Transformation of Oxygenate Components of Biomass Pyrolysis Oil on

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Ind. Eng. Chem. Res. 2004, 43, 2610-2618

KINETICS, CATALYSIS, AND REACTION ENGINEERING Transformation of Oxygenate Components of Biomass Pyrolysis Oil on a HZSM-5 Zeolite. I. Alcohols and Phenols Ana G. Gayubo,* Andre´ s T. Aguayo, Alaitz Atutxa, Roberto Aguado, and Javier Bilbao Departamento de Ingenierı´a Quı´mica, Universidad del Paı´s Vasco, Apartado 644, 48080 Bilbao, Spain

The effects of temperature and space time on the transformation over a HZSM-5 zeolite catalyst of several model components of the liquid product obtained by the flash pyrolysis of vegetable biomass (1-propanol, 2-propanol, 1-butanol, 2-butanol, phenol, and 2-methoxyphenol) have been studied. The transformation of alcohols follows a route similar to that of methanol and ethanol toward the formation of hydrocarbon constituents of the lumps of gasoline and light olefins. Phenol and 2-methoxyphenol have a low reactivities to hydrocarbons, and the deposition of coke of thermal origin caused by the condensation of 2-methoxyphenol is noticeable. The generation of catalytic coke and the deactivation by this cause attenuate as the space time and water content in the feed are increased. To avoid the irreversible deactivation of the HZSM-5 zeolite, operations must be carried out at a temperature below 400 °C. Above this temperature, the increase in product aromaticity is also significant. 1. Introduction The main interest in the upgrading of oxygenate hydrocarbons lies in their availability. Thus, these hydrocarbons are the constituents of the oil product of biomass pyrolysis, which is an inexhaustible potential source of hydrocarbons if the technological difficulties for their upgrading are solved.1,2 Biomass is the more interesting alternative raw material for oil from an environmental point of view, because of its negligible contents of sulfur, nitrogen, and metals and because its global balance of CO2 is neutral.3 The oil obtained from the flash pyrolysis of biomass is especially interesting because of its heating value (between 20 and 22 MJ kg-1), its easy handling, and its individual components. This oil contains more than 90 different components, mostly oxygenates, whose molecular structure ranges from simple chains to cycles with a great diversity of functional groups. Branca et al.4 published a review on the analyses of bio-oil in the literature,5-13 identifying 40-43% of its components, comprising 40 species classified in six lumps (major carbohydrates, minor carbohydrates, furans, phenols, guaiacols, and syringols). Branca et al.4 compared the composition of their oil with those of oils obtained in four commercial plants using reactors based on different technologiessa rotating cone reactor (BTG), a bubbling fluidized-bed reactor (Dynamotive), a moving-bed reactor (Ensyn), and a vacuum pyrolysis reactorsas well as raw materials of different composition and different pyrolysis temperatures. To use the bio-oil as fuel, its quality (composition, corrosiveness, stability) must be improved until its composition is that of the conven* To whom correspondence should be addressed. Tel.: 3494-6015414. Fax: 34-94-4648500. E-mail: [email protected].

tional fuel.14 The more interesting routes for reaching these objectives are catalytic transformations on an acid catalyst, with HZSM-5 zeolite being the most widely used in the literature.15-18 Because of the complexity of biomass-dervived pyrolysis oil, the establishment of suitable conditions for the catalytic process is difficult, given the different reactivities of the feed components. In this paper, the catalytic transformations over a HZSM-5 zeolite of several model components of biomass pyrolysis oil (1and 2-propanol, 1- and 2-butanol, phenol, and methoxyphenol) have been studied with the aim of ascertaining their reactivities by analyzing product distributions and, thereby, of advancing the knowledge of the kinetic schemes of product formation. Great attention has been paid in the literature to the catalytic transformations of the two oxygenate compounds methanol and ethanol to obtain fuels (gasoline and diesel) and light olefins.19-22 Methanol is the raw material of the MTG (methanol-to-gasoline) and MTO (methanol-to-olefins) processes. Whereas the MTG process is obsolete and requires technological improvements in the catalyst and in the reactor for economic viability, the MTO process has been industrially implemented because of the performance of the silicoaluminophosphate SAPO-34 in a fluidized-bed reactor with catalyst circulation.23-25 The catalytic transformation of ethanol to ethene, which gave rise to an incipient petrochemical industry in developing countries (especially India and Brazil) between the 1950s and the 1980s, is again being promoted by research into the BETE (bioethanol-toethene) and BTG (bioethanol-to-gasoline) processes.26-29 In these transformations of methanol and ethanol, the significant effects of process conditions (space time,

10.1021/ie030791o CCC: $27.50 © 2004 American Chemical Society Published on Web 05/01/2004

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Figure 1. Temperature-programmed desorption (TPD) of NH3 for the catalyst.

temperature, water content in the reaction medium) on the yield and distribution of products and on the deactivation of the catalyst have been demonstrated. The HZSM-5 zeolite undergoes reversible deactivation by coke, which is eliminated by combustion, but under severe conditions (high temperature and presence of steam in the reaction medium), it undergoes a dealumination process that decreases its activity upon reuse after coke combustion and makes the industrial process of transformation economically unfeasible. Consequently, for this process to become commercially viable, this irreversible deactivation must be avoided by delimiting the conditions under which it takes place. The catalytic transformations of other alcohols and oxygenates have been studied in an isolated way and generally as test reactions for evaluating the capacities of acidic catalysts.30-32 2. Experimental Section The HZSM-5 zeolite catalyst used in this work was prepared by agglomerating HZSM-5 zeolite (25 wt %) with bentonite (Exaloid, 30 wt %), using fused alumina (Martinswerk) as an inert charge (45 wt %). The HZSM-5 zeolite was synthesized with a Si/Al ratio, of 24, following Mobil patents.33,34 Calcination at a temperature of 570 °C for 2 h in a N2 stream is suitable for obtaining an acid structure that has been shown to be hydrothermally stable for the MTG (methanol-togasoline) process carried out after reaction-regeneration cycles.35 The physical properties of the catalyst, determined by N2 adsorption-desorption tests in an ASAP 2000 instrument from Micromeritics, are as follows: surface area, 131 m2 g-1; pore volume, 0.43 cm3 g-1; apparent density, 1.21 g cm-3; true density, 2.53 g cm-3. The contributions of pores of different sizes to the total pore volume are as follows: dp < 10-3 µm (micropores), 8.1%; 10-3 µm < dp < 10-2 µm (mesopores), 14.7%; 10-2 µm < dp < 2 µm (macropores), 77.2%. Figure 1 shows the results of the TPD (temperatureprogrammed desorption) of NH3. These results and those for the distribution of the catalyst acid strength were obtained by the adsorption-desorption of NH3 carried out in an SDT 2960 thermobalance (TA Instruments) connected on-line to a Thermostar (Balzers Instruments) mass spectrometer. These results demonstrate the uniformity of the catalyst acid structure. The Bro¨nsted/Lewis site ratio, determined by FTIR analysis

(Nicolet 740 spectrometer equipped with a Spectra Tech chamber) of adsorbed pyridine, is 3.4. The reaction equipment was described in detail elsewhere and is operated by means of a data acquisition and control program.28 The reactor is of 316 stainless steel, with a 9-mm internal diameter. It is provided with a fixed bed of catalyst diluted with alumina as the inert and operates in the isothermal regime. The gases are preheated and homogenized before they are fed into the reactor. The reaction products (as well as the possible formation of CH4, CO, and CO2) are periodically analyzed by gas chromatography (Hewlett-Packard 6890) with both thermal conductivity detection (TCD) and flame ionization detection (FID). The calibration of the chromatographic areas was carried out by using specific factors for each component, which were determined using commercial mixtures provided by Air Liquid and mixtures of model reactants with a reference compound (pentane) whose chromatographic factor is 1. To carry out a detailed identification of the products, samples were taken from the outlet stream throughout the reaction and were analyzed in a HP 5890 II gas chromatograph [equipped with a BPX5 phase capillary column (nonpolar) of 50 m] connected on-line to an HP 5989B mass spectrometer. The peaks were analyzed by comparing the corresponding spectra with those of the Wiley 6N library. For all of the feeds used, the liquid product was collected, and the gas flow rate was monitored throughout the reaction. These data allowed for mass balances to be performed, which gave a mass balance closure of (2% for the feeds corresponding to propanol, butanol, and phenol. In the case of 2-methoxyphenol, the difference between the mass at the inlet and outlet allowed for the percentage of the feed that had undergone thermal degradation to be determined (as will be discussed subsequently). The reaction conditions were as follows: space time, between 0.013 and 0.84 (g of catalyst) h (g of reactant)-1; pressure, atmospheric; temperature, between 200 and 450 °C, with experiments at both constant temperature (to quantify the deactivation rate of the catalyst) and following temperature ramps; water content in the feed, between 0 and 93% by volume. The maximum temperature studied, 450 °C, was established to avoid irreversible deactivation by dealumination of the HZSM-5 zeolite.36,37 The dilution of the reactants with water is justified by the fact that the pyrolysis oil has a high water content, which has a noticeable effect on both the kinetic behavior and the deactivation of the HZSM-5 zeolite. It has been shown that, in the transformation of methanol into hydrocarbons, the water in the reaction medium attenuates all of the steps of the kinetic scheme of the main reaction and coke formation.38,39 A similar effect of the water present in the reaction medium has been observed in the catalytic transformation of ethanol.28,40,41 This effect has also been observed in the transformation of methanol into hydrocarbons over other catalysts such as silicoaluminophosphate SAPO34 and is explained by the competition of the adsorption of water with the adsorption of intermediate reaction compounds and coke precursors.25 After each run, the catalytic bed was subjected to stripping with He at the reaction temperature, with the aim of eliminating the components of the reaction medium that might remain adsorbed on the catalyst.

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Figure 2. Effect of time on stream on the distribution of products at 400 °C for the transformations of (a) 1-propanol and (b) 2-propanol.

The total amount of catalyst contained in the bed was homogeneized to take a sample for coke measurement. This measurement was carried out by combustion of the coke in air at 550 °C in a simultaneous SDT 2960 thermobalance (TA Instruments). Prior to the combustion step, the samples of deactivated catalyst were subjected to a sweeping step with He at 550 °C for 1 h to attain the coke aging required for the results of combustion to be reproducible.42 3. Results Transformation of Alcohols. Figure 2 shows the evolution with time on stream of the distribution of products in the transformation of 1-propanol (graph a) and of 2-propanol (graph b) at 400 °C. This figure shows that the deactivation is low for both alcohols and takes place for low values of time on stream. In Figure 3, the evolution with reaction temperature of the distributions of products for the two alcohols is shown. The concentration is expressed as the mass fraction of each lump (or group of components with similar behavior in the kinetic scheme) per mass unit of organic components in the reaction medium (that is, water is not included). These results were obtained with a temperature ramp of 0.5 °C min-1. The total duration of each experiment was 8.3 h. For both alcohols, it can be observed that, at low temperatures, the main reaction is dehydration to propene, which is the primary product that then leads

Figure 3. Effect of reaction temperature on the distribution of products in the transformations of (a) 1-propanol and (b) 2-propanol.

to the formation of the remaining hydrocarbons at higher temperatures. These hydrocarbons are formed at moderate temperatures (between 250 and 400 °C) by reactions of oligomerization, alkylation, hydrogen transfer, cyclization, and isomerization, but above 400 °C, cracking also occurs. The higher the molecular weight, the more probable the cracking reaction, according to the following order: olefins > paraffins > aromatics. Dehydration is more rapid for 2-propanol than for 1-propanol (the fact that the run for 2-propanol corresponds to one-half the space time of the run for 1-propanol must be taken into account). Likewise, the maximum yield of propene is reached at a lower temperature (250 °C) in the dehydration of 2-propanol (Figure 3b) than in the dehydration of 1-propanol, in which the maximum is reached at 265 °C (Figure 3a). Furthermore, the yield of propene obtained from 2-propanol (almost 100%) is higher than that obtained from 1-propanol (86%). Above 250 °C, the transformation of propene into higher hydrocarbons starts, and the first species to appear are olefins, butenes, and C5+, of which hexenes resulting from propene dimerization prevail. The formation of C5+ olefins reaches a maximum at approximately 310 °C and decreases at higher temperatures. This decrease occurs simultaneously with a significant formation of C5+ paraffins (more evident in the case of 1-propanol, which is explained by the longer space time)

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Figure 6. Distributions of acid strength in the catalyst when fresh and after being used at 300 and at 400 °C.

Figure 4. Effect of space time on the distribution of products at 400 °C for the transformations of (a) 1-propanol and (b) 2-propanol.

Figure 5. Reaction scheme for 1-propanol and 2-propanol.

and a much lesser proportion of aromatics, mainly toluene and xylenes. The concentration of butenes remains approximately constant as the temperature is increased. Above 310 °C, the amount of propene in the reaction medium increases again, and at temperatures above 400 °C, ethene is formed. At temperatures above 400 °C, the amounts of C5+ paraffins and aromatics remain constant, which is explained by the cracking of these heavy components to lighter components and, consequently, the attenuation of their growth with temperature. It must be taken into account that, because of the long duration of these dynamic runs, the data on composition at high temperature might include the effects of catalyst deactivation. For this reason, the evolution of the reaction products with time was analyzed in runs at constant temperature, and it was found that the deactivation was low in the transformation of each alcohol. In Figure 4, the effects of space time on the distribution of products are shown for feeds of 1-propanol (graph a) and 2-propanol (graph b). These results correspond to 400 °C. On the basis of the preceding results, the scheme of Figure 5 was established for the transformations of 1and 2-propanol over HZSM-5 zeolite catalyst. As space time is increased, the medium composition evolves toward the right of the scheme.

Propene is the first product obtained in the dehydration of these alcohols. Below 300 °C, olefins, butenes, and C5+ olefins (mainly hexenes) are almost exclusively formed, even for high values of space time, and the amounts of C5+ paraffins and aromatics are always lower than 6% by weight (per mass unit of organic components). Above 400 °C, the amounts of C5+ paraffins and aromatics (these latter in a lower proportion) increase considerably as the space time is increased, and a significant amount of ethene is also formed (increasing asymptotically to values of less than 10 wt %). Propene never disappears from the reaction medium, which is evidence that it is a final product, together with ethene and butenes, obtained from the cracking of mainly heavy olefins. C5+ olefins are clearly intermediate products in the global reaction scheme, from which C5+ paraffins and aromatics (these in lower proportion) are obtained. The paraffins are obtained by alkylation, isomerization, and hydrogen-transfer reactions, and the aromatics by cyclization-dehydrogenation-aromatization reactions. The intermediates in the formation of these heavier lumps might be butenes, which, in turn, are cracking products. Both C5+ paraffins and aromatics are cracked, which is evidence that ethene, propene, and butenes are also final reaction products. The reaction scheme of Figure 5 is in agreement with the observed effect of time on stream on the disribution of products (Figure 2), so that the deactivation of the catalyst has generally the same effect as a decrease in space time. Thus, both displace the distribution of products to the left of the kinetic scheme. This effect is observed in Figure 2 for the transformations of 1-propanol (graph a) and 2-propanol (graph b). These results correspond to 400 °C. As is observed in Figure 6, in which the distribution of the acid strength is plotted for the fresh catalyst and for that deactivated in runs at 300 and at 400 °C, the deactivation is the result of coke deposition for the first few minutes of reaction on the sites of higher strength of the HZSM-5 zeolites (Bro¨nsted sites) in very accessible positions within the crystalline structure and on the external surface of the crystals. These highly active sites are a very small fraction of the acid sites in the HZSM-5 zeolite, which is characterized by homogeneity in the acid strength of its sites [between 130 and 140 kJ (mmol of NH3)-1]. Figure 6 shows that the total acidity undergoes a significant decrease from 0.12

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Figure 7. Effect of reaction temperature on the distribution of products in the transformations of (a) 1-butanol and (b) 2-butanol.

(mmol of NH3)(g of catalyst)-1 for the fresh catalyst to 0.09 and 0.06 (mmol of NH3)(g of catalyst)-1 for the catalyst deactivated in runs at 300 and 400 °C, respectively. Moderate acid sites are required in the dehydration of alcohols, whereas stronger acid sites are required in the reaction steps leading to heavier products and, especially, in the cracking of these products. These selective requirements for the different reaction steps also explain the evolution of the product distribution with time on stream toward the left of the reaction sheme of Figure 5. This evolution is a consequence of both the decrease in total acidity and the rapid elimination of strongly acidic sites through their blockage by coke. In Figure 7, the evolution with temperature of the compositions of products obtained by feeding 1-butanol (graph a) and 2-butanol (graph b) is shown. Temperature was increased linearly, with a heating ramp of 0.5 °C min-1, so that the total duration of each experiment was 8.3 h. The results correspond to a feed of pure alcohol, with the same value of space time for both alcohols. It is observed that, generally, 2-butanol is more reactive than 1-butanol and, at 230 °C, the former is almost completely dehydrated (total dehydration occurs at 250 °C) with 100% selectivity to butenes (Figure 7b). There is also a temperature range, between 230 and 270 °C, in which dehydration is complete and the butenes formed are not transformed into products of higher molecular weight. For the space time studied, the

Figure 8. Effect of time on stream on the distribution of products at 400 °C for the transformations of (a) 1-butanol and (b) 2-butanol.

dehydration of 1-butanol (Figure 7a) has a maximum net yield of butenes of 86% (which is attained at ∼300 °C), and their transformation into hydrocarbons starts at this temperature. Nevertheless, in the transformation of 2-butanol, the transformation of butenes starts 40 °C lower, which is due to the fact that the presence of butenes is significant at lower temperature. Above 275 °C for the transformation of 2-butanol and above 300 °C for that of 1-butanol, it is observed that butenes are transformed into higher olefins (pentenes and C6+ olefins), and propene and C4+ paraffins are also obtained in significant amounts. The formation of heavier olefins rapidly reaches a well-defined maximum in the 320-350 °C range, which is evidence of their nature as intermediate products in the reaction scheme, whereas the concentration of propene continues to increase and that of C4+ paraffins reaches a constant value at temperatures above 400 °C, ethene formation also being noted at these temperatures. These results are evidence that both propene and ethene are final products, although propene is also formed from butenes. The amount of butenes, which are primary products in the dehydration, initially decreases because of their oligomerization to give higher olefins, but their concentration is approximately constant above 350 °C, which is evidence that they are also final reaction products obtained when heavier olefins (and to a lesser degree aromatics and paraffins) crack at high temperatures. Figure 8 shows the effects of time on stream on the distribution of products in the transformations of 1-butanol (graph a) and 2-butanol (graph b). Both sets of

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Figure 10. Reaction scheme for 1-butanol and 2-butanol.

Figure 11. Evolution with temperature of the reaction products in the transformation of phenol.

Figure 9. Effect of space time on the distribution of products at 400 °C for the transformations of (a) 1-butanol and (b) 2-butanol.

results correspond to 400 °C. In this figure, the lumps of pentenes and C6+ olefins are grouped into a single lump (C5+ olefins) to facilitate the visualization of the graph, given that, as shown in Figure 7, the two lumps follow the same kinetic behavior. The deactivation is observed to be more severe than in the case of 1- and 2-propanol (Figure 5) and to affect both the first step of dehydration and the subsequents steps in the transformation of butenes. As time on stream increases, the following reaction steps are affected in succession: (i) cracking of large molecules (mainly C5+ olefins), which explains the initial decrease in the concentration of propylene and butenes simultaneously with the increase in the concentrations of C5+ olefins and C4+ paraffins; (ii) oligomerization-aromatization of C3-C5 olefins, which explains their progressive slight increase, the slight decrease in the concentration of aromatics, and the more significant decrease in the concentration of C4+ paraffins. By extrapolating the composition-time results obtained in runs at constant temperature to zero time on stream, the values of composition vs space time shown in Figure 9 for the transformations of 1-butanol (graph a) and of 2-butanol (graph b) at 400 °C were obtained. In virtue of these and the aforementioned results, the reaction scheme of Figure 10 was proposed for the transformations of 1- and 2-butanol. As was mentioned previously for the transformations of 1- and 2-propanol, as space time is decreased and time on stream is increased, the distributions of products from the transformations of 1- and of 2-butanol cor-

Figure 12. Effect of time on stream on the distribution of products at 450 °C for the transformation of phenol.

respond to a displacement toward the left of the scheme in Figure 10. Transformations of Phenol and Methoxyphenol. Figure 11 shows composition of the products obtained in the transformation of mixtures of phenol and water (92% water by mass to dissolve the phenol) against temperature for a run carried out with a temperature ramp of 0.5 °C min-1. It is observed that the conversion is low, and low amounts of butenes and of propene are obtained above 250 °C. These concentrations hardly increase with temperature. Figure 12 shows the evolution with time on stream of the concentrations of products at 450 °C for a space time of 1.19 (g of catalyst) h (g of phenol)-1. It is observed again that, although the space time is high, the conversion is low compared to that obtained for the oxygenate compounds studied previously. The deactivation observed is low, but the fact that the water content in the reaction medium is high and contributes to attenuating coke deposition must be taken into account.39-41

2616 Ind. Eng. Chem. Res., Vol. 43, No. 11, 2004 Table 1. Coke Content in the Catalyst for Different Reactants and Experimental Conditions water/ oxygenate mass ratio

Figure 13. Evolution with temperature of the reaction products in the transformation of 2-methoxyphenol.

Given the quantitatively significant presence of 2-methoxyphenol in the pyrolysis liquid, the transformation of this compound diluted in toluene (90% by mass of toluene, given that 2-methoxyphenol is not soluble in water) was studied. The results of Figure 13 correspond to runs carried out with a temperature ramp of 0.5 °C min-1 and a space time of 0.06 (g of catalyst) h (g of methoxyphenol)-1. It is also observed that the reactivity of this compound in the presence of an aromatic compound is very low and that a small amount of aromatics, probably generated by the decarboxylation of 2-methoxyphenol, is formed only at temperatures above 450 °C. Toluene is not reactive under these conditions. Nevertheless, 2-methoxyphenol undergoes thermal transformation to a carbonaceous material that initially is deposited in the heating zone of the reactor, then within the catalyst particles, and subsequently within the catalyst matrix. By means of a material balance, it was determined that the carbonaceous material deposited corresponded to approximately 50 wt % of the methoxyphenol fed, independently of the reaction temperature in the 200-450 °C range. Although this deposition does not influence catalyst deactivation as does catalytic coke, which is deposited on the acid sites, it does cause severe problems by blocking gas flux within the reactor. The low reactivities of phenol and substituted phenols (alkoxyphenols and alkylphenols) were also noted by Adjaye and Bakhshi,16,17 who obtained a maximum phenol conversion of 9.3% by mass at 370 °C with alkylated phenols as the main reaction products. In the transformations of methoxy- and alkylphenols, the conversion is also very low, and the gas produced is mainly made up of C1-C5 hydrocarbons and only traces of carbon oxides, which means that capability of phenol deoxygenation to form carbon oxides is very low. The well-known low reactivity of the ether group of the molecule 2-methoxyphenol on HZSM-5 zeolite differs from the behavior of alkyl ethers, whose reactivity is similar to that of alcohols.32 The low level of deoxygenation might be due to the stable nature of the molecules of aromatic ethers.43 Consequently, when the catalytic transformation of the pyrolysis liquid is carried out at 400 °C to obtain a high yield of olefins, it is convenient to separate phenol and the phenolic products of the pyrolysis liquid according to the method proposed by Chum et al.44 The

T (°C)

space time [(g of catalyst) h (g of oxygenate)-1]

time on stream (h)

coke content (wt %)

1.23 1.23 1.23 0 1.23 1.23 1.23

300 300 300 400 400 400 400

1-Propanol 0.026 0.103 0.411 0.103 0.026 0.103 0.411

6 6 6 6 6 6 6

1.31 1.26 1.17 1.55 1.58 1.46 1.36

1.27 1.27

300 400

2-Propanol 0.425 0.425

6 6

0.33 1.01

0 0

300 400

1-butanol 0.051 0.051

6 6

0.56 1.66

0 0

300 400

2-Butanol 0.013 0.013

6 6

0.64 1.74

400 450

Phenol 1.19 1.19

2.4 6.8

0.87 1.43

4.2 4.2

10.58 3.98

13.28 13.28 0 0

200-450 200-450

2-Methoxyphenol 003 0.06

phenolic compounds of the pyrolysis liquid are commercially interesting products in the formation of phenolic resins. Coke Deposition. The viability of transforming oxygenates into hydrocarbons is limited by the catalyst deactivation by coke, which explains the evolution of the product distribution with time on stream. The amount of coke deposited on the catalyst (Table 1) is a measure of the state of deactivation of the catalyst. In addition to the nature of the reactant, coke deposition is highly affected by the operating conditions: the water content in the reaction medium, the temperature, the space time, and the time on stream. When the water content in the feed is increased, the coke content undergoes a significant decrease. This effect has been studied in the transformation of methanol and of ethanol into hydrocarbons28,38-40 and is a consequence of competition between water and the hydrocarbon intermediates of the reaction for adsorption on the acid sites, which leads to the attenuation of the kinetics of reactant transformation as well as the mechanisms of coke evolution on the acid sites. These mechanisms are well described in the literature, and their precursors are the first olefins produced by dehydration. These olefins evolve toward polyaromatic structures by oligomerization-condensation.45 The polyaromatic structures are irreversibly retained in the channels of the HZSM-5 zeolite. These reactions that give way to coke evolution with time on stream are favored by increasing temperature, particularly above 400 °C. The low coke content in Table 1 for the transformation of phenol is in agreement with both the aforementioned low reactivity of this compound under the reaction conditions and the high water content in the feed. Furthermore, the simultaneous increase of the water content in the reaction medium and of the temperature is limited by the combined effect of the two variables on the dealumination of the HZSM-5 zeolite.36,37 By operating in reaction-regeneration cycles (by combustion of the coke with air at 550 °C), it has been shown

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that, when the reaction is carried out at 400 °C, the catalyst maintains its activity unaltered once 10 cycles have been carried out. Nevertheless, when the reaction is carried at 450 °C under conditions in which the water content (partly fed and also reaction product) is higher than 75 wt %, deterioration in the catalyst activity is significant, even though coke is completely eliminated. This result is important for the scaling up of this process, in which the water content and/or temperature will have to be limited or a more equilibrated catalyst than that of this study will have to be used. Such a catalyst can be attained by subjecting the HZSM-5 zeolite to a severe treatment of steaming, which unfortunately causes this zeolite to undergo a significant loss of activity. Table 1 also shows that, when the space time is increased, the coke content decreases, which is a consequence of the decrease in the concentration of the corresponding reactant. Although the results of Table 1 are for the average coke content in the bed, it was experimentally determined that, for all reactants investigated, the coke content is higher near the inlet of the reactor and decreases toward the outlet. This result is a consequence of the fact that the mechanisms of coke evolution occur in parallel with the main reaction, via the intermediate compounds adsorbed on the active sites and without reaction of the products already desorbed with the growing coke molecules. 4. Conclusions This work demonstrates that alcohols dehydrate rapidly to the corresponding olefins beginning at low temperature (around 200 °C). Iso-alcohols dehydrate more rapidly than linear alcohols. Subsequently, the olefin obtained in the dehydration is transformed into higher olefins (either butenes or C5+ olefins) above 250 °C. At temperatures higher than 350 °C, the olefins are transformed into C4+ paraffins (which are formed to a greater extent from 1 and 2-butanol) and aromatics (which are formed in very low proportion). Ethene, propene, and butenes are also final reaction products obtained by the cracking of aromatics and C5+ paraffins. Consequently, the catalytic transformation of C3-C4 alcohols on a HZSM-5 zeolite catalyst can be considered as complementary to the transformation of methanol and ethanol from the point of view of obtaining a high yield of light olefins (with a higher yield of butenes in the transformation of butanol) or a gasoline with a high octane index. Phenol has a low reactivity on HZSM-5 zeolite and produces only small amounts of propylene and butenes. The catalytic transformation of 2-methoxyphenol has a low reactivity to hydrocarbons and, furthermore, generates thermal coke, which, although it does not deactivate the catalyst, does block the catalytic bed. These results favor the separation of phenols prior to the catalytic valorization of the pyrolysis liquid. The rate of deactivation by coke deposition is low for the transformation of alcohols and phenols and decreases as the water content in the feed is increased. The results of coke deposition on the catalyst are evidence of the significant effect of the operating variables, and the attenuating effect of water in the reaction medium and the increase of coke content when temperature is raised above 400 °C are noteworthy. This result and the irreversible deactivation of the catalyst at higher temperatures when the water content in the

reaction medium is high recommend operation at temperatures below the limit of 400 °C in the valorization of the liquid product obtained by the flash pyrolysis of biomass. Acknowledgment This work was carried out with financial support from the University of the Basque Country (Project 9/UPV 00069.310-13607/2001) and the Ministry of Science and Technology of the Spanish Government (Project PPQ2000-0231). Nomenclature dp ) pore diameter, µm Fo ) mass flow rate of the oxygenate compound in the feed, g h-1 W ) catalyst mass, g Xi ) mass fraction of component i in the product stream XWo ) water/oxygenate mass ratio in the feed

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Received for review October 24, 2003 Revised manuscript received March 23, 2004 Accepted March 24, 2004 IE030791O