Methanol to Propylene over Foam SiC-Supported ZSM-5 Catalyst

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Kinetics, Catalysis, and Reaction Engineering

Methanol to propylene over foam SiC supported ZSM-5 catalyst: Performance of multiple reaction-regeneration cycles Zuwei Liao, Ting Xu, Yuntao Jiang, Binbo Jiang, Jingdai Wang, Yongrong Yang, yilai jiao, zhenming yang, and Jinsong Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02114 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 10, 2018

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Industrial & Engineering Chemistry Research

Methanol to propylene over foam SiC-supported ZSM-5 catalyst: Performance of multiple reaction-regeneration cycles Zuwei Liao,+ Ting Xu,⊥ Yuntao Jiang,⊥ Binbo Jiang,⊥ Jingdai Wang,+ Yongrong Yang,*,+ Yilai Jiao,# Zhenming Yang# and Jinsong Zhang# +

State Key Laboratory of Chemical Engineering, College of Chemical

and Biological Engineering, Zhejiang University, Hangzhou, 310027, PR China ⊥

Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering

Manufacture Technology, College of Biological Engineering, Zhejiang University, Hangzhou, 310027, PR China #

Shenyang National Laboratory for Materials Science, Institute of Metal

Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China Abstract: Due to the superior performance in thermal conductivity and mechanical strength, foam SiC-supported ZSM-5 catalyst is favorable in fixed bed methanol-topropylene (MTP) reactions, where temperature distribution and pressure drop are strictly controlled. However, its performance and service life in multiple reactionregeneration cycles had not been proved yet. Both the activity and selectivity of the catalyst will be tested in six reaction-regeneration cycles in a fixed bed reactor. In addition, the properties of the catalyst during the fourth cycle were characterized by various techniques (SEM, BET,

27

Al NMR). The results show that as the reaction

progresses, the catalyst continuously deposits carbon; and the local high temperature of

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Industrial & Engineering Chemistry Research 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

carbon burning during regeneration leads to the removal of aluminum from zeolite framework. Therefore, the catalyst life in one single process increases firstly then decreases. At the same time, the propylene selectivity also experiences rapid rising, steady rising and rapid descent period, the steady rising period is the best MTP reaction area of operation.

1. INTRODUCTION Propylene, as one of the main sources of modern chemical industry, its downstream products cover almost all areas of manufacturing, such as polypropylene, acrylonitrile, propylene epoxide and acetone.1-2 Therefore, it is important for our daily life. Propylene is mainly produced by petroleum processing. The increasing demand for propylene and the shortage of oil resources, it is desired to develop a non-oil-based propylene production method. Methanol, which is currently derived from catalytic of syngas (H2, CO), is believed to be competitive for petroleum substitution in the production of propylene because of the abundant raw materials. In this way, methanol is controllably converted into desired products, such as olefins/propylene (MTO/MTP), aromatics (MTA), and gasoline (MTG), on solid acid zeolites.3 The light olefins are produced through methanol-to-hydrocarbon (MTH) or methanol-to-olefin (MTO) processes.4 Consequently, due to the high products selectivity, methanol-to-propylene (MTP) process has been attractive. System efficiency can be improved by process optimizations,5-7 while reaction efficiency can be improved by selecting the suitable catalysts. In recent years, ZSM-5


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catalysts have been widely investigated due to their excellent properties, such as high surface area, high hydrothermal stability, intrinsic acidity, well-defined micro-porosity, 1, 3

which facilitates a specific technology targeting propylene: methanol-to-propylene

(MTP) process.8 However, this catalyst is generally used in a random packed bed, which have several disadvantages, such as limited heat- and mass transfer, high pressure drops.9 To overcome these disadvantages, zeolite coated structured catalysts have been popular in recent years. There are many support materials, such as Al2O3,10 glass,11 stainless steel12-13 or SiC.14-16 Among these support materials, SiC exhibits the necessary intrinsic properties required to become a valuable candidate as a zeolite support: high thermal conductivity17 and mechanical strength, resistance to oxidation, chemical inertness, and ease of shaping.18 All these advantages indicate that SiC support can replace classical supports such as alumina, silica and carbon, especially in highly endothermic and exothermic reactions.19-24 Ivanova et al. 15 synthesized the catalyst by ZSM-5 coating deposited on macroscopic β-SiC foams. They found the existence of a nanoscopic layer of silica and silicum oxycarbide on the surface of the supports ensures the strong anchoring of the zeolite crystal on the β-SiC substrate. The use of the catalyst in the methanol-to-olefins reaction presents substantial activity/selectivity improvements compared to the conventional zeolite. Ivaova25 also found a new concept in the preparation of zeolite materials named in situ zeolite synthesis. The silica superficial layer from the support itself was used as a silica source so that this route avoids addition of any external silicon source. As prepared structured catalysts have been tested in the conversion of methanol to light olefins, which



exhibited different performance in terms of activity and selectivity, including high propylene to ethylene ratio. Jiao et al.16 synthesized a ZSM-5 crystals with an average crystal size of about 3.5 μ m. The ZSM-5/SiC foam composite catalyst showed substantial stability and yield improvement in comparison to zeolites in the methanol to propylene reaction. The advantages of the SiC-structured catalysts have been shown in the methanol to propylene reactions. However, their performance and lifetime in multiple reactionregeneration cycles have not yet been demonstrated. In this paper, we will investigate the carbon deposition and dealumination deactivation of structured catalysts during the multiple reaction-regeneration cycles. Through multiple-cycles reaction-regeneration test, not only the total catalyst life can be obtained, but also the changes of catalyst activity under different cycles can be observed. Combing with the physical characterization of catalyst, the reasons for the changes of catalyst activity during multiple cycles can be revealed. 2. EXPERIMENTAL SECTION 2.1 Catalysts ZSM-5/SiC foam catalysts (referred to hereafter as a structured catalyst(SiC)) were provided by Shenyang Institute of Metal Research, Chinese Academy of Sciences (IMR, CAS). The synthesis method has been described in our previous publication.16 The ZSM-5/SiC foam catalysts are in the concentric perforated cylindrical structure. Both the outer diameter and height of the cylinder are 25 mm, while its central hole diameter is 7 mm. The pore size of structured catalyst is about 1.6±0.2 mm. Each


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structured catalyst is loading ZSM-5 zeolites about 1.3 g (28.3wt%). The thickness of zeolites coating is about 15 μm. The macroscopic properties of the catalyst are listed in the Table 1. Table 1 Macroscopic properties of structured catalyst catalyst

structure catalyst

compressive strength void volume molecular sieve heap density(g/ml） silica-alumina ratio(SiO2/Al2O3）

24-27(MPa） 70% 0.146 200

aMacroscopic

properties of the catalyst were provided by the catalyst supplier.

2.2 Catalysts Characterization The surface morphology of the samples was observed by Hitachi S-4700 Scanning Electron Microscope (SEM). The surface areas and pore volumes of the samples were measured by nitrogen sorption at 77K using Quantachrome Autosorb IQ after evacuation at 623K for 12 h. The framework structure of the zeolites was detected by Bruker’s AVANCE III 500WB NMR spectrometer (27Al MAS NMR). 2.3 Catalysts Testing The MTP reaction is carried out in a fixed bed reactor. Fig. 2 shows the flow chart of the whole reaction evaluation system, which consists of feed system, reaction system and analysis system. The feed system includes two gas feed (nitrogen and air) and two liquid feed (methanol and water). Before the gas/liquid enter the two reaction tubes, they are superheated and gasified by the corresponding preheaters (H1, H2). The reaction system consists of two isothermal fixed bed reactors in series. The reaction tube (R1 or R2) is placed in an electric heating furnace, which is divided into five sections from the top to bottom to ensure the isothermal effect of the reactor. The structured catalyst is paced with quartz wool on its outer surface to eliminate the wall



effect. At the end, the analysis system is in the form of an all-component on-line analysis, the outlet of R1, R2 reaction tubes are connected to a gas chromatograph (GC).

Figure 1 Experimental setup for MTP reaction process

The MTP process was divided into two sections26: etherification (DME) and oxygenate to propylene (OTP), referred to as “two-steps method”. Methanol was first subject to the etherification (DME) reaction in the reaction tube R1. The outlet stream of R1 was mixed with nitrogen. Then the mixture entered the reaction tube R2 to perform the OPT reaction. Nitrogen is served as diluent gas in R2. The temperature of DME reaction and OTP reaction were controlled at 543K and 743K respectively. The other reaction conditions of DME were the same as that of the OTP segment. The total system pressure was 140kPa. The catalyst loading was four pieces, methanol mass space velocity was 0.7 h-1, molar ratio of water to methanol was 2. The inlet partial pressure of methanol was 15kPa and the partial pressure of nitrogen was 95kPa. Since the product hydrocarbons can be converted from either methanol or its direct product dimethyl ether, the actual reactants in R2 are the mixture of methanol and dimethyl ether. We denote this mixture as MDOH. Once the conversion of MDOH dropped to 70 c%, we terminate the reaction. The in-situ air regeneration of deactivated catalysts was


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carried out for 50 h, the regeneration gas flow was 300 mL/min. The regeneration temperature was controlled at 723-748K. After the regeneration was completed, the next reaction-regeneration cycle was started with the same reaction and regeneration conditions. Note that the regeneration progress was not counted in the continuous operation time (TOS), and six reaction-regeneration cycles were continuously performed. 3. RESULTS AND DISCUSSION 3.1 The Changes of Catalysts Activity in Multiple Reaction-Regeneration cycles of MTP The reaction activity of the foamed SiC structured catalysts during the six reactionregeneration cycles was investigated. The MDOH conversion versus TOS is shown in Fig. 2. 110

MDOH Conversion(c%)

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


100

1

2

0

500

3

4

1000

1500

5

6

90 80 70 60 50

2000

2500

3000

TOS (h)

Figure 2 MDOH conversion versus TOS in MTP reaction with 6 reaction-regeneration cycles

As shown in Fig. 2, the MDOH conversion trends can be divided into two periods: cycle 1 to cycle 4 are steady cycles, while cycle 5 and cycle 6 are fast decent cycles. The stability of the catalyst increased within the steady cycles, then decreased in the fast decent cycles. At the sixth cycle, the conversion of MDOH directly declined even



without any plateau. This was specifically caused by “retained carbon” in the zeolite catalysts. When the conversion of MDOH was reduced to 90%, the corresponding TOS was taken as the one-process life span. The catalyst lifetime of all the six cycles are shown in Fig. 3. 700

658

600

568

500 Lifetime(h)

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

400

412 373

362 319

300 200 100 0

1

2

3

4

5

6

Reaction-Regeneration Cycle Number

Figure 3 single process life of the structured catalyst at each MTP reaction-regeneration cycle

From Fig. 3, it is obvious that the stability begins to decrease after the fourth reaction-regeneration cycle. Let’s characterize the catalyst samples of this cycle to further understand the reason behind the performance differences between cycles. We collected three kinds of catalyst samples, the fresh catalysts, the fourth-cycle deactivated catalysts and the fourth-cycle regenerated catalysts. They are labeled as SiC-Fresh, SiC-4C, SiC-4R respectively. Fig. 4 shows the SEM images of the three samples. The outer surface of SiC-4C was covered by coke, compared to the unreacted fresh catalysts, and then the coke was burned off through regeneration. The catalyst surface of Fig. 4-b was significantly rougher than that of the Fig. 4-a and 4-c. This is the result of surface coke deposition. In addition, the surface image of Fig. 4-a and 4-c are similar, which implies the reaction and regeneration process do not change the


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catalyst surface roughness at this scale.

(a)

(b)

(c) Figure 4 SEM images of the structured catalysts before and after reaction-regeneration: a. SiC-Fresh; b. SiC-4C; c. SiC-4R.

Table 2 listed the results of the physical adsorption characterization of the three



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samples. Note that the SiC support was also included in the physical adsorption test. Therefore, the surface area and the total pore volume were comprehensive information for both ZSM-5 zeolites and the SiC support. They must be lower than the intrinsic parameters of the ZSM-5 zeolites. Since SiC support contained no micropores, the micropore information was all from the ZSM-5 zeolite. Table 2 Textural properties of the SiC-Fresh, SiC-4C, and SiC-4R. catalyst

SiC-Fresh

SiC-4C

SiC-4R

specific surface area (m2/g)a

88.1

71.4

68.8

micropores area (m2/g)b

82.2

71.1

61.5

external pore area(m2/g)

5.9

0.3

7.3

the total pores volume (cm3/g)b

5.2×10-2

3.8×10-2

4.6×10-2

micropores volume (cm3/g)c

4.0×10-2

3.2×10-2

3.1×10-2

a BET

method; b t-plot method; c single point method, pore diameters smaller than 400 nm at P/P0=0.99495.

From table 2, the micropore surface and micropore volume of the fourth-cycle reaction catalyst showed a significant decline, compared to the fresh one, which meant that the coke deposited on the outer surface and the micropores of catalyst. The difference of the micropore area between SiC-4C and SiC-4R was 9.7 m2/g, indicating that the regeneration process may cause the removal of framework aluminum from zeolite;

18

while the external pore volume increased, indicating that the removal of

framework aluminum may create some mesopores in zeolite coating. The total pores volume of the three samples almost experienced firstly decreasing and then increasing,


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which meant that the regeneration process would burn coke, refreshing the catalyst activity.

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


SiC-Fresh

SiC-4R

-40

-20

0

20

40

60

80

100 120

Chemical Shift (ppm)

Figure 5 27Al MAS NMR spectra of the SiC-Fresh and SiC-4R.

According to the 27Al MAS NMR spectra most of the Al in the fresh SiC catalyst adopt tetrahedral coordination as indicated by the peak between 40 and 80 ppm chemical shift (Fig. 5). Upon reaction the decrease in the intensity of the peak corresponding to tetrahedral Al was compensated by the increase in the intensity for the peak related to Al in octahedral coordination as indicated by the peak at around 0 ppm chemical shift.27 From Fig. 5, we can see that after four reaction-regeneration cycles, the ratio of catalyst extra-framework aluminum to framework aluminum was as high as 0.415. In other words, the proportion of extra-framework aluminum to total aluminum was 29.3%, which meant a large number of the framework aluminum was transformed into extra-framework aluminum. It also was the evidence of the decline of catalyst activity and stability at the fifth cycle. The following conclusions were made combined with the above characterizations. The single process life of the catalyst firstly increases and then decreases with the



increase of reaction-regeneration cycles. As we know, coke not only covers the acidic sites, but also blocks the pores leading to catalyst deactivation. However, coke on the acidic catalyst mainly comes from the side reactions, such as the polycondensation and aromatization of olefins at the strong acidic sites. These oligomers and polycyclic aromatic hydrocarbon (PAHs) gradually transform into larger molecular weights, more stable activity coke. The stronger the acidic catalyst, the higher its activity and the faster the coke deposits. At the first two cycles, single process life of the catalyst didn’t change, even a bit lower. Under the continuous reaction-regeneration operation, the catalyst framework aluminum gradually collapsed, the acid strength and density of the catalyst decreased, which led to the decline of catalyst acidity and the initial coke deposition rate. Therefore, the single process life of catalyst began to rise from the third cycle and reached the maximum value (658 h) at the fourth cycle. Then the effect of irreversible deactivation due to the dealumination of catalysts increasing, the single process life and the stability of catalyst decreased rapidly. Thus, the catalyst acid strength and density for its single process life have a two-side effect. When the catalyst acidity is much enough, coke deposition rate controls the life; otherwise, acid mounts control the life. 3.2 Catalyst Selectivity Changes During the Multiple Reaction-Regeneration Cycles 3.2.1 Propylene and P/E Ratio Changes During the Multiple ReactionRegeneration Cycles The changes of the zeolites’ structure and the acidity can affect not only the single process life, but also its selectivity. The changes of propylene and P/E ratio in the


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multiple reaction-regeneration cycles were described in Fig. 6. As we can see, the propylene yield increased firstly and then decreased with each cycle. With the increase of operating cycles, the highest propylene yield in each cycle also increased firstly and then decreased. Similar to the single process life, the highest propylene yield appeared at the fourth cycle, as high as 45.4%. Secondly, with the increase of operating cycles, the rising period of propylene yield in each cycle was gradually shortened. At the sixth cycle, the rising period was basically negligible. This was mainly due to the incomplete conversion of MDOH. 60

14 12

Propylene Yield (c%)

50

1

2

3

4

5

6 10

40

8

30

6

P/E

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


4 20 2 10

0

500

1000

1500

2000

2500

3000

0

TOS (h) Figure 6 propylene yield and P/E ratio versus TOS in MTP reaction with 6 reaction-regeneration

Different from the propylene yield, the P/E ratio in each cycle continued to increase versus TOS, without a significant downward trend, even when the methanol conversion reduced. With the number increasing of reaction cycles, the irreversible deactivation of zeolites framework aluminum aggravated. It also showed that the lower acidity lead to lower concentration of aromatics and thus lower concentration of methylbenzenzes, and



the methylbenzene cycle was inhibited leading to lower ethylene formation. As the ratio of higher olefins and propylene didn’t shift very significantly, the secondary reaction of olefins seems to be the most prominent, so the olefin cycle was inhibited less. 3.2.2 Catalysts Selectivity Changes in Each Single Cycle To further understand the effect of catalyst coke on catalyst selectivity in each single process, Fig. 7 showed the selectivity changes of olefins, paraffins and aromatics versus TOS over six cycles. According to the variation of propylene selectivity in the first cycle, the single process could be divided into three stages, namely the rapid rise, the slow rise and the rapid decline of propylene selectivity. The three stages of propylene selectivity change gradually leveled off as the number of cycles increases, while the rapid rise and the rapid decline disappeared. The initial propylene selectivity increased versus cycles, which was due to the change of the initial acid structure of catalyst during the reaction-regeneration cycles. When the tetrahedral framework aluminum collapsed during the reaction-regeneration cycles (Fig. 5), the acid strength and density of the initial catalyst surface were reduced, the initial activity of olefin secondary reaction decreased, thus the activity of propylene transforming reactions decreased. As a result, propylene selectivity increased. Finally, the rapid rise and the slow rise periods gradually disappear. During the rising stage of propylene selectivity, aromatics selectivity decreased at a corresponding rate. The aromatics selectivity decreases along TOS for all the six cycles. In addition, the initial aromatics selectivity as well as its decreasing rate decreases as the cycle number increases. After the fourth cycle, the aromatics selectivity


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remains almost unchanged during the fifth and sixth cycle. This phenomenon agrees with the catalyst change depicted in Fig. 5, which shows 29.3% of framework aluminum reduction for the fifth reaction process. Framework aluminum reduction will weaken the acidic strength of catalyst, resulting decreasing of aromatics selectivity. This phenomenon also agrees with the MDOH conversion curve shown in Fig. 2, where the MDOH conversion of the fifth and sixth cycles drop faster than the first four cycles. From Fig. 7 we can see that the change of ethylene selectivity is similar to that of the aromatics: they decreased in each single reaction process while the decrease rate slows down among cycles. This is because the activation barriers of ethane formation of cracking reaction need strong acid strength.28

(a)

(b)

(c)

(d)



(e)

(f)

Figure 7 Products selectivity versus TOS in MTP reaction at the six reaction-regeneration cycles: a. the first period; b. the second period; c. the third period; d. the fourth period; e. the fifth period; f. the sixth period

4. CONCLUSIONS 1. The 29.3% of framework aluminum was transformed to the extra-framework aluminum through the first four reaction-regeneration cycles, leading to the acid strength and acidity of the initial catalyst both decreasing, which inhibited the initial activity of olefin secondary reaction, so that the three stages of propylene selectivity were not obvious in the following cycles, causing that the catalyst single-process life firstly increased and then decreased. Proper reduction of the fresh catalyst Si/Al can effectively increase the total lifetime. 2. The steady rise period of propylene selectivity was the best operating area for MTP reactions. In this period, MDOH can be fully converted. Secondly, the propylene selectivity was at the highest values. Also, the yields of ethylene, alkanes and aromatics were the lowest. Therefore, for MTP industrial process development, catalyst activity should be kept to the best operating area whatever possible.


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3. Whether the high temperature hydrothermal environment of reaction process or the local high temperature caused by the coke burning during the regeneration process, the molecular framework aluminum was removed, which on the one hand reduced the catalyst acid strength and acidity, on the other hand altered the pore structure, reducing the content of micropores. The completely collapse of the molecular sieve framework was the main reason for the irreversible catalyst deactivation.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The financial support provided by the project of National Natural Science Foundation of China (21822809&61590925), the National Science Fund for Distinguished Young Scholars (21525627) are gratefully acknowledged.

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Methanol to Propylene over Foam SiC-Supported ZSM-5 Catalyst

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