Presentation of achievements
It is a great challenge to regulate the precise position of confined metal species and assemble specific structures at the atomic level. Based on this,Professor Wu Peng, Associate Professor Guan Yejun and Associate Professor Xu Hao of East China Normal University, Researcher Zhang Jiangwei of Inner Mongolia University, Professor Han Yu of King Abdullah University of Science and Technology, Associate Professor Song Weiyu of China University of Petroleum (Beijing) (co-corresponding author) and othersHydrothermal thermally stable UTL-type germanium silicate (GE-UTL) obtained by acidic treatment has been reported as an alternative payload for confined PT species, utilizing its oversized 14-membered ring pore (14-MR), high content of stable skeleton of GE-species, and specific D4R subunits. pt@ge-UTL catalysts were prepared by simple wet impregnation and H2 reduction using GE-UTL support and H2PTCL6 as raw materials.
Differential phase contrast scanning transmission electron microscopy, in-situ X-ray absorption fine structure, 19F spin NMR, full-range synchrotron analysis of the distribution function G(R) and density functional theory (DFT) calculations show that the PT clusters of an average of 4 atoms are firmly isolated in the 14-membered ring channel. PT is selectively and directionally anchored to the unique secondary structure of the UTL zeolite by a PT-O-GE bond, which has a double quaternary ring (D4R) structure and abundant GE components. The test found that PT4-GE2-d4r@utl promoted high propane dehydrogenation activity, high propylene selectivity and long-term stability. This study demonstrates the application of germanium silicate in the design and synthesis of high-performance propane dehydrogenation catalysts.
Background:
Platinum (PT)-based catalysts are widely used in propane dehydrogenation to propylene, but there are problems such as high cost, uncontrollable dispersion, easy aggregation and rapid inactivation at high temperatures. Precisely dispersing a single atom or sub-nano cluster of a metal within a confined space is one way to maximize the use of atoms, but when the active sites of metals are dispersed at the scale of sub-nano clusters or even individual atoms, the surface free energy of the metals increases, thus exacerbating their agglomeration. Therefore, more efficient methods need to be sought to anchor and disperse individual atoms or sub-nanometer clusters to specific sites of the carrier. An efficient propane dehydrogenation (PDH) process relies on a tendency to break the C-H bond while inhibiting the cleavage of the C-C bond. Among them, dispersed PT particles or species tend to aggregate during catalyst preparation and subsequent reactions, resulting in reduced selectivity and accelerated inactivation of propylene. Although the addition of the second metal is beneficial for improving selectivity and stability, it requires sacrificing PDH activity and can even lead to irreversible deactivation of the catalyst. Therefore, the development of encapsulated PT clusters with unalloyed structures is of great significance to overcome the activity-stability equilibrium of PDH.
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According to the variation curves of propane (C3H8) conversion and propylene (C3H6) selectivity with time (TOS) at 500 different catalysts, it was found that the PT-UTL-C (47)-R catalyst is almost inactive against PDH reaction, and the propane conversion rate is negligible. However, the propane conversion rate of the catalyst supported by pickling-stabilized UTL zeolite was significantly improved, which was even much higher than that of the PT-Al2O3 benchmark catalyst. With the increase of the remaining GE content in the structurally stable UTL vector, the PDH activity and stability gradually increased.
Among them, the PT-A-2H(31)-R catalyst has the highest propane conversion rate and the best stability. The propylene selectivity was slightly lower (87%) in the initial phase, but gradually increased to 99% as the PDH reaction progressed. As the reaction progresses, these acid sites are covered with carbon, indicating that the selectivity of propylene eventually exceeds 99%. The PT-A-2H(31)-R catalyst was stably reactive in PDH for 200 h and 240 h, respectively. After 900 h of stabilization, the propane conversion and propylene selectivity were fully restored to their previous levels. In conclusion, PT-A-2H(31)-R is an ultra-stable catalyst that can stably and continuously catalyze PDH reactions for more than 4200 h, regardless of the changes in partial pressure, propane space velocity, and reaction temperature.
Figure 1Dispersion of PT species in UTL zeolites with different Si Ge ratios
Figure 2Catalytic performance of PDH reactions
Figure 3Location of sub-nanoPt species in UTL topological channels
Figure 4Characterization of the chemical state and coordination environment of Pt and GE
Figure 5The position of GE atoms in the support and PT-restricted catalysts
The authors constructed a series of models of encapsulated PT clusters within 14-MR or 12-MR channels and calculated their energies. Among them, the most stable structure in the 14-MR channel is (III), where two Pt atoms coordinate with the framework O to form the PT-O-Ge group; In addition, the most stable structure within the 12-MR channel is (viii) where the Pt4 cluster does not form any chemical bonds with the zeolite and therefore has higher energy. The authors also construct a series of structures in which PT4 clusters are located at different locations in the UTL topology. The results show that the lower the energy, the better the stability of the pt4@utl structure, and the stability of the Pt4 clusters immobilized on the D4R units with PT-O-Ge bonds is better than that of the Pt4 clusters immobilized on the non-D4R units with Pt-O-Ge bonds. Therefore, the structure in Figure 6b(iii) has the lowest energy, suggesting that the structure is more plausible to represent the true active site in the pt4@ge-utl.
Figure 6Calculation results of the theoretical structure
Figure 7Confirm the PT4-GE2-d4r@utl structural model
On the PT(111) surface, the first dehydrogenation step yields the intermediate C3H7* (* indicates adsorption on the surface) and the separated H* bond at the top position of the PT atom. The second H* separates from the methyl bond at a bridge site on PT(111) and must be overcome782 kJ mol 1 barrier, with subsequent steps including desorption of C3H6*, H* transfer, and H2 binding to regenerate the catalyst surface. On the surface of the Pt4-Ge2-d4r@utl, the coordination number of PT is 3, which is much lower than that in the Pt(111) plate, which helps to reduce the potential barrier of the first step of dehydrogenation. After the C-H bond is broken, the two separated H atoms form bonds at the bridge site coordinated with the two Pt atoms, respectively. After C3H6 desorption, due to the short distance, the remaining two H* atoms can directly combine to form H2 without H* transfer.
For pt(111) surfaces, the first step of dehydrogenation must overcome the maximum energy barrier. For the Pt4-Ge2-d4r@utl site, the second dehydrogenation has a higher potential barrier and H2 generation has the highest energy barrier, but the step that determines the rate is the first dehydrogenation, as the reaction rates of the second dehydrogenation and H2 generation steps are limited by the concentrations of C3H7* and H*, respectively. Therefore, the PT4-GE2-d4r@utl catalyst can effectively reduce the energy potential of the first step of dehydrogenation, which is the rate-determining step of the whole PDH reaction, thereby increasing the PDH reaction rate.
Figure 8DFT calculations for theoretical PDH processes
Bibliographic information
germanium-enriched double-four-membered-ring units inducing zeolite-confined subnanometric pt clusters for efficient propane dehydrogenation. nature catalysis,