Development Of Mesoporosity Research Articles

Enhanced catalytic isomerization of α-pinene over mesoporous zeolite beta of low Si/Al ratio by NaOH treatment

Small crystal zeolite beta with Si/Al ratio of 12 was subjected to post-synthesis treatment with NaOH solutions. The effect of alkaline concentration on the crystallinity, pore structures, compositions and acidic properties of zeolite beta was studied by XRD, N2 sorption, XRF, and FTIR techniques. The catalytic performance of zeolite beta before and after alkaline treatment was investigated in terms of liquid phase isomerization of α-pinene. No distinct change in the crystallinity was observed after the zeolite was treated by 0.1–0.4M NaOH solutions. However, mesoporosity was evidently developed; the mesopore surface area up to 349m2g−1 was obtained when 0.4M NaOH solution was employed. The acidic properties of zeolite beta were mostly preserved when it was treated by 0.1–0.2M NaOH solutions, but the amount of strong Brönsted acid sites was to some extent reduced when 0.4 and 0.7M NaOH solutions were employed. The catalytic activities of the samples increase with the development of mesoporosity. The best catalytic performance at 70°C and 30min reaction time was observed on zeolite beta containing the largest mesopore volume of 0.69cm3g−1: 100% conversion of α-pinene with 85.4% selectivity into camphene, terpinolene, α- and γ-terpinene as desired products. At the same conditions only 20.3% conversion was obtained over parent zeolite beta containing only 0.23cm3g−1 mesopore volume. This result indicates that the enhanced catalytic activity for α-pinene isomerization is mainly attributed to the mesopore formation resulting from alkaline treatment.

Sustainable and hierarchical porous Enteromorpha prolifera based carbon for CO2 capture

Nitrogen-containing porous carbon was synthesized from an ocean pollutant, Enteromorpha prolifera, via hydrothermal carbonization and potassium hydroxide activation. Carbons contained as much as 2.6% nitrogen in their as-prepared state. Physical and chemical properties were characterized by XRD, N2 sorption, FTIR, SEM, TEM, and elemental analysis. The carbon exhibited a hierarchical structure with interconnected microporosity, mesoporosity and macroporosity. Inorganic minerals in the carbon matrix contributed to the development of mesoporosity and macroporosity, functioning as an in situ hard template. The carbon manifested high CO2 capacity and facile regeneration at room temperature. The CO2 sorption performance was investigated in the range of 0–75°C. The dynamic uptake of CO2 is 61.4mg/g and 105mg/g at 25°C and 0°C, respectively, using 15% CO2 (v/v) in N2. Meanwhile, regeneration under Ar at 25°C recovered 89% of the carbon's initial uptake after eight cycles. A piecewise model was employed to analyze the CO2 adsorption kinetics; the Avrami model fit well with a correlation coefficient (R2) of 0.98 and 0.99 at 0°C and 25°C, respectively.

Porosity Development in Carbon Nanofibers by Physical and Chemical Activation

In this Work we Have Compared the Effects of Physical Activation with CO2and Chemical Activation with KOH on Porosity Development in Vapor Grown Carbon Nanofibers (CNFs). both Physical and Chemical Activations Result in Micro- and Mesoporosity Development in the Studied Cnfs. under this Work’s Conditions, Chemical Activation with KOH Was More Efficient than Physical Activation with CO2in Terms of Surface Area Increase Regarding the Fresh Material (7.5-Fold versus 4-Fold, Respectively, under the Optimal Conditions Found for each Type of Activation). Atomic Force Microscopy Indicated that, although the CNF Samples Retained their Fibrous Morphology upon both Physical and Chemical Activation, the Latter Treatment Brought about Noticeable Changes in their Nanometer-Scale Structure. Likewise, an Appreciable Decrease in Nanofiber Diameter Following both Types of Activation Was Noticed. However, such Diameter Reduction Could Not Account for the Increase in Specific Surface Area of the Activated Materials, which Has to Be Attributed to Porosity Development. X-Ray Diffraction Studies Showed that both Physical as Chemical Activation Take Place Mainly on the Disordered Skin of the Cnfs but in a Different Way. Thus, Physical Activation Removes the More Amorphous Areas from the CNF Skin by Gasification (which Increases their Structural Order), while upon Chemical Activation with KOH, the Carbon Material Is Oxidized to a Carbonate, and the Alkali Hydroxide Is Reduced to Metallic Potassium, which Becomes Intercalated between the Graphene Layers of the Carbon Material, Leading to a Certain Expansion of the Structure.

Hydrothermal and conventional H3PO4 activation of two natural bio-fibers

Two phosphoric acid activation procedures; Activation after Hydrothermal Impregnation (recently published) and Activation after Incipient Wetness Impregnation instead of conventional impregnation are analyzed in two natural bio-fiber precursors: banana pseudostem and coconut fiber matting. Both procedures are compared analyzing, in both precursors, the influence that variables such as H3PO4/precursor ratio, activation temperature and impregnation time have on the resulting activated carbons (ACs) properties. The work also pays special attention to the mesoporosity development and the application of these ACs to adsorb gasoline vapors.Both H3PO4 activation procedures develop activated carbons having suitable activation yields and porosity developments, giving the Activation after Incipient Wetness Impregnation method better results than the Activation after Hydrothermal Impregnation. Both natural bio-fibers are good precursors, rendering the coconut fiber matting better results than the banana pseudostem. The variables studied affect the porosity development, being precursor and H3PO4/precursor ratio the variables that most affect. By a suitable selection of these variables, activated carbons having high adsorption capacities (BET above 2500m2g−1 and micropore volume above 1.00cm3g−1) and well developed mesoporosity (reaching 1.41cm3g−1), can be prepared. Most of the samples prepared perform very well for adsorbing gasoline vapors, showing a linear relationship with their resulting volumes.

Aromatization of alkanes over Pt promoted conventional and mesoporous gallosilicates of MEL zeolite

Aromatization of hexane and propane was investigated over Pt promoted mesoporous gallium-containing HZSM-11 with controlled mesoporosity generated by desilication. Prepared catalysts were characterized by nitrogen adsorption, X-ray powder diffraction, scanning electron microscopy, Fourier transform infrared of chemisorbed pyridine, and NH 3 temperature programmed desorption confirming the development of intracrystalline mesoporosity of Ga-containing HZSM-11. The catalytic activities, which were compared in the aromatization of n-hexane and propane, increased upon desilication. The aromatization of n-hexane decreased in the following order, Pt/mesoporous GaZSM-11 ≫ Pt/conventional GaZSM-11 ≫ mesoporous GaZSM-11 > conventional GaZSM-11. Hexane conversion reached 70.1% over mesoporous Pt/GaZSM-11 with Si/Ga of 61, as compared with 29.6 and 24.9% for corresponding mesoporous and conventional GaZSM-11 (Si/Ga = 94), respectively, for experiments at liquid hour space velocity of 3.6 h −1, and 540 °C. Comparison of BTX (benzene–toluene–xylene) selectivity at the conversion level of ∼21.0% revealed that Pt/mesoporous GaZSM-11 is more selective than corresponding mesoporous and conventional GaZSM-11. The BTX selectivity over Pt/mesoporous GaZSM-11 (Si/Ga = 94), which showed strong dependence on the conversion, reached 28.2%, whereas over corresponding mesoporous and conventional GaZSM-11catalysts reached 19.1% and 5.5%, respectively. A higher conversion and better selectivity can be attributed to the improved accessibility to the active extra-framework Ga species owing to the generation of mesopores inside the zeolite particles and shortening the contact time. It is worth mentioning that the prepared catalysts exhibited quite low activity in propane aromatization but exhibiting similar trends as for hexane aromatization.

Effect of the activating agent on physico-chemical and electrical properties of activated carbon cloths developed from a novel cellulosic precursor

Different chemical reagents (phosphoric acid, boric acid, ammonium citrate, sodium dihydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate and trisodium phosphate) were employed to develop activated carbon cloths (ACC) by chemical activation of a lyocell precursor, in an attempt to explore their effect on main physico-chemical characteristics and electrical behaviour of the resulting ACC. The activating agent markedly influenced yield, elemental composition, and textural properties of the ACC. The ACC obtained with phosphoric and boric acids were essentially microporous, whereas those developed with the other reagents presented mesoporosity development. Phosphoric acid-derived samples showed the highest specific surface area (976 m 2/g). The results also highlight the relevance of correcting the external surface adsorption in order to obtain reliable estimates of micropore volume. All the ACC were electrically conductive, their resistivity being also strongly dependent on the nature of the activating agent. The electrical resistivity of the ACC obtained with all the phosphorous compounds was successfully correlated with their C/H ratio and micropore volume.

Phosphoric acid activation of recalcitrant biomass originated in ethanol production from banana plants

Abstract Biomass samples have been studied as activated carbon precursors. Two, denoted as recalcitrant biomass, are ethanol production wastes, one from acid hydrolysis of starch-rich banana ( Musa acuminate ) fruit and one from enzymatic hydrolysis of banana pseudostem (lignocellulosic material) and a third one is raw banana tree pseudostem. The samples were characterized by proximate, ultimate, forage, morphological and structural analyses. They all have volatile matter contents above 50% but present some remarkable differences. The sample coming from the acid hydrolysis has too high ash content (mostly calcium sulfate) and low cellulose and lignin contents that prevents its use as an activated carbon precursor. Contrarily, the other two samples have lower ash contents, higher carbon percentages and higher cellulose and lignin contents. The activation with phosphoric acid at 450 °C has been investigated. The results are quite reproducible, showing good porosity developments and a strong dependence on the precursor, the H 3 PO 4 concentration and the H 3 PO 4 /sample weight ratio used. The activated carbons have quite high apparent surface area, porosity and yield. Interestingly, the phosphoric acid activated carbons prepared in this work have quite well developed mesoporosity which is especially useful for some applications (e.g. for gasoline evaporative control). Our preliminary comparative study, carried out with a well known commercial activated carbon used to control automobile hydrocarbons emissions, has shown that these activated carbons perform very well, being their performance a function of their mesoporosity development. Their oxygenated surface complexes (assessed from TPD experiments) are considerably high.

Effect of counteranion of ammonium salts on the synthesis of porous nanoparticles (NH 4) 3[PMo 12O 40

Porous ammonium phosphomolybdate ((NH 4) 3[PMo 12O 40]) nanoparticles have been synthesized by the titration method using several ammonium salts. The purpose of this work is to analyze the role of the counteranion in the development of the pore texture. The crystalline structure was quite similar in all the synthesized (NH 4) 3[PMo 12O 40] nanoparticles, as revealed by the similarity of the obtained powder X-ray diffraction patterns and the electron microscopy images. However, adsorption studies have shown pronounced differences in the pore texture of the different (NH 4) 3[PMo 12O 40] nanoparticles prepared. From these studies, it can be deduced that the counteranion of the ammonium salt plays an important role in the development of porosity during the synthesis of (NH 4) 3[PMo 12O 40]. It has been found an effect on both, the specific micropore area and the pore size distribution. The basic character of the counteranion has shown to be the main property affecting the development of mesoporosity in (NH 4) 3[PMo 12O 40] samples. On the other hand, the local concentration of [NH 4] + during the precipitation of (NH 4) 3[PMo 12O 40] seems to promote a higher porosity.

Oxidation of activated carbon by dry and wet methods: Surface chemistry and textural modifications

The effects of dry and wet oxidation treatments of activated carbon (AC) on the surface chemistry and porous structure are studied. Using cherry stones (CS), AC was first prepared by carbonization at 900 °C for 2 h in N 2 and activation at 850 °C for 2 h in CO 2. Then, the resulting AC was oxidized in O 2(air) or O 3 atmosphere and with HNO 3 and H 2O 2 solutions. The acidic–basic surface sites were analyzed by FT-IR spectroscopy, Boehm method, and pH of the point of zero charge (pH pzc) and the porous structure by N 2 adsorption and mercury porosimetry. It has been found that the oxidizing agent, under specific reaction conditions, rather than whether it was a gas or a solute in aqueous solution, is the main factor that controls the changes produced in the surface chemistry and porous structure of AC. O 3 and HNO 3 are the most effective oxidants to form acidic oxygen surface groups. However, the content of basic groups decreases for the four oxidants, the effect being much stronger for HNO 3. A microporosity reduction is also observed, which is more important for O 2(air) and especially for HNO 3 than for O 3 and H 2O 2. The percentage of microporosity loss is as high as 43.3 for HNO 3. Mesoporosity significantly develops, whereas macroporosity usually remains practically unchanged. Dry oxidation of AC at 100 °C in O 3 has proved to be the most promising method to increase the content of acidic oxygen surface groups in the material without greatly decreasing the content of basic sites and microporosity and with a significant mesoporosity development.

Mesopore formation in zeolite H-SSZ-13 by desilication with NaOH

A zeolite H-SSZ-13 material with CHA topology and a Si/Al ratio of 14 was treated with sodium hydroxide solutions of various concentrations. The post synthesis treatment led to desilication of the framework accompanied by mesopore formation. N 2-physisorption measurements showed that the mesopore volume increased while the micropore volume decreased with increasing severity of the treatment. By applying FTIR spectroscopy with CO as probe molecule, it was shown that silanol groups and Lewis acid sites were created. The number of Brønsted acid sites decreased whereas the acid strength of the maintained Brønsted acid sites was preserved. The mesoporous areas could be visualized by following the oligomerization reaction of 4-methoxystyrene with confocal fluorescence microscopy. The development of mesoporosity started from the crystal surfaces and continued progressively into the crystal with increasing alkaline solution strength. After treatment with 0.4 M alkaline solution, the fluorescence diminished, indicating a decreased catalytic activity in the mesopores. The activity of the micro- and mesopores was probed by testing the material as catalyst for the Methanol-to-Olefins (MTO) reaction. In contrast to the case of ZSM-5, mesoporosity created by alkaline leaching does not enhance the catalytic properties of H-SSZ-13 in the MTO reaction. Especially, a decrease in methanol conversion capacity due to a decreased surface area and number of acid sites was observed. GC–MS and UV/Vis analyses revealed that the same type of coke was formed in both the microporous and the mesoporous H-SSZ-13 materials. Furthermore, the deactivated mesoporous samples contained less coke than the purely microporous H-SSZ-13.

Activation of Waste Tire Char upon Cyclic Oxygen Chemisorption−Desorption

Activation of waste tire char upon cyclic oxygen chemisorption−desorption permits a controlled development of porosity versus burnoff using air as feed gas for the activation process. A slow but monotonical increase of BET surface area is obtained from cycle to cycle. Initially the process led to the development of mesoporosity without generating micropores and then the micropore volume is increased whereas a decrease of narrow mesopore (20−80 nm) volume is observed, probably as a consequence of mesopore widening. Although the SBET reaches relatively low values (below 250 m2/g) even after 15 cycles, this surface development is associated with low burnoff values (around 22% for indicated BET surface area) and corresponds in an important percentage (up to about 50%) to external (nonmicropore) area. Temperatures around 210 and 550 °C for chemisorption and desorption, respectively, have been found as optimum for the purpose of preparing granular mesoporous carbons which can be interesting candidates as cataly...

Mesoporous metallosilicate zeolites by desilication: On the generic pore-inducing role of framework trivalent heteroatoms

Desilication in alkaline medium is demonstrated as a general route to synthesize hierarchical metallosilicate zeolite crystals with preserved crystallinity. The lattice trivalent cation (not only Al 3+ but also Ga 3+, Fe 3+, and B 3+) regulates the alkaline-assisted hydrolysis of silicon towards controlled mesoporosity development. This finding widens the family of modified zeolite compositions that integrate in a single crystalline material micropores and mesopores and enables better accessibility in catalytic applications compared to the purely microporous counterparts.

Accelerated generation of intracrystalline mesoporosity in zeolites by microwave-mediated desilication

Microwave-mediated desilication was successfully used to fabricate hierarchically structured zeolites combining micro- and mesoporosity. The application of microwave radiation during alkaline treatment of commercial ZSM-5 samples in aqueous NaOH solution at 338 K spectacularly accelerates the development of intracrystalline mesoporosity compared to the standard treatment (conventional heating). This is attributed to the more efficient transfer of thermal heating to the zeolite solution, thus enhancing the rate of silicon extraction. Very short exposure times (3-5 min) resulted in hierarchical zeolites with mesopore surface areas up to 230 m(2) g(-1) and pores centred around 10 nm, while preserving the crystallinity of the parent sample. The benefits of this treatment can be extrapolated to other zeolite structures.

Cover Picture: Tailored Mesoporosity Development in Zeolite Crystals by Partial Detemplation and Desilication (Adv. Funct. Mater. 1/2009)

Cover Picture: Tailored Mesoporosity Development in Zeolite Crystals by Partial Detemplation and Desilication (Adv. Funct. Mater. 1/2009)

Tailored Mesoporosity Development in Zeolite Crystals by Partial Detemplation and Desilication

AbstractPartial detemplation of zeolites followed by desilication in alkaline medium is demonstrated as a powerful and elegant approach to design hierarchical zeolites with tailored degree of mesoporosity. This achievement, illustrated for large beta crystals, is based on the fact that the template‐containing zeolite is virtually inert to Si leaching upon treatment in aqueous NaOH solutions. Partial removal of the structure‐directing agent creates regions in the crystal susceptible to mesopore formation by subsequent desilication, while template‐containing regions are protected from silicon extraction. Variation of the calcination temperature in the range 230–550 °C determines the amount of template removed and enables control of the extent of mesopore formation in the zeolite (20–230 m2 g−1) upon alkaline treatment. The functionality of the introduced mesoporosity in the hierarchical beta crystals is demonstrated by the improved performance in the catalytic pyrolysis of low‐density polyethylene. The partial detemplation–desilication treatment enhances the tuning options of this demetallation method.

Activated carbons from peach stones and pine sawdust by phosphoric acid activation used in clarification and decolorization processes

AbstractBACKGROUND: A comparative study of the influence of preparation variables in the activation of pine sawdust and peach stones with phosphoric acid is presented. Adsorptive properties of these materials were tested for the clarification of apple juice and the decolorization of red wine. The main objective of this study was to identify the influence of the textural and chemical properties of activated carbons on clarification and decolorization processes.RESULTS: The role of impregnation conditions in porosity development was more important for activated carbons originating from peach stones than in those originating from pine sawdust, mainly due to their morphological differences. The amount of phosphoric acid retained on the precursors was found to be a function of the impregnation variables. Furthermore, porosity development dependence on phosphoric acid content per gram of precursor was confirmed. For both precursors, the best microporosity development was obtained at a heat treatment temperature of 450 °C. Independently of the preparation variables used, activated carbons from pine sawdust and peach stones had the same type of surface functional groups: mainly phenolic, quinonic and phosphoric ester‐like structures.CONCLUSIONS: From the results of clarifications and decolorization tests, it was confirmed that to obtain good performance in these assays, the development of mesoporosity is crucial. However, when activated carbons had similar mesoporosities, the presence of surface functional groups contributed to improved performance in clarification and decolorization process. Copyright © 2008 Society of Chemical Industry

Selective production of propylene from methanol: Mesoporosity development in high silica HZSM-5

The mesoporosity development in high silica HZSM-5 was carried out by alkaline desilication treatment and soft template method, and its relationship with the catalytic performance of the modified catalysts in methanol-to-propylene reaction was studied. High propylene selectivity (42.2%) and propylene/ethylene ratio (10.1) were observed on the high silica HZSM-5 catalyst modified by alkaline desilication. The enhanced catalytic performance can be attributed to the newly created open mesopores on the surface of the zeolite crystals together with the low Brønsted acidity. A greater amount of mesoporosity could be readily formed in HZSM-5 crystals via the soft template route. However, the mesopores formed in this method are randomly distributed in the zeolite crystals and play a minor role in the molecular transport of the reaction, so the improvement in propylene selectivity and propylene/ethylene ratio of the catalyst is less evident. The higher propylene selectivity and propylene/ethylene ratio on modified HZSM-5 (especially by alkaline treatment) than unmodified HZSM-5 could be also related to different contributions of the methylaromatics route and olefins methylation/cracking route in MTP reaction.

The role of different parameters of synthesis in the final structure of Ti-containing mesoporous materials

Ti-containing mesoporous materials have been prepared by hydrothermal synthesis at 373 K and compared to pure siliceous MCM-41 synthesized in the same conditions. A detailed and correlative study about the effect of different synthesis parameters on the physicochemical properties of the material has been carried out. All the samples showed typical XRD patterns of a kind MCM-41 structure. It seems that the synthesis time and the surfactant chain length are critical variables to obtain good structures. Mesoporous structure allowed the incorporation of different degrees of Ti loading, although a lower degree of ordering was observed with the increase of metal content above 1%. Moreover, a high incorporation of Ti into the structure seems responsible for the development of a secondary mesoporosity confirmed by TEM and associated with a pronounced hysteresis loop in the N 2 isotherm. DRUVA-Vis and XPS studies revealed that the Ti species are mainly found isolated and tetrahedral coordinated into the framework. The superego-Ti(IV) species generated by the interaction of Ti-MCM-41 with aqueous H 2O 2 have been studied by EPR spectroscopy and correlated to DRUVA-Vis and XPS measurements.

Development of mesoporosity during phosphoric acid activation of wood in steam atmosphere

Oak and birch were used as precursors to produce the activated carbons (ACs) with well-developed mesoporosity by phosphoric acid-promoted activation in a steam atmosphere. The effect of experimental variables such as the amount of activating agent, the soaking time and the type of wood on the development of porous structure upon heating at 480°C was investigated. The materials were characterized by N2 adsorption at 77K, mercury porosimetry and elemental analysis. It was demonstrated that increasing impregnation ratio favors the development of micropores and small mesopores of 2–5nm, whereas the soaking time promotes the creation of large mesopores, between 10 and 50nm. Compared to birch, the oak activation using phosphoric acid in the same conditions gives ACs with lower mesopore volume and higher contribution of small mesopores that reflects the differences between both precursors in their biopolymer composition. The presence of steam in the H3PO4 activation process compared to nitrogen facilitates the development of mesoporosity to much higher extent for the birch than that of oak. The ACs prepared in this work show the BET surface area ranging from 800 to 2250m2g−1, the total pore volume of 0.35–2.04cm3g−1 with mesopore fraction between 0.06 and 0.68.

Alkaline-mediated mesoporous mordenite zeolites for acid-catalyzed conversions

The preparation of mesoporous mordenite zeolite and its subsequent application in the liquid-phase alkylation of benzene with ethylene is discussed. Mesoporous mordenite was obtained by controlled silicon extraction on alkaline treatment of successfully synthesized high-silica mordenite zeolites with molar Si/Al ratios in the range of 20–30. Besides substantial mesoporosity development, the combined microporous and mesoporous zeolites show preserved Brønsted acidic properties, which are highly attractive when acid-catalyzed reactions are targeted. Catalytic testing of the mesoporous mordenite found superior performance in benzene alkylation due to the unique interplay between improved physical transport in the shortened micropores and the preserved high density of acid sites.