Catalytic Study and Kinetic Modeling of the n-Heptane Isomerization

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Catalytic study and kinetic modelling of the n-heptane isomerization over Pt/Al-HMS/HZSM-5 hybrid catalysts Nastaran Parsafard, Mohammad Hassan Peyrovi, and Mohammad Jarayedi Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 15 May 2017 Downloaded from http://pubs.acs.org on May 23, 2017

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Catalytic study and kinetic modelling of the n-heptane isomerization over Pt/Al-HMS/HZSM-5 hybrid catalysts N. Parsafard a,*, M. H. Peyrovi b,* and M. Jarayedi b a

b

Kosar University of Bojnord, Department of Applied Chemistry, North Khorasan, Iran

Faculty of Chemistry and Petroleum Sciences, Department of Petroleum Chemistry and Catalysis, University of Shahid Beheshti, Tehran, 1983963113, Iran

* Corresponding authors E-mail addresses: [email protected] (N. Parsafard) and [email protected] (M. H. Peyrovi) Tel.: +98 21 29902892; Fax: +98 21 22431663. Address: Faculty of Chemistry and Petroleum Sciences, University of Shahid Beheshti, Tehran, Iran

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Abstract Pt/Al-HMS/HZSM-5 catalysts with various amounts of Si/Al ratios were evaluated for n-heptane isomerization reaction at 200–350 oC. To study the catalyst characterizations, XRD, XRF, FTIR, UV–vis DRS, NH3-TPD, Py-IR, H2 chemisorption, nitrogen adsorption-desorption and TGA techniques were done. Kinetic of n-heptane isomerization was investigated under various hydrogen and n-heptane pressures. For more study, two kinetic models have also been selected and tested to describe the kinetics for this reaction. Both used models, the power law and Langmuir-Hinshelwood, provided a good fit towards the experimental data and allowed to determine the kinetic parameters. According to these studies, Pt/Al(45)-HMS/HZSM-5 catalyst has better properties than other prepared catalysts for the isomerization reaction.

Keywords: n-Heptane isomerization; Kinetics; Power law model; Langmuir-Hinshelwood model.


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1. Introduction The contents of aromatic compounds are restricted in gasolines for environment protection. Unfortunately, by reducing the amount of aromatics, octane number of the gasoline as an important parameter is also reduced [1-5]. To avoid this problem, catalytic isomerization is employed as one of the most effective and economic methods. It usually has better performance at lower temperatures, due to the use of catalysts. The choice of these materials for the isomerization reaction depends on the feed, the reaction conditions such as temperature, pressure and many other parameters that can affect the catalyst activity, selectivity and its stability against deactivation. In recent years, many researchers have been reported for the catalytic isomerization reaction [6-8]. In a previous study, a new kind of micro/meso porous catalysts via the use of HMS and HZSM-5 precursors has been developed by our group [9]. Dual micro- and meso porous properties of these materials make them to show good activity and selectivity for selective isomerization reaction. In order to improve the quality and promote the acidic properties of these materials for more selective isomerization reaction, the modification treatment with Al was studied in the present work. We prepared a series of novel Pt/AlHMS/HZSM-5 hybrid catalysts with 40 wt% of HZSM-5 and various amounts of aluminum. The physicochemical properties of platinated catalysts were characterized by XRF, XRD, FTIR, BET, NH3-TPD, IR-Py, UV-vis DRS and TGA techniques and were studied in catalytic activity, selectivity and stability performances. Following of these evaluations, a detailed kinetic study of this reaction was undertaken in order to understand the reaction mechanism. The power law is the first mathematical model that was used in this work. Another model is the Langmuir model. This model consists of two mechanisms, Langmuir–Hinshelwood mechanism and Eley–Rideal


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mechanism [10]. These two kinetic models (power law and Langmuir) were evaluated and compared together. 2. Experimental 2.1. Catalyst preparation The Al(x)-HMS/HZSM-5 catalysts with 40 wt% of HZSM-5 and various molar ratios of Si/Al were prepared by adding 1 g of HZSM-5 (Zeolyst international, Si/Al=14) to Al(x)-HMS colloidal precursors. In this preparation method, four solutions were prepared as follows. Solution A: 0.18 g aluminum isopropoxide (as an aluminum source) was added to 5.8 g isopropyl alcohol by continuous stirring. Solution B: 1.1 g dodecyl amine (as a surfactant resource) and 5 mL isopropyl alcohol were added to 17 mL distilled water. Solution C: 3.5 g tetraethyl ortho silicate was added to solution A and 6.7 mL distilled water by continuous stirring. Solution D: 10 mL HNO3 was added to 1 g HZSM-5. Each of these solutions was stirred separately for a certain time period. Then, Solutions B, C, D and 6.7 mL water were mixed together and stirred for 4 h at room temperature. The resulting solution left to settle during 20 h for crystallization. The filtered solid product was dried at room temperature overnight and calcined at 600 oC for 6 h in the flowing air. This method was used for preparation of the catalyst with Si/Al=20. Other catalysts were synthesized similarly with different material amounts. Moreover, Pt (0.6 wt%) catalysts were prepared by impregnating the support with appropriate concentration of H2PtCl6 using Al(x)-HMS/HZSM-5 as a support. After evaporation of the solvent and drying, the Pt catalysts were calcined in air at 300 oC for 4 h. It


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should be noted that all materials were purchased from Merck company and were used as received without any further purification. The Pt doped Al(x)-HMS/HZSM-5 catalysts were labeled Pt/Al(x)-HZ, where x represents the nominal Si/Al ratios (that are equal to 5, 20, 35 & 45). 2.2. Characterization Si/Al ratios were measured by X-ray fluorescence (XRF) with an XRF-8410 Rh apparatus and a voltage of 60 kV. Powder X-ray diffraction (XRD) patterns were recorded on an X-PERT diffractometer using Ni-filtered Cu kα radiation at 45 kV and 50 mA with a 0.06° 2θ-step and 1 s per step. Fourier transform infrared (FTIR) spectroscopy was used for identification of chemical bonds in the prepared hybrid catalysts. This analysis was performed on a BOMEM FTIR spectrophotometer model Arid-Zone TM, MB series in a wave number range of 400–4000 cm–1 with appropriate amounts of the catalysts and KBr for preparing the transparent tablets. The FTIR of adsorbed pyridine (Py-IR) was used to evaluate the type of acidic sites by a Nicolet 170 SX spectrophotometer. The self-supported wafer was introduced into a glass Pyrex IR cell having greaseless stopcocks and CaF2 windows. The samples were outgassed at 450 oC for 4 h and cooled down to 200 oC prior to contact with pyridine. After evacuation of physically adsorbed pyridine, IR spectra were recorded at 200 oC for 30 min. To investigate the amounts of acidic sites, temperature programmed desorption of NH3 (NH3-TPD) was carried out on a TPD/TPR analyzer (2900 Micromeritics) instrument. 0.2 g of each sample was pretreated at a heating rate of 10 oC min-1 to 600 oC for 1 h under helium flow (40 mL min-1). After cooling to 100 oC, 10 vol.% of NH3–He flow was introduced until the acid sites of the catalysts were


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saturated with NH3. Afterwards, desorption profiles were recorded from 25 oC to 800 oC with a heating rate of 10 oC min-1. Nitrogen adsorption isotherms at -196 oC were recorded on an ASAP-2010 micromeritics (USA) apparatus. For this analysis, calcined catalysts were outgassed for 3 h at 300 oC. The volume of the adsorbed monolayer (Vp) and the specific surface area (SBET) were evaluated by the Brunauer–Emmett–Teller (BET) equation and by assuming a N2 molecule to cover 0.162 nm2, respectively. The Barret–Joyner–Halenda method (BJH) was also used for calculating the average pore diameter (dp). Uv-visible diffuse reflectance spectra (Uv-vis DRS) were recorded on a Shimadzu UV-2100 UV-vis spectrophotometer using BaSO4 as a reference. The powder catalysts were evaluated in 200–800 nm at room temperature. For evaluating the metals' dispersion, H2 pulse chemisorption experiments were conducted on a TPD/TPR analyzer (2900 Micromeritics) equipped with a TCD detector. Briefly, 0.2 g of each catalyst was subjected reduction for 1 h at 450 oC followed by purging in argon flow at 500 oC for 1 h. After cooling to room temperature, the pure hydrogen (14 mL/min) was injected in pulses until no further adsorbed hydrogen on the catalysts. Thermogravimetric and differential thermal analysis (TG/DTA) were performed on a STA503 M instrument for evaluating the coke depositions and stability of prepared catalysts. 20 mg of samples were measured under air atmosphere with 5 vol.% O2/N2 gas mixture (60 mL/min) and heated from 25 oC to 800 oC with 10 oC min-1. 2.3. Catalytic tests and calculation methods 2.3.1. Activity evaluation 1 g of each catalyst was loaded in a continuous fixed-bed Pyrex reactor, operated under isothermal conditions and connected to an on-line gas chromatograph (Agilent Technologies


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7890A equipped with a flame ionization detector) by a controller. The test was carried out at the atmospheric pressure, the reaction temperature ranges from 200 to 350 oC with a heating rate of 50 oC h-1, n-heptane flow rate of 2 mL h−1 and hydrogen flow rate of 40 mL min−1. Previous to starting the run, the catalyst was kept in the hydrogen stream at 450 oC for 2 h in order to prereduce the metallic function. Catalytic activity is expressed in terms of conversion. Conv. (%) = percentage of n − C transformed into products

(1)

The following equation was used for calculating the selectivity. (%) =

!"#$%!&'( ) % " *'! ") +!%(,* % "- "&%, %$ *%.'! '(

× 100

(2)

2.3.2. Stability study The major problem in the catalytic processes is the coke deposition on the catalysts' surfaces and their deactivation. So the stability of prepared catalysts for further study about the influence of aluminum amounts in the catalytic performances was also examined under the operating conditions similar to activity performance at a selected constant temperature (300 oC) for 72 h on stream. The products were also identified with an on-line gas chromatograph. 2.3.3. Mass transfer limitations Mass transfer limitations were checked by performing several experiments at reaction temperature in the range of 200 to 350 oC. To determine the influence of the amount of catalysts in the external diffusion limitations, the contact times were changed with varying the amounts of catalysts ranging from 0.5 to 1.5 g. The height of the catalytic bed was maintained constant by the addition of inert quartz. The quartz is used only to dilute the catalysts and according to our investigations has no effect on the conversion. For estimating the internal diffusion effect, the mean particle size of the catalysts was controlled using sieves in the range of 50–350 µm. 7 ACS Paragon Plus Environment

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2.4. Kinetic study Two kinetic models were used to investigate the kinetic of reaction on our prepared catalysts. The partial orders of reaction and other kinetic parameters were calculated based on the simplest model (power law model). 2.4.1. Power law model (PL) Power law model was chosen as a simplest kinetic equation. The kinetic expression as a function of the n-heptane and hydrogen pressures is as follows: 2 3

&%4.#

: < 5 = 6789 7;

where 2: reaction rate (

(3) ); 6: rate constant; 789 : partial pressure of hydrogen (7=); 7; : partial

&%4.#

pressure of n-heptane (7=), n and m: rate exponents of reaction [6,8]. The rate constant (6) follows Arrhenius equation: 6 = > ?

@ACD ABB EF

(4)

"* where G"++ : apparent activation energy (&%-); R: gas constant (&%- J); >: pre-exponential factor, HI

HI

and K: reaction temperature (L). The reaction rates were measured under low conversion conditions ( (mol/gs)

∆x; (kJ/mol) >; (atmu )

Langmuir-Hinshelwood model 90.7

98.2

83.1

81.9

8.9×10-7

2.8×10-6

1.1×10-6

1.6×10-5

-4.49

-4.48

-4.50

-4.51

73.32

73.51

73.13

72.95

-54.85

-43.75

-43.93

9.82×107

6.13×109

5.83×109

∆x89 (kJ/mol) -53.31 >89 (atmu )

9.44×108


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Figure Captions Fig. 1. XRD patterns of Pt/Al(x)-HMS/HZSM-5 catalysts that are named as Pt/Al(x)-HZ (x = Si/Al ratio), in (a) low angle (1-10o) and (b) high angle (10-80o) regions. Fig. 2. FT-IR spectra for powder catalysts with different degrees of promotion by aluminum. Fig. 3. UV-vis DRS of the prepared catalysts. Fig. 4. N2 adsorption–desorption isotherms of Pt supported catalysts at -196 oC. Fig. 5. NH3-TPD profiles of Pt-supported Al(x)-HZ catalysts. Fig. 6. Py-IR spectra of different catalysts. Fig. 7. The stability of Pt/Al(x)-HZ catalysts in 72 h on stream (TOS) at 300 oC for the isomerization reaction; (a) conversion and selectivity to (b) MOB isomers, (c) MUB isomers and (d) i-C7 (MOB+MUB) isomers. Fig. 8. (a) Arrhenius plots calculated from the experimental data for different prepared catalysts, (b) double-log plots of the isomerization reaction rates versus the partial pressures of nC7 and H2, (c) estimated data by power law model and (d) estimated data LangmuirHinshelwood model.


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Pt/Al(5)-HZ Pt/Al(20)-HZ Pt/Al(35)-HZ Pt/Al(45)-HZ

0

2

4

6

8

(b) Absolute intensity

(a) Absolute intensity

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10

10

30

50

70

2 Theta (degree) 2 Theta (degree) Fig. 1. XRD patterns of Pt/Al(x)-HMS/HZSM-5 catalysts that are named as Pt/Al(x)-HZ (x = Si/Al ratio), in (a) low angle (1-10o) and (b) high angle (10-80o) regions.


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Pt/Al(5)-HZ Pt/Al(35)-HZ

Pt/Al(20)-HZ Pt/Al(45)-HZ

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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3600

2800

2000

Wavenumber

1200

400

(cm-1)

Fig. 2. FT-IR spectra for powder catalysts with different degrees of promotion by aluminum.


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Absorbance (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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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200

400

600

800

Wavelength (nm)

Fig. 3. UV-vis DRS of the prepared catalysts.


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Volume adsorbed (cm3/g STP)

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0.1


0.3

0.5

0.7

Relative pressure (P/P0)

0.9

Fig. 4. N2 adsorption–desorption isotherms of Pt supported catalysts at -196 oC.


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Ammonia desorption (a. u.)

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Pt/Al(5)-HZ

Pt/Al(20)-HZ

Pt/Al(35)-HZ

Pt/Al(45)-HZ

0

200

400

600

Temperature (oC) Fig. 5. NH3-TPD profiles of Pt-supported Al(x)-HZ catalysts.


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Pt/Al(5)-HZ

Absorbance (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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Pt/Al(20)-HZ

Pt/Al(35)-HZ

Pt/Al(45)-HZ

1700

1600

1500

Wavenumber

1400

(cm-1)

Fig. 6. Py-IR spectra of different catalysts.


(a)

Conversion (%)

75

60

Pt/HZ Pt/Al(5)-HZ Pt/Al(20)-HZ Pt/Al(35)-HZ Pt/Al(45)-HZ

45

(b)

45

30

15

30

0 0

20

40

60

0

20

TOS (h) 18

40

60

TOS (h)

(c)

(d) 60

i-C7 selectivity (%)

MUB selectivity (%)

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MOB selectivity (%)

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12

6

40

20

0

0 0

20

40

0

60

20

40

60

TOS (h)

TOS (h)

Fig. 7. The stability of Pt/Al(x)-HZ catalysts in 72 h on stream (TOS) at 300 oC for the isomerization reaction; (a) conversion and selectivity to (b) MOB isomers, (c) MUB isomers and (d) i-C7 (MOB+MUB) isomers.


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R² = 0.9999

ln k

10

-4.2


-4.8

R² = 0.9984

-4.2

(b)

-4.6

ln (rate H2)

20

-3.6

ln (rate C7)


(a)

-5.0 0

-5.4

R² = 0.9838 R² = 0.9652

-10 0.0015

-6.0 0.0017

0.0019

0.0021

-5.4 -18

-15

1/T (k-1) 85

(c)

72

ln P -12

-9

(d)

76

Calculated i-C7 selectivity (%)

Calculated i-C7 selectivity (%)

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61

59 Pt/Al(5)-HZ Pt/Al(20)-HZ Pt/Al(35)-HZ Pt/Al(45)-HZ

46


46

33

31 35

45

55

65

75

31

Experimental i-C7 selectivity (%)

46

61

76

Experimental i-C7 selectivity (%)

Fig. 8. (a) Arrhenius plots calculated from the experimental data for different prepared catalysts, (b) double-log plots of the isomerization reaction rates versus the partial pressures of nC7 and H2, (c) estimated data by power law model and (d) estimated data LangmuirHinshelwood model.


Catalytic Study and Kinetic Modeling of the n-Heptane Isomerization

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