Post-synthesis alumination of MCM-41: Effect of the acidity on the HDS activity of supported Pd catalysts

Applied Catalysis A: General 383 (2010) 211–216

Contents lists available at ScienceDirect

Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata

Post-synthesis alumination of MCM-41: Effect of the acidity on the HDS activity of supported Pd catalysts A.M. Venezia a,∗ , R. Murania b , V. La Parola a , B. Pawelec c , J.L.G. Fierro c a b c

Istituto dei Materiali Nanostrutturati (ISMN-CNR), via Ugo La Malfa, 153, Palermo I-90146, Italy Dipartimento di Chimica Inorganica ed Analitica “Stanislao Cannizzaro”, Università di Palermo, Viale delle Scienze – Parco d’Orleans, Palermo I-9012, Italy Institute of Catalysis and Petrochemistry, CSIC, Cantoblanco, Madrid 28049, Spain

a r t i c l e

i n f o

Article history: Received 10 March 2010 Received in revised form 28 May 2010 Accepted 1 June 2010 Available online 10 June 2010 Keywords: Thiophene HDS Pd/MCM-41 Alumina modification

a b s t r a c t Siliceous MCM-41 with different amount of alumina, from 0.25 up to 4.0 wt%, were prepared by impregnation of the MCM-41 with aqueous solution of Al(NO)3 ·9H2 O. The modified mesoporous silicas were then used as supports for Pd catalysts prepared by wet-impregnation from PdCl2 precursor. Supports and corresponding Pd catalysts were characterized by XRD, XPS and NH3 -TPD. The catalytic behavior was tested in the hydrodesulfurization (HDS) reaction of thiophene. An increase of the hydrodesulfurization activity with increasing alumina amount up to 0.5 wt% was observed. On the basis of the acidity change of the support and the structural modification underwent by the deposited palladium, the improved catalytic behavior was associated to the increased acidity of the supports and also to its effect on the palladium dispersion. A bi-functional mechanism, implying that metallic palladium activates the hydrogen and the thiophene molecules are adsorbed on the acid sites of the support, could be operative. However, the increased acidity was also responsible for the larger catalyst deactivation which limited the beneficial effect up to a certain amount of alumina content. © 2010 Elsevier B.V. All rights reserved.

1. Introduction As alternative to the traditional CoMo systems, noble metals supported on mesoporous silicates have been recently used as HDS catalysts for petroleum feedstocks [1–3]. Due to their elevated hydrogenation activity, the Pd and Pt based catalysts are particularly suitable for deep HDS in which hydrogenation of the aromatic hydrocarbon is a preliminary step to the C–S cleavage [4]. As compared to the traditional CoMo or NiMo catalysts, those based on Pd and Pt allow to work at milder conditions in terms of temperature and pressure. However, a major drawback in their use is represented by their limited lifetime due to the easy poisoning of the active sites by sulfur [5–7]. The noble metals supported on acidic carriers have been reported to be more sulfur tolerant [1,8–10]. The acidity of the support has been shown to affect positively the performance of palladium catalysts supported over Beta-H zeolites, and mesoporous ZSM-5 zeolite in the hydrogenation reaction of aromatic compounds and also in the desulfurization of the 4,6dimethyldibenzothiophene [11,12]. This effect has been attributed to the partial electron transfer from the metal particles to the acidic sites of the supports. Then, the interaction between the electron depleted metal particles and the H2 S would decrease [5] producing

∗ Corresponding author. Tel.: +39 0916809372; fax: +39 0916809399. E-mail address: [email protected] (A.M. Venezia). 0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.06.001

more stable catalysts. Mesoporous materials are particularly suited as catalyst supports due to their high surface area, well defined pore structure and reasonable thermal stability. All siliceous mesoporous materials are characterized by a low acidity of the silanol groups on the surface. Just as in zeolites, substitution of Si4+ by trivalent ions in the inorganic framework of mesoporous silica creates more acidic groups [11]. Among different cations, Al3+ is the best candidate for incorporation in the silica framework. The insertion of aluminum induces the highest acidity with formation of both Lewis and Brønsted acid sites. Increasing the amount of the trivalent ions into the mesopore structure generates an increase of the acid site number but at the same time a decrease in the well defined hexagonal order and a significant contraction of the pore diameter by calcination [13]. In order to preserve the thermal stability while creating high acidity, efforts have been made for post-synthesis alumination of silica mesoporous materials [2,13,14]. According to this procedure, the Al3+ are deposited by simple impregnation [2]. The stability and the incorporation of Al3+ in the inorganic walls depend strongly on the aluminum source, i.e. aluminum isopropoxide, aluminum nitrate and aluminum chlorohydrate. Particularly, it has been shown that impregnation of MCM-41 with Al(NO3 )3 yields both Lewis and Brønsted acid sites with a mild acid strength and with the number of both types of acid sites increasing with increasing Al/Si molar ratio [2]. Moreover, the impregnation method improves accessibility since the acid sites are located on the surface (not in the walls) while maintaining the uniformly hexag-

212

A.M. Venezia et al. / Applied Catalysis A: General 383 (2010) 211–216

onal framework of the MCM-41. Several studies have been focused on the effect of the alumina on the catalytic activity of Pt in hydrogenation reaction and also in the hydrodesulfurization of thiophene [1,10]. However similar investigation over Pd supported on mesoporous silica has not yet been reported. In the present paper the effect of the post-alumination of MCM-41 on the hydrodesulfurization (HDS) activity of supported Pd catalysts is evaluated. Different concentrations of Al3+ are employed, aiming to establish a correlation between the catalytic behavior and the structural modification induced by the trivalent ions and to identify the best catalyst formulation. Thiophene is used as probe molecule. 2. Experimental 2.1. Preparation of supports MCM-41, Al2 O3 -modified MCM-41 and catalysts The MCM-41 was prepared by the hydrothermal procedure reported by Choma et al. [15]. Accordingly, the surfactant, hexadecyl trimethylammoniumbromide (CTAB), was dissolved in a solution of aqueous ammonia (28 wt%), distilled water and ethanol. After stirring for 10 min, the tetraethoxysylane (TEOS) was added. The molar composition of the gel was: 0.09 TEOS:1.12 NH4 OH:4.86 EtOH:0.04 CTAB:15.62 H2 O. It was homogenized by stirring for 2 h at room temperature before heating at 100 ◦ C for 5 days under static conditions. Thereafter the product was calcined at 650 ◦ C for 6 h in air. Attainment of the ordered mesoporous structure was confirmed by the XRD pattern, showing the typical reflection at 2 ≈ 2◦ , and by the typical type IV N2 adsorption–desorption isotherm [15,16]. Al2 O3 -modified MCM-41 supports were prepared by impregnation method [17]. The Al2 O3 loadings were x = 0.25, 0.5, 1 and 4 wt% corresponding to the atomic ratios Si/Al = 339, 169, 84 and 20. The MCM-41 was impregnated with aluminum nitrate (Al(NO3 )3 ·9H2 O, Aldrich) aqueous solution. After impregnation, the samples were dried at 120 ◦ C and then calcined at 500 ◦ C for 4 h. The supports are labelled as xAl/MCM-41 where x indicates the percentage of Al2 O3 . The Pd/xAl/MCM-41 catalysts were obtained by impregnating the xAl/MCM-41 supports with an aqueous solution of palladium chloride (Aldrich) followed by drying at 120 ◦ C and calcination at 400 ◦ C for 2 h [18]. The chemical composition of the samples was determined by X-ray fluorescence and accordingly the amount of palladium was 1 wt%. 2.2. Sample characterization XRD patterns were measured with a Bruker goniometer using Ni-filtered Cu K˛ radiation. A proportional counter and 0.05◦ step sizes in 2 were used. The assignment of the crystalline phases was based on the JPDS powder diffraction file cards [19]. From the line broadening of the main reflection peaks, using the Sherrer equation, particle sizes above the detection limits of 3 nm were determined [20]. Small angle X-ray scattering measurements (SAXS) were performed with BRUKER AXS NANOSTAR with step sizes of 0.02◦ in 2. The microstructural characterization was performed with a Carlo Erba Sorptomat 1900 instrument. The fully computerised analysis of the adsorption isotherm of nitrogen at liquid nitrogen temperature, allowed obtaining, through the BET approach, the specific surface area of the samples. By analysis of the desorption curve, using the BJH calculation method, the pore size volume distribution was also obtained [16]. The X-ray photoelectron spectroscopy analyses were performed with a VG Microtech ESCA 3000 Multilab, equipped with a dual Mg/Al anode. The spectra were excited by the unmonochromatised Al K␣ source (1486.6 eV) run at 14 kV and 15 mA. The analyser

operated in the constant analyser energy (CAE) mode. For the individual peak energy regions, a pass energy of 20 eV set across the hemispheres was used. Survey spectra were measured at 50 eV pass energy. The sample powders were analysed as pellets, mounted on a double-sided adhesive tape. The pressure in the analysis chamber was in the range of 10−8 Torr during data collection. The constant charging of the samples was removed by referencing all the energies to the C 1s set at 285.1 eV, arising from the adventitious carbon. The invariance of the peak shapes and widths at the beginning and at the end of the analyses ensured absence of differential charging. Analyses of the peaks were performed with the software provided by VG, based on non-linear least squares fitting program using a weighted sum of Lorentzian and Gaussian component curves after background subtraction according to Shirley and Sherwood [21,22]. Atomic concentrations were calculated from peak intensity using the sensitivity factors provided with the software. The binding energy values are quoted with a precision of ±0.15 eV and the atomic percentage with a precision of ±10%. The acidity of the oxide catalysts was determined by measurements of temperature-programmed desorption of ammonia (NH3 -TPD). The samples (0.03 g) were outgassed in a He flow (Air Liquide, 99.996%) at 350 ◦ C for 0.5 h. This was followed by ammonia-saturation by flowing 5% NH3 /He stream at 80 ◦ C for 0.5 h. After equilibration in He flow for 0.5 h at this temperature, the catalyst was heated in a linear rate of 10◦ /min to 1050 ◦ C, and the ammonia desorption was continuously monitored by the TCD. In order to determine the total acidity of the catalyst from its NH3 desorption profile, the area under the curve was integrated. A semiquantitative comparison of the strength distribution was achieved by Gaussian deconvolution of the peaks. Mild and strong acidities were defined as the areas under the peaks at the lowest and highest temperatures, respectively. The ZPC of the various supports was determined by mass titration [23]. According to this method, the variation of pH of a water solution containing increasing amount of solid was monitored until the steady state value of pH (ZPC) was reached. X-ray fluorescence was performed using the Bruker S2 Ranger spectrophotometer. 2.3. HDS reaction The hydrodesulfurization of thiophene was carried out in the vapour phase using a continuous flow microreactor and according to the procedure described previously [24]. An amount of 200 mg of catalyst (sieved fraction 210–430 ␮m), diluted with inert particles of SiC, was used for each test. The samples were sulfided in situ with a mixture of 10 vol% H2 S/H2 , at 50 ml/min, while raising the temperature up to 400 ◦ C at a rate of 7 ◦ C/min and were maintained at this temperature for 2 h. After purging with nitrogen, the HDS of thiophene was carried out at 340 ◦ C with 5.3 vol% thiophene in H2 and WHSV = 7500 h−1 . The reaction products were analysed by on-line gas chromatography (Carlo Erba GC 8340 gas chromatograph). Fractional conversions were calculated from the ratio of the peak area of the C4 products over the sum of the peak areas of the products and thiophene. The reaction rate for HDS (kHDS ) was calculated from the fractional conversion obtained after 6 h on stream, assuming a first order reaction in thiophene. The percentage of deactivation, % d, was calculated as % d = 100(xi − xf )/xi where xi and xf were the initial conversion and the conversion after 6 h, respectively. 3. Results and discussion In Table 1 the catalytic data in terms of thiophene HDS reaction rate and percent of deactivation for the Pd/xAl2 O3 -modified

A.M. Venezia et al. / Applied Catalysis A: General 383 (2010) 211–216

213

Table 1 Thiophene HDS reaction rate (k), and percentage of deactivation (% d) of the supported catalysts. Reaction conditions: 5.3 vol% thiophene in H2 ; WHSV = 7500 h−1 ; temperature = 340 ◦ C. Samples

k (␮mol g−1 s−1 )

%d

Pd/MCM-41 Pd/0.25Al/MCM-41 Pd/0.5Al/MCM-41 Pd/1.0Al/MCM-41 Pd/4.0Al/MCM-41

0.61 1.60 2.07 1.58 0.84

20 41 43 60 69

MCM-41 catalysts are summarized. The inertness of the supports in the reaction was confirmed by the absence of any activity in catalytic tests performed with the supports alone. The catalytic activity went through a maximum, with a three times increase of activity with respect to the alumina free catalyst, in correspondence of the 0.5 wt% Al2 O3 content. The extent of the enhancement of the activity was quite surprising, considering the very small amount of alumina. Similar trends were observed in the case of benzene hydrogenation with zeolite supported platinum catalysts as a function of the support surface acidity, with and without thiophene [10]. The enhanced activity with the increased acidity was attributed to a higher metal dispersion and to the presence of an optimum concentration of acid sites in the support. In Fig. 1 the conversion as a function of time are given for three selected samples. As shown in the figure, the attainment of a steady state eventually occurred after different time on stream, depending on the samples. The percentages of deactivation, calculated as the percentage difference between the initial conversion and the conversion after 6 h are summarized in Table 1. They increased with the increasing amount of alumina in a non-linear fashion. In order to ascertain the structural modification induced by the addition of alumina, small angle X-ray scattering measurements were performed. The SAXS patterns of the bare and of the Al doped supports in the range 2 = 1–4◦ are shown in Fig. 2. All the sample patterns contained one peak in the range of 2 = 2.64–2.74◦ due to the (1 0 0) reflection of the long range hexagonal ordering. The samples containing alumina maintained the hexagonal symmetry exhibiting a slight shift of the peak to higher 2 values with respect to the pure MCM-41. Correspondingly, the d1 0 0 spacing representing the plane distance, calculated by the Bragg’s law decreased slightly, from 3.34 nm as observed in the bare support to 3.24 nm as observed for the alumina doped samples. It is worth mentioning that such difference did not increase with the alumina loading. The unit cell dimension (a0 ) corresponding to the adjacent

Fig. 1. Plot of conversion as a function of time for three selected samples. Reaction conditions: 5.3 vol% thiophene in H2 , WHSV = 7500 h−1 , and temperature = 340 ◦ C.

Fig. 2. SAXS pattern of bare and alumina modified MCM-41 supports.

Table 2 BET surface area (SSA), pore size (D), zero point charge (ZPC) and lattice parameter (a0 ) of the various supports. SSA (m2 /g)

Samples MCM-41 0.5Al/MCM-41 1.0Al/MCM-41 4.0Al/MCM-41

1030 950 930 876

D (nm)

ZPC (pH)

a0 (nm)

2.6 2.6 2.6 2.5

5.6 4.6 4.6 4.6

3.9 3.7 3.7 3.7

pores centre–centre distance assuming hexagonal symmetry could √ be calculated by means of the equation: a0 = 2d1 0 0 / 3. According to the values listed in Table 2, a small contraction of 0.2 nm was occurring in all of the alumina loaded samples. Similar changes of the lattice parameter were reported in previous studies of postsynthesis aluminated MCM-41 [25]. The small contraction could be attributed to a shrinking of the structure caused by the replacement of the Si4+ with the coordinative unsaturated Al3+ ions. On the other hand, severe decrease of the lattice parameter was observed in the case of alumina doped MCM-41, prepared by sol–gel technique [26]. Inherently to the particular synthesis procedure, several reasons were proposed to explain the shrinkage, like a strong interaction between the surfactant and the framework aluminum or formation of smaller diameters micelles [26]. In Fig. 3 the typical nitrogen adsorption-desorption isotherm obtained with the pure MCM-41 sample is shown. The same curve

Fig. 3. Nitrogen adsorption–desorption isotherm of MCM-41.

214

A.M. Venezia et al. / Applied Catalysis A: General 383 (2010) 211–216 Table 3 Acid properties of the naked supports and oxide precursors as determined by TPDNH3 . Sample

Fig. 4. TPD-NH3 profiles of calcined Pd/(x)Al)/MCM-41 catalysts. Typical deconvoluted plot for calcined Pd/MCM-41 sample is also shown.

was obtained with all the alumina doped samples. According to the IUPAC classification, they all exhibit the same type IV isotherms typical of the mesoporous materials. All nitrogen isotherms have two capillary condensation steps: (i) the first hysteresis loop starts at a partial pressure about 0.2, indicating the presence of framework mesoporosity and (ii) the second hysteresis loop starting at a partial pressure of about 0.95 is due to textural inter-particle macroporosity. The sharp inflection of the adsorption branch confirms the high quality of materials with uniform mesoporosity. The rise of sorption at relative pressure of less than 0.05 corresponds to the volume filling of micropores. The BET derived values of the surface area and the average pore size are listed in Table 2. While no significant change in the pore size was occurring, the surface area decreased with the increasing loading of alumina. Thus, the specific surface area decreases from 1030 m2 g−1 for MCM-41 to 876 m2 g−1 for 4.0Al/MCM-41. The acidity of the supports was studied by the temperature programmed desorption of ammonia (NH3 -TPD). The technique provided information on the total acidity of the solids, without distinguishing between Brønsted and Lewis acidity but accounting for the different strength of acid sites. Ammonia is indeed a suitable probe molecule due to its small size and its basicity which allows it to interact with the majority of the acid sites. Thus, the amount of ammonia desorbed at some characteristic temperatures was taken as measurement of the number of acid centres, while the temperature ranges in which the ammonia was desorbed were indicator of the strength of the acid sites [13]. The acid site distribution pattern could be classified into weak to medium acidity, related to ammonia desorption in the range 150–400 ◦ C and strong acidity related to desorption in the range 400–600 ◦ C [27]. Total acidity was the sum of amount of ammonia desorbed from the entire temperature region. Typical TPD-NH3 profiles are shown in Fig. 4 for selected Pd/xAl/MCM-41 catalysts. The integration of the curves represent-

Amount of acid sites (mmol NH3 gcat −1 ) T = 150–400 ◦ C

T > 400 ◦ C

Total

Supports MCM-41 0.25Al/MCM-41 0.5Al/MCM-41 1.0Al/MCM-41

0.01 0.09 0.06 0.07

0.01 0.10 0.23 0.25

0.02 0.19 0.29 0.32

Catalysts Pd/MCM-41 Pd/0.25Al/MCM-41 Pd/0.5Al/MCM-41 Pd/1.0Al/MCM-41 Pd/4.0Al/MCM-41

0.11 0.16 0.08 0.13 0.16

0.10 0.14 0.17 0.20 0.23

0.21 0.30 0.25 0.33 0.39

ing the amount of NH3 desorbed as a function of the temperature in different temperature ranges provided us with the data compiled in Table 3. The peaks were treated mathematically using Gaussian functions. The bare siliceous MCM-41 had negligible acidity. The data in Table 3 indicate that the number of sites available to NH3 chemisorption increases with the increase of alumina percentage in the hybrid (x)Al/MCM-41 materials. Indeed, the isomorphous substitution of Si with Al produces Brønsted acid sites due to the formation of bridging Si(OH)Al groups [28]. At the same time Lewis acidity associated to the coordinatevely unsaturated Al3+ cations is also formed [29]. According to the data, the large proportion of increase was due to the strong strength acid sites, corresponding to the NH3 desorption in the temperature range above 400 ◦ C. It is worth noticing that the increase was not linear since by doubling the alumina amount from 0.5 to 1 wt% only a 10% increase of the total acidity was obtained. This suggests that only part of the Al3+ was incorporated into the silica framework while the rest was likely present as a non-structural aluminum rich species [25]. In line with this observation, the existence of limiting factor for Al incorporation into MCM-41 framework was observed previously by Mokaya and Jones. The authors proposed that the maximum amount of incorporated aluminum is limited by the number of available and accessible silanol groups [30]. As seen in Table 3, the subsequent deposition of the metal causes a slight increase of the total acidity, increasing mostly the weak medium strength acidity. Such effect especially appreciable in the less acidic samples may be due to some chlorine left from the catalyst precursor [31]. In Fig. 5 the thiophene HDS rate constant k versus the strong strength acidity of the calcined Pd catalyst are plotted. Following the trend of Table 1, the curve presents a maximum in correspondence of the sample Pd 0.5Al/MCM-41. Moreover, by plotting the percentage of deactivation in the HDS of thiophene as a function of the strong strength acid sites of the oxide catalyst precursors, as shown in Fig. 6, a straight line is obtained, pointing out to a direct involvement of the acidity in the catalyst deactivation. The diffractograms of the calcined Pd supported catalysts are shown in Fig. 7. The patterns exhibit at 2 = 34◦ the characteristic (1 0 1) reflection of the crystalline PdO [18,19]. From the line broadening, using the Scherrer equation, the PdO particle sizes are estimated. As shown from the values listed in Table 4, addition of alumina increases the dispersion of the supported oxides. XRD analyses of the spent catalysts were also performed. However, the pattern exhibiting only a small feature at ∼40◦ 2 corresponding to metallic palladium, did not allow any particle size calculation. The surface atomic composition and the surface electronic state of the calcined Pd catalysts were investigated by X-ray photoelectron spectroscopy. In Fig. 8 the Pd 3d experimental spectra with the fitted components of the calcined samples containing

A.M. Venezia et al. / Applied Catalysis A: General 383 (2010) 211–216

215

Table 4 XRD derived PdO crystal size (dPdO ) and XPS results in terms of Pd 3d5/2 binding energies and Pd/Si atomic ratios of the catalyst materials in the calcined state. Samples

dPdO (nm)

Pd 3d5/2 (eV)

Pd 3d3/2 (eV)

Pd/Si

Pd/MCM-41 Pd/0.5Al/MCM-41 Pd/1.0Al/MCM-41 Pd/4.0Al/MCM-41

8.5 6.0 n.a.a 4.0

338.0 337.7 337.3 337.2

343.3 342.9 342.5 342.5

0.001 0.002 0.002 0.003

a

Fig. 5. Variation of the rate constant, k, obtained at the temperature of 340 ◦ C, for the HDS of thiophene over Pd/(x)Al//MCM-41 catalysts versus strong strength acid sites of the catalyst oxide precursors (from Table 3).

Fig. 6. Correlation between the strong strength acid sites of the oxide precursors (from Table 3) and the catalyst deactivation in thiophene HDS reaction at temperature of 340 ◦ C over Pd catalysts supported on siliceous MCM-41 modified by aluminum ions.

Fig. 7. XRD patterns of calcined (a)Pd/MCM-41; (b) Pd/0.5Al/MCM-41; (c) Pd/4.0Al/MCM-41.

Not available.

different amount of alumina are shown. Beside the two spin-orbit components, Pd 3d5/2 and Pd 3d3/2 , ∼5.3 eV apart, an extra peak attributable to a “ghost” line arising from Mg anode contamination is also visible. Therefore for quantitative purpose the clean Pd 3d3/2 component was considered. The spectra in Fig. 7 and the binding energy values in Table 3 are characteristic of a highly oxidised palladium. As observed before [18], a Pd 3d5/2 value above 337 eV is more typical of Pd 4+ as in PdO2 formed by oxygen incorporation into the PdO during the calcination and stabilized by the MCM41 structure [32]. With addition of Al3+ ions, the energy of the Pd 3d photoelectron level shifts towards lower values and the shift is larger in the samples containing more alumina. The decrease of the Pd 3d binding energy may be attributable to the loss of the stabilization of the above mentioned Pd 4+ species upon the addition of alumina and to the formation of the more stable PdO. According to the XPS quantitative analysis of the surface samples, the increase of the Pd/Si atomic ratios with the increase of the alumina content in the support confirmed the improvement of the palladium dispersion, in agreement with the XRD results. Considering that the surface ratio was always lower than the bulk (analytical) ratio, (Pd/Si)bulk = 0.005, part of palladium was likely located into the mesoporous channels. The addition of alumina could have a twofold action over the palladium deposition. One may be just a physical impediment, by preventing Pd from depositing more deeply into the channel, the second one could be due to the direct interaction of the Pd with the generated Brønsted acid sites, therefore favoring a better dispersion on the support surface. XPS analyses of the spent catalysts were also performed, however in all samples, the Pd 3d signal was barely present and it was not possible to detect any partial electron transfer from the noble metal particles to the acidic sites on the support as suggested in literature [5,11]. Deterioration of the XPS spectra after reaction is not unusual and may be attributed to carbon deposit and/or to diffusion of the reduced particles into the mesoporous support matrix.

Fig. 8. Pd 3d spectra of (a) Pd/MCM-41; (b) Pd/0.5Al/MCM-41; (c) Pd/1.0Al/MCM41; (d) Pd/4.0Al/MCM-41.

216

A.M. Venezia et al. / Applied Catalysis A: General 383 (2010) 211–216

4. Conclusion The modification of the siliceous MCM-41 by post-synthesis alumination had a beneficial effect on the hydrodesulfurization of the supported palladium catalysts. The maximum of activity was observed with the sample containing little amount of alumina corresponding to the large atomic ratio Si/Al = 169. Upon addition of Al3+ ion, the structural and surface analyses allow to detect a small contraction of the porous channels, a decrease of the PdO particle sizes and an increase of the surface palladium dispersion. In analogy with mechanisms proposed earlier for the hydrogenation of aromatic compounds, a bi-functional mechanism may explain the improved catalytic activity of the alumina doped samples in the hydrodesulfurization of the thiophene. The decrease of activity observed above a certain amount of alumina is attributed to coke formation favored by the Brønsted acidity. Acknowledgements Support by European Community, Network of Excellence (NoE) IDECAT (Integrated Design of Catalytic Nanomaterials for Sustainable Production) and COST D36 action is acknowledged. References

Fig. 9. Suggested bi-functional mechanism of HDS of thiophene over Pd catalyst supported on siliceous MCM-41 modified by aluminum ions.

The increase of activity with the addition of alumina may only partially be attributed to the increased Pd dispersion, since no direct correlation between the dispersion and the activity is found. Similarly to the mechanism proposed long time ago by Vannice and co-workers [33], for the benzene hydrogenation over Pd catalysts on acidic support, the increase of the thiophene HDS activity could also be attributed to the presence of additional sites for the adsorption of the thiophene and to the destabilization of the aromatic ring. These alumina induced additional sites are the acid sites which would interact with the electron rich molecule of thiophene. If the adsorbed molecule is near the activated hydrogen over the Pd particles, the hydrogenation by spillover hydrogen, followed by the breaking of the C–S bond, would occur [9]. This process is obviously favored by the increased number of acid sites created upon alumina addition. As suggested in the case of Pt catalysts promoted by alumina [1], a bi-functional mechanism, as sketched in Fig. 9, where the acidity of the supports and the metal dispersion play the important roles may be invoked. Nevertheless, the mechanism does not account for the decline of activity observed in the samples with alumina loading ≥0.5 wt% as shown in Fig. 5. In fact, as shown in Fig. 6 a direct correlation between the percentage of deactivation and the acidity of the oxide catalysts exists. A theoretical investigation of the thiophene cracking mechanism over pure Brønsted acid zeolites has demonstrated that the molecule interaction with the Si(OH)Al group (Brønsted acid centre) favors the cracking through the rupture of the C–S bond [34]. Moreover, a TPD study of the thiophene decomposition on acidic zeolite supports, has shown carbonaceous species, formed by decomposition and polymerization of thiophene on the Brønsted acid sites [35]. Based on these theoretical and experimental studies, in the present case, above a certain support acidity, it is reasonable to assume that the spillover of the hydrogen atoms from the metal particles is not able to counteract the formation of coke on the acid sites, leading to the catalyst poisoning.

[1] Y. Kanda, T. Kobayashi, Y. Uemichi, S. Namba, M. Sugioka, Appl. Catal. A: Gen. 308 (2006) 111–118. [2] D. Trong On, D. Desplantier-Giscard, C. Danumah, S. Kaliaguine, Appl. Catal. A 253 (2003) 545–602. [3] A. Corma, A. Martinez, V. Martinez-Soria, J. Catal. 169 (1997) 480–489. [4] H. Topsøe, B.S. Clausen, F.E. Massoth, in: J.R. Anderson, M. Boudart (Eds.), Hydrotreating Catalysis, Springer-Verlag, Berlin, 1996. [5] B.H. Cooper, B.B.L. Donnis, Appl. Catal. A: Gen. 137 (1996) 203–223. [6] J. Barbier, E. Lamy-Pitara, P. Marecot, J.P. Boitiaux, J. Cosyns, F. Verna, Adv. Catal. 37 (1990) 279–318. [7] J.C. Rodríguez, J. Santamaría, A. Monzón, Appl. Catal. A: Gen. 165 (1997) 147–157. [8] R.M. Navarro, B. Pawelec, J.M. Trejo, R. Mariscal, J.L.G. Fierro, J. Catal. 189 (2000) 184–194. [9] A. Niquille-Röthlisberger, R. Prins, Catal. Today 123 (2007) 198–207. [10] L.J. Simon, J.G. van Ommen, A. Jentys, J.A. Lercher, J. Catal. 203 (2001) 434– 442. [11] T. Tang, C. Yin, L. Wang, Y. Ji, F.-S. Xiao, J. Catal. 257 (2008) 125–133. [12] Y. Sun, R. Prins, Angew. Chem. Int. Ed. 47 (2008) 8478–8481. [13] M. Gòmez-Cazailla, J.M. Mèrida-Robles, A. Gurbani, E. Rodriguez-Castellòn, A. Jimenez-Lòpez, J. Solid State Chem. 180 (2007) 1130–1140. [14] L.Y. Chen, Z. Ping, G.K. Chuah, S. Jaenickle, G. Simon, Micropor. Mesopor. Mater. 27 (1999) 231–242. [15] J. Choma, S. Pikus, M. Jaroniec, Appl. Surf. Sci. 252 (2005) 562–569. [16] S.J. Gregg, K.S. Sing, Adsorption, Surface Area and Porosity, 2nd ed., Academic Press, San Diego, 1982. [17] A. Papp, A. Molnár, A. Mastalir, Appl. Catal. A: Gen. 289 (2005) 256–266. [18] A.M. Venezia, R. Murania, G. Pantaleo, G. Deganello, J. Catal. 251 (2007) 94– 102. [19] JCPDS Powder Diffraction File. Int. Centre for Diffraction Data, Swarthmore; File No. 42-1467. [20] H.P. Klug, X-ray Diffraction Procedure for Polycrystalline and Amorphous Materials, Wiley, New York, 1954. [21] D.A. Shirley, Phys. Rev. B5 (1972) 4709–4714. [22] P.M.A. Sherwood, in: D. Briggs, M.P. Seah (Eds.), Practical Surface Analysis, Wiley, New York, 1990, p. 181. [23] S. Subramanian, J.S. Noh, J.A. Schwarz, J. Catal. 114 (1988) 433–439. [24] A.M. Venezia, V. La Parola, V. Nicolì, G. Deganello, J. Catal. 212 (2002) 56–62. [25] S. Kawi, S.-C. Shen, Mater. Lett. 42 (2000) 108–112. [26] J. Aguado, D.P. Serrano, J.M. Escola, Micropor. Mesopor. Mater. 34 (2000) 43–54. [27] S.K. Jana, H. Takahashi, M. Nakamura, M. Kaneko, R. Nishida, H. Shimizu, T. Kugita, S. Namba, Appl. Catal. A 245 (2003) 33–41. [28] Q. Huo, D.I. Margolese, G.D. Stucky, Chem. Mater. 8 (1996) 1147–1160. [29] G. Busca, Catal. Today 41 (1998) 191–206. [30] R. Mokaya, W. Jones, J. Mater. Chem. 9 (1999) 555–561. [31] M. Lewandowski, Z. Sarbak, Appl. Catal. A: Gen. 156 (1997) 181–192. [32] Y. Bi, G. Lu, Appl. Catal. B: Environ. 41 (2003) 279–286. [33] P. Chou, M.A. Vannice, J. Catal. 107 (1987) 140–153. [34] B. Li, W. Guo, S. Yuan, J. Hu, J. Wang, H. Jiao, J. Catal. 253 (2008) 212–220. [35] L.J. Simon, M. Rep, J.G. van Ommen, J.A. Lercher, Appl. Catal. A 218 (2001) 161–170.