Article pubs.acs.org/IECR
Production of Light Hydrocarbons from Syngas Using a Hybrid Catalyst Davy L. S. Nieskens,*,† Aysegul Ciftci,† Peter E. Groenendijk,† Marco F. Wielemaker,† and Andrzej Malek‡ †
Dow Benelux B.V., Herbert H. Dowweg 5, 4542 NM Hoek, The Netherlands The Dow Chemical Company, 1776 Building, Midland, Michigan 48674, United States
‡
ABSTRACT: A hybrid catalyst consisting of a Cu-ZnO/Al2O3 methanol synthesis catalyst and a SAPO-34 molecular sieve was shown to convert syngas into a narrow mixture of short chain paraffins centered at C2−C3 with little methane and C5+ made. This product mix is expected to be an excellent cracker feedstock for the production of olefins. The hybrid catalyst system closely couples sequential reactions on each of the two independent catalysts. The function of each of the components was unraveled by performing experiments in which the reactor was loaded with discrete layers of the individual catalysts. Methanol (MeOH) synthesis over Cu-ZnO/Al2O3 is followed by methanol conversion to dimethyl ether (DME) + steam over the acidic SAPO-34 or Cu-ZnO/Al2O3 component. Simultaneously, the so generated water feeds into the water gas shift (WGS) reaction over Cu-ZnO/Al2O3, producing H2 + CO2, and the DME/MeOH undergoes methanol to olefins (MTO) conversion over SAPO-34. The olefins are subsequently hydrogenated to paraffins over the Cu-ZnO/Al2O3 and SAPO-34 catalysts using hydrogen from the feed or hydrogen produced in the WGS reaction. Comparison of fully mixed vs layered catalyst bed systems indicated that a mixed catalyst bed is preferred to give a high CO conversion and selectivity to desired paraffin products. This scheme enables high single stage conversion by consuming methanol, thereby removing the thermodynamic constraint on that reaction step. The interactions between all the reactants and catalysts in this system create a complex relationship that is probed in this paper.
1. INTRODUCTION Light olefins are the essential building blocks for the modern petrochemical industry and are indispensable to production of plastics, consumer goods, and construction materials, among many others. Today, they are sourced mainly from steam cracking of naphtha and increasingly shale gas ethane and natural gas condensate, with most of the balance generated from petroleum refining. The immense volatility of petroleum product pricing in recent decades, along with the North American shale gas developments, continues to provide incentive to enable diversification of light olefin resourcing from alternative feedstocks. A very versatile feedstock for the production of hydrocarbons is syngas (a mixture of carbon monoxide and hydrogen), which can be produced from a wide variety of carbon sources, such as natural gas, coal, and biomass. The Fischer−Tropsch (FT) process is an example of a direct route where olefins are made from syngas in a single reaction step. The direct routes typically face low yields to olefins, resulting in large separation and recycle costs as compared to some of the indirect routes. The Methanol To Olefins (MTO) process (optionally coupled with the Olefin Cracking Process (OCP)) is an example of an indirect route, practiced on large scale, where olefins are made © XXXX American Chemical Society
from syngas in several reaction steps, all taking place in dedicated reactors: (1) syngas generation from methane or coal, (2) methanol formation from syngas, (3) methanol to olefins reaction, and, optionally, (4) the Olefin Cracking Process (OCP) to convert higher olefins into additional ethylene + propylene. The process is able to produce light olefins in high yield from syngas and is currently implemented on a large scale in China. As such, it is a practical example of a syngas based route to hydrocarbons. In this paper we describe a process that uses a single reactor to convert syngas into light hydrocarbons using a so-called “hybrid catalyst”: see Figure 1. Chang et al. were the first to introduce this concept of a bifunctional or hybrid catalyst for the conversion of synthesis gas, where a methanol synthesis component (typically a (mixed) metal (oxide)) is coupled to a methanol conversion component (typically a molecular sieve).1−3 These hybrid catalysts have been used to directly convert syngas to products where methanol only acts as an Received: November 30, 2016 Revised: February 9, 2017 Accepted: February 9, 2017
A
DOI: 10.1021/acs.iecr.6b04643 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 1. Hybrid catalyst process concept.
of this process. The performance of each of the catalysts components at the hybrid catalyst process conditions will be demonstrated together with a systematic study of the impact of a progressively better mixed catalyst bed. We will explore the interactions between all the reactants and catalysts in this system and propose a reaction network.
intermediate species being produced and consumed in situ. The continuous removal of the methanol ensures that no thermodynamic constraints are imposed on the overall syngas feed conversion. This kind of mechanism is called a “drain-off” mechanism.4 There are many examples in the literature of the use of this hybrid catalyst concept targeting different products. The product slate can, in general, be influenced by choosing the appropriate molecular sieve/zeolite component within the hybrid catalyst system. DME can be made using a wide variety of zeolites, such as SAPOs,5 ZSM-5,6−10 Ferrierite,11 and Alumina.6 C2−C4 hydrocarbons are typically targeted using SAPO-34 as the molecular sieve component within the hybrid catalyst mix.12−16 C3−C4 hydrocarbons (LPG) are favored using BETA17−19 or USY20,21 type frameworks. Liquid hydrocarbons in the gasoline range can be made using ZSM522−34 or Ferrierite.35,36 A similar, yet distinctly different process concept is the approach where the syntheses of methanol, dimethyl ether (DME), and gasoline are integrated into one synthesis loop with the individual reactions still taking place in two separate reactors.37,38 The methanol synthesis component of the hybrid catalyst is typically a conventional copper−zinc methanol synthesis catalyst. However, as implied by Chang et al.1−3 the drain-off mechanism can rely on any composition capable of syngas to methanol or dimethyl ether (DME) conversion. Typically, the methanol synthesis component is a mixed metal oxide. One notable example alternative to copper−zinc is the chromium− zinc high temperature methanol synthesis catalyst recently studied by Jiao et al.,39 which has particularly low olefin hydrogenation activity, allowing for reasonable yields of olefins. Also a Zr−Zn based system allows for reasonable yields of olefins, as recently shown by Cheng et al.40 We envision a process that uses a hybrid catalyst to convert syngas into a mix of short chain paraffins where these paraffins can be used as a cracker feedstock for the production of olefins. The current paper focuses on the particular combination of a Cu-ZnO/Al2O3 based methanol synthesis catalyst and a SAPO34 molecular sieve to produce this desired mix of short chain paraffins. The very low loss of carbon to byproduct methane and higher carbon compounds adds to the economic potential
2. EXPERIMENTAL SECTION 2.1. Methanol catalyst. A commercially available methanol synthesis catalyst was used: HiFUEL R120. This is a material developed by Johnson Matthey and sold by Alfa Aesar (a Johnson Matthey company) in the form of 5.2 mm × 3.0 mm pellets. These pellets were crushed and sized to 40−80 mesh (180−425 μm). The catalyst is composed of copper and zinc oxide supported on alumina; see Table 1. Table 1. Elemental Composition of the HiFUEL R120 Methanol Synthesis Catalyst According to the Certificate of Analysis Provided by Alfa Aesar Component
Quantity (wt %)
CuO ZnO Al2O3 MgO Na2O Fe2O3 S (as SO3) Cl NiO
64.1 24.4 9.7 1.4 0.075 0.013 0.003 0.001
