B. S. Jai Prakash

Surfactant-modified clay as adsorbent for chromate

Abstract The adsorption of oxyanion of chromate was not given much attention, perhaps due to the fact that clays are negatively charged and unmodified clay mineral surface shows no affinity for chromate. After modification by cationic surfactant, clay was found to adsorb considerable amounts of chromate. Hexadecyl trimethylammonium (HDTMA) bromide was used to modify the surface of clay minerals such as kaolinite, montmorillonite, and pillared montmorillonite. Montmorillonite adsorbed a quantity of HDTMA equivalent to the cation exchange capacity. Kaolinite and pillared clay adsorbed relatively small amounts of the surfactant. The amount of chromate adsorbed was maximum at and below pH 1 and proportional to the amount of HDTMA fixed on motmorillonite. The amount of chromate adsorbed between pH 2 and 6 was one half of that at pH 1, and above pH≈8 the adsorption was negligible. The pH dependence of chromate adsorption was attributed to the pH dependent-equilibria HCr2O7−⇋Cr2O72−⇋CrO42−. The adsorption data obtained was well described by Langmuir adsorption isotherm. Kinetics of chromate adsorption (diffusion constant) was calculated and a mechanism for the adsorption is proposed.

Microwave rehydrated Mg–Al-LDH as base catalyst for the acetalization of glycerol

Acetalization of glycerol with aldehydes to form cyclic acetals is an industrially important reaction and is generally carried out using acid catalysts. Base catalysts such as LDH can bring about microwave-assisted acetalization of glycerol and aldehydes to form 5-membered and 6-membered cyclic acetals. Among the different LDHs used, Mg–Al-LDH was found to exhibit maximum conversion of glycerol into 5-membered cyclic acetal. Modification of Mg–Al-LDH involving calcination at 450 °C and subsequent microwave-assisted rehydration showed improved glycerol conversion rate under similar reaction conditions. Rehydration of calcined Mg–Al-LDH by microwave irradiation was found to result in LDH regaining its layered structure with higher basicity possibly exposing more hydroxyl ions responsible for basicity. Multiple use of methanol washed spent catalyst showed good repeatability for conversion up to three cycles which subsequently showed a marginal decrease in the conversion. Further dehydration followed by rehydration of the spent LDH catalyst under microwave irradiation was found to rejuvenate the catalytic activity to its initial level.

Microwave-activated p-TSA dealuminated montmorillonite – a new material with improved catalytic activity

Abstract We report a montmorillonite material with enhanced surface area but with very little alteration in cation exchange capacity (CEC) upon dealumination with para toluene sulphonic acid (p-TSA). The new material shows higher catalytic activity in comparison with mineral-acid-treated clay. Montmorillonite clay was treated with p-TSA for 10 minutes under microwave irradiation. The resulting clay was characterized by CEC, X-ray diffraction (XRD), BET analysis, Fourier transform infrared spectroscopy (FT-IR), temperature programmed desorption (TPD) of ammonia and cyclic voltametry (CV) techniques. XRD patterns show an unchanged structure of pristine matrix after the acid action. BET analysis revealed an increase in the surface area and pore volume on p-TSA treatment, indicating formation of voids in the octahedral layer which suggests dealumination. Nitrogen adsorption-desorption curves showed the creation of new micro porous regions, possibly in the octahedral sheets. In contrast to mineral acid treatment, p-TSA treated clay samples showed similar CEC which shows the absence of dissolution of isomorphously substituted Mg and Fe ions present in the octahedral layer. CV studies confirm the formation of an Al-p-TSA complex, suggesting dissolution of aluminium octahedral sheets. The complex subsequently hydrolyses, replacing interlayer cations with Al3+ ions. Similar treatment with mineral acid resulted in clay with enhanced surface area but with reduced CEC, evidently due to the removal of isomorphously substituted Fe and Mg. Further, the p-TSA treated clays showed relatively higher esterification activity under solvent-free microwave irradiation. The p-TSA treated clay retained its activity even after three subsequent runs and thus can be exploited for practical applications.

Pillaring of smectites using an aluminium oligomer: A study of pillar density and thermal stability

Two smectite samples having different layer charges were pillared using hydroxy aluminium oligomers at a OH/Al ratio of 2.5 and at pH 4.3 to 4.6. Pillaring was carried out at different conditions such as ageing, temperature and base addition time of the pillaring solution, and also in the presence of nonionic surfactant polyoxyethylene sorbitanmonooleate (Tween-80). The primary objective of preparing at different conditions was to introduce varied quantities of aluminium oligomer between the layers and to study its effect on the properties of the pillared products. A simple method has been followed to estimate the amount of interlayer aluminium. A quantity called pillar density number (PDN) based on the ratio of interlayer Al adsorbed to CEC of the parent clay has been effectively used to evaluate the nature of the resulting pillared product. PDN, for a given clay, was found to correlate well with the sharpness of the d(001) peaks for the air dried samples. The calculated number of pillars, varied from 3.00 x 10(18) to 5.32 x 10(18) per meq charge. The present study shows that a higher value of PDN is indicative of better thermal stability. Pillar density number may be conveniently used as a measure of the thermal stability of pillared samples.

The active site accessibility aspect of montmorillonite for ketone yield in ester rearrangement

The removal of Al from the octahedral layer of montmorillonite by organic acid treatment results in increased microporosity generating a material with different surface features from the virgin clay. The micropores thus generated are found to be responsible for ester to ketone transformation. Various parameters of the newly generated pores such as the pore diameter, pore structure, pore volumes and acidity by pyridine-FTIR were evaluated with respect to ester to ketone formation. The best correlation was found between the volume accessibility factors (VAF) and the ketone yield. The VAF reflects the extent of volume space available around the acid site for the reactant to orient in a specific way for the transformation to occur within the pore to form a specific product. This acid site accessibility aspect was further verified by extending to micropores with large VAF generated by treatment with phenoldisulfonic acid (PDSA).

Dealuminated BEA zeolite for selective synthesis of five-membered cyclic acetal from glycerol under ambient conditions

BEA zeolite was treated with phenoldisulfonic acid (PDSA) of different concentrations from 0.1 M to 1.0 M to obtain dealuminated zeolites. The treated BEA samples were characterized by XRD, 27Al-MAS NMR, SEM-EDX, TPD NH3, pyridine-FTIR and BET measurements. Modified zeolites showed an increase in surface area and pore volume and a decrease in acid amount with an increase in PDSA concentration. An untreated BEA sample catalyzed the reaction between glycerol and acetone to form glycerol acteals to yield two products, five-membered dioxalane and six-membered dioxane. Under optimized reaction conditions, the untreated BEA sample showed 83% dioxalane and 17% dioxane while the modified samples showed an increase in dioxalane selectivity with an increase in the acid concentration used for the treatment. The 1 M PDSA treated sample exhibited 100% selectivity for dioxalane. This is attributed to an increase in pore volume and decrease in acid amount of the modified BEA samples with an increase in PDSA concentration. The combined effect of pore volume and acidity in determining dioxalane selectivity is discussed.

Chemically modified clays as recyclable adsorbents for iodine

The adsorption behaviour of iodine on chemically modified swelling type of clays has been studied. Chemical modification was brought about by interacting the clay with surfactants such as tween-80 and polyethylene glycol. Adsorption of iodine was found to increase by several orders of magnitude on chemical modification which remained constant between pH 1 and 10. The adsorption isotherms were non-linear and fitted the Freundlich equation for swelling clays. Scatchard analysis of the data indicated minimum two types of active sites with the tween-80 modified clay and one type with the polyethylene glycol modified one. The iodine sorbed on the surface was found to get desorbed almost completely on leaching with water. Modification of the clay surface with surfactant thus offers a method of designing a recyclable adsorbent for iodine.

Volume accessibility of acid sites in modified montmorillonite and triacetin selectivity in acetylation of glycerol

Organic acid treatment enhances the acidity and surface characteristics of montmorillonite clay by dealumination. Dealuminated and Al-clays were used for acetylation of glycerol with acetic acid. All clays used had comparable acidity but pore characteristics were different. Though glycerol conversions were similar but triacetin selectivity was different. Al-clay and acid treated clays showed poor and improved selectivity, respectively. This was attributed to increased pore volume around the acid sites, which facilitates the multiple acetylation of glycerol. The generated space around acid centers, termed as ‘volume accessibility’, helps glycerol to interact with acylium ions formed on the acid sites more effectively leading to formation of triacetin. Correlations were made between the changed characteristics of clays and triacetin yield. Among the different correlations, triacetin selectivity correlates well with the volume accessibility. The latter is quite useful in predicting the catalytic performance.

Re-usability of zeolites and modified clays for alkylation of cyclohexanol a contrast study

Organic sulfonic acid treatment of montmorillonite results in micro-pores on the surface of modified clays providing access to acid sites in the interlayer. Performance of modified clay catalysts for alkylation of para-cresol with cyclohexanol were compared with microporous zeolites. Phenoldisulfonic acid treated clay catalyst showed maximum activity comparable with that of beta-zeolite. Modified clay- and zeolite-catalysts on reuse for the alkylation exhibited different behaviour. Modified clays showed same activity as in the first cycle while zeolites showed significantly reduced catalytic activities on reuse. Differences in behaviour were further investigated by characterizing the reused samples with TGA, acidity and surface measurements. Used zeolite samples showed considerable variations in acidity, surface characteristics and TGA pattern, which was attributed to the formation of coke. Absence of coke in clays and deposition of coke in zeolites is attributed to difference in their pore architecture.

Zeolitic behaviour of paratoluenesulfonic acid-modified clay in Friedel-Crafts synthesis of raspberry ketone

Abstract During solventless alkylation of phenol with 4-hydroxy-2-butanone under microwave irradiation, paratoluenesulfonic acid (pTSA)-modified montmorillonite clays gave, regioselectively, 4-(4´-hydroxyphenyl)-2-butanone (raspberry ketone). The duration for this reaction under microwave irradiation is much shorter than that of the conventional method. A comparative study of the alkylation reaction over a montmorillonite clay sample treated with 0.5 M-pTSA (0.5 M-pTSA clay) with that of Al-exchanged montmorillonite (Al3+-Mont) and beta-zeolite (HB) was carried out. The results show that the reaction time to reach equilibrium and the product distribution pattern for the reaction over 0.5 M-pTSA clay were similar to those values for the HB. Micropores formed on the clay surface during the pTSA treatment were found to enhance the rate of formation of C-alkylation. Micropores appear to enable better access to the active sites during the course of reaction.