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Dr. Wei Zhou Email: wzhou@nist.gov
Phone : (301) 975-8169
Fax: (301) 921-9847
NIST Center for Neutron Research
National Institute of Standards and Technology Gaithersburg, MD 20899-6102
Department of Materials Science and Engineering
University of Maryland
College Park, MD
Wei Zhou is a materials chemist at NCNR/NIST, and a research faculty at UMD. His research interests are in the areas of novel porous materials, computational materials design, and neutron spectroscopy. His recent work focuses on developing advanced materials for gas sorption and storage (CH4, H2, CO2 etc.), by combining various experimental techniques and electronic structure calculations. The materials under investigations include porous metal-organic frameworks (MOFs), hydrides etc. Shown below are some highlights of his recent work.
Research id

( Current h-index: 40 )


Hydrogen and Methane Storage in Metal-Organic Frameworks
       
04/02/2010  Mechanistic insights into high-capacity methane storage in MOFs
MOFs are a novel family of physisorptive materials that have exhibited great promise for methane storage. So far, detailed understanding of their methane adsorption mechanism is still scarce. Here we report a comprehensive mechanistic study of methane storage in three milestone MOF compounds (HKUST-1, PCN-11, and PCN-14), whose CH4 storage capacities are among the highest reported so far. Combining neutron diffraction measurements, Grand Canonical Monte Carlo simulations, and density functional theory calculations, we unambiguously revealed the exact locations of the stored methane molecules in these MOF materials. Our results suggest that further, rational development of new MOF compounds for methane storage applications should focus on enriching open metal sites, increasing the volume percentage of accessible small cages and channels, and minimizing the fraction of large pores.
 


H. Wu, J. M. Simmons, Y. Liu, C. M. Brown, X.-S. Wang, S. Ma, V. K. Peterson, P. D. Southon, C. J. Kepert, H.-C. Zhou, T. Yildirim, and W. Zhou,* Chem. Eur. J., 16, 5205–5214 (2010); W. Zhou,* Chem. Rec., 10, 200-204 (2010).

       
03/10/2009  Open metal sites found to play an important role for CH4 storage in MOFs
We found that MOF compounds M2(dhtp) (also known as MOF-74 analogues) possess exceptionally large densities of open metal sites. By adsorbing one CH4 molecule per open metal, these sites alone can generate very large methane storage capacities, approaching the DOE target for material-based methane storage at room temperature. Our adsorption isotherm measurements at 298 K and 35 bar for the M2(dhtp) compounds yield excess methane adsorption capacities roughly equal to the predicted, maximal adsorption capacities of the open metal sites. Our neutron diffraction experiments clearly reveal that the primary CH4 adsorption occurs right on the open metals. DFT calculations show that the binding energies of CH4 on the open metal sites are significantly higher than those on typical adsorption sites in classical MOFs.
 
H. Wu,
W. Zhou,* and T. Yildirim, J. Am. Chem. Soc., 131, 4995 (2009).
       
01/27/2009  CH4 adsorption sites  in ZIF-8 and MOF-5 revealed by neutron diffraction studies
Using neutron powder diffraction, we have directly determined the methane sorption sites in two prototypical MOF materials: ZIF-8 and MOF-5. The primary methane adsorption sites are associated with the organic linkers in ZIF-8 and the metal oxide clusters in MOF-5. Methane molecules on these primary sites possess well-defined orientations, implying relatively strong binding with the framework. With higher methane loading, extra methane molecules populate the secondary sites and are confined in the framework. The confined methanes are orientationally disordered and stabilized by the intermolecular interactions. An unusual reversible methane-induced structural phase transition in MOF host lattice is observed at ~60 K in both ZIF-8 and MOF-5 due to strong intermolecular interaction between confined methane molecules in the pores of the host lattice.
 
H. Wu,* W. Zhou and T. Yildirim, J. Phys. Chem. C, 113, 3029 (2009).
       
10/25/2008  H2 binding strength found to depend strongly on open metal species in MOFs
MOFs with open metal sites exhibit much stronger H2 binding strength than classical MOFs. Yet, how the binding strength varies with different open metal species was previously unknown. We conducted a systematic study of the H2 adsorption on a series of isostructural MOFs, M2(dhtp) (M=Mg, Mn, Co, Ni, Zn). The experimental Qst for H2 of these MOFs range from 8.5 to 12.9 KJ/mol, with increasing Qst in the following order: Zn, Mn, Mg, Co, and Ni. The H2 binding energies derived from DFT calculations follow the same trend. We also found a strong correlation between the metal ion radius, the M-H2 distance and the H2 binding strength, which provides a viable, empirical method to predict the relative H2 binding strength of different open metals.
 
W. Zhou
,* H. Wu, and T. Yildirim,
J. Am. Chem. Soc., 130, 15268 (2008).
       
04/30/2008  Nature of open metal-H2 interaction in MOFs elucidated
It's well-known in coordination chemistry that transition metal-H2 interactions are often of so-called "Kubas-type". MOF compounds with exposed transition-metal sites exhibit impressive heats of adsorption of H2, thus the open metal-H2 interaction was also widely believed to be of "Kubas-type". Our first-principles calculation clearly shows that the H2 binding on the open metal site in Mn4Cl-MOF (a representative MOF compound with open metal sites) is not of the expected Kubas-type. Instead, the major contribution to the overall binding comes from the classical Coulomb interaction, which is not screened due to the open-metal site. We also show that the orientation of H2 has a surprisingly large effect on the binding potential, reducing the classical binding energy by almost 30%.
 
W. Zhou
,* and T. Yildirim, J. Phys. Chem. C, 112, 8132 (2008).
       
10/05/2007  High-quality H2 and CH4 adsorption isotherm data reported for MOF-5 and ZIF-8
We reported H2 and CH4 adsorption isotherms in two prototypical MOF compounds (MOF-5 and ZIF-8) over a large temperature (30-300 K) and pressure (up to 65 bar) range using a fully computer-controlled Sieverts apparatus. At low temperatures, the maximal excess adsorption capacities of H2 and CH4 in MOF-5 are found to be 10.3 wt % and 51.7 wt %, respectively, while they are only 4.4 wt % and 22.4 wt % in ZIF-8. From the temperature-dependent isotherm data, the isosteric heat of adsorption (Qst) was also estimated. The excess Qst’s for the initial H2 and CH4 adsorption in MOF-5 are ~4.8 kJ/mol and ~12.2 kJ/mol, respectively. We obtained similar Qst’s for ZIF-8. The detailed isotherm curves reported here over a large temperature and pressure range will be a critical test for future grand canonical Monte Carlo simulations and force-field models.
[download the isotherm data in this paper]
 
W. Zhou
, H. Wu, M. R. Hartman, and T. Yildirim,* J. Phys. Chem. C, 111, 16131 (2007).
       
03/01/2007  Neutron diffraction study reveals where H2 is stored in prototypical ZIF-8 compound
Zeolitic Imidazolate Frameworks (ZIFs), as a new subfamily of metal-organic framework compounds, possess the exceptional chemical stability and rich structural diversity of zeolites, and show great promise for H2 storage. Using the difference-Fourier analysis of neutron powder diffraction data along with first-principles calculations, we successfully revealed the H2 adsorption sites and the H2 binding energies within the nanopore structure of ZIF-8. Surprisingly, the two strongest adsorption sites are both directly associated with the organic linkers, instead of the ZnN4 clusters, in strong contrast to classical MOFs, where the metal-oxide clusters are the primary adsorption sites. These observations are important and hold the key to optimizing this new class of ZIF materials for practical H2 storage applications.
 
H. Wu, W. Zhou, and T. Yildirim,* J. Am. Chem. Soc., 129, 5314 (2007).
       

Novel Physical Properties of Metal-Organic Frameworks
       
11/14/2008  Methyl groups in ZIF-8 exhibits interesting quantum rotation behavior
The reorientational motion of the methyl group is an intriguing physics phenomenon. It can be well described by classical random jumps at high temperature, whereas at low temperature it is dominated by quantum-mechanical rotational tunneling. Using neutron inelastic scattering and diffraction, we have studied the quantum methyl rotation in ZIF-8. The rotational potential for the CH3 groups in ZIF-8 is shown to be primarily 3-fold in character. The ground-state tunneling transitions at 1.4 K of 334 ± 1 μeV for CH3 groups in hydrogenated ZIF-8 and 33 ± 1 ueV for CD3 groups in deuterated ZIF-8 indicate that the barrier to internal rotation is small compared to almost all methylated compounds in the solid state and that methyl-methyl coupling is negligible.
 
W. Zhou
,* H. Wu, T. J. Udovic,* J. J. Rush, and T. Yildirim, J. Phys. Chem. A, 112, 12602 (2008).
       
08/20/2008  Origin of the exceptional negative thermal-expansion in MOF-5 identified
Metal-organic framework-5 possesses an exceptionally large negative thermal-expansion (NTE) coefficient. Our direct experimental measurement, in the temperature range of 4 to 600 K, shows that the linear thermal-expansion coefficient is ~ -16×10-6  K-1. From first-principles lattice dynamics calculations, we deciphered the origin of this large NTE behavior. We found that almost all low-frequency lattice vibrational modes (below ~23 meV) involve the motion of the benzene rings and the ZnO4 tetrahedra as rigid units and the carboxyl groups as bridges. These  rigid-unit modes  exhibit various degrees of phonon softening and thus are directly responsible for the large negative thermal expansion in MOF-5.
 
W. Zhou
,* H. Wu, T. Yildirim, J. R. Simpson, and A. R. Hight Walker,
Phys. Rev. B, 78, 054114 (2008).
       
11/14/2006  Mechanical properties and lattice dynamics of MOF-5 studied in detail
By combining neutron inelastic scattering (NIS) and first-principles calculations, we investigated the lattice dynamics of MOF-5. The structural stability of MOF-5 was evaluated by calculating the three cubic elastic constants. We find that the shear modulus, c44=1.16 GPa, is unusually small, while two other moduli are relatively large (i.e., c11=29.42 GPa and c12=12.56 GPa). The phonon dispersion curves and phonon density of states were directly calculated and our simulated NIS spectrum agrees very well with our experimental data. Several interesting phonon modes are discussed, including the softest twisting modes of the organic linker.
 
W. Zhou
* and T. Yildirim,
Rhys. Rev. B, 74, 180301(R) (2006).