Clay catalyst support

Because the size of the interlayers can be easily varied by incorporation of complex moieties of different sizes, these clays (montmorillonite, hectorite) may nevertheless compete as catalyst supports with zeolites which have a rigid, predetermined cavity size. 

The dispersion and solid-state ion exchange of ZnCl2 on to the surface of NaY zeolite by use of microwave irradiation [17] and modification of the surface of active carbon as catalyst support by means of microwave induced treatment have also been reported [18]. The ion-exchange reactions of both cationic (montmorillonites) and anionic clays (layered double hydroxides) were greatly accelerated under conditions of microwave heating compared with other techniques currently available [19.]... 

A new trend in the field of oxidations catalyzed by metalloporphyrin complexes is the use of these biomimetic catalysts on various supports ion-exchange resins, silica, alumina, zeolites or clays. Efficient supported metalloporphyrin catalysts have been developed for the oxidation of peroxidase-substrates, the epoxidation of olefins or the hydroxylation of alkanes. 

Mesoporous synthetic clays synthesis, characterization, and use as HDS catalyst supports... 

The use of clays as supports for hydroprocessing has been reported and summarized [9-11], Dibenzothiophene (DBT) diluted with hexadecane (0.75 wt% S) was the liquid feed for HDS tests. The pore diameter of the MSC catalysts is seen to have a strong effect on both the HDS activity and selectivity (Figure 4). A commercial catalyst (Crosfield 465, Co/Mo alumina) was also measured under these conditions where it gave 77% DBT conversion and 61% BP selectivity. In a previous study [12], other synthetic hectorites were compared using these conditions except that a 1 wt% S feed was utilized. One sample was a control made without template that consisted of only micropores. The DBT conversion and BP selectivity were very low for this microporous material. The Crosfield material has significant macroporosity (42% of the pore volume) in addition to a broad distribution of mesoporosity, and has clearly been optimized to perform well under these HDS conditions. 

Recently Milczak et al.[57] have reported the nitration of o-xylene using 100% nitric acid over silica supported metal oxide solid acid catalysts with high yields (up to 90 %) but low selectivity to 4-o-NX (40-57 %). Choudary et a/. 5X 591 performed the nitration of o-xylene and other aromatic hydrocarbons by azeotropic removal of water over modified clay catalysts achieving low yields of 4-o-NX and a selectivity of 52%. Better results were obtained when HBeta zeolite was used as catalyst, performing the reaction in dichloromethane at reflux temperature.[60] Conversions of 40 % and maximum selectivity 68 % of 4-o-NX were obtained. Similar conversions and higher selectivities for 4-o-NX (65-75 %) were reported by Rao et al M 1 using a nanocrystaUine HBeta sample and working at 90 °C in the absence of solvent. 

The book explores various examples of these important materials, including perovskites, zeolites, mesoporous molecular sieves, silica, alumina, active carbons, carbon nanotubes, titanium dioxide, magnesium oxide, clays, pillared clays, hydrotalcites, alkali metal titanates, titanium silicates, polymers, and coordination polymers. It shows how the materials are used in adsorption, ion conduction, ion exchange, gas separation, membrane reactors, catalysts, catalysts supports, sensors, pollution abatement, detergency, animal nourishment, agriculture, and sustainable energy applications. 

The chapters in Characterization and Catalyst Development An Interactive Approach, assembled from both academic and industrial contributors, give a unique perspective on catalyst development Some chapters thoroughly characterize the catalyst prior to plant evaluation, whereas others utilize characterization to explain performance variances. Some new types of catalysts incorporated into this volume include the preparation of novel catalyst supports based on alumina and hydrous titanates. Attrition-resistant catalysts and ultrafine ceramics were prepared by modified spray-drying methods. New catalyst compositions based on vanadium-containing anionic clays were proposed for oxidation. A recently commercialized catalyst based on magnesium spinel was proposed for use in the abatement of sulfur oxide pollutants in fluid... 

Over the past 15-20 years, there has been a renewed and growing interest in the use of clay minerals as catalysts or catalyst supports. Most of this interest has focused on the pillaring of smectite clays, such as montmorillonite, with various types of cations, such as hydrated metal cations, alkylammonium cations and polycations, and polynuclear hydroxy metal cations (1-17). By changing the size of the cation used to separate the anionic sheets in the clay structure, molecular sieve-like materials can be made with pore sizes much larger than those of conventional zeolites. 

In order to improve the textural properties of particle-clay nanohybrids, bulky organic cations are intercalated as a kind of template into particle-intercalated clays before stabilization procedures. Intercalation of the organic cations results in the removal of some of the intercalated nanoparticles and/or in their rearrangement. Subsequent calcination leads to formation of additional pore space that is highly correlated to the geometry and size of the templates. This technique allows fine tuning of textural properties in the preparation of particle-clay nanohybrids. The clay nanohybrids intercalated with metals, oxides, and complexes have a broad range of applications. In particular, metal oxide particle-pillared clays have excellent potentials as catalysts, catalyst supports, selective adsorbents, etc. " ... 

Spent acids are used where feasible to neutralize alkaline waste streams, or are neutralized with purchased lime, or caustic, and are then routed to the normal effluent treatment system for further cleanup before discharge [61]. Waste solids from refinery operations include such materials as spent clay from decolorization of lubricating oils or waxes, sand used as a catalyst support, filter aid, or filter base, and exhausted Claus catalysts (primarily ferric oxide on alumina). These are disposed of by landfilling by the majority of Canadian refineries. However, some use landfarming for disposal of these materials. One refinery recovers spent Claus catalysts for regeneration into new catalyst. 

The necessity to develop hydrotreating catalysts with enhanced activity stimulates the search for alternative catalyst supports. It was shown that clay-supported transition metal sulfides can efficiently catalyze hydrodesulfurization (HDS) of thiophene [1-3]. However, the large scale application of the catalysts based on natural clays is still hampered, mainly due to the difficulties in controlling the chemical composition and textural properties. Synthetic clays do not suffer from these drawbacks. Recently, a novel non-hydrothermal approach was proposed for the synthesis of some trioctahedral smectites, namely saponite... 

The decrease in the activity of the rhodium catalyst supported on the pillared clay (Rh/BENPIL) after the reaction was carried out at 100 °C is significant. 

The activity of rhodium catalyst samples was monitored at 100 C for 7 hours, during which period the activity declined. All the catalysts reach a constant activity after 200 minutes fi om the start of the reaction approximately (Fig. 3). The percentage of residual activity (percentage of the final activity versus the maximum value reached) for the catalysts supported on zeolites (Rh/ZEDIP, 71% Rh/ZESEP, 83 % Rh/ZEDIX, 76% and Rh/ZESEX, 57%) indicates the resistance to poisoning. The deactivation of Rh/BENPIL (36%) is relatively rapid particularly during the first hour of reaction as mentioned, and this can be related to the formation of heavy secondary products formed in the MIBK formation route [20]. These molecules remain within the pores of the pillared clay sample, and therefore they block the access of the reactant to the active centres, hence causing a decrease in the activity of the catalyst. 

The PIL-clay and zeolitic materials synthesized are suitable as support for preparing heterogeneous catalysts by anchoring an organometallic complex by ion exchange. The structure of the materials used as supports had a great influence on the catalyst prepared. A higher metal content was achieved in the supports with lamellar structures, while better dispersion was shown by the catalysts supported on zeolitic structures. 

By the reaction of alcohols (in excess) with epoxidised vegetable oils in the presence of acids as catalysts - liquid polyols are formed (reaction 17.22). For example by the alcoholysis of epoxidised soybean oil with methanol, at the reflux temperature of methanol (the boiling point of methanol is 64.7 °C), in the presence of an acidic catalyst (H2S04, p-toluene sulfonic acid, HBF4 [31, 34, 44, 45], solid acidic clays [39], supported acidic catalysts), liquid soybean oil based polyols are obtained, with an hydroxyl number of around 170-173 mg KOH/g, a functionality of about 3-4 OH groups/mol and a viscosity of around 4,000-7,000 mPa-s at 25 °C. After the neutralisation of the acidic catalyst or by the filtration of solid acid catalysts, the methanol is distilled under vacuum and recycled back into the process. 

The smectite clays do, however, have some important features which make them particularly attractive as catalyst supports. In addition to their high intrinsic surface area, their laminar structure may confer size and shape selectivity to the resultant catalysts. Another important feature is the negative charge on the silicate layers which may be able to polarise reactant molecules and enhance catalytic activity. Finally the intrinsic acidity of clay minerals provides the catalyst with bifunctionality. This may be useful for example in stabilising intermediate carbocations which would otherwise deprotonate. 

The use of clay-based supported reagent catalysts in Friedel-Crafts reactions adds a new dimension to this area and has resulted in successful application on an industrial scale. This is discussed in Chapter 4. 

Clays and clay-based supported reagents (see Chapter 4) have become established catalysts in organic synthesis and it is not suprising that they have also become commonly associated with the clean synthesis of organic compounds. Some interesting recent examples are described below. 

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