The aim of this work is to synthesize Nb-V oxide catalysts by sol gel route starting from metal alkoxides using different H2O/V ratios. Dried samples are characterized by X-ray diffraction (XRD), atomic absorption, and Brunauer-Emmett-Teller (BET) analysis. Calcination of dried materials up to 600°C in flowing air has been followed with Thermogravimetry-Fourier-transform infrared spectroscopy (TG-FTIR) spectroscopic analysis. The catalytic properties of calcined materials are tested in the oxidative dehydrogenation of ethane at 600°C. An improvement of the catalytic performances of vanadium for the gel prepared with the higher H2O/V ratio is found with respect to those of the supported catalyst due to the better interaction between vanadium and niobium

Blanco I.

Catauro M.

Naviglio D.

Poggetto G. D.

English

Archivio Istituzionale della Ricerca - Università degli Studi della Campania "Luigi Vanvitelli"

FULL PAPERwww.ms-journal.deSurface Characterization of Composite Catalysts Preparedby Sol-Gel RouteMichelina Catauro, Ignazio Blanco, Daniele Naviglio, and Giovanni Dal Poggetto*The aim of this work is to synthesize Nb-V oxide catalysts by sol gel routestarting from metal alkoxides using different H2O/V ratios. Dried samples arecharacterized by X-ray diffraction (XRD), atomic absorption, andBrunauer-Emmett-Teller (BET) analysis. Calcination of dried materials up to600°C in flowing air has been followed with Thermogravimetry-Fourier-transform infrared spectroscopy (TG-FTIR) spectroscopic analysis. Thecatalytic properties of calcined materials are tested in the oxidativedehydrogenation of ethane at 600°C. An improvement of the catalyticperformances of vanadium for the gel prepared with the higher H2O/V ratio isfound with respect to those of the supported catalyst due to the betterinteraction between vanadium and niobium.1. IntroductionNiobia-based systems are active in the oxidative dehydrogenation(ODH) of light alkanes, particularly in the conversion of propaneto propene. More particularly, pure niobium pentoxide is a veryselective catalyst for the ODH of propane, but its activity appearsrather low. In recent years, it has been shown that promotingniobium pentoxide with elements such as vanadium, chromiumand molybdenum could significantly improve the catalytic activ-ity maintaining a satisfactory selectivity.[1]It is well known that the catalytic behavior of vanadium oxide(supported or mixed with other metal oxides) depends on the in-teraction occurring between the two metals.[2,3] Moreover, it hasbeen reported that vanadium functionality is modified by the ad-dition of P,Mg,Mo, or Nb leading to catalysts with better catalyticperformances. In particular, the addition of hardly reducibleM. CatauroDepartment of EngineeringUniversity of Campania “Luigi Vanvitelli”via Roma 29, Aversa I-81031, ItalyI. BlancoDepartment of Civil Engineering and Architecture and UdR-CataniaConsorzio INSTMUniversity of CataniaViale Andrea Doria 6, Catania 95125, ItalyD. NaviglioDepartment of Chemical SciencesFederico II University of NaplesVia Cintia 21, Naples 80126, ItalyG. D. PoggettoEcoricerche srl Via Principi Normanni 36Capua 81043, ItalyE-mail: giogiodp@hotmail.itDOI: 10.1002/masy.202000197metal, such as Nb, provides an enhancedselectivity in the ODH of light alkanesalso depending on the used preparationmethod.[4,5] Therefore, the key point toreach high catalytic performance appearsto be the ability to tune the catalyst archi-tecture, by controlling the nature and in-terdispersion of the oxide phases presentin these catalysts. Topography and surfacechemistry are in fact increasingly studiedand characterized because they can greatlyinfluence adsorption process and there-fore the catalytic activity.[6–9] The architec-ture design of the specific catalyst can beaccomplished by implementing adequatepreparative procedures such as the sol-gel methods, which havebeen found to be particularly powerful to control the intimacy ofmolecular scale in mixed oxides.[10–12] Sol–gel chemistry is basedon inorganic polymerization reactions, for whichmetal alkoxidesare generally used as startingmaterials, obtaining amacromolec-ular network via hydrolysis and hydroxyl condensation. Transi-tion metal alkoxides with a d° configuration, such as Ti(IV) orNb(V), are very reactive towards hydrolysis, addition of watercausing formation of precipitates insoluble hydroxides that donot display the textural properties of a genuine gel. Therefore,unlike silicon systems, the sols must be stabilized in order toprevent precipitation; this can be achieved by using nucleophilicchemical additives such as carboxylic acids or other chelatingagents, which control the reaction rates by modifying the reactiv-ity of the precursors, allowing sequential formation of sols andgels.[13–16]In this paper, bulk vanadium-niobium oxides samples withhigh surface area have been prepared via sol–gel method. Theirphysico-chemical properties have been characterized. These sam-ples have been tested as catalysts for the ODH of ethane.2. Experimental SectionTwo samples containing V2O5 and Nb2O5 with a V/Nb atomicratio 1/5 were prepared. Vanadium-niobium gels were pre-pared using Nb(OC2H5)5 and VO[(CH3)2CHO]3 analytical grade(Aldrich) reagents as starting materials. Water-free ethanol, ob-tained by distillation with metallic sodium of commercial an-hydrous ethanol was used. Bidistillated water was used for thehydrolysis reaction. Due to the high reactivity of niobium pen-taethoxide toward moisture, the alcoholic solution was handledinside a glove box at room temperature. The two gels differ fromeach other in the H2O/V ratio (Table 1). A clear homogeneous so-lution resulted. The container was sealed with parafilm and keptMacromol. Symp. 2021, 395, 2000197 © 2021 Wiley-VCH GmbH2000197 (1 of 3)www.advancedsciencenews.com www.ms-journal.deTable 1. Synthesis parameters, composition, and surface area.Material Gelation time (day) H2O/Vmolar ratio V2O5/Nb2O5Molar ratio Surface area before thermaltreatment (m2 g−1)Surface area after thermaltreatment (m2 g−1)Nb gel-1 6 4 1/5 149 11Nb gel-2 6 0.04 1/5 70 3at ambient conditions for gelation. Depending upon the amountof the H2O added, several days were required for a complete gela-tion. The gelled system was washed for 1 day more at room tem-perature before drying. The gel was fully dried in air at 50°C in anelectrical oven for 2 days. After these treatments, an amorphouspowder was obtained.The niobium and vanadium content in the dried gel wasevaluated by inductively coupled plasma-emission spectrometer(ICPE-9000 Shimadzu, Tokyo, Japan). Before ICPE analysis, thesamples were acidified with HNO3 solution to pH= 2. The amor-phous or crystalline nature of the dried and calcined gels, re-spectively, were ascertained by X-ray diffraction using a Philipsdiffractometer. Powders of each sample were scanned from 2ϴ5–60° using Cu K𝛼 radiation. All samples have been calcined inflowing air at 600°C for 3 h. Porosity and specific surface areaparticles were evaluated by BET (Fisons Instruments) analyses,which were performed at liquid nitrogen temperature and usinggaseous N2 to evaluate the specific surface of powders.The TG/FTIR measurements were carried out using a SE-TARAM 92-16.18 TG apparatus equipped with 250 µL aluminacrucibles filled with about 100–150 mg of sample at 10°C min1,under argon flow (40 mL min−1), in the temperature range 25–600°C for the analysis of the released gases. TG analysis was car-ried out by heating the sample at 10°C min−1, in air flow up to600°C holding the final temperature for 2h as for the calcina-tion treatment. The catalytic tests were carried out in a fixed bedquartz micro-reactor operating under atmospheric pressure asreported in a previous investigation.[5] The feed composition was4% C2H6 and 2% O2 in a balance of He. The reaction tempera-ture was 600°C, the space velocity was 0.13 g s N cm−3. Carbonbalance was closed within 3% error in all experiments.3. Results and DiscussionThe gelation is the result of the hydrolysis and condensationreactions. At room temperature, the hydrolysis is much fasterthan the condensation reactions so that the number of Nb-O-Nbbridges formed is not sufficient to give gelation and precipitationof hydrated niobium oxide–alkoxide aggregate occurs. Mixing ata lower temperature (−10°C) allows the control of the hydrolyticreactivity of niobium pentaethoxide, so that soluble polymericintermediates are obtained, which then undergo further poly-merization to form a gel. Both the gels were clear, transparent,and light yellow in color. Chemical analysis of melted gel showedthat a good composition control was provided by sol–gel process(Table 2). The analyzed and theoretical value are in fairly goodagreement. The BET analysis shows that the gels have a veryhigh value of the surface area as it was expected using the sol-gelpreparation method.[12]Table 2. Chemical analysis of gels.Constituent Analyzed (wt.%) H2O/Vmolar ratioV2O3 10.10 10.10Nb2O3 89.80 89.57The severe thermal treatment at 600°C, however, results in astrong reduction of surface area of the gels due to the crystal-lization of amorphous materials in different phases VNbgel-1.These results show signals of monoclinic Nb2O5 (JCPDS 27–1313) whereas, in addition to these peaks, VNbgel-2 shows alsosignals of n-Nb2O5 (JCPDS 27–1311) and of themixed vanadium-niobium oxide (JCPDS 16–132). This finding could suggest thatthe presence of a larger amount of water during the synthesisof gels results in the formation of niobium oxide particles likelycovered by a vanadium oxide layer, the formation of a mixedcompound prevails when water is almost absent. It has been ver-ified that the strong reduction of the surface area already occursat quite low temperature by evaluating the values of BET area ofVNbgel-1 sample treated at 200°C and 400°C, respectively, underthe same conditions used for the treatment at 600°C. Calcinationat 600°C starting from a lower value as dried sample, maintains agood value of surface area as calcined. The TG analysis of the cat-alyst reproducing the calcination conditions. All catalysts reach aconstant weight after the complete thermal treatment. The totalweight loss is 17.8% and 11.4% for the VNbgel-1 and the VNbgel-2. After a first release of physisorbed water, occurring at lowtemperature, all samples lose rapidly weight up to 230–250°C.After that, the impregnated sample slowly reaches a constantweight which does not change during the following 3 h treatmentat 600°C, whereas a weight increase starting at about 400°C isobserved for the two gels. During the isothermal step, the gelsstart to lose weight again. The weight increase observed for thetwo gels is not associated to any IR-detectable species and, there-fore, could reasonably be related to the oxidation of vanadiumlikely partially reduced during the synthesis. The catalysts havebeen employed in the ODH of ethane in order to compare theirperformances with those of other vanadium- niobium-basedcatalysts.[13] The reaction products are ethylene, CO, and CO2. InTable 3, the values of ethane consumption and ethylene forma-tion rate, evaluated under differential conditions, are reported.VNbgel-1 has high more selectivity of VNbgel-2. Moreover, ifsurface area is taken into account, VNbgel-1 is also significantlymore active than the impregnated sample.[14–17] A very low activ-ity, also associated to a very poor selectivity, has been shown byVNbgel-2. These results are in very good agreement with thoseobtained by the physico-chemical characterization of the calcinedmaterials and confirm the different nature of the gel synthesizedMacromol. Symp. 2021, 395, 2000197 © 2021 Wiley-VCH GmbH2000197 (2 of 3)www.advancedsciencenews.com www.ms-journal.deTable 3. Rate of ethane consumption referred to sample weight (rwC2H6)and sample surface area (rsC2H6) and rate of ethylene formation (rC2H4)evaluated from catalytic tests.Material rwC2H6(µmol s−1 g−1)rsC2H6(µmol s−1m−2)rC2H4(µmol s−1 g−1)VN-b gel-1 1.55 0.13 0.61VN-b gel-2 0.34 0.085 0.17with a low H2O/V ratio. The higher activity of VNbgel-1 couldbe likely associated to a larger surface exposure of vanadiumoxide phase. On the contrary, for VNbgel-2 it can be hypothe-sized that vanadium, more involved in the formation of a bulkmixed compound with niobium, is less available for the ODHreaction.4. ConclusionsPreparation of V-Nb gels through hydrolysis and polycondensa-tion of niobium ethoxide with vanadium oxytriisopropoxide leadsto the formation of high surface area, amorphous solids contain-ing residues that are lost during heating. The H2O/V ratio usedduring the synthesis strongly influences the main feature of theproduct obtained after calcination at 600°C, necessary to use thematerials as catalysts for the ODH of ethane, leading to niobiumoxide particles covered by amorphous vanadium oxide. The for-mer materials exhibit a much higher catalytic activity and selec-tivity towards ethylene formation.AcknowledgementsThe work was financially supported by V:ALERE 2019 grant support fromUniversità degli studi della Campania “L. Vanvitelli” of CHIMERA.Conflict of InterestThe authors declare no conflict of interest.Keywordscatalyst, Sol–gel synthesis[1] F. Barbieri, D. Cauzzi, F. De Smet, M. Devillers, P. Moggi, G. Predieri,P. Ruiz, Catal. Today 2000, 61, 353.[2] M. Catauro, C. Pagliuca, L. Lisi, G. Ruoppolo, Therm. Acta 2002, 381,65.[3] S. Albonetti, F. Cavani, F. Trifiro, Catal. Rev. Sci. Eng. 1996, 38, 413.[4] S. A. Korili, P. Ruiz, B. Delmon, Catal. Today 1996, 32, 229.[5] P. Ciambelli, G. Ruoppolo, L. Lisi, G. Russo, J. C. Volta, Stud. Surf. Sci.Catal. 1997, 110, 285.[6] G. Raffaini, F. Ganazzoli, Langmuir 2013, 29, 4883.[7] G. Raffaini, F. Ganazzoli, J. Biomed. Mat. Res. A. 2006, 76A, 638.[8] L. De Nardo, G. Raffaini, E. Ebramzadeh, F. Ganazzoli, InternationalJournal of Artificial Organs 2012, 35, 629.[9] G. Raffaini, S. Elli, F. Ganazzoli, J. Biomed. Mat. Res. A. 2006, 77, 618.[10] M. Catauro, M. G. Raucci, A. G. Ausanio, J. of Mater. Sci.: Mater. inMedicine 2008, 19, 531.[11] M. Catauro, F. Barrino, G. Dal Poggetto, F. Pacifico, S. Piccolella, S.Pacifico,Mater. Sci. and Eng. C 2019, 100, 837.[12] R. Gonzales, T. Lopez, R. Gomez, Catal. Today 1997, 35, 293.[13] M. Catauro, E. Tranquillo, G. Dal Poggetto, M. Pasquali, A. Dell’Era,S. Vecchio Ciprioti,Materials 2018, 11, 2364.[14] M. Catauro, M. G. Raucci, F. De Gaetano, A. Marotta, J. Mater. Sci2003, 38, 3097.[15] M. Catauro, F. Barrino, G. Dal Poggetto, G. Crescente, S. Piccolella,S. Pacifico,Materials 2019, 12, 148.[16] M. Catauro, F.Bollino Journal of Applied Biomaterials and FunctionalMaterials 2013, 11, 172.[17] M. Catauro, R.A Renella, F. Papale, S. Vecchio Ciprioti,Mater. Sci. Eng.C 2016, 61, 51.Macromol. Symp. 2021, 395, 2000197 © 2021 Wiley-VCH GmbH2000197 (3 of 3)

Surface Characterization of Composite Catalysts Prepared by Sol-Gel Route

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Surface Characterization of Composite Catalysts Prepared by Sol-Gel Route

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