Thangaraj Selvam

Zeolitic Materials with Hierarchical Porous Structures

During the past several years, different kinds of hierarchical structured zeolitic materials have been synthesized due to their highly attractive properties, such as superior mass/heat transfer characteristics, lower restriction of the diffusion of reactants in the mesopores, and low pressure drop. Our contribution provides general information regarding types and preparation methods of hierarchical zeolitic materials and their relative advantages and disadvantages. Thereafter, recent advances in the preparation and characterization of hierarchical zeolitic structures within the crystallites by post-synthetic treatment methods, such as dealumination or desilication; and structured devices by in situ and ex situ zeolite coatings on open-cellular ceramic foams as (non-reactive as well as reactive) supports are highlighted. Specific advantages of using hierarchical zeolitic catalysts/structures in selected catalytic reactions, such as benzene to phenol (BTOP) and methanol to olefins (MTO) are presented.

Reactivity and applications of layered silicates and layered double hydroxides

Layered materials, such as layered sodium silicates and layered double hydroxides (LDHs), are well-known for their remarkable adsorption, intercalation and swelling properties. Their tunable interlayers offer an interesting avenue for the fabrication of pillared nanoporous materials, organic-inorganic hybrid materials and catalysts or catalyst supports. This perspective article provides a summary of the reactivity and applications of layered materials including aluminium-free layered sodium silicates (kanemite, ilerite (RUB-18 or octosilicate) and magadiite) and layered double hydroxides (LDHs). Recent developments in the use of layered sodium silicates as precursors for the preparation of various porous, functional and catalytic materials including zeolites, mesoporous materials, pillared layered silicates, organic-inorganic nanocomposites and synthesis of highly dispersed nanoparticles supported on silica are reviewed in detail. Along this perspective, we have attempted to illustrate the reactivity and transformational potential of LDHs in order to deduce the main differences and similarities between these two types of layered materials.

Hydrothermal transformation of a layered sodium silicate, kanemite, into zeolite Beta (BEA)

Hydrothermal transformation of a layered sodium silicate, kanemite, in the presence of tetraethylammonium hy- droxide (TEAOH, 30%), into a commercially important zeolite Beta (BEA) has been investigated. During the hy- drothermal transformation of kanemite, zeolite Na-P1 was formed as an intermediate product and subsequently transformed into highly crystalline and phase-pure zeolite Beta as indicated by different physico-chemical characteri- sation techniques. The influence of various parameters such as SiO2/Al2O3 ratio, temperature, and TEAOH/SiO2 ratio were examined. For comparisons, hydrothermal transformation of other layered silicates such as Na-magadiite and Na- ilerite were also studied under similar reaction conditions. The catalytic activity of a zeolite Beta sample obtained by the hydrothermal transformation of kanemite was also demonstrated in the cracking of n-hexane at atmospheric pressure in a fixed bed reactor.

Solid state transformation of TEAOH-intercalated kanemite into zeolite beta (BEA)

The work reported herein involves the synthesis of zeolite beta by solid state transformation of TEAOH-intercalated kanemite, at 140 °C, under autogenous pressure. A series of TEAOH-intercalated kanemite samples were prepared by intercalation of kanemite with TEAOH at room temperature followed by centrifugation and drying at 35 °C for 24 h. The XRD, TG-DTA and chemical analyses of the TEAOH-intercalated kanemite samples revealed that either the TEAOH molecules were intercalated into the interlayer spaces or strongly adsorbed onto the outer surfaces of kanemite. Zeolite beta samples in high yields (71–97%) were obtained by solid state transformation of TEAOH-intercalated kanemite samples having moderate amounts of TEAOH (TEAOH/SiO2 = 0.11–0.23). The XRD measurements showed that the zeolite beta samples obtained from the present method were phase-pure and highly crystalline. SEM studies revealed that the zeolite beta samples were composed of agglomerated crystallites. For the zeolite beta samples, surface areas of as high as 586 m2 g−1 were observed by the BET method. In addition, the zeolite beta sample (SiO2/Al2O3 = 12) obtained from TEAOH-intercalated kanemite was found to be active for the cracking of n-hexane and its performance was comparable to a standard zeolite beta sample (SiO2/Al2O3 = 20).

Natural Cuban zeolites for medical use and their histamine binding capacity

Abstract The work reported herein involves the characterization of natural zeolites from two different mines (San Andrés and Tasajeras; Cuba) using XRD, SEM, TG-DTA, ICP and surface area measurements. In addition, the chemical composition of zeolites, the heavy metal and environmental organic toxins content, the ion exchange rates, stability under biological conditions using simulated body fluids as well as the binding capacity for histamine have been investigated with a view to using them as medical products. The investigated zeolites contain clinoptilolite and mordenite as major phases. Furthermore, the samples are apparently free from fibrous materials according to SEM observations. In particular, the San Andrés zeolite binds remarkable amounts of histamine which are nearly irreversible under acidic (pH = 1; 12.4 mg/g) and neutral conditions (pH = 7; 15.7 mg/g), respectively. Thus, the San Andrés sample may well be applied as a medical product due to its excellent binding capacity for histamine along with its remaining optimum physico-chemical characteristics.

Stability of AlPO and SAPO molecular sieves during adsorption-desorption cycles of water vapor investigated by in-situ XRD measurements

Abstract To gain some information about the hydrolytic stability of AlPO and SAPO molecular sieves structural changes during adsorption and desorption cycles of water vapor have been studied by in-situ XRD diffractometry.

Cetyltrimethylammonium bromide–silica membrane for seawater desalination through pervaporation

A simple preparation of mesostructured cetyltrimethylammonium bromide (CTAB)–silica membrane is reported. It effectively desalinates seawater to pure water through pervaporation separation process. The membrane thickness was of nanometer-length-scale obtained by deposition of CTAB–silica colloids on porous polysulfone support. Scanning electron microscopy (SEM), atomic force microscopy (AFM) and transmission electron microscopy (TEM) studies were performed to characterize the membrane while the structure of the colloids in coating solution was probed by small-angle neutron scattering (SANS). The prepared membranes exhibited excellent salt rejection efficiency of 99.9% in desalination of synthetic seawater of 40,000 ppm NaCl by pervaporation at 25∘C. The pure water flux was in the range of 1–2.6 kg m−2 h depending upon the membrane configuration and thickness. The flux could be greatly enhanced by operating the process at higher temperatures of 40–80∘C but it was at the cost of decreased salt-rejection efficiency. The initial rejection efficiency and flux of the membrane was found to be restored upon cooling the membrane back to room temperature.

In-situ ultrasonic methods for monitoring the hydrothermal synthesis of LTA-type zeolite from colloidal solutions

Abstract In the present study, we report the successful use of ultrasound device as a real-time, in situ diagnostic tool for monitoring the crystallization progress of zeolite A from homogeneous colloidal solutions with the composition 0.4 Na2O : 10 SiO2 : 1.4 Al2O3 : 16 (TMA)2O: 650 H2O. The results demonstrate that there exist good correlation between the crystallisation progress of zeolite A in terms of yield and the attenuated ultrasonic signal. It is also revealed that the kinetic information like activation energy of crystallisation reaction could be easily calculated from the measured ultrasonic signals data. In the studied case, activation energy of Ca. 30 kJ/mol was obtained.

Hydrothermal transformation of porous glass beads into porous glass beads containing zeolite beta (BEA)

Abstract Hydrothermal transformation of porous glass beads into porous glass beads containing zeolite Beta (BEA) is achieved, at 160 °C, using tetraethylammonium hydroxide (TEAOH) as the templating agent. The products were characterised by various techniques. The XRD results indicate that the transformation of porous glass beads into phase-pure zeolite Beta occurs within the SiO 2 /Al 2 O 3 ratios of 50–100. The Light microscopic images indicate that the shape of the starting porous glass beads is preserved during the hydrothermal treatment. SEM reveals that the outer surface of the porous glass beads is completely covered by zeolite Beta crystals. N 2 adsorption isotherm reveals the microporous nature of the zeolite Beta sample obtained from porous glass beads.

Defect-free zeolite membranes of the type BEA for organic vapour separation and membrane reactor applications

Abstract In this work defect free BEA type zeolite membranes with varying Si/Al ratio (18 to 33) were prepared on stainless-steel supports using a “multiple in -situ crystallization” technique. The quality of the membranes was evaluated using p-/o-xylene isomer separation, and 1,2,4-/1,3,5-trimethyl benzene isomer separation. A maximum p-xylene permeance of 1.5 × 10 −7 mol s −1 m −2 pa −1 and p-/o-xylene separation factor of 3.3 was obtained while in the trimethyl benzene separation experiments permeances were lower but the separation factor was comparatively higher (~ 4.8).