ZSM-22: Controllable microporous Brönsted acidity distribution and shape-selectivity

Journal Pre-proof N-dodecane Hydroisomerization over Pt/ZSM-22: Controllable Microporous Br¨onsted Acidity Distribution and Shape-Selectivity Xiangyu Wang, Xiangwen Zhang, Qingfa Wang

PII:

S0926-860X(19)30490-9

DOI:

https://doi.org/10.1016/j.apcata.2019.117335

Reference:

APCATA 117335

To appear in:

Applied Catalysis A, General

Received Date:

23 July 2019

Revised Date:

5 October 2019

Accepted Date:

30 October 2019

Please cite this article as: Wang X, Zhang X, Wang Q, N-dodecane Hydroisomerization over Pt/ZSM-22: Controllable Microporous Br¨onsted Acidity Distribution and Shape-Selectivity, Applied Catalysis A, General (2019), doi: https://doi.org/10.1016/j.apcata.2019.117335

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N-dodecane Hydroisomerization over Pt/ZSM-22: Controllable Microporous Brönsted Acidity Distribution and Shape-Selectivity

Xiangyu Wang1, Xiangwen Zhang1, 2*, Qingfa Wang1, 2*

1. Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of China.

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2. Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, China. *Corresponding author. Tel. /fax: +86 22 27892340.

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E-mail address: [email protected]; [email protected] (Q. Wang)’

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Graphical abstract

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Highlights

 Different microporous Brönsted acidity distribution in ZSM-22 was achieved.

 Shape-selectivity

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ZSM-22

was

microporous Brönsted acidity distribution. 1

related

to

 Synergistic effect of template protective and NH4+ exchange strategy was found.

Abstract The controllable acidity distribution in micropore for ZSM-22 zeolites is successfully tailored by using

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a new template protective NH4+ exchange strategy. The location and density of microporous Brönsted acidity is tailored by controlling the degree of detemplation and the concentration of NH4+ aqueous solution, respectively. The distribution of microporous Brönsted acidity of these Pt/ZSM-22 catalysts

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obviously affects the shape-selectivity in the hydroisomerization of n-dodecane. Weak microporous acidity leads to the formation of centrally mono-branched isomers, while strong microporous Brönsted

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acidity near the pore mouth favors the formation of multi-branched isomers, and strong microporous

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Brönsted acidity deep inside the pore mouth favors cracking through (s, p) β-scission. By selectively tailoring microporous Brönsted acidity near the pore mouth with lower acid density, the isomer yield

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over Pt/ZSM-22 was increased from 39.8% to 63.7%. This enhanced catalytic performance comes from the preferable formation of centrally mono-branched isomers and multi-branched isomers through

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key-lock model.

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Keywords: ZSM-22; partial detemplation; hydroisomerization; acidity distribution; shape-selectivity

1. Introduction

Hydroisomerization has been applied to enhance the low-temperature performance of fuels and lubricant oils[1, 2]. Through the hydroisomerization reaction, n-paraffins are converted into corresponding isomers which have a lower pour point and higher viscosity index. Hydroisomerization 2

reaction is normally carried over bifunctional catalysts[3-8]. Generally, these bifunctional catalysts consist of noble metal sites such as Pt or Pd for dehydrogenation/hydrogenation, and supports such as zeolites to supply acid site for skeletal isomerization reaction. However, besides these two kinds of active sites, the shape-selectivity of the support zeolites also plays a critical role in the catalysis of n-paraffin hydroisomerization. As isomerization and cracking reactions have similar activation energy, cracking always act as a primary side reaction in hydroisomerization[9-11]. Moreover, the isomers with

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more than one side-chains are more prone to crack. As a result, the hydroisomerization reactions carried out with poor shape-selectivity often suffer from low isomer yields.

ZSM-22 zeolites, with one-dimensional 10-membered ring microporous structure, has been found

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to favor the formation of methyl-isomers in the hydroisomerization of n-paraffins and avoid

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over-cracking probably through the shape-selectivity towards intermediate products[12, 13].

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Pore-mouth model and key-lock model have been established to explain the shape-selectivity of this microporous structure[6, 7, 14, 15]. According to these models, the Brönsted acid sites in microporous

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ZSM-22 zeolites are commonly classified into three types: the internal Brönsted acid sites in the micropores, the external Brönsted acid sites at the pore-mouths, and the external Brönsted acid sites on

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the lateral surfaces of rod-like ZSM-22 crystals[15]. It has been claimed that the micropores of ZSM-22 at a size of 0.45×0.55 nm are too narrow for the iso-paraffins to diffuse through[14]. Only

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n-paraffins may diffuse deep into these micropores, and usually crack into small molecules through (s, p) β-scission at the microporous Brönsted acid sites with severe confinement. This (s, p) β-scission cracking was believed as a parallel reaction with isomerization[15], and could be restrained by reducing microporous Brönsted acidity. The pore-mouths offer enough space for the skeletal isomerization for n-paraffins, and the Brönsted acid sites in pore-mouth provide a proper 3

shape-selectivity. As no pore-mouths locate on the lateral surfaces of ZSM-22, the lateral surfaces are believed no shape-selective, and cause cracking of branched isomers through (s, s) β-scission, which was regarded as a consecutive reaction of isomerization. In order to enhance the performance of Pt/ZSM-22 in the hydroisomerization, two kinds of strategies have been widely applied. One is to introduce more pore-mouth into ZSM-22 support through reducing the crystal length of ZSM-22 rods[16], or through post-synthesis methods such as

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alkali treatment[17-20]. The other strategy is to restrain consecutive cracking reactions by reducing

total and external Brönsted acid sites[21-24]. For example, Liu et. al synthesized Fe-substituted ZSM-22 with a reduced total Brönsted acidity[25]. Highly siliceous ZSM-22 with less total Brönsted

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acidity also showed high selectivity for n-dodecane hydroisomerization[26]. Chen et. al synthesized a

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core-shell ZSM-22@SiO2 to passivate the external Brönsted acidity and reduce consecutive cracking in hydrodeoxygenation of methyl palmitate[27]. In one of our previous work, the reduction of total

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Brönsted acid sites of ZSM-5 was achieved by simply tailoring the concentration of NH4+ aqueous

ZSM-22.

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solution[28]. This offer us a brief and effective method upon reducing total Brönsted acid sites in

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In the case of reducing microporous Brönsted acid sites, researchers tried to partially block the micropores of ZSM-22 recently[29, 30]. This pore-blockage method was proved very effective in

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enhancing the isomer selectivity by restraining the diffusion of n-paraffins inside the micropores. More centrally mono-branched isomers were formed according to key-lock adsorption mode instead of pore-mouth adsorption mode. More multi-branched isomers were also formed, probably because the formed mono-branched isomers was easier to diffuse out of the pore-mouths. However, it should be noted that this pore-blockage method may have limited application in processing oxygen-containing 4

stocks such as methyl palmitate, in which the deposited carbon for micropore blockage may be removed through oxidation[31]. The lifetime of these micropore-blocked catalysts may also be shortened[32, 33]. As a result, a more facile and universal method to reduce the microporous Brönsted acid sites is still called for. Moreover, as far as we know, few discussion has been made upon restraining the parallel cracking reaction through (s, p) β-scission. In present work, we reduced the microporous Brönsted acidity of ZSM-22 through tailoring their

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distribution by a new template protective and NH4+ exchange strategy. A series of pristine ZSM-22 zeolites were partial-detemplated before NH4+ exchange. While after the exchange step, these ZSM-22 were fully detemplated, leaving a totally open micropore structure. As a result, these series of ZSM-22

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zeolites had identical external Brönsted acidity, while their microporous Brönsted acidity was tailored,

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resulting in different microporous Brönsted acid sites location and reduced microporous Brönsted acidity. Another series of fully-detemplated ZSM-22 zeolites were exchanged in various concentrations

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of NH4+ aqueous solution to reduce total Brönsted acidity as well as microporous Brönsted acidity in

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ZSM-22. These series of ZSM-22 zeolites had Brönsted acidity all over the external surfaces as well as in the micropores, but the acidity density was reduced. After loaded with Pt, the catalysts showed

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enhanced isomer yield. By selectively tailoring the microporous Brönsted acidity location of ZSM-22 supports, the shape-selectivity of n-paraffin hydroisomerization over Pt/ZSM-22 was also tailored from

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favoring pore-mouth model to key-lock model. A reduction of the microporous Brönsted acidity deep inside the micropores significantly reduced cracking through (s, p) β-scission. While by reducing the density of total Brönsted acidity and microporous Brönsted acidity, the isomer yield was enhanced with least change of the shape-selectivity. A synergetic effect was found through the template protective NH4+ exchange strategy. The as-prepared Pt/ZSM-22 catalysts showed tailored microporous Brönsted 5

acidity near the pore mouth with lower acid density. The improved performance in the hydroisomerization of n-dodecane was achieved over this catalyst, due to more favored formation of centrally mono-branched and multi-branched isomers, as well as less cracking. 2. Experimental 2.1. Preparation of microporous ZSM-22 Commercial ZSM-22, bought from Dalian Keteli Chem. Co., was grind and sieved by a 40 mesh

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sieve and dried at 120 °C for 12 h. A part of ZSM-22 powders were partial-detemplated at 230 °C for 12 h. The partial-detemplation temperature was determined according to the thermogravimetric result of pristine ZSM-22 according to our previously reported[34], and the thermogravimetric result was

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shown in Figure S1. The obtained sample were ion exchanged with 1.0 M NH4Cl (Benchmark, 99.5

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wt%) solution at 80 oC for 5 h, filtrated, washed, dried at 120 oC overnight, and calcinated at 550 oC in air for another 12 h in order to fully remove the remained template. The obtained sample was named as

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230AN1, where 230 stands for partial-detemplation temperature, AN stands for NH4+ exchange, and

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the number 1 stands for the concentration of NH4Cl solution (1M). For comparison, a dried pristine ZSM-22 was directly ion exchanged without partial-detemplation step, and fully detemplated afterward.

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This sample was named as AN1.

Another part of the ZSM-22 powders were fully detemplated at 550°C for 12 h, followed by an

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ion exchange step. The concentrations of the NH4Cl solutions applied here were 0.2 M, 0.5 M and 1M. These samples were then filtrated, washed, dried at 120 oC overnight, and calcinated at 550 oC in air for 4 h. The obtained samples were named as 550AN0.2, 550AN0.5 and 550AN1, respectively. A fully detemplated ZSM-22 sample without being ion exchanged was also provided as blank sample named 550 detemplated. 6

2.2. Catalyst preparation Aqueous solution of H2PtCl6 (Aldrich) was applied as the Pt precursor. The incipient wetness impregnation method was applied for Pt loading. 0.5 wt% Pt was supported on the as-prepared ZSM-22 samples. After impregnation, the Pt loaded samples were dried at 120 °C for 12h, and further calcinated at 450 °C for 4 h in dry air. The prepared catalysts were named correspondingly as Pt/AN1, Pt/230AN1, Pt/550AN1, Pt/550AN0.2, Pt/550AN0.5. A Pt/550 detemplated catalyst was also prepared for blank

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control. 2.3. Characterizations

The crystal form and phase purity of the samples were determined by the X-ray diffraction (XRD)

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patterns recorded using a D8 advance X-ray diffractometer with (Cu Kα) radiation at 40 KV and 140

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mA. The morphology of the Pt loaded catalyst crystals were obtained on an EM-2010 FEF

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transmission electron microscope (TEM) with a field-emission gun operating at 200 kV. The textural properties of the samples were measured by a Micromeritics Model ASAP 2020 volumetric instrument

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at liquid N2 temperature (-196 °C) for N2 adsorption-desorption isotherms. Before measurement, all samples were degassed under vacuum at 150 °C overnight. BET method was applied to determine the

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total surface area (SBET) and total pore volume (Vtotal). The t-plot method was used to determine the micropore surface area (Smicro), external surface area (Sext) and micropore volume (V micro).

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Temperature-programmed desorption of NH3 (NH3-TPD) was carried in a Chemisorption Physisorption Analyzer (AMI-300, Altamira Instruments) equipped with a thermal conductivity detector (TCD) to determine the number of total acid sites of the samples. Before the NH3 adsorption, the samples were pretreated for 1h at 450 °C in He. Then the samples were cooled to 100 °C for NH3 adsorption in a mixture of NH3 and He (20% NH3 in He) for 10 min. To remove weakly absorbed NH3, the samples 7

were then treated under a flow of He for 2 h. TPD of NH3 was recorded in the range of 100-700 °C. The Si/Al ratio of bulk phase was determined on a S4 Pioneer X-ray fluorescence spectroscopy (XRF). The amount of total and external Brönsted acid sites was determined by Py-IR and 2,6-DTBPy-IR spectra, respectively. A Bruker Fourier transform infrared spectroscopy was used. Before the adsorption of Py or 2,6-DTBPy, the powder sample (ca. 20 mg) was grinded and pressed to a self-supported wafer, and pre-treated under a residual pressure below 10 -4 Pa for 1 h at 400 °C. Adsorption of pyridine or

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2,6-DTBPy was carried out at 60 °C. Desorption of pyridine and 2,6-DTBPy was carried out at 200 °C. A resolution of 4 cm-1 by collecting 64 scans was applied for a single spectrum of IR spectra. The adsorption bands around 1450 cm-1 and 1540 cm-1 in the Py-IR spectra represented the total Lewis and

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Brönsted acid sites, and the adsorption bands around 1616 cm-1 in the 2,6-DTBPy spectra represented

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external Brönsted acid sites, respectively.

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CO chemisorption was carried out also in the Chemisorption Physisorption Analyzer (AMI-300, Altamira Instruments) applied in NH3-TPD. The sample was firstly reduced in flowing H2 at 400 °C for

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4h, and purged with He at 450 °C for 0.5h afterward. Then the sample was cooled to 50 °C, and 10% CO/90% He was injected at regular intervals until the chemisorption was saturated. The dispersion of

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Pt was calculated by assuming that each surface Pt atom adsorbed a CO molecule. 2.4. Catalytic hydroisomerization of n-dodecane

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The hydroisomerization of n-dodecane was conducted as we previously reported[34], except the

reaction temperature was set at 240 °C – 320 °C with a 10 °C interval. Detailed description was also given in the SI. For the Pt/550 detemplated, the reaction temperature was raised to 300 °C – 350 °C. 3. Results and discussion 3.1. Textural structure and morphology of as-prepared ZSM-22 zeolites 8

Figure 1 shows the XRD patterns of as-prepared ZSM-22 zeolites. Typical TON-type structure with five characteristic peaks at 8.14°, 20.34°, 24.24°, 24.62° and 25.70° [35]are observed in all the samples. These samples presented almost identical XRD patterns as well as relative degree of crystallinity (listed in Table 1). This result indicates that the partial-detemplation & ion-exchange step did not significantly affect the TON-type crystal form of ZSM-22 zeolites. The TEM images of the Pt loaded samples are shown in Figure 2. All samples showed similar TON-type morphology and Pt

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dispersion. The rod-like ZSM-22 crystals had an average size of about 200 nm in length, as shown in

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Table 1. The Pt nano-particles dispersed well on the surface of the crystals. .

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Figure 1. XRD patterns of all samples.

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Figure 2. TEM images of Pt loaded samples. (A) Pt/AN1; (B) Pt/230AN1; (C) Pt/550AN0.2; (D) Pt/550AN0.5; (E) Pt/550AN1.

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N2 adsorption-desorption isotherms were measured to carefully determine the microporous structure. The isotherms of as-prepared samples are shown in Figure 3. The derived textural properties are listed in Table 1. After fully detemplation, all samples showed similar isotherms of typical

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microporous structure. The micropore volumes of these samples were about 0.09 cm3/g. This means

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that all the as-prepared samples shared the same ZSM-22 microporous structure. Further analysis verified the fact that the NH4+ exchange step almost did not affect the retained microporous templates

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in present work[15], as shown in Figure S2 and Table S1. As a result, the NH 4+ exchange was

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restrained at the exposed microporous surface. After NH4+ exchange before fully detemplation, the AN1 and 230AN1 samples showed a micropore volume of 0.001 and 0.007 cm3/g (Table S1),

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respectively. Based on their fully-detemplated micropore volume (0.090 and 0.094 cm3/g, Table 1) and average crystal length (220.7 and 213.2 nm, Table 1), it could be estimated that the NH4+ exchange was

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restrained at about 2.45 nm deep inside the micropores of AN1, and about 15.9 nm of 230AN1.

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Figure 3. N2 isotherms for as-prepared samples.

Average Crystal

Specific Surface Area (m2/g)

Volume (cm3/g)

Length (nm)

SBET

Smicro

Smeso

Vmicro (cm3/g)

Vmeso(cm3/g)

95.7

220.7

210.65

182.85

27.80

0.090

0.15

230AN1

100.0

213.2

222.56

192.02

30.54

0.094

0.14

550AN0.2

95.8

212.0

215.40

191.52

23.88

0.094

0.16

550AN0.5

97.8

203.8

209.74

180.14

29.60

0.088

0.13

550AN1

98.3

202.8

213.83

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Table 1. Textural properties for all samples

182.82

31.02

0.090

0.14

Samples

RCa(%)

AN1

Relative Crystallinity. 230AN1 was used as a standard (100%) to the calculation of RC.

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3.2. Acidity

NH3-TPD was applied to characterize the amount and strength of acidity. All NH 3-TPD curves are

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shown in Figure 4. The acidity amount and corresponding desorption peak temperature are listed in Table 2. The temperature of NH3 desorption peak indicates the strength of acidity. All samples showed

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two NH3 desorption peaks at a lower temperature (~200 °C) and a higher temperature (~ 400 °C),

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indicating the existence of both weak and strong acidity. However, it was clear in Figure 4 that the 550 detemplated sample showed the lowest desorption temperature and peak height compared with other samples. It has been reported that intrinsic acidity of ZSM-22 is too scarce to show enough activity for hydroisomerization[24]. In order to enhance its acidity, NH4+ exchange is necessary. The 550 detemplated sample showed the weakest acidity, with two desorption peaks at the temperature around 177.7 °C and 387.0 °C. All partial-detemplated samples showed a similar strength of acidity with two 11

desorption peaks at the temperature at about 190 °C and 420 °C. However, for the various NH4+ exchanged samples, the strength of strong acid increased as the NH 4+ aqueous concentration increased. The temperature of desorption peaks indicating strong acidity increased from 408.1 °C to 419.8 °C, when the NH4+ aqueous concentration applied increased from 0.2 M to 1.0 M. As shown in Table 2, the acidity amount follows the order of 550 detemplated < AN1 < 550AN0.2 < 230AN1 < 550AN0.5 < 550AN1. For the series of partial-detemplated samples and the various NH4+ exchanged samples, this

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result is in accordance with the increase of exposed micropore volumes and the increase of NH4+ aqueous concentrations, respectively. The highest acidity was obtained over the 550AN1 sample, which

was fully-detemplated and NH4+ exchanged in 1 M NH4+ aqueous concentration. All samples had Si/Al

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ratios at about 48, characterized by XRF and listed in Table 2. This result indicated that the preparation

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process of these samples did not affect the framework Si or Al, and the variation of acidity was simply

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derived from the different partial-detemplation and NH4+ exchange strategy.

Figure 4. Temperature-programmed desorption of ammonia from all samples Table 2. Acidity properties and Si/Al for all samples 12

Aciditya (μmol/g) Samples

Si/Al Weak aciditya

B/Lb

Strong aciditya

Brönsted

Brönsted

acidity

acidity

(Py)b

(DTBPy)c

(μmol/g)

(μmol/g)

AFd (%)

550 detemplated

241.5 (177.7 °C)

173.1 (387.0 °C)

47.9

--

--

--

--

AN1

276.7 (191.1 °C)

259.6 (418.0 °C)

47.7

6.3

152.6

81.9

53.6

230AN1

324.3 (190.5 °C)

286.4 (420.0 °C)

47.9

9.0

245.0

80.3

32.8

550AN1

338.7 (191.7 °C)

324.1 (419.8 °C)

48.2

13.0

407.3

78.7

19.3

550AN0.2

276.0 (189.1 °C)

267.2 (408.1 °C)

48.3

6.8

116.2

23.0

19.8

550AN0.5

313.6 (192.2 °C)

305.7 (415.4 °C)

47.9

10.7

298.0

59.9

20.1

a

Acidity, weak acidity and strong acidity was calculated from the relative area of the deconvoluted peak

b c

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obtained in NH3-TPD. Calculated from Py-IR spectra evacuated at 200°C.

Calculated from DTBPy-IR spectra evacuated at 200°C.

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AF for Brönsted acidity was calculated by Brönsted acidity (DTBPy) and Brönsted acidity (Py).

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Py-IR and DTBPy-IR were applied to differentiate Brönsted acidity, Lewis acidity and external

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Brönsted acidity. For Py-IR, absorption peaks located at 1545 cm-1 and 1450 cm-1 represent the amount of Brönsted acidity and Lewis acidity, respectively. For DTBPy-IR, the absorption peak located at 1616

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cm-1 represents the amount of external Brönsted acidity, as shown in Figure 5. The IR-derived results are listed in Table 2. For partial-detemplated samples, the amount of total Brönsted acidity increased as

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the exposed micropore volume increasing. For the AN1 sample that only exposed 0.001 cm3/g micropore volume after NH4+ exchange, the total Brönsted acidity calculated by Py-IR was only 152.6

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μmol/g. The total Brönsted acidity increased to 245.0 μmol/g for 230AN1, and further increased to

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407.3 μmol/g for 550AN1. For the various NH4+ exchanged samples, the total Brönsted acidity also increased from 116.2 μmol/g to 298.0 μmol/g and 407.3 μmol/g, as the concentration of NH4+ in aqueous solution increased from 0.2 M to 0.5 M and 1.0 M, respectively. In the case of the external Brönsted acidity, all the partial-detemplated samples showed similar results of about 80 μmol/g. It is in accordance with the fact that for these samples the external surfaces were exposed for the same NH4+

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aqueous concentration (1.0 M) in the ion-exchange step. For the various NH4+ exchanged samples, the amount of external Brönsted acidity varied with the NH4+ aqueous concentration applied, from 23.0 μmol/g to 59.9 μmol/g and 78.7 μmol/g. The accessibility factor (AF), defined as the ratio between the amount of external Brönsted acidity and total Brönsted acidity, was proposed to illustrate the shape-selectivity of zeolites[10]. It also described the distribution of Brönsted acid sites. This factor was calculated and listed in Table 2. It is clearly observed that in the case of partial-detemplated

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samples, the AF decreased as the volume of exposed micropores increased. An AF as high as 53.8%

was achieved on AN1, and decreased to 32.8% and 19.3% for 230AN1 and 550AN1, respectively. This

result indicated a higher distribution of microporous Brönsted acidity in the more detemplated ZSM-22

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samples. In the case of the various NH4+ exchanged samples, however, the AF values were similar. This

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indicated that NH4+ exchange happened evenly over the exposed ZSM-22 microporous and external

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surface. As a result, by simply lowering the NH4+ aqueous concentration in the exchange step, the acid density of ZSM-22 was tailored, while the acid distribution was remained. The acidity of Pt/ZSM-22

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catalysts was also characterized and the results are shown in Figure S3. After loaded with Pt, the height of desorption peak at around 400 °C increased a little, indicating a slightly increase of strong acidity.

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No signification variation was found.

Nevertheless, it should be noted that although pore-mouth and key-lock model has been widely

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accepted in describing the hydroisomerization of n-paraffins over ZSM-22, there are still no available means to differentiate pore-mouth Brönsted acidity from external Brönsted acidity[14, 15, 21, 29, 36]. Normally, the discussion upon pore-mouth Brönsted acidity is coupled with the variation of pore structure, such as the introduction of pore-mouths by alkali treatment[17, 18]. In present work, as the pore structure remains unchanged for the as-prepared samples, and the NH4+ exchange happens evenly 14

over the exposed ZSM-22 microporous and external surface, it is reasonable to suppose that the amount

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of pore-mouth Brönsted acidity alters in accordance with external Brönsted acidity.

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Figure 5. 2, 6-DTBPy-adsorbed FT-IR spectra (200 °C) of all samples.

Figure 6. Correlation between Micropore Exposed and total Brönsted acidity & AF (a), and correlation between NH4+ aqueous concentration and total Brönsted acidity & AF (b), and schematic acidity distribution.

Figure 6 shows the correlations between exposed micropore versus total Brönsted acidity and NH4+ aqueous concentration versus AF values for two series catalysts. For the partial detemplated 15

samples shown in Figure 6a, the total Brönsted acidity increased quickly as the exposed micropore volume increased from 0.001 to 0.007 cm3/g, and then increased slowly with further increase of exposed micropores. This result indicated a relatively higher microporous Brönsted acidity near the pore mouth. For the various NH4+ exchanged samples shown in Figure 6b, the density of Brönsted acidity was well tailored by the concentration of NH4+ aqueous solution. Based on the results, a diagrammatic figure of acidity distribution of representative as-prepared ZSM-22 samples (230AN1,

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550AN0.5 and 550AN1) is shown in Figure 6. The AN1 and 230AN1 samples showed similar pore mouth Brönsted acidity and external Brönsted acidity as 550AN1, while their microporous Brönsted acidity was tailored by the restrained microporous templates during the NH 4+ exchange process. AN1

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showed least microporous Brönsted acidity, while 230AN1 showed more microporous Brönsted acidity

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near pore mouth. For the various NH4+ exchanged samples, they all showed similar pore mouth,

altered.

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3.3. Metallicity

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external and microporous Brönsted acidity distribution, while the density of Brönsted acidity was

Pt dispersion and calculated Pt particle size are listed in Table 3. It was found that all the catalysts

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showed a similar Pt dispersion around 48%, and the size of Pt particles was about 2.4 nm, in accordance with the TEM results. It has been reported that the Pt dispersion on zeolites is less affected

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by external acidity, but easily affected by the morphology and pore structure of zeolites[20, 25, 36]. From the TEM characterization and textural properties, these as-prepared ZSM-22 supports had nearly identical morphology and pore structure, and a similar Pt dispersion. It should be noted that the Pt particles of a size about 2.4 nm was too large to penetrate into the 0.45×0.55 nm micropores of ZSM-22. As a result, these particles located mostly on the external surfaces of the ZSM-22 16

crystals[14]. Table 3. Results of CO chemisorption for Pt loaded catalysts Samples

Pt dispersion (%)

Pt particle size (nm)

Pt/AN1

47.9

2.48

Pt/230AN1

48.5

2.45

Pt/550AN1

48.2

2.47

Pt/550AN0.2

48.8

2.44

Pt/550AN0.5

48.0

2.48

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3.4. Catalytic performance The n-dodecane hydroisomerization was carried out over the as-prepared zeolites supported Pt catalysts. The conversions vs. temperatures are shown in Figure 7. For the partial-detemplated samples,

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the activity was Pt/AN1 < Pt/230AN1 < Pt/550AN1, in accordance with their total Brönsted acidity

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shown in Table 2. For the various NH4+ exchanged samples, however, similar activity was observed over Pt/550AN0.2, Pt/550AN0.5 and Pt/550AN1, despite the fact that they had different total Brönsted

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acidity. It is clear that the AF value, rather than total Brönsted acidity, is more relevant to the activity of as-prepared catalysts. The activity increased as the AF values decreased from 53.6% (Pt/AN1) to 32.8%

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(Pt/230AN1) and 19.3% (Pt/550AN1), and the similar AF values for Pt/550AN0.2, Pt/550AN0.5 and

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Pt/550AN1 leaded to similar activity. This result indicated that the microporous Brönsted acidity and acidity distribution had significant influence on the catalyst activity.

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It is also observed that at the reaction temperature around 270 °C and 280 °C, the activity of Pt/550AN0.2 and Pt/550AN0.5 exceeded Pt/550AN1. These results could be attributed to a diffusion effect[6, 10, 21, 30]. It has been reported that the slow diffusion inside the micropores was detrimental to the catalyst activity[30]. For the various NH4+ exchanged samples, the decrease of external Brönsted acidity probably enhanced the diffusion opportunity inside the micropores[29, 37], and lowered the

17

activity of Pt/550AN0.2 and Pt/550AN0.5 at lower temperature. However, at higher temperature, the adsorption mode of intermediate products varied from favoring pore-mouth mode to key-lock mode[21]. The microporous diffusion in Pt/550AN0.2 and Pt/550AN0.5 was therefore limited, and higher activity was achieved. This varition of adsorption mode of intermediate products could also explain that the difference of activity among Pt/550AN1, Pt/230AN1 and Pt/AN1 diminished as the

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reaction temperature increased to higher than 280 °C, due to their similar external Brönsted acidity.

Figure 7. Conversion vs. Temperature curves for all catalysts. Reaction conditions: H2/oil ratio: 400;

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LHSV: 2.0 h-1; Reaction pressure: 2.0 MPa.

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The isomers yields vs. n-dodecane conversions are shown in Figure 8, and the results at the conversion about 85% are listed in Table 4. All catalysts showed maximum yield higher than 50%

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except for Pt/550AN1 (40%). This result indicated that high Brönsted acidity leaded to severe cracking. For the various NH4+ exchanged samples, maximum isomer yield was Pt/550AN0.2 > Pt/550AN0.5 >

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Pt/550AN1, in accordance with the fact that cracking depends on Brönsted acidity. However, for the partial-detemplated samples, maximum isomer yield was Pt/230AN1 > Pt/AN1 > Pt/550AN1, against their total Brönsted acidity. This result indicated that the microporous Brönsted acidity near pore mouth probably favored skeletal isomerization rather than cracking, and the microporous Brönsted acidity deep inside the micropores facilitated cracking. In order to further clarify the relationship between acid 18

distribution and the selectivity between mono-isomers and multi-isomers, the selectivity of mono-branched isomers in all branched isomers against n-dodecane conversions is plotted in Figure 9. For the partial-detemplated samples, higher selectivity of multi-branched isomers is observed at high conversion. For the various NH4+ exchanged samples, higher selectivity of mono-branched isomers are observed. It has been reported that multi-branched isomers could only crack at external Brönsted acid sites through (s, s) β-scission[19]. As all the partial-detemplated samples (Pt/AN1 and Pt/230AN1)

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showed higher external Brönsted acidity than various NH 4+ exchanged samples (Pt/550AN0.2 and

Pt/550AN0.5), the cracking of multi-branched isomers would be more favored. The higher selectivity of multi-branched isomers over the partial-detemplated samples, therefore, could be due to the favored

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formation of multi-branched isomers through key-lock mode. The reduced microporous Brönsted

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acidity of these catalysts probably hindered the diffusion in the micropores by avoiding cracking

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through (s, p) β-scission, and enhanced the diffusion between adjacent pore-mouths[37]. As a result, more key-lock adsorption happened, and formed more multi-branched isomers. Pt/230AN1 showed

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even higher selectivity towards multi-branched isomers than Pt/AN1, because of the strong chemisorptions of intermediate products on strong microporous Brönsted acid sites near pore

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mouths[21]. These strong chemisorptions caused further isomerization of mono-branched carbonium ions into multi-branched carbonium ions. As a result, a multi-branched isomer yield of 11.2% and a

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mono-/multi- ratio of 3.9 was achieved on Pt/230AN1 at the conversion of 87.1%, as shown in Table 3. For the various NH4+ exchanged samples, the formation of mono-branched isomers were favored. The selectivity of multi-branched isomers over these catalysts followed the order as Pt/550AN0.2 > Pt/550AN0.5 > Pt/550AN1. This result is due to more and more external Brönsted acidity enhanced the cracking of branched isomers through (s, s) β-scission, and the multi-branched isomers were more 19

prone to cracking than the mono-branched isomers. The reduction of microporous Brönsted acidity of the various NH4+ exchanged samples also reduced the cracking in the micropores, and this would be discussed below. The reduction of pore-mouth Brönsted acidity of these samples almost did not affect

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their pore-mouth shape-selectivity.

Figure 8. Product yields vs. n-dodecane conversions for all catalysts. Reaction conditions: H2/oil ratio : 400; LHSV : 2.0 h-1; Reaction pressure: 2.0 MPa.

Table 4. Results of n-dodecane isomerization over all catalysts Catalysts

Pt/AN1

Pt/230AN1

Pt/550AN1 20

Pt/550AN0.2

Pt/550AN0.5

Temperature (°C)

300

300

290

290

280

Conversion (%)

82.6

87.1

87.8

89.9

85.1

ST (%)

63.9

62.6

46.4

63.9

62.6

YTotalb

(%)

52.8

54.5

40.7

57.4

51.4

YMono (%)

45.9

43.3

36.4

49.6

46.7

YMultib

6.9

11.2

4.3

7.8

4.7

6.7

3.9

8.5

6.4

9.9

a

b

(%)

mono-/multia

ST was calculated as total isomer yield divided by conversion.

b

Ytotal, Ymono and Ymulti was calculated as addition of the yield of corresponding isomers.

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Reaction conditions: H2/oil ratio : 400; LHSV : 2.0 h-1; Reaction pressure: 2.0 MPa.

Figure 9. Mono-branched isomers selectivity in all branched isomers. Reaction conditions: H2/oil ratio:

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400; LHSV: 2.0 h-1; Reaction pressure: 2.0 MPa.

Figure 10 showed the compositions of mono-branched isomers against conversions of n-dodecane.

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The compositions of mono-branched isomers over these samples were different. The Pt/AN1 showed obviously lower 2-methyl isomer fraction (47% - 32%) and higher 3-, 4- and 5-/6- methyl isomers

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fractions at the conversions from 10% - 80%. The Pt/230AN1 showed a little higher 2-methyl isomer

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fraction (56% - 33%), while the Pt/550AN1, Pt/550AN0.2 and Pt/550AN0.5 showed obviously higher 2-methyl isomer fraction (58% - 40%), and lower other methyl isomers fractions. It has been reported that the pore-mouth model favored the formation of 2-methyl isomer, and the key-lock model favored the formation of centrally mono-branched isomers (4- and 5-/6-methyl isomers)[21]. As discussed above, the hydroisomerization of n-paraffins favored the key-lock model on the partial-detemplated samples, and formed more multi-branched isomers on Pt/230AN1. From the results of Figure 10 (a) (b) 21

and (c), it could be proposed that the favoring of key-lock model on Pt/AN1 mainly enhanced the formation of centrally mono-branched isomers. On the other hand, reducing the external and internal acidity evenly did not affected the shape-selectivity for mono-branched isomers on microporous

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ZSM-22, as observed in Figure 10 (c) (d) and (e).

Figure 10. Compositions of mono-branched isomers against conversions of n-dodecane. (a) Pt/AN1; (b) Pt/230AN1; (c) Pt/550AN1; (d) Pt/550AN0.2; (e) Pt/550AN0.5. Reaction conditions: H2/oil ratio: 400; LHSV : 2.0 h-1; Reaction pressure: 2.0 MPa.

In order to investigate the cracking through (s, p) β-scission, the mole ratio of (C3+C4)/(C8+C9) 22

cracking products at the conversion of about 85% is analyzed. Without secondary cracking, the mole yield of (C3+C4) and (C8+C9) should be asymmetric and the ratio should equals to 1, as for each C 12 molecule the complementary fragments should be formed in equal amount [19]. As shown in Figure 11, For partial-detemplated samples and various NH4+ exchanged samples, this ratio increased as the total Brönsted

acidity increased.

Pt/550AN0.2,

Pt/550AN0.5

and

Pt/550AN1

samples

showed

(C3+C4)/(C8+C9) ratios of 1.12, 1.27 and 1.70, respectively. The partial-detemplated samples showed

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lower (C3+C4)/(C8+C9) ratios than the Pt/550AN samples. The Pt/AN1 sample and the Pt/230AN1

sample showed ratios as low as 1.03 and 1.08, respectively. This result clearly shows that the secondary cracking mainly happens at microporous Brönsted acidity though (s, p) β-scission deep inside the

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micropores, where the diffusion of intermediate products is severely restrained. Pt/AN1 catalyst with

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least microporous Brönsted acidity almost eliminated cracking through (s, p) β-scission. The

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microporous Brönsted acidity near pore mouth also did not cause heavily secondary cracking, for the intermediate products could still easily diffuse out of the pore mouth, and avoid further contacting with

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Brönsted acidity.

Figure 11. Mole ratio of cracking products (C3+C4)/(C8+C9) at about 85% conversion. Reaction conditions: H2/oil ratio: 400; LHSV: 2.0 h-1; Reaction pressure: 2.0 MPa. 23

In order to investigate the synergistic effect of template protective and NH4+ exchange in this new strategy, the samples of Pt/AN0.2 and Pt/230AN0.2 were further prepared. The preparation process of these two samples was similar with Pt/AN1 and Pt/230AN1, except the NH4+ concentrations in the solutions applied for these samples at 0.2 M. As shown in Figures S5 to S9, and Tables S2 to S4, a NH4+ exchange step is necessary for the proper activity of Pt/ZSM-22 catalyst, and these two samples maintained similar textural structure and morphology as the aforementioned samples. They showed

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lower Brönsted acidity compared with corresponding 1.0 M NH4Cl exchanged samples, and the AF

values were maintained. The conversion vs. temperature curves for these two samples are plotted in Figure S9. Pt/230AN0.2 showed slightly higher activity than Pt/AN0.2, and these two samples both

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showed lower activity than Pt/230AN1 & Pt/AN1, in accordance with their AF & total Brönsted acidity.

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Higher maximum isomer yield was achieved over these catalysts, as shown in Figure S10. To give a

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further insight into these results, the yield of different mono-branched and multi-branched isomers is

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shown in Figure 12 and Table S5.

Figure 12. Yield of mono-branched isomers (A) and multi-branched isomers (B) for all samples at the maximum yield. Reaction conditions: H2/oil ratio: 400; LHSV: 2.0 h-1; Reaction pressure: 2.0 MPa.

The Pt/AN0.2 showed an improved isomer yield of 61.2% at the conversion of 91.7%, while the Pt/230AN0.2 showed a maximum 63.7% yield at the conversion of 87.8%, higher than the Pt/AN1 24

(52.8%), Pt/230AN1 (54.5%) and Pt/550AN0.2 (57.4%). This enhancement of total isomer yield could be ascribed to the synergistic effect of template protective and NH4+ exchange. The yield of 2-, 3- and 4-methyl isomers that were formed according to pore-mouth model over Pt/AN0.2 and Pt/230AN0.2, was overall higher than Pt/AN1 and Pt/230AN1. This is because lower concentration of NH4+ exchange leaded to less total Brönsted acidity and less cracking. In the case of centrally mono-branched isomers, i.e. 5-/6- methyl isomers, the Pt/AN0.2 showed substantially higher yield than other catalysts, due to

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the template protective strategy greatly reduced microporous Brönsted acidity. As a result, the

hydroisomerization of n-dodecane favored more key-lock mode, and formed more centrally mono-branched isomers. The Pt/230AN0.2 showed similar mono-branched isomers yield as Pt/AN0.2,

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while a higher multi-branched isomers yield was observed, similar as Pt/230AN1. This result

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confirmed the chemisorption of the reactant intermediates near the pore-mouth Brönsted acid sites, and

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this chemisorption leaded to the formation of more multi-branched isomers through key-lock mode. As a result, a total isomer yield of 63.7% was achieved over the Pt/230AN0.2 sample, due to its tailored

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microporous Brönsted acidity near the pore-mouths and reduced total Brönsted acidity, coming from the synergistic effect of template protective and NH4+ exchange strategy.

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4. Conclusions

In this work, series of microporous ZSM-22 with different Brönsted acidity amount and

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distribution were prepared through a template protective and various NH4+ exchange strategy. The different acidity distribution of these samples loaded with Pt showed different selectivity towards different isomers. Microporous Brönsted acidity located inside the micropores near pore-mouths may help favor the formation of multi-branched isomers through key-lock model and a mono-/multi- ratio as low as 3.9 was obtained. Those Brönsted acidity located deep inside the micropores mainly caused 25

secondary cracking. Elimination of microporous Brönsted acidity would facilitate the formation of centrally mono-branched isomers through key-lock mode, and greatly restrain secondary cracking through (s, p) β-scission. By selectively reducing microporous acidity, the shape-selectivity of microporous ZSM-22 could be tailored. On the other hand, by reducing total Brönsted acidity and keeping the acidity distribution, the isomer yield could be enhanced and cracking restrained, while the shape-selectivity was less affected. A synergistic effect was found between the template protective and

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NH4+ exchange strategy. By selectively tailoring microporous Brönsted acidity near the pore mouth while reducing total Brönsted acidity, the maximum isomer yield could be enhanced to 63.7%. This

higher isomer yield resulted from a favored formation of central mono-branched isomers and

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multi-branched isomers through key-lock mode, coupled with less cracking.

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Acknowledgements

The financial supports by National Natural Science Foundation of China (Grant No. 21476169,

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21476168) are gratefully acknowledged.

26

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Electronic supplementary information

N-dodecane Hydroisomerization over ZSM-22 : Controllable Microporous Acid Distribution and Shape-Selectivity Xiangyu Wang1, Xiangwen Zhang1, 2*, Qingfa Wang1, 2* 1. Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of China. 2. Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, China.

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*Corresponding author. Tel. /fax: +86 22 27892340.

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Fig. S1 TGA (solid line) and DTG (dashed line) profiles of pristine ZSM-22.

As shown in the TGA profile, the transition at 170-270 °C (2.7 wt%) could be due to the

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gasification of weak bounded diaminohexane (DAH) templates. The transition at 370-470 °C (4.5 wt%) was assigned to the pyrolysis of strong bounded DAH templates. The significant weight loss at above 470 °C (4.2 wt%) could be ascribed to the oxidation of oligomeric products from incomplete pyrolysis

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in the former steps. Two peaks at 230 °C and 430 °C were observed in the DTG profile. In the present work, only 230 °C was applied, for the pyrolysis of DAH templates at 430 °C may cover the

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microporous surfaces, resulting an unsuccessful NH4+ exchange.

S1

Electronic supplementary information

Catalytic hydroisomerization of n-dodecane The hydroisomerization of n-dodecane was conducted in a stainless steel continuous down-flow fixed-bed reactor. The reaction temperature was controlled by three thermocouples on the reactor wall and monitored with a thermocouple in the catalyst bed. Typically, 2.0 g of the catalyst was pressed into pellets at 400 bar, crushed, sieved (20-40 mesh), diluted with SiC and then loaded in the reactor. The feedstock was injected into the reactor with a high-pressure pump. The loaded catalysts were reduced in situ at 400 °C under flowing H2 for 4 h, and then the temperature was cooled down to reaction temperature. The reaction pressure was set to 2.0 MPa. H2/n-dodecane volume ratio was fixed to 400,

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while liquid hourly space velocity (LHSV) was fixed to 2.0 h -1. Reaction temperature was set at the range of 240-320 °C in order to change the reactant conversion. At each temperature the reaction was

stabilized for 1 h. After the reaction reached stable, a sample by collecting the products for 30 min was withdrawn. The product from the reactor outlet was cooled with a heat exchanger, and further separated

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in a separator into gas and liquid fractions. The gaseous products were analyzed with an online

chromatograph equipped with a TCD using three columns (molecular sieve, plot U and alumina). The

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liquid products were analyzed with an offline gas chromatograph (GC) equipped with a flame ionization detector and a DB-1HT capillary column (30 m × 0.25 mm × 0.1 μm). The conversion and

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yield were calculated according to the equations below, where G feed and Gproduct stands for the weights of n-dodecane in the feed and product, and Gisomer stands for the weight of corresponding isomer in the products.

(Gfeed −Gproduct )

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Conversion =

∑ Gisomer

(2)

Gfeed

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Yieldisomer =

(1)

Gfeed

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Electronic supplementary information

Table S1. Textural properties for all samples Samples

Specific Surface Area (m2/g)

Volume (cm3/g)

SBET

Smicro

Smeso

Vmicro (cm3/g)

Vmesob(cm3/g)

3.98

22.84

0.002

0.10

37.61

2.13

35.48

0.001

0.12

26.82

AN1 exchanged without detemplation

32.99

5.50

27.49

0.003

0.11

230AN1 exchanged without fully detemplation

45.51

14.99

30.52

0.007

0.18

550°C detemplated

207.49

187.06

20.43

0.081

0.15

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230°C detemplated

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Pristine ZSM-22

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Fig. S2 N2 isotherms for detemplated samples before and after NH4+ exchange.

For AN1 sample that had been exchanged and before fully detemplation, the NH 4+ exchange step

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caused a little drop of micropore volume, from 0.002 to 0.001 m3/g. For 230AN1 sample, the NH4+ exchange step caused an increase of micropore volume, from 0.003 to 0.007 m3/g. This could be due to

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that a calcination under 230 °C transformed the retained micropore template into more soluble carbonizations. Still, most micropores of 230AN1 samples were occupied by retained micropore templates until fully detemplation. For 550 samples, the detemplation at 550 °C leaded to a fully removal of microporous templates, and micropores of the 550 samples were totally exposed to exchange.

S3

NH4+

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Electronic supplementary information

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Fig. S3 Temperature-programmed desorption of ammonia from Pt loaded catalysts

S4

Electronic supplementary information

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Fig. S4 XRD patterns of AN0.2 and 230AN0.2.

S5

Electronic supplementary information

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Fig. S5 TEM images of Pt loaded samples: (A) Pt/AN0.2 and (B) Pt/230AN0.2.

S6

Electronic supplementary information

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Fig. S6 N2 isotherms for AN0.2 and 230AN0.2.

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Electronic supplementary information

Table S2. Textural properties for AN0.2 and 230AN0.2. Specific Surface Area (m2/g)

Volume (cm3/g)

SBET

Smicro

Smeso

Vmicro (cm3/g)

Vmesob(cm3/g)

AN0.2 exchanged without detemplation

20.61

2.33

18.28

0.001

0.13

AN0.2

212.83

186.47

26.36

0.091

0.16

230AN0.2 exchanged without fully detemplation

45.05

14.53

30.52

0.007

0.14

230AN0.2

212.28

178.87

33.41

0.088

0.15

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Samples

S8

Electronic supplementary information

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Fig. S7 Temperature-programmed desorption of ammonia from AN0.2 and 230AN0.2.

S9

Electronic supplementary information

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Fig. S8 2, 6-DTBPy-adsorbed FT-IR spectra (200 °C) of AN0.2 and 230AN0.2.

S10

Electronic supplementary information

Table S3. Acidity properties for AN0.2 and 230AN0.2. Aciditya (μmol/g) Samples

Si/Al

b

B/L

Brönsted acidity (Py)b (μmol/g)

Brönsted acidity (DTBPy)c (μmol/g)

AFd

Weak aciditya

Strong aciditya

AN0.2

250.3 (194.6 °C)

201.1 (413.3 °C)

47.1

4.3

43.7

21.8

49.9

230AN0.2

264.1 (184.6 °C)

227.7 (412.3 °C)

47.7

6.8

67.6

22.5

33.3

(%)

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Acidity, weak acidity and strong acidity was calculated from the relative area of the deconvoluted peak obtained in NH3-TPD. b Calculated from Py-IR spectra evacuated at 200°C. c Calculated from DTBPy-IR spectra evacuated at 200°C. d AF for Brönsted acidity was calculated by Brönsted acidity (DTBPy) and Brönsted acidity (Py).

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Table S4. Results of CO chemisorption for Pt/AN0.2 and Pt/230AN0.2 Pt dispersion (%)

Pt particle size (nm)

Pt/AN0.2

48.0

2.48

Pt/230AN0.2

47.7

2.49

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Samples

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Fig. S9. Conversion vs. Temperature curves for Pt/AN0.2 and Pt/230AN0.2 and Pt/550 detemplated. Reaction conditions: H2/oil ratio : 400; LHSV : 2.0 h-1; Reaction pressure: 2.0 MPa.

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Fig. S10. Product yields vs. n-dodecane conversions for Pt/AN0.2 and Pt/230AN0.2. Reaction conditions: H2/oil ratio : 400; LHSV : 2.0 h-1; Reaction pressure: 2.0 MPa.

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Electronic supplementary information

Table S5. Detail yield of isomers for all catalysts. Catalysts

Pt/AN0.2

Pt/AN1

Pt/230AN0.2

Pt/230AN1

Pt/550AN0.2

Pt/550AN0.5

Pt/550AN1

Conversion (%)

91.7

82.6

87.8

87.1

89.9

85.1

86.7

2-Methyl isomer

14.5

14.1

13.8

13.0

16.4

20.4

13.8

3-Methyl isomer

12.5

11.1

11.6

10.7

12.5

10.8

8.6

4-Methyl isomer

9.2

7.7

9.0

7.3

7.4

5.1

5.2

5-/6- Methyl isomers

18.9

13.0

20.0

12.3

13.3

10.4

8.3

Mono-branched isomers

55.1

45.9

54.4

43.3

49.6

46.7

35.9

3,8-Dimethyl isomer

0.1

0.3

0.4

0.5

0.3

0.1

0.1

2,8-Dimethyl isomer

1.6

1.5

2.3

2.4

1.7

0.9

0.9

2,9-Dimethyl isomer

1.8

1.7

2.7

2.6

2.3

1.5

1.3

2,7-Dimethyl isomer

0.7

1.0

1.2

1.5

1.0

0.5

0.5

2,6-Dimethyl isomer

1.3

1.5

2.0

2.2

1.4

0.8

0.7

2,5-Dimethyl isomer

0.4

0.5

0.6

0.8

0.5

0.2

0.2

Multi-branched isomers

5.9

6.5

9.2

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Yield (%)

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Reaction conditions: H2/oil ratio : 400; LHSV : 2.0 h-1; Reaction pressure: 2.0 MPa.

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3.7