Alumina hydrothermal stability

Under hydrothermal conditions, alumina suffers from increased rates of hydration and hydrolysis of the oxides, resulting in the weakening of conventional supports according to Equation 1. Silica can suffer a similar hydrothermal fate. Process pH can exacerbate the problem (Fig. 34.5). Hence hydrothermal stability is a paramount criterion. 

In summary, the main goal of the present work is the development of a hydrothermally stable microporous silica membrane with prescribed transport properties. Preferably, these steam stable membranes should have very high permselectivities. Because the permselectivity of a molecular sieving silica membrane will drop to the Knudsen value of the y-alumina supporting membrane when the silica membrane deteriorates under steam reforming conditions, a selectivity of the silica layer higher than the Knudsen selectivity is sufficient. In this way the measurement of the permselectivity is a powerful tool to assess the hydrothermal stability of a supported microporous membrane. 

Somewhat surprisingly, however, only a very limited amount of literature is available on hydrothermal stability of even the most commonly applied mesoporous membrane type, namely y-alumina membranes on OC-AI2O3 supports. These mesoporous y-alumina membranes are the common supports for the microporous silica membranes to be used in membrane steam reformers. In the investigations that finally led to the present study, delamination of the y-alumina membrane from the OC-AI2O3 supports in hot steam was found to be a major compli-... 

Recent literature shows a growing trend to include free alumina in the formulation of fluid catalytic cracking (FCC) products. Over the last dozen years, FCC catalysts containing free alumina have been cited in the open and patent literature for benefits including (1) enhanced catalyst reactivity and selectivity (1-3). (2) more robust operation in the presence of metals in the petroleum feedstock (4-7). (3) improved attrition resistance (8.9). (4) improved hydrothermal stability against steam deactivation during regeneration (2.8). (5) increased pore volume and decreased bulk density (8), and (6) reduction of SOx emissions (10). 

In the early days, a conventional single stage turbulent bed (TB) regenerator was used, because the amorphous silica alumina microspheric catalyst used at that time was insensitive to carbon content and its hydrothermal stability was poor. Regeneration temperature was kept at... 

Several attempts were made to prepare pillared smectites with sufficient hydrothermal stability for use as active components in catalysts for catalytic cracking of heavy oil fractions. Although improvements were made, none of the attempts resulted in pillared materials stable enough to withstand the hydrothermal conditions found in the regenerator of a commercial FCC. One type of materials studied, i.e. alumina-montmorillonites, may be attractive alternatives to the active matrices, often alumina, currently used in FCC-catalysts designed for cracking of heavy oils. The alumina-montmorillonites can, perhaps, not be considered to be bona fide pillared smectites as they have considerably larger pores and a wider pore-size distribution than what is characteristic for pillared smectites. 

Influence of method of washcoat preparation on hydrothermal stability of alumina support... 

It is generally accepted that thermal and especially hydrothermal treatment of aluminas and other catalytic materials results in deterioration of porous structure, i.e. increase in average pore radius and diminishing in specific surface area [1-4]. It is very important that such alumina materials as some catalyst washcoats and membranes have to be exploited at higher temperatures and at atmosphere of large humidity. Therefore it is necessary to improve their thermal and hydrothermal stabilization by application of new binder materials or additives. Such additives as silica, ceria or zirconia are known as thermal stabilizers. The aim of this work was to determine the influence of addition of the selected stabilizers on hydrothermal stability of alumina material in the temperature range 150 - 225 °C and time up to 72 hours. 

The additional impregnation with silica sol of alumina earlier stabilised by TEOS does not change noticeably good hydrothermal stability of alumina. 

In comparison to TEOS hydrothermal stabilization efficiency of alumina by cerium oxide is low. 

Alumina membranes. It has been established that several phases of alumina exist and a particular phase of alumina is determined not only by the temperature it has experienced but also by the chemical path it has taken. For commercial membrane applications, the alpha- and gamma-phases of alumina are the most common. Alpha-alumina membranes are well known for their thermal and hydrothermal stabilities beyond 1,000 C. In fact, other transitional forms of alumina will undergo transformation towards the thermodynamically stable alpha-alumina at elevated tcmjxratures beyond 900 C. On the contrary, commercial gamma-alumina membranes are typically calcined at 400-600 C during production and are, therefore, subject to potential structural changes beyond 600°C. Moreover, alumina chemistry reveals that phase transition also occurs beyond that temperature [Wefers and Misra, 1987]. 

The FCC particle consists basically of about 35 wt % zeolite Y [12] incorporated into a matrix usually consisting of sihca, flich acts as a "ghie", and alumina, ch smves to crack the large molecules. The matrix often includes clay to provide the desired doi. One of the roles of the matrix is to increase the reristance of the catalyst to metal poisoning. This will be discussed in Section 2.4. Increasing the matrix content can also increase the hydrothermal stability of the catalyst. 

Compared with the high-alumina zeolites NaCaA or NaY, the relatively low hydrothermal stability of high-silica faujasites DAY-S and DAY-Tg result from the attack of water molecules at the silanol groups and the energy-rich Si-O-Si bonds of the crystal surface. The kinetics of this dissolution is significantly more rapid than that of the hydrolysis of aluminosilicates. 

Hydrothermal stability is a critically important property of the constituents of FCC catalysts accordingly prototype and reference materials were subjected to a hydrothermal deactivation treatment at a temperature of 788°C, 4 and 8 hours and 100% steam. Results are summarized in Table 3. Synthetic silico-aluminate (MX-0994) and alumina/kaolin (MM-0894) base matrices, which are used in the preparation of commercialFCC catalysts designed to crack heavy feedstocks, were used as references. 

Through studies over the last few decades, several methods have been developed for improvement of the stabilities of mesoporous silica materials. These strategies were shown to increase either the wall thickness or the degree of polymerization of the silica wall. Incorporation of metal oxides, such as zirconia, alumina, and vanadium oxides, was also demonstrated to be an effective method to improve the hydrothermal stability. 

C.H. Chang, R. Gopalan and Y.S. Lin, A comparative study on thermal and hydrothermal stability of alumina, titania and zirconia membranes. /. Membr. Sci., 91 (1994) 27-45. 

In response to this competitive pressure, Davison introduced a new molecular sieve catalyst in June of 1965. It was called XZ-25 and it was priced at 450 per ton. This was based on rare earth-exchanged Type X zeolite in a high-alumina semi-synthetic matrix, and had a lower zeolite content and a little less hydrothermal stability than the XZ-15. Acceptance was very rapid, and within half a year it was being used in 36 units, in eight of which it displaced the Filtrol Grade 800, reputedly on the basis of advantages in activity, selectivity and stability. Ultimately, XZ-25 was used in more than 116 units (67). 

A second approach makes use of oxides, such as AI2O3, Ti02, ZrOg (and others), as supports, promoted with base metal cations. Among such cations, Co, Ni, Cu, Fe, Sn, Ga, Au, In, and Ag have all been tested, and their activity in the SCR of NO by hydrocarbons (HCs) has been reviewed. These studies have mainly focused on alumina supported systems, which ensure hydrothermal stability up to high operating temperatures (1073 K), promoted with gii2,i42,i43 jj i44-i46 active components, as they appear as the... 

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