GB2391224A

GB2391224A - Activated carbon treated with silicon-containing compounds

Info

Publication number: GB2391224A
Application number: GB0217391A
Authority: GB
Inventors: Thomas Anthony Ryan; Harry Sharrock
Original assignee: KH Technology Corp; Sutcliffe Speakman Carbons Ltd
Current assignee: KH Technology Corp; Sutcliffe Speakman Carbons Ltd
Priority date: 2002-07-26
Filing date: 2002-07-26
Publication date: 2004-02-04
Also published as: GB0217391D0

Abstract

An activated carbon comprises a silicon-containing compound eg a silane or siloxane preferably in amount 0.01-30 wt%. The carbon may be sprayed with a liquid silicon compound or treated with a vapour. Suitable compounds are trimethylsilyl ethanol, hexamethyldisiloxane, methyl- or propyl trimethoxysilane and isobutyl- or octyl-triethoxysilane The treated carbon is more hydrophobic and may be used to adsorb toxic or radioactive components of a mixture in a humid environment.

Description

1 2391 224

IMPROVEMENTS IN ACTIVATED CARBON

Field of the invention

This invention relates to the field of activated carbon and,

in particular, to the treatment of activated carbon to improve its properties.

Background to the invention

Activated carbon may be derived from a range of materials, such as wood, coal and coconut shells by controlled carbonization and steam activation. This treatment results in a relatively inert carbon-based material having a highly porous honeycomb-like structure containing pores with a wide range of dimensions.

Pores of width less than 20 A are termed "micropores", those of width between 20 and 500 are termed "mesopores" and those of width greater than 500 A are termed "macropores". It is thought that the adsorption of substances within activated carbon occurs mainly within the micropores.

There are many applications in which activated carbon may be employed in a moist or humid environment. These include solvent recovery, removal of radioactive contaminants in nuclear installations, gas masks for civil and military use, carbons used for odour abatement, volatile organic compound (VOC) abatement and environmental protection.

However, activated carbon has an affinity for water, particularly at high relative humidity. This propensity to adsorb water prevents the full potential for adsorption of gases or vapours in humid environments from being realized. This problem is particularly relevant when the carbon is highly activated, i.e. where the carbon has a high surface area and relatively large micropore volume. There is a relationship between the activity of an activated carbon and the degree of water adsorption. This activity is most readily measured by the "carbon tetrachloride capacity" (CTC), the surface area and the pore volume of the activated carbon. Accordingly, highly activated carbons used in demanding applications are susceptible to high

( water uptake and retention, which adversely affects their practical use.

Accordingly, there exists a need to improve the performance of activated carbons in the presence of water.

Sumarv of the invention According to the present invention, there is provided an activated carbon comprising a silicon-containing compound.

Preferably, the silicon-containing compound is present in an amount from about 0.01 wt to about 30 wt %.

Conveniently, the silicon-containing compound is present in an amount from about 1 wt % to about 20 wt %.

Advantageously, the silicon-containing compound is present in an amount from about 3 wt % to about 10 wt %.

Preferably, the silicon-containing compound has the formula; RIR2R3Si(CH2),,-OH RlR2R3Si-Hal RARER IS i - X - S i - R4RsR6 R,Si(OR:)( 03) ( ORE) R1R2R IRE S. i where n -0-20 x = 0, S or NR Hal = halogen and each of R., Rid Rim Rat Rs and R6 is independently selected from the group consisting of, hydrogen, optionally substituted, branched and unbranched alkyl, alkenyl, alkynyl or aryl groups.

Conveniently, the silicon-containing compound is selected from; trimethylsilyl ethanol (TMSE) hexamethyldisiloxane (HMDSO) methyltrimethoxysilane (MTMS) propyltrimethoxysilane (PTMS) isobutyltriethoxysilane (i-BTES) octyltriethoxysilane (OTES) Advantageously, the activated carbon is derived from coal, wood or coconut shells.

According to a further aspect of the present invention, there is provided a method of treating an activated carbon comprising the step of treating an activated carbon with a silicon-containing compound.

Preferably, the method comprises the step of spraying a liquid comprising the silicon-containing compound onto the activated carbon.

Conveniently, the method comprises the step of adsorbing a vapour comprising the silicon-containing compound onto the activated carbon.

Advantageously, the method comprises the step of reacting the siliconcontaining compound with the activated carbon.

According to another aspect of the present invention, there is provided a method of purification of a mixture comprising the step of passing the mixture over an activated carbon of the invention, whereby a component of the mixture is at least partially adsorbed by the activated carbon.

Preferably, the component of the mixture is radioactive, coloured and/or toxic.

Conveniently, the component is methyl iodide-131.

The present invention will now be described, by way of example, with reference to the accompanying drawing in which: Figure 1 is a graph of water adsorption isotherms of treated and untreated activated carbon.

It has been found that the treatment of activated carbons with certain agents increases their hydrophobicity, ie that they adsorb less water than untreated carbons, at a certain relative humidity. In particular, the treatment of carbons with silicon-containing compounds can increase their hydrophobicity.

Suitable silylating agents include both activated compounds (eg silyl halides and esters) and non-activated compounds (eg silyl alcohols) with a range of substituents, and include having the following general formulae:

( RlR2R3Sl-(CH7) n OH RR:RSi-Hal RlR:R3Si-X-Si-R4R5R,, RlSi(OR2)(OR$)(OR) RtR2R3R4Si where n = 0-20 x = 0, S or NRZ Hal = halogen and each of Rib R. Rot Rat Rs and Ray is independently selected from the group consisting of; hydrogen, optionally substituted, branched and unbranched alkyl, alkenyl, alkynyl or aryl groups.

Specific examples of such R groups include H. Me, Et, Pr, n-Bu, iso-Bu, tert-Bu, allyl, phenyl etc. and specific examples of suitable silylating agents include: trimethylsilyl ethanol (TMSE) hexamethyldisilaxane (HMDSO) methyltrimethoxysilane (MTMS) propyltrimethoxysilane (PTMS) isobutyltriethoxysilane (i-BTES) octyltriethoxysilane (OTES) The silylating agent or agents may be incorporated in the carbon in a number of ways. The carbon may be dipped into a solution of the silylating agent and then dried, sprayed with the silylating agent, the silylating agent may be vapourised and adsorbed onto the carbon, or the carbon may be chemically activated prior to or during these steps, for example by treatment with a catalyst or base and treatment with an activated compound such as tertbutyldimethylchlorosilane. These steps may be performed at, below or above ambient pressure and temperature.

Examples of the incorporation of sllylatinq agents into activated carbons will now be described, along with properties of the resultant treated carbons.

( Experimental All of the starting materials and reagents used herein are commercially available or have published syntheses. For example, a range of activated carbons are available from Sutcliffe Speakman Carbons Ltd of Lockett Road, Ashton-in-Makerfield,

Lancashire, WN4 ODE, UK. The silicon-containing compounds are available from such sources as the Sigma-Aldrich Company Ltd of The Old Brickyard, New Road, Gillingham, Dorset, SP8 4BR, UK.

Oranosilane Impregnation Method quantity of granular activated carbon, which had been pre-conditioned at 120 C for a minimum of 4 hours in a fan assisted drying oven then cooled to ambient conditions in a desiccator over silica gel, was accurately weighed into a glass dish fitted with a ground glass lid. The glass dish and weighed activated carbon were placed into a temperature controlled heated oven together with the separate ground glass sealing lid. The oven temperature set-point was dependent upon the respective boiling point of the organosilane (or organosilyl) impregnant.

An oven temperature of 20 C higher than the corresponding organosilane boiling point was required.

The amount of organosilane compound to be added was calculated such that it would produce the degree of impregnation required with respect to the weighed activated carbon. The required amount of organosilane-based liquid was weighed into a small glass phial which was then carefully placed, separately, with the heated activated carbon within the glass dish. The heated dish and contents were then sealed by the lid and replaced into the heated oven. Frequent visual inspection determined when full vaporlsation/adsorption of the organosilane compound onto the heated adsorbent carbon had been achieved. The sealed glass dish and contents were removed from the hot oven and cooled. The increase in carbon weight was determined.

Percentage weight/weight organosilane-based addition was calculated by:

( % w/w additive = A x 100/(C + A) where C = original weight of carbon where A = weight increase of carbon Example 1. Trimethylsilylethanol (TMSE) 3% w/w addition Activated carbon: Highly activated coconut shell Dried carbon weight: 1.9400g Boiling point TMSE: 73 C Oven temperature: 93 C Weight increase: 0.06g % w/w TMSE: 0.06 x 100/(1.94 + 0. 06) = 3.0% Example 2. Hexamethyldisiloxane (HMDSO) 6% w/w addition Activated carbon: Highly activated coconut shell Dried carbon weight: 1. 8800g Boiling point HMDSO: 101 C Oven temperature: 121 C Weight increase: 0.12g % w/w HMDSO: 0.12 x 100/(1.88 + 0.12) = 6.0% w/w addition Determination of Water Uptake as a Function of Relative Pressure at 25 C In determining the property of water adsorption, a dynamic method involving the adsorption of water vapour from a generated air stream was considered as the most suitable which would represent adsorption conditions envisaged in practice. An experimental rig, as briefly described below, was constructed to generate air/water mixtures. "Pyrex" glassware and standard ground glass joints were employed throughout. A flow of air was dried and purified by passage through a series of towers containing activated carbon, silica gel, and silica gel + soda lime respectively. The conditioned air stream was divided to pass through two calibrated flow rotameters. The two air streams from the flowmeters were directed though separate glass spirals, which

were immersed in a thermostatically controlled water bath at 25 C. One air stream was saturated with water by passage through a series of two bubbler saturators. The water saturated air stream was then mixed with a controlled flow of air from the second flowmeter in a mixing bottle immersed in the water bath at 25 C. The mixed air stream was passed through a sorption tube at 25 C that contained a weighed quantity of predried activated carbon. By controlling and measuring the air flows through the flowmeters it was possible to calculate the partial pressure of water vapour in the resulting mixed air stream.

If al is the flowrate through the water saturators and a2 that of the pure air stream admixed,and a3 the increase in rate due to the water vapour then the vapour pressure of water, P. is given by: P = (a3 x PA) /al + a2 + a3)and Ps = (a3 x PA)/(al + a2), where PA is atmospheric pressure and Ps the saturation vapour pressure of water at the temperature of the test. Combining the two equations, the relative pressure of the water vapour is given by: P/Ps = al/(al + a2) - (a2Ps/PA) In practice about 0.5g of pre-dried carbon sample was accurately weighed into a previously weighed sorption tube, which was attached to the mixed flow of humidified air aL a set partial pressure. The carbon was allowed time to attain equilibrium with the water vapour air stream. At intermittent times the sorption tube was removed, stoppered and weighed. This procedure was repeated until constant weight had been achieved. Flow rates were then altered to produce a higher partial pressure of water vapour and so on until sufficient points had been obtained to construct the adsorption isotherm. The carbon was then subjected to an airstream flow saturated with water until equilibrium had been achieved. In constructing the adsorption curve the water uptake as g

uptake/lOOg carbon was plotted against the relative pressure of water vapour Pips (ie % Relative Humidity).

Results and Discussion Table 1: Trimethylsilylethanol additions- Effect upon Water Uptake _ Reference No. 1 (control) 2 3 Trmethylsilylethanol/%w/w None 2.1% 3.0% P/Po R.H Water Uptakes /% O O O O O

_ 0.407 40.7 5.2 3.2 T.9

_ 0.499 49.9 10.1 7.1 3.0

_ 0.606 60.7 21.7 16.3 6.5

0.696 69.6 34.9 23.0 10.0

0.805 80.5 64.0 35.4 19.5

0.901 90.1 64.9 53.2 39.8

_. 1.0 100 67.0 55.7 41.4 _

{The carbon used was highly activated coconut shell) Table 1 shows that water uptake has been reduced to around a third of the original value between 60 and 80 % R.H. Measurement of the CTC value after impregnation with 3.0 % of the TMSE showed a value of 9S % relative to an original CTC value of Ill %. This is a small reduction compared to the reduction in water uptake.

Figure l is an illustration of the tabulated data and clearly demonstrates that impregnation of the TMSE significantly reduces water uptake throughout the range of relative humidities O - 100 %. Further exemplification is presented in Table 2. 'this shows the diminution in water uptake for the activated carbon impregnated with a range of organosilyl compounds at 60 and 80 'I, relative humidities:

TMSE Trimethylsilyl ethanol ((CH3)3-Si-CH?CH.OH) HMDSO Hexamethyldisiloxane (CH3) 3-Si-o-Si- (CH,).

MTMS Methyltrimethoxysilane CH3-Si-(OCH3)< PTMS Propyltrimethoxysilane CH3CH?CH2-Si-(OCH<).

i-BTES iso-butyltriethoxysilane (CH3)2CHCH2-Si-(OC,H,)< OTES Octyltriethoxysilane CH(CH:),-Si-(OC H); Table 2: Water Uptake at 60 & 80 % RH | Ref no. Compound Compound 60 % RH | 80 % RH Addition I (% wow) Uptake g/lOOg 4 (control) INone 21.7 164.0 _ _ 5 TMSE 2.1 16.3 35.4

_ 6 TMSE 3.0 6.5 19.5

i7 HMDSO 1.6 14.1 33.5 8 HMDSO 3.1 9.8 32.1

|9 jHMDSO 5.6 10.0 125.2 j10 MTMS 5.2 7.2 l.5 11 PTMS 5.2 7.3 j29.9 i |12 1i-BTES 5.8 5.1 25.0 l 13 TOTES 6.1 j6.3 L29.1 It is to be appreciated that these water-adsorption data are in respect of the equilibrated adsorption of water onto the carbon.

It was noted that the dynamic rate of adsorption of water onto the treated carbons was reduced with respect to that of the untreated carbons.

Impregnated activated carbons are used extensively in the nuclear power industry as a safe-guard media designed for the effective trapping of radioactive fission vapours in the event

f of accident conditions.

In particular, it is desirable to prevent the accidental atmospheric discharge of methyl iodide-131, which has a half-life of 8.05 days. In the event of nuclear leakage the methyl iodide-131, which is thought to represent approximately 3 of the iodine present in the gaseous phase, should be trapped by use of an activated carbon. In these circumstances, the vapours would have high humidity.

Extensive studies have indicated that coal-based carbons which have been chemically impregnated with potassium iodide, potassium tri-iodide, or triethylenediamine compounds (or combinations thereof) provide the most efficient means of trapping methyl iodiode-131 under the demanding conditions of high humidity. A standard test method, as described in UKAEA Report TRG 2497W, is used to evaluate the retention property for radioactive methyl iodide-131 and provides a means of determining the efficiency of an impregnated carbon. The standard test method quantifies the fractional penetration (breakthrough) of a loading of methyl iodide through a carbon bed under the extreme high humidity conditions a plant filter is likely to encounter. The test method is briefly described below.

A test carbon sample is initially equilibrated in an airflow of 96 % relative humidity for 16 hours and the increase in weight then determined. The equilibrated sample is then subjected to a flow of labelled methyl iodide (prepared by methylation of sodium iodide-131 with dimethyl sulphate) calculated to achieve a contact time of 0.2 seconds. The methyl iodide flow is maintained for ten minutes to achieve a typical loading of 50 micrograms of methyl iodide per gram of carbon. The high humidity airflow is then reconnected and maintained through the loaded carbon for an additional two hours. Any iodine-131 species that break through the test adsorbent bed are effectively trapped "dowr.sC ream" by using a series of impregnated carbon traps. Gamma ray spectrometry is used to determine the activity remaining on carbon sample bed and of the penetrated radiator-. on the carbon

( traps. The 'K' value of the sample represents the fractional penetration (i.e. fractional breakthrough) of the carbon bed and can be calculated per the following example: Activity remaining on carbon bed: 40000 counts minute Breakthrough radiation: Trap (1) 150 counts / minute Trap (2) 10 counts / minute Contact time: 0.2 seconds (t) Decontamination Factor (DF): DF= 1 / Fractional penetration of carbon bed = 1 / (150 +10) /40160)

DF = 1 /( 160 / 40160) = 251

K value = logs: DF/ t = 2.39/ 0.2 - 11.9 (seconded) Experimental A test series of samples A, B. C and D was prepared which represented various hexamethyldisilaxane (HMDSO) additions to a 1.5- potassium iodide (KI) impregnated carbon.

Sample references A-D: A: nuclear carbon with 1.5% w/w KI impregnant B: nuclear carbon with 6% w/w HMDSO and 1.5% w/w KI C: nuclear carbon with 1. 5 o w/w KI and 6 w/w HMDSO D: nuclear carbon with 1.5% w/w KI and 12 % w/w HMDSO Sample A represents a typical grade of coal based nuclear carbon impregnated with KI only. The two test samples B and C were formulated to determine the effect of 6 % HMDSO additions both before and after 1.5 KI impregnation, and sample C represents a 12 % w/w organosilane addition applied after a KI

( impregnation. The activated carbon used in the following examples was a steam activated, coal based carbon with a particle size range of 2.0 to 1. 18 mm (i.e. 10 x 16 US mesh) which had been oven dried at 120"C for a minimum of 8 hours and cooled over silica gel prior to impregnation with KI.

Sample reference: A (1.5 % w/w KI impregnation) Activated Carbon: 98.5g Potassium iodide: 1.5g Distilled water: 45 ml Sample reference: B (6% w/w HMDSO impregnation followed by 1.5 w/w KI addition) Activated carbon: 92. 5g HMDSO weight: 6.0g Potassium iodide: 1.5g Distilled water: 45 ml Sample reference: C (1.5% w/w KI impregnation followed by 6.0% w/w HMDSO addition) Activated carbon: 92.5g Potassium iodide: 1.5g Distilled water: 45 ml HMDSO weight: 6.0 g Sample reference: D (1.5% w/w KI impregnation followed by 12% w/w HMDSo addition) Activated carbon: 86.5g Potassium iodide: 1.Sg Distilled water: 45 ml HMDSO weight: 12.0 g

( Sample Preparation: Impregnation techniques.

The KI impregnation method involved dissolving a calculated weight of potassium iodide (to represent 1.5U6 w/w) in sufficient distilled water which would just "wet" a weighed amount of pre-dried activated nuclear base carbon. The KI solution was added to the weighed carbon and thoroughly mixed until the moisture content appeared to be evenly distributed. The carbon / water / KI formulation was oven dried at 105C'C for a minimum of 8 hours prior to either testing or additional impregnation with organosilane compound.

Organosilane addition was achieved by a spraying technique in which a calculated weight of organosilane compound (HMDSO) was finely sprayed onto a weighed quantity of activated carbon which had been previously impregnated with 1.5 % KI by the "soaking" technique described above and oven dried.

Results and Discussion The four prepared samples (A,B,C,D) were tested by the standard 'K' value method described above. Table 3 gives corresponding test values for water uptake (I) and calculated 'K' value. Table 3

= j 9.4 9.8 10.5 _ The test results indicated that an improvement in 'K' value resulted from additions of HMDSO and was to some degree dependent

upon the mode of preparation. K' values for test samples B and C were 4 and 9 higher respectively, than sample A which had no additional organosilane impregnation.

Claims

t CLAIMS:

1. An activated carbon comprising a silicon-containing compound.

2. An activated carbon according to claim 1 wherein the silicon-containing compound is present in an amount from about 0.01 at % to about 30 wt %.

3. An activated carbon according to claim 1 or wherein the siliconcontaining compound is present in an amount from about 1 wt % to about 20 wt %.

4. An activated carbon according to claim 1, 2 or 3 wherein the siliconcontaining compound is present in an amount from about 3 wt % to about 10 wt %.

5. An activated carbon according to any one of the preceding claims, wherein the silicon-containing compound has the formula; RR2R3Si-(CH) n OH RR:R3Si-Hal R:RR3Si-X-Si-R4R5R RSi(OR)(OR3)(OR4) R,R?R3R4Si where n =0x = 0, S or NR Hal = halogen and each of R., R;, R3, R4,Rsand R6 is independently selected from the group consisting of, hydrogen, optionally substituted, branched and unbranched alkyl, alkenyl, alkynyl or aryl groups.

6. An activated carbon according to any one of the preceding claims, wherein the silicon-containing compound is selected from; trimethylsilyl ethanol (TMSE) hexamethyldisiloxane (HMDSO) methyltrimethoxysilane (MTMS) propyltrimethoxysilane (PTMS) iso-butyltriethoxysilane (i-BTES) ocLyltriethoxysilane (OTES)

r (

7. An activated carbon according to any one of the preceding claims which is derived from coal, wood or coconut shells.

S. A method of treating an activated carbon comprising the step of treating an activated carbon with a silicon-containing compound.

9. A method according to claim 8 comprising the step of spraying a liquid comprising the siliconcontaining compound onto the activated carbon.

10. A method according to claim 8 comprising the step of adsorbing a vapour comprising the silicon-containing compound onto the activated carbon.

11. A method according to claim 8 comprising the step of reacting the silicon-containing compound with the activated carbon.

12. A method according to any one of claims 8 to 11 wherein the treated activated compound comprises the silicon-containing compound in an amount from about 0.01 wt % to about 30 wt %.

13. A method according to any one of claims 8 to 12 wherein the treated activated compound comprises the silicon-containing compound in an amount from about 1 wt % to about 20 wt. I'd.

14. A method according to any one of claims 8 to 13 wherein the treated activated compound comprises the silicon-containing compound in an amount from about 3 at % to about 10 at %.

15. A method according to any one of claims 8 to 14, wherein the siliconcontaining compound has the formula; RIR2R3Si-(CH),'-OH R,R2R1Si-Hal RlR2R3Si -X-Si -R4R5R6 RSi(OR,)(OR3)(OR4) R.R?RR4Si where n =0-20 x - O. S or NR Hal = halogen and each of Rib, R;!, R<, R4, R,and R6 is independently selected from the group consisting of,

hydrogen, optionally substituted, branched and unbranched alkyl, alkenyl, alkynyl or aryl groups.

16. A method according to any one of claims 8 to 15, wherein the siliconcontaining compound is selected from; crimethylsilyl ethanol (TMSE) hexamethyldisiloxane (HMDSO) methyltrimethoxysilane (MTMS) propyltrimethoxysilane (PTMS) iso-butyltriethoxysilane (i-BTES) octyltriethoxysilane (OTES)

17. A method of purification of a mixture comprising the step of passing the mixture over an activated carbon as defined in any one of claims 1 to 7 or as produced by the method of any one of claims 8 to 16, whereby a component of the mixture is at least partially adsorbed by the activated carbon.

18. A method according to claim 17 wherein the component of the mixture is radioactive, coloured and/or toxic.

19. A method according to claim 17 or 18 wherein the component is methyl iodide-131.

20. An activated carbon substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

21. A method of treating an activated carbon substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

22. A method of purification of a mixture substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

23. Any novel feature or combination of features disclosed herein.