SiO2 Catalysts Embedded in ... - ACS Publications

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Nano-array Cu/SiO2 Catalysts Embedded in Monolithic Channels for the Stable and Efficient Hydrogenation of CO2-derived Ethylene Carbonate Mingming Zhou, Yifeng Shi, Kui Ma, Siyang Tang, Changjun Liu, Hairong Yue, and Bin Liang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04478 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018

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Industrial & Engineering Chemistry Research

Nano-array Cu/SiO2 Catalysts Embedded in Monolithic Channels for the Stable and Efficient Hydrogenation of CO2-derived Ethylene Carbonate Mingming Zhou1, Yifeng Shi1, Kui Ma1, Siyang Tang1, Changjun Liu1,2, Hairong Yue1,2*, Bin Liang1,2 1

Multi-phases Mass Transfer and Reaction Engineering Laboratory, School of

Chemical Engineering, Sichuan University, Chengdu 610065, China 2

Institute of New Energy and Low-Carbon Technology, Sichuan University, Chengdu

610207, China *Corresponding author: [email protected], TEL/FAX: (+86) 22-85997677

1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 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

Abstract: The unstable copper active sites and diffusion limitations of the current Cu-based catalysts are the key issues for the industrial ethylene carbonate (EC) hydrogenation reaction. Monolithic Cu-based catalysts were synthesized with the Cu/SiO2 nanoarrays embedded in cordierite channels via an in-situ hydrothermal process for catalytic EC hydrogenation to methanol (MeOH) and ethylene glycol (EG). The monolithic catalysts exhibited superb activity, with an EC conversion of 99% along with EG and MeOH selectivity of 97% and 50%, respectively. The characterization indicated that the high activities are attributed to the high dispersion of Cu species, appropriate proportion of surface Cu+ and Cu0, and negligible influence of inner diffusion, while the high stabilities are ascribed to the regular nano-array geometric structure and strong interactions between Cu species and SiO2. The Cu/SiO2 nano-array monolithic catalysts represent a new idea combining the properties of nanocatalysts and engineering of the practical industrial catalysts for EC hydrogenation.

Keywords: nano-array monolithic catalyst, ethylene carbonate, hydrogenation, diffusion

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1. Introduction The catalytic hydrogenation of CO2 to chemicals and alternative fuels such as methanol (MeOH) is an attractive approach for CO2 utilization and the chemical storage of hydrogen. 1 Due to the thermodynamic stability and inert kinetic nature of CO2, the direct catalytic hydrogenation of CO2 is usually carried out under harsh conditions (220~300ºC, 50~100 atm), which makes this reaction with a high energy consumption a tremendous challenge for large-scale industrial applications.1,

2

Balaraman3 and Han4 have developed an alternative route for indirect CO2 conversion to methanol using CO2-derived ethylene carbonate (EC) as the intermediate and subsequent EC hydrogenation to methanol and ethylene glycol (EG). The current reported work for this process primarily focused on the development of catalytic systems, i.e., homogeneous and heterogeneous, and investigated the catalyst supports for the EC hydrogenation reactions. Ding et al. developed a homogeneous PNP/RuⅡ pincer complex catalyst with excellent activity under the conditions of 140°C and 5 MPa.4 Li et al. reported a heterogeneous hydrogenation system over a copper chromite catalyst with a methanol selectivity of 60% and an ethylene glycol selectivity of 93% under the conditions of 180°C and 5 MPa.5 Chen et al. investigated the heterogeneous system both in batch and fixed-bed continuous flow reactors using B2O3-promoted copper-silica catalysts with methanol and EG yields of >95%.6 Dai et al. reported a continuous fixed-bed catalytic system using a Cu-HMS catalyst with an EC conversion of 100% and a methanol selectivity of 74%.7 Li et al. prepared a series of mesoporous silica (SBA-15, KIT-6, etc.) supported copper catalysts for this hydrogenation reaction, and the Cu/SBA-15 showed an excellent reactivity with turnover number values of 22 for EG and 11 for methanol.8 Our recent work developed Cu/MCM-41 catalysts for the EC hydrogenation reaction with high activities and emphasized the synergistic effect between the active sites. However, when we explored scaling up the catalysts for industrial use, the packed-bed granular catalyst with a size of 40-60 mesh showed a good reactivity, while the catalysts with sizes of φ2 and φ3 mm exhibited much lower activities because of diffusion 3



limitations. Therefore, the development of highly active and stable catalysts in which the effects of internal diffusion can be eliminated for an industrial preferred heterogeneous catalysis system is desired and crucial for large-scale industrial applications. Compared with traditional packed-bed catalysts, monolithic industrial catalysts have the advantages of lower mass-transfer resistance, higher mass-transfer efficiency and smaller scale effects,9-11 and they have been widely applied in the absorption of automobile exhaust, treatment of VOCs, removal of NOx from diesel engines and desulfurization of flue gases.10, 12 Coating the as-prepared catalysts on the monolithic substrate is the traditional preparation method, whereas, it has the disadvantages of non-uniform coating, being easy to desquamate and the blocking of the active sites with binder.13 In order to overcome these drawbacks, researchers have developed in-situ synthesis of catalysts on monolithic substrates.14-16 Recently, nano-array structured catalysts have also been synthesized via an in-situ hierarchical growing process with the nanostructures of nanowires, nanorods, nanotubes and etc. for CO and propane oxidation reactions.17-20 It has the merits of high surface area, excellent stability and superb mechanical adhesion. Meanwhile, enhanced activities and stabilities can be achieved compared with traditional coated monolithic catalysts. Herein, we investigated the key issues of the Cu/SiO2 catalysts when scaled up for industrial application and developed a novel Cu/SiO2 nano-array monolithic (CuSi-NAM) catalyst via an in-situ hydrothermal synthesis for enhancing the catalytic activity and stability in EC hydrogenation. The activities of CuSi-NAM catalysts and packed-bed granular catalysts (i.e., sizes of 40-60 mesh, φ2 and φ3 mm pellets) are evaluated in a continuous fixed bed reactor. These catalysts have been characterized by several technologies, and the mechanical and thermal stabilities of CuSi-NAM catalysts were also explored in reaction conditions. Furthermore, the effects of external and internal diffusion of the CuSi-NAM catalysts and packed-bed granular catalysts were evaluated adopting Carberry number and Wheeler-Weisz criterion, respectively. 4


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2. Experimental 2.1. Catalyst preparation 2.1.1. Materials The primary materials included the cylindrical cordierite monoliths (diameter of 5mm and length of 50mm; Jiangxi Antian High Tech Materials Co. Ltd); 10 wt% HNO3 (AR, Chengdu Kelong Chemicals Co. Ltd.); 30 wt% colloidal silica (Qingdao Yijida Chemical Co. Ltd); Cu(NO3)2•3H2O (AR, Chengdu Kelong Chemicals Co. Ltd.) and 25 wt% NH3•H2O (AR, Chengdu Kelong Chemicals Co. Ltd.). 2.1.2. Preparation of CuSi-NAM catalysts The preparation diagram of the CuSi-NAM catalysts is shown in Figure 1. The cordierite monoliths were first pretreated in a 10 wt% HNO3 solution by ultrasonic vibration to remove the surface impurities of channels. After thorough drying, the cordierites were calcined at 773 K for 2 h. The pretreated cordierite substrates were dipped into 30 wt% colloidal silica for several minutes, then, the residual liquid was blown away with pressurized air and the samples were dried thoroughly. The process was repeated several times until the required SiO2 coating weight (0.1~1.6g) was achieved. Finally, the coated monoliths were dried and calcined. The CuSi-NAM catalysts were prepared by in-situ hydrothermal synthesis. Amino-copper solution was prepared by adding ammonia solution (2.5 mL, 3.5 mL and 6 mL) dropwise into a certain amount of 1 mol/L Cu(NO3)2 solution. Then, three SiO2-coated monoliths were put into the amino-copper solutions with pH of approximate 11 for 30 min at room temperature. The monoliths and solutions were transferred into 150 mL Teflon-lined autoclaves. After heating in an oven at 413 K overnight, the products were washed with deionized water to pH of 7. Finally, the monolithic catalysts were dried, frozen thoroughly, and calcined at 773 K for 4 hours. The obtained catalysts are denoted as 1.0-CuSi-NAM, 3.1-CuSi-NAM and 6.0-CuSi-NAM (1.0, 3.1 and 6.0 denote the Cu loadings via ICP-OES). 2.1.3. Preparation of the granular and 3.0-CuSi-CM catalysts 5



In order to compare the granular and traditional coated monolithic catalysts, Cu/SiO2 powder was first prepared by a hydrothermal method as follows. 10g of 30 wt% colloidal silica diluted with 80 mL of deionized water was dispersed into a conical flask with ultrasonic dispersion for 20 min. Then, 11.72 mL of Cu(NO3)2 solution was added dropwise with mild agitation, followed by the addition of 6 mL of ammonia solution to a pH of approximately 11. The amino-copper solution was transferred into a 150 mL autoclave, and heated in an oven at 413 K for 24 h. Finally, the catalyst was washed several times with deionized water, dried at 393 K for 40 min and calcined at 773 K for 4 h. The granular Cu/SiO2 catalysts were extruded using the as-prepared Cu/SiO2 powder with sizes of 40-60 mesh, φ2 mm and φ3 mm. The Cu/SiO2 coated monolithic (CuSi-CM) catalyst was prepared by a dip-coating method. The Cu/SiO2 powder was mixed with deionized water (weight ratio of 3:10) and milled for several hours. Then, the pretreated cordierite was dip-coated several times to obtain the desirable weight (0.85g Cu/SiO2), followed by thorough freeze-drying. Finally, the catalyst was dried in an oven at 393 K for 40 min and calcined at 773 K for 4 h. The above-mentioned catalysts are named as 3.0-CuSi-CM, CuSi-40-60, CuSi-φ2 and CuSi-φ3, respectively. 2.2. Catalytic activity test The test of the catalytic activity was carried on in a fixed-bed reactor with a three-step thermal controller, and the reaction tube (inner diameter of 15 mm and height of 500 mm) was filled with about 10 mL catalysts. All the catalysts were first reduced in pure hydrogen at 573 K for 2 h, and the catalytic reaction was evaluated under the conditions of 3 MPa H2 and liquid hourly space velocity (LHSV) varying from 0.2 to 1.8 h-1 at 453 K. EC was dissolved in 1,4-dioxane with a mass ratio of 1:9 and injected into the reactor by a high-pressure pump, and the flow of H2 was controlled by a gas mass flowmeter. A condenser was applied to condense the gas-phase products, which were detected by a SP-2100A gas chromatograph using a flame ionization detector (FID). The tail gases were analyzed online using a thermal conductivity detector (TCD) with a six-way valve gas sampler. For comparison, the 6


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activities of CuSi-40-60, CuSi-φ2 and CuSi-φ3 were also investigated with crushed bare cordierite monolith and quartz pellets to retain the same void fraction and bed height as the monolithic catalysts. In addition, the mechanical adhesions of nano-array monolithic catalysts were explored by ultrasonic vibration at 40 Hz for 120 min (separated for six times), with weighing to determine the mass loss after thorough drying. The thermal stability was evaluated over reduced 3.0-CuSi-CM and 3.0-CuSi-NAM under the certain conditions (453 K, 3 MPa, LHSV of 0.2 h-1 and H2/EC of 200). After heat treatment in high purity nitrogen at 623 K for 24 h, the catalytic activities of the heat-treated catalysts were tested under the same conditions as those of reduced catalysts. 2.3. Catalyst characterization Morphological structures of monolithic catalysts were examined by a scanning electron microscope (SEM) (JEOL 7610F) and a transmission electron microscope (TEM) (FEI-TECNAI-G20). The samples for TEM characterization were distributed in ethanol under ultrasonic conditions, and the microscope was operated at 100 kV. Composition information was obtained from an inductively coupled plasma optical emission spectrometer (ICP-OES) (Thermo Fisher, ICAP 7400). The samples were prepared as follows: 0.05 g of sample was placed in a platinum crucible, a certain proportion of nitric acid and hydrofluoric acid were added for heat decomposition, and the sample was diluted with water and dissolved in a 100 mL volumetric flask. Fourier transform infrared spectroscopy (FT-IR) was recorded on a Thermo Fisher Nicolet 6700 spectrometer with 2 cm-1 resolution and a scanning scope from 4000 cm-1 to 400 cm-1. The sample was finely ground and then mixed with KBr to achieve uniform dispersion before characterization. X-ray diffraction (XRD) (PANalytical-Empyrean) was applied to obtain the crystallographic data of the catalyst. The instrument was operated with Cu Kα radiation (λ=0.15406 nm). The XRD patterns were acquired with scanning angles (2θ) of 10-90°, scanning steps of 0.026°, a voltage of 35 mA and a current of 40 kV. H2 Temperature programmed reduction (H2-TPR) (Quantachrome Instrument 7



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ChemStar TPx) was carried out to collect more information about the catalysts. Catalyst powder (30 mg) was placed into the tube and swept under Ar at 423 K for 60 min, followed by cooling to 323 K. After that, the sample was treated with 10% H2-Ar at a heating rate of 10 K/min up to 873 K. The consumption of H2 was detected by a TCD. XPS was carried on an X-ray photoelectron spectrometer (Axis Ultra DLD, Kratos Analytical Ltd., UK) to measure the valence state of the catalysts. All of the catalysts were tested using a monochromatized Al Kα X-ray radiation source (hν = 1486.7 eV) with a power of 150 W (15 kV voltage, emission current of 10 mA). The Cu species dispersion was measured by N2O titration. The samples (~10 mg Cu) were pretreated with Ar at 473 K for 2 h, and then were reduced with 10% H2-Ar. The temperature increased at a heating rate of 10 K/min up to 773 K. The reduced samples were treated with pure N2O to oxidize the metallic copper: 2Cu + N2O→ Cu2O + N2. Then, the samples were treated with 10% H2-Ar at a heating rate of 10 K/min from 323 K to 623 K. The Cu dispersion (DCu), the specific area of metallic copper (SCu) and mean Cu particle size (dCu) were calculated according to the previous work.6 2.4. Evaluation of the mass transfer The effects of external and internal diffusion play important roles on the reaction rate in the heterogeneous gas-solid reaction. It is necessary to eliminate the limitations of external and internal diffusion to guarantee the hydrogenation of EC in the reaction kinetics region. The Carberry number

21-23

(Ca) and Wheeler-Weisz value21, 22, 24-26

(W-W, ηφ2) are usually used to evaluate the effects of external and internal diffusion, respectively. The Carberry number is defined as follows：11, 21 =

∗

(1)

Where robs is the reaction rate (mol L-1s-1); kg is the gas-solid mass-transfer coefficient in mg3ms-2s-1; a is the specific surface area (m2g-1); and Ci* is the concentration of species i at the bulk of catalyst bed (mol m-3). If the Carberry number 8


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value is less than 0.05, it can be concluded that the rate of diffusion of reactants at the catalyst interface does not significantly affect the reaction kinetics, and the influence of external diffusion can be neglected.21-23 The kg is associated with a dimensionless mass transfer factor JD. JD is defined as follows 27 = / /

(2)

Where Sc is the dimensionless Schmidt number and can be written as = μ/(ρD, ) (3) Where µ is the dynamic viscosity in Pa s; ρ is the gas density in kg m-3; G is the gas mass flow rate in kg m-2s-1; and DB,i is the bulk diffusion coefficient of species i in m2s-1. JD is related to the flowing gas state. Thodos found that in the scope of Re=0.8~2130 and Sc=0.6~1300, JD is just the function of the Reynolds number through data analysis on the measurement of the packed-bed transfer characteristics. =

".$%

&' (.)* +".,%

(4)

The Re is defined by the catalyst particle size, that is -. = /0 /1

(5)

The ds is the specific area equivalent diameter of catalyst particles in m. The Wheeler-Weisz value (W-W, ηφ2) has been used to measure the effect of internal diffusion. It can be expressed as follows 11, 28 23 =

45

677, ∗

(6)

Where Deff,i is the effective diffusion coefficient (m2s-1) and L is the diameter of the pellet or thickness of the nanoarray in m. If the value of ηφ2 is less than 0.1, it indicates that the rate of diffusion of the reactants into the pores of the catalyst does not significantly affect the reaction kinetics, and the influence of internal diffusion can be neglected. 11 The effective diffusivity (Deff,i) is related to the pore diffusivity (DP) as follows 9



:

8'99, = ; 8

(8)

@,

The DK and DB,i can be determined as follows, respectively F

8A = 48.5/< E G ,

@,

8L =

,

= ,+H ∑KLM

,×,"OP F P/5

(9)

H

[

,

J

* *

SiO2 Catalysts Embedded in ... - ACS Publications

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