On the hydrothermal stability of MCM-41. Evidence of capillary tension-induced effects

MCM-41's limited hydrothermal stability has been often related to the hydrolysis of Si-O-Si bonds due to the low degree of condensation, its thin walls or a combination of them. In this work, evidence for an additional factor is provided; a physical effect that occurs during the drying of the hydrothermally treated calcined material due to the intense capillary stress exerted in water. Depending on both physical (i.e. mechanical) and chemical (i.e. hydrolysis) resistances, the structure undergoes differently. Three MCM-41 samples with different degree of condensation were investigated. The most remarkable results are found with un-aged TEOS based material, which gets fully disordered and shrunk for all applied hydrothermal temperatures in water. Comparison between water and a low-surface-tension-solvent drying revealed that capillarity is responsible for the loss of ordering (and shrinkage) at moderate hydrothermal temperatures. The material's structure is hexagonal and shrinkage-free under the low-surface-tension-solvent route. At a high hydrothermal temperature, hydrolysis is extensive and responsible for the loss of ordering. The other remarkable finding regards the aged MCM-41 mesostructure that maintains the hexagonal features at all applied temperatures in water, and it is more stable against capillarity at high temperature. The Na-metasilicate based material is mechanically very stable and gets disordered at high temperature due to hydrolysis.

Hot-spots during the calcination of MCM-41:a SAXS comparative analysis of a soft mesophase

The phenomenon of microscopic hot-spots, during the calcination of MCM-41, was investigated by quantifying the magnitude of the temperature increase during the calcination of a soft MCM-41 mesophase using a SAXS comparative study. This was performed by thermally treating a soft material that was detemplated via Fenton chemistry followed by equilibrating and drying in a low-surface-tension solvent (n-butanol or N,N-dimethylformamide); these samples have limited structural shrinkage. The resulting samples were thermally treated at increasing temperatures, and the structural shrinkage was compared with that of the directly calcined material. By comparing the structural shrinkage, it was found that the microscopic temperature increase would fall between 190 and 250 C, as deduced from N,N-dimethyl-formamide and n-butanol. The order of magnitude of the temperature increase appears to be consistent with the well-known glow effect. It is, however, substantially higher than the experimentally determined macroscopic temperature increase. Several aspects are discussed to interpret this difference. © 2013 Elsevier B.V.

Fast pyrolysis of cassava rhizome in the presence of catalysts

Cassava rhizome was catalytically pyrolysed at 500 °C using analytical pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) in order to investigate the effect of catalysts on bio-oil properties. The catalysts studied were zeolite ZSM-5, two aluminosilicate mesoporous materials Al-MCM-41 and Al-MSU-F, and a proprietary commercial catalyst alumina-stabilised ceria MI-575. The influence of catalysts on pyrolysis products was observed through the yields of aromatic hydrocarbons, phenols, lignin-derived compounds, carbonyls, methanol and acetic acid. Results showed that all the catalysts produced aromatic hydrocarbons and reduced oxygenated lignin derivatives, thus indicating an improvement of bio-oil heating value and viscosity. Among the catalysts, ZSM-5 was the most active to all the changes in pyrolysis products. In addition, all the catalysts with the exception of MI-575 enhanced the formation of acetic acid. This is clearly a disadvantage with respect to the level of pH in the liquid bio-fuel.

Evaluation of catalytic pyrolysis of cassava rhizome by principal component analysis

Rhizome of cassava plants (Manihot esculenta Crantz) was catalytically pyrolysed at 500 °C using analytical pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS) method in order to investigate the relative effect of various catalysts on pyrolysis products. Selected catalysts expected to affect bio-oil properties were used in this study. These include zeolites and related materials (ZSM-5, Al-MCM-41 and Al-MSU-F type), metal oxides (zinc oxide, zirconium (IV) oxide, cerium (IV) oxide and copper chromite) catalysts, proprietary commercial catalysts (Criterion-534 and alumina-stabilised ceria-MI-575) and natural catalysts (slate, char and ashes derived from char and biomass). The pyrolysis product distributions were monitored using models in principal components analysis (PCA) technique. The results showed that the zeolites, proprietary commercial catalysts, copper chromite and biomass-derived ash were selective to the reduction of most oxygenated lignin derivatives. The use of ZSM-5, Criterion-534 and Al-MSU-F catalysts enhanced the formation of aromatic hydrocarbons and phenols. No single catalyst was found to selectively reduce all carbonyl products. Instead, most of the carbonyl compounds containing hydroxyl group were reduced by zeolite and related materials, proprietary catalysts and copper chromite. The PCA model for carboxylic acids showed that zeolite ZSM-5 and Al-MSU-F tend to produce significant amounts of acetic and formic acids.

Catalytic pyrolysis of agricultural residues for bio-oil production

Agricultural residues from Thailand, namely stalk and rhizome of cassava plants, were employed as raw materials for bio-oil production via fast pyrolysis technology. There were two main objectives of this project. The first one was to determine the optimum pyrolysis temperature for maximising the organics yield and to investigate the properties of the bio-oils produced. To achieve this objective, pyrolysis experiments were conducted using a bench-scale (150 g/h) reactor system, followed by bio-oil analysis. It was found that the reactor bed temperature that could give the highest organics yield for both materials was 490±15ºC. At all temperatures studied, the rhizome gave about 2-4% higher organics yields than the stalk. The bio-oil derived from the rhizome had lower oxygen content, higher calorific value and better stability, thus indicating better quality than that produced from the stalk. The second objective was to improve the bio-oil properties in terms of heating value, viscosity and storage stability by the incorporation of catalyst into the pyrolysis process. Catalytic pyrolysis was initially performed in a micro-scale reactor to screen a large number of catalysts. Subsequently, seven catalysts were selected for experiments with larger-scale (150 g/h) pyrolysis unit. The catalysts were zeolite and related materials (ZSM-5, Al-MCM-41 and Al-MSU-F), commercial catalysts (Criterion-534 and MI-575), copper chromite and ash. Additionally, the combination of two catalysts in series was investigated. These were Criterion-534/ZSM-5 and Al-MSU-F/ZSM-5. The results showed that all catalysts could improve the bio-oils properties as they enhanced cracking and deoxygenation reactions and in some cases such as ZSM-5, Criterion-534 and Criterion-534/ZSM-5, valuable chemicals like hydrocarbons and light phenols were produced. The highest concentration of these compounds was obtained with Criterion-534/ZSM-5.

Catalytic pyrolysis of rice husk for bio-oil production

Catalytic pyrolysis experiments have been carried out on Brunei rice husk (BRH) to obtain bio-oil using a fixed-bed pyrolysis rig. ZSM-5, Al-MCM-41, Al-MSU-F and Brunei rice husk ash (BRHA) were used as the catalysts for the catalytic pyrolysis experiments and comparison was done to analyse the changes in the bio-oil properties and yield. Properties of the liquid catalytic and non-catalytic bio-oil were analysed in terms of water content, pH, acid number, viscosity, density and calorific value. The bio-oil chemical composition shows that ZSM-5 increases the production of aromatic hydrocarbons and light phenols, whilst Al-MCM-41 reduces the acetic acid production. The catalytic runs increased the calorific value and water content in the bio-oil, whilst viscosity, density and acid number is decreased. © 2012 Elsevier B.V. All rights reserved.

Catalytic intermediate pyrolysis of Brunei rice husk for bio-oil production

Rice husks from Brunei were subjected via intermediate pyrolysis for bio-oil production. Two main objectives were set out for this study. The application of intermediate pyrolysis on Brunei rice husk for the production of bio-oil is the main objective of this experiment. Characterisation of the rice husks was inclusive as a pre-requisite step to assess the suitability as feedstock for production of liquid fuels. Following on from the characterisation results, a temperature of 450°C was established as the optimum temperature for the production of bio-oil. A homogenous bio-oil was obtained from the pyrolysis of dry rice husk, and the physicochemical properties and chemical compositions were analysed. The second objective is the introduction of catalysts into the pyrolysis process which aims to improve the bio-oil quality, and maximise the desired liquid bio-oil properties. The incorporation of the catalysts was done via a fixed tube reactor into the pyrolysis system. Ceramic monoliths were used as the catalyst support, with montmorillonite clay as a binder to attach the catalysts onto the catalyst support. ZSM-5, Al-MCM-41, Al-MSU-F and Brunei rice husk ash (BRHA) together with its combination were adopted as catalysts. Proposed criterions dictated the selection of the best catalysts, subsequently leading to the optimisation process for bio-oil production. ZSM-5/Al-MCM-41 proved the most desirable catalyst, which increases the production of aromatics and phenols, decreased the organic acids and improved the physicochemical properties such as the pH, viscosity, density and H:C molar ratios. Variation in the ratio and positioning of both catalysts were the significant key factor for the catalyst optimisation study.

Interdependent lateral interactions, hydrophobicity and acid strength and their influence on the catalytic activity of nanoporous sulfonic acid silicas

A series of propylsulfonic (MCM-SOH) and octyl co-functionalised propylsulfonic (MCM-Oc-SOH) catalysts have been prepared by post modification of MCM-41 with mercaptopropyltrimethoxysilane (MPTS) to achieve SOH surface coverages spanning the range 0.12-1 monolayer. Within the MCM-Oc-SOH series, samples with submonolayer MPTS coverages were further grafted with octyltrimethoxysilane to cap bare hydroxyl sites and tune the hydrophobicity of the support. For the MCM-SO H series NH calorimetry revealed acid strength increases as a function of sulfonic acid loading, with -ΔH(NH ) increasing from 87 to 118 kJ mol. In contrast, MCM-Oc-SOH exhibits a dramatic enhancement of acid strength for submonolayer SOH coverages, with -ΔH(NH ) found to increase to 103 kJ mol. In line with these acid strength measurements the per-site activity of the MCM-SOH series in the esterification of butanol with acetic acid was found to increase with SOH content. Incorporation of octyl groups further promotes esterification activity of all the samples within the MCM-Oc-SOH series, such that the turn over frequency of the sample with the lowest loading of SOH more than doubles. Molecular dynamic simulations indicate that the interaction of isolated sulfonic acid groups with the pore walls is the primary cause of the decrease in acid strength and activity of submonolayer samples within the MCM-SOH series. Incorporation of octyl groups results in a combination of increased hydrophobicity and lateral interactions between adjacent sulfonic acid head groups, resulting in a striking enhancement of acid strength and esterification activity. © 2010 The Royal Society of Chemistry.

Green preparation of tuneable carbon-silica composite materials from wastes

Bio-oil has successfully been utilized to prepare carbon-silica composites (CSCs) from mesoporous silicas, such as SBA-15, MCM-41, KIT-6 and MMSBA frameworks. These CSCs comprise a thin film of carbon dispersed over the silica matrix and exhibit porosity similar to the parent silica. The surface properties of the resulting materials can be simply tuned by the variation of preparation temperatures leading to a continuum of functionalities ranging from polar hydroxyl rich surfaces to carbonaceous aromatic surfaces, as reflected in solid state NMR, XPS and DRIFT analysis. N2 porosimetry, TEM and SEM images demonstrate that the composites still possess similar ordered mesostructures to the parent silica sample. The modification mechanism is also proposed: silica samples are impregnated with bio-oils (generated from the pyrolysis of waste paper) until the pores are filled, followed by the carbonization at a series of temperatures. Increasing temperature leads to the formation of a carbonaceous layer over the silica surface. The complex mixture of compounds within the bio-oil (including those molecules containing alcohols, aliphatics, carbonyls and aromatics) gives rise to the functionality of the CSCs.