Effect of Addition of K, Rh and Fe Over Mo/HZSM-5 on Methane Dehydroaromatization Under Non-oxidative Conditions

Article abstract of DOI:10.1007/s10562-019-02697-8

Methane dehydroaromatization was studied over a series of K, Rh and Fe promoted 10?wt% Mo/HZSM-5 catalysts with different promoter loadings of 0.5, 1 and 1.5?wt% at 750?°C in a recirculating batch reactor. All the catalysts were reduced in H₂ at 750?°C prior to methane activation. K, Rh and Fe- promoted Mo/HZSM-5 catalysts were prepared by sequential impregnation. N-propylamine-temperature programmed desorption confirmed the significant modification in the acidity of the catalyst upon addition of K. Compared to 10?wt% Mo/HZSM-5, the conversion of CH₄ remained nearly unchanged for 1?wt% K-promoted catalyst but decreased by ~ 46% for 1?wt% Rh promoted catalyst and by ~ 4.3% for Fe-promoted catalyst after 255?min of reaction. The conversion of CH₄ further decreased with increase in K and Rh loading but increased with increase in Fe loading. Compared to Rh and Fe-promoted catalysts, K-promoted catalyst exhibited better selectivity for C₆H₆ after 255?min of reaction. The temperature programmed oxidation results revealed that K promoted catalyst significantly reduced coking. 1?wt% K added to?10?wt% Mo/HZSM-5 exhibited optimum performance, where the conversion of CH₄ was ~ 28%, selectivity of C₆H₆ was ~ 50% while the selectivity of carbon was ~ 47% after 255?min of reaction. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02697-8

Catalysis Letters
https://doi.org/10.1007/s10562-019-02697-8
Effect of Addition of K, Rh and Fe Over Mo/HZSM-5 on Methane
Dehydroaromatization Under Non-oxidative Conditions
1
1
1
1
Vaidheeshwar Ramasubramanian · Daniel J. Lienhard · Hema Ramsurn · Geoffrey L. Price
Received: 28 November 2018 / Accepted: 31 January 2019
©
Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
Methane dehydroaromatization was studied over a series of K, Rh and Fe promoted 10 wt% Mo/HZSM-5 catalysts with
different promoter loadings of 0.5, 1 and 1.5 wt% at 750 °C in a recirculating batch reactor. All the catalysts were reduced
in H at 750 °C prior to methane activation. K, Rh and Fe- promoted Mo/HZSM-5 catalysts were prepared by sequential
2
impregnation. N-propylamine-temperature programmed desorption confirmed the significant modification in the acidity of
the catalyst upon addition of K. Compared to 10 wt% Mo/HZSM-5, the conversion of CH remained nearly unchanged for
4
1
wt% K-promoted catalyst but decreased by ~46% for 1 wt% Rh promoted catalyst and by ~4.3% for Fe-promoted catalyst
after 255 min of reaction. The conversion of CH further decreased with increase in K and Rh loading but increased with
4
increase in Fe loading. Compared to Rh and Fe-promoted catalysts, K-promoted catalyst exhibited better selectivity for C H
6
6
after 255 min of reaction. The temperature programmed oxidation results revealed that K promoted catalyst significantly
reduced coking. 1 wt% K added to 10 wt% Mo/HZSM-5 exhibited optimum performance, where the conversion of CH was
4
~
28%, selectivity of C H was ~50% while the selectivity of carbon was ~47% after 255 min of reaction.
6 6
Graphical Abstract
Keywords Methane dehydroaromatization · Promoters · K-Mo/HZSM-5 · H pretreatment · β−Mo C
2
2
*
Hema Ramsurn
hema-ramsurn@utulsa.edu
1
Russell School of Chemical Engineering, The University
of Tulsa, 800 South Tucker Drive, Tulsa, OK 74104, USA
Vol.:(0
1
123456789)
3

V. Ramasubramanian et al.
1
Introduction
periodic CH4–H2 switch mode. It was found that promot-
ing metals like Fe, Co, Ru and Pd enhanced the CH con-
4
The low cost of natural gas has created an interest in its
conversion to high value chemicals and fuels. The major
component of natural gas is methane which generally con-
sists of about 70–90% by volume of the total. This meth-
ane can be converted into valuable chemicals by both
direct and indirect routes. The indirect conversion of meth-
ane by Fisher-Tropsch to methanol followed by methanol-
to-gasoline is currently a more commercially viable pro-
cess than the direct conversion of methane to aromatics
and has been practiced on large scales for decades [1, 2].
Direct methane conversion technologies include oxidative
coupling of methane, partial oxidation of methane and
methane dehydroaromatization (MDA). These processes
have barriers to commercialization including limited con-
versions, poor selectivity to desired products and catalyst
deactivation. Direct conversion of methane to aromatics
under non-oxidative conditions was first studied by Wang
et al. [3] in 1993. MDA effectively occurs over a transi-
tion-metal incorporated in a bifunctional zeolite catalyst.
Past studies investigated various metals which include Mo
version, while metals like Cu and Zn had insignificant
improvement on CH4 conversion and metals like Cr and
Mn decreased the catalyst’s activity. Their studies claimed
that the promotional effect of Fe is due to the formation of
Fe induced carbon nanotubes. During the H2 flow mode,
the presence of Fe enhanced the surface coke removal
resulting in better stability of the catalyst [27]. The same
study also concluded that the size of zeolite support is
significantly important during the catalytic activity. Fe
promoted 5% Mo/HZSM-5 (different Fe loadings of 0.3,
0.5, 1 and 2%) based on nanosized zeolites improved the
activity and stability of the catalyst while the Fe promoted
5% Mo/HZSM-5 based on microsized zeolites had negli-
gible promotional effect. The improved activity and stabil-
ity of the nanosized zeolites were due to the disagglomera-
tion of the microsized crystals caused by the growth of Fe
induced carbon nanotubes. Though the activity of the cata-
lyst was enhanced, the selectivity of benzene was lower in
Fe promoted 5% Mo/HZSM-5 on both nanosized and
microsized zeolites, compared to the unmodified 5% Mo/
HZSM-5 [36]. Sun et al. [37, 38] doped the Mo/HZSM-5
with nanosized Fe by mechanical ball milling and investi-
gated its effect on MDA and reported an increase in meth-
ane conversion upon addition of nanosized Fe particles.
Abdelsayed et al. [39] studied the promotional effect of Fe
and Zn on Mo/HZSM-5 and reported that when Fe (0.3%)
was used as a promotor, the benzene formation rate was
increased by 35% while the Zn (1%) promoted catalyst
showed a 10% increase in benzene formation rate. How-
ever, using both Zn (1%) and Fe (0.3%) simultaneously as
promotors, reduced the benzene formation by 31%. Aboul-
Gheit et al. [29, 40] replaced half the Mo concentration in
the catalyst with Zn in 6% Mo/HZSM-5 and noticed an
enhancement in benzene selectivity. Their results claimed
that promoters with higher electronegativity decreased the
selectivity of benzene and naphthalene. Their work also
included the investigation of oxygen free natural gas con-
version over 6% Mo/HZSM-5 and 3% Fe, 3% Co and 3%
Ni substituted 3% Mo/HZSM-5 prepared by mechanical
mixing. Results revealed that 6% Mo/HZSM-5 exhibited
better aromatization activity when compared to group VIII
metals promoted Mo/HZSM-5. Zeng el at [7]. studied the
promotional effect of Zn over W/HZSM-5 and Mo/
HZSM-5 prepared in the pH range of 2–3 stabilized by
H2SO4 and reported that addition of Zn (1.5%) improved
the catalyst activity and selectivity of benzene in both Mo
and W/HZSM-5 at 800 °C. Similarly, Xiong et al. [6, 41]
studied the effect of adding Zn (1.5%) on W/HZSM-5 and
reported an increase in catalyst activity and benzene selec-
tivity. Tshabalala et al. [31, 42] investigated the effect of
adding Pt and Sn to Mo/HZSM-5. Addition of Pt (0.5%)
[
4], Zn [5], W [6, 7], Re [8] and Ga [9] over HZSM-5.
MDA was also studied over Mo supported on different
zeolites like HSAPO-34 [10], HY [10], HZSM-5 [11],
HMCM-36 [12] and HMCM-49 [13]. The most active
catalysts were found to be molybdenum loaded on
HZSM-5 [4] and HMCM-22 [14] with high selectivity of
5
0–80% for benzene. Though Mo/HZSM-5 was found to
be a promising catalyst for MDA, the significant amount
of coke formation during the MDA reaction rapidly deac-
tivates the catalyst thus affecting the stability of the cata-
lyst and selectivity of desired aromatics. To overcome
catalyst coking and to improve the selectivity of desired
hydrocarbons, several techniques have been investigated
including structural modification of the surface of zeolite
support [15–17], use of composite catalyst (Mo/HZSM-5
along with Gd doped CeO ) [18], addition of gases like
2
H , H O [19], CO [20], CO [21, 22] during the CH acti-
2
2
2
4
vation, modifying the pretreatment conditions of the cata-
lyst [23], addition of promotors [24] and periodic switch-
ing between CH and H during the MDA reaction [25]. In
4
2
literature, it has been suggested that addition of a second
metal to Mo/HZSM-5 as a promotor can suppress the coke
formation and enhance the aromatic yield [26]. Among the
transition metals, Fe [27, 28] and Zn [29] are the most
commonly used promotors for Mo/HZSM-5. Apart from
these metals, other metals like Ga [30], Sn [31], Mg [32],
Ru [33, 34], Li [4] were also used as promotors for Mo/
HZSM-5 to enhance the MDA reaction. Xu et al. [35] stud-
ied the effects of transition metal promoted Mo/HZSM-5
prepared by co-impregnation on MDA at 800 °C in
1
3

Effect of Addition of K, Rh and Fe Over Mo/HZSM-5 on Methane Dehydroaromatization Under…
increased the methane conversion and aromatic selectivity
while increase in tin (0.05–0.2%) loading decreased the
methane conversion but increased the selectivity of aro-
matics. Their studies also revealed that the effect of prepa-
ration method highly influenced the aromatic and coke
selectivity and sequential impregnation of Sn followed by
Pt over Mo/HZSM-5 exhibited high benzene selectivity
and low carbon deposits. Similarly, Tan et al. [43] claimed
that Pt-Mo/HZSM-5 prepared by impregnating Pt (0.3%)
on Mo/HZSM-5 showed better activity than Pt-Mo/
HZSM-5 prepared by impregnating HZSM-5 with a mix-
ture of Pt and Mo precursors. Effects of addition of Ga to
Mo/HZSM-5 was studied by Liu et al. [30, 44] and their
results concluded that adding Ga (0.1–1%) improved the
performance of the catalyst, increased the yields of C –C
2 Experimental
2.1 Catalyst Preparation
ZSM-5 (SiO /Al O = 30) from zeolyst international was
2
2
3
used as a standard support for preparing all the cata-
lysts. Mo/HZSM-5 was synthesized by incipient wetness
impregnation. Required amount of (NH ) Mo O .4H O
4
6
7
24
2
(Ammonium Hepta Molybdate, AHM) salt (equivalent to
10 wt% Mo/HZSM-5) was dissolved in a known amount of
deionized water sufficiently enough to completely wet the
zeolite support and this aqueous AHM solution was added
drop by drop to the zeolite support. The resulting mixture
was dried at 120 °C for 5 h and calcined at 500 °C in air for
5 h. A series of K, Rh and Fe promoted Mo/HZSM-5 cata-
lysts were prepared by sequential impregnation method.
K CO .1.5H O, RhCl .3H O and Fe(NO ) .9H O were
2
11
hydrocarbons and decreased the coke. It was also observed
that Ga helps in effective adsorption of CO and CO while
2
2
3
2
3
2
3 2
2
improving alkene formation. Shu et al. [45] reported an
enhancement in catalytic activity for a 2 wt% Mo/HZSM-5
incorporated with Ru with a Mo/Ru ratio between 0.3 and
used as the precursors for K, Rh and Fe promotors respec-
tively. In this case, Mo/HZSM-5 was first prepared by fol-
lowing the above-mentioned procedure. After calcining
the catalyst at 500 °C it was cooled to room temperature
and then it was impregnated with the corresponding pro-
moting metal salt solution and finally the resulting mixture
was dried at 120 °C for 5 h and calcined at 500 °C in air
for 5 h. Finally, the catalysts were pressed and sieved to
particle size in the range of 20–40 mesh. The prepared
catalysts were denoted as x P-10 Mo/HZSM-5 where x
corresponds to the weight percent (0.5, 1, 1.5 wt%) of the
promoter (P).
0
.7. Addition of Ru decreased the acidic sites in the zeolite
and promoted the reduction of Mo species. Martínez and
Peris [46] studied the effect of replacing the acidic protons
in the zeolite with alkali and alkaline-earth cations. Com-
+
+
pared to Mo/HZSM-5, Mo loaded on alkali (Na andCs )
2
+
2+
and alkaline (Ca and Mg ) ions modified zeolite exhib-
ited less catalytic activity and enhanced benzene selectiv-
ity. In contrary, Cheng et al. [32] reported that Mg (1%)
promoted Mo/HZSM-5 prepared by impregnation method
improved both the activity of the catalyst and the selectiv-
ity of benzene. Wang et al. [47] compared the promotional
effects of W (0.5%), V (1%) and Zr (0.5–2%) over Mo/
HZSM-5 at 650 °C and reported that addition of W and Zr
increased the catalyst activity and benzene selectivity
while addition of V decreased the activity and benzene
selectivity. Kojima et al. [48] examined the effects of add-
ing noble metals like Pt and Rh (molar ratio of noble
metal/Mo = 0.2) on Mo/HZSM-5 and Mo/HMCM-22 by
co-impregnation and reported that these noble metals sup-
pressed coke formation on the Bronsted acid sites. Moreo-
ver, in the presence of Pt and Rh, α−MoC was formed
2.2 Catalytic Reaction System
The schematic representation of the batch circulation reac-
tor system is shown in Fig. 1. The working principle of
the reactor system, catalytic reduction and reaction proce-
dure and material balance considerations were explained
in detail in our previous work [49]. All the catalytic reac-
tion experiments were carried out at 750 °C and the reac-
tion products were analyzed using 5880 HP GC-FID. The
amount of carbon which is not detected by the GC-FID is
also included in the carbon balance, considering the miss-
ing carbon as “carbon” (carbon with molecular weight of
12) which includes the carbon consumed for active site
formation, dissociation of CH into CH species and the
1
−x
as the active site instead of Mo C. Though significant
2
research has been done on the addition of the promoting
metals Fe and Zn to zeolite supported Mo catalyst, results
based on addition of an alkali metal like K and noble metal
like Rh are scantily reported in literature. Thus, this work
primarily focuses on studying the individual promotional
effects of K and Rh added to Mo/HZSM-5 along with Fe
4
x
coke formed by agglomeration of heavier aromatics [30].
The conversion of methane is given by:
N −N
o
t
Conversion of methane % =
(
)
× 100where N is the
o
No
(
well-known promoter) on MDA at 750 °C and 1 atm using
initial moles of methane and N is the moles of methane
t
hydrogen as the reduction medium for the catalyst in a
recirculating batch reactor.
available at time t.
The selectivity of benzene is given by:
1
3

V. Ramasubramanian et al.
Fig. 1 Schematic diagram of
catalytic reactor system
N(B)
t
15 min. Once a clear solution was obtained, the samples
were diluted before analyzing in ICP-OES.
Selectivity of Benzene (%) = N(M) × 100where N(B) is
t
t
the total moles benzene formed at time t and N(M) is the
t
total moles of methane converted at time t.
2
.3.3 Scanning Electron Microscopy (SEM)
2
.3 Catalyst Characterization
FEI Inspect 60 scanning electron microscope was used to
study the surface morphology of the catalyst samples.
2.3.1 Brunauer–Emmett–Teller Surface Area
2
.3.4 X-ray Diffraction (XRD)
The surface area of the prepared catalysts was measured by
BET method using nitrogen as adsorbate on a Quantachrome
autosorb one unit. The degassing temperature was 300 °C for
Rigaku miniflex 600 diffractometer with PDXL software
was used to obtain X-ray diffraction patterns and perform
data analysis. The X-ray source was operated at 40 kW and
2
4 h and isotherms were measured at −196 °C.
1
5 mA (600 Watts) with Cu Kα-1 X-rays. The diffractom-
2.3.2 Inductively Coupled Plasma-Optical Emission
eter was equipped with a monochromator to reduce iron
Spectrometry (ICP-OES)
fluorescence. The samples were analyzed in the range of
◦
2
θ=10–80 by 0.04°/step with a count time of 2 s per step.
Agilent 5110 ICP-OES was used to quantify the elements
present in the catalyst. Acid digestion of the catalyst was
performed to prepare sample solution for the analysis.
Approximately 100 mg of catalyst was taken in a digestion
2.3.5 n-Propylamine–Temperature Programmed
Desorption (NPA–TPD)
vessel and trace metal grade HNO (4 ml), HCl (4 ml) and
NPA–TPD measurements were performed on Mo loaded
on HZSM-5 catalysts using a thermogravimetric analyzer
(Perkin-Elmer Pyris-1 TGA). The results will help in
3
HF (2 ml) were added. The mixture in the vessel was heated
to 180 °C at a rate of 10 °C/min and held at 180 °C for
1
3

Effect of Addition of K, Rh and Fe Over Mo/HZSM-5 on Methane Dehydroaromatization Under…
understanding the effect of Mo addition on acidic proper-
ties of the zeolite support. Approximately 20 mg of the dry
sample was placed in a Pt sample pan in the TGA and the
weight was recorded. The sample was dried at 350 °C in He
weakly adsorbed material. Weakly adsorbed material was
determined by evacuating a fully covered material and meas-
uring the uptake again at room temperature.
and cooled to 50 °C before treatment in 20% H /80% He by
2
heating to 750 °C to match the experimental condition. The
sample was then cooled to 50 °C before exposure to NPA.
Once the sample attained a constant weight, NPA supply
was stopped and the sample was purged with He for 10 min.
Finally, the sample was heated to 550 °C at a rate of 5 °C/
min in He and weight loss due to the desorption of NPA was
recorded as the function of temperature.
3 Result and Discussion
3.1 Catalyst Characterization
3.1.1 Surface Area
Table 1 shows the results of BET surface area and the pore
volume of the pure HZSM-5 compared with Mo/HZSM-5
and K, Rh and Fe-promoted Mo/HZSM-5 catalysts. The sur-
face area and the pore volume of Mo/HZSM-5 decreased
when compared to HZSM-5. High dispersion of Mo species
on the surface of HZSM-5 could possibly be the cause for
significant decrease in the surface area (Sect. 3.1.6). Surface
area and pore volume of promoted Mo/HZSM-5 shows that
addition of a promoter metal had affected the dispersion of
Mo species to some extent as the surface areas of K, Rh and
Fe-promoted catalysts were slightly higher than that of Mo/
HZSM-5. The increase in surface area of the catalyst upon
addition of Fe is in accordance with the findings Xu et al.
[36]. From Table 1, it is evident that impregnating metal spe-
cies on ZSM-5 did not significantly affect the pore volume
and average pore size (D) of the catalyst. Thus, addition of
Mo species and other promoter metals has no serious effect
on the pore structure of the zeolite.
2.3.6 Temperature Programmed Reduction (TPR)
Catalyst reduction conditions were established by perform-
ing TPR experiments using thermogravimetric analyzer
(
Perkin-Elmer Pyris-1 TGA). To dry the sample, approxi-
mately 20 mg of sample was placed in a Pt sample pan in
the TGA and heated to 350 °C in He atmosphere. Later the
sample was cooled to 50 °C and the weight was recorded.
TPR was performed in a 20% H /80% He atmosphere by
2
heating from 50 to 750 °C and the corresponding weight loss
was recorded. The temperature range where reduction took
place was used to plan the catalyst reduction procedure for
catalytic reaction studies.
2.3.7 Temperature Programmed Oxidation (TPO)
Carbon deposits in the spent catalysts were quantified by
TPO measurements using thermogravimetric analyzer (Per-
kin-Elmer Pyris-1 TGA). To dry the spent catalyst, approxi-
mately 20 mg was placed in a Pt sample pan in the TGA and
heated to 350 °C in He atmosphere. Once the drying was
complete, the sample was cooled to 50 °C and the weight
3.1.2 Inductively Coupled Plasma-Optical Emission
Spectroscopy (ICP-OES)
Table 2 compares the expected and actual metal loadings in
the catalysts analyzed using the ICP-OES. In the sequential
impregnation method where molybdenum was loaded first
followed by the promoter metal, the catalysts were calcined
twice. Thus, during the second stage calcination, sublima-
tion of Mo species in air around 500 °C is possible. This
accounts for loss of Mo and the actual Mo loading was less
compared to the expected Mo loading.
was recorded. Finally, in a 20% O /80% He atmosphere, the
2
dried sample was heated from 50 to 750 °C and the corre-
sponding weight loss was recorded.
2.3.8 CO-Chemisorption Studies
The CO-chemisorption studies were performed in a volu-
metric adsorption system. Approximately 500 g of sample
was dried under vacuum at 350 °C. The sample was then
Table 1 Physical properties of HZSM-5 based catalysts
reduced in H at 600 °C for 90 min and at 750 °C for 30 min
2
Catalyst
Surface area Pore volume Avg pore
to match the experimental pretreatment conditions. Later,
2
3
(
m /g)
(cm /g)
size (D)
(Å)
the system was evacuated to remove the H and the sample
2
was cooled to room temperature. A water bath was used
as a thermostat during the CO chemisorption. Volumetric
measurements were used to determine the amount of CO
chemisorbed in a conventional volumetric adsorption device.
The CO uptake was taken as the difference between the total
amount adsorbed at room temperature and the amount of
HZSM-5
353
237
243
247
266
0.16
0.15
0.13
0.13
0.15
30.69
30.62
30.60
30.56
30.58
1
1
1
1
0 Mo/HZSM-5
.5 K-10 Mo/HZSM-5
.5 Rh-10 Mo/HZSM-5
.5 Fe-10 Mo/HZSM-5
1
3

V. Ramasubramanian et al.
Table 2 Quantitative analysis of
Sample
Expected wt%
Actual wt%
elements in the catalysts
Mo
K
Rh
Fe
Mo
K
Rh
Fe
1
1
1
1
0 Mo/HZSM-5
10
10
10
10
–
1.5
–
–
–
1.5
–
–
–
–
1.5
8.9
8.6
8.9
9.5
–
1.3
–
–
–
1.5
–
–
–
–
1.6
.5 K–10 Mo/HZSM-5
.5 Rh–10 Mo/HZSM-5
.5 Fe–10 Mo/HZSM-5
–
–
3.1.3 X-ray Diffraction (XRD) Analysis
The crystallinity of K, Rh and Fe-promoted Mo/HZSM-5
catalysts was investigated by X-ray diffraction analysis.
Figure 2a shows the XRD patterns of the freshly prepared
ο
ο
catalysts calcined at 500 °C in air. Peaks at 2θ=12.9 , 27.4
corresponding to the ꢀ-MoO orthorhombic phase are appar-
3
ent in all the catalysts. In case of K promoted Mo/HZSM-5
catalysts, increase in K loading didn’t significantly affect
the intensity of MoO species. But in case of Rh and Fe
3
promoted catalysts with increase in Rh and Fe loadings, the
intensity of MoO decreases. Thus, the dispersion of MoO
3
3
on the zeolite has been modified with the addition of Fe and
Rh species. Figure 2b shows the XRD peak of 10 wt% Mo/
HZSM-5 catalyst reduced in H at 750 °C. The diffraction
2
ο
peak at 2θ value of 40.51 corresponds to elemental molyb-
denum phase. Thus, it was evident that the MoO species
3
were reduced to metallic Mo during the H reduction.
2
3.1.4 Temperature Programmed Reduction (TPR)
TPR is the most appropriate method to study the metal sup-
port interaction in terms of metal reduction as a function
of temperature. Figure 3 shows the weight changes of the
catalysts during microbalance reduction. Before reduction,
the catalyst samples were dried in the microbalance at about
3
50 °C in He. In case of Mo/HZSM-5 (Fig. 3, (5)), the first
reduction peak was observed at around 550 °C which cor-
responds to the reduction of MoO species to MoO spe-
3
2
cies and the second reduction peak was observed at around
7
15 °C which corresponds to the reduction of MoO to Mo
2
species which is in accordance with the findings of Tsha-
balala et al. [42]. The K and Fe–promoted Mo/HZSM-5
catalysts (Fig. 3, (6), (8)) exhibited similar reduction
trends for Mo species where reduction of MoO to MoO
3
2
2
occurred between 550 and 575 °C and reduction of MoO
to Mo occurred between 720 and 750 °C. In case of Rh–Mo/
HZSM-5 (Fig. 3, (7)) the first reduction peak was observed
in the range of 400–550 °C and the second reduction peak
was observed around 750 °C. On a quantitative basis, for a
Fig. 2 XRD patterns of a fresh catalyst (1) 10 wt% Mo/HZMS-5 (2)
0
.5 K–10 Mo/HZSM-5 (3) 1 K–10 Mo/HZSM-5 (4) 1.5 K–10 Mo/
1
0 wt% Mo/HZSM-5 catalyst, the expected relative weight
HZSM-5 (5) 0.5 Rh–10 Mo/HZSM-5 (6) 1 Rh–10 Mo/HZSM-5 (7)
1.5 Rh–10 Mo/HZSM-5 (8) 0.5 Fe–10 Mo/HZSM-5 (9) 1 Fe–10 Mo/
HZSM-5 (10) 1.5 Fe–10 Mo/HZSM-5 and b 10 wt% Mo/HZSM-5
reduced catalyst
loss (TA curve in Fig. 3) after the reduction of MoO to
3
Mo is ~5%. The experimental weight loss for 10 wt% Mo/
HZSM-5, K and Fe-promoted 10 wt% Mo/HZSM-5 (Fig. 3,
1
3

Effect of Addition of K, Rh and Fe Over Mo/HZSM-5 on Methane Dehydroaromatization Under…
Fig. 4 NPA-TPD profiles of pure HZSM-5 [(1) TA, (6) DTA],
Fig. 3 TPR profiles of 10 wt% Mo/HZSM-5 [(1) TA, (5) DTA],
.5 K–10 Mo/HZSM-5 [(2) TA, (6) DTA], 1.5 Rh–10 Mo/HZSM-5
(3) TA, (7) DTA] and 1.5 Fe–10 Mo/HZSM-5 [(4) TA, (8) DTA]
1
0 wt% Mo/HZSM-5 [(2) TA, (7) DTA], 1.5 K–10 Mo/HZMS-5 [(3)
1
TA, (8) DTA], 1.5 Rh–10 Mo/HZSM-5 [(4) TA, (9) DTA] and 1.5
Fe–10 Mo/HZSM-5 [(5) TA, (10) DTA]
[
(
1), (2), and (4)) catalysts were 4.64%, 4.18% and 4.95%
in the TA curve of HZSM-5, which is in the temperature
range 300–350 °C, is characteristic of weakly bound NPA
which were equivalent to ~ 9.37%, ~ 8.44%, ~ 10% of Mo
loading respectively. These results were consistent with the
results of ICP-OES analysis (Sect. 3.1.2). For Rh-promoted
Mo/HZSM-5 catalyst, the weight loss shown in the TA curve
+
having been desorbed and only a 1/1 NPA to H adsorbed
species (an n-propylammonium cation) remains. Desorption
of the amine from these sites results in a band observed at
about 400 °C [50]. In the TA (Fig. 4, (2)) and DTA (Fig. 4,
(7)) spectra of Mo/HZSM-5 catalysts, the plateau region is
difficult to observe as the plateau and the band at 400 °C
have shifted when compared to the pure HZSM-5. Moreover,
addition of promoters further modified the plateau region
and the intensity of the band at 400 °C. Rh (Fig. 4, (4) and
(9)) and Fe (Fig. 4, (5) and (10)) -promoted Mo/HZSM-5
catalysts showed similar trends on NPA desorption but addi-
tion of K (Fig. 4, (3), (8)) to Mo/HZSM-5 exhibited sig-
nificant changes in plateau region and the band at 400 °C.
The adsorbed NPA species on HZSM-5 almost completely
desorbed at the end of the experiment. From Fig. 4, the TA
curve (1) reached close to 100 wt%, which is the initial
weight of the sample after drying. However, each spectra
of the promoted catalyst showed a significant remnant of
undesorbed material at 520 °C at the end of the experiment,
(
Fig. 3, (3)) was only 2.9% which is equivalent to 5.85% of
Mo loading, whereas the ICP-OES result reported 8.9% of
Mo. This implies that not all the MoO species were reduced
3
to Mo. It can be postulated that either a portion of Mo spe-
cies has been covered by the Rh species or agglomeration of
MoO species occurred in the presence of Rh and hindered
3
the completed reduction of all the Mo species on the cata-
lyst as discussed in Sect. 3.1.6. Only the portion of MoO
3
reduced to MoO , were further reduced to Mo at around
2
7
50 °C as shown by two distinct peaks in the DTA curve of
Rh-promoted catalyst (Fig. 3, (7)).
3
.1.5 N-propyl Amine-Temperature Programmed
Desorption (NPA–TPD)
Mo/HZSM-5 acts as a bifunctional catalyst where the
+
Bronsted acid (H ) sites in the zeolite plays a crucial role
in MDA reaction. To assess the effect on acidic properties
of HZSM-5 on addition of metals, NPA–TPD studies were
performed. Figure 4 shows the TA (Thermal analysis) and
the corresponding, DTA (derivative) curves for catalysts
subjected to NPA-TPD studies. The TA (Fig. 4, (1)) and
DTA (Fig. 4, (6)) curves for pure HZSM-5 were similar
to the results reported by Price et al. [50–52], where the
intensity of the observed bands depends on the nature and
density of the acid sites in the zeolite. The “plateau region”
Table 3 CO uptake by Mo/HZSM-5 and promoted Mo/HMZS-5
Catalyst
CO uptake
(µmol/g)
1
1
1
1
0 Mo/HZSM-5
287.08
159.59
62.52
.5 K–10 Mo/HZSM-5
.5 Rh–10 Mo/HZSM-5
.5 Fe–10 Mo/HZSM-5
212.36
1
3

V. Ramasubramanian et al.
Fig. 5 Conversion of CH at 750 °C over 10 wt% Mo/HZSM-5 com- ▸
4
pared with promoted 10 wt% Mo/HZSM-5 with different promoter
loadings of a 0.5 wt%, b 1 wt%, c 1.5 wt%
indicative of a carbonaceous residue likely from metal cata-
lyzed polymerization of NPA as previously reported by
Price et al. [51]. This indicates that the metal species have
migrated into the zeolite lattice and modified the acidity of
the zeolite.
3.1.6 CO-chemisorption Studies
Table 3 shows the amount of CO chemisorbed by Mo/
HZSM-5 and metal promoted Mo/HZSM-5 catalysts at
room temperature. For Mo/HZSM-5, the CO uptake was
2
87.08 µmol/g and the metal dispersion (the fraction of Mo
atoms on the surface of the Mo crystallites) was 0.55, assum-
ing dissociative adsorption [53] (one CO per two Mo atoms
at room temperature). In case of promoted Mo/HZSM-5
materials, though the Mo loading remains the same, the CO
uptake is reduced, especially for Rh–Mo/HZSM-5. This is
possibly due to the decrease in the number of exposed Mo
atoms as the sequentially added promoter might have cov-
ered a portion of the surface of the Mo species or due to the
sintering and agglomeration of the Mo species (reduced Mo
dispersion) during the second stage calcination. Also, in case
of promoted Mo/HZSM-5 materials, since CO was adsorbed
on both Mo and the promoted metal species, the calculation
of actual Mo dispersion is not possible.
4
Catalytic Results
4
.1 Effect of Adding K, Rh and Fe as Promoters
on Mo/HZSM‑5
4
.1.1 Conversion of CH on Promoted Mo/HZSM-5 Catalyst
4
at 750 °C
The effect of adding promoters (K, Rh and Fe) with different
loadings of 0.5 wt%, 1 wt% and 1.5 wt% over 10 wt% Mo/
HZSM-5 on methane aromatization was studied in detail.
Figure 5 shows the conversion of CH on promoted Mo/
4
HZSM-5 catalyst compared with the 10 wt% Mo/HZSM-5 at
7
50 °C. When compared to the 10 wt% Mo/HZSM-5, addi-
tion of 0.5 wt% promoter (Fig. 5a) exhibited an improved
conversion in case of K-promoted catalyst. For the 0.5 wt%
K-promoted catalyst, the conversion of CH was ~ 16.5%
4
and ~32% at the 10 and 255 min reaction time, respectively.
This corresponds to an increase of ~ 13.5% and ~ 16.5%
when compared to 10 wt% Mo/HZSM-5 for the same reac-
tion times. For 0.5 wt% Fe-promoted catalyst, the conver-
sion of CH was ~16% and ~5.8% less than that of 10 wt%
4
1
3

Effect of Addition of K, Rh and Fe Over Mo/HZSM-5 on Methane Dehydroaromatization Under…
Fig. 6 Selectivity of C H at 750 °C over 10 wt% Mo/HZSM-5 com- ▸
6
6
pared with promoted 10 wt% Mo/HZSM-5 with different promoter
loadings of a 0.5 wt%, b 1 wt%, c 1.5 wt%
Mo/HZSM-5 at 10 and 255 min of reaction time respec-
tively. The Rh-promoted catalyst exhibited the least CH
4
conversion. The conversion at 10 and 255 min of reaction
was 25% and ~5% less when compared to the 10 wt% Mo/
HZSM-5. Increasing the promoter loading exhibited differ-
ent trends for each promoter metal as shown in Fig. 5b, c.
The conversion of CH decreased with increasing loading
4
of K and Rh. The conversion of CH for 1 wt% K-promoted
4
catalyst was almost similar to that of 10 wt% Mo/HZSM-5
but the conversion dropped by ~25% and ~12% at 10 min
and 255 min of reaction when the K loading was increased
to 1.5 wt%. Increasing the Rh loading to 1 wt% and 1.5 wt%
decreased the conversion by ~35% and ~46% by ~6.8% and
~
15.5%, at 10 and 255 min, respectively. Increasing the Fe
loading to 1 wt% and 1.5 wt% improved the conversion of
CH at 10 min of reaction by 18.7% and 20.1% respectively.
4
At 255 min, the conversion of CH decreased by ~4.3% and
4
increased by ~4.7% for 1 wt% and 1.5 wt%, respectively.
4
.1.2 Selectivity of C H , C2, and Carbon on Promoted Mo/
6 6
HZSM-5 Catalyst at 750 °C
Figure 6 compares the selectivity of C H on 10 wt% Mo/
6
6
HZSM-5 and promoted 10 wt% Mo/HZSM-5 at 750 °C. The
0 wt% Mo/HZSM-5 exhibited the maximum selectivity of
80% for C H within the first 45 min of reaction but the
1
~
6
6
selectivity of C H decreased with increasing reaction time
6
6
and at 255 min of reaction the selectivity of C H was less
6
6
than ~30%. In case of 0.5 wt% K-promoted catalysts, though
the selectivity increased during the first 80 min of reaction
and reached a maximum of ~60%, it gradually decreased to
~
40% after 255 min of reaction. Rh exhibited the least selec-
tivity for C H during the first 10 min of reaction but the
6
6
selectivity of C H increased with increasing reaction time
6
6
and was stable at ~40% after 255 min reaction. At 0.5 wt%
promoter loading, the initial selectivity of C H for Fe-
6
6
promoted catalyst was ~70% but it steadily decreased with
increasing reaction time and dropped to ~25% after 255 min
of reaction. With increase in promoter loading, the selectiv-
ity of C H increased in case of K-promoted catalyst. For
6
6
1
wt% and 1.5 wt% K loadings, the C H selectivity reached
6 6
a maximum of ~ 66% and ~ 76% and it was stable around
50% and 60% respectively after 255 min of reaction. In
case of Fe-promoted catalyst, the C H selectivity decreased
~
6
6
with increasing promoter loading. The C H selectivity of
6
6
1
0 min and 255 min of reaction time was ~44% and 19%
for 1 wt% Fe and ~ 33% and ~ 9% for 1.5 wt% Fe. 1 wt%
and 1.5 wt% Rh showed trends similar to that of 0.5 wt%
Rh where the initial C H selectivity was least compared
6
6
1
3

V. Ramasubramanian et al.
Fig. 7 Selectivity of C2 at 750 °C over 10 wt% Mo/HZSM-5 com- ▸
pared with promoted 10 wt% Mo/HZSM-5 with different promoter
loadings of a 0.5 wt%, b 1 wt%, c 1.5 wt%
to the catalysts with same loadings of other promoters and
the selectivity steadily increased and was stable around
~
35–45% after 255 min of reaction. Thus, after 255 min
of reaction, comparing all three promoters, K is the only
promoter which on addition to Mo/HZSM-5 has improved
the selectivity of C H .
6
6
Figure 7 compares the selectivity of C2 hydrocarbons on
0 wt% Mo/HZSM-5 and promoted 10 wt% Mo/HZSM-5 at
50 °C. In MDA studies, generally the selectivity of C2
1
7
hydrocarbons is significantly less when compared to the
selectivity of aromatics. Thermodynamic limitations favor
methane aromatization over methane dimerization at MDA
reaction temperatures of around 700–750 °C. Methane
dimerization is feasible at 1350 °C where the Gibbs free
ꢀ
ꢁ
energy of the reaction is zero ΔG = 0 whereas methane
r
ꢀ
ꢁ
aromatization is feasible at 1075 °C. ΔG = 0 [54, 55].
r
Compared to 10 wt% Mo/HZSM-5, the selectivity of C2
increased with the addition of K and Rh as promoters to
Mo/HZSM-5 catalyst and decreased in the presence of Fe.
With increase in K loading the selectivity of C2 increased.
In case of Fe and Rh-promoted catalysts, the selectivity of
C2 decreased with increase in promoter loading. Though the
selectivity of C2 with respect to reaction time increased for
Rh-promoted catalysts during the first 45 min of reaction,
the selectivity of C2 decreased with increase in reaction time
for all the catalysts.
Figure 8 compares the selectivity of carbon on 10 wt%
Mo/HZSM-5 and promoted 10 wt% Mo/HZSM-5 at 750 °C.
The carbon selectivity after 10 min of reaction was ~20%
for 10 wt% Mo/HZSM-5 and it increased in the presence
of a promoter. Since all the catalysts were reduced in H
2
at 750 °C, the K, Rh, Fe and Mo species were reduced to
metallic state before introducing CH . Hence along with Mo,
4
the promoter also activates CH . Thus, significant amounts
4
of CH species are formed but dimerization of the activated
x
methane species is possible only in the presence of active
Mo C sites (Sect. 4.2). In case of Rh-promoted catalyst,
2
the high selectivity of carbon after the first 10 min of reac-
tion could possibly be because of the activation of CH by
4
Rh forming CH species. Furthermore, poor dispersion of
x
Mo species in the presence of Rh (discussed in Sect. 3.1.4)
decreased the CH conversion (Sect. 4.1.1) and limited the
4
availability of active sites for dimerization. Similarly, in
case of Fe-promoted catalyst, though the conversion of CH
4
increased with increase in Fe loading (Sect. 4.1.1), possibly
due to the activation of CH by both Fe and Mo, the car-
4
bon selectivity increased with increase in both Fe loading
and reaction time. After 255 min of reaction, Fe-promoted
catalysts exhibited the maximum carbon selectivity. This is
1
3

Effect of Addition of K, Rh and Fe Over Mo/HZSM-5 on Methane Dehydroaromatization Under…
Fig. 8 Selectivity of carbon at 750 °C over 10 wt% Mo/HZSM-5 ▸
compared with promoted 10 wt% Mo/HZSM-5 with different pro-
moter loadings of a 0.5 wt%, b 1 wt%, c 1.5 wt%
also in accordance with the TPO analysis of spent catalysts
(
Sect. 4.4). The SEM analysis of spent catalysts (Sect. 4.3)
showed the presence of carbon nanostructures in the Fe-
promoted catalyst, explaining the high carbon selectivity for
Fe. The decrease in selectivity of C H along with the for-
6
6
mation of carbon nanostructures suggest that the presence of
Fe increases the formation of carbon species at the expense
of desired aromatic species [36]. In case of 10 wt% Mo/
HZSM-5, K and Rh-promoted catalysts, the carbon selec-
tivity initially decreased with increase in reaction time and
then it increased. This increase is due to the coke formed
through agglomeration of heavier hydrocarbons like C H .
6
6
In contrary to Rh and Fe-promoted catalysts, for K-promoted
catalyst, the selectivity of carbon decreased with increasing
K loading. 1.5 wt% K-promoted catalyst exhibited the lowest
carbon selectivity of ~38% after 255 min of reaction which
is ~ 54% less than the 10 wt% Mo/HZSM-5. Thus, K as a
promoter inhibits the coke formation due to agglomeration
of heavier hydrocarbons. It can be postulated that addition
of K has significantly modified the acidity of the zeolite
+
(
Sect. 3.1.5) where availability of H sites for agglomeration
of hydrocarbons to form coke is limited.
4.2 Active Species for MDA Reaction
Samples of all the spent catalysts were subjected to XRD
analysis to study the active phases available for MDA reac-
tion. From Fig. 9, it is evident that all the samples show
ο
ο
ο
ο
sharp peaks at 2θ values equal to 34.5 , 37.9 , 39.4 , 52
which corresponds to β−Mo C [56]. Mo C is the only active
2
2
phase present in all the catalysts. This is in contrast to the
results of Kojima et al. [48] who stated that addition of a
noble metal like Rh formed α−MoC phase as an active
1
−x
site. The catalyst pretreatment condition significantly influ-
ences the formation of active sites. Kojima et al. [48] pre-
treated Rh promoted catalysts in a mixture of CH and H .
4
2
Under this pretreatment condition, Rh was reduced to its
metallic state before the reduction of MoO to MoO and
3
2
this metallic Rh acted as a catalyst to activate methane and
formed MoO H C (precursor for α−MoC ). Based on our
x
y
z
1−x
previous study [49] and in this work, all the catalysts were
reduced to metallic state by H before introducing CH to
2
4
reduce the induction time on C H formation. Thus, for all
6
6
catalysts, including the Rh-promoted catalyst, Mo C was
2
formed as the active phase.
1
3

V. Ramasubramanian et al.
4.3 Scanning Electron Microscope (SEM) Analysis
Figure 10 shows the SEM images of all the spent catalysts.
In case of 10 wt% Mo/HZSM-5, K and Rh-promoted cata-
lysts there is no significant morphological difference in the
surface of the spent catalyst. In case of Rh and K promoted
Mo/HZSM-5, with increase in promotor loading, the parti-
cles tend to agglomerate on the catalyst surface (Fig. 10d
and Fig. 10g). This agglomeration of metal species leads
to decrease in the dispersion of active sites (Mo C sites)
2
and could possibly be the cause for decrease in catalyst
activity for CH conversion as discussed in Sect. 4.1.1. In
4
case of Fe-promoted Mo/HZSM-5, no significant agglom-
eration was observed and the conversion of CH increased
4
with increase in Fe loading (Sect. 4.1.1). Moreover, in
Fe-promoted Mo/HZSM-5 catalyst, carbon nanostructures
were formed, and the diameter of the carbon nanostruc-
tures increased with increasing Fe loading. These carbon
nanostructures were not found in unmodified or K and
Rh promoted Mo/HZSM-5 spent catalysts. Thus, Fe as
a promotor in Mo/HZSM-5 facilitated the formation of
carbon nanostructures as reported by the studies of Xu
et al. [27, 35, 36].
Fig. 9 XRD patterns of spent catalyst (1) 10 wt% Mo/HZMS-5 (2)
0
.5 K–10 Mo/HZSM-5 (3) 1 K–10 Mo/HZSM-5 (4) 1.5 K–10 Mo/
HZSM-5 (5) 0.5 Rh–10 Mo/HZSM-5 (6) 1 Rh–10 Mo/HZSM-5 (7)
1
.5 Rh–10 Mo/HZSM-5 (8) 0.5 Fe–10 Mo/HZSM-5 (9) 1 Fe–10 Mo/
HZSM-5 (10) 1.5 Fe–10 Mo/HZSM-5
weight loss peak possibly attributed to the sublimation of
Mo species was observed. Table 4 shows the relative weight
loss of the catalyst due to the removal of carbon and it was
evident that the least amount of coking was observed over
the surface of K–Mo/HZSM-5. The Fe-promoted catalyst
showed significant weight loss due to the removal of carbon
nanostructures.
4
.4 Temperature Programmed Oxidation Of Spent
Catalysts
Figure 11 shows the temperature programmed oxidation
of spent catalysts subjected to MDA at 750 °C. The DTA
curves show four regions in the TPO profile. In region I,
before 350 °C, there was no significant weight change as the
samples were dried at 350 °C in He. In region II, between
3
80 and 480 °C, considerable increase in weight was
5 Conclusion
observed, corresponding to the oxidation of molybdenum
carbide to molybdenum oxide [32]. In region III, between
MDA studies over H reduced, K, Rh and Fe-promoted Mo/
2
5
00 and 700 °C, significant weight loss was observed in all
HZMS-5 catalysts revealed that increasing the Fe loading
the catalysts, attributed to the removal of coke. In case of
Mo/HZSM-5 and K-Mo/HZSM-5, two distinct weight loss
peaks around 510 °C and 580 °C were observed. The low
temperature peak around 510 °C corresponds to removal
of hydrogenated carbon species and the high temperature
peak around 580 °C corresponds to the removal of hydrogen
deficient carbon deposits [39]. In case of Rh–Mo/HZSM-
improved the CH conversion whereas increasing the K and
4
Rh loading decreased the conversion of CH when com-
4
pared to Mo/HZSM-5. When the catalysts were reduced in
H before methane activation, no significant change was
2
observed in the phase of active sites (Mo C) formed in K,
2
Rh and Fe-promoted catalyst when compared to Mo/HZSM-
5. The K-promoted Mo/HZSM-5 catalysts exhibited better
selectivity for C2 hydrocarbon and C H compared to all
5
, a single weight loss peak around 525 °C was seen. The
6
6
high temperature peak attributed to the hydrogen deficient
carbon species was not present for this catalyst. As for the
Fe–Mo/HZSM-5, the low temperature peak was not well dis-
tinguished from the high temperature peak and the peak cor-
responding to maximum weight loss was observed around
other catalysts. Rh-promoted catalyst prepared by sequential
impregnation exhibited poor catalytic activity for CH con-
4
version and C H selectivity. Fe-promoted catalyst enhanced
6
6
the CH conversion but the selectivity of C H declined sig-
4
6
6
nificantly in the presence of Fe. Compared to Mo/HZMS-5,
K and Rh-promoted catalysts reduced the carbon deposition
on the catalyst, but Fe enhanced the formation of carbon
species. The improved selectivity of C2 and C H with less
6
10 °C. This peak was attributed to the removal of carbon
nanostructures formed during the methane activation as
reported by Xu et al. [36]. In region IV, around 715 °C, a
6
6
1
3

Effect of Addition of K, Rh and Fe Over Mo/HZSM-5 on Methane Dehydroaromatization Under…
Fig. 10 SEM images of spent
catalysts of a 10 wt% Mo/
HZMS-5 b 0.5 K–10 Mo/
HZSM-5 c 1 K–10 Mo/HZSM-5
d 1.5 K–10 Mo/HZSM-5 e
0
.5 Rh–10 Mo/HZSM-5 f 1
Rh–10 Mo/HZSM-5 g 1.5
Rh–10 Mo/HZSM-5 h 0.5
Fe–10 Mo/HZSM-5 i 1 Fe–10
Mo/HZSM-5 j 1.5 Fe–10 Mo/
HZSM-5
1
3

V. Ramasubramanian et al.
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