Adsorption of Biomass-Derived Polyols onto MetalâOrganic

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Adsorption of biomass-derived polyols onto MOFs from aqueous solutions Hua Jin, Yanshuo Li, and Weishen Yang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01372 • Publication Date (Web): 08 Aug 2018 Downloaded from http://pubs.acs.org on August 8, 2018

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Industrial & Engineering Chemistry Research

Adsorption of biomass-derived polyols onto MOFs from aqueous solutions Hua Jin, † Yanshuo Li,*,† and Weishen Yang *,‡ †

School of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211,

China ‡

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of

Sciences, Dalian 116023, China

ABSTRACT Adsorption of biomass-derived polyols from dilute aqueous solutions as an alternative to energy-intensive distillation has a significant impact on the economics of biorefinery. A series of metal organic frameworks which differ in chemical functionality and topology (SOD and RHO) are screened as adsorbents in this study. It has been demonstrated that SIM-1 (Zn (almeIm)2, SOD topology, almeIm=4-methylimidazole-5-carbaldehyde) outperforms other studied MOFs, namely ZIF-8, ZIF-90, ZIF-93 and ZIF-97, in terms of the adsorption capacity. The hydrophobic/hydrophilic balance and aperture/pore size of adsorbents are two key factors controlling the adsorption capacity. The adsorption of C2-C3 polyols onto SIM-1 increased in the order of glycerol < ethylene glycol < 1,3-propanediol, which has a positive correlation with their octanol-water partition coefficients (Kow). In addition, SIM-1 exhibited preferential adsorption of 1,3-propanediol over glycerol, indicating the promising application of SIM-1 as adsorbent in the fermentation process of glycerol to 1,3-propanediol.

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1. INTRODUCTION Due to rising oil prices and greenhouse gas emissions, renewable biomass resources have been proposed as an ideal alternative to petroleum for the production of fuels and chemicals.1-4 Among the bio-based platform chemicals, biomass-derived polyols (e.g., ethylene glycol and glycerol) are widely used in manufacturing of polymer and fine chemicals.5,6 Currently, one of the main challenges that limits the utilization of biomass-derived polyols is their recovery from dilute aqueous system (i.e., the polyol concentration is usually less than 70 g/L), which required the development of high-efficiency polyol-water separation technology.7 Distillation, as a traditional separation process, is especially energy-intensive for separating the target product polyol with low vapour pressure by removing the large quantities of water. Moreover, the high temperature used in distillation is not suitable for oxygen rich bio-based polyols due to the high reactivity. Adsorption, that can recover polyols from water directly at ambient temperature, has been considered to be an economically attractive approach in view of its easy operation and recyclable adsorbents. Several zeolites such as MFI, MOR, BEA and FAU have been investigated for adsorption of polyols from aqueous solutions at dilute concentrations.8-10 The results demonstrated that the polyol adsorption is mainly related with the dispersion forces derived from the fit of the polyols in the zeolite pores and the hydrophobic/hydrophilic properties adjusted by zeolite Al content and silanol defects. Nevertheless, the precise control and fine tuning of the structure and property of zeolites to obtain excellent polyol adsorption ability is not feasible. Over the last decade, metal organic frameworks (MOFs), consisting of metal ions or clusters coordinated to organic linkers,11,12 have received considerable interest as promising porous materials with potential for separation,13-15 storage16 and other applications.17 MOFs offer greater structural diversity and chemical varieties as compared to zeolites due to the large choice of building blocks. Besides, MOFs can further be modified and functionalized by 2 ACS Paragon Plus Environment

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post-synthetic approaches,18-20 which endows them the designability and adjustability for specific separation process. Recently, MOFs have been gradually applied for adsorption and purification of biobased compounds from aqueous media with the growing number of chemically stable and water-stable MOFs.21-23 Saint Remi et al. demonstrated that butanol can be efficiently recovered from aqueous mixtures by ZIF-8 in the presence of ethanol and acetone, and the capacity of ZIF-8 for butanol significantly exceeds that of active carbon or silicalite.24 The recovery of various bio-based compounds (e.g., methanol, ethanol, propanol, butanol isobutanol, furfural and HMF) from aqueous solutions have been further evaluated by MOFs.25-28 It was demonstrated that hydrophobic MOFs possess relatively high affinity for the above-mentioned adsorbates. Alongside the experimental studies, the adsorption of water and C1-C4 alcohols in MOFs has also been investigated by molecular simulation to better elucidate the adsorption behaviour, which is helpful to screen and design novel adsorbents rapidly and efficiently.29,30 To the best of our knowledge, the separation of polyol-water mixtures using MOFs have never been reported up to now. In contrast to monohydric alcohol-water separation, polyol-water separation is much more challenging because polyols have stronger interactions with water via hydrogen bonding. Hence, choosing the appropriate polyol-MOF pair is extremely difficult. In this work, the adsorptive separation of polyols mainly 1,3-propanediol from aqueous solutions by MOFs was systematically investigated. Several MOFs which differ in chemical functionality and topology were employed to evaluate the structure-performance correlations (Figure 1).31-34 Among them, ZIF-8, ZIF-90 and SIM-1 have a SOD topology while ZIF-93 and ZIF-97 have a RHO topology. The imidazolate linkers for ZIF-8, ZIF-90, SIM-1 (or ZIF-93) and

ZIF-97

are

2-methylimidazole

4-methylimidazole-5-carbaldehyde

(mIm),

(almeIm)

and

imidazole-2-carboxaldehyde

(Ica),

4-hydroxymethyl-5-methylimidazole

(hymeIm), respectively. The adsorption isotherms of 1,3-propanediol onto the five MOFs were 3 ACS Paragon Plus Environment


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measured. In addition, the most effective material SIM-1 is selected for further investigation, containing the regeneration performance, performance comparison with traditional zeolites and the adsorption capacity for other C2-C3 polyols.

Figure 1. Crystal structures and organic links of MOFs used in this study. The yellow ball within the cage represents the free space. The aperture diameter and pore diameter of each MOF were given in the following format: MOF name [aperture diameter, pore diameter]. 2. EXPERIMENTAL SECTION 2.1. Chemicals All chemicals were used as received without further purification. Zn(NO3)2·6H2O, Zn(AC)2·2H2O, 2-methylimidazole (Hmim) and 4-methylimidazole-5-carboxaldehyde (almeIm) were obtained from Sigma-Aldrich, USA. Imidazole-2-carboxaldehyde (ICA) was from Alfa Aesar, USA. 4-Hydroxymethyl-5-methylimidazole (hymeIm) was provided by TCI (Shanghai) Development Co., Ltd., China. Methanol, ethanol, N,N-dimethylformamide (DMF), 1,3-propanediol, ethylene glycol and glycerol were purchased from Sinopharm Chemical Reagent Co., Ltd., China. 2.2. Synthesis of MOF Adsorbents The synthesis of ZIF-8, ZIF-90 and ZIF-93 were based on the methods reported by Jin et 4 ACS Paragon Plus Environment

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al.28 ZIF-97 and SIM-1 crystals were prepared according to the literature protocol with slight modification. As for ZIF-97, a solution of Zn(AC)2·2H2O (0.304 g) in 16 mL DMF and a solution of 4-hydroxymethyl-5-methylimidazole (0.538 g) in 20 mL DMF were combined and heated in an oven at 100 °C for 12 h. The resulting white powder was recovered by centrifugation, washed with methanol, and dried at 333 K. In a typical synthesis of SIM-1, a mixture of Zn(NO3)2∙6H2O (1.624 g), 4-methyl-5-imidazolecarboxaldehyde (2.409 g) and DMF (40 mL) was heated in an oven at 358 K for 72 h. The product was collected by centrifugation and washed with plenty of methanol. Then, the samples are dried at 353K overnight under vacuum. 2.3. Characterization Powder X-ray Diffraction (PXRD) patterns were obtained on Rigaku D/MAX 2500/PC instrument using Cu Kα radiation (λ=0.154 nm at 40 kV and 200 mA). Morphological features were observed by Scanning Electron Microscopy (Quanta 200 FEG, FEI Co., 30 kV). Nitrogen physisorption isotherms were measured at 77K, on a Quantachrome Autosorb Automated Gas Sorption instrument. Water adsorption/desorption isotherm of five studied MOFs were measured on a Micromeritics 3Flex Surface Characterization Analyzer. Contact angle measurements were carried out on a Drop Shape Analyzer (Kruss DSA 100). The MOF samples were firstly crushed with a mortar, and then pressed into a 14 mm diameter disk by pelleting machine. A drop of water was slowly dropped onto the sample with a microsyringe and the contact angle was measured using the corresponding software package. 2.4. Adsorption and Desorption Experiments Liquid-phase batch adsorption experiments were performed as follows: 0.05 g of adsorbents was added into an aqueous solution of single compound of polyol with a volume of 0.50 mL, and the mixture was kept at constant temperature for approximately 24 h. Then the mixture was centrifuged and the concentrations of polyol in the supernatants were determined 5 ACS Paragon Plus Environment


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by a high performance liquid chromatography (HPLC, Agilent 1260) equipped with a Shodex SC1011 column and an RI detector. The equilibrium uptake was calculated using mass balance equation qe 

V  (C0  Ce ) , where qe is the equilibrium adsorption capacity of polyol on m

MOFs (mg g-1), V the volume of the used polyol solution (mL), m the mass of used MOFs (g), and C0 and Ce the initial and equilibrium concentrations of the polyol solution (g L-1), respectively. Equilibrium isotherm studies were conducted with polyol aqueous solutions with the concentrations varying from 0.5-25 wt. % at 25 oC. The effect of temperature on the adsorption of MOFs for 1,3-propanediol was studied at 1.5-100 oC with the 5.0 wt. % 1,3-propanediol aqueous solution. The competitive adsorption of 1,3-propanediol and glycerol on SIM-1 was investigated at 25 oC using aqueous solutions containing 1,3-propanediol and glycerol with 1:1 mass ratio. The desorption of 1,3-propanediol from porous adsorbents was carried out by two conventional regeneration methods. During the solvent assisted regeneration, the spent adsorbents were washed with clean methanol at room temperature. As to thermal regeneration, the 1,3-propanediol loaded adsorbents were heated at 120 oC under vacuum. 3. RESULTS AND DISCUSSION 3.1. Characterization of the As-synthesized MOFs As determined by PXRD patterns (Figure 2a), the as-synthesized MOF samples exhibited the characteristic peaks in consistence with the previous reports. SEM images reveal that the particle size of ZIF-8, ZIF-90 and ZIF-93 is in nanometer scale while that of SIM-1 and ZIF-97 in micrometer scale (Figure 2b-f). The textural properties of the synthesized MOFs were examined by nitrogen sorption technique. The BET surface area (pore volume) of each framework was found to be 1339 (0.598), 1202 (0.543), 363 (0.248), 1185 (0.477), 401 m2 g-1 6 ACS Paragon Plus Environment

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(0.323 cm3 g-1) for ZIF-8, ZIF-90, SIM-1, ZIF-93 and ZIF-97, respectively.

Figure 2. (a) PXRD patterns of ZIF-8 (blue), ZIF-90 (black), SIM-1 (pink), ZIF-93 (wine), and ZIF-97 (olive). (b - f) SEM images of the MOFs mentioned above.

3.2. Adsorption of 1,3-Propanediol on MOFs The adsorption isotherms of 1,3-propanediol on the prepared MOFs are given in Figure 3. It can be seen that the isotherms on ZIF-90, ZIF-93, ZIF-97 and SIM-1 are of type I, with strong adsorption at low concentrations. However, the isotherm on ZIF-8 is of S-shaped type V, indicating the adsorption of weakly interacting adsorbate in the microporous framework. Table 1 gives the textural parameters and adsorption characteristics of five studied MOFs. The adsorption capacity of MOFs was proven to be independent with their BET surface area and total pore volume. The adsorption uptake of 1,3-propanediol at dilute concentrations (100 g L-1) increased in the sequence of ZIF-8 < ZIF-90 < ZIF-93 < ZIF-97 < SIM-1, which was found to be in agreement with their hydrophilic property (shown in Figure 4 and Figure 5). The equilibrium contact angles of water on ZIF-8, ZIF-90 and ZIF-93 are about 76 °, 65 ° and 52 °, 7 ACS Paragon Plus Environment


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respectively. The contact angle of water on ZIF-97 changed from 54 ° to 41 ° during the test process. The contact angle of water on SIM-1 cannot be measured because water was absorbed into SIM-1 within 3 seconds). The resulting order of hydrophilicity of MOFs was as follows: ZIF-8 < ZIF-90 < ZIF-93 < ZIF-97 < SIM-1. In addition, the hydrophilicity of MOFs could be reflected by uptake rate of water at low relative pressure from its water ad-/desorption isotherms (Figure 5). The obtained result was consistent with water contact angle measurement. Therefore, the adsorption capacity of MOF adsorbents for 1,3-propanediol is inseparable from their hydrophilic character. As can be deduced from the subtle difference between ZIF-90 and ZIF-8 (or ZIF-93 and ZIF-97) for the adsorption of 1,3-propanediol at low concentrations, the functional group within MOFs might not a significant influence. SIM-1 with minimum pore volume and pore size gave the best 1,3-propanediol adsorption performance, which is likely due to the confinement effects, i.e., the confinement of 1,3-propanediol in the MOFs pores is the driving force as well. These findings are consistent with those identified for the adsorption of polyols onto zeolites. It was worth noting that the the adsorbed 1,3-propanediol molecular in per cage of studied MOFs except ZIF-90 were fundamentally the same, demonstrating the same adsorption configuration of 1,3-propanediol in the pore of MOFs.


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Figure 3. Adsorption isotherms for 1,3-propanediol over the five adsorbents: ZIF-8 (black), ZIF-90 (blue), SIM-1 (olive), ZIF-93 (purple) and ZIF-97 (orange). 1,3-Propanediol adsorption was measured at least three times for each value given and the standard deviation is given as error bars.

Table 1. Textural Parameters and Adsorption Characteristics of Five Studied MOFs.

MOFs SIM-1 ZIF-97 ZIF-93 ZIF-90 ZIF-8

Surface Pore area volume (m2 g-1) (cm3 g-1) 363 401 1185 1202 1339

0.248 0.323 0.477 0.543 0.598

Pore size (Å) 9.1 15.8 17.9 11.2 11.6

Maximum Aperture Adsorption Surface adsorption size amount a decoration amount (Å) (mg g-1) (mg g-1) 3 -CHO 123 130 3.3 -OH 54 58 3.6 -CHO 47 53 3.5 -CHO 26 32 3.4 None 24 148

n 2.9 2.63 2.37 0.64 2.57

a

The adsorption amount measured at the 1,3-PDO concentration of 100 g L-1. n represents the adsorbed 1,3-propanediol molecular in per cage of MOFs.

Figure 4. The contact angle images of water droplets on MOFs: ZIF-8, ZIF-90, SIM-1, ZIF-93 and ZIF-97.



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Figure 5. Water ad-/desorption isotherms of MOFs at 25 oC: ZIF-8 (black), ZIF-90 (blue), SIM-1 (olive), ZIF-93 (purple) and ZIF-97 (orange). The adsorption isotherms of 1,3-propanediol on ZIF-90, SIM-1, ZIF-93 and ZIF-97 were fitted by the two commonly used isotherm models, Langmuir and Freundlich models.35 Langmuir isotherm: Ce  qe

C 1  e kL qmax qmax

1 n

Freundlich isotherm: log qe  log K F  log Ce where Ce (g L-1) is the equilibrium concentration of the 1,3-propanediol, qe (mg g-1) is the equilibrium adsorption amount, qmax is the maximum adsorption amount (mg g-1), k L (L g-1) is the Langmuir constant that characterizes the adsorption affinity between 1,3-propanediol and MOF adsorbents, K F (mg Ln g-(1+n)) represents the Freundlich constant that relates to the adsorption capacity of the adsorbent, and n denotes a measure of adsorption intensity. Normally, n great than unity implies chemisorptions process. The obtained parameters of Langmuir and Freundlich adsorption isotherm models were given in Table 2. As can be seen from the correlation coefficients (R2), the Langmuir model 10 ACS Paragon Plus Environment

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exhibited better fit for the 1,3-propanediol adsorption onto MOFs than the Freundlich model. The results indicated the occurrence of monolayer adsorption on the surface of MOFs with homogenously active adsorption sites. The value of k L calculated from the intercept increased in the following order: ZIF-90

Adsorption of Biomass-Derived Polyols onto MetalâOrganic

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