Production of Benzoic Acid through Catalytic Transformation of Renewable Lignocellulosic Biomass

CHINESE JOURNAL OF CHEMICAL PHYSICS VOLUME 30, NUMBER 5 OCTOBER 27, 2017 ARTICLE Production of Benzoic Acid through Catalytic Transformation of Renewable Lignocellulosic Biomass Yi-heng Zhang a, Ming-hui Fan b, Rui Chang a, Quan-xin Li a a. Department of Chemical Physics, Key Laboratory of Urban Pollutant Conversion, Chinese Academy of Sciences, Anhui Key Laboratory of Biomass Clean Energy, University of Science and Technology of China, Hefei 230026, China b. Anhui Key Laboratory of Tobacco Chemistry, China Tobacco Anhui Industrial, Co., Ltd., Hefei 230088, China (Dated: Received on March 22, 2017; Accepted on May 31, 2017) In the present work, we reported a novel route for the conversion of lignocellulosic biomass (sawdust) to a high-value chemical of benzoic acid under atmospheric pressure. The transformation involved the catalytic pyrolysis of sawdust into aromatics, the decomposition of heavier alkylaromatics to toluene, and the liquid-phase oxidation of toluene-rich aromatics to benzoic acid. The production of the desired benzoic acid from the sawdust-derived aromatics, with the benzoic acid selectivity of 85.1 C-mol% and nearly complete conversion of toluene, was achieved using the MnO 2 /NHPI catalyst at 100 C for 5 h. The influence of adding methanol on the catalytic conversion of sawdust to the core intermediate of toluene was also investigated in detail. Key words: Lignocellulosic biomass, Benzoic acid, Catalytic pyrolysis, Dealkylation, Oxidation I. INTRODUCTION Concerns about the steady decline in fossil fuel reserves and the growing increase in carbon dioxide emissions have accelerated advances in alternative fuels and chemicals using biomass feedstocks [1 3]. Lignocellulosic biomass, mainly containing cellulose, hemicellulose and lignin, is considered as a promising, renewable and vast resource for the production of bio-fuels as well as bio-chemicals [4 8]. Among various developed biomass conversion routes, catalytic pyrolysis (or catalytic fast pyrolysis) could be an effective method for producing aromatic chemicals from biomass [9, 10]. Especially, catalytic pyrolysis of biomass over zeolites has been widely investigated, producing a variety of aromatics such as benzene, toluene, xylenes, alkylbenzenes, naphthalenes, and indenes [11, 12]. The conversion of lignocellulosic biomass to aromatics via catalytic pyrolysis mainly involved the formation of oxygenated organic compounds by biomass depolymerization, followed by the formation of low carbon aromatics (mainly C 6 C 8 monocyclic aromatic hydrocarbons) by further deoxygenation, catalytic cracking and aromatization when a zeolite catalyst is used [11, 12]. Coke as well as somewhat amount of polycyclic aromatics Author to whom correspondence should be addressed. E-mail: liqx@ustc.edu.cn, Tel.: +86-551-63601118, FAX: +86-551- 63606689 is also formed through the oligomerization of aromatics during the catalytic pyrolysis of biomass. However, the transformation of lignocellulosic biomass into the desired high-value aromatic chemicals (such as benzoic acid) still remains a problem. Benzoic acid is an important chemical, widely used in the production of pharmaceuticals, dye intermediates, plasticizers, spices, food preservatives and so on [13, 14]. There are three main industrial production methods for the synthesis of benzoic acid now, namely, the liquid-phase air oxidation of toluene, the hydrolysis of trichlorotoluene and the decarboxylation of phthalic anhydride [14]. Toluene can be oxidized to several oxygenates like benzoic acid, benzaldehyde, and benzyl alcohol [15 18], and the yield of a desired product derived from the oxidation process depends on catalysts, oxidizers and reaction conditions [16, 19]. For example, the gas-phase catalytic oxidation of toluene [20, 21] and the liquid-phase catalytic oxidation of toluene [16 19], have been developed for producing benzoic acid or benzaldehyde. The gas-phase catalytic oxidation of toluene was usually performed at middle temperature using air or oxygen as oxidants over transition metal oxide catalysts (like vanadium oxide, iron oxide and cobalt oxide) or composite metal oxide catalysts [20 23]. The gas-phase oxidation process is more favorable for simplifying product separation process as compared with liquid-phase catalytic oxidation process, but has some disadvantages including lower selectivity of benzoic acid or benzaldehyde, and the deactivation of the catalysts [20, 24]. 588

Chin. J. Chem. Phys., Vol. 30, No. 5 Production of Benzoic Acid through Catalytic Transformation 589 Alternatively, the liquid-phase catalytic oxidation of toluene to benzaldehyde and benzoic acid has also attracted much attention [14 16, 19, 25]. Typically, soluble metal salts like cobalt or manganese-based halides are used as catalysts for the liquid phase oxidation of toluene, together with the addition of bromide as a catalyst promoter [26, 27]. The bi-metallic catalysts [28], metal complex catalysts [29], and metal-molecular sieve composite catalysts [15, 18, 30] have also been investigated for the liquid phase catalytic oxidation of toluene. Generally, complex products, consisting of benzyl alcohol, benzaldehyde, and benzoic acid, are formed from the liquid phase oxidation of toluene, since the primary products like benzaldehyde can undergo the second oxidation reactions such as the oxidation of benzaldehyde to benzoic acid. For example, Zhong et al. reported a maximum toluene conversion of 24.7% with benzoic acid selectivity of 79.5% and benzaldehyde selectivity of 8.3% using 6 wt% MnO x /SBA-15 [15]. Sun et al. found a mixed-node MOF catalyst of Ag-Cu-BTC exhibited a high benzaldehyde selectivity of 90.6% with a low toluene conversion of 20.1% [31]. Compared with the gas phase oxidation method, the liquid phase oxidation of toluene can be carried out at lower reaction temperature with a higher selectivity of benzaldehyde or benzoic acid. But increasing the desired product yield still remains a major challenge. Herein, the production of benzoic acid from lignocellulosic biomass (sawdust) was achieved by a proposed transformation process under atmospheric pressure, giving the high selectivity of the targeted product (benzoic acid). II. EXPERIMENTS A. Material and characterization Sawdust used in this work mainly consisted of 46.20 wt% carbon, 6.02 wt% hydrogen, 47.30 wt% oxygen, and 0.48 wt% nitrogen, which was the same biomass raw material as that reported in our previous work [11]. Chemical reagents were purchased from Sinopharm Chemical Reagent Company Ltd. (Shanghai, China). All zeolite catalysts used were obtained from Nankai University catalyst Co., Ltd. (Tianjin, China), and treated at 550 C for 4 h at nitrogen atmosphere. The Zn-modified HZSM-5 zeolite for the catalytic pyrolysis of sawdust was prepared by the impregnation method with the same procedure reported in Ref.[12]. In addition, the catalysts used for the synthesis of benzoic acid, including MnO 2, CoCl 2, MnCl 2, CuCl 2 and the promoter of N-Hydroxyphthalimide (NHPI), were purchased from Sinopharm Chemical Reagent Company Limited (Shanghai, China). The composite catalysts (MnO 2 /NHPI, CoCl 2 /NHPI, MnCl 2 /NHPI, and CuCl 2 /NHPI) were prepared by the blend of the transition metal salt or oxide with NHPI, and dried in a dry box at 110 C until the water was completely re- TABLE I Main properties of the catalysts. Si/Al: the ratio of silicon to aluminum in the zeolites, S BET : Brunauer- Emmett-Teller surface area in m 2 /g) and V p : pore volume in cm 3 /g. Catalysts Si/Al S BET V p Total acidity a HZSM-5 25 413.2 0.27 580.6 1%Zn/HZSM-5 25 396.1 0.25 549.5 3%Re/HY 5 532.6 0.37 982.8 a The acid density was estimated by the Gaussian fitting of NH 3 -TPD profiles (µmol (NH 3 )/g(catalyst)). moved. The catalysts were characterized by the inductively coupled plasma and atomic emission spectroscopy (ICP/AES), the N 2 adsorption/desorption and the ammonia temperature-programmed desorption (NH 3 - TPD) analyses, as the same procedures described in our previous work [12]. Main properties of the catalysts are summarized in Table I. B. Experimental setup and product analysis The transformation of sawdust into benzoic acid was carried out by the catalytic pyrolysis of sawdust, the decomposition of heavier aromatics and the liquid-phase oxidation of toluene-rich aromatics. The catalytic pyrolysis of sawdust was performed in the fixed-bed reactor installed with a feeder, two condensers and a gas analyzer [11], and the experimental procedures were the same as described in our previous work [12]. Typically, sawdust was mixed with the 1%Zn/HZSM-5 catalyst (catalyst/sawdust weight ratio of 2). The mixture was fed into the reactor by the feeder with a feeding rate of about 6 g sawdust/h. In the case of co-feeding biomass with methanol, the methanol was synchronously fed by a multisyringe pump (TS2-60, Baoding longer precision pump) with a given feeding rate (6 g/h). The organic liquid products (catalytic pyrolysis oil, CPO) were collected by the condensers, weighed and analyzed by a GC-MS mass spectrometer. For the decomposition of heavier aromatics to toluene, CPO was further treated in a fixed-bed reactor using the Re/HY catalyst. Typical reaction condition for this process was the temperature of 530 C, the space velocity of 0.5 h 1, and the methanol content of 30 wt% in the CPO/methanol mixture. The resulting organic liquid products (catalytic dealkylation oil, CDO) were analyzed by a similar procedure mentioned above. Finally, the oxidation of toluene-rich aromatics (namely CDO) to benzoic acid was investigated by low-temperature liquid-phase reactions using the catalysts of MnO 2, NHPI, MnO 2 /NHPI, CoCl 2 /NHPI, MnCl 2 /NHPI, and CuCl 2 /NHPI. The oxidation reactions were carried out in a two neck flask reactor under the following reaction conditions: the mass ratio of CDO to acetic acid solvent=1:5, the mass ratio of catalyst to CDO=1:10, tem-

590 Chin. J. Chem. Phys., Vol. 30, No. 5 Yi-heng Zhang et al. FIG. 1 Influence of methanol on the catalytic pyrolysis of sawdust into aromatics over 1%Zn/HZSM-5. Reaction conditions: the weight of catalyst to sawdust of 2 at 450 C. CPO: catalytic pyrolysis oil. (a) Overall yields, (b) distribution of CPO derived from the catalytic pyrolysis of sawdust, (c) distribution of gas. perature at 60 100 C and time for 1 5 h. The oxygen was used as oxidant with a flow rate of 50 cm 3 /min, controlled by a mass-flow controller. The products were separated from the catalysts by the filtration method. The gas, liquid and solid products obtained in each test were analyzed using a gas chromatograph (GC- SP6890, Shandong Lunan Co., Ltd., China), GC- MS (Thermo Trace GC/ISQ MS, USA; FID detector with a TR-5 capillary column) and the TGA analysis (Q5000IR thermogravimetric analyzer, USA) respectively [11, 32]. The conversion (C), yield (Y ), selectivity (S), and distribution (D) of the products were calculated based on Eqs.(1) (4). Mass of gas, liquid or solid products Y = 100% Mass of feed (1) Mass of a liquid product D= 100% Total mass of organic liquid (2) Moles of reactant reacted C= 100% Moles of reactant fed in (3) Carbon moles in an aromatic product S= 100% Carbon moles in aromatics (4) III. RESULTS AND DISCUSSION A. Catalytic pyrolysis of sawdust Catalyst deactivation caused by the coke deposition on the catalysts stood as a serious problem for the catalytic pyrolysis of biomass [11]. Here, it was found that co-feeding sawdust with methanol was able to obviously decrease the coke formation. As shown in FIG. 1, the yield of coke from the direct catalytic pyrolysis of sawdust was 32.8 wt%, and significantly reduced to 18.5 wt% for co-feeding sawdust with methanol (the mass ratio of 1:1). The CPO yield was also improved to 23.2 wt% in the presence of methanol. On the other hand, the addition of methanol into sawdust increased the contents of toluene and xylenes, accompanied by the reduction of polycyclic aromatics (like naphthalenes). Considering the aromatization of methanol [33, 34], the final distribution of the liquid products obtained from co-feeding sawdust with methanol was mainly formed by the catalytic pyrolysis of sawdust and the aromatization of methanol. The reaction temperature has also strong impact on the production of aromatics by the catalytic pyrolysis of the sawdust. As can be seen from FIG. 2, increasing the temperatures increased the gas yield and decreased the CPO yield, attributed to the fact that high temperatures enhanced depolymerization and deoxygenation of biomass, as well as increased biomass gasification. The downward trend in the coke yield suggests that high temperatures are not beneficial to the polymerization of aromatics. With increasing temperature, the contents of benzene and toluene increased and the heavier aromatics decreased due to the decomposition of the heavier aromatics at higher temperatures. The total mass percentage of low carbon aromatics reached about 90.1 wt%, including 23.7 wt% benzene, 37.2 wt% toluene, and 29.2 wt% xylenes at 550 C. Small amounts of polycyclic aromatics (like naphthalene and methylnaphthalene) due to the oligomerization reactions of low carbon aromatics presented a downward trend with increasing the temperature. Moreover, the gaseous alkenes and alkanes from the catalytic pyrolysis decreased with increasing the temperature, because the decomposition and aromatization of light alkenes and alkanes were enhanced at higher temperatures [35, 36]. B. Decomposition of heavier aromatics For the decomposition of heavier aromatics to toluene, CPO was further treated using the Re/HY catalyst. As shown in FIG. 3, adding methanol into CPO effectively changed the distribution of the liquid products (CDO) and inhibited the formation of coke during the catalytic treatment process. The liquid products obtained from CPO contained 36.8 wt% benzene, 43.9 wt% toluene, 16.2 wt% xylenes, and the heavier aromatics of 3.1 wt%. The compositions in the liquid products derived from the CPO/methanol mixture (ratio of 1:1) became 23.8 wt% benzene, 51.4 wt% toluene, 22.7 wt% xylenes and the heavier aromatics of 2.1 wt%, mainly caused by the decomposition of heavier aromat-

Chin. J. Chem. Phys., Vol. 30, No. 5 Production of Benzoic Acid through Catalytic Transformation 591 FIG. 2 Effect of temperature on the catalytic pyrolysis of sawdust to aromatics over 1%Zn/HZSM-5. Reaction conditions: the ratio of sawdust to methanol of 1:1 and temperature of 450 550 C. (a) Overall yields, (b) distribution of CPO derived from the catalytic pyrolysis of sawdust, (c) distribution of gas. FIG. 3 Influence of methanol (MeOH) on transformation of CPO to toluene-rich aromatics over the Re/HY catalyst. Reaction conditions: T =530 C and WHSV=0.5 h 1. CPO was obtained by the catalytic pyrolysis of sawdust at 450 C with sawdust/methanol of 1:1. CDO: catalytic dealkylation oil. (a) Overall yields, (b) distribution of CDO derived from the catalytic dealkylation of CPO, (c) distribution of gas. FIG. 4 Transformation of CPO into toluene-rich aromatics over the Re/HY catalyst. Reaction conditions: CPO:MeOH=0.7:0.3, T =490 570 C and WHSV=0.5 h 1. CPO was obtained by the catalytic pyrolysis of sawdust at 450 C with sawdust/methanol of 1:1. (a) Overall yields, (b) distribution of CDO derived from the catalytic dealkylation of CPO, (c) distribution of gas. ics and the aromatization of methanol. FIG. 4 shows the effect of temperature on the catalytic treatment process. Increasing the reaction temperature significantly reduced the yield of liquid products due to the increase in the gas yield by the decomposition of aromatics. For the liquid distribution, the content of benzene sharply increased from 19.0 wt% to 72.1 wt% with increasing the temperature from 490 C to 570 C, which clearly proved that high temperatures increased the decomposition of heavier aromatics. The formation of toluene presented the maximal value of 51.7 wt% near 510 C, caused by the competitive reaction processes between the decomposition of heavier aromatics (C 8 and C 8 + aromatics) and the demethylation of toluene. C. Oxidation of toluene-rich aromatics Finally, the synthesis of benzoic acid was performed by the low-temprature liquid-phase oxidation of the toluene-rich aromatics (namely CDO) using oxygen as an oxidant. The different catalysts, including MnO 2, NHPI, MnO 2 /NHPI, CoCl 2 /NHPI, MnCl 2 /NHPI and CuCl 2 /NHPI, have been investigated for the oxidation of CDO to benzoic acid. As can be seen from

592 Chin. J. Chem. Phys., Vol. 30, No. 5 Yi-heng Zhang et al. TABLE II Production of benzoic acid by the oxidation of sawdust-derived aromatics (CDO) over the different catalysts. Reaction conditions: CDO:solvent (acetic acid)= 1:5, catalyst:cdo (in mass ratio)=1:10, T =80 C, t=4 h. CDO contained 46.1 wt% benzene, 45.2 wt% toluene, 6.8 wt% xylenes, 1.9 wt% other aromatics. MnO 2 NHPI MnO 2/NHPI CoCl 2/NHPI MnCl 2/NHPI CuCl 2/NHPI Conversion/C-mol% Benzene 0.2 1.5 1.0 0.8 0.7 0.5 Toluene 7.8 19.5 87.1 94.5 92.1 51.7 Xylenes 9.7 28.5 94.5 97.2 95.8 75.2 Overall 4.3 11.4 46.3 49.7 48.5 28.7 Selectivity of products/c-mol% Benzoic acid 52.8 27.7 77.7 81.8 78.6 56.0 Benzaldehyde 20.3 34.0 3.9 1.7 2.9 14.6 Benzyl alcohol 1.3 7.1 0.6 0.1 0.6 3.5 Phenol 1.3 5.5 0.7 0.6 0.7 0.5 Catechol 0.1 0.4 0.1 0.1 Toluic acid 14.6 12.5 14.0 13.2 13.4 15.1 Tolualdehyde 2.8 4.2 0.6 0.3 0.2 2.4 Others 6.8 8.6 2.4 2.3 3.6 7.8 Products distribution/wt% Benzoic acid 54.8 29.7 78.5 82.3 79.3 57.8 Benzaldehyde 18.3 31.7 3.4 1.5 2.5 13.1 Benzyl alcohol 1.2 6.7 0.5 0.1 0.5 3.2 Phenol 1.2 5.3 0.6 0.5 0.6 0.5 Catechol 0.1 0.5 0.1 0.1 Toluic acid 14.8 13.1 13.8 13.0 13.2 15.2 Tolualdehyde 2.5 3.9 0.5 0.3 0.2 2.1 Others 7.1 9.1 2.6 2.3 3.7 8.0 FIG. 5 Effect of temperature on the production of benzoic acid by the oxidation of CDO over the MnO 2 /NHPI catalyst. Reaction conditions: catalyst/cdo (in mass ratio)=1:10, T =60 100 C and t=4 h. CDO contained 46.1 wt% benzene, 45.2 wt% toluene, 6.8 wt% xylenes, 1.9 wt% other aromatics. (a) Conversion of aromatics, (b) selectivity of products, (c) distribution of products. Table II, the MnO 2 catalyst shows very low oxidation activities of aromatics. Adding the promoter of N- hydroxyphthalimide (NHPI) into the reaction system (MnO 2 /NHPI) effectively enhanced the catalytic activities for the oxidation of CDO, especially for the oxidation of toluene and xylenes. The conversion of toluene reached 87.1 C-mol% under the typical condition (80 C, 4 h) using MnO 2 /NHPI, and the main products obtained were benzoic acid and toluic acid with the selectivity of 77.7 C-mol% and 14.0 C-mol% respectively. NHPI could mainly play a key role as an initiator or promotor in the oxidation of aromatics, similar to the previous investigation on the oxidation of alkanes [37]. Moreover, the catalysts containing the transition metal halide such as CoCl 2 /NHPI, MnCl 2 /NHPI, and CuCl 2 /NHPI also exhibited high activities for the CDO oxidation. However, these catalysts may be environmentally harmful because of halide-containing compositions. Accordingly, MnO 2 /NHPI was selected for the production of benzoic acid, which was investigated in details under different conditions. FIG. 5 presents the influence of temperature on the the oxidation of CDO to benzoic acid over the selected MnO 2 /NHPI catalyst. The conversions of toluene and

Chin. J. Chem. Phys., Vol. 30, No. 5 Production of Benzoic Acid through Catalytic Transformation 593 FIG. 6 Effect of reaction time on the production of benzoic acid by the oxidation of CDO over the MnO 2/NHPI catalyst. Reaction conditions: catalyst/cdo (in mass ratio)=1:10, T =100 C. CDO was the same sample as described in FIG. 5. (a) Conversion of aromatics, (b) selectivity of products, (c) distribution of products. FIG. 7 Comparision of the selectivity for the oxidation of different aromatics over the MnO 2 /NHPI catalyst. (a) Oxidation of benzene, (b) oxidation of toluene, and (c) oxidation of xylenes. Reaction conditions: catalyst/reactants (in mass ratio) of 1:10 at 100 C for 4 h. xylenes in CDO are greatly higher than the conversion of benzene, indicating that alkylaromatic compounds have higher reactivity as compared with benzene. With increasing reaction temperature from 60 C to 100 C, the conversion of toluene gradually increases from 50.2 C-mol% to 97.1 C-mol%, indicating that increasing temperature favors to overcome the activation energy required for the the oxidation of CDO to benzoic acid, together with increasing the collision probability. The main products essentially consisted of benzoic acid and smaller amount of other by-products such as benzaldehyde, benzyl alcohol, tolualdehyde, toluic acid, and phenols. For the product selectivity, increasing temperature significantly improved the selectivity of the benzoic acid. Because the oxidation of toluene in CDO is a consecutive reaction process, the primary products can generally undergo the second oxidation reactions, such as the oxidation of benzaldehyde to benzoic acid and/or the oxidation of benzyl alcohol to benzoic acid. Thus, increasing the reaction temperature enhanced the deep oxidation of toluene, leading to the enhancement of the benzoic acid formation at higher temperatures. The reaction time also observably influenced the conversion of the aromatic reactants and the selectivity of products during the oxidation of CDO (FIG. 6). The conversion of all aromatics presented a rising tendency as increasing the reaction time. Nearly all of toluene was converted into benzoic acid after the oxidation at 100 C for 5 h. The selectivity of benzoic acid increased with increasing reaction time, along with the decrease in other products. The benzoic acid selectivity reached a maximum value of 85.1% at 100 C for 5 h. Obviously, increasing reaction time enhanced the second oxidation reactions of toluene and improved the selectivity of the desired benzoic acid. In addition, it was noticed that the toluene conversion was more sensitive to reaction time, as compared with benzene or xylenes. The dependence of the benzene conversion on reaction time was weak due to the low oxidation reactivity of benzene. Considering that the content of xylenes in CDO was much lower than that of toluene, the xylenes conversion rapidly reached saturation. To further understand the reaction process of the CDO oxidation, the following comparative experiments among the oxidation of benzene, toluene and xylenes were performed (FIG. 7). The oxidation of benzene presents the lowest conversion compared with that of toluene or xylenes using the MnO 2 /NHPI catalyst under the same condition (100 C, 4 h). Very low conversion of benzene could be attributed to its high stability and low oxidation reactivity when oxygen was used as the oxidant in the low-temperature liquid-phase oxidation process. The oxidation of benzene with oxygen mainly produced phenol (91.2 C-mol%) and catechol (6.3 C-mol%), originating from the primary oxidation of benzene and the secondary oxidation of phenol. For the oxidation of toluene, the products primarily included 0.6 C-mol% benzyl alcohol, 8.1 C-mol% benzaldehyde, and 88.7 C-mol% benzoic acid, in which benzoic acid was considered as the secondary oxidation product of benzyl alcohol and benzaldehyde. Besides,

594 Chin. J. Chem. Phys., Vol. 30, No. 5 Yi-heng Zhang et al. the oxidation of xylenes mainly produced toluic acid and tolualdehyde, with the selectivity of 71.5 C-mol% and 26.4 C-mol% respectively. Thus it can be seen that the oxidation of toluene to form benzoic acid plays a core role in the oxidation of sawdust-derived aromatics, attributing to its higher oxidation reactivity as well as its higher content in CDO. Moreover, the total yield of benzoic acid under optimized conditions was 207.8 g/kg sawdust (sawdust/methanol mass ratio of 1:1.2). IV. CONCLUSION The transformation of sawdust (a lignocellulosic biomass) to high-value chemical of benzoic acid was successfully realized with a controlled reaction pathway. The production of the core intermediate of toluene was conducted by the catalytic pyrolysis of sawdust using the Zn-modified zeolite and the decomposition of heavier aromatics using the Re/HY zeolite catalyst, the maximal mass percentage of 51.7 wt% toluene was obtained. 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