Characterization of shape selective properties of zeolites via hydroisomerization of n-hexane

Microporous and Mesoporous Materials 164 (2012) 71–81

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Characterization of shape selective properties of zeolites via hydroisomerization of n-hexane C.Y. Chen ⇑, X. Ouyang, S.I. Zones, S.A. Banach, S.A. Elomari, T.M. Davis, A.F. Ojo Chevron Energy Technology Company, Richmond, CA 94802, USA

a r t i c l e

i n f o

Article history: Available online 20 July 2012 In honor of Professor Jens Weitkamp for his 70th birthday Keywords: Zeolites Hydroisomerization of n-hexane Shape selectivities

a b s t r a c t In this work we use the hydroisomerization of n-hexane as a catalytic test reaction to investigate the shape selective properties of a series of zeolites (mordenite, Y, ZSM-5, ZSM-12, TNU-9, SSZ-25, SSZ-26, SSZ-32, SSZ-33, SSZ-56, SSZ-57 and SSZ-75). The yield ratio of mono-branched isomers (2- and 3-methylpentane) to di-branched isomers (2,2- and 2,3-dimethylbutane) and the yield ratio of 2,3- to 2,2dimethylbutane provide a useful tool for characterizing the effective pore sizes of zeolites. The high octane isomer 2,3-dimethylbutane (RON 101.0) can be selectively produced over 2,2-dimethylbutane (RON 91.8) by choosing zeolites (e.g., TNU-9 and SSZ-75) which have the appropriate pore sizes, demonstrating some new examples of the shape selectivities of zeolite catalysis. This catalytic test reaction is complementary to the vapor phase hydrocarbon adsorption for the investigation of the shape selective properties and effective pore sizes of zeolites. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction This paper is dedicated to Prof. Jens Weitkamp in celebration of his 70th birthday. As exemplified by his many publications [1–40] in the past decades, Prof. Weitkamp made many important contributions to the zeolite science and our catalysis community. Here we report our recent studies on the hydroisomerization of n-hexane over a series of zeolites and present some new examples of the shape selectivities of zeolite catalysis to appreciate Prof. Weitkamp’s pioneering work in the area of applying catalytic test reactions for characterizing the shape selective properties and effective pore sizes/volumes of zeolites [12,18,20–23,25–28,32,35]. The principle of catalytic test reactions is as follows: a catalytic test reaction is first investigated over various zeolites with known crystalline structures to establish a correlation between the shape selective properties and the effective pore sizes/volumes of these zeolites. If such a correlation exists, the reaction is subsequently applied to zeolites with unknown crystalline structures in order to formulate an estimate of their effective pore structures on the basis of the shape selective properties determined from this test reaction. As shown in the literature, catalytic test reactions by themselves or in combination with vapor phase hydrocarbon adsorption serve as powerful tools for the characterization of the shape selective properties and effective pore sizes of zeolites and provide useful information for the determination of zeolite structures [12,22,23,25–28,32,35,41–66]. ⇑ Corresponding author. E-mail address: [email protected] (C.Y. Chen). 1387-1811/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2012.07.003

The hydroisomerization of light alkanes is an important industrial process in mineral oil refining [67–69]. In this process, the light normal alkanes (i.e., n-butane, n-pentane and n-hexane) are isomerized to their more branched counterparts in the presence of hydrogen over a bifunctional catalyst containing both acidity (e.g., zeolites) and hydrogenation/dehydrogenation function (e.g., Pt or Pd). Examples of such catalysts are Pt on chlorinated alumina, Pt on zeolite H-mordenite or Pt on sulfated zirconia. The octane number of light alkanes increases with the degree of branching. For example, the RON (Research Octane Number) of n-hexane is only 24.8 while the RON’s of 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane are 73.4, 74.5, 91.8 and 101.0, respectively. The hydroisomerization represents one of the efficient ways to enhance the octane number of gasoline. The hydroisomerization is a reaction controlled by the thermodynamic equilibrium. It is well known and can be also elucidated according to [70] that the equilibrium shifts at higher reaction temperature towards the lower octane isomers (e.g., from dimethylbutanes via methylpentanes to n-hexane). Therefore, it’s desirable to develop a more active catalyst to carry out this reaction at a lower temperature. One task of our research is to find a zeolite catalyst which has high activity and selectivity at low temperature to produce more high-octane components such as 2,2- and 2,3dimethylbutane. The advantage and benefit of a successful zeolite based hydroisomerization catalyst over other catalysts (e.g., Pt/ Al2O3 promoted by chloride) are that it is easy for regeneration and environmentally benign (chloride-free). On the other hand, for another task of the present studies, we investigate the hydroisomerization of light alkanes over various zeolites and make use of the

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shape selective properties of zeolites with this reaction to characterize their structure-property relationship. In this paper we will place our emphasis on the second task and discuss our results from n-hexane hydroisomerization over a series of Pt- or Pd-loaded zeolites. 2. Experimental The zeolites investigated in this work (Table 1) were either synthesized in our laboratory according to the published literature (ZSM-12 [71], TNU-9 [72–75], SSZ-25 [76,77], SSZ-26 [75,78], SSZ-32 [79], SSZ-33 [75,78], SSZ-56 [80], SSZ-57 [60,81,82] and SSZ-75 [58,83,84]) or received from commercial vendors (CBV3024E ZSM-5, CBV-21A mordenite and CBV-720 Y, all from Zeolyst; Ca-A from Fluka). The zeolites powder samples synthesized in our laboratory were calcined at 595 °C in air to remove the organic structure directing agent molecules occluded in their channel systems. Their calcined versions were ion exchanged with aqueous 1 N NH4NO3 solution at 80 °C three times for 4 h each to create their NH4-forms. The three commercial zeolite powder samples (ZSM-5, mordenite and Y) were already in the NH4-form when received. Each zeolite was then ion exchanged with aqueous (NH3)4Pt(NO3)2 or (NH3)4Pd(NO3)2 solution at room temperature for 12 h to load with various amounts of Pt or Pd (e.g., approximately 0.5 wt.% Pt, 0.27 or 0.5 wt.% Pd, 0.25 wt.% Pt plus 0.14 wt.% Pd). The resulting catalysts were subsequently calcined in air at 350 °C. The Pt- or Pd-containing zeolites were then pelletized, crushed and sieved. The 24-42 Tyler mesh (0.35–0.71 mm) particles were used for the catalytic experiments. The reactions were carried out in a flow type fixed bed reactor with pure n-hexane as feed at temperatures between 204 and 343 °C (400–650 °F), pressure of 1480 kPa (200 psig), LHSV (Liquid Hourly Space Velocity) of 1 h 1 and molar H2 to hydrocarbon ratio of 6:1. Prior to each catalytic test reaction, the catalyst was reduced in situ in hydrogen at 350 °C and 1480 kPa for 3 h. Then the reactor temperature was lowered to 204 °C to start the catalytic experiment. Subsequently the reaction was continuously carried out by increasing the temperature incrementally by 5.6 °C (10 °F) toward 343 °C. The reaction products were analyzed with on-line GC equipped with a 60 m long HP-1 capillary column. Each GC analysis took 20 min and all the C1–C6 alkanes were well separated. The amounts of the cycloalkanes and benzene in the products were negligible. For each reaction temperature, the reaction was conducted for at least 1 h in order to acquire at least three on-line

GC data points for each reaction temperature. The results indicate that these catalytic data under each set of conditions were well duplicated. For each catalytic run, we also checked the catalyst stability at its maximum isomer yield as well as re-checked the catalyst performance by returning to certain reaction temperatures. The results from these experiments indicate that no noticeable deactivation occurred with these zeolite catalysts under the conditions applied in this work. The parent zeolites and the prepared Pt- or Pd-zeolite catalysts were characterized via XRD, N2 adsorption and elemental analyses. The Pt- or Pd-zeolite catalysts were also analyzed with H2 chemisorption to determine the Pt or Pd dispersion. 3. Results and discussion Results from XRD and N2 adsorption reveal that the Pt- or Pdloaded catalysts retained the structural features of the parent zeolites. Results from elemental analyses indicate that essentially all the Pd or Pt was exchanged from the solution into the zeolites. The Pt and/or Pd contents studied here were in the range of approximately 0.25–0.6 wt.% Pt and 0.14–0.6 wt.% Pd. The Pt or Pd dispersion was >50% according to H2 chemisorption. As described by Weitkamp et al. [3,33], the rates of hydroisomerization and hydrocracking of alkanes depend strongly on the numbers of carbon atoms in the feed molecules. Based on Refs. [3,33], Fig. 1 depicts various types of b-scissions of alkylcarbenium ions in the hydrocracking of alkanes. Since their relative stability increases in the order of primary < secondary < tertiary alkylcarbenium ions, alkanes with higher numbers of carbon atoms in their molecules undergo more readily cracking reactions via b-scissions which involve tertiary alkylcarbenium ions or even the secondary ones. That is the reason why the C7+ alkanes are not purposely included in the feeds to the hydroisomerization processes in the refineries where the easy cracking of their high octane, multibranched isomers could lead to a significant loss of the liquid yield. In contrast, as explained by Fig. 1, n-hexane can crack only slowly via the secondary carbenium ions, leading to a high selectivity to hydroisomerization. Furthermore, n-hexane produces only four paraffinic isomers of different kinetic dimensions as its hydroisomerization products (i.e., 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane) and provides a relatively simple case for studying the shape selective properties of zeolites which have effective pore sizes close to the molecular ki-

Table 1 Zeolites studied in this work and some of their properties. Zeolite

IZA code

Channel dimensionality

Pore features (Å)

Molar ratio (nSi/nAl)

Pt or Pd loading (wt.%)

ZSM-5 SSZ-32 SSZ-75

MFI MTT STI

3D 1D 1D

15 18 30

0.57 Pd 0.27 Pd 0.52 Pt

TNU-9 SSZ-25 SSZ-57 SSZ-56

TUN MWW ⁄SFV SFS

3D 2D 3D 2D

14 16 25 40

0.56 0.51 0.23 0.53

SSZ-26

CON

3D

17

0.52 Pd

SSZ-33

CON

3D

15

0.53 Pd

ZSM-12 Mordenite

MTW MOR

1D 1D

35 10

0.52 Pt 0.55 Pd

Y A

FAU LTA

3D 3D

10-ring: 5.1  5.5 and 5.3  5.6 10-ring: 4.5  5.2 10-ring: 4.7  5.0 (8-ring: 2.7  5.6) 10-ring: 5.5  5.6, 5.4  5.5 and 5.1  5.5 10-ring: 4.0  5.5 and 4.1  5.1; with 12-ring cages 10-ring: 5.4  5.4 and 5.5  5.6; with 12-ring cages 12-ring: 5.9  8.4 10-ring: 4.8  5.5 12-ring: 6.4  7.0 and 5.9  7.0 10-ring: 4.5  4.5 12-ring: 6.4  7.0 and 5.9  7.0 10-ring: 4.5  4.5 12-ring: 5.6  6.0 12-ring: 6.5  7.0 (8-ring: 2.6  5.7) 12-ring: 7.4 (8-ring: 4.1)

15 1

0.27 Pd 0.50 Pt

Pd Pd Pd Pt

Notes: The 8-ring channels are not accessible for the branched isomers of n-hexane and, therefore, their dimensions are given in parentheses. Pt/A is essentially inactive under the catalytic conditions employed here. The IZA code for SSZ-57 is ⁄SFV where the preceding asterisk (⁄) denotes a framework containing intergrowth.

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C.Y. Chen et al. / Microporous and Mesoporous Materials 164 (2012) 71–81

m<5

m>6

m>7

m>7

m>8

R1

R2

R1

R2

R1

R2

R1

R2

R1

sec.

prim.

sec.

sec.

sec.

tert.

tert.

sec.

tert.

tert.

R2

R1

R2

R1

R2

R1

R2

R1

R2

R1

R2

Fig. 1. b-Scissions of alkylcarbenium ions with different numbers (m) of carbon atoms in the corresponding alkane molecules, after Refs. [3,33].

100

100

90

90 n-Hexane 80

70

70 Isomerization

60

50

40

Isomer Distribution, mol%

Conversion or Yield, mol%

Conversion 80

60

50

40 2-Methylpentane

30

30

20

20

10

10

3-Methylpentane

2,2-Dimethylbutane Cracking

2,3-Dimethylbutane

0 220 230 240 250 260 270 280 290 300 310 320 330 340 350

0 220 230 240 250 260 270 280 290 300 310 320 330 340 350

Temperature, °C

Temperature, °C

Fig. 2. Hydroisomerization of n-hexane over Pd/mordenite.

netic dimensions of these C6 isomers. Therefore, we chose the hydroisomerization of n-hexane as a catalytic test reaction in this study. Since hydroisomerization plays a predominant role over hydrocracking when n-hexane is used as feed (see Figs. 2–10 and Supporting information Figs. S1–S3), our focus in this paper is placed on the investigation of the shape selectivities related to the hydroisomerization of this short chain alkane feed.

First, we used here zeolites mordenite and Y as the representatives of 12-ring zeolites with wide pore openings. It is worthwhile to note that the 8-ring channels present in mordenite (and other zeolites such as TNU-9, SSZ-75 and A) are too narrow for the monoand di-branched isomers of n-hexane to diffuse through. The results of n-hexane hydroisomerization from mordenite are shown in Fig. 2. The conversion of n-hexane increases with the increasing

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C.Y. Chen et al. / Microporous and Mesoporous Materials 164 (2012) 71–81

100

100

90

90

80

70

70

Isomer Distribution, mol%

Conversion or Yield, mol%

n-Hexane

Conversion

80

60

50 Isomerization 40

60

50 2-Methylpentane 40

30

30

20

20

3-Methylpentane

Cracking 10

10

2,3-Dimethylbutane 2,2-Dimethylbutane

0 220 230 240 250 260 270 280 290 300 310 320 330 340 350

0 220 230 240 250 260 270 280 290 300 310 320 330 340 350

Temperature, °C

Temperature, °C

Fig. 3. Hydroisomerization of n-hexane over Pd/SSZ-32.

100

100

90

90 n-Hexane

80

70

70

Isomer Distribution, mol%

Conversion or Yield, mol%

Conversion

80

60 Isomerization

50

40

60

50

40 2-Methylpentane

30

30

3-Methylpentane

20

20

0 200

2,3-Dimethylbutane

10

10

Cracking

210

220

230

240

250

260

270

280

290

0 200

2,2-Dimethylbutane

210

220

Temperature, °C Fig. 4. Hydroisomerization of n-hexane over Pt/ZSM-12.

230

240

250

260

Temperature, °C

270

280

290

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C.Y. Chen et al. / Microporous and Mesoporous Materials 164 (2012) 71–81

100

100

90

90 n-Hexane

80

70

70

Isomer Distribution, mol%

Conversion or Yield, mol%

Conversion

80

Isomerization

60

50

40

60

50 2-Methylpentane

40

30

30

20

20

10

10

3-Methylpentane

2,3-Dimethylbutane

2,2-Dimethylbutane

Cracking

0 170 180 190 200 210 220 230 240 250 260 270 280 290 300

0 170 180 190 200 210 220 230 240 250 260 270 280 290 300

Temperature, °C

Temperature, °C

Fig. 5. Hydroisomerization of n-hexane over Pd/TNU-9.

100

100

90

90 n-Hexane

Conversion

80

80

70

Isomer Distribution, mol%

Conversion or Yield, mol%

70 Isomerization

60

50

40

60

50 2-Methylpentane

40

30

30

20

20

3-Methylpentane

2,3-Dimethylbutane

Cracking

10

10

2,2-Dimethylbutane

0 200

210

220

230

240

250

260

270

280

290

0 200

210

220

Temperature, °C Fig. 6. Hydroisomerization of n-hexane over Pt/SSZ-75.

230

240

250

260

Temperature, °C

270

280

290

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C.Y. Chen et al. / Microporous and Mesoporous Materials 164 (2012) 71–81

100

100

90

90 n-Hexane

80

70

70

Isomer Distribution, mol%

Conversion or Yield, mol%

Conversion

80

Isomerization

60

50

40

60

50 2-Methylpentane

40

30

30

20

20

3-Methylpentane

10

10

2,3-Dimethylbutane

Cracking 2,2-Dimethylbutane

0 200 210 220 230 240 250 260 270 280 290 300 310 320 330

0 200 210 220 230 240 250 260 270 280 290 300 310 320 330

Temperature, °C

Temperature, °C

Fig. 7. Hydroisomerization of n-hexane over Pd/SSZ-25.

100

100

90

90

80

70

70

Isomer Distribution, mol%

Conversion or Yield, mol%

Conversion

80

Isomerization

60

50

40

n-Hexane

60

50 2-Methylpentane

40

30

30

20

20

10

10

3-Methylpentane

2,3-Dimethylbutane

Cracking

0 200

210

220

230

240

250

260

270

280

290

0 200

2,2-Dimethylbutane

210

220

Temperature, °C Fig. 8. Hydroisomerization of n-hexane over Pd/SSZ-57.

230

240

250

260

Temperature, °C

270

280

290

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C.Y. Chen et al. / Microporous and Mesoporous Materials 164 (2012) 71–81

100

100

90

90

80

70

70 Isomerization

60

50

40

Isomer Distribution, mol%

Conversion or Yield, mol%

Conversion

80

n-Hexane

60

50

40

2-Methylpentane

30

30

3-Methylpentane

20

20

10

10

2,3-Dimethylbutane

Cracking

2,2-Dimethylbutane

0 200 210 220 230 240 250 260 270 280 290 300 310 320

0 200 210 220 230 240 250 260 270 280 290 300 310 320

Temperature, °C

Temperature, °C

Fig. 9. Hydroisomerization of n-hexane over Pd/SSZ-33.

100

100

90

90 Conversion

n-Hexane

80

70

70 Isomerization

60

50

40

Isomer Distribution, mol%

Conversion or Yield, mol%

80

60

50

40 2-Methylpentane

30

30

3-Methylpentane

20

20

10

10 Cracking

2,3-Dimethylbutane

2,2-Dimethylbutane

0 200 210 220 230 240 250 260 270 280 290 300 310 320

0 200 210 220 230 240 250 260 270 280 290 300 310 320

Temperature, °C

Temperature, °C

Fig. 10. Hydroisomerization of n-hexane over Pt/SSZ-56.

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Table 2 Hydroisomerization of n-hexane over various zeolites at 10 mol.% isomer yield. Zeolite

Temp. (°C)

ZSM-5 SSZ-32 SSZ-75 TNU-9 SSZ-25 SSZ-57 SSZ-56 SSZ-26 SSZ-33 ZSM-12 Mordenite Y At thermodynamic equilibrium [70]

204 254 210 199 210 205 221 210 215 216 243 238 227

Distribution (mol.%) 2,2-DMBu

2,3-DMBu

2-MPn

3-MPn

Total

0 0 0.1 0.5 0 0.3 0 1.0 0.5 0.4 1.5 0.9 35.1

0.4 0 2.6 4.6 1.3 2.4 1.0 5.8 3.5 2.8 3.5 1.0 16.5

69.1 77.6 67.4 64.1 68.8 62.6 63.5 57.7 59.5 58.0 55.8 60.4 35.2

30.5 22.4 29.9 30.8 29.9 34.7 35.5 35.5 36.5 38.8 39.2 37.7 13.2

100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

Note: DMBu stands for dimethylbutane and MPn for methylpentane.

Table 3 Hydroisomerization of n-hexane over various zeolites at 20 mol.% isomer yield. Zeolite

Temp. (°C)

ZSM-5 SSZ-32 SSZ-75 TNU-9 SSZ-25 SSZ-57 SSZ-56 SSZ-26 SSZ-33 ZSM-12 Mordenite Y At thermodynamic equilibrium [70]

210 266 221 210 221 210 227 216 221 227 249 249 227

Distribution (mol.%) 2,2-DMBu

2,3-DMBu

2-MPn

3-MPn

Total

0 0 0.1 0.5 0 0.5 0.1 1.4 0.7 1.0 3.1 2.2 35.1

0.5 0.3 3.0 5.7 1.4 3.3 2.1 7.6 5.1 3.8 5.0 1.8 16.5

68.9 75.6 66.2 61.5 66.7 61.8 61.2 56.3 58.3 57.7 54.7 58.7 35.2

30.6 24.1 30.7 32.2 31.9 34.4 36.6 34.7 35.9 37.5 37.2 37.3 13.2

100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

reaction temperature. At low temperatures, the hydroisomerization is the only reaction. When the reaction temperature increases, the hydrocracking reaction takes off and its yield and selectivity go up. With the competing cracking reaction occurring, as the temperature increases, the yield and selectivity to hydroisomerization products increase initially, proceed to a maximum and then decrease. This is a common phenomenon which occurs with the hydrocracking and hydroisomerization of normal alkanes (e.g., in Ref. [34]) and is also demonstrated with other zeolites in Figs. 3– 10 and Supporting information Figs. S1–S3. The cracking products consist predominantly of propane because the formation of methane and pentanes as well as ethane and butanes involves primary carbenium ions and are energetically less favorable than the formation of propane according to Fig. 1. The distribution of the C6 isomers reveals how the hydroisomerization proceeds vs. the reaction temperature. Fig. 2 indicates that n-hexane is isomerized only to 2- and 3-methylpentane at low temperatures. As the temperature increases, 2- and 3-methylpentane are further isomerized to 2,2- and 2,3-dimethylbutane toward the thermodynamic equilibrium. Limited by the thermodynamics under the conditions employed in our studies, the yield of each individual branched isomer of n-hexane has the following order at and above the temperature for the maximum total isomer yield (78.6 mol.% at 293 °C for mordenite): 2-methylpentane (RON 73.4) > 3-methylpentane (RON 74.5) > 2,2-dimethylbutane (RON 91.8) > 2,3-dimethylbutane (RON 101.0). It is important to point out that the yield ratio of 2,2- to 2,3-dimethylbutane is about 2 total isomer yield for mordenite (Fig. 2 and Table 4). For the hydroisomerization of n-hexane, zeolite Y studied in this work be-

haves similarly to mordenite. Due to the limit to the length of this paper, the detailed results from zeolite Y regarding the n-hexane conversion as well as product yields and distributions vs. the reaction temperature are reported in Supporting information Fig. S1. As also summarized in Tables 2–4, the 12-ring zeolites represented by mordenite and Y do not appear to have spatial restrictions to the formation of the two relatively bulky di-branched isomers, 2,2and 2,3-dimethylbutane, in their channel systems. In contrast to zeolites Y and mordenite, the formation of the bulky 2,2- and 2,3-dimethylbutane is essentially hindered by the narrow pores in the 10-ring zeolites ZSM-5 and SSZ-32 (see Tables 2–4). The results from SSZ-32 containing 0.27 wt.% Pd are shown in Fig. 3. Very similar results were also obtained from SSZ-32 containing 0.5 wt.% Pd or 0.5 wt.% Pt, indicating together with other zeolites (e.g., SSZ-57) loaded with different amounts of Pt, Pd or Pt/Pd that the type and amount of Pt and/or Pd in the range of approximately 0.25–0.6 wt.% Pt and 0.14–0.6 wt.% Pd do not have noticeable impact on the catalytic results of this catalytic test reaction under the conditions applied in this work. Therefore, we report in this paper our results from either Pt- or Pd-loaded zeolites and think they are essentially equivalent. The results from ZSM-5 are reported in Supporting information Fig. S2. ZSM-12 is a 1-dimensional 12-ring zeolite with a fairly narrow ring opening. As shown in Fig. 4 and Table 4, its yield ratio of 2,2- to 2,3-dimethylbutane is 16.2:9.6 at the maximum isomer yield (72.7 mol.% at 260 °C) as compared to 2 for zeolites mordenite and Y. This sensitive shape selectivity is more impressively exhibited by two other 10-ring zeolites, namely, TNU-9 and SSZ-75. With TNU-9, the yield ratio of 2,2- to 2,3-dimethylbutane is re-

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C.Y. Chen et al. / Microporous and Mesoporous Materials 164 (2012) 71–81 Table 4 Hydroisomerization of n-hexane over various zeolites at maximum isomer yields. Zeolite

Temp. at Max. isomer yield (°C)

Max. isomer yield (mol.%)

ZSM-5 SSZ-32 SSZ-75 TNU-9 SSZ-25 SSZ-57 SSZ-56 SSZ-26 SSZ-33 ZSM-12 Mordenite Y At thermodynamic equilibrium [70]

260 304 254 260 271 260 266 260 271 260 293 304 277

74.4 68.5 63.8 77.4 74.5 76.3 65.3 78.8 79.1 72.7 78.6 79.5 –

duced to 2.6:13.1 at its maximum isomer yield (77.4 mol.% at 260 °C, see Fig. 5 and Table 4). With SSZ-75, the yield ratio of 2,2- to 2,3-dimethylbutane is further reduced to 0.6:9.1 at its maximum isomer yield (63.8 mol.% at 254 °C, see Fig. 6 and Table 4). The results from zeolites ZSM-12, TNU-9 and SSZ-75 indicate that their effective pore sizes become increasingly critical for the formation and diffusion of the molecules of 2,2-dimethylbutane over 2,3-dimethylbutane which is only slightly less bulky than the former. Furthermore, we investigated SSZ-25 and SSZ-57 for n-hexane hydroisomerization. These two zeolites have 10-ring channels containing 12-ring cages which are accessible only via the 10-ring channels except for those located on the external surface of the crystals. With SSZ-25, the yield ratio of 2,2- to 2,3-dimethylbutane amounts to 0.5:5.1 at its maximum isomer yield (74.5 mol.% at 271 °C, see Fig. 7 and Table 4). Similarly, the yield ratio of 2,2- to 2,3-dimethylbutane over SSZ-57 amounts to 3.2:9.9 at its maximum isomer yield (76.3 mol.% at 260 °C, see Fig. 8 and Table 4). When comparing these two sets of results with those from zeolites mordenite and Y, we think that the influence of the terminal 12ring cups (stemming from the 12-ring cages) located on the external surfaces of the crystals of these two zeolites and, in general, the influence of the catalytic activities on the external surfaces of zeolite crystals are minimum for this catalytic test reaction. Although it’s expected that, inside the zeolite crystals, 2,2-dimethylbutane can be more readily formed in these 12-ring cages than in the 10-ring channels of these two zeolites, the diffusion of the bulky molecules of 2,2-dimethylbutane out of 12-ring cages is apparently hindered by the 10-ring windows. Based on our other experimental results, many of the possible external cups in SSZ-25 disappear when we calcine the material to make the 3D zeolite which has an external surface area likely around 30–50 m2/g. Matias et al. studied the catalytic role of the pore systems of a SSZ-25 type zeolite (HMCM-22, MWW) with n-heptane transformation [85] and showed that the supercages are responsible for 97% of n-heptane transformation while the sinusoidal channels for only 3% probably due to the diffusion limitations. They also found that the protonic sites of the external cups are completely inactive. In addition, we also tested a Pt/A (8-ring zeolite Ca-A first exchanged with NH4OH and then loaded with 0.5 wt.% Pt via the procedures described in the Section 2) for n-hexane hydroisomerization. At a high temperature of 288 °C, n-hexane conversion is only 5 mol.% over this Pt/ A catalyst, indicating again that the surface reaction is negligible although the exterior of zeolite A is most likely not the best model for SSZ-25. In short, at least to a large degree, the shape selectivities observed in our present work with SSZ-25 and SSZ-57 reflect the preferred formation of 2,3-dimethylbutane over 2,2-dimethyl-

Distribution (mol.%) 2,2-DMBu

2,3-DMBu

2-MPn

3-MPn

Total

0.2 0.1 0.6 2.6 0.5 3.2 6.4 9.9 11.9 16.2 21.5 21.9 37.8

3.0 2.1 9.1 13.1 5.1 9.9 9.4 12.8 12.4 9.6 10.8 10.0 18.5

59.6 58.9 55.0 51.8 57.3 54.0 50.9 47.0 46.0 44.8 40.7 41.1 30.6

37.2 38.9 35.3 32.5 37.1 32.9 33.3 30.3 29.7 29.4 27.0 27.0 13.1

100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

butane in the 10-ring channel systems of these two zeolites. It is also interesting to point out that, based on the results from 2,2dimethylbutane adsorption [64] and the shape selective properties derived from n-hexane hydroisomerization, we recognized that SSZ-57 is a multi-dimensional medium pore zeolite before its crystallographic structure [82] was determined, exhibiting an example of the usefulness of catalytic test reactions as well as hydrocarbon adsorption method for evaluating the zeolite structures. The last group of zeolites which we investigated for n-hexane hydroisomerization consists of zeolites SSZ-26, SSZ-33 and SSZ56 which have intersecting 12- and 10-ring channel systems. The results from SSZ-33 and SSZ-56 are shown in Figs. 9 and 10 and Tables 2–4. SSZ-26 and SSZ-33 are described in Table 1 under the same structure code CON. As described by Lobo et al. [86,87], they both consist of intergrowths of the same two polymorphs. SSZ-26 and SSZ-33 differ in that they have different relative abundance of these two polymorphs. Due to the close structural similarities between SSZ-26 and SSZ-33, similar results were obtained from SSZ26 and SSZ-33 in this work. Therefore, the results from SSZ-26 are reported in Supporting information Fig. S3. The yield ratio of 2,2- to 2,3-dimethylbutane amounts to 9.9:12.8 (for SSZ-26 at 260 °C for the maximum isomer yield of 78.8 mol.%), 11.9:12.4 (for SSZ-33 at 271 °C for the maximum isomer yield of 79.1 mol.%) and 6.4:9.4 (for SSZ-56 at 266 °C for the maximum isomer yield of 65.3 mol.%), respectively. Based on the discussion above on the results from various 12- and 10-ring zeolites, we attribute the selectivities of zeolites SSZ-26, SSZ-33 and SSZ-56 to the presence of both 10- and 12-ring channels in their structures. The more hindered formation and diffusion of 2,2-dimethylbutane in their 10ring channels is clearly reflected by the lower yield of 2,2-dimethylbutane vs. 2,3-dimethylbutane over these three zeolites. When comparing the shape selectivities of these zeolites in a catalytic test reaction, it is always important to refer the catalytic properties to a certain reaction parameter at the same or similar value level. Tables 2–4 summarize the results from n-hexane hydroisomerization over the zeolites discussed above in connection with Figs. 2–10 and Supporting information Figs. S1–S3, together with the thermodynamic equilibrium distributions of isomers calculated according to [70]. In Tables 2–4, we choose to compare the distributions of the branched isomers of n-hexane at 10 and 20 mol.% and maximum isomer yields, respectively. At lower conversions before reaching the maximum isomer yields (e.g., at 10 or 20 mol.% conversions in Tables 2 and 3), the formation of dimethylbutanes is still influenced by the reaction kinetics whereas the reactions are increasingly influenced by the thermodynamics at higher conversions after reaching the maximum isomer yields. Therefore, we found that the maximum isomer yields

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C.Y. Chen et al. / Microporous and Mesoporous Materials 164 (2012) 71–81

reported in Table 4 provide a most useful platform for such a comparison with n-hexane hydroisomerization as a test reaction. The vapor phase physisorption of various hydrocarbons of different kinetic dimensions (e.g., n-hexane and 2,2-dimethylbutane) in zeolites which we reported previously [64] is complementary to the present work with n-hexane hydroisomerization as a catalytic test reaction for the investigation of the shape selective properties and effective pore sizes of zeolites. Fig. 4 of Ref. [64] exhibits the time dependence of the 2,2-dimethylbutane adsorption capacity vs. the adsorption time for a number of zeolites in a gravimetric microbalance at room temperature and P/P0 of 0.3. The results indicate that 2,2-dimethylbutane adsorption occurs rapidly in 12ring zeolites (e.g., SSZ-31 and SSZ-42, both not included in the present work) while the 10-ring pores of zeolites (e.g., ZSM-5 and SSZ-57) become hindering to the diffusion of 2,2-dimethylbutane molecules. Due to its very narrow pore opening (see Table 1), SSZ-32 does not adsorb 2,2-dimethylbutane. The results from 2,2dimethylbutane adsorption in SSZ-33 show the influence from both 12- and 10-ring pores: the initial quick uptake is attributed to the 12-ring pores while the subsequent slow adsorption is related to the 10-ring pores. Our currently ongoing adsorption work with n-hexane clearly demonstrates that n-hexane adsorption proceeds much faster than 2,2-dimethylbutane in these 10-ring zeolites. The results from these adsorption experiments are in excellent agreement with the results from n-hexane hydroisomerization of the present work. The n-hexane adsorption work will be reported in our next publication.

4. Conclusions In this paper we have reported our work on using n-hexane hydroisomerization as a catalytic test reaction to characterize the shape selective properties and effective pore sizes of a series of zeolites which include: (1) Y, mordenite and ZSM-12 (12-ring), (2) ZSM-5, SSZ-32, SSZ-75 and TNU-9 (10-ring), (3) SSZ-25 and SSZ-57 (10-ring with 12-ring cages) and (4) SSZ-26, SSZ-33 and SSZ-56 (intersecting 12/10-ring). Due to the shape selectivities associated with effective pore sizes of these zeolites, the yield ratio of mono-branched isomers (2- and 3-methylpentane) to dibranched isomers (2,2- and 2,3-dimethylbutane) and the yield ratio of 2,3-methylbutane to 2,2-dimethylbutane at the maximum isomer yields are valuable for distinguishing between 12-ring zeolites (mordenite and Y) and 10-ring zeolites (SSZ-32 and ZSM-5). Mordenite and Y do not appear to have spatial restrictions to the formation of the two relatively bulky di-branched isomers (2,2and 2,3-dimethylbutane) of n-hexane in their channel systems. In contrast, the formation of the bulky 2,2- and 2,3-dimethylbutane is essentially hindered by the narrow pores in the 10-ring zeolites ZSM-5 and SSZ-32. When the effective pore sizes of the zeolites vary between those of these two groups of zeolites, these ratios change accordingly. For example, the effective pore sizes of 12-ring ZSM-12 as well as 10-ring TNU-9 and SSZ-75 become increasingly critical to the formation and diffusion of the molecules of 2,2dimethylbutane over 2,3-dimethylbutane which is only slightly less bulky than the former. As a result, the high octane isomer 2,3-dimethylbutane (RON 101.0) can be selectively produced over 2,2-dimethylbutane (RON 91.8) by choosing zeolites (e.g., TNU-9 and SSZ-75) which have the appropriate pore sizes. Although 10ring zeolites SSZ-25 and SSZ-57 contain 12-ring cages, these 12ring cages are accessible only via the 10-ring windows and the shape selectivities observed from these two zeolites also reflect the preferred formation of 2,3-dimethylbutane over 2,2-dimethylbutane in their 10-ring channel systems. The reactions on the external surfaces of their crystals which contain terminal 12-ring cups appear to be negligible in our present studies. The selectivities

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