Catalytic properties of microporous zeolites in the catalytic cracking of m-diisopropylbenzene

Molecular Catalysis 461 (2018) 80–85

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Catalytic properties of microporous zeolites in the catalytic cracking of mdiisopropylbenzene Byung-Geon Parka, Kyong-Hwan Chungb, a b

T

⁎

Department of Food and Nutrition, Kwangju Women’s University, 165 Sanjung-dong, Gwangju, 62396, Republic of Korea Department of Environmental Engineering, Sunchon National University, 255 Jungang-ro, Sunchon, Jeonnam, 57922, Republic of Korea

A R T I C LE I N FO

A B S T R A C T

Keywords: Catalytic cracking m-Diisopropylbenzene MWW zeolite Cumene Benzene

The catalytic cracking of m-diisopropylbenzene, which is produced as a byproduct of the phenol production process, was studied over various microporous zeolite catalysts. The zeolites had different acidity and pore structures. The highest conversion of m-diisopropylbenzene in catalytic cracking was achieved on the MWW zeolite catalyst. BEA and FAU zeolite catalysts also showed considerably high conversion. The MWW zeolite exhibited a high selectivity for benzene and cumene compared to the other zeolites. The conversion of m-diisopropylbenzene on a MWW zeolite catalyst exceeded 90% at 400 °C. The yield of cumene plus benzene was approximately70% in catalytic cracking over MWW zeolite. The deactivation of MWW zeolite catalysts by carbon deposition on the catalyst could be neglected at high reaction temperatures, whereas it was quite serious on the zeolite catalysts with the MOR structure. Benzene might be produced by a secondary reaction of intermediates in the large pore channels during the retention time.

1. Introduction Aromatic compounds, such as benzene, toluene and xylene, are important raw materials in the petro-chemical and fine chemical industry. On the other hand, their demand is not always met by the supply. The amount of benzene produced is insufficient to meet the demand, whereas the production of toluene exceeds its demand. To control the proportion between demand and supply, benzene from pxylene is produced from selective toluene disproportionation [1–3] and the alkylation of aromatic compounds [4–6]. In other methods, benzene is supplied from the catalytic cracking of aromatic byproducts formed in the naphtha cracking or phenol production process [7–9] Many processes have been developed as industrial syntheses for the production of phenol, but only the Hock process for cumene oxidation [10,11] and the Dow process for toluene oxidation [12–14] are important industrially. The other processes have been rejected because of their high cost. The cumene-phenol process is derived from the discovery of cumene hydroperoxide and its cleavage to acetone and phenol [15,16]. Phenol is produced predominantly by this cumene-phenol process in plants. The process consists of the cumene production process and phenol production process. Two byproducts were also produced during all processes, which reached approximately 10% against the total amount of phenol production [17]. Cumene is produced from benzene alkylation with propene on phosphoric acid or aluminum ⁎

chloride catalysts [18]. In the process of cumene production, wasteful byproducts are obtained, which consist of isomers of diisopropylbenzene (DIPB), triisopropylbenzene (TIPB), butylbenzene, and heavy unknown aromatics [19]. m-Diisopropylbenzene (m-DIPB) contains two isopropyl groups on benzene and is formed in higher quantities than other DIPB isomers and TIPB in the byproduct. DIPBs react with benzene and can be used as an intermediate for the phenol production process. The byproduct can be used as a fuel oil in the industrial field without treatment, but it has a problem because aromatic compounds in the byproduct do not burn completely. The catalytic cracking behaviors of aromatic compounds depend on the acidity and pore structure of the catalyst. The strength of acidity affects the cracking behavior and catalyst deactivation by the deposition of polymerized high boiling hydrocarbons. The cracking behavior on zeolite catalysts, which have linear pore channels, exhibit high paraselectivity, but serious catalyst deactivation can occur in the pore channels, which block the pores easily by high-boiling aromatic compounds. On the other hand, MWW zeolite has a pillow-type structure formed with two 10-membered rings, which are multidimensional channels [20]. One of them is sinusoidal and the other containing large supercages is defined by the 12-membered rings with a 7.1 Å inner diameter and an 18.2 Å height [21]. In MWW zeolite, no high-boiling aromatic products were produced due to restrictions by the pore

Corresponding author. E-mail address: [email protected] (K.-H. Chung).

https://doi.org/10.1016/j.mcat.2018.10.008 Received 25 June 2018; Received in revised form 11 October 2018; Accepted 14 October 2018 2468-8231/ © 2018 Published by Elsevier B.V.

Molecular Catalysis 461 (2018) 80–85

B.-G. Park, K.-H. Chung

compositions of the zeolites were analyzed by energy dispersive X-ray (EDS; Norans, S-MAX 400) as an attachment to the scanning electron microscope. The N2 isotherms were recorded using an automatic volumetric adsorption apparatus (Miraei SI, Porosity-QX) at the liquid N2 temperature. Evacuation of the samples was carried out at 130 °C for 1 h before N2 adsorption. Gravimetric measurements of o-xylene adsorption-desorption were performed on a highly sensitive microbalance (quartz glass spring balance). After the zeolite catalysts (0.1 g) were evacuated at 550 °C for 2 h under a pressure of less than 10−4 Torr, the measurements were taken at 120 °C and at an o-xylene pressure of 10 Torr. The BrunauerEmmett-Teller (BET) surface area of the catalysts before and after the cracking reaction was measured from the N2-adsorption isotherm at liquid nitrogen temperature. NH3-temperature programmed desorption (NH3-TPD) was performed according to the typical method. The catalysts were evacuated at 550 °C for 1 h and then exposed to NH3 gas flow at room temperature for 30 min. The temperature was increased to 550 °C at a constant heating rate of 10 °C/min under helium gas flow. The relative amount of ammonia desorbed from the catalysts was determined by TCD.

entrance, so carbon deposition was difficult. Thus, MWW zeolite has been used in the reaction process using relatively large molecular reactants, such as the alkylation of isopropy1naphtalene [22], skeletal isomerization of 1-butene [23], and disproportionation of toluene [24]. In this study, the catalytic cracking behaviors of m-DIPB and the byproduct were examined on various zeolite catalysts, which have different pore structures and acidity. Variation of the product distribution by the difference in pore structure of the zeolite catalysts was also investigated based on the adsorption-desorption behavior of o-xylene. The product distribution suggested a potential route for the recovery of profitable raw materials from the byproducts formed in the isopropylation of benzene as a step in phenol production. 2. Experimental 2.1. Materials m-Diisopropylbenzene (m-DIPB, Aldrich, 96%) and byproducts were provided from Kum-Ho P&B Co. and used as reactants. MWW (MCM-22; Si/Al = 13) zeolite was prepared according to the methodology reported elsewhere [25]. A gel containing hexamethyleneimine was employed as the structure directing agent. The gel was crystallized in a Teflon-lined autoclave at 150 °C for 9 days with stirring. Na-form zeolites were transformed to the H-form by ion exchange with 0.2 M NH4NO3. Hydrothermally synthesized MFI (ZSM-5; Si/Al = 13) was applied after ion exchange into the H-forms. MOR (Si/Al = 10, Tosoh Co.), FAU (Si/Al = 14, Tosoh Co.), and BEA (Si/Al = 13, PQ Co.) zeolites were also introduced in the reaction. The Si/Al ratios of the catalysts were adjusted to 10 to 14 to exclude the influence of the different acidity. Table 1 lists the Si/Al molar ratios and physical properties of the zeolites.

3. Results and discussions 3.1. Physicochemical properties of the zeolites Fig. 1 presents XRD patterns of the zeolites. The characteristic peaks of the zeolites were in accordance with those reported elsewhere [26]. Fig. 2 presents SEM images of the zeolites. The MWW zeolite was larger than 10 μm, whereas that of BEA zeolite was less than 0.2 μm. The particle sizes of FAU and MFI were uniform at 0.5 μm. The type of N2isotherms of the zeolites is represented as a Langmuir isotherm, which was derived from a uniform micropore. The BET surface areas were 200–700 m2/g, as listed in Table 1. Fig. 3 presents the NH3-TPD profiles of the zeolites. The profile of FAU zeolite revealed a single desorption profile. In contrast, the profiles of the other zeolites were obtained as double profiles at 200 °C and 300 °C–500 °C, respectively. The first profile appeared at approximately 200 °C due to the desorption of NH3 adsorbed physically on the surface of the zeolites. The second profile was assigned to the desorption of NH3 on the acid sites of the zeolites [27]. The NH3-TPD profiles of MFI and MOR zeolites indicated the presence of acid sites on the structure. In

2.2. Catalytic cracking of m-DIPB The catalytic cracking reactions were performed in a fixed bed and continuous flow reactor at atmospheric pressure. After the catalyst was pretreated at 500 °C for 1 h, the reactant (m-DIPB) was fed from a microfeeder with nitrogen gas as a carrier at the desired W (catalyst weight) / F (flow rate of total gas) ratio and temperature. The eluted compounds were trapped by an ice bath and analyzed by gas chromatography (GC, Donam Co., FID, column; DB-1, J&W Scientific Co.). Conversion was defined as 1-(rate of m-DIPB recovery) / (rate of mDIPB feed). The yield was defined as the (rate of formation of the discussed compound) / (rate of m-DIPB feed). 2.3. Characterization of the zeolites

MWW

X-ray diffraction (XRD, D-MAXS-3000/PC, Rigaku) was carried out using nickel filtered Cu Kα X-rays (40 kV, 40 mA) with a 2°/min scan rate. The morphologies and particle sizes of the zeolites were estimated by scanning electron microscopy (SEM, Hitachi S-4800). The Si and Al

Intensity (a. u.)

MFI

Table 1 Physical properties of the zeolite catalysts used in this work. Zeolite

Si/Al molar ratio (-)

Pore diameter (Å)

BET Surface area (m2/g)

Micropore volumea (cm3/ g)

Source

MWW BEA

13 13

420 690

0.15 0.19

synthesized PQ Co.

FAU MOR

14 10

700 410

0.24 0.13

JRC-Z-HY5.5 JRC-Z-HM10

MFI

14

5.6 × 5.6 7.6 × 6.4, 5.5 × 5.5 7.4 × 7.4 6.5 × 7.0, 2.6 × 5.7 5.3 × 5.6, 5.1 × 5.5

260

0.12

synthesized

a

MOR

FAU

BEA 5

10

15

20

25

30

35

2 Fig. 1. XRD patterns of MWW, BEA, MOR, FAU, and MFI zeolites.

determined from t-plot method. 81

40

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B.-G. Park, K.-H. Chung

Fig. 2. SEM images of MWW, BEA, MOR, FAU, and MFI zeolites.

Fig. 4 presents the adsorption behavior of o-xylene on MWW, BEA, MOR, FAU, and MFI zeolites. The behaviors indicate the diffusion of oxylene inside the pores of the zeolites. o-Xylene adsorbed rapidly on BEA and FAU, which have a wide pore space. The levels of o-xylene adsorption on MOR and MWW were smaller than those on the BEA and FAU zeolite catalysts. The adsorption of o-xylene on MFI zeolite, which

contrast, the amount of NH3 desorbed at lower temperatures was larger on the BEA and FAU zeolites. The acid strength can be evaluated by the maximum peak temperature (Tmax) of desorption peak relating to the activation energy for desorption of ammonia [28]. The acid strength of MWW zeolite was similar to that of MFI zeolite. The order of the acid strength was FAU≒BEA < MWW≒MFI < MOR. 82

Molecular Catalysis 461 (2018) 80–85

B.-G. Park, K.-H. Chung

MWW MOR MFI

TCD response (a. u.)

BEA FAU

Tmax

Tmax

0

400

200

Temperature

600

200

0

Tmax

600

400

Temperature

(oC)

(oC)

Fig. 3. NH3-TPD profiles from MWW, BEA, MOR, FAU, and MFI zeolites.

has a narrow pore entrance and bent pore structure, was slower than those of the other zeolites. This suggests that the stereo hindrance by different pore structures of the zeolites might be influenced in the reactions.

BEA

250

FAU 200

3.2. Influence of the pore structure of the zeolites in cracking

-

150

100

Conversion and selectivity are influenced by the acidity and pore structures of the catalysts in the catalytic reaction using porous solid acid catalysts. Therefore, the Si/Al molar ratios of the zeolite catalysts were adjusted to10-14 to reduce the effect of acidity of the catalysts. The catalytic properties of the zeolites might be influenced by the difference in pore structure and acidities derived from different zeolite structures. Fig. 5 shows the conversion and chemical composition of the products from the catalytic cracking of m-DIPB on the zeolites. The results differed remarkably according to the zeolite species. Conversions on the MWW and BEA zeolite catalysts were as high as 80% at the initial reaction. Deactivation of MWW and BEA zeolites occurred slowly as the reaction proceeded. In contrast, conversions on the MOR and FAU zeolites were ca. 50%, and the conversions were reduced to below 20% after a 2 h process time. The conversion on MFI zeolite was excessively low.

MWW

50

MFI MOR

0 0

60

30

90

120

Fig. 4. Adsorption of o-xylene on MWW, BEA, MOR, FAU, and MFI zeolites.

m-DIPB

p & o-DIPB

Cumene

Benzene

Others

MWW

100

MFI

FAU

MOR

BEA

MWW

80

Catalysts

BEA

60

MOR

40 FAU

20 MFI

0 0

1

2 3 4 Time on stream (h)

0

5

20

40

60

80

100

Composition (%)

Fig. 5. Changes in the conversion with time on stream (A) and chemical composition of outlet gas (B) in the dealkylation of m-DIPB on various zeolites at 350 °C W/ F = 5.90 g·h·mol−1 and loading amount of catalyst = 0.2 g. Product was collected for 60 min after 2 h of time on stream. 83

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B.-G. Park, K.-H. Chung

Fig. 6. Changes in the conversion with time on stream (A) and chemical composition of outlet gas (B) in the dealkylation of m-DIPB on MWW zeolites. W/ F = 5.90 g·h·mol−1 and loading amount of catalyst = 0.2 g. Product was collected for 60 min after 2 h of time on stream. m-DIPB

p & o-DIPB

Cumene

Benzene

Others

zeolites because MOR zeolite has a linear type pore structure. Heavy carbon deposition also occurred on the FAU zeolite because large molecular products might be generated in their supercage. Carbon deposition on the catalysts led to the deactivation of the reaction. Conversion on MFI zeolite was low due to the narrow pores inside the MFI zeolite, which suppressed the diffusion of m-DIPB inside its pore. In contrast, the catalytic activities of BEA and MWW zeolite were maintained without deactivation because their pore structure hindered the formation of an intermediate polymer. Their yield of the products was also high. In particular, the yield of benzene increased on MWW zeolite. Benzene might be formed due to cumene, which is generated by the dealkylation of m-DIPB inside the pores.

0.05 g

Loading amount

0.1 g

0.2 g

3.3. Catalytic cracking of m-DIPB on MWW zeolite 0.3 g

Cumene, benzene, and p-DIPB were the main products in the mDIPB cracking reaction. Light hydrocarbons consisting of C3∼C5, triisopropylbenzene (TIPB), and o-DIPB were produced in much smaller quantities. As shown in Fig. 5(A), high conversion was observed on MWW, FAU, and BEA zeolite, which have a relatively large pore size compared to MFI zeolite. The pore structure of MWW zeolite is composed of a 10-oxygen member ring pore entrance and a 12-oxygen member ring pore inside the channel [25]. Fig. 6 shows the variation of conversion of m-DIPB and the chemical composition of the products in the dealkylation of m-DIPB on MWW zeolite. The conversion of m-DIPB increased and zeolite deactivation decreased with increasing reaction temperatures, as shown in Fig. 6(A). The intermediates of the aromatic compounds are formed easily at high reaction temperatures. The product distribution also varied with the reaction temperature, as shown in Fig. 6(B). p-DIPB and o-DIPB were generated mainly by the isomerization of m-DIPB at 250 °C. At 350 °C, the main product was cumene formed by the dealkylation of m-DIPB. The production of benzene and light hydrocarbons increased with increasing reaction temperature. Fig. 7 presents the chemical composition of products with the loading of MWW zeolite in the dealkylation of m-DIPB at 350 °C. The conversion of m-DIPB was improved and the yields of cumene plus benzene were also enhanced as the catalyst loading was increased. In particular, the conversion of m-DIPB exceeded 90%, and the yield of benzene plus cumene was ca. 70% under a 0.4 g catalyst loading at 350 °C.

0.4 g

0

20

40

60

80

100

Composition (%) Fig. 7. Chemical composition of products with the loading amount of MWW zeolite in the dealkylation of m-DIPB at 350 °C. Product was collected for 30 min after 30 min of time on stream.

The product distribution varies with the species of zeolite in the reaction. Fig. 5(B) shows the composition of products obtained in catalytic cracking. The product obtained from the isomerization and cracking reaction was less than 10% on the FAU and MFI zeolite catalysts. o-DIPB and p-DIPB were generated mainly by the isomerization of m-DIPB on MOR zeolite. In contrast, small amounts of benzene and cumene were produced by the dealkylation of m-DIPB. On the BEA and MWW zeolites, the products of the dealkylation of m-DIPB were larger in quantity than those by the isomerization of m-DIPB. The yields of benzene plus cumene were as high as ca. 50% on MWW zeolite compared to the other zeolites. Carbon deposition inside the pores took place easily on the MOR 84

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4. Conclusion [12]

The conversion of m-DIPB and product distribution were influenced by the pore structure of the zeolites in the catalytic cracking of m-DIPB. MWW zeolite showed higher conversion than the other zeolites as well as longer catalytic activity with slow deactivation. The selectivity of the MWW zeolite for benzene and cumene was higher than those of the other zeolites. The pillow-type pore structure of MWW zeolite could suppress secondary reactions, such as isomerization, which led to high yields of cumene and benzene. The catalytic behavior of MWW zeolite in the catalytic cracking of m-DIPB was superior to that of the other zeolites.

[13]

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[15] [16] [17]

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