Investigation on n-Alkane Hydroisomerization, a Comparison of IM-5

Subscriber access provided by University of Sunderland

Kinetics, Catalysis, and Reaction Engineering

Investigation on n-alkane hydroisomerization, a comparison of IM-5 to ZSM-5 zeolites Qianqian Yu, Zhigang Huang, Houxiang Sun, Lei Li, Xiaochun Zhu, Shenyong Ren, and Baojian Shen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03918 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 9, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Investigation on n-alkane hydroisomerization, a comparison of IM-5 to ZSM-5 zeolites Qianqian Yu, Zhigang Huang, Houxiang Sun, Lei Li, Xiaochun Zhu, Shenyong Ren, Baojian Shen * State Key Laboratory of Heavy Oil Processing; the Key Laboratory of Catalysis of CNPC; College of Chemical Engineering, China University of Petroleum, Beijing 102249, P. R. China

*Corresponding Author: Baojian Shen TEL: +(8610)89733369 FAX: +(8610)89733369 EMAIL: [email protected]

1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Porosity and acidity of zeolites are key factors for the high efficient catalysts design. Herein, we investigate the isomerization performance of a high-silica zeolite IM-5 which has unique limited 2.5nm “nanoslab” pore structure. Compared with the reference ZSM-5, the higher adsorption amount and rate of IM-5, proven by adsorption experiments of n-heptane and toluene, resulted in a higher isomers’ selectivity in n-C7 or n-C16 hydroisomerization reactions, which increased relatively by 19.5% and 91.5%, respectively. Furthermore, IM-5 also shows higher n-C7 or n-C16 conversion due to its more strong acid sites. Besides, mesopore introduction into IM-5 significantly increase isomers’ selectivity, especially for long chain n-C16. The selectivities of C7 isomers and C16 isomers of mesoporous M-IM-5 increase relatively by 73.3% and 216.1% compared with microporous IM-5. As mesopores and reduced diffusion length facilitate the mass transport of bulky molecules, then decreases subsequent cracking side reaction that consumed branched isomer products.

2


Page 2 of 42

Page 3 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION It is well known that selective hydroisomerization of linear alkanes over solid acid catalysts is a very important process in petrochemical industry. 1-4 As the octane number of gasoline and the low temperature fluidity of diesel or lubricant can be improved by converting n-alkane into branched isomers rather than undesired olefins or aromatics.

5-10

Among the catalysts developed so far, Pt or Pd loaded ZSM-22,

ZSM-5, SAPO-11, or Beta zeolites are commonly used as bifunctional catalysts in the hydroisomerization reaction due to their high surface area, strong acidity and high metal dispersion.

11-16

The reaction mechanism of these bifunctional catalysts is that

the metal components provide dehydrogenation functions to generate intermediate olefins, and zeolites provide acid sites to generate carbenium ions from intermediate olefins for isomerization. 17-18 When the hydrogenation-dehydrogenation function of metal components and the acidity function of zeolites achieve a balance, the bifunctional catalysts can reach the highest activity and the best isomers’ selectivity. 19 Pt loaded zeolites with various acidity such as Pt/ZSM-5, Pt/Beta, and Pt/SAPO-11 have

significantly

different

isomerization

selectivity

in

the

n-alkane

hydroisomerization process. 5,18 When the hydrogenation-dehydrogenation function is active enough, the reactivity and isomers’ selectivity of the bifunctional catalysts will depend on the acidic function and pore structure. The cracking and reactivity will decrease with decreasing acid amount. 10 On the other hand, the pore structure of zeolites also affects the isomerization performance of the catalysts.

20-22

The pore structure of zeolites can influence the 3



Page 4 of 42

formation and diffusion of intermediates and products, and consequently alter the different products distribution. MEL-type zeolite has higher isomers’ selectivity due to their higher selectivity for adsorbing linear rather than branched paraffins at high paraffin loading than MFI-type zeolite.

23

Compared with MCM-22 and ZSM-5, Beta

zeolite has the highest isomers’ selectivity due to its larger channel.

5

Besides, the

introduction of mesopore in zeolites is also an effective method to improve isomerization performance. For example, the Pt/Pd loaded nano-zeolites and mesoporous zeolites prepared through post treatments such as acid, alkaline treatment, or steaming exhibited dramatically improved of catalytic activity and isomers’ selectivity in the n-alkane hydroisomerization. 24-27 IM-5 with IMF topology is a high-silica shape selective zeolite, it was first successfully synthesized by using 1,5-bis(methyl-pyrrolidinium) pentane as template agent by Benazzi in 1998.

28

The pore structure of IM-5 was unknown until

Baerlocher illuminated the complex structure in 2007 by using charge-flipping structure-solution algorithm method. IM-5 has unique pore structure, which is consisted of two-dimensional ten-membered ring pore structure and limited 2.5 nm three-dimensional pore system. The distinctive three-dimensional pore system enables IM-5 to accommodate bulky intermediates and the two-dimensional structure retain the diffusion limited effect in the catalytic reactions.

29-30

Besides, IM-5 displays

excellent thermal and hydrothermal stability and it has higher amount of strong acid sites, which is even better than that of ZSM-5.

31

Thus, IM-5 has a promising

application as a shape selective zeolite in various catalytic reactions. 4


32-33

In recent

Page 5 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


years, many efforts were devoted to investigate the catalytic performance of IM-5 in petrochemical industry.

34-39

Corma reported that the pore structure of IM-5 was

consists of a unidirectional 10 MR with side pockets or crossing 10 membered ring pores by using different catalytic reactions and hydrocarbon adsorption measurements. 30

Lee has investigated the catalytic performance of IM-5 on the skeletal isomerization

of 1-butene reaction and the results indicated that IM-5 exhibited a very high 1-butene conversion but lower yield of i-butene compared with ZSM-35, which maybe due to its larger pore structures that allow undesired side reactions to occur such as 1-butene dimerization.

40

Besides, IM-5 also shows the excellent performance in the fluid

catalytic cracking and the NOx selective catalytic reduction.

41-42

However, the

purpose of these studies is to infer the pore structure of IM-5 through different catalytic reactions. And to our best knowledge, the influence of pore structure and acidity of IM-5 on the hydroisomerization reaction of n-alkane remains unclear. In order to make up for the lack of knowledge in this field and provide a more detailed understanding of IM-5, we focus our investigation on the effects of porosity and acidity of IM-5 on the hydroisomerization of n-C7 or n- C16 in this paper. And for comparison, ZSM-5 was also synthesized for the same reaction. All the catalysts with the same Pt loading (0.5 wt%) were prepared by incipient impregnation. The XRD, Nitrogen physical adsorption, Py-IR, SEM, XRF, TEM and IGA characterizations were chose to study the different properties of zeolites used in this paper. Furthermore, we also investigate the effect of the introduction of mesopore on the isomerization performance of IM-5. We would like to explore the application of IM-5 for the 5



Page 6 of 42

isomerization reaction and provide new knowledge for the development of high performance hydroisomerization catalysts.

2. EXPERIMENTAL 2.1. Preparation of Catalysts 2.1.1. Preparation of the Zeolites IM-5 was obtained by traditional hydrothermal synthesis as reported in the literature

23

and calcined in flowing air at 823 K for 4 h to remove the template.

Then the samples were twice ion exchange in NH4Cl solutions for 3 h and then calcined at 823 K for 4 h to H form IM-5. This zeolite is denoted as IM-5. The mesoporous IM-5 was prepared by alkali treatment (0.25M NaOH solutions, 363 K, 2 h) and was twice ion exchange in NH4Cl solutions for 3 h and then calcined at 823 K for 4 h to achieve H form mesoporous IM-5. The mesoporous IM-5 is denoted as M-IM-5. In order to investigate the effect of pore structure on n-heptane or

n-hexadecane isomerization performance over IM-5 and ZSM-5, we prepared another IM-5 sample that has similar acidity compared to that of ZSM-5. This zeolite is denoted as L-IM-5. L-IM-5 sample was also obtained by traditional hydrothermal synthesis as reported in the literature 23 and calcined in flowing air at 823 K for 4 h to remove the template. Then the calcined samples were ion exchanged only once in NH4Cl solutions for 2 h and then calcined at 823 K for 4 h to achieve H form L-IM-5. ZSM-5 zeolite was obtained by traditional hydrothermal synthesis and the crystallization was carried out in stainless steel autoclaves with PTFE-lined.

43

Then

the calcined samples were twice ion-exchanged with NH4Cl solution for 3 h, and then 6


Page 7 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


calcined at 823 K for 4 h to obtain H form ZSM-5. This sample is denoted as ZSM-5. 2.1.2. Platinum Loading All the Pt/H-zeolite used in this study have same Pt loading (0.5 wt%), which could reach good metal/acidity balance in the present bifunctional catalysis.

11,44

All

the catalysts were prepared by incipient impregnation with H2PtCl6•6H2O (37.68 wt% Pt) as the following steps. First, a certain amount of H2PtCl6•6H2O was dissolved in deionized water and the solution was dropped on the zeolites with the given concentration to ascertain 0.5 wt% loading of Pt. Then, the samples were kept at room temperature overnight, dried at 393 K for 12 h and calcined at 773 K for 4 h. Finally, the catalysts were pressed and sieved to obtain particles in the range of 40 – 60 mesh before they were loaded into the reactor.

2.2. Characterization of Materials X-ray diffraction (XRD) patterns of all samples were recorded on a powder X-ray diffractometer (X’Pert, PANalytical, Netherlands) using Cu Kα radiation (40 kV, 40 mA). The samples were scanned from 5 ° to 50 ° of 0.013 ° , with a scanning duration 30 sat each angle. The Si and Al content of the samples was determined with an X-ray fluorescence (XRF) spectrometer (AxiosmAX, PANalytial, Netherlands). Physical adsorption/desorption of nitrogen on the samples was performed with an adsorption analyzer (TriStar 3020, Micromeritics, USA). The samples were vacuum-out degassed and dehydrated at 623 K for at least 8 h prior to the measurements. The total surface area of each sample was determined by the Brunauer–Emmett–Teller (BET) method, and the micropore volume and external 7



surface area were determined by the t-plot method. The mesopore size distributions profile and the mesopores volume of the samples were calculated by the Barrett– Joyner–Halenda (BJH) adsorption branch of the isotherms. The micropore size distributions of samples were analyzed by the SF method (Autosorb-iQ3-XR, Quantachrome, USA). Chemical adsorption of NH3 on the samples was carried out with an adsorption analyzer (Autochem 2920, Micromeritics, USA), the samples were activated at 873 K for 0.5 h under He flow before measurement. The NH3 (1 V%) -N2 gas was used as adsorbent and desorbed from 373 K to 873 K (10 K/min) under He flow, the NH3 desorption was detected by a TCD detector. Pyridine infrared (Py-IR) spectra in the range of 1700 – 1300 cm−1 was recorded on a Fourier transform infrared FT-IR spectrometer (Nicolet iS10, Thermo Fisher, USA) with a resolution of 0.35 cm−1. According to the absorbance observed near 1540 and 1450 cm−1, the amounts of Brønsted (B) and Lewis (L) acid were calculated from the molar extinction coefficients of IMEC(B) = 1.88 cm2 µmol−1 and IMEC(L) = 1.42 cm2 µmol−1 , respectively. 45 The morphologies of all samples were measured by using a scanning electron microscope (SEM; Quanta200F, FEI, USA) at an accelerating voltage of 20 kV. Transmission electron microscopy (TEM) was carried out on a microscope (F20, FEI, USA) with point resolution of 0.24 nm and operated at 200 kV. The diffusion properties of zeolites were investigated in a computer-controlled intelligent gravimetric analyzer (IGA) (Hiden Analytical Ltd., Warrington, UK) by using

n-heptane and toluene as adsorbates. The zeolites were first degassed under a vacuum of less than 10-5 Pa at 673 K for 3 h before the adsorption measurement. The 8


Page 8 of 42

Page 9 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


adsorption measurement experiments were performed at 298 K, the mass of zeolite is 100 mg, the adsorption pressure is 1 mbar, and the pressure rate is 0.5 mbar/min.

2.3. Catalytic Testing The hydroisomerization reaction of n-heptane or n-hexadecane was carried out in a continuous-flow fixed-bed reactor with a 11.0 mm internal diameter. First, 1g of catalysts (40 – 60 mesh) were pre-reduced by leading a flow of H2 (60 ml/min) at 673 K for 4 h, then decreased to the reaction temperature. The hydrogen and reactants were simultaneously introduced into the reactor. The catalytic reaction was tested in the range of 473 K – 533 K, the molar ratio of H2 to n-heptane was 10, the reaction pressure was 2 MPa and weight hourly space velocity (WHSV) was 6 h−1. The flow rate of reactants was controlled with a double column pump, and H2 was controlled with a mass flow-meter. The products were analyzed by GC-MS (7890A, Agilent, USA) equipped with FID detector, the column was HP-5. The n-alkane conversion and isomers’ selectivity was defined as follows:

n-alkane conversion = (1－M/M0) × 100%; isomers’ selectivity = Mi / conversion × 100%; where M is the content of n-alkane in the corresponding product; M0 is the total content of n-alkane in the corresponding feed; Mi is the total content of C7 isomers or C16 isomers in the corresponding product.

3. RESULTS AND DISCUSSION 3.1. Characteristics of the Zeolites All the IM-5 samples prepared are phase-pure zeolites which exhibit the typical diffraction patterns of IMF structure (Supporting Figure S1). And the typical 9



Page 10 of 42

diffraction pattern of the prepared ZSM-5 sample is MFI type structure and no other phase appear (Supporting Figure S2). Besides, IM-5 has better hydrothermal stability than that of ZSM-5, as the retained crystallinity of IM-5 after hydrothermal treatment is higher than that of ZSM-5 as shown in Supporting Table S1. The silica to alumina mole ratio and acidity of all samples are listed in Table 1. The acidity situation of the zeolites were investigated by FT-IR spectra of adsorbed pyridine as shown in Figure 1. The absorption peaks around 1540 and 1450 cm-1 are respectively attributed to the characteristic absorption peaks of pyridine molecules adsorbed on the Brønsted acid sites and the Lewis acid sites 46, and the absorption peaks appearing at around 1490 cm−1 is attributed to pyridine chemisorption on both Brønsted and Lewis acid sites. 47 It is believed that the optimal catalysts for LSR (Light Straight Run) isomerization have a moderate SiO2/Al2O3 mole ratio which is between 20 and 40.

48-49

Thus, all

samples in our study have an appropriate SiO2/Al2O3 mole ratio (Table 1) for n-alkane hydroisomerization. Interestingly, IM-5 has higher amount of total acid sites and strong acid sites than that of ZSM-5 sample, despite they have same SiO2/Al2O3 ratio, which is due to its more framework Al species and more Al, H-sites with high acidity. 31, 50

For comparison, L-IM-5 sample with lower acid strength and less amount of acid

sites than that of IM-5 was prepared by controlling the extent of ion-exchange. Furthermore, compared to microporous IM-5, mesoporous M-IM-5 shows less amount of Brønsted acid sites but higher amount of Lewis acid sites. This is caused by the removal of some aluminum from the IM-5 framework during the alkaline treatment and its partial conversion into extra-framework Al. The acidity situation of all samples which were analyzed by NH3-TPD in Supporting Figure S3. Among all of the samples, IM-5 has the strongest acid strength and the highest amount of total acid sites and strong acid sites. L-IM-5, M-IM-5 and ZSM-5 samples have similar acid 10


Page 11 of 42

strength and amount of acid sites, these results are consistent with Py-IR results in Table 1.

Table 1. The chemical composition and acid properties of all samples.

a

Sample

SiO2/Al2O3 a

IM-5

473 K (µmol g-1)

623 K (µmol g-1)

B

L

Total

B

L

Total

33

173

116

289

98

42

140

L-IM-5

30

160

88

248

85

36

121

M-IM-5

21

129

123

252

63

57

120

ZSM-5

32

145

93

238

81

36

117

The chemical composition was determined by XRF measurement.

IM-5 L-IM-5 M-IM-5 ZSM-5

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1550

1500

1450

Wavenumber (cm-1)

Figure 1. Py-IR spectra of all IM-5 and ZSM-5 samples under 623K

The crystal size of zeolites also has a significant influence on the catalytic performance as well as the product distribution. Thus, to diminish the effect of the difference in crystal size on the isomerization performance, the samples with similar crystal size and crystal shape were prepared as seen in Figure 2. SEM images (Figure 11



2a-d) show that the crystal size of all samples is in the range of 300 ~ 500 nm. TEM images indicate that the mesoporous M-IM-5 sample (Figure 2g) has abundant mesopores as compared with microporous IM-5 (Figure 2e), and the four zeolites have similar morphology which were consist of single crystals with complete morphology. This indicates that mesopores were formed due to the desilication in the alkaline solution, in the meantime, the micropore structure of zeolites was well preserved, the results are supported by the micropores volume data (Table 2). Besides, the micropores volume of IM-5, L-IM-5 and ZSM-5 (Table 2) and SEM images (Figure 2a-c) indicate that all samples have good crystallinity.

12


Page 12 of 42

Page 13 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. SEM images of IM-5(a), L-IM-5(b), M-IM-5(c), ZSM-5(d) and TEM images of IM-5(e), L-IM-5(f), M-IM-5(g), ZSM-5(h).

The N2 adsorption/desorption isotherms and mesopore size distributions of all samples are shown in Supporting Figure S4. The isotherms of IM-5, L-IM-5, and 13



Page 14 of 42

ZSM-5 samples are typical for microporous materials. The corresponding textural properties are listed in Table 2. Clearly, the high micropore volume of all materials indicates that all samples have high crystallinity, and the mesopores volume were similar for IM-5, L-IM-5, and ZSM-5. Since L-IM-5 and IM-5 are prepared by the same synthesis conditions, and the only difference between them is the ammonium exchange conditions, the pore size and structure of the two samples are the same. Hence, we chose L-IM-5 and ZSM-5 for comparison of the pore structure. The micropore size distributions of L-IM-5 and ZSM-5 zeolites were analyzed by the SF method in Figure 3. Obviously, the micropore size distributions of L-IM-5 and ZSM-5 are very similar, both centered at around 0.58 nm, which is the effective pore size of IMF and MFI, respectively. However, L-IM-5 contains a broader micropore size distribution than ZSM-5. The M-IM-5 shows type IV adsorption isotherm, which is typically assigned to the combined presence of micropores and mesopores. The textural properties show that M-IM-5 has much larger mesopores volume and external surface areas than those of IM-5, L-IM-5, and ZSM-5. Furthermore, the mesopore size distributions of M-IM-5 shows that the generated mesopores centers around ~ 6 nm. Table 2. Textural properties of all samples. Sample

Pore volume (cm3 g-1)

Totala

Microporeb

External

Totalc

Microporeb

Mesopore

IM-5

338

295

43

0.257

0.142

0.115

L-IM-5

332

291

41

0.262

0.142

0.120

M-IM-5

381 371

250 286

131 85

0.412 0.232

0.120 0.136

0.292 0.096

ZSM-5 a

Surface area (m2 g-1)

The total surface area was obtained using the BET method.

14


Page 15 of 42

b

The micropore surface area and micropore volume were obtained using the t-plot method.

c

The total pore volume is assumed to be the volume of adsorbed nitrogen at P/P0 =0.99.

0.5

0.4

Volume (mL g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


L-IM-5 ZSM-5

0.3

0.2

0.1

0.0 0.3

0.6

0.9

1.2

1.5

1.8

Pore Diameter (nm)

Figure 3. The micropore size distributions analyzed by the SF method of L-IM-5 and ZSM-5 samples.

The TEM images in Figure 4 show the Pt particle size and distributions of the catalysts. It can be concluded that the Pt particles are uniformly dispersed on the external surface and the Pt particle size is uniform within the range of 3 ~ 8 nm for all catalysts. Thus, considering the same Pt loading (0.5 wt%) and size distributions, we argue that the influence of metal Pt on n-alkane hydroisomerization reaction can be neglected. Therefore, the differences in catalytic performance can be related to the differences of zeolites .

15



a

25

Percentage / %

20

15

10

5

0 2

4

6

8

10

the Pt partical size distribution (nm)

b

25

Percentage / %

20

15

10

5

0 0

2

4

6

8

10

8

10

8

10


c

25

Percentage / %

20

15

10

5

0 0

2

4

6


d

25

20 Percentage / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 42

15

10

5

0 0

2

4

6


Figure 4. TEM images of Pt/IM-5(a), Pt/L-IM-5(b), Pt/M-IM-5(c), Pt/ZSM-5(d), and the Pt particle size distributions of the corresponding samples.

16


Page 17 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.2. Diffusion Properties of the Zeolites The influence of zeolite pore structure on diffusion properties was investigated by comparing the adsorption rate and adsorption capacity of different adsorbates at 298 K by IGA analysis. The adsorption capacity of n-heptane in IM-5, L-IM-5, ZSM-5 and M-IM-5 were showed in Figure 5a. Clearly, the adsorption rate and adsorption amount of n-heptane in IM-5 and L-IM-5 are very similar, and both higher than that of ZSM-5, indicating that IM-5 possesses better diffusion property. As the micropore size of L-IM-5 and ZSM-5 are very similar (Figure 3), we conclude that the pore structure of L-IM-5 is more conducive to the adsorption and diffusion of

n-heptane. Moreover, the adsorption rate of n-heptane by M-IM-5 is much higher, and the amount of n-heptane is also higher as compared with those of IM-5, L-IM-5 and ZSM-5. The excellent adsorption and diffusion properties of M-IM-5 sample proves that the mesopores and reduced diffusion length promote the mass transport of

n-heptane and increase adsorption capacity. Figure 5b shows the adsorption of larger molecules of toluene, further explaining the difference between the four zeolites. Comparing with n-heptane, the lower adsorption amount of toluene were observed for these four samples, due to its larger kinetic diameter. The total adsorption amount decreases in the order M-IM-5 ＞ L-IM-5 ≈ IM-5 ＞ ZSM-5. The introduction of mesopores can also significantly increase the adsorption amount and adsorption rate of toluene on M-IM-5. The adsorption rate of IM-5, L-IM-5 and ZSM-5 samples are similar, but IM-5 and L-IM-5 have both higher adsorption amount of toluene. Furthermore, the diffusion coefficient of n-heptane and toluene over IM-5 are 1.5 and 17



1.8 times larger than that of ZSM-5 in Table 3, respectively, further confirming the better diffusion property of IM-5 zeolite. And the diffusion coefficients of n-heptane and toluene of mesoporous IM-5 are 7.2 and 8.5 times larger than that of microporous IM-5, respectively. Thus, considering the above discussion, we can conclude that the diffusion properties of IM-5 are better than ZSM-5, and can be further improved by the introduction of mesopores.

0.12

a Q (mL/mg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.08

0.04

0.00 0

5

10 15 Time (min)

18


20

Page 18 of 42

Page 19 of 42

0.04

b 0.03 Q (mL/mg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



0.02

0.01

0.00 0

10

20 30 Time (min)

40

Figure 5. The adsorption experiments of n-heptane (a) and toluene (b ) on IM-5, L-IM-5, ZSM-5, and M-IM-5 samples versus time by IGA.

Table 3. The diffusion coefficient (D) of all IM-5 and ZSM-5 samples in IGA experiments. Sample

IM-5

L-IM-5

M-IM-5

ZSM-5

Dn-heptane (cm2 S-1)

7.21e-14

7.05e-14

5.17e-13

4.84e-14

Dtoluene (cm2 S-1)

4.24e-15

4.31e-15

3.60e-14

2.40e-15

3.3. Hydroisomerization Performance Over the Pt/H-zeolite Catalysts 3.3.1. Effect of Pore Structure The acid properties of IM-5 and ZSM-5 zeolites are quite different. In order to eliminate the influence of acid properties on isomerization performance, we selected L-IM-5 and ZSM-5 samples with the same acid properties to investigate the effect of zeolite pore structure on the hydroisomerization reaction. In this study, n-C7 and n-C16, which have different chain lengths, were used as model compounds to investigate the 19



isomerization performance of L-IM-5 and ZSM-5. Figure 6 shows the n-C7 conversion and selectivities of C7 isomers of L-IM-5 and ZSM-5 catalysts. It can be seen that the n-C7 conversion of L-IM-5 and ZSM-5 catalysts both increased with increasing reaction temperature and exhibited very similar n-C7 conversion under different temperatures. However, the selectivities of C7 isomers were quite different for L-IM-5 and ZSM-5. L-IM-5 shows higher C7 isomers selectivity than ZSM-5. The different selectivity of C7 isomer of ZSM-5 and L-IM-5 zeolite samples can be attributed to the different pore structure of zeolites, as the two samples have same amount of strong acid sites.

100

40 L-IM-5 ZSM-5

i-C7 selectivity (wt%)

L-IM-5 ZSM-5

80 n-C7 conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 42

60

40

30

20

10 20 480

500

520

540

20

Reaction temperture (K)

40 60 n-C7 conversion (%)

80

100

Figure 6. The n-C7 conversion and selectivities of C7 isomers over Pt/L-IM-5 and Pt/ZSM-5 catalysts.

In order to clearly clarify the effect of pore structure of L-IM-5 and ZSM-5 on their isomerization performance, the detailed product distributions over the Pt/H-zeolite at approximately 60% of n-C7 conversion is given in Table 4. The major isomer products were 2-methylhexanes, 3-methylhexanes as well as mono-branched C7 alkanes for all the Pt/H-zeolite catalysts. Compared with ZSM-5, the selectivity of 20


Page 21 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C7 isomers for L-IM-5 increased from 30.8% to 36.8%. Furthermore, there is no dimethyl-pentane and ethyl-pentane formation for ZSM-5 catalyst as compared with L-IM-5. The micropore size distributions of L-IM-5 and ZSM-5 (Figure 3) indicate that the two zeolites has very similar pore size, the only difference is the pore structure. So, we speculate that the pore structure of IM-5 is more conducive to the formation and diffusion of C7 isomers than that of ZSM-5, which can be further proved by the better diffusion properties of L-IM-5. Besides, propane and i-butane were the main cracking products both for L-IM-5 and ZSM-5 samples. The yield of cracking products were higher than that of C7 isomers due to acidity and shape selectivity of these two zeolites, C7 isomers can be formed, but its outward diffusion is hindered to some extend, and a significant cracking side reaction occurred. As shown in Table 3, the iso / n molar ratios of cracking products were always smaller than 1 for the two catalysts, which reveals that the main cracking route is β-scission of the isomer products, not direct cracking of n-C7. 11,44,51 As is known to all, hydroisomerization and hydrocracking reactions occur simultaneously in the hydroconversion of n-alkanes. The isomers’ selectivity depends on the relative rates of diffusion and consecutive reactions (hydrocracking or hydroisomerization). Hollo used kinetic studies to investigate the hydroisomerization performance of Pt/MOR catalyst with various n-alkanes as feedstock and found that the n-alkanes (C > 6) hydroisomerization reaction is limited by diffusion at higher pressures

52

. Accordingly, zeolite structure will affect the molecular diffusion inside

the zeolite pores, especially for the bulkier branched isomers. Even though having 21



similar pore openings of ~ 0.54 nm, the distinct difference of pore structures between ZSM-5 and IM-5 is that ZSM-5 contains straight and Zig-Zag channels, while IM-5 contains limited 2.5 nm thick three-dimensional “nano-slab” pore channel and triple cell volume than ZSM-5. 29 The kinetic diameter of n-C7 molecules is 0.43 nm, it can easily enter the channels of L-IM-5 and ZSM-5 zeolites. However, the dibranched isomers products formed in the intersections of ZSM-5 will be rapidly cracking because they are too bulky to diffuse out of the channels. Thus, given the pore structure and there is no dibranched C7 isomers in the products in Table 4, the reaction path for ZSM-5 zeolite can be descried as: first n-C7 is hydroisomerized into mono-branched C7 isomers, then some mono-branched C7 isomers undergo cracking reaction subsequently, as shown in Supporting Scheme S1. For L-IM-5, limited 2.5 nm thick three-dimensional “nano-slab” pore system make it possess large pore structure features. 29 This distinctive pore structure enables IM-5 to accommodate bulky intermediates in catalytic reactions.Therefore, the mono-branched and di-branched C7 isomers can be formed and quickly diffuse out of the “nano-slab” pore channel, alleviating subsequent cracking that would consume the branched C7 isomers. Thus, as the data shown in Table 4, the reaction path for L-IM-5 is the n-C7 is first hydroisomerized into mono-branched C7 isomers, then some monobranched C7 isomers continue to be hydroisomerized into di-branched C7 and some mono-branched C7 isomers undergo cracking reaction, as shown in Supporting Scheme S2. Table 4. Product distributions at approximately 60% n-C7 conversion over Pt/H-zeolite. 22


Page 22 of 42

Page 23 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Sample

IM-5

L-IM-5

M-IM-5

ZSM-5

Reaction temperature, K Conversion, % i-C7 selectivity, wt% Product distribution, wt% C2 C3 i-C4 n-C4 C5 C6 2-MC6 3-MC6 2,4-DMC5 3,3-DMC5 EC5

513 66 32.9

513 61 36.8

513 54 57.0

513 56 30.8

0.2 33.5 31.2 1.3 0.5 0.3 16.7 13.9 1.2 0.7 0.4

0.2 32.2 28.2 1.2 0.9 0.5 18.3 15.7 1.6 0.5 0.8

0.1 21.2 18.9 0.5 1.4 0.8 28.3 22.9 3.6 0.3 1.9

0.4 34.8 30.1 1.5 0.8 0.5 16.9 13.9 0.0 0.0 0.0

MC6 : Monomethyl-hexane;

DMC5 : Dimethyl-pentane;

EC5 : Triethyl-pentane;

C7 isomers = mono + di-branched isomers

Moreover, the chain length of n-alkane also has influence on the hydroisomerization reaction. Generally, the longer the chain of n-alkanes, the more prone to occurring the hydrocracking reactions.

19,53

So we also compared the long

chain-length n-C16 hydroisomerization performance for L-IM-5 and ZSM-5 catalysts. It is worth noting that there is no C15 nor C14 hydrocarbons formation and only little ethane was observed in our experimental results, which indicates that the hydrogenolysis was excluded under experimental conditions. 54 As shown in Figure 7, the trend of n-C16 conversion with increaseing reaction temperature is the same as that of n-C7 under the same reaction conditions, but the n-C16 conversion over these two catalysts are lower than that of n-C7 conversion. Clearly, the L-IM-5 and ZSM-5 samples showed very similar n-C16 conversion under different temperature despite their difference in pore structures. The acidity data in Table 1 shows that the L-IM-5 23



and ZSM-5 zeolits have the same amount of strong acid sites and similar acid strength. Hence, we conclude that the n-alkane conversion is mainly controlled by the acidity of L-IM-5 and ZSM-5 zeolites.

14 100

L-IM-5 ZSM-5

12 i-C16 selectivity (wt%)

n-C16 conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 42

80

60

40

20

L-IM-5 ZSM-5

10 8 6 4 2

0 500

520

540

0

560

20


40 60 80 n-C16 conversion (%)

100

Figure 7. The n-C16 conversion and selectivity of C16 isomers over Pt/L-IM-5 and Pt/ZSM-5 catalysts.

It is believed that hydrocracking ability depends on the acid strength of catalysts, especially for long chain-length n-alkane.

18

Therefore, in the case of n-C16, cracking

is the main reaction even at very low conversion over these two catalysts as shown in Figure 7. In addition, due to the shape selectivity of 10-ring zeolites, the rearrangement of alkyl carbons seems to be limited. It has been demonstrated that larger channel and weak acidity rather than medium pores are desired for long chain-length n-alkane.

55-58

Table 5 lists the product distribution of all catalysts at

approximately 35% conversion in n-C16 hydroisomerization reactions. The main product is cracking product and the highsest selectivities of C16 isomers of L-IM-5 and ZSM-5 zeolites were only 12.64% and 8.22%, respectively. However, unlike n-C7 hydroconversion, the selectivity of C16 isomers decreases gradually with the n-C16 24


Page 25 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


conversion increases under the present reaction conditions. The reason for this result is the more severe cracking reaction caused by the higher reaction temperature. Compared with ZSM-5, the selectivity of C16 isomers for L-IM-5 significantly increased from 4.01% to 7.68% (increased 91.5%) due to its unique limited 2.5 nm thick “nanoslab” pore structure. These is no di-branched C16 isomers were detected and the major isomer is 6-MC15 ( 6-methylpentadecane ) both for the two catalysts, which is above 40% of the overall isomers yield. The yield of different C16 isomers both increases in the order of 7-MC15 < 4-MC15 < 3-MC15 < 2-MC15 < 6-MC15. So, considering the medium pore structure and no bi-branched C16 isomers were observed, the n-hexadecane hydroisomerization for IM-5 and ZSM-5 zeolites were mainly “ pore mouth ” reaction mechanism. Besides, compared with L-IM-5, the yield of cracking products below C9 of ZSM-5 was higher whereas lower for above C9. Among the cracking products, the total yield of i-(C9 – C13) were 26.66 wt% for L-IM-5 and 18.43 wt% for ZSM-5. These data indicate that ZSM-5 prefers cracking reaction due to its three-dimensional 10-ring cross-channel. However, the reaction path of long-chain alkanes n-C16 will be different from that of short-chain alkanes n-C7 over IM-5 and ZSM-5 zeolites due to significant diffusion limitation in the medium pore system and strong acidity. 55,58 Comparing the data shown in Figure 6 and Figure 7, the isomers’ selectivity decreases drastically with increasing chain length of reacant, indicating the increasing tendency of cracking reaction. In the process of n-C16 hydroconversion, mono-branched C16 isomers and cracking products were both primary reaction products, but cracking is the main 25



Page 26 of 42

reaction even at low n-C16 conversion. The products of primary cracking undergo further transformations in the channels of L-IM-5 and ZSM-5 zeolites, because di-branched isomers cannot be formed directly from n-C16, but only from the mono-branched isomers. No di-branched C16 isomers was observed even at high conversions (data in Supporting Table S2), indicating that reaction path of n-C16 hydroconversion over L-IM-5 and ZSM-5 zeolites can be described as: first some

n-C16 is hydroisomerized into mono-branched C16 isomers, and some n-C16 is cracked into smaller molecules, then some mono-branched C16 isomers undergo cracking reaction subsequently, as shown in Supporting Scheme S3.

Table 5. C16 isomers distribution at approximately 35% n-C16 conversion over Pt/H-zeolites. Sample

IM-5

L-IM-5

M-IM-5

ZSM-5

Reaction temperature, K Conversion, % i-C16 selectivity, wt% 2-MC15 3-MC15 4-MC15 5-MC15 6-MC15 7-MC15

513 43 4.67 0.71 0.59 0.00 0.00 2.78 0.59

513 32 7.68 1.56 1.14 0.00 0.00 3.89 0.95

513 28 14.76 2.68 2.1 1.83 2.18 4.97 1.00

513 33 4.01 0.82 0.69 0.49 0.00 1.85 0.16

Product distribution, wt% n-C2 n-C3 n-C4 i-C4 n-C5 i-C5 n-C6 i-C6 n-C7 i-C7 n-C8 i-C8

0.02 1.50 5.80 0.61 6.32 4.79 9.5 5.14 8.64 7.17 3.47 8.82

0.03 1.38 5.42 0.55 6.54 4.80 7.81 3.43 7.65 5.89 3.43 8.24

0.01 1.02 5.05 0.25 5.65 3.06 4.77 2.88 5.23 4.40 5.32 7.66

0.05 1.13 7.12 2.46 6.25 4.51 9.48 5.04 9.55 8.02 6.28 8.71

26


Page 27 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


n-C9 i-C9 n-C10 i-C10 n-C11 i-C11 n-C12 i-C12 n-C13 i-C13

3.78 7.68 2.01 6.39 1.65 5.54 1.08 3.36 0.41 1.61

3.96 7.56 2.24 6.65 1.82 6.37 1.55 3.89 0.69 2.19

3.98 6.16 3.34 7.55 2.15 7.74 2.09 4.34 0.88 2.71

4.15 6.80 2.55 4.94 1.41 3.32 0.76 2.34 0.29 1.03

MC16 : Monomethyl-pentadecane; C16 isomers = mono-branched isomers.

3.3.2. Effect of Acidity To illustrate the effect of acidity on the isomerization performance of IM-5 zeolite, different chain length n-alkanes were used as feedstocks in the hydroconversion reaction over IM-5 and L-IM-5 catalysts. For the n-C7 hydroisomerization reaction (Figure 8), IM-5 catalyst has a higher n-C7 conversion than that of L-IM-5 at different temperature reaction. Furthermore, IM-5 catalyst also shows a higher catalytic activity for the conversion of n-C16 (as shown in Figure 9). The acidity properties are main reason leading to the different n-alkane conversion for IM-5 and L-IM-5 catalysts, as they have the same pore structure. Theremore, the acidity properties listed in Table 1infer that n-alkane hydroconversion of these catalysts are strongly related to their concentration of strong acid sites. Thus, IM-5 catalyst which has higher amount of strong acid sites (Table 1), shows the highest conversion in n-alkane hydroisomerization reaction (Table 3 and Table 4). Besides, Figure 8 shows that the selectivity of C7 isomers increase in the beginning and then decrease for both catalysts with increasing the conversion of n-C7. The maximum selectivity of C7 isomer for IM-5 and L-IM-5 were 32.9% and 36.8%, respectively. 27



The slight increase for i-C7 selectivity over L-IM-5 as compared with that of IM-5 can be attributed to its lower amount of strong acid sites. However, for the n-C16, the selectivity of isomers over both catalysts decreases dramatically as n-C16 conversion increases, which is different from the n-C7 hydroconversion. And the highest selectivity of C16 isomers for L-IM-5 was only 12.64%, which is much lower than C7 isomers (36.8%). The reason for the difference is the strong acid strength and 10-ring selectivity of IM-5 zeolite, leading to severe cracking of long chain n-C16. 55-57 Bi et al. found that the activity of Pt/ZSM-22 catalyst on n-C16 hrdroisomerization reactions increases linearly with increasing the acid strength and the concentration of acid sites, but higher acid strength of zeolite can decrease the selectivity of C16 isomers even at an initial conversion and that lower concentration of acid sites on zeolite is beneficial to the improvement of isomers yield

56

. In addition, the detail isomers product

distributions of n-C7 and n-C16 hydroconversion were similar for IM-5 and L-IM-5, which indicate that the reaction path were same for the two catalysts.

40

100

IM-5 L-IM-5

IM-5 L-IM-5

80


n-C7 conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 42

60

40

30

20

10

20 480

500

520

20

540


40

60 n-C7 conversion (%)

Figure 8. The n-C7 conversion and selectivity of C7 isomers over Pt/IM-5 and Pt/L-IM-5 catalysts.

28


80

Page 29 of 42

14 100 12 80


n-C16 conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


IM-5 L-IM-5

60 40 20

IM-5 L-IM-5

10 8 6 4 2

0 500

520

540

0

560

0


20

40 60 80 n-C16 conversion (%)

100

Figure 9. The n-C16 conversion and selectivity of C16 isomers over Pt/IM-5 and Pt/L-IM-5 catalysts.

3.3.3. Effect of Mesoporosity In order to further improve the isomerization performance of IM-5 zeolite, mesoporosity was introduced into the IM-5 zeolite and its effect on the isomerization performance was also investigated. Unfortunately, the introduction of mesoporosity in IM-5 did not increase the n-C7 conversion (Figure 10), which is due to the decrease of strong acid sites. The L-IM-5 and M-IM-5 samples have similar n-C7 conversion, despite their significant differences in mesopore volume. It could be further explained that the n-C7 conversion strongly dependent on the amount of strong acid sites over IM-5 zeolite and seemed to be irrelevant to mesoporosity. The data in Table 3 shows that the maximum selectivity of C7 isomer increases in the order of IM-5 < L-IM-5

Investigation on n-Alkane Hydroisomerization, a Comparison of IM-5

Recommend Documents