A facile method to deposit zeolites Y and L onto cellulose fibers

A facile method to deposit zeolites Y and Lonto cellulose fibers

David Vu a, Manuel Marquez b,c, Gustavo Larsen a,*

a Department of Chemical Engineering, University of Nebraska-Lincoln, Lincoln, NE 68588-0126, USAb NanoteK R&D, Kraft Foods, Inc. 801 Waukegan Rd., Glenview, IL 60025, USA

c Los Alamos National Laboratory, Chemical Sci. and Technol. Division, Los Alamos, NM 87545, USA

Received 13 August 2001; received in revised form 26 April 2002; accepted 2 May 2002

Abstract

Zeolite (Y and L)/cellulose composites are synthesized from natural cellulose fibers pre-treated with NaOH, KOH or

Na2SiO3, and preformed zeolite powders. Several techniques such as diffuse reflectance infrared spectroscopy, X-ray

diffraction, scanning electron microscopy and BET specific surface area measurements were used to characterize these

materials. Successful synthesis depends on extent of water removal from open reactors, fiber pre-treatment and reaction

temperature, reaction time, and initial water:zeolite:fiber ratio. One important trait of the materials reported in this

study is that their zeolite content becomes stable on washing with water at 373 K for several hours.

� 2002 Published by Elsevier Science Inc.

Keywords: Zeolite; Fiber; Cellulose; Filter; Zeolite/cellulose composite

1. Introduction

Synthetic and natural zeolites, a family of alu-minosilicates with pores and cavities in the range4–18 �AA, are well known heterogeneous catalystsand sorbents. The preparation of a soft membranewith molecular sieving properties is attractive fromboth practical and fundamental standpoints. Onegeneral approach is to combine a crystalline, yetporous solid such as zeolites, with a flexible, cheapand abundant organic matrix such as natural cel-

lulose [1]. Reporting on a new route to preparesuch zeolite/cellulose composites is the main sub-ject of this contribution. Once a stable zeolite/cellulose material (in its ‘‘loose fibers’’ state) issynthesized, carton- or paper-like zeolite/cellulosefilters can be produced by contacting the com-posite with distilled water to incipient wetness,followed by applying pressure to 2–5 ton/cm2 at383 K for a few minutes. A porous zeolite/cellulosefilter paper manufactured this way will have mo-lecular sieving properties and as such, having somedegree of perm-selectivity makes it a membrane bydefinition. The issues of zeolite/cellulose filter pa-per manufacturing and the specific applications weare currently targeting will not be dealt with here.Rather, this contribution is a report on how to

Microporous and Mesoporous Materials 55 (2002) 93–101

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*Corresponding author. Tel.: +1-402-4729805; fax: +1-402-

4726989.

E-mail address: [email protected] (G. Larsen).

1387-1811/02/$ - see front matter � 2002 Published by Elsevier Science Inc.

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load and stabilize the zeolite phase in the precur-sor-type, loose fibers state.

Zeolite/cellulose and cellulose acetate compos-ites have already been proposed for numeroususes. For example, they have been tested formedical applications (medical antibacterial mate-rials, denture plaque control [2]), fragrance prod-ucts (tissue conditioner [2], fabric softeners [3]),deodorizers (removal of NH3, H2S [4], and EtNH2

[5]), and filtrations (ultrafiltration membrane [6],dewatering sludges [7], tobacco smoke filters [8],water purification [9], air purification [10], filtra-tion membranes for aerated water and wine [11],adsorbent filter for decaffeinating processes [12]).Other applications related to adsorption processeshave also been proposed. These are absorbentpads (antimicrobial water-absorbing sheet forfood [13], adsorbent sheets [14], disposable dia-pers, sanitary napkins, panty shields, underarmshields and incontinence pads [15]), gas and liquidseparations (CO2/CH4, O2/N2 [16], olefin/paraffin[17], monosaccharide/polysaccharide [18]), and re-fining processes (decoloration and decalcifying ofsugar syrup [19]).

In our particular case, we are exploring anumber of zeolite/cellulose and imprinted silica/cellulose papers for removal of a variety of harm-ful or undesirable molecules from edible liquids(liquid-phase adsorption) and solid food (gas-phase adsorption) products as a consumer tool i.e.,purification is to be done by the consumer or at thepackaged product, not at the food processing plantduring product manufacturing. The use of a ma-terial with a paper-like macroscopic appearance isviewed by industry as more marketable than oneconsisting of porous pads filled with the solid ad-sorbent, or any other configuration where thepresence of inorganic solid particles is immediatelyapparent. From a practical standpoint, an arbi-trary stability criterion for these materials needs tobe defined. For the several potential applicationsthat we are currently exploring at Kraft, a zeolite/cellulose composite is deemed stable if leaching ofthe zeolite phase on contact with water at 373 Kdoes not take place.

The prior art is that zeolite/cellulose compositematerials can be prepared by using adhesive

polymers [20–24], electret technology [12] or in situzeolite crystallization [1,25,26]. With regard to thelast strategy, after a considerable volume of ex-perimentation we noticed that untreated cellulosefibers have a marked tendency, at different tem-peratures and Si/Al starting ratios, to inducecrystallization of zeolite 5A particles on their sur-face [26]. Thus, it does not surprise us that thesparse work reported in the open literature onin situ zeolite crystallization over cellulose revolvesaround 5A/cellulose composites [1,25].

The adhesive polymer method was used in in-dustry for several decades. Mintova and Valtchev[25] indicated that there were several disadvan-tages associated with this process, the most im-portant ones being the weak zeolite/celluloseinteraction, and the uneven distribution of zeolitecrystallites on the cellulose surface. In addition,incorporating an adhesive (a third chemical entity)in the zeolite/cellulose composite might be deemedunacceptable for certain applications, especially ifthe polymer is unstable or soluble in the filtrationmedium. The so-called ‘‘electret’’ method requiresthe use of high voltages, which might damage sub-stances and poses an occupational risk. The thirdsynthetic approach normally yields by-products,and materials that are unstable on washing withboiling water [26]. On the other hand, the strategyproposed in this contribution appears to be rathergeneral, i.e., it should a priori be applicable tozeolites other than Y and L, which are the onesemployed in this study. Our method makes use ofcommercial zeolite powders and cellulose sources,produces stable zeolite/cellulose composites uponprolonged boiling water washing, and it is easy tocarry out.

A number of characterization experiments, in-cluding scanning electron microscopy (SEM), dif-fuse reflectance infrared spectroscopy (DRIFTS),X-ray diffraction (XRD), and BET specific surfacearea (SSA) measurements were carried out tomonitor how the properties of these zeolite/cellu-lose materials change with the different synthesisand post-treatment variables. Some of these arethe amount of water, zeolites, fibers, and NaOH(KOH and Na2SiO3 were also briefly investigated),and temperature.

94 D. Vu et al. / Microporous and Mesoporous Materials 55 (2002) 93–101

2. Experimental

2.1. Pre-treatment of the cellulose fibers with NaOH(KOH or Na2SiO3)

Sodium hydroxide (0.9 g, Mallinkrodt, 98þ%)was dissolved in 40 ml of distilled water, andplaced in a 150 ml Teflon reactor (cylindrical, 2 in.ID, 2.5 in. OD). Subsequently, 4 g of loose cellu-lose fibers (CelectTM Bleached Kraft pulp from theCelgar Pulp Co., British Columbia, Canada) weresuspended in this NaOH solution. The resultingsuspension was stirred until it reached a macro-scopically homogeneous appearance (�5 min).The loaded, open reactor was placed in an ovenwith forced air circulation at 373 K, and the mix-ture on the reactor was occasionally stirred duringthe first 4.5 h, or just until the point in which onlydry solids remained. The NaOH-cellulose solidmixture was then removed from the reactor andcontacted with excess distilled water in a house-hold blender. This vigorous ‘‘washing’’ cycle wasintended to remove physically adsorbed or trappedNaOH from the fibers. The cellulose material wasthen rinsed to neutrality, and placed back in theoven at 373 K for another 24 h. Since this NaOH-treated cellulose precursor is freed from reversiblyadsorbed hydroxide via thorough washing withdistilled water, the action of NaOH is limited toattack of the cellulose surface and/or irreversibleadsorption on the fibers. This precursor mate-rial was labeled as ‘‘NaOH-Fiber’’. Identical fiberpre-treatment experiments, using Na2SiO3 (penta-hydrate, Fluka, 97þ%) and KOH (Aldrich, semi-conductor grade, 99.99%), were con- ducted. Afterextensive experimentation, we found such fiberpre-treatment approach to be essential to ulti-mately produce zeolite/cellulose materials that arestable in the presence of boiling water. At thispoint, it should also be mentioned that the com-mercial source of cellulose utilized in this study hasa very low-lignin content (�0.1%, according to themanufacturer). Lignin has been shown to ad-versely affect zeolite deposition on cellulose duringzeolite in situ crystallization [1,25]. On prolongeddrying (>1 day) at 473 K, slow ‘‘browning’’ of thematerial is observed, which suggests that cellulose

dehydroxylation imposes an upper temperaturebound for utilization of these materials.

2.2. Zeolite deposition

In the same Teflon reactor as the one used in thepreceding step, a suspension of 12 ml of distilledwater and 12 g of zeolite Y (sodium form, Si/Al ¼ 5, Aldrich) was placed and contacted with1.92 g of NaOH-Fiber (KOH-Fiber, or Na2SiO3-Fiber). This solid–liquid suspension was stirred for15 min, until it achieved macroscopic homogene-ity. After that, the loaded reactor was placed in theoven with (or without) a screw-on cap with an O-ring seal at 373 K for 24 h. The resulting zeolite Y/cellulose mass was washed with distilled watermany times to pH 6.5–7.0, and finally oven-driedat 373 K for 24 h. This cold-water washing pro-cess, while tedious, also allows for the separationof the zeolite that has not attached to the cellulose,since the former tends to settle at the bottom of thewashing container in every washing step. As ex-plained later, by subsequently washing the mate-rial for several hours in boiling water, the zeoliteweight eventually stabilizes. While (physical) re-tention of a small fraction of colloidal zeoliteparticles cannot be entirely ruled out, the materialsstabilized by hours of washing in boiling water willremain stable from a practical viewpoint. Theproprietary applications we are investigating withthese composites do not subject them to conditionsharsher than the prolonged boiling water washingstep. Blank experiments using untreated cellulose,water and zeolites following exactly the same cold-water/hot-water washing protocol showed that noirreversible deposition of zeolite onto cellulose wasachieved.

Despite the procedure described above beingoptimum for zeoliteY/cellulose products, uponextensive experimentation, we found that zeolite L/cellulose composites are better produced by KOHactivation of the fibers, instead of NaOH. ZeoliteL was purchased from Tosoh Corporation, in itsKþ form and with Si/Al ratio of 3. In both cases(Y- and L-loaded fibers), several experiments wererun with closed or open reactors, the latter beingmuch more effective. This issue, which obviously

D. Vu et al. / Microporous and Mesoporous Materials 55 (2002) 93–101 95

adds water evaporation rates into the wholeequation, will be discussed in more detail in Sec-tion 3. It should also be mentioned that prior tozeolite loading and after thorough washing withdeionized water, the NaOH- (KOH- or Na2SiO3-)treated fibers were also characterized.

To study the stability of the zeolite deposits inthe presence of boiling water, several washing ex-periments were carried out. Typically, 0.3 g ofzeolite-loaded sample were contacted with 100 mldeionized water, and the resulting suspension wasplaced in an air-tight PVC container. The PVCbottles were then put in an oven at 373 K fordifferent periods of time (0.5, 1, 2, 4, and 8 h).After that, the zeolite fibers were filtered andwashed several times with distilled water. Thesezeolite-loaded fibers were finally oven-dried at373 K for 24 h. Since the occurrence of a zeo-lite crystallite anchoring mechanism involvingbasic surface sites was suspected, the stability ofthe zeolite/fiber materials was also tested by acidattack with 0.1% (w/w) HCl. Remarkably, thesecomposites, which showed to be stable after waterwashing at 373 K for several hours, undergo quan-titative removal of the zeolite coating on contactwith dilute HCl, as discussed later.

As mentioned earlier, since the best results wereobtained using open reactors, conducting waterevaporation loss experiments became necessary.Five reactors were loaded with reactants as de-scribed in the previous sections, and placed simul-taneously in the oven at 373 K with forced aircirculation. At a chosen time, all five reactors werethen taken out of the oven, and their water lossesrecorded by weight difference. This provides onewater loss data point that is actually an average offive measurements. The same procedure was re-peated using different times, until no mass changein the reactors was apparent.

2.3. Characterization methods

XRD using a Rigaku DMAX/IIB diffractome-ter with CuK-a radiation was used as a rapid,qualitative tool to determine whether stable zeoliteloadings were achieved on prolonged washing withboiling water. The spectra were recorded in the2h ¼ 2–35� range, scanning at a rate of 2�/min for

zeolite identification purposes and in the 2h ¼2–19� window at a 1�/min rate for quantificationpurposes. The step size was 0.02�.

SEM studies were done in the bright field modewith a JEOL MEM2010 instrument at 200 keVbeam energy. The sample was coated with a thinfilm of gold using a standard SEM specimenhandling protocol to improve the quality of theimage since the zeolite fibers are non-conducting.

The IR studies were carried out at room tem-perature on a Nicolet 20 SXB Fourier transforminfrared spectrometer equipped with an opticaland temperature control system, and DRIFTScell from Spectratech�. Spectral resolution was4 cm�1.

On washing the zeolite-loaded fibers with boil-ing water, BET SSA measurements were con-ducted in a custom-built glass system, equippedwith greaseless 3-O-ring stopcocks, mechanicaland diffusion pumps, and a Baratron� pressuretransducer. Prior to nitrogen adsorption mea-surements, samples were heated from room tem-perature to 383 K for 30 min, and then kept at thesame temperature for 1 h under vacuum to removeadsorbed moisture. The main goal of the adsorp-tion experiments is to complement XRD data onthe issue of zeolite removal by prolonged exposureto water at 373 K.

3. Results and discussion

By means of DRIFTS, it was confirmed thatfiber pre-treatment does not appear to affect thebulk chemical properties of the fibers. However,their surface was clearly affected by NaOH, orNa2SiO3 attack. Fig. 1a shows a SEM image ofthe NaOH-treated cellulose substrate. Crater-likeholes are created, a feature that is completely ab-sent in untreated fibers (Fig. 1b). Treatment withNa2SiO3 was also found to promote surfaceroughness in the cellulose substrate (not shown).Fibers from the source used in this study like theones shown in Fig. 1 were found to have diametersaround 20–60 lm. The full width of the images inFig. 1 is 60 lm.

The surface morphology changes observed onNaOH treatment are expected to be accompanied


by some level of chemical modification of thesurface. Cellulose is in essence a natural carbohy-drate-based polymer and a polyvalent alcohol,where the free hydroxyl groups in its monomericunits form hydrogen bonds with adjacent chains[27]. In addition, the proposal that hydroxylgroups in oligomeric silicates can disrupt the inter-chain hydrogen bonds in cellulose, was first madeby Mintova and Valtchev [25]. Damage of thecellulose surface to the extent shown in Fig. 1 isexpected to go beyond NaOH interaction with theexposed –OH groups at the surface of the fiber.Instead, such interaction should either thoroughlydisrupt the hydrogen bonding mechanism betweenadjacent polymer chains, or even partially hydro-lyze the polymer structure. For example, the use ofdiluted NaOH has long been known to promoteswelling and digestability of lignocellulosic mate-rials [26]. SSA analysis showed that the availablesurface of the fibers increases from 23 to 30 m2/gafter NaOH treatment, and to 26 and 41 m2/g afterKOH and Na2SiO3 attack, respectively. In addi-tion to cellulose surface activation, the presence ofwater will show to play a crucial role.

Fig. 2 shows an example of the effect of time onthe zeolite loading step, justifying the adoption of

24 h as standard in our most successful syntheses.Thus, unless otherwise indicated, all experimentsreported here on were based on a reaction timeequal to 24 h, which also applied to KOH andNa2SiO3 pre-treatments.

Since water at 373 K is unable to removestrongly bound zeolite particles but diluted HCl

Fig. 2. NaOH-treated fiber loaded with zeolite Y: (a) 14 h re-

action time (‘‘fresh’’ sample, only rinsed with cold water), (b) 24

h reaction time (only rinsed with cold water after reaction), and

(c) without water (solid/solid reaction, after separating the

phases and rinsing with cold water).

Fig. 1. (a) SEM image of a NaOH-treated cellulose fiber after removal of reversibly adsorbed hydroxide with deionized water. (b)

Untreated cellulose fiber.


was found very capable of doing so, it is clear thatthe binding mechanism involves participation ofsurface basic sites. In the presence of water, zeo-lites will adsorb water molecules in its pore (in-ternal) structure, but they will also do so via theirexternal hydroxyl groups, thereby creating a waterenvelope around the zeolite particles. Experimentsin the absence of water, i.e., using mechanicalmixtures of zeolites and treated cellulose fibers,gave no solid evidence for the occurrence of cel-lulose/zeolite solid/solid reaction pathway. Thus,the presence of adsorbed water to facilitate thebinding of zeolite crystallites to cellulose appearsto be essential. In all cases, the qualitative featuresshown in Fig. 2 were observed.

It is interesting to note that the closed reactorsdid not yield good results. It is apparent thattimely removal of water from the reaction mediumhelps the irreversibly adsorbed alkali on the cel-lulose to activate the zeolite crystallites. Analysisof the amount of water removed from the reactorshowed that no more water is released after 190min (Fig. 3). The treated cellulose/zeolite/waterslurry maintains neutrality throughout the zeoliteanchoring process. This indicates that partial dis-solution of the zeolite crystallites followed by re-crystallization at the cellulose surface is a highlyimprobable mechanism for composite formation.

Fig. 4 shows the XRD patterns of as-synthe-sized zeolite Y-loaded NaOH-Fiber, 2.5 h washedzeolite Y-loaded NaOH-Fiber and 2.5 h washedzeolite Y Na2SiO3-Fiber. The latter, despite re-sulting in a lower zeolite loading, still gave positiveresults. If the relative intensities of the broad cel-

lulose signals (2h � 16� and 23�) are compared tothat of the low-angle zeolite Y reflection (2h �6:2�), it is observed that approximately half thezeolite is removed after the 2.5 h washing cyclewith hot water at 373 K. It is clear that the as-synthesized material, i.e., one that has only beenthoroughly washed with cold water after synthesis,is not yet satisfactory in light of our adopted sta-bility criterion. However, (a) if the zeolite washingprocess in the presence of boiling water eventuallyplateaus, and (b) if the residual zeolite:cellulosemass ratio is still acceptable, the synthesis of astable composite should still be possible.

The peak ratio of the zeolite reflection men-tioned above, and that of cellulose (2h ¼ 16:45�)was always used as a relative measure of the zeolitecontent on materials washed for different amountsof time. As discussed later, the zeolite:cellulosemass ratios can be inferred from surface areameasurements. Emphasis was made on the NaOH-treated cellulose material, rather than on theNa2SiO3 case, because the latter resulted in smallerzeolite loadings. Fig. 5 is a SEM image of the ze-olite Y-loaded NaOH-Fiber material after 2.5 hwashing in boiling water. As stated earlier, KOHpre-treatment of the fibers gave better results (ul-timately higher zeolite loading) than NaOHtreatment for effectively anchoring zeolite L crys-tallites, which points to these zeolites (Y and L)natural cation preferences. Zeolite L is commer-Fig. 3. Water loss profile from reactor.

Fig. 4. (a) 24 h zeolite Y loading reaction on NaOH-treated

fiber, (b) same as (a), but after hot-water (373 K) washing for

2.5 h, (c) Y-loaded Na2SiO3-treated fiber after stabilization by

2.5 h hot-water washing.


cially produced in its potassium form, unlike Ythat is generally made from NaOH-based gels.

The effect of temperature was also investigated.The results showed that the NaOH-treated fibersat 353 K gave higher zeolite loadings than thoseconditioned at 373 K. The reaction temperaturemight also play an important role in the zeolitesloading step. Table 1 shows that increasing thereaction temperature did not improve the zeoliteloadings. It is apparent that water removal fromthe reactor was one of the key variables in oursynthetic approach. Table 1 also shows that low-ering the water amount gave better results, but as

stated earlier, the use of water cannot be elimi-nated altogether since it is essential for zeolite-fiberbinding.

Analysis of the relative intensities of the zeoliteand cellulose XRD signals show that the zeolitecontent is stabilized after 2.5 h hot-water washing(Fig. 6). In addition to a much higher stabilitytoward hot-water leaching than with conventionalin situ crystallization techniques, an importantaspect of our materials is that the zeolite Y contentafter prolonged hot-water washing was approxi-mately three times higher than those obtainedfrom extensive in situ crystallization studies donein our laboratory over the same type of cellulosematerial [26]. Fig. 7 shows the XRD pattern of

Table 1

Zeolite-to-cellulose XRD peak ratios as a function of prepara-

tion conditions

373 K 383 K

NaOH-Fiber Blank

Fiber

NaOH-Fiber

Amount

of water

1 ml 2 ml 3 ml 1 ml 2 ml

Fresh 4.1 1.9 1.5 0.63 1.2

Washed 2.1 0.85 0.67 – 0.51

‘‘Washed’’ refers to the materials exposed to water at 373 K for

2.5 h.

Fig. 6. Zeolite Y-to-cellulose XRD reflections ratios as a

function of hot-water washing time.

Fig. 7. Fresh zeolite L/cellulose composite XRD pattern.

Fig. 5. SEM image of zeolite Y-loaded NaOH-Fiber material,

after stabilization by hot-water washing for 2.5 h.


as-synthesized zeolite L KOH-treated fibers. Afterwashing in boiling water at 373 K for 30 min, astable material was obtained (Fig. 8). In all cases,and in accordance to an observation made byMintova and Valtchev [25], diluted HCl treatmentquantitatively removes the zeolite deposits. SinceY zeolite is known to be stable in dilute HCl,disruption of the cellulose/zeolite interaction ra-ther than dissolution of zeolite crystallites is theexpected mechanism for HCl-mediated decompo-sition of these materials.

Within scattering of data, Table 2 shows howleaching in boiling water affects zeolite loadingsuntil stable values are reached. Prolonged washingresults in removal of about 50–60% of the zeolitedeposits relative to the as-synthesized samplesfor both L- and Y-based composites. On theassumption that both phases, cellulose and zeo-lite, contribute to the overall SSA of the compos-ite independently, one can gain some insightinto the issue of mass fractions of each phasefrom nitrogen adsorption data. Specifically, the

simple equation, SSAcomposite ¼ XzeoliteSSAzeolite þð1�Xzeolite)SSAcellulose, indicates that �18% of theweight in the Y zeolite/NaOH-treated cellulosecomposite after 8 h washing is effectively Y zeolite,while 8 wt.% is the zeolite loading in the 8 hwashed L-zeolite/ KOH-treated cellulose material.

4. Conclusions

Pre-treatment of natural, low-lignin cellulosefibers with alkali provides a simple route for an-choring preformed zeolite crystallites onto thecellulose surface. In light of stability data of thesecomposites in water at 373 K, L- and Y-loadedcellulose materials resistant toward leaching of thezeolite phase in aqueous media can be prepared byexposing as-synthesized samples to boiling waterfor several hours. Typical zeolite loadings in stablecomposites fall in the 5–20% range.

Acknowledgements

Support from Kraft Foods R&D is gratefullyacknowledged.

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Table 2

BET surface area of zeolite Y and L as a function of washing

time in water at 373 K

BET

(m2/g)

Pure

zeo-

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Washed zeolite-fiber in hot

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OH-

treated

Fiber0 0.5 1 2 4 8

Y 550 246 154 153 118 142 126 30

L 250 75 64 50 56 51 44 26

Fig. 8. Zeolite L-to-cellulose XRD reflections ratios as a

function of hot-water washing time.


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A facile method to deposit zeolites Y and L onto cellulose fibers

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Transcript of A facile method to deposit zeolites Y and L onto cellulose fibers