CA1081715A - Preparation of linear alkylbenzenes - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Process for the preparation of alkylbenzenes by partial chlorination of linear C9 - C15 paraffins, catalytic alkylation of benzene with the resulting mixture and fractionation by distillation of the alkylation products.
At least a part of the unreacted paraffins recove-red by fractionation and to be recycled to the chlorin-ation stage is submitted, continuously or intermittently, to a purification treatment with molecular sieves.
The content in undesired by-products of the alkyl-benzenes can thus be maintained at negligibly low values.

Description

, Th~ present in~ention rel~tes to .imprvvements in the preparation Or linear alkylben~.enc3 in t~rhich the alkyl radical consists oI a linear chain contairling~ rrom g to 15 atoms of car~on, Such prod.ucts~ as is known, are Or great i.mportance in the preparation o~ b:i.ologically-degradable detergents.
Alkylbenzenes ha~ing alkyl subs*ituents consisting - of a linear paraffi.n chain are normally preparcd, on an industrial scale, by means of chlorination of the linear.
para~fins, allcylation of benzene, in the presence of ..
-- - --.Friodel Crafts cat~lysts, with the chlorinated paraffins .thus obtained, and ~r~cti.onation of the alkylated product, a~ter sepQration of the catalyst~ into thc component members o~ the product by me~ns o.f disti.llation.
In such processes, in ~rder to obt~in the ma~ num yield of monochloroparaffinsS the chlorination stage is carried ~ .
. , - - out with a high para~fin/chl.orine molar ratio.~As a r~sul~

part Or bhe~paraffins does not react.

The~mixture of chlcrinated paraffins~and unreaoted ~ parafrins thus obtained, given the dif~lculties Or separation, : : : is~con~eyed directly to.the alkylation st-agre. The unleacted ~.
.
~ ~- : paraffins nre~ subsequently`reco~ered by distillatio~ Or tbe ~
: ~: : :, .
nlkylation products and recycled to ths chlorinati.on stage.

` In ~he;processes under ~isoussion, there i9 a ~radual : ~ ~ ; accumulation of b~-~roducts5 both in tho r~cyc3.ed para~ins and ~n tho alkylbenzenes produc~d, .
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In particular, the accumulation Or ch~orinated products in the alkylbenzenes is such that the chlorine content may reach undesirable values; more~ver, due to the presence of aromatic by-products,the alkylbengene~ often give disagreeable odours.
Similarly, with regard to the impurities accumulating in the.recycled paraffins, t~le most harmruJ and consistent are due to chlorinat.ed and unchlorinated aromatic by-productsO
These impurlties, when they are recycled together ~ith the parafflns, may undergo further modifications in the chlori-nation and akylation stages, ~ith ~ormation of heavier products which separate in the subsequellt distillatioll ;
stage with the alkylbenzenes.
In each case~ the alkylbenzenes obtained are rich in lmpurities, especially chlorinated products, which orten give disagreable odours and have other undesirable chal~ac-teristics, Fspecially corrosive and toxic properti~s~ All this creates problems which are difficult to solve~ gi~en ~ :
that it is nec ssary for the alkylbenz~ne~ used in cornmer- :
cial deter~ent rormulations to be practicall~ to~ally free of chlorine and disagreeable~odours.
A process for the production Or linear:alkylbcnzen~s by alkyla~tion o~ benzene uith chlorinated pararflns, br means Or which it is possible to a~oid the disad~an1:a~es descri.bed a~o~e t ha~ ~lready forn]ed the subject of Italian Patent Specification ~o~.83q,022. n particular, : , :
. : . ' .

, by mearls of t}liS procoss, it is possible to obtain linoar alkylbcn7.enes wit}i an ext:rcmely low content of by-products, especially chlorinated products, and ~ree ~rom undesirable odours.
~ re precisely, according to this process, the recycled products, consisting essentjally Or pararfins, a~e fiub~ected to a hot treatment with sulplluric acid, oleum or sulphur trloxide, and the product obtained is decanted, after cooling, so as to separate the acldic sludgeis. The paraffi~s . , .
finally isolated are recycled to the chlorination stage.
Thls process allows hi-gh quality~ llnear alkyl~e~elles to be obtained dir~ctly wlthout the necessity o~ further treatments ~f th~ alkylbenzelles. It should bc noted in tiliS connection thatt to obtain high quality~ llncar allcyl-ben~enes directly in a oontinuous proces5~ it io-not surricient to use pure~ or practi~cally pure~ ~resll~para~fins but it is essential to:subject the rec~cled paraffins to :
the~treatn~ent described:above. ~his proce-ss9 however~ ~ -results in a loss, if only less than .S~ of the paraffins which~are subjected to the acid treatment and requires9 moroover~ a series of operative stages. ~
W~;have now found that it is possible to obtain~ in a more:econornic wa~, and with a simpl~r method;o~operation, linear Qlkylbenzen~s havln~ still furtller irnproved char~c-$erist-ics-without substa~tial loss of product.~
`Thus, the invention provides a proc~ss ror the preparation .
~ 4~

~;: ~ ' :

'7~ 5 of alkylben~enes by partial chlorination of linear paraffins having from 9 to 15 carbon atoms per moleculo, catalytic alkylation of benzene using the resulting mixture of chlori-na*ed and unreacted p~raffins, .fractionation by distillntion, after separation of the catalyst~ of the alkylati.on products thus obtained and recycling of the~reacted paraffins recovered from thc fractionatlo.n stage to the partiai chlorination stage, characterized in that at least a part of the unreac-ted paraffins to be recycled to t}le partial chlorinat.ion stage is submitted, continuously or intermittent~y, to a.
purlfi.cation treatment with molecular sieves.
This treatment may be caxried out, in a conti.nuous ..
or a discontinuo~1s manner~ on the whole of the recycled paraffins or only on part of these,-so as to elimi.nate the . impurities either substalltially completely or so as:to keep them below the critical limits.
~In every case, by means of the processof the present , invention~ lt is possible to obtain linear alkylbsnzenes .~ of high:quallty, ~ree from disngreeable odours, with a -chlorine content less than 100 ppm and ~ith a content of : other impurities normally present~ in particular aromatic - ~ ..._.
by-products~which is substantia~lly irrelevant.
In practice, the recycled paraffi~ls~ rich ln~impurities, are passcd through one or more beds of molecular sieves.

- - :: ~: : - ~ : . .
.GenerAlly:ri~Ad bods are used. ~lolecular s~ieves based on zeolites have be~n showrl to ~e particularly useful. These '7~5 are kno~n comm~rcially, for example, as molecular sieve6 of the type X and of the type Y, providcd, for example, -3y t}le Union Carbide Corporation Linde Division and have porc sizes varying between about 9 and 10 Anclstrom.
The molecular siev~s of the type X have the composition M20 A123 2 7 Si2 or M O.Al203.2.7 SiO2 - where M may be:
~ a (sieves type 13 x), Ca (sieves 10 x), K , Mg , Ba , NH4+ or in general, an alkali or alkaline earth metal.
Nevertheless molecular sieves o~ the type X in which the alkali metal cation is substituted by other metals such as Ag , Cu +, Co or metals of the rare earth group may also be used with profit. In the molecular sieves of the type X the Si~Al atomic ratio has a value close to 1.
. . : : .
The molecular sieves of the type Y are, however, much more rich in silica. The Si/Al atomic ratio is generally from 1.5 to 3. ~ ~
. - - .
- A typical composition of a molecular sieve of the type Y~is in fact the followinq (where the percentages are by weight)~

~Na20 13.5% Al203 21.3~ SiO2 65.~2~
The shapes and dimensions of the molecular sieves ysed are not a critical factor. ~The aforesaid adsorbents X and Y~may hence be used ln the form of powders, of small spheres or ~o~ cylindrical extrùsions of variable sizes, such as~are . : ~

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~ 5 available comlllercially.
It hasalso been ascertained that the avera~e ad~orbent capacity of the molecu]~r sieves used, expre~sed in grams adsorbed per grams of adsorbent, varics from 8,~ to 15/0 for the aromatic by-products and the other types of secondary impurities present ill the paraffins.
~ he temperature at which the passage of the paraffins through the bed of molecular sieves is carried Ollt may be chosen-within a relatively ~ide range. The best result~
ha~e been obtainsd by operating at temperatures of from amblent temperature to 200C. l`he operating pressure is -preferably from 1 to l0 atmospheres. -Th~ a~era6e residence tirr.e of the paraffins on tllemolecular sieves is generally from 5 minutes to 60 minlltes.
0f particular irnportance i~ the fact~that~ ~y o~erating according to the ~anner described above, praotically cornplete purification of the recycled paraffins may be obtained and the residual contents ofi chlorinated and unclllorinated .
aromQtic~compowlds and of all the otller seoondary impul~ities c~ntained~thereill may be reduced to levels lower than those .
deteo~able ~y flal~e eas-ollromatography.
It should also be noted thQt, di~feri~ne from sevoral kn~wn methods for the purification of recycled~paraffins, the method of;thc invelltion does not present pro~lem6 Or mixing and pollution of the~said p~raf`fins with chemical agents whicl~ ~ust then bc removed, nor problems of corrosiOn . :

s of' the plant or pollnt:ior~ Or t~le environmcnt by di s~har~e o f harm ful by-prt~ duc t s .
Moreover~ the efficlency of the purlfication ls ~rac-ticall~ constant whatever the initial concentration of the impuri ti e 4 in the pa~a ffins .
In practice, as already indicated, the purification treatment is carried out by passing the flow of paraffins to be purified over one or rnore fixed or moving beds of molooular sie~res which, once e.~austed, may convenientlybe regenerzte~ by means of' therm~l stripping or of displacelllent by elutiol of the adso~.~bed si~bstanccs. A pre-treatmont .of stripping with inert ~as is ~enerally carri~d out l~efore the regeneration treat~ncnt, to recover the unreacted parafflns retained in the molecular sievcs.
In th'e more frequent case in which it is desired' to oarry out the purification continuously, it is .evident ~ :
tllat the installatio~ of .at least. t.~o .absorptio.n colulT~ns or ~eviccs which work alternate]~ is neoessary.
-- ~ The purificatiol~ treatinent may be carri.ed out as follows:

.: ~ a) ~ passa~e of the paraffins tllrougll ~the absorbent untll ~ the~ ~latter is e~ austcd~ ~rhich will be indicated by the ,. : , .
reappearance of tlle impuri-tie~3 in thc para~ffi.ns which lea~e the abso~rpt;ion apparatus; and deviation of tho pn~affill - flow to: be purif'ie~l towards new, "freshi' molecular si.-sves;
b) stripping wi.th inert gas at a temperature of from ;an~Dient temperature to 1 SO~C, and at a pressnre of from 1 :- ~

7.~5 .~

to 10 atmospheres, Or the paraffin~ retnined mechanically bet~een the partlcles of the exhausted absorbent~
c) periodical regeneration of the exhausted molecular sieves.
Tho purified paraffins leaving the absorption apparatus are recycled directly to the chlorination reactor. The chlo~i-nation mixture is hence fed directly to the alkylation stage to give the linear alkylbenzenes.

.
With regard to the regenelation rnentloned at polllt (c), it is possible to use, wlth particular advantage~
- the three procedures which will be descrlbed hereinafter~
- - All the three operating methods give e~tremely satisfactory and practically equi~alent results although each one of these procedures offers decided~ specific advantages.
1) First method~
a strlpping treatment for the molecular sieves at a temperature Or from 250 to 400oG a~d at pressures o~ from - 1 to 10 atmospheres by me~ns of an inert gas.
` 2~ Second method: ;
an elution treatment with a light aromatic hydrocarbon~
- solvent~ such as, for example~ benzene, toluene-, chloroben-z~ne, ethylbe3lzene or the lilce. The pre~erred eluent ls ben~ene~
`
It ha-s been found that the most suitable~temperatures~
or the elutlon treatment are from amblent telbperature to tho boiling point Or the eluent at the workin6 pressure.

~ ~ 9 1C~ 5 Tho latter is ~d~antagcously maintained at a value of from t to 10 atmospheres.
The treatment of the molecular sleves by elution ia ~ generally followed by a stripping treatment of the elu~nt with an inert gas. This stage is carried out advantageousl~
- at a temperature of from 150C to Z50C, alld prererably from 170 to 200C and at a pressure of from 1 to 10 -- atmospheres.
3) Third method:
an elution treatment carried out under the conditions ~ ' .
- - described above at point (2). This elution is ~ollowed by the direct resumption of the absorption cycle of the recycled impure paraffins on the moleF~Ilar sie~es, carried out undcr~
the usual operation conditions.
In this manner the impurities (a~omatic by~products, , ~ etc~ contained in the paraffins displace the eluent from .
the ~olecular sieves to be absorbed in their turn. Obviousl~
, the firat paraffin fractlon léa~ing~the absorpti~on apparatus wi~ll oontain a certain quantlty Or eluent which ~.ay eQsily be removed by means o~ conventional ~et~ods~preferably by means Or ~istillation, s already stated abo~e, all the~three systems described :: ~
result in ef~ective re~eneration of the molecular sieves~

vhich reccver their adsorbent capacity practically completely .
both in a q~lantitative a~d a qunlitati~e s;ense,~The choice of~ono or the~other Or thc rogelleration systems hence depe~nds -; - ~ , , , , ~ ' . `, :, . . ; ! . . . ~ i . . ' . ' ' ,.i '~

~rom time to t~me only on inci.dental crltcria, in partl-cular economic convenience, du~, fbr example, to the a~aila'~ility or lack OA~ avai.lability OA~ the chemical reagen~s, of thc heatin~ ~eans, apparatus, ctc.
With re6ard to the o.thffr steps OA~ the proces6, these are carried out in a conventiorlal manner. In a preferred embodiment:
- -(a) the chlorination of the linear C~ - C15 paraAffins (recycled and unrecycled) is carried out by operating - ~ith gaseous chlorine with a paraffin/chlorine molar rati.o .. of from 2.:1 to 10:1 and at a temperature of from 50 to . 200C. The mixture OAf p~oducts from the chlorination .
- - (chlorinatcd and unchlorin~.ted pararAfins), after degassillgr to remo~e the hycdro~en chloride, is conveycd directly to . the aIkylation stn~re.
(b) the alkylation Or benzene is carried;out with the benze~e 11l exce~s~o~er the chlorin~ted paraffi.n:, in the~ ~ :
~ presence:or ~lCl3, at a temperature of from 40 to 1~0C
:_ and for a period of from 30 minutes to 3 hour (c) tho alkylation products, after decanting to separate ~ ~ ~ the catalytic sl.ud~g~ and washin~ with aqueous alkali UA~ to : ~ : neutrality, are fractionatec1 by dist~.llatlon. Greater details :
- will be.given in Example l.
Example : :Into a~tubular reactor there are red about 230 Kg/hour Or ~as~ou~ chlori.ne and 1~9650 ~/houx Or lin~ear C9 - C15 ~. :
' 7~5 paraffins consisting of 70~ by weight of recycled paraffins and for the re~nQining 3 ~ of fresh, con~lerci.al paraffins.
l`he composition in perentage by wei~ht of the n-paraf-:. fins, determined by gas-chromatographic analysis, ls th~
following:
Cg _ trace ~ C10 = 11,1%
C11= 32~ C12 =32,1p~
Cl 3= 22 ~ 11% C14 = 2.4,6 C15= 0. 02%
The chlorination of the paraffins is carrled out at - a temperature of about 110C.
. The products of chlorinatiorl, freed from the hydrogel1 chloride by degassing~ are tllen fed to the alkylation stage, togcther with 250 l~g/hour of catalytie sludgc.. Thi.s latter ~ ~ consists of the exhausted sludge dischar~ed fror~ the reactor and enriched with alu~inium tri.chloride ln cluantitie~ equal~
. : to about 10Cto by ~7eight with respect to the sludge.
~ In thio stage two agltated reactors~plaoed in series : are used~:so as to avoid the presence of unreacted chloro-paraffins in the mixture leaving the alkylation stage,~ due ; ~ ~to inevitable "short cirouit" phenon1ena. ~:
The~reaction temperature is 600C and the volume c~
, ~ - the-two.reac.tors allows a total stay time of about 1 hour , in the al'~ylation stage.

The proc3ucts leaving the alky].ation stage are then :: :
decanted to separate the catalytic sludge, ~ashed wi1:h . . - ~
: 12--- : . : :

, :~
.. ., ,, - . . ..
. - . .~ ~ . :

5 wt.~o soda solution and then with wnter up to neutrality, and finally fractionated to separate the benzcne~ the unreacted paraffins alld the alky]benzen~s. ~bout 1,150 Kg/hour of paraffins which nre-intended ~or recycling and about 610 Xg/hour Or al~ylbenzenes are recovered. The l~eavy products are recovered at the foot of the distilla*ion columll for the al~ylbenzenes in ~uantities equal to about 90 ~g/hour.
By proceeding under these conditions, a gradual increase ln the chlorine content of the linear alkyl~enzenes produced and in the content or secondary products of the recycled paraffins are noted. - -en the chlorine content of the alkylbenzen~s has reached a value of 200 ppm and their content of aromatic ~ -by-produots has reached a val~e of 5;' by weight~ the unreac-ted paraffins recovered ln the distillatlon atage ln quan-tltics equal to about 1,150 Kg/hours are no ]onger recycled~
dlrectly to~ the chlorination sta~e but are fed to an absorp-.
tion column ~fllled with molecular sleves Or the t~pe 13~ Xdescrlbed abovc~ previously acti~ated for 6 hours with . nlt~ogen at 30~ - 350C and at atmosplleric pressure.
ThQ paraffins are pnssed through the col~u~,n ~rooi the bo~tom towards the top~ The absorption columll, havirlg a .
dl~meter of 3~ cm and a }iel~rht of 4 metres~ cont;ains 185 ~g O~r moleoular gio~es in the rorm of cylindricAl extrusion of diam~ter 0.~ cm. Durir~i~r th~-whole operation a temperatllre : : : : ~ : , ; ~ -13_ : ` : : : :

-7~S

of 120C and a pressuro equul to atmospherlc are maintainedin the column; the avera~e stay tlmc of the paraffins on the molecular sie~es is about 15 minutes.
T~e paraffills enterillg the absorption apparatu~
contain about 1o,b by weigllt of aromatic by-products and 700 ppm of organic chloriIIe.
In the paraffins leaving the absorption apparatus, the average content of aromatic by-product~ is reduced to o.6C~S b~ ~eight and that of organic chlorine to 400 ppm.
-The paraffins leaving the absorption apparatus are bhen directl~ recycled, t~ether ~ith 30~o of fresh commer-. cial paraffins, to the chlorlnation stage. There is thus obtained, in the linear alkylbenzenes produced by this continuous process, a lowering of the chlo~rine content - -down to about 50 ppm and o~ the content of aromatlc products down to negligible leYels.
The linea~ alkylbenzene 6 produced are, moreov~er, free from d.isagreeable odours and are of high quality.
A~ter about 7 hours o~ continuous working the currellt .~ of recycled paraffin to be purified is deviated~ to~ard.s a second~absorption colùmn tB)~containil~g the ~same type of~
molecular sieves and placed in parallel ~ith the identical 9 fi~st column~ Ae~cribed abo~e.
The exhausted molccular sieves Or absorption column (Aj are~then subjected to a re~eneration treatment.
;T~is operation~consists of a pre-treatment with a :

current Or nitrogen at a temperature of 100C and at atmospheric pressure, for a period Or time of two hours, to remove the paraffins remaining in the interstices - between the molecular sieves. This is then followed by &
stripping s~ith nitrogen at a temperature of 300C, a~
.

atmospheric prcssure, for a period o~ time of about 2 hours, to remove the adsorbed impurities in the molecular - sieves.
~- When~ in continuation of the continuous process, the : - - regenerated absorption column (~) is reinserted, in~place - ~ of tho colwlm ~B), in the purlfication cycle for the - -reoycled para~fins, this has rea~uired practically completely ; its initial absorbent capncity~ both in the qualitative and in the quantitative sense, given that tlle impurities in the purified parafrills leaving it and in the final linear al~ylbenzene products of the process are maîntained con-.
~ stantly at the minimum values enumerated above. The process .
- being c~rried out continuously, after eight compIete cycles~

in which the absor~tion columns t~) and tBj are used alter-- -- nately ~or the purification Or the recycled ~ara~fins and hence are from time to time regener2ted, no appreciable variations in the results already given are noted.
- - ' , :
; Exam~le 2 ~ ~
-The run Or Example 1 s~as faithfully repcated, using, - as the sole variant, a different method for the regclleration -of the molecular sie~es. The two columns w~re alternately :
- - :
.. . . . .

.~B~7~5 subjected to the following thr~e successi~e treatments:
(~) dryin~ nt 79C, at atmospheric pressure, for - two hours, with nltro~en; ._.
(b) elution, still at 79C, at atrnospheric pressure, with b~nzene passed through the column from the bottom towards the top at a rate of 300 l/hour. ~f~er 2 hours the ben~ene issuing from the column was free from aromatic by-- products and from organic chlorine, and its feed was stopped;
(c3 stripping treatment wlth nitrogen at 180C and at - atmosplleric pressure~ for 3 hours.- In the course of the whole continuous opcration results practically identical to those described in Exarnple 1 were obtai~ed constantly.
E~am~le 3 The run of Example 2 was~faithfully repeat~d with the variant that, in the alternate regeneration of the molecular - ~
. . sievesj the treatment described at point (c):was abolislled.
- -stead, the passa~e of the impure recycled paraffins was reswned directl~
- ;The first fraction of the paraffins leaving the newly regenerclted absorption~colulnll ob~iously contained z certain :
quantity~or bcnæene~ which was removed by distillation before this yaraffin fraction was recycled, together with ~
the remai.nin~ part of the purified recycled paraffins, to the - :~- chlorinc~tion sta~e.
: : --. ... . . :

~ '7~5 In the course of the wholc continuous operation, results practically identi.cal to ths~se obt~incd in E~camples 1 ~nd 2 ~sre ol~tained continu~usly.

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Claims (12)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a process for the preparation of alkylbenzenes by partial chlorination of linear paraffins having from 9 to 15 carbon atoms per molecule, catalytic alkylation of benzene using the resulting mixture of chlorinated and unreacted paraffins, fractionation by distillation, after separation of the catalyst, of the alkylation products thus obtained and recycling of the unreacted paraffins recovered from the fractionation stage to the partial chlorination stage, the improvement which comprises sub-mitting at least a part of the unreacted paraffins to be recycled to the partial chlorination stage, continuously or intermittently, to a purification treatment with molecular sieves selected from the group consisting of X and Y zeolites at a temperature ranging from ambient temperature to 200°C and at a pressure ranging from 1-10 atmospheres to produce alkylbenzenes free from odor, having a chlorine content less than 100 ppm.

2. The process of claim 1, wherein said purification treatment is carried out at a temperature of from ambient temperature to 200°C, at a pressure of from 1 to 10 atmospheres and with an average residence time of the said unreacted paraffins on the molecular sieves of from 5 to 60 minutes.

3. The process of claim 1, wherein the molecular sieves are periodically regenerated by removing the substances absorbed in said molecular sieves by thermal stripping or by displacement by elution.

4. The process of claim 3, wherein said molecular sieves are regenerated when the amount of absorbed substances reaches a value of from 8 to 15% by weight with respect to the weight of said molecular sieves.

5. The process of claim 3, wherein the molecular sieves are subjected, before their periodic regeneration, to a pre-treatment of stripping with inert gas at a temperature of from ambient temperature to 150°C and at a pressure of from 1 to 10 atmospheres, to recover the unreacted paraffins retained between the particles of said molecular sieves.

6. The process of claim 3, wherein the periodic regeneration is carried out by stripping with an inert gas at a tempera-ture of from 250 to 400°C and at a pressure of from 1 to 10 atmospheres.

7. The process of claim 3, wherein the periodic regeneration is carried out by elution with a light aromatic hydrocarbon solvent, at a temperature of from ambient temperature to the boiling point of the said solvent the working pressure, said working pressure being from 1 to 10 atmospheres.

8. The process of claim 7, wherein said elution is followed by a stripping treatment with inert gas at a temperature of from 150 to 250°C and at a pressure of from 1 to 10 atmospheres.

9. The process of claim 7, wherein said elution is followed by the direct resumption of the passage of the unreacted paraffins through the molecular sieves thus treated, the elution solvent present in the first fractions of unreacted paraffins submitted to purification after said direct resumption being removed by conventional methods before recycling to the partial chlorination stage.

10. The process of claim 7, wherein said elution solvent is benzene.

11. The process of claim 1, wherein said molecular sieves are of the type 13X.

12. The process of claim 1, wherein said molecular sieves have pore sizes varying between about 9 and 10 angstroms.