Selective Desilication, Mesopores Formation, and MTO Reaction

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Kinetics, Catalysis, and Reaction Engineering

Selective desilication, mesopores formation and MTO reaction enhancement via citric acid treatment of zeolite SAPO-34 Wenlong Jin, Baojie Wang, Pengfei Tuo, Cheng Li, Lei Li, Hongjuan Zhao, Xionghou Gao, and Baojian Shen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00632 • Publication Date (Web): 13 Mar 2018 Downloaded from http://pubs.acs.org on March 16, 2018

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Industrial & Engineering Chemistry Research

Selective desilication, mesopores formation and MTO reaction enhancement via citric acid treatment of zeolite SAPO-34

Wenlong Jin1, Baojie Wang2, Pengfei Tuo1, Cheng Li1, Lei Li1, Hongjuan Zhao2, Xionghou Gao2, Baojian Shen1*

1

State Key Laboratory of Heavy Oil Processing; the Key Laboratory of Catalysis of

CNPC; College of Chemical Engineering, China University of Petroleum, Beijing 102249, China 2

Lanzhou Petrochemical Center, Petrochemical Research Institute, PetroChina Company

Limited, Lanzhou 730060, China *

Corresponding author E-mail: [email protected]

Abstract Adjusting the silica composition/structure and thus to tune the acidity besides introducing hierarchical pore structure is the key factor in zeolite SAPO-34 research. In this work, hierarchical zeolite SAPO-34 with well-organized mesopores was prepared via citric acid treatment. The mesopores presented as novel slit shape, which start from the 1

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crystal surfaces then end at the central of crystal. It was found that, after citric acid treatment, silicon species of Si(OSi)1(OAl)3 in the SAPO-34 framework were extracted selectively, at the same time both of the total Lewis acid amount (by NH3-IR) and the Brønsted acid amount on the external surface (by Pyridine-IR) of SAPO-34 zeolite obviously increased. Consequently, the hierarchical SAPO-34 sample exhibited superior catalytic performance in the MTO reaction with about 2.5 times prolonged catalytic lifetime and nearly 8% improvement of selectivity for ethylene and propylene, this is very important for industrial process.

Keywords: Zeolite SAPO-34, Citric acid treatment, Well-organized mesopores, Selective desilication, Methanol to olefins reactions

1. Introduction Benefit from the micropore structure and controllable acidity, zeolites had been widely used in many chemical processes.1-4 Especially, zeolite SAPO-34 has small 8-ring pore opening (3.8 × 3.8 Å), large CHA cavity (9.4 Å in diameter) and medium acidity and exhibits excellent catalytic performance than the others zeolites in methanol to olefins (MTO) reactions.5,6 But the microporous structure of SAPO-34 would lead to the restriction in mass transfer, thus the superiority in MTO process was achieved at the expense of rapid deactivation. The feasibility of introducing hierarchical pore structure into zeolites to enhance its 2


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transport properties has been confirmed.7,8 Basically, there are two methods to prepare hierarchically porous zeolites. One is the bottom-up method which through adding an additive like macromolecule9-11 or carbon materials12-14 into the zeolite synthesis gel, hierarchically porous zeolite could be obtained by combustion after one-step hydrothermal synthesis. The other is the top-down method in which conventional microporous zeolites are firstly synthesized and mesopore and/or macropore is then introduced by controllable extraction of the framework elements.15-18 In the case of zeolite SAPO-34, the bottom-up method was the first choice to introduce secondary larger pores,19-21 when the conventional dealumination or desilication post-treatment is feeble because of its poor resistance upon acid or alkali medium. However, recently research confirmed that SAPO-11 zeolite with lower silicon content (Si/Al+Si+P = 0.05 or 0.06) remains fully crystalline when treated by acid media, while SAPO-34 zeolite with higher silicon content (Si/Al+Si+P = 0.14) strongly amorphize.22 That means the acid resistance of silicoaluminophosphate zeolites might strongly depend on the silicon content and lower silicon content might lead to higher resistance, thus a suitable

initial

silicon

content

could

make

the

preparation

of

hierarchical

silicoaluminophosphate zeolites via acid treatment possible. In addition to meso/macropore-creating, the top-down method could adjust the composition of zeolites and consequently modulate the acidity via selective extraction of framework elements. As we know, the introduction of silicon is the source of acidity in silicoaluminophosphate zeolites and the silicon content and distribution decides the 3



acidity. So, adjusting the silicon species state besides introducing hierarchical pore structure is the core aim of our research. In our work, hierarchical zeolite SAPO-34 was prepared via facile acid treatment, when we used SAPO-34 with low silicon content (Si/Al+Si+P = 0.10) as parent sample and citric acid (an organic weak acid) as the acid medium. On this basis, the physico-chemical properties and MTO reaction performance changes were investigated.

2. Experimental 2.1 Preparation of parent and hierarchical SAPO-34 zeolite Parent zeolite SAPO-34 was synthesized by hydrothermal synthesis with a gel molar composition of 1 Al2O3: 1 P2O5: 0.6 SiO2: 3 R: 60 H2O, where pseudoboehmite (70.5 wt.%, Al2O3), orthophosphoric acid (85 wt.%, H3PO4), alkaline silica sol (30.4 wt.%, SiO2), triethylamine (TEA, 99 wt.%, C6H15N) and tetraethylammonium hydroxide (TEAOH, 25 wt.%, C8H21NO) were employed as the aluminum, phosphorus, silica sources and organic templates, respectively. The as-synthesized products were filtered, washed with deionized water and then dried at 120 °C overnight, followed by calcination at 550 °C for 8 h to remove the organic templates and the sample was named as SAPO-34-P. Hierarchical SAPO-34-H zeolite was prepared by treating the parent SAPO-34-P with citric acid solution in water-bath at 80 °C for 3 h, and followed by filtration, washing with deionized water and drying overnight at 120 °C. 4


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2.2 Characterization The X-ray powder diffraction (XRD) patterns were recorded on a Panalytical X’Pert Powder diffractometer using Cu Kα radiation at 40 kV and 40 mA. N2 adsorption/desorption measurements were performed on a TriStar II 3020 instrument. Prior to the measurement, the samples were outgassed at 350 °C for 8 h. The total surface area was determined by the BET method. The t-plot method was applied to obtain the micropore / external surface area and micropore volume. The total pore volume was single point adsorption volume of pores less than 345.8 nm at P/P0 = 0.9944. Mercury intrusion experiments were analyzed on a Micromeritics Autopore IV 9500 apparatus to a final pressure of 228 MPa. Prior to measurements, samples were dried at 150 °C for 2 h. Transmission electron microscopy (TEM) images were determined by a JEM-2100 electron microscope operating at an acceleration voltage of 200 kV. A Panalytical Petro-AxiosmAX X-ray fluorescence (XRF) spectrometer was used for the measurement of the bulk compositions of the SAPO-34 zeolites. The temperature-programmed desorption of ammonia (NH3-TPD) experiments were carried out using a Micromeritics AutoChem II 2920 automated chemisorption analysis unit with a thermal conductivity detector (TCD) under helium flow. The FT-IR spectra of samples were recorded using a Thermo Fisher Nicolet IS10 spectrometer with a resolution of 4 cm-1 and co-addition of 32 scans. Prior to the measurements, 30 mg of the dehydrated sample powders was pressed into a 5



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self-supporting wafer (1.33 cm2) under 5 MPa pressure and degassed in the pressure-tight cell at 400 °C for 2 h in a dynamic vacuum (10-6 - 10-7mbar). For NH3 adsorption, the wafer was cooled down to room temperature and saturated adsorbed by NH3 for 20 min. The IR spectra of the wafers collected at room temperature after desorption of NH3 at 100 °C for 1 h under the dynamic vacuum (10-6 - 10-7 mbar). For pyridine adsorption, the wafer was cooled down to room temperature and saturated adsorbed by pyridine for 20 min. The IR spectra of the wafers collected at room temperature after desorption of pyridine at 200 °C for 1 h under the dynamic vacuum (10-6 - 10-7 mbar). 27

Al,

29

Si and

31

P MAS NMR spectra were obtained on a Bruker AVANCE III

400 WB spectrometer at a resonance frequency of 104.2, 79.5 and 161.9 MHz, respectively. Chemical shift was referenced to 1.0 M Al(NO3)3 for 2,2-dimethyl-2-ilapentance-5-sulfonate sodium (DSS) for for

27

Al,

29

Si and 85 wt.% H3PO4

31

P and the spinning rates of the samples at the magic angle were 10, 4 and 6

kHz for 27Al, 29Si and 31P, respectively. 2.3 Catalytic performance in MTO reactions The methanol to olefins (MTO) reactions was performed in a fixed bed reactor with a continuous-flow system at atmospheric pressure. The SAPO-34 catalyst (1.0 g, 40-60 mesh) was activated at 450 °C in a N2 flow of 30 ml min-1 for 2 h and then the reactor temperature was adjusted to 400 °C. During the reaction, nitrogen was used as inert carrier gas (30 ml min-1) and absolute methanol was fed and vaporized at 120 °C, which gave the 6


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weight hourly space velocity (WHSV) was 1 h-1. The reaction products were analyzed using an online gas chromatograph (SHIMADZU GC-2014C), equipped with a flame ionization detector (FID) and Plot-Q column (CB-Plot-Q, 50 m× 530 µm × 20 µm).

3. Results and Discussion 3.1 Textural properties of parent and hierarchical SAPO-34 The X-ray diffraction patterns shown in Figure 1 revealed that the two samples all had typical SAPO-34 characteristic diffraction peaks and no impurity peaks appeared before and after citric acid treatment. Furthermore, the peaks intensity of sample SAPO-34-H decreased when comparing with sample SAPO-34-P, suggesting some crystallinity loss.

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


SAPO-34-H

SAPO-34-P

0

10

20

30

40

50

2 Theta (Deg.)

Figure 1. The X-ray diffraction patterns of sample SAPO-34-P and sample SAPO-34-H 7



The textural data of parent SAPO-34-P and hierarchical SAPO-34-H were collected by nitrogen physical absorption method and mercury intrusion method. In Figure 2, the N2 adsorption/desorption isotherm of sample SAPO-34-P was almost horizontal and it is a standard type Ι isotherm typical of zeolitic materials, which indicated the parent SAPO-34-P zeolite was microporous structure. Meanwhile, sample SAPO-34-H exhibits obvious hysteresis in the N2 adsorption/desorption isotherm, which indicated the existence of mesopores in sample SAPO-34-H. Furthermore, the shape of hysteresis revealed the mesopore was slit shape and the size of mesopores was uniform. The pore size distribution curve of sample SAPO-34-H showed the mesopore diameter was around 30nm. The results of N2 adsorption and desorption analysis are summarized in Table 1. The micropore volume of the parent SAPO-34-P (0.26 cm3 g-1) indicates a high crystallinity. The citric acid treated sample SAPO-34-H shows a lower micropore volume (0.19 cm3 g-1), i.e. a 24 % loss. This decrease is in good agreement with the crystallinity loss detected by XRD method. The micropore volume decrease is coupled with an increase in external surface area (2 to 33 m2 g-1) and in mesopore volume (0.01 to 0.08 cm3 g-1). Mercury intrusion method was used to analysis the macropore volume of parent and hierarchical SAPO-34 zeolites. Apparently, the mercury intrusion cumulative pore volume (50-500 nm pore size range) increased from 0.07 cm3 g-1 to 0.61 cm3 g-1, which indicates an increase in macropore volume after citric acid treatment.

8


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200 SAPO-34-P SAPO-34-H

150

3

Quantity Adsorbed (cm /g STP)

0.10

100 3

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


50

0.05

0.00 1

10

100

Pore Diameter (nm)

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/Po)

Figure 2. The N2 adsorption/desorption isotherms and pore size distribution curves of sample SAPO-34-P and sample SAPO-34-H

Table 1. Textural properties of sample SAPO-34-P and sample SAPO-34-H

a

SBET,

Smicro,

Sext,

Vtotal,

Vmicro,

Vmeso,

VHga,

m2 g-1

m2 g-1

m2 g-1

cm3 g-1

cm3 g-1

cm3 g-1

cm3 g-1

SAPO-34-P

529

527

2

0.27

0.26

0.01

0.07

SAPO-34-H

415

382

33

0.27

0.19

0.08

0.61

Mercury intrusion cumulative pore volume (50 ~ 500 nm pore size range).

The nitrogen absorption/desorption results clearly showed that citric acid treatment introduces slit shape mesopore into zeolite SAPO-34 and it was supported by the TEM analysis. Figure 3a clearly revealed the crystal of sample SAPO-34-P was uniformly pervious to light and no structure defects appeared in the interior of the crystal. The 9



transparency of crystal in sample SAPO-34-H, shown in Figure 3b, was uneven because of the existence of hierarchical pore structure produced from the citric acid treatment. It’s interesting that the mesopores in sample SAPO-34-H were a series of parallel straight channels, which start from the crystal surfaces and end at the central of crystal (Figure 3b).

Figure 3. The TEM images of sample SAPO-34-P (a) and sample SAPO-34-H (b)

3.2 Chemical compositions and acidity properties of parent and hierarchical SAPO-34 Table 2 listed the bulk compositions and acidity data of sample SAPO-34-P and sample SAPO-34-H. The bulk compositions got from XRF analyses show that the hierarchical SAPO-34-H zeolite possess less silicon compared to the parent microporous SAPO-34-P sample, where the Si/(Al+Si+P) ratio reduce from 0.10 (SAPO-34-P) to 0.06 (SAPO-34-H).

10


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Table 2. The chemical compositions and acidity data of sample SAPO-34-P and sample SAPO-34-H Total acid amountsb , mmol g-1

Surface acid amountsc , mmol g-1

Brønsted

Lewis

Brønsted

Lewis

acid

acid

acid

acid

Chemical Sample compositions

a

Total

Total

a

SAPO-34-P

Al0.54Si0.10P0.36O2

2.398

0.083

2.481

0.007

0.000

0.007

SAPO-34-H

Al0.56Si0.06P0.38O2

1.228

0.590

1.818

0.019

0.023

0.034

XRF method, b NH3-IR method. c Py-IR method

The acidity situation of the zeolites was investigated in more detail by FT-IR spectra of adsorbed NH3 and pyridine. The NH3-IR data showed in table 2 indicated that the total acid amount decreased after citric acid treatment, which was consistent with the NH3-TPD results (shown in Fig. 4), and the main reason of this decrease was the 49% loss of Brønsted acid. At the same time, it’s obvious that the Lewis acid amount of hierarchical SAPO-34-H sample was higher than the parent sample SAPO-34-P. The size of pyridine is larger than the 8-ring diameter of CHA structure, thus the IR spectra of adsorbed pyridine (Table 2) related to the acid site on the external surface of SAPO-34 zeolites. The surface acid amount of SAPO-34-P is negligible, on contrary, such sites can be probed for SAPO-34-H because of the higher external surface area and the less diffusion limitation springs from the introduction of hierarchical pores.

11



169 389 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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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167

SAPO-34-P

348

SAPO-34-H

100

200

300

400

500

600

Temperature (℃ )

Figure 4. The NH3-TPD profiles of sample SAPO-34-P and sample SAPO-34-H

3.3 Selectivity of silicon extraction during citric acid treatment As we know, acid treatment would lead to dealumination. However, the XRF results of sample SAPO-34-P and SAPO-34-H indicated the silicon content obviously decreased after citric acid treatment. In order to investigate this peculiar phenomenon, we compared the MAS NMR spectrum and OH-IR spectra of sample SAPO-34-P and SAPO-34-H. Solid state

27

Al,

29

Si and

31

P MAS NMR spectra were presented in Figure 5. In

27

Al

MAS NMR, the well resolved signal at 38 ppm is assigned to tetrahedrally coordinated framework aluminum atoms. A signal at -10 ppm clearly appeared in samples SAPO-34-H, which attributed to octahedrally coordinated extra framework aluminum species, and this species was the main source of the Lewis acid site. In addition, a weak signal at 15 ppm, due to pentacoordinated aluminum atoms, is observed.23 The 29Si MAS NMR spectrum of parent and hierarchical SAPO-34 zeolites exhibit signals ranging from 12


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-90 to -100 ppm correspond to Si(OSi)n(OAl)(4-n) (n=0-2) species respectively.24 It is surprisingly that the signal at -95 ppm attributed to Si(OSi)1(OAl)3 species almost disappeared after citric acid treatment and the intensity of signal at -100 ppm attributed to Si(OSi)2(OAl)2 species in sample SAPO-34-H was relatively increased, that is to say, the Si(OSi)1(OAl)3 species were selectively extracted. The change in

31

P MAS NMR

spectrum was less pronounced. All samples only exhibit well resolved signal at about -30 ppm attributed to tetrahedrally coordinated phosphorus atoms bound to four aluminum atoms, which meant no extra framework phosphorus species generation during the citric acid treatment process.25

38 (a) 15

-10

SAPO-34-H

SAPO-34-P

80

60

40

20

0

-20

-40

Chemical shift (ppm)

13


-60


-90

(b)

-95

-100

SAPO-34-H

SAPO-34-P

-40

-50

-60

-70

-80

-90

-100

-110

-120

-130

Chemical shift / ppm

-30 (c)

SAPO-34-H

SAPO-34-P

40

20

0

-20

-40

Chemical shift (ppm)

Figure 5. The

27

Al (a),

29

Si (b) and

31

P (c) MAS NMR spectra of sample

SAPO-34-P and sample SAPO-34-H

The OH infrared spectra were illustrated in Figure 6. The two peaks at 3600 and 3625 cm-1 were attributed to the T-OH-T groups in the hexagonal prisms and ellipsoidal cavities, respectively.26 The decrease of the peak intensity was corresponding to the decrease of Brønsted acid amount of sample SAPO-34-H. Especially, a peak at 3676 cm-1 generated in sample SAPO-34-H, this peak was attributed to the terminal P-OH groups23 which came from the fracture of P-O-Al bond.

14


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3625 3600

SAPO-34-P

3676 SAPO-34-H

3750

3700

3650

3600

3550

3500

Wave number / cm-1

Figure 6. OH infrared spectra of sample SAPO-34-P and SAPO-34-H

As showed before, extra framework alumina species and terminal P-OH groups generated, and Si(OSi)1(OAl)3 species disappeared after citric acid treatment, which indicated the citric acid tend to attack the special domain of P-Al-Si where enrich of Si(OSi)1(OAl)3 species. We deduce that this kind domain may prefer locates at the edge of the silica-alumina phase in SAPO-34 structure, thus the silica-alumina phase would fall off as the domain extraction. As comparing with other domain in the zeolite crystal, this kind of silica is concentrated but alumina not, that is why the XRF results indicated that silica content sharp decrease after citric acid treatment but alumina not. 3.4 Catalytic performance of parent and hierarchical SAPO-34 in MTO reactions Catalytic tests of methanol conversion were performed in a fixed bed reactor at 400 °C over the parent and hierarchical SAPO-34 catalysts. Fig. 7 shows the methanol conversions versus time-on-stream (TOS) over the SAPO-34 catalysts. The corresponding data including lifetime and selectivity are shown in Table 3. The methanol 15



conversion variation with TOS shows that all catalysts convert the methanol feed completely at the initial stage of the reaction, but the lifetime of the catalysts changes evidently over the hierarchical SAPO-34-H catalysts and the parent one. The methanol conversion over the hierarchical SAPO-34-H catalysts exhibits remarkably enhanced catalyst lifetime, and the lifetime of SAPO-34-H is 720 min, which is much longer than 300 min for the parent SAPO-34-P. The fast deactivation of the conventional SAPO-34-P can be explained by the coke formation that easily occurs near the external surface of the catalyst particles, and then block the diffusion path to the inner core of the catalysts.27 In contrast, the hierarchical SAPO-34-H with meso/macrochannels can greatly enhance the diffusion path inside the crystals, thus inhibit the catalyst deactivation. At the same time, as shown in Table 3, the lower hydrogen transfer index (HTI, C3H8/C3H6) and coking rate (Rcoke, coke content/reaction time) indicated the hierarchical SAPO-34-H with lower acid amount can decrease the secondary transformation of olefin product and retard the coke formation and thus prolong the catalyst lifetime.11 Meanwhile, the selectivity of ethylene, propylene and the sum of the hierarchical SAPO-34-H (35.11%, 39.66% and 74.77%) were much higher than the parent one, which gave 31.81%, 35.03% and 66.84%, respectively, which can be contributed to the decrease of the acidity.28

16


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100

Methanol conversion, %

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


SAPO-34-P SAPO-34-H

80

60 0

100

200

300

400

500

600

700

800

900

Time on stream, min

Figure 7. Methanol conversion variation with time-on-stream over sample SAPO-34-P and sample SAPO-34-H. Experimental conditions: WHSV = 1 h-1, T = 400 °C, catalyst weight = 1 g

Table 3. The life time a and the products distribution b of sample SAPO-34-P and SAPO-34-H in MTO reactions Life time,

C 2=,

C 3=,

C 2= + C 3=,

HTI,

Rcokec,

min

wt.%

wt.%

wt.%

C3H8/C3H6

mg min

SAPO-34-P

300

31.81

35.03

66.84

0.255

0.636

SAPO-34-H

720

35.11

39.66

74.77

0.003

0.254

Sample -1

a

Life time is defined as the reaction duration with >99 % methanol conversion.

b

Based on the average selectivity of ethene and propene under >99 % methanol conversion.

c

Rcoke = coke amount/reaction time, and the coke content was determined using a TG and DTA

analyzer up to 800 °C measured after the MTO reaction.

17



4. Conclusions Hierarchical pore structure was introduced into SAPO-34 crystals and the acidity of zeolite SAPO-34 was adjusted, and thus the MTO reactions performance was enhanced via citric acid treatment. N2 adsorption/desorption and mercury intrusion data showed the mesopore and macropore volume increased from 0.01 and 0.07 cm3 g-1 to 0.08 and 0.61 cm3 g-1, respectively. TEM images revealed the mesopores were slit shape, and started from the crystal surfaces then ended at the central of crystal. MAS NMR, XRF and OH-IR results indicated the citric acid treatment tend to attack the special domain of P-Al-Si where enrich of Si(OSi)1(OAl)3 species, and thus result in selective desilication of this kind of silica in the framework of SAPO-34. NH3-IR and pyridine-IR characterization indicated that total Lewis acid amount and the Brønsted acid amount on the external surface of SAPO-34 zeolites obviously increased although the total acid amount decreased. Consequently, the hierarchical SAPO-34 samples exhibit superior catalytic performance in methanol to olefins reactions with longer lifetime (720 min vs. 300 min) and nearly 8% improvement of selectivity of light olefins (C2H4 + C3H6) than the parent SAPO-34-P.

Acknowledgements This work was supported by National Natural Science Foundation of China (U1462202, 21776304), National Key R&D Program of China (2017YFB0306602), and PetroChina.

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Selective Desilication, Mesopores Formation, and MTO Reaction

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