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Green synthesis of hydrotalcite from untreated magnesium oxide and aluminum hydroxide

The influence of reaction temperature and time on the hydrothermal dissolution-precipitation synthesis of hydrotalcite was investigated. Untreated MgO, Al(OH)3 and NaHCO3 were used. An industrially beneficial, economically favourable, environmentally
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  Full Terms & Conditions of access and use can be found at Green Chemistry Letters and Reviews ISSN: 1751-8253 (Print) 1751-7192 (Online) Journal homepage: Green synthesis of hydrotalcite from untreatedmagnesium oxide and aluminum hydroxide F. J. W. J. Labuschagné, A. Wiid, H. P. Venter, B. R. Gevers & A. Leuteritz To cite this article:  F. J. W. J. Labuschagné, A. Wiid, H. P. Venter, B. R. Gevers & A. Leuteritz(2018) Green synthesis of hydrotalcite from untreated magnesium oxide and aluminum hydroxide,Green Chemistry Letters and Reviews, 11:1, 18-28, DOI: 10.1080/17518253.2018.1426791 To link to this article: © 2018 University of Pretoria. Publishedby Informa UK Limited, trading as Taylor &Francis Group The Author(s). Published byPublished online: 24 Jan 2018.Submit your article to this journal Article views: 69View related articles View Crossmark data  Green synthesis of hydrotalcite from untreated magnesium oxide and aluminumhydroxide F. J. W. J. Labuschagné  a , A. Wiid a , H. P. Venter  a , B. R. Gevers  a and A. Leuteritz b a Department of Chemical Engineering, University of Pretoria, Hatfield, South Africa;  b Leibniz-Institut für Polymerforschung Dresden e.V.,Dresden ABSTRACT The influence of reaction temperature and time on the hydrothermal dissolution-precipitationsynthesis of hydrotalcite was investigated. Untreated MgO, Al(OH) 3  and NaHCO 3  were used. Anindustrially beneficial, economically favourable, environmentally friendly, zero effluent synthesisprocedure was devised based on green chemistry principles, in which the salt-rich effluenttypically produced was eliminated by regenerating the sodium bicarbonate in a full recycleprocess. It was found that the formation of hydromagnesite dominates at low temperaturesindependent of reaction time. With an increase in reaction time and temperature, hydromagnesitedecomposes to form magnesite. At low temperatures, the formation of hydrotalcite is limited bythe solubility of the Al(OH) 3 . To achieve a hydrotalcite yield of 96%, a reaction temperature of 160°C for 5h is required. A yield higher than 99% was achieved at 180°C and 5h reaction time,producing an layered double hydroxide with high crystallinity and homogeneity. ARTICLE HISTORY Received 21 October 2017Accepted 8 January 2018 KEYWORDS LDH; hydrotalcite; greensynthesis; dissolution-precipitation; hydrothermalsynthesis 1. Introduction Layered double hydroxides (LDHs) are layered anionicclays with the general formula [ M II1 − x M IIIx  (OH) 2 ][ X q − x / q · nH 2 O ] with [M II1 − x M IIIx  (OH) 2 ] describing the composition of theLDH layers consisting of trivalent (M III ) and divalent(M II ) metal cations, and [X q − x / q · nH 2 O] representing thecomposition of the anionic interlayer ( 1 , 1021). Hydrotal-cite is a naturally occurring LDH form with the formulaMg 6 Al 2 (OH) 16 CO 3 ·4H 2 O ( 2 ). In its synthetic form, hydro-talcite can exist in Mg:Al ratios ranging from 1:1 to 3:1( 1 , 1024). Hydrotalcite has a wide range of applicationswhich include the use as polymer stabilizers, flame-retardants for polymers and anion scavengers ( 3 ;  4 ;  5 ; 6 ) as well as a wide range of environmental applicationswhich are comprehensively discussed in the Handbook of Clay Science ( 7  ) and include applications as basic cat-alysts and water purification agents.Various synthesis methods for hydrotalcite have beendeveloped, the most widely used being co-precipitation,urea hydrolysis and sol – gel ( 8 ). Although these methodscan yield high-quality material, they also present majordrawbacks: .  Hydrotalcite produced by these synthesis methodsusually requires post-synthesis treatment such ashydrothermal aging to increase the crystallinity ( 9 ); .  Metals salts used for the synthesis are expensive; © 2018 University of Pretoria. Published by Informa UK Limited, trading as Taylor & Francis GroupThis is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial License ( ), whichpermits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the srcinal work is properly cited. CONTACT  B.R. Gevers GREEN CHEMISTRY LETTERS AND REVIEWS, 2018VOL. 11, NO. 1, 18 – 28  .  Metal salts used as reactants in these methods invari-ably result in the production of salt-rich effluentstreams that are harmful to the environment;and .  Expensive effluent treatment prior to disposal isrequired.In order to eliminate some of these drawbacks, alterna-tive synthesis methods have been developed  –  one of which is a hydrothermal synthesis route which offers analternative to existing synthesis methods ( 7  ). In the hydro-thermal synthesis route,metal oxides/hydroxidesareusedas reactants and a dissolution-precipitation mechanism istypicallyemployedtoformtheLDHphase( 10 ).Hydrother-mal synthesis offers a greener production approach byeliminating the additional processes required to treatharsh salt-containing effluentproducedduring co-precipi-tation and urea hydrolysis. The hydrothermal synthesisroute does, however, present one major drawback: themetal oxides/hydroxides required have a very low solubi-lity. This necessitates the establishment of the correct syn-thesis conditions for the formation of LDH at the desiredquality.Dyer et al. ( 11 ) discussed the parameters that influencethe solubility (and therefore the dissolution/precipitationreactions) of metal oxides/hydroxides/carbonates; theseinclude: particle size, crystallinity, reaction temperature,pH and time. Castet et al. ( 12 ) indicated thatthe hydrolysisof boehmite (AlO(OH)) is favored with an increase intemperature. A possible shift of boehmite solubility withan increase in temperature and pH was observed. Theseresults were confirmed by Bénézeth et al. ( 13 ). Therefore,itcanbe concludedthatanalkalineaqueous environmentwould favor the solubility of some metal species such asAl(OH) 3  at higher temperatures which in turn is favorableforLDHformation.ApHabove10ispreferableforthepre-cipitation of MgAl-CO 3  LDH ( 8 ).In previous investigations concerning the hydrothermalsynthesis of hydrotalcite, great care was taken to enhancethe reactivity of reactants prior to synthesis. In general, thepre-treatment entailed calcination of magnesium- andaluminum salts to produce a fine-powdered oxidemixture ( 14 ). Xu and Lu ( 10 ) investigated the formationmechanism of hydrotalcite. They stated that magnesiumnitrate and aluminum hydroxide should be calcined toproduce magnesium oxide and alumina (Al 2 O 3 ) with ahigh surface area to yield highly reactive reactants. Thesewere then reacted hydrothermally for long periods (5 – 10 days) at temperatures near boiling. Hydrotalcite withslight brucite contamination was formed.Mitchell et al. ( 15 ) investigated the production of specific poly-types of hydrotalcite with hydroxide anionsin the interlayer. Budhysutanto et al. ( 9 ) used reactantgrade magnesium oxide and partially amorphous tran-sition alumina prepared by flash calcination of gibbsitein hydrothermal synthesis between 100°C and 240°C forup to 1h, reaching high conversions after approximately30min with slight brucite impurities. They found thatother pre-treatment methods include blending, ultrasonictreatment and wet grinding to increase surface area andsubsequently enhance reactivity. These pre-treatments  – especially calcination  –  are generally energy intensivedue to the high temperatures required ( 9 ).It is evident that most synthesis techniques are cost-and time intensive, require extensive pre- or post-treat-ment and produce undesirable effluent streams. Theobjective of this study, therefore, was to find an environ-mentally friendly synthesis route capable of producing ahigh-quality hydrotalcite without the use of expensivereactants. To improve the economic viability andcomply with green synthesis principles, processingsteps, such as pre-treatment of reactants and productpost-treatment, should be eliminated. In addition, theproduction of treatment-intensive effluent streamsshould be avoided. Finally, the optimum reaction con-ditions using this environmentally friendly synthesisroute should be found. 2. Experimental  2.1. Materials Chemically pure (CP) grade reactants were used through-out. Magnesium oxide light 96% (CAS nr. 1309-48-4),aluminum hydroxide 99% (CAS nr. 21645-51-2) andsodium hydrogen carbonate 99% (CAS nr. 144-55-8)were all obtained from Merck KGaA  –  South Africa. Alca-mizer 1 used for comparative purposes was obtainedfrom Kisuma Chemicals.  2.2. Synthesis A laboratory scale version of the green synthesis route byLabuschagné et al. ( 16 ;  17  ) was followed to producehydrotalcite samples with a Mg:Al molar ratio of 2:1.This method is based on hydrothermal dissolution-pre-cipitation. The procedure was designed to comply withgreen-synthesis principles as discussed in the introduc-tion. A schematic of the devised synthesis procedure isshown in Figure 1.All reactants were used without pre-treatment. Theuntreated metal oxide/hydroxide solution for eachexperiment was prepared by stoichiometrically weighingoff the required chemicals according to Equation (2) andadding them to separate beakers containing distilledwater under constant stirring. A 20%wt total solids GREEN CHEMISTRY LETTERS AND REVIEWS 19  loading was used.4MgO + 2Al(OH) 3 + 2NaHCO 3 + 5H 2 O − Mg 4 Al 2 (CO 3 )(OH) 12 · 3H 2 O + Na 2 CO 3 The mixture was stirred for 5min before addition to a 450mlSeries4560Parrautoclavereactorfittedwithapressuregauge and a fixed tungsten-iron thermocouple. Tempera-ture was controlled with a Parr 4843 controller. Thesamples were reacted at temperatures ranging between120°C and 180°C for 1 –  5h. After each run, the reactorwas allowed to cool down by natural convection andwas drained as soon as the temperature reached 50°C.Each sample was washed with distilled water until a fil-trate pH of 8 was reached. The filter cake was dried in aconvection oven at 70°C for 8h. All filtrate was collected.The sodium bicarbonate (NaHCO 3 ) was regenerated bybubbling CO 2  through the sodium carbonate (Na 2 CO 3 ) fil-trate solution from a previous run  –  thus eliminating theneed for expensive salt effluent treatment and onlyneeding small amounts of sodium carbonate as part of a make-up stream if required.Possible side reactions of magnesium were investi-gated by reacting magnesium oxide and the carbonatesource according to Equation (2) in the absence of alumi-num hydroxide over a temperature range of 120 – 180 °Cfor the upper time limit of 5h.  2.3. Characterization Wide angle X-ray scattering (WAXS) measurements wereperformed with the X ’ pert-Pro diffractometer systemfrom PANalytical with a continuous scanning using astep size of 0.001° between the ranges 5.01° and 89.99°2 θ . Co-K  α  radiation was generated with a wavelengthof 0.1789 nm at 50mA and 35kV by a diffractometer.Theirradiatedlength was keptat 15mmwithaspecimenlength of 10mm. The distance focus-diverge slit waskept at 100mm and the measuring time was kept at D t  = 14 . 52s for each point.A SPECTRO ARCOS model Inductively Coupled PlasmaOptical Emission Spectrometer (ICP-OES) was used forthe determination of the LDH aluminum and magnesiumratio. For analysis, 1g of each LDH sample was dissolvedin 20ml of 37% hydrochloric acid solution and dilutedwith distilled water in a 1:10 ratio. A multi-element stan-dard ICP grade from Merck was used for comparison. Theanalysis was performed using argon gas plasma. ICPanalysis was done in triplicate.Scanning electron microscopy (SEM) was used toconfirm the morphological characteristics expected forthe hydrotalcite phase. The samples were coated withgraphite (three coats) using a Polaron h/v coater. Thecoated samples were then analysed using Ultrahigh Res-olution Field Emission SEM (JEOL 6000F) at 3kV. A GatanDigital Micrograph imaging system was used.Thermogravimetric analysis (TGA) was used to confirmthe expected two-step decomposition of hydrotalcite. AMettler Toledo A851 simultaneous TGA/DTA setup wasused for differential thermal analysis and TGA. A smallamount of the powder sample (ca. 10mg) was placed inan open 70 μ l alumina pan and heated from 25°C to1000°C at a rate of 25°C/min in air flow (50mlmin − 1 ). 3. Results and discussions It is believed that the formation of hydrotalcite via thereaction of insoluble or low solubility metal oxides,hydroxides or carbonates occurs through a Figure 1.  Block diagram of effluent free hydrotalcite production showing recycle routes and processing steps required. 20 F. J. W. J. LABUSCHAGNÉ ET AL.  dissociation – deposition – diffusion mechanism ( 10 ). Inorder to commence the reaction, the metal oxides/ hydroxides must dissolve to at least to some extent.Magnesium oxide loaded into the reactor must dissolveand react with water to precipitate as brucite (mag-nesium hydroxide) which is the base layer of the LDHmaterial. Aluminum hydroxide must dissolve to makealuminum ions available for isomorphic replacementof some of the magnesium ions in the brucite layersto form the positively charged layers required as partof the desired hydrotalcite structure. The anion interca-lated into the structure balances the positively chargedlayers and causes hydrotalcite to precipitate; crystalwater is trapped in the interlayer during this process( 10 ). If hydrotalcite is the least soluble species at thesystem temperature and pressure, it precipitates andremoves the relevant ions from solution, causingmore metal ions from metal oxides/hydroxides fedto the reaction to dissolve following Le Chatelier ’ sPrinciple.Of course, given the right conditions, other speciescan also form. For example, according to Hartmann-Petersen et al. ( 18 ), magnesite (MgCO 3 ) is a highlystable carbonate form of magnesium that is producedby bubbling CO 2  through a magnesium hydroxideslurry; the precipitation occurs relatively rapidly.Furthermore, when an excess of CO 2  is bubbledthrough a magnesite solution, carbonic acid is formedcausing the solution to become more acidic. This aidsin the solubility of the magnesite and forms hydratedmagnesium carbonates (such as hydromagnesite4MgCO 3 ·Mg(OH) 2 ·2(H 2 O)), a more soluble form of mag-nesium ( 19 ). The formation of hydrated magnesium car-bonates generally occurs at low temperatures in anexcess carbonate environment and is referred to as “ the magnesite problem ”  ( 20 ).Although no CO 2  was bubbled through the reactionmixture per se, CO 2  released from the aqueous bicarbon-ate environment as a result of temperature, pressure andpH decomposition ofcarbonic acid would haveled totheformation of hydromagnesite as observable in Figures 2and 3. These figures show experimental results for theexperiments conducted. All reflections on the XRD pat-terns were identified. Quantitative results based on Riet-veld refinement for all the experiments conducted areshown in Table 1. The experimental results will now bediscussed further. 3.1. Reactions without an aluminum source As previously mentioned, the purpose of these exper-iments (results shown in Figure 2) was to observe the for-mationof side products atthe reaction conditions tested. 3.1.1. Formation of Al-based products The formation of hydrotalcite was observed at 180°C, ascan be seen from Figure 2, and is indicated in bold inTable 1. This is impossible as no aluminum source wasadded to the reactor intentionally. Unintended contami-nation must, therefore, have occurred. 3.1.2. Formation of Mg-based complexes Trace amounts of MgO (less than 1%) were detected inthe samples prepared at temperatures greater or equalto 140°C. Typically, it is expected that all caustic MgOadded to the reactor reacts with water to form Mg(OH) 2  at all reaction temperatures. The results were,therefore, surprising and it was concluded that a possibleexplanation for this observation could be the presence of a highly crystalline and unreactive form of MgO impurityin the feed material. Nevertheless, the XRD and Rietveldrefinement data confirmed the presence of large quan-tities of brucite in all the samples besides this MgOimpurity.Due to the known  “ magnesite problem, ’  it was also of interest which reaction path is followed when adding aNaHCO 3  to the mixture and in which form the mag-nesium precipitates at a given temperature: magnesiteor hydromagnesite. Magnesite, which is a frequentlyfound precipitate when adding carbonate to a solutionof brucite and water, was not detected for the synthesisperformed at 120°C; hydromagnesite was found. Whenusing intermediate reaction conditions  –  a temperatureof 140°C and 5h reaction time  –  both hydromagnesiteand magnesite were formed. When increasing the reac-tion temperature further to 160°C and 180°C, no hydro-magnesite was detected but magnesite was found.As described in the literature ( 20 ), it is evident fromFigure 2 and Table 1 that the formation of magnesite is favored at higher temperatures, while hydromagnesiteformation is favored at lower temperatures. Their for-mation was found not to be completely driven by thecarbonate concentration. 3.2. Reactions with an aluminum source The discussion will now refocus on the formation of theLDH phase when adding an aluminum source to thereaction. 3.2.1. Products formed at 120°C  When looking at the experimental series conducted at120°C (Figure 3) several things can be observed. Firstly,a large reflection situated at a 2 θ  value of 21° which cor-responds to the primary reflection of crystalline,unreacted gibbsite left in the samples, can be noted, GREEN CHEMISTRY LETTERS AND REVIEWS 21
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