The Catalyst Review April 2020 - 14

EXPERIMENTAL
B-MWW Zeolite: The Case Against Single-Site Catalysis...

Zeolites are based on TO4 tetrahedra, where T is an aluminum or silicon atom (phosphorus in aluminophosphates). They are among
the most widely used industrial catalysts and, recently, boron-containing zeolites have been identified as highly selective catalysts
for the oxidative dehydrogenation (ODH) of alkanes to olefins. Although it has been demonstrated that the surface of these boroncontaining ODH catalysts oxidize and hydrolyze under reaction conditions, forming an amorphous B2(OH)xO(3-x/2) (x = 0-6) layer, the
precise nature of the active site(s) remains elusive.
Figure 1. 1D 11B MAS SSNMR spectra of (lower)
fresh and (upper) spent B-MWW. The solid
lines correspond to the experimental spectra
and the dashed lines correspond to analytical
simulations.

Figure 2. a) 2D 11B→1H D-RINEPT spectra of B-MWW fresh acquired with 0.64 ms (blue)
and 2.56 ms (red) of total SR421 heteronuclear dipolar recoupling. b) (black) Comparison
of 1D 11B spin echo NMR spectrum and projections from the 2D 11B→1H D-RINEPT spectra
acquired with 0.64 ms (blue) and 2.56 ms (red) of total heteronuclear dipolar recoupling. c,
d) 11B→1H D-RINEPT dipolar recoupling buildup curves for the 1H slice at c) 3.6 ppm and d)
2.7 ppm (dashed black lines in Figure 2a). The black diamonds represent experimental data
points and the solid lines correspond to numerical simulations (SIMPSON) for heteronuclear
dipolar couplings corresponding to the indicated 1H-11B internuclear distances. All spectra
were recorded at 9.4 T (v0(1H)=400 MHz) with 25 kHz MAS.

Herein, the authors report the results of the catalytic testing for the ODH of propane using zeolite MCM-22 isomorphously
substituted with boron (B-MWW). Surprisingly, the B-MWW material showed no conversion above the background observed with
a blank reactor. To investigate the source of ODH inactivity in B-MWW, the authors performed 1H and 11B MAS SSNMR spectroscopy
experiments to directly probe the structure and distribution of boron atoms in the zeolite framework. The 11B chemical shifts suggest
that in B-MWW, nearly all boron atoms are incorporated into the framework. Also, it appears that these sites are well separated
(>3-4 A apart) from one another (Figure 1).
To better understand the boron speciation and confirm the assignments of the distinct 11B NMR signals for B-MWW, the authors
then performed 1H -11B correlation NMR experiments (Figure 2). Their findings suggest that catalytically active B/B-MWW, B/SiO2,
and boron nitride ODH catalysts contain aggregated boron sites that restructure under reaction conditions. In contrast, B-MWW is
stable under reaction conditions and shows no restructuring of the boron atoms. The difference in boron sites in B-MWW and these
other boron materials indicates that boron-catalyzed ODH of propane does not proceed over an isolated BO3 site. Instead, active
boron-based ODH catalysts likely require boron species with some degree of B-O-B connectivity. Source: Altvater NR, Dorn RW,
Cendejas MC, et al. (2020). Angew. Chem. Int. Ed. DOI: 10.1002/anie.201914696.
Decarbonylative Olefination of Aldehydes to Alkenes...

Although the Wittig carbonyl olefination of ketones or aldehydes is widely employed due to its utility and reliability, the efficiency of
the reaction is hindered by the need to pre-form phosphorus ylids and by the production of stoichiometric waste. Herein, the authors
describe an atom-economical olefination of carbonyls via aldol-decarbonylative coupling of aldehydes using robust and recyclable
supported Pd catalysts, producing only CO and H2O as waste. To this end, they synthesized and screened Pd-doped hydrotalcites
(Pd-HT), Pd-Al2O3, Pd-SiO2, Pd-C, and Pd-MgO as catalysts for the reaction shown in Scheme 1. Initial findings showed that Pd-HTs
facilitate the conversion of enolizable aldehydes under neat conditions to the α, β-unsaturated carbonyls, which decarbonylate
to afford internal alkenes. The reactions require low Pd loading (0.1 mol% Pd) and proceed at 1500C or above using microwave or
conventional heating. Heptanal thus afforded (E)-tridec-6-ene, n olefin with (2n − 1) carbons, and small amounts of the intermediate
3 in Scheme 1.
They then explored the substrate scope of the decarbonylative olefination. Enolizable aldehydes with no α-branching afford excellent
yields (Table 1, entries 1−3), while those with α-branching react more slowly (entries 4 and 5) due to slower aldol condensation.
14

The Catalyst Review

April 2020

The Catalyst Review April 2020

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