Propylphenol to Phenol and Propylene over Acidic Zeolites: Role of

Jul 30, 2018 - This contribution studies the steam-assisted dealkylation of 4-n-propylphenol (4-n-PP), one of the major products derived from lignin, ...
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Research Article Cite This: ACS Catal. 2018, 8, 7861−7878

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Propylphenol to Phenol and Propylene over Acidic Zeolites: Role of Shape Selectivity and Presence of Steam Yuhe Liao, Ruyi Zhong, Ekaterina Makshina, Martin d’Halluin, Yannick van Limbergen, Danny Verboekend,* and Bert F. Sels* Center for Surface Chemistry and Catalysis, KU Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium

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S Supporting Information *

ABSTRACT: This contribution studies the steam-assisted dealkylation of 4-npropylphenol (4-n-PP), one of the major products derived from lignin, into phenol and propylene over several micro- and mesoporous acidic aluminosilicates in gas phase. A series of acidic zeolites with different topology (e.g., FER, TON, MFI, BEA, and FAU) are studied, of which ZSM-5 outperforms the others. The catalytic results, including zeolite topology and water stability effects, are rationalized in terms of thermodynamics and kinetics. A reaction mechanism is proposed by (i) analyzing products distribution under varying temperature and contact time conditions, (ii) investigating the dealkylation of different regio- and geometric isomers of propylphenol, and (iii) studying the reverse alkylation of phenol and propylene. The mechanism accords to the classic carbenium chemistry including isomerization, disproportionation, transalkylation, and dealkylation, as the most important reactions. The exceptional selectivity of ZSM-5 is attributed to a pore confinement, avoiding disproportionation/transalkylation as a result of a transition state shape selectivity. The presence of water maintains a surprisingly stable catalysis, especially for ZSM-5 with low acid density. The working hypothesis of this stabilization is that water precludes diphenyl ether(s) formation in the pores by reducing the lifetime of the phenolics at the active site due to the high heat of adsorption of water on H-ZSM-5, besides counteracting the equilibrium of the phenolics condensation reaction. The water effect is unique for the combination of (alkyl)phenols and ZSM-5. KEYWORDS: catalysis, lignin valorization, ZSM-5, dealkylation, propylphenol and phenol, shape selectivity

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extensively studied.37−41 Acidic zeolites, especially ZSM-5, exhibited the most selective catalytic performance.42 The process is attained in the presence of precious metals under H2 atmosphere in order to prevent catalyst deactivation by coking.41,42 As a consequence, low-value short alkanes are formed from the side chain rather than the more preferable short alkenes such as ethylene and propylene.41,43 In contrast to that of alkylbenzenes, literature on alkylphenol dealkylation is scarce. For instance, dealkylation of alkylphenols, especially ethylphenols (EPs), has been investigated over 5 wt % of aluminum fluoborate on γ-Al2O3.44,45 A simple extrapolation of the dealkylation chemistry from alkylbenzene to alkylphenol may not be justified. For example, phenolics have an hydroxyl group that may affect the electron density at distinct locations in the aromatic ring and that may direct the interaction with the acid sites of the catalyst. Similarly, the polarity differences between alkylphenol and alkylbenzene may affect pore concentration as well as kinetics and thermodynamics of adsorption, and therefore also the conversion rate. For instance, the heat of adsorption of (alkyl)phenol is much lower than that of (alkyl)benzenes, viz. 10−20 kJ mol−1 vs. 60−80 kJ mol−1.46−49

INTRODUCTION Intensive usage of fossil energy leads to a shortage of cheap oil, and hence it may impact base chemicals production in the long-term. Early identification of potential alternatives, ideally from renewable resources, is therefore mandatory in research to secure the chemical industry’s resources in the future circular bioeconomy.1−12 One crucial base chemical group of intense interest, which contains the most important base precursors and monomers for the production of plastics, rubbers, and other synthetic fibers and materials, is the aromatics.13 Lignin, a waste product in the pulp and paper and biorefinery industries, constitutes a phenolic-rich polymer structure, which upon selective depolymerization may offer an attractive source of aromatics.9−11,14−19 In addition, aromatics can be extracted from coal tars, obtained from gasification of coal.20 These bio-derived and coal-derived alternative phenolics are short alkyl substituted aromatics,20−36 which have no direct use in today’s chemical industry. Hence, there is a need for a defunctionalization strategy that selectively removes, e.g., the short alkyl chain, to ultimately produce bulk chemicals such as phenol. Preferably, alkenes should be formed from the side-chain to improve the economics and sustainability of the chemical process. Dealkylation of aromatics such as alkylbenzenes (e.g., cumene and ethylbenzene) using acid catalysts has been © XXXX American Chemical Society

Received: April 21, 2018 Revised: June 12, 2018

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DOI: 10.1021/acscatal.8b01564 ACS Catal. 2018, 8, 7861−7878

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ACS Catalysis

The hydrated samples were prepared by exposing the sample under saturated water vapor (ca. 295 K) for 24 h. Catalytic Testing. Dealkylation and alkylation experiments were conducted in a fixed-bed reactor equipped with 4 parallel quartz tubes (inner diameter of 3 mm) under ambient pressure. Briefly, a 120 mg catalyst (0.125−0.250 mm, 30 mg each tube) was loaded and held by two layers of quartz wool, obtaining a catalyst bed height of ca. 13 mm. Alkylphenols and water were introduced into the gas phase by flowing nitrogen gas through their saturators, their partial pressures being controlled by the saturator temperature. Afterward these flows were mixed in a mixer (473 K), obtaining a gas mixture of molar composition: 0.86/0.12/0.02 (N2/H2O/alkylphenol) and fed over the reactor. In the absence of water the molar flow composition was 0.98/0.02 (N2/alkylphenol) by compensating the flow of water with N2. The high WHSV experiments were performed by reducing the catalyst amount, while maintaining the total gas flow and catalysts bed height by diluting the catalyst with (inert) quartz powder of similar mesh size (0.125−0.250 mm). The product line before the cold trap was heated at 473 K to avoid products condensation. An online gas chromatograph (GC HP4890D), equipped with a flame ionization detector (FID) and a HP1 column, was connected to the line between the outlet of the reactor and cold trap to analyze the products in the effluent gas. For the alkylation reaction, the N2 flow passed through the saturator (of phenol and water) was similar to the flow of the dealkylation experiments, while propylene was introduced into the gas phase directly without dilution. The high boiling point (>313 K) products were collected at 273 K−278 K and analyzed by using a GC (Agilent Technologies 7890A) equipped with a HP1 column and a FID, and a mass spectrometer. The reproducibility of the catalytic test is shown in Figure S1a-b. It suggests an error of less than 3%. The thermodynamic equilibrium compositions of PPs were calculated by the Gibbs reactor of ASPEN PLUS (V8.6). The catalytic activity decay of ZSM-5 over time on stream was fitted by the generally accepted empirical formula:54

It was only recently demonstrated that acidic zeolite catalysts are capable of removing the alkyl substituent from 4-ethylphenol (4-EP) and 4-n-propylphenol (4-n-PP) into phenol and the associated olefins.50,51 ZSM-5 showed a stable catalytic performance, up to at least 3 days with unchanged dealkylation selectivity and minor coking. Remarkably, unlike in the case of alkylbenzene dealkylation, the stable catalytic performance was attained in the absence of H2 and metal. Instead, water was used to achieve stable alkylphenol conversion. Although it was not investigated in-depth, water was proposed to assist the desorption of phenol from the zeolite, avoiding secondary reactions and pore blockage.50,52 The superior catalytic performance of ZSM-5 among the tested zeolites indicates an essential role of microporosity, but a precise explanation of this superiority is absent. Herein, we present a comprehensive catalytic study of the dealkylation of linear and branched propylphenols (PPs) into phenol and propylene in the presence of steam over acidic catalysts in a broad temperature range. After describing the thermodynamics in order to understand the reaction outcome and to justify the reaction conditions for phenol production, several crystalline and amorphous catalysts are evaluated using 4-n-PP as a prime substrate, ultimately showing the catalytic superiority of acidic zeolites. The impact of the zeolite pore structure on the catalytic activity and selectivity is demonstrated in different temperature and contact time ranges and related to mechanistic details. The products distribution as a function of temperature in the conversion of different propylphenol regio- and geometric isomers and phenol alkylation with propylene is used to support the proposed reaction mechanism. The beneficial role of water on the reaction kinetics and catalyst service time is related to the zeolite porosity and cokes formation and is unique for ZSM-5 among the tested catalysts. The results enable a thorough insight in zeolite-based catalysis of alkylphenols, the main products for instance from reductive lignin depolymerization,20−36 to phenol at elevated temperature and thus may contribute to the challenging valorization of lignin.

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Xt = ae−bt − ct + d X 0.25

EXPERIMENTAL SECTION Materials. All the commercial materials are listed in Table S1. All the zeolites were transformed into protonic form before the catalytic test. The sodium (Na) form zeolites were first transferred into ammonia form by three consecutive ionexchange procedures with 0.1 M NH4NO3 at room temperature for 12 h (concentration of zeolite: 10 g L−1). All the materials (including oxides, ammonia-form zeolites) were calcined for 5.5 h at 823 K (ramping rate 5 K min−1) in static air prior to catalysis. Catalyst Preparation. beta-P was prepared by post modification of beta-19 with citric acid according to the reported methods.53 Postsynthetic modifications of ZSM-5 (Si/Al = 40, code: ZSM-5-P) were conducted by contacting batches of zeolites with the aqueous NaOH or HCl solutions. Typically, zeolite samples were added to a vigorously stirred solution at a desired temperature within a certain time. Afterward, the suspensions were quenched by icy water and the resulting solids were filtered, extensively washed with distilled water, and dried at 338 K overnight. The obtained zeolite samples were brought into the protonic form prior to the catalytic evaluation by ion exchange to the ammonium form followed by a calcination (vide supra). The modification and ion exchange conditions are summarized in Table S2.

(1)

where Xt represents the conversion of n-PPs at time t, X0.25 is the conversion of n-PPs at 0.25 h, a is the parameter related to the total number of effective active sites participating in the dealkylation (rather than the total number of acid sites due to the presence of a second componentwater), b is the parameter related to the deactivation rate of the effect active sites caused by coking, c is the parameter related to the coke/ large product formation rate at the pore mouth and in the pores, narrowing and ultimately blocking the pore channel, and d is the parameter related to the total number of active sites that never deactivate. The kinetic analysis in this contribution uses catalytic data at conversions below 5% and exceptionally below 10%. The external and internal (including intracrystal and intercrystal) diffusion limitations are evaluated by the Mears and Weisz− Prater criteria, respectively (see Supporting Information and Scheme S1).55,56 The effective diffusion coefficient DeC of alkylphenols in the zeolite crystal was estimated based on reported DeC of similar aromatics such as benzene, phenol, npropylbenzene, and n-butylbenzene, and their mutual trends (Table S3). Assuming the estimated DeC value of 4-n-PP is 7862

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Figure 1. (a) Most important reactions of 4-n-PP in the dealkylation process. (b) Variation of the free Gibbs energy (ΔG) of the possible reactions in the dealkylation of 4-n-PP as a function of temperature (T). 2,4-Dipropylphenol (2,4-diPP) was selected as an example of diPPs; 2-n-PP and 2iso-PP were selected as examples of isomers. The hypothesis of calculation is that the ΔS of the reaction is independent of the temperature (namely, ΔS = ΔS0 (293 K)), and ΔH = ΔH0 (293 K). ΔG = ΔH − T × ΔS. The parameters of all chemicals are adapted from MOL-Instincts DATABASES.59 (c) The calculated products distribution in phenol alkylation with propylene (molar ratio = 1:1) reaction and (d) the ratio of n-PP isomers as a function of temperature (T). The products distribution calculation is based on the assumption that the alkylated products are only monoalkylated products. The thermodynamic free energies of formation are used for the calculation. Flows of phenol and propylene are 1 kmol s−1.

larger than 10−14 m2 s−1, the calculations suggest the absence of diffusion limitation for ZSM-5, beta, and USY zeolites used in this work. For practical reasons, light-off experiments were used to study the temperature dependency of the dealkylation catalysis. For these light-off experiments (increasing temperature from 473 to 748 K with a rate of 1 K min−1), the reliability was first checked by control experiments such as performing the lightoff experiments with both forward and backward temperature changes. Individual control experiments at fixed temperature over time on stream were also done and compared; both results are presented in Figure S1c−d. As illustrated (Figure S1c−d), insignificant variations of conversion and selectivity were thus measured by both controls, proving the reliability of the temperature light-off experiments in our case. Note that the high stability of the dealkylation catalysis in the presence of steam is responsible for this. The online GC cannot distinguish 3-n-PP from 4-n-PP and 3-iso-PP from 4-iso-PP. To identify 3-n-PP, a silylation treatment of the collected reaction products was applied prior to off-line GC analysis.57 n-PPs involve 4-n-PP, 3-n-PP, and 2-n-PP. iso-PPs include 4-iso-PP, 3-iso-PP, and 2-iso-PP. Cresols include 2-, 3-, and 4-methylphenol. The ratio of water/ 4-n-PP is expressed as molar ratio. Catalysts Characterization. Fourier-transformed infrared (FT-IR) spectra of spent catalyst, phenol, diphenyl ether, and

ZSM-5-P (2 wt % in dried KBr) were recored in vacuo on a Bruker IFS 66v/S instrument. Nitrogen-sorption (N2-sorption) was measured with a Micromeritics TriStar 3000 instrument at 77 K. Samples were pretreated overnight under a N2 flow at 573 K before the measurements. The N2-sorption isotherms of the aluminosilicates used in this work are shown in Figure S2. The surface area was calculated by the BET method. The pore size distribution was obtained by applying the BJH model to the adsorption branch of the isotherm (Figure S2). The t-plot method was used to determine the micropore volume. Brønsted and Lewis acid sites were analyzed using pyridine as probe molecule in FT-IR equipment (Nicolet 6700 spectrometer equipped with a DTGS detector). Samples were pressed into self-supporting wafers and degassed at 673 K for 1 h under vacuum prior to measurements. Afterward, 25 mbar of pyridine probe was introduced to the sample cell at 323 K until saturation. Then, the cell was heated to 423, 523, or 623 K to degas the sample respectively, followed by measuring at 423 K. The absorptions at 1450 and 1550 cm−1 correlated to the amount of Lewis and Brønsted acid sites, respectively. The extinction coefficients (ε(B) = 1.67 cm μmol−1 and ε(L) = 2.94 cm μmol−1) were determined by Emeis.58 Ammonia temperature-programmed desorption (NH3TPD) was measured in a flow apparatus with a mass spectrometer for the desorbed gas (NH3). After pretreatment 7863

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ACS Catalysis of the samples (100 mg) in helium flow at 673 K for 2 h, adsorption of NH3 was conducted at 473 K for 0.5 h. Afterward, the sample was flushed by helium for 0.5 h at 473 K. NH3-TPD profiles were obtained by heating the sample in a helium flow to 973 K with a ramping rate of 10 K min−1. The NH3-TPD profiles are presented in Figure S3. The properties of catalyst are summarized in Table S4. Liquid Phase Adsorption. Adsorption of phenolics such as 4-n-PP, 2-n-PP, 2-iso-PP, 4-iso-PP, 4-n-butylphenol (4-nBP), 4-sec-butylphenol (4-sec-BP), 4-tert-butylphenol (4-tertBP), and phenol was conducted by loading 0.5 g zeolite into a fixed concentration (typically 10 and 20 mg mL−1) in toluene (10 mL) for 12 h. The liquid and zeolites were separated by centrifugation (4000 rpm for 5 min) after the adsorption. The uptake by zeolites was determined by measuring the concentration difference before and after adsorption via GC. m-Xylene was used as the internal standard.

mechanism, are acid catalysts, but certain alkaline oxides such as ZnO and MgO also exhibit such catalytic activity.61,62 A range of catalysts with distinct acid and base properties was therefore briefly explored in the dealkylation of 4-n-PP from 473 to 743 K, in an effort to obtain both high catalytic conversion rate and phenol-propylene selectivity. Water was added to enable stable catalysis. The results presented as conversion rate (per catalyst weight) are shown in Figure 2a.

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RESULTS AND DISCUSSION Thermodynamic Aspects. The temperature dependency of the free Gibbs energy of most relevant reactions for 4-n-PP dealkylation is presented in Figure 1. The parameters of all chemicals are adapted from MOL-Instincts DATABASES.59 The target depropylation reactions, R1 and R5 for 4-n-PP and diPPs, respectively (Figure 1), are endothermic with 93.4 kJ mol −1 and 192.5 kJ mol −1 reaction enthalpy (ΔH), respectively, and therefore thermodynamically favored at high reaction temperature. Because the Cα−Cβ bond energy of 301 kJ mol−1 is lowest among the C−C bonds of the propyl side chain,60 the free Gibbs energy (ΔG) of deethylation (R3, Figure 1), forming cresol and ethylene, is also calculated. This reaction (R3, Figure 1) shows very comparable thermodynamic behavior with similar reaction enthalpy (ΔH), viz. 93.6 kJ mol−1. The equilibrium of the multiple dealkylation (R5) is most strongly related to the temperature. Besides dealkylation, the thermodynamics of isomerization (R7 and R9, Figure 1) and disproportionation (R11, Figure 1; to form e.g., di-n-PPs) were investigated. In contrast to cresol and ethylphenol (EP), 4-n-PP can undergo not only positional (R9, Figure 1) but also skeletal (R7, Figure 1) isomerization. The Gibbs free energies (ΔG) of isomerization (R7 and R9, Figure 1) and disproportionation (R11, Figure 1) are negative and close to zero, indicating that these reactions may proceed already at low temperature. In addition, this result suggests that the thermodynamics of isomer dealkylation is similar to that of 4-n-PP dealkylation. 4-n-PP can also undergo transalkylation with other PPs to produce dipropylphenols (diPPs) and phenol. The thermodynamic of transalkylation is very similar to that of disproportionation and therefore not shown. The thermodynamics and products distribution of phenol alkylation with propylene, the opposite reaction of the depropylation, is calculated in Figure 1c, assuming only monoalkylated products. The product composition at low temperature contains mainly propylated products including the geometric-isomers and regioisomers. The calculated thermodynamic ratio of (4-n-PP+3-n-PP) to 2-n-PP remains constant over the 473 to 773 K temperature range, suggesting an equilibrium ratio of ca. 2 (solid squares, Figure 1d). With increasing temperature, depropylation to phenol and propylene (the reverse reaction) becomes dominant. Exploring the Nature of the Active Sites. The leading catalysts for dealkylation between an aromatic ring and alkyl substituent, following a retro-Friedel−Crafts C−C cleavage

Figure 2. (a) Conversion rate of n-PPs as a function of temperature (T) in the dealkylation of 4-n-PP using distinct types of oxide catalysts. (b) Selectivity (S) to phenol and propylene as a function of conversion (X) of n-PPs (including the isomers). The conversion was changed by increasing the temperature from 473 to 743 K with a ramping rate of 1 K min−1, WHSV = 3.7 h−1, water/4-n-PP = 6.

ZrO2, MgO, and ZnO catalysts show very low rates of 4-nPP conversion (Figure 2a) due to limited acidity for all three samples and/or low surface area in the case of ZrO2 and ZnO (Figure S4). Therefore, they were not studied further here. The substantial activity of amorphous silica−alumina (ASA) and γ-Al2O3 pinpoints the importance of acid catalysis. Lewis acid sites are active as demonstrated by the catalytic results of γ-Al2O3, which contains exclusively Lewis acid sites (CLewis = 103 μmol g−1,Table S4). Although Lewis acid sites can be transformed into Brønsted acid sites in the presence of water,63−66 γ-Al2O3 shows very similar catalytic performance in the absence of water (vide inf ra, Figure 12d). The crystalline aluminosilicate ZSM-5-P (40 and 177 μmol g−1 Lewis and Brønsted acidity, respectively) also catalyzes 4-n-PP with high conversion rate (per catalyst weight). Thus, both Lewis and Brønsted acid sites are capable of converting 4-n-PP. Since the isomers of 4-n-PP can also be dealkylated into phenol and propylene (vide infra), the selectivity to phenol and propylene is compared for ZSM-5-P and γ-Al2O3 at varying 7864

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Figure 3. Yield (Y) of the phenol and propylene, 2-n-PP, 3-n- and 4-n-PP, n-PPs, diPPs, cresols, and other products as a function of temperature (T) for dealkylation of 4-n-PP over different zeolites and γ-Al2O3. WHSV = 3.7 h−1; the temperature was raised using a ramp rate of 1 K min−1, water/4-n-PP = 6, (a) ferrierite (FER-P), (b) ZSM-22-P, (c) ZSM-5-P, (d) beta-P, (e) USY-P, and (f) γ-Al2O3. The legend in (a) applies to the entire figure.

Topology Effect of Zeolites on the Dealkylation Chemistry. The zeolite topology can strongly influence not only the conversion rate but also the selectivity for certain chemical reactions.67−71 Therefore, the effect of topology, including pore size, ellipticity, and dimensionality, of zeolites on the catalytic performance of 4-n-PP dealkylation was evaluated in more depth first. Ferrierite (2D, 4.2 × 5.4 Å, 3.5 × 4.8 Å), ZSM-22 (1D, 4.6 × 5.7 Å), ZSM-5 (3D, 5.5 × 5.1 Å, 5.3 × 5.6 Å), beta (3D, 6.6 × 6.7 Å, 5.6 × 5.6 Å) and USY (3D, 7.4 × 7.4 Å)72 zeolites with similar Si/Al ratio (ca. 40 mol mol−1) to minimize the effect of acid properties, were tested using light-off experiments. The reliability of the light-off experiments was tested using two independent control

conversion of n-PPs (including all isomers) in Figure 2b. The temperature was used here to increase the conversion. Though the data may be anecdotal at this stage of the study, the graph already indicates considerably higher selectivity in the presence of the ZSM-5-P zeolite. The selectivity of γ-Al2O3 is low at low conversion and reaches 80% at full conversion (Figure 2b), which is much lower than the selectivity of ZSM-5-P (around 95% in the entire conversion range). The superior selectivity of ZSM-5-P suggests that either the acid site properties (e.g., type of acid sites and acid strength) or the occurrence of a molecular pore confinement may influence the 4-n-PP dealkylation selectivity. Therefore, a series of zeolites (rich in Brønsted acid sites) are further investigated. 7865

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ACS Catalysis experiments (see Experimental Section and Figure S1c-d). γAl2O3 was included as an acidic non-microporous reference catalyst. The results are shown in Figure 3. The product distribution and its temperature dependency were investigated in the presence of the different zeolites at 3.7 h−1 WHSV. Depending on the temperature and zeolite topology, 4-n-PP indeed undergoes not only dealkylation but also isomerization to form 2-n-PP and 3-n-PP and disproportionation (and transalkylation) to form diPPs at intermediate temperature. As predicted by the thermodynamics, formation of phenol and propylene is predominant at high reaction temperature. Closer inspection of the results in Figure 3 shows clear differences among the tested zeolites. Ferrierite (FER-P) displays low conversion rates, even at 743 K (Figure 3a). Phenol and propylene yields below 10% were obtained. ZSM22-P shows higher conversion rates in comparison to FER-P (Figure 3b), yielding 54% phenol and propylene at the highest temperature, albeit significantly lower than the three-dimensional 10-membered ring zeolite ZSM-5-P (Figure 3c). For ZSM-5-P, conversion occurs already at low temperature, showing complete conversion around 613 K. The formation of 2-n-PP during 4-n-PP conversion at intermediate temperatures is notable for this catalyst. The sharp increase of the conversion rate in the presence of ZSM-5-P with increasing temperature between 488 and 608 K (Figure 3c) suggests a high apparent activation energy (vide inf ra). By raising the temperature from 638 to 743 K, the yield toward side-products such as cresols and (alkyl)benzenes remains very low (Figure 3c and Table S5), ultimately leading to an excellent 95% phenol and propylene yield. Zeolite beta-P shows a similar trend as ZSM-5-P. However, the yield of cresols and other products increases almost linearly with temperature (Figure 3d), consequently leading to a lower selectivity to phenol and propylene in comparison to ZSM-5-P. Formation of 2-n-PP is less pronounced with beta-P and appears at somewhat higher temperature compared to that of ZSM-5-P. Besides, a small amount of diPPs, products of disproportionation (and transalkylation), was produced (Figure 3d), whereas they were absent with ZSM-5-P. A maximum phenol and propylene yield of 87% is obtained over beta-P at 675 K. Compared to the other four zeolites, USY-P shows a totally different temperature-dependent product distribution profile (Figure 3e). USY is very active in converting n-PPs, but there is a pronounced formation of diPPs, in addition to isomerization. A plateau of the yield of 4-n-PP is observed in the 548 to 598 K temperature range. The sudden increasing appearance of 4-nPP with decreasing diPPs from 563 to 608 K is an indication of a switch of the chemistry from disproportionation (and transalkylation) toward dealkylation of the diPPs to remake 4-n-PP (Figure 3e). Ultimately, a maximum phenol and propylene yield of about 80% is obtained with USY-P. The yield vs. temperature profile of γ-Al2O3 (Figure 3f) is very comparable with that of USY-P. A tentative summary of the catalytic results is plotted in Figure 4, presenting the phenol and propylene selectivity as a function of conversion of n-PPs (at 3.7 h−1 WHSV) for the different zeolites. The selectivity continuously decreases with increasing n-PPs conversion over FER-P and ZSM-22-P, which is ascribed to increased possibility of (thermal and catalytic) cracking to form cresol and other products at high temperature. Beta-P, USY, and γ-Al2O3 show a strong increase in

Figure 4. Selectivity (S) to phenol and propylene as a function of conversion (X) of n-PPs for dealkylation of 4-n-PP over different zeolites and γ-Al2O3. The different conversions were obtained by increasing the reaction temperature, and the data were derived from Figure 3. The different selectivity of USY-P at the conversion range from 40% to 45% was due to the diPPs dealkylation into n-PPs. The hollow points are the results from time on stream test at fixed temperature.

selectivity with temperature (also conversion here). Disproportionation/transalkylation products (diPPs) dominate here in the low to intermediate temperature (conversion) range, whereas dealkylation is dominant at the high temperatures (conversion). However, the high phenol and propylene selectivity of ZSM-5 (ca. 95%) is not reached with these acid catalysts due to the bimolecular reactions to form cresols (vide infra), while ZSM-5 inhibits the bimolecular pathways of the carbenium chemistry leading to diPPs at low to intermediate temperatures and side-products such as cresols at high temperatures (vide infra). To illustrate the high selectivity of ZSM-5-P, GC patterns at selected temperature (ca. 563 K) are displayed and compared in Figure 5; their comparison clearly illustrates the high phenol

Figure 5. Online gas chromatograms of ZSM-5-P, USY-P, and γAl2O3 during dealkylation of 4-n-PP at 563 K and 3.7 h−1 WHSV.

and propylene selectivity over ZSM-5-P. The formation of bulky products such as n-diPPs is avoided by using ZSM-5. In the case of USY-P and γ-Al2O3, diPPs also undergo isomerization, but a thorough product study is not within the scope of this study. 7866

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ACS Catalysis Many parameters could be invoked to explain the selectivity difference such as the acid properties (acid site type, strength, density) despite the similar Si/Al ratio of the zeolites, and/or the microporosity of the catalysts. The acid properties of the tested aluminosilicates were therefore characterized by NH3TPD (for acid strength) and pyridine FTIR (for details: see Figure S3 and Table S4). One might explain the low selectivity of γ-Al2O3 by the pure Lewis acid character of active sites, but USY-P, containing predominantly Brønsted acid sites (Table S4) also gives very low phenol and propylene selectivity (Figure 4). Beta-P contains similar Lewis acid density compared to γ-Al2O3, but it shows a higher selectivity. These two observations pinpoint that the cause of low selectivity of γ-Al2O3 is unlikely due to the presence of Lewis acidity. Further, ZSM-22-P and ZSM-5P have similar acid strength (NH3-TPD, Figure S3), density, and type of site (Table S4), and therefore the lower selectivity of ZSM-22-P cannot be explained by the acid properties; more likely pore structural differences are playing a vital role in determining the selectivity. Similarly, ZSM-5-P contains somewhat stronger acid sites and higher acid density compared to the commercial USY-P (Figure S3 and Table S4) and gives the highest phenol and propylene selectivity. Beta-P contains similar acid strength and much higher density (both Brønsted and Lewis acid sites) compared to USY-P, and the selectivity of beta-P is higher than that of USY-P. The total number of acid sites (both Brønsted and Lewis of the same strength) in beta-P is somewhat higher compared to ZSM-5-P, but the selectivity is considerably lower than that of ZSM-5-P. All these mutual comparisons suggest that the selectivity difference between zeolites is caused by differences in the micro(meso)porosity, while the acid properties only play a secondary role. The above results prove the vital role of topology in the dealkylation selectivity, indicating that ZSM-5-P may behave as a “shape selective” catalyst with superior dealkylation selectivity at both low and high temperature. Inhibition by pore confinement of the various bimolecular reactions of the carbenium chemistry such as disproportionation, transalkylation (at low to intermediate temperatures), and catalytic C−C cracking (at high temperatures) (vide infra) is the most likely origin of the high selectivity of ZSM-5-P. Kinetics and Proof of “Shape Selectivity”. The conversion rates (per acid site) of the different zeolites and γ-Al2O3 were investigated and compared at low conversion (Figure 6a), and the temperature dependency was analyzed. In the low temperature region, the similar activity of ZSM-5-P and γ-Al2O3 indicates that both Brønsted and Lewis acid sites are active for the dealkylation of n-PPs (Figure 6a). The apparent activation energy of γ-Al2O3, viz. 153 ± 2 kJ mol−1, is higher than that of the zeolites. Besides that, all tested zeolites show a similar apparent activation energy, such as 98 ± 6 kJ mol−1 (ZSM-5-P), 93 ± 4 kJ mol−1 (FER-P), or 107 ± 11 (USY-P), although their activity clearly changes with the micropore (window) size and ellipticity (Figure 6). This observation suggests a space constraint effect. FER-P and ZSM-22-P show very low activity per site. Likely n-PP cannot enter the micropore as the kinetic diameter of the aromatic ring is too large. Therefore, n-PP can only access to the active sites located at the external surface. For ZSM-5-P and the other zeolites, n-PPs are able to enter the micropores, and therefore acid sites in the micropores are accessible through diffusion and adsorption. The sorption experiments show increasing uptake from FER-P to ZSM-5-P. ZSM-22-P has a much higher

Figure 6. (a) Measured turnover frequency (TOF) of n-PPs conversion on different aluminosilicates catalysts (all rates are calculated at low conversion 2-iso-PP, is different though from that obtained over fluoborate catalysts in that the ortho isomer was always reported to be more reactive than its corresponding para-isomers. This difference might be due to reagent shape selectivity in the case of the bulky ortho iso 7870

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Scheme 1. (a) Proposed Reaction Pathway of Dealkylation, Isomerization, Disproportionation, Transalkylation, and Condensation of 4-n-PP and Condensation of Phenol over Brønsted Acid Sites; (b) Proposed Reaction Pathway of Cresol Formation in the Conversion of 4-n-PP over Aluminosilicates

initial formation of the propyl carbenium at the active site, followed by attack to another 4-n-PP. This pathway may explain the very small amounts of n-propyl-phenol-iso-propyl isomers. The bimolecular route (Route 2 in Scheme 1a) involves a transfer of the n-propyl group, through the formation of an diphenylpropane intermediate, to another 4n-PP by the α-C atom. Di-n-propyl-phenol isomers are the dominant products in this case. C−C cracking in the propyl chain occurs, especially at the higher temperature, explaining the formation of cresols and trace amount of EPs. Although the formation of toluene and ethylbenzene in the dealkylation of cumene over zeolites is a bimolecular mechanism,88 the formation of cresols and traces of EPs in the dealkylation of 4-n-PP can also proceed according to a monomolecular reaction by catalytic and thermal cracking of the side chain (Scheme 1b). The yield of cresols is much higher over the wide pore zeolites, suggesting that the bimolecular transition complex is the main pathway of cresols and EPs formation. Similar conclusions were drawn in studies of cracking of alkyl aromatics over zeolites.42,89 The diphenylpropane intermediates of the bimolecular reactions to cresols are different from the intermediates of bimolecular reactions to diPPs. In the former case, the diphenylpropane intermediates were formed by activation of β or γ carbon in the propyl tail (Scheme 1b), whereas in the case of diPPs, α carbon activation occurs (Scheme 1a). The intermediates of the bimolecular reactions to cresols cannot diffuse out of the micropores of zeolites, and therefore they are subjected to cracking to form cresols and trace amount of EPs. The cresol also undergoes isomerization (not shown in the reaction pathway). The bimolecular reactions, disproportionation and transalkylation, are not possible with ZSM-5-P due to pore space restriction, while these reactions forming diPPs are competitive

different for all tested zeolites, similar apparent activation energy (both at high and low temperature) was shown over the different zeolites. This is likely attributed to similar activation of 4-n-PP via protonation over Brønsted acid sites as the joint precursor state of the other reactions (Scheme 1a). The bond energies of the benzene ring-alkyl Car−Cα, alkyl Cβ−Cγ, and alkyl Cα−Cβ are 389, 335, and 301 kJ mol−1, respectively.60 These values indicate that the scission of Cα-Cβ should be favored over Car−Cα and Cβ−Cγ as the main dealkylation pathway, and therefore cresol and ethylene are expected as the main products. However, the selective formation of phenol and propylene demonstrates that Car− Cα cleavage is the dominant pathway of n-PP dealkylation using acid catalysis, rather than Cα−Cβ cleavage (Scheme 1a). This behavior is different from that of 4-n-PP thermolysis, and steam and hydroconversion of monoalkylated benzene, in which alkyl C−C (Cα−Cβ or Cβ−Cγ) cleavage is usually dominant.85,86 Therefore, it can be inferred that in the presence of acid catalysts, the formation of the α-alkylphenol carbenium ion is the initial step at the acid sites. The subsequent surface reactions such as isomerization, disproportionation, and dealkylation are most likely rate-determining. The formation of n-PPs and not iso-PPs in the isomerization of 4-n-PP is in favor of an intramolecular propyl shift mechanism, avoiding the formation of an isolated propyl carbenium ion that realkylates a phenol molecule. More specifically, 4-n-PP undergoes a 1,2-propyl shift to yield 3-n-PP first, and then 2-n-PP, similar to that of cresol and xylene isomerization (Scheme 1a).81,87 The formation of diPPs is the result of a competitive bimolecular disproportionation, viz. two 4-n-PP molecules react to phenol and diPP, and transalkylation, viz. one 4-n-PP with one other isomer reacts to phenol and diPP. There is also an insignificant monomolecular disproportionation (Route 1 in Scheme 1a), inferring the 7871

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Figure 12. (a) Yield (Y) of phenol and propylene, 2-n-PP, 3-n- and 4-n-PP, n-PPs, cresols, and other products as a function of temperature (T) for dealkylation of 4-n-PP over ZSM-5-P; (b) Productivity of phenol and propylene as a function of temperature (T) for dealkylation of 4-n-PP over ZSM-5-P; (c) Conversion (X) of n-PPs as a function of time on stream (TOS) for dealkylation of 4-n-PP over ZSM-5-P, 658 K for the high conversion (solid symbols). For the low conversion stability test, the temperatures are 608 and 523 K for no water (WHSV = 55 h−1) and water (WHSV = 3.7 h−1), respectively. (d) Conversion rate of n-PPs as a function of temperature (T) for dealkylation of 4-n-PP over γ-Al2O3; Arrhenius plots of 4-n-PP transformation over ZSM-5-P (e) and γ-Al2O3 (f) (triangles symbols: no water; squares symbols: water) with calculated activation energies. WHSV = 3.7 h−1 (except the one mentioned in (e)), the temperature was raised using a ramp rate of 1 K min−1 (a, b, d), water means water/4-n-PP = 6, and no water means water/4-n-PP = 0.

S8). At high temperature, the bimolecular reactions to form cresols are competitive with dealkylation to phenol in the case of large pore zeolites and γ-Al2O3. Condensation of two (alkyl)phenols to (alkyl)diphenyl ethers and water is also possible in the presence of acid catalysts, such as ZSM-5 (vide infra).

with isomerization at the low to intermediate temperature range in the case of large pore zeolites such as USY-P and γAl2O3. This shape selective effect is in accordance with the deviation from unity of the molar ratio of propylene to phenol for these acid catalysts at the low to intermediate temperature range, whereas equimolar amounts of phenol and propylene are formed in the dealkylation of 4-n-PP over ZSM-5-P (Figure 7872

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over ZSM-5. A beneficial effect of water on the conversion rate was for instance observed in the alkylation of phenol with propylene over beta zeolites.92 This positive effect was attributed to the density changes of Brønsted and Lewis acid sites in the zeolite in the presence of water, as ascertained by FT-IR spectroscopy. However, it is most likely that water influences the kinetics through an indirect effect, that is by stabilizing the dealkylation chemistry. In order to investigate this hypothesis thoroughly, the evolution of the conversion rate on stream was evaluated by increasing water-to-4-n-PP molar ratios from zero to six. The results are reported in Figure 13a. All time profiles display

The initial catalyst exploration also showed that Lewis acidity is able to catalyze the reactions. A proposed reaction mechanism over such sites is likely initiated via a hydride ion abstraction over Lewis acid sites to form the α-n-PP carbenium ion intermediate. Further reactions from here can also explain the formation of most products (Figure S9). Role of Water on the Kinetics. We have reported that the co-feed of water maintains a stable time-on-stream dealkylation activity. The role of water was tentatively attributed to a competitive adsorption phenomenon between water and phenol.50 If so, one might expect that the beneficial effect is pore structure dependent, and therefore this observation may not be universal for all acid catalysts. Besides, the more specific effect of water on the reaction kinetics is unexplored. The product distribution as a function of temperature for dealkylation of 4-n-PP over ZSM-5 in the absence of water is presented in Figure 12a. A comparison of this graph with the data in Figure 3c reveals the effect of water. The products (due to isomerization and dealkylation) are similar, but the corresponding reactions proceed faster in the presence of water. Figure 12b clearly shows that the high phenol and propylene yield can be achieved at lower temperature in the presence of water. In the absence of water, similarly high selectivity for phenol and propylene (ca. 95%) can be obtained initially at complete conversion with ZSM-5-P, but the catalysis is unstable (Figure 12c). Note that the stability of the dealkylation catalysis shows the same trend at low conversion, i.e. significant deactivation in the absence of water (hollow triangle symbols in Figure 12c). Similar profiles of the products distribution as a function of temperature were constructed for the other zeolites and γAl2O3 in the absence of water (Figures 12d and S10). The presence of water has no significant positive effect on the reaction rates of these acid catalysts, when compared to the reactions with ZSM-5. The conversion rate in the presence of γ-Al2O3 is only slightly affected (Figure 12d), and co-feeding of water reduced the dealkylation rate over FER (Figure S10a). Also in the case of USY-P, water has no positive effect on the catalytic performance (Figure S10b). Figures 12e and f show the effect of water on the kinetics (taking at low conversion) in the presence of ZSM-5-P and γAl2O3. The apparent activation energy of dealkylation is larger in the presence of water, viz. 98 ± 6 to 103 ± 8 vs. 67 ± 4 to 68 ± 1 kJ mol−1, for ZSM-5-P (Figure 12e) in the temperature domain of interest (and therefore faster catalysis is apparent). Co-feeding water in the presence of γ-Al2O3 has no effect on the apparent activation energy (∼153 ± 2 kJ mol−1) but actually slightly reduces the rate of reaction (Figure 12f). To explain the above water effect, the adsorption heats of water and phenolics on ZSM-5 are considered first. The heat of adsorption of water on H-ZSM-5 (Si/Al = 35) is 32−40 kJ mol−1 at high coverage (ca. 2 molecules/acid site),90,91 while the value is around 50−55 kJ mol−1 at low coverage (ca. 0.7 molecular/acid site).90 The heat of adsorption of phenol on HZSM-5 is 10−12 kJ mol−1, and this value is comparable or only slightly higher for the alkylphenols.46 The presence of water therefore reduces the number of effective active sites for activation of alkylphenol due to competitive adsorption, and therefore the rate of reaction is expected to be lower. This explains the rate lowering effect of water in the case of γ-Al2O3, but not the higher activity of 4-n-PP conversion over ZSM-5-P. Local structural changes perhaps provide a way to explain the positive water effect on the alkylphenol dealkylation rate

Figure 13. (a) Conversion rate of n-PPs as a function of time on stream (TOS) for dealkylation of 4-n-PP over ZSM-5-P at different water/4-n-PP ratio; (b) Temporal feeding water during the dealkylation of 4-n-PP over ZSM-5-P. WHSV = 3.7 h−1, 578 K.

a similar behavior, showing a fast initial activity drop, followed by a slower deactivation. A generally accepted fitting of deactivation according to an empirical exponential formula (eq 1 in Experimental section), presented as solid lines in Figure S11, corroborates the partial loss of active sites due to different mechanisms.93 Two deactivation phenomena may be distinguished: the first occurring exponentially in initial stages and the second proceeding linearly at longer time on stream. The fitting analysis reveals that increasing the amount of water reduces the total amount of effect active sites participating in the dealkylation (related to parameter a in eq 1), in accord with the high adsorption heat and the competitive adsorption of water for the sites of ZSM-5. This indicates low alkylphenol coverage in the zeolite in the presence of water. The rate of 7873

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ACS Catalysis both the fast (related to parameter b in eq 1) and slow (related to parameter c in eq 1) deactivation lowers with presence of water and appears to level off at high water contents. The amount of active sites that are not impacted by deactivation (related to parameter d in eq 1) is higher in the presence of water. Considering the fast initial deactivation of dealkylation in the absence of water with ZSM-5, the low apparent activation energy (Figure 12e), almost half that in the presence of water, may well be explained by a pore diffusion issue, ultimately leading to a lower amount of effective sites active in the steady state operation in the absence of water, suggesting a higher catalytic activity when operating under steam conditions. Whatever is causing deactivation of ZSM-5, the catalysis can be easily restored in the presence of water. This is for instance illustrated in Figure 13b. The fast drop in conversion rate in the absence of water is indeed counteracted by adding water, showing almost complete restoration of the exponential activity loss and a conversion versus time profile, in line with the insignificant linear deactivation of water presence. It is anticipated that the major cause of the exponential loss of the absence of water is a dehydration reaction of the (alkyl)phenol into (alkyl)diphenyl ether in the pores of ZSM5, inducing pore diffusion and blockage. Production of such chemicals has been described, e.g. diphenyl ether from phenol, and seems particularly effective with phenol (compared to alkylphenols) in the presence of large pore zeolites such as USY and MOR under similar reaction conditions.94−99 Though the formation of diphenyl ether was not analyzed in the product stream over ZSM-5 in this study, infrared analysis of a spent ZSM-5 catalyst (without water) indicates its characteristic vibrations (at 1497 and 1580 cm−1 assigned to C−C of two aromatic rings and C−C of one aromatic ring,100 Figure 14), proving its formation.

Adsorption experiments indeed suggest that water promotes the desorption of phenol from ZSM-5 rather than from USY or γ-Al2O3 (Figure S12). In summary, the formation of the diphenyl ether(s) could be the main cause of the initial fast deactivation in the absence of water, but this deactivation path is mostly prevented in the presence of water. The water effect is strongly dependent on the topology and substrate of the dealkylation: it appears only in the case of ZSM-5 and alkylphenols. Water improves the conversion by preventing pore blocking with side-products such as diphenyl ether in ZSM-5, but the situation is different, for instance, for the large pore zeolites such as USY. Here, due to the absence of transition state shape selectivity, diPPs are formed in the pores of large micropore zeolites. The heat of adsorption of diPPs might be higher than that of n-PPs as the heat of adsorption of diethylbenzene is much higher than that of ethylbenzene,47 which might be higher or comparable to the heat adsorption of water. Thus, water is unable to assist the desorption of diPPs. Moreover, since dehydration reactions forming diphenyl ether(s) are not occurring in the case of cumene dealkylation, such fast exponential loss of activity due to pore diffusion is not observed in reported data.101 Also, the activity of n-propylbenzene dealkylation is not influenced by water because the heat of adsorption of n-propylbenzene is much higher than that of water, viz. 60 and 32−40 kJ mol−1,48,90,91 respectively, in accord with the data in Figure S13. Relation of Deactivation Rate with Acidity and LongTerm Catalytic Stability. Figure 15a shows that deactivation of ZSM-5 (in the presence of water) depends on the density of the acid sites; the higher the density, the more pronounced is the initial deactivation of the surface acid sites. The deactivation analysis with the empirical formula, presented by the full lines in Figure S14, agrees with a higher initial total number of effect active sites for the high acid site density ZSM5 (related to a in eq 1), but the deactivation rate per site is irrespective of the acid site density (related to b in eq 1). Besides, the acid-dense ZSM-5 shows a higher coke formation rate that narrows or blocks the pore (mouth) of the ZSM-5 structure (related to c in eq 1). The number of active sites (related to d in eq 1) that will never deactivate increases with decreasing acidity. Besides the zeolite activity and selectivity, the long-term stability of zeolites is crucial for industrial application.102 USYP, ZSM-22-P, and beta-P zeolites prove unstable in this reaction, while only ZSM-5-P, and especially ZSM-5-140, is catalytically stable under the given reaction circumstances (Figures S15 and 16a). As water did not have a strong influence on the catalytic performance of γ-Al2O3, the catalytic stability of γ-Al2O3 was tested without water in the stream for the duration of 72 h (Figure 15b). The resulting performance shows that γ-Al2O3 deactivates very rapidly at the initial stage (0−10 h), followed by a slight deactivation after 10 h. The selectivity to phenol and propylene also shows a similar trend. The selectivity to diPPs obtained from disproportionation and transalkylation sharply increased from 3% to 36%, and the selectivity was kept at 36−40% from 10 to 72 h. Therefore, γAl2O3 deactivation has a strong influence on the cracking of C−C bonds (including Car−Cα, Cα−Cβ, and Cβ−Cγ), but not on the disproportionation/transalkylation and isomerization reactions (Figure 15c). Considering the weak acid of γ-Al2O3, it indicates that dealkylation requires stronger acid sites than disproportionation/transalkylation and isomerization. Similar

Figure 14. Fourier-transform infrared spectra of diphenyl ether, phenol, ZSM-5-P, and spent ZSM-5-P from reactions in the presence and absence of water.

Adding water to such spent catalyst reverses the reaction toward phenol formation, as we observed in a previous work.50 The presence of water during alkylphenol dealkylation likely prevents diphenyl ether formation in ZSM-5 by facilitating phenol desorption as a result of the difference in heats of adsorption, viz. 30−40 vs. 10−12 kJ mol−1 of water and phenol (Figure 14),46,90,91 respectively, in addition to disequilibrating this reaction. As a result, narrowing and blocking of pores are avoided, and thus pore diffusion limitation is not at play. 7874

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CONCLUSIONS Water-assisted dealkylation of alkylphenols provides an attractive route to produce phenol and alkenes from lignocellulosic biomass and coal tar. At low temperature (473−573 K) isomerization and disproportionation (and transalkylation) are the primary reactions and dealkylation dominates at high temperature (>573 K) as this process is endothermic. The intramolecular propyl shift, occurring at low temperature, is the main route of isomerization, which gives rise to 2-n-PP (>20% yield over ZSM-5) rather than iso-PPs. Besides, the isomerization mechanism is independent of the microporosity of the aluminosilicates. The reactivity of 4-n-PP and selectivity to phenol and propylene are strongly dependent on the zeolite topology. ZSM-5 is by far the most selective (around 95%) catalyst, which is a consequence of a transition state shape selectivity effect, avoiding carbenium-type bimolecular reactions such as disproportionation (transalkylation) at low temperature (473−573 K) to form diPPs and C−C cracking at high temperature (>573 K) to form cresols. ZSM-5 with low acidity (Si/Al = 140) is the most stable catalyst, which achieves a constant conversion rate after the insignificant deactivation in the first 5 h with a 72 h test. Water is essential to secure the stable catalysis of ZSM-5. Water inhibits the formation of dehydrated products in the pores as the result of its high heat of adsorption (32−40 kJ mol−1) compared to that of phenol (10 kJ mol−1), preventing loss of sites and pore diffusion. These results enable the design of acid catalyzed dealkylation catalysis and its process conditions, highlighting the challenges and opportunities that processing of second generation lignocellulosic biomass and waste coal tar may offer.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.8b01564. Diffusion analysis; supplier of materials; conditions for zeolite modification; porosity and acid properties of zeolites; N2 isotherms and pore size distribution; NH3TPD profiles; supporting catalytic data and adsorption results; error analysis (PDF)

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Figure 15. (a) Conversion rate of n-PPs and selectivity (S) to phenol and propylene as a function of time on stream (TOS) for dealkylation of 4-n-PP over ZSM-5 with different Si/Al. (b) Conversion rate of nPPs and selectivity (S) to phenol and propylene as a function of time on stream (TOS) for dealkylation of 4-n-PP over ZSM-5-140 and γAl2O3; (c) The products distribution in the dealkylation of 4-n-PP over γ-Al2O3. WHSV = 3.7 h−1, ZSM-5-15 (543 K), ZSM-5-P (578 K), ZSM-5-140 (638 K) and γ-Al2O3 (653 K).

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Bert F. Sels: 0000-0001-9657-1710 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.L. acknowledges funding from China Scholarship Council (CSC) for a doctoral fellowship (201404910467). D.V. acknowledges funding from the Research Foundation Flanders (FWO) for a postdoctoral fellowship. The authors are grateful to W. Vermandel for technical assistance during the catalytic testing. This work was performed in the framework of the IWT-SBO project ARBOREF and BioWood, INTERREG project BIOHArT, and Excellence of Science (EoS) project BIOFACT.

results are obtained in the transformation of cumene over aluminosilicates.103 ZSM-5 only deactivates slightly at the initial stage, but after that a constant conversion rate is obtained. The selectivity to phenol and propylene remains unaltered around 97% over the whole 72 h reaction course in the presence of water (Figure 15b). 7875

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