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<record><TEI><teiHeader><fileDesc><titleStmt><title xml:lang="en">Brønsted acid sites based on penta-coordinated aluminum species</title><author><name sortKey="Wang, Zichun" sort="Wang, Zichun" uniqKey="Wang Z" first="Zichun" last="Wang">Zichun Wang</name><affiliation><nlm:aff id="a1"><institution>Laboratory for Catalysis Engineering, School of Chemical and Biomolecular Engineering, Sydney University</institution>, Chemical Engineering Building J01, Sydney, New South Wales 2006,<country>Australia</country></nlm:aff></affiliation></author><author><name sortKey="Jiang, Yijiao" sort="Jiang, Yijiao" uniqKey="Jiang Y" first="Yijiao" last="Jiang">Yijiao Jiang</name><affiliation><nlm:aff id="a2"><institution>Department of Engineering, Macquarie University</institution>, Sydney, New South Wales 2109,<country>Australia</country></nlm:aff></affiliation></author><author><name sortKey="Lafon, Olivier" sort="Lafon, Olivier" uniqKey="Lafon O" first="Olivier" last="Lafon">Olivier Lafon</name><affiliation><nlm:aff id="a3"><institution>Univ. Lille, CNRS, UMR 8181-UCCS, Unité de Catalyse et de Chimie du Solide</institution>, F-59000 Lille,<country>France</country></nlm:aff></affiliation></author><author><name sortKey="Trebosc, Julien" sort="Trebosc, Julien" uniqKey="Trebosc J" first="Julien" last="Trébosc">Julien Trébosc</name><affiliation><nlm:aff id="a3"><institution>Univ. Lille, CNRS, UMR 8181-UCCS, Unité de Catalyse et de Chimie du Solide</institution>, F-59000 Lille,<country>France</country></nlm:aff></affiliation></author><author><name sortKey="Duk Kim, Kyung" sort="Duk Kim, Kyung" uniqKey="Duk Kim K" first="Kyung" last="Duk Kim">Kyung Duk Kim</name><affiliation><nlm:aff id="a1"><institution>Laboratory for Catalysis Engineering, School of Chemical and Biomolecular Engineering, Sydney University</institution>, Chemical Engineering Building J01, Sydney, New South Wales 2006,<country>Australia</country></nlm:aff></affiliation></author><author><name sortKey="Stampfl, Catherine" sort="Stampfl, Catherine" uniqKey="Stampfl C" first="Catherine" last="Stampfl">Catherine Stampfl</name><affiliation><nlm:aff id="a4"><institution>School of Physics, Sydney University</institution>, Sydney, New South Wales 2006,<country>Australia</country></nlm:aff></affiliation></author><author><name sortKey="Baiker, Alfons" sort="Baiker, Alfons" uniqKey="Baiker A" first="Alfons" last="Baiker">Alfons Baiker</name><affiliation><nlm:aff id="a5"><institution>Institute for Chemical and Bioengineering, Department of Chemistry and Applied Bioscience, ETH Zürich, Hönggerberg, HCI</institution>, CH-8093 Zürich,<country>Switzerland</country></nlm:aff></affiliation></author><author><name sortKey="Amoureux, Jean Paul" sort="Amoureux, Jean Paul" uniqKey="Amoureux J" first="Jean-Paul" last="Amoureux">Jean-Paul Amoureux</name><affiliation><nlm:aff id="a3"><institution>Univ. Lille, CNRS, UMR 8181-UCCS, Unité de Catalyse et de Chimie du Solide</institution>, F-59000 Lille,<country>France</country></nlm:aff></affiliation></author><author><name sortKey="Huang, Jun" sort="Huang, Jun" uniqKey="Huang J" first="Jun" last="Huang">Jun Huang</name><affiliation><nlm:aff id="a1"><institution>Laboratory for Catalysis Engineering, School of Chemical and Biomolecular Engineering, Sydney University</institution>, Chemical Engineering Building J01, Sydney, New South Wales 2006,<country>Australia</country></nlm:aff></affiliation></author></titleStmt><publicationStmt><idno type="wicri:source">PMC</idno><idno type="pmid">27976673</idno><idno type="pmc">5172364</idno><idno type="url">http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5172364</idno><idno type="RBID">PMC:5172364</idno><idno type="doi">10.1038/ncomms13820</idno><date when="2016">2016</date><idno type="wicri:Area/Pmc/Corpus">000783</idno><idno type="wicri:explorRef" wicri:stream="Pmc" wicri:step="Corpus" wicri:corpus="PMC">000783</idno></publicationStmt><sourceDesc><biblStruct><analytic><title xml:lang="en" level="a" type="main">Brønsted acid sites based on penta-coordinated aluminum species</title><author><name sortKey="Wang, Zichun" sort="Wang, Zichun" uniqKey="Wang Z" first="Zichun" last="Wang">Zichun Wang</name><affiliation><nlm:aff id="a1"><institution>Laboratory for Catalysis Engineering, School of Chemical and Biomolecular Engineering, Sydney University</institution>, Chemical Engineering Building J01, Sydney, New South Wales 2006,<country>Australia</country></nlm:aff></affiliation></author><author><name sortKey="Jiang, Yijiao" sort="Jiang, Yijiao" uniqKey="Jiang Y" first="Yijiao" last="Jiang">Yijiao Jiang</name><affiliation><nlm:aff id="a2"><institution>Department of Engineering, Macquarie University</institution>, Sydney, New South Wales 2109,<country>Australia</country></nlm:aff></affiliation></author><author><name sortKey="Lafon, Olivier" sort="Lafon, Olivier" uniqKey="Lafon O" first="Olivier" last="Lafon">Olivier Lafon</name><affiliation><nlm:aff id="a3"><institution>Univ. Lille, CNRS, UMR 8181-UCCS, Unité de Catalyse et de Chimie du Solide</institution>, F-59000 Lille,<country>France</country></nlm:aff></affiliation></author><author><name sortKey="Trebosc, Julien" sort="Trebosc, Julien" uniqKey="Trebosc J" first="Julien" last="Trébosc">Julien Trébosc</name><affiliation><nlm:aff id="a3"><institution>Univ. Lille, CNRS, UMR 8181-UCCS, Unité de Catalyse et de Chimie du Solide</institution>, F-59000 Lille,<country>France</country></nlm:aff></affiliation></author><author><name sortKey="Duk Kim, Kyung" sort="Duk Kim, Kyung" uniqKey="Duk Kim K" first="Kyung" last="Duk Kim">Kyung Duk Kim</name><affiliation><nlm:aff id="a1"><institution>Laboratory for Catalysis Engineering, School of Chemical and Biomolecular Engineering, Sydney University</institution>, Chemical Engineering Building J01, Sydney, New South Wales 2006,<country>Australia</country></nlm:aff></affiliation></author><author><name sortKey="Stampfl, Catherine" sort="Stampfl, Catherine" uniqKey="Stampfl C" first="Catherine" last="Stampfl">Catherine Stampfl</name><affiliation><nlm:aff id="a4"><institution>School of Physics, Sydney University</institution>, Sydney, New South Wales 2006,<country>Australia</country></nlm:aff></affiliation></author><author><name sortKey="Baiker, Alfons" sort="Baiker, Alfons" uniqKey="Baiker A" first="Alfons" last="Baiker">Alfons Baiker</name><affiliation><nlm:aff id="a5"><institution>Institute for Chemical and Bioengineering, Department of Chemistry and Applied Bioscience, ETH Zürich, Hönggerberg, HCI</institution>, CH-8093 Zürich,<country>Switzerland</country></nlm:aff></affiliation></author><author><name sortKey="Amoureux, Jean Paul" sort="Amoureux, Jean Paul" uniqKey="Amoureux J" first="Jean-Paul" last="Amoureux">Jean-Paul Amoureux</name><affiliation><nlm:aff id="a3"><institution>Univ. Lille, CNRS, UMR 8181-UCCS, Unité de Catalyse et de Chimie du Solide</institution>, F-59000 Lille,<country>France</country></nlm:aff></affiliation></author><author><name sortKey="Huang, Jun" sort="Huang, Jun" uniqKey="Huang J" first="Jun" last="Huang">Jun Huang</name><affiliation><nlm:aff id="a1"><institution>Laboratory for Catalysis Engineering, School of Chemical and Biomolecular Engineering, Sydney University</institution>, Chemical Engineering Building J01, Sydney, New South Wales 2006,<country>Australia</country></nlm:aff></affiliation></author></analytic><series><title level="j">Nature Communications</title><idno type="eISSN">2041-1723</idno><imprint><date when="2016">2016</date></imprint></series></biblStruct></sourceDesc></fileDesc><profileDesc><textClass></textClass></profileDesc></teiHeader><front><div type="abstract" xml:lang="en"><p>Zeolites and amorphous silica-alumina (ASA), which both provide Brønsted acid sites (BASs), are the most extensively used solid acid catalysts in the chemical industry. It is widely believed that BASs consist only of tetra-coordinated aluminum sites (Al<sup>IV</sup>) with bridging OH groups in zeolites or nearby silanols on ASA surfaces. Here we report the direct observation in ASA of a new type of BAS based on penta-coordinated aluminum species (Al<sup>V</sup>) by <sup>27</sup>Al-{<sup>1</sup>H} dipolar-mediated correlation two-dimensional NMR experiments at high magnetic field under magic-angle spinning. Both BAS-Al<sup>IV</sup> and -Al<sup>V</sup> show a similar acidity to protonate probe molecular ammonia. The quantitative evaluation of <sup>1</sup>H and <sup>27</sup>Al sites demonstrates that BAS-Al<sup>V</sup> co-exists with BAS-Al<sup>IV</sup> rather than replaces it, which opens new avenues for strongly enhancing the acidity of these popular solid acids.</p></div></front><back><div1 type="bibliography"><listBibl><biblStruct><analytic><author><name sortKey="Corma, A" uniqKey="Corma A">A. 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<front><journal-meta><journal-id journal-id-type="nlm-ta">Nat Commun</journal-id><journal-id journal-id-type="iso-abbrev">Nat Commun</journal-id><journal-title-group><journal-title>Nature Communications</journal-title></journal-title-group><issn pub-type="epub">2041-1723</issn><publisher><publisher-name>Nature Publishing Group</publisher-name></publisher></journal-meta><article-meta><article-id pub-id-type="pmid">27976673</article-id><article-id pub-id-type="pmc">5172364</article-id><article-id pub-id-type="pii">ncomms13820</article-id><article-id pub-id-type="doi">10.1038/ncomms13820</article-id><article-categories><subj-group subj-group-type="heading"><subject>Article</subject></subj-group></article-categories><title-group><article-title>Brønsted acid sites based on penta-coordinated aluminum species</article-title></title-group><contrib-group><contrib contrib-type="author"><name><surname>Wang</surname><given-names>Zichun</given-names></name><xref ref-type="aff" rid="a1">1</xref></contrib><contrib contrib-type="author"><name><surname>Jiang</surname><given-names>Yijiao</given-names></name><xref ref-type="aff" rid="a2">2</xref></contrib><contrib contrib-type="author"><name><surname>Lafon</surname><given-names>Olivier</given-names></name><xref ref-type="aff" rid="a3">3</xref></contrib><contrib contrib-type="author"><name><surname>Trébosc</surname><given-names>Julien</given-names></name><xref ref-type="aff" rid="a3">3</xref></contrib><contrib contrib-type="author"><name><surname>Duk Kim</surname><given-names>Kyung</given-names></name><xref ref-type="aff" rid="a1">1</xref></contrib><contrib contrib-type="author"><name><surname>Stampfl</surname><given-names>Catherine</given-names></name><xref ref-type="aff" rid="a4">4</xref></contrib><contrib contrib-type="author"><name><surname>Baiker</surname><given-names>Alfons</given-names></name><xref ref-type="aff" rid="a5">5</xref></contrib><contrib contrib-type="author"><name><surname>Amoureux</surname><given-names>Jean-Paul</given-names></name><xref ref-type="corresp" rid="c1">a</xref><xref ref-type="aff" rid="a3">3</xref></contrib><contrib contrib-type="author"><name><surname>Huang</surname><given-names>Jun</given-names></name><xref ref-type="corresp" rid="c2">b</xref><xref ref-type="aff" rid="a1">1</xref></contrib><aff id="a1"><label>1</label><institution>Laboratory for Catalysis Engineering, School of Chemical and Biomolecular Engineering, Sydney University</institution>, Chemical Engineering Building J01, Sydney, New South Wales 2006,<country>Australia</country></aff><aff id="a2"><label>2</label><institution>Department of Engineering, Macquarie University</institution>, Sydney, New South Wales 2109,<country>Australia</country></aff><aff id="a3"><label>3</label><institution>Univ. Lille, CNRS, UMR 8181-UCCS, Unité de Catalyse et de Chimie du Solide</institution>, F-59000 Lille,<country>France</country></aff><aff id="a4"><label>4</label><institution>School of Physics, Sydney University</institution>, Sydney, New South Wales 2006,<country>Australia</country></aff><aff id="a5"><label>5</label><institution>Institute for Chemical and Bioengineering, Department of Chemistry and Applied Bioscience, ETH Zürich, Hönggerberg, HCI</institution>, CH-8093 Zürich,<country>Switzerland</country></aff></contrib-group><author-notes><corresp id="c1"><label>a</label><email>jun.huang@sydney.edu.au</email></corresp><corresp id="c2"><label>b</label><email>jean-paul.amoureux@univ-lille1.fr</email></corresp></author-notes><pub-date pub-type="epub"><day>15</day><month>12</month><year>2016</year></pub-date><pub-date pub-type="collection"><year>2016</year></pub-date><volume>7</volume><elocation-id>13820</elocation-id><history><date date-type="received"><day>09</day><month>03</month><year>2016</year></date><date date-type="accepted"><day>03</day><month>11</month><year>2016</year></date></history><permissions><copyright-statement>Copyright © 2016, The Author(s)</copyright-statement><copyright-year>2016</copyright-year><copyright-holder>The Author(s)</copyright-holder><license license-type="open-access" xlink:href="http://creativecommons.org/licenses/by/4.0/"><pmc-comment>author-paid</pmc-comment>
<license-p>This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article's Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit <ext-link ext-link-type="uri" xlink:href="http://creativecommons.org/licenses/by/4.0/">http://creativecommons.org/licenses/by/4.0/</ext-link></license-p></license></permissions><abstract><p>Zeolites and amorphous silica-alumina (ASA), which both provide Brønsted acid sites (BASs), are the most extensively used solid acid catalysts in the chemical industry. It is widely believed that BASs consist only of tetra-coordinated aluminum sites (Al<sup>IV</sup>) with bridging OH groups in zeolites or nearby silanols on ASA surfaces. Here we report the direct observation in ASA of a new type of BAS based on penta-coordinated aluminum species (Al<sup>V</sup>) by <sup>27</sup>Al-{<sup>1</sup>H} dipolar-mediated correlation two-dimensional NMR experiments at high magnetic field under magic-angle spinning. Both BAS-Al<sup>IV</sup> and -Al<sup>V</sup> show a similar acidity to protonate probe molecular ammonia. The quantitative evaluation of <sup>1</sup>H and <sup>27</sup>Al sites demonstrates that BAS-Al<sup>V</sup> co-exists with BAS-Al<sup>IV</sup> rather than replaces it, which opens new avenues for strongly enhancing the acidity of these popular solid acids.</p></abstract><abstract abstract-type="web-summary"><p>Until now, it has been believed that Brønsted acid sites in amorphous silica-alumina are formed from only tetra-coordinated (Al<sup>IV</sup>) sites. Here, the authors use <sup>27</sup>Al-{<sup>1</sup>H} correlation NMR experiments to identify a new Al<sup>V</sup>-based Brønsted acid site, with implications for increasing the acidity of solid acid catalysts.</p></abstract></article-meta></front><body><p>The need for efficient and environmentally benign chemical processes has forced the replacement of harmful and corrosive liquid acids by solid acids in various fields of catalysis, including fine chemistry<xref ref-type="bibr" rid="b1">1</xref><xref ref-type="bibr" rid="b2">2</xref><xref ref-type="bibr" rid="b3">3</xref>, renewable energy production<xref ref-type="bibr" rid="b4">4</xref><xref ref-type="bibr" rid="b5">5</xref><xref ref-type="bibr" rid="b6">6</xref>, oil refining and petrochemical industries<xref ref-type="bibr" rid="b7">7</xref><xref ref-type="bibr" rid="b8">8</xref>. Silicon- and aluminum-based mixed oxides provide moderate and strong Brønsted acidity and are among the most popular solid acids used in current chemical processes<xref ref-type="bibr" rid="b7">7</xref><xref ref-type="bibr" rid="b9">9</xref>. Briefly, the solid acid catalysts can protonate hydrocarbon molecules to form carbocations and drive important reactions, such as cracking, hydrocracking, isomerization, alkylation and aromatization<xref ref-type="bibr" rid="b10">10</xref><xref ref-type="bibr" rid="b11">11</xref><xref ref-type="bibr" rid="b12">12</xref><xref ref-type="bibr" rid="b13">13</xref><xref ref-type="bibr" rid="b14">14</xref>, through surface complexes or transition states<xref ref-type="bibr" rid="b15">15</xref>.</p><p>Crystalline zeolites and amorphous silica-alumina (ASA) are two main types of solid acids that contain Brønsted acid sites (BASs). It has been widely believed that only tetra-coordinated aluminum (Al<sup>IV</sup>) atoms are able to contribute to the formation of BASs in nature<xref ref-type="bibr" rid="b16">16</xref>. In crystalline zeolites, the BASs are formed by protons, which compensate the negatively charged oxygens induced by the substitution of Si atoms by Al<sup>IV</sup> in the framework. The structure of these sites is well known as the bridging Si(OH)Al<sup>IV</sup> model (<xref ref-type="fig" rid="f1">Fig. 1a</xref>)<xref ref-type="bibr" rid="b15">15</xref><xref ref-type="bibr" rid="b16">16</xref><xref ref-type="bibr" rid="b17">17</xref>. Replacing Si atoms by more Al<sup>IV</sup> species can enhance the density of BASs, but it reduces the mean electronegativity of the framework, which thus leads to a decrease of the overall acid strength of BASs<xref ref-type="bibr" rid="b18">18</xref><xref ref-type="bibr" rid="b19">19</xref>. Similarly, Al<sup>IV</sup> species incorporated into the amorphous silica network are able to generate BASs on ASA<xref ref-type="bibr" rid="b9">9</xref><xref ref-type="bibr" rid="b20">20</xref><xref ref-type="bibr" rid="b21">21</xref>. The proximity between Al<sup>IV</sup> and silanol sites in ASA has recently been observed by nuclear magnetic resonance (NMR) correlation experiments between <sup>29</sup>Si and <sup>27</sup>Al nuclei, the sensitivity of which was enhanced by dynamic nuclear polarization<xref ref-type="bibr" rid="b22">22</xref>. However, the strength of these BASs is generally lower than that on crystalline zeolites<xref ref-type="bibr" rid="b7">7</xref> and thus the presence of bridging OH groups (<xref ref-type="fig" rid="f1">Fig. 1a</xref>) in ASA is still strongly under debate<xref ref-type="bibr" rid="b22">22</xref><xref ref-type="bibr" rid="b23">23</xref>.</p><p>A flexible coordination between the Al<sup>IV</sup> atom and the neighbouring silanol oxygen atom (<xref ref-type="fig" rid="f1">Fig. 1b</xref>)<xref ref-type="bibr" rid="b9">9</xref><xref ref-type="bibr" rid="b21">21</xref><xref ref-type="bibr" rid="b24">24</xref><xref ref-type="bibr" rid="b25">25</xref> or a pseudobridging silanol (PBS) with a nearby Al atom (<xref ref-type="fig" rid="f1">Fig. 1c</xref>)<xref ref-type="bibr" rid="b26">26</xref><xref ref-type="bibr" rid="b27">27</xref> have been proposed<xref ref-type="bibr" rid="b22">22</xref>, to account for the longer Al-O distances (2.94–4.43 Å) in ASA<xref ref-type="bibr" rid="b26">26</xref>, with respect to those in the crystalline zeolite framework (1.88–2.0 Å)<xref ref-type="bibr" rid="b28">28</xref>. So far, most efforts focus on tuning the concentration of Al<sup>IV</sup> as the main route to increase the Brønsted acidity on zeolites or silica-alumina<xref ref-type="bibr" rid="b29">29</xref><xref ref-type="bibr" rid="b30">30</xref><xref ref-type="bibr" rid="b31">31</xref><xref ref-type="bibr" rid="b32">32</xref><xref ref-type="bibr" rid="b33">33</xref>. However, Al<sup>IV</sup> species tend to condense, to form an alumina phase at high Al/Si ratios<xref ref-type="bibr" rid="b34">34</xref><xref ref-type="bibr" rid="b35">35</xref><xref ref-type="bibr" rid="b36">36</xref>, leading to the decrease of Brønsted acidity. For ASA containing solely BASs based on Al<sup>IV</sup> species (BAS-Al<sup>IV</sup>), the maximum Brønsted acidity has been obtained at 30 wt% Al loading<xref ref-type="bibr" rid="b37">37</xref><xref ref-type="bibr" rid="b38">38</xref>. In spite of the different BAS models, only Al<sup>IV</sup> species have been experimentally confirmed to contribute to the formation of BASs in these catalysts. Al<sup>V</sup> and Al<sup>VI</sup> species have been shown to act as Lewis acid sites on ASA and zeolites, but, to the best of our knowledge, no experimental evidence of BASs involving these sites has been reported so far<xref ref-type="bibr" rid="b39">39</xref><xref ref-type="bibr" rid="b40">40</xref><xref ref-type="bibr" rid="b41">41</xref><xref ref-type="bibr" rid="b42">42</xref>.</p><p>Herein, we provide the direct experimental evidence for a new type of BAS-Al<sup>V</sup> in ASA by dipolar-mediated heteronuclear multiple quantum correlation (<italic>D</italic>-HMQC) two-dimensional (2D) NMR experiments, which allow the detection of protons via <sup>27</sup>Al nuclei, hereafter noted <sup>27</sup>Al-{<sup>1</sup>H}, hence probing the spatial proximities between different Al species and surface protons<xref ref-type="bibr" rid="b43">43</xref><xref ref-type="bibr" rid="b44">44</xref><xref ref-type="bibr" rid="b45">45</xref>. These experiments show that ASA can contain a large amount of Al<sup>V</sup> species located near SiOH groups. The acidity of these surface BAS-Al<sup>V</sup> sites has been demonstrated in this research by the adsorption of basic ammonia molecules, which react with BAS-Al<sup>V</sup> to form surface ammonium ions.</p><sec disp-level="1"><title>Results</title><sec disp-level="2"><title>Probing the connectivity between Al<sup>V</sup> species and SiOH groups</title><p>The ASAs used in this work (see <xref ref-type="supplementary-material" rid="S1">Supplementary Methods</xref>) have been prepared according to a previously described procedure<xref ref-type="bibr" rid="b9">9</xref>, which generates ASA nanoparticles with a large amount of Al<sup>V</sup> species. The ASA powders are designated as SA/<italic>X</italic>, where <italic>X</italic> is 10 or 50, indicating the molar fraction of Al in the precursor with respect to the total amount of Al and Si atoms. The obtained ASAs have tunable BAS acidity strengths ranging from moderate (SA/10 has an acidity close to zeolite H-X) to large (SA/30–70 have stronger BASs than zeolites H-Y and ZSM-5), depending on the aluminum content, as confirmed by both <sup>13</sup>C magic angle spinning (MAS) NMR investigation with probe molecule acetone and ammonia-temperature program desorption (TPD)<xref ref-type="bibr" rid="b9">9</xref>. The ASAs exhibited excellent catalytic performances for the conversion of phenylglyoxal with various alcohols, better than that of dealuminated zeolite Y, which hitherto was considered to be the most active solid acid in phenylglyoxal conversion<xref ref-type="bibr" rid="b2">2</xref>.</p><p>The formation of BAS requires the aluminum atoms to be close to SiOH groups. Such proximity induces a dipolar coupling between <sup>27</sup>Al and <sup>1</sup>H nuclei, which can efficiently be probed by <italic>D</italic>-HMQC NMR 2D experiments based on coherence transfers via the <sup>1</sup>H-<sup>27</sup>Al dipolar couplings<xref ref-type="bibr" rid="b46">46</xref><xref ref-type="bibr" rid="b47">47</xref>. As shown in <xref ref-type="fig" rid="f2">Fig. 2</xref>, the correlation at <italic>δ</italic><sub>27A</sub>=50 p.p.m. and <italic>δ</italic><sub>1H</sub>=1.9 p.p.m. in the <sup>27</sup>Al-{<sup>1</sup>H} <italic>D</italic>-HMQC 2D spectrum of dehydrated SA/50 indicates a close proximity between Al<sup>IV</sup> species and the proton of SiOH groups. This correlation is ascribed to the Si-OH···Al<sup>IV</sup> coordination: the typical BAS-Al<sup>IV</sup> often described for ASA (<xref ref-type="fig" rid="f1">Fig. 1c</xref>)<xref ref-type="bibr" rid="b16">16</xref>. A very weak correlation at <italic>δ</italic><sub>27Al</sub>=10 p.p.m. and <italic>δ</italic><sub>1H</sub>=1.1 p.p.m. is assigned to the non-acidic terminal Al<sup>VI</sup>OH groups often observed on the surface of silica-alumina or zeolites, whereas the low-field broad hump at <italic>ca</italic>. 6 p.p.m. in the <sup>1</sup>H dimension could be caused by the small fraction of hydrogen-bonded AlOH groups<xref ref-type="bibr" rid="b16">16</xref>. Nevertheless, the most intense correlation is observed between Al<sup>V</sup> species (<italic>δ</italic><sub>27A</sub>=30 p.p.m.) and SiOH protons (<italic>δ</italic><sub>1H</sub>=1.9 p.p.m.), which indicates the close proximity between SiOH groups and Al<sup>V</sup> species, and the presence of Si-OH···Al<sup>V</sup> coordination (<xref ref-type="fig" rid="f2">Fig. 2</xref>) in dehydrated SA/50.</p></sec><sec disp-level="2"><title>Al<sup>V</sup>-based BASs</title><p>In zeolites, the substitution of a framework Si atom by an Al<sup>IV</sup> one to form one SiOHAl acid site (<xref ref-type="fig" rid="f1">Fig. 1a</xref>) can shift the <sup>1</sup>H NMR signal of SiOH from <italic>ca</italic>. 1.8 to 3.6–5.2 p.p.m. (ref. <xref ref-type="bibr" rid="b16">16</xref>). For these catalysts, BAS could be directly evidenced by the cross-peak in <sup>27</sup>Al-{<sup>1</sup>H} <italic>D</italic>-HMQC 2D spectrum between Al<sup>IV</sup> species (<italic>δ</italic><sub>27Al</sub>=60 p.p.m.) and the bridging OH groups (<italic>δ</italic><sub>1H</sub>=4.3 p.p.m.)<xref ref-type="bibr" rid="b43">43</xref>. However, previous works have shown that the Al atoms with neighbouring SiOH groups (<xref ref-type="fig" rid="f1">Fig. 1b,c</xref>) do not produce such a shift of the <sup>1</sup>H MAS signal of these groups<xref ref-type="bibr" rid="b5">5</xref><xref ref-type="bibr" rid="b6">6</xref><xref ref-type="bibr" rid="b9">9</xref><xref ref-type="bibr" rid="b16">16</xref><xref ref-type="bibr" rid="b20">20</xref><xref ref-type="bibr" rid="b21">21</xref>. <xref ref-type="supplementary-material" rid="S1">Supplementary Fig. 1a,b</xref> show that the <sup>1</sup>H signal of SA/10 and SA/50 is centred around 1.9 p.p.m., thus indicating a majority of flexible or PBS coordination rather than zeolitic bridging coordination between SiOH groups and either Al<sup>IV</sup> or Al<sup>V</sup> species.</p><p>Experiments using probe molecules have confirmed the role of flexible or PBS Si-OH···Al<sup>IV</sup> coordination as BAS in ASA<xref ref-type="bibr" rid="b5">5</xref><xref ref-type="bibr" rid="b6">6</xref><xref ref-type="bibr" rid="b9">9</xref><xref ref-type="bibr" rid="b16">16</xref><xref ref-type="bibr" rid="b20">20</xref><xref ref-type="bibr" rid="b21">21</xref>. Similar methods using ammonia probe molecules were applied here to demonstrate the acidity of the Si-OH···Al<sup>V</sup> coordination observed in dehydrated SA/10 and SA/50 (ref. <xref ref-type="bibr" rid="b16">16</xref>). For these samples loaded with ammonia, the <sup>1</sup>H signal of ammonium ions was observed at <italic>δ</italic><sub>1H</sub>=6.7 p.p.m., as shown in <xref ref-type="supplementary-material" rid="S1">Supplementary Fig. 1c,d</xref>, and commented in <xref ref-type="supplementary-material" rid="S1">Supplementary Note 1</xref>. The formation of these ions shows that ammonia reacts with BAS of SA/10 and SA/50.</p><p><sup>27</sup>Al-{<sup>1</sup>H} <italic>D</italic>-HMQC experiments were also carried out to determine the nature of BAS, which protonate the ammonia molecules. Such a strategy based on <sup>1</sup>H-<sup>27</sup>Al correlations has been applied for [Al]MCM-41 loaded with ammonia. For such catalysts, ammonium ions (<italic>δ</italic><sub>1H</sub>=6.7 p.p.m.) were only coupled to Al<sup>IV</sup> species (<italic>δ</italic><sub>27A</sub>=56 p.p.m.)<xref ref-type="bibr" rid="b44">44</xref>. Hence, there was only evidence for BAS-Al<sup>IV</sup> on the surface of [Al]MCM-41, which protonated ammonia to ammonium ions. As seen in <xref ref-type="fig" rid="f3">Fig. 3</xref>, a correlation between NH<sub>4</sub><sup>+</sup> ions (<italic>δ</italic><sub>1H</sub>=6.7 p.p.m.) and Al<sup>IV</sup> (<italic>δ</italic><sub>27Al</sub>=50 p.p.m.) is also observed in <sup>27</sup>Al-{<sup>1</sup>H} <italic>D</italic>-HMQC spectra of SA/10 and SA/50, showing that the BAS-Al<sup>IV</sup> sites are also present on the surface of ASAs (<xref ref-type="fig" rid="f4">Fig. 4a</xref>). Interestingly, these spectra also exhibit cross-peaks between Al<sup>V</sup> species (<italic>δ</italic><sub>27Al</sub>=30 p.p.m.) and NH<sub>4</sub><sup>+</sup> ions (<italic>δ</italic><sub>1H</sub>=6.7 p.p.m.) in both SA/10 and SA/50. As seen in <xref ref-type="fig" rid="f3">Fig. 3e</xref>, the intensity of this Al<sup>V</sup>-NH<sub>4</sub><sup>+</sup> cross-peak is comparable to that of the Al<sup>IV</sup>-NH<sub>4</sub><sup>+</sup> one. Given the BAS density ranging from 0.16 to 0.36 H<sup>+</sup> nm<sup>−2</sup> in the investigated ASA samples (<xref ref-type="supplementary-material" rid="S1">Supplementary Table 1</xref>), each ammonia molecule only interacts with one BAS. The distance between the aluminum atom and the neighbouring silanol oxygen in ASA ranges from <italic>ca</italic>. 2.94 to 4.43 Å<xref ref-type="bibr" rid="b26">26</xref> and the N–H bond length in ammonia is only 1.02 Å<xref ref-type="bibr" rid="b48">48</xref>. As the heteronuclear coherence transfer in <sup>27</sup>Al-{<sup>1</sup>H} <italic>D</italic>-HMQC is only effective up to a few angstroms, the protons of Si-O<sup>−</sup>(NH<sub>4</sub>)<sup>+</sup>···Al environment only interact with the neighbouring Al. The observation of an Al<sup>V</sup>-NH<sub>4</sub><sup>+</sup> cross-peak in <xref ref-type="fig" rid="f3">Fig. 3</xref> at (30, 6.7) p.p.m. directly confirmed that ammonia is protonated on a BAS containing Al<sup>V</sup>: the Si-OH···Al<sup>V</sup> group.</p><p>The comparison of <xref ref-type="supplementary-material" rid="S1">Supplementary Fig. 1c,d</xref> shows that more ammonia molecules are protonated on BAS-Al<sup>V</sup> in SA/50 than in SA/10. Combined with quantitative <sup>1</sup>H NMR investigations (<xref ref-type="supplementary-material" rid="S1">Supplementary Fig. 1</xref>) and the quadrupolar parameters (<xref ref-type="supplementary-material" rid="S1">Supplementary Table 2</xref> and <xref ref-type="supplementary-material" rid="S1">Supplementary Note 2</xref>) obtained from <sup>27</sup>Al one-dimensional MAS (<xref ref-type="supplementary-material" rid="S1">Supplementary Fig. 3</xref>) and 2D multiple quantum MAS (<xref ref-type="supplementary-material" rid="S1">Supplementary Fig. 2</xref>) NMR experiments, the analysis of <sup>27</sup>Al cross-peak intensities in <sup>27</sup>Al-{<sup>1</sup>H} <italic>D</italic>-HMQC spectra (<xref ref-type="supplementary-material" rid="S1">Supplementary Fig. 4</xref> and <xref ref-type="supplementary-material" rid="S1">Supplementary Note 3</xref>) revealed that the population densities of both BAS-Al<sup>IV</sup> and -Al<sup>V</sup> on SA/50 (0.078 and 0.053 mmol g<sup>−1</sup>) were both higher than those of SA/10 (0.058 and 0.039 mmol g<sup>−1</sup>). This result suggests that BAS-Al<sup>IV</sup> and -Al<sup>V</sup> can co-exist on the surface rather than replacing each other and the population of both acid sites can be amplified by increasing Al content. Thus, this observation is promising for enhancing the population of BAS on ASA without limitation imposed by the Al contents. It should be noted that the <bold>F</bold><sub><bold>2</bold></sub> projections of <sup>27</sup>Al-{<sup>1</sup>H} <italic>D</italic>-HMQC 2D spectra are almost identical for dehydrated and ammonia-loaded SA/50 (see <xref ref-type="supplementary-material" rid="S1">Supplementary Fig. 5</xref> and <xref ref-type="supplementary-material" rid="S1">Supplementary Note 4</xref>). Therefore, Si-OH···Al<sup>V</sup> and Si-OH···Al<sup>IV</sup> coordinations remained unchanged after the protonation of ammonia (as shown in <xref ref-type="fig" rid="f4">Fig. 4</xref>). No Si-OH···Al<sup>IV</sup> have been transferred to Si-OH-Al<sup>V</sup> permanently after the adsorption of ammonia. In other words, the NMR results do not show the formation of permanent covalent bridges between silicate and Al<sup>IV</sup> or Al<sup>V</sup> sites in ASA samples after the deprotonation of BAS reacting with ammonia. Ammonia partially interacting with surface Al<sup>IV</sup> or Al<sup>V</sup> sites (<xref ref-type="fig" rid="f3">Fig. 3b</xref> and <xref ref-type="supplementary-material" rid="S1">Supplementary Fig. 6</xref>) was also observed, which has been assigned to ammonia adsorbed on Lewis sites (<xref ref-type="supplementary-material" rid="S1">Supplementary Note 5</xref>).</p><p>As shown in <xref ref-type="fig" rid="f1">Fig. 1</xref>, a surface bridging SiOHAl (<xref ref-type="fig" rid="f1">Fig. 1a</xref>), a flexible coordination of SiOH and Al (<xref ref-type="fig" rid="f1">Fig. 1b</xref>), or a pseudo-bridge between SiOH and Al atom (<xref ref-type="fig" rid="f1">Fig. 1c</xref>) have been proposed for the formation of BAS-Al<sup>IV</sup> on ASAs. By analogy, similar structures might also contribute to the formation of BAS-Al<sup>V</sup>. The PBS model permits an explanation of the observation of the <sup>1</sup>H NMR signal of SiOH at 1.9 p.p.m. in both <xref ref-type="fig" rid="f2">Fig. 2</xref> and <xref ref-type="supplementary-material" rid="S1">Supplementary Fig. 1</xref>, whereas this <sup>1</sup>H signal of bridging OH groups (<xref ref-type="fig" rid="f1">Fig. 1a</xref>) should occur at 3.6–5.2 p.p.m. If bridging OH groups are present in the investigated ASAs, their concentration must be below the limit of detection of the one-dimensional NMR MAS spectra of <xref ref-type="supplementary-material" rid="S1">Supplementary Fig. 1</xref>. Nevertheless, the current NMR data cannot rule out, in addition to PBS, the presence of bridging silanol groups in low concentration in ASA samples. These elusive strong BASs may also contribute to the catalytic activity in spite of their low concentration. The identification of all catalytic BASs in ASAs is beyond the scope of the present study, which is mainly to report the existence of BASs based on Al<sup>V</sup> environments. A final assessment of the local structure of BAS-Al<sup>V</sup> will require further experimental and theoretical work.</p><p>In summary, a new type of BAS-Al<sup>V</sup> has been directly observed by <sup>27</sup>Al-{<sup>1</sup>H} <italic>D</italic>-HMQC NMR spectroscopy. Hitherto, it was widely accepted that Al<sup>V</sup> sites only provide Lewis acidity<xref ref-type="bibr" rid="b39">39</xref><xref ref-type="bibr" rid="b40">40</xref><xref ref-type="bibr" rid="b41">41</xref><xref ref-type="bibr" rid="b42">42</xref>, and that solely Al<sup>IV</sup> ones contribute to the formation of BASs in aluminosilicate. However, we prove here by NMR experiments that similar to Al<sup>IV</sup> sites, Al<sup>V</sup> ones interact with neighbouring SiOH groups in ASA and behave as BASs in agreement with the PBS model. BAS-Al<sup>IV</sup> and -Al<sup>V</sup> seem to be structurally similar and show comparable acidity to protonate ammonia. Finally, a very important implication emerging from this work is that both Al<sup>IV</sup> and Al<sup>V</sup> species can co-exist on the surface of ASA. This feature facilitates that the total population density of BAS can be increased up to 70% by increasing the Al content, an amount much higher than the maximum Al loading of <italic>ca</italic>. 30% at which maximum acidity on ASA containing exclusively BAS-Al<sup>IV</sup> is achieved<xref ref-type="bibr" rid="b37">37</xref><xref ref-type="bibr" rid="b38">38</xref>. Hence, our findings not only report the existence of a new type of BAS in nature, but also open new avenues for creating high-performance solid acid catalysts containing Al<sup>V</sup> species, which will be promising for sustainable oil-refining and many industrial chemical processes.</p></sec></sec><sec disp-level="1"><title>Methods</title><sec disp-level="2"><title><sup>27</sup>Al-{<sup>1</sup>H} <italic>D</italic>-HMQC 2D experiment</title><p>All NMR experiments were recorded on a Bruker Avance III 18.8 T (<sup>1</sup>H Larmor frequency of 800 MHz) spectrometer equipped with a 3.2 mm double-resonance MAS probe, in which rotors were spun at <italic>ν</italic><sub>R</sub>=20 kHz. In the <italic>D</italic>-HMQC sequence, we have detected the <sup>27</sup>Al nuclei to benefit from their fast longitudinal relaxation times and the <sup>1</sup>H-<sup>27</sup>Al dipolar couplings were reintroduced by applying a SR<inline-formula id="d32e1066"><inline-graphic id="d32e1067" xlink:href="ncomms13820-m1.jpg"></inline-graphic></inline-formula> recoupling on the <sup>1</sup>H channel<xref ref-type="bibr" rid="b49">49</xref>. The <sup>1</sup>H radiofrequency amplitudes for the 90° pulses and the SR<inline-formula id="d32e1077"><inline-graphic id="d32e1078" xlink:href="ncomms13820-m2.jpg"></inline-graphic></inline-formula> recoupling were equal to <italic>ν</italic><sub>1</sub>=62.5 and 40 kHz, respectively. The central transition selective pulse lengths on <sup>27</sup>Al were 8 and 16 μs for 90° and 180° pulses, respectively, that is, radiofrequency field amplitude <italic>ν</italic><sub>1</sub>=10 kHz. The total dipolar recoupling time, <italic>τ</italic><sub>rec</sub>, ranged from 700 to 1,000 μs depending on the sample. The 2D spectra resulted from the accumulation of 512 transients for each of 20 <italic>t</italic><sub>1</sub> increments with Δ<italic>t</italic><sub>1</sub>=50 μs and a recycle delay=1 s, that is, a total experiment time of about 3 h. Additional details about NMR experiments are given in the <xref ref-type="supplementary-material" rid="S1">Supplementary Methods</xref>.</p></sec><sec disp-level="2"><title>Data availability</title><p>The data that support the findings of this study are available upon request from the corresponding author J.H. and J.-P.A.</p></sec></sec><sec disp-level="1"><title>Additional information</title><p><bold>How to cite this article:</bold> Wang, Z. <italic>et al</italic>. Brønsted acid sites based on penta-coordinated aluminum species. <italic>Nat. Commun.</italic><bold>7,</bold> 13820 doi: 10.1038/ncomms13820 (2016).</p><p><bold>Publisher's note:</bold> Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.</p></sec><sec sec-type="supplementary-material" id="S1"><title>Supplementary Material</title><supplementary-material id="d32e18" content-type="local-data"><caption><title>Supplementary Information</title><p>Supplementary Figures, Supplementary Tables, Supplementary Methods, Supplementary Notes and Supplementary References.</p></caption><media xlink:href="ncomms13820-s1.pdf"></media></supplementary-material></sec></body><back><ack><p>J.H. and C.S. acknowledge the financial supports from Australian Research Council Discovery Projects (DP150103842). J.H. thanks Faculty's Energy and Materials Clusters and MCR scheme, and the International Project Development Funding at the University of Sydney. 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Ohio) <volume>96</volume>, CRC Press (<year>2015</year>).</mixed-citation></ref><ref id="b49"><mixed-citation publication-type="journal"><name><surname>Brinkmann</surname><given-names>A.</given-names></name> & <name><surname>Kentgens</surname><given-names>A. P. M.</given-names></name><article-title>Proton-selective <sup>17</sup>O−<sup>1</sup>H distance measurements in fast magic-angle-spinning solid-state NMR spectroscopy for the determination of hydrogen bond lengths</article-title>. <source>J. Am. Chem. Soc.</source><volume>128</volume>, <fpage>14758</fpage>–<lpage>14759</lpage> (<year>2006</year>).<pub-id pub-id-type="pmid">17105257</pub-id></mixed-citation></ref></ref-list><fn-group><fn><p><bold>Author contributions</bold> J.H., Y.J. and C.S. designed the study. Y.J. and A.B. prepared the samples. Z.W., O.L., J.T. and J.H. performed the NMR experiments and structural assignation. J.H. and J.-P.A. supervised the scientific work. J.H. and Z.W. contributed to writing the paper, and O.L., J.-P.A., A.B. and C.S. revised it.</p></fn></fn-group></back><floats-group><fig id="f1"><label>Figure 1</label><caption><title>Proposed models for BASs in silica-alumina catalysts.</title><p>(<bold>a</bold>) BAS consisting of a bridging silanol site bonded to Al<sup>IV</sup> site (Si(OH)Al) in zeolites<xref ref-type="bibr" rid="b15">15</xref>. (<bold>b</bold>) BAS consisting of the flexible coordination between silanol oxygen and neighbouring Al<sup>IV</sup> (ref. <xref ref-type="bibr" rid="b24">24</xref>). (<bold>c</bold>) BAS consisting of PBS interacting with Al<sup>IV</sup> site<xref ref-type="bibr" rid="b27">27</xref>. In that later case, the dotted line does not denote a covalent bond but only the close proximity between O and Al atoms.</p></caption><graphic xlink:href="ncomms13820-f1"></graphic></fig><fig id="f2"><label>Figure 2</label><caption><title><sup>27</sup><bold>Al-{</bold><sup><bold>1</bold></sup><bold>H}</bold><italic><bold>D</bold></italic><bold>-HMQC 2D spectrum of SA/50</bold>.</title><p>The sample was dehydrated at 723 K for 12 h under vacuum and recorded at 18.8 T with a MAS frequency of <italic>ν</italic><sub>R</sub>=20 kHz and <italic>τ</italic><sub>rec</sub>=1.0 ms. The spectrum reveals that the proximity between SiOH groups and Al<sup>V</sup> species is dominant.</p></caption><graphic xlink:href="ncomms13820-f2"></graphic></fig><fig id="f3"><label>Figure 3</label><caption><title><sup>27</sup><bold>Al-{</bold><sup><bold>1</bold></sup><bold>H}</bold><italic><bold>D</bold></italic><bold>-HMQC 2D spectra of ammonia-loaded ASA samples.</bold></title><p>The dehydrated SA/10 (<bold>a</bold>) and SA/50 (<bold>b</bold>) samples were loaded with ammonia and evacuated at 373 K for 1 h, and the spectra were recorded at 18.8 T with <italic>ν</italic><sub>R</sub>=20 kHz and <italic>τ</italic><sub>rec</sub>=900 μs. The <sup>1</sup>H slices at the shifts of Al<sup>IV</sup> and Al<sup>V</sup> sites of SA/10 extracted from the 2D spectrum (<bold>a</bold>) are displayed in subfigures (<bold>c</bold>,<bold>d</bold>), respectively. The subfigure (<bold>e</bold>) shows the <sup>27</sup>Al slice at the shift of NH<sub>4</sub><sup>+</sup> protons in SA/50 extracted from the spectrum (<bold>b</bold>).</p></caption><graphic xlink:href="ncomms13820-f3"></graphic></fig><fig id="f4"><label>Figure 4</label><caption><title>Proposed proton transfer between BAS and ammonia molecules.</title><p>(<bold>a</bold>) Ammonia protonated on conventional acidic BAS-Al<sup>IV</sup>. (<bold>b</bold>) BAS-Al<sup>V</sup> formed on ASA is able to transfer hydroxyl proton to ammonia, showing similar acidic properties as BAS-Al<sup>IV</sup>.</p></caption><graphic xlink:href="ncomms13820-f4"></graphic></fig></floats-group></pmc></record>Pour manipuler ce document sous Unix (Dilib)
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