Multi-scale complexities of solid acid catalysts in the catalytic fast pyrolysis of biomass for bio-oil production – A review

Highlights

• Aromatic hydrocarbon yield is correlated with shape selectivity of acid catalysts.

• Acid catalysts exhibit deactivation primarily because of coke formation.

• Metals in a bifunctional catalyst increase catalytic activity by creating Lewis acid.

• Controlled templating is one of the efficient strategies for hierarchical catalysts.

• Multi-functional catalyst could be a solution of complexities over acid catalysts.

Abstract

Despite remarkable progress in catalytic fast pyrolysis, bio-oil production is far from commercialization because of multi-scale challenges, and major constraints lie with catalysts. This review aims to introduce major constraints of acid catalysts and simultaneously to find out possible solutions for the production of fuel-grade bio-oil in biomass catalytic fast pyrolysis. The catalytic activities of several materials which act as acid catalysts and the impacts of Bronsted and Lewis acid site on the formation of aromatic hydrocarbons are discussed. Considering the complexity of catalytic fast pyrolysis of biomass with acid catalysts, in-depth understandings of cracking, deoxygenation, carbon-carbon coupling, and aromatization for both in-situ and ex-situ configurations are emphasized. The limitation of diffusion along with coke formation, active site poisoning, thermal/hydrothermal deactivation, sintering, and low aromatics in bio-oil are process complexities with solid acid catalysts. The economic viability of large-scale bio-oil production demands progress in catalyst modification or/and developing new catalysts. The potential of different catalyst modification strategies for an adequate amount of acid sites and pore size confinement is discussed. By critically evaluating the challenges and potential of catalyst modification techniques, multi-functional catalysts may be an effective approach for selective conversion of biomass to bio-oil and chemicals through catalytic fast pyrolysis. This review offers a scientific reference for the research and development of catalytic fast pyrolysis of biomass.

Introduction

Energy plays a vital role in economic development while adequate and sustainable energy is crucial for industrialization in any country. Global energy consumption by the entire human civilization is gradually increasing, and the energy consumption is comparatively higher in Asia and Pacific regions [1]. Fossil fuels meet a large part of energy demand, and according to a statistical review of global energy, the fractions of total energy are oil (32%), coal (27%), gas (22%), biomass (10%) and electricity (9%) [2]. Due to extensive utilization of fossil fuel resources, these non-renewable resources will not last forever as oil, coal and gas will be depleted in about 45, 120 and 60 years, respectively. Furthermore, dependence on imported fossil fuels poses economic threats to the sustainability of a country [3]. On the other hand, open burning is a common practice to deal with waste materials especially in Asian countries. Approximately, 7.3 × 10⁸ tons of biomass are burned per year in Asia as crop residue burning, forest fires, and grassland fires, etc [4]. Crop residue burning is not only common in Asia but also worldwide, which is a concern for air pollutant emissions and public health risks [5,6]. Among environmental pollutants, polycyclic aromatic hydrocarbons (PAHs) and alkyl polycyclic aromatic hydrocarbons (APAHs) are reported to create mutagenic and carcinogenic substances, and to cause developmental, reproductive, cardiology, hematology, neuro and immune-toxicities in the human body [7,8].

Concerns over fossil fuel depletion and environmental pollution with greenhouse gas emission are challenging issues to address climate change and mitigate energy consumption [9,10]. The promotion of renewable energy production is essential for sustainable development. Abundant and low-cost renewable energy sources, such as lignocellulosic biomass, organic wastes and polymers (plastic materials) have created a wide interest to produce biofuels and bio-based chemicals [3,11]. In the renewable fuels standard program of the United States (US), it has been proposed that 1.36 × 10⁸ L of renewable fuel can be generated from lignocellulosic biomass by 2022, and 56.9% production is expected from the agricultural residues [12].

A number of pathways for the conversion of biomass feedstock and waste materials to fuels or chemicals have been studied and some are still under development. Broadly, conversion pathways exist in two types: thermochemical and biological [13]. Different thermochemical approaches, like combustion, gasification, liquefaction and pyrolysis are applied. Among them, pyrolysis has received increased attention due to carbon neutrality and relatively inexpensive technology for the production of renewable energy [14,15]. Based on the heating rate, the pyrolysis process can be classified into three types: slow (0.1-1 °C/s), intermediate (1-200 °C/s) and fast pyrolysis (200-1000 °C/s or flash pyrolysis) [16]. Fast pyrolysis enhances bio-oil yield suppressing the char formation compared to slow and intermediate pyrolysis [16], and introducing catalysts in fast pyrolysis has become a prominent technology because of its potential to enhance the bio-oil quality [17], [18], [19].

The application of solid acid catalysts in pyrolysis of a wide range of feedstocks is of increasing importance because of efficient catalytic activity in response to aromatization, deoxygenation, reusability and thermal stability [20,21]. In last decades, various types of zeolites (ZSM-5, Y zeolite, MCM-41 zeolite, β-zeolite, SBA-15 zeolite and novel zeolite) [22], [23], [24], [25], [26], hierarchical catalysts [27,28], solid acid (silica-alumina) [29,30] and inexpensive solid materials (bentonite clay and activated red mud) [31], [32], [33] have been studied extensively in biomass pyrolysis and upgrading of bio-oil. Simultaneously, basic catalysts, like CaO and MgO have been applied in biomass catalytic fast pyrolysis considering their excellent de-acidification, resulting in lower acidity and higher heating value of bio-oil [34]. Fermoso et al. observed that MgO is an active catalyst for ketonization of carboxylic acid in catalytic fast pyrolysis of Eucalyptus woodchips [35]. Generally, basic catalysts are non-toxic and low-cost materials than solid acid catalysts [36] and have the potentials to crack large-molecular oxygenates, leading to an increase in smaller-molecular compounds in bio-oil [37]. The use of CaO was found effective to eliminate a significant amount of sugars and nitrogen-containing compounds and increase the aliphatic hydrocarbon in the bio-oil, however, less effective for aromatic hydrocarbon production than solid acid catalyst at the same operating conditions of pyrolysis [38,39].

Shape selectivity of solid acid catalysts is a crucial factor for catalytic fast pyrolysis that promotes the cleavage of C-C and C-O bonds on acid sites and increases the selectivity to hydrocarbons in bio-oil. Until recently, much research has been undertaken to determine effects of tuning the shape selectivity of catalysts in terms of particle size, shape, surface area, porosity and acidity in order to produce chemically stable and high-quality bio-oil with larger fractions of aromatics from catalytic fast pyrolysis of biomass [40,41]. Despite several improvements, a number of constraints exist including low yield and poor quality of bio-oil, diffusional complexity of catalyst, quick deactivation and poisoning of active sites of catalyst because of coke deposition. These limitations imply the necessity to further develop catalysts and catalytic fast pyrolysis [42], [43], [44], [45].

A number of review papers are focused on biomass structural complexity [46], pyrolysis reactor [16], operating conditions [24,47,48], catalytic pyrolysis mechanism [14,[49], [50], [51], pyrolysis kinetics [52], multi-scale complexity [23] and modeling of pyrolysis [53]. To the best of our knowledge, a review of multi-scale complexities of acid catalysts in fast pyrolysis is limited in the literature, which motivated us to write this paper. In the early part of this review paper, the potential of each configuration has been studied regarding the catalytic activity, product distribution and deactivation of catalysts.

It is necessary to study various acid catalysts before their application in catalytic fast pyrolysis of biomass. In order to give a brief idea, acidic zeolite and other heterogeneous acid catalysts including low-cost mesoporous materials (bentonite clay and acid-activated red mud) have been studied in this paper. The role of Bronsted and Lewis acid sites of acid catalysts on catalytic activity has been presented with reaction chemistry. Major chemical routes including cracking, carbon-carbon coupling reactions, steam reforming and aromatization have been presented to provide useful insights about the chemistry with an acid catalyst during catalytic fast pyrolysis of biomass. Several process complexities with acid catalysts including low catalytic activity, diffusional limitation along with coke formation, active site poisoning, sintering and thermal/hydrothermal deactivation have been discussed. The regeneration of spent catalysts is important from the point of view of catalyst expense and thus, much attention is given to this factor. Also, various approaches for catalyst modification towards efficient biomass conversion are outlined. At the end of this review, special attention is paid to the importance of multi-functional catalysts in pyrolysis. Based on the above review, conclusions and recommendations are offered to solve the current complexities of acid catalysts. Therefore, the review paper provides an in-depth understanding of the complexities of acid catalysts and simultaneously addresses catalytic activity for biomass catalytic fast pyrolysis.