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Valpos As Efficient Catalysts for Glycerol Conversion to Methanol

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Valpos As Efficient Catalysts for Glycerol Conversion to Methanol catalysts Article VAlPOs as Efficient Catalysts for Glycerol Conversion to Methanol 1, , 2, 2 2 Gheorghit, a Mitran * y, Florentina Neat, u y,S, tefan Neat, u , Mihaela M. Trandafir and Mihaela Florea 1,2,* 1 Department of Organic Chemistry, University of Bucharest, Biochemistry & Catalysis, Faculty of Chemistry, 4-12, Blv. Regina Elisabeta, 030018 Bucharest, Romania 2 National Institute of Material Physics, 405A Atomi¸stilor, P.O. Box MG 7, 077125 Măgurele, Romania; florentina.neatu@infim.ro (F.N.); stefan.neatu@infim.ro (S, .N.); mihaela.trandafir@infim.ro (M.M.T.) * Correspondence: [email protected] (G.M.); mihaela.florea@infim.ro (M.F.) Authors with equal contribution. y Received: 24 April 2020; Accepted: 28 June 2020; Published: 1 July 2020 Abstract: The catalytic activity of a series of vanadium aluminophosphates catalysts prepared by sol-gel method followed by combustion of the obtained gel was evaluated in glycerol conversion towards methanol. The materials were characterized by several techniques such as X-ray diffraction (XRD), UV-vis, Fourier-transform infrared (FTIR), Raman and X-ray photoelectron (XPS) spectroscopies. The amount of vanadium incorporated in aluminophosphates framework played an important role in the catalytic activity, while in the products distribution the key role is played by the vanadium oxidation state on the surface. The sample that contains a large amount of V4+ has the highest selectivity towards methanol. On the sample with the lowest vanadium loading the oxidation path to dihydroxyacetone is predominant. The catalyst with higher content of tetrahedral isolated vanadium species, such V5APO, is less active in breaking the C–C bonds in the glycerol molecule than the one containing polymeric species. Keywords: vanadium aluminophosphates; glycerol oxidation; glycerol dehydration; methanol synthesis; dihydroxyacetone; acetaldehyde 1. Introduction The conversion of glycerol in added value products has been the subject of extensive research due to its excess in the world market, whereas approximately 100 kg of glycerol is produced for each ton of biodiesel [1]. Glycerol is very cheap and could be harnessed by dehydration, esterification, oxidation, hydrogenolysis, reforming and polymerization, and, therefore, glycerol is expected to become a renewable building block chemical to be efficiently converted into various more added value products. The selective oxidation of glycerol to high-value chemicals with molecular oxygen has been extensively studied [2–5]. The catalytic glycerol oxidation occurs by a complex mechanism and conducts to many products such as formic acid [6], acrolein [7], acrylic acid [8,9], glycolic acid [10], glyoxylic acid, glyceric acid [11], glyceraldehyde and dihydroxyacetone [12,13] (see Scheme1)[ 6]. These products have many uses in the chemical industry, e.g., dihydroxyacetone can be used in cosmetics industry; glyceric acid for detergents [14]; formic acid is used as a hydrogen carrier [15]; and acrylic acid is used mainly in the production of acrylic esters and resins used in coatings and adhesives [16]. Catalysts 2020, 10, 728; doi:10.3390/catal10070728 www.mdpi.com/journal/catalysts Catalysts 2020, 10, 728 2 of 20 Catalysts 2020, 10, x FOR PEER REVIEW 2 of 23 SchemeScheme 1. 1.Parallel Parallel and and series series reaction reaction network network in in glycerol glycerol transformation. transformation. Therefore,Therefore, the the glycerol glycerol oxidation oxidation has has been been extensively extensively studied studied over over numerous numerous catalysts catalysts yielding yielding variousvarious byproductsbyproducts [3[3,17,18].,17,18]. WangWang etet al.al. [19[19]] obtained,obtained, mainly,mainly, acroleinacrolein andand acetaldehyde acetaldehyde overover vanadiumvanadium phosphatephosphate oxides,oxides, Ning et et al. al. [20] [20 ]repo reportedrted the the selective selective oxidation oxidation of ofglycerol glycerol to 1,3- to 1,3-dihydroxyacetonedihydroxyacetone over over Pt cataly Pt catalystssts promoted promoted with bismuth with bismuth and antimony, and antimony, Hamid Hamidet al. [21] et observed al. [21] observeda superior a superiorselectivity selectivity to glyceric to acid glyceric using acid nanosized using nanosized Pd-based Pd-based catalysts catalystssupported supported on activated on activatedcarbon (Ac), carbon hydrotalcite (Ac), hydrotalcite (HTc) and (HTc)activated and carbon–hydrotalcite activated carbon–hydrotalcite composite (Ac–HTc). composite Glyceric (Ac–HTc). acid Glycericwas also acid obtained was also over obtained CuNiAl over hydrotalcites CuNiAl hydrotalcites (HTs) [22], (HTs) while [22 acrylic], while acid acrylic was acid mainly was a mainly product a productobtained obtained by gas-phase by gas-phase oxidation oxidation of glycerol of glycerol over over W–V–Nb–O W–V–Nb–O complex complex metal metal oxides oxides having having a astructure structure like like that that of orthorhombicorthorhombic MoMo33VOx [[23].23]. The successfulsuccessful employmentemployment ofof didifferentfferent ceriaceria nanocatalystsnanocatalysts for for glycerol glycerol oxidation oxidation reaction reaction has has been been also also demonstrated demonstrated [ 4[4].]. The The authors authors concluded concluded thatthat the the presence presence of of reduced reduced Ce Ce3+3+ speciesspecies and,and, also,also, thethe interactionsinteractions ofofthe the reactants reactants with with di differentfferent crystallographiccrystallographic surfaces surfaces play play a a major major role role in in the the catalytic catalytic activity activity of of these these materials materials [4 [4].]. However,However, there there are are several several identified identified limitations limitations such such as: as: the the catalysts catalysts su sufferffer deactivations, deactivations, make make useuse of of alkaline alkaline conditions conditions and sometimesand sometimes lead to poorlead selectivitiesto poor selectivities [2,24,25]. Therefore, [2,24,25]. the Therefore, development the ofdevelopment highly active of and highly selective active heterogeneous and selective hetero catalystsgeneous for base-free catalysts glycerol for base-free oxidation glycerol remains oxidation a big challengeremains a for big the challenge scientific for community. the scientific community. AA much much more more interesting, interesting, but but more more challenging, challenging, alternative alternative is to is convert to convert glycerol glycerol to methanol, to methanol, due todue the to fact the that fact methanol that methanol is a major is a raw major material raw formaterial the production for the production of various of added various value added chemicals, value thereforechemicals, its therefore production its fromproduction glycerol from attracted glycerol the researcher’sattracted the interest.researcher’s Commercially, interest. Commercially, glycerol can beglycerol converted can tobe methanol converted by to a two-step methanol process: by a two- catalyticstep transformationprocess: catalytic of crudetransformation glycerol to of syngas crude (Hglycerol2/CO), followedto syngas by(H2 the/CO), methanol followed synthesis by the methanol process. synthesi However,s process. there are However, studies there demonstrating are studies thatdemonstrating the direct glycerol that the transformation direct glycerol to transformation methanol is possible. to methanol Forinstance, is possible. glycerol For instance, can be directlyglycerol convertedcan be directly to methanol converted in hydrogen to methanol with high in selectivityhydrogen overwith a Pdhigh/Fe 2selectivityO3 co-precipitated over a Pd/Fe catalyst2O [326 co-]. Furthermore,precipitated Hutchingscatalyst [26]. et al.Furthermore, [27] showed Hutchings that crude et glycerol al. [27] can showed be reacted that crude with water glycerol over can very be simplereacted basic with or water redox over oxide very catalysts, simple basic such asor MgOredox or oxide CeO catalysts,2, to produce such methanol as MgO or in CeO high2, yieldsto produce in a one-stepmethanol low-pressure in high yields process. in a one-step low-pressure process. HeterogeneousHeterogeneous catalysts catalysts based based on on vanadium vanadium oxides oxides are are well well known known as as active active catalysts catalysts for for the the oxidationoxidation processprocess due due to to their their multiple multiple oxidation oxidation states. states. SelectiveSelective oxidationoxidation reactionsreactions overover these these catalystscatalysts are are influenced influenced by by many many factors factors such such as (i) as lattice (i) lattice oxygen, oxygen, (ii) the (ii) strength the strength of vanadium-oxygen of vanadium- bonds,oxygen (iii) bonds, environment (iii) environment of oxygen atoms,of oxygen (iv) redoxatoms, properties (iv) redox of properties vanadium of and vanadium (v) multi-functionality and (v) multi- offunctionality the active sites of the [28 ].active sites [28]. Meanwhile,Meanwhile, microporousmicroporous materials,materials, aluminophosphatealuminophosphate frameworksframeworks (AlPO),(AlPO), havehave acquiredacquired attentionattention lately lately due due to to their their structural structural and and compositional compositional diversity diversity being being analogous analogous with with zeolites zeolites and and following their catalytic applicability trend, a multitude of metals have been placed in neutral AlPO framework. VAlPO-5 and VAlPO-11 were the most studied aluminophosphates molecular
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