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Upgrading of heavy crude oils: scientific advances in the description of catalyst supports

November 2015

In recent years, improvements in oil extraction processes have led to an increase in the proportion of heavy crudes in global production. Yet these crudes are ill-suited to the fuel market and by cracking them, to obtain lighter fractions that are easier to use, the production process becomes more efficient thereby reducing the need to exploit this fossil resource.

Amorphous aluminosilicates (ASAs) are strategic materials for use as catalyst supports in the refining industry. Their Brønsted acidity1, which is lower than that of crystalline zeolites, makes them ideal for the selective conversion of heavy oil feeds into middle distillates (diesel, kerosene) via hydrocracking.

These mesoporous inorganic oxides are known as "mixed" oxides due to the simultaneous presence of silica and aluminum atoms, which explains the acidity properties observed, properties that partly determine the catalytic performance targeted. The methods employed to prepare them are numerous, leading to a wide range of materials, particularly with respect to the possible distribution of silica and aluminum atoms within the network of oxygen atoms (for example, the formation of pure (SiO2, Al2O3) and/or mixed "Si-O-Al" phases over varying areas of material). Their surface properties - and in particular their acidity properties - are thus extremely diverse, further complicating the optimization of their performance.

Previously, this intrinsic complexity of ASAs, combined with their diversity, had made it impossible to develop a unified model to describe the structure of the "SiAl" interface on an atomic scale and thereby correlate the latter with their specific Brønsted acidity.

Maxime Caillot's thesis (2010 – 2013)2, dedicated to understanding the acidity properties of ASAs, and subsequent research, conducted in close partnership with ETH Zurich paved the way for moving a step closer to the development of this model in a field that has been the focus of much research and debate within the scientific community.

The objectives of the work program carried out were threefold:

1) synthesize a model family of ASAs whereby the structure of the "SiAl" is controlled, promoting, for example, the formation of a mixed Si-O-Al phase,

2) closely characterize the surface structure using advanced analytical methods

3) correlate the structure thus revealed with the Brønsted acidity generated.

All three objectives were met. Hence:

  • model materials were developed by grafting molecular precursors onto simple oxide (Si/Al2O3 and Al/SiO2), adjusting the temperature and water content conditions of the synthesis medium, to control the atomic structure presents on the surface [1];
  • the combination of specific characterization methods - such as NMR3 analysis (27Al, 29Si), ToF-SIMS4 analysis and thermogravimetry analysis (TGA) of ethanol adsorption - led to a first description of surface structure [2-4];
  • the dehydration of ethanol to ethylene, during the preceding TGA, also made it possible to quantify the number of Brønsted acid sites and evaluate their reactivity in terms of turnover frequency5, which varies depending on the nature of the ASA, but always remains below that of a zeolite [2-4].

This study also revealed an original, sequenced and selective mechanism of grafting of the silica precursor to the surface of the alumina, in specific operating conditions (Figure 1).

 Figure 1 : Sequenced grafting of silica species onto the surface of gamma alumina.

More recently, an in-depth study using DNP SENS6 further elucidated the surface structure [5]. For example, for the very first time, the c coupling of this analysis with quantum calculations of 1H, 27Al and 29Si NMR chemical shifts, using ASA surface models previously developed at IFPEN8, provides a unique model associating surface state with the supposed atomic structure of Brønsted acid sites (Figure 2).

The innovative characterization methodologies employed in this research pave the way for the optimized preparation of ASAs, with a view to enhancing their catalytic performance for all the targeted refining processes.

 

Figure 2 : Atomic description of surface state and ASA Brønsted acid sites by combined DNP-SENS and DFT. [5]

Scientific contacts: Alexandra Chaumonnot and Mathieu Digne.

  1. Capacity to donate H+ protons
  2. Thesis supervisor Prof. Jeroen van Bokhoven (ETH Zurich), IFPEN supervisors: Alexandra Chaumonnot, Mathieu Digne
  3. Nuclear magnetic resonance
  4. Time-of-Flight Secondary Ion Mass Spectroscopy
  5. Turnover frequency: parameter reflecting standardized intrinsic catalytic activity by catalytic site
  6. Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy
  7. by Density Functional Theory (DFT)
  8. Céline Chizallet and Pascal Raybaud, Angew. Chem. Int. Ed., 2009, 48, 2891

Publications

[1] “Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions”, M. Caillot, C. Poleunis, D. P. Debecker, A. Chaumonnot, M. Digne, J. A. van Bokhoven, Microporous and Mesoporous Materials, 2014, 185, 179.

[2] “The variety of Brønsted acid sites in amorphous aluminosilicates and zeolites”, M. Caillot, A. Chaumonnot, M. Digne, J. A. van Bokhoven, J. Catal., 2014, 316, 47.

[3] Creation of Bronsted Acidity by Grafting Aluminum Isopropoxide on Silica under Controlled Conditions: Determination of the Number of Bronsted Sites and their Turnover Frequency for m-Xylene Isomerization”, M. Caillot, A. Chaumonnot, M. Digne, J. A. van Bokhoven, ChemCatChem, 2014, 6, 3, 832.

[4] “Quantification of Brønsted acid sites of grafted amorphous silica-alumina compounds and their turnover frequency in m-xylene isomerization”, M. Caillot, A. Chaumonnot, M. Digne, J. A. van Bokhoven, ChemCatChem, 2013, 5, 3644.

[5] “Atomic Description of the Interface between Silica and Alumina in Aluminosilicates through Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy and First-Principles Calculations”, M. Valla, A. J. Rossini, M. Caillot, C. Chizallet, P. Raybaud, M. Digne, A. Chaumonnot, A. Lesage, L. Emsley, J. A. van Bokhoven, C. Copéret, J. Am. Chem. Soc., 2015, 137, 33, 10710.


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