Hydrogen Production by Catalytic Supercritical Water Gasification of Nitriles

international journal of hydrogen energy 35 (2010) 4474–4483

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Hydrogen production by catalytic supercritical water gasiﬁcation of nitriles

Yang Guo, Shuzhong Wang*, Donghai Xu, Yanmeng Gong, Xingying Tang, Jie Zhang

State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China article info abstract

Article history: Supercritical water gasiﬁcation (SCWG) of nitriles was studied in a tubular ﬂow reactor at Received 20 December 2009 different temperatures. This article focuses on the product distributions and corresponding

Received in revised form reaction pathways inﬂuenced by addition of Na2CO3 catalyst. Results showed that gas yield

18 February 2010 for both acetonitrile and acrylonitrile can be greatly enhanced by adding Na2CO3 catalyst. Accepted 20 February 2010 Especially, H2 gasiﬁcation efﬁciency can reach 55.4% and 123.3% at 550 C, respectively. But

Available online 29 March 2010 the catalytic effect on the gas yield of benzonitrile was relatively insignificant. Na2CO3 can also accelerate the hydrolysis of cyanogen and amido as a base catalyst. Benzene and Keywords: acetic acid were the primary intermediate products during the SCWG of benzonitrile and Supercritical water gasification acetonitrile, respectively. The conversion of acrylonitrile was more complicated because of Catalyst the activity of double bond. It is possible that 3,30 iminodipropionitrile was formed by Benzonitrile Na2CO3 catalyzed in the range of 490–520 C, which dominated two thirds of pathways for Acetonitrile the subsequent formation of acetic acid. Ammonia-nitrogen content in the liquid effluent Acrylonitrile was limited by the hydrolysis degree of cyano-group and the possible polymerization

Hydrogen reaction of intermediate products. There was no obvious trend to reveal that NH3 was converted into nitrogen under our experimental conditions. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1. Introduction [7,8] and some real organic compounds [9,10,17]. These studies have acquired a fundamental understanding of SCWG. Supercritical water (SCW, Tc ¼ 374 C, Pc ¼ 22.05MPa) as According to these investigations, high hydrogen yield can a reaction medium has attracted substantial attention due to only obtained at the temperatures above 600 C due to ther- its several advantages over liquid water, such as density, modynamic reasons [11]. In order to moderate reaction viscosity, dielectric constant, diffusivity, hydrogen bonding, temperature, increase reaction rate and enhance hydrogen ion product and heat-transporting property [1]. These features selectivity, much attention has been focused on the applica- make it a perfect solvent for non-polar organic compounds tion of catalysts to SCWG process in recent years. and gases. Reaction can be conducted in single-phase, and Kruse et al. [12] have done some pioneering research and higher reaction rate is achievable. reported that hydrogen production in SCWG of pyrocatechol SCWG is a promising technology because of its potential with adding KOH, which is considered as a catalyst, was utility as an alternative method for renewable resources improved by the catalytic effect on the water–gas shift recovery and harmless treatment. For instance, it can be used reaction. This hypothesis is supported by many researchers. to gasify industrial and agricultural biomass waste with high In the experiments of industrial organic wastewaters [13], the water content to produce H2. Much research has been carried highest amount of H2 was obtained in the presence of KOH. It out on SCWG of glucose [2–5], methanol [6], cellulose, lignin can be concluded that addition of alkali salts, probably as

* Corresponding author. E-mail address: [email protected] (S. Wang). 0360-3199/$ – see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.02.108 international journal of hydrogen energy 35 (2010) 4474–4483 4475

a base catalyst, increased the rate of the water–gas shift 2. Experimental section reaction. Schmieder et al. [14] also reported that H2 rich gas was obtained at 600 C and 25 MPa that all biomass compounds were completely gasified by addition of KOH. 2.1. Materials Watanabe et al. [15] investigated the catalytic effects of NaOH The reagents used in this investigation were benzonitrile and ZrO2 for partial oxidation gasification of n-hexadecane (purity 98.5%, Sinopharm Chemical Reagent Co.), acetonitrile and lignin in SCW. They found that H2 yield with NaOH was 4 times higher than that without catalyst. These results (purity 95%, BASF (China) Co.) and acrylonitrile (purity 99%, suggested that base catalyst has a positive effect for the Shanghai ShanPu Chemical Co.). Aqueous solutions of the gaseous production, especially hydrogen production. above reagents were prepared by resolving appropriate amount of reagents into deionized water and their concen- The catalytic effects of K2CO3 and Na2CO3 also have been confirmed by several reports. Sınag˘ et al. [16] gasified glucose trations were 2 wt.%, 2 wt.% and 1.6 wt.%, respectively. Na2CO3 (purity 99.8%, Tianjin Jinbei Chemical Co.) was added in SCW with K2CO3 and hypothesized the catalytic mechanism. In detail, a formate salt (HCOOKþ) is formed by adding to the solution and served as a catalyst for this investigation. potassium carbonate as shown by Eq. (1)–(2). 2.2. Apparatus and procedures

K2CO3 þ H2O/KHCO3 þ KOH (1) The experimental setup employed in this work was the same KOH þ CO/HCOOK (2) one used in our previous study [21]. Briefly, the reactor was a tubular flow reactor (1.7 m length and 12.3 mm i.d.) made of Hydrogen yield depends on the reaction of formate salt with Hastelloy C-276, which was designed for temperature of 600 C water (Eq. (3)). Then the catalytic cycle is finished by the and pressure of 40 MPa. The outer wall of preheater decomposition of KHCO3 (Eq. (4)) and water–gas shift reaction (1 m length and 15 mm i.d.) and reactor tube was packed with is precisely the overall reaction as shown in Eq. (5). insulating course and heated to the desired temperature by electrical heating wires which was controlled by thermode- HCOOK þ H O/KHCO þ H (3) 2 3 2 tector. The nitriles-water solution from the feed reservoir to the preheater and reactor was pumped by high-pressure 2KHCO /H O þ K CO þ CO (4) 3 2 2 3 2 metering pump, and the flow rate was controlled by trans- ducer. After reaction, the products were cooled down in a heat 4 4 H2O þ CO HCOOH H2 þ CO2 (5) exchanger and depressurized by means of a back pressure

Kruse et al. [17] also pointed out that the addition of K2CO3 regulator to normal temperature and pressure. The gas–liquid can not only decrease the formation of CO but also increase mixture was separated in a glass liquid-vapor separator. The the gasiﬁcation efﬁciency. The catalytic effect of K2CO3 and gaseous product was collected in gas bag, and liquid product

Trona (NaHCO3 Na2CO3 2H2O) on hydrothermal gasiﬁcation of was collected in conical beaker. Before gaseous product was natural biomass was investigated [18]. The results indicated collected, its volumetric ﬂow rate was determined by a wet that the catalytic activity of Trona was similar to that of test meter. Before each run, the residues from previous

K2CO3. experiments were rinsed by deionized water to minimize their Nitriles as vital reactive intermediates and solvent are possible effects on the following experiments. commonly found in the industry of petrochemical, polymers and plastics, pharmaceutical, and pesticides. Nitriles show 2.3. Analysis acute toxicity with prolonged exposure resulting in meta- bolic cyanide formation [19]. Such cyano-containing compounds exist in industrial and agricultural byproduct (a) Gaseous products. can set off a series of environmental- and health-related The gas composition was determined by using a gas chro- problems. Klein and co-workers [19] investigated the effect matograph (Shanghai, GC-112A) equipped with a thermal of pressure (ranging from 128 to 2600 bar) on the rate of conductivity detector (TCD) and a 3 m 3 mm TDX-01 packed butyronitrile hydrolysis in high-temperature water. In their column; helium served as the carrier gas and the ﬂow rate and further research [20], hydrothermal reaction (<300 C) the pressure were 18.5 ml/min and 0.42 MPa respectively; the pathways of saturated and unsaturated nitriles have been column temperature was maintained at 80 C and the TCD analyzed. detector temperature was maintained at 100 C. The method

The most of these published investigations have been of analysis is simple and effective for the detections of H2,CO2, examined under sub-critical conditions. In addition, little CO, CH4,N2, and O2. The content of NH3 in gas phase can be information has been published concerning the production of neglected for the reason that the solubility of NH3 in water is

H2 by SCWG of nitriles with catalyst. In this study, benzoni- extremely high (approximately 1:700) in room temperature. trile, acetonitrile and acrylonitrile were chosen as model compounds to be gasiﬁed in SCW. Their SCWG characteristics (b) Liquid Products. and gaseous product distributions with or without Na2CO3 The residual total organic carbon (TOC) content in the liquid were probed at different reaction temperatures. Correspond- efﬂuent was measured by a commercial TOC analyzer ingly, the reaction pathway for each model compound was (Shanghai EURO TECH, ET1020A). A high performance liquid proposed. chromatography equipped with a UV3000 detector and a C18 4476 international journal of hydrogen energy 35 (2010) 4474–4483

column (35 C) was used to determine the aromatic following decomposition involved decarboxylation reaction of compounds in the liquid efﬂuent. Methanol(chromatographic benzoic acid to benzene proceeds easily with simultaneous purity) and deionized water were used as mobile phases with expulsion of CO2 [22]. The benzene is thought to be the biggest 220 nm for detection wavelength. Measurements of ammonia- obstacle to the formation of gases due to its conjugated nitrogen (NH3–N) in the liquid product were conducted by p-bond which could be still highly stable even under extreme individual Merck cell test via the Spectroquant NOVA 60 condition in SCW. minpectrophotometer. In the case without catalyst, total gaseous yield of acetonitrile was low, but it can be remarkably improved (approxi- 2.4. Data interpretation mately 10–15 folds at 490–500 C) by adding 0.1 wt.% Na2CO3.

In the case of SCWG of benzonitrile with 0.1 wt.% Na2CO3, little Data obtained from the gas chromatograph was used to appreciable improving effect was observed. Total gas yield calculate the yields of gaseous products, hydrogen gasification increased as amount of catalyst increased. However, we found efficiency, hH, and carbon gasification efficiency, hC, in order that 0.5 wt.% addition was easier to cause a plug in the reactor to characterize the SCWG of nitriles. because its depositing on reactor wall under supercritical Hydrogen gasification efficiency, hH, is defined as: condition reflected in the pressure difference exists between before and after the reactor tube. Hydrogen amount in the product gas h ¼ 100% (6) Fig. 2 shows the TOC removal rate for SCWG of benzonitrile H Hydrogen amount in the feed and acetonitrile at different temperature. The increasing

Carbon gasification efficiency, hC, is defined as: tendencies were demonstrated for both SCWG of benzonitrile

and acetonitrile with increasing temperature. Na2CO3 catalyst Carbon amount in the product gas h ¼ 100% (7) showed an improving effect and the catalytic effect of 0.1 wt.% C Carbon amount in the feed adding dosage was more significant than that of 0.5 wt.% in The TOC removal rate is defined as: SCWG of benzonitrile. It is indicated that excessive addition of catalyst may accelerate the polymerization of intermediate TOCin TOCre TOC removal rate ¼ 100% (8) products. But it is confused that there is a contradiction TOCin between the low gas yield and high TOC removal rate during where, TOCin is initial TOC content in the feed, TOCre is the SCWG process of benzonitrile. The authors calculated the residual TOC content in the liquid product. At ambient carbon content in gaseous and liquid products and found that temperature and pressure, i.e., produced gases, after pressure the carbon balance was difficult to be achieved. It is hypoth- relief, can be considered as ideal gases. esized that char/coke was formed during the preheating process due to the high C/H ratio of benzonitrile molecule. This phenomenon is consistent with our unpublished data 3. Result and discussion about SCWG of phenol obtained by current continuous apparatus. Model compounds of benzonitrile, acetonitrile and acryloni- In the temperature range of 450–490 C, TOC removal rate trile selected in our investigation are considered to be the for SCWG of acetonitrile with 0.1 wt.% Na2CO3 increased simplest compounds of aromatic, saturated and unsaturated rapidly and reached a first maximum value of 95.6% at 490 C, nitriles, respectively. The reaction chemistry of these reac- while that of the other experiment without catalyst decreased tants defines major reaction pathways that can be generalized to a minimum value. It is likely that acetic acid was formed in to their respective classes. Table 1 summarizes the experi- this temperature range, which was known to be thermally mental conditions employed in this study along with the resistant [8,23]. It is one of the most important rate-controlling results we obtained. We estimate the uncertainties in the intermediates in SCW reaction [24]. In contrast, Na2CO3,as molar yields during the gas collection and measuring process indicated, had a promotive action on the thermal decompo- to be about 3–5% of the reported value. The temperatures sition of acetic acidinto lower molecular products, such as H2, mentioned in the following sections are the temperatures of CO, CH4 and CO2. It is supposed that Na2CO3 not only as reactor outer wall. a heterogeneous catalyst but also as a homogeneous catalyst had a complicated effect on SCWG reaction. 3.1. SCWG of benzonitrile and acetonitrile Sınag˘ et al. [16] found that the concentration of alkali metal ion was much lower than those under normal condition, Experimental results of total gas yields for SCWG of benzo- which is due to the relative low dielectric constant of SCW. nitrile and acetonitrile at different temperature are shown in This means that the solubility of Na2CO3 is low and phase

Fig. 1. It can be seen that the total gas yields of both benzo- interface can be formed in SCW. The great majority of Na2CO3 nitrile and acetonitrile increased as the temperature existing in the reactor tube was in solid form as a heteroge- increased. The gas yield for SCWG of benzonitrile was lower neous catalyst. It would be partially attached to the inner wall than that of acetonitrile, and almost no gas was produced of the reactor and partially ﬂowed with reacting ﬂuid out of even at temperature higher than 500 C. The cyano-group is the reactor. It is possible that, in this process, catalytic active the only vulnerable spot in chemical structure of benzonitrile, sites could be provided for the decarboxylation reaction of which is easier to be attacked by H2O as a nucleophilic reagent acetic acid, further led to the formation of gases. The to form benzamide. The benzamide can further undergo presumed catalytic mechanism is demonstrated in Fig. 3.On a hydrolysis reaction to yield benzoic acid and NH3. The the contrary, the rest of soluble Na2CO3 could play an international journal of hydrogen energy 35 (2010) 4474–4483 4477

Table 1 – Summary of operating conditions and results from the experiments. Feed and Temperature Pressure Catalyst Mole % of gaseous productsa Other Experimental concentration (C) (MPa) (wt.%)b (%) date (wt.%) H2 N2 CO CH4 CO2 yield yield yield yield yield (%) (%) (%) (%) (%)

Benzonitrile 390 30 – 0 0 0 0 0 0 Apr. 17–18, 28, (2 wt.%) 419 30 – 27.2 0 4.2 31.5 17.6 19.5 2009 456 30 – 30.6 0 3.9 30.5 25.0 10.0 506 30 – 40.6 0 4.8 25.4 18.7 10.5 547 30 – 43.1 0.5 2.7 20.7 19.5 13.6 412 30 0.1 37.9 0 4.1 30.9 14.7 12.5 Apr. 22, 2009 454 30 0.1 47.7 0 3.2 18.5 19.7 11.1 506 30 0.1 49.6 6.7 2.6 4.7 20.3 16.2 550 30 0.1 48.6 11.8 2.2 0.4 14.4 22.8 402 30 0.5 32.5 0 0.1 25.6 15.8 26.0 Apr. 20–21, 2009 461 30 0.5 50.0 0 0 0.3 14.0 35.7 502 30 0.5 37.3 19.4 0 5.2 12.5 25.7 550 30 0.5 36.3 14.3 0 12.5 14.9 22.0 Acetonitrile 410 25 – 0 0 0 0 0 0 May. 27, 2009 (2 wt.%) 458 25 – 54.7 0.2 1.6 33.7 6.6 3.3 490 25 – 57.4 2.7 1.5 27.4 8.6 2.4 520 25 – 59.3 4.8 0.1 24.4 9.9 1.5 550 25 – 62.7 3.7 0.5 23 9.1 1.0 400 25 0.1 52.4 0 0.8 31.6 14.1 1.1 May. 31, 2009 450 25 0.1 57.5 0 1.0 22.8 18.2 0.6 490 25 0.1 57.7 0 1.3 16.2 22.3 2.6 520 25 0.1 52.7 0.36 0.2 15.5 29.7 1.5 550 25 0.1 62.1 1.1 0.4 6.3 30.1 0.1 Acrylonitrile 450 25 – 49.4 0 19.6 0.3 18.3 12.5 Sep. 5–6, 2009 (1.6 wt.%) 490 25 – 46.2 0 17.1 0.4 19.8 16.5 520 25 – 35.3 3.6 14.4 0.7 22.1 23.9 552 25 – 36.6 27.4 9.5 1.6 14.6 10.4 450 25 0.1 45.3 0 20.3 0.7 22.5 11.3 490 25 0.1 44.6 0 7.9 1.6 39.9 6.1 520 25 0.1 47.0 0 1.2 2.6 28.2 21.0 550 25 0.1 45.5 0 0.8 4.8 27.3 21.6

a The mole % of gaseous product was measured under ambient condition.

bNa2CO3 served as a catalyst in this investigation.

Fig. 1 – Total gas yields for SCWG of benzonitrile and Fig. 2 – TOC removal rates for SCWG of benzonitrile and acetonitrile. acetonitrile. 4478 international journal of hydrogen energy 35 (2010) 4474–4483

provided by water even without addition of acid and base catalyst [33]. It indicates that the nucleophilic addition tocyano-groupcan be accelerated in sub-critical or near- critical condition during in the course of heating. Here, water behaves like a catalyst under this condition. The base catalytic effect can be reinforced by adding base catalyst, such Fig. 3 – Heterogeneous catalytic mechanism for as sodium carbonate. According to Ding et al. [24], heterolytic decarboxylation reaction of acetic acid. The ‘‘M’’ represent reaction involves a nucleophilic substitution step can be the catalytic active sites. enhanced by the addition of salt, acid and base. Meanwhile, the coupling mechanism during the hydrolysis can be inhibited. This assumption was conﬁrmed by the difference of important role as a homogenous catalyst. Excess addition ammonia-nitrogen contents between the cases with and would also enhance the content of dissolved Na2CO3.Itis without Na2CO3 catalyst as demonstrated in Fig. 4. possible to provide a strong base-catalyzed reaction condition Fig. 5 shows the yields of different gaseous products for for aldol condensation between aldehydes during the process. SCWG of acetonitrile. The main gaseous products were CO2,

In this circumstance, a large number of condensation prod- CH4 and H2. The recovered amount of H2 and CO2 per mole of ucts, such as cyclic aliphatic ketones, aromatic hydrocarbons, acetonitrile increased with increasing temperature, while that and aromatic aldehydes could be formed, and which may be of CH4 was negatively correlated with the reaction tempera- more difﬁcult to decompose in SCW [25]. It is consistent with ture. The yields of CO were very low at both conditions. In the the SCWG results of benzonitrile with two different ratios of case with 0.1 wt.% catalyst, according to experimental data, catalyst as shown in Fig. 2. CO yield slightly increased and reached a maximum value of SCWG is a process associated with hydrolysis and pyrolysis 9.5 103 mol/mol at temperature 490 C and further reaction [1]. Cyanogen exist in nitriles is easier to undergo decreased in the range of 490–550 C. This disappearance of a hydrolysis reaction to convert to amidogen and subse- CO was likely due to the water–gas shift reaction with the quently lead to ammonia in sub-critical water condition [26]. assistance of Na2CO3 catalyst. The conversion of ammonia-

However, ammonia is also known to be a ‘‘refractory nitrogen to N2 was difﬁcult to be observed even at high substance’’ which is still stable in SCW even above 600 C temperature (550 C). These results were consistent with our

[27,28]. In order to investigate the catalytic effect on cyanogen original estimation. The other gases, such as C2þ, were not conversion during the SCWG processes, the ammonia- observed because of the quantitative constraint of carbon nitrogen content in liquid effluent for SCWG of benzonitrile atoms in acetonitrile molecule. and acetonitrile is depicted in Fig. 4. The variation tendencies Fig. 6 shows the hydrogen and carbon gasification effi- were complicated, but it can be confirmed that Na2CO3 cata- ciencies for SCWG of acetonitrile. The hydrogen gasification lyst had obvious improving effects on ammonia-nitrogen efficiency was lower than the carbon gasification efficiency. production for both starting reactants. But the catalyst Na2CO3 had a remarkable improving effect on the both gasi- loading effect on ammonia-nitrogen production for SCWG of fication efficiencies. The carbon gasification efficiency shows benzonitrile is negligible between 0.1 wt.% adding and a complicated variation tendency, decreased first and hence 0.5 wt.% adding in high temperature range. increased. It may be that the acetic acid produced at relatively The ion product of water as it approaches the critical point low temperature, which prohibited the transmission of is about 3 orders of magnitude higher than it is for ambient carbon atoms from acetonitrile molecules into the carbon- liquid water. So the high concentration of Hþ and OH can be

Fig. 4 – Ammonia-nitrogen contents for SCWG of Fig. 5 – Yields of different gaseous products for SCWG of benzonitrile and acetonitrile. acetonitrile. international journal of hydrogen energy 35 (2010) 4474–4483 4479

catalytic decomposition of benzene was obviously lower than that of acetic acid as a result of its stable molecular structure.

The major gaseous products are CH4,CO2 and H2. The absence of CO indicates that a water–gas shift reaction from CO and

H2OtoH2 and CO2 (Eq. (5)) proceeded easily by Na2CO3 catalyzed [16].

3.2. SCWG of acrylonitrile

The SCWG of unsaturated nitrile was probed further through experiments conducted with the acrylonitrile as a starting reactant. The gaseous product distributions are in the form of gas yields versus temperature at 25 MPa as shown in Fig. 8.

The H2 and CO2 yield increased as temperature increased.

Na2CO3 catalyst had a signiﬁcant improving effect on

hydrogen yield. A small amount of N2 can only be produced at the temperature higher than 490 C with 0.1 wt.% Na2CO3.Itis noteworthy that, different from SCWG of benzonitrile and

acetonitrile, a large quantity of C2þ gases was produced, Fig. 6 – Hydrogen and carbon gasiﬁcation efﬁciencies for especially at the condition with adding catalyst. It is possible SCWG of acetonitrile. that unknown intermediate products were produced by

catalysis. And the yield of C2þ gases may be attributed to the incomplete decomposition of these intermediate products. The data of TOC removal rate for SCWG of acrylonitrile shown containing gases. Here, the hydrogen gasification efficiency in Fig. 9 confirm this speculation. As can be seen from Fig. 9, just reached 54.4% at 550 C. A possible explanation for this is the TOC removal rate rose to a maximum value of 82.1% near that a portion of hydrogen atoms transferred into CH4 mole- 490 C and hence decreased to 51.8% at 520 C, the higher cules during decarboxylation reaction, as shown in Eq. (9). temperatures are driving the reaction further to completion. However, hydrogen could be regenerated via steam reforming It indicates that more complicated intermediate products may reaction at higher temperature as shown in Eqs. (10) and (11). form by polymerization reaction in the range of 490–520 C, which may be directly related to the reactivity of double bonds CH3COOH/CH4 þ CO2 (9) in acrylonitrile molecules. For saturated nitriles, as mentioned above, cyano-group is CH4 þ H2O/CO þ 3H2 (10) the only option for nucleophilic addition reaction. However, unsaturated nitriles, such as acrylonitrile, can provide double CH4 þ 2H2O/CO2 þ 4H2 (11) bond as another option for electrophilic addition reaction [20]. Fig. 7 demonstrates the reaction mechanisms for benzoni- Water can play a role as predominant electrophilic reagent for trile and acetonitrile SCWG. Cyanogen as the only active group alkene hydration of acrylonitrile under SCW condition. for nucleophilic addition reaction leads to the similar linear Following Markovnikov rule, 3-hydroxypropiontrile is the reaction pathways for both starting reactants. Benzene and primary product. A polymerization reaction between the two 3- acetic acid highlighted by dashed frame are confirmed to be hydroxypropiontrile molecules may further proceed to form 2 the main intermediate products which control the yield of cyano ethyl ether. The both reactions mentioned above require gases for SCWG of benzonitrile and acetonitrile, respectively. acid as catalyst with high concentration under ambient þ Na2CO3 showed a significant base catalytic effect on hydro- condition. SCW or near-critical water can knock off H with lysis reaction of cyanogen and amido. From gas yield, the high concentration, which provides a perfect environment for

Fig. 7 – Reaction pathways for SCWG of benzonitrile and acetonitrile. (The term ‘‘(cat)’’ designates reaction pathways inﬂuenced by the presence of catalyst. The dashed frame highlighted substances designate key intermediate products). 4480 international journal of hydrogen energy 35 (2010) 4474–4483

further to form 3-aminopropionitrile reﬂected in the decreasing trend of ammonia-nitrogen content in the range of 490–520 C (see Fig. 9). This high yield of 3-aminopropionitrile would then present an opportunity for a polymerization reaction with acrylonitrile to produce 3,30 iminodipropionitrile. This possible polymerization reaction was considered to be the most important rate-controlling step of SCWG of acrylonitrile in our experiments. Our experimental data

seems to show that Na2CO3 as a catalyst had high selectivity for 3,30 iminodipropionitrile. But the detailed catalytic mechanisms are not clear and further investigations are required. Fig. 10 shows hydrogen gasiﬁcation efﬁciency for SCWG of

acrylonitrile. It can be observed that Na2CO3 catalyst had a strong improving effect on hydrogen gasification efficiency which increased as temperature increased and reached 123.3% at 550 C. It indicates that SCW can not only behave as a benign solvent but also play a role as resource of hydrogen, Fig. 8 – Yields of different gaseous products for SCWG of which makes it a significant influence on the distribution of acrylonitrile. pyrolytic reaction [1,31]. Here, the water may take part in the reaction and provide hydrogen mainly via hydrolysis reaction and water–gas shift reaction. And both of these reactions acid catalyzed reaction [33]. However, in the presence of could be catalyzed by Na2CO3. Na2CO3 catalyst, as indicated, 3-hydroxypropiontrile is merely Based on the previous research about hydrolysis of nitriles considered to be a logical intermediated product due to the in near-critical water [19,20] and SCWG of some model base-catalyzed re-arrangement. Klein et al. [29,30] also found compounds with alkali metal catalysts [12–18], the reaction that ether linkage was unstable and easily hydrolyzed to two pathways for gasification of acrylonitrile with Na2CO3 catalyst alcohols in SCW. were developed summarized in Fig. 11. Acetic acid and 3,30 The ammonia-nitrogen content versus temperature for iminodipropionitrile highlighted by dashed frame are SCWG of acrylonitrile is likewise shown in Fig. 9. It is clear that considered to be the primary intermediate products which the variation tendency of ammonian production was corre- may influence the final product distributions directly or lated with that of TOC removal rate in the presence of Na2CO3 indirectly. Acetic acid conversion was the final controlling catalyst. This phenomenon is understandable because the step for production of gases in the whole reaction, which was process of transforming from cyano-group to ammonia was partial dominated by the thermal decomposition of 3,30 also restrained by the formation of intermediate products. iminodipropionitrile. The 1,2-nucleophilic addition of water to cyano-group is As demonstrated by Fig. 12, there were three possible the other pathway for hydrolysis of acrylonitrile. Acrylamide pathways for the formation of acetic acid. The first was the can be first formed and subsequently converted to acrylic acid conversion of 3-hydroxypropionic acid to form one formal- and ammonia. It is possible that a portion of ammonia can dehyde molecule and one acetic acid molecule. The second subsequently compete with water to take part in the electrophilic addition reaction on the double bond of acrylonitrile,

Fig. 9 – TOC removal rate and ammonia-nitrogen content Fig. 10 – Hydrogen gasiﬁcation efﬁciency for SCWG of for SCWG of acrylonitrile. acrylonitrile. international journal of hydrogen energy 35 (2010) 4474–4483 4481

Fig. 11 – Reaction pathway for SCWG of acrylonitrile. (The term ‘‘(cat)’’ designates reaction pathways inﬂuenced by the presence of catalyst. The dashed frame highlighted substances designate key intermediate products). pathway was the hydrolysis of acetonitrile which was regar- catalyst, similar to benzonitrile and acetonitrile, strong base- ded as decomposed product of 3-hydroxypropionitrile. The catalyzed reaction condition had a responsibility for accel- third pathway involved the hydrolysis of propane diacid with eration of cyanogen and amido’s hydrolysis in SCWG of simultaneous decarboxylation. Here, the internal condensa- acrylonitrile. The cannizzaro reaction of formaldehyde could tion of diprotonic acid was negligible because electron- also be base-catalyzed and proceed readily in SCW to form withdraw group was attached to a-carbon atom. It is found methanol and formic acid [32] according to Eq. (12). Then that two thirds of them would be affected by the formation of methanol may undergo a fragmentation reaction to form CO 0 3,3 iminodipropionitrile. Meanwhile, the ammonia-nitrogen and H2 as shown in Eq. (13). And formic acid may decompose contents in the liquid production were also restrained. into gaseous products via decarboxylation or dehydration The routes marked with ‘‘(cat)’’ represented the reaction reaction [17] as shown in Eqs. (14) and (15), respectively. pathways may affected by Na2CO3 catalyst. Based on the

þH2 O above analysis, Na2CO3 showed a promotive action on the 2HCOH / CH3OH þ HCOOH (12) formation of 3,30 iminodipropionitrile. As a homogeneous ðcatÞ

CH3OH/CO þ 2H2 (13) Acrylonitrile

HCOOH/H2 þ CO2 (14)

3,3’iminodipropionitrile HCOOH/H2O þ CO (15)

in addition to these, a similar catalytic mechanism for decarboxylation reaction of acetic acid proceeded in SCWG of Propane diacid Acetonitrile 3-hydroxypropionic acid acetonitrile may directly contribute to the large gas yield.

4. Conclusions Acetic acid The SCWG of benzonitrile, acetonitrile and acrylonitrile were Fig. 12 – The possible degradation processes from investigated in a tubular ﬂow reactor at different tempera- acrylonitrile to acetic acid. tures. The gas yield for both acetonitrile and acrylonitrile can 4482 international journal of hydrogen energy 35 (2010) 4474–4483

be greatly enhanced by adding Na2CO3 catalyst, which may be [7] Yoshida T, Matsumura Y. Gasification of cellulose, xylan, and due to the acceleration for decarboxylation reaction of inter- lignin mixtures in supercritical water. Ind Eng Chem Res mediate products. However, the similar improvement was not 2001;40:5469–74. [8] Furusawa T, Sato T, Sugito H, Miura Y, Ishiyama Y, Sato M, observed for that of benzonitrile may be attributed to the et al. Hydrogen production from the gasification of lignin stable molecular structure of benzene ring. Char/coke may be with nickel catalysts in supercritical water. Int J Hydrogen formed during the preheating process. Energy 2007;32:699–704. The major gaseous products for SCWG of acetonitrile and [9] Xu X, Antal Jr MJ. Gasification of sewage sludge and other acrylonitrile were CO2,CH4 and H2. Most of CO converted to biomass for hydrogen production in supercritical water. Environ Prog 1998;17(4):215–20. CO2 via water–gas shift reaction by Na2CO3 catalyzed. The hydrogen gasification efficiency for both reactants increased [10] Antal MJ, Allen SG, Schulman D, Xu XD. Biomass gasification in supercritical water. Ind Eng Chem Res 2000; with increasing temperature and reached 55.4% and 123.3% at 39(11):4040–53. 550 C in the presence of Na2CO3, respectively. SCW can play [11] Kruse A. Supercritical water gasification. Biofuel, Bioprod a role as hydrogen source for SCWG of acrylonitrile via Biorefin 2008;2:415–37. hydrolysis and water–gas shift reaction. [12] Kruse A, Meier D, Rimbrecht P, Schacht M. Gasification of

Na2CO3 as a base catalyst can accelerate the hydrolysis of pyrocatechol in supercritical water in the presence of cyano-group. Benzene and acetic acid were likely to be the key potassium hydroxide. Ind Eng Chem Res 2000;39:4842–8. [13] Garcıá Jarana MB, Sa´ nchez-Oneto J, Portela JR, Nebot intermediate products for SCWG of benzonitrile and acetoni- Sanz E, Martinez de la Ossa EJ. Supercritical water trile, respectively. SCWG of acrylonitrile was more compli- gasification of industrial organic wastes. J Supercrit Fluid cated because of the activity of double bonds. It is possible 2008;46:329–34. 0 that 3,3 iminodipropionitrile was formed by Na2CO3 cata- [14] Schmieder H, Abeln J, Boukis N, Dinjus E, Kruse A, Kluth M, lyzed, which dominated 2/3 pathways for the subsequent et al. Hydrothermal gasification of biomass and organic formation of acetic acid. The yield of C2þ gases may be caused wastes. J Supercrit Fluid 2000;17:145–53. by incomplete decomposition of 3,30 iminodipropionitrile. [15] Watanabe M, Inomata H, Osada M, Sato T, Adschiri T, Arai K. Catalytic effects of NaOH and ZrO for partial oxidative Ammonia-nitrogen contents in the liquid effluent were 2 gasification of n-hexadecane and lignin in supercritical limited by the hydrolysis degree of cyanogen and possible water. Fuel 2003;82:545–52. polymerization reaction of intermediate products. There was [16] Sınag˘ A, Kruse A, Schwarzkopf V. Key Compounds of the no obvious trend to reveal that NH3 was converted into hydropyrolysis of glucose in supercritical water in the nitrogen under our experimental conditions. presence of K2CO3. Ind Eng Chem Res 2003;42:3516–21. [17] Kruse A, Krupka A, Schwarzkopf V, Gamard C, Henningsen T. Influence of proteins on the hydrothermal gasification and liquefaction of biomass. 1. Comparison of different Acknowledgments feedstocks. Ind Eng Chem Res 2005;44:3013–20. [18] Yanik J, Ebale S, Kruse A, Saglam M, Yuksel M. Biomass gasification in supercritical water: II. Effect of catalyst. The authors wish to acknowledge the financial supports from Int J Hydrogen Energy 2008;33:4520–6. the National High Technology Research and Development [19] Iyer SD, Klein MT. Effect of pressure on the rate of Program of China (Grant No. 2006AA06Z313), National Basic butyronitrile hydrolysis in high-temperature water. Research Program of China (Grant No. 2009CB220000) and the J Supercrit Fluid 1997;10:191–200. Program for New Century Excellent Talents in University of [20] Izzo B, Klein MT, LaMarca C, Scrivner NC. Hydrothermal Chinese Education Ministry (Grant No. NCET-07-0678). reaction of saturated and unsaturated nitriles: reactivity and reaction pathway analysis. Ind Eng Chem Res 1999;38: 1183–91. references [21] Xu DH, Wang SZ, Hu X, Chen CM, Zhang QM, Gong YM. Catalytic gasification of glycine and glycerol in supercritical water. Int J Hydrogen Energ 2009;34:5357–64. [22] Houser TJ, Tsao CC, Dyla JE, Van Atten MK, McCarville ME. [1] Guo Y, Wang SZ, Xu DH, Gong YM, Ma HH, Tang XY. Review The reactivity of tetrahydroquinoline, benzylamine and of catalytic supercritical water gasification for hydrogen bibenzyl with supercritical water. Fuel 1989;68:323–7.

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