Methane dehydroaromatization over Mo/HZSM-5 catalysts in the absence of oxygen: effects of...

Methane dehydroaromatization over Mo/HZSM-5 catalysts:

The reactivity of MoCx species formed from MoOx associated

and non-associated with Bronsted acid sites

Hongmei Liu, Wenjie Shen, Xinhe Bao, Yide Xu *

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,

457 Zhongshan Road, P.O. Box 110, Dalian 116023, PR China

Received 8 April 2005; received in revised form 29 July 2005; accepted 4 August 2005

Abstract

The catalytic performances of methane dehydroaromatization (MDA) under non-oxidative conditions over 6 wt.% Mo/HZSM-5 catalysts

calcined for different durations of time at 773 K have been investigated in combination with ex situ 1H MAS NMR characterization.

Prolongation of the calcination time at 773 K is in favor of the diffusion of the Mo species on the external surface and the migration of Mo

species into the channels, resulting in a further decrease in the number of Bronsted acid sites, while causing only a slight change in the Mo

contents of the bulk and in the framework structure of the HZSM-5 zeolite. The MoOx species associated and non-associated with the

Bronsted acid sites can be estimated quantitatively based on the 1H MAS NMR measurements as well as on the assumption of a stoichiometry

ratio of 1:1 between the Mo species and the Bronsted acid sites. Calcining the 6 wt.% Mo/HZSM-5 catalyst at 773 K for 18 h can cause the

MoOx species to associate with the Bronsted acid sites, while a 6 wt.% Mo/SiO2 sample can be taken as a catalyst in which all MoOx species

are non-associated with the Bronsted acid sites. The TOF data at different times on stream on the 6 wt.% Mo/HZSM-5 catalyst calcined at

773 K for 18 h and on the 6 wt.% Mo/SiO2 catalyst reveal that the MoCx species formed from MoOx associated with the Bronsted acid sites are

more active and stable than those formed from MoOx non-associated with the Bronsted acid sites. An analysis of the TPO profiles recorded on

the used 6 wt.% Mo/HZSM-5 catalysts calcined for different durations of time combined with the TGA measurements also reveals that the

more of the MoCx species formed from MoOx species associated with the Bronsted acid sites, the lower the amount of coke that will be

deposited on it. The decrease of the coke amount is mainly due to a decrease in the coke burnt-off at high temperature.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Methane dehydroaromatization; Mo/HZSM-5; 1H MAS NMR; Bronsted acid sites

www.elsevier.com/locate/apcata

Applied Catalysis A: General 295 (2005) 79–88

1. Introduction

As one of the promising routes for the direct

conversion of methane into high value-added chemicals,

methane dehydroaromatization (MDA) in the absence of

gas-phase oxygen has received considerable attention

recently [1–5]. Up to now, the Mo/HZSM-5 catalyst has

been proved to be the best one among the tested catalysts;

it shows a methane conversion of ca. 10% and a selectivity

to aromatics of ca. 70–80% at 973 K and a methane space

* Corresponding author. Tel.: +86 411 4379189; fax: +86 411 4694447.

E-mail address: [email protected] (Y. Xu).

0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2005.08.011

velocity of around 1500 ml/g h [6–13]. It is well accepted

that the Mo/HZSM-5 is a bifunctional catalyst. The MoCx

species are created via the reduction of MoO3 by CH4 in

the early stage of the reaction and are regarded as active

sites responsible for methane dehydrogenation and

oligomerization into C2Hy species (y < 4). Meanwhile,

the Bronsted acid sites of the HZSM-5 zeolite are

responsible for aromatization of the C2 species [7–13].

Since either ethylene or ethane aromatization proceeds

easily in the temperature range 573–873 K on HZSM-5 or

transition metal (Zn, and/or Ga) modified HZSM-5 [14–

19], such a bifunctional description of the Mo/HZSM-5

catalysts also suggests that methane dehydrogenation and

H. Liu et al. / Applied Catalysis A: General 295 (2005) 79–8880

dimerization on the MoCx species are rate-determining

steps.

However, more detailed descriptions of the bifunction-

ality of the Mo/HZSM-5 catalyst are missing, or still under

debate. Since ammonium heptamolybdate (AHM) is

generally used as the starting material, most of the Mo

species are supposed to locate on the external surface during

impregnation, while part of the Mo species would migrate

into the channels during calcination. Therefore, during the

early research stage of this topic, the Mo species were

physically divided into two types, i.e., Mo species located on

the external surface and the ones residing in the zeolite

channels. Some of the researchers believed that the Mo

species located on the external surface were active species

for the MDA reaction [20–32].

The chemical nature of the Mo species and their

location, as well as the interaction between the Mo species

and the Bronsted acid sites are important areas of

research, and have attracted many researchers to deal with

them. Recently, several research groups have reported that

MoOx species migrating into the channels can replace the

Bronsted acid sites and anchor there. Iglesia and

coworkers have demonstrated that the Mo species located

on the external surface are less active and less stable in the

MDA reactions performed over their Mo/HZSM-5

catalysts prepared by solid-state reaction between

MoO3 powder and HZSM-5 zeolite [33]. Su et al. have

also pointed out that the Bronsted acid sites are the driving

force for the MoOx species to migrate into the channels

during the calcination stage [34]. If the Si/Al ratio of the

HZSM-5 used is as high as 125, i.e. if the number of

Bronsted acid sites per unit cell is less than 0.76, the

MoOx species cannot migrate into the channels. There-

fore, the catalytic activity of the Mo/HZSM-5 catalyst

prepared with such a high Si/Al ratio of the HZSM-5 is poor.

Moreover, based on the results obtained on Mo-based and

steam-treated HZSM-5 catalysts, Bao and Lin et al. found

that if the HZSM-5 zeolite having a Si/Al ratio of 25 was

pretreated by steam (designated as HZSM-5(ST)), then the

Mo/HZSM-5(ST) catalysts so prepared were more active

and stable [35,36]. The authors suggested that the more the

Mo species migrate into the channels, the fewer the

remaining free Bronsted acid sites are left over, and the

better the reactivity and stability of the Mo/HZSM-5

catalysts. Recently, Xu and Bao et al. have also reported that

post-steam-treatment of the Mo/HZSM-5 catalyst is

another effective approach to improve the reactivity and

stability of the Mo/HZSM-5 catalysts [37,38]. The

improvements are attributed to the fact that there are more

Mo species migrating into the channels on one hand, and

part of the Bronsted acid sites are eliminated on the other

hand.

With the above-mentioned results and problems in mind,

we have considered recently that the MoCx species formed

from MoOx closely associated with the Bronsted acid sites

may play a more important role in the MDA reaction. This

idea also stimulated us to think that it may be more

reasonable to chemically distinguish the MoCx species on

the Mo/HZSM-5 catalysts into two types: one is formed

from MoOx closely associated with the Bronsted acid sites

and the other is formed from MoOx non-associated with

the Bronsted acid sites. Moreover, if we could find a way

to get more MoOx species to migrate into the channels, at

the same time no obvious changes would happen to the

framework of the HZSM-5 zeolite, we may have some

quantitative data to characterize the difference in catalytic

behaviors of the MoCx species formed from MoOx closely

associated with the Bronsted acid sites and those formed

from MoOx non-associated with the Bronsted acid sites.

In order to enhance the diffusion and/or migration of

the MoOx species after they have been impregnated onto

the HZSM-5 zeolite, the calcination temperature and the

calcination duration of time are two crucial parameters. It

has been reported that calcination of the Mo/HZSM-5

catalysts at 773 K is a suitable choice, as the extraction of

the framework Al by MoOx species to form inactive

Al2(MoO4)3 species and the sublimation of the MoOx

species are negligible at this temperature [31,32]. There-

fore, most researchers have prepared the Mo/HZSM-5

catalysts by impregnation, while employing a calcination

time of 4–6 h at 773 K. Since the diffusion and/or

migration of the MoOx species and their trapping by the

Bronsted acid sites are both relatively slow processes, we

considered that 4–6 h may not be long enough to get as

many MoOx species as possible to associate with the

Bronsted acid sites. On the other hand, it is reasonable to

consider that a 6 wt.% Mo/SiO2 sample can serve as a

catalyst in which all Mo species are non-associated with

the Bronsted acid sites.

In this work, the 1H MAS NMR technique was used to

characterize the interaction between the MoOx species and

the Bronsted acid sites, as well as to quantitatively

distinguish the MoOx species into two types i.e., MoOx

species associated and non-associated with the Bronsted

acid sites on 6 wt.% Mo/HZSM-5 catalysts calcined at

773 K for different durations of time. The results were

further correlated with their catalytic behaviors in MDA

and with the TPO results, so as to get deeper and more

detailed insights into the nature of the MoCx species

formed from MoOx associated and those formed from

MoOx non-associated with the Bronsted acid sites.

2. Experimental

2.1. Sample preparation

The HZSM-5 zeolite with a Si/Al ratio of 25 was supplied

by Nankai University (Tianjin, China), and the relevant

structural data have been reported previously [39–41]. Mo/

HZSM-5 catalysts having a Mo loading of 6 wt.% were

prepared by the conventional impregnation method as

H. Liu et al. / Applied Catalysis A: General 295 (2005) 79–88 81

described in Ref. [1]. In brief, the HZSM-5 zeolite was

impregnated with aqueous solutions containing given

amounts of ammonium heptamolybdate (AHM). The

samples were first dried at room temperature for 12 h,

then dried at 393 K for 2 h and calcined in air at 773 K for a

desired period of time. After calcination, the samples were

crushed and sieved to granules of 40–60 meshes for

catalytic evaluation. The 6 wt.% Mo/HZSM-5 catalysts are

hereafter denoted as 6Mo/HZSM-5(t), where 6 is the

nominal Mo content in weight percent and t is the

calcination time in hours. For comparison, a 6Mo/SiO2

catalyst was prepared with the same procedure. The SiO2

was provided by Qingdao Yinhai Chemical Co. Ltd.

(Qingdao, China).

2.2. Catalyst characterization

Surface areas and micropore volumes of the zeolites and

catalysts were measured by the BET method on a

Micromeritics ASAP-2000 instrument, basing on adsorption

isotherms at 77 K, and using 0.162 nm2 for the cross-

sectional area of the nitrogen molecules. All of the samples

were outgassed at 573 K for 2 h before measurements. The

data were processed and analyzed by a computer system.

FT-IR spectra were recorded at room temperature on a

Fourier transform infrared spectrometer (Nicolet Impact

410) with a resolution of 4 cm�1 and 64 scans in the region

from 4000 to 400 cm�1. Before the measurements, the

samples were diluted with KBr (ca. 1 wt.% of the samples

used) and pressed into wafers.

X-ray diffraction (XRD) patterns were obtained in air on

a Rigaku 200B diffractometer using Cu Ka radiation

(l = 1.5418 A) at room temperature, with instrumental

settings of 40 kV and 50 mA. Powder diffractograms of the

samples were recorded over a range of 2u values from 5 to

508 at a scanning rate of 58/min. The relative crystallinities

of the samples were calculated basing on the intensity of the

peaks of angle 2u = 22–258 in the XRD pattern [42].

X-ray fluorescence spectroscopy (XRF) experiments were

performed on a Philips MagiX X-ray Fluorescence spectro-

meter, and an IQ+ quantitative software was employed for the

elemental analysis of the 6Mo/HZSM-5 catalysts calcined in

air at 773 K for different durations of time.

All NMR spectra were recorded at room temperature on a

Bruker DRX-400 spectrometer with a BBO MAS probe and

using 4 mm ZrO2 rotors, as reported in our previous studies

[29,37,38]. 29Si MAS NMR spectra were collected at

79.5 MHz using a 0.8 ms (p/4) pulse with a 4 s recycle delay

and 1024 scans. 1H MAS NMR spectra were recorded at

400.1 MHz using a 1 ms (p/10) pulse with a 4 s recycle delay

and 200 scans. The chemical shifts were referenced to a

saturated aqueous solution of 4,4-dimethyl-4-silapentane

sulfonate sodium (DSS). Prior to the 1H MAS NMR

experiments, the samples were first dehydrated at 673 K for

20 h on a homemade apparatus for removing the water

adsorbed, and then were put into the NMR rotors for

measurement without exposing them to air. The Bruker

software WINNMR was employed for deconvolution, using

fitted Gaussian–Lorentzian line shapes. The 1H MAS NMR

technique allows us to measure the concentration of different

types of surface hydroxyl groups, since the intensity of the

corresponding 1H MAS NMR signals is directly propor-

tional to the concentration of the resonating nuclei [43,44].

The number of Bronsted acid sites per unit cell was

estimated based on the framework Si/Al ratio measured by29Si MAS NMR spectra and the chemical formula of the unit

cell of the HZSM-5 zeolite [45]. The number of Bronsted

acid sites per unit cell of the 6Mo/HZSM-5 catalysts after

calcinations for different durations of time was estimated by

comparing the peak areas of the 1H MAS NMR spectra with

those of the corresponding parent zeolite.

2.3. Catalytic evaluation

Catalyst evaluations were carried out in a fixed-bed reactor

at 973 K and atmospheric pressure, as reported in our previous

studies [6]. Briefly, ca. 0.5 g of the catalyst was charged into a

10.0 mm i.d. quartz tubular reactor. Catalytic reactions were

usually performed at 973 K at a space velocity of 1500 ml/

gcat h. Online analysis of the effluent was performed with a

Varian Star CP-3800 gas chromatograph using the Varian Star

5.5 data handling software. Ten percent N2 was added to the

methane feed as an internal standard. Therefore, the methane

conversion, the selectivity of hydrocarbon products and the

coke formation could be evaluated according to the carbon

mass balance. The depletion rate of methane and all product

formation rates, such as the formation rates of BTX aromatics

and the formation rate of naphthalene, were based on the

methane molecules converted, and are expressed in milli-

moles of methane per gram of catalyst per second. The TOF

data were further calculated on the results of the catalytic

evaluations, and the 1H MAS NMR characterization as well as

the XRF measurements.

2.4. Characterization of carbonaceous deposits

TGA profiles were recorded on a Perkin-Elmer TG 1700

instrument. The used catalyst of ca. 0.02 g was heated in an

air stream (30 ml/min) from 313 to 1023 K at a heating rate

of 10 K/min. The temperature-programmed oxidation

(TPO) measurements were carried out in a U-shaped quartz

tubular micro-flow reactor. Each used catalyst of 0.1 g was

heated in a He stream to remove the adsorbed water, and

flushed with a mixture stream of 10% O2/He (30 ml/min) at

room temperature for 1 h. Then, TPO was conducted from

room temperature to 1073 K in a mixture stream at a heating

rate of 20 K/min. The products were detected and analyzed

with a Balzers QMS-200 online multi-channel quadruple

mass spectrometer. During the temperature ramp, the

evolved species were monitored at m/e = 28 (CO), 32

(O2) and 44 (CO2), respectively. The data from TGA and

TPO were calculated and analyzed by the methods described


in our previous paper [46,47]. Briefly, the data at m/e = 28

recorded originally was corrected by subtracting the

contribution from the CO2 signal to get the data of CO.

Then, the data of CO was multiplied by its response

parameter, and was added to the data of CO2 to get the data

of total carbon oxides.

3. Results

3.1. BET, XRF, XRD, FT-IR and 29Si MAS NMR

measurements

The surface areas and micropore volumes of the HZSM-

5 zeolite and the 6Mo/HZSM-5(t) catalysts calcined for

different durations of time are listed in Table 1. As expected,

introduction of the Mo species on/into the HZSM-5 zeolite

led to an obvious decrease in both the BET surface areas

(from 342 to 284 m2/g) and micropore volumes (from 0.17

to 0.15 cm3/g). In coincidence with these, the relative

crystallinity of the zeolite also decreased to ca. 80%. On the

other hand, regardless of the loading of a 6 wt.% Mo species

on the HZSM-5 zeolite, no peaks characteristic of the MoO3

crystallites were observed in the XRD pattern of each 6Mo/

HZSM-5(t) catalyst, indicating that the crystallite size of

the MoO3 species on the zeolite surface was smaller than

5 nm, as reported previously [20,28].

The bulk Si/Al ratios and the actual weight content of Mo

in each sample were analyzed by the XRF technique, and the

results are also listed in Table 1. It can be seen that the Mo

contents of the 6Mo/HZSM-5(t) catalysts decreased slightly

(from 5.7 to 5.4 wt.%) with the prolonging of the calcination

time, and the Mo/Si ratios in the bulk of all the samples were

about 0.04, indicating that negligible sublimation occurred

during the process of calcination at 773 K.

The shape and position of all the structure-sensitive

bands in the IR spectra of the HZSM-5 zeolite and the five

samples of the 6Mo/HZSM-5 catalysts calcined for various

durations of time are similar, suggesting that the funda-

mental framework structures were not obviously affected by

the loading of the 6 wt.% Mo species and by the subsequent

calcination at 773 K, even when calcination lasted as long as

Table 1

Results of BET, XRF and XRD measurements

Samples Surface

areaa (m2/g)

Micropore

volumea (cm3/g)

Mo

cont

HZSM-5 342 0.17 –

Mo/HZSM-5(3 h) 284 0.15 5.72

Mo/HZSM-5(6 h) 281 0.15 5.68

Mo/HZSM-5(12 h) 273 0.15 5.59

Mo/HZSM-5(18 h) 272 0.16 5.40

Mo/HZSM-5(30 h) 287 0.15 5.42

a Based on the date of BET measurements.b Based on XRF results.c Based on XRD data.

30 h. 29Si MAS NMR measurements on the 6Mo/HZSM-5

catalysts calcined for 3 and 18 h also indicated that the

framework Si/Al ratios are 23.5 and 23.3, respectively, as

compared with the framework Si/Al ratio of the parent

HZSM-5 zeolite of 22.2, confirming that a long duration

calcination at 773 K did not cause a serious change the Si/Al

ratio of the zeolite framework.

3.2. 1H MAS NMR measurements

The 1H MAS NMR spectra of the HZSM-5 and the 6Mo/

HZSM-5(t) catalysts are shown in Fig. 1. As illustrated,

there are five types of characteristic resonance lines

appeared after deconvoluting the 1H MAS NMR spectrum

of the dehydrated parent HZSM-5 zeolite. The high-field

signal with a chemical shift of 1.7 ppm can be ascribed to

external Si–OH groups, while the second peak at

d = 2.4 ppm is associated with extraframework Al–OH

[48–51]. The low-field signal at d = 3.8 ppm belongs to the

bridging OH groups in the form of free Al–OH–Si groups

locating at the intersections of the channels of the zeolites,

which are the so-called free Bronsted acid sites [48–52]. And

the broad resonance peak at about 6.0 ppm can be due to

another kind of Bronsted acid sites, which are affected by

additional electrostatic interaction of the oxygen atoms in

the zeolite framework, as described in Refs. [43,53]. The

resonance signal of the water adsorbed on Lewis acid sites

exhibited a chemical shift of 4.7 ppm [54].

The variations in the number of different kinds of

hydroxyl groups per unit cell on the calcined 6Mo/HZSM-

5(t) catalysts are listed in Table 2. The number of hydroxyl

groups per unit cell in the parent zeolite was calculated from

the corresponding unit cell composition [43]. The number of

hydroxyl groups per unit cell of the 6Mo/HZSM-5(t)

catalysts after calcination was estimated by comparing the

peak areas of the 1H MAS NMR spectra with the

corresponding parent zeolite. Iglesia and co-workers have

investigated the location of the supported Mo species on the

Mo/HZSM-5 catalyst; they found that the Mo species would

interact with and replace the Bronsted acid sites of the

HZSM-5 zeolite with a stoichiometry of 1:1, as illustrated in

Scheme 1 [55–58].

entb (wt.%)

Mo/Si

in bulkb

Si/Al

in bulkb

Relative

crystallinityc (%)

– 23.4 100

0.04 23.5 81

0.04 23.1 82

0.04 23.2 83

0.04 23.4 79

0.04 23.1 80


Fig. 1. 1H spin–echo MAS NMR spectra of parent HZSM-5 zeolite and 6Mo/HZSM-5 catalysts calcined for different durations of time: (a) parent HZSM-5; (b)

6Mo/HZSM-5 calcined for 3 h; (c) 6Mo/HZSM-5 calcined for 6 h; (d) 6Mo/HZSM-5 calcined for 12 h; (e) 6Mo/HZSM-5 calcined for 18 h; (f) 6Mo/HZSM-5

calcined for 30 h.

Following the suggestion proposed by Iglesia and

coworkers, one can estimate the amount of Mo species

associated and non-associated with the Bronsted acid sites

from the change in the Bronsted acid sites; and the

calculation results are also shown in Table 2. It is interesting

to notice that, with the 6Mo/HZSM-5 catalyst calcined at

Table 2

Results of 1H MAS NMR experiments on 6Mo/HZSM-5 catalysts calcined at 77

Sample The number of hydroxyls per unit cell

B2

(6.0)

Water

(4.7)

B1

(3.8)

Al–OH

(2.4)

Si–OH

(1.7)

HZSM-5 1.9 1.9 2.1 0.50 0.54

Mo/HZSM-5(3 h) 0.6 0.3 0.7 0.25 0.08

Mo/HZSM-5(6 h) 0.5 0.2 0.7 0.22 0.08

Mo/HZSM-5(12 h) 0.5 0.3 0.6 0.23 0.07

Mo/HZSM-5(18 h) 0.2 0.2 0.4 0.14 0.03

Mo/HZSM-5(30 h) 0.2 0.2 0.4 0.15 0.03

a The number of Bronsted acid sites per unit cell = B1 + B2; the number of Bro

unit cell formula on the basis of the Si/Al ratio, which is measured by 29Si MAb The decrement in the number of Bronsted acid sites per unit cell of the Mo/HZ

the amount of Mo species associated with the Bronsted acid sites can be calculate

replaces one Bronsted acid site, as suggested in Refs. [55–58].c The amount of Mo species non-associated with the Bronsted acid sites can be

Mo species associated with the Bronsted acid sites.

773 K for 3 h, the Mo species associated with the Bronsted

acid sites are about 80% of all the Mo species, and become

ca. 100% for the 6Mo/HZSM-5 catalyst calcined at 773 K

for 18 h. When the calcination time was further increased to

30 h, there was almost no increase in the amount of the Mo

species associated with the Bronsted acid sites. On the other

3 K for different durations of time

Number of B

acid sites per u.c.aMo associated with

B acid sitesb

(mmol/gcat)

Mo non-associated

with B acid sitesc

(mmol/gcat)

4.0 – –

1.3 0.48 0.12

1.2 0.49 0.11

1.1 0.51 0.08

0.6 0.57 0

0.6 0.56 0

nsted acid sites per unit cell of the HZSM-5 zeolite was calculated from the

S NMR.

SM-5(t) catalysts is due to the MoOx species migrating into the channels, so

d based on the 1H MAS NMR measurements, assuming that one Mo atom

calculated based on the total Mo content listed in Table 1 and the amount of


Scheme 1.

hand, there are ca. 0.6 of the Bronsted acid sites per unit cell

on the 6Mo/HZSM-5 catalysts calcined at 773 K for 18 and

30 h. The remaining Bronsted acid sites may serve as the

active sites for aromatization of the C2 intermediate species.

3.3. Catalytic evaluation of MDA over the 6Mo/HZSM-

5 catalysts

The catalytic performances of the 6Mo/HZSM-5(t)

catalysts at 973 K and 1500 ml/gcat h are shown in

Fig. 2. Catalytic performances of the 6Mo/HZSM-5 catalysts calcined for differen

calcined for 6 h; (!) for the sample calcined for 12 h; (& ) for the sample calc

Fig. 2. Fig. 2a illustrates that there is no remarkable

difference in the depletion rate of methane among the five

Mo/HZSM-5(t) catalysts in the initial period of the reaction.

Meanwhile, for all these samples, the ability to activate

methane molecules decreases with the increase of reaction

time, and the decrements of different samples are distinct

and depend on the calcination time. The depletion rate of

methane over the 6Mo/HZSM-5(3 h) catalyst decreased

from 2.7 � 10�3 to 1.4 � 10�3 mmol/g s after running the

reaction for 10 h. With the prolonging of the calcination

t durations of time ((*) for the sample calcined for 3 h; (~) for the sample

ined for 18 h and ($) for the sample calcined for 30 h).


time, the catalytic reactivity of the 6Mo/HZSM-5 catalyst

increased. The 6Mo/HZSM-5(18 h) catalyst displayed the

highest stability among all the samples of this series, and the

depletion rate of methane decreased from 2.7 � 10�3 to

1.7 � 10�3 mmol/g s after 10 h on stream. No further

improvement could be detected over the 6Mo/HZSM-

5(30 h) catalyst. Meanwhile, the formation rate of mono-

cyclic aromatics conformed to the same trend of the

depletion of methane. During a 10 h reaction, the formation

rate of BTX on the 6Mo/HZSM-5(3 h) sample decreased

from 1.3 � 10�3 to 0.8 � 10�3 mmol/g s. However, over the

6Mo/HZSM-5(18 h) or the 6Mo/HZSM-5(30 h) catalyst, the

formation rate of BTX decreased from 1.5 � 10�3 to

1.3 � 10�3 mmol/g s when the reaction had run on stream

for 10 h.

3.4. TPO and TGA characterization of the

carbonaceous deposits

Fig. 3 shows the corresponding TPO profiles (after

correction) of the coked 6Mo/HZSM-5(t) catalysts after

running the reaction for 10 h. For all the coked catalysts

with different calcination times, each profile presents two

peaks, with the peak temperatures at about 741 and 816 K,

respectively. The profiles of total carbon oxides were

deconvoluted (only the deconvolution results of the TPO

profile for 6Mo/HZSM-5(3 h) sample are illustrated in

Fig. 3), and the peak temperatures on the TPO profiles as

well as the corresponding amounts of coke formed on the

five 6Mo/HZSM-5(t) catalysts calcined for different

durations of time were estimated; the results are listed in

Fig. 3. TPO profiles recorded from the coked 6Mo/HZSM-5 catalysts

calcined for different durations of time.

Table 3. The total amount of coke decreases with the

prolongation of the calcination time, and the changes

mainly depend on the decrement of the carbonaceous

deposits, which are burnt-off at a high temperature (at

816 K). However, the amount of coke burnt-off at a low

temperature (at 741 K) is independent of the calcination

time, and the values show little change for the five used

catalyst samples.

4. Discussion

4.1. The location of the MoOx species after calcination

Iglesia and coworkers have studied the location of the

Mo6+ species on the 4 wt.% Mo/HZSM-5 prepared by solid-

state reaction method, and have found that, after calcination

at 773 K under specified conditions, all of the Mo species

would replace the Bronsted acid sites of the HZSM-5 zeolite

with a stoichiometry of 1:1, and would exist in the channels

of the HZSM-5 zeolite [55–58]. A kind of Mo2O52+ dimer

species was assumed to exist for the interaction between the

Mo species and the Bronsted acid sites.

Recent studies on the MDA reaction over Mo/HZSM-5

catalysts prepared by an impregnation method have revealed

that the Mo species migrating into and residing in the

channels are more effective for the reaction, and that only a

small fraction of the Bronsted acid sites is required to

accomplish the aromatization [31,32]. Excessive free

Bronsted acid sites will cause severe carbonaceous deposits

under non-oxidative conditions at a temperature as high as

973 K [35,36].

Moreover, it was reported that the reducibilities of

different Mo species located at various positions on/in the

HZSM-5 zeolite are not all the same, i.e., the MoOx species

which are non-associated with the Bronsted acid sites can be

fully and easily reduced into Mo2C by methane, while the

Mo species associated with the Bronsted acid sites can only

be partially reduced by CH4 [24].

Our present results of the 1H MAS NMR experiments

demonstrated again that the introduction of 6 wt.% of

molybdenum species could cause a significant reduction

in the amount of the Bronsted acid sites (Table 2). For the

6Mo/HZSM-5(3 h) catalyst, the concentration of the

Bronsted acid sites decreased to 32% of that of the

parent HZSM-5. With the prolonging of the calcination

time, the Bronsted acid sites remaining on the 6Mo/

HZSM-5(t) catalyst decreased gradually. In the 6Mo/

HZSM-5(18 h) catalyst, only 15% of the original Bronsted

acid sites were left over (the number of Bronsted acid sites

per unit cell was ca. 0.6). Apparently, most of the Mo

species are located on the Bronsted acid sites after the

sample is calcined at 773 K, and the amount of this kind

of Mo species increases with the prolonging of calcination

time from 3 to 18 h. Therefore, it is reasonable to

distinguish the Mo species into two types, i.e., Mo species


Table 3

Peak temperatures of the TPO profiles and the amount of coke formed on the used 6Mo/HZSM-5(t) catalysts after 10 h of reaction

Sample Peak temperature (K) The amount of coke (mmol/gcat)

Low temperature High temperature Low temperature High temperature Totala

6Mo/HZSM-5(3 h) 740 816 1.3 1.8 3.1

6Mo/HZSM-5(6 h) 741 816 1.3 1.7 3.0

6Mo/HZSM-5(12 h) 741 814 1.3 1.5 2.8

6Mo/HZSM-5(18 h) 742 816 1.3 1.3 2.6

6Mo/HZSM-5(30 h) 742 816 1.3 1.3 2.6

a Measured by TGA.

associated and non-associated with the Bronsted acid

sites.

4.2. The reactivity of MoCx species formed from the

MoOx species associated and non-associated with

Bronsted acid sites

The TOF values of the Mo species associated with the

Bronsted acid sites referring to the 6Mo/HZSM-5(18 h)

catalysts and the Mo species non-associated with the

Bronsted acid sites referring to the 6Mo/SiO2 catalyst were

calculated, based on the results of catalytic evaluations and

of the 1H MAS NMR experiments, assuming that the

stoichiometry of the Mo species interacting with the

Bronsted acid sites is 1:1. The variation in TOF values

for methane depletion with time on stream on the Mo

species associated and non-associated with the Bronsted

acid sites is shown in Table 4. At the first 30 min on

stream time, the TOF of the Mo species associated with

the Bronsted acid sites was about 17.1, while it was about

12.0 for the Mo species non-associated with the Bronsted

acid sites. After running the reaction for 10 h, the TOF

was about 10.8 for the Mo species associated with

Bronsted acid sites, and about 1.2 for the Mo species non-

associated with Bronsted acid sites. Therefore, it is clear

that the Mo species associated with the Bronsted acid sites

are more active and more stable than the Mo species non-

associated with the Bronsted acid sites, indicating that the

former Mo species play a more important role in the MDA

reaction.

The experimental results of the depletion rate of methane

on the 6Mo/HZSM-5 catalysts calcined at 773 K for

different durations of time were further compared with

Table 4

TOF (CH4/Mo h�1) values of Mo species associated and non-associated with the

Catalyst/active sites Time on s

30

6Mo/HZSM-5(18 h)/Mo species associated with B acid sitesb 17.1

6Mo/SiO2/Mo species non-associated with B acid sitesc 12.0

a The calculation is based on the assumption that the activation and dehydroge

MDA reaction.b All of the Mo species are associated and no Mo species is non-associated wi

shown in Table 2.c All of the Mo species are non-associated with the Bronsted acid sites on 6M

the results calculated on the basis of the TOF values listed in

Table 4. The two series of data on the 6Mo/HZSM-5

catalysts calcined at 773 K for different durations of time fit

very well with each other, as listed in Table 5. This also

demonstrates the reasonableness of our previous suggestion

that the 6Mo/SiO2 catalyst can be recognized as a model of

the catalyst in which all of the Mo species are non-associated

with the Bronsted acid sites.

4.3. The chemical nature of coke formed during the

MDA reaction

Lunsford and coworkers [59] have characterized the

surface carbon formed during the conversion of CH4 to C6H6

over Mo/HZSM-5 catalysts by XPS. Three different types of

surface carbon species, denoted as species A, B, and C,

respectively, were identified. Species A is mainly present in

the zeolite channel system; Species B is due to carbidic-like

carbon in Mo2C and is mainly located on the outer surface of

the zeolite. Species C is a hydrogen-poor sp type or a pre-

graphitic type of carbon which mainly covers the Mo2C

species.

Our early 13C NMR experiments also showed that

carbon species did form on Mo and Bronsted acid sites,

respectively [28]. Ichikawa and coworkers attributed the

low-temperature peak to a reactive coke associated with

Mo2C and the high-temperature one to irreversible or inert

coke [60]. In our continuing studies on the carbonaceous

deposits from the TPO profiles of the 6Mo/TiO2 and the

6Mo/MCM-22 after MDA reaction at 973 K for 3 h

[61,62], we found that the 6Mo/MCM-22 gave a doubled

peak located at 742 and 830 K, respectively. Meanwhile,

only one peak corresponding to the carbonaceous deposits

Bronsted acid sites at different on-stream timesa

tream (min)

60 120 240 360 480 600

15.8 14.4 12.5 11.6 11.1 10.8

10.5 7.5 3.8 2.3 1.5 1.2

nation of methane on Mo carbide species is the rate-determining step in the

th the Bronsted acid sites in the case of the 6Mo/HZSM-5(18 h) catalyst as

o/SiO2 catalyst.


Table 5

Experimental and calculated depletion rates of methane on the 6Mo/HZSM-

5 catalysts calcined for different durations of time at 773 K (10�3 mmol/g s)

Catalyst Time on stream (min)

30 60 120 240 360 480 600

6Mo/HZSM-5(3 h)

Experimental valuea 2.74 2.35 2.05 1.82 1.62 1.53 1.37

Calculated valueb 2.68 2.46 2.17 1.79 1.62 1.53 1.48

6Mo/HZSM-5(6 h)



6Mo/HZSM-5(12 h)



a The experimental depletion rates of methane on the 6Mo/HZSM-5(t)

catalysts are from the results of catalytic evaluations (see Fig. 2a).b The calculated depletion rates of methane on the 6Mo/HZSM-5(t)

catalysts are based on the amounts of Mo species associated and non-

associated with Bronsted acid sites (see the data of the 8th and 9th columns

in Table 2) and the TOF values listed in Table 4.

on molybdenum carbide at 772 K was found, since the

6Mo/TiO2 catalyst has no Bronsted acid sites. Therefore,

the present TPO results of the 6Mo/HZSM-5(t) catalysts

may be ascribed to the fact that the coke burnt-off at a

lower temperature (ca. 741 K) was mainly located on the

MoCx species, while the coke burnt-off at a higher

temperature (ca. 816 K) was associated with the Bronsted

acid sites on the HZSM-5 zeolite.

It is interesting to notice that the amount of coke at low

temperature is almost the same, whereas the amount of coke

at high temperature decreased with the calcination durations

of time, as shown in Table 3. This also gives us a clue that the

Bronsted acid sites can provide active sites not only for the

formation of aromatics products, but also for the deposition

of carbonaceous species, which is a crucial factor leading to

the deactivation of the Mo/HZSM-5 catalysts. Obviously, it

is most important to control the number of available

Bronsted acid sites in order to obtain a good catalyst for the

MDA reaction.

5. Conclusion

Prolonging of the calcination time at 773 K is in favor

of the diffusion and migration of the Mo species from the

external surface of the 6 wt.% Mo/HZSM-5 catalyst into

the channels. It results in a further decrease in the number

of Bronsted acid sites per unit cell, as measured by the 1H

MAS NMR, but only causes a slight change in the Mo

content of the bulk as well as in the framework structure

of the HZSM-5 zeolite. The 1H MAS NMR technique can

quantitatively distinguish the Mo species into those

associated with and those non-associated with the

Bronsted acid sites on 6 wt.% Mo/HZSM-5 catalysts.

The MoCx species formed from MoOx species associated

with the Bronsted acid sites are more active and stable

than those formed from MoOx species non-associated with

the Bronsted acid sites under the MDA reaction

conditions.

Acknowledgments

Financial supports from the Ministry of Science and

Technology of China under the contract G1999022406, from

the National Natural Science Foundation of China under the

contract 20473086 and from the BP-CAS (China) Joint

Center are gratefully acknowledged.

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