Influence of organic species on surface area of bismuth molybdate catalysts in complexation and spray drying methods

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Influence of organic species on surface area of bismuth molybdate catalysts in complexation and spray drying methods
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  Applied Catalysis A: General 267 (2004) 227–234 Influence of organic species on surface area of bismuth molybdatecatalysts in complexation and spray drying methods M.T. Le a , b , ∗ , W.J.M. Van Well c , I. Van Driessche a , S. Hoste a a  Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281-S3, Gent 9000, Belgium b  Department of Petrochemistry, Hanoi University of Technology, Dai Co Viet Street 1, Hanoi, Viet Nam c  Department of Chemical Engineering, Aalborg University Esbjerg, Niels Bohrs Vej 8, Esbjerg 6700, Denmark  Received in revised form 5 March 2004; accepted 5 March 2004Available online 17 April 2004 Abstract With this report we describe the increase in specific surface area of high purity batches of three phases of bismuth molybdate catalysts bythe combination of two techniques: a complexation method and spray drying. Thermal decomposition of the citric acid in the precursor is thesrcin of the increase in surface area of the catalysts. The optimal amounts of added citric acid were established for both complexation andspray drying methods. Furthermore, the spray drying was improved by adding sorbitol or changing the pH of the precursor solutions. Theobtained high surface area catalysts obviously exhibit higher catalytic activity, opening promising applications of the methods.© 2004 Elsevier B.V. All rights reserved. Keywords:  Bismuth molybdates; Complexation; Spray drying; Surface area; Citric acid; Selective oxidation; Propylene 1. Introduction The three phases of bismuth molybdate catalysts(  -Bi 2 Mo 3 O 12 ,   -Bi 2 Mo 2 O 9 , and   -Bi 2 MoO 6 ) have beenprepared by solid state reaction and precipitation methodsfor many years [1–5]. Recently, a complexation method to synthesize these catalysts has been reported by a smallnumber of authors [6,7]. Considered as useful methods to synthesize materials owing to their homogeneity and stoi-chiometry [8], the complexation methods usually result in high purity of the obtained products. As we have alreadygained some experience in the synthesis of pure bismuthmolybdate catalysts using spray drying [9], it becomesinteresting to compare these results with those obtainableusing a complexation-based synthesis.The major characteristic of the complexation synthesis isthe use of a complexant. There are many suitable organiccomplexants but citric acid is the most frequently used. Theconsequential gelation process yields better intermixed andmore highly dispersed oxides through a three-dimensional ∗ Corresponding author. Tel.:  + 32-92-64-4440; fax:  + 32-92-64-4983.  E-mail addresses:  minhthang.le@rug.ac.be,lmthang-petrochem@mail.hut.edu.vn (M.T. Le). organic network of metallic components which is respon-sible for pore structure and surface area of the catalyst[10]. Indeed, experiments showed that bismuth molybdatesprepared by the complexation method exhibit increasedsurface area in comparison with other methods (precipita-tion, solid state reaction, and spray drying without citricacid). Obviously, the phenomenon should be attributedto the presence of citric acid, which is burnt off duringthe calcination. Since specific surface areas of pure singlephases of bismuth molybdates prepared by common meth-ods are usually low (approximately 1–2m 2  /g) [4,11,12],it is interesting to increase the surface areas of the prod-ucts. Consequently, the influence of different amounts of citric acid in precursor solutions on the surface area of theobtained products is the scope of this article. This influ-ence is studied in both methods used here: complexation(in which gelation occurs) and spray drying (in whichthe drying time is too short to allow significant gelation).Adding citric acid (or other organic species) into the spraydrying precursor solutions offers an approach to clarifyhow this complexant effects the surface area of the finaloxides. Catalytic activities for oxidation of propylene intoacrolein by samples with different surface areas were alsocompared. 0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.apcata.2004.03.007  228  M.T. Le et al./Applied Catalysis A: General 267 (2004) 227–234 2. Experimental The precursor solutions for the complexation methodand spray drying were prepared from aqueous solutionsof 0.14M (NH 4 ) 6 Mo 7 O 24 · 4H 2 O (Fluka, p.a.) (solution A)and 0.67M Bi(NO 3 ) 3 · 5H 2 O/HNO 3  (Fluka, p.a.) (solutionB). Fifty milliliters of solution A was slowly added into anequivalent amount of solution B (49, 73, and 146ml corre-sponding to Bi/Mo molar atomic ratio is 2/3, 1/1, and 2/1,respectively), and concentrated HNO 3  was continuouslyadded in order to preserve the high acidity of the mediumand to prevent the precipitation of bismuth molybdates.Citric acid was added as a solution of 10wt.% citric acidmonohydrate (Fluka, p.a.) (69, 103, and 205ml for Bi/Mo = 2/3, 1/1, and 2/1, respectively). In the spray drying syn-thesis, these obtained solutions were diluted with distillatedwater until the final total concentration of solutes reached0.4063mol/l in order to eliminate the influence of dilutionand viscosity to the particle size. The citric acid/Bi molar ra-tio is 1 in the above described solutions. The final solutionswere homogeneous, transparent and stable for up to 24h.Dimethyl oxalate, malic acid, malonic acid, and sorbitolwere used in the same manner to explore the influence of the type of organic additives in the spray drying synthesis.Spray drying solutions with pH 7 were prepared usingthe same the procedure. Here, the final pH was adjusted byaddingNH 4 OH.Clearandstablesolutionscouldbepreparedat this pH value.Inthecomplexationsynthesis,theobtainedsolutionswereaged in a furnace at 60 ◦ C for about 24h, until they werecompletely gellyfied. The transparent yellow gels were thendried at 150 ◦ C for 2h, and the spongy solid precursorsobtained were crushed.In the spray drying synthesis, the precursor solutions werespray dried using a Büchi 190 laboratory spray dryer with a0.5mm nozzle and a feeding rate of 5ml/min at a tempera-ture of about 225 ◦ C.The thermal decomposition of precursor powders in airatmosphere was examined using simultaneous TGA/DTA ona Stanton–Redcroft 1500 thermobalance at a heating rate of 5 ◦ C/min.Powders obtained after gelation and spray drying weredirectly calcined in air at 500–600 ◦ C. In order to ensurea good contact with the surrounding air and the completecombustion of the organic additive, the powder was repeat-edly manually mixed during the calcination procedure. Theobtained products were characterized using XRD (SiemensD5000 diffractometer, Cu K   radiation), IR (Matson 5020instrument), and Raman (Varian Carey 82 spectrometer)techniques.The morphology of the bismuth molybdates was exam-ined using a Philips 501 scanning electron microscope. Spe-cific surface area of powders was measured by the BETmethod using nitrogen gas with a Micromeritics Gemini de-vice. Since the surface areas of bismuth molybdates are low,sufficient amounts of products (5–7g) were used. The resultsare reproducible within a standard deviation of   ± 0.3m 2  /gfor five measurements with two independent samples.Catalytic activities were measured in a conventionalfixed-bed reactor with an internal diameter of 0.5cm. Acatalyst of 0.2g was used with a total gas flow rate of 0.04mmol/s at a pressure of 1atm. The volume composi-tion of the gas flow was C 3 H 6  /O 2  /N 2  = 2.5/2.5/95 (%) andthe reaction temperature was maintained at 375–400 ◦ C.Analysis of the products was performed using an on-lineGC-17A Shimazu gas chromatograph with a column of 80/100 chromosorb and carbowax 20M. 3. Results 3.1. Synthesis of bismuth molybdates by complexation incomparison with standard aqueous solution-based spaydrying The three phases of bismuth molybdates were preparedby the complexation method (molar atomic ratio of citricacid per bismuth atom is 1) and by the standard aqueoussolution-based spray drying (without citric acid in the pre-cursor solutions). Good gels were formed even at a low pHvalue (pH 1) in the precursor solutions demonstrating thatlow pH is suitable for the gelation of bismuth molybdatesolutions.The DTA curves of the precursor powders (Figs. 1and 2) show that the thermal phenomena of the samplesprepared by the complexation method are completely dif-ferent from those of the spray dried samples. Instead of thewell known decomposition of nitrate and ammonia groupsfrom starting materials at 230, 300, and 350 ◦ C (Fig. 1) [9], the “auto-ignition” (or spontaneous combustion) due to thethermally induced oxidation–reduction reaction between thecitrate and nitrate ions [8] occurs. This is characterized bya sharp exothermic peak at about 200 ◦ C in the DTA curves(Fig. 2). The other sharp exothermic peak at about 440 ◦ Cis believed to be due to combustion reactions of excessorganic anions [13]. Due to the late burning of excess or-ganic anions at high temperature, calcination temperaturesshould be above 500 ◦ C in the complexation synthesis inorder to burn all nitrate, ammonia, and citrate groups in theprecursors. Above 500 ◦ C, the weight of the complexatedsamples becomes constant. The weight losses are approxi-mately equal to the sum of the amounts of nitrate, ammoniagroups, and citric acid in the precursors, demonstrating thatall of the citric acid was burnt.Based on our previous published work on the prepara-tion of pure bismuth molybdates [9] and the thermal anal- ysis data of precursors, the alpha phase was calcined at500 ◦ C, the beta phase at 600 ◦ C, and the gamma phase at550 ◦ C for 1h. The XRD patterns of the three phases of bis-muth molybdate prepared by the complexation method andspray drying are presented in Figs. 3–5. The XRD patternsshow that pure phases of gamma, beta, and alpha of bismuth   M.T. Le et al./Applied Catalysis A: General 267 (2004) 227–234  229Fig. 1. TGA–DTA curves of 1/1 standard aqueous solution-based spray drying precursor powder (without citric acid, pH 1).Fig. 2. TGA–DTA curves of 1/1 complexation precursor powder (C/Bi = 1, pH 1). molybdates were formed by both complexation and spraydrying. There were only small amounts of gamma and al-pha phases contamination in the beta phase in 1/1 samplesprepared by both methods. This may be due to the decompo-sition of the beta phase at 540 ◦ C as known in the literature[14]. Compared to conventional synthesis methods (precipi-tation, solid state reaction), the complexation and spray dry- Fig. 3. XRD patterns of 2/1 complexation (C/Bi = 1, pH 1) and standard aqueous-based spray drying samples (C/Bi = 0, pH 1). ing methods result in bismuth molybdates of higher purity.This is due to the fact that the complexation in the com-plexation synthesis allows subsequent chemical reactions toproceed slowly and without physical discontinuity from ahomogeneous solution of catalyst precursors into a homo-geneous, amorphous phase. Thus, both spray drying [9] and complexation synthesis proceed via amorphous but highly  230  M.T. Le et al./Applied Catalysis A: General 267 (2004) 227–234 Fig. 4. XRD patterns of 1/1 complexation (C/Bi = 1 . 3, pH 1) and standardaqueous-based spray drying samples (C/Bi = 0, pH 1).Fig. 5. XRD patterns of 2/3 complexation (C/Bi = 0 . 9, pH 1) and standardaqueous-based spray drying samples (C/Bi = 0, pH 1). mixed intermediates, which is the srcin for their higherpurity.Table 1 summarizes the composition and surface area of the products obtained. Obviously, surface areas of bismuthmolybdates prepared by the complexation method are higherthan those obtained by spray drying of standard aqueoussolutions. The following parts will help to clarify the ques-tion whether the presence of citric acid (organic complexantwhich burns at high temperature) might be the reason. 3.2. Influence of the quantity of citric acid on surfacearea in the complexation synthesis The influence of the molar ratio of citric acid to bismuthatoms (C/Bi) on the surface areas of the bismuth molybdatesis shown in Fig. 6 (the citric acid to bismuth ratio is usedsince bismuth is presents as Bi 3 + cations in an aqueous so-lution, while molybdenum is present as (Mo 7 O 24 ) 6 − cluster Table 1Composition and surface area of bismuth molybdates prepared by the complexation and standard aqueous solution-based spray dryingMethod Atomic Bi/Mo  = 2/1 Atomic Bi/Mo  = 1/1 Atomic Bi/Mo  = 2/3Composition  S   (m 2  /g) Composition  S   (m 2  /g) Composition  S   (m 2  /g)Complexation    2.6    +   (small)  +   (small) 3.5    1.9Spray drying    1.8    +   (small)  +   (small) 1.4    1.5Fig. 6. Influence of different amounts of citric acid in the complexationmethod (pH 1). anions [15]). A remarkable observation is that the surface area shows a maximum at a specific C/Bi ratio (around 3)with the    and    phases. When the quantity of citric acidis higher, the surface areas decrease. In the case of the   phase, no significant influence of the quantity of the citricacid could be observed.Thedecreaseofsurfaceareasofgammaandalphabismuthmolybdateswithhighquantitiesofcitricacidisinagreementwith other results reported in the literature [16,17] where itis shown that the excess of citric acid in gels may result inan inhomogeneous structure of the Me–O–Me network anda decrease of the BET surface area. In fact, a small numberof precipitates were visible in the complexation precursorsusing a high ratio of citric acid to bismuth ratios.The maximum value of the surface area of the alphaphase is highest (12.2m 2  /g, 450% increase) compared to thegamma phase (5.4m 2  /g, 200% increase) and the beta phase(4.1m 2  /g, 193% increase). 3.3. Influence of the quantity of citric acid on surfacearea in spray drying synthesis The same study procedure as applied with the complex-ation synthesis is repeated for the spray drying synthesis.However, the effect of citric acid addition in the spray dryingsynthesis is completely different. When the amount of cit-ric acid increases, the surface area increases up to a certainvalue, after which it remains constant (Fig. 7). Efforts to addmore citric acid (C/Bi above 3) met with the difficulty thatspray dried powders became sticky and ignite even at thewall of the cyclone of the spray dryer. Although the surfaceareas of bismuth molybdates increase 100–200% compared   M.T. Le et al./Applied Catalysis A: General 267 (2004) 227–234  231Fig. 7. Influence of different amounts of citric acid in spray drying method(pH 1). to the samples without citric acid, these increases are muchsmaller than in the complexation synthesis.SEM images of samples spray dried with and without cit-ric acid are shown in Fig. 8. Particles of the alpha phaseprepared without citric acid are much bigger than those ob-tained using citric acid as an additive. These particles alsolook more porous because citric acid is playing its role asa pore forming agent. The SEM image of the alpha sam-ple synthesized using the complexation method in Fig. 9also shows a more porous structure while the shape of thethree-dimension network formed during the gellation is stillmaintained. Fig. 8. SEM images of alpha samples prepared by spray drying. (a) Alphaphase, C/Bi  =  0, pH 1, 600 ◦ C, 10h; (b) alpha phase, C/Bi  =  3, pH 1,600 ◦ C, 10h.Fig. 9. SEM images of alpha phase prepared by complexation, C/Bi = 1,pH 1, 600 ◦ C, 10h. 3.4. Other types of additives in the spray drying synthesis Different organic compounds were added to the spray dry-ing precursor solutions in order to fully understand the in-fluence on the surface area. The chosen compounds have tobe soluble in water in order to provide homogeneous pre-cursor solutions—the same starting conditions as in the caseof citric acid. Moreover, they should not decompose dur-ing the spray drying process. Therefore, malic acid (MM = 134), malonic acid (MM  =  104), dimetyl oxalat (MM  = 118), and sorbitol (MM  =  182) were used. The amountsof organic species used were calculated equivalently to theamounts of citric acid. In Fig. 10, the surface areas of al-pha bismuth molybdates (calcined at 500 ◦ C for 10h) usingdifferent organic compounds are given. Thus, not only cit-ric acid (MM = 192), but also other organic species addedinto the precursors appear to increase the surface area of final products. Obviously, from this we may state that, ingeneral, when a precursor contains any substance which canbe burned during calcination, the surface area of the finalproduct can be increased. However, this increase appears todepend on the molecular mass of the additive regarding withdimethyl oxalate, malonic acid, and malic acid. Indeed, atrend between molecular mass and surface area was found.The higher the molecular mass, that is, roughly correspond-ing to the overall molecular size, the higher the surface area. Fig. 10. Influence of organic compounds on surface area (S.D., pH 1).
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