Hydrothermal catalytic

Kong, L., Li, G., Wang, H., He, W., Ling, F. Hydrothermal catalytic conversion of biomass for lactic acid production. J Chem Technol Biotechnol 2008, 83, 383-388. 

Bai, X., et al., 2014. Hydrothermal catalytic processing of pretreated algal oil a catalyst screening study. Fuel 120 (0), 141—149. 

Catalytic Properties. In zeoHtes, catalysis takes place preferentially within the intracrystaUine voids. Catalytic reactions are affected by aperture size and type of channel system, through which reactants and products must diffuse. Modification techniques include ion exchange, variation of Si/A1 ratio, hydrothermal dealumination or stabilization, which produces Lewis acidity, introduction of acidic groups such as bridging Si(OH)Al, which impart Briimsted acidity, and introducing dispersed metal phases such as noble metals. In addition, the zeoHte framework stmcture determines shape-selective effects. Several types have been demonstrated including reactant selectivity, product selectivity, and restricted transition-state selectivity (28). Nonshape-selective surface activity is observed on very small crystals, and it may be desirable to poison these sites selectively, eg, with bulky heterocycHc compounds unable to penetrate the channel apertures, or by surface sdation. 

The properties of the zeolite play a significant role in the overall performance of the catalyst. Understanding these properties increases our ability to predict catalyst response to changes in unit operation. From its inception in the catalyst plant, the zeolite must retain its catalytic properties under the hostile conditions of the FCC operation. The reaclor/regenerator environment can cause significant changes in chemical and structural composition of the zeolite. In the regenerator, for instance, the zeolite is subjected to thermal and hydrothermal treatments. In the reactor, it is exposed to feedstock contaminants such as vanadium and sodium. 

MicrocrystalUne zeolites such as beta zeolite suffer from calcination. The crystallinity is decreased and the framework can be notably dealuminated by the steam generated [175]. Potential Br0nsted catalytic sites are lost and heteroatoms migrate to extra-framework positions, leading to a decrease in catalytic performance. Nanocrystals and ultrafine zeolite particles display aggregation issues, difficulties in regeneration, and low thermal and hydrothermal stabilities. Therefore, calcination is sometimes not the optimal protocol to activate such systems. Application of zeolites for coatings, patterned thin-films, and membranes usually is associated with defects and cracks upon template removal. 

Recently, it is reported that Xi02 particles with metal deposition on the surface is more active than pure Ti02 for photocatalytic reactions in aqueous solution because the deposited metal provides reduction sites which in turn increase the efficiency of the transport of photogenerated electrons (e ) in the conduction band to the external sjistem, and decrease the recombination with positive hole (h ) in the balance band of Xi02, i.e., less defects acting as the recombination center[l,2,3]. Xhe catalytic converter contains precious metals, mainly platinum less than 1 wt%, partially, Pd, Re, Rh, etc. on cordierite supporter. Xhus, in this study, solutions leached out from wasted catalytic converter of automobile were used for precious metallization source of the catalyst. Xhe XiOa were prepared with two different methods i.e., hydrothermal method and a sol-gel method. Xhe prepared titanium oxide and commercial P-25 catalyst (Deagussa) were metallized with leached solution from wasted catalytic converter or pure H2PtCl6 solution for modification of photocatalysts. Xhey were characterized by UV-DRS, BEX surface area analyzer, and XRD[4]. 

Titanium containing hexagonal mesoporous materials were synthesized by the modified hydrothermal synthesis method. The synthesized Ti-MCM-41 has hi y ordered hexa rud structure. Ti-MCM-41 was transformed into TS-l/MCM-41 by using the dry gel conversion process. For the synthesis of Ti-MCM-41 with TS-1(TS-1/MCM-41) structure TPAOH was used as the template. The synthesized TS-l/MCM-41 has hexagonal mesopores when the DGC process was carried out for less than 3 6 h. The catalytic activity of synthesized TS-l/MCM-41 catalysts was measured by the epoxidation of 1-hexene and cyclohexene. For the comparison of the catalytic activity, TS-1 and Ti-MCM-41 samples were also applied to the epoxidation reaction under the same reaction conditions. Both the conversion of olefins and selectivity to epoxide over TS-l/MCM-41 are found hi er flian those of other catalysts. 

Alumina - Alumina forms a variety of oxides and hydroxides whose structures have been characterized by X-ray diffraction (16). From the catalytic viewpoint y-alumina is the most important. This is a metastable phase that is produced from successive dehydration of aluminum trihydroxide (gibbsite) to aluminum oxide hydroxide (boehmite) to y-alumina, or from dehydration of boehmite formed hydrothermally. y-alumina is converted into a-alumina (corundum) at temperatures around 1000 C. 

Further experiments by Huber and Wachtershauser on chemoautotrophic biogenesis under hydrothermal conditions have shown that a number of a-amino acids and a-hydroxyacids could have been formed, subsequent to the binding of carbon (in the form of CO and CN ) to catalytically active transition metal precipitates. The general structure of such compounds is R-CHA-COOH, with R = H, CH3, C2H5 or HOCH2 and A = OH or NH2. 

Beta/montmorillonite composite was prepared under dynamic hydrothermal conditions. Firstly, montmorillonite calcined at 800 °C were added to a diluted solution of sodium hydroxide, potassium chloride and TEAOH in distilled water and the resulting mixture was vigorously stirred for 1 h secondly, silica sol was added into the above uniform mixture to allow at least 3 h stirring finally, the gel was moved into stainless steel autoclaves (1L) and heated at 413 K for 48 h. The samples were characterized by XRD, N2 adsorption-desorption, FT-IR and SEM-EDS. The catalytic assessment experiments were carried out in a flowing-type apparatus designed for continuous operation. 

Zeolites though possess high hydrothermal stability and enhanced catalytic activity, the merits are not fully exploited for industrial applications because micropores hinder the diffusion of bulkier molecules. This led to the development of new kind of silicious materials like that of MCM and SBA with large pore volumes which can be accessed by bulkier molecules. But these suffer with the limitation of week catalytic activity and poor hydrothermal stability which hampers their use for various industrial applications... 

Nowadays synthesis of mesoporous materials with zeolite character has been suggested to overcome the problems of week catalytic activity and poor hydrothermal stability of highly silicious materials. So different approaches for the synthesis of this new generation of bimodal porous materials have been described in the literature like dealumination [4] or desilication [5], use of various carbon forms as templates like carbon black, carbon aerosols, mesoporous carbon or carbon replicas [6] have been applied. These mesoporous zeolites potentially improve the efficiency of zeolitic catalysis via increase in external surface area, accessibility of large molecules due to the mesoporosity and hydrothermal stability due to zeolitic crystalline walls. During past few years various research groups emphasized the importance of the synthesis of siliceous materials with micro- and mesoporosity [7-9]. Microwave synthesis had... 

Sn-S-4, Sn-ST-4 (Si/Sn = 106) showed better conversion results (38-42% 64-41%) with 100% selectivity to corresponding lactone with respect to 2-methyl and 4-methyl cyclohexanones compared with high conversions in case of bulkier bicyclic ketones. Even the hydrothermal catalysts gave lower than that in case of Sn-ST-4 which was also supported by the UV and XPS studies which showed that the incorporation of Sn in tetrahedral site is more and thus leading to the more catalytic activity. 

The preparation methods of aluminum-deficient zeolites are reviewed. These methods are divided in three categories (a) thermal or hydrothermal dealumination (b) chemical dea-lumination and (c) combination of thermal and chemical dealumination. The preparation of aluminum-deficient Y and mordenite zeolites is discussed. The structure and physico-chemical characteristics of aluminum-deficient zeolites are reviewed. Results obtained with some of the more modern methods of investigation are presented. The structure, stability, sorption properties, infrared spectra, acid strength distribution and catalytic properties of these zeolites are discussed. 

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