Highly Selective Conversion of Cellobiose and Cellulose to Hexitols

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Highly Selective Conversion of Cellobiose and Cellulose to Hexitols by Ru-Based Homogeneous Catalyst under Acidic Condition Guozhen Wang, Xuefeng Tan, Hui Lv, Mengmeng Zhao, Min Wu, Jinping Zhou, Xumu Zhang, and Lina Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00518 • Publication Date (Web): 15 Apr 2016 Downloaded from http://pubs.acs.org on April 19, 2016

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Industrial & Engineering Chemistry Research

1

Highly Selective Conversion of Cellobiose and Cellulose to

2

Hexitols by Ru-Based Homogeneous Catalyst under Acidic

3

Condition

4

Guozhen Wang,† Xuefeng Tan,† Hui Lv,† Mengmeng Zhao,‡ Min Wu,‡ Jinping Zhou,

5

†

Xumu Zhang,*

,†

and Lina Zhang ,†

6 7 8 9 10

†

College of Chemistry and Molecular Sciences, Wuhan University, 430072, Wuhan,

China. ‡

Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, 100190,

Beijing, China

* To whom correspondence should be addressed. Phone: +86-27-87219274. Fax: +86-27-68754067.E-mail: [email protected], [email protected] (L. Zhang); [email protected] (X. Zhang).

ACS Paragon Plus Environment


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ABSTRACT：The catalytic transformation of the most abundant cellulose to valuable

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platform chemicals is one of the significant issues to overcome the shortage of fossil

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fuels. Herein, we reported the first example of Ru-based homogeneous catalyst for the

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highly selective conversion of cellobiose and ball- milled cellulose to hexitols

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(including sorbitol, mannitol and 1, 4-sorbitan) under an acidic condition with the

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yield of 94.5 and 56.4%, respectively. The main features of this catalytic system were

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the high conversion efficiency of biomass, mild reaction condition (100 C) and low

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catalyst loading, which was 1/20 of the related Ru/C heterogeneous catalyst. This

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work opened up a new avenue for the transformation of cellulose to hexitols under

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mild conditions.

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KEY WORDS: conversion of biomass, cellobiose, cellulose, hexitol, Ru-based

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homogeneous catalyst

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

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To decrease the dependence on fossil fuels, much attention has been paid to develop

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the ultraclean technologies such as hydrogen, wind, water and solar energy to resolve

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the energy issue.1-4 However, no organic compounds are produced in these processes.

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The conversion of renewable and sustainable biomass 5-6 is emerging as an important

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strategy to produce valuable organic materials and platform c hemicals.7-12 Cellulose, a

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linear polymer composed of glucose units linked by β-(1,4)-glycoside bonds, is the

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most abundant and non-edible lignocellulosic biomass on earth. 13 The annual net yield

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of photosynthesis production is 1.7 trillion tons for biomass, approximately 35–50%

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of which, is cellulose.14

INTRODUCTION

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It is noted that yet at present biorefineries are primarily concerned with the

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efficient conversion of cellulose into fuels and chemicals. 15 To address this point, the

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catalytic conversion of cellulose into organic monomers and platform chemicals has

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attracted much attention, and many excellent results have been reported. 16-24 For

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example, Fukuoka et al. studied the hydrolytic hydrogenation of cellulose into sorbitol

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and mannitol over a series of supported metal catalysts with a sorbitol yield of 58%

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by Ru/AC (AC = activated carbon) at 190 C in water.25 Zhang et al. developed a less

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expensive Ni-promoted W2 C/AC hydrogenation catalyst for the transformation of

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cellulose to ethylene glycol in aqueous system at 245 C.26 Tsubaki and co-workers

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prepared Pt nanocatalysts loaded on the reduced graphene oxide (Pt/RGO) for the

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selective hydrogenation of cellulose (or cellobiose) into sorbitol.27 The yield of

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sorbitol from cellobiose was 91.5% and from cellulose was 58.9%, respectively. Sels

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et al. studied the bifunctional Ru/H-USY catalysts for the conversion of cellulose in

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hot liquid water, and up to 95% hexitol yield could be obtained.24 As a result of the

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difficulty in the cellulose conversion, these systems were carried out under harsh



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conditions (e.g., 200–500 C, 30–100 atm H2 ) with relatively large amount of

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heterogeneous catalysts.28 It was worth noting that environmental- friendly

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homogeneous catalysts usually exhibited higher catalytic efficiency under mild

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reaction condition. One case was the reduction of aldehydes and ketones to alcohols

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using a Ru-diamine-diphosphine catalytic, which has been extensively studied by

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Noyori and others.29-30 However, most of the efficiently catalytic reaction for the

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reduction of carbonyls worked under the basic condition, which was unfavourable for

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the hydrolysis of hemiacetals in β-1, 4-glycosidic bonds. These catalysts could also be

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easily deactivated by multi- functional groups, such as amino and hydroxyl groups.

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For the hydrogenation of cellulose to hexitols, it may be highly desirable for an

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acid-assisted

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homogeneous catalyst in the conversion of cellulose to hexitols has never been

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reported. An effort should be made to develop a worthwhile homogeneous catalytic

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hydrogenation system, which is capable of efficiently and selectively transforming

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cellulose into hexitols with low catalyst loading under mild condition.

and

multi- functional

group-tolerant

catalyst.

However,

such

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In our previous work, an easily available ruthenium (II) catalyst, 31 which was

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coded as Ru1 (chemical structure is shown in Scheme 1), has been synthesized as a

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new compound for the catalytic hydrogenation of various aldehydes including glucose

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with excellent performance. As the rate-determining step for the hydrogenation of

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various aldehydes, Ru1 displayed high chemoselectivity in the presence of C=C bond

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and ketone group. Different from the situation in the hydrogenation of aldehydes, acid

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is essential for the cleavage of the glycosidic bond between the glucose units in

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cellobiose or cellulose. So the acidic medium was selected for the conversion of

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biomass. Herein, a novel homogeneous ruthenium catalyst/H2 SO 4 system for the

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hydrolysis and hydrogenation of cellobiose and cellulose to hexitols was studied. To


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achieve a high chemo-selectivity, the mild reaction condition (100 C) was used for

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the transformation of cellulose to hexitols. The catalytic transformation of the most

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abundant cellulose to valuable platform chemicals is very important to solve the

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shortage of fossil fuels.

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

EXPERIMENTAL SECTION

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Materials. Raw cellulose (cotton linter pulp) was supplied by Hubei Chemical

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Fiber Co. Ltd. China. To decrease the molecular weight and the crystallinity of the

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raw cellulose, ball- milling experiments were performed in batches of 10 g cellulose

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using several ZrO 2 balls (mass of 1.8 kg and the diameter of 2 cm) in a 2000 mL ZrO 2

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bottle for 30 h to obtain the ball- milled cellulose. The rolling speed was set at 540

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rpm. Cellobiose, H2 SO4 and BaCO 3 were of analytical grade and were purchased from

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Aladdin. Ru/C (5% Ru) and Pd/C (10% Pd) were purchased from Sigma-Adrich. All

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the chemical reagents were used without pretreatment.

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Catalyst Ru1 was synthesized according to our previous method.31 To screen out

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the best catalyst, another two catalysts, coded as Ru2 and Ru3 (chemical structure are

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also shown in Scheme 1), were prepared according to the references. 32-33 The catalysts

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inc luding Ru1, Ru2 and Ru3 were used in the homogeneous catalytic

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hydrogenation system of cellobiose and cellulose.

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General Procedure for the Hydrolysis/Hydrogenation Reaction. In a typical

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reaction, the desired amount of cellobiose or cellulose, catalyst, H2 SO 4 , water and

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iso-propanol were placed into a 100 mL stainless steel autoclave. The autoclave was

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purged by three cycles of pressurization/evacuating with N 2 , and then by three

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cycles of pressurization/venting with H2 (10 atm) before pressurized with H2 (50

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atm). The reaction mixture was stirred at a given temperature in an oil bath for the



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desired time. After reaction, the mixture was cooled to room temperature and the

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products were centrifuged. The resultant solution was neutralized with BaCO 3 and

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analysed by the high-performance liquid chromatography [HPLC; Agilent 1100 Series,

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refractive index detector (RID), Hi-Plex Ca column (300 × 7.7 mm), the mobile phase:

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ultra-pure water, the flow rate: 1.0 mL min-1 , column temperature: 80 C]. The

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conversion of cellobiose was determined by HPLC. The conversion of cellulose was

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calculated by the weight difference before and after the reaction. 34 The yield of each

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product was calculated as follows:35

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Yield  %  =

100%   mols of carbon in each product  mols of carbon in charged cellulose or cellobiose

(1)

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Characterizations. The morphology of cellulose was observed using scanning

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electron microscopy (SEM) with a field emission scanning electron microscopy

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(FESEM, Zeiss, SIGMA), with an accelerating voltage of 5 kV. The samples were

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coated with Au for the SEM observation.

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Solid-state

13 C

NMR spectra of the raw cellulose and the ball- milled cellulose 13 C

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were recorded on a BRUKER 500WC spectrometer. It was operated at a

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frequency of 125.88 MHz using the combined technique s of magic angle spinning

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(MAS) and cross-polarization. The spinning speed was set at 10 kHz for all samples.

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The contact time, acquisition time and recycle delay were 2 ms, 8 ms and 4s,

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respectively. A typical number of 10000 scans were acquired for each spectrum.

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X-ray diffraction (XRD) patterns were measured with a WAXD diffractometer

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(D8-Advance, Bruker, USA). The diffracted intensity of Cu Kα radiation (λ =

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0.15405 nm) was operated at 40 kV and 30 mA. The samples were measured in the 2θ

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region from 10 to 50°with a scanning rate of 2°/min. The crystallinity index (Icr) of

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cellulose was indicated on each curve and calculated as follows 36


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ICr = (I002 -Iam)/I002 ×100

(2)

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where I002 was the diffraction intensity of the crystalline plane (002) of cellulose at 2θ

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= 22.6°, and Iam was the intensity of the amorphous peak (2θ =18°), respectively.

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The intrinsic viscosity ([η]) of cellulose in LiOH/urea aqueous solution was

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measured at 25 °C by using an Ubbelohde capillary viscometer. The original

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concentration of cellulose was 3×10−3 gmL-1 . The viscosity molecular weight of

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cellulose was calculated according to the Mark-Houwink equation.37

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[η] = 3.72×10-2 Mw0.77 mLg-1

(3)

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 RESULTS AND DISCUSSION Hydrogenolysis of Cellobiose.

Cellobiose, a glucose dimer connected by one

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glycosidic bond, is the simplest model molecule of cellulose. 38 It was selected as the

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model substrate for the degradation of cellulose. 27,

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photographs of the products of the hydrolytic hydrogenation from cellobiose and

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cellulose. Obviously, the conversion of cellulose and cellobiose into hexitols by

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homogeneous catalyst under the optimal conditions (0.1 mol% Ru1, 0.5 mol·L-1

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H2 SO 4 , 50 atm H2 , 100 C, 16 h for cellobiose and 0.2 mol% Ru1, 1.5 mol·L-1 H2 SO4 ,

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50 atm H2 , 100 C, 20 h for cellulose) was successfully realized. Clear and transparent

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solution was obtained from cellobiose (Figure 1a), whereas brown and transparent

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supernatant liquid was obtained from cellulose (Figure 1b). Figure 1c performs the

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HPLC profiles for the hydrolytic hydrogenation products from cellobiose and

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cellulose by using Ru1 catalyst under the optimal hydrogenation conditions. It was

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interesting that there was only three major kinds of product (glucose, sorbitol and 1,

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4-sorbitan) generated in the cellobiose and cellulose systems. Trace amount of

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by-products mainly including 5- hydroxymethylfurfural (5-HMF) has been detected

39-41


Figure 1a and b show the


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which was partially contributed to the discoloration of the supernatant from cellulose.

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And the yield of 5-HMF in all samples were less than 0.1%. These results indicated

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that the Ru-based homogeneous catalyst had highly selectivity on the conversion of

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cellobiose and cellulose with high hexitol yield. It was worth noting that there was no

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mannitol generated in our homogeneous catalytic system, confirmed by HPLC

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profiles compared with the retention time of standards. This suggested that no

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epimerization of sorbitol occurred.

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The catalyst loading (catalyst/substrate mol%) played an important role in the

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hydrogenation of cellobiose. As listed in Table 1, when the catalyst loading increased

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from 0.01 to 0.1 mol%, the yields of sorbitol and 1, 4-sorbitan remarkably increased

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since the produced glucose was immediately converted into sorbitol. Once the catalyst

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loading was higher than 0.1 mol%, the yield of hexitol hardly changed. Because the

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catalyst reached up to saturation and almost equal amounts of sorbitol had been

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generated. More sorbitol had time to convert into 1, 4-sorbitan with higher catalyst

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loading. Therefore, 0.1 mol% catalyst loading was chosen for the hydrogenation of

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cellobiose. The results of cellobiose degradation at different H2 SO 4 concentration are

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shown in Figure 2. It was revealed that the hydrolysis of cellobiose strongly depended

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on the acid concentration, and 0.5 mol·L-1 H2 SO4 was the optimal condition. Initially,

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increasing the acid concentration could promote the cleavage of the β-(1, 4)- glycoside

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bond to produce more hexitols. When the acid concentration was too high, the sorbitol

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could be dehydrated to form a significant amount of 1, 4-sorbitan. Interestingly,

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cellobiose could be converted into sorbitol with high selectivity by three

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homogeneous catalysts (Ru1-Ru3), as shown in Figure 3. And Ru1 displayed higher

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catalytic activity than Ru/C when using identical amount of Ru in the same conditions.

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When the catalyst loading of Ru1 was 1/20 of Ru/C, almost the same yield of hexitols


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was obtained, as listed in Table 2. As shown in Figure 3, with the same loading of

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catalyst (Ru or Pd: 0.1 mol %), the hexitol yield was as follows: Ru1 > Ru2 > Ru/C >

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Ru3> Pd/C. Higher hexitol yield of Ru1 was as a result of the higher hydrogenation

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catalytic activity of Ru1. Once the glucose produced under the acidic condition, it

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was immediately converted into sorbitol with Ru1. The highest glucose yield was

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obtained when the Pd/C catalyst was used. It indicated that Pd/C performed the lowest

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hydrogenation activity for the produced glucose under the same conditions. The

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phosphine ligand of Ru1-Ru3 played an important role in the catalysts activity. The

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bidentate phosphine ligands, e.g. DPPP (Ru1) and BINAP (Ru2), were better than a

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monophosphine ligand, e.g. PPh3 (Ru3). Additionally, the more electron-rich of Ru1

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performed better than Ru2. It was found that no hexitol generated in the

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hydrogenation reaction in the absence of catalyst (Table 2, entry 3). The cellobiose

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was almost converted into glucose, but could not be further transformed into sorbitol

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as a result of the absence of ruthenium catalyst. Generally, the trace amount of

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homogeneous Ru-based catalysts was not recovered in organic synthesis which can

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avoid the complicated recovery process in industry. Similar to this situation, for the

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first employment of homogeneous Ru-based catalysts in the conversion of cellobiose

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and cellulose, we didn’t recover these catalysts.

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Effects of the Ball-Milling on Morphology and Structure of Cellulose. To

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increase the accessibility and the reaction activity of the cellulose hydrogenation,

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ball- milling was used for the cellulose pretreatment.42-45 Figure 4 shows the

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photographs and SEM images of cellulose before and after the ball- milling.

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Obviously, the raw cellulose retained fibrous shape with 10 μm width and 100-200

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μm length, whereas the ball- milled cellulose changed to small particles with

Highly Selective Conversion of Cellobiose and Cellulose to Hexitols

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