Conclusions Results

Methanol aromatization (MTA) is developing due to foreseen shortage of aromatics by shale gas revolution. HZSM-5 is omnivorous for aromatics synthesis because of its zeotype and solid acidity. Reducing diffusion limitation and incorporating dehydrogenation promoter (e.g., Ga or Zn) in HZSM-5 can enhance aromatics yields: the former reduce aromatics cracking, and the latter improve alkanes/alkenes dehydrogenation to aromatics. This study intends to investigate the synergy of alkaline-treatment (desilication) and Ga doping in HZSM-5 for methanol aromatization.

Conclusions Results

Background

This study was supported by Taiwan’s Ministry of Science and Technology (projects 104-2628-E-006-009-MY2) and the Ministry of Economic Affairs (The Development and Application of High Value-added Chemicals project).

Related publications

 Lai, P.-C., Chen, C.-H., Hsu, H.-Y., Lee, C.-H. Lee, Lin, Y.-C.*

Methanol aromatization over Ga-doped desilicated HZSM-5.

RSC Adv 2016, 6 (71), 67361-67371.

 Lai, P.-C., Chen, C.-H., Lee, C.-H. Lee, Lin, Y.-C.* Methanol conversion to aromatics over Ga-supported HZSM-5 with evolved meso- and microporosities by desilication.

ChemistrySelect 2016, 1 (20) 6335-6344.

400 450 500 0

1 2 3 4 5

400 450 500 0

1 2 3 4 5

T ( oC ) T ( oC ⁾

T ⁽ oC ⁾ T ( oC ⁾

T ⁽ oC ⁾

T ( oC )

T ( oC )

T ( oC ⁾ T ( oC )

CH 4/ %

T ( oC )

CH 4/ %

400 450 500 14

21 28

C 2H 4/ % C 2H 4/ %

R AT-0.01 AT-0.05 AT-0.1 AT-0.2

400 450 500 10

20 30 40

C 3H 6/ % C 3H 6/ %

400 450 500 5

10 15 20 25

is o b u ta n e/ % is o b u ta n e/ %

400 450 500 20

30 40 50 60

ar om at ics/ % ar om at ics/ %

400 450 500 10

15 20 25 30

Ga/R Ga/AT-0.01 Ga/AT-0.05 Ga/AT-0.1 Ga/AT-0.2

400 450 500 10

20 30 40

400 450 500 5

10 15 20 25

400 450 500 20

30 40 50 60

200 300 400 500 600 700 800 900

10% 19%

32% 12%

71%

56%

37% 45% 18%

33% 48% 19%

58% 21%

Ga/AT-0.2 Ga/AT-0.1 Ga/AT-0.05 Ga/AT-0.01

Int ens it y ( a .u. )

T ^{( o} C ⁾

Ga/R

21%

(GaO) ⁺ -Brønsted site

10 20 30 40 50 10 20 30 40 50

96%

95%

100%

78%

70%

71%

89%

102%

102%

R Ga/R AT-0.01

AT-0.05 AT-0.1

In ten s it y (a .u .)

2 Theta

AT-0.2 Ga/AT-0.2

Ga/AT-0.1

Ga/AT-0.05

Ga/AT-0.01 100%

2 Theta

100 200 300 400 500 600 700

AT-0.2 (0.56/0.15)

AT-0.1 (0.67/0.18)

AT-0.05 (0.69/0.19)

Bronsted

Int ens it y (a .u. )

T(

^o

C)

Lewis

AT-0.01 (0.68/0.19)

R

(0.57/0.16)

Ga/AT-0.2 (0.54/0.14)

Ga/AT-0.1 (0.54/0.16) Ga/AT-0.05 (0.63/0.18) Ga/AT-0.01

(0.65/0.18)

Ga/R (0.57/0.16)

XRD patterns

NH ₃ -TPD profiles

Porosities of tested catalysts

TPR profiles for Ga-doped catalyst

Initial selectivities of methanol aromatization Each catalyst still maintains its MFI structure

after desilication. However, for high alkaline concentration-treated ZSM-5 catalysts, their low crystallinities suggested destruction, at least partially, ZSM-5 structure.

Both of Ga-free and Ga-doped catalysts have maximum of NH ₃ uptake amount at AT-0.01 and AT0.05. The desilication can enhances both the Lewis and Brønsted acid strength. A small hump at approximately 600 ^o C was discovered for Ga/AT-0.1 and Ga/AT-0.2. It is likely from desorption of strong Lewis bound NH ₃ on Ga ₂ O ₃ .

With enhancing alkalinity in desilication, surface area of Ga-free and Ga-doped catalysts first increase from R and Ga/R to AT-0.1 and Ga/AT-0.05, then decrease to AT-0.2 and Ga/AT-0.2, respectively. The increase of surface area is growth of mesopore with slightly changed micropore from R to AT-0.1, and from Ga/R to Ga/AT-0.05. The sudden drops reflect a partial collapse of microporous architecture.

S

_BET

[m

²

g

^-1

]

S

_micro

[m

²

g

^-1

]

S

_meso

[m

²

g

^-1

]

V

_total

[cm

³

g

^-1

]

V

_micro

[cm

³

g

^-1

]

V

_meso

[cm

³

g

^-1

]

R 395 300 95 0.21 0.12 0.09

AT-0.01 396 275 116 0.25 0.12 0.13

AT-0.05 425 293 132 0.28 0.12 0.16

AT-0.1 444 291 153 0.33 0.12 0.21

AT-0.2 409 177 232 0.52 0.09 0.43

Ga/R 344 255 89 0.22 0.12 0.10

Ga/AT-0.01 366 252 114 0.23 0.12 0.11

Ga/AT-0.05 409 269 140 0.29 0.12 0.17

Ga/AT-0.1 383 247 146 0.31 0.11 0.20

Ga/AT-0.2 338 182 156 0.47 0.07 0.40

The three reduction peaks from low to high temperatures small Ga ₂ O ₃ , (GaO) ⁺ -Brønsted and large Ga ₂ O ₃ , respectively. The growing mesopores promote formation of (GaO) ⁺ - Brønsted sites due to reduced diffusion limitation of Ga precursor.

For Ga-free catalysts, the sharp decrease of aromatics at 500 ^o C for R and AT-0.01 suggest that aromatics dealkylation prevailed. Catalysts with higher mesoporosity, which allowing heavy aromatics to diffuse out, didn’t follow this tendency. All Ga-doped catalysts had higher aromatics than those of Ga-free counterparts. This indicates that Ga incorporation promotes aromatization by consuming C ₃ ⁺ olefins and C ₄ ⁺ alkanes. Ga/AT-0.05 has highest aromatics selectivity (60.1%) among all catalyst at 500 ^o C.

1) Improved mesoporosity not only reduces the transfer resistance of gallium precursor in ZSM-5 but increases the concentration of (GaO) ⁺ phase.

2) The quantity of coupled (GaO) ⁺ -Brønsted site depends heavily on the desilication condition. Severe alkaline treatment greatly raises mesoporosity but erodes Brønsted acid.

3) It can be concluded that the using moderate alkalinity (NaOH=0.05) in Si extraction with 1 wt% Ga impregnation for HZSM-5(Si/Al=11.5) can obtain maximal aromatics selectivity (60.1%) at 500 ^o C.

Acknowledgements and references

Ga-supported HZSM-5 with Evolved Meso- and Microporosities by Desilication for Methanol Aromatization

Yu-Chuan Lin* and Po-Chen Lai

Department of Chemical Engineering, National Cheng Kung University Tainan 70101, Taiwan

E-mail: [email protected]