Design of Novel Sulfated Nanozirconia Catalyst for Biofuel Synthesis

Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES

Article

Design of novel sulfated nanozirconia catalyst for biofuel synthesis Sana Labidi, Mounir Ben Amar, Jean-Philippe Passarello, Bernard Le Neindre, and Andrei V. Kanaev Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03448 • Publication Date (Web): 20 Jan 2017 Downloaded from http://pubs.acs.org on January 26, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Design of novel sulfated nanozirconia catalyst for biofuel synthesis

S. Labidi, M. Ben Amar, J.-P. Passarello, B. Le Neindre, A. Kanaev # Laboratoire des Sciences des Procédés et des Matériaux C.N.R.S., Institut Galilée, Université Paris 13, 93430 Villetaneuse, France

#

Corresponding author: [email protected] ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2 Abstract We report on the elaboration of nanoparticulate SO42--ZrO2 solid acid catalyst for conversion of free fatty acids into biofuel. Three different morphological modifications of zirconia were prepared with the sol-gel method and compared as acid supports: polydispersed nanopowder (PNP), monodispersed nanopowder (MNP) and monodispersed nanoparticles coating (MNC). The size-selected zirconia-oxo-alkoxy (ZOA) nanoparticles of 3.6 nm diameter were prepared in a rapid micromixing reactor and either precipitated (MNP) or deposited on glass beads (MNC). The polydispersed 4/100/500 nm ZOA particles were prepared in non-parent alcohol for realization of PNP powders. The grafting of SO42- groups was realized on the three ZOA supports by wet impregnation method and three catalysts were prepared after the subsequent drying and thermal treatment stages. The temperature of thermal treatment strongly affects the stability of the solid acid complex and serves to be an important factor of the preparation process. A strong retention of carbon on the surface of smallest zirconia nanoparticles induces their sintering, thus reducing the active area; therefore the supported size-selected nanoparticles appear to be the best solution for realization of an efficient catalyst. The best acid complex stability in terms of reuse number was obtained with the thermal treatment temperature of ~580 °C and the specific activity (normalized on catalyst mass) of MNC catalyst towards palmitic acid conversion to methyl palmitate was found almost two orders of magnitude higher than that of PNP and MNP catalysts.

Keywords: ZrO2 nanoparticles, nanopowders, nanocoatings, sulfate group grafting, solid acid catalysis, palmitic acid, esterification process, kinetics.

ACS Paragon Plus Environment

Page 2 of 44

Page 3 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3 1. Introduction Solid acid catalyst is an effective solution towards renewable green energy by biodiesel production

using

heterogeneous

esterification

and

transesterification

reactions.

The

heterogeneous catalytic process is investigated in order to replace currently widely used homogeneous liquid catalysis, thus reducing toxic wastes and avoiding costly purification stage after reaction. Recent studies in this field have proven effectiveness of the proposed concept with a variety of catalytic media with grafted acidic complexes on solid supports: sulfated metal oxides, H-form zeolites, sulfonic ion-exchange resins, sulfonic modified mesostructured silica materials, sulfonated carbon-based catalysts, heteropolyacids etc. [1]. Several inconveniences of solid acid Friedel-Craft catalysts [2] related to their sensitivity to water, corrosion, toxic wastes can be solved using oxide solids based in general cases on zirconia. Different metal oxides materials ZrO2, SiO2, Al2O3, WO3 and SnO2 with high specific surface area are suitable for esterification and transesterification reactions. The performance of TiO2ZrO2, sulfated ZrO2, W-ZrO2, K-ZrO2 and SiO2-ZrO2 have been evaluated in a continuous-flow fixed-bed reactor on transesterification of soybean oil in methanol at 250 °C [3]. The solid acids SO42--ZrO2 and SO42--TiO2 have been previously synthesized and studied for the biodiesel manufacturing process [4-5]. Sulfated zirconia is known as a superacid solid [5, 6] with Hammet function H0 ≤ -14.52 [6-7] largely inferior to sulfuric acid Hammet function (-12). This property allows sulfated zirconia to replace Friedel-Craft catalysts for n-butane isomerisation at ambient temperature as shown by Hino et al. [6, 8]. SO42--ZrO2 acidity has been investigated with different methods as NH3 programmed thermal desorption (NH3 TPD) [9, 10], UV spectroscopy for bases adsorption [11, 12], proton NMR [13-15] and results indicate a multitude of sulfated zirconia structures having in common Lewis-Brønsted acid sites, which enable the solid catalyst



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

4 activity [3, 16-19]. Among different heterogeneous catalysts, sulfated zirconia, first realized by Holm and Bailey [20], is considered today as one of the most suitable for biofuel synthesis owing to its high acidity, boiling point, strength, toughness and good corrosion resistance in acidic and alkaline environments, stability in aqueous solutions [10, 19]. The supported catalyst generally demonstrates higher activity and better stability in the esterification and transesterification processes compared to the non-supported catalyst [21, 22]. Although sulfated zirconia has shown to possess a significant activity, the acid complex stability in aqueous and alcoholic solutions related to the sulfate group hydrolysis [23] and catalyst lifetime in biofuel process require further improvements. It is generally recognized that control of material purity, crystalline structure and particle size improves its quality [24]. In particular, variations of catalytic properties of zirconia nanopowders in CO oxidation could be associated with changes of their fractal properties [25]. Despite a large scope of the synthesized materials, more studies are required to optimize the mass transport and to increase acidity and stability of the surface complex. These catalytic properties sensitively depend on the support material morphology and nanoparticulate materials are supposed to be a good base for developing new solid acids with an improved performance. Recently, we have reported on a new method of preparation of macroscopic quantities of size-selective zirconia-oxo-alkoxy (ZOA) nanoparticles [26]. By analogy with earlier studied titanium oxo-alkoxy (TOA) nanoparticles [27], they can be easily immobilized onto hydrophilic substrates forming nanocatalyst [28]. In this work we realized nanostructured supported catalysts of ZOA nanoparticles and compared their performance with that of monodispersed and polydispersed powders of nanoparticles. The reaction kinetics of palmitic acid esterification in


Page 4 of 44

Page 5 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5 methanol solvent and catalyst deactivation in repetitive process cycles after the solids recovery were measured. The goal of this study was to assess the optimal nanoscale catalyst morphology.

2. Experiment 2.1 Catalyst preparation The preparation of nanoparticulate SO42--ZrO2 catalyst used in the present study included four main steps of the (1) nanoparticles synthesis, (2) immobilisation, (3) functionalization and thermal treatment. In step (1), the size-selected 3.6 nm zirconium oxo-alkoxy (ZOA) nanoparticles were prepared in a sol-gel reactor with a rapid micromixing as described in ref. [29]. The main part of this reactor is a T-mixer of Hartridge and Roughton type with eccentric inputs for reacting fluids operating at Reynolds number Re=6·103 (Re = 4Qρ/ ηd, where Q, ρ, and η are the fluid flow rate, density and dynamic viscosity), which ensures the turbulent flow of sufficient intensity to thinner and dissipate eddies on millisecond timescale, before the chemical reactions begin. The two stock solutions, each of 50 ml volume, respectively contain: (i) 0.30 mol/l zirconium n-propoxide precursor (70 wt% supplied by Interchim) in n-propanol (≥99.5% supplied by Sigma-Aldrich) and (ii) 0.60 mol/l distilled and twice-filtered water (syringe filter 0.1 mm porosity PALL®Acrodisc) in n-propanol at 20.0 °C. The hydrolysis ratio H=[H2O]/[Zr]=2.0 was chosen according to Labidi et al. [26] for realization of nanocoatings, in order to guarantee the long-term stability of the ZOA nanoparticle colloids. The ZOA colloids become unstable for H>2.0 and precipitate after induction period. Accordingly, for producing polydispersed nanopowders, high hydrolysis ratio H>2.5 was used, at which the colloid rapidly precipitate. We used for this preparation zirconium n-butoxide precursor (80 wt.% supplied by Sigma-Aldrich) in ethanol



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

6 solvent (≥99.8 % supplied by Fulka), which results in a strongly polydispersed colloids with multimodal size distribution around mean size 2R≈4, 100 and 500 nm [26]. In step (2), the immobilization of 3.6 nm ZOA nanoparticles was realized by chemical colloid deposition method in a LABstar high-quality glove box workstation MBraun (purified from O2 gas and H2O vapour; H2O vapour pressure below 0.5 ppm) in order to avoid any contamination with atmospheric humidity. The colloid was set in contact with borosilicate glass beads of 1 mm diameter (preliminarily cleaned with acid and ultrasound water bath) for ~10 min, during which the chemically active nanoparticles reacted with surface hydroxyls forming mechanically stable surface coating. The recovered beads were dried in the glove box during 48 hours at room temperature and then at 80 °C overnight in the ambient atmosphere. Except for the monodispersed nanocoatings (MNC) on glass beads, we have prepared as reference catalytic supports monodispersed nanopowders (MNP) after precipitation and drying of monodispersed nanoparticles issued from the micromixing reactor and polydispersed nanopowders (PDP) after precipitation and drying of the manually prepared colloids in ethanol solvent. In step (3), the functionalization and thermal treatment processes were carried out. The sulfation of zirconia was performed in a 1N sulfuric acid solution. The calcination of the materials was performed in an oven under oxygen flow at of 0.5 bar static pressure at temperatures between 500 and 1200 °C during 4 hours. The obtained catalysts were conserved in a glove box. Before each use, the prepared catalysts were kept in a glove box to protect from atmospheric humidity.


Page 6 of 44

Page 7 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


7 2.2. Catalyst structural characterization The nanoscale morphology of the prepared samples was characterized using JEOL2011 high resolution transmission electron microscopy (TEM) operating at 200 KeV with LaB6 emission source of electrons. Element maps (Zr, S and C) of the samples were obtained using energy filtered transmission electron microscopy (EFTEM) with Gatan Imaging Filter 2000 system connected to TEM. The energy and spatial resolutions of the system were respectively 1 eV and 1 nm. The scanning electron microscopy (SEM) images were realized with a Zeiss Supra 40 VP SEM-FEG installation. The microscopy was performed in high vacuum mode and low acceleration voltage 1-30 kV. Non-conducting samples were carbon coated of 6-20 nm thickness (Precision Etching Coating System) before measurements. The samples were structurally characterized by X-ray diffraction (XRD) method on INEL XRG 3000 installation using Co-Kα (λ=1.788976 Å) radiation source. The thermogravimetric and differential thermal analysis (TGA-DTA) were performed with SETARAM TG92 installation in the temperature range between 50 and 1200°C with temperature increment of 2 °C/min under oxygen gas flow. The measurements of specific surface area by Brunauer, Emmet and Teller method (BET) were performed with BET COULTER SA3100 installation using nitrogen gas at liquid nitrogen temperature. The elemental analysis of the sample composition was achieved with ICP-OES (Inductive Coupled Plasma - Optical Emission Spectrometry) method with the Thermo Scientific iCAP 6000 Series installation. The surface species analysis was performed by DRIFT spectroscopy. The spectra were recorded with 4 cm-1 resolution and 30 scans accumulation using a Shimadzu IRPrestige-21 spectrophotometer equipped with a standard DLATGS detector. A Pike DiffusIRTM accessory containing an environmental chamber equipped with a KBr window was adapted on the FTIR



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 44

8 spectrophotometer. This chamber can contain a cup with approximately 10 mg of catalyst and can be heated up to 500 °C, at atmospheric pressure. Before each experiment, the catalyst sample was heated up to 400 °C under pure N2 gas flow (100 ml/min) for 2h. Then, the sample was cooled down to a room temperature and its spectrum was recorded after the background subtraction. 2.3. Catalytic activity The catalyst activity was studied on the reaction of palmitic acid esterification into methyl palmitate in methanol solvent: k

C15 H 31COOH + CH 3OH → C15 H 31COOCH 3 + H 2O

(1)

The esterification process was conducted under continuous stirring with 94 rpm in a batch type vessel reactor of 100 ml volume, connected to a condenser, with the molar ratio acid-to-methanol of 1:100 at the atmospheric pressure and temperature 65°C controlled with a bulb thermometer. Depending on the experimental series, 200 mg of the powder-like SO42-ZrO2 solid acid catalysts, MNP or PNP, or less than 12 mg of MNC catalysts deposited on glass beads (0.4 g total mass with the supported material) was inserted into the reaction medium. The vessel including reaction medium and catalyst was maintained homogeneous by mechanical axial rotation around an axis inclined by 30° relative to the vertical. The sampling (1 ml) of the reactive solution was taken periodically each 30 minutes and cooled down to room temperature by mixing with 2 ml of methanol in a glass tube to block reactions. When using MNP and PNP powder catalysts, preliminary centrifugation was applied to separate solid particles from the reaction medium. The concentrations of species were determined using Nicolet 6700 Fourrier Transform Infra Red (FTIR) Advanced Gold Spectrometer with DTGS (Deuterated Tri Glycine Sulfate) KBr detector and CaF2 beam splitter. The spectra were measured in the range of relatively strong absorption bands of palmitic acid (ν=1720 cm-1) and methyl palmitate (ν=1700-1750 cm-1) [30]


Page 9 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


9 not screened by a methanol solvent absorption. For each acquisition, methanol spectrum was taken as the background. The measured absorbance is expressed as: A(ν ) = ∑ k i (ν ) ⋅ C i

(2)

i

where ki(ν) and Ci are respectively its absorption coefficient and concentration. The reference spectra of palmitic acid (PA) and methyl palmitate (MP) were measured with different concentrations of 0.100, 0.075, 0.050 and 0.025 mol/l in methanol solution (see Fig. 1a) and showed a good linearity, which permits to fix ki(ν) of pure reagents in Eq. 2 in the spectral range 1600≤ν≤1850 cm-1. With knowledge of these coefficients, the concentrations Ci were monitored during the esterification process. Because of a weak water absorption in this spectral range, its contribution in Eq. 2 was neglected. The relative error of the concentration measurements was ±0.005 mol/l. The agreement between the measurements and fit using Eq. 2 is illustrated in Fig. 1b for an asymmetric mixture PA:MP=0.02:0.08 mol/l.

3. Results and discussion In step (3), the order of the functionalization and thermal treatment processes was chosen by a comparison of the surface acid complex stability using DRIFT method. The DRIFT spectra of ZrO2 powders sulphated (1N sulfuric acid solution during 1 hour) before and after heart treatment at 500 °C are shown in Fig. 2c and 2b. The absorption bands between 1000 and 1400 cm-1 are commonly ascribed to the sulfated group [31-36]. Pure zirconia shows no structural absorption in this spectral range (Fig. 2a). We have observed no significant modification of the absorption in this range after sulfation of crystallised zirconia (Fig. 2b), which indicates a low efficiency of this process. In contrast, sulfation of the amorphous zirconia produces absorption



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 44

10 bands between 1200 and 1400 cm-1 characteristic of the surface complex, which withstands the thermal treatment (Fig. 2c). Consequently, for the catalyst preparation we chose the sulfation of ZOA powders and coatings before thermal treatments. In contrast to crystalline zirconia, the ZOA phase is readily dissolved in strong acids. As a consequence, the sulfation competes with the material dissolution. This especially concerns MNC support because of an extremely small thickness of the deposited layer ~10 nm. To minimise the related mass losses, we treated 5g glass beads covered with MNC zirconia in 50 ml of 0.125 N, 0.25 N, 0.5 N, 0.75 N and 1 N solutions of sulfuric acid for 30 min and 17 hours. The remaining MNC zirconia mass from each sample was measured with the ICP method and expressed as thickness of a uniform coating. The results are presented in Fig. 3. A prolonged sulfation was privileged to increase the surface complex number density. Besides, mass losses of the most concentrated acid solutions were considered as excessive. We did not measure in this work kinetics of the dissolution and sulfation processes. As compromise, we used the treatment with 0.5 N sulfuric acid solutions during 17 hours, which results in a 30 % mass loss of the deposited ZOA material. The sulfated material was removed on the Büchner filter and dried during 24 hours at 80 °C. The MNP and PDN zirconia powders were sulfated in the same conditions as the nanocoatings. After sulfation, the powders were centrifuged and air dried at 80 °C overnight. Both nanocoatings and nanopowders undergone sulfation in similar conditions, which corresponds to the catalyst atomic ratio S/Zr=2.2 favourable for formation of the bisulfatederived strong Brønsted acid sites [37]. The comparison of different MNP, PNP and MNC catalysts was performed by measurements of the reaction rates of Eq. 1 defined as

r=−

1 dCi δ δ PA δ M ⋅ = k ∏ C j j = kCPA CM α i dt j


(3)

Page 11 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


11 where αi and δi are respectively stoichiometry coefficient and reaction order of i-th reagent. Because methanol solvent (indexed M in Eq. 3) is in large excess (99 mol %) and the stoichiometry coefficients are 1, Eq. 3 can be reduced to Eq. (4), which solution depends on the reaction order. dC PA δ = k ' C PAPA dt

(4)

The first reaction order δPA=1 of Eq. 1 was confirmed in present work for different selected powder and coating catalysts. Fig. 4a shows an example of the reaction kinetics in semilogarithmic frame in presence of MNC catalyst treated at 500 °C (called in following MNC500 catalyst) of 0.123 wt% loading with respect to PA reagent mass. As already mentioned in experimental part, the loaded masses of powder and coating catalysts varied in present experiments. The reaction constant of heterogeneous catalytic process k' in Eq. 4 commonly depends on the catalyst mass. In order to enable a comparison of catalysts with different loadings, we verified the esterification reaction kinetics dependence on the loaded mass and found k ' ∝ mcat as shown in Fig. 4b. These results allow expressing the specific reaction constant as

 CPA (t )   / mcat t K m = − ln  CPA (t = 0) 

(5)

in units min-1g-1 and compare different catalysts activities on this basis.

3.1. Comparison between powder MNP and PNP catalysts The results of TGA-DTA measurements of the zirconia powders before and after sulfation are shown in Fig. 5a and 5b respectively. The non-sulfated MNP and PNP powders exhibit similar mass loss curves, which stabilize at temperatures above 400 °C where crystallization



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 44

12 begins, as evidence strong narrow exothermic heat flow peaks in Fig. 5a. In the same time, the total mass loss of PNP was found significantly smaller (11.2 wt%) compared to MNP (20.9 wt%). This can be a consequence of a stronger retention of surface alkoxy groups by 3.6 nm sizeselected nanoparticles constituting MNP powder. In contrast, hydrolysis-polycondensation reactions resulting in elimination of alkoxy groups are more complete in PNP powders. The monodispersed nature of MNP powder reflects the narrow heat flow peaks while that of polydispersed nanoparticles PNP is broader. We notice that the small shift by 9 °C to high temperatures of the crystallization point in PNP compared to MNP (Fig. 5a) can be explained by different nature of alkoxy groups (respectively butoxy and propoxy) at the particles surface. The sulfation of PNP and MNP powders strongly modifies the TGA-DTA curves, where two mass loss regions below (region I) and above 650 °C (region II) were observed, as depicted in Fig. 5b. We assume that the mass loss in the temperature range 50-650 °C is related to the elimination of physisorbed and chemisorbed water and burnt carbon species originating from surface alkoxy groups of ZOA nanoparticles. At higher temperatures, the surface sulfate species are decomposed.The partial / total mass losses of sulfated MNP and PNP respectively are 14.7 wt% / 28.2 wt% and 11.4 wt% / 20.8 wt%. In contrast to PNP, a net decrease of the adsorbed water and carbonaceous species was observed in MNP after sulfation from 20.9 to 14.7 wt%. This can be explained by the surface group’s reactivity. Indeed, the surface exchange during sulfation involves the replacement of alkoxy and hydroxyl groups per sulfate: OH ...Zr−−OH + SO42 − → ...Zr−− SO42 − + 2OH −

(6)

OR ...Zr−−OR + SO42 − → ...Zr−− SO42 − + 2OR −

(7)

Apparently, reaction (6) is complete in both PNP and MNP samples. In contrast, reaction (7) involving surface butoxy groups in PNP is not effective, while that involving propoxy groups of


Page 13 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


13 MNP partially proceeds, reducing the mass of surface carbonaceous species. The smaller quantity of butoxy groups in PNP (11.2 wt%) compared to propoxy groups in MNP (20.9 wt%) is explained by their partial hydrolysis in preparation conditions with higher water content; consequently, PNP is more enriched with surface hydroxyls that MNP. The remaining mass loss above 650 °C (region II) of sulfated PNP sample 9.4 wt% (or 11.9 wt% with respect to pure ZrO2) belongs to sulfated species, which is lower compared to 13.5 wt% (18.8 wt% with respect to pure ZrO2) of MNP sample. This assignment and measurements of specific surface areas of MNP σ MNP =306 m²/g and PNP σ PNP =251 m²/g powders allow an estimation of the sulfate group density on zirconia surface:

n SO 2 − = χσ −1 4

NA M SO 2 −

(8)

4

where NA and M SO 2 − are the Avogadro number and molar mass of SO42-. Using Eq. 8 we 4 estimate the number density of sulfate group on PNP and MNP surface to be 3.0 and 3.9 per nm2. Consequently, the sulfation process of zirconia is very efficient providing more surface groups onto the MNP material. According to the above discussion, the decomposition of sulfate groups in MNP takes place at temperatures above 650 °C, which is confirmed by a strong endothermic peak at 700 °C. In PNP such narrow feature is absent, which indicates a size-specific nature of this phenomenon. At the same time, mass losses in both MNP and PNP samples continue until temperatures as high as 1000 °C, which indicates thermally induced transformations of the surface sulfate groups. The x-ray diffraction patterns of sulfated MNP and PNP powders calcinated at 500 °C are shown respectively in Fig. 6a and 6b. They are strongly different from that of the pure powders, one example of which is shown in Fig. 6c, where the monoclinic phase dominates although the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 44

14 tetragonal phase is also present. The Scherrer size 12 nm of tetragonal ZrO2 crystallites in sulfated PNP is smaller than 22 nm in non-sulfated PNP zirconia and both are considerably larger than 4 nm in sulfated MNP material, which is close to the size 2R=3.6 nm of ZOA nanoparticles. This can be related to a very small subcritical size 3.6 nm of ZrO2 nanoparticles [38], which constitute a net majority of MNP and a small but not negligible fraction of PNP [26]. This explains why the amorphous-tetragonal phase transition affects only a fraction of the smallest ZOA nanoparticles and not larger 100 and 500 nm particles. The appearance of pure tetragonal zirconia in sulfated PNP and MNP materials confirms previous conclusion about this phase stability for a high surface coverage by sulfate groups [37]. Both sulfated PNP and MNP materials generate tetragonal phase of zirconia, which enhances the material acidity. At the same time, crystallization of tetragonal phase zirconia begins favorably with the smallest 3.6 nm sulfated nuclei, which suggests the MNP morphology to be a preferable solution for the catalyst design. The DRIFT spectra of sulfated MNP after thermal treatment at different temperatures are shown in Fig. 7 and demonstrate the formation of the solid acid complex SO42 − . One distinguishes the principal vibration modes of this complex at 1070 cm-1 of S=O stretch, 1143 cm-1 of O=S=O and 1270 of S-O stretch shown in Fig. 7c by vertical marks. We also observed a band at ν1=1390 cm-1, which has been tentatively assigned in previous studies to the stretching mode of surface sulfate species containing a single S=O group. This band first appears at temperatures above 600 °C, shifts to 1405 cm-1 at 700 °C and then strongly intensifies at 750 °C. At the same time, the characteristic bands in the frequency range 1000-1300 cm-1 disappear at temperatures below 600 °C and above 700 °C. We relate these modifications with the formation of the stable surface sulfate acid complex. Another band was observed at 1465 cm-1 at 650 and


Page 15 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15 700 °C, which origin is not yet clear. However, it can be related to a presence of carbonic residues, which could survive on the zirconia surface after the calcinations [39]; these species decompose at T≥750 °C. Babou et al. [32] have previously observed that the 1390 cm-1 band is intense upon complete dehydration. When water content increases, this band weakens and shifts to 1322 cm-1, than disappearing at a strong surface saturation by water. This behavior correlates with that of another band in the range of 1600-1650 cm-1, which appears at 1600 cm-1 and progressively intensifies and shifts to 1634 cm-1 with an increase of the catalyst surface coverage by water until saturation. Depending on the preparation conditions, this band develops one or two maxima that have been assigned to bending modes of H2O [36]. According to this assignment, the band at 1640 cm-1 observed in our spectra of the sulfated samples treated at different temperatures from 500 to 750 °C, indicates a moistened catalyst surface. However, the ν1 band remains intense and even dominates DRIFT spectra at 750 °C where the band at 1640 cm-1 exhibits partially resolved structure characteristic of the very early hydration stage [31]. This apparently indicates that the catalyst surface is far from saturation with water molecules. Another peculiarity concerns the spectral position of ν1 band. According to Platero et al. [36], this band has components: one at frequency above 1400 cm-1 has been assigned to polynuclear species, most likely disulfates: [(Zr-O)2(SO)]2-O [36] and another at frequencies lower than 1400 cm-1 is expected for the S=O stretching mode of isolated (Zr-O)3-S=O. Under this assumption, higher temperatures are expected to decrease the number of sulfate groups at the zirconia surface and, therefore, shifts this band to lower frequencies. In contrast, we observed the spectral shift to higher frequencies with an increase of the calcinations temperature. Moreover, the observed increase of the 1000-1300 cm-1 bands contrast when going from 650 to 700 °C (see Fig. 7b-d)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 44

16 indicates, according to Ecormier et al. [37], a decrease of the surface sulfate number density due to their thermal decomposition. We observed the complete disappearance of the ν1 band after thermal treatment at 1200 °C, also in agreement with Platero et al. [36]. We distinguished ν 1+ (1405 cm-1) and ν 1− (1390 cm-1) components of the ν1 band shifted respectively to higher and lover frequencies, in agreement with [36]. Consequently, the complete conversion of ν 1− to ν 1+ above 700 °C may be related to the lower stability of sulfate group relative to disulfate. Moreover, the intensification of ν 1+ band suggests surface reorganization of sulfates resulting in the formation of disulfates. The ν 1− component may not be necessary related to the complete dehydration of the catalytic surface or that the stable sulfate and disulfate complexes do not attract water bound to zirconia surface. We furthermore discuss these features below in a section devoted to catalytic activity of the prepared materials. Specific kinetic constants Km defined by Eq. 5 (expressed in min-1g-1 units) of the palmitic acid esterification process using sulfated PNP and MNP catalysts calcinated in the range 500-700 °C are presented in Fig. 8. One can conclude about a strong difference in their behaviors. While the specific constant of PNP gradually decreases with an increase of the calcination temperature, that of MNP exhibits a maximum near ca 650 °C followed by a complete deactivation at higher temperatures. The specific constants of both catalysts have almost identical values at 650 °C; at higher and lower temperatures that of PNP is higher. This behavior could be only partially explained by sulfate losses, which according to our TGA-DTA data take place at T > 650 °C. Below 650 °C, the evolutions of Km(T) of PNP and MNP are completely different. This striking difference can be explained by the catalyst specific surface area variation with temperature. The results of BET measurements of specific surface area of sulfated PNP ( σ PNP ) and MNP


Page 17 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


17 ( σ MNP ) are shown in Fig. 8 (linked to the right y-axis). The specific surface area of PNP exhibits a common behavior of powder-like materials, decreasing with an increase of temperature, which reaches the highest value of 225 m2/g (251 m2/g in pure non-sulfated PNP). In contrast, the behavior of MNP is abnormal, since one would expect higher specific surface area of MNP composed of much smaller 3.6 nm nanoparticles. Indeed, pure MNP shows a higher specific surface area 306 m2/g than pure PNP. However, it considerably decreases down to 163 m2/g after sulfation: σ MNP < σ PNP . As one can see, sulfation does not affect the specific surface area of PNP while strongly decreases that of MNP. At higher temperatures of 500-520 °C, σ MNP drops to ~6 m2/g, which cannot be assigned to nanoparticles. SEM images presented in Fig. 9 show large connected grains in MNP and a highly porous structure of PNP samples. At the same time, the elementary building unit of MNP is 3.6 nm ZOA nanoparticule, which is not resolved in this picture. In order to better understand the thermal modifications, we performed TEM measurements of the sulfated nanopowders. Fig. 10 shows that MNP powder calcinated at 500 °C is composed of much larger particles and of better crystallinity than those of PNP. Moreover, these particles are strongly linked with their surfaces that can be clearly seen in Fig. 10d. The EELS elemental analysis of these powders in Table 1 indicates their significantly higher contamination with carbon and sulfur at 500 °C (respectively 6 and 3 times) compared with PNP. At 650 °C this difference between MNP and PNP vanishes. This supports a conclusion about a stronger retention of dopants (C, S) by the smallest 3.6 nm ZOA nanoparticles. On the other hand, these dopants serve to be glue between nanoparticles initiating their sintering and themselves reducing the specific surface area of MNP powder. The strongly polydispersed PNP powders are more resistant to this dense association of particles because of a weaker retention (below critical one) of surface dopants. We conclude that it is the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 44

18 surface carbon that collapses the specific surface area of MNP as a similar effect has been observed in non-sulfated MNP treated at 500 °C in Fig. 10d. Moreover, the pure effect of carbon on MNP is stronger in absence of sulfur species since the particles in Fig. 10d are even larger compared to Fig. 10a. The conclusion about the strong retention of elements on the surface of smallest zirconia nanoparticles is supported by MNP samples coloration. The initially white color of size-selected ZOA powders (80 °C) turns to dark gray at 500-520 °C and becomes white again at 580 °C in non-sulfated MNP. In sulfated MNP, the white-purple-white changes followed the same temperatures and indicate a partial replacement of surface carbon by sulfur in agreement with Eq. 7. At temperatures beyond 580 °C, the specific surface area of MNP powder increases because of the glued nanoparticles separation. Together with the nanoparticles coarsening during the thermal treatment, this process enables the similar evolution of specific surface areas of MNP and PNP with temperature. We notice that unlike MNP, PNP powders remain white over a whole range of the calcinations temperatures, which corroborates the absence of glued particles and common evolution of the specific surface area with temperature. The specific surface areas of sulfated MNP and PNP catalysts are plotted in Fig. 8 (right yaxis) and successfully reproduce trends in the reaction constants dependence on calcination temperature. For this reason we conclude on its key contribution to the powder activity. This result permits the catalyst comparison using modified Eq. 5 renormalized on the catalyst active area ( s = mcat σ ):

Ks = −

 CPA (t )  1  ln mcat tσ  C PA (t = 0) 

(9)

The specific reaction constant Ks (normalized on catalyst area) of sulfated PNP and MNP


Page 19 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


19 catalysts are shown in Fig. 11. This figure indicates a higher specific activity of MNP catalyst compared to PNP one, which is probably related to higher sulfate retention by the surface of its smallest size-selected ZOA nanoparticles.

3.2. Comparison with coating MNC catalysts As we discussed in previous section, the smallest nanoparticles possess high sulfur retention. Unfortunately, their specific area collapses during the heat treatment due to surface carbon. An optimum solution that allows taking benefit of a strong sites activity by preserving large catalytic area could be nanocoatings prepared of the preformed nanoparticles. These nanocoatings were deposited on silica gel beads from stable colloids of 3.6 nm ZOA nanoparticles followed by the sulfation and thermal treatment steps. These coatings have a very small thickness ~10 nm, which corresponds to almost three layers of 3.6 nm ZOA nanoparticles, which guarantee a permeability and full accessibility of active catalyst surface. The specific kinetic constants Km defined by Eq. 5 (expressed in min-1g-1 units) of the palmitic acid esterification process using sulfated MNC catalysts calcinated in the range 500-700 °C are presented in the semi-logarithmic frame in Fig. 12. The comparison with Fig. 8 evidences much higher activity of nanocoatings compared to powder catalysts. In particular, MNC catalysts show activity ~60 times higher compared to PNP, heat treated in the similar conditions of T=500650 °C; the most efficient MNP was obtained at 650 °C. The reuse of the MNC catalysts (after additional steps of separation in drying) in the esterification process decreases its activity. However even after three reuses, it remains significantly higher than the freshly prepared powder catalysts PNP and MNP (e.g. factor of six at 650 °C). An estimation of a model describing MNC kinetics could be quite useful at this stage of the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 44

20 preliminary data evaluation. Obviously, the single exponential model, which describes a single type of decaying states, disagrees with experimental data in Fig. 12. Therefore, we suggest that solid acid sites on the catalyst surface have different stabilities and that their relative population changes during repetitive uses. For the sake of simplicity, we consider two groups of such sites on the zirconia surface: one of which (ns) is quasi stable for a small number i of uses and another (nu) progressively degrading with i: n = nu + n s . The losses of unstable species are due to the adsorption-desorption equilibrium established in the reactive medium, while the losses of stable species are neglected. In the framework of this model, the total number of sulfur species n(i) after i-th utilization in the esterification process can be expressed as:

n(i ) = α i nu + n s

(10)

The least-squared fit of the experimental data with Eq. 10, shown in Fig. 12 by dashed line, provides the following solution: ns=0.040, nu=1.51, α=0.0176 (500 °C); ns=0.062, nu=0.73, α=0.0887 (580 °C) and ns=0.013, nu=0.09, α=0.117 (700 °C). According to these results, the population of unstable species decreases steadily with an increase of temperature, while the equilibrium coefficient α increases showing higher stability of the residuals. In contrast, the population of stable species reaches a plateau at ~600 °C and begins to decrease at 700 °C. Although the results cannot be considered as conclusive because of a limited number of experimental points, these results indicate the trend of an improved stability of the sulfate complex after heat treatment at ~600 °C. The inset in Fig 12 depicts the relative number of stable sulfates versus calcinations temperature. An important conclusion from this simplified model is that while the total number of sulfur species decreases with the temperature increase, the fraction of stable active sulfates attains its maximum at ~600 °C. Considering (as it follows from our TGA-DTA data at 580 °C) the total sulfur number n(1)~3.9 at/nm2, the stable sulfur species cover


Page 21 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


21 zirconia surface with number density ns≈0.3 per nm2. This latter value is above the threshold enabling superacid sulfate sites [37]. The decrease of the catalyst activity corresponds to the ν 1− → ν 1+ component conversion observed in our DRIFT spectra. According to Ref. [36] and our discussion, formation of single sulfate complex is expected at temperatures 600-650 °C, while disulfate complex is formed at 700 °C. Single and disulfate complexes enable respectively weak and strong Brønsted sites, which could affect the reaction channels differently. In particular, strong and weak acid sites have been reported to promote respectively limonene and camphene products of α-pinene isomerisation [37]. Since the kinetics of palmitic acid esterification decreases with an increase of the catalyst calcination temperature, we assume that weak Brønsted sites act more effectively. The comparison of our nanoparticulate zirconia activity with that reported in previous studies is not evident because of the lack of quantitative data about reaction kinetics. Relevant available data related to sulfated zirconia in esterification and transesterification reactions are presented in Table 2. The best of them suggest the conversion of 95% product in 10 min, however at a much higher reaction temperature 250 °C as used in the present work (65 °C). By comparing experimental data obtained in the same temperature range of reaction media, our data show somewhat faster kinetics by employing significantly smaller acid / alcohol molar ratio and/or catalyst / reagent weight ratio. This places thin monolayer coatings prepared from smallest zirconia nanoparticles between the best supports for preparation of solid acid catalysts. To allow quantitative comparison between different synthesized catalysts, we suggest evaluating the material activities using specific reaction constants given by Eqs 5 and 9.

4. Conclusion



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 44

22 We studied nanoparticulate SO42--ZrO2 solid acid catalysts in the esterification process of palmitic acid conversion into methyl palmitate. Size-selected 3.6 nm and polydispersed 3.6/100/500 nm zirconia-oxo-alkoxy (ZOA) nanoparticles, to be used as building blocks of the acid supports, were prepared via sol-gel method in a rapid micromixing reactor. Three modifications of zirconia substrates with different nanoscale morphologies were realized: polydispersed nanopowder (PNP), monodispersed nanopowder (MNP) and coatings of monodispersed nanoparticles (MNC). Grafting the sulfate groups was achieved in 0.5 N sulfuric acid solutions on the three ZOA supports followed by drying and thermal treatments in the range 500-750 °C to obtain three PNP, MNP and MNC solid acid catalysts. The calcinations temperature strongly affects the acid complex stability and is an important factor of the preparation process. The best complex stability in terms of reuse number was obtained with the processing at temperatures ~600 °C. The specific activity (normalized on catalyst mass) of the prepared MNC catalyst was found almost two orders of magnitude higher than that of PNP and MNP catalysts. The MNC activity decreases with the number of reuses in the esterification process; however, it remains substantially higher than those of freshly prepared powder catalysts. The catalytic activity of PNP and MNP catalysts correlates with the specific surface area and powders of the monodispersed nanoparticles surprisingly showed much smaller specific surface area than polydispersed nanopowders, because of the sintering assisted by residual surface carbons. A comparison with available literature data suggests using thin monolayer coatings prepared from the smallest zirconia nanoparticles to design the most effective solid acid catalysts. To allow the quantitative comparison between different synthesized catalysts, we suggest evaluating the material activities using specific reaction constants normalized on the catalyst mass and surface


Page 23 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


23 area.

Acknowledgments. ANR (Agence Nationale de la Recherche) and CGI (Commissariat à l’Investissement d’Avenir) are gratefully acknowledged for their financial support of this work through Labex SEAM (Science and Engineering for Advanced Materials and devices) ANR 11 LABX 086, ANR 11 IDEX 05 02.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 44

24

References (1)

Su, F.; Guo, Y., Advancements in solid acid catalysts for biodiesel production. Green Chem. 2014, 16 (6), 2934-2957.

(2) Rueping, M.; Nachtsheim,B. J.; A review of new developments in the Friedel–Crafts alkylation – From green chemistry to asymmetric catalysis. Beilstein J. Org. Chem. 2010, 6 (6), 1-24 (3) Furuta, S.; Matsuhashi, H.; Arata, K., Biodiesel fuel production with solid amorphouszirconia catalysis in fixed bed reactor. Biomass Bioenergy 2006, 30 (10), 870-873. (4) Yamaguchi, T., Recent progress in solid superacid. Appl. Catal. 1990, 61 (1), 1-25. (5) Hattori, H., Solid base catalysts: Generation, characterization, and catalytic behavior of basic sites. J. Jpn. Pet. Inst. 2004, 47 (2), 67-81. (6) Hino, M.; Kobayashi, S.; Arata, K., Solid catalyst treated with anion. 2. Reactions of butane and isobutane catalyzed by zirconium oxide treated with sulfate ion. Solid superacid catalyst. J. Am. Chem. Soc. 1979, 101 (21), 6439-6441. (7) Hino, M.; Arata, K., Synthesis of solid superacid catalyst with acid strength of H0–16.04. J. Chem. Soc. Chem. Commun. 1980, 18, 851-852. (8) Olah G. A., Shen J., Schlosberg R. H., Electrophilic reactions at single bonds. XI. Hydrogen-deuterium exchange of molecular hydrogen and deuterium in superacids involving isomeric triatomic (hydrogen, deuterium) (+) ions. J. Am. Chem. Soc. 1973, 95 (15), 4957-4960. (9) Ben Hamouda, L.; Ghorbel, A.; Figueras, F., Study of acidic and catalytic properties of sulfated zirconia prepared by sol-gel process: Influence of preparation conditions. Studies in Surf. Sci. Catal. 2000, 130, 971-976.


Page 25 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25 (10) Deshmane, V. G.; Adewuyi, Y. G., Mesoporous nanocrystalline sulfated zirconia synthesis and its application for FFA esterification in oils. Appl. Catal. A 2013, 462463, 196-206. (11) Drago, R.S.; Kob, N., Acidity and reactivity of sulfated zirconia and metal-doped sulfated zirconia. J. Phys. Chem. B 1997, 101 (17), 3360-3364. (12) Umansky, B.; Engelhardt, J.; Hall, W. K., On the strength of solid acids. J. Catal. 1991, 127 (1), 128-140. (13) Adeeva, V.; De Haan, J. W.; Jänchen, J.; Lei, G. D.; Schûnemann, V.; Van de Ven L. J. M.; Sachtler, W. M. H.; Van Santen, R. A., Acid Sites in Sulfated and Metal-Promoted Zirconium Dioxide Catalysts. J. Catal. 1995, 151 (2), 364-372. (14) Armendariz, H.; Sierra C. S.; Figueras, F.; Coq, B.; Mirodatos, C.; Lefebvre, F.; Tichit, D., Hydrogen Exchange between Sulfated Zirconias and per Deutero- Benzene as Characterization of the Surface Acidity. J. Catal. 1997, 171 (1), 85-92. (15) Armendariz, H.; Coq, B.; Tichit, D.; Dutartre, R.; Figuéras, F., Influences of Some Synthesis Parameters and Activation Procedures on the One-Step Sol–Gel Synthesis of Sulfated-Zirconia Catalysts, Followed by TG-DSC and Mass Spectrometry. J. Catal.

1998, 173 (2), 345-354. (16) Arata, K., Solid Superacids. Adv. Catal. 1990, 37, 165-211. (17) Arata, K.; Hino, M., Solid catalyst treated with anion: XVIII. Benzoylation of toluene with benzoyl chloride and benzoic anhydride catalysed by solid superacid of sulfatesupported alumina. Appl. Catal. 1990, 59 (1), 197-204. (18) Arata K., Preparation of superacids by metal oxides for reactions of butanes and pentanes. Appl. Catal. A 1996, 146 (1), 3-32.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 44

26 (19) Parera, J. M., Promotion of zirconia acidity by addition of sulfate ion. Catal. Today

1992, 15 (3-4), 481-490. (20) Holm, V. C. F.; Bailey, G. C. US Patent 3 032 599, 1962. (21) Chen, X. R.; Ju, Y. H.; Mou, C. Y., Direct Synthesis of Mesoporous Sulfated SilicaZirconia Catalysts with High Catalytic Activity for Biodiesel via Esterification. J. Phys. Chem. C 2007, 111 (50), 18731-18737. (22) Jacobson, K.; Gopinath, R.; Meher, L. C.; Dalai, A. K., Solid acid catalyzed biodiesel production from waste cooking oil. Appl. Catal. B 2008, 85 (1-2), 86-91. (23) Omota, F.; Dimian, A. C.; Bliek, A., Fatty acid esterification by reactive distillation: Part 2 - kinetics-based design for sulphated zirconia catalysts. Chem. Eng. Sci. 2003, 58 (14), 3175-3185. (24) Piticescu, R.; Monty, C.; Millers, D., Hydrothermal synthesis of nanostructured zirconia materials: Present state and future prospects. Sens. Actuators B 2005, 109 (1), 102-106. (25) Trypolskyi, A. I.; Gurnyk, T. M.; Strizhak, P. E., Fractal dimension of zirconia nanopowders and their activity in the CO oxidation. Catal. Commun. 2011, 12 (8), 766771. (26) Labidi, S.; Jia, Z.; Ben Amar, M.; Chhor K.; Kanaev, A., Nucleation and growth kinetics of zirconium-oxoalkoxynanoparticles. Phys. Chem. Chem. Phys. 2015, 17 (4), 2651-2659. (27) Azouani, R.; Soloviev, A.; Benmami, M.; Chhor, K.; Bocquet, J.-F.; Kanaev, A., Stability and growth of titanium-oxo-alkoxy TixOy(OiPr)z clusters. J. Phys. Chem. C 2007, 111 (44), 16243-16248.


Page 27 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


27 (28) Pucher, P.; Benmami, M.; Azoauni, R.; Krammer, G.; Chhor, K.; Bocquet, J.-F.; Kanaev, A. V., Nano-TiO2 sols immobilized on porous silica as new efficient photocatalyst. Appl. Catalysis A 2007, 332 (2), 297-303. (29) Azouani, R.; Michau, A.; Hassouni, K.; Chhor, K.; Bocquet, J.-F. ; Vignes, J.-L. ; Kanaev, A., Elaboration of pure and doped TiO2 nanoparticles in sol–gel reactor with turbulent micromixing: Application to nanocoatings and photocatalysis. Chem. Eng. Res. Design,

2010, 88 (9), 1123-1130. (30) Mistry B. D., A handbook of spectroscopic Data Chemistry, Oxford Book, Jaipur, India,

2009. (31) Morterra, C.; Cerrato, G.; Pinna, F.; Signoretto, M.; Strukul, G., On the acid-catalyzed isomerization of light paraffins over a ZrO2/SO4 system: The effect of hydration. J. Catal.

1994, 149 (1), 181-188. (32) Babou, F.; Coudurier, G.; Vedrine, J. C., Acidic proporties of sulfated zirconia: an infrared spectroscopy study. J. Catal. 1995, 152 (2), 341-349. (33) Holmgreen, E. M.; Yung, M. M.; Ozkan, U. S., Pd-based sulfated zirconia prepared by a single stepsol–gel procedure for lean NOx reduction. J. Mol. Catal. A. 2007, 270 (1-2), 101111. (34) Li, B.; Gonzalez, R. D., An in situ DRIFTS study of the deactivation andregeneration of sulfated zirconia. Catal. Today 1998, 46 (1), 55-67. (35) Yampaguchi, T.; Jin, T.; Ishida, T.; Tanabe, K., Structural identification of acid sites of sulfur-promoted solid superacid and construction of its structure on silica support. Mater. Chem. Phys. 1987, I7 (1-2), 3-19.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 44

28 (36) Platero, E. E.; Mentruit, M. P., IR characterization of sulfated zirconia derived from zirconium sulphate. Catal. Lett. 1995, 30 (1), 31-39. (37) Ecormier, M. A.; Wilson, K.; Lee, A. F., Structure–reactivity correlations in sulphatedzirconia catalystsfor the isomerisation of α-pinene. J. Catal. 2003, 215 (1), 57-65. (38) Navrotsky A., Energetics of nanoparticle oxides: interplay between surface energy and polymorphism. Geochem. Trans. 2003, 4 (6), 34-37. (39) Shi, G.; Yu, F.; Wang, Y.; Pan, D.; Wang, H.; Li, R., A novel one-pot synthesis of tetragonal sulfated zirconia catalyst withhigh activity for biodiesel production from the transesterification ofsoybean oil. Renew. Energy 2016, 92, 22-29. (40) Kiss, A. A.; Omota, F.; Dimian, A. C.; Rothenberg, G., The heterogeneous advantage: biodiesel by catalytic reactive distillation. Top. Catal. 2006, 40 (1), 141. (41) Jitputti, J.; Kitiyanan, B.; Rangsunvigit, P.; Bunyakiat, K.; Attanatho, L.; Jenvanitpanjakul, P., Transesterification of crude palm kernel oil and crude coconut oil by different solid catalysts. Chem. Eng. J. 2006, 116 (1), 61-66. (42) Kansedo, J.; Lee, K. T., Transesterification of palm oil and crude sea mango (Cerberaodollam) oil: The active role of simplified sulfated zirconiacatalyst. Biomass bioenergy 2012, 40, 96-104. (43) Chen, G.; Guo, C. Y.; Qiao, H.; Ye, M.; Qiu, X.; Yue, C., Well-dispersed sulfated zirconia nanoparticles as high-efficiencycatalysts for the synthesis of bis(indolyl)methanes and biodiesel. Catal. Comm. 2013, 41, 70-74. (44) Laosiripojana, N.; Kiatkittipong, W.; Sutthisripok, W.; Assabumrungrat, S., Synthesis of methyl esters from relevant palm products in near-critical methanol with modified-zirconia catalysts. Bioresource Technology 2010, 101 (21), 8416-8423.


Page 29 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


29 (45) Saravanan, K.; Tyagi, B.; Shukla, R. S.; Bajaj, H. C., Esterification of palmitic acid with methanol over template-assistedmesoporous sulfated zirconia solid acid catalyst. Appl. Catal. B 2015, 172-173, 108-115.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 44

30

Tables Table 1:

Elemental composition of PNP and MNP Zr-S catalysts heat treated at 500 and 650 °C. Atomic ratio (at./Zr)

Element S Zr C

Table 2:

TE / 200

TE / 150 E / 60 E / 95 E, TE / 250

E / 65 (1)

PNP 650 °C 0.07 ± 0.002 1 0.47 ± 0.02

500 °C 0.095 ± 0.006 1 0.29 ± 0.02

650 °C 0.14 ± 0.01 1 0.38 ± 0.02

Comparison of catalytic performances of different sulfated zirconia powders with MNC catalyst prepared in this study.

reaction type(1) / temperature (°C) E /140

E / 60

MNP 500 °C 0.81 ± 0.04 1 1.13 ± 0.05

Reaction medium

Catalyst fraction (wt.%)

Acid / alcohol (molar ratio)

Dodecanoïc acid in methanol Crude palm kernel oil in methanol Palm oil in methanol Soybean oil in methanol Palmitic acid in methanol Crude palm oil Refined palm oil Palmitic acid in methanol Palmitic acid in methanol

3

1/5

Conversion yield (%) / reaction time (h) 70 / 1

Reference

[40] 1

1/6

90 / 1 [41]

0.08

1/10

76 / 2

2

1/9

23 / 1.4

5

1/100

86 / 6

1

1/24

92 / 0.2

1

1/24

95 / 0.2

6

1/20

90 (7h)

0.1

1/100

90 (3h)

E = Esterification, TE = Transesterification


[42] [7] [43]

[44]

[45] This work

Page 31 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


31

Figure captions Figure 1.

Absorption spectra of PA, MP and H2O (a) and mixture PA:MP=0.02:0.08 mol/l (b).

Figure 2.

DRIFT spectra of ZrO2 powder heat treated at 500°C (a) and SO42--ZrO2 sulfated after (b) and before (c) heat treatment at 500 °C (4 hours).

Figure 3.

Thinning of ZOA coating in sulfuric acid solutions of different concentrations.

Figure 4.

PA transformation kinetics (a) and dependence of kinetic constant k' (Eq. 4) on mass (b) of MNC catalyst.

Figure 5.

TGA and DTA curves (as specified by arrows) of non-sulfated (a) and sulfated (b) zirconia. Solid and dotted lines correspond to MNP and PNP zirconia.

Figure 6.

XRD patterns of SO42--ZrO2 catalysts with MNP and PNP morphologies. The nonsulfated PNP zirconia is shown for a comparison. The calcination temperature is 500 °C in air atmosphere. Labels T and M indicate peaks of tetragonal and monoclinic phases.

Figure 7.

DRIFT spectra (absorbance) of sulfated MNP zirconia thermally treated at different temperatures indicated in figure.

Figure 8.

Specific reaction constant rate Km (Eq. 5) of the esterification reaction (filled ■, ●) and specific surface area (hollow □, ○) of MNP (■,□) and PNP (●, ○) catalysts, as specified by arrows.

Figure 9.

SEM images of sulfated MNP (a) and PNP (b) thermally treated at 500 °C.

Figure 10.

TEM images of sulfated MNP (a, c) and PNP (b) and non-sulfated MNP (d) zirconia thermally treated at 500 °C.

Figure 11.

Specific reaction constant Ks (normalized on catalyst area) of esterification reaction of MNP and PNP catalysts.

Figure 12.

Specific reaction constant rate Km (normalized on catalyst mass) of sulfated MNC catalysts vs number of uses for different calcination temperatures. Dotted lines represent the trend of the model given by Eq. 10. The relative number of stable sulfates for different calcination temperatures is shown in inset.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Absorption spectra of PA, MP and H2O (a) and mixture PA:MP=0.02:0.08 mol/l (b). figure 1 287x201mm (300 x 300 DPI)


Page 32 of 44

Page 33 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


DRIFT spectra of ZrO2 powder heat treated at 500°C (a) and SO42--ZrO2 sulfated after (b) and before (c) heat treatment at 500 °C (4 hours). figure 2 209x296mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Thinning of ZOA coating in sulfuric acid solutions of different concentrations. figure 3 288x201mm (300 x 300 DPI)


Page 34 of 44

Page 35 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PA transformation kinetics (a) and dependence of kinetic constant k' (Eq. 4) on mass (b) of MNC catalyst. figure 4 287x201mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TGA-DTA curves of MNP non-sulfated (a) and sulfated (b) zirconia. Dotted line shows heat flow (a) and mass loss (b) of PNP zirconia. figure 5 201x288mm (300 x 300 DPI)


Page 36 of 44

Page 37 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


XRD patterns of ZrO2-SO42- catalysts with MNP and PNP morphologies. The non-sulfated PNP zirconia is shown for a comparison. The calcination temperature is 500 °C in air atmosphere. Labels T and M indicate peaks of tetragonal and monoclinic phases. figure 6 297x209mm (150 x 150 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

DRIFT spectra (absorbance) of sulfated MNP zirconia thermally treated at different indicated temperatures. figure 7 209x297mm (150 x 150 DPI)


Page 38 of 44

Page 39 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Specific reaction constant rate Km (normalized on catalyst mass) of the esterification reaction (filled ■, ●) and specific surface area (hollow □, ○) of MNP (■,□) and PNP (●, ○) catalysts. figure 8 297x209mm (150 x 150 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

SEM images of MNP (a) and PNP (b) catalysts thermally treated at 500 °C. figure 9 244x90mm (150 x 150 DPI)


Page 40 of 44

Page 41 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TEM images of sulfated MNP (a, c) and PNP (b) and non-sulfated MNP (d) zirconia thermally treated at 500 °C. figure 10 180x181mm (150 x 150 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Specific reaction constant Ks (normalized on catalyst area) of esterification reaction of MNP and PNP catalysts. figure 11 297x209mm (150 x 150 DPI)


Page 42 of 44

Page 43 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Specific reaction constant rate Km (normalized on catalyst mass) of sulfated MNC catalysts vs number of utilizations for different calcinations temperatures. Dotted lines depict model tendencies given by Eq. 10. Relative number of stable sulfates versus calcination temperature is shown in inset. figure 12 296x209mm (300 x 300 DPI)



2− SO4

1.6

− ZrO2 catalyst

1.2

500 °C 580 °C 700 °C

-1

K, min g

-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 44 of 44

0.8

0.4

0.0

MNC

MNP


PNP

Design of Novel Sulfated Nanozirconia Catalyst for Biofuel Synthesis

Recommend Documents