Bismuth oxide is a visible-light activated photocatalyst that is adversely affected by a high rate of electron–hole recombination. To mitigate this, BiOBr/Bi2O3 composites were synthesized where BiOBr formed submicron thick platelets at the surface of the Bi2O3 particles. XRD measurements show the preferential formation of a (110)-facetted BiOBr overlayer which can be attributed to the commensurate structure of this plane with the (120) plane of Bi2O3. The photodegradation of p-cresol and RhB was studied as representative of an organic pollutant and a dye, respectively. The composite with 85% BiOBr/Bi2O3 exhibited the highest photoactivity for both molecules. Its higher activity compared to that of either Bi2O3 or BiOBr alone, or a mechanical mixture with the same composition, supports the hypothesis that the formation of an hetero-epitactic interface between BiOBr and Bi2O3 is instrumental in reducing electron–hole pair recombination. Interestingly, in mixtures of p-cresol and RhB, the rate of p-cresol photodegradation was enhanced but that for RhB was decreased compared to the pure solutions. This is not caused by competitive adsorption of the molecules but rather by excitation transfer from RhB to the co-adsorbed p-cresol. Therefore, the RhB degradation by deethylation, which is a surface reaction, is suppressed and only the reaction channel through attack of the OH· radical at the aromatic chromophore remains open.

•BiOBr is the most active BiOX photocatalyst for selective oxidation of primary amines.•BiOBr has a good balance of light absorption and oxidation ability.•Facets and surface area of BiOBr are important in photoactivity.•100% yield of N-benzylidenebenzylamine under atmospheric air and visible light.•BiOBr microspheres exhibited excellent stability and reusability.Bismuth oxyhalides BiOX (X = Cl, Br, I) are seldom applied as photocatalysts in organic synthesis. Herein, we investigated their potential for the aerobic oxidative coupling of benzylamine to N-benzylidenebenzylamine, as imines are important synthetic intermediates of pharmaceuticals and biologically active nitrogen-containing organic compounds. The influence of the halide and the exposed crystal facets on the photoactivity was investigated. BiOBr showed excellent photoactivity, surpassing BiOCl and BiOI, which have poor light absorption and oxidation ability, respectively. Three differently facetted BiOBr photocatalysts were synthesized. The {001}-facetted BiOBr exhibited higher intrinsic activity than the {010}- or {110}-samples due to efficient charge separation. However, surface area plays an important role as reactions usually occur at the catalyst surface. Because of their much larger surface area, the solvothermally-synthesized BiOBr microspheres with {110}-dominant exposed facet showed the highest photooxidative activity, with 100% conversion and 100% selectivity to N-benzylidenebenzylamine after 14 h visible light irradiation at room temperature using oxygen from atmospheric air. This work provides an economical, feasible, sustainable and green process for the synthesis of imines and illustrates the great potential of bismuth oxyhalides as photocatalysts for organic synthesis.Download high-res image (137KB)Download full-size image

A straightforward synthesis of 5-phenylthiazolamines by coupling thiourea with phenylacetones, phenylacetophenones, and β-tetralone has been developed. Thiourea acts as a substrate and an α-bromination shuttle by transferring a Br atom from CBrCl3 to the α-carbon of the carbonyl moiety. A series of steps are then triggered to reach the final product. Isolated yields from 80 to 95 % were obtained. Key features of this protocol include its minimal use of reagents (i.e., substrates, CBrCl3, and CsHCO3), its short reaction times under mild conditions (at 80 °C for 2–3 h), and its ease of scale up to prepare gram quantities of product.

3-Phenylimidazo[1,2-a]pyridines were synthesized through the perfluorohexyl iodide mediated coupling of 2-aminopyridines and phenylacetylenes. In situ iodination of the terminal alkyne by perfluorohexyl iodide reverses the polarity by generating a transient electrophilic iodoalkyne, and this alters the regioselectivity of the phenyl group. The reaction then proceeds by tandem electrophilic alkynylation and cyclization to form the fused-ring product. The protocol affords the 3-phenyl isomer with full regioselectivity and is complementary to reported methodologies for the synthesis of the 2-phenyl isomer starting from the same substrates.

•Simple, highly efficient and rechargeable bulk liquid membrane system.•Polar aprotic solvents support ionization of quaternary ammonium carrier.•A flow system for continuous remediation of Cr(VI).•Dip-in photometer for in-situ monitoring.Hexavalent chromium, Cr(VI), is highly toxic and harmful to living organisms due to its carcinogenic and mutagenic properties. Liquid membranes combine the processes of extraction and stripping in a single unit operation, and therefore have much potential in the context of process intensification. The liquid membrane separates two aqueous phases and normally contains carrier molecules to assist in the selective trans-membrane transport of the solute. Of the various quaternary ammonium salts investigated as Cr(VI) carriers, methyl trioctyl ammonium bromide showed the best performance. A mixture of o-xylene and sunflower oil was chosen as the liquid membrane. o-Xylene has a high boiling point, low solubility in water and relatively high polarity which supports the dissociation of methyl trioctyl ammonium bromide. The addition of sunflower oil improves the retention of the carrier in the organic phase. After passing through the liquid membrane, Cr(VI) was reduced to Cr(III) by acidified iron(II) sulfate. The process was demonstrated with a continuously operating flow system with online monitoring via a dip-in photometer. With a Cr(VI) feed concentration of 1.6 mmol/L, the removal efficiency was sustained at >98% over 4 h. The high removal efficiency can be maintained by replacing the liquid membrane at periodic intervals.

Grafting of 3-(2-aminoethylamino)propyltrimethoxysilane onto Ru/AlO(OH) resulted in an active and highly chemoselective heterogeneous catalyst for the transfer hydrogenation of α,β-unsaturated carbonyl compounds to the corresponding allylic alcohols. Potassium formate was used as a sustainable hydrogen donor. A range of substrates including cinnamaldehyde, α-amylcinnamaldehyde, citral, 3-methyl-2-butenal, trans-2-pentenal, and trans-hexenal were selectively hydrogenated at the CO moiety with >96% selectivity. In comparison, the unmodified 1 wt% Ru/AlO(OH) catalyzed hydrogenation of cinnamaldehyde at the CC bond, yielding 3-phenylpropanal as the product. Higher loaded samples with 2–10 wt% Ru exhibited 20–25% selectivity to cinnamyl alcohol. The results show that low coordination sites were more selective to hydrogenation of the internal CC than the terminal CO bond. Immobilization of the amine via chemical bonding with hydroxyl groups of the AlO(OH) support blocks adjacent exposed metal sites, increasing the chemoselective reduction of CO. Optimum results were achieved at an amine/Ru ratio of 6. The catalyst maintained high activity and chemoselectivity even after five cycles.

A versatile protocol for the synthesis of disubstituted 3-phenylimidazo[1,2-a]pyridines by coupling 2-aminopyridine with phenylacetophenones, phenylacetones, or β-tetralone has been developed. Isolated yields of up to 97% were obtained at 80 °C within 5 h. The 2-aminopyridine/CBrCl3 system acts as an α-bromination shuttle by transferring Br from CBrCl3 to the α-carbon of the carbonyl moiety. This triggers a series of steps with double C–N/C–N bond formation to the final product. The distinct advantages of this protocol include the use of commercially available inexpensive substrates, simplicity of a metal-free one-pot synthesis, and ease of scale-up to multigram quantities.

Supported silver catalysts were reported for the first time to be able to catalyze the coupling reaction between nitroarenes and alcohols via the borrowing hydrogen scheme. The recyclable, non-leaching catalyst is synthesized by the entrapment method, which allows entrapping of silver nanoparticles in an alumina matrix. Alcohols, acting as the reducing agents for nitro-groups, alkylated the resultant amines smoothly over these silver catalysts giving a yield of >98% towards the N-substituted amines. In this process, multiple steps were realized in one-pot over a single catalyst with very high efficiency. It offers another clean and economic way to achieve amination of alcohols.

Zr–Beta zeolite is a robust and active catalyst for the Meerwein–Ponndorf–Verley reduction of levulinic acid to γ-valerolactone, a versatile intermediate for bio-fuels and chemicals. In a batch reactor, γ-valerolactone was formed with a selectivity of >96%. In a continuous flow reactor, >99% yield of γ-valerolactone was obtained with a steady space-time-yield of 0.46 molGVLgZr−1 h−1 within 87 h, on a par with that of noble metal based catalysts. The high activity of this catalyst was attributed to the presence of Lewis acidic sites with moderate strength. Due to the relatively few basic sites, it is not poisoned by the acidic reactant. Its robustness in liquid and gas phase reactants coupled with good thermal stability makes Zr–Beta a green regenerable catalyst that can be used directly on levulinic acid without the need for derivatization.

Mesoporous Zr-SBA-15 platelets were prepared with different pore dimensions from 4 to 8 nm by the simple procedure of varying the amount of water in the synthesis gel. Narrow pore size distributions were obtained for water–tetraethoxysilane ratios between 208 and 639, but samples formed using a lower ratio of 100 had a broad pore size distribution. Thermogravimetric measurements showed that the interaction between the Pluronic template and the inorganic framework was affected by the amount of water in the synthesis gel. More zirconium was incorporated into the silica framework when the synthesis was conducted in a more dilute system. The Zr-SBA-15 obtained from this synthesis forms platelets with relatively short channels. The catalytic activity was tested for C–C-coupling (Prins reaction). The terpene alcohol Nopol, the product of an intermolecular Prins reaction between β-pinene and paraformaldehyde, could be obtained with excellent selectivity. The mesoporous structure of the catalyst together with the presence of zirconium in the silica framework, which confers strong Lewis acidity as well as weak Brønsted acidic sites, are essential for the activity and selectivity of the reaction.

Potassium triphosphate is a surprisingly active catalyst for the catalytic transfer hydrogenation of carbonyl compounds. A pretreatment at 600 °C is critical to obtain the active material. Temperature programmed desorption of CO2 indicates that sites of lower basicity were created after heat treatment for 5 h as compared to 1 h. With 2-propanol as hydride source, aromatic aldehydes were reduced to the corresponding alcohols with high rates. The presence of electron withdrawing chloro- and nitro-substituents at the benzene ring increases the rate of reaction compared with unsubstituted benzaldehyde, while electron donating methyl or 2-propyl groups has the opposite effect. Ketones were reduced with a lower rate of reaction than aldehydes. The role of K3PO4 as a heterogeneous catalyst is indicated by a complete lack of reaction in the absence of the solid base and by studies with competing substrates. The latter studies suggest that the catalytic transfer hydrogenation of benzaldehyde and chlorinated benzaldehydes takes place at different surface sites.Keywords: aldehydes; catalytic transfer hydrogenation; heterogeneous base; K3PO4; ketones; site differentiation; solid base;

A critical step in the synthesis of the rare sugars, L-lyxose and L-ribose, from the corresponding D-sugars is the oxidation to the lactone. Instead of conventional oxidizing agents like bromine or pyridinium dichromate, it was found that a heterogeneous catalyst, Pd–Bi/C, could be used for the direct oxidation with molecular oxygen. The composition of the catalyst was optimized and the best results were obtained with 5:1 atomic ratio of Pd:Bi. The overall yields of the five-step procedure to L-ribose and L-lyxose were 47% and 50%, respectively. The synthetic procedure is advantageous from the viewpoint of overall yield, reduced number of steps, and mild reaction conditions. Furthermore, the heterogeneous oxidation catalyst can be easily separated from the reaction mixture and reused with no loss of activity.

The methods for the formation of zirconia including precipitation from aqueous salts, sol–gel synthesis from zirconium alkoxides, and the templated synthesis using surfactants are described in this review. The surface areas obtained vary widely but invariably decrease upon prolonged calcination. Digestion of hydrous zirconia and incorporation of dopants such as lanthanum, yttrium, or sulfate ions can increase the surface area and thermal stability. However, these methods also affect the crystal phase of zirconia. The transformation from the metastable tetragonal to the monoclinic phase occurs during the cooling phase of calcination. Mechanisms for the stabilization of the tetragonal phase are discussed. Zirconia with well-ordered mesopores or in the form of hollow spheres can be prepared but lack thermal stability, unless doped with phosphates, silicates or sulfates.

The one-pot conversion of (±)-citronellal to menthol can be selectively catalysed by either a bifunctional Ni/Zr-zeolite beta catalyst or a dual catalyst system of Zr-beta and Ni/MCM-41, giving a high diastereoselectivity to (±)-menthol of 90–94%.

The stability of gamma alumina with respect to phase transitions and loss of surface area can be greatly improved by digestion of the aluminium hydroxide precursor. Pseudoboehmite (aluminium hydroxide) was formed by precipitating aluminium nitrate with ammonium hydroxide at pH 6. The freshly precipitated gel has a very low surface area, but the surface area increases after digestion of the hydroxide in the mother liquour. Digestion leads to a ‘drier’ hydroxide as shown by thermogravimetry. After calcination to 500°C, the resulting alumina has a surface area of 230 to 310 m2 g−1, depending on the length of digestion of the hydroxide. Aluminas prepared from aged precursors have better thermal stability than those prepared without the digestion step. The digested alumina was able to withstand calcination to 1200°C for 12 h, and maintained a surface area of ∼68 m2 g−1. The transformation to the alpha phase is delayed in the digested samples. The effect of digestion on the surface area and improved thermal stability are explained on the basis of a reduction in the number of defect sites responsible for surface diffusion.

The microstructure of hydrous zirconia (ZrO2) prepared by the hydrolysis of zirconium propoxide followed by digestion at pH 1, 3 and 9 and the ZrO2 derived from this precursor have been studied. The undigested hydrous oxides formed plate-like aggregates with low porosity. After digestion at pH 9, the hydrous oxides became highly porous with surface areas up to 550 m2/g. Calcination at 500°C for 12 h results in ZrO2 with surface areas of more than 380 m2/g. ZrO2 formed after digestion in acidic medium had lower surface areas of less than 100 m2/g and were mixed in tetragonal and monoclinic phases. The water/propoxide ratio during gel formation influences the surface area of the resulting ZrO2. The thermal stability of the material is remarkable: ZrO2 resulting from long digestion retained up to 100 m2/g after successive heating to 900°C. The observed microstructural properties are attributed to the nature of the alkoxide precursor formed under different water/alkoxide ratios and to the effects of digestion under varying pH conditions.

KF/γ-alumina is an effective catalyst for the aldol condensation of citral with acetone. Even at a relatively low acetone/citral ratio of 1–10, the selectivity to pseudoionones was high, ranging between 82% and 97%. X-ray diffraction shows the presence of KF and K3AlF6 crystallographic phases on the supported catalyst. At KF loadings of 3–6.75 mmol g−1, a pretreatment of the catalyst by heating to 450 °C was necessary to obtain an active catalyst. However, samples with KF loadings of 8.5 mmol g−1 and higher were active even without pretreatment, facilitating handling and use of the catalysts. The development of (1 1 1) planes revealed by in situ XRD during the thermal activation process correlates with the activity for aldol condensation.

The addition of nickel to Zr-beta gave a useful bifunctional catalyst that combines a high rate of cyclization of (±)-citronellal to isopulegols over zirconium sites and subsequent hydrogenation to menthols. The diastereoselectivity to the desired (±)-menthol was 90%. A loading of 4 wt% is optimal; lower loadings led to a low rate of hydrogenation, whereas a higher nickel loading appears to block the zirconium Lewis acid sites essential for the cyclization of citronellal. A mixture of Zr-beta and Ni/MCM-41 also formed an effective bifunctional catalyst system where the selectivity toward menthols remained high even with nickel loadings up to 15 wt%. The yield of (±)-menthol over the dual-catalyst system was 86–89% with <5% of byproducts, citronellol and 3,7-dimethyloctanol. Besides nickel, bifunctional Rh/Zr-beta catalysts also formed menthols with selectivity >93%, although the diastereoselectivity to (±)-menthol was lower (85%). In comparison, a 2% Pd/Zr-beta catalyst exhibited lower activity and selectivity to menthols, forming substantial amounts of 3,7-dimethyloctanal.

The hydrogenation of 4-tert-butylphenol and p-cresol was investigated over Zr-beta-supported rhodium catalysts. By designing a suitable bifunctional catalyst, the intermediate, 4-alkylcyclohexanone, formed by metal-catalyzed hydrogenation of 4-alkylphenol, could be reduced via the highly stereoselective Meerwein–Ponndorf–Verley reduction over zirconium Lewis acid sites. Thus, in the presence of 2-propanol as solvent and MPV reductant, a high stereoselectivity to cis-4-alkylcyclohexanol was observed. Over 0.5% Rh/Zr-beta, 4-tert-butylphenol, and p-cresol were hydrogenated to the cis-alcohols with 95 and 89% stereoselectivity, respectively. A higher metal loading or the use of solvents such as hexane or tert-butanol led to a lower stereoselectivity, as metal-catalyzed hydrogenation predominated. Similarly, the cis:trans alcohol ratio was lower for rhodium supported on zirconia or Al-beta. Compared with rhodium, palladium was less active in the hydrogenation of the 4-alkylphenols, requiring a higher hydrogen pressure and temperature. A two-step cascade reaction mechanism is proposed for the conversion of 4-alkylphenols to cis-4-alkylcyclohexanols.

Prins condensation of β-pinene with paraformaldehyde was carried out over MCM-22, delaminated ITQ-2 and silica pillared MCM-36. The mesopore-containing MCM-36 and ITQ-2 catalysts exhibit higher conversion of β-pinene due to more exposed acid sites. Lewis acid sites are responsible for Prins condensation while Brønsted acid sites favor the isomerization of pinene. The Brønsted acid sites can be removed mostly by ion-exchanging the zeolites with sodium cations. Thus, NaMWW zeolites had a higher selectivity towards Nopol. Of these, NaITQ-2 showed the highest activity and selectivity, and is a stable and reusable catalyst for production of Nopol.Download full-size imageResearch Highlights► The accessibility of acid sites is essential in the condensation reaction. ► Lewis acid sites are responsible for condensation while Brønsted acid sites favor isomerization. ► Brønsted acid sites on zeolites can be removed by ion-exchanging with sodium cations. ► NaITQ-2 is a good catalyst to produce Nopol by Prins condensation.

Supported silver catalysts were reported for the first time to be able to catalyze the coupling reaction between nitroarenes and alcohols via the borrowing hydrogen scheme. The recyclable, non-leaching catalyst is synthesized by the entrapment method, which allows entrapping of silver nanoparticles in an alumina matrix. Alcohols, acting as the reducing agents for nitro-groups, alkylated the resultant amines smoothly over these silver catalysts giving a yield of >98% towards the N-substituted amines. In this process, multiple steps were realized in one-pot over a single catalyst with very high efficiency. It offers another clean and economic way to achieve amination of alcohols.

A critical step in the synthesis of the rare sugars, L-lyxose and L-ribose, from the corresponding D-sugars is the oxidation to the lactone. Instead of conventional oxidizing agents like bromine or pyridinium dichromate, it was found that a heterogeneous catalyst, Pd–Bi/C, could be used for the direct oxidation with molecular oxygen. The composition of the catalyst was optimized and the best results were obtained with 5:1 atomic ratio of Pd:Bi. The overall yields of the five-step procedure to L-ribose and L-lyxose were 47% and 50%, respectively. The synthetic procedure is advantageous from the viewpoint of overall yield, reduced number of steps, and mild reaction conditions. Furthermore, the heterogeneous oxidation catalyst can be easily separated from the reaction mixture and reused with no loss of activity.

Grafting of 3-(2-aminoethylamino)propyltrimethoxysilane onto Ru/AlO(OH) resulted in an active and highly chemoselective heterogeneous catalyst for the transfer hydrogenation of α,β-unsaturated carbonyl compounds to the corresponding allylic alcohols. Potassium formate was used as a sustainable hydrogen donor. A range of substrates including cinnamaldehyde, α-amylcinnamaldehyde, citral, 3-methyl-2-butenal, trans-2-pentenal, and trans-hexenal were selectively hydrogenated at the CO moiety with >96% selectivity. In comparison, the unmodified 1 wt% Ru/AlO(OH) catalyzed hydrogenation of cinnamaldehyde at the CC bond, yielding 3-phenylpropanal as the product. Higher loaded samples with 2–10 wt% Ru exhibited 20–25% selectivity to cinnamyl alcohol. The results show that low coordination sites were more selective to hydrogenation of the internal CC than the terminal CO bond. Immobilization of the amine via chemical bonding with hydroxyl groups of the AlO(OH) support blocks adjacent exposed metal sites, increasing the chemoselective reduction of CO. Optimum results were achieved at an amine/Ru ratio of 6. The catalyst maintained high activity and chemoselectivity even after five cycles.

Mesoporous Zr-SBA-15 platelets were prepared with different pore dimensions from 4 to 8 nm by the simple procedure of varying the amount of water in the synthesis gel. Narrow pore size distributions were obtained for water–tetraethoxysilane ratios between 208 and 639, but samples formed using a lower ratio of 100 had a broad pore size distribution. Thermogravimetric measurements showed that the interaction between the Pluronic template and the inorganic framework was affected by the amount of water in the synthesis gel. More zirconium was incorporated into the silica framework when the synthesis was conducted in a more dilute system. The Zr-SBA-15 obtained from this synthesis forms platelets with relatively short channels. The catalytic activity was tested for C–C-coupling (Prins reaction). The terpene alcohol Nopol, the product of an intermolecular Prins reaction between β-pinene and paraformaldehyde, could be obtained with excellent selectivity. The mesoporous structure of the catalyst together with the presence of zirconium in the silica framework, which confers strong Lewis acidity as well as weak Brønsted acidic sites, are essential for the activity and selectivity of the reaction.