WO1998025851A1

WO1998025851A1 - Controlled-pore amorphous silicas and process for manufacturing the same

Info

Publication number: WO1998025851A1
Application number: PCT/GB1997/003305
Authority: WO
Inventors: David Richard Ward
Original assignee: Crosfield Limited
Priority date: 1996-12-10
Filing date: 1997-12-01
Publication date: 1998-06-18
Also published as: GB9625630D0; EP0960068A1; BR9713899A; CA2274099A1; AU5130598A

Abstract

A controlled-pore amorphous silica having a surface area of between 10 m2/g and 900 m2/g, a Macropore Diameter of between 1,000 and 10,000 Angstroms can be prepared under alkaline gel conditions.

Description

CONTROLLED-PORE AMORPHOUS SILICAS AND PROCESS FOR MANUFACTURING THE SAME Field of the invention

The present invention relates to controlled-pore amorphous silicas and to a process for manufacturing the same. The present invention more specifically relates to amorphous silicas having pore diameters of at least 1 ,000 Angstroms which are particularly useful as enzyme support in biocatalysis.

Background of the Invention

Enzyme supports play an essential role in biocatalysis. The effect of pore diameter and surface on enzyme efficiency is well known (see for example Use of

Hydrophobic Controlled - pore Glasses as Model Systems, John A Bosley and John

C Clayton, in Biotechnology and Bioengineering, vol. 43, pp.934-938 - 1994 John

Wiley & Sons). In this paper is stressed the importance of having supports presenting pores with diameter above 1 ,000 Angstroms. Available products, which exhibit this porosity, such as XWP Gels can be obtained from Grace Davison. They are described as extra wide silica gels for biotechnological and gene technological applications. They present a pore diameter of 300 to 1 ,500 Angstroms, a surface area of 170 rr /g (for a pore diameter of 300

Angstroms) to 20 m²/g (for a Pore diameter of 1 ,500 Angstroms). Up to now, it has not been possible to propose amorphous silicas having both a high surface area and a high mean pore diameter and there is a need for such a structure.

Another problem is that the production of such high porosity structure is a very difficult and expensive process which renders the product unaffordable for most of its potential applications. There is therefore a need for a process which enables the production of controlled-pore silicas exhibiting high mean pore diameters.

The formation of silica gel at both high and low pH is widely recognised and gelation at low pH has been practised for many years. However, nobody uses high pH systems for the formation of gels, this pH regime is that used for precipitate production. The main reason for this is the fact that the gel times under alkaline conditions are highly sensitive to small variations in acid flow rates and are thus difficult to control at reasonable concentrations. The formation of alkaline gels is recognised in the literature.

It has now been discovered that it is possible to prepare a series of gels with varying silica concentration and partial neutralisation such that a gel forms in a reasonable time. To these gels an electrolyte is added after the gel has set, the neutralisation process being then completed. Adding salt to precipitated silicas which are made under alkaline conditions would normally be expected to lead to the formation of dense low porosity structures. It is therefore surprising that adding salt actually increases the porosity. Tests and Definitions i Surface Area

Surface area is determined by standard nitrogen adsorption methods of Brunauer, Emmett and Teller (BET) using a multi point method with an ASAP 2400 apparatus supplied by Micrometrics of the USA. The samples are outgassed under vacuum at 120°C for at least 1 hour before measurement. Surface area is calculated from the adsorption data measured in the P/Po region from 0.05 to 0.3. The calculation is restricted to the linear region of the BET plot within this pressure range.

Pore Volume and Pore Diameter

The porosity of materials with pore sizes greater than about 500 Angstroms (50 nm) cannot be analysed using nitrogen sorption analysis because of severe theoretical and practical limitations of the method. The maximum pore diameter that can be measured by nitrogen is about 1000 Angstroms and this would be insufficient to allow complete characterisation of a 500 Angstrom pore size material if it had a normal pore size distribution. The best alternative method for the characterisation of such macroporous materials is mercury porosimetry which has an analytical range from about 300 microns to 35 Angstroms.

In the present invention, calculations are based on the procedure of Ritter and Drake using a contact angle of 140 degrees and an interracial tension for mercury of 485 dynes/cm.

Porous materials have a number of regions of porosity when measured using mercury intrusion analysis. The two main identifiable regions are the inter- and intra-particle regions which can be identified from the cumulative intrusion curves. The inter-particle porosity is dependant on the particle size of the material, the particle shape and the packing geometry. The intra-particle porosity is the porosity of interest in the present invention and this too can have more than one component depending on the nature of the material. Such porosity can exist in micro, meso, or macro pore sizes, as defined by the IUPAC convention. In the present invention, the Intra-particle Pore Volume (IPV) is defined as the volume of pores in the region less than 5 microns as measured by mercury porosimetry and the Macropore Volume (MPV) is defined as the contribution to the intra-particle pore volume in the region between 5 microns and 0.05 microns (500 Angstroms) as measured by mercury porosimetry. The pore size of particular relevance in the present invention is in the macropore region, namely greater than 500 Angstroms (or 50 nm). The

Macropore Diameter (MPD) is determined as the pore diameter position of the maximum of the first differential of the cumulative intrusion curve plotted between 5 microns and 500 Angstroms. iii) Degree of Neutralisation (D of N) Sodium silicate can be regarded as a mixture of sodium oxide (Na.O) and silica (Si0₂) and is available with various molar ratios of silica and sodium oxide. When reacted with acid the sodium oxide is converted to a sodium salt and water as illustrated by the reaction: Na₂0 + 2HX = 2NaX + H.O The silica is deposited from solution as an amorphous solid. Knowing the quantity of sodium silicate which is to be neutralised, and the silica to sodium oxide ratio of the silicate, it is possible to calculate the amount of acid needed to complete that neutralisation. If this value is taken as 100% then the Degree of Neutralisation (D of N) is the fraction of that amount of acid which is added in the initial mixing stage expressed as a percentage. Thus at 50% D of N only half of the available alkali is reacted during the initial stage of the process.

Summary of the Invention

According to one aspect of the present invention there is provided a controlled-pore amorphous silica having a surface area of between 10 m²/g and 900 m²/g, a Macropore Diameter of between 1 ,000 and 10,000 Angstroms, the Surface

Area (SA) and the Macropore Diameter (MPD) satisfying the following equation:

MPD > 2300 -(25 x SA).

Preferably, the controlled-pore amorphous silica of the invention has a surface area of between 52 m²/g and 900 m²/g. Preferably the controlled-pore amorphous silica of the invention has an

Intra-particle Pore Volume of between 1 and 3.5 cc/g.

Preferably also, the controlled-pore amorphous silica of the invention has a

Macropore Volume of between 1 and 3.0 cc/g. Most preferably, the Macropore volume represents at least 80% of the Intra-particle Pore Volume. It has been found that the amorphous silica of the invention can be used as a metal support in catalysis. When metals are deposited on a support, due to the high price of the metals used, great attention is paid to the fact that only a limited amount of metal is deposited on sites which are not going to be accessible during the reaction. Therefore, it is important to avoid, as much as possible, the deposition of metal in micropores. It has been found that it was possible to calcine the amorphous silica of the invention, thence eliminating the micropores while keeping the macropores intact. A product is then obtained with almost unchanged pore volume and macropore diameter but in which, the micropores having disappeared, the surface area has considerably decreased. In a preferred embodiment of the invention, the amorphous silica of the invention presents a surface area of between 52 m²/g and 200 m²/g, preferably below 150 m²/g.

According to a second aspect of the present invention there is provided a process for manufacturing controlled-pore amorphous silicas wherein:

• sodium silicate and sulphuric acid are mixed together, the resultant sol being allowed to set as a gel,

• the gel is subsequently broken up and mixed with a solution of electrolyte,

• the neutralisation of sodium silicate is completed later by adding further sulphuric acid,

• the resulting product is then filtered, washed and optionally dried.

Preferably the electrolyte is sodium chloride but sodium sulphate, potassium chloride and potassium sulphate have also been used.

If it is necessary to decrease the microporosity, the washed and dried product is subsequently calcined by heating in air at a temperature between 70ff and 1000°C, preferably between 700° and 800^CC.

A hydrothermal ageing step can be optionally introduced between the gelation and electrolyte addition steps, the electrolyte addition and the second acid addition steps, between the second acid addition and filtration and washing steps and/or any combination of these ageing steps.

The most interesting products for the proposed applications are those made at relatively high silica concentration and low D of N. Typically, the sol presents, before the electrolyte addition, a Degree of Neutralisation of 25% to 30% and a silica content of 12% to 15% by weight. These materials set within a few hours to form an easily processable solid. This is then slurried with an equal volume of electrolyte solution (eg 200g/1 NaCI) and stirred at room temperature for a few minutes.

It is the electrolyte addition that is the key to this process. The electrolyte is added such that the concentration of cations is between 1 and 3.5 Molar with respect to the cation. It should be noted that the electrolyte concentration referred to in the examples is expressed as the concentration in the liquid which is added to the gel in order to form a processable slurry and not the concentration in the final slurry. The electrolyte may be added either as a solution to the milled gel or as a dry solid to the gel after slurrying. The electrolyte causes the gel to form localised clusters which create the macropore matrix.

The electrolyte also promotes the formation of siloxane bonding and increased strength through sodium ion bridging while the gel is in the alkaline condition and thus these wide pores have significant wall strength and do not collapse easily on drying.

Macropore Diameters achieved through this process vary between a few hundred and 10,000 Angstroms (measured by mercury intrusion). The surface areas of the products can be much higher than might be predicted on the basis of the porosity alone using the well known (4,000 * Pore volume/Surface Area) rule. In the examples which have been prepared the surface area covers a wide range. The surface area is a feature of the smaller pore structures in the walls that surround the macropores. The presence of these meso and micro-pores within the structure therefore makes it possible to effect a significant surface area reduction by calcination at temperatures between 700° and 800°C without a significant loss of either the macropore size or macropore volume and the small reduction in pore volume that is observed is associated mainly with the loss of the micro and mesopore volumes. Specific Description of the Invention The present invention will be further illustrated in the following examples.

Example 1

Sulphuric acid and sodium silicate (Na-,0 : 3 Si0₂) were mixed together by pumping the acid and silicate through an in line high speed, high shear mixer head (in line Silverson mixer). The flow rates and solution concentrations were such that the resultant sol had a silica concentration of 15 % (wt/wt) and a D of N of 30%. The sol was allowed to set and had a gel time of about 15 minutes. The gel time is defined as the point at which the resultant sol behaves as a single mass rather than a viscous suspension/solution. It can be identified using a small sample collected in a beaker. When the beaker is tilted the attainment of the gel point is identified by the fact that the sol peals away from the side of the beaker rather than flowing across the beaker. A clearly defined margin corresponding to the position of the meniscus at the beaker wall will be discernible on the inclined gel surface.

The sol was allowed to set and harden up for 48 hours before being coarsely broken up by forcing it through a 3.5 mm stainless steel mesh. A weighed amount of the disintegrated gel was added to an equal weight of demineralised water and solid sodium chloride was added slowly over a period of about 10 minutes with continuous stirring. The amount of salt added was sufficient to create a 200g/1 solution in the added water. The resultant slurry was stirred for 10 minutes at ambient temperature before adding sufficient sulphuric acid solution to complete the neutralisation and achieve a pH of 3.

The white particulate solid that results was isolated and washed with demineralised water using a plate and frame filter press. The material obtained was freeze dried by cooling the sample quickly in liquid nitrogen and placing the frozen solid in an SB4 laboratory freeze drier supplied by Chemlab Instruments. The water was then removed from the sample by sublimation at reduced pressure over a period of 72 hours.

A second sample of the filter cake was dried over night in a laboratory fan oven at 120°C. The freeze dried material had a total intra-particle pore volume of 2.38 cc/g, a macropore volume of 2.09 cc/g and a macropore diameter of 7000 Angstroms and a BET surface area of 674 nrf/g. The oven dried material had a total intra-particle pore volume of 2.17 cc/g, a macropore volume of 1.87 cc/g, and a macropore diameter of 4800 Angstroms and a BET surface area of 536 rrf/g. Example 2

In a second preparation a sample was prepared as above with a silica concentration in the gel of 15% and a D of N of 20%. The gel time was 3 hours 15 minutes and the material was allowed to harden for 24 hours. 400 g of the gel broken into particles was slurried with 400g of a 200g/1 sodium chloride solution. After stirring for 10 minutes the excess alkali was neutralised and the pH adjusted to 3 before filtering and washing using a Buchner filter.

The resultant product was freeze dried and had a macropore diameter of 4200 Angstroms and a total intra-particle pore volume of 3.22 cc/g, a macropore volume of 2.64 cc/g, and a BET surface area of 306 rrfVg. Example 3

This preparation was the same as in Example 2 above with a silica concentration in the gel of 15% and a D of N of 20% but after contact with the sodium chloride solution the slurried gel was aged in a stirred vessel under reflux at 90C for 1 hour. The resulting material after washing and freeze drying had a total intra-particle pore volume of 3.28 cc/g, a macropore volume of 2.98 cc/g, a macropore diameter of 5000 Angstroms and a BET surface area of 78 rrfVg. Example 4

This material was prepared as in Example 1 above with a silica concentration in the gel of 15% and a D of N of 30%. The gel was slurried with a solution of sodium chloride at 100 g/1 and then aged in a stirred vessel at ambient temperature for 1 hour before completing the neutralisation. The product after filtering, washing and oven drying had a total intra-particle pore volume of 1.64 cc/g, a macropore volume of 1.40 cc/g, a macropore diameter of 3000 Angstroms and a BET Surface Area of 704 m²/g.

A freeze dried sample of the material had a total intra-particle pore volume of 2.43 cc/g, a macropore volume of 2.11 cc/g, a macropore diameter of 3000 Angstroms and a BET Surface Area of 857 rrf/g. Example 5

An alkaline gel was prepared as described in Example 1 above with a silica concentration of 15% and a D of N of 30%. The gel was allowed to harden for 10 days before contacting with electrolyte solution. 250 g of roughly milled gel was slurried with 250 g of demineralised water and 50 g of sodium chloride was added. The slurry was stirred at ambient temperature for 1 hour before adjusting to pH 3 with sulphuric acid. The solid was filtered off using a Buchner filter and washed by reslurrying 3 times with demineralised water and filtering. The solid produced was split into two samples and part freeze dried and part oven dried at 120°C. The freeze dried material had a total intra-particle pore volume of 1.36 cc/g, a macropore volume of 1.11 cc/g, a macropore diameter of 3300 Angstroms and a BET surface area of 475 rrrVg. The oven dried material had a total intra-particle pore volume of 1.14 cc/g, a macropore volume of 1.03 cc/g, a macropore diameter of 4200 Angstroms and a BET surface area of 99 rrf/g. Example 6

This sample was prepared as described in Example 5 above but the sodium chloride was omitted and replaced by 63.75g of potassium chloride.

The freeze dried material had a total intra-particle pore volume of 1.89 cc/g, a macropore volume of 1.74 cc/g, a macropore diameter of 5000 Angstroms and a BET surface area of 580 rrf/g. The oven dried material had a total intra-particle pore volume of 1.78 cc/g, a macropore volume of 1.64 cc/g, a macropore diameter of 5000 Angstroms and a BET surface area of 99 πf/g. Example 7

This sample was prepared as described in Example 5 above but the sodium chloride was omitted and replaced by 74.5g of potassium sulphate.

The oven dried material had a total intra-particle pore volume of 1.42 cc/g, a macropore volume of 1.25 cc/g, a macropore diameter of 4200 Angstroms and a BET surface area of 249 n /g. Example 8 This sample was prepared as described in Example 5 above but the sodium chloride was omitted and replaced by 60.72g of sodium sulphate.

The freeze dried material had a total intra-particle pore volume of 1.83 cc/g, a macropore volume of 1.63 cc/g, a macropore diameter of 5100 Angstroms and a BET surface area of 463 rrf/g. The oven dried material had a total intra-particle pore volume of 1 .80 cc/g, a macropore volume of 1 .65 cc/g, a macropore diameter of 5200 Angstroms and a BET surface area of 487 rrfVg. Example 9 250 kg of alkaline gel, with 30% degree of neutralisation and 15% silica, was made into 210 litres kegs by mixing a 9.5% (wt/wt) solution of sulphuric acid, at a flow rate of 0.172 litre/min, with a 19.6% (as SiO, wt/wt) sodium silicate (3.3 ratio) solution, at a flow rate of 0.49 litre/min. This was left for 24 hours before an equal volume of water was added to each keg and the gel was broken up with a paddle. The resulting slurry was added to a 400 litres vessel and the vessel agitator (straight six-blade turbine) was used to break the gel down into a slurry.

42 kg of sodium chloride was added to the slurry over approximately 10 minutes, followed by sulphuric acid (over approximately 15 minutes) to lower the pH value to 3. The slurry was dropped into a plate and frame filter-press and washed with demineralised water. The resultant filter-cake was oven-dried at 120PC. The oven dried material had a total intra-particle pore volume of 1 .49 cc/g, a macropore volume of 1.32 cc/g, a macropore diameter of 1500 Angstroms and a BET surface area of 318 m²/g. Samples of this oven dried material were calcined in a static furnace by heating from room temperature to 700°C and 800°C and holding at these temperatures for 1 hour. These materials had the following properties:

After calcination at 700°C the material had a total intra-particle pore volume of 2.19 cc/g, a macropore volume of 1 .96 cc/g, a macropore diameter of 1300 Angstroms and a BET surface area of 163 rrf/g.

After calcination at 800°C the material had a total intra-particle pore volume of 1.86 cc/g, a macropore volume of 1 .81 cc/g, a macropore diameter of 1700 Angstroms and a BET surface area of 34 rrf/g.

Claims

1 Controlled-pore amorphous silica having a surface area of between 10 rrf/g and 900 m²/g, a Macropore Diameter of between 1 ,000 and 10,000 Angstroms, the Surface Area (SA) and the Macropore Diameter (MPD) satisfying the following equation:

MPD > 2300 -(25 x SA).

2 Controlled-pore amorphous silica according to claim 1 having a surface area of between 52 m /g and 900 m²/g.

3 Controlled-pore amorphous silica according to claim 1 having an Intra-particle Pore Volume of between 1 and 3.5 cc/g.

4 Controlled-pore amorphous silica according to claim 1 , 2 or 3 having a surface area of below 200 m²/g, preferably below 150 m²/g.

5 Controlled-pore amorphous silica according to claim 1 , 2 or 3 having a surface area of below 150 m²/g. 6 Process for manufacturing controlled-pore amorphous silicas wherein:

• the gel is subsequently broken up and mixed with a solution of electrolyte,

• the neutralisation of sodium silicate being completed later by adding further sulphuric,

• the resulting product is then filtered, washed and optionally dried.

7 Process according to claim 6 wherein the sol presents, before the electrolyte addition, a Degree of Neutralisation of 25% to 30% and a silica content of 12% to 15% by weight.