Zhihengyu Chen

All Publications

• Node Distortions in UiO-66 Inform Negative Thermal Expansion Mechanisms: Kinetic Effects, Frustration, and Lattice Hysteresis JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Vornholt, S. M., Chen, Z., Hofmann, J., Chapman, K. W. 2024

  Abstract

  In metal-organic frameworks (MOFs) the interplay between the dynamics of individual components and how these are constrained by the extended lattice can yield unusual emergent phenomena. For the archetypal Zr-MOF, UiO-66, we explore the cooperative dynamics of a Zr-node transformation that gives rise to negative thermal expansion (NTE). Using in situ synchrotron X-ray scattering, with powder diffraction and pair distribution function (PDF) analyses, we identify lattice hysteresis and a thermal ramp-rate-dependence of the thermal expansion. Specifically, kinetic trapping of distorted node states formed at high temperature, leads to broad variability in the apparent thermal expansion which ranges from large positive to large negative thermal expansion with coefficients of thermal expansion (CTE) from +45 to -80 × 10-6K-1. Time-resolved relaxation studies at selected temperatures suggest that when equilibrated UiO-66 is intrinsically NTE, with a CTE of -35 × 10-6K-1. Kinetic trapping of the node-distorted state following high temperature activation has broad implications for characterization and applications of these Zr-MOFs; the nonequilibrium node state depends on the thermal history of the sample with quench vs slow cooling likely to impact gas binding, pore volume, and accessible catalytic sites.

  View details for DOI 10.1021/jacs.4c05313

  View details for Web of Science ID 001249071800001

  View details for PubMedID 38874381

• Atomically Precise Single-Site Catalysts via Exsolution in a Polyoxometalate-Metal-Organic-Framework Architecture JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Chen, Z., Rabbani, S., Liu, Q., Bi, W., Duan, J., Lu, Z., Schweitzer, N. M., Getman, R. B., Hupp, J. T., Chapman, K. W. 2024: 7950-7955

  Abstract

  Single-site catalysts (SSCs) achieve a high catalytic performance through atomically dispersed active sites. A challenge facing the development of SSCs is aggregation of active catalytic species. Reducing the loading of these sites to very low levels is a common strategy to mitigate aggregation and sintering; however, this limits the tools that can be used to characterize the SSCs. Here we report a sintering-resistant SSC with high loading that is achieved by incorporating Anderson-Evans polyoxometalate clusters (POMs, MMo6O24, M = Rh/Pt) within NU-1000, a Zr-based metal-organic framework (MOF). The dual confinement provided by isolating the active site within the POM, then isolating the POMs within the MOF, facilitates the formation of isolated noble metal sites with low coordination numbers via exsolution from the POM during activation. The high loading (up to 3.2 wt %) that can be achieved without sintering allowed the local structure transformation in the POM cluster and the surrounding MOF to be evaluated using in situ X-ray scattering with pair distribution function (PDF) analysis. Notably, the Rh/Pt···Mo distance in the active catalyst is shorter than the M···M bond lengths in the respective bulk metals. Models of the active cluster structure were identified based on the PDF data with complementary computation and X-ray absorption spectroscopy analysis.

  View details for DOI 10.1021/jacs.4c00523

  View details for Web of Science ID 001185383600001

  View details for PubMedID 38483267

• Humidity-Responsive Polymorphism in CALF-20: A Resilient MOF Physisorbent for CO₂ Capture ACS MATERIALS LETTERS Chen, Z., Ho, C., Wang, X., Vornholt, S. M., Rayder, T. M., Islamoglu, T., Farha, O. K., Paesani, F., Chapman, K. W. 2023; 5 (11): 2942-2947

  View details for DOI 10.1021/acsmaterialslett.3c00930

  View details for Web of Science ID 001081834800001

• Pair distribution function analysis of discrete nanomaterials in <i>PDFgui</i> JOURNAL OF APPLIED CRYSTALLOGRAPHY Chen, Z., Beauvais, M. L., Chapman, K. W. 2023; 56: 328-337

  View details for DOI 10.1107/S1600576723000237

  View details for Web of Science ID 000966717600001

• Node Distortion as a Tunable Mechanism for Negative Thermal Expansion in Metal-Organic Frameworks. Journal of the American Chemical Society Chen, Z., Stroscio, G. D., Liu, J., Lu, Z., Hupp, J. T., Gagliardi, L., Chapman, K. W. 2023; 145 (1): 268-276

  Abstract

  Chemically functionalized series of metal-organic frameworks (MOFs), with subtle differences in local structure but divergent properties, provide a valuable opportunity to explore how local chemistry can be coupled to long-range structure and functionality. Using in situ synchrotron X-ray total scattering, with powder diffraction and pair distribution function (PDF) analysis, we investigate the temperature dependence of the local- and long-range structure of MOFs based on NU-1000, in which Zr6O8 nodes are coordinated by different capping ligands (H2O/OH, Cl- ions, formate, acetylacetonate, and hexafluoroacetylacetonate). We show that the local distortion of the Zr6 nodes depends on the lability of the ligand and contributes to a negative thermal expansion (NTE) of the extended framework. Using multivariate data analyses, involving non-negative matrix factorization (NMF), we demonstrate a new mechanism for NTE: progressive increase in the population of a smaller, distorted node state with increasing temperature leads to global contraction of the framework. The transformation between discrete node states is noncooperative and not ordered within the lattice, i.e., a solid solution of regular and distorted nodes. Density functional theory calculations show that removal of ligands from the node can lead to distortions consistent with the Zr···Zr distances observed in the experiment PDF data. Control of the node distortion imparted by the nonlinker ligand in turn controls the NTE behavior. These results reveal a mechanism to control the dynamic structure of MOFs based on local chemistry.

  View details for DOI 10.1021/jacs.2c09877

  View details for PubMedID 36538759

• Mechanistic Insights into Nanoparticle Formation from Bimetallic Metal-Organic Frameworks. Journal of the American Chemical Society Chen, Z., Chen, Z., Farha, O. K., Chapman, K. W. 2021; 143 (24): 8976-8980

  Abstract

  Understanding and controlling nanomaterial structure, chemistry, and defects represents a synthetic and characterization challenge. Metal-organic frameworks (MOFs) have recently been explored as unconventional precursors from which to prepare nanomaterials. Here we use in situ X-ray pair distribution function analysis to probe the mechanism through which MOFs transform into nanomaterials during pyrolysis. By comparing a series of bimetallic MOFs with trimeric node different compositions (Fe3, Fe2Co, and Fe2Ni) linked by carboxylate ligands in a PCN-250 lattice, we demonstrate that the resulting nanoparticle structure, chemistry, and defect concentration depend on the node chemistry of the original MOF. These results suggest that the preorganized structure and chemistry of the MOF offer new potential control over the nanomaterial synthesis under mild reaction conditions. In the case of Fe2Ni-PCN-250, selective extraction of one Ni ion from each node without collapsing the framework (i.e., node-ligand connectivity) leaves a metal-deficient MOF state that may provide a new route to post-synthetically tune the chemistry the MOF and subsequent nanomaterials.

  View details for DOI 10.1021/jacs.1c04269

  View details for PubMedID 34115476