Pyrolysis of the Cellulose Fraction of Biomass in the Presence of Solid Acid Catalysts: An Operando Spectroscopy and Theoretical Investigation

Tuning the Number of Active Sites and Turnover Frequencies by Surface Modification of Supported ReO4/(SiO2–Al2O3) Catalysts for Olefin Metathesis

“…The model supported 5SiAl mixed-oxide support reveals that SiO x selectively anchors at basic Al–OH (3787, 3768, 3728 cm –1 ) and neutral Al–OH–Al (3694 cm –1 ) surface hydroxyls. Both terminal Si–OH and Al–(OH) + –Si Brønsted acid sites are present for 5SiAl, as was previously shown. − The addition of rhenia to the 5SiAl mixed-oxide support (3Re5SiAl) shows that the ReO x predominantly anchors at residual basic Al–OH (3743 cm –1 ), acidic Al 3 –OH (3670 cm –1 ) and the more acidic Al–(OH) + –Si surface hydroxyls (broad negative band at ∼3580 cm –1 ; Figure S1). The SiO 2 support possesses terminal Si–OH (3742 cm –1 ) and geminal Si–OH**Si–OH (a broad band from 3727 to 3755 cm –1 ) surface hydroxyls with surface metal oxides generally anchoring to the terminal Si–OH surface hydroxyls, but the decrease in the intensity of the Si–OH vibration was too small to be noticeable in Figure .…”

“…The Raman band for the ν s (ReO) stretch is sharpest for the supported 3ReSi catalyst, reflecting preferential anchoring of the rhenia only on the isolated Si–OH sites. The Raman band for the ν s (ReO) stretch at ∼1011 cm –1 for the surface AlO x modified SiO 2 support is much broader because of the presence of multiple anchoring sites (Si–OH, Al–OH, Al–OH–Al, and Al–(OH) + –Si) on this mixed-oxide support . Additionally, the absence of the Si–OH Raman band indicates that alumina anchors at the Si–OH surface hydroxyls (see Figure S7), and the absence of Si–OH bonds for the supported 3Re5AlSi catalyst suggests that rhenia is also anchoring at the surface Al–OH surface hydroxyls.…”

“…The effectiveness of the anchoring surface hydroxyls on Al 2 O 3 for activating the surface rhenia sites varies in the order Al 3 –OH > Al–OH–Al ≫ Al–OH, which tracks with the acidity of the anchoring surface hydroxyls. Many studies have proposed that surface acidity plays an important role in the olefin metathesis reaction. − For example, the Al 2 O 3 –SiO 2 mixed-oxide supports are known to introduce surface Lewis and Brønsted acid sites and are also highly effective oxide supports for activating surface ReO x sites for olefin metathesis. − These studies, however, have not provided supporting data for the conclusions that the surface acidity plays an important role in olefin metathesis and have not even investigated the nature of the surface AlO x , SiO x , and ReO x sites present in these mixed-oxide supports.…”

“…x modified SiO 2 support is much broader because of the presence of multiple anchoring sites (Si−OH, Al−OH, Al− OH−Al, and Al−(OH) + −Si) on this mixed-oxide support 15. Additionally, the absence of the Si−OH Raman band indicates that alumina anchors at the Si−OH surface hydroxyls (see FigureS7), and the absence of Si−OH bonds for the supported 3Re5AlSi catalyst suggests that rhenia is also anchoring at the surface Al−OH surface hydroxyls.…”

In Situ Spectroscopic Insights on the Molecular Structure of the MgO/SiO2 Catalytic Active Sites during Ethanol Conversion to 1,3-Butadiene

“…The structure of the native OH groups of all of the catalysts was investigated using DRIFTS and shown in Figure . There are four major peaks in the spectrum in addition to a broad peak at ∼3550 cm –1 assigned to germinal and vicinal OH groups of the silica support. , The peak at 3745 cm –1 was assigned to both isolated silanol groups of the SiO 2 support, , which decreased in intensity when MgO was wet-kneaded, and to an isolated OH group of MgO that depends on the coordination number of Mg . An intense peak at 3680 cm –1 was previously assigned to a magnesium silicate phase, lizardite .…”