Highly Efficient Conversion of Methane to Olefins via a Recycle-Plasma-Catalyst Reactor

Article abstract of DOI:10.1007/s10562-016-1846-y

Abstract: Spark discharge plasma is an efficient means to activate methane. In this work methane was converted to olefins via a two stage recycle-plasma-catalyst reactor. A recycle spark discharge plasma reactor was employed in the first stage to actively convert methane to a mixture comprising mainly acetylene and hydrogen. The mass transfer in the pulsed spark discharge channel was strengthened by recycling part of plasma effluent to the entrance, which improved the reactivity and energy efficiency of the plasma reactor. Pd and Ni based MgAl₂O₄ supported catalysts were employed in the second stage to selectively hydrogenate acetylene in the plasma reactor effluent. With the conversion of methane of 73 % the integration of recycle plasma and catalyst reactor may obtain a ethylene yield of 55 and 35 % on 0.3?wt% Pd-0.6?wt% Ag/MgAl₂O₄ and 2.5?wt% Ni-7.5?wt%-Zn/MgAl₂O₄ catalysts, respectively. Other than ethylene about 23 % of methane was transformed to C₃–C₅ light olefins over cheap 2.5?wt% Ni-7.5?wt% Zn/MgAl₂O₄ catalysts. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-016-1846-y

Catal Lett
DOI 10.1007/s10562-016-1846-y
Highly Efficient Conversion of Methane to Olefins via a Recycle-
Plasma-Catalyst Reactor
Bo Wang1 · H. M. Guan1
Received: 8 April 2016 / Accepted: 10 August 2016
© Springer Science+Business Media New York 2016
Graphical Abstract
Abstract Sparkꢀ dischargeꢀ plasmaꢀ isꢀ anꢀ efficientꢀ meansꢀ
to activate methane. In this work methane was converted
toꢀolefinsꢀviaꢀaꢀtwoꢀstageꢀrecycle-plasma-catalystꢀreactor.ꢀ
A recycle spark discharge plasma reactor was employed
inꢀtheꢀfirstꢀstageꢀtoꢀactivelyꢀconvertꢀmethaneꢀtoꢀaꢀmixtureꢀ
comprising mainly acetylene and hydrogen. The mass trans-
fer in the pulsed spark discharge channel was strengthened
byꢀrecyclingꢀpartꢀofꢀplasmaꢀeffluentꢀtoꢀtheꢀentrance,ꢀwhichꢀ
improvedꢀtheꢀreactivityꢀandꢀenergyꢀefficiencyꢀofꢀtheꢀplasmaꢀ
reactor. Pd and Ni based MgAl2O4 supported catalysts were
employed in the second stage to selectively hydrogenate
acetyleneꢀinꢀtheꢀplasmaꢀreactorꢀeffluent.ꢀWithꢀtheꢀconver-
sion of methane of 73% the integration of recycle plasma
and catalyst reactor may obtain a ethylene yield of 55 and
35% on 0.3 wt% Pd-0.6 wt% Ag/MgAl2O4 and 2.5 wt%
Ni-7.5 wt%-Zn/MgAl2O4 catalysts, respectively. Other than
ethylene about 23% of methane was transformed to C3–C5
lightꢀolefinsꢀoverꢀcheapꢀ2.5ꢀwt%ꢀNi-7.5ꢀwt%ꢀZn/MgAl2O4
catalysts.
Keywordsꢀ Methaneꢀ·ꢀOlefinꢀ·ꢀPlasmaꢀ·ꢀCatalystꢀ·ꢀ
Selective hydrogenation
1 Introduction
With abundant reserves natural gas comprising mainly
methane will substitute for diminishing petroleum as a
dominant energy source in the near future. However, due
toꢀ itsꢀ chemicalꢀ inertness,ꢀ efficientꢀ utilizationꢀ ofꢀ naturalꢀ
gas, i.e. conversion of methane into useful chemicals and
transportationꢀfuels,ꢀisꢀstillꢀaꢀchallenge.ꢀInꢀthisꢀfieldꢀmanyꢀ
research work and accomplishments in both industry and
academiaꢀhaveꢀbeenꢀfulfilled.ꢀGasꢀtoꢀliquidꢀprocessꢀviaꢀaꢀ
syngas intermediate generated by reforming of methane by
various means such as partial oxidation, steam reforming
and autothermal reforming have been established in indus-
try [1–3]. This process needs to operate at a large scale
toꢀgainꢀeconomicꢀbenefits.ꢀItꢀwasꢀestimatedꢀthatꢀtheꢀsyn-
thesis of syngas accounts for about 50–60% of the total
ꢀ Bo Wang
bwang@upc.edu.cn
1
Department of Chemical Engineering, China University of
Petroleum Huadong, Qingdao, Shandong 266580, China
1 3

2
B. Wang, H. Guan
investmentꢀcost.ꢀVariousꢀ meansꢀtoꢀactivateꢀmethaneꢀeffi-
ciently have stimulated researchers much interest world-
wide. Direct activation of methane has been reported by
means of pyrolysis, oxidative coupling, plasma, chlorina-
tion/oxychlorination, etc. [4–6]. Pyrolysis of methane sub-
Transformationꢀofꢀmethaneꢀtoꢀtheseꢀlightꢀolefinsꢀisꢀhighlyꢀ
desirable.
In this work methane was converted by a recycle spark
discharge plasma reactor to a gas mixture comprising mainly
acetyleneꢀandꢀhydrogen.ꢀToꢀobtainꢀhighlyꢀdesirableꢀolefinsꢀ
jectsꢀmethaneꢀtoꢀveryꢀhighꢀtemperatureꢀupꢀtoꢀ2500ꢀKꢀtoꢀ a selective hydrogenation reactor was combined with meth-
crack it into acetylene and hydrogen. The drawbacks of the
process are highly energy intensive and problems associ-
ated with coke formation due to high temperature in the
cracker. Oxidative coupling of methane to ethylene suffers
from low yield of ethylene due to low conversion of meth-
ane. The single pass conversion of methane is around 20%
with the selectivity for ethylene of about 70%. Oxychlo-
ane plasma activation. Selective hydrogenation of acetylene
in a C2 cut from a thermal cracking unit to ethylene is an
established process in industry [11]. Pd/alumina eggshell
catalysts are widely used in industry. In view of the high
price of Pd, in this work Ni was also investigated. A Ni-
containing hydrogenation catalyst, promoted with a second
metalꢀZn,ꢀwasꢀsynthesizedꢀinꢀthisꢀwork.ꢀWithoutꢀprovidingꢀ
rinationꢀemploysꢀchlorideꢀtoꢀactivateꢀmethane.ꢀInꢀtheꢀfirstꢀ extra hydrogen acetylene could be selectively hydrogenated
step methane is oxychlorinated to methyl chloride and the
resulting methyl chloride is converted in the second stage
to gasoline range hydrocarbons over ZSM-5 catalyst. This
process suffers from a number of disadvantages such as
multiple reaction processes, formation of unwanted poly-
chloromethanes as well as corrosion problems associated
with HCl and chloromethanes. Recently, methane activa-
tion by non-thermal plasmas such as corona discharge,
spark discharges, dielectric barrier discharges (DBD) have
been investigated at atmospheric pressure and ambient
temperature [7–9]. It was reported that plasma discharge
on both Pd-Ag/MgAl2O4 and Ni-Zn/MgAl2O4 catalyst to
olefinsꢀbyꢀusingꢀhydrogenꢀgeneratedꢀbyꢀplasmaꢀreactor.ꢀItꢀ
was observed that the integration of plasma activation and
selective catalytic hydrogenation is a good means to achieve
highlyꢀefficientꢀconversionꢀofꢀmethaneꢀtoꢀolefins.
2 Experimental
2.1 Catalyst Synthesis
techniquesꢀhaveꢀaꢀgreatꢀeffectꢀonꢀmethaneꢀconversionꢀandꢀ Methaneꢀ(99.99ꢀ%)ꢀwasꢀpurchasedꢀfromꢀXinyuanꢀGasꢀCom-
product selectivities. Jeong et al. studied methane conver-
sion by pulsed DC DBD [7]. The maximum methane con-
version was about 25% and ethane selectivity was about
pany. Magnesium aluminate spinel, MgAl2O4, was prepared
through the thermal decomposition of hydroxides co-preci-
patedꢀfromꢀaqueousꢀsolutionsꢀofꢀmagnesiumꢀandꢀaluminumꢀ
70–80ꢀ%.ꢀTheꢀeffectꢀofꢀaluminaꢀpelletsꢀfilledꢀinꢀgasꢀgapꢀwasꢀ nitrates [12]. The starting materials used were magnesium
also discussed and it was found that they played a role in
enhancing ethane selectivity. Zhu et al. studied methane
conversion to C2 hydrocarbons and hydrogen by different
nitrate hexahydrate, Mg(NO3)2 6H2O (Sinopharm 99.9+%)
and aluminum nitrate nonahydrate, Al2(NO3)3 6H2O (Sino-
pharm 99.0+%). In a typical preparation, stoichiometric
dischargeꢀtechniquesꢀatꢀatmosphericꢀpressureꢀandꢀambientꢀ amount of magnesium nitrate and aluminum nitrate were
temperature [8]. In the streamer discharge and pulsed spark
discharge processes, acetylene is the dominant C2 prod-
uct. The highest yields of acetylene and H2 reach up to 54
and 51% respectively at methane conversion of 69% on
a needle-to-plate reactor under pulsed spark discharges. In
theꢀDBDꢀprocesses,ꢀethaneꢀisꢀtheꢀmajorꢀC2 products and
the pulsed DC DBD process provides the highest ethane
yield. Recently, Zhu et al. reported the conversion of meth-
ane to ethylene in a plasma followed by catalyst reactor
[9]. A packed bed of Pd-Ag/SiO2 catalyst was used after
plasma reactor to hydrogenate acetylene to ethylene. Kado
et al. investigated the activation of methane using low tem-
perature plasmas such as DBD, corona and spark discharge.
dissolvedꢀinꢀdeionizedꢀwaterꢀtoꢀmakeꢀaꢀmixtureꢀsolution.ꢀ
The precipitant, a 25 wt% NH4OH solution, was then added
dropwise to the nitrate solution under continuous stirring,
andꢀpHꢀvalueꢀofꢀtheꢀfinalꢀsuspensionꢀwasꢀadjustedꢀtoꢀ9.ꢀAfterꢀ
precipitation, the slurry was stirred for another 30 min and
thenꢀrefluxedꢀatꢀ80ꢀ°Cꢀforꢀ24ꢀh.ꢀTheꢀmixtureꢀwasꢀthenꢀcooledꢀ
toꢀroomꢀtemperature,ꢀfilteredꢀandꢀwashedꢀwithꢀde-ionizedꢀ
water.ꢀTheꢀfinalꢀproductꢀwasꢀdriedꢀatꢀ110ꢀ°Cꢀforꢀ12ꢀhꢀandꢀ
calcinedꢀatꢀ600ꢀ°Cꢀforꢀ5ꢀh.
MgAl2O4 supported metal catalysts, 0.3 wt% Pd-0.6 wt%
Ag/MgAl2O4 and 2.5 wt% Ni-7.5 wt% Zn/MgAl2O4, were
prepared by incipient wetness impregnation method. The
impregnation solution contained both metal precursors
Theꢀenergyꢀefficiencyꢀinꢀsparkꢀdischargeꢀwasꢀmuchꢀhigherꢀ such that the metals were preferably applied to the sup-
than that in DBD and corona discharge. In point to point
spark discharge, acetylene was produced with the selectiv-
ity higher than 85% and small amount of deposited car-
bon [10].ꢀ Lightꢀ olefinsꢀ suchꢀ asꢀ ethylene,ꢀ propyleneꢀ andꢀ
port MgAl2O4 together at the same time. The impregnated
samplesꢀwereꢀdriedꢀatꢀ110ꢀ°Cꢀovernightꢀbeforeꢀcalcinationꢀinꢀ
staticꢀairꢀatꢀ450ꢀ°Cꢀforꢀ4ꢀh.
The structure properties of the samples were investigated
buteneꢀareꢀmajorꢀbuildingꢀblockꢀofꢀpetrochemicalꢀindustry.ꢀ by X-ray diffraction (XRD, X’Pert Pro MPD) using a CuKa
1 3

HighlyꢀEfficientꢀConversionꢀofꢀMethaneꢀtoꢀOlefinsꢀviaꢀaꢀRecycle-Plasma-CatalystꢀReactor
3
monochromatizedꢀ radiationꢀ sourceꢀ andꢀ aꢀ Niꢀ filterꢀ inꢀ theꢀ to the downstream catalytic reactor, which was a 15 mm
rangeꢀofꢀ2θꢀ=ꢀ5–75°.ꢀTheꢀcontentꢀofꢀcrystallineꢀphasesꢀwasꢀ o.d.×12 cm i.d.×400 mm long ss tube reactor packed with
evaluated from the relative intensity of the strongest dif-
fraction peaks of standard pure materials. The surface areas
catalyst.ꢀTheꢀ reactorꢀ axialꢀ temperatureꢀ profileꢀ wasꢀ moni-
tored by a sliding thermocouple inserted into the catalyst
(BET) were determined by nitrogen adsorption at −196ꢀ°Cꢀ bed.
usingꢀanꢀautomatedꢀgasꢀadsorptionꢀanalyzerꢀ(ASAP2020-
A N2ꢀflowꢀaddedꢀintoꢀtheꢀcatalyticꢀreactorꢀeffluentꢀwasꢀ
M).ꢀ Theꢀ poreꢀ sizeꢀ distributionꢀ wasꢀ calculatedꢀ fromꢀ theꢀ used as an internal standard for methane and hydrocar-
adsorption branch of the isotherm by the Barrett, Joyner and
Halenda (BJH) method.
bonꢀ productsꢀ quantification.ꢀ Theꢀ reactionꢀ productsꢀ wereꢀ
analyzedꢀ byꢀ twoꢀ onlineꢀ gasꢀ chromatographsꢀ (GC).ꢀ Oneꢀ
HP5890ꢀGCꢀequippedꢀwithꢀaꢀflameꢀionizationꢀdetectorꢀ(HPꢀ
PLOT/A12O3 capillary column) and a thermal conductivity
detector (5A packed column, N2 carrier gas) was used to
analyzeꢀhydrocarbonꢀcomponentsꢀupꢀtoꢀC12 and hydrogen.
AnotherꢀGCꢀequippedꢀwithꢀaꢀthermalꢀconductivityꢀdetectorꢀ
(13X column, H2ꢀcarrierꢀgas)ꢀwasꢀusedꢀtoꢀanalyzeꢀmethaneꢀ
and nitrogen. The combination of a 6-way and an 8-way
valvesꢀwereꢀusedꢀtoꢀaccomplishꢀtheꢀworkꢀofꢀsampling,ꢀinjec-
2.2 Reactor Setup
Figure 1 shows the plasma-catalyst reactor setup, which
consists of gas feed, recycle plasma-catalyst two stage reac-
tor and product analysis sections. The gas feeds (CH4, H2,
N2ꢀandꢀair)ꢀwereꢀcontrolledꢀbyꢀmassꢀflowꢀcontrollers.ꢀH2,
N2 and Air were used for the regeneration of the deacti-
vatedꢀcatalysts.ꢀTheꢀplasmaꢀreactorꢀwasꢀmadeꢀofꢀaꢀquartzꢀ tionꢀ andꢀ backꢀ flushingꢀ offꢀ C2+ components during TCD
tube of 22 mm o.d.×18 mm i.d.×120 mm long. A 1.5 mm
o.d.×1.0 mm i.d. ss tube were used as the high voltage
electrode. A rotary ss plate with a diameter of 16 mm and
a thickness of 3 mm was used as the ground electrode.
Multiple grooves were made on the plate to improve mix-
ing of reactant in the discharge channel. The distance from
analysis. For accurate gas chromatography analysis, the
transfer line from the reactor exit to the gas chromatograph
and the sampling valve were heat traced to keep the reactor
effluentꢀinꢀgasꢀstate.
Based on mass balance of carbon and hydrogen, the con-
version of methane (Xm) and the selectivities of individual
theꢀneedleꢀtoꢀtheꢀplateꢀwasꢀfixedꢀatꢀ7ꢀmm.ꢀTheꢀhigh-voltageꢀ hydrocarbon products (SHC) were calculated using the inter-
power supply source, model CTP-2000 of Corona Lab,
was used to generate pulsed spark discharges between the
two electrodes. The discharge power was measured via the
areaꢀ ofꢀ theꢀ voltage/chargeꢀ Lissajousꢀ figuresꢀ onꢀ anꢀ oscil-
loscope (TDS1012B-SC, Tektronix). A recycle pump was
installed at the outlet to provide a means to recycle part of
theꢀeffluentꢀbackꢀtoꢀtheꢀentranceꢀofꢀtheꢀplasmaꢀreactor.ꢀTheꢀ
recycleꢀratioꢀcanꢀbeꢀadjustedꢀbyꢀaꢀcontrolꢀvalveꢀmountedꢀatꢀ
theꢀpumpꢀoutlet.ꢀTheꢀplasmaꢀreactorꢀeffluentꢀwasꢀdirectedꢀ
nal standard method as follows.
FN 2
Fout
CN 2
=
Fmin − Fmout
Fmin − FoutCmout
Xm
=
×100% =
×100%
Fmin
Fmin
FN 2Cmout






= 1−
×100%
FminCN


2
Fig. 1 Experimental setup
vent
N2
MFC
ground
CH4
MFC
H2
Pump
MFC
Flow
N2
Meter
MFC
Air
MFC
GC
high voltage
electrode
Recycle plasma reactor
Hydrogenation reactor
1 3

4
B. Wang, H. Guan
F Cout
Fmin XmCN
out
HC
Fmin Xm
Fout
C
accelerate electrons to a high energy in a short distance.
Methane molecule is thus activated by collision with high
N2 HC
SHC = x×
×100% = x×
×100%
·
2
energyꢀelectrons,ꢀwhichꢀsequentiallyꢀdecomposesꢀtoꢀCH3 ,
·
Hydrogenꢀwasꢀquantifiedꢀbyꢀtheꢀexternalꢀstandardꢀmethod.
CH2 , CH· and hydrogen radicals as follows [13]:
2FoutCHout
4Fmin Xm
0.5FN CHout
Fmin XmCN
CH4 e→CH·3 + H· e→CH2 + H2
2
2
2
2
SH 2
=
×100% =
×100%
e→CH· + H· + H2 e→C + 2H2
The rates of these dehydrogenation reactions are very fast,
InꢀtheꢀaboveꢀequationsꢀFin and Foutꢀdenoteꢀtheꢀmolarꢀflowꢀ which leads to the high concentration of carbon. The con-
rate at the inlet and outlet of the reactor, respectively, and C
represents molar fraction.
As the hydrogenation reaction is highly exothermic,
0.5ꢀgꢀcatalystꢀwasꢀdilutedꢀwithꢀ5ꢀgꢀinertꢀα-Al2O3 to keep
the packed bed isothermal during the reaction. Prior to reac-
centration of CH formed by hydrogenation of atomic carbon
and C2 formed by coupling of atomic carbon are considered
to be high. Both them are the precursor of acetylene, which
give high selectivity for C2H2. It is shown that other than
acetylene small amounts of C3–C5 light hydrocarbons are
tion,ꢀcatalystsꢀwereꢀactivatedꢀbyꢀfirstꢀremovalꢀofꢀmoistureꢀatꢀ formed. Along with the formation of unsaturated hydrocar-
500ꢀ°Cꢀforꢀ4ꢀh,ꢀandꢀthenꢀwasꢀreducedꢀbyꢀaꢀmixtureꢀofꢀhydro-
genꢀandꢀnitrogenꢀatꢀmolarꢀratioꢀofꢀ1:9ꢀatꢀ500ꢀ°Cꢀforꢀ4ꢀh.
bons, e.g. acetylene, ethylene and propylene, hydrogen is
also produced. It is shown in Fig. 3 that the hydrogen yield
at methane conversion of 72% reaches up to 51%. The co-
product, hydrogen free of oxygen, in this process is highly
desirable for proton exchange membrane fuel cell [14, 15].
3 Results and Discussion
3.1 Plasma Activation of Methane
3.1.1 Effect of Spark Discharge Power
3.1.2 Effect of Recycle Ratio
Theꢀeffluentꢀfromꢀtheꢀplasmaꢀreactorꢀwasꢀreturnedꢀbackꢀtoꢀ
theꢀentranceꢀatꢀdifferentꢀrecycleꢀratio,ꢀwhichꢀwasꢀdefinedꢀasꢀ
theꢀratioꢀofꢀtheꢀvolumetricꢀflowꢀrateꢀofꢀtheꢀstreamꢀreturnedꢀ
to the reactor entrance to the stream leaving the system.
Although the feed rate of methane to the plasma reactor
wasꢀ fixedꢀ atꢀ 40ꢀ ml/min,ꢀ theꢀ increaseꢀ ofꢀ theꢀ recycleꢀ ratioꢀ
strengthened the mass transfer of reactants and improved
the contacting between reactants and discharge channel.
The reactor may be model as a continuously stirred tank
reactor. The larger the recycle ratio, the better is the mixing
ability. Methane was activated by spark discharge between
high voltage needle and ground plate. It was observed
that the maximum diameter of the discharge channel was
The effect of spark discharge power on methane activa-
tion was investigated by varying the plasma power input at
pureꢀmethaneꢀflowꢀrateꢀofꢀ40ꢀml/min.ꢀAsꢀshownꢀinꢀFig.ꢀ2,
the conversions of methane increases from 27 to 72% with
increasing spark discharge power input from 8.0 to 24.0 W.
With the increase of input energy more methane molecule
may be activated and decomposed to radicals, resulting in
the increased conversion. It is observed that acetylene is
the dominant product with selectivity up to 85% at 8.0 W.
With the increase of plasma power input the selectivity for
acetylene decreases slightly. High discharge power gener-
atesꢀaꢀstrongꢀelectricꢀfieldꢀinꢀtheꢀplasmaꢀreactor,ꢀwhichꢀmayꢀ estimated to be 1.5–2.5 mm. This indicated that some of
Fig. 2 Effect of discharge power input on methane conversion and
product selectivity
Fig. 3 Effect of discharge power input on product yields
1 3

HighlyꢀEfficientꢀConversionꢀofꢀMethaneꢀtoꢀOlefinsꢀviaꢀaꢀRecycle-Plasma-CatalystꢀReactor
5
Fig. 4 Effect of recycle ratio on reactivity of methane in recycle
plasma reactor
methane might pass through the discharge region without
contacting the discharge channel. Therefore, the improve-
ment of the contacting between reactants and discharge
channel increased the reacting ability of methane. As shown
in Fig. 4, the conversion of methane increases from 66 to
74% with increasing the recycle ratio from 0 to 4, then starts
to decrease to 69% with further increasing the recycle ratio
to 6. The activation of methane by pulse spark discharge
plasma consists of a complex reaction network, involv-
ing various carbon-containing radicals, hydrogen radical
and short-chain hydrocarbons. The maximum formation
of C2H2, a intermediate, has an optimum recycle ratio for
this type of reaction network. The energy cost for methane
conversionꢀdecreasesꢀfromꢀ10.6ꢀev/moleculeꢀwithꢀnoꢀefflu-
ent recycle to 9.3 ev/molecule with optimal recycle ratio of
Fig. 5ꢀ XRDꢀ patternꢀ ofꢀ theꢀ synthesizedꢀ MgAl2O4 support. a Pure
MgAl2O4 phase. b Pure MgAl2O4 phase. c MgAl2O4 with trace amount
of MgO phase
certain effect on the purity of MgAl2O4ꢀphasesꢀsynthesized.ꢀ
As shown in Fig. 5,ꢀsamplesꢀaꢀandꢀbꢀsynthesizedꢀbyꢀaddi-
tion of precipitant NH4OH solution to the mixed solution
of MgSO4 and Al(NO3)3 gave pure MgAl2O4 spinel phase
only. Whereas, besides MgAl2O4 phase, trace amounts of
MgOꢀphaseꢀwasꢀalsoꢀgeneratedꢀwhenꢀtheꢀadditionꢀsequenceꢀ
was reversed in the case of sample c. So sample a was cho-
sen as support for the preparation of selective hydrogena-
tionꢀcatalyst.ꢀItꢀwasꢀcharacterizedꢀbyꢀBETꢀthatꢀsampleꢀaꢀhasꢀ
4.ꢀTheꢀenergyꢀefficiencyꢀofꢀpulsedꢀsparkꢀdischargeꢀplasmaꢀ a surface area of 37.02 m2/g and a pore volume of 0.25 ml/g,
reactorꢀwasꢀimprovedꢀbyꢀtheꢀmeansꢀofꢀrecyclingꢀeffluentꢀtoꢀ respectively.
the entrance of the reactor.
3.2.1 Selective Hydrogenation on Pd-Ag/MgAl2O4
3.2 Selective Hydrogenation of Acetylene
Catalysts
Acetylene and hydrogen were the dominant product in the
recycle spark discharge plasma reactor. In view of the co-
existence of acetylene and hydrogen in the product it is pos-
sible to transform acetylene and hydrogen to value-added
olefinsꢀviaꢀselectiveꢀhydrogenationꢀreactionꢀonꢀaꢀhydroge-
nation catalyst without any extra addition of hydrogen from
outside sources. So, the integration of plasma and selective
hydrogenation reactors are investigated in this work.
Theꢀoperatingꢀconditionsꢀwereꢀfixedꢀforꢀrecycleꢀplasmaꢀreac-
tor,ꢀwhereas,ꢀtheꢀeffluentꢀwasꢀdirectedꢀtoꢀaꢀsecondꢀcatalyticꢀ
reactor, which was packed with selective hydrogenation cat-
alystꢀtoꢀconvertꢀacetyleneꢀtoꢀvaluableꢀolefinsꢀselectively.ꢀTheꢀ
selectiveꢀhydrogenationꢀofꢀacetyleneꢀinꢀtheꢀeffluentꢀofꢀtheꢀ
recycleꢀplasmaꢀreactorꢀwasꢀinvestigatedꢀfirstlyꢀonꢀ0.3ꢀwt%ꢀ
Pd-0.6 wt% Ag/MgAl2O4 catalyst under temperature in the
rangeꢀfromꢀ75ꢀtoꢀ150ꢀ°C.ꢀItꢀisꢀobservedꢀinꢀFig.ꢀ6 that the
Asꢀ acidꢀsitesꢀcouldꢀcatalyzeꢀoligomerizationꢀofꢀalkyneꢀ conversion of methane remains almost constant with tem-
andꢀalkene,ꢀα-aluminaꢀsupport,ꢀfreeꢀofꢀacidꢀsites,ꢀisꢀwidelyꢀ perature. With the combination of hydrogenation reaction
usedꢀinꢀselectiveꢀhydrogenationꢀtoꢀavoidꢀoligomerizationꢀofꢀ atꢀ75ꢀ°Cꢀtheꢀselectivityꢀforꢀacetyleneꢀdecreasesꢀdramaticallyꢀ
olefins.ꢀMgAl2O4 spinel has gained much interest as sup-
port due to its chemical inertness, high thermal stability
and mechanical resistance in heterogeneous catalysis. In
from 83% with plasma alone to 36%. In the meantime, the
selectivity for ethylene increases from 1 to 36%. The selec-
tivity for acetylene decreases further from 36% to 0 with
this work MgAl2O4ꢀ wasꢀ synthesizedꢀ byꢀ co-precipitationꢀ increasingꢀtemperatureꢀfromꢀ75ꢀtoꢀ125ꢀ°C,ꢀthenꢀlevelsꢀoff.ꢀ
of a mixed solution of MgSO4 and Al(NO3)3ꢀ withꢀ fixedꢀ Whereas, the selectivity for ethylene increases from 43 to
Mg2+/Al3+ ratio using diluted NH4OH solution. It was
found that the contacting pattern of these two solutions has
76%. With the increase of temperature, the selectivity for
ethane only slightly increased from 3 to 4%. The catalyst is
1 3

6
B. Wang, H. Guan
Ni-7.5 wt% Zn/MgAl2O4 catalyst, whereas the feed to the
catalytic reactor was the same as before. Figure 7 shows
the effect of reaction temperature on the selective hydroge-
nationꢀofꢀacetyleneꢀinꢀtheꢀeffluentꢀofꢀrecycleꢀplasmaꢀreac-
tor. The selectivity for acetylene decreases dramatically
from 83% in the recycle plasma reactor to 37% with the
combinationꢀ ofꢀ hydrogenationꢀ reactionꢀ atꢀ 125ꢀ°C.ꢀ Inꢀ theꢀ
meantime, the selectivity for ethylene increases from 1 to
24%. The selectivity for acetylene decreases further from
37ꢀtoꢀ1ꢀ%ꢀwithꢀincreasingꢀtemperatureꢀfromꢀ125ꢀtoꢀ175ꢀ°C,ꢀ
while the selectivity for ethylene increases from 24 to 48%.
Other than selective formation of ethylene, selectivity for
C3–C5 hydrocarbons increases from 7 to 39%, probably due
toꢀ theꢀ dimerizationꢀ ofꢀ ethyleneꢀ andꢀ propylene.ꢀ Theꢀ opti-
mal hydrogenation temperature for 2.5 wt% Ni-7.5 wt%
Zn/MgAl2O4ꢀcatalystꢀisꢀ175ꢀ°C,ꢀaboutꢀ25ꢀ°Cꢀhigherꢀthanꢀthatꢀ
on Pd-Ag/MgAl2O4. It is shown in Fig. 8 that the selectivi-
ties for C3, C4 and C5ꢀolefinsꢀatꢀ175ꢀ°Cꢀareꢀ4,ꢀ23ꢀandꢀ4ꢀ%,ꢀ
respectively,ꢀleadingꢀtoꢀtheꢀsumꢀofꢀlightꢀolefinꢀselectivityꢀ
in C3–C5 up to 31%. In sum, the integration of recycle
plasma reactor and hydrogenation catalyst MgAl2O4 leads
to the yields of ethylene and C3–C5ꢀolefinsꢀofꢀ35ꢀandꢀ23ꢀ%,ꢀ
respectively.
Fig. 6 Comparison of methane conversion and product selectivities
in plasma reactor alone and the two stage reactor with the hydroge-
nation catalyst of 0.3 wt% Pd-0.6 wt% Ag/MgAl2O4 under different
reactionꢀtemperature.ꢀReactionꢀconditions:ꢀmethaneꢀflowꢀrate:ꢀ40ꢀml/
min, recycle ratio: 4
highly selective with most acetylene converted to ethylene.
The integration of plasma and hydrogenation reactor is very
efficientꢀtoꢀconvertꢀmethaneꢀtoꢀethylene.ꢀTheꢀyieldꢀofꢀethyl-
ene can reach 55% at methane conversion of 73%.
3.2.2 Selective Hydrogenation on Ni-Zn/MgAl2O4
Catalysts
The disadvantage of Pd-containing catalysts is the price of
Pd.ꢀThereꢀisꢀaꢀneedꢀforꢀaꢀcommerciallyꢀattractiveꢀandꢀeffi-
cient selective catalyst. Ni has been widely used as hydro-
genation catalyst, and it has been reported in literature that
4 Conclusion
Itꢀwasꢀobservedꢀthatꢀpulsedꢀsparkꢀdischargeꢀwasꢀanꢀefficientꢀ
means to convert methane into acetylene and hydrogen. A
significantꢀ improvementsꢀ inꢀ theꢀ selectivityꢀ toꢀ ethyleneꢀ recycle plasma reactor was employed to activate methane
over Ni/MgAl2O4 resulted from the addition of promoters,
Zn [16]. In this work we investigated MgAl2O4 supported
in this work. The recycling of part of spark discharge reac-
torꢀeffluentꢀtoꢀtheꢀentranceꢀstrengthenedꢀmassꢀtransferꢀandꢀ
Niꢀ catalystꢀ andꢀ adjustedꢀ itsꢀ hydrogenationꢀ performanceꢀ improved contacting between reactants and pulsed spark
by alloying with a second metal, Zn. The catalyst was
changed from 0.3 wt% Pd-0.6 wt% Ag/MgAl2O4 to 2.5 wt%
discharge channel, which improved the reactivity of meth-
ane. It was found that both spark discharge power input
and recycle ratio affected the conversion of methane. The
Fig. 7 Comparison of methane conversion and product selectivities
in plasma reactor alone and the two stage reactor with the hydroge-
nation catalyst of 2.5 wt% Ni-7.5 wt% Zn/MgAl2O4 under different
reactionꢀtemperature.ꢀReactionꢀconditions:ꢀmethaneꢀflowꢀrate:ꢀ40ꢀml/
min, recycle ratio: 4
Fig. 8ꢀ TheꢀdistributionꢀofꢀolefinsꢀandꢀparaffinsꢀinꢀC3–C5 light hydro-
carbon product after recycle plasma-catalyst two stage reactor. Reac-
tion conditions: 2.5 wt% Ni-7.5 wt% Zn/MgAl2O4ꢀcatalyst,ꢀ175ꢀ°C,ꢀ
methaneꢀflowꢀrate:ꢀ40ꢀml/min,ꢀrecycleꢀratio:ꢀ4
1 3

HighlyꢀEfficientꢀConversionꢀofꢀMethaneꢀtoꢀOlefinsꢀviaꢀaꢀRecycle-Plasma-CatalystꢀReactor
7
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a selectivity of acetylene of around 85%, the conversion of
methane varied from 27 to 74%. The yield of hydrogen free
of COx, varied from 20 to 52% depending on conversion,
was also highly desirable for PEM fuel cell.
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Acknowledgments The support of this work by the Fundamental
ResearchꢀFundsꢀforꢀtheꢀCentralꢀUniversitiesꢀ(GrantꢀNo.ꢀ24720094028)ꢀ
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