Methane dehydroaromatization on Mo/ZSM-5 catalysts: structure of active sites and

carbonaceous deposits during catalytic cycle

E.V. Matus1, Z.R. Ismagilov1, V.I. Zaikovskii1, L.T. Tsikoza1, N.T. Vasenin1, O.B. Sukhova1, I.Z. Ismagilov1,2, M.A. Kerzhentsev1, V. Keller2, N. Keller2, F. Garin2

1 Boreskov Institute of Catalysis SB RAS, Novosibirsk, Russia 2 LMSPC, Laboratoire des Matériaux, Surfaces et Procédés pour la Catalyse,Strasbourg, France

EUROPACAT – Vll«CATALYSIS: A KEY TO REACHER AND CLEANER SOCIETY»

28 August – 1 September, 2005, Sofia Bulgaria

6СН4 ⇒С6Н6 + 9Н2 bifunctional catalyst

MoOx => Mo2C (or MoOyCz)

2CH4 => C2H6 => C2H4 on formed Mo2C (or MoOyCz),

3C2H4 => C6H6, etc. on HZSM-5

equilibrium CH4 conversion to benzene under non-oxidativeconditions at 720°C is about 12%.

Methane dehydroaromatization (DHA) over Mo/ZSM-5 catalysts- a new perspective environmentally-friendly way to obtain both

valuable aromatics and hydrogen

CH4

- What is the nanostructure and localization of the active molybdenum species ?

- What is the nature of unwanted carbonaceous deposits formed during the reaction and how to minimize their formation ?

- Reaction mechanism

- How to make the DHA of methane a commercially applicable process ?

Methane dehydroaromatization (DHA) over Mo/ZSM-5 catalysts:main questions

Approach - systematic study of the state of Mo at all stages of Mo/ZSM-5 preparation and catalytic cycle:

- selection of molybdenum precursor

- preparation of impregnation solutions

- catalyst after treatment in air at 110oC, 500oC and 700oC

- catalyst after pretreatment in Ar at 720oC

- catalyst after the CH4 DHA reaction at 720oC

- past reaction catalyst after treatment in O2 at 600oC

depending on Si/Al ratio in the parent H-ZSM-5 and on Mo content

Preparation of Мо/ZSM-5 catalysts Method of incipient wetness impregnation of zeolite H-ZSM-5 by solution of ammonium heptamolybdate ((NH4)6Mo7O24*4H2O, AHM)at controlled value of solution pH

• concentration of AHM solution• pH of AHM solution• Si/Al atomic ratio of

H-ZSM-5 zeolite

VARIATION of CONDITIONS

impregnation thermal treatment• temperature • composition of medium (air, Ar)

⇓ ⇓

⇓

PHYSICO-CHEMICAL PROPERTIES and ACTIVITY ofМo/ZSM-5 CATALYSTS

?

Мо/ZSM-5 catalyst

Investigation of Mo/ZSM-5 catalysts by group of methods

N2 physisorptionXRD

HRTEM, EDXDTA

XPS

ESR

Drying,Calcination- 120 -700oС,- O2, Ar

Reaction DHA of СН4(t h, %СН4, ToC)

Oxygen treatmentafter reaction

PreparationpH, С, Si/Al

TPO, TPH

TPR

Laboratory flow setup for CH4 DHA

Test conditions: load = 1.0 cm3 (0.6 g); fraction = 0.25-0.5 mmflow = 13.5 cm3/min; GHSV = 810 h-1

fumehood

toFID

torefe-re-nce

toTCD

FID TCD

com-pressedair

fumehood

NaX

14b

23

SKT

Chro-matonN

17a 17b GC(165 C)

GC(room T)

Catalytic activity of Мо/ZSM-5 catalysts (Si/Al=17)in DHA of methane: dependence on Mo content

0 2 4 6 8 100

2

4

6

8

10

Molybdenum Content, wt. %M

etha

ne C

onve

rsio

n to

Ben

zene

, vol

. %

The maximum catalytic activity (CH4 conversion ~ 14%) and benzene formation selectivity (70%) were observed for the samples with 2-5% Mo.

0 50 100 150 200 250 300 350 4000

2

4

6

8

10

0.6 % Mo

10% Mo

1% Mo

5% Mo

2% Mo

Met

hane

Con

vers

ion

to B

enze

ne, v

ol. %

Reaction Time, min

0

2

4

6

8

10

12

C10H8C7H8C2H6C2H4

COC6H6

H2

conc

entra

tion,

vol

. %

0,00

0,05

0,10

0,15

0,20

0,25 C10H8

C7H8

C2H6

C2H4

CO

conc

entra

tion,

vol

. %

Composition of gaseous reaction products (2% Мо/ZSM-5 catalyst (Si/Al=17))

The benzene and hydrogen are main products.

Catalytic activity of 2% Мо/ZSM-5 catalysts in DHA of methane: dependence on Si/Al ratio

The activity and selectivity increase with Si/Al change from 45 to 17.

0 50 100 150 200 250 300 350 4000

5

10

15

20

25

30

35Si/Al ratio in the parent H-ZSM-5.Si/Al 17 30 45 - total conversion - conversion to benzene

Conv

ersi

on, v

ol.%

Reaction Time, min. 0 50 100 150 200 250 300 350 400

0

20

40

60

80

100Si/Al=17

Si/Al=30

Si/Al=45Benz

ene

Sele

ctiv

ity, %

Reaction Time, min.

Catalytic activity of 2% Мо/ZSM-5 catalyst (Si/Al=17)in DHA of methane: stability

The oxygen treatment with following methane re-admission leads to recovery of catalyst performance.

0 5 10 15 20 25 30 35 40 45 50 55 60 650123456789

101112

O2, 600oC/2 h

Reaction Time, hM

etha

ne C

onve

rsio

n to

Ben

zene

, vol

. %

0 2 4 6 8 10 12 14 16 18 20 22 2402468

101214161820

- total convercion - conversion to benzene

Reaction Time, h

Met

hane

Con

vers

ion,

vol

. %

The catalyst performance is decreasing with time on methane stream.

XRD

2θ

(NH4)6Mo7O24*2H2O/ZSM-5 ⇒ MoO3/ZSM-5 ⇒ Мо9О26 or Мо4О11/ZSM-5

Мо9О26 or Мо4О11/ZSM-5 ⇐ η-MoC or η-Mo3C2 /ZSM-5

О2/500оС Ar/720oC

CH4/720оСО2/520оС

0 10 20 30 40 50

intencity of main zeolite lines after O2 treatment is decreasedhalo at 18-28o

halo at 35-42ohalo at 22-25o

halo at 18-28o

**

*

O2/520oC/2h

CH4/720oC/6h

Ar/720oC/1hO2/500oC/4hO2/110oC/4h

10%

Mo/

ZSM

-5

Si/A

l=17

initi

alco

mpo

unds

H/ZSM-5

MoO3

(NH4)6Mo7O24*4H2O

*

degrees

HRTEM (2% Мо/ZSM-5)

During the reaction, particles of β-Mo2Cwith sizes 2-15 nm are forming on the ZSM-5 surface.

The surface of Mo2C particle is covered by graphite layers with thickness of ~ 2 nm.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 170

10

20

30

40

50

rela

tive

num

ber,

%

size of particle, nm

β-Мо2С

С, graphite

CН4/720oC/6hAr/720oC/1чSurfaceМо-O-clusters1-5 nm

EDX

The particle number density on a flat projection of zeolite crystalgrows with an increase of crystalthickness, and these particles areabsent on the side projections of zeolite crystal.

During the reaction, Mo-containing clusters with sizes ~ 1 nm are forming in the ZSM-5 channels.

HRTEM (10% Мо/ZSM-5 , CН4/720oC/6h)

HRTEM (2% Мо/ZSM-5, CН4/720oC)

The content of friable carbonaceousdeposits is increasing with timeon stream. 5 nm

During the reaction, carbonaceousdeposits are forming on the ZSM-5surface in the form of friabledisordered layer with thicknessup to 3 nm.

after 6 h of reaction

after 20 h of reaction

The treatment of the Мо/ZSM-5 catalyst in oxygen stream at 520oC for 2 hleads to formation of Mo-O-contained clusters on zeolite surface similar tothose in parent Mo/ZSM-5 catalysts.

HRTEM (2% Мо/ZSM-5, О2/520oC/2h)

150 200 250 300 350 400 450 500 550 600 650

0.5h

6h

-1%

-3%

500oC

450oC

Temperature, oC

150 200 250 300 350 400 450 500 550 600 650

+2%

+3.5%

0.5h

6h

-1%

-1.5%

445oC

290oC

Temperature, oC

СxHy +(x+y/4)O2 = xCO 2 + y/2H2O (weight loss)

Mo2C (204 g/mol) + 4 O2 = 2 MoO3 (288 g/mol) + CO2 (weight gain)

For the 2 and 10% Mo/ZSM-5 catalysts and reaction duration between 0.5 and 6 h, carbonaceous deposits in the catalysts with Si/Al = 17 are characterized by asingle maximum of the exothermal burn-out effect. The content of carbonaceousdeposits and extent of their condensation degree (C/H ratio) are increasing with time-on-stream.

DTA

2% Mo 10% Mo

The Mo-containing clusters localized in zeolite channels can be the active centers for DHA of CH4.

С, graphite

β-Мо2С

Conclusions:

1. The maximum values of Mo/ZSM-5 catalytic activity (total CH4 conversion (14%) and C6H6 formation selectivity (70%) are achieved at Mo content 2-5% and are increasing upon the lowering of zeolite Si/Al ratio from 45 to 17.

2. Using the HRTEM it was shown, that during the reaction molybdenum carbide β-Mo2C is formed on the ZSM-5 surface, and it is characterized by the lattice parameters d002 = 0.235 nm, d400 = 0.26 nm and the particle size of 2-15 nm. It was also demonstrated, that during the reaction Mo-containing clusters with sizes ~ 1 nm are forming in the ZSM-5 channels.

3. In the course of reaction, the carbonaceous deposits are formed both on the surface of Mo2C particles (in the form of graphite layers with lattice parameter d002 = 0.35 nm and thickness of approximately 2 nm) and on the zeolite surface (in the form of friable disordered layer with thickness up to 3 nm).

4. The content of carbonaceous deposits and extent of their condensation (C/H ratio) are increasing with time-on-stream.

5. The oxygen treatment with following methane re-admission leads to recovery of catalyst performance.

Acknowledgements:

The authors are grateful to Dr. V.A. Ushakov for the XRD

measurements and discussion, Dr. G.S. Litvak for the DTA

measurements.