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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 Downloaded from http://pubs.acs.org on November 21, 2016
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ACS Combinatorial Science
Fast optimization of LiMgMnOx/La2O3 catalysts for the oxidative coupling of methane 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 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 combinatorial approach can be used for ultra-fast optimization of La2O3-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 LiMgMnOx-La2O3 catalysts in a 64*8*8*8*8 = 262144 compositional space were fabricated by IJPA 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-La100Ox with an external C2 yield of 10.87 % at 700 oC. 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.
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INTRODUCTION
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The oxidative coupling of methane (OCM) to C2 hydrocarbons (C2H4, C2H6) represents an attractive, yet challenging way to monetize natural gas – provided the reaction can be made to work economically with attractive conversions and 1 yields. However, at present the achievement of high selectivity and conversion for OCM reaction remains a challenge (an upper bound of the yield to C2 is about 25 %). Non-selective surface oxidation of methane and subsequently formed hydrocarbon products to carbon dioxide result in the poor selectivity of C2 hydrocarbons associated with high methane 2 conversions. Over the past 30 years, a large variety of catalysts have been developed to achieve the yield target (25 %) for the industrial applications of OCM reaction. The active catalysts have three or more metal oxide components with more than 25 different elements. These elements have no certain relationship and no equally compositional distribution, and are almost based on the researchers’ expected and 3 interested area.
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Although OCM has been extensively studied over 30 years, the mechanistic details of OCM reaction are still unclear, which makes a rational catalyst design for OCM unrealistic. Therefore, new ways are needed to develop or explore catalyst with good performance. However, the statistics from the past research result and hypothesize according to the recognized knowledge did not make much progress for this reaction. Combinatorial and screening from difference elements can be suitably used to find catalyst with good performance, and thus unveil the reaction mechanism. Early in 1980s, La2O3was firstly reported to be active and 4 selective in OCM by Otsuka et al. In the next thirty years, the values of methane conversion and C2 selectivity (C2H4 +
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C2H6) in OCM over La2O3–based catalyst were 19 – 35 % and 5 30 – 56 % in different conditions. Choudhary et al. found catalytic performance of La2O3 have a good correlation with 6 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 La2O3 catalysts. The o catalyst Sr-La2O3 shows the best performance (at 800 C, -3 -1 -1 ratio CH4/O2 = 4.0, space velocity = 102 000 cm g h , C2 yield = 17.1 %) in the OCM because of the narrow acid strength distribution and intermediate strength acid sites. They also found that the surface basicity of the catalyst depends on preparation method, coprecipitation method show7 ing a higher value than physical mixing method. Besides, the catalysts prepared by coprecipitation method with the nitrates of La and Ca exhibited higher catalytic performance than that by sodium carbonate and ammonium carbonate. Faria et al. found the Ce-doped La2O3 catalyst by citrate and solvothermal methods also show different catalytic perfor8 mance. They link electrophilic oxygen species on catalyst surface with the catalytic performance. The C2 selectivity of sample prepared by the citrate method was 10 % higher to o that by the solvothermal method at 750 C. And the former 2mainly contained O and O2 species, while the latter only had O species. Metiu et al. also found that the surface properties including the surface oxygen species, basicity and surface defect, strongly affect the catalytic property of La2O3 8-9 catalysts as predicted by DFT calculation. LeVan et al. observed strontium doping of La2O3 at high temperature (850 o 900 C) also maintain high C2+ selectivity ( > 65 % ) because doped strontium could be textural, retarding the sintering of these highly selective La2O3 oxide particles. Yuhan Su et al. elaborately explored the shape effects of La2O3 nanocatalysts for oxidative coupling of methane reaction. They found that
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the La2O3 nanorods show the higher C2 selectivity and CH4 o conversion towards OCM at low temperature (450 – 650 C) o compared to La2O3 nanoparticle. At 6o0 C, CH4 conversion was about 28 %, C2 selectivity was about 40 % on La2O3 nanorods, while the La2O3 nanoparticles displayed 22 % and 28 %, respectively. Their study indicates that the size and morphology of the catalyst have great effect on the perfor10 mance of catalyst in OCM. Recently, La2O3 nanowire libraries with varied morphologies and compositions are generated and screened for excellent performance through high 11 throughput work flow by Siluria technology. These nanowires prepared with filamentous bacteriophage template are applied in a variety of catalytic reactions like OCM. Typically, 50 mg of La2O3 nanowire catalyst in 4 mm ID quartz reaction tube shows that the value of methane conversion, C2 o selectivity and C2 yield from 600 to 700 C are 27 %, 54 %, -1 and 14 %, respectively (CH4/O2 ratio = 4, GHSV = 130000 h ). In general, most of the studies are mainly about the size, morphology, surface properties of La2O3 and reaction condi11 tions in OCM. However, only a few reports studied multicomponent La2O3-based catalyst, which is similar to a well12 known Na-Mn-W-SiO2 system. The main reasons are their versatile ratios, multi-step precise synthesis and complex factors. All the factors that are very important for OCM reaction 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. Taking into account the previous research on the OCM reaction, 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 combinations of these elements. State-of-the-art, for preparing very small quantities of catalyst, high-throughput inorganiccatalyst synthesis have been highly successful and are reported to provide thousands of catalysts per day containing up to four components by thin film deposition and solution13 based synthetic methods. However, many high-throughput syntheses mainly on preparing very small quantities (< 1 mg) of catalyst did not consider the scale effect of catalytic reac14 tion. In addition, even with these techniques it will easily cost several years of hard work and substantial associated materials consumption when it is applied to make and test tens of millions of possible catalysts for the OCM reaction. In our group, we have developed a new combinatorial approach that combines IJP-A and m-GT. The effectiveness of the new method has been validated in catalytic reactions like quaternary catalyst of photo-catalytic hydrogen evolution 15 and ternary metal oxide catalyst for n- hexane oxidation. Here, this new combinatorial approach is used for ultra-fast optimization of La2O3-based multi-component metal oxide catalysts for OCM reaction. Conventionally, rare earth oxides (REOs) particularly the sesquioxides, such as La2O3 and Sm2O3, have been investigated as promising catalysts in the 16 OCM at relatively low temperature. Li additive can improve 17 the performance of rare earth oxides in the OCM reaction 18 and catalyst of Li/MgO is classical for the OCM. Mn additive is considered as a promoter for activity. However, the multicomponent system of LiMgMnOx-La2O3 has never been investigated so far. In this study, LiMgMnOx-La2O3 is taken as an example to validate the effectiveness of our combinatorial approach because of its unique composition. A series of LiMgMnOx-La2O3 with thousands different formulas have been precisely and quickly fabricated via IJP-A. Combined
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the m-GT method, 2048 catalysts were evaluated in 32 tests. o The optimized catalyst shows 10.87 % C2 yield at 700 C. The excavated LiaMgbMncOx-La2O3 catalyst display a C2 yield
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even as high as 15 % at a appropriate GHSV of 40,000,
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which is comparable to the performance reported in the Silu11 ria Patent. The good performance of the optimized catalyst formulation has been confirmed in the manual preparation. The precise mass production of the catalyst and rapid screening speed up the discovery of new catalyst for OCM reaction.
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EXPERIMENT
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Lanthanum oxide substrate preparation
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The quantitative filter paper (FP, Whatman, Grade 2589c) as printed-ink acceptor was uniformly coated with 2 mL lan-1 thanum nitrate ethanol solution (0.25 mol∙L ). The theoretical final mass of La2O3 was 0.0815 g if one paper was calcined in air. The detailed process is listed as follows: A quantitative filter paper was clamped between two square glass molds. Then, 2 mL lanthanum nitrate ethanol solution was transferred on the quantitative filter paper by a pipette (1000 µL) in two times. The coated quantitative filter papers were dried o at 65 C in an oven in the air for 12 h.
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Ink preparation
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The preparation of metal precursor ink is similar to previous 15b report. In the fabrication of LiaMgbMncOx-La2O3 library, three kinds of metal precursor inks were formulated separately. The metal precursor inks developed here are noncorrosive and nontoxic. Taking Li ink as an example, 0.6 mmol lithium acetate (CH3COOLi∙H2O, Shanghai Feng Shun Fine Chemicals, AR) and 4 g glycerol (Sinopharm Chemical Reagent) were dissolved in 60 mL ethanol under vigorously stirring 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 -1 surface tension (21 - 22 dyn∙cm ) of the metal precursor inks meet the strict fluid rheological property requirements of IJP-A.
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Table 1. The properties of ink solution for ink-jet printing synthesis. metal precursor
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a
Size
Viscosity
ST
(nm)
(mpa∙s)
(dyn∙cm )
-1
CH3COOLi∙2H2O
