Co Bimetallic

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Steam catalytic cracking of n-dodecane over Ni and Ni/ Co bimetallic catalyst supported on hierarchical BEA zeolite Mohamed H.M. Ahmed, Oki Muraza, Anas Karrar Jamil, Emad N. Shafei, Zain H. Yamani, and Ki-Hyouk Choi Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 20 Apr 2017 Downloaded from http://pubs.acs.org on April 21, 2017

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Page 1 of 31 1 2 3 4 5 6 7 8 9 n-dodecane 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

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Medium conversion High paraffins Less isomers Less olefins Quick coking

n-dodecane

Lower conversion Higher conversion High paraffins Less paraffins More isomers High isomers Less olefins High olefins Fast coking n-dodecane Less coking

M

M M

NaOH

MNO3

M

M M

Desilication Ion exchange

M

M

Created mesopores M= Ni or Co

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Steam catalytic cracking of n-dodecane over Ni and Ni/Co bimetallic catalyst supported on hierarchical BEA zeolite Mohamed H.M. Ahmeda, Oki Muraza*a, Anas K. Jamila, Emad N. Shafeib, Zain H. Yamania, Ki-Hyouk Choib a

Center of Excellence in Nanotechnology and Chemical Engineering Department,

King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia b

Research and Development Center, Saudi Aramco, Dhahran 31311, Saudi Arabia ⃰Corresponding author, E-mail: [email protected].

Abstract Steam catalytic cracking was performed over metal modified BEA zeolite. Nickel and cobalt were introduced to the desilicated BEA zeolite by substitution treatment. The method of metal incorporation used here allowed to incorporate the metal(s) into framework matrix of BEA in tetrahedral form together with the external surface. The successful incorporation of nickel/cobalt into BEA framework in tetrahedral form was confirmed from FTIR spectra and UV-vis. The quantity of metals attached to BEA zeolite was calculated by EDX. The changes and defects on BEA structure were studied from XRD patterns and SEM micrographs. The metal incorporation was significantly increased the total acidity as confirmed by NH3-TPD and pyridine FTIR analysis. The attachment of metals, considerably increased the conversion of n-dodecane as compared to the parent sample. The stability of BEA zeolite was also enhanced significantly. However, the addition of Co to Ni incorporated BEA zeolite had negatively effect on both n-dodecane conversion and BEA stability. Keywords: Ni and Ni/Co bimetallic catalysts; hierarchical BEA; steam catalytic cracking; ndodecane.

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1. Introduction Recently, the demand of middle distillate fuel products especially gasoline and diesel is growing rapidly 1. Consequently, the catalytic cracking of heavy hydrocarbon is getting more attention to meet the demand on these products

2-5

. Zeolites are the common catalyst for

petroleum and petrochemical industry due to their unique properties (ex. high surface area, controllable acidity and flexible pore sizes) 6. However, there are many different metals can further improved the catalytic properties of zeolite such as Ni, Zr, Nd, Pt and La. Bifunctional catalyst (metal/acid) is the most suitable catalyst for heavy hydrocarbon cracking 7. The metal site is required for hydrogenation and dehydrogenation while the acid site is required for the cracking and isomerization 8. The most critical part which determines the stability, activity, and selectivity of the catalyst is the balance between the metal and acid sites as well as the position and the distribution of the metals on the zeolite substrate9-11. Nickel incorporated into zeolite showed a promising result in different catalytic applications especially hydrogenation/dehydrogenation and hydrocracking

12, 13

. There are mainly two practical

routes which are commonly being used for metal incorporation with zeolites

14, 15

.

Impregnation of the metals to the zeolite surface is the preferred route because it offers a combination of the acid properties of zeolite with the metal catalytic properties 16. However, this procedure has usually negatively impact on the microporosity by blocking most of these micropores. The next common route is ion exchange; which represents also an efficient way to attach the metals to zeolite. In this route, the metals mainly connected to the zeolite surface and/or inside the pores, which may also affect on the diffusion

17

. In this work, we tried a

third method of metal incorporation with zeolite in which highly charge empty vacant sites were created in the framework of BEA zeolite by extraction of some silicon atoms using NaOH solution followed by a treatment with nickel nitrate and cobalt nitrate to allow to Ni and Co atoms to be internally connected with the zeolite matrix and substitute the extracted 2 ACS Paragon Plus Environment

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Si. The developed bi-functional catalysts were characterized to investigate the changes happened on BEA zeolite and also was evaluated in conversion of n-dodecane. The ndodecane was selected as a model compound of heavy naphtha 18. 2. Experimental 2.1 Synthesis of Parent BEA BEA zeolite crystals were hydrothermally prepared by mixing 30 mL of distilled water with 34.8 g of colloidal silica (SNOWTEX, 40% SiO2) over a magnetic stirrer at a rate of 800 rpm. A 30 g of tetraethyleammonium hydroxide (TEAOH, Sigma Aldrich 40% in H2O) was added to the solution. The solution stirred for 10 min, then a 0.76 g of sodium aluminate (NaAlO2, Sigma Aldrich) was added. The gel was kept for 1 h in stirrer at room temperature. The molar composition of the gel was 1SiO2:0.02Al2O3:0.034Na2O:0.35TEAOH:11.6H2O. The gel placed into 100 mL autoclave and transferred to oven at 150 oC for 72 h. The autoclave was cooled down and the products were washed several times with distilled water and dried overnight at 110 oC. The powder was then calcined at 550 oC for 12 h to remove the template. The parent sample was named as BEA parent. 2.2 post treatment of BEA zeolite with Ni The BEA sample was first desilicated by treating with 0.1 M NaOH. The treatment was carried out by adding 1 g of BEA zeolite to 30 mL of NaOH solution at 65 oC for 15 min. The powder was later cooled and washed with distilled water. The desilicated sample was denoted as Dsi-BEA. The desilicated BEA sample was subjected to treatment with Ni(NO3)2 with different concentrations. The treatment process was performed at 65 oC for 15 min. A 30 mL solutions of Ni(NO3)2 with 0.2 and 0.6 M was used and the samples were called as 0.2NiDsi-BEA and 0.6Ni-Dsi-BEA, respectively. One other sample was prepared by treating the


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desilicated sample with 0.2M of Co(NO3)2, the sample called as 0.2Co-Dsi-BEA. Another sample was prepared by coupling two treatment steps. The first treatment was with 0.2 M of Ni(NO3)2 followed by another treatment with 0.2 M of Co(NO3)2. Both treatments were performed at 65 oC for 15 min. The sample was named as 0.2Ni-0.2Co-Dsi-BEA. Later, the entire samples were calcined at 550 oC for 12 h in a muffle furnace. All the samples including the parent subjected to ion exchange with 2M ammonium nitrate (NH4NO3). The treatments were performed by adding 1 g of solid catalysts to 20 ml of NH4NO3 solution at 85 oC for 20 min. The solids were later washed separated and dried for 12 h at 110 oC. 2.3 Characterization Miniflex, a Rigaku diffractometer with Cu Kα radiation was used to obtain XRD patterns of the dried solid powder products. The analysis performed in the range of 5 to 50o of 2θ with a scan step of 0.03o and a counting time of 4 s for each step. Field-emission scanning electron microscopy (FE-SEM) was used to study the morphology and chemical composition of the samples (LYRA 3 Dual Beam Tescan) equipped energy dispersive X-ray spectrometry (EDX, Oxford Instruments) operated at an acceleration voltage

of 30 kV.

The

N2

adsorption/desorption was measured using Micromeritics ASAP 2020 porosimeter. Prior to measurement, the samples were degassed at 350 oC for 12 h to remove any possible adsorbed gasses. Thermogravimetric analysis and differential scanning calorimetry (TGA/DSC), experiments were carried out under Argon gas using a heating rate of 10 oC /min up to 700 o

C. Pyridine adsorption followed by an infrared (IR) spectroscopy (Nicolet 6700

Spectrometer) in transmission mode. Spectra were recorded at 4 cm−1 spectral resolution, an under sampling ratio of 4, and a speed of 20 kHz. Samples of fresh catalysts (200 mg) were first pressed into thin wafers and then activated in situ in the IR cell under secondary vacuum (10−6 mbar) at 500 oC for 30 min. After that the sample cooled down to 150 oC and the


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pyridine introduced to the cell for 30 min. The vacuum was applied again for 10 min to remove the excess pyridine then the spectrum was recorded. 2.4 Steam catalytic cracking of n-dodecane Catalytic performance of BEA zeolite catalysts were evaluated in steam catalytic cracking (SCC) of n-dodecane using a steam compatible packed bed reactor. All reaction tests were performed at 400 oC and LHSV of 4 h-1. The experimental setup for the SCC process is presented in Fig. 1. The SCC process was carried out using a tubular flow system at atmospheric pressure. In addition, the n-dodecane and water used during the process were introduced using electric driven syringe pumps. The flow rates of n-dodecane and water were maintained at 3.6 cm3 h−1and 0.4 cm3 h−1, respectively, to achieve dodecane to steam ratio of 9 v/v. Nitrogen gas was employed as carrier gas. The products were directly injected and analyzed using a GC-MS. Gas chromatography of mass spectrometry detector (GC-MSD) from Agilent was used in order to evaluate the hydrocarbon conversion. The GC-MS column is used Agilent J&W HP-5ms length of 30 m, the internal diameter 0.25 mm, film thickness 0.25 µm, and gas splitter ratio was applied 1:100 at inlet temperature of 250 oC. The initial temperature was set at 40 oC for 1 minutes than the oven was ramped at 10 oC/min in order to reach 280 oC. The GC-MS is used to classified product yield from reactor into five classes as follow: paraffin, iso-paraffins, olefins, naphthalene or cylco-paraffins and aromatics which is the PIONA. 3. Results and Discussion 3.1 Structure defects and textural properties The desilication of BEA zeolite with 0.1 M of NaOH strongly affected the crystallinity of BEA as observed by XRD patterns in Figure 1. The desilicated samples lost almost two thirds of their crystallinity as compared with the parent BEA based on the calculations of the area 5 ACS Paragon Plus Environment

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under the peaks. Moreover, the Ni and Co treatments affected a more decrease in the crystallinity as shown in Table 1. Please insert Figure 1 here. However, SEM micrographs presented in Figure 2 are not showing any significant damages on the treated samples. Therefore, this SEM observation forbids the hypothesis of attribution the crystallinity decreases to any possible structure collapse due to the desilication. Consequently, the attribution of crystallinity decrease to the deposition of amorphous Al species is more reasonable due to high Al content of the parent BEA sample. It was also observed from the SEM images that the surface roughness of the desilicated is higher as compared to the parent sample which is an indication for the external mesopores created upon desilication. Please insert Figure 2 here. The extraction of Si from BEA structure together with the introduction of Ni and Co was confirmed by the EDX results as tabulated in Table 1. The Si/Al ratio decreased from 24 in the parent sample to 16 after the desilication for Dsi-BEA sample. However, after the treatment with 0.2 M of Ni(NO3)2 (0.2Ni-Dsi-BEA), the Si/Al ratio was a bit decreased to 15 which can be explained by the slight desilication caused by alkaline pH of the treatment solution. This treatment also allowed to incorporate Ni to the BEA zeolite with a molar ratio of 170 Si/Ni as observed by EDX results. The treatment with higher concentration of Ni(NO3)2 (0.6 M) allowed more Ni to be attached with molar ratio of 86 Si/Ni in the case of 0.6Ni-Dsi-BEA. In contrast, the treatment of desilicated sample with 0.2 M of Co(NO3)2 yielded to more incorporation of Co to BEA framework (Si/Co = 60) as compared with the same condition for the case of Ni(NO3)2. The sample with sequential Ni(NO3)2 and Co(NO3)2 treatments (0.2Ni-0.2Co-Dsi-BEA) showed a less Ni content as compared to single step 6 ACS Paragon Plus Environment

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Ni(NO3)2 treatment along with successful incorporation of Co to BEA structure with Si/Co ratio of 213. The incorporation of Ni and Co metals to BEA zeolite framework was investigated by FTIR presented by the spectra in Figure 3. The Ni treated samples showed a peak at 614 cm-1 which was not observed in the Dsi-BEA sample. This peak is assigned to the stretching mode of NiO bond

19, 20

. Additional peak was also observed at 674 cm-1 , which can be assigned to the

tetrahedral symmetry of Ni

21, 22

. The intensity of these two peaks (614 and 674 cm-1)

confirms that 0.6Ni-Dsi-BEA sample has a higher Ni content, which is good agreement with EDX result as well as higher tetrahedral Ni appeared in this sample as compared with 0.2NiDsi-BEA. This finding confirms the successful incorporation of Ni inside the structure of BEA zeolite in tetrahedral structure in the form of Si-O-Ni-O-Si instead of external attachment by applying ion exchange only without desilication. Please insert Figure 3 here. More investigation to confirm the position of Ni was performed by UV-Vis analysis as presented in Figure 4. The results show the appearance of broad peak in Ni treated samples which was not existed in the parent BEA sample. The peak is located at 401 nm wavelength which is attributed to Ni-O tetrahedral coordination according to the literature

23, 24

.

Moreover, the order of peaks intensities is in good agreement with FTIR results which showed that 0.6Ni-Dsi-BEA had more tetrahedral incorporated Ni ions than 0.2Ni-Dsi-BEA sample. Please insert Figure 4 here. Table 1. Chemical composition and crystallinity of metal treated BEA samples. Sample

Si/Al*

Si/Ni*

Si/Co*

Si (%)*

Relative 7

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crystallinity (%)

BEA Parent

24

-

-

35.3

100

Dsi-BEA

16

-

-

32.2

32

0.2Ni-Dsi-BEA

15

170

-

30.1

36

0.2Co-Dsi-BEA

14

-

60

32.0

40

0.6Ni-Dsi-BEA

15

86

-

28.5

28

15

158

213

27.7

25

0.2Ni-0.2Co-DsiBEA *

Metals composition was measured by EDX.

The textural properties were calculated based on nitrogen adsorption-desorption isotherms as shown in Figure 5. The parent BEA has the highest microporosity surface area and lowest mesoporosity surface area as compared with all other desilicated sample which is reasonable because the extraction of Si from the structure created vacant (mesoporosity) and decreased the microporosity which is in good agreement with what reported before on BEA desilication by Groen et al

25

. Introducing the Ni treatment on the desilicated sample reduced the

micropore volume as well as microporosity surface area but on the other hand, the mesoporosity surface area increased due to the little desilication observed which consistent with EDX results. Please insert Figure 5 here. The pore size distribution results in Figure 6 show that the mesopore size was around 2.5 nm in all BEA samples. However, the distribution of mesopore around 2.5 nm became more pronounced after the desilication which confirms the successful creating of mesopores. Moreover, by Ni and Co treatments, the population of mesopores was little increased which supporting the fact that there was mild desilication happened. The evidences for this 8 ACS Paragon Plus Environment

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desilication were explored by the Si content of each sample (results in Table 1) which show a decrease after the sample subjected to Ni and Co treatments Please insert Figure 6 here. Table 2. Textural properties of metals incorporated BEA zeolite. Sample

Smicro (m2/g)

Smeso(m2/g)

Vmicro(cm3/g)

Vmeso(cm3/g)

BEA Parent

430.4

74.2

0.214

0.054

Dsi-BEA

322.4

245.2

0.161

0.158

0.2Ni-Dsi-BEA

288.8

291.9

0.145

0.193

0.2Co-Dsi-BEA

296.2

318.3

0.141

0.254

0.6Ni-Dsi-BEA

299.2

282.8

0.150

0.182

0.2Ni-0.2Co-Dsi-BEA

283.3

307.0

0.142

0.202

3.2 Acidity distribution of metal incorporated BEA zeolite The acidity of parent and metal modified BEA zeolite was measured by NH3-TPD as shown in Figure 7 and Table 3. These results show that the parent and desilicated BEA samples have only moderate acidity which is located at ~ 200 oC. The amount of moderate acidity was not significantly changed after desilication as shown in Table 3. On the other hand, the addition of Ni significantly increased the amount of moderate acidity from 0.255 mmol/g in the case of Dsi-BEA sample to 0.341 and 0.423 mmol/g in the case of 0.2Ni-Dsi-BEA and 0.6Ni-DsiBEA samples, respectively. Please insert Figure 7 here. Interestingly, the incorporation of Ni contributed to add strong acidity which was observed at 615 oC. The sharp peak in Ni treated samples as shown in Figure 7 is an indication for the small amount of strong acidity appeared as compared with the large broad peak of moderate 9 ACS Paragon Plus Environment

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acidity. The addition of Co to Ni significantly increased both moderate and strong acidity in large way. This increase in the total acidity can be attributed to the tendency of Co to form tetrahedral CoO4-2 arrangement especially in the case mesoporous structure, which offered more acidity in the zeolite 26. On the other hand, pyridine-FTIR analysis was also performed to evaluate the amount of Brønsted and Lewis acid sites presented in the modified samples as shown in Figure 8 and Table 3. The results are shown that the amount of Brønsted acidity of Dsi-BEA was considerably decreased while the Lewis acidity was remarkable increased as compared with BEA parent. The change especially the increase in Lewis acidity can be explained as the desilication created some extra-framework Al species which are known as strong Lewis acid site 27. Please insert Figure 8 here. The introduction of Ni after the desilication allowed for significant increase in the Brønsted and Lewis acidity due to deposition of Ni in the vacant sites upon desilication as confirmed by BET results as well as in the on the external surface. The Brønsted acidity was increased from 0.077 mmol/g in Dsi-BEA sample to 0.263 and 0.482 mmol/g in 0.2Ni-Dsi-BEA and 0.6Ni-Dsi-BEA, respectively. This observation confirms that the amount of tetrahedral Ni in 0.6Ni-Dsi-BEA sample is much higher as compared with 0.2Ni-Dsi-BEA. The incorporation of Co gave considerable increase in both Brønsted and Lewis acidity. The increase of Lewis site was much higher in the case of Co as compared to Ni because the amount of incorporated Co was much higher than Ni as confirmed by EDX results. The introduction of both Ni and Co together to the desilicated BEA was slightly different. Since not the whole attached Co get a chance to internally incorporate to the BEA structure


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because most of the vacant sites were already equipped by Ni ions. Therefore, some of Co was just attached on the external surface of BEA zeolite which was responsible to the increase of Brønsted acidity of this sample from 0.263 mmol/g in 0.2Ni-Dsi-BEA to 0.675 mmol/g in 0.2Co-0.2Ni-Dsi-BEA sample. However, for the Lewis acidity, there was a decrease from 0.473 to 0.390 mmol/g which can be explained by the less amount of Ni attached in this sample as compared to 0.2Ni-Dsi-BEA sample. The increase of Bronsted acidity after Co introduced to 0.2Ni-Dsi-BEA samples is an indication that some of Co is externally attached to BEA framework. In literature, many works proof that when transition metals (like Zr, Co, Ni, Sn, etc) are internally incorporated to zeolite framework they are offering Lewis acidity

28-30

, however, when these metals are externally connected they offer

Bronsted acidity. Therefore, the increase in Bronsted acidity in 0.2Ni-0.2Co-Dsi-BEA is a sign for some Co ions which were externally attached. Table 3. Acid sites distribution of metals modified BEA samples. Moderate

Strong

Brønsted

acidity

acidity

acid

Lewis acid Sample

B/L mmol/g

mmol/g

mmol/g

mmol/g

BEA Parent

0.254

-

0.278

0.013

21

Dsi-BEA

0.255

-

0.077

0.169

0.45

0.2Ni-Dsi-BEA

0.341

0.002

0.263

0.473

0.55

0.2Co-Dsi-BEA

0.651

0.011

0.303

0.729

0.41

0.6Ni-Dsi-BEA

0.423

0.006

0.482

0.627

0.77

0.2Ni-0.2Co-Dsi-BEA

0.934

0.180

0.675

0.390

1.73

3.3 Cracking of n-dodecane over modified BEA zeolite


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The catalytic performance of BEA and metal modified BEA zeolite was evaluated in conversion of n-dodecane to lower hydrocarbons (Figure 10) in a fixed bed reactor at 400 oC. The conversion of n-dodecane over BEA parent sample was 22% in the first hour, which is relatively low conversion as shown in Table 4. Table 4. Conversion of n-dodecane and coke produced over metal modified BEA. Conversion, % Time on stream Samples BEA Parent

1h

2h

3h

4h

5h

22

17

3

3

0

Dsi-BEA

11

3

0

0.2Ni-Dsi-BEA

44

29

24

26

15

0.2Co-Dsi-BEA

73

66

65

64

0.6Ni-Dsi-BEA

50

30

29

0.2Ni-0.2Co-Dsi-BEA

25

15

4

6h

Coke Amount rate of coke (wt.%/h) wt.% 12.9

6.5

9.2

9.2

15

12.9

2.15

64

63

5.1

0.85

23

18

15

14.6

2.4

0

0

0

8.8

4.4

Unfortunately, after 3 h on stream the conversion dropped to 3% with considerable amount of coke produced ~13% as measured by TGA. The large amount of coke is perhaps the main cause for catalyst deactivation in this sample. Based on the pyridine FTIR results in Section 3.2, BEA parent sample has a high amount of Brønsted acidity which inclines to donate hydrogen and promote cracking to light hydrocarbons (mainly paraffin’s which is known by primary reaction) and favored the formation of coke in high temperatures (400 oC as example)

31

. On the other hand, there was no enough Lewis acidity which can accept

hydrogen and endorse the formation of olefins as well as aromatics which is known by secondary reaction. This hypothesis is confirmed by the selectivity results summarized in Table 5. The results are shown that the dominant product is paraffins with selectivity of 72% of the product stream, which can be attributed to Brønsted acidity, while the selectivity of olefins was very low. 12 ACS Paragon Plus Environment

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Table 5. Products selectivity of parent of metal modified BEA samples after 1 h on stream. Conversion (%)

Selectivity (%) Naphthenes

Paraffins

i-Parraffins

Aromatics

Olefins

Samples BEA Parent

22

0

71.3

15.4

12.1

1.07

Dsi-BEA

11

0.4

60.6

24

8.8

5.56

0.2Ni-Dsi-BEA

44

0.20

45.86

21.75

13.25

18.94

0.2Co-Dsi-BEA

73

4.8

13.3

30.2

23.0

28.6

0.6Ni-Dsi-BEA

50

0.72

37.38

25.91

13.92

22.07

0.2Ni-0.2CoDsi-BEA

25

0

57.33

17.47

8.73

16.47

Please insert Figure 9 here. In the case of Dsi-BEA sample there was more Lewis than Brønsted acidity due to the desilication. The conversion of this sample was 11% which is lower than the parent because there is no enough Brønsted to perform continues and stable cracking of n-dodecane to more light paraffins (primary reaction). Again, the selectivity results support this assumption as the paraffins selectivity results decreased to 60% while the olefins selectivity increased. Similar to Brønsted, the availability of more Lewis acidity can promote the formation of coke due to extensive dehydrogenation especially at higher temperature which is the main reason for having 9% coke on this sample. It worth to mention here that the existence of mesoporosity enhances the formation of paraffins isomers since the selectivity increased from 15 to 24% as compared to BEA parent. Although there was considerable mesoporosity created on this sample via desilication, however the cracking performance of Dsi-BEA was lower than the parent sample which indicates that the there is no significant diffusion limitation while the


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role of acidity is more important here. The introduction of Ni in this sample offered better balance between the Brønsted and Lewis acidity as observed in FTIR results. Moreover, the Ni species on the outer surface played important dehydrogenation role

32

of n-dodecane

33

which offers more activity for this acid-metallic catalyst. The balance in acidity together with the external Ni species were positively effect on the balance between hydrogenation and dehydrogenation which was the main reason for higher conversion and longer stability of 0.2Ni-Dsi-BEA and 0.6Ni-Dsi-BEA samples

34

. The conversion of n-dodecane over 0.2Ni-

Dsi-BEA sample was 44% which is double the parent sample. The selectivity results of this sample show that the paraffins selectivity decreased from 72% in the parent to 46% in the same time the olefins was considerably increased from 1% in the parent to 19% here, while the selectivity to aromatics was maintained the same. This is very convincing results since there was enough amount of Brønsted to perform cracking as well as enough amount of Lewis to form olefins and aromatics. From mechanism point of view, the n-dodecane is getting hydrogenated and cracked to lower paraffins over Brønsted sites. Later, these lower paraffins imposed to dehydrogenation to form olefins over the Lewis sites. The importance of balance between hydrogenation and dehydrogenation is originated from the need of continuous substitution for the consumed proton over the Brønsted (hydrogenation process) sites which were being achieved by dehydrogenation over Lewis sites

35, 36

. Under these

circumstances, the chance of quick coking upon fast hydrogenation and dehydrogenation is lower, which gives this sample longer life time and better stability as 13% coke was formed in 7 h as compared with same amount but was formed in 5 h over the parent sample. At higher Ni content, the sample 0.6Ni-Dsi-BEA gave better conversion which was increased up to 50% as compared with 44% over 0.2Ni-Dsi-BEA. The selectivity results of this sample strongly support our hypothesis since the paraffins selectivity decreased from 46% in 0.2NiDsi-BEA to 37%, meanwhile the olefins selectivity increased from 19% to 22 and aromatics


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

Page 16 of 31

selectivity was preserved ~14%. Referring to pyridine FTIR results, the increase of Ni concentration from 0.2 to 0.6 M offered an overall increase in both Brønsted and Lewis. However, the ratio of B/L was 0.77, which is higher than the case of 0.2 M Ni concentration. This enhancement of Brønsted acidity offered mainly an increase in i-paraffins selectivity 22 to 26% and without any improvement in catalyst stability as compared with 0.2Ni-Dsi-BEA sample. The amount of coke produced over 0.6Ni-Dsi-BEA sample is 15%. This ratio is higher than the ratio observed on 0.2Ni-Dsi-BEA which can be directly attributed to the increase of Brønsted acidity. The incorporation of Co in the case of 0.2Co-Dsi-BEA sample strongly supports the hypothesis about the importance role of Lewis acidity in dehydrogenation reaction. Dodecane conversion reached to 73% in the first hour, which is the maximum conversion among other sample. Moreover, the selectivity’s of olefins and aromatics reached to 28.6 and 23.0% (the yield reached to 21 and 17% as presented in Tables S1 and S2) respectively, meanwhile the paraffin’s selectivity reached to the lowest amount 13.3%. The presence of enough Lewis active sites as confirmed by FTIR pyridine adsorption promoted the dehydrogenation of paraffin’s to olefins. Moreover, the created mesoporosity endorsed the aromatization of olefins to give more aromatics by implying more dehydrogenation. The stability performance of this sample (0.2Co-Dsi-BEA) was exceptional as compared to other metal treated samples. The conversion of dodecane just dropped 10% from the first hour to the sixth hour as presented in Table 4, which is in good agreement with the percentage of coke produced per hour. The addition of Co to 0.2Ni-Dsi-BEA sample was extremely harmful to its performance as the conversion decreased from 44 to 25%. The incorporation of nickel and cobalt to BEA zeolite was expected to give good performance since they were already gave good results previously when they used without zeolite support 16. The performance of 0.2Co-0.2Ni-Dsi15 ACS Paragon Plus Environment

Page 17 of 31 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

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BEA sample was almost similar to the parent BEA sample since both samples have higher Brønsted acidity than Lewis. However, the main differences were observed on this sample was the less paraffin’s selectivity and higher olefins selectivity as compared with parent BEA. This difference can be attributed to higher Lewis amount in 0.2Co-0.2Ni-Dsi-BEA sample as compared with parent BEA. Please insert Figure 10 here. Moreover, to understand the mechanism of deactivation; the products selectivity was measured with time-on-stream as shown in Table 6. In all metal modified samples (0.2NiDsi-BEA, 0.6Ni-Dsi-BEA and 0.2Ni-0.2Co-Dsi-BEA), there was a considerable decrease in aromatics selectivity with time-on-stream, which can be explained as coke begins to be formed inside the pore and the pore size became smaller which eliminate to diffuse out the aromatics. Since these aromatics were trapped inside the pores that allowed for more dehydrogenation to take place and increase the formation of coke and consequently quick deactivation was observed. In the meantime, there was an increase in paraffins selectivity which indicated a decrease in dehydrogenation capacity attributed to aromatics that occupied the Lewis sites.


Energy & Fuels 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 18 of 31

Table 6. Products selectivity over BEA modified samples at different times-on-streams.

Samples

TOS (h)

0.2NiDsi-BEA

0.2NiDsi-BEA

0.6NiDsi-BEA 0.2Ni0.2CoDsi-BEA

Conversion %

Naphthenes %

Paraffins %

iParraffins %

Aromatics %

Olefins %

1 2 4 6 1 2 3 6 1 2 3

43.9 28.6 24 14.8

0.2 0 0 0.45

45.86 42.1 41.85 54.82

21.75 24.69 21.51 20.26

13.25 6.38 8.42 1.08

18.94 26.82 28.29 23.38

72.6 66 64.8 62.4 49 29.6 28.6

5.1 10.5 7.1 5.6 0.72 0 0.37

14.0 13.5 19.6 18.9 37.38 50.31 58.45

31.8 30.6 27.8 31.8 25.91 20.85 17.69

18.9 12.2 11.4 9.5 13.92 8.59 3.92

30.1 33.2 34.1 34.2 22.07 20.25 19.57

1

25

0

57.33

17.47

8.73

16.47

2

14.8

0

62.27

17.56

3.16

17.01

Conclusions Successful internal incorporation of nickel and cobalt to BEA zeolite matrix structure was achieved by sequential desilication and wet treatment. Based on FTIR spectra and pyridine FTIR results, not all the nickel and cobalt ions get the chance to be internally connected and forming tetrahedral form. Therefore, there was an amount of Ni and Co just connected to the surface of BEA zeolite which caused a wide range of changing Brønsted and Lewis acidity. The addition of nickel and cobalt significantly enhanced the catalytic cracking of ndodecance by maintaining a suitable balance between Brønsted and Lewis sites which preserved the continuous dehydrogenation and hydrogenation process. The entrapping of aromatics inside the pores accelerated the formation of coke by extensively dehydrogenation of aromatics over the Lewis sites. The reduction of Lewis active sites disturbed the balance between Brønsted and Lewis sites and consequently deactivated the catalyst. 17 ACS Paragon Plus Environment

Page 19 of 31 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

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Acknowledgments The authors would like to thank the funding provided by Saudi Aramco for supporting this work through project contract number 6600011900 as part of the Oil Upgrading theme at King Fahd University of Petroleum and Minerals. References 1. Zhang, W.; Smirniotis, P. G., Effect of zeolite structure and acidity on the product selectivity and reaction mechanism forn-octane hydroisomerization and hydrocracking. Journal of catalysis 1999, 182, (2), 400-416. 2. Akhmedov, V. M.; Al-Khowaiter, S. H., Recent advances and future aspects in the selective isomerization of high n-Alkanes. Catalysis Reviews 2007, 49, (1), 33-139. 3. Chang, J.; Fujimoto, K.; Tsubaki, N., Effect of initiative additives on hydro-thermal cracking of heavy oils and model compound. Energy & fuels 2003, 17, (2), 457-461. 4. Arandes, J. M.; Torre, I.; Azkoiti, M. J.; Erena, J.; Bilbao, J., Effect of atmospheric residue incorporation in the fluidized catalytic cracking (FCC) feed on product stream yields and composition. Energy & Fuels 2008, 22, (4), 2149-2156. 5. Wang, B.; Han, C.; Zhang, Q.; Li, C.; Yang, C.; Shan, H., Studies on the preliminary cracking of heavy oils: the effect of matrix acidity and a proposal of a new reaction route. Energy & Fuels 2015, 29, (9), 5701-5713. 6. Corma, A.; Martinez, A.; Pergher, S.; Peratello, S.; Perego, C.; Bellusi, G., Hydrocrackinghydroisomerization of n-decane on amorphous silica-alumina with uniform pore diameter. Applied Catalysis A: General 1997, 152, (1), 107-125. 7. Lugstein, A.; Jentys, A.; Vinek, H., Hydroisomerization and cracking of n-octane and C8 isomers on Ni-containing zeolites. Applied Catalysis A: General 1999, 176, (1), 119-128. 8. Lugstein, A.; Jentys, A.; Vinek, H., Hydroconversion of n-heptane over bifunctional HZSM5 zeolites influence of the metal concentration and distribution on the activity and selectivity. Applied Catalysis A: General 1998, 166, (1), 29-38. 9. Fang, K.; Wei, W.; Ren, J.; Sun, Y., n-Dodecane hydroconversion over Ni/AlMCM-41 catalysts. Catalysis letters 2004, 93, (3-4), 235-242. 10. Martens, J. A.; Tielen, M.; Jacobs, P. A., Attempts to rationalize the distribution of hydrocracked products. III. Mechanistic aspects of isomerization and hydrocracking of branched alkanes on ideal bifunctional large-pore zeolite catalysts. Catalysis Today 1987, 1, (4), 435-453. 11. Li, D.; Li, F.; Ren, J.; Sun, Y., Rare earth-modified bifunctional Ni/HY catalysts. Applied Catalysis A: General 2003, 241, (1), 15-24. 12. Fang, K.; Ren, J.; Sun, Y., Effect of nickel precursors on the performance of Ni/AlMCM-41 catalysts for n-dodecane hydroconversion. Journal of Molecular Catalysis A: Chemical 2005, 229, (1), 51-58. 13. Kinger, G.; Lugstein, A.; Swagera, R.; Ebel, M.; Jentys, A.; Vinek, H., Comparison of impregnation, liquid-and solid-state ion exchange procedures for the incorporation of nickel in HMFI, HMOR and HBEA: Activity and selectivity in n-nonane hydroconversion. Microporous and mesoporous materials 2000, 39, (1), 307-317. 14. Romero, M. D.; Calles, J. A.; Rodríguez, A., Influence of the preparation method and metal precursor compound on the bifunctional Ni/HZSM-5 catalysts. Industrial & engineering chemistry research 1997, 36, (9), 3533-3540.


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15. Canizares, P.; De Lucas, A.; Dorado, F.; Duran, A.; Asencio, I., Characterization of Ni and Pd supported on H-mordenite catalysts: Influence of the metal loading method. Applied Catalysis A: General 1998, 169, (1), 137-150. 16. Lugstein, A.; Jentys, A.; Vinek, H., Hydroconversion of n-heptane over CoNi containing HZSM5. Applied Catalysis A: General 1997, 152, (1), 93-105. 17. Gervasini, A., Characterization of the textural properties of metal loaded ZSM-5 zeolites. Applied Catalysis A: General 1999, 180, (1–2), 71-82. 18. Al-Shammari, A. A.; Ali, S. A.; Al-Yassir, N.; Aitani, A. M.; Ogunronbi, K. E.; Al-Majnouni, K. A.; Al-Khattaf, S. S., Catalytic cracking of heavy naphtha-range hydrocarbons over different zeolites structures. Fuel Processing Technology 2014, 122, 12-22. 19. Ermakova, M. A.; Ermakov, D. Y., High-loaded nickel–silica catalysts for hydrogenation, prepared by sol–gel: Route: structure and catalytic behavior. Applied Catalysis A: General 2003, 245, (2), 277-288. 20. de Paiva, J. A. C.; Graça, M. P. F.; Monteiro, J.; Macedo, M. A.; Valente, M. A., Spectroscopy studies of NiFe2O4 nanosized powders obtained using coconut water. Journal of Alloys and Compounds 2009, 485, (1–2), 637-641. 21. Clause, O.; Kermarec, M.; Bonneviot, L.; Villain, F.; Che, M., Nickel (II) ion-support interactions as a function of preparation method of silica-supported nickel materials. Journal of the American Chemical Society 1992, 114, (12), 4709-4717. 22. Guang-She, L.; Li-Ping, L.; Smith Jr, R. L.; Inomata, H., Characterization of the dispersion process for NiFe2O4 nanocrystals in a silica matrix with infrared spectroscopy and electron paramagnetic resonance. Journal of Molecular Structure 2001, 560, (1–3), 87-93. 23. Śrębowata, A.; Baran, R.; Łomot, D.; Lisovytskiy, D.; Onfroy, T.; Dzwigaj, S., Remarkable effect of postsynthesis preparation procedures on catalytic properties of Ni-loaded BEA zeolites in hydrodechlorination of 1,2-dichloroethane. Applied Catalysis B: Environmental 2014, 147, 208-220. 24. Baran, R.; Kamińska, I. I.; Śrębowata, A.; Dzwigaj, S., Selective hydrodechlorination of 1,2dichloroethane on NiSiBEA zeolite catalyst: Influence of the preparation procedure on a high dispersion of Ni centers. Microporous and Mesoporous Materials 2013, 169, 120-127. 25. Groen, J. C.; Peffer, L. A. A.; Moulijn, J. A.; Pérez, R.; amp; x; rez, J., On the introduction of intracrystalline mesoporosity in zeolites upon desilication in alkaline medium. Microporous and Mesoporous Materials 2004, 69, (1–2), 29-34. 26. Rutkowska, M.; Piwowarska, Z.; Micek, E.; Chmielarz, L., Hierarchical Fe-, Cu- and Co-Beta zeolites obtained by mesotemplate-free method. Part I: Synthesis and catalytic activity in N2O decomposition. Microporous and Mesoporous Materials 2015, 209, 54-65. 27. Tarach, K.; Góra-Marek, K.; Tekla, J.; Brylewska, K.; Datka, J.; Mlekodaj, K.; Makowski, W.; Igualada López, M. C.; Martínez Triguero, J.; Rey, F., Catalytic cracking performance of alkalinetreated zeolite Beta in the terms of acid sites properties and their accessibility. Journal of Catalysis 2014, 312, 46-57. 28. Tang, B.; Dai, W.; Sun, X.; Wu, G.; Guan, N.; Hunger, M.; Li, L., Mesoporous Zr-Beta zeolites prepared by a post-synthetic strategy as a robust Lewis acid catalyst for the ring-opening aminolysis of epoxides. Green Chemistry 2015, 17, (3), 1744-1755. 29. Moliner, M., State of the art of Lewis acid-containing zeolites: lessons from fine chemistry to new biomass transformation processes. Dalton Transactions 2014, 43, (11), 4197-4208. 30. Dapsens, P. Y.; Mondelli, C.; Pérez-Ramírez, J., Design of Lewis-acid centres in zeolitic matrices for the conversion of renewables. Chemical Society Reviews 2015, 44, (20), 7025-7043. 31. de la Puente, G.; Souza-Aguiar, E. F.; Zotin, F. M. a. Z.; Camorim, V. L. D.; Sedran, U., Influence of different rare earth ions on hydrogen transfer over Y zeolite. Applied Catalysis A: General 2000, 197, (1), 41-46. 32. Rane, N.; Kersbulck, M.; van Santen, R. A.; Hensen, E. J. M., Cracking of n-heptane over Brønsted acid sites and Lewis acid Ga sites in ZSM-5 zeolite. Microporous and Mesoporous Materials 2008, 110, (2–3), 279-291. 19 ACS Paragon Plus Environment

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33. Weitkamp, J., Catalytic hydrocracking—mechanisms and versatility of the process. ChemCatChem 2012, 4, (3), 292-306. 34. Escola, J. M.; Serrano, D. P.; Aguado, J.; Briones, L., Hydroreforming of the LDPE Thermal Cracking Oil over Hierarchical Ni/Beta Catalysts with Different Ni Particle Size Distributions. Industrial & Engineering Chemistry Research 2015, 54, (26), 6660-6668. 35. Thybaut, J. W.; Laxmi Narasimhan, C.; Denayer, J. F.; Baron, G. V.; Jacobs, P. A.; Martens, J.; Marin, G. B., Acid-metal balance of a hydrocracking catalyst: Ideal versus nonideal behavior. Industrial & engineering chemistry research 2005, 44, (14), 5159-5169. 36. Degnan, T.; Kennedy, C., Impact of catalyst acid/metal balance in hydroisomerization of normal paraffins. AIChE journal 1993, 39, (4), 607-614.


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0.2Ni-0.2Co--Dsi-BEA

0.6Ni-Dsi-BEA 0.2Co-Dsi-BEA Intensity [a.u]

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

Page 22 of 31

0.2Ni-Dsi-BEA Dsi-BEA

BEA Parent

5

15

25

35

45

2θ Figure 1. XRD patterns of parent and Ni/Co modified BEA zeolites. ACS Paragon Plus Environment

Page 23 of 31

Energy & Fuels

BEA Parent 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

Dsi-BEA

500 nm

500 nm

0.2Co-Dsi-BEA

0.6Ni-Dsi-BEA

500 nm

0.2Ni-Dsi-BEA

500 nm

0.2Ni-0.2Co-Dsi-BEA

500 nm

ACS Paragon Plus Environment Figure 2. SEM images of parent and metal modified BEA zeolite.

500 nm

Energy & Fuels

Transmittance [%]

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

Page 24 of 31

0.2Ni-Dsi-BEA 0.6Ni-Dsi-BEA 614

0.2Ni-0.2Co--Dsi-BEA Dsi-BEA

Wavenumber [cm-1] Paragon Plus Environment Figure 3. FTIR spectra of ACS desilicated and metal modified BEA zeolite.

Page 25 of 31

0.2

BEA parent

0.2Ni-Dsi-BEA 0.16

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

Energy & Fuels

0.6Ni-Dsi-BEA

0.12

0.08

0.04

350

365

380

395

410

425

440

455

470

485

500

Wavelength [nm] Figure 4. UV-vis absorption spectra of parent and Ni modified BEA zeolite. ACS Paragon Plus Environment

BEA Parent

240

Pore volume [cm3/g]

1220 2 3200 4 5180 6 7 8160 9 10 140 11 12 120 13 14 100 15 16 17 18 19300 20 21 250 22 23 24200 25 26150 27 28 100 29 30 31 50 32 33 0 34 35 36 37 38 39 40 41

0

0.5

0.5

220

200

200

180

180

160

160

140

140

120

120

100

100 0

0.5

1

0.6Ni-Dsi-BEA

240

1

Page 26 of 31 0.2Ni-Dsi-BEA

240

220

1

0.2Co-Dsi-BEA

0

Dsi-BEA

Energy & Fuels

240

220

200

200

180

180

160

160

140

140

120

120

100

100 0.5

1

0.5

1

0.2Ni-0.2Co-Dsi-BEA

240

220

0

0

0

0.5

1

P/Po [-]

ACS Paragon Plus Environment Figure 5. Nitrogen adsorption-desorption isotherms of parent and metal modified BEA samples.

Page 27 of 31

0.025

Incremental Pore Volume (cm³/g)

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

Energy & Fuels

BEA Parent

0.02

Dsi-BEA 0.015

0.2Ni-Dsi-BEA 0.2Co-Dsi-BEA

0.01

0.6Ni-Dsi-BEA 0.005

0

0

20

40

60

80

100

Pore width [Å] Figure 6. BJH pore size distribution of parent and modified BEA zeolite. ACS Paragon Plus Environment

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0.08

BEA Parent Dsi-BEA

0.07

0.2Ni-Dsi-BEA

0.06

TCD signal [a.u.]

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

Page 28 of 31

0.2Co-Dsi-BEA 0.6Ni-Dsi-BEA

0.05

0.2Ni-0.2Co-Dsi-BEA

0.04 0.03 0.02 0.01 0 100

200

300

400

500

600

700

Temperature [oC] Figure 7. NH3-TPD profiles for parent and metal modified BEA zeolite catalysts. ACS Paragon Plus Environment

Page 29 of 31

0.2Ni-0.2Co-Dsi-BEA

0.6Ni-Dsi-BEA

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

Energy & Fuels

0.2Co-Dsi-BEA

0.2Ni-Dsi-BEA

Dsi-BEA

BEA Parent Wavenumber [cm-1] ACS Paragon Plus Environment Figure 8. Pyridine-FTIR spectra of parent and metal modified BEA zeolite samples.

Energy & Fuels

100

Weight loss [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 100 18 19 20 2195 22 23 2490 25 26 27 85 28 29 30 3180 32 33 3475 35 36 37 38 39 40 41

Page 30 of 31

100

BEA Parent

Dsi-BEA

95

95

90

90 85 85

80

80

75 0

100

200

300

0.2Ni-Dsi-BEA

400

500

600

0

100

0.6Ni-Dsi-BEA

100 95 90 85 80 75 70

0 100 200 300 400 500 600 700

700

0

200

300

100 95 90 85 80 75 70 65 60 55 100 200 300 400 500 600 700 0

400

500

600

700

0.2Ni-0.2Co-Dsi-BEA

100 200 300 400 500 600 700

Temperature [oC]

Paragon Plus Environment Figure 9. TGA profiles ofACS parent and metal modified BEA zeolite samples.

Page 31 of 31

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Conversion and Selectivity [%]

100% 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

90% 80% 70%

Olefins

60%

Aromatics

50%

i-Parraffins

40%

Paraffins

30% 20%

Naphthenes Conversion

10% 0%

Figure 10. Dodecane conversion and products selectivity over Ni-Co modified BEA zeolite at -1 and TOS of 1 h. 400 °C ,ACS LHSV of 4 h Paragon Plus Environment

Co Bimetallic

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