Catalysis Science & Technology Blog

In the field of heterogeneous catalysis, the characteristics of the support can make a substantial difference in the operating conditions of a system and on its performance. Boron nitride (BN), in its hexagonal, graphite-like morphology, has proven to be a promising support for a number of catalysts and processes, including the synthesis of ammonia over barium/ruthenium catalysts thanks also to its higher chemical and thermal stability.

Chinese researchers led by Liqiang Xu focused on the synthesis of such supports, developing ultrathin nanosheets  (<20 layers) with high specific surface area that could potentially be complementary to their carbon counterparts.

The group developed a convenient solid phase synthesis starting from boron oxide, zinc powder and N₂H₄ 2HCl that yielded nanosheets with thickness varying from 2 to 6 nm. Notably, when zinc was absent, no BN was produced. Replacing zinc with iron or manganese resulted in lower yields or thicker nanosheets.

To test the properties of the new boron nitride support, the nanosheets were impregnated with platinum and employed in the oxidation of carbon monoxide with interesting results. Not only did the system reached almost full conversion with catalyst concentration as low as 2, 1 and 0.04%, but the respective temperature at which it was achieved (respectively 165, 210 and 310 °C) was lower than with conventional BN or alumina supports.

The material was also functionalised with gold nanoparticles, but to obtain interactions strong enough to anchor them to the surface, a supplementary functionalisation of the material was required. After hydrothermal treatment of the nanosheet with hydrogen peroxide, the Au nanoparticles dispersed more efficiently and allowed for higher loading.

In the Authors` view, these Au/BN nanosheet could find applications in optical materials, sensors and photocatalysis.

Find the full communication here.

Convenient synthesis and applications of gram scale boron nitride nanosheets

Liancheng Wang, Changhui Sun, Liqiang Xu and Yitai Qian

Catal. Sci. Technol., 2011, Advance Article

DOI: 10.1039/C1CY00191D

Titanium doxide is an extremely versatile photocatalyst that can be employed in a wide range of reactions used in environmental decontamination, such as, CO₂ and nitric oxide oxidation among many. Not surprisingly new ways to employ this flexible system are developed every year.

Recently, a large amount of research in this field focused on improving the characteristics of the support used for the catalyst, with the aim to enhance its performances or adapt to specific reaction conditions.

In this same context, the group led by Tsuyoshi Ochiai developed a procedure to easily fabricate catalytic filter material based on chemically etched titanium sheets impregnated with titanium dioxide for applications in air- and water-decontamination.

This new titanium mesh was fabricated by applying a layer of resist on both sides of a titanium sheet followed by chemical etching. The pattern of the pores could easily be controlled during the application of the resist layer, allowing to great control in the mesh structure. The finished structure was further modified to be finally coated with TiO2 anatase sol-gel.

The material was then tested against commercial titanium mesh coated following the same procedure in the photocatalytic (UV light promoted) degradation of acetaldehyde at room temperature and atmospheric pressure, revealing the superior performances of the new material, the measured rate constant of the group`s titanium mesh being 2.5 times higher than the commercial one.

The activity of the material was also assessed in water and determined with a mehylene blue decolorisation test. The purification performance both in air and water did not decrease over repeated tests, indicating a good adhesion of the TiO₂ nanoparticles onto the surface of the mesh. Tests performed without irradiation resulted in no reaction, proving the photocatalytic nature of the process.

The  increased activity of the new coated titanium mesh has been ascribed by the authors to the good morphology of their material, with its highly ordered structure of interconnected macropores and the crystallinity of the TiO₂ coating.

Continue reading this communication here or access the ESI here.

Fabrication of a TiO2 nanoparticles impregnated titanium mesh filter and its application for environmental purification
Tsuyoshi Ochiai, Toru Hoshi, Houda Slimen, Kazuya Nakata, Taketoshi Murakami, Hiro Tatejima, Yoshihiro Koide, Ammar Houas, Takuji Horie, Yuko Morito and Akira Fujishima
Catal. Sci. Technol., 2011, Advance Article
DOI: 10.1039/C1CY00185J, Communication

The oxidation of alcohols to carbonyl compounds holds an important place in the transformations of organic molecules, especially the selective oxidation to aldehydes or ketones without overoxidation. The importance of these transformations lies in the usefulness and reactivity of carbonyl compounds as synthons in organic synthesis. Many techniques and reagents are available for this reaction, but they tend to suffer from the need for stoichometric amounts of oxidants, as in the case of permanganate and chromate based oxidants.  Ideally, the most environmentally friendly and efficient process of transforming alchools in their respective carbonyl compounds would be by employing metal catalysts that can exploit molecular oxygen as the oxidant. Although several systems have been developed that achieve this goal, they generally require additives, ligands, sacrificial substrates or high pressures of oxygen.

With these limitations in mind, Ma and Lei have been focusing their efforts on the development of simpler systems to achieve selective oxidations of alcohols to aldehydes and ketons (a short selection of their recent publications can be found here and here) , and recently discovered a simple methodology that only requires AlBr₃ 6H₂O and athmospheric oxygen to efficiently convert primary benzylic alcohols in their corresponding aldehydes with high selectivity and yields.

Employing a catalyst loading of 30% in dioxane in a batch reactor at the mild temperature of 70 °C afforded conversions of primary benzylic alcohols up to 100% with total selectivity for aldehyde in several cases, with 89% being the minimum selectivity measured. Running the reaction in an argon athmosphere instead of air resulted in only trace amounts of benzaldehydes and the use of anhydrous conditions did not change the outcomes of the transformation, suggesting that moisture in solvents or gases does not affect the reaction; in addition, nitro and ether moyeties on the ring were well tolerated.

In order to gather insights into the mechanism of the reaction, a set of isotope-labelling experiments have been performed and suggested that the oxygen atom is derived from the oxidant and not from a rearrangement of the original molecule.

When the conditions were applied to secondary benzylic alcohols,  the conversion rate of several diphenylmethanols into the corrisponding benzophenones suroassed 94%  with selectivities up to 80%, with the main side-products being the brominated derivatives.

In summary, Ma and Lei developed a simple system that does not use chlorinated solvents, environmentally unfriendly metals or complex ligands and conditions but still provided good yields and conversions with the substrates tested. Further study on a  broader substrate scope, refinement of conditions and mechanisms are underway.

Read this interesting communication in Catalysis Science & Technology.

AlBr3·6H2O catalyzed oxidation of benzylic alcohols
Yun-Mei Zhong, Heng-Chang Ma, Jin-Xia Wang, Xiao-Jie Jia, Wen-Feng Li and Zi-Qiang Lei
Catal. Sci. Technol., 2011, Advance Article DOI: 10.1039/C1CY00165E, Communication

The beginning of the new century, in chemistry, has been often linked to the renaissance of organocatalysis as enthusiastically reported by articles appearing on Angewandte Chemie and Chemical Society Reviews in the past few years. Although presenting several advantages over traditional metal-based catalysts, such as high stereoselectivity, ease of synthesis and handling, and the general feeling of being more environmentally benign, they yet have to persuade industry into their large scale application.

Some of the reasons behind the fall-out of industry for organocatalysts lie in the lower efficiency they exhibit, the major difficulties of catalyst separation, recovery and recycling. Attempts to covalently bind organocatalysts on polymeric and other supports addressed the problems of separation but often resulted in a loss of activity triggered by modifications of structure of the catalyst.

In an interesting perspective appeared on Cat. Sci. Technol., Luo, Zhang and Chen presented an overview of alternative immobilisation techniques based on non-covalent bonds.

As stated by the authors, the loss of activity on covalently bound organocatalysts is likely to depend on the changes in the structure and chemical properties that these small organic molecules undergo when bound to the support. Non-covalent immobilisation, on the other hand, seems to circumvent this problem and, despite other shortcomings, might be a promising field of study.

Examples of acid-base immobilisation presented include the use of solid acid like polyoxometalates or polystyrene sulfonic acids as the support, used with chiral amines-based catalysts in the aldol reaction and Michael additions, where recovery of the catalyst could be achieved by precipitation with diethyl ether. Other immobilisation strategies presented are the incorporation of the organocatalyst into a phase transfer catalysts (PTC) and the immobilisation into clays. The latter is achieved on materials such as montmorillonite, which comprises negatively charged layers  alternated with layers of Na+ species.Using a cation exchange process, molecules such as proline and proline derived structures could be immobilized in the interlayers and the resulting material successfully used in aldol reactions. Another family of supports is represented by ionic liquids, where an organocatalysts is used as the anionic partner in the structure, creating chiral ionic liquid s that had been used to catalyse Mannich-type and aza-Diels-Alder reactions. Other supports mentioned are self-supported gel-type organocatalysts, biphasic immobilisation and techniques based on hydrophobic interactions, using cyclodextrines as the supports.

Several detailed examples and references for each category, inviting the interested reader to further reading.

In the authors` words  – the non-covalent immobilisation is anticipated to provide a viable solution to enhance the applications of organocatalysis with “practical” credentials.

Read the full article here

Perspective
Non-covalent immobilisation of asymmetric organocatalysts
Long Zhang, Sanzhong Luo and Jin-Pei Cheng
Catal. Sci. Technol., 2011, Advance Article
DOI: 10.1039/C1CY00029B

Pollutants enter the ecosystem through several pathways. Dangerous substances are released into the atmosphere in many ways, for instance by car exhausts, from industrial fumes and drainage effluents and through degradation of pesticides to name but a few.

A contaminant that appears to be ubiquitous and released through all these pathways is phenol. Phenols are highly toxic by inhalation, causing lung edema, through ingestion, damaging kindneys, brain and liver, and it is also an extreme irritant to the skin, causing severe burns. Some phenols are reported carcinogens.

Degrading and removing phenols from contaminated water is essential before allowing it to enter the ecosystem. Many techniques have been developed and applied in order to do this, such as the use of polymeric resins, microbial or enzymatic  degradation, adsorption and other catalytic chemical transformations.

A recently published work by Yao, Shi and Sui focused on the latter.

The group focused on the use of supported titania for the photocatalytic degradation of phenols under UV light in aqueous solutions. The catalyst was prepared by anchoring TiO2 onto fly ash deriving from municipal waste combustion: a less usual support compared to the more widespread silica, alumina and zeolites.

The fly ash was treated with acid, washed  and dried before the titanium oxide coating was applied as a sol-gel and the material calcinated. Several calcination temperatures were tested, finding that 500 °C provided the ideal condition to obtain high degree of crystallinity in the catalyst.

The catalytic loading was also explored in the range 10-30 g/L to determine the highest concentration achievable without preventing light from reaching the bulk of the solution; the catalyst also proved robust and reusable, retaining 90% of its photocatalytic activity even after 4 cycles. Several tests were peformed to ensure that the activity of the system was due to the photocatalyst, including test runs with untreated  fly ash, only titania and UV irradiation alone.

To better understand the characteristics of the system, a kinetic study of the degradation reaction was also performed, revealing a first order-like behaviour; also, a three-step mechanism for the fly ash-supported catalyst was proposed.

Using the optimal conditions found during the study, the group achieved  94 % of phenol degradation with a catalyst loading of 20 g/L over 4 hours of irradiation, which, with its reusability, could make the catalyst potentially employable for continuous use in water treatment.

Read more about this catalyst here.

Application of fly ash supported titanium dioxide for phenol photodegradation in aqueous solution

Zhongliang Shi, Shuhua Yao and Chengcheng Sui
Catal. Sci. Technol., 2011, Advance Article
DOI: 10.1039/C1CY00019E

It is a fact that the world’s reservoirs of conventional oil are diminishing at a fast pace, forcing mankind to look for alternatives – possibly cleaner, sustainable and more environmentally friendly than fossil fuels.

While the technologies to exploit renewable sources of energy, like wind, tidal or solar power, mature to the point of replacing old-fashioned sources of energy, the use of sustainable fuels may bridge the gap between us and a greener future.

Biodiesel, a mixture of methyl esters of fatty acids, is an alternative to normal diesel that burns producing consistently less greenhouse gases and sulphurated compounds. Biodiesel can be produced via trans-esterification of vegetable or animal oils with methanol generally under acidic conditions; however, the process is as yet not cost-competitive with conventional diesel fuel.

A way to reduce costs is to use cheaper oils, containing free, unesterified fatty acids in the production process introducing an extra esterification step in the process.

Lingaiah et al., in a recently published paper, presented their work on a solid-phase catalyst for the acidic conversion of fatty acids into their correspondent methyl esters that simultaneously catalyses trans-esterification, avoiding the need for a separate reaction.

To provide the necessary acidity, the group employed 12-tungstophosphoric acid (TPA), a heteropolyacid, supported on tin oxide to increase its thermal stability and to reduce catalyst leakage in the reaction media. A full study of the effect of parameters like TPA loading, reaction temperature, stirring speed and catalyst concentration resulted in and optimised TPA loading of 15 wt%  to provide ideal surface area and acidity.

The activity test were performed using palmitic acid as the model substrate at 65 °C with a total catalyst loading of 25%, in the presence of an excess of methanol (1:14) to improve conversion (since the esterification is a reversible reaction). After 4 hours, the palmitic acid conversion reached exceeded 70%, and tests showed the catalyst could be washed and reused 5 times without significant loss of activity.

To prove its industrial viability, simultaneous esterification/trans-esterfication in the presence of mixtures of triglycerides and free fatty acids was successfully performed.

Read the full paper here.

Efficient solid acid catalysts for esterification of free fatty acids with methanol for the production of biodiesel
K. Srilatha, Ch. Ramesh Kumar, B. L. A. Prabhavathi Devi, R. B. N. Prasad, P. S. Sai Prasad and N. Lingaiah
Catal. Sci. Technol., 2011, Advance Article
DOI: 10.1039/C1CY00085C, Paper