Catalytic mechanism of sulfuric acid in cellulose pyrolysis: A combined experimental and computational investigation

Highlights

• Theoretical and experimental methods are used to reveal H₂SO₄ catalytic mechanism.

• H₂SO₄-assisted reactions are investigated via density functional theory calculation.

• H₂SO₄ accelerates all pyrolytic reactions by decreasing the activation energies.

• Original inconspicuous dehydration becomes very important in the catalytic process.

• Promoted dehydration greatly affects char yield and volatile products distribution.

Abstract

Sulfuric acid (H₂SO₄) is widely used as a strong acid catalyst in biomass pyrolysis process, and exhibits prominent catalytic effects on the pyrolytic reactions and product distribution. In this study, the fundamental catalytic mechanism of H₂SO₄ on cellulose pyrolysis process was investigated via combined experimental and computational methods. Both thermographic analysis (TGA) and pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS) experiments were performed to reveal the pyrolytic characteristics and product distribution in the H₂SO₄-catalyzed pyrolysis of cellulose. In addition, quantum chemistry methods were employed to build the reaction models and investigate the major H₂SO₄-assisted reactions in initial cellulose pyrolysis process, i.e., depolymerization of the cellulose chain, bridged dehydration, ring-opening, ring-contraction and dehydration of the free hydroxyl groups. The results indicate that the H₂SO₄-assisted depolymerization via C1-O1 bond scission and bridged dehydration reactions take place in a two-step mechanism involving a sulfate ester intermediate. While other reactions occur via a one-step mechanism involving only hydroxyl group or both hydroxyl and sulfonic groups of H₂SO₄. The activation energies of above reactions are all decreased by adding H₂SO₄, and thus lowering the degradation temperature of cellulose. Among all initial pyrolytic reactions, dehydration reactions which are inconspicuous in the non-catalytic process, become very important in the catalytic process, because their activation energies are decreased dramatically by adding H₂SO₄ (from 322.7 to 372.1 to 152.2–237.7 kJ/mol). The promoting effect on the dehydration reactions plays a vital role in the increased char yield and significant change of organic volatile product distribution. Through the promoted dehydration reactions, unsaturated structures of cellulose are formed, which is favorable for the formation of certain dehydrated products at the expense of depolymerized and ring scission products.

Introduction

Conventional fast pyrolysis of biomass is a non-selective thermal conversion process to obtain a very complex liquid product known as bio-oil. The complex chemical composition and poor properties of crude bio-oil significantly limit its utilization in current industries [1]. In order to solve this problem, catalytic pyrolysis has been proposed as a promising way to selectively control the biomass pyrolysis process towards specific bio-oils [2,3]. Inorganic acids have been widely utilized to catalyze the biomass pyrolysis process, including H₂SO₄ [4,5], phosphoric acid (H₃PO₄) [6,7] and nitric acid (HNO₃) [8]. These acid catalysts are very effective to significantly alter the biomass pyrolysis characteristics as well as product distribution, and can be utilized to prepare value-added chemicals [7,8].

H₂SO₄ is a common strong inorganic acid, and has been widely employed for catalytic pyrolysis of biomass and its primary components (cellulose, hemicellulose and lignin). The catalytic effect of H₂SO₄ on cellulose pyrolysis has been preliminarily elucidated as summarized by the following three aspects. Firstly, the initial decomposition temperature of cellulose will be significantly decreased by H₂SO₄. This fact has been confirmed by previous work [[7], [8], [9], [10]] according to TGA studies on the H₂SO₄ impregnated cellulose. Secondly, the promoting effect of H₂SO₄ on dehydration and charring reactions changes the yields of solid, liquid and gas products. Char and water yields increase, while that of organic liquid compounds (organic fraction of bio-oil) decreases. Such catalytic effects will be promoted along with the increase of H₂SO₄ content [[8], [9], [10], [11]]. Thirdly, pyrolytic product composition will also be altered by H₂SO₄. In regard to the liquid products, H₂SO₄ can significantly decrease the yield of levoglucosan (LG) which is the typical depolymerized product and also the most abundant pyrolytic product in bio-oil. Meanwhile, yields of several dehydrated products can be increased, such as 1,4:3,6-dianhydro-α-d-glucopyranose (DGP), furfural (FF) and levoglucosenone (LGO). Detailed H₂SO₄-catalyzed product distribution generally differed in previous studies due to different impregnation methods, pyrolytic conditions and H₂SO₄ contents [7]. However, the basic conclusions were not controversial based on the fact that H₂SO₄ could promote dehydration and cross-linking reactions [12].

Currently, the published studies on H₂SO₄-catalyzed pyrolysis of cellulose mainly paid attention to the catalytic behaviors of H₂SO₄, neglecting deep insight into the catalytic mechanism. In regard to the catalytic center of H₂SO₄ (proton or acid ion), different viewpoints were raised. Nimlos et al. [13,14] proposed the proton-catalyzed mechanism for ethanol pyrolytic reactions based on the quantum chemistry calculation. It was found that protonated ethanol/carbohydrates would readily undergo dehydration reactions with relatively low activation energies. On the contrary, Julien et al. [8] regarded that the pyrolysis reactions were mainly catalyzed by the sulfate ion. In comparison of catalytic pyrolysis of cellulose over different inorganic acids [[4], [5], [6], [7], [8]], both similarities and distinctions exist, indicating that both the proton and the acid ion should play vital roles in influencing the pyrolytic reactions of cellulose. In addition, the stability of ions is questionable under a fluent gas condition in the pyrolysis process, especially the carrier gases (usually N₂ and He) having low dielectric constants [15]. Therefore, it is necessary to consider the global effect of the acid molecule on the cellulose pyrolysis.

In the present study, catalytic effect of H₂SO₄ on cellulose pyrolysis was investigated in detail using computational chemistry methods. Three model compounds, i.e.,1,4-dimethyl-glucose, 4-methyl-glucose and 1-methyl-glucose were selected to investigate the effect of H₂SO₄ on major cellulose initial decomposition reactions, i.e., depolymerization of cellulose chain, bridged dehydration, ring-opening, ring-contraction and dehydration of the free hydroxyl groups, as shown in Fig. 1. Structural analyses and kinetic analyses were conducted based on the computational results. Meanwhile, TGA and Py-GC/MS experiments were performed to combine with the computational investigation to give an insight into the catalytic mechanism of H₂SO₄ on cellulose pyrolysis.