The Effect of Water Layer in Microreactor on the Low-temperature

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The Effect of Water Layer in Microreactor on the Lowtemperature Synthesis of High-activity Cu/ZnO Catalysts Xinchao Chen, Shuaishuai Chen, Xin Jiang, Chen Ling, and Zhongbiao Wu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b03122 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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Industrial & Engineering Chemistry Research

The Effect of Water Layer in Microreactor on the Low- temperature Synthesis of High-activity Cu/ZnO Catalysts Xinchao Chen a, Shuaishuai Chena, Xin Jiang*a, Chen Ling a and Zhongbiao Wub (a Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, P. R. China b College

of Environmental and Resource and Sciences, Zhejiang University, Hangzhou 310058, P. R. China)

Corresponding author: Xin Jiang, E-mail address: [email protected]

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ABSTRACT： The effect of water layer on precipitation process in a three-channel microreactor at low temperature was investigated. FT-IR, XRD, TG, XPS, TPR and BET were employed for studying structural evolution of the intermediate products during the preparation of both zincian georgeite-derived and zincian malachite-derived catalysts and catalytic activities were measured for methanol synthesis from syngas. It is manifested that the effect of uniform precipitates acts consecutively on subsequent aging process, Zn incorporation in precursors, thermal decomposition, reduction and catalytic performance of the catalysts. Numerical simulation revealed the change of species properties and reaction rates at varying temperatures can lead to different regulations of water layer, denoting disparate ratios of water layer were required for obtaining uniform precipitates under different conditions, which further suggests the key role of uniformity of the precipitates in preparing high activity Cu/ZnO catalysts.

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1. INTRODUCTION Methanol, as an important industrial intermediate,1-3 has a growing demanding for consumption worldwide. The most widely employed catalysts for synthesizing methanol are Cu/ZnO catalysts4-6 whose synthetic method is processed mainly by co-precipitation.7-9 Starting from precipitates, a series of microstructural changes will occur with consecutive effect during their evolution process when preparing Cu/ZnO catalysts.8 During the aging process, uniform precipitates will transform into zincian malachite with high Zn incorporation, making for forming porous structure of the catalysts.10,11 Highly dispersed Cu-Zn mixed oxides are obtained after a further calcination from precursors and ultimately reduced to active Cu/ZnO catalysts, while formation of Cu-Zn interface at this stage is verified.12 High performance of the catalysts originating from the increasing Cu-Zn interfaces is reported in literatures.13-15 Numerous studies have investigated the effect of aging16-19 and calcination processes20-22 on final activities of the catalysts, however, studies about precipitation process are rarely concerned. Given that the structural evolution of Cu/ZnO catalysts originates from initial precipitates23,24 and special “chemical memory” phenomenon was early proved during the process for preparing Cu/ZnO catalysts,25 namely the sensibility of reaction characteristics to reaction conditions including changes of temperatures, solution concentrations and pH values,26-28 researches on precipitation process are thus deemed very important. The changes of pH values and temperatures are considered the most important factors influencing the precipitation process. Precursors mainly consisting of zincian malachite can be obtained in the temperature range of 20~80 ℃ by controlling the pH value as nearly 7 at which Cu and Zn elements can be completely precipitated.26,27 Precipitation temperatures in literatures are commonly used as given parameters and set at nearly 70 ℃.8,9,12 Researches on low temperature precipitation are rarely reported except for a small amount of studies using the same temperatures for both precipitating and aging processes,27,28 which was convenient and natural progression for semi-batch reactor where precipitation and aging processes are carried out in the same reaction tank. Nevertheless, a great influence of aging on structures of the catalysts was reported previously16 and aging processed at low temperature is very different from that processed at high temperature. Therefore, effects resulting from lowering precipitation temperature are hard to distinguish, since the simultaneous decrease of temperatures in both precipitation and aging will bring up mutual interference. Recently, the continuous preparation of nanoparticles has attracted increasing attention due to its high efficiency and wide application.29,30 In this study, we hope to carry out the precipitation process in the microreactor while aging process was carried out in a separated aging section. In our previous research,24 a three-channel microreactor was used for synthesizing Cu/ZnO co-precipitating catalysts and precipitates with uniform Cu-Zn distribution were obtained utilizing diffusion process for counterbalancing the different reaction rates via the introduction of water layer. Notably, the microreactor has already been reported as a new facility for preparing Cu/ZnO catalysts with uniform Cu-Zn distribution due to its

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remarkable mixing effect.8 It was evidenced that uniform precipitates will lead to positively structural evolution of the catalysts and better catalytic performance. Obviously, both diffusion coefficients and reaction rates change with varying temperatures and will inevitably affect the precipitation process, which implies different regulations at different temperatures due to the relative change between diffusion and reaction processes. By comparing the regulations at different conditions, a more profound understanding of the mechanism of co-precipitation could be established. Comparing the co-precipitation inside the microreactor at low and high temperature, this investigation will give more insight into the evolution of Cu/ZnO catalysts.

2. EXPERIMENTAL METHODS 2.1 Sample preparation Preparation of Cu/ZnO co-precipitation catalysts was carried out in a three-channel microreactor (all channels are in shape of square with the size of 0.6 mm*0.6 mm) as shown in Figure 1. A 0.3 mol /L mixed solution of copper nitrate and zinc nitrate (Cu:Zn=7:3) together with the same concentrated Na2CO3 solution at a total flow rate of 41 mL/min was pumped into the lateral channels of the microreactor while deionized water was introduced into the middle channel simultaneously(residence time≈32 ms). Before entering the microreactor, three reactant solutions were preheated to 20 ℃ with the microreactor also submerged in water bath at 20 ℃. After initial precipitation, the suspension passed through an extension section bathing in water at 80 ℃ and then flow into a 250 mL flask. The pH value was adjusted to 6.9-7.1 by regulating the relative flow rates of reactant solutions.31 The suspension was then divided into two parts. One was washed three times using anhydrous ethanol (before entering the extension section) to remove most of the water and dried in 110 ℃ for 24 h to get the blue-colored precipitate. Calcination of the precipitates skipping aging process was then carried out at 350 ℃ for 4 h to get the mixed oxides which were finally reduced under 10 vol% H2 in N2 at 320 ℃ for 3h to obtain zincian georgeitederived catalysts (Cu/ZnO-G). The other part was aged in water bath at 80 ℃ for 2 h and washed three times using deionized water followed by same drying conditions to get the green-colored precursors. Precursors were then processed likewise including calcination and reduction to obtain the zincian malachite-derived catalysts (Cu/ZnOZM). Similar to previous research,24 the effect of water layer on precipitation was studied by changing the flow rates of the deionized water at low temperature. While increasing the ratio of water layer, the corresponding flow rates of the reactant solutions were diminished and therefore the solution concentrations were raised accordingly to keep the total flow rate and the total moles of reactants unchanged. Four different ratios of water were set to 1/20, 1/10, 1/6, 4/9 with the corresponding solutions concentrations regulated as 0.31 mol/L, 0.33 mol/L, 0.36 mol/L, 0.54 mol/L. Thus, four samples were named as TR-1-20, TR-1-10, TR-1-6, TR-4-9. Specially, samples for preparing zincian georgeite-derived catalysts were named as TR(G)-1-20, TR(G)-1-10, TR(G)-1-6, TR(G)-4-9.

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Figure 1. Preparation process of Cu/ZnO catalysts by the three-channel microreactor 2.2 Characterization X-ray powder diffraction (XRD) was carried out using a X'Pert PRO MPD X-ray diffractometer employing Cu-Kα radiation (λ=0.15406 nm, 5°/min, 10°-80°). The FT-IR analysis was performed using a Nicolet 5700 Fourier transform instrument. Energy dispersive spectrometer (EDS) was performed using an X-MaxN 80 T with samples highly dispersed on carbon-supported molybdenum grids. Thermal gravimetric analysis (TG) was performed using a Mettler Toledo thermo balance with both precipitates and precursors (10 ℃/min,50 ℃-800 ℃, N2). X-ray photoelectron spectroscopy (XPS) measurement was performed on oxides using a Physical Electronics PHI-5400 ESCA work station equipped with a magnesium anode (Mg Ka=1.254

eV) at a power of 240 W (12

kV and 20 mA).Temperature programmed reduction (H2-TPR) was performed on oxides (40-60 mesh) using a quartz tube reactor (Quantachrome ChemTPR) via mixed reducing gas (10 vol% H2 in Ar, 40 mL/min) at a heating rate of 10 ℃/min up to 400 ℃. Specific surface areas were measured by BET method using a Quantachrome Autosorb IQ (N2 physisorption). 2.3 Catalytic activity measurement Catalytic performance measurement was carried out in a fixed-bed reactor (ID-4 mm) under 5 MPa loaded with 0.1 g catalyst (40~60 mesh) mixed in 2.0 g quartz sand. Syngas constituted of CO and H2 (CO:H2=1:2) using 3.7 % Ar as the internal standard gas was introduced into the reactor with a total flow of 9 mL controlled by mass flowmeters. The products and CO were detected by online GC-1102 (FID detector) and GC-9790 (TCD detector). CO conversion was determined using internal standard method while the measurement of methanol production used external standard method. Catalytic performance was expressed using the amount of methanol produced per hour on per unit mass of the catalyst (g MeOH/g catalyst/h). Five reaction temperatures including 150, 200, 230, 250, 260, 270 ℃ were tested respectively. 2.4 Model simulation setting Flow state simulation with specific calculated Reynold numbers was studied in our former research 33 and the results showed the dominance of laminar flow in our three-channel microreactor. In this study, an identical simulation was carried out to investigate the flow state inside the microreactor when the ratio of water layer was 5



shrunk to 1-20 and state close to laminar flow can also be verified (see Section 2 in ESI). Therefore, simple numerical simulation was applied using ideal plug flow model (Figure 10) in MATLAB programming to study the diffusion-reaction process starting from where three inlet flows joins.

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RESULTS AND DISCUSSION Classical preparation of Cu/ZnO catalysts is processed through precipitation, aging, calcination and reduction,

during which intermediates including precipitates, precursors, mixed oxides and final catalysts are obtained. However, some recent literatures32-34 evidenced the preparation of catalysts with high activities by direct calcination and reduction from precipitates without aging process. It is due to the presence of zincian georgeite in precipitates, which was previously reported as precursors (without aging process) for preparing high-activity catalysts. However, pure zincian georgeite is not favored under traditional high temperature conditions based on the fast transformation from zincian georgeite into zincian malachite as reported in literatures.32 Therefore, part of the precipitates obtained applying low temperature conditions were directly calcined skipping high temperature aging process for acquiring Cu/ZnO catalysts and corresponding catalysts prepared by this method were named as zincian georgeite-derived catalysts (Cu/ZnO-G in short). For comparison, rest of the precipitates undergoing further aging process were transformed into precursors containing no zincian georgeite but mainly zincian malachite (Zn

The Effect of Water Layer in Microreactor on the Low-temperature

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