Liquid-Phase Selective Oxidation of Propane on Silica-Supported Nafion Catalysts

Transcription

1 Journal of Natural Gas Chemistry 11(2002) Liquid-Phase Selective Oxidation of Propane on Silica-Supported Nafion Catalysts Francesco Frusteri 1, Lorenzo Spadaro 1, Claudia Espro 2, Adolfo Parmaliana 2, Francesco Arena 1,2 1. Istituto CNR-TAE, Via Salita S. Lucia 39, I S. Lucia - Messina (Italy) 2. Dipartimento di Chimica Industriale e Ingegneria dei Materiali, Universita degli Studi di Messina, Salita Sperone c.p. 29, I S. Agata - Messina (Italy) [Manuscript received September 03, 2002; revised December 18, 2002] Abstract: The liquid-phase partial oxidation of propane in the presence of the Fe n+ /H 2O 2 Fenton system at 70 and 1.4 atm on silica supported Nafion catalysts has been investigated. The reaction proceeds via a radical reaction path the efficiency of which is improved by silica-supported Nafion catalysts. Because of the direct relationship between reaction rate and concentration of sulphonic acid sites of Nafion catalysts, it is inferred that the active phase enahnces the kinetics of propane conversion by promoting the rate of active radicals generation. Key words: nafion catalyst, silica carrier, propane, partial oxidation, liquid-phase, fenton system 1. Introduction To overcome both economic and environmental drawbacks of many industrial processes based on the employment of mineral acid catalysts, a great deal of research effort has been devoted during the last few years to the design and development of solid acids which could mimic the catalytic action of homogeneous systems. Progresses achieved in this research area have afforded the application of solid acids like HY and ZSM-5 zeolites [1] and heteropolyacid [2] to some actual industrial processes. However, a considerable interest has been in recent years focused on ion-exchanged resins, such as perfluorinated polymers, since these can feature a broad range of acidic strength along with peculiar physico-chemical properties [3]. These solid acids, commercially available in the form of resinsulfonic acids like Nafion, constitute a very interesting class of catalysts having a potentially huge number of applications because of the versatility with which their acidic properties can be tuned by proper design of the macromolecule [4,5]. Namely, varying the relative position and density of fluorine and/or sulphonic groups, the electron-withdrawing effect on the acidic functional groups can be easily controlled and acidic strengths comparable or even superior to that of 100% sulphuric acid, coupled to a liquid-like proton mobility, can be attained [5]. In spite of such potential advantages, however, a major drawback to be overcome for a practical exploitation of these engineered materials lies in the poor extent of surface area generally resulting in a relatively limited dispersion and availability of active sites [3]. Using conventional oxide carriers, like silica and carbon, and various preparation routes [4], a significant enhancement in both dispersion and stability of Nafionbased systems, yielding a stabilisation of very small aggregates of Nafion chains (<100 nm) on high surface area ( m 2 /g) catalytic systems, has been realized [3,4]. Potential applications of acidic polymeric resins for several industrially relevant transfor- Corresponding author. Tel : ; Fax: ; frusteri@itae.cnr.it.

2 Journal of Natural Gas Chemistry Vol. 11 No mations (e.g. alkylations, olefin isomerization, olefin oligomerisation, acylations, esterifications, hydrationdehydration, etc.) have also been documented [5]. Furthermore, we found that carbon-supported Nafion catalytic membranes, in the presence of the Fe II -H 2 O 2 Fenton system, effectively promote the selective oxidation of light alkanes under rather mild (T<120 ; P<2 atm) reaction conditions [6-7]. Yet, the relatively high level of metal impurities in carbon-based membranes, implying high H 2 O 2 decomposition rates and consequently poor H 2 O 2 yields [6,7], prompted us to investigate the suitability of a chemical inert carrier, like silica, for the preparation of Nafion-based catalysts driving the selective oxidation of light alkanes in the presence on the Fenton system under mild conditions. Therefore, activity data of Nafion/SiO 2 catalysts in the liquid-phase selective oxidation of propane to oxygenates, in the presence of the Fe n+ -H 2 O 2 Fenton system, are reported here. The promoting role of solid catalysts on such a peculiar three-phase reaction is discussed. 2. Experimental 2.1. Catalyst Nafion-H was used in the form of perfluorinated ion-exchanged 5% isopropanolic solution (Aldrich product), while several commercial silica samples, namely precipitated Si 4-5P (Akzo product, 380 m 2 /g surface area), silica gel CS-1020 (Cabot Corporation, 200 m 2 /g surface area) and fumed LM-50 (PQ Corporation, 130 m 2 /g surface area), were used as carriers. Catalysts were prepared by incipient wetness of silica samples with a diluted ethanolic solution containing the designed amount of Nafion with subsequent drying at 90 at atmospheric pressure. After impregnation, all catalysts were treated in air at 150 for 12 h. The list of the catalysts, with the relative code, Nafion loading and H + -exchange capacity, as determined by Na-exchange measurements, is reported in Table 1. Table 1. List of catalysts Code Composition Nafion loading (wt%) Acid capacity (meq./g) N-1 Nafion-1100/ SiO 2 (Si4 5P) N-2 Nafion-1100/ SiO 2 (Si4 5P) N-3 Nafion-1100/ SiO 2 (Si4 5P) N-4 Nafion-1100/ SiO 2(LM 50) N-5 Nafion-1100/ SiO 2 (CS 1020) N-6 Nafion-900/SiO 2 LM Proton exchange capacity of the various catalysts was evaluated by performing a liquid-phase shift of the acidic protons with a NaCl (0.04 M) aqueous solution and a subsequent NaOH (0.001 M) titration of the acid released using the phenolphthalein as an indicator Catalyst test Catalyst test in the selective oxidation of propane was performed at 70 in a batch mode using a glass autoclave operating at a C 3 H 8 pressure of 1.4 ata with a stirring speed of 800 rpm. The reactor was loaded with 80 ml of an aqueous solution with a ph of ca. 3 (by H 2 SO 4 ), unless otherwise specified, containing 1% (vol/vol) of H 2 O 2, a concentration of Fe 2+ /Fe 3+ (ex sulphate) between 10 and 350 µmol/l and a catalyst mass sample of 0.2 or 0.6 g. Reaction products in the solution were periodically analysed by a GC equipped with a Carbopack B 3% SP-1500 column at 45 connected to a flame ionization detector. Changes in the H 2 O 2 concentration were monitored by performing a periodic titration of the reaction solution with a standard KMnO 4 solution (10 2 M), and GC analysis of the gas-phase did not reveal any presence of CO or CO 2 products at any time during the reaction. Catalytic data were expressed in terms of reaction rate values (nmol/(s l) and nmol/(s g)) after 90 min, which is the time required to attain a stationary conversion rate, because

3 182 Francesco Frusteri et al./ Journal of Natural Gas Chemistry Vol. 11 No at this time the effect of substrate conversion (<1%) on reaction kinetics became practically negligible. 3. Results and discussion 3.1. Mechanistic evidence of the three phases of propane selective oxidation Previous evidence on the activity of C-containing Nafion membranes in the partial oxidation of C 1 C 3 alkanes mediated by the H 2 O 2 -Fe II Fenton system in a three-phase membrane reactor [6,7] pointed to the enhancement of the H 2 O 2 yield as one of the routes for catalyst optimisation. Namely, since the poor H 2 O 2 yield of such catalysts is connected to the presence of transition metal ions on carbon carriers [6,7], this work was undertaken in order to explore the reactivity of a class of Nafion catalysts supported on an inert silica matrix. The first step of this study dealt with a series of blank tests carried out in the presence of the conventional Fe 2+ /H 2 O 2 Fenton system and various silica samples used as catalyst supports. The experimental results, summarized in Table 2 in terms of reaction rates and product selectivity values after 90 min of reaction time, indicate that the partial oxidation of propane to oxygenated products proceeds even in the absence of solid catalysts (blank test) with a rate that, under the adopted conditions, equals 163 nmol/(s l). A H 2 O 2 decomposition rate of 10 µmol/(s l) signals a H 2 O 2 utilization efficiency, expressed as a ratio of propane to the H 2 O 2 conversion rate (R), of only 1.6%. The product distribution indicates in acetone (66%) and i-c 3 H 7 OH the main reaction products, though n-c 3 H 7 OH and C 2 H 5 CHO are found at the expected, relatively high (10% 15%) levels (Table 2). Evidently, such findings point to a free-radical reaction path yielding an unselective attack of the hydroxy-radicals generated by the Fenton system on both the primary and secondary carbon of the substrate molecule. However, thermodynamic factors preferentially drive the oxy-functionalization of the latter. Table 2. Sample Activity data of silica carriers in the liquid-phase selective oxidation of propane under mild reaction conditions W a reaction rate rate H2 O 2 R a Selectivity (%) (g) nmol/(s 1) nmol/(s g) µmol/(s l) (%) i-c 3 H 7 OH CH 3 COCH 3 n-c 3 H 7 OH C 2 H 5 CHO SiO 2 (Si4-5P) SiO 2 (Si4-5P) SiO 2 (LM-50) SiO 2 (LM-50) SiO 2 (CS-1020) Reaction conditions: T=70 ; P=1.4 atm. (a) reaction rate and selectivity values after 90 min of time on stream. Standard reaction tests in the presence of 0.2 g of the various bare silica samples do not indicate any positive effect of these inert materials on the kinetics of the propane conversion, even a negative influence depending on mass and silica samples (Table 2). Indeed, reaction rate values of nmol/(s l)(65-80 nmol/(s g)), comparable with that recorded for the homogeneous system, were recorded on the Si 4-5P and LM-50 silicas, whereas it was considerably lower (36 nmol/(s l)) for the CS-1020 silica sample. A comparable H 2 O 2 conversion rate ( nmol/(s l)) results in a reaction efficiency (R) of 1.2 and 2.9% on Si 4-5P and LM-50 silicas, dropping to a value of 0.4% on the CS-1020 silica. In spite of such different reactivity, no significant changes in the product distribution with respect to the blank test were recorded (Table 2). Reaction tests in the presence of a higher mass sample (0.6 g) of the Si 4-5P and LM-50 silicas, provide evidence of a generalised negative effect of any silica sample on reaction kinetics with a lowering of the rate value from to nmol/(s l)(5 8 nmol/(s g)). Moreover, an average R value of ca. 1% (Table 2) perhaps also signals a negative influence of the high mass loading on the efficiency of H 2 O 2 con-

4 Journal of Natural Gas Chemistry Vol. 11 No version, while the product distribution remains unchanged (Table 2). On the whole, these findings point to the major role of the Fenton system in driving the liquid-phase activation of propane according to a radical path, the efficiency of which is obviously depressed by silicas to an extent which depends on the exposed surface area and the solid mass loading. In other words, we can infer that the title reaction proceeds via a homogeneous radical mechanism and, in agreement with the recognised pattern of such reactions, is negatively affected by unreactive surfaces [8-11] Reactivity of Nafion/SiO 2 catalysts in the liquidphase selective oxidation of propane The results of standard activity tests carried out at 70 in the presence of 0.2 g of Nafion/SiO 2 catalysts (Table 1) and the Fe II /H 2 O 2 Fenton system are summarised in Table 3 in terms of propane and H 2 O 2 conversion rates and product selectivity values. The results of a reaction test in the presence of the N-2 sample without Fe ions in the solution, included in Table 3, indicate that, in the absence of Fe ions, the reaction proceeds at a rate (62 nmol/(s l)) lower than that of the silica carrier in the presence of the Fenton system though negligible changes in the product selectivity were observed. However, a higher efficiency of the H 2 O 2 utilization (R, 3.7%) was recorded as compared with that run in Table 2. Table 3. Catalyst Activity data of silica-supported Nafion catalysts in the liquid-phase selective oxidation of propane under mild reaction conditions a reaction rate rate H2 O 2 R a Selectivity (%) nmol/(s 1) nmol/(s g) µmol/(s l) (%) i-c 3 H 7 OH CH 3 COCH 3 C 2 H 5 CHO n-c 3 H 7 OH N-2 b N N N N N N-6 1, Reaction conditions: T=70 ; P=1.4 atm. (a) selectivity values after 90 min of time on stream; b) catalytic test performed in the absence of Fe ions. Contrary to observations of unpromoted silicas (Table 2), Nafion catalysts in the presence of the Fenton system exert an appreciable (two-threefold) promoting effect on reaction kinetics, to an extent which depends both on Nafion loading and the silica support (Table 3). Namely, the positive effect of the Nafion loading in the 10%-17% range entails a proportional increase of the reaction rate from 300 (N-1) to 538 nmol/(s l)(n-2), whilst a further rise in the loading (N-3) yields no further improvements with a plateau of nmol/(s l)( nmol/(s g)) being attained for Nafion loading in the 17% 25% range (Table 3). At the above optimum loading (17%), the support induces a different reactivity of the active phase resulting in an activity scale (N-6>N-4>N-2>N-5) which practically mirrors that of the respective silica carriers (Table 2). The reaction rate results are the highest on the N-6 sample (1,125 nmol/(s l)) and the lowest (350 nmol/(s l)) on the N-5 one, but a generalised increase of the R values (Table 3) signals that the extent of the promoting effect exerted by Nafion catalysts on reaction kinetics parallels the rise of the H 2 O 2 utilization efficiency. In fact, at the same loading, the highest activity of the N-6 catalyst is related to the lower average molecular weight of the Nafion- 900 used as active phase, implying a higher surface density of sulphonic groups well evidenced by the highest proton-exchange capability (Table 1). Then, taking into account the N-2, N-4 and N-5 samples, it emerges that, in terms of relative activity, the N- 2 (538 nmol/(s l)) and N-4 (660 nmol/(s l)) catalysts feature an activity which is ca. three times higher than that of the corresponding carrier, while the counterpart N-5 has a rate larger by ca. one order of mag-

5 184 Francesco Frusteri et al./ Journal of Natural Gas Chemistry Vol. 11 No nitude than that of the CS 1020 silica. This result evidently arises from the poor reactivity of the CS 1020 silica carrier, which likely plays a negative effect on the reactivity of the active phase resulting in the least reactive system. In other words, experimental results suggest that the active phase acts as a co-catalyst improving the efficiency of the Fenton system towards the peroxide activation resulting in higher propane converison rates. Plotting reaction rate and R values of the various catalysts (Table 3) against the relative H + -proton-exchange capacity (Table 1), which reflects the availability of sulphonic functionalities at the catalyst surface, results in two exponential-like relationships, as shown in Figure 1. Evidently, such trends indicate that an increase in the concentration of sulphonic groups yields a parallel rise in both the H 2 O 2 utilization efficiency and the rate of propane conversion. According to Kiwi et al., it is speculated that the sulphonic sites of Nafion molecules synergistically enhance the rate of radical generation via an electrostatic interaction with Fe ions which speeds up the rate of the electron-transfer process(es) to H 2 O 2 molecules [10]. Accordingly, it is inferred that the rate determining step of the title reaction is the process of hydrogen peroxide decomposition with the consequent generation of radicals enabling the conversion of the substrate Effects of reaction parameters (ph and Fe II/III concentration) In order to strengthen the primary role of the homogeneous path on the title reaction, the effects of Fe II /Fe III ions, their concentration ( µmol/l) and the solution ph (1 and 3) on the reaction rate of the N-2 catalyst (0.2 g) were investigated. The experimental data, summarized in Figure 2, indicate that a rise in the concentration of Fe II and Fe III ions at ph=3 exerts a comparable promoting effect on the reaction rate, which results in a peculiar increasing trend with [Fe n+ ] attaining a plateau of ca. 220 nmol/s of C 3 H 8 at [Fe n+ ] higher than ca. 100 µmol/l. At ph=1, the reaction rate is much lower, and a minor influence of the concentration of Fe ions is found. In this case for [Fe II ] or [Fe III ] higher than ca. 100 µmol/l maximum rate values of ca. 63 and 88 nmol/s of C 3 H 8 respectively are attained. Such findings account for the known reactivity pattern of the Fenton-system, for which an optimum efficiency at a ph value of ca. 3 is generally found [9,11]. On the other hand, it is likely that high Fe n+ concentrations have a negative effect on the reactivity of the system because of the fact that the occurrence of secondary reactions between radical oxo-species and iron ions would compete with the activation of the organic substrate molecules dissolved in the liquid medium [9,11]. Figure 1. Liquid-phase selective oxidation of propane under mild reaction conditions (T, 70 ; P, 1.4 atm). Reaction rate (1) and efficiency of H 2 O 2 utilization (2) vs. proton-exchange capacity of silica-supported Nafion catalysts. Figure 2. Liquid-phase selective oxidation of propane under mild reaction conditions on the N-2 catalyst (T, 70 ; P, 1.4 atm). Effect of Fe II/III concentration and ph: (1) Fe 2+ (ph, 3); (2) Fe 3+ (ph, 3); (3) Fe 2+ (ph, 1); (4) Fe 3+ (ph, 1).

6 Journal of Natural Gas Chemistry Vol. 11 No Conclusions This study documents that Nafion/SiO 2 catalysts effectively drive the selective oxidation of propane to oxygenates mediated by the Fe n+ /H 2 O 2 Fenton system under mild reaction conditions. Bare silicas depress the perfomance of the Fenton system, while silica-supported Nafion catalysts enhance its reactivity and also improve the efficiency of the H 2 O 2 utilization. The title reaction proceeds via a homogeneous radical path involving a synergetic action of Fe n+ ions and sulphonic sites of the active phase, which likely promote the process of electron-transfer and consequently the rate of active radical generation. References [1] Izumi Y, Urabe K, Onaka M. Zeolite, Clay and Heteropoly Acid in Organic Reactions. New York: Kodansha, Tokyo/VCH, 1993 [2] Pope M T. Heteropoly and Isopoly Oxometalates. New York: Springer Verlag, 1983 [3] Palinko I, Totok B, Surya Prakash G K, Olah G A. Appl Catal A, 1998, 174: 147 [4] Harmer M A, Sun Q, Farneth W E. J Am Chem Soc, 1996, 118: 7708 [5] Harmer M A, Sun Q. Appl Catal A, 2001, 221: 45 [6] Frusteri F, Arena F, Bellitto S, Parmaliana A. Appl Catal A, 1999, 180: 325 [7] Espro C, Frusteri F, Arena F, Parmaliana A. J Mol Catal A, 2000, 159: 359 [8] Strukul G. Catalytic Oxidation with Hydrogen Peroxide as Oxidant. The Netherlands: Kluwer Academic Publishers, 1992 [9] Jones C W. Application of Hydrogen Peroxide and Derivatives. Cambridge: The Royal Society of Chemistry, 1999 [10] Fernandez J, Bandara J, Lopez A et al. Langmuir, 1999, 15: 185 [11] Gozzo F. J Mol Catal A, 2001, 171: 1