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Fermentable sugars recovery from lignocellulosic waste-newspaper by catalytic hydrolysis a

b

c

ad

e

Angela M. Orozco , Ala'a H. Al-Muhtaseb , David Rooney , Gavin M. Walker , Farid Aiouache cf

& Mohammad Ahmad a

The QUESTOR Centre, School of Chemistry and Chemical Engineering, Queen's University Belfast, Belfast, Northern Ireland, UK b

Petroleum and Chemical Engineering Department, Faculty of Engineering, Sultan Qaboos University, Muscat, Oman c

Centre for the Theory and Application of Catalysis, School of Chemistry and Chemical Engineering, Queen's University Belfast, Belfast BT9 5AG, Northern Ireland, UK d

Department of Chemical and Environmental Sciences, Materials Surface Science Institute, University of Limerick, Limerick, Ireland e

Engineering Department, Lancaster University, Lancaster, Lancashire, UK

f

Department of Mechanical Engineering, Faculty of Engineering and Architecture, American University Beirut, Beirut, Lebanon Published online: 14 May 2013.

To cite this article: Angela M. Orozco, Ala'a H. Al-Muhtaseb, David Rooney, Gavin M. Walker, Farid Aiouache & Mohammad Ahmad (2013) Fermentable sugars recovery from lignocellulosic waste-newspaper by catalytic hydrolysis, Environmental Technology, 34:22, 3005-3016, DOI: 10.1080/09593330.2013.798002 To link to this article: http://dx.doi.org/10.1080/09593330.2013.798002

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Environmental Technology, 2013 Vol. 34, No. 22, 3005–3016, http://dx.doi.org/10.1080/09593330.2013.798002

Fermentable sugars recovery from lignocellulosic waste-newspaper by catalytic hydrolysis Angela M. Orozcoa∗ , Ala’a H. Al-Muhtasebb∗ , David Rooneyc , Gavin M. Walkera,d , Farid Aiouachee and Mohammad Ahmadc,f a The

QUESTOR Centre, School of Chemistry and Chemical Engineering, Queen’s University Belfast, Belfast, Northern Ireland, UK; and Chemical Engineering Department, Faculty of Engineering, Sultan Qaboos University, Muscat, Oman; c Centre for the Theory and Application of Catalysis, School of Chemistry and Chemical Engineering, Queen’s University Belfast, Belfast BT9 5AG, Northern Ireland, UK; d Department of Chemical and Environmental Sciences, Materials Surface Science Institute, University of Limerick, Limerick, Ireland; e Engineering Department, Lancaster University, Lancaster, Lancashire, UK; f Department of Mechanical Engineering, Faculty of Engineering and Architecture, American University Beirut, Beirut, Lebanon

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b Petroleum

(Received 1 March 2013; final version received 16 April 2013 ) The urgent need for alternative renewable energies to supplement petroleum-based fuels and the reduction of landfill sites for disposal of solid wastes makes it increasingly attractive to produce inexpensive biofuels from the organic fraction of the municipal solid waste. Therefore, municipal waste in the form of newspaper was investigated as a potential feedstock for fermentable sugars production. Hydrolysis of newspaper by dilute phosphoric acid was carried out in autoclave Parr reactor, where reactor temperature and acid concentration were examined. Xylose concentration reached a maximum value of 14 g/100 g dry mass corresponding to a yield of 94% at the best identified conditions of 2.5 wt% H3 PO4 , 135◦ C, 120 min reaction time, and at 2.5 wt% H3 PO4 , 150◦ C, and 60 min reaction time. For glucose, an average yield of 26% was obtained at 2.5 wt% H3 PO4 , 200◦ C, and 30 min. Furfural and 5-hydroxymethylfurfural (HMF) formation was clearly affected by reaction temperature, where the higher the temperature the higher the formation rate. The maximum furfural formed was an average of 3 g/100 g dry mass, corresponding to a yield of 28%. The kinetic study of the acid hydrolysis was also carried out using the Saeman and the two-fraction models. It was found for both models that the kinetic constants (K) depend on the acid concentration and temperature. The degradation of HMF to levulinic acid is faster than the degradation of furfural to formic acid. Also, the degradation rate is higher than the formation rate for both inhibitors when degradation is observed. Keywords: lignocellulosic waste; newspaper; fermentable sugars; catalytic hydrolysis; phosphoric acid; xylose

1. Introduction Rising energy costs coupled with increasing concerns about global warming related to CO2 emissions and the reduction of landfill sites for disposal of solid waste resulted in increasing interest in alternative, low, and non-carbonbased energy sources,[1] such as the idea of producing biofuel from the fraction of the municipal solid waste. Biomasses for fuel production can generally be divided into two categories, organic wastes and dedicated energy crops. A further division includes starch or sugar crops and ligno-cellulosic biomass.[2,3] Lignocellulosic biomass is the most abundant renewable source of organic carbon on earth and the only one of low enough cost.[4–6] It is composed of cellulose, hemicelluloses, and lignin, from which sugars can be produced.[7] Ethanol from lignocellulosic material in the form of municipal solid waste is regarded more favourably due to the sustainable availability in large quantities, carbon dioxide neutral sources, and substantially will reduce the amount of wastes that would otherwise exert pressure on municipal

landfills.[8,9] Lignocellulose is also not digestible for human beings, which is an advantage over sugars and starch since the use of edible carbohydrates for the synthesis of bioethanol fuel has competed with the food production.[10] For example, Pereira and Ortega [11] found that ethanol production from sugarcane is not renewable; therefore, it cannot be sustained in the long term. About 35% of municipal solid waste (before recycling) by weight is paper and paper products which contain up to 70% polymeric sugars.[12] The world annual consumption of paper products has been estimated to be around 400 million tonnes in 2010, and the annual bioethanol production from waste paper to be around 80 billion litres.[13] This gives waste paper significant potential as a feedstock for bioethanol production. Chu and Feng [14] studied the conversion of newspaper and office paper to fermentable sugars via the enzymatic hydrolysis. They showed that the degree of sugar release increased with hydrolysis time but was not affected by the enzyme or acid pretreatment. The maximum sugar yield

∗ Corresponding authors. Emails: [email protected]; [email protected] This article was originally published with erroneous pagination. This version has been corrected. Please see Erratum (http://dx.doi.org/ 10.1080/09593330.2013.869394).

© 2013 Taylor & Francis

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from office paper was around 0.82 g of reducing sugar per gram of paper which is five times higher than the maximum sugar release from the newspaper substrate. Kim et al.[15] investigated the effects of surfactants in the pretreatment and/or enzymatic hydrolysis stages of newspaper hydrolysis. They reported a glucan conversion efficiency of 50%. Cellulosic ethanol production, however, requires a significant amount of processing to make the sugar monomers available for fermentation by microorganisms.[16] To break down cellulose and hemicelluloses polymers in lignocellulosic biomass to form individual sugar molecules which can be fermented into ethanol dilute or concentrated acid catalysed hydrolysis has been widely employed.[17,18] The interest in the use of H3 PO4 acid for the hydrolysis of lignocelluloics materials is that after neutralization with NaOH, it yields sodium phosphate which will remain in the hydrolysates and can subsequently be used as a nutrient by microorganisms in down-stream fermentation media, etc. Therefore, filtration is not required with the consequent advantage of improving the economics of the process; it is also friendly to the environment.[19] Dilute phosphoric acid, on hydrolysates from olive tree pruning, has shown hemicelluloses conversion rates of 77% with glucose and reducing sugar concentrations being observed as 89% of the hemicellulosic sugars contained in the raw material at conditions of 8% acid concentration at 90◦ C for 240 min.[20] Similarly, on hydrolysates from municipal bio-waste wood shavings, a xylose concentration of 17 g/100 g dry mass corresponding to a yield of 100% at the best identified conditions of 2.5 wt% H3 PO4 , 175◦ C and 10 min reaction time is shown.[19] The maximum yield of reducing sugars from grass clippings using dilute phosphoric acid was obtained at 7.5% (w/v) and 160◦ C, corresponding to 60% of the total sugars possible with a reaction time of 5 min.[21] Therefore, the aim of this work was to establish the optimum conditions for dilute acid hydrolysis of waste cellulosic biomass (newspaper) using an autoclave Parr reactor. Process parameters investigated include variation in reaction temperature (135–200◦ C) and acid catalyst concentration (2.5–10% w/v). The concentration levels of the monomers (mainly xylose and glucose) were studied. The main degradation products (e.g. acetic acid, furfural, and 5-hydroxymethylfurfural (HMF)) were also measured throughout to investigate their effect on sugar yield. The results of this study may benefit government and private sectors that are interested in the production of biofuel from waste materials.

2. Materials and methods 2.1. Experimental aim For the purposes of this research work the conditions to be varied were temperature and acid concentration. The raw material was hydrolysed at 135◦ C, 150◦ C, 175◦ C, and 200◦ C using phosphoric acid at 2.5%, 5%, 7.5%, and

Table 1.

Components of newspaper.

Component Cellulose Xylan Klason lignin Ashes Water Carbon Hydrogen Nitrogen Oxygen

Weight % (g/100 g dry mass) 32.4 ± 0.8 13.29 ± 0.7 22.98 ± 0.7 4.06 ± 0.1 27.3 ± 0.2 43.28 ± 0.1 5.16 ± 0.2 0.13 ± 0.08 51.4 ± 0.2

10% (w/w) acid concentrations. The raw material substrate being studied was newspaper. 2.2.

Newspaper

The compositional analysis of local newspaper sample on a dry weight basis (% w/w) was: cellulose 32.0; xylan 13.0; klason lignin 24.0; ashes 4.0; water 27.0; carbon 43.3%; hydrogen 5.2%; nitrogen 0.2%; oxygen 51.3%, and the calorific value was 17.90 MJ/kg (Table 1). 2.3. Analytical methods 2.3.1. Sample preparation Newspaper samples were cut in squares of 0.5 cm × 0.5 cm using a paper shredder and then a mixture of a known concentration of newspaper and water was blended for about 3 min until getting slurry. 2.3.2. Cellulose and hemicellulose analysis Three 0.3 g samples were weighed into three test tubes and to each is added 3 ml of 85% sulphuric acid that has been cooled to 15◦ C. The samples were stirred thoroughly before being placed in a water bath at 30◦ C. This temperature was maintained for 2 h, stirring the samples every 10 min. After a total time of 2 h the mixture was washed from the vial into an Erlenmeyer flask and made up to 89.11 g with distilled water. The dilute solution was autoclaved at 1.5 bar steam pressure and 121◦ C for 1 h. The sample was then cooled and vacuum filtered to remove un-reacted lignin.[22] The filtrate was then syringed through a 0.45 μm filter, before being analysed by high-performance liquid chromatography (HPLC). With 100% conversion assumed, the composition of glucose is recognized as cellulose and that of xylose, arabinose, and manose can be recognized as hemicellulose. 2.3.3. Other analyses Having hydrolysed the cellulose and hemicellulose components of the biomass, the composition of lignin can be determined quite easily. The process of vacuum filtering the samples results in the separation of the hydrolysate

Environmental Technology with the remaining solid depositing. This deposit is made up of mainly lignin and ash components. The glass filter crucibles which had been used in the vacuum filter were dried overnight in an oven at 110◦ C before having their weight recorded. They were then placed in a muffle furnace at 550◦ C for 3 h to burn off the remaining organic deposits. The weight was then recorded again.[22] The proportion of acid-insoluble residue, mainly lignin, can be calculated using Equation (1).[22] (1)

between them it was not possible to quantify the concentrations of each of these species. Herein we will group these three sugars together and refer to them as xylose throughout. The peak area was calculated using the software ChemStation from LC systems. The sugars produced in the hydrolysis reactions were identified using the retention times of standards sugars such as D-xylose (Alfa Aesar), D-galactose (Sigma), L-arabinose (Sigma), D-mannose (Alfa Aesar), D-glucose (BDH AnalaR), and, D-cellobiose (Sigma) and correlations coefficients for each standard was R2 > .99.

where W1 = weight of newspaper sample (g); W2 = weight of filter crucible after ignition in muffle furnace – ash sample (g); W3 = weight of filter crucible after vacuum filtration – lignin and ash (g); T110 = received sample conversion factor.

2.4.3. Potential monomer and yield calculations The potential concentration of each sugar, furfural, and HMF, P0 was calculated by assuming total conversion [25,26]:

Percentage of lignin =

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W2 − W3 × 100, W1 × T110

P0 = F × CPn0 , 2.3.3.1. Moisture analysis The moisture content of the newspaper sample was measured by weighing out a recorded amount of sample and placing it in an oven at 110◦ C until the dry weight of the sample is constant over a 2 h period. The sample was then cooled and its weight was recorded. Moisture content was determined according to the following equation [23]: MC = 1 −

dry weight . initial weight

(2)

2.3.3.2. Ash analysis The ash content was calculated by dividing the weight of the filter crucible, after it has been ignited in the muffle furnace W2 , by the initial weight of the sample W1 times the conversion factor T110 .[24] 2.4. Sugar analysis 2.4.1. Sample preparation The hydrolysates were filtered through a filtering crucible. The pH was measured and recorded using a pH meter ‘4330 Jenway’ and an electrode ‘Philips Harris’; the pH was adjusted to approximately 4.0 with calcium hydroxide, Ca(OH)2 . Samples were centrifuged at 13,000 rpm for 15 min in a Micro Centaur MSE centrifuge, and after, the sample was filtered with a cellulose nitrate membrane filter ‘Whatman’ of 0.45 μm pore size and 25 mm diameter. The samples were kept in the fridge at 4◦ C. 2.4.2. HPLC with refractive index detector An HPLC with refractive index detector ‘Agilent Technologies 1200 Series’ was used with a ‘Bio-Rad Aminex HPX-87H’ column for analysis of sugars and decomposition products, respectively. Samples were run at 40◦ C and eluted at 0.6 ml/min with 5 mM H2 SO4 . When using this column the peaks for xylose, mannose, and galactose eluted together. As the technique used could not differentiate

(3)

where P0 is the maximum concentration possible of monomer (g/100 g of dry mass); F is the stoichiometric factor of hydration of sugars during the hydrolysis (Fpentoses is 150/132 and Fhexoses is 180/162); also, F for dehydration of sugars into degradation products (Ffurfural is 96/132 and FHMF is 126/162). CPn0 is the composition of the raw material for the polysaccharide Pn (g/100 g dry mass). The yield of each sugar or degradation product was calculated as follows: Cmi , (4) Y = 100 × P0 where Y is the sugar yield or degradation product yield in %, Cmi is the concentration of product at time i in g/100 g dry mass, and P0 is the potential concentration of each product calculated using Equation (3). 2.5. Experimental procedure 2.5.1. Autoclave Parr reactor A 1-L continuously stirred batch reactor (Parr Instrument Company, USA) was employed for the experimental programme. The reactor operates at a temperature range of −10◦ C to 350◦ C up to 130 bar pressure. Operating conditions were modulated by a controller unit. The total contents of the reactor constitute 700 g of which 5% (w/w) will be the raw material newspaper. The newspaper samples were dried and milled to 16 mesh or 1 mm diameter particles. The remaining 95% (w/w) content of the reactor was made up of the dilute acid concentration. The acid concentration was not initially added to the reactor but instead is delivered through the acid reservoir during the initialization of the reaction. For acid concentrations of 2.5%, 5%, and 7.5% (w/w) this was made by preparing a 70 g sample made up of the 85% phosphoric acid required to achieve the desired acid concentration for the reaction and distilled water. The remaining distilled water required to achieve this dilution

A.M. Orozco et al.

(a)

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Results and discussion

3.1. Effect of temperature and acid concentration Diluted acid hydrolysis of newspaper was carried out at phosphoric acid concentration range of 2.5–10 wt% and temperatures between 150◦ C and 200◦ C.

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To initialize the reaction, the phosphoric acid must be delivered to the reaction vessel from the acid reservoir. The reservoir was first pressurized by opening the nitrogen valve, thus pressurizing it to 20 bar. The acid inlet valve was then opened, causing a pressure differential between the reservoir and the reaction vessel which allowed the acid to be delivered to the vessel. The pressure gauge of the vessel was monitored for any increase in pressure; once this was observed it indicated that all the acid had been delivered. At this point the inlet valve was closed and the stop watch started simultaneously. Sampling occurred at time intervals of 2, 4, 8, 15, 30, 45, 60, 75, and 90 min. A sample tube was secured to the sample line; the sample outlet valve was opened allowing a maximum of 5 ml of solution to be collected. The sample tube was cooled using cold water to further reduce the reaction rate of the solution by rapid cooling and then cleared using compressed air to prevent contamination from subsequent samples. Samples were sealed and placed in ice to reduce impact of further reaction. Although contamination of the solution by newspaper particles was severely reduced by the presence of the gauze mesh it was not eliminated; therefore purification by vacuum and syringe filtration was performed prior to HPLC analysis.

Yield (%)

2.5.2. Reaction procedure

3.1.1. Xylose formation Xylose formation reached a maximum value of 14 g/100 g dry mass corresponding to a yield of 94% at the best identified conditions of 2.5 wt% H3 PO4 at 135◦ C in 120 min reaction time; 2.5 wt% H3 PO4 at 150◦ C in 60 min reaction time; 2.5 wt% H3 PO4 at 175◦ C in 10 min reaction time, and 5 wt% H3 PO4 at 150◦ C in 5 min reaction time. The effect of reaction temperature on xylose formation is shown in Figure 1(a) at H3 PO4 concentration of 5 wt% (all graphs are not shown here). The lower the temperature the more xylose was produced. Xylose degradation was noticed in all reactions carried out at 175◦ C and 200◦ C, generating furfural. Its decomposition was more severe when the temperature was increased. At a temperature of 200◦ C and an acid concentration of 10 wt%, the xylose formed was very low. This is most likely due to the high reaction speed and temperature increasing the yield of xylose formation but also xylose degradation. The effect of acid concentration on xylose formation is shown in Figure 1(b) at 175◦ C (all graphs are not shown here), where the yield of xylose formation increases with the acid concentration except at the 10 wt% concentration. The same behaviour was observed at 135◦ C. At temperatures of 175◦ C and 200◦ C, the highest xylose production was generated at the lowest acid concentrations and reaction

Xylose (g/100 g dry mass)

was mixed with the newspaper and charged to the Parr reactor vessel. The sample tube was then fitted with a gauze mesh to restrict the solid sample from blocking it. The reactor was secured tightly by six bolts to maintain the operating pressure within the vessel during the reaction. The vessel was then attached with the heating jacket and the agitator impellor was connected to begin mixing. The sample line and acid reservoir were bolted tightly to the reactor. The nitrogen line was then attached to the acid reservoir. Finally, the thermocouple which provides feedback to the 4843 controller was inserted and the temperature set point was entered. The controller then ramped up the jacket heating to achieve and maintain the required operating temperature set point. Depending on the weather the temperature output required is 135◦ C, 150◦ C, 175◦ C, or 200◦ C; it took between 30 and 60 min to reach the desired temperature. The impellor was initiated at the same point as the jacket heating element and remained constant for all experiments at the controller maximum rpm rate of 632. This was to ensure that by the time the reaction commences the dispersion of newspaper sample will be constant throughout the vessel. Once the desired temperature set point was at steady state the reaction was commenced.

Xylose (g/100 g dry mass)

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0

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Figure 1. Effect of temperature at 5 wt% H3 PO4 (a) and H3 PO4 concentration at 175◦ C (b) on xylose formation in the dilute acid hydrolysis of newspaper.

Environmental Technology

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3.1.2. Glucose formation The maximum glucose formation was an average of 9 g/100 g dry mass corresponding to a yield of 26% at the best identified conditions of 2.5 wt% H3 PO4 , 200◦ C and 30 min reaction time, and at 5 wt% H3 PO4 , 200◦ C and 10 min reaction time. Due to its crystalline structure, it was not possible at the reaction conditions to obtain a higher yield of glucose, and mainly the glucose formed is due to the glucan presence in the hemicelluloses.[27] Phosphoric acid is not strong enough to break the bonds between the molecules of glucose. Glucose degradation was observed at the highest temperature (200◦ C). The effect of reaction temperature can be seen in Figure 2(a) at a 5 wt% H3 PO4 concentration and at different reaction temperatures. Glucose concentration increased with temperature. It can be seen that at 200◦ C the generation of glucose was always higher than the others temperatures.

Glucose (g/100 g dry mass)

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temperatures, demonstrating once again that hemicellulose is very easily hydrolysable. Acid concentration catalysed the xylose degradation to furfural. At the highest acid concentration (10 wt%) a very low xylose formation was detected (3 g/100 g dry mass). This is due, as explained before, to the reaction being very fast and also xylose degradation being catalysed by the acid.

100

Time (min) 2.5%

5%

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Figure 2. Effect of temperature at 5 wt% H3 PO4 (a) and H3 PO4 concentration at 175◦ C (b) on glucose formation in the dilute acid hydrolysis of newspaper.

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Glucose degradation to HMF was produced at 200◦ C at a very high rate (data are not shown here). In Figure 2(b), the effect of acid concentration on glucose formation at a fixed temperature, 175◦ C, is shown. When the reaction temperature increased, glucose formation increased. When the reaction temperature was 200◦ C, the yield of glucose formation was very low due to the sharp increase of glucose degradation to HMF. Glucose degradation is not as severe with reaction temperature and acid concentration as xylose during the time. It only showed degradation when the reaction conditions were severe as 10 wt% and/or temperature of 200◦ C. 3.1.3. Total sugars formation The maximum total sugars formation was an average of 13 g/100 g dry mass corresponding to a yield of 35%, of the total sugars available, at the best identified conditions of 2.5 wt% H3 PO4 , 135◦ C, and 90 min reaction time; 2.5 wt% H3 PO4 , 150◦ C, and 15 min reaction time; 2.5 wt% H3 PO4 , 175◦ C, and 5 min reaction time; 5 wt% H3 PO4 , 135◦ C, and 120 min reaction time; 5 wt% H3 PO4 , 150◦ C, and 30 min reaction time; and at 5 wt% H3 PO4 , 175◦ C, and 15 min reaction time. Despite almost all hemicellulose being converted to its monomers, the low yield is due to the maximum sugar being obtained at different times and also due to sugar degradation which was catalysed by both reaction temperature and acid concentration. Figure 3(a) and 3(b) shows the effect of reaction temperature and acid concentration on the total sugar formation. The higher the reaction temperature and acid concentration the more sugars were formed, but as mentioned before, sugars, especially xylose, degraded quicker due to the severity of the reaction conditions. 3.1.4. Total inhibitors During acid hydrolysis, sugar degradation is a prominent feature. HMF is formed from the dehydration of hexoses, and furfural is formed from the dehydration of pentoses. These by-products have an inhibiting effect on the rate of reaction during the fermentation process, which is required to convert these low value sugars into high-value biofuel products. The compounds damage the yeast and other microorganisms by slowing down their metabolism and reducing enzymatic activity. They enter the cell’s nucleus and attaching to replicating DNA and severely hinder reproduction and growth of the cell culture. Obtaining high yields of sugar during hydrolysis can be offset by the concentration of inhibitors within the sugar slurry. These along with acetic acid, formed during a number of secondary reactions, retard the fermentation of hydrolysates and may require further processing to remove or, possibly, dilute to reduce their effect on the efficiency of fermentation.[19] Figure 4 shows the acetic acid, furfural, and HMF formation at the reaction condition of 5 wt% H3 PO4 and 150◦ C.

A.M. Orozco et al.

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Figure 3. Effect of temperature at 2.5 wt% H3 PO4 (a) and H3 PO4 concentration at 150◦ C (b) on total sugar formation in the dilute acid hydrolysis of newspaper.

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Figure 5. Effect of temperature at 2.5 wt% H3 PO4 (a) and H3 PO4 concentration at 175◦ C (b) on total inhibitors formation in the dilute acid hydrolysis of newspaper.

2.4 Inhibitor (g/100 g dry mass)

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Figure 4. Inhibitors formation at the acid hydrolysis of newspaper using 5 wt% H3 PO4 and 150◦ C.

Acetic acid does not seem to be affected by reaction temperature or acid concentration; it is formed quickly until it reached a constant value of 1.4 g/100 g dry mass average. This was also reported by Rodríguez-Chong et al.[28] For the conditions at 200◦ C and 10 wt% H3 PO4 , a small decrease in the concentration of acetic acid was noticed. Furfural formation was clearly affected by the reaction temperature, where the higher the temperature the higher the furfural formation. When the temperature was fixed above 150◦ C and the acid concentration was increased, the furfural formation was also increased, being more evident at higher

temperatures. The maximum furfural formed was an average of 3 g/10 g dry mass, corresponding to a yield of 28%. This maximum yield was obtained at the best identified conditions of 2.5 wt% H3 PO4 , 175◦ C, and 90 min reaction time; 2.5 wt% H3 PO4 , 200◦ C, and 15 min reaction time; 5.0 wt% H3 PO4 , 175◦ C, and 60 min reaction time; 7.5 wt% H3 PO4 , 150◦ C, and 60 min reaction time and at 10 wt% H3 PO4 , 200◦ C, and 5 min reaction time. Furfural was also degraded to formic acid when the reactions were carried out at 175◦ C and 200◦ C; degradation was more severe at the higher temperatures. Acid concentration does not seem to have a strong effect on HMF formation at constant temperature. The degradation of HMF to levulinic acid is also affected by reaction temperature, the higher the temperature the higher the HMF degradation, especially when the temperature is 200◦ C. Figure 5(a) and 5(b) shows the effect of reaction temperature at 2.5 wt% H3 PO4 and 175◦ C, respectively. It is evident that reaction temperature has a profound influence on inhibitor reaction kinetics. As temperature increases, the concentration of inhibitors increases rapidly but also their degradation is catalysed by it. These inhibitors are broken down further into various degradation products, many of which are also inhibitors. The degradation reaction which occurs during this reaction reduces HMF to levulinic

Environmental Technology acid, which is itself an inhibitor. Therefore, it appears as though the total inhibitors have been diminished as the reaction nears completion. It must be noted that the inhibitors degraded are almost entirely converted to other less-potent inhibitors which have not been studied as they fall outside the scope of this work. With respect to the acid concentration, the formation of inhibitors was not affected but the degradation did.

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3.2.

Kinetic study for the dilute acid hydrolysis of newspaper Several reactions take place during the hydrolysis of sugar polymers with dilute acids. The substrate is in the solid phase and the catalyst in the liquid phase. Up to the present, the Saeman model [29] and the two-fraction model [30] have been extensively accepted as good ways of describing the kinetics of cellulose hydrolysis. The Saeman model predicts the sugar concentration as a function of time: M = M0 e−K2 t + P0

K1 (e−K1 t − e−K2 t ), K2 − K 1

(5)

where M and P0 are the concentrations of monomer and polymer expressed in (g/100 g dry mass); M0 is the initial monomer concentration; K1 (min−1 ) is the rate of release of sugar; K2 (min−1 ) is the rate for sugar decomposition; and t is the time (min). The two-fraction model proposes that xylose is released by the hydrolysis of xylan and it further decomposed to other groups under severe conditions. Both of the release and decomposition are generally considered to be first-order reactions.[31] The rate equation that dominates the kinetics of xylan hydrolysis and thus the two-fraction model can be

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written as follows [31]: M = α.P0

K1 (e−K1 t − e−K2 t ), K2 − K 1

(6)

where α is the mass fraction of the susceptible polymer in the raw material (g/g). 3.2.1. Kinetics of xylose formation Table 2 shows the kinetic constants and correlation coefficient R2 for xylose formed during the hydrolysis of newspaper. The values of R2 are higher than .9 for almost all of the reactions and both models with the exception of the reactions carried out at 10 wt% and 135◦ C, where the value of R2 was .864 for the fitting of the Saeman model. In general, it can be said that both models gave good fitting of the experimental data. Figure 6 shows the values of K1 and K2 for both models graphically. It can be seen that K1 increased when the temperature increased. Also, when the acid concentration increased, a slight increase in K1 is observed. The values of α in the two-fraction model vary from 0.405 to 1.000. These values are within the range reported in literature for this kind of material. Also, it is evident that the α values were affected by xylose degradation due to the severity of the conditions and the fast reaction rate of xylose formation made it impossible to measure the data for calculating K1 , as explained before. As it can be seen, α values are 1 in all the reactions carried out at 2.5–5.0 wt%. Values of K1 obtained for the Saeman model are lower than the values obtained when fitting the experimental data to the two-fraction model. This is due to the fraction value, α, which affects the value of K1 (kinetic constant for sugars formation). The α value depends on the fraction that is easy to hydrolyse. The change in the values of K2 (kinetic

Table 2. Kinetic parameters of the fitting of the Saeman model and the two-fraction model for xylose formation on the dilute acid hydrolysis of newspaper using H3 PO4 . Operating conditions H3 PO4 (wt%) 2.5 2.5 2.5 2.5 5.0 5.0 5.0 5.0 7.5 7.5 7.5 7.5 10 10 10 10

The Saeman model

The two-fraction model

Temperature (◦ C)

K1 (min−1 )

K2 (min−1 )

R2

K1 /K2

α (g/g)

K1 (min−1 )

K2 (min−1 )

R2

K1 /K2

135 150 175 200 135 150 175 200 135 150 175 200 135 150 175 200

0.027 0.063 0.324 1.413 0.039 0.123 0.473 1.116 0.043 0.079 0.344 2.000 0.039 0.073 0.386 0.464

0.000 0.000 0.011 0.067 0.001 0.001 0.020 0.175 0.003 0.004 0.088 0.276 0.008 0.017 0.171 1.230

.993 .934 .996 .998 .968 .982 .997 .990 .963 .982 .967 .999 .899 .864 .970 .998

– – 29.5 21.1 39.0 123 23.7 6.4 14.3 19.8 3.9 7.2 4.9 4.3 2.3 0.38

1.000 1.000 1.000 1.000 1.000 1.000 1.000 0.903 0.856 1.000 0.676 0.997 0.649 0.612 0.665 0.405

0.027 0.063 0.324 1.413 0.039 0.123 0.473 5.748 0.054 0.079 0.762 5.023 0.080 0.204 0.796 15.38

0.000 0.000 0.011 0.067 0.001 0.001 0.020 0.140 0.001 0.004 0.044 0.276 0.002 0.007 0.097 0.397

.993 .934 .996 .998 .968 .982 .997 .991 .969 .982 .998 .999 .968 .988 .988 .997

– – 29.5 21.1 39.0 123 23.7 41.1 54.0 19.8 17.3 18.2 40.0 29.1 8.2 38.7

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Figure 6.

Variation of K1 and K2 for xylose formation on the dilute acid hydrolysis of newspaper using H3 PO4 .

Table 3. H3 PO4 .

Fitting the Saeman model and the two-fraction model for glucose formation on the dilute acid hydrolysis of newspaper using

Operating conditions H3 PO4 (wt%) 2.5 2.5 2.5 2.5 5.0 5.0 5.0 5.0 7.5 7.5 7.5 7.5 10 10 10 10

The Saeman model

The two-fraction model

Temperature (◦ C)

K1 (min−1 )

K2 (min−1 )

R2

K1 /K2

α (g/g)

K1 (min−1 )

K2 (min−1 )

R2

K1 /K2

135 150 175 200 135 150 175 200 135 150 175 200 135 150 175 200

0.001 0.003 0.007 0.027 0.001 0.004 0.010 0.050 0.001 0.002 0.006 0.059 0.002 0.004 0.005 0.080

0.004 0.021 0.022 0.041 0.008 0.022 0.026 0.084 0.003 0.014 0.039 0.216 0.028 0.037 0.063 0.600

.981 .965 .919 .969 .993 .949 .937 .986 .982 .982 .964 .969 .891 .865 .959 .960

0.25 0.14 0.32 0.66 0.13 0.18 0.38 0.60 0.33 0.14 0.15 0.27 0.07 0.11 0.08 0.13

0.133 0.301 0.181 0.334 0.263 0.501 1.000 1.000 0.198 0.380 0.141 0.462 1.000 1.000 1.000 0.231

0.005 0.009 0.060 0.134 0.004 0.007 0.010 0.050 0.003 0.006 0.042 0.113 0.002 0.004 0.005 0.176

0.000 0.010 0.000 0.011 0.005 0.016 0.026 0.084 0.000 0.009 0.005 0.113 0.028 0.037 0.063 0.176

.981 .962 .959 .945 .994 .942 .937 .986 .982 .982 .965 .979 .891 .865 .959 .900

– 0.39 – 12.2 0.8 0.44 0.38 0.60 – 0.67 8.4 1.0 0.07 0.11 0.08 1.0

constant for sugars degradation) obtained with both models is not significant when α is equal to 1. When α is a value other than 1, K2 values for the two-fraction model are smaller than the values for the Saeman model, due to mathematical regression and the influence of this parameter. 3.2.2. Kinetics of glucose formation Table 3 shows the kinetics and correlation coefficients R2 for glucose formed during the hydrolysis of newspaper. The values of R2 are higher than .9 for almost all of the reactions,

except for the reaction carried out at 10 wt% and 150◦ C, which was .865 for both models. Figure 7 shows the values of K1 and K2 for both models. It can be seen that K1 increases when the temperature increases for both fitting methods. For the Saeman model, when the reaction temperature is 200◦ C, K1 increased with the increase in acid concentration. This is in contrast with the other temperatures, where K1 remains constant when the acid concentration changes. In the twofraction model, K1 is not affected by the acid concentration. The values of α in the two-fraction model vary from 0.133 to 1.000. These values are within the values reported in

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Environmental Technology

Figure 7.

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Variation of K1 and K2 for glucose formation on the dilute acid hydrolysis of newspaper using H3 PO4 .

Table 4. Fitting the Saeman model for acetic acid formation on the dilute acid hydrolysis of acetic acid using H3 PO4 . H3 PO4 (wt%) 2.5 2.5 2.5 2.5 5.0 5.0 5.0 5.0 7.5 7.5 7.5 7.5 10 10 10 10

Temperature (◦ C)

Aco (g/100 g dry mass)

K1 (min−1 )

R2

135 150 175 200 135 150 175 200 135 150 175 200 135 150 175 200

1.489 1.326 1.785 1.707 1.239 1.331 1.456 1.221 1.382 1.564 1.151 1.700 1.329 1.291 0.845 0.960

0.058 0.116 0.166 0.198 0.120 0.212 0.388 0.582 0.104 0.167 0.503 0.300 0.254 0.604 0.146 1.603

.993 .962 .930 .886 .909 .964 .976 .011 .897 .906 .198 .910 .857 .627 .868 .044

literature for this kind of material. Also, it is worth to note that the α values were very small for the reactions carried out at 2.5 wt% of acid and a little higher with the concentration of 5 wt%. For the remaining acid concentration the value of α was 1. This indicates that the α parameter depends on the acid concentration. The α value obtained at 200◦ C and 10 wt% was very low, 0.231, due to the reaction rate of glucose formation being very high and also degradation, as explained previously. Comparing the values of K1 , those obtained for the Saeman model are lower than for the two-fraction model when

Figure 8. Variation of K1 for acetic acid formation on the dilute acid hydrolysis of newspaper using H3 PO4 .

the acid concentrations were 2.5 wt% and 5 wt%. This is due to the fraction value, α. Only a small variation is observed in the values of K2 obtained with both models. When the values of K2 are compared, the values are smaller for the two-faction model than for the Saeman model when the values of α are different than 1. This is in contrast to the K1 values. 3.2.3. Kinetics of acetic acid formation The values for the potential concentration of acetic acid is in the range of 0.845–1.785 g/100 g dry mass; these values are lower than those reported for the hydrolysis of sorghum straw using phosphoric acid [27] which were between 0.94 and 1.16 g/l. In Table 4, the values of the potential concentration of acetic acid and also the kinetic constant K1 and the

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correlation coefficient R2 are shown. R2 values are higher than .9 for almost all of the reactions, except for the reactions carried out at 5.0 wt% at 200◦ C, 7.5 wt% at 175◦ C, and 10 wt% at 200◦ C. Figure 8 shows the values of K1 and K2 for both models. It can be seen that K1 increases when the acid concentration increases for the formation of acetic acid. The same behaviour is observed for the increase in temperature, with exception of the conditions of 7.5 wt% and 10 wt% at 200◦ C, where the rate of acetic acid formation decreases. This behaviour of the acetic acid has not been

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Table 5.

2.5 2.5 2.5 2.5 5.0 5.0 5.0 5.0 7.5 7.5 7.5 7.5 10 10 10 10

Figure 9.

3.2.4. Furfural and HMF formation The values of K1 , K2 , and R2 for furfural and HMF formation are shown in Table 5. The model gave a good fitting since all R2 values are higher than .9 for both inhibitors. It can be seen from Figure 9 that K1 for furfural and HMF formation increases with the acid concentration, and also

Fitting the Saeman model for furfural and HFM formation on the dilute acid hydrolysis of newspaper using H3 PO4 .

Operating conditions H3 PO4 (wt%)

reported, in that acetic acid is being affected by temperature or acid concentration.

Furfural

HFM

Temperature (◦ C)

K1 (min−1 )

K2 (min−1 )

R2

K1 /K2

K1 (min−1 )

K2 (min−1 )

R2

K1 /K2

135 150 175 200 135 150 175 200 135 150 175 200 135 150 175 200

0.000 0.001 0.009 0.032 0.001 0.002 0.015 0.040 0.001 0.003 0.024 0.109 0.001 0.003 0.032 0.137

0.000 0.000 0.012 0.045 0.000 0.000 0.020 0.058 0.000 0.000 0.028 0.195 0.000 0.000 0.040 0.220

.988 .979 .997 .902 .961 .989 .975 .971 .926 .982 .918 .946 .946 .997 .986 .953

– – 0.75 0.71 – – 0.75 0.67 – – 0.86 0.56 – – 0.80 0.62

0.000 0.000 0.003 0.016 0.000 0.001 0.007 0.053 0.000 0.001 0.008 0.037 0.000 0.001 0.012 0.058

0.000 0.000 0.030 0.075 0.000 0.008 0.061 0.227 0.000 0.016 0.089 0.229 0.017 0.021 0.113 0.291

.954 .990 .990 .964 .962 .995 .988 .880 .943 .947 .921 .932 .956 .984 .953 .985

– – 0.10 0.21 – 0.13 0.11 0.23 – 0.06 0.09 0.16 – 0.05 0.11 0.20

Variation of K1 and K2 for furfural and HFM formation on the dilute acid hydrolysis of newspaper using H3 PO4 .

Environmental Technology

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with the increase in temperature. K1 for furfural is higher than K1 for HMF, which is most likely due to the high rate and high yield of xylose formation. K1 is zero at 135◦ C for HMF verifying that glucose degradation does not occur at this temperature and the yield of glucose is very low. Regarding the degradation of the inhibitors, the values of K2 for furfural are null for the temperatures of 135–150◦ C at any acid concentration, and for HMF at 135◦ C from 2.5 to 7.5 wt% acid concentration. The degradation of HMF to levulinic acid is faster than the degradation of furfural to Formic acid. Also, the degradation rate, K2 , is higher than the formation rate for both inhibitors when degradation is observed. 4. Conclusions The dilute acid hydrolysis of newspaper using H3 PO4 gave a good performance for sugar production. A very good yield was achieved (94%) at the best identified conditions of 2.5 wt% H3 PO4 , 135◦ C, 120 min reaction time, and at 2.5 wt% H3 PO4 , 175◦ C, and 10 min reaction time. However, for glucose, an average yield of 26% was obtained at 2.5 wt% H3 PO4 , 200◦ C, and 30 min reaction time. The maximum yield for the total sugars formation was an average of 13 g/100 g dry mass corresponding to 35% of the total sugars available. Furfural formation was affected by the reaction temperature with a maximum yield of 28%. In taking all the investigated experimental parameters into account, it can be concluded that ‘milder’ reaction conditions (low acid concentration and reaction temperature) within our system can provide xylose yields approaching 94%, albeit at longer reaction times. Acknowledgements The authors are very grateful for the support of QUESTOR and CenTACat research centres at Queen’s University Belfast, UK.

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