Glycothermal Synthesis and Catalytic Properties of

Authors: Wiktoria Walerczyk Mirosław Zawadzki Hanna Grabowska

Publish Date: 2011/01/15

Volume: 141, Issue: 4, Pages: 592-601

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Abstract

Zn1−x Co x Al2O4 x = 0 05 10 spinels were synthesised under microwaveassisted solvothermal conditions using 14butanediol as reaction medium The results of XRD and HRTEM have indicated nanocrystallinity of prepared materials average crystallite size in the range 6–16 nm and N2 adsorption–desorption measurements have revealed high feature of their surface area and mesoporous nature Acid–base properties of prepared materials were determined using TPDNH3 method and cyclohexanol test All studied spinels that underwent examination were active in phenol methylation but Cosubstituted zinc aluminate Zn05Co05Al2O4 was found to be the most efficient in the selective formation of orthomethylated products It could be ascribed to its dominant basic character and the presence of weak and strong acid centres and the highest surface area as wellSpinels are mixed oxides with general formula AB2O4 where A represents a divalent metal cation that usually occupies a tetrahedral site while B represents trivalent metal cation that normally occupies the octahedral sites of a cubic crystal structure In normal spinel structure A2+ and B3+ occupy 8 tetrahedral and 16 octahedral sites respectively while in the inverse structure B3+ ions may occupy both octahedral and tetrahedral sites depending on the degree of inversion Moreover so called random spinels are also possible in which there is statistical distribution of ions between both sites The physicochemical properties of spinels depend mainly on cations distribution and their nature which may be changed or modified by the introduction of the third metal to the spinel structure Various substitutions may be incorporated into it so adequate selection of substituting ion and appropriate chemical composition are of the utmost importance in order to achieve desired properties of material including its catalytic performances Synthesis method is also a significant factor influencing the properties of the obtained material such as crystallite size specific surface area or porosity 1 2 3Common spinel compounds known as aluminate family including ZnAl2O4 have been already studied as catalysts and catalysts’ carriers because of their combination of desirable properties such as high mechanical resistance high thermal stability low sintering temperature low surface acidity and high ability of cations diffusion Therefore they are mainly used as high temperature ceramics but also as useful material for optical and catalytic applications In our earlier studies we elaborated the high active aluminate catalysts which enable obtaining methylated phenols 4 synthesis of 1methylimidazole 5 and efficient methylation of 2 and 4hydroxypyridine 6 7Aluminates may be obtained by many different methods eg solid state reaction ceramic mechanochemical method 8 or wet chemical routs such as coprecipitation 9 sol–gel 10 solvothermal methods 11 and some other methodologies like molten salts synthesis 12 combustion in various solutions 3 polymeric precursors method 13 etc Glycothermal method being one type of the solvothermal process offers some advantages over most other methods like short reaction time the possibility of using various precursors and reaction media Different reaction parameters such as pressure temperature synthesis time and even heating method can affect the reaction products 14 Mixed oxides obtained by glycothermal synthesis are usually nanocrystalline materials with wellcontrolled chemical composition and very interesting textural properties high surface area mesoporosity 15Phenol belongs to compounds characterised by high reactivity which alkyl derivatives can be obtained during alkylation with olefins or alcohols under relatively mild conditions in the liquid as well as gas phase Usually the reaction of phenol with methanol is performed in the gas phase in the presence of catalyst Reaction conducted in the gas phase has many advantages especially when used catalyst is highly active and selective In the process carried out in such a way the troublesome separation of reaction products is eliminatedThe most important commercial alkylphenols are the methyl derivatives the cresols and xylenols among them orthocresol and 26xylenol belong to the most crucial because of their further applications They are the most sought after for the further production of agrochemicals antioxidants plastics pharmaceuticals and very useful polymers etc There is always competition between C and Oalkylation and the strength of acid centres profoundly affects the product selectivity 16 The surface of catalysts should possess both basic and acidic mainly Lewis properties to secure a high yield of Cmethylated phenol derivatives The catalyst selectivity is attributed to a presence of acidic centres possessing weak and medium strength 17 For catalysts exhibiting mainly Brönsted acidic sites especially strong the selectivity towards Cmethylated products decreased with temperature 16 It is accepted that only Brönsted acidity is responsible for coke formation Suitable catalysts for the gas phase methylation of phenol with methanol have been reported in the literature Catalysts having basic properties such as MgO 18 19 20 or Fe2O3based 21 22 23 produce predominantly ring alkylated products with high orthoselectivity With the increase in the acidity of catalysts eg for Al2O3 or zeolites 24 25 26 the selectivity for Calkylated products decreases and products of Oalkylation are obtained Among active catalysts in the phenol methylation materials with spinel structure are very interesting eg ferrites 27 28 29 30 and also others 31 32 33 Issues concerning the mechanism of phenol alkylation have been presented in many publications Among them the works of Mathew et al 28 34 35 36 37 are worth to be quoted in which the authors have presented a proposal for the mechanism of phenol methylation occurring on wellcharacterised spinel catalysts They underlined the role of an acid–base pair sites as the active centres for the orthomethylation reactionsSome of the present authors have studied catalytic reaction of phenol with methanol leading to orthocresol and 26xylenol in the presence of ZnAl2O4 It was stated that zinc aluminate was an active and selective catalyst in the alkylation of phenol and other hydroxyarenes 4 6 7 38 The catalytic properties in methylation of phenol of zinc aluminate with incorporation of different metals eg copper and iron were also investigated 39 40The aim of the present work was to study the effect of substitution of Zn2+ by other divalent cation namely Co2+ in the framework of zinc aluminate spinel on phenol conversion and orthomethylated products selectivity during methylation Zn1−x Co x Al2O4 x = 0 05 1 spinels prepared under microwaveassisted glycothermal conditions were characterized using XRD TEM/SEM N2 adsorption–desorption TPDNH3 and CHOL test to find a relationship between the structure acid–base properties and the catalytic behaviour

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