Shape-Selective Catalysis Determined by the Volume of a Zeolite

Article pubs.acs.org/JPCC

Shape-Selective Catalysis Determined by the Volume of a Zeolite Cavity and the Reaction Mechanism for Propylene Production by the Conversion of Butene Using a Proton-Exchanged Zeolite Yasuyoshi Iwase, Yasuharu Sakamoto, Akinobu Shiga, Akimitsu Miyaji, Ken Motokura, To-ru Koyama, and Toshihide Baba* Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan ABSTRACT: The role of the zeolite cavity in the production of C3H6 from butene was investigated using various zeolites with pore structures of 8-, 10-, and 12-membered rings (MRs). The reaction mechanism is discussed on the basis of which octyl carbocations produced C3H6 at low conversions of butene. The selectivity for C3H6 was highly dependent on the volume of the zeolite cavity but not on the entrance pore diameter. The optimum cavity volumes of zeolites with 8-, 10-, and 12-MR entrance pore structures were similar, while the highest C3H6 selectivity among the zeolites was different. The most selective production of C3H6 can be accomplished by matching the volume of the specific octyl carbocation to that of the zeolite cavity. This concept can be employed to explain the selective production of C3H6 according to a proposed reaction model. Furthermore, the reaction mechanism for the production of C3H6 from C2H4 was also investigated at low conversion of C2H4. C3H6 was produced by the β-scission of the same specific octyl carbocations in the conversions of both butene and C2H4.

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INTRODUCTION The shape-selective catalysis that takes place in zeolites is not observed in other heterogeneous catalysts. This unique catalysis can be applied to various technologies, such as petrochemical processing, to produce a target compound with a high selectivity. It has been proposed that the observed effects can be explained by discerning among three types of shape selectivity: reactant, product, and restricted transition state shape selectivity.1,2 The restricted transition state selectivity occurs when certain reactions are prevented when their transition states require more space than is available in the cavity. Neither reactants nor potential product molecules are prevented from diffusing through the pores, and reactions requiring a smaller transition state proceed unhindered. When employing this shape-selective catalysis, it can be difficult to distinguish product selectivity from restricted transition state selectivity. Recently, we have found that various carbocations produced in the cavity of H+-exchanged zeolites are efficiently recognized by the cavity volume and that the β-scission of specific carbocations selectively produces certain reaction products. The selectivity for reaction products, such as C3H6, depends on the volume of the zeolite cavity but not on the entrance pore diameter of the zeolite.3 For example, when the conversion of 1-hexene was carried out over H+-exchanged zeolites with 8membered rings (MRs), whose pore diameters were nearly the same as the kinetic diameter of C3H6, it did not always show a high selectivity for C3H6. Furthermore, the production of C3H6 with a high selectivity was observed using 10- and 12-MR zeolites, which have pore diameters large enough to accommodate © 2012 American Chemical Society

the kinetic diameter of C3H6. This observed C3H6 selectivity cannot be explained using the concept of product selectivity. In the conversion of 1-hexene, the compositions of hexene isomers in the effluent gas from the reactor reached the equilibrium composition; however, the selective production of C3H6 was observed. This clearly indicates that the β-scission of 1 among hexyl cations (C6H13+) produced in the zeolite cavity selectively generates C3H6, as expressed by eq 1. This phenomenon corresponds to recognition of specific carbocations by a zeolite cavity, which is similar to the recognition of molecules by an enzyme.

It should be noted that reaction 1 selectively proceeded only when the volume of the hexyl carbocations 1 (146 Å3) matched that of the zeolite cavity. Therefore, the 8-MR, 10-MR, and 12MR zeolites that had the maximum selectivity for C3H6 had similar zeolite cavity volumes, even though their entrance pore diameters were different. In the unimolecular cracking of pentenes (2-pentenes and 2-methyl-2-butene) using various 10- and 12-MR zeolites, the same amounts of C3H6 and C2H4 were produced with a high selectivity by adjusting the volume of the zeolite cavity to match the volume of the pentyl carbocations.3 This shows that the Received: December 28, 2011 Revised: January 26, 2012 Published: January 30, 2012 5182

dx.doi.org/10.1021/jp212549j | J. Phys. Chem. C 2012, 116, 5182−5196

The Journal of Physical Chemistry C

Article

β-scission of 2 and/or 3 expressed by eqs 2 and 3 proceeded selectively.

X-ray diffraction patterns of the synthesized zeolites were in good agreement with previously reported diffraction data. To further identify the synthesized zeolites, the microscopic features of the synthesized materials were examined by scanning electron microscopy (SEM). The mean particle diameters of the materials were estimated by measuring the size of 150 particles in the SEM images. The zeolite shapes and their mean particle diameters are summarized in Table 1. 27 Al, 29Si, and 31P magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra were also measured to determine the positions of these cations in the lattice and/or extra framework of zeolites. The details of the above experimental methods (XRD, SEM, and MAS NMR measurements) were described in ref 3. (2). Elemental Analysis of Aluminosilicate Molecular Sieves. The amounts of Si, Al, and P in silicoaluminophosphate molecular sieves were determined by inductively coupled plasma atomic emission spectroscopy. For the aluminosilicate molecular sieves, the Si/Al ratio in the framework was determined by 29Si MAS NMR measurements. (3). Preparation of NH4+-Exchanged Aluminosilicate Molecular Sieves and Elemental Analysis. Before preparing NH4+-exchanged zeolites, the occluded organic materials in the pores of the synthesized zeolites were removed by heating at 723 K for 30 h in air under atmosphere. The zeolites were exchanged with Na+ in a NaCl solution, and then the final traces of organic materials were removed at 793 K in air. The zeolites were then converted to the NH4+ form by exchange in an NH4Cl aqueous solution. The amount of residual Na+ was determined by atomic adsorption analysis. The amounts of Si and Al were determined by inductively coupled plasma atomic emission spectroscopy, and the Si/Al ratio in the zeolite lattice was determined by 29Si MAS NMR measurements. To detect tetrahedral framework Al atoms and octahedral extra framework Al species, 27Al MAS NMR spectra were obtained. (4). Measurements of 1H MAS NMR Spectra. (a). Preparation of Samples for 1H MAS NMR Measurements. Each zeolite (∼0.20 g) was packed in a glass tube with side arms, each of which was connected to a glass tube used for 1H MAS NMR measurements. To convert the NH4+-type zeolite (aluminosilicate molecular sieves) to an H+-exchanged form, the zeolite was heated in a stream of dry air at a flow rate of 500 cm3 min−1. The sample temperature was increased from room temperature zto 393 K at a constant rate of 1 K min−1 and then held at 393 K for 2 h. The sample was further heated to 723 K at a constant rate of 0.5 K min−1 and then held at 723 K for 4 h. The sample was then evacuated and held at 723 K for 3 h. The silicoaluminophosphate molecular sieves containing an SDA were also heated in a stream of dry air at 913 K for 4 h to convert them to H+-exchanged molecular sieves. After heating the samples under dry air at 723 or 913 K, the sample was evacuated at 723 K for 3 h. The zeolite (H+-exchanged molecular sieves) thus prepared was transferred into a glass capsule under vacuum, filling the capsule completely and evenly. The neck of the capsule was then sealed with a microtorch while the sample temperature was maintained at 77 K. (b). 1H MAS NMR Measurements. 1H MAS NMR spectra were measured using sealed glass capsules to avoid the influence of humidity.27 1H MAS NMR spectra were recorded on a Bruker Avance III spectrometer operating at 400 MHz, equipped with a 4 mm CRAMPS probe. A sealed sample in a glass tube was inserted into a zirconia rotor. The spectra were recorded at 298 K. The rotation frequency of the glass capsule

We found that the selectivity of reaction products can be controlled by adjusting the volume of the zeolite cavity to accommodate the volume of specific carbocations. This concept is different from restricted transition state selectivity that was reported by Csicsery.1,2 We previously reported that the influence of the zeolite cavity volume during the conversion of C2H4 on the selectivity for C3H6 was also observed using various 8-, 10-, and 12-MR zeolites. In this case, the highest C3H6 selectivity was achieved by adjusting the volume of the zeolite cavity to that of the octyl carbocations.3 However, we could not determine the specific octyl carbocations that produce C3H6 or the reaction mechanism for the conversion of C2H4 into C3H6. According to our previous experimental results on selective C3H6 production by the conversion of C2H4, pentenes, and hexenes, the selective β-scission of specific carbocations should produce C3H6 with a high selectivity. On the basis of that concept, we hypothesized that a zeolite can select for specific carbocations on the basis of the zeolite cavity volume. This hypothesis may be applicable to the conversion of any alkenes except C2H4 for the production of C3H6. In the conversion of C2H4 over H+-exchanged zeolites, the initial C−C bond formation proceeds by the dimerization of C2H4 to produce n-butenes (1-butene and 2-butenes). In this work, the conversion of 1-butene was carried out using various zeolites with different cavity volumes to verify the above hypothesis. Thus, the effects of the zeolite pore structure and cavity volume on the selectivity for C3H6 from 1-butene were examined, and the reaction mechanism for the production of C3H6 was investigated. The carbocations that participate in the production of C3H6 at the initial stage were elucidated, and the shapes of the carbocations in the zeolite cavity were determined. The key role of the zeolite cavity volume in the selective production of C3H6 was revealed, while the reaction mechanism for the conversion of 1-butene into C3H6 were also discussed by comparing the reaction mechanism for the conversion of C2H4 into C3H6.

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EXPERIMENTAL SECTION

(1). Zeolite Synthesis and Identification. Various zeolites (aluminosilicate and aluminophosphate molecular sieves) were hydrothermally synthesized using various structure-directing agents (SDA) in Teflon-lined 50−300 mL autoclaves at approximately 450 K, according to procedures described in the literature. The physicochemical properties of these zeolites, such as the chemical mole fraction of Si or Si/Al and the 1H chemical shifts due to acidic protons, are summarized in Table 1. To confirm the structures of the synthesized materials, powder X-ray diffraction (XRD) patterns were obtained using a Rigaku Denki Multi Flex diffractometer with Cu Kα radiation at 40 kV and 40 mA, at a scan rate of 0.5° (2θ) per minute. The 5183


5184

b

CON AFI FAU

28 0.51 2.9

0.015 37 12 9.2 9.3

0.024 0.028 30 39 9.3 12 16 24 60

31 0.067 0.095 0.077 0.095 50 32 251 0.10 7.0 3.4 1.0

25

Si/Al or Si / (Si + Al+ P)

21  4.2

4.0 3.7, 4.8 3.8, 4.7

 4.1 4.2 4.0 4.1

3.7, 4.8 3.7, 4.6 4.2 4.1 4.1 4.2 4.1 4.1 3.8

  36 105 13 17 19 28 79  19 13 9.0 8.6

4.1 3.9, 3.7 3.8 3.8 3.8, 4.1 4.2 4.1 4.3 3.8, 3.2 4.1  3.8

3.4

chemical shift/ppm

H MAS NMR

1

48     29 55 235  5.1 4.8 1.0

22

Si/Al

Si MAS NMRc

29

100  68

 96 82 76 100

  100 100 86 100 100 100 94

100     94 100 92  99 97 

93

NH4+ exchange degreed/%

c

tapered-end quadragular prism rombohedron, 3 octahedron, 0.5∼2

hexagonal column, 0.3 × 5 rombohedron, 0.5 irregular shape, 0.5∼1 spherulites, 0.2 cuboid, 0.2

cuboids, 3∼4 rectangular prism, 0.2∼0.5 rodlike crystal, 0.05 × 0.3 rodlike crystal, 0.2 × 2 overlapped platelets, 1∼2 rice kernel shaped, 0.5 × 1 rectangular rod, 1∼2 rectangular prism, 3 overlapped platelets, 2∼3

rectangular platelets, 0.5 cuboids, 0.2 rombohedron, 2∼5 cuboids, 0.8 cuboids, 2.5 cuboids, 4∼5 rombohedron, 1∼2 needles, 2∼3 overlapped cuboids, 0.3 small spherulites, 0.3 cuboids, 0.3 cuboids, 3∼4

small spherulites, 0.2

639 215 840

218 324 648 516 495

218 175 312 320 369 385 390 442 499

229 563 567 701 736 631 338 528 692 513 583 524

29

surface area/ crystal shape, average size/μm m2 g−1

7.39 8.24 11.2

5.68 6.02 6.62 6.64 7.03

5.37 5.58 5.65 6.13 6.25 6.30 6.98 7.66 9.63

6.19 6.98 7.04 7.31 7.31 7.31 7.60 8.12 8.76 10.0 10.61 11.1

5.90

Die/Å

× × × × × × × × ×

× × × × × × × × × × × × 7.0 6.5 5.7 5.2 4.2 4.8 × 3.5 5.3 5.5 × 5.1 4.8 5.4 4.0

3.9 5.1 4.8 3.8 3.8 3.8 4.4 4.1 3.9 4.4 3.9 4.1

29

5.3 × 5.4 5.9 × 6.0 6.7 × 6.6 5.6 × 5.6 6.5 × 7.0 (8MR 2.6 × 5.7) 6.8 × 6.4 (10MR 5.8 × 5.2) (10MR 5.2 × 5.2) 5.1 × 4.5 7.0 × 5.9 7.0 × 6.4 7.3 × 7.3 7.4 × 7.4

4.1 4.0 4.6 4.5 5.4 5.6 5.7 5.3 5.5

3.8 3.6 3.8 3.8 3.8 3.8 3.6 3.8 3.9 3.6 3.9 4.1



crystallographic pore size/Å

pore structure characteristics

d

5.54 7.36 7.29

5.43 5.62 6.07 6.39 4.94

5.01 4.57 5.05 5.01 4.63 4.64 5.01 5.13 4.86

3.97 3.36 3.47 3.66 3.66 3.66 3.59 4.08 4.04 3.83 3.98 4.15

1.87

3 1 3

1 1 3 3 3

1 1 1 1 2 3 2 3 2

1 3 2 3 3 3 2 2 3 2 3 3

0

dmaxf/Å nDg

i

25 26

24

i

i

22 23

19 20 21

i

i

15 16 17 18

i

5 6 7 8 8 9 10 11 12 13 14

4

ref

Catalyst number. Chemical analysis by inductively coupled plasma (ICP) atomic emission spectroscopy. Determined Si/Al ratio in the zeolite lattice by Si MAS NMR measurements. Determined by Si MAS NMR measurements and the amounts of Na+. eMaximum included sphere diameter. fMaximum free sphere diameter. gPore dimensions. hMembered ring. iObtained from Tosoh Ltd.

29

a

AFO AEL TON MTT FER MFI MES MEL MWW

SAPO-41 SAPO-11 ZSM-22 ZSM-23 Ferrierite ZSM-5 Nu-87 ZSM-11 MCM-22

CIT-1 SAPO-5 Y

MTF ERI LEV CHA CHA CHA DDR RTH SAV UFI KFI LTA

MCM-35 SAPO-17 SAPO-35 SAPO-34 SAPO-34 SSZ-13 Sigma-1 RUB-13 SAPOSTA-7 UZM-5 ZK-5 Ca-A

ATO MTW BZA MOR MSE

RUT

Nu-1

27 28 29

zeolite

SAPO-31 ZSM-12 β Mordenite MCM-68

h

6-MR 1 8-MRh 2 3 4 5-a 5-b 6 7 8 9 10 11 12 10-MRh 13 14 15 16 17 18 19 20 21 12-MRh 22 23 24 25 26

no.a

structure code

ICPb

Table 1. Synthesized Zeolite Materials and Their Physicochemical Properties

The Journal of Physical Chemistry C Article



Article

was 5.0 kHz at each prescribed temperature. To reduce 1H background signals from the probe material, the DEPTH2 pulse sequence was used with a π/2 pulse width of 3.6 μs and a recycle delay of 60 s.28 (5). Surface Area Measurements. The surface areas of the zeolite catalysts, from which organic materials such as structuredirecting agents (SDA) were removed by calcination, were obtained using a dynamic N2 adsorption technique and a BELSORPmini automatic gas adsorption apparatus. The isotherms of nitrogen adsorption were measured at 77 K, using the same instrument. The surface areas of various zeolites are shown in Table 1. (6). Reaction Procedure for 1-Butene Conversion and Cracking of n-Butane. NH4+-exchanged aluminosilicate molecular sieves or silicoaluminophosphate molecular sieves containing an SDA were diluted with SiO2, which has a much lower surface area (6.2 m2 g−1) than the catalysts. The mixture was pressed, crushed, and sorted into grains by 16−32 meshes. The grains were packed into a reactor of silica tubing (10 or 6 mm i.d.) in a vertical furnace and heated under an air stream at a heating rate of 0.5 K min−1 from room temperature to a prescribed temperature. The final calcination temperature was 723 K for aluminosilicate molecular sieves such as ZSM-5 and 913 K for silicoaluminophosphate sieves such as SAPO-34. The catalyst was then calcined at the same temperature for 3 h. After calcination of the catalyst, the conversion of 1-butene was carried out in a continuous flow reactor at atmospheric pressure. 1-Butene (99.2% pure) containing trans-2-butene (0.4%) and cis-2-butene (0.4%) was fed with helium into the reactor through a mass flow meter, while 1-octene was injected into the preheating zone of the reactor using a motor-driven syringe. The effluent gas was withdrawn periodically and analyzed by gas chromatography. Helium served as both the carrier gas and an internal standard for determination of the amount of C2H4 by gas chromatography using an activated carbon column and a thermal conductivity detector. The amounts of methane, ethylene, ethane, propylene, and propane were determined using a Porapak Q column, and the amounts of hydrocarbons with more than three carbon atoms were determined using an OV-101 column. Analysis for butenes, such as 2-methyl propene, was performed using a Unicarbon A400 column. The hydrocarbon distributions were expressed on a carbon-number basis, excluding the coke remaining in the reactor. To examine the acid strength of each catalyst, the rate of cracking of n-butane, which is a typical acid-catalyzed reaction, was also measured using a continuous flow reactor. The reaction temperature was 803 K, and the pressure of n-butane was 3.3 kPa. (7). Estimation of Zeolite Entrance Pore Diameter. Each zeolite has a different pore window that is an open pore shape. This means that each zeolite has a characteristic pore diameter. Here, on the basis of the report by Foster and co-workers, we adopted the maximum free sphere diameter among the largestfree-shares to pass through a window along the crystallographic a, b, or c axis,29 which were da, db, and dc, respectively. Here, dmax, which was the largest of these, was defined as the maximum free sphere diameter. For example, SAPO-34 has a da, db, and dc, of 3.66, 3.66, and 3.66 Å, respectively, so the maximum free sphere diameter dmax was 3.66 Å. This diameter is almost the same as the crystallographic pore diameter (3.8 × 3.8 Å), as shown in Table 1. In the case of Y-zeolite, dmax is 7.29 Å, which is also almost the same as the crystallographic pore diameter of 7.4 Å. However, the dmax of ZSM-5 is 4.64 Å, which is slightly different from the crystallographic pore diameter of 5.3 Å. These results suggest that it is reasonable to consider dmax as the pore diameter in the zeolites used in this work.

(8). Estimation of Zeolite Cavity Volume. Each zeolite also has an individual cavity, whose shape depends on the zeolite. Delaunay triangulation is a technique used in computational geometry to determine the empty circumspheres of a set of points. Foster and co-workers reported that Delaunary triangulation provides a powerful computer-automated tool for the determination of the largest sphere that can be included in a zeolite framework.29,30 In this work, the diameter of the maximum included sphere in a zeolite framework is expressed as Di, in angstroms (Å). We can estimate the approximate pore volumes of various zeolites using the value of Di (cavity volume = 4/3π(Di/2)3). In ref 30, Treacy and Foster compared the pore volumes estimated from Di with the published pore volume data, which were determined by measuring the adsorption of rare earth gases such as Ar and Kr, and generally found that their calculations agreed with the experimental results. We therefore expect the estimated pore volumes to be reasonably close to the actual pore volumes of zeolites. We also determined the pore volume of several zeolites, such as Y-type zeolite, by measuring the amount of adsorbed Ar at 87 K, and observed similar results to those reported in ref 30. (9). Preparation of Zeolite with a Small Particle Size. We have reported the effect of the size of individual SAPO-34 crystals on C3H6 selectivity for the conversion of C2H4.31 In that report, particle size (the size of the crystal grains) of SAPO-34, which appeared to be cubic, was defined as the side length of a SAPO-34 cube. A particle size smaller than ca. 4 μm had no influence on the selectivity for C3H6. In this work, in the conversion of 1-butene, various zeolites with particle sizes smaller than 4 μm were also used as catalysts to minimize the influence of particle size on the selectivity for C3H6 and the rate of C3H6 production. The particle sizes and zeolite shapes are summarized in Table 1. (10). Shape of Octyl Carbocations Estimated Using an Ab Inito MO Method. A cluster model of CHA zeolite containing 186 atoms (Si36O90H36) was obtained from the DMol3 program from Accerlys Ltd., USA. CHA zeolites are SSZ-13 and SAPO-34. The dangling bonds of border silicon atoms were terminated with hydrogen atoms. A Brönsted acid site of CHA zeolite was generated by replacing one Si atom with one Al atom and putting a H atom on the O atom adjacent to the Al atom. An octane molecule in the zeolite cavity was prostrated by the hydrogen and bonded to the oxygen at the Brönsted acid site. The octyl carbocations were fixed at the positions of the Si atoms of CHA zeolite. The HF/3-21G calculations were performed with the Gaussian 03 program package.32 The carbocation volumes, which were calculated using density functional theory (DFT), were reported in ref 3.

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RESULTS AND DISCUSSION (1). Conversion of 1-Butene Using Various Zeolites and Product Distribution. To examine the catalytic performance of 8-, 10-, and 12-MR zeolites shown in Table 1, the conversion of 1-butene was carried out at 673 K using these zeolites as catalysts. In this paper, the conversion of 1-butene is defined as how many moles of butene were converted into various hydrocarbons other than butenes, such as 2-butenes. Thus, the conversion is expressed as “the conversion of butene” when 1-butene is used as a reactant. To compare the distribution of reaction products and to examine the rates of butene conversion and C3H6 production, the contact time (W/ F) was controlled to achieve ∼10% conversion of butene at 1 h time-on-stream. Here, W and F denote the catalyst weight (g) and the total molar flow (mol h−1), respectively. The results are summarized in Table 2. 5185


7.8

C4H8 conversion/%

5186

C4H8 conversion C3H6 production n-butane crackingb

Olefins C2H4 C3H6 C5H10 C6H12 C7H14 C8H16 Paraffins CH4 C2H6 C3H8 C4H10 C5H12 C6H14 C7H16 C8H18 C9+ Aromatics Diene C4H6

1.87

1.8 2.8 × 10−1 1.9 × 10−1 0.16

1.0 × 10−2 5.1 × 10−3 0.067

0.2 0.3 0.1 3.9  0.0 0.3 0.9  

0.9 0.3 0.8 12.0 0.9 0.0 0.0 0.8 5.3 6.9

1.7

10.4 51.6 26.9 0.9 0.6 2.1

3.1 35.9 29.1 1.1 0.0 1.2

8.0

3.97

6.19

5.90

Di/Å

dmax/Å

2

MCM-35

6-MR

1

Nu-1

catalyst

Catalyst No.

8.3 × 10−1 6.1 × 10−1 0.13

2.6

0.7  0.0 2.8      

5.6 55.2 33.1   

6.8

3.36

6.98

SAPO-17

3

2.7 × 10−1 1.7 × 10−1 0.17

2.7

0.2   0.1      

6.5 48.3 42.2   

5.9

3.47

7.04

SAPO-35

4

8.7 6.7 0.29

0.1

0.1  1.9 3.0 0.1 0.2    

3.9 64.6 24.1 2.0  

12.1

3.66

7.31

SAPO34

5a

5.5

3.66

7.31

SSZ-13

6

0.2 0.1 3.5 4.1 0.6   0.1  

7.2 57.0 25.1 1.2  0.6

0.1 0.3 Rate/mmol g−1 h−1 1.1 × 10 5.7 8.8 4.2 0.33 0.24

0.1  2.2 2.8 0.1 0.3    

3.8 65.2 24.0 1.4  

Distribution/C-atom %

10.9

3.66

7.31

SAPO-34

5b

Table 2. Catalytic Performance of Various Zeolites in the Conversion of Butenea

2.7 2.1 0.45

0.2

0.2 0.1 1.8 0.8 0.1  0.1   

10.6 58.9 26.7 0.3 0.2 

11.0

3.59

7.60

Sigma-1

8-MR

7

2.8 × 10−2 1.4 × 10−2 0.073

2.0

0.5 0.5 5.3 0.3  0.8 0.3 0.4  

16.8 38.9 6.1 0.6 0.5 27.0

4.3

4.08

8.12

RUB-13

8

1.9 1.6 0.15

0.3

0.2  5.1 0.4      

8.6 59.0 26.4   

4.8

4.04

8.76

SAPOSTA-7

9

4.0 2.1 0.34

0.3

0.1 0.1 0.3 3.7 0.2 1.0 1.3 0.9  0.5

0.6 39.9 42.8 4.9 1.4 1.8

11.4

3.83

10.0

UZM-5

10

7.6 4.7 2.5

1.7

0.3   5.4 3.8     

6.2 46.0 36.6   

4.4

3.98

10.61

ZK-5

11

3.2 × 10−3 1.2 × 10−3 0.043

5.8

6.6 5.8 4.5 12.2 0.1 1.3 4.6 4.9  

6.9 29.3 15.0 1.7 0.4 0.9

3.9

4.15

11.1

Ca-A

12



5187

a

0.2

7.1 × 10−1

2.9 × 10−1

0.039

0.8

2.5 × 10−1

1.2 × 10−1

0.046

3.2

2.2 × 10

4.2 × 10

1.0

6.7

17

5.2

3.1

4.9

0.2

0.2 0.1 0.8 9.5 0.1  0.2 0.1  

7.6 47.8 28.4 0.9 2.5 1.6

5.3

4.63

6.25

Ferrierite

10-MR

1.4 × 10

0.6

0.1  0.4 6.6 10.7 0.1 0.3 0.3  

  0.8 16.2 0.8 0.9 0.6 0.2   0.9

1.3 36.7 30.7 1.5 2.7 8.0

4.9

5.01

6.13

ZSM-23

16

1.2 38.8 33.7  4.1 1.8

4.8

5.05

5.65

ZSM-22

15

6.3

2.1

4.0

1.3

0.1   1.7  0.1  0.1  

0.7 42.0 49.9  1.3 2.8

6.3

4.64

6.30

ZSM-5

18

3.8

4.86

9.63

MCM-22

21

0.1   1.7      

0.6 46.1 51.1    0.1  0.1 5.3 0.1 0.4 0.3 0.6  0.7

0.9 36.8 45.2 1.2 0.9 6.6

Distribution/C-atom %

11.3

5.13

7.66

ZSM-11

20

3.6

6.8 × 10

4.4

5.1 × 102

4.6

5.6 × 10

0.4 0.8 Rate/mmol h−1 g−1 1.2 × 102 8.8 × 102 1.0 × 102

0.2

   2.7  0.3 0.1 0.2  

0.2 43.5 44.3 5.3 0.2 3.0

10.1

5.01

6.08

Nu-87

19

0.095

1.7 × 10−1

4.2 × 10−1

0.2

0.1 0.1  7.0   0.2 0.4 2.3 11.2

3.2 30.7 29.2 3.3 3.1 9.0

10.8

5.43

5.68

SAPO-31

22

0.35

3.8 × 10

7.7 × 10

0.3

  0.2 6.3 1.2   0.1 0.2 1.1

0.3 37.1 45.1 2.8 0.4 4.9

9.1

5.62

6.02

ZSM-12

23

2.0 × 102 1.9

25

2.4

1.4

2.8

1.6

0.1 1.5 1.5 7.7 0.5   0.1 0.0 0.4

1.5 40.2 41.9 2.7  0.3

9.3

6.39

6.64

26

3.2

8.9

1.7 × 10

0.1

  0.2 3.5 0.4     0.7

0.6 46.2 46.8 1.5  

10.3

4.94

7.03

MCM-68

12-MR Mordenite

3.0 × 102

0.1

  0.3 2.6 0.3 0.2   0.1 0.2

0.9 48.4 41.1 1.8 1.2 2.8

10.1

6.07

6.62

β

24

1-Butene conversion: reaction temperature, 673 K; 1-butene pressure, 3.3 kPa. bn-Butane conversion: reaction temperature, 803 K; n-butane pressure, 3.3 kPa.

C4H8 conversion C3H6 production n-butane crackingb

11.9

0.2   8.9  0.1 0.4 0.3  10.7

10.3

C4H8 conversion/%

4.57

0.4 0.1  9.6 0.4 0.1 0.1 0.2  22.5

5.01

dmax/Å

5.58

SAPO-11

1.0 32.2 38.7 2.2 1.2 3.9

5.37

Di/Å

14

0.5 35.9 23.1 2.8 1.3 2.2

SAPO-41

catalyst

Olefins C2H4 C3H6 C5H10 C6H12 C7H14 C8H16 Paraffins CH4 C2H6 C3H8 C4H10 C5H12 C6H14 C7H16 C8H18 C9 + Aromatics Diene C4H6

13

Catalyst No.

Table 2. continued

0.55

2.3 × 10−1

6.9 × 10−1

3.2

 0.1 0.8 21.7   0.1 0.2 0.7 4.4

1.5 35.3 23.4 1.3 1.1 6.2

8.6

5.54

7.39

CIT-1

27

0.17

1.4 × 10−2

9.0 × 10−2

0.1

0.1 0.1 2.5 26.9 1.9 1.5 0.6 0.6 0.5 4.2

2.4 39.8 16.6 1.0 0.1 1.1

5.5

7.36

8.24

SAPO-5

28

0.88

1.7

6.3

0.1

0.6 1.3  37.4 4.5 0.8 0.1 1.2 1.3 11.4

0.6 20.4 19.1 1.1  0.1

10.9

7.29

11.2

Y

29




Article

The product distribution was strongly dependent on the zeolite. When 8-MR zeolites were used, the products were mainly aliphatic hydrocarbons, while 12-MR zeolites produced both aromatic and aliphatic hydrocarbons. The 8-MR SAPO-34 (No. 5a and 5b) showed the highest selectivity for C3H6 of 64.6 and 65.2 C-atom %, respectively, while the 10-MR Ferrierite (No. 17) and 12-MR β (No. 24) showed the highest C3H6 selectivity of each ring size of zeolites: 47.8 C-atom % and 48.4 C-atom %, respectively. The product distributions of 8-, 10-, and 12-MR zeolites were compared with that of Nu-1, which has a 6-MR entrance pore that is so narrow that neither 1-butene nor many other hydrocarbons can pass through it. The conversion of butene proceeds on the external surface of Nu-1 (No. 1). This indicates that the pores of zeolites can be effectively used to produce C3H6, and the C3H6 selectivity was over 35.9 C-atom % using Nu-1. The rate of butene conversion and that of C3H6 production were also strongly dependent on the zeolite. ZSM-11 (No. 20) had the highest rates of butene conversion and C3H6 production: 8.8 × 102 and 5.1 × 102 mmol g−1 h−1, respectively. To examine the effect of acid strength on the rate of C3H6 production, the rate of C3H6 production in the conversion of butene was plotted against the rate of n-butane cracking, as shown in Figure 1. n-Butane cracking was an adequate test reaction to

Figure 2. Selectivity for C3H6 in the conversion of butene over various zeolites plotted against dmax. Catalyst No. is shown in Table 1. (Δ) 6MR, (○) 8-MR, (□) 10-MR, and (●) 12-MR zeolites. 1-Butene conversion: reaction temperature, 673 K; 1-butene pressure, 3.3 kPa.

roughly decreased in the order of 8-, 10-, and 12-MR zeolites, whose crystallographic pore diameter and dmax are larger than the kinetic diameters of C3H6 and 1-butene. As shown in Figure 2, 8-MR zeolites did not always give C3H6 with a high selectivity, and the selectivity drastically changed over a narrow range of dmax, even though the entrance pore diameters were almost the same as the kinetic diameter of C3H6. Furthermore, 10- and 12-MR zeolites, whose pore diameters are much larger than the kinetic diameter of C3H6, did not always give C3H6 with a low selectivity. The difference in C3H6 selectivity over 8-, 10-, and 12-MR zeolites cannot be fully explained by the difference in the zeolite entrance pore diameters. Therefore, the C3H6 selectivity cannot be simply explained by product shape selectivity. To examine the effect of acid strength on the selectivity for C3H6 in the conversion of 1-butene, the selectivity for C3H6 was plotted against the rate of n-butane cracking, though the figure was not shown. No good correlation between the acid strength of the zeolites and C3H6 selectivity was observed, indicating that the C3H6 selectivity did not depend on the acid strength. (3). Effect of Zeolite Cavity Volume on C 3 H 6 Selectivity. As reported previously, the volume of the zeolite cavity plays a key role in the selective production of C3H6 in the conversion of C2H4, pentenes, and hexenes.3 To examine the effect of Di on the C3H6 selectivity in the conversion of 1butene, the selectivity shown in Figure 2 was plotted against the Di of 8-, 10-, and 12-MR zeolites, in Figures 3(a), (b), and (c), respectively. The selectivity for C3H6 strongly depended on the Di of the zeolites in the conversion of 1-butene as well. The highest selectivities for C3H6 over 8-, 10-, and 12-MR zeolites were 64.6 and 65.2 C-atom % (SAPO-34), 47.8 C-atom % (Ferrierite), and 48.4 C-atom % (β), with Di values of 7.31, 6.25, and 6.62 Å, respectively. Therefore, the highest C3H6 selectivity of 8-, 10-, and 12-MR zeolites was observed at a Di of approximately 7 Å, indicating that the C3H6 selectivity depends

Figure 1. Rate of C3H6 production in the conversion of butene over various zeolites plotted against the rate of n-butane cracking. Catalyst No. is shown in Table 1. (Δ) 6-MR, (○) 8-MR, (□) 10-MR, and (●) 12-MR zeolites. 1-Butene conversion: reaction temperature, 673 K; 1butene pressure, 3.3 kPa. n-Butane cracking: reaction temperature, 803 K; n-butane pressure, 3.3 kPa.

evaluate the acid strength of the catalyst.33 As shown in Figure 1, the rates of C3H6 production using various zeolites had a tendency to increase with increasing n-butane cracking rate, indicating that the rate of C3H6 production was higher with stronger acids. We have already reported that the rate of C3H6 production in the conversion of C2H4 has a tendency to increase with increasing n-butane cracking rate.34 (2). Effect of Zeolite Entrance Pore Diameter on C3H6 Selectivity. To examine the effect of the zeolite entrance pore diameter on C3H6 selectivity, the selectivity for C3H6 was plotted against dmax, which is a measure of the zeolite entrance pore diameter, as shown in Figure 2. The selectivity for C3H6 5188



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on the volume of the zeolite cavity but not on the entrance pore diameter. We previously reported that the highest C3H6 selectivity with 8-, 10-, and 12-MR zeolites was also observed at a Di of about 7 Å in the conversion of C2H4 into C3H6.3 The similar Di dependence on C3H6 selectivity in the conversions of C2H4 and 1-butene suggests that β-scission of the same carbocations produces C3H6 in both conversions. This means that the selective production of C3H6 could be achieved by involving only those specific carbocations, which have a volume similar to that of the zeolite cavity (about 180 Å3), which corresponds to a Di of 7 Å. DFT calculations indicated that the volumes of C8H17+ were 152−223 Å3, which corresponds to a 6.6−7.5 Å sphere diameter. Therefore, the specific carbocation could be C8H17+. In our previous work, we proposed that the β-scission of C8H17+ resulted in the selective production of C3H6 in the conversion of C2H4, but we did not determine which specific C8H17+ species was involved.3 (4). Reaction Mechanism for the Conversion of 1Butene over MCM-68 at the Initial Stage. To investigate the reaction mechanism for C3H6 production in the conversion of butene at the initial stage, the product distributions were examined at a low conversion of butene using MCM-68 as a catalyst. MCM-68 is a 12-MR zeolite with a dmax of 4.94 Å, which is much larger than the kinetic diameters of isobutene and 2-methylbutene. The volume of the zeolite cavity (Di = 7.03 Å) was almost the same as that of the octyl carbocations. To determine the initial reaction products, the product distribution was plotted against the conversion of butene, when the reaction was carried out at 673 K with a 1-butene partial pressure of 3.3 kPa. The product distributions except for butenes and the fraction of butenes are shown as a function of butene conversion in Figures 4(a) and (b), respectively. At low butene conversion, the products were octenes, pentenes, C3H6, hexenes, and C2H4 along with isomerized butenes. The distribution of octenes drastically decreased with increasing conversion of butene, while the distributions of C3H6 and C5H10 increased. At butene conversions below about 1%, similar amounts of C3H6 and C5H10 were produced. The product distributions of C2H4 and hexenes (C6H12) were very low compared with those of C3H6 and C5H10. These results show that the initial reaction product is C8H16, which is then cracked into C3H6 and C5H10 at the initial stage in the MCM-68 cavity. To determine the isomerized butenes, their fractions were plotted against the conversion of butene, as shown in Figure 4(b). By extrapolating the fraction of butenes along the conversion curves to zero conversion, the fractions of butenes at the initial stage were obtained. At the initial stage, the butenes were 2-butenes (trans- and cis-2-butenes) and 1-butene, and isobutene was not observed. This indicates that octyl carbocations X and Y could be produced by the dimerization of butenes other than isobutene through catalysis by acidic protons (Brönsted acid sites) on zeolites, as shown in Scheme 1. The octenes produced from X would be mainly observed as 2-, 3-, and 4-methylheptenes, while the octenes produced from Y would be mainly observed as 2,3-, 2,5-, and 2,4-dimethylhexenes. The details of C3H6 production from the octyl carbocations X and Y will be discussed below. To estimate the octene fraction at the initial stage, the fractions of octenes were plotted against the conversion of butene, as shown in Figures 5(a) and (b). Both methylheptenes and

Figure 3. Effect of Di on C3H6 selectivity over (a) 8-MR and 6-MR zeolites, (b) 10-MR zeolites, and (c) 12-MR zeolites in the conversion of butene. Catalyst number: see Table 1. 1-Butene conversion: reaction temperature, 673 K; 1-butene pressure, 3.3 kPa. 5189



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Figure 5. Effect of butene conversion on the fraction of octene isomers using MCM-68: (●) 2,3-dimethylhexenes + 2,5-dimethylhexenes, (○) n-octenes, (□) 2,4-dimethylhexenes, (■) 2-methylheptenes, (Δ) 3-methylheptenes, (◊) 4-methylheptenes. 1-Butene conversion: reaction temperature, 673 K; 1-butene pressure, 3.3 kPa.

Figure 4. Effect of butene conversion on the product distribution using MCM-68: (Δ) C8H16, (○) C3H6, (●) C5H10, (□) C6H12, and (■) C2H4. 1-Butene conversion: reaction temperature, 673 K; 1butene pressure, 3.3 kPa.

hexenes + 2,5-dimethylhexenes), 0.40; 2,4-dimethylhexenes, 0.05; 3-methylheptenes, 0.28; 2-methylheptenes, 0.15; and 4-methylheptenes, 0.12, indicating that the rates of isomerization of X and Y into these octyl carbocations are much higher than those of C3H6 production by the β-scission of octyl carbocations. Here, the key point was to estimate which octyl carbocations can produce C3H6. It is difficult to produce primary pentyl carbocations and primary propyl carbocations by the β-scission of octyl carbocations. Therefore, the following β-scissions of octyl carbocations can produce secondary and tertiary pentyl carbocations and C3H6, as expressed by eqs 4−7.

Scheme 1. Reaction Scheme for the Production of the Key Octyl Carbocations X and Y by the Dimerization of Butenes

dimethylhexenes were observed, showing that both X and Y were generated from butenes. At zero conversion of butene in Figures 5(a) and (b), the fractions of the following octenes were not zero: (2,3-dimethyl5190



Article

isomerization by the protonated cyclopropane mechanism are slower than the rates of the hydride transfer and transmethylation reactions, at least within the zeolite cavity. From this point of view, the isomerization between methylheptyl carbocations and dimethylhexyl carbocations by the protonated cyclopropane mechanism shown in Scheme 2 would also proceed unfavorably. Therefore, methylheptenes and dimethylhexenes are mainly produced from X and Y, respectively. Very little production of 3,4-dimethylhexenes from Y was observed at low butane conversion, as shown in Figure 5(b). The fractions of 2,3- and 2,5-dimethylhexenes quickly decreased with increasing up to 1% butene conversion. In this range, equal amounts of C3H6 and C5H10 were produced. These results suggest that the rates of isomerization of Y to 2,4dimethylhexyl carbocation (R) and those of R to 2,3-dimethylhexyl carbocations (I, I1) and/or 2,5-dimethylhexyl carbocation (J) are much faster than the rates of isomerization of X to P and Q. Therefore, C3H6 is mainly produced through I, I1, and/or J from Y during the initial reaction stage. When the cracking of 1-octene was carried out using MCM68 at 673 K, butenes were produced along with pentenes and C3H6. For example, the distributions of butenes, pentenes, and C3H6 was 63.3, 15.3, and 17.7 mol %, respectively, at 24.6% conversion of 1-octene. Therefore, the β-scission of octyl carbocations can also produce butenes. The following reactions are possible.

3-Methylheptenes and 2,3-dimethylhexenes produced from Q and I, respectively, were observed in the effluent octenes from the reactor, while 3-ethylhexenes and 3,3-dimethylhexenes produced from L and S, respectively, were not observed (see Figure 5). Therefore, the β-scission of Q and I could have proceeded to produce C3H6 and C5H10 as expressed with eqs 4 and 6. The β-scission of octyl carbocations to produce pentenes and secondary propyl carbocations are as follows.

However, only 2-methylheptenes and 2,5-, 2,4-, and 2,3dimethylhexenes produced from P,J, R, and I1, respectively, were observed in the effluent octenes from the reactor. Therefore, the production of C 5 H 10 and secondary propyl carbocations could have proceeded by the β-scission of P, J, R, and I1, as expressed in eqs 8, 9, 10, and 11, respectively. These results indicate that the β-scission of methylheptyl carbocations P and Q and dimethylhexyl carbocations I, I1, J, and R could proceed to produce C3H6 and C5H10. The shortest isomerization routes to these octyl carbocations, P, Q, R, I, I1, and J, from X and Y are shown in Scheme 2. Thus, methylheptyl carbocation X, generated by the dimerization of butenes, would isomerize to P and Q by hydride transfer and transmethylation reactions. Dimethylhexyl carbocation Y would also isomerize to R, I, I1, and J. Interconversion between methylheptyl carbocations and dimethylhexyl carbocations could proceed by the protonated cyclopropane mechanism. As shown in Figure 5(b), the fraction of n-octenes reached zero during the initial stage, indicating that the rate of isomerization of methylheptyl carbocations to n-octyl carbocations, which is explained by the protonated cyclopropane mechanism, was very slow. This means that the rates of skeletal

However, trimethylpentenes related to W and Z were not observed. Thus, butenes were produced by reactions 15, 16, 17, and 19. Among these, the β-scissions expressed by eqs 15 and 17 produce n-butenes from X and Y, while isobutene was produced by the reactions of 16 and 19. As shown in Figure 4(b), the fraction of isobutene gradually increased with increasing butene conversion. Isobutene could be generated from the octyl carbocations V and R1. (5). Effect of Zeolite Cavity on the Shape of Carbocations. As mentioned above, the zeolite cavity was considered to be spherical, and Di was estimated. The volume of the zeolite cavity was calculated from the estimated Di, being (4/3)π(Di/2)3. On the other hand, the volumes of carbocations were calculated using density functional theory (DFT) in the previous work,3 where we assumed that the shape of 5191



Article

Scheme 2. Reaction Scheme of the Formation of Specific Octyl Carbocations for the Production of C3H6

(6). Reaction Mechanism for the Conversion of C2H4 into C3H6 at the Initial Stage. We have already reported that SAPO-34 shows the highest selectivity for C3H6 in the conversion of C2H4.34 The cavity of SAPO-34 has a Di of 7.31 Å, which is corresponds to almost the same volume as the octyl carbocations. This result made us confident that the octyl carbocations, which act as reaction intermediates to produce C3H6, are the same in the conversions of both C2H4 and butene. To further investigate the reaction mechanism by which C3H6 is produced at the initial stage in the conversion of C2H4, not only the effluent hydrocarbons but also the hydrocarbons remaining in the pores of SAPO-34 were qualitatively and quantitatively analyzed. The entrance pore diameter of SAPO34 is so narrow that branched hydrocarbons produced in the pores should remain there. When the conversion of C2H4 was carried out at 523 K, the effluent hydrocarbons from the reactor were analyzed with varying contact time (W/F g h mol−1). After C2H4 was fed for 10 min time-on-stream, the reaction was rapidly quenched in liquid nitrogen. To determine the amounts of hydrocarbons remaining in the pores of SAPO-34, the samples were removed from the reactor and placed in an HCl solution to dissolve the SAPO-34. Hydrocarbons with more than six carbon atoms could be analyzed with this procedure, but it was difficult to collect pentenes, pentanes (C 5 hydrocarbons), or hydrocarbons lower than C5 because the boiling points of these hydrocarbons are close to or lower than room temperature. Once separated from the catalyst, the hydrocarbons were extracted with diethyl ether, and then the identification of the products and their quantitative analysis was performed using GC-MS and gas chromatography, respectively. The amounts of hydrocarbons effluent from the reactor outlet were determined by gas chromatography. All extracted materials were aliphatic hydrocarbons with six to ten carbon atoms. Among these hydrocarbons, hexenes (C6H12) and octenes (C8H16) were mainly produced at lower W/F, as shown in Table 3, indicating that these olefins were produced initially. The C6H12 and C8H16 contents at very low conversions of C2H4 are also summarized in Table 3.

carbocations such as octyl carbocations in the zeolite cavity was spherical. In this work, we attempted to clarify whether or not the carbocations in the zeolite cavity can be considered spherical. The crucial problem is the shape of octyl carbocations in the zeolite cavity, where their interactions with the wall of the zeolite cavity may enable selective β-scission to produce C3H6. The shapes of octyl carbocations P and Q were selected for investigation since P and Q are key intermediates in the production of C3H6, as shown in Scheme 2. To further compare the shape of these with that of the linear octyl carbocation N, their shapes in both free space and in the cavity of CHA zeolite were calculated. The optimized configurations of P, Q, and N are shown in Figures 6(a), (b), and (c), respectively. In all cases, the carbon chain lengths of carbocations P, Q, and N in the cavity were shorter than in free space, and their shapes were nearly spherical within the cavity. For example, the carbon chain length of P in free space was 8.1 Å but was reduced to 6.5 Å in the cavity, and its shape was nearly spherical with a diameter of about 6.5 Å. Therefore, it is reasonable to consider the octyl carbocations generated by interaction with Brönsted acid sites and bonded to oxygen anions at the Brönsted acid sites to be spherical. According to DFT calculations in our previous work (ref 3), the volumes of the key intermediate octyl carbocations that produce C3H6 were: P (190 Å3), Q (186 Å3), R (190 Å3), I (190 Å3), I1 (190 Å3), and J (189 Å3). These volumes correspond to sphere diameters of 7.1−7.2 Å. As mentioned in Section (3), the 8-, 10-, and 12-MR zeolites that had the maximum selectivities for C3H6 in the conversion of 1-butene had Di values of 7.31 Å (SAPO-34, 8-MR), 6.25 Å (Ferrierite, 10-MR), and 6.62 Å (β, 12-MR). A Di of 6.25−7.31 Å corresponded to spheres with volumes of 128−204 Å3, which is almost the same size as the octyl carbocations. These experimental results indicate that highly selective production of C3H6 from butene can be accomplished by adjusting the volume of the zeolite cavity to match that of the octyl carbocations. 5192



Article

Figure 6. Configurations of octyl carbocations (P, Q, and N) in free space and in the cavity of CHA zeolite shown in (a), (b), and (c), respectively. The green circles are C, and the yellow circles are H in the octyl carbocations. The red circles are O; the purple circles are Al;, and the light brown circles are Si in CHA zeolite.

At a W/F of 0.32 g h mol−1, the conversion of C2H4 was 0.022%, and the hydrocarbon products were mainly C3H6 and C4H8 in the gas phase. The products in the cavity of SAPO-34 were only hexenes, and no octenes were observed. These results show that C3H6 is produced by the β-scission of hexyl carbocations. Under these reaction conditions, the concentration of butenes was very low, compared with that of C2H4, showing that the dimerization of butenes to produce octenes did not occur. However, butenes react with C2H4 to produce hexenes. It is quite reasonable to observe 3-methylpentenes because the reaction of secondary n-butyl carbocations with C2H4 produces 3-methylpentyl carbocations. When W/F increased from 0.32 to 0.80 g h mol−1, the production of octenes, such as 3-methylheptenes and 2,4- and 2,5-dimethylhexenes, began to be observed. This means that the corresponding octyl carbocations X, R, and J should have been produced. In the gas phase, 2-butenes were mainly produced. This suggests that the octyl carbocation Y can be produced by the dimerization of 2-butenes. However, the 3,4dimethylhexenes produced from Y were not observed, indicating that Y was quickly isomerized to R and J, which were observed in the cavity of SAPO-34, as shown in Table 3. This result is in agreement with the observed production of 2,3and 2,5-dimethylhexenes in the conversion of butene at the initial stage, as shown in Figure 5(b). The production of the octyl carbocations resulted in increased C3H6 production. When W/F further increased to 1.2 g h mol−1, the concentration of butenes increased, compared with that at a W/F of 0.80 g h mol−1. This increased the production of X and Y. Thus, Y was isomerized to R, J, I, and I1, while X was isomerized to P and Q. The formation of these carbocations was observed as the production of the corresponding octenes. The

increased amounts of octyl carbocations led to an enhanced rate of C3H6 production. At W/F ratios of 1.8 g h mol−1 and higher, octenes produced from various octyl carbocations other than P, Q, R, J, I, and I1 were also observed. When SAPO-34 was used as a catalyst and the conversion of butene (3.3 kPa) was carried out at 673 K, the rate of butene consumption was 23 mmol g−1 h−1. When the conversion of C2H4 (6.6 kPa) was carried out at 673 K, the rate of C2H4 consumption was 5.0 × 10−2 mmol g−1 h−1, which was about 400 times slower than the butene consumption. Under these reaction conditions, the rate of C3H6 production in the conversion of 1-butene was 21 mmol g−1 h−1, while that in the conversion of C2H4 was 6.0 × 10−3 mmol g−1 h−1. This production rate was 3500 times slower than in the conversion of butene. Furthermore, in the presence of both 1-butene and C2H4, the consumption rate of butene was 23 mmol g−1 h−1, and that of C2H4 was 3.8 mmol g−1 h−1, suggesting that the rate of dimerization of butenes to octenes was higher than that of the reaction of butene with C2H4 to produce hexenes. These results show that, even in the conversion of C2H4, C3H6 becomes mainly produced via octyl carbocation intermediates provided by the dimerization of butenes, as the concentration of butenes increases. (7). Selective Production of C3H6 by the β-Scission of Specific Octyl Carbocations. As mentioned in Section (4), during the initial reaction stage in the conversion of 1-butene using MCM-68, C3H6 is mainly produced by the cracking of octenes such as 2- and 3-methylheptenes and 2,4-dimethylhexenes, especially 2,3- and/or 2,5-dimethylhexenes at butene conversions lower than 1%. Therefore, the volume of the zeolite cavity is almost equal to that of any octyl carbocations, which must be suitable to provide C3H6 by the β-scission of the 5193



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Table 3. Amounts of Hydrocarbons Produced in the Cavity of SAPO-34 in the Conversion of C2H4a

a

Reaction conditions: reaction temperature: 523 K, 10 minutes of time-on-stream, C2H4 pressure: 34 kPa.

volume of the carbocations. Derouane and co-workers proposed that molecules and their direct curved framework environment in zeolites tended to reciprocally optimize their van der Waals interactions35 and that the van der Waals energy was proportional to the heat of sorption of hydrocarbons on zeolites. If this model is applied to the carbocations in a zeolite cavity, the relative van der Waals attractive energy Wr(s) can be calculated by eq 21.

octyl carbocations P, Q, R, and especially I, I1, and/or J. As shown in Figure 7, although the fraction of n-pentenes in C5H10 was almost 0.26 at butene conversions higher than 1%, it approaches 0.5 when the fraction vs conversion curve is extrapolated to zero conversion. This indicates that the production of C3H6 and n-pentene from octyl carbocation I1 is more favorable in the zeolite cavity than the production of C3H6 and isopentene from octyl carbocations I and/or J. When using SAPO-34, the reaction would take place via the same mechanism as with MCM-68. However, the molecular sizes of isobutene and the isopentenes are too large to pass through the SAPO-34 pores. Therefore, the distribution of C3H6 became higher, and SAPO-34 showed the highest selectivity for C3H6 among all of the zeolites examined. (8). Proposed Model for the Selective Production of C3H6: Role of Zeolite Cavity Volume. It was unclear why the selective production of C3H6 could be accomplished by adjusting the volume of the zeolite cavity to accommodate the

Wr(s) = W (s)/W (0) = (1 − (1/2)s)−3 = (1 − (1/2)(d mol /d zeol))−3

(21)

where W(s) and W(0) are the van der Waals energy for the flat surface and the zeolite cavity, respectively. We set s = dmol/dzeol, where dmol is the distance from the point of carbocation to the pore wall of the zeolite and dzeol is the diameter of the zeolite 5194



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diameter of the C6H13+ cation 1, with an estimated diameter of 6.35 Å. To examine this linear relationship between log(TOF) and Wr(s), the values of log(TOF) were plotted against Wr(s), as shown in Figures 8a and 8b. Figure 8a shows the relationship

Figure 7. Effect of butene conversion on the ratio of n-pentene isomers in C5H10 using MCM-68. 1-Butene conversion: reaction temperature, 673 K; 1-butene pressure, 3.3 kPa.

cavity. For example, Wr(s) is 8 when the carbocations completely fill the zeolite cavity because s = 1. To explore the role of the zeolite cavity, the conversion of 1-hexene was selected. This reaction is so simple that the rate of C3H6 production was easily measured. As mentioned in the Introduction, the selectivity for C3H6 depended on the volume of the zeolite cavity, which was estimated from Di. The rate of C3H6 production in hexene (n-C6H12) cracking is given by eq 22. Rate = k[H+][C6H12]in

(22)

Here [C6H12]in refers to the concentration of hexene in the zeolite cavity, while [H+] and k are the concentration of Brönstead acid sites in the zeolite cavity and the rate constant, respectively. Equation 23 can be rewritten using Ka, which is the equilibrium constant for the sorption of hexene as shown in eq 24. Rate = kKa[H+][C6H12]out

(23)

Rate/[H+] = TOF = kKa[C6H12]out

(24)

Here [C6H12]out refers to the concentration of hexene in the outer surface of the zeolite. Equation 23 can be rewritten to express the turnover frequency (TOF), as shown in eq 24. Derouane and co-workers also reported that Wr(s) is proportional to the heat of adsorption of various molecules on the zeolite cavity.35 The heat of adsorption is expressed as Qs

Q s = α1Wr(s)

Figure 8. Relationship between the logarithm of turnover frequency (log(TOF)) of (a) aluminosilicate molecular sieves and (b) aluminophosphate molecular sieves in the conversion of hexene and the relative van der Waals attractive energy Wr(s). Catalyst no. is shown in Table 1. (○) 8-MR, (□) 10-MR, and (●) 12-MR zeolites ((a) aluminosilicate and (b) aluminophosphate molecular sieves). 1-Hexene conversion: reaction temperature, 673 K; 1-hexene pressure, 3.3 kPa.

(25)

where α1 is a constant of proportionality. When the differential heat (−ΔH) is nearly equal to Qs, the following relation is obtained. Q s ≈ RT ln Ka = 2.303RT log Ka

(26)

log(TOF) ≈ α2Wr(s)

(27)

between log(TOF) of the aluminosilicate molecular sieves and Wr(s), while Figure 8b shows the relationship between log(TOF) of the aluminophosphate molecular sieves and Wr(s). In both cases, a linear relationship between log(TOF) and Wr(s) was observed, and the rate of C3H6 production increased upon reaching a dmol/dzeol of 1 (unity). This shows that the van der Waals attraction force depends on the ratio of the volumes of the carbocations formed in the zeolite cavity to

Here α2 is a constant of proportionality. Therefore, eq 27 shows that a linear relationship between log(TOF) and Wr(s) should be observed, if the pressure of hexene is constant. When Wr(s) is calculated, dzeol is equal to Di, and dmol is equal to the 5195



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(16) Schnabel, K. H.; Fricke, R.; Girnus, I.; Jahn, E.; Löffler, E.; Parlitz, B.; Peuker, C. J. Chem. Soc., Faraday Trans. 1991, 3569. (17) Kokotailo, G. T.; Schlenker, J. L.; Dwyer, F. G.; Valyocsik, E. W. Zeolites 1985, 5, 349. (18) Rohrman, A. C. Jr.; LaPierre, R. B.; Schlenker, J. L.; Wood, J. D.; Valyocsik, E. W.; Rubin, M. K.; Higgins, J. B.; Rohrbaugh, W. J. Zeolites 1985, 5, 352. (19) Shannon, M. D.; Casci, J. L.; Cox, P. A.; Andrews, S. J. Nature 1991, 353, 417. (20) Kokotailo, G. T.; Chu, P.; Lawton, S. L.; Meier, W. M. Nature 1978, 275, 119. (21) Leonowicz, M. E.; Lawton, J. A.; Lawton, S. L.; Rubin, M. K. Science 1994, 264, 1910. (22) Abbad, B.; Attou, M.; Kessler, H. Microporous Mesoporous Mater. 1998, 21, 13. (23) LaPierre, R. B.; Rohrman, A. C. Jr.; Schlenker, J. L.; Wood, J. D.; Rubin, M. K.; Rohrbaugh, W. J. Zeolites 1985, 5, 346. (24) Shibata, T.; Suzuki, S.; Kawagoe, H.; Komura, K.; Kubota, Y.; Sugi, Y.; Kim, J. H.; Seo, G. Microporous Mesoporous Mater. 2008, 116, 216. (25) Castaeda, R.; Corma, A.; Forns, V.; Rey, F.; Rius, J. J. Am. Chem. Soc. 2003, 125, 7820. (26) Flanigen, E. M.; Lok, B. M.; Patton, R. L.; Wilson, S. T. Pure Appl. Chem. 1986, 58, 1351. (27) Baba, T.; Komatsu, N.; Ono, Y.; Sugisawa, H. J. Phys. Chem. B 1998, 102, 804. (28) Cory, D. G.; Ritchey, W. M. J. Magn. Reson. 1969, 80, 128. (29) Foster, M. D.; Rivin, I.; Treacy, M. M. J.; Delgado Friedrichs, O. Microporous Mesoporous Mater. 2006, 90, 32. (30) Treacy, M. M. J.; Foster, M. D. Microporous Mesoporous Mater. 2009, 118, 106. (31) Iwase, Y.; Motokura, K.; Koyama, T.; Miyaji, A.; Baba, T. Phys. Chem. Chem. Phys. 2009, 11, 9268. (32) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision E. 01; Gaussian, Inc.: Wallingford, CT, 2004. (33) Rastelli, H. Jr.; Lok, B. M.; Duisman, J. A.; Earls, D. E.; Mullhaupt, J. T. Can. J. Chem. Eng. 1982, 60, 44. (34) Oikawa, H.; Shibata, Y.; Inazu, K.; Iwase, Y.; Murai, K.; Hyodo, S.; Kobayashi, G.; Baba, T. Appl. Catal. A: Gen. 2006, 312, 181. (35) Buchanan, J. S.; Santiesteban, J. G.; Haag, W. O J. Catal. 1996, 158, 279. (36) Derouane, E. G.; Andre, J. M.; Lucas, A. A. J. Catal. 1988, 110, 58.

the volumes of the zeolite cavities themselves. On the basis of this idea, it is natural that the differential heat of adsorption of the carbocations on Brönsted acid sites becomes larger when the carbocation-to-cavity volume ratio reaches 1 (s = 1), and −ΔG (the free energy difference of the adsorption of carbocations) should be larger. Therefore, to produce C3H6, the concentration of carbocations such as hexyl carbocations in the zeolite cavity should be increased by matching the volume of the zeolite cavity to that of the desired carbocations.

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CONCLUSION C3H6 selectivity in the conversion of 1-butene depends on the volume of the zeolite cavity and that of octyl carbocations. When the zeolite cavity has a volume of about 180 Å3 (Di of about 7 Å), the highest C3H6 selectivity was achieved and was independent of the pore structure in 8-, 10-, and 12-MR zeolites. In the initial stages of both conversion reactions, C3H6 is mainly produced by the β-scission of octyl carbocations, which are generated by the dimerization of butenes and subsequent isomerization. Thus, the β-scission of the octyl carbocations P, Q, R, and especially I, I1, and/or J can selectively proceed by closely fitting the carbocation volume to that of the zeolite cavity.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This study was supported by a Grant-in-aid for Scientific Research (A) (no. 21246120), Japan Society for the Promotion of Science, and by the Japan Petroleum Energy Center (JPEC) as a technological development project supported financially by the Ministry of Economy, Trade, and Industry.

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REFERENCES

(1) Csicsery, S. M. J. Catal. 1971, 23, 124. (2) Csicsery, S. M. Zeolites 1984, 4, 202. (3) Koyama, T.; Hayashi, Y.; Horie, H.; Kawauchi, S.; Matsumoto, A.; Iwase, Y.; Sakamoto, Y.; Miyaji, A.; Motokura, K.; Baba, T. Phys. Chem. Chem. Phys. 2010, 12, 2541. (4) Bellussi, G.; Millini, R.; Carati, A.; Maddinelli, G.; Gervasini, A. Zeolites 1990, 10, 642. (5) Rubin, M. K.; Pa, B. C. U. S. Patent 4 981 663 (1990). (6) Valyocsik, E. W.; von Ballmoos, R. U. S. Patent 4 778 780 (1998). (7) Prakash, A. M.; Hartmann, M.; Kevan, L. Chem. Mater. 1998, 10, 932. (8) Lok, B. M.; Messina, C. A.; Patton, R. L.; Gajek, R. T.; Cannan, T. R.; Flanigen, E. M. U. S. Patent 4 440 871 (1984). (9) Zones, S. I.; Van Nordstrand, R. A. Zeolites 1988, 8, 166. (10) Stewart, A.; Johnson, D. W.; Shannon, M. D. Stud. Surf. Sci. Catal. 1988, 37, 57. (11) Vortmann, S.; Marier, B.; Gies, H.; Daniels, P. Microporous Mater 1995, 4, 111. (12) Castro, M.; Garcia, R.; Warrender, S. J.; Slawin, A. M. Z.; Wright, P. A.; Cox, P. A.; Fecant, A.; Mellot-Draznieks, C.; Bats, N. Chem. Commun. 2007, 3470. (13) Blackwell, C. S.; Broach, R. W.; Gatter, M. G.; Holmgren, J. S.; Jan, D. Y.; Lewis, G. J.; Mezza, B. J.; Mezza, T. M.; Miller, M. A.; Moscoso, J. G.; Patton, R. L.; Rohde, L. M.; Schoonover, M. W.; Sinkler, W.; Wilson, B. A.; Wilson, S. T. Angew. Chem., Int. Ed. 2003, 42, 1737. (14) Corbin, D. R.; Pa, W. C.; Schwarz, S.; Del, N. U. S. Patent 5 344 989 (1993). (15) Venkatathri, N.; Srivastava, R. Indian J. Chem. 2004, 43A, 1039. 5196


Shape-Selective Catalysis Determined by the Volume of a Zeolite

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