Nb₂O₅·nH₂O as a Heterogeneous Catalyst with Water-Tolerant Lewis Acid Sites

Abstract

Niobic acid, Nb(2)O(5)·nH(2)O, has been studied as a heterogeneous Lewis acid catalyst. NbO(4) tetrahedra, Lewis acid sites, on Nb(2)O(5)·nH(2)O surface immediately form NbO(4)-H(2)O adducts in the presence of water. However, a part of the adducts can still function as effective Lewis acid sites, catalyzing the allylation of benzaldehyde with tetraallyl tin and the conversion of glucose into 5-(hydroxymethyl)furfural in water.

Dehydration of Glucose to 5‐Hydroxymethylfurfural Using Nb‐doped Tungstite

“…The results are shown in Table 4. The low HMF yield of 16 % at full glucose conversion for niobic acid is consistent with the work of Nakajima et al 13. The Nb x –WO 3 samples afforded higher HMF yields under similar reaction conditions.…”

“…This opens the possibility for a two‐step glucose conversion strategy consisting of isomerization on LAS and partial dehydration under weakly acidic conditions on BAS followed by dehydration of the intermediates by a mineral acid. This approach results in HMF yields of close to 60 %, which is comparable to other transition metal oxide systems,8, 13, 14, 15, 16, 17, 18 yet lower than results obtained with Sn‐Beta/HCl in THF/water 5. The advantage of the use of tungstite catalysts is that they can operate at lower temperature and the catalyst is much easier to prepare than Sn‐Beta.…”

“…For Nb 0.2 –WO 3 , the HMF yield decreased from 38 % to 30 %. The HMF yield for P/WO 3 was much lower, despite the complete conversion of glucose after 3 h. It has been reported that deactivation of the BAS of Nb 2 O 5 , Ta 2 O 5 , and ZrO 2 by phosphate groups increases the HMF yield during glucose conversion 13, 16, 17, 18. In our case, the decrease of the Brønsted acidity upon phosphation of Nb 0.2 –WO 3 and WO 3 results in decreased HMF yield, presumably because, different from the other materials, the rate‐controlling step is dehydration of a reaction intermediate.…”

“…A mechanism based on isomerization and dehydration was also proposed by Ståhlberg and co‐workers for the boric acid catalyzed glucose dehydration in ionic liquids 12. Furthermore, early transition metal oxides such as TiO 2 and Nb 2 O 5 are abundant and cheap materials with water‐tolerant and tunable acid–base properties with promising performance in sugar conversion reactions 8, 13, 14, 15. Among these, niobic acid (hydrated niobium pentoxide, Nb 2 O 5 ⋅H 2 O) has received considerable attention 13, 14, 15.…”

Catalytic Isomerization of Biomass‐Derived Aldoses: A Review

“…[7] Interestingly,b oths olid and soluble Lewis acids have been proposed for such one-pot reactions. For example, CrCl 3 , [81] AlCl 3 , [87] and lanthanide salts [88] have been intensively studied for the one-pot conversion of glucosei nto HMF.An umber of reports considerc ombinations of Brønsteda cids with solid Lewis acids including Sn-b, [7] Nb 2 O 5 ·n H 2 O, [89] Cr-based heteropoly acid ionic crystal, [90] Ti phosphates, [91] and dealuminated b zeolite. [92] Nevertheless, there are some concernsr egardingt he long-term stability of zeolitesi nh ot water in the presence of Brønsted acids and salts.…”