A series of (Cu, Mn)-Mg-Al hydrotalcite-like layered

double hydroxides (HTs) was synthesized by a co-precipitation

method.The  incorporation  of  Mn  into  the  Mg-Al  and  Cu-Mg-

Al HTs structure was investigated by employing powder X-ray

diffraction.   Calcination  of  the  precursors  resulted  in  destruc-

tion of the layered structure and led to low-crystalline MgO. Ob-

tained mixed metal oxides were tested as catalysts for the process

of total oxidation of methanol.The catalytic performance of the

Cu-containing  samples  was  better  comparing  to  the  Mn-based

catalysts and decreased in the following order:  Cu-Mg-Al

>

Cu-

Mn-Mg-Al

>

Mn-Mg-Al

>

Mg-Al.  Quantitative total oxidation

of methanol was achieved at 325

 

C with Cu-Mg-Al mixed oxides

Chmielarz, Lucjan

Jabłońska, Magdalena

Piwowarska, Zofia

Jagiellonian Univeristy Repository

Total oxidation of methanolat lowtemperatures on (cU, mN)-mG-aL mixed metaloxides derived from hydrotalcitesMagdalena JABŁOŃSKA, Zofia PIWOWARSKA,Lucjan CHMIELARZ∗Keywords: oxidation; volatile organic compounds;hydrotalciteAbstract: A series of (Cu, Mn)-Mg-Al hydrotalcite-like layereddouble hydroxides (HTs) was synthesized by a co-precipitationmethod.The incorporation of Mn into the Mg-Al and Cu-Mg-Al HTs structure was investigated by employing powder X-raydiffraction. Calcination of the precursors resulted in destruc-tion of the layered structure and led to low-crystalline MgO. Ob-tained mixed metal oxides were tested as catalysts for the processof total oxidation of methanol.The catalytic performance of theCu-containing samples was better comparing to the Mn-basedcatalysts and decreased in the following order: Cu-Mg-Al > Cu-Mn-Mg-Al >Mn-Mg-Al > Mg-Al. Quantitative total oxidationof methanol was achieved at 325◦C with Cu-Mg-Al mixed oxides.1. IntroductionHydrotalcite-like compounds are a group of naturally occurring anionic clays. Thestructure of these materials can be described with the general formula of[MII1−xMIIIx (OH)2]x+Az−x/z· nH2O, where MII and MIII are two- and trivalent cations,respectively, A - interlayer anions, while x is the molar ratio of the metal cations,whose value varies in the range of 0.15-0.34 (Cavani et al., 1991). It is possible tosynthesize a broad variety of hydrotalcite-like materials containing different divalent(e.g. Mg2+, Ni2+, Zn2+ etc.) and trivalent cations (e.g. Al3+, Fe3+, Cr3+ etc.) aswell as both organic or inorganic interlayer anions (e.g. CO2−3 , Cr2O2−7 , Mo7O6−24 etc.)(Millange et al., 2000; Valente et al., 2009). Hydrotalcite-like materials and mixedoxides obtained by their thermal decomposition have been used as active environmen-tal catalysts (Chmielarz et al., 2002, 2011) and they can be an alternative catalystsfor the total oxidation of mono-carbon VOCs (e.g. methane, methanol, and formicacid) (Chmielarz et al., 2012). Mn-containing oxides derived from hydrotalcite-likecompounds have been widely studied as potential catalysts for total VOCs oxida-tion. For example, the successful useof Mg-Mn-(Al)mixed metal oxides obtained from∗Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland42 PhD Interdisciplinary Journalhydrotalcite-like materials was reported in catalytic combustion of methane (Jira´tova´et al., 2002) or toluene (Velu et al., 1999). On the contrary only few reports are re-lated to the applications of Cu-based hydrotalcite-like derivatives.An example couldbe Cu-Mn-Almixed metal oxidestudied in the process of the total oxidation of toluene(Palacio et al., 2010; Zimowska et al., 2007).The temperature needed for 50% conver-sion of toluene to CO2 was among the lowest reported in literature (Kovanda et al.,2001; Lamonier et al., 2007; Mikulova´ et al., 2007; Zimowska et al., 2007).In the present study similar catalytic systems based on hydrotalcite-like materialsare studied and its catalytic performance in the total oxidation of methanol is carriedout. A series of Cu/Mn-containing LDHs was synthesized by a simpleco-precipitationmethod, whereas the changes in the physicochemical properties were investigatedemploying XRDand BET techniques.2. Experimental part2.1. Samples preparationThe Mg-Al, Cu(Mn)-Mg-Al, Cu-Mn-Mg-Al hydrotalcite-like samples with the in-tended molar ratios of 71.0:29.0, 5.0:66.0:29.0 or 2.5:2.5:66.0:29.0 were synthesizedbya co-precipitation method using 1 M aqueous solutions of the following metalnitrates: Cu(NO3)2·3H2O (Merck), Mn(NO3)2·4H2O (Lach-Ner), Mg(NO3)2·6H2O(Sigma) and Al(NO3)3·9H2O (Fluka). A solution of NaOH (POCh) was used as aprecipitating agent. Solution of metal nitrates and NaOH were simultaneously addedto a vigorously stirred solution containing a slight excess of Na2CO3 (POCh). ThepH was maintained at 9.0 ± 0.2 the constant value (with the exception of Mg-Al andCu-Mg-Al hydrotalcite-like materials, which were synthesized at pH = 10.0 ± 0.2)by dropwise addition of 1 M NaOH solution. The obtained slurry was aged at 60 ◦Cfor 1 h, filtered, washed with distilled water and dried at room temperature. Mn-containing hydrotalcite-like materials were synthesized with stirring under an argonatmosphere. The as-synthesized samples were calcined in static air at 600 ◦C for 12 h.For catalytic experiments, a fraction of the catalysts with a particle size in the rangeof 0.160-0.315 mm was selected.2.2. Samples characterizationThe crystalline structure of the as-synthesized and calcined samples was analysed bymeans of an X-ray powder D2 Phaserdiffractometer (Brucker) using Cu Kα radiation(λ = 1.54060 L´, 30 kV, 10 mA). Cell parameters were calculated using a positionof 110 reflection: (a = 2(d110)) and positions of basal reflections(c = [3(d003) +6(d006)]/2 ). The parameter a corresponds to the cation-cation distance within thebrucite-like layer, whereas the parameter c is related to the thickness of the brucite-like layer and interlayer distance. Crystal sizes were calculated from the Scherrerequation D = 0.89 λ/βcosθ, where D is the crystallite size, λ - the X-ray wavelength,β- the line broadening and θ- the Bragg angle.The specific area of the calcinedhydrotalcite-like materials was determined by low-temperature (-196 ◦C) N2 sorption using Quantasorb Junior sorptometer (Ankersmit).Prior to nitrogen adsorption the samples were out gassed at 250 ◦C for 2 h in a flow ofN2 (1.92 cm3/min). The measurements were performed in a flow of N2 (1.92 cm3/min)43and He (16.88 cm3/min).2.3. Catalytic studiesCalcined hydrotalcites were tested as catalysts of methanol incineration. The cat-alytic experiments were performed under atmospheric pressure in a fixed-bed flowmicroreactor system. The reactant concentrations were continuously measured usinga quadruple mass spectrometer RGA 200 (PREVAC) connected directly to the reac-tor outlet. Prior to the catalytic test each sample of the catalyst (100 mg) was outgassed in a flow of synthetic air 500 ◦C for 30 min. The isothermal saturator witha constant flow of synthetic air was used for supplying of methanol into the reactionmixture. The composition of gas mixture at the reactorinlet was [CH3OH] = 4.0vol.%, [O2]=19.0 vol.% and[N2]=77.0 vol.%. The reaction was studied in the rangefrom 100 to 500 ◦C with the linear temperature increase of 10◦C/min. Total flow rateof the reaction mixture was 20 cm3/min.3. Results and discussion3.1. Powder X-ray diffractionPowder XRD diffraction pattern obtained for the as-synthesized hydrotalcite-like ma-terials and their derivatives are presented in Fig. 1. The obtained precursors exhibit atypical crystalline hydrotalcite-like structure belonging to the space group R3m in thetrigonal symmetry (Węgrzyn et al., 2010). Cell parameters and crystallites sizes ofthe hydrotalcite-like samples, determined from XRD measurements, are presented inTab. 1. The cell parameter c for all precursors obtained in the study of about 2.3 nmis typical for hydrotalcites containing carbonate as the interlayer anion (Chmielarzet al., 2011). XRD patterns of the calcined samples contain only the typical patternof the poorly crystallized MgO-type mixed oxides (reflections at 36 ◦, 43 ◦ and 63 ◦)(Cavani et al., 1991).Tab. 1. Structural parameters of hydrotalcite-like precursorsSamplecodesPFWHM of(003) peak(2Θ)Cellparametera [nm]Crystallitesize Da[nm]Cellparameterc [nm]Crystallitesize Dc[nm]Mg-Al 1.1106 0.3046 24 2.2888 15Cu-Mg-Al 1.1142 0.3050 26 2.2868 15Cu-Mn-Mg-Al 1.6237 0.3054 26 2.3155 13Mn-Mg-Al 1.3920 0.3062 29 2.3121 153.2. Specific surface area measurementsThe specific surface area (SBET ) of the calcined samples are shown in Tab. 2. Specificsurface area of the samples depends on their chemical composition. It should be noted44 PhD Interdisciplinary Journalthat incorporation of copper and/or manganese into the hydrotalcite-like framework,reduces their surface area comparing to the calcined Mg-Al sample.Tab. 2. BET surface area of the calcined samplesSample codes BET surface area [m2/g]Mg-Al-600 173Cu-Mg-Al-600 121Cu-Mn-Mg-Al-600 118Mn-Mg-Al-600 1503.3. Catalytic activitiesCalcinedhydrotalcites were tested as catalysts for the complete oxidation of methanol.Carbon dioxide and water vapour were the only detected reaction products of methanoloxidation. Tab. 3 presents the results of methanol incineration.The activities of thecatalysts are frequently characterized by two parameters, T50 and T90. T50 refers tothe temperature needed to reach 50% conversion, and is widely used to compare cat-alytic activity, while T90 is the temperature necessary for 90% conversion of methanol.Fig. 1. X-ray diffraction patterns of (a) hydrotalcite-like precursors: Mg-Al, Cu-Mg-Al, Cu-Mn-Mg-Al, Mn-Mg-Al, (b) calcined materials: Mg-Al-600, Cu-Mg-Al-600, Cu-Mn-Mg-Al-600, Mn-Mg-Al-600; H – hydrotalcite-like materials, P –periclase (MgO)45Tab. 3. T50 and T90 temperatures of methanol oxidation over (Cu, Mn)-Mg-Al cata-lystsSample codes T50/◦C T90/◦CMg-Al-600 346 396Cu-Mg-Al-600 252 281Cu-Mn-Mg-Al-600 285 319Mn-Mg-Al-600 302 340Among all the studied catalysts, Cu-Mg-Al mixed oxides showed superior catalyticperformance over the others both at T50 and T90. The oxidation of methanol overthese catalysts started at about 225◦C, while its complete oxidation was possible at325◦C. The Mn-Mg-Al and Mg-Al mixed oxides were the least active catalyst in astudied series.4. ConclusionMg-Al, Cu-Mg-Al, Cu-Mn-Mg-Al and Mn-Mg-Al hydrotalcite-like materials were suc-cessfully synthesized, what was confirmed by the results of powder X-ray diffractionstudies. Calcination of the precursors at 600◦C resulted in destruction of the lay-ered structure and led to the formation of mixed metal oxides. Specific surface areastrongly depended on the composition of the sample and decreased after introductionof transition metal into the Mg-Al structure. Activity of the catalysts in the reactionof complete oxidation of methanol strongly depended on the kind of transition metalintroduced to the mixed oxides system. The Cu-containing catalysts were found to beactive catalysts for total methanol oxidation. Further studies using different copperconcentration, different calcination temperature and doping the material with somealkali or noble metals focused on the improvement of the catalytic activity of thestudied samples are planned.ReferencesCavani, F, F Trifiro and A Vaccari (1991), ‘Hydrotalcite type anionic calys: prepara-tion, properties and applications.’, Catalysis Today pp. 173–301.Chmielarz, L., A. Węgrzyn, M. Wojciechowska, S. Witkowski and M. Michalik (2011),‘Selective catalytic oxidation (sco) of ammonia to nitrogen over hydrotalcite origi-nated mg-cu-fe mixed metal oxides.’, Catalysis Letters pp. 1345–1354.Chmielarz, L., P. Kuśtrowski, A. Rafalska-Łasocha, D. Majda and R. Dziembaj(2002), ‘Catalytic activity of co-mg-al, cu-mg-al and cu-co-mg-al mixed oxides de-rived from hydrotalcites in scr of no with ammonia.’, Applied Catalysis B: Envi-ronmental pp. 195–210.Chmielarz, L., Z. Piwowarska, M. Rutkowska, M. Wojciechowska, B. Dudek, S.Witkowski and M. Michalik (2012), ‘Total oxidation of selected mono-carbon vocsover hydrotalcite originated metal oxide catalysts.’, Catalysis Communicationspp. 118–125.46 PhD Interdisciplinary JournalJira´tova´, K., P. Cˇuba, F. Kovanda, L. Hilaire and V. Pitchon (2002), ‘Preparation andcharacterisation of activated ni (mn)/mg/al hydrotalcites for combustion catalysis.’,Catalysis Today pp. 43–53.Kovanda, F., K. Jiratova´, J. Rymes and D. Kolousek (2001), ‘Characterization of ac-tivated cu/mg/al hydrotalcites and their catalytic activity in toluene combustion.’,Applied Clay Science pp. 71–80.Lamonier, J.F., A.B. Boutoundou, C. Gennequin, M. J. Pe´rez-Zurita, S. Siffert andA. Aboukais (2007), ‘Catalytic removal of toluene in air over co-mn-al nano-oxidessynthesized by hydrotalcite route’, Catalysis Letters pp. 165–172.Mikulova´, Z., P. Cuba, J. Balanova´, T. Rojka, F. Kovanda and K. Jiratova´ (2007),‘Calcined ni-al layered double hydroxide as a catalyst for total oxidation of volatileorganic compounds: Effect of precursor crystallinity.’, Chemical Papers pp. 103–109.Millange, F., R.I. Walton and D. O’Hare (2000), ‘Time-resolved in situ x-ray diffrac-tion study of the liquid-phase reconstruction of mg-al-carbonate hydrotalcite-likecompounds.’, Journal of Material Chemistry pp. 1713–1720.Palacio, L. A., J. Vela´squez, A. Echavarr´ıa and A. 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Total oxidation of methanol low temperatures on (Cu, Mn)-Mg-Al mixed metal oxides derived from hydrotalcites

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