic of an MFI zeolite is absent. The oxide bonds likely are relaxed resembling those in amorphous material. Besides, the length of a micropore in the BIPOM1 material of a few nanometers is hardly longer than that of the reacting hydrocarbon molecules. BIPOM3 on the contrary displays the XRD pattern of MFI zeolite and has more pronounced zeolite properties. The difference with a conventional zeolite is the presence of extra microporosity (enhanced BET surface area and high microporosity compared to commercial MFI samples (Table 1)). Product Distribution. Figure 5 presents a product distribution of 2,2,4-trimethylpentane cracking on faujasite CBV760 in terms of alkanes/alkenes and of linear/branched species, respectively. Typically, alkanes with carbon number ranging from 1 to 7 and alkenes with carbon number from 2 to 8 are observed. A negligible amount of cyclics and aromatics is detected at all experimental conditions; at most 1 mol % of cyclics (mainly cyclopentane) and 5 mol % of aromatics (mainly toluene and xylenes) are formed. The main reaction products of 2,2,4-trimethylpentane cracking on the faujasites over the whole conversion range investigated are isobutane, isobutene, 2-butenes, and propylene. Direct β-scission of the 2,2,4trimethylpentyl ion leads to isobutene and the isobutyl carbenium ion which can desorb from the surface via hydride transfer resulting in the formation of isobutane or via deprotonation in isobutene. The C4 fraction is present in the largest amount throughout the whole conversion range investigated. To produce linear hydrocarbons, the 2,2,4-trimethylpentyl ion has to isomerize prior to cracking. The unequal amounts of C3 and C5 hydrocarbons suggest that isomerization toward an iso-octyl carbenium ion followed by β-scission is not the only possible route toward their formation. Protolytic scission followed by β-scission or alkylation with a gas phase alkene followed by β-scission will also contribute. The C3 fraction consists mostly of propylene while the C5 fraction is almost entirely made up of branched products: isopentane and various isopentenes. Small amounts of typical protolytic scission products such as methane are formed especially at low conversion, confirming the necessity of taking protolytic scission into account as a starting step in the alkane cracking cycle on faujasites. Figure 6 depicts a typical product distribution obtained on MFI zeolite CBV3020E. Alkanes with 1-6 and alkenes with 2-8 carbon atoms are formed. A negligible amount of cyclics was detected at all experimental conditions; at most 2 mol %
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Figure 6. Product distribution per carbon number of 2,2,4-trimethylpentane cracking on fresh MFI (CBV3020E) at 10 mol % conversion, 748 K, 7 kPa partial pressure and space time 84 kg s mol-1, subdivided into (a) alkanes, alkenes, and aromatics and (b) unbranched and branched hydrocarbons.
(mainly cyclopentane) is formed. In contrast to that of faujasites, a substantial amount of C6-C9 aromatics is observed on MFI, mainly, toluene and xylenes. The main reaction products of 2,2,4-trimethylpentane cracking on MFI over the range of conversion studied are located in the C1-C3 fraction: methane, ethylene, propylene, and propane. Moreover, 2,2-dimethylpropane is detected, which is not formed on FAU. A detailed product distribution obtained on FAU (CBV760, Si/Albulk ) 30) and MFI (CBV3020E, Si/Albulk ) 15) under the same conditions (748 K, 7 kPa and 15 mol % 2,2,4-trimethylpentane conversion) is given in Table 3. At these conditions remarkably higher selectivities are found on MFI for methane, ethylene, propane, and neopentane (7 mol %, 2 mol %, 4 mol %, and 0.5 mol %) Obtained on FAU (Si/Albulk ) 30) and MFI (Si/Albulk ) 15) at 748 K, 7 kPa, and 15 mol % 2,2,4-Trimethylpentane Conversiona selectivity (mol %) hydrocarbon CN
alkane
alkene
1 2 3 4
methane ethane propane n-butane isobutane n-pentane isopentane neopentane
ethylene propylene n-butenes isobutene n-pentenes isopentenes
5
a
FAU alkane
MFI alkene
alkane
alkene
2 32 23 37 3 6
47 5 38 6 25 0.5 2 10
49 39 7 6 0.5 2
7 4 4 79 0.5 12
Site time yields on FAU and MFI are 1.7 10-2 s-1 and 2.2 10-3 s-1, respectively.
Table 4. Selectivities of Alkanes and Alkenes (>0.5 mol %) Obtained on FAU (Si/Albulk ) 2.6), MFI (Si/Albulk ) 40), and BIPOMs (both Si/Albulk ) 50) at 748 K, 7 kPa, and 2 mol % 2,2,4-Trimethylpentane Conversiona selectivity (mol %) hydrocarbon CN
alkane
alkene
1 2 3 4
methane ethane propane n-butane isobutane n-pentane isopentane neopentane
ethylene propylene n-butenes isobutene n-pentenes isopentenes
5
a
FAU alkane
MFI alkene
17 2 7 59 3
1 29 36 75 2 4
alkane 39 1 5 6 19 0.5 13
BIPOM1 alkene
alkane
BIPOM3
alkene
14 39 95 24 19 1 2
3 7 53 4 1
7 42 28 46 1 2
alkane 37 0.5 4 9 22 0.5 7
Site time yields on FAU, MFI, BIPOM1, and BIPOM3 are 2.2 10-4 s-1, 5.2 10-4 s-1, 1.1 10-3 s-1, and 9.7 10-4 s-1, respectively.
alkene 17 75 27 39 1 1
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Figure 7. Product selectivities as a function of 2,2,4-trimethylpentane conversion on five fresh FAU, three fresh MFI and two fresh BIPOM zeolites at 748 K and 7 kPa partial pressure. Experimental: FAU: [, Y62; 9, LZ-Y20; 2, CBV500; b, CBV720; 1, CBV760; MFI: ], CBV3020E; 0, CBV5524G; 4, CBV8014. BIPOM: ×, BIPOM1; +, BIPOM3. The two solid lines represent trendlines obtained by fitting all FAU and all MFI data to a second-order polynomial curve.
neopentane are indicative of MFI type behavior, while the relatively high isobutene selectivity suggests FAU type catalytic behavior. Moreover, the product selectivities of ethylene (17 mol % on BIPOM3, 1 mol % on FAU, 39 mol % on MFI) and neopentane (7 mol % on BIPOM3, < 0.5 mol % on FAU, 13 mol % on MFI) are in between those observed on FAU and MFI. From all these observations, it can be concluded that BIPOM3 shows a mixed FAU/MFI type catalytic behavior in 2,2,4-trimethylpentane cracking indicating that the active sites in BIPOM3 are rather evenly distributed among both micropore networks. The selectivity behavior of the BIPOMs observed in 2,2,4-trimethylpentane cracking is consistent with that observed in hydroisomerization and hydrocracking of n-decane.29,30 In BIPOM1, the abundance of wide pore mouths of zigzag channels relative to straight channels is proposed to be responsible for the selectivity behavior as a large pore zeolite.29,30 In BIPOM3, molecular shape selectivity ascribed to catalytic sites inside the ultramicropores was observed next to contributions by active sites located in sterically unconstrained environments assumed to be located in the supermicropores of this material. The different catalytic performance and active site location of both BIPOMs can be ascribed to the different synthesis procedure leading to 0.51 cm3 g-1 supermicropores in BIPOM1 and 0.05 cm3 g-1 in BIPOM3. Influence of Acid Properties and Pore Topology on Product Selectivity. The effect of acid properties and pore topology of the zeolites on the obtained product distribution can be observed from the product selectivity as a function of conversion for all fresh zeolites. Figure 7 presents the selectivities of propylene, isobutene, isobutane, and isopentane on all catalysts as function of conversion at 748 K and 7 kPa. It can be seen that all the experimental data gathered on FAU zeolites follow the same trendline. Similar plots are obtained for all individual reaction products and for other experimental conditions, indicating that the acid properties of the FAU zeolites do not influence the obtained product selectivities in 2,2,4-
trimethylpentane cracking. Similar results on the effect of zeolite acid properties on product selectivities has been observed in catalytic cracking of alkanes,11,43 hydrocracking of alkanes,52 and liquid phase alkylation of benzene with 1-octene.53 On MFI zeolites, the product selectivities obtained also follow a trendline, but a different one than the faujasites. Since this is observed for each product, it can be concluded that on MFI, the acid properties of the zeolite do not affect the product selectivities obtained in 2,2,4-trimethylpentane cracking either. However, topology does affect the obtained product selectivities because of the occurrence of shape selectivity. For BIPOM1, the product selectivities follow the same trend as observed for the faujasites, confirming the FAU type catalytic behavior of this new bimodal porous material. For BIPOM3, isobutane and isopentane selectivities follow the same trend as on MFI, while propylene and isobutene selectivities are situated between those on FAU and on MFI demonstrating its mixed FAU/MFI type catalytic behavior in 2,2,4-trimethylpentane cracking. Conclusions Catalytic cracking of 2,2,4-trimethylpentane was studied on a series of five commercial FAU zeolites, three commercial MFI zeolites, and two newly developed zeotype materials with bimodal pore structure. The catalysts vary in acid properties and framework topology. The influence of these catalyst properties on activity and selectivity was investigated. At equal experimental conditions and equal site time, conversion is a factor 3-4 higher on FAU than on MFI. The lower activity can be explained by slower diffusion of 2,2,4-trimethylpentane inside the pores of MFI as compared to FAU. The cracking activity of both BIPOMs is comparable to that of a commercial MFI zeolite with a similar Si/Al ratio. The main reaction products of 2,2,4-trimethylpentane cracking on faujasites are isobutane, isobutene, 2-butenes, and propylene.
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Hydride transfer, followed by β-scission, is the main reaction route responsible for their formation. The maximum in the product distribution at C4 on FAU shifts to C1-C3 on MFI. On MFI, the most abundant reaction products are methane, ethylene, propylene, and propane. Moreover, 2,2-dimethylpropane is formed, which is not detected on FAU zeolites. Protolytic scission is mainly responsible for the formation of these species, pointing to the occurrence of transition state shape selectivity in MFI. These product distribution characteristics can be used to identify FAU- or MFI-type behavior of the BIPOMs. The catalytic behavior of BIPOM1 in 2,2,4-trimethylpentane cracking resembles FAU, while BIPOM3 shows a mixed FAU/MFI type behavior. Hence, it can be concluded that the active sites in BIPOM1 are located primarily in the supermicropore network, while they are more evenly distributed among ultra- and supermicropore networks in BIPOM3. This is consistent with their different supermicropore volume. The acid properties of the faujasites do not affect the product selectivities obtained in catalytic cracking of 2,2,4-trimethylpentane. The same observation is valid for MFI zeolites. However, the zeolite topology does influence the obtained product selectivities because of shape selectivity. Therefore, it can be stated that within one framework type cracking activity is controlled by the acid properties of zeolites, while selectivity is governed by the framework topology. Acknowledgment The Institute for the Promotion of Innovation through Science and Technology in Flanders (IWT-Vlaanderen) is gratefully acknowledged for their financial support to this “BIPOM”project (SBO 030202). The Belgian government is acknowledged by the authors for supporting an IAP-PAI network. J.A.M. and G.B.M. acknowledge the Flemish government for longterm structural funding (Methusalem). A.A. acknowledges the Flemish FWO for a postdoctoral fellowship. Nomenclature Ai ) GC surface area of species i CFi ) GC calibration factor for species i CN ) carbon number Ct ) total molar concentration of active sites, kgcat s mol-1 Fi0 ) molar flow rate of species i, mol s-1 MWi ) molecular mass of species i, kg mol-1 NH+ ) number of acid sites, mol SBET ) BET surface area, m2 g-1 Si ) selectivity toward species i, mol % pi0 ) partial pressure of species i, Pa T ) temperature, K Vmicro ) total micropore volume, cm3 g-1 W ) catalyst mass, kgcat X ) conversion, mol % Superscript 0 ) initial Subscript DME ) dimethyl ether iC8 ) 2,2,4-trimethylpentane
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ReceiVed for reView October 30, 2009 ReVised manuscript receiVed January 15, 2010 Accepted January 25, 2010 IE901708M
