Direct formation of gasoline hydrocarbons from cellulose by hydrothermal conversion with in situ hydrogen

b i o m a s s a n d b i o e n e r g y 4 7 ( 2 0 1 2 ) 2 2 8 e2 3 9

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http://www.elsevier.com/locate/biombioe

Direct formation of gasoline hydrocarbons from cellulose by hydrothermal conversion with in situ hydrogen Sudong Yin a,*, Anil Kumar Mehrotra b, Zhongchao Tan c,d,** a

Centre for Environmental Engineering Research & Education (CEERE), Schulich School of Engineering, University of Calgary, Calgary, Alberta, Canada T2N 1N4 b Department of Chemical & Petroleum Engineering, Centre for Environmental Engineering Research & Education (CEERE), Schulich School of Engineering, University of Calgary, Calgary, Alberta, Canada T2N 1N4 c Department of Mechanical & Mechatronics Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 d Waterloo Institute of Sustainable Energy, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1

article info

abstract

Article history:

A new process based on aqueous-phase dehydration/hydrogenation (APD/H) has been

Received 21 November 2011

developed to directly produce liquid alkanes (C7e9), which are the main components of

Received in revised form

fossil gasoline, from cellulose in one single batch reactor without the consumption of

6 September 2012

external hydrogen (H2). In this new process, part of the cellulose is first converted to in situ

Accepted 13 September 2012

H2 by steam reforming (SR) in the steam gas phase mainly; and, in the liquid water phase,

Available online 23 October 2012

cellulose is converted to an alkane precursor, such as 5-(hydroxymethyl)furfural (HMF). In

Keywords:

Accordingly, this new process has been named SR(H2)-APD/H. Experimental results show

Hydrothermal conversion

that the volumetric ratio of the reactor headspace to the reactor (H/R) and an initial weakly

Alkane

alkaline condition are the two key parameters for SR(H2)-APD/H. With proper H/R ratios

the final reaction step, in situ H2 reacts with HMF to form liquid alkanes through APD/H.

In situ hydrogen

(e.g., 0.84) and initial weakly alkaline conditions (e.g., pH ¼ 7.5), liquid alkanes are directly

Biomass

formed from the SR(H2)-APD/H of cellulose using in situ H2 instead of external H2. In this

Cellulose

study, compared with pyrolysis and hydrothermal liquefaction of cellulose at the same temperatures with same retetion time, SR(H2)-APD/H greatly increased the liquid alkane yields, by approximately 700 times and 35 times, respectively. Based on this process, direct formation of fossil gasoline from renewable biomass resources without using external H2 becomes possible. ª 2012 Elsevier Ltd. All rights reserved.

1.

Introduction

Bio-oil from biomass is a promising energy source, featuring renewable and carbon-neutral properties compared with fossil energy resources. Since biomass is being continually

produced via photosynthesis, bio-oil from biomass has a renewable property. In addition, after releasing energy through the combustion of bio-oil, the emitted carbon dioxide (CO2) would be adsorbed by plants for photosynthesis. As a result, the use of bio-oil generates almost zero emissions of

* Corresponding author. Tel.: þ1 403 220 4199. ** Corresponding author. Department of Mechanical & Mechatronics Engineering, Waterloo Institute of Sustainable Energy, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1. Tel.: þ1 519 888 4567x38718. E-mail addresses: [email protected], [email protected] (S. Yin), [email protected] (Z. Tan). 0961-9534/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biombioe.2012.09.038

b i o m a s s a n d b i o e n e r g y 4 7 ( 2 0 1 2 ) 2 2 8 e2 3 9

CO2. Due to these attractive properties, bio-oil is receiving worldwide attention. The International Energy Agency envisions that bio-oil will provide 27% of the world’s transport fuel by 2050 [1]. Aqueous-phase dehydration/hydrogenation (APD/H) is an advanced technology for bio-oil production from biomass. Through APD/H, the produced bio-oil mainly consists of alkanes, which are the same components as petroleum [2]. Since current internal combustion engines are designed based on the properties of petroleum (alkanes), alkane bio-oil from the APD/H of biomass is compatible with existing engines [3]; therefore, modification of engines is not required. By comparison, hydrothermal liquefaction (HTL) and pyrolysis, the other two main thermal processes for bio-oil production, mainly produced furan derivatives, benzene derivatives, carboxylic acids and phenolic-rich bio-oils [4e8]. They are incompatible with current internal combustion engines [9]. To date, by APD/H, Dumesic and co-workers have formed chain alkanes from cellulose derivatives, including glucose, sorbitol and 5-(hydroxymethyl)furfural (HMF) [2,10,11]. Similarly, Kou et al. obtained cyclic alkanes from lignin monomers and phenols [12,13]. Recently, Huber and co-workers further improved and extended APD/H to real biomass samples and successfully converted pyrolysis bio-oil and hydrolysis products of maple wood to alkanes [14,15]. The essential reactions of APD/H are dehydration and hydrogenation [16]. The cooperation between these two reactions leads to the oxygen atoms in the biomass being removed as water, thereby producing alkanes [11]. Taking APD/H of phenol as an example (Fig. 1) [13], phenol is first treated by the hydrogenation reaction, forming carbonecarbon single bonds and a hydroxyl function group. Then, via the dehydration reaction, the oxygen atom in the hydroxyl group is removed as water, and a carbonecarbon double bond is formed. In the last step, carbonecarbon double bonds are hydrogenated, forming a cyclic alkane. The same reaction strategy applies to chain alkanes; however, the production of C1e6 alkanes and of >C6 alkanes follow different reaction pathways. For C1e6 alkane formation (Fig. 2), after hydrolysis of cellulose to glucose, glucose is first converted to sorbitol by hydrogenation and then from sorbitol to C1e6 alkanes [11]. For >C6 alkanes (Fig. 2), glucose is first converted to HMF rather than sorbitol. HMF then reacts with aldehydes via the aldol condensation reaction to make the number of carbon atoms in HMF derivatives higher than 6. Based on the HMF derivatives, APD/H generates >C6 alkanes [2]. Thus, through APD/H, a wide carbon range of alkanes can be formed from renewable biomass.

Hydrogenation OH

Phenol

H2

Dehydration

Hydrogenation

OH

H2

H2O

Cyclohexanol

Cyclohexene

Cyclohexane

Fig. 1 e General reaction pathways of aqueous-phase dehydration/hydrogenation of phenol to cyclic alkane.

229

APD/H is not, however, an independent process. The reaction pathways of APD/H (Figs. 1 and 2) show that H2 is required for the hydrogenation steps. Huber et al. [2] studied alkane formation from 5-HMF and its derivatives by APD/H. They found that 5-HMF and its derivatives mainly converted to coke instead of alkanes without external hydrogen. The coke then covered and easily deactivated catalysts. However, when external hydrogen was employed, coke formation was inhibited, and the alkane yield reached about 70% on carbon basis of the input 5-HMF. Similarly, Huber et al. [11] also studied the effect of hydrogen on the conversion of sorbitol to alkanes. The alkane yield was around 37% (carbon basis) without external hydrogen, while the alkane yield increased to 76% as external hydrogen was added. Hence, APD/H needs external hydrogen. However, it is important to note that the current dominant supply of H2 is from the natural gas/petroleum industry. With the depletion of non-renewable natural gas/petroleum resources, the application of APD/H to produce alkane bio-oil from biomass would be hindered. Hence, a reliable H2 source is one of the key factors in the development and commercialization of APD/H. In situ H2 from biomass can be a reliable H2 source for APD/ H. First, in situ H2 is produced from biomass, making it a renewable H2 source for APD/H. Second, in situ H2 is produced and then used for APD/H in the same reactor. No loading of H2 to reactors is needed for the APD/H. A remarkable advance in in situ H2 generation was recently reported in the literature. Aqueous-phase reforming (APR) converted biomass intermediates, such as sorbitol and hydrogenated pyrolysis bio-oil, to in situ H2. With the in situ H2 from APR, Huber and his co-workers converted sorbitol and updated pyrolysis bio-oil to C1e6 alkanes through APD/H, without adding external H2 [15,17]. It is, however, difficult for APR to produce in situ H2 from the key biomass intermediates, such as HMF and glucose, for >C6 liquid alkane generation (Fig. 2). With respect to gasification, HMF is quite stable in an aqueous solution. Even at the high temperatures of 400e500  C, small amounts of HMF are gasified to H2 in the aqueous phase [18]. APR of glucose can produce in situ H2 [10], but this reaction is only a fractional order of the glucose concentration; whereas, the reaction of glucose to HMF in the aqueous solution is of the first order [19]. The difference between the reaction rates indicates that in situ H2 from the APR of glucose cannot meet the H2 requirement for the APD/H of HMF to >C6 alkanes. Thus, for the APD/H of biomass to >C6 alkanes, it is necessary to develop another process of generating in situ H2. Steam reforming (SR) can be a potential approach to producing in situ H2 for APD/H. With SR, H2-rich gas can be directly produced from real biomass. Wei et al. studied the effects of steam to biomass mass ratios (S/B) on H2 production from the pyrolysis of legume straw and pine sawdust and reported that, as the S/B ratios increased from 0 to 1, the corresponding total gas yields increased from 70 mass% to 90 mass% [20]. The H2 concentrations also increased from 27 mol % to 35 mol%. Similarly, SR has also been used to produce H2-rich gas from coal [21]. The main reason for the high H2 yields by SR is that the presence of steam promotes not only an SR reaction, but also a water-gas shift, thereby leading to high H2 yields [22e24].

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A APR

Cellulose

in situ H2

H2 Sorbitol

C1-6 alkanes

Hydrogenation

APD/H H2

Glucose Dehydration

Aqueous phase

HMF HMF-derivatives > C6 alkanes Aldol condensation APD/H

General conversion pathway of cellulose to C1-6 and > C6 chain alkanes

B

H C -alkane Dehydration/ hydrogenation H H

Hydrogenation

Self condenstaiton

Hydrogenation HMF

Condenstaiton

H

H C -alkane Dehydration/

Hydrogenation

hydrogenation

HMF

Condenstaiton

H

Dehydration/ hydrogenation

H C -alkane Dehydration/ hydrogenation

Detail reaction pathways for the conversion of HMF and HMF derivatives to alkanes Fig. 2 e Pathways of aqueous-phase dehydration/hydrogenation of cellulose to C1e6 and >C6 chain alkanes [2].

During hydrothermal conversions, steam is automatically formed from water, because reaction temperatures are usually higher than 200  C. As such, SR can take place during hydrothermal conversion to form in situ H2, even without external steam. In addition to the formation of in situ H2, the SR of biomass has the advantage of not negatively influencing APD/H reactions. SR and APD/H occur in the steam gas and liquid water phases, respectively. By comparison, APR (in situ H2 formation) and APD/H (alkane formation) both occur in the liquid phase. APR and APD/H are competitive reactions [17,25]; and, it is hard to balance them in one phase for in situ H2 and alkane formation. Thus, SR is another potential method of producing in situ H2 for APD/H. A new process combining SR and APD/H for liquid alkane formation is proposed in this paper, as illustrated in Fig. 3. In

this process, part of the input biomass (e.g cellulose) is first converted to in situ H2 by SR in the steam gas phase; and, in the liquid water phase, the remaining biomass (e.g cellulose) is converted to alkane precursors (e.g., HMF) by hydrothermal decomposition. With APD/H, the in situ H2 then reacts with the alkane precursors to form liquid alkanes. Since in situ H2 is directly produced from the input biomass, this process is considered an independent conversion. This proposed process is named SR(H2)-APD/H. The purposes of this study, therefore, were the analysis and investigation of the proposed SR(H2)-APD/H process. First, the feasibility of this process was analyzed based on the literature, with a focus on determination of key conversion parameters. Secondly, the SR(H2)-APD/H process was investigated experimentally, with respect to the effects of the key conversion parameters on liquid alkane yields and compositions.

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Steam reforming (SR) (1) Biomass

Hydrogenation

in situ H2 Steam phase Dehydration

Liquid alkanes

Alkane precursors (2) Hydrothermal decomposition

Hydrogenation

(3)

Liquid phase

APD/H

Fig. 3 e General reaction pathways of SR(H2)-APD/H of biomass to liquid alkanes in a single reactor.

2. Feasibility analysis of SR(H2)-APD/H of biomass to liquid alkanes The feasibility of the SR(H2)-APD/H process depends on whether all three reaction steps can occur in one single reactor. As shown in Fig. 3, these steps are (1) the production of in situ H2 from the biomass, (2) the production of alkane precursors from the biomass, and (3) the production of alkanes from the alkane precursors in the presence of in situ H2. In this section, we analyze each step in terms of the conversion method based on the literature. By comparing the conditions of each step, the reaction conditions for the SR(H2)APD/H process are proposed. For the first step, as mentioned in Section 1, SR has been selected to produce the in situ H2 from the biomass. Compared with thermal reforming (pyrolysis), steam promotes not only SR reaction, but also water-gas shift [22]. As such, more H2 can be produced from the biomass [23,24]. Hydrothermal gasification (HTG) is another technology than can produce H2 from biomass. In fact, it is consistent with SR. Gao et al. [26] recently reviewed the HTG of biomass to H2 based on a number of experimental data and revealed that the overall HTG reaction is the same as SR. The SR reaction largely explains why more H2 is produced by supercritical HTG rather than by subcritical HTG. During supercritical HTG, no liquid water exists in a reactor, with all the water evaporated. Due to the homogeneous steam environment, SR is believed to produce more H2. However, during subcritical HTG, the majority of water does not vaporize. The limited steam vapor results in low H2 yields [27]. However, the H2 yields of subcritical HTG of biomass can be improved with the use of alkalis, which are catalysts for the SR reaction [28e30]. Therefore, although HTG is considered another technology for producing H2 from biomass, its essential reaction is still SR. Conventional gasification of biomass to H2 was not considered for the first step. The main reason is that gasification takes place in the presence of oxygen or air, and oxygen usually has negative impacts on the bio-oil yields from the hydrothermal liquefaction (HTL) of biomass [31]. Therefore, SR was selected for in situ H2 production. Steam and alkalis are considered the two important factors for this step. The purpose of the 2nd reaction step is the formation of alkane precursors. As reviewed in Section 1, the important

alkane precursors mainly include HMF, sorbitol and the monolignols of lignin. However, sorbitol and the monolignols of lignin are not considered for liquid alkane production in this present study, because they mainly produce gaseous and cyclic alkanes, respectively, rather than liquid chain alkanes, which are the main components of fossil gasoline [11,12]. HMF is a good alkane precursor from which liquid chain alkanes can be formed (Fig. 2) [2]. Thus, the specific aim of the second reaction step becomes the formation of HMF from the biomass. The conversion of biomass to HMF has been reported in literature [32,33]. Acidic conditions usually produce more HMF from biomass as compared with neutral and alkaline conditions. When water serves as the reaction media, HMF is only formed in the liquid water phase rather than in the steam gas phase. Kabyemela et al. [34] studied the reaction pathways of the hydrothermal decomposition of glucose and found that HMF was formed in the liquid phase; whereas, in steam, glucose was mainly converted to 1,6-anhydroglucose. Based on the above analysis, an acidic aqueous solution is identified as a key factor for the 2nd reaction step. In the 3rd reaction step, alkanes are formed by the reaction of HMF with in situ H2 through APD/H. As shown in Fig. 2, the main reactions of APD/H are hydrogenation / dehydration / hydrogenation. Davda et al. [25] conducted an extensive review of the APD/H process and pointed out that hydrogenation can be promoted by a metal catalyst, such as platinum (Pt) and palladium (Pd), and that dehydration by acidic aqueous conditions (pH ¼ 2, 3). Accordingly, the key conversion factors for the 3rd reaction step are a metal catalyst and acidic conditions. After comparing the conditions of these three reaction steps, however, it was found that the three reaction steps need contrasting conditions. As summarized in Table 1, in terms of the phase state of water, the first step needs steam; whereas, the other two steps need liquid water. Also, in terms of the pH level, the SR reaction prefers a pH > 7 instead of a pH < 7; however, the other two steps favor a pH < 7. This means that the feasibility of the SR(H2)-APD/H process specifically depends on the conditions that allow the coexistence of steam and liquid water phases, as well as the cooccurrence of alkaline and acidic conditions during the conversion (Table 1). The desired reaction conditions can be realized by carefully choosing the volumetric ratios of the reactor

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Table 1 e Reaction conditions for the three steps of SR(H2)-APD/H of biomass to alkanes. Step 1 Purpose Method Condition Desired condition for SR(H2)-APD/H

Step 2

In situ H2 HMF Steam reforming Hydrothermal decomposition Steam with a pH of 7 Liquid water with a pH of <7 1. Coexistence of steam and liquid water 2. Co-occurrence of alkaline and acidic conditions 3. Metal catalysts

headspace to the reactor (H/R) and the initial pH levels of the aqueous solutions. The H/R ratio influences the phase balance between steam and liquid water in a closed reactor, while the initial pH level affects the pH changes of the reaction media during conversion [35]. Since the conversion temperatures of hydrothermal conversion are around 300  C, with proper H/R ratios, a part of the input water evaporates to become steam, but some water still remains in liquid form. As such, SR with the steam gas phase, hydrothermal decomposition and APD/H in the liquid water phase could all occur to form alkanes (Fig. 4). The initial pH level of the aqueous solution is another key factor for SR(H2)-APD/H. By using initial weakly alkaline aqueous solutions, both alkali-catalyzed SR and acidcatalyzed HMF formation and APD/H can be promoted for liquid alkane formation. The main reason is that initial weakly alkaline solutions allow alkali-catalyzed SR to produce in situ H2 (Fig. 4). After the pH levels of the initial weakly alkaline solutions gradually decrease to lower than 7, due to the produced carboxylic acids from the biomass under the initial alkaline conditions [27,35,36], the acid-catalyzed HMF formation and APD/H take place to form liquid alkanes by reacting with the produced in situ H2. With proper H/R ratios and initial pH levels of aqueous solutions, it becomes possible to directly produce liquid alkanes from biomass through SR(H2)-APD/H. To further check the feasibility, the following sections present the investigation of SR(H2)-APD/H of cellulose to liquid alkanes with experiments.

Reactor

Step 3

Argon

Steam

in situ H2 (Steam reforming)

3.

Liquid alkanes Aqueous-phase dehydration/hydrogenation Liquid water with a pH of <7 and catalysts

Materials and methods

Cellulose (Sigma Aldrich, Cat. No. C6413) was used as the feedstock. Alkaline (pH ¼ 7.5, 8 and 9) aqueous solutions were prepared by adding sodium hydroxide (NaOH) to distilled water. The pH values of these solutions were monitored by a pH meter (Oakton, Ion 5) with an accuracy of 0.01. The catalysts used in this study included 1 wt.% platinum aluminum oxide (Pt/Al2O3, Sigma Aldrich, Cat. No. 205966), 5 wt.% Pt/Al2O3 (Sigma Aldrich, Cat. No. 205974) and 5 wt.% palladium aluminum oxide (Pd/Al2O3, Sigma Aldrich, Cat. No. 205710) and were used as received. All tests were carried out in a 69 mL stainless steel tubular reactor. The description of this reactor has been reported in another paper [27]. Briefly, it mainly consisted of a stainless steel tube. One end of this tube could be opened or closed by removing or securing a stainless steel cap. The other end was connected to a needle valve, by which the air in the reactor could be removed and replaced with an inert gas, such as argon. The tubular reactor was rated at 350  C and 5000 psig. In a typical test, a mixture of 3 g of cellulose and 11 of mL aqueous solution and sometimes with 0.2 g of a catalyst was first loaded to the tubular reactor through the end with a cap. After loading and sealing the reactor, air in the reactor was removed through the needle valve by using a vacuum and was then replaced by argon using a gas cylinder. Subsequently, the reactor was put into and kept vertical in a preheated muffle

Steam

in situ H2 (Steam reforming)

Liquid alkanes (APD/H)

Water HMF (Hydrothermal decomposition)

Biomass

Before conversion

During conversion (step 1)

During conversion (step 2)

During conversion (step 3)

At room temperature pH>7

At 300°C in this study pH>7

At 300°C in this study,

At 300°C in this study,

pH < 7 (due to produced carboxylic acids from biomass)

pH < 7 (due to produced carboxylic acids from biomass)

Reaction time Fig. 4 e Illustration of SR(H2)-APD/H of biomass to liquid alkanes in one single reactor (Conditions: proper volumetric ratios of headspace to reactor (H/R) and initial weakly alkaline solutions).

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4.

Results and discussion

4.1.

In situ H2 from steam reforming (SR) of cellulose

The relation between the in situ H2 yield and the H/R ratio is presented in Fig. 5. It can be seen that in situ H2 was produced and its yields increased with increasing H/R ratios. At a small H/R ratio of 0.6, the H2 yield was only 0.5 mmol, but with the higher H/R ratio of 0.95, the yield rose to 2.3 mmol. Furthermore, because the H/R ratios mainly determine the amount of generated steam at a given temperature and a closed batch reactor [38], Fig. 5 also shows that the H2 yields increased with increases in generated steam. This confirmed that steam benefited in situ H2 generation from biomass. However, the increases in the in situ H2 yields at the small H/R ratios of 0.60e0.68 were much smaller than those at the large H/R ratios of 0.80e0.95. This was mainly due to two reasons. First, small H/R ratios limited steam formation in the small headspace for SR, given the fixed saturated vapor pressure at 300  C. Second, the small H/R ratios meant relatively large input water volumes. Although a part of the input water vaporized at 300  C, most of the input water was still in liquid form and totally covered the cellulose particles. As a result, the steam could not easily contact cellulose, thereby producing low yields of in situ H2. Furthermore, by comparing the H2 yields at the small H/R ratios of 0.60 and 0.68, it is noted that the H2 yield at 0.60 was higher than that at 0.68. The main reason for this unexpected result was that at small H/R ratios, H2 was formed by reforming of glucose (the main decomposition product of cellulose) in liquid water phase, and the H2 selectivity usually decreased with increasing glucose concentrations in liquid water [19,25]. Because the relatively large H/R ratio of 0.68 meant less input water and a higher glucose concentration, the H2 yield formed at the H/R ratio of 0.68 was lower than of 0.6. For the large H/R ratios, the large headspace contained more steam at 300  C, and the reactor contained less of the input water. As such, the cellulose particles were not

3.0 Steam reforming

6.0 5.5

2.5

4.5

H2

4.0

1.5 Subcrticial hydrothermal gasification/liquefaction

3.5

1.0 3.0 0.5

2.5

Generated steam 0.0 0.55

Generated steam (g)

5.0 2.0

H (mmol)

furnace (at 300  C) for conversion. During experiments, we did not shake the reactor or use a stirrer. One main purpose of this study was to study the effect of phase changes on alkane biofuel formation. Agitation during the reaction would promote the mixing of solid, liquid and gas phases. As such, it was difficult for us to investigate the impacts of spontaneous phase changes at high temperatures on biofuel generation. The temperature inside the reactor was monitored by a Ktype thermocouple. Once the inside temperature of this reactor reached the desired temperature of 300  C, the reactor was heated for additional 15 min, which is defined as the reaction residence time. The reactor was then quenched to stop by moving out of the muffle furnace and immersing into tap water. After cooling down, the needle valve was opened to release the gas product. The gas composition was analyzed by micro gas chromatography (micro-GC, Varian CP-4900), in terms of H2, nitrogen (N2), oxygen (O2), carbon monoxide (CO), and CO2. This micro-GC contained two separation channels. Each channel was a separate GC with pneumatics, injector, column and thermal conductivity detector. The first channel was equipped with a Molsieve 5A plot column to detect N2, O2, H2, and CO. The second channel was equipped with a PPU column to detect CO2. During the micro-GC analysis, the injection temperature was 110  C, and the first and second columns were maintained at 100  C with 30 psig (argon) and 100  C with 20 psig (helium), respectively. The liquid products were collected and filtered to remove residual solids using Whatman No.1 filter paper. The residual solids on the filter paper were further heated to 120  C until their weight became constant. The mass of the residual solids was then determined using a scale. In cases where a solid metal catalyst was used, the weight of the added metal catalyst was first subtracted from the total residual solids, so that the actual residual solid yield was measured. The filtrate was extracted by a non-polar organic solvent of dichloromethane (CH2Cl2) at a volumetric ratio of 1e20 for 30 min in a 250 mL separation funnel. The extract was then treated at 40  C to remove the CH2Cl2 using a rotaryevaporator (BUCHI RE-121 Rotavapor with BUCHI 461 water bath). The remaining liquid was defined as bio-oil [37]. The alkane components of the bio-oil were analyzed by GC (Varian GC 430) equipped with a CP7717 column and a flame ionization detector (FID). Before analysis, this GC was quantitatively calibrated by using the ASTM D5307 crude oil standard (C49 alkanes, Sigma Aldrich, Cat. No. 48128). The detailed GC separation program was set up as follows: an injection temperature of 200  C, oven temperatures from 30  C for 3 min to 100  C with a heating rate of 20  C/min, and a final temperature of 230  C for 10 min. The alkane yield was defined as the carbon molar ratio of the produced liquid alkanes to the input cellulose. In addition, because HMF is the key alkane precursor, its concentration in the liquid product was also determined using GC, with the GC separation program as follows: an injection temperature of 230  C, oven temperatures from 30  C to 230  C with a heating rate of 10  C/min, and a final temperature of 230  C for 10 min. In this study, each test condition was repeated at least three times until the deviation between the results was less than 10%. The mean values are reported in this paper.

0.60

0.65

0.70

0.75

0.80

0.85

0.90

2.0 0.95

1.00

Volumetric ratios of reactor headspace to reactor (H/R)

Fig. 5 e In situ H2 yields from steam reforming of cellulose (Conditions: 3 g cellulose, reactor volume of 69 mL, 300  C, distilled water, 15 min reaction residence time, and 0 psig of argon as the processing gas).

234

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completely covered by the liquid (water) at 300  C. More steam directly reacted with the cellulose, thereby generating more in situ H2. Hence, the H/R ratio plays an important role in in situ H2 formation. Only with proper H/R ratios can steam first be automatically generated from water and then react with cellulose particles directly to form in situ H2 for the subsequent APD/H of the biomass to liquid alkanes.

4.2.

Liquid alkanes from SR(H2)-APD/H of cellulose

4.2.1.

H/R ratios

The effects of the H/R ratios on alkane yields are presented in Fig. 6. It is noted that the liquid alkane yields first increased with increasing H/R ratios and then decreased. For small H/R ratios, such as 0.60 and 0.64, the alkane yields were lower than 0.3%. When the H/R ratio was increased, the liquid alkane yield improved. The highest alkane yield of 1.1% was obtained at the H/R ratio of 0.84. However, at the higher H/R ratios of 0.88, 0.92 and 0.96, the alkane yield dramatically decreased. These results confirm that the H/R ratios play an important role in SR(H2)-APD/H and that, in the absence of external H2, liquid alkanes can be directly produced from cellulose via SR(H2)-APD/H. The low alkane yields at small H/R ratios were mainly attributed to two reasons. First, as mentioned in Section 4.1, low in situ H2 yields were formed at the small H/R ratios, thereby hindering the APD/H of the cellulose to alkanes. Second, with small H/R ratios, most of the input water existed in liquid form at the conversion temperature of 300  C. The liquid (water) diluted concentrations of the cellulose and cellulose derivatives, such as HMF. Therefore, the reaction possibilities of the APD/H between in situ H2 and HMF decreased, generating low alkane yields. Low alkane yields were also produced at the large H/R ratios, but were mainly caused by the low yields of HMF rather than of in situ H2. We measured the HMF concentrations produced at the large H/R ratios of 0.88, 0.92 and 0.96, and found that no HMF was formed (Fig. 7). At the same time, the alkane yields were all lower than 0.02% (Fig. 6).

As mentioned in Section 2, the formation of HMF from cellulose/glucose only occurs in the aqueous phase [34]. However, at large H/R ratios, the input water all vaporized at 300  C. With no liquid phase, no HMF was formed from the cellulose, resulting in alkane yields of almost zero at large H/R ratios. Based on the specific volume of saturated vapor at 300  C (0.0217 m3/kg) and the volume (69 mL) of this reactor, we calculated that 3.18 mL was theoretically the maximum volume of liquid water that could completely become steam at 300  C in this reactor [38]. Because 3.18 mL of water corresponds to the H/R ratio of 0.95, this value theoretically explains why almost no alkane was formed at the H/R ratio of 0.96. In practice, more than 3.18 mL of water is needed to maintain a liquid phase at 300  C in this reactor, because dry cellulose powders can absorb some water. Our experiments showed that, only when more than 10 mL of water was added to the reactor, which corresponded to the H/R ratio of 0.85, 3 g of cellulose totally dissolved in water at room temperature; and, some liquid (water) was still in the reactor at 300  C. Thus, the other large H/R ratios of 0.88 and 0.92 in this study also resulted in the low alkane yields. Therefore, the production of high yields of alkanes requires proper H/R ratios, so that both in situ H2 and HMF are formed in a single reactor. The in situ H2 can then react with HMF to form alkanes through APD/H. In addition, SR(H2)-APD/H could be more efficient than hydrothermal liquefaction in terms of utilization of in situ H2 for APD/H. Because SR(H2)-APD/H used less water than hydrothermal liquefaction, the mass transfer of in situ H2 to liquid water phase became easier than hydrothermal liquefaction. As such, although the in situ hydrogen yields from SR(H2)- APD/H were low, the hydrogen was efficiently used for alkane formation in liquid phase (see Fig. 5). The variation of the H/R ratios, however, did not apparently influence the alkane compositions. The main components of the produced liquid alkanes were heptane (C7-alkane), octane (C8-alkane) and nonane (C9-alkane). This result was consistent with other results in the literature. 10

2.5

1.5

8

6

0.9

0.6

Hydrothermal liquefaction

1.5 4 1.0 2

0.3

3

H2 HMF

2.0

HMF (mmol*10 )

1.2

H2 (mmol)

Alkane yields (carbon mol % of input cellulose)

SR(H2)-APD/H

Pyrolysis 0.5

0.0 0.56 0.60 0.64 0.68 0.72 0.76 0.80 0.84 0.88 0.92 0.96 1.00

Volumetric ratios of reactor headspace to reactor (H/R)

Fig. 6 e Effects of H/R ratios on alkane yields from SR(H2)APD/H of cellulose (Conditions: 3 g cellulose, reactor volume of 69 mL, 300  C, pH [ 7, 15 min reaction residence time, and 0 psig of argon as the processing gas).

0

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

Volumetric ratios of reactor headspace to reactor (H/R)

Fig. 7 e . Yields of in situ H2 and HMF from SR(H2)-APD/H of cellulose (Conditions: 3 g cellulose, reactor volume of 69 mL, 300  C, pH [ 7, 15 min reaction residence time, and 0 psig of argon as the processing gas).

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4.2.2.

2.5

Alkane yields (carbon mol % of input cellulose)

Huber et al. [2] studied APD/H of HMF to alkanes and also found that these three liquid alkanes were the main alkane components. Furthermore, as shown in Fig. 8, the carbon molar ratios of the produced heptane, octane and nonane were almost constant at around 2:5:3 although the H/R ratio varied from 0.60 to 0.84. The stability of the alkane compositions can be explained based on the reaction pathways of SR(H2)-APD/H. As shown in Fig. 3, alkanes are formed by the APD/H of alkane precursors with in situ H2. During this process, H2 mainly promotes hydrogenation, rather than removing and adding carbon atoms. Thus, the alkane compositions mainly depend on the compositions of the alkane precursors. Literature shows that the production and the compositions of alkane precursors from biomass are mainly determined by the conversion temperature, pressure, processing gas and reaction time [32]. In our study, since all the tests used the same conversion temperature, processing gas and reaction time, similar alkane precursors were produced at the different H/R ratios, leading to the stable alkane compositions.

2.0

1.5

1.0

0.5

0.0 No catalyst

5 wt.% Pd/Al2O3 5 wt.% Pt/Al2O3 1 wt.% Pt/Al2O3

Catalysts

Fig. 9 e Influences of Pd/Al2O3 and Pt/Al2O3 on the alkane yields from SR(H2)-APD/H of cellulose (Conditions: 3 g cellulose, 0.84 H/R ratio, 0.2 g catalyst, 300  C, pH [ 7, 15 min reaction residence time, and 0 psig of argon as the processing gas).

Catalysts

The use of the metal catalysts (Pd/Al2O3 and Pt/Al2O3) further improved the alkane yields from SR(H2)-APD/H. As shown in Fig. 9, in the presence of 5 wt.% Pd/Al2O3 and 5 wt.% Pt/Al2O3, the alkane yields at the H/R ratio of 0.84 increased to 2.4% and 2.1%, respectively, higher than the yield of 1.1% without any added catalyst. Pd and Pt catalysts promoted the hydrogenation reactions in the APD/H, in order to improve the alkane yields. Singh et al. [39] reviewed the hydrogenation reactions in the liquid phase with the presence of metal catalysts and pointed out that Pd and Pt were two effective catalysts for hydrogenation. Using Pd and Pt catalysts also improved the alkane yields from the

APD/H of sorbitol and lignin, respectively [11,12]. Furthermore, in terms of the capacity for hydrogenation, Pd is a little higher than Pt [2], thereby producing relatively more alkanes in this study (Fig. 9). The use of metal catalysts also increased the percentages of heavy liquid alkanes (i.e., nonane) in the alkane products. As shown in Fig. 10, with the use of 1 or 5 wt.% Pt/Al2O3, over 60% of carbons in the alkanes were nonane; whereas, without a catalyst, only 25% of alkane carbons were stored in nonane. The formation of heavy alkanes largely depends on the rate difference between the hydrogenation reaction and the carbonecarbon (CeC) cleavage [11]. When the rate of the

Nonane

Octane

Nonane

Heptane

100

80

60

40

20

0 0.56

0.60

0.64

0.68

0.72

0.76

0.80

0.84

Octane

Heptane

100

Alkane compositions (carbon mmol %)

Alkane compositions (carbon mol %)

120

0.88

Volumetri ratios of reactor headspace to reactor (H/R)

Fig. 8 e Effects of H/R ratios on alkane compositions from SR(H2)-APD/H of cellulose (Conditions: 3 g cellulose, reactor volume of 69 mL, 300  C, pH [ 7, 15 min reaction residence time, and 0 psig of argon as the processing gas; The total compositions was normalized to 100% based on C7e9 alkanes, the main produced alkanes).

80

60

40

20

0 No catalyst

5 wt.% Pd/Al2O3

5 wt.% Pt/Al2O3

1 wt.% Pt/Al2O3

Catalysts

Fig. 10 e Influences of Pd/Al2O3 and Pt/Al2O3 on the alkane compositions from SR(H2)-APD/H of cellulose (Conditions: 3 g cellulose, 0.84 H/R ratio, 0.2 g catalyst, 300  C, pH [ 7, 15 min reaction residence time, and 0 psig of argon as the processing gas; The total compositions was normalized to 100% based on C7e9 alkanes, the main produced alkanes).

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hydrogenation reaction is faster than CeC cleavages, the produced alkanes contain more carbon atoms. When no external catalyst was added to the reactor, iron (Fe), which is the main metal element of the reactor wall, may have played a role as a catalyst for APD/H [40]. However, the CeC cleavage rate of Fe is much faster than hydrogenation, thus resulting in low percentages of heavy alkanes [25]. In contrast, the hydrogenation rates of Pt and Pd are higher than their CeC cleavages rates, thereby producing relatively more heavy liquid alkanes [41]. Therefore, the use of Pt or Pd catalysts improved not only the alkane yields from the SR(H2)APD/H of cellulose, but also the percentages of heavy alkanes in the alkane products.

4.2.3.

Initial alkalinites

Fig. 11 shows that the alkane yields first increased with initial alkalinities and then decreased, regardless of the metal catalysts. For example, with Pt as the catalyst, the alkane yield produced from an initial solution with a pH ¼ 7.5 was 6.8%, much higher than the 2.1% from the initial solution of pH ¼ 7. However, in the initial solution with a pH ¼ 9, the alkane yield decreased again to 1.8%. Under initial weakly alkaline conditions (pH ¼ 7.5), the three main steps of SR(H2)-APD/H were all promoted, improving the alkane yields. The weakly alkaline solutions first promoted SR to generate more in situ H2, because the reforming reaction is an alkali-catalyzed reaction [29]. Second, in the liquid water phase, the weakly alkaline conditions converted some cellulose to carboxylic acids [42]; and, these produced carboxylic acids gradually reduced the pH levels of the aqueous solutions. Once the weakly alkaline condition changed to acidic, the acid-catalyzed decomposition of cellulose to HMF was promoted as well [27]. The initial strongly alkaline solutions (pH ¼ 9), however, did not improve alkane yields. Compared with weakly alkaline conditions (pH ¼ 7.5), the high alkali concentrations hindered

the pH decrease of the reaction media [35]. As a result, the acid-catalyzed conversion of cellulose to HMF occurred to a smaller extent and then the APD/H of HMF produced low alkane yields. According to our experimental data on HMF yields, with the initial solution of pH ¼ 9, the HMF yields from cellulose in the presence of Pt/Al2O3 and Pd/Al2O3 were only 3.22 mM and 2.37 mM, respectively. However, at pH ¼ 7.5, the HMF yields were as high as 17.06 mM and 12.80 mM, respectively. On the other hand, the use of either initial weakly or strongly alkaline solutions increased the percentages of heavy alkenes. As shown in Fig. 12 and Fig. 13, higher percentages of heavy alkanes (nonane) were formed at pH ¼ 7.5 and 8 than at pH ¼ 7. For instance, at pH ¼ 7.5 with the Pd catalyst, the nonane percentage reached 65%, based on the total produced carbons in alkanes. The main reason for this improvement was that, with the final mildly acidic pH levels under these conditions, the produced HMF did not tend to form char, but rather water-soluble HMF polymers with middle polymerization degrees. Since HMF polymers contained more carbon atoms than HMF, more heavy alkanes were formed from them via APD/H. In summary, the aforementioned experimental results confirmed the feasibility of SR(H2)-APD/H. By this approach, independent and direct production of fossil gasoline (liquid alkanes) from biomass resources becomes possible. Although the alkane yield was only 6.8%, SR(H2)-APR can become a process for practical application after process optimization. SR(H2)-APR is a process based on APD/H, which is well proved in terms of practical production of biofuel. In another word, research on APD/H has set up a solid foundation for practical application of SR(H2)-APR.

120

Alkane yields (carbon mol % of input cellulose)

7

Pd

Alkane compositions (carbon mmol %)

Nonane

Pt

6 5 4 3 2

Octane

Heptane

100

80

60

40

20

0

1

7

7.5

8

9

pH

0 3

7

7 .5

8

9

pH

Fig. 11 e Effects of initial alkalinities on the alkane yields from SR(H2)-APD/H of cellulose in the presence of catalysts (Conditions: 3 g cellulose, 0.84 H/R ratio, 0.2 g catalyst of 5 wt.% Pd/Al2O3 or 5 wt.% Pt/Al2O3, 300  C, 15 min reaction residence time, and 0 psig of argon as the processing gas).

Fig. 12 e Effects of initial alkalinities on the alkane compositions from SR(H2)-APD/H of cellulose in the presence of Pd/Al2O3 catalyst (Conditions: 3 g cellulose, 0.84 H/R ratio, 0.2 g 5 wt.% Pd/Al2O3, 300  C, 15 min reaction residence time, and 0 psig of argon as the processing gas; The total compositions was normalized to 100% based on C7e9 alkanes, the main produced alkanes.).

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120

Alkane compositions (Carbon mmol %)

Nonane

Octane

Table 2 e Comparison of SR(H2)-APD/H with petroleum formation.

Heptane

100

SR(H2)-APD/H

80

Products Liquid alkanes Conversion environments Water Yes Steam Yes pH 7.5 Final pH Around 3 for dehydration

60

40

Catalyst

20

0 7

7.5

8

9

pH

Fig. 13 e Effects of initial alkalinities on the alkane compositions from SR(H2)-APD/H of cellulose in the presence of Pt/Al2O3 catalyst (Conditions: 3 g cellulose, 0.84 H/R ratio, 0.2 g 5 wt.% Pt/Al2O3, 300  C, 15 min reaction residence time, and 0 psig of argon as the processing gas; The total compositions was normalized to 100% based on C7e9 alkanes, the main produced alkanes.).

5.

Implications

5.1. Relation between pyrolysis, SR(H2)-APD/H and hydrothermal liquefaction Most of the literature on thermochemical conversion of biomass to bio-oil shows that pyroysis and HTL (hydrothermal liquefaction) are two different processes. They have different reaction pathways and bio-oils with different compositions [5,8,43]. Based on our study, however, it is found that they are not totally independent from each other. They are connected by water. As shown in Fig. 6, when less input water is used (large H/R ratios), the conversion is essentially pyrolysis. However, when more input water (small H/R ratios) is loaded to the reactor, the conversion is chemically hydrothermal liquefaction. With proper amount of input water, SR(H2)-APD/H occurs, involving both gas-phase and liquid-phase reactions. Water is the connection among these three thermochemical conversion processes. Based on this connection, water partly explains why almost no alkane bio-oil was formed by HTL or pyrolysis (Fig. 6). HTL of biomass lacks in situ H2, because it converts biomass mainly in the liquid phase, producing alkane precursors (e.g., HMF) without H2. In contrast, for pyrolysis, it lacks alkane precursors (HMF). It converts biomass only in the gas phase and forms only in situ H2. Therefore, water plays a key role in thermochemical conversions of biomass, especially in terms of pathways and bio-oil qualities. This further indicates that, based on the properties of water and steam vapor, it is possible to develop a generic thermodynamic model for different

Pt

Petroleum formation at the bottom of the sea Liquid alkanes Yes Yes (sea geothermal steam) Around 7.5 in the seawater SiO2: solid acidic catalyst for dehydration (the Dominate component of earth crust) TRACE amount of metal catalysts

thermochemical conversions. As such, it would advance our understanding of thermochemcial conversion of biomass to bio-oil and to other biomass-based products.

5.2. Comparison of SR(H2)-APD/H with petroleum formation As presented in Table 2, some similarities do exist between SR(H2)-APD/H and petroleum formation. Both generate liquid alkanes, and both occur in liquid (water) and steam phases. The pH of the aqueous solutions used in SR(H2)-APD/H was 7.5, and the seawater pH at the bottom of the sea is around 7.5 too [44]. During SR(H2)-APD/H, the final pH of around 3.0 promoted dehydration. Similarly, at the bottom of the sea, silicon dioxide (SiO2), which is the main component of the Earth’s crust, is also an efficient solid acidic catalyst for dehydrdation [25,45]. Furthermore, according to SR(H2)-APD/H, the in situ H2 is formed by SR of the biomass and the hydrogen atoms in in situ H2 are from water [46]. In other words, it indicates that the hydrogen atoms in alkanes from SR(H2)-APD/H is mainly from water. Similarly, Lewan et al. studied the role of water in petroleum formation using immature petroleum rock and oil shale as feedstock and also found that water provides an important source of hydrogen [47,48]. These similarities partly explain why SR(H2)-APD/H can directly form liquid alkanes from biomass. On the other hand, it shows a connection between bio-oil and petroleum research.

6.

Conclusions

C7e9 liquid alkanes, which are the main components of fossil gasoline, were directly produced from cellulose via the SR(H2)APD/H process without using external H2. The first key operational point is the maintenance of a proper balance of steam and liquid water phases in the reactor. As such, in situ H2 can first be formed from cellulose by SR in the steam phase, and HMF from cellulose can then react with the in situ H2 to form alkanes via APD/H in the liquid water phase. The second key point is the use of weakly alkaline aqueous solutions, by which each reaction step of the SR(H2)-APD/H process is promoted to further increase liquid alkane yields.

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Compared with the APD/H for liquid alkane production from biomass, the SR(H2)-APD/H process not only reduces the dependence on external H2, but also directly uses real biomass (e.g., cellulose) instead of biomass derivatives (e.g., HMF) as conversion feedstock.

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