Effect of the SiO2 support on the catalytic performance of Ag/ZrO2/SiO2 catalysts for the single-bed production of butadiene from ethanol
Graphical abstract
For ETB over Ag/ZrO/SiO2 catalysts, the choice of the SiO2 support has a significant effect on the butadiene production.
Introduction
With an annual production of 11 million tons/year, butadiene is the most important conjugated diene, being the basis of a wide variety of synthetic rubbers, elastomers, and polymer resins upon polymerization by itself or in conjunction with other polymerizable monomers [1]. Currently, butadiene is primarily obtained as a byproduct of the ethylene production by steam cracking. The amount of butadiene produced from a steam cracker depends on the composition of the cracking feedstock used [2]. Heavier feedstocks such as naphtha produce more butadiene than lighter feedstocks such as ethane [2]. For example, an ethane steam cracker typically produces ∼2 lb of butadiene per 100 lb of ethylene, while a naphtha steam cracker produces ∼16 lb of butadiene per 100 lb of ethylene [3]. Currently, ethane is produced inexpensively from shale gas, which has become the preferred feedstock for steam cracking units in North America. Hence, butadiene co-production has been in decline over the past decade. In addition to the shift in lighter feedstocks, crude oil price swings have historically led to corresponding price fluctuations in the cost of butadiene which is not ideal for end users. Thus, alternative technologies for producing butadiene are highly desired [4].
Ethanol-to-butadiene (ETB) represents an attractive alternative technology. Ethanol is commercially produced from renewable biomass or waste sources. In addition, the ethanol “blend wall” coupled with advancements in production efficiency and feedstock diversification will potentially lead to excess ethanol at competitive prices available for production of a wide range of fuels and commodity chemicals [5]. Furthermore, in recent publications, Patel et al. described an early-stage assessment method [6,7]. They compared the bioethanol-based pathway for butadiene production with the naphtha-based route and suggested that the bioethanol pathway could be a promising alternative to the naphtha-based process [6,7].
Research on butadiene production was initiated in the early 20th century and numerous articles have since been published. One-step and two-step processes were developed in the former Soviet Union and the United States, respectively, for ethanol conversion to butadiene. A one-step catalytic process developed for ethanol conversion to butadiene was commercialized for some time to produce synthetic rubber but was abandoned later due to low-cost oil making more convenient the production of butadiene from petrochemical sources [8]. In principle, the one-step process offers greater simplicity and lower operating costs as compared to two-step processes. However, achieving high yields to butadiene at industrially relevant process conditions has been challenging. A large number of catalytic systems (e.g., doped Al2O3, promoted MgO-SiO2, sepiolites, ZrO2-Fe2O3, and zeolite-based catalysts) that are capable of converting ethanol to butadiene in one processing step have been reported [[9], [10], [11], [12], [13], [14]]. These catalytic systems have been well summarized in recent reviews [1,4,8]. A number of studies have reported achieving 70 to 80% selectivity to butadiene. However, these results are usually obtained at low single-pass conversion (e.g., <45%) or low catalyst space velocities (e.g., < 0.20 h−1). For the single-step process to be commercially realized higher single pass conversions coupled with high selectivity (≥70%) are likely required. Recently, several catalytic materials have distinguished themselves from others for their high catalytic performance. Using a Cu1.0Hf3.0Zn0.5/SiO2 catalyst operating at 360 °C, weight hourly space volume (WHSV) = 0.21gEtOH gcat-1 h−1 and atmospheric pressure, 72% butadiene selectivity at 99% conversion was obtained [15]. Under similar conditions (350 °C, WHSV = 0.3gEtOH gcat-1 h−1, P = 1 atmosphere), 75% butadiene selectivity of was obtained at 100% conversion for a 2%Zn/8%Y/beta catalyst. Ag/ZrO2 catalysts supported on SiO2 and Zeolite Beta also have shown very promising performance levels [14,16]. A 1%Ag/10%ZrO2/SiO2 catalyst has been reported with catalytic performance achieving 74% butadiene selectivity and 88% conversion at 320 °C, WHSV = 0.04gEtOH gcat-1 h−1, and atmospheric pressure [17]. Under similar conditions and at low conversion a 1%Ag/1%ZrO2BEA was found to be four times more active than a 1%Ag/1%ZrO2/SiO2. However, commercialization of zeolite-based catalysts can be challenging because of the nature of chemicals used in the synthesis (e.g., hydrofluoric acid) and the waste produced (e.g. hydrochloric acid). In addition, pore confinement also can lead to increased catalyst deactivation by pore blocking and coking.
In this study, we have focused our understanding and further development of Ag/ZrO2/SiO2 catalysts for the ETB reaction. Although very encouraging results were obtained for the previously reported 1%Ag/10%ZrO2/SiO2 catalyst [17], increasing catalytic activity while maintaining high selectivity to butadiene, thus enabling faster throughput, is highly desirable for commercial application. Therefore, we have further investigated the AgZrO2/SiO2 catalyst system with the goal of better understanding its structure-function relationship and improving yield to butadiene through improved catalyst design. We have studied the role that each individual component, Ag, ZrO2, and SiO2, has on the reaction mechanism. A supported Ag catalyst was compared to supported precious metal (i.e., Ir and Pt) catalysts. We comparatively evaluated several classes of silica supports studied how the nature of the SiO2 support affects catalytic performance. We also evaluated catalyst lifetime and regenerability.
Section snippets
Catalysts Synthesis
A series of xAg/yZrO2/SiO2 catalysts were synthesized by incipient wetness impregnation of SiO2 with silver nitrate powder and zirconyl nitrate solution dissolved in deionized water. A total of 11 different SiO2 materials were used as supports: 636, 645, 646, 923 (Davisil), silica gel large pores (alfa Aeser), silica gels KSKG-GOST 3956-76 and KSMG-GOST 3956-76 (JSC Karpov), mesoporous silicas SBA-15 and SBA-16 (ACS Materials), fumed silica Aerosil 380 (Degussa) and L90 (Cab-O-Sil). After
Mechanistic considerations
The reaction mechanism for ETB has been studied extensively and many reaction mechanisms have been proposed [1,[19], [20], [21]]. The generally accepted reaction pathway involves the cascading sequence of reactions shown in Fig. 1 which includes dehydrogenation, condensation, and dehydration, occurring over a single multifunctional catalyst [1]. To verify the reaction pathway for Ag/ZrO2/SiO2 a space velocity study was conducted over the 4 A g/4ZrO2/SiO2-646 catalyst. Fig. 2 shows the evolution
Catalyst lifetime and regenerability
Catalyst stability has been a long-standing challenge for ETB and catalyst deactivation due to coking is a major issue [39]. Although very limited research has been done, one study has proposed the use of a process initiator (i.e., hydrogen peroxide) as a potential solution to limit coking [40]. However, the use of a process initiator might not be applicable to industrial processes. The stability and regenerability results obtained for 1 A g/4ZrO2/SiO2-SBA16 are shown in Fig. 8. Conversion
Conclusions
Various Ag/ZrO2/SiO2 catalysts were studied for use in single-step ETB conversion. The catalyst composition and the role of each component in the composition were examined by varying parameters such as the choice of metal promoter (i.e., Ag, Ir, or Pt), Ag loading, ZrO2 loading, and characteristics of SiO2 supports (e.g., acidity, surface area). Operating conditions such as space velocity and feed gas composition (e.g., pure N2 or N2/H2 co-feed) also were investigated. Our study has yielded
Acknowledgments
This work was financially supported by the U.S. Department of Energy (DOE) Bioenergy Technologies Office and performed at the Pacific Northwest National Laboratory (PNNL). PNNL is a multi-program national laboratory operated for DOE by Battelle Memorial Institute. Catalyst characterization equipment use was granted by a user proposal at the William R. Wiley Environmental Molecular Sciences Laboratory, which is a national scientific user facility sponsored by the DOE Office of Biological and
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