Startups promising to power planes, ships and trucks with clean fuel are sputtering before they get off the ground, showing how hard it will be to wean many industries off oil and gas. A company backed by United Airlines that raised hundreds of millions of dollars to turn trash into jet fuel appears to have shut down. “The excitement of the early days has not lived up to the hype,” said Andy Marsh, chief executive of Plug Power a startup that recently opened one of the country’s first plants making green hydrogen, a potential replacement for fossil fuels in industries such as steel making and chemical production.