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Nanoscale intimacy in bifunctional catalysts for selective conversion of hydrocarbons

Abstract

The ability to control nanoscale features precisely is increasingly being exploited to develop and improve monofunctional catalysts1,2,3,4. Striking effects might also be expected in the case of bifunctional catalysts, which are important in the hydrocracking of fossil and renewable hydrocarbon sources to provide high-quality diesel fuel5,6,7. Such bifunctional hydrocracking catalysts contain metal sites and acid sites, and for more than 50 years the so-called intimacy criterion8 has dictated the maximum distance between the two types of site, beyond which catalytic activity decreases. A lack of synthesis and material-characterization methods with nanometre precision has long prevented in-depth exploration of the intimacy criterion, which has often been interpreted simply as ‘the closer the better’ for positioning metal and acid sites8,9,10,11. Here we show for a bifunctional catalyst—comprising an intimate mixture of zeolite Y and alumina binder, and with platinum metal controllably deposited on either the zeolite or the binder—that closest proximity between metal and zeolite acid sites can be detrimental. Specifically, the selectivity when cracking large hydrocarbon feedstock molecules for high-quality diesel production is optimized with the catalyst that contains platinum on the binder, that is, with a nanoscale rather than closest intimacy of the metal and acid sites. Thus, cracking of the large and complex hydrocarbon molecules that are typically derived from alternative sources, such as gas-to-liquid technology, vegetable oil or algal oil6,7, should benefit especially from bifunctional catalysts that avoid locating platinum on the zeolite (the traditionally assumed optimal location). More generally, we anticipate that the ability demonstrated here to spatially organize different active sites at the nanoscale will benefit the further development and optimization of the emerging generation of multifunctional catalysts12,13,14,15.

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Figure 1: Scheme of hydrocracking reactions that use a bifunctional catalyst.
Figure 2: Distribution of zeolite Y and alumina components within Y/A extrudates.
Figure 3: Controlled deposition of platinum (Pt) on either the zeolite Y or the alumina component of Y/A extrudates.
Figure 4: Impact of nanoscale intimacy on hydrocracking activity and selectivity.

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Acknowledgements

This work has been supported by the NRSC-C and the European Research Council, EU FP7 ERC Advanced Grant no. 338846. (J.Z. and K.P.deJ.), and by the Flemish government via the Methusalem program (J.A.M.). We thank J. D. Meeldijk for assistance with ultramicrotomy and electron microscopy; M. Rigutto (Shell Projects and Technology) for the zeolite–alumina extrudates; R. Oord and J. Ruiz-Martínez for ammonia TPD measurements; and M. De Prins and S. Radhakrishnan for help with the catalytic experiments.

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Authors and Affiliations

Authors

Contributions

J.Z. synthesized and characterized the samples, and drafted the manuscript. G.V. performed hydrocracking tests. K.P.deJ. and J.A.M. contributed to experimental design, data analysis and manuscript writing.

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Correspondence to Krijn P. de Jong or Johan A. Martens.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Textural analysis of Y/A extrudates.

a, N2 physisorption isotherm (adsorption and desorption), plotting the adsorbed N2 volume (in cm3 g−1), referred to a standard temperature and pressure (STP) of 0 °C and 1 atmosphere, against the relative pressure, P/P0. The isotherm shows the hysteresis loop that is indicative of the presence of mesopores (of diameter 2–50 nm). In addition, the quantity of N2 that is adsorbed at very low relative pressure indicates the presence of micropores (of diameter ~1 nm). b, The Barrett–Joyner–Halenda (BJH) pore-size distribution, derived from the adsorption branch of the isotherm in a, points to the broad size distribution of mesopores and macropores (macropores having a diameter greater than 50 nm). V is the pore volume; D is the pore diameter.

Extended Data Figure 2 Controlled deposition of platinum.

a, Left, an HAADF-STEM image of a 70-nm-thick section of the Pt-Y/A catalyst. Right, EDX elemental maps of the region outlined in green in the HAADF-STEM image. The EDX maps show the presence of Pt particles (yellow) in the zeolite region (green, with the dominant Si signal); the alumina region (red) is empty. ‘O’ denotes the oxygen signal. b, Left, an HAADF-STEM image of a 70-nm-thick section of the Pt-A/Y catalyst. Right, EDX elemental maps of the region outlined in red at the left. Pt particles (yellow) are present in the alumina region (red), while the zeolite crystal (green) contains no Pt particles. The bright spot and line in the HAADF-STEM image of Pt-A/Y, below the mapped region of interest, originate from prolonged electron-beam exposure. Scale bars in the EDX maps represent 50 nm.

Extended Data Figure 3 Three-dimensional structural analysis using electron tomography.

a, Pt-Y/A catalyst. One-pixel slices, equal to thicknesses of 0.34 nm (left) and 0.26 nm (right), from the middle of the electron-tomography reconstructions of zeolite crystal (left) and alumina aggregate (right) from the Pt-Y/A catalyst, show the presence of ~2.5-nm Pt particles inside the zeolite crystal (left). Within the alumina aggregate (right), only a very few Pt particles were detected, of which one is shown in the zoomed-in region. b, Pt-A/Y catalyst. Ten-pixel slices, equal to thicknesses of 3.6 nm (left) and 2.8 nm (right), from the middle of the electron-tomography reconstructions of zeolite crystal (left) and alumina aggregate (right) from the Pt-A/Y catalyst, show that Pt particles of ~3.5-nm diameter were located on the alumina platelets surrounding the zeolite crystal (left) and on the alumina platelets of the aggregate (right). No Pt particles were detected inside the zeolite crystal. For electron-tomography analysis, both catalysts were ground, dispersed in ethanol, and sonicated in order to break zeolite crystals and alumina aggregates apart and analyse them separately on the TEM grid.

Extended Data Figure 4 Comparing the acidity of the samples

. Temperature-programmed desorption of ammonia by the Pt-Y/A (green) and Pt-A/Y (red) catalysts displays peaks that indicate the presence of weakly acidic (at ~160 °C) and strongly acidic (at ~320 °C) sites within both catalysts, with Pt-A/Y showing slightly lower peak intensity in the region of the weakly acidic site. The total amount of ammonia desorbed was measured to be 14.3 cm3 g−1 (Pt-Y/A) and 14.7 cm3 g−1 (Pt-A/Y) STP.

Extended Data Figure 5 Hydrocracking product distribution at 35% cracking conversion.

a, n-Decane feed; b, n-nonadecane feed; c, pristane feed. Results obtained with Pt-Y/A and Pt-A/Y catalysts are represented with green squares and red triangles, respectively. Experiments were performed at a pressure of 0.45 MPa and H2/hydrocarbon molar ratio of 214 (n-decane); or a pressure of 0.65 MPa and a H2/hydrocarbon molar ratio of 14.6 (n-nonadecane and pristane).

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Zecevic, J., Vanbutsele, G., de Jong, K. et al. Nanoscale intimacy in bifunctional catalysts for selective conversion of hydrocarbons. Nature 528, 245–248 (2015). https://doi.org/10.1038/nature16173

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