Presented at 3rd International Symposium on Environmental Management, SEM – Towards Sustainable Technologies (SEM2011)

at University of Zagreb, Faculty of Chemical Engineering and Technology, 26–28 September, 2011, Zagreb, Croatia

Sol-gel Derived Mixed Metal Oxide Sorbentsfor High Temperature Gas Desulfurization

V. Mandiæ,a T. Oèko,b G. Matijašiæ,a and S. Kurajicaa

aFaculty of Chemical Engineering and Technology,University of Zagreb, Maruliæev trg 19, Zagreb, CroatiabVetropack StraÞa d.d. Hum na Sutli, Croatia

High temperature gas desulfurization (HTGD) is the main process for hydrogen sul-phide removal from hot gases. Currently leading regenerable desulfurization sorbents arebased on ZnO–TiO2 system. This work presents structural and textural modifications ofZnO–TiO2 based sorbents by Al2O3 addition in order to ensure high temperaturemechanical support. Sorbents with composition Zn(2-x)Al2xTi(1-x)O4 (x = 0, 0.25, 0.5, 0.75,1) were prepared by sol-gel process followed by thermal treatment. Nanocrystalline pow-ders were produced at moderate sintering conditions (500 °C, 2 h). Depending on the ini-tial ZnO:TiO2:Al2O3 molar ratio, zincite (ZnO), brookite (TiO2) as well as gahnite(ZnAl2O4) solid solution were formed. Al2O3 induced textural modifications, rising spe-cific surface area from 34 to 90 m2 g–1. The sorbent sulphidation performance was testedusing newly developed procedure comprising batch sulphidation process. Sorbent withcomposition Zn1.75Al0.5Ti0.75O4 was found to possess the highest sulphidation reactivity,i.e. sulphur removal efficiency.

Key words:HTGD, desulfurization, sorbent, ZnO–Al2O3–TiO2

Introduction

Integrated gasification combined cycle (IGCC)is one of the most promising processes for produc-ing electrical energy from coal due to the noticeableimprovement in thermal efficiency that it can pro-vide. This advanced power-generating system in-volves the direct contact of hot coal-derived gaseswith turbine blades. In order to ensure high envi-ronmental requirements, as well as corrosion andabrasion protection of the turbines and relatedequipment, precleaning of the coal gas from pollut-ants, especially hydrogen sulphide, H2S, is neces-sary. Economic as well as environmental aspects foradvanced power-generating systems mandate thehigh temperature gas desulfurization (HTGD) as themain process for the removal of the most abundantsulphur-containing compound in coal gas, hydrogensulphide, which can react readily with the oxides ofalkali earth and transition metals.1

The development of metal oxide materials asdurable, chemically, mechanically, and thermallystable regenarable sorbents, capable of supportingmultiple desulfurization-regeneration cycles, hasbeen the goal of several studies.2–5 In a vast numberof metal oxides proposed, zinc-oxide based sor-bents are recognized as those with the best cost-benefit ratio and are proven HTGD sorbents for re-moval of hydrogen sulphide.6–8

Despite the fact that zinc-oxide ensures effi-cient reduction of hydrogen sulphide content to theppm level, it is unable to provide a long-term stabledesulfurization-regeneration process. Namely,desulfurization conditions promote reduction ofZn2+ to Zn, and due to harsh temperature processconditions, the zinc evaporate from the sorbent.9

Another weakness of zinc oxide are its insufficientmechanical properties yielding wearing during flu-idization in the course of the sulphidation-regenera-tion process.10,11

In order to surpass these disadvantages, a com-bination of zinc oxide with other compounds was

V. MANDIÆ et al., Sol-gel Derived Mixed Metal Oxide Sorbents for High …, Chem. Biochem. Eng. Q. 27 (1) 43–49 (2013) 43

*Corresponding author: Vilko Mandiæ, e-mail: vmandic@fkit.hr

Original scientific paperReceived: September 18, 2012

Accepted: February 5, 2013

investigated.12–14 It was observed that combinationof Zn-based sorbents with other metal oxide com-pounds can greatly improve the performance ofmixed-metal sorbents in several aspects, especiallythe ones pointed out as disputable. Enhanced per-formance can be expected in terms of greater sulfurremoval efficiency, in decrease of the Zn2+ reduc-tion and in the suppression of sulfate formation, allwith retained mechanical strength, structure ofmicropores, and overall reactivity and stability ofsorbent particles. TiO2 and Al2O3 were emphasizedas most useful for combining. Combination withcompounds like titanium oxide can provide thermalstability of the sorbent, i.e. prevent evaporation ofthe zinc.15 On the other hand, by combination withAl2O3, which is proven as a high-temperature stableand durable material, mechanical wearing and attri-tion of the sorbent can be minimized. Also the hightemperature sintering process should be postponed,influencing the pore structure which should lead toan increase in specific surface. By the enlargedinteraction interface, a better desulfurization effi-ciency should be noticed.10,11

The ability to provide an efficient sorbentshould not only be focused on optimizing the chem-ical composition but also a proficient productiontechnique. A vast number of sorbents are preparedby high temperature techniques which could annulthe benefits achieved by chemical composition.6,16

Therefore a technique which offers the possibilityof achieving sorbent synthesis at lower tempera-tures, the sol-gel technique, should be applied.17

In this work several compositions of theZnO–Al2O3–TiO2 based sorbent were synthesizedby means of sol-gel process and investigated.Yielded sorbent performance was tested by newlydeveloped high-temperature and high-pressure sul-phidation process in batch reactor.

Experimental

Materials used to prepare precursor gels wereAluminum sec butoxide (Asb, Al(OsBu)3, 97 %,Acros organics, Belgium), Titanium n-butoxide(Tnb, Ti(OnBu)4, 98 %, Alfa Aesar, Germany), Zincnitrate hexahydrate (Zn(NO3) × 26H2O, 98 %, Ri-edel-de Haën, Germany). Ethyl acetoacetate (Eaa,C6H10O3, 99 %, Fluka, Germany) was used as acomplexing agent while Isopropyl alcohol(C3H7OH, 99 %, Carlo Erba Reagents, Italy) wasused as a solvent. All chemicals were used as re-ceived.

The appropriate amounts of Asb and Tnb weredissolved in Eaa and isopropyl alcohol solution in-dependently; also zinc nitrate hexahydrate was dis-solved in isopropyl alcohol. The molar ratio of Asb

and Tnb to Eaa was held 1/1 as it was previouslyexperimentally proven optimal.18 The mixtureswere stirred 30 minutes. The Tnb solution was thenadded dropwise to Asb solution and stirred for anadditional 30 minutes. After that the Zn nitrate so-lution was added in the same manner, the mixturewas stirred for 24 hours at room temperature duringwhich the gelation occurred. The obtained sampleswere subsequently ground to fine powder andstored. For the experiment five gels were preparedwith compositions 0.500 ZnO × 0.500 Al2O3, 0.556ZnO × 0.333 Al2O3 × 0.111 TiO2, 0.600 ZnO ×0.200 Al2O3 × 0.200 TiO2, 0.636 ZnO × 0.091Al2O3 × 0.273 TiO2 and 0.667 ZnO × 0.333 TiO2,corresponding to the ternary diagram intersectionsas shown in Fig. 1, and denoted accordingly.

IR spectra of the samples were acquired usingthe Fourier transform infrared spectrometer BrukerVertex 70 in ATR (attenuated total reflectance)mode. The samples were pressed on a diamond andthe absorbance data were collected between 400and 4000 cm–1 with spectral resolution of 1 cm–1

and 64 scans.The crystal phases were identified by powder

X-ray diffraction (XRD) using Shimadzu diffracto-meter XRD 6000 with CuK� radiation. Data werecollected between 5 and 70° 2� in a step scan modewith steps of 0.02° and scan speed of 2° 2� min–1.

The thermal behavior of powder precursor wascharacterized with Differential Thermal Analysis(DTA) and Thermo-Gravimetric Analysis (TGA)using simultaneous DTA/TGA analyzer NetzschSTA 409. For the thermal analysis approximately50 mg of material were placed in Pt crucibles andheated at a rate of 10 °C min–1 to 1000 °C in a syn-

44 V. MANDIÆ et al., Sol-gel Derived Mixed Metal Oxide Sorbents for High …, Chem. Biochem. Eng. Q. 27 (1) 43–49 (2013)

F i g . 1 – Samples molar composition and notation

thetic air flow of 30 cm3 min–1, �-alumina was usedas a reference.

Specific surface area of powders was deter-mined by a Brunauer–Emmet–Teller (BET) N2 gasadsorption-desorption isotherm obtained at 77 K ona Micromeritics ASAP 2000 equipment. Samplewas previously degassed at 400 °C under a dynamicvacuum of 1.3 · 10-2 Pa.

Sulphidation process was performed in a batchautoclave. Approximately 250 mg of all samplesZAT 1 – 5 and reference ZnO sample were placedin separate crucibles in the reactor. Separately, FeSgrains and 10 % HCl were prepared in order toproduce H2S. Reactor was heated at 200 °C for 2hours.

The extents of sulphidation of sorbents in theautoclave assembly were monitored as weightchanges during oxidation, i.e. regeneration process,by Thermo-Gravimetric Analysis (TGA) usingPerkin Elmer TGS-2. A small amount of sulphi-dated sorbent was placed in Pt crucible and heatedat a rate of 10 °C min–1 to 1000 °C in a synthetic airflow. The conversion of the sulphidation processwas calculated from the partial mass loss in thecourse of the oxidation process:

ZnS + 3/2 O2 � ZnO + SO2

Conversion was determined from the appropri-ate section of the thermogravimetric curve usingequations:

�mM M

MZnS ZnO

ZnSmax �

��100 %

conversion =�

�

m

mreg

max

�100 %

Results and discussion

From the FTIR investigation of the gels, infor-mation about the chelation process can be obtained.Infrared spectra of the samples ZAT 1 – ZAT 5dried gels are presented in Fig. 2. The FTIR spectraof Eaa-based chelates are assigned as described inliterature.19 Majority of alkoxides react readilywhen exposed to water (even air humidity). The hy-drolysis of some alkoxides, among which are Asband Tnb, occur rapidly. The role of Eaa, which wasadded to the system, is to stabilize the rate ofalkoxide hydrolysis by forming a chelate withalkoxide. Namely, ß-ketoesters like Eaa resonatebetween keto and enol form (keto-enol tautomer-ism), in which the keto form is favored in pure Eaa,while upon forming chelate with alkoxide the enolform dominates. Monitoring of the keto-enol equi-

librium enables the insight in the processes of hy-drolysis and condensation, i.e. in the sol-gel pro-cess.

In the IR spectra of all dried gels several char-acteristic bands occur which could be assigned toEaa. Both keto and enol form of Eaa yield withcharacteristic absorption bands in region 2980–2850 cm–1 originating from C–H stretching vibra-tions. Band at 2980 cm–1 results from the asymmet-rical stretching mode in which two C-H bonds ofthe methyl group are extending, while the third oneis contracting (�as CH3). Band at 2870 cm–1 arisesfrom symmetrical stretching (�s CH3) in which allthree C–H bonds extend and contract in phase. Theasymmetrical stretching (�as CH2) and symmetricalstretching (�s CH2) occur at 2940 and 2850 cm–1,respectively. Also, bands at 1475 cm–1 (as asym-metrical bending vibration involves out-of-phasebending of the C–H bonds, �as CH3) and 1369 cm–1

(as symmetrical bending vibration involves inphase bending of the C–H bonds, �s CH3) were no-ticed. The CH2 scissoring band in the spectrum oc-curs at 1470 cm–1. The absorption frequencies fromO–C–C stretching, –C–C–O stretching and C–Cstretching occur at 1040 cm–1, 1155 cm–1 and 1317cm–1, respectively.

Band characteristic exclusively for the ketoform of Eaa occurs at 1753–1718 cm–1 due to theC=O stretching vibration of two carbonyl groups.Instead of bands of the keto form, in the spectrumof enolic form characteristic bands are observed at1650 cm–1 due to hydrogen bonding between theester C=O and the enolic hydroxyl group, as well asabsorption frequencies of the alkene bond in conju-gation with a carbonyl group at 1630 cm–1. Eaa inchelate shows characteristic absorption bands at~1610 cm–1 due to a C–O in enolic form bonded to

V. MANDIÆ et al., Sol-gel Derived Mixed Metal Oxide Sorbents for High …, Chem. Biochem. Eng. Q. 27 (1) 43–49 (2013) 45

F i g . 2 – IR spectra of samples ZAT 1 – 5

Al, as well as absorption band at ~1525 cm–1 due toa C-C vibration of six membered ring of the com-plex. The major absorption bands due to a NO3– arelocated at 1305, 1445 and 1630 cm–1, mostly over-lapped with Eaa bands.

FTIR spectra presented in Fig. 2 confirm thatEaa, in all samples, i.e. for all Eaa/Asb/Tnb ratios,reacts with alkoxides and forms chelate. As can beseen, although the dominance of the enol form wasobserved, during the hydrolysis and polycondensa-tion processes in the course of the synthesis, a partof Eaa in the keto form was entrapped and re-mained even in dried samples.

In frequencies 400–600 cm–1 weak bands re-sembling [ZnO6] and [ZnO4] polyhedra are ob-served.20 At frequencies from 550–650 cm–1 bandsof [TiO6] are expected.20 Frequencies characteristicfor Al-O-Al linkages21 occur at ~700 cm–1. In IRspectra of the raw gels most of the bands character-istic for M-O bonds are highly overlapped, as bandsrelated to the organic component predominate andthus are difficult to observe.

The thermal behavior of all dried gels is char-acterized by multistep weight loss in the tempera-ture range of 100–450 °C as shown by DTGA mea-surement in Fig. 3. In the higher temperature regionthe weight loss is constant at approximately 55 %mass loss for all samples (TGA scan, not shown inFig. 3). DTA curves of all samples consist of a se-ries of endothermic and exothermic effects withvarious intensity and mutual superposition (notshown in Fig. 3) and correspond well to DTGA datain Fig. 3. In the temperature range below 200 °Cobserved phenomena are related to the subsequentreleases of adsorbed water. Chemisorbed water isremoved also up to the temperature of 300°C. Fur-thermore solvent degradation follows. The occur-rence temperature and intensity of these processes,and especially the following ones representing

chemical transformations are related to distinctchemical species present in the gels. Due to gradualchange of composition of the gels it is possible toobserve similarities in thermal effects. However,due to high level of mutual superposition if is diffi-cult to assign specific transformations. Despite,some processes accompanied with high mass losscan be observed overall in samples, such as the ni-trates decomposition process, occurring at about250 °C.22 At temperatures above 300 °C, the effectsoccurring can be related to crystallization processesin the system. A series of DTGA peaks centered at~400 °C with increasing intensity in samples con-taining more Zn can be attributed to the decomposi-tion of Zn(OH)2 (crystallization of ZnO). Having inmind the composition of the gels, several crystalphases may occur; however, to provide exact assig-nation of DTA effects, structural investigationsshould have been performed at various tempera-tures. Instead, crystal composition was monitoredafter thermal treatment at the temperature of 500 °Cand discussed in XRD section.

Samples were fired at temperature as low as500 °C for 2 hours in order to preserve high spe-cific surface area, as the temperature increasewould promote sintering and therefore cause the re-duction of the specific surface area. All ZAT sam-ples fired at 500 °C were subjected to XRD analysisin order to determine phases present in the studiedsystem (Fig. 4). All scans exhibit crystalline phases.Chemical composition of the sample ZAT 1 wastargeted to fit gahnite which was confirmed usingXRD. ZAT 5 should fit to Zn2TiO4, which was alsoconfirmed. Samples ZAT 2, 3 and 4 resemble thecomposition and phases chemically between thetwo mentioned (zinc alumotitanate solid solutions).In samples ZAT 2 – 5 zincite, ZnO, has been evi-denced, also with increasing intensities resembling

46 V. MANDIÆ et al., Sol-gel Derived Mixed Metal Oxide Sorbents for High …, Chem. Biochem. Eng. Q. 27 (1) 43–49 (2013)

F i g . 3 – DTGA scans of samples ZAT 1 – 5

F i g . 4 – Diffractograms of samples ZAT 1 – 5 fired at 500°C for 2 hours

Zn content increase. In sample ZAT 5 brookite,TiO2, crystallized.

To facilitate high efficiency of the sorbent, it isessential to provide reaction interface as immenseas possible during the whole cycle. The conversionof ZnO to ZnS during the desulfurization process,involves significant volume changes (molar volume+50 %). Small specific surface area would disableor significantly reduce the rate of the conversionprocess as the gas diffusion resistance would be in-creased on behalf of the reaction volume increase,especially in the later stage of the reaction. Smallerparticles can also ease up the diffusion resistanceduring the regeneration process of the sorbent sam-ples.23 To evaluate if the material preparation hasbeen successful in terms of achieving high specificsurface area, a series of BET N2 adsorption-des-orption isotherms were measured. The determinedspecific surface areas are shown in Table 1. Reason-ably high SBET have been obtained for all samples.As can be seen, the greater alumina content yieldsbetter surface properties, while the complete ab-sence of alumina yields significant decrease of spe-cific surface area. The opposite can be observed forsamples with higher ZnO loading. The aluminacontaining gels yield finer particles.24 Furthermore,ZnO loading enhances grain growth and promoteaggregation with crystallized spinel particles. Bothprocesses result in larger pores and thus smallerspecific surface area.25 It was shown that high sur-faces area is necessary for efficient gas removal;however it can also ensure greater mechanical dura-bility of the whole composite. Finer particles ensuremechanical strength necessary for harsh fluidizationconditions and also longer lifetime of the sorbent.Namely, repetitive sulphidation-regeneration cycleis accompanied with volume changes which gener-ate strain in the sorbent. Sorbent with limited con-version capabilities (low specific surface area)would not be able to withstand these forces andwould have poor lifetime expectancy.

In order to investigate the feasibility of thesamples as a regenerable high temperature desul-furization sorbents a whole sulphidation-regenera-tion cycle (ZnO–ZnS–ZnO) was simulated. Sam-

ples were partially converted to ZnS in a simulatedsulphidation process involving batch autoclave forH2S exposure, while thermogravimetric analysis insynthetic air simulated regeneration process. Gener-ally, setup for sulphidation/desulfurization experi-ment usually consist of feed gas dosing section,equipped with mass flow controller, heated quartzreactor tube, where sulphidation and regenerationoccur, and equipment for measurement of the com-position of the gas leaving the reactor.26 The basicflaws of described setup are complexity, the pres-ence of temperature and gas composition gradientand non-uniformity of the gas velocity.26 In order todevelop a simpler and more affordable procedurefor testing the potential of prepared sorbents forhot-gas desulfurization process, a new type of sul-phidation experiment setup has been designed. Theextent of the reaction between prepared sorbentsfine particles and H2S has been studied using tef-lon-lined stainless steel laboratory autoclave. Theautoclave is a closed stainless steel vessel with aninternal cup and lid made of teflon. The autoclavecan be charged with reagents, and tightly closed.Under external heating, the contents will be raisedto high temperatures and pressures. For the purposeof this experiment the autoclave has been modifiedby adding the internal rack splitting the vessel intwo parts as shown in Fig. 5. In lower part a hydro-gen sulphide gas has been produced by the reactionof iron sulphide with hydrochloric acid. Holes inrack enabled undisturbed passage of H2S to upperpart where powder samples were placed in smallglass crucibles. Under high temperature and pres-sure hydrogen sulphide reacts with samples form-ing metal sulphide. To the best of our knowledge,such experimental setup for sulphidation experi-ment has not been described previously. Unfortu-nately, although the described procedure is simplerand shows no usual disadvantages of flow reactor,

V. MANDIÆ et al., Sol-gel Derived Mixed Metal Oxide Sorbents for High …, Chem. Biochem. Eng. Q. 27 (1) 43–49 (2013) 47

T a b l e 1 – BET specific surface of samples ZAT 1 – 5

Sample Specific surface area / m2 g–1

ZAT 1 89.58

ZAT 2 88.98

ZAT 3 79.05

ZAT 4 67.74

ZAT 5 33.58

F i g . 5 – A schematic representation of experimental as-sembly

it is not flawless. While it is very useful for com-parison of powders sulphidation ability, it is not ad-equate for the kinetic investigation and all experi-ments involving variations in temperature, due tomutual dependence of temperature and pressure inthe vessel. After sulphidation process all sampleswere structurally analyzed by means of XRD analy-sis. The diffractograms of sample ZAT 3 before andafter sulphidation are shown as example (Fig. 6).All scans generally resemble very well the dif-fractograms before sulphidation with one major dif-ference; the occurrence of ZnS diffraction lines inall samples in various intensity. At the same timethe intensity of zincite and spinel diffraction lineswas reduced. Obviously, during the sulphidationprocess ZnO and spinel solid solution were partiallyconverted to ZnS. In such manner a change of thecrystal composition in the samples was establishedand the reliability of the batch sulphidation methodwas confirmed. By enabling sufficient conversionto ZnS, the procedure served as a satisfactorymethod for simulation of conditions that realsorbent would have to go trough in a HTGD pro-cess.

The regeneration results enabled the validationof pollutant removal efficiency of samples with dif-

ferent compositions. The amount of ZnS formedduring the sulphidation process could be calculatedfrom the mass loss during the regeneration processas shown in Fig. 7. The TGA curve of referencesample (pure zincite) was used to demonstrate theprocedure. Assuming that all of the formed ZnS re-generates back to ZnO during the TGA measure-ment (in synthetic air flow), the calculation of thesulphidation efficiency is enabled only from ratio ofmolar masses. The regenerated sorbents did notsinter during the regeneration process; moreover nozinc sulfate (which can significantly decrease sorb-ent sulphide removal efficiency) was formed duringthe regeneration. For samples ZAT 1 – 5 the resultsare given in Table 2. As can be seen, sample ZAT 4(composition Zn1.75Al0.5Ti0.75O4) exhibits the bestsulphidation efficiency with conversion of 33.90 %.Such efficiency is in accordance with already re-ported results. Namely, in the case of Zn-basedHTGD sorbents, quite uniform pollutant removalresults can be revealed from literature. The conver-sions in successful sulphidation-regeneration cycleare primarily function of the Zn content and weremostly reported to range from 5–22 %.16,27,28 Obvi-ously, it is proven that the Zn content provides amain contribution in removal activity. However, asalready mentioned, the Zn content has an inverseinfluence on sulphidation efficiency and mechani-cal durability. Both aspects have to be consideredwhen optimizing the composition for the synthesisof the sorbent with overall best functionality.

Conclusions

The novel zinc oxide based sorbents for hightemperature gas desulfurization (HTGD) were pre-pared by sol-gel synthesis. FTIR analysis confirmedoptimal synthesis parameters. When thermally

48 V. MANDIÆ et al., Sol-gel Derived Mixed Metal Oxide Sorbents for High …, Chem. Biochem. Eng. Q. 27 (1) 43–49 (2013)

F i g . 7 – TGA regeneration of sulphidated ZnO referencesample

F i g . 6 – Comparison of diffractograms of sample ZAT 3before and after sulphidation

T a b l e 2 – Mass loss during the regeneration in TGA appa-ratus and calculated conversions

Sample Mass loss / % Conversion / %

ZAT 1 3.97 24.08

ZAT 2 3.81 23.10

ZAT 3 4.11 24.92

ZAT 4 5.59 33.90

ZAT 5 3.95 23.95

treated at 500 °C samples yield several crystalphases: gahnite, zinc titanate, zinc alumotitanate,zincite and brookite, and show high specific surfaceareas.

A novel batch reactor for sulphidization of pre-pared sorbents was introduced. Monitoring of theZnO–ZnS–ZnO cycle was enabled, revealing theefficiency of prepared sorbents. Sorbent with com-position Zn1.75Al0.5Ti0.75O4 was found to possess thehighest sulphidation reactivity, i.e. sulphur removalefficiency.

ACKNOWLEDGEMENT

The financial support of the Ministry of Sci-ence, Education and Sports of Republic of Croatiawithin the framework of the project No. 125-1252970-2981 “Ceramic nanocomposites obtainedby sol-gel process” is gratefully acknowledged.

S y m b o l s

�as � asymmetrical stretching mode�s � symmetrical stretching mode�as � asymmetrical bending vibration�s � symmetrical bending vibration�mmax � theoretical mass loss, %�mreg � registered mass loss, %M � molar mass, g mol–1

SBET � specific surface area (obtained by a BET N2gas adsorption-desorption isotherm)

A b b r e v i a t i o n s

Asb � aluminum sec butoxideATR � attenuated total reflectanceBET � Brunauer–Emmet–Teller (N2 gas adsorption-

desorption isotherm)CPS � counts per secondDTA � differential thermal analysisDTGA � differential thermo-gravimetric analysisEaa � ethyl acetoacetateFTIR � Fourier transform infrared spectroscopyHTGD � high temperature gas desulfurizationIGCC � integrated gasification combined cycleTGA � thermo-gravimetric analysisTnb � titanium n-butoxideXRD � X-ray diffractionZAT � zinc alumotitanate sample

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