Synthesis of zirconium and cerium over HZSM-5 catalysts for light olefins production from naphtha

Accepted Manuscript Title: Synthesis of Zirconium and Cerium over HZSM-5Catalysts for Light Olefins Production from Naphtha Author: Forough Momayez Jafar Towfighi Darian Seyedeh Mahboobeh Teimouri Sendesi PII: DOI: Reference:

S0165-2370(15)00049-2 http://dx.doi.org/doi:10.1016/j.jaap.2015.02.006 JAAP 3415

To appear in:

J. Anal. Appl. Pyrolysis

Received date: Revised date: Accepted date:

29-6-2014 1-2-2015 1-2-2015

Please cite this article as: Forough Momayez, Jafar Towfighi Darian, Seyedeh Mahboobeh Teimouri Sendesi, Synthesis of Zirconium and Cerium over HZSM5Catalysts for Light Olefins Production from Naphtha, Journal of Analytical and Applied Pyrolysis http://dx.doi.org/10.1016/j.jaap.2015.02.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis of Zirconium and Cerium over HZSM-5 Catalysts for Light Olefins Production from Naphtha ForoughMomayeza,b, JafarTowfighiDariana*, SeyedehMahboobehTeimouriSendesib a. b.

Chemical Engineering Department, TarbiatModares University, P.O. Box 14115–143, Tehran, Iran. Young Researchers and Elite Club, Roudbar Branch, Islamic Azad University, Roudbar, Iran, P.O. Box 14579-44616

Abstract HZSM-5 samples modified with Ce and Zr, separately and simultaneously, were prepared for naphtha catalytic cracking at 650°C and 700°C. The physicochemical features of parent and modified catalysts were characterized by SEM, XRD, BET and, NH3-TPD. Separate loading of Ce and Zr increased the number of acid sites which was favorable for light olefins production. The maximum yields of ethylene (19.62 wt. %) and propylene (36.64 wt. %) were achieved over 8%Zr/HZSM-5 and 2%Zr/HZSM-5, respectively. Loading Ce and Zr, simultaneously, decreased the number of acid sites which caused a reduction of ethylene and propylene production. 1. Introduction High demand for light olefins, as petrochemical industry feedstock, has attracted a great deal of attention. Ethylene and propylene are considered as the main product and byproduct of thermal cracking, respectively [1] which is the major technology for production of light olefins. In contrast to the global propylene demand, which is much higher than ethylene demand, the selectivity of propylene in thermal cracking is quite low [2]. Besides, high reaction temperature and limitation for feedstock are other disadvantages of thermal cracking. Therefore, thermal catalytic cracking seems to be a good alternative due to higher yield of light olefins, reduction of

*

Corresponding author. Tel.: +98 912 2197906; fax: +98 21 8288 3311. E-mail address: [email protected](J. Towfighi).

energy consumption, lower greenhouse gas emission, reduction of the cost and number of equipments and also, accessibility to cheap feed supply [3-5]. Naphtha, as an intermediate product of heavy oil thermal catalytic cracking, is a favorable feed for light olefins production [6]. In addition, zeolites, owing to their unique properties such as hydrothermal stability, shape selectivity, non-corrosive and also, environmentally friendly have been extensively used in thermal catalytic cracking [7,8]. However, to achieve a high yield of light olefins, moderate acidity of zeolite is required. For this purpose, modification of zeolite is an alternative. Wang et al [5] investigated the effect of rare earth metal loading on the performance of HZSM-5 for butane cracking. They found that the selectivity of olefins was increased by rare earth metal modification. Among, catalysts, Ce/HZSM-5 showed the highest yield of light olefins whereas the greatest yield of propylene was achieved by Nd/HZSM-5 at 600° C. Catalytic cracking of nbutane over La/ZSM-5 and Pr/ZSM-5 was studied by Wakui at al [9]. It was found that the rare earth elements caused prohibition of bimolecular reactions and consequently, increasing of light olefins yield. Zhang et al [10] reported the influence of the La addition to catalytic performance of PtSnNa/ZSM-5 in propane dehydrogenation. The higherconversion of propane and the best catalytic stability was achieved by 1.4 wt. % of La loading while a further increase in La amount led to lower selectivity and stability. Xue et al [11] studied the performance of CePtSnNa/ZSM-5 for dehydrogenation of propane. The best catalytic performance was found in 1.1 wt. % of Ce. Lu et al [12] reported results of different amount of Cr loading on HZSM-5 for isobutene cracking. Trace amount of Cr (0.004 mmol/gr) caused changing the acidity towards suitable number and strength of acid sites which was favorable for light olefins production (56.1 wt. %). Wei et al [13] studied the effect of different modification on ZSM-5 for naphtha catalytic cracking inthe presence of N2 and steam streams. Some modifications such as P and Mg

increased light olefins yield as a result of reduction of Bronsted acid sites whereas La loading displayed different performance. In fact, higher yield of BTX and lower yield of light olefins was achieved over La/HZSM-5. The effect of different modifications on ZSM-5 for ethanol conversion to propylene was investigated [14]. Zr loading caused no changes in the number of strong acid sites whereas the number of weak acid sites increased which led to the higher result for light olefins yield. Teimouri et al [15] investigated the effect of either iron or phosphorous onthe catalytic behavior of HZSM-5 in cracking of naphtha. It was found that dealumination was diminished by a suitable amount of P. The best catalytic performance was obtained by 6 wt. % of Fe and 2 wt. % of P over HZSM-5 (25). The effect of Zr and Ce on theperformance of SAPO and HZSM-5 was studied by Zeinali et al [16]. The best catalytic behavior was achieved over 2%Ce2%Zr/HZSM-5 due to more carbenium ion mechanism as a result of more medium and strong acid sites. According to our previous study [16] which resulted in better catalytic performance of CeZr/HZSM-5 in comparison with various parent and modified SAPO-34 catalysts, in this work, for the first time, we intend to investigate the effect of Ce and Zr in different loading amount and also, in various reaction temperatures on light olefins production. 2. Experimental 2.1.Catalyst preparation HZSM-5 with aSi/Al ratio of 100 was purchased from ZEOCHEM Company, Switzerland. Parent catalyst was dried at 110°C for 15 min. Suitable amount of Ce(NO3)3.6H2O and/or Zr(NO3).H2O were solved in 10 cc of distilled water and subsequently, were added to HZSM-5 using the wet impregnation method. The suspension was evaporated in a rotary vacuum evaporator at 90°C with rpm=110. The residue was completely dried at 110°C for 1 hour and

then, calcinated at 750°C for 210 min in air flow. Therefore, catalysts with 2%Ce, 8%Ce, 2%Zr, and 8% Zr, 2% Ce2% Zr, 2% Ce8% Zr, 8% Ce2% Zr and 8% Ce8% Zr loading amount were prepared for experimental tests. 2.2.Characterization Scanning electron microscopy was carried out in a EM3200 with an accelerated voltage 25kv to 30kv. The crystallinity of samples was investigated using Philips Diffractometer (PW-1840) (Lump Cu Kα, λ =1.54˚ A at 40 kV and 30 mA). The BET specific surface areas of the catalysts were determined by Nitrogen adsorption/desorption at 77K using TriStar II 3020 instrument. TPD analyses are described elsewhere [17]. 2.3.Catalyst evaluation The catalytic cracking of naphtha was performed in a fixed bed reactor with 45 cm length and 1.35 cm diameter. Naphtha and water were injected into two consecutive preheaters by two separated pumps. Gasified feed then,entered into the reactor where reactions happened. The outlet gas stream was condensed in two condensers to analyze light compounds via an online GC (Hewlett Packard 5890 GC). The reactor tests were performed at 650 and 700°C,a steam ratio of 0.5gr/gr, WHSV 60hr-1 and also, 2gr of catalyst with 20-30 mesh fractions. Results and discussion 3.1 Structure properties According to Fig. 1.it is apparent that no changes happened to the morphology of catalysts after loading different amount of Ce and Zr. Spherical structure of parent catalyst and modified catalysts is crystal clear in SEM images.

Fig.1. SEM images of: (a) HZSM-5(100) and (b): 8%Ce8%Zr/HZSM-5(100)

The XRD patterns of parent and some modified catalysts are shown inFig.2. As it is clear, the similar XRD profile of modified samples with fresh one indicates that the structure of catalysts has remained approximately intact [18]. Also, based on Fig. 2., well dispersion of Ce and Zr is suggested as no sharp peak is appeared after impregnation [19,20]. The XRD patterns of modified catalysts indicate that zirconium oxide and cerium oxide have been formed in HZSM-5 structure.

Fig.2. XRD pattern of unmodified and modified catalysts

As shown inTable 1., BET results imply that compared to parent sample, surface area and pore volume of modified catalysts were decreased, noticeably. Besides, for samples 2%Zr/HZSM-5 and, 2%Ce2%Zr/HZSM-5 increasing Zr loading led to reduction of either SBET or VPwhich could be as a result of pore blockage by Zr. The same attitude was observed for 2%Ce/HZSM-5 and 2%Ce-2%Zr/HZSM-5 by increasing Ce loading amount. However, unlike the mentioned modified catalysts, theSBETand also, Vpof 8%Ce2%Zr/HZSM-5 increased due to increasing Zr loading which could be as a result of the creation of some secondary pores due to increase in micropores [3]. Table 1.The textural data of all samples

3.2 Acidity of catalysts NH3-TPD results of parent and modified catalysts are reported inFig.3.andTable 2. The NH3TPD curve of the standard sample (HZSM-5), as a comparable index, consists of three peaks. The temperature range of 140-320°C and 400-700°C are considered for weak and strong acid sites, respectively. Ce loading increased the number of both weak and strong acid sites. In addition, compared to HZSM-5, due to cerium adding the strength of weak acid sites has decreased slightly while the strength of strong acid sites has increased. Also, in comparison with the parent catalyst, Zr loading increased the number and the strength of either weak or strong acid sites. Increasing total number of acid sites could be due to the formation of some new basic acid sites [21]. Highest number of acid sites was obtained over 8%Ce/HZSM-5(100) (2.54 mmol NH3/g-catalyst). In contrast, sharp decrease of total acid sites was observed over 2%Ce8%Zr/HZSM-5 (0.41) and 8%Ce2%Zr/HZSM-5 (0.18). No desorption peak was observed for strong acid sites of 8%Ce2%Zr/HZSM-5 which is in agreement with other works [19]. Table 2. NH3-TPD results for parent and some modified catalysts

Fig.3.The NH3-TPD Spectra of parent and modified catalysts to investigate the effect of: (a) Ce loading, (b) Zr loading and, (c) Ce and Zr loading on acidity of catalysts.

3.3 Catalyst performance 3.3.1 Effect of temperature The results of experimental tests are shown inTable 3.andFig.4. By increasing temperature from 650°C to 700°C, in addition to increasing light paraffins yields, ethylene yield raisedslightly for

all samples suggested thermal cracking as the main reaction for ethylene production [3,12]. However, it was not the same for propylene production. Propylene is an intermediate product and moderate conditions are required to achieve higher yield of that [1, 14]. In fact, for catalysts with higher acidity like 8%Ce/HZSM-5 and 8%Zr/HZSM-5, by increasing reaction temperature, secondary reactions facilitated which caused reduction of propylene yield [3]. 3.3.2 Effect of Ce and Zr loading For all samples, thermal catalytic cracking led to higher yield of propylene due to more carboniumion formation and then, β-scission reaction [15]. Light olefins production increased owing to separate loading of Ce and Zr whereas a different trend was observed for the simultaneous loading of cerium and zirconium. In fact, according to NH3-TPD results, it could be assumed that decreasing tremendous amount of acid sites after simultaneous loading of Ce and Zr led to fewer active sites for cracking process which could be the main reason for such a trend. Among catalysts, 2%Zr/HZSM-5 showed the highest yield of propylene at 650°C. In contrast, 8%Ce/HZSM-5 with the highest number of acid sites (2.54 mmol NH3/gr catalysts), did not show the highest propylene yield. Hence, it could be suggested that moderate acidity and temperature are required for higher propylene yield [14]. Moreover, higher yield of ethylene was obtained over 8%Zr/HZSM-5 at 700°C.

Table 3. Naphtha thermal and catalytic cracking

Fig.4. Naphtha thermal and catalytic cracking XRD patterns of some catalysts after experimental tests are shown inFig.5.Decrease in crystallinity has happened in all used samples and implies the partial destruction of structure in

reaction operations. Catalysts with 2 wt. % of Zr loading had a slight destruction due to more hydrothermal stability. In other words, Zr presence in HZSM-5 structure prevented the destruction of catalyst as a result of delumination in the reaction environment [22]. Totally, the moderate acidity, better structural maintenance and moderate temperature were favorable for having highest total olefins yield.

Fig.5. XRD patterns of some samples after reactor tests Conclusion In this study, we found that Ce and Zr loading can promote ethylene and propylene yield. Simultaneous loading of Ce and Zr caused loss of acidity and is not suggested for light olefin production. Zr increased crystallinity and hydrothermal stability of parent catalyst and is favorable for naphtha catalytic cracking. It was found that moderate reaction temperature and acidity was favorable for higher propylene production. Acknowledgement Financial support ofChemical Engineering Center of ExcellenceatTarbiatModares University is highly appreciated.

Table 1. The textural data of all samples

SBET

Smicro

Sexternal

VP

Vmicro

Vmeso

(m2/g)

(m2/g)

(m2/g)

(cm3/g)

(cm3/g)

(cm3/g)

HZSM-5(100)

442.72

286.79

155.93

0.258

0.132

0.126

2%Ce/HZSM-5(100)

366.65

147.51

219.14

0.220

0.069

0.151

8%Ce/HZSM-5(100)

361.2

185.65

175.55

0.210

0.085

0.125

2%Zr/HZSM-5(100)

375.42

200.43

174.99

0.230

0.089

0.141

8%Zr/HZSM-5(100)

370.3

213.77

156.53

0.210

0.098

0.112

2%Ce2%Zr/HZSM-5(100)

386.42

134.26

252.16

0.237

0.062

0.175

2%Ce8%Zr/HZSM-5(100)

368.63

183.82

184.81

0.223

0.084

0.139

8%Ce2%Zr/HZSM-5(100)

351.86

200.72

151.14

0.195

0.092

0.103

8%Ce8%Zr/HZSM-5(100)

360.32

208.94

151.38

0.212

0.096

0.116

Catalyst

Table 2. NH3-TPD results for parent and some modified catalysts Acidity (mmol NH 3/g-catalyst)

Catalyst

Weak acid

Strong acid Total acid sites

sites

sites

HZSM-5(100)

0.40(33%)

0.82(67%)

1.18

2%Ce/HZSM-5(100)

0.72(46%)

0.85(54%)

1.57

8%Ce/HZSM-5(100)

1.05(41%)

1.49(59%)

2.54

2%Zr/HZSM-5(100)

0.53(34%)

1.03(66%)

1.55

8%Zr/HZSM-5(100)

0.71(35%)

1.33(65%)

2.04

2%Ce8%Zr/HZSM-5(100)

0.27(65%)

0.14(35%)

0.41

8%Ce2%Zr/HZSM-5(100)

0.18(100%)

0.00(0%)

0.18

Table 3. Naphtha thermal and catalytic cracking data Ethylene Temperature Samples

Methane Ethane

Ethylene

Propane

n-

+

butane

Propylen

Propylene

(°C)

e 650

8.9

2.69

13.3

0.56

12.68

0.13

25.98

700

9.65

3.88

14.2

0.6

13.5

0.18

27.7

650

8.47

3.01

15.87

1.1

24.85

0.14

40.72

700

10.41

3.71

19.29

1.35

26.64

0.18

45.93

650

9.34

4.04

16.99

2.2

29.82

0.19

46.81

700

9.81

4.25

18.84

2.31

28.59

0.18

47.43

650

7.21

3.85

17.85

2.84

33.11

0.12

50.96

700

9.92

4.3

19.06

2.91

30.47

0.09

49.53

650

6.8

3.14

18.93

2.4

36.64

0.17

55.57

700

6.94

3.21

19.1

2.45

31.96

0.17

51.06

650

6.6

3.53

17.91

2.57

34.00

0.1

51.91

700

6.66

3.56

19.62

2.6

31.6

0.21

51.22

650

6.74

3.03

15.51

1.73

28.7

0.16

700

6.65

3.07

16.31

1.82

32.89

0.15

49.02

650

7.87

2.9

15.04

1.18

25.17

0.18

40.21

700

8.21

3.03

15.69

1.23

29.12

0.15

44.81

650

6.32

3.33

13.24

2.43

17.24

0.18

30.48

700

6.89

3.63

14.34

2.65

21.66

0.18

36

650

5.4

2.34

13.43

1.43

18.09

0.15

31.52

700

7.13

3.1

15.1

1.88

20.86

0.16

No Catalyst

Parent

2%Ce/HZSM-5(100)

8Ce/HZSM-5(100)

2Zr/HZSM-5(100)

8%Zr/HZSM-5(100)

44.21

2%Ce 2%Zr/HZSM-5(100)

2%Ce 8%Zr/HZSM-5(100)

8%Ce 2%Zr/HZSM-5(100)

8%Ce 8%Zr/HZSM-5(100) 35.96

(a)

(b)

Fig.1. SEM images of: (a) HZSM-5(100) and (b): 8%Ce-8%Zr/HZSM-5(100)

Fig.2. XRD pattern of unmodified and modified catalysts

(a)

(b)

(c)

Error! Unknown switch argument. of: (a) Ce loading, (b) Zr loading and, (c) Ce and Zr loading on acidity of catalysts

Fig. 4. Naphtha thermal and catalytic cracking

Fig.5. XRD patterns of some samples after reactor tests

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