Micro/Mesoporous Zeolitic Composites: Recent Developments in Synthesis and Catalytic Applications

Recent Advances in the Seed-Directed Synthesis of Zeolites without Addition of Organic Templates

Zeolites have been widely employed in fields of petroleum refining, fine chemicals and environmental protection, but their syntheses are always performed in the presence of organic templates, which have many drawbacks such as high cost and polluted wastes. In recent years, the seed-directed synthesis of zeolites has been paid much attention due to its low-cost and environmentally friendly features. In this review, the seed-directed synthesis of Al-rich zeolites with homonuclear and heteronuclear features, the seed-directed synthesis of Si-rich zeolites assisted with ethanol and the utility of seed-directed synthesis have been summarized. This review could help zeolite researchers understand the recent progress of seed-directed synthesis.

Synthesis of Well-Ordered Mesoporous Aluminosilicates with High Aluminum Contents: The Challenge and the Promise

SBA-15 has recently emerged as a potential material for the catalytic conversion of large molecules. Usually, SBA-15 has a low content of aluminum due to the conventional acidic synthesis medium. Although a few approaches have been adopted to prepare Al-SBA-15 with a high alumina content, it is still challenging to prepare well-ordered Al-SBA-15 with a high alumina content. Here, we demonstrate a facile synthesis process in neutral mediums for the grafting of Al into the framework of SBA-15. This approach relies mainly on the dissociation of Si-O-Si bonds and the polymerization of Si-O-Si/Al bonds promoted by sodium persulfate (SPS) in neutral mediums. In this way, well-ordered Al-SBA-15 with a high aluminum content and enhanced acidity was obtained. Results of X-ray fluorescence spectroscopy (XRF) showed an n(SiO₂)/n(Al₂O₃) ratio of 13.7, much lower than that of the conventional sample (21.7) obtained in acidic medium. The characterization results indicated the presence of a well-ordered Al-containing mesophase with high hydrothermal stability. Notably, the Al content and the acidity of Al-SBA-15 can be tuned by changing the SPS amount.

The Potential of Cellulose as a Source of Bioethanol using the Solid Catalyst: A Mini-Review

One of the most important biofuels is cellulose ethanol which is a popular material for bioethanol production. The present cellulosic ethanol production is through the cellulolytic process and this involves the splitting of complex cellulose into simple sugars through the hydrolysis process of the lignocellulose pretreated with acids and enzymes after which the product is fermented and distilled. There are, however, some challenges due to the enzymatic and acid processes based on the fact that acid hydrolysis has the ability to corrode equipment and cause unwanted waste while the enzymatic hydrolysis process requires a longer time because enzymes are costly and limited. This means there is a need for innovations to minimize the problems associated with these two processes and this led to the application of solid catalysts as the green and effective catalyst to convert cellulose to ethanol. Solid catalysts are resistant to acid and base conditions, have a high surface area, and do not cause corrosion during the conversion of the cellulose due to their neutral pH. This review, therefore, includes the determination of the cellulose potential as feedstock to be used in ethanol production as well as the preparation and application of solid catalyst as the mechanism to convert cellulose into fuel and chemicals. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0). 

Fabrication of the Hierarchical HZSM-5 Membrane with Tunable Mesoporosity for Catalytic Cracking of n-Dodecane

Hierarchical HZSM-5 membranes were prepared on the inner wall of stainless steel tubes, using amphiphilic organosilane (TPOAC) and mesitylene (TMB) as a meso-porogen and a swelling agent, respectively. The mesoporosity of the HZSM-5 membranes were tailored via formulating the TPOAC/Tetraethylorthosilicate (TPOAC/TEOS) ratio and TMB/TPOAC ratio, in synthesis gel, and the prepared membranes were systematically characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), N2 adsorption–desorption, N2 permeation, inductively coupled plasma (ICP), in situ fourier transform infrared (FT-IR), ammonia temperature-programmed desorption (NH3-TPD), etc. It was found that the increase of the TPOAC/TEOS ratio promoted a specific surface area and diffusivity of the HZSM-5 membranes, as well as decreased acidity; the increase of the TMB/TPOAC ratios led to an enlargement of the mesopore size and diffusivity of the membranes, but with constant acid properties. The catalytic performance of the prepared HZSM-5 membranes was tested using the catalytic cracking of supercritical n-dodecane (500 °C, 4 MPa) as a model reaction. The hierarchical membrane with the TPOAC/TEOS ratio of 0.1 and TMB/TPOAC ratio of 2, exhibited superior catalytic performances with the highest activity of up to 13% improvement and the lowest deactivation rate (nearly a half), compared with the microporous HZSM-5 membrane, due to the benefits of suitable acidity, together with enhanced diffusivity of n-dodecane and cracking products.

Nature and Location of Carbonaceous Species in a Composite HZSM-5 Zeolite Catalyst during the Conversion of Dimethyl Ether into Light Olefins

The deactivation of a composite catalyst based on HZSM-5 zeolite (agglomerated in a matrix using boehmite as a binder) has been studied during the transformation of dimethyl ether into light olefins. The location of the trapped/retained species (on the zeolite or on the matrix) has been analyzed by comparing the properties of the fresh and deactivated catalyst after runs at different temperatures, while the nature of those species has been studied using different spectroscopic and thermogravimetric techniques. The reaction occurs on the strongest acid sites of the zeolite micropores through olefins and alkyl-benzenes as intermediates. These species also condensate into bulkier structures (polyaromatics named as coke), particularly at higher temperatures and within the meso- and macropores of the matrix. The critical roles of the matrix and water in the reaction medium have been proved: both attenuating the effect of coke deposition.