Non-Oxidative Methane Conversion Using Lead- and Iron-Modified Albite Catalysts in Fixed-Bed Reactor

Article abstract of DOI:10.1002/cjoc.201700800

Raw and modified albite catalysts, including Pb/Albite and Fe/Albite catalysts, have been investigated for methane conversion to C₂ hydrocarbons under non-oxidative conditions. Introduction of Pb to albite improved the activity and selectivity to non-coke products. Based on characterization, it was found that Pb entered into the alkali and alkaline-earth metal sites of albite, while partial Fe doped in the tetrahedron sites and the other loaded on the surface of albite. At the reaction temperature of 1073 K, methane gas hourly space velocity (GHSV) of 2 L·gcat^–1·h^–1, catalyst dosage of 0.25 g (300 mesh), the methane conversion catalyzed by raw albite in the fixed-bed micro reactor exhibited a methane conversion of 3.32%. Notably, introducing a Pb content of 3.4 wt% into albite greatly enhanced the conversion of methane up to 8.19%, and the selectivity of C₂ hydrocarbons reached 99% without any coke under the same reaction conditions. While Fe-doping could weakly heighten the methane conversion to 3.97%, and coke was formed. Thus, a comparison of Pb/Albite and Fe/Albite catalysts demonstrates that the catalytic activity of albite is mainly decided by alkali and alkaline-earth metal sites, and lead-modification can effectively improve the catalytic activity of albite.

Full text of DOI:10.1002/cjoc.201700800

Non-oxidative methane conversion using lead- and iron-modified
albite catalysts in fixed-bed reactor
Ye Chena,b,&, Xin Wanga,b,&, Xuegang Luob, Xiaoyan Lina,b, Yu Zhangb,*
aDepartment of Materials Science and Engineering, Southwest University of Science and
Technology, Mianyang 621010, Sichuan, China
bEngineering Research Center of Biomass Materials, Ministry of Education, Mianyang 621010,
Sichuan, China
Abstract: Raw and modified albite catalysts, including Pb/Albite and Fe/Albite catalysts, have
been investigated for methane conversion to C2 hydrocarbons under non-oxidative conditions.
Introduction of Pb to albite improved the activity and selectivity to non-coke products. Based on
characterization, it was found that Pb entered into the alkali and alkaline-earth metal sites of albite,
while partial Fe doped in the tetrahedron sites and the other loaded on the surface of albite. At the
reaction temperature of 1073 K, methane gas hourly space velocity (GHSV) of 2 liter·gcat-1·h-1,
catalyst dosage of 0.25 g (300 mesh), the methane conversion catalyzed by raw albite in the
fixed-bed micro reactor exhibited a methane conversion of 3.32%. Notably, introducing a Pb
content of 3.4wt% into albite greatly enhanced the conversion of methane up to 8.19%, and the
selectivity of C2 hydrocarbons reached to 99% without any coke under the same reaction conditions.
While Fe-doping could weakly heighten the methane conversion to 3.97%, and coke was formed.
Thus, a comparison of Pb/Albite and Fe/Albite catalysts demonstrates that the catalytic activity of
albite is mainly decided by alkali and alkaline-earth metal sites, and lead-modification can
effectively improve the catalytic activity of albite.
*Corresponding author.
E-mail address: zhangyuswust@163.com.
&These authors contributed equally to this work and should be considered co-first authors.
This article has been accepted for publication and undergone full peer review but has not
been through the copyediting, typesetting, pagination and proofreading process, which
may lead to differences between this version and the Version of Record. Please cite this
article as doi: 10.1002/cjoc.201700800
This article is protected by copyright. All rights reserved.

Keywords: Methane conversion; Albite; C2 hydrocarbons; Active site; Lead-modification
1. Introduction
The conversion of natural gas (typically 75% by weight methane) into other valuable higher
hydrocarbons, including C2 hydrocarbons, is of significant importance to the petrochemical industry.
Being an important chemical raw material, C2 hydrocarbons are increasingly demanded in both
developed and developing countries. Including ethylene, the most produced organic compound in
the world [1], C2 hydrocarbons have a large market and can be obtained by direct conversion of
methane [2-4]. Methane is an elementary building block for organic synthesis, and considerable
interest has been paid to convert methane into more commercially useful C2 hydrocarbons.
Therefore, how to activate CH4 and realize the conversion into high value-added products under
mild conditions has become one of the most challenging subjects in the field of chemical industry
[5,6].
In order to solve the problem, several different approaches based on catalysis and reaction
engineering have been proposed and tested. Oxidative coupling of methane (OCM) [7] is a
promising direct route for the production of C2 hydrocarbons by oxidation of methane, which is
thermodynamically feasible (exothermic process). However, the active sites in the coupling
catalysts also activate the C-H bond in C2H6 and C2H4 and result in the formation of CO2 by
combustion, which leads to a low selectivity towards C2 (50%-55%) and high selectivity of CO and
CO2 [8]. With respect to a higher selectivity of C2 hydrocarbons, methane can be converted to C2
hydrocarbons by the direct pyrolysis: 2CH4
C H +H
C2H4+2H2
C H +3H
→ →
2 2 2
→
→
2
6
2
2C+4H2 [9,10]. According to the thermodynamics, the methane cracking is weak at the temperature
of 1300 K below, and when the temperature increase to 2000 K above, methane can be pyrolyzed to
This article is protected by copyright. All rights reserved.

ethyne with a yield up to 85% [11]. Nevertheless, the increasing pyrolysis temperature will raise the
cost and is not conducive to industrialization. Fortunately, the cracking temperature of methane can
be reduced by adding suitable catalysts to activate the C-H bond [12], just as the catalyst showed in
this paper. Compared with these two approaches, non-oxidative conversion of methane to C2
hydrocarbons (NOCM) firstly defined by Belgued et al [13]. in 1990 avoids the deep oxidation of
the products without CO or CO2 and has achieved the direct and efficient conversion of methane to
ethane, so the process has attracted the attention of many researchers [14,15].
The catalysts utilized for NOCM are usually over monometallic catalysts that are composed of
or modified by compounding them together [16,17], where metals include precious metals,
rare-earth metals and transition metals, including Ru, Co [18], Re [19], Pt [20], Pd [21], and Ni [22],
then Al2O3 [23], SiO2 [24] and molecular sieve [25] are applied as general carriers. Unfortunately,
accumulation of coke on the catalyst surface results in deactivation and loss of selectivity from 91%
to 66% [23].
Up to date, the study of the single-atom active center catalyst has become one of the new
frontiers and hotspots in the field of catalysis [26]. The NOCM to C2 hydrocarbons can be
catalyzed by single site catalyst, which is generally supported catalyst [27]. As a result, the active
components are not stable under the condition of high temperature and easy to coke. Accordingly,
the reaction temperature should be below 873 K, which leads to methane conversion occurs at a
fairly low rate (<1%) [13]. Unlike the supported catalysts, Xinhe Bao et. al achieved a high
efficient conversion of methane under high temperature by the construction of Fe-Si bond in
Fe©SiO2, a single-atom active center catalyst, which did not deactivate in an 60-hour test [28]. In a
word, without any coke deposition, the catalytic activity of the catalyst can be heightened through
This article is protected by copyright. All rights reserved.

the construction of the single-atom active center catalyst, which probably solve the problem of
NOCM catalyst.
Based on above, a new catalyst for NOCM, which is employed under mild conditions and do
not produce any coke, has been considered in this paper. Albite, a natural mineral with a chemical
formula of A[T4O8] (major elements shown in Table 1), was studied as a catalyst for NOCM here
[29]. Albite consists of vertex-shared [SiO4] and [AlO4] tetrahedra that form tetra-atomic and
octatomic rings tunnels. Cationic Na(Ⅰ), K(Ⅰ), Ca(Ⅱ), Mg(Ⅱ) and so on occupy the interspace
between the tetra-atomic ring, forming A site (see Fig. 1) [30]. Owing to the natural structural
properties of albite, the alkali and alkaline-earth metal sites have not only positive and negative
charge attraction with [TO4] (T=Al, Si), but also a induced force with the oxygen bridge, so it can
keep stable at the high temperature. Alkali and alkaline-earth metal sites (A sites) can be
ion-exchanged with metal ions of large radius, offering a variety of possible active sites for
methane activation. Importantly, albite has a good cleavage in the direction of parallel to the chain.
As a result, it is likely that alkali metal sites are exposed on the external and become active, which
may directly contact with methane molecular to catalyze NOCM. In theory, alkali and
alkaline-earth metal sites in albite are the steady sites of coordinatively unsaturated metal center,
and can be introduced into reactive metal by ion-exchange to form the active center for NOCM.
Table 1. The ionic radius of the major elements in albite.
Site
T site
A site
Na (Ⅰ)
0.124
Element
Si (Ⅳ)
0.034
Al (Ⅲ)
K (Ⅰ)
Ca(Ⅱ) Mg (Ⅱ)
0.120 0.097
Radius(nm)
0.047
0.159
This article is protected by copyright. All rights reserved.

Fig. 1 Basic structure of albite.
T
This paper is devoted to the catalytic performance study of albite, and Pb and Fe elements with
higher activity were applied to make albite modified. The catalytic test of raw and modified albite
in thhe NOCM to C2 hydrocarbons was studied, which exhibits higher selectivity (>99%), activity,
and stability for production of C2 hydrocarbons without any coke. For comparison purpose, the
struccture, the morphology and the catalytic property of Pb- and Fe-modified albite catalysts for
NOCCM were examined in order to find the acting active sitee for non-oxidative methane
convversion.
2. Experimental
2.1 Reagents
TThe Pb/Albite (Pb/Ab) and Fe/Albite (Fe/Ab) catalysts were preepared by calcination in Muffle
furnace (SXW-5-17Z, Shanghai, China). Albite (100 mesh, Hebei, China), PbCl2 (AR, Kelon
Chengdu, China), Fe2O3 (Nano-scale, Aladdin, Shanghai, China), HF (39%, Kelon, Chengdu,
Chinaa), deionized water (laboratory homemade) were used in catalys
tt preparation.
2.2 Catalyst preparation
2.2.1 Pb/Ab catalyst
IIn a typical procedure, foremost the albite powder was calcined at 1373 K for 5 h to eliminate
impurrities in natural albite (see Fig. S1 in Supporting Material). Then it was milled in the ball mill
This article is protected by copyright. All rights reserved.

(QM-1SP2, Nanjing, China) with a speed of 5000 r/min for 2 h to obtain the fine albite powder
(300 mesh). Afterwards, special amount (mass ratio of PbCl2/albite was 0.1, 0.5 and 1) of PbCl2
was mixed with the albite powder. After an intermediate grinding, the powder was further calcined
at 773 K for 25 h in air to finish ion-exchange. Then the powder was adequately washed to remove
redundant PbCl2 and finally dried in an oven at 383 K to obtain the final Pb/Ab catalyst.
2.2.2 Fe/Ab catalyst
To be brief, special amount (depends on the specific catalyst; e.g. mass fraction of Fe was 0.5,
1 and 3 wt%) of nono-scale Fe2O3 was mixed with albite. Then the mixture was milled in the ball
mill with a speed of 5000 r/min for 2 h. Subsequently, the powder mixture was calcined at 1373 K
in air for 5 h to dope Fe into albite. After being crashed into 50-100 mesh by using universal
high-speed smashing machines (FW-80, Beijing, China), the powder was grinded into 300 mesh in
a mortar to obtain the final Fe/Ab catalyst.
2.3 Catalyst characterization
A series of ion-changed Pb/Ab catalysts were detected by inductively coupled plasma spectra
(ICP) (Agilent, 7700x) to measure the content of ion exchange between Pb and Na in albite. The
test conditions were RF power of 1450 W, pump speed of 0.1 rpm, atomizing chamber temperature
of 2 °C, sampling depth of 8.0 mm, plasma gas flow rate of 15 L/min and nebulizer gas flow rate of
0.75 L/min. Before measurement, all the obtained catalysts powder was digested using HF acid
according to the method in literature [31].
To determine the coordination state of the iron element, Fe/Ab catalysts were measured by
Brook EMX-10/12 Electron spin resonance (ESR) spectrometer with an operating frequency of
9.76 MHz.
This article is protected by copyright. All rights reserved.

The phase characterization of raw and modified albite catalysts were analyzed by X-ray
diffraction (XRD) analysis on a Panalytical X’Pert PRO diffractometer (The Netherlands). The
samples were scanned from 10 to 60° 2θ. Cu Kα radiation (λ = 0.15406 nm) generated at 40 kV and
30 mA was used as the X-ray source.
X-ray photoelectron spectroscopy (XPS) (Thermo SCIENTIFIC ESCALAB 250, USA)
analysis was performed to estimate the surface composition and binding energy between atoms of
catalysts. The instrument was equipped with a monochromatic Al Kα radiation source operated at a
power of 300 W. The binding energy was calibrated using the adventitious carbon (C 1s peak at
284.6 eV).
The morphology of modified albite was observed by Libra 200FE field emission transmission
electron microscopy (TEM), with an acceleration voltage of 200 KV.
The total amount of coke accumulated in the spent catalysts was obtained from the weight loss
in the 50-1050 K range determined by thermogravimetric analysis (TGA) in a TQ500 equipment
under a flow of air and heating from room temperature to 1050 K at a rate of 10 K/min.
2.4 Catalyst testing
The catalytic NOCM reaction was implemented at 873-1073 K and atmospheric pressure in a
fixed-bed flow reactor (see Fig. S2 in Supporting Information). Specifically, 0.25 g of Pb/Ab or
Fe/Ab catalyst (300 mesh) was mixed with 2 g quartz sand (50-100 mesh), then the mixture was
filled in the middle of quartz tube (7 mmi.d.) and both ends were fixed with silica wool. Firstly, the
catalyst was heated to a predetermined temperature with a speed of 4 °C/min at air atmosphere,
then the feeding gas (90vol% CH4/10vol% Ar) was continuously injected at a determined gas
hourly space velocity (GHSV) for 0.5 h as prereaction. After that, it was time to collect the reaction
This article is protected by copyright. All rights reserved.

gas. Subsequently the reaction gas was analyzed by Agilent7890A gas chromatography (GC)
equipped with FID detector and PLOT-Q capillary column. The activity of the Pb/Ab and Fe/Ab
catalysts was characterized by the yield of C2 hydrocarbons.
2.5 Analysis
The percentage of various components of the reaction gas was obtained by normalization
method [32,33], the formula is as follows:
Xi
= A
i
× f
i
(1)
Where Xi is the molar concentration of chemical i, Ai represents the peak area of chemical i,
and fi is the correction factor of chemical i.
Methane conversion is given:
X
(bef ) − X
CH4
CH4
CH4(conv)
=
(aft) ×100%
(2)
X
CH4 (bef )
Where CH4(conv) is the conversion of methane, X (bef ) and XCH (aft) are the initial and
CH4
4
final molar concentration of methane, respectively.
Selectivities of C2 hydrocarbons are as follows:
2× XC H
2
6
C2H6(sel)
C2H4(sel)
C2H2(sel)
=
=
=
×100%
×100%
×100%
(3)
(4)
XCH
− X
4 (bef )
CH4 (aft)
2× XC H
2
4
X
− X
CH4 (bef )
CH4 (aft)
2× XC H
2
2
(5)
(6)
X
− X
CH4 (bef )
CH4 (aft)
C2 (sel) = C2 H6(sel) + C2 H4(sel) + C2 H2(sel)
Where C2H6(sel), C2H4(sel), C2H2(sel), and C2(sel) represent the selectivity of ethane,
ethylene, ethyne, and C2 hydrocarbons, respectively; XC H6, XC H , and XC H are the molar
2
2
4
2 2
This article is protected by copyright. All rights reserved.

concentration of ethane, ethylene and ethyne, respectively.
Yield of C2 hydrocarbons is as below:
YC = CH4(conv) ×C2(sel)
(7)
2
Where YC is the yield of C2 hydrocarbons.
2
The quantitative method of coke amount is as below [34].:
−
mi m f
am
o
o
unt =
×100%
Coke
(8)
mi
Where mi represents the weight of coked catalyst after desorption of water and mf
represents the weight of coked catalyst after burning off the coke
Carbon balance is as below [35]:
−
=
+
+
) +
XC
2(X C H
(9)
X
X
X
X
(Bef )
( Aft)
CH4
CH4
C2H4
C2H2
oke
2
6
Where YCoke is the molar concentration of coke.
3
Results and discussion
3.1 Catalyst characterization
3.1.1 Structure analysis
Fig. 2 ICP results of ion-exchanged albite with different mass ratio
ss
of lead chloride and albite.
The contents of Pb(
Ⅱ) and Na( ) in albite can be observed intuitively from ICP results in Fig.
2. As seen, when the mass ratios of lead chloride and albite were 0, 0.1, 0.5 and 1, Na(
Ⅰ) contents
This article is protected by copyright. All rights reserved.

were 5.0, 4.6, 3.9 and 3.5wt% and Pb(
determined that the content of Na( ) decreases along with the increase of Pb(Ⅱ) content, which is
speculated that Na( ) was exchanged by Pb( ). In fact, in silicate mineral albite, alkali or
alkaline-earth metal ions will be compensated the excess negative charges generated when partial
Si( ) are replaced by Al( ). Balancing the electric interaction in the void, these cations can
Ⅱ) contents were 0.1, 1.5, 2.2 and 3.4wt%, respectively. It is
Ⅰ
Ⅰ
Ⅱ
Ⅳ
Ⅲ
participate in the ion-exchange reaction at the condition of that the crystal structure of albite could
not be destroyed due to the strong liquidity and small binding energy with crystal skeleton [36]. On
the other hand, the ion-exchange reaction occurs between dissociative lead ions and sodium ions in
albite in the vicinity of the melting point of lead chloride, resulting the production of soluble
sodium and Pb/Ab. Followed by the formation of Pb/Ab, the structure of albite is still not damaged,
which is consistent with the results of previous study [37].
Fig. 3 ESR spectra of Fe/Ab with different Fe contents.
The transition metal ions with unpaired electron have paramagnetic effect, and different
coordination states can cause difference of the ESR spectra of the fine structure. According to the
study of the paramagnetic iron atom, the coordination situation of iron atoms in albite can be
judged from the spectral split factor g. Two obvious signals appear in ESR spectra (Fig. 3), which
are attributed to the Fe(Ⅲ) of the coordination of four and eight at g=4.3 and 2.3, respectively [38].
This article is protected by copyright. All rights reserved.

It is depicted that the intensities of the two absorption peaks increase with the increasing Fe (Ⅲ)
content, and the peak height of g=2.3 significantly increases at 3wt%Fe. Especially, it might be
illustrated that the absorption peak of g=4.3 shows partial iron atoms doped into the albite
framework [TO4], forming a tetrahedral structure [39].
Fig. 4 XRD spetra of modified albite with different metal contents. (a) Pb/Ab; (b) Fe/Ab.
XRD spetra of Pb/Ab and Fe/Ab catalysts with different Pb or Fe contents are depicted in Fig.
4. As shown in Fig. 4a, the typical diffraction peaks at 2θ=20°‒34° indicates that the framework of
the albite was still retained without obvious changes of relative crystallinity in all the samples,
which can be indexed to pure albite Na(Al Si3O8) (JCPDS 99-0001). Futhermore, the absence of
peak of lead chloride or lead oxide indicates that Pb(Ⅱ) was well dispersed in albite and did not
appear in the albite framework so that it had no effect on albite structure. In this sense, lead element
successfully entered into the A site of the albite A[T4O8], and achieved the purpose of ion exchange,
which has borne out the ICP analysis.
In different iron-doped proportions, XRD spectra of albite did not change obviously (Fig. 4b),
which can be indexed to pure Albite Na(Al Si3O8) (JCPDS 99-0001). A comparision of the albite
spectra with different iron contents demonstrates that the diffraction peaks of Fe/Ab shift toward
the lower angle followed the iron content increasing. As report goes, when the heteroatom of larger
radius than Al and Si atom was introduced in the molecular sieve, heteroatom in molecular sieve
This article is protected by copyright. All rights reserved.

framework often caused lattice constants and unit cell volume become larger, resulting in that the
diffraction peaks of the sample shifted to lower angles than that of the undoped sample [40]. In
effect, albite and molecular sieve are both connected with silicon-oxygen or alumina-oxygen
tetrahedrons, so the diffraction peaks of Fe/Ab shifted toward the lower angle direction. This
further shed light on that Fe element successfully entered into the albite framework [TO4].
Specifically, only some of iron oxide doped into the tetrahedron framework [TO4] in albite and the
other failed, which is obvious for the new peak at 33.2° in XRD attributed to iron oxide (JCPDS
89-0599) in the spectrum of 3wt% Fe/Ab. This may be due to that the melting point of iron oxide
(1838 K) is much higher than that of albite (1373 K), resulting that partial iron oxide can not fully
melt into albite at 1373K.
Fig. 5 XPS patterns of modified albite with different modified-metal contents. (a) Pb/Ab; (b) Fe/Ab.
Fig. 5 demonstrates the XPS patterns of Pb/Ab and Fe/Ab catalysts with different Pb or Fe
contents. As shown, the peaks at 138.9 eV and 1072.1 eV should be attributed to Pb 4f7/2 and Na 1s,
respectively (Fig. 5a). Of these, the peak of Pb 4f7/2 at 138.9 eV is close to that of lead silicate. It
can be observed that the peak intensity of Pb 4f7/2 increases while Na 1s decreases. In combination
with ICP and XRD analysis, it is again verified that Pb(Ⅱ) successfully ion-exchanged with Na(Ⅰ)
in albite and entered into alkali metal site of albite [37,41].
The Fe 2P3/2 peak position of the albite moves to the direction of high binding energy, and the
This article is protected by copyright. All rights reserved.

lower the doping amount, the greater the amplitude of the shift (Fig. 5b). It is an obvious signal that
Fe successfully doped into the albite framework. This shift can be attributed to that Si(IV) is more
electronegative than Fe(III), resulting in an induced effect on Fe(III) by bridging oxygen bond. As a
consequence, the electric potential energy of skeleton Fe(III) increased, leading to the increase of
electron binding energy of skeleton iron. It is further corroborated that partial iron oxide is
successfully incorporated into T site of albite [41]. On the other side, the peak position of Fe 2P3/2 is
closer to that of Fe2O3 when the iron content reached 3wt%. Combining with the results of ESR and
XRD analysis, it may be ascribed to that after Fe-doping partial iron oxide did not enter into the
albite tetrahedron framework [TO4], which is more obvious in spectra of 3wt%Fe/Ab catalyst.
3.1.2 Morphology analysis
Fig. 6 TEM patterns of modified albite with different modified-metal contents. (a) 1.5wt% Pb/Ab; (b) 2.2wt%
Pb/Ab; (c) 3.4wt% Pb/Ab; (d) 0.5wt% Fe/Ab;(e) 1wt% Fe/Ab;(f) 3wt% Fe/Ab.
TEM patterns of Pb/Ab and Fe/Ab catalysts with different modified-metal contents are shown
in Fig. 6. As depicted in Figs. 6a, b and c, the morphologies of 1.5, 2.2 and 3.4wt%Pb/Ab catalysts
have no clear change and are still a plate structure, without other attachments on the surface after
high content of Pb modification. This result is in agreement with the conclusion above.
Apparently, 0.5 and 1wt%Fe/Ab is still a plate structure (Figs. 6d, e), which is accord with the
This article is protected by copyright. All rights reserved.

flaky dissociation of albite. The obvious appearance of spherical particle on the surface of
3wt%Fe/Ab indicates that partial iron oxide did not melt into albite and loaded on the surface of
albite (Fig. 6f). This is consistent with the XRD results, and also corroborates the reason why XPS
diagram of Fe 2P3/2 peak of 3wt%Fe/Ab moved to lower binding energy direction is that partial iron
oxide failed to enter into albite. Thus, it is reliably ascertained that partial iron oxide did not enter
into the albite flamework [TO4] after Fe-doping, which can be intuitively reflected in 3wt%Fe/Ab
catalyst.
Compared with the characterization of Pb/Ab and Fe/Ab catalysts, it was found that Pb(Ⅱ)
smoothly entered into A site of albite, while partial Fe(Ⅲ) successfully doped into T site of albite
and the remaining loaded on the surface of albite.
3.2 Catalytic performance on NOCM
3.2.1 Study on catalytic properties of albite
First of all, the catalytic effects of raw albite under different temperatures and space velocities
were investigated, which are depicted in Fig. 7. The albite dosage was 0.25 g, and the reaction time
was 1 h.
This article is protected by copyright. All rights reserved.

Fig. 7 Catalytic activity of raw albite with different conditions including (a) GC chromatography (b) temperatures
ranging from 873 to 1073 K, (c) methane GHSV varied from 1to 3 liter·gcat-1·h-1 at temaperature of 1073 K. (d)
catalytic performance of quartz sand at temaperature of 1073 K and methane GHSV of 2 liter·gcat-1·h-1.
Fig. 7a shows the GC spetra of the products of methane conversion catalyzed by albite at
different temaperatures. Intuitively, the products contain C2 hydrocarbons (ethane, ethylene and
ethyne) and C3 hydrocarbons (propane and propylene). Of these, ethane and ethylene were the main
products. What’s more, the content of C3 hydrocarbons was so little that the study focus has been
C2 hydrocarbons in this paper.
Fig. 7b demonstrates the calculation results of GC pattern at Fig. 7a. As shown in Fig. 7b, the
conversion of methane increased apparently from 0.86% to 3.32% followed the reaction
temaperature increasing from 873 K to 1073 K. This might be attributed to that the increasing
reaction temaperature improved the activity of catalytic site in albite, which was favor to methane
conversion. Meanwhile, at the reaction temperature ranging among 873 K and 1073 K, ethane
selectivity decreased from 90.1% to 51.4%, while ethylene selectivity increased from 9.9% to 47.8%
This article is protected by copyright. All rights reserved.

and ethyne selectivity increased from 0 to 0.6% without any coke. It might be ascribed that ethane
is so unstable in high temperature that would further dehydrogenate to produce ethylene and ethyne
[10]. Remarkably, the methane conversion at 1073 K was greatly higher than that of 973 K.
The effects of methane GHSV (from 1 to 3 liter·gcat-1·h-1) on the catalytic performance of
albite for NOCM are depicted in Fig.7c. It can be seen that the conversion of methane decreased
from 4.62% to 1.05% while the selectivity of C2 hydrocarbons did not decrease obviously when
methane GHSV increased from 1 to 3 liter·gcat-1·h-1 at temaperature of 1073 K. Specifically,
besides a bit of ethyne, the selectivity of ethane was improved from 30.4% to 61.6%, while the
selectivity of ethylene decreased from 66.5% to 38.1%. This could be explained that when the feed
gas spacevelocity increased, the residence time of the product gas in the high temperature region
was shortened, so methane molecule had less time to be converted and the continuous
dehydrogenation of C2 hydrocarbons was weakened.
By comparison, a blank experiment (an empty reactor with no catalyst) under the same
conditions showed a methane conversion of only 0.75% (Fig. 7d). A test with quartz sand as the
catalyst yielded virtually the same result. Therefore, quartz sand had no obvious effect on methane
conversion, and it can be mixed with finer albite powder to make methane gas pass through.
In conclusion, the best catalytic effect of raw albite for NOCM is that the methane conversion
rate was up to 3.32% and the main products were ethane and ethylene without any coke at the
optimal experimental conditions of the temperature of 1073 K, methane GHSV of 2 liter·gcat-1·h-1,
reaction time of 1 h and catalyst dosage of 0.25 g.
3.2.2 Pb/Ab catalyst
This article is protected by copyright. All rights reserved.

Fig. 8 Catalytic activity of Pb/Ab with different Pb contents. Reaction conditions: the reaction temperature of 1073
K, the methane GHSV of 2 liter·gcat-1·h-1, and the catalyst dosage of 0.25 g (300 mesh).
The NOCM tests of modified-albite were implemented under the optimal experimental
conditions for the highest catalytic activity of albite. The results of catalytic activity test of Pb/Ab
with different Pb contents are decpited in Fig. 8. As shown, when the Pb content increased from 0
to 3.4wt%, the conversion of methane increased from 3.32% to 8.19%. Meanwhile, C2
hydrocarbons yield increased from 3.31% to 8.11% (Fig. 8a). It shows that Pb-modification can
significantly enhance the catalytic activity of albite. Combined with the characterizations above,
this may be attributed to that Pb(Ⅱ), an ion with higher activity than Na(Ⅰ), ion-exchanged with
Na(Ⅰ) to enter into A site and improved the catalytic activity of A site, which was favourable for
methane activation.
Containing trace ethyne (about 0.2%), the main components of the C2 hydrocarbons were
ethane (about 53.4%) and ethylene (about 45.8%) after reaction (Fig. 8b). Fairly accurately, lead-
modification could weakly improve the selectivity of ethylene, but the overall selectivity of C2
hydrocarbons did not decrease, conforming the reaction process did not produce coke. This may be
ascribed to that the ion-exchange between Pb(Ⅱ) and Na(Ⅰ) in A site of albite enhanced the
activity of albite for methane conversion, but did not change the albite structure so that it had a little
effect on the selectivity of C2 hydrocarbons.
This article is protected by copyright. All rights reserved.

Fig. 9 The stability of Pb/Ab catalyst. Reaction conditions: the reaction temperature of 1073 K, the methane
GHSV of 2 liter·gcat-1·h-1, and the catalyst dosage of 0.25 g (300 mesh).
Futhermore, the stability of Pb/Ab catalyst was tested. Fig. 9 shows the catalytic activity of
albite for NOCM during the reaction time from 1 h to 25 h. Obviously, with the extension of
reaction time, the conversion of methane and the selectivity of the ethane, ethylene and ethyne had
only a small fluctuation and maintained at a stable value of 8.17%, 53.7%, 45.5% and 0.3%,
respectively. This illustrates that Pb/Ab catalyst showed the relatively steady catalytic performance
on NOCM.
This article is protected by copyright. All rights reserved.

Fig. 10 TEM (a) (b), XRD (c) and TGA (d) spectra of Pb/Ab catalyst before and after reaction for 25 h.
The TEM image, XRD and TGA curves of Pb/Ab catalyst before and after reaction for 25 h
were measured. The TEM image of the Pb/Ab catalyst after the reaction (Fig. 10a,b) shows that the
morphology of the Pb/Ab catalyst did not change significantly after the reaction and remained a
layered structure with no other attachments, demonstrating that no coke was produced. As can be
seen from Fig. 10c, the XRD pattern of the Pb/Ab catalyst before and after reaction did not change
significantly, indicating that no obvious structural changes occurred in the Pb/Ab catalyst during the
NOCM reaction, and no carbon deposition occurred. Fig. 10d shows that the weight loss rate of
Pb/Ab catalyst before reaction was 0.31% at 300 K-1050 K, and the weight loss rate of Pb/Ab was
0.73% after NOCM reaction for 25 h. It can be seen that there is such little change of the weight
loss rate of Pb/Ab catalyst before and after 25 h reaction, so that it could be ignored. Meanwhile,
there is no weight loss peak due to coking at around ~700 K and ~750 K [43,44]. Combined with
XRD and TEM analysis results, it was indicated that Pb/Ab catalyst did not form coke during the
reaction.
3.2.3 Fe/Ab catalyst
Fig. 11 Catalytic activity of Fe/Ab with different Fe contents. Reaction conditions: the reaction temperature of
1073 K, the methane GHSV of 2 liter·gcat-1·h-1, and the catalyst dosage of 0.25 g (300 mesh).
The conversion rate of methane increased slightly followed the iron-doping increasing (Fig.
This article is protected by copyright. All rights reserved.

11a). Specificly, the doping amount increased from 0 to 3wt% followed by the conversion rate of
methane increased from 3.32% to 3.97%. Instead, the yield of C2 hydrocarbons was reduced from
3.31% to 3.06%.
With a shred of ethyne, the main components of products were ethane, ethylene and coke (Fig.
11b). The selectivity of C2 hydrocarbons gradually decreased from 99% to 75.6% followed
iron-doping increasing from 0 to 3wt%. According to the characterization results above, it is
attributed to that a part of iron oxide entered into the albite framework [TO4] and weakly raised the
activity of albite for methane conversion. Meanwhile, the other part of iron oxide failed to enter
into albite framework [TO4], but loaded on the surface of albite in the modification process,
resulting the heavy coke production and the lower selectivity of C2 hydrocarbons. This is
conformed with the metal-doping catalysts, which were widely used in methane dehydrogenation to
obtain nanocarbon and hydrogen [45-47].
3.3 Mechanism of NOCM
As dicussed above, albite has an obvious effect on the methane conversion under
non-oxidative condition (Fig. 7). In order to enhance catalytic effect, two ions, Fe(Ⅲ) (0.057 nm)
and Pb(Ⅱ) (0.137 nm), whose activities are higher than native ions in albite and radii are close to
that of A and T sites in Table 1 were chosen to make albite modified at different sites.
Fig. 12 Schematic diagram of alkali-metal sites modified.
Pb(Ⅱ) substituted Na(Ⅰ) in albite to enter into the alkali-metal site in the void between
This article is protected by copyright. All rights reserved.

octatomic ring, forming Pb/Ab catalyst, as shown in Fig. 12. With unoccupied d orbitals, a lead ion
shows higher activity and can be coordinated by one Si (Al) and two methane molecules. In other
words, Pb was introduced into alkali-metal site to form the Pb/Ab catalyst with higher catalytic
activity, which greatly promoted the conversion of methane (Fig. 8). This further proves that the
alkali and alkaline-earth metal sites are the active sites. On the one side, the tetra-atomic ring chain
parallelling to the direction of chain elongation is stablely combined in albite structure, so the
crystal along this direction is not easily broken. On the other side, although oxygen bridge
connection exists among chains perpendicular to the direction of chain growth, some rely on a
coordination bond between metal ions and O2- ions, so the combination is much weaker than that of
the former. As a result, albite has a good cleavage in parallel to the direction of the chain (Fig. 6).
Then alkali and alkaline earth metal ions will be exposed to the external of albite after cleavage,
forming coordinatively unsaturated metal center to stablely exist at high temperatures and become
the center of methane activation, which has been proved in the catalyst characterization section.
Fig. 13 Schematic diagram of modified [TO4] structure.
Fe, an element with higher activity than Na, partly doped into T site of albite framework [TO4]
to form the Fe/Ab catalyst, which weakly promoted the conversion of methane (Fig. 11). It is
implied that T site is not the active site of albite. Actually, reactants need to react with catalyst to be
activated in heterogeneous catalytic reaction, and methane activation requires that the methane
molecules are chemically adsorbed on the catalyst surface and needs the interaction between
methane and metal site [19,25]. But in albite structure, each silicon or aluminum atom is
This article is protected by copyright. All rights reserved.

surrounded by four oxygen atoms, forming a silicon-oxygen or aluminum-oxygen tetrahedron (Fig.
13). So does the iron-oxygen tetrahedron[37]. Furthermore, with the smaller ionic radius than that
of the surrounding oxygen, the intermediate silicon, aluminum and iron are completely coated by
oxygen. Due to the intermediate metal site is closed by tetrahedral structure and methane molecules
are unable to contact with the active metal, so that it can not be activated on the catalyst.
Accordingly, [TO4] is clearly not consistent with such a structure, which could explain why the
doping of transition metal iron did not significantly improve the activity of albite.
In this paper, the process of methane conversion occured at the catalytic site of albite is similar
to catalytic cracking of methane, which can be speculated based on the previous studies [9-11] and
the experimental study. The possible reaction process is given:
2CH4 ⎯c⎯at→ 2CH3⋅+H2 C H +H
C H +2H
C H +3H
2C+4H
→
2
→
→
→
2
6
2
2
4
2
2
2
2
First of all, alkali-metal sites of albite activates and initiates methane molecules
dehydrogenation by generating methyl radicals at high temperature, and then ethane is generated by
the combination of two methyl radicals [28]. Next, the dehydrogenation of ethane and its
subsequent dehydrogenation at varied reaction temperatures and methane spacevelocites are
consistent with the thermal decomposition process of alkanes. Ethane is so unstable at high
temperature that continued to produce ethylene by dehydrogenation [10]. If the temperature
continues to rise or the residence time of ethylene in high temperature area is lengthening, ethylene
will continue to be converted into ethyne by dehydrogenation. Then coke is formed by the
dehydrogenation of ethyne, which is only present in Fe/Ab catalyst due to the poorly dispersed iron
oxide.
4. Conclusion
This article is protected by copyright. All rights reserved.

Here we show that albite can be employed in methane conversion under non-oxidative
conditions. The conversion of methane catalyzed by raw albite was 3.32%. After introducing a Pb
content of 3.4wt% to albite, a conversion as high as 8.19% was obtained at 1073 K and a space
velocity of 2 liter gcat-1 hour-1, with a selectivity to C2H4 of 47.4%. It is worth mentioning that the
Pb/Ab catalyst as a single-active-center-like catalyst exhibited a higher performance on NOCM
than conventional catalysts, due to that total carbon selectivity to the three C2 hydrocarbons
remained >99%, and no deactivation was observed even after reaction for 25 h in this paper.
Furthermore, the active site of albite, alkali and alkaline-earth metal site, was determined by the
comparision of Pb-introduction and Fe-doping, which provides direction for further modification.
Deficiently, the specific adsorption and decomposition processes of methane on albite surface have
not been studied owing to the limitations of study conditions. The finding opens up new
possibilities for fundamental studies of direct, non-oxidative conversion of methane, the main
component of natural gas. It is anticipated that combining a catalyst such as this one with an
efficient reactor technology may enable the development of methane-based routes to transform C2
hydrocarbons into high-value-added chemicals.
Acknowledgements
This work was financially supported by the National Transformation Program for Agricultural
Science and Technology (grant number 2012GB2G200469) and Postgraduate Innovation Fund
Project by Southwest University of Science and Technology (grant number 17ycx008).
Supplementary Information
This article is protected by copyright. All rights reserved.

Supplementary material includes Figs. S1 and S2 as described in the text, which include XRD
patterns of albite before and after calcination, and the fixed-bed flow reactor for NOCM utilized in
this paper.
Conflict of Interest
The authors declare that they have no conflict of interest.
References
[1] Gholipour, Z.; Malekzadeh, A.; Hatami, R.; Mortazavi, Y.; Khodadadi, A.J. Nat. Gas Chem.
2010, 19, 35‒42.
[2] Holmen, A. Catal. Today 2009, 142, 2‒8.
[3] Yuliati, L.; Hattori, T.; Itoh, H.; Yoshida, H. J. Catal. 2008, 257, 396‒402.
[4] Radoiu, M.-T.; Chen, Y.G.; Depew, M.C. Appl. Catal. B 2003, 43, 187‒193.
[5] Gharibi, M.; Zangeneh, Yaripour, F.T.F.; Sahebdelfar, S. Appl. Catal. A 2012, 443‒444, 8‒26.
[6] Tang, P.; Zhu, Q.J.; Wu, Z.X.; Ma, D. Energy Environ. Sci. 2014, 7, 2580‒2591.
[7] Fleischer, V.; Littlewood, P.; Parishan, S.; Schomacker, R. Chem. Eng. J. 2016, 306, 646-654.
[8] Alexiadis, V.I.; Thybaut, J.W.; Kechagiopoulos, P.N.; Chaar, M.; Van Veen, A.C.; Muhler, M.;
Marin, G.B. Appl. Catal. B 2014, 150‒151, 496‒505.
[9] Guéret, C.; Daroux, M.; Billaud, F. Chem. Eng. Sci. 1997, 52, 815‒827.
[10] Keramiotis, C.; Vourliotakis, G.; Skevis, G.; Founti, M.A.; Esarte, C.; Sánchez, N.E.; Miller,
A.; Bilbao, R.; Alzueta, M.U. Energy 2012, 43, 103‒110.
[11] Holmen, A.; Olsvik, O.; Rokstad, O. A. Fuel Process Technol. 1995, 42, 249‒267.
[12] Fau, G.; Gascoin, N.; Steelant, J.J. Anal. Appl. Pyrol. 2014, 108, 1‒11.
[13] Belgued, M.; Pareja, P.; Amariglio, A.; Amariglio, H. Nature 1991, 352, 789‒790.
[14] Liu, S.Y.; Mei, D.H.; Shen, Z.; Tu, X.J. Phys. Chem. C 2014, 118, 10686‒10693.
[15] Nahreen, S.; Praserthdam, S.; Beltran, S.P.; Balbuena, P.B.; Adhikari, S.; Gupta, R.B. Energy
& Fuels 2016, 30, 2584‒2593.
[16] Guczi, L.; Sarma, K.V.; Borkó, L. Catal. Lett. 1996, 39, 43‒47.
[17] Guczi, L.; Sarma, K.V.; Borkó, L. React. Kinet. Catal. Lett. 1999, 68, 95‒104.
This article is protected by copyright. All rights reserved.

[18] Guczi, L.; Sarma, K.V.; Borkó, L. J. Catal. 1997, 167, 495‒502.
[19] Bazin, D.; Borkó, L.; Koppány, Zs.; Kovács, I.; Stefler, G.; Sajó, L.I.; Schay, Z.; Guczi, L.
Catal. Lett. 2002, 84, 169‒182.
[20] Gerceker, D.; Motagamwala, A.H.; Rivera-Dones, K.R.; Miller, J.B.; Huber, G.W.; Mavrikakis,
M.; Dumesic, J.A. ACS Catal. 2017, 7, 2088‒2100.
[21] Moya, S.F.; Martins, R.L.; Ota, A.; Kunkes, E.L.; Behrens, M.; Schmal, M. Appl. Catal. A
2012, 411‒412, 105‒113.
[22] Moya, S.F.; Martins, R.L.; Schmal, M. Appl. Catal. A 2011, 396, 159‒169.
[23] Borkó, L.; Guczi, L. Top Catal. 2006, 39, 35‒43.
[24] Szeto, K.C.; Norsic, S.; Hardou, L.; Le Roux, E.; Chakka, S.; Thivolle-Cazat, J.; Baudouin, A.;
Papaioannou, C.; Basset, J.M.; Taoufik, M. Chem. Commun. 2010, 46, 3985‒3987.
[25] Xu, Y.D.; Bao, X.H.; Lin, L.W. J. Catal. 2003, 216, 386‒395.
[26] Soulivong, D.; Norsic, S.; Taoufik, M.; Copéret, C.; Thivolle-Cazat, J.; Chakka, S.; Basset, J.
M.; Am. Chem. Soc. 2008, 130: 5044‒5045.
[27] Lin, X.F.; Xi, Y.Y.; Zhang, G.D.; Phillips, D.L.; Guo, W.Y. Organometallics 2014, 33,
2172‒2181.
[28] Guo, X.G.; Fang, G.Z.; Li, G.; Ma, H.; Fan, H. J.; Yu, L.; Ma, C.; Wu, X.; Deng, D.; Wei, M.;
Tan, D.; Si, R.; Zhang, S.; Li, J.; Sun, L.; Tang, Z.; Pan, X.; Bao, X. Science 2014, 344,
616‒619.
[29] Pelte, S.; Flamant, G.; Flamand, R.; Gauthier, D.; Beche, E.; Berjoan, R.; Miner. Eng. 2000, 13,
609‒622.
[30] Baur, W.H.; Joswig, W.; Müller, G.J. Solid State Chem. 1996, 121, 12‒23.
[31] Souza, S.D.O.; Costa, S.S.L.D.; Santos, D.M.; Pinto, J.D.S.; Garcia, C.A.B.; Alves, J.D.P.H.;
Geovanny, R. Spectrochim Acta. B 2014, 96, 1‒7.
[32] Caravaca, A.; Lucas-Consuegra, A.D.; González-Cobos, J.; Valverde, J.L.; Dorado, F. Appl.
Catal. B 2012, 113‒114, 192‒200.
[33] Caravaca, A.; Ferreira, V. J.; Lucas-Consuegra, A.D.; Figueiredo, J.L.; Faria, J.L.; Valverde,
J.L.; Dorado, F. Int. J. Hydrogen Energy 2013, 38, 3111‒3122.
[34] Coelho, A.; Caeiro, G.; Lemos, M.A.N.D.A.; Lemos, F.; Ramôa Ribeiro F. Fuel 2013, 111,
449‒460.
This article is protected by copyright. All rights reserved.

[35] Zhao, G.B.; John, Sani; Zhang, J.J.; Wang, L.N. ; Muknahallipatna, S.; Hamannb, J.C.;
Ackerman, J.F.; Argyle, D.M.; Plumb, O. A. Chem. Eng. J. 2006, 125, 67–79.
[36] Deubener, J.; Sternitzke, M.; Müller, G. Amer. Mineral 1991, 76, 1620‒1627.
[37] Liu, R.; Lu, A. H.; Qin, S. Acta Geol Sin (English Edition) 2006, 80, 175‒179.
[38] Tatumi, S.H.; Kinoshita, A.; Fukumoto, M.E.; Courriol, L.C.; Kassab, L.R.P.; Baffa, O.;
Munita, C.S. Appl. Radiat. Isot. 2005, 62, 231‒236.
[39] Calis, G.; Frenken, P. Zeolites 1987, 7, 319‒326.
[40] Chou, B.; Tsai, J.L.; Cheng, S. Micropor. Mesopor. Mat. 2001, 48, 309‒317.
[41] Farquhar, M.L.; Vaughan, D.J.; Hughes, C.R.; Charnock, J.M.; England, K.E.R.; Geochim.
Cosmochim. Acta. 1997, 61, 3051‒3064.
[42] Seyama, H.; Soma, M.J. Electron, Spectrosc, Relat, Phenom. 1987, 42, 97‒101.
[43] Tempelman, C.H.L.; Rodrigues, V.O.; Eck, E.R.H.; Magusin, P.C.M.M.; Hensen, E.J.M.
Micropor. Mesopor. Mat. 2015, 203, 259–273.
[44] Ismagilov, Z.R.; Matus, E.V.; Tsikoza, L.T. Energy Environ. Sci. 2008, 1, 526–541.
[45] Wang, G.W.; Jin, Y.; Liu, G.J.; Li, Y.D. Energy & Fuels 2013, 27, 4448‒4456.
[46] Fakeeha, A.H.; Ibrahim, A.A.; Naeem, M.A.; Khan, W.U.; Abasaeed, A.E.; Alotaibi, R.L.
Catal. Sustain. Energy 2015, 2, 71‒82.
[47] Ashik, U.P.M.; Wan Daud, W.M.A.; Hayashi, J.I. Renew. Sustain. Energy Rev. 2017, 76,
743‒767.
This article is protected by copyright. All rights reserved.