Recent Advances in Catalysts for the Conversion of Ethanol to Butadiene.

Butadiene is an important monomer for synthetic rubbers. Currently, the annual demand of ∼16 million tonnes is satisfied by butadiene produced as a byproduct of steam naphtha cracking where ethylene and propylene are the main products. The availability of large amounts of shale gas and condensates in the USA since about 2008 has led to a change in the cracker feed from naphtha to ethane and propane, affecting the amount of butadiene obtained. This has provided the impetus to look into direct processes for butadiene production. One option is the eco-friendly conversion of (bio) ethanol to butadiene (ETB). This process had been developed in the 1930s in the then Soviet Union. It was operated on a large scale in USA during World War II but has since been abandoned in favour of petroleum-based processes. The current trend, driven both by the availability of the raw material and ecological considerations, may make this process feasible again, particularly if the catalytic systems can be improved. This critical review discusses recent catalysts for the ETB process with special focus on the development since 2014, benchmarking them against earlier systems with a large database of operational experience.

Ion-exchange Properties of γ-Zirconium Phosphate.

γ-Zirconium phosphate (γ-ZrP) has potential as a very useful inorganic ion-exchanger due to its stability under thermal and acidic conditions. We recently reported its facile synthesis using a modified solventless method, which could make the exploitation of its ion-exchange properties for cleanup of radioactive materials and extraction of valuable ions feasible. The adsorption isotherms of Rb+ and Sr2+ over γ-ZrP are described well with the Langmuir model, with maximum adsorption capacity of 1.52 and 1.31 mmol/g, respectively. Both ions adsorb on γ-ZrP following pseudo-second order kinetics with a much faster uptake of Rb+ than of Sr2+ . γ-Zirconium phosphate shows very high affinity for Rb+ , allowing its enrichment from low-concentration solutions. Notably, Rb+ is selectively removed from complex solutions containing large amounts of Na+ , Mg2+ , Ca2+ , or transition metals such as Cu2+ , Ni2+ , Fe2+ , Fe3+ , Co2+ and Zn2+ . The used sorbent can be fully regenerated by nitric acid, enabling the recovery of Rb+ .

One-Pot Synthesis of Layered Disodium Zirconium Phosphate: Crystal Structure and Application in the Remediation of Heavy-Metal-Contaminated Wastewater.

Inorganic ion exchangers offer advantages whenever operation at high temperatures or in oxidizing environments is required. A novel two-dimensional disodium zirconium phosphate, Zr(NaPO4)2·H2O, was reported and investigated as an ion exchanger for heavy metals. The material was synthesized by a novel minimalistic solventless approach, and its solid-state structure was determined from powder X-ray diffraction data. Zr(NaPO4)2·H2O crystallizes in the space group P21/c with cell parameters a = 8.7584(1) Å, b = 5.3543(1) Å, c = 18.1684(3) Å, ß = 109.053 (1)°, and Z = 4. Its layered structure is similar to that of α-zirconium phosphate, Zr(HPO4)2·H2O. However, unlike α-zirconium phosphate which is limited in practical applications by its narrow interlayer spacing (d = 7.6 Å), the disodium zirconium phosphate has a larger spacing of 8.6 Å between planes. The material with inherent structural advantages displays excellent sorption for heavy metals such as Pb2+, Cu2+, Cd2+, and Tl+, maintaining its high selectivity with distribution coefficients, Kd, of 104-105 mL/g even in the presence of a large excess of Na+, K+, Mg2+, and Ca2+, which are commonly present in underground water. In particular, the maximum sorption capacity for the highly toxic Tl+ is a record high, 5.07 mmol/g (1036 mg/g). The fast reaction kinetics indicate that the exchangeable positions in Zr(NaPO4)2·H2O are readily accessible, in contrast to Zr(HPO4)2·H2O. The ease of preparation, benign nature, and advantageous ion-exchange properties make Zr(NaPO4)2·H2O a highly promising sorbent for the treatment of water polluted with heavy metals.

Method for Visible Light-Induced Photocatalytic Degradation of Methylparaben in Water Using Nanostructured Ag/AgBr@m-WO3.

An efficient method of photocatalytic degradation of methylparaben in water using Ag nanoparticles (NPs) loaded AgBr-mesoporous-WO3 composite photocatalyst (Ag/AgBr@m-WO3 ), under visible light is presented. In this process, quantification of methylparaben in water was carried out by high-performance liquid chromatography (HPLC) and the HPLC results showed a significant reduction of methylparaben in water due to the enhanced of photocatalytic degradation efficiency of Ag/AgBr@m-WO3 . For the material synthesis, highly ordered mesoporous-WO3 (m-WO3 ) was initially synthesized by sol-gel method and AgBr nanoparticles (NPs) were subsequently introduced in the pores of m-WO3 , and finally, the Ag nanoparticles were introduced by light irradiation. The enhanced photocatalytic degradation of methylparaben in water is attributed to the formation of surface plasmonic resonance (SPR) due to the introduction of Ag NPs on the surface of the catalyst. Also, the formation of heterojunction between AgBr and mesoporous-WO3 in Ag/AgBr@m-WO3 significantly inhibited the recombination of light-induced electron-hole pairs in the semiconductor composite. The morphological and optical characterizations of the synthesized photocatalysts (Ag/AgBr@m-WO3 ) were carried out using SEM, TEM, XDR, N2 adsorption-desorption, UV-VIS diffuse reflectance spectroscopy (DRS). Also, the photocatalytic studies using radical scavengers were carried out and the results indicated that O 2 · - is the main reactive species.

A Dual-Functional Catalyst for Cascade Meerwein-Pondorf-Verley Reduction and Dehydration of 4'-Methoxypropiophenone to Anethole.

Anethole is an ingredient in many flavours, fragrances and pharmaceutical formulations. To reduce the dependence of its supply on natural oils, a green route for anethole synthesis was designed on the basis of Meerwein-Pondorf-Verley (MPV) reduction and dehydration of 4'-methoxypropiophenone. The one-pot cascade reactions were heterogeneously catalysed by dual-functional Zr-MSU-3, a predominantly Lewis-acidic catalyst with a Si/Zr ratio of 10 and pores with sizes in the range of 3.2-4.2 nm. The use of 2-pentanol as solvent and hydrogen donor for the MPV reduction was advantageous, as its high boiling point enhances the rate of the reactions, especially the dehydration of the MPV product, 1-(4-methoxyphenyl)-propan-1-ol. This dispenses with the need for a strong acid catalyst that could result in by-products of acid-catalysed reactions. Anethole yields of 91 % with a trans/cis isomer ratio of about 92:8, similar to that of natural anethole, were obtained. In comparison, microporous Zr-beta (Si/Zr 12.5) gave lower activity owing to pore-size constraints. Hence, through design of the reactions and catalyst, 4'-methoxypropiophenone can be efficiently converted to anethole in a sustainable and green manner.

Mechanochemistry-Based Synthesis of Highly Crystalline γ-Zirconium Phosphate for Selective Ion Exchange.

Highly crystalline γ-zirconium phosphate has been synthesized by a novel minimalistic approach and investigated as a selective ion exchanger for cesium, ammonium and potassium. In contrast to current solution-based preparations, the mechanochemistry-based synthesis provides easy access to γ-zirconium phosphate with short synthesis times and low crystallization temperature. The addition of NaF as a mineralizer increases the crystallinity of γ-zirconium phosphate, which forms micrometer-sized uniformly shaped rectangular platelets. The crystalline material has extremely high selectivity to cesium even in the presence of 1000- or 500-fold excess Na+ or Ca2+, respectively. The removal efficiency was >98% in the pH range of 2-5.5. As an ion exchanger for purification of dialysate, crystalline γ-zirconium phosphate shows higher uptake of ammonium and potassium ions than the amorphous gel compound currently used in sorbent cartridges. This sustainable protocol opens up opportunities for many practical applications of γ-zirconium phosphate.

Efficient photodegradation of chlorophenols by BiOBr/NaBiO3 heterojunctioned composites under visible light.

Forming heterojunctioned composites is an effective way to develop visible-light-driven photocatalysts. A series of BiOBr/NaBiO3 composites were synthesized by surface transformation of NaBiO3 with hydrobromic acid. Commensurate planes of BiOBr and NaBiO3 enabled the formation of a closely bound interface. Composites with <20wt.% BiOBr exhibited excellent photocatalytic activity towards the degradation of chlorophenols under low intensity visible light (λ>400nm). The best photocatalyst was 9% BiOBr/NaBiO3 with a quantum yield of 0.365. No photocorrosion was observed after three cycles. Using radical scavengers and inert atmosphere, holes, superoxide and hydroxyl radicals were found to be involved in the photoactivity of the BiOBr/NaBiO3 composite. Hydroxylated and open-ring diacid molecules were identified as intermediates in the mineralization of 4-chlorophenol.

Minimalistic Liquid-Assisted Route to Highly Crystalline α-Zirconium Phosphate.

Zirconium phosphates have potential applications in areas of ion exchange, catalysis, photochemistry, and biotechnology. However, synthesis methodologies to form crystalline α-zirconium phosphate (Zr(HPO4 )2 ⋅H2 O) typically involve the use of excess phosphoric acid, addition of HF or oxalic acid and long reflux times or hydrothermal conditions. A minimalistic sustainable route to its synthesis has been developed by using only zirconium oxychloride and concentrated phosphoric acid to form highly crystalline α-zirconium phosphate within hours. The morphology can be changed from platelets to rod-shaped particles by fluoride addition. By varying the temperature and time, α-zirconium phosphate with particle sizes from nanometers to microns can be obtained. Key features of this minimal solvent synthesis are the excellent yields obtained with high atom economy under mild conditions and ease of scalability.

Reagent-controlled regiodivergent intermolecular cyclization of 2-aminobenzothiazoles with ß-ketoesters and ß-ketoamides.

Two regiodivergent approaches to intermolecular cyclization of 2-aminobenzothiazoles with ß-ketoesters and amides have been developed, controlled by the reagents employed. With the Brønsted base KOt-Bu and CBrCl3 as radical initiator, benzo[d]imidazo[2,1-b]thiazoles are synthesized via attack at the α-carbon and keto carbon of the ß-ketoester moiety. In contrast, switching to the Lewis acid catalyst, In(OTf)3, results in the regioselective nucleophilic attack at both carbonyl groups forming benzo[4,5]thiazolo[3,2-a]pyrimidin-4-ones instead.

Synthesis of Disubstituted 3-Phenylimidazo[1,2-a]pyridines via a 2-Aminopyridine/CBrCl3 α-Bromination Shuttle.

A versatile protocol for the synthesis of disubstituted 3-phenylimidazo[1,2-a]pyridines by coupling 2-aminopyridine with phenylacetophenones, phenylacetones, or ß-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.

Alumina-entrapped Ag catalyzed nitro compounds coupled with alcohols using borrowing hydrogen methodology.

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 heterogeneous Pd-Bi/C catalyst in the synthesis of L-lyxose and L-ribose from naturally occurring D-sugars.

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.

On the synergistic catalytic properties of bimetallic mesoporous materials containing aluminum and zirconium: the Prins cyclisation of citronellal.

Bimetallic three-dimensional amorphous mesoporous materials, Al-Zr-TUD-1 materials, were synthesised by using a surfactant-free, one-pot procedure employing triethanolamine (TEA) as a complexing reagent. The amount of aluminium and zirconium was varied in order to study the effect of these metals on the Brønsted and Lewis acidity, as well as on the resulting catalytic activity of the material. The materials were characterised by various techniques, including elemental analysis, X-ray diffraction, high-resolution TEM, N(2) physisorption, temperature-programmed desorption (TPD) of NH(3), and (27) Al MAS NMR, XPS and FT-IR spectroscopy using pyridine and CO as probe molecules. Al-Zr-TUD-1 materials are mesoporous with surface areas ranging from 700-900 m(2) g(-1), an average pore size of around 4 nm and a pore volume of around 0.70 cm(3) g(-1). The synthesised Al-Zr-TUD-1 materials were tested as catalyst materials in the Lewis acid catalysed Meerwein-Ponndorf-Verley reduction of 4-tert-butylcyclohexanone, the intermolecular Prins synthesis of nopol and in the intramolecular Prins cyclisation of citronellal. Although Al-Zr-TUD-1 catalysts possess a lower amount of acid sites than their monometallic counterparts, according to TPD of NH(3), these materials outperformed those of the monometallic Al-TUD-1 as well as Zr-TUD-1 in the Prins cyclisation of citronellal. This proves the existence of synergistic properties of Al-Zr-TUD-1. Due to the intramolecular nature of the Prins cyclisation of citronellal, the hydrophilic surface of the catalyst as well as the presence of both Brønsted and Lewis acid sites synergy could be obtained with bimetallic Al-Zr-TUD-1. Besides spectroscopic investigation of the active sites of the catalyst material a thorough testing of the catalyst in different types of reactions is crucial in identifying its specific active sites.

Zr-zeolite beta: a new heterogeneous catalyst system for the highly selective cascade transformation of citral to (+/-)-menthol.

The transformation of citral to menthols involves hydrogenation steps as well as cyclisation of the intermediate, citronellal. The ability of Zr-zeolite beta to catalyse the cyclisation with high diastereoselectivity to (+/-)-isopulegol is the critical step in this cascade transformation. Bifunctional catalysts containing nickel or rhodium supported on Zr-zeolite beta gave menthols in yields of 87-89% and an excellent diastereoselectivity of 94% for the desired (+/-)-menthol. Dual catalyst systems of Zr-zeolite beta and nano-dispersed Ni on an MCM-41 support were equally effective and have the added advantage that the rates of the acid- and hydrogenation-catalysed steps can be independently varied. By applying a pressure ramp of 0.2-2 MPa, the yield of menthols could be increased to 95%, with 94% diastereoselectivity for (+/-)-menthol. The low initial pressure minimises the rates of competing hydrogenation reactions to byproducts such as citronellol and 3,7-dimethyloctanol.

Synergy between Brønsted acid sites and Lewis acid sites.

Synergy between Brønsted acid sites and Lewis acid sites in mesoporous Al-Zr-TUD-1 was demonstrated to exist in Brønsted acid catalysed reactions, but not in Lewis acid catalysed reactions.

Dynamic kinetic resolution of secondary alcohols combining enzyme-catalyzed transesterification and zeolite-catalyzed racemization.

Hydrophobic zeolite beta containing low concentrations of Zr or Al was found to be a good catalyst for the racemization of 1-phenylethanol. The formation of styrene as a side product could be minimized by reducing the metal concentration in the zeolite beta. Combined with an immobilized lipase from Candida antarctica, the dynamic kinetic resolution of 1-phenylethanol to the (R)-phenylethylester can be achieved with high yield and selectivity. The reaction was best conducted in toluene as solvent at 60 degrees C, with higher temperatures leading to a loss in the enantioselectivity of the formed ester. By using high-molecular-weight acyl-transfer reagents, such as vinyl butyrate or vinyl octanoate, a high enantiomeric excess of the product esters of 92 and 98 %, respectively, could be achieved. This is attributed to a steric effect: the bulky ester is less able to enter the pore space of the zeolite catalyst where the active sites for racemization are localized. Close to 100 % conversion of the alcohol was achieved within 2 h. If the more common acyl donor, isopropenyl acetate, was used, the enantiomeric excess (ee) of the formed ester was only 67 %, and the reaction was considerably slower.

Domino-cyclisation and hydrogenation of citronellal to menthol over bifunctional Ni/Zr-Beta and Zr-beta/Ni-MCM-41 catalysts.

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%.