KOH: An Efficient Heterogeneous Catalyst for

Jun 5, 2019 - Hydroxyl groups in hemicellulose can react with sodium ... All chemicals were purchased from Sigma-Aldrich and Fluka and used as receive...
0 downloads 0 Views 6MB Size
Download PDF
Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 11680−11688

pubs.acs.org/IECR

Natural Clinoptilolite/KOH: An Efficient Heterogeneous Catalyst for Carboxymethylation of Hemicellulose Mohammad A. Khalilzadeh,†,‡ Hasan Sadeghifar,‡ and Richard Venditti*,‡ †

Department of Chemistry, Qaemshahr Branch, Islamic Azad University, Qaemshahr, Iran Department of Forest Biomaterials, College of Natural Resources, North Carolina State University, Raleigh, North Carolina 27607, United States

Downloaded via GUILFORD COLG on July 19, 2019 at 18:35:48 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

ABSTRACT: Potassium hydroxide supported on natural zeolite clinoptilolite (CP) is found to be an efficient catalyst for the carboxymethylation of hemicellulose. Adsorption and dissociation of KOH on the surface of CP were investigated by pH measurement and theoretical modeling analysis using density functional theory. Indeed, the adsorption of KOH on the surface of the CP porous structure provides a cheap solid catalyst with increased basicity. Different ratios of KOH:CP were examined, and the 1:2 ratio gave the best results with almost two times higher efficiency than standard KOH solution, even with shorter reaction time and lower temperature. The carboxymethylated sample with KOH:CP (1:2) indicated less degradation than KOH as catalyst investigated by viscosity measurements. Thermal analysis of the original and modified hemicellulose indicated that the degree of substitutions of the sample calculated through thermal analysis was close to the results from the titration method.

1. INTRODUCTION

Primary and secondary hydroxyl groups are the main functional reactive sites in the hemicellulose. Hydroxyl groups in hemicellulose can react with sodium monochloroacetate under alkali condition and convert to carboxylic groups using water or water/organic solvent mixtures.11−15 Many conventional methods have been applied for hemicellulose carboxymethylation using standard reagents to create proper degree of substitutions (DS). However, conventional methods indicated several disadvantages including excessive base consumption, depolymerization of hemicellulose12,13 long reaction times, and high temperature for reaction.11 Therefore, introducing a method with higher reaction efficiency, less base consumption, and more protection of the polymer structure is important for hemicellulose carboxymethylation. Using assisted ultrasonic and microwave techniques with conventional reaction conditions has been reported to improve reaction efficiency.16 Recently, we developed a simple catalytic system using potassium fluoride impregnated on clinoptilolite (KF:CP) which indicated various organic transformations.17,18 The KF:CP system demonstrated more basicity compared to potassium fluoride. This system, similar to other heterogeneous catalysts such as SiO2, Al2O3, clay, and zeolite, indicated greater reaction selectivity, enhanced reaction rates, and

Chemical modification is one of the most important and practical methods for natural polymers to increase their potential applications.1 Partial hydrolysis, graft polymerization, oxidation, reduction, etherification, and esterification of hydroxyl groups are commonly reported as chemical modifications.2 Carboxymethylation, a process to convert hydroxyl group into a carboxymethyl group, is one of these key methods for the natural biopolymers modification. The modification improves biopolymers solubility in water and their potential applications.3,4 Hemicellulose is an important carbohydrate polymer with abundance second only to cellulose in the wood structure and nonwoody plants. Hemicellulose is a byproduct of the pulping process in the pulp and paper industries. It is usually combusted with lignin as part of the black liquor in the pulping recovery process.4 In recent modified pulping processes, hemicellulose can be recovered using presteaming to produce a value-added product. Depending on the plant source, the chemical structure of hemicelluloses contains different types of sugars including glucose, xylose, mannose, arabinose, and galactose with partial uronic acid functional group.5 The proportions and connections of the sugars in the polymer structure are different in various plant sources. Chemical modifications can improve the hemicellulose potential for various applications like hydrogels preparation,6 food additives and emulsifiers,7 binders in paper making,8 wound dressings,9 and biobased fuels.10 © 2019 American Chemical Society

Received: Revised: Accepted: Published: 11680

April 25, 2019 May 28, 2019 June 5, 2019 June 5, 2019 DOI: 10.1021/acs.iecr.9b02239 Ind. Eng. Chem. Res. 2019, 58, 11680−11688

Article

Industrial & Engineering Chemistry Research

distilled water (50 mL) and mixed with 9 g of clinoptilolite. The mixture was stirred for 1 h. The water was evaporated at 60 °C using a vacuum oven for 30 h. The dried catalyst was powdered using a mortar and stored in a desiccator until use. 2.4. Hemicellulose Carboxymethylation. A mixture of extracted hemicellulose (0.50 g, ∼3.5 mmol sugar) and 0.63 g of KOH:CP(1:2) were added into a 50 mL two-necked bottle. Ten milliters of isopropanol/water (25:75, v/v) was added, and the mixture was stirred at room temperature for 5 min. Then, 0.51 g (4.375 mmol) of sodium monochloroacetate (MCA) was added and heated at 50 °C for 30 min. The reaction mixture was first filtered to remove KOH:CP catalyst, and the filtrate was neutralized with diluted acetic acid. Modified hemicellulose solution in water was precipitated with the addition of 80% methanol (v/v) solution and washed two times with 95% ethanol. The product was first air-dried for 12 h and then further dried at 45 °C in a vacuum oven for 24 h. 2.5. Determination of Degree of Substitution. The substitution degree of carboxymethyl hemicellulose (CMHC) was determined using a back titrimetric procedure according to a published method.31,32 CMHC (1.0 g) was dispersed in acetone and 6 M HCl and stirred for 30 min. The acid transformed the sodium form of the carboxyl group to the hydrogen form (H-CMHC). H-CMHC first was washed with 80% (v/v) methanol and after drying was dissolved in 20 mL of 0.2 M NaOH solution, and then 50 mL of DI water was added. Then, the solution was titrated using 0.05 M HCl, and the volume of consumed HCl was measured using a phenolphthalein color change indicator. The number of moles of carboxymethyl groups (CCOOH) was calculated with the following equation:

environmental compatibility with less base consumption in etherification reactions.17,18 Clinoptilolite is one of the most abundant and cheapest natural zeolites which can absorb various cations such as cesium, potassium, and ammonium.19−21 Clinoptilolite presents a negative charge due to the interaction of aluminum tetrahedra (Al3+) and the bridging oxygens (O2−), which are counterbalanced by cations.21 The affinity of cation absorption by clinoptilolite has been reported,22 in the order of23 Cs+ > Rb+ > K+ > NH4 + > Ba 2 + > Sr 2 + > Na + > Ca 2 + > Fe3 + > Al3 + > Mg 2 + > Li+

In this research, we utilized natural clinoptilolite as a heterogeneous catalyst for carbohydrate modifications. Zeolites have high adsorption and ion-exchange capacity simultaneously due to their porous and chemical structure. To confirm the positive effect of CP on KOH basicity and improved catalyst efficiency, we measured amounts of KOH attached to the surface of CP using pH measurements and also used a computational model study with Density Functional Theory (DFT). DFT has recently garnered much attention as a strong method to predict the electronic structure of systems upon interaction.24−28 The DFT study provides a deeper understanding of the structural and electronic properties of the clinoptilolite zeolite upon KOH interaction and confirms the effect of CP on the increased basicity of the catalyst. The prepared cocatalyst of potassium hydroxide supported on the natural zeolite clinoptilolite with various ratios of KOH:CP was then demonstrated to have improved efficiency over KOH alone for the carboxymethylation of hemicellulose extracted from hardwood.

nCOOH =

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. All chemicals were purchased from Sigma-Aldrich and Fluka and used as received. The natural clinoptilolite (CP) sourced from the Semnan region in northeast Iran was purchased from Afrand Touska Company (Isfahan, Iran). A Vector 22- Bruker FT-IR was used for Fourier transform infrared (FT-IR) in attenuated total reflectance (ATR) in the range of 400−4000 cm−1 at room temperature. Thermogravimetric analysis (TGA) was carried out on a Mettler Toledo TGA instrument using N2 and O2 as carrier gas with a temperature ramp of 10 °C/min from room temperature to 650 °C. Samples viscosities in water were measured using a Brookfield viscometer (DC-II + Pro) using spindle #61 at 22 °C at different speeds. 2.2. Isolation of Hemicelluloses. Hemicellulose was extracted from dried hardwood bleached pulp of a southeast U.S. source. The pulp was soaked in 10% KOH solution (1:20 pulp to liquor ratio) for 10 h at 50 °C. The filtrate was first neutralized with glacial acetic acid and then precipitated using 70% ethanol. The precipitated hemicellulose was washed with pure ethanol and dried in a vacuum oven at 45 °C.29 The sugar composition of the extracted hemicellulose was evaluated by HPLC analysis (Agilent 1200, Agilent, Santa Clara, CA) equipped with an Agilent guard column, degasser, pump, and refractive index detector (RID), following a modified NREL Laboratory Analytical Procedure.30 2.3. Preparation of KOH:CP Catalyst. The KOH:CP catalyst was prepared according to our previously reported procedure.17 To prepare different mixture ratios of KOH:clinoptilolote, 3.0, 4.5, and 9.0 g of KOH were dissolved in

Vb − V × C HCl 1000

(1)

where Vb is the average volume of HCl used to achieve color change in the blank (in mL); V is the volume of HCl used to achieve color change in titration of the sample (in mL); and CHCl is the concentration (molarity) of the HCl solution in mol/L. The DS was calculated using following equation: DS =

140 × nCOOH mds − 58 × nCOOH

(2)

where 140 g/mol was used as the average molar mass of the hemicellulose according to the HPLC analysis that indicated 70% C5 sugar (130 g/mol) and 30% C6 sugar (160 g/mol) composition; nCOOH (in mol) is the amount of COOH in the sample (calculated above); 58 g/mol is the net increase in the mass of a sugar unit for each carboxymethyl group substituted; and mds (in g) is the mass of dry sample that was used for the test 2.6. Computation Method. The reacted and unreacted clinoptilolite zeolite optimizations were studied with the Omega b97xd (ωB97XD) density functional theory using 631G level of theory mode as implemented in the Gaussian 09 suite of the program.33 To consider the dispersion forces, all parameters were considered using ωB97XD.34 The adsorption energy (Ead, kJ/mol) of KOH on clinoptilolite zeolite is calculated with the following equation: Ead = ECZ−KOH − (ECZ + E KOH) 11681

(3)

DOI: 10.1021/acs.iecr.9b02239 Ind. Eng. Chem. Res. 2019, 58, 11680−11688

Article

Industrial & Engineering Chemistry Research

Figure 1. Effect of KOH:CP ratio on the released pH and amount of KOH bonded on the surface of the CP (0.5 g of KOH in all samples). (pH of CP alone in water was 7.5−8.)

Figure 2. Top (left) and side (right) views of the interaction part of clinoptilolite zeolite: optimized structure in water solution at the 6-31G/ ωb97xd level of theory.

where ECZ−KOH is the total energy of adsorbed KOH on the surface of the clinoptilolite zeolite, and ECZ and EKOH are the total unreacted energies of clinoptilolite zeolite and KOH, respectively. For systems with one dimension that is small such as clinoptilolite zeolite, the basis set/functional 6-31G/wb97xd provides good response with excellent accuracy.35

were measured and compared with a control solution (just KOH). Figure 1 indicates the effect of KOH:CP ratio on the released pH and amount of KOH bonded on the surface of the CP. The results indicate a strong correlation between the amount of clinoptilolite and KOH absorption or released pH. With increasing the ratio of CP, more KOH were bonded on the surface of the CP, and this bond is strong enough to prevent KOH releasing in the solution. In the catalyst with ratio of 1:1 KOH:CP, around 58% of the KOH remained on the CP surface and the rest was released. With increasing the ratio of KOH:CP up to 3, only 5.6% of the KOH was released and 94.4% of the KOH remained on the CP surface. This is important since the bonded KOH on the surface of the CP has a higher basicity than soluble KOH in the water and can perform more effective catalyst activities. 3.2. Computational Studies. To confirm the effect of CP on KOH dissociation and an associated increase in its basicity, a computational modeling study was carried out. A simplified structure of the real zeolite was used to avoid the complexities of the system. In this model of clinoptilolite zeolite, the element aluminum (Al) is surrounded by four oxygen (O) atoms where each atom is connected to a silicon atom. The terminal oxygen atoms are capped by hydrogen atoms to neutralize their electric charges. To have a zeolite with neutral

3. RESULTS AND DISCUSSION 3.1. KOH:CP Catalyst Preparation, Performance, and Analysis. The KOH:CP catalyst was prepared according to a previously reported procedure.17 CP is a naturally occurring zeolite mineral with an open aluminosilicate framework structure. It has high internal surface area and indicated broad applications in chemistry and other industries. CP has high cation exchange capacity especially for potassium cation. It is believed that CP can absorb the metal ion of a base like potassium strongly and weaken the bond between the hydroxyl group and the metal.17 The weaker bond between the anion and the metal enables the anion to act as a more effective base for reaction. To demonstrate the effect of clinoptilolite on KOH base and prove its effect of stronger basicity of the final catalyst, experimental and calculation modeling methods were carried out. Three ratios of KOH:CP catalyst were prepared (weight ratio of 1:1, 1:2, and 1:3) with a constant amount of KOH in each sample (0.5 g). The pH values of the prepared catalysts 11682

DOI: 10.1021/acs.iecr.9b02239 Ind. Eng. Chem. Res. 2019, 58, 11680−11688

Article

Industrial & Engineering Chemistry Research charge, sodium ion (Na+) was placed near the Al atom to neutralize the overall charge of the zeolite (Figure 2). For an understanding of the KOH dissociation on the zeolite, a unit of KOH was placed in different positions relative to the surface of the zeolite, and the system was then permitted to be fully optimized (minimum energy) under the aforementioned level of theory. Despite having different initial positions as inputs, the output of all of the systems was the same, indicating that the system is finding a single preferred configuration, the most energetically favorable mode during the optimization. This output is known as the relaxed structure. Figure 3 shows the relaxed structure of CP with

This charge changes to the less positive value (+1.522 e) upon interaction with KOH. This is because the oxygen of the KOH, which is rich in electron density, shares some of its charges toward the Al atom of the zeolite, resulting in a decrease in its negative charge from −1.096 e for the isolated KOH to −0.926 e upon interaction. This result is in accordance with the outcome of energy adsorption that confirms the case of chemisorption. 3.3. Investigation of Surface Morphology. The surface morphologies of clinoptilolite zeolite and prepared catalyst, KOH:CP(1:2), were studied using scanning electron microscopy (SEM). The SEM images of CP in Figure 5A and 5B are indicative of a porous spongy structure. The SEM image of the prepared catalyst containing CP and KOH is shown in Figure 5C and 5D. In comparison to the SEM image of CP, the KOH:CP(1:2) image has lower porosity with interconnected rooms inside which is due to the insertion of the K cation into the porous clinoptilolite. These observations are in agreement with pH measurements and computational studies which showed the higher basicity in the reaction media. This can be related to the existence of the K cation in the pores of the clinoptilolite structure and the decreasing interaction between K cation and hydroxide anion, which increases the basicity of the reaction media. Creating negative charges on the CP surface through loading KOH can change the electrostatic repulsion on the CP surface and alter its structure.36 3.4. Hemicellulose Extraction and Its Sugar Analysis. HPLC analysis of the purified hemicellulose indicated a sugar composition including xylose (66.07 ± 1.1%), glucose (6.66 ± 0.3%), arabinose (7.0 ± 0.6%), and mannose (20.27 ± 0. 8%). Xylose and arabinose are five-carbon sugars with the molecular weight of ∼130 g/mol and contain two secondary hydroxyl groups for each sugar unit. Glucose and mannose are sixcarbon sugars with molecular weight around 160 g/mol and have three hydroxyl groups in each sugar unit (two secondary and one primary). Considering the weighted average of sugars in the extracted hemicellulose, the average molecular weight of the sugars in the extracted hemicellulose is around 140 g/mol and has a total hydroxyl groups per sugar unit around 2.3 OH/ sugar. 3.5. Optimization of Hemicellulose Carboxymethylation Reaction Using KOH:CP Catalyst. The main function of the water in the etherification reaction is the dissolution of hemicellulose, sodium monochloroacetate (SMCA), and sodium hydroxide. However, to increase reaction basicity, an alcohol such as isopropanol is needed. The proposed reaction

Figure 3. Tube type (left) and ball and bond type (right) of the relaxed structure of adsorbed KOH onto clinoptilolite zeolite: optimized structure in water solution at the 6-31G/ωb97xd level of theory.

KOH in a tube and ball representation and a bond type representation. The model indicates that the Al atom forms new bonds. The oxygen of KOH is positioned close to the Al of the zeolite whereas the potassium of KOH tends to be adsorbed onto the neighboring O atom bonded to the Al. Based on the model calculations, the bond length of K−O in an isolated KOH in water (2.38 Å) was increased when KOH was absorbed on the surface of the zeolite (2.64 Å), an approximate 11% increase. The value of adsorption energy (Ead) is calculated to be ∼−309 kJ/mol, which is an indication of chemisorption. However, the bond length of O−H in KOH was not significantly changed (0.97 Å) after interaction with zeolite (0.96 Å). This result supports our claim that the zeolite can enhance the OH− catalytic activity by capturing its K ion. This indicates an increase in the basicity of the KOH. The charge distribution of zeolite before and after interaction with KOH is indicated in Figure 4. It can be concluded that all charge allocations of atoms are changed upon the interaction. The charge of the Al atom in the unreacted zeolite is +1.575 e, indicating electron deficiency.

Figure 4. Mullikan charge distribution of an isolated clinoptilolite zeolite (A), isolated KOH (B), and interacted clinoptilolite zeolite with KOH (C). Calculations are carried out in water solution at the 6-31G/ωb97xd level of theory. 11683

DOI: 10.1021/acs.iecr.9b02239 Ind. Eng. Chem. Res. 2019, 58, 11680−11688

Article

Industrial & Engineering Chemistry Research

Figure 5. SEM images of (A and B) CP and (C and D) KOH:CP(1:2).

scheme of carboxymethylation in the presence of CP is the following: CP + K+−OH → CP−K:−OH

Table 1. Effect of Various Bases Utilized on the Degree of Substitution for the Carboxymethylation of Hemicellulosea

(4)

CP−K:−OH + (CH3)2 CHOH V CP−K:−OCH(CH3)2 + H 2O

(5)

Hem−OH + CP−K:Base → Hem−O−:CP−K + Base−H (6)

Hem−O−:CP−K + ClCH 2COONa → Hem−OCH 2COONa + CP + KCl

(7)

In addition, there is also a reaction byproduct (sodium glycolate):

entry

catalyst

1 2 3 4 5 6

hemicellulose control sample KOH KOH:CP (1:1) KOH:CP(1:2) KOH:CP (1:3)

DS 0.02 0.02 0.12 0.17 0.33 0.21

± ± ± ± ± ±

0.01 0.01 0.02 0.01 0.02 0.03

a

Measurements performed in triplicate, and standard deviation is indicated.

KOH + ClCH 2COONa → HOCH 2COONa + KCl (8)

Potassium hydroxide supported on clinoptilolite in all ratios indicated better results than the standard KOH solution. Among the catalysts with different ratios of KOH:CP, the ratio of 1:2 indicated the highest DS (0.33). Figure 1 indicates that in the ratio of 1:2 KOH:CP around 83% of the KOH remains on the CP surface and 17% is released after adding the catalyst into water. The KOH bonded on KOH:CP ratios of 1:1 and 2:3 are 58.6% and 94.4%, respectively. It seems that having a balanced amount of KOH in the solution and on the catalyst surface is a key parameter for better reaction performance. At low ratio of KOH:CP, almost all KOH is released into the water and behaves the same as the standard solution of KOH. At the high ratio KOH:CP (1:3), very little KOH is in the water, and almost all of the KOH is attached to catalyst surface. At a very high ratio of KOH:CP, almost all reaction happens through physical contact of hemicellulose with CP surface which reduces the reaction efficiency. Generally having a

Sodium glycolate is a nonactive side product that increases the chemical consumption and has negative effect on reaction performance and DS. Due to weaker nucleophilicity of alkoxide ion adsorbed on CP ((CH3)2CHO−:CP−K) than the hydroxide ion, using an alcohol rather than water would reduce the side reaction.37 However, due to the insolubility of hemicellulose in high concentration of alcohols, only a mixture of water and alcohol is an option to reduce side reaction and increase DS. To confirm the KOH:CP catalyst performance relative to the standard catalyst (KOH), different ratios of KOH:CP catalyst were used for the carboxymethylation reaction (2 h reaction at 70 °C in water using sugar:catalyst ratio of 1:1 and sugar:chloroacetate ratio of 1:1). The DS results are summarized in Table 1. No reaction was obtained when the reaction was carried out without catalyst. The standard KOH solution indicated low reaction efficiency with DS = 0.17. 11684

DOI: 10.1021/acs.iecr.9b02239 Ind. Eng. Chem. Res. 2019, 58, 11680−11688

Article

Industrial & Engineering Chemistry Research

0.4 for standard KOH. To reach a DS = 0.4 with KOH solution, the mole ratio of KOH:sugar was 1.75, whereas using KOH:CP, the DS = 0.62 was obtained at much lower mole ratio (1.25). Higher basicity of the base attached to the clinoptilolite is thought to be the reason for the higher DS with this base at lower concentration of base/sugar than the standard KOH solution.36 Effect of the molar ratio of the MCA to sugar on the DS for three levels of the molar ratio of the base to sugar is indicated in Figure 6B. A maximum DS of 0.67 was obtained using the 1.25 molar ratio of sodium monochloroacetate (MCA) to hemicelluloses (MCA:sugar) and 1.5 molar ratio for the base catalyst per sugar. To compare our results with published research reports, a series of data are organized in Table 3. All of the published results reported lower DS values than the DS achieved in the present study (DS 0.66). There are few reports in the literature of the carboxymethylation of hemicellulose, and no report has been published for carboxymethylation of hemicellulose with KOH or KOH-supported zeolite. 3.6. Analysis of Carboxymethylated Hemicellulose. 3.6.1. FT-IR Spectra of Carboxymethyl Hemicellulose. FT-IR spectra of the hemicellulose and their carboxymethylated derivatives are illustrated in Figure 7. The absorbance around 3359, 2884, 1637, 1380, 1248, 1037, and 890 cm−1 are associated with the unmodified hemicellulose chemical structure.41,42 A sharp band at 897 cm−1 is assigned to βglucosidic linkages between the sugar units, indicating that βform bonds link the xylose residues forming the backbone of the macromolecule.42 The region between 1412 and 1049 cm−1 relates to the frequencies of C−H and C−O stretching bonds. At 3359 cm−1 a strong broad band belongs to hydroxyls with hydrogen bonding, and at 2915 cm−1 a symmetric band relates to C−H stretching.42 In the spectrum of carboxymethylated hemicellulose with KOH (Table 3, entry 5) and with KOH:CP (1:2) as a catalyst (Table 3, entry 6), the presence of new bands occurs relative to the unmodified hemicellulose. A strong signal at 1595 cm−1 for KOH:CP catalyst and KOH modified hemicellulose is related to the salt form of the COO− group in carboxymethylated hemicellulose. These new bands in the spectra confirmed carboxymethylation of the hemicelluloses. The intensity of the carboxyl band in the sample prepared using KOH:CP catalytic is much stronger than the sample prepared using standard

balance of KOH bonded with CP and free in the solution leads to the best reaction condition. As a standard method for evaluation of the effect of different solvents mixed with water on reaction performance and to compare the results with reported articles, three different alcohols with ratio percentages of 10, 25, and 35 were used, and the results are tabulated in Table 2. Table 2. Effect of Solvents and Their Mixing Ratio on the Degree of Substitution for the Carboxymethylation of Hemicellulose with KOH:CP (1:2)a entry

water/alcohol

ratio (v/v)

DS

1 2 3 4 5 6 7 8 9

water water/t-butanol water/ethanol

100 75/25 100/00 75/25 65/35 90/10 75/25 65/35 90/10

0.23 ± 0.03 0.25 ± 0.01 0.32 ± 0.03 0.29 ± 0.02 0.28 ± 0.02 0.12 ± 0.01 0.45 ± 0.03 0.35 ± 0.02 0.33 ± 0.02

water/isopropanol

a

Measurements performed in triplicate, and standard deviation is indicated. T = 70 °C; t = 2.0 h; sugar:cat (1:1); sugar:MCA (1:1).

Similar experiments with standard catalysts (NaOH) using pure alcohol without water presented poor results probably due to the insolubility of hemicellulose in the pure alcohols. Addition of water into the alcohol improved the reaction DS for all conditions. t-Butanol indicated poor results probably due to its low solubility in water. Isopropanol indicated the best reaction DS at a 25% (v/v) level with water, suggesting a good balance of flower water concentration but still reasonable hemicellulose solubility. The results indicate that the presence of alcohol improves KOH:CP performance through increasing basicity, similar to standard catalyst. The catalyst concentration, ratio of carboxymethylation reagent to sugar, and temperature have been reported to have major effects on the reaction yield, side reaction, and chemical properties of the final product.13 The effect of ratio of the catalyst base to sugar on the DS for the standard KOH and KOH:CP catalyst systems is indicated in Figure 6A. For both systems, the DS increased with increasing mole ratio of base and reached a maximum DS of 0.62 for KOH:CP (1:2) and

Figure 6. (A) Effect of base concentration on DS, T = 70 °C; t = 2.0 h. (B) Effect of the molar ratio of the MCA to sugar on the DS for three levels of the molar ratio of base to sugar (T = 70 °C; t = 2.0 h). 11685

DOI: 10.1021/acs.iecr.9b02239 Ind. Eng. Chem. Res. 2019, 58, 11680−11688

Article

Industrial & Engineering Chemistry Research

Table 3. Comparison of the KOH:CP and KOH (Present Work) with Other Reported Catalysts for the Carboxymethylation Reaction entry

catalyst

sugar

solvent

temp (°C)

reaction time (min)

1 2 3 3 4 5 5 6

NaOH NaOH NaOH/μ.w NaOH NaOH NaOH KOH KOH:CP

xylan hemicellulose hemicellulose xylan xylan xylan hemicellulose hemicellulose

H2O EtOH/H2O EtOH/H2O i-PrOH/H2O i-PrOH/H2O i-PrOH/H2O i-PrOH/H2O i-PrOH/H2O

70 65 65 55 55 55 70 50

120 75 40 60 300 300 120 45

Figure 7. FT-IR spectra of unmodified hemicellulose and carboxymethylated hemicellulose with KOH as catalyst (Table 3, entry 5) and with KOH:CP as catalyst (Table 3, entry 6).

molar ratio OH/AGU

molar ratio MCA/ AGU

DS

ref

1.5 1.5 1.0 4.1 1.5 1.5 1.5 1.5

1.5 1.5 2.0 1.5 1.5 2.5 1.25 1.25

0.21 0.56 0.39 0.50 0.23 0.58 0.44 0.66

11 13 16 38 39 40 this work this work

−

Figure 8. Viscosity of carboxymethylated hemicellulose versus shear rate.

method (KOH solution). This confirms the titration evidence for the better performance of the KOH:CP catalyst for hemicellulose carboxymethylation. 3.7. Viscosity of Carboxymethylated Hemicellulose. The effects of carboxymethylation on the chemically modified polymer’s molecular weight and viscosity were reported.43,44 In more stable polymers like cellulose, samples with higher DS were reported to have higher viscosity.45 However, with branched natural polymers, generally achieving high DS has a negative effect on molecular weight and viscosity.46 To understand the effect of the carboxymethylation process used herein on hemicellulose molecular structure, the viscosity of the samples and the effect of shear rate on the viscosity were determined. The viscosities of the carboxymethylated hemicellulose prepared using KOH catalyst (DS = 0.44, Table 3, entry 5, and DS = 0.21) and the KOH:CP catalyst (DS = 0.66, Table 3, entry 6, and DS = 0.21) were measured using a Brookfield viscometer (DC-II + Pro, spindle #61 at 22 °C and 4% solids concentration) (Figure 8). The results indicate that the products prepared with the KOH:CP catalyst have 2.5−3 times higher viscosity than those prepared with the KOH alone. Based on the DS results of the products, the effect of the type of catalyst significantly outweighs the effect of DS on viscosity. This indicates that less degradation occurred with the KOH:CP catalyst relative to the standard KOH solution. In addition, these materials appear to be shear thinning, with about 20−25% viscosity drop over the conditions evaluated. 3.8. Thermal Analysis of Carboxymethylated Hemicellulose. Differential thermogravimetric analysis data of the hemicellulose sample and its modified samples using KOH (DS = 0.44, Table 3, entry 5) and the KOH:CP catalyst (DS = 0.66, Table 3, entry 6) are presented in Figure 9. All samples were heated to 550 °C at 10 °C/min under nitrogen gas, and

Figure 9. TGA of hemicellulose and carboxymethylated hemicellulose before (A) and after (B) acid wash. All samples were heated using 10 °C/min to 550 °C under nitrogen gas, and then the gas was switched to oxygen and the samples heated up to 700 °C.

11686

DOI: 10.1021/acs.iecr.9b02239 Ind. Eng. Chem. Res. 2019, 58, 11680−11688

Article

Industrial & Engineering Chemistry Research then the gas was switched to oxygen and heated at 10 °C/min to 700 °C. The unmodified hemicellulose (Figure 9A) displays a main mass loss of degradation in a range from 275 to 325 °C. At the end of the ramp to 550 °C under nitrogen gas, around 20% char remains. After switching to oxygen gas, the remaining char burns; however, around 3% of the total weight remains which is an indication of inorganic ash content in the unmodified hemicellulose sample. For the carboxymethylated samples, the main degradation and weight loss occurred in the 285−325 °C range, Figure 9A. The remaining mass at the end of the degradation under nitrogen gas to 550 °C was around 30% and then after heating to 700 °C in oxygen was reduced to around 20%, which is about 17% more than the unmodified hemicellulose sample. Higher residual materials after burning carboxymethylated samples relative to the unmodified hemicellulose sample can originate from the presence of potassium in salt form of the modified hemicellulose. To evaluate this conclusion, the same thermal analysis of acid-washed carboxymethylated samples was carried out (Figure 9B). The results indicated that the acid-washed residual mass was much lower than the non-acid washed sample and very similar to the unmodified and nonacid washed hemicellulose. Assuming 17% of the residual mass is sodium in the nonacid washed samples, the DS of the product can be roughly estimated to be around 0.72 which is in general in the range near the DS calculated by titration. The DS from TGA was roughly estimated as 0.17*195/46 where 0.17 is the mass fraction of ash after oxygen burning, 195 is the estimated molecular weight of the carboxymethylated hemicellulose (DS = 0.66), and 46 is the molar mass of sodium in Na2O after oxidation.

ORCID

Richard Venditti: 0000-0002-7986-4092 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge the Qaemshahr Branch, Islamic Azad University, for providing the essential financial support for their visit to the Laboratories of Professor Richard Venditti in the Forest Biomaterials Department at NCSU.

■

(1) Crini, G. Recent developments in polysaccharide-based materials used as adsorbents in wastewater treatment. Prog. Polym. Sci. 2005, 30, 38−70. (2) Leszczyńska, A.; Radzik, P.; Szefer, E.; Mičušík, M.; Omastová, M.; Pielichowski, K. Surface modification of cellulose nanocrystals with succinic anhydride. Polymers 2019, 11, 866−889. (3) Chen, Y.; Wan, J.; Dong, X.; Ma, Y. Fiber properties of eucalyptus kraft pulp with different carboxyl group contents. Cellulose 2013, 20, 2839−2846. (4) Farhat, W.; Venditti, R. A.; Hubbe, M.; Taha, M.; Becquart, F.; Ayoub, A. A Review of water-resistant hemicellulose-based materials: processing and applications. ChemSusChem 2017, 10, 305−323. (5) Li, Z.; Pan, X. Strategies to modify physicochemical properties of hemicelluloses from biorefinery and paper industry for packaging material. Rev. Environ. Sci. Bio/Technol. 2018, 17, 47−69. (6) Yang, J.; Fang, L.; Tan, T. Synthesis and characterization of superabsorbent hydrogels composites based on polysuccinimide. J. Appl. Polym. Sci. 2006, 102, 550−557. (7) Gonzalez, J.; Cruz, J. M.; Dominguez, H.; Parajo, J. C. Production of antioxidants from Eucalyptus globulus wood by solvent extraction of hemicellulose hydrolysates. Food Chem. 2004, 84, 243− 251. (8) Srivastava, U.; Kawatra, S. K.; Eisele, T. C. Study of organic and inorganic binders on strength of iron oxide pellets. Metall. Mater. Trans. B 2013, 44, 1000−1009. (9) Melandri, D.; De Angelis, A.; Orioli, R.; Ponzielli, G.; Lualdi, P.; Giarratana, N.; Reiner, V. Use of a new hemicellulose dressing (veloderm) for the treatment of split-thickness skin graft don or sites. Burns 2006, 32, 964−972. (10) Galbe, M.; Zacchi, G. A review of the production of ethanol from softwood. Appl. Microbiol. Biotechnol. 2002, 59, 618−628. (11) Konduri, M. K. R.; Fatehi, P. Synthesis and characterization of carboxymethylated xylan and its application as a dispersant. Carbohydr. Polym. 2016, 146, 26−35. (12) Ren, H.; Omori, S. Comparison of hemicelluloses isolated from soda cooking black liquor with commercial and bacterial xylan. Cell. Chem. Technol. 2014, 48, 675−681. (13) Ren, J. L.; Sun, R. C.; Peng, F. Carboxymethylation of hemicelluloses isolated from sugarcane bagasse. Polym. Degrad. Stab. 2008, 93, 786−793. (14) Saghir, S.; Iqbal, M. S.; Hussain, M. A.; Koschella, A.; Heinze, T. Structure characterization and carboxymethylation of arabinoxylan isolated from Ispaghula (Plantago ovata) seed husk. Carbohydr. Polym. 2008, 74, 309−317. (15) Petzold-Welcke, K.; Schwikal, K.; Daus, S.; Heinze, T. Xylan derivatives and their application potential. Carbohydr. Polym. 2014, 100, 80−88. (16) Peng, X. W.; Ren, J. L.; Zhong, L. X.; Cao, X. F.; Sun, R. C. Microwave-induced synthesis of carboxymethyl hemicelluloses and their rheological properties. J. Agric. Food Chem. 2011, 59, 570−576. (17) Khalilzadeh, M. A.; Hosseini, A.; Pilevar, A. Potassium fluoride supported on natural nanoporous zeolite: a new solid base for the synthesis of diaryl ethers. Eur. J. Org. Chem. 2011, 2011, 1587−1592.

■

CONCLUSIONS A new catalyst was developed using a mixture of clinoptilolite (CP) with KOH. Depending on the KOH:CP ratio, a portion of the KOH adsorbed on the surface of the CP. The fraction of the KOH adsorbed on the CP surface increased with increased KOH:CP ratio. Up to 80% of the KOH remained on the surface of the catalyst with a KOH:CP ratio of 1:2 after mixing with water. The prepared catalyst indicated better efficiency in comparison with standard KOH solution for the hemicellulose carboxymethylation reaction. The catalyst with KOH:CP ratio of 1:2 displayed about two times greater reaction efficiency than the KOH solution. The expected carboxymethylation reaction was confirmed using FTIR, chemical titrations, and thermogravimetric analysis. The maximum obtained DS using the KOH:CP catalyst was DS = 0.66 using 1.25 molar ratio of sodium monochloroacetate to hemicellulose (MCA:sugar) and 1.5 molar ratio for catalyst per sugar unit. For the KOH solution as catalyst, a lower maximum DS = 0.4 was achievable for 1.75 molar ratio of base:sugar unit. The viscosity of the carboxymethyl hemicellulose with the KOH:CP catalyst was around two times greater than that of the product prepared with the KOH solution, indicating less degradation of the hemicellulose during reaction. These results indicate that the KOH:CP has significant potential for improving the carboxymethylation reaction of hemicellulose and potentially other carbohydrates, using less base with less degradation.

■

REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 11687

DOI: 10.1021/acs.iecr.9b02239 Ind. Eng. Chem. Res. 2019, 58, 11680−11688

Article

Industrial & Engineering Chemistry Research (18) Khalilzadeh, M. A.; Keipour, H.; Hosseini, A.; Zareyee, D. KF/ Clinoptilolite, an effective solid base in Ullmann ether synthesis catalyzed by CuO nanoparticles. New J. Chem. 2014, 38, 42−45. (19) Choo, H.; Kevan, L. Catalytic study of ethylene dimerization on Ni(II)-exchanged clinoptilolite. J. Phys. Chem. B 2001, 105, 6353− 6360. (20) Howery, D. G.; Thomas, H. C. Ion exchange on the mineral clinoptilolite. J. Phys. Chem. 1965, 69, 531−537. (21) Johnson, M.; O’Connor, D.; Barnes, P.; Catlow, C. R. A.; Owens, S. L.; Sankar, G.; Bell, R.; Teat, S. J.; Stephenson, R. Cation exchange, dehydration, and calcination in clinoptilolite: In situ X-ray diffraction and computer modeling. J. Phys. Chem. B 2003, 107, 942− 951. (22) Mumpton, F. A. La roca magica: uses of natural zeolites in agriculture and industry. Proc. Natl. Acad. Sci. U. S. A. 1999, 96, 3463−3470. (23) Caputo, D.; Pepe, F. Experiments and data processing of ion exchange equilibria involving Italian natural zeolites. Microporous Mesoporous Mater. 2007, 105, 222−231. (24) Shokuhi Rad, A.; Zardoost, M. R.; Abedini, E. First-principles study of terpyrrole as a potential hydrogen cyanide sensor: DFT calculations. J. Mol. Model. 2015, 21, 273−279. (25) Rad, A. S.; Ayub, K. Coordination of nickel atoms with Al12 × 12 (X = N, P) nanocages enhances H2 adsorption: A surface study by DFT. Vacuum 2016, 133, 70−80. (26) Gholami, S.; Shokuhi Rad, A.; Heydarinasab, A.; Ardjmand, M. Adsorption of adenine on the surface of Nickel-decorated graphene; a DFT study. J. Alloys Compd. 2016, 686, 662−668. (27) Padash, R.; Rahimi-Nasrabadi, M.; Shokuhi Rad, A.; SobhaniNasab, A.; Jesionowski, T.; Ehrlich, H. A Comparative computational investigation of phosgene adsorption on (XY)12 (X = Al, B and Y = N, P) nanoclusters: DFT investigations. J. Cluster Sci. 2019, 30, 203− 218. (28) Mirmotahari, M.; Sani, E.; Shokuhi Rad, A.; Khalilzadeh, M. A. Calcium-doped single-wall nanotubes (Ca/SWCNTs) as a superior carrier for atropine drug delivery: a quantum-chemical study in gas and solvent phases. J. Biomol. Struct. Dyn. 2019, 1. (29) Bian, J.; Peng, F.; Peng, X. P.; Xu, F.; Sun, R. C.; Kennedy, J. F. Isolation of hemicelluloses from sugarcane bagasse at different temperatures: Structure and properties. Carbohydr. Polym. 2012, 88, 638−645. (30) Sluiter, A.; Hames, B.; Ruiz, R.; Scarlata, C.; Sluiter, J.; Templeton, D.; Crocker, D. Determination of structural carbohydrates and lignin in biomass; NREL/TP-510-42618; National Renewable Energy Laboratory Analytical Procedure: Golden, CO, 2008. (31) Lawal, O. S.; Lechner, M. D.; Kulicke, W. M. Single and multistep carboxymethylation of water yam (Dioscorea alata) starch: Synthesis and characterization. Int. J. Biol. Macromol. 2008, 42, 429− 435. (32) Li, X.; Gao, W. Y.; Huang, L. J.; Wang, Y. L.; Huang, L. Q.; Liu, C. X. Preparation and physicochemical properties of carboxymethyl Fritillaria ussuriensis Maxim. starches. Carbohydr. Polym. 2010, 80, 768−773. (33) Frisch, M. J.; et al. Gaussian 09, revision E.01; Gaussian, Inc.: Wallingford, CT, 2009. (34) Chai, J. D.; Head-Gordon, M. Long-range corrected hybrid density functionals with damped atom−atom dispersion corrections. Phys. Chem. Chem. Phys. 2008, 10, 6615−6620. (35) Shokuhi Rad, A.; Zareyee, D.; Pouralijan Foukolaei, V.; Kamyab Moghadas, B.; Peyravi, M. Study on the electronic structure of Al12N12 and Al12P12 fullerene-like nano-clusters upon adsorption of CH3F and CH3Cl. Mol. Phys. 2016, 114, 3143−3149. (36) Montegut, G.; Michelin, L.; Brendle, J.; Lebeau, B.; Patarin, J. Ammonium and potassium removal from swine liquid manure using clinoptilolite, chabazite and faujasite zeolites. J. Environ. Manage. 2016, 167, 147−155. (37) Togrul, H.; Aralan, N. Production of carboxymethyl cellulose from sugar beet pulp cellulose and rheological behaviour of carboxymethyl cellulose. Carbohydr. Polym. 2003, 54, 73−82.

(38) De Mattos, N. R.; Colodette, J. L.; De Oliveira, C. R. Alkaline extraction and carboxymethylation of xylans from corn fiber. Cellulose 2019, 26, 2177−2189. (39) Petzold, K.; Schwikal, K.; Heinze, T. Carboxymethyl xylansynthesis and detailed structure characterization. Carbohydr. Polym. 2006, 64, 292−298. (40) Alekhina, M.; Mikkonen, K. S.; Alén, R.; Tenkanen, M.; Sixta, H. Carboxymethylation of alkali extracted xylan for preparation of bio-based packaging films. Carbohydr. Polym. 2014, 100, 89−96. (41) Fang, J. M.; Sun, R. C.; Tomkinson, J.; Fowler, P. Acetylation of wheat straw hemicellulose B in a new non-aqueous swelling system. Carbohydr. Polym. 2000, 41, 379−387. (42) Dace, R.; Mcbride, E.; Brooks, K.; Gander, J.; Buszko, M.; Doctor, V. M. Comparison of the anticoagulant action of sulfated and phosphorylated polysaccharides. Thromb. Res. 1997, 87, 113−121. (43) Ding, J.; Wang, Y.; Xiong, S.; Zhao, S.; Huang, Q. Optimised methodology for carboxymethylation of (1,3)-β-D-glucan from yeast (Saccharomyces cerevisiae) and promotion of mechanical activation. Int. J. Food Sci. Technol. 2013, 48, 253−259. (44) El-Sheikh, M. A. Carboxymethylation of maize starch at mild conditions. Carbohydr. Polym. 2010, 79, 875−881. (45) Varshney, V. K.; Gupta, P. K.; Naithani, S.; Khullar, R.; Bhatt, A.; Soni, P. L. Carboxymethylation of α-cellulose isolated from Lantana camara with respect to degree of substitution and rheological behavior. Carbohydr. Polym. 2006, 63, 40−45. (46) Wendling, R. J.; Christensen, A. M.; Quast, A. D.; Atzet, S. K.; Mann, B. K. Effect of carboxymethylation on the rheological properties of hyaluronan. PLoS One 2016, 11, 1−15.

11688

DOI: 10.1021/acs.iecr.9b02239 Ind. Eng. Chem. Res. 2019, 58, 11680−11688