Kannan Srinivasan

One-Step Hydroxylation of Benzene to Phenol Over Layered Double Hydroxides and their Derived Forms

Phenol, an important bulk organic compound, has diverse applications encompassing both industry and society. Commercially, it is produced through energy intensive three-step cumene process operating at relatively low yield with the co-production of acetone. Several attempts were made for producing phenol through challenging one-step direct hydroxylation of benzene using different oxidants like O2, N2O and H2O2. Liquid phase hydroxylation of benzene using H2O2 found to be more attractive due to its low reaction temperature and environmentally friendly nature (as water is only formed as by-product). The hydroxylation reaction occurs through Fenton’s mechanism; however along with phenol several other products are also formed due to higher reactivity of phenol compared to benzene. Our research group has been working on this reaction for nearly a decade using layered double hydroxides (LDHs) and their derived forms as heterogeneous selective oxidation catalyst. Screening of different LDHs having different metal ions in the layers revealed the necessity of copper for hydroxylation in pyridine. Addition of co-bivalent metal ion along with copper was made in an endeavour to improve the activity that revealed the promising results for CuZnAl LDHs. Efforts were then made to shift from pyridine to environmentally benign solvent, water, for this reaction that showed reasonably good yields with very high selectivity of phenol. Addition of small amount of sulfolane as a co-solvent increased the selectivity for phenol further. The reusability difficulty faced while using as-synthesized LDHs was overcome when calcined LDHs were used. Structure–property-activity relationships were deduced to understand the results observed. The present review besides covering our work also provides the state-of-art on this reaction using different oxidants with emphasis on H2O2.

CoCuAl layered double hydroxides – efficient solid catalysts for the preparation of industrially important fatty epoxides

CoCuAl ternary layered double hydroxides (LDHs) with a (Co + Cu)/Al atomic ratio of 3.0 and varying Co/Cu atomic ratios were synthesized by co-precipitation. Catalytic epoxidation of ethyl linoleate was carried out over these materials using aqueous tert-butyl hydroperoxide (70 wt.%) as the oxidant. Co30Cu70Al-LDH (Co : Cu atomic ratio of 30 : 70) showed maximum conversion of 85% with 100% selectivity for epoxide at 110 °C in 4 h. Higher activity of this catalyst was attributed to synergism between cobalt and copper as inferred through physicochemical techniques. The catalyst was reusable six times without significant loss. The study was extended to vegetable oils (edible, non-edible and used cooking oils) and showed 50–70% conversion with 100% selectivity. Comparison of the activity between oils and fatty acid methyl esters (FAME) showed lesser activity for the former probably due to hindered access. The process was successfully scaled up (50 g) for sunflower oil and the physical properties were markedly varied for epoxidized oil, suggesting promise for industrial applications.

Co3O4 microcubes with exceptionally high conductivity using a CoAl layered double hydroxide precursor via soft chemically synthesized cobalt carbonate

Cubic microparticles of Co3O4 spinel were synthesized by calcination of CoCO3 obtained using CoAl layered double hydroxide (LDH) as a unitary precursor through soft-chemical decomposition. The obtained cobalt spinel showed an exceptionally high electrical conductivity at room temperature. This is attributed to high concentrations of charge carriers (Co4+), unique morphology, high reduction temperature and low activation barrier.

Preparation of functionalized castor oil derivatives with tunable physical properties using heterogeneous acid and base catalysts

Functionalized castor oil derivatives namely ring-opened glyceryl ricinoleates, epoxy alkyl ricinoleates, and ring-opened alkyl ricinoleates were successfully prepared through two reaction chemistry viz., ring opening and transesterification using epoxidized castor oil (ECO) as a raw material. Amberlyst 15, the most active catalyst among several acid catalysts screened, showed a maximum conversion of 82% for ring opening of ECO with methanol. In another chemistry, 91% yield of epoxy methyl ricinoleate was achieved through transesterification of ECO with methanol using CaAl-layered double hydroxide (LDH) derived oxides as base catalyst. The scope is extendable to many nucleophiles and alcohols for both reactions respectively. Ring-opened alkyl ricinoleates were prepared both in two-pot and one-pot reactions using both acid and base catalysts together. The catalysts were recyclable and were successfully scaled at 25 g. The physical properties of these castor-based derivatives bestow the opportunity to design tailor-made materials suiting industrial needs.

Novel biomolecule-assisted interlayer anion-controlled layered double hydroxide as an efficient sorbent for arsenate removal

The synthesis of pure nitrate-containing layered double hydroxides (LDHs) via biomolecule-assisted methods is difficult to achieve without producing substantial waste. For the first time, we demonstrated the synthesis of LDHs with a controlled interlayer anion composition using an environmentally friendly L-arginine-assisted hydrothermal method with zero waste disposal. The mechanism of LDH formation was revealed through PXRD, FT-IR, XPS and ion chromatographic (IC) analyses. At low synthesis temperatures (90–110 °C), arginine-mediated water decomposition led to OH− and [Arg+]-NO3− formation and thus produced pure NO3−-containing LDHs. Conversely, at temperatures above 115 °C, L-arginine decomposition occurred and produced NH4+ and CO2, which resulted in CO32−-bearing LDHs. The FT-IR spectra of the solid residues, which were obtained at lower temperatures, indicated that several amino acids were functionalized on the surface of the LDHs and replaced by CO32−, which was produced at higher temperatures. The sorption of arsenate from an aqueous solution on the resulting LDHs showed maximum sorption capacity values of 1.675 and 1.972 mmol g−1 for Mg2.3Al-LDH and Mg2Al-LDH synthesised at 100 °C, respectively. The arsenate sorption capacity was enhanced by the functionalization of L-arginine compared with conventionally prepared LDHs. The mechanism of arsenate sorption was based on the ion-exchange of interlayer NO3− and functionalized arginine molecules. In summary, the chemical precursor L-arginine (utilized in this study) acts as a multifunctional reagent, including (i) a precipitant for the synthesis of LDH, (ii) an engineer for interlayer anion control, (iii) a functional reagent and (iv) a scavenger for free NO3− that is present in the synthesis medium. The current synthesis method did not utilize a hazardous base during synthesis, and the [Arg+]-NO3− byproduct can be used as a chemical source for health/skin care formulations with zero waste disposal, which offers great benefits.

Double bond isomerization of ethyl linoleate and vegetable oils to conjugated derivatives over an LDH supported ruthenium catalyst

MgAl-layered double hydroxides (LDHs) with different Mg/Al atomic ratios were prepared by co-precipitation and studied for the catalytic double bond isomerization of ethyl linoleate. Among them, MgAl4-LDH (Mg/Al atomic ratio of 4.0) showed maximum conversion (15%) and yield of isomerized products (15%) at 90 °C in the presence of 1-butyl-3-methyl imidazolium chloride using toluene as solvent in 12 h. The catalytic performance improved significantly upon supporting 5% Ru on MgAl4-LDH which gave 42% conversion with 29% yield of isomerized products under similar conditions. However, while reusing, the catalyst showed a reduced yield (18%). CO2-TPD studies revealed an increase in the amount of weak basic sites along with a decrease in the amount of high strength medium basic sites is probably the reason for this drop. Under the optimized conditions, vegetable oils were also converted to their corresponding conjugated oils though with lesser conversion and yield.