Review articleFrom synthesis gas production to methanol synthesis and potential upgrade to gasoline range hydrocarbons: A review
Introduction
Gasoline is an important liquid hydrocarbon-based fuel derived primarily from fractional distillation of petroleum fractions. It can be produced in a variety of grades depending on the demand and applications. The commodity with major application as fuel for internal combustion engines, comprised mainly of light to medium alkanes (straight chains and isomers), with certain concentrations of aromatics as octane enhancers (Erofeev et al., 2014, Song et al., 2015, Galadima et al., 2012), although their usage have been banned by environmental agencies in the recent times, due to associated environmental and health consequences (Ou et al., 2015, Agarwal et al., 2015). Globally, gasoline is popularly employed as a major commodity for transportation and fuel and petrochemicals-based industrial applications (Chang et al., 1976, McGillivray, 1976). While its global demand was projected to rise for many world regions, particularly due to increase in the number of automobiles and industrial-based internal combustion engines, the available crude oil reserves are on the decline (Owen et al., 2010, Campbell and Laherrère, 1998, Edwards, 1997). One major alternative given consideration today is the production from non-fossil sources. High octane gasoline can be produced from the hydrotreatment and subsequent hydroisomerization of vegetable oils. This technology is already under commercial consideration by the global refineries (Milne et al., 1990, Saxena and Viswanadham, 2014, Malleswara Rao et al., 2012). Several researches have also been published and are underway for this technology. Recently, García-Dávila et al. (2014) demonstrated the potential of jatropha oil for upgrade to linear and isomerized alkanes, with composition in the gasoline range, using supported nickel catalysts. Their hydrodeoxygenation-hydrocracking approach involved the initial conversion of the oil into high molecular weight alkanes that were subsequently cracked into gasoline range compounds. Several other authors have also employed different catalysts, vegetable oils and reaction conditions for this process (Maher and Bressler, 2007, Furimsky, 2000, Kubička et al., 2009, Huber et al., 2007, Liu et al., 2011, Charusiri et al., 2006). Another modern technology for gasoline production is the catalytic upgrading of methanol, a process otherwise called “methanol to gasoline” (MTG). Methanol has been successfully converted into a range of olefinic and aromatic hydrocarbons using different solid acid catalysts like zeolites and phosphate based catalysts (Aghamohammadi and Haghighi, 2015, Yaripour et al., 2015, Aghaei and Haghighi, 2015). The technology is therefore being modified towards limiting the reaction selectivity to these compounds with enhanced selectivity to gasoline range alkanes. One important issue of interest is the possibility of obtaining the feedstock at economical scale from known sources. Methanol can be derived from synthesis gas (H2, CO), which in principle can be produced from the reforming of abundant natural gas reserves or biomass-based materials.
The reforming process, which involved the reaction of methane gas derived from biomass or natural gas with carbon dioxide or water, is achieved catalytically at normally high reaction temperatures, under controlled pressure conditions to produce hydrogen-rich synthesis gas (Edwards and Maitra, 1995, Johnsen et al., 2006). The process was initially viewed as a future alternative for the production of clean hydrogen fuel. But recent studies, indicated a reasonable shift to its linkage as a background route for producing synthetic raw materials required by the energy and petrochemical industries, because the gaseous products could be upgraded to fuels and petrochemicals, especially via the Fisher–Trospch reaction (Surisetty et al., 2012, Hoek et al., 1985, Dry, 2002). Therefore, methanol is one of the key industrially-derivable petrochemicals from the reforming technology. Although methanol had been employed previously for a variety of applications, covering the production of pigments, plastics and paints, as solvent, wastewater denitrification, biodiesel production and in electricity generation by driving turbines (Kaeding and Butter, 1980, Pretzer and Kobylinski, 1980, Hamnett, 1997, McNicol et al., 1999), the current status of the energy industry gave a considerable emphasis to hydrocarbon production. The methanol to gasoline (MTG) technology is a forefront catalytic route under exploration. For these reasons, the global methanol production is reasonably high (Fig. 1) (The University of York, 2014), and the demand had been predicted to be on the increase (Herder and Stikkelman, 2004, Olah, 2005). The paper therefore presents a review on the MTG process with emphasis to updates on the catalyst systems under consideration. Important parameters such as the reaction conditions and reaction mechanisms have also been adequately reviewed. Recent updates on syngas production and methanol preparation from syngas were also initially discussed.
Section snippets
Syngas production
The modern methanol production technologies involved two major catalytic processes; the production of synthesis gas followed by upgrading the synthesis gas (i.e. syngas) into methanol. The principal raw materials for synthesis gas production are natural gas, methane gas from associated petroleum, shale gas, coal and biomass. Synthesis from fossil sources can be successfully achieved by the reforming technologies (i.e. dry and steam reforming) whereas pyrolysis and gasification processes for the
Methanol to gasoline (MTG)
Modern processes for the conversion of methanol to gasoline gave emphasis to the methanol production from syngas before subsequent upgrade to gasoline in the MTG operation unit. Unlike other synthetic methods for gasoline production, the methanol to gasoline route produce gasoline with compatible octane properties that is also free from impurities. At the industrial scale, the MTG process, which can be achieved at complete conversion, is exothermic with heat of reaction of 1.74 MJ/kg methanol (
Conclusions
There are indications that shale gas have good potential to compete with natural gas or biomass, as raw material for syngas production. However, cost-effective and efficient shale gas purification processes must be developed, considering its chemical compositions in comparison to other feedstocks. The reforming (i.e. dry and steam) technologies are considered to be very effective syngas production processes, with supported and/or promoted Ni-based systems as the main catalysts. However, the
Acknowledgments
The authors would like to acknowledge the funding provided by King Abdulaziz City for Science and Technology (KACST) through the Science & Technology Unit in Center of Research Excellence in Nanotechnology at King Fahd University of Petroleum & Minerals (KFUPM) for supporting this work through project No. 13-NAN1702-04 as part of the National Science, Technology and Innovation Plan.
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