Fast optimization of LiMgMnOx/La2O3 catalysts for the oxidative coupling of methane

Article abstract of DOI:10.1021/acscombsci.6b00108

The development of efficient catalyst for oxidative coupling of methane (OCM) reaction represents a grand challenge in direct conversion of methane into other useful products. Here, we reported that a newly developed combinatorial approach can be used for ultrafast optimization of La₂O₃-based multicomponent metal oxide catalysts in OCM reaction. This new approach integrated inkjet printing assisted synthesis (IJP-A) with multidimensional group testing strategy (m-GT) tactfully takes the place of conventionally highthroughput synthesis-and-screen experiment. Just within a week, 2048 formulated LiMgMnO_x-La₂O₃ catalysts in a 64.8.8.8.8 = 262 144 compositional space were fabricated by IJP-A in a four-round synthesis-and-screen process, and an optimized formulation has been successfully identified through only 4.8 = 32 times of tests via m-GT screening strategy. The screening process identifies the most promising ternary composition region is Li_0-0.48Mg_0-6.54Mn_0-0.62-La₁₀₀O_x with an external C₂ yield of 10.87% at 700 °C. The yield of C₂ is two times as high as the pure nano-La₂O₃. The good performance of the optimized catalyst formulation has been validated by the manual preparation, which further prove the effectiveness of the new combinatorial methodology in fast discovery of heterogeneous catalyst.

Full text of DOI:10.1021/acscombsci.6b00108

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Article
Fast optimization of LiMgMnOx/La2O3
catalysts for the oxidative coupling of methane
Zhinian Li, Lei He, Shenliang Wang, Wuzhong Yi, Shihui Zou, Liping Xiao, and Jie Fan
ACS Comb. Sci., Just Accepted Manuscript • DOI: 10.1021/acscombsci.6b00108 • Publication Date (Web): 18 Nov 2016
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Fast optimization of LiMgMnO /La O catalysts for the oxidative cou-
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Zhinian Li, Lei He, Shenliang Wang, WuZhong Yi, Shihui Zou, Liping Xiao, Jie Fan*
Department of Chemistry, Zhejiang University, HangZhou 310027, China
KEYWORDS High-throughput; oxidative coupling of methane; multidimensional group testing; ink-jet printing
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ABSTRACT: The development of efficient catalyst for oxidative coupling of methane (OCM) reaction represents a grand
challenge in direct conversion of methane into other useful products. Here, we reported that a newly developed combina-
torial approach can be used for ultra-fast optimization of La O -based multi-component metal oxide catalysts in OCM
reaction. This new approach integrated ink-jet printing assisted synthesis (IJP-A) with multidimensional group testing
strategy (m-GT) tactfully takes the place of conventionally high-throughput synthesis-and-screen experiment. Just within
a week, 2048 formulated LiMgMnO -La O catalysts in a 64*8*8*8*8 = 262144 compositional space were fabricated by IJP-
A in a four-round synthesis-and-screen process, and an optimized formulation has been successfully identified through
only 4*8 = 32 times of tests via m-GT screening strategy. The screening process identifies the most promising ternary
composition region is Li 0-0.48Mg0-6.54Mn0-0.62-La100O with an external C yield of 10.87 % at 700 C. The yield of C is two
times as high as the pure nano-La O . The good performance of the optimized catalyst formulation has been validated by
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the manual preparation, which further prove the effectiveness of the new combinatorial methodology in fast discovery of
heterogeneous catalyst.
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INTRODUCTION
45 C H ) in OCM over La O –based catalyst were 19 – 35 % and
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30 – 56 % in different conditions. Choudhary et al. found
catalytic performance of La O have a good correlation with
the surface acidity and basicity in the OCM process. In their
study, alkaline earth promoter (Mg, Ca, Sr and Ba) strongly
influenced the acidity and basicity of the La O catalysts. The
catalyst Sr-La O shows the best performance (at 800 C,
ratio CH /O = 4.0, space velocity = 102 000 cm
yield = 17.1 %) in the OCM because of the narrow acid
strength distribution and intermediate strength acid sites.
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The oxidative coupling of methane (OCM) to C hydrocar-
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bons (C H , C H ) represents an attractive, yet challenging
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way to monetize natural gas – provided the reaction can be
made to work economically with attractive conversions and
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yields. However, at present the achievement of high selectiv-
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ity and conversion for OCM reaction remains a challenge (an
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h , C2
upper bound of the yield to C is about 25 %). Non-selective
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surface oxidation of methane and subsequently formed hy-
drocarbon products to carbon dioxide result in the poor se-
55 They also found that the surface basicity of the catalyst de-
pends on preparation method, coprecipitation method show-
lectivity of C hydrocarbons associated with high methane
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conversions. Over the past 30 years, a large variety of cata-
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57 ing a higher value than physical mixing method. Besides, the
58 catalysts prepared by coprecipitation method with the ni-
lysts have been developed to achieve the yield target (25 %)
for the industrial applications of OCM reaction. The active
27 catalysts have three or more metal oxide components with
28 more than 25 different elements. These elements have no
29 certain relationship and no equally compositional distribu-
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trates of La and Ca exhibited higher catalytic performance
than that by sodium carbonate and ammonium carbonate.
Faria et al. found the Ce-doped La O catalyst by citrate and
solvothermal methods also show different catalytic perfor-
mance. They link electrophilic oxygen species on catalyst
surface with the catalytic performance. The C selectivity of
sample prepared by the citrate method was 10 % higher to
that by the solvothermal method at 750 C. And the former
mainly contained O and O species, while the latter only
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30 tion, and are almost based on the researchers’ expected and
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interested area.
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Although OCM has been extensively studied over 30 years,
33 the mechanistic details of OCM reaction are still unclear,
34 which makes a rational catalyst design for OCM unrealistic.
35 Therefore, new ways are needed to develop or explore cata-
lyst with good performance. However, the statistics from the
past research result and hypothesize according to the recog-
nized knowledge did not make much progress for this reac-
tion. Combinatorial and screening from difference elements
can be suitably used to find catalyst with good performance,
and thus unveil the reaction mechanism.
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had O species. Metiu et al. also found that the surface prop-
erties including the surface oxygen species, basicity and sur-
face defect, strongly affect the catalytic property of La O
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catalysts as predicted by DFT calculation. LeVan et al. ob-
72 served strontium doping of La O at high temperature (850 -
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73 900 C) also maintain high C2+ selectivity ( > 65 % ) because
74 doped strontium could be textural, retarding the sintering of
75 these highly selective La O oxide particles. Yuhan Su et al.
76 elaborately explored the shape effects of La O nanocatalysts
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77 for oxidative coupling of methane reaction. They found that
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Early in 1980s, La O was firstly reported to be active and
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selective in OCM by Otsuka et al. In the next thirty years,
the values of methane conversion and C selectivity (C H +
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the La O nanorods show the higher C selectivity and CH
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65 the m-GT method, 2048 catalysts were evaluated in 32 tests.
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conversion towards OCM at low temperature (450 – 650 C)
66 The optimized catalyst shows 10.87 % C yield at 700 C. The
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compared to La O nanoparticle. At 6o0 C, CH conversion
67 excavated Li Mg Mn O -La O catalyst display a C yield
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was about 28 %, C selectivity was about 40 % on La O na-
68 even as high as 15 % at a appropriate GHSV of 40,000,
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norods, while the La O nanoparticles displayed 22 % and
69 which is comparable to the performance reported in the Silu-
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28 %, respectively. Their study indicates that the size and
morphology of the catalyst have great effect on the perfor-
70 ria Patent. The good performance of the optimized catalyst
71 formulation has been confirmed in the manual preparation.
72 The precise mass production of the catalyst and rapid screen-
73 ing speed up the discovery of new catalyst for OCM reaction.
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mance of catalyst in OCM. Recently, La O3 nanowire li-
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braries with varied morphologies and compositions are gen-
erated and screened for excellent performance through high
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74 EXPERIMENT
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throughput work flow by Siluria technology. These nan-
owires prepared with filamentous bacteriophage template
are applied in a variety of catalytic reactions like OCM. Typi-
cally, 50 mg of La O nanowire catalyst in 4 mm ID quartz
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75 Lanthanum oxide substrate preparation
76 The quantitative filter paper (FP, Whatman, Grade 2589c) as
77 printed-ink acceptor was uniformly coated with 2 mL lan-
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reaction tube shows that the value of methane conversion, C2
78 thanum nitrate ethanol solution (0.25 mol∙L ). The theoreti-
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selectivity and C yield from 600 to 700 C are 27 %, 54 %,
79 cal final mass of La O was 0.0815 g if one paper was calcined
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and 14 %, respectively (CH /O ratio = 4, GHSV = 130000 h ).
80 in air. The detailed process is listed as follows: A quantitative
81 filter paper was clamped between two square glass molds.
82 Then, 2 mL lanthanum nitrate ethanol solution was trans-
83 ferred on the quantitative filter paper by a pipette (1000 µL)
84 in two times. The coated quantitative filter papers were dried
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In general, most of the studies are mainly about the size,
19 morphology, surface properties of La O and reaction condi-
20 tions in OCM. However, only a few reports studied multi-
21 component La O -based catalyst, which is similar to a well-
22 known Na-Mn-W-SiO system. The main reasons are their
23 versatile ratios, multi-step precise synthesis and complex
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85 at 65 C in an oven in the air for 12 h.
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Ink preparation
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factors. All the factors that are very important for OCM reac-
tion but not easy to control make it difficult to develop an
efficient catalyst. So, a fast, accurate and straightforward
synthesis and measurement technique are extremely needed.
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The preparation of metal precursor ink is similar to previous
15b
report. In the fabrication of Li Mg Mn O -La O library,
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three kinds of metal precursor inks were formulated sepa-
90 rately. The metal precursor inks developed here are noncor-
91 rosive and nontoxic. Taking Li ink as an example, 0.6 mmol
92 lithium acetate (CH COOLi∙H O, Shanghai Feng Shun Fine
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Taking into account the previous research on the OCM re-
action, assuming there is a promising industrial catalyst in 30
suitable candidate elements in the periodic table or their
combination, for a quaternary system using of 21 discrete
concentrations, there will be tens of millions of combina-
tions of these elements. State-of-the-art, for preparing very
small quantities of catalyst, high-throughput inorganic-
catalyst synthesis have been highly successful and are re-
ported to provide thousands of catalysts per day containing
up to four components by thin film deposition and solution-
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93 Chemicals, AR) and 4 g glycerol (Sinopharm Chemical Rea-
94 gent) were dissolved in 60 mL ethanol under vigorously stir-
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ring for 12 hours. The properties of all the printing ink are
listed in Table 1. The addition of glycerol was optimized to
make the ion size (< 3.2 nm), viscosity (5 - 10 mPa∙s) and
surface tension (21 - 22 dyn∙cm ) of the metal precursor inks
meet the strict fluid rheological property requirements of
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00 IJP-A.
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based synthetic methods. However, many high-throughput
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01 Table 1. The properties of ink solution for ink-jet
02 printing synthesis.
syntheses mainly on preparing very small quantities (< 1 mg)
of catalyst did not consider the scale effect of catalytic reac-
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41 tion. In addition, even with these techniques it will easily
42 cost several years of hard work and substantial associated
43 materials consumption when it is applied to make and test
44 tens of millions of possible catalysts for the OCM reaction.
a
Size
nm)
Viscosity
(mpa∙s)
5-10
ST
metal precursor
CH COOLi∙2H O
-1
(
(dyn∙cm )
<0.6
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In our group, we have developed a new combinatorial ap-
Mg(NO ) ∙6H O
5-10
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46 proach that combines IJP-A and m-GT. The effectiveness of
47 the new method has been validated in catalytic reactions like
48 quaternary catalyst of photo-catalytic hydrogen evolution
Mn(CH COO) ∙4H O
1.3
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ST: Surface tension
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49 and ternary metal oxide catalyst for n- hexane oxidation.
104 Design of MMOs/support libraries
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Here, this new combinatorial approach is used for ultra-fast
optimization of La O -based multi-component metal oxide
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05 It is like looking for a needle in a haystack that exploring an
06 optimized MMOs/support catalyst in the full-formulation
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catalysts for OCM reaction. Conventionally, rare earth oxides
(REOs) particularly the sesquioxides, such as La O and
107 space with high formulation precision (0.01). So, a few of
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108 strategies like combinations have been adopted to find out
Sm O , have been investigated as promising catalysts in the
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109 the optimal solution quickly and accurately. In these study,
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OCM at relatively low temperature. Li additive can improve
the performance of rare earth oxides in the OCM reaction
and catalyst of Li/MgO is classical for the OCM. Mn addi-
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10 multidimensional group testing (m-GT) have been success-
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11 fully introduced in optimizing MMOs/support catalyst. In
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12 fact, m-GT is based on bisection method which is used for
tive is considered as a promoter for activity. However, the
multicomponent system of LiMgMnO -La O has never been
13 finding a root of a continuous function in math and combi-
14 nation. Bisection method was used to design MMOs/support
15 libraries. For the colorimetric values of CMYK in a picture,
x
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investigated so far. In this study, LiMgMnO -La O is taken
x
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as an example to validate the effectiveness of our combinato-
rial approach because of its unique composition. A series of
16 the colorimetric values (A ) can range from 0 to 100. The dif-
i
17 ferent color channels can be regarded as different ink chan-
18 nels. The colorimetric values for controlling the output value
19 of diverse color in a picture can be used to control the
63 LiMgMnO -La O with thousands different formulas have
64 been precisely and quickly fabricated via IJP-A. Combined
x
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amount of printed ink. So, the range of each colorimetric was
55 Scheme 2 shows a schematic diagram of the experimental
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firstly divided into 0 - 50 and 50 - 100 according to the bisec-
tion method. Three element inks were equivalent to three
different color. Then, each divided colorimetric are in per-
mutation and combination. There are 8 samples with differ-
ent colorimetric ranges of different color. In order to im-
prove the efficiency of preparing different catalyst, we select
m (m=4) colorimetric values points with reasonable interval
for each element in each group. That is to say, the range of 0-
50 or 50-100 was further divided four intervals by selecting
four points like 13, 25, 38, and 50 in 0 - 50,or 63, 75, 87 and
100 in 50 - 100. The each four colorimetric values in two in-
tervals also were permuted and combined, which produce 64
56 setup. The catalytic activity for oxidative coupling of me-
57 thane was measured using a modified catalytic platform (So-
58 lution 210, Shanghai Solution Analysis Technology Co.Ltd)
59 with conventional quartz fixed bed reactor, a 250 mm of
60 straight cylindrical tubing with an internal diameter of 8 mm
61 at atmospheric pressure. The complex surface/gas phase
62 mechanism of OCM reaction has strong impact on the reac-
63 tion engineering. Beck et al. have found the significant con-
64 tribution of the gas phase reaction under high oxygen pres-
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sure condition. To avoid the contribution of gas phase
reaction, the catalyst bed was placed between quartz wool
plugs, which in the remaining space of the reactor minimize
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formulas in each sample. So, there are 2 ∙4 compositions in
each array. The performance of sample was the average con-
tribution of the 64 formulas. According to the definition of
any reaction that might occur in the post-catalytic volume.
All the catalysts were pelletized and sieved to 40-60 mesh. In
the test, 0.2 g of catalysts diluted with 0.2 g quartz sands
were placed in the reactor. Methane and oxygen were co-fed
at (methane) / (oxygen) = 3 (mol/mol) with 40 % nitrogen.
The total flow rate of the reactant mixture was 66 mL∙min
with a gas hourly space velocity (GHSV) of 20000 mL∙h g .
The OCM reaction temperature was rose to 700 C at a ramp
rate of 10 C∙min . The reaction temperature was controlled
using a thermocouple attached to the outer wall of the reac-
21
the statistical threshold criterion, samples with good per-
formance above a statistical threshold should be chosen as
the candidates for the next iteration. The statistical thresh-
-1
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old is derived from mean and standard deviation. Thus, the
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21 range has been narrowed down, and the intervals should be
22 divided, permuted and combined again. So the different pic-
23 ture should be designed again.
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24 IJP-A synthesis
78 tor, centered along the catalyst bed. The thermocouple well
79 with diameter of 6 mm o.d. inset into the quartz reactor was
80 not only for the hot spot characterization but also for further
81 reducing the free space volume and corresponding residence
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IJP-A was accomplished by C2Fast-II high-throughput syn-
thesis system (Manufactured by Hangzhou Nanonov Inc.).
Firstly, the three ink channels and cartridges were filled with
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time. The picture of real reactor was in the right part in
Scheme 2. The products were analyzed using an online gas
chromatography system (GC-2010) equipped with a flame
ionization detector (FID) and a thermal conductivity detec-
tor (TCD). Once the reaction temperature was attained, the
catalyst was allowed to stabilize for 30 min before data was
collected. The result showed the experiment error (< 5 %)
was allowable to laboratory apparatus. Generally, the carbon
mass balance can be achieved up to 97 %. The blank exper-
iment confirm the empty reactor display little activity to-
28 Mg ink, Mn ink or Li ink. Six quantitative filter papers with
29 lanthanum substrate were paved on the platform of printer
30 in correct location. Second, the designed picture for
31 Li Mg Mn O -La O library were inputted in the printing
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32 system. The number of printing times was set to be 15 and
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the color of C, M, and Y in picture should be consistent with
Mg ink, Mn ink, and Li ink, respectively. Once the procedure
of printing ended, the printed quantitative filter papers were
o
transferred into a 65 C oven and aged for 12 h. They were
o
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calcined at 650 C in air for 5 hours (ramp rate 1 C /min) to
remove the fibre of quantitative filter paper. The oxides with
64 kinds of compositions were tested with a series of physical
characterizations. Finally, the oxides were tableted, crushed,
sieved to a size of 40 - 60 mesh catalyst and tested in OCM
reaction.
ward methane conversion (< 0.5%) and no C products has
2
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Scheme 1. The procedure of the preparation of
Li Mg Mn O -La O library by IJP-A.
a
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46 Characterization
Scanning electron microscopy (SEM) coupled with energy-
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48 dispersive X-ray (EDX) analyses were taken on a Hitachi SU-
49 4810 microscope with a field-emission electron gun operating
50 at 15 KV. Wide-angle-X-ray diffraction (WXRD) patterns
51 were recorded on a Rigaku Ultimate IV with Cu K radiation
52 at 40 mA and 40 KV. ICP-MS was recorded on PerkinElmer
9
9
5
6
Scheme 2. The Scheme of Experiment Setup.
Verified experiment
α
9
9
9
7
8
9
The optimized formulation of Li Mg Mn O -La O catalyst
a
b
c
x
2
3
was confirmed by manual preparation that used nano-La O3
2
53
NexION 300X ICP-MS.
as the starting material (50 nm, Aladdin) and normal im-
54 Catalysis evaluation
1
00 pregnation procedure.
A mixture of lithium acetate
101 (CH COOLi∙2H O), magnesium nitrate (Mg(NO ) ∙6H O),
3
2
3
2
2
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Mn(CH COO) ∙4H O, nano-La O , and ethanol in a molar
3 2 2 2 3
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
ratio of 0.0031 : 0.0605 : 0.0030 : 1.0000 (Li / Mg / Mn / La),
was stirred at room temperature. After removing the ethanol
o
by evaporation, the powder was dried at 65 C for 12 h and
o
o
calcined at 650 C for 5 h (ramp rate 1 C / min). The catalytic
test of the catalyst was identical to IJP-A samples.
7
RESULT AND DISSCUSION
8
9
0
The homogeneity of Lanthanum oxide substrate. To
evaluate the homogeneity of the La O coating process. The
2
3
1
La O -coated substrate was divided into four parts with equal
2 3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
11 area. The weight of each parts after the calcination were
12 listed in Table 2. The small relative standard deviation (<
13 0.6 %) of the mass difference of each parts in three parallel
14 experiments indicates that a good homogeneity has been
15 achieved in La O -coating process.
2
3
16 Table 2.Three parallel experiments and the weight of
17 each quarter for the calcined lanthanum substrates.
18 The quoted RSDs are relative standard deviations.
4
4
4
2
3
4
Figure 1. The working curves of elements. (a) The designed
cyan patterns with different colorimetric value (Ai). (b); (c)
¼ mass of FP (g)
Times
1
RSD%
0.52
45 and (d) The molar ratio percents of M/La (M = Li, Mg, Mn)
1
2
3
4
4
4
4
4
6
7
8
9
(verified by ICP-MS) are as linear function of the AM value
determined by designed color picture in (a). (Magenta star:
the optimized sample verified by ICP-MS/G1-1-1; red rectan-
gular: the optimized area of 0-25)
0.1239 0.1185 0.1312 0.1259
2
3
0.1256 0.1200 0.1257 0.1239
0.1262 0.1180 0.1217 0.1235
0.26
0.34
5
5
5
5
5
5
5
5
0
1
2
3
4
5
6
7
As for the A value calculation, take the G1-1-1 catalysts li-
brary for example. The designed image for G1-1-1 is given in
1
2
9
0
IJP-A synthesis of LiMgMn-La O library. To validate the
2 3
effectiveness and precise composition control of IJP-A in the
Fig. 2, which has special A , AMg, AMn values as the yellow,
Li
21 preparation of Li Mg Mn O -La O catalyst libraries, the ex-
a
b
c
x
2
3
cyan, and magenta. The A , AMg, AMn values are diverse in 0,
Li
22 periments are implemented to relate the printing parameters
23 to the real constitutes. Prior to the printing, cyan patterns
24 with colorimetric value (Ai) of 100, 70, 50, 20, and 10 were
8, 17, 25 according to bisection method. So, after permutation
and combination of A , AMg, AMg, there are 64 different for-
Li
mulations. Each formulation corresponds to one composi-
tion in this sample.
2
2
2
2
2
3
3
3
3
3
3
3
3
5
6
7
8
9
0
1
2
3
4
5
6
7
designed by Photoshop in Fig 1 (a). All parameters of printing
including times, ink channels, ink concentrations and the
process of IJP-A were the same as designing experiment pro-
cess. After printing, the mole ratio of metal/ La can be calcu-
lated by the printed volume and the calcined weigh, which
was verified by ICP-MS. As a result, a good linear of mole
ratio of metal / La to the A value range between 0 and 100
were obtained in Fig. 1 (b); (c) and (d). The values of linear
dependent coefficient in M/La (M = Li, Mg, Mn) are respec-
tively 0.9751, 0.9965, and 0.9944. The good linear correlation
demonstrates that the method of IJP-A are accurate and fea-
sible in preparing Li Mg Mn O -La O libraries. In addition,
5
5
6
8
9
0
For a sample with a number of average A values, the defi-
nition based on additivity of colorimetric should be given as
follow,
ꢀꢁꢂꢄꢅ ꢆꢄꢀꢁꢂꢄꢇ ꢆ ⋯ ꢀꢁꢂꢄꢈ
ꢀꢁꢂ
ꢃ
ꢉ
ꢀ
ꢊꢋꢄꢅ ꢆꢄꢀꢊꢋꢄꢇ ꢆ ⋯ ꢀꢊꢋꢄꢈ
ꢀꢊꢋ
ꢃ
ꢉ
ꢊꢌꢄꢇ
ꢀꢊꢌꢄꢅ ꢆꢄꢀ
ꢆ ⋯ ꢀꢊꢌꢄꢌ
ꢀꢊꢌ
ꢃ
ꢄ
a
b
c
x
2
3
ꢉ
precise and flexible adjustment of the metal composition of
38 Li Mg Mn O -La O libraries can be realized according to the
39 A values of working curves. The actual composition can also
40 be predicted by the working curve after the A value of the
41 group calculated like the one illustrated bellow.
61
A is the colorimetric value designed in a picture (image of
62 G1-1-1), n is the number of total formulations in a picture.
a
b
c
x
2
3
6
6
6
3
4
5
According to the calculation formulas above, the average
values of A , AMg, and AMn are 12.5, 12.5 and 12.5 for G1-1-1.
Li
Predicting the composition by working curves, the mole ratio
66 percents of Li/La, Mg/La, Mn/La were 0.24 %, 3.27 %, and
67 0.31 %. Subsequently, the average mole ratio percent of Li/La,
68 Mg/La, and Mn/La in this sample was measured by ICP-MS,
6
7
7
7
7
9
0
1
2
3
which are 0.31 %, 4.05 % and 0.29 %. The errors between the
theory and measured value are within 0.78 %, which verified
the high precision and reproducibility of IJP-A at the same
time. This excellent accuracy should be sufficient screening
the metal oxide too.
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mainly peaks located at 2θ = 13.o, 22.o, 29.o, and 40.0 corre-
spond to the (020), (110), (130), and (060) crystal planes of
1
2
3
4
5
6
7
8
9
29 La O CO . There are a number of weakly diffraction peaks
30 that can be well indexed to LiMnO , which is consistent with
31 the low concentration ink and few volume spout out. No
32 peaks ascribed to MgO phase was detected, suggesting an
2
2
3
2
3
3
3
3
4
5
intimate mixture of Mg and La components. This also
demonstrates that the printed elements of IJP-A are well
dispersed.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
2
3
Figure 2. Designed image and AMg, AMn, A value of G1-1-1
catalysts library.
Li
36
3
3
7
8
Figure 4. XRD patterns of G1-1-1(Li0-0.48Mg0-6.54Mn0-0.62-
La100O ) synthesized by IJP-A.
x
3
4
4
4
4
4
4
4
4
4
4
9
0
1
2
3
4
5
6
7
8
9
Catalytic test. To select a suitable reaction condition, the
basic characterization of this reaction should be explored in
both literatures and experiments. It is extensively suggested
that the temperature, feed composition and space velocity
24
have significant effects on the reaction of OCM. Fig. 5
shows the results of the catalytic activity of an unmodified
lanthanum oxide catalyst on different temperature, feed
composition and space velocity. It was evident that the con-
version of methane significantly increased from 3 to 26 % as
the reaction temperature was increased from 500 to 700 °C,
and then it leveled off in Figure 5(a). With the temperature
50 further increased to 800 °C, it got up to 29 %. The oxygen
51 conversion also increased from 10 to 91 % in the reaction
52 temperature from 500 to 700°C; then, it slowly reached to 97 %
53 when the temperature rose to 800 °C. The main products of
54 the OCM reaction are С H , С H , СО and CO. With tem-
4
5
6
7
Figure 3. Scanning electron microscopy and EDX elemental
mapping of the quaternary G1-1-1 library synthesized by the
IJP-A.
2
6
2
4
2
5
5
5
5
5
6
6
6
6
6
6
6
6
5
6
7
8
9
0
1
2
3
4
5
6
7
perature increasing, the selectivity of C (С H +С H ) and the
2 2 6 2 4
8
Compositional and structural characterization.
ratio of C H /C H enhanced, leading to the continuously
2
4
2
6
9
0
1
G1-1-1 (Li0-0.48Mg0-6.54Mn0-0.62-La100O ) was taken as an exam-
x
increase of C2 yield. The increasing C H /C H ratio with
2
4
2
6
1
1
ple to show the typical morphology of quaternary metal ox-
ides prepared by the IJP-A technique. The SEM image
increasing temperature suggests that it is favorable for CH4
activation and C H dehydrogenation in relative high tem-
2
6
12 showed irregular shape for the catalysts. But the EDX result
13 showed uniform distribution of La, Mn and Mg in printed
14 sample. The homogeneous atomic compositions prepared by
15 IJP-A synthesis indicates the synthesis is precise and efficient
perature. This means parts of products like C H and C H
6
2
4
2
were over-oxidized on a higher temperature. So, the temper-
ature for OCM was chosen at 700 °C. The dependence of the
feeding composition (CH /O ratio) was also studied in Fig.
4
2
1
1
1
1
2
2
6
7
8
9
0
1
in preparing well-disperse sample. Because the relative atom-
ic weight and the content of the lithium are too small, it is
too difficult to collect its signal. But the existence of Li was
confirmed by XRD and ICP-MS. Besides, the concentration
of Li within G1-1-1 was in good agreement with the theoreti-
cal amount (from the result of standard curve).
5(b). The decrease of methane conversion and increase of C2
selectivity is observed when increasing the ratio, which is
9b
typical for OCM reaction. As shown in Fig. 5(c), there is no
significant change of the methane conversion and eth-
68 ylene/ethane ratio when increasing space velocity. However,
69 there is a steady increase of C selectivity and corresponding
70 C yield as the space velocity (GHSV) increases from 10,000
71 mL∙h g to 40,000 mL∙h g before leveling off. Both nano-
2
2
2
2
2
2
2
3
4
5
6
For G1-1-1 group, the X-ray diffraction pattern (Fig. 4) was
2
o
-
1
-1
-1 -1
obtained after calcination at 650 C. The diffraction peaks at
26.1o, 29.1o, 30.0o, 39.6 o, 46.3o, and 52.1 could be well in-
dexed to (100), (002), (101), (102), (110) and (103) plane reflec-
72
La O and optimal Li Mg Mn O -La O catalysts (shown in
2 3 a b c x 2 3
Fig. 7b) show the similar GHSV-dependence. It is worthy of
73
tions of hexagonal La O (JCPDS card number: 05-0602). The
2
3
74 noting that the chosen reaction condition (reaction tempera-
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ture, space velocity, feeding ratio) is only serve as an identi-
17 space. The optimized process starts by dividing the selected
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
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4
4
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6
cal reaction for the purpose of screening large number of
catalysts, which do not stands for an optimized condition for
a given catalyst. Meanwhile the reaction condition has also
been applied in other researchers. It help us to compare our
18 space into eight subspace by bisection method. (As descrip-
19 tion in design of MMOs/support libraries). Each subspace
20 contains 64 formulas. The catalytic evaluation can validate
21 the average performance of the 64 formulas. The good
22 groups with better catalytic activity than the threshold calcu-
1
6
result with their result.
2
2
2
2
2
2
2
3
3
3
4
5
6
7
8
9
0
1
lated from the statistical threshold criterion are chosen to be
next iteration. Then, the offspring groups with reassignment
molar ratio subinterval by the bisection method are evaluat-
ed and the screening process starts again. If the catalytic
activity of the offspring groups were enhanced, it means that
the screening process is effective. Then, the screening pro-
cess should be repeated until no better subgroup can be
found. From the perspective of mathematics, it is conver-
gent to find a supper catalyst.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
32
3
3
3
4
Scheme 3. The screening schematic diagram by m-GT
3
3
5
6
Fig. 7 shows the catalytic performance of all the sample
fabricated by IJP-A at 700 C. The detailed data are shown in
o
37 Table 3. In the first screening, the different composition cata-
38 lysts show great difference in OCM. The C yield of OCM in
2
39 G1 (C yield = 7.6 %) was higher than other catalyst libraries,
2
40 which was also higher than threshold (6.8 %). Others were
41
no more than 6.0 %. G7 displayed the lowest C yield of 2.8 %.
2
42
So G1 was selected for the second round screening.
43
In the second screening, we shrinked the screening space
44 of G1 by halving the ink concentration. So the G1 was divided
45 into eight smaller cubes which the interval is 0 - 100. The
4
4
4
4
5
6
7
8
9
0
statistical threshold of the second screening was 8.3 %. G1-1
shows the best C2 yield of 8.5 %. Compared with the first
screening, the threshold of C2 yield was increased by as
much as two percent. This implies that screening can’t be
ended.
5
5
5
5
5
5
1
2
3
4
5
6
In the third iteration, we further narrowed space to 0-50
and optimized. The yield of all the group increased except
G1-1-3 and G1-1-4 compared with the second screening. The
threshold (8.7 %) was improved a little bit. As shown in Fig.
6 (c), the spaces of Li, Mg and Mn of the best group are 0-25,
0 - 25 and 0 - 25 with 10.8 % C yield.
2
57
We rescaled the intervals of the G1-1-1 to the range of 0-25.
7
58 Five hundreds printed Li Mg Mn O -La O catalysts with
a b c x 2 3
5
6
6
6
6
9
0
1
2
3
high formulation precision were synthesized and tested. It
shows that five subgroups of catalysts have the similar cata-
lytic performance as that for G1-1-1, and three subgroups (G1-
1-1-3, G1-1-1-4 and G1-1-1-6) exhibits a little bit lower C2 yields
in Fig. 6(d). Since there is no group with performance above
8
9
Figure 5. The OCM reaction characteristics of commercial
nano-La O (50 nm, Aladdin) as a function of (a) temperature
2
3
10 (b) CH /O ratio (c) space velocity. (solid☆) O conversion; (
4
2
2
11 solid □) CH conversion; (◇) C H /C H ratio; (△) C yield;
4
2
4
2
6
2
12 and (solid ○) C selectivity
2
64 the calculated threshold, the optimization process stops at
this round. The optimal composition of catalyst for OCM was
66 excavated finally.
65
13 m-GT screening. The m-GT methodology (scheme 3) is
14 used to efficiently optimize composition of Li Mg Mn O -
15 La O , which establishes a close and straight-forward corre-
a
b
c
x
6
6
7
8
In the whole screening shown in Fig. 6 (e), there is a ten-
dency that the catalytic performance of groups improved
2
3
16 spondence between the optimization results and screening
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with the screening space narrowed, which means that the
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
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4
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4
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4
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5
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5
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higher yield of C was obtained in OCM reaction, with the
2
fewer amounts of Li, Mg, Mn additives in Li Mg Mn O -
a
b
c
x
La O catalysts. In other words, La O provides the major
2
3
2
3
contribution of OCM activity in Li Mg Mn O -La O cata-
a
b
c
x
2
3
lysts, while the small additives lead to some modified surface
structure, which could enhance the activity and stability to
some extent. It is well accepted that surface oxygen species
25
like oxygen vacancies is required for methane activation.
A
1
1
1
1
1
1
1
1
1
surface with an oxygen vacancy which have two unpaired
electrons is a Lewis base. And small low-valence dopants like
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
26
Li, Mg turned the La O surface into a Lewis acid. Because
2
3
of the acid-base pair interaction, they can further lower the
energy of oxygen vacancy formation and increase the energy
of the dissociative adsorption of CH thus lower the activa-
4
,
26-27
tion energy for breaking the C-H bond.
According to DFT
calculations, Derk et al. also found the presence of a lower-
valence dopant (e.g., Fe, Cu, Mg, Zn) in the surface of La O3
2
19 lowers the energy of oxygen-vacancy formation very substan-
20 tially. The Mn dopant can increase the amount of oxygen
21 vacancies, promoting O activation. Despite of this, design-
22 ing a good oxidation catalyst by doping, especially for selec-
23 tive oxidation, the “moderation principle” is very important.
28
29
2
2
2
2
2
2
2
3
3
3
3
3
3
4
5
6
7
8
9
0
1
2
3
4
5
Therefore, an appropriate amount of Li, Mg and Mn is suffi-
cient to adjust the competition between selective reaction
(C ) and deep oxidation (CO ) by generating active oxygen
2+
x
species. The result from screening suggests a small amount
of additives can boost the performance of La O -based cata-
2
3
lysts, whereas the subgroups with the high contents of addi-
tives exhibit little improvement of the catalytic performance.
In each round test, the subgroups (G1-4, G1-1-4, G1-1-1-4),
which has a relatively high Li and Mn contents show inferior
performance than others. This may relate to the formation of
LiMnO as confirmed by XRD characterization, which de-
2
creases the oxygen vacancy of the La O surface. As the space
2
3
36 narrowed constantly, it turned out to be better. Meanwhile,
37 the presence of high Mg contents may help the dispersion of
38 Li and Mn components, resulting in a better performance of
39 G1-1-1-8 as compared to G1-1-1-4. Despite of this, we believe
40 that a more systematic and comprehensive study of the
4
4
4
4
1
2
3
4
structure/composition-to-performance correlation is neces-
sary to fully reveal the relationship between the composi-
tions and activities to understand the mechanism of high
activity, which is ongoing and will be reported elsewhere.
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1
1
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1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
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5
5
5
6
Table 3 Detailed catalytic results of all sample prepared by IJP-A.
(
Li, Mg, Mn)
Calculated average
mole ratio
CH4
C2
C2
No.
Parameters
Conversion % Selectivity % Yield %
G1
G2
(0-100), (0-100), (0-100)
(100-200), (0-100), (0-100)
(0-100), (0-100), (100-200)
(100-200), (0-100), (100-200)
(0-100), (100-200), (0-100)
(100-200), (100-200), (0-100)
(0-100), (100-200), (100-200)
(100-200),(100-200), (100-200)
(0-50), (0-50), (0-50)
Li0.12Mg16.48Mn1.56-La100Ox
Li3.14Mg16.48Mn1.56-La100Ox
Li0.12Mg16.48Mn4.02-La100Ox
Li3.14Mg16.48Mn4.02-La100Ox
Li0.12Mg42.52Mn1.56-La100Ox
Li3.14Mg42.52Mn1.56-La100Ox
Li0.12Mg42.52Mn4.02-La100Ox
Li3.14Mg42.52Mn4.02-La100Ox
Li0.60Mg8.24Mn0.78-La100Ox
Li1.57Mg8.24Mn0.78-La100Ox
Li0.60Mg8.24Mn2.01-La100Ox
Li1.57Mg8.24Mn2.01-La100Ox
Li0.60Mg21.26Mn0.78-La100Ox
Li1.57Mg21.26Mn0.78-La100Ox
Li0.60Mg21.26Mn2.01-La100Ox
Li1.57Mg21.26Mn2.01-La100Ox
Li0.24Mg3.27Mn0.31-La100Ox
Li0.71Mg3.27Mn0.31-La100Ox
Li0.24Mg3.27Mn0.91-La100Ox
Li0.71Mg3.27Mn0.91-La100Ox
Li0.24Mg9.68Mn0.31-La100Ox
Li0.71Mg9.68Mn0.31-La100Ox
Li0.24Mg9.68Mn0.91-La100Ox
Li0.71Mg9.68Mn0.91-La100Ox
Li0.15Mg1.57Mn0.12-La100Ox
Li0.37Mg1.57Mn0.12-La100Ox
Li0.15Mg1.57Mn0.47-La100Ox
Li0.37Mg1.57Mn0.47-La100Ox
Li0.15Mg4.97Mn0.12-La100Ox
22.87
21.92
22.35
20.9
33.04
26.47
22.41
28.77
25.86
26.49
14.87
24.72
39.06
35.84
34.31
32.27
36.28
38.42
34.31
30.61
42.32
33.95
33.34
28.02
32.29
35.98
34.13
39.59
32.21
7.56
5.8
0
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9
0
1
2
3
4
5
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7
8
9
0
G3
5.01
G4
6.01
5.5
1st
Round
G5
21.26
20.87
18.82
23.61
21.83
21.31
G6
5.53
2.8
G7
G8
5.84
8.53
7.64
7.81
G1-1
G1-2
(50-100), (0-50), (0-50)
(0-50), (0-50), (50-100)
(50-100), (0-50), (50-100)
(0-50), (50-100), (0-50)
(50-100), (50-100), (0-50)
(0-50), (50-100), (50-100)
(50-100), (50-100), (50-100)
(0-25), (0-25), (0-25)
G1-3
20.93
20.7
G1-4
6.68
8.09
8.24
6.96
6.27
10.87
8.81
7.09
6.15
2nd
Round
G1-5
22.29
21.46
20.28
20.48
25.69
25.95
21.25
21.96
24.13
24.62
23.61
21.58
33.57
33.42
31.90
30.97
33.69
G1-6
G1-7
G1-8
G1-1-1
G1-1-2
G1-1-3
G1-1-4
G1-1-5
G1-1-6
G1-1-7
G1-1-8
G1-1-1-1
G1-1-1-2
G1-1-1-3
G1-1-1-4
G1-1-1-5
(25-50), (0-25), (0-25)
(0-25), (0-25), (25-50)
(25-50), (0-25), (25-50)
(0-25), (25-50), (0-25)
(25-50), (25-50), (0-25)
(0-25), (25-50), (25-50)
(25-50), (25-50), (25-50)
(0-13), (0-13), (0-13)
3rd
Round
7.79
8.86
8.06
8.54
10.81
9.90
8.61
8.06
10.85
(13-25), (0-13), (0-13)
29.62
26.99
26.03
32.21
4th
Round
(0-13), (0-13), (13-25)
(13-25), (0-13), (13-25)
(0-13), (13-25), (0-13)
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G1-1-1-6
G1-1-1-7
G1-1-1-8
(13-25), (13-25), (0-13)
(0-13), (13-25), (13-25)
(13-25), (13-25), (13-25)
Li0.37Mg4.97Mn0.12-La100Ox
Li0.15Mg4.97Mn0.47-La100Ox
Li0.37Mg4.97Mn0.47-La100Ox
31.51
33.04
28.33
26.16
30.81
36.74
8.25
10.18
10.41
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Figure 7. (a) The results of G1-1-1 group synthesized by dif-
28 ferent methods. Reaction condition: 200 mg catalysts with
29 200 mg quartz, CH : O : N =3 : 1 : 2.6, GHSV =20000 h . T =
30 700 C (b) The reaction data as a function of space velocity
31 obtained over optimized catalyst at 700 C; CH : O : N =3 : 1 :
-1
4
2
2
o
o
1
4
2
2
3
2
2.6.
However, The 10.87 % C yield is still only about 80% of
2
3
4
5
6
7
8
9
0
1
2
3
Figure 6. The fast screening result of Li Mg Mn O -La O
a b c x 2 3
3
3
2
libraries for oxidative coupling of methane. (Yield of C as
2
34 the Siluria benchmark. As shown in Fig. 7(b), the C yield
35 increase with the increase of space velocity according to Fig.
36 5(c). We found the optimal Li Mg Mn O -La O catalyst dis-
2
evaluation index). (a) The 1st round screening: the best sub-
space (G1) were subjected to next iteration. (b) The 2nd
round screening: the G1-1 with best activity was sent to fol-
lowing iteration. (c) The 3rd round screening: the optimized
composition was G1-1-1. (d) The 4th round screening: no dis-
tinct improvement was found. Reaction condition: 200 mg
catalysts with 200 mg quartz, CH : O : N =3 : 1 : 2.6, GHSV =
a
b
c
x
2
3
3
3
3
4
4
4
4
7
8
9
0
1
2
3
play a C yield as high as 15.2 % at a higher GHSV of 40,000
2
-1 -1
mL∙h g . Although this value is comparable to Siluria
o
benchmark (14 %), the reaction temperature is 100 C higher
than that reported in the patent. It is possible to further im-
prove the catalytic performance of the Li Mg Mn O -La O
3
1
1
1
1
4
2
2
a
b
c
x
2
-1 -1
o
20000 mL·h g . T = 700 C (e) The direction and process of
catalyst if the composition optimization is based on some
specific La O nanomaterials, which have shown excellent
screening was summarized and the location of the optimized
2
3
10b
catalyst can be found immediately in the 3D A space.
i
44 activity at low reaction temperature.
45 CONCLUSION
1
1
1
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2
4
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0
1
2
To verify the effectiveness of the IJP-A synthesis and m-GT
methodology, the optimized formulation of G1-1-1 catalyst
was confirmed by manual preparation with impregnation
4
4
6
7
A series of Li, Mg, Mn on La O catalysts have been prepared
2 3
by IJP-A synthesis, and evaluated towards the oxidative cou-
method and nano-La O3 as the starting material (50 nm,
2
48 pling of methane with m-GT algorithm. High-throughput
Aladdin). The performance of the catalysts were shown in Fig.
7(a). The performance of the manual G1-1-1 shows a yield of
4
9
(HT) screening based on IJP-A and m-GT is successfully in-
troduced to the system of OCM for the first time. Compared
5
0
10.56 %, whose yield of C is almost as same as the sample
2
51 to other HT experiments, the procedures of preparation and
52 testing are simple without the preparation of special plate
53 and the quantity of catalysts are relatively in a mass. More
excavated by IJP-A and m-GT, which doubles the C yield of
2
the nano-La O . This result of the optimized group repro-
2
3
23 duced by the manual sample manifests that optimization via
24 IJP-A and m-GT are reliable and valid.
5
4
important, the whole library is explored and the numbers of
validation tests can be greatly reduced. The optimal catalyst
5
5
2
5
56 with the yield of C
exploring 2048 different candidates of LiMgMnO -La O
3
2
up to 11 % has been screened out after
5
7
x
2
58 within 32 catalytic tests. Further adjusting the reaction con-
59 dition, the optimal Li Mg Mn O -La O catalyst display a C2
a
b
c
x
2
3
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yield as high as 15 %, which is comparable to the best yield of
La O nanowires reported in Siluria patent. Above all, the
optimized composition by HT screening are reproduced and
validated by manual preparation. We believe that the feasi-
bility and validity of IJP-A and m-GT will be applied in explo-
ration of other functional materials beyond catalysis.
62 their performance in oxidative coupling of methane. J. Chem.
63 Technol. Biotechnol. 1998, 72 (2), 125-130.
64 (7). Rane, V. H.; Chaudhari, S. T.; Choudhary, V. R., Oxidative
1
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coupling of methane over La-promoted CaO catalysts: Influence
of precursors and catalyst preparation method. J. Nat. Gas Chem.
2010, 19 (1), 25-30.
(8). Ferreira, V. J.; Tavares, P.; Figueiredo, J. L.; Faria, J. L., Ce-
2 3
Doped La O based catalyst for the oxidative coupling of me-
7
thane. Catal. Commun. 2013, 42, 50-53.
(9). (a) Kuś, S.; Otremba, M.; Taniewski, M., The catalytic per-
8
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AUTHOR INFORMATION
Corresponding Author
72 formance in oxidative coupling of methane and the surface ba-
73 sicity of La , Nd , ZrO and Nb ☆. Fuel 2003, 82 (11), 1331-
1338; (b) Gao, Z.; Shi, Y., Suppressed formation of CO and H
in the oxidative coupling of methane over La /MgO catalyst
0
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O
3
2
O
3
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2 5
O
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2
O
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* Corresponding author: jfan@zju.edu.cn
Notes
The authors declare no competing financial interest.
2 3
O
by surface modification. J. Nat. Gas Chem. 2010, 19 (2), 173-178;
(c) Li, B.; Metiu, H., DFT Studies of Oxygen Vacancies on Un-
doped and Doped La O Surfaces. J. Phys. Chem. C 2010, 114 (28),
2 3
12234-12244; (d) Lei, Y.; Chu, C.; Li, S.; Sun, Y., Methane Activa-
13 ACKNOWLEDGMENT
tions by Lanthanum Oxide Clusters. J. Phys. Chem. C 2014, 118
1
1
1
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4
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7
We thank Dr. C. Mesters, Dr. A. Vandermade, and Dr S.
Venkatraman for the very helpful discussion, Prof. L.J. Chou,
and Dr. J. Yang on the help of building up OCM reaction sys-
tem. This work was supported by Shell Global Solutions In-
81 (15), 7932-7945; (e) Song, J.; Sun, Y.; Ba, R.; Huang, S.; Zhao, Y.;
82 Zhang, J.; Sun, Y.; Zhu, Y., Monodisperse Sr-La hybrid nano-
fibers for oxidative coupling of methane to synthesize C hydro-
2 3
O
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carbons. Nanoscale 2015, 7 (6), 2260-2264; (f) Ghose, R.; Hwang,
H. T.; Varma, A., Oxidative coupling of methane using catalysts
synthesized by solution combustion method. Appl. Catal. A -
Gen. 2013, 452, 147-154; (g) Chen, M. L.; Hou, Y. H.; Xia, W. S.;
Weng, W. Z.; Cao, Z. X.; Zhou, Z. H.; Wan, H. L., Dimeric 1,3-
propanediaminetetraacetato lanthanides as the precursors of
18 ternational B.V. (PT37712), National Natural Science Founda-
19 tion of China (21271153, 21373181, 21222307, U1402233), Major
20 Research Plan Of National Natural Science Foundation of
21 China (91545113) , Fok Ying Tung Education Foundation
2
2
(131015) and the Fundamental Research Funds for the Central
Universities (2014XZZX003-02).
2
3
90 catalysts for the oxidative coupling of methane. Dalton Trans.
91 2014, 43 (23), 8690-8697.
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