Microwave synthesis and characterization of MOF-74 (M = Ni, Mg) for gas separation

Microwave synthesis and characterization of MOF-74(M =Ni,Mg)for gas

separation

Xiaofei Wu a ,Zongbi Bao b ,Bin Yuan a ,Jun Wang a ,Yingqiang Sun a ,Hongmei Luo a ,Shuguang Deng a ,b ,⇑

a Chemical Engineering Department,New Mexico State University,Las Cruces,NM 88003,USA

b

Key Laboratory of Biomass Chemical Engineering of Ministry of Education,Department of Chemical and Biological Engineering,Zhejiang University,Hangzhou 310027,China

a r t i c l e i n f o Article history:

Received 4May 2013

Received in revised form 10June 2013Accepted 12June 2013

Available online 25June 2013Keywords:MOF-74

Microwave-assisted synthesis Adsorption Selectivity

Gas separation

a b s t r a c t

Isostructural MOF-74(M =Ni,Mg)were successfully synthesized with both hydrothermal method (1and 3)and microwave-assisted method (2and 4).These MOF-74samples were characterized with scanning electron microscopy for crystal structure,powder X-ray diffraction for phase structure,and nitrogen adsorption for pore textural properties.The experimental results showed that MOF-74samples synthe-sized by the microwave-assisted method had a smaller particle size with relatively more uniform particle size distribution.The microwave effects also helped to produce a larger specific surface area and micro-pore volume,with a similar median pore diameter.Adsorption equilibrium and kinetics of various gases (CO 2,CH 4,N 2,C 2H 4,C 2H 6,C 3H 6and C 3H 8)on these MOF-74samples were determined at 298K and gas pressures up to 1bar.Adsorption equilibrium selectivity (a ),combined equilibrium and kinetic selectiv-ity (b ),and adsorbent selection parameter for pressure swing adsorption processes (S )were estimated.The relatively high values of adsorption selectivity indicates the potential to separate CO 2/CH 4,CO 2/N 2,C 2H 4/C 2H 6,C 3H 6/C 3H 8and C 3H 6/C 2H 4pairs in a vacuum swing adsorption process using the MOF-74as adsorbent.The microwave-assisted method was found to improve MOF-74with a larger adsorption capacity and somewhat higher selectivity for gas separation.

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1.Introduction

As a new class of microporous/mesoporous materials,metal–or-ganic frameworks (MOFs)are currently investigated as potential candidates for gas storage and separation,[1–8]due to their supe-rior properties such as exceptionally high specific surface area,well-characterized crystalline architectures,controlled pore size,and suitable chemical functionalities [9–13].Among the hundreds MOFs being synthesized and characterized in the past decades,a group of isostructural MOF compounds with open metal ions,M/DOBDC (DOBDC 4À=2,5-dihydroxyterephthalic acid,also donated as M-MOF-74,M 2(dhtp),or CPO-27-M,where M =Zn,[14]Ni,[15]Co,[16]Mn,[17]Mg,[18]and Fe [19]),have been found many promising applications,especially in hydrogen storage,[15,20]car-bon dioxide capture,[21–26]and separations of CO 2/CH 4/H 2,[27]CO 2/CH 4,[28,29]and hydrocarbon,[30–32]both in computational and experimental studies.The MOF-74structure is based on coor-dinated carboxyl and hydroxy groups.A 3-D hexagonal packing of helical O 5M chains is connected with 2,5-dihydroxyterephthalte linkers,showing a 1-D arrangement of parallel hexagonal channels of dimensions 10.3Â5.5Å2[14,33].The coordinated solvent mol-ecule (H 2O or DMF)can be easily removed by a thermal treatment under a vacuum,resulting in an activated stable framework struc-ture with a high concentration of coordinatively unsaturated metal cations.These unsaturated metal centers (UMCs),also known as open metal sites,can offer extra binding sites to the guest gas mol-ecules (like H 2[2]and CO 2[34]).Strong interactions between the UMCs and various guest gas molecules (CO 2,H 2,NO,etc.)are ex-pected and observed.Thus,a significant amount of gas could be ad-sorbed even at a low pressure,leading to high adsorption capacities and possibly a high selectivity as well.

The traditional solvothermal method for synthesizing MOF-74is time-consuming with a reaction time from 20h to 3days.[14–19]Actually,the relative long synthesis time is always a problem for the solvothermal method.In the past few years,several new synthesis techniques including ultrasound (US),[35]surfactant-as-sisted method,[36]and microwave-assisted method [37–41]have been applied in the MOFs synthesis pared to the con-ventional solvothermal method,the microwave-assisted method has attracted a great attention because it sharply reduces the over-all processing time,increases the product yield and improves the quality of the product.The successful applications of microwave-assisted method in both organic and inorganic synthesis have well been documented [42,43].The effect of microwave irradiation on organic synthesis could be attributed to a combination of thermal

1387-1811/$-see front matter Ó2013Elsevier Inc.All rights reserved./10.1016/j.micromeso.2013.06.023

⇑Corresponding author at:Chemical Engineering Department,New Mexico State University,Las Cruces,NM 88003,USA.Tel.:+15756464346;fax:+15756467706.

E-mail address:sdeng@ (S.Deng).