US-12623207-B2 - Process for the preparation of high surface area alpha alumina and the use thereof

Abstract

The present invention refers to a process for the preparation of a high surface area nanoparticulate alpha alumina.

Inventors

• Amol AMRUTE
• Ferdi Schueth
• Hannah SCHREYER

Assignees

• STUDIENGESELLSCHAFT KOHLE MBH

Dates

Publication Date

20260512

Application Date

20200121

Priority Date

20191024

Claims (1)

1. 1 . A method comprising: (a) preparing a high-surface area nanoparticulate alpha alumina, having a BET surface area of at least 130 m 2 /g and having a particle size in the range of 1 to 50 nm measured by TEM, by a process comprising subjecting a precursor consisting of (A) γ-AlOOH·xH 2 O with x in the range of 0.1≤x≤0.5 and optionally (B) a metal and/or a metal compound to a milling process in a ball mill with a milling jar and balls in a weight ratio of balls to said γ-AlOOH·xH 2 O of 1 to 200 in a temperature range below the conversion temperature of nanocrystalline γ-AlOOH·xH 2 O to γ-Al 2 O 3 to yield said high-surface area nanoparticulate alpha alumina as a milling product; and (b) using the high-surface area nanoparticulate alpha alumina prepared as catalyst or catalyst support for metal catalysts or in ceramics applications.

Description

This application is a 371 of PCT/EP2020/051445, filed Jan. 21, 2020, which claims foreign priority benefit under 35 U.S.C. § 119 of European Patent Application No. 19154220.8, filed Jan. 29, 2019; and German Patent Application No. 10 2019 216 426.9, filed Oct. 24, 2019, the disclosures of which are incorporated herein by reference. The present invention refers to a process for the preparation of a high surface area nanoparticulate alpha alumina from boehmite Aluminium oxide is an important class of material with a wide range of technological applications, e.g. in ceramics, catalysis, skin-care and biomedical products, owing to its outstanding mechanical, chemical, electrical, and optical properties. Besides, alumina has a rich crystal chemistry due to its existence in seven crystallographic forms, namely, chi (χ), kappa (κ), eta (η), gamma (γ), delta (δ), theta (θ), and alpha (α). The first six forms are called as transition aluminas as they can be converted from one to another by annealing and the last one, called as corundum, is the thermodynamically most stable form of alumina. These aluminas are typically prepared by the thermal route which includes calcination of aluminum hydroxide (Al(OH)3, commonly known as gibbsite, produced by Bayer process of bauxite refining) or oxide hydroxide (γ-AlOOH, called as boehmite) at different temperatures (Ivanova, Kinetics and Catalysis 2012, 53, 425-439) as stipulated in ⁢Equation⁢⁢1Al⁡(OH)3⁢→180⁢-⁢300⁢⁢C.⁢γ⁢-⁢AlOOH⁢→550⁢°C.⁢γ⁢-⁢Al2⁢O3⁢→850⁢°⁢⁢C.⁢δ⁢-⁢Al2⁢O3⁢→1050⁢°⁢⁢C.⁢θ⁢-⁢Al2⁢O3⁢→1200⁢°⁢⁢C.⁢α⁢-⁢Al2⁢O3(Eq.⁢1) The formation of transition aluminas in this route occurs via dehydration at mild-high temperature (i.e. up to 550° C.) to form γ-Al2O3, followed by the topotactic transformation of the latter to δ-Al2O3 and θ-Al2O3. Thus, transition aluminas, particularly gamma phase, generally possess a high specific surface area (70-150 m2/g) and are attractive for applications such as in catalysis and adsorption. However, they have poor hydrothermal stability. For example, γ-Al2O3 is widely applied as a catalyst support in Fischer-Tropsch catalyst formulation (Aad et al., ChemCatChem 2017, 9, 2106-2117). However, since water is produced as the byproduct of the Fischer-Tropsch synthesis process; it causes the hydration of γ-Al2O3 to form boehmite according to Equation 2: γ-Al2O3→γ-AlOOH   (Eq. 2) This has an adverse effect on the catalyst and process stability. In contrast, corundum possesses an exceptional hydrothermal stability and can withstand the undesired hydration process, but it has a very low surface area because of its highly energy-uphill nucleation from transition alumina (Apparent activation energy=485 kJ/mol, Steiner et al. Journal of the American Ceramic Society 1971, 54, 412-413) requiring temperature above 1200° C., which leads to uncontrolled crystal growth owing to sintering (Shelleman et al., Journal of Non-Crystalline Solids 1986, 82, 277-285). The α-Al2O3 materials obtained in such a way usually have a low surface area of <10 m2/g. Besides, the thermal route leads to vermicular microstructures of α-Al2O3 which do not achieve a full densification. This is critical for ceramic applications (Yarbrough et al., Journal of Materials Research 1987, 2, 494-515). Thus, it is intriguing to produce nanocrystalline α-Al2O3 with a high surface area (>100 m2/g). This is only possible by reducing the formation temperature of the corundum phase. A method known in the art involves the obtainment of corundum from diaspore (α-AlOOH). The latter is a crystallographic polymorph of boehmite and has a hexagonal close packing of oxygen atoms, alike the α-Al2O3 structure. Thus, this material can be transformed topotactically to corundum at a much lower temperature (500-600° C.) and thus can preserve high surface area (McHale et al., Science 1997, 277, 788-791; Perrotta et al., Materials Research Innovations 1998, 2, 33-38). However, the preparation of stable diaspore is very energy demanding (Yanagida and Yamagichi, Journal of the Ceramic Association of Japan 1966, 74, 94-100; Tsuchida and Kodaira, Journal of Materials Science 1990, 25, 4423-4426), as it involves the hydrothermal treatment of boehmite powder with the seeded growth method (i.e. adding seeds of natural diaspore to a mixture of boehmite and water and optionally a base as well) at 450° C. and 1200 bar for 35 days. This makes the overall process unattractive. Another approach reported in the literature comprises a liquid-feed flame spray pyrolysis of nano-transition-aluminas to obtain nano-α-Al2O3 (Laine et al. Nature Materials 2006, 5, 710-712). However, it suffers from the incomplete transformation to α-Al2O3 and requires a long premixing (24 h milling followed by ultrasonication) of 1-10% alumina in ethanol and subsequent combustion, releasing green-house gas. Besides, from the perspective of applications in catalysis, minor amounts of transition alumina could be a source of catalyst deactivation due to a possibl