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Control of the Al Distribution in the Framework of ZSM-5 Zeolite and Its Evaluation by Solid-state NMR Technique and Catalytic Properties Toshiyuki Yokoi, Hiroshi Mochizuki, Seitaro Namba, Junko N Kondo, and Takashi Tatsumi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b03289 • Publication Date (Web): 11 Jun 2015 Downloaded from http://pubs.acs.org on June 14, 2015

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The Journal of Physical Chemistry C 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.

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The Journal of Physical Chemistry

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Control of the Al Distribution in the Framework of ZSM-5 zeolite and

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Its Evaluation by Solid-state NMR Technique and Catalytic Properties

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Toshiyuki Yokoi*, Hiroshi Mochizuki, Seitaro Namba, Junko N. Kondo and Takashi Tatsumi

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Chemical Resources Laboratory, Tokyo Institute of Technology,

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4259 Nagatsuta, Midori-ku,Yokohama 226-8503, Japan.

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*

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Fax number: +81-45-924-5282, E-mail address: [email protected]

Corresponding author:

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Abstract

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The effects of the organic structure-directing agents (OSDAs) and Na cations for the synthesis of

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ZSM-5 on the location of Al atom in the framework as well as the acidic and catalytic properties were

16

investigated.

17

organic structure-directing agents (OSDAs) including tetrapropylammonium hydroxide cations,

18

dipropylamine, cyclohexylamine and hexamethyleneimine with or without Na cations. In-situ FT-IR

19

spectroscopy using CO as probe molecule was applied to the evaluation of the acid property of the

20

ZSM-5 zeolites.

21

NMR techniques.

22

sites in the micropores.

23

transition-state shape-selectivity through the cracking of n-hexane and 3-methylpentane.

24

the cracking of various types of paraffins and the conversion of aromatic compounds were conducted to

25

clarify the acid site distributions.

To achieve these purposes, ZSM-5 zeolites were synthesized by using four kinds of

The location of Al atoms was examined by high resolution 27Al MAS and MQMAS The constraint index (CI) has been also used to estimate the distribution of acid The location of acid sites was investigated based on the difference in the Furthermore,

26 27

KEYWORDS: MFI-type zeolite, organic structure-directing agents, Al distribution in the framework,

28

constraint index, 27Al MAS and MQMAS NMR

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1. Introduction

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Zeolites are widely used as heterogeneous catalysts in industrial chemical processes, because

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of their strong acidity and shape selectivities.

The acidic properties of aluminosilicate-type zeolites

4

originate from the presence of protons balancing the negative charge induced by the framework Al

5

atoms in tetrahedral sites (T sites).

6

as pore structure, acid strength and acid amounts.

7

and distribution of Al atoms in the zeolite framework have been recognized as an important factor for

8

activity and selectivity, because they would profoundly affect the accessibility of molecules to acid

9

sites and the spatial constraints of the reaction field in the pores [1-13].

The catalytic properties of zeolites depend on various factors such In addition to these factors, recently, the location

Furthermore, the relationship

10

between the distribution of Al atoms in the framework and the acid strength has not been fully

11

understood.

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framework Al atoms and their control in the pores, thus challenging issues have not been completely

13

resolved to date.

Although zeolite researchers have seriously tackled the estimation of the distribution of

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It is well recognized that, to balance the charge, Al3+ species are located near cations

15

including inorganic cations, e.g., Na+, K+ or organic ones, such as quaternary ammonium ions as

16

organic structure-directing agents (OSDAs).

17

and the location of Al atoms may be dependent on their size and type.

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of the distribution of Al in the FER-type zeolite by using different organic SDAs in the presence or

19

absence of Na cations has been developed [2-7, 10]. Recently, the relationship between the size of

20

cyclic amine in the synthesis gel and the distribution of acid sites has been reported [6, 7].

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recently, we have revealed that the Al distribution over the RTH-type framework was clearly

22

dependent on the type of the cations by the high-resolution 27Al MAS NMR and 27Al MQMAS NMR

23

techniques [14]. Namely, the location of Al atoms in zeolites can be controlled by rational choice of

24

the cations.

Hence, the number of the cations affects the Al content, A synthesis strategy to control

Very

25

We have tackled the control of the of Al atoms in the MFI-type aluminosilicate zeolite

26

(ZSM-5), which has been widely used as solid acid catalyst in numerous petrochemical catalytic

27

processes such as cracking, isomerization, aromatization and alkylation processes [15].

28

by controlling the location of Al atoms in the pore, we have aimed to develop the ZSM-5 catalyst that

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shows a high catalytic performance in the catalytic cracking of naphtha to selectively produce light

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olefins such as ethylene and propylene, which are important basic raw materials for the petrochemical

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industry.

2 ACS Paragon Plus Environment

In particular,

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The MFI structure has crystallographically distinct 12 T sites and consists of parallel and

2

straight 10-membered ring (MR) channels intersected by sinusoidal 10-MR channels [16].

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the sizes of both 10-MR channels are similar to that of the aromatic ring (ca. 5.5 Å), the intersections

4

of these 10-MR have large spherical spaces (ca. 10 Å in diameter).

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T6 are not facing to the intersections (Scheme 1). ZSM-5 zeolite with a wide variety of Si/Al ratios

6

ranging from 10 to ∞ has been hydrothermally synthesized with tetrapropylammonium (TPA) cation

7

[17-21].

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extending into both the straight and sinusoidal channels [18-21].

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Al atoms in ZSM-5 synthesized with TPA cations in the absence of Na cations are selectively located

10

at the intersections, and that the Al atoms in H-ZSM-5 synthesized with both TPA and Na cations are

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located not only at the intersections but also in narrow straight and/or sinusoidal channels.

12

Meanwhile, there are many reports on the synthesis of ZSM-5 using various organic molecules such as

13

amines and alcohols [22-26].

14

H-ZSM-5 zeolite can be controlled by the types of organic and inorganic cations used.

15

Although

Among the 12 T sites, T1, T4 and

The TPA cation species are located at the channel intersections with the propyl chains Thus, it has been assumed that the

We have expected that the locations of Al atoms in the pores of

Methods for the evaluation of the distribution of the acid site in the pores have attracted a

16

considerable interest and also been extensively investigated [10].

17

FER-type zeolite was investigated by the adsorption of pyridine on SiOHAl groups [4].

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Dedecek and his co-workers reported that the Al distribution of ZSM-5 was estimated based on Co(II)

19

ion exchange capacity in combination with multinuclear MAS NMR analyses [8, 9].

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crystallographic techniques based on the Rietveld refinement have also been applied to the evaluation

21

of distribution of Al atoms in the framework of Si-rich zeolites [10].

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co-workers have applied a combination of extended X-ray absorption fine structure (EXAFS) analysis

23

and

24

estimate the Al distribution of Beta [12].

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the micropore based on the constraint index (CI), which is defined as the cracking rate of n-hexane to

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that of its isomer 3-methylpentane, and expressed as the following equation (1) [27-33].

27

27

Location of Al atoms in the In 2012,

Recently,

Very recently, Lercher and his

Al MAS NMR spectroscopy supported by DFT-based molecular dynamics simulations to In this study, we estimated the distribution of acid sites in

CI = kn-hexane / k3-methylpentane = log [1 - Xn-hexane] / log [1 - X3-methylpentane]

(1)

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where kn-hexane and k3-methylpentane are the rates of the cracking of n-hexane and 3-methylpentane,

29

respectively.

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“monomolecular cracking” via the penta-coordinated carbonium ion and/or the classical “bimolecular

31

cracking” route via the carbenium ion/β-scission mechanism involving a hydride transfer reaction

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(Scheme 1) [31-33].

Haag and Dessau have reported that the cracking of paraffin proceeds through the

In the bimolecular cracking, the transition state for 3-methylpentane is 3 ACS Paragon Plus Environment


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significantly larger than that for n-hexane, meaning that large space is required for the bimolecular

2

cracking compared to the monomolecular cracking [28].

3

carbenium ion/β-scission mechanism, the reactivity of paraffins having a tertiary carbon atom is higher

4

than that of n-paraffins.

5

and increased with a decrease in the size of void space [27-30].

6

impose more severe steric constraint on the bulky transition state of 3-methylpentane than on that of

7

n-hexane.

8

located in the intersections exhibits a lower CI value, and also that the distributions of acid sites should

9

affect catalytic properties for the reactions involving bulky transition states.

In general, in the cracking via the

However, in medium pore (10-MR) zeolites, the Cl value is larger than unity This would be because narrow spaces

Therefore, it is expected that the H-ZSM-5 zeolite with a larger amount of acid sites

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Here we report on the synthesis of ZSM-5 zeolite catalysts with the location of Al atoms in

11

the pores controlled by using different organic molecules, including tetrapropylammonium cation,

12

dipropylamine, cyclohexylamine and hexamethyleneimine, with or without Na cations.

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distribution of acid sites derived from the Al atoms in the framework was estimated from the CI value

14

as well as

15

catalysts were applied in the cracking of other paraffins and the conversion of aromatic compounds to

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clarify the influence of the distributions of acid sites on the catalytic performance.

27

Al MAS NMR and

27

Al MQMAS NMR techniques.

The

Finally, thus prepared ZSM-5

17 18 19

2. Experimental

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2.1. Synthesis of calcined silicalite-1 seeds

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Silicalite-1 used as seeds for preparing ZSM-5 was synthesized by a previously reported

22

procedure as follows [34].

The gels with compositions of 1 SiO2: 0.25 TPAOH: 8.3 H2O were

23

prepared from tetraethyl orthosilicate (TEOS, Tokyo Kasei, >96 %), and tetrapropylammonium

24

hydroxide (TPAOH, 40 % aqueous solution, Alfa Aesar).

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crystallized at 443 K for 24 h.

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sample in an oven at 823 K.

The gel was stirred at 80 ºC for 24 h and

The silicalite-1 was obtained by calcination of the as-synthesized

27 28

2.2. Synthesis of zeolite catalysts

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ZSM-5 zeolites were synthesized using four kinds of organic molecules including

30

tetrapropylammonium hydroxide (TPA) cations, dipropylamine (Dpa, Tokyo Kasei, >99.0 %),

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cyclohexlyamine (Cha, Sigma-Aldrich, 99 %) and hexamethyleneimine (Hmi, Sigma-Aldrich, 99 %) 4 ACS Paragon Plus Environment

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with or without Na cations.

For example, the samples synthesized by using TPA cation with or

2

without Na cations were denoted by [TPA, Na] and [TPA], respectively.

3

The [TPA, Na] and [TPA] samples were synthesized from the mother gels with the molar

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compositions of 1 SiO2: 0.01 Al2O3: 0.25 TPAOH: 0.1 or 0 NaCl: 8.3 H2O and 1 SiO2: 0.01 Al2O3: 0.5

5

TPAOH: 8.3 H2O [35, 36].

6

sodium chloride (Wako, 99.5 %) were used as Al and Na sources, respectively.

7

[TPA, Na] and [TPA] were stirred at 80 ºC for 24 h before crystallization, and then hydrothermally

8

treated at 443 K for 3 and 7 days, respectively.

For the syntheses, aluminium nitrate nonahydrate (Wako, 99.9 %) and The mother gels of

9

For the synthesis of [DPa, Na] and [Cha, Na], the mother gels with the molar compositions

10

of 1 SiO2: 0.01 Al2O3: 0.4 Dpa or Cha: 0.1 Na2O: 25 H2O were prepared from colloidal silica (Ludox

11

HS-40, Sigma-Aldrich), DPa or Cha, aluminium nitrate nonahydrate and sodium hydroxide (Wako,

12

97 %).

13

calcined silicalite-1 was added to the mixture as a seed.

14

crystallized at 443 K for 2 days.

The prepared gels were stirred at ambient temperature for 30 min.

Then, 5 wt% of the

Thereafter, the resultant gels were

15

For the synthesis of [HMi, Na], the gel with the molar compositions of 1 SiO2: 0.01 Al2O3:

16

0.5 HMi: 0.1 Na2O: 25 H2O: 0.07H2SO4 was prepared from colloidal silica, Hmi, aluminium nitrate

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nonahydrate, sodium hydroxide and sulfuric acid (Wako, 95 %), and were stirred at ambient

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temperature for 30 min. After the addition of 5 wt% of the calcined silicalite-1 to the mixture, the

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prepared mother gel was crystallized at 433 K for 2 days.

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All of the as-synthesized ZSM-5 zeolites were calcined in an oven at 823 K to remove

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OSDAs.

The NH4-type ZSM-5 (NH4-ZSM-5) zeolites were obtained by ion-exchanging the calcined

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ZSM-5 zeolites with 1 M NH4NO3 aq. at 353 K for 3 h twice.

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converted to the H-type ones (H-ZSM-5) by calcination at 823 K for 10 h.

The NH4-ZSM-5 samples were

24

As a control, NH4-ZSM-22 and NH4-ZSM-12 with a Si/Al ratio of ca. 50 were synthesized

25

by hydrothermal synthesis and the following ion exchange using an NH4NO3 solution [37].

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H-mordenite with a Si/Al ration 45 is a JRC reference catalyst (JRC-Z-HM90).

27 28

2.3. Characterizations

29

XRD patterns were collected on a Rint-Ultima III (Rigaku) using a Cu Kα X-ray source (40

30

kV, 20 mA). Nitrogen adsorption measurements to determine the BET surface area (SBET), external

31

surface area (SEXT) and micropore volume (Vmicro) were conducted at 77 K on a Belsorp-mini II (Bel

32

Japan).

SEXT and Vmicro were estimated by the t-plot method. 5 ACS Paragon Plus Environment

Field-emission scanning electron


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microscopic (FE-SEM) images of the powder samples were obtained on an S-5200 microscope

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(Hitachi) operating at 1-30 kV.

3

Co.) without any metal coating.

4

inductively coupled plasma-atomic emission spectrometer (ICP-AES, Shimadzu ICPE-9000).

5

content of the samples was determined by using atomic absorption spectrometer (AAS, Shimadzu

6

AA-6200).

7

weight loss from 573 to 1073 K in a thermogravimetric (TG) profile, which was performed on a

8

thermogravimetric-differential thermal analyzer (TG-DTA, RigakuThermo plus EVO II).

9

analysis data were obtained from a Vario MACRO cube (Elementar Analysensysteme GmbH).

10

The sample was mounted on a carbon-coated microgrid (Okenshoji The Si/Al ratio of the samples was determined by using an The Na

The amounts of OSDAs and coke formed during the reaction were determined from the

The high-resolution

27

Elemental

Al MAS NMR, and 27Al 3Q MQMAS NMR spectra of the NH4-type

11

ZSM-5 were obtained on a JEOL ECA-600 spectrometer (14.1 T) equipped with an additional 1 kW

12

power amplifier.

13

samples were spun at 17 kHz by using a 4 mm ZrO2 rotor.

14

recorded by using a single pulse, the pulse width was set at 0.1 µs and 10000 scans were accumulated

15

at a sample spinning rate of 17 kHz.

16

enough to permit quantitative analysis of zeolite samples.

17

excitation pulse and the 3Q-1Q conversion pulse were 5.5 and 2.1 µs, respectively, and z-filter was 0.2

18

ms.

19

The

27

Al chemical shift was referenced to AlNH4(SO4)2·12H2O at -0.54 ppm and For 27Al MAS NMR spectra, which were

A 100 ms relaxation delay was determined so as to be long For 27Al 3Q MQMAS NMR spectra, the 3Q

The relaxation delay time was 10 ms. Temperature-programmed ammonia desorption (NH3-TPD) profiles were recorded on a

20

Multitrack TPD equipment (Japan BEL).

21

mL min-1) for 1 h and then cooled to 423 K.

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at 423 K for 1 h. Approximately 2500 Pa of NH3 was allowed to make contact with the sample at 423

23

K for 10 min.

24

temperature for 30 min.

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ramping rate of 10 K min-1 in a He flow (50 mL min-1).

26

desorbed NH3 (m/e = 16).

27

Typically, 30 mg catalyst was pretreated at 923 K in He (50 Prior to the adsorption of NH3, the sample was evacuated

Subsequently, the sample was evacuated to remove weakly adsorbed NH3 at the same Finally, the sample was cooled to 423 K and heated from 423 to 1073 K at a A mass spectrometer was used to monitor

The amount of acid sites was determined by using the area in the profiles.

Fourier transform infrared (FT-IR) spectra were obtained at a resolution of 4 cm-1 by using a

28

Jasco 4100 FTIR spectrometer equipped with a mercury cadmium telluride (MCT) detector.

A total

29

of 64 scans were averaged for each spectrum.

30

153 K to obtain background spectra.

31

diameter, 30 - 60 mg) and placed in an IR cell attached to a closed-gas circulation system.

32

sample was pretreated by evacuation at 773 K, followed by adsorption of 2-30 Pa CO at 153 K.

IR spectra of the clean disk were recorded in vacuo at

The sample was pressed into a self-supporting disk (20 mm


The Then,

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the sample was evacuated at the same temperature for 30 min.

The spectra were recorded at 153 K.

2

The IR spectra resulting from the subtraction of the background spectra from those with CO adsorbed

3

are shown unless otherwise noted.

4 5 6

2.4. Catalytic reaction Catalytic reactions were performed by using a fixed-bed reactor equipped with an on-line

7

gas-chromatograph.

The catalytic reactions were carried out in a 6 mm quartz tubular flow

8

microreactor loaded with 10 to 200 mg of 50/80 mesh zeolite pellets without a binder.

9

was centered at the reactor in a furnace.

Ar was used as a carrier gas.

The catalyst

The hydrocarbon products

10

were analyzed with an on-line gas chromatograph (Shimadzu GC-2014) with an FID detector.

11

Hydrogen was analyzed by a gas chromatograph (Shimadzu, GC-2014) with a TCD detector.

12

The cracking of 1,3,5-triisopropylbenzene (1,3,5-TIPB, Tokyo Kasei, 95.0%) was conducted

13

at 673 K in the presence or absence of 2,4-dimethylquinoline (2,4-DMQ, Tokyo Kasei, >95.0 %) to

14

poison selectively the acid sites on the external surfaces [38].

15

Ar at 823 K for 1 h prior to the reaction, and then cooled to the desired reaction temperatures.

16

initial partial pressure of 1,3,5-TIPB was set at 0.5 kPa with the W/F (W: amount of catalyst /g, F: total

17

flow rate /mol h-1) of 0.2 g h moltotal-1.

The catalyst was activated in flowing The

18

The cracking of C6 paraffins including n-hexane (Sigma-Aldrich, >99.0 %) and

19

3-methypentane (Tokyo Kasei, >99.0 %) was carried out at 623-673 K in the presence of 2,4-DMQ.

20

The catalyst was activated in flowing Ar at 823 K for 1 h prior to the reaction, and then cooled to the

21

desired reaction temperatures.

22

W/F, in particular the amount of the catalyst, was adjusted so as to obtain < 20 % of the C6 paraffin

23

conversion.

24

individual product) / (mol number of C6 paraffins reacted).

25

kinetics plot, the reaction rate constant was calculated by assuming that the paraffin conversion follows

26

a first-order kinetic model using 3-4 points [39].

27

methylcyclohexane (Tokyo Kasei, >99.0 %) were also conducted at 923 K.

28

pressure of the paraffins was set at 40 kPa with the W/F of 2.7 g-cat h moltotal-1.

29

The initial partial pressure of the C6 paraffins was set at 40 kPa.

The

The selectivities and the yields were expressed as mol% defined as (mol number of each From the slopes of the first-order

The cracking reactions of n-hexane and The initial partial

The conversions of toluene and m-xylene were conducted at 573-648 K in the presence of

30

2,4-DMQ.

The initial partial pressure of the aromatic compounds was set at 20 kPa.

31

conversion of toluene, the W/F was adjusted so as to obtain < 3 % of the conversion, and the reaction

32

rate was calculated from the dependence of W/F on the conversion under differential reaction 7 ACS Paragon Plus Environment

In the


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conditions.

In the conversion of m-xylene, the W/F was adjusted so as to obtain < 35 % of the

2

conversion, and the reaction rate constant were calculated by the slopes of the first-order kinetics plot.

3

[40].

4 5

2.5. Estimation of constraint index

6

The estimation of constraint index (CI) value was carried out at 673 K by using

7

single-component feeds in order to unambiguously assess the true ratio of the rates of the cracking of

8

n-hexane and 3-methylpentane.

9

The W/F was adjusted to obtain 15 -20 % of the conversion.

The initial partial pressure of the C6 paraffins was set at 40 kPa.

10 11 12

3. Results and discussion

13

3.1. Synthesis and physicochemical properties of H-ZSM-5

14

The XRD patterns of the products synthesized with various organic molecules indicate that

15

all of the samples were identified as the MFI structure with a high crystallinity (supporting

16

information, Fig. S1).

17

revealed by the FE-SEM images, the crystallite sizes of [TPA] and [TPA, Na] were ca. 100 nm, that of

18

[HMi, Na] is 200 - 300 nm, and larger sized crystals were formed when DPa and Cha were used as

19

OSDAs (ca. 500 - 800 nm) (Fig. S2).

20

There is a marked difference in the crystallite size of the products.

As

The Al contents determined by ICP analysis were 0.31-0.38 mmol/g for all of the samples

21

(Table 1), being close to those of the gels for synthesis.

22

except for the [TPA] sample was lower than that of Al atoms, indicating that the negative charges of

23

Al atom were balanced by the used organic cation in addition to the Na cations.

24

The amount of Na species in the samples

The content of the organic molecules in the as-synthesized samples are summarized in Table

25

1.

The C/N atomic ratios, which were calculated from the CHN elemental analysis, for [TPA], [TPA,

26

Na] [DPa, Na] [Cha, Na] and [HMi, Na] are 12.3, 12.3, 6.0, 6.2 and 6.1, respectively, being similar to

27

the C/N ratios of these molecules (12 for TPA and 6 for the others).

28

of the as-synthesized samples showed that the molecular structures of all of the OSDAs were

29

completely retained (Fig. S3).

30

in diameter), the organic molecule used would be located in the micropores with its molecular

31

structure intact.

The 13C CP MAS NMR spectra

Considering the size of the intersection of the MFI-structure (ca. 9 Å


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1

Based on the TG-DTA and CHN elemental analyses, the numbers of the organic molecule

2

per unit cell of the MFI-structure were estimated at approximately 4.2, 4.1, 4.6, 3.5 and 6.5 for [TPA],

3

[TPA, Na], [DPa, Na], [Cha, Na] and [HMi, Na], respectively.

4

of four intersections, one can suppose that all of the intersections are fully occupied by the organic

5

molecules.

6

were in the framework.

7

samples.

8

was calculated at 0.5-1.6.

9

that of Al atoms, implying that the excess of positive charges introduced by the organic and Na cations

10

would be balanced by structural defects, namely ≡SiO- groups. The presence of the defect sites was

11

supported by the FT-IR spectra of the samples after the evacuation (Fig. S4).

The

27

Considering that one unit cell consists

Al MAS NMR spectra (Fig. 2) revealed that all of the Al species in the zeolites The number of Al atom per unit cell was calculated at 1.8 - 2.2 for all the

For [TPA, Na], [DPa, Na], [Cha, Na] and [HMi, Na], the content of Na cations per unit cell Hence, the total of the contents of organic and Na cations were higher than

12

The physicochemical properties of H-ZSM-5 samples are listed in Table 2.

The N2

13

adsorption and desorption isotherms for all of the H-ZSM-5 zeolites exhibited a typical patterning of

14

microporous materials with a plateau at high relative pressures (type I, IUPAC).

15

area (SBET) and the external surface area (SEXT) of [TPA], [TPA, Na] and [HMi, Na] were slightly larger

16

than those of [DPa, Na] and [Cha, Na].

17

volume (Vmicro) (ca. 0.16 - 0.18 cm3 g-1) and the internal (micropore) surface area, which is defined as

18

the difference between SBET and SEXT (ca. 380 - 400 m2/g), among the H-ZSM-5 samples.

19

findings indicated that the prepared H-ZSM-5 samples are similar in structural qualities.

20

amounts estimated by NH3-TPD measurement are approximately consistent with the amount of Al in

21

the bulk, which was determined by ICP analysis, indicating that almost all of the Al atoms are

22

incorporated into the framework and work as acid sites.

The BET surface

However, there is no marked difference in the micropore

These

The acid

23 24 25

3.2. Acidic property

26

Katada et al. reported that the local structure in zeolites is correlated with Brønsted acid

27

strength [41]; the acidic strength must be dependent on the location of Al atoms in the framework.

28

In-situ FT-IR spectroscopy using CO as probe molecule was applied to the evaluation of acidic

29

property of the zeolite samples.

30

molecular diameter and high sensitivity of the IR band frequency to the strength of the acid sites [42,

31

43].

The advantages of CO as probe are its very weak basicity, small



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1

Page 10 of 39

The ν(OH) and ν(CO) and regions of the CO-adsorbed FT-IR spectra are shown in Figs. 1 All of the samples exhibited the strong band at 2175 cm−1 (Fig. 5 (b)),

2

(a) and (b), respectively.

3

which corresponds to the stretching mode of ν(C≡O) on Brønsted acid site [42, 43]. Note that the

4

band at 2230 cm−1, which corresponds to the stretching mode of ν(C≡O) on Lewis acid site, was hardly

5

observed.

6

different organic molecules with or without Na cations and they have an extremely low concentration

7

of Lewis acid sites.

Therefore, Brønsted acid sites predominate in the H-ZSM-5 zeolites prepared by using

8

In general, when CO molecules are introduced into the FT-IR system, the band of acidic OH

9

groups was shifted e.g., from 3620 to 3306 cm−1 due to the interaction of CO with acidic OH groups,

10

and this shift is related to the acidic strength [42, 43].

However, in our case, the difference of ∆OH

11

was hardly observed in the H-ZSM-5 zeolites (Fig. 4 (A)).

12

Brønsted acidity of the H-ZSM-5 zeolites is not significantly affected by either the type of the organic

13

molecules or presence of Na cations.

These findings suggest that the strength of

14 15 16 17

3.3. Al state in framework Solid-state NMR techniques with a high magnetic field (600 MHz, 14.1 T) were utilized to 27

18

investigate the environment of Al species in detail.

19

the NH3-ZSM-5 samples, indicating that the spectra of all the samples exhibited an intense peak at ca.

20

55 ppm, which is assigned to tetrahedrally coordinated Al in the framework.

21

assigned to octahedral coordinated Al atom was not observed in any samples.

22

Figure 2 shows the

Al MAS NMR spectra of

The peak at 0 ppm

Furthermore, 27Al MQMAS NMR was measured to characterize the state of Al atoms in the MFI

23

framework.

24

distribution of Al species in the framework.

For all the samples, only tetrahedrally coordinated Al

25

atoms were observed in the MQMAS spectra.

Figure 3 shows the representative 27Al MQMAS NMR

26

spectra in the region of framework Al species of the [TPA] and [TPA, Na] samples.

27

cross-sections indicated by the arrows 1, 2, 3, 4 and 5 were observed in both spectra; at least five

28

crystallographically distinct Al species were present in the zeolites.

29

The number of cross-sections in the MQMAS NMR spectrum allows us to estimate the

In general, the resolution of

27

Five

Al MAS NMR spectra is not high enough to detailedly

30

characterize Al species because a quadrupolar interaction at Al atom leads to a broadening of the peak

31

[44-47]. In this study, for obtaining high-resolution 27Al MAS NMR spectra at 14.1 T, the pulse

32

width was set at 0.1 µs and 10000 scans were accumulated at a sample spinning rate of 17 kHz. 10 ACS Paragon Plus Environment

A

Page 11 of 39

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1

100 ms relaxation delay could be regarded as long enough to permit quantitative analysis of the Al

2

species with different states based on the peak area.

3

ranging from 45 to 65 ppm attributed to the framework Al atoms were divided into five peaks at

4

around 52, 53, 54, 56 and 58 ppm and they are designated as Al(a), Al(b), Al(c), Al(d) and Al(e),

5

respectively (Fig. 4).

6

significant differences in the proportion between the samples; the proportion of Al(d) was low and that

7

of Al(c) was high for [TPA] compared to the other samples, that of Al(d) for [TPA, Na] was the

8

highest and that of Al(b) for [Cha, Na] was the highest.

9

proportions of Al(a).

Based on the MQMAS spectra, the broad peak

The proportions of these peaks are listed in Table 3. Note that there are

There are no significant difference in the

It is considered that the chemical shift of

27

Al MAS NMR spectrum of the

10

MFI-type zeolite is shifted to the higher magnetic field as the mean T-O-T angle is increased [48, 49].

11

The difference in the proportion would be caused by the different distribution of Al atoms over twelve

12

distinct T sites of the MFI structure.

13 14 15

3.4. Estimation of the distribution of acid sites in the framework

16

3.4.1. Constraint index (CI)

17

“Constraint index (CI)” was employed to estimate the distribution of acid sites derived from

18

the Al atoms in the framework.

First, the CI values of the typical H-type zeolites with the TON, MFI,

19

MTW and MOR topologies were estimated.

20

identified as zeolites with their own crystal phases with a high crystallinity (Fig. S5).

21

molar ratio of these zeolites was around 50 and the amount of acid sites estimated from NH3-TPD was

22

almost similar to the Al contents (Table S1). H-ZSM-22 with the TON topology and H-ZSM-12 with

23

the MTW topology have 10- and 12-MR 1-dimensional straight channels without a large cavity,

24

respectively.

25

side-pockets [16].

From their XRD patterns, all of the samples were The Si/Al

H-mordenite with the MOR topology has 12-MR straight channels and 8-MR

26

In the cracking of n-hexane (n-Hx) and 3-methylpentane (3-MP), the amount of catalysts

27

and the total flow rate of reactant gas (W/F) were adjusted so as to obtain 15 - 20% of the conversion.

28

Figure 5 (a) shows the CI values for the zeolites with different topologies at the reaction temperature of

29

673 K, indicating that the CI value is strongly dependent on the structure of the zeolite.

30

[TPA], the CI values were in good agreement with those reported in literature [27-30], and they were

31

decreased with an increase in the size of the pore.


Except for


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1

Page 12 of 39

The CI values of the H-ZSM-5 zeolites synthesized with different organic cations and/or

2

presence of Na cations were measured (Figure 5 (b)).

To avoid the influences of the acid sites on the

3

external surface on the catalytic performance, the cracking reactions of C6 paraffins were carried out

4

in the presence of 2,4-DMQ; the reaction occurred inside the micropores of ZSM-5; the difference in

5

the catalytic performance was caused by the difference in the distribution of Al atoms in the

6

framework, not the difference in the acid sites on the external surface.

7

Note that the CI value for [TPA, Na] (ca. 5.2) was remarkably higher than that for other

8

H-ZSM-5 zeolites although their structures and acidic properties are identical (Table 4), meaning that

9

the number of the acid sites located at the intersection in [TPA, Na] is very low compared to other

10

samples.

On the other hand, the CI values for [TPA], [DPa, Na] and [HMi, Na] were low, implying

11

that the acid sites are predominantly located at the intersection.

12 13

3.4.2. Total selectivities to methane, ethane and hydrogen (SCH4+C2H3+H2)

14

Hydrogen and hydrocarbons below C3 are scarcely formed in the bimolecular cracking,

15

because these products are only formed through energetically unfavorable primary carbenium ions.

16

In contrast, in the monomolecular cracking, a penta-coordinated carbonium ion formed by the

17

protonation of paraffins decomposes into a carbenium ion and hydrogen or lower paraffins containing

18

methane and ethane (Scheme 2).

19

(SCH4+C2H6+H2) can be regarded as a parameter of the contribution of the monomolecular cracking [31,

20

50], and are listed in Table 4, indicating that SCH4+C2H6+H2 in 3-MP cracking was decreased with an

21

increase in the size of the pore, and that the bimolecular cracking hardly occurred over H-ZSM-22; a

22

wider reaction spaces would be required for the bimolecular cracking of 3-MP that proceeds via a

23

bulky transition state.

24

Hence, the total selectivities to methane, ethane and hydrogen

In general, the monomolecular cracking requires a higher activation energy than the

25

bimolecular cracking [32, 33].

Therefore, to clarify the effect of the distribution of acid sites on the

26

reaction mechanism of the cracking of n-Hx and 3-MP over the H-ZSM-5 zeolites, the activation

27

energy and product distributions were examined.

28

selectively poison the acid sites on the external surfaces of ZSM-5 [38].

29

the acid sites on the external surface was confirmed by the cracking of 1,3,5-triisopropylbenzene

30

(1,3,5-TIPB), which is too large to enter the pore of H-ZSM-5 and will be cracked only by the acid site

31

on the external surface [51]; H-ZSM-5 poisoned with 2,4-DMQ were no longer active in the

32

1,3,5-TEPB cracking, indicating the complete poisoning of its external surface (Fig. S6).

First, the zeolites were treated with 2,4-DMQ to


The complete poisoning of

Page 13 of 39

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1

The Arrhenius plots for the cracking of n-Hx and 3-MP over the [TPA, Na] and [TPA]

2

samples from 623 to 673 K are shown in Fig. S7.

3

and activation energy were almost the same; the activation energies of [TPA] and [TPA, Na] were

4

estimated at 66.3 and 65.7 kJ mol-1, respectively.

5

the same (ca. 13 mol%, Table 4).

6

monomolecular cracking to the bimolecular cracking, and the bimolecular cracking mechanism

7

predominate predominant over the monomolecular cracking in [TPA, Na] and [TPA].

8

the cracking of 3-MP, the activation energy for [TPA] was about half of that for [TPA, Na].

9

addition, SCH4+C2H6+H2 for [TPA, Na] (ca. 56.4 mol%) was much higher than that for [TPA] (ca. 31.3

10

In the cracking of n-Hx, the reaction rate constant

Moreover, SCH4+C2H6+H2 were found to be almost

Hence, [TPA, Na] and [TPA] were quite similar in the ratio of the

In contrast, in In

mol%, Table 5), indicating that the bimolecular cracking predominates on [TPA].

11

The finding that the bimolecular cracking was not observed in the cracking of 3-MP over

12

H-ZSM-22 (Table 5) strongly suggests that the cracking of 3-MP over acid sites located at straight or

13

sinusoidal channels of H-ZSM-5 proceeds solely via the monomolecular cracking mechanism.

14

spaces of straight and sinusoidal channels of the MFI structure are too small to accommodate the

15

bimolecular transition state consisting of 3-MP and its carbenium ion.

16

are large enough to cause the bimolecular reaction, which could occur predominantly.

17

most of the acid sites of [TPA] would be located at the intersections.

18

The

In contrast, the intersections Therefore, the

The relationship between the activation energy and SCH4+C2H6+H2 at 10 % of the conversion

19

for cracking of n-Hx and 3-MP over the H-ZSM-5 samples are shown in Fig. 6.

In the cracking of

20

n-Hx, the activation energy and SCH4+C2H6+H2 among these H-ZSM-5 samples were almost the same.

21

In contrast, the activation energy for the cracking of 3-MP varied depending on the samples, and there

22

is a positive correlation between activation energy and SCH4+C2H6+H2. Haag and co-workers have

23

reported that the crystallite size does not affect the activity in the cracking of either of the C6 alkanes

24

over H-ZSM-5 at conventional cracking temperatures (600-800 K) [28].

25

the activation energy and SCH4+C2H6+H2 in the cracking of 3-MP among the H-ZSM-5 zeolites are not

26

caused by the diffusional restriction of 3-MP molecules, but the difference in the distribution of acid

27

sites in the pores.

Therefore, the differences in

28 29

3.4.3. Consideration on the results of the 27Al MAS NMR spectra and the cracking activity

30

Interestingly, we found that there was a correlation between the proportion of the five peaks 27

31

in the

Al MAS NMR spectra (Fig. 4 and Table 3) and the cracking activity.

32

with the highest proportion of Al(c) and the lowest proportion of Al(d) gave both the lowest CI value 13 ACS Paragon Plus Environment

The fact that [TPA]


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Page 14 of 39

1

and SCH4+C2H6+H2 suggests that the acid sites derived from Al(c) are mainly located at the intersections,

2

contributing to the bimolecular cracking of 3-MP, and that Al(d) contains the T sites not facing the

3

intersections (i.e., T1, T4 and T6, Scheme 1), enhancing the monomolecular cracking.

4

contains the T sites not facing the intersections because [Cha, Na] exhibited a high activity for the

5

bimolecular cracking of 3-MP.

Al (b) also

6 7 8

3.5. Effect of the location of Al atoms on the catalytic performance

9

3.5.1. Conversion of aromatic compounds

10

The conversions of toluene and m-xylene have been used as a test reaction to investigate the

11

size of reaction space in zeolites [52].

12

[TPA] and [TPA, Na] on these reactions at 623 K was representatively examined.

13

of [TPA] was higher than that of [TPA, Na] (Fig.7 (a)).

14

xylene and benzene along with the W/F, indicating that the molar ratio of benzene/xylene over [TPA,

15

Na] and [TPA] were almost the same (ca. 1.1).

16

over [TPA] and [TPA, Na] were totally different; the disproportionation of toluene was predominant

17

compared to the alkylation of toluene.

18

H-ZSM-5 zeolite proceeds via bulky diphenylmethane-like transition states [52].

19

H-ZSM-5 catalyst with the acid sites preferentially located at the intersections is able to be active in

20

the disproportionation of toluene.

21

performance than [TPA, Na].

22

Hence, the influence of the distribution of the acid sites on The reaction rate

Fig 7 (b) shows the change in the yields of

However, the formation rate of xylene and benzene

It has been reported that the disproportionation of toluene over Hence, the

This is the reason why [TPA] exhibited a higher catalytic

In contrast, the overall reaction rates for the conversion of m-xylene over [TPA] and [TPA, Na]

23

were almost the same (Fig. 8 (a)).

In this reaction, a large amount of xylene isomers and a small

24

amount of toluene were formed (Fig. 8 (b)).

25

disproportionation, and the rate of isomerization is significantly faster than that of the

26

disproportionation.

27

disproportionation of m-xylene over [TPA] was faster than that that over [TPA, Na] (Fig. 8 (b)).

28

the other hand, there was no marked difference in the reaction rate of the isomerization of m-xylene

29

between [TPA] and [TPA, Na].

30

reaction, it proceeds without any steric constraints [53, 54].

31

by the acid site distribution in the pores.

The isomerization proceeds simultaneously with the

It is noteworthy that, like the disproportionation of toluene, the rate of the On

Since the isomerization of xylene proceeds via the monomolecular Thus, the isomerization was not affected


Page 15 of 39

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1

The Arrhenius plots for the conversions of toluene and m-xylene over [TPA, Na] and [TPA]

2

revealed that [TPA] showed a smaller activation energy for the conversion of toluene than [TPA, Na]

3

(Fig. S8); activation energies for [TPA] and [TPA, Na] were calculated at 60.0 and 78.9 kJ mol-1,

4

respectively.

5

the intersections proceeded with a lower activation energy than that over acid site located at narrow

6

space, resulting in the different overall activation energies for [TPA] and [TPA, Na].

7

hand, in the conversion of m-xylene, there are no marked differences in the reaction rate constant and

8

activation energy between [TPA and [TPA, Na]; activation energies for [TPA] and [TPA, Na] were

9

30.2 and 28.6 kJ mol-1, respectively, in agreement with the finding that the isomerization of xylene is

10

The disproportionation of toluene via bulky transition states over acid sites located in

On the other

not affected by the distribution of acid sites.

11

We conclude that the location of Al atoms in the framework of H-ZSM-5 zeolite

12

significantly affects the reactions via bulky transition states such as bimolecular cracking of branched

13

hexanes and the disproportionation of aromatic compounds.

14 15

3.5.2. Influence of the location of Al atoms on the deactivation

16

Finally, the influence of the distribution of acid sites of H-ZSM-5 zeolite on the deactivation for

17

the cracking of n-Hx and methylcyclohexane (MCH) was examined.

The change in the conversions

18

of n-Hx and methylcyclohexane (MCH) over [TPA] and [TPA, Na] along with TOS was examined

19

under severe conditions, initial pressure of hydrocarbons of 40 kPa and reaction temperature of 923 K,

20

which could cause a faster deactivation.

21

constants for n-Hx and MCH cracking at 923 K between [TPA] and [TPA, Na] (Table S2).

There was no marked difference in the reaction rate

22

In the cracking n-Hx over [TPA] and [TPA, Na], the initial conversion of n-Hx on both

23

catalysts was ca. 100 % as shown in Figs. 9 (a) and (b) and the conversions were decreased along with

24

TOS.

25

between [TPA] and [TPA, Na].

26

dealumination, hardly occurred in either catalyst during the reaction.

27

deactivation should be caused by coking.

28

during the reaction and the decrease in the micropore volume due to coke formation, respectively.

29

There were no marked differences in the coke formation rates and the degree of the decrease in the

30

micropore volume either; the micropore volumes of [TPA, Na] and [TPA] after the TOS of 25 h were

31

found to be 0.09 and 0.07 cm3 g-1, respectively.

32

than 100 nm, the reactant and products involving coke precursors may easily diffuse out from the pores,

There were no marked difference in the deactivation rate and the products distributions 27

Al MAS NMR revealed that the change in the Al conditions, e.g., Therefore, thus observed

Figures 9 (c) and (d) shows the amount of deposited coke

We have reported that when the crystallite size is less



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Page 16 of 39

1

and that the coke is mainly deposited on the external surface of H-ZSM-5 [35].

2

deactivation for n-Hx cracking over H-ZSM-5 zeolite was not drastically affected by the distribution of

3

acid sites.

4

Thus, the

In contrast, in the cracking of MCH, there was a remarkable difference in the deactivation

5

behavior between [TPA] and [TPA, Na] (Figs. 10 (a) and (b)).

6

and the products distributions at 100 % of the conversion were almost the same.

7

was drastically deactivated in comparison with [TPA, Na]; the conversion for [TPA] at 10 h of TOS

8

was below half of that for [TPA, Na] (ca.15 % and ca. 41 %, respectively).

9

micropore volume of [TPA] was much lower than that of [TPA, Na] despite similar coke formation

10

rates (Figs. 10 (c) and (d)).

11

more seriously on [TPA] than on [TPA, Na].

The initial conversion reached 100 % However, [TPA]

Furthermore, the

These results indicated that the pore plugging by coke formation occurred

12

Masuda and his co-workers reported that the coke deposition in the cracking of MCH

13

occurred inside the pores as well as on the external surface, causing the pore plugging, while that of

14

n-Hx occurred at the external surface [55].

15

involving bulky transition sate such as alkylation of aromatics, cyclization and hydrogen-transfer

16

reaction [56].

17

would be ascribed to the difference in the amount of coke inside the pores.

18

located in the intersection would enhance the formation of coke from MCH inside the pores, resulting

19

in a rapid deactivation.

20

sites in H-ZSM-5 zeolites.

The coke formation proceeds through several reactions

Therefore, the difference in the deactivation behavior between [TPA] and [TPA, Na] For [TPA], the acid sites

Thus, the catalyst lifetime can be accounted for by the distribution of acid

21 22 23

4. Conclusions

24

The ZSM-5 catalysts with the location of Al atoms controlled were successfully synthesized

25

by using tetrapropylammonium cations, dipropylamine, cyclohexylamine or hexamethyleneimine as

26

OSDAs with or without Na cations.

27

techniques revealed that the location of Al atoms in the framework was dependent on the cations

28

and/or the amines used.

29

cracking that proceeded via a small-sized transition state.

30

zeolites in 3-MP cracking were markedly different. From the considerations of the CI values and

31

SCH4+C2H6+H2, we can conclude that the bimolecular cracking in the cracking of 3-MP proceeds via a

32

bulky transition state over the acid sites located at the channel intersections.

The high-resolution

27

Al MAS NMR and

27

Al MQMAS NMR

All of the prepared H-ZSM-5 zeolites exhibited a similar activity in n-Hx In contrast, the activities of these H-ZSM-5


Accordingly, the Al

Page 17 of 39

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1

atoms on the H-ZSM-5 synthesized by using TPA cations in the absence of Na cations are

2

predominantly located in large spaces, namely the channel intersections.

3

of 3-MP, the distribution of acid sites affects the catalytic activity for disproportionation of aromatic

4

compounds, which proceed bulky transition state, while the activity for m-xylene isomerization, which

5

proceeds without any steric constraints, was not affected by acid site distributions.

6

found that the distribution of acid sites affects the catalytic lifetime for the cracking of MCH; the

7

zeolite with Al atoms located at the intersection showed a shorter catalytic lifetime.

8

contribute to the development of various zeolite catalysts with the location of Al atoms in the pores

9

controlled to improve the catalytic performance in not only for cracking reaction but also for other

10

In addition to the cracking

Furthermore, we

Our findings will

several acid-catalyzed reactions.

11 12 13

ASSOCIATED CONTENT

14

Acknowledgment.

15

This work was partly supported by the green sustainable chemistry project of New Energy and

16

Industrial Technology Development Organization (NEDO) and Grant-in-Aid for Scientific Research

17

on Innovative Areas “Exploration of nanostructure-property relationships for materials innovative”

18

(grant number 26106508).

19

Functional Materials with Advanced Properties by Hyper-nano-space Design”, Japan Science and

20

Technology Agency (JST).

This study was also partly supported by CREST “Creation of Innovative

21 22

Supporting Information Available:

23

The information regarding the characterization of the zeolites used in this study and the results of

24

cracking reaction is available free of charge via the Internet at http://pubs.acs.org/.

25 26

AUTHOR INFORMATION

27

Corresponding Authors

28

*E-mail: [email protected]

29



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1

References

2

1.

3 4

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Pinar, A. B.; Gómez-Hortigüela, L.; Pérez-Pariente, J. Cooperative Structure Directing Role of the

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Cage-Forming Tetramethylammonium Cation and the Bulkier Benzylmethylpyrrolidinium in the

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Synthesis of Zeolite Ferrierite. Chem. Mater. 2007, 19, 5617-5626

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Pinar, A. B.; Pérez-Pariente, J.; Gómez-Hortigüela, L. Method for Preparation of an

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Aluminosilicate with Ferrierite Structure from Gels Containing Tetramethyl Ammonium and

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Benzylmethylpyrrolidine, and Uses Thereof. WO2008116958-A1, 2008.

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Pinar, A. B.; Márquez-Álvarez, C.; Grande-Casas, M.; Pérez-Pariente, J. Template-Controlled Acidity and Catalytic Activity of Ferrierite Crystals. J. Catal. 2009, 263, 258-265.

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Márquez-Álvarez, C.; Pinar, A.B.; García, R.; Grande-Casas, M.; Pérez-Pariente, J. Influence of Al

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Distribution and Defects Concentration of Ferrierite Catalysts Synthesized From Na-Free Gels in

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the Skeletal Isomerization of n-Butene. Top. Catal. 2009, 52, 1281-1291.

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Gómez-Hortigüela, L.; Pinar, A.B.; Cora, F.; Perez-Pariente, J. Dopant-Siting Selectivity in

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Nanoporous Catalysts: Control of Proton Accessibility in Zeolite Catalysts Through the Rational

17

Use of Templates. Chem. Commun. 2010, 46, 2073–2075.

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7.

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Román-Leshkov, Y.; Moliner, M.; Davis, M. E. Impact of Controlling the Site Distribution of Al Atoms on Catalytic Properties in Ferrierite-Type Zeolites. J. Phys. Chem. C 2011, 115, 1096-1102.

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Dedecek, J.; Balgová, V.; Pashkova, V.; Klein, P.; Wichterlová, B. Synthesis of ZSM‑5 Zeolites

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with Defined Distribution of Al Atoms in the Framework and Multinuclear MAS NMR Analysis

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of the Control of Al Distribution. Chem. Mater. 2012, 24, 3231-3239.

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9.

Dedecek, J.; Sobalík, Z.; Wichterlová, B. Siting and Distribution of Framework Aluminium Atoms

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in Silicon-Rich Zeolites and Impact on Catalysis. Catalysis Reviews: Science and Engineering,

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2012, 54, 135-223.

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10. Pinar, A.B.; Gómez-Hortigüela, L.; McCusker, L.B.; Pérez-Pariente, J. Controlling the Aluminum

27

Distribution in the Zeolite Ferrierite via the Organic Structure Directing Agent. Chem. Mater. 2013,

28

25, 3654-3661.

29

11. Janda, A.; Bell, A.T. Effects of Si/Al Ratio on the Distribution of Framework Al and on the Rates

30

of Alkane Monomolecular Cracking and Dehydrogenation in H‑MFI. J. Am. Chem. Soc., 2013,

31

135, 19193−19207


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12. Vjunov, A.; Fulton, J.L.; Huthwelker, T.; Pin, S.; Mei, D.; Schenter, G. K.; Govind, N.; Camaioni,

2

D.M.; Hu, J. Z.; Lercher, J.A. Quantitatively Probing the Al Distribution in Zeolites. J. Am. Chem.

3

Soc., 2014, 136, 8296-8306.

4

13. Pinar, A.B.; Verel, R.; Pérez-Pariente, J.; van Bokhoven, J.A. Direct Evidence of the Effect of

5

Synthesis Conditions on Aluminum Siting in Zeolite Ferrierite: A

6

Microporous Mesoporous Materials, 2014, 193, 111-114.

27

Al MQ MAS NMR study.

7

14. Liu, M.; Yokoi, T.; Yoshioka, M.; Imai, H.; Kondo, J.N.; Tatsumi, T. Differences in Al

8

Distribution and Acidic Properties between RTH-type Zeolites Synthesized with OSDAs and

9

without OSDAs. Phys. Chem. Chem. Phys. 2014, 16, 4155-4164.

10 11

15. Vermeiren, W.; Gilson, J.-P. Impact of Zeolites on the Petroleum and Petrochemical Industry. Top. Catal. 2009, 52, 1131-1161.

12

16. International Zeolite Association, Structure Commission, http://www.iza-structure.org/databases/.

13

17. Flanigen, E.M.; Bennett, J.M.; Grose, R.W.; Cohen, J.P.; Patton, R.L.; Kirchner, R.M.; Smith, J.V.

14

Silicalite, a New Hydrophobic Crystalline Silica Molecular Sieve. Nature 1978, 271, 512-516.

15

18. Price, G.D.; Pluth, J.J.; Smith, J.V.; Bennett, J.M.; Patron, R.L. Crystal Structure of

16

Tetrapropylammonium Fluoride-Containing Precursor to Fluoride Silicalite. J. Am. Chem. Soc.

17

1982, 104, 5971-5977.

18 19 20 21

19. Chao, K.J.; Lin, J.C.; Wang, Y.; Lee, G.H. Single Crystal Structure Refinement of TPA ZSM-5 Zeolite. Zeolites 1986, 6, 35-68. 20. Chang, C.D.; Bell, A.T. Studies on the Mechanism of ZSM-5 Formation. Catal. Lett. 1991, 8, 305-316.

22

21. Burkett, S.L.; Davis, M.E. Mechanism of Structure Direction in the Synthesis of Si-ZSM-5: An

23

Investigation by Intermolecular 1H-29Si CP MAS NMR. J. Phys. Chem. 1994, 98, 4647-4653.

24

22. Lok, B.M.; Cannan, T.R.; Messina, C.A. The Role of Organic Molecules in Molecular Sieve

25 26 27 28 29

Synthesis. Zeolites 1983, 3, 282-291. 23. Van der Gaag, F.J.; Jansen, J.C.; van Bekkum, H. Template Variation in the Synthesis of Zeolite ZSM-5. Appl. Catal. 1985, 17, 261-271. 24. Araya, A.; Lowe, B.M. Effect of Organic Species on the Synthesis and Properties of ZSM-5. Zeolites 1986, 6, 111-118.

30

25. Schwarz, S.; Kojima, M.; O'Connor, C.T. Effect of Tetraalkylammonium, Alcohol and Amine

31

Templates on the Synthesis and High Pressure Propene Oligomerisation Activity of ZSM-type

32

Zeolites. Appl. Catal. 1991, 73, 313-330. 19 ACS Paragon Plus Environment


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26. Sang, S.; Chang, F.; Liu, A.; He, C.; He, Y.; Xu, L. Difference of ZSM-5 Zeolites Synthesized with Various Templates. Catal. Today 2004, 93-95, 729-734. 27. Frilette, V.J.; Haag, W.O.; Lago, R.M. Catalysis by Crystalline Aluminosilicates: Characterization of Intermediate Pore-size Zeolites by the “Constraint Index”. J. Catal. 1981, 67, 218-222. 28. Haag, W.O.; Lago, R.M.; Weisz, P.B. Transport and Reactivity of Hydrocarbon Molecules in a Shape-selective Zeolite. Faraday Discuss. Chem. Soc. 1981, 72, 317-330. 29. Zones, S.I.; Harris, T. The Constraint Index Test Revisited: Anomalies Based upon New Zeolite Structure Types. Micropor. Mesopor. Mater. 2000, 35-36, 31-46. 30. Carpenter, J.R.; Yeh, S.; Zones, S.I.; Davis, M.E. Further Investigations on Constraint Index Testing of Zeolites that Contain Cages. J. Catal. 2010, 269, 64-70. 31. Haag, W.O.; Dessau, R.M. in 8th International Congress of Catalysis, Proceedings, Verlag Chemie, Weinheim, Dearfield Beach, Basel, DECHEMA, Frankfurt/Main 1984, 2, 305-316. 32. Haag, W.O.; Dessau, R.M.; Lago, R.M. Kinetics and Mechanism of Paraffin Cracking with Zeolite Catalysts. Stud. Surf. Sci. Catal. 1991, 60, 255-265. 33. Krannila, H.; Haag, W.O.; Gates, B.C. Monomolecular and Bimolecular Mechanisms of Paraffin Cracking: n-butane Cracking Catalyzed by HZSM-5. J. Catal. 1992, 135, 115-124. 34. Watanabe, R.; Yokoi, T.; Tatsumi, T. Synthesis and Application of Colloidal Nanocrystals of the MFI-type Zeolites. J. Colloid Interface Sci. 2011, 356, 434-441.

19

35. Mochizuki, H.; Yokoi, T.; Imai, H.; Watanabe, R.; Namba, S.; Kondo, J.N.; Tatsumi, T. Facile

20

Control of Crystallite Size of ZSM-5 Catalyst for Cracking of Hexane. Microporous Mesoporous

21

Mater. 2011, 145, 165-171.

22

36. Mochizuki, H.; Yokoi, T.; Imai, H.; Namba, S.; Kondo, J.N.; Tatsumi, T. Effect of Desilication of

23

H-ZSM-5 by Alkali Treatment on Catalytic Performance in Hexane Cracking. Appl. Catal. A 2012,

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449, 188-197.

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37. Wang, Q.; Cui, Z.M.; Cao, C.Y.; Song, W.G. 0.3 Å Makes the Difference: Dramatic Changes in

26

Methanol-to-Olefin Activities between H-ZSM-12 and H-ZSM-22 Zeolites. J. Phys. Chem. C 2011,

27

115, 24987-24992.

28 29 30

38. Namba, S.; Nakanishi, S.; Yashima, T. Behavior of Quinoline Derivatives as Poisons in Isomerization of p-xylene on HZSM-5 Zeolite. J. Catal. 1984, 88, 505-508. 39. Abbot, J. Cracking Reactions of C6 Paraffins on HZSM-5. Appl. Catal. 1990, 57, 105-125.


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40. Richter, M.; Fiebig, W.; Jerschkewitz, H.G.; Lischke, G.; Ohlmann, G. Refined Application of the

2

m-xylene Isomerization to the Characterization of Shape-selective Zeolite Properties. Zeolites,

3

1989, 9, 238-246.

4 5 6 7 8 9

41. Katada, N.; Suzuki, K.; Noda, T.; Sastre, G.; Niwa M. Correlation between Brønsted Acid Strength and Local Structure in Zeolites. J. Phys. Chem. C 2009, 113, 19208–19217 42. Lercher, J.A.; Gründling, C.; Eder-Mirth, G. Infrared Studies of the Surface Acidity of Oxides and Zeolites using Adsorbed Probe Molecules. Catal. Today, 1996, 27, 353-376. 43. Zecchina, A.; Spoto, G.; Bordiga, S. Probing the Acid Sites in Confined Spaces of Microporous Materials by Vibrational Spectroscopy. Phys. Chem. Chem. Phys. 2005, 7, 1627-1642.

10

44. Alemany, L.B. Critical Factors in Obtaining Meaningful fast MAS NMR Spectra of Non-integral

11

Spin Quadrupolar Nuclei. A Review with Particular Emphasis on 27Al MAS NMR of Catalysts and

12

Minerals. Appl. Magn. Reson. 1993, 4, 179–201.

13 14 15 16

45. Lippmaa, E.; Samoson, A.; Magi, M. High-resolution Aluminum-27 NMR of Aluminosilicates. J. Am. Chem. Soc. 1986, 108, 1730-1735. 46. Kolodziejski,W.; Zicovich-Wilson, C.; Corell, C.; Perez-Pariente, J.; Corma, A.,

27

Al and

29

Si

MAS NMR Study of Zeolite MCM-22. J. Phys. Chem. 1995, 99, 7002-7008.

17

47. Fyfe, C.A.; Feng, Y.; Grondey, H.; Kokotailo, G.T.; Gies, H. One- and Two-dimensional

18

High-resolution Solid-state NMR Studies of Zeolite Lattice Structures. Chem. Rev. 1991, 91,

19

1525-1543.

20 21 22

48. Derouane, E.G.; Hubert, R.A. Resolution and Assignment of Framework Sites in Zeolite ZSM-5 by Correlation of X-ray and NMR Measurements. Chem. Phys. Lett., 1986, 132, 315-318. 49. Han, O.H.; Kim, C.-S.; Hong, S.B. Direct Evidence for the Nonrandom Nature of Al Substitution

23

in Zeolite ZSM-5: An Investigation by

24

2002, 41, 469-472.

27

Al MAS and MQ MAS NMR. Angew. Chem. Int. Ed.,

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50. Kubo, K.; Iida, H.; Namba, S.; Igarashi, A. Selective Formation of Light Olefin by n-heptane

26

Cracking over HZSM-5 at High Temperatures. Micropor. Mesopor. Mater. 2012, 149, 126-133.

27

51. Namba, S.; Inaka, A.; Yashima, T. Effect of Selective Removal of Aluminium from External

28 29 30 31 32

Surfaces of HZSM-5 Zeolite on Shape Selectivity. Zeolites 1986, 6, 107-110. 52. Xiong, Y.S.; Rodewald, P.G.; Chang, C.D. On the Mechanism of Toluene Disproportionation in a Zeolite Environmen. J. Am. Chem. Soc. 1995, 117, 9427-9431. 53. Corma A.; Cortes, A.; Nebot, I.; Tomas, F. On the Mechanism of Catalytic Isomerization of Xylenes. Molecular Orbital Studies. J. Catal. 1979, 57, 444-449. 21 ACS Paragon Plus Environment


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1

54. Martens, J.A.; Pérez-Pariente, J.; Sastre, E.; Corma, A.; Jacobs, P.A. Isomerization and

2

Disproportionation of m-xylene : Selectivities Induced by the Void Structure of the Zeolite

3

Framework. Appl. Catal. 1988, 45, 85-101.

4

55. Konno H.; Tago T.; Nakasaka Y.; Ohnaka, R.; Nishimura, J.; Masuda, T. Effectiveness of

5

Nano-scale ZSM-5 Zeolite and its Deactivation Mechanism on Catalytic Cracking of

6

Representative Hydrocarbons of Naphtha. Micropor. Mesopor. Mater. 2013, 175, 25-33.

7 8

56. Guisnet M.; Magnoux, P. Coking and Deactivation of Zeolites: Influence of the Pore Structure. Appl. Catal. 1989, 54, 1-27.

9 10


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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 2

TOC

3 4 5 6 7 8 9 10 11



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

Page 24 of 39

Table 1 Chemical analysis and organic content of the ZSM-5 samples.

2 Al contenta)

Na contentb)

C/Nc)

mmol g-1

mmol g-1

molar ratio

wt %

mmol g-1

molar ratio

[TPA]

0.31

n.d.

12.3

14.3

0.70

2.3

[TPA, Na]

0.31

0.28

12.3

13.9

0.68

3.1

[DPa, Na]

0.38

0.28

6.0

7.7

0.76

2.8

[Cha, Na]

0.32

0.09

6.2

5.9

0.59

2.1

[HMi, Na]

0.34

0.11

6.1

10.5

1.06

3.4

Sample

3 4 5 6 7

TGAd)

OSDA content

a)

Determined by ICP. Determined by AAS. c) Determined by elemental analysis.

b)

d)

Weight loss in the range 573 to 1073 K as measured by thermogravimetric analysis.

8


(Na+OSDA)/Al

Page 25 of 39

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


Table 2 Physicochemical properties and acid amount of H-ZSM-5 catalysts

2 Sample

Si/Ala)

2

m g

3 4 5 6 7 8 9

a) b) c) d) e)

SEXTc)

SBETb) -1

2

m g

-1

Vmicrod) 3

cm g

-1

Acid amounte) mmol g

[TPA]

53

438

47

0.18

0.29

[TPA, Na]

52

424

43

0.17

0.30

[DPa, Na]

42

412

13

0.17

0.37

[Cha, Na]

51

406

5

0.17

0.31

[HMi, Na]

48

433

41

0.16

0.34

Si/Al: Si/Al atomic ratio in the sample determined by ICP. SBET: BET surface. SEXT: External surface area estimated by the t-plot method. Vmicro: Micropore volume estimated by the t-plot method. Acid amount: Estimated by the NH3-TPD.

10


-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

1

Page 26 of 39

Table 3 Relative peak areas of the 27Al MAS NMR spectra of the NH4-type ZSM-5 zeolites

2 Al(e)

Al(d)

Al(c)

Al(b)

Al(a)

Chemical shift / ppm

58

56

54

53

52

[TPA]

14

34

22

18

13

[TPA, Na]

15

48

10

18

10

[DPa, Na]

19

42

15

15

11

[Cha, Na]

14

44

10

22

10

[HMi, Na]

17

43

13

14

13

3


Page 27 of 39

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

Table 4 Conversion, total selectivity to methane, ethane and hydrogen and CI values for C6 paraffins

2

cracking over various zeolites Catalyst

Conversion

Selectivities to CH4+C2H6+H2

%

moles (100 moles cracked)

n-Hx

a

CI

-1

3-MP

b

n-Hx

a

3-MP

b

H-ZSM-22

15

1.6

35

99

9.9

H-ZSM-5 [TPA, Na]

15

3.1

13

69

5.2

H-ZSM-5 [TPA]

17

10.5

13

28

1.7

H-ZSM-5 [DPa, Na]

18

9.0

15

39

2.1

H-ZSM-5 [Cha, Na]

16

5.0

16

62

3.3

H-ZSM-5 [HMi, Na]

16

7.0

15

42

2.4

H-ZSM-12

19

8.4

7.4

37

2.4

H-mordenite

15

14.7

4.8

5.5

1.0

3 4 5 6 7 8 9 10 11

a) b)

n-hexane cracking 3-methylpentane cracking

Reaction conditions W/Ftotal: 2.0 - 11.2 g h moltotal-1, Partial pressure of C6 paraffins: 40 kPa, Reaction temperature: 673 K, Date at 10 min on stream. n-Hexane cracking and 3-methylpentane cracking were carried out under the same conditions over each zeolite.

12 13 14



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

Page 28 of 39

Table 5 Results of cracking of C6 paraffins over H-ZSM-5 zeolites

2 Reactant

Catalysts

kC6

a)

Conv.

b)

Selectivity to H2 + CH4 + C2H6

2

-1

10 mol g h n-Hxd)

3-MPe)

-1

Ea

c)

b)

-1

%

moles (100 mol cracked C6)

-1

kJ mol

[TPA,Na]

10.5

9.9

13.1

66

[TPA]

11.3

10.2

12.8

66

[TPA,Na]

1.7

9.5

56.4

81

[TPA]

5.6

11.7

31.3

47

3 4 5 6 7 8 9 10 11 12 13

a)

Reaction rate constant at 673 K Total selectivity to methane, ethane and hydrogen below 10% conversion at 673 K c) Activation energy d) n-Hexane e) 3-Methylpentane

b)

Reaction conditions W/Ftotal: 1.0 - 6.2 g h moltotal-1, Partial pressure of C6 paraffins: 40 kPa, Reaction temperature: 623 - 673 K, Date at 10 min on stream, 2,4-Dimethylquinoine : 1.1 µLmin-1

14 15


Page 29 of 39

0.1

0.1

e)

e)

d) d)

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


c) c) b) b) a) a) 3800

1 2 3 4 5

3600

3400

3200

3000 2250 Wavenumber /cm-1

2200

2150

2100

Figure 1 Differential FT-IR spectra for (a) ν(OH) and (b) ν(CO) regions of the CO-adsorbed H-ZSM-5 samples: a) [TPA], b) [TPA, Na], c) [DPa, Na], d) [Cha, Na] and e) [HMi, Na].

6 7 8



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

2 3 4 5 6 7

Figure 2 27 Al MAS NMR spectra of the NH4-type ZSM-5 zeolites: a) [TPA], b) [TPA, Na], c) [DPa, Na], d) [Cha, Na] and e) [HMi, Na].

8 9


Page 30 of 39

Page 31 of 39

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 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Figure 3 27 Al MQMAS NMR spectra of the NH4-type ZSM-5 zeolites, (a) [TPA] and (b) [TPA, Na].



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 32 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Figure 4 Curve fittings of the 27Al MAS NMR spectra of the NH4-type ZSM-5 zeolites: a) [TPA], b) [TPA, Na], c) [DPa, Na], d) [Cha, Na] and e) [HMi, Na].

21


Page 33 of 39

1 2 1 3 2 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 3 24 25 4 26 5 27 6 28 7 29 8 30 9 31 10 32 33 11 34 12 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


(a)

(b)

Figure 5 Constraint index values for (a) zeolites with different topologies and (b) the ZSM-5 catalysts synthesized using various OSDAs with or without Na cation at 673 K. Reaction conditions: W/Ftotal: 2.0 - 11.2 g h moltotal-1, Partial pressure of n-hexane and 3-methylpentane: 40 kPa, Reaction temperature: 673 K, Data at 10 min on stream.



60 Selectivity to CH4+C2H6+H2 /moles (100 mol cracked C6)-1

1 2 1 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 2 27 28 3 29 4 30 5 31 32 6 33 7 34 8 35 9 36 10 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 34 of 39

[TPA, Na] [HMi, Na] [Dpa, Na]

40

[Cha, Na]

[TPA] 20

0 40

50

60

70

80

90

Activation energy / kJ mol-1 Figure 6 Relationship between activation energy and total selectivities to H2 + CH4 + C2H6 at 10 % conversion* for n-hexane (open symbol) and 3-methylpenatne (solid symbol) cracking over H-ZSM-5 zeolites. Reaction conditions: W/Ftotal: 1.0 - 2.7 g h moltotal-1, Partial pressure of C6 paraffins: 40 kPa, Reaction temperature: 623 - 673 K, Data at 10 min on stream, 2,4-Dimethylquinoine : 1.1 µLmin-1


Page 35 of 39

1 2 1.6

3 A

B (b)

(a) Yield / mol %

2.5

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


2 1.5 1

1.2

0.8

0.4 0.5 0

0 0

3

4

8

0

12

4

8

12

W/F / g h mol-1

4 5 6 7 8 9 10 11 12 13

Figure 7 Catalytic conversions of toluene over [TPA] (open symbol) and [TPA, Na] (solid symbol) with W/F varied; (a) the conversion of toluene and (b) the yields of benzene (◆) and xylene (▲). Reaction conditions: W/Ftotal: 2.1 - 10.7 g h moltotal-1, Partial pressure of toluene paraffins: 20 kPa, Reaction temperature: 623 K, Date at 10 min on stream, 2,4-Dimethylquinoine : 1.1 µLmin-1

14 15 16



35

A

Xylene isomers yield / mol %

(a)

30 25 20 15 10 5

5 B(b)

30

4

25 20

3

15

2

10 1

5 0

0 0

1

2

0 0

3

Toluene yield / mol %

35

Conversion / %

1 2 1 3 2 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 3 24 25 4 26 5 27 6 28 29 7 30 8 31 9 32 10 33 34 11 35 36 12 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 36 of 39

1

2

3

W/F / g h mol-1

Figure 8 Catalytic conversions of m-xylene over [TPA] (open symbol) and [TPA, Na] (solid symbol) with W/F varied; (A) the conversion of m-xylene and (B) the yields of xylene isomer (◆) and toluene (▲). Reaction conditions: W/Ftotal: 1.0 – 2.7 g h moltotal-1, Partial pressure of m-xylene: 20 kPa, Reaction temperature: 623 K, Date at 10 min on stream, 2,4-Dimethylquinoline: 1.1 µLmin-1


Page 37 of 39

1

Coversion and yield / mol %

(a)

100

A

80 Conversion 60

Olefin

40

Paraffin

20 BTX

(b)

B

80 60 40 20 0

0 0

5

10

15

20

50 40 30 20 10 0 0

5 10 15 20 Time-on-stream / h

5

10

15

20

0.2

C

(c)

0

25

Micropore volume / cm3 g-1


100

Coke formation / %

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


25

(d)

25 D

0.16 0.12 0.08 0.04 0 0

5

10 15 20 Time-on-stream / h

25

2 3 4 5 6 7 8 9 10 11 12 13

Figure 9 Cracking of n-hexane at high 923 K over [TPA, Na] and [TPA]. Changes in the conversion and product yieldsa) against time on stream of (a) [TPA, Na] and (b) [TPA], the (c) relationship between reaction time and coke amountb) and (d) relationship between reaction time and micropore volume for [TPA, Na] (solid symbol) and [TPA] (open symbols). a) Olefin: Total yield of ethylene, propylene and butenes. Paraffin: Total yield of ethane, propane and butanes. BTX: Total yield of benzene, toluene and xylenes Reaction conditions W/Ftotal: 2.7 g h moltotal-1, Partial pressure of n-hexane: 40 kPa, Reaction temperature: 923 K



1

100

A

Conversion

80

(a)



100

Olefin

60 40 BTX

20

Paraffin

0 4

6

8

80 60 40 20

10 C

(c) 20 15 10 5 0 0

2


0

2

4

6

8

0.2

Micropore volume / cm3 g-1

2

25

2

(b)B

0 0

Coke formation / %

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(d) D

0.16 0.12 0.08 0.04 0 0

10

10

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10

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Figure 10 Cracking of methylcyclohexane at high 923 K over [TPA, Na] and [TPA]. Changes in the conversion and product yieldsa) against time on stream of (a) [TPA, Na] and (b) [TPA]. (c) Relationship between reaction time and coke amountb). (d) Relationship between reaction time and micropore volumeb). a) Olefin: Total yield of ethylene, propylene and butenes. Paraffin: Total yield of ethane, propane and butanes. BTX: Total yield of benzene, toluene and xylenes b) [TPA] (open symbol), [TPA, Na] (solid symbol) Reaction conditions W/Ftotal: 2.7 g h moltotal-1, Partial pressure of methylcyclohexane: 40 kPa, Reaction temperature: 923 K


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Scheme 1 Crystallographically distinct 12 T-sites in the MFI structure. Among the 12 T sites, T1, T4 and T6 are not facing the intersections; T4 is located at the sinusoidal 10-MR channels and T1 and T6 are located at the straight 10- MR channels.

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Scheme 2 Reaction mechanism of monomolecular cracking of C6 paraffins [23].

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