WO2001074713A1

WO2001074713A1 - Improved mesoporous catalysts

Info

Publication number: WO2001074713A1
Application number: PCT/AU2001/000371
Authority: WO
Inventors: Gao Qing Lu; Huai Yong Zhu
Original assignee: The University Of Queensland
Priority date: 2000-04-03
Filing date: 2001-04-03
Publication date: 2001-10-11
Also published as: AUPQ665000A0

Abstract

A method of preparing activated alumina catalysts, which have large pore size and greater resistance to the effects of high temperature. The method includes the steps of; forming an aqueous aluminium hydroxide precipitate from mixing an acidic solution and a basic solution; mixing it with a low molecular weight, water dispersable surfactant; forming a homogeneous composite of aluminium hydroxide and surfactant through keeping the aluminium hydroxide and surfactant in a closed environment at approximately 100 °C; and calcining the aluminium hydroxide alumina precipitate and surfactant mixture at greater than 100 °C to eliminate the surfactant and form the activated alumina (AI2O3).An alumina catalyst having a mesoporous structure, formed from nanometer particles of alumina arranged so as to create interstices.

Description

"IMPROVED MESOPOROUS CATALYSTS" The invention relates to a method of preparing improved catalysts having high surface area and defined pore structure.

BACKGROUND TO THE INVENTION

Materials such as activated alumina are widely used as catalysts, catalyst supports and adsorbents. Activated alumina is currently produced by thermal decomposition of hydrated aluminas. The main uses of activated alumina are for drying gases (air, He, H₂, natural gas, etc), drying liquids (hydrocarbons, petroleum products, alcohols, etc), Claus catalyst (for sulfur recovery from H₂S), dehydration of alcohols, isomerization, cracking and reforming of hydrocarbons, autoexhaust catalysts, and chromatography. The world production of activated alumina exceeds a few million tons a year. The pore structure of the activated alumina affects the performance of the catalysts or adsorbents. It is desirable to have controllable average pore size and a controllable pore size distribution. Control of these properties has not been possible with the known techniques for producing catalysts. The inventors are aware of United States patent number

US2697066, in the name of Sieg, which describes a method of producing gel-type inorganic oxide catalysts that involves the addition of large molecular weight organic material. Sieg states that organic material having a molecular weight in the range 1x10³ to 1x10⁷ is required. Furthermore, Sieg claims that organic materials possessing a molecular weight of below 1 x 10³, such as simple sugars and the like, do not effect any material improvement in the available surface area of the catalyst. As a result Sieg only demonstrated the use of large organic/naturally occurring polymers having an effect on catalyst porosity. Sieg achieved this higher porosity through the larger size of the organic polymer molecules being incorporated in to the hydrogel framework of the catalyst prior to a drying step. In addition, the material used by this process is not recoverable from the process and does not allow for good control of catalyst characteristics. The use of water-soluble polymers for the control of pore size and pore size distribution in alumina and silica gels has also been reported (Basmadjian et. al., J. Catalysis, 547-563, 1962). The paper describes the use of synthetic polymers in place of the natural polymers used in the Sieg process. Basmadjian specifically mentions that the prior art processes (including Sieg) were seriously limited by the availability of soluble polymers and suggests that the controllability of catalyst characteristics was unacceptable.

The Basmadjian process works in a similar manner as the Sieg US Patent. However both Basmadjian and Sieg produce catalysts with a broad pore size distribution and less controllability.

Despite a number of researchers proposing methods of producing catalysts with controlled characteristics, the problems have not been adequately solved.

OBJECT OF THE INVENTION

It is an object of the present invention to provide an improved method of preparing mesoporous catalysts, particularly activated alumina.

It is also an object to provide improved catalysts and catalyst supports prepared by the method.

Further objects will be evident from the following description.

DISCLOSURE OF THE INVENTION In one form, although it need not be the only or indeed the broadest form, the invention resides in a method of preparing activated alumina including the steps of: forming an aqueous aluminium hydroxide precipitate from mixing an acidic solution and a basic solution; mixing the aqueous aluminium hydroxide precipitate with a low molecular weight, water dispersable surfactant; forming a homogeneous composite of aluminium hydroxide and surfactant by keeping the aluminium hydroxide and surfactant in a closed environment at approximately 100°C; calcining the aluminium hydroxide alumina precipitate and surfactant mixture at greater than 100°C to eliminate the surfactant and form the activated alumina (AI₂O₃).

In preference the surfactant is a short chain polyethylene oxide having a molecular weight in the range 400-730. Preferably the water dispersable surfactant forms irregular micelles within the aqueous aluminium hydroxide precipitate.

More preferably, the acidic solution has glycerol added to assist in the precipitation of smaller particles of aluminium hydroxide.

The step of calcining is preferably at a temperature of greater than 200°C. The method may further include the step of collecting and recycling surfactant eliminated in the drying step and the calcining step.

A feature of the resultant catalyst from the above method is not only more controllable pore size and pore size distribution but also that the catalyst can withstand treatment, use or regeneration under high temperatures without any significant affect to the catalyst structure or activity.

In another form of the invention provides an activated alumina catalyst or catalyst support having a mesoporous structure, formed from particles of alumina, which are arranged to create interstices, wherein the particles of alumina have their largest dimension in the nanometer range.

Preferably the interstices of the activated alumina catalyst or catalyst support have a diameter in the range of 10 to 400 A. More preferably the interstices have a mean diameter in the range of 80 to 160 A. The activated alumina catalyst or catalyst support suitably has interstices with a mean diameter in the range of 120 to 220A, when subjected to heat treatment at temperatures greater than 900°C.

More suitably the activated alumina catalyst or catalyst support has a surface area equal or greater than 200 m²/g and/or has porosity between 1.4 and 2 cm²/g.

BRIEF DETAILS OF THE DRAWINGS To assist in understanding the invention preferred embodiments will now be described with reference to the following figures in which: FIG 1 is a flowchart showing a method of forming an activated alumina;

FIG 2 is a plot of pore size distribution for a first example of the method of FIG 1 ;

FIG 3 is a plot of pore size distribution for a second example of the method of FIG 1 ;

FIG 4 is a TEM photo of the alumina catalyst of the example

FIG 5 is a sketch of the first stage in the production of an activated alumina; and

FIG 6 is a sketch of a later stage in the production of an activated alumina.

DETAILED DESCRIPTION OF THE DRAWINGS

In the drawings, like reference numerals refer to like parts.

Referring to FIG. 1 there is shown a flowchart that outlines the steps for producing an activated alumina with a well-controlled pore size distribution. Aluminium hydroxide is precipitated in the conventional manner from an acid and a base, but not dried. A suitable amount of the wet aluminium hydroxide precipitate is mixed with a surfactant and stirred until a high degree of mixing is achieved. The mix is the kept in a closed environment at 100°C to form a homogeneous composite of the aluminium hydroxide and the surfactant. Preferably the closed environment is a sealed container.

A supernatant liquid of mainly surfactant and water remains after this stage. It may be collected and recycled.

The resultant aluminium hydroxide and surfactant cake is then calcined at above 100°C to eliminate surfactant from the aluminium hydroxide and convert the aluminium hydroxide to activated alumina, thus forming a catalyst. The catalyst of this process is found to have a larger average pore size and narrower pore size distribution than prior art catalysts. Furthermore, the pore size and pore size distribution can be controlled by careful control of the base, acid and surfactant combination.

The person skilled in the art would appreciate that the term catalyst is used to generically describe the resultant activated alumina. It would also be appreciated that the final product of activated alumina (the catalyst) can be used as a catalyst per se or a catalyst support.

EXAMPLES Example 1 A series of catalysts were prepared from NaAIO₂ and acetic acid with varying quantities of polyethylene oxide as the surfactant. A base solution was prepared by dissolving 37.6g of NaAIO₂ in 100ml of water. An acid solution was prepared by adding 20g of glycerol into 100ml of 5N acetic acid solution. The base solution was added dropwise to the acid solution under vigorous stirring. White precipitate (alumina) was formed during the addition. The pH of the final solution was 8.8.

The precipitate was separated by centrifugation at 2500rpm for 15 minutes. To remove Na⁺ ions, the precipitate was washed three times by dispersion in deionised water and centrifugation. The washed precipitate was divided into five equal portions and each portion was mixed with a different amount of low molecular weight polyethylene oxide surfactant.

The surfactant used is known as Tergitol 15-TS-7 and is available from Aldrich Chemical Company. It is a polyethylene oxide (PEO) material having a molecular weight about 500.

The precipitate and surfactant were stirred for one hour and then transferred to an autoclave and kept at 100°C for two days. Supernatant liquid was collected and the remaining white cake allowed to dry in atmosphere. The supernatant liquid is primarily surfactant with some water content. The liquid can be collected and reused in the process. This feature has advantage in industrial-scale applications by reducing reagent costs.

The dried white cake is calcined at 500°C for 20 hours. The calcination temperature was raised from ambient to 500°C at a rate of

2°C/min. The PEO surfactant is evaporated in the temperature range between 150°C and 250°C. The surfactant can be collected in a cooling trap during this phase and reused.

The specific surface area and porosity of the five samples were measured by N₂ adsorption-desorption at liquid nitrogen temperature

(-196°C) on a surface area and porosity analyzer (NOVA-1200) using

0.1~0.2g samples after a degassing period of three hours at 300°C.

The following table presents the results of the measurements.

Table 1 These samples were then heated at 900°C for three hours. The surface area and porosity data of the products is shown in the following table.

Table 2 As can be seen from the two tables, the addition of the surfactant results in a controlled increase in the peak of the pore size distribution. The width of the pore size distribution is also well controlled as can be seen from FIG 2. FIG 2 shows the pore size distribution as a function of pore diameter for a subset of the preparations in Table 1. In FIG 2, MR is the molar ratio of surfactant used in preparing the samples. Example 2

A base solution was prepared from 26.0g of NaAIO₂ dissolved in 500ml of water. An acid solution was prepared by dissolving 241.5g of AICI₃»6H₂O in 500ml of water. 10g of glycerol was added to 70ml of the acid solution and 350ml of the base solution was added drop-wise under stirring. White precipitate was formed during the addition. The final pH of the suspension was controlled to about 6. The precipitate was then separated by centrifugation at 2500rpm for 15 minutes.

The precipitate was washed three times by dispersing in deionised water and separating by centrifugation to remove Na⁺ and Cl^" ions. The washed cake was mixed with polyethylene oxide surfactant, Tergitol 15-TS-7. The amount of surfactant was varied from Og to 214g to obtain a range of samples. After prolonged stirring of one hour the mixture was transferred into an autoclave and kept at 100°C for two days. Supernatant liquid was poured out and the white cake at the bottom was dried in atmosphere. The dried white cake is calcined at 500°C for 20 hours. The calcination temperature was raised at a rate of 2°C/min from ambient to 500°C.

The specific surface area and porosity was measured in the manner described above. The following table summarises the results.

These samples were then heated at 900°C for three hours. The surface area and porosity data of the products is shown in the following table.

FIG 3 shows the pore size distribution as a function of pore diameter for a subset of the preparations in Table 1. As with FIG 2, it can be seen that a desired pore size distribution can be achieved by careful selection of the molar ratio of surfactant to aluminium. Example 3

A number of alumina catalyst samples were produced in a similar manner to Example 1 and heated to 1200°C to determine the effect of calcining temperature on the pore size of the resultant catalyst. The results are summarized in the table below.

Table 5 It can be seen from Table 5, when heated to 1200°C, the sample prepared with the surfactant retained significantly larger specific surface area and porosity, when compared with those samples prepared without surfactant, ie the surface area and pore volume of an alumina prepared with surfactant are over 10 times of those samples prepared without surfactant. The samples prepared with surfactant have significant resistance to high temperature heating. This is very important when the solids are used as the supports of catalysts working at high temperatures, for instance, combustion catalysts. Materials with this property are rare and normally difficult to make and therefore are expensive.

The mesoporous structure of the alumina catalysts of examples 1 to 3 is shown in the TEM photo of FIG 4. As can be seen from the tables and plots, a desired pore size distribution and total pore volume can be achieved by control of the quantity of surfactant used. Control of catalyst characteristics is achieved to a greater degree than has been possible with prior art methods. The inventors believe that this is due to the mechanism of formation of the catalyst.

The inventors speculate that the mechanism of formation of the catalyst is as depicted in FIG 5 and FIG 6. The white precipitate forms particles in the nanometer range that stack together to form small interstices that are filled with water as shown in FIG 5. Adding surfactant forces the interstices to enlarge as they fill with surfactant and water as shown in FIG 6. During the first heating at 100°C, part of the water is squeezed out of the enlarged interstices by the surfactant micelles, which remain and therefore maintain the enlarged interstices. The aluminium hydroxide particles set in the positions during the heating step thereby forming a composite meso-structure of aluminium hydroxide. The subsequent heating at higher temperatures eliminated the surfactant and strengthens and forms the catalyst or mesoporous alumina structure.

The pore size and total pore volume is determined by the quantity of surfactant rather than the size of the surfactant molecules, as is the case in the prior art techniques that utilise high molecular weight polymers.

It has been found that when the same weight of surfactant is used, the use of the higher molecular weight surfactants result in an alumina of a lower porosity. This surprising result over what is taught in the prior art, particularly the Sieg US Patent, is believed to occur because there are more molecules of the lower molecular weight surfactant within the solution prior to addition to the alumina precipitate and the formation of micelles by the surfactant.

The dispersable surfactant forms micelles of irregular shape within the dispersed phase of a solution. These micelles are unevenly dispersed and incorporated within the hydrogel framework of the aluminium hydroxide and surfactant mixture. In the aluminium hydroxide precipitate and surfactant mixture, the aluminium hydroxide is aligned or orientated around the surfactant micelles. This arrangement also traps within it some water. After forming the homogeneous mixture of aluminium hydroxide and surfactant at 100°C over two days, most of the trapped water is driven out of the aluminium hydroxide-surfactant mixture. The physical structure of the aluminium hydroxide is stabilised due to the support provided by a number of the smaller surfactant molecules incorporated into the aluminium hydroxide precipitate and surfactant mixture. When the surfactant is finally removed in the calcining stage the aluminium hydroxide is converted to alumina, which maintains it's structural arrangement achieved after drying with minimal or no collapsing of its structure.

The use of glycerol in the acidic solution, to precipitate the aluminium hydroxide, is to form small particles of precipitate. The aluminium hydroxide has a strong affinity to the glycol and tends to be adsorbed onto the surface of the aluminium hydroxide particles. The resultant precipitate has a larger surface area and pore volume.

An additional advantage and result of the current process is to produce a catalyst, which can withstand treatment, use and regeneration at high temperatures, between about 700 - 900°C, without any significant affect to the catalyst structure or activity.

The resultant catalyst of mesoporous alumina, produced by the above method, has large porosity and greater surface area. The surface area is typically equal to or greater than 200m²/g, whilst the porosity is between 1.4 and 2cm³/g. This larger porosity is maintained after heat treatment at very high temperatures, typically 900 °C. This is in comparison to conventional prepared γ alumina with porosity of less than 0.5cm³/g and surface area of between 50 and 60m²/g.

It has been found that a copper catalyst supported on alumina produced by the above process, when calcined at high temperatures

(900°C) exhibits exceptional performance for NO_x reduction compared to that the catalyst supported on alumina's produced by other methods. The larger the surface area of the alumina support allows for more active sites of copper can be formed. Thus resulting in a better performing catalyst. In addition, when alumina type catalysts are used for the oil cracking process, the catalysts have to be regenerated at high temperatures (700-800°C). The structure of the mesoporous alumina prepared by the above process can withstand such heating treatments without serious damage to the structure and thus the performance of the catalysts is maintained.

Using the described method it is possible to obtain desired characteristics of a catalyst by selecting an appropriate acid, base and surfactant combination.

The inventors have found that the advantages of the method includes:

• More effective control of pore structure of mesoporous aluminas over a relatively wide pore size range;

• Products have high surface area and high porosity compared to prior art methods;

• Products maintain large porosity even after heating at very high temperatures;

• Surfactant can be recycled and reused to achieve economic savings and reduced pollution; and

• The preparation conditions are mild and simple. Throughout the specification the aim has been to describe the preferred embodiments of the invention without limiting the invention to any one embodiment or specific collection of features.

Claims

CLAIMS:

1. A method of preparing activated alumina including the steps of: forming an aqueous aluminium hydroxide precipitate from mixing an acidic solution and a basic solution; mixing the aqueous aluminium hydroxide precipitate with a low molecular weight, water dispersable surfactant; forming a homogeneous composite of aluminium hydroxide and surfactant by keeping the aluminium hydroxide and surfactant in a closed environment at approximately 100°C; and calcining the aluminium hydroxide alumina precipitate and surfactant mixture at greater than 100°C to eliminate the surfactant and form the activated alumina (AI₂O ).

2. The method of claim 1 , where in the surfactant is a short chain polyethylene oxide having a molecular weight in the range of 400-730.

3. The method of claim 1 , wherein the surfactant is a short chain polyethylene oxide having a molecular weight of 500.

4. The method of claim 2, wherein the surfactant forms irregular micelles within the aqueous aluminium hydroxide precipitate.

5. The method of claim 1 , wherein the acidic solution has glycerol added to assist in the precipitation of smaller particles of aluminium hydroxide.

6. The method of claim 1 , wherein the calcining step is carried out at temperatures greater than 200°C.

7. The method of claim 7, wherein the calcining step is carried out at temperatures equal to or greater than 500°C.

8. The method of claim 1 , further comprising the step of collecting and recycling surfactant eliminated in the drying step and the calcining step.

9. An activated alumina catalyst or catalyst support, when prepared by the method of claim 1.

10. An activated alumina catalyst or catalyst support having a mesoporous structure, formed from particles of alumina which are arranged to create interstices, wherein the particles of alumina have their largest dimension in the nanometer range.

11. The activated alumina catalyst or catalyst support of claim 10, wherein the interstices have a diameter in the range of 10 to 400 A.

12. The activated alumina catalyst or catalyst support of claim 10, wherein the interstices have a mean diameter in the range of 80 to 160 A.

13. The activated alumina catalyst or catalyst support of claim

10, wherein the interstices have mean diameter in the range of 120 to 220A, when subjected to heat treatment at temperatures greater than 900°C.

14. The activated alumina catalyst or catalyst support of claim 10, having a surface area equal or greater than 200 m²/g.

15. The activated alumina catalyst or catalyst support of claim 10, having porosity between 1.4 and 2 cm²/g.

16. The alumina catalyst of claim 10, having a surface area equal to or greater than 200 m²/g and porosity between 1.4 and 2 cm²/g.